Fulleride salts

Margadonna S, Iwasa Y, Takenobu T, Prassides K (2004) Structural and Electronic Properties of Selected Fulleride Salts 109 127-164 Maseras F, see Ujaque G (2004) 112 117-149 Mattson WD, see Rice BM (2007) 125 153-194... 

The first chemical transformations carried out with Cjq were reductions. After the pronounced electrophilicity of the fullerenes was recognized, electron transfer reactions with electropositive metals, organometallic compounds, strong organic donor molecules as well as electrochemical and photochemical reductions have been used to prepare fulleride salts respectively fulleride anions. Functionalized fulleride anions and salts have been mostly prepared by reactions with carbanions or by removing the proton from hydrofullerenes. Some of these systems, either functionalized or derived from pristine Cjq, exhibit extraordinary solid-state properties such as superconductivity and molecular ferromagnetism. Fullerides are promising candidates for nonlinear optical materials and may be used for enhanced photoluminescence material. 

Reductive transformations of fullerenes have not only been carried out to prepare fulleride salts. The fulleride ions themselves are reactive species and easily undergo subsequent reactions, for example, with electrophiles. Therefore, the anions provide a valuable synthetic potential for fullerene chemistry. 

The possibility of electrochemically producing CgQ anions in a defined oxidation state by applying a proper potential can be used to synthesize fulleride salts by electrocrystallization [39, 75-80]. An obvious requirement for this purpose is the insolubility of the salt in the solvent to be used for the electrocrystallization process. This can be achieved by choosing the proper solvent, the oxidation state of Cjq and the counter cation, which usually comes from the supporting electrolyte. 

The formation of crystalline fulleride salts at the electrode occurs when less polar solvents and bulky cations are used for the electrosynthesis. The first fulleride salt was synthesized by Wudl by bulk electrolysis of in o-dichlorobenzene with tetraphenylphosphonium chloride as supporting electrolyte [39, 80]. This black microcrystalline material with the composition (Ph4P )3(Cgg )(Cr)2 exhibits an ESR line with a g-value of 1.9991 and a line width of 45 G at room temperature. Single crystals of the slightly different salts (Ph4P )2(Cgg )(Cr) and (Ph4P )2(C50 )(Br ) could be obtained by electrocrystallization and their crystal structure was determined [82, 83]. Magnetic measurements showed the presence of unpaired spins. 

Another semiconducting fulleride salt, [Ru(bpy)3](C5o)2 with bpy = 2,2 -bipyridine, crystallizes on the Pt electrode surface out of dichloromethane solutions saturated with [Ru(bpy)3]PF5 within a few minutes [79]. The NIR spectra of benzonitrile solutions of this salt demonstrate that the only fulleride anion present is 55 . The temperature dependence of the conductivity is typical for a semiconductor, with the room temperature conductivity being 0.01 S cm and the activation energy 0.1 kj mol (0.15 eV). It was postulated that there is an electronic overlap between the two ions of this salt leading to a donation of electron density from the 55 to the ligand orbitals in the [Ru(bpy)3] " AI 0.7) [79]. 

Takenobu T, Muro T, Iwasa Y, Mitani T (2000) Antiferromagnetism and phase diagram in ammoniated alkali fulleride salts. Phys Rev Lett 85 381-384... 

R = H or C02Et and R = H, C02Et, 7-butyl, or CN) [164]. The experimental method to produce these derivatives elec-trosynthetically has been described in detail elsewhere [[83] and references therein], and the same procedure has been used more recently to prepare the C70 derivative C7o(C6H5CH2)2 [165]. In addition, several (inorganic) fulleride salts have also been obtained using electrosynthetic and electrocrystallization methods. These methods are also described in a recent review [166]. 

Structural and Electronic Properties of Selected Fulleride Salts... 

Fig. 7 a Magnetic moment versus temperature for a Q0 fulleride salt ([PPN] [C60].PhCN.THF (solidsymbol)) and for a C q fulleride salt ([PPN]3[C60].2CH3CN (open symbol)), b A representative ESR spectrum of Cfo showing the three typical signals. The spectrum was recorded at 85 K for the [PPN+] salt in benzonitrile. (From [9])... 

Direct evidence for a JTD in this phase has, however, been hard to establish. A thorough x-ray investigation has been performed for Rb4C60, which sets a maximum of 0.04 A for the anisotropy between the diameters of the molecule in different directions [41]. This error bar is of the order of magnitude of the distortion observed in fulleride salts, so it is not very conclusive in itself. The fact that no superstructure is observed in this study further means that JTD, if present, do not adopt a long-range order. Consequently, the insulating state is unlikely to result from a band property, like a gap opened by a commensurate cooperative distortion. The JTD is more likely dynamic, like in the Mott-JT scenario. 

In this review, we have argued that the metallicity is intimately connected to the presence of JTD. We could speculate that the population of many different JTD states at high temperatures introduces a form of disorder that could trigger a metal-insulator transition of the Anderson type. It reveals conversely that cooperation is required between JTD and the electronic motion to establish coherent band-like properties. The transition at 250 K in K4C60 shows that the nature of the distortion can change, a situation close to that of the fulleride salt. A similar transition in metallic compounds might be responsible for the de-stabilization of the metallic state. 

Doping of fullerenes proceeds similar to the doping of polymers. Electron donors are intercalated to the rather large octahedral and tetrahedral interstitial sites of the fee lattice and fill the originally LUMO derived band of the semicondeting pristine compound with electrons. The resulting materials are fullerides, salts of the fullerites. Basic information on the structure and properties of fullerites and fullerides can be obtained from the special... 

Following Jahn-Teller Distortions in Fulleride Salts by Optical Spectroscopy... 

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