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

Abstract

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 × 10^3 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/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 (R_nCp)_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 σ than σ^+; ρ = −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.