Thermodynamic Trends for Reduction of CO by Molecular Complexes

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, and 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 (ΔG°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, for which ΔG°H2 decreases moving up the group, Re >
Mn. Computational analysis indicates the trends can be described in
terms of electrostatic effects, where a low ΔG°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 a more
energetically accessible H2 addition, which will facilitate
the development of molecular catalysts for reduction of CO.