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Russian Chemical Reviews 70 (10) 809 ± 825 (2001) Nanochemistry of metals G B Sergeev Contents I. Introduction II. Synthesis and chemical reactions of metal nanoparticles III. Cryonanochemistry of metals IV. Ensembles involving nanoparticles V. Photochemistry and nanophotonics VI. Semiconductors and sensors VII. Catalysis on nanoparticles VIII. Nanoparticles in biology and medicine IX. Conclusion Abstract. metals of nanochemistry the on studies of results The The results of studies on the nanochemistry of metals published attention Primary generalised. are years recent in published in recent years are generalised. Primary attention is is centred and nanoparticles of synthesis the for methods the on centred on the methods for the synthesis of nanoparticles and their their chemical of stabilisation of means The reactions. chemical reactions.The means of stabilisation of nanoparticles nanoparticles which of atoms incorporate and metals individual involve which involve individual metals and incorporate atoms of several several metals physicochemical their as well as considered are metals are considered as well as their physicochemical properties. properties. Self-assembling described. are nanoparticles of processes Self-assembling processes of nanoparticles are described. The The prospects semiconductor in nanoparticles metal using of prospects of using metal nanoparticles in semiconductor devices, devices, catalysis, The discussed. are medicine and biology catalysis, biology and medicine are discussed. The bibliography bibliography includes references 165 includes 165 references. I.Introduction The last decade of the XXth century was marked by the increased attention of scientists in the fields of physics, chemistry, materials science, etc. devoted to nanoparticles, viz., to their synthesis, properties and different reactions. The reason for this lies in the fact that particles of nanometer sizes exhibit peculiar mechanical, optical, electrical and magnetic properties different from those of the corresponding macroparticles. Since 1990, the annual growth in publications devoted to both nanoparticles themselves and the prospects of their application in nanotechnologies has been observed. The methods of synthesis and stabilisation of nano- particles as well as their optical, magnetic and other properties have already been summarised.In particular, quite a number of books, collections of papers and surveys devoted to metal and semiconductor nanoparticles and materials fabricated on their basis have already been published.1 ±15 At the same time, studies on the chemical reactions that involve nanoparticles are still in their infancy, and even the keyword `nanochemistry' has appeared in reference journals relatively recently. This review is intended to make up for this gap. It mainly deals with chemical reactions that involve metal G B Sergeev Department of Chemistry, MV Lomonosov Moscow State University, Leninskie Gory, 119899 Moscow, Russian Federation. Fax (7-095) 939 02 83.Tel. (7-095) 939 54 42. E-mail: gbs@kinet.chem.msu.ru Received 28 March 2001 Uspekhi Khimii 70 (10) 915 ± 933 (2001); translated by T Ya Safonova #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n10ABEH000671 809 809 813 815 817 818 820 822 823 nanoparticles; however, a special section is devoted to the syn- thesis, stabilisation and physicochemical properties of nanopar- ticles based on individual metals, several metals and metal compounds. In this review, an attempt is undertaken to follow the main trends in the development of this new scientific field by consider- ing studies published mostly in 2000. Primary attention is given to the use of low temperatures in studies on the activities of metal nanoparticles and size effects.(The question of how the particle size affects its reactivity still remains one of most challenging and stubborn questions of modern chemistry.16) Self-assembling of nanoparticles and their application in catalysis and photocatalysis are discussed. Major problems of the chemistry of nanoparticles and nanomaterials are considered and approaches to their sol- ution are suggested. II. Synthesis and chemical reactions of metal nanoparticles Until recently, particles with sizes in the range from 1 to 100 nm were assigned to nanoparticles (in certain studies, this interval was even extended to 1000 nm), although, in accordance with the International System of Units (SI) (the prefix nano- is used for fraction units, viz., 1079), nanoparticles should involve only particles with sizes in the range from 1 to 10 nm, and it is more correct to consider 100-nm particles as ultradispersed ones and the materials based on them as macroscopic materials.Nanochemistry studies synthetic methods and chemical prop- erties of particles with sizes of 1 ± 10 nm. The most interesting problems of nanochemistry are concerned with the reactivity of nanoparticles. For example, chemical properties of 1-nm particles that contain*10 metal atoms can be changed by introduction of only one atom. This is why, it is important to know and correctly understand the chemical and physical regularities that govern the activities of particles with the sizes of an order of magnitude of 1 nm.Investigation of the dependence of chemical and physical properties of nanoparticles on the number of constituent metal atoms and the nature of ligands bound to nanoparticles represents a new direction in nanochemistry. At present, this field accumu- lates the knowledge which will allow one to understand the effects of the geometry, electronic properties and sizes of such systems on their reactivities. Properties of nanoparticles synthesised in the gas810 phase have been studied in more detail. The joint use of a supersonic jet, pulse lasers and mass spectrometry made it possible to obtain, identify and characterise `free' nanoparticles containing different numbers of metal atoms. The dependence of the chemical activity of nanoparticles in the gas phase on the number of the constituent atoms was studied in detail for niobium 17 and iron 18 particles.At present, attention is focused on the development of methods for the synthesis and stabilisation of nanoparticles in the liquid phase. Special attention is paid to the preparation of monodispersed particles. 1. Reduction in solution Methods of chemical reduction play an important role in the solution of the problem of synthesis of monodispersed particles. These methods were mainly used for the synthesis of gold and silver nanoparticles and less often for platinum, cobalt, nickel and copper nanoparticles. Spherical silver nanoparticles with sizes of 3.3 ± 4.8 nm were synthesised by reduction of silver nitrate by sodium borohydride in the presence of bis[(trimethylammoniodecanoylamino)ethyl] disulfide dibromide as a stabiliser.19 The particles obtained were characterised by active light absorption in the wavelength range near 400 nm, which corresponded to the silver plasmon peak and indicated the metallic nature of the particles.The environment effect on the stability of silver particles formed showed that the latter form aggregates in the presence of sulfuric and hydrochloric acids. The stability of silver particles was also shown to depend on the pH of the medium; at pH 5 ± 9 in aqueous media, the particles remained stable for a week. Both an increase and a decrease in the pH resulted in fast aggregation and precipitation of silver par- ticles.The effect of the pH on the stability of gold particles was less pronounced compared with silver particles. Metal nanoparticles of controllable sizes (1 ± 2 nm) were prepared using amphifillic poly(octadecylsiloxane) as a matrix.20 Hybrid matrices based on polyelectrolyte gels with oppositely charged surfactants were used as nanostructurised media for the reduction of various platinum salts by sodium borohydride and hydrazine. It was shown that with NaBH4 as the reductant small platinum particles with radii of *2 ± 3 nm were mainly formed, whereas the use of hydrazine resulted in particles of*40 nm.21 aq, hydroxyl radicals, hydrogen For cobalt nanoparticles in aqueous solutions, the formation mechanism, electronic spectra and reactions were studied.22 Radiation-induced chemical reduction of cobalt ions from aque- ous solutions of Co(ClO4)2 and HCOONa produced spherical cobalt particles with diameters of 2 ± 4 nm.Sodium polyacrylate with a molecular mass of 2100 was used as a stabiliser. Radiolysis produced solvated electrons e¡ atoms and CO2¡. radical ions H2O eaq ¡ , H., OH. , OH.(H. )+HCO¡2 H2O(H2)+CO¡2 .. 2 . radical ions reduced Co2+ ions 2. Reactions in micelles, emulsions and dendrimers Hydrated electrons and CO¡ producing cobalt nanoparticles which demonstrated an absorp- tion maximum in the 200 nm wavelength range. The use of pulse radiolysis has shown that the reduction of Co2+ with the formation of cobalt nanoparticles occurred by an autocatalytic mechanism.Radiation-induced chemical reduction of Ni2+ ions in aque- ous solutions of Ni(ClO4)2 containing propan-2-ol in the presence of polyethylene, polyacrylate and poly(vinyl sulfate) produced metal sols comprising spherical particles with diameters of 2 ± 4 nm. Nickel nanoparticles easily oxidisable by O2 and H2O2 form sufficiently stable Ni ±Ag nanosystems in the interaction with silver ions.23 Spherical copper particles measuring 20 ± 100 nm were obtained by g-radiolysis of KCu(CN)2 aqueous solutions in the presence of methanol and propan-2-ol as the acceptors of hydroxyl radicals.24 G B Sergeev Silver particles were obtained upon g-radiolysis of solutions of silver nitrate in water, ethanol and 0.01 M C12H25OSO3Na.25 The fractal dimensions of aggregates were 1.81, 1.73 and 1.70 in water, ethanol and C12H25OSO3Na, respectively.25 Stable nanoparticles (the average size of 1 ± 2 nm) of platinum, rhodium and ruthenium were prepared in organic media by heating the corresponding colloidal solutions of the metal hydroxides in ethanol.26 Silver particles measuring 2 ± 7 nm were obtained by electro- chemical dissolution of a metal anode (silver plate) in an aprotic solution of tetrabutylammonium bromide in acetonitrile.27 This process was shown to depend on such characteristics as the current density and the nature of the cathode material.Thus at high current densities under nonequilibrium conditions, particles of irregular shapes can be formed. In the interval from 71.35 to 76.90 nA cm72, the particle diameter changed from 60.7 to 1.70.4 nm.In the reduction of silver ions stabilised by tetrabu- tylammonium bromide, silver nanoparticles were formed and deposited on cathodes made of either platinum or aluminium (Fig. 1). The use of platinum cathodes resulted mainly in the formation of spherical silver nanoparticles, whereas only films were deposited on aluminium cathodes. An analysis of the optical spectra of nanoparticles measured during their syntheses led to the conclusion that this process includes an autocatalytic stage. It was also shown that the half-width of the surface plasmon peak of a particle linearly depends on 1/R (R is the particle radius) and the plasmon band shifts to lower frequencies with a decrease in the particle size.N+ N+ 1 N+ N+ N+ N+ N+ Ag N+ N+ N+ 2 Aluminium cathode Figure 1. Schematic illustration of the competition between two proc- esses. (1) Formation of silver particles and (2) deposition of particles and formation of a film.27 Micelles, emulsions and dendrimers,{ which can be considered as a sort of nanoreactors, which make it possible to synthesise particles of definite sizes, are used for preparing metal nanoparticles. Crystalline bismuth nanoparticles measuring less than 10 nm were obtained by reduction of aqueous solutions of bismuth salts inside reverse micelles based on sodium diisooctyl sulfosuccinate (usually designated as AOT).28 Mixing of AOT dissolved in isooctane with a certain volume of an aqueous solution of BiOClO4 resulted in the formation of reverse micelles.The {A dendrimer represents a highly branched macromolecule which includes a central core, internal repeating units and terminal functional groups. Dendrimers with small numbers of internal units exist in an `open' form, whereas those involving large number of units form spherical three- dimensional structures. Terminal groups of dendrimers can be modified by hydroxy, carboxy and alkyl groups. Platinum cathodeNanochemistry of metals micellar solution of NaBH4 was prepared in the same fashion with the same ratio w=[H2O] : [AOT]. Both solutions were mixed under argon. Bismuth particles precipitated after the mixture was kept for several hours at room temperature.The liquid phase was removed in vacuo, and the dry residue was dispersed in toluene. According to X-ray powder diffraction and electron spectroscopy data, a dark solution thus obtained contained bismuth particles measuring 3.20.35 nm for w=2 and 6.92.2 nm for w=3. When polymers were used for the protection of crystalline bismuth particles, the particle size increased to 20 nm and further.29 Reduction of rhodium salts in water in the presence of an amphiphilic block copolymer of styrene with ethylene oxide and an anionic surfactant (sodium dodecyl sulfate) produced rhodium particles with diameters of 2 ± 3 nm stabilised by the block copolymer.30 Luminescent nanomaterials based on yttrium oxide doped with europium were synthesised using non-ionic reverse microemulsions based on poly(ethylene oxide) and other ethers.31 At present, an active search is directed to macromolecules that can be used as matrices in syntheses of nanoparticles.In such methods, the stabiliser molecules interact with the surfaces of metal particles and affect their growth. For example, the reduction of bivalent copper in the presence of poly(N-vinylpyrrolidone) produced particles with the diameters of 71.5 nm at 11 8C and 102 nm at 30 8C.32 Studies on the temperature effect on the stability of preformed nanoparticles have shown that if copper particles formed at 11 8C were heated to 30 8C, their polymeric shell lost its protective properties, which enhanced aggregation and accelerated oxidation of metal particles.The opposite result was achieved by cooling to 11 8C the system formed at 30 8C. In this case, the particles do not aggregate, their sizes remain unchanged and the resistance to oxidation increases. The com- petition of different temperature-dependent processes is reflected not only in the stabilities of particles formed but also in their size distribution. Photochemical reduction of Ag+ ions in the presence of dendrimers with terminal amino and carboxy groups produced silver particles with an average diameter of *7 nm.33 Below, a possible mechanism of particle formation is shown. hn Ag0+[dendrimer]7CO Ag++[dendrimer]7CO¡ 2 2 [dendrimer] .+CO2 , hn Ag0+[dendrimer]7NHá Ag++[dendrimer]7NH2 2 .The particle size can be controlled by varying the nature of the 4 dendrimers. Currently, stabilisation of metal nanoparticles is performed in the presence of dendrimers based on polyamido- amines and their derivatives. The use of dendrimers as micro- reactors for the synthesis of metal nanoparticles was documented in Ref. 34. Monodispersed spherical polyamidoamine dendrimers are permeable to low-molecular reactants. Thus an addition of HAuCl4 to an aqueous solution of a dendrimer with primary and tertiary amino groups results in the appearance of a protonated dendrimer with AuCl¡4 as the counter-ion. Reduction of AuCl¡4anions by sodium borohydride have produced 1 ± 5-nm gold particles. By varying either the concentration ratio (D) of AuCl¡ counter-ions to terminal amino groups or the diameter (gener- ation) of the dendrimer, one can control the particle size.Reduc- tion of gold ions in a dendrimer of the ninth generation (G.9) resulted in the formation of spherical gold particles with the sizes of 2.5, 3.3 and 4 nm for D=1 : 4, 1 : 2 and 1 : 1, respectively. For D=1 : 1, gold particles measuring 2, 2.5, 3.2 and 4 nm were formed in dendrimers of the G.6, G.7, G8 and G.9 generations, respectively. Reduction of gold and silver salts in the presence of modified dendrimers produced particles with an average diameter of 2 ± 6 nm. The autocatalytic mechanism of the reaction has been revealed using spectral methods.35 811 The prospects of using mesopores for the preparation of different nanosize materials were discussed.36 Nanoparticles of silver and silver sulfide were obtained in nanosize cavities of perfluorinated ionomer membranes.37 Reduction of metal ions in the presence of aminodextran and styrene resulted in the formation of spherical polystyrene particles with diameters of *2.0 mm covered with gold and silver islets measuring from 5 to 200 nm.38 3.Particles of different shapes and films The sizes and shapes of particles often depend on the synthetic methods; however, the ratio of nucleation to growth rates of particles is also of great importance. Each of these processes depends in turn on variations in the reaction conditions such as temperature, the nature and concentrations of metal and ligand and the nature of stabiliser and reductant.The problems of nucleation and particle growth are surveyed in detail elsewhere.15 THF, 66 8C Pd particles. Pd(NO3)2+(n-C8H17)4N+(RCO2)7 The shapes of metal nanoparticles formed in the reduction of metal salts can be controlled using tetra-n-octylammonium car- boxylates (n-C8H17)4N+(RCO2)7 as the reductants and stabil- isers (see Ref. 39). Palladium particles measuring 1.9 and 6.2 nm were formed in the reaction Their sizes and shapes were determined by electron micro- scopy. It was found that if acetate, dichloroacetate, pivalate or pyruvate anions were used, the particles formed had largely spherical shape. However, if palladium nitrate was treated with an excess of (n-C8H17)4N+(HOCH2CO2)7, triangular particles with an average size of 3.6 nm were formed in addition to spherical ones.The shape changes observed were attributed 39 to the presence of the hydroxy group in the anion. A nickel compound Ni(COD)2 (COD is cycloocta-1,5-diene) was used for elucidating the role (reductant or stabiliser) the glycolic acid residue plays in the reduction.39 The reaction was as follows: H2 , 60 8C Ni particles. Ni(COD)2 + (n-C8H17)4N+(HOCH2CO2)7 Inasmuch as nickel in Ni(COD)2 is in its zerovalent state, the glycolate can act only as a stabiliser, whereas it is hydrogen that reduces COD to cyclooctane. By means of electron microscopy it was found that the reaction produces crystalline nickel particles with an average size of 4.5 nm preferentially shaped as triangles.In control (n-C8H17)4N+Br7 experiments using or (n-C8H17)4N+NO¡3 as stabilisers, spherical particles were formed. The triangular shape of particles was additionally confirmed by the results of scanning tunnelling microscopy. The glycolate effect on the morphology of particles formed seems to be associated with the selective adsorption of anions on growing nanocrystals, which can be traced by changes in the absorption spectra. During synthesis of nanoparticles, the band at 1621 cm71 which belongs to dissolved glycolate disappears, and a new band appears at 1604 cm71, which corresponds to adsorbed glycolate. It is believed 39 that the changes observed in the IR spectra recorded in situ agree with the proposed mechanism.The formation of spherical and cylindrical silver nanoparticles was observed upon photochemical reduction of silver salts in the presence of polyacrylic acid.40, 41 Polyacrylic acid and Ag+ ions form a complex which, being illuminated, produces silver nano- particles. According to electron microscopy and sedimentation analysis data, photoreduction of the complex produced spherical silver nanoparticles measuring 1 ± 2 nm. In the presence of a modified (partially decarboxylated) acid, apart from spherical particles, elongated particles (nanorods) with lengths of up to 80 nm characterised by light absorption in the 500 ± 800-nm range were also formed. Apparently, decarboxylation disturbs the co- operative mechanism of the bonding between polyacrylic acid and812 silver cations, making the stabilisation of spherical particles less effective and favouring the growth of nanorods.The sizes of metal particles formed in the presence of macro- molecular stabilisers depend on the conditions of formation of the polymeric protective shell. If the polymer involved is an insuffi- ciently effective stabiliser, a particle already bound to the macro- molecule may continue to grow. Changing the monomer and the corresponding polymer and varying the polymer concentration in solution, enables one to control the sizes and shapes of particles formed. An original method of changing the stabilising ability of a polymer was proposed.42 The authors studied how the conforma- tion of poly(N-isopropylacrylamide) affected the shape of plati- num nanopartilcles formed upon the reduction of K2PtCl4 by hydrogen.This polymer can change its conformation with temper- ature. Below 306 K, polymer molecules are hydrophilic and represent swelled globules; in this case, up to 60% of platinum nanoparticles formed have irregular shapes. At temperatures above 306 K, polymer molecules are hydrophobic and begin to collapse. The stabilising ability of such molecules decreases. At 313 K, the reduction of platinum ions proceeded on the most active (111) face of the growing nanocrystal, and cubic nano- particles were preferentially formed (in up to 68% yield). The morphology of the particles also depended on the concentration ratio of platinum salt and polymer in solution (however, to the lesser extent than on the temperature).The shapes and sizes of silver nanoparticles can be controlled when the methods of pulse sonoelectrochemistry (the use of ultrasound in electrochemistry) were applied.43, 44 Ultrasound allows one to clean and degasify the electrode surface, accelerate mass transfer and increase the reaction rate. Silver particles shaped as spheres, rods and dendrites were obtained by the electrolysis of aqueous solutions of AgNO3 in the presence of N(CH2COOH)3. These were characterised by electron micro- scopy, X-ray diffraction and electron spectroscopy. The shapes of particles were found to depend on the reagent concentration. Spherical particles had diameters of *20 nm. The diameters of rods were 10 ± 20 nm.In certain cases, their surfaces exhibited protrusions which could be developed into dendrites. An interesting method of successive layer-by-layer deposition of thin (100 ± 300 nm) films incorporating magnetic nanoparticles was considered.45 Alternating layers of magnetic nanoparticles, e.g., Fe3 O4, and poly(diallyldimethylammonium bromide) were first deposited on a glass plate covered with paraffin and acetyl cellulose. Upon reaching the desirable film thickness, the cellulose layer was removed, and the whole specimen was dissolved in acetone. The suspension thus obtained could be applied on any porous surfaces and dense supports. It was noted that the films which incorporate uniform magnetic nanoparticles measuring *10 nm, can be used in memory devices.46 At present, the control over the shapes and sizes of nano- particles seems to be the most important problem of nanochem- istry.47 Synthesis of uniform spherical and rod-like particles of metallic iron have been described.48 Nanoparticles were prepared by thermal decomposition of iron pentacarbonyl in the presence of stabilisers. Spherical particles measuring 2 nm were uniformly dispersible in solution and formed rods of 2-nm diameters and 11-nm lengths.These particles were amorphous, whereas the rods had the face-centred cubic structure of a-iron. Different supports with films of 4-mercaptobenzoic acid applied in high vacuum were used for the preparation of thin heterogeneous films with incorporated nanoparticles of gold, silver, and cadmium sulfate.The heterogeneous films were syn- thesised by successive immersion of supports in the corresponding solutions.49 High-resolution electron microscopy has shown that, being encapsulated into micelles, rod-like gold nanoparticles prepared by an electrochemical method grow along the [001] axis. Here, (100) faces of the particles remain stable, whereas (110) faces are unstable.50 G B Sergeev The possibility of structural changes in platinum nanopar- ticles supported on silicon and the microcrystalline structure of individual nanoparticles were studied by electron microscopy before and after the system was heated in vacuo and in hydrogen and oxygen atmospheres.51 In all the cases, the mass of particles remained unchanged, but the sizes of constituent crystals increased with temperature.This was attributed to surface fusion and self-diffusion of platinum particles. Studies of adhesion of platinum particles to silica which were carried out using atomic force microscopy have shown that heating changes the crystalline nature of particles and strengthens their adhesion to silica. More- over, platinum nanoparticles were shown to be stable both under oxidative and reductive conditions. 4. Reactions of oxides Like metals, oxides also find extensive application in practice. The reactivity of metal oxides is low compared with the corresponding metals; therefore, oxide formation is used for stabilising metal nanoparticles.Recently, metal oxide nanoparticles have been employed in reactions which are of interest for nanochemistry. An original application of nanocrystalline zinc oxide has been described,52 where zinc oxide was synthesised by a modified sol ± gel method by the reactions Zn(OBut)2+2C2H6 , ZnEt2+2ButOH Zn(OH)2+2ButOH, Zn(OBut)2+2H2O ZnO+H2O. Zn(OH)2 CO2+2 ZnCl2 . 2 ZnO+CCl4 The process of ZnO preparation involved three stages: syn- thesis, isolation and activation. The latter process, in turn, consisted of several successive stages of thermal treatment. First, the powder was slowly heated to 90 8C and kept at this temper- ature for 15 min. Then, the temperature was gradually increased to 250 8C, held at this level for 15 min and slowly decreased to room temperature. Zinc oxide thus obtained represented crystal- line nanoparticles measuring 3 ± 5 nm with a surface area of *120 m2 g71.Nanosize zinc oxide was used in the reaction The process was carried out at 250 8C, and CCl4 was intro- duced into the reaction vessel portionwise at 7-min intervals. Carbon dioxide and nonconsumed CCl4 were quantitated by gas chromatography. Nanocrystalline zinc oxide was shown to be more reactive than common commercial specimens. It was also shown that the adsorption of sulfur dioxide and the destructive adsorption of diethyl 4-nitrophenyl phosphate (a noxious organo- phosphorus compound) proceed with a higher efficiency on nanocrystalline zinc oxide.52 In these processes, nanocrystalline ZnO exhibited higher reactivity compared with commercial speci- mens.Nanocrystalline oxides of alkali-earth metals were success- fully used for deactivation of yperite (mustard gas) and other war gases. Autocatalytic dehydrohalogenation of 2,20-dichlorodiethyl sulfide on nanocrystalline calcium oxide was studied using NMR technique.53 The decomposition products of 2,20-dichlorodiethyl sulfide were found to involve divinyl sulfide (*80%) and thio- glycol and/or a sulfonium ion (*20%) with a hydroxyalkyl group, which seem to provide its binding to the surface of the alkali-earth metal oxide. In addition to the reaction of CaO with yperite, its reactions with organophosphorus compounds were studied. The kinetics of the reactions of all studied substances with calcium oxide was shown to be characterised by a rapid initial stage followed by a slow diffusion-controlled stage.An interesting application of transition metal oxides has been discussed.54 Particles of cobalt, nickel, copper and iron oxides measuring 1 ± 5 nm were used as the electrode materials in lithium power sources (electrochemical capacity of 700 mA h g71). Here, the formation and decomposition of Li2O and the accompanying reduction and oxidation of nanoparticles occur on electrodesNanochemistry of metals made of CoO nanoparticles. The scheme of reversible reactions is as follows: 2Li+ 2Li72e7 CoO+2 Li++2e7 Li2O+Co 1 CoO+2 Li Li2O+Co 2 The direct reaction (1) is possible and thermodynamically permissible.The reverse reaction (2) is unusual for electrochem- istry, because Li2O is traditionally considered to be electrochemi- cally inactive. Electrochemical decomposition of Li2O using mechanically ground Li2O and CoO powders could not be effected.54 Thus, the possibility of the reverse reaction is associ- ated with the use of nanoparticles and the increase in their electrochemical activity with a decrease in size. Currently, great attention has been drawn to the preparation and analysis of physicochemical properties of hybrid nanomate- rials of the core ± shell type and the particles which involve two and even three different metals. New nanocrystalline hybrid core ± shell materials such as TiO2 ±MoO3 were prepared and studied in detail.55 The TiO2 ± (MoO3)x particles were synthesised by co-nucleation of metal oxides on the surfaces of micelles.In such materials, the photoabsorption energy correlates with the particle size. With a decrease in the sizes of TiO2 ±MoO3 particles from 8 to 4 nm, the absorption energy decreased from 2.9 to 2.6 eV. For comparison, it may be noted that the forbidden gap energies of bulk TiO2 and a-MoO3 are 3.2 and 2.9 eV, respec- tively. Unfortunately, the materials obtained turned out to be less efficient in photocatalysed oxidation of acetaldehyde compared with common titanium dioxide produced by Degussa (France). To facilitate the electron transport in metal nanoparticles, it was proposed to use the pH-induced changes in the charges of stabilising shells which are bound to the particle surface. Mole- cules of rigid mercaptophenylacetylenes were shown to bind strongly gold and silver particles.56 It was found that electron transport in charged gold particles is better described in terms of classical rather than quantum-mechanical concepts.57 The use of nanoparticles of metals and their oxides in optical and electronic devices requires that new approaches be found to the solution of such problems as: establishment of reliable electrical contacts between individual nanoparticles; determina- tion of characteristics of electromagnetic interactions between particles in symmetrical well-organised aggregates of nanopar- ticles; elucidation of how the chemical properties of the surface of nanoparticles depend upon their size and its effect on their optical and electronic properties.58 5.Bi- and trimetallic particles Nanoparticles that involve two or more different metals are of particular interest for the development of materials with novel properties, because at the nanolevel it is possible to synthesise such intermetallic compounds and alloys that can never be formed for bulk specimens. Nanoparticles involving two 59 and even three 60 different metals were obtained by radiation-induced reduction of salt solutions. A two-step synthesis was used for the preparation of Au ±Hg particles. First, radiation-induced reduction produced 46-nm gold particles. Then, Hg(ClO4)2 and propan-2-ol were added to the gold sol, which resulted in deposition of mercury ions on the gold particles.Finally, mercury ions were reduced by free radicals formed upon radiolysis. Palladium particles measuring 4 nm with a narrow size distribution were prepared by the reduction of Na2PdCl4 by hydrogen in the presence of sodium citrate. The addition of K2Au(CN)2 to the sol of palladium particles in methanol followed by g-irradiation resulted in the reduction of gold ions. No individual gold particles were formed: all gold was deposited on 813 palladium particles and formed an outer layer.Asilver layer could also be deposited on the Pd ±Au particles obtained. These multi- layer clusters seem to be of interest for studying rapid (femto- second) electronic processes.61, 62 The use of silicates modified by organic compounds as matrices and stabilisers made it possible to synthesise bimetallic nanoparticles, their sols and gels by a single-step method.63 Scanning tunnelling microscopy studies of Pd ± Pt particles have shown the latter to consist of palladium cores covered with platinum shells.Thin silicate films with bimetallic nanoparticles were used in electrocatalytic oxidation of ascorbic acid. A sonochemical method was applied to produce bimetallic gold ± palladium nanoparticles.64 The particles were synthesised NaAuCl4 . 2H2O in and aqueous solutions of PdCl2 . 2NaCl . 3H2O with sodium dodecyl sulfate added. The latter functioned as both the stabiliser and the reductant.The bimetallic particles synthesised represented cores of gold atoms surrounded by shells of palladium atoms. The sizes of cores and shells were measured for different gold to palladium ratios using high-resolution electron microscopy. The use of X-ray spectro- scopy allowed the gold and palladium contents in the particles to be determined. With the known density, mass and initial ratio, one can estimate the sizes of cores and shells (Table 1). Using the sonochemical synthesis of bimetallic particles, one can control the sizes of cores and shells by varying Au3+ and Pd2+ concentra- tions. Bimetallic Au± Pd particles exhibited high catalytic activity in hydrogenation of pent-4-enoic acid. Table 1. Core diameter (gold) and shell thickness (palladium) in Au ± Pd bimetallic nanoparticles. Estimate Experiment Au : Pd ratio shell thickness core diameter shell thickness core diameter 0.8 1.6 6.4 4.8 1.0 1.5 6.0 5.0 1 : 1 1 : 4A comparative study of gold, nickel and bimetallic Au±Ni nanoparticles on amorphous carbon supports, which were obtained by laser ablation of the corresponding pure metals and the alloy, was carried out.65 Studies accomplished by different methods have shown the bimetallic particles to have, on average, diameters of 2.5 nm and a narrow size distribution, while their composition was found to correspond to that of the evaporated alloy. Nanoparticles of FeTiH were prepared by mechanical syn- thesis from a mixture of iron and titanium hydride.66 Nanosize iron ± tungsten composite particles with a tungsten concentration varying from 2 at.% to 85 at.% were synthesised by coreduction of a mechanical mixture of iron hydroxide (FeOOH .nH2O) and tungstic acid (H2WO4) in a flow of hydrogen at 740 8C for 1 h. The particles obtained were studied by X-ray diffraction and MoÈ ssbauer spectroscopy.67 III. Cryonanochemistry of metals The presence of stabilisers, the size distribution of nanoparticles, their different shapes and the interactions between nanosize particles and the medium complicate the studies of the reactions involving metal nanoparticles. In addition, high chemical activ- ities of metal atoms and nanoparticles they form require special approaches to their studies to be developed.One of such approaches is isolation of nanoparticles at superlow temperatures (4 ± 10 K) in inert-gas matrices. Co-condensation of a large amount of an inert gas, e.g., argon, and a small number of nanoparticles (more than 1000 inert gas species per metal particle) allows active particles to be effectively isolated from one another,814 while low temperatures hinder diffusion of these particles and provide their stabilisation in an inert matrix. Here, we will not analyse all the studies devoted to reactions of metal atoms and the corresponding nanoparticles at low and superlow temperatures and only consider the results of recent publications.68 ¡À 70 The specimens under discussion were prepared by co-condensation of vapours of metals and ligands on surfaces cooled either to 10 or 77 K.In certain experiments, the vapours were diluted by argon. The amount of metal deposited was determined using quartz-crystal microbalances. The co-conden- sates obtained were studied by spectroscopy in UV, visible and IR ranges. ESR spectroscopy was also used. Table 2 lists the products formed upon reactions of magnesium and samarium particles with different ligands in argon matrices at 10 ¡À 40 K.68, 69 Table 2. Reaction products of samarium and magnesium particles with different ligands in an argon matrix at 10 ¡À 40 K. Ligand Metal CO2 Mg Mg2 ¡À 4 Mgx Mg+CO¡¦2 , upon annealing of the matrix Mg+CO¡¦2 , upon co-condensation Mg+CO¡¦2 , upon annealing of the matrix Sm Sm+CO¡¦2 , CO, SmCO3, upon annealing of the matrix Sm2, Smx Sm+CO¡¦2 , CO, SmCO3, upon co-condensation Studies of properties of metal nanoparticles, in particular at low temperatures, face various difficulties.Therefore, theoretical simulation of the structures of metal nanoparticles, their elec- tronic, optical and other properties and the processes of their aggregation is gaining in importance. In theoretical studies of nanoparticles, valuable results can be obtained by ab initio quantum-chemical calculations which use the concepts developed by solid-state physics, and by calculations based on the molecular dynamics approach. At present, the Hartree ¡À Fock method is applied for the determination of proper- ties of atoms and small clusters.Particularly, this method was successfully employed for the calculation of the properties of nickel particles which involved n=4, 6, 8 and 13 atoms.71 The density functional method is also widely used for the simulation of the behaviour of metal nanoparticles.13 For example, the latter method was used for studying the structure and properties of Pt13 and Pt55 species.72 The results obtained were also compared with those of different ab initio and semi-empirical quantum-mechan- ical methods which are used in calculations of properties of metal nanoparticles. The density functional method was applied 73 for studying the products of carbon monoxide adsorption on palladium particles deposited on magnesium oxide. An extended Hu�� ckel method was used for studying bimetallic nanoparticles of Pt ¡À Fe alloys with a number of atoms ranging from 13 to 309.74 Figure 4.A temperature dependence of the relative activity (in arbitrary 2 . radical-ion pair. According to calculations, this dioxide.9 (1) Sm, l=501 nm; (2) Sm, l=557 nm; (3) Sm, l=520 nm; units) of samarium atoms and nanoclusters in the reaction with carbon The ab initio quantum-mechanical calculations have shown that the magnesium ¡À carbon dioxide system represents a meta- stable Mg+.CO¡¦ pair is stabilised by the argon matrix. The IR spectrum of the argon ¡À ethylene co-condensate have revealed absorption bands of ethylene dimers. As was shown by calculations, the potential CH3X (X=Cl, Br) C2H4 CH3MgX, upon irradiation (l>280 nm for cyclic Mg(C2H4)2 (see Fig.2 a), upon annealing Mg and l>300 nm of the matrix for Mg274 and Mgx) Sm(C2H4) and methane, upon co-condensation Sm(C2H4)2 complexes a b Mg 2.09 2.09 H C H1.55 H C C 1.56 H c O (70.91) 2.11 (+0.87)Mg 2.11 O(70.91) surface of the Mg. 2C2H4 complex demonstrates no global mini- mum corresponding to the C2u symmetry. The global minimum can be obtained if only all the restrictions on the system's symmetry are removed. In the M��ller ¡À Plessett (MP2) approx- imation, the minimum corresponded to a cyclic structure which involves an ethylene dimer and a magnesium atom. The intera- tomic distances in the structures of magnesium complexes with ethylene and carbon dioxide were calculated.Figure 2 shows the structures of these complexes. Studies of a samarium ¡À carbon dioxide co-condensate have shown that samarium and CO2 form a complex with an angular structure. In the reaction products, CO and CO2¡¦ were detected. Radical anions CO2¡¦. and a samarium carbonyl compound of an uncertain composition were also formed. Figure 3 shows the data on the reaction products identified and on their interconversions. Figure 4 shows the temperature dependence of the relative activity of samarium nanoparticles in the reaction with CO2. The results Sm+CO2 OT=35K CO+CO2¡¦ Figure 3. Schematic illustration of the reactions of samarium atoms with carbon dioxide molecules in an argon matrix. Straight arrows are the reactions occurring during co-condensation; curved arrows are the reac- tions occurring during heating of the co-condensate.68 1.0 0.8 0.6 0.4 0.20 15 (4) 41 nm; (5) Smx, l=600 nm; (6) Sm2, l=541 nm (l is the wavelength in the electronic spectrum).Relative activity (arb. u.) G B Sergeev Mg 2.07 1.87 H H C O C H H 1.55 1.55 1.34 H C C 1.21 1.56 1.09 H H O H 1.25 C(+0.95) 1.25 Figure 2. Bond lengths and the charges on the atoms (shown in parentheses) for magnesium com- pounds with (a) ethylene dimer, (b) ethylene and carbon dioxide and (c) carbon dioxide.68 3 T=35K Sm Sm+CO¡¦2C Sm+CO¡¦2 O Sm Sm(CO)n 3 123456 T /K 30 20 25Nanochemistry of metals shown suggest a higher activity of samarium nanoparticles com- pared with samarium atoms.A similar trend was observed for magnesium nanoparticles. According to IR spectroscopic data, in samarium ± ethylene co-condensates, `sandwich' p-complexes of samarium and ethylene SmC2H4 and Sm(C2H4)2 are formed rather than cyclic compounds typical of magnesium co- condensates. At ordinary temperatures, the reactions of bulk magnesium and samarium with alkyl halides occur in a similar way to give organometallic compounds. At low temperatures, atoms and small clusters of these metals react with haloalkanes (e.g., with halomethanes) by different routes. Magnesuim inserts into the C7Br and C7Cl bonds with the formation of Grignard reagents, whereas samarium, being more reactive, reduces CH3Cl and CH3Br to methane.Thus, on the nanoparticle level, magnesium and samarium behave differently. Another peculiarity in the behaviour of magnesium and samarium nanoparticles was observed in their reactions with mixtures of two ligands, viz., CO2 and C2H4. It was found that under comparable conditions magnesium and samarium react only withCO2 and do not react with ethylene. The results obtained demonstrate that no unambiguous relationship exists between the activity and selectivity of metal nanoparticles. The formation of complexes and organometallic nanostruc- tures was studied by an example of film co-condensates of the vapours of metallic silver and mesogenic cyanobiphenyls CnH2n+1C6H4C6H4CN (n=5 and 8) and CnH2n+1OC6H4..C6H4CN (n=8) in the temperature interval of 80 ± 350 K. Low-temperature co-condensation of silver particles with C5H11C6H4C6H4CN and C8H17C6H4C6H4CN produced p-com- plexes stable at low temperature.68 IR and UV spectroscopic studies have confirmed their formation and revealed their sand- wich structures and 1 : 2 composition; 1 : 1 complexes were also detected. The structure of p-complexes determined from IR spectroscopic data was confirmed by model quantum-chemical calculations.75 Vibrational frequencies of CN groups were also calculated and compared with the experimental ones. Spectral properties of the reaction products of mesogenic cyanobiphenyls with silver particles were discussed in detail elsewhere.76 Thermal degradation the complexes undergo on being heated to the room temperature results in the formation of nanosize (15 ± 30 nm) silver clusters and their subsequent aggregation with formation of anisotropic particles (more than 200 nm long) stabilised in a liquid-crystal line matrix.76 An analysis of the ESR spectrum of a co-condensate of silver particles with 4-pen- tyl-40-cyanobiphenyl at 90 K has revealed two spectral doublets.The values of the g-factor and the hyperfine coupling constant (a) for 107Ag and 109Ag are as follows: g(107Ag)=2.001, a(107Ag)=48.5 mT and g(109Ag)=2.003, a(109Ag)=72.75 mT. The occupation degree (rm) of valence s orbitals of the metal in the complex was estimated (rm=0.77).The value obtained suggests that electron density is shifted from a silver atom to the p-orbitals of the ligand. The ESR spectrum also revealed a broad anisotropic singlet which was tentatively attributed to zero-valent silver nanoparticles. Optical spectroscopy in the UV and visible ranges in combi- nation with the dynamic light scattering technique were used for studying the properties of polymer-stabilised silver nanopar- ticles.77 Co-condensation of vapours of silver and 2-dimethylami- noethyl metacrylate in vacuo occurred on the walls of a glass vessel cooled by liquid nitrogen. According to the electron microscopic data, heating of the co-condensate to room temperature resulted in the formation of polymer-stabilised silver nanoparticles meas- uring 5 ± 12 nm.Dynamic light scattering studies have shown that the size distribution of these particles is bimodal. Such a distribu- tion seems to suggest that both isolated silver particles and their aggregates are simultaneously present in the system. The depend- ence of the radius of a solvated particle on the nature of the solvent was studied. Among the solvents such as water, toluene and 815 acetone, the latter gave solutions containing silver particles with the smallest radii.78 Co-condensation of lead and a monomer polymerisable at low temperatures, viz., p-xylylene, vapours at 80 K, afforded films with incorporated lead nanoparticles. Conductivity of the films was studied in the course of co-condensation and subsequent heating.79 The metal ± polymer films obtained exhibited satisfac- tory sensor activity with respect to ammonia in air and ammonia in the presence of water vapour.Complexes of zero-valent samarium with 1,3,5-tri(tert-butyl)- 170 ± 210 K into the complex benzene were obtained by co-condensation of metal and ligand vapours at 80 K. Kinetics of thermal decomposition of these complexes was studied.80, 81 Transformations that occur in sama- rium complexes with mesogenic cyanobiphenyls were also studied at low temperatures.82 It was shown that the complex with the composition Sm:C5H11C6H4C6H4CN=1 : 2 transforms in the temperature range Sm:C5H11C6H4C6H4CN=1 : 1; this reaction is characterised by an effective activation energy of 8 kJ mol71.The reaction of magnesium particles with chloro-, bromo- and iodobutanes at low temperatures was studied.83 It was shown that the yield of octane (the WuÈ rtz reaction product) depends on the amount of magne- sium and the carbon ± halogen binding energy. The mechanism of the reaction of magnesium with alkyl halides and the earlier data on the cryonanochemistry of metal particles have been discussed elsewhere.2, 3 IV. Ensembles involving nanoparticles When metal nanoparticles are synthesised by the condensation method, the process begins with the formation of nanoparticles of individual metal atoms; this may be considered as `self-assembly' or `self-organisation' { of atoms with the formation of atomic ensembles.As applied to metal nanoparticles, it is common practice to consider both the ensembles of particles themselves and the ensembles of stabilising protective layers. Prime attention is given to the effect of the chemical nature of stabilisers on self- assembly processes. An analysis of studies devoted to self-assem- bly and performed before 2000 is given elsewhere.13 The synthesis of thiol-covered metal particles and their self-assembly into one-, two- and three-dimensional superlattices were surveyed in Ref. 85. The processes of assembly and self-assembly of nanoparticles were discussed most extensively by the example of silver and gold nanoparticles. Monodispersed silver nanocrystals were stabilised by chemisorption of dodecanethiol on the receptor sites of the crystal surface.86 The kinetics of aggregation and the structure of the formed ensembles of stabilised particles depend on the number of receptors on the nanocrystal surface.The formation of alternating positively charged gold particles and negatively charged silver particles and their self-assembly in the presence of 4-mercaptoaniline and 4-mercaptobenzoic acid on a glass surface were described.87 One-dimensional layers of gold particles on silicon supports were also formed.88 The latter process was realised using atomic force microscopy (AFM) for surface treatment and chemical deposition of gold particles. Self-assembly of gold particles measuring 4.6 nm results in aggregates with diameter of *30 nm.89 The initial gold particles were prepared by the reduction of HAuCl4 by NaBH4 in the presence of HSCH2COONa.Removal of the excess metal ions from the solution by dialysis resulted in the formation of coarser aggregates. Before the dialysis, the electronic spectra of particles revealed no plasmon peak typical of gold particles. A method of synthesising materials from `bricks and mortar' was first described in Ref. 90. Binding between a polymer and { General description of such self-organisation is given in a monograph by Lehn.84816 thymine-protected gold particles was provided by hydrogen bonds formed. The process of self-association of particles into aggre- gates, the sizes of the aggregates and their morphology were temperature-controlled.At 23 8C, the initial 2-nm gold nano- particles formed spherical aggregates with diameter of 9717 nm. At 720 8C, spherical aggregates with diameter of 0.5 ± 1 mm, which in turn comprised finer aggregates as individual subunits, were formed. At 10 8C, *50-nm-long chains built of spherical nanoparticles were formed. These self-association proc- esses were studied using X-ray small-angle scattering and trans- mission electron microscopy (TEM). It was assumed that the formation of aggregates at 10 8C is an intermediate stage of the formation of coarser ensembles at 720 8C, and the former aggregates can be used as the precursors of nanoensembles of different shapes and sizes. Self-assembly of rod-like gold particles with diameter of 12 nm and lengths of 50 ± 60 nm was studied using high-resolu- tion electron microscopy.91 By choosing concentrations of these particles, their size distribution, the conditions of solvent evapo- ration, its ionic strength and the surfactant concentration, one-, two- and three-dimensional structures can be obtained by elec- trolysis.Amixture of hexadecyltrimethylammonium bromide and tetraoctylammonium bromide was used as the water-soluble electrolyte.Tetraoctylammonium bromide provided the forma- tion of cylindrical particles, where the ratio of two surfactants determined the diameter to length ratio of the gold nanorods formed. The electrolysis was carried out for 4.5 min. Gold nano- rods were formed on a copper wire net partially immersed in solution (on net ± solution contacts).Presumably, the following two factors are most important for further self-assembly of the rods, viz., the induced convective transfer of particles from the solution to the thin film by evaporation of water and the interaction of particles within the film, which results in the formation of different structures.91 A possible reason for the parallel arrangement of rod-like particles is associated with capillary forces that act between parallel rods, although more accurate theoretical analysis is required to explain the anisotropy of the rod ensembles observed. Self-assembly of nanoparticles entails not only the transla- tional ordering but also the orientational one. The orientational ordering is typical of particles with well determined shapes.92 Structural changes in gold nanorods upon the action of femto- second and nanosecond laser pulses were studied by high-reso- lution electron microscopy.93 The energy of a pulse did not exceed the value necessary for melting the rods but was sufficient for inducing deformational processes which caused changes in the shapes of nanoparticles: a transition of nanorods into nano- spheres was observed.The size and shape distributions of nano- rods before and after their exposure to a laser beam were statistically analysed.94 In connection with the problem of the development of electronic nanodevices, the problem of synthesising nanoparticles protected by monolayers has attracted attention. The methods of preparation of such layers on gold particles by using solutions of hexanethiols and hexanedithiols in organic 95 and aqueous 96 media were developed. Stable gold films which involved particles measuring *4 nm were prepared 97 by the reaction of low-molecular-weight (M44000) polymers (such as polyethylenimine and poly-L- lysine) with gold particles covered with carboxylic acids.The film morphology was shown to depend on the nature of the acid which covers the particles. The use of mercaptododecanoic and mercaptosuccinic acids has provided films with higher degrees of ordering. Glass-supported thin porous gold films were prepared using colloidal gold crystals as the templates. These films were studied by scanning tunnelling microscopy.98, 99 Microstructures, wettabilities and thermal stabilities of self- assembly manolayers of partially fluorinated alkanethiols of the F(CF2)10(CH2)nSH (n=2, 6, 11, 17, 33) type on gold particles G B Sergeev were studied.100 Introduction of fluorocarbon chains as the terminal groups into alkanethiols enhanced the stability of self- assembly manolayers (a gold-supported monolayer of non-fluo- rinated alkanethiols loses its ordering at *100 8C).Fluorinated compounds with the numbers of methylene groups in a chain n=11, 17, 33 formed well ordered monolayers. For smaller numbers of methylene units, the degree of ordering decreased. Wettabilities of the layers were also shown to depend on the number of methylene units. Thermal stabilities of self-assembled gold monolayers increased with an increase in n, and for n=33 the monolayer films formed remained stable in air at 150 8C for 1 h.The effect of the nature of the support on the morphology of two- and three-dimensional superlattices formed by dodecane- thiol-covered silver sulfide nanocrystals of sizes of 5.8 nm were studied.101 Oriented pyrolylic graphite and molybdenum disulfide were used as the supports.The self-assembly process was moni- tored by scanning electron microscopy and AFM. It was found that self-assembly into superlattices depends on the nature of the support and is governed, in addition to capillary forces, by van der Waals interactions of the particle ± particle and particle ± support types. 