Proton affinity studies of nickel N₂S₂ complexes and control of aggregation

The thiolate ligands of [NiFe]-H₂ase enzymes have been implicated as proton-binding sites for the reduction/oxidation of H⁺/H₂. This study examines the ligand effect on reactivity of NiN₂S₂ complexes with an array of acids in methanol solution. UV-Vis absorption spectroscopy is utilized to observe the transformation from the monomeric species to a trimetallic complex that is formed after proton-induced ligand dissociation. Nickel complexes with a flexible (propyl and ethyl) N to N linker were found to readily form the trimetallic complex with acids as weak as ammonium (pK_a = 10.9 in methanol). A more constrained nickel complex with a diazacycloheptane N to N linker required stronger acids such as 2,2-dichloroacetic acid (pK_a = 6.38 in methanol) to form the trimetallic complex and featured the formation of an NiN₂S₂H⁺ complex with acetic acid (pK_a = 9.63 in methanol). The most strained ligand, which featured a diazacyclohexane backbone, readily dissociated from the nickel center upon mixture with acids with pK_a ≤ 9.63 and showed no evidence of a trimetallic species with any acid. This research highlights the dramatic differences in reactivity with proton sources that can be imparted by minor alterations to ligand geometry and strain.