Russian Chemical Reviews 68 (1) 19 ± 38 (1999) Donor ± acceptor complexes and radical ionic salts based on fullerenes D V Konarev, R N Lyubovskaya Contents I. Introduction II. Fullerenes III. Donor ± acceptor complexes and radical ionic salts of fullerenes IV. Structure and spectral characteristics of complexes and radical ionic salts of fullerenes V. Conclusion Abstract. The review generalises for the first time the published data on the synthesis and properties of donor-acceptor type of compounds based on fullerenes, various solvates and clathrates, inclusion compounds, molecular complexes and charge-transfer complexes both with inorganic donors and with organoelement donors of the tetrathiafulvalene, amine, metallocene and metal- loporphyrin series.Radical ionic salts of fullerenes with bulky cations and alkali metals obtained by intercalation or by direct synthesis in solution are discussed. Results of studies of fullerene compounds by IR, optical, ESR, X-ray photoelectron and 13C NMR spectroscopy, as well as their conducting (including super- conducting), magnetic and optical properties are discussed. The bibliography includes 208 references. I. Introduction The discovery of fullerenes, a new allotropic modification of carbon, in the mid-80's 1 confirmed the prediction of theorists 2, 3 on the possible existence of polyhedral carbon molecules with icosahedral symmetry. In the early 90's, a simple method to obtain fullerene C60 in gram amounts was found; this gave an impetus to more detailed studies of physical and chemical properties of the C60 clusters and compounds based on them.4 The number of works in this area grows steadily, and they cover an ever wider range of fundamental and applied branches of science and technology.In 1996, Kroto, Smalley and Curl were awarded the Nobel Prize in chemistry for D V Konarev, R N Lyubovskaya Institute for Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation. Fax (7-096) 515 35 88. Tel. (7-096) 517 18 52. E-mail: konarev@icp.ac.ru (D V Konarev), lyurn@icp.ac.ru (R N Lyubovskaya) Received 26 May 1998 Uspekhi Khimii 68 (1) 23 ± 44 (1999); translated by S S Veselyi #1999 Russian Academy of Sciences and Turpion Ltd UDC 593.194+547.435 19 20 21 30 35 the discovery of fullerene and for their great contribution to the development of this area.Generally, fullerenes display acceptor properties5±7 and can be regarded as p-acceptors. They can form various donor ± acceptor (D ±A) non-covalent compounds, similar to the well- known planar p-acceptors, e.g., tetracyanoquinodimethane 1 (TCNQ), tetracyanoethylene 2 (TCNE), p-quinone, etc.5, 8 NC CN CN NC NC CN CN NC 2 1 These D±A compounds are formed due to rather weak (in comparison with the standard covalent chemical bonds) van der Waals interactions and due to charge transfer from the donor to the acceptor.9 Charge transfer plays a very important role and results in qualitatively new physicochemical properties of these compounds.D±A compounds can be divided into several groups according to the degree of charge transfer (d), although there is no distinct boundary between these groups. The compounds with d close to zero can be attributed to molecular complexes. In the case of partial charge transfer from the donor to the acceptor (0<d<1), charge-transfer complexes (CTC) Dd+Ad7 are formed. For example, in the TTF ±TCNQ complex, where TTF is tetrathiafulvalene, different estimates give d=0.48 ± 0.67.8 A characteristic feature of CTC is the appearance of a new band in the absorption spectrum of the complex in the visible and near IR regions due to the transfer of an electron from the donor to the acceptor when absorption of a light quantum occurs.9 In the extreme case, i.e., in the complete charge transfer from the donor to the acceptor or in the formation of a compound from oppositely charged ions, radical ionic salts Dn+An7 are formed (in the case of radical ionic salts of fullerenes, n is an integer).The charge-transfer complexes and radical ionic salts based on planar polyconjugated heterocyclic p-donors and acceptors are being intensely studied lately. Many of them, the so-called organic metals, have unique conduction and magnetic properties.10, 11 Radical cationic salts of tetrathiafulvalene derivatives draw special attention. The majority of organic metals and super- conductors obtained to date correspond to this class of com- pounds.10, 11 The discovery of fullerene has given the researchers a new p-acceptor with a number of essential features that distinguish it from other acceptor molecules: larger size, spherical shape, unique electronic structure, high symmetry and polarisability.The resulting specific features of donor-acceptor interactions in com- pounds of the C60 fullerene enabled the design of materials with20 unusual physical properties. For example, it was found that intercalation of C60 by alkali metals results in superconductors with composition M3C60 (M=K, Rb, Cs) with rather high superconductivity transition temperatures (Tc) (184Tc4 440 K).12 ¡À 14 The salt of C60 with an organic donor, tetra- kis(dimethylamino)ethylene (TDAE), is a ferromagnetic with Tc=16.1 K,15 and apparently has superconducting properties.16 Combination of conducting polymers with fullerene enables efficient phototransfer and separation of charges with long lifetime, which increases considerably the photoconductivity of polymers and can be utilised in xerography and energy photo- transducers.17 By now, a considerable number of various D¡ÀA compounds of fullerenes have been obtained, including molecular complexes, CTC and radical ionic salts with different states of oxidation or reduction.The C60 fullerene molecule can accept up to 12 electrons13, 14, 18 and release one electron,7 i.e., the charge of the C60 molecule can vary from +1 to 712. As a result, C60 compounds manifest a broad spectrum of properties. The considerable interest of experts from different areas in fullerenes and compounds based on them is reflected in a number of books and reviews.19 ¡À 28 In particular, the reviews are devoted to chemistry,21 spectroscopy 22 ¡À 26 and magnetic properties 27 of fullerenes and compounds based on them, as well as to intercala- tion of fullerenes with alkali and alkaline-earth metals.13, 14, 28 The present review generalises for the first time the published data on the synthesis ofD¡ÀAcompounds based on fullerenes that are formed both through van der Waals interaction and charge transfer.These include solvate and clathrate compounds, inclu- sion compounds, molecular complexes, CTC and radical ionic salts of fullerenes with organic and organometallic donors and various metals.Data on IR, electronic, ESR, X-ray, photoelec- tron and 13C NMR spectroscopy are considered. The features of D¡ÀA complexes and radical ionic salts of fullerenes, including superconducting, magnetic and optical properties, are discussed. II. Fullerenes 1. Features of fullerene structure It is known that the molecule of the C60 fullerene has icosahedral symmetry (group Ih), and its surface consists of 20 six- and 12 five- membered rings.1 The molecule of the C70 fullerene consists of 25 six-membered and 12 five-membered rings. It has an elongated shape and lower symmetry (D5h). Higher fullerenes also represent polyhedral molecules and contain a larger number of hexagonal facets.25 All 60 carbon atoms in the C60 molecule are equivalent, which is confirmed by the presence of a single signal in the 13C NMR spectrum.29 The meanC7C distance in C60 (1.44 A �º ) is close to the C7C distance in graphite (1.42 A �º ).Each carbon atom in the C60 molecule is linked to three other carbon atoms by two longer 6 ¡À 5 bonds in five-membered rings and by one shortened 6 ¡À 6 bond, which is common to two fused six-membered rings. Thus, the carbon atoms in C60 have a near-sp2 hybridisation.25, 30 Usually, the symmetry axis of the p-orbital is orthol (Ysp=90 8) to the plane of the three s-bonds in the sp2 hybrid- isation. Because of the spherical shape of the C60 fullerene molecule, the four mutually linked carbon atoms do not lie in the same plane and the angle Ysp is 101.64 8 rather than 90 8.30 Thus, pyramidalisation of carbon atoms occurs in fullerenes, which results in significant strain in the fullerene molecule and changes the character of the p-orbitals; thus, there is certain contribution by a s-orbital to the p-orbitals of all fullerenes. Pyramidalisation affects strongly the electronic properties of fullerenes and prede- termines their high electron affinity, as the strain in the molecule is partially eliminated upon reduction of fullerenes.30 D V Konarev, R N Lyubovskaya 2.Donor ¡À acceptor properties and polarisability of fullerenes The ionisation potentials (IP),25 electron affinities (EA)6 and the corresponding redox potentials (EOx, ERed) 31 of the C60, C70 and C76 fullerenes are listed in Table 1.Table 1. Vertical electron affinity (EA),6 first and second reduction potentials of fullerenes (E1Red and E2Red),31 ionisation potentials (IP)25 and oxidation potentials of fullerenes (E1Ox).7 E2 IP /eV Fullerene EA/eV E1 E1Ox /Vb Red /Va Red /Va 7.58 C60 70.82 70.80 C70 +1.26 +1.20 +0.81 2.67 2.68 2.86 70.44 70.41 7 C76 aCH2Cl2, SCE, 0.05 M Bun4 NBF4; b relative to Fc/Fc+ (Fc is ferrocene). The first redox potentials of the C60 and C70 fullerenes in the polar solvents are close to each other. The redox potential of the C60/C ¡¦60. pair is70.44 V in dichloromethane and acetonitrile 5, 31 and 70.33 V in tetrahydrofuran 32 relative to the saturated calomel electrode (SCE). These values are 0.6 ¡À 0.7 V smaller than the corresponding redox potentials of TCNQ (+0.22 V) andTCNE (+0.28 V).5 Changing the polarity of the solvent shifts the redox potential of C60 insignificantly.For example, the redox potential in nonpolar benzene is70.36 V relative to SCE.32 The C60 fullerene is rather a weak acceptor. The EA of C60 in the gas phase is 2.67 eV.6 The adiabatic EA of C60 in solution (estimated from the energy of charge transfer and redox poten- tials) is much lower and equals 2.10 ¡À 2.20 eV.5 These values are comparable with the EA of such weak acceptors as 3,6-dibromo- 2,5-dimethylquinone (3) or 1,2,4,5-tetracyanobenzene (4) (2.1 eV) but are much lower than those of TCNQ (2.82 eV) and TCNE (2.90 eV).5 O Br Me CN NC Me Br CN NC 4 O3 One possible way to increase the EA is the introduction of strong acceptor substituents in fullerenes.It has been shown33, 34 that halogenated fullerenes have stronger acceptor properties than C60. For example, the theoretically calculated EA for C60Br8 is *4.0 eV.33 Fluorinated fullerenes, viz., C60F36, C60F48 and C70F54, have considerably more positive first redox potentials than the corresponding fullerenes (70.05, +0.51 and +0.76 V, respectively, relative to an Ag/Ag+ reference electrode in dichloromethane).34 The electron affinities of C60F36 and C60F48 are also higher than that of C60 (3.48 and 4.06 eV, respectively).34 Functionalisation results in significant changes in the symmetry and electron structure of the C60 fullerene.33 Fullerenes, especially C60, have high oxidation potentials.The ESR method detected the formation of the C60 radical cation in solutions containing strong oxidants,35 but individual compounds in which C60 is charged positively have not been isolated yet. Higher fullerenes possess higher electron affinities, and thus they are stronger acceptors. The C76 fullerene and other higher fullerenes undergo oxidation a little more readily than C60.7 For example, C76 is oxidised with hexabromocarboranyltris(2,4- dibromophenyl)ammonium (ERed=+1.16 V) to give a radical cationic salt with positive charge on the fullerene.7 The polarisability of the C60 fullerene molecule is high (a*85 A)36 and is several times higher than that of other p-acceptors.Therefore, polarisation van der Waals forces are essential in the formation of D¡ÀA complexes and radical ionic salts of fullerenes.Donor-acceptor complexes and radical ionic salts based on fullerenes 3. Features of the crystal structure of C60 fullerene At room temperature, C60 has a face-centred cubic (FCC) lattice.37 The shortest distance between the centres of the C60 molecules in a crystal is 10.02 A, which is smaller than the van der Waals diameter of the C60 molecule, considering the size of its p-electron cloud (10.18 A, Ref. 25), hence the p-orbitals of theC60 molecules in a crystal overlap little. The weakness of interactions between the fullerene molecules in the solid state is the reason for the fast isotropic rotation of the C60 molecules in the crystal.Below 255 K, C60 crystals undergo a phase transition from the face-centred cubic lattice to a simple cubic lattice.37 The rotation of the fullerene molecules in these crystals becomes anisotropic, it slows down, gets synchronised and occurs ratchet. In the case of such rotation, adjacent C60 molecules can be present in two different orientations relative to each other, which have different energies. In the first orientation, the shortened `double' bond of one C60 molecule is located above the electron-deficient pentago- nal facet, whereas in the second orientation, it