Utilizing Charge Effects and Minimizing Intramolecular Proton Rearrangement to Improve the Overpotential of a Thiosemicarbazonato Zinc HER Catalyst

Abstract

The zinc(II) complex of diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-hydrazonepyridine), ZnL¹ (<b>1</b>), was prepared and evaluated as a precatalyst for the hydrogen evolution reaction (HER) under homogeneous conditions in acetonitrile. Complex <b>1</b> is protonated on the noncoordinating nitrogen of the hydrazonepyridine moiety to yield the active catalyst Zn(HL¹)OAc (<b>2</b>) upon addition of acetic acid. Addition of methyl iodide to <b>1</b> yields the corresponding methylated derivative ZnL²I (<b>3</b>). In solution, partial dissociation of the coordinated iodide yields the cationic derivative <b>3'</b>. Complexes <b>1</b>-<b>3</b> were characterized by ¹H NMR, FT-IR, and UV-visible spectroscopies. The solid-state structures of <b>2</b> and <b>3</b> were determined by single crystal X-ray diffraction. HER studies conducted in acetonitrile with acetic acid as the proton source yield a turnover frequency (TOF) of 7700 s^-1 for solutions of <b>1</b> at an overpotential of 1.27 V and a TOF of 6700 s^-1 for solutions of <b>3</b> at an overpotential of 0.56 V. For both complexes, the required potential for catalysis, <i>E</i>_cat/2, is larger than the thermodynamic reduction potential, <i>E</i>_1/2, indicative of a kinetic barrier attributed to intramolecular proton rearrangement. The effect is larger for solutions of <b>1</b> (+440 mV) than for solutions of <b>3</b> (+160 mV). Controlled potential coulometry studies were used to determine faradaic efficiencies of 71 and 89% for solutions of <b>1</b> and <b>3</b>, respectively. For both catalysts, extensive cycling of potential under catalytic conditions results in the deposition of a film on the glassy carbon electrode surface that is active as an HER catalyst. Analysis of the film of <b>3</b> by X-ray photoelectron spectroscopy indicates the complex remains intact upon deposition. A proposed ligand-centered HER mechanism with <b>1</b> as a precatalyst to <b>2</b> is supported computationally using density functional theory (DFT). All catalytic intermediates in the mechanism were structurally and energetically characterized with the DFT/B3LYP/6-311g(d,p) in solution phase using a polarizable continuum model (PCM). The thermodynamic feasibility of the mechanism is supported by calculation of equilibrium constants or reduction potentials for each proposed step.