Photocatalytic Hydrogen Production Based on a Serial Metal‐Salen Complexes and the Reaction Mechanism

Abstract 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 (EY 2− ) as photosensitizers and trimethylamine (TEA) as sacrificial donors. In a Ni(II)‐salen/EY 2− /TEA system, a TON of 362 based on Ni(II)‐salen was achieved in 5.5 h with one time complement for EY 2− . The stability study revealed the quick photodecomposition of EY 2− in the presence of ether Ni(II)‐salen, TEA or both chemicals, and quick dehalogenation of EY 2− 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 7.62×10 6 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.