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Copper(II)‐Mediated Transformation of a Tridentate Non‐Innocent Ligand into a Tetradentate Salen‐Type Innocent Ligand
Author(s) -
Ghorai Samir,
Mukherjee Chandan
Publication year - 2014
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.201402868
Subject(s) - chemistry , ligand (biochemistry) , non innocent ligand , crystallography , electron paramagnetic resonance , imine , photochemistry , unpaired electron , moiety , stereochemistry , radical , inorganic chemistry , redox , organic chemistry , biochemistry , physics , receptor , nuclear magnetic resonance , catalysis
A non‐innocent ligand, H 4 L CH 2 NH 2, was synthesized by introducing a CH 2 NH 2 group at the ortho carbon atom to the aniline moiety of 2‐anilino‐4,6‐di‐ tert ‐butylphenol. The new ligand was characterized by IR and NMR spectroscopy and mass spectrometry techniques. Upon treatment with CuCl 2 ⋅ 2 H 2 O, this non‐innocent ligand provided a mononuclear four‐coordinate salen‐type Cu II complex by complete modification of the ligand backbone. The complex was characterized by IR spectroscopy, mass spectrometry, X‐ray single‐crystal diffraction, electron paramagnetic resonance (EPR) spectroscopy, and UV/Vis/near‐IR spectroscopy techniques. X‐ray crystallographic analysis showed an asymmetric environment around the Cu II center with a small (≈12°) twist between the two biting planes. Analysis of the X‐band EPR spectrum also supported the asymmetric environment and also indicated the presence of an unpaired electron on the d x 2 − y 2orbital. The UV/Vis/near‐IR spectrum showed strong absorption bands for metal‐to‐ligand charge transfer and ligand‐to‐metal charge transfer along with a Cu II ‐centered d–d transition. Mechanistic investigation of the formation of complex 1 indicated that modification of the ligand backbone proceeded through ligand‐centered amine to imine oxidation as well as through CN bond‐breaking processes. During these processes, 3,5‐di‐ tert ‐butyl‐1,2‐benzoquinone and 2‐aminobenzylidene were produced. Ammonia, generated in situ through hydrolysis of the imine to the aldehyde, reacted with 3,5‐di‐ tert ‐butyl‐1,2‐benzoquinone to form the corresponding 3,5‐di‐ tert ‐butyl‐1,2‐iminobenzoquinone moiety, which upon two‐electron reduction in the reaction medium formed 3,5‐di‐ tert ‐butyl‐1,2‐aminophenol. This aminophenol underwent condensation with the H 2 L5 ligand that was formed by self‐condensation of two molecules of 2‐aminobenzaldehyde and provided the modified ligand backbone.