Models of key intermediates in metallocene catalyzed alkene polymerizations: Zirconium-alkyl-alkene chelates

The β-allyl zirconacyclobutane complex Cp*₂Zr[CH ₂CH(CH₂CH=CH₂)CH₂] reacted rapidly with B(C₆F₅)₃ in CD₂Cl₂ at −78°C to form the zwitterionic d0 zirconium(IV)-alkyl-alkene chelate complex Cp*₂Zr[η1,η2-CH ₂CH[CH₂B(C₆F₅)₃]CH ₂CH=CH₂]. Low-temperature 1H, 13C, TOCSY1D, and NOESY1D NMR spectroscopy established the bonding of the tethered alkene to the d0 metal center. A dynamic NMR study of it's two diastereomers allowed measurement of the alkene dissociation energy (ΔG ‡ = 10.5 kcal mol−1) but the complex decomposed before the barrier for site epimerization at the zirconium center could be determined. Protonation of Cp*₂Zr[CH₂CH(CH₂CH=CH ₂)CH₂] with [(C₆H₅)₂(CH ₃)NH][B(C₆F₅)₄] led to the formation of two isomeric d0 zirconium(IV)-alkyl-alkene chelates Cp* ₂Zr[η1,η2-CH₂CH(CH ₃)CH₂CH=CH₂][B(C₆F₅) ₄]. This more thermally stable zirconium-alkyl-alkene complex allowed the measurement of barriers associated with decomplexation of the alkene (ΔG ‡ = 10.7 kcal mol−1) and site epimerization at the zirconium center (ΔG‡ = 14.4 kcal mol −1) by line shape analysis of variable temperature 1 H and 13C NMR spectra.; The binding energy of a 1,1-disubstituted alkene to the cationic d 0 zirconium center in [Cp*₂Zr{lcub}η1,η 2-CH₂CH(CH₃)CH₂C(CH₂CH ₃)=CH₂{rcub}][B(C₆F₅)₄] ( A) was determined to be ΔH° = 3.9 kcal mol−1 by direct observation of the equilibrium between alkene bound and alkene free complexes. The alkene binding energy of a monosubstituted alkene in Cp*₂Zr[η1,η2-CH₂ CH(CH₃)CH₂CH=CH₂][B(C₆F ₅)₄], (B) was determined indirectly by comparing the relative binding energies of tetrahydrofuran to both A and B. THF binds 2.0 kcal mol−1 more tightly to A than to B. The alkene binding energy of A is therefore ΔH° = 5.9 kcal mol−1.