Ross W. Hogue, Olivier Schott, Garry S. Hanan, Sally Brooker

17 Cobalt complexes - eleven dinuclear cobalt(II) complexes and three tetranuclear cobalt complexes (two mixed valent) of ditopic ligands with varying N-donor aromatic bridging moieties and pendant pyridine side arms, as well as three mononuclear cobalt(II) complexes of Schiff base macrocyclic ligands - have been screened for photocatalytic hydrogen evolution reaction (HER) activity. All 17 complexes are active catalysts for HER, in both DMF and aqueous solution, in tandem with the [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) photosensitiser. All are benchmarked to the literature standard [CoIII(dmgH)2(py)Cl] (dmg = dimethyl glyoxime; py = pyridine) in identical conditions. Two families, of dinuclear cobalt(II) complexes of bis-tetradentate ligands that provide a triazole bridging moiety and of mononuclear cobalt(II) complexes of tetradentate Schiff base macrocycles, were found to be the most active catalysts, outperforming [CoIII(dmgH)2(py)Cl] by two-to-three fold. Within these two families, the use of shorter alkyl linkers between the N donors, and hence smaller chelate ring sizes, was found to significantly enhance catalytic performance, while the variation of peripheral functional groups was found to have little effect. This last point will be convenient for subsequent surface immobilisation studies.