Synthesis and crystal structure of a new tetranuclear copper(II) complex with N-(2-hydroxyphenyl)-N′-(3-aminopropyl)oxamide ligand: cytotoxicity, DNA and BSA-binding studies

A new tetracopper(II) complex, [Cu4(papo)2 (dabt)2](NO3)2 2H2O, where H3papo and dabt stand for N-(2-hydroxyphenyl)-N0-(3-aminopropyl)oxamide and 2,20diamino-4,40-bithiazole, respectively, has been synthesized and characterized by elemental analyses, molar conductivity, infrared and electronic spectra, and single-crystal X-ray diffraction. The crystal structure reveals that the presence of a circular tetracopper(II) cation [Cu4(papo)2(dabt)2] 2? with an embedded center of inversion is assembled by a pair of cis-oxamido-bridged dicopper(II) units through l2-phenolato bridges, in which the Cu(II) atoms at the inner sites of papo have square-planar environments and those at the outer sites are in square-pyramidal geometries. The separations of the Cu(II) ions bridged by oxamide and phenolate groups are 5.2039(14) and 3.7159(12) A, respectively. In the crystal, two-dimensional supramolecular structures are formed by hydrogen bonds and p–p stacking interactions. An in vitro cytotoxicity experiment indicates that the tetracopper(II) complex exhibits cytotoxic effects against human hepatocellular carcinoma cell SMMC-7721 and human lung adenocarcinoma cell A549. The reactivity toward HS-DNA and protein BSA suggested that the tetracopper(II) complex can interact with HS-DNA in the mode of intercalation, and the complex binds to BSA responsible for quenching of tryptophan fluorescence by a static quenching mechanism.