is above the hexagonal facet of the C60 molecule. As the temperature is decreased, the number of pairs of molecules in the first orienta- tion increases and reaches 83% at 90 K.Below 90 K, the ratio of molecules in the first and second orientations remains constant, but ratchet rotation of the C60 molecules occurs in such a manner that each of two orientations is transformed only to the equivalent one.25 This results in ordering, in which the rotation of the C60 molecules in a crystal is synchronised completely. This kind of ordering can also occur in D±A complexes and radical ionic salts of fullerenes provided that the distances between the C60 mole- cules are short.38 ± 40 4. Conducting and magnetic properties of fullerenes The upper unoccupied and lower occupied levels of the C60 molecule are presented in Fig. 1. The highest occupied hu level is five times degenerate and is completely filled with ten electrons.The lowest free t1u and t1g levels are three times degenerate.25, 36 Energy, b 71 hu t1g t1u 0 hu 1 gg, hg Figure 1. Highest occupied and lowest unoccupied molecular p-orbitals of the C60 fullerene calculated by the HuÈ ckel method;36 b is resonance integral. Overlapping of p-orbitals of the adjacent C60 molecules in a crystal results in the formation of a valence band and a conduction band. The energy gap between the valence band and the con- duction band in C60 is estimated as 1.5 ± 1.8 eV,25, 36 hence crystalline fullerene is a semiconductor. The highest-energy edge of the valence band consists of the hu levels, while the lowest- energy edge of the conduction band consists of the t1u levels (see Fig.1). Magnetic measurements carried out with pure C60 and C70 fullerene specimens have shown that magnetic transitions are observed at 60 K.40 This is due to the fact that the currents of p-electrons in the six- and five-membered fullerene rings are different and create a very small but distinct magnetic moment in the fullerene molecule [for C60, m=70.3561079 A m2 (see Ref. 27)]. Cooling of specimens of pure C60 below 90 K results in the transition to ratchet synchronised rotation of the fullerene molecules. In this transition, the positions of 83% of the magnetic moments of the C60 molecules are frozen in an ordered fashion, but 17% of the magnetic moments are frozen in disorder. This results in a glassy magnetic state.40 21 When C60 specimens are placed in a magnetic field, the magnetic moments of the C60 fullerene molecules are oriented along the external magnetic field.Therefore, cooling of C60 specimens in a magnetic field below the temperature of transition to synchronised rotation of fullerene molecules results in freezing of the ordered positions of the magnetic moments of the C60 molecules and formation of the frozen glassy magnetic state. Heating of the specimens above the temperature of this transition results in violation of the synchronised rotation of fullerene molecules, and ordering of the magnetic moments of the C60 molecules disappears.40 III. Donor ± acceptor complexes and radical ionic salts of fullerenes 1. Methods of preparation Fullerene compounds are prepared using various methods: slow concentration of solutions,41 ± 43 diffusion methods,44 ± 52 cooling of saturated solutions,53, 54 precipitation with a solvent 48 ± 50, 55 and an electrochemical method.56 ± 61 Concentration of solutions containing a fullerene and a donor is the basic method for the preparation of molecular fullerene complexes.The concentration is carried out in an inert atmos- phere, as molecular oxygen can be absorbed on the surface of the fullerene molecule, which blocks the approach of the donor to the fullerene.62 As a rule, those solvents are used in which fullerenes are well soluble: carbon disulfide (7.9 mg ml71), benzene (1.5 mg ml71), toluene (2.9 mg ml71) and chlorobenzene (5.7 mg ml71).63, 64 Fullerene forms solvates of the type C60(Sol)x with many of these solvents.65 ± 74 Two competing reactions occur upon concentration in a reaction system `donor ±C60 ± solvent': Dn(C60)m(Sol)l C60+Sol+D C60(Sol)x Sol is a solvent, D is a donor; n=1 ± 6, m=1 ± 3, l=0 ± 4; x=0.4 ± 4.Shifting of the reaction equilibrium towards the formation of a complex (as the solvent is present in a considerable excess in comparison with the donor) sometimes requires a great molar excess of the donor relative to the fullerene (up to 100 : 1).75 ± 77 As the temperature is increased, the rate of displacement of the solvent by the donor from the solvent shell increases, thus the time of heating or refluxing of the reaction mixture is an essential factor.An increase in the temperature of concentration of fullerene solutions in benzene results in a decrease in the content of benzene in the crystal solvate [C60 ±C6H6], and above 40 8C the solvate is decomposed completely and virtually pure fullerene is formed.42 To avoid losses on the walls of the reaction vessel in the preparation of microquantities of complexes, special techniques for the concentration of the solvent in closed volume using a temperature gradient have been developed.41, 42 Fullerene compounds can be isolated from solutions by precipitation or cooling of saturated solutions. For example, radical anionic salts of fullerenes are isolated from solutions in pyridine and benzonitrile by precipitation with non-polar solvents (pentane or hexane).However, this results in polycrystalline specimens.48 ± 50, 55 Single crystals of the K3C60(THF)14 salt can be prepared by slow gradient cooling of a solution obtained by treatment of the C60 fullerene with potassium in tetramethylethy- lenediamine in the presence of tetrahydrofuran and diethyl ether.53 The diffusion method (in which the vessels with solutions of a fullerene and an appropriate donor are connected by a tube filled with a solvent) is most suitable for the preparation of single crystals of poorly soluble C60 compounds. Single crystals of the22 complexes (DAN)C60(C6H6)3 (DAN is dianthracene),44 the salt (TDAE)C60 [TDAE is tetrakis(dimethylamino)ethylene] 45, 46 and others have been obtained using this procedure.47 60 .anion is controlled by the exact molar ratio of the at 190 K,78 for C60[Br(CH2)3Br] at 190 K78 and for C60(Cl2C=CHCl), at 167 K.72 All these transitions are similar 60.(THF)3 is isolated from THF by precipitation with hex- to the phase transition in pure C60 at 255 K.37 The diffusion method is also used in the synthesis of fullerene radical anionic salts by cationic metathesis. The reaction is carried out in two stages. In the first stage, C60 is reduced with an excess of sodium in THF in the presence of dibenzo-18-crown-6 48 ± 50 or other crown ethers.51 The degree of reduction of the fullerene to the Cn¡ fullerene and the crown ether (1 : n). The salt [Na+(18-crown- 6)]C¡ ane.49 Similar C2¡ 60 , C360¡. and C460¡ salts are poorly soluble in THF and spontaneously precipitate from solutions.49, 51 In the second stage, the resulting sodium salts of C60 are dissolved in acetonitrile and placed in a vessel.A compound with a bulky cation, for example, bis(triphenylphosphoranylidene)ammonium chloride 5 (PPNCl), is placed in the other vessel.50, 51 Ph Ph + Ph P N P Ph Cl7 Ph Ph 5 The vessels are connected by a tube containing the pure solvent. Single crystals of (PPN)2C60 have been obtained by this method.50 Cationic metathesis of Rb3C60 with the salts Me4NCl, Et4NBr and Me4PCl in liquid ammonia can also be used; the specimens were isolated as powders.52 In the electrochemical method, the radical anions C60 ¡ .and CD can be used for medical purposes.95 Similar compounds are C70 ¡ . were obtained by reduction of neutral fullerenes in an formed upon dissolution of a mixture of fullerenes with p-tert- H-shaped cell on a platinum cathode. Compounds Ph4PCl or PPNCl with bulky cations were used as the supporting electro- lytes.56 ± 60 1,2-Dichlorobenzene or mixtures of dichloromethane with toluene and chlorobenzene with tetrahydrofuran served as the solvents. The fullerene salts crystallised on the cathode.56 ± 60 The salts MxC60(THF)y with alkali metals (M = Li, Na, K, x*0:4, y*2:2) were obtained by electrochemical reduction of C60 in the presence of a supporting electrolyte, viz., the corre- sponding tetraphenylborate. A mixture of chlorobenzene with THF served as the solvent.61 2. Solvate and clathrate compounds.Inclusion compounds Dissolution of C60 in various solvents results in formation of donor ± acceptor compounds.65, 66 The interaction of the solvent with C60 occurs basically through polarisation van der Waals forces. In fact, the solubility of fullerenes in solvents with high polarisability, especially in benzene (a*10 A3) and naphthalene derivatives (a*20 A3) is the highest.63, 64 Most of the complexes with solvents are unstable, but in some cases C60 solvates can be isolated as crystals by slow concentration of the solutions.67 ± 74 The interaction with the solvent molecules results in the orientational ordering of molecules compared to crystalline C60; in certain cases, this allows one to perform X-ray diffraction studies of these compounds.Solvate compounds have diverse crystal structures: C60(C6H6)CH2I2 has lamellar packing,67 C60(C6H6)4 forms a cage with hexagonal channels,68, 69 while C60(C6H12)2 70 and C60(1,2-Me2C6H4)2 71 have hexagonal layered packing. The crystal cells of the solvates have lower symmetry than that of pure C60. However, the crystal structure could not be determined for many solvates, such as C60(Cl2C=CHCl),72 C60(CS2) 73 and C60(CCl4)10,74 because of significant orienta- tional and structural disorder. The solvent molecules in clathrate compounds are located in the cavities between the bulky fullerene molecules. Clathrate compounds of C60 are obtained by precipitation of the fullerene from toluene with a large excess of the other solvent, viz., n-pentane, 1,3-dibromopropane, butanone, diethyl ether, ace- tone,78 n-heptane,79 etc.80, 81 The composition of the clathrates is C60(Sol)x.For solvents with small molecules, x=1. As the size of solvent molecules increases, x decreases. Precipitation of fullerene with isobutane results in pure C60.78 Clathrate compounds have D V Konarev, R N Lyubovskaya been obtained basically with the solvents in which the solubility of fullerene is very small; this suggests that the interaction between the molecules of the solvent and fullerene is weak.78 ± 81 Solvate and clathrate compounds are characterised by phase transitions related to orientational ordering of the fullerene molecule.For instance, phase transitions for C60(Me2CO) occur at 240 K,78 for C60(CS2)x at 230 K,78 for (TSeT)xC60(CS2)y (TSeT is tetraselenatetracene) at 203 ± 240 K,82 for C60(C5H12) Compound (TSeT)xC60(CS2)y contains up to 26% of carbon disulfide and only traces of the donor.82 The high content of the solvent probably results in orientational disorder and separates the C60 molecules from each other to such an extent that upon complete removal of carbon disulfide the fullerene is completely sublimed at *520 8C, whereas the maximum sublimation rate of pure C60 is reached only at 700 8C.82 Inclusion compounds of fullerenes are obtained by refluxing aqueous solutions of g-cyclodextrin 6 (g-CD) with finely dispersed fullerene.83 ± 85 There are two types of complexes of g-CD withC60: one C60 molecule forms van der Waals contacts with two g-CD molecules, or a van der Waals aggregate of several C60 molecules forms short contacts with several g-CD molecules.83 ± 85 The unique property of these compounds is their solubility in water, therefore such complexes can be utilised in reactions that occur in the presence of water.In particular, complexes of fullerene with g- butylcalix[8]arene (7) in toluene.86 Complexation with calixarenes has been proposed for efficient separation of fullerene mix- tures.86, 87 It is possible to isolate C60 of 99.5% purity by multiple recrystallisation of a mixture containing 85% C60 and 15% C70.86 It was shown that fullerene molecules are separated from each other C60 ± in crystal structures of compounds p-iodocalix[4]arene 88 and C60 ± p-iodocalix[5]arene.89 But OH O O HO 8 8 OH OH 6 7 3.Complexes with inorganic compounds Complexes of fullerenes with S8,41, 90 ± 92 P4,93 I2,94 S4N4 75, 95 and a number of other small molecules have the composition C60X2 or C60X(Sol). The crown-shaped S8 molecule is very flexible, and molecular sulfur readily forms complexes with fullerenes. The structures of compounds C60(S8)2 and C60S8(CS2) are cage lattices of the fullerene molecules with channels filled with the sulfur molecules.90, 91 In compounds C70(S8)6 and C76(S8)6, the fullerene molecules form loose corrugated hexagonal layers with distances between the fullerene molecules 10.2 ± 10.5 A.41, 92 All S8 ± fullerene compounds contain shortened S7C contacts [3.13 ± 3.52 A, which is shorter than the sum of the van der Waals radii of sulfur and carbon (3.7 A)].As a result of this strong interaction, the rotation of fullerene molecules ceases almost completely. This made it possible to establish the structure of the fullerenes C70 (see Ref. 92) and C76 (see Ref. 36) more precisely. The complex C60(P4)2 has