A new Pt(II)-based metalloviologen analog, [PtCl(Mepytpy)]2+ (abbreviated as PV2+; Mepytpy+ = 4′-(4-methyl-pyridinio)-2,2′ : 6′,2′′-terpyridine), was synthesized and its spectroscopic and electrochemical properties were investigated. The structures of Mepytpy+ and PV2+ were solved by single-crystal X-ray diffraction analysis. Electrochemical studies in DMF (N,N-dimethylformamide) revealed that both the first and second reduction potentials of PV2+ (E11/2 = −0.87 V vs. Fc/Fc+ for PV2+/PV+˙; E21/2 = −1.31 V vs. Fc/Fc+ for PV+˙/PV0) are nearly consistent with those of methylviologen (N,N′-dimetyl-4,4′-bipyridinium, MV2+) (E11/2 = −0.90 V vs. Fc/Fc+ for MV2+/MV+˙; E21/2 = −1.28 V vs. Fc/Fc+ for MV+˙/MV0), where the first reduction of PV2+ is assigned as the reduction at the Mepytpy+ ligand bound to the Pt(II) ion. Upon the addition of a reducing agent Na2S2O4 into an aqueous acetate buffer solution (pH = 5.0) of PV2+, the generation of one-electron-reduced radical species (PV+˙) was spectrophotometrically observed and was assigned based on the time-dependent density functional theory (TD-DFT) calculations. The H2-evolving activity of PV2+ was evaluated in the presence of a sacrificial electron donor (EDTA) in an aqueous acetate buffer solution (pH = 5.0). It was found that the stability of the metalloviologen PV2+ during the photolysis is much higher than that of the parent compound [PtCl(tpy)]+.
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