(17g) Quantum Mechanical Study of CO2 Conversion Catalyzed by Metal(Salen)-Dmap Complexes
Few chemical processes have proven successful in using CO2 as an inexpensive and renewable feedstock in chemical synthesis. While CO2 is abundant and nontoxic, its stability imposes a challenge in finding routes for its effective use. Some transition metal–salen complexes can catalyze the reaction of CO2 and epoxides to produce either cyclic carbonates or polycarbonates via two competing reactions. However, experimental studies have demonstrated that when 4-dimethylaminopyridine (DMAP) is incorporated into this system, the reaction temperature and pressure decrease, while the yields and selectivity increase.
In this study, density functional theory (DFT) calculations were used to evaluate the thermochemistry of plausible elementary steps for the coupling reaction of CO2 with ethylene oxide catalyzed by metal–salen complexes. In order to understand the role of the metal–salen catalyst on this system, we have performed a systematic analysis of the possible interactions of CO2 with metal–salen complexes consisting of Co, Cr, Mn, Fe, Zn, and Al. To determine the effect of the axial ligand on the energies of reaction, two axial ligands were considered: chlorine and DMAP. These calculations were performed using the unrestricted OPBE functional. Geometrical optimizations were carried out beginning with a variety of different configurations and frequency calculations were used to verify that structures lie in an energy minimum.
Our results demonstrate that, when chlorine is the axial ligand, the formation of CO2–salen complexes is an endothermic reaction for the six metal systems considered, even when the epoxide is included in the system. However, energies of reaction for the coupling of CO2 with metal–salen complexes decrease when DMAP is the axial ligand. Thermodynamically favorable intermediates were obtained for Cr and Al salen systems. The lowest energy complexes involve the interaction of CO2 with an opened-epoxide bonded to the metal–salen catalyst. The thermochemistry of these elementary steps was used to predict the most probable reaction mechanism on these systems.