a laminar structure.93 In compound C60I2(C6H5Me), iodine can act as an acceptor with respect to C60, as its EA (3.06 eV) is higher than that of fullerene (2.65 eV). This compound has a donor ± acceptor sandwich structure, in which the iodine molecules are located between the fullerene and toluene molecules.94 Tetrasulfur tetranitride S4N4 (8), like S8, has a crown shape and forms a number of molecular complexes with the C60 fullerene.Donor-acceptor complexes and radical ionic salts based on fullerenes S S S S N N N N 8 The following compounds were isolated from toluene: C60 .S4N4 and C60(S4N4)2.75 The similarity of the sizes of the S4N4 and benzene molecules results in their mutual replacements in the crystal lattice of the complex. For this reason, compounds of the series C60(S4N4)27x(C6H6)x were isolated from benzene, where x<2.75, 95 This replacement has a random character, and the use of an excess of the donor yields compounds with a higher content of S4N4. In the crystal structure of the complex C60(S4N4)1.33(C6H6)0.67, the densely packed layers of fullerene molecules alternate with layers of tetrasulfur tetranitride and benzene molecules.95 cluster and C60 The Pd6Cl12 form a complex C60(Pd6Cl12)2(C6H6)3 with cage structure, in which each fullerene molecule is surrounded by eight Pd6Cl12 molecules.96 4.Complexes with organic donors The interest in complexes of fullerenes with donors of the tetrachalcogenafulvalene class (9 ± 17) is primarily related to the fact that both tetrathiafulvalene and its derivatives are the main components in the production of organic metals and supercon- ductors.10 It is known97 that tetrachalcogenafulvalenes are strong donors with IPs ranging from 6.3 to 7.4 eV. The molecular polarisability of these donors is 15 ± 38 A.97 Tetrathiafulvalenes have planar structures, sometimes with a small deviation of the terminal groups from the conjugation plane.10 This structure allows arrangement of their salts in regular stacks or layers.A partial charge transfer (0.254d 41) from the donor to the acceptor is a prerequisite for the appearance of conductivity.8 Tetrathia(selena,tellura)fulvalenes 9 ± 17 are widely used as donors for the preparation of complexes with fullerenes. S S S S S S S S 9 (BEDT ± TTF) Table 2. Fullerene complexes with tetrachalcogenafulvalenes. Donor Bis(ethylenedithio)tetrathiafulvalene Octamethylenetetrathiafulvalene Dibenzotetrathiafulvalene Bis(ethylenethio)tetrathiafulvalene Hexamethylenetetratellurafulvalene Bis(dimethylthieno)tetratellurafulvalene Tetramethylenedithiodimethyltetrathiafulvalene Bis(methylthio)ethylenedithiotetrathiafulvalene Tetramethyltetraselenafulvalene a For the C70 fullerene. S S S S 10 (OM ± TTF) Abbreviation BEDT± TTF (9) OM±TTF (10) DB± TTF (11) BET ± TTF (12) HM± TTeF (13) BDM± TTeF (14) TMDTDM± TTF (15) C1TET ± TTF (16) TM± TSeF (17) SS 11 (DB ± TTF) Te Te 13 (HM± TTeF) Me SS Me 15 (TMDTDM± TTF) Me Se Se Me (TM ± TSeF) The (BEDT ± TTF)2C60 complex was the first to be obtained.98 Subsequently, about a dozen compounds of full- erenes with tetrachalcogenafulvalene derivatives have been syn- thesised (Table 2).It was found that the degree of charge transfer in these compounds is insignificant.5, 98, 103 ± 106 This is due both to the weak acceptor properties of fullerene and the steric factors unfavourable for charge transfer from the p-orbitals of the initially flat donors to the spherical t1u orbital of C60.106 The formation of complexes with spherical fullerene molecules results in strong distortion of the flat tetrachalcogenafulvalene mole- cules, which assume boat conformations: the dihedral angles between the flat central fragment E4C2 and the terminal groups of tetrachalcogenafulvalene derivatives are 20 ± 30 8,98, 101, 103, 105, 107 which results in some violation of p-conjugation in these molecules.108 The packing of fullerene molecules in the crystals of these complexes can be diverse: dense 5, 89, 100 or rather loose 107, 108 layers of C60 molecules,98, 100 double chains of C60 mole- cules 98, 102, 110 as well as island motifs with isolated C60 mole- Solvent without solvent C6H6 C6H6 C5H5N C6H6 C6H5Me C6H5Cl without solvent CS2 CS2 without solvent CS2 C6H6 SS Te Te S SS SMe Se Se Me 17 Donor : fullerene : solvent ratio 2 : 1 1 : 1 : 1 1 : 1 : 1 1 : 1 : 1 1 : 1 : 1 a 1 : 1 : 1 1 : 1 : 1 1 : 1 1 : 1 : 1 2 : 1 : 3 2 : 1 1 : 1 : 2 1 : 1 : 0.5 23 S S S S S S 12 (BET ± TTF) Me Me Te Te S S Te Te Me Me 14 (BDMT± TTeF) SMe S S SS S SMe S 16 (C1TET ± TTF) Ref.Structural type 98, 99 chain 5 100 100 100 101 101 102 layered island 77layered "" 103 104, 105 106 107 102 island layered double chains layered "24 Table 3.Complexes of C60 with organic donors. Solvent Abbreviation Donor TPDP (18) twin-TDAS (19) 2,20,6,60-Tetraphenyldipyranylidene 3,30,4,40-Tetrathiabis(1,2,5-thiadiazole) twin-BEDT ± TTF (20) CS2 without solvent CS2 CTV (21) BTX (22) DAN (23) C6H5Me CS2 C6H6 Bis(ethylenedithiotetrathiafulvaleno)[b,h]- 1,4,7,10-tetrathiacyclododeca-2,8-diene Cyclotriveratrylene 9,90-trans-Bi(telluraxanthenyl) (9,10,90,100)-Bi(9,10-dihydroanthrylene) (dianthracene) 2,3,6,7,10,11-Hexamethoxytriphenylene HMT (24) SbPh3 Au(PPh3)Cl Hydroquinone without solvent the same "" 23 (DAN) cules.101, 105, 111 The donor-acceptor interaction of tetrathiafulva- lene molecules with C60 occurs both by the n ± p type (the n-orbitals of the sulfur atom of the central E4C2 donor fragment are directed to the centre of the six-membered ring of one C60 molecule) and by the p ± p type (the central E4C2 donor fragment is almost parallel to the six-membered ring of the other C60 molecule).98, 105 The distance between heteroatoms of the donor and atoms of the C60 molecule is somewhat shorter than the sum of the van der Waals radii of the corresponding heteroatoms and carbon atoms.98 ± 107 Syntheses of tetrathiafulvalene complexes with halogenated fullerenes have been described. As halogenated fullerenes are stronger acceptors than the C60 fullerene, the oxidation of tetrathiafulvalenes with the formation of radical cationic salts is possible.33, 119 In addition to compounds of fullerenes with tetrachalcogena- fulvalene derivatives, compounds with donor molecules of other classes (Table 3) have also been obtained.S S N N O O S S N N S S 19 (twin-TDAS) 18 (TPDP) The molecules of compounds 21 (CTV),114 22 (BTX) 54, 111 and 23 (DAN)44 have three-dimensional shapes (BTX and DAN, of the `double butterfly' type, and CTV, of the hemisphere type). The structures of these molecules match well the spherical surface of the C60 molecule, which forms molecular complexes with them owing to numerous van der Waals contacts. DAN has a unique ability to undergo cocrystallisation with C60 almost quantitatively even from dilute solutions in benzene; this can be utilised for the isolation of C60 from various mixtures.44 The C70 fullerene matches less the spatial shape of such molecules as CTV and DAN and does not form complexes with these donors.44, 114 However, it forms complexes with composition 1 : 1 : 0.5 with BTX and CS2.119 Isolated packing of C60 molecules is character- istic of complexes with other donors, such as HMT,115 SbPh3,116 Au(PPh3)Cl 117 and hydroquinone118 (see Table 3).S S S S S S S S All of the complexes described above are dielectrics with a conductivity of*1076 S cm71 and less.5, 100, 103, 104 S S S S S S S 5. Composites of conducting polymers with the C60 fullerene S 20 (twin-BEDT ± TTF) MeO OMe Te MeO OMe The discovery of the photoinduced charge transfer in polyvinyl- carbazole ±C60 composites 17 stimulated the intense development of studies in this field.Composites with a dozen of different polymers have been studied by now.120 Films of polymer ±C60 composites are prepared by concentration of solutions of a polymer and a small amount of a fullerene (from 1% to 3% of the polymer weight) in aromatic hydrocarbons on a sub- strate.17, 120 ± 122 Te MeO 22 (BTX) OMe 21 (CTV) Only weak charge transfer from the polymer to the fullerene is observed in these composites in the ground state.121,122 For tration of C60 ¡ . (2.361018 spins g71) was detected in a composite example, an ESR signal corresponding to an insignificant concen- polyvinylpyrrolidone ±C60.122 The polymer photoexcitation occurs at energies higher than the difference between the HOMO and LUMO levels of the polymer.This is accompanied by fast transfer of an electron Donor : fullerene : solvent ratio 1 : 2 : 4 4 : 3 1 : 1 : 1 1 : 3 : 1 1 : 1 : 1 1 : 1 : 3 2 : 1 6 : 1 2 : 1 3 : 1 MeO MeO D V Konarev, R N Lyubovskaya Ref. Structural type 43, 109 113 layered cage 110 chain 114 54, 111 44 island "layered 115 island 116 117 118 """ OMeOMe OMe MeO24 (HMT)Donor-acceptor complexes and radical ionic salts based on fullerenes (<10712 s) from the photoexcited polymer molecule to the C60 molecule with formation of a complex (polymer+) ±C¡60. in excited state.Transition to the state with free separate charge carriers is now possible from this state.121 Charge separation in the excited state of CTC results in the generation of charge carriers and a strong increase in the photoconductivity of the polymer.121 The involvement of a fullerene in efficient charge separation in these composites is reduced to two factors. On the one hand, as an acceptor, the fullerene accepts electrons occupying the LUMO of the polymer upon photoexcitation with formation of vacancies (`holes') on the polymer. On the other hand, the recombination of photoexcited electrons and `holes' is considerably inhibited due to their spatial separation upon delocalisation of charges on the bulky fullerene molecule.121 As the rate of electron transfer is high and the rate of carrier recombination is relatively low, the quantum yield of formation of charge carriers increases consid- erably in the presence of a fullerene.121, 122 The possibility of using these composites in xerography, in solar energy phototransducers and in other devices is being studied.120, 121 6.Complexes and radical ionic salts of fullerenes with amines. Magnetic properties of the (TDAE)C60 salt 60. at 1070 nm in the complicates the study of these compounds and results in signifi- cant differences in the estimation of their magnetic properties. The compound (TDAE)C60 has been studied by IR,133 The detection of ferromagnetic transition with the highest temper- ature known for organic materials (Table 4) 15 in the salt formed by the C60 fullerene and tetrakis(dimethylamino)ethylene (25) (TDAE)C60 has stimulated strong interest in compounds of C60 with amines.Polycrystalline specimens of fullerene compounds with amines are obtained by precipitation of a fullerene solution in toluene with an excess of amine.15, 123 ± 128 Single crystals of (TDAE)C60 have been obtained by mutual diffusion of solutions of tetrakis(dimethylamino)ethylene 25 and C60 in toluene.45, 46 Other unsaturated amines, e.g., 26 and 27, react with fullerenes to give radical anionic salts having unusual magnetic properties (see Table 4).123 ± 128 Saturated amines, such as N,N,N0,N0-tetramethyl-p-phenyl- enediamine (TMPD) or triphenylamine (TPA) (see Table 4), possess weaker donor properties than the amine 25, and form only weak charge-transfer complexes.76, 77, 129 These compounds are obtained by concentrating solutions of fullerenes in chloro- benzene with a large excess of the donor (100 : 1).The existence of charge transfer in compounds of C60 with TMPD is confirmed by the presence of an absorption band of C¡ optical spectrum and a shift of the absorption band of the T1u(4) vibration of C60, which is sensitive to charge transfer onto the fullerene molecule, in the IR spectrum.76 Table 4. Donor-acceptor complexes and radical ionic salts of C60 with amines. Abbreviation Donor D:C60 ratio 1 : 1 Tetrakis(dimethylamino)ethylene Tetrakis(pyrrolidino)ethylene TDAE (25) TPYE (26) 1 : 1 TMBI (27) 2,2-Bi(1,3-dimethylimidazolidin-2-ylidene) 2,20-Bi(1,3-dimethylhexahydropyrimidin-2-yl) TMBH (28) 1,5-Diazabicyclo[4.3.0]non-5-ene 1,8-Diazabicyclo[5.4.0]undec-7-ene 72 : 1 2 : 1 DBN (29) DBU (30) 1 : 1 1 : 1 N,N,N0,N0-Tetramethyl-p-phenylenediamine TMPD TPA Triphenylamine 3 : 1 DAP 1,5-Diaminopentane a Number of spins per formula unit determined by ESR; b addition of amine to fullerene is possible.25 N N Me N Me N Me2N NMe2 N N NMe2 Me2N NMe Me N 27 26 25 Me N Me N N N N N Me N Me N 30 29 28 Amines with sterically unhindered nitrogen atoms add to fullerenes.132 Some amines with strong donor properties, such as 27, 29 or 30, reduce fullerene and add to it. The reaction of C60 with these amines in solution 123, 127, 128 can occur in two steps: k2 k1 Am+[C60] [Am+] [C 60 ¡ .] salt [Am+C60 ¡ . ] zwitter-ion Am is an amine. If the first reaction step is sufficiently fast (k1 is high) and the resulting salt is poorly soluble in the solvent (as in the case of TDAE), the reaction stops in the first step, and the salts [Am+][C ¡60.] can be isolated. If k1 is small and the formation of salts occurs slowly (as in the case of amines 29, and, especially, 27 and 30), and the resulting salts are rather soluble in the solvent, the amine adds to C60 in this stage to give diamagnetic zwitter- ions.123, 127, 128 These reactions are accompanied by recombina- tion of radicals, which results in a gradual decrease in the signal of the radical anionC¡60.. The addition products that are precipitated from the solution contain a small amount of the radical anion C60 ¡ .