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

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 (H2L(Bu,Bu): R1 = R2 = t-butyl; H2L(MeO,MeO): R1 = R2 = methoxyl; H2L(Bu,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). (C) 2014 Elsevier B.V. All rights reserved.