Well‐Defined Silica‐Supported Calcium Reagents: Control of Schlenk Equilibrium by Grafting

Abstract

Calcium reagents Ca(alpha-Me(3)Si-2-Me(2)N-benzyl)(2) x 2 thf (1) and Ca[N(SiMe(3))(2)](2) x 2 thf (2) reacted with silica partially dehydroxylated at 700 degrees C to afford materials that bear ([triple bond]SiO)Ca(alpha-Me(3)Si-2-Me(2)N-benzyl) x 1.6 thf (SiO(2)-1) and ([triple bond]SiO)Ca [N(SiMe(3))(2)] x 1.3 thf (SiO(2)-2) fragments, respectively. Due to the bulk of the supported complexes, the silanol groups are only partially metalated: 50% in SiO(2)-1 and 70% in SiO(2)-2. In the case of SiO(2)-2, a parallel SiMe(3)-capping side reaction affords in fine a silanol-free surface. The materials were characterized by IR spectroscopy, 1D and 2D solid-state high-field NMR spectroscopy, and elemental analysis. Reaction of 2 with one equivalent of the bulky silanol (tBuO)(3)SiOH, a silica-surface mimic, afforded the homoleptic bis-silyloxide calcium derivative through ligand exchange (Schlenk equilibrium), and a derivative was isolated and structurally characterized. Preliminary studies have shown that both grafted benzyl and amide derivatives are active in olefin hydrosilylation, intramolecular hydroamination, and styrene polymerization, with evidence showing that catalysis occurs through supported species. In styrene polymerization, a marked influence of the surface acting as a ligand on the stereoselectivity of the reaction was observed, as syndiotactic-rich polystyrene (88% of r diads) was obtained. These results illustrate that grafting of calcium benzyl or amide compounds on a silica surface is a new concept to prevent ligand exchange through the Schlenk equilibrium. Heteroleptic calcium complexes that cannot be synthesized as stable molecular species in solution can be obtained as silica-supported species which have been shown to be catalytically active.