Pyridylamido Bi-Hafnium Olefin Polymerization Catalysis: Conformationally Supported Hf···Hf Enchainment Cooperativity

Homobimetallic Hf(IV) complexes, L2-Hf2Me5 (3) and L2-Hf2Me4 (4) (L2 = N,N′-{[naphthalene-1,4- diylbis(pyr idine-6,2-diyl)]bis[(2-isopropylphenyl)- methylene)]bis(2,6-diisopropylaniline}), were synthesized by reaction of the free ligand L2 with the appropriate Hf precursor and were characterized in solution (NMR) and in the solid state (X-ray diffraction). In 3, L2 acts as a dianionic tridentate ligand for one Hf metal center and as a monoanionic bidentate ligand for the other, whereas in 4, both Hf units are tricoordinated to opposite sides of L2. In the solid state, the Hf···Hf distance is significantly different in 3 vs 4 (6.16 vs 8.06 Å, respectively), but in solution, the structural dynamics of the two linked metallic units in bis-activated complex 3 accesses conformers with far closer Hf···Hf distances (∼3.2 Å). Once activated with Ph3C+B(C6F5)4 − (B1) or PhNMe2H+B(C6F5)4 − (NB), 3 exhibits pronounced bimetallic cooperative effects in ethylene homopolymerization and ethylene +1-octene copolymerization vs the monometallic analogue L1-HfMe2 (1, L1 = 2,6-diisopropyl-N-{(2-isopropylphenyl)[6-(naphthalen-1- yl)pyridin-2-yl]methyl}aniline) and bimetallic 4, producing polyethylene with 5.7 times higher Mw and poly(ethylene-co-1- octene) with 2.4 times higher Mw and 1.9 times greater 1-octene enchainment densities than 1. The activation chemistry of 3 and 4 with 1 or 2 equiv of B1 and NB is characterized in detail by NMR spectroscopy. In sharp contrast to 1, which undergoes Hf− Cnaph protonolysis followed by naphthyl remetalation with NB as the cocatalyst, activation of 3 with B1 or NB proceeds by consecutive −CH3 protonolysis/abstractions at each Hf center, explaining the higher polymerization activity of 3/NB versus 1/ NB. All product polymers have narrow (2−3) PDIs, and this is explained by NMR evidence for very fast exchange of alkyl moieties between the two active Hf metal centers. Key experimental findings are supported by DFT analysis.