Fullerene alkali metal salt

Fullerenes, Cxy, have internal cavities that can act as containers for metal atoms. The metal reduces Cxy and is trapped in a fullerene cage (Saalfrank 1996). For example, Buck-minsterfullerene C6o can react with two potassium atoms, producing a dianion diradical that is a triplet in its ground state. Alkali metal salts of Buckminsterfullerene C6o possess the highest superconductivity transition temperatures (Tc) to date for organic materials, e.g., K3C60 with Tc = 19 K, Rt Ceo with Tc = 29 K, and Cs2RbC60 with / , 33 K (Haddon... 

Localization of the unpaired electrons at definite sites on the fullerene internal sphere is a promising property of these alkali metal salts. 

Addition of complex alkali metal salts to a high temperature system leads to negative ion formation in the vapour. KCMS-IME can be applied to electron affinities and the estimation of negative ion enthalpies of formation. The EA of fluorides and oxides of transition metals in their higher oxidation states were determined. Almost all of these compounds have EA in the range 3.5-7.0 eV. Application of this method to fullerene vapours has yielded the electron affinities for the series of higher fuller-enes C (n=70, 72. 106). 

Alkali metal salts of fullerenes C ) (M ), were also tested as initiators for polymerization in polar solvents. Only the hexa-anion was able to initiate reactive monomers like methyl methacrylate, but not non-polar monomers hke styrene. In all cases, the initiation proceeds through electron transfer to the monomer so that no fullerene is incorporated [95]. The reduced fullerenes initiate polymerization like classical radical anions or dianions of aromatic or conjugated molecules and therefore this synthetic route is of limited interest. 

It was found that the intercalation of Cgo fullerene by an alkali metal in stoichiometric ratio (1 1) gives rise to the formation of anion-radical salts, namely, KC50, RbCgg, and CsCgo (Bommeli et al. 1995, Btouet et al. 1996). On slow cooling of the intercalation products, [2 + 2] cycloaddition of the fullerene species that is neighboring a crystal lattice occurs. Linear chain fullerenic polymers are formed. These polymers are stable in air, insoluble in THF, and possess metallic conductivity. They depolymerize only on heating above 320°C. 

Interestingly, if the C50 fullerene doped by alkali metals is rapidly cooled down to the liquid nitrogen temperature, polymerization does not occur. Only monomeric anion-radical salts are obtained. Warming up these monomers to 80-160 K results in dimerization polymerization does not take place. The dimer (KCgo)2 is dielectric (Pekker et al. 1995). It has been shown that the tris(anion)-radical Cgo can polymerize too. Particularly, Na2CsCgo forms a polymer that maintains superconducting properties (Mizuki et al. 1994). 

Fullerenes can be easily chemically reduced by the reaction with electropositive metals [1, 97-99], for example, alkali- and alkaline earth metals. The anions Cjq"" (n = 1-5) can be generated in solution by titrating a suspension of in liquid ammonia with a solution of Rb in liquid ammonia [100], whereupon the resulting anions dissolve. Monitoring of this titration is possible by detecting the characteristic NIR absorption of each anion by UV/Vis/NIR spectroscopy. The solubility of the alkali metal fullerides in the polar solvent NHj demonstrates their salt character. 

The electrochemical method is also used for the synthesis of fullerene derivatives, among them C6o fullerene salts with alkali metals crystallized at the cathode [11, 12]. No evidence on the electrochemical deposition of fullerenes on the electrodes from organic solvents is available although this method for producing fullerene coatings on metals is of indubitable practical interest. 

The room-temperature conductivities of these compounds are usually about one or two orders of magnitude smaller than those shown in Table 12.2. Included in Table 12.3 are the superconducting (but very air-sensitive) alkali metal and alkaline earth fullerides these are compounds with three-dimensional superconductivity, where the alkali metal ions are just gegenions tucked in tetrahedral and octahedral holes in the cubic fullerene crystal structure [36]. The critical temperatures of ET salts seem to be stuck at... 

For all the prototypes of fullerene derivatives depicted in Fig. 1 several, in some cases a large number, of examples have already been realized (Fig. 1). Among the salts the superconductors M3C60 (M is, for example, an alkali metal) are the most prominent representatives [8, 9]. The first Qq derivative with an uncovered orifice was the ketolactam 1 [ 10]. The endohedral fullerene N Qo (2)... 

The alkali metals act here as donors, which make a metal of the semiconductor Ceo as acceptor, with an energy band gap of ca. 2.3 eV via half-filling of the conduction band. The system behaves similarly to the radical-anion salts which we have already treated (Sects. 9.2 and 9.3). Ceo is a good electron acceptor. It consists entirely of carbon atoms and is thus not actually an organic molecule. Superconducting Fullerene salts however have properties which are like those of the organic molecular salts. These are above all the important role played by the Jt electrons in charge transport, and the existence of relatively narrow bands with a low electron density. 

The chemistry of fullerenes is proving to be even more fascinating than their synthesis. Fullerenes have a high electron affinity and readily accept electrons from alkali metals to produce a new metallic phase—a buckide salt. One such salt, KsCgO) is a stable metallic crystal consisting of a face-centered-cubic structure of buckyballs with a potassium ion in between it becomes a superconductor when cooled below 18 K. Fullerenes have even been synthesized that have metal atoms in the interior of the carbon atom cage. 

In solution, the reduction of fullerenes is typically performed in etheral solvents (e.g., tetrahydrofuran, dimethoxyethane) [138] or liquid ammonia [139]. Using Li as a reducing agent it is possible to reach the highest reduction step, the hexa-anion. With the other alkali metals this was observed only when naphthalide salt was added [140]. The reduction of C o and all the higher fullerenes to their hexa-anions was first made possible by sonication with excess Li [16] and later by adding a small amount of 2 as an electron shuttle (vide supra). 

Very similar results were obtained from the CV studies of ( )-38 and ( )-39, but the observed anodic shifts of the first redox couples upon complexation with K+ were smaller (50 mV for ( )-38 and 40 mV for ( )-39). The reduction of the anodic shift from 90 mV (in ( )-37) to 40 mV (in ( )-38) can be explained by an increasing average distance between the cation bound to the crown ether and the fullerene surface, as the addition pattern changes from trans-1, to trans-2, and to trans-3 [55], Additionally, the effects of different alkali- and alkaline-earth-metal ion salts on the redox properties of ( )-37 were investigated. As expected, all electrochemical data clearly demonstrate a much larger interaction between crown-ether-bound cations with the negatively charged than with the neutral fullerene core [55],... 

---