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 (H(4)thBu) as bis(catecholate) ligand and two different ancillary ligands, 2,2'-bipyridine (bpy) or 2,2'-dipyridylamine (dpa). These compounds were characterized by (1)H NMR spectra, electrochemical measurements and UV -Vis spectra. In one case, [Co(2)(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 [Co(2)(dpa)(4)(thBu)](2+) using Ag(+) is observed to occur as a stepwise two-electron process forming [Co(2)(dpa)(4)(thBu(Cat,SQ))](3+) or [Co(2)(dpa)(4)(thBu(SQ,SQ))](4+) by UV -Vis spectrum. However, [Co(2)(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. (C) 2008 Elsevier B. V. All rights reserved.