Coordination driven self-assembly of discrete and polymeric metallosupramolecules from 3,5-bis(3-pyridyl)-1,2,4-oxadiazole and Cu(II) salts, and their antimicrobial activity study

Ligand orientation plays an important role in the overall geometry and structure of metal complexes, and semi-rigid ligands have dictated such characteristics in the formed complexes. In this study, we have shown that the rigid bidentate ligand 3,5-bis(3-pyridyl)-1,2,4-oxadiazole, L exhibits different orientations such as cisoid-up, cisoid-down, and transoid while forming different complexes with copper(II) salts. The cisoid-down orientation of ligand L exists in complexes {[Cu(L)(SO4)(H2O)3]•2H2O}n, 1 and [Cu2(L)2(NO3)4(CH3OH)2], 2; complex 1 is polymeric in nature and prepared from copper sulfate whereas complex 2 is a discrete metallocage prepared from copper nitrate. Ligand L shows transoid orientation in complexes {[Cu2(L)(OAc)4]•CH3OH}n, 3 and [Cu2(L)4(OAc)4]•H2O, 4 which are prepared from copper acetate using different solvents. Compound [Cu2(L)(OAc)4]n, 5 has been synthesized from copper acetate and ligand L in acetonitrile-water mixture where ligand L exists in cisoid-up orientation. This study shows anions and solvents determine formation of polymeric or discrete matallocage when ligand L and Cu(II) salts, in same molar ratio, combine to form complexes. Compounds 1 and 4 have shown effective antibacterial activities against S. aureus with MIC values 625 and 156 µg/mL, respectively.