3 Self-assembled monolayers of 40-hydroxy-4-mercaptobi- phenyl, 4-(4-mercaptophenyl)pyridine and their mixtures with 40-methyl-4-mercaptobiphenyl were deposited on the (111) faces of gold crystals and used as the templates for growing glycine crystals.102 The morphology of glycine crystals obtained depended on the properties of supporting surfaces and was determined by hydrogen bonds formed between glycine molecules in the growing layer and functional groups on the self-assembled monolayer.Hydrogen bonds formed between the CO¡2 and NHá3groups of glycine and a hydroxy group on the surface of a monolayer comprised of HOC6H4C6H4SH molecules was assumed to be stronger than the interaction between the NHá group of glycine and the nitrogen atom of pyridine on the surface of a monolayer built of NC5H4C6H4SH molecules.As a rule, self-assembly of monolayers of different substances on metal nanoparticles and films was studied using X-ray photo- electron spectroscopy, ellipsometry and diffraction techniques. Electronic spectroscopy was successfully used 103 in studying self- assembly of monolayers of metalloporphyrins and metallophtha- locyanines on ultrathin gold films. Gold films 1.3 ± 10-nm thick were prepared using high-vacuum evaporation on mica at low temperatures. The deposition rate was varied in the range 0.2 ± 0.4 A s71. The films were annealed at 250 8C for 2.5 ± 4 h. The resulting films were studied using scanning atomic force microscopy, electronic spectroscopy and X-ray diffraction techni- ques.It was shown that isolated structurised islets were formed on the mica surface the sizes and optical properties of which were governed by the conditions of evaporation and subsequent annealing. The electronic spectra of such films demonstrated a peak due to a gold surface plasmon with a wavelength shifting from 606 to 530 nm as the thickness of the film decreased from 10 to 1.3 nm. The kinetic data on the light absorption were used for making semiquantitative estimates of the nature of bonding and chemical and structural properties of the monolayers. It was shown that the absorption in the region of the gold surface plasmon band, which is caused by association of molecules of organometallic compounds on gold films, may be used for monitoring molecules devoid of chromophore groups.In addition to the sodium salt of AOT, its derivatives with other metals were used as the templates for self-assembly. To correctly interpret the template effect on the self-assembly of metal particles, phase diagrams of such systems are necessary.104 By the example of the Cu(AOT)2 ± iso-C8H18±H2O system used as a template, it was shown that the shapes of nanocrystalline particles can be dramatically changed under the effect of various salts in small concentrations.105 The development of methods for synthesising metal nano- particles of controllable sizes is of great interest. Apparently, theNanochemistry of metals majority of studies in this direction employed thiolates and were devoted to the preparation of gold nanoparticles. The use of alkanethiol monolayeres made it possible to synthesise gold particles stable both in solution and in the `dry' state.Introduction of functional groups in alkanethiols 57 extended the possibilities of such a synthesis. In the presence of alkanethiols, the formation of gold nano- particles and the appearance of a protective layer proceed in two steps 106 (AuISR)n (polymer), AuCl¡4 (in toluene)+RSHAux(SR)y . (AuISR)n+BH¡4 This reaction combines the processes of nucleation and passivation. An increase in the RSH : Au molar ratio and the addition of the reductant at the earlier stage result, on average, in the formation of particles with smaller metal cores. A short interruption of the reaction favours the formation of thicker coatings with very small cores (<2 nm).The dynamics of growth of gold cores in the presence of protective monolayer coatings was studied.106 The use of trans- mission electron microscopy made it possible to monitor slow changes in the sizes of gold particle cores which followed the active initial stage of the reaction. Thus it was found that with hexane- thiol as the stabilising coating, the core size of a particle based on gold atoms increased to 3 nm during the first 60 h of the reaction and then remained unchanged. The knowledge of the mechanisms of growth and annealing of nanoparticles will allow one to refine the methods for synthesising finer particles with narrower size distributions.V. Photochemistry and nanophotonics Recently, studies which deal with kinetics and dynamics of reactions that involve metal nanoparticles have appeared. A photochemical reaction which involves n-dodecanethiol-covered silver nanoparticles dispersed in cyclohexane was studied in a picosecond time interval.107 Figure 5 schematically shows the photoreaction mechanism. E* 0.5 ns 1.4 ns E** E* hn E th SS S SS 3.6 ns 70 ms Dissipation kT E0 S S S SS S SS S S SS S S Figure 5. Schematic illustration of the photoreaction involving AgxSC12H25 dispersed in cyclohexane.107 According to electron microscopic data, the average size of polyhedral AgxSC12H25 nanoparticles was *6.2 nm. Under short-term irradiation, the nanoparticles first decomposed to smaller particles (< 2 nm).However, an increase in the irradi- ation time to 9 min resulted in the appearance of larger (20-nm) particles. Photochemical transformations were detected from the time variations of the intensity and broadness of silver plasmon absorption lines (marked by heavy lines in Fig. 5). It was found that this absorption depends nonlinearly on the light intensity. The kinetics of photolysis of the particles was studied in the nanosecond range, and the corresponding kinetic constants were determined. Changes in the dielectric properties of the environ- ment occurred within 0.5 ns. In response to optical excitation, a part of alkanethiol molecules were split off over the time (3.6 ns) determined by the energy liberated as heat (Eth).The proportion of particles with split off thiol groups depended on the excitation- pulse energy. Within 40 ns (recombination time), the thiol-cov- ered particles could also undergo fragmentation. Subsequent exposure to light partially prevented fragmentation of nano- particles. In contrast to silver-based nanoparticles, which undergo light- induced chemical transformations, the behaviour of gold-based nanoparticles under similar conditions is characterised by changes in their shapes. Fragmentation of these nanoparticles is dramat- ically weakened both in polar solvents and with the changes in the length of the alkanethiol chain.108 Silver nanoparticles were used for carrying out photochemical transformations of phenazine and acridine.109 Being adsorbed on the surfaces of silver particles, both molecules were found to decompose under the action of light in a single-photon process with N7C bond cleavage.The reaction rate and the degree of photodecomposition depended on the light wavelength. Being exposed to short-wave laser light, phenazine decomposed with the formation of graphite. The decomposition rate of phenazine was higher compared to acridine, which apparently is due to different orientation of these molecules on the surfaces of silver particles. The reason for the decomposition these molecules undergo on the surfaces of silver nanoparticles remains unclear.109 The spectra of acridine adsorbed on nanoparticle surfaces revealed two new S S S S S S SSS S SSS S S S S S S S S S S Slow aggregation SSSS S S 817 S S S S S S SS Splitting off thiols + S S SSS S S S S S Fragmentation Recombination (40 ns) S S S S Reverse association S S (0.13 s) + SS S SS S SS S SG B Sergeev 818 p-nitrophenol on freshly prepared and aged powders.Upon long- term storage, the optical properties of powders remained unchanged. Highly concentrated aqueous solutions (up to *100 mmol litre71) of semiconducting particles were used as fluorescent biological labels. Multilayer films incorporating CdSe nanoparticles were syn- thesised.120 The particles measuring 1.7 ± 2.0 nm were obtained from a dimethylformamide solution of cadmium salts and sele- nium.The films incorporating CdSe particles were prepared by successive formation of layers on quartz or CaF2 plates. Layers of benzoic acid derivatives and polyvinylpyridine were first applied to the plate surface. The plate thus treated was placed in a dimethylformamide solution of CdSe in order to form a layer containing particles of cadmium selenide. Multilayer (up to five layers) films were obtained by severally adding polyvinylpyridine and cadmium selenide to the solution. Such films are interesting for the development of new types of light-emitting diodes and non-linear optical devices and can serve as conducting films. bands at 543 and 619 cm71, which are close to those observed for quinazoline adsorbed on a silver surface.110 It was noted that the rate and the extent of photoreactions described are not interre- lated.It was assumed that to be decomposed the molecules of phenazine and acridine should either be adsorbed on certain surface sites or form definite geometrical configurations; hence, only a small proportion of molecules can be involved in the reaction. Optical properties of gold nanoparticles with radii of 2.5, 9 and 15 nm prepared by g-radiolysis of an aqueous solution of KAuCl4 and stabilised in polyvinyl alcohol were studied.111 It was found that solutions containing 2.5-nm particles do not attenuate transmitting light, whereas those with larger particles severely decrease the intensity of a laser pulse with a wavelength of 530 nm.The effects observed were attributed to the appearance of a great number of light-diffusion centres formed as a result of evapora- tion of gold nanoparticles under the laser pulse. Light-diffusion centres were formed in several nanoseconds and largely relaxed to the initial state; upon the next pulse, they partially degraded to small particles. By means of time-resolved laser measurements, the elec- tron ± phonon interactions in metal particles based on Au particles with sizes ranging from 2 to 120 nm and in bimetallic particles based on Au, Ag and Pb were studied. Particles with Au cores and Ag shells and those with Au cores and Pb shells were studied in most details. All the particles were synthesised by radiolysis.61 It was shown that for Au, in contrast, e.g., to Ag, the time scale of the electron ± phonon interaction is independent of the particle size.112 Preparation of semiconducting nanowires of definite diame- ters by using colloidal nanoparticles of metal-catalysts was described.121 ± 124 As an example, we consider the results of controlled synthesis of GaP nanowires with diameters of 10, 20 and 30 nm and lengths exceeding 10 mm.124 The wires were obtained by laser ablation of a solid target made of GaP and gold (catalyst). For this purpose, a solution that contained gold particles was applied on a target.The latter was placed in a quartz tube with a solid ZnS target. The tube was located in a blown end of an oven, which was heated to 700 8C and the target was ablated for 10 min by means of an excimer laser.Simultaneous generation of nanoclusters of gold and of GaP semiconductor occurred; the latter particles formed semiconducting nanowires. Control experiments have shown that in the absence of gold particles no semiconducting wires form. A relationship between the sizes of gold nanoparticles and the diameters of semiconduct- ing GaP wires allowed the authors to conclude that the diameters of the wires obtained can be controlled. A similar method of governing the diameters of silicon nanowires was proposed.123 Finally, the studies dealing with the prospects of obtaining nanosize quantum dots, quantum wells and quantum wires in inorganic semiconductors 113 should be mentioned, as well as the works on nanophotonics.The latter studies optical properties of objects with much smaller sizes compared with the irradiation wavelength. The efforts were largely focused on investigation of linear optical effects, while the field of non-linear nanoscale optical phenomena is still under way.114 The problems and prospects of nanophotonics were analysed in Ref. 115. VI. Semiconductors and sensors The effect of the size of a semiconducting nanoparticle on its forbidden gap width was studied using CdSe particles measuring from 0.7 to 2 nm as an example.125, 126 All the particles were obtained by using the methods of organometallic chemistry so that their surfaces bore phosphorus atoms and phenyl and propyl groups.For example, the composition of one of the particles studied was Cd10Se4(SePh)12(PPr3)4. The atomic structure of the particles was determined using the X-ray diffraction technique. Figure 6 shows the dependence of the forbidden gap width of cadmium selenide nanoparticles on their sizes. It is seen that as the a b Eg /eV Eg /eVCd Cd Cd8 Cd8 4.0 4.0 Cd10 Cd17 3.5 3.5 Cd10 Cd17 Cd32 Cd32 3.0 3.0 Semiconducting nanoparticles are widely used in heterogeneous catalysis, they are also promising materials for laser engineering, in manufacturing flat displays, light-emitting diodes and sensors. The use of AOT-based reverse micelles for the preparation of ZnSe nanoparticles was first reported in Ref. 116.ZnSe particles measuring 5.7 nm were studied using X-ray diffraction, electronic spectroscopy, light scattering, electron microscopy and lumines- cence techniques. A method of stabilisation of nanoparticles of a semiconductor (exemplified by CdS, co-deposition of CdS ± ZnS) and a metal (exemplified by gold) was put forward by Japanese scientists.117 Reverse micelles were prepared and nanoparticles were stabilised by in situ polymerisation of (cetyl)dimethyl(p-vinylbenzyl)ammo- nium chloride which was initiated either by light or the addition of azobisisobutyronitrile. The polymer obtained was soluble in polar solvents and formed transparent films with incorporated nano- particles of cadmium sulfide measuring from 4.7 to 6.3 nm. The proposed method seems to be versatile.2.5 2.5 2.0 2.0 Internal cores of vesicles based on a-phosphatidylcholine can be used as `nanoreactors' for growing monodispersed (with*8% deviation) nanocrystals of CaS, ZnS, HgS with definite sizes.118 0.2 3 d /nm 0 2 1 0 0.4 1/(NCd)1/3 Figure 6. Forbidden gap thickness of CdSe nanoparticles as a function of their (a) diameter and (b) reciprocal radius.125 N is the number of atoms in a particle. Black points are experimental data, open points are literature data for larger particles (results by different authors). The arrow marks a bulk particle. A simple and inexpensive method of synthesising gram quantities of stable water-soluble nanocrystalline powders of zinc sulfide (particles measuring *6 nm) was put forward.119 Cysteine-covered ZnS particles were obtained by introduction of sulfide to predissolved zinc salt and cysteine.Following deposi- tion, redissolution and drying, crystalline ZnS powders were obtained, which remained stable for 30 months at 4 8C. Their stability was checked by carrying out catalytic decomposition ofNanochemistry of metals nanoparticle size increases the forbidden gap width (Eg) of a particle approaches the Eg value of CdSe bulk samples. Dynamic properties of charge carriers on the liquid ± semi- conductor interface are important for photocatalysis, solar energy conversion and photoelectrochemistry. The properties of charge carriers, viz., electrons and holes, were studied in a number of semiconducting nanosystems using laser techniques.127 Suspensions of cadmium sulfide nanoparticles were used for the initiation of acrylonitrile polymerisation.The polyacryloni- trile (PAN) obtained and the products of its partial hydrolysis were used as the templates which governed the shapes of CdS nanoparticles and CdS/PAN composite nanowires with diameters of 6 nm and lengths varying from 200 nm to 1 mm.128 A nanosize electronic switch was built of gold nanoclusters and compounds containing redox groups.129 Synthesis of N,N 0- di(10-mercaptodecyl)-4,40-bipyridinium dibromide containing a bipyridinium fragment as the redox group was described.130 When gold nanoparticles are bound to the film of bipyridinium dibro- mide molecules, the reduction of the bipyridinium group on a gold electrode proceeds readily and reversibly.bipy2++e7 bipy+.. Figure 7 schematically shows an electronic switch measuring less than 10 nm which is controlled by the voltage applied to the STM probe.} When a molecule involves bipyridinium in a reduced (bipy+. ) state, a noticeable tunnelling current flows in the nano- cluster ± molecule ± electrode circuit. If a certain threshold voltage is applied to the gold electrode, the tunnelling current decreases. The threshold voltage corresponds to the oxidation of bipy+. to } The scheme is taken from Ref. 131, in which the results of another study (Ref. 130) are commented upon. The system shown was assumed 130 to correspond to approximately 60 organic molecules and required no less than 30 electrons to operate.a b STMprobe STMprobe Gold nanoparticle Current No current S S 9 9 Thiol bridge N+ N+ Electron transfer Bipyridinium group e7 + N N Thiol bridge 9 9 S S Gold electrode Gold electrode Figure 7.Schematic illustration of the performance of an electronic switch.131 (a) Bipyridinium group is in the oxidised state and no current flows, (b) accepting an electron, the bipyridinium group is reduced and a current arises. 819 bipy2+. At present, such devices operate slowly and require an amplifier. However, they can be used in those fields where amplification is of less importance, e.g., as the chemical sensors for detection of single molecules or individual chemical reactions.Moreover, based on these systems, memory devices for computers can be developed.131 New possibilities in designing sensors and optoelectronic devices including those with nanoparticles have been pio- neered.132 It was shown that pore-free crystals of organoplatinum compounds can reversibly absorb and evolve sulfur dioxide with- out any losses in their crystallinity. Figuratively speaking, the crystals `breath'. Figure 8 schematically shows the corresponding transitions. SO2 is present SO2 is absent Cl Cl Pt Pt N N N SO2 N = = , . OH OH Figure 8. Crystals that reversibly absorb SO2.133 On being exposed to SO2 for one minute, the crystal acquires an orange colouring. The colour change is accompanied by a transition of planar square platinum complexes into square- pyramidal complexes which involve SO2 as the fifth ligand.In the process, the crystal increases its volume by 25% but retains the ordered crystal structure. Then, if the crystal is exposed to air, it breathes out SO2 and relaxes to its initial colourless SO2-free state. The process can be repeated many times without loss of the crystallinity. It was suggested to use such crystals as optical switches and SO2-sensors. It is possible that some other similar compounds can reversibly interact with chlorine, CO2 and other gases. There are strong grounds to believe that substances that bind gases in solution can also bind them in the solid state. Nanocrystalline films of semiconducting oxides can also be used as the sensitive units of gas sensors. The effects of ethanol and nitrogen dioxide vapours on the properties of SnO2 nanocrystals applied on single-crystal silicon and of SnO2 nanocrystals doped by nickel, palladium and copper were studied.134 Doped SnO2 nanocrystals were synthesised by pyrolysis of aerosols.According to X-ray diffraction data, their average size was 6 ± 8 nm. It was shown that diffusion of gases affects the voltammetric character- istics of heretostructures, which manifests itself in variations of the potential-barrier height on the heterogeneous boundary and in the changes in the tunnelling processes at the interface. A new generation of chemical sensors can be synthesised by using non-uniform nanosystems and unusual electronic and physicochemical properties of nanoparticles involved in such systems.In these systems, compact crystalline cores of nano- particles are surrounded by outer amorphous shells, and the electron transfer induced by adsorbed gas molecules proceeds through the nanoparticles arranged in ensembles and interacting with one another. The sensor properties of non-uniform nano- composite films were studied 135 for the oxide ± oxide (SnO2 ± TiO2), metal ± oxide (Cu ± SiO2) and metal ± polymer [Cu ± poly(p-xylylene)] systems. The sensor activities of the speci- mens with respect to hydrogen, wet air and ammonia were820 determined from conductivity variations. All specimens were synthesised under non-equilibrium conditions, which seems to be necessary for the preparation of effective sensor materials.VII. Catalysis on nanoparticles The studies devoted to using nanoparticles in the development of novel catalysts which were published before 2000 are surveyed in detail by Bukhtiyarov and Slin'ko.14 In this section, we consider only the studies published in 2000. Methane combustion in air proceeds steadily at temperatures above 1300 8C. However, at these temperatures, noxious nitrogen oxides evolve resulting in smog. In this connection, the search for new catalysts of methane combustion is urgent. A new catalyst was synthesised 136 which provided methane combustion at 400 8C. This was prepared using reverse microemulsions based on isooctane, water and surfactants such as the polyethylene oxide ± alcohol adducts.Alkoxides Ba(OC3H7)2 and Al(OC3H7)3 were dissolved in isooctane and mixed with the microemulsion at room temperature: the resulting solid crystalline nanosize barium hexaaluminate exhibited high catalytic activity in CH4 combus- tion. The advantage of barium hexaaluminate lies in the fact that the sizes and surfaces of its particles remain unchanged at high temperatures. Moreover, this can be modified by cerium, cobalt, manganese and lanthanum. Modification with cerium oxide produced a composite which enabled methane combustion at temperatures below 400 8C. The history of the search for new methane-combustion catalysts and their characteristics are described elsewhere.137 In aqueous AOT± n-heptane buffer microemulsions, a num- ber of reactions were catalysed by palladium nanoparticles.The reaction of N,N-dimethyl-p-phenylenediamine with Co(NH3)5Cl2+ catalysed by palladium nanoparticles was described.138 The catalytic oxidation of N,N,N 0,N 0-tetramethyl- p-phenylenediamine by Co(NH3)Cl2+ was also studied in similar microemulsions at pH 5.6.139 The limiting stage of the reaction was the adsorption of p-phenylenediamine on palladium particles of 2.5-nm radii within the microemulsion. This conclusion relies on an observation that the activation energy of the reaction changes from 97 at 15 8C to 39 kJ mol71 at 40 8C and on the results of electrochemical measurements. The specific features of the reaction were discussed in terms of the microreactor model.Synthesis of platinum, palladium, rhodium and iridium nano- particles and their use in catalytic hydrogenation of cyclooctene, dodec-1-ene and o-chloronitrobenzene were described.140 Nano- particles were synthesised by reduction of metal salts by alcohols and stabilised by an amphiphilic copolymer of 1-vinylpyrrolidone with acrylic acid (PVP ± AA). The diameters of particles were 0.74 (Ir), 1.93 (Rh), 2.2 (Pd) and 1.2 ± 2.2 nm (Pt), which were determined by electron microscopy. The introduction of nickel ions into the PVP ±AA± Pt cata- lytic system enhances the effeciency of the latter. Hydrogenation of chloronitrobenzene to chloroaniline at 330 K was carried out with a selectivity of 97.1% and 100% conversion. Introduction of Co2+ and Fe3+ ions reduced the hydrogenation selectivity to 78.1% and 72.1%, respectively.The analysis of IR spectra have shown that, on being exposed to hydrogen, the ions of nickel, iron and cobalt were not reduced to zero-valent metals. This was explained by the fact that the metal ions interact with two C=O groups in the PVP ±AA copolymer and only with a single C=O group in poly(1-vinylpyrrolidone).141 The problems of measuring sizes of metal particles applied on supports and size and surface-area distributions for particles with diameters smaller than 2.5 nm were discussed in Ref. 142. When chemisorption is used for the determination of a catalyst's properties, it is necessary to know how to follow catalyst operation with contaminated surfaces and make an allowance for strong interaction of the metal with the support.A systematic G B Sergeev study (in the temperature interval of 573 ± 973 K) of palladium particles of average size *2 nm applied on activated carbon has been accomplished.143 The use of two methods, viz., X-ray small- angle scattering and transmission electron microscopy allowed the effective sizes of palladium particles to be estimated and the results to be compared with the data obtained in studying CO chem- isorption on supported palladium. The agreement between the results of two aforementioned methods was observed for the stoichiometric ratio Pd :CO=2. A high catalytic activity of nanoparticles composed of metal cores and outer shells was demonstrated in several studies.144 ± 147 Gold nanoparticles measuring 2 and 5 nm surrounded by dec- anethiol molecules were used for electrochemical oxidation of carbon monoxide.145 Figure 9 schematically shows the electrode reaction.It is noteworthy that the use of thiol-covered gold nanoparticles suggests a co-operative increase in the catalytic activity. CO CO2 S S S S S S S S SS S S S S S S S S S S e7 S S S S S SSSS SSS S S S SSSS S S S e7 S S S S S S S S S S S SS S S S S S S S e7 Electrode Figure 9. Catalytic oxidation of carbon monoxide on an ensemble of thiol-covered gold nanoparticles on the electrode surface. The methods of preparation of optimal catalysts may involve varying the shapes of cores, the structures and properties of molecular shells and the types of core ± shell bonding.The latter factor affects active sites, defect packing and co-operative elec- tronic properties of nanoparticles. Platinum nanoparticles were obtained by reducing chloropla- tinic acid (H2PtCl6 . 6H2O) by ethanol in the presence of poly(N- vinylformamide), poly(N-vinylacetamide), poly(N-isopropylacry- lamide) or poly(N-vinylpyrrolidone). The particles synthesised had an average diameter of *2.0 ± 2.5 nm.148 The effects of additon of KCl and Na2SO4 and of temperature variations on the stability of nanoparticles were studied. The particles obtained were used in hydrogenation of allyl alcohol in water and in a salt solution (0.8 M Na2SO4).It was shown that catalytic platinum particles are stable in the salt solution and their activity is equal to that in water. Bimetallic Au± Pt/C(graphite) catalysts were prepared by selective deposition of gold on a platinum film on graphite as a support.149 The latter monometallic catalyst was obtained by the reduction of H2PtCl6 in anhydrous ethanol. The monometallic catalyst was modified by two methods. The first method used platinum as the reagent which reduced AuCl¡4 . In the other method, the monodispersed catalyst was first treated with hydro- gen and then HAuCl4 was added. At pH 1, bimetallic particles were formed according to the reaction 3 PtH+AuCl¡ Pt3Au+4 Cl7+3H+. 4 The average size of particles was *10 nm. The properties of particles and their reactions were studied by several techniques.Nanochemistry of metals Being chemically inactive, gold did not attract much attention as a catalyst until recently. However, gold ions Aun+ (14n43) in zeolites turned out to be active in the reaction H2+CO2 H2O+CO at 323 K.150 Lithographic systems comprising SiO2-supported platinum nanoparticles and Al2O3-supported silver nanoparticles, which were prepared using electronic beams, were used as model metallic catalysts.151 The particles with the average sizes of 20 ± 50 nm were spaced at 100 ± 200 nm.The thermal stability of the systems obtained was studied by means of electron microscopy and AFM. The system with platinum particles was stable up to 973 K, whereas the system with silver particles was stable up to 773 K in both vacuum and hydrogen and up to 623 K in oxygen.The catalytic effect of a model catalyst with SiO2-supported platinum nanoparticles was compared to that of a platinum foil in hydro- genation and dehydrogenation of cyclohexene at 100 8C.152 The reactivity of a system with platinum particles of 28 nm diameter was shown to be twice as high compared with that of the foil, and its selectivity was higher by a factor of 3. Isomerisation of dichlorobutanes catalysed by iron-contain- ing nanoparticles stabilised in polymeric matrices was studied.153 The catalytic activity was shown to depend on the nature of the matrix and the metal content. Photocatalytic reduction of bis(2-bipyridyl) disulfide (RSSR) to pyridine-2-thiol (RSH) by water selectively proceeded on the surface of titanium dioxide.The reaction rate substantially increased when silver nanoparticles (0.24 mass %) with sizes <1 nm were applied on TiO2.154 The following mechanism was assumed 154 for this reaction: Ka 2 RSAgTiO2 , Ij 2 RSAgTiO2(e7... h+), kdl 2 RSAgTiO2 , kcs 2 RSAg(e7)TiO2(h+), kd2 2 RSAgTiO2 , (1) RSSR+2 AgTiO2 (2) 2 RSAgTiO2 (3) 2 RSAgTiO2 (e7... h+) (4) 2 RSAgTiO2 (e7... h+) (5) 2 RSAg(e7)TiO2(h+) k0 (6) 2 RSAg(e7)TiO2(h+)+H2O 2 RSAg(e7)TiO2+2H++12O2 , kr 2RSH+AgTiO2 , ( 7) 2 RSAg(e7)TiO2+2H+ where Ka is the equilibrium adsorption constant, I is the light intensity, j is the intensity of light absorption.In the first stage, the selective adsorption of RSSR on the surfaces of silver nanoparticles is accompanied by the cleavage of the S7S bond. In the second stage, upon the excitation of nanoparticles in the forbidden gap range of TiO2, an electron (e7) ± hole(h+) pair is generated. Most of electron ± hole pairs recombine (stages 3 and 5). In stage 4, charge separation occurs: silver nanoparticles accept electrons, and holes are transferred to the TiO2 support. The positive potential of a hole is sufficient for water to be oxidised with the formation of H+ ion and O2 molecule (stage 6). In stage 7, the catalytic system is reduced producing RSH. As a result, the deposited silver nanoparticles seem 154 to cause the following effects: �acceleration of RSSR adsorption; � space separation of sites responsible for reduction (silver particles) and oxidation (TiO2 support), i.e., the effect of charge separation; �selective adsorption of the substance to be oxidised (RSSR) and the reducing agent (H2O) in the reduction and oxidation sites, which makes the process highly selective.821 Comparison of TiO2 and AgTiO2 catalytic systems has shown that both systems provide equal activation energies in the photo- catalytic reduction (301.5 kJ mol71), which suggests that the multistage process is limited by stage 6 rather than by photo- catalysis. Below, the reduction on TiO2 is shown. kr 2RSH+TiO2 . 2 RS...TiO2(e7) + 2H+ In recent years, semiconducting nanoparticles have been actively used in photocatalysis.In this connection, the catalytic activities of nanoparticles of different oxides and sulfides in aqueous and polar (acetonitrile) organic solvents were com- pared 155 using photooxidation of pentachlorophenol (a toxic compound used as a fungicide and a bactericide and for wood conservation) as an example. Figure 10 shows the relative con- centration of pentachlorophenol as a function of the time of its exposure to light in the presence of nanoparticles of different metal sulfides. c (arb. u.) 1.0 0.1 1234 t /min 600 400 200 0 Figure 10. Changes in the relative concentration of pentachlorophenol (c) in water with the time of its exposure to light of the wavelength 400<l<700 nm (1) in the absence and (2 ±4) in the presence of catalysts.155 (1) Without catalyst (or TiO2); (2) CdS powder (0.1 mg ml71), (3) MoS2 particles with diameters of 4.5 nm (0.036 mg ml71), (4) MoS2 particles with diameters of 3 nm (0.09 mg ml71).It is of note that the effect of the nanoparticle size on the reaction rate is extremely strong for MoS2 particles. This reflects the effect of the forbidden gap width of particles on the reaction and is associated with the corresponding change in the reduction potential. In contrast to a common photocatalyst, viz., titanium oxide, which is active in the UV region, molybdenum disulfide particles measuring 3 nmcatalyse the reaction in the visible range. When SnO2 nanoparticles measuring 26 and 58 nm were used, the nanoparticle size did not significantly affect the photooxidation of pentachlorophenol. One can predict the development of a new direction of nano- chemistry which is associated with realisation of chemical reac- tions by means of AFM and STM probes.Interesting examples of chemical modification of terminal functional groups in aggregates of organosilicon molecules were carried out by the catalytic action of an AFM probe covered (at open circuit conditions) with palladium.156 A minimum effort required for such chemical reactions to occur is*2.5 mN at a scanning rate below 5 mm s71. It should be noted that the typical activation energy of the bimolecular reactions shown is *50 kJ mol71. At the same time, according to estimates, the energy of surface deformation of an associate formed by organosilicon molecules is *340 kJ mol71, which far exceeds the activation energy.An approach to `chemical lithography', which consists in chemical modification of functional groups in the molecules arranged along definite preset lines in a sample, was first docu- mented in Ref. 157.822 O Si (CH2)6 O Si (CH2)6 O OOO Si Si Si Si (CH2)6 O O Si O Si O Si Si Si Si O Si O Si O OOOO OOO Si Si Si Si O O Si O Si O Si Si Si Si O Si (CH2)6 O Si (CH2)6 O OOOO OOO Si Si Si Si (CH2)6 O O Si O Si O Si Si Si Si OOOO Z is a benzyloxycarbonyl group. VIII. Nanoparticles in biology and medicine At present, a new direction of nanochemistry is being developed vigorously. The goal of these studies is the synthesis and applica- tion of systems comprising metal nanoparticles (mainly, gold and silver) and various biomolecules: DNA, peptides and oligonucleo- tides.The optical properties of aggregates of gold nanoparticles bound by DNA fragments containing from 27 to 72 nucleotide pairs were discussed.158 Alkanethiol-modified gold particles of diameter *15 nm were used. The effect of the length of an oligonucleotide chain on the optical properties of aggregates obtained was studied, and the position of the metal plasmon peak was found to depend on the chain length. The sizes of nanoparticle aggregates were determined by the kinetic method, because their growth rate depended on the lengths of oligonucleo- tide chains and on the distances between nanopes.Among N N N7 N HCO2H, HCO2Na N N7 Pd N N N7 NH2 (CH2)6 NH2 (CH2)6 NH2 (CH2)6 HNZ HNHCO2H, HCO2Na Z Pd HNZ NH2 NH2 NH2 CH2 CH2 HSiMe2(CH2)4NH2 Pd CH2 Me2 Si (CH2)4NH2 (CH2)7 Me2 Si (CH2)4NH2 (CH2)7 Me2 Si (CH2)4NH2 (CH2)7 G B Sergeev the factors that affect the growth rate of aggregates, the rate of binding ofDNAlinkers } with complementaryDNAmolecules on the surfaces of gold nanoparticles (k1) and the rate of aggregate growth (k2) are the most important. Optical changes were also observed upon annealing of structures formed by two longest nucleotides-linkers at temperatures below their melting point.Thus, DNA linkers can be used for kinetic control over aggregate growth. A new method of introduction of biomaterials into living cells was proposed 159 based on electrospraying of metal particles with large electrical charges and high velocities. If one disperses gene- covered metal particles using the equipment developed, these particles disintegrate in liquid drops under the external non- uniform field. Gene fragments thus obtained have charges of the same sign as the metal particles and are present in high concen- trations. Monkey fibroblasts were used as the living cells and either suspensions of plasmids { labelled with fluorescent peptides or plasmids with gold nanoparticles measuring 5 ± 10 nm were used as the biomaterials.UV fluorescent microscopic studies have shown plasmid suspensions with gold particles to penetrate into cells. The fact that gold-free plasmid suspensions also penetrated into the cells and got incorporated into its DNA is of prime importance. This result seems to be most significant, because this allows one to dispense with gold traditionally used for these purposes. It was noted that electrostatic spraying opens up new possibilities for transfection of genes and gene therapy. Nanoparticles doped with gadolinium ions were proposed for use as new contrast materials for NMR studies in medicine. Such nanoparticles with diameters of 120 nm are sufficiently small to penetrate easily into blood vessels.These have been used for obtaining the images of hearts and gastric channels in rats.160 Approaches to using biomolecules for identification of com- mon inorganic materials are being developed. These can be realised by applying the principles of selective binding known from molecular biology. Selective binding of peptides to different semiconductors was proposed as a method of preparation of nanocrystalline ensembles.161 It was shown that nanocrystalline semiconductors can be used for the isolation of certain peptides which bind specifically to the semiconductor surface. Five differ- ent faces of semiconducting single crystals were used, as the substrates, viz., GaAs(100), GaAs(111)Ga (with Ga atoms on the surface), GaAs(111)As (with As atoms on the surface), InP(100) and Si(100).Each substrate was found to select and bind certain amino acid sequences from a large number of random peptides. The nanocrystalline ensembles obtained were studied using antibodies labelled with 20-nm gold nanoparticles and the methods of transmission and fluorescent microscopy, photoelec- tron spectroscopy and atomic-force and scanning tunnelling microscopy. Figure 11 illustrates two different approaches to the develop- ment of biomolecular ensembles on surfaces of inorganic materi- als.162 Development of these approaches is a challenge to materials science of the XXIst century. In this context, attention should be drawn to a study 163 in which the prospects of synthesising two- and three-dimensional nanostructures based on biological principles were considered.It is believed that the strategy of synthesising complex ensembles from simpler ones will be developed in the new century. Biological structures may be used as surface detectors which organise bind- ing of large organic and inorganic building blocks. Indeed, in this approach, the principles developed in the course of evolution for building new functional structures may be used for synthesising new materials. } Linkers are nonhelical segments of a double-stranded DNA which interlink compact DNA segments stabilised by peptides and polyamines. { Plasmids are extrachromosome formations representing closed circles of double-stranded DNA.Nanochemistry of metals a Biomolecules b Biomolecules Figure 11.Two approaches to assembly of inorganic materials into complex structures with the use of biological molecules. (a) The use of complementarity of two biomolecules, (b) the use of biomolecule interaction with an inorganic material.162 IX. Conclusion The analysis of studies published in recent years shows that many aspects of nanochemistry of metals and semiconductors are gaining importance, and considerable progress has been made towards their solution. The factors that affect the sizes and shapes of nanoparticles during their synthesis, the processes of self-assembly of both nanoparticles themselves and ligands that stabilise them are being actively studied. The list of polymers, surfactants, reversed micelles and thiols, which are traditionally used for the stabilisa- tion of nanoparticles, was successfully replenished with den- drimers.The use of nanoparticles in catalysis has been extended, as well as the use of semiconducting particles in photocatalysis. The studies in which metal nanoparticles are used in carrying out unusual reactions are conducted more intensively. The problem of synthesising and studying the reactions of non-protected metal nanoparticles becomes increasingly pressing. Interest is shifting to reactions of metal nanoparticles measuring less than 1 nm. In this aspect, the use of inert gas at low and superlow temperatures as the matrices for the synthesis of nanoparticles seems to be very promising.Yet another challenging direction of nanochemistry may be developed at the boundary of such sciences as biology, organic and inorganic chemistry and materials science. Mutual `recogni- tion' and joint self-assembly of inorganic and biological molecules are the key problems which being solved, will provide new advanced materials.164 At the beginning of 2000, a new programme aimed at the development of fundamental and applied studies in nanotechnol- ogy was formulated in the USA.165 In 2001, this issue is addition- ally financed by 500 million dollars. The following directions were chosen as the most challenging: �the use of modern experimental equipment for studying the processes which involve nanoparticles or proceed on the surfaces of nanostructures; � elucidation of the roles nanoparticles play in such impor- tant processes as deposition of proteins, desorption of contami- nants, stabilisation of colloidal dispersions, aggregation of micelles and control over microorganism mobility; � development of experimental techniques and their com- puterisation for studying the role of nanoparticles in the processes that occur in the atmosphere and water sources; Inorganic material Inorganic material 823 � the use of mesoporous structures integrated with appro- priate microcomponents to provide highly sensitive and highly selective contamination detectors.In conclusion, it should be stated that at present the studies of nanoparticle properties form an interdisciplinary scientific field which largely determines the development of the other neighbour- ing fields.The author is grateful to his co-workers, post-graduate students and students of the Laboratory of Low-Temperature Chemistry for their assistance in the preparation of this review. 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ISSN:0036-021X    

