摘要:

Russian Chemical Reviews 69 (10) 821 ± 843 (2000) Quantum-size colloid metal systems V I Roldughin Contents I. Introduction II. Methods for the synthesis of metal nanoparticles III. Methods for calculation of the electronic properties of metal nanoparticles IV. The electronic and thermodynamic properties of small metal particles and thin films V. Optical properties of small particles VI. The influence of radiation on the stability of nanoparticles VII. Relaxation of electrons in small particles VIII. Electron transfer through nanoparticles IX. Self-assembly of nanoparticles X. Conclusion Abstract. metal nano-sized of preparation the for Methods Methods for the preparation of nano-sized metal particles the to approaches Theoretical considered. are particles are considered. Theoretical approaches to the analysis analysis of effects quantum-size with systems in behaviour electron of electron behaviour in systems with quantum-size effects are are outlined.the of investigations experimental of results The outlined. The results of experimental investigations of the ther- ther- modynamic and optical characteristics of small metal particles modynamic and optical characteristics of small metal particles and on experiments of data The presented. are films thin and thin films are presented. The data of experiments on the the electron nanoparticle and nanoparticles single in dynamics electron dynamics in single nanoparticles and nanoparticle ensem- ensem- bles to nanoparticles of self-assembly The described. are bles are described. The self-assembly of nanoparticles to form form supported The considered.is structures ordered supported ordered structures is considered. The bibliography bibliography includes references 343 includes 343 references. I. Introduction The interest of researchers in nano-sized systems has markedly enhanced in recent years. Two main lines have taken shape in the research and practical application of systems consisting of ele- ments with sizes from several nanometers to hundred nanometers. The first line includes studies of nano-structured materials. These materials are formed by a large number of nano-sized particles the individual properties of which show themselves in an indirect way � the change in the properties of individual structural units on passing to the nanometer region results in the appearance of new properties of the material.Thus, the ceramics produced from nanometer particles possess enhanced strength, plasticity and wear resistance; alloys of non-compatible metals can be formed; the electrical and magnetic properties of the composites are enhanced, and so on. The achievements concerning imparting new properties to nano-structured materials have been described in the literature fairly comprehensively.1, 2 Therefore, we do not consider these aspects here. The second line of research is related to studies of individual nanoparticles. The goals of these studies include both the develop- V I Roldughin Institute of Physical Chemistry, Russian Academy of Sciences, Leninsky prosp. 31, 117915 Moscow, Russian Federation. Fax (7-095) 952 53 08. Tel.(7-095) 955 46 47. E-mail: roldugin@phyche.ac.ru Received 22 May 2000 Uspekhi Khimii 69 (10) 899 ± 923 (2000); translated by Z P Bobkova #2000 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2000v069n10ABEH000605 821 822 825 829 832 834 835 836 837 839 ment of methods for the preparation of nanoparticles which would allow one to manipulate them as separate objects and investigation into the physicochemical properties of the nano- particles synthesised. The history of development of these inves- tigations is fairly long; the results are reflected in monographs.3, 4 However, after the publication of these monographs, many new results not reflected yet in review literature have been obtained. The greater part of the experimental data concerning the synthesis and study of the properties of nanoparticles appeared during the last decade. This enhancement of the interest in nanoparticles is due to several reasons � methods for controlling the size of nanoparticles have been developed; � new methods for the experimental investigation of nano- particle structures and electronic and optical characteristics have appeared; � procedures which enable isolation and stabilisation of individual nanoparticles together with the investigation of their physicochemical properties have been elaborated; � finally, new properties of nanoparticles were discovered which permit them to be regarded as promising elements of microelectronic devices.The progress attained in the synthesis and study of nanoparticles brought about new problems, which have also stimulated new experimental studies.In this review, we consider only some of the problems related to the `quantum-size' colloid metal systems. One should bear in mind the difference between nano-sized (or quantum-size) par- ticles and ultradisperse systems. The latter term has long been used in colloid chemistry. It refers to systems consisting of particles the size of which is smaller than 100 nm; in this case, the contribution of the surface to the thermodynamic characteristics becomes significant, which implies the possibility of changing the surface properties, for example, the surface tension (size corrections). The bulk properties of the particles are usually considered invariant. Meanwhile, the term nano- or quantum-size particles is used to imply colloidal systems of particles, the bulk characteristics of which undergo changes of one type or another caused by particle dimensions.It is this type of systems that is discussed below. Attention is focused on the preparation of metal nanoparticles in colloidal solutions and on the thermodynamic properties of nanoparticles and metallic films under conditions where quantisa- tion of the electron movement plays a noticeable role. Simulta-822 neously, the main methods used to calculate the electronic structures and thermodynamic characteristics of quantum-size metal systems are outlined. The features of electron dynamics in small metal particles are described and the collective effects in the colloidal ensembles of nanoparticles are considered.We wittingly restricted consideration to colloidal systems based on metal particles because other nano-sized systems as well as methods for their synthesis and study of properties have already been described in review literature. For example, the results of investigations into the synthesis and encapsulation of nanoparticles in various organic matrices have been reported 5, 6 and various aspects of the physical chemistry of semiconductor nanoparticles have been analysed comprehensively in a review.7 In addition, the experimental data concerning nano-sized systems are so numerous that it is impossible to embrace the whole body of information available from the literature in one review. II. Methods for the synthesis of metal nanoparticles There exist diverse methods for the preparation of nanoparticles and nanocomposites.In this Section, we shall consider several main methods used to prepare nanoparticles which ensure the possibility of isolating them and separating them from the ensemble. Nanoparticles can present interest as elements of microelectronic devices only when they have been isolated from the ensemble. Therefore, the problem of preparing individual nanoparticles is one of the central problems in today's investiga- tions. 1. Evaporation ± condensation method The evaporation ± condensation method has long served as a common method for the preparation of metal nanoparticles.3 According to this method, a metal is evaporated into a flow of an inert gas. Then the flow of the carrier gas with the metal vapour is fed to a chamber, while the temperature of the chamber walls varies according to a previously specified pattern.The pattern of temperature variation is usually calculated taking into account the heat and mass transfer equations and the kinetics of nucleation, in order to attain the necessary rate of condensation of the metal vapour such that the particles have time to grow to the required size befaturation is eliminated. Until recently, the particles thus prepared have been deposited on an inert substrate.3, 8 The use of a support was necessary in the gas-phase method. Only in this case, were individual particles produced because nano-sized particles are fairly mobile and stick together upon collision; hence, their aggregation has to be sup- pressed somehow.Detailed investigations 9 into the kinetics of aggregation of nanoparticles during their preparation by the evaporation ± con- densation method have shown that, for example, zinc nanometer particles aggregate over periods of 10 ± 100 ms depending on the conditions of particle growth. They form fractal aggregates with log-normal size distribution, the average size of the aggregates corresponding to the micrometer range. The evaporation ± condensation procedure gives rise to the purest metal particles. This is why it still remains attractive; however, search for the methods that would enable the prepara- tion of nanoparticles without using solid supports is in progress. Methods have been proposed 10 ± 14 in which the solid support was replaced by frozen argon 10 or by a solvent.11, 14 According to these (and other similar) techniques, the particles are co-precipitated with the solvent; thus, a composite containing nanoparticles gradually grows.By subsequent heating, the solid composite is converted into a colloidal dispersion of nanoparticles. Colloidal dispersions obtained in this way are not always stable. In order to obtain stable systems, the methods mentioned above have been modified,15, 16 namely, surfactants were intro- duced into the system by co-precipitation (or added subse- quently). This gave particles with sizes of up to 1 nm dispersed in V I Roldughin a solvent with a concentration of 1020 particles per m3. According to the data of electron microscopy, systems prepared in this way contained no particle aggregates.In early studies,15, 16 evaporation ± condensation experiments were performed with silver particles. This method was studied in more detail in another study in which Ag, Bi, Cu and Te nanoparticles were obtained.17 It was found that particles with the size from 1 to 10 nm can be obtained in any of the systems tested by varying the pressure and the composition of the carrier gas. The particle size varies as a linear function of pressure. The particles are characterised by a rather narrow size distribution, the corresponding standard deviation being 1.4 ± 1.6. Particles of Ag/M alloys (M=Co, Fe, Ni) with approxi- mately the same characteristics were also prepared by the evapo- ration ± condensation technique.18 An aerosol method for the preparation of silver nanoparticles coated with a dodecanethiol layer and subsequent introduction of the particles into an organic solvent have been reported.19, 20 Coating with dodecanethiol was accomplished in the vapour phase immediately after the nucleation of particles by mixing of an aerosol flow with a flow of an inert gas saturated by dodeca- nethiol vapour.This procedure provided nanoparticles with the best dispersity; however, this was associated with substantial procedural difficulties. Despite the progress achieved using the evaporation ± conden- sation method, it is mainly employed to produce composite materials and is seldom used to prepare nanoparticles. This is due, first, to the low productivity of the method and, second, to the fact that the size distribution of the resulting particles is too broad, according to the modern criteria.In addition, when using this method, it is difficult to control the size of the nanoparticles formed. As a consequence, standard colloidal methods for the preparation of nanoparticles proved to be more convenient; they have become more attractive owing to the unexpectedly successful use of special surfactants. 2. Reduction from solutions Chemical reduction is used most widely to synthesise colloidal particles.21 Metal salts can be reduced by various reducing agents in the presence of stabilisers (special ligands, polymers, surfac- tants), which suppress particle aggregation.22, 23 The reduction yields particles of different sizes. The size cannot be predicted because it is affected simultaneously by several factors: the nature and the concentration of the solvent and the stabilising agent used, reaction temperature, and reaction time.In addition, the particle size is determined to a large extent by the supersaturation developed, higher supersaturation resulting in smaller particles; hence, the size of the particles can be controlled by varying supersatruration.21, 24 The degree of supersaturation reached in the experiments is governed by the rate of the chemical reduction, temperature and the amount of the reduced metal spent for the particle formation and growth. The process of particle formation is normally connected with homogeneous (or, perhaps, heterogeneous) nucle- ation. As a rule, it is limited by the experimental conditions.For this reason, the main way of controlling the particle size is to use growth retardants, which play a dual role. On the one hand, they retard the transfer of the reduced metal from the solution to the particles and thus increase supersaturation. On the other hand, the retardants directly limit the rate of the increase in the particle size. Thus, the choice of the protective shell of the particles appears to be the crucial point in controlling the particle size. Alkanethiols are known to form dense self-assembled mono- layers on the surface of crystalline gold.25 However, until recently, this fact has not been used in the synthesis of nanoparticles. Only in a few publications,26, 27 were alkanethiols employed in the reduction of gold in a solution of HAuCl4 by NaBH4; it was found that this process yields virtually monodisperse particles.Originally, so-called two-phase method consisting of two stages has been used for reduction.26, 27 In the first stage, metal-contain-Quantum-size colloid metal systems ing reagents were transferred from the aqueous phase to the organic phase and then solutions of the surfactant and the reducing agent were added to the organic phase. Subsequently, a single-phase method was developed.28 ± 34 According to this method, solutions of the surfactant and the reducing agent were added simultaneously to a metal-containing solution. Presum- ably, in both cases, gold is reduced to the atomic state, which ends in the nucleation of atoms to give nanocrystals; alkanethiols are adsorbed on the surface of the crystals.The thermodynamics of formation of gold nanoparticles in the presence of alkanethiols has been considered in a study,30 where it was shown that the process of formation of nanoparticles is similar to the process of formation of drops of a water-in-oil microemulsion. The gold nanocrystals coated by an alkanethiol monolayer, which are formed upon reduction, consist of a metallic core comprising 10 to 5000 atoms and a dense surfactant shell. Almost all the particles exhibit a clear-cut plasmon band in the optical absorption spectrum. The position and the width of the band depend on the particle size and the properties of the adsorbed layer. The monolayer on nanoparticles is assumed to be self- assembled in the same way as on the surface of bulk samples.The size of nanocrystals depends on the length of the alkyl chain in alkanethiol and on the thiol to gold ratio in the solution.27, 32, 35 Examination by electron microscopy shows that gold micro- crystals are compact and faceted and have a face-centred cubic (FCC) lattice.32 The crystals are mainly octahedral or icosahedral. The inner area of the microcrystals is pure gold, the sulfur atoms being located on the surface and connected to alkyl chains. These results were obtained from spectroscopy and mass spectrome- try.32, 36, 37 Arnold and Reilly 37 reported a unique experiment. After the powder of gold nanoparticles coated by alkanethiols had been prepared, it was exposed to powerful laser radiation. As a result, the particles were `evaporated'. The flow of particles was again irradiated by a laser with the wavelength selected in such a way as to cleave the bond between sulfur and the alkyl chain.This yielded gold nanoparticles with an adsorbed layer of sulfur atoms. The resulting clusters were studied by mass spectrometry. Interpreta- tion of the mass spectra showed that the sulfur atoms form dense monolayers on the surface of gold microcrystals. This result fully confirmed the assumption concerning the formation of dense self- assembled monolayers of alkanethiols on the surface of gold nanoparticles. Astudy has been devoted to the dependence of the structure of alkanethiol adlayers coating nanoparticles on the particle size.38 Particles with sizes ranging from 1.2 to 5.2 nm were used in these experiments. The properties of the adlayer were studied by Four- ier transform IR spectroscopy, differential scanning calorimetry, wetting and thermal desorption mass spectrometry.The transition from a two-dimensional to three-dimensional packing of alkane- thiol molecules was also investigated. It was found that in the case of larger nanoparticles, more ordered conformations of the alkyl chains in the adsorbed alkanethiols are observed. The critical diameter corresponding to the change in the pattern of the adlayer packing was found to be 4.4 nm. For particles with larger diameters, a two-dimensional adlayer is formed, whereas in the case of particles with smaller diameters, elements of a three- dimensional packing show themselves in the adlayer. In recent years, more complex thiolates have been used for the production of gold nanoparticles,39 ± 43 because surfactants with more complicated structures may promote the formation of self- assembled monolayers of nanoparticles (see below).The method is being perfected;44 this allows preparation of particles of differ- ent metals according to the same procedure. In particular, alkanethiols have been employed to produce silver nanoparticles 45 ± 50 (by both single-phase and two-phase procedures). Simultaneous synthesis of gold, silver and platinum nanoparticles has also been accomplished.51 As for gold, in this case, too, the position of the plasmon band in the optical 823 absorption spectra depended on the method of preparation and the structure of the environment of metal particles. 3. Reduction in microemulsions Microemulsions of the `water-in-oil' type (reverse micelles) have been attracting increasing attention in recent years as micro- reaction media for the preparation of nanoparticles.Microemul- sions consist of nano-sized water (or solution) drops dispersed in oil (organic) phase and stabilised by surfactant molecules distrib- uted over the interface. Highly dispersed drops of water are perfect microreactors for the preparation of nanoparticles because the size of drops is a natural restrictor for the dimensions of the growing nanoparticles. Primarily, microemulsions have been used for the preparation of monodisperse microparticles. The synthesis of Pt, Pd, Rh and Ir particles by the reduction of the corresponding salts in water drops has been described.52 Since this publication appeared, micro- emulsions have been repeatedly used for the preparation of diverse particles of both individual metals and their compounds.In the 90s, the use of microemulsions to prepare silver,53 ± 55 gold,53 platinum, cobalt 56 and iron 57 nanoparticles was reported. Nanoparticles were prepared by reduction of the corresponding metal salts by sodium tetrahydroborate or hydrazine. The unique opportunities for controlling the size and shape of nanoparticles from microemulsions have been demonstrated in several stud- ies.58 ± 61 The high efficiency of microemulsions in preparing nanoparticles of specified dimensions and shapes was demon- strated most vividly in several studies by Pileni et al. 62 ± 67. In these works, a functional surfactant, copper bis(2-ethylhexyl)sulfosuc- cinate Cu(AOT)2 , was used to prepare copper nanoparticles.The surfactant played a dual role � on the one hand, it stabilised drops of water and, on the other hand, it served as a source of copper in the drops. To prepare nanoparticles, a microemulsion of water drops stabilised by Cu(AOT)2 and Na(AOT) in isooctane was mixed with a microemulsion of drops of a solution of sodium tetrahy- droborate stabilised by Na(AOT). After microemulsions have been mixed, the material exchange between the drops starts and copper is reduced. Due to the presence of natural restrictions, the resulting copper particles are nano-sized. The size and shape of particles are largely determined by the ratiow= âH2Oä âAOTä in the microemulsion, because the structure of the microemulsion also depends significantly on this ratio.When w<4, only spherical drops are present, their size being proportional to w. Reduction of copper in such a system gives rise to spherical nanoparticles 1 to 12 nm in diameter; their size is also propor- tional to w. Further increase in the content of water (4<w<5.5) changes the shape of microdrops�they become spheroidal. Correspond- ingly, reduction in the system affords spherical nanoparticles with diameters of 8.2 and 12 nm and cylindrical particles with a diameter of 12 nm and a length of 18.5 nm. When the relative water content is 5.5<w<11, the microemulsion has a structure of a bi-continuous phase in which nano-spheres with diameters of 6.7 and 9.5 nm and 22.6-nm long nano-rods 9.5 nm in diameter are formed.When the content of water increases further (w>11), the microemulsion is transformed into a lamellar structure in which only rods with lengths of 300 to 1500 nm and with diameters of 10 to 30 nm are present. According to electron microscopy data, nanoparticles synthesised in microemulsions (including the long rods mentioned above) have defect-free surfaces, which indicates their high quality. The process of reduction of nanoparticles in microemulsions was studied by numerical modelling by the Monte Carlo method.68 ± 70 The modelling data showed that the flexibility of824 the surfactant monolayer and the surfactant concentration in the system play an important role in the formation of particles in microemulsions.By varying the monolayer flexibility and the content of the surfactant, one can change appreciably the size distribution of nanoparticles and even prepare ensembles with a bimodal size distribution, which is apparently due to the character of evolution of the supersaturation of microemulsions with metal atoms.24 Since surfactants play an important role in the preparation of nanoparticles in microemulsions, the search for new, in particular, non-ionic surfactants which could be used in the synthesis of metal nanoparticles is currently in progress.71 Syntheses are carried out using non-traditional microemulsions such as water drops in supercritical carbon dioxide.72 In view of the significant dependence of the size distribution of particles on the character of the exchange of matter between the drops, the use of new methods for the synthesis of nanoparticles in microemulsions which exclude this exchange has started.For example, Revina et al.73, 74 prepared silver nanoparticles by radiation-induced reduction. Studies on the influence of external factors, for example, temperature,75, 76 on the size and shape of nanoparticles formed in microemulsions have been carried out (in the studies cited, these were gold nanoparticles). 4. Preparation of nanoparticles with a stoichiometric composition A method yielding metal nanoparticles with a definite composi- tion is most promising as regards investigation of the individual properties of these particles. Primarily, these particles were obtained as metal clusters containing less than ten and, later, more than ten metal atoms (a brief review of the history of preparation of these compounds has been reported 77).Substan- tial progress along this line has been achieved by Vargaftik et al.,78 ± 80 who identified unambiguously palladium clusters con- taining (in the ideal case) exactly 561 metal atoms. The cluster had the composition Pd561L60(OAc)180, where L is 1,10-phenanthro- line or 2,20-bipyridine and OAc groups form the ligand shell. The cluster was produced in a two-stage synthesis 78 ± 80 including the reduction of Pd(OAc)2 with hydrogen in an AcOH solution (1/n)[Pd4H4(OAc)L]n+AcOH Pd(OAc)2+L+H2 and subsequent treatment of the high-molecular-weight clusters with oxygen [Pd4H4(OAc)L]n+O2+AcOH Pd561L60(OAc)180+Pd(OAc)2+L+H2O. This cluster belongs to the category of `magic clusters',{ i.e.those containing strictly definite, `magic' numbers of metal atoms, namely, 13, 55, 147, 309, 561, ... . These numbers correspond to fully occupied atomic layers (shells) of cubooctahedral clusters having a FCC lattice. No clusters with three filled shells have been synthesised up to now;81 however, clusters with four 82 (Pt309Phen5O30 , Phen is o-phenanthroline) and five 78, 83 (Pd561Phen36O200) shells have been prepared. The existence of clusters with seven 84 (Pd1415Phen54O1000) and eight 85 (Pd2057Phen38O1600) shells has also been established. The clusters obtained were studied by various methods � high-resolution electron microscopy, EXAFS, MoÈ ssbauer spectroscopy, NMR and others. These studies demonstrated that metal atoms actually form a close cubic packing and the distance between the metal atoms virtually coincides with that found for bulk samples.The diameter of the metallic core of the clusters varies from 2.4 to 3.7 nm. { The matter concerns `magic' numbers in terms of the Mackay ± Chini crystal-chemical model. See A L Mackay Acta Cystallogr. 15 225 (1979); P Chini J. Organomet. Chem. 200 37 (1980); B K Teo, N J A Sloan Inorg. Chem. 24 4545 (1985). V I Roldughin The mechanism which provides the synthesis of clusters with a definite number of metal atoms according to the above-described scheme has not been ultimately elucidated. Investigations into the structures of the cluster precursors at different stages of evolution showed that they are shaped like polymeric chains, which are transformed into compact metal particles during heat treatment.86 In the case of the cluster with the Pd561 core, the metal particle forms a FCC lattice and takes on an icosahedral or twin shape at the final stage.87 ± 89 The methods for the preparation and investigation of clusters with a specified number of atoms are being constantly improved.Modern methods have been employed to perform unique inves- tigations of the thermodynamic and magnetic properties of these nanoparticles. It will be shown below that physical characteristics of `quantum'-size particles depend essentially on the number of collective electrons they contain. 5. Other methods for the preparation of metal nanoparticles Among other methods used to synthesise metal nanoparticles, apart from those described above, mention should be made of pulse radiolysis in solutions, which is employed fairly fre- quently.90 ± 97 Under the action of X-ray or ultraviolet radiation or of high-energy electrons, clustersMá2 , Má3 , ..., Mxn á are formed successively.98 Some of these are exceptionally stable. As a result, stable magic clusters are accumulated in the solution. The presence of magic clusters is indicated by the appearance of characteristic bands in the absorption spectra. The contribution of `non-magic' clusters was found to be insignificant because their lifetimes are fairly short. When using the radiolysis method to prepare nanoparticles, various stabilisers are also added, although in some cases, for example, for aqueous solutions of silver, the use of stabilisers is optional.The nanoparticles formed in the presence of stabilisers are characterised by a higher level of monodispersity and a smaller size. Polymers are used as stabilisers.99 The method of radiolysis is also applicable for the preparation of nanoparticles with a mixed composition.100, 101 This is done either by the reduction of different metal ions present simultaneously in the solution or by preparing first the particles of one metal and coating them subsequently with a shell of a different metal,102 which is also reduced by radiolysis. By varying radiation doses, one can control the size and the composition of nanoparticles. Apart from the radiation-chemical method, sonochemical prepa- ration of particles is also used.103 Metal nanoparticles are often prepared in the presence of polymers.104 ± 120 Soluble polymers,104 ± 109, 118 ± 120 block copoly- mers 115, 116 or cross-linked matrices are used.The role of the polymeric medium is usually to stabilise the particles. In addition, in the case of solutions of polymers or block copolymers subjected to microphase separation,116 they restrict the particle size. The extent to which the polymer influences the size of nanoparticles depends on the length of the polymer chain 119 and on the conditions of synthesis. The synthesis can require a fairly sophis- ticated procedure; for example, nanoparticles have been prepared using supercritical carbon dioxide.111 In another study,120 poly- mers were used to ensure high yields of nanoparticles of noble metals.One more problem was solved simultaneously, namely, transfer of the nanoparticles into a colloidal solution of any concentration, replacement of the solvent being possible. Various templates are used in the synthesis of nanoparticles in order to impart a specified shape to them and to restrict their size. The materials used for this purpose included polymeric mem- branes 121, 122 (this permitted the researchers to grow cylindrical particles), high-porosity oxide films 123 and specially prepared substrates,124 ± 126 which often provided the possibility of prepar- ing catalysts ready for use or ordered structures constructed from nanoparticles. In some cases, nanoparticles and composites based on them are prepared using the `sol ± gel' technology,127 ± 132 or by growing particles in biological 133, 134 and liquid-crystalline 135 systems. A method for the preparation of nanoparticles under electric discharge in solution has been proposed.136Quantum-size colloid metal systems III.Methods for calculation of the electronic properties of metal nanoparticles 1. The density functional method The ab initio Hartree ¡¾ Fock method has found wide use in the calculation of properties of many-electron systems. In this method, the properties of a many-particle system are determined by calculating the orbitals of separate electrons moving in the self- consistent field of other electrons and atomic nuclei. The Har- tree ¡¾ Fock method allows one to find the wave functions of electrons without invoking adjustable parameters.The main obstacle to the use of this method is the complexity of calculations; the time it takes to perform calculations increases as N4, where N is the number of electrons in the system. For this reason, the Hartree ¡¾ Fock method is mainly used to perform calculations for atoms or relatively small molecules and clusters. The greatest progress in calculations for many-electron sys- tems has been attained in the last decades using the density functional method. The time needed for calculations by this method increases with an increase in the number of electrons much more slowly; at present, this method is widely used for calculations for various systems with a large number of electrons including polyatomic molecules, complexes, clusters, and con- densed media.There are two approaches in the density functional method, namely, integral and local approaches. The former dates from an original study by Hohenberg and Kohn,137 while the latter is a modification of the theory proposed by Kohn and Sham.138 We shall consider these approaches in relation to a system of N non- relativistic electrons described by a Hamiltonian H=T+V+U. (1) Here T is the operator for the kinetic energy of electrons (2) Di , T a ¢§ 1 2mh2 i X m is the mass of an electron, Di is the Laplacian for an ith particle, V is the potential energy operator for a system of electrons in an external field and U is the operator of the electrostatic repulsion energy of electrons. It was shown 137 that the electrodensity distribution r(r) determines unambiguously all the main properties of a many- electron system including the wave function.The energy of a many-electron system in an external field v(r) is a functional of r(r) (3) O E [r(r)]: v(r)r(r)dr+G[r(r)]. The last term depends on the wave function C of the system of electrons, determined by the distribution of their density (4) O G[r(r)]= C*[r(r)](T+U)C[r(r)]dr (the character * means the complex conjugate). The minimum energy corresponds to the ground state of the system E [r(r)]5E [rg(r)], (5) where rg(r) is the electron density distribution in the ground state. However, the fundamentality and simplicity of the Hohenberg and Kohn theorem 137 are fraught with latent difficulties. Deter- mination of the electronic density and other characteristics of an electron system requires that the explicit form of functional (3) be known.However, the explicit form of this functional is unknown; thus, additional statements are needed to find the form of E[r(r)]. Functional (3) can formally be represented as follows: O E [r(r)]= v(r)r(r)dr+T0[r(r)]+ +12 Oe2rOrUrOr0U jr ¢§ r0j where e is the charge of an electron, r, r0 are space coordinates. The second term is the kinetic energy of non-interacting electrons T0 , and the third term is the energy of electrostatic interaction between them. The last term, Exc[r(r)], is an electron density functional depending implicitly on v(r). It is usually called exchange correla- tion energy of electron gas. Actually, equality (6) is the definition of this functional. Thus, a fundamental problem faced by the density functional method is to elucidate the form of the func- tional Exc[r(r)].There are several approximate methods for the calculation of this functional.139 h2 m Dj a vOrU a ja1 XN vxc(r)=dExcarOrUa , ja1 Exc= rexcdr , Minimisation of functional (3) in a one-electron approxima- tion results in a set of self-consistent equations (Kohn ¡¾ Sham equations) Oe2rOr0Udr0 ¢§1 a vxcOrU ¢§ Ej jr ¢§ r0j 2 jjOrU a 0: (7) Here, r(r) is the electron density r(r)= jj OrUjjOrU, (8) nxc(r) is the exchange correlation potential (9) drOrU Ej are single-electron energy levels, jj (r) are single-electron wave functions, j=1, ... ,N. The ground-state energy for a many-electron system is given in this approximation by the relation Oe2rOrUrOr0U XN O j ¢§ 1 drdr0 ¢§ v E= xcOrUrOrUdr .E jr ¢§ r0j 2 (10) Formally, the Kohn ¡¾ Sham equations are exact; however, there exists the problem of the relationship between vxc(r) and electron density. At present, it can be regarded proven that the potential vxc(r) can be determined unambiguously; in this case, the use of single-electron approximation does not play a crucial role.140 ¡¾ 142 It has also been proven that, by using the potential vxc(r), one can reproduce precisely the distribution of electron density in the system 143 and that the upper occupied level of an isolated system of electrons in the absence of an external field is related uniquely to the ionisation potential;144 in addition, the asymptotic variation of the electron density for a system with a finite number of electrons is entirely determined by the upper occupied single-electron function (orbital).As has already been noted, the use of the density functional method requires at least an approximate way of finding Exc[r(r)]. For a uniform electron gas, it can formally be written in the local density approximation that O (11) where exc is the local exchange correlation energy of electrons, equal to 79[(3/4p)r]1/3. For non-uniform systems, the local energies exc were found as the gradient expansion 145 ¡¾ 149 exc=gr(r),Hr(r),H2r(r). Other attempts to determine the exchange correlation energy as the electron density functional have also been under- taken.150, 151 In particular, it has been represented as a non-local functional.152 825 (6) drdr0+Exc[r(r)], (12)826 2.The method of statistical functions In the investigations of many-electron systems including nano- particles, considered in this review, the method of statistical distribution functions is also used. This method has been used successfully to study the surface properties of various quantum objects 153 ¡¾ 156 and the structures of clusters;157 it can also be used to investigate the dynamic processes in quantum systems.156 For this reason, we shall outline briefly the main principles of the distribution functions method. In quantum mechanics, the role of distribution functions is played by the density matrix 158 for Nparticles, rN(t) (t is time). In the coordinate representation, the density matrix of a system is a function of coordinates and spin variables of all particles (13) i hs0,R0|rN(t)|s,Ri=XPiCi OR0; s 0UCiOR; sU, whereRand s are sets of coordinatesR=(r1 , r2 , ..., rN) and spin variables s=(s1 , ... , sN), and Pi is the probability that the system occurs in a pure state i with the wave functionCi . By means of the density matrix, the average value of any operator ^ T can be written in the form (14) , s h ^ Ti aX dRT^hs 0;R0jr O Njs;Ri s 0as; R0aR the operator ^ T acting on the `non-primed' variables of the density matrix. After the action of the operator, the primed and non- primed variables are taken to be equal. The diagonal elements of the density matrix hs;RjrNjs;Ri determine the probabilities of distribution of the system over (15) i h q qt ¢§ ^Ha ^H0 hs0;R0jrNjs;Ri a 0. coordinates and spin states of the particles.The density matrix satisfies the Liouville quantum equation H^a ¢§ h2 2mDi a uOri ¢§ rjU , i 14i<j4N Here ^His the Hamiltonian for a system of particles acting on the non-primed variables, while ^H0 acts on primed variables. For a system of particles with pair interaction, in the absence of an external field, the Hamiltonian has the form X X where Di is the Laplacian operating on the coordinates of an ith particle and u(ri7rj) is the energy of pair interaction of particles. Wigner 159 introduced a special (mixed) representation for a density matrix in which the distributions of particles over coor- dinates R=(r1, ... , rN) and momenta P=(p1, ... , pN) appear. The density matrices in the Wigner representation and those in the coordinate representation are related as (16) 6hs;R ¢§ h O FOR;P; tU a O2pU3N dtaexpO¢§i tPUa6 1 2 tjrNOtUjs;R a h 2 ti, where ta Os1; ::: ; sNU;R ¢§ h 2 t a Or1 ¢§ h 2 s1; :::; rN ¢§ h 2 sNU and tP a Os1p1+...+ sNpN).(17) In the general case, the mixed representation F(R,P,t) can be used to derive separate distributions over coordinates and over particle momenta O FR(R,t)= F(R,P,t)dP, O FP(P,t)= F(R,P,t)dR. V I Roldughin Using the Wigner density matrix, the average value of any operator ^ TOR; PU depending on R and P is calculated from the formula (18) h ^ Ti a dRdPT^(R,P)F(R,P). O Thus, the mixed representation for the density matrix (16) is similar to the ordinary distribution function used in classical statistical mechanics; its standard expressions can be employed to find the average values of physical quantities.There is, how- ever, one fundamental difference between the classical and quan- tum descriptions: unlike the classical distribution function of particles over coordinates and momenta, the density matrix i p q m Fs0;sOP;RU a qri F(R,P,t) can acquire not only positive but also negative values. This is due to the fact that the function F(R,P,t) has no clear physical meaning because it is known that particle coordinates and momenta cannot be measured simultaneously in quantum mechanics. Only the distribution functions FR(R,t) and FP(P,t) have a physical meaning; they are positive. The Liouville quantum equation (15) in the mixed representa- tion has the form q qt a ia1 XNdt a 1 i h dp0i dp0j dOp0i a p0j ¢§ pi ¢§ pjU6 OO2pU3 i 6a j XO (19)

ISSN:0036-021X    

出版商:RSC    

年代:2000

数据来源: RSC

摘要:

Russian Chemical Reviews 69 (10) 845 ± 860 (2000) Heterocyclisation of trisubstituted formamidines containing an aryl or hetaryl substituent at the imine nitrogen atom G V Oshovsky, AMPinchuk Contents I. Introduction II. Pericyclic and cyclisation reactions initiated by nucleophilic addition to multiple bonds III. Intramolecular nucleophilic substitution IV. Other heterocyclisation reactions V. Conclusion Abstract. of cyclisation the on data Published Published data on the cyclisation of N1,N1-disubsti- tuted analysed, are and N2-aryl- -aryl- and N2-hetaryl-formamidines -hetaryl-formamidines are analysed, described The generalised. and systematically described systematically and generalised. The bibliography bibliography includes references 147 includes 147 references.I. Introduction Heterocyclisations play an important role in organic chemistry and provide key, sometimes irreplaceable, approaches to the preparation of heterocyclic systems. Since heterocyclic derivatives represent the most numerous class of organic compounds cover- ing about two-thirds of all natural and synthetic products, it is important to describe systematically the methods used to prepare them. At present, the syntheses of heterocyclic compounds are classified using three different approaches: in terms of the cyclisa- tion product (heterocyclic compound or a class of compounds), in terms of the type of ring-forming reaction, and in terms of the functional groups or classes of organic compounds involved in the construction of the heterocycle.The last-mentioned approach attracts special attention because it discloses the synthetic poten- tial and the advantages and disadvantages of heterocyclisation components and represents more vividly the methods suitable for solving sophisticated synthetic problems. Amidines are important starting compounds in the synthesis of heterocyclic systems of various types. There are several ways of construction of heterocycles with participation of amidines (a) cyclisation upon the reaction with the NH group of the amidine; (b) transformation of the reactive CH groups attached to the amidine carbon atoms; (c) pericyclic reactions; (d) reaction of the electrophilic amidine carbon atoms with external or internal nucleophiles. Only routes (a) and (b) have been considered comprehensively in the literature.1 ±5 In this review, we analyse the prospects for using N1,N1-disubstituted N2-aryl- and N2-hetarylformamidines (hereinafter, referred to as aryl- and hetarylformamidines, respec- tively) for the construction of simple and fused heterocycles.These G V Oshovsky, AMPinchuk Institute of Organic Chemistry, National Academy of Sciences of Ukraine, ul. Murmanskaya 5, 02094 Kiev, Ukraine. Fax (38-044) 573 26 43. Tel. (38-044) 551 06 79 (G V Oshovsky). E-mail: oshovsky@carrier.kiev.ua Received 21 April 2000 Uspekhi Khimii 69 (10) 924 ± 939 (2000); translated by Z P Bobkova #2000 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2000v069n10ABEH000600 845 845 849 856 858 compounds cannot react by routes (a) and (b); therefore, it becomes possible to focus attention on the key reactions of types (c) and (d) and to extend the existing generalisations and views on the role of amidines in the synthesis of heterocyclic systems.II. Pericyclic and cyclisation reactions initiated by nucleophilic addition to multiple bonds 1. Cycloaddition N1,N1-Dimethyl-N2-arylformamidines and N1,N1-dimethyl- N2-hetarylformamidines can play a dual role in cycloaddition: they either provide the imine fragment of the dimethylaminome- thylideneamino group N=CHNMe2 (hereinafter, the amidine fragment) for heterocyclisation or act as heterodienes in which the amidine fragmentN=CHis conjugated with the double bond of the substituent at the imine nitrogen atom.The latter pathway is realised only for N-hetarylamidines characterised by a high degree of alternation of single and double bonds. In these compounds, the double bond in a heterocyclic substituent is activated by the electron-donating amidine group;6 hence, the Michael addition to this bond is possible. The primary reaction products can cyclise to give compounds with the same structure as those formed upon concerted [2+4]-cycloaddition. In these products, recyclisation of heterocyclic fragments is possible, resulting in new heterocyclic compounds. N1,N1-Dimethyl-N2-phenylformamidine 1a and phenylimi- nopyrrolidinomethane 1b react with dehydrobenzene according to the [2+2]-cycloaddition pattern 7 giving rise to unstable benza- zetines 2a,b, which are converted into 2-phenylaminobenzalde- hyde 3 and acridine 4 under the reaction conditions.NR2 NPh R2NCH 1a,b N 2a,b Ph CHO NHPh 3 NR2 NR2 7R2NH N Ph N846 N4 (b). NR2=NMe2 (a), N The amidine 1a reacts with sodium azide according to the [2+3]-cycloaddition route to give 8 1-phenyltetrazole 5. Ph N NaN3, D, AcOH N N 7Me2NH PhN CHNMe2 1a N5 1-Nitrosovinylaryls 6a,b, generated in situ from halooximes, react with N1,N1-dimethyl-N2-arylformamidines 1a,c according to the [3+2]-cycloaddition route to give 1,4-diaryl-2-dimethyl- amino-3-oxy-2,5-dihydro-1H-imidazoles 7a,b.9, 10Ar1 N NO Me2N Ar1N CHNMe2+ Ar2 N + Ar2 6a,b 1a,c 7a,b O7 Ar1=Ar2=Ph (1a, 6a, 7a), 4-MeC6H4 (1c, 6b, 7b).In the case of N1,N1-dimethyl-N2-hetarylformamidines con- taining many alternating single and double bonds, heterocyclisa- tion involves the azomethine fragment of the amidine substituent and the neighbouring vinylene fragment of the heterocycle. The diene reactivity increases in the sequence N1,N1-dimethyl-N2- arylformamidines, N1,N1-dimethyl-N2-hetarylformamidines and N1,N1-dimethyl-N2-vinylformamidines. This is determined by the degree of participation of the vinylene fragment in conjuga- tion. Indeed, N1,N1-dimethyl-N2-phenylformamidine deriva- tives do not participate in cycloaddition reactions as dienes, whereas N1,N1-dimethyl-N2-vinylformamidines are quite reac- tive dienes. On passing from N-vinylformamidines to N-hetaryl- formamidines, the diene reactivity decreases.N1,N1-Dimethyl- N2-(isopropenyl)formamidine 8a and N1,N1-dimethyl-N2- (cyclohex-1-enyl)formamidine 8b readily react with alkenes and alkynes 11 to give 1,4-dihydropyridine and pyridine derivatives formed as [4+2]-cycloaddition products.R2 R1 C C 7HNMe2 HN R2 R1 R2 R1 N CHNMe2 8a,b C C 7HNMe2 N R1=R2=H (a); R17R2=(CH2)3 (b). The reactions of N1,N1-dimethyl-N2-(1,3-dimethyl-2,6- dioxo-1,2,3,6-tetrahydropyrimidin-4-yl)formamidine 9 with methyl vinyl ketone, methyl acrylate, acrylonitrile and dimethyl or diethyl maleate proceed in a similar way to give 1,3-dimethyl- 5,8-dihydropyrido[2,3-d ]pyrimidine-2,4-diones 10, which are formed from the products of primary [2+4]-cycloaddition 11.12 The high regioselectivity observed in the reactions with mono- substituted alkenes is in line with the electron-donating influence of the amidine substituent, which increases the electron density at the 5-position of amidinodimethyluracil 9.The compounds 10 G V Oshovsky, A MPinchuk were oxidised to the corresponding 1,3-dimethylpyrido[2,3-d ]pyr- imidine-2,4-diones 12 by refluxing in nitrobenzene. O R2 R1 MeN PhMe, 110 8C, 8 h O N CHNMe2 Me N9 R1 O R2 *H MeN 7Me2NH N O NMe2 NMe 11 O R1 R1 O R2 R2 MeN MeN PhNO2, D N O N O NMe H 10 NMe12 R1=H:R2=COMe, CO2Me, CN; R1=R2=CO2Et, CO2Me. N1,N1-Dimethyl-N2-5-(1-ethylpyrazolyl)formamidine 13 un- dergoes [2+4]-cycloaddition to (2-nitrovinyl)benzene 14a, 1-phe- nyl-3-(2-nitrovinyl)indole 14b or 2-(2-nitrovinyl)thiophene 14c on exposure to microwave radiation.13 The adducts 15 formed regioselectively are converted in situ into a mixture of 4-aryl-1- ethyl-5-nitropyrazolo[3,4-b]pyridines 16 and 4-aryl-1-ethylpyr- azolo[3,4-b]pyridines 17 as a result of abstraction of HNO2 with aromatisation.The ratio of the compounds 16 and 17 depends on the nature of the nitroalkene 14 but the nitro derivative 16 predominates in all cases. NO2 N +Ar N CHNMe2 13 NEt 14a ± c Ar Ar Ar NO2 NO2 N N + N N N N NMe2 NEt NEt NEt 17 16 15 Yield (%) Ar Nitroalkene 14 17 16 a Ph 64 8 b 9 60 Ph N 7 84 c S 2. Recyclisation In the reactions of heterocyclic derivatives of amidines with activated alkynes, Michael addition is the preferred reaction route, as opposed to the reactions of N-vinylformamidines.Indeed, the reaction of the compound 9 with dimethyl acetylene- dicarboxylate in acetonitrile (20 8C, 36 h) affords a mixture of dimethyl 1,3-dimethyl-2,4-dioxo-1,2,3,4-tetrahydropyrido[2,3-d ]- pyrimidine-5,6-dicarboxylate 18 and methyl 2,5-dimethyl-3-(3- methyl-1,3-diazabut-1-enyl)-1,4,6-trioxo-2,4,5,6-tetrahydropyrrolo- [3,4-c]pyridine-7-carboxylate 19 in 1 : 2 ratio. The reaction carried out in toluene (16 h) or chloroform (18 h) gives only the compound 19 inHeterocyclisation of trisubstituted formamidines containing an aryl or hetaryl substituent at the imine nitrogen atom 70% yield.It was suggested 14 that this compound results from recyclisation of the uracil residue in theMichael addition product 20 formed initially. MeCN 9+ MeO2CC CCO2Me O O CO2Me N CHNMe2 CO2Me MeN MeN NMe + O N O M Ne O MeO2C 19 18 O CO2Me H CO2Me MeN 7 MePh 9+MeO2CC CCO2Me + N O NMe2 NMe 20 + NCH O NMe2 H 7NMe MeN 7MeO7 O CO2Me CO2Me + O NCH NMe2 H MeN NMe 19 7H+ O O MeO2C A reactive dienophile, 4-nitro-2-phenyloxazole 21, reacts with amidinouracil 9 according to the Michael addition pathway, although its reactions with other dienes follow the [2+4]-cyclo- addition route.15 The intermediate 22 is converted into 6-benz- amido-7-dimethylamino-1,3-dimethyl-2,4-dioxo-1,2,3,4-tetrahyd- ropyrido[2,3-d ]pyrimidine 23 through opening of the oxazole ring followed by an electrocyclic reaction accompanied by aromatisa- tion.O7 N 7O O2N N O N H 9, CHCl3, 20 8C Ph O MeN Ph + O21 N O NMe2 NMe 22 O NHCOPh CH MeN NO2 7HNO2 N O CHNMe2 NMeO NHCOPh MeN O NMe2 NMe N23 3. Reactions with diketene and isocyanates The reactions of N-hetarylformamidines with diketene have been studied in relation to systems containing a pyridine nitrogen atom at the a-position relative to the amidine substituent, namely, N1,N1-dimethyl-N2-benzothiazol-2-ylformamidine 24a, N1,N1- dimethyl-N2-benzoxazol-2-ylformamidine 24b and N1,N1- dimethyl-N2-benzimidazol-2-ylformamidine 24c. These reactions 847 proved convenient for the construction of fused pyrimidine- containing heterocycles 25a ± c.16, 17 COMe O O H2C N N N CHNMe2 O 7Me2NH N X 25a ± c X 24a ± c X = S (a), O (b), NH (c).In the general case, N1,N1-dimethyl-N2-arylformamidines react with aryl isocyanates to give 1 : 2 adducts, 1,3,5-triazines 26.18, 19 However, apart from these products, the reactions afford solid substances with high melting points, which have been identified 20 ± 22 as 1,3,6,8,10-pentaazaspiro[4.5]decane-2,4,7,9-tet- raones 27. Presumably,20, 22 they are formed via bipolar inter- mediate 28, which regioselectively adds isocyanate giving rise to sym-triazinedione 26. Successive insertion of isocyanate into the C7N and C7H bonds results in the spiro compound 27. + NR 7 Me2NHC RNCO RNCO N ArN CHNMe2 O Ar 28 N(R)CONMe2 NMe2 NR ArN NR ArN RNCO RNCO O O O O RN RN O 26 RN RNH(O)C N(R)CONMe2 NR NR ArN OArN NR RNCO 7RNHCONMe2 O O RN O O 27 RN 4-Dimethylamino-2,6-dioxo-5-phenyl-4,5-dihydro-1,3,5-tri- azine-1,3-disulfonyl dichloride 29 is produced in the reaction of the amidine 1a with Graf's isocyanate 28.23 SO2Cl Me2N N ClSO2NCO (28) 1a O PhN N O SO2Cl 29 An unusual cycloaddition of methyl isocyanate to N1,N1- dimethyl-N2-arylformamidines has been proposed 24 as a method for the synthesis of 3-aryl-1,1-dimethyl-3-(1,3,5-trimethyl-4,6- dioxo-1,2-dihydro-1,3,5-triazin-2-yl)ureas 30, possessing herbici- dal properties.O MeN Rn NMeO NMe Rn N CHNMe2 MeNCO 0 ±30 8C N CONMe2 30 Rn=H, Cl, Cl2, 3-CF3 , 4-MeO, 4-Me, 3-Cl-4-MeO, 3-Cl-4-Me, 3-CF3-4-Cl.Some reactions of N-hetarylformamidines with isocyanates differ from the transformations of N-arylamidines because either heteroatoms or activated multiple bonds of the heterocycle are involved in the reaction. Thus the reaction of N1,N1-dimethyl- N2-2-pyridylformamidines 31 with phenyl isocyanate gave a [2+2]-cycloadduct, 4-dimethylamino-1-phenyl-3-pyridin-2-yl-1,3- diazetidin-2-one 32, in a high yield. At elevated temperature in the presence of phenyl isocyanate, this product is converted into848 3-phenylpyrido[1,2-a]-1,3,5-triazine-2,4-dione 33.25 3-Phenylpyr- idazino[1,6-a]-1,3,5-triazine-2,4-dione is prepared in a similar way.26 N N PhNCO CH2Cl2 N CHNMe2 N31 32 N C O PhNCO N Presumably,25 the cycloadduct 32 formed initially undergoes cyclodegradation at elevated temperatures.The subsequent [2+4]-cycloaddition of the resulting isocyanates gives rise to the bicyclic adduct 33. Depending on the temperature and the reactant ratio, the reactions of N1,N1-dimethyl-N2-2-(4,5-dihydrothiazol-2-yl)form- amidines 34 with phenyl isocyanate afford different products, 2-dimethylamino-3-phenyl-2,3,6,7-tetrahydrothiazolo[3,2-a]-1,3,5- triazin-4-one 35, N1,N-dimethyl-N2-(2-dimethylamino-4-oxo-1,3- diphenylhexahydrothiazolo[3,2-a]-1,3,5-triazin-8a-yl)formamidine 36 or 3-phenyl-6,7-dihydrothiazolo[3,2-a]-1,3,5-triazine-2,4-dione 37.27 In the opinion of the researchers cited, phenyl isocyanate reactswith the endocyclic imine nitrogen atom of the hetarylamidine 34 at room temperature; this gives rise to intermediate bipolar compound A.This compound either cyclises to yield compound 35 or reacts with a second phenyl isocyanate molecule, resulting in the amidine 36. At elevated temperatures, the attack by phenyl iso- cyanate on the exocyclic imine nitrogen atom of the amidine 34 proves to be the preferred reaction pathway; this gives initially intermediate B, which is converted into compound 37 upon the reaction with phenyl isocyanate. Apparently, an important stage of this transformation in the latter case is transamination, i.e. the exchange by the imine fragments between phenyl isocyanate and the amidine 34.S N S N PhNCO, 20 8C C N CHNMe2 34 N 7 NPh O N NMe2 S NPh N 35 O N Ph N NMe2 Me2NCHS PhNCO NPh N 36 O S N CH + PhNCO N NMe2 34 C 240 8C 7 NPh O B NMe2 S N NR 71a N O O D NPh 71a NMe2 O N NPh N 33 O CH + NMe2 A G V Oshovsky, A MPinchuk O N S S PhNCO N C O NPh N N 37 O 4. Electrocyclic reactions The transformation of phenyliminoaziridinomethane 38 into 1-phenyl-4,5-dihydro-1H-imidazoline 39 induced by nucleophiles or acids proceeds similarly to the symmetry-allowed synchronous isomerisation of vinylcyclopropane into cyclopentene. The rear- rangement is similar to the isomerization of vinylethyleneimine into pyrroline.The researchers suggested 28, 29 that the reaction of aziridine with phenyl isocyanide gives rise to thermodynamically unstable Z-isomer 38, the transformation of which into the imidazoline 39 proceeds via initial Z,E-isomerization. H N C N NH HCl D NPh N PhNC Ph N C N (Z)-38 Ph 39 H(E )-38 On heating or irradiation, N-hetarylamidines containing a substituent with an unsaturated fragment in the neighbouring position to the amidine group undergo sigmatropic transforma- tions giving rise to more complicated heterocyclic systems. Thermolysis of N1,N1-dimethyl-N2-6-(5-arylideneamino- 1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-yl)form- amidine 40 is accompanied by the aza Cope reaction. An intra- molecular pericyclic reaction followed by isomerisation and elimination of dimethylamine affords 6-substituted 1,3-dime- thylpteridine-2,4-dione 41.30 O O , 300 8C, 3 h Ar Ar N N SO2 MeN MeN 7Me2NH O O NMe2 NMe NMe N 40 N 41 O .Ar=Ph, 4-ClC6H4, 4-MeOC6H4, O Transformations of this type are also typical of systems containing an azo fragment in the neighbouring position to the amidine substituent, although they are accompanied by side processes. Thus N1,N1-dimethyl-N2-6-(1,3-dimethyl-2,6-dioxo- 5-phenylazo-1,2,3,6-tetrahydropyrimidin-4-yl)formamidine 42 is converted at 210 ± 220 8C into a mixture of 3-dimethylamino- 5,7-dimethyl-2-phenyl-1,5-dihydro-2H-pyrimido[4,5-e]-1,2,4-tri- azine-6,8-dione 43 and 8-dimethylamino-1,3-dimethylpyrine-2,6- dione 44.Apparently, the latter product is formed from 9-anilino- 8-dimethylamino-1,3-dimethylpurine-2,6-dione 45.31 O HN NPh MeN O O NMe2 N NMe N 43 MeN NPh D O NHPh O NMe2 N Me N MeN N 42 NMe2 O Me N N 45 O NH MeN NMe2 O Me N N 44Heterocyclisation of trisubstituted formamidines containing an aryl or hetaryl substituent at the imine nitrogen atom The reactions of amidinouracil 9 with azodicarboxylates proceed similarly to transformations of 6-amino- or 6-alkylami- nouracils to give adducts 46. These products are converted into pyrine derivatives 44 upon thermal cyclisation.14 CO2R HN O N MeN CO2R MePh D 44 9+RCO2N NCO2R O CHNMe2 NMe N 46 R=Me, Et. In the photolysis of formamidine 42, 1,5-cycloaddition is the predominant reaction pathway; 32, 33 8-phenylamino-1,3-dime- thylpurine-2,6-dione 47 is formed upon irradiation. A similar transformation takes place in the reductive heterocyclisation of the compound 42 induced by sodium dithionite.34 O NH MeN hn NHPh 42 O N 47 Me N The intramolecular 1,5-cycloaddition in N1,N1-dimethyl-N2- (5-nitrosopyrimidin-4-yl)formamidine 48 proceeds at room tem- perature without irradiation.35 NO NO N N DMF N N NH2 N CHNMe2 48 N N NMe2 NH N III.Intramolecular nucleophilic substitution 1. Reactions with participation of N-nucleophiles N1,N1-Dimethyl-N2-arylformamidines and N1,N1-dimethyl- N2-hetarylformamidines are convenient objects for heterocyclisa- tion, provided that a position adjacent to the amidine substituent is occupied by a reactive functional group (amino, carbonyl, ethoxycarbonyl, nitrile, iminophosphorane group and so on).The reactions in which an N-nucleophile reacts with form- amidine in the key stage of ring formation are summarised in Table 1. It should be emphasised that this approach is especially attractive for the synthesis of fused systems which include five-, six- and seven-membered nitrogen-containing heterocyclic frag- ments. The use of functionalised amidines in the synthesis of various fused pyrimidines has been studied most thoroughly. Cyclisation can either involve a nitrogen-containing nucleo- phile present in the molecule or occur upon the addition of ammonia, amines, hydroxylamine, their salts and hydrazine or guanidine.In some cases, the reaction is accompanied by deami- dination, i.e. transformation of the amidine substituent into an amino group. Aryl- and hetarylformamidines containing a free amino group in the position neighbouring to the amidine substituent are postulated as intermediates in several syntheses of imidazoles from 1,2-diamines. Meanwhile, N2-(2,4-diamino-6-hydroxypyri- midin-5-yl)-N1,N1-dimethylformamidine 49 is a stable com- pound because the electron-donating substituents in the pyrimidine ring decrease the electrophilicity of the formamidine carbon atom. Formamidines 49 can be converted into 2-amino-6-chloro- purine 50 on treatment with HCl and subsequent heating.36 Table 1. Products of heterocyclisation of aryl- and hetarylformamidines X formed with participation of N-nucleophiles.N CHNMe2 X Type of reaction a NH2 b CH2NHPh c CH(R)CH2NH2 d CN e CN f CN g CN h C(O)R i C(O)R1 j CO2Et k P+(Cl)(NR2)2Cl7 NH3 l CHO NH2 HO HO NH2 a N N NH2 (a) POCl3, DMF; (b) HCl, H2O; (c) D,7Me2NH. HCl. Hetarylamidines containing a free amino group are synthes- ised by selective reduction of nitro- or cyano-substituted amidines. Under the reaction conditions, they undergo heterocyclisation according to route a (see Introduction) giving rise to an imidazole fragment. Thus reduction of N1,N1-dimethyl-N2-(2-methyl-3- nitroquinolin-4-yl)formamidine 51 results in N2-(3-amino-2- 849 Reaction product Reagent or reaction conditions N D HN NPh D N R D NH NNHR N RNH2 NNHNNH2 H2NNH2 NNH2 N +O7 H2NOH NNH2 N (H2N)2C=NH N NH2 R N NH3 NR1+NR2 R2NH2 NO NR RNH2 N R2N NR2 P N N O7 N + NH2OH N N N CHNMe2 NH Cl NH2 b, c N N N N 50 NH2 49 NH2850 methylquinolin-4-yl)-N1,N1-dimethylformamidine 52, which undergoes intramolecular cyclisation in situ to give 4-methylimi- dazo[4,5-c]quinoline 53.37 N CHNMe2 N CHNMe2 NH2 NO2 Zn AcOH 7Me2NH Me N Me N 52 51 HN NMe N 53 The electron-donating effect of the dimethyluracil moiety accounts for the stability of N1,N1-dimethyl-N2-(1,3-dimethyl- 2,4-dioxo-6-phenylaminomethyl-1,2,3,4-tetrahydropyrimidin-5- yl)formamidine 54; it is converted into 1,3-dimethyl-7-phenyl-7,8- dihydropyrimido[5,4-d ]pyrimidine-2,4-dione 55 only on heating in toluene.38O O N N CHNMe2 MeN MeN D 7Me2NH NPh O O CH2NHPh NMe NMe 55 54 Cyclisation in situ of N1,N1-dimethyl-N2-[4-(2-amino-1-tri- methylsilyloxyethyl)-1-methylpyrazol-5-yl]formamidine 56, pre- pared by the reduction of N1,N1-dimethyl-N2-[4-(cyano- trimethylsilyloxymethyl)-1-methylpyrazol-5-yl]formamidine 57, is a key stage in the synthesis of 1-methyl-1,4,5,6-tetrahydropyr- azolo[3,4-d ]-1,3-diazepin-4-ol 58.39 ± 41 CHO CH(CN)OSiMe3 Me3SiCN H2, Cat 100 8C N MeN N MeN N CHNMe2 N CHNMe2 57 HO Me3SiO NH NH2 7Me2NH N MeN N MeN N N CHNMe2 58 56 In the synthesis of 3H-2-thia-3,5,6-triazaaceanthrylene 59, the key intermediate, formamidine 60, containing a free amino group, is prepared using the Gewald heterocyclisation of N1,N1- dimethyl-N2-(4-methyl-3-cyanoquinolin-2-yl)formamidine 61.42 Me CN S8,HN DMF N N CHNMe2 61 S S NH NH2 7Me2NH N N N N CHNMe2 59 60 The reactions of compounds containing cyano-, ester or cabonyl group in the position adjacent to the amidine substituent with ammonia derivatives (see Table 1, reactions d ± j ) are widely used to form a fused pyrimidine fragment.In some cases, the reaction with ammonia is accompanied by deamidination. G V Oshovsky, A MPinchuk On treatment with excess Vilsmeier reagent, reactive amino- substituted heterocyclic compounds are converted into formylhe- tarylformamidines.43 For example, 2-aminoindole 62 was used to synthesise N1,N1-dimethyl-N2-(3-formylindol-2-yl)formamidine 63.44 Refluxing this product with ammonium salts in ethanol induces cyclisation giving rise to salt 64 the subsequent reduction of which affords 3-substituted 2,3,4,4a-tetrahydropyrimido[4,5- b]indole 65. This type of cyclisation has been studied in relation to hetarylamidines derived from 3-aminoindole 45, 46 or 3- and 5-aminopyrazoles.47, 48 H O RNH2 .HX 7Me2NH POCl3 DMF N CHNMe2 NH2 63 62 NH NH NR NR + KBH4 N NHal7 65 64 NH NH X=Cl, Br; R=CH2COOH, Et, Ph, (CH2)2NMe2, (CH2)3OH.The behaviour of hetarylformamidines having an acetyl group in the position adjacent to the amidine substituent with respect to low-basicity nitrogen-containing nucleophiles is similar to the behaviour of formyl-substituted derivatives.49, 50 Thus N1,N1- dimethyl-N2-(5-acetyl-6-methylthio-2-phenylpyrimidin-4-yl)form- amidine 66 is converted into 5-methyl-4-methylthio-2-phenylpyr- imido[4,5-d ]pyrimidine 67 when heated with ammonium acetate in butyl alcohol.51 Ph Ph N N N N Me2NCH N N N AcONH4, BuOH, D 7Me2NH,7H2O Ac 66 SMe Me SMe 67 1,2-Disubstituted N1,N1-dimethyl-N2-(4-benzoyl-1H-benz- imidazol-5-yl)formamidine 68, containing a low-reactivity car- bonyl group, is transformed into substituted 9-phenyl-3H- imidazo[4,5-f ]quinazoline 69 only upon prolonged heating in an ethanol solution of ammonia in an autoclave.52 Ph N Me2NCH COPh N N N N R2 R2 NH3, EtOH 7Me2NH NR1 NR1 69 68 R1=CH2CH2OH, Ph; R2=H, Me.The carbonyl groups of anthraquinone can also be involved in the amidine cyclisations.53, 54 For example, 1,5-bis(3-methyl-1,3- diazabut-1-enyl)-9,10-dioxo-9,10-dihydroanthracene 70 is con- verted into 1,3,7,9-tetraazaperylene 71 on refluxing with ammo- nium acetate in ethylene glycol methyl ether.55 This reaction proved convenient for the synthesis of anthrapyrimidine dyes.56, 57 N N O N Me2NCH NH4OAc 7Me2NH.AcOH N N 71 70 O N CHNMe2 The reactions of alkylamines with hetarylamidines containing an ethoxycarbonyl group next to the amidine substituent provide the possibility of constructing a pyrimidone fragment. Thus ethyl 5-(3-methyl-1,3-diazabut-1-enyl)-1H-pyrazolyl-4-carboxylates 72 were used to synthesise 5-substituted pyrazolo[3,4-d ]pyrimidin-4-Heterocyclisation of trisubstituted formamidines containing an aryl or hetaryl substituent at the imine nitrogen atom ones 73.58 3-Benzyl-7-methylpyrido[2,3-d ]pyrimidin-4-one was prepared in a similar way from the corresponding pyridine derivative.59 O EtO2C R2N N R2NH2 7Me2NH N N Me2NCH N NR1 73 72 RN1 R1=H, Ph; R2=CH2Ph, CH2CH2Ph, MeCHCH2Ph.The reactions of nitriles with nitrogen-containing nucleophiles are widely used to synthesise heterocyclic compounds.60, 61 Thus the reactions of aryl- and hetaryl-formamidines containing a cyano group in the position adjacent to the amidine substituent with ammonia, amines or their salts were found to be convenient for the preparation of fused aminopyrimidines.This approach is used in combinatorial chemistry.62 7-Amino-2-methyl-2H-1,2,3-triazolo[4,5-d ]pyrimidine 74 was obtained by refluxing N1,N1-dimethyl-N2-(5-cyano-2-methyl- 2H-1,2,3-triazol-4-yl)formamidine 75 in an aqueous solution of ammonium acetate.63, 64 NH2 CN N N N MeN MeN NH4OAc 7Me2NH.AcOH N N N N CHNMe2 74 75 The same strategy was employed to prepare adenine 76,65 4-amino-6,8-di- 78,70 5,6,7,8-tetrahydro- 4-aminopyrazolo[3,4-d ]pyrimidine 77,66 ± 69 methoxy-5-hydrobenzo[e]pyrimido[4,5-b]azepin-10-one 4-aminopyrrolo[2,3-d ]pyrimidine 79,71 1,3,5,10-tetraazaphenanthren-4-ylamine 80,72 4-aminoquinazo- line 8173 and their derivatives.OMe NH2 NH2 NH2 N N N N MeO N NHN HN N N NH 78 O 77 76 NH2 NH2 N N N N NHHN N N N H2N 81 80 79 The transformation of N1,N1-dimethyl-N2-(3-cyano-5,6,7- trihydropyrrolo[1,2-a]imidazol-2-yl)formamidine 82 into substi- tuted 4-amino-5,6,7-trihydropyrrolo[2,1-f ]purines 83 has been studied and a mechanism has been proposed for this reaction.74 Presumably, the reaction starts with transamination at the form- amidine carbon atom, accompanied by evolution of dimethyl- amine. This is followed by an intramolecular attack by the NH group on the cyano carbon atom giving rise to 5-substituted 4-imino-5,6,7-trihydropyrrolo[2,1-f ]purines 84. On treatment with nucleophiles, the products 84 undergo the Dimroth rear- rangement; as a result, the aminopyrimidine fragment adds to the initial heterocyclic system.N N N N RNH2 7Me2NH N N CHNHR CHNMe2 82 CN CN 851 N N N N N N Nu or NuH CHNu NR NH RHN 84 HN N N N N RHN 83 Nu=OH7, EtO7; NuH=HOH, EtOH, RNH2 . Partial hydrolysis of hetarylformamidines containing a cyano group in the position adjacent to the amidine group is accompa- nied by heterocyclisation which gives a pyrimidone fragment. Thus treatment of 5-dimethylaminomethylideneamino-4-cyano- pyrazoles substituted at the 1-position yields pyrazolo[3,4-d ]pyr- imidin-4-ones 85. Apparently, the cyclisation proceeds via amides 86.75 O NC H2N HCl N NR N NR N Me2NCH Me2NCH N86 O HN N NR N 85 Cyclisation of hetarylformamidines can be used to prepare organophosphorus compounds.76, 77 For example, the phospho- rus salts 87 cyclise on treatment with ammonia to yield 3-methyl- 4,4-bis(amino)-1-phenylpyrazolo[4,5-e]-1,3,4l5-diazaphosphin- ines 88 via intermediate iminophosphonic diamides 89.78 Cl Me NR2 Me R2N + P (R2N)2P NH3 HN Cl7 7Me2NH N NPh N NPh N N Me2NCH Me2NCH 89 87 R2N NR2 Me P N N NPh N 88 O, NEt2.NR2=N When the amidine molecule incorporates two different reac- tion sites which occupy positions neighbouring the amidine group, the more reactive site participates predominantly in the cyclisa- tion. For instance, N1,N1-dimethyl-N2-(1-methoxy-7,8-di- methyl-4-oxo-6-cyanoisochroman-5-yl)formamidine 90 is con- verted into 6-methoxy-7,8-dimethyl-4H,6H-5-oxa-1,3-diazaphe- nalene-9-carbonitrile 91, the cyano group remaining intact during the reaction.79 N N O N Me2NCH NC NC NH4OAc O O Me Me Me OMe 91 Me OMe 90852 The same functional groups located in different positions of the heterocycle can differ substantially in reactivity.Thus hetero- cyclisation of dicyano-substituted 4-dimethylaminomethylide- neamino-2-oxo-1,2-dihydropyridines 92 occurs selectively giving rise to 8-cyano-4,7-dioxo-3,4,6,7-tetrahydropyrido[4,3-d ]pyrimi- dines 93.80 O O NC NC NR NR HCl N N Me2NCH 92 CN O NH93 R=Ph, 4-MeC6H4, 4-BrC6H4. The reaction of N1,N1-dimethyl-N2-(5,7-dicyano-2,3-dihydro- 1H-pyrrolizin-6-yl)formamidine 94a with benzylamine involves the 5-cyano group and results in 8-benzylamino-4-cyano-2,3- dihydro-1H-pyrrolo[10,20:5,1]pyrrolo[2,3-d ]pyrimidine 95a.The second cyano group does not participate in the reaction. In the case where different functional groups are present in the positions neighbouring the amidine substituent, the cyclisation pathway depends not only on the reactivity of each group but also on their positions in the ring. Indeed, in the cyclisation ofN1,N1-dimethyl- N2-(5-cyano-7-ethoxycarbonyl-2,3-dihydro-1H-pyrrolizin-6-yl)- formamidine 94b, the more reactive cyano group is involved in the reaction, which affords 8-benzylamino-4-ethoxycarbonyl-2,3-di- hydro-1H-pyrrolo[10,20:5,1]pyrrolo[2,3-d ]pyrimidine 95b.81 R R PhCH2NH2 N N N N CHNHCH2Ph CHNMe2 CN CN 94a,b R R N N N N Dimroth rearrangement N N HN CH2Ph PhCH2NH 95a,b R=CN (a), COOEt (b).The fact that the reaction follows only this pathway can appear unexpected because the a-position of the unsubstituted pyrrole ring is enriched in electron density, and the cyclisation would rather be expected to involve the electrophilic substituent located in the less electron enriched b-position. However, the presence of two electron-withdrawing groups induces redistrib- ution of the partial charges on the carbon atoms of the ring in both the ground and transition states. As a consequence, the electronic effects of the heterocyclic fragment of the molecule substantially change. In the above example, the reactivity of the nitrile group in the a-position of the pyrrole ring increases and, hence, cyclisation is highly regioselective.The assumption concerning electron density redistribution is supported by the fact that the ester groups at the 4-position of 2,4-pyrroledicarboxylic acid diesters are more stable against hydrolysis than the 2-ester groups.82 In the case of 5-benzoyl-7-cyano-6-(3-methyl-1,3-diazabut-1- enyl)-2,3-dihydro-1H-pyrrolizine 96, the carbonyl group is much more reactive than the nitrile group; therefore, cyclisation affords 4-cyano-8-phenyl-2,3-dihydro-1H-pyrrolo[10,20:5,1]pyrrolo[2,3-d ]- pyrimidine 97. The reaction is accompanied by deamidination, which gives compound 98.83 G V Oshovsky, A MPinchuk CN NH3 N MeOH N CHNMe2 96 COPh CN CN N N NH2 + N N COPh 98 (45%) Ph 97 (53%) Kadushkin et al.84 have studied the influence of the size of the ring fused to the pyrrole fragment on the pathway of cyclisation of polymethylenepyrrole derivatives, namely, 7-cyano-6-dimethyl- aminomethylideneamino-5-ethoxycarbonyl-2,3-dihydro-1H-pyr- rolizine 99a, 1-cyano-2-dimethylaminomethylideneamino-3- ethoxycarbonyl-5,6,7,8-tetrahydroindolizine 99b and 1-cyano-2- dimethylaminomethylideneamino-3-ethoxycarbonyl-6,7,8,9-tet- rahydro-5H-pyrrolo[1,2-a]azepine 99c.CN PhCH2NH2 N (H2C)n N CHNMe2 99a ± c CO2Et CN N (H2C)n N CHNHCH2Ph CO2Et CN Pathway a (H2C)n N N NCH2Ph 100a ± c O HN NCH2Ph Pathway b Dimroth rearrangement N (H2C)n N CO2Et PhCH2NH NN (H2C)n N 101a ± c CO2Et Compound n Yield (%) 100 101 <5 40 95 123 79 42 <5 abc In the case of the compound 99a, pathway b predominates, i.e.the reaction involves the nitrile group and gives compound 101a. When the ring size and, hence, steric overcrowding, increases, the heterocyclisation involves the ethoxycarbonyl group activated by the neighbouring heterocyclic fragment. The contribution of pathway a increases and, correspondingly, the compounds 100b,c are formed in higher yields.Heterocyclisation of trisubstituted formamidines containing an aryl or hetaryl substituent at the imine nitrogen atom Hydrazine hydrate is also used to perform cyclisation of hetarylformamidines containing a nitrile or carbonyl group in the position adjacent to the amidine substituent.Thus 5-cyano-4- dimethylaminomethylideneamino-2-methylthiopyrimidine 102 reacts with hydrazine hydrate being thus converted into 3-amino-4-imino-7-methylthio-3,4-dihydropyrimido[4,5-d ]pyrim- idine 103.85 However, in some cases, only deamidination is observed in these reactions instead of cyclisation.86 N N MeS N N MeS CHNMe2 N NNH2 H2NNH2 .H2O 7Me2NH N CN 103 102 NH Since the formation of a pyrimidine ring is more favourable than the formation of a 1,2,4-triazepine ring,87 only one nitrogen of the hydrazine molecule participates in the cyclisation.88 ± 90 The free amino group can be used in subsequent transformations. Thus the reaction of N1,N1-dimethyl-N2-(2-benzoyl-4-chloro- phenyl)formamidine 104 with hydrazine hydrate has served as the first stage in the synthesis of symm-triazolo[4,3-a]-1,4-benzo- diazepine derivatives 105.91 N CHNMe2 H2NNH2 .H2O O Cl 104 Ph N ClCH2COCl NNH2 Cl Ph OHCOCH2Cl NCH NNHCOCH2Cl 1) ClCH2CO2H 2) NH3 O Cl Ph N N N N Cl 105 Ph Treatment with hydroxylamine of hetarylformamidines con- taining a carbonyl or cyano group neighbouring an amidine group gives rise to pyrimidine N-oxides.44 ± 47, 92, 93 For instance, 5-chloro-3-dimethylaminomethylideneamino-4-formyl-1-phenyl- pyrazole 106 was converted into 3-chloro-5-oxy-2-phenylpyr- azolo[3,4-d ]pyrimidine 107, while 3-cyano-2-dimethylamino- methylideneaminopyrazine 108 was transformed into 4-amino-3- oxypteridine 109.N CHNMe2 N N N H2NOH + PhN PhN 7Me2NH N CHO O7 107 Cl 106 Cl CN N CN N H2NOH 7Me2NH N CHNHOH N N N CHNMe2 108 NH2 O7 N N + N N109 853 It should be emphasised that, unlike the reactions with ammonia, hydrazine or their salts, in this case, the initial product of replacement of the dialkylamino group 110 can be detected and, in some instances, isolated. This difference is due to the fact that the initial reaction product 110 is equilibrated with its tautomer 111, the latter predominating in the reaction mixture. Compounds of the type 111 do not tend to undergo heterocyclisation, which proceeds more slowly. NOH HetNHCH 111 HetN CHNHOH 110 N-Hetarylformamidines in which the amidine substituent occupies the neighbouring position with respect to the pyridine- type nitrogen atom 94 can be used to synthesise fused heterocycles containing a 1,2,4-triazole fragment.95 For example, 4-dimethyl- aminomethylideneamino-1,11,11-trimethyl-3,5,6-triazatricyclo- [6.2.1.02,7]undeca-2,4,6-triene 112 was converted into 11,14,14- trimethyl-3,4,5,7,9-pentacyclo[9.2.1.02,9.04,8]tetradeca-2,5,7,9-tet- raene 113 in a high yield.96 The triazole fragment (compound 114) is formed upon nucleophilic attack by a ring nitrogen atom on the nitrogen atom attached to the hydroxy group in the product 113 formed in the reaction with hydroxylamine.The cyclisation occurs on treatment with polyphosphoric acid (PPA) with heating.The high potential of this approach was also demonstrated in relation to derivatives of pyridine,97 pyrimidine,98 pyridazine,89 pyrazine,99 symm-triazine and some other polycyclic systems.73, 100, 101 Me MeH NN NH2OH. HCl, MeOH, 25 8C 7Me2NH Me N 112 N CHNMe2 Me Me MeH MeH N N PPA, 120 8C N N N Me Me N N N 114 113 HNCH NOH Apparently, cyclisation induced by hydroxylaminesulfuric acid follows a similar route.102, 103 Thus N1,N1-dimethyl-N2-2- pyridylformamidine derivatives 115 are converted into the corre- sponding 1,2,4-triazolo[1,5-a]pyridines 116. 