[4% and 2% for (DBU)C60 and (TMBI)C60, respec- tively 123, 128]. Fullerene mixed addition and reduction products are perhaps formed in the reaction. If the salt is rapidly precipi- tated from benzene, the addition cannot go to completion. In this case, the reaction product, viz., (DBN)C60, contains a large percentage of reduced C¡60. (14%) and manifests strong magnetic fluctuations down to 80 K.127 The addition of amines to C60 Raman,134 ESR,135 ± 137 NMR138 and X-ray photoelectron 139 spectroscopies. The temperature of ferromagnetic transition in (TDAE)C60 is 16.1 K. It has been shown39, 43, 135 that the onset of Ref.Magnetic properties Compound Sa type 1 15 125 salt 7 0.02 123 124 126 127 128 CTC see note b 7CTCb CTCb 0.14 0.04 ferromagnetic (Tc=16.1 K) superposition of paramagnetic and ferromagnetic phases weak paramagnetic ferromagnetic (Tc<140 K) antiferromagnetic weak antiferromagnetic only near magnetic order observed 77 76, 129 77 130, 131 CTC molecular complex 7 726 the ferromagnetic state in (TDAE)C60 specimens requires that they were kept for several days at 20 ± 100 8C (constant temper- ature). Without this procedure, the (TDAE)C60 specimens man- ifest only antiferromagnetic properties.39, 43, 135 Several mechanisms for the origin of the ferromagnetic state in (TDAE)C60 have been discussed.38, 140, 141 It has been shown by ESR137, 142 that in (TDAE)C60 complete charge transfer from the donor to the fullerene occurs.It was assumed that ferromagnetism is due to the presence of the radical anion C¡60.. However, other temperature.39,138 salts with the radical anion C60 ¡ . do not manifest any ferromag- netism.38, 141 It was suggested 38, 39 that ferromagnetism in (TDAE)C60 is also determined by the presence of the radical cation TDAE+.. The ESR spectrum of (TDAE)C60 contains only one line with g=2.0008, which is an average value between those for TDAE+. (g=2.0035) and C60 ¡ . (g=1.9960).38, 142 This is probably due to sphere, as is the case in C¡60. (see Ref.143). the strong exchange interaction between TDAE+. and C¡60. . The C60 molecules in crystalline (TDAE)C60 are packed in one- dimensional chains along the crystallographic axis c 7with short- ened distances between the centres (9.98 A)15, 45 (Fig. 2). The presence of shortened contacts between the C60 molecules allows the slowing down of the rotation of these molecules on cooling, with transition to synchronised ratchet rotation, as in crystals of pure C60 cooled below 90 K. In fact, the rotation of the C60 fullerene molecules in (TDAE)C60 slows down at temperatures below 150 K, as confirmed by broadening of the 13C NMR signal.38, 138 According to theoretical calculations, the negative charge of the radical anion C60 ¡ . is mostly concentrated in the by slowing down of the rotation of C60 molecules in this salt.45 equatorial area of the fullerene sphere.Because of this, long-range magnetic order can be formed in this salt upon transition to synchronised rotation of C60 molecules.38, 39 In this case, the spins of C60 ¡ . in the fullerene chain along the crystallographic axis c 7are superconducting phase with Tc=17.4 K may be present in Measurements on a SQUID magnetometer have shown that a ordered antiferromagnetically. The antiferromagnetic interaction can be transferred between the fullerene chains through the radical cation TDAE+. (see Refs 38 and 39). a 1 C60 C60 C60 c TDAE TDAE 2 C60 C60 C60 TDAE TDAE C60 C60 C60 Figure 2. Scheme for formation of ferromagnetic order in the salt (TDAE)C60 by the spin polarisation mechanism;38, 39 (1) is ferromagnetic interaction; (2) is antiferromagnetic interaction. According to theory,39 distortion of the radical cation TDAE+.may result in inhomogeneous spin density distribution on TDAE+. (Fig. 2). In fact, after keeping of the salt (TDAE)C60 at constant temperature,39 the 1H NMR spectrum displays two sets of lines (A and B) from the methyl groups of TDAE+. (see Ref. 138). This implies an asymmetrical spin density distribution in the radical cation TDAE+. . Asymmetrical spin density distribution results in violation of the antiferromagnetic order of the C¡60. spins (Fig. 2) and the onset of ferromagnetic transition highly symmetrical cationic environment. The phenyl substituents D V Konarev, R N Lyubovskaya below 16.1 K.Ferromagnetic ordering of the C¡60. spins is observed in the plane ab normal to the crystallographic axis c 7 (Fig. 2). Compound (TDAE)C60 manifests only antiferromagnetic properties without preliminary keeping at constant temperature. In this case, the spin density is uniformly distributed on TDAE+. , and the intensity ratio of linesAand B in the 1HNMRspectrum of (TDAE)C60 differs from that of the specimens kept at constant No ferromagnetic properties have been found for compounds of TDAE with higher fullerenes (C70±C96). This is apparently due to differences in the electronic structures of C¡60. and radical anions of higher fullerenes, as the negative charge in the mono- anions of higher fullerenes was found to be delocalised over the entire anion surface rather than in the equatorial area of the The conducting properties of some compounds of fullerenes with amines have been studied.45, 46, 123 The antiferromagnetic phase of the salt (TDAE)C60 displays semiconductor properties with a conductivity of about 1075 S cm71 and an activation energy (Ea) of 0.4 ± 0.8 eV.46 The conductivity of the ferromag- netic phase of (TDAE)C60 at room temperature is of the same order of magnitude (561075 S cm71).It is also of activated character and is due to tunnelling of electrons between the fullerene molecules. Electron transfer is largely affected by the rotation of fullerene molecules; for this reason, the decrease in the activation energy at 150 K from 0.3 to 0.14 eV is explained It was shown123 that the conductivity of (TMBI)C60 is 561074 S cm71.(TDAE)C60.16 The volume of the superconducting phase increases if the sample is cooled very slowly at *150 K, which temperature corresponds to the transition to hindered and synchronised rotation of the C60 molecules. 7. Complexes and radical ionic salts of C60 with metallocenes Table 5 lists the compounds of C60 with metallocenes. The donor properties of metallocenes vary over a wide range, and they can form compounds with different degrees of charge transfer with fullerenes (from molecular complexes to radical ionic salts containing C60 3¡.).50, 151 The cyclopentadienyl rings of metallo- cenes coordinated with fullerenes are parallel to the five-mem- bered fullerene rings.For example, the deviation from this plane in the structure [(C5Me5)2Ni]C60(CS2) is only 0.3 8 (see Ref. 149). Such a coordination ensures the maximum overlapping of the metallocene and fullerene p-orbitals and efficient charge transfer. Two types of structures are characteristic of compounds of metallocenes with fullerenes. In compounds of C60 with ferrocene and cobaltocene, dense layers of fullerene molecules alternate with layers of the metallocene molecules.47, 145 The layers of the fullerene molecules in compounds with substituted metallocenes are looser and also alternate with the layers of the donor.149 The physical properties of these compounds are little studied.It is known that the ESR spectra of compounds of C60 with nickelo- cene and decamethylferrocene contain a signal corresponding to C¡60. (see Ref. 147). The complexes [(C5H5)2Ni]C60 and [(C5H5)2Co]C60 display magnetic properties with an S-shaped magnetisation curve which has a hysteresis. However, these properties disappear on exposure of the specimens to air,140 which indicates that the radical anions C¡60. in these complexes are sensitive to oxygen. It has been shown that the conductivity of [(C5Me5)2Ni]C60(CS2) is rather high (1072 S cm71).149 8. Fullerene salts with bulky cations Radical anionic salts of C60 ¡ . and C¡70. with bulky cations, such as Ph4P+, PPN+ and others,56 ± 58, 152 ± 154 are stable in air (Table 6).Each radical anion of C60 in these ionic compounds is located in aDonor-acceptor complexes and radical ionic salts based on fullerenes Table 5. Compounds of C60 fullerene with metallocenes. Donor (C5H5)4Fe4(CO)4 (C5H5)2Fe (C5Me5)2Fe Biferrocenyl (C5H5)2Ni (C5Me5)2Mn (C5Me5)2Ni (C5H5)2Co (C6H6)2Cr (C5Me5)2Co (C5H5)(C6Me6)Fe a The compound gives a signal corresponding to C60 7. in ESR spectra; b the compound gives an S-shaped magnetisation curve with hysteresis.147 Table 6. Salts of fullerenes with bulky cations. Cation Ph4P+ Ph4As+ PPN+ (5) Ru(biPy)2á 3 of the cations are drawn together to the six-membered rings of the fullerene in such a manner that the phenyl groups surround completely the C¡ fullerene molecule is surrounded by 22 phenyl groups of the cations, which efficiently shield the charge of the C2¡ 60 anion.According to X-ray diffraction data, there are no shortened distances between the fullerene anions in the crystal; the shortest distance between the centres of fullerene molecules is *12.5 A.152, 153, 155 A study of the magnetic susceptibility of these salts 49 showed that they are paramagnetic.49, 50 Apparently, the bulky cations surrounding the C60 fullerene radical anions interfere with their magnetic interaction. The magnetic susceptibility is determined by the spin ground state of fullerene radical anions. The magnetic moment of salts containing the C¡60. and C360¡ anions is 1.8 mB at room temperature, which corresponds to the singlet ground state with S=1/2.At low temperatures, the magnetic susceptibility in C60 ¡ . salts decreases because of the weak antiferromagnetic a. Conducting properties of fullerene salts. Superconductivity interaction between the neighbouring fullerene molecules. The magnetic moment of C2¡ be 2.5 mB.49, 50 A study of the conductivity of these salts showed that all of them are semiconductors. The conductivity of the salt [Ru(bipy)3](C60)2 (Table 6) is 1072 S cm71, and this salt is a semiconductor with an activation energy of 0.15 eV.59 It has been shown56,153 that the conductivity of the salts of C60 ¡ . with the the degree of reduction x=1, the saltsM.C60 can display metallic strongly depending on the degree of the fullerene reduction.At Ph4P+ cation is from 1077 to 1074 S cm71. The low conductivity is apparently due to the large distances between the fullerene anions in these compounds.Solvent D:C60 : Sol ratio 1 : 1 : 0.3 C6H6 2 : 1 2 : 1 0.8 : 1 : 0.7 without solvent the same MeCN 1 : 1 1 : 2 1 : 1 : 1 1 : 1 : 1 1 : 1 : 1 1 : 1 1 : 1 : 1 without solvent the same CS2 PhCN CS2 without solvent PhCN Charge on the fullerene Composition (Ph4P)C60(Ph4PCl) (Ph4P)C60(Ph4PCl)2 (Ph4P)2C60(Ph4PBr) (Ph4P)2C60Ix (0<x<1) (Ph4P)C70(Ph4PI) 71 71 71 71 71 (Ph4As)C60(Ph4AsCl) 71 (PPN)C60(C6H5Cl) (PPN)2C60 (PPN)2C60(PPNCl)MeCN (PPN)3C60(MeCN)2 71 72 72 73 [Ru(biPy)3](C60)2 71 60.radical anion. In the salt C60(PPN)2,155 one to use them for the reduction of fullerenes. For example, The redox properties of metalloporphyrins also make it possible 60 . salts at room temperature was found to levels, which can be populated with up to six electrons.25, 36 Reduction of fullerene results in the population of the t1u energy 27 Ref. Magnetic properties Compound type 144 77 molecular complex the same CTCa, b CTC 145 146 147 superposition of paramagnetic and ferromagnetic phases displays magnetic properties b CTCa, b CTC CTCa s=1072 S cm71 displays magnetic properties b the same CTCa, b CTC salt Cn¡ 60 (n=1, 2, 3) 146 148 149 48 47 150 50 151 salt Cn¡ 60 (n=1, 2, 3) Ref. Method of synthesis 57 56 152 153 154 electrocrystallisation """" 152 " 48 50, 155 49 49 "diffusion cationic metathesis the same 59 electrocrystallisation 9.Salts of the C60 fullerene with metalloporphyrins compound Cr(II)(TPP), where TPP is tetraphenylporphyrin, has strong donor properties (EOx=70.86 V)156 and reduces full- erene in tetrahydrofuran to C¡60. to give the [Cr(TPP)]C60(THF)3 salt.156 The reaction is reversible; the addition of toluene shifts the equilibrium towards the formation of neutralC60. In pure toluene, the reduction of C60 does not occur. The [Cr(TPP)]C60(THF)3 salt is a paramagnetic with S=1/2.156 The Sn(I)(TpTP) complex, where TpTP is tetra-p-tolylpor- phyrin (EOx=71.17 V), in the presence of N-methylimidazole (N-MeIm) forms a salt with C60 with composition [Sn(TpTP)](N- MeIm)2(C60)2.N-Methylimidazole stabilises the Sn(TpTP)2+ cation and hence facilitates the formation of the complex.48 10. Metal salts of fullerenes Therefore, the compounds obtained can display conducting and superconducting properties if the degree of reduction x of the C60 fullerene ranges from 0 to 6.13, 14, 25 The character of conductivity in the salts Mx .C60 varies conductivity.61, 157 ± 159 The compounds M3C60 have higher conductivity (e.g., the conductivity of K3C60 at room temperature was shown to be28 2.561073 S cm71) than the compounds MxC60 with different stoichiometry. At low temperatures, the salts