出版商:RSC    

年代:2001

数据来源: RSC

摘要:

Russian Chemical Reviews 70 (10) 827 ± 863 (2001) The chemistry and application of carbon nanotubes E G Rakov Contents I. Introduction II. Filling of the inner cavities of carbon nanotubes III. Functionalisation of carbon nanotubes IV. Decoration of carbon nanotubes and their use as templates V. Substitution of the carbon atoms of nanotubes by atoms of other elements VI. Insertion of `guest' atoms and molecules into the intertubular space of multi-walled nanotubes and nanotube bundles VII. Adsorption and storage of gases in carbon nanotubes VIII. Potential fields of application of carbon nanotubes IX. Conclusion Abstract. physico- and chemical main the concerns review The The review concerns the main chemical and physico- chemical of modification of methods the and properties chemical properties and the methods of modification of carbon carbon nanotubes, and feasible The material.promising novel a nanotubes, a novel promising material. The feasible and potential potential fields outlined. are nanotubes carbon of applications of fields of applications of carbon nanotubes are outlined. The The bibliography references 573 includes bibliography includes 573 references. I. Introduction Nanotubes (nanotubulenes, hereafter NTs) were discovered by Iijima in 1991.1 A decade has passed since then; however, solid state chemists and physicists as well as materials scientists persist in giving increasing attention to NTs.2 Moreover, it is NTs that are believed to be a tool for passage from macromolecules to nano- technology and for transformation of the latter from a scientific concept into reality.Currently, we cannot say whether these dreams will come true or not. Intensive research into NTs continues to expand, the number of publications increases rapidly;3 some monographs, books of collected works and a textbook on NTs have been published.4± 10 Unfortunately, only a few out of the dozens of journal reviews concerning NTs were reported in the journals published in the Russian Federation.11 ± 16 In addition, at present these reviews fail to cover not only some recent advances, but also entire avenues of research into NTs. The author of this review strove to bridge this gap. The background of the NT problem was briefly outlined recently.15, 16 The structure, some physical properties and meth- ods for the preparation of NTs have also been reviewed.14, 16 To get acquainted with carbon NTs, we can also recommend a visit to the Website of the Michigan State University (USA) at URL http://www.pa.msu.edu/cmp/csc/nanotube.html.Here, it will only be remembered that perfect, defect-free NTs are formed by rolling planar atomic graphite sheets (graphenes) into seamless cylinders. Nanotubes can be multi-walled (MWNTs), comprising E G Rakov D I Mendeleev Russian University of Chemical Technology, Miusskaya pl. 9, 125047 Moscow, Russian Federation. Fax (7-095) 490 75 23. Tel. (7-095) 948 54 67. E-mail: rakov@rctu.ru Received 13 February 2001 Uspekhi Khimii 70 (10) 934 ± 973 (2001); translated by A M Raevsky #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n10ABEH000660 827 828 832 836 836 837 839 840 854 several coaxial cylinders, or single-walled (SWNTs).As a rule, the proportion of defects in SWNTs is less than in MWNTs. More- over, SWNTs can become defect-free after high-temperature annealing in inert media. The structure of NTs affects their electronic, mechanical and chemical properties; because of this, SWNTs and MWNTs behave in fundamentally different man- ners. The inner diameters of NTs vary between 0.4 nm and several nanometres. The volume of the inner cavity of NTs is sufficient for the molecules of other substances to occupy the cavity. Graphene sheets in SWNTs and in each shell of MWNTs can have different orientations of the primitive graphene lattice vectors.This affects the properties of NTs. The as-prepared NTs are usually closed by hemispherical or conical `caps' at their ends. A salient feature of NTs is their aggregation into rather stable bundles (bunches or ropes), which can be either bent or even closed (ring-like structures). The axes of individual NTs in the bundles are parallel to one another. The shortest distance between individual NTs is *0.32 nm. Nanotube bundles arise due to van der Waals forces. The intertubular space can be filled with `guest' molecules. Initially, most of NTs were synthesised by the arc method which involves graphite vaporisation in a dc arc struck between graphite electrodes in an inert gas flow.This technique was replaced by laser-induced thermal synthesis. This technique involves vaporisation of a graphite target under the action of a scanning laser beam in a furnace heated to a particular temper- ature and has a higher performance. In recent years, increasing attention has been given to pyrolytic synthesis from gaseous hydrocarbons (CH4, C2H2, etc.) and CO. Preference is given to the catalytic processes which allow the preparation of not only MWNTs, but also SWNTs in relatively high yields.16 It is thought that the processes proceeding in the presence of a `volatile catalyst' introduced into the reaction zone in the form of vapours along with the substances to be decomposed can provide the highest performance.Nanotubes can be prepared in different forms. Synthesis of NTs can give a tough precipitate of different density, e.g., a rubber-like material consisting of entangled NTs (compact `nano- paper', `buckypaper', `mats'), a web-like material and a textured material comprised of arrays of aligned NTs separated by some distance from one another (nanotube `forest', nanotube `towers').828 In addition to hollow cylindrical NTs, conical, helical, horn-like (`nanohorns'), bamboo-like (consisting of individual closed extended sections), different types of alternating conical NTs (`nanobells' and `fishbone') as well as L-, T- and Y-shaped NTs (both bent and branched) can also be formed. The synthesis of NTs is often accompanied by the formation of other forms of carbon (e.g., fullerenes, polyhedral particles and amorphous carbon).Therefore, in many instances purification of NTs (removal of side products) is a necessary stage. Before going to the main body of the text, mention should be made of the term `carbon nanotube chemistry', which is currently used in a rather broad sense.{ Carbon nanotube chemistry covers, first of all, the synthesis and purification of carbon nanotubes and their opening in reactions with various oxidants. In addition, the subject matters of carbon nanotube chemistry are modification of NTs (filling the inner cavity with different reactants, attachment of functional groups to the NT ends and side walls and decoration of these surfaces with particular molecules, replacement of some of the carbon atoms on the NT side walls by atoms of other elements, insertion of `guest' atoms and molecules into the intertubular space of NT bundles); the use of carbon NTs for template synthesis of nanowires, nanorods and nanotubes made of other substances; adsorption of gases on NTs and NT bundles and physicochemical properties of NTs.Research in the field of carbon nanotube chemistry also includes studies on the effect of the chemical modification on the electronic, magnetic, mechanical and other properties of NTs. With rare exceptions, in this review we will not dwell on the methods for the preparation and purification of NTs and filling their inner cavities. These problems have been considered in detail recently.16 Chemical transformations of fullerenes, graphite and NTs exhibit some common features.Nevertheless, it should be emphas- ised that nanotube chemistry differs essentially from both full- erene and graphite chemistry. This is due to the unique shape, structural peculiarities and small diameter of NTs. Nanotubes differ from fullerenes in the relatively large volume of the inner cavity (stable fullerene molecules are too small to contain more than three or four atoms of other elements within the inner cavity). Perfect (defect-free) NTs differ from graphite in very low percent- age or even the absence of chemically active dangling bonds. A feature of NTs is a variety of specific distances that are due to van der Waals forces (e.g., the separations between individual shells of MWNTs, between individual tubules in NT bundles, etc.).Each fullerene can be considered as a molecule which can form or be a constituent of molecular crystals. Graphite represents a typical layered polymeric crystal. In contrast to them, individual NTs cannot be thought of as individual molecules (SWNTs resemble molecules, while MWNTs are similar to carbon fibres) and NT bundles cannot be classified as conventional three-dimen- sional crystals (topologically, individual NTs and NT bundles represent one-dimensional and two-dimensional crystals, respec- tively.18). Graphite, NTs and fullerenes can form intercalation com- pounds with fundamentally different structures. Graphite forms two-dimensional structures, NTs usually form one-dimensional structures (formation of two-dimensional structures is possible for the rolled-up NTs), whereas fullerenes form [topologically] zero- dimensional structures.In addition, only NTs can form several types of intercalation compounds with different geometries. Sometimes, NTs exhibit quite unexpected chemical behav- iour. For instance, bent NTs contain topological defects (e.g., pairs of carbon pentagons and heptagons) and their reactivity differs from that of straight NTs. Carbon NTs also allow one to study the effect of the radius of curvature of the surface on the { This term was first used by the researchers from the Oxford University in 1996.17 Their communication concerned the synthesis, opening (removal of `caps'), filling and potential uses of NTs.E G Rakov strength of the bonding between the substances reacting on the NT surface and the carbon atoms. Modification of NTs can lead to essential changes in their electronic structure and functional properties and to creation of novel materials including those with unbelievable properties. The possibility of modification of fullerenes and NTs was first pre- dicted theoretically (see, e.g., Refs 19 and 20) and then confirmed experimentally. This review concerns the chemical properties of NTs and their role in the processes of fabrication of the NT-derived end products. In the largest part of the review (Section VIII), we consider the potential fields of application of NTs.This seems to be of great importance since NTs are no longer `exotic fruits'. II. Filling of the inner cavities of carbon nanotubes The inner cavities of NTs can be filled in the course of synthetic procedures or in a targeted manner after the preparation and purification of NTs. The latter strategy was found to be more versatile and rational. Targeted filling can be performed with melts, solutions or reactive gases that are either involved in the chemical vapour deposition reactions or form covalent chemical bonds with carbon atoms within the NTs. Filling the NT cavities with gases due to adsorption represents a special case. 1. Nanotube filling in the course of synthetic procedures Many substances can fill the inner cavities of NTs in the course of synthetic procedures.16 In this review, we will dwell on the results reported in some of the most recent reviews and original studies.According to Seraphin,21 in the case of arc synthesis of NTs the additives (B, Mg, Mn, Fe, Co, Ni, Cu, Y, Nb, Mo and Ta) can be encapsulated into the NTs in the form of carbides. The filling of NTs with Nb, Ta and Mo carbides has been studied in more detail.22 Nanotubes can also be filled with rare-earth element carbides and, in anH2 atmosphere, with metallic Cu and Ge which do not tend to form carbides. Two types of NT filling, a complete and incomplete one, have been described.23 For instance, Se, Sb, S and Ge fill NTs completely (up to several micrometres long), whereas Bi, B, Al and Te form short particles.No rationalisation was provided to this phenomenon. Sulfur is thought to play the key role in the formation of filled NTs in arc synthesis.24 Thorough removal of sulfur from the anodic material reduces the probability of the growth of filled NTs. The effect of sulfur on theNTgrowth mechanism in the case of arc synthesis was confirmed by Demoncy et al.25 who found that sulfur added to the anode material favours the filling of NTs with such metals as Co, Ni, Cr and Dy if the content of S is less than1% of the metal mass. Boron, a well-known graphitisation catalyst, also has a strong effect on the NT growth mechanism.26 A theoretical study of the effect of boron on the growth rate and morphology of NTs showed that boron stabilises the zigzag-shaped end of a NT, thus preventing it from `capping' and favouring the NT filling.27 Other substances also can act as promoters. The addition of both a catalyst (2% Co) and promoter (5% Bi) to the anode material (graphite) resulted in an increase in the diameter of the grown SWNTs, namely, nearly 50% of them were found to be 1.5 nm and more in diameter and a large proportion of the SWNTs had a diameter of 2 nm.28 It seems to be more interesting to consider the growth of NTs with encapsulated fullerene molecules.In this case, the encapsu- lated fullerene molecules must affect the properties of composites filled with such NTs, the electronic properties of the NTs and their filling with other substances.Smith et al.29 showed that the molecules of C60 and C70 and elongated capsules with diameters close to that of C60 can exist within NTs. It was also found that theThe chemistry and application of carbon nanotubes encapsulated fullerene molecules can move jumpwise and form pairs. The assumption of the possibility of filling the NT cavity with C60 molecules was first proposed in the study of the products of laser-induced thermal synthesis.30 After a short time, an exper- imental confirmation of this hypothesis was obtained, viz., nano- scale `peapod' structures were found among acid treated and in vacuo annealed (1100 8C) reaction products.31 Most of such structures have a NT diameter of 1.4 nm, which is twice as large as that of the C60 molecule (0.7 nm).They are characterised by yet another, third, type of specific separation that is due to the van der Waals forces (the above-mentioned two types are characteristic of MWNTs and NT bundles,32 see also Section I). This separation differs from the interlayer spacing in graphite but is close to this value (the shortest distance between the C60 molecule and the NT wall is *0.35 nm). The fact that the C60 molecules form pairs indicates that some other interactions occur in these systems in addition to the van der Waals forces. Two-walled NTs with the inner diameter equal to that of the C60 molecule were found to be less abundant than SWNTs.29 It was assumed that C60 molecules are formed in the gas phase independently of NTs and enter the NT cavities most likely through defects due to diffusion that occurs in the gas phase (or, which is less probably, due to the diffusion over the NT surface) in the course of in vacuo annealing.33 Then, the defects are `healed' and theC60 molecules self-assemble into chains up to 30 units long and coalesce into interior tubes at temperatures above 1100 8C.There is an optimum temperature for the C60 molecules adsorbed on the outer surface to pass inside the NTs. At low temperatures, the saturated vapour pressure of C60 is too low and filling the NT cavity takes a long time. At high temperatures, the saturated vapour pressure is, on the contrary, too high and the desorbed fullerene molecules move away from the NTs.The formation of interior elongated (several nanometres long) capsules was also explained by coalescence of the C60 encapsu- lated within NTs due to electron irradiation. The electron beam also damages the outer NT shell; however, this occurs at a lower rate than the inner coalescence.34 To prove that NTs are filed with C60 molecules, the NT walls were acid etched and the `peads' were suspended in toluene.35 The results of spectral studies of the suspension showed that the sample under study was characterised by a C60-filled tube length fraction of 4.5%. The `peapod' structures were found to be stable to at least 1200 8C (cf. 1000 8C for the transformation of pure cubic solid C60 into amorphous carbon). Initially, it was assumed that nanoscale `peapods' are solely comprised of NTs 1.3 to 1.4 nm in diameter and C60 molecules.More recently, this opinion was rejected. The C60 fullerene molecules can also be encapsulated within SWNTs obtained by arc method.36 In this case, molecular fullerenes were found to be present inside 5% to 10% of all of the observed NTs and to have different chemical compositions spanning the range from C36 to C120 (C60 was the most abundant). A clear correlation was also found between the NT diameter and the encapsulated fullerene size.Blockage of the filling of fullerene-containing NTs with other substances was observed.36 The inner cavities of NTs can be filled with carbides using not only the arc method, but also laser-induced thermal synthesis.The addition of boron to the material of graphite target resulted in the appearance of carbide particles inside the grown NTs near their ends.37 Filling of NTs also seems not to be improbable in the case of catalytic pyrolysis of hydrocarbons (see, e.g., Ref. 38). 2. `Targeted' filling of carbon nanotubes Filling pre-fabricated NTs requires that they be opened (in other words, `caps' at the NT ends should be removed). Most often, this can be done by selective oxidation, which, therefore, is a necessary stage of the process of `targeted' filling of NTs. Intercalation with CuCl2 or Br2 changes the character of the oxidation of MWNTs and allows their purification with O2.41 ± 44 The interaction of Br2 with unpurified NTs leads to intercalation of bromine molecules into graphite nanoparticles that are `swelled' due to an increase in the interlayer spacings, which b a 100 1 1 Relative mass of sample 829 a.Oxidation of carbon nanotubes `Caps' at the ends of closed NTs are built of carbon pentagons and hexagons. The former type of polygon is more reactive, which determines the possibility of its selective oxidation. This can be achieved by using gaseous reactants, aqueous solutions or melts. Processes in solutions and melts can be initiated or activated electrochemically. Reactions in gaseous media are sometimes initiated using electric discharge or laser irradiation. It should be kept in mind that NTs prepared via different synthetic routes can exhibit different behaviours in oxidation and other chemical reactions.For instance, the densities of materials produced by electric arc method and by laser-induced thermal synthesis can differ by an order of magnitude. Multi-walled NTs and SWNTs also behave in different manner, the latter being oxidised more readily. Air, oxygen,CO2 and oxygen plasma represent conventionally used gaseous oxidants. According to ab initio calculations,39 the oxygen atoms bound to the carbon atoms at the end of an open NT cause its stabilisation. However, the authors of this study 39 believe that the heat of desorption of the CO2 molecules changes its sign as the NT radius increases, so that an endothermic process is transformed into an exothermic one. Therefore, NTs with relatively large diameters are more prone to `capping'.An increase in temperature also favours the formation of closed NTs. Accord- ing to calculations, the oxidation energies of `caps' and lateral surfaces of NTs differ appreciably. The oxidation of a SWNT material containing carbonaceous impurities, Ni and Co begins at*370 8C (Fig. 1), a large fraction of NTs being oxidised along with other forms of carbon at relatively low temperatures 40 {see also information posted at the website of the Rice University (USA) at URL [http://www.ruf. rice.edu/*johnz/researchorig.html]D}.{ Exothermic oxidation of metals and amorphous carbon causes a local increase in temper- ature, which results in the oxidation of NTs at these `hot spots'. Noteworthy is the coincidence of the curves shown in Fig.1 b at temperatures above 450 8C. Probably, the conditions for prelimi- nary oxidation in HNO3 do not affect the amount of NTs consumed. (For unknown reasons, the curve 3 in Fig. 1 a and curve 2 in Fig. 1 b in the original study differ.) { In most cases, the original research results posted at the Webpages cited in this review are represented in the form of abstracts or in other popular forms. Detailed information can be obtained by visiting the Websites labelled as D. 80 2 60 40 3 2 3 200 200 400 600 T /8C 200 400 600 T /8C Figure 1. TGA curves for the oxidation of 1 ± 2 mg NT samples in air.40 The air flow rate was 100 cm3 min71 and the temperature was raised at a rate of 5K min71.The data for the refluxed material was normalised to 100% at 100 8C to compare dry weights; (a): purified NTs (1), unpurified NTs (2) and NTs after a 16 h reflux in 3 M HNO3 (3); (b): NTs after a reflux in 3 M HNO3 for 4 (1), 16 (2) and 48 h (3). (mass %)830 favours a more ready oxidation of the nanoparticles. The reaction with Br2 is completed after 96 to 240 h while the oxidation (4% O2 in He, 530 8C) takes a period of 48 to 72 h.42 The yield of purified NTs is determined by the O2 flow rate, the sample mass and the conditions for arc synthesis and can reach a value between 10% and 20%. Some of the Br2 molecules form complexes with the carbon atoms of the NT walls. Purification of NTs using CuCl2 is carried out as follows: Cathode precipitate Reduction of CuCl2 to Cu Intercalation of NTs with CuCl2 Product Acid treat- ment of NTs Oxidation of im- purities and removal from NTs This procedure is inapplicable to SWNTs since the yield of the pure product obtained using the Br2±O2 system is very low (only *3%).This is thought to be related to different curvatures of the MWNT and SWNT surfaces.45 Functionalisation of SWNTs reduces the rate of their oxida- tion. Zimmerman et al.45 studied purification of NTs with moist chlorine and proposed a mechanism which involves the formation of hydroxy- and chloride-functionalised NT `caps' with the C7Cl and C7OH groups that withstand further oxidation. The description of the experimental setup and material of a discussion concerning the purification mechanism can be obtained via the Internet [URL http://www.ruf.rice.edu/*johnz/ researchorig.html]D.Smith and Luzzi 46 reported the `closure' of NT ends in inert atmosphere at a very low temperature (*550 8C). This is con- sistent with the idea of the `closure' of dangling bonds at the open NT ends in O2, Cl2 or H2 atmospheres. The selectivity of impurity oxidation in the course of purifica- tion of SWNT bundles in a 1% O2 ±Ar mixture can be enhanced by using catalysts, e.g., ultrafine gold particles with a diameter of *20 nm, and benzalkonium chloride as surfactant.47 (The article contains no information on whether or not the NTopening occurs in the low-temperature oxidation at 350 8C.) After a short time the oxidation conditions were refined and the optimum concentra- tions were found to be 0.6 at.% for gold particles and 7 g litre71 for benzalkonium chloride.48 It was also shown that amorphous carbon burns out in the temperature range from 300 to 550 8C, the NToxidation temperatures span the range from 550 to 750 8Cand the residual graphite flakes are oxidised at temperatures above 730 8C.It was suggested that benzalkonium chloride facilitates breakdown of NT bundles. The list of reactants that can be used in the form of melts is also short. Opening of `capped' NTs was carried out in air using molten lead at 400 8C (the lead oxide that is formed in the course of the reaction acts as a catalyst).49, 50 The reaction in the presence of molten bismuth in air at 850 8C follows the same pattern (in this case, Bi is oxidised into Bi2O3).Experiments on NT opening can also be performed with V2O5 (see Ref. 51) and low-melting metal nitrates (see below). Aqueous solutions were found to be the most appropriate and efficient reagents for NT opening. High yields of open NTs can be achieved by prolonged refluxing of NTs in HNO3. The efficiency of aqueous solutions of other reagents (e.g., K2Cr2O7 , KMnO4 , H2O2 , OsO4 , RuO4 , HClO4, H3IO5) and various mixtures was also studied. It was shown that treatment with concentrated hydrochloric acid can lead to the opening of NTs.52 Relatively high yield of the open purified NTs obtained by pyrolytic technique was achieved using a KMnO4 solution in H2SO4.53, 54 Experiments with H2SO4 ±HNO3 mixtures were also reported.55 The mechanism of the interaction between various oxidants and NTs remains unclear as yet.It is known that NT shells with different chiralities are oxidised at different rates.56 E G Rakov Studies on the NT oxidation in the course of their purification were reviewed by Duesberg et al.57. They pointed out that the results obtained by different authors who used nitric acid treat- ment of NTs are close despite considerable differences in duration of the acid treatment, the concentration of nitric acid and the operating temperatures. After oxidation, acid-treated NTs are covered by small fragments of carbon particles which should be washed off with alkali solutions.Duesberg et al.57 also described mechanical and physicochemical methods for purification of NTs. b. Filling of carbon nanotubes from liquid media General methods of filling carbon NTs from liquid media can be conventionally divided into three groups. These are one-step, two- step and three-step methods. The one-step method uses the interaction of NTs with metal nitrates in the course of nitric acid assisted opening of NTs. Subsequent removal of HNO3 and thermal decomposition of the nitrates upon calcination of NTs leads to the formation of metal oxide crystallites inside the NTs.17, 58 The two-step method involves the interaction of pre-opened NTs with concentrated salt solutions. Preference should be given to this method in those cases where fillers are unstable or poorly soluble in acid.58 The three-step method involves an intermediate stage between the opening and filling of NTs, namely, in vacuo annealing at 2000 ± 2100 8C.59, 60 Surface tension is an important characteristic of liquids that fill NTs.Only those substances can be soaked in the NT cavities that have a surface tension of at most 150 mN m71 (see Ref. 61). On the other hand, this numerical value is a function of the NT diameter. For instance, the action of capillary forces allows the filling of NTs with a diameter of at least 4 nm for AgNO3 61 and *1 nm for PbO and V2O5. In air, NTs can also be filled with PbO2; e.g., a nanorod with a diameter of 4 nm and a length of more than 400 nm was obtained at 450 8C.59 Molten V2O5, Bi2O3, B2O3 and MoO3 also fill the inner cavities of NTs with ease to form continuous crystals along the entire nanotube length.51, 62 If the NT diameter is less than 3 nm, a disordered glassy phase is formed instead of the crystalline phase for unknown reasons.Filling of SWNTs with a diameter of at most 3.5 nm with bismuth occurs upon heating of mixtures of capped NTs with the metal nanoparticles to 400 8C in air.28, 63 The stability of the filled NTs under conditions of electron irradiation in vacuum was studied.64 Kiang et al.63 reported a study on the NT filling with bismuth (i) in the course of the arc synthesis of NTs (equal amounts of Bi and Co were introduced into the graphite anode material), (ii) from melt or vapours (a mixture of NTs and Bi was heated in air) and (iii) from aqueous solutions [boiling in 1 M solution of Bi(NO3)3 in HNO3 followed by reduction with H2].The abun- dance of filled NTs was found to be less than 1% for method (i), *10% for method (ii) and more than 30% for method (iii) provided that the experiments were performed with the same type of NTs. Since only a few low-melting metal oxides are known, recent studies were carried out using metal halides and nitrates. In particular, NTs were `doped' with ZrCl4 , UCl4 , AgCl, CdCl2 , CsCl, AgBr, with AgCl ± AgBr mixture and with eutectic mixtures KCl ± UCl4 and KCl ± ThCl4.36, 65, 66 Filling with uranium tetra- chloride was found to occur only in the presence of KCl, since the surface tension of UCl4 exceeds 370 mN m71 at 900 K.At temperatures above the liquidus temperature, the mixtures with the molar ratios KCl : UCl4=1 : 1 and 2 : 1 are characterised by a surface tension of 120 (750 K) and 68 mN m71 (950 K), respec- tively. The surface tension of silver chloride is 173 mN m71 (833 K) while that of silver bromide is 151 mN m71 (800 K). The structure of the metal halide `dopant' affects the crystal chemistry features of the product (for detailed information, we recommend visiting the Website at URL http://www.cpes/sussex.The chemistry and application of carbon nanotubes ac.uk/nanotec99/present/mlhg.htm). For instance, depending on the NT structure, layered CdCl2 and CdI2 encapsulated in NTs were found in different polymorphous modifications.Substances that form chain structures can either retain native polyhedral chain patterns (ZrCl4 , NbI4) or change them inside NTs. Bulky chain halides (ThCl4 , UCl4) form one-dimensional structures inside NTs. A 0.1 nm space resolution study of the structure of an ultra- small single crystal, a KI inclusion into a SWNT, obtained by heating open NTs with molten salt revealed considerable lattice distortion as compared to bulk crystals.67 ± 69 The cross-section of the KI crystal encapsulated within a 1.6-nm-diameter NT repre- sented a 363 atomic array. A longitudinal contraction of the KI crystal by 0.695 ± 0.705 nm along the h001i axis as compared to the `bulk' substance was determined. The coordination numbers (CN) of the face and edge atoms were found to be 5 and 4, respectively.Since the proportion of such atoms is rather large, one can expect that differences in the geometry and CN will affect the electronic properties of substances. For instance, typical metals can turn into dielectrics or vice versa. This opens up possibilities for the design of new materials. Even smaller KI single crystal was obtained in an 1.4 nm- diameter NT. The cross-section of this crystal represented a 262 atomic array. Each atom was found to have a CN of 4.68 Compared to the bulk phase, a strong tetrahedral distortion of the cubic crystal lattice was observed and the lattice parameter was found to be more than halved. As in the preceding case, the h001i direction was parallel to the NT axis.Lattice distortions and changes in the crystal lattice parame- ters of the substances encapsulated within NTs were also observed by other authors (see, e.g., Ref. 63); however, detailed study and discussion concerning the prospects for practical implementation of this phenomenon were first undertaken.67 Silver, cobalt and copper nitrates have low melting temper- atures and their melts are characterised by low surface tension. They are oxidants and can therefore be encapsulated into capped NTs. These compounds were used for the preparation of NTs with encapsulated continuous silver rods 4 nm in diameter and up to 47 nm in length and CoO rods 1.7 nm in diameter and 17 nm long.59 Govindaraj et al.70 used metal halide hydrates (in particular HAuCl4 .xH2O) as the reactants for filling open SWNTs and reported the conditions for filling NTs with molten PdCl2. b-Tin nanowires were obtained by electrolysis of a LiCl ± SnCl2 melt using graphite electrodes.22 Electrochemical preparation of Sn ± Pb nanowires 40 to 90 nm long inside NTs was reported.71 Usually, nanowires are 1.0 to 1.4 nm in diameter and up to 90 nm long. In some cases, it has been possible to prepare single- crystalline metal nanowires; however, most often they represent polycrystalline structures.70 Sometimes, filling of NTs was accom- panied by their decoration due to insertion of the metal atoms into the intertubular space of NT bundles.70 The thickness of the intercalated metal layers was 0.4 to 0.5 nm, which is somewhat larger than the van der Waals separation between individual NTs (*0.3 nm).Most often, carbon NTs are filled using aqueous solutions 17 since this method is easy to operate. The substances encapsulated into NTs can participate in chemical reactions. These are, e.g., thermal decomposition of nitrates resulting in the formation of oxides or metals; reduction of metal chlorides and oxides with hydrogen to obtain NTs filled with Cu,41 Ni, Fe, Ag, Au, Pd and Rh (see Ref. 58); reactions between chlorides and dioxygen to prepare NTs with encapsulated uranium oxychlorides 65) and alkali hydrolysis of SnCl2. Kyotani et al.72 studied the filling of carbon NTs with the acid H2PtCl6 .6H2O in solution and reduction of the acid by NaBH4 at room temperature.Sulfidising reactions of NTs with encapsu- lated CdO or AuCl3 in an H2S atmosphere to give CdS and AuSx were investigated.58 It was found that many substances encapsu- 831 lated into NTs can be dissolved inHNO3. Decomposition of silver chloride and silver bromide encapsulated within NTs and then exposed to light or electron beam has been reported. To prepare NTs with encapsulated SnO, they were opened, washed with HNO3, placed in a concentrated SnCl2 solution and then the pH value of the suspension was slowly increased to 10.2 using a solution of Na2CO3.73 Encapsulated crystallites were found to have a spherical or ellipsoidal shape and a diameter from 2 to 6 nm.Successful synthesis 74 of MWNTs with extremely small inner diameter (0.4 nm) opens up the prospects for the preparation of nanowires with uniquely small diameter by encapsulation into such NTs. c. Filling of carbon nanotubes from gaseous media Filling the inner cavity of NTs using chemical vapour deposition technique can be performed using, e.g., volatile metal compounds which can decompose on heating to give non-volatile substances. Using this method and Fe(C5H5)2, the inner cavities of the NTs grown in the pores of anodised aluminum (Fig. 2) were filled with Fe or Fe3O4 nanoparticles.75 The formation of the oxide inside NTs in the decomposition of ferrocene was explained by partial oxidation of the iron. A similar process was performed using Ni(C5H5)2 at 275 8C.76 Carbon NTs with the outer diameter of 30 nm were found to be filled with Ni wires 4 nm in diameter and up to 500 nm long.It is noteworthy that no nanowires were formed near the open ends of the NTs; this can hardly be rationalised. It is also unclear why the diameter of the nickel nanowires appeared to be much smaller than the inner diameter of the carbon NTs. Usually, the diameters of the nanowires prepared using other methods were only slightly different from the inner NT diameter (see, e.g., Refs 23 and 50). Filling of NTs with various compounds was studied by the molecular dynamics method. Simulation was performed assuming that filling occurs due to the capillary forces and that the model NT 2.74A in diameter comprised 56 carbon atoms.According to calculations, lead clusters can fill NTs along their entire length, whereas carbon and sulfur clusters are retained near the open ends of the NTs.77 Carbon NTs can also be filled with molten selenium which forms atomic chains.78 Targeted filling without pre-opening the NTs occurs in the course of catalytic pyrolysis of hydrocarbons in porous AlPO4.79 The porous material is preliminarily filled with cobalt which is then encapsulated into NTs. Experiments with NaY zeolite template also revealed the formation of NTs; however, no filling of the NTs with the catalyst particles was observed due to different character of the interaction between the catalyst and template. 2 3 1 Nanotubes Pores M30 nm Figure 2.A scheme illustrating the filling of NTs from the gas phase using a porous Al2O3 film.75 Propylene pyrolysis (1), chemical vapour deposition of the metal (2) and dissolution of the template in NaOH (3). The metal particles are labelled as M.832 A short review of recent advances in the field of the prepara- tion of filled NTs and metallic nanowires was reported by Terrones et al.80 III. Functionalisation of carbon nanotubes Chemical modification of activated carbons and carbon fibres has been studied in detail. Several methods for attachment of oxygen- containing groups to the NT surfaces have been developed.81, 82 Cahill and Rohling 83 showed that functional groups can bound to the NT side walls. The oxidised surfaces of NTs were found to be covered by the carboxylic, carbonyl and hydroxyl groups in the approximate ratio 4 : 2 : 1.53 The behaviour of acid groups on the surfaces of nitric acid treated NTs was studied by temperature-programmed decompo- sition and acid-base titration.84 The concentration of these groups on the NT surfaces was found to be *161020 g71, whereas graphite treated under the same conditions showed a lower concentration (5.361019 g71).Sonication of NTs prior to the functionalisation in acids increases the concentration of acid groups. Oxidation of NTs with a H2SO4 ±HNO3 mixture leads to a higher concentration of functional groups on the surface than in the case of treatment with HNO3.85 The formation of functional groups and their subsequent bonding to the metal ions can be described by the following mechanism: HNO3 H2SO4 OH HO2C O O H3O+ S O O HO2C O OH OH HO2C OH Pt2+/H2O HO2C OSO3H O OH O C OH O O Pt2+ H2 Pt2+ O O NTs with Pt C O O OH Prolonged acid treatment followed by heating in air is used for purification of SWNTs {see Ref.40 and material posted at URL [http://www.ruf.rice.edu/*johnz/researchorig.html]D}. After a 16 h reflux in 3 M HNO3 (1208C), washing and drying the mass loss was 18%, of which*6% were identified as metal impurities. The surfaces of acid treated NTs were found to be uniformly coated with polyaromatic hydrocarbons, which can be removed by oxidation in air at 550 8C for 30 min. The residue (*20% of the initial mass) represented high-purity pristine SWNTs.The impurity content was at most 2%. Oxidation of these NTs begins at 735 8C. This temperature is*125 8C higher than that reported earlier,55 which indicates that the NTs are defect-free and contain no dangling bonds. Long-term acid treatment of NTs leads to changes in the chemical composition of the material. Indeed, the uniform hydro- philic carbonaceous precipitate disappears and NTs randomly coated with both the precipitate `spots' and particle aggregates appear in addition to the pristine NTs after 48 h with 3 M HNO3. A large fraction of NTs was found to be damaged and shortened (`cut'). Being oxidised in air, this material behaves in a different manner compared to the material obtained after acid treatment for shorter times (see Fig.1) and the yield of purified NTs becomes nearly halved. It should be noted that SWNTs obtained by different methods exhibit different behaviour in the course of acid treatment and subsequent oxidation. Oxidation of nitric acid treated SWNTs with gases results in selective removal of thin NTs.86 Functional groups can be removed from the surface of NTs by heating. The process begins at temperatures above 623 K;87 however, complete removal is observed at 1073K (higher stability of functional groups up to a temperature of 1173K was also reported 83). The density of oxidised (defect) carbon sites in closed (10,10)-NTs after acid treatment and removal of CO2 and CO at 1273K can be determined using ozone titration.88 The fraction of defect carbon sites was estimated to be 5.5%.To enhance the possibilities of atomic force microscopes (AFM) with nanotube probe tips, the tip ends are functional- ised.89 ± 91 This can be done with both solutions 89 ± 91 and gases.92 ± 94 The latter approach is thought to be preferred since it involves a single-step treatment.92 A schematic representation of the AFM cantilever functionalisation process is shown in Fig. 3. The treatment procedure uses electric discharge inO2, H2, N2 and H2±N2 mixtures. Modified probes can be used for studying the surfaces of non-ionisable self-assembled layers terminated in OH a * X2 X2 X2Nb c X2 X2 X Nb Figure 3. Schematic representation of theAFMcantilever functionalisa- tion process. Carbon nanotube attached to a cantilever is oscillated above a sputtered niobium surface in the presence of gas, X2 (a); applying a potential produces electric discharge between the oscillating NT and the surface (b) and the nanotube probe tip functionalised with X (c).E G Rakov b * X2 Xá2X2 NbThe chemistry and application of carbon nanotubes a Adhesion /nN N OHOH OHOHOH OH + 1.5 NH 1.0 OHOH OHOHOH OH 0.50 b Adhesion /nN 2.0 HHHH H HHH H H 1.5 1.0 0.50 Fluorination of NTs at 500 8C for 4 h resulted in distortion of 4 6 8 pHaregular structure of NT bundles and in the formation of a white Figure 4. Adhesion of nanotubes to the surface of self-assembling monolayers as a function of pH (measured using an AFM).A probe with a nanotube tip modified by nitrogen (a) and hydrogen (b) atoms.92 groups. Modification of nanotube tips using a discharge in N2 atmosphere results in the formation of nitrogen-containing het- erocycles at the tip ends. Chemically modified nanotube tips can be used for measuring the pH values of different surfaces (Fig. 4). High degree of saturation of nanotube tips with functional groups (up to the atomic ratio O:C=0.2) is achieved by treat- ment with a low-pressure ammonia plasma followed by oxidation with a NaClO3 solution.93 According to calculations, bombardment of SWNT bundles 3 radicals with energies between 10 and 80 eV must result with CH in bonding the radicals or their fragments to the surface.94 A theoretical study on the hypothetical structure of NTs 1 nm in diameter and 5 nmlong with acid functionalities attached to the outer surface and on the structure of NTs linked through such functional groups has been reported.95 a. Fluorination of carbon nanotubes Fluorination plays a distinctive role in the modification of NTs and functionalisation of NTs.A review by Touhara and Okino 96 covers the latest studies of researchers from Japan in the field of fluorination of carbon materials. The authors of this review believe that fluorination belongs to the most efficient chemical methods of modification and control of the physicochemical properties of these materials. 833 They constructed a binding energy scale for the C7F bonds and inferred that these bonds can be of different nature and can vary from covalent to semi-ionic and thus be responsible for the variety of properties of fluorinated materials. Graphite fluorides and intercalation compounds based on them are known to be of paramount importance as compared to other graphite compounds (oxides and salts).They are readily available, exhibit relatively high chemical and thermal stability and are characterised by high energy content, which makes them the subject of great practical interest as promising materials for chemical current sources and components for solid lubricants. Studies on fluorination of NTs have led to some remarkable findings. Pioneering experiments on the fluorination of MWNTs were carried out by researchers from Japan in 1996.96, 97 It was shown that fluorine is first of all intercalated into the outer layers of NTs.Fluorinated graphene sheets were found to retain their tubular structure on heating up to 300 ± 400 8C, the spacing between them being increased to 0.53 and even to 0.65 nm at higher temper- atures. As in the case of graphite, nanotube fluorination is accompanied by buckling of graphene sheets and the formation of a polymeric structure 2 to 3 nm thick with covalently bonded fluorine atoms on the surfaces of NTs. By analogy with fluorina- tion of graphite, nanotube fluorination was assumed 97 to proceed as stepwise process. However, this hypothesis was not confirmed more recently.Experiments on fluorination of MWNTs 20 to 40 mm in diameter and up to 10 mm long obtained by catalytic pyrolysis 98 showed that these NTs are more reactive than graphite but less reactive than C60 and C70 .96 The reaction of the NTs with a F2±HF± IF5 mixture at room temperature resulted in the for- mation of a black compound of composition CF0.4 (fluorination of graphite under the same conditions resulted in graphite mono- fluoride). The reaction product was found to represent an inter- calate, though no intercalation of IF5 into the NTs was observed. The product retained the tubular morphology and sp2-hybrid- isation of the carbon atoms. compound with a chemical composition close to that of graphite monofluoride (at 600 8C).Studies of the electrochemical properties of fluorinated NTs 96, 99 revealed that the capacity of a nanotube-derived elec- trode material fluorinated at 480 8C and then used in batteries can be as high as 620 A h kg71.96 The results of earlier studies 98, 99 were to some extent con- firmed by the results of experiments on the fluorination of `buckypaper' fabricated from purified SWNTs with dilute F2 (9% F2 in He).100 It was shown that fluorine atoms can be attached to the side walls of NTs.100 The atomic ratio F :C was found to be merely*0.1 at 150 8C, ranged between 0.46 and 0.52 at 250 8C and within nearly the same limits at 325 8C and corresponded to the formation of a compound with approximate composition C2F, which retained its tubular structure. At temper- atures above 500 8C most of the NTs were destroyed and the formation of structures similar to MWNTs was observed at 500 8C.Fluorination was found to have a strong effect on the electronic properties of NTs, namely, the untreated NTs were conductors while the fluorinated NTs were insulators. Surprisingly, fluorination of NTs at T4325 8C appeared to be reversible. The interaction between fluorinated NTs with a composition close to C2F and anhydrous hydrazine resulted in removal of the fluorine atoms and in recovery of the initial structure of the NTs. This was accompanied by recovery of many (but not all) initial properties of the NTs. This was associated 100 with the reaction C(NTs)+nHF+n4 N2 . CFn (NTs)+n4 N2H4834 However, no structure recovery was observed if NTs were fluorinated at 400 8C.`Fluorocarbon' NTs were shown to react with such a strong nucleophile as sodium methoxide to give a relatively stable compound with an approximate composition C4.4F(OCH3)0.25. Sonication of fluorinated NTs in various alcohols leads to solvation of the NTs, the degree of solvation being much higher than that reported by Chen et al.102 Solvated NTs can form solutions, which is of great importance for the development of nanotube chemistry and some technological procedures, e.g., for nanotube sizing and purification. Mickelson et al.101 studied the fluorination of thoroughly purified NTs primarily with the (10,10) structure. The reaction product consisted of *70 at.% C and 30 at.% F and corre- sponded to the empirical formula C2.33F.The C7F bond ionicity was found to be much higher than in, e.g., alkyl fluorides. This allowed the preparation of metastable solutions of the NTs in methanol, ethanol, 2,2,2-trifluoroethanol, propan-2-ol, butan-2- ol, n-pentanol, n-hexanol, cyclohexanol and n-heptanol by soni- cation. These solutions were stable for 24 ± 168 h. Of the solvents used, propan-2-ol and butan-2-ol were found to be the best. The solubility of NTs in butan-2-ol was at least 1 mg ml71. It was assumed that solvation is accompanied by the formation of hydrogen bonds following the scheme R R R O O O H H HF F F C C C C C . Prolonged sonication of the fluorinated NTs (*10 min and longer) results in removal of some of the fluorine atoms from the surface of NTs.The solutions of the NTs in chloroform appeared to be much less stable. Attempts at solvating the NTs in perfluorinated liquids, acetic acid and aqueous acetone solutions also failed. Reactions in solutions also proved the role of hydrazine as defluorinating agent. The structures of the NTs precipitated out of the solution after adding anhydrous hydrazine and characterised by low fluorine content were only slightly different from those of the untreated NTs. Smalley et al.103 succeeded in replacing the fluorine atoms attached to the side walls of NTs by alkyl groups. Fluorinated NTs of stoichiometry C2.33F were treated with either a solution of alkyllithium in alkane or with Grignard reagents RMgBr (R=Me, Et, Bu,C6H13 , C8H17 orC12H25) in THF.The reactions were activated by sonication. It was found that the NTs alkylated with hexyl groups can be dissolved in chloroform (up to a concentration of*0.6 g litre71), THF (*0.4 g litre71) and methylene chloride (*0.3 g litre71). No flocculation was observed over a period of several weeks. Heating of the alkylated NTs in air can lead to recovery of the initial composition the NTs had before fluorination. The hexy- lated NTs decompose or are oxidised at 250 8C. Judging from the mass loss over a period of 1 h, there are 10 carbon atoms of the outer surface of the NT per hexyl group. Smalley et al.103 also reported convincing proofs of the chemical nature of the bonding between alkyl groups and the surface of NTs.They also found that alkylated NTs can form true solutions and various complex functional nanostructures linked through the atoms attached to their side walls. To establish the fluorination and functionalisation mecha- nisms, Kelly et al.104 studied fluorinated and butylated NTs by scanning tunnelling microscopy (STM) and carried out quantum- chemical calculations of the energies of different isomers. The observed STM images were consistent with the formation of the 1,4-isomer (Fig. 5), though the calculations predict that this isomer is only slightly more stable than the 1,2-isomer. The E G Rakov b a Figure 5. Structure of fluorinated nanotubes. 1,4-isomer (a) and 1,2-isomer (b).mechanism of fluorination involves the addition of fluorine atoms to the side walls of NTs around the circumference of the NTs in the intermediate stages. Fluorination of the external side walls of NTs affects their electronic properties. For instance the conductivity type of NTs can vary from semiconducting to metallic (and vice versa).105 The binding energies of one, two, three and four fluorine atoms to the side-wall carbon atoms of a (10,0)-NT have been calculated.106 The sum of the binding energies of two fluorine atoms to the carbon atoms of the NT is much higher than the binding energy in the F2 molecule. A group of researchers from Japan studied the process of fluorination of the inner cavity of NTs and the effect of changes in the chemical composition of the NTs on their properties.107 The starting material, open-end SWNTs, was produced in a porous Al2O3 template with a pore diameter of*30 nm and a pore length of 75 mm.After treatment with F2 under isothermal conditions at 50 to 200 8C and atmospheric pressure the template was dissolved to separate the fluorinated NTs (Fig. 6). The product was shown to retain the tubular structure. Fluorinated carbon nanotube membranes are thought to be a perfect material for electrochemical batteries or capacitors.96 1 2Carbon Al2O3 Al2O3 F F F F F FFF Carbon Carbon F F F F F F F F F F F Inner surface of NTs Carbon F F F F F F Carbon Carbon External surface of NTs Al2O3 Al2O3 FFF Figure 6.The preparation of nanotubes with fluorinated inner cavity using a porous Al2O3 film (deposition of the carbon layer is not shown).107 Fluorination (1) and dissolution in HF (2).The chemistry and application of carbon nanotubes Long-term (168 h) fluorination of MWNTs with `roll' struc- ture and nanoparticles obtained by arc method with BrF3 vapours at room temperature results in tubular or quasi-spherical C2F.108 This is accompanied by corrugation of the external shells of the NTs while the inner shells of the NTs retain the shape of coaxial cylinders. As the depth of fluorination increases, the interlayer spacings and the NT diameter also increase. At a critical fluori- nation depth (or, in other words, at some critical strain), a number of the outer shells are simultaneously unrolled, thus leading to the formation of multilayered plate-like particles.This phenomenon was first observed. The interlayer spacing for the untreated NTs was 0.342 nm (by 0.007 nm larger than for graphite), whereas that for the fluorinated NTs was 0.614 nm, which is indicative of a very weak interlayer coupling. However, the role of Br2 used in the form of (i) vapours to pre-saturate the NTs and (ii) solution to evaporate BrF3 is still to be clarified. b. Soluble carbon nanotubes The ability of NTs to dissolve in various solutions opens new possibilities for their purification, classification (selection of fractions with uniform structure and equal length) and modifica- tion.Pioneering studies on the attachment of organic functional groups to NT bundles by radiation-induced photolabilisation using biradicals and nitrene sources followed by treatment with dichlorocarbene were carried out by Smalley et al.109 They found that the interaction with dichlorocarbene leads to the appearance of chlorine within the NT bundles but failed to determine whether the reactant is attached to the NT walls or to amorphous carbon due to insufficient purity of the starting material.Dichlorocarbene was attached to the double bond of a NT using another reaction, namely, the interaction of NTs with PhHgCCl2Br.109 The reaction product was found to contain*2 at.% of chlorine. Solvation of SWNTs due to the attachment of octadecyl groups has been reported.102 Single-walled NTs with an average diameter of 1.38 nm were obtained by arc discharge technique, purified, cut to 100 to 300 nm, opened, acid-functionalised (in HCl in the final stage) and treated with SOCl2 containing a small amount of DMF for 24 h at 70 8C.The solid phase was separated from the solution, washed with anhydrous THF, dried and heated with octadecylamine at 90 ± 100 8C for 96 h. The solid residue obtained after removal of excess amine and four-fold washing with ethanol contained up to 60% of NTs that were extracted using a dichloromethane solution. It was found that the function- alised NTs can be dissolved in chloroform, benzene, toluene, chlorobenzene, dichlorobenzene and CS2. The solubility limit in dichlorobenzene and CS2 was greater than 1 g litre71.Dilute solutions were brown while nearly saturated solutions were black. Functionalisation was accompanied by noticeable swelling of NTs, which was tentatively related to the breakdown of the NT bundles. It should be noted that a*300-nm (10,10)-NT contains *50 000 carbon atoms and has a molecular mass of *600 000 Da. with SWNTs of Solutions solubilised attached CONH(CH2)17CH3 groups were studied by IR, Raman and UV spectroscopy 102 and by the 1H NMR (see Ref. 102) and ESR spectroscopy.110 The ESR signal with the parameters g=2.003 and DH=2.1 G observed by Chen et al.110 is characteristic of the cut NTs. Reactions of NTs with Br2 and I2, which can also proceed in solutions, were shown to cause changes in the electronic structure of the NTs.Dissolution of SWNTs via their functionalisation has been studied in detail.111 The SWNTs were first treated with SOCl2 and then the chloride groups were replaced by amino groups using the reaction with 4-CH3(CH2)13C6H4NH2. Solution-phase doping of the solubilised NTs with iodine and bromine resulted in pro- nounced changes in the vibrational spectra of the NTs and served as evidence that the reactions in the solutions do proceed. Compared to the cut NTs, the uncut functionalised NTs were 835 found to be much less soluble. Distinctions between the absorp- tion bands of semiconducting and metallic NTs and better solubility of the NTs of larger diameter (their fraction is small) in pyridine than in THF, CH2Cl2 and CS2 were also pointed out.Dissolution of the amine-functionalised NTs can be achieved using the amines with long alkyl radicals since the interaction with aniline was found to have almost no effect on the solubility of the NTs. Soluble NTs were also obtained by treating with poly(phenyl- acetylene).112 Short NTs helically wrapped by the polymer mole- cules (the NT content in the composite was 1.9 mass% to 6.0 mass %) were dissolved in THF, toluene, chloroform and 1,4-dioxane. The addition of poly(m-phenylene vinylene-co-2,5-dioctyl- oxy-p-phenylene vinylene) to powdered NTs allows their dissolu- tion in standard organic solvents (e.g., toluene) and purification from insoluble particles of turbostratic graphite.113, 114 The poly- mer is prone to form helices with a diameter of *20 nm and a pitch of *6 nm.The NTs under study were 15 to 20 nm in diameter and 0.5 to 1.5 mm long. Because of the correspondence between the NT diameter and the diameter of the polymer helix, the polymer molecules can wrap the NTs, though in some instances they were found inside the NTs. Noteworthy is that the interaction of bright yellow polymer with black NTs resulted in a dark green composite. The nanotube/polymer suspensions were stable for a period of at least 6 months. c. Colloidal solutions and dispersions of carbon nanotubes Solubilisation of NTs can also be achieved by colloid chemistry tools. Functionalised soluble NTs differ from surfactant-coated NTs in strength of the bonds they form (only in the former case these bonds are covalent).Suspensions of NTs are stabilised using common surfactants.91 Surfactant-stabilised dispersions of MWNTs and SWNTs are convenient for chromatographic puri- fication.115 ± 117 The most often used surfactants are DMF and N-methylpyr- rolidone. Dispersions of NTs in DMF are stable.103, 118 However, the authors of a more recent study 119 showed that these dis- persions tend to aggregate after a period of several days. One can assume that NTs tested by different authors exhibited different properties. The most stable are dispersions of NTs in strong Lewis bases which do not form hydrogen bonds.119 These are (in the descend- ing order of the optical density of NT dispersions) N-methyl- pyrrolidone, DMF, hexamethylphosphoric triamide, cyclopenta- none, tetramethylenesulfoxide and e-caprolactam. The optical spectra of NT dispersions in these solvents follow the same pattern.Dispersions of NTs in DMSO, acrylonitrile, 4-chloro- anisole and ethylisothiocyanate follow another spectral pattern, while NT dispersions in 1,2-dichlorobenzene, 1,2-dimethylben- zene, bromobenzene, iodobenzene and toluene follow yet another, third, spectral pattern. The third group of solvents is thought to be the best for C60 and C70 (see Ref. 120). Complexation with polyacrylic acid and surfactants allows solubilisation of MWNTs up to several micrometres long.121 Exact reproduction of the geometry of NT-derived nanoscale devices required studies on the behaviour of micellar dispersions of NTs on patterned surfaces 122 and preparation of the Lang- muir ± Blodgett films based on amphiphilic molecules and NTs.123 Such films were fabricated by dispersion of NTs in aqueous solution of lithium dodecyl sulfate, isolation of the residue by centrifugation and introduction of the residue into the subphase of the aqueous solution of poly(allylamine hydrochloride). Burghard et al.124 coated electrodes with NTs using surfac- tant-stabilised NT suspensions or Langmuir ± Blodgett films.At low temperatures, SWNT bundles in the Langmuir ± Blodgett films exhibit nonlinear current-vs.-voltage characteristics.125836 IV. Decoration of carbon nanotubes and their use as templates 2 Coating of the side walls of NTs can be achieved via their pre- treatment (functionalisation) or without it.The first route implies the chemical interaction as a necessary stage and is often called nanotube decoration. The second route implies that NTs are used only as templates for the deposition of other substances, though in this case the chemical interaction between the NTs and the substance to be deposited cannot often be ruled out. A clear correlation was established between the amount of adsorbed Pd2+ ions and the concentration of acid groups on the surfaces of both closed and open NTs and graphite.84 Adsorption of Cu2+, Co2+, Fe2+, Ni2+, Pr3+ and UO2á ions on the surface of functionalised open NTs was also studied. The adsorption capacity was found to range between 0.005 (UO2á 2 ) and 0.029 (Pd2+) mmol per gramme of carbon.To obtain metallic palladium using NTs, they were first reduced with H2 at 250 8C. Coating of acid-treated NTs with platinum clusters has been described.85 To show that NTs can be used as a template, they were oxidised in two steps,124 namely, by long-term (3 h) reflux in HNO3 followed by short-term treatment with a strongly acidic KMnO4 solution (the oxidant was decomposed after less than 1 min by adding acetic acid). The oxidised NTs acquired the ability to be readily dispersed in water and to attach copolymers with hydrophobic (butyl methacrylate) and hydrophilic (4-vinyl- pyridine) groups. The external surfaces of the functionalised NTs were then decorated with gold nanoparticles that form a conduct- ing layer.Copper nanoparticles with narrow particle size distribution can hardly be obtained by conventional techniques; because of this, they were synthesised using NTs as a template.126 The NTs used in this study were prepared by catalytic pyrolysis of CH4 . Changing the catalyst composition enabled production of NTs with different diameters and narrow diameter distributions (from 5 ± 10 to 25 ± 35 nm). In addition to variation of (i) the copper salt concentration in the aqueous solution and (ii) the initial ratio Cu : NTs, the procedure involved the reduction inH2 and washing from the template by sonication and resulted in either Cu nano- particles or nanofibres.The smaller the NT diameter and the lower the salt concentration in the solution, the smaller the diameter of the copper particles (with the smallest particle size between 5 and 10 nm). An increase in the salt concentration in the solution and the Cu : NTs ratio resulted in the formation of copper nanofibres from 100 nm to 5 mm in diameter and up to hundreds of micrometres long. Decoration of activated NTs with copper and nickel particles in the presence of Pd complexes using one-step and two-step activation-sensitisation has been reported.127 Both methods imply coverage of the NT surface with Pd or Pd ± Sn nuclei followed by the formation of a continuous coating of the NTs with copper and nickel. The coating density was determined by the activation conditions.The structure of the coating formed on the external surfaces of NTs interacting with molten V2O5 was studied and discussed by Ajayan et al.51, 62 This coating can be formed due to surface tension and be very thin. Vanadium oxide has a pseudolayered structure with pyramidal VO5 units held together by bridging oxygen atoms. These layers can uniformly cover the entire surface of NTs and form thin films. Oxidation of such films in air at 650 ± 675 8C resulted in removal of carbon and preparation of V2O5 flakes less than 1 nm thick and rod-like V2O5 . Atopotactic correspondence between the oxide layer and NTs should be pointed out, viz., the c axis of the oxide is perpendicular to the NTs axis. Mention was also made of preferred orientation of the crystallites found inside and outside the NTs (see, e.g., Ref. 52).The possibility of coordinating Cr(CO)6 to NTs has been reported.128 Molecular dynamics simulations showed that the E G Rakov chromium center of the Cr(CO)3 group is coordinated to the three double bonds of a benzene ring of the NT side wall. Nickel and cobalt with small additives of P and B can be chemically deposited onto the MWNT `forest' to form a contin- uous polycrystalline coating.129, 130 Similar experiments on the deposition of Ni on the NTs that were dispersed in SnCl2 and PdCl2 solutions by sonication and transferred then into the bulk solution were also carried by Li et al.131 who, however, failed to obtain continuous coating.Strong effect of the pH value of the bulk solution on the character of the coating and, in particular, on the degree of aggregation of the nickel particles was pointed out. Decoration of NTs with complex molecules including DNA is also possible. For instance, long-term storage of the solutions of such compounds with the NTs pre-treated to be covered with acidic centres resulted in immobilisation of `small proteins' metal- lothioneine, cytochrome C3 and b-lactamase I. The fact that most parts of the immobilised proteins remain catalytically active indicates the absence of pronounced conformational changes. The adhesion force between the proteins and the surface is rather strong and the proteins cannot be washed off with water.132 Multi-walled NTs coated with a rhodium phosphine complex were prepared.133 The use of gel techniques and, in particular, sol-gel process opens considerable opportunities for NT decoration.For instance, coating of NTs with zirconium propoxide followed by calcination and burning out of carbon resulted in zirconia nano- tubes.134 After functionalisation and interaction with Zr(OPrn)4 the NTs was washed with methanol and dried at 373 K. The gel was then calcinated at 723K and carbon was burnt out in air at 973 K. The zirconia nanotubes thus obtained were 40 nm in diameter, had the side walls 6 nm thick and represented a mixture of equal amounts of monoclinic and tetragonal zirconia phases. Fabrication of Y2O3-stabilised zirconia NTs via gelation of colloidal solutions of ZrOCl2 and Y(NO3)3 by addition of NH4OH followed by heat treatment is a considerable step forward. A similar procedure was used for the synthesis of hollow NTs or nanorods from SiO2, Al2 O3, V2O5, Sb2 O5 , MoO2 , MoO3, WO3 and RuO2.135, 136 Gels were prepared using (i) the vanadium, molybdenum or tungsten acids synthesised from corresponding salts by ion exchange technique and (ii) metal chlorides.TiO2-Coated NTs were also prepared (this report can be accessed via the Internet at URL http://www.fys.kuleuven. ao.be/vsm/spm/Ti_oxidation_fig11.html). It was established that physically deposited Ti, Ni and Pd coatings cover the surfaces of NTs by continuous or quasi- continuous layers, whereas Al, Au and Fe coatings only form isolated discrete particles on the NTs.137, 138 Calculations 139 of the adsorption of potassium atoms on the external and internal NT surfaces showed that the process is energetically favourable for small-diameter NTs.A theoretical study of the interaction of Ni and Ni2 with graphite and NT side walls 140 revealed considerable differences in bonding sites, magnetic moments and the direction of charge transfer. The effect of the NT surface curvature } on the reactivity of graphene sheets has been studied.141 Some conclusions drawn based on the results of quantum-chemical calculations were confirmed experimentally. V. Substitution of the carbon atoms of nanotubes by atoms of other elements The interest in carbon NTs in which some of the carbon atoms are partially or completely replaced by atoms of other elements, as well as the interest in the NTs made of other substances stems from the fact that these NTs can exhibit improved electronic properties as compared to the carbon NTs.For instance, the electronic } The study 141 concerns bending of NTs and has a new term in the title, `Kinky Chemistry'.The chemistry and application of carbon nanotubes characteristics of boron nitride NTs (hereafter, BN-NTs) are known to be independent of their structure.142 Young's modulus of BN-NTs is *1.2 TPa, which is a record value for fibrous dielectrics.143 Therefore, BN-NTs are thought to be the best material for encapsulation of conductors in their inner cavities. This type of NTs is promising for fabrication of low-voltage field emission devices and preparation of composites from coaxial sandwiches of carbon NTs and BN-NTs.144 The structure and electronic properties of nitrogen-containing NTs have been studied at different N:C ratios and different structure of isomers.145 It was found that, depending on the mutual arrangement of nitrogen and carbon atoms, NTs can exhibit either metallic or semiconducting properties with a narrow energy gap.However, BN-NTs as well as BxC-NTs or BxCNy-NTs are difficult to prepare using the procedures developed for production of carbon NTs. An original method of attacking the problem of preparation of BxC-NTs involves replacement of the C atoms in carbon NTs by B atoms.146, 147 Indeed, the oxidation of NTs by B2O3 vapours in Ar stream at 1373K resulted in BxC-MWNTs (x40.1) with the same diameter and length as those of the untreated NTs.2BxC(NTs)+3x CO. x B2O3+(2+3x)C(NTs) It was suggested that the low degree of substitution is related to the multi-walled nature of the starting material (the untreated NTs) and that mostly the outer shells were involved in the reaction. In addition to BxC-NTs, crystalline rod-like nanomate- rials of stoichiometry B4C and B13C2 were found among the reaction products. Preparation of (BN)xC-NTs from carbon NTs with an aver- age diameter of 10 nm and B2O3 powder in N2 stream at 1573K has been reported.148 In this case, the graphite crucible used for the synthesis of BxC-NTs should be replaced by a BN crucible.Noteworthy, C?B substitution was found to lead to a more ordered NT structure. The same procedure was also used for the synthesis of BN- SWNTs and carbon NTs doped with boron and nitrogen.149 The reaction 2BN(NTs)+3CO B2O3+3C(NTs)+N2 was conducted in an induction furnace at temperatures between 1523 and 1803K and resulted in BxC-NTs, BxC17x7yN-NTs and BN-NTs. The yield of the BN-NTs increased as the temperature increased. However, not all NTs were involved in the reaction. It is interesting that some of the BN-NTs were filled with octahedral BN particles, which resembles the formation of nanoscale `pea- pod' structures of carbon NTs. A more complicated procedure for increasing the yield of substituted NTs consists of MoO3-promoted synthesis using B2O3.150 The reaction was carried out at the mass ratio B2O3 :MoO3 : NTs=5 : 2 : 1 at 1773 K.The reaction was nearly complete after 30 min due to the stronger oxidative effect of the MoO3 additive compared to that of B2O3 and resulted in the formation of BN-NT bundles up to 1 mmlong. The authors of this study 150 suggest that the process begins with the oxidation of NTs following the reaction Mo+3 CO. MoO3+3C(NTs) As a result, the NTs are opened, thus enhancing their reactivity towards B2O3 andN2. TheMoatoms chemisorbed at the NT ends act as catalysts to preclude the formation of `caps'. This is accompanied by filling the NTs with Mo-containing clusters. Boron- and nitrogen-doped SWNTs containing up to 10 at.