Conversion of CO2 to cyclic carbonates by metal-ethylenediamine complexes in ionic liquid: A DFT mechanistic study

To curtail the accumulation of CO2 concentration in the atmosphere, different strategies have been adopted at present. Among them, one of the possible methods of reducing CO2 level involves transforming it into useful organic compounds. In this line, converting CO2 to cyclic carbonates is a topic of interest as cyclic carbonates are commercially important chemicals. Recently, Honores and co-workers have experimentally prepared cyclic carbonates from cycloaddition of CO2 and ethylene oxide/propylene oxide/Epichlorohydrin catalyzed by Co-ethylenediamine (en) complexes using [BMIm] [BF4] ionic liquid as solvent. This work explores the complete reaction mechanism of the cycloaddition reaction at B3LYP/LANL2DZ(6-31G*) level of density functional theory. Two mechanist routes were computed: first route deals with the reaction mechanism without the presence of ionic liquid and second route deals with the ionic liquid involved reaction mechanism. In both the routes, the ring closure step was calculated to be the rate-determining step with an activation barrier in the range of 47.87-52.23 kcal/mol in the absence of ionic liquid and 48.82 to 35.29 kcal/mol in the presence of ionic liquid. Interestingly, the involvement of ionic liquids in the reaction mechanism was found to remarkably reduce the activation free energy barriers by 2.12-19.31 kcal/mol in all the steps along the proposed mechanism. Geometrical analysis indicated that the ability of ionic liquids to form intermolecular hydrogen bonding interaction with the catalyst and substrate is the reason behind the decrease in the barriers. In order to quantify the strength of the hydrogen bonding interactions, NBO analysis was performed on key transition state geometries and n(B-F) -> sigma*(H-N) hydrogen bonding interactions between anionic part of IL and en ligand framework were found to possess significant second-order stabilization energies. Overall, this study sheds light on the mechanistic details of the cycloaddition reaction of CO2 and epoxide and also substantiates that the ionic liquids are efficient green solvents which can be employed in similar catalytic reactions to achieve more efficiency.