Mechanistic Pathways for N₂O Elimination from trans-R₃Sn-O-N═N-O-SnR₃ and for Reversible Binding of CO₂ to...

The rate and mechanism of the elimination of N₂O from trans-R₃Sn-O-N═N-O-SnR₃ (R = Ph (1) and R = Cy (1)) to form R₃Sn-O-SnR₃ (R = Ph (2) and R = Cy (2)) 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 1 and ΔH^⧧ = 22.7 ± 2.5 kcal·mol^-1 and ΔS^⧧ = -12.4 ± 6 cal·mol^-1·K^-1 for 1. 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 (1) 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 1 with protic acids HX are rapid and lead to formation of R₃SnX and trans-H₂N₂O₂. Reaction of 1 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 2 and 2 resulted in very rapid reaction to form the corresponding carbonates R₃Sn-O-C(═O)-O-SnR₃ (R = Ph (3) and R = Cy (3)) at room temperature. Evacuation results in fast loss of bound CO₂ and regeneration of 2 and 2. Variable temperature data for formation of 3 yield ΔH^o = -8.7 ± 0.6 kcal·mol^-1, ΔS^o = -17.1 ± 2.0 cal·mol^-1·K^-1, and ΔG^o_298K = -3.6 ± 1.2 kcal·mol^-1. DFT studies were performed and provide additional insight into the energetics and mechanisms for the reactions.