Metal Cation−Methyl Interactions in CB11Me12- Salts of Me3Ge+, Me3Sn+, and Me3Pb+
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Abstract
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 SE2 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.
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