Coordination chemistry of copper(II) complexes with N4, N4S2, and N4O2 donor macrocyclic ligands: Biological aspects—antifungal, synthesis, spectral studies, and magnetic moments

Abstract

Three macrocyclic ligands and their complexes with copper(II) salts (with anions Cl−, NO 3 − , and NCS−) were prepared and investigated using a combination of microanalytical analysis, melting point, molar conductance measurement, magnetic susceptibility measurement, and electronic, IR and ESR spectral studies. Ligands L1, L2, and L3 having N4, N4O2, and N4S2 core, respectively, and all the donor atoms of these ligands are bonded with Cu, which is confirmed by a seven-line pattern observed at half-field in the frozen (H2O: MeOH = 10: 1 at pH 10) solution ESR spectrum. The polycrystalline ESR data (g ∥ = 2.20–2.27, g ⊥ = 2.01–2.05, and A ∥ = 120–270) of all the complexes together with the high asymmetry geometry suggest that all complexes appear to be near the static distortion (CuN4O2 and CuN4S2 chromophore geometry). The electronic spectra of the complexes involve two bands at the same intensity corresponding to a cis-distorted octahedral geometry. A common structural feature of both ligand L2 and ligand L3 is that two different donor atoms at five-membered heterocyclic aromatic ring due to this N4O2 and N4S2 chromophore form stable six-membered chelate rings with metals via these two, Cu-O and Cu-S, new interactions comparatively to the first macrocyclic ligand, which has four-membered N,N′-chelate rings. The cyclic voltammetric studies point to a two-step electron transfer indicating the reduction of the two copper atoms to copper(I), i.e., Cu(III)Cu(II) ⇄ Cu(II)Cu(I) ⇄ Cu(I)Cu(0). The molar conductance for the complexes corresponds to 1: 2 and is nonelectrolyte in nature. The magnetic moment (μeff) of the complexes lie in the range between 1.80–1.96 μB. Finally, these complexes were screened for their antimicrobial activity against Aspergillus-niger of fungal strains.