Mono and tri-nuclear cobalt(III) complexes with sterically constrained phenol based N 2 O 2 ligand: Synthesis, structure and catechol oxidase activity Imran Ali a , Bikramaditya Mandal a , Rajat Saha b , Rajarshi Ghosh c , Mithun Chandra Majee d , Dhrubajyoti Mondal d , Partha Mitra d , Debdas Mandal a,⇑ a Department of Chemistry, Sidho-Kanho-Birsha University, Purulia 723104, West Bengal, India b Department of Chemistry, Kazi Nazrul University, Asansol, Paschim Bardhaman-713340, West Bengal, India c Department of Chemistry, The University of Burdwan, Burdwan 713 104, India d Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700032, India article info Article history: Received 8 December 2019 Accepted 1 February 2020 Available online 5 February 2020 Keywords: Mono and tri-nuclear Co(III) complexes Tetradentate ligand Syntheses X-ray structures Catecholase activity abstract Mono and tri- nuclear Co(III) complexes, [Co 3 (OMe) 4 L 2 ]ClO 4 (1) and [CoL(acac)] (2) have been synthe- sized using sterically constrained tetradentate ligand N,N / -dimethyl-N,N / -bis(2-hydroxy-3,5-dimethyl- benzyl)-ethylenediamine (H 2 L) with varying reaction condition. All complexes have been characterized by single crystal X-ray diffractometer and various spectroscopic tools. Complex 1 is tri-nuclear methoxo and phenoxo bridged cobalt complex with Co III – Co III – Co III linear core, whereas complex 2 is mononu- clear mixed ligand octahedral Co(III) complex. Both complexes show catechol oxidase activity in metha- nol as monitored by the UV–Vis spectroscopy of the aerial oxidation of 3,5-DTBC to 3,5-DTBQ. The kinetic parameters have been determined using Michaelis–Menten approach. The complexes are efficient cata- lysts with high turnover numbers 3.32  10 3 h 1 and 2.19  10 3 h 1 for 1 and 2, respectively. Mechanistic investigations of the catalytic behaviour by ESI-MS spectra and estimation of hydrogen peroxide forma- tion indicates that the catalytic reaction occurs through the reduction of Co(III) to Co(II). Ó 2020 Elsevier Ltd. All rights reserved. 1. Introduction Multinuclear transition-metal complexes are of current interest due to their wide range of applications in various fields such as catalysis, adsorption, storage, magnetism, molecular recognition, fluorescence, nonlinear optics, and sensors [1]. Bridging capability of phenolate donor site will be exploited judiciously to synthesize multinuclear metal complexes with aesthetically pleasing struc- tural frame works [2]. Using phenol-based ligands, we would like to assemble more metal ions in close proximity. Inspired by the above knowledge, we would like to synthesize new type of ligands having pre-organized donor set with biological relevance. We design ligand in such a way so that the ligands possess flexibility character. Flexibility in ligand environment introduces structural diversity in metal complexes giving interesting property. Coordination complex with catecholase like activity plays a pivotal role for the development of new bio-inspired catalyst [3]. Catechol oxidase is a copper containing type III active site protein which catalyzes the oxidation of catechol to the corresponding qui- none. The generated quinones are highly reactive compounds that undergo auto polymerization to produce melanin which is a brown polyphenolic pigments. Melanin may be responsible for protecting the tissues from damage against pathogens and insects [4]. The crystal structure of catechol oxidase, isolated from sweet potatoes was determined in 1998 which revealed that the active center of the catechol oxidase consists of a hydroxo bridged dicopper (II) center in which each copper(II) center is coordinated to three his- tidine ligands [5]. The ability of di-copper complexes to oxidize phenols and cate- chols is a topic of recent interest for the development of new bio inspired catalyst [6]. From the report of various research group, catechol oxidase activity of metal complexes with other metal ion such as Co(III), Fe(II), Ni(II), Mn(III) and Zn(II) is well estab- lished in recent time [7]. With the above mentioned two fold aims, namely, development of multinuclear transition metal complexes and catechol oxidase activity of cobalt(III) complexes derived from flexible phenol-based ligand, we have isolated two new cobalt complexes of composition [Co III 3 (OMe) 4 L 2 ]ClO 4 (1) and [Co III L(acac)] (2). These complexes are https://doi.org/10.1016/j.poly.2020.114429 0277-5387/Ó 2020 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: deb_mandal@yahoo.co.in (D. Mandal). Polyhedron 180 (2020) 114429 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly