Effect of nitrogen-donor ancillary ligand on oxidation behavior of Co(III) binuclear complexes with conjugated bis(catecholate) ligands

Two binuclear complexes of cobalt(III) have been prepared with 3,3′,4,4′-tetrahydroxy-5,5′-di-tert-butylbenzaldazine (H₄thBu) as bis(catecholate) ligand and two different ancillary ligands, 2,2′-bipyridine (bpy) or 2,2′-dipyridylamine (dpa). These compounds were characterized by ¹H NMR spectra, electrochemical measurements and UV–Vis spectra. In one case, [Co₂(dpa)₄(thBu)]²⁺, electrochemical oxidation of the complexes occurs at the bridges as two closely spaced one-electron couples (E_1/2 = 1 mV and 168 mV versus Fc/Fc⁺). Chemical oxidation of [Co₂(dpa)₄(thBu)]²⁺ using Ag⁺ is observed to occur as a stepwise two-electron process forming [Co₂(dpa)₄(thBu^Cat,SQ)]³⁺ or [Co₂(dpa)₄(thBu^SQ,SQ)]⁴⁺ by UV–Vis spectrum. However, [Co₂(bpy)₄(thBu)]²⁺ shows no change in electronic spectrum under the same conditions of oxidation. This illustrates the dependence of redox properties of the binuclear Co(III) complexes on the nature of the nitrogen-donor ancillary ligands. In this report we discuss the effect of two different nitrogen-donor ancillary ligands on the0 oxidation behavior of binuclear Co(III) complexes.