Reactions of group 4 metallocene dihydrides, (R nCp) 2MH 2 (R nCp = alkyl-substituted cyclopentadienyl; M = Zr, Hf), with olefins afford stable metallocene alkyl hydride complexes of the general formula (R nCp) 2M(CH 2CHR′ 2)(H) (R′ = H, alkyl). For sterically crowded, monomeric dihydrides, Cp* 2ZrH 2 (Cp* = η 5-C 5Me 5), Cp*(η 5- C 5Me 4H)ZrH 2, Cp*(η 5-C 5Me 4Et)-ZrH 2, Cp* 2HfH 2, and Cp*(η 5-C 5H 3-1,3- (CMe 3) 2)HfH 2, second-order rate constants for olefin insertion have been measured. For Cp* 2HfH 2, the relative rates of olefin insertion have been found to be 1-pentene> styrene ≫ cis-2-butene > cyclopentene > trans-2-butene > isobutene. The rate of isobutene insertion into Cp*(η 5-C 5Me 4H)ZrH 2 is 3.8 × 1Q 3 times greater than that for Cp* 2ZrH 2 at -63°C, demonstrating the striking steric effect for isobutene insertion imposed by a tenth methyl substituent on the two cyclopentadienyl ligands. A primary k H/k D of 2.4(3) at 23°C and a linear free energy correlation to σ (ρ = -0.46(1)) for para-substituted styrene insertion indicate that insertion into a Zr-H bond proceeds via rate-determining hydride transfer to coordinated olefin, with small positive charge buildup at the β-carbon of the inserting styrene. The rates of β-H elimination for the series (R n-Cp) 2Zr(CH 2CHR′)(H) have been measured via rapid trapping of the intermediate zirconocene dihydride with 4,4-dimethyl-2-pentyne. Key observations for β-H elimination are (a) primary kinetic deuterium isotope effects (k H/k D = 3.9-4.5) and (b) a linear free relationship for the phenethyl hydride series Cp*(η 5-C 5Me 4H)Zr(CH 2CH 2-p-C 6H 4-X)(H) (X = H, CH 3, CF 3, OCH 3), which correlates better to o than σ +; p = -1.80(5). The rate of β-H elimination slows with more substituted, hence more sterically crowded, cyclopentadienyl ligands. Equilibration of a series of Cp*(CpR n)Zr(CH 2CHMe 2)(H) and Cp*(CpR n)Zr(CH 2CH 2CH 2CH 3)(H) with free isobutene and 1-butene has established the relative ground-state energies of isobutyl and n-butyl complexes. These data, in combination with the free energies of activation for β-H elimination, allow free energy profiles to be constructed for insertion and β-H elimination for each olefin.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry