Bis(μ-alkoxo) bridged dinuclear CuII2 and ZnII2 complexes of an isoindol functionality based new ligand: Synthesis, structure, spectral characterization, magnetic properties and catechol oxidase activity

Abstract

Two new dinuclear copper(II) and zinc(II) complexes of an isoindol functionality based new dinucleating ligand, H3hdpa (H3hdpa=2-({[2-hydoxyethyl]-[2-hydroxy-3-(1-oxo-1,3-dihydro-isoindol-2-yl)-propyl]-amino}-methyl)-benzoic acid) have been synthesized and characterized. In methanol, the reaction of stoichiometric amounts of Cu(OAc)2·H2O and the ligand H3hdpa in the presence of NaOH at ambient temperature afforded a new dinuclear copper(II) complex, [Cu2(Hhdpa)2]·2CH3OH·6H2O (1). Similarly, in methanol, the reaction of stoichiometric amounts of Zn(OAc)2·2H2O and H3hdpa in the presence of NaOH yielded a new dinuclear zinc(II) complex, Na4[Zn2(hdpa)2](OAc)2 (2). Characterization of the complexes has been performed using various analytical techniques, including single crystal X-ray structure determination. The X-ray crystal structure of complex 1 reveals that two copper(II) centers adopt a five-coordinate square pyramidal geometry with a Cu–Cu separation of 2.910Å. The DFT optimized structure of complex 2 shows that two zinc(II) centers are in a distorted trigonal bipyramidal geometry with a Zn–Zn separation of 3.124Å. 1H and 13C NMR spectroscopic investigations authenticate the integrity of complex 2 in solution. Further, the mass spectroscopic analyses of complexes 1 and 2 reconfirm their dimeric nature, even in solution. Variable-temperature (2–300K) magnetic susceptibility measurements show the presence of antiferromagnetic interactions (J=−52.20cm−1) between the two copper(II) centers in complex 1. The catechol oxidase activity of complexes 1 and 2 has been investigated in methanol medium by the UV–Vis spectrophotometric technique using 3,5-di-tert-butylcatechol as a model substrate. Both complexes are active in catalyzing the aerobic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butyl-1,2-benzoquinone (3,5-DTBQ). A DFT calculation has been performed to find the Fukui functions at the metal centers in complexes 1 and 2 to predict the possible metal centers involved in the binding process with 3,5-DTBC during the catalytic oxidation reactions.