AGOSTIC ZR...C-BETA-SI INTERACTIONS IN CP'2ZR(CH2CH2SIME3)(L)+ COMPLEXES (L = THF, CH3CN, PME3) - EVIDENCE FOR AN ORGANOMETALLIC BETA-SILICON EFFECT

The reaction Of [CP'2Zr(H)(THF)][BPh4] (Cp' = C5H4Me) with vinyltrimethylsilane yields [Cp'2Zr(CH2CH2SiMe3)(THF)][BPh4] (3), which has been characterized by spectroscopic methods and single-crystal X-ray diffraction. Crystal data for 3: space group Pna2(1), a = 17.188(4) angstrom, b = 14.199(4) angstrom, c = 16.170(10) angstrom, V = 3930.3(4.5) angstrom3, Z = 4, R = 0.057, R(w) = 0.079. The alkyl group of 3 is highly distorted (Zr-Ca-Cbeta (84(1)-degrees), Zr-Cbeta 2.57(2) angstrom); however, the SiMe3 group lies in the O-Zr-Calpha-Cbeta plane, suggesting that strong agostic Zr...Hbeta interactions are absent. The alkyl group distortion of 3 is proposed to result from overlap of the back lobe of the Cbeta-Si bond with the Zr LUMO. This direct Zr...Cbeta-Si interaction is analogous to the stabilization of silyl-substituted carbocations by the ''gamma-silicon effect'' and appears to be stronger than the Zr...Hbeta agostic interactions observed previously for Cp'2Zr(CH2CH2R)(L)+ alkyls. Reaction of 3 with PMe3 and CH3CN yields [Cp'2Zr(CH2CH2SiMe3)(PMe3)][BPh4] (5) and [Cp'2Zr(CH2CH2SiMe3)(CH3CN)][BPh4] (6, two isomers), which, on the basis of spectroscopic parallels with 3, are assigned analogous Zr...Cbeta-Si structures. 3, 5, and 6 are resistant to beta-H elimination; however, 6 undergoes CH3CN insertion yielding CP'2Zr(N=C(CH2CH2SiMe3)(CH3)}(L)+ (7, L = CH3CN; 8, L = PMePh2, two isomers).