Ligand-Centered Hydrogen Evolution with Ni(II) and Pd(II)DMTH.

In this study, we report a pair of electrocatalysts for the hydrogen evolution reaction (HER) based on the noninnocent ligand diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-pyridinehydrazone) (HDMTH, HL). 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 p 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 diffusion-controlled, reversible ligand-centered events at -1.83 and -1.79 V (vs ferrocenium/ferrocene) for NiL and PdL, respectively. A quasi-reversible, ligand-centered event is observed at -2.43 and -2.34 V for NiL and PdL, respectively. The HER activity in acetonitrile was evaluated using a series of neutral and cationic acids for each catalyst. Kinetic isotope effect (KIE) studies suggest that the precatalytic event observed is associated with a proton-coupled electron transfer step. The highest turnover frequency values observed were 6150 s at an overpotential of 0.74 V for NiL and 8280 s at an overpotential of 0.44 V for PdL. Density functional theory (DFT) computations suggest both complexes follow a ligand-centered HER mechanism where the metals remain in the +2 oxidation state.