Effect of H-bond and molecular geometry towards innocent and non-innocent behavior of 3,5-di-tert-butyl-2-aminophenol units attached to a piperazine backbone: Co(III) and Cu(II) complexes

Abstract

A 2-aminophenol-based potential non-innocent ligand H4L have been synthesized by reacting 2,2′-(piperazine-1,4-diylbis(methylene))dianiline with 2.0 equivalent amounts of 3,5-di-tert-butyl-catechol. In the ligand, both 2-aminophenol units were connected by a piperazine unit. X-ray single crystal analysis of ligand H4L revealed that piperazine unit was present in the more stable chair conformation. The ligand provided six-coordinate, mononuclear, diamagnetic complex 1 {[CoIII(H2L)]Cl} upon reacting with CoCl2·6H2O. A binuclear biradical-containing complex 2 {[CuII2(L)Cl2]} was obtained by reacting ligand H4L with 2.0 equivalent amounts of CuCl2·2H2O. Both the complexes were characterized by using IR, mass, UV–Vis/NIR, and single crystal X-ray crystallographic techniques. X-ray crystallographic analysis revealed that in complex 1 the ligand unit retained its oxidation state, while one-electron oxidation of each 2-aminophenol unit to 2-iminosemiquinone unit was noticed in case of complex 2. Two strong intramolecular H-bond interactions were found to be responsible for the innocent character of the 2-aminophenol coordination units in complex 1. Herein, the boat conformation of the piperazine backbone and the octahedral geometry of the complex promoted the interactions. The ligand, upon reacted with an equivalent amount of Cu(ClO4)2·6H2O in the presence of Et3N under air in CH3CN, provided a green solid (complex 3). Spectroscopic characterization, and DFT–based computational study indicated that the green solid was a four-coordinate, mononuclear, paramagnetic Cu(II) complex {3; [CuII(L)]} with two coordinated iminosemiquinone radicals. The optimized structure revealed that the piperazine unit in the complex was in the boat conformation.