2-Pyridylcyanoa- mides 117 are formed as side products. Presumably, the dimethylamino group is initially replaced by hydroxylaminesulfu- ric acid giving rise to compound 118, which cyclises.The side product 117 is produced from tautomer 119. R R H2NOSO3H CH N N N N NHOSO3H CHNMe2 118 115 R N N N 116 R R NHCN NHCH N N NOSO3H 117 119 R=H, 2-Me, 3-Me, 4-Me. When hetarylamidines containing a cyano group near the amidine function are subjected to cyclisation on treatment with854 guanidine, two amino groups are incorporated simultaneously into the pyrimidine ring formed. This approach was used to prepare 2,4-diaminopyrimido[4,5-b]quinoxaline 120 from 3-cyano-2-dimethylaminomethylideneaminoquinoxaline 121.104 CN N HN C(NH2)2 N CHNMe N 121 2. Reactions involving C-nucleophiles Cyclisations of hetarylamidines in which the substitution of the amidine amino group gives rise to a C7C bond are presented in a general form in Table 2.As in the case of N-nucleophiles, this approach permits the construction of five-, six- and seven-membered heterocycles. Aryl- and hetarylformamidines containing a methyl group 2-dimethylaminomethylideneamino-3- next to an amidine group are converted into heteroannelated pyrroles on heating with sodium phenylmethylamide in methyl- aniline.105, 106 Thus Me X NaNMePh 180 8C Me N N X Ph 122a,b X NH X123a,b X = N (a), CH (b). Table 2. Products of cyclisation of hetarylamidines formed on treatment with C-nucleophiles. Type of Hetarylamidine reaction Me a N CHNR2 Me b N CHNR2 PhNCH2R2 c N CHNR12 COMe d N CHNR2 e N CHNR2 +P(CH2R2)(NR32 )2 Hal7 f N CHNR12 CH2COR2 N g N CHNR12 NH2 N N N N NH2 120 7 X CH2 Me N N X Ph 124a,b Reagent or re- Reaction action conditions product B or flash pyrolysis HNCHO POCl3 ±DMF HN Ph N D NH O B NH CF3CO2H N R32 N NR3 R2 P B NCOR2 N B N G V Oshovsky, A MPinchuk methylpyrazine 122a is transformed into 5H-pyrrolo[2,3-b]pyra- zine 123a.The indole 123b was prepared from N1,N1-dimethyl- N2-2-tolylformamidine 122b in a similar way. This process can be regarded as a modified Madelung synthesis of indoles.107 The reaction occurs in the presence of strong bases via intermediate carbanions 124a,b, or under conditions of vacuum flash pyrol- ysis.108 Treatment of heterocyclic compounds having methyl and amino groups in the vicinal positions with excess Vilsmeier reagent results in the formation of a pyrrole fragment containing a b-formyl group.109, 110 For example, 5-amino-4-hydroxy-2,6- dimethylpyrimidine 125 was converted into 4-hydroxy-2-methyl- 5H-pyrrolo[3,2-d ]pyrimidine-7-carbaldehyde 126.111 The first stage gives amidinium salt 127, which is then formylated at the methyl group by the Vilsmeier reagent.The CH-acidity of the methylene group in intermediate 128 is enhanced, which facilitates heterocyclisation. The amidinium salt 127 does not cyclise when heated in DMF in the absence of POCl3, which confirms the proposed mechanism.111 Me N Me 1) (MeO)2CHNMe2 2) HCl N NH2 125 OH + N Me CH2CH NMe2 Me Me N DMF OP(O)Cl¡2N N POCl3 CHNMe2 CHNMe2 NH+ H+N Cl7 128 OH Cl7 127 OH + CH NMe2 CHO N Me N Me X7 N N NH NH OH 126 OH X=Cl, OP(O)Cl2 .When hetarylamidines in which the pyridine nitrogen atom R2 occupies an adjacent position to the amidine substituent are made to react with halo ketones, fused imidazole-containing hetero- cycles are produced. For example, 3-(4-hydroxybenzoyl)-8-meth- ylimidazo[1,2-a]pyridine 129 was prepared by the reaction of N1,N1-dimethyl-N2-(3-methylpyridin-2-yl)formamidine 130 with 2-bromomethyl 4-hydroxyphenyl ketone. In this process, the pyridine nitrogen atom is alkylated to give intermediate 131. The high CH-acidity of the methylene group in this compound 112 ensures easy cyclisation, which proceeds via enolate anion 132.Therefore, the basicity of the amidine substituent is normally sufficient for the subsequent transformations. Me OH BrCH2CO N CHNMe2 N 130 Me N CHNMe2 +N Br7 B 7B ,7HBr O 2 HO 131Heterocyclisation of trisubstituted formamidines containing an aryl or hetaryl substituent at the imine nitrogen atom Me Me N CHNMe2 N N +N 7 7Me2NH O O HO HO 129 132 Despite the ambident properties of the enolate anion 132, the process follows only the route involving the attack of the C-nuc- leophile on the electron-deficient formamidine centre; after elim- ination of dimethylamine, this results in the formation of a fused imidazole ring. This reaction has been used to synthesise deriva- tives of imidazo[1,2-a]pyridine,113 imidazo[1,2-a]pyrimidine,114 imidazo[1,2-b]-1,2,4-triazine,115 imidazo[2,1-b]thiazole,116 imid- azo[2,1-b]-1,3,4-thiadiazole, pyrrolo[1,2-a]imid-azole 117 and so on.The enolate anion generated from hetarylamidines having an acetyl substituent in the position adjacent to the amidine fragment also acts as a C-nucleophile. Intramolecular heterocyclisation results in the annelation of a pyridone fragment.118 For example, 4-methylthio-2-phenyl-5-oxo-8H-pyrido[2,3-d ]pyrimidine was prepared from 5-acetyl-4-dimethylaminomethylideneamino-2- phenyl-6-thiomethylpyrimidine. Ph N N Me2N MeONa N Me SMe O Ph N N Me2N MeOH N CH27 Na+ SMe O Ph N Ph N HN HN Me2N N N AcOH 7Me2NH SMe O SMe O Cyclisation of 2-(3-methyl-1,3-diazabut-1-enyl)-4-trifluoro- methylacetophenone yields 4-dimethylamino-7-trifluoromethyl- quinoline 134 instead of the expected 4-hydroxy-7- trifluoromethylquinoline 133.119 This unusual behaviour was explained by assuming that oxygen is replaced by dimethylamine at the stage of enolisation.The decrease in the nucleophilicity on passing from the enolate anion to enamine is counterbalanced, in the opinion of the authors, by the increase in the electrophilicity of the amidine substituent due to protonation. OH O N F3C 133 Me NMe2 N CHNMe2 F3C H+ + HNMe2 CHNMe2 F3C + NH NMe2 NMe2 7H+, 7HNMe2 N F3C NMe2 F3C NH 134 855 Phosphonium ylides generated from pyrazolylphosphonium salts 135 undergo intramolecular heterocyclisation in situ giving rise to pyrazolo[5,4-b]-1,4l5-azaphosphinines 136.76, 78, 120 4-Nitrobenzylphosphonium salts cyclise much more easily than methylphosphonium salts because of their higher CH-acidity.CH2R1 Me R22 N + P R22 NHal7 EtONa 7EtOH, 7NaHal N NPh N Me2NCH 135 R22 N NR22Me R22 N NR22Me P R1 P R1HC 7Me2NH N NPh N N NPh N Me2NCH 136 O, NEt2. R1=H,Hal = I;R1=4-NO2C6H4, Hal=Br; NR22 = N Heating of N-methyl-(2-dimethylaminomethylideneamino- 5-nitrophenyl)-phenylmethylideneaminoacetamide 137 gives 3-methylaminocarbonyl-7-nitro-5-phenyl-1H-benzo[e]-1,4-diaz- epine 138 as the major product.121 In the opinion of the authors, cyclisation occurs through the substitution of the enolate anion for the dimethylamino group.N CHNMe2 D 7Me2NH NCH2C(O)NHMe O2N 137 Ph N HN N CONHMe+ O2N N O2N Ph MeN 139 138 (50% ± 70%) O Ph The side product, 1-methyl-9-nitro-10b-phenyl-1,10b-dihy- droimidazo[1,2-c]quinazolin-2-one 139, results apparently from an intramolecular cyclisation of the aza-Cope type of the product of rearrangement of the initial compound 137. The electrophilicity of the amidine substituent markedly increases upon protonation. This was used to synthesise 7,8- dimethoxy-1-methyl-3-phenylpyrazolo-[3,4-c]isoquinoline 140 and 6H-pyrrolo[3,4-c]b-carboline-1,3-dione 141 from the corre- sponding formamidines.122, 123 Cyclisation occurs upon refluxing in trifluoroacetic acid over a period of 8 ± 12 h.Me Me NNPh NNPh MeO MeO 1) CF3COOH, D N N 2) NH3, H2O MeO MeO 140 NR2 O O NH NH O O 1) CF3COOH, D N N 2) NH3, H2O NH NH141 NR2 O . NR2= N856 3. Reactions involving O- and S-nucleophiles N,N-Dimethylformamidine hydrochloride and dimethylamino- ethoxyacetonitrile are mild reagents used to transform the amino group into the amidine group. However, when they were made to react with 1,2-hydroxyamino or 1,2-sulfanylamino compounds, no amidines were produced; only cyclisation products � anne- lated oxazoles or thiazoles�were isolated instead. The formation of functionalised amidine intermediates 142 in these reactions was just postulated.2 Aminophenols, aminobenzenethiols or the cor- responding pyrimidine and pyridazine derivatives were used as the initial compounds.XH HN CHNMe2 . HCl or (Me2N)EtOCHCN NH2 XH X 7Me2NH N N CHNMe2 142 X=O, S. The systems containing an N-oxide fragment in the adjacent position to the amidine substituent are more stable; however, they undergo recyclisation on heating or on treatment with hydroxyl- amine salts.124, 125 For example, N-hydroxy-N0-(1,2,4-oxadiazol- 3-ylpyridin-2-yl)benzamidines 143a and N-hydroxy-N0-(1,2,4- oxadiazol-3-ylpyridazin-2-yl)benzamidines 143b were prepared from the corresponding formamidines 144a,b.HON Ph Ph N N N NH + H2NOH. HCl, MeOH 7Me2NH N X N X O7 CHNMe2 N O 144a,b 143a,b N X=CH (a), N (b). Treatment of hetarylformamidines containing a nitrile group next to the amidine group with hydrogen sulfide in the presence of bases is used to furnish a pyrimidinethiol fragment.126 This method was used, for example, to transform 5-cyano-4-dimethyl- aminomethylideneamino-2-methyl-2H-1,2,3-triazole 145 into 2-methyl-7-thioxo-2H-1,2,3-triazolo[4,5-d ]pyrimidine 146.SH CN N N N MeN MeN H2S, NaOH, EtOH, D 7Me2NH N N N N CHNMe2 146 145 A similar reaction was used 66 in the synthesis of 3-cyano-4- methylthiopyrazolo[3,4-d ]pyrimidine 147 from 3,4-dicyano-5- dimethylaminomethylideneamino-1H-pyrazole. It was suggested that 4-imino-3-thiocarbamoyl-1,4-dihydropyrazolo[3,4-d ]-1,3- thiazine 148 undergoes the Dimroth rearrangement to give intermediate 149.The compound 149 reacts with MeI to give the reaction product 147. S CN NH CNH2 NC NaOH S H2S, NaOH, EtOH, D 7Me2NH N NH N Me2NCH N N NH 148 G V Oshovsky, A MPinchuk S SH HN SH SMe CNH2 MeI N N NaOH 7MeSH N NH N N N NH 149 SMe CN N N N NH 147 IV. Other heterocyclisation reactions This Section surveys transformations of different types in which nucleophilic substitution at the formamidine carbon atom or cycloaddition involving the C=Nfragment are not the key stages in the synthesis of heterocyclic compounds. The leading role in these reactions is played by polyfunctional reagents which react with the amidine or by transformations of the heterocyclic frag- ment of N-hetarylformamidines.In the general case, during cyclisation of trisubstituted aryl- and hetarylformamidines with functionalised nitriles, first, the amidine amino group is replaced and subsequently N-nucleophilic heterocyclisation occurs. Thus the reaction of 6-dimethylamino- methylideneamino-9H-purine 150 with cyanamide starts with substitution of cyanamide for the dimethylamino group to give derivative 151a; heterocyclisation resulting in 3-substituted 7-imino-3H-1,3,5-triazino[2,1-i ]purine 152 occurs in the tautomer 151b. The compound 152 reacts in situ with excess cyanamide to be converted into 7-amidinoimino-3H-1,3,5-triazino[2,1-i ]purine.127 N Me2NCH N H2NCN N 7Me2NH NR N 150 NCN CHN N NCNHCH N N HN N NR NR N N 151b 151a H N N N N N H2NCN N N N N N H2N HN NR NR N N 152 Trisubstituted formamidines react with aminocyanoaceta- mide according to the transamination pattern.The resulting N,N0-disubstituted amidines 153a are equilibrated with their tautomers 153b; the latter undergo cyclisation to give imidazole derivatives 154 due to the reaction between the NH and CN groups.128 This strategy has also been employed in the synthesis of purine derivatives.129, 130 RN CHNMe2+H2N(NC)CHCONH2 7Me2NH RN CHNHC(CN)HCONH2 153a NH2 RN RNHCH NC(NC)HCONH2 153b CONH2 154 N R=Ph (1a), Bn.Heterocyclisation of trisubstituted formamidines containing an aryl or hetaryl substituent at the imine nitrogen atom Highly basic N1,N1-dimethyl-N2-phenylformamidine 1a per- forms two functions in the reaction with cyanoacetamide 155; first, it facilitates the transformation of cyanoacetamide into the carbanion on heating, and, second, it acts as the substrate in which this carbanion substitutes the dimethylamino group.The reaction yields 3-anilino-2-cyanoacrylamide 156, which is converted into pyrazole derivative 157 on refluxing in an aqueous solution of hydrazine hydrate for 1 h.131 CONH2 N2H4 .H2O 7Me2NH 1a+NCCH2CONH2 155 PhNHCH 156 CN CONH2 N NH2 NH 157 In this reaction, the amidine, like triethyl orthoformate, acts as a one-carbon synthon for the synthesis of pyrazole derivatives. WhenN-hetarylformamidines are heated with hippuric acid in acetic anhydride, nucleophilic substitution at the formamidine carbon atom is accompanied by heterocyclisation.This reaction has been performed for pyridine, pyrimidine, pyrazine, pyrida- zine, oxazole and benzo-2,1,3-thiadiazole derivatives.132 Com- pounds of the type 158 are the key intermediates in the synthesis of b-hetarylamino-a,b-dehydro-a-amino acids.133 ± 136 HetN CHNMe2+PhCONHCH2CO2H Ac2O, 70 8C 7Me2NH O Ph O N HetNHCH 158 Aromatic or heteroaromatic compounds containing more than two electron-donating substituents in the ring, including 1,2-diamino derivatives, behave unusually when made to react with dimethylformamide dimethyl acetal. These reactions can give bis- or even tetrakis-amidines, instead of fused imidazoles, for example, 1,2,4,5-tetrakis(dimethylaminomethylideneamino)ben- zene 159 and 2,4,5-tris(dimethylaminomethylideneamino)ben- zene 160.Apparently, the [2-aminoaryl(hetaryl)]amidines formed initially cannot undergo cyclisation because of the low electro- philicity of the formamidine carbon atom. The corresponding fused imidazole-containing heterocyclic compounds � benzo- [1,2-d ;4,5-d 0]diimidazole 161 and 2-dimethylaminomethylidene- aminopurine 162 � can be prepared by subsequent pyrolysis in the presence of Al2O3 or on heating in diethylene glycol.137, 138 NH2 H2N (MeO)2CHNMe2 NH2 H2N N N CHNMe2 Me2NCH N N D Al2O3 NH HN N N CHNMe2 Me2NCH 161 159 N Me2NCH N N CHNMe2 N N CHNMe2 N D N N HN N Me2NCH 162 160 The amidine substituent can also function as a leaving groterocyclisation.Thus 5-dimethylaminomethylideneamino-6- (2-dimethylaminovinyl)pyrimidine-2,4-dione 163 is converted into pyrrolo[3,2-d ]pyrimidine-5,7-dione 164 on treatment with ammo- nia.139 857 O O NH2 N CHNMe2 N CHNMe2 HN HN NH3 O O CH CHNMe2 CH CHNMe2 NH HN 163 O O N CHNMe2 NH NH HN HN 7HN=CHNMe2 O O NH164 NH According to this scheme, ammonia adds to the double bond of the heterocycle, being attached to the more electrophilic carbon atom at the a-position relative to the carbonyl group. A similar transformation has been used to prepare 5,7-dioxa- 6-aza-12,14-dione 165 from 2-amino-4-oxo-4H-chromenyl-3- carbaldehyde.140 O O O ON CHNMe2 NH N O O 7HNCHNMe2 O O O O OHC NH2 165 Partial deamidination occurs during the cyclisation of 40-dimethylaminomethylideneamino-2,20-dimorpholino-5-formyl- [4,50]bithiazole 166, giving rise to 2,7-dimorpholinothiazo- lo[4,5-b]thiazolo[4,5-d ]pyrimidine 167.141 R2N R2N S S N N CHO Ac2O, D N N CHNMe2 S S N N166 R2N 167 R2N O .NR2= N The ease of cleavage of the isoxazole ring at theN7Obond on treatment with nucleophiles or upon reduction accounts for the non-classical behaviour of isoxazolylamidines under heterocycli- sation conditions. For example, when 4-cyano-5-dimethylamino- methylideneaminoisoxazole 168 is treated with a hot aqueous solution of ammonia, instead of the expected formation of 4-aminoisoxazolo[5,4-d ]pyrimidine 169 or hydrolysis of the ami- dine substituent, recyclisation occurs giving rise to 4-amino-5- cyanopyrimidin-6-one 170.142 N ON N N Me2NCH NH3 NH2 169 ON N H2N NC O N R2NCH 168 N NC CN NC 170 O R=H, Me.858 The reduction of 4-dimethylaminomethylideneaminoisoxa- zol-5-ones 171 catalysed by palladium supported on carbon affords imidazoles 172 in high yields 143 (90 ± 95%).The key stage of the heterocyclisation is apparently intramolecular trans- amination at the formamidine carbon atom by the iminocarbonyl function, resulting from reduction of the N7O bond. R2 R1 N R1N CHNMe2 1) Pd/C (10%), H2, EtOAc, 20 8C, 0.5 h 2) HBr, AcOH N NH O R2 O 172 171 Formamidines react with trivalent phosphorus halides in the presence of bases to give molecules with a phosphorus7form- amidine carbon bond.144, 145 This approach has been used to synthesise phosphorus-containing heterocyclic compounds.For example, treatment ofN1,N1-dimethyl-N2-arylformamidines 173 with PBr3 in the presence of bases followed by treatment with secondary amines and heating affords 3H-benzaazaphospholes 174.146, 147 1) PBr3, Py, NEt3 N 2) HNR22 , NEt3 R1 NMe2 N CHNMe2 3) D P R1 174 173 NR22 R1=Me2N, MeO, Me, H, Br; R2=Me, Et, Pri. V. Conclusion The data considered here, which are concerned with heterocycli- sation of N1,N1-disubstituted N2-aryl- and N2-hetarylformami- dines, clearly demonstrate the high synthetic potential of these compounds and the prospects for using them to synthesise various heterocyclic systems, including those difficult to prepare other- wise.However, it should be noted that the synthetic capacities of these compounds are far from being exhausted. The diverse reactivity of amidines is determined by the electrophilicity of the amidine carbon atom, the strong electron-donating ability of the amidine substituent, and the possibility of being involved in nucleophilic reactions and in pericyclic reactions of various types. 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Russian Chemical Reviews 69 (10) 861 ± 882 (2000) Monoorganyl derivatives of tellurium(IV) A A Maksimenko, A V Zakharov, I D Sadekov Contents I. Introduction II. Methods of synthesis III. Reactions of s-telluranes RTeX3 IV. Structures of s-telluranes RTeX3 in the crystalline state and in solution. Electronic effects of TeX3 groups V. Conclusion Abstract. of structures and reactions synthesis, the on Data Data on the synthesis, reactions and structures of monoorganyl RTeX of derivatives monoorganyl derivatives of tellurium( tellurium(IV) (s-telluranes) -telluranes) RTeX3 are compounds these of use The generalised. and systematised are systematised and generalised. The use of these compounds in in preparative The considered. is chemistry organic preparative organic chemistry is considered.The bibliography bibliography includes references 238 includes 238 references. I. Introduction Derivatives of tetracoordinate tellurium RTeX3 (X is an anionoid substituent) belong to the so-called s-telluranes. Compounds R2TeX2 and tetraorganyltelluranes R4Te are also s-telluranes. s-Telluranes RTeX3 are important starting compounds in the synthesis of various organic derivatives of tellurium. Thus cyclisa- tion of aryl- and arylvinyltellurium trichlorides is a method for the preparation of various tellurium-containing heterocycles, which is specific to the chemistry of organotellurium compounds. In addition, detelluration and halodetelluration of s-telluranes RTeX3 are used in organic synthesis. The two last-mentioned fields of application of monoorganyl derivatives of tellurium(IV) are based on the electrophilic properties of the trichlorotellurium group and a low (compared to other chalcogens) energy of the C7Te bond.s-Telluranes are also characterised by higher stabilities compared to analogous sulfur and selenium derivatives because the higher energy levels of the p orbitals of tellurium are favourable for bonding.1, 2 Thus sulfur compounds RSCl3 decom- pose to give RSCl and Cl2 even at very low temperature, RSeCl3 are stable at room temperature but decompose to RSeCl and Cl2 on heating, whereas RTeCl3 are stable up to their melting points.3 Previously, s-telluranes RTeX3 have been considered in a review (see Ref. 4) and in the corresponding sections of mono- graphs (see Refs 3 and 5 ± 7).II. Methods of synthesis Most compounds of the RTeX3 type have been prepared from tellurium tetrachloride and diorganyl ditellurides as the starting compounds. Various derivatives of RTeX3 can also be synthesised by anion exchange in organyltellurium trihalides. Approaches A A Maksimenko, A V Zakharov, I D Sadekov Research Institute of Physical and Organic Chemistry of the Rostov State University, prosp. Stachki 194/2, 344090 Rostov-on-Don, Russian Federation. Fax (7-863) 228 56 67. Tel. (7-863) 228 08 94 Received 17 April 2000 Uspekhi Khimii 69 (10) 940 ± 962 (2000); translated by T N Safonova #2000 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2000v069n10ABEH000594 861 861 870 875 879 based on the use of hexahalotellurates, complexes of tellurium tetrachloride with aromatic amines, tellurinic acids and their anhydrides have been studied to a substantially lesser extent.The corresponding reactions were either examined for a few com- pounds or afforded a limited number of compounds. 1. Reactions of tellurium tetrahalides with heteroorganic compounds The most important preparative procedures for the synthesis of organyltellurium trihalides involve exchange reactions of tellu- rium tetrahalides with heteroorganic compounds. These reactions were used primarily for the preparation of aryltellurium trichlor- ides and aryltellurium tribromides. Poor solubility of TeI4 in non- polar solvents and its decomposition in polar solvents prevent its use in the synthesis of tellurium triiodides.In principle, a general procedure for the synthesis of organ- yltellurium trihalides could involve reactions of equimolar amounts of tellurium tetrahalides with organolithium or -magne- sium compounds. RTeX3 RM+TeX4 7MX M=Li, MgX; X=Cl, Br. However, these reactions afford complex mixtures of products due to a more profound exchange. Nevertheless, the reaction of 2-biphenylyllithium with TeCl4 gave rise to 2-biphenylyltellurium trichloride, albeit in low yield (16%).8 (N,N-Dimethylcarbamoyl)tellurium trichloride 1 was pre- pared by the reaction of TeCl4 with dimethylaminocarbonylso- dium and isolated as a complex with DMF.9 1) CCl4 Me2NC(O)Na +TeCl4 2) DMF Me2NC(O)TeCl3 .DMF 1 Organyltellurium trihalides can be synthesised by the reac- tions of tellurium tetrahalides with organic derivatives of Hg, Si, Sn or Pb.The procedure most generally employed involves the reaction of TeCl4 with organomercury compounds. This proce- dure has been used for the synthesis of aryltellurium trichlorides 2. Unlike telluration (see below) wich yields p-substituted aryltellu- rium trichlorides, this procedure is more versatile because it allows one to synthesise compounds with any substituents (including electron-withdrawing groups) at any position.8, 10 ± 21 ArTeCl3+HgCl2 ArHgCl+TeCl4 2862 Ar Ref. Ar 17 S Br NO2 14, 15 16 16 16 16 16 11, 16 11,16 C6F5 4-BrC6H4 4-ClC6H4 4-NO2C6H4 3-FC6H4 3-NO2C6H4 4-MeC6H4 Ph 13 Cl 12 Me 8 20 2-PhC6H4 4-PhNHC6H4 But Me O 10 The above-mentioned reaction is best carried out in anhyd- rous dioxane (sometimes CHCl3 is used) because the resulting mercury(II) chloride forms a 1 : 1 complex with dioxane, which is poorly soluble at room temperature.Generally, the aryltellurium trichlorides formed in admixture with HgCl2 and, hence, it is necessary to purify the reaction products by recrystallisation or to reduce them to diaryl ditellurides followed by chlorinolysis. Arylmercury chlorides were used as the starting compounds in the synthesis of aryltellurium tribromides 3. If molecules of the initial organic substrates contain amino groups 22 or the substrates represent pyridine 23, 24 or quinoline derivatives,24 their reactions with TeCl4 afford mixtures of unidentified compounds due apparently to side complexation reactions.In this case, TeBr4 is more suitable. ArTeBr3+HgBr2 ArHgCl+TeBr4 3 Ar Ref. Ar Br NH2 22 NH2 NO2 22 Br Me NH2 22 NO2 23, 24 N Organyltellurium trihalides can also be synthesised from silicon or tin derivatives or, less often, lead or germanium derivatives. The reactivities of heteroorganic derivatives R4E (E=Si, Ge, Sn or Pb) and R3EX (X=Cl or Br) increase as the atomic weight of the element increases. The derivatives of the R4E type are more reactive thanR3EX. Compounds R2EX2 andR3EX do not react with tellurium tetrahalides. The Ar7E bond in mixed derivatives ArEAlk3 is cleaved under the action of electrophilic tellurium tetrahalides.Thus aryltellurium trichlorides 2 were prepared in 65% ±80% yields upon refluxing a mixture of Ref. O CO2H 10 SPh 13 NHCOMe 21 NHCOMe 21 NHCOMe 19 NHCOMe 19 NHCOMe 19 N NPh 18 Ref. 24 NNHCOMe 21 NHCOMe 21 A A Maksimenko, A V Zakharov, I D Sadekov equimolar amounts of aryltrimethylsilanes and TeCl4 in toluene for 1 h.25, 26 D ArSiMe3+TeCl4 7Me3SiCl ArTeCl3 2 Ar=4-BrC6H4, Ph, 4-MeC6H4, 3-MeC6H4, 2-C6H4CH2Ph. In benzyltrimethylsilane, the Csp37Si bond is inert with respect to TeCl4. Refluxing of equimolar amounts of these compounds in toluene afforded a mixture of 4-Me3SiCH2C6H4 ± TeCl3 and (4-Me3SiCH2C6H4)2TeCl2.27 The corresponding aryltrimethylsilanes were used for the synthesis of bis(trichlorotelluro)benzenes.Thus 1,4-bis(trichloro- telluro)benzene 28 and 1,2-bis(trichlorotelluro)benzene 29 were prepared in *50% yields upon refluxing equimolar amounts of TeCl4 and 1,4-bis(trimethylsilyl)benzene or 1,2-bis(trimethyl- silyl)benzene, respectively, in 1,2-dichlorobenzene. 4-Hydroxyaryltellurium trichlorides 4 were synthesised by the reactions of aryl trimethylsilyl ethers with TeCl4 in boiling toluene in yields higher than 80%.30 OTeCl3 OSiMe3+TeCl4 R R5a,b HO TeCl3 R 4a,b R = H (a), Me (b). It was assumed 30 that the reaction proceeded via intermediate aryloxytellurium trichlorides 5 followed by rearrangement into compounds 4.This mechanism was supported by the synthesis of aryloxytellurium trichloride 5c from TeCl4 and the trimethylsilyl ether of 5-bromosalicylaldehyde.30 Apparently, this compound is stabilised due to coordination of tellurium to the carbonyl oxygen atom. H Br O O TeCl3 5c The reactions of TeF4 with silylated perfluoroalkanols 6 afforded aryloxytellurium trifluorides 7 in 76%± 82% yields.31 PhH, 50 ± 60 8C 7Me3SiF TeF4+Me3SiOCH2(CF2)nH 6 F3TeOCH2(CF2)nH 7 n=2, 4, 6. N-Trichlorotellurioamines 8,32 9 33 and 10 34 were synthesised by the reactions of the corresponding N-trimethylsilyl derivatives with TeCl4. NSiMe3 Ph NSiMe3 Ph +TeCl4 NTeCl3 CH2Cl2 7Me3SiCl N(SiMe3)2 8 Me3Si TeCl3 N Ph3P NSiMe3+TeCl4 Ph3P N PPh3 PhMe 7Me3SiCl 9 TeCl3 Ph2S NSiMe3+TeCl4 Py 7Me3SiCl Ph2S NTeCl3 .Py 10 Organotin compounds react with tellurium tetrahalides at room temperature.35 ± 38 The yields of organyltellurium trihalides vary from moderate to very high.Monoorganyl derivatives of tellurium(IV) PhH or PhMe RTeX3+R3SnX R4Sn+TeX4 Ref. Ref. R X R X Br Cl Me Et Pr 35 35 35 35 Ph 35 35 35 35, 36 37 Me Et Pr Ph 4-MeC6H4 PhH or CH2Cl2 RTeX3+R2SnX2 R3SnX+TeX4 Ref. Ref. R X R X Br Cl 38 38 38 36 Et Ph Et Ph The reaction of tributylphenylstannane with TeCl4 resulted in the cleavage of the C(Ar)7Sn bond to form phenyltellurium trichloride in 80% yield.37 Tetraarylgermanes react with tellurium tetrachloride like organotin derivatives.37 Ar4Ge+TeCl4 ArTeCl3+Ar3GeX 2 Ar=Ph, 4-MeC6H4. Germanium derivatives Ar3GeCl do not react with TeCl4.37 Tetraarylplumbanes react with TeCl4 to form diaryltellurium dichlorides even under mild conditions.39 Phenyltellurium tri- chloride was prepared with the use of Ph3PbCl.39 Dioxane PhTeCl3+Ph2PbCl2 Ph3PbCl+TeCl4 2.Reactions of TeCl4 with activated arenes The salt-like structure of tellurium tetrachloride, viz., [TeCl3]+Cl7 (see Refs 40 ± 44) or [TeCl3]+[TeCl5]7 (see Ref. 45), both in the solid state and in solutions was confirmed by IR and Raman spectroscopy 40 ± 45 and the data on conductiv- ity.41, 42 This compound exhibits moderate electrophilicity and can be involved in electrophilic substitution (telluration) with arenes containing electron-donor substituents.The telluration is highly regioselective. The trichlorotellurio group enters exclu- sively into the para position with respect to the electron-donor substituent in the ring.13, 16, 27, 46 ± 56 Heating of azobenzene with TeCl4 afforded 2-trichlorotellurioazobenzene.57 The yields of aryltellurium trichlorides 2 vary over a wide range. D R R +TeCl4 TeCl3 2 Ref. R Ref. R 16 16 56 16 16 13 46 OPr OBu OPh OAc SMe SPh NHAc 46 47 48, 49 27 16, 46, 50 ± 52 53 16, 46, 51, 54, 55 OH Br Me CH2SiMe3 OMe OCH2CO2Me OEtMorgan and Drew 55 were the first to demonstrate the high selectivity of telluration. He prepared bis(4-ethoxyphenyl) ditel- luride by reduction of the product of the reaction of TeCl4 with phenetole and demonstrated that the reaction of the ditelluride with nitric acid afforded exclusively 4-nitrophenetole.This selec- tivity was also proved by IR and 1H NMR spectroscopy for a large number of compounds. 863 However, C-telluration does not occur if the benzene ring of compounds contains such a strong electron-donor substituent as the dimethylamino group; instead, the complex 2Me2NPh . TeCl4 is formed.58, 59 Analogous complexes were also obtained from other aromatic amines.59 ± 62 The corresponding tellurium tri- chlorides can be isolated from these complexes by subsequent treatment (see below). A decrease in the basicity of nitrogen prevents complexation with TeCl4.For example, acetanilide underwent telluration in the para position.46 Organyltellurium trichlorides 11 ± 13 were synthesised from the corresponding derivatives of acridine or quinoline and TeCl4 in CCl4.63 TeCl3 RN TeCl3 N 11 12 (R=NH2, COOH) CO2H N Cl3Te 13 However, other quinolines and acridines,63 various pyri- dines,42, 45, 60, 63 ± 65 indoles, carbazoles,63 Schiff's bases 66 ± 69 and hydrazones 70 react with tellurium tetrachloride to form only molecular complexes. Aromatic compounds containing electron-withdrawing sub- stituents (nitrobenzene or 4-nitroanisole) do not react with TeCl4,50 whereas refluxing of TeCl4 with benzonitrile led to trimerisation of the latter yielding 2,4,6-triphenyltriazine.47 Under ordinary conditions, benzene does not react with TeCl4 due to which it can be used in telluration as the solvent along with CHCl3 and CCl4.The introduction of one methyl group into the benzene ring leads to only weak activation of the ring with respect to telluration. Although prolonged boiling (20 h) of a solution of TeCl4 in toluene afforded 4-methylphenyltellurium trichloride in 83% yield,49 toluene can be used as the solvent in certain reactions of TeCl4 with sufficiently reactive arenes.16 The TeCl3 group possesses strong electron-withdrawing prop- erties (see Section IV). Only one trichlorotellurium group can be introduced into the ring by telluration, unlike mercuration, which often gives rise to polymercurated compounds.Polynuclear hydrocarbons, for example, naphthalene, react with TeCl4 without additional activation of their nuclei by introducing electron-donor substituents. The reaction of TeCl4 with naphthalene in toluene under conditions of kinetic control afforded 1-trichlorotellurionaphthalene in 24% yield,16 which agrees with the usual direction of electrophilic substitution in naphthalene. However, prolonged heating of naphthalene with TeCl4 at 110 8C without a solvent (under conditions of thermody- namic control) gave rise to 2-trichlorotellurionaphthalene in 54% yield.71 The reaction of anthracene with TeCl4 afforded a mixture of 9-chloro- and 9,10-dichloroanthracenes.72 In the case of arenes containing two substituents, the TeCl3 group is introduced in the para position with respect to the stronger electron-donor substituent.16, 48, 50, 55, 73 ± 75 If this posi- tion is occupied, the introduction occurs in the ortho position to this substituent.50 R1 R1 D +TeCl4 TeCl3 R2 R2 14a ± i864 Ref.R2 R1 Compound 14 48 50 55 16 55 73 74 74 75 3-Me 3-Me 2-MeO 3-MeO 3-OH 3-Me 2-Cl 3-Cl 5-Me 4-Me 4-OMe 4-OMe 4-OMe 4-OMe 4-OH 4-OH 4-OH 2-OMe abcdefghi Telluration of dialkylphenols occurs in the para position with respect to the hydroxy group.75 R R HO HO +TeCl4 TeCl3 D 7HCl R R R=Me, But. The reactions of TeCl4 with 4-bromoanisole and 3-methoxy- toluene also gave rise to aryltellurium trichlorides.50 However, the position of the introduction of the trichlorotellurium group into the ring was not established.50 Acetamidophenyltellurium trichlorides 15 were synthesised by telluration of 3-mono- and 3,4-disubstituted acetanilides.19 NHCOMe NHCOMe R1 R1 CHCl3, D +TeCl4 R2 R2 TeCl3 15 R1=R2=Me; R1=Me, OH, OMe, SMe; R2=H.Aryltellurium tribromides and triiodides cannot be prepared by telluration of arenes with tellurium tetrabromide or tetra- iodide, respectively, even in the case of such active compounds as anisole, phenetole or veratrole. Apparently, this is associated with a decrease in electrophilicity of tellurium tetrahalides TeX4 on going from chlorine to iodine. The electrophilicity of tellurium tetrachloride can be increased by forming complexes of TeCl4 with Lewis acids.In particular, the complex [TeCl3]+[AlCl4]7 was formed with AlCl3.43, 76 Bergman 47 was the first to use this approach. However, prolonged boiling of the components in the presence of catalytic amounts of AlCl3 afforded a mixture of aryltellurium trichlorides and diaryltellurium dichlorides. A more convenient procedure was proposed by Gunther et al.77 The reactions are carried out with a two-to-threefold excess of AlCl3. The course of the reaction is monitored by measuring the amount of hydrogen chloride evolved, and the reaction is stopped after evolution of one equivalent of HCl. As mentioned above, direct telluration of arenes containing NR2 substituents (R=H or Alk) cannot be performed because these compounds form 2 : 1 complexes with tellurium tetrahalides, which are poorly soluble in ordinary solvents (benzene, toluene or chloroform).Prolonged boiling of these complexes in anhydrous MeOH under an inert atmosphere did lead to electrophilic substitution at the para position with respect to the NR2 group.61 4-R2NC6H4TeX3 2 (R2NPh) . TeX4 MeOH, D 7Me2NPh .HX R=H, Me; X=Cl, Br. However, attempts to synthesise 4-dimethylaminophenyltel- lurium trichloride by this procedure failed.59 It is believed 59 that 2 : 1 complexes of aromatic amines with TeCl4 can have different structures depending on the nature of the amines. In the com- plexes with N- and ortho-substituted amines 16, the tellurium- containing group is located at position 4 with respect to the NR1R2 group.Reduction of these complexes affords mixtures of A A Maksimenko, A V Zakharov, I D Sadekov 4-amino-substituted diaryl tellurides 17 and the corresponding diaryl ditellurides 18. The ditellurides 18 can be converted into the tellurides 17 by treatment of the reaction mixture with a copper powder or Pd/C. The yields of the diaryl tellurides 17 vary over a wide range (8% ± 72%).591) Na2S2O5 2) NaHCO3 [R1R2N(ArH)]2TeCl4 16 Cu(0) or Pd/C 17 (R1R2NAr)2Te+(R1R2NAr)2Te2 18 17 R1R2NAr=4-Me2NC6H4, 4-MeNHC6H4, 4-PhNHC6H4, 4-H2NC6H4, 4-H2N-3-MeC6H3, 4-H2N-3,5-Me2C6H2, 4-H2N-3-EtOC6H3, 4-H2N-3-CF3C6H3, 4-H2N-3-MeOCOC6H3. Reduction of 2 : 1 complexes of substituted anilines with TeCl4 (19) gives rise to elemental tellurium and the corresponding anilines.59 1) Na2S2O5 Te+H2NAr 2) NaHCO3 (H2NAr)2TeCl4 19 ArNH2=4-MeC6H4NH2, 3,5-Me2C6H3NH2, 3-O2NC6H4NH2, 2,4,6-Br3C6H2NH2.The synthesis of p- or o-substituted aminophenyltellurium trihalides was carried out also with the use of hexahalotellurates of amines, which have been prepared in high yields from amines and solutions of tellurium tetrahalides in hydrohalic acids.61, 62 Refluxing of these salts in anhydrous methanol was accompanied by elimination of HX to form the corresponding aminoaryltellu- rium trihalide. It was believed 62 that hexahalotellurates in meth- anolic solutions exist in equilibrium with free amines and tellurium tetrahalides and the electrophilic substitution occurs at the nucleus of the free amine.+ MeOH, D R2NArTeX3 (R2NHAr)2TeX2¡ 6 R2NAr=4-H2NC6H4, 4-MeNHC6H4, 4-Me2NC6H4, 4-H2N-3-MeC6H3, 2-H2N-5-MeC6H3, 2-H2N-3,5-Me2C6H2, 4-H2N-3,5-Me2C6H2; X=Cl, Br, I. 3. Reactions of TeCl4 with compounds containing active methylene groups The reactions of aliphatic compounds containing activated methyl or methylene groups with TeCl4 can proceed as electrophilic substitution at the saturated carbon atom. Generally, ketones containing at least one methylene group at the a position with respect to the carbonyl group react with TeCl4 (a solution in CHCl3) in a molar ratio of 2 : 1 to form mixtures of organyltellurium trichlorides 20 and diorganyltellurium dichlor- ides 21. The trichlorides can be isolated provided that they are poorly soluble and form precipitates.78 ± 82 In some cases, the trichlorides were obtained as the only reaction products in high yields.81, 82 It was suggested 78 that the reaction proceeded through the addition of TeCl4 at the double bond of the enol followed by elimination of HCl. Diisopropyl ketone did not react with TeCl4.78, 79 CHCl3, D R1COCH2R2+TeCl4 R1COCHR2TeCl3 + (R1COCHR2)2TeCl2 21 20 Ref.R2 Ref. R1 R2 R1 Ph 78 H 82 H 82 Ph 4-ClC6H4 2-HOC6H4 78 ± 80 78 80 Me Me H Me Et PrMonoorganyl derivatives of tellurium(IV) Ref. R2 Ref. R1 R2 R1 H 8281 81 78 ± 80 78, 80 78 Et HEt Pr But Ph 3-MeOC6H4 (CH2)6 (CH2)7 The reactions of TeCl4 with some polyfunctional ketones gave rise to organyltellurium trichlorides of the type 20 as the only products.83 Thus telluration of 2-acetylcyclohexanone, 3-acetyl-7- methoxycoumarin and 2,6-diacetylpyridine afforded trichlorides 22 ± 24, respectively. Apparently, the trichlorotellurium group in the compounds 22 ± 24 exhibits low electrophilicity due to the formation of intramolecular O?Te coordinate bonds, and the reaction stops at this stage.The presence of intramolecular coordinate bonds was confirmed by IR and NMR spectroscopy and X-ray diffraction study of the trichloride 24 (see Section IV). The ketones 22 ± 24 are more stable to hydrolysis than other organyltellurium trichlorides, which was also attributed to intra- molecular O?Te coordinate bonds.83 O O TeCl3 TeCl3 MeO O O O 22 23 O Me N O Te Cl3 24 The compounds 22 ± 24 have been used for the preparation of thermo- and photothermographic materials.84 Trichlorotellurio ketones 25 and 26 containing the trichlor- otellurium group at the b position with respect to the carbonyl group were prepared by the reactions of trimethylsilyloxycyclo- propanes 27 and 28, respectively, with TeCl4 (the yields were 71%± 93%).85 The compounds 25 and 26 were subjected to dehydrotelluration without isolation from the reaction mixture (see Section III).Based on the IR spectral data, Nakahira et al.85 assumed the presence of intramolecular O7Te coordinate bonds in the ketones 25 and 26. O TeCl3 Me3SiO CH2Cl2, 0 8C, 10 min 7Me3SiCl +TeCl4 R2 R1 R1 27 R2 25 R1=But, , Ph; R2=H, Me.OSiMe3 O TeCl3 R1 R1 CH2Cl2, 0 8C, 10 min 7Me3SiCl (CH2)n +TeCl4 (CH2)n R2 R2 26 28 R1=R2=H, n=2, 5;R17R2=(CH=CH)2, n=1. In b-diketones, the most acidic hydrogen atoms of the methylene group are replaced by the TeCl3 group only if the carbonyl groups bear aryl substituents. Thus the reaction of dibenzoylmethane with TeCl4 afforded very unstable (dibenzoyl- methyl)tellurium trichloride (PhCO)2CHTeCl3.86 The reactions of TeCl4 with b-diketones containing at least one hydrogen atom at each of the a- and a 0-carbon atoms give rise to 1,1-dichloro-1- telluracyclohexane-3,5-diones 29 as the major products. Diorga- nyltellurium dichlorides and organyltellurium trichlorides 30 can be isolated in rare cases and only in reactions carried out in ethanol-free CHCl3 as the solvent.87 ± 89 865 O O R2 Te R1 Cl3TeCH(R1)C(OH) CHCOR2 30 Cl Cl 29 R1, R2=Alk. Ref.R2 R1 87 88 89 H C6H13 H C8H17 Pr Pr The formation of organyltellurium trichlorides is favoured by the presence of two or three alkyl substituents at one of the a-carbon atoms and the presence of at least one hydrogen atom at the a 0-carbon atom. If the organyltellurium trichlorides formed bear at least one hydrogen atom at position 3 and chloroform used as the solvent has not been freed from EtOH, the reactions yielded tellurium trichlorides of ethyl enolates 31.86 ± 90 The position of the ethoxy group in these compounds has not been established unambiguously.However, taking into account the strong elec- tron-withdrawing properties of the trichlorotellurium group, it can be assumed that the ethoxy group is located at the b position with respect to the TeCl3 group.22 R1COCH=C(OEt)CR2R3TeCl3 31 Ref. R3 R2 R1 Me H H 90 H H 87 H Et 89 H H 87 Me Me 88 H H 86 H H 87 H H 86 H 87 Me Prn Prn Pri Pri But PriCH2 Ph Et The reactions of tellurium tetrachloride with acetic or pro- pionic anhydrides taken in a ratio of 1 : 2 in CHCl3 afforded mixtures of the corresponding a-trichlorotellurocarboxylic acids and anhydrides.50, 91 However, these trichlorides were converted without isolation into ditellurides 32 under the action of an aqueous solution of K2S2O5.CHCl3, D (RCH2CO)2O+TeCl4 R RCHCOOH RCHCO O CH Te2 + K2S2O5 H2O TeCl3 TeCl3 2 2 HO2C32 R=H, Me. Methylenebis(tellurium trichloride) was isolated in low yield from the mixture obtained by the reaction of TeCl4 with acetic anhydride.91 Later, this compound was prepared by refluxing a mixture of TeCl4 with Ac2O (taken in a molar ratio of 3 : 1) in CHCl3 in 52% yield.92 The mechanism of formation of methyl- enebis(tellurium trichloride) was proposed.92 (Cl3TeCH2CO)2O+HCl, (MeCO)2O+TeCl4 (Cl3TeCH2CO)2O+TeCl4 Cl3TeCH2CO2TeCl3+Cl3TeCH2COCl, CH2(TeCl3)2+CO2 . Cl3TeCH2CO2TeCl3 Attempts to perform telluration of anhydrides of higher aliphatic acids resulted only in elimination of tellurium to form resinous products.866 Of other compounds containing the active methylene group, nitromethane was subjected to telluration.93 D O2NCH2TeCl3 MeNO2+TeCl4 4.Addition of tellurium tetrachloride to multiple bonds The major and, apparently, the only procedure for the preparation of alkanes and alkenes containing the TeCl3 groups and the chlorine atoms at the vicinal positions involves the electrophilic addition of TeCl4 to double or triple bonds. a. Reactions of TeCl4 with alkenes Under mild conditions (0 ± 60 8C), TeCl4 adds to the double bond of acyclic or cyclic alkenes to form b-chloroalkyl- and b-chloro- cycloalkyltellurium trichlorides 33.94 ± 104 The reactions are gen- erally performed in CHCl3, CCl4 or MeCN. In some cases, bis(b- chloroalkyl)- and bis(b-chlorocycloalkyl)tellurium dichlorides are formed along with the corresponding trichlorides.The addition of TeCl4 proceeds regioselectively. Thus the reactions of TeCl4 with terminal alkenes occur as a Markovnikoff addition. Cl3Te(R1)CHCH(Cl)R2 R1CH=CHR2+TeCl4 33 Ref. R2 Ref. R1 R2 R1 Me 99, 100 99, 100 94 ± 96, 100, 102 ± 104 HHHH Me(CH2)3 (CH2)4 CH2CH(Me)(CH2)2 103, 104 99, 100 (CH2)6 97 96, 98 96 101 99, 100 99, 100 HMe Et Bn H C8H17 D C8H17 The direction of the reactions of TeCl4 with alkenes and primarily with cycloalkenes depends substantially on the temper- ature. Thus the reactions of cyclohexene and 4-methylcyclohexene with TeCl4 in CHCl3, CCl4 or MeCN at 25 8C gave rise to the adducts 33, whereas the reactions carried out with boiling afforded benzene and toluene, respectively, as the major prod- ucts.103, 104 Dienes containing isolated double bonds, viz., 1,4-cyclohex- adiene 104 and 1,5-cyclooctadiene,104, 105 also react with tellurium tetrachloride.In these cases, TeCl4 adds to only one double bond to form organyltellurium trichlorides 34 and 35 in 49% and 64% yields, respectively. Diallyl sulfide reacts analogously to give the trichloride 36, which is stabilised by an intramolecular S7Te coordinate bond (2.763 A).106 TeCl3 S TeCl3 Cl3Te Cl Cl Cl 35 34 36 Unlike dienes containing isolated double bonds, cyclic dienes with conjugated double bonds (1,3-cyclohexadiene and 1,3-cyclo- octadiene) react with TeCl4 to form primarily dichlorocycloal- kenes.104 The stereochemistry of the addition of TeCl4 to alkenes has been studied.100 Thus the reactions of (Z)- and (E)-but-2-enes