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^BISCu^IICl exhibits a distorted tetrahedral geometry, while L^BISCu^IIOAc is square pyramidal. In both solid state structures the ligand is coordinated to Cu^IIvia 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^BISCu^IICl and L^BISCu^IIOAc, respectively. Magnetic susceptibility measurements indicate that both complexes are diamagnetic due to antiferromagnetic coupling between the d⁹ 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^BISCu^IIX (X = Cl) is EPR-silent, chemical oxidation affords a species with an EPR signal consistent with ligand oxidation to form a d⁹ 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^BISCu^IIX–Cs₂CO₃ as the catalyst.