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

The zinc(II) complex of diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-hydrazonepyridine), ZnL (), was prepared and evaluated as a precatalyst for the hydrogen evolution reaction (HER) under homogeneous conditions in acetonitrile. Complex is protonated on the noncoordinating nitrogen of the hydrazonepyridine moiety to yield the active catalyst Zn(HL)OAc () upon addition of acetic acid. Addition of methyl iodide to yields the corresponding methylated derivative ZnLI (). In solution, partial dissociation of the coordinated iodide yields the cationic derivative . Complexes - were characterized by H NMR, FT-IR, and UV-visible spectroscopies. The solid-state structures of and 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 for solutions of at an overpotential of 1.27 V and a TOF of 6700 s for solutions of at an overpotential of 0.56 V. For both complexes, the required potential for catalysis, , is larger than the thermodynamic reduction potential, , indicative of a kinetic barrier attributed to intramolecular proton rearrangement. The effect is larger for solutions of (+440 mV) than for solutions of (+160 mV). Controlled potential coulometry studies were used to determine faradaic efficiencies of 71 and 89% for solutions of and , 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 by X-ray photoelectron spectroscopy indicates the complex remains intact upon deposition. A proposed ligand-centered HER mechanism with as a precatalyst to 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.