M3C60 display metallic conductivity and can pass into the superconducting state.The density of energy states at the Fermi level N(EF) (number of states eV71) is an important parameter determining the super- conductivity transition temperature (Tc). Because of the small overlapping of the p-molecular orbitals of the neighbouring C60 molecules in the compounds M3C60, the conduction band is half occupied and has a width of 0.2 ± 0.3 eV. At a small band width, the density of states at the Fermi level is rather high (25 states eV71 for K3C60 and 35 states eV71 for Rb3C60, see Ref. 25). It is this fact that explains higher Tc values in fullerene salts with alkali metals in comparison with other known organic superconduc- tors.10 In the compounds M3C60 with face-centred cubic (FCC) lattice, an increase in the size of the alkali metal atom results in an increase in the distance (d) between the fullerene molecules in the crystal cell and a linear increase in Tc.The explanation for the latter phenomenon is that the increase in d(C607C60) in M3C60 decreases the overlapping of the p-molecular orbitals of the neighbouring fullerene molecules and, accordingly, the width of the conduction band. The narrowing of the conduction band increases the density of states at the Fermi level (as the number of states at the Fermi level does not depend on the band width and is constant) and increases the temperature of superconductivity transition (Tc).13, 14, 28 However, if the distance between the centres of fullerene molecules in the crystal cell is more than 10.3 A, the compound becomes a Mott dielectric.161 It was found that theM2C60 andM4C60 salts, in which the C60 molecule accepts two or four electrons, display only semiconduc- tor properties with an activation energy of about 0.5 eV.160 The significant difference of these phases from the M3C60 phase may be due to a decrease in the symmetry of the fullerene molecule in these compounds.Partial removal of the degeneracy of the t1u orbital and splitting of the conduction band into two bands (completely filled and vacant) with an energy difference between them of*0.5 eV occurs inM2C60 andM4C60.160 At x=6, the conduction band is filled completely, andM6C60 compounds are dielectrics.13 If more than six electrons are introduced in a C60 molecule, filling of the t1g level starts; this level can accept six electrons. Therefore, metallic conductivity is displayed again and transition to the superconducting condition is possible at a degree of reduction of fullerenes, x, in C60 compounds from 6 to 12.13, 18 The conductivity of C70 fullerene salts is little studied.Calculations show that MxC70 phases, where M is an alkali metal and x=4, can have metallic conductivity, while at x=1.8 they can be semiconductors. For example, the K4C70 phase actually has metallic conductivity but is not a superconductor down to 1.35 K.13 b. Intercalation of fullerenes The crystals of the C60 fullerene have a densely packed FCC lattice 37 with relatively weak intermolecular bonds between separate molecules. This lattice contains two tetrahedral and one octahedral cavities per molecule with radii 1.10 A and 2.06 A, respectively, hence fullerene is a convenient object for intercala- tion.If the cavities are completely filled with the metal atoms, the composition of the compounds obtained has the formula M3C60. As the dopant size or the number of its atoms are increased (to n=6), the densely packed FCC-lattice is transformed to a less dense volume-centred cubic (VCC) lattice with six equivalent tetrahedral cavities per C60 molecule.13, 14, 28 Diverse methods for the intercalation of fullerenes have been developed. In the most popular procedure for the synthesis of MxC60 compounds, a fullerene and x equivalents of an alkali metal are placed in a quartz tube evacuated to 1072 ± 1075 Torr, which is then sealed and heated at 200 ± 500 8C.162 The intercala- tion of the C60 fullerene with alkali metal hydrides, borohydrides and azides and other reagents has also been described.13, 14 D V Konarev, R N Lyubovskaya The gas-phase intercalation of the C60 fullerene suffers certain drawbacks.In this case, it is difficult to control the degree of reduction of the fullerene; the formation of a mixture of MxC60 phases with different content of the metal is possible (for example, M3C60 contains the M6C60 phase, which is a dielectric). An inevitable heterogeneity of the specimens formed complicates the study of the crystal and electronic structure of these materials.with C60 ¡ . and C360¡. radical anions in solutions.153, 154 Single crystals of these compounds are obtained by syntheses Refluxing a fullerene solution with a 90-fold excess ofKor Rb in toluene results in a precipitate containing 1% of a super- conducting K3C60 phase and 7% of a superconducting Rb3C60 phase.163 The addition of 5%± 30% of benzonitrile to toluene favours the electron transfer from the alkali metal to the fullerene and interferes with the precipitation of intermediate compounds with the C60 ¡ . and C260¡. anions, which allows one to obtain specimens with high content (up to *50%) of the superconduct- ing M3C60 phase. Reduction of fullerene with an alkali metal in pure benzonitrile results inMxC60 compounds, where x=4, 5 and 6.162 A drawback of the method is the presence of an unconsumed alkali metal among the reaction products. Heating and stirring of a suspension of fullerene in Cu, Zn) resulted in the corresponding salts of C60 ¡ .and C260¡. . The N-methylimidazole with a high excess of a metal (Li, Na, Ba, Fe, and compounds have the formulas: [M(N-MeIm)x]C60 [M(N-MeIm)x]2C60, where x=4 ± 6.148 A series of compounds MxC60(THF)y, where x=0.4 ± 3 and y=1 ± 14, have been obtained by the reaction of C60 with alkali metals in THF in the presence of 1-methylnaphthalene,55 by cooling a C60 solution reduced with potassium in tetramethyle- thylenediamine 55 or by the reaction of C60 with K[Mn(C5Me5)2] in THF.164 Salts with small x (*0.4) and y (*2.2) values have been obtained by electrochemical reduction of fullerene in the presence of an alkali metal tetraphenylborate.61 60 .in the structure of K3C60(THF)14 are packed The anions C3¡ in linear chains;53 one of the K+ ions is coordinated with a five- membered ring of C60 and serves as a bridge between two fullerene radical anions. The other two K+ cations are located above and below the C3¡ 60 . anion, forming contacts with six-membered rings of C60. TheK+ cations are also coordinated with THF molecules. Judging by the arrangement pattern of the C3¡ 60 . anions, the structure of K3C60(THF)14 is similar to that of M3C60 super- conductors with an FCC lattice. However, the distance between theC3¡ 60 .ions is large, and this compound does not display metallic properties.53 A compound with a different stoichiometry, viz., MxC60(THF)y, where x*0.4 and y*2.2, has a conductivity of 50 S cm71 at room temperature, which increases with a decrease in temperature and is about 1000 S cm71 at 100 K. This is probably due to incomplete charge transfer to the fullerene (d=0.4) and the presence of small distances between C60 molecule centres along the crystallographic axis c (9.93 A) in the crystal.61 After removal of tetrahydrofuran from the salt K3C60(THF)7 by evacuation, keeping at constant temperature at 300 8C for 12 h and subsequent cooling to 103 8C, heat evolution in the sample is observed, which is explained by a transition to a more stable phase with an FCC lattice.As a result, the sample becomes a super- conductor containing 31% of the superconducting K3C60 phase.164 c. Fullerene polymerisation in MC60 andM3C60 salts The intercalation of C60 fullerene with alkali metals in stoichio- metric ratio (1 : 1) gave the radical anionic salts KC60, RbC60 and CsC60.157 ± 159 On slow cooling of the intercalation products, [2+2]-cycloaddition of the neighbouring fullerene molecules occurs, which results in the polymerisation of C¡60. into linear chains. The distance between the centres of fullerene molecules decreases to 9.11 ± 9.13 A.157 Polymeric compounds are stable in air,157 insoluble in tetrahydrofuran and depolymerise only on heating above 320 8C. A study of the conductivity of the resultingDonor-acceptor complexes and radical ionic salts based on fullerenes polymeric specimens showed that [KC60]n is a three-dimensional metal the conductivity of which slowly increases on decreasing the temperature to 4 K.158 Magnetic measurements also show159 that [KC60]n behaves as a three-dimensional metal down to liquid- helium temperatures.Unlike [KC60]n, the [RbC60]n and [CsC60]n polymers are one-dimensional metals and pass to the dielectric state at 50 K and 40 K, respectively. At 25 K, the [RbC60]n and [CsC60]n polymers undergo magnetic transition with antiferro- magnetic ordering of spins in the polymeric chain. The interaction between antiferromagnetic polymeric chains occurs through alkali metal atoms, which results in three-dimensional magnetic ordering of spins.159 The nature of conductivity in these compounds is not clear yet.It is possible that polymerisation of fullerenes results in p-conjugated bonds between the molecules and the conduction electrons can move through them along the fullerene chain. Another possible mechanism assumes that the carbon atoms that are not bound directly and correspond to the neighbouring fullerene skeletons approach each other with overlapping of the t1u orbitals of these skeletons, and the conduction electrons move through these orbitals.13 The second variant is more likely, as it has been shown158 that the decrease in the ionic radius of the metal on going from rubidium to potassium results in overlapping of the t1u orbitals of fullerenes belonging to different polymeric chains and to a change from one-dimensional to three-dimensional conductivity.If C60 doped by alkali metals is rapidly cooled by liquid nitrogen, polymerisation cannot occur and monomeric radical anionic salts are obtained, such as KC60, RbC60 and CsC60.157 Heating of monomeric KC60 above 77 K, or RbC60 and CsC60 above 160 K, results in dimerisation; the dimeric phaseK2(C60)2 is a dielectric.157 It has been shown that the radical anion C360¡. in compounds state is observed at different populations of the conduction band, M3C60 can polymerise as well.165 Na2CsC60 polymerises with transition to an orthorhombic phase at a pressure of 3 kbar; the resulting polymer maintains superconducting properties.165 d.Superconducting compounds of C60 To date, about thirty superconductors have been obtained based on C60. Their superconducting transition temperatures range within 2 ± 40 K (Fig. 3). Compounds with compositionM3C60, whereM=K, Rb or a combination of K, Rb and Cs, with an FCC lattice (Fig. 3, 1) have been studied in most detail.12 ± 14, 28 It was found for this series of compounds that Tc increases linearly with the size of the alkali metal atom and the distance between the centres of fullerene molecules, d(C607C60), in the crystal cell.13, 14, 28, 166 An increase in Tc is also observed in the reaction of the M3C60 salts with ammonia. For example, the intercalation of Na2CsC60 with Tc /K 5 40 metal 1 Mott dielectric 20 semiconductor dielectric 2 superconductor 3 4 0 10.5 10.3 9.7 d /A 10.1 9.9 Figure 3.Phase diagram for the salts M3C60. Dependence of Tc on the closest distance (d) between the centres of C60 molecules in crystalline state at room temperature:14 (1), experimental data for M3C60 (K3C60, K2RbC60, K2CsC60, KRb2C60, Rb3C60, Rb2CsC60 and RbCs2C60) with FCC lattice; (2), experimental data for the series Na2(RbxCs17x)C60 with simple cubic lattice; (3), Li2RbC60; (4), Li2CsC60; (5), Cs3C60 with A15 structure, which has superconductor properties under 15 kbar. 29 ammonia results in (NH3)4Na2CsC60 with a similar crystal lattice, while the distance between the C3¡ 60 . radical anions increases from 9.99 A to 10.23 A.This is accompanied by an increase in Tc from 10.5 K in Na2CsC60 to 29.6 K in (NH3)4Na2CsC60.166 However, it was found160 that the com- pound Cs3C60 with the A15-type structure and distances between the fullerenes molecule *10.35 A is the Mott dielectric and becomes a superconductor with Tc=40 K only at a pressure of 15 kbar.160 The salts MxC60 have the highest temperature of supercon- ductivity transition for x=3.28 If the stoichiometry deviates from this value in either direction, Tc starts to decrease, and at x42 and x54 the compounds no longer pass into the superconducting state. On transition from one crystal lattice type to the other, the character of interaction between the neighbouring fullerene molecules changes.Therefore, the compounds Na2MC60, where M=K, Rb and Cs, with simple cubic lattice have a different dependence of Tc on the distance between the fullerene molecules than the compoundsM3C60 with FCC lattice. A minor increase in this distance in compounds with a simple cubic lattice strongly increases the Tc (Fig. 3, 2).14 A weak covalent interaction Li7C in compounds Li2MC60 (Fig. 3, 3, 4) changes their electronic structure, and these com- pounds do not display superconducting properties.14 A group of superconductors AxC60 obtained by