Tetra silica surface-reaction

Scheme 18.5 Typical reactions between tin tetra(alkyl) or tin (tris-alkyl)(hydride) derivatives with a silica surface at temperature of 100°C or higher leading to the same tin(tris-alkyl)(monosiloxy) derivative.

The stoichiometry of these reactions can be controlled by modulating the concentration of hydroxyl groups on the surface of silica. When starting with the tetra-alkyl complex, subsequent reaction with an alcohol R OH (Equation(4)) is necessary this generally occurs under conditions mild enough to maintain the anchoring bond SiO—M. 

Bimetallic catalysts can be obtained by surface organometallic chemistry on metals. These catalysts are prepared by the controlled reaction under hydrogen between tetra n-butyl tin and silica supported rhodium particles. For a given amount of tin fixed, these solids exhibit increasing activities and selectivities for the conversion of acetophenone to phenylethanol. 

Surprisingly, if the surface of silica supports a platinum particle covered with hydrogen, the reaction of tetra-n-butyl tin leads aheady at room temperature to the platinum particle reported in the following equation, on which SnRs groups are grafted and the sihca support is untouched [62]. No reaction occurs on the support in such conditions. 

The reaction is usually performed at low temperature, typically 20-50 °C, in order to avoid a reaction of the tin complex with the hydroxy groups of the support, as shown by solid-state C CP-MAS NMR. Indeed, it has been shown that tetra-n butyl tin or tributyl tin hydride reacts, above ca. 150 °C with the hydroxy groups of silica, siUca-alumina or alumina with formation of a surface grafted =M-0-Sn(n- 4119)3 fragment and evolution of butane or hydrogen [61, 109-111] (Scheme 18.5). 

This reaction is analogous to olation and oxolation reactions between hydrolyzed species. It involves, at pH PZCi Oxo or hydroxo ligands from the surface acting as nucleophiles within the coordination sphere of the adsorbed ion. This reaction is also responsible for the adsorption of mixed aquo-amino complexes of nickel. Washing silica suspensions which have adsorbed hexamino nickel(ll) complexes leads to the formation of tetra-amino complexes. The shift in the UV-visible absorption towards lower frequencies makes it possible to follow the level of replacement of NH3 by H2O, and then of water by SiO [4,10] ... 

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