Design and investigation of a Ru(II) N-heterocyclic complex which undergoes proton coupled electron transfer

A novel tridentating ligand containing a single ionizable proton was designed for studying proton coupled electron transfer. The ligand was synthesized by derivatizing 2,2′-bipyridine at the 6-position with benzimidazole (bpy–bzimH), and it was used to prepare the compound [Ru(bpy–bzimH)2](PF6)2. Cyclic voltammetry was used to characterize the redox behavior in an aprotic (0.1 M TBAH–CH2Cl2) and protic (1:1 acetonitrile–water buffered solutions) solvent conditions where the latter was employed to characterize the pH-dependence of the Ru(III/II) couple. The redox potential as a function of pH was plotted and reveals a one-proton/one-electron transfer in two separate pH regions (1.39–2.58, and 5.92–7.97), while a two-proton/one-electron process was exhibited between 2.58 and 5.92.