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-hydrogen-evolving molecular catalyst (1), given by a coupling of Ru(bpy)(2)(5-amino-phen) and [PtCl(2)(4,4'-dicarboxy-bpy)] (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline), was investigated in detail. The H(2) 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 H(2) formation. A 2:1 coupling product of 1 and ethylenediamine (i.e., a {Ru(II) (2)Pt(II) (2)} 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 H(2) formation through the intermolecular path competes with the intramolecular path. Molecular orbital diagrams, conformational features, and PtH(water or acetic acid) hydrogen bonds were characterized by DFT calculations.