Fullerene metal complexes

Key point: The polyhedral fullerenes undergo reversible multielectron reduction and form complexes with d-metal organometallic compounds and with OsO₄.Reasonably efficient methods of synthesizing the fullerenes have been developed, and their redox and coordination chemistry have been extensively investigated. In keeping with the forma tion of alkali metal fullerides, C60 undergoes five electrochemi cally reversible electron transfer steps in nonaqueous solvents (Fig. 14.6). These observations suggest that the fullerenes ought to serve as either electrophiles or nucleophiles when paired with the appropriate metal. One illustration of this ability is the attack of electron-rich Pt (0) phosphine complexes on C₆₀, yielding com pounds such as (4), in which the Pt atom spans a pair of C atoms in the fullerene molecule (section 22.9). This reaction is analogous to the coordination of double bonds to Pt phosphine complexes. Although analogy with 6-benzenechromium complexes (Section 22.19) suggests that a metal atom might coordinate to a sixfold face of C₆₀ , that such hexa hapto complexes do not in fact form is attributed to the radial arrangement of the C₂pπ orbitals (5), which results in them having a poor overlap with d orbitals of a metal atom centred above a sixfold face of the molecule.

In contrast to the poor interaction of a fullerene sixfold face with a single metal atom, a larger array of metal atoms, the triruthenium cluster Ru3 (CO)₁₂ , reacts to form a Ru3 (CO)₉ cap on a sixfold face of C₆₀ . In the process, three CO ligands are  

Figure 14.6 The cyclic voltammogram of C₆₀ in toluene at 25C.

displaced (6). The relatively large triangle of three metal atoms provides a favourable geometry for overlap with the radially orientated C₂pπ orbitals.

The chemical properties of C₆₀ are not limited to its interaction with electron-rich metal complexes. Reaction with a strong electrophile and oxidant, OsO₄ in pyridine, yields an oxo bridge complex analogous to the adducts of OsO₄ with alkenes (7). In addition to complexes formed with the metal atom outside the fullerene cage, endohedral fullerenes are formed in which one or more atoms are accommodated inside the C₆₀ shell. Such complexes are denoted M@C₆₀ , indicating that the M atom is inside the C60 cage. Small inert gas atoms and molecules may be driven inside the cage at high temperatures (>600C) and pressures (>2000 atm) to give, for example, H₃@C₆₀ . Alternatively, the carbon cage can be formed around the endohedral atom by using a metal-doped carbon rod in an electric arc. Larger shells, such as La@C₈₂ and La₃ @C₁₀₆ , are often formed.

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مواضيع ذات صلة

Partially crystalline carbon

Carbon clusters

Diamond and graphite

Simple compounds

Occurrence and extraction

Thermodynamic considerations

The atom and its ions

Oxidation states

Occurrence

Enthalpies of atomization

Electronegativity

Ionization energies and electron affinities