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

The rhodium dicarbonyl {PhB(Ox)Im}Rh(CO) (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 RSiH 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 RSiH, with rate constants for oxidative addition of PhSiH and PhSiD revealing k/ k ∼ 1. The reverse reaction, reductive elimination of Si-H from {PhB(Ox)Im}RhH(SiHR)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·molK (for 1 ⇄ 2). The rate laws and activation parameters for oxidative addition (Δ H = 11 ± 1 kcal·mol and Δ S = -26 ± 3 cal·mol·K) and reductive elimination (Δ H = 17 ± 1 kcal·mol and Δ S = -10 ± 3 cal·molK), particularly the negative activation entropy for both forward and reverse reactions, suggest the transition state of the rate-determining step contains {PhB(Ox)Im}Rh(CO) and RSiH. 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 K for oxidative addition of arylsilanes is >1, whereas reductive elimination is favored for alkylsilanes.