PNNL

Abstract

Selective reduction of CO2 into fuels and chemical feedstocks is highly desirable to reduce our dependence on fossil fuels. Most molecular catalysts afford 2e- reduction products, such as CO or HCO2-, as opposed to more reduced products. Here we present an analysis of the thermodynamic limitations for reduction of the CO ligand in the form of a series of isostructural Group 6 carbonyl complexes, Cp*M(CO)3(P(OMe)3)+ (M = Cr, Mo, W). The free energy for stepwise transfer of a hydride (H-) and a proton (H+) to the CO ligand, resulting in a hydroxycarbene (CHOH) complex, was measured by equilibration with H-/H+ donors and acceptors with known hydricity or acidity. Together, these two reaction steps are equivalent to a net addition of H2 across the CO ligand. A large and unfavorable free energy for H2 addition (?G°H2) was measured for all three complexes, and decreases in the order Cr > Mo > W. The trend for these complexes is opposite to the trend previously reported for Group 7 carbonyl complexes, where H2 addition comes more favorable moving up the group, Re > Mn. Computational analysis indicates the periodic trends can be described in terms of electrostatic effects, where a low ?G°H2 is obtained in complexes that balance the atomic charges of the M–C=O fragment of the complex. These findings can be used to design metal carbonyl complexes with more energetically accessible H2 addition, which will facilitate the development of molecular catalysts for reduction of CO.