intercalation of C60 with alkaline-earth metals, i.e., Ca, Sr and Ba, is of interest.13, 18 Unlike M3C60 (M=K, Rb, Cs), in which the population of the t1u orbital occurs, the superconductivity in these compounds is due to the population of the t1g orbital.The Tc of compounds AxC60 ranges within 4 ± 8.5 Kand depends little on the distance between the fullerene ions.13 The superconducting but it is rather difficult to determine the exact extent of charge transfer (d) to the fullerene. In the series of compounds CaxC60 with x=3 ± 6, Ca5C60 has the maximum conductivity (the degree of charge transfer to C60 d equals 10) and passes in the super- conducting state at 8.4 K.13, 18 The compounds Ca3C60 (d=6) and Ca6C60 (d=12) (the conduction band is completely popu- lated) do not display metallic properties.13, 18 As opposed to Ca3C60 and Ca6C60, a weak covalent interaction of Ba and Sr with fullerene in Ba3C60, Ba6C60, Sr3C60 and Sr6C60 is possible because of the larger ionic radii of these metals.This results in a decrease in the actual charge transfer to the fullerene and in only partial population of the conduction band at x=3 and x=6.18 Therefore, Ba3C60 and Sr3C60 display metallic properties, and Ba4C60, Sr6C60 and Ba6C60 have superconducting properties at Tc=4, 7 and 4 K, respectively.13, 18 It has been shown166 that C60 fullerene intercalated with lanthanides can also be a superconductor (for example, Yb2.75C60 has Tc=6 K). e. Intercalation of molecular complexes of C60 with alkali metals Structural diversity of organic molecules allows one to create C60 compounds with different packing of the C60 molecules in the crystal, viz., one-dimensional chains, two-dimensional layers or three-dimensional arrangement of the C60 molecules.Fullerene compounds with one-dimensional and two-dimensional packing of the C60 molecules are of special interest for the study of conducting and superconducting properties. In addition, by using different donor molecules, it is possible to control the distance between the fullerene molecules in the crystal lattice in order to obtain materials with high Tc. This stimulated studies of the intercalation of C60 complexes with alkali metals. The intercalation of molecular complexes of C60 is accompanied by reduction of the fullerene according to the scheme: Red D(C n¡ 60 n Red+)Sol Dd+Cd¡ 60 Sol Red is reducing agent.30 The intercalation of C60 complexes with octamethylenetetra- thiafulvalene (10) (OM± TTF)C60(C6H6) or bis(ethylenedioxy)- tetrathiafulvalene [(BEDO ± TTF)*C60] with potassium and rubidium is carried out at 1074 ± 1075 Torr and 55 and 67 8C, respectively, as in the case of pure C60.168, 169 The intercalation of (OM± TTF)C60(C6H6) gave compounds with composition Kx(OM± TTF)C60(C6H6), where x41:8.Apparently, the sol- vent is retained in the compound.168 Because of their small radii, the alkali metal atoms occupy the cavities in the structure of the starting molecular complex; this increases somewhat the param- eters of its crystal cell.169 The intercalation of (OM± TTF)C60(C6H6) with potassium gives a superconducting phase with a transition temperature of 17 ± 18.8 K, and that with rubidium gives a superconducting phase with Tc=23 ± 26 K.The intercalation of [(BEDO ± TTF)*C60] with potassium yields a superconducting phase with Tc=15 K.168, 169 f. Intercalation of fullerenes and their complexes with halogens The intercalation of fullerenes with molecular iodine (I2) and interhalides, viz., IBr and ICl, gives the complexes (Hal2)xC60. The reaction is carried out at 100 ± 250 8C in evacuated tubes.40, 170, 171 The content (x) of the halogen in the sample can vary from 0.2 to 2,170 ± 172 depending on the conditions and duration of intercala- tion. A study of the structure of the (I2)2C60 complex has shown that it has a layered structure; the C7I distances (3.6 ± 4.0 A) are smaller than the sum of the van der Waals radii of carbon and iodine.170 There is no charge transfer from the fullerene to iodine in these compounds.Iodine is weakly bound to the fullerene and is removed from the compound at 200 8C.170 (I2)2C60 is a dielectric the conductivity of which is less than 1079 S cm71 (Ref. 170). A study of the magnetic properties of C60 complexes obtained by intercalation of fullerene with I2, ICl and IBr revealed magnetic transitions 40, 171 at 60 K, 30 K and 30 K, respectively. For pure C60 and C70 fullerenes, these are also are observed at 60 K.38 Apparently, these magnetic transitions, like those in the case of pure fullerenes, are due to transition of the specimens to the frozen glassy magnetic state,38 because synchronisation of rotation of the C60 molecules is also possible in the presence of shortened contacts between the C60 molecules in the crystals of these molecular complexes at low temperatures.Cooling of these specimens in a magnetic field below this temperature results in freezing of a completely ordered orientation of the magnetic moments of the fullerene molecules. The intercalation of the complexes (DB ± TTF)C60(C6H6), (TMDTDM± TTF)2C60(CS2)3 and TPDP(C60)2(CS2)4 with iodine results in compounds with a high content of iodine, viz., (DB ± TTF)C60I9, (TMDTDM± TTF)2C60I7.5 and TPDP. .(C60)2I10 .173, 174 The intercalation of C60 complexes with iodine is based on the solid-phase oxidation of the donor component of these complexes with the formation of a radical cation. The solvent, for example, CS2, is displaced by iodine.174 Ox (D n+ n Ox7)C60 Dd+Cd¡ 60 Sol Ox is oxidant.The intercalation is accompanied by noticeable changes in the ESR spectra due to the oxidation of the donor.173 The optical absorption spectra display a shift (up to 10 nm) of the absorption bands of the basic electron transition e0 at l=260 and 350 nm and an increase in absorption intensity in the region of 450 ± 620 nm.173 These changes can be due to the formation of a radical cation by the donor.173 The position of the absorption band of the T1u(4) vibrations of C60 (1429 cm71) in the IR spectrum of intercalated specimens is not changed, which indi- cates the absence of charge transfer to the fullerene molecule. The starting complexes are dielectrics. Intercalation results in an insignificant increase in the conductivity of the complexes (by a D V Konarev, R N Lyubovskaya factor of less than 100), which is apparently due to the large distances between the donor molecules in these complexes.174 IV.Structure and spectral characteristics of complexes and radical ionic salts of fullerenes 1. Specific features of the crystal structure The position of the C60 molecules in a crystal and the number of direct van der Waals contacts between the fullerene molecules make it possible to distinguish several structural types of fullerene compounds.175 1. Three-dimensional packing of the C60 fullerene molecules with distances between the centres of the molecules ranging from 9.8 to 10.3 A is observed in fullerene salts MxC60 (x=1 ± 6) with alkali and alkaline-earth metals.Simple cubic, cubic face-centred, cubic volume-centred and rhombic lattices with the number of closest neighbouringC60 molecules from 8 to 12 correspond to this type of packing.13, 14 2. Layered packing, in which two-dimensional dense or loose hexagonal layers of C60 molecules are formed. The number of closest neighbouring fullerene molecules ranges from 4 to 6. The layers of donor molecules in these structures also alternate with layers of C60 molecules. In the donor layer, e.g., in the (TMDTDM± TTF)2C60(CS2)3 complex,103 there are also short- ened contacts between theTMDTDM±TTF molecules.Ashift of hexagonal layers relative to each other with transition to simple hexagonal packing of layers can be observed in compounds with small donors.This refers to compounds of C60 with such molecules as I2,170 S4N4,95 P4,93 TMPD,129 OM± TTF,5 TPDP109 or DAN.44 The structure of the (DAN)C60(C6H6)3 complex with layered packing of C60 molecules is shown in Fig. 4. Each C60 molecule in this complex is surrounded by four fullerene molecules with a distance of 10.07 A between the centres. a c Figure 4. Projection of the crystal structure of the complex with grey spheres) along the crystallographic axis b 744 (schematic repre- (DAN)C60(C6H6)3 (the positions of the fullerene molecules are indicated sentation). The solvate benzene molecules (not shown) are located in the dianthracene layer.44 3. Cage packing: the fullerene molecules form various cavities or channels, in particular, hexagonal channels, which are filled with donor molecules. The number of the closest neighbouring fullerene molecules can vary from 4 to 8.This structure is represented by compounds of C60 with twin-TDAS,113 C6H6 68, 69 and S8.90, 91 4. Chain packing: the fullerene molecules form densely packed chains (with two closest neighbouring fullerene mole- cules) or double chains (with three closest neighbouring fullereneDonor-acceptor complexes and radical ionic salts based on fullerenes molecules). This structure is characteristic of complexes of full- erene with BEDT± TTF,98, 99 twin-BEDT ± TTF102 and C1TET ± TTF.110 5. Island mode of packing: in this case, there are no direct van der Waals contacts between the fullerene molecules, and all distances between the centres of C60 molecules are *12 A.This type of packing is observed in compounds with large donor molecules (HMT,115 Ph3Sb116) or cations (Ph4P+ (see Refs 57, 136 and 153) and PPN+ (see Refs 58 and 155). The structures of some fullerene complexes, for example with such donors as DBTTF105 and BTX, are intermediate between the cage and island structures.111 Figure 5 presents the structure of the DBTTF.C60 .C6H6 complex. It is characterised by isolated packing of C60 molecules, in which each of them is surrounded by six closest neighbouring fullerene molecules with a distance of 10.4 ± 10.5 A between the centres. This distance is larger than the van der Waals diameter of fullerene (10.18 A) but smaller than the distance characteristic of island structures (*12 A).Figure 5. Crystal packing of the complex (DB ± TTF)C60 .C6H6 (posi- tions of fullerene molecules are indicated by grey spheres; crystallographic axes are shown by straight lines).100 Table 7 lists the mean bond lengths of the C60 fullerene molecule in its compounds. It is evident that the bond lengths in C60 change with an increase in the degree of charge transfer to the fullerene molecule; in this case, the 6 ± 5 bonds shorten while the 6 ± 6 bonds elongate. The direction of changes in the bond lengths upon reduction of fullerene is due to the nature of the t1u orbital, which is anti-bonding with respect to the 6 ± 6 bonds and bonding with respect to the 6 ± 5 bonds.13 This results in elongation of the fullerene molecule, and the C60 sphere is distorted to become an ellipsoid.Table 7. Lengths of 6 ± 5 and 6 ± 6 bonds for D±A complexes and radical ionic salts of C60. Ref. Compound 6 ± 6 Bond length/A Charge 6 ± 5 Bond on C60 length/A C60 176 116 98 145 109 90 149 47 155 1.355(9) 1.383(4) 1.389(7) 1.387(6) 1.381(6) 1.340(8) 1.389(3) 1.384(8) 1.399(2) 1.400(4) 1.445(3) 1.467(2) 1.452(5) 1.452(1) 1.450(5) 1.451(6) 1.448(8) 1.449(3) 1.453(4) 1.446(2) 1.452(1) 1.432(1) C60(SbPh3)6 C60(BEDT ± TTF)2 C60[(C5H5)2Fe]2 (C60)2TPDP(CS2)4 C60(S8)2 C60[Ni(C5Me5)2]CS2 C60[Co(C6H5)2]CS2 C60(PPN)2 K3C60 K6C60 000000 71 71 72 73 76 13 13 31 2. Stability The stability of fullerenes with respect to atmospheric oxygen differs from the stability of their compounds.In solid state, fullerenes can adsorb oxygen on their surface.62 In solution, they can add oxygen under illumination to give epoxides C60On, where n=1, 2 and 3 (therefore, it is preferable to carry out reactions with fullerenes in the dark).19 Anionic fullerene compounds are particularly sensitive to oxygen because of the possible reaction Cn¡ C60+nO¡ 60 +nO2 2 . This reaction can also yield addition products C60O¡2 . at the 60. 