Reactivity of dialkoxydibutylstannanes toward carbon dioxide: A DFT study of electronic and steric effects
Résumé
DFT calculations were performed for the reaction of CO2 with the monomeric species, R′2Sn(OR)2, (R = R′ = CH3; R = CH3, CH2CH3, CH(CH3)2, R′ = n-Bu) for assessing the role of electronic and steric effects in the kinetics and thermodynamics of CO2 insertion into Sn-OR bonds. The reaction pathways are exothermic and involve the successive insertion into the two Sn-OR bonds. The driving force for insertion is ascribed to a charge-transfer between the HOMO of the complexes, mainly localized on the oxygen atom of the alkoxy ligands, and the LUMO of CO2. Interestingly enough, the energy barrier of the second insertion is much lower by around 27 kJ mol−1, and quite similar whatever the alkyl and alkoxy groups studied. The thus-formed alkylcarbonato complexes undergo rotation around the Sn-O bonds which involve energy barriers sensitive to the steric hindrance of the alkyl and alkoxy groups. In the n-butyl series, the energy barriers for rotation are significantly higher than those for CO2 insertion. As a result, the isopropylcarbonato ligand is thermodynamically and kinetically more stable toward extrusion of CO2, as found experimentally. The present study highlights that steric effects play a significant role on the reaction pathways for the successive insertion of CO2 into the two Sn-O bonds. Moreover, dialkylcarbonato tin (IV) species may play a key role in the formation of dialkyl carbonate which takes place experimentally under high CO2 pressure.