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

Reactions of group 4 metallocene dihydrides, (RnCp)(2)MH2 (RnCp = alkyl-substituted cyclopentadienyl; M = Zr, Hf), with olefins afford stable metallocene alkyl hydride complexes of the general formula (RnCp)(2)M(CH2CHR'(2))(H) (R' = H, alkyl). For sterically crowded, monomeric dihydrides, Cp*2ZrH2 (Cp*=eta(5)-C5Me5), Cp*(eta(5)-C5Me4H)ZrH2, Cp*(eta(5)-C5Me4Et)ZrH2, Cp*2HfH2, and Cp*(eta(5)-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*(eta(5)-C5Me4H)ZrH2 is 3.8 x 10(3) times greater than that for CP*2ZrH2 at -63 degrees 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 degrees C and a linear free energy correlation to sigma (p = -0.46(l)) 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 beta-carbon of the inserting styrene. The rates of beta-H elimination for the series (R-n-CP)(2)Zr(CH2CHR')(H) have been measured via rapid trapping of the intermediate zirconocene dihydride with 4,4-dimethyl-2-pentyne. Key observations for beta-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*(eta(5) -C5Me4H)Zr(CH2CH2-P-C6H4-X)(H) (X = H, CH3, CF3, OCH3 which correlates better to sigma than sigma(+); p = -1.80(5). The rate of beta-H elimination slows with more substituted, hence more sterically crowded, cyclopentadienyl ligands. Equilibration of a series of Cp*(CpRn)Zr(CH2CHMe2)(H) and Cp*(CpRn)Zr(CH2CH2CH2CH3)(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 beta-H elimination, allow free energy profiles to be constructed for insertion and beta-H elimination for each olefin.