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(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 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.