Self-Assembled Nickel-4 Supramolecular Squares and Assays for HER Electrocatalysts Derived Therefrom.

Solid-state structures find a self-assembled tetrameric nickel cage with carboxylate linkages, [Ni(NS'O)I(CHCN)] ([]), resulting from sulfur acetylation by sodium iodoacetate of an [NiNS] dimer in acetonitrile. Various synthetic routes to the tetramer, best described from XRD as a molecular square, were discovered to generate the hexacoordinate nickel units ligated by NS, iodide, and two carboxylate oxygens, one of which is the bridge from the adjacent nickel unit in []. Removal of the four iodides by silver ion precipitation yields an analogous species but with an additional vacant coordination site, [], a cation but with coordinated solvent molecules. This also recrystallizes as the tetramer []. In solution, dissociation into the (presumed) monomer occurs, with coordinating solvents occupying the vacant site [Ni(NS'O)I(solv)], ([Ni-I]). Hydrodynamic radii determined from H DOSY NMR data suggest that monomeric units are present as well in CDCl. Evans method magnetism values are consistent with triplet spin states in polar solvents; however, in CDCl solutions no paramagnetism is evident. The abilities of [] and [] to serve as sources of electrocatalysts, or precatalysts, for the hydrogen evolution reaction (HER) were explored. Cyclic voltammetry responses and bulk coulometry with gas chromatographic analysis demonstrated that a stronger acid, trifluoroacetic acid, as a proton source resulted in H production from both electroprecatalysts; however, electrocatalysis developed primarily from uncharacterized deposits on the electrode. With acetic acid as a proton source, the major contribution to the HER is from homogeneous electrocatalysis. Overpotentials of 490 mV were obtained for both the solution-phase [] and []. While the electrocatalyst derived from [] has a substantially higher TOF (102 s) than [] (19 s), it has a shorter catalytically active lifespan (4 h) in comparison to [] (>18 h).