A non-innocent ligand, H4L(CH2NH2), was synthesized by introducing a -CH2NH2 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 CuCl2⋅2H2O, 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 dx2-y2 orbital. 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 C-N 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 H2L5 ligand that was formed by self-condensation of two molecules of 2-aminobenzaldehyde and provided the modified ligand backbone.
Keywords: amines; copper; enzymes; non-innocent ligands; oxidation.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.