Unraveling the Role of Ligands in the Hydrogen Evolution Mechanism Catalyzed by [NiFe] Hydrogenases

DFT investigations have been carried out on the hydrogen evolution reaction (HER) mechanism followed by [NiFe] hydrogenases. Calculations on the active site of the [NiFe] hydrogenase from Desulfovibrio vulgaris str. Miyazaki F reveal that H-2 is formed as the final product through the singlet multiplicity pathway. Non-spontaneous reaction energies can be seen for both H+/e(-) additions to the reactive sulfur atom from the truncated cysteine residues, being the limiting steps of the whole reaction. In contrast, transfers toward the metal environment to produce the bridging hydride and the bonded H-2 molecule at the Ni-C and 12 steps, respectively, are spontaneous processes. Our DFT results highlight the role of the ligands attached to both the Ni and Fe centers. When the protein ligand environment is spatially confined, reaction energies for the HER are lower than those when the ligand carbons are able to freely adjust. In addition, larger changes can be seen on interchanging the [CN](-) and CO ligands on the Fe center; in particular, the energy profile dramatically changes as [CN](-) ligands are replaced by CO. These results may guide materials synthesis efforts toward optimized HER catalysts.