Electrochemically and Photochemically Induced Hydrogen Evolution Catalysis with Cobalt Tetraazamacrocycles Occurs Through Different Pathways

Abstract

Cobalt complexes containing equatorial tetraazamacrocyclic ligands are active catalysts for the hydrogen evolution reaction in pure aqueous conditions. We investigated the effect of different groups directly linked to the macrocyclic ligand (-NH-, -NCH₃ -, or -N(CH₂ OH)-). In electrochemically induced hydrogen evolution catalysis at pH 4, the rate determining step is the protonation of the reduced Co^I species that gives a cobalt hydride (Co^III -H), a key intermediate towards the H-H bond formation. In sharp contrast, under photochemical conditions using [Ru(bpy)₃ ]²⁺ (bpy=2,2'-bipyridine) as a photosensitizer and ascorbate as sacrificial electron donor, the formation of a Co⁰ species that quickly protonates to give a Co^II -H is proposed. In this scenario, the rate determining step is the H-H bond formation that occurs in an intermolecular fashion from the Co^II -H species and a water molecule. Both mechanisms are supported by DFT calculations, which allowed us to estimate the pK_a values of the Co^III -H and Co^II -H species and transition states based on intramolecular and intermolecular H-H bond formation from Co^II -H.

Keywords: catalysis; cobalt; hydrogen evolution; macrocyclic ligand; mechanism.