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

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)(2)Sn(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.