Synthesis, structure and behavior of vanadium(III) diphosphine complexes in the homo- and co-polymerization of ethylene with norbornene : the ligand donor strength and bite angle make the difference

A series of vanadium(III) complexes containing diphosphine ligands of the type R2P(CH2)2PR2[R = Me (dmpe, 1a), R = Et (depe, 1b), R = Ph (dppe, 1c)] were synthesized by reacting VCl3(THF)3with 1.3 equiv. of the corresponding ligand. The molecular structures of 1a and 1b were determined by X-ray crystallography, revealing that both are dimers of the type [VCl3(dmpe)]2(μ-P,P′-dmpe) and [VCl3(depe)]2(μ-P,P′-depe), respectively, having one chelating diphosphine ligand, and an additional diphosphine bridging the two vanadium metals. Upon activation with diethylaluminum chloride (Et2AlCl) and in the presence of ethyltrichloroacetate (ETA) the complexes exhibited good activity in the polymerization of ethylene, affording linear, semicrystalline polymers with melting temperatures of about 135 °C. All the complexes (1a–1c) showed from good to excellent activity in the copolymerization of ethylene with norbornene. The resultant copolymers are mainly alternating with a uniform composition and a norbornene incorporation depending on the ligand employed. The substituents at the phosphorus atoms strongly affect the catalytic activity, which increased in the order 1a < 1b < 1c with increasing the ligand donor strength, and the comonomer incorporation, while no significant differences were observed in terms of copolymers microstructure and molecular weight. The results are carefully compared with those obtained with the monodentate phosphine VCl3(PMePh2)2complex and VCl3(THF)3. Overall, we found that the use of diphosphine ligands gave less active catalysts likely because PMePh2and THF ligands are weakly coordinated to the metal, but (co)polymers with higher molecular weight due to the ability of chelating diphosphines to stabilize the electron-deficient catalytic intermediate, thus reducing the rate of β–hydrogen elimination and subsequent chain-transfer.