Synthesis and Spectroscopic Characterization of the d~0 Transition Metal-Alkyl-Alkene Complex Cp~*_2YCH_2CH_2C(CH_3)_2CH=CH_2

摘要

Coordination of an alkene to an electron-deficient metal— alkyl complex has been proposed as a requisite chain propagation step in Ziegler—Natta alkene polymerization. Calculations indicate that subsequent alkyl migration to the coordinated alkene via a concerted four-center transition state to generate a growing polymer chain has a very low activation barrier. The d~0 M(alkyl)(alkene) intermediates of group 3, group 4, and lanthanide metals are particularly unstable due to the lack of d-π~* back-bonding to the coordinated alkene. There are only a few examples of d~0 transition metal—alkene complexes, and none possess an additional metal-alkyl bond. Jordan recently reported the X-ray crystal structure of the zirconium(IV) pent-4-enyloxo chelate complex [Cp_2ZrOC(CH_3)_2CH_2CH_2CH=CH_2]~+ [MeB(C_6F_5)_3]~- (1), which was designed to model a Cp_2Zr-(alkyl)(alkene)~+ intermediate. Here we report the synthesis and spectroscopic characterization of the first d° transition metal-alkyl-alkene complex.