Metal complexes of a novel heterocyclic benzimidazole ligand formed by rearrangement-cyclization of the corresponding Schiff base. Electrosynthesis, structural characterization and antimicrobial activity

Abstract

The electrochemical oxidation of anodic metals (M = cobalt, nickel, copper, zinc and cadmium) in a solution of the ligand 1H-anthra[1,2-d]imidazol-6,11-dione-2-[2-hydroxyphenyl] [H₂L] afforded homoleptic [ML] compounds. The addition to the electrochemical cell of coligands (L′) such as 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen) allowed the synthesis, in one step, of heteroleptic [MLL′] compounds. The crystal structures of H₂L (1), [CoL(MeOH)]₂ (2), [CoL(phen)]₂ (3), [NiL(bpy)]₂ (4), [CuL(bpy)] (5), [CuL(phen)] (6) and [CdL(bpy)]₂ (7) have been determined by X-ray diffraction techniques. The crystal structures of 2, 3, 4 and 7 consist of dimeric species in which both metallic atoms are connected through two phenolate bridges in a penta-coordinated (2) or hexa-coordinated (3, 4 and 7) environment. Copper compounds 5 and 6 are monomeric species with the metal in a pentacoordinated [N₄O] environment. In all the compounds, the main interactions responsible for the crystal packing are classic (N–H⋯O, O–H⋯N and O–H⋯O) and non-classic (C–H⋯O and C–H⋯N) hydrogen bond interactions, and π interactions (π–π-stacking and C–H⋯π). All compounds were also characterized by microanalysis, IR spectroscopy, FAB mass spectrometry and ¹H NMR spectroscopy. Magnetic susceptibility data were measured for 2–4 over the temperature range 2–300 K, and their analysis has revealed the occurrence of intramolecular antiferromagnetic coupling for 2 (J = −2 cm⁻¹) and ferromagnetic coupling for 3 (J = 7.8 cm⁻¹) and 4 (J = 2.8 cm⁻¹) [J being the isotropic magnetic coupling parameter]. The nature of the magnetic coupling in 2–4 is correlated with the magnitude of the M–O_phenolate–M angle between the phenolate bridge and the metallic centers [M(II) = Co, Ni]. The in vitro antimicrobial properties of the novel ligand and its metal complexes were detected against Gram positive and Gram negative bacteria and fungi. [NiL(bpy)]₂ and all tested Cd(II) complexes were the most active compounds, showing the highest inhibitory effect against bacilli (MIC 1.5–3 μg mL⁻¹) and Sarcina, Streptococci and Haemophilus influenzae bacterial strains (MIC 12–50 μg mL⁻¹), while almost no antifungal properties were observed.