Photo-hydrogen-evolving molecular catalysts consisting of polypyridyl ruthenium(II) photosensitizers and platinum(II) catalysts: Insights into the reaction mechanism

The mechanism of photoinduced hydrogen evolution from water driven by the first photo-hydrogenevolving molecular catalyst (1), given by a coupling of [Ru(bpy)₂(5-aminophen)]²⁺ and [PtCl₂(4,4'- dicarboxybpy)] (bpy=2,2'-bipyridine, phen=1,10-phenanthroline), was investigated in detail. The H₂ evolution rate was found to obey Michaelis-Menten enzymatic kinetics with regard to the concentration of EDTA (ethylenediamine tetra-acetic acid disodium salt, sacrificial electron donor), which indicates that an ion-pair formation between the dicationic 1 and the dianionic form of EDTA (pH 5) is a key step leading to H₂ formation. A 2:1 coupling product of 1 and ethylenediamine (i.e., a {Ru^II₂Pt ^II₂} complex 2) was found to show significantly higher photo-hydrogen-evolving (PHE) activity than 1, which revealed the validity of the bimolecular activation proposed in our previous study. The PHE activity of 2 was also observed to be linear to the concentration of 2, which indicates that H₂ formation through the intermolecular path competes with the intramolecular path. Molecular orbital diagrams, conformational features, and Pt···H(water or acetic acid) hydrogen bonds were characterized by DFT calculations.