generally give rise to mixtures of isomers due to competitive syn- and anti-addition.p-Benzoquinone as a substrate favours the formation of a syn-addition product. It is believed that the stereospecific syn-addition occurs more or less according to a concerted mechanism. The free-radical reaction competes with this addition. The addition of aryltellurium trichlorides to alkenes, unlike that of TeCl4, occurs nonstereoselectively. A A Maksimenko, A V Zakharov, I D Sadekov The reactions of allylic derivatives with TeCl4 take different paths depending on the nature of the substituents at the sp3 carbon atom.107 The reactions of TeCl4 with allylic alcohols are generally accompanied by resinification of the reaction mixtures and the reaction products cannot be isolated.The reactions of TeCl4 with allyl ethers occur according to the Markovnikoff rule giving rise to trichlorides 37 in yields above 80%. Cl CHCl3 OR Cl3Te OR +TeCl4 37 R=Bn, 4-ClC6H4. The reaction of TeCl4 with N-allylbenzamide,107 like that with N-acetyldiallylamine,108 afforded zwitterionic dihydrooxazoli- nium salt 38. PhNH + O CHCl3 NHCOPh +TeCl4 7 38 Cl4TeH2C The addition of TeCl4 to allyl carboxylates proceeded anom- alously. In this case, 1,3-addition products, viz., b-acyloxy-g- chloropropyltellurium trichlorides 39, rather than 1,2-addition products were obtained.107 OCOR CHCl3 Cl Cl3Te OCOR +TeCl4 39 (73% ± 97%) R=Me, Ph, 4-MeC6H4, 4-ClC6H4, 4-O2NC6H4, (E)-CH=CHMe, (E)-CH=CHPh.The reaction conditions depend on the nature of the substitu- ent at the carbonyl carbon atom. Thus allyl benzoate and allyl 4-methylbenzoate reacted at room temperature, the reactions with allyl 4-chloro- and 4-nitrobenzoates proceeded only on boiling in chloroform, whereas allyl 2,4-dinitrophenylbenzoate did not react at all even on prolonged heating of the reaction mixture. The reactions of TeCl4 and TeBr4 with allyl cinnamate in a mixture of Et2O and H2O at730 8C occurred as 1,3-addition.109 According to the data from IR and 1HNMRspectroscopy,107 compound 39 (R=Me) in solution adopts the conformation A in which the TeCl3 group is located in the gauche position with respect to the acetoxy group.This conformation is stabilised through the intramolecular O7Te coordinate bond. Me O TeCl3 O H H H CH2Cl A Tellurium tetrachloride reacts with a-methylallyl a,a-dime- thylallyl and a-ethylallyl benzoates to give also 1,3-addition products.109 b. Reactions of TeCl4 with alkynes The reactions of TeCl4 with alkynes have been studied to a substantially lesser extent than the reactions with alkenes. The reactions of TeCl4 with alkynes in CCl4 proceed regio- and stereospecifically giving rise to 2-chlorovinyltellurium chlorides 40 in yields of 75% and higher.54, 110, 111 The Z configuration of the compounds 40 was established by halodetelluration yielding (Z)-chlorobromo(iodo)alkenes (see Section III).111Monoorganyl derivatives of tellurium(IV) CCl4 R1C CR2 +TeCl4 Ref.R2 R1 111 54, 110, 111 111 111 110, 111 H CH2 OH Ph Ph Ph Ph HMe Et Ph 5. Oxidation of diorganyl ditellurides A wide range of organyltellurium trihalides were prepared by oxidation of diorganyl ditellurides with chlor- ine,22, 26, 91, 112 ± 120 bromine,13, 26, 50, 51, 91, 113, 114, 116, 117, 120 ± 131 iodine 13, 51, 120, 121, 123, 127, 129 ± 132 or fluorine diluted with an inert gas 133 at room or reduced temperature. Organyltellurium tri- chlorides can be prepared with the use of sulfuryl chloride 112, 116 or thionyl chloride 114, 115, 117, 120 instead of chlorine.These reac- tions are used primarily for the preparation of alkyltellurium trichlorides, aryltellurium tribromides and aryltellurium triio- dides. X2 RTeX3 R2Te2 Ref. X R 133 118 119 91 114 114 112, 115 Br F Ph Cl Me Cl Et Cl HO2CCH2 Cl 3-FC6H4 Cl 4-FC6H4 Cl PhN 120 Cl S Cl 2-OCHC6H4 Cl 2-NH2-4-MeC6H3 Cl 2-NH2-4-BrC6H3 Cl 2-NH2-4-NO2C6H3 Cl 2-PhCH2C6H4 Cl 2-Me2NCH2C6H4 122 130 91 50 125 114 113 22 22 22 26 116 Cl 2-Me2NCH(Me)C6H4 117 Br Me Br Et Br HO2CCH2 Br MeCH(CO2H) Br 2-ClC6H4 Br 3-FC6H4 Br 4-FC6H4 Br Ph Br 2-MeC6H4 Br 4-MeC6H4 114, 125 124, 127 I 113, 125 51, 124, 127, 131 I N 120 Br S Br 2-Me2NCH2C6H4 116 Br 2-Me2NCH(Me)C6H4 117 Poly(ditellurides) are also oxidised with halogens.Thus treat- ment of poly(methylene ditelluride) with chlorine 91, 93 or bro- mine 91 afforded bis(trihalogenotellurio)methanes. R2 R1 Cl TeCl3 40 Ref. X R Br 4-MeOC6H4 Br 4-EtOC6H4 13, 50, 51, 123, 124, 127 13, 51, 121, 123, 124, 127 13 129 Br NMe2 128 Br Br 2-PhCH2C6H4 Br 4-PhOC6H4 Br 4-PhSC6H4 Br 2-PhC6H4 Br 4-PhC6H4 I Me I Et I Ph I 4-MeC6H4 I 4-MeOC6H4 4-EtOC6H4 26 13, 124, 127 13 126, 131 131 130 130 127 51, 127, 131 13, 51, 121, 123, 127 13, 51, 123, 127 13 129 I 131, 132 131 13, 127 13 2-PhC6H4 4-PhC6H4 4-PhOC6H4 4-PhSC6H4 IIII 867 CH2(TeX3)2 CH2TeTe n +X2 X=Cl, Br.The reaction of bis{2-[N-(p-tolyl)benzylaminomethyl]phenyl} ditelluride (41) with SO2Cl2 yielded the telluracycle 42.134 NR Te Te Te SO2Cl2 7HCl, 7SO2 Cl Cl CH2NHR RHNCH2 42 41 R=4-MeC6H4. The behaviour of dibenzyl ditelluride in the reactions with halogens is anomalous compared to other ditellurides. Treatment of the former with bromine in CCl4 led to the cleavage of both C7Te bonds giving rise to benzyl bromide and TeBr4.135 The reactions with the use of less than one equivalent of bromine afforded dibenzyltellurium dibromide as the major product. (PhCH2)2TeBr2 1 equiv. of Br2 7Te (PhCH2)2Te2 2 equiv. of Br2 PhCH2Br+TeBr4 Ditelluride 43 also behaved anomalously. Thus the reaction with bromine yielded the telluracycle 44.136 O O +TeBr4 +Br2 Te Te Te Br Br 44 43 Organyltellurium trifluorides were synthesised not only with the use of fluorine but also using XeF2 (see Refs 133, 137 and 138) or ClF.138, 139 Ditellurides containing perfluorinated organic groups were converted into organyltellurium trifluorides 45 under the action of XeF2,133, 138 whereas diphenyl ditelluride under the same conditions was converted into phenylpentafluoro- tellurium 46.137 XeF2 R2Te2 RTeF3 45a,b R=C2F5 (a), C6F5 (b).XeF2 Ph2Te2 PhTeF5 46 The data on the reaction of bis(pentafluoroethyl) ditelluride with ClF are contradictory. Desjardins et al.139 believed that the reaction afforded C2F5TeF3 as the major product (trans- C2F5TeClF4 and TeClF5 were isolated as by-products in low yields), whereas Lau et al.138 reported that a mixture of C2F5TeClxF37x, C2F5Cl and trans-C2F5TeClF4 formed.Some disulfides act as oxidising agents with respect to diaryl ditellurides. Thus thermochromic aryltellurium tris(dithiocarba- mates) 47 were prepared by the reactions of diaryl ditellurides with thiuram disulfides 48 in almost quantitative yields.140 ArTe SCNR1R2 R1R2NCS CHCl3 Ar2Te2+ S S 2 3 47 48 Ar=Ph, 4-MeOC6H4; R1=R2=Me, Et; R17R2=(CH2)5. Oxidation of diaryl ditellurides with lead tetraacetate afforded the corresponding aryltellurium triacetates.141 ± 143 The presence of these compounds in solutions was established by 1H NMR spectroscopy. However, attempts to isolate these compounds were unsuccessful due to rapid hydrolysis.868 6.Exchange reactions of anionoid substituents s-Telluranes RTeX3 containing different anionoid substituents can be synthesised with the use of organyltellurium trichlorides in which the chlorine atoms are readily replaced by more nucleo- philic anions X7. Exchange reactions were used for the synthesis of a series of organyltellurium tribromides 61, 73, 74, 82 and organ- yltellurium triiodides.13, 61, 73, 74, 82 ± 95 Generally, exchange reac- tions are performed in aqueous-methanolic (ethanolic) solutions under the action of potassium halides. RTeCl3+KX RTeX3 Ref. R R X X Ref. 82 13 Br, I 4-ClC6H4COCH2 I 4-HOC6H4 Cl 95 I Br, I 4-Me2NC6H4 Br, I 3-Me-4-HOC6H3 Br, I 2-Cl-4-HOC6H3 Br, I 3-Cl-4-HOC6H3 Br, I 2-HOC6H4COCH2 61 73 74 74 82 Br, I 3-MeOC6H4COCH2 82 The exchange reactions were also used for the synthesis of aryltellurium trifluorides 45c,d.51 RTeCl3+AgF RTeF3 45c,d R=4-MeOC6H4 (c), 4-EtOC6H4 (d).An analogous reaction was carried out with MeTeI3, but attempts to isolate the resultingMeTeF3 in the pure form failed.144 The exchange of chlorine atoms in organyltellurium trichlor- ides is used as the major procedure for the synthesis of s-telluranes RTeX3 containing such substituents as SC(S)NR2 or SP(S)(OR)2. Thus aryltris(thiocarbamoylthio)tellurium(IV) 47 was prepared by treatment of solutions of RTeCl3 in CH2Cl2, acetone or methanol with N-substituted sodium dithiocarb- amates.116, 117, 140, 145, 146 ArTeCl3+NaSC(S)NR2 ArTe[SC(S)NR2]3 47 Ar=Ph, 4-MeOC6H4 , 2-C6H4N=NPh, 2-Me2NCH(Me)C6H4 , 2-Me2NCH2C6H4; R=Me, Et, Bn.The s-telluranes 47 were also prepared by exchange reactions of aryltellurium trichlorides with Pb[SC(S)NEt2]2 in dry ben- zene.147 ArTe[SC(S)NEt2]3 ArTeCl3+Pb[SC(S)NEt2]2 Ar=Ph, 4-MeOC6H4 , 4-EtOC6H4 , 3-Me-4-MeOC6H3 . It should be noted that (according to the data from 125Te NMR and MoÈ ssbauer spectroscopy 146) 2-(phenylazo)phe- nyltris(thiocarbamoylthio)tellurium(II) 47 (Ar= 2-C6H4N=N± C6H4-2) is very unstable and dissociates in solution to form 2-(phenylazo)phenylbis(thiocarbamoylthio)tellurium(II) and [R2NC(S)S]2. The evidence for the possible existence of mixed organyltellu- rium trihalides of the RTeX2Y type is contradictory (see Section II.7).However, compounds of this type were prepared upon treatment of aryltellurium trichlorides with sodium dialkyl- dithiocarbamates. Thus bis(N,N-diethylthiocarbamoylthio)- (chloro)phenyltellurium(IV) (49a) was synthesised in high yield starting from PhTeCl3 and NaSC(S)NEt2.148 Treatment of the compound 49a with potassium O,O-diethyl dithiophosphate afforded the mixed s-tellurane 49b whose structure was estab- lished by X-ray diffraction analysis.148 Mixed s-telluranes of this type formed in the reactions of PhTeCl3 with salts containing the [SC(S)NEt2]7, [S(P)S(OR)2]7 or [SC(S)OEt]7 anions were studied in solutions by 13C, 31P and 125Te NMR spectroscopy.148 A A Maksimenko, A V Zakharov, I D Sadekov CH2Cl2 PhTeCl3+NaSC(S)NEt2 KSP(S)(OEt)2 PhTe[SC(S)NEt2]2[SP(S)(OEt)2] PhTe(Cl)[SC(S)NEt2]2 49b 49a Yet another approach to the synthesis of the mixed derivatives 49 is based on oxidative addition of alkyl iodides to bis(N,N- diethylthiocarbamoylthio)tellurium(II) (50).148 CH2Cl2 MeTe(I)[SC(S)NEt2]2 MeI+Te[SC(S)NEt2]2 50 The mixed derivatives 49c ± e were also synthesised by the reactions of aryltris(N,N-dialkylthiocarbamoylthio)tellurium with halogens.149 ± 151 ArTe[SC(S)NR2]3+X2 ArTe(X)[SC(S)NR2]2+[R2NC(S)S]2 49c ± e Ref.X R Compound 49 Ar cde 149 150 151 Et Me Et Ph 4-MeOC6H4 4-MeOC6H4 IBr I It is noteworthy that the reaction of the compound 49e with bromine was accompanied by partial replacement of the iodine atoms.The structure of the reaction product 49f was established by X-ray diffraction analysis.151 4-MeOC6H4Te(I)[SC(S)NEt2]2+Br2 49e 4-MeOC6H4Te(Br0.41)(I0.59)[SC(S)NEt2]2 49f The exchange of the chlorine atoms in aryltellurium trichlor- ides was also used for the synthesis of aryltris(dialkoxythiophos- phorylthio)tellurium (compounds 51 and 52).152 PhTeCl3+NH4SP(S)(OPri)2 ArTe[SP(S)(OR)2]3 51 O NH4S P 4-MeOC6H4TeCl3+ Me O S Me S O 4-MeOC6H4Te P Me O S Me 3 52 Compounds of the type 51 were prepared by the reactions of PhTe(OEt)3 (which has been prepared in situ from PhTeCl3 and EtONa in EtOH) with O,O-dialkyl dithiophosphates, e.g., HSP(S)(OPri2)3.152 As in the case of derivatives of dithiocarbamic acid, mixed derivatives can be synthesised.153 CS2, 20 8C ArTeX2[SP(S)(OR)2] ArTeX3+NH4[SP(S)(OR)2] Ar=Ph, 4-MeOC6H4; X=Br, Cl; R=Me, Et, Pri.Attempts to prepare s-telluranes RTeX3 containing SP(S)R2 ligands, unlike compounds containing SC(S)NR2 or SP(S)(OR)2 ligands, were unsuccessful. The reaction of PhTeCl3 with Na[SP(S)R2] gave rise only to [diorganyl(thiophosphorylthio)]- phenyltellurium(II) 53 due apparently to decomposition of inter- mediate s-telluranes of the type 54.154 PhTeCl3+Na[SP(S)R2] .2H2O PhTe[SP(S)R2]3 54 PhTe[SP(S)R2]+[R2P(S)S]2 53Monoorganyl derivatives of tellurium(IV) 7. Symmetrisation and reverse conversions Symmetrisation and reverse conversions are well known, have been studied in detail for organic derivatives of Groups II ± IV elements and are of preparative importance in the chemistry of organic derivatives of these elements.It was demonstrated 115 that the symmetrisation reverse reaction also occurs in the case of diaryltellurium dichlorides. 4-RC6H4TeCl3 (4-RC6H4)2TeCl2+TeCl4 R=OEt, OMe, H, Br. Donor substituents favour these reactions. 4-Methoxy- and 4-ethoxyphenyltellurium trichlorides were prepared in 93% and 81% yields, respectively, upon refluxing (5 h) of equimolar mixtures of the reagents. Phenyl- and 4-bromophenyltellurium dichlorides were synthesised under more drastic conditions (a twofold excess of TeCl4 and prolonged boiling). 8. Miscellaneous procedures In this section, we consider syntheses of s-telluranes RTeX3, which are exemplified in a few compounds and are of no preparative importance.Thus 4-hydroxy-3-methoxyphenyltellurium trichloride was prepared along with the corresponding dichloride by condensa- tion of o-cresol with TeOCl2.155 It was the only aryltellurium trichloride synthesised according to this procedure. Organyltellurium trihalides were prepared by the reactions of some organotellurium compounds, viz., organyltellurenyl halides, organyl tellurocyanates, diorganyl tellurides and organyltellurinic acids, with halides (see Section II). Oxidation of diorganyl ditellurides to form organyltellurium trihalides proceeds through intermediate formation of organyltel- lurenyl halides RTeX 55.131, 156 Compounds of this type, which do not contain coordinating substituents, are unstable and readily undergo disproportionation to form R2TeX2 and Te.157 Certain groups which can form strong intramolecular coordinate bonds with the Te atom through theNor O atoms stabilise the tellurenyl halides 55.For example, the NO2, CHO, COR, CH=N, N=N and CH2NR2 groups located at position 2 of the aryl or alkenyl substituent exert such a stabilising effect.158, 159 Compounds 55 can be synthesised not only by oxidation of ditellurides but also according to other procedures.160 Hence, oxidation of the organ- yltellurenyl halides 55 was used for the preparation of organyltel- lurium trihalides.161 ± 169 X2 RTeX3 RTeX 55 Ref. R X 164 165 ± 167 166, 167 166, 167 168 169 169 161 162 163 165 ± 167 166, 167 166, 167 166, 167 168 169 169 165 ± 167 166, 167 166, 167 166, 167 Cl Cl Cl Cl Cl Cl Cl Br Br Br Br Br Br Br Br Br Br IIII 2-PhCOC6H4 2-C6H4CH=NC6H4Me-4 2-C6H4CH=NPh 2-C6H4CH=NC6H4OMe-4 (2-C6H4CH=NCH2)2 PhCOCH=CH PhCOCH=CPh 2-NO2C6H4 2-HOCC6H4 2-PhCOC6H4 2-C6H4CH=NC6H4Me-4 2-C6H4CH=NPh 2-C6H4CH=NC6H4OMe-4 2-C6H4CH=NC6F5 (2-C6H4CH=NCH2)2 PhCOCH=CH PhCOCH=CPh 2-C6H4CH=NC6H4Me-4 2-C6H4CH=NPh 2-C6H4CH=NC6H4OMe-4 2-C6H4CH=NC6F5 869 The oxidative addition is often performed with the use of SO2Cl2 instead of chlorine.165 ± 167 Some tellurenyl iodides, for example, PhCOCH=C(Ph)TeI, were not oxidised with iodine to form the corresponding triiodides.169 Aryltellurenyl chlorides can be used for the preparation of mixed aryltellurium trihalides ArTeX2Y 56.18 NPh NPh N N Br2 TeCl TeClBr2 56 Attempts to synthesise compounds 56 starting from aryl 2-(halogenotellurenyl)vinyl ketones 57 were unsuccessful.169 Thus the reaction of tellurenyl bromide 57a with chlorine afforded a mixture of trichloride 58 and tribromide 59 in a ratio of 2 : 1.Treatment of tellurenyl chloride 57b with bromine gave rise to a mixture of the same compounds, but the ratio was 1 : 2.169 COPh H Cl2 CH2Cl2 COPh H COPh H TeBr Me 57a +Me Me TeBr3 TeCl3 COPh H 59 58 Br2 CH2Cl2 TeCl Me 57b On the contrary, oxidative addition of bromine to tellurenyl iodide 57c afforded vinyltellurium tribromide 59.169 H COPh COPh H Br2 + I2 Me TeI Me TeBr3 59 57c 2-(N,N-Dimethylthiocarbamoyltellurenyl)azobenzene react- ed analogously.146 NPh NPh N N Br2 +[Me2NC(S)S]2 TeBr3 TeSC(S)NMe2 The reactions of benzyl tellurocyanates with two equivalents of bromine were accompanied by the cleavage of the Te7CN bond yielding the corresponding benzyltellurium tribro- mides.170, 171 Apparently, s-telluranes 60 were formed in these reactions as intermediates.170 Br Br2 Br2 RC6H4CH2TeCN RC6H4CH2TeBr3 RC6H4CH2TeCN 7BrCN 60 Br R=H, 2-COOEt.It is known 3, 4, 6, 7 that diorganyl tellurides R2Te react with halogens to form diorganyltellurium dihalides R2TeX2. However, the reactions of some diorganyl tellurides with halogens proceed with the cleavage of one of the C7Te bonds giving rise to the corresponding organyltellurium trihalides.Thus butyltellurium tribromide was obtained from butyl-2-thienyl telluride.172 Br2 BuTeBr3+ Br TeBu S S Compounds 61 underwent scission under the action of bro- mine in CCl4 to form a,o-bis(tribromotellurio)alkanes 62.173 Br2, CCl4, 20 8C Br3Te(CH2)nTeBr3 62 4-EtOC6H4Te(CH2)nTeC6H4OEt-4 61 n=6 (42%), 10 (40%).870 Tellurium-substituted alkynes add bromine with simultane- ous elimination of organyltellurium tribromide.174 R1C CTeR2 R1C(Br) CBr2+R2TeBr3 Br2, CH2Cl2 20 8C, 20 min R1=Ph, C5H11, 4-BrC6H4, R2=Bu; R1=Ph, R2=4-MeOC6H4. The C7Te bond was cleaved in the reaction of telluraphtha- lide 63 with bromine.171 In the presence of alcohols, the reaction afforded 2-alkoxycarbonylbenzyltellurium tribromides 64.171 O CO2R Te Br2, ROH CCl4 CH2TeBr3 64 63 R=Me, Et.In the reactions with HI, tellurinic acids are converted into organyltellurium triiodides. Thus tellurinic acid as a component of the complexes Bu2TeO . 3BuTe(O)OH and 2(Bu2TeO) . . BuTe(O)OH, which have been prepared by oxidation of dibutyl telluride, was converted into butyltellurium triiodide under the action of HI.175 HI BuTe(O)OH BuTeI3 III. Reactions of s-telluranes RTeX3 The reactions of s-telluranes RTeX3 were studied primarily for organyltellurium trichlorides. Some of them proceed with reten- tion of the coordination number of the tellurium atom.{ 1. Transformations of s-telluranes RTeX3 into s-telluranes R2TeX2 Transformations of s-telluranes RTeX3 into compounds R2TeX2 proceed with retention of the coordination number of the tellu- rium atom, but are accompanied by the formation of an addi- tional C7Te bond. These reactions are of preparative importance in the synthesis of s-telluranes R1R2TeCl2 (for earlier data, see Refs 3, 4, 6 and 7). Since aryltellurium trichlorides exhibit electro- philic properties, they enter into reactions analogous to those of TeCl4. Thus these compounds react with different heteroorganic compounds to form symmetrical and non-symmetrical organyl- tellurium dichlorides R1R2TeCl2, are involved in electrophilic substitution with activated arenes to give diaryltellurium dichlor- ides Ar1Ar2TeCl2, react with compounds containing the activated methylene group to form s-telluranes (ArTeCl2CH2COR in the case of ketones) and add to double and triple bonds yielding derivatives of the types ArTeCl2CH(R1)CH(Cl)R2 and ArTeCl2C(R1)=C(Cl)R2, respectively.However, aryltellurium trichlorides are weaker electrophilic agents than TeCl4, and hence, the above-mentioned reactions proceed under more drastic conditions. Intramolecular cyclisation of aryl- and arylvinyltellurium trichlorides giving rise to different six- and five-membered tellu- rium-containing heterocycles is of particular interest.176 Thus thermal or AlCl3-catalysed cyclisation of aryltellurium trichlor- ides 65 afforded tetracoordinate tellurium derivatives, viz., phe- noxatellurine 66a,56 thiophenoxatellurine 66b,13 phenotellurazine 66c 20 and telluroxanthene 66d.26 Y Y D, AlCl3 Te TeCl3 Cl Cl 66a ± d 65 Y = O (a), S (b), NH (c), CH2 (d).{ Exchange reactions of anionoid substituents are considered in Section II.6. A A Maksimenko, A V Zakharov, I D Sadekov Heating of organyltellurium trichlorides 67 and 68 at high temperature gave rise to 1,1,3-trichloro-2-phenylbenzo[b]telluro- phene (69) 177 and 1,1-dichlorodibenzotellurophene (70),8 respec- tively. Cl Cl Ph D Ph TeCl3 Te 69 67 Cl Cl D Te TeCl3 Cl Cl 70 68 Aryltellurium trichlorides and vinyltellurium trichlorides can be converted into the corresponding dichlorides by symmetrisa- tion.Thus heating of solutions of 2-chloro-2-phenylvinyltellurium chloride in EtOH or AcOH resulted in elimination of the TeCl4 molecule.110 D [PhC(Cl) CH]2TeCl2+TeCl4 PhC(Cl) CHTeCl3 Aryltellurium trichlorides do not undergo spontaneous sym- metrisation. These compounds were converted into diaryltellu- rium dichlorides only upon boiling of their solutions in benzene in the presence of a copper powder (the yields were 60%± 70%).115 4-RC6H4TeCl3+Cu PhH, D (4-RC6H4)2TeCl2 R=OMe, OEt. Aryltellurium trichlorides were converted into diaryltellurium dichlorides also under the action of organylsilicon hydrides.178Ar- yltellurium trichlorides reacted with silicon hydrides (in a ratio of 3 : 4) with stirring at room temperature for 6 ± 8 h to afford diaryl ditellurides.When refluxed for 6 ± 10 h, the mixture of the reagents taken in a ratio of 1 : 2 gave rise to diaryltellurium dichlorides in 75%± 95% yields. D Ar2TeCl2+Te+R1 ArTeCl3+R1 nR2mSiCl47(n+m) nR2mSiH47(n+m) Ar=Ph, 4-MeOC6H4, 4-EtOC6H4; R1, R2=Ph, C6H13, Me; n=1, 2; m=1, 2. Certain reactions of s-telluranes RTeX3 are accompanied by a decrease in the coordination number of the Te atom to 2 (reduction) or 3 (hydrolysis). 2. Reduction of aryltellurium trihalides to aryltellurenyl halides { Reduction of aryltellurium trichlorides affords different products depending on the structure of the organic ligands. Aryltellurium trihalides containing coordinating substituents in the ortho posi- tion are reduced to aryltellurenyl halides, whereas organyltellu- rium trihalides in which such substituents are absent are reduced only to diorganyl ditellurides.hydrate 23, 24, 116, 117, 120 Aryltellurium trihalides containing CH=N,23, 24, 120, 168 N=N,18 NO2,161 CH2NMe2 (see Ref. 116) or CH(Me)NMe2 (see Ref. 117) groups in the ortho position were reduced to aryltellurenyl halides. Hydrazine NaBH4,18 NaHSO3 (see Ref. 161) and Na2S2O5 (see Ref. 168) were used as reducing agents. { Reduction of organyltellurium trihalides to ditellurides is not considered in the present review because these reactions have been surveyed in detail in a recent review.179Monoorganyl derivatives of tellurium(IV) R R [ArTeO]2O+H2O 73 TeX TeX3 Presumably, this equilibrium is shifted to the right only in a Ref.Reducing agent R strongly acidic medium. In the presence of weak acids (for example, AcOH), the anhydrides 73 were formed as the final hydrolysis products of aryltellurium trihalides.129, 180 ± 182 23, 24 N2H4 .H2O N N2H4 .H2O 24 N Aryltellurium trichlorides and tribromides undergo partial hydrolysis under the action of cold water to form aryltellurium oxohalides 74.46, 63, 180, 181, 183 Aryltellurium triiodides are inert to cold water. In most cases, hydrolysis at high temperature afforded unidentified compounds. Poor solubility of aryltellurium oxoha- lides in ordinary organic solvents was attributed 181 to the fact that they have a trimeric structure A. Attempts to isolate possible intermediate products of partial hydrolysis, viz., compounds (ArTeX2)2O, failed.18 161 116 117 N=NPh NO2 CH2NMe2 CH(Me)NMe2 NaBH4 NaHSO3 N2H4 .H2O N2H4 .H2O CH CH NCH2CH2N Na2S2O5 ArTeX3 H2O 72HX TeX3 X3Te CH CH NCH2CH2N X Compound 74 XTe TeX N N N2H4 .H2O TeCl Cl Cl Cl Cl Cl Cl TeCl3 abcdef S S g Cl h Cl In the case ofN-(2-tribromotelluriobenzylidene)-4-methylani- line (71a), both the TeBr3 and azomethine groups were reduced under the action of sodium borohydride,134 whereas the azome- thine group in sterically hindered N-(2-trichlorotelluriobenzyli- dene)-2,4,6-trimethylaniline (71b) remained unchanged. CH2NHC6H4Me-4 NC6H4Me-4 NaBH4 i Cl TeBr TeBr3 71a Me Me NaBH4 N N Me Me j Cl TeCl Me TeCl3 Me 71b 3.Hydrolysis of organyltellurium trihalides Alkaline hydrolysis of aryltellurium trihalides followed by acid- ification of the reaction mixture afforded aryltellurinic acids 72 in 70%± 100% yields. klmn Br Br Br Br H+ OH7 [ArTe(O)O]7 o Br ArTeX3 ArTe(O)OH 72 X=Cl, Br. Ref. Ar Ref. Ar Aryltellurium oxochlorides 74a,c,d were also synthesised by treatment of the corresponding aryltellurium trichlorides with 2-phenyliodoniobenzoate monohydrate.184 182 180, 182 113 4-BuOC6H4 4-PhOC6H4 2-HOCC6H4 180 113 46, 180 46, 180, 181 46, 180, 182 129, 180, 182 Ph 2-MeC6H4 4-HOC6H4 4-EtOC6H4 4-MeOC6H4 Alkyltellurium oxohalides RTe(O)X (R=Me andX=Cl, Br or I; R=Et and X=I) 185 and anhydrides [RTe(O)]2O (R=C8H17, C10H21 or C12H25) 186 were prepared according to a procedure analogous to that used for the synthesis of the aryl analogues 74a ± o.Acidification of an aqueous solution of the salt [Me3Te]+[MeTe(O)O]7 afforded a mixture of methyltellurinic acid and its anhydride.187 In some cases, acidification gives rise to anhydrides 73 rather than to tellurinic acids 72 due apparently to the existence of the following equilibrium in solution 871 ArTe(O)OH. 72 X Ar Te O O Ar Ar Te Te ArTe(O)X 74a ± o X X OARef. R 180 46 46, 180, 183 46, 180, 181, 183 183 180 Ph 4-HOC6H4 4-MeOC6H4 4-EtOC6H4 3,4-(MeO)2C6H3 4-PhOC6H4 180 CO2H 63 N C6H4-4 CO2H 63 N C6H4-4 63 N 180, 181 180 180, 181 180 Ph 4-MeOC6H4 4-EtOC6H4 4-PhOC6H4 180872 4.Synthesis of salts containing organyltetrahalogenotellurate anions Reactions yielding salts containing organyltetrahalogenotellurate anions and complexation reactions are accompanied by an increase in the coordination number of the tellurium atom. The reactions of different organyltellurium trihalides with onium salts in organic solvents (MeOH or CHCl3) give rise to salts containing organyltetrahalogenotellurate anions.181, 187 ± 192 RTeX3+M+Y7 R X Me Me Me Me Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-EtOC6H4 4-PhOC6H4 I Me3Te Br Br Me3Te Br Br I Me3Te Cl Br Et4N Cl Cl Et4N Cl Ph3PCH2Ph Cl Cl Cl Ph4P Cl Cl Ph4As Cl Cl Ph3Se Cl Ph2I Cl Cl Cl C5H5NH Cl Br Ph3PMe Cl Br Cl Br Ph3Te Cl Br cyclo-C7H7 Cl I Bu4N Br Me4N Br Br Br Et4N Br Br Ph3PMe Br Br Ph4P Br Br Br Br Ph3S Br Br Ph2I Br Br cyclo-C7H7 I Pr4N I I I Me3S I Cl Ph3PCH2Ph I Br cyclo-C7H7 Cl Cl Me4N Cl Cl C5H5NH Cl Cl Ph3PCH2Ph Cl Cl Ph4P Cl Cl Ph4As Cl Cl Ph3Se Cl Cl Ph2I Cl Br Ph3P Cl Br Cl Br cyclo-C7H7 Br Br Et4N Br Br Ph3PMe Br Br Br Br Ph4P Br Br Ph3S Br Br Ph2I Br Br cyclo-C7H7 Br Cl Ph3PCH2Ph I I Pr4 N I I Me3 S I Cl Ph3 PCH2Ph I cyclo-C7H7 Cl Tropylium 4-ethoxyphenyltetrabromotellurate was prepared by the reaction of the corresponding aryltellurium trichloride with tropylium perchlorate followed by anion exchange.190 M+(RTeX3Y)7 75 Ref.M Y 187 187 187 188 189 190 190 181, 189 189 189 181 190 Ph3PCH2CO2Et 190 190 190 191 189 189 190 190 Ph3PCH2CO2Et 190 189 189 190 189 189 190 190 189 181 190 190 189 189 189 190 Ph3PCH2CO2Et 190 190 189 189 Ph3PCH2CO2Et 189 189 189 189 190 190 189 189 190 190 192 Br Cl Et2NH2 A A Maksimenko, A V Zakharov, I D Sadekov ArTeCl3 Ph3C+ClO¡4[cyclo-C7H7]+ClO¡ cyclo-C7H8 4 Br7 [cyclo-C7H7]+[ArTeBr4]7 [cyclo-C7H7]+[ArTeCl3(ClO4)]7 Ar=4-EtOC6H4.The reactions of aryltellurium trihalides with onium salts in aqueous solutions of hydrohalic acids also gave rise to salts 75 in high yields.189, 190 M+Y7 ArTeCl3+HX H+[ArTeX4]7 M+[ArTeY4]7 75 Ar=Ph, 4-EtOC6H4; X=Cl, Br;M=cyclo-C7H7, Ph3PMe, Ph3PCH2CO2Et. 5. Complexation s-Telluranes RTeX3 act as Lewis acids in complexation. In contrast to tellurium tetrahalides which typically form 2 : 1 adducts,193 organyltellurium trihalides give rise mainly to 1 : 1 complexes. Aryltellurium trichlorides ArTeCl3 (Ar=Ph, 4-MeC6H4, 4-MeOC6H4, 4-EtOC6H4 or 4-PhOC6H4) 125 ± 128 form 1 : 1 com- plexes with pyridine, picolines,125 ± 128 pyridine N-oxide and pico- line N-oxide.194 ± 197 In solution in nitromethane, the complexes with pyridine and picolines act as electrolytes (1 : 1) and have, apparently, the structure [ArTeCl2 .L]+Cl7.194 The complexes with N-oxides are weak electrolytes of the formula ArTeCl3 . L.194 In addition to the complexes with the above-mentioned ligands, complexes of aryltellurium trichlorides with a number of other nitrogen-, phosphorus- and oxygen-containing bases, viz., with trimethyl(ethyl)amine,195 ± 197 a-naphthylamine,195 ethylene- diamine,195 benzimidazole,195 ± 197 benzothiazole,195, 196 2,2 0- bipyridyl,195, 197 1,10-phenanthroline,196, 197 triphenylphos- phine,195 triphenylphosphine oxide,195 ± 197 triphenylphosphine sulfide,195 triphenylarsine oxide 195 ± 197 and dimethyl sulfox- ide,195 ± 197 were isolated in the individual state as 1 : 1 complexes.The composition and the enthalpy of formation of the complex of PhTeCl3 with (C6H13)2SO were determined by calorimetric titra- tion.198 Judging from the enthalpies of formation, the acceptor abilities of tetracoordinate tellurium derivatives decrease in the series TeCl4>PhTeCl3>Ph2TeCl2.198 It should be noted that Garad 199 assigned the 1 : 1 and 2 : 1 compositions to complexes of aryltellurium trichlorides with bidentate ligands (bipyridyl or phenanthroline) and monoden- tante ligands [pyridine, pyridine N-oxide or (C8H17)3P=O], respectively, contrary to the above-considered data.194 ± 197 Organyltellurium trihalides exhibit acceptor properties also with respect to sulfur-containing donors.It was demonstrated 200 that organyltellurium trihalides 4-EtOC6H4TeX3 (X=Cl, Br or I), like tellurium tetrahalides, react with tetramethylthiuram disulfide. These reactions are accompanied by extrusion of one sulfur atom to form 2 : 1 complexes of aryltellurium trihalides with tetramethylthiuram sulfide. According to IR and 1H NMR spectral data, the 1 : 1 com- plexes of RTeCl3 and RTeBr3 (R=Me, Et, 4-MeOC6H4 or 4-EtOC6H4) with tetramethylthiourea 130, 195, 201 have a square- pyramidal structure with the organic substituents in apical posi- tions. Three halogen atoms and the S-coordinated ligand are located in the base of the pyramid.130, 201 The reactions of phenyl- tellurium trihalides with thiourea in aqueous-methanolic solu- tions were accompanied by reduction of Te(IV) to form the complexes PhTeX .(NH2)2CS.202 Tetramethyl- 124 and tetraethyldithiooxamides 123 belong to yet another type of sulfur-containing ligands giving the 1 : 1 complexes ArTeX3 . C(S)(NR2)C(S)(NR2) (R=Me or Et) with aryltellurium trihalides 4-RC6H4TeX3 (R=H, Me, MeO, EtO, PhO or PhS;X=Cl, Br or I). The long-wavelength IR spectra and Raman spectra provide evidence in favour of the octahedralMonoorganyl derivatives of tellurium(IV) structure of the complexes containing bidentate ligands coordi- nated through the sulfur atoms of the thiocarbonyl groups. Measurements of the dipole moments (in benzene) demon- strated that aryltellurium trichlorides form 1 : 1 complexes with donors of the Ph3P=X (X is a lone electron pair, O, S or Se) and PhP(O)Me2 types.203 The dipole moments are indicative of the donor-acceptor character of the bond between the components of the complexes. The coordination centres of the complexes adopt a pseudooctahedral configuration in which the donor is located in the cis position with respect to the lone electron pair of the tellurium atom and in the trans position with respect to the chlorine atom.It is noteworthy that the reactions of 4-RC6H4TeCl3 (R=H, OH or OMe) with thiosemicarbazones of benzaldehyde, acetophenone, salicylaldehyde and 2-hydroxy- acetophenone afforded ionic tricoordinate Te(II) complexes [ArTe(TSK)2]+Cl7 (TSK is the corresponding thiosemicarb- azone) in which the Te atom is coordinated through the sulfur atom.204 6 In the reactions with strong Lewis acids, e.g., with SbCl5, organyltellurium trichlorides RTeCl3 (R=Me, Et or 4-MeOC6H4) act as donors to give 1 : 1 complexes.119 Ionic structures were assigned to the complexes [RTeCl2]+SbCl¡ based on the facts that their IR spectra are similar to those of the ionic complexes RSeCl3 .AlCl3 and have bands characteristic of the SbCl¡6 ion. The enthalpies of formation of complexes of tetracoordinate tellurium derivatives with AlBr3 (1 : 1 composition) 198 are indica- tive of an increase in the donating ability of the tellurium atom in the series TeCl4<PhTeCl3<Ph2TeCl2. 6.Detelluration Elimination of tellurium (detelluration) is also characteristic of s-telluranes RTeX3. Some of these reactions are of particular preparative interest. Detelluration is performed by pyrolysis, photolysis or under the action of Raney nickel, Pd(II) salts, oxidants or some nucleophilic reagents. Heating of 2-chloroethyl- 97 or 2-chloropropyltellurium tri- chlorides 96 afforded Te, HCl, saturated (isopropyl chloride) and unsaturated (vinyl chloride and allyl chloride) halogen-containing derivatives and 1,2-dichloroalkanes. Pyrolysis of 2-chlorocyclo- hexyltellurium trichloride gave rise to cyclohexyl chloride, hexene and benzene along with Te and HCl.96 Photolysis of organyltellurium trihalides proceeds in a differ- ent way.101, 205 Irradiation of solutions of these compounds in benzene afforded organic halogen-containing derivatives, the halogen atoms occupying positions to which the tellurium-con- taining group was initially bound (ipso substitution).R TeX3 hn PhH X+R X+ R R X R=MeO, Me, H; X=Cl, Br, OAc. Thus irradiation of a solution of 4-methoxyphenyltellurium trichloride in C6H6 using a high-pressure mercury lamp at 20 ± 30 8C for 1 h afforded 4-chloroanisole in 70% yield and 2,4- dichloroanisole as a by-product (20%). Photolysis of 4-methoxy- phenyltellurium tribromide gave rise to 4-bromoanisole in 30% yield. Irradiation of a solution of 4-methoxyphenyltellurium triacetate [prepared in situ from Ph2Te2 and Pb(OAc)4 in AcOH] afforded 4-acetoxyanisole in 14% yield.In some cases, coupling products of organic radicals of s-telluranes with benzene were also obtained in trace amounts.101, 205 The yields of aryl halides decrease as the donor properties of substituents in the aryl groups of s-telluranes decrease. 873 The reactions of cycloalkyl- and vinyltellurium trichlorides proceed analogously.101, 205 In the latter case, the reaction pro- ceeds with retention of the geometric configuration of the initial alkene. Cl Cl hn PhH Cl TeCl3 Ph Ph H H hn PhH Cl Cl Cl TeCl3 Aryltellurium trichlorides undergo detelluration upon heating with Raney nickel (Ni/Ra) in diglyme to form biaryls.71 Appa- rently, this reaction proceeds through initial reduction of tellu- rium trichlorides to form ditellurides followed by elimination of tellurium.107 Detelluration of (b-acyloxy-g-chloroalkyl)tellurium trichlorides of the type 39 under the action of Raney nickel gave rise to chlorohydrin esters in 26%± 78% yields.107 TeCl3 Ni/Ra, D DiglymePhOCO R3 R3 PhOCO R1CHC(R2)CCl R1CH2C(R2)CCl Ni/Ra EtOH, D R4 R4 TeCl3 39 R4 R3 R2 R1 H H H H H Me H H H H Me H H H H Me H H Me Me Biaryls were also formed upon treatment of aryltellurium trichlorides with PdCl2 and NaOAc in AcOH49 or with Li2PdCl4 in MeCN.206 The latter reagent is more efficient than the former.The formation of biaryls is favoured by the presence of donor substituents in the aryl groups. Li2PdCl4,MeCN ArAr ArTeCl3 Ar=4-MeOC6H4 (54%), 4-BuOC6H4 (26%), 4-PhOC6H4 (29%), (78%).The reactions performed under a CO atmosphere afforded biaryls in approximately the same yields, but the reaction rates were substantially higher.206 However, good yields of biaryls were achieved only after treatment of the reaction mixture with a solution of NaOH. In the latter case, carboxylic acids were obtained as by-products. The reactions were accelerated due to the formation of active intermediates 76 ± 79 containing Pd7CO bonds.206 CO ArTeCl3 Li2Pd(CO)nCl4 Li2PdCl4 7LiCl Cl Cl3TePd(CO)nCl2 *Ar Ar Te Pd(CO)nCl3 Ar Cl 77 76 Ar (Cl4Te)Pd(CO)nCl (Cl4Te)Pd(CO)n NaOH 7TeO2,7H2O, 7NaCl,7CO Ar 78 Ar 79 ArAr [ArPdAr] 7Pd874 Treatment of a mixture of aryltellurium trichlorides, PdCl2 and NaOAc in AcOH with a tenfold excess of styrene afforded trans-stilbenes in rather low yields (7% ± 38%) along with trace amounts of biaryls.49 It should be noted that the yields of stilbenes in the reactions with aryltellurium trichlorides are lower than those obtained in the reactions with diaryltellurium dichlorides containing the same substituents.H Ph Pd(II) ArTeCl3+PhCH CH2 Ar H Ar=Ph, 4-MeC6H4, 4-MeOC6H4. Detelluration of different types of organyltellurium trihalides proceeds also under the action of halogens giving rise to the corresponding organyl halides (halodetelluration).96, 111, 207 Apparently, halodetelluration was observed for the first time in the treatment of 2-chloropropyltellurium trichloride with bro- mine.96 MeOH, KBr, 50 8C, 4 h MeCHClCH2TeCl3+Br2 MeCHBrCH2Br+MeCHBrCH2Cl+MeCHClCH2Br+ +MeCHClCH2Cl Iododetelluration of (Z)-2-chlorovinyltellurium trichlorides proceeded with retention of the configuration of the initial alkenes giving rise to (Z)-chloroiodoalkenes in 40% ±94% yields.111 R1 R1 R2 R2 MeCN or MeOH D, 2 h + I2 TeCl3 I Cl Cl R1=Ph, R2=H, Me, Et, Ph; R1=CH2OH, R2=H.Bromodetelluration occurs only upon treatment of vinyltellu- rium trichlorides with N-bromosuccinimide in the presence of AlCl3.111 Iododetelluration of aryltellurium trichlorides containing donor substituents under the action of iodine proceeded efficiently only in the presence of potassium fluoride, cesium fluoride, ammonium fluoride, mercury(II) chloride or SbCl5 to form aryl iodides in yields higher than 80%.207 However, the reactions of phenyl- and 4-bromophenyltellurium trichlorides with iodine under identical conditions gave rise to aryl iodides in very low yields.This fact is indicative of the electrophilic character of the reaction.207 Apparently, the catalytic action of fluoride ions is associated with the formation of the intermediates [ArTeCl3F2]7 due to which the electron density at the tellurium atom and at the C7Te bond increases, which favours the electrophilic attack of the halogen. The function of metal chlorides is to activate iodine by complexation or in situ formation of more reactive iodine monochloride.207 I R R TeCl3 + I2 F7 or Cl7 MeCN, D, 5 h R=Me, OMe.Bromodetelluration of aryltellurium trichlorides with bro- mine in MeCN, 1,4-dioxane or CCl4 gave rise to 2,4-dibromoar- enes in rather high yields, while 4-bromoarenes were obtained in trace amounts.207 The selective formation of dibromides was attributed to bromination of the initial 4-bromoarenes catalysed by organic derivatives of tellurium(IV).207 MeCN, 35 ± 45 8C, 20 h R TeCl3+Br2 Br Br+ R R Br R=H, Me, OMe. A A Maksimenko, A V Zakharov, I D Sadekov Chlorodetelluration of aryltellurium trichlorides under the action of different chlorinating agents (Cl2, SbCl5 or ButOCl) proceeded with very low yields even in the case of highly reactive 4-methoxyphenyltellurium trichloride.207 Cyanodetelluration of 4-methoxyphenyltellurium trichloride with CuCN in DMF also occurred in very low yield (5%).207 Halodetelluration of organyltellurium trichlorides proceeded rather readily under the action of tert-butyl hydroperoxide.101, 208 Different types of organyltellurium trihalides, viz., with 2-chloro- alkyl-, 2-chlorocyclohexyl-, alkenyl- and aryltellurium trihalides, enter into this reaction.In most of reactions with alkenyl derivatives, the initial configuration is retained.101 The yields of alkenyl halides are lower than those of alkyl and aryl halides. It was thus assumed 101 that the reactivities of organyltellurium trihalides in oxidative detelluration change in the order: alkyl>aryl>alkenyl. R1CHClCH(R2)X R1CHClCH(R2)TeX3+ButOOH 1,4-Dioxane D, 30 min R1=C8H17, PhCH2; R2=H; R17R2=(CH2)4; X =Cl, I.Ph R R Ph 1,4-Dioxane D, 30 min +ButOOH TeCl3 Cl Cl Cl R=H, Ph. ArX ArTeX3+ButOOH 1,4-Dioxane D, 30 min , Ar=4-MeC6H4, 4-MeOC6H4, ; X=Cl, Br. The possible reaction mechanism involving the formation of unstable organic derivatives of tellurium(VI) 80 is shown in the scheme.101 X R1 [O] C Te X R2 X R3 X R1 R1 X C X R2 C Te R2 7[Te(O)X2] X R3 R3 O 80 The reactions of aryltellurium trichlorides with Ni(CO)4 in DMF resulted in detelluration. Subsequent treatment of the reaction mixtures with water afforded carboxylic acids in 35%± 51% yields.209 Symmetrical ketones and diaryl tellurides were obtained as by-products (sometimes in yields of up to 15%).The reaction of 4-methoxyphenyltellurium trichloride with Fe2(CO)9 yielded bis(4-methoxyphenyl) telluride as the only product.209 1) Ni(CO)4, DMF, 70 8C, 24 h ArCO2H+Ar2CO+ArTeAr ArTeCl3 2) H2O . Ar=4-MeOC6H4, It is believed 209 that carbonylation of aryltellurium trichlor- ides proceeded according to a scheme involving intermediate aryltelluriocarbonylnickel dichloride 81. CO Ni(CO)4 ArTeNiCl ArTeCl3 81 Cl ArTeCl3 ArTeAr CO CO H2O ArCOOH Ni(CO4) NiCO4 ArCONiCl ArNiCl ArTeCl3 ArCOAr CO COMonoorganyl derivatives of tellurium(IV) In some cases, the direction of detelluration of organyltellu- rium trichlorides under the action of nucleophiles is governed by the presence of intramolecularO?TeCl3 coordinate bonds.Thus detelluration of b-trichlorotellurio ketones 25 and 26 proceeded smoothly under the action of basic agents, such as DMSO or amines.85 O O TeCl3 5 equiv. of DMSO, 0 8C, 10 min R R 25 , Ph. R=But, O O TeCl3 R1 R1 5 equiv. of DMSO, 0 8C, 10 min (CH2)n (CH2)n R2 R2 26 R1=R2=H, n=2, 5; R17R2=(CH=CH)2, n=1. In the reactions with NH4OH, s-telluranes 82a,b containing intramolecularO?Te coordinate bonds underwent detelluration with elimination of metallic tellurium to form a mixture of ketones 83a,b (the yields were 56%± 63%) and tellurenyl halides 84a,b (the yields were 13%± 36%).169 O TeCl3 R2 R1 82a,b O TeCl O O NH4OH + R2 R1 R1 R2 83a,b 84a,b O OMe MeOH±CH2Cl2 (1 : 1) +83a,b+84a,b Et3N R1 R2 85a,b R1=R2=Ph (a); R1=Ph, R2=Me (b).The reactions of the organyltellurium trichlorides 82a,b with Et3N in a 1:1 CH2 Cl2 ±MeOH mixture afforded enol methyl ethers 85a,b (a mixture of stereoisomers, the yields were 56%± 80%). The ketones 83a,b and the tellurenyl halides 84a,b (in trace amounts) were isolated as by-products.169 Detelluration of the ketones 82 proceeded under the action of other nucleophiles as well. Thus the reactions of the compounds 82a,c with sodium benzenethiolate afforded exclusively Z-isomers of vinyl ketones 86a,c in 74%± 82% yields, whereas the reaction of the ketone 82a with hydrazine gave rise to 3,5-diphenylpyrazole and the tellurenyl halide 84a in 44% and 22% yields, respec- tively.169 O H O TeCl3 PhSNa, CH2Cl2 R1 R2 R1 R2 82a,c SPh 86a,c R1=R2=Ph (a); R1=3-FC6H4, R2=Ph (c).NH N N2H4, CH2Cl2 82a +84a Ph Ph 875 IV. Structures of s-telluranes RTeX3 in the crystalline state and in solution. Electronic effects of TeX3 groups 1. Crystal structures of s-telluranes RTeX3 According to the polarity rule 210 extended to s-telluranes,211, 212 the axial positions in trigonal bipyramids in molecules of these compounds should be occupied by the most electronegative substituents. However, the real structures of a large number of s-telluranes RTeX3, being determined by the nature of both the electronegative substituents at the tellurium atom and the organic ligands, rarely correspond to trigonal bipyramids predicted theo- retically.Three major groups of s-telluranes RTeX3, which differ in the degree of association in the crystalline state and in the coordinate bonding to the tellurium atom, can be distinguished. Organyltellurium trihalides RTeHal3 whose organic ligands do not contain substituents capable of forming intramolecular coor- dinate bonds with tellurium atoms belong to the first group. The second group consists of organyltellurium trihalides containing a intramolecular O(N)?Te coordinate bond. The third group comprises s-telluranes RTeX3 containing bidentate (primarily, dithiocarbamate) ligands. The data on the bond lengths, bond angles and types of associates in organyltellurium trihalides RTeX3 whose organic ligands do not contain substituents capable of forming intra- molecular coordinate bonds with tellurium atoms are given in Table 1.Taking into account secondary interactions, the tellurium trihalides listed in Table 1 contain a pentacoordinate tellurium atom. The major structural unit in these compounds (CTeX4) is characterised by a square-pyramidal configuration of the bonds about the tellurium atom, the organic radical being located in the apical position. The tellurium atom lies virtually in the base of the square pyramid formed by four halogen atoms (the deviation from the plane is insignificant). Two halogen atoms form shorter (terminal) bonds, whereas the two remaining halogen atoms are involved in longer (bridging) bonds through which the tellurium atoms are linked in polymeric or dimeric structures.The dimers are linked to each other through weak secondary bonds, which are substantially longer than the Te7Xbridge bonds. s-Telluranes containing two TeHal3 groups bound to one carbon atom [for example, methylenebis(tellurium trichloride) 217] or to the vicinal carbon atoms [for example, 1,2-bis(tribromotellurio)cyclohex- ane 219] have monomeric structures. The bonds about each tellu- rium atom in these compounds adopt the square-pyramidal configuration, except that both bridging Te7X bonds are intra- molecular. 