Can Green Dimethyl Carbonate Synthesis be More Effective? A Catalyst Recycling Study Benefiting from Experimental Kinetics and DFT Modeling
Résumé
Dibutyldimethoxystannanes are known to catalyze the reaction between carbon dioxide and methanol leading to dimethyl carbonate. Despite similarities between din-butyl-and ditert-butyldimethoxystannane, the recycled complexes have different structural features. In the din-butyl series, a decatin(IV) complex has been characterized and is less active than the stannane precursor. Kinetic experiments likely indicate that all the tin centers are not active, which is confirmed in comparing with the related dinuclear 1,3-dimethoxytetran-butyldistannoxane complex. In the ditert-butyl series, the tritin(IV) complex isolated upon recycling features the steric effect of bulky tBu ancillary ligands. Interestingly enough, the SnOH...O(H)CH3 hydrogen bonding found in the structure prefigures Sn-OH/Sn-OCH3 interchange, a crucial step for closing the catalytic cycle. Density functional calculations highlight that the Sn-OH/Sn-OCH3 exchange is endothermic. Taken together, the results cast a clear light on the significant role of complexes of low nuclearity for dimethyl carbonate synthesis.