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 (E1/2 = 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(thBuCat,SQ)]3+ or [Co2(dpa)4(thBuSQ,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.

Graphical abstractTwo binuclear Co(III) complexes ([Co2(dpa)4(thBu)]2+ (1) and [Co2(bpy)4(thBu)]2+ (2)) 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). In (1), new absorbance peak appears nearly 543 nm after chemical oxidation. On the other hand, (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.Figure optionsDownload full-size imageDownload as PowerPoint slide