A macrocyclic N-heterocyclic carbene (NHC)–cobalt complex was found to act as an improved H2-evolving catalyst in a [Ru(bpy)3]2+-sensitized photosystem using methylviologen as a redox acceptor (MV2+ + e− → MV+˙, MV2+ = N,N′-dimethyl-4,4′-bipyridinium), which provides a driving force of only 150 meV for H2 evolution at pH 5.0.
