Catalytic performance and mechanism of Cu(II)-hydrazone complexes as models of galactose oxidase

• A new GO model system is established.
• The catalytic mechanism is proved via formation of phenoxyl radicals.
• Catalytic performance is related to the electron density on phenolates.

As simulants of galactose oxidase (GO), three mononuclear Cu(II) complexes with 1-((3-t-Bu,5-R1)-salicylidenehydrazono),2-((3-t-Bu,5-R2)-salicylidenehydrazono)-1,2-diphenylethane (H2LBu,Bu: R1 = R2 = t-butyl; H2LMeO,MeO: R1 = R2 = methoxyl; H2LBu,MeO: R1 = t-butyl, R2 = methoxyl) as ligands were synthesized. X-ray diffraction defined their structures, like GO, all having a distorted square N2O2-coordinated Cu(II) center, and catalytic experiments confirmed their abilities to enable the aerobic oxidation of benzyl alcohol to benzaldehyde under room temperature with turnover numbers up to 823–1036. Voltammetric measurements indicated that the cupric phenolates are electroactive, in the range of 0.3–0.9 V (vs EFc+/Fc), all giving two anodic peaks symbolizing the formation of +1 and +2 charged complex radicals. The presence of radicals was proven by the thianthrene perchlorate titration UV–Vis spectra of 1–3 with showing two new absorptions typical for phenoxy radicals and by the electronic spin resonance spectra with revealing an antiferromagnetic coupling of phenoxy radicals with Cu(II) (s = 1/2).

A comprehensive study on the responses of Cu(II)-hydrazone complexes to electric and magnetic fields, and UV–Vis light under oxidative condition proves their catalyzing the aerobic oxidation of benzyl alcohol through providing monophenoxy radicals to intermediate the hydrogen transfer.Figure optionsDownload as PowerPoint slide