摘要:

Russian Chemical Reviews 71 (4) 265 ± 282 (2002) Theoretical studies of transition metal complexes with nitriles and isocyanides M L Kuznetsov Contents I. Introduction II. Electronic structures of complexes and the nature of coordination bonds III. Spectroscopic properties of coordinated ligands IV. Reactivities and mechanisms of reactions with coordinated ligands V. Conclusion Abstract. with complexes metal transition of studies Theoretical Theoretical studies of transition metal complexes with nitriles electronic The reviewed. are isocyanides and nitriles and isocyanides are reviewed. The electronic structures structures and the nature of coordination bonds in these complexes are and the nature of coordination bonds in these complexes are discussed. structures electronic the between correlation The discussed.The correlation between the electronic structures of of transition and isocyanides and nitriles with complexes metal transition metal complexes with nitriles and isocyanides and their their structural and characteristics, spectroscopic properties, structural properties, spectroscopic characteristics, and reactiv- reactiv- ities are considered. The bibliography includes 121 references ities are considered. The bibliography includes 121 references. I. Introduction It is known that free nitriles (N:CR) and isocyanides (C:NR) containing no strong acceptor substituents R are poorly reactive or even inert in a number of processes, for example, in nucleophilic addition. If these molecules are involved as ligands in transition metal complexes, the situation reverses because complex forma- tion leads to activation of nitrile and isocyanide ligands. Reac- tions, which proceed under drastic conditions, if at all, with free nitriles and isocyanides, every so often occur under relatively mild conditions with coordinated ligands.In some reactions with coordinated ligands, the rate is increased by a factor of 1018 as compared to that in the reaction with the correspodning free ligand.1±4 Nitriles and isocyanides thus activated can be used in the synthesis of new compounds with the C7O, C7N, C7C, C7S and C7P bonds, which cannot be prepared without resort- ing to transition metals. Cycloaddition of the corresponding reagents to the inner-sphere nitrile and isocyanide ligands affords tetrazoles and oxadiazolines, respectively, which are widely used in pharmacology.The design of new antitumour drugs is yet another field of application of nitrile complexes. Thus the imino-ether compounds trans-[PtCl2{NH=C(OMe)Me}2] prepared by the nucleophilic addition of methanol to the [PtCl2(NCMe)2] complex exhibited high antitumour activity.5, 6 This result was unexpected because cis isomers of platinum complexes with N-donor ligands generally show higher antitumour activity as compared to the correspond- ing trans isomers. ML Kuznetsov Department of Chemistry, Moscow Pedagogical State University, Nesvizhskii per. 3, 119021 Moscow, Russian Federation. Fax (7-095) 246 77 66. Tel.(7-095) 246 88 56. E-mail: inorgchem@mtu-net.ru Received 26 December 2001 Uspekhi Khimii 71 (4) 307 ± 326 (2002); translated by T N Safonova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n04ABEH000708 265 265 274 276 281 Coordinated nitriles also find application in catalysis. For example, large-scale production of acrylamide and nicotinamide is based on catalytic (including, catalysis by metal ions 4, 7 ± 9) hydrolysis of nitriles. Metal-promoted hydrolysis of nitriles and isocyanides can be used for detoxication of industrial wastes containing these compounds. Finally, some nitrile complexes of transition metals possess nonlinear-optical properties.10, 11 Interesting properties of transition metal complexes with nitriles and isocyanides and their wide industrial applications have given impetus to a substantial increase in the number of publications devoted to these compounds. The results of exper- imental studies of their reactivities have been surveyed in reviews.1, 4, 12 ± 17 However, the data from theoretical investiga- tions of nitrile and isocyanide coordination compounds were not systematised. At the same time, transition metal complexes with other ligands have been extensively studied by quantum-chemical methods { in the last decade.These investigations were motivated primarily by the necessity of interpreting and systematising the ever-growing body of experimental information. Besides, theoret- ical methods are sometimes the only tool in studying unstable compounds and intermediates formed in the course of many chemical reactions. In this review, the results of theoretical investigations of transition metal complexes with nitriles and isoelectronic isocya- nides are generalised. The electronic structures and the nature of coordination bonds in these compounds are discussed.The correlations between the electronic structures of these complexes and their structural characteristics, spectroscopic properties, and reactivities are considered. Studies concerned with analysis of interactions between ligands and some transition metals are partially included for comparison. II. Electronic structures of complexes and the nature of coordination bonds Two models, viz., the ligand field theory and the Dewar ± Chatt ± Duncanson (DCD) model, are most commonly used for the qualitative description of the nature of coordination bonds in transition metal complexes. The latter model is applied to com- plexes containing ligands with multiple bonds.These theories are based on the molecular orbital method and treat the formation of coordination bonds as a result of interactions between the valence { The results of theoretical studies of transition metal complexes (primar- ily, carbonyl, carbene, carbyne and p-complexes) are surveyed in reviews.18 ± 24266 orbitals of the ligand and the metal atom. Of particular interest is the elucidation of the character of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals because they are primarily responsible for the chemical reactions and determine the spectroscopic and other physical properties of compounds.Systematic studies of nitrile and isocyanide complexes by quantum-chemical methods were started only recently and most of the publications date back to 1990s.{ The DCD model 25, 26 is commonly used for the description of these complexes. According to this model, the metal7ligand coordination bond involves the s-component, which originates from the transfer (donation) of the electron density from the occupied s-orbitals of the ligand to the unoccupied d-orbitals of the metal atom, and the p-component resulting from p-back donation. By p-back donation is meant the electron density transfer from the highest occupied d-orbitals of the metal atom to the free valence antibonding p*-orbitals of the ligand resulting in two-electron stabilisation of the frontier MOs of the complex.In addition, the p-component of the coordination bond often includes the contributions of the highest occupied p-orbitals of the ligand whose interactions with the occupied d-orbitals of the metal atom lead to destabilisation of the HOMOs (Fig. 1). a b E E d p* d p* d d p HOMO HOMO p Ligand Metal Complex Ligand Metal Complex Figure 1. Scheme of formation of the HOMO of complexes containing ligands with multiple bonds . (a) Ligands possessing strong p-acceptor properties, (b) ligands possessing weak p-acceptor properties. Apparently, the contributions of the p- and p*-MOs of the ligand to theHOMOof the complex are determined by the relative energies of the valence orbitals of the metal atom and the ligand.The d (M) ¡À p*-MO(ligand) interaction prevails in the HOMOs of the complexes with ligands possessing strong p-acceptor proper- ties [for example, the carbon(II) oxide or carbyne ligands] because the valence occupied d-orbitals of the metal atom are closer in energy to the p*-MOs than to the p-MOs of the ligand (Fig. 1 a), whereas the d (M) ¡À p-MO(ligand) interaction dominates the HOMOs of the complexes with ligands exhibiting weak p-acceptor properties, such as nitriles, because the energies of the highest occupied d-orbitals of the metal atoms are similar to those of the occupied p-MOs of the ligand (Fig.1 b). Until very recently, the extended Hu�� ckel theory (EHMO) was most commonly used in calculations of MOs of nitrile and isocyanide complexes. However, higher-level approximations, viz., the Hartree ¡À Fock method (HF), the functional density theory (DFT), the second-order M��ller ¡À Plesset theory (MP2), etc., have found application in the last few years. The next section covers the results of studies devoted to the electronic structures and the nature of coordination bonds in transition metal com- plexes with isocyanides and nitriles. { It should be noted that theoretical studies devoted to isocyanide complexes and, particularly, to nitrile complexes are much smaller in number than those dealing with carbonyl compounds or p-complexes.M L Kuznetsov 1. Electronic structures of isocyanide and nitrile complexes The nature of coordination bonds and reactivities of the molyb- denum diisocyanide complex trans-[Mo(CNMe)2(dppe)2] (dppe=Ph2PCH2CH2PPh2) and the rhenium isocyanide complexes trans-[Re(CNR)Cl(dppe)2] (R=H, Alk, Ar) were studied.27, 28 It was found that two of the highest occupied MOs of the molybdenum complex and the HOMOs of the rhenium complexes are formed via interactions of the d-orbitals of the metal atom with the p- and p*-MOs of isocyanide. The HOMOof the molybdenum complex (Fig. 2) is bonding for theMo7Cbond and antibonding for the C:N bond due to the predominant contribution of the p*-MO of the ligand to the HOMO respon- sible for the p-back donation effect.However, the p-component of the metal ¡À ligand bond is rather weak because of the small coefficients with which the AOs of the a-carbon atom are included into the HOMO of the complex under consideration. The con- tributions of the p- and p*-MOs of isocyanide to the HOMOs of the rhenium complexes are approximately equal (the HOMO is a nonbonding orbital with respect to the Re7C bond). Cg Cl Re Nb Ca The electronic structures of the [Pd2(CNR)2(m-Z3-L)2] (L is indenyl) and [Pd4(m-CNR)4(m-OAc)4] complexes, the model com- pounds [Pd2(m-CNH)2(OH)4] 4¡¦ 2 and [Pd2(m-CNH)2(OH)4] 27 and the [Ni2(CNH)(PH2CH2PH2)2] complex were examined at the EHMO level of theory.29, 30 The highest occupied MOs of the last two mentioned complexes consist of the pd -orbitals of the {Pd2(OH)4}27 or {Ni2(PH2CH2PH2)2} fragment and the p*-MO of isocyanide.It was noted that the interaction between these orbitals in the former complex is rather weak. The lowest unoccupied MO of the nickel complex is a purely p*(CNH)- orbital. For the indenyl complexes, the energy gap between the HOMO and LUMO was demonstrated to be rather large. As a consequence, these complexes exist in the singlet state, which accounts for the formation of the Pd7Pd bond. The MOs of the model binuclear complex [Pd2(CNMe)4Cl2] were calculated by the extended Hu�� ckel method 31 with the aim of obtaining qualitative information on the nature of the frontier MOs and examining the dependence of the compositions of the MOs on the internuclear Pd7Pd distance and the torsion angle y between the planes of two {Pd(CNMe)2Cl} fragments.The orbitals of the metal atom make the major contribution to the HOMOandLUMO, the orbitals of theCa atom of isocyanide also making a particular contribution (the antibonding character of the Pd7C bond), whereas the AOs of the Nb and Cg atoms of the isocyanide ligand contribute insignificantly to these orbitals. The composition of the MOs is virtually insensitive to variations in the torsion angle y and the internuclear Pd7Pd distance. The electronic structures and the nature of the coordination bonds in the isocyanide complexes M(CNBut)Me(CO)2. .(Z5-C5R5)] and the isomeric compounds [M(Z2-CMe=NR0)..(CO)2(Z5-C5R5)] and [M{Z3-CH2=CHNBut}(CO)2(Z5-C5R5)] (M=Mo, W; R=H, Me; R0=Pri, But) were discussed based on the data from photoelectron spectroscopy and the results of EHMO calculations of the molecular orbitals carried out for model compounds.32 In the isocyanide complexes, the two highest occupied MOs are centred on the metal atom but the p*-MOs of the carbonyl and isocyanide ligands also make particular contri- butions to these MOs giving rise to p-back donation [the p-MO(CNR) also makes a certain contribution to these MOs of the complexes]. Although the isocyanide ligands in many complexes possess noticeable p-acceptor properties, the contributions of the p- and p*-MOs of the ligand to the HOMOs of the complex are in some cases insignificant and theHOMOsare centred on the metal atom.Thus calculations for the frontier MOs of the model complexesTheoretical studies of transition metal complexes with nitriles and isocyanides E 10a 9a 8a7a 6aCN H3C Figure 2. Correlation diagram of the MOs in the trans-[Mo(CNMe)2(dppe)2] complex.27 [Ag2(CNMe)4X2] (X=Cl, Br, I) (1),33 [PtRh(CNBut)2. .(CN)2(dmpm)2]+ (dmpm=Me2PCH2PMe2) (2), trans- [Rh(CNBut)2(PPh3)2]+ (3) 34 and [M(CNH)4]+ (4) (M=Cu, Ag) 35 demonstrated that the HOMOs are localised predomi- nantly on the metal atoms. The orbitals of the halogen atoms in the complexes 1 and the s-orbitals of the carbon atom of the isocyanide ligand in the complexes 4 also make contributions to the HOMOs of these complexes.The lowest unoccupied MOs of the complexes 1 and 4 are the p*-orbitals of the CNR fragment, whereas the LUMOs of the complexes 2 and 3 consist of the pz(Rh), p*(CN), pz , dxz(P) and p(Pt) atomic orbitals. The compositions of the frontier MOs in the tetranuclear linear [M4(CNMe)8(PH3)2]2+ clusters (M=Pd, Pt) were studied by the EHMO method.36, 37 The highest occupied and lowest unoccupied MOs are the sd -orbitals of the metal atom. The next 18 occupied MOs of the clusters are centred on the metal atom, whereas 4 unoccupied MOs following after the LUMO are the p*-orbitals of CNMe. The introduction of other ligands exhibiting strong p-acceptor properties into isocyanide complexes often leads to weakening of p-back donation of the electron density from the d-orbitals of the metal atom to the p*-orbitals of isocyanide.This weakening is particularly noticeable when a strong p-acceptor is in the trans position with respect to the isocyanide ligand, which actually reflects the mutual trans effect of two p-acceptors. In contrast, the introduction of a ligand exhibiting pronounced donor properties (for example, the chloride ligand) in the trans position with respect to isocyanide leads to enhancement of the p-acceptor properties of 19ag 18ag 17ag 16ag 15ag CH3 N HP 2 H2 C P PH2 PH2 Mo CN H3C 15a1 14a1 6b2 6b1 13a1 2a2 CH3 NC HP2 HP 2 Mo PH2 PH2 isocyanide and strengthening of p-back donation. Thus several highest occupied MOs in the trans- and cis-[(PH3)2(CO)2W. .(CNMe)(:CCH=CH2)]+ complexes containing one carbyne ligand and two carbonyl ligands (strong p-acceptors) along with the isocyanide ligand are composed predominantly of the orbitals of the metal atom and the carbonyl and carbyne ligands with a very small contribution of the AOs of the nitrogen atom of isocyanide (Figs 3 and 4).38 The nature of the coordination bond in the isocyanide com- plexes [Cp2Ta(CNMe)(Z2-CH2=CH7CH=CH2)]+ (Cp is the cyclopentadienyl ligand) (5), [Cp2Ta(CNMe)2]+ (6) and [Cp2M(CNMe)] 0/+ (M=Ta, Zr) was examined based on the results of DFT calculations 39 with full geometry optimisation of different isomers of these complexes.Analysis of the theoretical and experimental structural data (the linearity of the isocyanide molecule, the equal CN bond lengths in free and coordinated isocyanides) and examination of the energy components of the coordination bond showed that p-back donation is of secondary importance in the interaction between isocyanide and tantalum in the complex 5, which contains other strong p-acceptors. In this case, the interaction between the metal atom and the ligand is predominantly Coulombic in character.In the complex 6, which contains no other p-acceptor ligands except for isocyanide, the Ta ? CNR p-back donation becomes essential. Three orbitals, viz., the d-orbital of the nd the p- and p*-orbitals of isocyanide, contribute to the p-component of the coordination bond between the metal atom and the isocyanide ligand.The interactions between these orbitals give rise to three 267268 E p*-MO + + CO CO C C W C N H3C H3C N H3P PH3 Figure 3. Correlation diagram of the MOs in the trans-[(PH3)2(CO)2W(CNMe)(:CCH=CH2)]+ complex.38 C N C C C C P P trans P P C C C C N C cis Figure 4. Composition of the HOMOs in the tungsten isocyanide com- plexes trans- and cis-[(PH3)2(CO)2W(CNMe)(:CCH=CH2)]+.38 + + CO CO CH2 CH C W CH H3P PH3 molecular orbitals two of which, viz., pd7p(L) (MO1) and p (MO3), are formed primarily from the d-orbitals of the metal atom and the p- or p*-orbitals of the ligand. These MOs produce the bonding and antibonding combinations for the metal ± ligand bond, respectively (Fig.5). In some cases, the third orbital (MO2) is the HOMO of the complex derived from a linear combination of the pd¡pÖLÜ- and pd7p*(L)-orbitals. The contribu- tion of the unoccupied p*-orbital of the ligand to the MO2 of the isocyanide complexes is often larger than the contributions of the occupied p-MOs of the ligand although the participation of the latter remains essential. As a result, the MO2 orbital either remains nonbonding with respect to the metal ± ligand bond or becomes weakly bonding. Actually, as can be seen from Fig. 6, the formation of the MO2 leads to a substantial decrease in the electron density on the Ca and Cg atoms, whereas the electron E pd¡pÖLÜ MO3 MO2 d MO1 pd7p(L) Metal M7CNR Figure 5.Three-orbital interaction between the frontier MOs of the metal atom and the ligand. M L Kuznetsov + CH2 d¡pÖLÜ p*(CN) p(CN) CNRTheoretical studies of transition metal complexes with nitriles and isocyanides + d p(CN) Figure 6. Polarisation effect of the nonbonding HOMO. density on the nitrogen atom increases resulting in polarisation of the corresponding MO. This polarisation of orbitals in cyanide, carbonyl and nitrogen complexes was described by R Hoff- mann 40 as early as 1977. At the same time, the contributions of the p- and p*-MOs of isocyanide to the HOMO are very small in the presence of other (in addition to CNR) ligands possessing strong p-acceptor properties. In the latter case, the properties of such complexes are not always correctly interpreted in terms of the qualitative DCD model.On the whole, the compositions of frontier MOs determining the p-component of the coordination metal ± ligand bond in nitrile complexes is similar to those in isocyanide compounds. The EHMO calculations were carried out for the model nickel nitrile complex [Cp(PH3)Ni(N:CC6H4NH2)]+ (7).10 Figure 7 shows the correlation diagram of the MOs describing interactions between the molecular orbitals of the [Cp(PH3)Ni]+ fragment and the nitrile ligand upon the formation of the complex 7. The most important result of this interaction is s-donation of the 3a0 a00 2a0 1a0 Ni+ H3P Figure 7. Scheme of the interactions between the MOs of the [Cp(PH3)Ni]+ and NCC6H4NH2 fragments upon the formation of the complex 7. For the nitrile ligand, only the orbitals belonging to the nitrile group are shown.10 + M Ca N Cg p*(CN) E 3a 00 8a 0 7a 0 2a 00 6a 0 5a 0 4a 0 3a 0 1a 00 2a 0 1a 0 + Ni N C H3P k-orbital of the ligand to the HOMO only insignificantly over- electron density from the occupied s-orbitals of the nitrile ligand (s1- and s2-MOs), which are bonding with respect to the CN group, to the unoccupied dz2-orbital of the metal atom. In addition to the above-mentioned two s-orbitals, the third lower-energy s-orbital of the nitrile ligand (it is omitted in the figure), which is antibonding with respect to the CN bond, also makes a contribu- tion to this interaction. The authors of the cited study believed that the last-mentioned contribution [which leads to a decrease in occupancy of the s*(CN)-orbital upon coordination] is mainly responsible for strengthening of the CN bond on going from the free ligand to the complex.In addition to the s-component, the coordination bond in the complex 7 also involves the p-component (interactions between the occupied d-orbitals of nickel and the p-orbitals of the nitrile ligand). Both p- and p*-MOs contribute to the resulting MOs of the complex. The highest occupied MO derived from a linear combination of three orbitals a 00+l1pk+l2pk is a nonbonding orbital with respect to theNand C atoms. The contribution of the prides the contribution of the pk -orbital from which it was concluded that the metal ± ligand p-interaction has its origin in very weak p-donation of the electron density from the ligand to metal.In transition metal complexes, nitriles exhibit both weak p-donor and weak p-acceptor properties, which is yet another important characteristic of the electronic structures of this class of pk p\ p\nb s2 pk s1 p\ H H N N C N H H 269270 coordination compounds. Hence, the total contribution of the p-orbitals of nitrile (both p and p*) to the frontier MOs of the complex is, generally, insignificant and the latter orbitals are often centred on the metal atom. The results of EHMO or HF calculations of the electronic structures of the model nitrile complexes [CpFe(NCMe)(PH3)2]+, [CpFe(N:CCH2C6H4O..{P3N3(OH)5}-p)(PH3)2]+ (see Ref. 41) and [CpFe(p-N:C. .C6H4NO2)(PH3)2]+ (see Ref. 11) indicate that the HOMOs of all complexes consist predominantly of the d-orbitals of the iron atom although the p*-orbitals of the nitrile ligand in the latter compound also make a particular contribution to the MO, which is the closest in energy to the HOMO, thus providing the p-dative interaction. Analogous results were obtained in the DFT calcu- lations of the molecular and electronic structures of the model nickel complexes [LNi{N:CN=C(NH2)2}(NH3)3] (8) and [(NH3)3Ni{N:CN=C(NH2)NHNi(NH3)2L}] (9) (L is 1,2- diazolidine).42 NH3 Ni N H3N H3N NH NH2 N C NC H2N 8 C C Cl Cl Cl C Figure 9.Compositions of the HOMOs in the complexes 10 ± 12 when they contain two linear acetonitrile ligands (a) and when one of the acetonitrile ligands has an angular sructure (b).44 NN Figure 8. Interactions between the orbitals of the metal atom and the nitrile ligand in the complex 9.42 The ground state of the complex 9 is quintuplet, whereas the NH3 Ni2 N H3N N H3N N NH3 C Ni1 NH3 N N C H 9 H2N energies of the triplet and singlet states are higher by 46.9 and 381.4 kJ mol71, respectively. The singly occupied MOs are cen- tred on the dz2 - and dxy-orbitals of Ni1 and the dz2¡y2 - and dx2± orbitals of Ni2. The lower-energy doubly occupied MOs are composed of the dyz- and dxz-orbitals of the Ni1 and Ni2 atoms with a particular contribution of the p*-MO of nitrile.The latter orbital is responsible for insignificant p-back donation of the electron density from the orbitals of the Ni2 and Ni1 atoms to the orbitals of the ligand. The side-to-side interaction between the orbitals of the Ni1 atom and the nitrile ligand (Fig. 8) is rather unusual for the high-spin Ni(II) ion. Kuznetsov et al.43, 44 reported the results of theoretical studies of the platinum nitrile complexes trans-[Pt(NCMe)2Cl2] (10), a b C C Cl N Cl N Cl trans-[Pt(NCMe)2Cl2] (10) C Cl C N Cl Cl Cl [Pt(NCMe)Cl5]7 (11) Cl C C N Cl C N Cl trans-[Pt(NCMe)2Cl4] (12) N Ni2 NN Ni1 N NN C N N C N C Cl N C C N C Cl C Cl Cl N Cl Cl C Cl Cl N C N C Cl Cl M L Kuznetsov N Ni2 N N N N N N Ni1 C N N N C N CCTheoretical studies of transition metal complexes with nitriles and isocyanides [Pt(NCMe)Cl5]7 (11) and trans-[Pt(NCMe)2Cl4] (12).Figure 9 presents the compositions of the HOMOs of the complexes 10 ± 12 when they contain two linear acetonitrile ligands (a) and when one of the acetonitrile ligands has a bent sructure (b). Calculations performed for the complexes 10 ± 12 at different levels of theory (HF and MP2) and with different basis sets demonstrated that several of the highest occupied MOs of these complexes are formed from the orbitals of the chlorine atoms or, in the case of trans-[Pt(NCMe)2Cl2], from the orbitals of the metal atom with- out a noticeable contribution of the orbitals of acetonitrile.Recently, similar results were obtained by Pombeiro and co-workers 38 for the tungsten complexes trans- and cis- [(PH3)2(CO)2W(NCMe)(:CCH=CH2)]+ (Fig. 10). P P C C C N C C N C C P C C P C trans cis Figure 10. Compositions of the HOMOs in the tungsten nitrile complexes trans- and cis-[(PH3)2(CO)2W(NCMe)(:CCH=CH2)]+.38 One of the highest occupied MOs of the {(PH3)2. .(CO)2W(:CCH=CH2)}+ fragments (the first orbital in the case of the trans isomers and the second orbital in the case of the cis isomers) is derived from a linear combination of the d-orbitals of the metal atom with the p- and p*-MOs of two CO ligands. Due to a large contribution from the p*-orbitals, the HOMO is bonding relative to theW7C bond and antibonding with respect to the C7O bond.This is indicative of the presence of p-back donation from the metal atom to the p*-MO of a strong p-acceptor such as CO. Two other highest occupied MOs of the {(PH3)2(CO)2W(:CCH=CH2)}+ fragments of the cis and trans isomers are composed of the d-orbitals of the metal, which also interact with the p- and p*-MOs of one of the CO ligands and with the orbitals of the carbyne ligand. Upon the formation of the nitrile complex [(PH3)2(CO)2W(NCMe)(:CCH=CH2)]+ from the {(PH3)2(CO)2W(:CCH=CH2)}+ and {NCMe} fragments, the composition of the highest occupied MOs of the former fragment remains virtually unchanged and the orbitals of the nitrile ligand make an insignificant contribution to the HOMO of the complex (see Fig.10). In the study,45 calixarene complexes of Ta and Nb, including those containing the coordinated acetonitrile molecule, were synthesised, their spectroscopic parameters and structural characteristics were determined, and the molecular orbitals were calculated by the Fenske ± Hall method.46 In the [Cp*Ta(NCMe)(calix[4]arene)] complex (Cp*=Z5-C5Me5), the bonding interaction between nitrile and metal involves the dz2 (Ta)- and pz(N)-orbitals via s-bonding (Fig. 11) and the HOMO of the complex is centred on the atoms of the calixarene ligand. The ab initio calculations for the tetranuclear copper(I) clusters [Cu4(NCMe)2I4L2] (L is 2,6-dimethylaniline, 6-ethyl-o- toluidine or p-toluidine) 47 demonstrated that the p-orbitals of the iodine atoms make major contribution to the HOMO of the complex, whereas the LUMO is formed from the p-orbitals of copper.It was noted that the orbitals of acetonitrile make no substantial contribution to the frontier MOs of the complex. 271 E s* dz2 p* p* (calix[4]arene) pz s [Cp*Ta(calix[4]arene)] [Cp*Ta(NCMe)(calix[4]arene)] NCMe Figure 11. Correlation diagram of the MOs in the nitrile complex [Cp*Ta(NCMe)(calix[4]arene)].45 The main difference between the nitrile and isocyanide com- plexes is associated with weaker p-acceptor properties of nitriles compared to those of isocyanides due to which the contributions of the p- and p*-MOs of the ligand to the MOs of the complex compensate each other and the corresponding molecular orbital is nonbonding.The total contribution of the p- and p*-orbitals of the ligand to the HOMOs in nitrile complexes is smaller than that in isocyanide complexes, which is also indicative of weaker p-acceptor properties of the nitrile ligands. As a result, the p-component of the coordination bond in nitrile complexes is often negligible as compared to the s-donor or Coulombic component (see below). Hence, the DCD model must be applied to the description of the properties of the nitrile complexes with caution. Finally, analysis of the compositions of the frontier MOs of acetonitrile and methyl isocyanide 48 demonstrated that nitriles are weaker s-donors than isocyanides due to substantially lower energy of the valence occupied s-orbitals of NCR compared to those of CNR.2. Structural features of metal complexes with nitriles and isocyanides Back p-donation of the electron density from metal to the ligand has a substantial effect on the geometric parameters of the complexes. Actually, an increase in the occupancy of the p*-anti- bonding orbitals of the ligand must lead to weakening of the multiple C:N bond in the ligand and, consequently, its length must slightly increase on going from the free ligand to the complex. To the contrary, the metal ± ligand bond becomes stronger due to additional p-interaction resulting in shortening of the metal ± ligand bond compared to those in the related complexes in which p-back donation is absent.In the early studies by Sarapu and Fenske,49, 50 the presence of p-back donation in isocyanide complexes was confirmed by the X-ray diffraction data. The Mn7C(CNMe) bond length in the [Mn(CNMe)2(CO)3Br] complex is 0.10 ± 0.14A smaller than the sum of the covalent radii of Mn and the sp-hybridised carbon atom, which indicates that this bond in the complex is partially double. The results of X-ray diffraction studies are in agreement with the data from theoretical calculations of MOs by the Fenske ± Hall method. In particular, the substantial occupancies of the p*-MOs of the isocyanide ligand in the [Mn(CNMe)n(CO)57nBr] (n=0 ± 4), [Mn(CNMe)n(CO)67n]+ (n=0 ± 6), [Mn(CN)(CO)5], [Mn(CN)6]57 and [Fe(CNMe)6]2+ complexes are indicative of the efficient electron density transfer from the d-orbitals of metal to the p*-MO of isocyanide in spite of272 the presence of such a strong p-acceptor as the CO ligand.A comparison of the occupancies of the p*-MOs of the isocyanide and carbonyl ligands shows that the latter is the stronger p-acceptor than isocyanide and that p-back donation of the electron density from the d-orbitals of metal to the p*-MOs of the carbonyl ligand is more pronounced than that to the p*-or- bitals of the isocyanide ligand. It does not always happen that the expected elongation of the C:N triple bond takes place on going from the free ligand to the complex. For example, the calculated C:N bond length in the trans- and cis-[(PH3)2(CO)2W(CNMe)(:CCH=CH2)] com- plexes (1.145 and 1.147A, respectively) is 0.011 ± 0.013A smaller than that in the free ligand.38 The interactions of the [MCp3]+ fragment (M=Ti, Zr, Hf) with such ligands as NCMe, CNMe and CO, which afford the corresponding [MLCp3]+ complexes, were examined.51 It was demonstrated that theM7L bond length calculated by the DFT method increases in the series Ti<Hf<Zr and CO<CNMe<NCMe.The authors of the cited study attributed the former tendency to the relativistic effects, which were taken into account for Hf. An increase in the M7L bond length on going from CO to NCMe correlates with weakening of the p-acceptor properties of the ligands in this series. The variations in the C:O and C:N bond lengths on going from the free ligand to the complex are less evident.These bond lengths depend on two opposite effects.52 One effect, viz., s-donation from the ligand to metal L?M, must lead to short- ening of the C:N(O) bond because the corresponding occupied s-orbital of the ligand, which is involved in the formation of the M7L s-bond, is antibonding. Consequently, coordination of the ligand must lead to a decrease in the electron density on the s*-orbital, which is antibonding for the C:N or C:O bond, resulting in its strengthening. The opposite effect, viz., p-back donation from metal to the ligand, must lead to elongation of the C:N or C:O bond due to an increase in the electron density on the p*-MO of the ligand upon coordination.Taking into account weakening of the p-acceptor properties of the ligands (more precisely, a decrease in the ratio between the p-acceptor and s-donor abilities) on going from CO to NCMe, one would expect a decrease in Dr=r(C:X)comp7r(C:X)free (X=O, N) in this series. However, the results of calculations demonstrated that the C:O bond length increases upon coordination, the C:N bond length in isocyanide complexes decreases, whereas the corre- sponding bond length in nitrile complexes remains virtually unchanged (Table 1). To account for this phenomenon, the energy characteristics of the M7L bond were analysed. The energy of this bond (Ebond) was divided into three components Ebond=7(DECoul.+DEPauli+DEoi), where DECoul.character- ises the Coulombic component of the M7L interaction, DEPauli Table 1. The M7L, C:N and C:O bond lengths in the [MLCp3]+ complexes (M=Ti, Zr, Hf; L=CO, CNMe, NCMe), the energies of the coordination bonds and their components. Bond lengths /A E /kJ mol71 Complex Ebond DEPauli DEoi M7L C:N DECoul. (C:O) [Ti(CO)Cp3]+ [Zr(CO)Cp3]+ [Hf(CO)Cp3]+ 476 414 261 365 345 223 448 457 186 (1.15) 1.17 1.16 (1.14) 1.17 1.16 (1.15) 1.17 1.16 2.09 [Ti(CNMe)Cp3]+ 2.15 [Ti(NCMe)Cp3]+ 2.18 2.25 [Zr(CNMe)Cp3]+ 2.30 [Zr(NCMe)Cp3]+ 2.32 2.22 [Hf(CNMe)Cp3]+ 2.25 [Hf(NCMe)Cp3]+ 2.28 7302 108 7246 169 7164 136 7247 110 7218 182 7155 151 7294 134 7274 207 7192 170 7282 7337 7233 7228 7309 7219 7288 7390 7264 Note.The calculated C:O and C:Nbond lengths in the free CO, CNMe and NCMe ligands are 1.14, 1.18 and 1.16A, respectively. M L Kuznetsov represents the exchange repulsion and DEoi accounts for the contribution of the bonding orbital interaction. The sum DEPauli+ DEoi represents the orbital energy (DEorb). The results of analysis showed that the maximum contribution of DECoul. to the energy of the bond Ebond is observed in the case of isocyanide complexes, whereas this contribution in the case of carbonyl and nitrile complexes is much lower (see Table 1), which is responsible for additional shortening of the C:N bond in the [M(CNMe)Cp3]+ complexes. The lowest orbital energies DEorb were found for nitrile complexes, which results in intermediate values of Dr. It should be noted that p-back donation sometimes leads to a bent distortion of the ligand.Thus the geometry of coordinated isocyanide can be described as intermediate between two struc- tures: R M C N . and C N R MApparently, a predominance of a particular form is deter- mined by the relative contributions of the p-acceptor and s-donor components to the metal7ligand bond. When the s-donor (L ? M) component substantially dominates over the p-acceptor (M ? L) component, the ligand adopts a linear conformation. In contrast, when p-back donation becomes dominant, the multi- plicity of theM7L bond increases, whereas the multiplicity of the C:N bond decreases, which could cause a bent distortion of the isocyanide ligand.39, 53 Actually, according to the X-ray diffraction data,54 ± 58 the isocyanide ligand has a linear structure in most of isocyanide complexes of transition metals in high oxidation states in which p-back donation is manifested to only a small extent, if at all.At the same time, the isocyanide ligand has an angular structure in complexes of transition metals in low oxidation states in which efficient electron density transfer occurs from the d-orbitals of metal to the p*-MO of the ligand,59, 60 the C7N7C angle being contracted to 140 8. The correlation diagrams of the MOs for linear and angular methyl isocyanide 48 show that the bent distortion of the ligand leads to an increase in the energy of the p-MO and this orbital becomes the highest occupied MO, whereas the energy of the p*-MOs decreases (Fig.12 a). Hence, the bent distortion leads to a noticeable enhancement of the p-acceptor properties of theCNMe ligand, its s-donor properties being virtually unchanged because the energy of the p-MO in the angular ligand is only 0.008 a.u. higher than the energy of the s-MO in the linear ligand and the p-MO is localised on the terminal carbon atom to a much lesser extent. a b E/a.u. E/a.u. py;z 0.23 0.20 p p z y;z pz py0.21 0.16 py 0.12 py;z py;z 70.49 s pz py 70.46 s s py;z 70.57 s 70.50 NCC angle CNC angle 180 8 165 8 123 8 180 8 Figure 12. Walsh diagrams for the bent distortions of the methyl iso- cyanide (a) and acetonitrile (b) molecules (the data from the study 48).Theoretical studies of transition metal complexes with nitriles and isocyanides Analysis of the linear and angular structures of the nitrile ligand NCMe (Fig. 12 b) demonstrated that acetonitrile exhibits very weak p-acceptor properties due to a small degree of local- isation of the p*-MO on the terminal N atom although the energy of the p*-MO of the nitrile ligand determined in the study 48 is comparable with the energy of the p*-MO of the isocyanide ligand. The bent distortion of the NCMe ligand must lead to weakening of its acceptor properties due to the total destabilisa- tion of the p*-MO.At the same time, the authors of the cited study 48 believed that essential localisation of the p-MO(NCMe) on the C and N atoms of the C:N bond makes acetonitrile a potentially strong p-donor ligand.These p-donor properties of the nitrile ligand are responsible for the deviation of the M7N7C fragment from linearity, which is frequently observed in exper- imental studies. However, as shown in Section II.1, the fact that the contributions of the p-orbitals of nitriles to the HOMOs of the complexes are small refutes this assumption. Somewhat different distributions of the frontier MOs for acetonitrile and the model NCH structure were found in the study 61 (Fig. 13). The authors believed that the bent distortion of the NCH ligand leads to mixing of the s- and py-orbitals and their reorientation so that overlapping of these orbitals with the orbitals of the metal atom is enhanced in the case of the lateral coordination of the nitrile ligand through the C:N triple bond.Hence, the bent distortions of the nitrile ligands lead to enhance- ment both of their donor and acceptor properties. The lateral coordination of nitrile leads to a decrease in the energy of the LUMOof the ligand and, consequently, p-back donation in nitrile p-complexes becomes substantial. Thus theNCCfragment of the ligand is linear in the case of the standard s-coordination of nitriles to transition metals, whereas a E 7a 4a 6a 3a 2a 1a 5a 4a 3a 2a 1a PH3PH3 PH3PH3 W W Cl Cl Cl Cl PH3 PH3 Figure 14. Correlation diagrams of the MOs in the [W(NCH)Cl2(PH3)3] (a) and [CpW(NCMe)(CO) {C(SiH3)=C=O}] (b) complexes.61 E py;z spy,z N C H Figure 13.Walsh diagram for the bent distortion of the NCH molecule and the frontier MOs of the angular acetonitrile molecule (the data from the study 61). the nitrile ligand undergoes a bent distortion due to substantial p-back donation in the case of the lateral p-coordination through the C:N triple bond. Actually, geometry optimisation of the model complex [CpW(Z2-NCH)(CO)(HC=C=O)] by the DFT method 61 gave the N7C7H angle of 135.6 8. Analysis of the electronic structures of the tungsten complexes [W(NCH)Cl2(PH3)3] and [CpW(NCMe)(CO) {C(SiH3)=C=O}] demonstrated that the p*-MOs of the ligand contribute signifi- cantly to the highest occupiedMOsof the complexes (in the case of [CpW(NCMe)(CO) {C(SiH3)=C=O}], to the highest occupied MO, which is the closest in energy to the HOMO) (Fig.14). The effect of the nature of the ligand, primarily, of its donor and acceptor properties, on the geometric parameters of other 4a 7a 0 3a 3a 00 2a 1a 6a 0 2a 00 5a 0 H O H C C C N C N H3Si OC pz p py spz py H N C N C b E 8a 7a 6a 5a 4a 3a 2a 1a OC N C H3Si W W C OC CH3 273 z py spz py CH3 3a 00 7a 0 6a 0 2a 00 5a 0NC CH3274 fragments in complexes was considered in the study.62 In the cited study, the results of ab initio calculations for a series of [TaLL0(OH)2H2]+ complexes with various L and L0 ligands, including NCMe, were discussed (the geometry optimisation was carried out by the MP2 method).The trans effect of the ligand L on the phosphine ligand (the Ta7P distance was used as a measure of the trans effect) was considered using the [TaL(PH3)(OH)2H2]+ complexes as an example. Depending on the nature of the ligand L, the Ta7P distance was found to decrease in the series CH¡3 >H7>NH¡2 > BF> CS > PH3 (A) > OH7>CO>CN7>F7>NC7 > Cl7>Br7>I7>NH3>NCMe>PH3 (B)>N2. The relative stabilities of the isomers A and B OH H OH H Ta L Ta L H3P H3PH H OH OH A B (DEAB=EA7EB) change in the following series: F7<OH7< Cl7<Br7<NC7<I7<CH37<CN7<NCMe<NH3< N2<H7<NH¡2 <PH3<CO<CS<BF (Table 2).The ligands exhibiting strong p-donor properties reside in the left side of this series, i.e., the p-donor ability of the ligands L decreases from left to right. From the above series it follows that there is no correlation between the s-donor properties (trans effect) and the p-donor (acceptor) properties of the ligands. It was found that the hydride atoms in the complexes A and B, which are located in the trans positions with respect to each other, are shifted from the stronger p-donor toward the stronger p-acceptor. If the ligands possess both weak p-acceptor and weak p-donor properties, the bent distortion is caused by the ligand exhibiting the strongest static trans effect and the hydride atoms are shifted toward the weaker s-donor. It was proposed 62 that the unusual bent distortion of the H7Ta7H fragment is determined by these p-donor and p-acceptor (rather than by s-donor) properties of the ligands L.From the above-considered data, it is evident that p-back donation sometimes has a substantial effect on the structural parameters of the complexes. One of the indicators of the efficient p-back donation is the bent distortion of the CNC or NCC fragment. However, in actual practice this distortion is uncom- mon even in the case of transition metal complexes in low oxidation states and, hence, the linearity of CNC or NCC cannot serve as proof of weak p-acceptor properties of the ligand in the complex. In the general case, the relative length of the C:N multiple bond in transition metal complexes with nitriles and isocyanides also cannot be considered as a reliable criterion for the degree or even existence of p-back donation because the length of this bond is substantially influenced not only by two opposite effects, viz., s-donation and p-back donation, but also by the Coulombic component.39, 51 A comparison of a series of related complexes showed that theM7L bond length is a more reliable criterion for the degree of p-back donation.Table 2. Relative energies of the A and B isomers of the [TaL(PH3). .(OH)2H2]+ complexes. L L L DEAB /kJ mol71 DEAB /kJ mol71 DEAB /kJ mol71 757.32 2 I7 CN7 NCMe 736.84 NH3 N2 H7 NH¡ PH3 CO CS BF 719.43 0.00 10.76 16.08 53.55 Cl7 7111.33 Br7 7101.24 NC7 790.43 789.47 768.91 735.21 724.99 719.93 CH¡3 M L Kuznetsov III.Spectroscopic properties of coordinated ligands 1. Vibrational spectra Stretching vibrational frequencies of the C:N and C:O bonds in transition metal complexes with nitriles and carbonyls } are influenced by several factors, including s-donation of the electron density from the ligand to the metal atom, p-back donation from metal to the ligand and the Coulombic component of the coordination bond. s-Donation from the ligand to metal leads to strengthening of the C:Nand C:O bonds and, consequently, to an increase in their vibrational frequencies, whereas p-back donation from metal to the ligand causes weakening of these bonds and, hence, a decrease in the corresponding vibrational frequencies.In complexes with strong p-acceptor ligands, such as CO (and in some cases CNR), the latter effect generally dominates over the former as a result of which the n(CN) and n(CO) frequencies for the complexes are lower than those for the free ligands. Actually, lowering of the n(CO) and n(CN) vibrational fre- quencies on going from the free ligand to the complex was predicted theoretically and observed experimentally for many carbonyl complexes as well as for a series of transition metal complexes with isocyanides (predominantly, with metals in low oxidation states).39, 59, 63, 64 However, complex formation with isocyanides possessing weaker p-acceptor properties as compared to CO often leads to an increase in the n(CN) frequency.Thus theoretical analysis of the force constants and n(CN) vibrational frequencies for the [Mn(CNMe)n(CO)57nBr] (n=0 ± 4), [Mn(CNMe)n(CO)67n]+ (n=0 ± 6) and [Fe(CNMe)6]2+ com- plexes demonstrated that the n(CN) frequency for the complexes is higher than that for the free ligand, which indicates that s-donation dominates over p-back donation.50 At the same time, detailed theoretical studies of the nature of coordination bonds in transition metal complexes demonstrated that the n(CN) and n(CO) stretching vibrational frequencies generally cannot serve as a reliable criterion for the degree of L?Mdonation andM ? L p-back donation even for complexes with such a strong p-acceptor as carbon(II) oxide.Examples are provided by the so-called `non-classical' carbonyl complexes, for instance, [Os(CO)6]2+ and [Ir(CO)6]3+. In their spectra, the n(CO) frequency is higher than that in the spectrum of the free ligand. The reason is that Coulombic interactions are of paramount importance in the formation of coordination bonds in non- classical carbonyl complexes and these interactions ultimately determine the n(CO) frequency.65, 66 In transition metal complexes with isocyanides, which are weaker p-acceptors than CO, Coulombic interactions can exert a stronger effect on the n(CN) frequency compared to that of p-donation, which often leads to an additional increase in this vibrational frequency upon complex formation.In nitrile complexes, the above-described tendencies are further enhanced because the nitrile ligands exhibit very weak p-acceptor properties and rather weak s-donor properties. In this case, the Coulombic component of the metal ± ligand interaction often exerts primary control over the n(CN) frequency. This fact is confirmed by the preliminary data from analysis of the natural bond orbitals performed for the cis-[Re(NCMe)Cl(PH3)4] and trans- and cis-[Re(NCMe)Cl(PH3)4]+/2+ complexes.67 Generally, the n(CN) vibrational frequencies for nitriles are shifted to the high-frequency region on going from the free ligand to the complex.68 ± 78 At the same time, there are examples of low- frequency shifts of this band upon coordination of nitriles with metals in the lowest oxidation states 79 ± 81 but these shifts are much smaller than those in the spectra of analogous isocyanide complexes. Finally, it should be noted that the low-frequency } Spectroscopic properties of carbonyl compounds are considered for comparison.Theoretical studies of transition metal complexes with nitriles and isocyanides shifts of n(CN) for nitrile Z2-complexes in which the ligand is coordinated through the C:N bond are rather high,82 ± 84 which indicates that this is virtually a double bond.The spectroscopic properties of nitrile complexes were exam- ined in several theoretical studies. Thus the vibrational spectra of the platinum complex [Pt(NCMe)Cl3]7 were calculated and described in detail.69 The n(CN) band in the spectrum of the complex is shifted by 113 cm71 to the high-frequency region as compared to its position in the spectrum of free acetonitrile. Interestingly, analogous high-frequency shifts of n(C:N) (95 ± 117 cm71) were found for the adducts of acetonitrile with Lewis acids BF3 and BCl3 (Ref.85) in which p-back donation is, in principle, impossible. It was demonstrated that the calculated n(CN) frequency decreases from 2209 cm71 for the free N:C7N=C(NH2)2 ligand to 2189 and 2143 cm71 upon coordination of the ligand to nickel (complexes 8 and 9, respectively).42 The calculated C:N bond length increases from 1.163A (in the free ligand) to 1.178 and 1.187A (in the complexes 8 and 9, respectively), which correlates with a change in the n(CN) vibrational frequency.However, the order in which the calculated n(CN) frequencies change is inconsistent with the experimental series 8> N:C7N=C(NH2)2>9. The vibrational spectra of acetonitrile coordinated to the Li+ and Na+ ions were calculated at the HF/6-31G and HF/6-31G** levels of theory. 