Electrocatalytic hydrogen evolution by macrocyclic cobalt and nickel complexes

A series of cobalt and nickel complexes with macrocyclic imine ligands such as difluoroboryldiglyxomime and [14]-Tetraene-N4 were synthesized for electrochem. hydrogen evolution catalysis. The complexes catalyzed the redn. of proton to dihydrogen at potentials as pos. as -0.26 V vs. SCE in acidic acetonitrile solns. The catalysis by the cobalt complexes was triggered by the redn. of cobalt(II) precursors to cobalt(I) species, while the catalysis by the nickel complexes was mediated by the redn. of nickel(II) precursors to nickel(II) stabilized ligand radicals. The influence of acid strengths on catalysis was discussed: for the cobalt mediated catalysis, using stronger acids as the proton sources gave higher catalytic rates; for the nickel mediated catalysis, the catalytic rates did not vary significantly when using different acids. Structure-function study of the catalysts was also described: more electron-deficient complexes catalyzed hydrogen evolution at more pos. potentials, yet with lower reaction rates.