Ionically dissociative hydrocarbons containing the C-60 skeleton

The coordination of tert-butylfulleride ion (t-BuC60-) with tropylium (la(+)), 1,4-di-tert-butyltropylium (1b(+)), and 1,4-dicyclopropyltropylium (1c(+)) ions yielded 1-tert-butyl-4-(3,6-R-2-2,4,6-cycloheptatrien-1-yl)-1,4-dihydrofullerenes (2a: R = H, 2b: R = l-Bu, 2c: R = cyc-;Jr) in isomerically pure form. The structures of 2a-c were determined from the H-1 and C-13 NMR spectra, which clearly indicate the absence of symmetry in the molecules, and from H-1 NMR NOE experiments. Spectrophotometric studies showed that 2a-c are unique hydrocarbons that regenerate t-BuC60- rind 1(+) in polar solvents by heterolysis of a carbon-carbon sigma bond. Although this facile heterolysis is due principally to the high thermodynamic stabilities of both t-BuC60- and 1a-c(+), the free energies of heterolysis are considerably lower than predicted from pK(HA) of t-BuC60- and pK(R+) of 1a-c(+) using Arnett's master equation. The results suggest that steric repulsion between the 1-tert-butyl group and the cycloheptatrienyl group on the surface of C-60 plays an important role in enhancing the heterolysis of 2a-c. (C) 1997 Elsevier Science Ltd.