70., is 70.4 V.31 With these values of redox potentials, 2 . couple becomes more positive due to stabilisation of fullerene 6 ± 6 or 6 ± 5 bonds.177 In aprotic media, the redox potential of the O2/O¡2 .couple is 70.8 V,48 while the first redox potential of fullerenes, C60/C ¡ and C70/C ¡ oxidation of fullerene radical anions C60 ¡ . and C¡70. is thermody- namically unfavourable, and hence radical monoanionic fullerene compounds should be stable when exposed to air. This is actually observed for salts containing theC¡60. radical anion (KC60, RbC60, CsC60) and salts with bulky cations.56 ± 58, 152 ± 154, 157 The insta- bility of salts ofC¡60. with metalloporphyrins,48 metallocenes 48, 147 and amines 15, 134 in the air can be due to the fact that protons or metal cations can stabilise the charge on O¡2 . . The redox potential of theO2/O¡ the O2¡.radical anion, which enables the oxidation of C¡60. . The second redox potential of fullerenes C¡60./C260¡ and C70 ¡ ./C270¡ is *70.8 V,31 hence the oxidation of fullerene di- anions with oxygen is thermodynamically favourable. Therefore, radical anionic salts containing fullerene dianions or Cn¡ 60 anions in higher degrees of reduction are very sensitive with respect to oxygen.82 3. Thermogravimetry Derivatography is generally used to study the thermal stability of both pure fullerene and its compounds.43, 44, 54, 75, 101, 104, 178 Heat- ing in air causes complete combustion of fullerene at 650 8C. In nitrogen, fullerene starts to sublime at 600 8C, and the maximum sublimation rate is reached at 700 ± 800 8C.178 It is possible to determine the content and the strength of binding of a solvent in a complex from derivatograms of molecular complexes of fullerenes.43, 44, 54, 75, 101, 104 Thermogravi- metric studies of fullerene complexes show that partial decom- position of donors occurs in these compounds.The decomposition temperatures of donors in complexes are close to those of the pure donors. In certain instances, the decomposition temperature of a donor increases owing to its stabilisation because of the donor-acceptor interaction with the fullerene.44, 54, 75 The presence of a large amount of a solvent in a molecular complex separates the fullerene molecules from each other. Therefore, the sublimation temperature of the fullerene in such complexes even after complete removal of the solvent is lower 43, 50, 103 than that of pure C60.178 4.Spectroscopy of compounds based on fullerenes A significant number of publications deal with the study of fullerenes and their compounds by spectroscopic methods (see, for example, Refs 5, 22 ± 26, 36 and 179 ± 195). a. Electronic spectroscopy Optical spectroscopy is a convenient method for the study of changes in the electronic structure of fullerenes upon formation of donor ± acceptor compounds.5, 9, 181 ± 185 The optical absorption spectrum of the C60 fullerene in solid state (Fig. 6) has been studied in detail.25, 66 In the ultra-violet region (250 ± 400 nm), two intense bands corresponding to sym- metry-allowed electronic transitions are observed.There is a rather strong band in the visible region (l=420 ± 540 nm) with a maximum at l=450 nm (2.7 eV), the origin of which is not quite clear. This band is absent in the absorption spectra of C60 solutions,25 but appears, for example, upon aggregation of several32 D 2.6 1.8 1.0 0.2240 800 600 400 l /nm Figure 6. The absorption spectrum of C60 in a KBr matrix.181 C60 molecules in a complex with g-cyclodextrin in aqueous solution.84 Therefore, this band is sometimes 179 related to intermolecular transfer of an electron from the HOMO of the C60 molecule to theLUMOof the neighbouringC60 molecule. The absorption at l=540 ± 620 nm (2.2 ± 2.0 eV) has low intensity and corresponds to the symmetry-forbidden hu?t1u transition from HOMO to LUMO of one C60 molecule (see Fig.1).25 The manifestation of this forbidden transition both in liquid and in solid phase is explained by a deviation of the symmetry of C60 molecules from Ih.25 The absorption edge of fullerene in optical spectra is in the region of 1.95 ± 1.75 eV, which corresponds to 640 ± 700 nm, and it is basically related to exciton transitions.180 The formation of molecular complexes does not induce considerable changes in the electronic system of fullerenes.181 Irrespective of the solvent, the absorption edge of fullerene in fullerene solvates shifts by 0.1 eV upfield in comparison with pure fullerene,180 which is explained 185 by separation of fullerene molecules from each other by the solvent.The spectra of complexes both in solution and in solid state display charge-transfer bands (CTB) from the donor to the fullerene. The process of charge transfer for complexes with an uncharged ground state upon absorption of a light quantum is described by the scheme: hn D(1+d)+A(1+d)7. Dd+Ad7 The dependence of the charge transfer energy (hnCT) on the donor IP for complexes of one acceptor with a series of donors is linear. This dependence is described by the equation:5, 9 hnCT=a(IP7EA)7EC, where a is a constant, IP is the ionisation potential of the donor, EA is the electron affinity of the acceptor, and EC is the energy of electrostatic interaction between the donor and acceptor radical ions in the excited state of the complex.Such a dependence for complexes of the C60 fullerene with substituted anilines 182 and naphthalenes 183 (IP=7.2 ± 8.13 eV) in toluene is shown in Fig. 7 and can be described by the expression: hnCT=0.91IP74.34 (eV). Table 8 lists the energies corresponding to the maximum of the charge-transfer band for a number of C60 complexes in the solid state. It is evident that hnCT decreases with a decrease in the redox potential (ERed/Ox) of the donors.5 Irradiation of a crystalline complex (TMPD)C60 with visible solvates,187, 188 violation of symmetry of the environment of the C60 fullerene molecules is observed, which results in partial light (He ± Ne laser) was found to cause a strong increase in the modification of symmetry-forbidden vibrations, hence they absorption at 1070 nm.76 This is related to the formation of a long-lived radical anion C60 ¡ .upon transfer of an electron from dislocations, admixtures or solvent molecules in the crystal appear in the IR spectra. This is due to the presence of defects, TMPD to C60. The lifetime of the C60 ¡ . radical anion in a crystal is structure of C60 and in crystal solvates. about 1 h, which is several orders larger than the lifetime in the case of similar electron transfer in solution.76 D V Konarev, R N Lyubovskaya hnCT /eV 9 3.0 7 8 4 5 6 2.6 2 3 1 2.2 IP /eV 8.0 7.8 7.4 7.2 7.6 Figure 7. Dependence of charge transfer energy hnCT on ionisation potentials of donors in CTC of C60 in toluene: (1), N,N-diethylaniline;182 (2), N,N-dimethylaniline;182 (3), N-methylaniline;182 (4), 1-methoxynaph- thalene;183 (5), 2,6-dimethylaniline;182 (6), o-toluidine;182 (7), 1-methyl- naphthalene;183 (8), aniline;182 (9), 1-chloronaphthalene.183 Table 8.Position of the charge-transfer band maximum in electronic absorption spectra of C60 complexes in solid state. Ref. CTC ERed/Ox of hnCT /eV the donor /V 70.086 +0.29 +0.39 +0.42 7+0.52 7 1475 1045 101 98 110 [(C5Me5)2Fe]2C60 (OM± TTF)C60(C6H6) (TMDTDM± TTF)2C60(CS2)3 (BEDO ± TTF)*C60 a (BET ± TTF)C60(C6H5Me) (BEDT ± TTF)2C60 (twin-BEDT ± TTF)C60(CS2) 1.13 1.35 1.38 1.51 1.55 1.65 1.65 a The exact complex composition has not been determined. Charge transfer from the donor to the fullerene in the ground state is related to the population of the t1u orbital of the C60 fullerene.This enables electronic transitions from the t1u orbital to vacant molecular orbitals with higher energies (see Fig. 1), which results in the appearance of new absorption bands in the near IR region. Their position corresponds to definite charge of the fullerene molecule: . C270¡ C460¡ C360¡ C260¡ C70 ¡ . Fullerene C60 ¡ . 1070 1165 1370 950 n (nm) 730 1195 88 780 1380 35 35 35 35, 48, 184 50 Ref. b. IR spectroscopy The change in the symmetry and redistribution of the electron density upon formation ofD±A complexes and radical ionic salts of fullerenes is reflected in their IR spectra.9, 25, 26, 184 ± 195 Due to its high symmetry (Ih), the C60 molecule has 46 characteristic normal vibrations.Four of these are active in the IR spectra [T1u(1 ± 4) vibrations with absorption bands at 527, 577, 1183 and 1429 cm71, respectively] and ten in Raman spectra; 32 normal vibrations in the C60 molecule are symmetry-forbidden in the dipole approximation.25, 26, 186 In the crystalline C60 fullerene 186 and some of its crystal At room temperature, the molecules of the C60 fullerene in crystals rotate quickly and isotropically, occupying positions with the Th symmetry.188 The T1u vibrations, which are active in IRDonor-acceptor complexes and radical ionic salts based on fullerenes spectra of theC60 fullerene, are threefold degenerate and appear as single bands.Cooling C60 crystals below 255 K results in an orientational-type phase transfer with freezing of the rotation of the fullerene molecules and a decrease in its position symmetry to S6.189 The degeneration of the T1u(4) vibration of C60 at 1429 cm71 is eliminated, and at 8 K it is split into three bands with wave numbers 1424.5, 1427.9 and 1431.2 cm71. The T1u(3) vibration at 1183 cm71 remains unsplit.189 Similar splitting of the T1u(4) vibration of C60 into three bands (Table 9) has been reported in compounds of C60 with amines, viz., (TMPD)C60 and (TPA)C60.190 This splitting is due to freezing of rotation of the C60 molecules in the crystal of the complex due to the intermo- lecular interaction with molecules of the donor and a decrease in the positional symmetry of the C60 molecules (in comparison with pure fullerene above 255 K).190 Table 9.Position of the T1u(4) vibration band of fullerene (n) and degree of charge transfer (d) estimated from Eqn (2) in donor ¡¾ acceptor complexes and radical ionic salts of C60. Ref. Compound n /cm71 Compound d type 189 C60 1424, 1428, 1431,a, b 1429 c 1429 75 S4N4C60 molecular 0 complex the same 077 (S8)2C60(C6H5Cl)0.5 (TPA)C60 (TMPD)C60 (C5H5)2CoC60(CS2) *1 1429 1425, 1428, 1433 b " 1422, 1425, 1427 b CTC 1411 (C5H5)2CoC60(C6H5CN) 1413 1407 1395 1394 1394 1392 (C6H6)2CrC60 [Na(18-C-6)]C60(THF)3 (Ph4P)C60(Ph4PCl) (Ph4P)C60(Ph4PI) RbC60 d CTC CTC CTC salt """ *1 1390 " (Ph4As)C60(Ph4AsCl) 194 190 190 *0.5 47 *0.5 48 *0.7 150 48 *1 184 *1 195 26, 191 *1 184 a Measured at 8 K; b splitting of the T1u(4) band of fullerene on freezing the rotation in the crystal; c measured at 293 K; d film.The transfer of electron density from the donor to the C60 fullerene in the ground state results in a shift of some of its bands in the IR and Raman spectra.26, 191, 192 This is caused by the population of the t1u orbital of the fullerene and interaction of the T1u vibrations with virtual electronic transitions from the t1u orbital to the higher t1g orbital.191 Figure 8 shows the charge dependence of the position of absorption bands (n) of C60 vibrations active in the IR spectrum.26, 191 The T1u(4) and T1u(2) vibrations are most sensitive to charge transfer: they are charac- terised by a linear increase in np and an almost linear shift of n.The A1g(2) vibration active at 1469 cm71 in the Raman spectrum also has a linear dependence of n on the degree of reduction of the fullerene molecule.26, 192 This relationship may be used for the determination of the degree of C60 reduction in salts,26 including that occurring during intercalation. The plasma frequency (op) (square root of the oscillator force) also changes linearly with the charge on the fullerene molecule (Fig. 9). The change in frequency of absorption bands in the IR spectrum of C60 vibrations in D¡¾A complexes makes it possible to estimate even a small degree of charge transfer (0<d<1).This method has already been used previously for the estimation of charge transfer in organic CTC.9, 193 degree of charge transfer (d), as the transition from C60 to the C60 ¢§ . The T1u(4) vibration is most suitable for determination of the radical anionic salts is accompanied by a strong shift of the absorption band of this vibration from 1429 cm71 to 1390 ¡¾ 1395 cm71 (see Refs 26, 48, 184, 191 and 195) (Table 9). 33 n /cm71 1480 1460 5 1440 1400 1360 43 1180 2 580 540 500 1 460 6 x 5 4 3 2 1 0 Figure 8. Wave numbers (n) for different degrees of reduction (x) of the C60 fullerene molecule.191 (1), T1u(1) vibrations; (2), T1u(2) vibrations; (3), T1u(3) vibrations; (4), T1u(4) vibrations; (5), A1g(2) vibrations.op /cm71 600 500 400 300 200 100 x 5 4 3 2 1 0 Figure 9. Plasma frequency (op) for different degrees of reduction (x) of the C60 fullerene molecule.191 (1), T1u(2) vibrations; (2), T1u(4) vibrations. The change in the positions of absorption bands of three other T1u vibrations active in the IR region on transition from C60 to C¢§60. is less marked.26, 192 Taking into account the linear dependence of the position of the absorption band of the T1u(4) vibrations on the degree of reduction of the fullerene molecule, it is possible to use Eqn (1) for the estimation of the degree of charge transfer in complexes:9, 193 (1) 2Dn d a n0O1 ¢§ n21 =n20 U , where n0 is the position of the absorption band of the T1u(4) C60 ¢§ .salts [1392.52.5 cm71, depending on the crystal structure vibration in neutral C60 (1429 cm71); n1 is its average position in (Table 9)], and Dn is the difference between the positions of absorption bands of T1u(4) vibrations in neutral C60 and in the corresponding complex. The use of these values in Eqn (1) gives Eqn (2): (2) d%0.03 Dn. The accuracy of estimation of charge transfer d, which is 0.03, depends on the accuracy of measurement of the positions of absorption bands in IR spectra (1 cm71). One can see from Table 9, which lists the positions of the T1u(4) vibration bands and the degree of charge transfer in C60 compounds estimated from Eqn (2), that the degree of charge transfer is close to zero (molecular complexes) for the majority of C60 compounds with organic donors.In radical ionic salts, the34 degree of charge transfer is close to unity. Only the complexes of C60 with TMPD,190 cobaltocene 47, 48 and dibenzenechromium 150 have intermediate degrees of charge transfer. c. ESR spectra The g-factors and DH of ESR signals of fullerenes and their compounds are listed in Table 10. In a discussion of the ESR spectra of C60 compounds, it is necessary to