Phenoxyl-copper(II) complexes: models for the active site of galactose oxidase

Abstract

The reaction of the macrocycles 1,4,7-tris (3,5-di-tert-butyl-2-hydroxy-benzyl)-1,4,7-triazacyclononane, L1H3, or 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxy-benzyl)-1,4,7-triazacyclononane, L2H3, with Cu(ClO4)2·6H2O in methanol (in the presence of Et3N) affords the green complexes [CuII(L1H)] (1), [CuII(L2H)]·CH3OH (2) and (in the presence of HClO4) [CuII(L1H2)](ClO4) (3) and [CuII(L2H2)] (ClO4) (4). The CuII ions in these complexes are five-coordinate (square-base pyramidal), and each contains a dangling, uncoordinated pendent arm (phenol). Complexes 1 and 2 contain two equatorially coordinated phenolato ligands, whereas in 3 and 4 one of these is protonated, affording a coordinated phenol. Electrochemically, these complexes can be oxidized by one electron, generating the phenoxyl-copper(II) species [CuII(L1H)]+ ·, [Cu(L2H)]+ ·, [CuII(L1H2)]2+ ·, and [CuII(L2H2)]2+ ·, all of which are EPR-silent. These species are excellent models for the active form of the enzyme galactose oxidase (GO). Their spectroscopic features (UV-VIS, resonance Raman) are very similar to those reported for GO and unambiguously show that the complexes are phenoxyl-copper(II) rather than phenolato-copper(III) species.