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

A tridentate benzoxazole-containing aminophenol ligand HLBAP 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. (LCuCl)-Cu-BIS-Cl-II exhibits a distorted tetrahedral geometry, while (LCuOAc)-Cu-BIS-O-II 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 (LCuCl)-Cu-BIS-Cl-II and (LCuOAc)-Cu-BIS-O-II, 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 (LCuX)-Cu-BIS-X-II (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 (LCuX)-Cu-BIS-X-II-Cs2CO3 as the catalyst.