Alkali Metal Fullerides

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 ... 

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. 

Synthesis and Properties of Alkali Metal Fulleride Solids... 

The discovery of superconductivity [105] in alkali metal doped attracted the attention of a broad cross-section of the scientific community. The composition of the first alkali metal doped fullerene was determined to be [106,107]. In this compoimd, the transition to a superconducting state occurs at = 19.3 K [106]. In a short time, a large variety of alkali metal fullerides M Cgo have been synthesized and studied with respect to their solid-state properties [1, 106-114]. Thereby, the stoichiometries of as well as the alkali metal with M = Na, K, Rb, Cs have... 

This direct method is a convenient route, especially for saturation-doped fullerides MgCgo-A facile control of the alkali metal fulleride stoichiometry of M Cgg (M = K,... 

Alkali metal doping of Cjq is also possible by solution-phase techniques [1,118-121]. K CgQ and Rb CgQ containing small fractions of the superconducting M3C50 phases were prepared by allowing toluene solutions of Cjq to react with the alkali metal [118, 119]. During the reaction, the alkali metal fullerides form a black precipitate. In another example, sonication of a solution of Cjq and excess potassium in TMEDA yields K3C5q(THF)j4 with a defined stoichiometry [104],... 

The occurrence of superconductivity [112-114] in alkali metal fullerides always requires the trianionic state of C50. In this case, the conduction band derived from the orbitals is half filled. This gives rise to the maximum conductivity of MjCjq... 

Alkali metal fullerides are not organometallic compounds in the normal sense, but their chemistry is in a number of respects similar to that of... 

The stractures of the alkali metal fullerides may be considered to be intercalation compounds of the fee lattice of the Ceo host formed by filhng the interstitial sites. Superconductivity has been investigated in detail for the alkali metal and mixed alkah metal phases of composition AgCeo where the conduction band is half filled. Critical temperatures as high as 31.3 K (in Rb2CsCeo) have been observed. A clear correlation between unit cell size and Tc is observed, and it has been suggested that the conduction bandwidth controlled by the separation between the molecules is the dominant factor in determining Tc. 

Gd + C6o-[CCOOH]io used as MRI contrast agents,l35] (b) the ionic unit cell for the superconductive alkali metal fulleride CsxRbj,C6o, and (c) the conjugate structure of Cgg covalently bound to the lung cancer drug PaclitaxelJs l... 

F191a R. M. Fleming et al., Preparation and Ko92b Structure of the Alkali-Metal Fulleride A4C60, Nature 352, 701-703 (1991) ... 

D. W. Murphy et ai, Synthesis and Characterization of Alkali Metal Fullerides AjjCeo, ... 

Ta92b K. Tanigsdci ei al, Superconductivity in Sodium- and Lithium-Containing Alkali-Metal Fullerides, Nature 356, 419-421... 

The endohedral metallofullarenes just described (and the alkali metal fullerides described on p. 285) are all formally examples of metal carbides, M cCy, but they have entirely different structure motifs and properties from the classical metal carbides and the more recently discovered metallacarbohedrenes (metcars) on the one hand (both to be considered in Section 8.4) and the graphite intercalation compounds to be discussed in Section 8.3. Before that, however, we must complete this present section on the various forms of the element carbon by describing and comparing the chemical properties of the two most familiar forms of the element, diamond and graphite. 

Apart from electrochemical oxidation and reduction, the fuUerenes electron affinity can also be used in wet-chemical processes. The formation of alkali-metal fullerides is among the most important reactions with reducing agents. Here, the respective elemental metal and C ) are reacted in solid phase, for example, by heating in a closed ampoule. The stoichiometry of the resulting products is controlled by the ratio of fuUerene and metal ... 

Mixed alkali-metal fullerides can also be obtained in this fashion ... 

An interesting offshoot in the context of carbon-based nanomaterials is the role of external perturbations in modulating their physical and chemical characteristics. These perturbations can include cations or neutral atoms. In this context, we examined the magnetic properties of exohedral fullerenes of alkali metal fullerides (Aj,-C6o. A = Na, K, Rb, Cs) [148-150] and the spin properties of endohedral fullerenes (A C6o, A = N, P, As, O, S) [128,151] (Fig. 34.11). The most interesting aspect of the experimental... 

Other derivatives of [60]fullerene that have been the subject of substantial attention are the alkali-metal fullerides - mainly since the discovery of superconductivity in K3C60 (66). Synthesis of these systems usually involves combination of stoichiometric quantities of alkali metal and Ceo- Solid-state synthesis required long reaction times, however - typically several days. Use of a microwave-induced argon plasma led to reaction times in the order of seconds [68]. 

It has been shown that fullerenes or their derivatives can exhibit very interesting chemical, electrical, magnetic, and mechanical properties. Besides, for example, the superconductivity, which has been experimentally verified for the alkali metal fullerides M3C60 with M = K or Rb [77], fullerenes can serve as a starting material for diamond synthesis [78,79] and may exhibit high hardness themselves under high pressure conditions [80]. 

The alkali metal fullerides, M3C60 (M = Na, K, Rb, and Cs), are a little different. The absence of the sign indicates that these are not endohedral fullerenes. Take K3C6O a representative example. It is prepared from stoichiometric amounts of solid C o potassium vapor. The three potassium atoms transfer their valence electrons to the fuUerene so that K3C60 more accurately written as (K )3(C o ) This special compound becomes a superconductor below the critical temperature, 7, of 19.3 K. (Below the 7, a superconductor is perfectly conducting—that is, it has zero resistance.) It has a face-centered cubic structure of C o anions with potassium cations in both the octahedral and tetrahedral holes as shown in Figure 15.9. (See p. 173 for more details on the positions and numbers of these holes.)... 

Fleming RM, Rosseinsky MJ, Ramirez AP, Murphy DW, Tully JC, et al. 1991. Preparation and strueture of the alkali-metal fulleride A4c60. Nature 352 701-703. 

---