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

The electrochemical oxidation of anodic metals (M = cobalt, nickel, copper, zinc and cadmium) in a solution of the ligand 1 H -anthra[1,2- d ]imidazol-6,11-dione-2-[2-hydroxyphenyl] [H 2 L] 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 H 2 L ( 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 [N 4 O] 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 1 H 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. One-pot electrochemical synthesis of metal complexes containing a novel heterocyclic benzimidazole ligand is reported and characterized.