Classical hydrogen bonding and stacking of chelate rings in new copper(II) complexes

Three new copper(ii) complexes, viz., [Cu(L)(NO)(HO)]HO 1, [Cu(L)(HO)]NO2 and [Cu(L)(pyrazine)](ClO)·4HO 3, were prepared using a biomimetic synthesis strategy [HL = 4-chloro-2-{(E)-[2-(pyridin-2-yl)hydrazinylidene]methyl}phenol]. Structural characterization revealed very different local geometries around copper(ii) ions, being square pyramidal for mononuclear complexes (τ = 0.021 for 1 and τ = 0.13 for 2) and square planar for homobinuclear pyrazine bridged complex 3 (τ = 0.06). In complex 3, stacking of chelate rings was observed. This insight was thought to provide new structural evidence for the stacking of the planar chelate rings. These π-interactions provide stability to the crystal structure of homobinuclear complex 3. Magnetic measurements of bulk materials 1, 2 and 3 revealed weak antiferromagnetic coupling in all complexes. The EPR spectra of complexes 1, 2 and 3 in polycrystalline state exhibited broad signals at g ≅ 2.15 owing to spin-spin interactions between two copper ions. The cyclic voltammograms of mononuclear complexes (1 and 2) in DMSO gave one Cu/Cu irreversible wave. On the other hand, pyrazine bridged binuclear complex 3 exhibited two waves, which correspond to CuCu/CuCu and CuCu/CuCu redox processes. Differential pulse voltammetry (DPV) experiments also exhibited the same reduction behavior. These complexes displayed effective antioxidant SOD activity and exhibit the outlines of a structure-based cyclic mechanism. On the basis of experimental and theoretical DFT studies, the structure-activity relationships of these complexes have also been discussed.