Photo-hydrogen-evolving activity of chloro(terpyridine)platinum(II): a single-component molecular photocatalyst

[PtCl(terpy)]Cl center dot 2H(2)O (terpy = 2,2':6',2 ''-terpyridine) (1Cl center dot 2H(2)O) is the first example serving as a bifunctional system promoting both photosensitization and hydrogenic activation as an H-2-evolving catalyst in aqueous media in the presence of a sacrificial electron donor (EDTA) under visible-light illumination. The rate of H-2 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 H-2 formation. It is suggested that the bimolecular mechanism operates at the photosensitization process through the formation of the so-called (MMLCT)-M-3 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 H-2 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 H-2 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)(2)(2+)) and the dianionic form of EDTA form an ion-pair adduct to facilitate the electron injection from EDTA during the photochemical processes.