Structural and functional mimic of galactose oxidase by a copper complex of a sterically demanding [N2O2] ligand

Abstract

A structural and functional mimic of the galactose oxidase (GOase) enzyme active-site by a copper complex supported over a sterically demanding ligand having [N₂O₂] donor sites is reported. Specifically, the binding of the histidine (496 and 581) and tyrosine (272 and 495) residues to the copper center in a square-pyramidal fashion in the active-site of galactose oxidase (GOase) enzyme has been modeled in a copper complex, {[(3-tert-butyl-5-methyl-2-hydoxybenzyl)(3′-tert-butyl-5′-methyl-2′-oxobenzyl)(2-pyridylmethyl)]amine}Cu(OAc)} (1b), stabilized over a sterically demanding ligand in which the two phenolate-O atoms mimicked the tyrosine binding while an amine-N and pyridyl-N atoms emulated the histidine binding to the metal center, similar to that in the enzyme active-site. Furthermore, the copper complex 1b is found to be an effective functional model of the enzyme as it efficiently catalyzed the chemoselective oxidation of primary alcohols to aldehydes in high turnover numbers under ambient conditions. An insight into the nature of the active-species was obtained by EPR and CV studies, which in conjunction with the DFT studies, revealed that the active-species is an anti-ferromagnetically coupled diamagnetic radical cation, ¹1b⁺, obtained by one electron oxidation at the equatorial phenolate-O atom of the ligand in the 1b complex.