869212-84-0Relevant articles and documents
Cyclopentadienyl and olefin substituent effects on insertion and β-hydrogen elimination with group 4 metallocenes. kinetics, mechanism, and thermodynamics for zirconocene and hafnocene alkyl hydride derivatives
Chirik, Paul J.,Bercaw, John E.
, p. 5407 - 5423 (2008/10/09)
Reactions of group 4 metallocene dihydrides, (RnCp) 2MH2 (RnCp = alkyl-substituted cyclopentadienyl; M = Zr, Hf), with olefins afford stable metallocene alkyl hydride complexes of the general formula (RnCp)2M(CH 2CHR′2)(H) (R′ = H, alkyl). For sterically crowded, monomeric dihydrides, Cp*2ZrH2 (Cp* = η5-C5Me5), Cp*(η5- C5Me4H)ZrH2, Cp*(η5-C 5Me4Et)-ZrH2, Cp*2HfH 2, and Cp*(η5-C5H3-1,3- (CMe3)2)HfH2, second-order rate constants for olefin insertion have been measured. For Cp*2HfH2, 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*(η5-C 5Me4H)ZrH2 is 3.8 × 1Q3 times greater than that for Cp*2ZrH2 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/kD 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 (Rn-Cp)2Zr(CH2CHR′)(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 (kH/kD = 3.9-4.5) and (b) a linear free relationship for the phenethyl hydride series Cp*(η5-C5Me4H)Zr(CH 2CH2-p-C6H4-X)(H) (X = H, CH 3, CF3, OCH3), 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*(CpRn)Zr(CH 2CHMe2)(H) and Cp*(CpRn)Zr(CH 2CH2CH2CH3)(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.
