Cobalt(II)–Salen Complexes for Photocatalytic Hydrogen Production in Noble Metal-Free Molecular Systems

Developing cost-effective hydrogen-production systems is always appealing in order to satisfy the future energy request. Here, two Co(II)–salen complexes 1 and 2 were firstly introduced into homogeneous photocatalytic hydrogen-evolution systems. Based on these two cobalt complexes, we developed noble-metal-free hydrogen production systems using xanthene dyes [such as Disodium salts of Eosin Y (EY 2− ), Rose Bengal (RB 2− ) and Fluorescein (Fl 2− )] as photosensitizers and trimethylamine (TEA) as sacrificial donor. Complex 1 presented the best catalytic performance for hydrogen production with a TON of 319 based on catalyst within 9 h irradiation, which is superior to other cobalt complexes in noble-metal-free systems. UV–Vis spectra studies demonstrate that EY 2− suffered quick decomposition, especially under the presence of complex 1 , which should be responsible for hydrogen-production deactivation. The thermodynamically favorable photo-induced electron transfer from 3 *EY 2− to complex 1 was supported by investigations involving fluorescence quenching and cyclic voltammetry studies. However, the immediate color change (within seconds) as the hydrogen production systems exposed under the irradiation indicates that EY 2− firstly suffered dehalogenation which functions as the real active dyes. Graphical Abstract