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) (H2DMTH, H2L1). The neutral complexes NiL1 and PdL1 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 pK a of this nitrogen was determined by spectrophotometric titration in acetonitrile to be 12.71 for NiL1 and 13.03 for PdL1. Cyclic voltammograms of both NiL1 and PdL1 in acetonitrile exhibit diffusion-controlled, reversible ligand-centered events at −1.83 and −1.79 V (vs ferrocenium/ferrocene) for NiL1 and PdL1, respectively. A quasi-reversible, ligand-centered event is observed at −2.43 and −2.34 V for NiL1 and PdL1, 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–1 at an overpotential of 0.74 V for NiL1 and 8280 s–1 at an overpotential of 0.44 V for PdL1. Density functional theory (DFT) computations suggest both complexes follow a ligand-centered HER mechanism where the metals remain in the +2 oxidation state.