From CO2 to dimethyl carbonate with dialkyldimethoxystannanes: the key role of monomeric species

Abstract

The formation of dimethyl carbonate (DMC) from CO₂ and methanol with the dimer [n-Bu₂Sn(OCH₃)₂]₂ was investigated by experimental kinetics in support of DFT calculations. Under the reaction conditions (357–423 K, 10–20 MPa), identical initial rates are observed with three different reacting mixtures, CO₂/toluene, supercritical CO₂, and CO₂/methanol, and are consistent with the formation of monomeric di-n-butyltin(IV) species. An intramolecular mechanism is, therefore, proposed with an Arrhenius activation energy amounting to 104 ± 10 kJ mol⁻¹ for DMC synthesis. DFT calculations on the [(CH₃)₂Sn(OCH₃)₂]₂/CO₂ system show that the exothermic insertion of CO₂ into the Sn–OCH₃ bond occurs by a concerted Lewis acid–base interaction involving the tin center and the oxygen atom of the methoxy ligand. The Gibbs energy diagrams highlight that, under the reaction conditions, the dimer–monomer equilibrium is significantly shifted towards monomeric species, in agreement with the experimental kinetics. Importantly, the two Sn–OCH₃ bonds are prompt to insert CO₂. These results provide new insight into the reaction mechanism and catalyst design to enhance the turnover numbers.