Synthesis and characterization of two copper(II) compounds with xanthurenic acid

Xanthurenic acid (8-hydroxy-4-oxo-1H-quinoline-2-carboxylic acid) is a well-known tryptophan metabolite involved in many biological processes. Metal ions bonded to xanthurenic acid can change its biological activity. The copper(II) compounds [Cu(Xan)(H2O)(2)] and [Cu(Xan)(H2O)(2)]H2O (Xan(2-) = xanthurenic acid dianion) were synthesized and characterized by single crystal X-ray diffraction, IR spectroscopy, thermal and elemental analysis. In both compounds, xanthurenic acid acts as a tridentate ligand forming a neutral mononuclear complex of square-pyramidal geometry with two water molecules. Neighboring molecules are linked together along the b axis by intermolecular hydrogen bonds. In the second complex, a molecule of water of crystallization was found in the asymmetric unit. The antioxidant activities of the compounds were investigated with the 2,2'-diphenyl-1-picrylhydrazyl and 2,2-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) methods. Using the former reagent both copper(II) compounds showed higher antioxidant activities than H(2)Xan, but with the latter method the free ligand and the prepared compounds did not show any significant difference.