Propene polymerization using ansa-metallocenium ions: Catalyst deactivation processes during monomer consumption and molecular structures of the products formed

Propene polymerization using ansa-metallocene dialkyl complexes [Me ₂C(Cp)IndMMe₂] (1a; M = Zr) and (1b; M = Hf) was studied using B(C₆F₅)₃ as a stoichometric activator. The initial rates of propene uptake as well as the steady-state rates of propene consumption are dramatically increased in the presence of excess borane using 1a and to a lesser extent 1b. ¹H and ¹⁹F NMR experiments at B:Zr ratios of 1:1 and 1.2:1 reveal that the by-products produced following consumption of monomer are also sensitive to B:Zr stoichiometry. Under the former conditions, initiation is inefficient and unreacted ion-pairs [Me ₂C(Cp)IndZrMe][MeB(C₆F₅)₃] (2a-b) effect C-H activation of the unsaturated oligomers that form to produce π-allyl complexes [Me₂C(Cp)IndZr(η³-2-R-C ₃H₄)][MeB(C₆F₅)₃] (4a) [R = -(CH₂CHMe)_nCH₂CH₂CH₃] and methane; these oligomeric complexes were identified by comparison to π-allyl complexes 4b (R = Me) and 4c (R = CH₂CMe₃) prepared from ion-pairs 2a-b and either isobutene or 2,4,4-trimethylpentene. Complex 4b was structurally characterized and presents a highly distorted π-methallyl ligand. In the presence of excess borane, initiation is efficient, and the principal by-products formed are Zr-borohydride complexes Me₂C(Cp)IndZrMe(μ-H)B(C₆F₅)₃ (5a-b). The major isomer present was identified by comparison to the spectroscopic data observed for its Hf-analog 6 with could be isolated from these reactions and was structurally characterized. Complexes 5 are detected after consumption of monomer is complete whereas complex 6 was formed competitively during consumption of monomer and both are relatively unreactive towards subsequent monomer insertion. Thus, the formation of these μ-borohydride complexes is a reversible deactivation pathway for some ansa-metallocenium ions, and that excess borane may serve to reactivate them towards further insertion.