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)](2+) and [PtCl2(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 {(Ru2Pt2II)-Pt-II) 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 Pt ... H(water or acetic acid) hydrogen bonds were characterized by DFT calculations.