Carbon Dioxide Cycloaddition to Epoxides Promoted by Nicotinamidium Halide Catalysts: A DFT Investigation

The utilization of CO₂ as building block for the production of cyclic carbonate is a promising route to simultaneously mitigate the global warming issue and obtain valuable commercial chemicals. In this work, the activity of nicotinamidium halide catalysts towards the CO₂ conversion into cyclic carbonate has been explored by means of density functional theory (DFT) calculations. DFT calculations support the ability, suggested experimentally, of the pyridium α-C-H proton of the catalysts to activate the epoxide ring via a hydrogen bond. Interestingly, DFT calculations underline the involvement of the n-octyl substituent of the pyridyl ring in the epoxide activation, while the hydrogen atom of the amide group N-H is rather involved in the stabilization of the iodide trough electrostatic interactions. Moreover, the replacement of the pyridium α-C-H proton with the bulkier methyl group leads to a different reaction mechanism. The calculated energy barriers well reproduce the experimental trends of the studied catalysts, and the computed activation barrier of 29.0 kcal/mol, relative to the ring opening step of the most active catalyst, is in line with the experimental working temperature of 80 °C. Those results shed light on the CO₂ fixation reaction contributing to the development of more efficient catalytic systems.