Cationic Zirconium and Hafnium Isobutyl Complexes as Models for Intermediates in Metallocene-Catalyzed Propylene Polymerizations. Detection of an α-Agostic Interaction in (C5Me5)2Hf(CH2CHMe2) (PMe3)+

Abstract

A series of cationic Zr and Hf isobutyl complexes has been prepared to model the Cp2M{CH2- CH(R)(R′)}+ and Cp2M{CH2CH(R)(R′)}(α-olefin)+ intermediates in metallocene-catalyzed α-olefin polymerizations (R′ = growing poly(α-olefin) chain). The cationic hydride [Cp′2Zr- (H)(THF)][BPh4] (5, Cp′ = C5H4Me) reversibly inserts isobutylene at 23 °C to afford [Cp′2- Zr(CH2CHMe2)(THF)] [BPh4] (6), in which the isobutyl group adopts a normal structure. The reaction of 6 with PMe3 yields [Cp′2Zr(CH2CHMe2) (PMe3)] [BPh4] (7) which adopts a β-agostic structure and undergoes β-H elimination above −13 °C. The reaction of the hafnacyclobutane Cp*2Hf(CH2CHMeCH2−) (10) with [HNBu3] [BPh4] in the presence of PMe3 yields [Cp*2Hf- (CH2CHMe2) (PMe3)] [BPh4] (12). NMR data, including isotope perturbation of resonance results for deuterium labeled analogues, establish that the isobutyl group of 12 is distorted by an α-agostic interaction. The reaction of 12 with THF, and the reaction of 10 with [HNBu3] [BPh4] in THF yield [Cp*2Hf(CH2CHMe2)(THF)] [BPh4] (14). Solution NMR data, solid state IR data, and X-ray crystallographic results establish that the isobutyl group of 14 is distorted (Hf‒C‒C angle 137.5(8)°) but that Hf…Hα agostic interactions are absent. Crystal data for 14: space group P21/c, a = 10.747(3) Å, b = 21.417(3) Å, c = 21.783(8) Å, β = 101.83(4)°, V = 5365(6) Å3, Z = 4, R = 0.053, Rw = 0.086. 14 undergoes predominant β-Me elimination at 58 °C. The structures of Cp′2Hf(nBu)(L)+ (17, L = THF, normal butyl group; 19, L = PMe3, β-agostic butyl group) are analogous to those of the corresponding Cp′2Zr(nBu)(L)+ complexes. The structures and reactivity of 6, 7, 12, and 14 are rationalized in terms of the steric and electronic properties of the C5R5− and L ligands. © 1994, American Chemical Society. All rights reserved.