86 Coordination leads to a substantial increase in the intensities of the n(C:N), n(C7C) and d(CH3) vibrations but calculations demonstrated that their frequencies change only slightly. Finally, mention may be made of the study 87 in which the vibrational spectra and the nature of the Fermi resonance for ruthenium isocyanide complexes 13 and 14 were analysed by the DFT method.71/0/+1/+2 R N C O OO Ru O O C N R Ru O O O O Ru O O O O C N R 13, 14 R=MeC6H4 (13), But (14). In the case of the neutral complexes and their reduced forms, Fermi resonance appears due to the interaction between the fundamental frequency n(C:N) [2094 (13), 2010 (137), 2131 (14) and 2028 cm71 (147)] and the vibrational overtone B2 (13 and 137) or E (14 and 147) involving deformation of the C:N7C fragment as the major component. In some experimental studies (see, for example, the review 4), it was found that the change in the n(C:N) frequency correlates with the reactivities of nitrile and isocyanide complexes with respect to nucleophilic and electrophilic addition. It was shown that complexes for which the parameter Dn(C:N)= n(C:N)comp7n(C:N)free is negative are readily subjected to electrophilic attack.If Dn(C:N)>0, complexes are involved in nucleophilic addition at the b-carbon atom. If the contribution of p-back donation to the coordination bond is essential, the b-atom of the ligand becomes more nucleophilic due to the electron density transfer from the d-orbitals of metal to the p*-MO of the ligand thus facilitating electrophilic attack. In contrast, if p-back donation makes only an insignificant contribution upon the formation of the coordination bond, the electron density transfer from the s*-orbitals of the ligand to the d-orbitals of metal leads 275 to enhancement of electrophilicity of the b-atom of the ligand, which facilitates the nucleophilic attack.However, detailed theo- retical studies providing the interpretation of this correlation are lacking. 2. Electronic spectra and excited states Theoretical and experimental investigations devoted to the elec- tronic spectra and the nature of excited states of nitrile and isocyanide complexes are rather scarce. Most of the calculations were carried out in a one-electron approximation. Several elec- tronic states of the bis(isocyanide) complexes of nickel(II) peroxo NiO2(CNH)2 were studied by the unrestricted HF and CI meth- ods.88 It was demonstrated that the singlet state 1A1 is the ground electronic state of the complex. The first excitation state is the triplet 3B1 with the energy parameter DE(3B171A1)= 47.60 kJ mol71.The authors of the cited study considered the electron density distribution (r) and its Laplacian (H2r) and analysed the natural orbitals. n Photophysical properties of the model mononuclear copper clusters CuLá (L=NH3, NCMe, C5H5N; n=1 ± 4) were described.89 These properties were interpreted with the use of the Hartree ± Fock method. The nature of the low-energy excited states was examined, their energies were determined, and the assignments of the metal-to-ligand charge transfer (MLCT) bands in the spectra of these complexes were made. The first excited state of the [Cu(NCMe)4]+ complex (3B3) is formed due to MLCT and the second excited state (3B1) is formed through electron transfer from the 3d- to 4s-orbitals of the metal (Table 3).The assignments of these bands in the emission spectra of these complexes were performed.89 Table 3. Energies of the first excitation states (DE) and changes in the Mulliken electronic populations of the 4s-, 4p- and 3d-orbitals of the copper atom with respect to the ground state for the [Cu(NCMe)4]+ complex. State DCu DE /a.u. total 3d 4p 4s 333 0.393 0.050 0.056 0.062 0.900 0.905 0.159 0.164 0.179 B3 B1 A1 70.331 0.156 0.135 70.786 70.796 70.827 The nature of the low-energy excited states of the [Ag2(dmb)2X2] complexes (X=Cl, Br, I; dmb is 1,8-diisocyano- p-menthane) was discussed based on the experimental data and the results of EHMO calculations.33 The charge-transfer bands in the electronic absorption spectra were assigned and their nature was studied.The broad band in the region of 250 ± 350 nm was assigned to the electron transfer from theHOMO(HOMO7n) to the LUMO (LUMO+n): d(Ag2)/p(X)?p*(CNR). A comparison of the photoelectron spectra of the [CpM(CNBut)(CO)2Me] and [CpM(CO)3Me] complexes (M=Mo or W) showed 32 that the replacement of the carbonyl ligand by isocyanide leads to destabilisation of the d-orbitals of the metal atom, which is manifested in shifts of stretching bands in the spectrum of the isocyanide complex to lower energies by 0.73 ± 0.83 eV. An analogous result was obtained in the study 90 dealing with iron pentacarbonyl Fe(CO)5 and the iron isocyanide complexes Fe(CNR)(CO)4 (R=Me, But, SiMe3, Ph).The higher energies of the occupied p-MOs of the isocyanide ligand compared to those ofCO(the electron ionisation energy for the p-MOs of the corresponding free ligands are 12.46 and 16.91 eV) leads to enhancement of the destabilising interaction between the valence occupied d-orbitals of metal and the occupied p-MOs of the isocyanide ligand and to weakening of the stabilising d(M) ± p*(L) interaction in the pentacarbonyl complexes resulting in an increase in the HOMO energy. Hence, it was concluded that276 E 74.53 (5p) (397 nm, 3.123 eV) DE=3.565 eV 712.73 (4d) Rh Figure 15. Correlation diagram of the MOs in the trans-[Rh(CNBut)2(PPh3)2]+ (a) and [PtRh(CNBut)2(CN)2(dmpm)2]+ (b) complexes.34 the isocyanide ligand is a better p-donor but a poorer p-acceptor as compared to the carbonyl ligand.Analysis and the assignments in the electronic spectra of the trans-[Rh(CNBut)2(PPh3)2]+ and [PtRh(CNBut)2(CN)2. .(dppm)2]+ complexes (dppm=Ph2PCH2PPh2) were carried out based on the results of EHMO calculations for the model com- pounds trans-[Rh(CNBut)2(PPh3)2]+ and [PtRh(CNBut)2. .(CN)2(dmpm)2]+ (Fig. 15).34 Two intense absorption bands at 397 and 317 nm in the spectrum of the former complex were assigned to the transitions dz2(Rh)?pz(Rh)/p*(CNR)/pz,dxz(P) and dyz(Rh)?pz(Rh)/p*(CNR)/pz,dxz(P), respectively. The bands at 473 and 348 nm in the spectrum of the latter compound were dz2(Rh)/dz2(Pt)? assigned to the transitions py(Rh)/p*(CNR)/pz,dxz(P) and dyz(Rh)?py(Rh)/p*(CNR)/ pz,dxz(P), respectively, the electron transfer from the d-orbitals of Rh to the p*-MO of the ligand contributing significantly to the former band.Among other studies devoted to examination of the spectro- scopic properties of the complexes under consideration, note- worthy is the investigation 91 in which the spin-spin coupling constants JHD for the [OsL(NH3)4(Z2-H2)](z+2)+ compounds (where L=Me2CO, H2O, MeCOO7, Cl7, H7, C5H5N, NCMe, NH3, NH2 OH or CN7) were calculated by the DFT method. For the acetonitrile complex, the constant JHD is 20.8 Hz. The dependence of JHD on the internuclear H7H distance was discussed. IV. Reactivities and mechanisms of reactions with coordinated ligands 1. Isomerism and coordination modes The coordination modes of ligands in complexes were discussed in theoretical studies.38, 67, 92 ± 94 The relative stabilities of the trinuclear platinum and palladium isocyanide clusters [M3(CNH)2(m-dppm)3]2+ (M=Pt, Pd) with different coordina- tion modes of the CNH ligand (isomers A±D) were studied based on the results of EHMO calculations: 92 E a 76.943 (dx27y2) 78.935 (pz, p*)(LUMO) (317 nm, 3.911 eV) DE=3.629 eV 712.500 (dz2) (HOMO) 712.564 (dyz) 712.723 (dxy) 712.757 (dxz) [Rh(CNBut)2(PPh3)2]+ 74.53 (5p) (473 nm, 2.621 eV) DE=2.919 eV 712.73 (4d) Rh [PtRh(CNBut)2(CN)2(dmpm)2]+ 2+ HN HNC C M M M M MCNH A B According to these results, the stability of the clusters A±D changes in the series A>B>C>D.This order is inconsistent with the experimental data, which provided evidence that the configuration C is most stable for the platinum complex. Appa- rently, more precise calculation methods are necessary for the correct description of the energy characteristics. The stabilities of the monodentate (A) and chelate (B) config- urations of the model complex [Li7O7PH2=CH7C:N] were analysed theoretically (HF/6-31G*).93 Li H O C C P H H N A Calculations demonstrated that the chelate configuration of the complex is 112.6 kJ mol71 more stable than the monodentate form in spite of the fact that it is characterised by the unusual Li7N:C angle (91 8). The reactions of the Li+ ion with the NCH and NCMe molecules were studied by the semiempirical MNDO method and EPR spectroscopy.94 Geometry optimisation of the LiNCH complexes revealed three minima corresponding to the structures A±C.b 76.943 (dx2¡y2 ) 79.312 (ps, pCN)(LUMO) (348 nm, 3.563 eV) DE=3.222 eV 712.231 (d s)(HOMO) 712.534 (dyz) 712.719 (dxz) 712.861 (dxy) 713.303 (ds) 2+ 2+ HNC M M M M CN CNH H CN Li O C C P H H H B M L Kuznetsov 75.866 (6p) 713.43 (5d) Pt 2+ HNCM M M CNH DTheoretical studies of transition metal complexes with nitriles and isocyanides Li+ Li+ 7 H C N Li C N7 C N H H C B A The heats of formation of these isomers are 46, 163 and 113 kJ mol71, respectively. The activation energy of the C? B conversion is 8 kJ mol71, which indicates that there is little likelihood that the structure C exists under the experimental conditions.The geometric isomerisation of transition metal complexes was examined in the studies.38, 67 The preliminary study of the rhenium(I) complexes trans- and cis-[ReLCl(PH3)4] (L=NCMe, CNMe or CO) revealed that the cis isomers are more stable in the presence of weak p-acceptor ligands (L=NCMe), whereas the trans isomers are more stable in the presence of strong p-acceptor ligands (L=CO and CNMe).67 PH3PH3 L PH3PH3 L Cl Re Re H3P H3P H3P Cl PH3 cis trans L=NCMe, CNMe, CO. The opposite tendency was found for the model tungsten complexes [(PH3)2(CO)2WL(:CCH=CH2)]+ (15) and [(H2PCH2CH2PH2)(CO)2WL(:CCH=CH2)]+ (16) (L=Cl7, NCMe, CNMe or CO),38 which contain such a strong p-acceptor as the carbyne ligand in addition to the nitrile or isocyanide ligands.If the ligand L is a strong p-acceptor (L=CO, CNMe), the cis isomers are more stable (L and the carbyne ligands are in the cis positions) (Table 4). In contrast, if the ligand L is a weak p-acceptor (L=NCMe) or an efficient s-donor (L=Cl7), the trans isomers become more stable. The relative stabilities of the cis isomers compared to those of the trans isomers increases in the series Cl7<NCMe<CNMe<CO in accordance with enhancement of the p-acceptor properties of the ligands (more precisely, with the net p-acceptor minus s-donor abilities of the ligand). Theoretical predictions about the relative stabilities of the geometric isomers were confirmed by the experimental data on the trans?cis isomerisation of the tungsten isocyanide complexes.+ + CO CO CO CH2 CO CH2 W C C L W C C H3P H3P H H3P H L PH3 cis trans L=Cl7, NCMe, CNMe, CO. + + CO CO CH2 CH2 CO W C C H2P CO W C C L H2P H H L H P 2 PH2 cis trans L=NCMe, CNMe, CO. Based on the results of theoretical investigations 38, 67 and experimental studies,95 ± 98 it can be assumed that configurations with s-donor ligands and strong p-acceptor (in particular, CNR) ligands or with weak p-acceptor ligands (for example, NCR) and strong p-acceptor ligands in the trans positions are thermody- namically more stable for octahedral transition metal complexes in low oxidation states.Structures containing two strong p-acceptors, a weak p-acceptor and a s-donor or two weak p-acceptors in the trans positions are less stable than the corre- sponding cis isomers. In contrast, for complexes with metals in 277 Table 4. Relative energies of the trans- and cis isomers of the complexes 15 and 16. L DE=Etrans7Ecis /kJ mol71 MP2//HF HF//HF [(PH3)2(CO)2WL(:CCH=CH2)]+ 727.05 714.49 10.63 35.71 726.08 712.56 11.60 35.71 Cl7 NCMe CNMe CO [(H2PCH2CH2PH2)(CO)2WL(:CCH=CH2)]+ 718.34 4.81 36.68 NCMe CNMe CO high oxidation states, structures with s-donor ligands and weak p-acceptor ligands in the trans positions or with s-donor ligands and strong p-acceptor ligands in the cis positions appear to be stable.The question of whether these conclusions have a general character calls for further investigations. 2. Nucleophilic addition reactions Theoretical studies of the reactivities of compounds, in general, and complexes, in particular, are usually carried out with the use of frontier orbital theory.99 According to this theory, if the reactivity of the compound is determined predominantly by the energies and compositions of the highest occupied and lowest unoccupied MOs (the so-called frontier orbitals) whose interac- tions are responsible for the formation of new bonds, the reaction is orbital-controlled. If the reaction (in particular, nucleophilic and electrophilic addition) is determined predominantly by the distribution of the effective charges on the atoms involved in formation of a new chemical bond, the reaction is charge- controlled.Among various reactions with coordinated nitriles, the nuc- leophilic addition at the nitrile C:N group is of paramount importance. These reactions were considered in numerous exper- imental studies and reviews. The reactions of such nucleophiles as water, amines, alcohols,4, 100 ± 104 thiols,105, 106 phosphines,1077109 hydrazines 110 and oximes 111 ± 114 with nitrile complexes lead to the addition of the nucleophile at the carbon atom of the C:N group, which is generally accompanied by migration of the proton from the nucleophile to the nitrile nitrogen atom. Examples of the reactions of coordinated nitriles with a number of nucleophiles are presented in Scheme 1 (all products are arbitrarily shown in the E configurations).It was noted that coordinated nitriles are more reactive in nucleophilic addition reactions than the free ligands, the complexes of metals in high oxidation states being more reactive than those bearing metals in low oxidation states. Theoretical studies concerned with activation of nitriles with metals and the reactivities of coordinated nitriles in nucleophilic addition reactions are few in number. The reactions of oximes with nitriles and hydrolytic processes have received the most study (see Section IV.3). Activation of nitriles upon complex formation is often qualitatively explained by a decrease in the electron density on the carbon atom of nitrile upon its coordination to the metal atom and, as a consequence, by the enhancement of electrophilicity of the b-carbon atom thus facilitating the attack of the nucleophile on this atom.However, theoretical studies 43, 44, 69 demonstrated that this interpretation is in doubt. Thus it was found 69 that the effective charges on the C and N atoms of the nitrile ligand, which were calculated according to Lowdin scheme, are increased on going from free acetonitrile to the [PtII(NCMe)Cl3]7 complex (Table 5), i.e., the electron density on these atoms decreases due to which the ligand becomes more accessible to nucleophilic attack. However, calculations of the278 effective charges according to Mulliken gave another charge density distribution and opposite results.Thus the effective charge on the carbon atom of nitrile was found to decrease on going from NCMe to [PtII(NCMe)Cl3]7. The reactivities and activation of platinum nitrile complexes in the nucleophilic addition at the C:N triple bond were exam- ined.43, 44 The ab initio calculations of the effective charge distri- butions in free acetonitrile and three platinum complexes, viz., trans-[PtII(NCMe)2Cl2] (10), [PtIV(NCMe)Cl5]7 (11) and trans- [PtIV(NCMe)2Cl4] (12), by the HF and MP2 methods with differ- Table 6. Mulliken effective charges on the atoms calculated for the free NCMe ligand and the complexes 10 ± 12. Atom NCMe Ntrans-[Pt(NCMe)2Cl2] [Pt(NCMe)Cl5]7 trans-[Pt(NCMe)2Cl4] CPt Cl NCPt Cl NCPt Cl NC 0.67 70.24 70.39 0.19 a Stuttgart quasi-relativistic pseudopotentials and the corresponding basis sets on the Pt, C, N and Cl atoms (for the hydrogen atoms, the 6-31G basis set was used).b Stuttgart quasi-relativistic pseudopotentials and the corresponding basis sets on the Pt atom. [M] N C R1 ECP a 70.02 70.16 0.75 70.52 70.16 0.03 0.82 70.40;70.34 70.14 0.04 0.84 70.31 70.17 0.07 Scheme 1 OH C H2O [M] N R1 H R1 C HNR2R3 [M] N N R2 R3 H R1 C R2OH [M] N OR2 H R1 C R2SH [M] N SR2 H R1 C HON CR2R3 O [M] NH N C R2 R3 6-31G b 3-21G b 70.32 0.12 70.51 0.31 0.59 70.41 70.39 0.18 0.51 70.44 70.77 0.64 0.09 70.26;70.14 70.36 0.17 0.32 70.33;70.21 70.72 0.63 0.44 70.19 70.39 0.20 0.59 70.26 70.81 0.69 M L Kuznetsov Table 5.Effective charges on the atoms in the NCMe and [Pt(NCMe)Cl3]7 molecules. NCMe [Pt(NCMe)Cl3]7 Atom or ligand Ma La Ma Lb Mb La 70.10 70.05 0.15 0 70.08 70.05 0.11 0 70.09 70.13 70.05 70.04 0.17 0 0.14 0 NCCMe NCMe Pt Cltrans 0.11 0.13 0.20 0.44 71.15 70.05 70.12 0.18 70.16 0.10 0.12 0.43 70.49 70.53 Clcis Note. M is the analysis of the Mulliken populations, L is the analysis of populations according to Lowdin. a Without considering the electron correlation. b Taking into account the electron correlation. ent basis sets demonstrated that the effective positive charge on the carbon atom of nitrile calculated according to Mulliken changes only slightly on going from the free ligand to the complex and is virtually independent of both the oxidation state of the metal atom and the total charge on the complex ion (Table 6).Activation of nitriles in the complexes 10 ± 12 can be partially accounted for by the orbital control. In the context of the theory of frontier orbitals, the formation of a new chemical bond upon nucleophilic addition results from the interaction between the HOMO of the nucleophile and the LUMO of nitrile. In this case, the reactivity must depend on the relative energies of these orbitals. If the reaction involves the same nucleophile, the reac- tivity must be governed by the energy of the LUMO of nitrile.Actually, calculations showed that the energies of the LUMOs of the complexes 10 ± 12 are 1.5 ± 3.5 eV lower than the energy of the LUMO (p*-MO) of acetonitrile, which provides an explanation for the activation of MeCN in complexes. At the same time, the energy of the LUMO changes non-monotonically and insignif- icantly on going from 10 to 12 in spite of the noticeable enhance- DHb 6-31G* b 6-31G(d-Cl) b MP2 6-31G(d-Cl) b HF 70.09 70.05 7 70.25 70.45 0.30 0.11 7 0.27 70.36 0.06 0.01 0.62 70.43 70.45 0.34 0.24 70.31 70.25 0.19 0.66 70.43 70.38 0.18 0.02 70.26;70.15 0.07 70.05 0.30 70.31;70.18 70.42 0.32 0.34 70.31;70.18 70.35 0.15 70.18 70.22;70.11 70.20 0.16 0.29 70.19 0.10 70.03 0.59 70.23 70.42 0.34 0.07 70.15 70.21 0.20Theoretical studies of transition metal complexes with nitriles and isocyanides ment of the reactivity in this series of compounds.In addition, the LUMOs of the starting nitrile complexes do not virtually involve the orbitals of acetonitrile although the nucleophilic attack occurs at the carbon atom of the nitrile group (see Fig. 9 a). The nucleophilic addition reactions of oximes with the com- plexes 10 ¡À 12 were accompanied by a change in the geometry of the acetonitrile fragment from linear to angular, which corre- sponds to a change in hybridisation of the b-C atom from sp to sp2. Apparently, the change in hybridisation must cause the rearrange- ment of the valence molecular orbitals and substantial variations in their composition and energy.Analysis of the Walsh dia- grams 44 demonstrated that the change of the conformation of coordinated acetonitrile from linear to angular leads to a decrease in the energy of the LUMOs of the complexes by 0.3 ¡À 0.8 eV and to variations in the compositions of the lowest occupied MOs such that the orbitals of the carbon and nitrogen atoms of nitrile contribute significantly to the LUMOs (Fig. 9 b). According to the theory of frontier orbitals, this bent distortion of acetonitrile facilitates the nucleophilic addition at the b-C atom. With the aim of providing the more rigorous description of the relative reactivities of nitriles and their platinum complexes 10 ¡À 12, the activation and reaction energies of the interaction of formaldoxime with these complexes were calculated.It was found that the activation energy decreases and the absolute value of the reaction energy increases in the series NCMe¡À 10 ¡À 11 ¡À 12 (Table 7), which is consistent with the enhancement of the reactivity of these compounds. Hence, activation of nitriles in complexes and their relative reactivities can be described on the basis of kinetic (a decrease in the activation energy) and thermo- dynamic (an increase in the reaction energy) factors. Table 7. Activation energies of the rate-limiting step (Ea /kJ mol71) and the energy effects (DE /kJ mol71) of the reactions of NCMe and the complexes 10 ¡À 12 with formaldoxime.Compound Ea DE 275.16 265.53 253.93 237.52 NCMe 10 11 12 73.85 766.61 775.32 791.73 The mechanism of nucleophilic addition of oximes to acetoni- trile and the complexes 11 and 12 was examined in detail.43, 115 The possible transition states and the reaction coordinates were calculated. Under non-catalytic conditions, these reactions pro- ceed through the formation of the four-membered cyclic transi- tion state A (Scheme 2). Nucleophilic and electrophilic addition to coordinated nitriles and isocyanides at the C:N triple bond are of great practical importance because these reactions allow one to prepare com- Scheme 2 H ON CH2 6�� Me O H C [Pt] N C Me + N O N [Pt] N 11, 12 H2C H CH2 A Me Me C C N O CH2 O [Pt] N [Pt] N N H H CH2 [Pt]=PtCl¡¦5 (11), PtCl4(NCMe) (12).279 pounds possessing valuable properties, which cannot be synthes- ised without the use of transition metals. Nowadays, these processes call for further extensive theoretical investigations. 3. Catalytic reactions One of the most promising and practically important directions in the theoretical chemistry of transition metal complexes is con- cerned with catalytic reactions whose mechanisms often cannot be studied in detail without application of theoretical methods. Actually, the idea of metal complex catalysis assumes that intermediates formed in the course of the reaction are highly labile due to which they often cannot be detected experimentally.The mechanisms of catalytic reactions with the use of tran- sition metal complexes have been the subject of much investiga- tion, but the studies devoted to reactions involving nitrile complexes are few in number. Thus the mechanism of hydrolysis of acetonitrile adsorbed on Zn2+ ion-exchange zeolite was studied in detail by the DFT method.116 The reaction involves three main steps, viz., hydration, isomerisation and desorption of the prod- uct. The first step includes two different processes, viz., the initial activation of the reagents and the primary nucleophilic attack of the water molecule on Zn or nitrile. Of eight possible mechanisms of the first step (involving one water molecule), two mechanisms (A and B) shown in Scheme 3 are preferable (in this scheme and in Scheme 4, the calculated heats of formation of intermediates (in kJ mol71) are given below the formulae in parentheses).116 The first mechanism involves activation of nitrile upon coordination followed by the nucleophilic attack of the water molecule on the oxygen atom of zeolite and concerted elimination of the OH group with the simultaneous nucleophilic attack at the carbon atom of nitrile.The second mechanism (in the presence of an excess of water) consists in primary adsorption of the H2O molecule on zeolite followed by the cleavage of the O7H bond Scheme 3 H3CCNZn Zn CH3CN H2O O O O O (A) O O Si Al Si Al ( ¡À 125.5) (0.0)H3C H3C H C H C O O N N H H Zn Zn Ea=66.9 kJ mol71 O O O O O O Al Si Al Si ( ¡À 95.3) ( ¡À 147.8) HO H Zn Zn CH3CN H2O O (B) O O O O O Si Al Si Al (0.0) ( ¡À 92.2) CH3 CH3 H C C O H N N O H H Zn Zn Ea=25.8 kJ mol71 O O O O O O Si Al Si Al ( ¡À 87.7) ( ¡À 100.3)Æ 280 and migration of the proton on the oxygen atom of zeolite, coordination of acetonitrile to the zinc atom and migration of the OH group from metal to the carbon atom of nitrile.The second step (isomerisation) involves conformational changes of the iminol fragment and migration of the proton from the OH group to the N atom through the formation of a four-membered transition state (or a six-membered state with the participation of the water molecule). The third step, viz., desorption of acetamide, consists in the formation of the hydrogen bond between the hydrogen atom of the OH group and the new water molecule followed by the proton transfer from the water molecule to the nitrogen atom (Scheme 4) and desorption of acetamide by itself, which either occurs directly or proceeds with the participation of acetonitrile or water mole- cules.Scheme 4 Me Me O O C C H O H H N H N H H Zn Zn Ea=1.43 kJ mol71 H2O O O O O O O Si Al Si Al ( ± 62.6) (0.0) Me O C H O H H N H O Zn Zn O O O O + Me C O O ±H2O NH2 Si Al Si Al ( ± 98.0) The mechanism of nickel-cysed hydrogenation of nitriles giving rise to amines was proposed.117 According to the results of EHMOcalculations, the addition of the first hydrogen atom most probably leads to the attack at the carbon atom of nitrile to produce imidoethylidenes CH3CH=N7[Ni].Then either the second proton adds to the b-carbon atom to form the nitrene intermediate CH3CH27N=[Ni] or the imidoethylidene products undergo dimerisation. 4. Other reactions Hydrogenation of quinoline (Q) catalysed by [Ru(NCMe)2(CO). .H(PR3)2][BF4] was studied by the CNDO/2 method.118 The bond between the metal atom and the acetonitrile ligand, which is located in the trans position with respect to the hydrogen atom, is weaker than that with another acetonitrile ligand located in the cis position with respect to this hydrogen atom. This arrangement facilitates the replacement of the former nitrile ligand in the reaction with Q although the reaction at the latter ligand also Table 8.Relative energies of formation of the compounds 17 ± 21 and the transition state (TS) (kJ mol71). PM3 Compound 6-31G*//6-31G* a 6-31+G//6-31+G a MP2/6-31+G*// MP2/6-31+G* b 0.0 742.96 9.42 46.81 746.39 34.29 0.0 745.97 10.26 40.74 740.44 51.37 0.0 7107.10 72.80 18.92 768.08 42.08 NCMe+17 19 20 PS 21 18 0.0 (0.0) 742.79 (747.23) 7 713.98 (711.30) 763.60 (767.70) 24.24 (29.64) a The Hartree ± Fock approximation; b the values, which were corrected for the zero-point energy scaled by a factor of 0.91, are given in parentheses. M L Kuznetsov can proceed to a considerable extent. Based on the results of calculations and taking into account the data from kinetic and chemical studies,119 the mechanism of this reaction was suggested. In the experimental and theoretical investigation of the coordination transformations of the indenyl (Ind) ligand in the [(Z5-Ind)MoL2(CO)2]0/+ complexes (L are various ligands, including NCMe and CNMe),120 the Z5?Z3-transformation of indenyl due to the addition of one more acetonitrile molecule to the [(Z5-Ind)Mo(NCMe)2(CO)2]+ complex was considered {this reaction did not proceed with the [(Z5-Ind)Mo(CNMe)2(CO)2]+ complex}.The driving forces for these processes were discussed based on the results of DFT and EHMO calculations. The mechanism of the reaction of acetonitrile with dimeric lithium amide [LiNH2]2 (17) was examined by the MP2 and DFT methods and by the semiempirical methods (PM3 and MNDO).121 The reaction involves deprotonation of acetonitrile accompanied by the formation of the dianionic [(CH2CN)/ (NH2)]27.2Li+ complex (18) (Scheme 5).Scheme 5 Li NH2 MeCN + H2N Li Me C N Li NH2 Li NH2 19 17 H H H NH2 H H H H H H C C C H NH2 H N Li C C C Li N N N NH2 NH2 Li Li Li Li C 20 NH2 21 Li NH2+NH3 H2C C N Li 18 The first step of the reaction affords the adduct of NCMe with the dimer of lithium amide (adduct 19). This adduct is trans- formed into the `open' dimeric complex 20, which requires energy consumption of 47.7 kJ mol71 (the MP2/6-31+G*//MP2/ 6-31+G* level, Table 8). Then, the migration of the proton from the CH3 group of acetonitrile to the amino group occurs through the formation of a transition state (TS) to produce complex 21.Finally, elimination of NH3 gives rise to the reaction product 18. Calculations by the PM3 method demonstrated that the intramolecular mechanism (including solvation) is 37.7 kJ mol71 energetically more favourable than the intermo- lecular mechanism involving two dimeric molecules of lithium amide. The driving forces for the chemical, electrochemical and photochemical activation of CO2 by transition metal (Ir or Ni) complexes containing the isocyanide ligands were discussed rely- ing on analysis of frontier molecular orbitals.30DGauss/DZVP A1 b B3LYP/6-311+G*// B3LYP/6-311+G* b 0.0 (0.0) 742.45 (744.25) 74.14 (73.06) 71.42 (6.11) 763.81 (768.75) 23.36 (27.63) 0.0 (0.0) 749.82 (751.79) 74.14 (74.10) 8.37 (0.75) 755.43 (759.62) 49.91 (47.90)Theoretical studies of transition metal complexes with nitriles and isocyanides V.Conclusion To summarise, theoretical methods can successfully be used for the solution of practically important problems, such as investiga- tions of electronic and geometric structures, spectroscopic proper- ties, reactivities and reaction mechanisms of coordination compounds containing nitrile and isocyanide ligands. However, in spite of the fact that transition metal complexes with nitriles and isocyanides belong to an important class of coordination com- pounds (from both fundamental and applied standpoints), the theoretical chemistry of these compounds remains a poorly studied field of the modern coordination chemistry and applied quantum chemistry due, apparently, to the difficulties in describ- ing coordination bonds in isocyanide and, particularly, in nitrile complexes.In most cases, complexes with strong p-acceptor ligands, such as carbonyls, in which p-back donation most often prevails, are successfully described with the use of the Dewar ± Chatt ± Dun- canson (DCD) model. However, this model must be applied to the interpretation of the properties of isocyanide complexes and, particularly, of nitrile complexes with caution because, being weak p-acceptors, nitriles and often isocyanides, form coordina- tion bonds in which p-back donation is of secondary importance.The s-component and Coulombic interactions make a large (and often the major) contribution to the coordination bonds in these compounds. As a result, there are often no direct correlations of the properties of these complexes with the p-acceptor ability of the ligand or with the difference between the p-acceptor and s-donor properties. Studies of the reactivities of coordination compounds are of paramount importance in modern chemistry. This problem is most efficiently solved by the combined employment of exper- imental and theoretical methods. Early in the development of applied quantum chemistry, theoretical analysis of the reactivities was restricted to examination of the electronic structures of the starting compounds. However, recent studies demonstrated that detailed data on the reaction mechanisms with consideration for all possible transition states and intermediates are necessary for the correct description of the chemical properties. A knowledge of the mechanisms and driving forces for chemical reactions helps essentially in the design of chemical experiments and target- directed syntheses.Modern computation tools and theoretical methods allow one to study the reactivities of complexes using high-level approximations. There is no doubt that theoretical studies of transition metal complexes, including those with nitrile and isocyanide ligands, will attract ever increasing interest in the near future. References 1.V Yu Kukushkin, A J L Pombeiro Chem. Rev. 102 1771 (2002) 2. 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ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC

摘要:

Russian Chemical Reviews 71 (4) 283 ± 294 (2002) Microwave radiation in analytical chemistry: the scope and prospects for application I V Kubrakova Contents I. Introduction II. Some aspects of the influence of electromagnetic fields on solutions III. Specific features of chemical reactions in a microwave field IV. Microwave-assisted sample preparation in chemical analysis V. Conclusion Abstract. the on microwaves of effect the of aspects key The The key aspects of the effect of microwaves on the physicochemical heterogeneous and solutions in processes physicochemical processes in solutions and heterogeneous sys- sys- tems of use the for prospects Good analysed. are tems are analysed. Good prospects for the use of microwave microwave radiation are interaction of types various intensifying for radiation for intensifying various types of interaction are demon- demon- strated organic of hydrolysis dissolution, to relation in strated in relation to dissolution, hydrolysis of organic and and inorganic in ions metal by formation complex substances, inorganic substances, complex formation by metal ions in solution solution and compounds, organic of oxidation phase, sorbent the in and in the sorbent phase, oxidation of organic compounds, and and synthesis Specific compounds.organic and coordination of synthesis of coordination and organic compounds. Specific fea- fea- tures treatment microwave with operations analytical of tures of analytical operations with microwave treatment are are considered analytical of development of trends the and considered and the trends of development of analytical methods methods using 137 includes bibliography discussed.are microwaves using microwaves are discussed. The The bibliography includes 137 references. I. Introduction In recent years, a breakthrough has taken place in the develop- ment of instrumental methods used in inorganic and organic analysis, such as electrothermal atomic absorption spectrometry (ETAAS), inductively coupled plasma atomic emission spectrom- etry (ICP AES), inductively coupled plasma mass spectrometry (ICP MS), chromatographic and GC/MS techniques, etc. This is due to the need for more stringent control of technological processes, development of the analytical groundwork for the search for new raw material sources, aggravation of the environ- mental problems related to industrial environment pollution.Meanwhile, traditional methods used to prepare samples for analysis, which are time-consuming, comprise many stages and require large amounts of reagents, held up the elaboration of modern quick analytical procedures and hampered implementa- tion of the most important parameters of instrumental methods. Development of more advanced methods for preparing a sub- stance for determination of its composition has become a highly topical task in modern analytical chemistry. The efficiency of analysis is largely determined by the rate and completeness of the underlying physicochemical processes. The search for ways of affecting these processes and controlling them in various fields of chemistry and technology has resulted in I V Kubrakova V I Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul.Kosygina 19, 119991 Moscow, Russian Federation. Fax (7-095) 938 20 54. Tel. (7-095) 939 70 45, (7-095) 137 71 52. E-mail: i_kubrakova@mail.ru Received 15 October 2001 Uspekhi Khimii 71 (4) 327 ± 340 (2002); translated by Z P Bobkova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n04ABEH000699 283 283 286 289 291 extensive use of physical fields 1 and in the design of photo- chemical, catalytic and ultrasonic methods. Nevertheless, an increase in temperature has the most crucial effect on reactions in solutions and heterogeneous systems.However, slow heat transfer from the source to the sample and within the sample creates insurmountable obstacles which preclude substantial acceleration of processes under conditions of conventional (ther- mal) heating. An alternative way of raising the temperature of reaction mixtures is fast bulk heating on exposure to electro- magnetic radiation of the microwave range. Microwave heating differs from the traditional one by the greater volume and time gradients involved and by the fact that solutions and components of heterogeneous systems differing in composition possess unequal abilities to absorb the energy of radiation. In addition, charged species and dipoles present in the solution become oriented in the electromagnetic microwave field, which has an influence on their interaction.2 This changes the yields of reaction products 3 and gives rise to some specific effects 2 observed only under microwave heating conditions.No unambig- uous interpretations of these phenomena have been proposed; an approach to investigation of chemical processes induced by microwave radiation (MW) includes comparison of the reaction rates, the compositions and yields of the resulting compounds, and the activation energies of reactions proceeding under thermal and microwave heating.4 This review describes systematically the available data on the use ofMWfor the intensification of chemical reactions, first of all, those which underlie analytical operations.II. Some aspects of the influence of electromagnetic fields on solutions Numerous chemical reactions proceed in aqueous solutions or, at least, in the presence of water.5 The processes taking place in solutions, in particular, in aqueous ones, are mediated by inter- actions of the reactants with the solvent. These electrostatic interactions result in the formation of ions, solvation shells and assembled structures; this may prevent the initial substances from reacting with each other. The application of external physical fields, including electromagnetic fields, not only activates diffu- sion and mass transfer in solutions but also gives rise to additional effects upon electrostatic interactions, namely, it changes the degree of ordering of the solvent and the structure of solvation shells of ions, deforms the electron shells of atoms and molecules, facilitates reactions between species, increases the reaction effi- ciency, and accelerates chemical reactions.These phenomena can be of different natures such as the influence of the field on284 electrostatic interactions in solutions (an additional effect of external field), an increase in the system's intrinsic energy due to heating or the direct effect on charged species. An increase in temperature and the electric field effect have the most pronounced influence on the structure of water and solutions. Under exposure to MW, these factors are closely interrelated. Now we consider the mechanisms of microwave heating in more detail.Microwave radiation is a form of non-ionising electromagnetic radiation whose frequency is intermediate between the infrared (IR) and radio-frequency ranges, namely, 300 to 30 000 MHz (wavelengths from 1 m to 1 cm). Since these waves are actively used in radio- and telecommunications, for industrial purposes, in science and in medicine, four frequencies are stipulated by international rules, a frequency of 2450 MHz (12.2 cm) being utilised most often. In recent years, radiation of this type is so widely employed to accelerate chemical processes and to solve applied analytical problems that some researchers have spoken about a new line of research called microwave chemistry;2, 6 nevertheless, the data on the chemical aspects of the action ofMWon reaction systems are sparse. The mechanism of interaction of MW with matter involves absorption of the electromagnetic radiation energy by the material and scattering of the energy as heat.Although the microwave frequency corresponds to the rotational movement of molecules, in the condensed state where no free molecular rotations take place, as opposed to the gas phase, the absorption of energy immediately results in its redistribution between molecules and homogenous heating of the medium. Thus, microwave heating is a bulk phenomenon; the physical aspects of the effect of MW on a substance are described in detail in monographs dealing with the theory of dielectrics (see references in Ref. 2) and in some other studies.7±9 As MW passes through solutions, the transfer of energy occurs by two mechanisms, namely, due to re-orientation of the solvent dipoles in an alternating electric field and due to migration of ions of the solute, i.e., through dipole rotation and ion conduction.The induced thermal motion is superimposed onto the solvent structure disordering, which is due to the orienting action of the field. The significance of each factor is dictated by the nature of the process and the reactants. It is worth noting that back in 1933, Bernal and Fowler,10 introduced the notion of `structural temperature', according to which the influence of ions (i.e., charged species which form some electric field) on water (solvent) can be likened to the action of an increase or decrease in temperature.These views can be useful for the understanding of the specific features of chemical reactions under microwave heating when the same outcome is attained at the same or even lower temperature than upon thermal heating.2 The mechanism of dipole rotation is the major one in the case of pure water or other solvents with low degrees of dissociation. The heating rate and the temperature that can be obtained largely depend on the radiation frequency, the initial temperature of the solution and on the solvent properties, in particular, the dielectric constant. Figure 1 shows the experimental curves characterising the microwave heating of water and some organic solvents. The dielectric properties of free and bound (present as aggregates in a substance) water molecules are somewhat differ- ent;11 as a consequence, they possess different capacities for absorbing the microwave radiation energy.This difference can be clearly seen when examining drying in a microwave oven of ferrimanganese concretion samples (FMC),12 which are layered structures binding water molecules in different ways. When dried under conventional conditions, FMC initially lose the absorbed water and then the water incorporated in the structure of the minerals.13 A similar stepwise removal of water also occurs in a microwave field, and the curves for the loss of weight by the samples during microwave heating for several minutes reveal the presence of different H2O forms.This selectivity in the absorption of the radiation energy by the solvent molecules bound in different ways has also been used 11, 14, 15 in microwave extraction techni- P /atm 76 1 54 2 32 1 0 1 2 0 40 80 60 0 20 P /atm 3 3 04 12 1086420 300 200 100 Figure 1. Effect of microwave radiation on the variation in the temper- ature (T) of solvents (1 ± 5) and the pressure (P) of the vapour (1 0 ± 5 0 ) ina closed vessel: (1, 1 0) dimethyl sulfoxide; (2, 2 0) water; (3, 3 0) propan-2-ol; (4, 4 0) dichloromethane; (5, 5 0) hexane (t is the time of microwave treatment). ques.