13.5.4: Fullerene Complexes

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The \(\pi\) systems of fullerenes (eg buckmisterfullerene, \(\ce{C60}\)) also act as ligands for metal complexes. Most transition metal-fullerene complexes are derived from \(C_{60}\). The structure of \(\ce{C60[IrCl(CO)(PMe3)2]2}\) is shown in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Structure of \(\ce{C60[IrCl(CO)(PMe3)2]2}\). doi=10.1021/ic00101a015. (Smokefoot, YEMVOB, CC BY-SA 4.0)

As ligands, fullerenes behave similarly to electron-deficient alkenes, and they prefer coordination to electron-rich metal centers (metal ions with softer character like those of the 4d and 5d transition metals). They almost always coordinate in a dihapto (\(\eta^2\)) fashion. The \(\eta^2\) metal binding most often occurs on the junction of two 6-membered rings, as shown for \(\ce{[[Ph3P]2Pt]6(\eta^2-C60)}\) on the left of Figure \(\PageIndex{2}\). In (\ce{Ru3(CO)9(C60)}\), the fullerene binds to the triangular face of the cluster as shown on the right in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Structures of two \(\eta^2\) fullerene complexes. Left, \(\ce{[[Ph3P]2Pt]6(\eta^2-C60)}\). Right, \(\ce{Ru3(CO)9(C60)}\). ( CMSherwood, Fullerene 4 and Fullerene 2, CC BY-SA 3.0)

Hexahapto and pentahato binding is possible, but is less common that dihapto coordination. Modification of the fullerene with phynyl substituents makes the fullerene a more electron rich ligand so that penta- and hexahapto coordination is more favorable. For example, the pentaphenyl anion, \(\ce{C60Ph5^-}\), binds to Fe in a pentahpto (\(\eta^5\)) fashion, similar to the interactions in ferrocene (Left, Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): Structures of two modified fullerenes. Left, a ferrocene-like complex of the pentaphynyl bucknisterfullerene anion (CMSherwood, Fullerene 3, CC BY-SA 3.0). Right, the structure of \(\ce{C60(OsO4)(4-t-butylpyridine)2}\), which was important for the original determination of the structure for \(\ce{C60}\)
(CC-BY-SA; Kathryn Haas).

The first X-ray structure that gave insight into the spherical structure of fullerenes was derived from an oxygen adduct of osmium tetroxide (Right, Figure \(\PageIndex{3}\)).