2-Biphenylyltellurium trihalides containing the bulky 2-biphe- nylyl radical also have monomeric structures with a trigonal- bipyramidal arrangement of the ligands about the tellurium atom.126, 132, 220 The axial positions are occupied by the halogen atoms.The angle between the axial bonds in 2-biphenylyltellu- rium tribromide is 178.5 8. The corresponding angles in the a and b modifications of the triiodide are 176.5 8 and 176.0 8, respectively. The axial Te7X bonds are substantially longer than the equato- rial bonds in accordance with the polarity rule.210 Short intra- molecular Te . . .C contacts are observed in 2-biphenylyltellurium trihalides [2.945 A in the tribromide and 3.32 and 3.18 A in the a and b modifications of the triiodide, respectively; the sum of the van der Waals radii of the tellurium and carbon atoms is 3.90 A (see Ref. 221)]. Due to these contacts, 2-biphenylyltellurium trihalides are readily converted into 5,5-dihalogenodibenzotellur- ophenes upon heating.8 Two modifications of 2-biphenylyltellu- rium triiodide are structurally similar and differ only in intermolecular contacts involving the heavy atoms. In the b modification, the I .. . I contacts (3.337 A) occur along with the secondary Te . . . I contacts,220 whereas two I . . . I contacts (3.239 and 3.772 A) are observed in the a modification.132 The difference in the character of these intermolecular interactions is reflected in the colour of the compounds, viz., the crystals of the a and b876 Table 1. Average bond lengths (A) and bond angles (deg) in s-telluranes RTeX3. Compound C2F5TeF3 ClCH2CH2TeCl3 PhTeCl3 (see a) 4-EtOC6H4TeCl3 4-PhOC6H4TeCl3 CH2(TeCl3)2 PhTeBr3 4-EtOC6H4TeBr3 2-PhC6H4TeBr3 TeBr3 TeBr3 4-MeOC6H4TeI3 2-PhC6H4TeI3-a 2-PhC6H4TeI3-b a Phenyltellurium trihalide PhTeCl1.7Br1.3 prepared by the reaction of equimolar amounts of TeCl4 and Ph2BBr also has a polymeric structure;215 b the Te7Xeq bond length; c the average Te7Xax bond length; d the lengths of the I .. . I secondary bonds are 3.239 and 3.772 A; e the lengths of the Te . . . I and I . . . I secondary bond are 3.703 and 3.337 A, respectively. modifications of 2-biphenylyltellurium triiodide are black and red, respectively. The degree of association and, what is more important, the molecular geometry of organyltellurium trihalides are changed substantially if the molecules contain substituents with donor Table 2. Average bond lengths (A) and bond angles (deg) in s-telluranes RTeX3 containing intramolecular coordinate bonds.Compound OEt TeCl3 87OEt TeBr3 88 Cl S TeCl3 89 SPh TeCl3 90 MeNMe2 TeCl3 91 NMe2 TeBr3 92PhO Ph TeBr3 93 aY=O, S or N;b the N?Te coordinate bond length; c the O?Te coordinate bond length; d the N7Te bond lengths are given; e in the complex with CH2Cl2. Bond length Te7Xbridge Te7Xterm 2.913 2.717 2.755 2.749 2.768 2.760 2.829 2.932 7 1.872 2.386 2.377 2.396 2.374 2.351 2.526 2.531 2.490,b 2.661 c 2.88 2.541 3.153 77 2.788 2.769,b 2.966 c 2.748,b 2.928 c Bond angle Bond length C7Te Te7X Y_Te a CTeX 93.7 2.172 2.505, 2.419 2.481, 2.355 95.1 2.29 87.9 2.665, 2.49 2.653, 2.502 2.139 2.555, 2.763 2.453, 2.424 90.02 2.108 2.324, 2.972 2.477, 2.497 92.6 2.078 2.538, 2.406 2.465, 2.447 92.4 2.12 2.758, 2.42 2.633, 2.632 95.0 2.175 2.661, 2.362 2.659, 2.513 Bond angle Te_X Te7C C7Te7Xterm 88.8 92.7 91.68 91.0 91.3 90.8 92.2 93.6 polymer """dimer monomer polymer dimer 7 7 monomer 2.20 2.164 2.122 2.12 2.111 2.120 2.140 2.10 2.136 77773.702 774.143 3.713 92.4 2.224 7 94.6 dimer 7 7 monomer 2.15 2.15 3.827 see d 2.153 see e 7 7 " atoms (oxygen, nitrogen or sulfur) capable of being involved in intramolecular coordination with the tellurium atom.The results of X-ray diffraction studies of these compounds are given in Table 2.Compound Ref. XTeX Me 105 176.2 OTeCl3 Me 94 222 172.9 N NPh TeCl3 106 168.9 95 N 223 174.9 TeBr3 96 Me N 117 175.4 O Te 97 Cl3 PhC 116 178.7 NSiMe3 TeCl3 NSiMe3 98 169 7 Ph2P NSiMe3 TeCl3 (see e) 2.185 d 2.496, NSiMe3 A A Maksimenko, A V Zakharov, I D Sadekov Ref. Type of the associate C7Te7Xbridge 77.6 82.5 85.46 88.2 87.2 75.6 85.4 88.4 138 98 213, 214 121 216 217 218 121 126 219 83.2 " 89.7 121 132 220Ref. Bond angle Bond length XTeX C7Te Te7X Y_Te a CTeX 224 172.8 92.4 2.135 2.516, 2.438 2.486, 2.364 57 171.7 92.9 2.114 2.491, 2.417 2.483, 2.406 24 172.4 92.5 2.110 2.673, 2.244 2.658, 2.589 83 171.3 O 2.129 2.500, 2.402,b 88.4 2.491, 2.878 c 2.438 32 2.096, 2.489,7 7 172.1 2.192 d 2.479, 2.434 225 2.056, 2.550,7 7 177.1 99 2.488Monoorganyl derivatives of tellurium(IV) 8-Ethoxycyclooct-4-enyltellurium trichloride (87), like other organyltellurium trichlorides, adopts a square-pyramidal config- uration.The carbon atom occupies the vertex of the pyramid. The base of the pyramid is formed by three chlorine atoms (the Cl7Te bond lengths have standard values; 2.335 ± 2.505 A) and the oxygen atom (in other s-telluranes RTeCl3, four chlorine atoms lie in the base of the pyramid, two of these atoms being bridging). The Te . . .O distance (2.419 A) is substantially smaller than the sum of the van der Waals radii of the tellurium and oxygen atoms (3.60 A).In the crystals, there are also very weak secondary Te . . . Cl bonds (3.558 A) through which the molecules are linked in dimers. Apparently, 2-ethoxycycloheptyltellurium tribromide (88),222 2-chloro-3-(allylthio)propyltellurium trichloride (89) 106 and 2-(phenylthio)phenyltellurium trichloride (90) 223 have anal- ogous structures. In organyltellurium trihalides 91 ± 96, the bonds at the tellu- rium atoms adopt an octahedral configuration. Two halogen atoms are located in the apical positions. The equatorial positions are occupied by the third halogen atom, the bidentate organic ligand and the lone electron pair of the tellurium atom. Some compounds of this type have monomeric structures. In crystals of other compounds, the molecules are linked in dimers through intermolecular Te .. .X bonds. However, the lengths of these bonds (3.498 A in the trichloride 91 and 3.596 ± 3.896 A in the tribromides 92, 93 and 96) are substantially larger than the lengths of the bridging Te7X bonds (see Table 1) and are close to the sums of the van der Waals radii of the corresponding elements. The N?TeX3 coordinate bond lengths are in the range of 2.24 ± 2.42 A. The hybridisation state of the nitrogen atom affects only slightly the coordinate bond lengths. The organic ligand in (6-acetyl-2-pyridyl)carbonylmethyltel- lurium trichloride (97) is polydentate, which results in a new structural type.83 The coordination polyhedron about the tellu- rium atom can be described as a distorted pentagonal bipyramid with two axial Te7Cl bonds (2.492 and 2.500 A).The equatorial positions are occupied by the Cl atom (Te7Cleq is 2.431 A), three atoms of the tridentate organic radical, viz., the carbon (Te7C is 2.129 A), nitrogen (Te . . .Nis 2.402 A) and oxygen atoms (Te . . .O is 2.878 A), and the lone electron pair of the tellurium atom. The two last-mentioned bond lengths, though larger than the sums of the covalent radii of the corresponding elements (the sum of the Te and N radii is 2.07 A and the sum of the Te and O radii is 2.03 A), are substantially smaller than the sums of the van der Walls radii of these elements (the sum of the Te and N 3.70 A and the sum of the Te and O radii is 3.60 A221).The presence of the Te?N and Te?Ocoordinate bonds was also confirmed by IR spectroscopy. Selected data for organyltellurium trichlorides containing N?TeCl3 bonds (compounds 98 and 99) are also given in Table 3. Average bond lengths (A) and bond angles (deg) in s-telluranes containingN,N-diorganyldithiocarbamate ligandsR1Te(S2CNR2)2X (100a ± g). R1 Bond length Xa R2 Compound 100 Te7C Te7S Te7X secondary bonds 2.124 2.14 2.145 2.115 2.153 2.606 ± 2.816;b 3.228 2.674 ± 2.700 c 2.623 ± 2.717 c 2.618 ± 2.723 c 2.625 ± 2.716 c Et Et Me Et Et Ph Ph 4-MeOC6H4 Me Ph S2CNEt2 S2P(OEt)2 Br d II abcde I 2.211 2.151 Et Et 2.626 ± 2.733 c 2.624 ± 2.728 c 4-MeOC6H4 4-MeOC6H4 fg I0.59Br0.41 (see f) aX is the ligand occupying the second axial position; b the range of the five Te7S bonds in the equatorial plane is given; c the range of the four Te7S bonds in the equatorial plane is given; d solvated with 0.5 equiv.of CH2Cl2; e the average Te7I bond length for two crystallographically independent molecules is given; f the compound was prepared by the reaction of bis(N,N-diethyldithiocarbamato)(iodo)(4-methoxyphenyl)tellurium(IV) with bromine; g the Te7I bond length; h the Te7Br bond length. 877 Table 2. The N?Te bond lengths in these compounds have close values (the differences are only 0.1 ± 0.12 A). The four- membered PN2Te ring in the compound 99 is virtually planar. The nitrogen atoms deviate from the plane of the ring by *0.05 A.225 s-Telluranes containing N,N-diorganyldithiocarbamate ligands (100a ± g) have analogous structures.Selected structural data for these compounds are listed in Table 3. Compounds 100a ± g have pentagonal-bipyramidal struc- tures. The equatorial positions of the bipyramid are occupied by five sulfur atoms (compounds 100a,b) or four sulfur atoms and one halogen atom (compounds 100c ± g). In the tris(dithiocarba- mate) derivative 100a, the axial positions are occupied by the organic group and the sulfur atom of a dithiocarbamate ligand.226, 227 In the bis(dithiocarbamato)(dithiophosphate) deriv- ative 100b, the position of the second axial ligand is occupied by a sulfur atom of the S2P(OEt)2 group.148 In the other compounds, the lone electron pair of the tellurium atom 148 or an atom of the adjacent molecule 149 ± 151 serves as the second axial ligand.The compounds 100a,b,d exist as monomers, whereas molecules 100c,e ± g 149 ± 151 are linked as dimers through weak intermolecu- lar Te . . .S or Te. . . Hal bonds. The reason for the presence of secondary bonds of different nature in the compounds 100c,g containing virtually identical substituents at the tellurium atom (one bromine atom in the compound 100c or bromine and iodine atoms in a ratio of 41 : 59 in the compound 100g) remains unclear. Thus the coordination polyhedron about the tellurium atom in the compound 100c is completed by the weakly p-coordinated dithiocarbamate ligand of the adjacent molecule,150 whereas the coordination polyhedron in the compound 100g is completed by the bromine (iodine) atom.151 The structure of (4-methoxyphenyl)tellurium [(dimethyloxy)- thiophosphorylthio)] dibromide, (4-MeOC6H4)Te[S2P(OMe)2]..Br2, differs from that of bis-(and tris)(dithiocarbamate) deriva- tives 100a ± g due to the presence of one bidentate ligand in the former compound. The molecule of this compound has a distorted square-pyramidal structure with the bromine atoms (Te7Br bond lengths are 2.616 and 2.677 A) in cis positions relative to each other.153 The sulfur atoms of the thiophosphorylthio ligand (the Te7S bond lengths are 2.632 and 2.728 A) lie in the base of the pyramid. The aryl group (the Te7Br bond length is 2.127 A) occupies the apical position.In the crystal, weak secondary Te . . . Br bonds (3.810 A) are also present. The structures of s-telluranes RTeX3 were also studied by NQR228 and MoÈ ssbauer spectroscopy.146, 229 ± 232 The isomer chemical shifts for aryltellurium trichlorides, tribromides and triiodides in the region of 0.9 ± 1.0 mm s71 are substantially larger than those for the corresponding diaryltellurium dihalides and are Ref. Bond angle C7Te7X 7 7 144.6 148.0 107.20 73.751(Te_C) 226, 227 148 150 7 7 148 149 2.708, 3.436 2.890 3.117 3.126 e 3.823, 3.598 (Te_S) 3.569 (Te_I) 3.558 (Te_I), 151 151 3.169 3.164,g 2.904 h 166.1, 154.8 176.7 174.9, 3.476 (Te_Br) 176.3878 similar to those for tellurium tetrahalides.229 The quadrupole splitting (5.9 ± 9.3 mm s71) decreases in the order: chlori- des>bromides>iodides.2. Structures of organyltellurium trihalides in solutions. 125Te NMR spectra and dipole moments The conductivities of solutions of organyltellurium trichlorides RTeCl3 in MeCN, PhNO2 and Me2CO73, 74, 194, 233 indicate that these compounds do not dissociate into ions. However, organ- yltellurium tribromides and triiodides in these solvents dissociate into ions (RTeXá2 and X7),73,74 although Chen and George 234 stated that MeTeBr3 in nitrobenzene is not an electrolyte. On the contrary, aryltellurium trihalides containing amino(dimethyl- amino) groups in the aryl fragments are 1 : 1 electrolytes in the above-mentioned solvents regardless of the nature of the halogen atom.61, 62 Apparently, this is due to the higher polarity of the Te7Hal bonds in the latter compounds owing to the presence of strong electron-donor groups.The results of cryoscopic studies of organyltellurium trihalides agree on the whole with the data on conductivity. In solutions, aryltellurium trichlorides [except for amino(dimethylamino) derivatives] exist as monomers and tend to form weak associates as the concentration is increased,74, 194, 233 whereas aryltellurium tribromides and triiodides partially dissociate.61, 62, 73, 74 In the case of methyltellurium trihalides, the monomer ± dimer equili- brium exists in benzene,118, 234 the degree of association of methyltellurium tribromide being higher than that of the corre- sponding trichloride.Table 4. Chemical shifts in the 125Te NMR spectra of s-telluranes RTeX3. X Solvent R R Ref. d a /ppm Me Me Cl3TeCH2 Et Et Pr Pr MeCOCH2 Ph Ph 4-BrC6H4 4-ClC6H4 4-MeC6H4 4-PhOC6H4 4-MeOC6H4 4-MeOC6H4 4-MeOC6H4 PhCOCH=CMe PhCOCH=CMe PhCOCH=CPh MeCOCMe2 EtCOCH2 EtCOCHEt EtCOCMe2 PrCOCH2 PrCOCHEt PrCOCMe2 PriCOCH2 BuiCOCH2 ButCOCH2 Br ICl Cl Cl Cl Cl BrI CH2 Cl2 ±DMSO (2 : 1) 1078 Cl Br Cl 4-MeC6H4COCH=CPh Br Cl Br Cl S2CNEt2 S2CNEt2 S2CNEt2 S2CNMe2 S2CN(CH2Ph)2 (S2CNEt2)2Cl (S2CNEt2)2Cl (S2CNEt2)2I CH2 Cl2 THFd, f (S2CNEt2)2Y CH2 Cl2 (S2CNEt2)2Y CH2 Cl2 (see d) (S2CNEt2)2Y CH2 Cl2 (see f) (S2CNEt2)2Y CH2 Cl2 (see c, f) (S2CNEt2)2Z CH2 Cl2 (see f) (S2CNEt2)2Z CH2 Cl2 (see f) 758 35 647 35 1198 235 b 900 35 849 35 920 35 1204 35 1372 79 1483 79 1488 79 1632 79 1374 79 1485 79 1488 79 1621 79 1376 79 1484 79 1490 79 1625 79 1373 79 1482 79 1373 79 1489 79 1364 79 1487 79 917 35 532 148 524 148 503 148 CH2Cl2 ±DMSO (2 : 1) 1229 236 892 35 PhH ± PhMe PhH ± PhMe DMSO PhH ± PhMe PhH ± PhMe PhH ± PhMe PhH ± PhMe CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 PhH ± PhMe THFc DMFd DMSO Cl Br Cl Cl Br Cl Br Cl MeCOCHMe Cl MeCOCHEt Cl Cl Cl EtCOCHMe Cl Cl Cl Cl PrCOCHMe Cl Cl Cl Cl PriCOCHMe Cl Cl BuiCOCHMe Cl Cl ButCOCHMe Cl Cl Cl Cl Cl Cl Br 1-C10H7COCH=CPh 1-C10H7COCH=CPh 2-PhN=NC6H4 Ph Ph Me 2-PhN=NC6H4 2-PhN=NC6H4 Ph Ph Me Ph Ph Ph Me Me Ph Me Ph Ph Ph Ph Ph Ph PhH ± PhMe Note: Y=S2P(OEt)2; Z=S2 COEt; a the chemical shifts are given relative to Me2Te; b for (Cl3TeCH2CO)2O, d125Te=864 ppm;235 c at 780 8C; d at740 8C; e Ph2Te was used as the standard (dMe2Te=dPh2Te+688); f the spectrum of a solution prepared by mixing of the corresponding substrates.A A Maksimenko, A V Zakharov, I D Sadekov The above-considered data agree with the results of spectral studies of organyltellurium trihalides.The 125Te NMR spectral data for s-telluranes RTeX3 are given in Table 4. The chemical shifts of alkyltellurium and aryltellurium triha- lides are in the ranges of 647 ± 1204 and 892 ± 1234 ppm, respec- tively. In the spectra of organyltellurium trichlorides with ligands containing carbonyl groups, the 125Te resonance is substantially shifted downfield (1372 ± 1632 ppm) compared to the spectra of alkyltellurium trichlorides. The highest deshielding of the 125Te nuclei is observed in b-aroylvinyltellurium trichlorides whose chemical shifts are in the range of 2047 ± 2104 ppm. The shielding of the 125Te nuclei in PhTe increases on going from phenyl- tellurium trichloride to phenyltellurium triiodide. It should be noted that the chemical shifts of organyltellurium trihalides depend substantially on the nature of the solvent.Thus the chemical shifts for PhTeCl3 and PhTeBr3 increase by 312 and 301 ppm, respectively, on going from the PhH ± PhMe system to the 2 : 1 CH2Cl2 ±DMSO system. Apparently, this is associated with the above-mentioned effect of the solvent on the degree of aggregation of organyltellurium trihalides in solution. The dipole moments of aryltellurium trihalides are given in Table 5. The dipole moments of aryltellurium tribromides are smaller that those of the corresponding trichlorides regardless of the substituents in the benzene ring,211 which is indicative of a weaker electron-withdrawing effect of the TeBr3 group compared to that of the TeCl3 group.Ref. Solvent X d a /ppm DMSO CH2Cl2 ±DMSO (2 : 1) 1193 CH2Cl2 ±DMSO (2 : 1) 1101 CH2Cl2 ±DMSO (2 : 1) 1208 CH2Cl2 ±DMSO (2 : 1) 1208 CH2Cl2 ±DMSO (2 : 1) 1234 CH2Cl2 ±DMSO (2 : 1) 1226 1246 CH2Cl2 ±DMSO (2 : 1) 1204 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CDCl3 THF CH2Cl2 CH2Cl2 CDCl3 CDCl3 THF THFc (S2CNEt2)Cl2 236 236 236 236 236 236 236 236 236 237 237 237 237 237 237 146 148 148 148 146 146 148 148 148 148 148 148 148 148 148 148 2053 e 2074 e 2047 e 2086 b 2053 e 2104 e 1278 1455 1472 1673 1225 1240 1622 1660 1892 1098 1563 1612 1768 1796 1510 1722Monoorganyl derivatives of tellurium(IV) Table 5.Dipole moments of aryltellurium trihalides RC6H4TeX3 (dioxane, 25 8C).211 mcalc R X mexp mcalc R X mexp 3.79 4.14 4.95 3.89 3.97 4.02 4.33 4.0 3.88 4.13 5.16 3.76 4.20 4.23 4.36 4.01 Br Cl Cl Cl Cl Cl Cl Cl 4-MeO 4-NO2 3-NO2 3-F 4-OH 4-MeS 3,4-(MeO)2 3,4-Me2 HH4-Me 4-Me 4-Cl 4-Cl 4-Br 4-Br 3.66 a 3.35 3.94 3.51 3.33 2.62 3.20 2.78 4.61 773.84 3.57 72.78 3.32 74.04 Cl Br Cl Br Cl Br Cl Br 4-MeO Cl a The dipole moment of PhTeCl3 in C6H6 (40 8C) is 2.90 D.238 3. Electronic effects of the TeX3 groups The inductive and mesomeric constants of the TeX3 groups were determined based on the chemical shifts in the 19F NMR spectra of m- and p-fluorophenyltellurium trihalides.114 Substituent sP sC dFmeta /ppm dFpara /ppm sI 0.66 0.62 0.11 0.11 0.55 0.51 TeCl3 TeBr3 76.62 76.43 73.28 73.05 The sI and sC values indicate that interactions between the TeX3 groups and the p-electron system of the benzene ring are predominantly inductive in character because the sC constants comprise *15% of the total electronic effect of the substituent. The sP constants of the TeX3 groups, unlike those of the Te(R)X2 groups (X=F, Cl, Br or I), decrease on going from chlorine to bromine.The orbital populations were calculated and the nature of the highest bonding and lowest unoccupied orbitals of PhTeCl3 was examined using the extended HuÈ ckel method.211 According to the results of calculations, the p-electron system of the benzene ring is only slightly affected by the substituent and retains six electrons.A large positive charge on the tellurium atom exists due to the fact that its p orbitals and the combination of the px and pz orbitals of the axial chlorine atoms with the appropriate symmetry are combined into the molecular orbital MOp (E1a).Calculations demonstrated that the three highest occupied MOs are s orbitals. The upper orbital is localised primarily at the tellurium atom, whereas the unoccupied orbital, which involves the dp orbital of the tellurium atom, (MO34) is far removed from the highest occupied p-MOs. These quantum-chemical data account for the absence of a noticeable effect of conjugation between the TeCl3 group and the aryl groups.V. Conclusion The data surveyed in the present review suggest that s-telluranes RTeX3 are of preparative importance in the chemistry of organo- tellurium compounds. s-Telluranes are used as the starting compounds in the synthesis of other classes of organic derivatives of tellurium, viz., of diorganyl ditellurides R2Te2, tellurinic acids RTe(O)OH, tellurium oxochlorides RTe(O)Cl, s-telluranes of the R2TeX2 type and pentacoordinate tellurium derivatives M+[RTeX4]7 (M is the onium group). Intramolecular electro- philic cyclisation of the corresponding organyltellurium trichlor- ides is used for the preparation of different five- and six-membered tellurium-containing heterocycles (benzotellurophenes, dibenzo- tellurophenes and telluroxanthenes) and six-membered hetero- cycles involving other heteroatoms along with the tellurium atom (phenoxatellurines, phenothiatellurines and phenotellurazines).These reactions are specific to the synthesis of tellurium-contain- ing heterocycles and have virtually no analogues in the chemistry of the corresponding sulfur and selenium derivatives. Halodetel- 879 luration of organyltellurium trihalides, which in some cases proceeds with rather high yields, can be used for the preparation of biaryls, different types of mono- and dihalogeno derivatives, stilbenes, carboxylic acids and usaturated ketones. 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Russian Chemical Reviews 69 (10) 883 ± 898 (2000) Polymer monolayers and Langmuir ± Blodgett films. Polythiophenes V V Arslanov Contents I. Introduction II. Oligothiophene monolayers and Langmuir ± Blodgett films III. Polythiophene monolayers and Langmuir ± Blodgett films IV. Mixed monolayers and Langmuir ± Blodgett films of oligo- and polythiophenes V. Prospects for the application of polythiophene Langmuir ± Blodgett films in molecular devices VI. Conclusion Abstract. Lang- and monolayers of properties and structures The The structures and properties of monolayers and Lang- muir polythiophenes and oligo- conducting of films Blodgett ± muir ± Blodgett films of conducting oligo- and polythiophenes are are analysed. number, the of influence the on data published The analysed.The published data on the influence of the number, size, size, and molecules polythiophene in substituents of composition and composition of substituents in polythiophene molecules and and of on chain the along distribution their of pattern the of the pattern of their distribution along the chain on the the monolayer quality film Blodgett ± Langmuir and stability monolayer stability and Langmuir ± Blodgett film quality are are generalised. description the to given is attention Considerable generalised. Considerable attention is given to the description of of the ± Langmuir and monolayers mixed of behaviour the behaviour of mixed monolayers and Langmuir ± Blodgett Blodgett films amphiphilic various and polythiophenes of composed films composed of polythiophenes and various amphiphilic com- com- pounds.chains polymer of lengths conjugation effective The pounds. The effective conjugation lengths of polymer chains and and conductivities films Blodgett ± Langmuir in polythiophenes of conductivities of polythiophenes in Langmuir ± Blodgett films and and cast Langmuir of use the of Examples compared. are films cast films are compared. Examples of the use of Langmuir ± Blodgett are purposes technological and scientific for films Blodgett films for scientific and technological purposes are given. given. The references 100 includes bibliography The bibliography includes 100 references. I. Introduction The Langmuir ± Blodgett method is a versatile technique for assembling multilayer planar ensembles.The scope of problems that can be solved with the use of this method has expanded considerably over recent years. Polymer monolayers and Langmuir ± Blodgett (LB) films play an important role in funda- mental and applied molecular architecture studies. In the author's review,1 the two-dimensional polymerisation of, mostly, tradi- tional monomers and their analogues with hydrocarbon substitu- ents incorporated in order to increase their amphiphilic properties was discussed. In the majority of cases, polymerisation in mono- layers and LB films has been carried out by photo-irradiation of unsaturated compounds such as acrylate and methacrylate lipids, alkyl fumarates and maleates, unsaturated fatty acids, etc. Within the latter group of systems, diacetylenes and polydiacetylenic amphiphilic compounds belonging to conducting polymers were considered.In another paper,2 the structure and properties of polymer monolayers and LB films have been analysed. This paper dealt with the characteristic features of the packing of macromolecule V V Arslanov Institute of Physical Chemistry, Russian Academy of Sciences, Leninsky prosp. 31, 117915 Moscow, Russian Federation. Fax (7-095) 952 53 08. Tel. (7-095) 955 44 89. E-mail: arslanov@servl.phyche.ac.ru Received 12 July 2000 Uspekhi Khimii 69 (10) 963 ± 980 (2000); translated by S S Veselyi #2000 Russian Academy of Sciences and Turpion Ltd on the surfaces of liquids and solids, the compatibility of high- and low-molecular compounds in monolayers, the influence of the degree of polymerisation, size and orientation of side groups on the phase states of monolayers, the correspondence between the architectures of Langmuir and LB monolayers, and many other issues.However, this paper did not touch upon the so-called special polymers which include conducting macromolecules with conjugated bonds, polymers synthesised for nonlinear optics, and highly stable multifunctional network polymers. The abundance of recent data on the properties of monolayers and LB films of only conducting polymers does not allow a detailed analysis of these systems to be made in the framework of one publication. Therefore, the author intends to present separate discussions on the most promising conducting polymers in the form of LB films that are studied most extensively.Polythiophenes occupy a particular place among these poly- mers (polydiacetylenes, polyaniline, polypyrrole, polyphenylenes, etc.), primarily because they are soluble in ordinary solvents, form rather stable monolayers on the surface of water in air and have relatively high stability and conductivity in a doped state. Extensive studies of polymers with internal conductivity started back in the 1970s when polydiacetylene was synthesised.3, 4 Conducting organic polymers have a quasi one-dimensional structure which, to some extent, is similar to that of charge- transfer complexes (CTC). In the conducting state, both types of materials exist as charged molecules.The conductivity of CTC is much higher along the axial direction of stacked molecules due to overlapping of the p-orbitals of the adjacent complexes. In the case of conducting polymers, charge transfer occurs along the chain due to overlapping of the p-orbitals of the adjacent mono- meric units. Due to the chain-like structures of the conducting polymers, strong interactions of the electronic states with the excited conformation states (solitons, polarons, bipolarons) exist, which is typical of one-dimensional systems. The relatively weak interchain interactions provide the diffusion of dopant molecules to the polymeric chain. A decrease in the intensity of interchain interactions in polythiophenes containing substituents at posi- tions 3 and 4 favours the solubility of the polymers in ordinary solvents.This is the most important feature of polythiophenes when deciding whether the Langmuir ± Blodgett technique should be used for their organisation. For example, it was noted 4 that the conductivity of non-substituted polythiophenes ranges from 50 to 100 S cm71, while it is as high as 500 S cm71 in the case of poly(3-methylthiophene) and is close to 1000 S cm71 for some substituted polythiophenes. DOI 10.1070/RC2000v069n10ABEH000612 883 884 885 890 895 897884 Polythiophenes are synthesised by chemical or electrochem- ical methods. The former method gives non-conducting polymers which can be transformed to the conducting state by chemical or electrochemical doping.In the latter case, doped (oxidised) polymers formed can be reduced chemically (de-doped) to the insulating state. The doping efficiency is determined from the position and intensity of the characteristic p ± p* transition absorption band and from the change in the polymer colour. In addition, optical absorption spectra enable determination of the effective conjugation length of the polymeric chains. The longer the conjugation length the higher the polymer conductivity. The conjugation length can be increased by chemical synthesis of polythiophenes with a regular structure. As a result, the electric conductivity can be increased by a factor of 100 (Ref. 3). The features of polythiophenes noted above are used for obtaining monolayers and LB films that are promising as metal and gas sensors, photoelectric and electrochromic devices, optical switches, solar batteries, etc .The Langmuir ± Blodgett method is used for the formation of planar ensembles of conjugated polymers in order to obtain ordered, similarly organised molecular chains, i.e., to reach the longest possible conjugation length so as to increase the intra- molecular conductivity and provide the transfer of charge carriers between the adjacent molecules. However, the potential of this method can only be realised if true monolayer films with concerted molecular orientation are formed on the surface of a liquid and if these monolayers can be transferred onto solid supports with the formation of multilayered ensembles.The success of these oper- ations is usually judged from the structural or optical spectro- scopy studies. The latter method makes it possible to estimate the zone structure features and evaluate the conjugation length of the system formed. Measurement of the conductivity of the LB films obtained is the final step. Depending on the ultimate goal of the studies, other properties of the systems, such as nonlinear optical, photo- and electrophysical properties, can also be determined. The majority of studies on the subject of this review have been carried out according to this scheme, hence it is reasonable to follow it when discussing their results. It might seem that one glance at the polythiophene chain (1) { would suffice to make sure that these polymers cannot form stable monolayers on the surface of water and, even more so, on solid supports.However, as shown below, not all researchers agree with this viewpoint. S S n S1 The problem of increasing the stability of monolayers of compound 1 can be solved at least by two methods: viz., by incorporation of groups that can enhance the surface activity of the molecule and by creation of mixed monolayers with amphi- philic compounds that form stable monolayers. The merits and drawbacks of these two methods for obtaining stable monolayers from molecules that are not `strictly amphiphilic' are considered elsewhere.2 It should be emphasised here that for polymers with distinct functions which depend, first, on the presence of certain reactive groups or conjugated bonds, and second, on realisation of a given conformation or structure of side chains determining the operating capabilities of molecules and their ensembles, incorpo- ration of a second component is undesirable because it decreases the density of the key elements (functional groups, molecules, domains) in the layer and, as a consequence, decreases the efficiency of devices based on these elements. Therefore, the efforts of researchers are primarily aimed at synthesising such derivatives of polythiophenes whose spreading does not require { Henceforth, poly(2,5-diylthiophene) is considered in this review.V V Arslanov dilution with other components, i.e. at designing mixed mono- layers with the matrix components possessing high surface activ- ities.Let us note the main directions of these studies. 1. Grafting of a polar group to an end of the oligothiophene chain which in this case plays the role of the hydrophobic component of the amphiphilic molecule. 2. Incorporation of a side alkyl chain in each thiophene unit. This method works successfully in the case of linear polymers.2 3. Incorporation of one or two hydroxyalkyl groups, which increase the polarity of the thiophene components, in each thiophene unit. 4. Copolymerisation (including block copolymerisation) of thiophene with other monomers that ensure distribution of the thiophene fragments over the subphase surface. In this case it is important that the second component also had conjugated bonds.5. Synthesis of regioregular poly(3-R-thiophenes), where R is an alkyl, alkoxy or some other group. However, these methods do not guarantee the desired result; therefore, mixed systems are studied, in which the functional capabilities of the key elements are preserved, provided that the component ratio remains at the optimum level. II. Oligothiophene monolayers and Langmuir ± Blodgett films The effect of a polar group on the compression isotherms of a monolayer is shown in Fig. 1 for three-membered oligothiophenes 2 ± 7 on the surface of water at 10 8C. One can see that compounds 2 and 3 do not form stable monolayers.2 The esters 4 ± 6 form condensed monolayers, in which the limiting area per molecule (A0) is 27±29 A2. It is believed that this area corresponds to the vertical arrangement of terthiophene units that are directed towards the air phase, since they are more hydrophobic than the polar components of the oligomer molecules.Taking the param- eters of the characteristic points in the compression isotherms of compounds 4 ± 6 into account, Nakahara et al.5 concluded that the monolayers of these oligomers are stable up to surface pressures (p) of 20 (4 and 5) and 30 mN m71 (6). X S S S 2 ± 7 X=CHO(2), CH2OH (3), CH2OCOMe (4), CH2OCOCH=CMe2 (5), CH2OCO(CH2)7Me (6), CH2OCO(CH2)16Me (7). For the long-chain derivative 7, the surface pressure for destruction is as small as several units, while the value of area per molecule extrapolated to zero surface pressure is no higher than 23 A2.In this case, a condensed monolayer is formed in p /mN m71 60 4 6 40 5 2 3 1 200 40 60 20 A /A2 (per molecule) Figure 1. Compression isotherms of terthiophene derivatives at 10 8C;5 (1) 2, (2) 3, (3) 4, (4) 5, 95) 6, (6) 7.Polymer monolayers and Langmuir ± Blodgett films. Polythiophenes which dense packing of hydrocarbon chains probably exists, whereas the less hydrophobic terthiophene group can be immersed in the aqueous phase. The U-shaped configuration of molecules in the monolayer can be ruled out because the areas of the molecules are very small. Presumably, the capability of these compounds to give stable monolayers is overestimated: the isotherms of compound 4 are shifted to smaller areas with increase in temperature.Moreover, the possibility of `immersion into water' of the thiophene groups is determined by interactions of their hydrocarbon substituents with water molecules rather than by the lengths of these substituents. Unfortunately, the data on the changes in the surface pressure following stopping of the mobile barrier of surface balances were not reported. Monolayers of oligothiophenes with a larger number of units could be obtained only by mixing them with eicosanoic (arachidic) acid.5 Monolayers and LB films were obtained from the six-mem- bered oligothiophene 1 (n=4, S6 ) using a strategy for the formation of one-dimensional conducting systems.6 The mono- layers were transferred onto a solid support with the surface pressure of 15 mN m71 and surface area per molecule (A) of 25 A2.The chain orientation found from the dichroic ratio was vertical relative to the support (Fig. 2). The eighteen-layered LB films were thermally polymerised (270 8C, 2.5 h). Comparison of the electronic spectra of LB films of the original oligomer S6 and its thermal polymerisation product showed that the conjugation length in the latter was larger. It was also noted that the S6 oligomer is completely soluble in nitrobenzene and trichloroben- zene, while the polymer synthesised in LB films is insoluble in these solvents. 