Synthesis, crystal structure, spectroscopic and electrochemical properties, and H2-evolving activity of a new [PtCl(terpyridine)]+ derivative with viologen-like redox properties

Abstract

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+) (E = -0.87 V vs. Fc/Fc(+) for PV(2+)/PV(+)˙; E = -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+)) (E = -0.90 V vs. Fc/Fc(+) for MV(2+)/MV(+)˙; E = -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(2)S(2)O(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)](+).