% B and up to 2 at.% N were produced in high yield using substitution reactions.151 The highest yield was achieved at 1553 K after 30 min synthesis.All the SWNTs synthesised were found to form bundles with an inter-tube spacing of *0.3 nm. A multistage substitution reaction was used for the template synthesis of GeO2 nanorods (NR).152 A mixture of powdered Ge, 837 Si and SiO2 with carbonMWNTs15 nm in diameter was heated in Ar atmosphere at 850 8C for 3 ± 4 h. The overall process is assumed to involve a number of reactions of Ge vapours, viz., partial exchange with SiO2 , a reaction with the carbon atoms on the NT surfaces and oxidation into GeO2 in the presence of O2 . GeO2(NR)+2 CO. Ge(gas)+2O2+2C(NTs) Nanotubes were used for the template synthesis of SiC,153, 154 TiC, NbC, Fe3C and BCx (see Ref.153); GaN,155 Si3N4 (see Ref. 156) and TaC 157 via reactions with volatile oxides (SiO, Ga2O, B2O3), chlorides (FeCl3) and iodides (TiIx , NbIx). Nanotubes can be coated with titanium.137, 138, 158 Reactions of the type MCx(solid) xC(solid)+M(solid,gas) do not belong to but are much the same as substitution reactions. The interaction of SWNTs and SWNT bundles with silicon begins at temperatures above 800 8C and is accompanied by the for- mation of b-SiC nanorods that retain the morphology of the carbon NT bundles (this holds even for long rods!).159 The reactions of individual NTs or NT bundles at 970 8C in ultrahigh vacuum resulted in structures with the Si7SiC(NR)7C(NTs) het- erojunctions.Similar structures with well-defined carbide ± metal interfaces were obtained in reactions of NTs with titanium. A schematic illustration of the method is shown in Fig. 7. b a NT NT MC M M c NT MC Figure 7. A schematical illustration of the method for fabricating MC±C(NTs) heterojunctions. Before reaction (a), formation of carbide in the reaction of the nanotube with the metal (b) and after reaction (c). Preparation of structures with NbC±C(NT) heterojunctions appeared to be a more difficult task due to peeling of NTs from the carbide on cooling. However, the reaction with niobium is convenient for cutting long NTs and making a uniform morphol- ogy of NT ends, which is of great importance for fabrication of field emitting devices.160 To this end, a perforated Nb wafer was coated with a suspension of NTs, the solvent was removed and the NT-coated wafer was kept in*1078 mm Hg vacuum at 950 8C.VI. Insertion of `guest' atoms and molecules into the intertubular space of multi-walled nanotubes and nanotube bundles Intercalation compounds of carbon materials are of considerable interest in many respects. Lithium-intercalated graphite and related substances are commercially used in lithium batteries; the theoretical limit for the specific energy capacity of these materials equals 372 mA h g71 and corresponds to the composition LiC6 (see Ref. 161). Sometimes, the second electrode in lithium chem-838 ical current sources is fabricated from fluorinated graphite (see, e.g., Ref.162). Fullerenes and NTs can also be used for these purposes.163 Theoretically, NTs must possess a higher degree of intercalation than fullerenes. Intercalation of MWNTs and SWNTs are two basically different processes. In the case of MWNTs, the intercalated particles penetrate into the space between the nested shells through topological defects and stay there.164, 165 Single-walled NTs cannot be intercalated in this manner, but the `guest' atoms or molecules can penetrate into the intertubular space of the SWNT bundles. Intercalated NTs are also fundamentally different from full- erene intercalation compounds. For instance, fullerene C60 can form charge-transfer complexes only with electron donors.166, 167 On the other hand, the results obtained by Raman 168 and optical 169 spectroscopy and from conductivity measurements 170 show that SWNT bundles exhibit the double behaviour typical of graphite and can be doped with either electron donors or accept- ors.Charge transfer in SWNTs was also observed in many other experimental studies. This is characteristic of both metallic and semiconducting SWNTs.169 Charge transfer in MWNTs also does occur. The conductivity temperature coefficient of crystalline bun- dles of single-walled (10,10)-NTs is positive, thus indicating their metallic properties. Intercalation with Br2 or K reduces the electrical resistance of the NTs at 300 K by a factor of 30 and extends the temperature range in which the conductivity temper- ature coefficient is positive.This points to the fact that Br2- or K-doped NTs (C52Br2 and C116Br2) belong to the synthetic metals.170 Intercalates are ionic compounds and intercalation leads to an increase in the charge carrier density. Optical spectrometry studies 171 proved the possibility of controlled shift of the Fermi level of individual SWNTs by doping them with various substan- ces. The results of electrochemical measurements also point to reversible doping of SWNTs (`nanopaper') with Li+ ions in organic radical anions (naphthalene, benzophenone, fluorenone, anthraquinone and benzoquinone) and to shift of the Fermi level.20 No intercalation stages typical of graphite intercalation reactions were observed.It was found that the chemical compo- sition of the doped NT samples can be varied smoothly. Absorption spectra of intercalated SWNTs in the visible region exhibit complex patterns which depend on the dopant concentration.172 Conclusions drawn by Iijima et al.173 contradict the known idea that NT intercalation is accompanied by a rather high degree of charge transfer. The results obtained in this study should be more thoroughly analysed because of the high scientific authority of the authors. 1. Intercalation of electron donors Multi-walled NTs 164, 165 and SWNT bundles 170, 174 are charac- terised by a different structure of the van der Waals spacings between individual NTs. However, both types of NTs can be intercalated with alkali metal vapours up to the compositionMC8 (M=K, Rb, Cs).The capacity of these materials is comparable to that of graphite which also forms compositions of stoichiometry MC8 upon intercalation. Multi-walled NT bundles increase their mass by 33% upon intercalation with potassium and by 260% upon intercalation with FeCl3. The intercalation is accompanied by changes in the NT shape, volume and morphology and by swelling and forma- tion of necks. Intercalation products can be readily hydrolysed; however, subsequent washing leads to recovery of their initial state. Different types of MWNTs (i.e., with the `Russian doll' and `roll' structure) are believed to exhibit different intercalation behaviours.175, 176 In vacuo intercalation of SWNT bundles with Cs and K vapours at room temperature is reversible and leads to structural disordering of NT bundles.177 The chemical composition of the E G Rakov intercalation products corresponds to the empirical formulae KC24 and CsCx (x=8 ± 24).Disordered structure of SWNT bundles prevents locating the metal sites.177 Disordering is due to presence of the metal atoms in the intertubular space rather than filling of individual NTs (the samples used in the experiments were `capped'). Electrochemical intercalation of SWNTs with alkali metals involves three stages characterised by different rates. These are charge transfer through the macroscopic electrolyte ± electrode interface, diffusion through the mesopores of the electrode and diffusion in the NT bundles.178 A number of studies 168, 170, 179 were carried out using `mat'- likeSWNTstructures.However, NT`mats' consist of a mixture of metallic and semiconducting NTs and the results obtained appeared to be hardly interpretable. Chemically doped semiconducting NT bundles change their hole-type conductivity to electron conductivity.180 A correlation between the concentration of the intercalated potassium and the charge carrier density was found. Typically, the electron density varies between 100 and 1000 mm71. The effective mobility of electrons was found to be 20 to 60 cm2 V71 s71, which is in agreement with the corresponding numerical estimate obtained for holes.181 Intercalation of SWNTs and SWNT bundles with potassium also changes their transport properties.182 ± 184 Pichler et al.184 found that at 425 K the C:K ratio in potassium-intercalated NT bundles approaches 7, which corresponds to the formation of a stage-1 intercalation compound, KC8.According to Pichler et al.,184 intercalation leads to a relatively small increase in the intertubular spacing and the total conductivity of the intercalated bundles is somewhat lower than that of the untreated NT bundles. Molecular dynamics simulation 185 predicted the most stable structures of K-doped single-walled (10,10)-NTs. These were found to be KC16 (filling of the intertubular space) and KC10 (filling of both the inner NT cavities and intertubular space). The optimum structure corresponds to the compositions Kexo 5 C80 for the former type of NT filling and Kexo endoC80 for the latter. 5 K3 2.Intercalation of electron acceptors As mentioned above, charge transfer occurs between the NTs and intercalated Br2. Photoelectron spectra 42 point to the formation of surface complexes C± Br2. The nature of the interaction between Br2 and the NTs was studied by high-resolution trans- mission electron microscopy.186 A preferential orientation of the bromine atoms was found, which is consistent with the results obtained in other studies. It appeared quite surprising that the region of Br2 accumulation in MWNTs extends perpendicular to the NTs axis, which means that the liquid Br2 behaves to some extent like a solid. The complexes formed were found to be not too stable and Br2 can be washed off with CCl4.Br2-Intercalated NTs retain tubular structure, though the arrangement of the nested carbon shells becomes somewhat disordered (the NTs are `amorphised'). After removal of Br2, the NTs were more readily oxidised with oxygen than the unintercalated NTs.} The interaction of molten iodine with NT bundles in a sealed ampoule at 140 8C results in disordering within the NT bun- dles.187 The two-dimensional crystal lattice of the NT bundles expands (the cross-section area increases). In the intertubular space the iodine atoms are grouped and form (I5)7 chains separated by *2.1 nm at moderate intercala- tion degrees. Intercalation reduces the electrical resistance and thermal electromotive force (emf) of the NTs.The iodine-doped } A somewhat arbitrary use of the term `intercalation' for the description of the above-mentioned processes should be pointed out. Strictly speak- ing, no increase in the interlayer spacing occurs in this case and the crystal lattice parameter increases insignificantly. Liquid Br2 is adsorbed on the outer surfaces of NTs and is soaked into the inner cavity after opening the NTs due to wetting.The chemistry and application of carbon nanotubes NTs are stable in air. More recently,188 the poly(iodide) chains formed inside SWNTs were found to have a helical shape. Bower et al.189 studied the interaction of 70% HNO3 with SWNT bundles and found that (i) intercalation occurs reversibly and (ii) the intertubular spacing increases upon the insertion of the acid molecules.The starting material was a `mat'-like substance, or `nanopaper', with the NT content of about 70%. The inter- calation occurred at room temperature and, in contrast to the interaction with graphite, took a rather short time due to the capillary effect of the NT bundles and was accompanied by an increase in the intertubular spacing within the bundles. The crystallinity of the NT bundles remained after intercalation; however, an increase in the lattice parameter from *1.7 to 2.0 nm was observed. No proton abstraction from the HNO3 molecule was suggested. The structure of the HNO3/SWNTs intercalate is shown in Fig. 8. In vacuo heating of the samples to 500K caused the onset of the reverse process.Extention of intercalation time to 12 h and longer times resulted in the loss of order within the NT bundles and in partial or complete break- down of the NT bundles into individual NTs upon removal of HNO3. A study of electrochemical oxidation of SWNT bundles by H2SO4 revealed some remarkable features.190 A spontaneous charge-transfer reaction was observed in the NT bundles. In contrast to the graphite ±H2SO4 system, this reaction proceeds even before applying the potential difference. On the other hand, both systems share some common features. Under the action of electric current the NT bundles behave similarly to graphite, namely, the reaction H2SO4+e HSO4¡+12 H2 proceeds by the `charge exchange' mechanism and there is an `overoxidation' regime (irreversible formation of covalent bonds C7O).FeCl3-Intercalated MWNT bundles exhibit metallic conduc- tivity.175, 176, 191 The temperature dependence of this parameter was studied by Baxendale and Amaratunga.192 Intercalation of NTs with CuCl2 followed by reduction of the chloride to metallic Cu can be used for purification of the NTs.41 An interesting reaction was studied by Hsu et al.193 The main goal of the study 193 was to obtain diamond from NTs via treat- ment of potassium-intercalated NTs with CCl4 to give KCl and excess carbon. This reaction seems to be of no interest for large- scale implementation; however, it opens new prospects for syn- thetic chemistry. The authors hoped that the reaction 4 KCl+C 4K+CCl4 Figure 8.A model of the structure of the HNO3/SWNTs intercalate. The C±O distance was assumed to be the same as in the case of HNO3 intercalated graphite, the distance between the centres of the nanotube cross-sections was set to*2.0 nm. 839 would result in cross-linking of adjacent graphite basal planes and in replacement of the sp2-bonds by sp3-bonds within the inter- tubular space of the MWNTs. The reaction was carried out in an autoclave at 200 8C; however, no diamond was produced. Anal- ysis of the reaction products revealed the loss of ordering (`amorphisation') of the tubular structure and the formation of KCl crystallites within the space between graphite shells with presumably distorted structure.No KCl crystallites were found (i) within the NTs with the outer diameter of less than 10 nm and (ii) far from the outer surface of the MWNTs. VII. Adsorption and storage of gases in carbon nanotubes Nanotubes can adsorb gases at high pressures or under conditions of electrochemical saturation. Adsorption can occur on the internal surfaces of NTs (within the innner cavities), on the external surfaces of free-standing NTs and in the intertubular space of NT bundles. Pioneering calculations of the adsorption of gases on NTs were carried out by Maddox et al.194 Model calculations of the sorption of hydrogen, oxygen and chlorine atoms by SWNTs and graphite revealed three types of surface sites and provided an explanation for high efficiency of hydrogen storage in NTs.195 Compared to fluorine atoms, attachment of hydrogen atoms to the side walls of NTs is less probable; the attachment of two or four hydrogen atoms to a (10,0)-NT is an endothermic process.108 Adsorption of a Lennard-Jones gas mixture on single-walled arm-chair NTs with inner diameters of 6.125, 7.482 and 8.16 A was theoretically studied by Ayappa.196, 197 He showed that at T5388.9 K adsorption of large particles occurs for the most part.These particles interact relatively strongly with the NTs walls. As the temperature decreases, the equilibrium is shifted towards smaller particles which completely replace the large particles at T4303 K. Condensation of the small particles occurs in pores. A series of theoretical studies on the adsorption of gases in NTs was carried out by Stan et al.198 ± 204 In the case of adsorption of inert gases on NTs the bonding between the adsorbate and the surface is stronger than in the case of graphite.198 At low surface coverages, the adsorption strongly depends on the NT structure and size.A salient feature of the process is its one-dimensional character. Helium and neon readily fill the intertubular space of NT bundles. They are strongly bound to the NT side walls even under those conditions where almost no adsorption occurs on the basal planes of graphite.200 The condensed phase is strongly aniso- tropic. Since adsorption leads to the formation of a quasi-one- dimensional system, the properties of this system differ from those of standard systems.201 According to calculations, adsorption of small atoms occurs both inside NTs and in the intertubular space of NT bundles, whereas the adsorption of relatively large atoms and molecules with rare exceptions occurs only inside the NTs.203 Helium atoms are strongly bound to the side walls within the intertubular space of NT bundles. The system can behave like a lattice gas or condensed phase; despite the fact that the content of the adsorbate atoms is at most 2% they contribute much more to the specific heat capacity than the `host' molecules at low temperatures.202 In the case of adsorption of a 3He ± 4He mixture the partial pressure of each component is determined by the ratio of their concen- trations within NT bundles; the situation looks like adsorption is controlled by some rule similar to Raoult's law.204 A study on the desorption of 4He at 14 ± 23 K showed 205, 206 that the process can be described by the equation for one-dimen- sional adsorption of neutral particles with very high binding energy estimated as 330 K (at 14 ± 16 K), which is very close to the calculated value (340 K).200 Channels in the intertubular space of NT bundles can be represented by cylinders 0.21 nm in840 diameter and their cross-sections can be filled by at most one helium atom.At temperatures above 16 K, the experimental adsorption capacity was higher than the theoretical value for unknown reasons. The binding energies of other noble gases (neon and xenon) and CH4 to the external surfaces of closed SWNT bundles are nearly 75% higher than to the flat graphite surface.207, 208 How- ever, neither of the gases can be adsorbed in the intertubular space of the NT bundles.Enhanced adsorption of xenon inside SWNTs at 95 K in the presence of defects, especially after removal of carboxylic groups from the NT surface was observed.87 The experimentally deter- mined activation energy of xenon desorption (26.8 kJ mol71) was found to be close to the calculated value (22.6 kJ mol71).198 Experimental study of N2 adsorption on MWNTs that were open at one end, with mesopores of width 4.00.8 nm, at 77 K showed 209 that nitrogen molecules are adsorbed on both the external and inner NT surfaces.It was found that adsorption on the external surface is five times greater than that on the inner surface and that the corresponding adsorption isotherms are considerably different. Adsorption in mesopores was described in terms of the classical theory of capillary condensation and the calculated pore diameter was found to be 4.5 nm. The adsorption isotherms exhibited no hysteresis, in stark contrast to the curves reported by Kyotani et al.210 who studied adsorption in mesopo- rous NTs open at both ends. These differences were rationalised by accessibility of only one NT end. Single-walled NT bundles also readily adsorb N2. Unpurified NTs prepared by the arc method were characterised by an inner specific surface of 233 m2 g71 and an external specific surface of 143 m2 g71 (see Ref.211). Acid treatment of the NTs resulted in an increase in the total specific surface from 376 m2 g71 to 483 (HCl) and 429 m2 g71 (HNO3) and enhanced the adsorption capacity towards benzene and methanol. Adsorption of gases affects the electronic 212 and emission 213 properties of NTs. The effect of O2 adsorption on the electronic and magnetic properties of NTs has been studied.214 VIII. Potential fields of application of carbon nanotubes Increasing interest in NTs is due to great potentialities of their efficient use in industry.2, 18, 215 ± 219 Small size and unique struc- ture of NTs determine their unusual mechanical and electronic properties (these can be controllably varied over a wide range).In contrast to graphite, NTs are free from dangling bonds and are therefore chemically inert. Nanotubes represent a highly rigid and, at the same time, elastic material that exhibits the ability to undergo reversible swelling (buckling up) and folding (compres- sion, flattening). The fields of application of NTs can be arbitrarily divided into two groups. The first group of applications utilises NTs in the form of relatively large devices or parts (many NTs `function') such as light, strong, conducting (if necessary) and impact energy absorbing fillers for various composites, materials for chemical current sources and accumulators of gases. The second group of applications uses NTs in mini- and microcomponents and devices (individual NTs `operate').These are electronic components and devices including ultrasmall and ultrafast computers, cathodes for electron field emitters, indestructible nanoscale scanning micro- scope probes, high-frequency resonators, `nanostraw' and nano- pipettes for introduction into living cells to study their chemical nature. 1. Composites a. Chemical composition and properties Nanotubes can serve as ideal reinforcing fillers for composite materials with matrices of any chemical nature, i.e., polymeric and inorganic (metals, ceramics). Many characteristics of NT-filled composite materials can reach record values. Of great importance E G Rakov is also the fact that NTs have low density. This allows one to obtain light composite materials.However, it should be kept in mind that the properties of NTs depend on their origin. For instance, the flexural rigidity of graphitised arc-grown NTs is *1 TPa, which is two orders of magnitude higher than that of the NTs synthesised by chemical deposition of hydrocarbons.220 Development of methods for obtaining uniformly distributed and oriented fillers is a vital issue in the fabrication of filled composite materials. Nanotubes are characterised by large aspect ratios (1000 and more), which makes possible reinforcement of composite materials. At the same time, NTs are very short and therefore standard equipment for the production of polymers can be utilised. Using conventional long fibres imposes restrictions on the shape of finished products (carbon fibres are brittle).In the case of NTs these restrictions are removed since products of any shape can be produced by moulding. Nanotubes are known to be highly rigid. They exhibit high axial strength 221 and a record value of Young's modulus, which approaches 1.25 TPa.144 This value remains unchanged on going from SWNTs to MWNTs, since it is determined by the strength of the C7C bonds in individual shells.222, 223 Not without reason theoretical estimates of possible mechanical properties of some composite materials were obtained assuming that the Young moduli of SWNTs (along the NTs axis) and diamond are comparable in magnitude.224 The tensile strength of MWNTs can be as high as 63 GPa, which is 50 to 60 times greater than that of high-quality steels (this material can be accessed via the Internet at URL http://wupa.wustl.edu/record/archive/2000/02-03-00/articles/nanotube.html). The limiting pressure for NTs approaches 100 GPa. This is two orders of magnitude higher than for other fibres and allows the use of NTs for production of bullet-proof jackets, car and truck bumpers and for construction of earthquake-proof build- ings.225 According to ab initio calculations, NTs can undergo plastic deformation.226 Experimental studies of NTs confirmed the possibility of creating NT-derived accumulators of mechanical energy.227 Compression of purified, oriented, highly crystalline SWNTs is accompanied by exceptionally large and reversible volume reduction. The properties of the NTs resemble those of an ideal flat spring.Under load the NT density increases smoothly to reach that of graphite, which was related to compression and flattening of entangled NT bundles. This phenomenon was also observed in studies of MWNTs. A theoretical study on the strain and bending behaviour of NTs and the influence of defects on their transport properties was reported.228 The lack of the filler materials precluded extensive research into reinforcement of polymers by filling them with NTs. Never- theless, pioneering experiments 229, 230 showed that design of various types of composite materials is possible. Single-walled NTs appeared to be the best fillers, since the inner shells of MWNTs are not involved in reinforcement.231 Potential advan- tages and disadvantages of this type of composite materials have been discussed by Ajayan et al.232 Tensile test of an epoxy resin/MWNTs (5 mass %) composite showed that only the outer MWNT shells were loaded.233 On the other hand, the results of compression test of the same composite indicated loading of all the MWNT shells.Mechanical tests of composite materials 234 ± 236 revealed that mechanical strength of the composites filled with MWNTs is an order of magnitude higher than that of materials filled with conventionally used fibres. The ultimate compression strength of NT-filled composite materials was found to be*60 GPa, which is nearly two orders of magnitude higher than that of standard composites.235 The bonding between the matrix and filler in composite materials is known to be neither too weak nor too strong.In this respect the situation with NTs was unclear. The effect of the origin and concentration of SWNTs on their interaction with theThe chemistry and application of carbon nanotubes polymer matrix was studied taking PMMA-based composites filled with NTs as examples.237 Mixtures with the NT content ranging from 1 to 20% were also studied.238 It was found that at low NT concentrations (2.5% and 5%) the NT bundles remain unchanged, though one can assume that they do interact with the matrix since the Raman spectra indicate an increase in the intertubular spacings within the bundles.238 One of the nondestructive methods for increasing the surface energy of NTs consists in attachment of functional groups to the NT surfaces.231 Chen et al.102 studied solubilisation of NTs and showed that they can be dispersed in polymeric matrices without cutting.The bonding between NTs and matrix can also be increased by using defect NTs, though this can lead to some decrease in the strength of the composite material. Of particular interest are conjugated polymer/NT composites. A salient feature of this class of polymers is the presence of an extended p-electron system that is due to the formation of a specific electronic structure with energy gaps 1.5 to 2.5 eV wide. Usually, these polymers exhibit low hopping conductivity. Intro- duction of NTs into composites must increase their conductivity, mechanical strength, thermal conductivity and resistance to optical damage. Indeed, a composite material with interesting optoelectronic properties was obtained from electroluminophor poly(m-phenylene vinylene-co-2,5-dioctyloxy-p-phenylene vinyl- ene) and MWNTs.113, 114 The electrical conductivity of this material was measured to be eight orders of magnitude higher than that of the starting polymer.Polymer chains were found to wrap themselves around the NTs, thus making the composite material extremely strong and allowing the NTs to be suspended in the excess polymer solution. Introduction of low concentrations of NTs had little effect on the luminescent properties of the polymer. Moreover, it was found that NTs act as a nanoscale heat sink, thus preventing the composite material from over- heating and destruction (under the action of, e.g., laser beams).Coleman et al.239 studied the properties of poly(m-phenylene vinylene-co-2,5-dioctyloxy-p-phenylene vinylene)/NT composites with the NT content ranging from 0.5% to 36%. It is noteworthy that at low NT concentrations an increase in the NTs content is accompanied by insignificant increase in the conductivity of the composite material. At the NT concentrations in the range between 7% and 10% the conductivity of the material increases jumpwise by ten orders of magnitude and then again increases insignificantly at higherNT content. This behaviour is also typical of composites filled with other carbonaceous materials, e.g., soot.It was found that charge transfer occurs between the polymer and NTs; however, the effective mobility of the charge carriers decreases. A study on the percolation character of the conductiv- ity of composite materials was reported.240 At low NT concen- trations, no aggregation of MWNTs in the polymeric system occurs despite the weak interaction between the polymer and NTs.241 Single-walled NTs in conjugated polymers with hole-type conductivity can trap charge carriers.242 Methods for preparation and fields of application of composite materials comprised of conjugated polymers and C60 or NTs were reviewed by Dai.243 Wrapping of polymer molecules around NTs was also observed by Seifert et al.105 who polymerised phenylacetylene in the presence of NTs to prepare NTs that could be dissolved in organic solvents.The use of polymers for dissolution of NTs was studied by M in het Panhius et al.244 Nanotube solutions can be used as optical filters while composite materials as converters of light into electricity.245 Ago et al.246 reported fabrication of photovoltaic devices based on NT-filled composites. Raman spectroscopy provides the largest body of information in studies of the structure of novel carbon nanomaterials.247 This method has also been successfully applied to composites. Raman spectroscopy allowed the researchers to follow tension due to load of an epoxy resin/NT composite material and to show that reinforcement with fibrous fillers is usually more pronounced than reinforcement with particles.248 Wood et al.249 identified the 841 components responsible for the appearance of particular lines in the Raman spectrum of an SWNT-filled polymer, studied the effect of temperature on the spectral patterns, revealed the lines caused by thermal stress and established a stress ± strain correla- tion.Fragmentation of MWNTs in an urethane ± diacrylate poly- meric matrix under load has been studied 250 (the content of NTs was not reported).Composites with NTs can be used for production of aligned NT arrays. By cutting thin slices (50 ± 200 nm) of a composite material based on epoxy resin with random orientation of NTs one can obtain partial alignment of the NTs on the surface of the slice.229 A more simple and efficient procedure involves the use of composite materials based on thermoplastic polymers.Aligned NT arrays can also be produced by mechanical tension on heated composite material followed by cooling under load.251 The desired effect was observed only in the experiments with MWNTs. Nanotube alignment by applying mechanical shear to a composite material sandwiched between two pieces of glass slides was reported by Tang and Xu.112 Composite materials based on NTs and polyvinyl alcohol 252 and polypyrrole 253 were prepared. An increase in the NT content to 10 mass% is accompanied by a sharp increase in the electrical conductivity. At higher NT concentrations, the increase in the conductivity is less pronounced.252 Introduction of 1% MWNTs leads to an increase in the tensile strength and elasticity modulus of the polystyrene films by 36%± 42% and *25%, respec- tively.254 Nanotube fillers can be used for production of high-strength composite fibres, ribbons and coatings with enhanced wear resistance, which can find application in military, aerospace and automotive industries and pharmacy.For instance, Andrews et al.255 introduced SWNTs (Carbolex, USA) into fibres based on petroleum pitch. The filler was added to a hot pitch solution in quinoline, the solvent was removed and fibres containing 1 mass%or 5 mass%NTs (extrusion at higher NT concentrations becomes difficult) were extruded and carbon- ised at 1100 8C. Tests showed that the*18-mm-diameter compo- site fibres with 5% NTs exhibited improved tensile strength, elasticity modulus and conductivity as compared to those of the unfilled fibres (by 90%, 150% and 340%, respectively).An original method for fabrication of macroscopic composite fibres and ribbons incorporating flow-aligned SWNTs by forming a net from solution of NTs in a polymer followed by directed alignment was proposed.256 Such fibres with regularly aligned NTs exhibit an elasticity modulus of 15 GPa, which is lower than that of free-standing NTs but much higher than that of, e.g., `nanopaper' (1 GPa). The fibres can be bent, crumpled and even knotted. Reinforced PMMA fibres filled with oriented NTs were prepared.257 Two methods for production of composite materials based on commercially sold polymers and NTs have been developed.258 To synthesise polymer-coated NTs, poly(ethylene glycol), poly(2- vinylpyridine), poly(4-vinylpyridine), poly(4-vinylphenol) or poly(ethylene oxide) were dissolved in DMF and NTs were added.To obtain polymer-grafted NTs, the NTs were mixed with a solution of SOCl2 in C6H6 , kept in N2 atmosphere for 120 h, dried in vacuo and then triethylamine and poly(ethylene oxide) were added (140 8C, N2 , 48 h). Poly(ethylene oxide) was washed off with DMF. Thin films of the composite materials and NT solutions exhibited remarkable nonlinear optical properties. Conducting NT-containing composite materials can be used in car body panels to replace metallic car bodies, since they allow spray painting by giving the spray electrostatic charge.231 In this respect the properties of a composite material based on polyphe- nylene ether/polyamide mixture containing 10% NTs were found to be quite interesting and useful.245 The conductivity of the epoxy resin increases by several orders of magnitude upon introduction842 of minute amounts of NTs (merely 0.1 vol.%) and becomes higher than that measured after addition of the same amount of soot.259 It is assumed that lightweight composite materials will find use in the preparation for human Martian mission.245 Nanotubes can be used as additives to polymeric electro- luminophors.Colouration of these luminophors occurs by giving them electrostatic charge, which makes possible painting of metallic rather than dielectric surfaces.It is believed that these limitations can be removed by introducing NTs as conductivity- enhancing additives into thermoplastic polymers.240 Research efforts aimed at creating chemoresistive sensors based on NT/polymer composites for a multichannel `electronic nose' are being undertaken. Sun and Crooks 260 reported the preparation of NT-filled polymeric membranes for sensing large neutral molecules. Composite materials comprised of a dielectric matrix and conducting filler in the form of small extended particles can be used for electromagnetic shielding, in aerials, waveguides, etc. Nanotubes can serve as ideal fillers (http://www.sbirsttr.com/ SbirMisc/alld991.htm). The introduction of SWNTs 10 nm in diameter (23 mass%) into ethyl methacrylate matrix leads to a nearly 35-fold increase in the dielectric constant of thick films at 500 MHz.261 At low NT concentrations (4% to 7%), composite materials based on NT-filled thermoplastic polymers can exhibit relatively high and uniform electrical conductivity (a short communication was posted in the Internet at URL http://www/rtpcompany.com/ news/press/nanotube.htm).Nanotubes coated with conducting polypyrrole represent a special class of materials.262 They were prepared by polymer- isation of pyrrole immediately on the NT surfaces. No chemical interaction between the polymer and NTs was observed; never- theless, the modification led to pronounced changes in the proper- ties of the NTs.Methods for purification of NTs can employ stability of NT solutions in polymers 113, 114, 263 or the difference in precipitation rates between NTs and impurities in polymer solutions.264, 265 The proportion of short and thin NTs increases upon purification.263 A NT/crystalline C60 composite was prepared (see Ref. 266). Research into metal/NT composites has been less intensive compared to studies on polymeric composite materials. Xu et al.267 studied the morphology, mechanical and electric properties of NT/aluminium composites obtained by hot pressing of powder- like mixtures. Introduction of NTs leads to a slight increase in the electrical resistance of aluminium at room temperature and to a sharp drop of the resistance at 80 K.Related composites were prepared and investigated by researchers from Japan (this mate- rial can be accessed via the Internet at URL http://www.mrs.org/ publications/jmr/jmra/1998/sep/013.html). It was found that hot pressing or hot extrusion do not damage NTs and that the NTs do not form products of chemical interaction with aluminium. In contrast to aluminium produced by powder metallurgy, mechan- ical properties of the aluminium/NT composites remained virtu- ally unchanged upon annealing (873 K). Introduction of NTs into a titanium matrix substantially increases its hardness.268 A composite NTs(3%)/metallic glass Fe82P18 was prepared by pressing a mixture of powdered starting components with NTs, melting and quenching of the melt on a copper wheel rotating at a linear speed of 30 m s71.Composite ribbons 40 mm thick were characterised by uniform distribution of the NTs that retained their initial structure.269 Some of the NTs `capped' at both ends were filled with iron alloy. The number of studies on the composites with ceramic matrices is a little more than those concerning the composites with metallic matrices. Introduction of NTs into composites with Bi2Sr2CaCu2O87d (a Bi-2212 superconductor) has the same effect as irradiation of the Bi-2212 by heavy ions and favours an increase E G Rakov in the current density.270 Nanotube composites with supercon- ductors of other chemical composition were also studied.271 ± 273 To prepare NT/Fe/Al2O3 composites, Laurent et al.274 ± 276 calcined amorphous solid solution Al1.8Fe0.2O3 at 1025 ± 1100 8C and then reduced it with a 88% H2 ± 12% CH4 mixture at 10508C.Calcination at 1100 8C resulted in the formation of a solid solution a-Al272xFe2xO3 (x<0.1) and trace amounts of the Fe- enriched phase a-Al272yFe2yO3. Iron particles isolated upon reduction of this phase catalysed the growth of flattened (rib- bon-like) arrays of NT bundles. It was found that homogenenous solid solution a-Al1.8Fe0.2O3 with large specific surface area (this favours an increase in the NT yield and improves NT quality) can be obtained only at temperatures between 1025 and 1050 8C. This method results in two-walled NTs with an average inner diameter of *2 nm as the main product.The proportion of SWNTs was nearly 20%. The same authors reported the synthesis of powder-like NT/Co/MgO mixtures.277 b. Space cable In 1910 F A Tsander put forward an idea of connecting the Earth and the Moon by a cable. More recently this idea was transformed into the concept of a transport cable connecting the Earth and a geostationary artificial satellite {for references, please visit the Internet Website at URL [http://www.isd.net/anowicki/ SPBI120.HTM]D}. Conventional space rocket mass transporta- tion systems would travel a predetermined length to transfer cargo and then mass transportation would be performed along the cable due to the action of centrifugal force. However, no materials suitable for such a cable have been developed as yet.Polymers are vulnerable to space radiation while steel has too high density so a steel cable must have enormous weight. The best commercially available fibrous material based on specially prepared polyethylene fibres, `Spectra 1000', has a characteristic length of 315 km. (Characteristic length is the maximum length of a cable that can carry its own weight). Recently, NASA's specialists returned to the idea of implementa- tion of a space elevator using a NT-based fibre. The calculated characteristics of this fibre and other materials are listed in Table 1. Table 1. Properties of particular fibrous materials (http://www.isd.net/ anowicki/SPBI1MA.NTsM). Density Material Young's Longitudinal modulus /kg m73 speed of sound /m s71 Tensile strength /Pa /GPa 1 ± 5 3.3 3.5 77 5000 12870 12778 5760 5170 5339 5860 9502 7900 1870 2450 2200 2320 2540 2490 1440 200 310 400 73 62 72.4 85.5 130 2.4 4.5 3.6 170 365 3.0 5.8 2 ± 5 13239 15199 11600 ± 21200 22014 970 1580 250 ± 830 1850 1300 630 Steel Beryllium fibre Boron fibre Fused silica Pyrex glass E-Glass fibre S-Glass fibre Kevlar-49 (aramid fibre) `Spectra 1000' PBOa Carbon fibre Buckytube cable 150 (theoretical data) aPBO is poly(p-phenylene benzobisthiazole), a plastic fibre.As can be seen, none of the other materials is comparable to the NT cable in tensile strength. The NT cable also must have the greatest characteristic length.The chemistry and application of carbon nanotubes c. Artificial muscles { As long ago as 1940s it was found that fibres made of polymeric gel-like materials compress in acidic solutions and extend in alkali solutions.The phenomenon can be used in practice by changing the direction of direct electric current. This idea provides the basis for the development of actuators and artificial muscles. For some time, conducting polymers were thought to be the most promising material for electrochemical actuators.278 However, recent stud- ies 279, 280 revealed that NTs are even more promising. To function, most of the previously studied materials required application of a voltage of at least 30 V, whereas the operating voltage for `nanopaper' varies from 1 and 4 V.279, 280 The new material is characterised by a deformation (changes in linear dimensions) of *1% and is capable of developing a maximum force of*36107 J m73 per cycle, which is about 30 times greater than the characteristics of the best ferroelectric, electrostriction and magnetostriction materials and is even greater than the force developed by human muscles.Schematic representation of the `nanopaper' test device is shown in Fig. 9 and the relative elongation-vs.-voltage curve of this actuator is presented in Fig. 10. 1 7 + 2 3 4 5 Figure 9. A scheme for a `nanopaper' test device. Mirror (1), optical sensor (2), electrolyte (3), `nanopaper' ribbons (4) and insulator, a poly(vinyl chloride) film (5).`Nanopaper' was studied electrochemically.281 Samples with apparent densities of 0.30 to 0.40 g cm73 containing NT bundles 1.2 to 1.4 nm in diameter were tested in various electrolytes, in the presence of both light and heavy ions, over a wide range of pH values and at different voltage scan rates. It was found that, in contrast to other forms of porous carbon, the capacity of `nano- paper' depends only slightly on most of the parameters studied (except for the regions characterised by very low and very high pH values). Typically, the `nanopaper' capacity varies between 18.0 and 40.7 F g71 in 1.0 MNaCl solution at 0.4 V and a scan rate of 50 mV s71. These results show that pores inside the NT bundles are readily accessible to ions of different size and charge and that, at least for small samples, penetration of the electrolyte into the pores is not controlled by diffusion.The charging mechanism is described by the equation COH. C=O+H++e The results obtained in these studies are of great importance for further development of not only electromechanical actuators, but also supercapacitors, sensors, etc. { Information presented in this Section does not take into account the results obtained for many new chemical systems that are thought to be potentially used for the creation of artificial muscles. 843 0.04 0.020 70.02 1.0 0.5 0 71.0 70.5 Applied voltage /V Figure 10. A relative elongation-vs.-voltage curve for a `nanopaper' actuator.279 Che et al.282 experimented with an actuator made of a `nano- structured carbon material' (most likely, NTs).Potential fields of application of artificial muscles cover a rather wide range from mechanical actuators that operate under severe conditions (as, e.g., `windshield wipers' for space vehicles, in jet engines at 1000 8C, etc.) to prostheses, though the last- named task requires considerable research and development efforts and is quite time-consuming. The reverse process, i.e., conversion of mechanical energy into electric energy, can draw attention in the context of development of electric generators that use the energy of sea waves. 2. Fields of application of nanotubes in electronics { Miniaturisation (the characteristic size of electronic chips is halved every 3 years) and improvement of performance of sili- con-based electronic devices should sooner or later stop (see, e.g., Ref.283). On going from micrometre (0.3 ± 1.0 mm) to nanometre scale (*10 nm) electronics comes into the world of quantum effects and materials change their behaviour. Industry will make this step in the coming decade, this makes research aimed at developing new electronics vitally important. The use of single molecules or quantum dots as functional devices can be considered the ultimate point of miniaturisation process. However, electrical contacts can hardly be attached to molecules. Therefore, the most promising candidates for practical implementation in molecular electronic devices are NTs, which can serve as the basis for creation of a new generation of integrated circuits (ICs).Search for suitable chemical substances for future electronics has been carried out since the mid-1990s. Recently, research plans were corrected and focussed on NTs.284 ± 287 Depending on their structure, NTs possess metallic or semi- conducting properties (see, e.g., Refs 288 and 289). For this reason alone they can be used for fabrication of heterostructures with metal ± metal, metal ± semiconductor and semiconduc- tor ± semiconductor junctions. In this respect particular emphasis is placed on SWNTs. Nanotube conductivity is of quantum character.290 ± 293 Defect-free metallic NTs appeared to be ballistic conductors of electric current that flows without heat release.The current density can reach a colossal value of 107 A cm72 and, were NTs classical conductors, they would be immediately vaporised. Ohm's law is not valid for ideal ballistic conductors, namely, their resistance is independent of length and approaches a theoretical (quantum) limit of 6500 O. The conductivity in NTs differs from that in two- and three-dimensional systems and can be described in the framework of the Luttinger liquid theory.294, 295 { In this Section, we present the results of selected research studies in physics and technology that are of prime interest for chemists. Relative elongation (%)844 Nanotube conductivity also differs from conductivity in conducting polymers where free electrons are supplied by dop- ants.Graphite is characterised by delocalisation of one of the four valence electrons. Graphene is an electronic hybrid; it is neither a dielectric nor a conductor, and also not a metal as well. Rather, it is a zero-gap `semimetal' or semiconductor. Therefore, the proper- ties of graphene depend on some extra conditions, in particular, the manner of rolling into NTs. Of course, there is no need to discard silicon-based electronics with billion-dollars investments and start new, NTs-based elec- tronics at the very beginning, the more so as many problems (in particular, high surface resistance of contacts, etc.) are still to be solved. On the other hand, production of various devices exploit- ing unique electronic properties of NTs that will function along with `old' electronic is thought to be a short distance in the future.This can be a new generation of electronic and logical microchips and mechanical and electromechanical devices. Theoretically, the switching rate of NT-derived electronic devices can be as high as 10 THz. It is believed that the first step of new electronics will be fabrication of hybrid circuits. Therefore, Si-supported NTs have been the subject of intensive studies. a. Diodes and transistors Semiconductor diodes (two-electrode elements with one junction) and transistors (three-electrode elements with two of more junc- tions) are main building blocks of ICs. It is these components that first of all appeared to be the focus of research activity. Junctions can be formed in several ways, namely, by matching two NTs (in particular, NTs of different diameters) with different electronic properties; by bending NTs; by partial filling of the inner NT cavities with those substances that give the filled parts of the NTs distinct electronic properties; by branching NTs or by making them three-terminal (Y- or T-shaped NTs).Matching two NTs with different structures requires insertion of pair defects (pentagon ± heptagon) into their hexagonal atomic sheets. If the pair defects form a `belt' along the circumference of the junction, the NTs keep their coaxial structure. If the defects are arranged opposite each other, the NTs become bent (Fig. 11). Bent NTs do exist and can be found among products of catalytic pyrolysis of hydrocarbons, as was reported by Han et al.296 who observed NTs bent at angles from 18 to 34 8.Service et al.297 reported the first, theoretical, study on the subject. Insertion of pair defects into the NT structure was studied 298 ± 302 and their energy characteristics were dis- cussed.303, 304 5 7 Figure 11. Schematical representation of a bent nanotube with topolog- ical defects. E G Rakov Rectification of alternating electric current by a molecular diode comprised of semiconducting SWNTs and impurity mole- cules has been studied by Antonov and Johnson.305 A number of theoretical studies is devoted to the properties of different junctions. 306 ± 309 Nanotube junctions can function as Schottky barriers. A prototype of single-electron transistor was first fabricated from SWNT bundles.290 This type of transistor can also be assembled from free-standing NTs.291, 310 Yao et al.311 found that a bent NT with metal ± semiconductor junction functions as diode rectifier.Among a total of nearly 500 NTs they found four nanotubes with one junction and one NT with two junctions and studied their current-vs.-voltage characteristics and the temper- ature dependence of conductivity. A prototype of single-electron MWNT transistor was fabri- cated 312 {for references, please visit the Website at URL [http:// haithabu.fy.chalmers.se/abstracts/031-html]D}. Researchers from Japan reported high operational characteristics of diodes based on NTs with homo- and heterojunctions.313 Nanotube-based field effect transistors were called TUBUFETs by analogy with the abbreviation MOSFET (Metal Oxide ± Semiconductor Field-Effect Transistor).314 Characteris- tics of the TUBUFET transistors fabricated from aligned SWNT arrays approach those of silicon-based MOSFET transistors. IBM Corp.(USA) also reported successful development of SWNT- and MWNT-based field effect transistors (for references, visit IBM's Website atURL http://www.research.ibm.com/nano- science/nanotubes.html). Researchers at IBM believe that NT- based transistors will soon be quite competitive with silicon transistors. Junctions consisting of two crossed SWNTs (a metallic NT and a semiconducting one) were studied experimentally.315 This type of junction can be readily fabricated and they can serve as the basis for large-scale production of useful devices.Fabrication of integrated circuits of NTs requires enabling controlled growth and arrangement of NTs in the space between electrodes. A feature article by Dai et al.316 concerns methods for the synthesis of NTs and integrated NT-based devices; particular emphasis was placed on SWNTs. The authors of this study pointed to the fact that, in contrast to traditional architectures of microelectronic devices and ICs based on the `top ± down' fab- rication techniques, the methods being developed for NTs are based on the `bottom ± up' approach. Lefebvre et al.317 briefly outlined different techniques for fabrication of ICs incorporating SWNTs. Using electron beam lithography and nanomanipulation with an AFM, they showed the possibility of fabricating diodes, devices with junctions between matched individual SWNTs, single-electron transistors and field effect transistors. This study seems to be the first communication that concerns the effect of chemical environment, i.e., the adsorbate molecules, on the characteristics of electronic devices, since adsorption can change the electronic properties of substances when fabricating nanometre size NT-based devices.Mention was also made of the effect of mechanical deformation on the functional properties of NTs. Very recently, strong limitations on the use of NTs in electronic devices were found.318 They are due to specific noise that appears as electric current flows through NTs.Impurities adsorbed on the external surface of NTs are thought to be the source of the noise (or one of the noise sources). If so, NTs will enable production of very sensitive sensors.319 Noise reduction can be achieved by simply cleaning the NT surfaces. In a continuation of their research 318 Collins et al.320 studied the effect ofO2 impurity on the electrical conductivity and thermal emf of NT bundles and films. It was found that adsorption of O2 (i) changes the n-type conductivity of the NTs to p-type conduc- tivity, (ii) increases the NT conductivity by 10% to 15% as compared to the value in vacuum and (iii) changes the thermal emf from710 to +20mV K71.The chemistry and application of carbon nanotubes Model calculations predict that bonding of O2 molecules to SWNTs will affect their electronic and magnetic properties.214 Electronic devices can be fabricated by controlled deposition of individual SWNTs with attachedCO2Hgroups through a mask on the surface that was chemically pre-functionalised with NH2 groups.118 Another closely related technique 321 involves plasma etching prior to the adsorption of functionalised NTs.Both procedures allow a space resolution of several micrometres. Schematic representation of a chemical process of fabrication of integrated circuits incorporating nanotubes by pyrolysis ofCH4 on a Fe ±Mo catalyst patterned by electron beam lithography is shown in Fig. 12.322 Recent developments in this field allow fabrication of metal ± horizontally aligned NT arrays ± metal structures 323 or three-dimensional Si ± vertically aligned NT arrays ± metal aggregates 324 on silicon substrates using lithogra- phy and chemical vapour deposition.12345 Figure 12. A scheme illustrating the order of operations in the production of integrated circuits with embedded nanotubes.322 Patterning with Ti ±Al alloy (1), coating withPMMAwith Petri dishes (2), catalyst deposition on the Petri dishes and dissolution of PMMA (3), chemical vapour deposition of nanotubes by catalytic pyrolysis ofCH4 (4) and coating with metallic electrodes (5). Tests for the best ohmic contact between NTs and metallic electrodes were carried out with Ag, Al, Au, Ca, Cr, Mg, Nb, Ni and Ti.316 The lowest contact resistance was found for titanium.One can also use Sc and V in addition to Ti, whereas Fe, Co, Ni and Cu exhibit high contact resistance.325 A procedure for improving the stability of contacts with Ti ±Al electrodes involves short-term (30 s) heating at 600 ± 800 8C.326 Special research into MWNT-based electrical contacts carried out to develop a fab- rication procedure resulted in a rather simple method consisting in placing a tungsten wire 4.3 mm in diameter (a shadow mask) crosswise on the MWNTs, deposition of a conducting layer and removal of the wire.327 Since chemical methods for fabrication of NT-based devices are currently in a primitive state of development, the problem of manipulating NTs is usually solved by using scanning probe microscopes.312, 313, 328 Some rather exotic types of NTs have been studied.Molecular dynamics simulation revealed the possibility of fabricating met- al ± semiconductor ± metal junctions in three-terminal NTs taking T-shaped 329 and Y-shaped 330 NTs as examples. Electronic prop- erties of these systems were also studied theoretically.331 ± 333 Branched NTs (a mixture of Y-, T- and L-shaped NTs) were first obtained in low yield by the arc method.334 Formation of Y-shaped NTs in the course of hot wire activated CVD synthesis of diamond was reported.335 Higher yields of this type of NTs can be obtained by other methods, e.g., CVD in the course of pyrolysis of hydrocar- bons 336 ± 338 and transition metal catalysed decomposition of C60.339 Branched NTs were also obtained by pyrolysis of acetylene followed by deposition on an Al2O3 template with electrochemi- cally etched, Y-shaped channels, which was dissolved after 845 completion of the process.336 Microwave plasma enhanced pyro- lysis of CH4 diluted with H2 was catalysed by nanocrystalline Pd on the surface of porous silicon.337 Yet another procedure for the synthesis of Y-shaped NTs involves pyrolysis of a gaseous nickelocene ± thiophene mixture at 10008C.338 The yield of*40 nm-diameter, Y-shaped NTs with an angle of nearly 90 8 between the junctions was as high as 70%.Asymmetry of the current-vs.-voltage curves of these NTs with respect to the bias current points to the fact that the NTs function as rectifiers.These synthetic results were found outstanding.340 A chlorophyll-containing protein in spinach is capable of not only receiving photons of light, but also transmitting electric signals in one direction at a rate 100 times faster than in silicon photodiodes. A nanodiode 10 nm long was fabricated from the molecules of this protein. Using 2-mercaptoethanol, the protein molecules were oriented perpendicular to the surface of a gold support and then connected by NTs to give a biomolecular device. This biochip was first demonstrated in spring 1999. Illumination by a pulse of light caused the device to conduct an electric current {this material can be accessed via the Internet at URL [http:// www.ornl.gov/ORNLReview/rev32_3/brave.htm]D}.b. Field emission devices Cathode-ray tubes (CRT) with hot-cathode electron sources appeared more than 50 years ago. Now it is timely to replace CRT by compact flat panel displays with field (or tunnel) emitters.341, 342 Field emission, i.e., the emission of electrons from highly curved surfaces (tips, blades, etc.) under influence of an applied voltage, is usually observed only in ultrahigh vacuum at high voltages and the emission current is limited to several mA. Fabrication of field emitters is complicated and expensive since tips are made of single-crystalline silicon or diamond pyramids. However, these devices offer some clearly seen advantages over the hot-cathode sources, such as operation in the cold emission regime, small size, small energy expenditure in the operating mode and high density of the emission current.Because of this, field emitters have been the subject of intensive current research. For instance, it was found that*7 mm-diameter carbon fibres used as field emitters operate under somewhat lower vacuum than other materials do (see, e.g., Ref. 343). Graphite is characterised by a relatively high work function of electrons (4.4 eV) and many graphite-based materials (in partic- ular, micro-coarse, powder-like, ion-irradiated graphite) can be used for fabrication of field emitter cathodes. However, all of them have a great drawback consisting in rapid degradation under operating conditions. Nanotubes were proposed to be used as field emitters based on similarity of the field emission mechanisms from graphite materials and NTs.344 Large aspect ratio, small radius of curvature of the NT end, high electrical and thermal conductivity and chemical stability of NTs make them promising for field emission applications. Not only individual NTs, but also NT bundles can serve as field emitters and function in moderate vacuum.Schematic representation of an NT-based field emitter is shown in Fig. 13.345 4 3 I 2 + 1 V7 1 mm Figure 13. A scheme of a nanotube-based field emitter. A film comprised of nanotube arrays grown perpendicular to the support surface (1), insulator (mica) (2), grid (3) and anode (4).846 Let us consider the properties of NTs as field emitters.For purified MWNTs, the work function for electrons is 4.3 eV and increases up to 4.8 eV on oxidation of the NT surface.346 The work function for NT-containing films is much lower than for graphite, though it depends on the film surface relief.347 Pioneering experimental studies of the emission properties of NTs were carried out by Chernozatonskii et al.348 in the Russian Federation and by Ugarte et al.349, 350 (in Western countries) who fabricated NT-based field emitters. An NT-based cathode can have a surface area of up to several hundreds of square centi- metres, it is cheap and stable in air. However, the procedure for fabrication of aligned NT arrays (suspension, filtering and fix- ation of precipitate) proposed by Ugarte et al.349, 350 was found to be too complicated to be used in large-scale production. Nanotubes are characterised by emission current densities of up to 10 mA cm72 at a rather low gate electric field (0.8 V mm71).351, 352 The gate fields for SWNTs are lower than for MWNTs; however, the latter have longer lifetimes.353 Both open and closed NTs can emit electrons.The effect of opening of the NT ends on the emission properties has been studied.354 Free-standing MWNTs 10 to 15 nm in diameter were produced by arc method and opened by oxidation in air at 650 8C or by laser ablation. It was found that (i) opening of NTs enhances field emission and (ii) opening by oxidation in air is more efficient than laser ablation. In their short review Schonenberger and Forro 222 pointed out that closed NTs are more efficient emitters.Theoretically, open NTs must produce higher emission currents than closed NTs; however, impurities (e.g., oxygen atoms) attach themselves to the free dangling bonds at the end of the NT and the emission current reduces. Nevertheless, in both cases the emission current reaches colossal values for such small-size emitters. Emitters based on aligned arrays of `capped' MWNT were found to exhibit the most reproducible properties and the longest lifetimes. Nanotubes operating in the field electron emission mode emit light.355, 356 In a dark room, the glow can be seen by the naked eye. This phenomenon is of great importance for understanding the emission mechanism.Nanotubes produced by different methods and having differ- ent structures possess markedly different emission properties (Table 2).357 Table 2. Average field amplification coefficients (b ), trigger and threshold electric field strengths (Et and Ethr /V mm71) and average NT lifecycles (t /h) at a current density of 0.2 mA cm72 for different NT samples. NT Ethr Et t b Multi-walled nanotubes open closed catalytic Single-walled nanotubes 20 120 10 12.9 302.2 143.9 4.5 1.1 5.6 1.5 1100 1600 830 3400 Kim et al.359 fabricated field emitters with improved charac- teristics (b=17 000 ± 33 000, Et41.0 V mm71, the brightness of the green phosphor was 1800 cd m72) using highly dense (5 ± 10 mm71), well-aligned NT arrays.Diamond-`capped' silicon whiskers are characterised by record values of the turn-on (Et) and threshold (Ethr) fields; however, parameters of the best samples of NT-based field emitters approach them.358 Characteristics of field emitters are closely related to the methods and conditions for the synthesis and purification of NTs.44, 360 Experiments with NTs grown by plasma-assisted CVD showed that the emission current can change by an order of magnitude upon variation of the procedure for coating the catalyst (at different substrate ± solution contact times).361 Information reported in studies on the field emission from NTs is often contradictory and the mechanism of the process is E G Rakov still to be clarified.The limiting field emission currents differ by orders of magnitude. Some contradictions were eliminated by Dean and Chalamala 213 who showed that saturation of the emission current is due to the presence of adsorbed impurities (the effect of saturation is observed at voltages between 1600 and 1800 V and leads to deviation of the emission currents by 2 to 4 orders of magnitude from that expected from the Fowler ± Nord- heim theory) . On the one hand, the presence of impurities favours an increase in the emission current. On the other hand, the impurities are removed as the emission current increases and after some time the dependence follows another pattern. Pre- desorption of impurities by heating samples in vacuo eliminates the saturation effect and reduces the emission current.Electron energy distribution for emitting n-Si-supported SWNTs has been studied.362 The observed current densities were as high as 25 mA cm72. The emitter appeared to be stable with time and exhibited well-reproducible properties. A similar study on SWNT bundles was carried out.363 Emission from NT bundles gives a bright structureless spot on a phosphor screen. Unexpectedly, emission from a free-standing NT with an open end gave a conical spot. The cone angles were estimated at *0.2 rad for the external and 0.05 rad for the internal parts of the cone.364, 365 Field emitters can also be fabricated from entangled NT arrays. The emission current density achieved in experiments with a material incorporating randomly oriented NTs (according to Collins and Zettl,353 this favours elimination of defects) was as high as 400 mA cm72 at 200 V.Cathodes for field emitters can be fabricated from NTs sintered under mild conditions (2273 K, 25 MPa).366 Dai and Mau367 discussed some of the known methods for fabrication of aligned NTs. These methods can be arbitrarily divided into two groups. The first group comprises two-step procedures that involve individual stages of NT synthesis and alignment , namely, the above-mentioned cutting of thin compo- site slices,229, 368 passage of a NT dispersion through a micro- porous filter 350 and rubbing a NT-coated plastic surface with a thin Teflon plate or Al foil.349 The second group comprises one-step methods that involve synthesis-induced alignment of NTs and are of much greater importance.These are template synthesis using microporous substrates and synthesis with patterned catalyst coated on the substrate surface (possible patterns are strips, squares, etc). Usually, experiments are carried out using Al2O3 membranes with parallel pores obtained by anodisation of Al, mesoporous SiO2 and porous silicon as microporous substrates. The catalyst is either deposited on the bottom of micropores by CVD or sputtered onto the substrate surfaces that were pre-treated by lithography or programmed etching. Precipitation of catalyst from solutions is also used. Nanotube synthesis using porous Al2O3 Fabrication of ordered structures from NTs by thermal decomposition of silicon carbide represents a special case.369, 370 has been described 129, 350, 371 ± 375 (some of these studies do not concern fabrication of field emitters).The diameter, length and degree of ordering of NT arrays are completely determined by the proper- ties of the template and can be varied over a rather wide range. Synthesis of NTs using mesoporous SiO2 has also been reported.376 ± 378 Fan et al.379 succeeded in growing a `forest' of NTs with uniform length distribution on the surface of porous n+-Si substrate with an area of *4 cm2 by catalytic pyrolysis of C2H4. Catalytic iron particles were deposited on the silicon surface through a shadow mask. It is thought that this method can be applied to larger surface areas (Fig.14). The MWNT `forest' was grown perpendicular to the substrate and presented regularly arranged, square (262 mm) NT arrays, or towers, of the same height varied in the range from 10 to 240 mm. Each of the 15 samples fabricated by the authors of this study 379 exhibited stable low-voltage electron emission. Considerable advantages ofThe chemistry and application of carbon nanotubes a 1 b 2 Fe Fe Fe c 3 d Figure 14. A scheme illustrating the process of fabrication of aligned NT layers on the surface of porous silicon. Anodisation (1), sputtering of iron (2) and growth of NTs (3); n+-silicon (a), porous silicon (b), mask (c) and product (d). porous silicon in the synthesis of oriented NTs were pointed out.