{ The stepwise pattern of variation of the absorptivity of aqueous solutions of alcohols, indicating the presence of different molecular structures, has also been noted in other publica- tions.16, 17 Yet another mechanism of the microwave heating is related to the effects of conduction, which may be due to electrophoretic ion migration in solution or the transfer of charged species in a solid upon application of an electromagnetic field.The contribution of the ion conduction mechanism in solutions is determined by the sizes, charges and the electrical conductivity of ions;8 in addition, this value depends on the interaction of ions with solvent molecules. The ion conductivity depends on the concentration, ion mobility and the solution temperature. Compounds with high dissociation constants absorb efficiently the microwave energy. The relative contribution of each of the two mechanisms of transformation of the MW energy (dipole rotation and ion conduction) depends substantially on the temperature.For small molecules such as water molecules or molecules of other solvents, the dielectric loss caused by the contribution of the dipole rotation { The term `microwave extraction' is often used in the literature to describe the extraction of useful components of different natures by any solvent on exposure to MW. Here, this term is used to imply the extraction of organic components by organic solvents. I V Kubrakova T /8C250 200 150 100 50 0 100 120 t /s T /8C 4 0 5 5 0 200 150 100 50 0 t /s 400Microwave radiation in analytical chemistry: the scope and prospects for application decreases with an increase in the temperature.8, 18 Conversely, the dielectric loss due to ion conduction increases with an increase in the sample temperature, and the effect ofMWon charged species becomes more significant.Due to the diversity of factors which influence the warming-up of a solution exposed toMW,modelling of this process is currently considered to be impossible,2 although a physical model that allows the calculation of the change in the solution temperature in an open system has been proposed, the mechanisms of gaining and losing energy through the solution have been analysed,19 and the theoretical aspects of the microwave heating of solids have been considered.20, 21 It is generally believed among the analysts who use laboratory microwave systems mainly for the routine digestion of samples that fast heating is the major or the only consequence of action of a microwave field on solutions.Indeed, theMWfrequency approx- imately corresponds to the frequency of the rotation of molecules but the energy of a quantum per mole (*1 J mol71) is too low to cleave not only chemical bonds (the energy of a C7H bond is 350 kJ mol71) but also hydrogen (*20 J mol71) and even van der Waals bonds (2.5 J mol71).11 However, regarding chemical transformations, the changes in the structures of the solvent and solvated ions are especially important because, as noted above, it is the solvation shells that hamper the occurrence of reactions involving ions and molecules; the electron density redistribution in molecules and conformational changes induced by an electro- magnetic field are equally important.It can be seen from Fig. 2 that an electromagnetic field brings about a substantial change in the state of the ion shell; as a consequence, the ion should become much more reactive and accessible for a nucleophilic attack. The electron density shifts very rapidly (in 10714 s) 23 (electron polarisation), and then it is `fixed' in the field with the given frequency over a period of 2.561079 s; the positions of the ions and molecules themselves change to a much lesser extent. During the indicated period, a field-activated ion present in aqueous solution can participate in 10 ± 103 collisions. This is important, for example, when micro- wave heating is used to accelerate substitution in the inner coordination spheres of complexes with inorganic ligands or in the molecules of organic compounds where steric factors and configurations of the reacting species are rather significant.In particular, acceleration of complexation caused by microwave 7 7 7 7 7 + 7 7 + + + 7 7 + 7 7 7 Figure 2. Change in the state of the ionic environment in a microwave field (a) and electrophoretic effect (b).22 a + 7 7 7 77 +7 7 b + + + + 77 285 treatment has been observed for various elements and sorbents in different sorption regimes at the same temperature as in the case of traditional heating.24 ± 26 Note that the displacement of the elec- tron density in an electromagnetic field does not depend on the temperature and can be attributed to so-called non-thermal effects.Direct evidence for the change in the state of solvated (hydrated) species in a microwave field has also been obtained. Circular dichroism measurements provided experimental data on the change in the spatial structure of protein molecules, which is known to be largely formed due to hydration of polar groups in the polypeptide chains by water molecules. Experiments carried out { at different temperatures with either thermal or microwave heating of an aqueous solution of erythropoietin demonstrated that thermal heating at 65 8C is equivalent to exposure toMWat 45 8C over a period of time which makes only 7% of the overall experiment duration (recall the Bernal and Fowler's structural temperature).We have also observed destruction of colloid particles of soil in anMWfield; thus the microstructure of the particles formed by hydrated layers was identified; the solution temperature did not exceed 50 8C. Microwave treatment can induce additional thermal effects related to the nature of the influence of the radiation on the system. In the case of conventional heating, the number of boiling centres in the solution bulk is much smaller than that near the surface. The microwave heating takes place from the inside, uniformly throughout the whole bulk; therefore, the temperature of the solution is higher than the temperature of the surroundings (vessel walls, the gas phase above the solution, etc.).27 As a result, the solution can be heated to a temperature exceeding its boiling point under atmospheric pressure, which has been shown exper- imentally using various physical methods 2, 3 (Table 1).Table 1. Change in the boiling point (b.p.) of liquids of different natures under microwave radiation conditions.2, 3 b.p. /K Solvent Db.p. /K microwave heating traditional heating 377 357 376 373 354 328 380 354 448 368 443 358 373 338 352 355 339 313 354 329 435 351 426 342 Water Methanol Ethanol Propan-2-ol Tetrahydrofuran Dichloromethane Acetonitrile Acetone Diglyme Ethyl acetate Dimethylformamide Diisopropyl ether 4 19 24 18 15 15 26 25 13 17 17 16 One more mechanism of interaction ofMWwith the matter is possible in heterogeneous systems, which is due to interface phenomena (interfacial polarisation). The thermal effects thus arising are used to heat mixtures of dielectrics with compounds that poorly absorb the MW energy.28 The non-uniformity of heating caused by different abilities of substances to absorb the MW energy gives rise to non-trivial results upon bulk removal of water, synthesis on solid surfaces and the preparation and use of catalysts.2, 29 The difference in the ability of the initial reactants {We carried out these studies together with N V Bovin (Institute of Bioorganic Chemistry, Russian Academy of Sciences).286 and reaction products to absorb the MW energy can give rise to self-catalysed reactions.30 The course of chemical processes and the composition of the resulting products are influenced by one more important factor, the time gradient of heating (DT/Dt), which is much higher in the case of microwave treatment than in the case of conventional heating of solutions. When several competing reactions with different rate constants take place simultaneously, the probabil- ities of formation of particular products in the very narrow temperature range under the action ofMWcan differ appreciably from those observed under conventional heating.The same is true for equilibrium processes in which the rate constants for the forward and back reactions are different.This fact has been considered 31 as the reason for the higher selectivity of some processes of organic synthesis under microwave heating. III. Specific features of chemical reactions in a microwave field This Section covers the main types of reactions widely used in analytical practice. 1. Hydrolysis of inorganic and organic compounds The effect of MW on reaction systems is most pronounced in the case where the radiation energy is absorbed directly by a reactant. In some cases, a reactant acts simultaneously as the reaction medium. Evidently, MW should have a substantial influence on hydrolysis because water efficiently absorbs energy of electro- magnetic radiation. Hydrolysis induced by MW has been used to synthesise organic compounds,32, 33 to cleave the peptide bonds in proteins when determining their amino acid composition,34, 35 and to study the composition of a number of other natural compounds.35, 36 Hydrolysis of inorganic compounds in a microwave field has been studied 37 in relation to the exceptionally stable polynuclear iridium sulfate complexes.The uncertainty in elucidation of the composition of these complexes is the main cause for the analyt- ical problems arising during determination of iridium in sulfate solutions by various methods. The mononuclear chloride complex of iridium was prepared upon hydrolysis and destruction of the iridium sulfate complex on exposure to MW with subsequent transformation of the resulting aqua hydroxo complex into the chloride complex on treatment with NaCl in HCl.Microwave irradiation accelerates deprotonation and hydrolysis. The degree of transformation of sulfate complexes of iridium into the chloride complexes is linearly dependent on temperature, which indicates a substantial contribution of thermal effects to acceleration of the reaction. However, it has not yet been explained why the metal is stabilised in the higher oxidation state under the action of MW. The rigorous conditions of hydrolysis, namely, concentrated acid and elevated temperature, ensure the cleavage of most of hydrolysable bonds in organic compounds. However, in many cases, these conditions also promote undesirable side processes, in particular, destruction of either initial compounds or reaction products.The efficiency of using microwave treatment for performing such reactions was first demonstrated by Majetich andHicks.38 Of special importance is the choice of conditions for mild and, simultaneously, selective hydrolysis for the removal of protective groups. Under microwave heating, the occurrence of these reactions is markedly facilitated.33 Microwave-enhanced hydrolysis has been used for analyt- ical purposes in the synthesis of macrocyclic compounds 39 that have found application as reagents for determination of metals. Acid hydrolysis of p-toluenesulfonyl protective groups under conditions of thermal or microwave heating has been studied in relation to a number of macrocyclic compounds and their linear fragments.40, 41 The possibility of stepwise hydrolysis under the action of radiation and, hence, targeted control of reaction products has been demonstrated.Compounds have been syn- thesised that could not be prepared in the given reaction system I V Kubrakova upon thermal heating. The achieved effects are caused, most likely, by the sharp increase in the temperature of the reaction mixture under conditions of microwave heating (to 200 8C for 2 min), which is due, to some extent, to the absorption of the radiation energy by the substrate;4 according to some views,42 this substantially activates the reactions. 2. Complexation of metal ions with organic reagents The kinetic inertness of the coordination compounds of a number of metals towards substitution is often an obstacle hampering the practical use of complexation reactions.43The use ofMWfor the solution of various problems related to complexation has been described only in a few publications.For example, it was demonstrated that coordination compounds of transition and noble metals, in particular, chromium, ruthenium, iridium, platinum and gold, with organic ligands can be synthesised efficiently under microwave heating.44, 45 The analytical aspects of complexation of inorganic ions in solutions and in the phase of complexing sorbents under the action of MW (sorptional collec- tion of palladium and rhodium 46 and chromium26 using nitro- gen-containing sorbents, the formation of complexes by the platinum metal and chromium in solutions) have been consid- ered.25, 47 ± 50 A reaction with microwave heating between cad- mium or cobalt ions and immobilised reagents, bromothiazo- and 1-(2-pyridylazo)-2-naphthols,51 prepared using MW, has been described.52 The possibility of synthesising coloured com- pounds of platinum metals for spectrophotometric determina- tion of metals in a flow system on exposure to radiation has been demonstrated.53, 54 A study55 has been devoted to the targeted synthesis of binuclear tetraacetate, the initial compound for the coulometric determination of rhodium, in the solution to be analysed.The general regularities of MW-assisted complexation identified for the above-listed systems are considered below.a. Complex formation in solutions The reactions of metal ions with nitrogen-, sulfur- or oxygen- containing reagents under routine conditions can involve ligand desolvation and conformational changes, hydrolysis and desolva- tion of metal ions, changes in the metal oxidation state (usually, reduction to a more labile state), and the formation of a complex (metal ± ligand interaction), which is complicated in some cases by the formation of polynuclear complexes.47, 49, 56 For complexation under microwave heating conditions, it is most often impossible to separate the contributions of thermal effects and the field effects on the electrostatic interactions of particles. According to the Arrhenius equation, the rate constants of the slowest reactions increase most appreciably as the temperature rises.The orienting action of the field can play a crucial role in the case of sterically hindered transformations, for example, those involving large molecules of organic compounds which react with metal ions according to the nucleophilic substitution mechanism. The most comprehensive information on the intensification of complexation under the action of MW has been obtained for platinum metal compounds (Table 2) some chemical forms of which are extremely inert towards ligand substitution.43 It was found that radiation substantially intensifies the reduction of metals (at the same temperature),25, 50, 57 markedly shortens the reaction time and often makes it possible to exclude the use of stabilising additives.The composition of complexes thus formed does not change. The labilising action of MW is so pronounced that the efficiency of complexation does not depend on the initial form of the metal. This allows the use of MW for reagent-free activation of inert coordination compounds.58 b. Complex formation in the sorbent phase The sorption of metal ions on sorbents with fixed complexing groups depends not only on the chemical reaction but also on the diffusion of metal ions both from the solution to the sorbent (outer diffusion) and in the sorbent pores (inner diffusion). It was noted that in the reaction of cobalt ions with the modified silicicMicrowave radiation in analytical chemistry: the scope and prospects for application Table 2.Effect of microwave radiation on the time (t ) of metal complexation with organic reagents in solution. Reagent Metal Hypothetical composition of the complex PdR2 unknown the same """[Ir(H2O)R3]2+ [OsR3]2+ [RuR3]2+ [RuCl4R2]7 [AlR3]3+ chlorophosphonazo-mN 5-chloro-2-pyridylazo-2,4-diaminobenzene the same """2,20-bipyridine 1,10-phenanthroline the same sulfochlorophenolazorhodanine sulfonaphtholazorhodanine Pd(II) Rh(III) Ru(III) Os(IV) Ir(IV) Pt(IV) Ir(IV) Os(IV) Ru(IV) Ru(IV) Al(III) a Complexation in the presence of a reducing agent. b Complexation in the presence of an acetate buffer solution. acid xerogel, microwave heating increases the rates of both the outer and inner diffusion.51 Acceleration of diffusion processes on treatment with physical fields of various natures has also been described in other publications.20, 21, 59 ± 61 The extraction of noble metals by the POLYORGS IV sorbent containing 3(5)-methylpyrazole groups has been studied under static conditions.12, 46 The action of MW substantially accelerates the process (which takes place at the solution boiling point) both for rhodium, which reacts with sorbent groups by a complex-formation mechanism,62 and for palladium and plati- num (which react by the ion-exchange mechanism }).Similar results have been obtained for sorption on POLYORGS XI-H,24 the degree of rhodium collection in a microwave field being higher than that with conventional heating.The observed acceleration of sorption has been explained by the influence of the electromag- netic field on the state of the complex ion and electrostatic interactions in solution. Under conventional conditions, similar results can be attained by adding alkali metal salts, which destroy the structure of the solvation shells of ions; however, the effect of a microwave field is much greater.12 Microwave treatment has markedly enhanced the efficiency of collection of chromium ions, especially Cr(VI), by the DETATA sorbent.26, 63 Almost complete separate extraction of Cr(III) and Cr(VI) from solutions with pH 7 and 3, respectively, can be attained only using MW. An EPR study of the possible changes in the Cr(III) coordination sphere during the reaction with the aminocarboxy groups of the sorbent showed that the spectra of the complexes obtained at room temperature with conventional heating and in a microwave field are identical.This indicates that the same complex spieces of Cr(III) are formed under different conditions and that the factors influencing the reaction rate make the predominant contribution to the increase in the efficiency of sorption extraction of chromium on exposure to MW. Obviously, the same factors increase the efficiency of preconcentration of chromium ions by sorption in a flow regime and make it possible to increase the solution flow rate from 0.4 ± 0.5 to 10 ml min71. Similar results have been obtained in a study of the reaction of cadmium ions with silicic acid xerogels modified by organic reagents.51 3.Oxidation of organic compounds by inorganic acids Oxidation of an organic matrix with nitric acid is a widely used analytical operation. Study of the mechanism of oxidation of organic compounds including natural products in a microwave field 64 ± 67 showed that, as in the case of traditional heating, the } Acceleration of cation exchange processes has also been described by Mingos and Baghurst.7 287 Ref. t /min microwave treatment usual conditions 54 53 53 53 53 53 47 ± 49 47 47 25, 48 25, 48 0.5 0.5 a 0.5 a 0.5 a 0.5 a 0.5 a 10 15644 1 does not react the same """220 180 45 30 b 60 b major reaction products include, apart from carbon dioxide and water, nitro-substituted aromatic acids, which decompose at temperatures above 260 8C.The same composition of the oxida- tion products obtained in the conventional and microwave heat- ing modes suggests that the reactions follow the same pathways. However, the rates of oxidation are substantially different: decomposition of organic matrices at the same temperature is much faster with microwave heating than with traditional heating both in open and closed systems.67, 68 No unambiguous interpre- tation of this fact can be found in the literature; therefore, it seems important to compare the quantitative characteristics, for exam- ple, the activation energies (Ea) of the processes that take place under different conditions.The first attempt to solve this compli- cated experimental problem was made by Pratt,69 who showed that the Ea values for the oxidation of amino acids (phenylalanine and tryptophan) with nitric acid under conditions of conventional and microwave heating are equal.} Bacci et al.71 were the first to use heating of the reaction system by microwave radiation when determining the thermodynamic functions and kinetic parameters. Microwave heating at a rate of 6.0 ± 7.5 K min71 was combined with simple photometric record- ing of the absorbance of the resulting complex. This technique appears efficient because the experiment can be carried out quickly and no temperature gradients exist in the solution; however, special experimental equipment is required.In recent years, continuous temperature variation 72 has been used to estimate the activation energies of various processes ranging from enzymic reactions to evaporation of atoms in electrothermal atomic-absorption spectrometry (dynamic method). This method is based on a logarithmic dependence of a parameter proportional to the concentration of an element or a substance in the analytical volume on the reciprocal temperature. This dependence is used to calculate the activation energy of the process using the Arrhenius equation from the slope of the straight line plotted in the lnP ± T71 coordinates, where P is the sought- for parameter (in particular, pressure) and T is absolute temper- } Only sparse data on determination of Ea under conditions of microwave treatment can be found in the literature.The researchers' opinions concerning the coincidence of the activation energies of processes carried out with traditional and microwave types of heating are different. For example, the activation energy for sintering of alumina calculated for microwave heating 70 is several times lower than the Ea value for this process carried out with traditional heating. Apparently, this is due to the specific character of microwave heating, namely, high temperature and time gradients and a change in the ratio of contributions of various reactions to the overall process under these conditions. A refined mathe- matical model describing the temperature ± time distribution in solid metal oxides heated byMWhas been reported.20288 ature. An important aspect in using the dynamic method is substantiation of the applicability of experimental results to the construction of the Arrhenius plot and determination of the thermodynamic characteristics of the processes. Modern commercially produced laboratory microwave sys- tems are equipped with fibre-optics sensors for direct measure- ment of the temperature of the reaction mixture, by monitoring systems of the pressure in the vessel and facilities for data record- ing in graphical and digital forms at 1 ± 30 s intervals.Hence, this equipment can be used to study processes during which the pressure in the system varies together with the temperature, in particular, due to the formation of gaseous reaction products.This formed the basis of a mathematical model for the kinetics of a chemical reaction 4 proceeding in a closed vessel under the action of MW, which was developed in the simple approximation of homogeneous distribution of the temperature and pressure in the system. The above-considered dynamic method for determination of the activation energy for the oxidation of organic compounds by nitric acid from the experimental exponential dependence of the system pressure on the reciprocal temperature of the reaction mixture was substantiated. Microwave oxidation of individual organic compounds (amino acids, stearic acid, monosaccharides) forming the basis for the most important natural products (proteins, fats and carbohydrates) has been studied.73, 74 Figure 3 shows the temper- ature and pressure variation in the system vs.time during the oxidation of phenylalanine by nitric acid; Figure 4 presents the Arrhenius dependence of the total pressure in the system, which is proportional, in the chosen approximation,4 to the rate constant T /8C P /atm 20 200 1 10 150 2 100 0 t /s 400 200 Figure 3. Effect of microwave radiation on the variation of temperature (1 ) and pressure (2) in a closed vessel during oxidation of phenylalanine by nitric acid.74 ln P 42 1000/T 2.4 2.2 Figure 4. Arrhenius dependence of pressure on temperature during oxidation of phenylalanine by nitric acid under conditions of microwave heating (Ea=50.1 kJ mol71).74 I V Kubrakova Table 3.Experimental values of the activation energy (Ea) for the oxidation of organic compounds determined under microwave heating conditions.74 Compound Class Ea /kJ mol71 Sugars Amino acids 73.3 73.5 24.0 50.1 (50.0 a) 32.0 (29.8 a) 76.4 saccharose galactose lysine phenylalanine tryptophan stearic acid Fatty acids a The data were obtained with electrochemical detection of the products of decomposition of amino acids at constant temperatures (static method).69 for the oxidation, on the reciprocal temperature. The data for the calculation of the activation energies for the oxidation of some model compounds and natural products are listed in Table 3. These values fall in the normal range of the Ea values typical of reactions of this type carried out with traditional heating (40 ± 120 kJ mol71) 75 and are correlated with the strengths of chemical bonds in the molecules of these compounds.The approach to the determination of the activation energy of oxidation used in this work was implemented within the frame- work of formal kinetics and the views of the collision theory in which the pre-exponential factor A=zN in the Arrhenius equa- tion k=Aexp [7Ea/(RT)] reflects the influence of the steric factors (N) and the number of collisions (z). In addition, this approach does not contradict interpretation of the results obtained in terms of the transition state theory. In this case, the activation energy correlates with the enthalpy of formation of the activated complex, and the pre-exponential factor is correlated with entropy.The former correlation leads tacitly to a fundamen- tally important conclusion stating that the reaction pathway remains invariant irrespective of whether it is induced by micro- wave or traditional thermal heating, and the latter correlation attests to an important role of the entropy factor, i.e., the orienting influence of the electrical component of a microwave field on the reacting species. Thus, acceleration of chemical reactions by microwave radiation is due to kinetic factors. Hence, microwave heating can be used not only to affect physicochemical processes in solutions or heterogeneous systems but also for fast determi- nation of the quantitative characteristics of reactions and model- ling of kinetically hindered transformations. 4.Synthesis of organic compounds Within the scope of this review, which is mainly devoted to the processes used in the chemical analysis and accelerated by micro- waves, it is also pertinent to touch upon some aspects of organic synthesis. First, for many reactions involving organic compounds, the reaction mechanisms, the roles of the solvent and the compo- sition of the reaction mixture and reaction conditions have been studied in sufficient detail. This makes easier the identification of the effects caused by the additional external treatment and promotes the understanding of the general mechanisms of the microwave influence on chemical transformations.Most of the results unexplainable in terms of thermal effects alone and most studies analysing the patterns of MW influence on the course of reactions refer to the processes of organic synthesis.2, 3, 42 Second, diverse compounds of the organic nature (solvents, extractants, polymer-based sorbents, complexing reagents including macro- heterocycles) play an enormous role in analytical research, and development of methods for preparing them remains a topical task. The extensive use of MW for synthetic purposes started in 1986.76, 77 Acceleration in a microwave field of many organic reactions such as elimination, esterification, cycloaddition, iso- merisation, substitution, hydrolysis, dehydration, condensation, etc., has been noted.2, 3, 42 In some cases, the reaction timeMicrowave radiation in analytical chemistry: the scope and prospects for application determined on the basis of the product yield can be shortened by three orders of magnitude.78 The reaction conditions are excep- tionally diverse � at atmospheric or elevated pressure, in a flow, in aqueous or non-aqueous solutions, on a solid surface 2, 42, 79 and in the solid phase.80 A laboratory setup has been elaborated for microwave organic synthesis 81 carried out at high pressures (up to 100 atm) and at a temperature of up to 260 8C.The decrease in the solution viscosity, heating of the solution to a temperature higher than the boiling point, change in the electron density distribution in the molecules of organic com- pounds and in the contribution of the products of competing reactions are, apparently, the main factors inducing the effects observed when organic reactions are carried out under microwave radiation.It has been shown 82 that the kinetics and the order of reactions do not depend on the method of heating. However, some abnormal features may arise on exposure to MW.83 In competing reversible reactions,31 the rate of warming-up of the reaction mixture plays the most important role because the ratios of the products formed are different at different temperatures. Fast microwave heating allows one to perform the reaction at a strictly specified temperature and to minimise the duration of the temper- ature variation step and, hence, to avoid conditions that may bring about e formation of undesirable products.In some cases, the abnormally high selectivity and substantial yields of the reaction products for the reactions taking place in the presence of catalysts or on supports after the removal of a solvent, can be explained by the selective heating of separate sections of the surface on which the reaction takes place, due to the microwave irradiation.Asharp increase in the yield (from 2% to 92%) of the product of intra- molecular rearrangement of unsaturated ketones has been noted 84 (the results were obtained from conventional and microwave heating, respectively, at the same temperature; the publication cited contains no kinetic data 84). The synthesis of acetylene in a yield of >90% by microwave cracking of benzene has been reported 85 (the thermal cracking of benzene scarcely takes place).The reaction mode also has a certain influence on the product yields. For example, the increase in the yields of products in some organic reactions induced by microwave radiation was reported to be more pronounced in the flowmode than in closed vessels.86 This fact was attributed 2 to the higher surface area of the reaction mixture in the case of flow systems, because the action ofMWon the surface increases the contribution of the interfacial polar- isation mechanism. It has been suggested in a review 42 generalising the results of a large number of synthetic studies dealing with the use ofMWthat, if the radiation energy is absorbed only by the solvent, the acceleration of reactions with respect to those proceeding by conventional heating will not be significant.If the radiation interacts with either the reactants or intermediate compounds, the increase in the reaction rate can be appreciable. The absorp- tion of the radiation energy by not only the solvent but also the reacting substances is observed, in particular, for polymerisation processes.87 ± 89 This makes it possible to control the conditions of synthesis with high accuracy and to reduce the energy expenditure by a factor of 10 ± 20. The polymers produced in this way often possess better physical properties, for example, higher strengths,3 than the polymers synthesised with conventional thermal heat- ing,42 although the structures of both types of polymer are similar.The conditions used to conduct reactions are diverse, and the outcome depends appreciably on the type of the process, the nature of the solvent and the reactants, the radiation power and theway of irradiation,42 because these factors determine the degree of action of MW on the reaction system. For example, variation of the conditions of emulsion polymerisation of acrylates allows, apart from shortening of the polymerisation time by a factor of 60,90 the synthesis of poly(p-nitrophenyl acrylates) with molecular masses ranging from 40 000 to 180 000. 289 IV. Microwave-assisted sample preparation in chemical analysis The preparation of samples for the measurement of the analytical signal from a particular component can include drying, ashing, mineralisation, decomposition (dissolution) and preconcentration (matrix separation).The sample pretreatment operations are based on various chemical reactions and physicochemical inter- actions involving components of the sample. Many of these reactions (hydrolysis, oxidation, complexation) have been consid- ered above; their typical features observed under microwave heating also remain valid for sample pretreatment processes. Despite the obvious similarity of the occurrence of chemical reactions under thermal and microwave heating, significant addi- tional effects can be attained in the latter case by using the specificity of warming-up induced by radiation.The development of microwave-assisted sample preparation techniques has been the subject of hundreds of papers, two monographs 2, 8 and several reviews.11, 15, 18, 58, 63, 91 ± 95 1. Drying Owing to the direct absorption of the MW energy by H2O molecules, heating and vaporisation of water present in the pores of a material proceed much faster than under conventional conditions. Unlike the situation with traditional heating, warm- ing-up of the material becomes less pronounced as water is removed, and by the end of the process, the sample temperature decreases. It is expedient to carry out microwave-induced drying for individual substances, for example, in the preparation of inor- ganic specimens,96 or in those cases where the accuracy of analysis is not subject to stringent requirements.This is due to the fact that it is difficult to select conditions for the microwave drying of materials with mixed or variable composition 97 because water molecules bound to the matrix in different manners absorb the MWenergy to different extents. If a determination reproducibility equal to several percent is admissible, drying in microwave ovens has a substantial effect.2, 12, 98 The results of early studies on the use of microwave drying are summarised in a monograph,2 and the most recent achievements along this line have been reported.99 An interesting way of using MW for drying of silicic acid gels in the preparation of modified xerogels has been proposed.51 2.Ashing and fusion These operations involve no direct action ofMWon the substance under analysis; as in the case of conventional heating, the material is warmed up by the muffle furnace walls, which are heated extremely rapidly (in several minutes) to 1200 8C due to the efficient absorption of the radiation energy by silicon carbide, which is the material of the internal walls.8 As a consequence, the time required for the furnace to be heated is markedly shortened and the efficiency of the method increases. The duration of the ashing decreases several-fold because of the use of forced con- vection; fusion can also be carried out 1.5 ± 2.0 times faster. 3. Digestion One of the most important applications of microwave systems in analysis is to decompose samples having highly diverse composi- tions and origins.Since the first publication 100 dealing with the use of a microwave oven to accelerate the sample pretreatment appeared in 1975, numerous expedients and techniques of micro- wave decomposition suitable for virtually any type of object have been developed; they are covered in the above-mentioned mono- graphs 2, 8 and reviews.18, 58, 63, 91 ± 95 We shall note only typical features of this method for sample digestion. Decomposition of substances on treatment with inorganic acids is a heterogeneous reaction the rate of which can be determined by either diffusion processes or interactions at the solid ± solution interface. In both cases, treatment with MW substantially accelerates the processes involved.The regularities290 considered above established for individual reactions (shortening of the reaction time, decrease in the reactant ratio down to stoichiometric amounts, an increase in the completeness of reactions) are clearly manifested in decomposition processes. Microwave digestion is carried out using the same reagents (most often, acids and their mixtures) as that induced by conven- tional heating.{ Treatment can be carried out both in usual open vessels made of materials which slightly absorbMW(glass, quartz or fluoroplastic) and in closed systems at elevated temperatures and pressures. This allows the mixture under analysis to be heated to a temperature markedly exceeding the boiling point of the used acid under normal pressure; therefore, both the rate and the degree of transfer of the sample into a solution become much higher.An important advantage of microwave-assisted digestion in closed vessels is low pressure with a relatively high temperature (see Figs 1, 3), due to the fact that radiation causes heating of the liquid but not of the gas phase. Since the temperature of decomposition dictates the range of applicable acids, an increase in this temperature often makes it possible to replace high-boiling perchloric, sulfuric or phosphoric acid by hydrochloric or nitric acid, which are more convenient for digestion. In some cases, the higher degree of sample dissolution at higher temperatures makes it possible to eliminate the stage of fusion of the residue after sample dissolution in acids.It is fairly useful to apply microwave heating for decomposi- tion of organic matrices (biological and clinical objects, pharma- ceutical preparations, foodstuffs, forages, food raw materials and ecosystem components). In this case, the time of analysis can be appreciably reduced. Decomposition of organic matrices can be carried out in both closed and open systems. The use of open systems is recommended for working with large amounts (up to 10 g) of materials when considerable amounts of gaseous prod- ucts of oxidation are evolved during the process. In combination with modern, highly sensitive methods of analysis (for example, atomic spectroscopy techniques such as ETAAS, ICP AES or ICP MS) in which determination of elements at a level of 1076%±1075% can be accomplished using fractions of a gram of the material, closed or flow systems are used more frequently.Flow setups represent a step towards automation of the analysis, attained by connecting a flow system for sample digestion to the injecting device of a spectrometer. The samples are commonly introduced in the systems as suspensions in acids.101, 102 The publications describing the use of microwave heating for decom- position of organic matrices are devoted, most of all, to determi- { However, due to specific features of microwave heating and the change in the kinetics of chemical reactions, the conditions chosen for thermal digestion can be regarded only as the starting point in microwave procedures.Table 4. Comparison of the times of digestion of various samples in the conventional and microwave heating modes.4, 22 Microwave digestion conditions Sample type t1 /h a HNO3 or its mixtures with HF and HCl, closed systems HNO3, HCl, closed systems HF and HCl, HF and HNO3, closed or open systems HNO3, closed systems 864 ± 16 8 ± 16 2 ± 6 H2SO4, open systems H2SO4, H3PO4, closed systems HCl, HF, HNO3, closed systems HF, HNO3, closed systems HF, HNO3, closed systems HNO3, closed systems 24188 ± 24 6 Biological objects (trace components) Geological objects (macrocomposition) Geological objects (trace components) Objects with an organic matrix (trace components) Biological objects, Kjeldahl determination of nitrogen Alumina Steels Soils, dust High-purity compounds Waste water a Time of traditional sample pretreatment.b Time of microwave pretreatment. I V Kubrakova nation of trace elements and identification of the species contain- ing them in environmental and biological objects by instrumental methods.18, 26, 65 ± 67, 103 ± 106 Decomposition of inorganic matrices often requires drastic conditions; therefore, for this purpose, it is expedient to use closed vessels, which are applicable up to a temperature of*350 8C and a pressure of 200 atm.107 ± 109 These conditions may ensure dis- solution of the most complex geological specimens (for example, those containing noble metals), poorly soluble oxides (such as niobium and tantalum oxides), metals, alloys or catalysts.18, 110, 111 The use of microwave setups including automated or robotised ones acquires special significance in the analysis of radioactive specimens.112 Data on the time required for preparation of some substances for the analysis using traditional and microwave heating procedures are compared in Table 4.The main causes for the intensification of sample digestion in the case of microwave treatment include high rate and uniformity of heating of the reaction mixture; exceeding of the boiling point corresponding to the given pressure; and local effects (warming- up of the solvent in the sample pores, i.e., actually, in a confined space, and sample destruction under high pressure; selective heating of components of heterogeneous systems; reflection of the radiation to the near-surface layer of the solution in the case of digestion of metallic samples).Indeed, according to our data, the dissolution of gold alloys under microwave heating is twice as fast as that at the same temperature achieved by conventional heating. The specific features of microwave digestion have a substan- tial influence on the metrological characteristics of the results. Due to good reproducibility of decomposition conditions, it is possible to obtain solutions of virtually identical compositions from identical samples, which is favourable for the reproducibility of the subsequent determination. The work with stoichiometric or nearly stoichiometric reactant ratios leads to a considerable decrease in the blank correction. A decrease in this value is also attained owing to use of closed or flow systems, which eliminates the loss of the analyte as volatile compounds and the ingress of contaminants from air; in combination with the use of high-purity chemicals, this brings the analysis closer to the `clean room' conditions.113 In addition, simplification of the composition of the reaction mixture allows some sample pretreatment operations to be avoided, which also improves the metrological character- istics of the analysis as a whole.Mention should be made of high productivity and economic efficiency of the microwave techni- ques, and their flexibility in combination with preconcentration and instrumental determination.2, 4, 58 The most promising approach to the substantiated selection of digestion conditions consists in modelling with the use of individ- ual compounds of various classes.This approach was used, for example, to establish the conditions for the oxidation of model organic compounds by nitric acid. Kingston and Jassie 114 showed t1 / t2 t2 /min b 1.5 ± 30 4 10 ± 20 10 ± 20 16 ± 100 90 24 ± 32 24 ± 64 3 ± 6 20 401 ± 2 20 12 10 36 45 24 40 ± 120 60Microwave radiation in analytical chemistry: the scope and prospects for application that carbohydrates, proteins and fats are oxidised on exposure to MW at 140, 150 and 160 8C, respectively. Later, the activation energies for the oxidation of a number of model organic com- pounds were determined; these data were shown to be suitable as the basis in choosing for conditions for decomposition of natural products.74 The principles of selection of the conditions for microwave decomposition depending on the sample weight, the rate of temperature rise, the volume of the reaction mixture and the total volume of the vessel have been considered in detail by Knapp.115 4.Preconcentration of traces of elements Exposure to radiation permits fast preconcentration of trace amounts of metals by evaporation 99 or by using organic sorbents that react with metal ions according to complex-formation or ion- exchange mechanism. Preconcentration by sorption is carried out in either the static or dynamic mode.Preconcentration of plati- num group metals and chromium on nitrogen-containing com- plexing sorbents has been reported.104, 110 5. Identification of the element forms (speciation) Analysis with the use of MW, which provides simultaneously specific chemical treatment and fast heating of the system, permits selective isolation of analytical forms of elements without chang- ing their chemical composition in the sample. The compounds are collected using organic solvents or mixtures based on inorganic acids.116 This expedient was used for mass-spectrometric identi- fication of chemical species containing mercury,117 tin 118 and heavy metals.119 6. Extraction of organic impurities Of special interest are recent studies dealing with the selective microwave-assisted extraction of toxic organic compounds into organic solvents.11 These studies would undoubtedly serve as a significant stage in the development of organic analysis methods, first of all, chromatographic techniques.The advantages (consid- erable acceleration and an increase in the degree of extraction of the organic impurities to be determined) are caused in this case by the increase in the temperature of the extraction system by several tens of degrees. A highly important specific feature of microwave extraction is the fact that this method allows selective collection of components either through selective heating of the phase or a separate component of the system (for example, free water molecules in plant cells), depending on its dielectric properties, or through choosing an organic solvent that would selectively extract one or another component.In addition, the consumption of organic solvents and, hence, the amount of toxic wastes markedly decrease, which is beneficial from the environmental standpoint.14, 15 An interesting version of microwave-assisted extraction is the withdrawal of volatile organic compounds to the gas phase through microwave heating of a matrix containing a polar solvent (water). The gas phase which does not absorbMWdoes not warm up, and under certain conditions, it virtually does not contain solvent vapour due to the non-equilibrium character of the system. Determination of the impurities of relatively volatile organic compounds in solid matrices such as soils is also possible; the most polar substances are the first to be evaporated.Upon irradiation of solid materials containing moisture, the selectivity of determination is due to the separate transition to the gas phase of components characterised by different vapour pressures.11, 14 The principles of selection of the conditions for the extraction of organic impurities (biologically active nutritional compounds, antioxidants, UV stabilisers of plastics, pesticides, polyaromatic hydrocarbons, phenols, organometallic compounds and other contaminants) in microwave systems and comparison of the results with traditional methods have been reported.11, 14, 15 291 7.Determination of the contents of total nitrogen and phosphorus Under conditions of traditional heating, sample digestion, which is the limiting step in determination of the total nitrogen, takes 2 to 6 h. Since sulfuric acid, used in the Kjeldahl method, efficiently absorbs microwave radiation, the time it takes for the reaction mixture to warm up to 400 8C can be shortened from 45 min to 3 ± 6 min. The introduction of an additional oxidant (Caro's acid) and the use of special techniques make it possible to reduce both the duration of decomposition of the organic matrix and the time of cooling the mixture and also to simplify titration. The problems of the traditional Kjeldahl determination of nitrogen and the capabilities of the microwave modification of this analysis have been considered quite comprehensively in a publication.120 The results of application of microwave methods for determination of nitrogen 121 and phosphorus 122 in the analysis of environmental objects have been generalised.V. Conclusion The significance of MW for analytical chemistry grows continu- ously and rapidly. In the 1970s, there were only seven publications devoted to analysis using MW; in the 1980s, the number of publications increased to 132, while over a period of only four years, from 1991 to 1995, according to the data of a review,18 166 more studies were published. About 1000 studies related to the application ofMWin analysis have now been published. The main topics considered in these studies are related to sample digestion, drying and ashing under the action of MW.The microwave- assisted sample preparation has underlain new-generation ana- lytical techniques�fast and highly efficient methods having good metrological characteristics; this reduces the gap between the existing power of instrumental determination of elements and the level of sample pretreatment techniques and extends the range of objects that can be analysed by routine methods. Due to the versatility of microwave methods, it is possible to design inte- grated analytical sequences consisting of inseparably connected operations and meant for the determination of a wide range of substances and materials, instead of separate procedures. This field of analytical chemistry is being vigorously developed.The design of multipurpose analytical schemes on the basis of minimisation of the number of operations and integration of operations in space and in time 123 implies, first of all, improve- ment of the sample preparation step and convenient combination of this step with instrumental determination. The most promising lines of development of inorganic analysis with the use of MW were outlined by Matusiewicz and Sturgeon.91 They believe that the goals of usingMWshould include the decrease in the number of analytical operations and the time required to perform them through intensification of chemical processes, integration of operations, development of on-line analysis and speciation methods and implementation of rational hybrid methods.Gen- erally, these trends correspond to the general principles of the development of analysis discussed in a number of publica- tions.123 ± 129 Attempts to develop general approaches to the design of analytical schemes based on the microwave sample pretreatment, to improve individual stages and to implement integrated sequen- ces of operations in a microwave field within a single system have been undertaken.6, 130, 131 An approach proposed in our study 12 and developed further under the name `total microwave sample preparation' (which implies sample pretreatment in which all operations are carried out under the action of MW, are interre- lated and often combined with one another) 132 is regarded to be one of the major directions in the microwave-assisted sample preparation.2 In the development of hybrid methods and analyt- ical schemes for analysing unique samples by high-sensitivity instrumental methods, non-traditional combinations (in particu- lar, those with the use of micromethods, i.e., techniques and expedients for working with small amounts of the analyte) are292 fairly promising. The methods of sample preparation with the use of MW are suitable for complex analytical objects, both for elements and matrices such as geological (ores, rocks), environ- mental (foodstuffs and raw materials, biological preparations, natural water, soil), industrial (high-purity substances, concen- trates, metals, alloys, second-hand raw materials) containing heavy, rare and noble elements as macro- and micro- components.The use ofMWsubstantially improves characteristics of analysis, not only that carried out by advanced instrumental methods but also of the analysis performed by well-known traditional methods, thus providing an increase in the potential of the existing ana- lytical equipment. Owing to the microwave pretreatment, it is possible to reduce several-fold the duration of analysis, decrease the consumption of chemicals and energy expenditure, and to reduce the required man-hours. In addition to ultra-trace analysis of components, which is associated with specific conditions (clean chemistry), one more important analytical problem that has appeared quite recently is elaboration of methods for determination of the forms in which elements exist.Microwave methods, which possess great potential due to the diversity of operations carried out in a microwave field and the flexibility of combinations with instrumental methods, are undoubtedly promising in this respect. Microwave systems are used to increase the efficiency of multistep procedures of succes- sive selective acid extraction of various chemical forms of metals, sample digestion and extraction with organic solvents. Sample derivatisation, i.e., preparation of a derivative convenient for the subsequent instrumental determination of the element, is also carried out directly in the microwave oven; this is done in open, closed or flow systems.2, 116, 117 By combining microwave pretreat- ment with atomic and mass spectrometry or liquid and capillary gas chromatography, hybrid methods have been developed, which shorten the time of analysis by a factor of 20 ± 100.130, 131, 133, 134 Yet another important feature of microwave methods is the possibility of preparing metals as convenient analytical forms and as complexes that are difficult to prepare by traditional methods due to kinetic restrictions.A simple procedure for preparation of the required analytical forms under the action of MW increases the efficiency of rather long-lasting clsical spectrophotometric, fluorescence, luminescence, sorptional, electrochemical and other methods that are sensitive to the form of element existence in solution.The correlation between the specified conditions of reactions and the quantitative and qualitative composition of the products provides the possibility of targeted synthesis of both inorganic complexes and macrocyclic reagents 40, 41 or poly- mers.4, 90 The embodiment of the potential of microwave methods is determined by the development of instrumentation, increase in the efficiency of the control of process parameters, and the decrease in the cost of equipment. Thus an increase in the speed of analysis can be achieved by two methods: by increasing the number of samples treated simultaneously [in closed systems, this can be 52 (see Ref. 135)] and by using flow systems in which, according to the data of CEM company, 180 samples can be prepared over a period of 3 h.The metrological characteristics of analysis can be improved, for example, by using steel autoclaves in combination with microwave heating by focused radiation; in Matusiewicz's opinion,108 this ensures faster heating, a higher pressure, and a better reproducibility of results. A promising technique is the use of microwave flow setups for digestion and preconcentration coupled with automated determination. The scopes of the two approaches are analysed in a publication.66 Flow setups appear to be the optimal devices for the design of automated systems, although other facilities (for example, open and robotised devi- ces 2, 136, 137) can also serve as parts of these systems used to solve particular analytical problems.Undoubtedly, with the advent of new technologies and materials, novel engineering solutions for the implementation of microwave heating will also arise (the ways of radiation supply; the design of the vessels, flow systems, automated injection units and types of coupling with instrumental I V Kubrakova determination) and new ways of controlling the parameters (temperature, pressure) and of the `parameter ± process' feedback will appear. This will give rise to a new generation of laboratory microwave systems. The possibility of conversion of microwave devices from a fairly advanced facility used for routine sample pretreatment into a tool for investigation of the regularities of chemical transforma- tions appears to be of fundamental importance.69, 71, 73, 80 The change in the kinetic parameters of reactions on exposure to radiation with maintenance of invariable thermodynamic proper- ties of the system allows quantitative investigations of the slow physicochemical processes, simulation of various reactions in model systems and, thus, provides a way for predicting the results of processes taking place under real conditions.This is of interest from both theoretical and practical standpoints, for example, for the perfection of complex technological processes involving a series of reactions with participation of many components. The development of the theoretical basis of microwave sample pretreatment is a topical subject. 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出版商:RSC    

