Hydrogen transfer between ligands: A density functional study of the rearrangement of M(eta(6)-C7H8)(2) into M(eta(7)-C7H7)(eta(5)-C7H9) [M = Mo, Mo+, Zr]

Abstract

The electronic structure of the family of sandwich complexes M(eta(6)-C7H8)(2) and M(eta(7)-C7H7)-(eta(5)-C7H9) (M = Mo, Mo+, Zr) was investigated using density functional theory, and their geometries were accounted for. The mechanism for conversion of M(eta(6)-(CH8)-H-7)(2) into M(eta(7)-C7H7)(eta(5)-C7H9) by hydrogen transfer was investigated, and the experimental trends were reproduced. The neutral Mo species was calculated to have an activation free energy 13 kJ mol(-1) lower than the cationic Mo+. The rate-limiting step for the transfer in the neutral molybdenum species is breaking of the C-H bond to form a molybdenum hydride intermediate, which rapidly converts into the rearranged product. The transition state is late on the reaction coordinate, and C-H bond breaking occurs after ring slippage to form an (eta(6), eta(4)) 16-electron species. The cationic molybdenum species has a similar reaction profile. The zirconium species is found to have two competing pathways both with higher activations energies: one with an (eta(7), eta(5)) Zr-H intermediate, the other occurring by direct transfer of the hydrogen without forming Zr-H species. The effect of adding PH3 to the zirconium system has also been studied, which has the effect of lowering the barrier to the direct transfer of hydrogen between the ligands.