(A special issue of the journal `Physica Status Solidi A' 380 was dedicated to the preparation, properties and applications of porous silicon). The substrate surface can be patterned with the catalyst either lithographically or by vaporising a fraction of pre- coated catalyst. Fabrication of field emitters from SWNTs is a more compli- cated problem. Nevertheless, the use of pyrolytic method and substrates patterned with 1 to 5 mm wide catalytic islands resulted in the synthesis of NTs that were 0.7 to 4.0 nm in diameter and bridged adjacent islands.322, 381, 382 A special procedure was devel- oped for the deposition of NTs on top of Si `towers' pre-coated with catalyst using a contact printing technique.383 The NT structures obtained resembled power lines with suspended SWNT bridges.Related NT structures of some larger size were fabricated by researchers from the Russian Federation. Nanotubes for field emitters were grown by catalytic pyrolysis of acetone or by disproportionation of CO on silicon whiskers.384 Enhanced performance of acetylene pyrolysis and an increase in the surface area for NT deposition can be achieved by pattern- ing the substrate with the catalyst using microcontact printing technique.385 Experiments were carried out using hydrophilised polydimethylsiloxane as `stamp' and solutions of iron, cobalt and nickel nitrates or their mixtures in ethanol as `ink'. The method allows one to obtain NT strips *10 mm wide, grow NTs 5 to 15 mm high and control the NT density on the surfaces. Fabrication of NT structures by pyrolysis of hydrocarbons in the presence of volatile catalyst or by pyrolysis of the chemical NT precursors containing both the carbon source and metal cata- lyst 386 ± 390 represents a special case.[To some extent, decomposi- tion of a Fe(CO)5±C2H2 mixture 391 also falls in this category]. There are two ways for fabricatingNT structures on thermally unstable substrates, in particular, polymeric materials. These are transfer of the precipitate obtained on thermally stable materials to polymer and reduction of the pyrolysis temperature by intro- ducing particular additives into the chemical composition of the starting gas mixture followed by physical activation of the process.For instance, pyrolysis of C2H2 in excess NH3 allowed NT deposition immediately on the low-melting glass.392, 393 Polymeric substrates are coated with NT `forests' using NT deposition on quartz glass followed by dissolution of the substrate and oriented transfer of the precipitate.321, 388 847 Hot filament activated pyrolytic synthesis of NTs on different substrates was developed by researchers from France.394 By varying the reaction conditions they grew carbon nanostructures with different morphologies and emission characteristics. It was shown that a coral-like material containing NTs and nanopar- ticles with reasonable emission properties can be deposited immediately on the glass surface. Catalytic pyrolysis was used for NT deposition from gaseous hydrocarbons and fabrication of field emitters from entangled randomly oriented NTs on large surface areas.395 Emission characteristics of oriented NTs grown by glow discharge method from a CH4±H2 mixture were studied by researchers from Moscow and Novosibirsk, Russian Federation {this material can be accessed via the Internet at URL http:// carbon.phys.msu.su/MRS/Mrs.htm]D}.Emission properties of an NT `forests' grown pyrolytically from C2H2 on oxidised Si surfaces coated with nickel, cobalt or a nickel ± cobalt alloy were reported.396 ± 400 Swiss researchers grew NTs on a silicon substrate and showed that field emission is observed without preliminary treatment of the deposited film and even in the presence of catalyst.401 The work function for electrons was estimated to be 5.3 eV.Field emitters are produced in the USA, Japan, Russian Federation, Australia, in EC and South-East Asia. It was shown that NT-based field emitters can exhibit improved technological characteristics (density of emission current, gate voltage, bright- ness and lifetime) compared to analogous devices made of other materials and hot-cathode emitters. For instance, experiments at the Oak Ridge National Laboratory (USA) showed that NT- based field emitters are 10 times more efficient than diamond ones {this material can be accessed via the Internet at URL [http:// www.ornl.gov/ORNLReview/rev32_3/brave.htm]D}. MWNT-Based cathode tubes for elements of large-area out- door displays were produced in Japan.402 ± 404 The tube diameter and length were 20 and 74 mm, respectively.The devices exhibited stable electron emission and long lifetimes (continuous glow over a period of 3 months). Their brightness was twice as high as that of conventional hot-cathode devices. The results of research into NT-based field emitters for CRT displays and light sources carried out in Japan were reviewed by Saito and Uemura.405 A flat plane display with a brightness of up to 800 cd cm72 at 5 V mm71 was fabricated in Taiwan.406 A method for fabrication of field emitters from SWNT bundles was developed by research- ers from the PR China.407 Nanotubes represent a new kind of goods on the world market. The emission properties of commercially sold SWNTs and MWNTs were studied by Monteiro et al.408 An SWNT-based display 11.4 cm in diagonal with a bright- ness of 1800 cd cm72 was fabricated (see Ref.409). In 1998 and 1999, Ulvac Japan Ltd. and ISE Electronics Corp. (both from Japan) and Samsung Corp. (South Korea) demonstrated elements ofCRTand prototypes of MWNT-based displays (these materials can be accessed via the Internet at URLs http://www.jnmr.com/ intro/nanotubes.html and http://www.itron-ise.co.jp/english/ nano/). In the late 1999, ISE Electronics Corp. had test-produced these display devices (this material can be accessed via the Internet at URL http://www.jnmr.com/intro/nanotubes.html). Research at Samsung Advanced Institute of Technology led to development of an NT-based 9 inch colour flat panel display with a resolution of 5766242 lines (this material was presented by J M Kim at the American Physical Society Conference in March 2000 and can also be accessed via the Internet at URLs http://www.eps.org/aps/ meet/MAR00/baps/abs/S2090003.html and http://fuji.stanfor- d.edu/seminars/spring00/slides/kimSlides.html).Samsung Corp. plans to start commercial production of NT-based colour displays in 2001 ± 2002. Large-scale industrial production of NT-based displays requires solving two major problems, viz., to make NTs cheaper848 and to develop methods for fabrication of large-area NT-based emitting surfaces with uniform nanotube characteristics. Nanotube-based field emitters can find applications not only in flat panel displays, but also in production of light sources, in such devices as vacuum pressure probes, microtriodes, klystrons, etc.c. Memory devices No NT-based memory devices have been fabricated so far. Nevertheless, some proposals that NTs can be used for this purpose have been made. Such memory elements can consist of, e.g., K@Cá60 complex encapsulated into short, closed 1.4 nm- diameter (10,10)-NTs (any other alkali metal can be used instead of K). TheK@Cá60 `molecule' can change the equilibrium position from the `bit 0' to the `bit 1' ends of the capsule under the action of an electric current. The time taken to reach the opposite end of the NT is only 4 ps, which is ten times shorter than the switching time of conventionally used memory elements.410 The system resembles a kind of nanoscale `abacus' or a nanoscale `shuttle'.The authors of this study 410 have discussed some approaches to large-scale fabrication of these structures; however, solving this problem faces great difficulties at present. Nanotube structures with encapsulated neutral C60 molecules (nanoscale `peapods') 31 are formed by cyclic raising and lowering the temperature. The yield of these structures can be increased by varying the reaction conditions.33 It was stated that this method allows mass production of the nanoscale `peapods'.34 Prospects for fabrication of NT-based memory devices for molecular computers have been outlined.411 Memory arrays built of NTs are expected to store 10 000 to 30 000 times larger amounts of information and operate 1000 to 10 000 times faster than modern memory chips.d. Nanolithography Currently, intensive research has been carried out into fabrication of semiconductor devices with a characteristic size of 100 nm and less. Optical lithography using sources of short-wave UV radia- tion (l=193 nm) is limited by the lack of optically transparent materials at wavelengths shorter than 190 nm. This requires development of alternative methods for nanoscopic etching that use, e.g., a new type of microscope probes. The invent of scanning tunnelling microscope (STM) dates back to the early 1980s. The AFM, which is based on measure- ment of the adhesive force, is known since 1986. Usually, the STMs provide better resolution than the AFMs which are capable of operating without conducting substrate. The invention of the STM and AFM offered the possibility of manipulating individual atoms and molecules and moving them nanometre-scale distan- ces.412 ± 420 The greatest drawback of both types of instrument consists in frequent metallic probe tip `crashes' with the surface.The probe lifetime can be extended by using a carbonNT attached to the end of a silicon cantilever as the probe tip.421 Molecular dynamics simulations demonstrated that diamond surface can be selectively etched using an NT tip with attached C2 groups.422 It was concluded that NTs with the unmodified ends with strong covalent C7C bonds can be used for atomic-scale etching of semiconductor surfaces with relatively weak bonds (e.g., Si and Ge).Another simulations performed by the same method revealed the possibility of nanoscale etching and inden- tation of the Si(001) surface using NT ends (this material can be accessed via the Internet at URL http://www.foresight.org/ Conferences/MNTs6/Papers/Dzegilenko/). Local anodisation of hydrogenated Si(100) surface and litho- graphic transfer of silicon dioxide in the form of lines 10 nm wide at a speed of *0.5 mm s71 in the experiments with an AFM equipped with a MWNT probe tip were reported.423 The litho- graphic writing speed achieved using the MWNT probe tip was 5 times higher than with other probes. It was shown that the NT tips present a solution to the tip-wear problem and are impervious E G Rakov to breakdown in high electric field.Theoretical foundations of the method were considered. Nanoscale lithography experiments with polysilane films using an STM equipped with an NT probe tip showed that the NT tip provides higher resolution, larger groove depth-to-width ratio at higher scanning speeds and extended tip lifetime than the probe tips made of other materials.424 Impressive experiments were carried out on the manipulation and deposition of individual atoms on a surface using the STM probe tip 425, 426 and the possibility of atomic-level modification of nanostructures was shown.415 This gave an impetus to the study by Kra'l and Toma'nek 427 who tried to eliminate the major draw- back of modern STM, that is, manipulation of a single atom.They theoretically substantiated the possibility of design of a `molecular pump'. Open NTs serve as ideal material for such a `pump'. The atoms filling the inner cavities of NTs can be pumped through the NTs by irradiating with two pulsed lasers. New lithographic methods are expected to provide fabrication of ICs with the circuit density 8 times higher and performance 16 times higher than those of the ICs produced using optical lithography technique by 2004. e. Amplifiers and generators for cell phone communication Using the ability of NTs to emit electrons upon application of relatively low voltages (this requires small energy expenditures but the current density remains high), researchers from the North Carolina University (USA) developed an NT-based microwave generator.428 This property of NTs can find applications in wire- less communication devices.Usually, cell phones generate weak signals that are amplified at a central office. Unique properties of NTs allow production of the central offices characterised by smaller size and longer service life. A prototype of such central office has been developed. The property of NTs to change their electric characteristics under the action of mechanical stress was proposed for use in a new generation of phones.428 f. Other applications Tsukagoshi et al.429 obtained rather encouraging results concern- ing the use of NTs for design of electron spin devices. Injection of spin-polarised electrons into MWNTs resulted in coherent elec- tron spin transport.Nanotubes have unusually high thermal conductivity which approaches or even reaches some record values.430 ± 433 This suggests the possibility of using NTs as heat sink for ICs and electric motors (this material can be accessed via the Internet at URL http://composite.about.com/industry/composite/library/ PR/2000/blupenn1.htm). According to model calculations,430 individual nanotubes in NT bundles interact with one another rather weakly and thermal vibrations occur solely along the NT axis.Prospects for design of an NT-based molecular-scale digital computer and a new chemical approach to production of ICs have been discussed (this material can be accessed via the Internet at URL http://marble.he.net/*foresite/Conferences/MNT7/ Abstracts/Ellenbogen2/index.html).To reduce the size of electrical contacts in future ultraminia- ture ICs and electromechanical nanosystems, researchers at ORNL proposed to replace the wires and pins by NTs placed both on and off the chip. It is assumed that carbon NTs outside the chip will communicate with the on-chip NTs through field- emitted electron beams (this material can be accessed via the Internet at URL http://www.ornl.gov/ORNLReview/rev32_3/ brave.htm). Nanotube-based electrodes can find efficient application in gas-discharge tube protector units which are used in telecom network interface device boxes and central office switching gears to provide protection from lightning and AC power cross faults.Tests showed that characteristics of experimental NT-based gas- discharge tubes (mean DC breakdown voltage and breakdownThe chemistry and application of carbon nanotubes reliability) were better than those of commercial products (this material can be accessed via the Internet at URL http://www. eps.org/aps/meet/MAR00/baps/abs/S4890/html; communication M10.003). 3. Fields of application of nanotubes in power engineering a. Lithium batteries Research on current sources with high specific (per unit mass and unit volume) energy capacity is an important avenue of develop- ment of `small' power engineering. Here, the most widely used are lithium current sources in which lithium is intercalated into graphite or other carbon materials (see, e.g., Ref.434). Nearly immediately after their discovery, fullerenes appeared to be the subject of intensive studies in this field. This also holds for NTs. If the capacity of graphite is limited to one lithium atom per six carbon atoms, the capacity of NT bundles must be higher, since lithium atoms can fill both the intertubular space of the NT bundles and the inner cavities of individual NTs. According to ab initio calculations, the content of lithium in SWNT bundles can be much higher than in graphite and correspond to the composition LiC2.435 Intercalation of lithium and other alkali metals follows the same pattern and is accompanied by charge transfer without structural distortions.Single-walled NTs recovering their initial structure after de-intercalation similarly to NT bundles seem to be of greatest interest for the use in rechargeable batteries.174 Single-walled NT bundles were intercalated with Li not only by vapour transport, but also electrochemically.178, 436 The limit- ing intercalation degree corresponded to the composition Li1.7C6 , which is much higher than the ideal value for graphite and that determined for MWNTs (LiC6).437 ± 439 The irreversible specific capacity was as high as 952 mA h g71. The reversible specific capacity was measured to be 447 mA h g71; however, it reduced to 237 mA h g71 and lower after fifth charge ± discharge cycle. For comparison, the reversible capacity of lithium cells with graphite electrodes lies between 280 and 330 mA h g71 (see Ref.434). As a rule, electrochemical intercalation of lithium into SWNT bundles is performed using a pressedNTmat placed on a platinum plate as the working electrode and a 1 M LiAsF6 solution in ethylene carbonate and diethyl carbonate (1 : 1) as electrolyte. Heat treatment of NTs improves their texture and reduces the oxidation ± reduction overvoltage difference but has a drawback consisting in reduction of capacity. High intercalation degrees (up to Li2.7C6) were achieved using impact grinding pre-treatment of purified SWNTs followed by electrochemical intercalation in 1 M LiClO4 solution in a mixture of ethylene carbonate and dimethyl carbonate taken in equal volumes.161 Mechanical treatment for some time resulted in the loss of ordering of the NT bundles and in an increase in the average intertubular spacing.However, the mechanism of such a strong enhancement of the NT capacity to lithium remains unclear. Most likely, the metal atoms fill not only the intertubular space, but also the inner cavities of NTs.161 The metal content in the product can be increased by increas- ing pressure.440 Intercalation of lithium into NTs at high pressures resulted in a material with the atomic ratio Li :C=2. The NTs retained their structure and the lattice parameter along the c axis increased to 0.411 nm.441 Another method involves coating of NTs with copper, their oxidation in air for 12 h at 160 8C and subsequent electrochemical intercalation with lithium.442 Carbon NTs obtained in this man- ner can reversibly store the amount of lithium (per gramme of carbon) corresponding to a capacity of 700 mA h (cf.268 mA h per gramme of CuO for composites). Intercalation of lithium into and deintercalation from CuO require applying a voltage ranging from 1.7 to 1.0 V and from 2.3 to 2.5 V, respectively (vs. lithium electrode). The reactions proceed by the following mechanism CuO+xe+x Li CuOLix . 849 Yet another, relatively recently proposed route to enhance- ment of the capacity of lithium current sources consists in the use of membranes with vertically aligned NT arrays as anodes. (The NTs were prepared by template synthesis followed by removal of the template, as shown in Fig.2). Che et al.374 used a 60 mm thick Al2O3 template with 200 nm pores. Nanotubes were deposited within the pores. The NTs were held together by the thin layer of carbon precipitate formed on the external surface of the template. Multistage treatment of the product followed by dissolution of the Al2O3 template resulted in a pure carbon NT membrane consist- ing of the NTs of relatively large diameter which replicated the structure of the template. The inner cavities of this membrane were filled with iron catalyst (from solution), the CVD procedure was repeated to obtain thin helical NTs inside the large-diameter NTs and then the system was electrochemically intercalated with lithium. It was found that the well-developed membrane surface allowed the capacity to lithium to be nearly doubled as compared to that of a membrane without `internal' NTs.The use of NTs in lithium current sources is thought to be promising. Recently fabricated SWNT- and MWNT-based Li anodes are characterised by a discharge capacity of 640 and 385 mA h g71, respectively, which is much higher than the parameters of industrially produced samples. b. Hydrogen accumulators The development of lightweight and safe systems for H2 storage is necessary for wide use of highly efficient H2 ± air fuel elements in, e.g., transportation vehicles. According to calculations, for a car with rechargeable hydrogen `reservoir' to travel 500 km, 3.1 kg of H2 is required. To this end, the `reservoir' must have a capacity of 62 kg m73 and the concentration ofH2 in the hydrogen-saturated material must be at least 6.5% (these values represent the U.S.Department of Energy targets for hydrogen storage for fuel cell vehicles). Hydrogen fuel is more environmentally safe than hydro- carbon fuel, since the only combustion products are water vapour. For long, industry looked to hydride-forming metals and alloys as the basis for high-capacity and safe H2 storage systems.443 On the other hand, a number of studies on the use of fibrous and tubular forms of carbon and SWNTs has been published recently.444 ± 449 The first experimental data on the hydrogen sorption on NTs were reported by Dillon et al.444 in 1997. In studies of temper- ature-programmed desorption of H2 from a material containing only 0.1% of SWNTs 1.2 nm in diameter and obtained by arc method in the presence of Co catalyst using the Siverts apparatus, they inferred that the adsorption capacity of nanotubes to hydro- gen varies between 5 mass% and 10 mass% or 20 kg m73 at 7140 8C and 40 kPa.Since the measurements were performed for unpurified material, the authors had to make a far-reaching extrapolation to NTs of purity 99%, so the results obtained can hardly be considered precise. It was found that the adsorption capacity of NTs 2.0 nm in diameter (up to 50 kg m73) approaches the desired characteristics (Fig. 15). Ye et al.450 studied pristine SWNTs and found that hydrogen adsorption on the NTs was 8.25 mass%at 80 Kand a pressure of 12 MPa.Single-walled NTs were prepared by laser-induced thermal synthesis in the presence of catalysts. Individual NTs were 1.3 nm while NT bundles were 6 to 12 nm in diameter. The specific surface area of the material was 285 m2 g71. At pressures above 4 MPa, theSWNTmaterial undergoes a transition to a new state of hydrogen coverage. Here, adsorption occurs not only on the external surfaces of the NT bundles, but also on the surface of individual NTs (according to calculations, the specific surface area of the NTs was 1600 m2 g71). The cohesive energy of the SWNT bundles was estimated at 4.5 meV per carbon atom. The behaviour of sonicated material differs from that of the untreated material. Bacsa et al.451 prepared NTs with a specific surface area of 790 m2 g71 and suggested even higher adsorption capacity of their material.850 60 2.0 1.63 40 1.22 1234567 1.36 D D C 20 C B B A A 0 10 8 6 4 2 Gravimetric density (mass %) Figure 15.Characteristics of different materials as hydrogen accumula- tors.444 H2 in the C/polymer composite cylinders at different pressures (1), H2 in the glass fibre/Al composite cylinders at different pressures (2), H2 produced in the reaction of Fe with H2O (3), liquiefied H2 (4), H2 on activated carbon (5),H2 inSWNTs (figures denote the nanotube diameters given in nm) (6) and the region of desired parameters (7). The region of metal hydrides is shown within a rectangle. Pressure /MPa: 20 (A), 24.8 (B), 40 (C) and 60 (D).Single-walled NTs of relatively large diameter (1.85 nm) were synthesised by the arc technique. The reaction was performed in an apparatus with a rotating 400 mm-diameter cylindrical anode with drilled radial openings filled with graphite powder and a catalyst.448 To obtain cheaper NTs, the synthesis was carried out in a H2 ±Ar rather than H2 ±He mixture using S-containing additives. Nanotubes were produced at a rate of several grammes per hour. Specially treated NTs 1.85 nm in diameter can store up to 4.2% of H2 with respect to their own mass (the atomic ratio H:C=0.52) at room temperature and a pressure of 10 MPa. It was found that*80% H2 can be desorbed and room temperature at atmospheric pressure. Complete desorption of H2 requires some heating of the material.449 The samples under study were pre-annealed at 773 K for 2 h, which favoured enhancement of the adsorption capacity.The capacity was nearly unchanged after four charge ± discharge cycles. The NT content in the material was varied between 50% and 60%, so its purification was suggested to result in considerable enhancement of the adsorption capacity. Chen et al.447 reported sensational results, according to which lithium-doped MWNTs can adsorb up to 20% of H2 with respect to their own mass over a period of 2 h at 380 8C while the adsorption capacity of potassium-doped MWNTs can be as high as 14% at room temperature. According to calculations, a 20% adsorption corresponds to 3 hydrogen atoms per carbon atom while the fuel necessary for a car to travel 500 km can be stored in a 18 litres `reservoir' of carbon NTs (this material can be accessed via the Internet at URL http://www.nus.edu.sg/INTsRO/ newsletters/issue_20/CurrRes.htm). The results obtained by Chen et al.447 were considered doubtful.452 Indeed, attempts at reproducing these results using NTs produced by the same methods inferred that such a large increase in mass was due to the presence of water vapour in H2.453 Alkali metals do enhance the adsorption capacity of NTs, but to a much lesser extent than was reported by Chen et al.447 Great interest was attracted to the study by Rodriguez et al.446 who synthesised graphite fibres capable of storing up to 67% of their own mass of H2 , which corresponds to a H:C ratio of 24.These results met initially with wide spread scepticism, since no experimental details were reported. Park et al.454 to some extent clarified the situation. Unusual behaviour of the fibres was related Hydrogen capacity /kg m73 Table 3. H2 Storage properties of various systems. Material Single-walled NTs (low purity) Single-walled NTs (high purity) Single-walled NTs (50% mass pure) Nanofibres (tubular) Nanofibres (herringbone) Nanofibres (platelet) No structure given Graphite Li ± NTs Li ± graphite K± NTs K± graphite FeTi ±H NiMg ±H Sorbent saturated at low temperature Octane gasoline a a Given for comparison. to their structure with a specific surface area of 1000 m2 g71 and specific volumes of microscopic and macroscopic pores of 0.36 and 0.41 m2 g71, respectively.However, this issue requires fur- ther investigations. The data listed in Table 3 characterise the hydrogen storage properties of NTs and other hydrogen accumulators and the state- of-the-art in this field. Hydrogen storage in NTs was reviewed by Fischer 457 and Tarasov et al.458 Electrochemical storage of hydrogen in unpurified MWNTs 2 to 15 nm in diameter and SWNTs 0.7 to 1.2 nm in diameter was studied.459 Relatively reliable results were obtained only in the experiments with SWNTs whose specific energy capacity was found to be rather high (110 mA h g71, or 0.39%). The reaction proceeded reversibly. Electrodes with open SWNTs provided reversible electro- chemical charging of up to 2.9 mass% of hydrogen, which corresponds to a specific energy capacity of up to 800 mA h g71 (see Ref.460). As the charge current changed from 10 to 100 mA, the capacity changed only by 50 mA h g71. This differs NTs from metal hydrides whose specific capacities are strongly dependent on the charge current. The maximum specific capacity was achieved after 20 charge ± discharge cycles and was kept constant for the next nearly 50 cycles. A large number of theoretical studies on H2 adsorption has been published. Most of them were cited by Park et al.454 Pederson and Broughton 461 were the first to propose that NTs can be filled with gas molecules due to the action of capillary forces.The results of model calculations of H2 adsorption 199 differ appreciably from experimental data. Insignificance of quantum effects at temperatures above 50 Kand low hydrogen coverages of theNTsurfaces was pointed out. The calculated storage capacities were found to be lower than those obtained from gravimetric measurements.462 ± 464 Hydrogen adsorption in SWNT bundles was calculated.465 The bundles consisted of square arrays of SWNTs 0.7 to 1.96 nm in diameter with an intertubular spacing of 0.334 nm. The effect of variation of the intertubular spacing on the NT storage capacity was studied taking 1.174-nm-diameter NTs as an example. It was shown that the highest H2 storage capacity at room temperature T/K Max. mass%H2 133 5 ± 10 80 8.25 300 4.2 298 11.25 298 298 298 ± 773 298 473 ± 673 473 ± 673 <313 <313 >263 >523 *77 67.55 53.68 0.4 4.52 20.0 14.0 14.0 5.0 52.0 54.0 *5 >233 17.3 E G Rakov Ref.P/MPa 444 0.040 450 7.18 10 ± 12 449 446 11.35 446 446 447 446 447 447 447 447 455 456 456 11.35 11.35 0.101 11.35 0.101 0.101 0.101 0.101 2.5 2.5 2.0 456 0.1The chemistry and application of carbon nanotubes (10.7 kg m73) is achieved at an intertubular spacing of 0.7 nm. However, the manner in which the intertubular spacing in the NT bundles can be changed remains unclear. The possibility of electrochemical storage of H2 in SWNTs was confirmed by Lee et al.466, 467 who also performed a theoret- ical study of the process.Several adsorption sites in and on the SWNTs were found. According to Raman spectroscopy data, the inner NT cavities are filled with H2 molecules. Molecular dynam- ics simulations showed that the highest hydrogen storage capacity of (10,10)-NTs is 14.3 mass% (160 kg H2 m73) and increases linearly with the NT diameter. Multi-walled NTs exhibit a lower hydrogen storage capacity. In this case, hydrogen is adsorbed on the external MWNT surfaces and their capacity is independent of diameter. Based on the above-mentioned experimental results, yet another group of theoretical studies was carried out. For instance, it was found that the desired 62 kg m73 capacity (3.1 kg ofH2) of the NT `reservoir' at 77 K can be achieved using SWNTs 0.6 nm in diameter at an intertubular spacing greater than 1.0 nm.Nanotubes carrying an electric charge were predicted to exhibit an enhanced hydrogen storage capacity (by 10% to 20% at 298 K and by 15% to 30% at 77 K).468 Williams and Eklund 469 attempted to bridge the gap between the results of theoretical and experimental studies. They pointed to the fact that all calculations performed so far used a model of infinite three-dimensional SWNT crystal lattice, whereas electron microscopy data show that SWNT bundles form a close-packed triangular `honeycomb' lattice and have finite diameters and length. Mention was also made of inadequacy of the approach relating enhanced adsorption to arbitrarily changed symmetry or increased intertubular space.c. Electrolytic capacitors Thin NT films coated on metallic surfaces are of considerable practical interest for production of supercapacitors characterised by high capacity and the ability to survive a large number of charge ± discharge cycles. The capacity of capacitors with an electrical double layer is known to increase linearly with the specific surface area that is rather large in the case of NTs (more than 300 m2 g71).470 In addition, it is important to use low- resistance electrode materials and to know the resistance of the electrolyte in the porous electrode structure. Carbon or graphite provide no possibility for fabrication of highly porous, `mat'-like electrodes, which is necessary for obtaining an optimum pore structure; however, this can be readily done using NTs.The specific capacitance of catalytically grown NTs of size *8.0 nm in 38% H2SO4 reaches 102 and 49 F g71 at 1 and 100 Hz, respectively. A single cell device had a power density exceeding 8000 W kg71 (see Ref. 471). Cyclic voltammetry measurements 472 revealed an effective capacitance of 283 F g71 at 0.5 V (vs. silver reference electrode), which is twice as large as the maximum value for carbon electrodes in non-aqueous media (120 F g71). The material was stable on potential cycling and no differences were observed after continu- ous cycles over 30 min at 50 mV s71. Capacitors with NT block electrodes and specific capacitance of 90 F g71 were produced.473 Nanotube-derived capacitors can be used for laser power supplies, wrist watches, flash lamps, starters for electric motors, etc.They can also find military applications in, e.g., bird- or insect-size reconnaissance airborne vehicles.470 4. Fields of application of nanotubes in analytical instruments a. Sensors Single-walled NTs are rather sensitive to gas atmosphere and can change their electrical resistance and thermal emf upon adsorp- tion of N2 or He.212 This was the starting point for work on an `electronic nose', or a chemical gas sensor. SWNT-Based chemical NO2 and NH3 sensors are character- ised by extremely short response time, thus being different from 851 conventionally used sensors.474 The sensitivity of the new sensors is three orders of magnitude higher than that of standard solid- state devices.A 0.02% concentration of NO2 increases the electrical conductivity by 3 orders of magnitude after 10 s expo- sure, while exposure to 1% NH3 decreases it by two orders of magnitude after 2 min. To reset the new sensors, they must be heated because of the low relaxation rate at room temperature. These sensors are very small (their size does not exceed several micrometres), simple in design (an NT connecting two conductors), cheap and operate at room temperature. A CO2 sensor has been developed (this material can be accessed via the Internet at URL http://www.newscientist.com/ns/19991225/ newsstory13.html). It should be mentioned that providing the possibility of sensing particular gases in complex mixtures selec- tively remains a moot question.Nanotubes are thought to be of use for development of miniature sensors that each soldier might carry to detect poison gas or biological weapons (this material can be accessed via the Internet at URL http://www.post-gazette.com/healthscience/ 1999101nanotubes1.asp). Frequency shift in the Raman spectra upon immersion of SWNTs in various liquids can find practical application in the development of sensors to test liquids. The frequency shift magnitude depends on the surface tension of a particular liquid, which can be used for the determination of its chemical composi- tion.249, 475 Construction and characteristics of items fabricated from individual MWNTs were reported.476 Nanotubes 80 to 120 nm in diameter and 15 to 50 mm long were attached to a platinum tip using conducting epoxy resin.The NT side walls were insulated with polyphenol. Cyclic voltammograms measured at different electrode immersion depths in electrolyte solutions were obtained. Nanotubes can be used as electrode material.477 Redox- proteins immobilised on the surface and inside NTs exhibited well reproducible cyclic voltammograms. Immobilisation ofDNA treated with I2 and [Pt(NH3)2]2+ and other proteins on NTs was also carried out.478 The possibility of attachment of metal porphyrins (including enzymes) to NTs allows fabrication of biosensors for studying different biomolecules and other biological objects.In particular, a proposal was made that NTs can be used as microelectrodes for the determination of glucose and cholesterol concentrations. The use of NTs can lead to basic change in the blood count procedure, so a drop will be enough to perform complete blood count over a very short time. The results reported by Balavoine et al.479 who obtained ordered helical shells of protein molecules on the NT surfaces can be considered as the first step towards the develop- ment of new biosensors. b. Microscope probes Usually, STM probes are made of tungsten or platinum while AFM cantilevers are made of silicon or Si3N4. Typically, the radius of curvature of theAFMcantilever tips varies between tens and hundreds of A Ê ngstroÈ m.The use of NTs as probe tips was first reported by Dai et al.421 who attached SWNTs to a pyramidal silicon cantilever, tested the device and obtained encouraging results. The use of chemically inert, conducting, hydrophobic, thin and flexible NTs makes the research process more simple; allows penetration into narrow grooves; improves resolution and pro- vides the possibility of performing non-destructive studies of delicate biological objects. A detailed instruction for attachment of NTs to commercial STM probes was posted on the Internet at URL {[http://cnst. rice.edu/mount.html]D}. Nanotube growth immediately on commercialSTMprobes by pyrolysis of hydrocarbons was reported.480 ± 483 This method is considered to be much more simple than attachment of arc-grown NTs.Yet another technique consists in attachment of NTs in an electric field.484 ± 486 However, the simplest and most closely related to large-scale production conditions method 487 involves852 a combination of (i) CVD of NTs on a special cartridge by catalytic pyrolysis 384 and (ii) transfer of the grown NT from the cartridge to the microscope probe tip in an electric field. The properties of `capped' NTs as materials for tunnelling microscope probes have been described by Harrison et al.488 The radius of curvature of an SWNT probe tip is 3 to 6 nm (some- times, even 0.2 to 0.5 nm), which is several times smaller than for conventional probe tips.489 Using an AFM with an NT probe tip, Choi et al.490 obtained images of individual NTs on mica surface, while Zha et al.491 studied chromium-filled NTs with an STM.491 2 Wong et al.89, 90, 92, 492 oxidised NTs in air at 700 8C (98% of the carbon material was burnt out), functionalised their tips with carboxylic groups and obtained world's first chemically sensitive probe microscope capable of analysing different substances at the molecular and atomic levels.Then they performed amino, hydro- carbon and bioactive functionalisation of the carboxy-modified probe tips by the formation of chemical bonds between CO¡ groups and one of the amino groups of ethylenediamine molecule, amide bonds with benzylamine molecule and amide bonds with biotin derivatives, respectively. The first (carboxy-functionalised), second and third probe tips possessed acidic, basic and hydro- phobic properties, respectively.The fourth probe tip was sensitive to streptavidine. In principle, CO¡2 groups can form bonds with a broad spectrum of functional groups, which allows fabrication of probe tips for various applications. Wong's `constructions' exhibit some advantages over the previously used probes. The point is that earlier functionalisation of the Si3N4 or SiO2 cantilevers resulted in attachment of func- tional groups not only to the tip, but also to the side walls, which damaged the objects under study. In contrast to this, no damages were observed in the experiments with NTtips since the functional groups were attached only to the `cap'. In addition, NT probes are characterised by small effective radius and provide higher reso- lution.Finally, 1.4-nm-diameter (10,10)-NTs contain only 20 atoms at the open end, so it is an easy matter to choose the conditions for fabrication of a probe tip functionalised with a single group with known structure and orientation even at random distribution of the NTs with different number of carbox- ylic groups at the end. Specifically functionalised NT probe tips can be used for selective removal of atoms from the surface. Probe microscopes were used to obtain the first images of DNA molecules.489 Microscopes with NT probe tips allow short- ening the time taken to analyse large DNA molecules.493 Atomic force microscopes with NT probe tips were used for the determination of the structure of protein complexes 494 and for manipulating DNA molecules.At particular voltages, the DNA molecules were adsorbed on NTs and arranged on their surfaces in a predetermined manner (this material can be accessed via the Internet at URL http://www.eps.org/aps/meet/MAR00/baps/ abs/S4890/html; communication M10.010). A well-resolved image of right-hand helical DNA molecule has been reported.495 Nanotube-modified cantilevers allow improving the resolu- tion of electrostatic force microscopes.496 Improved AFMs with NT probe tips can be used in studies of both rigid and soft surfaces in aqueous solutions. According to recent predictions, these microscopes will serve as the basis for a breakthrough in biological studies.497 Nanotube probe tips can also be used as the `reading' probes in memory storage devices of supercomputers.A possible candi- date would be a system consisting of layering H and F atoms on a diamond surface and a pyridine molecule attached to the probe tip end. Theoretically, the system can store 1015 bytes cm73 (this material can be accessed via the Internet at URL http:// www.msu.edu/*hungerf9/nanotube.html). 5. Nanotubes as catalyst and sorbent supports The idea of using NTs as catalyst supports was proposed nearly immediately after the discovery of this form of carbon.498, 499 This E G Rakov application of NTs is based on their chemical stability and well- developed surface. Catalysts can be obtained by the filling of the inner cavities of NTs or in the course of their decoration or, in particular, in the course of the NT synthesis.Only one study concerning SWNTs was cited in a short review on the application of fullerenes and NTs as catalyst supports.500 After long-term storage in hydrogen at 673 Ka material grown by arc method in the presence of cobalt and platinum exhibited high selectivity in the liquid-phase hydrogenation of cinnamalde- hyde.501 Multi-walled NTs were also studied as catalyst supports.ARu catalyst coated on arc-grown MWNTs with a specific surface area of 27 m2 g71 allowed an increase in the yield of hydrocinnamic alcohol obtained by hydrogenation of cinnamaldehyde to 80%. It was found that reduction selectivity increased from 30%± 40% to 92%.502 A nanotube-supported rhodium catalyst was found to be efficient in the thermal decomposition reaction.503 Complete decomposition of NO occurs at 600 8C on NTs without catalyst, at 450 8C on NTs containing 1 mass% Rh and at 300 8C on the catalyst pre-treated with H2 at 300 8C or higher temperatures.After 2 h exposure to NO at 500 8C the Al2O3 ±Rh (1%) catalyst contained rhodium only in the form of Rh2O3 , while the NTs ±Rh catalyst contained metallic Rh (8 nm particles). The efficiency of different supports was compared taking the reaction of catalytic hydroformylation of propene as an exam- ple.133 It was found that MWNT-supported ruthenium phosphine complex is not only much more active than other supports, but also exhibits high regioselectivity towards butyraldehyde (the reaction product) despite relatively small specific surface areas (155 and 237 m2 g71) and pore volume (0.46 and 1.33 cm3 g71) and relatively small pore diameter (3.2 ± 3.6 and 2.4 ± 3.2 nm) of the MWNTs used.Nanotubes were compared with supports characterised by much larger specific surface area (up to 1210 m2 g71, a carbon molecular sieve TDX-601), larger pore volume (up to 1.66 cm3 g71, a polymeric support GDX-102), and a larger (20 ± 100 nm, GDX-102) or smaller pore diameter (1.4 ± 2.0 nm, activated carbon). Efficient catalysts were obtained by filling the inner cavities of the above-mentioned carbon membranes (see Section VIII.3.a) 374 with catalytically active metals. For instance, solution-coated platinum enhanced the rate of electrocatalytic reduction of O2 , while a Pt ± Ru mixture enhanced the reduction of CH3OH, which means that these catalysts are promising to be used for fuel elements.A study of the electrocatalytic reduction of dissolved O2 using NTs showed that the exchange current density in the H2SO4 solution (pH 2) was five times higher than in the reaction carried out on graphite. This suggests possible use of NTs in fuel batteries.504 The rates of electrochemical processes occurring at platinum- and ruthenium-coated NT electrodes are higher than at platinated glass electrodes. `Nanohorns', a recently found kind of NTs, appeared to be an excellent adsorbent of liquid ethanol. (The properties of `nano- horns' were studied by Murata et al.505). The oxygen adsorption capacity of `nanohorns' is 3.5 times higher than that of other forms of carbon with the largest specific surface area.506 More- over, `nanohorns' catalyse oxidation of ethanol into acetaldehyde.6. Application of nanotubes in optical devices Molecules with high third-order optical nonlinearity and second- order hyperpolarisability are considered promising for use in photonic devices, e.g., optical switches, data processing systems, optical filters and sensors. However, most of the third-order nonlinear optical materials have low second-order hyperpolaris- ability. Exotic electronic structure of NTs allowed researchers to hope for their application in nonlinear optical devices. A theoretical model for SWNTs was proposed by Xie.507 ± 511 who also reported a third-order optical nonlinearity in chiral carbon NTs.512 Chiral NTs as nonlinear optical materials areThe chemistry and application of carbon nanotubes quite competitive with conducting polymers. Large nonlinear optical effects are typical of zigzag and B-(N-)doped NTs.513 Second-order hyperpolarisability of NTs was predicted.514, 515 The optical properties of NTs are strongly affected by the NT symmetry, the presence or absence of the `cap' and the number of constituent carbon atoms.516 A theoretical expression for the third-order nonlinear optical susceptibility of NTs, related to the third harmonic generation, was derived by Margulis et al.517 who pointed out that the resonant optical susceptibility of NTs is several orders of magni- tude higher than that of C60 or C70 .The properties of SWNTs 1.3 nm in diameter and *160 nm long in a DMF solution were studied.518 The ultrafast second- order hyperpolarisability was estimated at 2.1610728 esu. Absorbing cells filled with a suspension of SWNTs in water were used as protection from laser radiation.519 The effect of optical limitation was observed at 1064 and 532 nm. It was found that an increase in the radiation intensity by three orders of magnitude can reduce the optical transparency by two orders of magnitude. Due to their unique properties, SWNTs rank first as nonlinear optical switches among all the materials used in this field, though the mechanism of the phenomenon is still to be clarified. Studies on the optical limitation of SWNT suspensions in water, ethanol and ethylene glycol showed that the magnitude of the effect can differ substantially in different liquids.520 Multi-walled NTs also exhibit remarkable nonlinear optical properties 258, 521, 522 that are comparable with those of soot and C60 suspensions.This type of NTs combines the advantages of both soot and C60 . Similarly to soot particles, NTs are relatively large. Similarly to fullerenes, NTs can be readily modified by, e.g., filling to acquire desired optical properties. Third-order optical nonlinearity of MWNTs was measured experimentally.523 However, more recently it was found that incorrect experimental design led to observation of unusually high and unstable nonlinear optical response.Nanotubes are poorly soluble; however, their composites with polymeric matrices can be used for protection from laser radia- tion. Laser irradiated solutions of NT solubilised in poly(pheny- lacetylene) exhibited unusual behaviour, viz., they turned opaque with an increase in the radiation intensity.112 Nanotube additives preclude optical damage to the polymer even at such high fluences as 10 J cm72. Soluble complexes of MWNTs with polyacrylic acid and surfactants exhibit nonlinear optical properties.121 Dispersions of NTs in ethanol can be used for alignment of NTs under the action of electric current; they possess anisotropic properties and can change the direction of polarisation of the incident laser beam.524 Conclusions drawn by the authors of this study deserve further confirmations, since all the measurements were carried out using insufficiently purified material. Nanotube-containing composites can find application in photoelectric (photovoltaic) devices and light emitting diodes (LEDs).113, 114, 246 Filling conjugated polymers with fullerenes was shown to be promising in the early 1990s.More recently, NTs became the subject of research. For instance, attempts at fabricating LEDs with microfiltration-aligned NTlayer have been undertaken. However, the method involving preparation of an NT film by centrifugation and subsequent coating of the NT film with a poly(p-phenylene vinylene) film led to the best results.246 Multi-walled NTs on the surface of polymer films somewhat reduce photoluminescence.The efficiency of NTs as hole-collect- ing electrodes of photovoltaic devices appeared to be twice as large as that of standard indium ± tin oxide electrodes (the quantum yield at 2.9 to 3.2 eV was 1.8%). Thin films of this composite can be used as emitting layer of LEDs and glow at lower current densities compared to the untreated polymer.113 853 7. Molecular nanotechnology Nanotechnology is a field in which unique properties of NTs can best be used. Molecular nanotechnology concerns fabrication of functional structures and devices by atom-by-atom or molecule- by-molecule assembling using programmed robots (assemblers) capable of self-replication.According to calculations, the mass of an assembler equipped with a molecular computer and actuator can be at most 109 amu (this material can be accessed via the Internet at URL http://www.public.iastate.edu/*bhein/ FAQ.html). The assembly will occur consistent with the chemical laws; however, under conditions of `position' synthesis, where an atom or a molecule is supplied exactly to the place, these laws act in another manner compared to conventional chemical reactions conditions; if necessary, the overcoming of activation barriers can be achieved by consuming mechanical energy supplied from outside. After the invention of STM and AFM, molecular nano- technology is no longer only a scientific concept. There remains only some disagreement about how long its development will take {[this material can be accessed via the Internet at URLs http://www.nanomedicine.Com/2.1.html]D, .../2.2.html]D, .../2.3.html]D and .../2.4.html]D}.In 1998, Zivex LLC (USA) demonstrated the possibility of three-dimensional manipulating NTs inside a scanning electron microscope with a resolution approaching that of the scanning probe microscopes and near real-time video control. The develop- ment of this manipulator and especially its improvement to provide the possibility of simultaneous operation of several independent manipulators inside one microscope is a great stride in the development of nanotechnology. Probe microscopes allow one to manipulate NTs and move (`push around') molecules over the surface.Using a `nanoplotter', one can draw lines up to 30 nm wide. However, these devices have a strongly confined action range. New possibilities for manipulating nanoscale objects and studying their properties are opened after the development of the scanning probe microscope with `nanotweezers', that is, two tiny plates of gold separated by a thin glass layer and attached to the tips of 100-nm-diameter NTs.525 By applying an electric field it is possible to close and open the NTs and to grab and move submicrometre-size clusters and nanowires. Nanotubes of smaller diameter should make it possible to manipulate objects only 2 nm in size, i.e., single molecules and structures inside biological cells, thus modifying them.Initially, the greatest interest of experts in nanotechnology was attracted to various organic substances, especially a number of products of biosynthesis. It is assumed that molecular com- puters will be the first commercial fruit of the development of nanotechnology {this material can be accessed via the Internet at URL [http://www.resonance-pub.com/e-print%201.htm]D}. Currently, more definite hopes are associated with fullerenes and especially NTs. In particular, severe requirements imposed on the chemical and thermal stability of structural elements of molecular machines for aerospace industry make the use of NTs for fabrication of these elements justified. Experts at NASA's Research Center (Ames, USA) also point out that NT-based nanotechnology can provide a way to reduce the cost of space missions.526 ± 529 Bearings, springs and gears are important components of future nanomechanical systems. Their development is one of the main research avenues.In 1992, benzene was found to react with C60 by the 2,4- cycloaddition mechanism. According to calculations (1997), such reactions on the external side walls of NTs are possible. That is why one of the pioneering studies on the development of NT- based nanomechanical devices was concerned with molecular dynamics simulation of a NT-based gear with benzene gear teeth {see Ref. 530 and material posted on the Internet at URLs [http://www.nas.nasa.gov/Groups/Nanotechnology/publications/ MGMS_EC1/simulation/]D and [...MGM_EC1/quantum/index.854 Figure 16.A nanotube gear with attached C6H6 molecules. html]D}. Calculations were carried out for systems containing up to 2000 atoms. The angular momentum was transferred via two (14,0)-NTs 1.1 nm in diameter (Fig. 16). According to calcula- tions, the gear could operate at frequencies up to 100 GHz in vacuo at room temperature. More recently, systems comprised of hundreds of thousands of atoms were calculated. Other types of molecular gears were also calculated (this material can be accessed via the Internet at URLs http://www.nas.nasa.gov/Groups/Nanotechnology/publications/ MGMS_EC1/simulation/paper.html and ...MGMS_EC1/index. html) and the behaviour of molecular devices in helium and neon atmosphere was studied.A phenomenological model of a laser- driven molecular motor was studied. The motor supplied power to a nanotube gear.531 Several video clips and animations illustrating the operation of various nano-gear devices can be accessed via the Internet at URL http://science.nas.nasa.gov/Groups/Nanotechnology/pub- lications/MGMS_EC1/simulation/data/index.html. Calculations of gears consisting of internal and external NTs were carried out. A concept of fullerene motor consisting of two concentric graphite cylinders (a shaft and a clutch) was proposed. Possible conditions for coaxial arrangement of two NTs of different diameter (10 carbon atoms around the circumference of the shaft and 30 or 34 atoms around the circumference of the clutch) were calculated.532 Pioneering experimental studies attracted considerable atten- tion.A team from the California University at Berkeley (USA) and Lawrence National laboratory (Berkeley, USA) managed to elongate MWNTs by pulling out the inner shells and then to shorten them by reducing the force exerted.533 No indications of the `nanospring' fatigue or wear were observed after 10 to 20 in/out cycles. The author of a popular essay on the subject predicts a passage from microelectromechanical systems that are on the scale of a human hair to nanoelectromechanical systems (NEMS) that are so small that 10 000 bearings for such systems can be stretched across the diameter of a human hair (this material can be accessed via the Internet at URL http://www.berkeley.edu/ news/media/releases/2000/07/27_nano.html). According to expert evaluations, these systems will be developed in the nearest two years.At first, these will be hand-made devices and then some methods for their mass production are expected to be developed. Based on the results of their studies on the behaviour of MWNTs under load, Yu et al.534 ± 536 pulled out inner shells of MWNT using an AFM and estimated the magnitudes of forces that act in this system. The proposed model of the process included the shear interaction, a specific type of `capillary' effect (an interaction between two solids) and boundary effect (the influence of dangling bonds at the end of the inner NT `immersed' in the outer shell). The development of NT-based nanotechnology is associated with the breakthrough studies on nanolithography,423 develop- ment of `nano-fountain pen',427 fabrication of functionalised E G Rakov probe tips and theoretical investigation of selective removal of hydrogen atoms from diamond surface.422 A rather large number of communications on NTs were reported at the 8th Conference on molecular nanotechnology held in the USA in November 2000 and called `foresight'.In particular, new results were obtained on the character of con- ductivity in the `arm-chair' NTs (http://www.foresight.org/ Conferences/MNTs8/Abstracts/White/index.html); on the possi- bility of fabricating all-carbon electronic devices (.../Fuhrer/ index.html); on the application of SWNTs of different chemical compositions (including metal-doped NTs) for fabrication of ICs (.../Darsey/index.html); on the stability of composite (cluster- type) structural elements made of NTs and diamond; on the possibility of fabricating heterojunctions for nanoscale logical and memory devices and NT-based field emitters `capped' with diamond (.../Shenderova2/index.html); on theoretically studied gas separation processes using NTs and on the effect of NT filling with gases and C60 molecules on the mechanical properties of the NTs (.../Sinnott/index.html); on the thermodynamic properties of polyatomic liquids inside NTs and on the stability of NTs in aqueous solutions (.../Walther/index.html); on the quantum- mechanically studied effect of the surface curvature on the reactivity of NTs and fullerenes (.../Srivastava/index.html). A recently published short review `Carbon nanotubes.From macromolecules towards nanotechnology' 537 contains almost no information on applications of NTs in nanotechnology. IX. Conclusion Nanotube chemistry becomes a rather large division of general chemistry. Currently, it includes inorganic, colloid, polymer, physical and analytical chemistry of nanotubes (analytical chem- istry of NTs also includes sensorics, the subject of which is development of chemical and biochemical sensors). Nanotube chemistry is closely related to nanotube physics, materials science and quantum chemistry. Some problems of NT chemistry are common to electronics and photonics. Nanotube chemistry is also related to biochemistry.Some of the potential fields of application of NTs in biology were pointed out by Guo et al.478 These are vessels for pharmaceuticals, biomimetic systems, which simulate biological objects and proc- esses, catalysts prepared from immobilised enzymes, sensors, selective electrodes, biomolecule sensors, etc. Studies on NT filling with various compounds can lead to applications of NTs as specific type of chemical reactors. One can expect that chemical reactions in the NT cavities will proceed in a significantly different manner compared to processes that are on different scale, since even such phenomena as melting of crystals and crystallisation of liquids or glasses inside NTs are character- ised by some salient features.538 ± 540 Nanotube chemistry is expected to become an integral part of nanotechnology which undergoes rapid development.Close rela- tion between methods for fabrication of devices incorporating NT-derived elements and nanotube chemistry is also evident. Many applied research avenues have demands for NTs with uniform structure and size, and require chemical modification of NTs, etc. Nanotubes continue to surprise researchers by their unusual properties. Among new discoveries are, e.g., the superconductor proximity effect (a change in the electronic properties of a conven- tional conductor in the immediate vicinity of a superconduc- tor);541 superconductivity in *1-nm-diameter NTs at 4 K; magnetic flux trapping by a MWNT `forest';542 and oriented growth of SWNTs prepared by catalytic pyrolysis of CH4 on the surface of single-crystalline Si.543 For instance, NTs grown on the Si surface are oriented parallel to the surface in two perpendicular directions for Si(100) and in three directions at angles of 60 8 for Si(111). Probably, in the future this discovery will allow simplifi- cation of the fabrication process of hybrid NT structures and devices based on them.The chemistry and application of carbon nanotubes However, the period when the results of almost all studies on NTs were considered fundamentally important (or at least of great importance) and when the search for new methods for the NT synthesis and the studies of their properties were the focus of research activity is now over.The focus of current research has been applied studies. On the other hand, the synthesis of NTs with reproducible properties is still a challenge. For instance, a semi-continuous setup with a rotating anode for the arc synthesis of NTs has been reported;448 an installation with rotating cathode for preparation of NTs under microgravitation conditions has been developed,544 a method for continuous preparation of NTs using an arc immersed in liquid nitrogen was proposed,545 a technique for the preparation of mostly two-walled NTs was developed;546 and the performance of the setups for production of NTs by laser-induced thermal synthesis was improved and is now as high as 20 g per 24 h.New synthetic approaches were developed. These are syn- thesis of NTs under hydrothermal conditions,547 by exposure of a mixture of graphite and iron nanoparticles to atomic hydrogen,548 by irradiation of amorphous carbon (fly ash) with Ar ions (this material can be accessed via the Internet at URL http:// www.jnmr.com/intro/nanotubes.html) and thermal decomposi- tion of aminoazines (in the presence of catalysts) 549 and saccha- rose.550 A plasma jet method for the preparation of NTs was tested.551 A joint team from Osaka Gas Co. (Japan) and Labo- ratory for New Technology developed a new technique for the synthesis of NTs, based on electron irradiation of a solid residue obtained by decomposition of fluorocarbons. This method allows the obtaining of a product containing *50% of SWNTs.The process requires the use of powerful electron accelerators and is expected to be readily scaled up (this material can be accessed via the Internet at URLs http://www.jnmr.com/intro/nanotubes. html and http://www.kippo.or.jp/news/1998-e/0617.htm). Researchers from the Russian Federation were involved in the R&D of a method for the preparation of NTs by vaporisation of carbon at gas mixture pressures up to 1300 atm,552 while research- ers from Belorussia undertook considerable efforts aimed at developing the synthesis of NTs at temperatures below room temperature.553 However, the greatest advances were made in the preparation of NTs by the pyrolytic method, in particular, by decomposition of CO.The most remarkable is the development of a setup `Marc II' 554 for production of highly pure SWNTs (up to 99%) by catalytic decomposition of CO. A jet of a mixture of CO and a metal carbonyl flows out of a high-pressure (tens of atmospheres) chamber and is heated to 900 ± 1000 8C using a laser. The performance of the setup is expected to be *100 g of NTs per 24 h.554 A process of catalytic decomposition of CO under pressure (trade mark HiPCOTM) was proposed to be of use for commercial production of SWNTs. It is expected that NTs will be produced at a rate of up to 10 kg per day in 2002 and at rates of hundreds or thousands of kilogrammes per day in 2003 ± 2004 (this material can be accessed via the Internet at URL http://www.cnanotech.com; htth://carbonnanotech.com/CNI_ home.html). A mixture of CO with 25% H2 and iron carbonyl can be used for the synthesis of SWNTs at atmospheric pressure at 1100 8C (this material can be accessed via the Internet at URL http:// fy.chalmers.se/f3a/Fullerenes/Nanotubes/projects/Ntproduction. html). Catalytic pyrolysis of hydrocarbons at temperatures above 1000 8C was proposed to be a method for NT production at a rate of several hundreds of kilogrammes per day. The National Institute of Materials and Chemical Research (JNMR, Japan) and the Showa Denko Co. (Japan) work on a pilot plant since 1999 (this material can be accessed via the Internet at URL http:// www.jnmr.com/intro/nanotubes.html). Currently, various MWNT-based products are produced.These are NT powders, `spaghetti', suspensions in various sol- 855 vents and substrates covered withNT`forest' arrays. An industrial process for the preparation of MWNTs of purity 95% is based on pyrolysis of hydrocarbons performed according to the method proposed by Ren et al.392 The assortment of NTs and price lists are available in the Internet at URL http://www.nano-lab.com/ SubPages/prices.html. In 1999, the market price was 100 USD per gramme for unpurified NTs and 1400 USD per gramme for purified SWNTs.245 In March 2001, the price of purified SWNTs was reduced down to 60 to 100 USD per gramme (this material can be accessed via the Internet at URL http://www.carbolex. com/). The pyrolytic method is expected to allow realisation of the idea of NT synthesis (build-up) on a `seed' (a short NT with particular structure), thus presenting a solution to a complex problem of sorting the synthesised products by structure or shape (this material can be accessed via the Internet at URL http:// cnst.rice.edu/VARENNA.html). The idea of existence of a critical size for a catalytic particle is well known.It is believed that the use of catalytic particles of larger size makes preparation of NTs impossible.555 This gave an impetus to search for novel approaches to development of catalysts. A JNMR's technology for mass production of MWNTs to be used in cathodes of television displays is based on the reversed micelle method. (this material can be accessed via the Internet at URL http://www.jnmr.com/dez/online/nlpage1013.html). Flame preparation of NTs represents a kind of pyrolytic synthesis of NTs.556 ± 558 This process has much in common to an industrial processes of soot production and can be adapted to large-scale synthesis of NTs. Pyrolysis of sputtered solution of Fe(C5H5)2 in C6H6 is yet another pyrolytic technique.557 In Russian Federation research into NTs are well-based (see, e.g., recently published reviews 559, 560 covering studies in a close field of catalytic pyrolysis of hydrocarbons to prepare carbon filaments).Asetup for catalytic decomposition ofCOoperating at a rate of kilogramme quantities of products per day was tested. * * * Anumber of studies on the subject of this review were published in the course of preparation of the manuscript.The most important and interesting results are outlined below. A special issue of the journal `Physics World' 561 dedicated to research into NTs was published. Hirahara et al.562 reported the synthesis of unique compound, (Gd@C82)n@NTs. Structurally, it represents a nanoscale `pea- pod' consisting of a SWNT with encapsulated electrically charged C82 `peas' with a Gd atom inside each `pea'. This `doubly endohedral' substance was synthesised by the interaction of Gd@C82 vapours with pre-open NTs. Electrical resistance meas- urements revealed electron transfer from the metal atom not only to the C82 molecule, but (tentatively) to the NTs. This opens new possibilities for modification of NTs and brings the advent of unique memory devices closer.Two studies 563, 564 carried out independently concern chem- ical modification of particular sections of NTs, which can lead to the formation of intramolecular heterojunctions and creation of molecular electronic devices. Researchers from the Stanford University (USA) studied the adsorption of potassium atoms on semiconducting and p-type conducting NTs.564 Theoreticians from the N S Kurnakov Institute of General and Inorganic Chemistry (Moscow, Russian Federation) calculated the proper- ties of fluorinated NTs.563 Their results are in agreement with those reported by Kelly et al.104 who showed that fluorinated isomers capable of forming ring-like fluorinated sections should be the most stable.Ouyang et al.565 experimentally and theoretically studied the atomic structure and electronic properties of metal ± semiconduc- tor and metal ± metal heterojunctions in SWNTs. This appeared856 to be a step towards fabrication of intramolecular heterojunc- tions. The first biological study of the structure of malaria-infected erythrocytes using an AFM with an NT probe tip was reported by Nagao et al.566 The space resolution obtained using an NT probe tip was four times better than with the Si probe. Mattson et al.567 proposed the use of functionalised NTs as substrates for neural growth. Nanotubes appeared to be excellent sorbents for purification of waste gases from low concentrations of highly stable carcino- genic dioxins.568 In this respect, NTs are more efficient than activated carbon and sorbents based on porous graphite.New procedures for preparation of NTs at moderate temper- atures have been developed. One of them consists in reduction of hexachlorobenzene with potassium in the presence of bimetallic Co ±Ni catalyst at 350 8C,569 while the second involves hydrolysis of Al(OBus)3 in the presence of acetylacetone or acetic acid and Co(NO3)2 .6H2O, followed by gelation, drying of the gel and heating to 300 ± 400 8C.570 A method for quantitative determination of the NT content in the reaction products using conjugated polymer filtration was first developed.571 The method involves preparation of a NTs ± pol- ymer composite dispersion of the composite in toluene and storage until complete precipitation of impurities for at least 48 h.A method for obtaining stable colloidal solutions of carboxy- lated SWNTs in water, ethanol, acetone and DMF was pro- posed 572 and some of their nonlinear optical properties were measured. These solutions were shown to be promising for the use in optical information processing systems. 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ISSN:0036-021X    

