Electrocatalytic Proton Reduction by a Dicobalt Tetrakis-Schiff Base Macrocycle in Nonaqueous Electrolyte

A series of dicobalt complexes, Co2L2+ and Co2LAc+, where L is a N6O2 coordinating bis­(phenolate) tetrakis-Schiff base ligand, have been synthesized and characterized via electrochemical and spectroscopic techniques. [Co2LAc]­(ClO4) crystallizes in the monoclinic space group P21/n, and the structure reveals a highly distorted octahedral geometry for the CoII ions, which are bridged by an acetate with a Co–Co distance of 3.2 Å. Cyclic voltammetry (CV) of Co2L2+ and Co2LAc+ in anhydrous acetonitrile reveals large anodic/cathodic peak splitting for the CoII/III redox transitions and a multielectron wave for the CoII/I reductions. The CVs for Co2L2+ and Co2LAc+ were also compared to those of Zn2LAc+ and H4L2+ to identify the ligand-center oxidations and reductions. Addition of trifluoroacetic acid (TFA) or acetic acid (AcOH) to the electrolyte solutions of Co2L2+ results in an irreversible reduction wave that is consistent with electrocatalytic H+ reduction. The catalytic rate law shows a first order dependence on [catalyst] and a second order dependence on [acid]. Using TFA as the acid source, the electrocatalytic H+ reduction rate constant for Co2L2+ was determined to be 138 M–2 s–1, while coordination of acetate slows the rate to 63 M–2 s–1 for Co2LAc+. Controlled potential electrolysis of Co2L2+ with AcOH generated H2 in 72–94% Faradaic efficiency as determined by gas chromatography. Initial studies suggest CoI2 as the catalytically active form of the complex. These complexes represent a new class of Co-based electrocatalytic H+ reduction catalysts that utilize a bimetallic active site.