Co Displacement In An Oxidative Addition Of Primary Silanes To Rhodium(I)

The rhodium dicarbonyl {PhB(Ox(Me2))(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 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 k(H)/k(D) similar to 1. The reverse reaction, reductive elimination of Si-H from {PhB(Ox(Me2))(2)Im(Mes)}RhH(SiH2R)CO (2), is also first-order in [2] and depends on [CO]. The equilibrium concentrations, determined over a 30 degrees C temperature range, provide AH =-5.5 0.2 kcal/mol and AS =-16 1 cal"mo1-11C-1 (for 1 t 2). The rate laws and activation parameters for oxidative addition (OH* = 11 1 kcal-morl and AS* = -26 3 cal"mo1-1"K-1) and reductive elimination (OH* = 17 1 kcal-mol' and AS* =-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(Mes))(2)/mMes}Rh(CO)(2) and RSiH3. Comparison of a series of primary silanes reveals that oxidative addition of arylsilanes is ca. 5x 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.