Co(III) and Ni(II) complexes of an anthracene appended aroyl hydrazone: Synthesis, crystal structures, DNA binding and catecholase activity Satyajit Mondal a , Bholanath Pakhira a , Alexander J. Blake b , Mike G.B. Drew c , Shyamal Kumar Chattopadhyay a,⇑ a Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, India b School of Chemistry, The University of Nottingham, University Park, Nottingham, Nottinghamshire NG7 2RD, UK c Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK article info Article history: Received 25 January 2016 Accepted 18 May 2016 Available online 7 June 2016 Keywords: Co(III) complex Crystal structure Voltammetry DNA binding Catecholase activity abstract The Co(III) and Ni(II) complexes [Co(L 2 ) 2 (bpy)]Cl (1) and [Ni(L 2 ) 2 (phen)] (2) (where L 2 is a monoanionic bidentate hydrazone ligand and bpy = 2,2 0 -bipyridine, phen = 1,10-phenanthroline) have been synthesized and characterized by infra-red, UV–Vis, ESI-MS spectroscopies and elemental analyses. Complexes 1 and 2 were structurally characterized by single crystal X-ray diffraction. Structural analysis reveals a distorted octahedral coordination environment around the metal center with an MN 4 O 2 chromophore [M = Co(III) and Ni(II)], with the hydrazone ligand acting as a monoanionic bidentate N, O-donor. Binding of complexes 1 and 2 with calf thymus DNA (CT-DNA) was investigated by UV–Vis absorption, fluorescence spectroscopy and viscosity measurements. Complex 1 shows excellent catecholase mimicking activity with 3,5-di-tert-butylcatechol (3,5-DTBCH 2 ) as the substrate. Kinetic measurements suggest the rate of catechol oxidation follows saturation kinetics with respect to the substrate and the k cat value was found to be quite high, with a value of 1.00  10 5 h 1 . Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction The coordination chemistry of nitrogen–oxygen donor Schiff base ligands is a fertile area of inorganic research [1]. Aroyl hydra- zones are quite interesting as they present a combination of donor sites, such as a protonated/deprotonated amide oxygen atom, imine nitrogen atom of the hydrazone moiety and an additional donor site (usually N or O) provided from the aldehyde or ketone forming the Schiff base [2–4]. Hydrazones form wide variety of complexes with chemical, structural, biological and industrial importance [2–10]. They have a variety of applications in biological and clinical fields, such as anti-bacterial, anti-fungal, anti-convuls- ing, anti-inflammatory, anti-malarial, analgesic, anti-platelets, anti-tuberculosis, anti-cancer and insecticidal activities [11–17]. These activities are attributed to the formation of stable chelate complexes with transition metals which catalyze physiological processes [18,19]. Recent studies have shown that a variety of transition metal complexes have significant potential as probes for sequence- and structure-specific DNA binding. Significant attention has centered upon metal complexes capable of binding DNA by intercalation, due to their strong DNA binding affinity and ease of detection of such binding using the change in lumines- cence properties. Metal complexes that exhibit interactions with DNA have been studied with the goal of developing probes for nucleic acid structures as well as chemotherapeutic agents [20]. Again, catechol oxidase is a copper containing enzyme whose activity is similar to that of tyrosinase, a related class of copper oxi- dase. Catechol oxidase carries out the oxidation of 1,2-diphenols, such as catechol, to o-quinones, using dioxygen (O 2 ). Hydrogen atoms removed from catechol combine with the oxygen to form water. The crystal structures of met- and deoxy- forms of catechol oxidase have enhanced the understanding of the mechanism of catecholase activity of tyrosinase and catechol oxidase. The mecha- nism of catechol oxidation by the natural enzyme was proposed by several workers, including prominent proposals by Solomon and Kerbs [21–24]. Apart from the large number of Cu(II) model complexes mimicking catecholase activity that have been reported in the literature, quite a number of Co(III/II) and Ni(II) complexes have also been reported to show catecholase-like activity, though their mechanistic aspects are not as clearly understood as that of the Cu(II)-model complexes [25]. In this paper we describe two mononuclear Co(III) and Ni(II) complexes of a hydrazone ligand, containing 2,2 0 -bipyridine (bpy) http://dx.doi.org/10.1016/j.poly.2016.05.052 0277-5387/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: shch20@hotmail.com (S.K. Chattopadhyay). Polyhedron 117 (2016) 327–337 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly