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

Homobimetallic Hf­(IV) complexes, L²-Hf₂Me₅ (<b>3</b>) and L²-Hf₂Me₄ (<b>4</b>) (L² = <i>N</i>,<i>N</i>′-{[naphthalene-1,4-diylbis­(pyridine-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 <b>3</b>, L² acts as a dianionic tridentate ligand for one Hf metal center and as a monoanionic bidentate ligand for the other, whereas in <b>4</b>, both Hf units are tricoordinated to opposite sides of L². In the solid state, the Hf···Hf distance is significantly different in <b>3</b> vs <b>4</b> (6.16 vs 8.06 Å, respectively), but in solution, the structural dynamics of the two linked metallic units in bis-activated complex <b>3</b> 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), <b>3</b> exhibits pronounced bimetallic cooperative effects in ethylene homopolymerization and ethylene +1-octene copolymerization vs the monometallic analogue L¹-HfMe₂ (<b>1</b>, L¹ = 2,6-diisopropyl-<i>N</i>-{(2-isopropylphenyl)­[6-(naphthalen-1-yl)­pyridin-2-yl]­methyl}­aniline) and bimetallic <b>4</b>, producing polyethylene with 5.7 times higher <i>M</i>_w and poly­(ethylene-<i>co</i>-1-octene) with 2.4 times higher <i>M</i>_w and 1.9 times greater 1-octene enchainment densities than <b>1</b>. The activation chemistry of <b>3</b> and <b>4</b> with 1 or 2 equiv of B₁ and NB is characterized in detail by NMR spectroscopy. In sharp contrast to <b>1</b>, which undergoes Hf–C_naph protonolysis followed by naphthyl remetalation with NB as the cocatalyst, activation of <b>3</b> with B₁ or NB proceeds by consecutive −CH₃ protonolysis/abstractions at each Hf center, explaining the higher polymerization activity of <b>3</b>/NB versus <b>1</b>/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