[2 + 2]-type Reaction of Metal-Metal σ-Bond with Fullerene Forming an η1-C60 Metal Complex: Mechanistic Details of Formation Reaction and Prediction of a New η1-C60 Metal Complex.

C60[CpRu(CO)2]2 is only one transition-metal fullerene complex with pure η1-coordinated bonds, which was recently synthesized through the reaction between dinuclear Ru complex [CpRu(CO)2]2 and C60. Though new properties can be expected in the η1-coordinated metal-fullerene complex, its characteristic features are unclear, and the [2 + 2]-type formation reaction is very slow with a very small yield. A density functional theory study discloses that the η1-coordinated bond is formed by a large overlap between the Ru dσ orbital and C pσ one involved in the lowest unoccupied molecular orbital (LUMO) (π*) of C60 unlike the well-known η2-coordinated metal-fullerene complex which has a π-type coordinate bond with metal dπ orbital. The binding energy per one Ru-C bond is much smaller than those of η2-coordinated Pt(PMe3)2(C60) and IrH(CO)(PH3)2(C60) because the Ru d orbital exists at low energy. The formation reaction occurs via Ru-Ru bond cleavage on the C60 surface followed by a direction change of CpRu(CO)2 to afford C60[CpRu(CO)2]2 in a stepwise manner via two asymmetrical transition states to avoid a symmetry-forbidden character. The calculated Gibbs activation energy (ΔG°‡) is very large and the Gibbs reaction energy (ΔG°) is moderately negative, which are consistent with a very slow reaction rate and very small yield. The charge transfer from CpRu(CO)2 to fullerene CT(Ru → C60) is important in the reaction, but it is small due to the presence of the Ru d orbital at low energy, which is the reason for the large ΔG°‡ and moderately negative ΔG°. The use of Li+@C60 is theoretically predicted to accelerate the reaction and increase the yield of Li+@C60[CpRu(CO)2]2, because the CT(Ru → C60) is enhanced by the low energy LUMO of Li+@C60. It is also predicted that Li+@C60[Re(CO)4(PMe3)]2 is a next promising target for the synthesis of the η1-coordinated metal-fullerene complex, but syntheses of C60[Co(CO)4]2, C60[Re(CO)5]2, Li+@C60[Co(CO)4]2, and Li+@C60[Re(CO)5]2 are difficult. The use of nonpolar solvent is another important factor for the synthesis of the η1-coordinated metal complex with Li+@C60.