A Copper Complex of a Noninnocent Iminophenol-Amidopyridine Hybrid Ligand: Synthesis, Characterization, and Aerobic Alcohol Oxidation

Reaction of the noninnocent iminophenol-iminopyridine hybrid ligand HLIPIP, where L-IPIP denotes [2-((E)-{(E)-2-[(E)-pyridin-2-ylmethyleneamino] benzylidene} amino)-4,6-di-tert-butylphenolate], with copper acetate afforded a copper complex, (LCuII)-Cu-APIP, in which one of the imine functional groups is oxidized to an amide during metal complexation. The new Cu-II complex is capable of catalyzing efficient aerobic alcohol oxidation under mild conditions. The crystal structure of (LCuII)-Cu-APIP exhibits a square-planar geometry with the Cu-II center coordinated by three nitrogen atoms and one oxygen atom. Electrochemical studies were conducted to evaluate the redox-active behavior of the complex, and the results showed a quasireversible reduction and a ligand-based oxidation process. The neutral species of (LCuII)-Cu-APIP is EPR active, which is consistent with a paramagnetic electronic ground state (d(9), S = 1/2), whereas the one-electron oxidized complex was X-band EPR silent. One-electron chemical oxidation of (LCuII)-Cu-APIP gave a new species that can be attributed to a Cu-II-phenoxyl radical complex. Based on EPR measurements in conjunction with density functional theory calculations, [(LCuII)-Cu-APIP](+) is proposed to have a triplet electronic ground state, exhibiting a weak ferromagnetic interaction between the Cu-II center and the coordinated phenoxyl radical.