The co-crystal of copper(II) phenanthroline chloride complex hydrate with p-aminobenzoic acid: structure, cytotoxicity, thermal analysis, and DFT calculation

Complex [Cu(phen)2Cl]Cl∙2PABA∙4H2O has been isolated, where phen ligand is 1,10-phenanthroline and PABA is p-aminobenzoic acid. The complex crystallizes in the centrosymetric (monoclinic) space group P2(1)/n. The unit cell dimensions are a = 10.479(4) A, b = 22.396(7) A, and c = 16.212(6) A. The basic unit structure of complex is distorted trigonal bipyramidal. Two hydrogen-bonded PABA molecules are enclosed through π−π interactions in filled aryl box FAB motif made by juxtaposition of four phen from two opposing cations. While π−π interaction between phen causes the formation of offset face-to-face overlap OFF primary motif. The result is formation of extended (OFF-FAB)n chains. Cyclic voltammetry showed one reversible oxidation reduction process followed by one irreversible oxidation process. Thermal analysis indicated successive loss of crystalline water and crystalline PABA molecules, chloride ions, and phen ligands. DFT study indicates that PABA interacts with phen with binding energy 68.87 kJ mol−1 reflecting strong π-stacking. The charge density in HOMO is localized on the metal halogen bond, whereas in LUMO, it is spread over two phen. The vibrational spectrum of complex was calculated at M05-2X/6-31G(d) and compared with the experimental vibrational spectrum data. The inhibitory concentrations IC50 against carcinoma cells A549 (lung adenocarcinoma) and MDA-MB-231 (breast adenocarcinoma) are 3.4 and 4.5 µM, respectively. IC50 values of new complex are comparable to those of cisplatin and smaller than the values for [Cu(phen)2Cl]Cl∙6.5H2O.