年代:2002

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Russian Chemical Reviews 71 (4) 295 ± 314 (2002) Titanocene derivatives containing titanium ± carbon s-bonds V A Knizhnikov, N A Maier Contents I. Introduction II. Methods for the synthesis of titanocene derivatives containing Ti7C s-bonds III. Reactions of titanocene derivatives containing Ti7C s-bonds Abstract. the for methods major the on data published The The published data on the major methods for the synthesis titanocene of conversions chemical and synthesis and chemical conversions of titanocene derivatives derivatives containing bib- The systematised. are ± titanium containing titanium ± carbon carbon s-bonds -bonds are systematised. The bib- liography includes 288 references liography includes 288 references. I. Introduction The chemistry of dicyclopentadienyltitanium (titanocene) deriva- tives is one of the most rapidly developing branches of the chemical science.The interest in titanocene derivatives is partly due to their ability to catalyse various chemical processes, e.g., polymerisation, isomerisation and hydrogenation of alkenes, cyclotrimerisation of acetylene derivatives, fixation of molecular nitrogen under mild conditions and many other reactions. These compounds acquire increasing popularity as additives to fuels and high-temperature lubricants, rubber accelerators, lacquer and water-repellent components, antiknock compounds, etc. Among other compounds of the titanocene series, titanocene derivatives containing Ti7C s-bonds are still the least known due to the thermal instabilities of the majority of representatives of this class and, to some extent, their high sensitivities to oxygen and atmospheric moisture.High reactivities of titanocene derivatives containing Ti7C s-bonds with various organic and inorganic substrates and the presence of several reaction centres in some of them open broad prospects for their application in both organic synthesis and in the preparation of diverse organometallic com- pounds including novel representatives of the known types of titanocene derivatives. The published data on the major methods for the synthesis of titanocene derivatives containing Ti7C s-bonds isolated from various reaction mixtures in individual state and the most typical chemical transformations are consid- ered. The principal characteristics of titanocene derivatives (col- our, solubility, air resistance, thermal stability, structure, etc.) have been described in the majority of the original papers and therefore fall outside the scope of this review.V A Knizhnikov, N A Maier Institute of Physical Organic Chemistry, National Academy of Sciences of Belarus, ul. Surganova 13, 220072 Minsk, Belarus. Fax (37-517) 284 16 79. Tel. (37-517) 284 16 38 (V A Knizhnikov), Tel. (37-517) 284 23 72. E-mail: maier@ifoch.bas-net.by (N A Maier) Received 16 October 2001 Uspekhi Khimii 71 (4) 341 ± 362 (2002); translated by R L Birnova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n04ABEH000701 295 295 304 II. Methods for the synthesis of titanocene derivatives containing Ti7C s-bonds 1.Reactions of titanocene halides with organic derivatives of non-transition metals Reactions of titanocene dihalides with organolithium, -sodium, -magnesium or -aluminium compounds constitute a classical method for the synthesis of titanocene s-derivatives; depending on the reagent ratio, both mono- and dialkyl- or diaryl derivatives can be prepared. Dimethyltitanocene (1) was prepared by the reaction of titanocene dichloride (2) with an excess of methyllithium, methyl- magnesium halides or lithium tetramethylaluminium at reduced temperatures. Diethyl ether is the most convenient solvent for such syntheses.1±5 Et2O Cp2TiMe2 Cp2TiCl2+MeX 2 1 Cp=C5H5; MeX=MeLi, MeMgHal, LiAlMe4 .Dimethyltitanocene derivatives 3 containing various substitu- ents in p-bound cyclopentadienyl ligands are prepared from the corresponding dichlorides. Cp1Cp2TiCl2+MeX Cp1Cp2TiMe2 3 MeX=MeLi, MeNa, MeMgHal, Me3Al. Cp1 Ref. Cp2 C5Me5 C5H2(SiMe3)3 C5H5 C5Me4(CH2)2NMe2 C5H4CMe2C13H9 C5H4Me C5H4Me 6 C5Me5 (Cp*) C5Me5 2, 6 1,2,3-Me3C5H2 1,2,3-Me3C5H2 6 C5Me4H C5Me4H 6±8 C5Me4(CH2CH2CH=CH2) C5Me4(CH2CH2CH=CH2) 9 C5H5 10 C5H3(SiMe3)2 11 1,3-But2C5H3 12 C5Me5 13 C5Me5 14 Dimethyltitanocene derivatives with bridges between the cyclopentadienyl ligands, such as compounds 4,15 ± 19 5a 20 and 5b,21, 22 are known.296 R34 H2C R1R2X TiMe2 TiMe2 H2C R34 5a 4 R1 R2 R3 X Ref. Me2Si TiMe2 Me2Si Me Me H C 15 H BunH C 16 Me Me H Si 17, 18 Me Me Me Si 19 5b The binuclear titanocene-like complex 623 with a bridging (dimethylsilylidene)dicyclopentadiene ligand and the complex 724 in which the role of a bridge is played by biphenyl-2,2 0-diyl have been synthesised.Me Me Si Me Ti Me Cp*TiMe2 Me2TiCp* 6 ()-7 The reaction of titanocene dichloride (2) with phenyllithium or phenylmagnesium halides gave diphenyltitanocene Cp2TiPh2.4, 25 ± 28 Diphenyltitanocene derivatives containing substituents in both p-bound cyclopentadienyl ligands, such as compound 8, and s-bound phenyl rings, such as compounds 9, have been described.Cp1 Cp1 TiPh2 TiAr2 Cp2 Cp2 9 8 Ref. Ar Cp2 Ref. Cp1 Cp2 Cp1 C5H5 C5H5 107, 8 7, 8 C5H5 C5H5 C5H5 C5H5 C5H5 C5H5 C6F5 C5Me5 C5HMe4 C5Me5 C5Me5 C5Me4 19 Me2Si 3(4)-MeC6H4 27, 28 2-Me2NC6H4 274, 29, 307, 8 7, 8 C5HMe4 C5HMe4 4-MeC6H4 4-MeC6H4 C5Me5 C5H5 C5Me4 [(C5Me4)2SiMe2]Ti(CH2SiMe3)2,19 The reaction of titanocene dichloride (2) or its derivatives with benzylmagnesium chloride or benzyllithium results in dibenzyl- titanocene derivatives Cp2TiBn2,31 ± 33 (C5H4Me)2TiBn2,33 (C5H4SiMe3)2TiBn2 33 and [(C5H4)2SiMe2]TiBn2.16 Syntheses of titanocene derivatives in which s-bound organic radicals contain a heteroatom, e.g., Cp2 Ti(CH2SiMe3)2,34 ± 37 [(C5H4)2SiMe2]Ti..(CH2SiMe3)2,21 Cp2Ti(CH2GeMe3)2,37 Cp2Ti(CH2SPh)2,38, 39 Cp2Ti(CH2PPh2)2 ,40 Cp*(Cp)Ti(CH2PPh2)2,41 etc., have been described.Thiophene 42 or trichlorothiophene (compounds 10a,b) can be used as s-bound substituents.43 X3 S Cp2Ti S X3 10a,b X = H (a), Cl (b). V A Knizhnikov, N A Maier The reaction of titanocene dichloride (2) with cyclopentadie- nylsodium afforded tetracyclopentadienyltitanium in which two cyclopentadienyl ligands were linked by p-bonds, whereas the other two ligands, by s-bonds.44 ± 47 Presumably, this complex underwent rapid (on anNMRtime scale) p>s rearrangement of the ligands at 20 8C.46, 48 The reaction of an excess of ferrocenyllithium with titanocene dichloride (2) results in diferrocenyltitanocene Cp2TiFc2 (Fc=C5H5FeC5H4).49 The reaction of titanocene dichloride (2) with an excess of 2-(dimethylaminomethyl)ferrocenyllithium (11) yields the symmetrical complex 12.Under similar conditions, bis(pentamethylcyclopentadienyl)titanium dichloride yields the monoferrocenyl-substituted complex 13.50 NMe2 FcN Li Fe Cp2Ti Cp2TiCl2+ 2 FcN 12 11 FcN Cp2 Ti Cp2 TiCl2+11 Cl 13 NMe2 Fe . FcN= The reaction of titanocene dichloride (2) with o(m)-carboran- 1-ylmethylmagnesium bromides leads to carboranylmethyl-sub- stituted titanocenes 14a,b and 15.51, 52 Dicyclopentadienylbis(o- carboranylmethyl)titanium 14a, contrary to the majority of tita- nocene derivatives containing Ti7C s-bonds, is stable in air at 20 8C over a long period of time and is decomposed only on melting at 185 8C. CR CH2C B10H10 Cp2Ti Cp2TiCl2+ RC CCH2MgBr 2 B10H10 CR CH2C 14a,b B10H10 R = H (a), Me (b).CH2CB10H10CMe Cp2Ti Cp2TiCl2+MeCB10H10CCH2MgBr 2 CH2CB10H10CMe 15 Titanocene dichloride (2) reacts with an excess of sodium, lithium or magnesium acetylenide to give titanocene diacetyle- nides Cp2Ti(C:CR)2 , where R=Ph,53 ± 55 SiMe3 ,56, 57 CF3 ,58 Et, Prn, Bun, But(see Ref. 56), cyclo-C6H11, Ph2 CHCH2 , n-C6H13,55 Fc, Rc (Rc=C5H5RuC5H4);59 (C:CR)2= (C:C72-C6H47C:C72-C6H47C:C).60 Similar com- pounds were prepared from titanocene dichloride derivatives containing substituents in cyclopentadienyl rings, viz., Cp02Ti(C:CR)2, where Cp0=C5H4Me: R=Ph, cyclo-C6H11, n-C6H13, Ph2 CHCH2 ,55 SiMe3 ;56 Cp0=C5H4SiMe3 : R=Ph, SiMe3 ,56 C:C7C:CEt,61 Fc,59, 62 Rc;59 Cp0=C5H2Me3 , R=SiMe3 ;63 Cp0=Cp*, (C:CR)2=(C:C72-C6H47 C:C72-C6H47C:C);60 Cp0=C5H4CH2CH2NMe2: R= SiMe3 ;64 Cp0=C5H4X (X=Ph2P, Ph2P=O, Ph2P=S), R=But (see Ref. 65).2 (see Ref. 18), have been synthesised. Symmetrical dicyclopentadienyltitanium derivatives with other s-bound organic groups, e.g., Cp2TiBun2 (see Ref. 66), Cp2TiEt2 ,67 Cp2Ti[C(Ph)=CMe2]2 ,68 Cp2Ti(CF=CF2)2 69 and [Me2Si(C5H4)2]TiBun The reactions of titanocene dichloride (2) or its derivatives containing substituents in the cyclopentadienyl ligands withTitanocene derivatives containing titanium ± carbon s-bonds organodilithium or -dimagnesium compounds yield metalla- cycles. Thus titanaindans 18a,b,70 19a 18 and 19b 19 were prepared from the dichlorides 2, 16, 17a,b and 1,2-bis(chloromagnesio- methyl)benzene.R TiCl2 + 2, 16 R R = H (2, 18a), SiMe3 (16, 18b). R4 TiCl2+ Me2Si R4 17a,bR4 Me2Si Ti R4 19a,b R = H (a), Me (b). The dilithium derivative 20 was used for the preparation of substituted titanaindan 21.71 SiMe3 Li Li Cp2TiCl2+ 2 20 SiMe3 Titanaindene 22 is produced by the reaction of titanocene dichloride (2) with the dilithium derivative 23.72 Li Li Cp2TiCl2+ 2 Ph Bun 23 The reaction of titanocene dichloride (2) with 2,2 0-dilithiobi- phenyl 72 or 2,2 0-dilithiooctafluorobiphenyl 73 gave titanafluor- enes 24a,b. X4 Li Li Cp2TiCl2+ 2 X4 X = H (a), F (b). Diverse titanacyclic compounds Cp2TiL2 have been described in which the cyclic structures are formed owing to the ligands L2.R MgCl Ti MgCl 18a,b R MgCl MgCl Me3Si Cp2Ti Me3Si 21 Cp2Ti Ph Bun 22 X4 Cp2Ti X4 24a,b 297 Ref. Ref. L2 L2 77, 78 (CH2)4 Bun N 74 79 CH2CH(Me)CH2 C6H4-2 C6H4-2 80 NBun CH2SiMe2XSiMe2CH2 (X=CH2, O, SiMe2) 80 CH2SiMe2SiMe2CH2 75, 76 CH2 CH2 Titanocene dichloride (2) reacts with 1,1-dimethyl-1-sila-3- magnacyclobutane (25) to give thermally stable 3,3-dimethyl-1,1- dicyclopentadienyl-1-titana-3-silacyclobutane (26).81 SiMe2 SiMe2 Cp2Ti Mg Cp2TiCl2+ 2 26 25 1-Titana-3-metallacyclobutanes 27 and 28 were prepared by the reactions of bis(bromomagnesiomethyl)titanocene with dichlorodimethylgermane 82 or titanocene, zirconocene and haf- nocene dichlorides.83 Me2GeCl2 Cp2Ti GeMe2 CH2MgBr 27 Cp2Ti CH2MgBr Cp2MCl2 Cp2Ti MCp2 28 M=Ti, Zr, Hf.1,1-Dicyclopentadienyl-1-titana-2-methylidenecyclobutane (29) was synthesised by the reaction of titanocene dichloride (2) with vinyllithium in THF in the reagent ratio of 1 : 2.84, 85 Presumably, this reaction proceeds via divinyltitanocene, which is rearranged into 2-methylidenetitanacyclobutane 29 at temper- atures above778 8C. THF Cp2TiCl2+ Cp2Ti Li 2 29 Monoalkyl- or monoaryl titanocene chlorides are formed upon treatment of titanocene dichloride (2) with equimolar amounts of organolithium or -magnesium compounds. Thus methyltitanocene chloride derivatives 30 were prepared using methyllithium, methylmagnesium chloride or trimethylalumi- nium.Cp1Cp2Ti(Me)Cl Cp1Cp2TiCl2+MeX 30 MeX=MeLi, MeMgCl, Me3Al. Ref. Cp2 Ref. Cp1 Cp2 Cp1 12 86 ± 89 C5H5 C5H5 C5H5 1,3-But2C5H3 C5H4CH(Me)Ph 90 C5H5 91 C5H4Me C5H5 17, 18 Me2Si 91 C5H4Me C5H4Me complexes where Cp1Cp2Ti(Me)ClAlMe2Cl The Cp1=C5H5, Cp2=C5H4Me, C5H4SiMe3; Cp1=Cp2= C5H4SiMe3 , have also been synthesised.92298 Reactions of titanocene dichloride (2) or its derivatives con- taining substituents in the cyclopentadienyl ligands with trime- thylsilylmagnesium chloride (1 : 1) yield the trimethylsilylmethyl derivatives 31a ± d. Compounds 31e ± h were prepared in a similar way.Cp1Cp2Ti(CH2EMe3)Cl 31a ± h Ref. Cp2 Cp1 E Compound 31 33, 93, 94 33 33 C5H5 C5H4Me C5H4SiMe3 abc Si Si Si C5H5 C5H4Me C5H4SiMe3 C5H4 Me2Si d 17, 18 Si C5H4 e C5H5 C 9393 93 33 C5H5 C5H5 C5H5 C5H4Me Ge Sn C C5H5 C5H5 C5H4Me fgh The reaction of titanocene dichloride (2) or its analogue containing a bridging dimethylsilylene group with an equimolar amount of benzylmagnesium chloride gave Cp2Ti(Bn)Cl (see Ref.86) and [Me2Si(C5H4)2]Ti(Bn)Cl.17 Chlorotitanocene derivatives Cp2TiCl(L) containing bulky ligands [e.g., L=CH2CH2CHR1CR2=CH2 , where R1= R2=H; R1=H, R2=Me; R1=Ph, R2=H; L= CH2CMe2CHCR1=CR2CR3=CH2, where R1=R2=R3=H; R1=R2=H, R3=Me; R1=Me, R2=R3=H; R1=R3=H, R2=Me; R1=R2=Me, R3=H] were synthesised by the reac- tion of titanocene dichloride (2 ) with the corresponding organo- magnesium compounds.95 Titanocene dichloride derivatives containing substituents in the cyclopentadienyl ligands react with sodium acetylenides (charge ratio 1 : 1) to give the monoacetylenides Cp02TiCl(C:CR) [Cp0=C5H4Me, C5H4SiMe3, R=Ph, SiMe3 (see Ref.56); Cp0=C5H4SiMe3, R=C:CEt (see Ref. 61)]. The reaction of titanocene dichloride (2) with pentafluoro- phenyllithium in ether yields bis(pentafluorophenyl)titanocene (32) and pentafluorophenyltitanocene chloride (33).29 The monopentafluorophenyl titanocene derivative 34 was prepared by treatment of cyclopentadienyl(tert-butylcyclopentadienyl)tita- nium dichloride with pentafluorophenylmagnesium bromide.96 Cp2Ti(C6F5)2+Cp2Ti(C6F5)Cl 33 Cp2TiCl2+C6F5Li 2 32 Cp Cp Ti(C6F5)Cl TiCl2+C6F5MgBr 34 ButC5H4 ButC5H4 The chlorotitanocene derivatives 35 containing various alkyl-, aryl-, alkenyl- and other s-bound ligands were synthesised by reactions of equimolar amounts of the corresponding dichlorides with organomagnesium or -lithium compounds.Cp1 Ti(R)Cl Cp2 35 Ref. R Cp2 Cp1 86 86, 97 86 40 Ph Et Prn CH2PPh2 C5H5 C5H5 C5H5 C5H5 C5H5 C5H5 C5H5 C5H5 98 C5H5 C5H5 V A Knizhnikov, N A Maier Cp1 Ref. R Cp2 99 69 CH2OMe CF2=CF C5H5 C5H5 C5H5 C5H5 Fe 100 C5H5 C5H5 CH2NMe2 C5H5 41 99 13 CH2PPh2 CH2OMe CH2=CH C5Me5 C5H4Me C5H4Me C5Me5 C5Me4(CH2)2NMe2 The bromotitanocene derivative Cp2TiBr(CH2MgBr) was prepared from titanocene dibromide and CH2(MgBr)2.101 The alkyl- and aryl(dicyclopentadienyl)titanium(III) deriva- tives 36 ± 38 have been described.They are prepared by treating dicyclopentadienyltitanium(III) chloride or its derivatives con- taining substituents in the cyclopentadienyl ligands with organo- lithium, -sodium or -magnesium compounds at778 8C. TiR 36 Ref. R Ref. R 95 105 105, 106 107 108 CHCMe2CH2CH2C(R)=CH2 (R=H, Me) 2(3,4)-MeC6H4 2,6-Me2C6H3 C(Me)=CHMe Bn Me 102 ± 104 Et 102 Bun 102 Ph 105 2,4,6-Me3C6H2 105 C6F5 104, 105 Me5 TiR Me5 37 Ref. R Ref. R Ref. R C2D5 C3D7 108 108 CMe=CHMe 109 108 108 CH2=CH 108 108 Me Et CH2But Ph 108, 109 Prn 108, 109 Bn 108 108 CD3 R1nRef. R2 R1n TiR2 110 110 111 Me Ph Me Me4 Me4 1,3-But2 R1n 38 The dicyclopentadienyltitanium(III) derivatives 39a ± e con- taining a heteroatom in the alkyl substituent have been obtained.Ref. R Compound 39 TiCH2R SiMe3 PPh2 39a ± e 104 40 (CH2)2OMe 112 112 112 (CH2)2SMe CH2OMe abcdeTitanocene derivatives containing titanium ± carbon s-bonds It is assumed that the titanium atom in compounds 39b ± e is additionally coordinated by the heteroatom of the s-bound ligand.40, 112 Treatment of the dicyclopentadienyltitanium(III) chloride dimer or its derivatives containing substituted cyclopentadienyl ligands 40a ± d with lithium or sodium phenyl- or trimethylsilyl- acetylenides at778 8C yields the paramagnetic complexes 41a ± d which are converted into the dimagnetic complexes 42a ± d upon heating.17, 54, 57, 113, 114 CR Cp1 Cp1 C Cp1 Ti RC CM TiCl Ti Cp2 C Cp2 2 Cp2 RC 41a ± d 40a ± d Cp1 CR C Ti Cp1 Cp2 Ti C RC Cp2 42a ± d M=Na, Li.Ref. Cp2 Compound Cp1 R 54 113 57 Ph Ph SiMe3 C5H5 MeC5H4 C5H5 abc C5H5 MeC5H4 C5H5 C5H4 Me2Si d 17, 114 Ph C5H4 monoacetylenides are Dicyclopentadienyltitanium(III) formed as side products upon treatment of titanocene dichloride (2) with an excess of acetylenides EC:CR (E=BrMg, Na, Li; R=Ph, Et, Prn, Bun, But, SiMe3).56 The formation of a dicyclo- pentadienyltitanium(III) derivative Cp2Ti(2-MeOCH2C6H4) proceeds through preliminary reduction of Ti(IV) to Ti(III) in the reaction of titanocene dichloride (2) with an equimolar amount of bis(2-methoxymethylphenyl)magnesium in THF.This is the first Ti(III) derivative in which the titanium atom is coordinated by oxygen. A similar reaction with bis(2-N,N-dimethylamino- methylphenyl)magnesium 115 affords the Ti(III) complex Cp2Ti(2-Me2NCH2C6H4). Reactions of Cp2TiCl2 with sterically hindered lithium alkyls devoid of b-hydrogen atoms, such as LiCHPh2 or LiCH(SiMe3)2 yield the Ti(III) complexes Cp2TiR [R=Ph2CH, (Me3Si)2CH]. Dicyclopentadienyltitanium(III) chloride was found to be the intermediate product of these reactions.116 Treatment of titanocene dichloride (2) with n-butyllithium yields Cp2TiH which adds to allylamines 43 and 44 to give 1-titana-2-azacyclopentanes 45 and 46.117Cp2Ti BunLi N N(Bun)R Cp2TiCl2+ 2 43 Bun R 45 Bu N Cp2Ti TiCp2 BunLi N N Cp2TiCl2+ 2 Bun Bun 46 NBu 44 The binuclear complex 47 was prepared by the reaction of dicyclopentadienyltitanium(III) chloride with N,N0-bis(o-lithium- phenyl)-N,N-di(n-butyl)ethylenediamine 48.74 299 Bun NC6H4Li-2 TiCp2 Cp2Ti [Cp2TiCl]2+ N N NC6H4Li-2 Bun Bun Bun 47 (40%) 48 The reaction of dicyclopentadienyltitanium(III) chloride with lithium tetramethylaluminate in toluene at 0 8C was used to prepare the complex Cp2Ti(m-Me)2AlMe2 containing two bridging methyl groups.118 The titanocene derivatives Cp2Ti..(m-Cl)(m-CH2)AlMe2 (see Ref. 119) and Cp2Ti(m-CH2)2PPhR (R=Me, Ph) 120 containing bridging methylene groups have also been synthesised.The reaction of titanocene dichloride (2) with isopropylmag- nesium chloride in a nitrogen atmosphere at temperatures below 778 8C affords [Cp2TiPri]2N2 with a coordinated nitrogen molecule.121 The complexes [Cp2TiR]2N2 [R=CH2But, CH2SiMe3 , 3-MeC6H4, 2,6-Me2C6H3,122 C(Me)=CHMe (see Ref. 107)] were prepared from dicyclopentadienyltitanium(III) chloride and organomagnesium compounds under similar con- ditions. The phosphine titanocene derivatives 49 containing s-bound methyl- or trimethylsilylmethyl groups were synthesised by the reaction of the phosphine complexes 50 with organolithium compounds.123 R Cl RLi Ti Ti Me2Si Me2Si PMe2Ph PMe2Ph 49 50 R=Me, Me3SiCH2. It was shown that the complexes [Cp2TiR2]M (M=Li, Na; R=Me, Et, Pri) stable only at low temperatures are formed upon treatment of titanocene dichloride (2) with an excess of organo- lithium or -sodium reagents at temperatures below 770 8C.124 The [Z5-(C5HMe4)2Ti(Z1-C:C7C:CSiMe3)2]7 complex [Li(THF)2]+ was isolated and characterised by X-ray diffraction analysis. This is the first complex in which the Li atom is located between the inner triple bonds of the ethynyl ligands.125 2.Synthesis of titanocene s-derivatives using unsaturated organic compounds Reactions resulting in metallacyclic titanocene complexes con- taining Ti7C s-bonds can arbitrarily be divided into three groups: 1. Formation of a metallacycle by cross-linking of two un- saturated compounds including those p-coordinated by a metal atom; 2.Formation of a titanacycle by incorporation of an unsatu- rated compound into the Ti7C s-bond; 3. Addition of molecules containing several multiple bonds to dicyclopentadienyltitanium derivatives. The first type of reactions include reduction of titanocene dichloride (2) with naphthalenesodium 126 or metallic magne- sium 127, 128 in the presence of diphenylacetylene resulting in 1,1- dicyclopentadienyl-2,3,4,5-tetraphenyl-1-titanacyclopentadiene (51). Mg or Na/C10H8 PhC CPh Cp2Ti Cp2TiCl2 2300 CPh Cp2Ti CPh 52 It is believed 129 that this reaction proceeds through the reduction of titanium(IV) to titanium(II) with subsequent forma- tion of the tolan ± titanocene p-complex 52. Supporting evidence is provided by the fact that 1,1-dicyclopentadienyl-2,3,4,5-tetra- phenyl-1-titanacyclopentadiene 51 is formed as a result of inter- action of the complex 52 with tolan,129 while treatment of the intermediate compound 52 with water gives the binuclear complex 53.127 Titanacyclopentadienes 54a,b were synthesised by the reac- tion of the tolan ± titanocene complex 52 with phenylacetylene or hex-3-yne.130 52+ R1C CR2 R2 54a,b R1=Ph, R2=H (a); R1= R2=Et (b).The dimerisation of alkynes and the formation of titanacyclo- pentadienes 51 and 55a ± d occur in the reaction of the diphosphi- ne ± titanocene complex 56 with acetylene, dimethylacetylene or methylacetylene or of the titanocenedicarbonyl 57 with tolan or diacetoxyacetylene. Cp2Ti(PMe3)2 56 Cp2Ti(CO)2 57 Compound R1 HMe Me 55a 55b 55c 55d 51 131 ± 133 134 133 OC(O)Me OC(O)Me 135 135 Ph 1,1-Dicyclopentadienyl-2,3,4,5-tetraphenyl-1-titanacyclopen- tadiene (51) was prepared by treatment of the titanocene complex with bis(trimethylsilyl)acetylene 58136 or of the p-allyl titanium(III) complex 59 137 with an excess of tolan.The titana- cycle 51 is also formed in a photochemical reaction of dimethylti- tanocene (1) with tolan in pentane.138 ± 140 SiMe3 C Cp2Ti C 58 SiMe3 Cp2Ti 59 hn C5H12 Cp2TiMe2 1 Ph Ph PhC CPh Cp2Ti Ph Ph 51 CHPh C(Ph) H2O Cp2Ti O TiCp2 C(Ph) CHPh 53 Ph Ph Cp2TiR1 R1 R2 R1C CR2 Cp2Ti R2 R1 51, 55a ± d Ref. R2 HMe HPh PhC CPh 51 V A Knizhnikov, N A Maier In contrast to the reaction of the bis(trimethylsilyl)acetyle- ne ± titanocene complex 58 with tolan, its reaction with mono- substituted acetylenes yields alkenyltitanocene acetylenides.141 CHR CH SiMe3 C +RC CH Cp2Ti Cp2Ti CR C 58 CSiMe3 R=Ph, Me3Si, Me(CH2)3 .The reaction of titanocene with bis(trimethylsilyl)butadiyne gives the titanacyclopentadiene derivative 60.142 SiMe3 Me3Si C C Cp2Ti SiMe3 C C Me3Si 60 Cross-linking of ethylene and acetylene ligands and the formation of 1,1-bis(pentamethylcyclopentadienyl)-1-titanacyc- lopent-2-ene derivatives 61a,b take place in the reaction of bis(pentamethylcyclopentadienyl)(ethylene)titanium (62) with methyl- or dimethylacetylene.143 R Me CH2+RC CMe Cp2 Ti Cp2 Ti CH2 62 61a,b R=H (a),Me (b).Compounds containing a titanacyclopentene fragment were prepared by reactions of (acetylene)(trimethylphosphine)di(cyclo- pentadienyl)titanium with ethylene 144 and by reactions of (di- phenylacetylene)di(cyclopentadienyl)titanium with tricyclo- [5.2.1.02,8]deca-2,5,8-triene.145 Acetylene ± titanocene complexes enter into heterocyclisation reactions with carbonyl compounds. Thus the tolan ± titanocene complex 52 reacts with an equimolar amount of acetone 146, 147 or benzaldehyde 147 to yield titanadihydrofuran metallacycles. Ph Ph Ph C R1C(O)R2 Cp2Ti Cp2Ti R1 THF O R2 CPh 52 R1=R2=Me; R1=Ph, R2=H. The titanadihydrofuran derivatives 63a,b are formed by reactions of acetone or acetaldehyde with dimethylacetylene ± ti- tanocene (64) 134 or acetylene(trimethylphosphine) ± titanocene (65) 144 complexes.Me Me Me C R1R2CO Cp2Ti Cp2Ti R1 R2 64 CMe O63a PMe3 R1R2CO Cp2Ti R1 Cp2Ti O R2 CH 63b HC 65 R1=H, R2=Me; R1=R2=Me. The reaction of carbon dioxide with alkyne ± titanocene p-complexes results in the binuclear s-vinylcarboxylate com- plexes 66. Their oxidation by atmospheric oxygen gives titanadi- hydrofuranones 67.Titanocene derivatives containing titanium ± carbon s-bonds R R R R RCCO2 O2 Cp2Ti Cp2Ti Cp2Ti 20 8C O O CR O O 67 66 Cp2Ti R=Ph,148 ± 150 SiMe3,149, 150 Me.134 Titanadihydrofuranone (67) (R=H) was prepared by the reaction of carbon dioxide with the trimethylphosphinacetylene ± titanocene complex 65.144 The reaction of the ethylene ± titano- cene complex 68 with CO2 yields titanatetrahydrofuranone.143 PMe3 Cp2Ti CO2 Cp2Ti O CH HC O 67 65 CH2 CO2 Cp2Ti Cp2Ti CH2 O 68 O The reaction of dimethyltitanocene (1) with alkynes in boiling benzene in the dark is a typical reaction of the second type, which consists in the formation of titanacycles due to incorporation of unsaturated compounds into the Ti7C s-bond. In the first step, methyl(vinyl)titanocene complexes 69a ± e are formed; in the case of R=Ph 151 and C6F5 (see Ref.152), these complexes were isolated in an individual state. In the second step, methane is abstracted from the complexes 69a ± e resulting in the titanacyclo- butene derivatives 70a ± e.151, 153 In-depth studies of these reac- tions have been carried out.154 Me Cp2Ti 7CH4 Cp2TiMe2+RC CR 1 CRMe CR 69a ± e R R Cp2Ti70a ± e R=Et (a), Prn (b), Ph (c), Me3Si (d), C6F5 (e).Titanacyclobutene derivatives were prepared by the reaction of bis(pentamethylcyclopentadienyl)titanium chloride with prop- argyl bromides in the presence of samarium diiodide with sub- sequent treatment of the reaction mixture with an equimolar amount of the corresponding halide.155, 156 R1 1) SmI2 2) R2X R2 Cp2 Ti Cp2 TiCl+R1C CCH2Br R1=H;R2=Bn; R1=Me, R2= MeC CCH2, Bn; R1=Ph, R2=Bn, PhC CCH2; X=Hal. Diphenyl-substituted titanacyclobutene 70c was synthesised by the reaction of titanocene dichloride (2) with tolan and trimethylaluminium. This reaction proceeds via the intermediate vinyl derivative 71.156 Cl Me3Al Cp2Ti Cp2TiCl2+PhC CPh 2 CPhMe CPh 71 Ph Ph Cp2Ti 70c Thermal reactions of alkynes with diphenyltitanocene are also accompanied by insertion of alkyne into the Ti7C s-bond.It is believed 157, 158 that the phenylenetitanium complex 72 is formed in the first step as a result of splitting of the benzene molecule from diphenyltitanocene. Subsequent insertion of alkyne into one of the Ti7C s-bonds results in titanaindenes 73a ± c. D Cp2TiPh2 Cp2Ti 7PhH R1=R2=Ph (a), R1=R2=C6F5 (b), R1=SiMe3, R2=Ph (c). Analogous reactions yielding mixtures of substituted titana- indenes 74a ± d occur on heating of m- or p -tolyltitanocenes in the presence of alkynes.159 The formation of a complex mixture of reaction products in the case of the m-tolyl derivative is explained by the fact that this compound yields a mixture of isomeric phenylenetitanium complexes 75 and 76 upon heating.159 PhH Cp2TiC6H4Me-4 D R R Cp2Ti Me 74a PhH Cp2TiC6H4Me-3 D 74a+74b+ Me Titanaindene 73a was prepared by reduction of titanocene dichloride (2) with magnesium in the presence of tolan and o-bromofluorobenzene.128 Heating of diphenyltitanocene derivatives in the presence of alkenes results in titanaindenes 77.160 R1 Ph CH2 Ti Ph R1 R1=H, Me; R2=H, Me, Bun.The 1,1-dicyclopentadienyl-1-titanacyclobut-2-ene deriva- tives 70c,d,f ± h were prepared by the reaction of alkynes with the titanium ± aluminium complex 78 in the presence of pyri- dine 161, 162 or with carbene ± titanocene complexes 79.163 ± 165 301 R1 R2 Cp2Ti R1C CR2 72 73a ± c RC CR Cp2Ti Me 75 R R Cp2Ti + 74b MeMe RC CR 75+ Cp2Ti 76 R R R R Cp2Ti Cp2Ti + Me 74d 74cR1 R2 Ti CHR2 D R1 77302 Cl R2 AlMe2 Cp2Ti R2C CR3 R3 Cp2Ti 70c,d, f ± h 3 CH2 78 Cp2Ti CH2 .PR1 79 R13 =Me3, Me2Ph, Et3; R2=R3=Ph (70c), SiMe3 (70d); R2= CMe CH2, R3=Et (70f); R2=Et, R3=CMe CH2 (70g); R2=CEt CH2, R3=Me (70h).Alkylidenetitanacyclobutenes 80 are formed in reactions of alkynes with the titanocenevinylidene complexes 81.166 CHR1 R2 Cp2Ti Cp2Ti C CHR1+R2C CR3 81 80 R3 R1=Ph, Prn, n-C5H11; R2=H, Prn, n-C5H11, Ph; R3=Prn, Ph, SiMe3.The titanacyclobutane derivatives 82a ± d result from reac- tions of trisubstituted alkenes with the carbene ± titanocene com- plexes 79.163 ± 165 Titanacyclobutanes 83a ± c were prepared by the reaction of terminal alkenes with titanium ± aluminium complexes in the presence of 4-dimethylaminopyridine (DMAP).91, 167 The titanium ± aluminium complex 78 yields the titanabicyclopentane derivative 84 in the reaction with 3,3-dimethylcyclopropene.168 R2 R3 CR3R4 Cp2Ti Cp2Ti CH2 .PR13 +R2CH R4 79 82a ± d R13 =Me3, Me2Ph, Et3; R2=R3=H;R4=Bun (a); R2=R3=H,R4=But (b), R2=H, R3=R4=Me (c); R2=R4=Me; R3= H (d). Cl DMAP R1 Cp1Cp2Ti CR1R2 Cp1Cp2Ti AlMe2+CH2 R2 CH2 83a ± c Ref.R2 Compounds 83 R1 Cp2 Cp1 Pri abc Me But But C5H5 C5H4Me C5H4Me C5H5 C5H4Me C5H5 167 H 91 H 91 Me Me Me Cl Me AlMe2+ Cp2Ti Cp2Ti CH2 78 84 Titanacyclobutane derivatives react with alkynes to give titanacyclobutenes and the corresponding alkenes. Thus titana- cyclobutenes 70c,i were synthesised from diphenylacetylene and titanacyclobutanes 83c ± e. Ph Cp1Cp2Ti R+PhC CPh Ph Cp1Cp2Ti 7RCH CH2 83c ± e 70c,i Ref. R Cp2 Cp1 Titanacyclo- butenes Titanacyclo- butanes 70c 70c 70i 83e 83d 83c 169, 170 169, 170 91 But Pri But C5H5 C5H5 C5H4Me C5H5 C5H5 C5H5 V A Knizhnikov, N A Maier Alkylidenetitanacyclobutanes 85 react with alkynes at 70 8C in benzene or toluene to give alkylidenetitanacyclobutenes 80.171, 172 R2 R1 R2 R1 R3 Cp1Cp2Ti Cp1Cp2Ti +R3C CR4 7C2H4 85 80 R4 Cp1=Cp2=C5H5: R1=R2=Me; R1=Me, R2=Pri; R3=R4=Me, Et; Cp1=Cp2=C5Me5: R1=R2=H, R3=R4=H, Me, Ph, SiMe3 , SiBun3 ; R3=Prn, R4=H; R3=Me, R4=Et; R3=Et, R4=Me; R3=Me, R4=MeC:C; R3=Ph, R4=PhC:C; R3=Me, R4=Ph; R3=Ph, R4=Me; R3=Ph, R4=SiMe3 .Reactions of titanacyclobutane complexes with 1,1-dimethyl- and 1-isopropyl-1-methylallenes result in the formation of alkyli- denetitanacyclobutene derivatives 80 (R1=R2=Me, R1=Me, R2=Pri; R3=R4=H).172, 173 Some reactions of titanacyclic complexes with unsaturated or carbonyl compounds are accompanied by expansion of the metal- lacycle.Thus 1,1-dicyclopentadienyl-3,4-diphenyl-1-titanacyclo- pentadiene reacts with phenylacetylene to give a nine-membered titanacycle 86. 1,1-Dicyclopentadienyl-2,3,4,5-tetraethyl-1-tita- nacyclopentadiene forms a similar cyclic derivative 87 in a reaction with diethylacetylene.174 Ph Ph Ph +PhC CH Cp2Ti Cp2Ti Ph Ph Ph 86 Et Et Et Et Et Et +EtC CEt Cp2Ti Cp2Ti Et Et Et Et Et Et 87 Insertion of aliphatic ketones into the Ti7C s-bond of the titanacyclobutene derivatives 70 results in six-membered metalla- cycles 88.175R1 R2 R1 R2+R3R4CO Cp2Ti Cp2Ti O 70 88 R4 R3 R1=R2=Ph: R3=R4=Me; R3=Me, R4=Ph, CH2=CH; R3=Ph, R4=cyclo-C3H5; R1=Ph, R2=Me: R3=Me, R4=Me, Ph. Reactions of 1,1-dicyclopentadienyl-1-titana-2,3-diphenylcy- clobut-2-ene 70c with aldehydes yield mixtures of isomeric titana- dihydropyrans 88 and 89.175 Ph Ph Ph Ph Ph Ph+RCHO Cp2Ti Cp2Ti Cp2Ti + O O 70c R R 89 88 R=Me, Ph, Bn. The expansion of the metallacycle also takes place in reactions of 1,1-bis(pentamethylcyclopentadienyl)-1-titanacyclopropane 90 with methylidenecyclopropane or 1-methylidene-2-phenylcyclo-Titanocene derivatives containing titanium ± carbon s-bonds propane 176 and the ketene ± titanocene complex 91 with ethyl- ene.177 Cp2 Ti + Cp2 Ti 90 R H R R=H, Ph.C2H4 Cp2Ti Cp2Ti O O 91 The metallacyclic compounds 92 were prepared by reactions of the carbene ± titanocene complex with ketenes.178 Cp2Ti CH2+R2C C O CR2 Cp2Ti O92 R=H, Ph. Reactions of bis(trimethylphosphine)titanocene (56) with 1,2- diphenylcyclopropene,179 d-enones and d-inones belong to the third type of reactions leading to the formation of titanacyclic compounds.180 Ph Ph Ph Ph Cp2Ti 70cMe R O O MeCCH2CHCH2CH CH2 R Cp2Ti(PMe3)2 Cp2Ti 56 R=H, PhMe O MeC(O)(CH2)3C CR Cp2Ti R R=H, Me. A reaction of the bis(trimethylsilyl)acetylene ± titanocene complex 58 with diynes was used for the preparation of bicyclic titanacyclopentadiene derivatives 93.181 R SiMe3 C ( )n71 +RC C(CH2)nC CR Cp2Ti Cp2Ti C 58 SiMe3 93 R n=2, R=Me, Bu; n=4, R = Et.The reaction of the bis(trimethylsilyl)acetylene ± titanocene complex 58 with 1,4-diphenylbuta-1,3-diyne results in titanacy- clocumulene 94,182 while the reaction with acetone azine gives the 1-titana-2,3-diazacyclopent-3-ene derivative 95.183, 184 Ph 58+PhC C C CPh Cp2Ti Ph 94 303 Pri N N Cp2Ti 58+Me2C N N CMe2 C Me 95 The reaction of di(carbonyl)di(cyclopentadienyl)titanium (57) with 1,1,3,3-tetramethyl-2-thiocarbonylcyclohexane gives a monomeric alkylidenetitanathiirane containing a Z2-(C7S)- bound thioketone ligand.185 The reaction of di(carbonyl)di(cyclo- pentadienyl)titanium (57) with bis(trifluoromethyl)thioketene yields a titanacycle in which the titanium atom is bound to two thioketene ligands.186 F3C CF3 S S Cp2Ti(CO)2+ (CF3)2C C S Cp2Ti 57 CF3 F3C 3.Miscellaneous methods for the synthesis of titanocene s-derivatives In some cases, addition of various substrates to p-bound organic ligands of titanocene derivatives results in the formation of Ti7C s-bonds.Treatment of di(Z5-cyclopentadienyl)(Z2-ethylene)titanium (68) with alcohols, phenols187, 188 and water 188 gave mono- and binuclear derivatives of ethyltitanocene. Et ROH Cp2Ti OR CH2 Cp2Ti Et Et CH2 68 H2O TiCp2 Cp2Ti O R=Me, Et, Ph. The reactions of acetylene titanocene complexes with alcohols or phenols yielded s-alkenyl complexes 96.187, 188 OR2 PMe3 R2OH Cp2Ti Cp2Ti CHR1 CR1 CR1 R1C 96 R1=Ph, Me; R2=Me, Et, Ph. Bis(Z5-pentamethylcyclopentadienyl)(Z3-allyl)titanium (97) reacts with allyl bromide to give the titanacyclobutane complex 98.189 CHCH2Br +CH2 CH2CH CH2 Cp2 Ti Cp2 Ti 98 97 Reactions of the cationic p-allyl complex 101 with benzylmag- nesium chloride or propiophenone enolate lead to the titana- cyclobutane derivatives 99 and 100.190 BnMgCl Bn Cp2 Ti + 99 BF¡4 Cp2 Ti MeCH CPh 7 O 101 CHMeCPh Cp2 Ti 100 O304 Bis(pentamethylcyclopentadienyl)titanium hydride reacts with alkenes to give the s-alkyl derivatives Cp2 TiCH2CH2R (R=H, Me).108 Reactions of bis(trimethylphosphine)titanocene (56) with an equimolar amount of 3,3-disubstituted cyclopropenes afford the titanocene(vinylcarbene) complexes 102.191 The complex 102 (R1=R2=Ph) was prepared together with the complex Cp2Ti(PMe3)=C=C=CPh2 by treatment of titanocene dichlor- ide with two equivalents of butyllithium in the presence of trimethylphosphine and 3,3-diphenylcyclopropene.192 R2 R1 PMe3 Cp2Ti Cp2Ti(PMe3)2+ 56 CHCH CR1R2 102 R1=R2=Me, Ph; R1=Me, R2=Ph. Reduction of titanocene dichloride (2) by isopropylmagne- sium halides in the presence of methylidenecyclopropanes, meth- ylidenecyclobutane or methylidenecyclopentane results in the di(cyclopentadienyl)(cycloalkylmethyl)titanium(III) complexes 103 and 104.193 R2 R1 PriMgBr R3 R1 Cp2TiCH2 Cp2TiCl2+ 2 103 R4 R3 R4 R2 R1, R2, R3, R4=H, Me.PriMgX Cp2TiCH2 n n Cp2TiCl2+ 2 104 n=1, 2; X=Cl, Br. Titanium enolates of the formula Cp2 TiMe(OCR1=CR2R3) were prepared by heating bis(pentamethylcyclopentadienyl)di- methyltitanium(IV) in the presence of epoxides, e.g., cyclohexene oxide, cis- and trans-but-2-ene oxides.194 The titanocene sulfide and titanocene oxide complexes 105 enter into [2+2]-cycloaddition reactions with alkynes to give titanaoxa- (106a) and titanathiacyclobutenes (106b).195, 196 X X R1C CR2 C R2 Cp2Ti Cp2 Ti Py C 105 R1 106a,b 106a: X=O; R1=H; R2=Me, But, Ph, 4-MeC6H4; R1=R2=Ph; 106b: X=S; R1=R2=H; R1=H, R2=Ph, Me3Si.The azatitanacycles 107 and 108 are prepared by the reaction of the complex 109 with acetylene or ethylene, respectively. Reactions of the complex 109 with phenyl- or trimethylsilylacetyl- ene yield acetylenides 110.197 HC CH Cp2Ti 107 Ph N C2H4 Cp2Ti Cp2Ti NPh 109 108 NPh NHPh RC CH Cp2Ti CR C 110 R=Ph, Me3Si. V A Knizhnikov, N A Maier Some titanocene derivatives containing Ti7C s-bonds were prepared by the reduction of titanocene dichloride (2) in the presence of unsaturated compounds.Thus its reduction by magnesium in the presence of Schiff's bases prepared from a,b- unsaturated aldehydes yields titanazacyclopentenes 111.198 RN1 Mg Cp2Ti Cp2TiCl2+ R1N CHC(R2) CHPh 2 R2 Ph 111 R1=But, 4-MeC6H4, R2=H; R1=cyclo-C6H11, R2=Me. Reduction of bis(pentamethylcyclopentadienyl)titanium dichloride by magnesium in the presence of 1,4-bis(trimethylsi- lyl)buta-1,3-diyne (112) results in the Z3-enyne complex 113.199 Mg Cp2 TiCl2+Me3SiC C C CSiMe3 112 Me3SiC SiMe3 C C Cp2Ti C 113 SiMe3 Reduction of bis(trimethylsilyltetramethylcyclopentadienyl)- titanium dichloride by magnesium in THF in the presence of bis(trimethylsilyl)acetylene gives Z5 :Z1-[C5Me4Si-Me2CH2]. .[Z5-C5Me4(SiMe3)]Ti(III).200 Reactions of ylides Me37n(Me2N)nP=CH2 (n=1, 2) with titanocene dichloride yield complexes Cp2TiCl(CH=PR1R22 ), where R1=NMe2, R2=Me; R1=Me, R2=NMe2.201 Reactions of the titanocene(dicarbonyl) 57 with alkyl iodides gave the acyl complexes Cp2TiI[C(O)R], where R=Me, Et, Pri , Bun (see Refs 202, 203).Acyl titanocene halide derivatives are also formed upon treatment of the titanocene(dicarbonyl) 57 by organic acid chlorides 202, 203 or by treatment of alkyl(titanocene) halides with carbon monoxide.204 III. Reactions of titanocene derivatives containing Ti7C s-bonds Reactions of titanocene derivatives containing Ti7C s-bonds are little studied.The published data on the reactivities of these compounds are conventionally divided into two groups, viz., reactions involving Ti7C s-bonds and those with retention of these bonds. 1. Reactions with retention of Ti7C s-bonds Reactions of halide derivatives of titanocene s-complexes with organometallic derivatives of non-transition metals belong to the most typical reactions of this type. Thus reaction of alkyl- or aryltitanocene chlorides with organolithium compounds was used for the preparation of the complexes 114, which contain two different s-bound organic radicals. LiR2 Cp1Cp2Ti(R1)R2 Cp1Cp2Ti(R1)Cl 114 Ref. R2 R1 Cp2 Cp1 C5H5 C5H5 C5H5 C5H5 C5H5 C5H5 C5H5 205 205 205 206 206 206 207 13 Ph 4-MeOC6H4 4-MeC6H4 Me Ph C:CPh C:CXa Me Me Me Me 2-thienyl 2-thienyl 2-thienyl CH2SiMe3 CH=CH2 C5H5 C5H5 C5H5 C5H5 C5H5 C5H2 C5H5 C5Me5 C5Me4(CH2)2NMe2Titanocene derivatives containing titanium ± carbon s-bonds Ref.R2 R1 Cp2 Cp1 ButC5H4 CH2CH=CH2 Me C6F5 CH=CH2 C5H5 C5Me5 C5Me5 208 209 aX=SiMe3 , 3,5-(Me2NCH2)2C6H3 , 4-NCC6H4 , 4-C5H4N, Fc, 4-I-3,5- (Me2NCH2)2C6H2 . In some cases, synthesis of this type of compound is compli- cated by side reactions. Thus the attempts to prepare non-sym- metrical ethyltitanocene derivatives by the reaction of bis(pentamethylcyclopentadienyl)ethyltitanium chloride with methyllithium, benzylpotassium or vinyllithium yielded only the ethylene complex Cp2 Ti(Z2-C2H4) concomitantly with the libera- tion of the corresponding hydrocarbon. Treatment of bis(penta- methylcyclopentadienyl)vinyltitanium chloride 115 with benzylpotassium or dimethylphosphinomethyllithium gave the complex (Z5-C5Me5)(C5Me4CH2)Ti7CH=CH2.209 The reaction of the complex 115 with vinyllithium yields 1,1-bis(pentamethyl- cyclopentadienyl)-1-titana-2-methylidenecyclobutane (116).209 Presumably, bis(pentamethylcyclopentadienyl)divinyltitanium (117), which is formed in the first step of this reaction, is decomposed to give the derivative 118.The latter adds the ethyl- ene molecule at the Ti=C bond to give the final product 116. Cl +H2C CHLi Cp2 Ti CH CH2 115 CH CH2 Cp2 Ti 7C2H4 CH CH2 117 Cp2 Ti C2H4 Cp2 Ti C CH2 118 116 CH2 Apparently, reactions of phenyltitanocene chloride with butyl-, vinyl- or benzyllithium afford the intermediates Cp2Ti(Ph)R (R=Bun, CH=CH2, Bn) which are further con- verted into phenyldicyclopentadienyltitanium(III).209 Treatment of alkyl- or aryltitanocene chlorides with alkali metals alcoholates 90, 96, 210 ± 213 or thiolates 87, 214 or with lithium amides 215 results in the replacement of chlorine atoms.R2OM Cp1Cp2Ti(R1)OR2 Cp1Cp2Ti(R1)Cl M=Na, Li; Cp1=Cp2=C5H5: R1=C6F5, R2=Me, Et; R1=Me, R2=CH=CH2; R1=2-thienyl, R2=2,4,6-(NO2)3C6H2 , 3,4-(NO2)2C6H3; Cp1=C5H5, Cp2=C5H4CH(Me)Ph, R1=C6F5, R2=2-MeC6H4; R1=R2=Me. Cp2Ti(R1)Cl R2SLi Cp2Ti(R1)SR2 R1=Me, R2=4-NH2C6H4; R1=2-thienyl, R2=Bn, 2-thienyl.LiNR2 S S Cp2Ti Cp2Ti Cl NR2. NR2=NPh2, , N N Reactions of 2-thienyltitanocene chloride with potassium 2,4- dichlorophenoxyacetate, mercury(I) benzoate or silver trifluoro- acetate were used for the preparation of the corresponding 2-thienyltitanocene acylates (119).216 305 MCO2R S S Cp2Ti Cp2Ti OC(O)R Cl 119 M=K, R=2,4-Cl2C6H3OCH2;M=Hg, R=Ph;M=Ag, R=CF3 . The reaction of pentafluorophenyltitanocene chloride with sodium hydroxide occurs with the retention of the Ti7C s-bond and results in the substitution of the hydroxy group for the chlorine atom.211 Some transformations of dicyclopentadienyltitanium(III) derivatives under the action of various oxidants, which may even include dimethylzinc, dimethylcadmium and PbCl2, occur with the retention of the Ti7C s-bond.Thus reaction of ethyl-, phenyl- or vinylbis(pentamethylcyclopentadienyl)titanium with dimethylzinc 217, 218 or dimethylcadmium 217 afforded methyltita- nocene derivatives along with metallic zinc or cadmium. R 1MMe2 2 Cp2 Ti R Cp2 Ti +12M Me R=Et, Ph, CH CH2; M =Zn, Cd. The methylbis(pentamethylcyclopentadienyl)titanium deriva- tives Cp2 Ti(Me)X, where X= Cl, OMe, N=CHBut, are also formed in the oxidation of the corresponding Cp2 TiX derivatives by dimethylzinc or -cadmium.217, 218 It is noted 217 that dimethyl- mercury does not enter into such reactions. Alkyl- or acyldicyclopentadienyltitanium(IV) chlorides were prepared by oxidation of s-alkyl- or s-acyldicyclo- pentadienyltitanium(III) derivatives by lead dichloride.109, 219 Treatment of dicyclopentadienyltitanium(III) derivatives with diphenyl disulfide is also accompanied by the oxidation of Ti(III) to Ti(IV) with the retention of the Ti7C s -bond.102, 103, 107 R1 Ph2S2 Cp2Ti Cp2Ti R1 SPh R1=C(O)CMe CHMe; 2,6-Me2C6H3N CR2, R2=Me, Et, Bun.CR1 I2 Cp2Ti Cp2Ti In some cases, reaction of dicyclopentadienyltitanium(III) s-derivatives, e.g., complexes 120 and 121, with iodine also occurs with the retention of the Ti7C s-bond resulting in titanium(IV) derivatives.103, 220 CR1 NR2 NR2 I 120 R1=Me, R2=2,6-Me2C6H3; R1= Ph, 2-MeC6H4 , R2=2-MeC6H4, 2,6-Me2C6H3. R1 R1 I2 Cp2Ti Cp2Ti I C NR2 121 R1=Ph, 2-MeC6H4; R2=2-MeC6H4, 2,6-Me2C6H3.Sometimes, reduction of titanocene s-derivatives occurs with the retention of the metal ± carbon s-bond. Thus the reaction of alkyltitanocene chlorides with zinc or magnesium yields titanium(III) complexes.40, 104 R M Cp2Ti R Cp2Ti Cl M =Zn,Mg; R=C6F5, Me, CH2SiMe3, CH2PPh2.306 Reaction of titanaindan 122 with naphthalenesodium occurs with the opening of the metallacycle and the formation of the complex 123.221 Me3SiC5H4 C10H8, Na Ti Me3SiC5H4 122 Me3SiC5H4 CH2Na TiCH2 123 Me3SiC5H4 In some cases, the role of reducing agents for titanocene s-derivatives is played by organolithium compounds. Thus the reaction of di(cyclopentadienyl)diphenyltitanium(IV) with phe- nyllithium in ether results in the reduction of Ti(IV) to Ti(III) and the formation of diphenyl(cyclopentadienyl)titanium(III) and diphenyltitanium(II).Diphenyl(cyclopentadienyl)titanium(III) is formed first. An excess of phenyllithium causes the decomposition of the Ti(III) complex, the diphenyltitanium being the final product.222, 223PhLi [Cp2TiPh3]7Li+ Cp2TiPh2 7Ph CpTiPh2+CpLi [Cp2TiPh2]7Li+ CpTiPh2+PhLi [CpTiPh3]7Li+ 7Ph TiPh2+CpLi Reactions with retention of the Ti7C s-bond include the formation of complex bimetallic compounds based on titanocene s-derivatives. Thus reactions of diphenyltitanocene with diphe- nylzinc or triethylaluminium yield the binuclear complexes (C5H4)4Ti2Ph2Zn (see Ref. 224) and [Cp(C5H4)TiHAlEt2]2 in which Ti and Al atoms are linked through a bridging hydride ligand (see Ref.225). It is of note that the derivatives of the type Cp2TiX2AlR2 (X=Cl, I, Br; R=Alk, Ar) are the most well-studied titanium ± aluminium complexes;226 however, they do not contain Ti7C s-bonds and are therefore not considered in this review. The bimetallic Ti(III) complexes in which the alkali metal ion was incorporated between the acetylenide fragments (tweezer complexes) were synthesised from titanocene acetylenides and its derivatives containing substituents in the cyclopentadienyl ligands. Thus the complexes 124 were prepared by the reaction of (Z5-C5HMe4)2Ti(Z1-C:CSiMe3)2 with alkali metals in tol- uene.63, 227 CSiMe3 Me4 C M+ Ti C Me4 CSiMe3 124 M=Li, Na, K. 2 A similar complex [(Z5-C5HMe4)2Ti(Z1-C:CC:CSiMe3)¡ [Li(THF)2]+ (see Ref.125) was prepared by treatment of bis- (tetramethylcyclopentadienyl)(4-trimethylsilylbuta-1,3-diynyl)ti- tanium in THF with an excess of 1-lithio-4-(trimethylsilyl)buta- 1,3-diyne. Bis(alkynyl)titanocenes are very good chelating ligands owing to their ability to form bimetallic complexes with copper(I), iron(II) and silver salts. Thus reactions of bis(trimethyl- silylcyclopentadienyl)bis(trimethylsilylethynyl)titanium with CuCl, FeCl2 and various silver salts yield the complexes 125a ± g. V A Knizhnikov, N A Maier CSiMe3 CSiMe3 C C MXn MXn (Me3SiC5H4)2Ti (Me3SiC5H4)2Ti C C 125a ± g CSiMe3 CSiMe3 Ref. M X n Compounds 125 228, 229 228, 230 230 230 230 230 230 Cl Cl Cl CN SCN NO2 ClO4 Cu Fe Ag Ag Ag Ag Ag abcdefg 1211111 In the reaction of the complex 125a with lithium acetylenides, the chlorine atom is replaced by the ethynyl group to give complexes of the general formula (Z5-C5H4SiMe3)2Ti..