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

The rhodium dicarbonyl {PhB(Ox Me 2 ) 2 Im Mes }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 RSiH 3 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 3 , with rate constants for oxidative addition of PhSiH 3 and PhSiD 3 revealing k H / k D ∼ 1. The reverse reaction, reductive elimination of Si-H from {PhB(Ox Me 2 ) 2 Im Mes }RhH(SiH 2 R)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 -1 K -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 -1 K -1 ), particularly the negative activation entropy for both forward and reverse reactions, suggest the transition state of the rate-determining step contains {PhB(Ox Me 2 ) 2 Im Mes }Rh(CO) 2 and RSiH 3 . 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 e for oxidative addition of arylsilanes is >1, whereas reductive elimination is favored for alkylsilanes.