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

The boat conformation of the piperazine backbone in octahedral Co(III) complex of ligand H4L facilitated H-bonding interactions, which rendered non-innocent character of 2-aminophenol units. The absence of such interactions in the corresponding Cu(II) complexes promoted iminosemiquinone radical stat...

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Published in:Inorganica Chimica Acta 2019-02, Vol.486, p.213-220
Main Authors: Paul, Ganesh Chandra, Sarkar, Prasenjit, Mukherjee, Chandan
Format: Article
Language:English
Subjects:
Aminophenol
Antiferromagnetic coupling
Backbone
Catechol
Chemical bonds
Coordination compounds
Crystallography
Cu-radical complex
Diamagnetism
Equivalence
H-bonding
Ligands
Methylene dianiline
Oxidation
Redox non-innocent ligand
Single crystals
Valence
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Summary:The boat conformation of the piperazine backbone in octahedral Co(III) complex of ligand H4L facilitated H-bonding interactions, which rendered non-innocent character of 2-aminophenol units. The absence of such interactions in the corresponding Cu(II) complexes promoted iminosemiquinone radical state of the coordinating 2-aminophenol units. [Display omitted] •Innocent and non-innocent behavior of 2-aminophenol coordination unit.•Conformation of piperazine ligand backbone.•H-bonding interactions.•DFT-based geometry optimization. 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.
ISSN:0020-1693
1873-3255
DOI:10.1016/j.ica.2018.10.048