出版商:RSC    

年代:2001

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摘要:

Russian Chemical Reviews 70 (10) 865 ± 884 (2001) State-of-the-art coordination chemistry of radioactive elements B I Kharisov,MA Mendez-Rojas Contents I. Introduction II. General notion of technetium and actinide complexes III. Actinide alkoxides and other oxygen-containing complexes and salts IV. Halide complexes of actinides V. Actinide p-complexes with allyl, cyclopolyene, arene and related ligands VI. Hydride and hydroborate complexes of actinides VII. Actinide complexes with macrocyclic ligands VIII. Actinide complexes with nitrogen-containing ligands IX. Complexes with phosphorus-containing ligands X. Sulfur-containing actinide complexes XI. Technetium complexes Abstract. coordination of synthesis the for procedures Modern Modern procedures for the synthesis of coordination and technetium and actinides of compounds organometallic and organometallic compounds of actinides and technetium and and the surveyed.are compounds these of properties the properties of these compounds are surveyed. Experimental Experimental techniques, actinide of synthesis the for methods including techniques, including methods for the synthesis of actinide and and technetium complexes from elemental metals (oxidative dissolu- technetium complexes from elemental metals (oxidative dissolu- tion carbonyl halide, and salts electrosynthesis), direct and tion and direct electrosynthesis), salts and halide, carbonyl and and other includes bibliography The generalised. are complexes other complexes are generalised.The bibliography includes 283 283 references. I. Introduction Among compounds of natural and artificial radioactive elements (Tc, Pm, Po, Fr, Ra, Ac and actinides), only organometallic compounds and complexes of technetium and actinides (An) have been well studied. Complexes of these metals attract interest because technetium, which is available in multikilogram quanti- ties, is widely used for medical and technical purposes and actinides find application in nuclear industry and power engineer- ing. Over a period of the last two decades, a large number of experimental studies, reviews and monographs were devoted to Tc and An complexes.1±16 The aim of the present review is to give a comprehensive survey of modern synthetic procedures for the preparation of coordination and organometallic compounds of radioactive elements.The data are systematised according to the nature of the ligands in these complexes. B I Kharisov Facultad de Ciencias Quimicas, Universidad Auto'noma de Nuevo Leo'n, 66450 San Nicola's de los Garza 18-F, Nuevo Leo'n, Me'xico. Fax (52-8) 374 07 09. Tel. (52-8) 375 30 68. E-mail: bkhariss@ccr.dsi.uanl.mx MA Mendez-Rojas Centro de Investigaciones Quimicas, Universidad Auto'noma del Estado de Hidalgo, Carretera Pachuca-Tulancingo Km. 4.5, 42073 Pachuca, Hidalgo, Me'xico. Fax (52-7) 717 20 00, ext. 5075. Tel. (52-7) 717 20 00, ext. 4881. E-mail: mmendez@uaeh.reduaeh.mx Received 22 December 2000 Uspekhi Khimii 70 (10) 974 ± 995 (2001); translated by T N Safonova #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n10ABEH000646 865 865 867 869 870 873 874 874 875 876 877 II.General notion of technetium and actinide complexes Actinium (atomic number 89) and all elements with atomic numbers from 90 to 103 (actinides) are radioactive.17 Only four actinides (Th ± Np) and actinium by itself were found in nature, whereas the remaining elements were prepared artificially by irradiation ofUand other elements with neutrons or by bombard- ment with heavy atoms. Actinides are analogues of lanthanides. They belong to the same IIIB Group but reside in the next row. Technetium (atomic number 43) has several isotopes two of which are characterised by large half-lifes (2.126105 and 1.56106 years for 99mTc and 98Tc, respectively) and is a rhenium analogue.18 The electronic states of the actinide atoms and ions differ substantially from those of the lanthanide atoms and ions.In both series, the f level is successively filled up to the f 14 configuration; however, this filling in actinides begins with Th only formally because Th does not bear f electrons and is an electronic analogue of hafnium.16 In contrast to lanthanides, actinides can exist in various oxidation states. Actinides in the oxidation state +3 are analogues of lanthanides, whereas actinides in the oxidation state +4 are analogues of Hf(IV) and Ce(IV). 2 Actinides produce the Anm+ (m=2 ± 4) and AnOmá (m=1 or 2) ions whose highest occupied level is filled only with f electrons.In the series of the An3+ anions, the actinide contrac- tion is observed, this contraction changing in parallel with the lanthanide contraction. The 5f and 6d energy levels of the Anm+ ions as well as the 4f and 5d energy levels of the Lnm+ ions correlate with each other. The difference in the correlations for the An3+ and Ln3+ ions leads to the noticeable difference in the magnetic properties and electronic spectra of their compounds.17 Technetium (the 4s24p64d 55s2 or 4s24p64d 65s1 electronic config- uration) exists in the oxidation states from +1 to +7, the oxidation states from +4 to +7 being most stable.18 Spin ± orbit coupling (J ) in the An3+ ions is very strong (2000 ± 4000 cm71) and it is *1000 cm71 stronger than J in the Ln3+ ions.In contrast to lanthanides, the splitting due to spin ± orbit coupling in actinides is comparable with the crystal field splitting and, hence, the J value is no longer a good quantum number. Since the 5f and 6d orbitals have close energies and the866 thermally accessible excited states of actinides are filled, the equation for the effective magnetic moment me=g[J(J+1)]1/2 is less appropriate than in the case of lanthanides.17, 19 Actinide organometallic complexes contain actinide ± carbon p bonds, actinide ± carbon s bonds or bonds of both types. Organometallic complexes of all early actinide elements from thorium to californium are known.20 However, most data have been obtained for the chemistry of organometallic compounds of Th andUdue to the extremely large half-lifes of natural 232Th and 238U (1.4161010 and 4.4686109 years, respectively).The first actinide organometallic compound, viz., Cp3UCl, was isolated by Reynolds and Wilkinson in 1956.21 The actinide atoms possess rather large atomic and ionic radii and large (up to 14) coordination numbers.22 Examples of actinide compounds characterised by various coordination numbers and oxidation states are listed in Table 1. The maximum coordination numbers were found for poly- meric uranium hydroborates, whereas the presence of bulkier Table 1. Coordination numbers (CN) and coordination polyhedra in actinide compounds.17 CN Oxidation state +7 6 +6 678 +5 6789 +4 45678 Note: acac is acetylacetonate, Hacac is acetylacetone, HMPA is hexamethylphosphoramide, dmed is N,N0-dimethylethylenediamine, TMPO is trimethylphosphine oxide, tta is thenoyltrifluoroacetonate, TOPO is tri-n-octylphosphine oxide, tfa is trifluoroacetylacetonate, sal is salicylate, bipy is bipyridyl.Coordination polyhedron octahedron octahedron pentagonal bipyramid hexagonal bipyramid octahedron pentagonal bipyramid cube tricapped trigonal prism tetrahedron trigonal bipyramid octahedron trigonal prism pentagonal bipyramid monocapped octahedron dodecahedron square antiprism Examples of compounds Li5(AnO6) (An=Np, Pu) (NEt4)(PaOCl5), AnF6 (An=U, Np, Pu), trans-UO2Cl2(OPPh3)2 [UO2Cl2(acac)2] .Hacac, PuO2(C2O4) .3H2O, K3UO2F5 Cs2[AnO2(MeCO2)3] (An=Np, Pu, Am), (UO2)(NO3)2(H2O)2 CsAnF6 (An=U, Np, Pu) PaCl5 Na3(AnF8) (An=Pa, U, Np) M2(PaF7) (M=NH4, K, Rb, Cs) U(OAr)4 . [N(SiMe3)2]3 .THF U2(NEt2)8 . [Li(THF)4] . [U(OAr)5] Na2PuCl6 , cis-UCl4 .2Ph3PO, trans-UBr4 .2Ph3PO, UCl4 . 2HMPA U4(dmed)3 UBr4, K3UF7 [UCl(TMPO)6]Cl3 Np(HCOO)4, Th(tta)4, An(S2CNEt2)4 (An=Th, U, Np, Pu) K7Th6F3, An(acac)4 (An= Th, U) ligands, for example, of NPh2, leads to a decrease of the maximum coordination number to 5 or even to 4.17 Like lanthanides, the spherical An3+ and An4+ ions have the highest coordination numbers (generally, 8 or 9) and often form isomorphous com- plexes in which the polyhedron type is determined by repulsions between the ligands (steric factors) or packing factors rather than by the electronic effects.23 For example, the square antiprism in the rhodanide complexes Cs4An(NCS)8 and U(NCS)4(Ph3PO)4 is transformed into the dodecahedron in the case of Th(NCS)4[(Me2N)2CO]4 or into the cube in the case of (NEt4)4An(NCS)8.17 The nature of the bond between the p-donor ligand and the actinide centre has been discussed.24, 25 The authors emphasised that there is no simple answer to the question as to the nature of the bond in organoactinide complexes. Examples of almost completely ionic binding of the Cp7 anions or cyclooctatetraene (COT) withU4+ (see Ref.26) as well as of covalent binding of the metal atom with the aromatic ring of the ligand in uranocene accompanied by essential ligand-to-metal electron transfer are available in the literature.24, 27, 28 It was concluded 24, 29 that the CN Oxidation state 8 +4 9 10 11 12 145 +3 6896 +2 8 0 B I Kharisov, MA Mendez-Rojas Examples of compounds Coordination polyhedron cube (NEt4)4[An(NCS)8] (An=Th, Pa, U, Np) Th(acac)4 [UCl2(Me2SO)6](UCl6) bicapped trigonal prism bicapped octahedron tricapped trigonal prism monocapped square antiprism bicapped square antiprism irregular structure icosahedron (NH4)3(ThF7), LiUF5, Th(tta)4 .TOPO Li3ThF7, Th(tfa)4 .2H2O, [C(NH2)3]5[Th(CO3)3F3] U(C2H3O2)4 .2H2O, An(MeCO2)4 (An=Th, U), Na6[Th(CO3)5] .2H2O An(NO3)4 .5H2O (An=Th, Pu) (PPh4)[Th(NO3)5(OPMe3)2], An(BH4)4 (An=Np, Pu) [An(BH4)4]n (An=Th, U) bicapped hexagonal antiprism trigonal bipyramid octahedron dodecahedron bicapped trigonal prism tricapped trigonal prism [K(THF)2]2[U(NHAr)5] .THF [UCl6]3¡ UCl3 . 3DMSO AnBr3 (An=Pu, Am, Cm, Bk), AnI3 (An=Pa, U, Np, Pu) NaPuF4, AnCl3 (An=U, Np, Pu, Am, Cm), Am(sal)3 .H2O Th6Br12 framework structure cube U(bipy)4State-of-the-art coordination chemistry of radioactive elements covalent binding of the metal atom with the ligand in U(COT)2 involves primarily the 6d orbitals of uranium, while the 5f orbitals are of secondary importance. As mentioned above, actinides show various oxidation states in aqueous solutions.Stable oxidation states vary from +3 for Ac to +6 for U and Np and then gradually decrease to +3 for Am and the following elements, except for No (+2). For Ac, Th, Pa, U, Md and Lr, the maximum oxidation states are stable; Np and Pu can occur in the oxidation state +7; Am can exist in the oxidation state +6; Cm, Bk, Cf, Es and Fm can occur in the oxidation state +4; the oxidation state +3 is possible for No. In aqueous solutions, virtually all actinides can exist in the unstable oxidation state +2.17 The hydrated An2+, An3+, An4+, AnO2+ and AnO2a ions behave as BroE nsted acids. 30 2 An(OH)(n71)++H+. Ann++H2O The An4+ ions are typical of actinides from thorium to californium (however, U4+ is readily oxidised). As for Th, only Th4+ ions exist in solutions.The acidity decreases in the series Pa4+ 44 U4+>Pu4+>Np4+>Th4+.17 Individual ions occur only in very dilute solutions. These ions tend to form polynuclear species as the concentration increases 2 mAn4++nH2O Anm (OH)n O4m¢§nUa +nH+ . The acidity of the Ann+ ions depends on their charge and radius. The An4+ and AnO2a ions are much stronger acids than the An3+ and AnOa2 ions, respectively.17 The redox behaviour of actinides is complicated by their high radioactivity, which, in particular, results in the formation of hydrogen peroxide in aqueous solutions. 2 , The data on the complex formation of the actinide ions An3+, An4+, AnOa2 and AnO22a (An=U,Np or Pu) with chlorides were surveyed in several comprehensive reviews.30 ¡¾ 32 The results of recent studies of hydrated and chloride complexes of UO2a NpO2+, Np4+, Pu3+, etc.by XAFS (X-ray absorption fine structure) spectroscopy were covered in Refs 33 and 34. In particular, using hydrated ions and fluoride complexes of U(IV) and Th(IV) as examples, it was demonstrated that the coordina- tion number of these hydrated ions is 10 and theM7O distances for U(IV) and Th(IV) are in the ranges of 2.420.01 and 2.450.01A, respectively.34 The results of physical and chemical studies of hydrated complexes of uranium were published in the literature.35, 36 MoE ssbauer spectroscopy is very convenient to use for studying the oxidation states and the symmetry of the ligand environment. The MoE ssbauer effect is observed for 232Th, 231Pa, 238U, 240Pu, 243Am and, particularly, 237Np generated upon the 237U b7-decay.The maximum isomer shifts (up to 770 mm s71) are observed for Np(VII) compounds; for Np(III) compounds, these shifts decrease to +30 mm s71 (see Ref. 37). III. Actinide alkoxides and other oxygen- containing complexes and salts 2)4,39 Uranium alkoxides are similar to alkoxides of transuranium elements. Their chemistry has been surveyed in the recent review. 37 Homoleptic alkoxides An(OR)n are known for n=3 (U and Pu), 4 (Th, U, Np and Pu), 5 (Pa and U) and 6 (U). A great quantity of suchUand Th compounds are available, whereas only a few Pa and Pu complexes were prepared.Alkoxides of uranium in the the oxidation states +3, +4, +5 and +6 are known. Complexes characterised by mixed oxidation states, such as [U(OPh)3(THF)]2[UO2(THF)2]2(m-OPh)4(m3-O)2, were also pre- pared.38 Compounds of U(IV) are analogous to Th(IV) com- pounds. Most actinide alkoxides are oligomeric {like [U3O(OBut)10]}. However, derivatives of bulky alcohols and 2,6-disubstituted phenols are monomeric,17 for example, U(OCHBut U(OEt)5,37 U(OMe)6, U2(OEt)10, U(OAr)4 and U(OAr)3 (Ar=2,6-ButC6H3).37, 40, 41 The influence of electronic factors 867 on the structure and stability of complexes of uranium tri-tert- butylmethoxide was examined.42 Monomeric alkoxides can react with each other 43 C6H14 U2(OEt)10 . U(OEt)4+U(OEt)6 Anionic alkoxide complexes, such as (But4N)[U2(OBut)9] and [Li(THF)4][U(OAr)5], were also obtained.44 Treatment of [U(COT)(BH4)2] with alcoholsROH(R=Et, Pri or But) afforded the alkoxide derivatives [U(COT)(BH4)(OR)].45 In a toluene solution of the Th(OCHPri2)4 complex, a mono- mer ¡¾ dimer equilibrium occurs; however, only the dimeric form crystallises (with trigonal-bipyramidal geometry).46 Actinide alkoxides are very readily hydrolysed 17 PhMe 3U2(OBut)8(HOBut)+2H2O 2U3 O(OBut)10+7HOBut.The actinide ions have large radii and, consequently, high coordination numbers, which are often responsible for oligomer- isation of homoleptic alkoxide complexes of f metals 37, 47 ¡¾ 49 giving rise to dinuclear (1), tetranuclear (2, 3) or higher oligo- mers.47 H Pri O OPri PriO OPri Ce Ce Pri OO PriO OPri OPri O Pri Pri H 1 Py OR RO RO OR Th RO Nd RO OR Nd OR RO OR RO Th OR Th OR RO OR RO OR RO RORO OR OR OR Nd Nd Th OR RO OR RO Py 2 (R=Pri) OR 3 (R=CH2But) Dimeric actinide complexes are exemplified by the {U[O(2,6-Pri2C6H3)]3}2 complex (4), which contains the unusual p-arene bridge and is sterically hindered due to the presence of the isopropyl groups.47, 49 This complex is stabilised through p-arene interactions both in the solid state and in solution.Pri O ArO ArO M Pri Pri M OAr O OAr Pri 4 Ar=2,6-Pri2C6H3;M =La, Nd, Sm, Er, U. Some actinide complexes can occur both in the monomeric and dimeric forms. Thus treatment of metallacycle 5 with iso- propyl alcohol afforded the homoleptic Th2(OCHPri2)8 complex.SiMe3 N (Me3Si)2N Th SiMe2 +4HOCHPri2 (Me3Si)2N 5 1/2 Th2(OCHPri2)8+3 HN(SiMe3)2 6 In the solid state, the latter exists as dimer 6 (the coordination number of thorium is 5) consisting of two ThO5 fragments (with trigonal-bipyramidal geometry), which are linked via a shared868 axial-equatorial edge.44 At room temperature, this complex in non-coordinating solvents exists as monomer 7.46, 47 2)4 2)8 2 Th(OCHPri 7 Th2(OCHPri 6 The dimeric halide-alkoxide complex Th2I4(OPri)4(HOPri)2 was prepared by the reaction of metallic thorium with 2 equi- valents of iodine in isopropyl alcohol.50 This reaction provides an example of the direct synthesis from elemental metals.The Th2I4(OPri)4(HOPri)2 complex (the triclinic system, space group P1 ) has a bioctahedral structure (the octahedra are linked via a shared edge), the isopropyloxy anions occupying the bridging positions. This complex is isostructural to its uranium analogue U2I4(OPri)4(HOPri)2 (see Ref. 51) and is structurally similar 50 to a number of alkoxide and halide-alkoxide compounds of lantha- nides and early transition metals, such as M2(OPri)8(HOPri)2 (M=Zr or Ce) 52 and Ti2Cl4(OR)4(HOR)2 (R=CH2CH2Cl).53 It was assumed that the [AnI(OPri)2(HOPri)4]I intermediates (An=U or Th) were initially formed, which then underwent dimerisation.50 Other alkoxide complexes of actinides, for exam- ple Th2(OBut)8(HOBut),54 (TBA)U2(OBut)9 (TBA is tetrabutyl- ammonium) and KU2(OBut)9,55 were also characterised.It was demonstrated that the K+ cations in the latter complex in solutions remain associated with the dimeric anions, the structures of the molecular unit in the crystalline state and solution being, apparently, identical. Oxidation of the uranium(III) aryloxide complex U(OAr)3 (8) (OAr is 2,6-di-tert-butylphenoxide) afforded various oxide and halide-oxide complexes of U(IV).50 e FUIV(OAr)3 BrUIV(OAr)3 f I2U(OAr)2 (ArO)3USU(OAr)3 (10) g U(OAr)3 8 ClUIV(OAr)3 IUIV(OAr)3 abcd h U(OAr)4 (ArO)3UOU(OAr)3 (9) (a) AgBF4 or AgPF6; (b) CI4; (c) PCl5; (d) O2; (e) AgBr, CBr4 or PBr5; (f) COS or Ph3P=S; (h) N2O, NO, Me3NO or pyridine N-oxide. In the presence of an appropriate source of chalcogenide, oxidation gave rise to bridged binuclear uranium(IV) complexes 9 or 10.50 Alkoxide complexes of cyclooctatetraenyluranium(IV) were synthesised by the reactions of cyclooctatetraenyluranium pre- cursors with alcohols.56 The unusual monomeric complex U(OTeF5)6 can be consid- ered as an analogue of metal alkoxides.57 In spite of the large molecular weight, this complex sublimes at 333 K (1.3 Pa).57 Actinide alkoxides are generally prepared from dialkylamides and alcohols.44 The homoleptic UX4(MeCN)n complex can serve as a precursor in the synthesis of mixed halide-alkoxide com- pounds of uranium.The coordination environment in the reaction product depends on the nature of the halogen. Thus the reaction of UBr4(MeCN)4 with 2 equivalents of KOAr in THF afforded the Br2U(OAr)2(THF) .4THF complex, whereas UCl4 produced the anionic [K(THF)4][UCl3(OAr)2] complex. In solution, mixed halide-aryloxide complexes do not undergo ligand exchange. Light halides of uranium(IV) are reagents of choice in the meta- thesis because UI4(MeCN)4 is thermally unstable.58 A series of aryloxides and alkoxides of (triamidoamine)uranium(IV) were described.59 One of the approaches to the synthesis of methoxide deriva- tives of actinides of type 11 involves the reactions of Cp2 MH2 (Cp is pentamethylcyclopentadienyl; M=U or Th) with trimethyl phosphite in pentane.60 P(OMe)3 Cp2 Th(OMe)2+[Cp2 Th(OMe)2](m-PH) Cp2 ThH2 7H2 11 B I Kharisov, MA Mendez-Rojas The corresponding thorium derivatives can also be prepared by alcoholysis of alkyl complexes, for example, of Cp2 ThMe2, the reactions of Cp2 ThCl2 with alkali metal hydroxides, the insertion of ketones at the Th7Alk bond (see the review 37 and references cited therein) PhMe Cp2 ThCl(OCMe3) , Cp2 ThCl(Me) +Me2CO and hydrogenation of Z2-acyl complexes 37 H2 Cp2 ThCl(Z2-OCH2CH2But).Cp2 ThCl(Z2-OCCH2But) 3 Crystallisation of a Pu(IV) salt (0.15 mol litre71) from a solution of Na2CO3 (2.6 mol litre71) afforded single crystals of [Na6Pu(CO3)5]2 . Na2CO3 . 33H2O (the space group is P21/c).61 In the solid state, the asymmetric unit contains a complex framework consisting of [Pu(CO3)5]67 anions and Na+ cations linked through interactions between the CO2¡ ligands and the H2O molecules. The [Pu(CO3)5]67 anion can be considered as a pseudohexagonal bipyramid with three carbonate ligands in the equatorial plane and two carbonate ligands in the axial positions.Its structural unit is very similar to that of the related [Th(CO3)5]67 anion involved in the crystal structures of Na6[Th(CO3)5] . 12H2O and [C(NH2)3]6[Th(CO3)5] .4H2O (see Ref. 61 and references cited therein). Clark et al.61 believed that the well-known coordination polyhedron (hexagonal bipyramid) in the [PuO2(CO3)3]47 complex (12) 62 ± 64 is analogous to that of the [Pu(CO3)5]67 anion (13). O O O O O O O OO Pu OO Pu O OO OO O O O O O O O O O 12 O 13 n The structure of yet another carbonate complex, viz., (Me4N)4[NpO2(CO3)3] .8H2O, consists of alternating anionic and cationic layers.65 The water molecules of crystallisation are involved in the formation of the {[NpO2(CO3)3] .8H2O}4n¡ anionic layers.Analogous uranium and uranyl complex ions in carbonate,66, 67 sulfate,68 nitrate,69 phosphate,70 silicate 70 and citrate complexes 71 as well as in other oxygen-containing com- pounds were also studied. In particular, the behaviour of carbo- nate complexes of uranium in CO2 ±HCO¡3 solutions of various ionic strengths (I=0.5, 1.0, 2.0 or 3.0 mol litre71 of NaClO4) at 25 8C was examined by UV/Vis spectrophotometry.65 The authors believed that tetracarbonate complexes exist in an equili- brium with pentacarbonate complexes [U(CO3)5]67 .[U(CO3)4]47+CO2¡ 3 Citric acid forms mixed Fe and U complexes analogous to the citrate complex of uranium. The mixed Fe ±U citrate complex is stable to biodegradation.71 Among other oxygen-containing actinide compounds, ura- nium peroxo complexes with various ligands, viz., with tri- and tetradentate Schiff's bases,72 amines or aminocarboxylic acids,73 were described. The einsteinium(II) complexes with crown ethers [Es(18-C-6)]L2 (L=ClO¡4 and BF¡4 , BPh¡4 ) were prepared.74 The behaviour of einsteinium(II) in THF was examined by co-crystal- lisation. It was found that the coefficients of co-crystallisation of Es(II) with solid [Sr(18-C-6)]I2 in THF depend on the ClO¡4 and BF¡4 ions as additives and are independent of the presence of the BPh¡4 ions.Very stable actinide complexes with b-diketones [for example, with An(acac)4 or AnO2(acac)2] are used for isolation and separation of actinides by extraction. These complexes are pre- pared by the reactions of metal or actinyl halides with theState-of-the-art coordination chemistry of radioactive elements Table 2. Preparation of actinide alkoxides and related oxygen-containing complexes. Ref. Products Reaction system Starting compound or metal Synthesis from elemental metals U anode Hbac, UO2(bac)2(Hbac)0.5, 84 O2 or N2 (see a) UO2 (bac)2 Hacac, O2 or N2 (see a) UO2 (acac)2 . Hacac Synthesis from salts UO2(MeCO2)2 Based on the data on the electronegativities obtained earlier (see references cited in the study 78), the U7O bond orders in the complexes 14, UO2 (acac)2, UO2 (Hdbm)2 and UO2(HBTF)2 U(acac)4, 84 (Hdbm is dibenzoylmethane, HBTF is 1,1,1-trifluoropentane- 2,4-dione) were calculated. The results of the calculations indi- cated that the equatorially coordinated 2-hydroxy-1-naphthalde- hyde ligand exerts a pronounced effect on the oxygen atoms of the UO2á anion.78 75 ± 77, 85 Hacac Hdbm HBTF in EtOH 37, 38, 86 NaOPri, HOPri 37, 87 NaOEt, EtOH UO2(Hacac)2 UO2(Hdbm)2 UO2(HBTF)2 Th(OPri)4(HOPri)x Pa(OEt)5 Np(OR)4 37, 88 ThCl4 PaCl5 NpCl4 LiOR, ROH (R=Me, Et) Synthesis from halide complexes 89 KOAr UBr4(MeCN)4 Br2U(OAr)(THF) .(THF)4 44, 90 ThBr4(THF)4 2,6-But2C6H3OK ThBr2(OAr)2(THF)2 (see b) Py2PuCl6 Pu(OPri)4, 91 Pu(OPri)4(Py) NH3, PhH, PriOHc Synthesis from b-diketonates or alkoxides CHBut2 Me O U 92 MeLi U(OCHBut2)4 Li But2HCO But2HCO OCHBut2 93 Th(acac)4 [Th(acac)4]7 94, 95 electrochemical reduction d Synthesis from p-complexes (MeCp)3U(THF) alcohols or thiols (MeCp)3UR (R=OMe, OPri, OPh, SPri) 96 EtOH Cp3 UOEt Cp3 UH 96 EtOH (C5H4But)3UOEt HU(C5H4But)3 (C5H4SiMe3)3UOEt 96 HU(C5H4SiMe3)3 EtOH a Direct electrochemical synthesis; b alkylation of the product afforded Th(OAr)2(CH2SiMe3)2; c recrystallisation from hot isopropyl alcohol gave rise to Pu(OPri)4(HOPri); d the product lost acac7 to form Th(acac)3; the known Th(III) complexes are few in number.corresponding diketone in the presence of a base. In the case of An(IV) actinides, only fluorine-containing diketonates form com- plexes with Lewis bases; diketonates of actinyls AnO2 (An=Np or Pu) generally exhibit the Lewis acid properties and are stabilised through the formation of adducts. The fluorinated complex UO2(hfa)2 (hfa is hexafluoroacetylacetonate) exhibits pronounced acidic properties and its adducts with water and alcohols can be sublimed without decomposition.17 Uranyl acetylacetonate,75, 76 other actinide b-diketonates 77 and uranyl complexes with 2-hydroxy-1-naphthaldehyde (14) 78 and 2-hydroxybenzaldehyde 79 were studied in detail by spectro- scopic methods. These investigations made it possible to establish the structure of the complex 14 synthesised previously.80 869 O O O U O O O14 2Based on the spectra of neptunium b-diketonates,77 the Np(IV) complexes with acetylacetonate, dibenzoylmethane, ben- zoylacetone (Hbac), benzoyltrifluoroacetone (Hbtfa) and the- noyltrifluoroacetone (Htta) can be divided into two groups according to the type of coordination polyhedron about the central atom.In Np(acac)4, Np(dbm)4 and Np(tta)4, the coordi- nation polyhedron is a tetragonal antiprism, whereas Np(bac)4 and Np(btfa)4 (dbm is dibenzoylmethanate, bac is benzoylaceto- nate and btfa is benzoyltrifluoroacetonate) have bitetrahedral structures. Apparently, when Np(IV) b-diketonates are dissolved in benzene, the bidentate ligands are oriented about the central atom so that the oxygen atoms form the tetragonal-pyramidal coordination.77 It should be noted that uranium b-diketonates were also prepared by direct electrochemical synthesis using a dissolving anode.Thus electrochemical oxidation of uranium in the presence of b-diketones afforded chelate complexes of the UL4 or UO2L2 type (L is diketonate).81 ± 83 The reaction with benzoylacetone gave rise to a compound with composition UO2(bac)2(Hbac)0.5 along with the above-mentioned complexes.82 In our opinion, the structure 15, which was assigned to this compound on the basis of the IR spectroscopic data, needs further investigation. Ph Me Ph Me O O U U O C(Ph)CH2C(Me) O OO2 O O O2O O O Me Ph Ph Me 15 Selected methods for the synthesis of actinide alkoxides are listed in Table 2.IV. Halide complexes of actinides Light actinides (U, Np and Pu) react with iodine or bromine in donor solvents 97, 98 to give complexes of trivalent actinides AnX3L4 (X=Br or I). 0 8C U+1.5 I2+nL UI3 Ln 16a ± c n=2, L=DME (a); n=4, L=THF (b), Py (c). This reaction is a convenient and highly efficient procedure for the preparation of halide complexes of actinides in quantitative yields, which does not require special equipment. Complexes of uranium triiodide with Lewis bases are used as intermediates in the synthesis of other trivalent uranium com- pounds. For example, the UI3(THF)4 adduct (16b), which can be prepared in large amounts, is a soluble form of UI3 convenient for subsequent use in synthesis.98 Reactions involving solvate com- plexes of other uranium halides or alkoxides can be complicated, for example, by the formation of mixtures of products.870 The mononuclear complex UI3(THF)4 (16b) crystallises in the space group P21/c.The coordination environment about the central U atom is a pentagonal prism. The complex 16b is stable up to 75 8C. At a higher temperature, the THF molecules are successively eliminated; at 162 8C, UI3 is finally formed. were also Other adducts of uranium halides UX3 described.99 ± 103 Actinide tetrahalides readily react with Lewis bases to give complexes containing two or four donor atoms. The compositions of some resulting products differ from usual AnCl4 .2L or AnCl4 .4 L. For example, the AnCl4 . nL complexes, where n=2.5, 5 [L is N,N-dimethylacetamide (DMA)] or 6 (L=DMSO or TMPO), were prepared. Complexes with compo- sition AnCl4 . 2HMPA (An=Th or U) are extremely volatile.17 The coordination polyhedron typical of the AnCl4 . 2 L complexes is a trans-octahedron.17 The cationic uranium(IV) complexes UX2L4Y2 [X=Cl, Br or I; L is a bulky neutral O-donor ligand, such as tris(pyrrolidin-1- yl)phosphine oxide; Y=ClO4 or BPh4] were described.102 The reactions of UI4 with a series of sulfoxide donor ligands in non-aqueous media were investigated. The behaviour of UI4 was compared with that of UCl4 and UBr4 in the presence of the same ligands. It was demonstrated that UI4 was readily oxidised with dimethyl or diisobutyl sulfoxide at*20 8C.Only complexes with compositions UI4(DMSO)8 and UI4(DIBSO)6 (DIBSO is diiso- butyl sulfoxide) appeared to be stable and were isolated.104 All actinide halides tend to accept Hal7 ions to form anionic complexes. The stability of the complexes decreases in the series F 44 Cl>Br 44 I. Adducts of actinide trihalides are ionised to a large extent. Thus uranium trichloride crystallises from dimethyl sulfoxide as the solvate UCl3 . 3DMSO. Its structure consists of the dodecahedral [U(OSMe2)8]+ cations and the octahedral [UCl6]7 anions. X-ray diffraction study demonstrated that ameri- cium chloride hexahydrate has an ionic structure built of the [AmCl2(H2O)6]+ cations and [Cl(H2O)6]7 anions, which are linked via hydrogen bridges.17 Uranium and protactinium pentahalides generate complexes of the AnX5 .L type (X=Cl or Br; L=R3PO or HMPA). Dissolution of UO3 in thionyl chloride afforded the UCl5 . SOCl2 adduct, whereas dissolution of Pa(V) hydroxide gave rise to the ionic complex (SO)(PaCl6)2. Generally, actinide hexahalides do not react with Lewis bases (except for the UCl6 . bipy complex), whereas actinyl halides readily form complexes with composition AnO2X2 . nL (n=1, 1.5, 2, 3 or 4).17 Uranium fluorides UF5 and UF6 give complexes with 2-fluo- ropyridine (F-Py) or bipy.105 Thus UF6 in CH2Cl2 produced the UF4 . (F-Py) and U2F12 . bipy complexes, respectively. However, in the case of the UF6 ± bipy system, reduction of UF6 was the predominating process and the formation of U2F12 . bipy {the authors believed that this complex has the structure [UF4(bipy)2]2+ .[UF7]¡2 .UF6} can be considered as the first stage of this reduction. In the case of UF5, two products were obtained and structur- ally characterised. These are the extremely moisture-sensitive UF5 . bipy and the ionic derivative [(bipy)2H]+[UF6]7. The AnX3(THF)4 complexes serve as synthetic precursors of a number of inorganic and organometallic complexes due, in particular, to their high solubility in toluene and THF. The use of UI3(THF)4 can also provide a convenient approach to the synthesis of many other compounds of trivalent uranium.97 Salts of organic ligands with alkali metals react with AnX3(THF)4 to give the corresponding aryloxides, amides, etc.This reaction can serve as a highly efficient procedure for the preparation both of the known and new complexes of trivalent actinides. Thus reduction of uranium tetrachloride with NaH or Na/Hg in THF afforded poorly soluble UCl3(THF)n.106 This compound is sometimes used for the synthesis of other uranium complexes. However, this procedure is of limited application due to the formation of by-products. For example, the synthesis of U[N(SiMe3)2]3 from UCl3(THF)n and NaN(SiMe3)2 often B I Kharisov, MA Mendez-Rojas give rises 97 to a mixture of a U(III) complex and metallacycle the viz., derivative, a uranium(IV) [(Me3Si)2N]2U(CH2SiMe2NSiMe3).107 Selected procedures for the synthesis of halide complexes of actinides are listed in Table 3.Table 3. Synthesis of halide complexes of actinides. Ref. Products Reaction system Starting compound or metal Synthesis from elemental metals 97, 98 U X2 (X=Cl, Br, I), UX3(THF)4 THF X2 (X=Br, I), Th 44 ThX4(THF)4 ThBr4(MeCN)4 108 UCl4(THF)3 (see b) 109 AnX3(THF)4 (see c) 97 THF Th anode a Br2, MeCN U HgCl2, THF An (U, Np, Pu) Br2 or I2, THF Synthesis from salts or halide complexes 104 DMSO UI4 DIBSO 104 NaH 44 UCl4 UI4(DMSO)n (n=6, 8) UI4(DIBSO)n (n=6, 8) UCl3 . nTHF Th(NPh2)4 .THF 110 ThBr4(THF)4 111 20, 98 KNPh2, THF KNMePh, THF Cp*MgBr(THF) KCp*, THF NaN(SiMe3)2, 97, 112 K[Th(NMePh)5] Cp*ThBr3(THF)3 Cp*UI2(THF)3 An[N(SiMe3)2]3 THF ThBr4(THF)4 UI3(THF)4 AnI3(THF)4 (An=U, Np, Pu) a Direct electrochemical synthesis; b a large amount of a uranium amalgam was obtained as a by-product; c air-sensitive.V. Actinide p-complexes with allyl, cyclopolyene, arene and related ligands Considerable recent attention has been given to actinide com- plexes with allyl, cyclopolyene and arene ligands, particularly, with cyclopentadiene and its derivatives (see review 20). Allyl complexes can be synthesised from AnCl4 and the Grignard reagent 44 Et2O AnCl4+AllMgCl An(Z3-All)4 . <0 8C According to the data of low-temperature NMR spectro- scopy,20 the allyl groups in these complexes are Z3-coordinated and can be replaced under the action of HX or alcohols to produce, for example, U(C3H5)3X or [U(C3H5)3(OR)]2.44 Thus the dimeric tert-butyloxyallyl complex [U(C3H5)3(OR)]2 (17) was synthesised.20OBut But U OU O But OBut 17State-of-the-art coordination chemistry of radioactive elements Of actinide p-complexes, the uranium(III, IV) and thorium(III, IV) complexes with cyclopentadiene and its deriva- tives were obtained in the widest range.Uranium(V, VI) complexes 20 and neptunium(VI) and califor- nium(III) sandwich complexes were also prepared.44 These com- plexes can be compositionally divided into the following groups: CpnAn (n=3 or 4), CpnAnX (n=2 or 3), Cp2AnXn (n=1 or 2) and CpAnXn (n=2 or 3),X being generally Hal. The correspond- ing cyclopentadienyl alkyls, carbonyls, alkoxides, etc.are also known.20 All Cp4An complexes are poorly soluble in organic solvents. The U7C distances in the Cp4U complex are 2.81(2) A. The bonds between the An atom and the ligand are covalent in character.20, 44 The Cp3An complexes are strong Lewis acids and form complexes with various Lewis bases (see Ref. 24 and references cited therein). The Cp3UCl complex, which was the first synthesised organo- actinide compound,21, 44 serves as a precursor of compounds containing the non-bridging metal ± metal bond between, for example, uranium and iron or ruthenium [Cp3U7MCp(CO)2, M=Fe or Ru] or between uranium and germanium (Cp3UGePh3).20, 113 The first organouranium(III) com- plexes,24, 114, 115 viz., tris[(cyclopentadienyl)uranium] and some its derivatives of the Cp3UL type, are also used for the synthesis of bimetallic compounds.24 a Cp3U=N b Cp3U=N c Cp3UO C UCp3 d NPh UCp3 Cp3US C S Cp3U(THF) e Cp3UTeUCp3 f Cp3USUCp3 g Cp3USeUCp3 N R h UCp2 Cp2U NR (a) 1,4-(N3)2C6H4; (b) 1,3-(N3)2C6H4 (c) PhNCO; (d) CS2; (e) TePBu3; (f ) COS or Ph3P=S; (g) Ph3P=Se; (h) Cp3U=NR; R=Ph, SiMe3.Voltammetric studies 116 demonstrated that oxidation of (RC5H4)3UCl (R=H, Me, But or Me3Si) was accompanied by disproportionation. NMR studies revealed electron transfer and ligand exchange in Cp3UX (X=Hal, BH4 or Alk).24, 117 e¡ Cp3UX Cp3UX¡, ¡ e¡ Cp3U*(THF)+Cp3U*X, Cp3UX+Cp3U*(THF) Cp3U*(THF)+Cp3U*X¡.Cp3UX¡ +Cp3U*(THF) The equilibrium constants of the ligand exchange in com- plexes with substituted cyclopentadienyl ligands were deter- mined. 24, 118 It was found that binding of ligands with (MeCp)3U is reduced in the series of the ligands PMe3> P(OMe)3>Py>tetrahydrothiophene>THF>quinuclidine> CO, whereas binding of (Me3SiC5H4)3U with the EtNC ligand is stronger than with the EtCN ligand, which is indicative of substantial p-back bonding with uranium in the case of p-acceptor ligands.24 The results of calculations of the electronic structure of the model Cp3U(CO) complex showed substantial back bonding N=UCp3 N=UCp3 871 between the 5f orbitals ofUand the 2p orbitals of CO, which leads to stabilisation of the 5f atomic orbitals of uranium.20, 119 Some uranium complexes containing carbonyl ligands were described in the study.120 The relativistic effective core potentials of tris(cyclo- pentadienyl) actinide complexes were calculated by the ab initio quantum-chemical method.These complexes were examined also by gas-phase UV photoelectron spectroscopy.121, 122 The Cp3AnR1 complexes tend to incorporate the CO, CO2 and CNR2 ligands to form Z2-acyl (18), Z2-carboxylate (19) and Z2-iminoalkyl (20) complexes, respectively.20 O R1 R1 C AnCp3 C AnCp3 R1C AnCp3 O N O 18 19 R2 20 The Cp3UX compounds containing the U7P, U7N, U7Si, U7Sn or U7Ge bonds were prepared from Cp3UCl by the replacement of the chloride ligand.24 In the crystal structures of Cp3U(OPh) and Cp3U(OSiPh3), which were established by X-ray diffraction analysis, the U7O distances are shortened [2.119(7) and 2.135(8) A, respectively] and the U7O7C and U7O7Si angles are 159.4(5) 8 and 172.6(6) 8, respectively.These geometric parameters are indicative of strong p-bonding between the U and O atoms in oxygen-containing uranium complexes.24 The bimetallic oxygen-bridged complex, viz., m-O-bis[tris(cyclopenta- dienyl)uranium(IV)] was investigated.123 The absolute enthalpies of the cleavage of the uranium ± ligand bonds in complexes containing the U7S bonds of the L3U7SR type (L=C5H4Bun, C5H4SiMe3 or C9H6SiMe3; R=Et or Bu) were determined.124 The heteroatomic compound UCp2(m-Cl)2Li[(Me2NCH2CH2)2. .NMe] containing two chloride bridges provides yet another example of bimetallic complexes.125 The electronic structure and configuration of the ground state of the monomeric alkyl uranium(III) complex Z5-Cp2 UCH(SiMe3)2 were studied.126 The ground state of the molecule is 4A00[(a0)1(a00)1(a00)1], which corresponds to the 5f 3 configuration of the uranium atom; its energy is close to that of the higher-lying 4A00(5f 26d1) state. The uranium(III) ions free of ligands can adopt two different electronic configurations, viz., 5f 3 and 5f 2d 1 (see Ref.126 and references cited therein). The ground state of the U3+ ions in the gaseous phase has the 5f 3 config- uration. According to the results of an EPR study of the Th[Z5-C5H3(SiMe3)2]3 complex in the gaseous state, Th(III) in the ground state adopts the 6d1 configuration, whereas the free ion has the 5f 1 configuration.127 Cp Z2-acyl complexes Cp The insertion of CO or CO2 into the An7R bond in the 2AnR2 complexes gave rise to complexes 21 ± 23 or carbene-like 2An(COR)2 (24, R=Alk).20, 128 ± 130 R R R O O R O O O Cp2 An AnCp AnCp 2 Cp2 An 2 O O O O O R 22 (R=CH2SiMe3) R R R 23 (R=Me, Bn) 21 (R=Me)O R Th R O 24872 The dynamic behaviour of CpUCl3L2 in solutions was exam- ined.24, 131 In toluene, rapid isomerisation and the ligand exchange were observed.24 Cp L Cl Cl Cp U , Cl U L Cl L Cl Cl L Cp Cp L* L Cl L Cl Cl 7L L* U U Cp U .Cl L 7L* L* Cl L Cl Cl L Cl Cl L The tribromide complex Cp*ThBr3(THF)3 (see Ref.111 and Table 3) is a convenient compound for studying the chemistry of mono(pentamethylcyclopentadienyl) derivatives of thorium. Treatment of this complex with bases or Grignard reagents afforded aryloxide or alkyl derivatives.111 THF Cp*ThBr2(OAr)(THF) +KBr, Cp*ThBr3(THF)3+KOAr 18 h THF Cp*ThBr2(OAr)2+2 KBr, Cp*ThBr3(THF)3+2KOAr 18 h Cp*MeTh(OAr)2+MgBr2. Cp*ThBr2(OAr)2+MeMgBr In 1971, the first indenyl derivative of actinides was prepared upon treatment of uranium tetrachloride with indenyl anions.132 The indenyl uranium(III) complex U(C9H7)3 was synthesised from UCl3 and indenylsodium.133 Indenyl actinide(IV) complexes 25 were prepared from the corresponding halides and indenylpotas- sium.20 X K(C9H7) U AnX4 THF 25 An=Th, U, Np; X=Hal. Uranium complexes 26 ± 28 containing phosphacyclopenta- dienyl ligands were described.24, 134 Table 4.Selected actinide complexes with allyl, cyclopolyene, arene, alkyl and carbonyl ligands. Ref. Reaction product Reagent Starting compound Allyl complexes AnCl4 C3H5MgCl, Et2O An(C3H5)4 20, 44 20 [U(C3H5)2(OR)2]2 HOR, Et2O U(C3H5)4 Cyclopentadienyl complexes Cp3An(THF) Cp3AnCl Cp3AnX NaC10H8, THF or Na/Hg, THF M0Cp, DME (M0=Na, K, Tl) 24, 140 20, 44, 141 142 MX4 (M=Th, U, Np; X=Cl, Br, I) (MeC5H4)3UBut (MeC5H4)3U(But)L (L=C2H4, CO) Cp3UCl 143 ± 145 20, Cp3UX HSnPh3 Cp3U(SnPh3) 24 BkCl3 24 C2H4 or CO in PhMe LiL (L=PPh2, Cp3 UL NEt2, SiPh3) 2,6-Me2C6H3NC Cp3U.(X=NEt2, [Z2-C(X)=N . 145, PPh2, SiPh3) C6H3Me2-2,6] 146 Cp3U(NEt2) Cp3U=C(H)PPh2Me HC:CPh Cp3UC:CPh BH4 P C4Me4PK U U(BH4)4 BH4 P 26 Na/Hg TeBH4 P U BH4 C4Me4PK ArU(BH4)3 P U BH4 P Nu=THF, OPPh3 . 7 The structure of the first cycloheptatrienyl sandwich complex [K(18-C-6)][U(Z-C7H7)2] was established.135 The authors believed that the cycloheptatrienyl ligand can be formally considered as C7H3¡ and the uranium atom occurs in the oxidation state +5.135, 136 It was demonstrated that the C7H7 ligands in this compound are planar and are located parallel to each other and perpendicular to the axis passing through the uranium atom and the centres of the rings.135 Generally, cyclooctatetraene complexes of actinides have the composition (COT)2An or (COT)AnX2 (X=Hal). Compounds of the mixed type (COT)AnCp can also be obtained.Some of these complexes contain solvent molecules, for example, THF.20 Com- pounds of this type can be prepared, for example, according to the following procedures: An(COT)2+4 KCl , AnCl4+2K2(COT) An=Th, Pa, U, Np; Pu(COT)2+4 KCl +2 PyHCl , 2 (COT)ThCl2(THF)2 , (COT)UCl2(THF)2+2 NaCl +H2. (PyH)2PuCl6+2K2(COT) ThCl4+Th(COT)2 UCl4+COT+2NaH Cyclooctatetraene complexes of uranium, thorium, protacti- nium, neptunium and plutonium [(Z-COT)An(m-SPr)2]2 with Reagent Starting compound Cyclopentadienyl complexes (MeCp)3U(THF) Me3SiN3 or PhN3 I2 L3UH, (L=Cp*, C5H4But, C5H4SiMe3) Cp3AnCl (An=Th,U,Np) Cp4U, Cp3UBun or Cp3UBui RMgX in THF or RLi in Et2O BunCl, ButCl, BnCl CpRu(CO)2Na Cp2 ThX2 (X=Cl, Br) LiR, Et2O Cp2 AnR2 Cp2 AnCl2 Na/Hg, THF (Me3Si)2C5H3UX2 Cp2Be B I Kharisov, MA Mendez-Rojas P P BH4 Nu U Nu P 28 27 Ref.Reaction product 20 (MeCp)3U=NR (R=Me3Si, Ph) L3UI 96 20 Cp3AnR 147 Cp3UBun, Cp3UCH2CMe2Cl, Cp3UBn, Cp3UCl or CpUBn3 24 Cp2 (X)Th . Ru(Cp)(CO)2 20 (R=Alk) 20 (Me3Si)2C5H3 . UX(THF) [Cp2BkCl]2 20State-of-the-art coordination chemistry of radioactive elements Table 4 (continued). Ref. Reaction product Reagent Starting compound Arene complexes 24 UCl4 C6Me6, [U3 (m3-Cl)2(m2-Cl)3 . AlCl3, Al 98 Al, AlCl3, PhH 98 (m1,Z2-AlCl4)3 . (Z6-C6Me6)3](AlCl4) (PhH)U(AlCl4)3 Zn, AlCl3, C6Me6 [(C6Me6)2U2Cl7]+.