270 8C 2.5 h Figure 2. Organisation of six-membered thiophenes 1 (S6) in LB films before and after thermal polymerisation.6 The solubility of oligothiophenes in various solvents and the possibility of coordination of their sulfur atoms with metals 7 are the properties that attract the most attention in relation to the studies in the field of self-assembled monolayers,8± 10 i.e., systems very similar to LB films.A study of this kind was carried out for systems containing (oligo)thiophene unit(s) with 2-chlorotetrame- thyldisilanyl and phenylethynyl (compound 8) or oct-1-ynyl substituents.11, 12 Me Me S Si Si Cl S S S S S S 4 4 S S S S S S 4 4 S S S S S S 4 4 Support Me(CH2)5 Me Me S S S S S S Support 4 4 S S S S S S 4 4 S S S S S S 4 4 885 Me Me S Si Si Cl n Me Me 8 n=1, 2, 4 The adsorption of the oligothiophene 8 from its solution in toluene was carried out onto a SiO2 surface (3000 A)/Si.The authors succeeded in obtaining self-assembled monolayers the density of which depended on the number of thiophene rings in the molecule and increased with the increase in the length of the oligomeric chain. Obviously, lateral conductivity requires a high density of the surface layer and at least six or seven thiophene units in the oligomer molecule are required for obtaining high trans- verse conductivity.13 ± 17 III. Polythiophene monolayers and Langmuir ± Blodgett films One attempt at obtaining monolayers and LB films from individ- ual polythiophene derivatives has been undertaken by Logsdon et al.18 Poly(3-dodecylthiophene) was synthesised by electrochem- ical and chemical polymerisation.Electrochemical polymerisation of 3-dodecylthiophene was carried out under galvanostatic con- ditions at an anodic current density of 2 mA cm72 at 5 8C in an argon atmosphere using nitrobenzene as the solvent.19 The average molecular mass of the chloroform-soluble fraction of the polymer 9 is 105 Da. The chemical synthesis included three steps, viz., synthesis of 3-dodecylthiophene,20 its conversion to diiodo-3- dodecylthiophene 21 and polymerisation.22 The molecular mass of the final product (10) determined by gel-permeation chromato- graphy was 2.36103 Da. Me Me (CH2)11 (CH2)11 I I n n S S 9 (n=400) 10 (n=9) The limiting areas per monomeric unit were 14.4 and 11.2 A2, respectively, for monolayers of poly(3-dodecylthiophenes) syn- thesised chemically and electrochemically. The authors believe that, since the first value correlates with that of 14.7 A2 calculated using the structural model for thiophene rings arranged at a right angle to the substrate surface, this particular orientation exists in the polythiophene monolayers synthesised chemically.If this is the case, the role of the hydrocarbon chain, the cross-section of which is *18.5 A2, remains unclear. We shall return to this problem in the analysis of the structure of monolayers of other thiophene derivatives. It should be noted here that both specimens of poly(3-dodecylthiophene) spread poorly and do not form stable true monolayers on the surface of water.2 The limiting area of the polythiophene monolayer synthesised chemically was larger than that of the thiophene monolayer obtained electrochemically, but the former monolayers were less stable and could not be trans- ferred onto solid supports.The Langmuir ± Blodgett vertical method `did not work' in the case of the electrochemically polymerised poly(3-dodecylthiophene), either. However, the horizontal method at a surface pressure of 12 mN m71 made it possible to transfer the monolayers and to obtain X-type films several tens layers thick on a solid support hydrophobised with trichlorooctadecylsilane. Unfortunately, diffraction methods were not used for structural studies of the resulting films, therefore the structure quality of these films cannot be judged. Obviously, the films considered above may be, with caution, classified as LB films with distinct molecular orientation in the layers and alternation of the layers.This should be taken into account when considering the results of analysis of poly(3-do-Df /kHz 2 0.15 2.0 1.0 0 0 N 30 10 Figure 3. Dependence of optical absorption (1) and change in resonance frequency (f) of a quartz crystal (2) on the number of monolayers (N) of polythiophene 9.18 decylthiophene) LB films presented below. Figure 3 shows the dependences of the optical absorption and the change in the quartz resonator frequency on the number of layers of the chemi- cally polymerised poly(3-dodecylthiophene). The small inflexion on the plot of absorption versus the number of layers at N=10 can be explained either by incomplete transfer of thick films onto the support due to the non-uniformity of the morphology of the surface formed or by a decrease in the structural correlation due to disordering of the polymeric segments.The monolayer thickness in LB film calculated from optical absorption data was found to be 35 A. This value is closer to the result of ellipsometric thickness measurement (30 A) than to the monolayer thickness determined using the structural model (19 ± 24 A). The linear dependence of the quartz resonator frequency on N indicates that the mass of the monolayer transferred onto the solid support is constant (the mass of one layer per 1 cm2 is 350 ng) and that changes in the slope of the optical absorption on N are due to the orientation effects rather than to changes in the mass of the monolayers transferred.The specific electric conductivity of the non-doped film does not depend on the number of layers for N440 and equals 9610710 S cm71. Doping with iodine increases the conductivity by 8 orders of magnitude, and its minimum value depends on the number of layers: the conductivity for N=1, 20 and 40 equals 0.014, 0.20 and 0.51 S cm71, respectively. The curves of changes in sk (lateral conductivity) and absorption at 760 nm during the doping in iodine vapour are presented in Fig. 4. During the first 460 s, sk increases by 6 orders and then reaches saturation in 4300 s. Flushing the cell with argon, which removes iodine from the system, restores the initial value of conductivity.The cubic susceptibility [w(3)] of the non-doped film is as high as 161079 electrostatic units. After doping, w(3) becomes 10% less and acquires its original value upon removal of iodine from the system.23Absorption 886 0.30 4.0 3.0 1 Absorption 7log sll (S cm71) Ar 0.2 0 21 3 0.1 2 6 1 9 0 7000 t /s 1000 2000 5000 Figure 4. Dependences of the specific electric conductivity (1) and optical absorption (2) of a 40-layer LB film of polythiophene 9 on the time of exposure in iodine vapour and changes in these parameters after removal of iodine with a flow of argon.18 V V Arslanov p /mN m71 60 1 30 2 2 0 0 10 20 A /A2 (per molecule) Figure 5.Compression isotherms of monolayers of polythiophene 12 (1) and its monomer 13 on the surface of water (2) and a 1073 M CdCl2 solution (2 0).25 It should be emphasised that complex studies of poly(3- dodecylthiophene) LB films did not reveal any specific features of multilayer structures which would be untypical of this con- jugated polymer in the bulk. A decrease 24 or increase 25 in the length of the alkyl chain at position 3 of the thiophene ring in the polymer (from C12 to C8 or to C17) virtually does not affect the ability of the polymer to form stable monolayers. The limiting area per unit for poly(3-octyl- thiophene) (11) monolayer does not exceed 10 A2, while that for poly(3-heptadecylthiophene) (12) does not exceed 12 A2 (Fig.5). The small value of the limiting monolayer area obtained in the latter case is explained 25 by detachment of some fraction of molecules or their units from the surface of the aqueous subphase (incomplete spreading). According to ellipsometry, the layer thickness is 50% larger than that calculated for the monolayer, hence the small value of A0 can also be due to the formation of a tilted bilayer structure.25 Figure 5 also shows the compression isotherms for the 3-heptadecylthiophene monomer (13). One can see that transition to the monomer, which decreases the intrachain cohesion due to cleavage of the polymer covalent bonds, improves insignificantly the spreading of thiophene and results in a liquid monolayer with low decomposition pressure.Me Me Me (CH2)16 (CH2)16 (CH2)7 n S S n S 13 11 12 Thus, incorporation of an alkyl chain in the thiophene unit enhances insignificantly the spreading ability of the polymer due to weakening of the cohesion interactions between the thiophene rings. However, the hydrophilicity of the thiophene units remains rather low and does not ensure the formation of a stable mono- layer on the surface of water. Incorporation of substituents at positions 3 and 4 of the thiophene ring, for example, of an alkoxy group to give poly(3-octyloxy-4-methylthiophene) (14), does not allow a true monolayer to be obtained on the surface of water.26, 27 Me Me Me (CH2)7 (CH2)3 (CH2)3 O O O Me n n S 15 S 14Polymer monolayers and Langmuir ± Blodgett films.Polythiophenes The idea of hydrophilisation of each thiophene ring in poly- thiophene was implemented by Callender et al.,28 who have synthesised and studied the properties of monolayers of poly(3,4-dibutoxythiophene) (15). Polythiophene 15 was synthes- ised from 3,4-dibromothiophene and sodium butoxide in the presence of copper oxide and platinum iodide with subsequent chemical oxidation of the monomer with iron trichloride. The compression isotherms of compound 15 at two temper- atures are presented in Fig. 6. The monolayer destruction at 11 8C occurs above a surface pressure of 20 mN m71 , which is believed to be due to folding of the monolayer and formation of a bilayer film. Destruction at higher temperatures is observed at lower p.The Langmuir ± Blodgett films of polythiophene 15 were obtained at 11 8C and a surface pressure of 12 mN m71. Under these conditions, changes in p and A were negligibly small within 2 h and the limiting area per residue was 20 A2. It was found 29 that the surface of the thiophene chains, whose long axes are arranged at a right angle to the subphase surface and have a dihedral angle of 90 8, is 28 A2. However, it was shown 18 that a poly(thiophene) molecule, whose chains are parallel to the surface of water and the rings are oriented perpendicularly to this surface, requires a surface of 14.7 A2 per residue. In this case, the area per residue equal to 20 A2 found for polythiophene 15 should correspond to a polymer with an axis directed parallel to the surface of water and a dihedral angle between repeating units ranging from 0 8 to 90 8.The resulting twisted conformation is formed due to steric interactions between the bulky alkoxy substituents at positions 3 and 4. The absorption spectrum of the 28-layer LB film of polythiophene 15 does not differ from the spectrum of this polymer in the solid state; the absorption intensity in the max- imum (the band at 462 nm) increases linearly with an increase in the number of layers (N430). Optical spectroscopy was used to analyse the stability of polythiophene 15 LB films stored under various conditions:28 in the dark, under daylight and under artificial light with a wavelength of >500 nm. After ageing for 30 days, absorption at 462 nm did not change for the first sample, but it decreased by 35% and 24% for the second and third samples, respectively. The absorption maximum of polythiophene 15 LB films is shifted towards shorter wavelengths relative to the absorption maxima of solid poly(3-alkylthiophenes) (*500 nm) 30 and poly(3-butoxy-4-methylthiophene) (545 nm).26 These data suggest that steric interactions between the two bulky side chains and the polymer chain favour the formation of such a polymer conformation where the thiophene units are turned relative to each other.This decreases the effective conjugation length. X-Ray diffraction data 28 indicate that the bulk polymer 15 is semicrystal- line and has a helical conformation, which is apparently preserved in solution (chloroform).Since the polymer chain conformation and the LB film structure determine the electric 27 and nonlinear- optical properties to a considerable extent, structural studies of these systems have to be carried out. p /mN m711 2 40 200 20 10 A /A2 (per unit) Figure 6. Compression isotherms of monolayers of polythiophene 15 on the surface of water at 11 (1) and 26 8C (2).28 887 After their failure to obtain stable monolayers and LB films of poly(3-alkylthiophenes), Bolognesi et al.31 synthesised polythio- phenes 16 ± 18 containing ether groups at position 3. Et Et Me O O O (CH2)4 (CH2)10 (CH2)10 n n n S 18 S 17 S 16 Figure 7 shows the compression isotherms of monolayers of polythiophenes 16 ± 18.One can see that the curves differ notice- ably; the regions of area per residue exceeding 14.7 A2 (the minimum area occupied by the thiophene monomer) are the most interesting. The limiting areas per residue are 20, 15.5 and 15.3 A2 for the polymers 16 ± 18, respectively. Presumably, in all three cases the oxygen atoms in the monolayers are in contact with the aqueous support, whereas the differences in the areas are explained by different slopes of the hydrocarbon spacers in the side chains. The angle between the adjacent monomeric units should also be taken into account; the area per residue can vary from 28 to 14.7 A2 as a function of this angle. The pressures for monolayer collapse differ considerably as well: these are 20.0, 41.0 and 21.5 mN m71 for compounds 16, 17 and 18, respectively. The big difference between the collapse surface pressures (pc) for polythiophenes 16 and 17, whose side chains differ only in one CH2 group, is unexpected. This fact was explained 31 by the higher polarity of OMe-substituted polythiophene.However, this would imply that the more polar polymer 18 would form a more stable monolayer. In reality, the opposite phenomenon is observed (see Fig. 7). The viewpoint of Bolognesi et al.31, 32 regarding the orientation of polythiophene 17 molecules in the monolayer is also unclear. They consider that the main chain is parallel to the surface of water, the side chain is almost perpendicular to this surface, while the oxygen atom (at the free end of this chain) contacts water.Obviously, the latter two orientations cannot exist simultaneously. In the case of the polymer 17, both Y-transfer to a hydro- phobic support and Z-transfer to a hydrophilic surface were carried out. In the former case, the transfer ratios were 0.9: and 0.8; (the arrow indicates the direction of support movement: : means upwards from the subphase, while ; means downwards to the subphase) for the first 8 ± 10 cycles, while in the latter case they were 1: and 0.2 ± 0.5; for the first 12 ± 14 cycles. The transfer ratio decreased as the number of cycles increased. A ten-layer LB film of polythiophene 17 was analysed by optical spectroscopy in combination with a cast film. The difference in positions of the absorption maxima for the LB film (lmax=472 nm) and for the film obtained from solution (lmax=460 nm) is explained by a p /mN m71 45 30 15 3 2 1 0 20 10 A /A2 (per unit) Figure 7.Compression isotherms of monolayers of polythiophenes 16 (1), 17 (2) and 18 (3).31888 larger conjugation length in the case of the LB film. IR spectro- scopy was used to demonstrate the higher planarity of the main polythiophene chain in the LB film. Similar data were also obtained for the polymer 16.33 These data differed from those of Callender et al.28 who had also used optical spectroscopy but did not find differences between the spectra of LB films and those of a solution of polythiophene 15. The discrepancy between the data obtained by both research groups were explained 31 by the absence of a spacer in the side group of the polymer 15 between the thiophene ring and the oxygen atom.The ordered structure of polythiophene 17 in LB films was confirmed by polarisation optical spectroscopy and by electron microscopy.32 In addition, alternating LB films containing monolayers of 17 and cadmium alkadiynoate were obtained. Polymerisation of the diacetylene was carried out after transferring the monomer onto a solid support.32 It was shown that polymerisation does not result in destruction of polythiophene, and the spectrum of the Lang- muir ± Blodgett hetero-film does not show a shift of the exciton band of polydiacetylene, indicating the absence of interactions between the polydiacetylene exciton and the polythiophene mac- romolecules.Thus, incorporation of ether groups in the side chain of polythiophene enhances the stability of monolayers in compar- ison with that of poly(3-alkylthiophenes). However, the problem of orientation of monomeric units both in monolayers and in LB films requires a more thorough analysis. An increase in the polarity of monomeric units of polythio- phene by incorporation of an ester group at position 3 enhances polymer spreading in comparison with that of polyalkylthiophene and polythiophene containing ether groups in the side chains.24 Figure 8 presents the plots of surface pressure vs. the monomeric unit area for poly[3-(2-propanoyloxyethyl)thiophene] (19), poly[3-(2-decanoyloxyethyl)thiophene] (20) and, for comparison, poly(3-octylthiophene) (11).The short polar side chain in com- pound 19 `stacks' the thiophene ring parallel to the surface of water: the limiting area per residue in this region is *28 A2. Further, the rings are detached from the surface and the increase in pressure is observed in the region of closest packing of vertically arranged thiophene rings. The more complex isotherm shape in the case of polythiophene 20 monolayer is explained by the contribution of the comparatively long hydrocarbon chain arranged so that the polar group in each monomeric unit faces the water surface. The first inflexion on the isotherm results from detachment of these chains from the subphase surface.Although the monolayers of compounds 19 and 20 are stable, Kim et al.24 were unable to transfer them onto solid substrates and to obtain LB films. This problem was solved for mixed monolayers of these polythiophenes with octadecylamine (see below). p /mN m71 40 20 1 2 3 0 60 40 20 A /A2 (per unit) Figure 8. Compression isotherms of monolayers of polythiophenes 11 (1), 20 (2) and 19 (3).24 V V Arslanov Me Et (CH2)8 C O CF3 C O O O (CF2)5 (CH2)3 (CH2)2 (CH2)2 n n n S 21 S 20 S 19 The possibility of an increase in the polarity (hydrophilicity) of the side chains of polyalkylthiophenes for enhancement of the stability of monolayers has already been considered. However, the same result can be achieved by means of a considerable increase in the hydrophobicity of the side chain, as was demonstrated by Robitaille et al.34 for poly[3-(4,4,5,5,6,6,7,7,8,8,9,9,9-tridecafluo- rononyl)thiophene] (21).Monolayers on the surface of water were from a solution of the polymer 21 in octafluorotoluene. The compression isotherm of polythiophene 21 at 9 8C presented in Fig. 9 was obtained after several preliminary `compression ± expansion' cycles in the surface pressure range from 0 to 25 mN m71. Extrapolation of the curve to p=0 gives the limiting area per residue of 34 A2, which agrees with the area occupied by a fluorinated fatty acid molecule in the monolayer. The thermo- chromic properties of polythiophene 21 were studied using its LB films obtained at p=10, 14 and 18 mN m71.Unfortunately, good LB films were not formed under these conditions: the transfer ratio onto silanised glass was *0.6:. Transfer did not occur at all when the support moved to the subphase. Figure 10 shows the temperature dependences of optical absorption in the UV and visible spectral regions for the monolayer and a film obtained from solution on a rotating support. Robitaille et al.34 believe with good reason that comparison of spectra of the two samples makes it possible to understand the nature of the thermochromism of compound 21. Heating of the samples shifts the absorption maximum at 488 nm to 396 nm. It is believed 35, 36 that this optical effect results from conformational transition of the main polymer chain.It should be noted that the spectra for the two- (monolayer) and three-dimensional (film from solution) states of polythiophene 21 virtually coincide: in both cases, a coplanar structure existing at low temperature is transformed to a non-planar one at high temperature. Since the effects for the two- and three-dimensional systems are identical (Fig. 10 a and b), it is concluded that this thermally induced conformational transition is determined by the polythiophene chain itself and does not depend on interchain interactions. However, it should be taken into account that this conformational transition is possible only if it is not prohibited by steric hindrance from the substituents at position 3. In the case of polythiophene incorporating rather p /mN m71 40 200 40 20 A /A2 (per unit) Figure 9.Compression isotherm of fluorinated polythiophene 21 at 9 8C.34Polymer monolayers and Langmuir ± Blodgett films. Polythiophenes 300 400 500 Figure 10. Absorption spectra of a monolayer (a) and a coating obtained from solution (b) of fluorinated polythiophene 21 at various temper- atures.34 The numbers at the curves indicate the temperatures (8C). bulky side chains, such a transition is possible only if regioregular regions exist in the polymer (`head-to-tail' diads); however, the existence of such regions was not reported.34 The problem of the regioregularity of poly(3-R-thiophenes), including the formation of stable molecular ensembles on liquid and solid supports, has made progress only in recent years after poly(3-R-thiophenes) with regioregularity of up to 100% had been synthesised and their study had begun.37 ± 44 Two conforma- tions of poly(3-alkylthiophene) are shown below: coplanar (`head- to-tail') (22a) and twisted (`head-to-head') (22b).R R S S 22a R As shown by McCullough et al.,41 ± 43 an increase in the regioregularity of poly(3-alkylthiophene) increases considerably the electric conductivity of the films and results in greater changes in the physical parameters of the polymers than for the samples synthesised with the use of the standard chemical or electro- chemical polymer oxidation procedures. Similar conclusions were drawn 39, 40 in studies of the photoluminescence of poly(3-hexylth- iophene) films with different regioregularity obtained from sol- utions.Obviously, the use of such an efficient organisation method as the Langmuir ± Blodgett technique for obtaining ultra-thin films of structurally uniform poly(3-R-thiophenes) will make it possible to increase their uniformity and improve the parameters of various devices constructed from these films. Ochiai et al.45 compared the properties of monolayers of regioregular poly(3-{2-[(S)-2-methylbutoxy]ethyl}thiophene) (23) containing more than 93% of `head-to-tail' conformers Absorption Absorption a 40 80 100 120 140 160 b 40 80 100 120 140 160 600 700 l /nm S S 22b R 889 (23a, Mw=2.56104) and polythiophene containing as little as 46% of `head-to-head' conformers (23b,Mw=2.056104).Et CHMe CH2 O (CH2)2 S n 23 A monolayer of compound 23a forms a condensed phase with a collapse surface pressure of*18 mN m71 (Fig. 11). The limit- ing area per residue is 17 A2, which corresponds to an almost perpendicular arrangement of thiophene rings with respect to the surface; the head groups of these rings are oriented towards the subphase. A monolayer of compound 23b has a larger limiting area per residue (19 A2), but the low collapse pressure of this monolayer (*10 mN m71) indicates its low stability and random orientation of the molecules. Due to these characteristics of monolayers of 23b, they do not form LB films. Analysis of the polarised optical spectra in the region of 400 ± 650 nm for LB films of the polymer 23a showed that its molecules are oriented parallel to the surface of the substrate and the chain orientation correlates with the direction of movement of the support when the mono- layers are transferred. Comparison of these spectra with those for films deposited from solution showed that in the case of LB films the molecules have a higher degree of orientation and larger conjugation lengths.This increases the lateral conductivity of polythiophene 23a LB films doped with NOPF6 or FeCl3 to 1 ± 5 S cm71, which is 4 to 5 orders higher than the conductivity of polythiophene 23b films obtained from solution. In addition, LB films of the polymer 23a have anisotropy of lateral conductiv- ity and high cubic susceptibility.45 Unfortunately, there are as yet few studies of this kind within the Langmuir ± Blodgett method, and for individual polythio- phenes only the studies cited above are available.Solutions, films obtained from solution, and solid polythiophenes are being studied thoroughly. These systems are not covered in the present review dealing with monolayers and LB films; however, we have to dwell on some general issues of the relationship between the structure, composition and properties of polythiophenes in sol- utions and in the bulk. It is known 46 that electrochemical synthesis of poly(3-alkyl- thiophenes) gives units with substituents at positions 2 and 4, whereas chemical synthesis results in residues with substituents at positions 2 and 5.Both methods give irregular polymers. Natu- rally , these `defects' affect the planarity of the main polymer chain p /mN m71 50 1 20 2 20 10 A /A2 (per unit) Figure 11. Compression isotherms of monolayers of polythiophenes 23a (1) and 23b (2) at 10 8C.45890 and hinder both the intrachain and the interchain transport of charge carriers. As a result, poly(3,4-dialkylthiophenes) and poly(3,30-dihexyl-2,20-dithiophenes) mostly containing `head-to- head' conformers have a small conjugation length and low conductivity.47, 48 There are prerequisites for increasing the poly- mer regularity by polymerisation of long-chain 3-alkylthio- phenes.49, 50 However, in this case the polymer contains structural defects as well 48 [C(2)7C(40) bonds and branchings]. It is believed 51, 52 that regular polymers with high conductivities can be obtained from 3,4-disubstituted thiophenes if steric hindrance is removed. This assumption was checked 50 by the synthesis of poly(3-alkoxythiophenes), poly(3-alkoxy-4-methyl- thiophenes) and poly(3,4-dialkoxythiophenes) by chemical oxida- tion of monomers with anhydrous FeCl3 (see Refs 48, 53). The effects of the position, size and nature of substituents on the molecular mass, positions of characteristic absorption bands and conductivities of the resulting polymers were determined.It was shown that the absorption band of 3,4-disubstituted polyalkyl- thiophenes is strongly (by *200 nm) shifted towards the blue region in comparison with the bands of the other polythiophenes synthesised.This implies a non-planar conformation of chains due to strong steric interactions between the substituents and the main chain. As a result, gaps remain between the conjugated chains of the adjacent molecules; these gaps decrease the probability of charge carrier transfer according to the hopping mechanism.52 Replacement of alkyls by alkoxy groups in poly(3,4-dialkylthio- phenes) increases considerably the conjugation length and con- ductivity of the polymer, which is explained by the smaller van der Waals radius of the oxygen atom (1.4 A) in comparison with the methylene group radius (2.0 A). This conclusion can be very important in the synthesis of Langmuir monolayers, since incor- poration of a polar group enhances the ability of polythiophenes to spread over an aqueous subphase surface.Studies on the synthesis of regular polythiophenes with various quantitative and qualitative compositions of substituents and their use for obtaining monolayers and LB films are already under way. For example, works have been published on the synthesis and properties of polythiophenes, polybithiophenes and polyterthiophenes containing such substituents as 3-cyclo- hexyl, 3-methyl-4-cyclohexyl, 3-alkylphenyl, etc.54 ± 57 IV. Mixed monolayers and Langmuir ± Blodgett films of oligo- and polythiophenes Analysis of the behaviour of monolayers and LB films of simple oligo- and polythiophenes showed that the formation of stable films on the surface of water requires adjustment of the hydro- philic-lipophilic balance of thiophenes towards enhancement of the hydrophilicity of the head group.In addition, a side group, most commonly an alkoxy group, is to be introduced into each thiophene ring of the main chain (in most cases, at position 3) in order to weaken the interchain interactions responsible for aggregation of macromolecules on the surface of water and incomplete spreading of the compound to form a monolayer. An easier method to obtain stable monolayers containing polythio- phenes involves formation of mixed monolayers with an amphi- philic compound which forms stable monolayers with high collapse pressure. The problem of the role of intra- and interchain charge transfer is solved using mixed monolayers of conjugated polymers with compounds which are inert with respect to electric conductivity (fatty acids, alcohols, polymers).Obviously, incor- poration of an inert component should not hinder the interchain charge transfer in the case of mixed films. A five-membered oligomer of thiophene 1 (n=3, S5) was mixed with stearic acid 29 or eicosanoic acid 58 and placed on the surface of aqueous solutions containing CdCl2. Compression isotherms obtained for monolayers of mixtures with different component ratios were used to determine the limiting area per molecule of oligothiophene S5, which was 26 A2 for mixtures of S5 with stearic acid 29 and 19 A2 for mixtures of S5 with eicosanoic V V Arslanov acid.58 Such a large difference was explained by the higher pH used in the former study 29 and better correspondence between the lengths of the molecules of S5 and cadmium stearate (25.8 A) than of S5 and cadmium eicosanoate (28 A).According to calcula- tions,29 the area of an S5 molecule arranged at a right angle to the film surface and having a dihedral angle of 90 8 is 28 A2. There- fore, the area per S5 molecule equal to 26 A2 should correspond to vertical orientation of oligothiophene with the dihedral angle ranging between 20 8 and 50 8. Figure 12 shows the dependence of the specific electric con- ductivity of iodine-doped LB films obtained from a mixture of S5 and stearic acid (the content of S5 was 30 mol.%) on the number of layers (N) in a Y type film.29 One can see that for N47, the conductivity increases rapidly and reaches a steady-state value of log sk (S cm71) 72 74 76 15 0 N 5 Figure 12.Dependence of the lateral conductivity on the number of layers in an LB filmobtained from a mixture of oligothiophene 1 (S5) (30 mol.%) and stearic acid.29 k for iodine-doped 0.1 S cm71 atN>15. The dependences of the lateral conductivity on the mole fraction of S5 in the mixture for 1-, 3- and 11-layer, iodine-doped LB films are presented in Fig. 13. Two regions are observed in the curves. For the 11-layer LB film, the conductivity reaches a plateau at a mole fraction [S5]50.35. The limiting conductivity (0.2 S cm71) corresponds to s bulk polythiophene (0.01 ± 0.1 S cm71) 59 and exceeds the sk for the spray-deposited, iodine-doped S5 film.The lateral conductiv- ity of non-doped LB films does not exceed 1079 S cm71, while the transverse conductivity s\ is lower than 10712 S cm71. The maximum conductivity can be reached by doping for a period of two days. The doping process is partially reversible: for the 21-layer LB film obtained from a mixture of S5 with stearic acid (1 : 1), sk and s\ were 3.061073 and 1.561077 S cm71, respec- tively, after removal of iodine. Since neither the oligothiophene chain length nor the film ordering affect noticeably the electric conductivity of oligomers log sk (S cm71) 3 72 2 74 1 76 78 0 0.8 [S5] /mole fraction 0.4 Figure 13.Dependence of the lateral conductivity of LB films consisting of monolayers of oligothiophene 1 (S5) and stearic acid on the mole fraction of S5 for an LB film containing 1 (1), 3 (2) and 11 layers (3).29Polymer monolayers and Langmuir ± Blodgett films. Polythiophenes doped with iodine,29 it was concluded that the electric conductiv- ity of polymers and oligomer ensembles is determined by the intermolecular transfer of charge carriers. There are many more studies of mixed monolayers and LB films containing polythiophenes than of those dealing with monolayers of oligothiophenes and films of individual com- pounds. It is believed 60 that pure poly(3-alkylthiophenes) are unsuitable for obtaining organised ensembles for at least two reasons.First, they do not form true monolayers; when they spread over the surface of water, they form islets visible by naked eye. The same situation is observed for poly(3-alkylthiophenes) containing up to 18 carbon atoms in the side chain. Second, monolayers can be transferred onto solid supports only by the horizontal method. The films obtained in this manner have low degree of organisation. As in the case of oligothiophenes, these drawbacks can usually be overcome by using fatty acids such as stearic and eicosanoic acids.61 ± 65 Figure 14 shows the p ±A plots for mixed monolayers with various compositions obtained from mixtures of poly(3-hexylth- iophene) and stearic acid.60 It is evident that the mixture contain- ing up to 80 mol.