Synthesis, crystal structure, spectroscopic and electrochemical properties, and H-2-evolving activity of a new [PtCl(terpyridine)](+) derivative with viologen-like redox properties

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+ (E-1/2(1) = -0.87 V vs. Fc/Fc(+) for PV2+/PV+; E-1/2(2) = -1.31 V vs. Fc/Fc(+) for PV+/PV0) are nearly consistent with those of methylviologen (N,N'-dimetyl-4,4'-bipyridinium, MV2+) (E-1/2(1) = -0.90 V vs. Fc/Fc(+) for MV2+/MV+; E-1/2(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 H-2-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)](+).