[NiFe]-hydrogenases use a nickel active site to catalyze production of hydrogen, a carbon-neutral fuel, with high rates at low overpotentials. This makes them attractive models for synthetic catalysts since it may be possible to design mimics to be more robust in industrial, electrocatalytic applications than enzymes. Inspired by the active site of [NiFe]-hydrogenase enzymes, we synthesized and characterized model complexes, [Ni(bdt)(dppf)], where the nickel atom is coordinated by ferrocenyl(diphenyl)phosphine (dppf) and benzenedithiol (bdt) ligands. To investigate the impact of aromatic substituents on the electronic environment of the nickel and concomitant catalysis, this project compares the electrocatalytic hydrogen production activity of complexes with bdt and methyl-functionalized bdt (see figure). The complex with the electron-donating, methyl-substituted bdt has an electrocatalytic hydrogen production rate that is decreased by an order of magnitude compared to the electron-neutral analogous complex. Density-functional theory (DFT) calculations were used to compare the electronic structures of the two [Ni(bdt)(dppf)] complexes and evaluate the impact of the methyl substituent. The electron-donating methyl substituent in the bdt ligand increases electron density and nucleophilicity at both bdt sulfur sites, and this modification substantially impacts progression through the catalytic cycle. This work provides valuable insights for future development of synthetic hydrogenase mimics and organometallic electrocatalysts.

Aromatic substituent effects modulate catalytic activity of synthetic [NiFe]-Hydrogenase mimics

2019 Fall National Meeting

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