[AlCl4]7 Alkyl complexes 20 UCl4 Li2UR6L8 148 ThCl4 LiR, L (L=THF, Et2O) LiMe, TMEDA, [Li(TMEDA)]3 . ThMe7 .TMEDAa Et2O Cycloheptatriene complexes 96 UCl4 C7H8, THF [U(Z-C7H7)2]¡ UX4 K[X3U. K(C7H9) 149 (X=NEt2, BH4) (m-Z7:Z7-C7H7)UX5] Cyclooctatetraene complexes K2(COT) U(COT)2 UCl4 150, 151 a In this complex, six of sevenCH3 groups are involved in coordination to the Li(TMEDA)+cations, two groups being coordinated to each of the cations; TMEDA is tetramethylenediamine. mixed ligands were prepared by treatment of An(COT)(BH4) with thiol.137 Mono(cyclooctatetraene) amide uranium complexes K[U(COT)(NEt2)2] and U(COT)(NEt2)2(THF) were synthesised from tetrakis(diethylamine)uranium and bis(cyclooctatetra- ene)uranium, respectively.Oxidation of these complexes afforded a series of uranium(V) derivatives.138 As can be seen from Table 4, actinide p-complexes containing the cyclopolyene or arene ligands (see also Ref. 139) are more abundant than actinide complexes with other ligands. VI. Hydride and hydroborate complexes of actinides The first organoactinide hydrides were prepared by hydrogena- tion of Cp2 AnR2.20 Cp2 AnH2+2RH Cp2 AnR2+H2 Both monomeric and oligomeric hydride complexes of acti- nides are available. Generally, these complexes contain OAlk, 1,2- bis(dimethylphosphino)ethane (dmpe) or cyclopentadienyl as additional ligands. Thus uranium and thorium tetrahydroborates are polymeric compounds, whereas neptunium and plutonium tetrahydroborates are monomeric.Methyltrihydroborate deriva- tives An(MeBH3)4 always occur as monomers. 0 8C An(BH4)4+2 AlF2(BH4) AnF4+2 Al(BH4)3 The volatility of the complexes increases and their stability decreases on going from thorium to plutonium.17 Thorium, neptunium, plutonium and protactinium hydro- borates and uranium tris(tetrahydroborate) possess similar prop- erties. However, Th, Np, Pu and Pa hydroborates have a number of characteristic structural features.44 The UH(BH4)3(DME) complex contains the U7H bond. The U(BH4)3[Ph2P(Py)]2 complex was also described in the cited study.44 873 Ref. Reaction product Reagent Starting compound Cyclooctatetraene complexes [K(solv)][An(COT)2] 152, 153 K2(COT), solvent (solv) COT, NaH 24 (COT)AnCl2(THF)2 24 COT (COT)U(BH4)2 20 Pu(COT)2 K2(COT), THF AnX3 (An=U, Np, Pu, Am) AnCl4 (An=U, Th) U(BH4)4 (PyH)2PuCl6 (COT)ThCl2(THF)2 155 Cp*U(COT)(THF) UI3(THF)4 Cp*MgCl(THF), (COT)ThCp*Cl(THF) 154 PhMe 1) KCp*, THF 2) K2(COT) Carbonyl complexes CO, Ar (4 K) U (vapour) 156 U(CO)x CO [(Me3Si)2C5H3]U(CO) 20 [(Me3Si)2C5H3]U CO Cp3M(CO)R 44, 157 Cp3MR (M=U, Th, Np) Trimeric thorium complex 29 provides a rare example of hydride complexes of early actinides, which are stabilised only by the aryloxide ligands.90H2, PhP, 1.5 atm, 7 days Th(OAr)2(CH2SiMe3)2 7SiMe4 ArO OAr Th H H H H OAr ArO Th Th OAr ArO HH 29 Ar=C6H3But2-2,6 The complex 29 exhibits moderate catalytic activity in hydro- genation of hex-1-ene.158 Alkoxide tetrahydroborate complexes of uranium(IV) were also described.159 Organoactinide hydrides are formed from the ArnAnR com- plexes through metathesis of s-bonds.20 d7 d+ H H ArnAnH+RH ArnAnR+H2 ArnAn Rd7 d+ The structure of complex 30 was established.20 The geometric parameters [the H7Th7Th and Th7H7Th angles are 58(1) 8 and 122(4) 8, respectively; the distance between the thorium atoms is 4.007(8) A] indicate that the metal ± metal interaction in this complex is weak.H Th Th HH H 30874 The unusual electron-deficient cyclopentadienyl complexes and were (C5H4BH3)2U(BH4)2 (C5H4PPh2)2U(BH4)2 described.160 Other cyclopentadienyl and related hydroborate complexes, for example, [Na(THF)6][Cp*U(BH4)3]2,161 were also synthes- ised.161, 162 Selected examples of hydride and hydroborate complexes of actinides are listed in Table 5.Table 5. Selected hydride and hydroborate complexes of actinides. Ref. Reaction product Reagent Starting compound U(BH4)3(THF)3 44 44 UH3 U(BH4)4 B2H6, THF thermal decom- U(BH4)3 position in solution THF 44 U(BH4)4(THF)2 44 Et2O [U(BH4)4(Et2O)]? 163 Li2C2B9H11 [Li(THF)4]2 . Th(Z5-C2B9H11)2X2 164 CpMgBr(THF) CpThBr3(THF)3 90 H2 Th3(m3-H)2(m2-H)4 . (OAr)6 ThX4 (X=Cl, Br) ThBr4(THF)4 Th(OAr)2 . (CH2SiMe3)2 UBr4(MeCN)2 165 Li2C2B9H11, Li2 [UIV(Z5-C2B9H11)2 . MeCN Br2] 165 UI3(THF)4 Li2C2B9H11, TMEDA [Li(TMEDA)] .[U(C2B9H11)I2(THF)2] 20 dmpe Cp2 U(H)Cl Cp2 UH2 166 (Me3XC5H4)3UH (Me3XC5H4)3UCl KHBEt3 (X=C, Si) 96 Ph3PBH3 Cp3 UBH4 Cp3 UH 96 (C5H4But)3UH (C5H4But)3UBH4 Ph3PBH3 96 (C5H4SiMe3)3UH Ph3PBH3 (C5H4SiMe3)3UBH4 167 [K(THF)2] . [ReH6(PPh3)2] K[Cl(Z-C5Me5)2UH6 . Re(PPh3)2] or [U(Z-C5H4R)3UH6 . Re(PPh3)2] 44 H2 Cp2M(H)(m-H)2 . M(H)Cp2 168 [U(Z-C5Me5)2 . Cl(THF)] or [U(Z-C5H4R)3Cl] (R=H, Bu, SiMe3) Cp2MR2 (M=U, Th) [(Z5-C2B9H11)2 . UIVBr2] . 2 [Li(THF)4] electrochemical [(Z5-C2B9H11)2UIIIBr . reduction and (THF)] . 2 [Li(THF)x] reduction (x=2± 4) with Na/Hg VII. Actinide complexes with macrocyclic ligands Data on several actinide complexes with crown ethers and other macrocyclic ligands were published in the literature.Thus treat- ment of uranium and thorium tetrakis(diethylamides) [An(NEt2)4] with free porphyrin (TPP) afforded the diporphyrin non-planar sandwich complexes An(TPP)2.17 Ultrafast electronic deactivation and the vibrational dynamics for the excited states of the uranium(IV) porphyrin sandwich complexes were exam- ined.169 General procedures for the preparation of actinide complexes with macrocyclic Schiff's bases involve direct binding of the metal ion with the corresponding ligand and cyclisation in the presence of the metal ion. Thus phthalodinitrile reacted with anhydrous uranyl dichloride in DMF to give a `superphthalocyanine' uranyl complex. Condensation of ethylenediamine with 2,6-dicarbonyl- B I Kharisov, MA Mendez-Rojas 2 pyridine in the presence of UO2á gave rise to a hexaazamacrocy- clic complex.17 When irradiated with neutrons, the phthalocyanine complex ThPc2 (Pc is phthalocyanine) entered into the nuclear reaction to give the corresponding protactini- um(IV) complex.17 233 232 PaPc2 ThPc2(n,g)233ThPc2 The phthalocyanine complexes AnPc2 were described in detail in a number of publications.170 ± 173 Complexes with other polydentate ligands, for example, with chelating agents, can be used for efficient binding of actinide ions.These complexes are soluble both in water and organic solvents.17 The higher the dentation of the ligand the more stable the complex. In the An4+(EDTA) complexes (EDTA is ethylenedia- minetetraacetic acid), the ligand is hexadentate and adopts a twist conformation.17 Binding of plutonium and other radionuclides can be carried out with the use of cyclic and linear catechol- amines.174 2 Actinide complexes with crown ethers are generally prepared from actinide salts.Complexes only with tri- and tetravalent actinide ions are known {for example, [UCl3(18-C-6)]2 . [UO2Cl3(OH)H2O]}. The hydrogen bonds between the oxygen atoms of the ether and the water molecules of coordination were found in the UO2á complexes.17 The IR spectra and the isotope effects for uranium complexes with crown ethers and the crystal structures of some of these compounds were published in the literature.175 Actinide complexes with crown ethers can be prepared also from organometallic compounds. Thus Cp3UCl reacted with 18-C-6 in the presence of a sodium amalgam in THF to yield (Cp3UCl)[Na(18-C-6)] (see Ref.24 and references cited therein). VIII. Actinide complexes with nitrogen-containing ligands Most of N-donor bases possess low affinity for actinides and act predominantly as proton acceptors. The An7N bonds with mono- and even bidentate N-donor ligands are rather weak.17 The complex UO2(hfa)2 exhibiting high acidity forms a 1 : 1 complex only with ammonia. Like actinide alkoxides, actinide amides occur generally in the associated form. For example, these compounds react with chelating N,N'-dimethylethylenediamine to give the linear trimeric [U(dmed)2]3 complex and the square tetrameric [U(dmed)2]4 complex.17, 176 Dialkylamide and related complexes, such as U[N(SiMe3)2]3, [U(NEt2)3]BF4, U(NPh2)4, UO2[N(SiMe3)2]2 .(OPPh3)2 and UO2CrO4 . 2MeCONEt2, were described in the literature.97, 177 ± 182 The structure of the U[N(SiMe3)2{N(SiMe3)[SiMe27 CH2B(C6F5)3]}] complex was established by X-ray and neutron diffraction analysis. It was demonstrated that the electron defi- ciency on the uranium atom is effectively compensated by the formation of multicentre bonds between the U atom and the SiCH2 groups of the amine ligands. The X-ray diffraction data are unambiguously indicative of m3-coordination of the BH4 groups.183 The structure of the unusual uranium(III) complex with a tripodal aromatic amine, viz., tris[(2,2 0-bipyridin-6- yl)]amine, was established by X-ray diffraction analysis.184 Protolysis of the U7N bond in amide complexes of uranium affords cationic complexes.This procedure was used for the synthesis of compounds containing the cations [U(Z-Cp)3(THF)]+, [U(Z-Cp*)(NEt2)2(THF)2]+, [(U-Z-C5R5)2 . (NEt2)(THF)]+, [U(Z-C5R5)(Z-COT)(THF)2]+ (R=H, Me),185 [U(NEt2)3]+, [U(NEt2)2(THF)3]2+, etc.177 The resulting Th and U complexes with the silylated N(CH2CH2NSiMe3)3 ligand con- tain bonds between the metal atoms and the chlorine, carbon, hydrogen or oxygen atoms.186 Chloro, pentamethylcyclopenta- dienyl and tetrahydroborate complexes of uranium containing the N(CH2CH2NSiMe3)3 ligand were characterised by the X-ray diffraction method.186 The reactions of actinide amides were surveyed in a recent review.187State-of-the-art coordination chemistry of radioactive elements Only several imide complexes of actinides are known: Me3SiN=UV[N(SiMe3)2]3, Me3 SiN=UVIF[N(SiMe3)2]3, Cp2 ..U(NC6H2But3-2,4,6) and U[N(CHMeCH=PPh2Me)]Cp3.188, 189 In complexes of the (C5H4Me)3U=NR type (R=Ph or SiMe3), both lone electron pairs of nitrogen are involved in binding with uranium.24, 190 Of the simplest nitrogen-containing ligands, the azide anion gives very stable complexes with uranium (Kdiss=561073). The stabilities of the latter complexes are comparable with those of fluoride complexes.17 The binuclear (U7Mo) heterocomplexes with N2 were prepared.191 IR spectro- scopic and quasirelativistic theoretical studies of the coordination and activation of N2 with the uranium and thorium atoms were carried out virtually at the same time.192 The mononuclear UCp2 (NHR)2 complexes (R=2,6- Me2C6H3, Et or Bu) were synthesised, their structures were established and these complexes were demonstrated to catalyse hydroamination of terminal alkynes with ammonia.193 The data on complex formation of uranium(VI) with neutral N-donors in DMSO were reported. 194 An analogue of the uranyl ion was found in the crystal structure of the red complex PPhá4 {UOCl4[NP(C6H4Me-3)3]}7 (31).195 The air-stable complex 31 was prepared from compound 32 by the replacement of the chloride ligand in the [UOCl5]7anion accompanied by elimination of Me3SiCl.CH2Cl2 PPhá4 [UOCl5]7+Me3Si[NP(C6H4Me-3)3] 7Me3SiCl 32 PPhá4 {UOCl4[NP(C6H4Me-3)3]}7 31 According to the data of X-ray diffraction analysis, the {UOCl4[NP(C6H4Me-3)3]}7 ion contains the linear O=U=N group coordinated by four chlorine atoms.195 The U7O distance (1.76 A) is typical of the uranyl ion (see Ref. 195 and references cited therein). TheU7Ndistance (1.90 A) indicates that the bond order is no less than 2. The U7N7P group is nearly linear (171.9 8), which suggests that the p electrons of the nitrogen atom are completely involved in bonding with uranium. The orders of the bonds between the uranium atom and each of the oxygen atoms in the uranyl ion can be equal to 3 due to the fact that the uranium atom bears simultaneously f and d valence orbitals.195, 196 Other anionic actinide complexes can be prepared immedi- ately from salts.For example, UF5 and UF6 react with nitrogen bases to give molecular or ionic compounds, such as [(bipy)2H]+[UF6]7, UF4(2-FC5H4N) and U2F12(bipy).197 Thermolysis of the thorium derivative Th(OSO2CF3). .[N(SiMe3)2]3 in the presence of 1 equiv. of pentamethylcyclopen- tadiene afforded binuclear complex 33.111 OTf 110 8C, 8 days Th N(SiMe3)2+2 Cp*H (Me3Si)2N PhMe N(SiMe3)2 CF3 O O CF3 S S O OTh Th OOO N(SiMe3)2 N Me3Si O CH2 S Si Me O CF3 Me 33 The actinide complex with 1,4-di-tert-butyl-1,4-diazabuta- diene (DAB) 34 was prepared for the first time from the complex containing N(CH2CH2NHSiMe3)3 (tren) as a ligand.198 THF [(tren)ThCl]2+2 Li[But2(DAB)2] 780 8C 2 (tren)Th[But2(DAB)] +2 {Li[But2(DAB)]}Cl 34 The complex 34 was obtained as red paramagnetic crystals P21/n).(monoclinic, The the space group is N(CH2CH2NHSiMe3)3 ligand ensures the optimum spatial envi- ronment for stabilisation of this complex. Since thorium(IV) compounds are diamagnetic, the diazabutadiene ligand in the paramagnetic complex 34 is subjected to one-electron reduction, i.e., this ligand is present as the [But The complex formation of neptunium(VI) with 3,30-bis(diazir- idine) derivatives 199 and of uranium(VI) with 8-hydroxyquinoline and its 5-halogen derivatives was studied.200 In the latter case, the UO2L and UO2L2 complexes (where L is a 8-hydroxyquinoline derivative) were obtained.200 Considerable recent attention has been given to uranium complexes with Schiff's bases (see, for example, the review 201).Uranium(VI) complexes with Schiff's bases are used as organic oxidants (the catalytic modifications of these reactions were also developed).202 Polystyrene-supported chelating resins incorporat- ing U(VI) complexes with Schiff's bases are available.203, 204 Salicylaldehyde 205 and triethylenetetramine,203 3-formylsalicylic acid and o-hydroxybenzylamine 204 or salicylaldehyde and 1-amino-2-hydroxynaphthalene-4-sulfonic acid 206 were used as precursors of Schiff's bases. Uranium(VI) complexes with Schiff's bases exhibit photochromic properties. The kinetics and the mechanisms of photochromic conversions were examined.207 Examples of actinide complexes with nitrogen-containing ligands are given in Table 6.Table 6. Selected actinide complexes with nitrogen-containing ligands. Starting compound UO2Cl2 ThBr4(THF)4 UCl3(THF)3 U(NPh2)4 (NSiMe2)2U. CH2SiMe2NSiMe3 UCl4 HMe2Si NSiMe3 Th N(SiMe3)2 (Me3Si)2N (Me3Si)2N[ThN . (SiMe3) . (SiHMe2CH)] Cp3U=C(H)PPh2Me HNPh2 (C5H4Me)3U(THF) Cp2U(NEt2)2 IX. Complexes with phosphorus-containing ligands A large number of actinide complexes, which bear oxygen- containing phosphorus ligands and in which the metal atom is 875 2DAB]¡ radical anion. Ref. Reaction products Reagents 208 (THF)2 KN(SiMe3)2, UO2 [N(SiMe3)2]2 . THF KNR2 44, 110 112 Th(NPh2)4(THF) or K[Th(NMePh5)] U[N(SiMe3)2]3 NaN(SiMe3)2, THF 180 U(NPh2)4 .L L(L=Py, Et2O, THF, (EtO)3PO) 44 U(NPh2)4 Ph2NH 44 U(NEt2)4 LiNEt2 110 HNMePh Th[N(SiMe3)2]2 . (NMePh)2 208 1) CF3SO3H 2) CpH 24 Cp[(Me3Si)2N]Th . (m2-OSO2CF3)3 . Th[N(SiMe)3 . (SiHMe2CH)]Cp Cp3UNPh2 (C5H4Me)3U=NR 190 RN3 (R=Me3Si, Ph) NEt2NR Cp2U 44 RNC NEt2876 coordinated by the oxygen atom, are available.17 Examples of phosphine complexes containing the An7P bond are few in number.209 Thus thorium(IV) and uranium(IV) tetrahalides react with trimethylphosphine to give the 1 : 2 complexes MCl4(PMe3)2 (M=U or Th).17, 210 The U[Me2PCH2CH2PMe2]4 complex con- taining an eight-coordinate uranium atom, which is isostructural to the thorium analogue, was described.211 Uranium complexes with phosphorus donor ligands, procedures for their synthesis, structures and properties were considered in the study. 212 Ligands of the phosphacyclopentadienyl type serve as bridges, for example, in the dimeric uranium(III) complex 27.24, 134, 213 The reactions of lithium salts of the potentially tridentate diphosphinoamide ligands N(CH2CH2PR2)2¡¦ (R=Et or Pri) with uranium or thorium tetrachlorides afforded diphosphinoa- mide complexes of actinides. Ligands of this type are suitable for the preparation of various complexes of uranium(IV) (for exam- ple, {UCl2[N(CH2CH2PEt2)2]2}2), uranium(V) and thorium(IV). Depending on the reaction conditions, these ligands can be mono-, bi- and tridentate.The character of the coordination compounds can be radically changed by varying the substituents at the neutral phosphine centre.214 Thorium and uranium complexes with diphosphazane diox- ides were synthesised.215 The crystal structure of the UO2(NO3)2[Ph2P(O)N(Ph)P(O)Ph2] complex was established. It was demonstrated that the metal atom is coordinated by diphos- phazane dioxide in a bidentate fashion. Some other organo- phosphorus and related compounds of uranium were described in the studies.216 ¡À 219 Actinide complexes containing simultaneously the phosphine and cyclopentadienyl ligands are few in number. Only the Cp2ThX2(Me2PCH2CH2PMe2) complexes (X=Cl, Me or Bn) 179 and monomeric hydride complex of uranium(III) 35 were prepared.220 H Me2 P PhMe, H2 U Cp2 UR2+Me2PCH2CH2PMe2 autoclave,720 8C P Me2 35 R=Me, CH2SiMe3 .The reactions of U(BH4)3(THF)x with some diphosphine ligands afforded the U(BH4)3(L)2 complexes [L is dimethylphos- phinoethane 221 or 2-(diphenylphosphino)pyridine 222]. In these compounds, dynamic transformations are observed within the NMR time scale even at low temperature. Table 7. Selected actinide complexes with phosphorus-containing ligands. Ref. Product Reagent Substrate AnCl4 211, 214 MP(CH2CH2PMe2)2 An[P(CH2CH2PMe2)2]4 An=Th, U M=Li, K ThCl4 [N(CH2CH2PPri2]2¡¦ . ThCl2[NCH2CH2PPri0:5]2 211, 214 Li�¢2 UCl4 Li�¢2 211, 214 189 Me3Si[NP . [UOCl5]¡¦ [N(CH2CH2PEt2]2¡¦ . {UCl2[N(CH2 .CH2PEt2)2]2}2 [UOCl4NP. (C6H4Me-3)3] (C6 H4Me-3)3]¡¦ (see a) KOBut 181 UO2Cl2 . (Ph3PO)2 181 NaN(SiMe3)2 UO2(OBut)2 . (Ph3PO)2 UO2[N(SiMe3)2]2 . (Ph3PO)2 HNPPh3 Cp2 UCl2(HNPPh3) b Cp2 UCl2 223 a The red salt containing the Ph4P+ cation is air-stable and is soluble in CH2Cl2 and MeCN; b the first complex of the f element with the phosphineimino ligand. B I Kharisov, MA Mendez-Rojas X. Sulfur-containing actinide complexes UCl4+4Li2(SCH2CH2S) DME [Li(DME)]4[U(SCH2CH2S)4] .DME 36 Examples of actinide complexes with phosphorus-containing ligands are given in Table 7. Being hard acids, actinides usually do not form stable complexes with S-donor ligands, which are soft bases. Only a few examples of complexes with sulfur-containing ligands are available in the literature, viz., UCl4(dmte)2 (dmte is 1,2-dimethylthioethane), An(S2CNEt2)4 (An=Th, U, Np or Pu), (NEt4)[Np(S2CNEt2)4], (NMe4)[UO2(S2CNEt2)3],17 U(SBun)4, [U(SBun)6]27 and [U(SPh)6]27;224 [Na(18-C-6) .(THF)][U(COT)(C4H4S4)2] is the only known uranium(V) complex containing the metal ¡À sulfur bond.225 The reaction of UCl4 with Li2(SCH2CH2S) in dime- thoxyethane afforded 226 the first homoleptic dithiolate complex of an f element (36; the space group is P21/n). The nature of the U7S bond in the complex 36 was estimated by the semiempirical Hu�� ckel method. It was found that the 6d, 7s and 7p orbitals of uranium are responsible for interactions with sulfur-containing ligands. The involvement of 5f orbitals is insig- nificant, and hence, the p interaction between uranium and sulfur is weak.226 The authors also gave a brief review of the state of the art in the chemistry of actinide ions with sulfur-containing ligands.226 Derivatives of uranium(IV) tetrathiolate were prepared by treatment of U(NEt2)4, U(BH4)4 or U(SBun)4 with thiols and by oxidation of metallic uranium with disulfides.227 It was found that the reactions of the U(SBui)4 complex with acids, I2 and CS2 proceeded at the U7S bond.227 The neptunium(V) complex with DMSO with composition [(NpO2)2(DMSO)7(H2O)2](ClO4)2 .H2O was structurally charac- terised.228 The structure contains the dimeric complex [(NpO2)2(DMSO)7(H2O)2]+ cations, perchlorate anions, coordi- nated water molecules and water molecules of crystallisation.The IR and electronic spectra are indicative of the non-equivalence of the dioxo cations. Thus Np(1)O�¢2 acts as a monodentate ligand, whereas Np(2)O�¢2 serves as the coordination centre.228 Treatment of the U(COT)(BH4)2 complex with thiols or sodium thiolates (RSH or RSNa, where R=Bun, Prior But) afforded first organouranium compounds containing the bridging disulfide {[U(COT)(m-S)2]2} and thiolate {[U(COT)(m-SR)2]2} groups. The structures of these compounds differ from those of their alkoxide analogues, which contain only two bridging OR groups.229 The reaction of U(NEt2)4 with PriSH and OP(NMe2)3 gave rise to the first structurally characterised tetrathiolate complex of uranium U(SPri)4[OP(NMe2)3]2.229 The uranium sulfur cluster U3(m3-S)(m3-SBut)(m2-SBut)3(SBut)6 was also described.230 The Cp2 U(dddt) complex (dddt is 5,6-dihydro-1,4-dithiin-2,3- dithiolate) 231 and the homoleptic uranium(IV) complexes (THF)3Na(m-SR)3U(m-SR)3Na(THF)3 (R=But or Ph) were syn- thesised.232 Recently, the first heteroleptic uranium(V) complex 38 was prepared by oxidation of complex 37 with AgBPh4 in THF.233 (COT)UX2(THF)n+Na2(dddt) AgBPh4 THF [(COT)UIV(dddt)2][Na(18-C-6)]2 37 U S S S S S S S S 38 X=BH4, n=0; X=I, n=1.State-of-the-art coordination chemistry of radioactive elements Notwithstanding the fact that the available sulfur-containing actinide complexes are few in number, these compounds are of great interest in the coordination chemistry 230 and can find use for extraction of actinides.232 Examples of sulfur-containing uranium complexes are given in Table 8.Table 8. Selected sulfur-containing uranium complexes. Ref. Product Reagents Starting compound 232 UCl4 or U(BH4)4 UI4 104 (THF)3Na(m-SR)3U. (m-SR)3Na(THF)3 UI4(DMSO)n (n=6, 8) UI4(DIBSO)n (n=6, 8) 230 Cp2 U(dddt) Cp2 UCl2 233 U(SPri)4[OP(NMe2)3]2 U(NEt2)4 NaSR (R= But, Ph), THF DMSO DIBSO Na2(dddt) PriSH, (Me2N)3PO XI. Technetium complexes The chemistry of technetium becomes increasingly important, particularly, due to the fact that the short-lived (T1/2 6.015 h, g irradiator) nuclide 99mTc is used in diagnostic medicine. This nuclide has been used over many years in bone scanning. Recently, 99mTc has found use in the treatment of various diseases of the heart, brain, kidney, liver and other organs as well as of tumour tissues. Technetium complexes are also of interest in the radio- pharmaceutical industry.234 Owing to the ideal energy of g radiation (140 keV), lack of particulate radiation dose, the small half-life T1/2 and the fact that 99mTc is readily accessible, this isotope is a radionuclide of choice for obtaining images in diagnostic nuclear medicine.235, 236 1.Complexes with nitrogen-containing ligands Complexes of Tc(III), Tc(II) and Tc(I) with pyridine ligands were prepared.237 The authors were interested to prepare a coordina- tively unsaturated low-valence electron-rich Tc metal centre surrounded by very weak p-donor ligands.Tl+, Py, D trans-[TcCl2(Py)4]+ PPh3trans-[TcCl2(Py)3(PPh3)]+ Cl MeCN Cl Cl tpy, DME, 20 8C PPh3 mer-TcCl(tpy) tpy is terpyridine. The TcCl2(Py)4, TcCl3(PPh3)2(TMEDA), TcCl3[But3(tpy)] and [Tc(tpy)(Py)3]Cl compounds were characterised by electro- chemical and spectroscopic methods and by X-ray diffraction analysis.237 According to the data of this investigation, p-back bonding is observed in the Tc(II) and Tc(I) complexes, unlike the Tc(III) complexes. In particular, the Tc7N bonds in pyridine complexes of Tc(II) are shortened by 0.04 ± 0.06 A and the Tc7N(internal) bonds in terpyridine complexes of Tc(I) are shortened by 0.09 A compared to those in the analogous Tc(III) complexes.237 These effects are favourable for stabilisation of low oxidation states of the metal atoms. The pyridine Tc(III) com- plexes give Knight-shifted 1H NMR spectra.The visible regions of the spectra show transitions, which were empirically assigned to ligand-to-metal charge transfer transitions; multiple reversible electrochemical redox couples were found.237 Reduction of pertechnates (as well as perrhenates or permo- lybdates) with 2-hydrazinopyridine hydrochloride in methanol afforded complexes containing the M(Z1-NNC5H4NHx)(Z2- HNNHyC5H4N) core (M=Tc, Re or Mo).238 In particular, the TcCl3(NNC5H4NH)(HNNC5H4N) complex was prepared. The 877 latter was used for the synthesis of the Tc(C5H4NS)2..(NNC5H4N)(HNNC5H4N) complex (C5H4NS is pyridine-2-thi- olate), which is a precursor of 99mTc-containing peptide reagents for medical investigations. The bifunctional hydrazine ligands used in the study 238 are efficient and versatile linkers for labelling antibodies and protein fragments. Other organic hydrazine com- plexes of 99mTc were also prepared. The reactions of [TcOCl4]7 with hydrazine hydrochloride (PhMe, 20 8C or CH2Cl2, 36 8C) gave [TcCl2(C8H5N4)(PPh3)2] . 0.75 PhMe rise to and [TcNCl2(PPh3)2] . 0.25CH2Cl2, respectively.239 The exchange reaction of Tc(MeCN)(PPh3)2Cl3 with 2-hydrazinopyridine afforded Tc(III) complex 39.240 N PPh3Cl Tc Cl N PPh3 N 39 Complexes with compositions mer-[Cl3(pic)3Tc] and mer- [Cl3(pic)(PMe2Ph)2Tc] (pic is 4-picoline) were prepared by the reactions of [TcOCl4]7 with a series of phosphine ligands in 4-picoline.The process was accompanied by the oxygen atom migration. The resulting compounds were characterised by spec- troscopic methods and X-ray diffraction analysis.241 Complexes of high oxidation state technetium containing the [Tc:N]2+ core are more stable than the corresponding complexes of technecyl [Tc=O]3+ and their properties resemble those of technecyl derivatives. The technetium(V) complex [TcN(L)(H2O)] .2H2O (L is the tetraazamacrocycle) was prepared by the reaction of TcNCl2(PPh3)2 with the tetraazamacrocycle.242 A complex with the [Tc:N]2+ core was synthesised using ancillary polydentate phosphorus- and nitrogen-containing ligands.The reaction of [TcNBr4]7 with 2,20-bipyridyl in ethanol afforded the cis-octahe- dral [TcNBr(bipy)2]2(TcBr4) complex with the [Tc:N]2+ core containing the tetrahedral tetrabromo technetate(II) cation.243 Other chelate nitride complexes of technetium(V) with the N2S2 ligand and the [Tc:N]2+ core were prepared by the reactions of TcNCl2(PPh3)2 with (HSCR2CH2NRCH2)2 (R=Me or Et).244 Since the chemical behaviour of the MoVO complexes is similar to that of the TcVIN complexes, pertechnate complexes can be synthesised according to the procedure developed for the preparation of analogous MoVO complexes. Thus chelate nitride complexes of technetium(V) with ligands of the N2S2 type and the [Tc:N]2+ core, [TcN(S2CNEt2)]2(m-O)2, [TcN.viz., .(S2CNC4H8)]2(m-O)2, (AsPh4)2{[TcN(CN)]2(m-O)2} and (AsPh4)2. .{[TcN(edt)]2(m-O)2} [edt=S(CH2)2S], were prepared by the reactions of {[TcN(OH2)3]2(m-O)2}2+ or Cs2(TcNCl5) with pre- cursors of the corresponding ligands in solutions of Na4P2O7.245 The reaction of TcNCl2(PPh3)2 with the piperidinium ferro- cenyldithiocarboxylate gave rise to nitride complex 40. According to the data of cyclic voltammetry,246 two iron(II) atoms in the complex 40 act as independent redox centres linked through the [Tc:N] core. Chelate complexes 41 and 42 containing the same [Tc:N] core were also synthesised.247 S + N S7 H2N Cl Cl Tc +2 Fe PPh3 Ph3P N S S Tc S S Fe Fe 40878 N Cl Ph3P +2Na2(SPPh2NPPh2S) Tc Ph3P N S PPh2 N Tc PPh3 Ph2P S N Ph2P S S PPh2 41 NaN(SPPh2)2 TcNCl2(PPhMe2)2 TcN(Cl)(PPhMe2)[N(SPPh2)2] 42 The possibility of radiopharmaceutical application of nitride technetium complexes with nitrogen-substituted amino acids, viz., derivatives of 2,5-dimethyldithiocarbamic acid, was reported.248 Complexes with composition TcN(Ln)(PPh3), where Ln=Z-Gly- dtc (n=1), Z-Ala-dtc (n=2), Z-Phe-dtc (n=3), Z-Val-dtc (n=4) or Z-Leu-dtc (n=5), were synthesised. All these com- plexes were characterised by spectral methods and X-ray diffrac- tion analysis. Binuclear complexes of Tc(VII) 43 and Tc(VI) 44 were syn- thesised by reduction of complexes 45 and 46, respectively, with sodium.249 Na Tc(NC6H3Me2-2,6)3I 45 Tc2(NC6H3Me2-2,6)4(m-NC6H3Me2-2,6)2 43 Na Tc(NC6H3Pri2-2,6)6 Tc(NC6H3Pri2-2,6)3I 44 46 The crystal structure of the complex 43 consists of tetrahedra linked via shared edges, whereas the complex 44 has an `ethylene- like' structure.249 The reaction of the pertechnate ion with the salt of 3,6-bis(20-pyridyl-1,2,4,5-tetrazine) (bptz .2 HCl) in methanol or ethanol afforded binuclear complexes of the general formula (m-bptz)(TcO3X)2 (X=Cl, OMe or OEt) containing Tc(VII), Tc(V) and the bridging bptz ligand. 4-Phenyl-3,6-bis(20-pyridyl)- pyridazine (pppz) was used as a ligand for the preparation of mononuclear complexes from pertechnate and TcOCl4 in an aqueous-ethanolic solution of HCl.250 The binuclear polypyridyl oxo-bridged technetium(III) complex {(tpy)[Me2(bipy)]Tc7 O7Tc(tpy)[Me2(bipy)]}(OTf)4 was prepared by the reaction of TcCl3(tpy) with thallium triflate in the presence of water.251 The synthesis of diazene technetium complexes was described.252 Thus the TcCl(NNR)2(PPh3)2 complex (R=C6H4Cl-4), which was prepared from TcOCl¡4 and mono- substituted hydrazine RNHNH2 in methanol, reacted with the bidentate S2CNR2 ligand and maltol to give Tc(NNR)L2(PPh3) and TcCl(NNR)L(PPh3) (L=S2CNR2) in high yields.252 The template synthesis from tetrabutylammonium pertech- nate (NBu4)(TcOCl4) or (NH4)2(TcXO) (X=Cl or Br) followed by reduction by divalent tin afforded seven-coordinate Tc(III) complexes, for example, such as the monocapped adduct of boric acid with technetium(III) trisdioximate or TcX(dioxime)3BR (X=Cl or Br; dioxime=dimethylglyoxime or cyclohexanone dioxime; R=Me or Bu).253 A technetium(III) complex containing acetonitrile, viz., TcCl3(MeCN)(PR3)2 (R=Ph or C6H4Me-3), was prepared by reduction of TcCl4(PPh3)2 with zinc in acetonitrile in the presence of PPh3.This complex is a convenient intermediate in the synthesis of other Tc(III) compounds. Its reactions with bipy, 1,10-phenan- throline (phen) and tpy afforded dicationic Tc(III) complexes, viz., [Tc(bipy)3]2+, [Tc(phen)3]2+ and [Tc(tpy)3]2+, respectively, as salts with BPh¡4 or PF¡6 .254 2. Complexes with sulfur- and oxygen-containing ligands Reduction of [Tc(OH)O(dmpe)2]2+ in the presence of an excess of toluene-3,4-dithiol (H2tdt) gave rise to the thiolate Tc(IV) complex [Tc(tdt)(dmpe)2](PF6) (tdt=MeC6H3S2-3,4).255 Its structure was established by spectroscopic methods and X-ray diffraction analysis.The coordination polyhedron about technetium is inter- mediate between the octahedron and the trigonal prism [the Tc7S B I Kharisov, MA Mendez-Rojas and Tc7P distances are 2.318(6) A and 2.902(7) A, respectively; the S7Tc7S angle is 84.49(4) 8]. Refluxing of the salt (NBu4)(TcOCl4) in alcoholic solution with dihydrooxazoles and dihydrothiazoles, for example, with 2-(2-hydroxyphenyl)-4,5-dihydrooxazole, 2-(2-hydroxy-3-meth- ylphenyl)-4,5-dihydrooxazole or 2-(2-hydroxyphenyl)-4,5-dihy- drothiazole, yielded the six-coordinate complexes TcOClL2 (L is the corresponding (hydroxyphenyl)dihydrooxazole or -thiazole).256 The reaction of ammonium pertechnate with 3,5-di-tert- butylpyrocatechol (DBCat) in methanol afforded a mixture of the TcVI(DBCat)3 and TcVI(DBCat)2(DBAP) complexes (DBAP is the amidophenoxide ligand).257 The amidophenoxide ligand is formed through condensation of ammonia (from ammonium pertechnate) with pyrocatechol to give the Schiff's base.Accord- ing to the data of EPR spectroscopy and X-ray diffraction analysis, technetium in the complex exists in oxidation state +6, which is very untypical. The DBCat ligand serves both as a reducing and chelating agent. 3. Complexes with nitrogen- and sulfur-containing ligands The HTcO(cysteine)2 complex (47) and its barium salt Ba[TcO(cysteine)2]2 were synthesised.258 These compounds are of importance in biology and medicine. The properties of these complexes were studied.NH2 . HCl NH4OAc, pH 7.4 Bu4NTcOCl4+ HSCH2CHCO2HO H2 N H2 O N OH Tc S S O O 47 The complex 47 can be synthesised starting from ammonium pertechnate and cysteine. All previous attempts to prepare tech- netium complexes with cysteine afforded products contaminated by polymeric compounds.258 It was also reported 259 ± 263 that chelation of technetium with polyfunctional ligands was accom- panied by the formation of polymeric by-products. It is believed that an excess of the ligand causes decomposition of the initially formed chelate. The use of S-protective groups (for example, benzyl, acetylaminomethyl or benzoylaminomethyl) prevents the formation of undesirable polymeric products.258 The technetium (99mTc) analogue of oxorhenium bis-cystei- nate possesses valuable biological properties.In particular, it is accumulated in kidneys,264 which is useful in diagnostics of the morphological status of this organ.258 Potential radiopharmaceuticals for diagnostics of the renal function were synthesised with the use of chelating carboxyl- containing ligands bearing N- and S-centres . The carboxyl groups favour the renal uptake of these compounds.264 The 99TcO(ECH3) complex was prepared based on the ammonium saltNH4(TcOX4) (X=Hal) and (2R,7R)-2,7-dicarboxy-3,6-diaza-1,8-octanedi- thiol (ECH3). In the isostructural rhenium(V) complex, one of the carboxyl groups in the trans position with respect to the oxo ligand is coordinated in an unexpected fashion.265 The choice of other ligands, viz., Schiff's bases, was governed by their tendency to be coordinated to technetium in various unusual oxidation states.The chemistry of technetium(I) com- plexes is poorly known. Only a few compounds were identified and characterised. The reaction of (PPh3)2(CO)3Cl with the lithium salt of Schiff's base, viz., N-ortho-hydroxybenzylidene-2- thiazolylimine, in boiling THF gave rise to the Tc(I) complex with composition Tc(PPh3)2(CO)2[(C3H2NS)N=CHC6H4O]. This six-coordinate complex has a distorted octahedral structure with trans-PPh3 and cis-CO groups and one chelating bidentate anion.266 Complexes containing biologically active molecules, such as peptides, proteins or antibodies, as ligands can be used as uniqueState-of-the-art coordination chemistry of radioactive elements target-specific radiopharmaceuticals in diagnostics.The reaction of [TcO4]7 with SnCl2, sodium gluconate and RP294 produced the 99Tc(V) oxo complex with RP294, which exists as the syn and anti isomers. The study of the crystal structure of the isostructural rhenium(V) complex demonstrated that the isomerism results from the positions of the CH2OH groups in the serine residues. At room temperature, the isomers in solution undergo intercon- version. The 99mTc and rhenium complexes with RP294 exhibit similar chemical properties.267 The 99Tc and 99mTc complexes with new tetradentate N2S2 and NS2 ligands were prepared by refluxing a methanolic solution of (TcOCl4)(NBu4) with the corresponding NS3H3 proligands.The technetium(V) complex [TcO(NS3)](NBu4) (48) was obtained in high yield. However, compounds of this type are unstable and decompose in a matter of several hours or days. By analogy with the crystal structure of the related rhenium oxo complex, these compounds can be considered as square-pyramidal complexes containing the oxygen atom in the apical position.268 O S Tc S S S 48 4. Complexes with phosphorus-containing ligands Phosphine 99mTc-labelled derivatives can be used for obtaining images of internal organs, for example, heart images. Complexes of the [TcO2(PR3)3](BPh4) (R=Et or Pr) type 234 have distorted trigonal-bipyramidal structures with two oxo ligands located in a single plane. Salts of the [TcO2(PR3)3]+ cations are convenient starting compounds for the synthesis of other complexes with mixed ligands.Py [TcO2(PR3)2(Py)2]+ [TcO2(PR3)2]+ MeOH The trimethylphosphine derivative [TcO2(PMe3)2(Py)2]+ can be prepared according to a one-pot procedure directly from pertechnates [TcO4]7. Apparently, this procedure can be adapted for the preparation of 99mTc radiopharmaceuticals in hospitals. NH4(TcO4) Py, PMe3 [TcO2(PMe3)2(Py)2]+ MeOH The [TcO2(PR3)2(Py)2]+ derivatives are diamagnetic, which is indicative of a noticeable deviation of their geometry from the ideal octahedron.234 The reactions of pertechnate with derivatives of phosphino- carboxylic acids afforded the TcL3 complexes [L is 2-(diphenyl- phosphino)benzoic acid, 3-(diphenylphosphino)propionic acid or (diphenylphosphino)acetic acid].According to the results of spectroscopic studies and X-ray diffraction analysis, these com- plexes possess a distorted octahedral geometry with the mer configuration in which the donor atoms of the same type are in trans positions with respect to each other and the phosphorus atom is in the trans position with respect to the oxygen atom.269 Complexes of the short-lived 99mTc isotope were prepared analogously. Their physical and chemical properties are similar to those of 99Tc complexes. Biological assays revealed significant brain uptake of these compounds. Chelate technetium(V) complexes with bis(o-hydroxyphenyl)- phenylphosphine and (o-hydroxyphenyl)diphenylphosphine were prepared by metathesis of the corresponding Tc(V) precursors or by reduction ± ligand exchange with ammonium pertechnate.270 These complexes combine a soft phosphine donor site and two hard phenoxide centres.It was expected that this fact would stabilise technetium in intermediate oxidation states. 879 The [Tc(tu-S)6]Cl3 .4H2O complex (tu-S is thiourea) was used as the precursor of [Tc(dppe)2(ButNC)2](PF6) (dppe is diphenyl- phosphinoethane).271 This complex can be prepared by mixing both ligands and a source of technetium(III) in ethanol followed by refluxing of the reaction mixture. This procedure is more convenient than the method involving a sodium amalgam, dppe and TcCl4(PPh3)2.In the [Tc(tu-S)6]Cl3 .4H2O complex, the coordination polyhedron about the technetium atom is a distorted tetrahedron, the isocyanide ligands being in trans positions with respect to each other. Treatment of [TcO4]7 with an excess of the corresponding S,P-bidentate phosphinothiolate ligands, such as 2-(diphenyl- phosphino)ethanedithiolate, 2-(diphenylphosphino)propanethio- late or 2-(diphenylphosphino)thiophenoxide, yielded neutral technetium(III) complexes.272 These complexes have a trigonal- bipyramidal geometry, two phosphorus donor centres (consisting of two chelate rings) occupying axial positions and being in trans positions with respect to each other. 5. Complexes with the metal ± metal bond, carbonyl complexes The data on complexes containing multiple Tc7Tc bonds are scarce. Thus several diamagnetic phosphine ditechnetium(II) complexes of the Tc2Cl4(PR3)4 type (49) were described.160 These complexes contain a triple bond between the metal atoms and provide the first examples of phosphine complexes possessing a multiple technetium ± technetium bond. PR3 Cl Cl PR3 PhMe, 50 ± 55 8C or Tc TcCl4(PR3)2 PhH, PhH, utrasonics Cl TcR3P Cl PR349 (>90%) PR3=PEt3, PPrn3 , PMePh2, PMe2Ph. n The polymeric chain structure of [Tc2Cl6]2n¡ also contains triple metal ± metal bonds.273 The triple Tc:Tc bond was found in the a and b forms of the Tc2Cl4(dppe)2 complex.274 The a isomer 50 adopts an eclipsed conformation; the Tc7Tc distance is 2.15(1) A.In the b isomer 51, the twist angle is 35(2) 8 and the Tc7Tc distance is 2.117(1) A. These isomeric complexes were prepared by refluxing Tc2Cl4(PR3)4 (PR3=PEt3 or PMe2Ph) in toluene in the presence or in the absence of an excess of dppe, respectively. L X L X L X L X Tc Tc Tc Tc X L X L X L L X 51 50 L=dppe, X=Cl. Complexes containing the multiple Tc7Tc bond of order 3.5 were synthesised in high yields by one-electron chemical oxidation of the Tc2Cl4(PMe2Ph)4 complex with ferrocenium hexafluoro- phosphate in acetonitrile.275 In this study, the cationic [Tc2Cl4(PMe2Ph)4](PF6) complex was prepared. 275 Oxidation in the presence of bis(triphenylphosphine)iminium produced the neutral complex.275 Tc2Cl5(PMe2Ph)3 The complexes Tc2Cl4(PR3)4 49 (PR3=PEt3, PMePh2 or PMe2Ph) reacted with molten diphenyl- or di-p-tolylformamidine to give mixtures of formamidine-bridged complexes of the general formula Tc2(L)4Cln (n=1 or 2) in moderate yields.276 Complexes with the triple Tc:Tc bond, for example [Tc2(MeCN)10](BF4)4,277 in an acetonitrile solution can undergo photodissociation to give the [Tc(MeCN)6]2+ ions in almost quantitative yields.278 A decaace- tonitrile binuclear complex with the triple Tc:Tc bond was obtained in good yield by acidification of a solution of Tc2Cl4(PR3)4 in a mixture of acetonitrile and dichloromethane using HBF4 .Et2O.277 The [Tc2Cl8]n7 (n=2, 3 or 4) and880 [Mo2Cl8]m7 (m=4 or 5) clusters were studied by theoretical methods.279 Of other types of Tc complexes, noteworthy are Tc carbonyl derivatives.280 ± 282 The Tc(CO)5I complex, which is isostructural to the Mn(CO)5I complex, exists as orthorhombic crystals. Its crystal structure consists of Tc(CO)5I molecules. The crystals of [Tc(CO)4I]2 are monoclinic and are also built of individual [Tc(CO)4I]2 molecules. Complexes with composition [M(CO)4X]2 (M=Mn, Tc or Re; X=Cl, Br or I) are isostruc- tural. The main types of technetium complexes are given in Table 9. * * * In spite of the difficulties associated with operations with radio- active elements, their complexes are being studied intensively. The development of the coordination chemistry of these compounds, particularly, of those containing radioactive elements in unusual oxidation states,24 is dictated both by the possibility of their Table 9.Synthesis of technetium complexes with oxygen-, sulfur-, nitrogen- and phosphorus-containing ligands. Reagent and reaction conditions Starting compound Complexes with oxygen- and sulfur-containing ligands But But OH, MeOH KTcO4 OH LiOC6H4CH=N(CSNC3H2), THF Tc(PPh3)2(CO)3Cl (1) Bu3SnH or Zn (2) MeCN, HBF4 Tc(CO)5X H2O (X=Cl, Br, I) Complexes with nitrogen-containing ligands Tc2Cl4(PR3)4 (PR3=PEt3, PMe2Ph, PMePh2) [TcCl6]27 (1) Bu3SnH or Zn (2) MeCN, HBF4 Cys b (Bu4N)(TcOCl4) TcCl3(PPh3)2(MeCN) But3(tpy), DME , MeOH NH4TcO4 N NHNH2 . HCl Complexes with phosphorus-containing ligands Tc2Cl4(PMe2Ph)4 (Cp2Fe)(PF6) NH4(TcO4) PR3 , MeOH (R=Et, Pr) (see d) [TcO2(PR3)3]+ (R=Et, Pr) TcCl3(PPh3)2(MeCN) TcCl4(PPh3)2 PR3, MeOH, Py (R=Et, Pr) TMEDA, PhMe, DME Py PR3 (R=Et, Prn), THF Zn, PhH, utrasonics TcCl4(PEt3)2 a The yield was 50%, the Tc7Tc multiple bond is present; b Cys is an aqueous solution of cysteine chloride monohydrate; c the multiple Tc7Tc bond is present; d the yield was 60% ± 70%; a *10-fold excess of the ligand was used; there was no need for the use of reducing agents; the reaction was accompanied by the formation of NH3.B I Kharisov, MA Mendez-Rojas practical use and theoretical interest. Thus considerable recent attention has been given to bimetallic complexes of 5f elements in connection with a search for new unusual properties and molec- ular structures.Investigations and a search for new uranium(V) complexes remain to be of great importance. Information 166, 167 on the unique activities of alkylcyclopentadienyl derivatives Cp2 AnR2 (An=Th or U; R=H or Alk) as catalysts of alkene hydrogenation and polymerisation gave impetus to studies in this field of chemistry. These compounds proved to be ten times more active than traditional Pt/SiO2 catalysts. Hence, actinide com- plexes are of interest from the viewpoint of extension of the scope of their use in catalysis, as the starting compounds in organo- metallic synthesis 17, 24 and for separation and extraction of actinides.283 We are grateful to Professor M Ephritikhine (Service de Chimie Moleculaire, France) for permission to reproduce schemes from his articles and Professors A D Garnovskii (Rostov State University, Russia) and S S Berdonosov (Moscow State Univer- sity, Russian Federation) for valuable advice. Ref.Products 257 Tc(DBCat)3, Tc(DBCat)2(DBAP) 266 Tc(PPh3)2(CO)2 . 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ISSN:0036-021X    

