Computational Investigation of the Mechanism of an Octahedral Ni(II) Proton Reduction Catalyst and Importance of Intramolecular Hydrogen Bonding

Water-splitting to make hydrogen gas is of extreme importance in the field of alternative energy research. Transition-metal complexes are capable of catalyzing the conversion of water to hydrogen at higher pH, with low overpotential. Our research focuses on the importance of intramolecular hydrogen bonding (H-bonding) on the pKa and thermodynamic stability of the catalytic intermediates of a well-known proton-reduction catalyst, nickel (II) tris-pyridinethiolate. Density Functional Theory (DFT) calculations on the parent catalyst and eleven derivatives demonstrate geometric isomer formation after the protonation step of catalysis. These isomers differ in the relative thermodynamic stabilities and pKa values, which can be attributed to the difference in the strength of a hydrogen-bonding interaction between the proton on the pyridyl nitrogen atom and a sulfur atom from a neighboring ligand. The H-bond strength is directly proportional to the thermodynamic stability and properties of the protonated intermediates, which is well-understood in bio-macromolecules (proteins), but relatively unexplored in small-molecule catalysts. This research demonstrates the significance of considering isomer formation while modeling the mechanism related to octahedral complexes. This work also indicates the prospect of ligand modification to manipulate H-bonding in homogeneous catalysis to simultaneously target both pKa and reduction potential values.