Quantum mechanical study of the reaction of CO2 and ethylene oxide catalyzed by metal–salen complexes: effect of the metal center and the axial ligand

• Published in: Reaction kinetics, mechanisms and catalysis mechanisms and catalysis, 2015-12, Vol.116 (2), p.351-370
• Main Authors: Santiago-Rodríguez, Yohaselly, Curet-Arana, María C.
• Format: Article
• Language: English
• Subjects: Catalysis; Chemistry; Chemistry and Materials Science; Industrial Chemistry/Chemical Engineering; Physical Chemistry
• ISSN: 1878-5190; 1878-5204
• DOI: 10.1007/s11144-015-0904-6

Density functional theory calculations were used to evaluate the thermochemistry of plausible elementary steps for the reaction of CO 2 with ethylene oxide catalyzed by metal–salen complexes consisting of Co, Cr, Mn, Fe, Al and Zn. The effect of the axial ligands on the energies of reaction was studied using chlorine and 4-dimethylaminopyridine (DMAP). This analysis was performed using UOPBE/LANL2DZ to screen the plausible reaction intermediates. The most favorable intermediates were then studied with B3LYP/6-311 g**/LANL2DZ in gas phase and in dichloromethane to evaluate the method and solvent effect on their reaction energies and geometries. Our results demonstrated that when chlorine is the axial ligand, the formation of CO 2 -epoxide-salen complexes was an endothermic reaction for all metal–salen complexes considered. However, the energies of reaction decreased for these complexes with DMAP as axial ligand. While differences were found in the reaction energies with the two methods, both methods yield similar minimum energy structures on the intermediates that were analyzed. The most energetically favorable intermediates for the coupling of CO 2 and ethylene oxide obtained when the solvent was taken into account were the complexes with Al as metal center and DMAP as axial ligand. However, the inclusion of dichloromethane as a solvent did not have a significant effect in the reaction energies compared to the vacuum system. Our results indicated that the axial ligand impacts the electronic properties of the catalyst, such as the HOMO and LUMO surfaces, LUMO energies, hardness and electronegativity. Energies of reaction were correlated with the hardness of the catalysts.