Mononuclear to Polynuclear Transition Induced by Ligand Coordination: Synthesis, X-ray Structure, and Properties of Mono-, Di-, and Polynuclear Copper(II) Complexes of Pyridyl-Containing Azo Ligands

Reactions of two hydrated cupric salts (CuCl2·2H2O and Cu(ClO4)2·6H2O) with three azopyridyl ligands, viz.
2-[(arylamino)phenylazo]pyridine [aryl = phenyl (HL1a), p-tolyl (HL1b), and 2-thiomethyl phenyl (HL1c)], 2-[2-(pyridylamino)phenylazo]pyridine (HL2), and 2-[3-(pyridylamino)phenylazo]pyridine (HL3), afford the mononuclear
[CuClL1] (1), dinuclear [Cu2X2L22]n+ (X = Cl, H2O, ClO4; n = 0, 1; 2, 3), and polynuclear [CuClL3]n (4) complexes,
respectively, in high yields. Representative X-ray structures of these complexes 1−4 are reported. X-ray structure
analysis of 4 reveals an infinite 1D zigzag chain that adopts a saw-tooth-like structure. Variable-temperature
cryomagnetic measurements (2−300 K) on the complexes 2−4 have revealed weak magnetic interactions between
the copper centers with J values −1.04, 9.88, and −1.31 cm-1, respectively. Positive ion ESI mass spectra of the
soluble complexes 1−3 are studied which provide the evidence for the integrity of the complexes also in solution.
Visible range spectra of the complexes 1−3 in solution consist of intense and broad transitions in the range 700−600 nm. The solid-state spectrum of the insoluble copper complex 4, on the other hand, shows a structured band
near 700 nm. The intensities of the transitions of the dinuclear complexes are much higher than those of the
corresponding mononuclear copper complexes. Redox properties of the present copper complexes are reported.
Notably, the dinuclear complex, 3, displays two successive redox processes:  CuIICuII ⇌ CuIICuI ⇌ CuICuI. It
catalyzes aerial oxidation of L-ascorbic acid. The catalytic cycle is most effective up to H2A/3 (H2A = L-ascorbic
acid) molar ratio of 20:1.