Borane-Induced Dehydration of Silica and the Ensuing Water-Catalyzed Grafting of B(C6F5)3 To Give a Supported, Single-Site Lewis Acid, SiOB(C6F5)2
journal contributionposted on 11.01.2012, 00:00 by Ying-Jen Wanglee, Jerry Hu, Rosemary E. White, Ming-Yung Lee, S. Michael Stewart, Philippe Perrotin, Susannah L. Scott
A supported, single-site Lewis acid, SiOB(C6F5)2, was prepared by water-catalyzed grafting of B(C6F5)3 onto the surface of amorphous silica, and its subsequent use as a cocatalyst for heterogeneous olefin polymerization was explored. Although B(C6F5)3 has been reported to be unreactive toward silica in the absence of a Brønsted base, we find that it can be grafted even at room temperature, albeit slowly. The mechanism was investigated by 1H and 19F NMR, in both the solution and solid states. In the presence of a trace amount of H2O, either added intentionally or formed in situ by borane-induced dehydration of silanol pairs, the adduct (C6F5)3B·OH2 hydrolyzes to afford C6F5H and (C6F5)2BOH. The latter reacts with the surface hydroxyl groups of silica to yield SiOB(C6F5)2 sites and regenerate H2O. When B(C6F5)3 is present in excess, the resulting grafted boranes appear to be completely dry, due to the eventual formation of [(C6F5)2B]2O. The immobilized, tri-coordinate Lewis acid sites were characterized by solid-state 11B and 19F NMR, IR, elemental analysis, and C5H5N-TPD. Their ability to activate two molecular C2H4 polymerization catalysts, Cp2ZrMe2 and an (α-iminocarboxamidato)nickel(II) complex, was explored.