CO Displacement in an Oxidative Addition of Primary Silanes to Rhodium(I).

Abstract

The rhodium dicarbonyl {PhB(OxMe2)2ImMes}Rh(CO)2 (1) and primary silanes react by oxidative addition of a nonpolar Si-H bond and, uniquely, a thermal dissociation of CO. These reactions are reversible, and kinetic measurements model the approach to equilibrium. Thus, 1 and RSiH3 react by oxidative addition at room temperature in the dark, even in CO-saturated solutions. The oxidative addition reaction is first-order in both 1 and RSiH3, with rate constants for oxidative addition of PhSiH3 and PhSiD3 revealing kH/ kD ∼ 1. The reverse reaction, reductive elimination of Si-H from {PhB(OxMe2)2ImMes}RhH(SiH2R)CO (2), is also first-order in [2] and depends on [CO]. The equilibrium concentrations, determined over a 30 °C temperature range, provide Δ H ° = -5.5 ± 0.2 kcal/mol and Δ S ° = -16 ± 1 cal·mol-1K-1 (for 1 ⇄ 2). The rate laws and activation parameters for oxidative addition (Δ H⧧ = 11 ± 1 kcal·mol-1 and Δ S⧧ = -26 ± 3 cal·mol-1·K-1) and reductive elimination (Δ H⧧ = 17 ± 1 kcal·mol-1 and Δ S⧧ = -10 ± 3 cal·mol-1K-1), particularly the negative activation entropy for both forward and reverse reactions, suggest the transition state of the rate-determining step contains {PhB(OxMe2)2ImMes}Rh(CO)2 and RSiH3. Comparison of a series of primary silanes reveals that oxidative addition of arylsilanes is ca. 5× faster than alkylsilanes, whereas reductive elimination of Rh-Si/Rh-H from alkylsilyl and arylsilyl rhodium(III) occurs with similar rate constants. Thus, the equilibrium constant Ke for oxidative addition of arylsilanes is >1, whereas reductive elimination is favored for alkylsilanes.