emphasise the presence of two ESR signals at room temperature in specimens of the initial, `pure' fullerene, one with g=2.0025 ± 2.0021 and DH=2 G196, 197 and the other with g=2.0006 ± 2.0012 and DH=0.5 ± 2 G;196 their widths virtually do not change down to liquid-helium temperatures.These signals originate from defects, i.e., paramagnetic admixtures formed upon oxidation of fullerene with oxygen.177, 196 The intensity of the former signal increases by an order of magnitude on heating the specimen in air at 623 K for 2 h and by two orders on heating for 24 h.196 The intensity of this signal changes considerably depending on the way the fullerene has been obtained or stored. Unlike the signals related to the oxidation of fullerenes, the ESR signals of the Cn¡ 60 anions have a considerably larger line width (DH=20 ± 60 G), which depends strongly on temperature. The ground state of the radical anion C¡60. is a singlet (S = 1/2) with g=1.997 ± 1.999. This value is smaller than the g-factor of a free electron.56, 56, 61, 153 Larger g-factors (2.0008) are observed only in salts of C60 with amines.38,142 The C2¡ 60 anion displays a signal with g=2.0010 and DH=10 ± 30 G at room temperature.This corresponds to the triplet state (S=1) with forbidden splitting D%0: two electrons with parallel spins are so distant from each other that they behave similarly to electrons with independent spins, therefore their interaction is not displayed in the ESR spectra.50 The radical anion C3¡ 60 . is in a singlet basic state (S=1/2) at room temperature, and its g-factor equals 2.0012 ± 2.0017, and DH=10 ± 30 G.49 In the compounds M3C60, the ESR parameters (the g-factor and DH) of the C3¡ radical anion depend on the nature of the metal.13, 14 The ESR signals of all of the Cn¡ 60 anions become much narrower as the temperature is decreased.58, 153, 154 Due to the Jahn ± Teller effect, the presence of fullerene radical anions in C60 salts with bulky counter-ions results in violation of the Ih symmetry of C60.48 ± 50, 149 The effect can be both dynamic Table 10.g-Factors and line widths (DH) for ESR signals in D-A complexes and radical ionic fullerene salts. Compound C60 C60 (Ph4P)C60(Ph4PCl)2 (PPN)C60(C6H5Cl) (Ph4P)2C60I0.35 KC60(THF)5 Na0.4C60(THF)2.2 (TDAE)C60 (TDAE)C70 (TDAE)C84, C90, C96 (DBU, DBN)C60 [(C5H5)2Co]C60(C6H5CN) (Ph4P)C70(Ph4PI) SbCl3+C60 (CB11H6Br6)C76 154 357 a The signal is observed below the temperature T; b the ESR signal was measured at two different temperatures T(T1); c the signal is observed above the temperature T; d ESR signal after transition of the compound to ferromagnetic state.Charge on fullerene 00 71 71 71 70.4 71 71 71 71 71 71 +1 +1 60 . g-Factor 2.0021 2.0023 2.0006 ± 2.0012 1.9991 1.9992 2.0007 1.9979 1.9987 1.999 2.0008 2.0017 ± 2.0030 2.0022 2.0020 ± 2.0022 71.9969 2.000 2.047 2.0029 2.0030 and static on the ESR time scale.198 At room temperature, a dynamic effect with fast transition from one static Jahn ± Teller configuration to the other is observed. Based on the ESR data, the frequency of this pseudo-rotation is estimated as*1012 Hz.198 If the temperature is decreased, pseudo-rotation is hindered, the ESR signal narrows, and transition to the static Jahn ± Teller effect is observed.In the salt (Ph4P)C60(Ph4PCl)2, this transition occurs when the temperature is decreased to 70 K;199 this is accompanied by abrupt narrowing of the signal. On further decreasing the temperature, its width almost does not change. The static Jahn ± Teller effect at low temperatures (4 ± 70 K) results in transformation of the isotropic ESR signal to aniso- tropic. For instance, the Na(18-crown-6)C60(THF)3 salt displays an anisotropy of the g-factor: g\ =1.9968, gk=2.0023.48 60. radical anion decreases from Ih to As the symmetry of the C¡ D5d, the T1u state splits into two states, viz., E1u and A2u (Fig. 10). The difference between the energies of the E1u and A2u states is small and equals 1 kcal, therefore thermal population of the over- laying A2u state can occur, and an additional `high-temperature' signal with g=2.000 appears in the ESR spectrum of C¡ 2T1u C60 7.(Ih) Figure 10.Scheme of partial removal of degeneration of the t1u orbital in the C60 7. radical anion upon lowering the symmetry from Ih to D5d.48 The radical anion C¡70. in the (Ph4P)2C70I salt gives a broad ESR signal with g=2.047,154 as does that in the salt (TDAE)C70, with g=2.0022.136, 137 Higher fullerenes display a narrow signal with g=2.0023,143 which is close to the g-factor for the free electron. 60. radical anion in some fullerene The ESR signal of the C¡ complexes has been used to determine the degree of charge T /K <300 a <300 a <300 a 300 300(77) b >50 c 300(113) b 300 300 300 <16 a, d <300 a <300 a 300(5) b 130(4.5) b >24 c 300(4.2) b 300 300 D V Konarev, R N Lyubovskaya 60..48 2A2u 1 kcal 2E1u C60 7.(D5d) Ref. DH /G 196 197 56 58 153 55 61 38, 142 136, 137 143 127, 128 48 0.3 ± 1.0 0.5 ± 2.0 0.5 ± 2.0 45 35(5) 1 ± 2 50(14) 34 30 22 30 10 1.85 ± 2.40 40(5) 24(6) 3600(1.5) 1.5 0.5Donor-acceptor complexes and radical ionic salts based on fullerenes transfer from the donor to C60 (based on the number of C¡60. spins per formula unit).123, 127, 128 d. X-ray photoelectron spectroscopy X-Ray photoelectron spectroscopy (XPS) is a sensitive method for the determination of the valence state of elements in thin (0.5 ± 4.0 nm) surface layers.Based on the position of lines corresponding to internal electronic shells of heteroatoms of the donors contained in the complex and their shift relative to the lines of the individual donor, it is possible to estimate the redistribution of the electron density upon formation of a D±A compound and the atomic composition of the compound.200 ± 205 The C(1s) X-ray photoelectron spectrum of the C60 fullerene consists of the main singlet peak with an energy of 285 eV. The higher energy region contains a satellite shifted by 5.9 eV from the main C(1s) peak. It originates from excitation of a p-plasmon, i.e., coordinated vibrations of p-electrons of the C60 molecules in a crystal.206 The density of valence electrons is determined from the electron energy loss spectra (EELS).The loss function has two peaks in the case of the C60 fullerene,203, 206 which correspond to the p-plasmon with a maximum at 5.8 eV originating from excitation of plasma vibrations of p-electrons and the (p+s)- plasmon with a maximum at 26.1 eV originating from excitation of all valence electrons in C60.203, 206 The position of the C(1s) peak in the XPS spectra of molecular C60 complexes and salts of fullerenes remains unchanged, but some changes in its satellite structure occur.44, 205 The disappearance of p ± p* transitions of phenyl substituents of the donors in the compounds, e.g., satellite in structure of (DAN)C60(C6H6)3 44 and in salts with bulky cations,205 has been reported.This is due to the strong interaction of the phenyl substituents of the donor with the fullerene or to the fact that, upon formation of a compound with fullerene, the p ± p* tran- sitions in the donor molecule become less favourable than the excitation of plasma vibrations of p-electrons of the fullerene itself.44, 205 In many compounds, a decrease in the energy of the p+s-plasmon is observed: 24.0 eV in (S8)2C60,203 25.2 eV in (BTX)C60(CS2) 54, 119 and 25.5 eV in TPDP(C60)2(CS2)4.43 The shift in the position of the S(2p), N(1s) and Te (3d5/2) lines of the donor heteroatoms in various fullerene complexes by 0.1 ± 1.0 eV towards higher energies can be caused 200, 201, 204 by the electron density shift from the donor to the fullerene.However, in some cases a similar shift in fullerene complexes in comparison with the individual donors can also be due to the calibration of spectra of the complex and the donor with respect to the C(1s) line, as the exact position of this line in the spectra of the donor and C60 can differ.200 A shift in the position of the S(2p) peak in compound (S8)2C60 towards lower energies by 0.4 eV has been reported.203 60 e. 13C NMR spectroscopy The 13C NMR spectrum of crystalline C60 fullerene at room temperature contains a narrow singlet at d 143.29 This is caused by fast rotation of the C60 molecule and isotropic averaging of the signal.As the temperature is decreased, the rotation of fullerene molecules is hindered, and broadening of the signal is observed.22 The observed phase transitions in fullerene at 255 and 90 K result in stepwise changes in the line width. The formation of molecular complexes does not change the position of the 13CNMR signal of the fullerene.93, 98, 114 The formation of fullerene anions, Cn¡ (n=1, 2, 3), causes a shift of the 13C NMR signal towards lower magnetic field, which can be due to the paramagnetic state of these 60 ions.50, 207, 208 However, the salts containing the C60 ¡ . and C260¡. anions have close chemical shifts of signals in the 13C NMR spectra (d 187 and 183, respectively), though their magnetic susceptibilities differ strongly.207, 208 A shift of the 13C NMR signal to d 156 is also observed in the diamagnetic state of the C6¡ anion.208 35 V.Conclusion Based on the survey of the most important results concerning the synthesis and properties of the D±A complexes and radical ionic salts of fullerenes obtained over the last years, one can distinguish the most important directions of the development in this field and evaluate some possibilities of using fullerene compounds, both for obtaining new materials and for solving fundamental problems. The ability of molecules of the C60 fullerene, its molecular complexes and salts to synchronise their rotation in crystals results in the appearance of unusual magnetic properties in these compounds. The existence of the frozen glassy magnetic state of fullerenes and molecular complexes of fullerenes with halogens has been established; radical anionic salts of C¡60.with unsatu- rated amines possess ferromagnetic properties with the highest Tc among organic materials; various magnetic properties are dis- played by complexes of C60 with metallocenes. Obviously, syn- thesis of D±A complexes of fullerenes with strong organic and organometallic donors, viz., unsaturated amines, metallocenes and metalloporphyrins, and the study of their structure and properties will lead to new interesting results. Another important direction includes the synthesis of con- ducting and superconducting materials based on fullerene com- pounds. By now, several dozens of superconductors with Tc440 K have already been obtained based on C60 and some specific features of their superconductivity have been rationalised.Superconducting phases can exist in fullerene compounds with alkali or alkaline-earth metals, lanthanides, and in salts of C60 with strong organic donors: they can be obtained both by direct chemical synthesis and by intercalation in the gas phase. Fullerenes are weak acceptors, and the range of donors capable of reducing them to radical anions is limited. In addi- tion, the essential drawback of both superconducting and ferro- magnetic compounds of fullerenes is their instability in air. This restricts considerably the possibilities of obtaining and applying the fullerene-based materials with specific conducting and mag- netic properties.One possible way to solve these problems is the synthesis of three-component systems. The systems `organic donor ± fullerene radical anion ± alkali metal cation' include a wider range of ionic compounds of fullerenes. In some of them, the fullerene radical anion can be stabilised because the bulky organic donor sterically hinders the approach of oxygen molecules to the fullerene radical anion. In the majority of fullerene complexes that have been obtained, in particular in the complexes with tetrathiafulvalenes, charge transfer is insignificant because of the weak acceptor properties of C60 and C70. However, in a three-component system `donor radical cation ± neutral fullerene ± halide anion', fullerene compounds with radical cations of the donors can be obtained.Similar compounds can also possess conducting and magnetic properties. Three-component systems can be obtained by intercalation of fullerene complexes (with either alkali metals or halogens) or by direct synthesis in solution. In the latter case, it is probably possible to obtain single crystals of these compounds. Synthesis of complexes of chemically modified fullerenes, in particular, brominated, chlorinated and fluorinated ones, seems to be a promising direction. Unlike the C60 and C70 molecules, these derivatives have strong acceptor properties and can appa- rently yield molecular complexes and radical ionic salts with strong organic donors (e.g., tetrathiafulvalenes).The significant delocalisation of electrons in C60 upon photo- induced electron transfer results in the formation of free charge carriers and high photoconductivity. In the near future, this phenomenon can find application for the development of energy phototransducers and other devices that use photoconductivity. Therefore, the study of electron photoconduction in fullerene compounds is an important direction. 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