Electrochemical generation of hydrogen from acetic acid using a molecular molybdenum-oxo catalyst

We recently reported the catalytic generation of hydrogen from water mediated through the in situ reduction of the molybdenum(iv)-oxo complex [(PY5Me₂)MoO]²⁺ (1; PY5Me₂ = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine) at a mercury electrode. To gain further insight into this unique molecular motif for hydrogen production, we have now examined the competence of this complex for the catalytic reduction of protons on an alternative electrode material. Herein, we demonstrate the ability of the molybdenum-oxo complex 1 to reduce protons at a glassy carbon electrode in acidic organic media, where the active catalyst is shown to be diffusing freely in solution. Cyclic and rotating disk voltammetry experiments reveal that three reductive electrochemical processes precede the catalytic generation of hydrogen, which occurs at potentials more negative than -1.25 V vs. SHE. Gas chromatographic analysis of the bulk electrolysis cell headspace confirms that hydrogen is generated at a Faradaic efficiency of 99%. Under pseudo-first order conditions with an acid-to-catalyst ratio of >290, a rate constant of 385 s^-1 is calculated for the reduction of acetic acid in acetonitrile. Taken together, these data show that metal-oxo complex 1 is a competent molecular motif for catalytic generation of hydrogen from protons under soluble and diffusion-limited conditions.