Cobalt Complex of a Tetraamido Macrocyclic Ligand as a Precursor for Electrocatalytic Hydrogen Evolution

Hydrogen (H-2) is a clean fuel that can potentially store renewable energy and overcome some of the environmental problems that arise from fossil-fuel consumption. One attractive approach is to produce H-2 from water electrocatalytically using molecular complexes that can be systematically improved through ligand modifications. We report cobalt and nickel complexes supported by tetraamido macrocyclic ligands (TAMLs), which exclusively consist of earth-abundant elements. Although TAML systems are well established in high-valent transition-metal chemistry, little is known about their reactivity in reductive catalysis despite the electron-rich nature of the tetraanionic TAML. Thus we explored the utility of these nucleophilic -ate complexes as potential electrocatalysts for H-2 evolution using water as the proton source. Controlled potential electrolysis experiments were performed, and the cobalt TAML variant exhibited catalytic H-2 evolution activity in acetonitrile containing 1.0 M water but was inactive in purely aqueous solutions. Further investigation revealed that cobalt metal nanoparticles were electrodeposited as the active catalyst for H-2 evolution. We propose that these disparities in reactivity arise from the different number of water molecules coordinated to the cobalt center, with intermediate concentrations favoring a square pyramidal structure with labile ligands, whereas high concentrations of water result in a kinetically inert octahedral complex with no empty coordination sites.