Mechanistic Pathways for N2O Elimination from trans-R3Sn-O-N═N-O-SnR3 and for Reversible Binding of CO2 to R3Sn-O-SnR3 (R = Ph, Cy)

Abstract

The rate and mechanism of the elimination of N2O from trans-R3Sn-O-N═N-O-SnR3 (R = Ph (1Ph) and R = Cy (1Cy)) to form R3Sn-O-SnR3 (R = Ph (2Ph) and R = Cy (2Cy)) have been studied using both NMR and IR techniques to monitor the reactions in the temperature range of 39-79 °C in C6D6. Activation parameters for this reaction are ΔH⧧ = 15.8 ± 2.0 kcal·mol-1 and ΔS⧧ = -28.5 ± 5 cal·mol-1·K-1 for 1Ph and ΔH⧧ = 22.7 ± 2.5 kcal·mol-1 and ΔS⧧ = -12.4 ± 6 cal·mol-1·K-1 for 1Cy. Addition of O2, CO2, N2O, or PPh3 to sealed tube NMR experiments did not alter in a detectable way the rate or product distribution of the reactions. Computational DFT studies of elimination of hyponitrite from trans-Me3Sn-O-N═N-O-SnMe3 (1Me) yield a mechanism involving initial migration of the R3Sn group from O to N passing through a marginally stable intermediate product and subsequent N2O elimination. Reactions of 1Ph with protic acids HX are rapid and lead to formation of R3SnX and trans-H2N2O2. Reaction of 1Ph with the metal radical •Cr(CO)3C5Me5 at low concentrations results in rapid evolution of N2O. At higher •Cr(CO)3C5Me5 concentrations, evolution of CO2 rather than N2O is observed. Addition of 1 atm or less CO2 to benzene or toluene solutions of 2Ph and 2Cy resulted in very rapid reaction to form the corresponding carbonates R3Sn-O-C(═O)-O-SnR3 (R = Ph (3Ph) and R = Cy (3Cy)) at room temperature. Evacuation results in fast loss of bound CO2 and regeneration of 2Ph and 2Cy. Variable temperature data for formation of 3Cy yield ΔHo = -8.7 ± 0.6 kcal·mol-1, ΔSo = -17.1 ± 2.0 cal·mol-1·K-1, and ΔGo298K = -3.6 ± 1.2 kcal·mol-1. DFT studies were performed and provide additional insight into the energetics and mechanisms for the reactions.