Light-Induced H2 Evolution with a Macrocyclic Cobalt Diketo-Pyrphyrin as a Proton-Reducing Catalyst.

Cobalt complexes are well-known catalysts for photocatalytic proton reduction in water. Macrocyclic tetrapyridyl ligands (pyrphyrins) and their CoII complexes emerged in this context as a highly efficient class of H2 evolution catalysts. On the basis of this framework, a new macrocyclic CoII complex consisting of two keto-bridged bipyridyl units (Co diketo-pyrphyrin) is presented. The complex is synthesized along a convenient route, is well soluble in water, and shows high activity as a water reduction catalyst (WRC). In an aqueous system containing [Ru(bpy)3]Cl2 as a photosensitizer and NaAscO as a sacrificial electron donor, turnover numbers (TONs) of 2500 H2/Co were achieved. Catalysis is terminated by a limited electron supply and decomposition of the photosensitizer but not of the WRC, highlighting the distinct stability of Co diketo-pyrphyrin.