Tris(2,2′-azobispyridine) Complexes of Copper(II): X-ray Structures, Reactivities, and the Radical Nonradical Bis(ligand) Analogues

Tris(abpy) complexes of types mer-[Cu-II(abpy)(3)][PF6](2) (mer-12(+)[PF6-](2)) and ctc-[Cu-II(abpy)(2)(bpy)][PF6](2) (ctc-2(2+)[PF6-](2)) were successfully isolated and characterized by spectra and single-crystal X-ray structure determinations (abpy = 2,2'-azobispyridine; bpy = 2,2'-bipyridine). Reactions of mer-12(+) and ctc-2(2+) ions with catechol, o-aminophenol, p-phenylenediamine, and diphenylamine (Ph-NH-Ph) in 2:1 molar ratio afford [Cu-I(abpy)(2)](+) (3(+)) and corresponding quinone derivatives. The similar reactions of [Cu-II(bpy)(3)](2)(+) and [Cu-II(phen)3]2(+) with these substrates yielding [Cu-I(bpy)(2)](+) and [Cu-I(phen)(2)](+) imply that these complexes undergo reduction-induced ligand dissociation reactions (phen = 1,10-phenanthroline). The average -N-N- lengths in mer-12(+)[PF6-](2) and ctc-2(2+)[PF6-](2) are 1.248(4), while that in 3(+)[PF6-]center dot 2CH(2)Cl(2) is relatively longer, 1.275(2) angstrom, due to dCu -> pi(azo)* back bonding. In cyclic voltammetry, mer-12(+) exhibits one quasi-reversible wave at -0.42 V due to Cu-II/Cu-I and abpy/abpy(center dot-) couples and two reversible waves at -0.90 and -1.28 V due to abpy/abpy(center dot-) couple, while those of ctc-2(2+) ion appear at -0.44, -0.86, and -1.10 V versus Fc(+)/Fc couple. The anodic 32(+)/3(+) and the cathodic 3(+)/3 redox waves at +0.33 and -0.40 V are reversible. The electron paramagnetic resonance spectra and density functional theory (DFT) calculations authenticated the existence of abpy anion radical (abpy(center dot-)) in 3, which is defined as a hybrid state of [Cu-I(abpy(0.5 center dot-))(abpy(0.5 center dot-))] and [Cu-II(abpy(center dot-))(abpy(center dot-))] states. 32(+) ion is a neutral abpy complex of copper(II) of type [Cu-II(abpy)(2)](2)(+). 3 exhibits a near-IR absorption band at 2400-3000 nm because of the intervalence ligand-to-ligand charge transfer, elucidated by time-dependent DFT calculations in CH2Cl2.