出版商:RSC    

年代:2001

数据来源: RSC

摘要:

Russian Chemical Reviews 70 (10) 885 ¡À 895 (2001) Recent trends in the chemistry of sulfur-containing reducing agents S V Makarov Contents I. Introduction II. The structure of molecules of sulfur-containing reducing agents III. Classification of sulfur-containing reducing agents and the fields of their application IV. Methods of synthesis of sulfur-containing reducing agents V. The kinetics and mechanisms of reactions involving sulfur-containing reducing agents VI. Conclusion Abstract. reactivity and stability synthesis, structure, the on Data Data on the structure, synthesis, stability and reactivity of D bonds r with agents reducing sulfur-containing of sulfur-containing reducing agents with C7S o S or S7S bondsD sodium and hydroxymethanesulfinate sodium dithionite, sodium dithionite, sodium hydroxymethanesulfinate and thiourea thiourea oxides and anaerobic of Reactions surveyed.are D oxides D are surveyed. Reactions of anaerobic and aerobic aerobic decomposition are agents reducing sulfur-containing of decomposition of sulfur-containing reducing agents are discussed. discussed. The of studies the in compounds these of applications The applications of these compounds in the studies of non-linear non-linear phenomena in chemical kinetics and in guanidine syntheses are phenomena in chemical kinetics and in guanidine syntheses are considered. references 165 includes bibliography The considered. The bibliography includes 165 references. I. Introduction Compounds containing an S7S or C7S bond, namely, sodium dithionite, sodium hydroxymethanesulfinate (HMS, commercial name rongalite) and thiourea dioxide (TUDO), have long been used in chemistry and chemical technology as reducing agents.The traditional fields of application including printing and dyeing of textiles,1 production of synthetic rubber,2 manufacture of uranium and transuranium element compounds,3 preparative organic 4 and inorganic 5 chemistry have been covered fairly comprehensively in monographs.6, 7 In recent years, some new fields of application appeared and the above-mentioned fields (mainly, organic synthesis 8, 9) have been further developed. In particular, increasing numbers of publications are devoted to the use of these compounds in biochemistry,10 ¡À 12 in organofluorine chemistry,13 and in investigations of non-linear phenomena in chemical kinetics.14, 15Apromising field of application of thiourea dioxides and the products of their oxidation, trioxides, is the synthesis of guanidines.16 ¡À 20 The researchers' interest in the guanidine properties, which has increased in recent years,21, 22 is due to the discovery of an important biological function of nitrogen oxide, the precursor of which in an organism is L-arginine (2-amino-5-guanidinovaleric acid), and to the search for new medical drugs.18, 20 Thus, there is a need for a review that would reflect the state-of-the-art of the chemistry of sulfur-containing reducing agents.S V Makarov Ivanovo State University of Chemistry and Technology, prosp.F Engelsa 7, 153460 Ivanovo, Russian Federation. Fax (7-093) 241 79 95. Tel. (7-093) 232 73 97. E-mail: makarov@icti.ivanovo.su Received 13 February 2001 Uspekhi Khimii 70 (10) 996 ¡À 1007 (2001); translated by Z P Bobkova #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n10ABEH000659 885 885 886 888 888 893 II. The structure of molecules of sulfur-containing reducing agents 4 6 5 6 44 4 4 5 6 The molecular structures of sodium dithionite, HMS and TUDO have been studied in detail by IR and Raman spectroscopy and by X-ray diffraction analysis. The structure of sodium dithionite has aroused the greatest interest. Seemingly, the structure of the S2O2¡¦ ion should resemble the structures of well-known sulfur-and- oxygen anions with the S7S bonds, e.g., S2O2¡¦ and S2O2¡¦ (see Ref.23). One might expect that S2O2¡¦ and S2O2¡¦ ions would correspond to D3d and C2h point groups of symmetry, respec- tively. This prediction came true only for the latter anion, while for sodium dithionite, unexpected results were obtained. According to X-ray diffraction data,24, 25 the S2O2¡¦ ion belongs to the C2u point group and contains an abnormally long S7S bond (2.39 A). Later, it was shown by Raman spectroscopy 26, 27 that in aqueous solutions of sodium dithionite, the structure of the S2O2¡¦ ion changes, namely, it becomes centrosymmetrical and corresponds to the C2h point group. The influence of the nature of the salt cation on the structure of the dithionite ion has been studied.23 It was shown that the `abnormal' eclipsed conformation (C2u) of S2O2¡¦ can be found in salts with small cations, for example, with sodium. In the salts with large cations, in particular, in tetraethyl- ammonium dithionite, this anion is centrosymmetrical both in the solid state and in solution.Thus, the abnormal structure of sodium dithionite in the solid phase is due to the influence of the cation and to the associated packing effects 23 rather than to the properties of the S2O2¡¦ 4 anion itself. These experimental data were substantiated theoretically by the results of LCAO-Xa density functional calculations.28 It was shown that the curve for the total energy vs. S7S bond length for dithionite ion, unlike these curves for S2O2¡¦ or S2O2¡¦ ions, has a very broad minimum.This indicates that the S7S bond length in S2O2¡¦ 4 depends appreciably on external factors, in particular, on the packing effects in the crystal. It was also found that the barrier to rotation in the dithionite ion is very low. This accounts for the easy dehydration of Na2S2O4 . 2H2O and the concomitant change in the dithionite structure (for example, the S7S bond length increases from 2.298 A in the dihydrate to 2.393 A in the anhydrous salt). Therefore, it can be expected that the properties of solid dithion- ites should depend on the nature of solvents from which they have been crystallised. Indeed, it has been demonstrated in relation to lithium dithionite 29 that the products isolated from aqueous and tetrahydrofuran solutions have different properties; in particular, the latter is extremely susceptible to oxidation and to self-ignition in air.When lithium dithionite is stored in a sealed capillary, its886 2 2 2 crystal lattice is transformed giving rise to defects, and this accelerates solid-phase processes. A similar transformation but taking place at elevated temperatures was observed for the sodium salt. Thus, conditions used to prepare dithionites affect appreci- ably their properties. A substantial dependence of the stability on the nature of the solvent used for recrystallisation is also typical of solid sodium hydroxymethanesulfinate.30 Due to the long and rather weak S7S bond, dithionites tend to undergo homolytic decomposition giving rise to the SO¡ radical anions.The structural parameters of the SO¡ radical anion have been determined;31 the (r) S7O distance is 1.5230.02 A, the O7S7O angle (aO7S7O) is 115.62.0 8. The structures of the SO¡ radical anion and the NaSO2 charge transfer complex were also determined by ab initio calculations.32 According to X-ray diffraction studies,33 HMS is the sodium salt of hydroxymethanesulfinic acid, HOCH2SO2Na. The crystal lattice parameters of HMS and the interatomic distances in the HOCH2SO¡2 anion were determined; the C7S bond was found to be the longest one (1.838 A). X-Ray diffraction analysis of TUDO was first performed by Sullivan and Hargreaves;34 later, the data were refined.35, 36 In the solid phase, TUDO exists as (NH2)2CSO2.The CSO2 group is pyramidal, the S7O and C7N bond lengths are 1.496 and 1.296 A, respectively, and the C7S bond (1.867 A) is much longer than this bond in the thiourea (1.716 A) molecule. The bond lengths in theTUDOmolecule found from X-ray diffraction data are in good agreement with the results of ab initio calcu- lations.36 It was suggested that TUDO is a combination of two zwitter-ion forms O O7 H2N + and C S . H2N +C +S O7 O7 H2N H2N In these ions, the negative charge is concentrated on the oxygen and nitrogen atoms and the positive charge is on the hydrogen, carbon and sulfur atoms, mainly, on sulfur. The results of calculations indicate that nucleophilic attack is directed at the carbon and sulfur atoms.The structure of thiourea dioxide depends substantially on the solvent. It was found 37 that after dissolution of TUDO in water, the acidity increases, a constant pH value being established very slowly. It was found in special experiments that these changes are not related to decomposition of TUDO or to the presence of oxygen. The 1H NMR spectra of aqueous 38 and dimethyl sulfoxide 39 solutions of TUDO were found to vary with time. The peak observed initially in the TUDO spectrum is rapidly split into a doublet, these changes taking place only in the first minutes after dissolution of thiourea dioxide. Quantum-chemical calculations by the AM1 method for TUDO in the (NH2)2CSO2 and NH2C(=NH)SO2H (aminoiminometha- nesulfinic acid) forms 40 showed that solvation with water or dimethyl sulfoxide decreases substantially the total energy of the system.Note that unlike the gas phase, in aqueous solutions, aminoiminomethanesulfinic acid is the thermodynamically more stable form. Thus, quantum-chemical calculations imply the possibility of TUDO tautomerisation in aqueous solutions. The tautomeric transformations of TUDO in aqueous solutions have been studied theoretically using the method of critical points on the potential energy surface.41 The most probable mechanism of tautomerisation includes stages of successive intermolecular pro- ton transfer in TUDO oligomers and decomposition of the oligomers to give solvated monomers of aminoiminomethanesul- finic acid.The structure of thiourea trioxide (TUTO) has also been studied.42 It was found that the C7N bonds in TUTO are virtually equivalent (1.298 and 1.297 A) and are much shorter than the typical C7Nsingle bond (1.470 A), while the C7S bond length (1.815 A) is slightly greater than the sum of the covalent radii of the S and C atoms (1.790 A). The tetrahedral S atom forms three nearly equivalent S7O bonds (1.439, 1.431, and S V Makarov 2 1.446 A). The structures of thiourea di- and trioxides are stabilised by intermolecular hydrogen bonds.36, 42 The unusually high den- sity calculated 42 for TUTO, 1.948 g cm73 (the density of thiourea dioxide is 36 1.70 g cm73) points to a highly efficient packing of (NH2)2CSO3 molecules in the crystal lattice.Apparently, the structure of TUTO is similar to that of TUDO. The research into the structures of sodium dithionite, HMS and TUDO showed that each of these molecules contains an abnormally long bond, which is, hence, highly prone to rupture (S7S bond in the dithionite and C7S bonds in HMS and TUDO). It is this structural feature that accounts for the high reducing activities of these compounds. As will be shown below, the chemical reactivities of dithionites, HMS, TUDO and their analogues are governed by transformations of the same inter- mediates, namely, sulfoxylic acid H2SO2 or its anions and the SO¡ radical anion. This fact, together with the similarity of methods for the use of these compounds explain why they have been combined in one group.III. Classification of sulfur-containing reducing agents and the fields of their application The sulfur-containing reducing agents considered here (Table 1) can be classified into dithionites, a-hydroxyalkanesulfinates, and thiourea dioxides. Thiourea trioxides are also included in Table 1. Data on a-aminoalkanesulfinates not reflected in the Table can be found in a monograph.7 Note that the properties of most of compounds listed in Table 1 have been little studied. The only exceptions are sodium dithionite, HMS and TUDO, which have found wide practical use. Sodium dithionite and hydroxymethanesulfinate are used most often for the reduction of vat dyes in textile industry.1 The processes involving these reducing agents form the basis of all industrial printing and vat dyeing techniques.The textile industry also makes use of zinc hydroxymethanesulfinates, which possess a lower reducing activity than HMS.6 Conversely, a-hydroxyetha- nesulfinate is a stronger reducing agent than HMS.7 An important application of HMS and TUDO is the manu- facture of synthetic rubber.2 For instance, a mixture of HMS with isopropylbenzene hydroperoxide and ethylenediaminetetraaceta- toferrate(III) initiates copolymerisation of butadiene with styrene. The Fe2+ ±TUDO±H2O2 system is used to initiate polymer- isation of various vinyl monomers.59 Other effective initiating systems are TUDO±KBrO3 60 and TUDO±KMnO4.61 In addi- tion, TUDO is used for waste paper processing 62 and for wool bleaching.63 Numerous publications have been devoted to the use of the reducing agents in question, mainly sodium dithionite, in biochemistry.10 ± 12, 64 ± 66 Sodium dithionite is used for the reduc- tion of many natural and synthetic electron transfer agents including nicotinic acid derivatives, flavins and cytochromes;64, 65 TUDO is used for reduction of ferredoxin, cytochrome C and other substrates 66 and for inactivation and modification of cytidine triphosphate synthase.11, 12 N,N0-Dimethylthiourea is a highly efficient antioxidant, which has found wide use in bio- chemical and medical research.67 Thus the reaction of hydrogen peroxide with N,N0-dimethylthiourea, resulting in the dioxide, can be used to determine the content of H2O2 in biological systems.55 Sulfur-containing reducing agents are widely used in organic synthesis.Thus sodium dithionite serves as the reagent for the reduction of nitro, nitroso, azido and azo compounds, aldehydes, ketones, keto esters, benzyls, quinones, heterocycles, and com- pounds with double bonds as well as for the synthesis of sulfones.4 Hydroxymethanesulfinate and thiourea dioxide are used in sim- ilar processes, in particular, in the preparation of a light-stabiliser for plastics, Benazol P,68 a stabiliser for motor oils called Diafen FP 69 and sulfones.70 ± 74 In recent years, the possibility of using these reducing agents in the chemistry of organofluorine com- pounds has been studied intensively.13, 75 Thiourea dioxide is also employed to reduce organosulfur compounds (sulfylimines, sulf-Recent trends in the chemistry of sulfur-containing reducing agents Table 1.Sulfur-containing reducing agents. Name of compound Dithionites Sodium dithionite Potassium dithionite Lithium dithionite Zinc dithionite Tin dithionite Tetraethylammonium dithionite a-Hydroxyalkanesulfinates Sodium hydroxymethanesulfinate (rongalite) Disodium hydroxy- methanesulfinate Zinc hydroxymethanesulfinate Zinc hydroxyl hydroxy- methanesulfinate Zinc hydroxymethanesulfinate, disubstituted Calcium hydroxymethanesulfinate, disubstituted Titanium hydroxymethane- sulfinate Barium hydroxymethanesulfinate, disubstituted Lead(II) hydroxymethanesulfinate, disubstituted Mercury(II) hydroxy- methanesulfinate, disubstituted Sodium a-hydroxyethanesulfinate Sodium a-hydroxytrifluoro- ethanesulfinate Potassium a-hydroxyethanesulfinate MeCH(OH)SO2K 6 Sodium a-hydroxypropanesulfinate EtCH(OH)SO2Na Potassium a-hydroxy-2-propane- sulfinate Sodium a-hydroxy-n-butane- sulfinate Thiourea dioxides Thiourea dioxide N-Methylthiourea dioxide N-Propylthiourea dioxide N-Isopropylthiourea dioxide N-Butylthiourea dioxide N-But-2-ylthiourea dioxide N-tert-Butylthiourea dioxide N-Neopentylthiourea dioxide N-Hydroxymethylthiourea dioxide N-Hexylthiourea dioxide N-Dodecylthiourea dioxide N-Phenylthiourea dioxide N-p-methylphenylthiourea dioxide N-o-hydroxyphenylthiourea dioxide N-o-methoxyphenylthiourea dioxide N-2,6-Dimethylphenylthiourea dioxide Name of compound Ref.Formula Thiourea dioxides N-2,6-Diethylphenylthiourea dioxide N-2,6-Diisopropylphenyl- thiourea dioxide N-Guanylthiourea dioxide 7 43 29 44 25 45 Na2S2O4 K2S2O4 Li2S2O4 ZnS2O4 Sn(S2O4)2 (Et4N)2S2O4 7 HOCH2SO2Na 6 NaOCH2SO2Na 46 47 (HOCH2SO2)2Zn HOCH2SO2Zn(OH) OCH2SO2Zn OCH2SO2Ca (HOCH2SO2)4Ti N-Diphenylmethylthiourea dioxide N-Triphenylmethylthiourea dioxide N,N-Dimethylthiourea dioxide N,N-Dipropylthiourea dioxide N,N-Dibutylthiourea dioxide N,N-Di(2-methylpropyl)thiourea dioxide N,N0-Dimethylthiourea dioxide N,N0-Dibut-2-ylthiourea dioxide N,N0-Diphenylthiourea dioxide N,N0-Di-o-methylphenylthiourea dioxide N,N0-Dicyclohexylthiourea dioxide (cyclo-C6H11NH)2CSO2 OCH2SO2Ba Thiourea trioxides OCH2SO2Pb 666666 OCH2SO2Hg 48 49 MeCH(OH)SO2Na CF3CH(OH)SO2Na Thiourea trioxide N-Methylthiourea trioxide N-Propylthiourea trioxide N-But-2-ylthiourea trioxide N-tert-Butylthiourea trioxide N-Allylthiourea trioxide 76 PrnCH(OH)SO2Na N-Phenylthiourea trioxide Me2C(OH)SO2K 6 N-Benzylthiourea trioxide N-o-Methylphenylthiourea trioxide N-o-Chlorophenylthiourea trioxide N-p-Fluorophenylthiourea trioxide N-2-Isopropylphenylthiourea trioxide 36 50 50 50 N-2,6-Dimethylphenylthiourea trioxide N-2,6-Diethylphenylthiourea trioxide (NH2)2CSO2 MeNHC(NH2)SO2 PrnNHC(NH2)SO2 Me2CHNHCSO2 NH2 BunNHC(NH2)SO2 Et(Me)CHNHC(NH2)SO2 Me3CNHC(NH2)SO2 Me3CCH2NHC(NH2)SO2 HOCH2NHC(NH2)SO2 50 50 50 50 51 N-2,6-Diisopropylphenylthiourea trioxide N-2,6-Dichlorophenylthiourea trioxide N,N-Dimethylthiourea trioxide 52 52 51 53 C6H13NHC(NH2)SO2 C12H25NHC(NH2)SO2 PhNHC(NH2)SO2 p-MeC6H4NHC(NH2)SO2 51 o-HOC6H4NHC(NH2)SO2 51 50 o-MeOC6H4NHCSO2 NH2 2,6-Me2C6H3NHCSO2 NH2 2,6-Cl2C6H3NHCSO3 NH2 Me2NC(NH2)SO3 N-Ethyl-N-phenylthiourea trioxide Ph(Et)NC(NH2)SO3 N,N0-Diethylthiourea trioxide N,N0-Diisopropylthiourea trioxide (Me2CHNH)2CSO3 N,N0-Dibut-2-ylthiourea trioxide N,N0-Di-tert-butylthiourea trioxide (Me3CNH)2CSO3 N,N0-Dineopentylthiourea trioxide (Me3CCH2NH)2CSO3 N,N0-Dicyclohexylthiourea trioxide (cyclo-C6H11NH)2CSO3 N,N0-Diphenylthiourea trioxide N,N0-Dibenzylthiourea trioxide 887 Ref.Formula 50 2,6-Et2C6H3NHCSO2 NH2 50 2,6-Pri2C6H3NHCSO2 NH2 NH2CNC(SO2H)NH2 54 NH Ph2CHNHC(NH2)SO2 50 50 Ph3CNHC(NH2)SO2 Me2NC(NH2)SO2 Prn2 NC(NH2)SO2 Bun2 NC(NH2)SO2 (Me2CHCH2)2NCSO2 50 50 50 50 NH2 (MeNH)2CSO2 [Et(Me)CHNH]2CSO2 (PhNH)2CSO2 (o-MeC6H4NH)2CSO2 55 50 51 53 50 17 56 56 56 56 56 (NH2)2CSO3 MeNHC(NH2)SO3 PrnNHC(NH2)SO3 Et(Me)CHNHC(NH2)SO3 Me3CNHC(NH2)SO3 NHCSO3 NH2 PhNHC(NH2)SO3 PhCH2NHC(NH2)SO3 o-MeC6H4NHC(NH2)SO3 56 56 17 17 o-ClC6H4NHC(NH2)SO3 16 p-FC6H4NHC(NH2)SO3 56 56 17 2-Me2CHC6H4NHCSO3 NH2 2,6-Me2C6H3NHCSO3 NH2 2,6-Et2C6H3NHCSO3 NH2 56 2,6-Pri2C6H3NHCSO3 NH2 17 (EtNH)2CSO3 [Et(Me)CHNH]2CSO3 (PhNH)2CSO3 (PhCH2NH)2CSO3 56 57 57 56 56 56 56 56 57 58888 oxides, disulfides),76, 77 for the preparation of a number of com- pounds of selenium and tellurium 78, 79 and deoxygenation of various heteroaromatic N-oxides;8 in addition, it serves as a convenient reductant in phase transfer catalysis.76, 80 Sodium hydroxymethanesulfinate is also utilised in the syntheses of various compounds of selenium and tellurium 81, 82 and for the preparation of sultins.9 An important application of thiourea oxides is the synthesis of guanidines and their derivatives,16 ± 20, 83 ± 85 which consists in the reaction of thiourea oxides with primary 83 or heterocyclic secon- dary amines, for example, morpholine,85 or with amino acids 84 and other amino compounds. The reactions of thiourea oxides with amines are carried out in organic solvents (methanol,83 acetonitrile 85), while the reactions with amino acids take place in aqueous solutions.84 The mechanism proposed for this reaction includes the addition of the nucleophile (i.e., amine) to thiourea oxide and decomposition of the intermediate giving rise to guanidine.85 The use of trioxides is more convenient because it permits the preparation of guanidines 85 and guanidine acids 84 in high yields.Currently, thiourea trioxide is among the most important guanidylating agents used in the syntheses of guani- dines with various structures.18 ± 20, 86 ± 98 Unfortunately, the kinetics of the reactions of thiourea oxides with amino compounds has not yet been studied, despite the fact that the interest in guanidines is very high due to the discovery of the crucial role of nitrogen oxide in functioning of the vascular system.99 ± 101 It was found that it is the guanidine fragment of L-arginine that is oxidised in vivo to give citrulline and nitrogen oxide (in a recent publication,87 the possibility of using thiourea trioxide in the syntheses of L-arginine derivatives is demonstrated).Guanidines have proved efficient in the treatment of cardiovascular dis- eases.100 Thus chemists of the Du Pont company have developed the DuP 714 preparation,18 which prevents thrombus formation; this synthesis was a success only when thiourea dioxide was used to introduce the guanidine group.The attempts to synthesise this preparation using other guanidylating agents failed. Thiourea trioxide 86 ± 98 and N-phenylthiourea trioxide 16 have also been employed for the preparation of other medicinal drugs. Thiourea trioxides are the initial compounds in the syntheses of amino- iminoethanenitriles, 5-aminotetrazoles, N-cyanoguanidines and N-hydroxyguanidines.57 The reaction of (NH2)2CSO3 with methyl anthranylate affords 5H-quinazolino[3,2-a]quinazoline-5,12(6H)- dione.102 Numerous studies have been devoted to the use of reduction reactions involving HMS and TUDO to produce metals � silver,103 cadmium,104 technetium,105 nickel,106 and metallic coat- ings on fibres.107 IV.Methods of synthesis of sulfur-containing reducing agents Metal dithionites are prepared by the reduction of sulfur dioxide with zinc, sodium formate 44 or borohydrides.43 A method for the synthesis of extra pure sodium dithionite containing 99% of the required substance has been developed.108 A procedure for the preparation of tetraethylammonium dithionite using ion exchange resins is documented.45 Several methods for the synthesis of sodium hydroxymetha- nesulfinate are known.6 The preparation ofHMSby the reduction of sodium hydroxymethanesulfonate with disperse zinc is used most often. Thiourea mono-, di- and trioxides are produced upon the oxidation of thiourea with hydrogen peroxide in a neutral medium.7 The synthesis of TUDO is performed at 0 ± 58C.Unlike the dioxide, thiourea monoxide is quite unstable. More stable monoxides are formed from thiourea derivatives containing bulky substituents at the nitrogen atoms. Thus N-phenyl- and ethyl- enethiourea monoxides have been prepared.109 The formation of these compounds in solution was proved by the test reaction with FeCl3, typical of mono-S-oxygenated thioamides and thiocarba- S V Makarov mates. Neutral aqueous solutions of the monoxides synthesised are stable over a period of 24 h at 0 8C; however, these compounds are exceptionally unstable in the solid state. N-Phenyl-, N,N0- diphenyl-, N-o-hydroxyphenyl- and N-o-methoxyphenylthiourea dioxides have been prepared by using sodium molybdate 51 to catalyse the reaction between the corresponding thiourea deriva- tive and H2O2.Dioxides of other N-aryl-substituted thioureas have also been prepared.53 Syntheses of N-aryl- and N-alkyl- substituted thiourea dioxides in 80%± 90% yields without cata- lysts have been reported.50, 110 In order to prepare N-phenyl- thiourea dioxide containing *100% of the required substance, the synthesis should be carried out at 77 to 12 8C.17 An attempt to prepare N,N0-diphenylthiourea dioxide by a procedure pro- posed previously 53 failed.17 Thiourea trioxides can be synthesised by treatment of thioureas or thiourea dioxides with three or one equivalent of peracetic acid, respectively.56 An electrochemical procedure for the synthesis of thiourea trioxides has been pro- posed.111 The major products formed in the reaction of thiourea with hydrogen peroxide in strongly acidic media and in the presence of metal ions are formamidine disulfide salts.112 There is no con- sensus of opinion on the mechanism of the reaction between thiourea and H2O2.The kinetics and the mechanism of the reactions of thiourea and N,N0-dialkylthiourea with hydrogen peroxide in acid media have been studied.113 Presumably, in this case, these thioureas act as nucleophiles by replacing the peroxide oxygen, i.e., the reaction does not involve free radicals. However, the use of spin traps, namely, N-benzylidene-tert-N-butylamine N-oxide and 5,5imethyl-1-pyrroline N-oxide (DMPO) allowed detection of the carbamidinothiyl radical NH2(=NH)CS upon oxidation of thiourea with hydrogen peroxide.114 The stability of this radical was found to depend substantially on the medium pH, the most intense EPR signal being observed at 2.5 ± 3.0.V. The kinetics and mechanisms of reactions involving sulfur-containing reducing agents Decomposition of sulfur-containing reductants and their reac- tions with oxidants are closely interrelated because they proceed via the formation of the same intermediates. Therefore, we shall first consider the results of research into decomposition of sulfur- containing reducing agents. The stability of these compounds in aqueous solutions depends appreciably on the pH. Sodium dithionite and HMS are less stable in acid media.Oxygen-free decomposition of sodium dithionite in acid solutions has been studied in detail.115, 116 Polarography was used to measure the time variation of the concentrations of dithionite and the products of its decomposition, which include sulfide, sulfite, thiosulfate, and active sulfur (Sa); the term `active sulfur' implies atomic sulfur, the S .H2O hydrate, the HSOH thioperoxide, the polymeric biradical and sulfur in a polysulfide chain. Decomposition was found to be autocatalytic, active sulfur and sulfide functioning as the catalysts. The influence of sulfide, sulfite, thiosulfate and sulfur dichloride additives (and also of mixtures of these compounds) on the rate of dithionite decom- position has been studied.115 On the basis of the results obtained, it was concluded that the following reactions are responsible for the dithionite decomposition: �non-catalysed reaction proceeding during the induction period, (1) Sa+3SO2+2H2O; 2H2S2O4 � autocatalytic reaction mainly proceeding at the fast decom- position stage, (2) H2S+5SO2+2H2O.3H2S2O4 Simultaneously, a series of side reactions take place. On the basis of experimental data,115 a mathematical model of the dithionite decomposition was proposed.116 Calculations per- formed in terms of this model made it possible to estimate theRecent trends in the chemistry of sulfur-containing reducing agents contribution of individual homogeneous reactions and to demon- strate that the proposed two-stage scheme is adequate to describe the real mechanism of decomposition.Non-catalysed and catalysed decomposition of sodium dithionite in weakly acidic solutions in the presence of sulfur- containing admixtures has been investigated.117 Particular atten- tion is devoted to the influence of sulfite. In the opinion of the researchers cited,117 in the case of a high sulfite concentration in the solution, decomposition of S2O2¡ 4 can follow two mechanisms, a heterolytic and a homolytic one. The former includes the following stages: (3) [O2S(O2)S7SO2OH]37, S2O2¡ 4 +HSO¡3 [O2S(O2)S7SO2OH]37+H+ S3O2¡ 6 +H2O. (4) In this case, reaction (3) is the rate-determining stage. The 2 trithionate formed in reaction (4) participates, together with the SO¡ radical ion, in the decomposition by the homolytic mecha- nism (5) 2 SO¡ S2O2¡ 2 , 4 (6) S2O5H2¡ , HSO¡3 +SO¡2 (7) S2O5H2¡ +S3O2¡ 6 SO2+HSO¡3 +SO23 ¡ +S2O¡3 , (8) S2O2¡ +SO2.3 S2O¡3 +SO¡2 The reaction between two dianions [reaction (7)] determines the rate of the whole homolytic process. When the sulfite concentration in the solution is low, the process follows a third pathway (9) [O2S(O2)S7OSOOH]37 , (10) S2O2¡ +HSO¡2 , 5 (11) 2 (12) S2O2¡ 4 +HSO¡3 [O2S(O2)S7OSOOH]37 2HSO¡ S2O2¡ 3 +H2O , S2O2¡ 2HSO¡3 . 5 +H2O Unlike the heterolytic mechanism mentioned above, in this pathway, the initial reaction is the transfer of an oxygen atom from the bisulfite ion to the dithionite sulfur atom.A serious drawback of the mechanisms proposed by Holman and Bennett 117 [see reactions (3) ± (12)] is that nothing is said about the role of sulfur or sulfides in the dithionite decomposition. This is explainable for a large excess of sulfite present because in acid media, HSO¡3 rapidly reacts with sulfur and sulfides, which virtually cancels out their effect on the dithionite decomposition. However, in the case of low initial concentrations of sulfite in the solution, the influence of sulfur or sulfide, which are the main products of dithionite decomposition in acid media, cannot be neglected. Decomposition of sodium hydroxymethanesulfinate in acid media has been studied.118, 119 This process is autocatalytic, being accelerated by active sulfur.The catalytic effect of Sa is attribut- able to its influence on the decomposition of sodium dithionite, which is one of the main intermediate compounds formed upon decomposition of HMS. The following sequence of reactions was proposed as taking place during oxygen-free decomposition of HMS in acidic aqueous solutions: 2 HSO¡2 +CH2O, 2 S2O2¡ 3 +H2O, Sa+SO2¡ 3 , 3 HSO¡3 , (13) (11) (14) (15) (16) S2O2¡ 4 +H2O, (5) HOCH2SO¡ 2HSO¡ S2O2¡ SO2¡ 3 +H+ HSO¡2 +HSO¡3 S2O2¡ 2SO¡2 . 4 An important role in the mechanisms of decomposition of sodium dithionite as well as HMS and TUDO is played by reaction (5), i.e., the homolytic cleavage of S2O2¡ 4 . 889 The equilibrium constant of reaction (5) at 298 K is equal to 1.461079 mol litre71 (see Ref.120). Studies of the temperature dependence of this equilibrium constant gave quite contradictory results. Thus it has been found 121 that an increase in the temper- ature from 298 to 353 K results in a 10-fold increase in the concentration of the SO¡2 radical anion, which entails an increase in the equilibrium constant of reaction (5) by a factor of 10. A similar pattern of dependence of the constant on the temperature was also indicated by other researchers.122 However, according to some other studies,120, 123, 124 the increase in the equilibrium constant upon an increase in the temperature is much more gradual. In view of the inconsistency of the data on the temperature dependence of the equilibrium constant, the thermodynamic characteristics of reaction (5) were determined using other methods. x The standard enthalpies of formation (DfH8) of the SO¡ (x=2 ± 4) radical anions were calculated.125 The DfH8(SO¡x , 298) values in an aqueous solution were determined on the basis of thermodynamics data for liquid-phase decomposition of ions of the [R7R]27 type including the dithionite ion: (17) R¡ +R¡ .[R7R]27 The standard enthalpy of formation of the product of this reaction can be estimated using the following relation: DfH8(R¡ , 298) = = 0.5 {DrG8(298)+TDrS8(298)+DfH8([R7R]2¡, 298)}, (18) 4 where DrG8(298) and DrS8(298) are the change in the standard Gibbs energy and the standard entropy in reaction (17) at 298 K, respectively, DfH8([R7R]2¡, 298) is the standard enthalpy of formation of the [R7R]2¡ ion at 298 K.The DrG8(298) (52.31.3 kJ mol71) and DfH8(S2O2¡ 4 , 298) (7753.7 kJ mol71) values for the decomposition of S2O2¡ are known. The uncertainty in TDrS8(298) is due to the absence of data on the standard entropy of formation of the SO¡x anion at 298 K [S f (SO¡x , 298)]. These values were estimated using the empirical dependence of the standard entropy of the [R7R]2¡ oxy anion on its mass (M), charge (Z), the distance between the central atom and the peripheral oxygen atoms (r), and the empirical factor ( f) that takes into account the ion geometry (19) S f (298)76.3 R(lnM) = 2767338.6 (Z/r) , 2 have not been determined yet. 2 4 where R is the universal gas constant, r is an empirical variable equal to r/f .For SO¡2 , the r value was found proceeding from the values r=0.151 nm, f=0.870.11. The S f (SO¡2 ) value at 298 K found on the basis of relation (19) amounted to 13425 J mol71 K71 [for the dithionite ion, S f (S2O24 ¡)= 104.5 J mol71 K71]. The DrS 8(298) value for reaction (5) corre- sponding to these parameters is equal to 163.550 J mol71 K71. The DfH8(Sxcl;2 , 298) value determined from Eqn (18) amounts to 7326.58 kJ mol71. The researchers cited 125 did not compare the resulting DrH8(298) (100.717 kJ mol71) with the values found from the temperature dependence of the equilibrium con- stant of reaction (5). It is noteworthy that this value is close to DrH8(298)=89.1 kJ mol71, presented in a publication.122 Unfortunately, the standard enthalpies and entropies of forma- tion of the sulfoxylate ion SO2¡ Equilibrium (5) in aprotic media has been studied using tetraethylammonium dithionite.45 It was shown by EPR and UV spectroscopy that the S2O2¡ dimer dissociates to give the SO¡ radical anions in DMF, DMSO and acetonitrile, the equilibrium constants in non-aqueous solvents being 107 times as high as that in water.Thus, the equilibrium constant for reaction (5) depends appreciably on the nature of the solvent. Unlike HMS or sodium dithionite, TUDO is much less stable in alkaline solutions than in neutral or acidic solutions. Comparative investigation of the composition of the products of decomposition of thio- and N-phenylthiourea di- and trioxides890 and N,N0-diphenylthiourea trioxide in neutral and alkaline sol- utions has been carried out.17 Ureas are the major products of thiourea dioxide decomposition in alkaline media.Conversely, decomposition of thio- and N-phenylthiourea trioxides affords mainly cyanamides (pH 13 ± 14) (N,N0-diphenylthiourea trioxide is mainly converted into N,N0-diphenylurea). In less alkaline solutions (pH 10), the percentage of ureas in the products of dioxide decomposition decreases, while the content of cyanamides increases. Under similar conditions, thiourea trioxide gives mela- mine in 52% yield; the formation of cyanoguanidine (dicyanodia- mide) was also detected (the formation of substituted triazine related to melamine has been observed previously in the decom- position of N-propylaminoiminomethanesulfonic acid in the presence of tert-butylamine 16).Presumably, a product of self- condensation of TUTO is formed intermediately in the reaction of melamine formation. The essentially different decomposition mechanisms of thio- urea dioxides and trioxides and, as a consequence, different compositions of the reaction products have been attributed,17 first of all, to the different stabilities of NH2NHRCSO2 and NH2NHRCSO3 in alkaline media (R=H, Ph). The trioxide is assumed to be much more stable in solution than the dioxide; however no experimental evidence supporting this assumption is available. No data on the sulfur-containing products of decom- position of thiourea oxides are available either. 2 It has been assumed 126 that an important role in the decom- position of TUDO is played by dithionite ions and the SO¡ radical anions.The appearance of these species in alkaline solutions ofTUDOwas detected by polarography and interpreted as being due to the reaction (20) (NH2)2CSO2+OH7 HSO¡2 +(NH2)2CO, as well as reactions (5) and (16) presented above. Presumably,126 decomposition of TUDO gives sulfoxylate as the primary product [however, it is not indicated what process is responsible for the formation of sulfite needed for the formation of dithionite by reaction (16)]. Yet another mechanism of TUDO decomposition has also been proposed;104 according to this mechanism, the strong reducing properties of TUDO in alkaline media are due to the homolytic decomposition of the H2N(=NH)CSO¡2 anion to give reactive species, SO¡2 and (NH2)2C OH.It has also been suggested 127 that the SO¡2 radical anion (in the aqueous alkali ± ethanol system) is formed as the primary product of TUDO decomposition. Unfortunately, in most of the above-mentioned studies, no integrated approach is used to investigate the processes of decom- position of sulfur-containing reducing agents. This is especially true for thiourea oxides. The transformations of their `sulfur' and `nitrogen' components have been studied independently by chem- ists working in different fields. The former, `sulfur' part has been studied by inorganic chemists and specialists in chemical kinetics, while the latter, `nitrogen' part has been an object of investigations of organic chemists and biochemists. These investigations have not been connected with one another and, moreover, the results have been published in journals of different disciplines. The fewness of kinetic data concerning the processes of decomposition of sulfur-containing reducing agents also appears surprising.Data on the decomposition mechanisms are fragmentary and often contradictory. Unfortunately, in many publications, no attention is paid to the role of oxygen in the decomposition of the compounds in question. Often, it is even impossible to grasp whether the reaction was carried out in an inert atmosphere or in air (this is especially typical of papers in organic chemistry).It will be shown below that it is the effect of oxygen on the composition of the intermediate and final products in the decomposition of sulfur-containing reducing agents that is the main reason for the contradictions mentioned above. Let us consider the reactions of sulfur-containing reductants with oxygen. Creutz and Sutin,128 who confirmed the previously proposed 120 mechanism of the dithionite reaction with oxygen, S V Makarov (5) 2SO¡ S2O2¡ 2 , 4 (21) products, SO¡2 +O2 showed also that the process rate is determined by reaction (5), the rate constant for the forward reaction being virtually independent of the pH: k=2.5 (pH 6.5) and 1.8 s71 (pH 13).The researchers also determined the lower limit for the rate constant for reaction (21): k21516108 litre mol71 s71. Later, the value k21=2.46109 litre mol71 s71 was found.129 The kinetics of the reaction of sodium dithionite with hydro- gen peroxide has been studied.128 As in the case of oxygen, the SO¡2 radical anion rather than the S2O24 ¡ dithionite ion enters into the reaction with the oxidant. The influence of oxygen on the decomposition ofHMSandTUDOhas been studied.119, 130 It was found that in the HMS decomposition under aerobic conditions, the appearance of dithionite is preceded by an induction period, which is missing when the experiment is carried out under an inert atmosphere.119 Study of decomposition ofHMSin the presence of superoxide dismutase, catalase and sodium formate showed that active forms of oxygen (superoxide, peroxide, hydroxyl radicals) have little influence on the rate of dithionite formation. Sulfite additives exert a much more pronounced influence under both anaerobic and aerobic conditions.Decomposition of air-saturated alkaline solutions of thiourea dioxides is also accompanied by the formation of dithionite, preceded by an induction period.130 However, unlike HMS, sulfite additives do not affect decomposition of TUDO. In addition, decomposition of thiourea dioxides under anaerobic conditions does not give dithionite. Thus, the S2O2¡ 4 ion and the SO¡2 radical anion occurring in equilibrium arise upon the reaction of a sulfur- containing product of thiourea dioxide decomposition with oxy- gen.Apparently,130 this sulfur-containing product is the sulfox- ylate ion SO2¡ 2 . This species reacts with oxygen to give the SO¡2radical anion, which in turn reacts with oxygen [reaction (23)] and dimerises [back reaction (5)]: (22) SO¡2 +O¡2 , SO2¡ 2 +O2 (23) SO2+O¡2 , SO¡2 +O2 (5) 2SO¡ S2O2¡ 2 4 These reactions occur during the induction period. Since dithionite ion is accumulated after the completion of the induction period, i.e., after oxygen has disappeared, the sulfur-containing species react with superoxide or with the product of its dismuta- tion, peroxide. However, it is known that superoxide is quite stable in strongly alkaline media 131 (most experiments were carried out in a solution of NaOH with a concentration of 0.5 mol litre71) and is unlikely to undergo dismutation over the period of TUDO decomposition.Thus, the peroxide can be formed only upon the reduction of superoxide. This was proved by an independent research of the reaction of TUDO with potassium superoxide in the absence of oxygen. The shape of the resulting kinetic curves was similar to that typical of the curves of TUDO decomposition and dithionite accumulation in the pres- ence of oxygen. The same regularities were also observed in the reaction of TUDO with peroxide under anaerobic conditions; this is due to reactions of the sulfoxylate (arising upon decomposition of TUDO deprotonated in a strongly alkaline medium) and the SO¡2 radical anion with peroxide, giving rise to a hydroxyl radical [represented in Eqns (25) and (26) as the deprotonated form O¡ ]: SO2¡ 2 +NH=C=NH, SO¡2 +O¡ +OH7, (24) (25) (26) SO2+O¡ +OH7, (5) 2 , 4 (27) (NH2)2CO.(NH)2CSO2 SO2¡ 2 +HO¡2 SO¡2 +HO¡2 S2O2¡ 2SO¡ NH=C=NH+H2O Thus, study of the kinetics of reactions of thiourea dioxide with oxygen and its reduced forms made it possible to prove thatRecent trends in the chemistry of sulfur-containing reducing agents 2 2 the sulfoxylate ion SO2¢§ rather than the SO¢§ radical anion, as had been proposed previously,104 is the primary sulfur-containing product of TUDO decomposition in strongly alkaline media. It is the formation of sulfoxylate that accounts for the strong reducing properties of TUDO in alkaline media.The aerobic decomposition of N-methyl- and N,N0-dime- thylthiourea dioxides follows regularities similar to those consid- ered above for TUDO.130 N,N0-Dimethylthiourea dioxide is the most reactive reductant, apparently due to the much higher stability of the intermediate formed primarily in its decomposi- tion, namely, N,N0-dimethylcarbodiimide, compared to the stabilities of carbodiimide and N-methylcarbodiimide.132 Con- versely, the rates of the reactions of oxygen and its reduced forms withN-methylthiourea dioxide (MTUDO) are lower than those in the case of TUDO. Among the compounds considered, MTUDO is the most stable. N-Phenylthiourea dioxide (PTUDO) also exhibits a higher stability in solutions than TUDO.133 Data on the reactivity of thiourea dioxides can be important for interpret- ing the toxicity of thioureas.Many thioureas, especially N-sub- stituted ones, are known to be highly toxic. Thus lethal doses of a-naphthyl- and N-phenylthioureas amount to 0.5 and 5 mg kg71, respectively.17 The toxicity of thioureas is supposed to be due to their oxidation 134 ¡¾ 137 and the tendency of the resulting oxides to undergo desulfurisation;17 the higher this tendency, the more toxic the compound. However, data of some publications 130, 133 are at variance with this conclusion:17 the rates of desulfurisation of N-methyl- and N-phenylthiourea dioxides are lower than that of thiourea dioxide; nevertheless, N-methyl- and N-phenylthioureas are much more toxic than the unsubsti- tuted thiourea.Apparently, there are also other reasons for the high toxicity of N-substituted thioureas. Therefore, the formation and the properties of adducts produced by thiourea oxides and proteins are being vigorously studied.137 2 In weakly alkaline solutions of thiourea dioxides, dithionite ions are accumulated much more slowly. Among other reasons, this is due to the sharp increase in the reactivity of hydrogen peroxide upon a decrease in the pH, which results in a change in the mechanism of interaction of H2O2 with TUDO. It was found that at pH<13, TUDO reacts directly with the peroxide to give thiourea dioxide radicals and hydroxyl radicals.138 The latter have been detected by EPR in the presence of DMPO.Yet another possible reason for the appreciable decrease in the rate of dithionite formation in solutions of thiourea dioxides is a change in the mechanism of their decomposition. To verify this, a comparative kinetic study 138 of the processes of TUDO decom- position and its reactions with oxygen at various pH has been carried out. The rate of the reaction of thiourea dioxide with oxygen at [TUDO]0 44 [O2]0 was found to be described by the equation u=k [TUDO]. In nearly neutral and in strongly alkaline media, the rate constants for both processes virtually coincide. However, in the pH range of 9 ¡¾ 12, substantial differences are observed: the rate constant for TUDO decomposition increases much more rapidly and reaches a constant level at pH 10, whereas the rate constant for the reaction of thiourea dioxide with O2 continues to grow even in the pH 10 ¡¾ 13.7 range.The non- coincidence of the mechanisms of TUDO decomposition at pH 10 and 13 has been also discovered by other authors. Thus it has been found 139 that the reaction of glycine with TUDO in concentrated ammonium hydroxide solutions affords guanidino- acetic acid in 36% yield. When the reaction is carried out in a solution of K2CO3 with a concentration of 1 mol litre71 (pH 10), guanidineacetic acid is not formed.17 In addition, ammonia was detected upon decomposition ofTUDOin this solution 17 (ammo- nia is also formed when thiourea trioxide decomposes in weakly acidic media 42). These facts led to the conclusion 138 that decom- position of thiourea dioxide at pH 9 ¡¾ 12 follows two pathways.According to one of them, ammonia is eliminated (as this takes place, TUDO loses the guanidylating capacity towards glycine) but the C7S bond is not cleaved, the SO2¢§ anion is not formed, and the reaction with oxygen does not occur. Conversely, the 891 2 second pathway includes a stage of formation of the SO2¢§ anion and the C7N bond remains intact; therefore, TUDO actively reacts with oxygen and glycine. It was found138 that both path- ways have a stage of TUDO ionisation. Since the dependence of the rate constant for the reaction of thiourea dioxide on pH is shifted to the region of high pH, the pathway involving the formation of sulfoxylate is assumed to result from decomposition of TUDO dianions.In the TUDO monoanion, the C7N bond is apparently much weaker than the C7S bond; hence, the primary decomposition stage yields ammonia. In acid media, thiourea dioxide is very stable and barely decomposes at room temperature.7 On heating in glacial acetic acid, TUDO and some its analogues decompose to give formami- dine acetate and sulfur dioxide.110 The data presented here indicate that oxygen and active forms of oxygen (AFO) play an important role in the reactions with sulfur-containing reductants. Let us consider published data dealing with the reaction of the TUDO precursor, thiourea, with AFO (oxidation with peroxide has been mentioned above, in the discussion of the methods of synthesis of thiourea oxides).The strongest oxidant among the AFO is the hydroxyl radical.140 The reaction of thiourea with the OH radicals proceeds at a very high rate 141 (k=1.261010 litre mol71 s71). The hydroxyl radicals also oxidise very effectively N,N0-dimethylthiourea 55 and tetra- methylthiourea.141 The reactivity of the O¢§2 superoxide is substantially lower than that of OH radicals. In aqueous solvents, superoxide reacts with thiourea to give cyanamide.142 It is assumed that TUDO is formed as an intermediate of this process. The reaction of super- oxide with diarylthiourea follows a different pathway 143 giving rise to a substituted guanidine. In nonaqueous solutions, super- oxide reduces sulfur dioxide (28) SO2+O¢§2 SO¢§2 +O2 .2 2 2 4 2 The equilibrium constant for this reaction, calculated on the basis of redox potentials (E8) of the O2/O¢§ and SO2/SO¢§ couples, equals 4.8;144 however, experimental data indicate that reaction (28) proceeds virtually to completion. This is due to the subsequent transformations of the SO¢§ radical anion, namely, dimerisation to give S2O2¢§ and complexation with SO2. In the aqueous medium, superoxide does not reduce sulfur dioxide: the redox potential of the O2/O¢§ couple at pH 7 and pH 14 equals 70.16 V; that at pH 0 is +0.12 V;145 E8 of the SO2/SO¢§2 pair is 70.26 V.146 In some studies,147 ¡¾ 149 electron transfer reactions in the O2/O¢§2 ¡¾ metal complex systems are considered in terms of the Marcus theory.The equations derived (29) k12 = k11k22K12f12 , (30) pAAAAAAAAAAAAAAAAAAAAAAAAA OlogK12U2 log f12 a 4 logOk11k22=z2U , where z is a factor taking into account the frequency of collisions of the reacting species in solution, were used to determine the rate constant for the electron exchange in the O2/O¢§2 system (k22) on the basis of known constants for electron exchange in metal complexes (k11) and the experimental rate constants for the reduction of metal complexes by superoxide (k12), for example(31) Co(NH3)2a +O2 Co(NH3)3a +O¢§26 6 [in Eqns (29), (30), K12 is the equilibrium constant for reaction of type (31)]. However, the k22 values found in these calculations,147 based on the data on the kinetics of reactions of O¢§2 with various metal complexes, varied in the range of 1078 ¡¾ 105.7 litre mol71 s71.In the opinion of the researchers cited,145 the unsuitability of the Marcus theory for the calculation of k22 was due to the unequal solvations of O2 and O¢§2 . Later,148 a different approach was used to determine the k22 values, namely, the kinetics of the reaction of metal complexes with oxygen892 (32) CrL3a +O¢§ CrL2a +O2 2 , 3 3 rather than with superoxide was studied (L is 2,20-bipyridine, 1,10- phenanthroline and their derivatives). The use of this approach provided the k22 values in the range of 1 ¡¾ 10 litre mol71 s71 (in a later review,150 a value of 10 litre mol71 s71 was recommended). The reasons for the so great difference between the k22 values calculated from the data on the kinetics of reduction of complexes with superoxide remain unknown.No corresponding data concerning the superoxide reduction to peroxide can be found from the literature. 2 2 2 Comparison of the rate constants for the reactions of the O¢§ and SO¢§2 radical anions with identical oxidants, metal complexes, published in a study is of interest.149 The ratio of the rate constants for the reactions involving SO¢§ and O¢§ was found to be approximately 103; it does not depend on the nature of the oxidant, which is consistent with the Marcus theory and the equation derived from it (33) sAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA kOSO2=SO¢§2 U kOO2=O2¢§ U 1016:9 DE, kOSO¢§ kOO2¢§ U a 2 U 2 2 where kOSO¢§2 U and kOO¢§2 U are the rate constants for the reaction of SO¢§ and O¢§ with the same substrate; DE is the difference between the reduction potentials ofO2 (70.16 V) and SO2 (70.26 V), and kOSO2=SO¢§2 U and kOO2=O¢§2 U are the self-exchange rate constants of the SO2/SO¢§ If we assume that kOSO¢§ 2 and O2/O¢§2 systems, respectively. 2 U/kOO¢§2 U=103, the kOSO2=SO¢§2 U/ 2 3 2 kOO2=O¢§2 U ratio should be equal to*104.In most studies devoted to the electron exchange in the SO2/ SO¢§ couple, dithionite served as the source of SO¢§2 .149 The kOSO2=SO¢§2 U values calculated for the outer-sphere oxidation of CrL2a (L is bipyridine, phenanthroline, and their substituted derivatives) with sulfur dioxide were compared with the values determined previously in experiments using SO¢§ reagent.151 It was found that, unlike the k(O2/O¢§ and SO¢§2 as the 2 ) values, the kOSO2=SO¢§2 U constants calculated from the kinetic data for reactions involving SO2 are rather close: kOSO2=SO¢§2 U=(1 ¡¾ 18)6104 litre mol71 s71 at 298 K.These values are approximately 104 times as high as the value kOO2=O¢§2 U=10 litre mol71 s71 reported in a review,150 which is consistent with the results cited above.149 2 2 . Thus, the published data indicate that the Marcus theory is applicable to the SO2/SO¢§2 and O2/O¢§2 couples. It is noteworthy that the SO¢§2 /SO22 ¢§ couple in which SO¢§2 is the oxidised form has not been studied. Apparently, this is due to the difficulty of reduction of the SO¢§ radical anion.Oxidation of sulfoxylate would be the preferred variant. The above data indicate that thiourea dioxides can be used as sources of SO2¢§ Let us consider data on the reactions of thioureas with yet another active form of oxygen, namely, singlet oxygen 1O2. Thus oxidation of thiourea with photochemically generated singlet oxygen has been studied.152 It was shown previously that solutions of thiourea do not undergo photolysis either in the absence or in the presence of oxygen even on prolonged exposure to UV light. Photooxidation of thioureas was observed when their alcohol solutions containing oxygen and a sensitiser (dye) were exposed to the visible light. The composition of photooxidised products depends on the nature of thiourea and, to a lesser extent, the dye.The major products formed in the reaction of thiourea with 1O2 include SO2, sulfur andTUDO (the sensitisers used are Methylene Blue or chlorophyll). In the presence of Rose Bengal, the for- mation of dicyanediamide was also observed. Photochemical destruction of thiourea was discovered when titanium dioxide was used as the sensitiser.153 The reaction yields cyanamide and dicyanamide. The acid HOCl is also an active form of oxygen.55 The kinetics of reactions of thiourea with oxohalogen compounds draws special attention.14, 15, 154, 155 Until recently, studies of non-linear phenomena in chemical kinetics have been mainly related to the S V Makarov chemistry of halogens.However, at present, oscillators based on sulfur compounds are becoming more and more important. The combination of halogen and sulfur compounds in one reaction is rather attractive. However, data on the kinetics of such reactions are scarce. One of the first chemical oscillators based on sulfur-contain- ing compounds 156 is a mixture of sulfide, sulfite, oxygen and the Methylene Blue dye. It has been reported that the reaction between chlorite and thiourea is accompanied by oligo-oscilla- tion.154 Diverse non-linear effects have been discovered in the reactions of thiourea with iodate, bromate, chlorine dioxide, iodine and bromine.155, 157, 158 However, the greatest attention of researchers is still attracted by the reaction with chlorite.Never- theless, there is no consensus of opinion concerning the mecha- nism of the reaction between thiourea and chlorite. Thus it has been assumed 155 that the protonated form of the chlorite HClO2 is much more reactive than the deprotonated form. However, other researchers 159 hold to the opposite opinion. They proposed a mechanism for the reaction of ClO2 and (NH2)2CS, according to which two ClO2 molecules successively transfer the oxygen atoms to HOSCNHNH2 to give HO3SCNHNH2. However, other researchers 155 believe that the reaction between ClO2 and the thiourea monoxide is accompanied by electron transfer giving rise to ClO¢§2 and H2NNHCSO (it has been assumed 60 that the reaction of thiourea dioxide with bromate also follows a radical mechanism).It was shown experimentally 155 that the rates of reactions of formamidine disulfide and thiourea dioxide with ClO2 depend substantially on the time elapsed after the prepara- tion of solutions of the sulfur-containing compounds. This was attributed 155 to accumulation of reactive aminoiminomethane- sulfenic acid (thiourea monoxide) in the solution. However, this explanation can hardly be considered to be convincing because thiourea monoxide is exceptionally unstable at 298 K and can hardly be expected to accumulate in solution. It was shown 158 that, among other reasons, non-coincidence of the kinetic data obtained by different researchers may be due to the influence of impurities of metal compounds, especially copper and iron compounds (Cu and Fe compounds are known to catalyse efficiently the processes of reduction by thiourea).How- ever, the main reason for the inconsistency mentioned above is that no kinetic data concerning individual stages of the multistage reaction between thiourea and oxohalogen compounds have been available until recently. Only in 1993 ¡¾ 1995, were reactions between an intermediate of (NH2)2CS oxidation, i.e. thiourea dioxide, with iodate,160 bromate 15 and chlorite 14 studied. The kinetic parameters thus found made it possible to correct (in some cases, the corrections were quite significant) the rate constants for the individual stages of thiourea oxidation published previously. However, the reported 14 ratios of the rate constants for reactions of thiourea di- and trioxides with the same oxidants raise doubts.Thus the rate constants for the reactions of TUDO and TUTO with HOCl are nearly equal (9.56103 and 6.56103 litre mol71 s71) but those for the reaction with ClO2 differ by a factor of 60 (66102 and 10 litre mol71 s71, respectively). The reasons for this discrepancy have not been elucidated. The interpretation 161, 162 of the results of kinetic studies of the reactions of hydroxymethanesulfinate with bromate and chlorite in acid media is even more objectionable. Presumably, the reactions of HMS with any oxohalogen compounds either present initially or formed at intermediate stages (BrO3, HBrO2, HOBr, ClO¢§2 , HOCl, ClO2, Cl2 O2) afford hydroxymethanesulfo- nate.161, 162 As a result of oxidation, this product gives sulfate, i.e., as in the case of reactions involving thiourea, the C7S bond rupture takes place after the appearance of sulfonate.Meanwhile, it is known that hydroxymethanesulfonate is very stable in acid media and does not tend to participate in redox reactions.7 Conversely, HMS rapidly decomposes in acid solutions, the products of its decomposition exhibiting strong reducing proper- ties. Unfortunately, in the studies cited above,161, 162 no mention is even made of theHMSdecomposition or the role of its products�¢Recent trends in the chemistry of sulfur-containing reducing agents sulfoxylate and formaldehyde (later, the same researchers demonstrated 163 that formaldehyde reacts with chlorite at a rather high rate).The results of study of Makarov et al.164 made it possible to introduce substantial changes to the schemes of reactions of halogens and oxohalogen compounds with thiourea, thiourea dioxide and hydroxymethanesulfinate. It was shown 164 that the rates of oxidation of sulfonates (thiourea trioxide and hydroxymethanesulfonate) are much lower than the rates of oxidation of sulfinates (thiourea dioxide and hydroxymethane- sulfinate) and even thiourea. Thus, it appears that thiourea trioxide and hydroxymethanesulfonate are not intermediates of the reactions of TUDO, thiourea and HMS with halogens and oxohalogen compounds. During the oxidation of sulfinates, rupture of the C7S bond takes place before the formation of sulfonates.164 The active reducing agent thus formed, sulfoxylate, is then oxidised to sulfite and sulfate.VI. Conclusion The data on the properties of sodium dithionite, sodium hydrox- ymethanesulfinate and thiourea dioxides considered in the review provide a number of conclusions. An important feature common to these compounds is decomposition in solutions to give strong reducing agents. However, the opposite patterns of dependence of the stability of TUDO and HMS on the pH entail differences between the processes of their decomposition. Indeed, in solutions of HMS, the dithionite ion is formed predominantly according to an `anaerobic' mechanism, whereas in solutions of TUDO, this occurs by an `aerobic' mechanism.Sulfur-and-oxygen ions (sul- fite, thiosulfate) barely affect the TUDO decomposition, whereas the mechanism of decomposition of hydroxymethanesulfinate largely depends on admixtures. Thus, in the case of HMS decom- position, the subsequent reactions of sulfur-containing com- pounds are much more significant than in the decomposition of TUDO. Decomposition of HMS is affected most appreciably by dithionite ions. The very low stability of these ions in acid media and the capability of decomposing in aqueous solutions by an autocatalytic mechanism account for the autocatalytic mode of HMS decomposition as a whole. The unique character of dithion- ite is due to the presence of the very long S7S bond. Therefore, insertion of monoatomic sulfur into the S7S bond becomes possible and, as a consequence, decomposition of dithionite and hydroxymethanesulfinate is accelerated. Conversely, in the thio- urea dioxide molecule, direct insertion of sulfur into the shorter C7S bond is difficult.No conjugate pathway for dithionite destruction exists in the decomposition of TUDO (in alkaline solutions of thiourea dioxide under aerobic conditions, dithionite formation does take place but it is much more stable in these media than TUDO). The following important circumstance should also be men- tioned. Thiourea dioxides are the most convenient precursors of the little studied sulfoxylate ion (SO2¡ 2 ). 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ISSN:0036-021X    

出版商:RSC    

年代:2001

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