Ruthenium and iron complexes with benzotriazole and benzimidazole derivatives as simple models for proton-coupled electron transfer systems

Iron and ruthenium complexes of the type [M-LH](n) (where M = Ru-II,Ru-III( NH3)(5)(2+, 3+), Ru-II,Ru-III(edta)(2-,-) [edta = ethylenedinitrilotetraace tate], or Fe-II,Fe-III(CN)(5)(3-,2-) and LH = benzotriazole or benzimidazol e) were prepared and characterized in aqueous solutions by means of electro chemical and spectroelectrochemical methods. Special emphasis was given to the pH-dependent redox processes, exhibited by all the investigated complex es. From their related Pourbaix diagrams, which displayed a typically Nerns tian behavior, the pK(a) and formal reduction potential values were extract ed. In addition, these E-1/2 versus pH curves were also used to illustrate the partitioning relationship concerning the redox and acid-base species, a nd their interconversion equilibria. The active area in which the dependenc e of the M-III/M-II couple on the pH takes place, as delimited by pK(a)(III ) and pK(a)(II), was taken into account in order to evaluate the usefulness of such simple complexes as models for proton-coupled electron transfer (P CET). The results were interpreted in terms of the acceptor/donor electroni c character of the ligands and sigma,pi -metal-ligand interactions in both redox states of the metal ion.