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

[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.