Proton affinity studies of nickel N2S2 complexes and control of aggregation

Abstract

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 (pKa\u2009=\u200910.9 in methanol). A more constrained nickel complex with a diazacycloheptane N to N linker required stronger acids such as 2,2-dichloroacetic acid (pKa\u2009=\u20096.38 in methanol) to form the trimetallic complex and featured the formation of an NiN2S2H+ complex with acetic acid (pKa\u2009=\u20099.63 in methanol). The most strained ligand, which featured a diazacyclohexane backbone, readily dissociated from the nickel center upon mixture with acids with pKa\u2009≤\u20099.63 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.Graphical abstract