Metal Cation−Methyl Interactions in CB₁₁Me₁₂^- Salts of Me₃Ge⁺, Me₃Sn⁺, and Me₃Pb⁺

Oxidation of Me₆M₂ (M = Ge, Sn) and Me₄Pb with the CB₁₁Me₁₂^• radical in alkane solvents produced the insoluble salts Me₃M⁺CB₁₁Me₁₂^-, characterized by CP-MAS NMR and EXAFS. The cations interact with methyl groups of CB₁₁Me₁₂^- with coordination strength increasing from Pb to Ge. Density functional theory (DFT) calculations for the isolated ion pairs, Me₃M⁺CB₁₁Me₁₂^- (M = Ge, Sn), revealed three isomers with the cation above methyl 2, 7, or 12, and not above a BB edge or a BBB triangle. The interaction has a considerable covalent component, with the cation attempting to perform a backside S_E2 substitution on the methyl carbon. In a fourth less favorable isomer the cation is near methyl 1, inclined toward methyl 2, and interacts with hydrogens. DFT atomic charge distributions and plots of the electrostatic potential on the surface of spheres centered at the CB₁₁H₁₂^- and CB₁₁Me₁₂^- icosahedra display the effects of uneven charge distribution within the anion and contradict the common belief that the negative charge of the cage anion is concentrated primarily on the cage boron atoms 7−12; in CB₁₁Me₁₂^-, roughly half is on the cage carbon and the rest on methyls 7−12.