Studies on galactose oxidase active site model complexes: effects of ring substituents on Cu(II)-phenoxyl radical formation

Abstract Copper(II) complexes of new N 3 O- and N 2 O 2 -donor tripodal ligands bearing one or two o -substituted phenol moieties have been synthesized as models for the galactose oxidase active site. The complexes of 2-[ N -(1-methyl-2′-imidazolylmethyl)- N -(6″-methyl-2″-pyridylmethyl)-aminomethyl)]-4-methyl-6-methylthiophenol (MeSL), [Cu(MeSL)Cl], and N -(6-methyl-2-pyridylmethyl)- N , N -bis(2′-hydroxy-3′,5′-di- tert -butylbenzyl)amine (t-buL2mepy), [Cu(t-buL2mepy)(H 2 O)], have been revealed by X-ray structural analysis to have a square-pyramidal structure with one and two phenolate oxygens in the basal plane, respectively. [Cu(MeSL)Cl] was converted into a Cu(II)- o -methylthiophenoxyl radical species by electrochemical or Ce(IV) oxidation. An o -methoxyphenoxyl radical in a similar complex was considerably more stable than the 2,4-di( tert -butyl)phenoxyl radical. While t-buL2mepy reacted with Cu(ClO 4 ) 2 to give [Cu(t-buL2mepy)(H 2 O)] without disproportionation, an N2O2-donor ligand containing an o -methoxyphenol, a 2,4-di( tert- butyl)phenol, and an N -methylimidazole moiety gave a phenoxyl radical complex exhibiting the characteristic absorption peak at 478 nm as a reddish powder by the reaction with Cu(ClO 4 ) 2 as a result of spontaneous disproportionation. It exhibited a quasi-reversible redox wave at E 1/2 =0.34 V (vs. Ag/AgCl) in CH 3 CN, which is lower than the potentials of the copper complexes of various N 3 O-donor ligands, and oxidized ethanol to acetaldehyde with a low turnover number.