CATIONIC ZIRCONIUM AND HAFNIUM ISOBUTYL COMPLEXES AS MODELS FOR INTERMEDIATES IN METALLOCENE-CATALYZED PROPYLENE POLYMERIZATIONS - DETECTION OF AN ALPHA-AGOSTIC INTERACTION IN (C5ME5)2HF(CH2CHME2)(PME3)+

A series of cationic Zr and Hf isobutyl complexes has been prepared to model the Cp2M{CH2CH(R)(R')}+ and Cp2M{CH2CH(R)(R')}(alpha-olefin)+ intermediates in metallocene-catalyzed alpha-olefin polymerizations (R' = growing poly(alpha-olefin) chain). The cationic hydride [Cp'2-Zr(H)(THF)][BPh4] (5, Cp' = C5H4Me) reversibly inserts isobutylene at 23-degrees-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 beta-agostic structure and undergoes beta-H elimination above 13-degrees-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 alpha-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)degrees) but that Hf...H(alpha) agostic interactions are absent. Crystal data for 14: space group P2(1)/c, a = 10.747(3) angstrom, b = 21.417(3) angstrom, c = 21.783(8) angstrom, beta = 101.83(4)degrees, V = 5365(6) angstrom3, Z = 4, R = 0.053, R(w) = 0.086. 14 undergoes predominant beta-Me elimination at 58-degrees-C. The structures of Cp'2Hf((n)Bu)(L)+ (17, L = THF, normal butyl group; 19, L = PMe3, beta-agostic butyl group) are analogous to those of the corresponding Cp'2Zr((n)Bu)(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.