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 PV ²⁺; 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 PV ²⁺ 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 PV ²⁺ (E11/2 = -0.87 V vs. Fc/Fc ⁺ for PV ²⁺/PV ⁺; E21/2 = -1.31 V vs. Fc/Fc ⁺ for PV ⁺/PV ⁰) are nearly consistent with those of methylviologen (N,N′-dimetyl-4,4′-bipyridinium, MV ²⁺) (E11/2 = -0.90 V vs. Fc/Fc ⁺ for MV ²⁺/MV ⁺; E21/2 = -1.28 V vs. Fc/Fc ⁺ for MV ⁺/MV ⁰), where the first reduction of PV ²⁺ is assigned as the reduction at the Mepytpy ⁺ ligand bound to the Pt(ii) ion. Upon the addition of a reducing agent Na ₂S ₂O ₄ into an aqueous acetate buffer solution (pH = 5.0) of PV ²⁺, 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 PV ²⁺ 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 PV ²⁺ during the photolysis is much higher than that of the parent compound [PtCl(tpy)] ⁺.