SnIV Metalloporphyrin/CoIII Complex: An All-Abundant-Element System for the Photocatalytic Production of H2 in Aqueous Solution

A new, molecular system for the light-driven production of hydrogen in aqueous solution was developed by combining a water-soluble tin porphyrin ([(SnCl2TPPC)-Cl-IV], A) acting as photosensitizer with a cobalt-based proton-reduction catalyst ([(CoCl)-Cl-III(dmgH)(2)(py)], C). Under visible light illumination and with triethanolamine (TEOA) as electron source, the system evolves H-2 for hours and is clearly catalytic in both dye and catalyst. A detailed analysis of the relevant redox potentials in combination with time-resolved spectroscopy resulted in the development of a Z-scheme type model for the flow of electrons in this system. Key intermediates of the proposed mechanism for the pathway leading to H-2 are the porphyrin dye's highly oxidizing singlet excited state (1)A* (E similar to, +1.3 V vs NHE), its strongly reducing isobacteriochlorin analogue (E similar to +0.95 V), and the Co-I form of C (E similar to -0.8 V), acting as catalyst for H-2 formation. Among other results, the suggested reaction sequence is supported by the detection of a shortened excited-state lifetime for singlet (1)A* (tau similar to 1.75 ns) in the presence of TEOA and the ultraviolet visible detection of the Sr-IV isobacteriochlorin intermediate at lambda = 610 nm. Thus, a molecular, conceptually biomimetic, and precious-metal-free reaction chain was found which photocatalytically generates H-2 in a 100% aqueous system from an electron donor with a high oxidation potential (E(TEOA) similar to +1.1 V). On the other hand, at identical conditions, this photoreaction chain yields H-2 markedly slower than a system using the photosensitizer [Re-I(CO)(3)(bpy) (py)](+), probably due to the much longer excited-state lifetime (tau similar to 120 ns) of the rhenium dye and better electron-transfer rates caused by its simple single-electron photoreduction chemistry.