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-[CuII(abpy)3]­[PF6]2 (mer-12+[PF6–]2) and ctc-[CuII(abpy)2(bpy)]­[PF6]2 (ctc-22+[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-22+ ions with catechol, o-aminophenol, p-phenylenediamine, and diphenylamine (Ph–NH–Ph)
in 2:1 molar ratio afford [CuI(abpy)2]+ (3+) and corresponding quinone
derivatives. The similar reactions of [CuII(bpy)3]2+ and [CuII(phen)3]2+ with these substrates yielding [CuI(bpy)2]+ and [CuI(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-22+[PF6–]2 are 1.248(4), while
that in 3+[PF6–]·2CH2Cl2 is relatively longer, 1.275(2) Å, due to dCu →
πazo* back bonding. In cyclic voltammetry, mer-12+ exhibits one quasi-reversible
wave at −0.42 V due to CuII/CuI and abpy/abpy•– couples and two reversible waves at −0.90
and −1.28 V due to abpy/abpy•– couple,
while those of ctc-22+ 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•–) in 3, which is defined as a hybrid state of [CuI(abpy0.5•–)­(abpy0.5•–)] and [CuII(abpy•–)­(abpy•–)] states. 32+ ion
is a neutral abpy complex of copper­(II) of type [CuII(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.