Quantum Chemical Determination of Stable Intermediates on CO2 Adsorption Onto Metal(Salen) Complexes

Coupling reactions of CO₂ and epoxides to produce either cyclic carbonates or polycarbonates are environmentally friendly reactions that allow the use of an inexpensive and renewable feedstock while achieving carbon efficiency. In this study, density functional theory calculations were used to understand the role the metal(salen) catalyst on CO₂ adsorption. We have performed a systematic analysis of the plausible interactions of CO₂ with metal(salen) catalysts and ethylene oxide/metal(salen) complexes. Adsorption reactions were analyzed on six metal(salen) complexes: Co, Cr, Mn, Fe, Zn, and Al, using the unrestricted OPBE functional. Geometry optimizations were carried out beginning with a variety of different conformations and frequency calculations were used to verify that structures lie in an energy minimum. Our results demonstrate that CO₂ does not bind to the metal atom of the bare metal(salen). The adsorption of CO₂ onto metal(salen) complexes is an endothermic reaction and the lowest energy adsorbed complex involves the interaction of CO₂ with the adsorbed opened-epoxide.