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

Abstract

Homobimetallic Hf(IV) complexes, L 2 -Hf 2 Me 5 (3) and L 2 -Hf 2 Me 4 (4) (L 2 = 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 L 2 with the appropriate Hf precursor and were characterized in solution (NMR) and in the solid state (X-ray diffraction). In 3, L 2 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 L 2 . 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 Ph 3 C + B(C 6 F 5 ) 4 − (B 1 ) or PhNMe 2 H + B(C 6 F 5 ) 4 − (NB), 3 exhibits pronounced bimetallic cooperative effects in ethylene homopolymerization and ethylene +1-octene copolymerization vs the monometallic analogue L 1 -HfMe 2 (1, L 1 = 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 M w and poly(ethylene-co-1-octene) with 2.4 times higher M w and 1.9 times greater 1-octene enchainment densities than 1. The activation chemistry of 3 and 4 with 1 or 2 equiv of B 1 and NB is characterized in detail by NMR spectroscopy. In sharp contrast to 1, which undergoes Hf− C naph protonolysis followed by naphthyl remetalation with NB as the cocatalyst, activation of 3 with B 1 or NB proceeds by consecutive −CH 3 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.

Bimetallic Zr,Zr-Hydride Complexes in Zirconocene Catalyzed Alkene Dimerization

“…In broad understanding, this case can be considered as an example of biszirconium catalysis similar to the alkene polymerization in the presence of Group 4 bimetallic complexes [62][63][64]. Further development of this work into establishing the activation mechanism of the biszirconium hydride complexes can be fruitful in uncovering the reasons for the selective formation of alkene diand oligomers.…”

Mono‐ and binuclear nickel catalysts for 1‐hexene polymerization

“…Also, the poly (1‐hexene) obtained in the presence of binuclear catalysts showed less chain branching than that obtained in the presence of mononuclear catalyst (MN). This result can be explained by the optimum bulkiness and synergistic effect owing to the presence of a second metal centre and plausible agostic interaction between the polymer chain and second centre, along with the greater chain walking (Scheme ) …”

Copolymerization of Ethylene and Long-Chain Functional α-Olefins by Dinuclear Zirconium Catalysts

“…Multimetallic catalysts have been pursued as a method for synthesizing functionalized polyethylene via a direct copolymerization of ethylene and polar monomers . Examples of bimetallic catalysts with rigid linkers that feature improved performance include enhanced incorporation of functionalized norbornenes using dinickel catalysts ( A ; Figure ), higher incorporation of ester-containing comonomers and acrylic anhydride using macrocyclic frameworks to support “double-decker” dinickel or dipalladium catalysts with phenoxy-imine ( B ) or α-diimine ( C ) donors, − and increased tolerance to amine additives and incorporation of amino olefins by terphenyl-supported bis­(phenoxy-imine)­dinickel catalysts ( D ). − Bimetallic catalysts displaying early transition metals have been demonstrated to affect performance by increasing the incorporation of α-olefins ( E and F ), − though not those containing polar groups. A bimetallic mechanism of α-olefin incorporation was proposed where distal C–H bonds interact with the second metal to facilitate enchainment. − ,− An analogous bimetallic mechanism for comonomer incorporation is particularly appealing for polar olefins.…”