Phenoxyl Radical Complexes of Zinc(II)

Abstract

A series of phenoxyl radical complexes of zinc(II) have been generated in solution and, in one instance, isolated as solid material (5) in order to study their spectroscopic features by EPR, resonance Raman, and UV−vis spectroscopy. They serve as model complexes for the active form of the copper containing fungal enzyme galactose oxidase. The complexes [Zn(L1H2)]BF4·H2O (1), [Zn(L2H2)]BF4·H2O (2), [Zn(L2H)] (2a), [Zn(L3)(Ph2acac)] (3), [Zn(L4)(Ph2acac)] (4), and [Zn(L4)(Me-acac)] (6) were synthesized from solutions of Zn(BF4)2·4H2O and the corresponding ligand (L1H3 = 1,4,7-tris(3,5-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane; L2H3 = 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane; L3H = 1,4-dimethyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane; L4H = 1,4-dimethyl-7-(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, Ph2acac- = 1,3-diphenyl-1,3-propanedionate, and Me-acac- = 3-methyl-2,4-pentanedionate). Complexes 2, 3·0.5 toluene·1n-hexane, and 4 were structurally characterized by single-crystal X-ray crystallography. An electrochemical investigation of these complexes in CH3CN and/or CH2Cl2 solution revealed that the coordinated phenolate ligands undergo reversible one-electron oxidations with formation of coordinated phenoxyl radicals. Synthetically, the microcrystalline, paramagnetic (μeff = 1.7 μB), solid material of [Zn(L4)(Ph2acac)]PF6 (5) was produced by one electron oxidation of 4 by 1 equiv of ferrocenium hexafluorophosphate in dry CH2Cl2. Oxidation of coordinated phenol pendent arms in 1, 2, and 2a occurs at significantly higher potentials and is irreversible. Electronic (UV−vis), electron paramagnetic resonance (EPR), and resonance Raman (RR) spectra of the radicals have been studied in solution and allow the description of the electronic structure of these coordinated phenoxyl radical complexes.