Fullerides

Weaver J H 1992 Fullerenes and fullerides photoemission and soanning tunneling miorosoopy studies Accou/rfs. Chem. Res. 25 143-9... 

Weaver J H 1992 Eleotronio struotures of Cgg, C g and the fullerides—photoemission and inverse photoemission studies J. Phys. Chem. Solids 53 1433... 

Xie Q, Perez-Cordero E and Echegoyen L 1992 Electrochemical detection of and enhanced stability of fullerides in solution J. Am. Chem. Soc. 114 3978-80... 

Fig. 11. Composition dependence of the resistivity p x) for thick films of Cbo doped with Na, K, Rb, and Cs. Points indicate where exposure to the alkali-metal source was stopped and x-ray and ultraviolet photoemission spectra were acquired to determine the concentration x. The labels indicate the known fulleride phases at 300 K. The minima in p x) occur for stoichiometries corresponding to NaQCeo, K.iCeo and Cs,.,.Cfio[ll3].

Reduction of fullerenes to fullerides — Reversible electrochemical reduction of Ceo in anhydrous dimethylformamide/toluene mixtures at low temperatures leads to the air-sensitive coloured anions Qo" , ( = 1-6). The successive mid-point reduction potentials, 1/2, at -60°C are -0.82, -1.26, -1.82, -2.33, —2.89 and —3.34 V, respectively. Liquid NH3 solutions can also be used. " Ceo is thus a very strong oxidizing agent, its first reduction potential being at least 1 V more positive than those of polycyclic aromatic hydrocarbons. C70 can also be reversibly reduced and various ions up to... 

Chemical reduction by alkali metals leads to solid fullerides which are sometimes solvated. 

Thus, fullerides M C6o are known for n = when M = Rb, Cs and forn = 2, 3,4 and 6 when M = Na, K, Rb and Cs. An important alternative route treats Cgo in toluene with a solution of Na[Mn(tj -C5Me5)2] in thf to give an 80% yield of the dark-purple, air- and moisture-sensitive crystalline solvate NaCeo-Sthf. " ... 

Carbon forms binary compounds with most elements those with metals are considered in this section whilst those with H, the halogens, O, and the chalcogens are discussed in subsequent sections. Alkali metal fullerides and encapsulated (endohedral) metallafullerenes have already been considered (pp. 285, 288 respectively) and met-allacarbohedrenes (metcars) will be dealt with later in this section (p. 300). Silicon carbide is discussed on p. 334. General methods of preparation of metal carbides are ... 

Brouet V, Alloul H, Garaj S, Forro L (2004) NMR Studies of Insulating, Metallic, and Superconducting Fullerides Importance of Correlations and Jahn-Teller Distortions 109 165-199... 

Gunnarsson O, Han JE, Koch E, Crespi VH (2005) Superconductivity in Alkali-Doped Fullerides 114 71-101... 

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... 

Saito S, Umemoto K, Miyake T (2004) Electronic Structure and Energetics of Fullerites, Fullerides, and Fullerene Polymers 109 41-57 Sakata M, see TakataM (2004) 109 59-84... 

The appearance of free fulleride reoxidations in the backscan (starred peaks) is accounted for by the following ECE mechanism ... 

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. 

A broadening and red-shifting was observed for the 330 nm absorption and also for the 255 nm absorption [11], which can also be accounted by a diminution in symmetry [33], In the visible region, spectral changes produce the colors of the different fulleride anions, which are dark red-purple for Cgg , red-orange for and dark red-brown for [17]. 

In solid-state NMR [1,51-64], the magnetic coupling between the fullerene anions has to be taken into accoimt. In the case of metal intercalated fullerides that have metallic properties a contribution from the conduction electrons must be added, a phenomenon called the Knight shift . Even if this additional shift affects the C-chemical resonance, the correspondence between extended and discrete systems of comparable Cjq oxidation state is quite close [1]. 

X-ray structures of CgQ crystals often show disorder in the arrangement of the CgQ-anions. The first crystallographically ordered structure of a discrete fulleride was found in the dianionic salt [PPN ]2[Cgo ] [73], The first example of a highly ordered monoanion was [Ni(C5Me5)2 ][Cgo ]CS2, which was prepared by chemical reduction with decamethylnickelocene [74],... 

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. 

Fulleride anions are often more soluble, especially in more polar solvents, than the parent fullerenes. For example, in bulk electrolysis experiments with tetra-n-butylammonium perchlorate (TBACIO4) as supporting electrolyte, carried out in acetonitrile where Cjq is completely insoluble, fairly concentrated, dark red-brown solutions of 50 can be obtained [81]. Upon reoxidation, a quantitative deposition of a neutral Cjq film on the surface of a gold/quartz crystal working electrode takes place. This Cjq film can be stepwise reductively doped with TBA, leading to (Cjo )... 

Whereas Cjq is insoluble and inert in liquid ammonia without any cosolvent, the fulleride anions Cjq"" n = 1 ), generated electrochemically with KI as supporting electrolyte, dissolve completely in this polar medium [15]. Further reductions lead to the ammonia-insoluble potassium salts of the penta- and hexaanions. 

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]. 

The electrochemical generation of fulleride anions can also be used to synthesize covalent organofullerene derivatives by quenching the anions with electrophiles. This was exemplified in the synthesis of dimethyldihydro[60]fuUerene, the simplest dialkyl derivative of [87]. For this purpose benzonitrile solutions of Cjq and tetra-tert-butylammonium perchlorate (TBACIO4) where exhaustively electroreduced in a dry-box to yield a dark red solution of Treatment of this solution with an... 

---