FULL PAPER Some divalent metal(II) complexes of novel potentially tetradentate Schiff base N,N′‐bis(2‐carboxyphenylimine)‐2,5‐ thiophenedicarboxaldhyde: Synthesis, spectroscopic characterization and bioactivities Ayman A. Abdel Aziz | Ibrahim S.A. El‐Sayed | Mostafa M.H. Khalil Department of Chemistry, Faculty of Science, Ain Shams University, 11566 Cairo, Egypt Correspondence Ayman A. Abdel Aziz, Department of Chemistry, Faculty of Science, Ain Shams University, 11566 Cairo, Egypt. Email: aymanaziz31@sci.asu.edu.eg A novel tetradentate dianionic Schiff base ligand, N,N′‐bis(2‐carboxyphenylimine)‐ 2,5‐thiophenedicarboxaldhyde (H 2 L) and some first row d‐transition metal chelates (Co(II), Cu(II), Ni(II) and Zn(II)) were synthesized and characterized using various physicochemical and spectroscopic methods. The spectroscopic data suggested that the parent Schiff base ligand coordinates through both deprotonated carboxylic oxy- gen and imine nitrogen atoms. The free Schiff base and its metal chelates were screened for their antimicrobial activities for various pathogenic bacteria and fungi using the agar well diffusion method. The antibacterial and antifungal activities of all the newly synthesized compounds are significant compared to the standard drugs ciprofloxacin and nystatin. The antioxidant activities of the compounds were deter- mined by reduction of 1,1‐diphenyl‐2‐picrylhydrazyl and compared with that of vita- min C as a standard. DNA binding ability of the novel Schiff base and its complexes was investigated using absorption spectroscopy, fluorescence spectroscopy, viscosity measurements and thermal denaturation. The obtained results clearly demonstrate that the binding affinity with calf thymus DNA follows the order: Cu(II) complex > Ni(II) complex > Zn(II) complex > Co(II) complex >H 2 L. Furthermore, the DNA cleavage activity of the newly synthesized ligand and its metal complexes was investigated using supercoiled plasmid DNA (pUC18) gel electrophoresis. KEYWORDS antioxidant activity, DNA binding, DNA cleavage, metal complexes, Schiff base, spectroscopic characterization 1 | INTRODUCTION The design of Schiff bases and related complexes represents a relevant and qualified goal in the development of coordina- tion chemistry. [1,2] Due to relative ease of preparation, synthetic flexibility and the very good binding nature of azomethine linkage par- ticipating in chelation processes, Schiff bases play an impor- tant role in inorganic chemistry [3,4] due to their ability to form stable complexes with most transition metal ions. Moreover, the free electron pair of nitrogen in imine group offers signif- icant advantages due to its ability to chelate various metal ions, leading to a wide range of applications in selective removal of metal ions from waste and sea water [5] or as spec- troscopic sensors for various ions. [6,7] Transition metal complexes derived from Schiff base ligands with potential biological activity are the focus of extensive investigations in coordination chemistry, due to Received: 31 October 2016 Revised: 4 December 2016 Accepted: 10 December 2016 DOI 10.1002/aoc.3730 Appl Organometal Chem. 2017;e3730. https://doi.org/10.1002/aoc.3730 Copyright © 2017 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/aoc 1 of 16