Synthesis and proton-coupled electron-transfer reaction of self-assembled monolayers of a ruthenium(II) complex containing tridentate 2,6-bis(benzimidazol-2-yl)pyridine on a cold surface: Comparison of acid/base chemistry with bulk solution chemistry

The self-assembly and electrochemical properties of a novel Ru(bimpyH(2)) c omplex functionalized by disulfide on a polycrystalline gold electrode (bim pyH(2) = bis(benzimidazol-2-yl)pyridine) were characterized by means of MAL DI-TOF mass spectrometry, XPS, and cyclic voltammetry. The Ru complex monol ayers exhibit a quasireversible Ru(mm) oxidation at +0.66 V vs SCE in 0.1 M HClO4 aqueous solution. The shape of the cyclic voltammograms for Ru(III/I I) couple is strongly dependent on the surface coverage, and is close to th e ideal voltammogram when the Ru neat monolayers are diluted by octanethiol . The oxidation potential of the Ru complex mixed monolayers with octanethi ol shows a linear dependence on solution pH, indicating that the proton-cou pled oxidative reactions occurred an the gold surface. The pK(a) values for the Ru mixed monolayers on the gold surface were obtained as pK(a1) = 6.0 and pK(a2) = 7.8 for the Ru(II) state and similar to 1.5 and 3.4 for the Ru (III) state from the E-1/2-pH plots. The pK(a) values of the Ru(bimpyH(2)) group on the gold surface are a little smaller than those of [Ru(dmbimpy)(b impyH(2))](2+) (dmbimpy = bis(1-methylbenzimidazol-2-yl)pyridine) in the bu lk solution. These disulfide-functionalized Ru(bimpyH(2)) complex monolayer s on the gold surface can act as an electrochemical pH probe on the surface .