(Z1-C:CSiMe3)2CuC:CR, where R=SiMe3, But, Ph.229 The structurally similar copper and silver complexes (Z5-C5H4SiMe3)2Ti(Z1-C:CSiMe3)2MR, where M=Cu, Ag and R=2,4,6-Me3C6H2, have been synthesised.231 Bis(trimethylsilylcyclopentadienyl)bis(ferrocenylethynyl)tita- nium (126) reacts with nickel tetracarbonyl or tetra- kis(triphenylphosphine)palladium to give the complexes 127 69 and 128.232 CFc C Ni(CO)4 NiCO (Me3SiC5H4)2Ti CFc C C 127 CFc (Me3SiC5H4)2Ti C C 126 CFc Pd(PPh3)4 (Me3SiC5H4)2Ti C 128 CFc PdPPh3 CFc Reaction of the complex 126 with silver hexafluorophosphate yields the complexes 129a ± c in which the silver ion is bound to the four triple bonds of the acetylenic groups.233 Compounds 129d,e were prepared analogously.R2C CR2 C C Ag+ (R1C5H4)2Ti Ti(C5H4R1)2 PF¡6 C C R2C CR2 129a ± e R1=H,R2=Fc (a); R1=Me3Si, R2=Fc (b); R1=H,R2 = (C C)2Fc (c); R1=Me3Si, R2=Ph (d); R1=Me3Si, R2=Rc. Bis(trimethylsilylcyclopentadienyl)(trimethylsilylethynyl)tita- nium and bis(trimethylsilylcyclopentadienyl)(hexa-1,3-diynyl)- titanium chlorides form the complexes [(C5H4SiMe3)2Ti(R)Cl]. .CuBr [R=C:CSiMe3,234 C:C7C:CEt (see Ref. 61)] with copper(I) bromide.The complexes 130 were synthesised from CR2 C MXn (R1C5H4)2Ti C 130 CR2 R1=Me2NCH2CH2, R2=Me3Si, MXn=AgCl; R1=Ph2P, Ph2P(O), Ph2P(S); R2=But, MXn=CuCl, Mo(CO)4; R1=Me3Si, R2=Ph, MXn=CoCO.Titanocene derivatives containing titanium ± carbon s-bonds dialkynyltitanocene derivatives containing various substituents in the cyclopentadienyl ligands.65, 235 The reaction of the triyne titanacycles 131 with bis(cycloocta- diene)nickel Ni(COD)2 yields the binuclear complexes 132.60 C C Cp1 Cp1 C C C C Ni(COD)2 Ti Ti Ni C C Cp2 Cp2 C C C C 132 131 Cp1=Cp2=C5H5, C5Me5. Dicyclopentadienylbis(diphenylphosphinomethyl)titanium and bis(dicyclopentadienyldiphenylphosphinomethyltitanium) oxide form the complexes [Cp2Ti(CH2PPh2)2Rh(CO)Cl]2 (see Ref.40) and O[Cp2TiCH2PPh2]2Rh(CO)Cl 236 with dicarbonyl- rhodium chloride. The cationic complex [Cp2Ti(CH2PPh2)2Rh. .(COD)]+[BPh4]7 was prepared by treatment of dicyclo- pentadienylbis(diphenylphosphinomethyl)titanium with the rhodium complexes [L2Rh(COD)]+[BPh4]7, where L= Ph3P, COD.237 The reaction of titanacumulene (133) with dicyclopentadie- nylzirconium occurs with the cleavage of the central C=C bond and results in the formation of the heterodimetallic titanocene ± zirconocene complex 134 with a s,p-alkynyl bridge.142 But CBut C +Cp2Zr Cp2Ti ZrCp2 Cp2Ti ButC C 134 But 133 2. Insertion into Ti7C s-bonds Transformations of titanocene derivatives occurring with inser- tion of various fragments into Ti7C s-bonds have been described.Thus isocyanides are inserted into the Ti7C s-bonds of alkyl- or aryltitanocenes 36 to give the titanacycles 135 at 20 8C. At the same time, at 778 8C the reaction of the complexes 36 with isocyanides yields exclusively the adducts Cp2TiR1.R2NC. R2 C R3NC (R15 C5)2TiR2 (R15 C5)2Ti 20 8C N 36 135 R3Ref. R3 R2 R1 Et Bu Me Me Ph Ph 2-MeC6H4 2-MeC6H4 102 102 102, 103 109 220 220 220 220 2,6-Me2C6H3 2,6-Me2C6H3 2,6-Me2C6H3 2,6-Me2C6H3 2-MeC6H4 2,6-Me2C6H3 2-MeC6H4 2,6-Me2C6H3 HHHMe HHHH Insertion into the Ti7C s-bond takes place in the reaction of cyclohexyl isocyanide with dimethyltitanocene (1) 238 or the meth- ylidenetitanacyclobutene complexes 136 169 as well as in reactions of tert-butyl isocyanide with 1-oxa-5-titanacyclopentanes 137 239 or bis(methylethynyl)titanocene 138 in the presence of dimethy- lanilinium tetraphenylborate.240 307 Me Me C N C6H11-cyclo cyclo-C6H11NC Cp2Ti Cp2Ti Me Me 1 C6H11-cyclo R N R cyclo-C6H11NC R Cp2 Ti Cp2 Ti R 136 R=Me, Ph.R O R O Cp2 Ti ButNC Cp2 Ti N 137 But R=Me, Et, Pri. + CMe CMe C C ButNC C BPh¡ Cp2Ti 4 [PhNMe2H]+BPh¡4C Cp2Ti NBut 138 CMe C NBut As in the case of isocyanides, reactions of alkyltitanocenes with nitriles at low temperatures give the adducts Cp2TiR1 .R2CN (R1=Ph, 2-MeC6H4, Bn, C6F5; R2=Me, But, Ph, 2-MeC6H4, 2,6-Me2C6H3).241 An increase in the temperature leads to dimer- isation of the nitrile ligand.242 R1 R1 R1 R2CN D R2 R2 TiCp2 Cp2Ti Cp2Ti Cp2TiR1 778 8C N N C C N CR2 R1=Ph, 2-MeC6H4, Bn, C6F5; R2=Ph, Me, 2-MeC6H4.Treatment of methyltitanocene chloride with equimolar amounts of nitriles in the presence of sodium tetraphenylborate gives the complexes 139 which further yield the azaalkylidene derivatives 140 due to the insertion of the nitrile molecule into the Ti7C bond.243, 244 + Me Me RCN RCN BPh¡ Cp2Ti Cp2Ti 4 NaBPh4 N CR Cl 139 + CR N BPh¡ R Cp2Ti 4 C N Me 140 R=Me, Pr, Ph, But. In this reaction, trimethylsilyl cyanide is isomerised into the corresponding isocyanide, which forms an Z2-iminoacyl complex [Cp2Ti(CMe=NSiMe3)(C:NSiMe3)]+BPh¡4 with methyltitano- cene chloride (see Ref.244). Reactions of the titanaluminium complex Cp2Ti(m-Cl). .(m-CH2)AlMe2 or 2-tert-butyl-1,1-bis(cyclopentadienyl)-1-tita- nacyclobutane with tert-butyl cyanide in the presence of trime- thylphosphine or 4-(N,N-dimethylamino)pyridine gives first the vinylimide complexes 141, which react with an excess of nitrile to yield the 2,6-diaza-1-titanacyclohexa-2,5-diene derivative 142.245, 246308 CH2 Cp2Ti AlMe2 Cl Cp2Ti But But N Cp2Ti N 142 But L=PMe3, 4-Me2NC5H4N. 2,6-Diazatitanacyclohexa-2,4-dienes 142 (R=But, Ph) are formed upon refluxing of tert-butyl or phenyl cyanide with dimethyltitanocene (1) in benzene.151 Me RCN Cp2Ti Cp2Ti PhH, D Me 1 R=But, Ph. The direction of reactions of titanocene s-derivatives with carbon monoxide depends on the reaction conditions and the structure of the organometallic compound.Thus treatment of alkyl-, aryl- and vinyldicyclopentadienyltitanium(III) derivatives as well as ethyl- or methyltitanocene chlorides with carbon monoxide leads to the insertion into the Ti7C s-bond eventually resulting in the acyltitanocene complexes 143.109, 204, 247, 248 Cp1 Cp1 CO Ti R Cp2 Cp2 Cp2 Cp1 RMe Et Me n-C5H11 C(Me)=CHMe 2-MeC6H4 C5Me5 C5Me5 C5Me5 C5Me5 C5H5 C5H5 C5H5 C5H5 C5H5 C5H5 C5H5 C5H5The products of CO insertion into the Ti7C bond (144 ± 146) were prepared by treating acetonitrile(methyl)titanocene tetra- phenylborate 243 or the corresponding titanacyclopentane and titanaoxacyclopentane derivatives 77, 239 with carbon monoxide. R + C(O)Me BPh¡ Cp2Ti CMe N 144 Reactions of chromium and tungsten hexacarbonyls with the methyl(vinyl)titanocene derivative 147 occur as the insertion of CO into the Ti7C s-bond and the formation of the bimetallic complexes 148.13 L ButCN ButCN Cp2Ti L N C CH2 141 But R NHN R 142 Ti C R O 143 Ref.T /8C 770 77000 106 106 247 247 106, 248 106, 248 20 20 O 4 Cp2Ti Cp2 Ti O O 145 146 R=Me, Et, Pri. V A Knizhnikov, N A Maier O Me Cp1 Cp1 Ti Ti +M(CO)6 M(CO)5 7CH4 CH Cp2 Cp2 CH2 148 147 Cp1=C5Me4(CH2)2NMe2, Cp2=C5Me5; M=Cr, W. In contrast to alkyldicyclopentadienyltitanium(III) deriva- tives, pentafluorophenyltitanocene reacts with carbon monoxide (25 8C, 1 atm) with the formation of the carbonyl complex Cp2Ti(CO)C6F5.247 Photochemical reactions of dimethyl- or diphenyltitanocene with carbon monoxide 25, 140 or treatment of dibenzyltitanocene 204, 249 and titanaindan derivatives 18a,b 70 in benzene with carbon monoxide gave the titanocenedicarbonyl Cp2Ti(CO)2. It was noted that the reaction of carbon monoxide with dibenzyltitanocene in pentane is sometimes associated with the intermediate formation of the benzylacyl complex Cp2Ti(COBn)Bn.249 Reactions of some titanocene derivatives with carbon dioxide also occur by insertion into the metal ± carbon s-bond. Thus the photochemical reaction of dimethyltitanocene with carbon diox- ide in pentane (16 8C, 20 h) results in methyltitanocene acetate Cp2Ti(Me).[OC(O)Me].250 Passage of carbon dioxide through a solution of dimethyltitanocene in toluene at 80 8C leads to the insertion ofCO2 simultaneously into two Ti7Cs-bonds resulting in the titanocene diacetate Cp2Ti(OC(O)Me)2.251 Treatment of diphenyltitanocene with carbon dioxide in xylene at 80 ± 90 8C gives the titanacyclic compound 149.251, 252 Ph CO2 80 ± 90 8C Cp2Ti Cp2Ti Cp2Ti 7PhH Ph 72 O C 149 O It is believed that compound 149 is formed as a result of insertion of carbon dioxide into the phenylenetitanium complex 72 formed upon splitting of the benzene molecule from diphe- nyltitanocene. Reactions of Z2-iminoacyl complexes with carbon dioxide occur by insertion of CO2 into the Ti7Alk bond and yield dicyclopentadienyltitanium(III) acylates 150.102 R C CO2 Cp2Ti(OCOR) Cp2Ti N 150 C6H3Me2-2,6 R=Me, Et, Bun.The product of insertion into the Ti7C s-bond 151 was obtained in the reaction of the binuclear complex 152 withCO2.253 O Bui Bui C O 3 3 N N Cp2Ti Cp2Ti CO2 PhH, 10 8C Cp2Ti N N Cp2Ti C O Bui Bui 3 3 O 151 152 The insertion into the Ti7C s-bond and the formation of O-sulfinates take place in the treatment of dimethyl-, diphenyl- or bis(2-thienyl)titanocenes 254 ± 256 and methylchloro- or methyl- (pentafluorophenyl)titanocenes 254, 257, 258 with sulfur dioxide.Titanocene derivatives containing titanium ± carbon s-bonds R1 R1 O R2 O S R2 SO2 Ti Ti O R2 S R2 O R1 R1 R1=H,R2=Me, Ph, 2-thienyl; R1=R2=Me.R1 R1 O O S Me Me SO2 Ti Ti R2 R2 R1=CH(Me)Ph, R2=C6F5; R1=H,R2=Cl, C6F5. The reaction of carbon disulfide with vinyldicyclopentadie- nyltitanium(III) gives the paramagnetic insertion product Cp2Ti[SC(S)CMe=CHMe].107 3. Cleavage of Ti7C s-bonds Whereas the p-bond between the cyclopentadienyl ligand and the titanium atom is stable in acidic media, the Ti7C s-bonds are easily cleaved by acids. Thus dialkyl- and diaryltitanocene deriv- atives react with an excess of hydrogen chloride to give the corresponding titanocene dichlorides. HCl Cp1Cp2TiCl2 Cp1Cp2TiR2 Ref. Cp2 R Cp1 Ph Me Fc 259 260 49 C5H5 C5H5 C5H5 C5H5 C5H5 C5H5 C5H4 Me2C Me 260 C5H4 56 56 C5H4Me C5H4SiMe3 C5H4Me C5H4SiMe3 C:CRa C:CRa aR=Et, Prn, Bun, But, Me3Si, Ph.Titanocene dichloride is prepared by treatment of titana- cycles, such as 1,1,-dicyclopentadienyl-1-titanacyclopentane,78 1,1-dicyclopentadienyl-1-titanafluorene,73 titanacyclopenta- dienes 54a,b130 or titanabicyclopentane 84, 168 with an excess of hydrogen chloride. The complexes Cp1Cp2TiCl2 containing two different p-bound cyclopentadienyl ligands were prepared by the reaction of hydrogen chloride with the titanocene derivatives Cp12 Ti(Cp2)Cl in which the two cyclopentadienyl ligands were linked by p-bonds and one cyclopentadienyl ligand, by s-bonds.261, 262 Reactions of dialkyl- or diaryltitanocenes with equimolar 2Ti(R)Cl, where Cp1=C5H5, 4 was prepared by the reaction of dimethyl- amounts of hydrogen chloride result in the cleavage of only one Ti7C s-bond and yield alkyl- or aryltitanocene chlorides Cp1Cp2Ti(R)Cl and Cp2 Cp2=C5Me4H: R=Me, Ph, 4-MeC6H4;7 Cp1=Cp2=C5H5, R=2-thienyl,214 3,4,5-trichloro-2-thienyl.43 The cationic com- plex [Cp2TiMe]+BF¡ titanocene with an equimolar amount of fluoroboric acid.263 In contrast to the majority of titanocene s-derivatives, bis(o- carboranylmethyl)- and bis(pentafluorophenyl)titanocenes are relatively stable in acidic media and are converted into titanocene dichloride only after prolonged boiling with hydrochloric acid.24, 45 The retention of the s-bond between the titanium 309 atom and the perfluorophenyl ligand was also observed in the treatment of methylsulfinate and perfluorophenyltitanocene alco- holates Cp(C5H4CHMePh)Ti(C6F5)(O2SMe) (see Ref.257) and Cp(C5H4CHMePh)Ti(C6F5)OR (R=Me, 2-MeC6H4) with hydrogen chloride.96 The cleavage of Ti7C s-bonds takes place upon treatment of alkyl- or aryltitanocenes with organic acids. Thus dimethyltitano- cene reacts with equimolar amounts of acetic,205 benzoic or 2-phenylpropionic acids 264 with cleavage of one of methyl groups eventually resulting in the corresponding methyltitanocene acyl- ates. O C R RCO2H Cp2Ti Cp2TiMe2 O Me R=Me, Ph, CH(Me)Ph. Treatment of dimethyltitanocene with an excess of phenyl- propiolic or trifluoromethanesulfonic acid results in the replace- ment of two s-bound methyl groups by acid residues.265, 266 The reaction of pyridine-2,6-dicarboxylic acid with dimethyltitano- cene yields the complex 153.260 OC(O)N .Cp2Ti OC(O) 153 The formation of titanocene diacylates takes place in the reaction of bis(2-thienyl)titanocene 10a with 3,5-dinitrobenzoic, perfluorobenzoic and trifluoro-, trichloro- and tribromoacetic acids.42 O C R S RCO2H Cp2Ti Cp2Ti S O OC R O 10a R=3,5-(NO2)2C6H3, C6F5, CF3, CCl3, CBr3. The reaction of methyltitanocene chloride with benzoic or phenylacetic acids afforded mixtures of titanocene dichloride with the corresponding titanocene diacylates Cp2Ti[OC(O)R]2 ; their formation is due to symmetrisation of the intermediate chloroti- tanocene acylates Cp2TiCl[OC(O)R].267 The formation of a Cp2TiMe2, Cp2 TiMe[OC(O)CF3] and Cp2Ti[OC(O)CF3]2 mix- ture is also observed upon treatment of 1,1,3,3-tetracyclopenta- dienyl-1,3-dititanacyclobutane with trifluoroacetic acid.167 Reactions of (2-thienyl)titanocene chloride with trichloro- and tribromoacetic, perfluorobenzoic or 2-nitrobenzoic acids gave the corresponding chlorotitanocene acylates.216 O C R RCO2H S Cp2Ti Cp2Ti O Cl Cl R=CCl3, CBr3, C6F5, 2-NO2C6H4.It is of note that treatment of some methyltitanocene alcohol- ates Cp2TiMe(OR1) (R1 is benzyl, menthyl, bornyl) with benzoic or diphenylacetic acids results in the cleavage of the alkoxide ligand (rather than the methyl ligand) and affords the methyltita- nocene acylates Cp2Ti(Me)OC(O)R2, where R2=Ph, CHPh2.267 Ti7C s-bonds are cleaved upon the action of hydrogen chloride on alkyl- or aryl[dicyclopentadienyltitanium(III)] and their complexes with molecular nitrogen or310 nitriles.103, 105, 112, 241, 268 The titanocene monochloride formed at low temperatures (778 8C) is oxidised to titanocene dichloride at 20 8C.The Ti7C bond is also cleaved under the action of some alcohols and phenols on titanocene derivatives. Thus reactions of dimethyltitanocene with an equimolar amount of alcohols bearing bulky radicals gave the methyltitanocene alcoholates Cp2Ti(Me)OR [R is benzyl, menthyl, bornyl,267 CHMeC8H17, CMePh2, CHPrnPh, CHPhBn, CH2CH=CHPrn (see Ref. 264)]. Titanocene trifluoromethanesulfonate alcoholates Cp2Ti(O3SC- F3)(OR), where R= Me, Me3Si, Me2ButSi, are formed upon treatment of methyltitanocene trifluoromethanesulfonate with the corresponding alcohols.265 Reactions of dimethyltitanocene with (+)-(2R,3R)-dimethyl and (+)-(2R,3R)-dibutyl tartrates 269 and reactions of diphenyltitanocene with 2,2-dimethylpropane- 1,3-diol 270 yielded the binuclear complexes 154 and 155, respec- tively.Me Ph OTiCp2 RO2C Me OTiCp2 OTiCp2 CO2R Cp2TiO Me 155 Ph Me 154 R=Me, Bun. A bimetallic complex was also formed when chromotricarbo- nyl(hydroxymethyl)benzene was used.271 R CH2OH Cp2Ti + Cr Br OC CO CO O CH2 Cp2Ti Cr Br CO OC CO The reaction of bis(2-thienyl)titanocene (10a) or (2-thien- yl)titanocene chloride with picric acid, 4-hydroxybiphenyl or 3,4- dimethylphenol afforded the corresponding titanocene dipheno- lates and chlorotitanocene phenolates.213 OR S ROH Cp2Ti Cp2Ti S OR 10a OR ROH S Cp2Ti Cp2Ti Cl Cl R=2,4,6-(NO2)3C6H2, 4-PhC6H4, 3,4-Me2C6H3.The cleavage of the Ti7C s-bond by propane-1,3-dithiol has been described.272 S(CH2)3S HS(CH2)3SH Cp2Ti Cp2TiPh2 TiCp2 S(CH2)3S Treatment of dialkyl- or diaryltitanocene derivatives with halogens results in the cleavage of the metal ± carbon s-bonds to give the corresponding titanocene dihalides Cp2TiX2 (X=Cl, Br, I).31, 42, 56, 73, 78, 79, 273, 274 It was shown that the reaction of V A Knizhnikov, N A Maier (2-thienyl)titanocene derivatives with iodine proceeds in a step- wise manner; the s-bound thienyl group is split off in the first step.275, 276 I I2 S Cp2Ti Cp2Ti X X X=3,4-Me2C6H3O, PhS, Ph2N, Cl, 2-thienyl.Treatment of alkyl- or aryldicyclopentadienyltitanium(III) derivatives with an excess of halogens occurs as the cleavage of the Ti7C s-bond and the oxidation of the titanium atom leading to the formation of titanocene dihalides.105, 112, 268 Substitution of chlorine atoms for s-bound alkyl or aryl ligands takes place in the reaction of dialkyl- or diaryltitanocenes with mercury dichloride. HgCl2 Cp2TiCl2+RHgCl Cp2TiR2 Ref. R Ref. R 36 66 Bn o-carboranylmethyl 251, 277 278, 279 Me Ph The cleavage of s-bound pentafluorophenyl ligands in bis- (pentafluorophenyl)titanocene under the action of TiCl4 (see Ref. 255) or BCl3 was observed.35 The cleavage of Ti7C s-bonds and the formation of titanocene dichloride takes place upon treatment of dicyclopentadienyl-2,4-bis(trimethylsilyl- ethynyl)-3,5-bis(trimethylsilyl)titanacyclopenta-2,4-diene with S2Cl2,280 as well as upon treatment of Cp2Ti[C:CFc]2 with nickel chloride.62 Exchange reactions of methyl and phenyl groups for halogen atoms upon treatment of dimethyl- or diphenyltitanocene with titanocene dihalides have been studied.251 Cp2Ti(R)X Cp2TiR2+Cp2TiX2 R=Me, Ph; X=F, Cl, Br, I.It was found that reaction rates decrease in the following order: I>Br>F>Cl for dimethyltitanocene and I>Cl> Br>F for diphenyltitanocene. Heating of dimethyl- or diphenyltitanocene with chloroform or carbon tetrachloride is also accompanied by the cleavage of Ti7C s-bonds resulting in titanocene dichloride.277, 279 Heating of diphenyltitanocene with benzyl chloride yields a titanocene dichloride ± (cyclopentadienyl)titanium trichloride mixture.281 1,1-Dicyclopentadienyl-1-titanacyclobutane reacts with benzyl or allyl chloride to give alkyltitanocene chlorides Cp2Ti(CH2R)Cl (R= Bn, All).282 Cleavage of Ti7C s-bonds takes place upon treatment of diphenyltitanocene with benzoyl peroxide.283 ± 285 When this reaction is carried out in isopropyl alcohol, titanocene dibenzoate Cp2Ti(O2CPh)2 is formed as the major product.The dicyclopen- tadienyltitanium fragment is destroyed in ether or benzene to give the cyclopentadienyltitanium dibenzoate CpTi(O2CPh)2. In con- trast to reactions of diphenyltitanocene with benzoyl peroxide, treatment with 3- or 4-nitrobenzoyl peroxides is not accompanied by the cleavage of the titanocene fragment and the corresponding titanocene bis(nitrobenzoates) are formed.286 The Ti7Cs-bonds of methyl- or butyltitanocenes are split off under the action of pyridine derivatives.287 Cp2TiR1+R2C5H4N R2 N Cp2Ti R1=Me, Bu; R2=2-Me, 2-Ph, 2-CH=CH2.Titanocene derivatives containing titanium ± carbon s-bonds Reactions of thienyltitanocene chloride with acetylacetone, benzoylacetone or dibenzoylmethane gave the corresponding chlorotitanocene b-diketonates.288 R1 Cl O R1COCH2COR2 S Cp2Ti Cp2Ti O Cl R2 R1=R2=Me, Ph; R1=Ph, R2=Me.* * * The data presented demonstrate high reactivities of titanocene derivatives containing Ti7C s-bonds. 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Russian Chemical Reviews 71 (4) 315 ± 327 (2002) Chemical aspects of implantation of high-energy ions into polymeric materials D V Sviridov Contents I. Introduction II. Peculiarities of the interaction of high-energy ions with polymeric target III. Chemical processes occurring during ion bombardment of polymers IV. Peculiarities of the morphology and properties of ion-implanted layer V. Prospects for practical applications of ion-implanted polymers VI. Conclusion Abstract. ion accelerated of influence the of study the on Data Data on the study of the influence of accelerated ion beams mechanisms The generalised. are materials polymeric on beams on polymeric materials are generalised. The mechanisms of of chemical upon matrix polymer a in occurring processes chemical processes occurring in a polymer matrix upon ion ion implantation the in changes crucial in resulting and implantation and resulting in crucial changes in the electrical electrical conductivity, other and hydrophilicity microhardness, conductivity, microhardness, hydrophilicity and other character- character- istics Main discussed.are surface polymer irradiated the of istics of the irradiated polymer surface are discussed. Main fields fields of applications of ion-implanted polymeric materials are consid- of applications of ion-implanted polymeric materials are consid- ered. references 188 includes bibliography The ered. The bibliography includes 188 references. I. Introduction Long-term ion bombardment of a polymer causes important transformations of the material, accompanied by the formation of specific modifications of amorphous hydrogenated carbon which exhibits conducting properties. Ion irradiation is also responsible for an increase in the conductivity of initially dielectric polymers (e.g., polyimide and polyethylene) by 10 to 15 and even 18 orders of magnitude.Studies carried out in the last decade allowed one to gain a deeper insight into the mechanisms of ion bombardment induced processes in polymeric materials and showed that varying the main parameters of ion irradiation (energy and mass of implanted ions, implantation dose, ionic current) has a significant effect on the formation of a carbona- ceous phase in the implanted layer. These studies also revealed the possibility of effective control of not only conducting and optical, but also tribological, hydrophilic and hydrophobic properties of the irradiated polymer surface.Salient features of ion implantation into polymeric targets have been reviewed.1 ±6 However, the emphasis was placed on systematisation of the data on the nature of the conducting properties of ion-implanted polymers. This review (i) concerns the character and mechanisms of chemical reactions occurring in the polymer matrix during ion irradiation to high fluences and (ii) analyses the role of these processes in the formation of the properties of ion-implanted layers formed at the surface of polymeric materials. D V Sviridov Institute of Physicochemical Problems, Belarussian State University, ul.Leningradskaya 14, 220050 Minsk, Belarus. Fax (37-517) 226 55 67. Tel. (37-517) 209 51 80. E-mail: sviridov@bsu.by Received 31 January 2002 Uspekhi Khimii 71 (4) 363 ± 377 (2002); translated by AMRaevsky #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n04ABEH000710 315 315 316 319 323 324 II. Peculiarities of the interaction of high-energy ions with polymeric target Specific effects of ion irradiation on polymeric samples are first of all due to very high density of energy transferred to the irradiated target. Energy loss of incident ions in polymer matrices can be as high as tens and even hundreds of electron-volts per 1A of track length. Energy deposition occurs within a very small volume along the ion track and takes a very short time (*10713 s).7 Taking into account the fact that the bond dissociation energies in polymers do not exceed 10 eV (e.g., 4.3 eV for the C7H and 9.3 eV for the C=N bond), the energy deposited by the stopping projectile is high enough to break a large number of chemical bonds within the track volume even in the case of implantation of ions with medium energies (from tens to hundreds of kiloelectron-volts).This, in turn, creates prerequisites for essential transformations in the irradiated polymer during ion bombardment. The character of chemical processes induced in the polymer matrix by ion implantation is strongly affected by two fundamen- tally different mechanisms of energy transfer from the incident ions to the polymer, namely, by inelastic (electronic excitations) and elastic (nuclear) collisions.7 The former mechanism leads to excitation and ionisation of polymer molecules and (also as a result of fast subsequent neutralisation) to the formation of a large number of mobile radical species (first of all, H.radicals) and macroradicals. Nuclear collisions are accompanied by massive rupture of chemical bonds due to displacement of atomic nuclei and by transfer of rather high energy to the polymer matrix in the form of phonons.7 Once the deposited energy is sufficiently high, the displaced atoms can, in turn, create their own track cas- cades.1, 7 The contribution of each mechanism to the overall energy loss of an incident ion is characterised by its stopping power, Sn (for `nuclear') and Se (for `electronic' energy loss), which represents the change in the ion energy caused by its interaction with the polymer matrix per unit path length.The Sn and Se values depend on the energy and mass of incident ions. High-energy light ions deposit their energy mostly by electronic excitations, while heavy ions lose their energy nearly completely by nuclear collisions.7, 8 As the ion slows down, the contribution of each mechanism to the overall energy loss substantially changes (e.g., a feature of the implantation of light-element ions is an increase in the relative weight of `nuclear' energy loss at the end of the ion track 7, 8). Thus, stopping of ions is accompanied by deposition of rather high energy within the ion tracks and is followed by the formation of radicals, secondary ions and electrons.A certain fraction of316 electrons (the so-called d-electrons) have energies high enough to escape the primary ion track.7, 9 The tracks of d-electrons (secon- dary tracks) form a region called `penumbra' 10 in which the excitation and ionisation of the polymeric material also occurs. For ions with energies of the order of 100 keV to 1 MeV per nucleon, the radius of the primary track lies between 5 and 10A 11 while the penumbra radius can reach 50 to 100A (this estimate was obtained for water which differs only slightly in stopping power from most polymers).10, 11 Compared to the ion track, the density of energy deposited in the penumbra is an order of magnitude lower 1, 9 ± 11 and the processes occurring in this region can be considered as pure radiation processes.Energy deposition occurs mainly within the track core which is thus heated to very high temperatures. Heating is due to electrons confined within the track by a strong electric field 12 perpendicular to the track axis and to direct transfer of vibrational energy to polymer by nuclear collisions.7 The `thermal spike' model allows estimation of tem- perature within a cylindrical track as a function of the radial distance (r) and time (t) elapsed after passage of an ion as follows:13 ± 16 0Ü2 0 0 , ¡ Ör=r 1 á 4dt=r2 T (r,t)=1 á 4dt=r2 exp T0 S T0=prCVr20 , where T0 is the initial temperature (at the instant the thermal spike is created), d is the thermal diffusivity of the condensed medium in which the ion slows down, r0 is the radius of the ion track, S is the overall stopping power for the incident ion, r is the density of the polymer and CV is the heat capacity of the polymer.The T0 values corresponding to the implantation of ions with energies ranging from 0.1 to 1 MeV into various polymers can be as high as *104 K, while quenching of the track down to the temperature of a polymeric target under ion irradiation [*100 ± 150 8C (see Refs 17 and 18)] takes 1079± 10710 s. This formula provides only a rough estimate of the parameters of the thermal pulse produced upon passage of a high-energy ion; nevertheless, it allows one to estimate the main parameters (including the time parameters) of the thermal effects accompanying ion implantation.It should be noted that the experimental estimate of the temperature of the electron cloud created during ionisation of the atoms within the ion track can be obtained from analysis of the C (KVV) spectrum of the Auger electrons generated in the ion bombarded amor- phous carbon (this is closely related to the conditions in heavily implanted polymeric materials); the theoretical estimate of the T0 value is in reasonable agreement with the experimental results of these experiments.19 Build up of the dose of implanted ions leads to basic changes in the character of ion-induced processes.Initially, a pristine poly- mer is ion bombarded until accumulation of a certain dose Df corresponding to complete coverage of the surface of the polymer sample with ion tracks. Futher implantation occurs into the polymer exposed to ion irradiation. In this case, the processes in the polymeric target represent a specific kind of pulse thermo- radiolysis since the products of radiochemical reactions formed within and in the vicinity of the track region and those accumu- lated in the polymer matrix during previous irradiation are involved in the thermally induced reactions occurring in the thermal spike. III. Chemical processes occurring during ion bombardment of polymers Studies of the effect of ion irradiation on various polymeric materials showed that implantation to high doses leads to carbon- isation of the surface of the polymer sample and to the formation of specific surface forms of amorphous hydrogenated car- bon.20 ± 24 Carbonisation occurs efficiently if the energy loss per D V Sviridov unit track length (or the linear energy transfer, LET) of incident ions exceeds a particular threshold value (15 to 20 eV A71 for aliphatic polymers 1).Accumulation of carbon in the implanted layer is accompanied by evolution of a large amount of dihydro- gen (usually, the major gaseous product of ion implantation), low- molecular-mass hydrocarbons (first of all, acetylene) and CO and CO2 in the case of oxygen-containing polymers (Table 1).1, 24 ± 28 Table 1. Relative yields of molecular products formed during irradiation of polymer surface with 200 keV He+ ions.1, 24 Polyimide Polystyrene Molecular species Polyethylene (fragment) 1 b 0.01 0.53 0.004 0.013 1 a 0.02 0.11 0.01 7 170.05 7721 H2 CH4 C2H2 C3H5 C6H6 CO CO2 Note.All values were normalised to the yield of dihydrogen. a The absolute yield is 4.8 hydrogen molecules per 100 eV adsorbed by the polymeric target.1 b The absolute yield is 1.15 hydrogen molecules per 100 eV adsorbed by the polymeric target.1 The amount of gaseous products formed in ion-induced, radiochemical pyrolytic processes exponentially decreases as the surface of a polymeric target is covered by ion tracks:1, 5 Q=Q0 exp(7eD), where Q0 and Q are the initial and current yields of the detected gaseous product, respectively; e is the cross-section of the reaction accompanied by gas evolution (based on the e value, one can estimate the reaction volume in which ion-induced thermoradiol- ysis occurs); and D is the implantation dose.Clearly, the e71 value allows estimation of Df for a polymer irradiated under particular implantation regime.1, 5 For ions with energies of the order of hundreds of kiloelectron-volts, the cross-sections of ion-induced reactions accompanied by gas evolution are *10± 14± 10715 cm2 (see Refs 1, 5, 29). Since the radiation processes accompanied by evolution of dihydrogen also occur within the penumbra, the cross-section of the reaction of H2 liberation is somewhat larger than those of the reactions of formation of hydrocarbons (CO and CO2) released immediately in the intra-track region.30 The fact that radiochemical processes in the penumbra allow cross-linking of polymer chains is of great importance.For instance, the yield of cross-linking for electron beam irradiated polyethylene (this regime is analogous to the action of d-electrons) is 3 to 5 times higher than the chain scission yield.31 In this case, radiation-induced cross-linking of the polymer occurs even at low fluences, which are two orders of magnitude lower than Df for medium-range energy ions.5, 32 This must strongly affect the course of ion-induced pyrolytic processes. This conclusion is, in particular, supported by the fact that, in contrast to pristine polyethylene, thermal degradation of radiochemically cross- linked polyethylene leads to the formation of a solid carbonaceous phase with pronounced semiconductor properties rather than a mixture of liquid hydrocarbons.33 Radiolysis products are involved in the intra-track pyrolytic processes or are accumulated within the implanted layer to be consumed at later stages of ion irradiation.High-temperature pyrolysis in the track and its immediate vicinity is accompanied by scission of polymer chains. This provides the possibility of cyclisation of radiochemically generated unsaturated linear chain fragments by the intramolecular Diels ± Alder mechanism (aromatic hydrocarbons are much more stable than linear unsa-Chemical aspects of implantation of high-energy ions into polymeric materials turated hydrocarbons at temperatures above 800 8C).As the track is cooled down, the newly formed aromatic fragments and analogous fragments present in the pristine polymer and released upon thermoradiolysis are linked together, thus being stabilised due to extension of the conjugation system.3 According to data from mass-spectrometric analysis, the exhaust gases evolved during ion implantation include hydrogen and hydrocarbons with different molecular masses.1, 24, 34 Usually, the most abundant product is acetylene which is the most stable acyclic unsaturated hydrocarbon at high temper- atures. Large amounts of saturated hydrocarbons (methane, ethane) are produced by ion irradiation of polypropylene and polybutylene.34 Despite the fact that the exhaust rate of the gases evolved can be as high as hundreds of metres per second,16 most of them have no time to leave the heating zone during the dispersal time of the thermal spike.Evidently, a fraction of these hydro- carbons can react before escaping the track region; however, the contribution of such processes to the formation of the end products of thermoradiolysis is still to be clarified. In deeper stages of ion implantation, polynuclear compounds (in essence, they represent primary carbon-containing cluster structures) and other thermoradiolysis products (unsaturated compounds, stable macroradicals) formed in the track region are involved in the growth of carbon clusters.For both aliphatic and aromatic polymers irradiated by ions with medium energies (hundreds of kiloelectron-volts) carbonisation of their surfaces nearly goes to completion at implantation doses of 561015 to 1016 cm72 and subsequent ion bombardment causes only slight changes in the composition of the implanted layer.35, 36 Note- worthy is that no full carbonisation of the polymer surface occurs. For instance, implantation of polyethylene with 150 keV F+ ions (Se/Sn&5) leads to an increase in the concentration of carbon in the implanted layer from 33% (in the pristine polymer) to 40%.35 The extent of dehydrogenation of ion irradiated polymers rapidly increases with increasing energy transferred from the incident ions by nuclear collisions.In particular, saturation of carbon concentration in the implanted layer is reached at 65% for implantation of polyethylene with 150 keV As+ ions (Se/Sn&0.3).35 If the electronic stopping mechanism dominates, dehydrogenation is of low efficiency. This can be rationalised by delocalisation of excitations over sufficiently large carbon clus- ters. Direct cleavage of chemical bonds upon nuclear collisions is more efficient to provide deep dehydrogenation of the polymer matrix. The extent of carbonisation that can be reached in this case is first of all controlled by the trapping of hydrogen radicals by broken carbon bonds. A profound dehydrogenation (concen- tration of bound hydrogen less than 10%) can be achieved with electronic deposition mechanism when ions with energies of several tens of MeV are implanted in a polymer target.21 In this case, the formation of broken bonds, due to migration of excitations, is concentrated at the edges of growing carbon clusters and carbonaceous phase produced by high-fluence implantation exhibits the Raman spectrum similar to that of ion-implanted graphite.21 Implantation of high-energy ions can provide prerequisites for intra-track formation of fullerenes C60 and C70 .This is supported by the results of mass-spectrometric analysis of gaseous products evolved during implantation of poly(vinylydene chloride) with ions with energies of the order of several tens of megaelectron- volts 37 and by the data of chromatographic analysis of the products extracted from the ion-implanted layer on the surface of polyimide irradiated with 5 MeV Li+ ions.38, 39 Taking into account modern concepts of the mechanism of fullerene forma- tion in plasma (vapour phase),40 ± 45 one can assume that the synthesis of bulky carbon structures involves coalescence of polycyclic radicals (e.g., combining of six perdehydronaphthalene radicals to yield the C60 molecule 45).This must be favoured by shrinking of the reaction volume upon track `collapse'.44 Full- erene formation can also occur via polyynes 43 (products of deep dehydrogenation of polymers or intra-track conversion of 317 acetylene) which lose their stability as the molecular chain is lengthened. This reaction is to be exothermic, since the enthalpy of formation of polyynes (27 kcal mol71 per carbon atom 46) is much larger than that of fullerene (9.08 kcal mol71, see Ref.47). Accumulation of alkynyl groups in the polymer matrices irradi- ated with high-energy ions 48 suggests the possibility of polyyne formation during ion implantation. The composition of the polymer matrix can strongly affect the composition of the products of ion-induced thermoradiolysis. This can be best exemplified by halogenated polyolefins. Ion irradiation of perfluorinated polyolefins results in lower unsatu- rated perfluorocarbons (C2F4, C2F6) as major gaseous products since the energy of theC7F bond is higher than those of theC7H and C7C bonds.24 Because of this, detectable carbonisation of ion irradiated polytetrafluoroethylene (PTFE) can be reached only at very high doses 49 while the implantation is accompanied by erosion of the polymer sample 50 (up to perforation 24).Ion bombardment of partially halogenated aliphatic polymers causes radical-induced dehydrohalogenation1,18, 23, 27 accompanied by dehydrogenation and evolution of acetylene (Table 2). Accumu- lation of double bonds in the polymer matrix accelerates dehy- drohalogenation and the newly formed polyenes are involved in the formation of the carbonaceous phase. It is believed that efficient ion-induced carbonisation of poly(vinyl chloride) 1, 18 is favoured by the fact that thermolysis of this polymer can occur at low temperatures (*200 8C, see Ref.51). Table 2. Relative yields of molecular products formed during irradiation of polytetrafluoroethylene 24 and Tefzel (copolymer of ethylene and tetrafluoroethylene) 23 with 200 keV Si+ ions. Tefzel PTFE Product 0.35 a 7 4.34 7 0.87 7 3.91 1 1 2.05 a H2 C2H2 HF C2F4 C2F6 Note. All values were normalised to the yield of C2F4 . a Determined from the yield of the CFá3 fragment detected in the mass spectrum. Ion-induced thermoradiolysis of organosilicon polymers leads to gradual accumulation of {SiO4} clusters at the irradiated surface due to the lack of efficient mechanisms of silicon sub- limation. The loss of most of methyl groups in polymethylsiloxane as the result of bombardment with 5 keV Ar+ leads to the formation of carbon ± silicate composite, the composition of which varies from Si1.3CO2.0 at the surface to Si1.5CO2.5 in the depth, with silicon atoms in this composite being directly bound to oxygen ones.52 Noteworthy is that implantation of a copolymer of polydimethylsilane and methylphenylsilane, [*Si(CH3)27 SiPhCH3*]n, with 2 MeV Ar+ ions leads to the formation of silicon carbide as a result of cleavage of the Si7Si and C7H bonds and by elimination of methyl side groups.53 Depending on the mechanism of energy deposition by incident ions to the polymer matrix, strongly different thermoradiolysis can be induced.The distinctions are due to the fact that nuclear collisions involve direct nonselective (in essence, random) break- age of chemical bonds, whereas an electronic excitation can migrate over a rather long distance during the lifetime of the excited state (*10712 s) (according to calculations for an ali- phatic chain,54 the mean path length of electronic excitation can amount to 100 monomer units) and therefore energy deposition by `electronic' stopping implies scission of the weakest bonds.This makes possible selective initiation of chemical reactions in the polymer matrix, especially in the case of ion irradiation of heterochain polymers.318 Implantation of polyimide with 90 keV N+ ions or 150 keV Ne+ ions (Se/Sn>6.5) leads to cleavage of bonds between aromatic rings 55 and to transformation of imido groups into amido groups without fragmentation.56, 57 O O C H CO+ C NH N CO This process occurs simultaneously with dehydrogenation of polyimide and causes the loss of some carbonyl groups 58 accom- panied by the evolution of CO (major gaseous product formed during ion irradiation of polyimide).24 A feature of the irradiation of aromatic poly(ether sulfone) under analogous conditions (Se 44 Sn) is the fast transformation of sulfone groups into sulfoxide ones and then, at high fluences, into sulfide groups in the reactions involving mobile radical species H..6, 59 O O SO O S O OOH O SO OH O SHO During ion irradiation of aliphatic polysulfones the reduction of sulfone groups and their elimination in the form of sulfur dioxide occur in comparable yields.60 The formation of hydroxy groups in the polymer matrix was revealed in the study of low-dose ion implantation of poly(ethyl- ene terephthalate) (PETP) under conditions where the electronic stopping prevails.61 O O O O C C CH2 CH2 O OH+ C O CH2 This allows rapid accumulation of unsaturated groups in the ion-implanted layer on the PETP surface [these groups are accumulated in PETP faster than in, e.g., poly(butylene tereph- thalate) 62].Ion-induced processes occurring during bombard- ment of PETP by high-energy ions (with energies of the order of several tens of megaelectron-volts and, correspondingly, with large Se values) proceed by the same mechanism.63 Implantation of this material with ions with energies of the order of several hundreds of megaelectron-volts leads to profound dehydrogen- ation of the irradiated polymer, accompanied by accumulation of alkynyl groups in the ion-implanted layer.48 Peculiar to polystyr- ene and polyimide implanted in this regime is the formation of H +CO CH groups containing triple bonds [e.g., isocyanate (7N=C=O) and cyanate (7O7C:N) end groups for polyimide].48 Thus, if energy transfer to the polymeric target is mainly by electronic excitations, the bridging bonds are broken and func- tional groups undergo transformations (as a rule, towards the reduction of heteroatoms), whereas aromatic fragments remain intact and can be involved in the formation of polycyclic fused structures.Heterocycles with asymmetrical delocalised p-electron systems are much less stable. For instance, experiments on electron beam irradiation of poly(2-vinylpyridine) (in this case, energy is deposited in the polymeric target solely by electronic excitations) showed that the pyridine ring rapidly breaks down with the formation of amino groups 64 in contrast to the aromatic rings in polyimide that are stable under these conditions.6, 64 A consequence of the increased contribution of a collision term to the overall energy loss of the stopping ions is the change in the character of processes occurring in the polymeric target.In particular, destruction of highly stable aromatic rings is made possible. For instance, irradiation of aromatic poly(ether sulfone) with 50 keV As+ ions (Sn/Se=5.6) causes not only the trans- formation of sulfone groups, but also the release of acetylene and the formation of 1,4-substituted butadiene.65 Similarly, implanta- tion of polyimide with ions that lose their energy mostly by nuclear collisions is accompanied by breakage of aromatic rings and formation of acetylene 24 and by nonselective degradation of imido groups resulting in the formation of various products (Fig. 1).6, 66 ± 68 Similar products were also detected during in vacuo thermal decomposition of polyimide.67 However, their concentrations are substantially different from those in the ion- implanted layer (see Fig.1) because these products are formed in two different ways, viz., in the course of several concurrent radical processes (ion implantation) and in consecutive chemical trans- formations of the polymer matrix (thermal decomposition).The overall picture of the processes occurring during