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Propene polymerization using ansa-metallocene dialkyl complexes [Me2C(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 [Me2C(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-C3H4)][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. |
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