Synthesis, structure and catalytic activities of nickel(II) complexes bearing N 4 tetradentate Schiff base ligand Saikat Sarkar a, *, 1 , Sanat Kumar Nag b , Asoke Prasun Chattopadhyay b , Kamalendu Dey b , Sk. Manirul Islam b , Avijit Sarkar c , Sougata Sarkar d a Department of Chemistry, Chakdaha College, Chakdaha, 741222, West Bengal, India b Department of Chemistry, University of Kalyani, Kalyani, 741235, West Bengal, India c Department of Chemistry, Bhairab Ganguly College, Kolkata, 700056, West Bengal, India d Department of Chemistry, Ramakrishna Mission Vivekananda Centenary College, Rahara, West Bengal, India article info Article history: Received 19 October 2017 Received in revised form 13 January 2018 Accepted 14 January 2018 Keywords: Schiff base Spectroscopy Catalytic activity Crystal structure Theoretical study abstract Two new nickel(II) complexes [Ni(L)Cl 2 ](1) and [Ni(L)(NCS) 2 ](2) of a neutral tetradentate mono- condensed Schiff base ligand, 3-(2-(2-aminoethylamino)ethylimino)butan-2-one oxime (L) have been synthesized and characterized using different physicochemical techniques e.g. elemental analyses, spectroscopic (IR, Electronic, NMR) methods, conductivity and molecular measurements. The crystal structure of complex (2) has been determined by using single crystal X-ray diffraction method and it suggests a distorted octahedral geometry around nickel(II) having a NiN 6 coordinating atmosphere. The non-coordinated OeH group on the ligand L remain engaged in H-bonding interactions with the S end of the coordinated thiocyanate moiety. These H-bonding interactions lead to OeS separations of 3.132 Å and play prominent role in crystal packing. It is observed that the mononuclear units are glued together with such OeH…S interactions and finally results in an 1D supramolecular sheet-like arrangement. DFT/ TDDFT based theoretical calculations were also performed on the ligand and the complexes aiming at the accomplishment of idea regarding their optimized geometry, electronic transitions and the molecular energy levels. Finally the catalytic behavior of the complexes for oxidation of styrene has also been carried out. A variety of reaction conditions like the effect of solvent, effect of temperature and time as well as the effect of ratio of substrate to oxidant were thoroughly studied to judge the catalytic efficiency of the Ni(II) coordination entity. © 2018 Elsevier B.V. All rights reserved. 1. Introduction The Schiff base ligands and their metal complexes are still being vividly pursued for their non-stop development in the area from fundamental scientific study to cutting edge research [1e4]. Such ligands having oxime moiety [5e7] are of current interest because of their indispensable analytical applications and capability to form a large variety of polynuclear systems [8e10] of significant importance. The application of Schiff base metal complexes in the field of material-chemistry has expanded their use as magnetic, spin-crossover (SCO) and bistable molecular materials [11e 13]. The acid-base equilibria of oxime Schiff bases and the complex forma- tion equilibria with such ligands are also of modern day's research interest [14]. The advancement in the field of coordination chem- istry of the transition metal Schiff base complexes have been noticed with the discovery of many metal containing enzymes and activated inhibitors for enzymes and proteins [15e17]. The green approach using sonochemical method for especially the nano-scale synthesis of Schiff base metal complexes with interesting metal- organic-frameworks (MOFs) are remarkable because of their effi- cient utilities and may be compared with other such metal-organic coordination systems [18e23]. The applications of Schiff base complexes of V, Cr, Mn, Fe, Co, Cu etc. as highly active homo- and heterogeneous catalysts to achieve selectivity in many organic transformations are very rich [24,25]. Thus the catalytic activities of the Schiff base metal complexes including artificial Schiff base metalloenzymes [26,27] and their fascinating structural varieties [28,29] become also interesting areas of cutting edge research. * Corresponding author. E-mail address: saikat_s@rediffmail.com (S. Sarkar). 1 Previous address: Department of Chemistry, Santipur College, Santipur 741404, W.B. Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc https://doi.org/10.1016/j.molstruc.2018.01.035 0022-2860/© 2018 Elsevier B.V. All rights reserved. Journal of Molecular Structure 1160 (2018) 9e19