[PtCl(terpy)]Cl·2H2O (terpy = 2,2′:6′, 2″-terpyridine) (1Cl·2H2O) is the first example serving as a bifunctional system promoting both photosensitization and hydrogenic activation as an H2-evolving catalyst in aqueous media in the presence of a sacrificial electron donor (EDTA) under visible-light illumination. The rate of H2 formation has turned out to be quadratic to the concentration of 1, suggesting that a bimolecular path determines the overall reaction rate for the photoinduced H2 formation. It is suggested that the bimolecular mechanism operates at the photosensitization process through the formation of the so-called 3MMLCT excited state and thus formed dinuclear photosensitizer itself provides a site for the hydrogenic activation (MMLCT = metal-metal-to-ligand charge transfer). The stability of the complex during the photolysis was successfully confirmed by ESI-TOF mass spectrometry. The photolysis was carried out in both the absence and the presence of mercury to rule out the formation of colloidal platinum. The rate of H2 evolution considerably decreases when the photolysis was carried out by using acetate, propionate, or phosphate as a buffer reagent instead of MES (MES = 2-morpholinoethanesulfonic acid), which was used in typical experiments. The major chemical species in MES, acetate, propionate, and phosphate buffer solutions were respectively ascertained to be [PtCl(terpy)]+, [Pt(acetato)(terpy)]+, [Pt(propionato)(terpy)]+ and [Pt(H2PO4)(terpy)] + by ESI-TOF mass spectrometry. It is concluded that the original chloro species (i.e., [PtCl(terpy)]+) plays a crucial role in the photochemical H2 formation. The saturation kinetics for the H 2 formation with regard to the EDTA concentration was observed, revealing that the dimer of 1 (i.e., (1)22+) and the dianionic form of EDTA form an ion-pair adduct to facilitate the electron injection from EDTA during the photochemical processes.
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