Proton-coupled electron transfer and Lewis acid recognition at self-assembled monolayers of an oxo-bridged diruthenium(III) complex functionalized with two disulfide anchors.

A new μ-oxo-bis(μ-acetato)diruthenium(III) complex bearing two pyridyl disulfide ligands {Ru2(μ-O)(μ-OAc)2(bpy)2(L(py-SS))22 (OAc = CH3CO2(-), bpy = 2,2'-bipyridine, L(py-SS) = (C5H4N)CH2NHC(O)(CH2)4CH(CH2)2SS) (1)} has been synthesized to prepare self-assembled monolayers (SAMs) on the Au(111) electrode surface. The SAMs have been characterized by contact-angle measurements, reflection-absorption surface infrared spectroscopy, cyclic voltammetry, and reductive desorption experiments. The SAMs exhibited proton-coupled electron transfer (PCET) reactions when the electrochemistry was studied in aqueous electrolyte solution (0.1 M NaClO4 with Britton-Robinson buffer to adjust the solution pH). The potential-pH plot (Pourbaix diagram) in the pH range from 1 to 12 has established that the dinuclear ruthenium moiety was involved in the interfacial PCET processes with four distinct redox states: Ru(III)Ru(III)(μ-O), Ru(II)Ru(III)(μ-OH), Ru(II)Ru(II)(μ-OH), and Ru(II)Ru(II)(μ-OH2). We also demonstrated that the interfacial redox processes were modulated by the addition of Lewis acids such as BF3 or Al(3+) to the electrolyte media, in which the externally added Lewis acids interacted with μ-O of the dinuclear moiety within the SAMs.