Group 4 Post-Metallocenes Supported by [OCH2N,C(σ-aryl)] Auxiliaries Bearing a Seven-Membered Metallacycle: Synthesis, Characterization, and Catalysts for Olefin Polymerization

A series of pyridine-2-phenolate-6-(sigma-aryl) [(ON)-N-CH2,C] group 4 bis(benzyl) precatalysts, featuring a flexible O,N-donor chelate, have been prepared and characterized by multinuclear NMR spectroscopy. These complexes adopt C-1 symmetry, and variable-temperature H-1 NMR experiments for a difluoro-substituted sigma-aryl Ti(IV) derivative indicate a fluxional process involving inversion of the seven-membered chelate ring accompanied by conformational changes for the benzyl groups (Delta G(double dagger) = 62.6 kJ mol(-1)). Scrutiny of the molecular structure of a Zr(IV) relative, determined by X-ray crystallography, revealed severely distorted quasi-meridional coordination for the [(ON)-N-CH2,C] ligand and virtually perpendicular orientation of the benzyl moieties. Olefin polymerization studies have been undertaken in conjunction with methylaluminoxane (MAO) and trityl borate. Using dried MAO (250 equiv) as cocatalyst and 7 atm of ethylene pressure, a CF3-substituted Ti(IV) catalyst displays good activity (680 g mmol(-1) h(-1) at 75 degrees C; M-w = 3.4 x 10(2) kg mol(-1), M-w/M-n = 2.6), while the difluoro-substituted analogue produces high molecular-weight polymers (M-w = 1.34 x 10(3) kg mol(-1) at 50 degrees C; M-w/M-n = 2.4). For the latter derivative, a weak C-H center dot center dot center dot F-C interaction between a CH2(benzyl) proton and the fluorine atom ortho to the Ti-C(sigma-aryl) bond is detected by [H-1,F-19]-HMQC experiments. These results open up the possibility that such stabilizing interactions could potentially occur for the catalytic species to suppress beta-H elimination and hence chain termination processes.