KGaA, Weinheim Three metal-salen (Ni, Cu, Zn) complexes were firstly introduced into homogeneous photocatalytic hydrogen-evolution systems. Based on these complexes, we developed noble-metal-free hydrogen production systems using disodium salts of Eosin Y (EY2−) as photosensitizers and trimethylamine (TEA) as sacrificial donors. In a Ni(II)-salen/EY2−/TEA system, a TON of 362 based on Ni(II)-salen was achieved in 5.5 h with one time complement for EY2−. The stability study revealed the quick photodecomposition of EY2− in the presence of ether Ni(II)-salen, TEA or both chemicals, and quick dehalogenation of EY2− was observed in the presence of TEA under irradiation. Besides, relatively slower photodecomposition of Ni(II)-salen was also found. These factors should be responsible for the deactivation of the photolysis system. Investigation of electrocatalytic proton reduction by Ni(II)-salen showed a CECE mechanism. An overall catalytic rate constant of 7.62×106 M−2 s−1 was achieved. Combined with the stability analysis, photo-induced electron transfer and electrochemistry investigation, for the first time, the photocatalytic hydrogen production mechanism for metal-salen complexes (e. g. Ni(II)-salen) was proposed
