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 P2(1)/n, and the structure reveals a highly distorted octahedral geometry for the Co-II ions, which are bridged by an acetate with a Co-Co distance of 3.2 angstrom. Cyclic voltammetry (CV) of Co2L2+ and Co2LAc+ in anhydrous acetonitrile reveals large anodic/cathodic peak splitting for the Co-II/III redox transitions and a multielectron wave for the Co-II/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 add 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 H-2 in 72-9496 Faradaic efficiency as determined by gas chromatography. Initial studies suggest Co-2(I) 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.