Electrocatalytic proton reduction by a dicobalt tetrakis-Schiff base macrocycle in nonaqueous electrolyte

Abstract

A series of dicobalt complexes, Co2L(2+) 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 Co(II) ions, which are bridged by an acetate with a Co-Co distance of 3.2 Å. Cyclic voltammetry (CV) of Co2L(2+) 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 Co2L(2+) and Co2LAc(+) were also compared to those of Zn2LAc(+) and H4L(2+) to identify the ligand-center oxidations and reductions. Addition of trifluoroacetic acid (TFA) or acetic acid (AcOH) to the electrolyte solutions of Co2L(2+) 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 Co2L(2+) 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 Co2L(2+) with AcOH generated H2 in 72-94% Faradaic efficiency as determined by gas chromatography. Initial studies suggest Co(I)2 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.