Theoretical Studies on the Catalysis of the Reverse Water−Gas Shift Reaction Using First-Row Transition Metal β-Diketiminato Complexes.

The reverse water−gas shift reaction CO2+ H2→ H2O + CO has been investigated using a set of homogeneous catalyst models L′MI(L′ = β-diketiminate, C3N2H5−; M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn). The thermodynamics of prototypical reaction pathways were simulated at two levels of theory: B3LYP/6-311+G(d) and B3LYP/aug-cc-pVTZ. The modeled catalytic reaction has been considered in the following steps: coordination of CO2by the catalyst to generate a carbon dioxide complex, L′M(CO2); scission of L′M(CO2) to yield L′M(CO) and L′M(O); L′M(O) hydrogenation to form L′M(H2O). The final products, H2O and CO, were obtained from the dissociation of L′M(H2O) and L′M(CO). All of the reactants, intermediates, and products were modeled, where different possible conformers and multiplicities were identified and considered as potential minima. The reaction enthalpy ΔH, of all steps for each catalyst as a function of transition metal have been determined. The Mn and Fe catalysts show more thermodynamically accessible pathways than the other catalyst models studied. The overall reaction enthalpy has been determined not only by B3LYP/6-311+G(d) and B3LYP/aug-cc-pVTZ but also via a more rigorous ab initio electron-correlation-based approach, the correlation consistent Composite Approach (ccCA). [ABSTRACT FROM AUTHOR]