Proximity and cooperativity effects in binuclear d ⁰ olefin polymerization catalysis. Theoretical analysis of structure and reaction mechanism

This contribution focuses on the distinctive center-to-center cooperative catalytic properties exhibited by bimetallic "constrained geometry catalysts" (CGCs), and analyzes metal-metal proximity effects on ethylene polymerization processes mediated by (μ-CH ₂-3,3′) {(η ⁵-indenyl)[1-H ₂Si( ^tBuN)](ZrMe ₂)} ₂ (Zr ₂)- derived catalysts using density functional theory. Precatalyst geometries are first discussed, and then ion- pair formation/heterolytic dissociation processes involving the binuclear bis(borane) cocatalyst 1,4-(C ₆F ₅) ₂BC ₆F ₄B(C ₆F ₅) ₂ (BN ₂), are analyzed and compared with those in the parent mononuclear analogue. It is found that, on proceeding from the mononuclear to binuclear catalyst system, ion-pair dissociation energies increase due to the stronger catalyst center-counterdianion interactions. Moreover, in the binuclear case, the interaction energies are markedly sensitive to geometrical matching between the binuclear bis(borane) and the precatalyst Zr-methyl positions. Binuclear catalytic effects between the metal centers are then explored, with the specific contribution from the proximity of the second metal center. Possible agostic interactions of α-alkenes π-coordinated to one Zr center with the second Zr center of the binuclear catalyst are scrutinized for the case of 1-octene. It is argued that these agostic interactions are at least partly responsible for the unusual enchainment properties of the bimetallic catalysts. In particular, the greater polyethylene product branch densities found experimentally for the bimetallic catalysts can be correlated with an intramolecular reinsertion process, assisted by agostic interactions. Moreover, these same agostic interactions involving a chain growing at one metal site with the second metal site of the binuclear catalyst modify the environment to increase propagation/termination rate ratios, in turn favoring increased product molecular weight (M _n). These effects are observed experimentally at closer Zr · Zr proximities in olefin polymerizations mediated by binuclear CGC catalysts.