Alkyl rearrangement processes in organozirconium complexes. Observation ofinternal alkyl complexes during hydrozirconation

Isotopically labeled alkyl zirconocene complexes of the form (CpRn)(2)Zr(CH 2CDR'(2))(X) (CpRn = alkyl-substituted cyclopentadienyl; R' = H, alkyl grou p; X = H, D, Me) undergo isomerization of the alkyl ligand as well as excha nge with free olefin in solution under ambient conditions, Increasing the s ubstitution on the Cp ring results in slower isomerization reactions, but t hese steric effects are small. In contrast, changing X has a very large eff ect on the rate of isomerization. Pure sigma-bending ligands such as methyl and hydride promote rapid isomerization, whereas pi-donor ligands inhibit beta-H elimination and hence alkyl isomerization. For (eta(5)-C5H5)(2)Zr(R) (Cl), internal alkyl complexes have been observed for the first time. The r ate of isomerization depends on the length of the alkyl group: longer alkyl chains (heptyl, hexyl) isomerize faster than shorter chains (butyl). The t ransient intermediate species have been identified by a combination of isot opic labeling and H-1, H-2, and C-13 NMR experiments. The solid-state struc ture of the zirconocene cyclopentyl chloride complex, Cp2Zr(cyclo-C5H9)(Cl) , has been determined by X-ray diffraction.