Peripheral Bromination for Strongly Affecting the Structural, Electronic, and Catalytic Properties of Cobalt Corroles

The feasibility of a hydrogen-based economy critically depends on the development of catalysts for the hydrogen evolution reaction (HER) that do not rely on Pt or other noble metals. Contemporary efforts are focused on developing first-row transition metal complexes that will be operative at low overpotentials and catalyze the reaction with high efficacy and turnover rates. We now report a surprisingly strong effect of bromide substituents on the structure, coordination chemistry, electronic spectrum, reduction potentials, and catalytic activity of an already electron-poor cobalt corrole. The six-coordinate cobalt(III) complex of the brominated corrole displays a very nonplanar macrocycle, its axial pyridines are perpendicular to each other, the maxima in the electronic spectrum are red-shifted by almost 70 nm, and it is reduced by 600 mV less negative potentials. It catalyzes the HER from an organic acid in an organic solvent with a very low onset potential of -0.96 V vs. the Fc⁺/Fc couple and has been used for the preparation of a catalyst-modified cathode to produce hydrogen gas from acidic water with 97% Faradaic efficacy at potentials as low as -0.4 V vs. RHE.