In this study we report a pair of homogenous electrocatalysts for the hydrogen evolution reaction (HER) based on the non-innocent ligand diacetyl-2-(4-methyl-3-thiosemicarbazonate)-3-(2-pyridinehydrazonato) (H2DMTH, L). The neutral complexes NiL and PdL were synthesized and characterized by spectroscopic and electrochemical methods. The complexes contain a non-coordinating, basic hydrazino nitrogen that is protonated during the HER. The pKa of this nitrogen was determined by spectrophotometric titration in acetonitrile to be 12.71 for NiL and 13.03 for PdL. Cyclic voltammograms of both NiL and PdL in acetonitrile exhibit two reversible events that are under diffusion control. The first event is ligand centered with an E1/2 of -1.83 V for NiL and -1.79 V for PdL (vs Fc+/Fc). The second event is a metal centered reduction with an E1/2 of -2.43 V for NiL and -2.34 V for PdL. Evaluation of HER activity in acetonitrile was investigated using a series of neutral and cationic acids for both catalysts. Kinetic isotope effect studies suggest the pre catalytic event observed is associated with a proton coupled electron transfer step. The highest turnover frequency values observed were 6150 s-1 at an overpotential of 0.74 V for NiL and 8280 s-1 at an overpotential of 0.44 V for PdL. Both complexes undergo a ligand centered reduction of hydrogen unlike the similar NiATSM which undergoes metal assisted ligand centered reduction.

Modulating the mechanism of HER with a redox active ligand

ACS Spring 2022

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