ion- induced thermoradiolysis of heteroatom-containing polymers can be described as follows. Until build up of a certain implantation dose corresponding to Df (a single track regime) the polymer matrix undergoes dehydrogenation and some other unambigu- ously identifiable chemical transformations. The pathways of Relative concentration of nitrogen (%) a 100 80 60 40 2001013 1014 1015 D /cm72 Figure 1. Changes in relative concentration of nitrogen-containing O O N N groups in polyimide O O implantation with 5 keV Ar+ ions (a) and thermal decomposition (b) (according to XPS data).6, 67 Total nitrogen content (1), imide groups (2), 7C=N7 groups (3), pyridine groups (4) and tertiary amines (5).D V Sviridov b 12345 600 300 900 T /8C O duringChemical aspects of implantation of high-energy ions into polymeric materials these processes are first of all determined by the ratio of energy transfer from the implanted ions by electronic excitations and by nuclear collisions. Within this implantation dose, the overall concentration of heteroatoms in the ion-implanted layer (nitrogen atoms in irradiated polyimide 6, 67 or sulfur atoms in irradiated polysulfone 6, 59) can remain virtually constant (Figs. 1 and 2) while the energy of the implanted ions is expended for structural rearrangement of the polymer.6 Aromatic fragments released in this stage are fused to form `pre-carbon' structures.These proc- esses can occur involving the heteroatoms incorporated into the pyridine, pyran and thiopyran rings. Because of this, a large fraction of heteroatoms (first of all, sulfur and nitrogen) is retained in the implanted layer in chemically bound form even at high implantation doses.6 100 80 1234 60 40 20 01012 1013 1014 1015 D /cm72 Figure 2. Changes in relative concentration of sulfur-containing groups in poly(ether sulfone) during implantation with 150 keV Ar+ ions.6 Total sulfur content (1); sulfone (2), sulfide (3) and sulfoxide (4) groups. IV. Peculiarities of the morphology and properties of ion-implanted layer Electron microscopy studies of ion-implanted polymers revealed the formation of carbon aggregates of a size up to several tens of nanometres in polymeric targets implanted with high doses.23 Using the small-angle neutron scattering technique, it was estab- lished that these aggregates are comprised of small carbon clusters.69 In particular, it was found that high-dose irradiation of PETP films with 50 MeV B+ ions leads to the formation of nanoclusters of size 6 to 7 nm separated by *13 nm and united into spherulites of diameter up to 50 nm in the implanted layer.69 This is consistent with the results obtained by optical spectro- scopy, according to which the build up of implantation dose is accompanied by a shift in the edge of the optical absorption band of the irradiated polymer towards the long-wavelength region due to extension of the conjugated system in the growing carbon clusters. The magnitude of the shift indicates that the size of the clusters formed during implantation with medium energy ions is at most several nanometres.70 ± 72 Using an approach developed for amorphous carbon,73 the mean size of a carbon cluster grown in the implanted layer can be estimated as follows: Eg=2jbjN70.5, where Eg is the optical gap, b is the resonance integral (according to EHT calculations, b=2.9 eV) and N is the number of rings in the compact two-dimensional carbon cluster (fragment of graph- ite sheet). As the implantation dose D increases, the Eg value exponentially decreases 71, 72 Relative concentration of sulfur (%) 319 Eg=Eg 0+Aexp (7ZD), where Eg0 is the energy gap corresponding to the largest clusters that can grow under the given implantation regime, Z is the cross- section of the growth reaction of carbon clusters and A is a constant.The growth rate of the carbon nanophase and the Z value depend first of all on the nature of the polymer,71 while the Eg0 value is mainly determined by the mass and energy of ions implanted.3, 72 Implantation of light ions with energies ranging from 100 to 300 keV into polyolefins and polystyrene saturates the Eg values at 0.6 eV,70 ± 72 which corresponds to clusters with a mean size of 2.7 nm (*75 rings). Peculiar to the irradiation of polyamide-6 under analogous conditions is the formation of smaller clusters (N*60, Eg=0.76 eV, see Ref.72). The fact that there exists a limiting size of carbon clusters for a given implantation regime can be rationalised 3, 72 by delocalisation of the energy of incident ions over the carbon network and by expenditure of this energy for cleavage and formation of carbon bonds accompanied by trapping of the H. radicals. Not counting subtle details, this produces no significant variations in the medium-range order of the implanted layer. In this case, an increase in the implantation dose causes gradual growth of small carbon clusters up to the limiting size.70 For polyamide-6, the formation of smaller carbon clusters is a result of the incorpo- ration of a heterocyclic nitrogen atom into the growing cluster (due to, e.g., the formation of pyridine rings).72 The outermost heteroatoms cannot play the role of nucleation sites of the carbon clusters and their presence in the clusters reduces the size of extended regions of the p-electron conjugation system and, as a consequence, the effective size of carbon clusters determined from electronic spectra.3, 72 An increase in the size and concentration of carbon clusters in the ion-implanted layer with the implantation dose is accompa- nied by an increase in the concentration of paramagnetic centres (radicals stabilised by delocalisation of their p-electrons over the p-electron system of the carbon nanophase).25, 65, 74, 75 There is a correlation between the increase in the ESR signal during implan- tation and completion of dehydrogenation of the polymer matrix.17, 29 At high implantation doses where the possibilities of further carbonisation are exhausted the prerequisites are created for compensation of the terminated carbon bonds by their recombination that results in the decrease of spin concentra- tion.17, 29, 65, 74, 76, 77 This effect can be readily traced for poly- imide,29 polyacrylonitrile 74 and other polymers containing a large number of groups that can be efficiently fused during thermo- radiolysis. Aliphatic polymers are characterised by a much lower yield of carbon clusters compared to polymers with conjugated bonds.In this case, the spin concentration plotted as a function of implantation dose exhibits no extremum (Fig.3) even at high fluences (1017 cm72).75, 78 As the dose increases, the ESR line becomes narrower due to extension of the p-electron conjugated system in the ion-implanted layer.29, 74, 78 Studies of temperature dependence of the ESR linewidth and of the effect of oxygen pressure on the ESR spectrum revealed the formation of quasi- two-dimensional degenerate electron gas in the carbonaceous layer on the surface of heavily implanted polymers (D>261016 cm72 for polyethylene and polyimide irradiated with 100 keV B+ ions).75, 78, 79 Asalient feature of the morphology of the ion-implanted layer is the presence of nanopores that are formed along the trajectories of projectiles.Pore mouths can be clearly seen in, e.g., atomic force microscopy images of the PETP surface irradiated with low doses of heavy high-energy ions.80 Implantation of low-energy (100 ± 200 keV) ions is also accompanied by the formation of a developed pore system. This is indicated by the possibility of spontaneous incorporation of molecular iodine from the vapour phase 81, 82 and Pb2+, Fe3+, K+ and Cl7 ions (see Refs 83 ± 85) and some bulky molecules (a carborane complex of cobalt,86 amino acids 87) from aqueous solutions into the implanted layer and by the possibility of extension of the oxidation of radical320 a N 80 60 40 200 b Cs /spin g71 1019 1018 1017 1016 1015 Figure 3. Dependences of the average number of carbon rings (N) in carbon cluster and of the dc conductivity (s) (a), concentration of unpaired spins (Cs) and ESR linewidth (b) on the implantation dose for polyethylene irradiated with 100 keV B+ ions.75 centres across the whole width of the implanted layer.35, 88 ± 90 Sometimes, changes in the polymer density along the track of the implanted ion are insufficient to ensure the pore formation.In this case, slow oxygen diffusion via ion-induced defects occurs along the so-called latent tracks (the regions in which locally reduced density appeared in the polymer matrix during passage of an ion). For ion irradiated polypropylene, the oxidation front propagates in the implanted layer with a velocity which does not exceed 461075 nms71 and complete oxidation takes several weeks,35 whereas the oxidation of polyethylene irradiated under the same regime is completed virtually immediately after the vacuum ± open air transition.35 According to IR spectroscopy data, oxygen fixation in the ion-implanted layer is accompanied by the for- mation of carbonyl 88, 91 and hydroxy groups.92 Trapped oxygen is accumulated in the domains with high density of the broken carbon bonds formed during implantation; therefore, the character of oxygen distribution in the irradiated polymer contains information on the structure of the implanted layer.3, 35, 89, 93 Rutherford backscattering spectroscopy studies showed that the concentration of trapped oxygen in the irradiated polyethylene monotonically increases during ion irradiation 90 and reaches a maximum value at fluences of (1 ± 5)61014 cm72 for heavy ions 35, 93 and at 561015 cm72 for light ions.88 The total amount of oxygen assimilated within the implanted layer can be rather large (the concentration of oxygen in polyethylene irradi- ated with 150 keV F+ions at a fluence of 561015 cm72 is 0.24 per carbon atom 88).Using the depth profiles of oxygen trapped in the implanted layer, it was found that the role played by the `electronic' and `nuclear' stopping mechanisms in the generation of dangling carbon bonds is dependent on the implantation dose.88, 93 If a pristine or slightly dehydrogenated polymer is ion irradiated, the formation of free valences in the growing carbona- ceous phase is first of all a result of `electronic' energy loss.If a partially carbonised polymer is implanted, the carbon bonds are s /O71 cm71 1.2 0.9 0.6 0.3 0 DH1/2 /G 1284 1017 D /cm72 Concentration (rel.u.) D V Sviridov mainly broken by nuclear collisions, whereas the energy trans- ferred to the irradiated polymeric target by electronic excitations dissipates in carbon structures or is expended for their rearrange- ment. Therefore, even for irradiation with heavy ions character- ised by large Sn/Se ratios the oxygen depth profile reconstructed from the Rutherford backscattering spectra corresponds to the `electronic' loss distribution at low fluences (*1014 cm72) and to the `nuclear' loss distribution at higher fluences.3, 88, 93 At high fluences, a well-developed carbonaceous phase can form by linking individual carbon clusters.In this case, the concentration of oxygen scavengers substantially reduces and the concentration of oxygen in the implanted layer and the yield of carbonyl groups per implanted ion rapidly decrease as fluence increases.93 The minimum concentration of trapped oxygen in the implanted layer corresponds to maximum enrichment of this layer with carbon as a result of ion-induced thermoradiolysis.89 It should be noted that in polyethylene and polyamide-6 heavily implanted with medium-range energy (100 ± 200 keV) light (e.g., B+, see Refs 94 and 95) and heavy (e.g., Sb+ and As+, see Refs 3, 89, 93) ions the spatial region of maximum carbonisation is beneath the polymer surface.75, 95, 96 This can be illustrated by the depth profile of the relative concentration of carbon in the near-surface layer and by the presence of an additional maximum of the concentration of trapped oxygen on the surface of irradi- ated samples (in this case, the oxygen depth profile exhibits two peaks, see Fig.4). The `buried' carbonaceous layer is separated from the surface by a layer of non-carbonised, radiation damaged polymer.75, 94, 96 This effect can be rationalised by higher heat release at the track ends of implanted ions and by lateral spread of the thermoradiolysis zone within the collision cascade region.3 2 32 1 10 d /nm 250 150 50 Figure 4. Depth profiles for trapped oxygen (1 ) and for an increase in the carbon concentration (2) in polyethylene irradiated with 150 keV As+ ions at a fluence of 561016 cm72 (according to Ref.89). Rutherford backscattering studies of the implant distribution (for implantation with heavy ions) and neutron depth profiles (for implantation with light ions) showed that sometimes high pene- trability of the implanted layer allows the implant to diffuse out of the irradiated polymer. The pore system produced by the tracks of implanted ions provides conditions for specific gas transport reactions. In these situations the implant is either not detected 97, 98 or the experimental depth profile differs from the corresponding calculated depth profile to such a high extent 99, 100 that this can be explained by neither changes in the polymer density due to carbonisation and ion-induced erosion of the surface of the sample nor by backscattering of ions at high fluences.These effects are observed if the implant is incapable of bonding to the polymer matrix or if the interaction between the implant and matrix results in volatile products (here, heating of the polymeric target to moderate temperatures is sufficient for complete sub- limation of the implant). This is characteristic of, e.g., implanta-Chemical aspects of implantation of high-energy ions into polymeric materials tion of noble gas ions 97, 98 and F+ ions in polyolefins.88 In the latter case, volatile fluorocarbons are formed. Volatile com- pounds are also formed during implantation of PTFE with Sb+ ions, which is accompanied by sublimation of antimony fluo- rides.49 However, if the flux of implanted ions is rather low, the noble gas ions are retained in the polymeric target (e.g., Xe+ ions in poly(ether sulfone),21 Kr+ ions in polyimide 101), but they diffuse out of the ion-implanted layer with time.102 In the case of implantation of polymeric targets cooled down to 90 K, the depth profile of noble gas ions (in particular, xenon 103) matches the corresponding calculated depth profile to an accuracy of 10%.Diffusion of low-molecular-mass compounds formed in the interaction of thermalised implanted ions with the polymer matrix (including the interaction on storage) can have a significant effect on the spatial distribution of the implant.For instance, phospho- rus implanted into a molecular film based on a dianthrylidene derivative of g-substituted pyrone diffuses towards the surface of the irradiated sample (probably, in the form of phosphines). The interaction with dioxygen causes phosphorus fixation in the form of corresponding oxides.104 As a result, the phosphorus depth profile in the implanted layer appears to be bimodal. The first maximum corresponds to medium projected range of the implanted ions while the second maximum is at the surface of the sample.104 High mobility is also characteristic of implanted lithium (the form in which lithium is transferred is unknown). A fraction of lithium ions diffuses towards the surface of irradiated polymers.105, 106 Substantial redistribution of boron implanted into various polymers is illustrated by Fig.5. According to XPS data, the boron escaped from the polymer bulk is oxidised.108 The formation of volatile alkylboron compounds during implantation of boron ions has been suggested;3 this is indicated by the fact that, according to cryochemical atomic-beam experiments,109 elemen- tal boron readily reacts with hydrocarbons. The interaction with dioxygen and atmospheric moisture results in volatile oxidised alkylboron compounds and alkylboric acids. This is responsible for the possibility of thermally induced desorption of implanted boron 110 and for complete disappearance of boron from the implanted layer upon long-term storage.97 Obtaining mobile implant at low fluences implies that the stopping ions transfer to the polymer a rather high energy (15 ± 20 eV A71, see Ref.105) which is sufficient for occurrence of efficient thermoradiolysis and provides a necessary decrease in the density of the material of the polymeric target within the track region of implanted ions. Under these conditions, diffusion occurs 5 Rp 0 d /nm 600 400 200 Figure 5. Boron depth profiles for polyethylene (1) and polyamide (2) irradiated with 100 keV B+ ions (D=561016 cm72, see Refs 3 and 93) and calculated depth profile for boron implanted into polyethylene (3) (results of Monte Carlo simulation using the TRIM code 107). Rp is the theoretical projected range of the implanted ion. Boron concentration (rel.u.) 15 3 1 2 10 321 nearly completely along ion tracks.106 On the other hand, the formation of developed carbonaceous phase in heavily irradiated polymer samples creates a barrier to diffusion of the implant towards the surface.111 The reduction conditions which are created in the surface layer during the bombardment of a polymeric target by metal ions favours the formation of metal nanophase.112 This process was studied taking polymethylphenylsiloxane and epoxy resins implanted with Ag+, Fe+ and Co+ ions as examples (see Refs 113 ± 118).It was found that the morphology of the newly formed metal nanophase is to a great extent controlled by the viscosity of the polymer matrix. Ion implantation of polymers in the viscous-flow state is accompanied by the formation of spher- ical nanoparticles with narrow size distribution, while transition of the polymer matrix to the glassy state leads to the formation of facet metal structures in the implanted layer .114 Spherical metal nanoparticles are also formed in polyimide implanted with 150 keV copper, zinc and silver ions in high doses [D = (1 ± 5)61017 cm72],119, 120 whereas the implantation of polyimide with W+ ions causes the formation of a tungsten carbide nano- phase.121 A consequence of essential structural changes in the polymer matrix, caused by ion bombardment is a substantial change in the polarity of the polymer surface (Fig.6). This can be due to both degradation of initial (if they exist) and formation of new polar groups.The latter can be formed either immediately during implantation (for heteroatom-containing polymers) or as a result of the oxidation of radical centres due to contact with dioxygen. For instance, transformation of an imide group into an amido group during low-dose implantation of 90 keV N+ ions into polyimide leads to a more than twofold decrease in the polar component of the surface free energy.56 A decrease in the surface polarity was also observed in the course of ion irradiation (Sb+, 150 keV) of polyamide-6,49 whereas ion irradiation of PETP (N+, 90 keV) accompanied by transformation of ester groups into hydroxy groups leads to an insignificant (by 30%) increase in the surface polarity.61 These rearrangements of the polymer surface occur at low irradiation doses.Polarity of heavily implanted samples first of all depends on the presence of carbonyl groups formed involving uncompensated bonds of carbon atoms at the boundaries of the carbonaceous phase. For aliphatic polymers (polyethylene, polypropylene) the formation of carbonyl groups is the main factor responsible for a substantial increase in the polarity of the polymer surface at any dose.49, 88 An increase in the surface polarity of polysiloxanes after ion implantation is due to the formation of nanodispersed SiO2 phase inclusions in the implanted layer.52 The sorption properties of irradiated polymers are affected by the developed system of nanopores in the implanted layer and by ion-exchange character of the sorption sites inside the pores (the gp /mJ m72 gp /mJ m72 16 24 1 2 15 20 14 16 13 12 1016 D /cm72 1014 1015 Figure 6.Polar component of the surface free energy (gp) of polyimide (1) and PETP (2) films irradiated with 90 keV N+ ions plotted as a function of the implanted dose (according to Refs 56 and 61).322 adsorption capacity of irradiated polyethylene and polypropylene with respect to cations is 2 to 5 times higher than with respect to anions 84, 85). Carbon-containing species formed during ion implantation reinforce the polymer surface. In some instances the microhard- ness of the irradiated surface can be substantially increased (up to 50 to 70 times 122 ± 126). These effects were observed for various polymers, namely, polyimide,28, 123, 126 polystyrene,122, 123 poly- ethylene,92, 127 PETP,28 polycarbonate {[*OC6H4C(CH3)27 C6H4OCO*]n},128 partially polyethylene,23 fluorinated poly(ether ether ketone) {[*OC6H4OC6H4COC6H4*]n} 129 and some other polymers irradiated with different (mostly light and not too heavy) ions with energies ranging from 100 keV to 1 MeV.The microhardness of carbonaceous products is first of all determined by the character of interconnectivity within the carbon cluster aggregates.20 High microhardness requires that the inci- dent ions deposit their energy mainly by electronic excitations (this provides the formation of carbon clusters) and the energy loss by nuclear collisions be relatively small.Though nuclear collisions are responsible for the formation of random links between the clusters, a general trend is towards an increase in the microhardness with an increase in the relative weight of energy loss by electronic excitations (Fig. 7).122, 130 However, since the displacement energy of an sp3-hybridised carbon atom (80 eV for diamond) is much higher than that of an sp2-hybridised carbon atom (30 eV for graphite),5 one can expect that nuclear collisions lead to gradual accumulation of domains with predominant sp3- hybridisation of carbon atoms and increased microhardness of the ion-implanted layer. b a One of the most important consequences of ion implantation is the onset of conductivity of initially dielectric polymers.It is of great importance that the conductivity of irradiated polymers is due to the formation of carbon-containing structures in the implanted layer rather than ion doping. Therefore the conductiv- ity, which is proportional to the optical absorption of the carbona- ceous phase,131 only indirectly depends on the nature of implanted ions, being mainly determined by the energy loss during the stopping of ions.1, 17, 21, 129 Since the conducting phase in the ion- implanted layer is formed by discrete carbon clusters and their associates, the onset of conductivity in an irradiated polymer upon build up of implantation dose has a threshold character (see Fig. 3) and the transition from dielectric to conductor can be considered as percolation transition.5, 17, 29, 132 The dependence of the dc conductivity, s, on the implantation dose is given by the formula:1, 17, 29, 132 Microhardness /GPa 12 1 8 2 4 3 0 2 1 2 4 10715D /cm72 Se/Sn Figure 7.Microhardness of a polystyrene surface irradiated with Ar+ ions plotted as a function of irradiation dose (a) and of the relative weight of energy transferred from the incident ion to polymer by electronic excitations and nuclear collisions (b).122 Ion energy /keV: 1000 (1), 500 (2) and 200 (3). D V Sviridov s*(D7Dc) t, where Dc is the critical dose corresponding to the percolation threshold. Thermoradiolysis in the implanted layer occurs under con- ditions of gradual coverage of the surface of polymeric target with ion tracks.Nevertheless, the formation of an infinite conducting cluster is, as a rule, observed at implantation doses corresponding to a smaller fraction of the surface covered with ion tracks compared to those predicted by the two-dimensional percolation problem. For instance, the percolation threshold for polyimide irradiated with 500 keVN+ ions corresponds to a coverage factor of 0.3 (see Ref. 29), whereas theory predicts that percolation in an ideal two-dimensional system must occur at a critical coverage of *0.45. This suggests some ordering in the carbonaceous phase and is also supported by large values of the exponent t in the above-mentioned formula for the polymers that are prone to form carbon clusters [polyimide,29 polyacrylonitrile,17 poly(2,6-dime- thylphenylene oxide),17 molecular films based on perylene deriv- atives 18].In this case, the t values estimated from the slope of the linear dependence logs*log(D7Dc) can be as high as 4 ± 5 if energy deposition during implantation occurs mainly by nuclear collisions and even as high as 7 ± 8 if the electronic stopping prevails. On the other hand, for poorly carbonised polyethylene irradiated with 100 keV B+ ions the percolation threshold corre- sponds toDc=1.561015 cm72 (i.e., in this case,Dc>Df) and the exponent t in the above-mentioned formula does not exceed a value of 2.75, 133 As the irradiation dose increases, a system of conducting `islands' separated by potential barriers is formed within the implanted layer.1, 18, 134 The nanostructured carbonaceous phase formed in the immediate vicinity of the percolation threshold acquires semiconductor properties.135 Studies of temperature dependence of the conductivity of some irradiated polymers [polyethylene, poly(ether ether ketone),21, 136 etc.] showed that the formation of developed carbonaceous phase at high implan- tation doses causes transition from one-dimensional to three- dimensional conductivity. A similar phenomenon was observed for polyimide heavily implanted with 90 keV N+ ions 56 and for polyamide-6 irradiated with 150 keV Sb+ ions.49 For polymers which are prone to undergo carbonisation during irradiation [e.g., poly(p-phenylenebenzobisoxazole), poly(p-phenylenebenzobis- thiazole), poly(benzimidazobenzophenanthroline)], the con- ductivity of the ion-implanted layer formed at high implantation dose (Kr+, 200 keV, D=461016 cm72) is close to the metal conductivity.137, 138 At implantation doses close to Dc , the conductivity of implanted polyethylene can be increased by 3 orders of magnitude by diffusion doping with molecular iodine.80 However, the doping effect is unstable and ion-implanted polyethylene takes the initial value of the electrical resistance much faster than changes in the concentration of the trapped iodine diffusing out of the implanted layer occur.139 It should be noted that it is difficult for molecular iodine vapours to penetrate the implanted layer in heavily irradiated polymers due to destruction of the system of track channels caused by the formation of the carbon nanophase.81, 139 Doping of irradiated polyethylene with FeCl3 84 and treatment of ion-implanted polyimide with molecular chlorine lead to an increase in the conductivity by two orders of magnitude (despite the fact that addition of chlorine to some unsaturated bonds occurs simultaneously in the latter case).140 The conductivity of the implanted layer is strongly dependent on the peculiarities of the morphology of the carbonaceous phase (the size of carbon clusters, character of their linking in aggre- gates, the presence of heteroatoms). As a result, the electrical resistance of different polymers that were ion implanted under the same conditions can differ by several orders of magnitude, e.g., the specific resistance of polyethylene and polyimide irradiated with 1 MeV Xe+ ions (D=1016 cm72), is 6.861073 and 30 O cm, respectively (see Ref.55). The conductivity of the implanted layerChemical aspects of implantation of high-energy ions into polymeric materials is also determined by the implantation regime and, in particular, by the energy of the implanted ions, by the presence (or absence) of forced cooling of the polymer sample (polyimide films irradiated in the absence of forced cooling have lower electrical resistance than those irradiated in the presence of forced cooling 141), film thickness (thick films exhibit lower conductivity due to efficient cooling of the implanted layer during irradiation 25). Nevertheless, a general trend is towards an increase in the conductivity of irradiated polymers with increasing energy of implanted ions.The highest conductivity is observed if energy deposition by incident ions occurs mostly by an `electronic' mechanism.21, 65, 123, 130, 142 V. Prospects for practical applications of ion- implanted polymers Initially, interest in studies of the effect of ion irradiation on polymers was due to fabrication of resists (both negative and positive) for lithographic processes with submicron resolution. Detailed reviews concerning the cross-linking ± chain scission effects observed at very low implantation doses (1011 ± 1012 cm72) and responsible for changes in the solubility of irradiated polymers have been reported.1, 143 On the other hand, the possibility of initiation of essentially different radiolytic and thermoradiolytic processes (cross-linking ± fragmentation of polymer chains at low implantation doses, transformations of functional groups at implantation doses comparable with Df and carbonisation of polymers at high implantation doses) by ion irradiation allows control of the sorption, tribological, optical and conducting properties of the polymer surface.This opens up new fields of practical applications of polymeric materials. Implantation of PETP with heavy high-energy ions is accom- panied by the formation of latent tracks in the polymer. This makes possible perforation of polymer films 10 ± 20 mm thick by hydrolytic etching and fabrication of membranes with a control- lable number of identical pores.144 ± 146 The pore diameter can be varied over a wide range (0.01 ± 5 mm) by varying the duration of etching. Studies of the character of energy loss of a-particles in perforated PETP films obtained using this procedure allowed one to trace the changes in the shape of membrane channels in different stages of etching.147 It was found that the channels have rather long cylindrical sections (up to 2/3 of the overall thickness of the polymer film) that are transformed into conical pore mouths.147 Cylindrical channels with smooth walls can also be formed by etching ion irradiated polyimide in the presence of hydrogen peroxide 148 and hypochlorites.149 On the other hand, typical of many polymers [e.g., poly(methyl methacrylate),148 polycarbonate,146 etc.] is the formation of channels with irregular shape and broad size distribution.High uniformity of etching channels in PETP is due to some features of latent track struc- ture.146, 150, 151 The track core, which is at most several nanometers in radius, represents a degraded, rapidly soluble polymer contain- ing a large number of isolated pores 152 and surrounded by a shell of radiochemically cross-linked polymer.80, 146, 150 The shell diam- eter is 50 to 60 nm for implantation of xenon ions (1 MeV per nucleon) and is much larger than the penumbra diameter.80, 150 This can be explained by the formation of cross-links due to recombination of macroradicals formed in the interaction of polymer chains with knock-on hydrogen atoms 146 (including those formed as a result of hydrodynamic shock accompanying the passage of high-energy heavy ions 153).Probably, high spatial selectivity of etching is to some extent due to ordering of the polymer structure as a result of reorientation of the macro- molecules along the ion tracks.63 It should be noted that forced orientation of polymer molecules along the direction of ion flux can be observed not only during implantation of high-energy ions, but also in the case of ion irradiation with energies of the order of several kiloelectron-volts 154 at a fluence of*561015 cm72. The formation of surface pores, changes in the polar compo- nent of the surface free energy and the appearance of specific adsorption sites as a result of ion bombardment can have both 323 positive 52, 155 ± 162 and negative 156 effects on the efficiency of adsorption of living cells on the polymer surfaces (e.g., polystyr- ene,156, 158, 159 polyurethane,160, 161 polymethylsiloxane,52 colla- gen,156, 162 etc.).This opens up new possibilities of control of biocompatibility of polymeric materials. As a rule, cell adsorption efficiency nonmonotonically depends on the implantation dose (i.e., it is determined not only by the effects of changes in hydro- philicity) and reaches a maximum value at implantation doses that are somewhat lower thanDf.158, 160 It was also shown that changes in the dynamics of plasma protein adsorption on the surface of silicon rubber induced by irradiation with 150 keV Há2 , Ná2 , Na+ and Ne+ ions leads to nearly complete suppression of thrombo- genicity.163 At low ion implantation doses, the refractive index, n, of a polymer increases with the polymer density.In some instances a relative increase in the refractive index can be substantial. In particular, poly(methyl methacrylate) is known to lose methoxy groups upon ion irradiation and undergo gradual transformation into an analogue of cross-linked polyethylene.164 For this poly- mer, the Dn/n ratio can be as high as 0.3 (see Refs 165 and 166) and the dependence of the refractive index on the implantation dose is nearly linear up to D&1015 cm72. At higher doses, the polymer becomes turbid due to carbonisation.166 This allows the formation of light guides and other optical devices (couplers, interferome- ters) on the polymer surface.167 ± 169 It is of great importance that the use of high-energy ions characterised by a shift of maximum energy loss towards the track end allows fabrication of `buried' (including crossed) light guide structures at a depth from several to several tens of micrometres under the surface of a polymer sample (depending on the energy of implanted ions).167, 170 Fabrication of nonlinear optical devices in the form of metal particles dispersed in a dielectric matrix 171 by implantation of polymer with corresponding metal ions is a difficult task since the optical density of the polymeric host increases with the implanta- tion dose.On the other hand, one can expect that the formation of a metal nanophase during ion irradiation of the polymers con- taining chemically bound metal ions will occur at low implanta- tion doses. As mentioned above, carbonisation of polymers at high implantation doses leads to a substantial increase in their micro- hardness,28 which can be as high as 22 GPa 123 (cf. a value of less than 12 GPa for martensite steel) and is comparable with the hardness of diamond-like carbon films. The best results were obtained in the case of simultaneous implantation of ions with different mass and energy, i.e., characterised by different pene- tration depth and responsible for different degrees of carbon- isation of polymers.28 An increase in hardness of the surface of implanted polymers [polyimide,123, 126, 172 polystyrene,122, 123 polycarbonate,128, 173 pristine 92, 127 and partially fluorinated poly- ethylene,23 poly(2,6-dimethylphenylene oxide),174 etc.] is accom- panied by reduction of their electrical resistance 122 and causes substantial improvement of wear resistance.122, 124 The latter parameter nonmonotonically depends on implantation dose since an increase in hardness of the implanted layer makes it more brittle and causes a gradual decrease in its adhesion to the polymeric support.The dramatic decrease in the friction coeffi- cient upon ion irradiation observed for polycarbonate 173, 175 and poly(2,6-dimethylphenylene oxide) 174 seems to be to a great extent due to an increase in hydrophilicity of the polymer surface, which favours the formation of a surface layer of chemisorbed water with good antifrictional properties.The carbonised surface layer substantially improves the chem- ical stability of irradiated polymers towards organic solvents; however, this layer is porous and therefore provides no perfect protection of the polymeric support.176 High-dose implantation of low-energy (25 ± 40 keV) Si+, Al+, Y+, Y2+ and Y3+ ions leads to suppression of erosion of the surface of polyimide, PETP and some other polymers exposed to a flux of oxygen atoms (under conditions corresponding to Earth orbit space environment) due to the formation of a composite oxide-carbon coating.177324 Polymers irradiated to high doses represent an unusual electrode material, the surface properties of which can vary depending on the ratio of the concentrations of sp2- and sp3- hybridised carbon atoms in the implanted layer.Since the carbon clusters responsible for conductivity of the implanted layer are small, their oxidation under anodic polarisation observed in the absence of depolarisers is accompanied by irreversible loss of the electrochemical activity of irradiated polymers.75 On the other hand, for initially conducting polymers (polyaniline,178 polypyr- role,179 polythiophene,180 polybithiophene,181 poly- ethoxythiophene,182 polyethylenedioxythiophene 182) that lose their electrochemical activity as a result of degradation of the p-electron system at low implantation doses (D<1015 cm72) 180 the surface of the polymeric electrode exhibits recovery under anodic polarisation due to electrochemical oxidation of the carbonaceous phase.179, 181, 183 The possibility of ion-induced passivation of conducting polymers towards the processes of electrochemical polymerisation 181, 183 and electrochemical depo- sition of metals 179, 180, 182 allows one to change the surface profile of polymeric electrodes 181 and to form metal patterns with micrometre resolution.179, 180, 182 The possibility of varying the conducting properties of implanted polymers opens up new prospects for fabrication of microelectronic systems including the formation of conducting strips on the surface of polymeric materials,184 fabrication of planar resistors with variable electrical resistance which can be varied over a wide range,2 thermistors and bolometers.2 Ion implantation can be used for development of more complex polymer-based electronic devices, namely, diodes and transistors.It is of great importance for fabrication of polymeric electronic devices (in particular, p ± n-junctions 185, 186 and conducting chan- nels of field-effect transistors 187) that many of the initially conducting polymers with well-developed p-electron conjugation systems [e.g., poly(p-phenylenevinylene), polyacetylene] 186, 187 exhibit resistance towards ion-induced dehydrogenation. There- fore, they can be doped by implantation of ions with medium- range energies (*30 keV).Under these conditions, purely chem- ical doping of p-type for implantation of iodine ions and of n-type for implantation of alkali metal ions can be accompanied by the effects associated with the formation of structural defects in the polymer matrix (e.g., as a result of electron capture from the p valence band of polyparaphenylene by the lower-lying sp2-elec- tron energy levels corresponding to dangling bonds of carbon atoms 188). The possibility of fabrication of unusual electronic switching devices (analogues of field-effect transistors 3, 75, 94, 96) based on implanted, initially dielectric polymers (polyethylene, polyamide-6) with a `buried' carbonaceous phase separated from the surface by a dielectric layer *10 nm thick was also demon- strated.This layer can be made more uniform by electrochemical polymerisation of dielectric poly(o-phenylenediamine) in the pores of the implanted layer.75 The spatial characteristics of this structure allow the `buried' layer to act as a conducting channel of the field-effect transistor. The ac resistance of such a channel changed by more than four orders of magnitude upon applying an external electric field.3, 75, 94 VI. Conclusion The above-mentioned experimental results indicate that ion irradiation of polymers causes some specific phenomena that currently should be considered within the framework of a special division of high-energy chemistry. Ion-induced processes in poly- meric targets represent a kind of pulse thermoradiolysis which occurs in the subnanosecond time scale and sometimes under conditions of massive (`non-thermodynamic') breakage of chem- ical bonds by nuclear collisions.The energy deposited by the incident ions in the polymer matrix is rather high; however, carbonisation of the polymer surface observed in this case cannot be reduced to simple `graphitisation' of the polymer within the track of the stopping ion. D V Sviridov The formation of the carbonaceous phase on the polymer surface with an increase in the implantation dose involves some consecutive stages essentially differing in character of the corre- sponding ion-induced reactions. These stages include (1) dehy- drogenation of a polymer (this occurs, as a rule, simultaneously with the radiation-induced cross-linking of the polymer) and transformation of functional groups, which results in the forma- tion of `pre-carbon' structures; (2) growth of carbon clusters, the limiting size of which is determined by the energy of implanted ions; (3) aggregation of carbon clusters and the formation of conjugated and non-conjugated bonds.The energy loss of high- energy ions affects the composition of the newly formed nano- structured carbonaceous phase more than the nature of the irradiated polymer. On the other hand, subtle variations of the short- and medium-range order of the carbonaceous nanophase, which determine such fundamental properties as conductivity and microhardness, depend on the mechanism of ion-induced ther- moradiolysis and are very sensitive to the composition of the irradiated polymeric target.By and large, this type of influence of the polymer nature on the properties of ion implantation products can be considered as specific memory effect. Figure 8 presents a diagram which qualitatively outlines main consequences of ion irradiation of polymers. By varying the irradiation dose one can control the rheologocal, sorption, con- ducting, electrochemical and optical properties of polymers and their microhardness, wear resistance and biocompatibility. Onset of electrical conductivity Enhancement of microhardness Changes in hydro- philicity and biocompatibility Changes in solubility Cross-linking ± chain scission Transformations of functional groups Growth of carbon clusters Formation of a-C:H phase 1017 1016 1015 1012 1014 1013 1011 Fluence /ions per cm2 Figure 8.Principal transformations in the implanted layer and related changes in the physical and chemical properties of the polymer surface irradiated with different ion doses at energies between 0.1 and 1 MeV. Thus, ion irradiation is a promising method of targeted modification of polymer surfaces. Build up of implantation doses causes processes of fundamentally different nature which can be controlled by varying the energy and masses of the implanted ions. All potentialities of this experimental technique and detailed mechanisms of physicochemical processes accompa- nying ion implantation of polymeric materials require further study.The author expresses his gratitude to the Basic Research Foundation of Belarus for financial support of the experiments on the synthesis of nanostructured carbon materials by ion implantation techniques.Chemical aspects of implantation of high-energy ions into polymeric materials References 1. T Venkatesan, L Calcagno, B S Elman, G Foti, in Ion Beam Modification of Insulators (Eds P Mazzoldi, G W Arnold) (Amsterdam: Elsevier, 1987) p. 301 2. R E Giedd,M G Moss, J Kaufmann, Y Q Wang, in Electrical and Optical Polymer Systems (Eds D L Wise, G E Wnek, D J Trantolo, T M Cooper, J G Gresser) (New York: Marcel Dekker, 1998) p. 1011 3. 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ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC

摘要:

