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 Gold Surface: Comparison of Acid/Base Chemistry with Bulk Solution Chemistry

Abstract

The self-assembly and electrochemical properties of a novel Ru(bimpyH2) complex functionalized by disulfide on a polycrystalline gold electrode (bimpyH2 = bis(benzimidazol-2-yl)pyridine) were characterized by means of MALDI-TOF mass spectrometry, XPS, and cyclic voltammetry. The Ru complex monolayers exhibit a quasi-reversible Ru(III/II) oxidation at +0.66 V vs SCE in 0.1 M HClO4 aqueous solution. The shape of the cyclic voltammograms for Ru(III/II) couple is strongly dependent on the surface coverage, and is close to the ideal voltammogram when the Ru neat monolayers are diluted by octanethiol. The oxidation potential of the Ru complex mixed monolayers with octanethiol shows a linear dependence on solution pH, indicating that the proton-coupled oxidative reactions occurred on the gold surface. The pKa values for the Ru mixed monolayers on the gold surface were obtained as pKa1 = 6.0 and pKa2 = 7.8 for the Ru(II) state and ∼1.5 and 3.4 for the Ru(III) state from the E1/2−pH plots. The pKa values of the Ru(bimpyH2) group on the gold surface are a little smaller than those of [Ru(dmbimpy)(bimpyH2)]2+ (dmbimpy = bis(1-methylbenzimidazol-2-yl)pyridine) in the bulk solution. These disulfide-functionalized Ru(bimpyH2) complex monolayers on the gold surface can act as an electrochemical pH probe on the surface.