Synthesis, characterization and catalytic activity of copper(ii) complexes containing a redox-active benzoxazole iminosemiquinone ligand

Abstract

A tridentate benzoxazole-containing aminophenol ligand HL(BAP) was synthesized and complexed with Cu(II). The resulting Cu(II) complexes were characterized by X-ray, IR, UV-vis-NIR spectroscopies, and magnetic susceptibility studies, demonstrating that the ligand is oxidized to the o-iminosemiquinone form [L(BIS)](-) in the isolated complexes. L(BIS)Cu(II)Cl exhibits a distorted tetrahedral geometry, while L(BIS)Cu(II)OAc is square pyramidal. In both solid state structures the ligand is coordinated to Cu(II)via the benzoxazole, as well as the nitrogen and oxygen atoms from the o-iminosemiquinone moiety. The chloride, or acetate group occupies the fourth and/or fifth positions in L(BIS)Cu(II)Cl and L(BIS)Cu(II)OAc, respectively. Magnetic susceptibility measurements indicate that both complexes are diamagnetic due to antiferromagnetic coupling between the d(9) Cu(II) centre and iminosemiquinone ligand radical. Electrochemical studies of the complexes demonstrate both a quasi-reversible reduction and oxidation process for the Cu complexes. While L(BIS)Cu(II)X (X = Cl) is EPR-silent, chemical oxidation affords a species with an EPR signal consistent with ligand oxidation to form a d(9) Cu(II) iminoquinone species. In addition, chemical reduction results in a Cu(II) centre most likely bound to an amidophenoxide. Mild and efficient oxidation of alcohol substrates to the corresponding aldehydes was achieved with molecular oxygen as the oxidant and L(BIS)Cu(II)X-Cs2CO3 as the catalyst.

Oxidation of sulfides including DBT using a new vanadyl complex of a non‐innocent o‐aminophenol benzoxazole based ligand

“…The IR spectra of the complex exhibit a sharp band at 970 cm −1 , which is characteristic of the ν V=O stretch, the strong and sharp ν O‐H stretch of the ligand disappears. The monomeric complex isolated herein differs from copper and cobalt complexes of a similar o ‐aminophenol ligand recently investigated …”

“…2,4‐di‐tert‐butyl‐6‐(2‐(5,7‐di‐tert‐butylbenzo [d] oxazol‐2‐yl) phenylamino) phenol, was synthesized according to a modified literature procedure …”

Copper catalysis with redox-active ligands

“…Inspired by the pioneering works by Stack [10], Wieghardt and Chaudhuri on GAO mimics [11][12][13], Safaei and Storr have reported Cu(II) complexes bearing a non-innocent amidophenolate type benzoxazole ligand which was fully characterized by X-ray crystallography and other spectroscopic techniques (Scheme 1) [14]. These complexes were found to perform the aerobic catalytic oxidation of alcohols into aldehydes, thus mimicking the reactivity of galactose oxidase.…”

A biradical oxo-molybdenum complex containing semiquinone ando-aminophenol benzoxazole-based ligands

“…The redox-active aminophenolate ligand H 2 L BAP was synthesized from 1 : 2 molar ratio of 2-aminobenzyl amine and 3,5-DTBQ. 14 The synthesis of MoOL BIS L SQ complex was performed in 40% yield by reuxing CH 2 Cl 2 solution of a ligand H 2 L BAP and Et 3 N with MoO 2 (acac) 2 (Scheme 2). The complex was puried by crystallization from CH 3 OH/CH 2 Cl 2 1 : 1 mixture and single Scheme 1 Proposed routes for the synthesis of adipic acid (AA).…”