% of the polymer (with respect to the repeating thiophene unit) forms a stable condensed monolayer.The phase transition in this monolayer is observed at high surface pressures (25 mN m71). For all component ratios, p increases with a decrease in the area until the transition region is reached where the surface pressure remains constant. A new abrupt increase in p starts at monolayer areas close to A for a monolayer of pure stearic acid. Obviously, the plateau corresponds to displacement of the polymer molecules from the liquid surface and formation of a bilayer film. This picture is observed for all alkyl-substituted polythiophenes except for poly(3-octadecylthiophene) in which strong interactions of the alkyl chains with the chains of similar length of stearic acid prevent polymer displacement from the interface.Let us consider the properties of LB films obtained from these mixtures. The LB film thicknesses after one passage of the support through a mixed polythiophene ± acid (2 : 1) monolayer at transfer pressures below (20 mN m71) and above (28 mN m71) the phase transition (plateau) were 32 and 44 A, respectively (see Fig. 14). According to the optical spectroscopy data, the bilayer structure formed on the surface of water at high p is transferred onto the solid support. However, more precise X-ray diffraction data showed that the bilayer thickness in LB films correlates with the size of two stearic acid molecules and does not depend on the transfer pressure. The formation of high-quality LB films from a mixture of poly(3-hexylthiophene) with stearic acid was evidenced by linear dependences of the absorption intensity at the maximum of the characteristic absorption band of polythiophene on the p /mN m71 60 40 20 5 1 2 3 4 0 50 60 30 20 40 A /A2 (per molecule) Figure 14.p ±A Dependences for mixed monolayers of poly(3-hexyl- thiophene) and stearic acid for the molar component ratios: 0 : 1 (1 ); 1 : 2 (2); 1 : 1 (3); 2 : 1 (4) and 5 : 1 (5).60 Cd2+ poly(3-alkyl- Intensity 891 1234 8.00 3.00 13.00 2Y /deg Figure 15. X-Ray diffraction diagrams for LB films consisting of mixed monolayers of poly(3-hexylthiophene) and stearic acid for the molar component ratios: 1 : 2 (1); 1 : 1 (2); 2 : 1 (3) and 5 : 1 (4).60 number of layers for various component ratios and surface trans- fer pressures and by a linear relationship between the inverse capacity of LB films and the number of layers.The layered structure of LB films was confirmed by X-ray diffraction (Fig. 15). The X-ray diffractogram displays distinct Bragg peaks with an interlayer distance of 50 A, which corresponds to a bilayer of stearic acid. Near-edge X-ray absorption fine structure analysis showed a high degree of organisation of stearic acid hydrocarbon chains in the bilayers and an irregular orientation of poly(3- hexylthiophene) chains. A model of a mixed LB film structure was built on the basis of the experimental data (Fig. 16). The effect of the content of poly(3-octylthiophene) in a mixture with eicosanoic acid on the properties of monolayers and LB films was studied.65 No plateau was found on the compression isotherms of monolayers (the mixture containing 60% of the polymer was an exception).As in other studies, an increase in the polymer fraction in the mixture resulted in an increase in the monolayer limiting area. The high stability of mixed monolayers [unlike the monolayers of pure poly(3-octyl- thiophene)] and the high surface collapse pressure (>50 mN m71) made it possible to form a 45-layer LB film at a stearic acid thiophene) 50A Ê Figure 16. Amodel of the structure of an LB filmobtained from a mixture of poly(3-hexylthiophene) and stearic acid.60892 transfer ratio close to 1.Despite the instability of pure poly(3- octylthiophene) monolayers, the respective LB films were obtained and their thicknesses were determined by ellipsometry. The thickness of one layer in an LB film was 40 A, which is 3 A thicker than that of one monolayer in the LB film obtained from a mixture of eicosanoic acid and poly(3-octylthiophene) measured by the same method. These data were not discussed, but it is easy to understand the origin of the larger thickness of the pure polythiophene LB film if its `monolayers' were transferred at area per unit parameters insignificantly in excess of 5 A2. It should be noted that the thickness of one LB film layer of pure poly(3- octylthiophene) and its mixtures with eicosanoic acid specified above is considerably larger than the thickness of a densely packed monolayer of eicosanoic acid.In addition, ellipsometry, unlike X-ray diffraction, does not allow determination of the structure spacing parameters and hence does not provide data on the film layered structure. Such data were reported.60, 66 It was found for mixed LB films of stearic acid with poly(3-hexylthiophene) in the ratios from 2 : 1 to 1 : 5 that the structure spacing corresponded to the thickness of a stearic acid bilayer.60 It was also found for LB films obtained from binary mixtures (1 : 10 and 1 : 3) of poly(3- heptylthiophene), poly(3-decylthiophene) or poly(3-undecylthio- phene) with cadmium eicosanoate that the structure spacing of all three systems corresponds to the thickness of a cadmium eicosa- noate bilayer (55.2 A).66 Two reflections were observed for rather high contents of poly(3-decylthiophene) in a mixture with cad- mium eicosanoate (3 : 1); these reflections corresponded to inter- layer distances of *70 and *50 A.This result and the broadening of the diffraction bands indicated a decrease in the system ordering. Despite the high content of behenic acid in a mixed monolayer with poly(3-hexadecylthiophene) (5 : 1 w/w) and its high stability (pc= 60 mN m71 and A0=13.7 A2 per unit), only a Z-type LB film (20 layers) could be obtained.67 It was found in a study of these films by attenuated total internal reflection IR spectroscopy and reflection at grazing light angles that the LB film obtained from a mixture has a less organised structure than the LB film of pure behenic acid. Mixed monolayers of poly(3-octylthiophene) and unsaturated oleic acid having the same hydrocarbon chain length as stearic acid were studied.68 It was shown that an increase in the content of polythiophene in the mixture from 10% to 90% decreased the mean molecular area of the monolayer upon its destruction from 42 to 5 A2.Let us remind that in the case of saturated acids, the monolayer area generally increases with an increase in the polymer fraction.60, 69 Exposure of the solution used for the formation of mixed monolayers of oleic acid and poly(3-octyl- thiophene) to daylight shifted the dependence of the monolayer area on the polymer content towards larger areas. The shoulder at 590 nm found 68 in the absorption spectra of LB films composed of mixed monolayers is explained by the formation of a charge- transfer complex between oleic acid and the thiophene rings.The study of mixed monolayers of poly(3-butylthiophene) and stearic acid,70 like yet another study 65 of the poly(3-octylthio- phene) ± eicosanoic acid system, did not show a plateau on the compression isotherms for the polymers with molecular masses differing by an order of magnitude (2.56105 and 1.56104). For these systems, as for the majority of other mixtures of fatty acids with polythiophenes, the area per monomer unit in a monolayer increases with an increase in the polymer content in the mixture.65 The main differences in the shape of the compression isotherms for polythiophene samples with different molecular masses were observed in the region of destruction of mixed monolayers, i.e.in the transition from a two-dimensional to a three-dimensional state of the material. The surface potential of monolayers is `insensitive' to these differences and to the obvious differences of the aggrega- tion states of polymer monolayers with significantly different molecular masses. In both cases (as for the majority of monolayers of low- and high-molecular-mass compounds), the surface poten- tial starts to increase at monolayer areas considerably exceeding those where the increase in the surface pressure starts. The transfer V V Arslanov of mixed monolayers onto a glass surface by the vertical method was studied in detail;70 it was found that the transfer ratio is close to unity and that the resulting LB films of the Y type have homogeneous structures.It was noted above that important information on the proper- ties of conjugated systems can be obtained from absorption spectra of LB films and their comparison with the spectra of polymers in solutions. High quality LB films of the Y type with various ratios of stearic acid and poly(3-hexylthiophene) were described.60 Differences between the absorption spectra of LB films of individual polythiophenes 16 and 17 and their films obtained from solution were observed;31 ± 33 however, no differ- ences were found between similar spectra of LB films obtained from a mixture with stearic acid and that of a film obtained from a solution of pure poly(3-hexylthiophene).60, 69, 70 The position of the p ± p*-transition maximum in the absorp- tion spectra of LB films consisting of mixed monolayers of poly(3-alkylthiophenes) and cadmium eicosanoate depends on the length of the polymer alkyl chain.66 For example, for LB films of poly(3-decylthiophene) and poly(3-undecylthiophene), the absorption maximum of a 1 : 3 polymer : salt mixture was observed at 520 and 515 nm, respectively, whereas in the case of a film obtained from solution, the absorption maxima were at 477 and 500 nm.For poly(3-heptylthiophene), no differences between the absorption spectra of mixed LB films and a film formed from solution were found.These data are believed 66 to be consistent with other data.60, 71 It was therefore concluded that the conjuga- tion length in poly(3-decylthiophene) and poly(3-undecylthio- phene) is larger in LB films than in films formed from solution. This effect was explained by interactions between the hydro- carbon chains of the acid and poly(3-alkylthiophenes). For poly(3-heptylthiophene), no increase in the conjugation length in LB films was observed. The absorption spectra of LB films containing poly(3-octyl- thiophene) and eicosanoic acid displayed a red shift of the absorption band maximum by 60 nm in comparison with the position of the absorption band maximum of a solution of the polymer in chloroform.65 As in yet another study,36 this effect was explained by conformational reorganisation of the chain in LB films.Ageing of the film shifted this absorption band towards the short-wave region. Doping of LB films was carried out in SbCl5 or iodine vapour. As a result, the LB film colour changed from red to blue, while the conductivity increased from 261079 to 261075 S cm71 for iodine and to 161074 S cm71 for SbCl5.65 The conductivity of a mixed 34-layer LB film doped with nitrosyl hexafluorophosphate depends on the component ratio.60 An increase in the poly(3-hexylthiophene) content in a mixture with stearic acid from 1 : 2 to 5 : 1 increases the electrical con- ductivity from 0.02 to 2.0 S cm71. Similar results were obtained 72 for LB films consisting of binary mixed monolayers (2 : 1) of stearic acid with poly(3-butylthiophene), poly(3-octylthiophene) or poly(3-octadecylthiophene); the maximum electric conductiv- ity was 5.0 S cm71.Doping also results in significant changes in the absorption spectra of mixed LB films. The intensity of the strong absorption band corresponding to p ± p*-transition of the conjugated main chain (lmax=500 nm) decreases considerably and two new bands appear at 800 and 2200 nm, which correspond to transitions between the valence band and the new bipolar charged states that appear due to doping.73 Attempts to incorporate ions in mixed LB films consisting of poly(3-octylthiophene) with stearic acid (1.4 : 1) were made in order to increase the conductivity of these films.69 However, even small contents of e.g., N+ (561014 ions per cm2) violated the LB film layer structure, apparently due to destruction of stearic acid.The conductivity increased to only 1073 S cm71 with increase in the N+ content by 2 orders of magnitude and resulted from formation of a conducting form of carbon. This not only destroyed completely the layer structure of the film but also destroyed the conjugated polymer structure (optical spectroscopy data).Polymer monolayers and Langmuir ± Blodgett films. Polythiophenes Mixed monolayers and LB films of poly(3-hexylthiophene) or poly(3-dodecylthiophene) with poly(isobutyl methacrylate) were studied.71 Taking account of the deviation from linearity of the relationship between the monolayer limiting area and composi- tion, it was concluded in this study that polyalkylthiophenes and poly(isobutyl methacrylate) are compatible.Comparison of the optical spectra of mixed LB films with those of films with the same component ratio cast from solution showed that the conjugation efficiency for LB films containing poly(3-dodecylthiophene) is higher than for cast films but it is lower for films containing poly(3-hexylthiophene). The aggregation states of macromole- cules are also different for these two film types. The conductivity of mixed LB films doped with sulfuric acid ranged from 0.05 to 0.1 S cm71; doping changed the LB film colour from red to blue. Polymerisation of thiophenes can occur in organised planar systems, resulting in the formation of anisotropic macromole- cules.74 In view of this, the properties of pure thiophene 24 and its mixtures with viologen were studied.75 COOH (CH2)14 S 24 It was shown by X-ray photoelectron spectroscopy with angular resolution 76 ± 78 that an LB film of compound 24 has a layered structure (bilayers of the Y type) with a distance of 4.6 nm between the thiophene sublayers.In mixed monolayers and in LB films containing thiophene 24, phase separation does not occur due to interactions between the positive charges of viologen and the deprotonated carboxyls of thiophene resulting in donor- acceptor diads. In addition to mixed monolayers and LB films of polythio- phenes with insulating molecules (mostly, fatty acids), of signifi- cant interest are thin planar systems consisting of mixtures of a conducting polythiophene with a conducting compound mani- festing a surfactant property in an aqueous medium.Rikukawa et al.79 used mixed monolayers of poly(3-hexylthiophene) with nickel (25) and iron (26) tetra-tert-butylphthalocyanines. X X N N N M N N N N N X 25, 26 X M=Ni (25), Fe (26); X=But. It is considered 80, 81 that, along with unique electrical proper- ties, phthalocyanines have an important advantage of higher stability of their monolayers in comparison with the stability of monolayers of fatty acids. The behaviour of pure phthalocyanine monolayers is essentially different: the limiting area of a phthalo- cyanine 26 monolayer is 75 and that of phthalocyanine 25 monolayer is only 34 A2 per molecule.In the former case, the molecular planes are oriented perpendicularly to the surface of water, and in the latter case the monolayer consists of aggregates whose thickness exceeds the size of one molecule. Incorporation of polythiophene expands slightly the monolayer of compound 25 and increases considerably the area of the monolayer of com- pound 26. Thus, in the latter case the compatibility of monolayer components was higher than in the former, and displacement of polythiophene molecules occurred with much greater difficulty. 893 The structural organisation of mixed monolayers and films on a solid substrate and the character of interactions between the components were determined more accurately by comparison of the optical spectra of two mixed LB films with the spectra of phthalocyanines in solution and with the spectra of mixed LB films of phthalocyanines with stearic acid.It was found that the sulfur atoms of the thiophene rings tend to coordinate with the iron atoms of phthalocyanine. This limits the possibility of formation of phthalocyanine dimers and trimers and increases the extent of molecular mixing. For mixed LB films of compound 25 with polythiophene (as with stearic acid), the interactions between the molecules are not so strong and aggregation of molecules of compound 25 is much stronger than that in mixed LB films containing compound 26. Interesting results of structural studies of mixed films carried out by small-angle X-ray scattering and by optical spectroscopy were reported.In mixed LB films of phthalocyanine 25 with polythiophene, as in the LB films of pure compounds 25 and 26, the majority of phthalocyanine molecules form one-dimensional linear structures in which the ring planes are oriented perpendicularly to the substrate surfaces. In mixed films of phthalocyanine 26 and polythiophene, the phthalocya- nine molecules exist mainly as monomers and have random organisation because of interaction with the thiophene units. The conjugated polymer chains in both mixed films are oriented parallel to the film surface, while the side chains have no preferential orientation. It should be noted that irreversibility of thermochromic properties was reported only for mixed LB films of poly(3-hexylthiophene) with phthalocyanine 26:79 heating from 25 to 120 8C changed the colour of the film from dark brown to green.The absorption spectrum of the film changed correspond- ingly. It should be noted that mixed films with the same compo- nent ratio cast from solution on a solid substrate did not display thermochromic properties. Doping of mixed LB films was carried out in iodine and SbCl5 vapours.79 As a result, electrical con- ductivity increased in comparison with the electrical conductivity of the LB films of pure phthalocyanines (1075 ± 1076 S cm71 ) by several orders and reached 1071± 1072 S cm71. Unlike LB films doped with iodine, the films doped with SbCl5 were stable in vacuo and in an atmosphere of nitrogen. According to optical spectro- scopic data, both components of the mixed film underwent doping.Figure 17 shows the dependences of the lateral conduc- tivity of mixed LB films on the content of polythiophene compo- nents. It is evident that the maximum conductivity is reached at 80 mol.% of the polymer in the mixture; the conductivity of the system with phthalocyanine 26 is by an order of magnitude higher than that for the systems with phthalocyanine 25. This effect is explained by coordination between the molecules of polythio- phene and iron phthalocyanine. Of considerable interest are the alternating Langmuir ± Blodgett heterofilms consisting of materials with different con- ductivity band widths.These films are convenient models of quantum wells in organic systems.82, 83 In addition, these alter- nating systems can be used for studies of energy transfer in log sk (S cm71) 71 1 72 2 73 74 75 c (mol.%) 60 20 Figure 17. Dependence of the lateral conductivity of SbCl5-doped Lang- muir ± Blodgett films consisting of mixed monolayers of poly(3-hexylth- iophene) and iron (1) and nickel (2) phthalocyanines on the polymer content (c).79894 400 500 Figure 18. Absorption spectra of an alternating Langmuir ¡À Blodgett film obtained from a mixture of polythiophene 17 and alkadiynoic acid at different irradiation times (light with the wavelength of 254 nm).32 Time /min: (1) 0.25; (2) 2; (3) 12; (4) 24; (5) 56. polymeric materials.For this purpose, Bolognesi et al.32 chose alternating mixed LB films consisting of poly(3-methoxydecylth- iophene) 17 and cadmium salt of the alkadiynoic acid H(CH2)12C:CC:C(CH2)8COOH. UV irradiation of the LB films formed from a polymer: monomer mixture at a wave length of 254 nm gave a film that contained alternating layers of two conjugated polymers. Figure 18 shows the absorption spectrum of a Langmuir ¡À Blodgett heterofilm for various irradiation times determining the degree of polymerisation of diacetylenic acid. The band at 640 nmcorresponds to exciton absorption of the extended polydiacetylenic chain. The band at 540 nm overlapping with the neighbouring bands originates from absorption of short polydia- cetylenic chains.84 It should be noted that irradiation of the Langmuir ¡À Blodgett heterofilm does not shift the polythiophene absorption band at 505 nm nor decreases its intensity.On the other hand, laser irradiation of the original heterofilm near the polythiophene absorption (488 nm; light with this wavelength does not cause polymerisation of pure diacetylenic acid or its salt) results not only in polymerisation of the amphiphilic diacetylene but also in a shift of the maximum of the polythiophene absorp- tion band to higher energies and in a decrease in its intensity. This result suggests photooxidation of the polythiophene component of the hetero structure and its subsequent sensitising effect on the photopolymerisation of the diacetylene. The identity of the absorption spectra of Langmuir ¡À Blodgett homo- and heterofilms containing polydiacetylene indicates the one-dimensional exciton nature in the structures of both films, i.e.the absence of inter- actions between the polydiacetylene exciton and the polythio- phene macromolecules.32 Mixed monolayers of poly(3-hexylthiophene) and pentacosa- 10,12-diynoic acid were studied by Tsumura et al.85 Stable monolayers were obtained for various component ratios; after transferring them onto a solid substrate by the vertical method they were polymerised by exposure to light. It was found that polythiophene does not affect the polymerisation of the diacety- lenic acid and the LB film has a layered structure with oriented molecules of both components; the area per molecule in the mixed monolayer and the structure spacing in the LB film indicate the formation of bilayer films.85 In this LB film variant, the bilayer consists of identical mixed monolayers rather than of alternating monolayers of different polymers (as in the previous case).A detailed study of the effect of thickness, temperature and electric field on the conductivity mechanism in LB films consisting of mixed (5 : 1) monolayers of poly(3-hexylthiophene) and 3-decanoylpyrrole was carried out.62 The dependence of the lateral conductivity and charge mobility in external electric field for an non-doped LB films, an LB film doped with NOPF6 and a Absorption 54132 600 l /nm V V Arslanov a b s /S cm71 1072 1074 1073 1074 1075 D1 D2 1075 D3 1076 1076 0 10 20 N 0 10 20 N Figure 19.Dependences of specific electric conductivity (a) and charge mobility in electric field (b) for LB films consisting of mixed layers (5 : 1) of poly(3-hexylthiophene) and 3-octadecanoylpyrrole on the number of layers.62 Film: (1) non-doped; (2) doped with NOPF6; (3) dedoped. dedoped LB film (a sample which was allowed to relax after doping to a certain high conductivity) on the number of layers is presented in Fig. 19. It is evident that doping increases the conductivity by 5 orders of magnitude, while the mobility is increased by 4 orders. In view of this, a conclusion on the primary role of charge mobility in the LB film conductivity was drawn.62 The thickness of LB films has but an insignificant effect on these parameters, except for the region between the 3- and 11-layer LB films.The decrease in conductivity upon keeping the samples in air (preparation of the dedoped system) is due to the loss of activity of the PF¡¦6 anions because of their reaction with moisture. Obviously, this affects the thinner film most strongly (see Fig. 19). It was shown in the same study that the dependence of conductiv- ity (and charge mobility) on temperature for the non-doped film has the form log s*T71/4. This means that the hopping mecha- nism is the main mechanism of conductivity.86 Based on this conclusion and on the experimental dependences of conductivity on external electric field, the mean hopping length and energy were calculated as 230 Aand 0.41 eV, respectively, using the Mott equation.87 In the case of doped and dedoped samples, the temperature dependence of conductivity had the form log s*T71/4 (Fig.20). Extrapolation of the straight line obtained for the doped LB films to room temperature gave a conductivity of 0.1 S cm71. After dedoping, the plot slope and hence the activation energy changed, but the dependences still remained linear. Similar relationships are observed in the case of metal and metal-ceramic systems consist- ing of grains;88, 89 in view of this, some possible mechanisms describing the conductivity in these systems were considered.62 Based on comparative analysis, the conclusion was drawn that the tunnel mechanism with charge transfer operates in poly(3-alkyl- thiophene) films doped with NOPF6 (see Ref. 90).It was estab- lished on the basis of these concepts that dedoping results in an increase in the size of conducting grains (domains) in LB films from 30 to 50 A. The conducting domains are formed by the main conjugated polymer chains separated by the side alkyl groups. Interesting data on changes in the conductivity mechanism were obtained for 11-layer LB film samples exposed to external electric fields with field strengths up to 0.5 MV cm71. Transition to the Fowler ¡À Nordheim tunnelling mechanism was observed in this case.91, 92 Analysis of the properties of monolayers and LB films of individual regioregular polythiophenes synthesised in recent years showed the advantages of these structurally homogeneous poly- mers assembled in organised planar ensembles over the systems Mobility /S2 V71 s71Polymer monolayers and Langmuir ± Blodgett films.Polythiophenes log s (S cm71) 1 74 76 78 2 3 710 12 102 T71/2 /K71/2 8 Figure 20. Temperature dependence of specific electric conductivity of a 25-layer LB film consisting of mixed (5 : 1) monolayers of poly(3-hexylth- iophene) and 3-octadecanoylpyrrole. Film: (1) non-doped; (2) moderately doped with NOPF6; (3) dedoped. containing poly(3-R-thiophenes) with random orientation of substituents. The effect of the structural homogeneity of the polymer chain on the properties of monolayers and LB films also manifests itself in the case of mixed monolayers.93 This is evident if one compares the compression isotherms of mixed monolayers of stearic acid with regioregular poly(3-hexylthiophene) containing up to 100% of `head-to-tail' diads presented in Fig.21 with the compression isotherms of a mixture of stearic acid with poly(3-hexylthiophene) having a random distribution of the chain substituents (see Fig. 14). Both monolayers consist of two phases, but the larger area and the smoother transitions between the phase states in the range of 20 ± 30 mN m71 for the former monolayer (see Fig. 21) suggest the better compatibility of stearic acid with the regular polythiophene. A similar trend was observed 94 in a study of the effect of the chain length of the alkyl substituent in polythiophene on the behaviour of monolayers and LB films obtained from its mixture with stearic acid. Smoothing of compression isotherms in the phase change region observed for the mixture of fatty acid with poly(3-hexylthiophene) is explained by better compatibility in this system than in the system with shorter alkyl substituents in polythiophene.The mixed monolayers of stearic acid with regioregular poly(3-hexylthiophene) were transferred by the horizontal method at a surface pressure of 20 mN m71 to give the Y type LB films (20 layers) on a hydrophobised glass plate surface.93 It was shown by X-ray diffraction for LB films containing 2 : 1 polythiophene and stearic acid in monolayers that the film has a distinct layered structure with an interlayer distance of 17 A.The same structure spacing found previously for poly(3-hexylthio- phene) films cast from solution 42, 95 corresponds to the distance between the polymer chains stacked parallel to the substrate p /mN m71 60 40 20 3 1 2 0 80 40 A /A2 (per unit) Figure 21. Compression isotherms of monolayers of regioregular poly(3- hexylthiophene) and stearic acid with component molar ratios of 0 : 1 (1); 2 : 1 (2); 5 : 1 (3).93 895 surface. However, the interlayer distance determined by X-ray diffraction for LB films composed of mixed (1 : 2) monolayers of stearic acid and irregular poly(3-hexylthiophene) corresponded to a bilayer of cadmium stearate, i.e., 50 A.96 Figure 16 illustrates the structure of such an LB film.No similar scheme for LB films consisting of stearic acid and regular poly(3-hexylthiophene) has been reported in the literature; however, it can be expected that the stearic acid molecules in this system either form a tilted phase or are distributed along the polythiophene chains. Rikukawa et al.93 compared the UV and visible absorption spectra for 2 : 1 mixtures of regioregular and irregular poly(3- hexylthiophene) and stearic acid. According to the spectra pre- sented in Fig. 22, the p ± p*-transition energy of the mixture with the regular polymer is lower than that in the system with irregular polythiophene. This implies that the molecules of the former polythiophene in LB films have a more stretched structure and have a larger conjugation length than the molecule of the latter polythiophene.The lateral conductivity of LB films of mixed (5 : 1) monolayers of regioregular poly(3-hexylthiophene) with stearic acid doped with NOPF6 was 50 ± 100 S cm71, which is almost 1000 times higher than the lateral conductivity of a similar system containing irregular poly(3-hexylthiophene). It should be noted that the conductivity of LB films obtained from a mixture of regioregular poly(3-hexylthiophene) and stearic acid is several tens times higher than the conductivity of LB films obtained from the pure regioregular compound.45 Absorption 1 0.2 2 0.10 300 500 l /nm Figure 22. Absorption spectra of a 20-layer LB film consisting of mixed (2 : 1) monolayers of regioregular (1) and irregular (2) poly(3-hexylthio- phene) and stearic acid.93 Thus, the regioregularity of polythiophene favours an increase in compatibility of the polymer with stearic acid and results in a sharp increase in electric conductivity of thin films consisting of these components.These features of polythiophene can intensify the development of molecular devices. In addition, comparison of systems containing polythiophenes with different degrees of regularity enables deeper insight into the relationship between the structure and properties of conducting polymers. V. Prospects for the application of polythiophene Langmuir ± Blodgett films in molecular devices It has been noted in many publications that LB films of poly- thiophenes and their mixtures with other substances can be used for practical purposes, nevertheless only a few works describing molecular devices based on these compounds are available.The first study with mixed monolayers of oligothiophene 1 (S5) with eicosanoic acid in a Langmuir ± Blodgett heterofilm (alternating mixed monolayers and pure eicosanoic acid mono- layers) was carried out as early as in 1974 58 (see footnote {). A cell was designed for photocurrent measurements, in which the LB film contained a mixed monolayer with oligothiophene S5 located { This was the first publication of Kuhn and his colleagues who laid the foundation of rapid development of studies in molecular design combined with the Langmuir ± Blodgett technique.close to one of the electrodes (aluminium or mercury) or at a certain distance set by the number of spacer monolayers of pure eicosanoic acid. By measuring the dependences of photocurrent on the applied voltage, temperature, and the arrangement of the conducting and insulating layers, the photophysical parameters of the device and the mechanism of generation of thermo- and photocurrent were determined.The authors suggested the main directions in the design of photoelectric devices. However, to date, scientific literature contains no information on the development of these kinds of devices based on polythiophene LB films. Nevertheless, metal ± insulator ± semiconductor transistors 97 and light-emitting devices 39, 40, 55, 56, 98 suitable for inexpensive flat colour displays and adjustable lasers from oligo- or polythiophene films deposited from solution or spray-deposited on electrodes are being designed.Adsorption /nmol 896 Ng et al.99 used LB films of mixed monolayers of poly [3-(6- hydroxyhexyl)thiophene] (27) or poly(3-octylthio-2,20-bithio- phene) (28) and octadecanol as a sensor for heavy metal ions. A quartz crystal microbalance was used as the recording device. The high quality of the LB films obtained (up to 40 monolayers) was confirmed by the facts that the change in frequency of the quartz resonator depended linearly on the number of LB film layers and that the transfer ratio of monolayers was equal to unity. Figure 23 shows the dependence of adsorption of metal ions on a 10-layer LB film from mixed (3 : 1) polythiophene 27 : octa- decanol monolayers deposited on the quartz resonator on the time of LB film exposure to 20 ppm solutions of ions at neutral pH and 25 8C.Similar results were obtained for mixed (1 : 3) polythio- phene 28 : octadecanol LB films. One can see in the figure that the LB films of pure octadecanol does not adsorb metal ions under the same conditions. Complexing of metal ions with the polythio- phene sulfur atoms is the driving force of adsorption. The selectivity in the series of ions Cu2+>Ni2+>Co2+> Zn2+>Fe2+ correlates with the ionisation potentials and ionic sizes in the Irving ± Williams series. The higher adsorption of Ag+ and Hg2+ in comparison with the other ions is explained by the high affinity of these ions to the sulfur atom.An important feature of the sensor is its reversibility: complete regeneration of the sensor upon exposure of the resonator with LB films in 1 mM ethylenediaminetetraacetic acid solution occurred in 5 min. An increase in the number of layers in the LB film increased the Figure 23. Dependence of the adsorption of metal ions on a 10-layer LB film consisting of mixed (3 : 1) polythiophene 27 : octadecanol monolayers on the time of exposure to solutions containing ions: Ag+ (1); Cu2+ (2); Hg2+ (3); Ni2+ (4); Co2+ (5); Zn2+ (6); Fe2+ (7); Cu2++octadecanol LB film (8). The concentrations of the ions are 20 ppm (see Ref. 99). SC8H17 C6H12OH S S n n S 28 27 1.2 1234 0.8 576 0.4 8 0 6 3 t /min V V Arslanov absolute value of the change in the quartz resonator frequency Df (the mass of the absorbed ions increases). This result indicates that the diffusion of ions in LB films does occur. However, for example, the 40-layer LB film was unstable and delaminated from the substrate after 7 ± 10 measurements, whereas the 20- layer LB film withstood 50 measurements.The concentration dependences of Df for Ag+ and Hg2+ ions are shown in Fig. 24. It is evident that a linear dependence is preserved up to the 100 ppm concentration. At higher concen- trations, the frequency is unchanged because of saturation of the LB film with ions. b a Df /Hz Df /Hz 1200 1 1 1200 800 2 800 2 400 400 0 0 90 120 90 120 60 30 60 30 [Hg2+], [Ag+] /ppm [Hg2+], [Ag+] /ppm Figure 24.Concentration dependences of changes in the frequency of a quartz resonator with a 20-layer mixed LB film after exposure to a solution containing Hg2+ (1) and Ag+ (2). Film: (a) polythiophene 27 : octadecanol (3 : 1); (b) polythiophene 28 : octadecanol (1 : 3).99 The time dependence (Fig. 25) shows the lower detection limit of Hg2+ ions by the 20-layer LB film. Adsorption where Df exceeds the `noise' (30 Hz) by a factor of three is chosen as the criterion. Each addition of Hg2+ ions with a concentration of 0.1 ppm to the solution where the resonator with the LB film is located produces a sharp change in the resonator frequency.The three changes in frequency shown in Fig. 25 corresponded to changes in the Hg2+ mass by 45, 38 and 36 ng. It was noted 99 that the sensor sensitivity can be increased at least twofold. The sensor selectivity was tested for the 5 ppm concentration of the Hg2+ ions. It was found that such cations as Na+, K+, Ca2+, Pb2+, Cr2+ , Co2+ and Ni2+ (but not Ag+ and Cu2+) interfere with the determination of Hg2+ if their concentrations in solution were twice as high as that of mercury. The same concentrations of anions such asCO2¡ 3 , ClO¡4 , SO24 ¡, PO34 ¡ and Cl7do not affect the Df /Hz 0 760 71200 16 32 t /min Figure 25. Dependence of the frequency of a quartz resonator with a 20- layer LB film consisting of mixed (3 : 1) polythiophene 27 : octadecanol monolayers on time of exposure in a solution containing Hg2+.The arrows indicate the instants of addition of Hg2+ ions with a concentration of 0.1 ppm (see Ref. 99).Polymer monolayers and Langmuir ± Blodgett films. Polythiophenes determination of Hg2+, but anions such as Br7, I7, CrO¡4 and CNS7 form stable complexes with mercury. Thus, although the sensitivity and selectivity for many cations and anions are high, the problem of selectivity remains unsolved for a number of anions. Mixed (1 : 1) binary monolayers of polythiophenes 11, 19 and 20 and octadecylamine were studied with the purpose of obtaining a sensor for NO2.24 The compression isotherms for the mixtures have the same character as those of the pure compounds but with lower compressibility and a considerably smaller difference in the monolayer areas; the p ±A plot for polythiophene 11 is shifted to larger areas: the limiting monolayer area per thiophene unit increased from 9 to 20 A2.Figure 26 shows the dependences of quartz resonator frequency on the exposure time of a 14-layer LB film composed of mixed binary monolayers of polythiophenes 11, 19 and 20 and octadecylamine in NO2 (550 ppm). The differences in sensitivity of films containing the three polythiophenes cannot be caused by differences in the initial mass of the deposited substance as the monolayer areas corresponding to the surface pressure used for their transfer (30 mN m71) differed insignif- icantly. Taking into account the identity of the head groups of these compounds, one can state that the observed differences in sensitivity are caused by the side chains.Unfortunately, the sensor reversibility was low: treatment of the films in vacuo at 40 8C for one hour changed the frequency by no more than 10% (see Fig. 26).24 NO2 can be determined using a reported principle 100 for a gas sensor (for NO2, H2S, NH3 and others) in which a thin film (not LB film) of electropolymerised polythiophene is used as the working element. Changes in electrical resistance of this sensor upon exposure to NO2 (an electron acceptor) occur due to chemical doping, whereas in the case of electron-donating gases (for example, H2S), the resistance changes due to a decrease in the number of charge carriers in the polythiophene film.Df /Hz II I 0 12 740 780 3 7120 0 20 t /min 10 Figure 26. Dependence of frequency change on the time of exposure of a 14-layer LB film consisting of mixed (1 : 1) polythiophene : octadecylamine monolayers in NO2 (550 ppm).24 Polythiophene: (1) 20; (2) 19; (3) 11. The vertical lines indicate the times of turning on (I) and off (II) the NO2 generator. VI. Conclusion The above analysis of the properties of conducting polythiophenes in monolayers and LB films shows that these objects are of significant interest for theoretical study and for practical applica- tion. The solubility of polythiophenes in ordinary solvents and formation of rather stable monolayers in one-component systems and definitely stable layers in multicomponent systems (in mix- tures with inert and reactive amphiphilic compounds) and their resistance against moisture and oxygen in the initial and doped states make these polymers superior to the known conjugated polymers (for example, polydiacetylene, polypyrrole, etc.). Incor- poration of substituents at positions 3 or 3 and 4 of the thiophene monomer, control of the chain length and polarity of the sub- stituents and the regularity of their arrangement along the 897 polymer chain make it possible to control the structural, phys- icochemical and electrophysical characteristics of polythiophenes in organised planar systems.This primarily corresponds to the control of the effective conjugation length of the polythiophene main chain and to the synthesis of uniformly oriented chains separated by distances which provide charge transfer between the adjacent macromolecules. In certain cases, creation of mixed monolayers of polythiophenes with surfactants (fatty acids, alco- hols, amines, etc.) increases the stability of monolayers, ensures their transfer onto solid substrates and formation of high-quality multilayer LB films.In these cases, the conducting characteristics of polythiophene films are not inferior to those of similar systems containing pure polythiophenes. The use of dopants allows one to reach the electrical conductivity of substituted polythiophenes in the range from 500 to 1000 S cm71. The practically important properties of polythiophenes also include the photo- and electro- chromic properties as well as the ability to form complexes with metals and gases.These properties enable the use of polythio- phenes for creating thin-film light-emitting devices, sensors, etc. The Langmuir ± Blodgett method has good prospects in this respect as it provides organised ultra-thin films with larger conjugation lengths than those of the films formed from solutions, and hence higher functional capabilities of molecular devices. Based on the analysis carried out above, we can suggest the directions and priorities in the study of these systems. First, it is necessary to synthesise novel polythiophenes with functional substituents which are regularly distributed along the chain but are not included in each thiophene unit.These substituents can be represented, for example, by azo compounds which can undergo photoisomerisation disturbing the macromolecule conformation, which, in turn, should switch the conjugation length and hence change the conductivity and the chromatic characteristics of the systems. Second, more detailed studies of the structure of mono- layers on the surface of water with the use of such methods as optical spectroscopy, Brewster angle microscopy and synchrotron X-ray reflectometery should be carried out. Third, the problem of monolayer stability requires a detailed study. In this connection, studies of the relaxation properties of two-dimensional systems would be rather useful.Fourth, it is necessary to study more intensely the structures of LB films comprising both individual and mixed monolayers. In the latter case, it would be desirable to obtain a consistent model of the distribution of film components in the layers. And, fifth, much greater attention should be given to the development of molecular devices based on LB films consist- ing of polythiophenes. This review was financially supported by the Russian Foundation for Basic Research (Project No. 99-03-32400). References 1. V V Arslanov Usp. Khim. 60 1155 (1991) [Russ. Chem. Rev. 60 584 (1991)] 2. V V Arslanov Usp. Khim. 63 3 (1994) [Russ. Chem. Rev. 63 1 (1994)] 3. F Garnier La Recherche 193 1306 (1987) 4. A O Patil, A J Heeger, F Wudl Chem. Rev. 88 183 (1988) 5.H Nakahara, J Nakayama, M Hoshino, K Fukuda Thin Solid Films 160 87 (1988) 6. S Isz,HPerez, S Palacin,ARuaudel-Teixier Synth.Met. 71 2017 (1995) 7. A Soukopp, C Seidel, R Li,M Bissler, M Sokolowski, E Umbach 8. P Kohli, K K Taylor, J J Harris, G J Blanchard J. Am. Chem. Soc. Thin Solid Films 284/285 343 (1996) 120 11962 (1998) 9. 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ISSN:0036-021X    