Russian Chemical Reviews 71 (4) 329 ± 346 (2002) The state-of-the-art and prospects for the development of rechargeable lithium batteries AMSkundin, O N Efimov, O V Yarmolenko Contents I. Introduction II. The main problems in the development of rechargeable batteries with lithium anodes. III. Practical developments in the field of metallic lithium cells. IV. Materials alternative to metallic lithium V. Lithium-ion rechargeable batteries VI. The main problems in the development of lithium-ion batteries. VII. Polymeric electrolytes for lithium-ion batteries VIII. Conclusion Abstract. develop- the into investigations of state-of-the-art The The state-of-the-art of investigations into the develop- ment of class promising most the of perfection and ment and perfection of the most promising class of chemical chemical power is batteries, lithium rechargeable namely, sources, power sources, namely, rechargeable lithium batteries, is consid- consid- ered.a with batteries the designing of problems main The ered. The main problems of designing the batteries with a metallic metallic lithium of use the and formulated are electrode lithium electrode are formulated and the use of alternative alternative negative paid is attention Special substantiated. is electrodes negative electrodes is substantiated. Special attention is paid to to the performance the of principles the with dealing studies the studies dealing with the principles of the performance of of lithium-ion the for directions key the as well as batteries lithium-ion batteries as well as the key directions for the perfection perfection of of elaboration the concern mainly which devices, these of these devices, which mainly concern the elaboration of new new materials is section separate A batteries.lithium-ion for materials for lithium-ion batteries. A separate section is devoted devoted to lithium for electrolytes polymeric of consideration the to the consideration of polymeric electrolytes for lithium and and lithium-ion references 390 includes bibliography The batteries. lithium-ion batteries. The bibliography includes 390 references. I. Introduction A new type of chemical power source, namely, lithium cells with aprotic electrolytes appeared for the first time in the early 1970s and later gained wide acceptance.1, 2 Thus, a long-standing idea of scientists, viz., to build a chemical power source with the most active reductant, i.e., an alkali metal, was materialised.The use of such a reductant allowed one to enhance both the operational voltage of a power source and its energy density. In contrast to primary cells (one-shot cells) with lithium anodes, which were developed relatively quickly and successfully and are used now as the power sources in portable devices of every-day and special equipment, the history of the development of rechargeable lithium batteries has been very dramatic.3, 4 AMSkundin A N Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Leninsky prosp. 31, 119991 Moscow, Russian Federation.Fax (7-095) 952 08 46. Tel. (7-095) 955 40 20. E-mail: skundin@gol.ru O N Efimov, O V Yarmolenko Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation. Fax (7-096) 524 44 01. Tel. (7-096) 522 18 87. E-mail: efimov@icp.ac.ru (O N Efimov) Received 10 December 2001 Uspekhi Khimii 71 (4) 378 ± 398 (2002); translated by T Ya Safonova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n04ABEH000706 329 329 330 330 331 332 337 341 II. The main problems in the development of rechargeable batteries with lithium anodes. Primary cells designed for operation under ordinary conditions can be divided into two classes: cells with liquid and solid cathodes.For liquid cathodes-oxidants, thionyl chloride and sulfur dioxide, which simultaneously form the basis of the electro- lytes, are used most often. Exhibiting high values of power and energy densities and operating in a wide range of temperatures, the cells with liquid oxidants represent, however, a fire hazard and are explosive and, hence, are used only in special equipment. For cells with solid cathodes, oxides of manganese, copper and certain other metals serve as the active cathodic materials, together with iron sulfide and fluorocarbons. Electrolytes in such cells represent solutions of certain lithium salts (perchlorate, hexafluoroarsenate, tetrafluoroborate, etc.) in organic nonaqueous aprotic solvents (propylene carbonate, dimethoxyethane, tetrahydrofuran, g-butyrolactone and various mixtures).Depending on the catho- dic material type, the working voltage of the cells can approach 3 V (for manganese dioxide and fluorocarbons) or 1.5 V (for copper oxide and iron sulfide); in the latter case, lithium cells and conventional cells based on the manganese dioxide ± zinc system are interchangeable. It is the primary cells with aprotic electrolytes that are considered as the analogues of rechargeable (secondary) cells. The reactions which proceed in primary cells with aprotic electrolytes are similar to those in secondary cells. In both types of cells, the discharge involves the same processes, viz., the anodic dissolution of lithium and the cathodic insertion of lithium into the lattice of the positive electrode material.The charging in a secondary cell represents the reverse processes. The materials for positive electrodes on which the cathodic insertion and anodic extraction (in other words, cathodic intercalation and anodic deintercalation) of lithium proceed reversibly were found back in the late 1970s. Titanium and molybdenum disulfides can serve as examples of such compounds. The main problem arisen concerns the choice of the negative electrode material. Its discharge, i.e., cathodic deposition of lithium poses complications typical of electroplating practice. It is known that the lithium surface in aprotic electrolytes is coated with a thin passive film (its thickness does not exceed several nanometres) as a result of chemical reactions with the electrolyte components, namely, the organic solvent and anions.This film has the properties of a solid electrolyte with conduction in lithium330 ions and adequately protects lithium from the reactions with the electrolyte (i.e., prevents self-discharge). On the cathode, the deposition of lithium results in the formation of dendrites and, in many cases, the film formed during charge ± discharge cycles envelopes individual lithium micropar- ticles preventing their electric contact with the support. Such a phenomenon called `encapsulation' manifests itself in the fact that every charging step removes a part of lithium from the further operation. That is why, the secondary cells with metallic lithium electrodes should contain lithium in amounts exceeding (from 4- to 10-fold) its stoichiometric value; hence, the specific capacity decreases from the theoretical value of 3828 to 380 ± 800 mA h g71.Moreover, the formation of dendrites increases the probability of short-circuiting and, hence, makes the cells inflammable and explosive. Various methods for the treatment of the cathodic surface and a search for additives to the electrolyte that would prevent the dendrite formation during cathodic deposition of lithium yielded positive results. For example, it was shown that, being added to a propylene carbonate electrolyte, even trace amounts of hydrogen fluoride favour lithium deposition as a dense fine-grain continu- ous layer rather than in the form of dendrites.5, 6 The beneficial effect achieved by treating the lithium surface with carbon dioxide (or, simply, by adding either CO2 as such or substances evolving CO2 into the electrolyte upon their reactions with lithium) has been repeatedly mentioned.7, 8 It was suggested 9 to modify the lithium surface by applying monomolecular layers of non-ionic surfactants, particularly, dimethyl ether of poly(ethylene glycol) and a dimethylsiloxane ± propylene oxide copolymer. Electrolytes that prevent the dendrite formation even in the absence of additives were also found.For example, a solution of lithium hexafluoroarsenate in 1,3-dioxolane (this solution also contained tributylamine which prevents dioxolane polymerisation).10 ± 12 III.Practical developments in the field of metallic lithium cells. Despite the fundamental obstacles in the development of secon- dary cells with metallic lithium electrodes, attempts were made to establish their commercial production. Thus in 1987, Moli Energy Ltd (Canada) started serial production of AA-size batteries (which corresponds to the Russian size of 316) { with positive electrodes made of molybdenum disulfide. The cells possessed the following characteristics: an initial voltage of 2.3 V, a capacity of 600 mA h for a discharge at a current of 0.2 A up to the final voltage of 1.3 V. In certain cases, the discharge to the final voltage of 1.1 V allowed one to reach a discharge capacity of 800 mA h.According to the reports by manufacturers, their cells had a cycle life of several hundred charge ± discharge cycles; however, these results were disputed by some experts and consumers. In fact, a cycle life of 100 charge ± discharge cycles could be claimed. In 1989, a modernised version of a secondary cell was developed which contained a positive electrode of molybdenum sulfide with a composition of Mo6S8. This cell had a higher discharge voltage (from 2.3 to 1.8 V) and a higher capacity (up to 1000 mA h) and allowed a high-current discharge (up to 1 A). The characteristics of the secondary cells fabricated by Moli Energy Ltd could have been considered adequate even despite a considerable voltage decrease during the discharge and a short real cycle life, but the reliability of the cells proved to be { In Russia, the sizes of cylindrical cells may be designated in accordance with three types of nomenclature: the GOST (State Standard) (three-digit numbers: 283, 286, 314, 316, 373, etc.), the recommendations by the International Electrotechnical Commission (R06, R03, R4, R6, R20, etc.) and the USA nomenclature (AAA, R, AA, D, etc.).Thus a cell with a diameter of 14.5 mm and a height of 50.5 mm may be designated as AA, 316 and R6; a cell of a diameter of 34.2 mm and a height of 61.5 mm may be designated as D, 373 and R20. Tables comparing different designations of cell sizes are available (e.g., in Ref. 2, p. 216). AMSkundin, O N Efimov, O V Yarmolenko insufficient. In the summer of 1989, incidences of ignition of primary lithium cells were observed, as well as failures of the secondary cells produced by this firm, which made them stop the production.At the Jet Propulsion Laboratory of the California Institute of Technology (USA), secondary cells with positive electrodes of titanium disulfide have been developed; however, no attempt has been made to organise their serial production mainly due to their low cycle life, namely, 100 ± 200 charge ± discharge cycles. The same situation was observed with the cells developed by Honey- well, Eveready Battery Co. Inc. (USA) and the United Design and Technology Bureau `Orion' at the Novocherkassk Polytechnical Institute (Russia). Engineers of Matsushita Electric Industrial Co.(Japan) have developed AA-size rechargeable batteries of a higher quality using manganese dioxide as the active substance for positive electrodes. The batteries had a discharge voltage in a range from 3.2 to 2.0 V and a capacity of 800 mA h at the beginning of cycling and 600 and 450 mA h after 60 and 200 charge ± discharge cycles, respectively. At present, Tadrian (Israel) is engaged in the development of rechargeable batteries with matallic lithium electrodes; however, the prospects of their application are still vague. It was reported 13 that, for AA-size cells, the gravimetric power density of 125 ± 140 W h kg71 and the volumetric power density of 280 ± 315 W h dm73 at a service life of 250 full depth charge ± discharge cycles were achieved.13 [The depth of discharge (dod) or the cycling depth of a battery is the capacity fraction realised during the discharge from the total capacity designed into a rechargeable battery.] The reliability of these cells was not confirmed, but it was reported that explosions are ruled out even for substantial overcharges. However, in certain cases, the cells mildly leaked (without explosions) due to overheating.IV. Materials alternative to metallic lithium The attempts to solve the problems associated with the use of metallic lithium in negative electrodes involved substitution of lithium alloys for lithium. Lithium ± aluminium alloys were assumed to be the most promising candidates. On such an electrode, the lithium concentration decreases during the dis- charge (lithium is etched out of alloys) and increases during the charge.The chemical activity of lithium in an alloy is somewhat lower as compared with that of metallic lithium; hence, the potential of an alloy-based electrode is more positive (by *0.2 ± 0.4 V) than the potential of an electrode of pure lithium. This results, on the one hand, in a decrease in the working voltage and, on the other hand, in a weakening of the alloy interaction with the solvent, i.e., reduces the probability of self-discharge. Disk-shaped rechargeable batteries of the VL 2020 model developed by Matsushita Electric Industrial Co. involved negative electrodes made of a lithium ± aluminium alloy and positive electrodes of vanadium oxide.Such batteries were characterised by a rated capacity { of 20 mA h being discharged from 3.2 to 2.0 V, their cycle life was *1000 charge ± discharge cycles for a 10% dod and *450 cycles for a 20% dod. At the Jet Propulsion Laboratory of the California Institute of Technology (USA), specimens of cells were built with advanced characteristics: their cycle life reached 600 charge ± discharge cycles at 50% dod. Indeed, the small dod values typical of these batteries make their specific characteristics inferior as compared with secondary cells with lithium electrodes. A principal drawback of lithium ± aluminium alloys consists in the fact that they change substan- tially their specific volumes during the cycling. A deep discharge can result in embrittlement and crumbling of the electrode. That is why, no further progress was observed in the field using lithium ± aluminium alloys as the material for negative electrodes.{ The rated capacity is a capacity realised in a 5 h discharge to a final voltage of 2.5 V.The state-of-the-art and prospects for the development of rechargeable lithium batteries Lithium alloys with heavy metals (such as the Wood's alloy) change their specific volumes during cycling to a lesser extent as compared with lithium ± aluminium alloys. Matsushita Electric Industrial Co. has developed several versions of secondary cells with electrodes based on lithium alloys with heavy metals. Specific characteristics of these batteries were very low, and these alloys were considered to be of little promise.V. Lithium-ion rechargeable batteries 1. General information The publications reporting the secondary cells with negative electrodes of carbon materials developed in Japan marked a revolution in the field of rechargeable lithium batteries.14�17 Carbon was shown to be a very convenient matrix for intercala- tion of lithium. The specific volumes of many graphitised materi- als changed by no more than 10% upon insertion of sufficiently large amounts of lithium.18 The potential of carbon electrodes which contain moderate amounts of intercalated lithium can be 0.5 ± 0.8 V more positive than the lithium electrode potential. To make the cell's voltage sufficiently high, Japanese researchers used cobalt oxide as the active material for positive electrodes. The potential of lithiated cobalt oxide was*4 V vs.lithium electrode; the working voltage of the cell was 3 V. During the discharge of this cell, lithium is deintercalated from the carbon material (on the negative electrode) and is intercalated into the oxide (on the positive electrode). The charge involves the reverse processes. Thus, the system contains no metallic (zero- valent) lithium, and its charge and dischargprocesses simply involve the transfer of lithium ions from one electrode to another. These rechargeable batteries were named lithium-ion,14 `rocking- chair' 19 and `swing' 20 cells. Figure 1 shows the scheme of performance of a lithium-ion battery. On the left, a negative graphite electrode is shown.Its structure is characterised by the presence of layers (conditionally shown as hexagonal fragments) between which lithium ions (black points) can be inserted. On the right, the positive electrode of lithium manganese spinel with a structure that can intercalate lithium ions is shown. Lithium salts in non-aqueous solvents are used as the electrolyte. During the charging of the cell, in the external circuit, an electronic current flows from the positive electrode to the negative one, whereas inside the cell, an ionic current flows also from the positive electrode to the negative one. Since the appearance of first publications on the development of lithium ion batteries,14 ± 17 active research into the intercalation of lithium into carbon and the other materials commonly used for the positive electrodes has started.To date, the publications on e7 e7 A + 7 e7 e7 Li+ e7 e7 Li+ e7 e7 Li+ e7 Li+ e7 Electrolyte LixC6 Li(17x)Mn(17y)O2 Discharge Charge Figure 1. A scheme of performance of lithium-ion rechargeable bat- teries. 331 this subject number in the hundreds and involve several reviews.19, 21 ± 28 2. Examples of lithium-ion batteries In 1991, Moli Energy Ltd elaborated the specimen of lithium-ion batteries shaped as disks (coins).29 A cell of the 1225 size } had a capacity of*7 mA h and a cycle life of *300 charge ± discharge cycles at a discharge current of 1 mA and a charge current of 0.5 mA. Of Japanese firms, the leading position in the production of lithium-ion cells belongs to Sony which produces lithium-ion cells of various types, not only coin cells but also cylindrical ones of a spiral design and prismatic cells.Its annual production exceeds 10 million cells.30 Sanyo Electric Co. holds the second position. Matsushita Electric Industrial Co. also has a large-scale produc- tion. In addition, six to eight other Japanese firms have just entered the market of lithium-ion batteries. Since 1994, Sony has arranged the production of rechargeable batteries with sufficiently stable characteristics. The cells of AA size (according to Japanese nomenclature, US 14500) with a weight of 19 g, a rated capacity of 500 mA h at a voltage of 3.6 V are most representative.The manufacturer guarantees their performance in temperature ranges from 720 to +60 8C (dis- charge) and from 0 to +45 8C (charge). The energy density of these cells was *95 W h kg71 or 222 W h dm73. Cells of larger sizes had somewhat higher energy densities: e.g., for cells of size 16 630 and 18 650 the energy densities were 105 and 126 W h kg71 (or 250 and 290 W h dm73), respectively. Under the usual storage conditions, the self-discharge of the cells was *10% in three months.21 The greatest rated capacity equal to 2800 mA h was demon- strated by a cylindrical cell of the size 26 650 weighing 83 g. A discharge under a 1 h high-current mode reduced the capacity of Sony cells only insignificantly. The manufacturer guarantees cycle life of 500 charge ± discharge cycles at only a 20% capacity fade.In Japan, in addition to the development of lithium-ion cells of small sizes, attention has focused on elaboration of large-size rechargeable batteries, which can be used for both electrical vehicles and load distribution between autonomic nets. Since 1992, the studies have been carried out in accordance with the `New Sunshine' program, which was intended to be accomplished in *10 years with a total financing of *$140 mln (USA). This programme scheduled the development of two versions of rechargeable cells, viz., batteries with medium energy densities (up to 140 W h kg71) designed for long-term service (the cycle life of 3500 charge ± discharge cycles) and batteries intended for a short cycle life (500 charge ± discharge cycles) but displaying enhanced energy densities (up to 180 W h kg71).31, 32 In other countries, the industrial output of lithium-ion bat- teries ranks far below that in Japan; however, certain companies intend to substantially increase the production of these secondary cells.In the USA, the engineers of Rayovac Corp. have developed specimens of both coin and cylindrical cells.22 Coin cells of the 1225 size were characterised by energy density of 35 W h kg71 or 115 W h dm73. These characteristics rank substantially below those of the cylindrical cells produced by Sony, which is explained by the fact that the effectiveness of using the cell's mass and volume are different for small and large batteries (the share of casings and other structural elements in the cell's total weight is higher in coin cells as compared with cylindrical cells).The monthly self-discharge in these cells is*5%, which is comparable with the characteristics of Japanese cells. Engineers of Rayovac } The designations of sizes of coin cells comprise the numbers that signify their diameters (in millimetres) and heights (thickness, in tenths of a millimetre). For instance, a cell of the 1225 size has a diameter of 12 mm and a thickness of 2.5 mm.332 Corp. intend to organise the production of prismatic cells with an energy density approaching*130 W h kg71. Being designed for a long-term discharge (and charge) by small currents, the first version of these cells should demonstrate an enhanced capacity.The second version is designed to have a lower capacity and withstand high-current discharges and charges. Bellcore (USA) has undertaken great efforts in the develop- ment of lithium-ion batteries.33, 34 Their cell prototypes of the AA size with a capacity of 380 mA h (i.e., lower as compared with similar batteries made by Sony) were, however, capable of discharging in a 20-min mode } without substantial capacity loss (<20%). Prismatic lithium-ion batteries developed by Yardney Corp. (USA) 35 exhibited capacities of 3 A h and energy densities of 87 W h kg71 or 225 W h dm73. SAFT (France) have completed the preliminary stage in the production of lithium-ion batteries to be used in both communi- cation facilities and electric vehicles.36, 37 The energy density of such a battery was 125 ± 150 W h kg71 with discharge by low currents and 90 ± 135 W h kg71 at a high-current discharge.In the latter case, the power density was 190 ± 200 W h kg71. A decrease in the temperature to 730 8C resulted in a 20% ±25% decrease in the capacity, the cycle life was assessed to be *400 charge ± discharge cycles. Large-size lithium-ion batteries (capacity from 40 to 100 A h) are being elaborated by engineers of VARTA (Germany).38 The characteristics of the developed prototypes are still not too high: the energy density of a cell with a capacity of 100 A h was 50±70 W h kg71 for a 2 h discharge mode and *30 W h kg71 for a 30 min discharge mode.The capacity of the cells substan- tially decreased with cycling, which, apparently, is associated with the scale factor. VI. The main problems in the development of lithium-ion batteries. 1. Materials for negative electrodes The lithium-into-graphite intercalation compounds (layered graphite compounds) have been known for a long time (see, e.g., Ref. 39). In the 1970 ± 1980s, the reversible intercalation of lithium into graphite and certain other materials from lithium salt solutions in aprotic solvents was extensively studied.40 ± 46 The very first studies on the reversible lithium intercalation have shown that lithium intercalation into carbon materials is a complex process with the mechanism and kinetics largely depend- ent on the choice of carbon material and the electrolyte. Whereas the effect itself was quite evident, however, the results of different authors on its mechanism appear to be extremely contradictory. Even now, an important question remains open as to which properties (structure, electronic structure, the presence of impur- ities, etc.) of carbon materials determine the intercalation and deintercalation processes.Despite the uncertainty mentioned above, certain simple relationships and phenomena have been established unambigu- ously. Thus it was found that, upon the first contact of an aprotic electrolyte with a carbon material (irrespective of its structure), the latter acquires a potential in the range from 2.5 to 3.5 V vs. lithium electrode, the value being determined by the presence of compounds with certain chemical groups (first of all, oxygen- containing groups) in the electrolyte.At a cathodic polarisation, the potential shifts to negative values, and slow intercalation of lithium begins. Simultaneously, the electrolyte is reduced (both the solvent and different impurities), and this reaction proceeds much more easily (at less negative potentials) on the surface of carbon material as compared with the metallic lithium surface. }A discharge with such a current that the total designed capacity is realised in 20 min. AMSkundin, O N Efimov, O V Yarmolenko Being partially insoluble, the reduction products form a film of the carbon material surface. This film is similar in composition and properties to the passive film formed on the lithium surface.Thus, during the first charge, electricity is consumed in both lithium intercalation and passive-film formation. A completely formed film prevents direct contact between the electrolyte and carbon, thus virtually eliminating further reduction of the electrolyte. The second undeniable fact is that the carbon material structure affects the mechanism of lithium intercalation. As a rule, intercalation of lithium into well pronounced graphite structures results in the formation of a thermodynamically stable compound, namely, LiC6. In this compound, the lithium activity is equal to unity, i.e., the potential of such a compound is equal to that of a lithium electrode.The majority of authors represent intercalation ± deintercalation by the following equation: (1) x Li++xe + 6C LixC6 . The dependence of the LixC6 potential on the degree of intercalation x (i.e., the charge curve of a graphite electrode) was shown to have several almost horizontal steps corresponding to the following compounds: LiC6, Li0.5C6, Li0.33C6, etc.45 Interca- lation of lithium into graphite structures occurs at negative potentials (the main part of lithium is intercalated at a potential more negative than 0.5 V), i.e., the activity of lithium in such intercalates is sufficiently high. On the one hand, this is an advantage, because the cell voltage increases; however, on the other hand, this is a drawback, because the reduction of electro- lytes on these intercalates proceeds more actively than on non- graphitised samples.Moreover, intercalation of lithium into graphite structures is often accompanied by co-intercalation of the solvent, which leads to swelling and exfoliation of graphite. Intercalation of lithium into amorphous structures occurs at more positive potentials, as a rule, in the range from 0.9 to 0.0 V. In this case, x vs. potential plots represent smooth curves typical of uniform systems. Nongraphitised structures do not exfoliate during intercalation of lithium. Numerous studies were devoted to elucidation of how the potential variation depends on the degree of intercalation x (thermodynamics of the intercalation process) (see, e.g, Refs 18, 29, 45 ± 48).It was shown that the dependence of the activity of intercalated lithium on its content has a complex form, its mechanism still being a subject of speculation. Different scientists tried to find a correlation between the structure of carbon materials and their abilities to intercalate lithium reversibly. This correlation is still not established con- clusively. Controversial assumptions have been drawn on this subject. For instance, it was supposed 49 that reversible intercala- tion is possible exclusively into materials with the well pronounced graphite-type crystal structure and is unfeasible for amorphous materials. In contrast, it was also proposed 50 that lithium is intercalated only into amorphous materials. Probably, the opti- mum materials should possess amorphous matrices with meso- phase inclusions, viz., nuclei of graphite crystals.These materials are represented by various cokes, pyrographite and pyrolysis (carbonisation) products of different polymers. This assumption was confirmed by the results of studying the possibility of lithium intercalation into the same substances preliminarily pyrolysed at different temperatures. The optimal pyrolysis temperature was found to correspond exactly to the formation of the hypothesised mesophase materials.51, 52 The research practice employs various carbon materials, whereas in the industry only certain specific materials which ensure adequate characteristics of negative electrodes for lith- ium-ion cells are used.Of the latter, a material produced by Osaka Gas Co. (Japan) which is called mesocarbon microbeads (MCMB) was employed most extensively. This material is the product of pitch carbonisation carried out under certain temper- ature conditions.53, 54 It was assumed that fibrous materials are also promising for fabrication of anodes.55 ± 59The state-of-the-art and prospects for the development of rechargeable lithium batteries Carbon materials (obtained by pyrolysis of certain organic substances, e.g., poly(p-phenylene), phenol ± formaldehyde resin, soluble phenol ± formaldehyde resin and even sucrose) with the intercalation capacity exceeding 372 mA h g71, which corre- sponds to the LiC6 limiting composition, have been described. Their real intercalation capacity could reach 600 ± 700 mA h g71 (see Refs 60 ± 71).Naturally, elucidation of the reason for such an enhanced capacity became a challenge for scientists. It was found that lithium ions intercalated into graphite can occupy different sites in the graphite lattice, viz., in interstitials (proper intercalation), at the edges and on surfaces of graphite crystals.72 These sites differ in the types of bonds formed between the lithium ions and carbon. In amorphous matrices, lithium appears to be dissolved and uniformly distributed. Materials of the MCMB type are consid- ered as the mixtures of pure graphite and a certain amorphous (turbostratic) material.67, 73 Furthermore, it was assumed that excessive lithium ions can be incorporated into nanosize micro- cavities.56, 65, 74 Strongly disordered structures contain isolated graphite sheets (a `house of cards' structure) both sides of which can adsorb the incorporated lithium.Such an intercalation mechanism doubles the intercalation capacity. Moreover, a defi- nite correlation has been found between the intercalation capacity and the content of residual hydrogen in the carbon material.75, 76 The formation of hydrogen ± lithium bonds was hypothesised.65 Further studies have shown that the characteristics of disordered materials gradually deteriorate so that their enhanced intercala- tion capacity is observed only at the beginning of their operation. These studies are being continued, however, without any definite hopes for success.77 ± 80 Passive films formed on the carbon material surface during the first cathodic charge have a great effect on intercalation ± deinter- calation processes.The problem of formation of passive films on carbonaceous materials has been extensively studied, being prob- ably the central question in the development of lithium-ion cells.A vast number of studies were devoted to the elucidation of the formation mechanism, composition, structure and properties of these films. As was stressed above, the passive films on carbon material resemble those formed on the surface of pure lithium with respect to their composition and properties (see, e.g., Refs 21, 81 ± 89). They result from irreversible reduction of elec- trolyte components and serve as a natural barrier that prevents the further process.Thus, the carbon material surface catalyses the electrolyte reduction. The data on the kinetics of this process are contradictory. 0 Propylene carbonate is reduced most actively, hence, it is recommended not to use propylene carbonate-containing electro- lytes in lithium-ion batteries. The electrolyte reduction during the first discharge consumes a charge which is comparable with the specific capacity of the material and represents the irreversible capacity of the negative electrode. To reduce this quantity, it is desirable to add different substances to the electrolyte, e.g., carbon dioxide, crown ethers, sulfur dioxide, ethylene sulfite, nitrogen dioxide, vinylene carbonate, etc.83, 90 ± 96 Later, different procedures of surface treatment were proposed for carbon materi- als themselves, which includes deposition of certain protective coatings.For instance, it was proposed to oxidise 65, 97 the graph- ite material surface, silver-plate its particles 98 and vacuum spray fine tin films.99 Coating of fine graphite particles with still finer nickel particles (in essence, coating of graphite with a porous nickel film) seems more practicable.10 Such a treatment favours a decrease in the irreversible capacity from 310 to 40 mA h g71 and a simultaneous increase in the reversible capacity. A similar result was achieved by applying a thin layer of non-graphitised carbon on the surface of graphite particles.101, 102 Judging from the deposition method used, viz., a fluidised bed treatment of graphite with a mixture of nitrogen and toluene vapour at 1000 8C, the resulting coating represented pyrolytic carbon.It was shown 102 that such a coating eliminates completely the irreversible capacity in such electrolytes as propylene carbonate (PC) and ethylene 333 ± carbonate (EC). Finally, an original proposal to precoat the surfaces of graphite materials with protective films which resem- ble the film usually formed during the first cathodic polarisation in the working electrolyte deserves mentioning. Such protective films can be formed using chemical (treatment with butyllithium) 103 and electrochemical 104 procedures. It follows from above that in the elaboration of negative electrodes, attention is mainly focused on carbon materials; however, attempts have been made to study different substances.At one time, carbon compounds with nitrogen and boron were assumed to be promising.105±109 It was found that although their starting capacities (the capacity in the first cycles) are comparable with those of carbon materials, these decreased substantially during the cycling. The most promising results looked with carbon±silicon compositions.110 ± 112 In the late 1990s, the attention of many researchers has been attracted by the materials based on tin oxides.113 ± 128 When negative electrodes made of such materials were used, lithium was intercalated into metallic tin formed during the initial cathodic polarisation of the electrode (2) Sn+2 Li2O SnO2+4Li++4e7 rather than into the tin oxide.Subsequent insertion of lithium into tin formally proceeds in the same fashion as its intercalation into carbon (3) x Li++Sn+xe Lix Sn, but, for tin, the coefficient x can be sufficiently high, i.e., reach 4.4 (for intercalation into graphite, x=1/6). For x=4.4, the theo- retical specific capacity of a tin electrode is 991 mA h g71 or 7234 mA h cm73. Such high values of specific capacity (especially the volumetric one) are responsible for the keen interest paid to these materials. Even being calculated with respect to the initial oxide SnO2 rather than to tin (into which lithium is intercalated), the resulting values of specific capacity still remain sufficiently high, viz., 781 mA h g71 and 5428 mA h cm73. However, the use of tin oxides and other tin compounds entails a number of problems.The common (and the main) drawback of all metal electrodes is the substantial change in their specific volumes upon lithium intercalation. During lithium intercalation, the specific volume of an alloy formed (or, more precisely, a binary system) substantially increases, which results in high internal stresses and, as a conse- quence, the electrode cracking. In turn, this results in the appear- ance of a high ohmic resistance at the boundaries of individual metal particles and a sharp polarisation rise. While, for primary cells, this factor cannot play any significant role, for a secondary cell (rechargeable battery), it is the determining factor.(It should be borne in mind that the specific volume changes induced by cycling were the reason for the failure of numerous attempts to create a negative electrode based on aluminium and certain alloys like the Wood alloy.) Tin is a remarkable material because it can be obtained in a fine-grain form with a characteristic particle size of several nano- metres using different technological procedures. Dispersion of tin occurs in particular owing to its formation in the lithium oxide matrix by Eqn (2). In several studies,115 ± 190 this was considered to be the key factor. Yet another principle for the preparation of highly dispersed particles is the use of tin alloys and intermetallic com- pounds.121 ± 130 However, in this method too, the stability of a finely dispersed phase of either tin or its intermetallic compound is favoured by the presence of another (inactive) phase.A relatively complex procedure of fabrication of negative electrodes based on tin, viz., chemical deposition of tin into pores of a polymeric membrane has been proposed.127 Such a membrane (a polycar- bonate filter) has a system of monodispersed cylindrical pores with diameters of 50 nm. Prior to deposition of tin, the membrane was applied on a current collector made of foil. In the end, the membrane was burnt out to give a brush of tin hairs, which are334 oxidised in air to give crystalline SnO2. The electrodes thus prepared exhibited highly stable characteristics and were capable of discharging by huge currents (a complete discharge in 1 min!).A two-phase system consisting of mixed powders of tin and a tin ± antimony intermetallic compound (SnSb) was suggested 128 for the fabrication of negative electrodes. The ultradispersed powders can be obtained by chemical reduction using boro- hydride. Being inserted into such a two-phase system, lithium occurs first in the intermetallic compound, the tin phase playing the role of a stabilising matrix. Following completion of this process, intercalation of lithium into tin begins. Thus, the tin `matrix' is not inert (in contrast to Li2O) but makes an additional contribution to the capacity. The use of these electrodes allowed a specific capacity of 360 mA h g71 to be maintained for 200 charge ± discharge cycles.It was proposed 126 to prepare a Ni3Sn2 intermetallic compound by a mechanochemical method, viz., by processing nickel and tin powders in a ball mill. Electrodes based on such an intermetallic compound exhibited specific capacities up to 327 mA h g71 or 2740 mA h cm73. The same method was proposed for fabricating an intermetallic compound of indium with antimony (InSb).129 Despite their adequate characteristics, the electrodes made of this compound have several drawbacks, viz., the high cost of indium and the toxicity of antimony. Later, copper was substituted for indium and tin was substituted for antimony.130, 131 ACu6Sn5 intermetallic compound was also shown to be quite plausible as a matrix for intercalation of lithium.In the potential range from 2.0 to 0.2 V, lithium intercalated successfully to give a Li13Cu6Sn5 compound. True enough, such an intercalation was accompanied by a noticeable increase in the specific volume; however, this effect can be eliminated by decreasing the size of intermetallic compound particles. A proposal 132 to prepare a microcomposite material from the Cu6Sn5 intermetallic com- pound and metallic copper was more successful. This material was prepared by the so-called mechanical alloying of an equi- atomic mixture of copper and tin powders in a high-speed planetary mill in an argon atmosphere. The electrodes made of this material withstand more than 80 charge ± discharge cycles with a specific capacity of 200 mA h g71.An interesting approach was suggested 133 for the preparation of a Sn ± LiO2 nanocomposite by a reaction of tin oxide with lithium nitride in a ball mill in an argon atmosphere. This reaction (4) 2Li3N+3SnO 3Sn+3Li2O+N2 : was carried out for 5 days. However, in reward, this method produced a ready-for-use composite; hence, there was no need to spend the charge in the primary reduction of tin by reaction (2). Amorphous oxides of silicon and tin SiO . SnO (see Ref. 134) are also considered as promising materials for negative electrodes. Powders of these oxides were also obtained in a planetary ball mill from the starting monoxides. Electrodes of these materials had the initial capacity of *600 mA h g71 which decreased to 350 mA h g71 during the initial 20 charge ± discharge cycles.Recently, in addition to most popular tin-containing com- pounds, other materials were proposed for use as negative electrodes in lithium-ion batteries. The following examples may be mentioned: the KC8 intercalation compound;135 magnesium silicide Mg2Si (the starting capacity of 1370 mA h g71 which, however, smoothly decreased to 100 mA h g71 in the initial 10 charge ± discharge cycles);136 a complex nitride Li2.6Co0.4N (the starting capacity was 760 mA h g71 and also decreased with time);137 a sodium ± molybdenum bronze of the Na0.25MoO3 composition which exhibited an intercalation capacity in lithium of 940 A h kg71 (or 4000 mA h cm73).138 However, the applica- tion field of lithium-ion batteries with electrodes of this bronze is rather narrow, because their potential changes within a very wide range, viz., from 3 to 0.005 V during the discharge.It was proposed to use a material based on Li4Ti5O12 (Refs 139, 140) and a lithium ± zinc alloy 141 for negative electrodes. The latter material can be obtained by controlled vapour codeposition of AMSkundin, O N Efimov, O V Yarmolenko individual metals, the vapour mixture being condensed on a titanium current collector cooled by dry ice. The microstructure formed makes it possible to carry out cycling at high current densities. Lithium is known to form alloys with silicon up to the Li4.4Si composition, which corresponds to a theoretical specific capacity of 4000 mA h g71.However, intercalation of lithium into silicon is accompanied by great changes in the specific volume. A laser- induced method was applied for the preparation of silicon powder from silane.142 The particles measuring only *80 nm did not destroy during cycling. Electrodes fabricated from a mixture of such a silicon powder with carbon black demonstrated the capacity values of 1700 mA h g71 after the tenth cycle and 1300 mA h g71 after the twentieth cycle. Nanocomposites of silicon microparticles in a titanium nitride matrix were also used for negative electrodes.143 In this case, lithium gets intercalated into silicon. For a 33% silicon content, the composite capacity reached 300 mA h g71 and remained unchanged for 20 cycles (no data on long-term cycling is avail- able).The Si/TiN nanocomposites were also fabricated in a high- speed ball mill. Mention should be made of important problems of the technology of negative electrodes such as the difficulties in the development of binding materials and current collectors, the imperfect technology for the fabrication of the electrodes them- selves that gives no way of providing their uniform (small) thick- ness, the questions associated with corrosion of electrodes, etc. 2. Materials for positive electrodes The choice of the material for the positive electrode in recharge- able lithium-ion batteries is restricted to lithiated cobalt, nickel and manganese oxides. Electrodes based on lithiated cobalt oxide are characterised by the lowest polarisation and the highest specific capacity.The performance of a positive electrode is reduced to deintercalation of lithium during the cell discharge and intercalation of lithium during the charge (5) Li+ + e+CoO2 . LiCoO2 The operational characteristics of high-potential electrodes (i.e., the electrodes with open-circuit potentials of *4 V vs. lithium electrode, which maintains the discharge voltage at a level above 3 V) are approximately the same. The choice of the active substance for positive electrodes was arbitrary to a certain extent. Japanese scientists prefer cobalt oxides which are much more expensive than other materials; however, their industrial synthesis is relatively simple and reproducible.In France and Canada, preference is given to cheaper nickel oxide. Lithiated nickel oxides operate at a less positive potential than cobalt oxide. This mitigates to a certain degree the require- ments of the stability of the electrolyte to oxidation during charging. At the same time, the discharge curve of a nickel-oxide electrode is steeper, i.e., the voltage variations during the dis- charge in a cell with nickel-oxide electrodes are greater as compared with cells equipped with electrodes of manganese and cobalt oxides. The USA industry mainly uses the materials based on manganese oxides (lithium ± manganese spinels) which seem to be more attractive economically and environmentally safe (see, e.g., Ref.144). Electrochemical characteristics of positive electrodes depend to a great extent on the technology used in their synthesis. Most publications were largely devoted to technological problems, and the questions of mechanisms of processes on these electrodes have received less attention. The technology of synthesis of lithiated oxides is based on the use of various high-temperature (sintering) and low-temperature (sol ± gel, ion exchange, deposition from solutions) processes. Lithiated cobalt oxide was mainly prepared by a low-temperature technology,145 whereas lithiated nickel oxide was fabricated by sintering.146, 147 Numerous studies aimed at the perfection of positive electro- des for lithium-ion cells are largely limited to improvement of theirThe state-of-the-art and prospects for the development of rechargeable lithium batteries extremely expensive cobalt.163, 164 First specimens contained low (y<0.1) iron fractions; however, later, materials with y=0.24 were developed. Unfortunately, the resulting materials were characterised by too fast a capacity fade during cycling.The main drawback of lithium nickelates used as the materials for positive electrodes is the impossibility of complete deinterca- lation of lithium. As a rule, compounds LixNiO2 are cycled only in the range of 0.5<x<1.0. A deeper deintercalation results in irreversible structural changes. At the same time, it was found that a material with a violated stoichiometry with the composition LixNi1.02O2 can be cycled reversibly in the whole range of x (i.e., 0<x<1).165 structures.This is achieved by both technological methods and the introduction of different modifiers. For example, it was shown that lithium cobaltate with the O2 structure has a more pronounced layered structure (suitable for reversible intercalation of lithium) as compared with conventional lithium cobaltate with the O3 structure.148 The modification with the O2 structure can be prepared, for instance, by ion exchange from a solution of sodium cobaltate (Na2/3CoO2) and lithium bromide in hexanol. Although the materials with the O2 and O3 structures hardly differ in many parameters, the former material is more cycling-stable. Yet another technology used in the prepara- tion of lithium cobaltate with a good texture (preferential crystal orientation along a lithium intercalation direction) consists in radiofrequency magnetron sputtering of thin films on various conductive supports,149 which also facilitates the reversible cycling, i.e., increases the battery life.To obtain highly dispersed LiCoO2 powders with enhanced reversibility degrees, an original method of supercritical aqueous synthesis was put forward.150 For this purpose, the aqueous solutions of LiOH, Co(NO3)2 andH2O2 were fed under a pressure of 30 MPa to an electric furnace at 400 8C, where the synthesis was completed in 30 s. An extension of the cycle range for nickelates can be also achieved by using mixed oxides. Lithiated nickel and cobalt mixed oxides (at a small content of the latter, see, e.g., Refs 166 ± 168) are gaining in acceptance. A mixed oxide material with a layered O2 structure and a composition described by the formula Li2/3(Ni1/3Mn2/3)O2 has been proposed.169 In the structural aspect, this material resembled the layered cobaltate modifica- tion 148 and was also obtained by ion exchange from a sodium bronze Na2/3(Ni1/3Mn2/3)O2 [or, more generally, from Na2/3(NiyMn17y)O2].The discharge capacity of this material was *180 mA h g71 but the discharge curve demonstrated two steps: at a potential of *3.9 V, which corresponded to electron transfer to the 3d level of nickel, and at 2.9 V where manganese changes its valence from +4 to +3. A method of emulsion drying was used for the preparation of highly dispersed LiCoO2 powders.151 As was mentioned above, a sufficiently high discharge poten- tial of a positive electrode at which the lithiation occurs is one of the merits of lithium cobaltate.However, this property is simulta- neously a drawback, because the charging of a positive electrode occurs at still higher positive potentials at which the electrolyte can be oxidised on the catalytically active surface of cobaltate (or on a carbon additive used for increasing the active-layer conduc- tivity). To overcome this disadvantage, an original method for modifying the electrode surface by applying a very thin (no thicker than 0.1 mm) layer of diamond-like carbon by plasma spraying from ethylene has been proposed.152 The resulting electrodes could be cycled in the potential range from 3.0 to 4.2 V with a specific capacity above 200 mA h g71.A series of magnesium-modified lithium nickelates with the general composition Li1+xNiMgyO2+2y has been described.170 An introduction of magnesium resulted in stabilisation of the nickelate structure, which, on the one hand, allows its deeper delithiation, i.e., increases the specific capacity and, on the other hand, reduces the capacity fade during cycling. A material with x=0.15 had a specific capacity of *145 mA h g71, which even increased a little during the first 30 cycles. Lithiated nickel ± magnesium