Fullerene coordination compounds

A further indication of the rapid advances that have occurred in the chemistry of the elements during the past 15 years can be gauged from the several completely new sections which have been added to review work in what were previously both nonexistent and unsuspected areas. These include (a) coordination compounds of dihapto-dihydrogen, (b) the fullerenes and their many derivatives, (c) the metcars, and (d) high-temperature oxide superconductors. 

Coordination Compounds with Transition Metals Coordination compounds formally do not count among the covalently bound structures. Still they shall be discussed in this chapter as, compared to other noncovalent variants, they are rather related to the covalently modified nanotubes. One can presume that nanotubes like fullerenes form coordination compounds with transition metals. Yet the tendency to do so is much less pronounced which is, among other reasons, due to the lack of five-membered rings. Hence, firstly, the delocalization of double bonds is less disturbed and, secondly, the energy gap between HOMO and LUMO is wider. The latter effect hinders an efficient backbonding and thus decreases the affinity of the nanotubes toward electron-rich metal systems. The cyclopenta-dienyl units as existent in fullerenes can stabilize the complexes once they come into being, whereas their absence in nanotubes hampers the formation of T -complexes. 

Bis(zinc porphyrin)-oxoporphyrinogen [(ZnTPP)2-OxP] is an example of a donor-acceptor-type triad. Excitation of the zinc porphyrin gronps leads to an efficient energy or electron transfer in the triad and this process depends on the polarity of the solvent used. Further elaboration of this system was accomplished by complexing this compound and other derivatives with fullerene-containing compounds capable of coordinating at the porphyrin zinc metal ions. The structures of the complexes are shown in Figure 14. 

In fact, it is just the unsaturated central double bond of the pyracylene unit which commonly becomes the reactive site for coordination to metal fragments. This is the case not only for [( 2-C6O) (K20,0)-Os(O4)(4-terf-butylpyridine) ], but for example also for C60[Pt(PPh3)2]25 and [Ir(CO)( 2-C6o)Cl(PPh3)2].26 In both compounds the C1-C2 bond length ( 1.50 A) is considerably longer than the value (1.38 A) that the 6 6 C = C bond should have in the absence of coordination (in the strictly related [Pd( 2-C60)(PPh3)2] the C1-C2 distance is 1.45 A27). At variance with [O72-C6o) (K20,0)-Os(O4)(4-terr-butylpyridine) ], in the latter cases the metal atom is directly linked to the fullerene unit. 

Such specific ligand systems as catenanes [968], calixarenes [969], fullerenes [970a,b], and cyclodextrines [971], and functionalized dendrimers [972a-d] are of great interest. Thus, on the basis of fullerenes C60 and C80 were obtained metal-cyclo-pentadienyl and metal-carbonylcyclopentadienyl compounds with rj1 - and r 5-coordination bonds [970b]. 

From the perspective of structural chemistry, the modes of bonding, coordination, and the bond parameters of a particular element in its allotropic modifications may be further extended to its compounds. Thus organic compounds can be conveniently divided into three families that originate from their prototypes aliphatic compounds from diamond, aromatic compounds from graphite, and fullerenic compounds from fullerenes. 

A final question to be addressed is that of the limits of the descriptor system with regard to the structures it can be applied to. In a stricter sense, it should be valid for all fullerenes and their derivatives having a cage framework that is unaltered with respect to the number of atoms and their coordination within the core. In practice, however, as various core-modified fullerenes have been synthesized,41 49 it may appear convenient to apply it to all compounds that are relatively closely related to the carbon spheres and to which fullerene nomenclature can be easily applied. 

A variety of -hydrofullerene complexes having the Re(CO)fragment coordinated to a ) -Cp like Ceo derivative has been disclosed. These compounds are made by Re2(CO)io-assisted hydrogenation of the fullerenes. ... 

The heteroatom cy clopentadienyl analog 1,4,2-P2 SbC2 Bu 2 has been coordinated to Tl(l) via equation (2), producing zigzag polymer (2). Thallium coordination has played an important role in crystallographic characterization of metal-fullerene complexes, providing the first structurally characterized pentahapto metal complexes of Ceo (3) and C70 (4). The key to the formation of these monothallium(l) compounds was the arylation of the fullerenes. Methylated Ceo forms a thallium complex as well. ... 

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