Well-Defined Silica-Supported Zirconium-Benzyl Cationic Species: Improved Heterogenization of Single-Site Polymerization Catalysts

摘要

Zr(CH_2Ph)_4 (1) was grafted onto a recently disclosed hybrid material based on amorphous silica that features unique phenol grafting sites {[(≡SiO)_2(AlOC_6H_4OH)(Et_2O)] (H)}. A monopodal tribenzyl surface species, structurally similar to the silica-supported species previously disclosed [≡SiOZr(CH_2Ph)_3] (2), was obtained and fully characterized as [(≡SiO)_2(AlOC_6H_4OZr(CH_2Ph)_3)(Et_2O)] (3). The activation of both these species by B(C_6F_5)_3 proceeded by benzyl abstraction to yield the inner-sphere ion pairs [≡SiOZr- (CH_2Ph)_2]+[(PhCH_2)B(C_6F_5)_3]~- (4) and [(≡SiO)_2(AlOC_6H_(4-) OZr(CH_2Ph)_2)(Et_2O)]~+[(PhCH_2)B(C_6F_5)_3]~- (5), respectively. These surface species were fully characterized by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, elemental analyses, and ~1H, ~(13)C, ~(11)B, and ~(19)F solid-state NMR spectroscopy. These isolated cationic catalysts displayed increased ethylene polymerization activity compared to the neutral species. Most importantly, the heterogenization of the cationic complex with the phenol spacer (5) led to a fourfold increase in productivity compared to that of the silica counterpart 4, in agreement with reduced surface interactions and improved electrophilicity.