oxides with layered structures (and approximately the same characteristics) have been described.171, 172 It is known that, being heated, lithium cobaltate decomposes to evolve oxygen.This reaction is especially fast for highly dispersed powders. To enhance the cycling stability of the lithium cobaltate structure, the latter was coated with a SnO2 film using a sol ± gel technology and then annealed.153 Lithiated nickel oxides are much cheaper than lithiated cobalt oxides; however, partial substitution of iron for nickel is believed to be economically warranted. Attempts 173, 174 for such a sub- stitution were made, which, however, were not too successful: only the materials with the LixNi17yFeyO2 composition, where y<0.1, could be cycled reversibly. A similar method was used for modifying a LiSr0.002Ni0.9Co0.1O2 mixed compound.154 A powder of this compound was coated with a Mg(OH)2 layer and then annealed.The annealing resulted in the formation of a MgO coating which prevented electrolyte oxidation during the discharge, increased the discharge capacity from 145 to 160 mA h g71 and enhanced the cycling stability (the capacity fade did not exceed 0.4 mA h g71 per cycle). A LiMn2O4 coating was applied on the LiCoO2 powder to protect the electrolyte from oxidation during the charge and to enhance the thermal stability of lithium cobaltate.155 Among lithiated manganese oxides, spinels with compositions similar to LiMn2O4 were used most often for fabrication of positive electrodes. Intercalation of lithium into such a material (i.e., during a discharge of the lithium-ion battery) resulted in formation of compound Li1+xMn2O4.Such electrodes had a rated potential value of*3 V. In the lithium deintercalation (the formation of Li17xMn2O4), the potential value approached *4 V. Various technologies for the preparation of spinels of different compositions (LiMn2O4, Li2 Mn4O9, Li4 Mn5O12, etc.) were described.175 ± 184 The best results were obtained using low- temperature `wet' technologies which allow finely dispersed mate- rials to be obtained,177, 183, 184 although plausible high-temper- ature versions are known.179 In studying spinels, the main attention was paid to their lattice parameters. Intercalation of lithium into a spinel was accompa- nied by a decrease in the a0 parameter of the cubic lattice 180, 182, 185 and a linear decrease in the phase-transition temperature, which was attributed to disproportionation of the spinel and a loss of a certain amount of oxygen.The use of mixed oxides, particularly, cobalt and nickel oxides, substantially improves the characteristics of lithiated oxides. Although the materials for such compounds were pro- posed for this use long ago, they are still being refined.156 ± 159 Complex mixed oxides are also of great interest. Thus in addition to LiSr0.002Ni0.9Co0.1O2 mentioned above, a material with the composition Li8(Ni5Co2Mn)O16 remarkable for its sufficiently high specific capacity (*150 mA h g71) and cycling stability (a capacity fade was*0.4 mA h g71 per cycle) deserves mention.160 This material can be obtained by fine synthesis from solutions of salts of the corresponding metals in methanol and tetrahydro- furan, which is followed by controlled annealing.Materials of the compositions LiAlyCo17yO2 have also attracted attention of scientists.161, 162 The promising mixed materials with a composition described A substantial decrease in the capacity during cycling, espe- cially at high temperatures, is a drawback of lithium ± manganese spinels (as well as their low specific capacity as compared with cobaltates and nickelates). The reasons for deterioration of characteristics of spinel electrodes were discussed in Ref. 186. Fourier-transform IR spectroscopic studies 187 have shown that self-discharge of spinel electrodes is accompanied by simultaneous oxidation of the solvent on particles of a carbon conducting additive.by the general formula LiCo17yFeyO2 should be also mentioned. The cost of lithium-ion batteries based on these materials could be substantially decreased by partial substitution of cheap iron for 335336 Recently, attention of researchers has been attracted to non- spinel metastable and disordered lithium ± magnesium structures including their monoclinic and orthorhombic modifica- tions.188 ± 195. Good characteristics of these materials are associ- ated particularly with the high defect densities at their grain boundaries and the small sizes of grains themselves (5 ± 20 nm). Doping spinels with small amounts of other elements, such as aluminium 192 ± 195 and potassium was recommended 196 to obtain such structures.For example, a monoclinic modification of LiAl0.25Mn0.75O2 had a capacity of 130 ± 150 mA h g71, being cycled in a potential range from 2.0 to 4.4 V.192 A material of the composition LiAl0.05Mn0.95O2 had a specific capacity of 150 mA h g71 for its orthorhombic modification and *200 mA h g71 for the monoclinic modification.194 Moreover, for these materials, the capacity faded by *0.05% per cycle, whereas for common spinels this value exceeds*0.5% per cycle. Yet another way of enhancing the cycling stability of positive- electrode materials is the synthesis of amorphous (or amorphised) materials, e.g., complex compounds such as Li1.5Na0.5MnO2.85I0.12 (see Refs 197 ± 199).Its first 40 cycles not only showed no sign for the capacity fade but also resulted in its increase from 120 to 125 mA h g71. It was suggested 200 to use an amorphous modification of manganese dioxide LixMnO2 as a starting material that can be cycled to the depth of over 400 mA h g71, which corresponds to a change in x from 0 to 1.6. It is evident that the capacity of these electrodes should be lower but are quite acceptable (260 mA h g71 in a range from 4.0 to 2.5 V). Yet another approach to the preparation of stable materials is the use of sol ± gel technology. Its version using supercritical drying is the most interesting. For example, a technology was described 201 in which, first, a hydrogel was obtained from an aqueous solution of potassium permanganate and lithium fuma- rate, after which water was exchanged for acetone and then hexane.The hexanogel thus obtained was dried under conditions close to critical ones. In the process, the surface tension was eliminated, and an extremely fine-grain structure retained. An electrode made of this material with the composition Li0.9MnO2.5 had a capacity of *440 mA h g71 at a discharge up to 2.9 V. It should be noted that such electrodes could be discharged by very high currents. Thus a discharge with a current of 270 mA g71 gave a capacity of 200 mA h g71 and a discharge at 100 mA g71, i.e., in a 2.5C mode,{ resulting in a capacity of 150 mA h g71 (the maximum load tested for these electrodes corresponded to the 7.5C mode, i.e., a total discharge took 8 min). A material with the composition LiAl0.24Mn1.76O3.98S0.02 was obtained 202 using the sol ± gel method.Having a well defined spinel structure, this material could be cycled without any noticeable capacity loss; its capacity was 230 mA h g71. A similar material (with sulfur partially substituted for oxygen) has been described.203 A LiAlxMn27yO47zFz material 204 exhibited a high cycling stability; this was attributed to the presence of a small amount of fluorine in the oxygen sublattice. In addition to lithium ± manganese spinels doped with alumi- nium, materials with chromium atoms partly substituted for manganese were reported to show promise for using them in lithium-ion batteries.205 ± 207 A sample of lithium ± manganese spinel with a well pro- nounced crystalline structure (with coarse crystals) was synthes- ised.208 This material exhibited cycling stability, although its capacity was not too high (less than 80 mA h g71).As was noted above, cobaltate particles were coated with a thin layer of manganese oxides to protect electrolytes from oxidation and enhance the thermal resistance of electrode materi- als.155 The same group of authors have proposed a `reverse' { If a cell is completely discharged in 1 h, this mode is designated as 1C; if a cell is discharged in 2 h, the mode is designated as 0.5C or C/2; if the total discharge takes 0.5 h, then the mode is 2C, etc. AMSkundin, O N Efimov, O V Yarmolenko version, viz., deposition of lithium cobaltate on the particles of a lithium ± manganese spinel.209 Such a coating was applied as a gel consisting of lithium and cobalt acetates, which was followed by annealing of the specimen. This procedure allowed the authors to reduce the capacity fade from 1.1 to 0.06 mA h g71 per cycle.The use of certain cobalt ± manganese mixed spinels provides a discharge at very high potential ratings, viz., *5 V. The use of such material poses very severe requirements to the electrolyte (which should not be oxidised at a potential of *5.3 V). In principle, the possibility of enhancing the battery voltage is a challenge. `Five-volt' materials are being elaborated for several years.210 ± 212 To date, two of these materials have been described, viz., Li2CoMn3O8 and LiCoMnO4.In addition to lithiated cobalt, nickel and manganese oxides, other materials were suggested for use as positive electrodes, e.g., lithiated chromium oxides (the capacity of *200 mA h g71),213 LiCoyNi17yVO4,214 LiVOPO4 215 and LiFePO4.216 ± 219 The latter compound exhibits a theoretical specific capacity of 170 mA h g71. The prospects of using these materials are still unclear, although the use of such a cheap compound as LiFePO4 as the active material for positive electrodes seems to be challeng- ing.The materials based on vanadium oxides exhibited sufficiently high specific capacities; however, their voltage changed with the discharge in a very wide range, which cannot be tolerated in chemical cells designed for power supply of electronic equipment.3. Electrolytes for lithium and lithium-ion rechargeable batteries In lithium-ion rechargeable batteries, liquid electrolytes, which represent solutions of lithium salts in aprotic organic solvents, were used.220, 221 As was mentioned above, in rechargeable batteries with metallic lithium electrodes, the use of liquid electro- lytes poses certain problems; hence, the efforts of researchers were focused on the search for solid polymeric electrolytes (SPE). The processes of dendrite formation and encapsulation were assumed to be eliminated at the metal lithium/SPE interface. At the same time, the possibility of using polymeric electrolytes in lithium-ion batteries was also widely discussed. An electrolyte for a lithium-ion rechargeable battery should exhibit high conductivity.As a rule, liquid electrolytes have a conductivity in the range from 0.001 to 0.01 S cm71. The electro- lyte should be resistant toward oxidation and reduction, in other words, have a wide `electrochemical window'.{ The potential range of electrolyte stability is *5 V. As was mentioned above, the stability of electrolytes largely depends on the formation of passive films on the electrode surface. These films are conductive with respect to lithium ions and do not prevent their intercalation and deintercalation. At the same time, the films should exhibit a minimum electronic conductivity to avoid corrosion (self-dis- charge) of the electrode.For this very reason, the film material should not dissolve in the electrolyte. Moreover, these films should be sufficiently elastic to avoid destruction during electrode volume changes in the discharge and charge processes. Various combinations of solvents (both individual and mixed) and lithium salts intended for use as the electrolytes were described; however, only some of them are applied in practice. Propylene and ethylene carbonates (PC and EC) are com- monly used as the solvents. Solvents based on PC are reduced on graphite negative electrodes; hence, EC was proposed as the substitute for PC. At the same time, PC has a number of advantages over EC: the solubility of lithium salts is higher in PC, its freezing point (754 8C) is substantially lower than the freezing point of EC (+36 8C).Moreover, in certain cases, the use of PC is preferential for improvement of positive electrode characteristics. As a rule, PC and EC are used as their mixtures { The difference between the potential of the beginning of anodic oxida- tion and the potential of the beginning of cathodic reduction.The state-of-the-art and prospects for the development of rechargeable lithium batteries with less viscous, although less polar, solvents such as 1,2- dimethoxyethane, dimethyl carbonate and diethyl carbonate. Lithium hexafluorophosphate and tetrafluoroborate are used most often as lithium salts; the possibilities of using lithium hexafluoroarsenate and perchlorate were also examined. Merck (Germany) produces ready-for-use electrolytes for lithium-ion batteries of the following types: LP20, LP30 and LP40, where electrolytes represent 1 MLiPF6 solutions in the following solvent mixtures: PC± diethyl carbonate (1 : 4), EC± dimethyl carbonate (1 : 1) and EC± diethyl carbonate (1 : 1), respectively.In recent years, the following lithium salts were proposed for use as lithium-ion batteries, viz., lithium trifluoromethanesulfo- nate (triflate), bis(trifluoromethylsulfonyl)imide and tris(tri- fluoromethylsulfonyl)methanide and their derivatives. The derivatives involved such compounds as cyclic imides Li[N(SO2)(CF2)n(SO2)], where 1<n<4,222, 223 an asymmetrical Li[C(SO2CF3)2(SO2C4F9)] and methanide bismethanide Li2[(C2(SO2CF3)4(S2O4C3F6),224 imides with long fluoroalkyl chains,225 lithium hexafluorotrimethylenebis(sulfonyl)di[bis(tri- fluoromethylsulfonyl)methanide],226 etc.Boron-containing compounds, particularly, different borates and chelatoborates were used in lithium-ion batteries as the additives that extend the potential range of electrolyte stability, increase the solubility of salts and decrease the corrosive action of the electrolyte on the current lead material (e.g., alumi- nium).227 ± 235 Of great interest is the hexamethoxycyclotriphos- phazene [NP(OCH3)2]3 additive, which favours a decrease in the inflammability of lithium-ion batteries.236 VII. Polymeric electrolytes for lithium-ion batteries Among solid ionic conductors which can be used at ambient temperature, the SPE with a relatively fast ion transfer are most promising.Thin films fabricated of these polymeric electrolytes are characterised by elasticity (mobility), adequate mechanical properties, a low electronic conductivity (10714 S cm71 at room temperature), a wide electrochemical window of stability (3 ± 4 V) and are compatible with the electrodes of alkali metals. At the same time, the use of SPE is limited due to the following reasons: first, they have a low ionic conductivity at ordinary temperatures (from 1075 to 1078 S cm71); second, not only lithium ions, but also anions take part in the charge transfer, which can give rise to concentration polarisation; third, like liquid electrolytes, they can form a transition layer with a high resistance upon contact with the electrode (both positive and negative) material.Armand et al.237 were the first to propose the use in lithium cells of a complex of a lithium salt with a polymer [poly(ethylene oxide), PEO]. The conductivity of such a PEO ± LiSCN complex system at room temperature was 1078 S cm71.237, 238 These rechargeable batteries could operate only at temperatures above 60 8C, because it was only under these conditions that the conductivity of the electrolyte solution had acceptable values for practice. Nonetheless, the interest in these electrodes was so high that the extensive research undertaken has soon produced poly- meric electrolytes with conductivities of *1073 S cm71 at room temperature, which is comparable with the conductivity of liquid electrolytes. An important requirement to SPE is their mechanical strength, because, to decrease the resistance, these electrodes are mainly used as thin films with thicknesses of 0.0025 ± 0.005 cm commensurable with the thicknesses of porous separators based on polypropylene and polyethylene, which divide the electrodes in lithium batteries with liquid electrolytes.A solid polymeric electrolyte should also withstand the pressures induced by mor- phological changes in the electrode material during the lithium cell cycling. Thus, SPE, on the one hand, should provide ionic conduction typical of liquid electrolytes and, on the other hand, 337 should be spatially stable, i.e., retain their solid shapes.Modern SPE fulfil all these requirements. We can single out two large classes of polymeric electrolytes with conductivities above 1075 S cm71 at room temperature. The first class involves `solution-free' SPE. In addition to polymers, these SPE contain lithium salts. The second class comprises SPE `with solvents', which, in addition to polymers and lithium salts, contain liquid solvents called plasticisers. These two SPE classes may have polymeric matrices with different structures. 1. Solid electrolytes with the polymer ± lithium salt composition Solid electrolytes based on crystalline polymers have low con- ductivities, whereas those based on amorphous polymers exhibit higher conductivities. Thus, the SPE conductivity can be increased by decreasing the degree of crystallinity of a polymer by changing its structure.Table 1 lists the characteristics of certain polymers; Table 2 lists the SPE conductivities. Table 1. Properties of certain polymers used as the electrolyte matrices.239 Polymer unit Polymer a m.p. /8C Tg /8C PEO PPO POO 7CH2CH2O7 764 65 7CH(Me)CH2O7 760 see b 7(CH2O)7(CH2CH2O)7 766 13 OCH2CH2OCH2CH2OMe P N see b MEEP 783 OCH2CH2OCH2CH2OMe PDMS PAN PMMA PVC PVDF 7Si(CH3)2O7 7127 740 7CH2CH(CN)7 125 317 7CH2C(Me)(CO2Me)7 see b 105 7CH2CHCl7 see b 82 7CH2CF27 740 171 a The following abbreviations were used: PEO, poly(ethylene oxide); PPO, poly(propylene oxide); POO, poly(oxymethylene oligooxyethylene); MEEP, poly{bis[2-(2-methoxy)ethoxy]ethoxy}phosphazene; PDMS, poly(dimethylsiloxane); PAN, poly(acrylonitrile); PMMA, poly(methyl methacrylate); PVC, poly(vinyl chloride); PVDF, poly(vinylidene di- fluoride).b Amorphous polymer. Table 2. Composition and bulk conductivity with respect to Li+ ions for solid polymeric electrolytes.237, 238, 240 Electrolyte Conductivity at 20 8C /S cm71 1075 1078 (PEO)8LiClO4 (PPO)8LiClO4 (POO)25LiCF3SO3 (MEEP)4LiBF4 361075 261075 Back in 1979, it was pointed out 238 that PEO exhibits good solvating ability towards those Li+ ions and ions of other metals located in a favourable position relative to the donor oxygen atoms of the polymeric chain. By changing the oxygen atom position in the chain, the solvating ability and the temperature transitions in the polymer can be changed.For example, PEO can form polymeric electrolytes with salts of alkali and transition metals.241 Moreover, the existence of various PEO complexes with LiBF4 , LiPF6 and LiB4,242 LiAsF6,243 LiSCN,244 LiCF3SO3 and LiClO4,245 LiN(CF3SO2)2246, 247 has been reported. The use of inorganic additives in the form of fine powders of Al2O3 or LiAlO2 was proposed to enhance the mechanical strength of an electrolyte based on PEO and a lithium salt.248 ± 254 Such additives, first, prevent crystallisation of the polymer and, second, can absorb water, which allows one to338 reduce the lithium electrode passivation. At room temperature, the ionic conductivity of an electrolyte with the composition (PEO)8LiClO4 was 1075 S cm71, whereas the conductivity of the (PEO)8LiClO4+10 mass% LiAlO2 composition was 1074 S cm71.It was proposed 255 to modify a polymeric electrolyte based on PEO and LiBF4 using finely dispersed Al2O3 powders with *10 mm particles as well as by modified particles measuring 1.361072 mm. The conductivity of the latter SPE was higher by an order of magnitude as compared with SPE modified by 10 mm particles. Introduction of g-LiAlO2 to a polymeric electrolyte based on PEO and LiN(CF3SO2)2 resulted in a substantial decrease in the degree of crystallinity and an increase in the stability of the Li/electrolyte interface.256 The effect of various types of fine powders added to a polymeric electrolyte on the conductivity and mechanical strength of PEO-based SPE was also studied.257, 258 Using LiClO4 complexes with a number of aliphatic polyesters (PEst) synthesised by a reaction of poly(ethylene glycol) (PEG) (Mw=300, 1000) with diacid chlorides (ClOC[CH2]nCOCl, n=3 ± 8, 10) as the examples, the effect of the polymer structure on its ionic conductivity was studied.259 The general composition of these complexes can be expressed by the formula PEst(m,n), where m is the number of ethylene groups and n is the number of methylene groups; e.g., PEst(6,8) is [7(OCH2CH2)6O7 CO(CH2)8CO7].In contrast to the PEO± LiClO4 system with a degree of crystallinity of 0.4, the PEst(m,n) ± LiClO4 systems are totally amorphous, and their ionic conductivity varies in the range from 2.561075 to 4.161075 S cm71.The degree of crystallinity of PEO can be reduced by shortening its chain and decreasing its molecular mass;260 however, low-molecular PEO with short chains represent viscous liquids even after the formation of complexes with lithium salts and could not form SPE. With the aim of decreasing the degree of crystallinity, comb- like polymers with short PEO side-chains attached to the main chain were synthesised, e.g., poly[2-(2-methoxy)ethoxy]ethyl gly- cidyl ether O CH CH2 n CH2(OCH2CH2)2OMe. A complex of this amorphous polymer (Tg=757 8C) with LiClO4 had a conductivity of an order of magnitude of 1075 S cm71 at room temperature. A search for amorphous polymers has produced materials with highly mobile inorganic skeletons based on polysiloxanes (7SiR2O7)n and polyphosphazenes ( P=N7)n . Siloxane poly- mers can be exemplified by poly(dimethylsiloxaneoligoethylene oxide) Me SiO HC CH n O m , Me poly{[o-methoxyoligo(ethylene oxide)ethoxy]methylsil- and oxane}Me SiO n HC CH OCH2CH2 OMe.m O A maximum conductivity achieved using electrolytes of these types (which also represent viscous liquids) is 1074 S cm71 (Refs 261, 262). Cross-linking of polymers decreased the conduc- tivity to 1075 S cm71 at room temperature. The drawback of these polymers is the sensitivity of Si7O7C bonds to hydrolysis and their structural degradation with time. This problem can be AMSkundin, O N Efimov, O V Yarmolenko solved, e.g., by synthesising copolymers with stable Si7C bonds, which has been achieved by grafting PEO to polysiloxanes.263 Electrolytes based on such a polymer (777 8C< Tg<754 8C) exhibited conductivities of 1075 S cm71 at room temperature.A modified polymeric electrolyte based on MEEP (see Table 1) was described.264 Complexes of MEEP with lithium salts had a low stability. The authors proposed to add LiAlCl4 to such an electrolyte, which allowed them to fabricate MEEP± (LiAlCl4)n films. The mechanical properties of spatially unstable MEEP± lithium salt complex systems were substantially improved as a result of formation of complexes with such polymers as PEO, PPO, poly(ethylene glycol diacrylate) and poly(vinylpyrrolidone). An electrolyte containing 55 mass% MEEP+45 mass% PEO ± [LiN(CF3SO2)2]0.13 had a conductiv- ity of 6.761075 S cm71 at room temperature. Such electrolytes remained stable up to 4.5 V.A Li/TiS2 cell with these electrolytes had a cycle life exceeding 200 charge ± discharge cycles. Results 265 obtained in line with these studies should be considered as less successful. The authors have obtained a number of polysilane polymers: 1a ± c, 2 and 3 (CH2)4O(CH2)2OEt Si R n 1a ± c R=Me (a), Pr (b), C8H17 (c). (CH2)4O(CH2)2OEt Si (CH2)4O(CH2)2OEt n 2 (CH2)4O(CH2)2OEt (CH2)4O(CH2)2OEt Si Si Pr (CH2)4O(CH2)2OEt 2 3 n 3 Polymers 1b and 2 with 8 oxygen atoms per lithium ion exhibited a maximum ionic conductivity equal to 1.961077 S cm71. The degree of crystallinity can be reduced by cross-linking the polymers.Such polymeric nets exhibited relatively high conduc- tivities (1.561075 S cm71 at 25 8C for a LiClO4 salt) and an acceptable mechanical strength. This approach was put forward in Refs 266 ± 268 and involved cross-linking of low-molecular PEO (Mw=600, 1000, 2000) and block-copolymers PEO± PPO ±PEO with urethane and cyclosiloxane. Polymeric electrolytes based on PEO with Mw=2000 which contained either LiCF3SO3 or LiN(CF3SO2)2 and electrolytes with the composition PEG± LiCF3SO3 with an addition of poly(ethylene glycol dimethyl ether) (PEGDME) with Mw=500 were studied.269 The following electrochemical properties of these three electrolytes were examined: ionic conductivity, electrochemical stability win- dow, reduction and the resistance and stability of the lithium electrode/electrolyte interface.Moreover, prototypes of lithium cells with composite cathodes based on LiMn2O4 were tested with the aim of studying the performance of these electrolytes in solid lithium batteries. In SPE containing LiN(CF3SO3)2, a conductiv- ity of 1074 S cm71 required for their practical application was achieved only at 60 8C. An addition of PEGDME decreased the resistance of the lithium electrode/electrolyte interface and trebled the specific capacity (90 mA h g71). A new method of SPE preparation by dissolving the polymers with high glass transition temperatures (Tg>90 8C) in a LiCF3SO3 melt has been described.270, 271 Such polymers as poly(N,N-dimethylacrylamide), poly(acrylonitrile) (PAN), poly-The state-of-the-art and prospects for the development of rechargeable lithium batteries (vinylpyrrolidone) and its copolymer with vinyl acetate were studied.At 45 8C, the conductivity of a PAN± LiCF3SO3 (75 mass %) electrolyte did not exceed 1076 S cm71. 2. Plasticised polymeric electrolytes In contrast to SPE with the polymer ± salt composition, plasticised polymeric electrolytes contain organic solvents which are con- fined within the swollen polymeric matrix so that these electrolytes resemble rubber in their consistency. Plasticised electrolytes are also called polymeric gel-electrolytes (PGE). They can be obtained by adding plasticisers, viz., polar aprotic solvents with high boiling points and high dielectric constants to SPE.Table 3 shows typical solvents-plasticisers. Table 3. Certain organic solvents used as plasticisers.272 Solvent-plasticiser Viscosity /1073 Pa s m.p. b.p. Dielectric /8C /8C constant at 25 8C 95.3 65.1 39 36.7 5.5 248 240 204 153 85 Ethylene carbonate 36 Propylene carbonate 770 g-Butyrolactone 743.5 Dimethylformamide 761 1,2-Dimethoxyethane a 770 1.9 2.53 1.75 0.8 0.46 a Used only as mixtures with high-boiling plasticisers added to decrease the viscosity. The conductivity of PGE depends on the solvent content, its viscosity and dielectric constant. Polar solvents with low viscos- ities such as N-dimethylformamide (DMF) and g-butyrolactone (BL) substantially increase the conductivity in lithium ions.The activation energy of conductivity decreases dramatically with an increase in the solvent-to-lithium salt molar ratio. In fact, lithium ions travel in the liquid electrolyte confined by a polymeric matrix. Hence, to obtain a PGE with an acceptable conductivity, the amount of DMF required should be only a third of that required for more viscous PC. Solvents-plasicisers solvate Li+ ions facilitating their trans- port (this phenomenon was not observed for polyethers in which an ion travels along the polymer chain). The `degree of assistance' to the ionic transport probably depends on the rate at which the ions form complexes with the polymeric chain rather than with the solvents-plasticisers. The structures of plasticised polymeric electrolytes are still not clearly understood.The PEG structure in which Li+ forms a complex both with the polymeric network and the solvent has been hypothesised 273 for electrolytes based on PAN (Y=CN) and PVC (Y=Cl) in a mixture of propylene carbonate ± ethylene carbonate (PC/EC) solvents CH2CH(Y) n Li+ [(PC)m(EC)k]l . To date, a large number of PGE that consist of lithium salts and plasicising organic electrolytes are known. The important advantage of PGE as compared with SPE is their high ionic conductivities at room temperature, which approaches the con- ductivity of liquid electrolytes. However, in practical applications of PGE, a number of other characteristics should be taken into account, e.g., their ability to confine electrolyte, mechanical strength and conductivity in a wide temperature range.In partic- ular, for multicomponent PGE, solvent losses due to leakage and vaporisation can hardly be avoided.274, 275 This results in a decrease in the cell resistance and impairs the electrolyte ± elec- trode contact. The main parameter that characterises the solvent 339 confinement in the PGE is the affinity between the polymeric matrix and the organic solvent. This parameter also affects the mechanical strength of the PGE film and its conductivity in a wide temperature range, especially in thin-film cells which contain electrodes with high surface areas. Low degrees of affinity between a polymer and an electrolyte can result in separation of PGE into predominantly polymeric and solvent-enriched microphases.As a consequence, the mechanical strength can increase due to the formation of a polymeric network. The electrolyte electrochemical stability in lithium chemical power sources can be increased if highly polar, easily freezing solvents are used (e.g., ethylene carbonate with m.p.=36 8C). It may be assumed that an increase in the degree of solvent ± polymer affinity will allow one to eliminate the problem of the increase in the PGE resistance with a decrease in the temperature, because the interaction of solvent molecules with the polymer will prevent their ordering during freezing. Properties of PGE which contained PVDF, a copolymer of vinylidene fluoride with hexafluoropropylene (PVDF ± HFP), PAN (Mw=150 000) and PMMA (Mw=12 000) as the poly- meric components were compared,276 which revealed the follow- ing features.The solvent-loss rate increased in the order PMMA4 PAN 55 PVDF±HFP4PVDF in accordance with the increase in the solvent affinity to the polymer and the increase in the degree of crystallinity. Polymers with low degrees of crystallinity swelled more easily and better confined the solvent, the latter being uniformly distributed throughout the polymeric matrix. In some studies,277, 278 PGE based on chemically cross-linked PEO and LiBF4 and LiN(CF3SO2)2 were plasticised by PC and BL, then their ionic conductivities were compared to the con- ductivities of liquid electrolytes without polymers. Although the degree of dissociation of salts in PGE was higher, the conductivity appeared to be lower due to specific capture of lithium ions by the polymeric matrix.27 Itoh et al.9 dissolved LiN(CF3SO2)2 in PEGDME (Mw=400 ± 500) and then added g-LiAlO2. The solution obtained was mixed with PEO (Mw=4 000 000) and a liquid electrolyte was added. The compositions thus prepared contained different amounts of the following liquid components: dimethyl carbonate (DMC), diethyl carbonate (DEC), EC and PC. These reagents were used for increasing the amorphous phase fraction in the polymeric composite membrane. Impedance measurements have shown that the maximum stability with respect to Li electro- des was observed for the following compositions (mass %): 13.6 PEO+77.3 lithium salt solution in PEGDME+9.1 g-LiAlO2 (I) and 18.2 PEO+54.5 lithium salt solution in PEGDME+18.2 DMC+9.1 g-LiAlO2 (II). At the same time, composition (II) displayed a high ionic conductivity, viz., 261073 S cm71 (for composition I, the ionic conductivity was 0.0961073 S cm71).The main disadvantage of using PC and oligomeric ethers as the plasticisers is the low stability of the resulting films due to the solubility of the linear PEO in these solvents. To solve this problem, two approaches were suggested: (1) plasticisation of cross-linked polymeric nets which involve 7CH2CH2O7 units; (2) plasticisation of polymers which do not involve such units but are either insoluble or poorly soluble in plasticisers.It was shown 277 that the problem of solubility of chemically linked PEO in PC can be solved using the first approach. Although the cross-linked polymer studied was amorphous and contained the 7CH2CH2O7 unit, a non-plasticised polymeric electrolyte on its basis with addition of LiCF3SO3 had a conductivity of 1.461076 S cm71 at 25 8C. Addition of 50 mass%PC increased the conductivity of the system to 861074 S cm71, while the mechanical properties allowed its application as a film. A new plasticiser, viz., a modified carbonate (MC), was synthesised,280 by addition of three ethylene oxide groups to propylene carbonate.340 (OCH2CH2)3OMe O O O The conductivity of a PEO ± LiCF3SO3 complex rapidly increased to 561075 S cm71 at room temperature upon intro- duction of 50 mass% MC.It was assumed that this plasticiser increases the conductivity throughout the complex system bulk, in contrast to common plasticisers that provide conduction only in the liquid phase. It was assumed that MC favours a deeper dissociation of the salt as compared with PC. A large number of studies were devoted to PAN-based PGE obtained by heating the polymer, plasticiser and lithium salt to 130 ± 150 8C, which resulted in cross-linking along the C7N bondsCH2 CH2 n 150 8C N N N CN Although the amount of PAN did not exceed 20 mass %, the electrolyte obtained resembled rubber in its consistency. Table 4 shows the conductivities of PGE based on PAN with additions of lithium salts such as LiClO4, LiAsF6, LiCF3SO3, LiN(CF3SO2)2 and LiPF6 (Refs 273, 281).PC, EC, BL, N-meth- ylpyrrolidone (N-MP) and their mixtures were used as plasticisers. Table 4. Conductivities of electrolytes based on PAN at 20 8C. Electrolyte composition (mol.%) Conductivity at 20 8C /S cm71 21 PAN+38 EC/33 PC+8 LiClO4 21 PAN+38 EC/33 PC+8 LiAsF6 16 PAN+68 PC+16 LiClO4 21 PAN+18 BL/20 EC/33 PC+8 LiClO4 21 PAN+40 EC/36 PC+3 LiN(CF3SO2)2 21 PAN+61 EC/13 PC+5 LiCF3SO3 21 PAN+61 EC/13 PC+5LiBF4 21 PAN+33 PC/30 BL/8 N-MP+8 LiPF6 1.761073 2.161073 8.661074 2.361073 1.561073 1.161073 3.561074 2.261073 Polymeric gel-electrolytes based on PAN were also modified by introducing crown ethers such as 15-crown-5 and benzo-15- crown-5.282 Earlier, the effect of these crown ethers as well as 2,4- dioxo-16-crown-5 on the conductivity of PEO-based solid electro- lytes was studied.283 It was found that the introduction of 15-crown-5 and 2,4-dioxo-16-crown-5 increases the ionic conduc- tivity of a film electrolyte approximately by an order of magni- tude, whereas the addition of benzo-15-crown-5 does not affect this electrolyte characteristic.A gel-electrolyte with the following composition: 8.5 mass% PAN+81 mass% PC+3 mass% LiClO4+7.5 mass% 15-crown-5 had a conductivity of 1073 S cm71 at 20 8C.282 Films prepared of a composite polymeric electrolyte involving zeolite powder dispersed in a PAN-based gel with an addition of LiAsF6 were studied.284 Gel-electrolytes based on a mixture of PC/EC andPAN with addition of LiAsF6 had ionic conductivities exceeding 1073 S cm71 at room temperature.The addition of zeolite powder increased the ionic conductivity at low temper- atures due to high degrees of amorphousness of the composites. The lithium electrode/electrolyte interface was studied using the impedance spectroscopy technique. The resistance of charge transfer across the Li/PGE interface was found to decrease, which indicates a decrease in the area occupied by the passive film on the surface of lithium. According to results of cyclic voltammetry, for Li/LiCoO2 cells, an introduction of PAN and zeolite powder into a PC/EC electrolyte containing LiAsF6 did AMSkundin, O N Efimov, O V Yarmolenko not change the window of electrochemical stability, and, at a current density of 0.5 mA cm72, the capacity loss during cycling did not exceed the corresponding values for liquid electrolytes.A highly conducting PAN-based PGE was prepared 285 using EC/DMC solutions of either LiPF6 or LiC(CH3SO2)3 as the plasticisers. The PGE preparation involved the following stages: (1) dissolution of a lithium salt in EC/DMC; (2) addition of PAN to the solution obtained and stirring for several hours at room temperature; (3) casting of the resulting liquid solution on an aluminium support preheated to 90 8C for no longer than 30 s for cross- linking of PAN; (4) cooling of the gel obtained to room temperature. A LiPF6+EC+DMC+PAN composition with a molar ratio of 4 : 60 : 20 : 16 had a conductivity of 5.961073 S cm71 and a break-down voltage of 4.5 V.Ionic conductivity of a polymeric electrolyte based on PAN and lithium methacrylate (PAN ± LiMA) with an addition of EC- dissolved LiClO4 was 2.461074 at 10 8C and 1.961073 S cm71 at 25 8C.286 A copolymer of acrylonitrile with butadiene had an ultimate strength of 3.0 MPa and a conductivity of 1073 S cm71.287 A new copolymer of PAN with bis[2-(2-methoxyethoxy)- ethyl]itaconate (PANI) was synthesised.288 The maximum con- ductivity, namely 1.961073 S cm71, was found for a gel-electro- lyte of the following composition: 25 mass% PAN+10 mass% PANI+50 mass% EC/BL+15 mass% LiClO4. Polymeric electrolytes were also prepared based on PVC.289 They were obtained by dissolution of PVC in THF and subse- quent addition of plasticisers with lithium salts.Then, the solution was cast on a support and dried at room temperature up to complete evaporation of THF. For an electrolyte of the following composition: 15 mass% PVC+40 mass% PC+40 mass% EC+5 mass% LiClO4, the conductivity of 1.261073 S cm71 was reached at room temperature. Attempts to prepare an electro- lyte by dissolving PVC directly in PC and EC have failed,290 because these polymers do not dissolve PVC even at elevated temperatures. This obstacle was bypassed using THF which dissolves all the electrolyte components. The use of PVDF and PC-solutions of LiClO4 as the poly- meric matrix and the plasticiser, respectively, made it possible to obtain PGE with a conductivity in the range from 1075 to 1073 S cm71 at room temperature.291 In recent years, the active search for polymeric composites as well as binary and ternary copolymers which can be used as polymeric matrices for electrolytes has been in progress.292 ± 299 It was shown 282, 300, 301 that, being used as effective protective coatings for lithium electrodes, the polyacetylene (PA)/porous polyethylene (PE) and PA/polypropylene composites play the additional role of electrolyte carriers and conductors, because PA present in their compositions exhibit both ionic and electronic conductivity. A constantly increasing number of publications devoted to synthesis of new PGE using photopolymerisation have appeared in recent years.302, 303 For example, this method was used for the synthesis of poly(ethylene glycol diacrylate).Synthesis of a gel- electrolyte by UV-irradiation of a mixture of oligo(urethane methacrylate) and poly(ethylene glycol monomethacrylate) has been reported;304, 305 the PGE conductivity was of an order of magnitude of 1073 S cm71. An ethylene oxide/propylene oxide (80 : 20) copolymer with acrylate groups at the macromonomer's termini that could be cross-linked under the action of UV-radiation was synthesised.306 EC/PC (1 : 1, by volume); EC/BL (1 : 1, by volume); PC; PC/BL (1 : 1, by volume); BL and LiBF4 (1 M) were used as the plasti- cisers. The polymeric films obtained exhibited good mechanical properties in a wide range of concentrations of additives to liquid electrolytes.Their shear moduli decreased with an increase in the liquid component content, however, remaining sufficiently high, viz., 10 MPa even for a liquid content of 90 mass %. The ionicThe state-of-the-art and prospects for the development of rechargeable lithium batteries conductivity of a composition with 1 MLiBF4/BL decreased from 3 to 161073 S cm71 with a decrease in the temperature from +20 to720 8C. Recently, it was proposed to use ternary copolymers as the polymeric gel-electrolytes.307 For example, ternary polymeric electrolytes were prepared by casting from a solution of a homogeneous mixture containing 18 mass% poly[(acrylonitrile)(methyl methacrylate)styrene] [poly(AMS)], 73 mass% 1 M LiClO4 solution in EC/DMC (1 : 1, by volume) and 9 mass% silica.The Li/PGE/carbon cell had a reversible capacitance of 305 mA h g71 in a voltage range of 0.01 ± 1.5 V and a cycling efficiency of 99%. The next work by the same authors 308 dealt with a PGE which contained 12 mass% poly(AMS), 75 mass% plasticiser and 13 mass% SiO2. The plasticiser represented a 1 M solution of LiBF4 in either EC/DMC (1 : 1, by volume) or EC/DMC/BL (5 : 4 : 1, by volume) (BL was added to increase the ionic conduc- tivity). LiCoO2 was used as the cathodic material. Indeed, the LiBF4+EC+DMC+g-BL electrolyte added to the polymer has demonstrated a somewhat higher ionic conductivity (6.161073 S cm71) as compared with the LiBF4+EC+DMC electrolyte (5.261073 S cm71).The PGE films obtained were 60 ± 80 mm thick and had satisfactory mechanical properties. Composite electrolytes based on a mixture of lithium-ion- conducting ceramics Li1.3Al0.3Ti1.7(PO4)3 and a poly(ether ure- thane) electrolyte have been proposed.309 Electron microscopy studies have shown the ceramic and polymeric phases to be divided by *1 mm interlayers. The conductivity of a dry compo- site did not exceed that of the initial polymeric electrolyte. Under the action of solvent vapour (DMF, acetonitrile, water), the conductivity increased substantially as compared with a polymer subjected to a simpler plasticisation procedure. Treatment of the ceramics with a compatibilising agent Me O O Ti C O O Me 2 improved the adhesion on the polymer ± ceramics interface but decreased the total conductivity of the specimen studied.A polymeric matrix has a strong effect on the dendrite formation in PGE. Inhibition of lithium dendrite formation in a PAN-based electrolyte was studied.310 The electrolytes used were obtained by dissolving PAN (up to 17 mass%) in a PC/EC solvent mixture (1 : 1, by volume) containing LiClO4 (1 M) at 120 ± 140 8C with subsequent cooling to 715 8C for 16 h. Upon electrodeposition of lithium for 1 h from liquid electrolytes containing up to 3 mass% PAN, dendrites were observed to be formed on the lithium/electrolyte interface, whereas in a gel- electrolyte (5 mass%± 17 mass% PAN) their formation was suppressed. Taking into account the increase in the bulk resistance with an increase in the PAN content, its optimal concentration in the electrolyte was found to be 5 mass%± 10 mass %.A metallic lithium electrode in contact with a gel-like PAN- based electrolyte which contained two lithium salts (LiBF4 and LiPF6) was precycled to decrease the bulk resistance.311 The cycling was carried out in the range from +0.5 to 70.5 V at 20 8C immediately after the Li/PGE/Li cell was assembled. For cells with LiBF4-containing PGE, the impedance values measured after storage for 230 days at 20 8C were below 100 O. Cells with LiPF6 needed to be precycled at the potentials >0.5 V and at 60 8C. The impedance values at 10 kHz that determine the bulk resistance of the gel-electrolyte were the same for both types of cells and remained unchanged during storage. This was also confirmed by the example of a Li/PGE/(1,3,4-thiadiazole-2,5- dithiol + polyaniline-based composite cathode) cell.According to XPS data, only a LiF ± LiOH layer was retained upon cycling in a gel-electrolyte containing LiBF4, whereas, in a gel-electrolyte containing LiPF6, a dense and thin LiF interlayer was `incorpo- 341 rated' into the LiF ± LiOH mixed layer. It was assumed that the layer containing LiF and LiOH formed during preliminary cycling stabilises the Li/PGE interface. Electrolyte based on the following polymers were studied:312 dimethyl ether of tetra(ethylene glycol) (tetraglyme), dimethyl ether of poly(ethylene glycol) (Mw=400) and poly(methylene ethylene oxide).The study was aimed at elucidation of the kinetics of decomposition of liquid and solid polyether electrolytes, which contained either LiPF6 or complexes based on this salt and exhibited high thermal stability and advanced electrochemical properties resembling those of a pure salt. Lithium bis(diglyme) hexafluorophosphate 313 and lithium (pentamethyldiethylenetri- amine) hexafluorophosphate 314 were shown to exhibit advanced thermal stability and conductivity in nonaqueous media.315 In recent years, attention is focused on electrolytes based on the PVDF±HFP copolymer called Kynar in the industry. Using LiClO4, LiPF6 316 and LiN(CF3SO2)2,317 as well as the EC/ethyl methyl carbonate plasticising mixtures, the PVDF±HFP electro- lyte films with advanced mechanical and electrochemical proper- ties were obtained.In another study,318 PVDF±HFP membranes were first cast using such solvents as acetone, THF, ethyl methyl ketone and N-MP and then impregnated with a 1 M solution of LiBF4 in PC. The polymeric electrolytes obtained had a conduc- tivity of an order of magnitude of 1073 S cm71. Syntheses of new gel-electrolytes based on PVDF3197321 and PVDF7HFP322 ± 327 have been reported. The search for new polymeric electrolyte compositions for lithium-ion cells is in progess. About 100 publications on this subject appear annually. For a deeper insight into the state-of-the- art of this field, the readers are directed to the reviews 239, 328, 329 and the book.242 VIII. Conclusion In the past decade, lithium-ion rechargeable batteries have occu- pied a significant place in the power source market.They are extensively used in portable electronic devices. At present, active research on the development of large batteries, particularly, for electric vehicles is in progress. Success achieved in elaboration of lithium-ion cells is determined by the progress in the research fields of intercalation processes and development of high-per- formance intercalation materials. Despite the evident achieve- ments, many problems remain to be solved. While this review was in the stage of preparation, interesting publications devoted to lithium rechargeable batteries have appeared. For example, interesting information can be found in the Proceedings of the 10th International Meeting on Lithium Batteries which took place in Como (Italy) in 2000 (see Refs 330 ± 353) as well as from the materials of the Joint Meeting of the International Society of Electrochemistry and the Electro- chemical Society (USA) held in San-Francisco (USA) in 2001.354 ± 390 The studies that deal with nanomaterials (such as fullerenes and nanotubes) used for negative electrodes in lithium- ion cells 384 and, as a consequence, the new developments in the technology of nanosize materials 341, 346, 354, 373, 390 deserve atten- tion.384 The research aimed at the development of new materials for both negative 347, 349, 350, 364, 382, 386, 387 and positive electro- des 332, 360 ± 362, 368, 370 as well as for electrolytes 330, 345, 362, 375 is noteworthy.Finally, generalising reviews 329, 335, 340, 376 should be mentioned. References 1. 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ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC