Bioinspired Design and Computational Prediction of SCS Nickel Pincer Complexes for Hydrogenation of Carbon Dioxide

Inspired by the structures of the active site of lactate racemase and H-2 activation mechanism of mono-iron hydrogenase, we proposed a series of sulphur-carbon-sulphur (SCS) nickel complexes and computationally predicted their potentials for catalytic hydrogenation of CO2. Density functional theory calculations reveal a metal-ligand cooperated mechanism with the participation of a sulfur atom in the SCS pincer ligand as a proton receiver for the heterolytic cleavage of H-2. For all newly proposed complexes containing functional groups with different electron-donating and withdrawing abilities in the SCS ligand, the predicted free energy barriers for the hydrogenation of CO2 to formic acid are in a range of 22.2-25.5 kcal/mol in water. Such a small difference in energy barriers indicates limited contributions of those functional groups to the charge density of the metal center. We further explored the catalytic mechanism of the simplest model complex for hydrogenation of formic acid to formaldehyde and obtained a total free energy barrier of 34.6 kcal/mol for the hydrogenation of CO2 to methanol.