出版商:RSC    

年代:2000

数据来源: RSC

摘要:

Russian Chemical Reviews 69 (10) 899 ± 909 (2000) Application of radical cage reactions in studies of microstructures of polymers and composites { E Ya Davydov, A P Vorotnikov, V P Pustoshnyi, G B Pariiskii Contents I. Introduction II. The choice of model reactions for the investigation of structural-physical properties of polymers III. Assessment of the structural-physical heterogeneity of polymers from the kinetics of thermal decay of carbenes IV. Estimation of structural characteristics of filled polymers from the kinetics of cage reactions V. Conclusion Abstract. for tool a as reactions cage radical of potentialities The The potentialities of radical cage reactions as a tool for investigating are polymers of structures chemical the investigating the chemical structures of polymers are considered.considered. The kinetic regularities of triplet carbene reactions in the matrix- The kinetic regularities of triplet carbene reactions in the matrix- isolated reveal to analysed are clusters in and state isolated state and in clusters are analysed to reveal structural structural features of possibility The surroundings. cage the of features of the cage surroundings. The possibility of determina- determina- tion from polymers filled of characteristics structural of tion of structural characteristics of filled polymers from kinetic kinetic data references 103 includes bibliography The discussed. is data is discussed. The bibliography includes 103 references. I. Introduction The first ideas about the relationship between chemical kinetics and molecular organisation of media were formulated by acad- emician N M Emanuel back in the 1960s.In particular, he initiated studies on the influence of the physical structure of polymeric systems on the kinetics and mechanisms of reactions which occur in them. These studies were aimed, on the one hand, at creating a theory of processes of polymer ageing 1 and, on the other hand, at the development of scientifically substantiated methods for their stabilisation and prediction of their lifetime under different operation conditions.2 Numerous studies carried out in this direction have shown that kinetic regularities of the reactions occurring in solid polymers are functions of their physical-structural properties. In particular, the kinetics of such reactions is associated with the kinetics of molecular motions of macromolecules.Simple kinetic laws were found to be as a rule inapplicable for the description of chemical processes occurring in solid polymers at temperatures below { the glass transition.3±5 The formal kinetic regularities of a chemical process in a condensed medium depend on the rapidity of changes in local medium properties, which influence the reactivity of the species involved, i.e., on the relationship of three temporal parameters, viz., the characteristic time of a chemical transformation (tch), the lifetime (tl) of the process-controlling active centre and the correlation time (tc), which is sufficient for all the reactive species present in a given medium to pass through all possible states where they have different reactivities.6 If the condensed medium is represented by solid polymers, the correlation time is much longer E Ya Davydov, A P Vorotnikov, V P Pustoshnyi, G B Pariiskii NMEmanuel Institute of Biochemical Physics, Russian Academy of Sciences, ul.Kosygina 4, 117334 Moscow, Russian Federation. Fax (7-095) 137 41 01. Tel. (7-095) 939 74 46 (E Ya Davydov), (7-095) 939 71 03 (G B Pariiskii) Received 16 May 2000 Uspekhi Khimii 69 (10) 981 ± 992 (2000); translated by V D Gorokhov #2000 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2000v069n10ABEH000604 899 899 901 904 908 than the reaction time (tc 44 tch), i.e., dispersion of species as regards their reactivities is observed (such species are kinetically non-equivalent).6 The observed process rate depends on the distribution pattern of the reacting species and its variation with time.In this context, the kinetics of even an elementary reaction in solid polymers is determined by a set of rate constants which characterise the reactivity of a species in a particular surrounding. The effect of local structural-physical properties of solid polymers on the kinetics of reactions occurring in them is the reason for the structural `memory' of the reacting species on their initial distri- bution as a function of reactivity. Therefore, the kinetics of chemical reactions in polymers depend on the sample prehistory.7 Despite the complexity of the analysis of experimental results, kinetic studies of the reactions occurring in solid polymers are of a tremendous theoretical and applied interest.These offer a unique opportunity for elucidating peculiarities of the microstructures of these subjects, since the reaction kinetics is dependent on the initial polymer state and on changes in the distribution of reacting species over local structural parameters. The present review considers the validity of the approach to the estimation of structural-physical and molecular-dynamic characteristics of solid polymers based on the kinetic analysis of a series of pseudoelementary radical cage reactions, in particular, low-tem- perature decay of triplet carbenes, thermal dissociation of dimers of stable radicals as well as dissociation of macroradicals induced by nitrogen dioxide and salts of transition metals.II. The choice of model reactions for the investigation of structural-physical properties of polymers Certain difficulties are related to the choice of versatile model cage reactions, the kinetics of which may be purposefully used for structural studies of polymers. Apparently, this objective can be hardly achieved using complex free-radical processes, such as thermo- and photooxidation including migration of free valence, decay of macroradicals and reactions of oxidation products. The point is that it is difficult to isolate the stages sensitive to definite structural-physical properties of the matrix in these multistage { Dedicated to the blessed memory of academician N M Emanuel (1915 ± 1984).{ As the temperature increases (above Tg) the intensity of molecular motions is enhanced and the `solid-phase' formal kinetics approaches the conventional liquid-phase kinetics.900 processes on the basis of measured kinetic parameters. Even the simpler reaction of thermal decay of radicals cannot always be used as a model. Thus, the death of macroradicals generated by low-temperature photolysis, radiolysis or mechanical impacts on polymers cannot be regarded as the versatile model suitable for the estimation of structural-physical properties, because in this case the mechanism of macroradical decay is rather complicated and may be accompanied by migration of the free valence over considerable distances as the result of diffusion and chemical relay-race.Numerous studies established a characteristic feature of this reaction in solid polymers, viz., its kinetic arrest at constant temperature or its `stepwise' character.6, 8 The nature of this phenomenon was rationalised from different stands taking into account the structural-physical heterogeneity of the matrix,9, 10 differences in the interradical distances,11 increased concentration of radicals in structural defects,12 etc. However, the kinetic regularities observed in this process can be regarded only as an indirect indication to the dependence of the reactivity-based distribution of radicals on the structural-physical properties of polymers.Direct analysis of the relationship between kinetic parameters and structural features of the nearest cage environments is confined to those reactions which are not complicated by migra- tion of active centres over considerable distances. These types of processes include, for example, thermal decay of radical pairs (RP) in a cage. In most cases, RP are formed in low-temperature photolysis and g-radiolysis of frozen solutions or polymers.13 The radical pairs arising in radiolysis are sufficiently stable at the temperature of their generation, and processes of their recombi- nation, disproportionation and their exit from the cage can be investigated in the thermal annealing of samples. Kinetic studies of the RP formation during photodisintegra- tion of a number of low-molecular-weight additives can be used, e.g., for the establishment of the mechanism of radical initiation in the solid phase.The radical pairs offer a unique opportunity for studying structural-physical properties of the medium. The inter- radical distances in such pairs can be measured directly by EPR spectroscopy. Thus it was established from the EPR spectra 14 that the mean distance between radicals in the RP generated in the photolysis of a solution of 3,6-di-tert-butyl-o-benzoquinone and diphenylamine in chlorobenzene changes from 0.62 to 0.64 nm (the accuracy of measurements was 0.005 nm) in the course of RP accumulation, which may be indicative of different geometry of molecular complexes from which these RP were formed.15 The radical pairs formed under the action of light in monocrystalline matrices in the course of redox reactions involving molecular complexes of quinonediazides with catechols 16, 17 also proved to be kinetically non-equivalent, as followed from the scatter of the rates of their decay during annealing.The solid-phase transfer of hydrogen atoms accompanied by the formation of RP stable at room temperature has been detected in quinone ¡À catechol poly- crystalline mixtures under the action of elastic wave pulses.18 ¡À 21 It was shown that in these systems the polymeric matrix decreases the threshold of the elastic wave strength required for the formation of paramagnetic products and enhances the RP stabil- ity.It was established 22 that the dispersion with respect to the interradical distances in RP is proportional to the difference between the maximum and minimum activation energies of their decay. The kinetic non-equivalence of RP may be determined not only by the differences in the interradical distances, but also by the structural distinctions of the nearest surroundings.23, 24 Thus the kinetics of thermal dissociation of RP in oriented polyethylene is described by several rate constants, though the mean distance between the radicals (rm=0.53 nm) remains constant throughout the process. The formation of four types of RP was established in the low-temperature photolysis of single crystals of tetraphenyl- hydrazine.25 Their formation results from disintegration of mol- ecules differently oriented in a magnetic field.All the four types E Ya Davydov, A P Vorotnikov, V P Pustoshnyi, G B Pariiskii are characterised by identical magnetic properties but different rate constants of their decay. The kinetics of transformation of the RP generated upon g-irradiation of dimethylglyoxime and dimethylglyoxime-d2 had been used for the determination of the constants of their decay, the kinetic isotope effect (KIE), the direction of the free valence migration and the distance of the hydrogen atom transfer.26 ¡À 28 Characteristic features of the free valence migration are its stereo- specificity, anisotropy of the valence migration directions and sensitivity to weak molecular interactions.The process of RP recombination does not require relocations of radicals to considerable distances but is rather sensitive to local molecular motions.29 This allows the use of RP reactions for studies of structural transitions and characteristic frequencies of small-scale molecular mobility in a polymeric matrix. Thus in the temperature range from 160 to 260 K, the curves of RP defrosting in polyethylene revealed two structural transitions in the crystal- line regions of the matrix. Structural and phase transitions can also be registered from the mean distance between radicals in RP over a wide temperature range.13 Kinetic measurements of the exit of radicals from the cage allow determination of the frequency of translational motions.23, 30 In principle, the abstraction of a hydrogen atom from the molecules of the medium by active radicals with the formation of a radical matrix can be used as a model of cage reactions.25, 30 Taking into account that the distance of hydrogen atom transfer is *0.3 nm, one can presume that the kinetic features of this reaction should reflect directly the structural effect of the nearest cage surroundings.However, examples of kinetic studies of the RP reactions and hydrogen abstraction by radicals are mainly confined to the investigations of frozen solutions of low-molec- ular-weight compounds as well as of mono- and polycrystalline substances (see, e.g., Refs 31 ¡À 33). The effect of structural peculiarities of a polymeric matrix at the level of reaction cages on the kinetics of the reactions occurring in the matrix can be studied using the approach proposed by Denisov et al.34 ¡À 37 where the reaction of spatially hindered phenoxyl radicals with the hydroperoxide groups of macromole- cules has been used for the development of a model of cage reactions PhO.+ROOH (1) PhOH+RO2., products. (2) PhO.+RO2. Reaction (1) is the rate-limiting one. This reaction is slow and is not limited by the diffusion of reagents in the case of regular distribution of radicals in the matrix.The phenoxyl radical and the hydroperoxide group enter the reaction cage and react only when they possess sufficient energy E and have a favourable mutual configurations. However, the walls of the cage prevent the achievement of such configuration, and the more `rigid' the walls the higher the activation energy of the elementary reaction (1).To study the kinetics of this reaction, a stable nitroxyl radical as a spin probe was added along with a phenoxyl radical to the polymer. In this case, the rate constant of the reaction (1) correlated linearly with the frequency of the spin probe rotation (nr). Thus the activation energies of the nitroxyl radical rotation in polyethylene, polypropylene and chlorobenzene measured using the EPR spec- tra at 290 ¡À 370 K were 28, 32 and 12 kJ mol71, respectively. The difference in the activation energies of the nitroxyl radical rotation in polymers and chlorobenzene virtually coincided with those for the reaction (1) in the polymer and chlorobenzene.(3) k �� 6n n KABPrPexp The results obtained permitted expression of the rate constant of the cage bimolecular reaction (1) in the form ¡¦Er �¢ E , RT where n and n are the frequency of collisions and the number of `neighbours' in the cage, KAB is the equilibrium constant of theApplication of radical cage reactions in studies of microstructures of polymers and composites A+B pair formation in the cage, Pr exp (7Er/RT) is the proba- bility of rotation of the reagents in the cage resulting in the activated complex configuration and P is the steric factor of the reaction in the liquid phase.36 Thus, the rotation frequency of the reagents in the cage and the frequency-related reaction rate constant characterise the cage surrounding of reagents in poly- mers.Of special interest for studies of the microstructures of polymeric objects are versatile cage reactions which can be initiated in various matrices over a wide temperature range. In this connection, many possibilities are provided, e.g., by radical processes such as low-temperature decay of triplet carbenes, macroradical dissociation reactions induced by nitrogen dioxide and photoreduction of transition metal salts and thermal dissoci- ation of dimers of stable radicals. III. Assessment of the structural-physical heterogeneity of polymers from the kinetics of thermal decay of carbenes Carbenes are most often generated in thermal, thermocatalytic and photochemical degradation of diazo compounds.38 The reactivities of the carbenes formed depend on their spin states.39, 40 Insertion into ordinary chemical bonds in a one-step mechanism and also stereospecific addition to double bonds with formation of cyclopropanes are typical reactions of carbenes in a singlet state.The triplet carbenes are also characterised by insertion reactions but these are multistep reactions. Let us consider the reaction of low-temperature decay of triplet carbenes in solid polymeric matrices. 3C C CHC CHC +HC C (4) In the first step, the transfer of hydrogen atom } from the surrounding macromolecules to a matrix-isolated triplet carbene results in the formation of a short-lived RP in the triplet state which then undergoes triplet ¡¾ singlet conversion. The RP in the singlet state resulting from this conversion can recombine with formation of the final product of carbene insertion into the C7H bond of the macromolecule.The dynamics ofHatom transfer and RP recombination in polymers depends on the polymer structure. The frequency of the triplet ¡¾ singlet transition in RP is *108 s71 (Ref. 41). It is known that the frequency of small- scale molecular motions associated with rotational oscillations of the side groups of macromolecules is of the same order.42 Thus, the spin evolution of RP generated in reaction (4) is within the same time scale as the molecular dynamics, which determines dislocations and rotations of the side groups of macromolecules in the cage.Numerous studies have shown that if the ground state of carbenes is triplet, then the efficiency of carbene transformations by the `abstraction ¡¾ recombination' mechanism (4) in the tran- sition from the liquid to the solid phase with a decrease in temperature is drastically increased.38, 43, 44 The effective con- stants of the decay rate of triplet carbenes determined experimen- tally at cryogenic temperatures are very small. For example, they amount to 1073 ¡¾1074 s71 for different frozen organic matrices (Ref. 45). One should note the principal distinction of the short-lived RP, appearing as active intermediates in the process of triplet carbene decay from the RP which can be registered owing to the characteristic EPR spectra in frozen solutions of low-molecular- } The transfer of anHatom requires the approaching of carbene andC7H bond of the macromolecule to a distance of 0.3 ¡¾ 0.4 nm and it is these distances that are realised in the cage.901 weight compounds and polymers.13 The former type includes the short-lived species whose lifetime in the cage is comparable with the time of their spin conversion (*1078 s71). They cannot be recorded by the usual EPR method. However, the existence of such species is confirmed by the influence of the magnetic field on the decay rate of the triplet di-tert-butylcyclohexadienone carbene (BCHC) (at 77 K) in hexaethyldisiloxane, ethyl acetate, po- ly(methyl methacrylate) (PMMA) and cellulose triacetate (CTA).46, 47O But But BCHC The high decay rate of the intermediate RP formed in this process is determined by a small interradical distance (*0.3 nm) and retardation of rehybridisation of chemical bonds 48 in the radicals of the ion pair due to difficulties in structural relaxation.As a result of relaxation processes, such pairs can be transformed into RP of the second type (r>0.3 nm) which are comparable with free radicals in their thermal stabilities. 1. The effect of spatial orientation factors on the decay kinetics of carbenes in polymers A characteristic feature of cage reaction (4) of low-temperature decay of carbenes in solid polymers is the kinetic non-equivalence of species.49 The time dependence of carbene concentration (N) is described by the following expression: (5) NOtU a N0 rOkU expO¢§ktUdk, O kmax kmin where r(k) is the distribution function of carbenes over decay rate constants.It is natural to presume that the reactivities of carbenes depend on local orientations of the carbene centre and the C7H bonds of the macromolecular matrix in the cage, with a definite decay rate constant corresponding to each orientation. In the system of reaction cages, the distribution r(k) changes with the gradual disappearance of carbenes which are best suited for the H atom transfer onto them. Because there is strong restraint of molecular motions in the solid matrix at low temperatures, the favourable orientations of carbenes are not recovered. This structural effect was illustrated by the dependence of the initial decay rate of diphenylcarbenes (DPC) in propanol at 77 K on the duration of their photoregeneration from diphenyldiazomethane (DDM).50, 51 The results reported by Bauschlicher et al.52 can be used for the theoretical substantiation of the decisive effect of carbene distribution over spatially oriented parameters on their kinetic non-equivalence in the insertion into the C7H bonds of the solid matrix. These authors calculated the ideal geometric configura- tion of the classical transition state, which corresponds to the minimum height of the potential barrier of the reaction of triplet methylene with a hydrogen molecule and methane. It was shown that hydrogen atom transfer over the trajectory lying in the methylene plane and passing along the bisecting line of the H7C7H angle corresponds to this configuration: HC H CH3 2CH3.H In this case the calculated height of the potential barrier was 103 kJ mol71. Upon changes in the geometric configuration of the transition state, i.e., the deviation of the trajectory of the migrating hydrogen atom from the ideal one by 20 ¡¾ 40 8, the height of the potential barrier increased to 110 kJ mol71. The decay kinetics of cyclohexadienone carbenes (CHC) in PMMA at 77 ¡¾ 160 K is well described by the equation 53902 (6) N(t)=N0 exp (7at1/2), (7) ¢§ a2 4k rOkU a a k3=2 exp . where a is the temperature-dependent parameter. This equation also describes the decay of other carbenes in the glassy and polycrystalline matrices of low-molecular-weight compo- unds.51, 54, 55 Kinetic equation (6) is derived from Eqn (5) where the distribution function r(k) is expressed as:56, 57 The distribution function (7) has a maximum at k=a2/6=k 0.The latter value is the most probable magnitude of the rate constant. Thus, the observed kinetic regularities of the CHC decay in PMMA are explained by the distribution of decay rate constants over the species. A different kinetic law was established for the decay of BCHC in PMMA.58 In this case, the kinetic curves are linearised in the coordinates N/N0 vs. lnt. It is this kinetic description that is characteristic of the processes with a kinetic arrest, for which the function of distribution over rate constants has usually the form 59 (8) rOkU a k lnOkmax=kminU , 1 where kmax and kmin are the boundary values of rate constants.Function (8) corresponds to a rather wide distribution of carbenes with respect to their reactivities, while the kinetic dependence satisfying this distribution is expressed as (9) lnOkmintU 0 . NOtU a N ¢§lnOk max=kminU Yakimchenko and Degtyarev 57 noted that the distribution function (7) is approximated by function (8) for the values of k>k 0, i.e., where the distribution of species over rate constants is rather narrow: ln(kmax/kmin)<10. In this case, the k 0 value correlates with kmin, whereas the kmax value may be determined from the initial decay rate of carbenes.The effect of the spatial orientation factor on the kinetic non- equivalence of carbenes results directly from comparison of the reactivities of CHC and BCHC. Thus at 77 K, the width of distribution regarding the rate constants of CHC decay ln(kmax/kmin) is equal to 5, whereas the same parameter for BCHC is 13. Apparently, introduction of bulky tert-butyl sub- stituents into the carbene structure increases the set of the carbene centre orientations relative to the C7H bonds of the surrounding macromolecules and largely expands the distribution of carbenes regarding their reactivities. The temperature dependences of the rate constants of the H atom transfer to carbenes in polymers (e.g., upon decay of DPC in PMMA) confirm the decisive effect of spatial orientations in the cages on the kinetic non-equivalence of carbene species, which is evidenced by an obvious deviation of these dependences (Fig.1, lines 1 and 2) from the Arrhenius law (dashed lines in Fig. 1). The effective activation energies decrease with a decrease in temper- ature from 12 ¡¾ 16 kJ mol71 to an abnormally low value of *5 kJ mol71 (see Refs 49, 53, 58, 60 ¡¾ 62). The qualitatively similar temperature dependences were also observed for the cage reaction of hydrogen atom abstraction from the solid matrix molecules by free radicals following the tunnelling mechanism.31, 63 This mechanism takes into account the intermo- lecular oscillations resulting in changes in the height and width of the potential barrier of the process.31, 64, 65 The temperature dependences of rate constants in both processes can be expressed by the exponential law (10) k(T)=a exp(bT ), where a and b are the parameters characterising the properties of the species and the matrix.E Ya Davydov, A P Vorotnikov, V P Pustoshnyi, G B Pariiskii 90 130 T /K 170 7ln k 1 0 5 1 2 0 15 2 20 5 9 13103 (1/T) /K71 Figure 1. Temperature dependences of the rate constants of DPC decay in PMMA kmax (1, 1 0) and kmin (2, 2 0);60 (1) and (2) dependences of the form7ln k*T; (1 0) and (2 0) dependences of the form7ln k*(1/T). It should be noted that the dependence (10) is well described by the tunnelling mechanism of H atom transfer for the entire series of carbenes, as indicated by the similarity of temperature dependences for kmax and kmin.The corresponding representa- tions of kmax and kmin in the reaction of DPC decay inPMMA are presented in Fig. 1 (curves 1 0 and 2 0). These results show that the kinetic non-equivalence of carbenes in polymers is due to the same reason, viz., the scatter of parameters (the width and height of the potential barrier) of the tunnelling transfer of the hydrogen atom. The width of carbene distribution with respect to these parameters is in turn determined by geometric heterogeneities of the reaction cages. According to the tunnelling model of H atom transfer, a bond of the hydrogen-bond type is formed between the C7H bond of the matrix molecule and carbene. C .. . H C Molecular oscillations in this complex result in changes in the permeability of the potential barrier to H atom transfer. The higher the temperature the larger the amplitude of intermolecular oscillations and the closer the reagents can come to each other and, consequently, the more probable is the tunnelling process. Thus, the effective potential barrier decreases as the temperature increases, and the rate of the process increases accordingly. Vorotnikov et al.61, 66, 67 determined the values of parameters for the tunnelling of H atom to CHC and DPC in the same polypiperylene matrix. The height U and the width x of the potential barrier as well as the equilibrium distance (R0) between carbon atoms of the active centre of carbene and the C7Hgroups of polypiperylene in the carbene ¡¾ polypiperylene complex were determined from the kinetic data obtained at 77 ¡¾ 133 K.The boundary values of these parameters, which correspond to the maximum and minimum rate constants of carbene decay, are presented in Table 1. These results indicate that the width and height of the potential barrier decrease with the increase in temperature. These values are somewhat higher for DPC than for CHC, which is indicative of the higher CHC activity in the reaction of H atom abstraction in the same polymeric matrix. Also worthy of note is the proximity of parameters of the hydrogen atom tunnelling from macromolecules to triplet car- benes and the corresponding values of parameters of the H atom tunnelling to macroradicals in their low-temperature decay by the relay-race mechanism.65 The frequencies of intermolecular oscillations in a polymer which activate the H atom transfer were estimated from the KIE determined for the low-temperature decay of BCHC and DPC in common and perdeuterated PMMA and polystyrene (PS).TheApplication of radical cage reactions in studies of microstructures of polymers and composites Table 1. Parameters of the hydrogen atom tunnelling from the polypiper- ylene molecules to CHC and DPC.67 xmax Rmax 0 Rmin 0 Carbene T /K Umin a Umax a xmin /nm /nm /nm /nm CHC DPC 0.400 0.400 0.400 0.370 0.370 0.370 0.375 0.375 0.375 0.360 0.360 0.360 0.140 0.115 0.100 0.130 0.118 0.110 0.115 0.100 0.094 0.120 0.110 0.103 208 150 121 188 159 138 150 125 108 167 138 121 *0 77 133 *0 77 133 a In kJ mol71.KIE was shown to be dependent on the reactivities of carbenes. Ensembles of carbenes characterised by higher decay rate con- stants manifest lower KIE at a given temperature. At T>100 ¡¾ 115 K, the kinetic isotope effect virtually disappears. The regu- larities observed are explained in terms of the model of H atom tunnelling across a potential barrier which oscillates due to intermolecular vibrations. It was shown that for a temperature increase in the range h 4Ok < T < mhM o O2 C¢§Hk Y1=2R0 2OmD1=2 ¢§ mH1=2U 2h ln kH a ¢§ kD 2h2MO2 [O is the characteristic frequency of intermolecular vibrations in the complex, M is the normalised mass of the carbene ¡¾CH (or CD) group complex] the KIE should decrease linearly in reactions of hydrogen atom transfer in conformity with the equation YR20 kTOmD ¢§ mHU , (11) where Y=mHo2C7H=mDo2C7D is the parameter character- ising the rigidity of the C7H and C7D bonds; R0 is the distance between the potential wells for the H and D atoms.At higher temperatures T> hMO2 moC¢§Hk KIE varies in conformity with the T72 law: 2 1 1 h0 ln kH . (12) & ¢§ kD 2Y1=2 mD1=2 mH1=2 MO2R kT Analysis of KIE in the reaction of BCHC decay in PMMA66, 67 was performed using relations (11), (12) and the condition T&hMO2/moC7Hk, under which the linear temper- ature dependence is transformed into a T72 dependence.The derived values of characteristic frequencies of intermolecular vibrations (O&53 cm71&1013 s71) correlate with the Debye frequencies for molecular crystals.68 The difference of distances DR0 between potential wells in the reaction of H(D) transfer in different kinetic ensembles of carbenes was also determined from the temperature dependence of KIE.48, 66, 69 It turned out that the decrease in the rate constant of BCHC decay by one order of magnitude corresponds to DR0&0.006 nm. Such a drastic rate dependence on the distance between reagents is in agreement with the tunnelling mechanism of H atom transfer to triplet carbenes.Unique information about the nature of the kinetic non- equivalence of carbenes in solid polymers can be obtained from studies of the magnetic field effect (MFE). Numerous literature data (see, e.g., Refs 70 ¡¾ 72) demonstrate the possibility of using this effect for the establishment of a detailed mechanism of 903 W(H)/W(H=0) 1.0 0.6 12 0.2 H /mT 0 400 200 Figure 2. Effect of magnetic field on the rate of BCHC decay in PMMA (1) and CTA (2) at 77 K.67 processes involving RP. The application of a magnetic field was shown 46, 67, 69 to influence noticeably the rate of low-temperature decay of BCHC in PMMA and CTA (Fig. 2); however, the character of this influence is different. Based on experimental dependencesW=f(H), the effect of magnetic field is explained by the strong exchange interaction J between unpaired electrons and short-lived RP.71 From the character of MFE, one can draw a conclusion about the existence in polymers of a set of RP differing in energies of exchange interactions. Poly(methyl methacrylate) is more homogeneous than cellulose triacetate, which is reflected in a smaller set of cages with different exchange energies in the radical pairs.The distribution of RP with respect to J corresponding to the scale of the observed effect is determined by the orientational factor associated with different angles of rotation of the axes of the orbitals of unpaired electrons relative to each other in RP.73 The effect of magnetic field on the decay of carbenes in solid polymers is analogous to its influence on their decay in low- molecular-weight organic glasses; however, the scale of the MFE is different.This provides evidence that the extent of differences in the character of MFE is determined not only by the nature of RP, but also by the nature of the matrix. In low-molecular-weight glasses, the reaction cages are more homogeneous with regard to the structural-physical parameter J compared to polymeric matri- ces. 2. Determination of molecular-dynamic characteristics of polymers from the transformation kinetics of carbenes in clusters Diazo compounds, like many low-molecular-weight additives, are irregularly distributed in solid polymers due to the existence of defects in the packing of macromolecules.74 This leads to a higher concentration of diazo compounds in the regions with a decreased packing density of macromolecules, which can result in the formation of clusters.Thus EPR spectroscopy revealed that photolysis (at 77 K) of DDM in common and perdeuterated PMMA, PS and polycarbonate (PC) resulted in carbenes Ph2C: formed in the matrix-isolated state and also biradicals 1 75, 76 generated in the regions with a decreased packing density of macromolecules due to conversion of DPC dimers (2) in clusters according to the scheme H kT hn (13) PhC Ph2C 2Ph2C N2 7N2 CPh2 Ph2C 2 1 The EPR spectrum of biradicals generated in reaction (13) is shown in Fig. 3 a. It represents an anisotropic signal with the parameters of cleavage in a zero field D\=11 mT and Dk=22 mT.The mean distance between unpaired electrons is 0.63 nm. The formation of biradicals in reaction (13) is confirmed by the increase in their yields at higher DDM concentrations. At904 a 10 mT H D\ Dk Figure 3. EPR spectra of biradicals generated in the photolysis of polymers supplemented with DDM at 77 K.75 [DDM] (mol kg71): (a) 0.1; (b) 0.2. sufficiently high [DDM] (*0.2 mol kg71) a signal of biradicals comprising three or more DPC fragments appears in the central part of the EPR spectrum (Fig. 3 b): PhC CPh Such species arise in bulkier clusters, their proportion in PC and PMMA does not usually exceed 20% of the total quantity of biradicals. Reactions of carbenes localised in clusters are interesting in two respects: the yield of biradicals 1 reflects the irregularity of the distribution of carbene precursors as regards their concentrations and this can also be used for the estimation of distribution of other low-molecular-weight additives as regards their concentrations in solid polymers.For example, an analogous distribution of bimo- lecular, trimolecular and larger clusters should apparently be expected for a number of photostabilisers, antioxidants and photosensitisers (salicylates, bisphenols, benzophenones). In addition, the kinetics of biradical formation may be used to characterise the intensity of molecular mobility of the cluster surrounding. The kinetics of biradical generation in clusters at 77 ¡¾ 120 K is well described by the logarithmic dependence on time.Analysis of experimental kinetic dependences performed by Davydov et al.66, 75 made it possible to establish the distribution pattern of rate constants as regards the Arrhenius parameters. In general case, the reactivities of carbenes in clusters can be determined from the distribution with respect to both effective activation energies of reaction (13) and preexponents.77 Figure 4 shows the integral distribution functions Y(E) and Y(k0) for the formation of biradicals involving DPC in polycar- bonate. As the dependences shown in Fig. 4 are virtually linear, the functions of carbene distribution in clusters with respect to the Arrhenius parameters may be regarded as `rectangular': dYOEU a dE OEmax ¢§ EminU , 1 dYOk0U a log d logk0 kmax 0 kmin 0 0 The limit values of the Arrhenius parameters in PC are as follows: Emax=38 kJ mol71, Emin=26 kJ mol71, log k max= 14.4, log k min=12.8.The values of the activation energy are typical of the relaxation processes in RP determined by motions of monomer units of macromolecules. These values are in good b HCPh2 Emin4E4Emax ; , 0 log kmin4log k04log kmax 0 . 0 E Ya Davydov, A P Vorotnikov, V P Pustoshnyi, G B Pariiskii 12 14 13 logk0 Y 0.8 0.4 32 24 40 E /kJ mol71 Figure 4. Integral functions of DPC distribution with respect to the Arrhenius parameters of biradical generation in PC.75 agreement with the activation energies of the spin probe rota- tion.78, 79 Thus, it may be concluded that the rate of reaction (13) in PC is controlled by the vibrational mobility of carbenes in clusters. Indeed, the values of k0=1013¡¾ 1014 s71 are comparable with the frequencies of motions of low-molecular-weight species in polymers.One should note a relatively narrow dispersion of the Arrhenius parameters in PC: DE=12 kJ mol71, log k0=1.6. InPMMA,the limiting values of the Arrhenius parameters are 0 0 0 Emax=50 kJ mol71, as Emin=22 kJ mol71, follows: log k max=16.0, log k min=10.5 kJ mol71. The mean values of these parameters (Emean=36 kJ mol71, log k mean=13.25) cor- relate with the activation energy of rotation and the correlation time of the spin probe in PMMA (E=38 ¡¾ 42 kJ mol71, log t=13.2).78 Of definite interest is a rather broad dispersion of parameters (DE=28 kJ mol71, Dlog k0=5.5), which seems to be due to a large set of molecular motions in PMMA at given temperatures.Indeed, the relevant calculations 80 show that the activation energy of rotational motions in the ester groups of PMMA with concomitant vibrations of the main chain segments near the equilibrium positions is 21.6 kJ mol71. This value correlates with the minimum activation energy of biradical formation. The activation barrier to rotation of the ester groups increases with the deviation of rotation angles of the main chain segments from their equilibrium values. Relevant calculations give a maximum potential barrier of 68 kJ mol71 for the torque vibrations about the C7C bonds of the main chain.80 Therefore, the increase in the activation energy of reaction (13) to 50 kJ mol71 is indicative of the existence of ensembles whose reactivities are controlled by the mobilities of segments of the main macromolecular chain.The results obtained 75, 76 make it possible to consider the reaction of biradical formation in DPC clusters as a specific probe for studying the molecular dynamics of macromolecules at low temperatures. The conventional method based on the use of a paramagnetic probe 79 is little suited for studies of molecular mobility in polymers at temperatures below the glass transition point. For T<Tg, EPR spectroscopy makes it possible to record only a part of the high-frequency spectrum of vibratory motions of the probe.The method based on measurement of the kinetics of DPC transformation in clusters extends considerably the temper- ature range for studies of the small-scale dynamics of macro- molecules. This allows determination of molecular-dynamic characteristics of polymers in the temperature range 90 ¡¾ 220 K where the use of other methods is problematic. IV. Estimation of structural characteristics of filled polymers from the kinetics of cage reactions Structural-physical modification of polymers at the macroscopic level has a strong effect on the reactivities of radicals in the cage. One of the most popular methods for the modification of the physical structure of polymers is to fill them with inorganic additives.Numerous studies have shown that the effect of polymer filling is associated with the formation of boundary layers 81 ¡¾ 85 with structures noticeably different from the bulk structure ofApplication of radical cage reactions in studies of microstructures of polymers and composites polymeric medium. The effective width of the boundary layer measured experimentally was found to reflect the behaviour of the filled system in respect of the property under study, i.e., this characteristic can be changed depending on the technique used for its measurement. Information about the boundary layers is as a rule derived from physicochemical parameters which are essen- tially dependent on the degree of filling, e.g., from the heat capacity (in the range of glass transition temperatures),86 den- sity 83 and viscosity.87 Due to a close relationship between the structural-physical properties of polymers and the kinetics of reactions occurring in them, investigation of kinetic dependences may also be regarded as an informative method for the determi- nation of structural characteristics of filled polymers, in particular of the effective thickness of the boundary layers.There is a large body of literature data devoted to the analysis of regularities of the thermal decay and thermal oxidation of filled polymers (see Bryk's monograph 88 and references cited therein). The available information points to a substantial role played by the filler in these processes.However, at high temperatures the kinetics can be influenced, in addition to the structural-physical factor, by the catalytic properties of the filler surface. From this viewpoint, the use of cage radical reactions occurring at low and moderate temperatures offers definite advantages for the estima- tion of the effect of the structural factor. The kinetics of these reactions should reflect peculiarities of the molecular organisation of boundary layers and the reactions must not be complicated by the interaction of radicals with the filler surface. 1. Effect of the filler on the kinetics of low-temperature decay of matrix-isolated carbenes Filling ofPMMAand TFC with Aerosil decreases the rate of low- temperature decay of BCHC in these polymers.66, 89 This effect was observed at 100 ¡¾ 140 K.The increase in the filler content decreases the limiting rate constants kmax and kmin; however, the width of the distribution as regards the rate constants log (kmax/kmin) remains invariable and amounts to 6.8 in PMMA and 5.5 in CTA at 100 K. As the temperature increases, the distribution with respect to the constants becomes narrower. The constancy of the distribution with respect to the reaction rate constants under isothermal conditions is also confirmed by the approximation of the stepped kinetic curves to the equation N N0 0tU¢§1=n, a O1 a nk where n is the parameter characterising the distribution width of carbenes as a function of their reactivities and k0 is the mean effective rate constant.90 Despite different contents of Aerosil, the kinetics of low-temperature BCHC decay in PMMA and CTA is characterised by the parameter n which is constant at a given temperature.69, 89 Figure 5 shows the dependence of k0 on the distance l between the filler particles.One can see from the graphs presented in Fig. 5 that k0 varies little at l>20 nm, though it decreases by more than one order of magnitude at l<20 nm. Such a drastic drop of the process rate is related to changes in the physical structure of the polymeric matrix upon transition to the state of boundary layers. The structural rearrangement changes the ratio of the rates of individual stages of BCHC decay as well as the relative population density of the triplet and singlet states of the intermediate RP resulting from changes in their configurations and, consequently, in the exchange energies.All these factors lead to a lower k0 constant. The kinetic data obtained indicate that the effective thickness of the boundary layers of the filler in PMMA and CTA ranges from 15 to 20 nm. a 103 k0 /s71 42 60 40 20 b 103 k0 /s71 146 40 20 60 Figure 5. Dependence of the mean rate constant of BCHC decay on the distance between Aerosil particles in PMMA (a) and CTA (b);89 (a) T=100 (1), 113 (2), 125 K (3); (b) T=100 (1), 125 (2), 135 K (3). 2. Effect of the filler on the kinetics of carbene transformation in clusters In the example of filled PC, the reaction (13) of generation of biradicals in DPC clusters is shown to be noticeably influenced by the presence of a filler.75, 76 Figure 6 shows the temperature dependence of the molecular mobility (the `defrosting' curves), which reflects the reactivity of carbenes and biradicals in DPC clusters, on the Aerosil content in PC.The maximum quantity of biradicals is formed in the Aerosil-free PC at 130 ¡¾ 150 K, while the molecular mobility is unfrozen for all ensembles of carbenes. In the filled samples, the maximum is shifted towards lower temperatures (T&120 K), because at T>120 K the biradicals N/N0 1.5 1.0 100 80 Figure 6. Temperature dependences of the yield of biradicals in the Aerosil-free (1) and Aerosil-filled (2, 3 ) PC.75 The content of the filler (mass %): 50 (2), 65 (3).905 32180 l /nm 102 k0 /s71 2 14 3 6 1 80 l /nm 123 140 T /K 120906 are thermally destroyed in these samples. The different time course of the unfreezing curves is determined by a considerable decrease in the thermostability of biradicals with the increase in the degree of polymer filling. The kinetics of biradical decay in PC at different filler contents may be used for the estimation of the effective thickness of boundary layers.75 We give below the initial decay rates of biradicals (BR) in the filled PC at 146 K, relative to their initial concentration depending on the proportion (vol.%) of Aerosil Va. 40 15 Va (%) (W0 /[BR]0)6104 /s71 309 223 00.25 The non-additive character of the initial rate dependence on Va and its dramatic increase at Va>20% are determined by the transition of the polymer to the structural state of the boundary layer.The boundary layer thickness was found to be 12 nm. This magnitude is rather close to the value derived from the filler effect on the decay kinetics of the matrix-isolated carbenes. 3. Effect of the filler on the transformation kinetics of macroradicals Structural modification of polymers upon their filling is reflected in the kinetics of other elementary reactions, in particular of the monomolecular decay of macroradicals. Thus it was found that the kinetics of the radical decay of polyvinylpyrrolidone (PVP) initiated by photoreduction of Fe(III) additives at 77 K49, 91, 92 is dependent on the presence of a filler.CHCH2 CHCH2 + N N O hn O , + FeCl3 FeCl2+Cl7+ 3 CHCH2 CCH2 + N N O O kT +H+. (14) 4 Degradation of the radical cations 3 in the thermal stage results in the formation of macroradicals 4 which are identified from their EPR spectra. The effect of the Aerosil filler on the efficiency of generation of the radicals 4 is reflected by the unfreezing curves shown in Fig. 7. The maximum relative quan- tity of the radicals 4 is registered at 180 ± 200 K. This characterises the proportion of their precursors, viz., radical cations 3, which are stabilised in the low-temperature photolysis of samples. It follows from the plots shown in Fig. 7 that in highly filled PVP only 10% of the radical cations 3 are stabilised at 77 K, whereas their content reaches*70% in non-filled samples.The non-linear character of variation of the radical concentration at a given c / c0 1 2.6 2 1.8 3 1.0 0.2 140 260 T /K 200 Figure 7. Temperature dependences of the yield of macroradicals 4 in the Aerosil-free (1) and Aerosil-filled (2, 3) PVP. The filler content (mass%): 15 (2), 50 (3).92 E Ya Davydov, A P Vorotnikov, V P Pustoshnyi, G B Pariiskii temperature as a function of the filler content is determined by the structural transition occurring in PVP upon addition of definite amount of Aerosil, i.e., upon transition of the polymeric phase to the boundary layer state. The effect of structural modification on the efficiency of transformation of the radical cations 3 in reaction (14) is particularly strong where the amount of the filler is 50 mass%, which makes the distance between the filler particles equal to *25 nm.This distance may be regarded as the effective thickness of the boundary layer. The observed effect seems to be caused by the formation of a looser molecular packing compared to that in the polymer bulk. The decrease in the molecular packing density favours the lowering of the energetic barrier to rehybrid- isation of the carbon atom 48 in macroradicals 4 from the sp3 to the sp2 configuration in the reaction (14). As a result, the activation energy of reaction (14) is decreased in the case of filled PVP compared to that in the non-filled polymer and the concentration of the radicals 4 increases accordingly in the photoinitiation at 77 K.The application of the above reactions to the structural studies is limited to rather low temperatures. The expansion of the temperature range requires further search for radical reactions, the kinetics of which reflect the structural effect under conditions of usual and elevated temperatures and can be measured in a fairly simple manner. From this standpoint, highly interesting are the reactions in which active radicals may be transformed into stable radicals which can be recognised by their characteristic EPR spectra. Thus dissociation of the radicals 4 leads to the pyrrolidone ring opening, which is accompanied by the formation of radicals 5.CCH2 CCH2 N N O O (15) 5 4 However, because of their low stabilities, radicals 5 could not be registered by EPR spectroscopy. For this reason, the course of reaction (15) was monitored by following the transformation of the radicals 5 into stable nitroxyl radicals under the action of nitrogen dioxide on the initial and Aerosil-filled (50 mass %) PVP.93, 94 Under these conditions, free-radical transformations of PVP occur according to the following scheme:95 CHCH2 CCH2 CHCH2 N N N O O hn O+2HNO2, +2NO2 + 5 6 H2O+NO2+NO, 2HNO2 CCH2 CCH2 CCH2 N NO N R O NO NRNO , O O 5 7 CHCH2 CHCH2 CHCH2 N N N O ON O ON O NO NO2 7HNO2 8 6 CCH2 N O . R =These reactions result in the formation of two stable nitroxyl radicals, viz., 7 and 8.The EPR spectrum recorded after exposure of PVP in NO2 represents superimposition of the signals from these radicals (Fig. 8 a). The content of the radicals 7 is markedlyApplication of radical cage reactions in studies of microstructures of polymers and composites ab 2 mT H 2AN\ 8 8 2ANk 2AN 7 k Figure 8. The EPR spectra of polyvinylpyrrolidone after its irradiation in the presence of NO2.94 (a) Aerosil-free PVP; (b) PVP with 50 mass%of Aerosil. increased in highly filled PVP (Fig. 8 b). This fact points to an increase in the efficiency of formation of their precursors, viz., radicals 5, in reaction (15) owing to a decreased density of molecular packing upon formation of boundary layers. Thus, the proportion of the signal from the radicals 7 in the overall EPR spectrum may be regarded as an indirect characteristic of the depth of the structural-physical modification of the filled polymer.4. Kinetic features of the dissociation of radical dimers in filled polymers Structural characteristics of filled polymeric compositions can be estimated at relatively high temperatures using the cage reaction of thermal dissociation of diphthaloylethane (DPE) into two stable radicals.96 ¡¾ 98 Me2N O O O O (16) NMe2 DPE O 2 NMe2 O R. The reaction (16) is an equilibrium one in liquid state. However, kinetic studies of the DPE-free radicals system in CTA at 320 ¡¾ 370 K pointed to a more complex mechanism of this reaction in the solid polymer matrix.99, 100 In the temperature range 320 ¡¾ 334 K this process follows the scheme k1 k2 2R.DPE R_R k71 k3 In the first step, dissociation of DPE yields RP. The rigid cage surrounding it prevents the exit of radicals from the cage. There- fore, some radicals recombine in the cage, while those released into the bulk of the system are transformed into initial DPE as a result of diffusive collisions. At higher temperatures (344 ¡¾ 364 K), the reaction scheme is different, which is related to the change in the mechanism of radical decay. In this case, the linear loss of radicals occurs instead of the quadratic loss. k1 k2 k 0 R. 3 DPE R_R k71 Taking account of the steady-state concentrations of RP and provided k71 44 k2, the equations for the steady-state concen- trations of radicals are expressed as [R.]c a sAAAAAAAAAAAAAAAAAAAAAAAA k1k2aDPEa0 , k¢§1k3 [R.]c a k1k2aDPEa .k¢§1k03 Structural modification of CTA upon its filling with Aerosil (up to 60%) influences the reactivity of the radicals R.. This effect is demonstrated in Fig. 9, which shows the temperature depend- ences of the steady-state concentration of radicals in the original and Aerosil-filled CTA. At T>340 K, the concentrations ofR. in the filled samples are higher than those in the Aerosil-free samples. However, the reverse temperature dependence is observed in the high-temperature range. The increase in the steady-state concen- tration of radicals in the filled CTA at 320 ¡¾ 344 K is associated with the decrease in the efficiency of radical decay and lower values of the parameters k71k3 and k71k 03 relative to the parameter k1k2 in equations (17) and (18).This is also evidenced by the results of measurement of the temperature dependence of the correlation time of the spin probe rotation.99, 100 The effective activation energy of the probe rotary mobility increased from 4.6 kJ mol71 in non-filled CTA to 7.6 kJ mol71 in CTA contain- ing 60 mass%of Aerosil, i.e., filling has a substantial effect on the molecular dynamics and hence on the ratio of rate constants in individual stages of the radical process. Figure 9 shows that the temperature coefficient of the change in [R.] variation becomes negative for the highly filled CTA at T>340 K.In CTA containing 30 mass% of Aerosil, an inter- mediate situation is realised in which only a decrease in the efficiency of radical formation was observed as the temperature was increased. These facts indicate that structural modification upon filling influences most effectively the decay of radicals. It is for this reason that the inversion of the dependence shown in Fig. 9 is observed in the region of transition of the quadratic decay of radicals to the linear one. The mean distance between the filler particles in CTA with 60 mass% of Aerosil is estimated as *10 nm. This distance corresponds to the overlapping of boun- [R.]c6104 /mol kg71 16 1284 340 320 Figure 9.Temperature dependence of the mean radical concentration upon dissociation of DPE in Aerosil-free (1) and Aerosil-filled (2, 3) CTA.100 Aerosil content (mass %): 30 (2), 60 (3). 907 products. (17) (18) 123T /K 360908 dary layers and transition of the entire polymer to a new structural state. Thus, the exit of stable radicals from the cage in the process of thermal dissociation of DPE is rather sensitive to changes in the molecular organisation upon filling of the polymer and can characterise the degree of its structural-physical modification. V. Conclusion The kinetics of radical cage reactions demonstrates their clear-cut dependence on the physical structure of solid polymers and filled compositions. A distinctive feature of the processes considered above is their versatile character as regards their possible imple- mentations in diverse polymers and composites over a wide temperature range (77 ± 350 K), viz., from cryogenic to relatively high temperatures.A strong dependence of kinetic parameters on structural factors makes it possible to use such reactions as a sensitive tool for estimating the structural-physical modification of polymers. The results of kinetic studies of radical reactions in polymers can also be of independent significance in designing kinetic models of chemical processes characterised by their spatial-temporal disorder.101 ± 103 Based on the results obtained up to now, it may be concluded that the kinetic regularities of the processes considered reflect physical structures of polymers both on local levels as well as on the scale which is much larger than the sizes of reaction cages.This is evidenced primarily by the results obtained in the studies of the reactions of triplet carbenes. For example, the effect of magnetic field on the kinetics of their low-temperature decay is determined by the distribution of intermediate RP with respect to geometric configurations defined by the nearest cage surrounding. It should be noted that the kinetics of this cage process is rather sensitive to relatively large-scale structural changes which involve the 15 ± 20 nm thick interface layers in filled polymers. The use of the above `kinetic' approach in studies of the physical structures of polymers is undoubtedly limited. Today, based on the results obtained, one can speak solely about the possibility of comparative estimation of the extent of structural- physical modification reached in a definite way in a given polymeric material.Thus the dependences of kinetic parameters on the content of a filler may be instrumental in the establishment of polymer transition to a qualitatively different structural state (highly filled polymer) in which the boundary layers begin to overlap. Analogous conclusions about changes in the physical structure may be drawn by measuring yields of definite radicals in the reactions under study. 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ISSN:0036-021X    

出版商:RSC    

年代:2000

数据来源: RSC