Syntheses and characterization of Co(III) binuclear complexes with bis(catecholate) ligands containing an acetylene linker

Three binuclear Co(III) complexes with 5,5′-(buta-1,3-diyne-1,4-diyl)bis(3-tert-butylcatechol) (L1), 5,5′-(2,5-dimethoxy-1,4-phenylene)bis(ethyne-2,1-diyl)bis(3-tert-butyl-catechol) (L2) and 5,5′-(4,4′-(buta-1,3-diyne-1,4-diyl)bis(2,5-dimethoxy-4,1-phenylene))bis(ethyne-2,1-diyl)bis(3-tert-butyl-catechol) (L3) have been prepared. The triple bond-containing L1, L2 and L3 ligands were synthesized by a cross-coupling reaction. These complexes were characterized by elemental analyses, electrochemical measurements, 1H NMR and UV–Vis spectra. In [Co2(bpy)4(L1)]2+, electrochemical oxidation of the complexes occurs at the bridges as two closely spaced one-electron couples. UV–Vis spectra reveal that chemical oxidation of [Co2(bpy)4(L1)]2+ using Ag+ occurs as a two-electron process forming [Co2(bpy)4(L1Cat,SQ)]3+ or [Co2(bpy)4(L1SQ,SQ)]4+. On the other hand, [Co2(bpy)4(L2)]2+ and [Co2(bpy)4(L3)]2+ exhibit different oxidation behavior under the same experimental conditions. In this report we discuss the role of the distance between the two metal atoms on the oxidative behavior of binuclear Co(III) complexes.