Non‐Innocent Property of Tridentate Ligand in Novel Cobalt Complex : Crystal Structure and Evidences for Cobalt(II) Phenoxyl Radical Complex Formation

Abstract

A phenoxyl radical complex [Co II (L)(L + )]NO 3 (where LH = (E)-2,4di-tert-butyl-6-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol and H = dissociable proton) was synthesized at room temperature in aerobic condition and was characterized. Crystal structure, UV-Vis-NIR and 1 H-NMR spectra, magnetic properties and electrochemical studies authenticated the presence of Co (II) phenoxyl radical complex.Generation of phenoxyl radical complexes and their reactivity studies are significantly important in the context of structural and functional modelling of galactose oxidase enzyme and are important areas in chemical research. [1, 2] Although phenoxyl radical complex of copper exhibit vital role in the oxidation chemistry of galactose by the native enzyme, phenoxyl radical complexes of cobalt gained considerable current interest because of their essential roles in the catalytic cycle performed by Jacobsen catalyst. [3][4][5][6] Cobalt complexes derived from salen type ligand exhibit crucial role in epoxide formation and CO 2 / polymerization reactions. [5,6] It has been found out that ligand centred oxidation and radical formation are important events in the catalytic cycle. Hence in the literature there are reports of cobalt(II) as well as cobalt(III) phenoxyl radical complexes. [2][3][4][5][6][7] Herein we report a facile synthesis and characterization of novel cobalt(II) radical complex [Co(L)(L + )]NO 3 and molecular structure of the complex was determined by X-ray crystallography.Recently, Fujii and co-workers reported Co(salen)(X) complex with the 40 % character of Co II (salen * + )(X) and 60 % character of Co III (salen)(X). [4] Thomas and co-workers have shown similar kind of cobalt(II) salen complex and one electron oxidized complex was found to have 70 % electron density over the cobalt ion and remaining 30 % electron density spreading over the ligand. [8] In another report by the same group, phenolato donor was replaced by the anilido donor and the resultant complex was found to have 75 % contribution of metal ion in SOMO. [6] Hence, to the best of our knowledge there is no report of phenoxyl radical complex of cobalt(II) in the literature in a ligand system other than salen and salen type ligands. We would like to mention here that Benisvy and co-workers synthesized octahedral cobalt(II) complex bearing imidazole-phenolato moiety and oxidation of this complex afforded a cobalt (II) phenoxyl radical complex; [12] however no crystal structure was described in this report. We have been working with a tridentate ligand [9] LH (shown in Figure 1) and in present study, deprotonated ligand having NNO donors was treated with cobalt(II) nitrate to afford a green colored complex. The resultant cobalt complex provided several interesting properties which are described below.Presence of phenolato function in the ligand frame and synthesis of complex [Co II (L)(L + )]NO 3 , one could simply expect the formation of a diamagnetic Co(III) complex with formula [Co III L 2 ]NO 3 however we have solved the molecular str...