Computational Investigation into Heteroleptic Photoredox Catalysts Based on Nickel(II) Tris-Pyridinethiolate for Water Splitting Reactions

This work demonstrates a strategy to fine-tune the efficiency of a photoredox water splitting Ni(II) tris-pyridinethiolate catalyst through heteroleptic ligand design using computational investigation of the catalytic mechanism. Density functional theory (DFT) calculations, supported by topology analyses using quantum theory of atoms in molecules (QTAIM), show that the introduction of electron donating (ED) −CH₃ and electron withdrawing (EW) −CF₃ groups on the thiopyridyl (PyS^–) ligands of the same complex can tune the pK_a and E⁰, simultaneously. Computational modeling of two heteroleptic nickel(II) tris-pyridinethiolate complexes with 2:1 and 1:2 ED and EW −CH₃ and −CF₃ group containing PyS^– ligands, respectively, suggests that the ideal combination of EW to ED groups is 2:1. This work also outlines the possibility of formation of a large number of isomers after the protonation of one of the pyridyl N atoms and suggests that to acquire unambiguous computational results it is necessary to carefully account for all possible geometric isomers.