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

Abstract

Reactions of two hydrated cupric salts (CuCl₂·2H₂O and Cu(ClO₄)₂·6H₂O) with three azopyridyl ligands, viz. 2-[(arylamino)phenylazo]pyridine [aryl=phenyl (HL^1a), p-tolyl (HL^1b), and 2-thiomethyl phenyl (HL^1c)], 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: Cu^IICu^II⇔ Cu^IICu^I ⇔Cu^ICu^I. 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.