Synthesis and properties of diphenoxo-bridged Co^II, Ni^II, Cu^II, and Zn^II complexes of a new tripodal ligand: generation and properties of M^II-coordinated phenoxyl radical species

Abstract

Four dinuclear complexes of composition [M^II₂(L)₂]·xS [M = Co, x = 0.5, S = 1,4-dioxane (1·0.5 1,4-dioxane); Ni, x = 0 (2) [single crystals have x = 2, S = diethyl ether (2·2 diethyl ether)]; Cu, x = 0 (3); Zn, x = 0.5, S = 1,4-dioxane (4·0.5 1,4-dioxane)] have been synthesized using a new tripodal ligand [2,4-ditert-butyl-6-{[(2-pyridyl)ethyl](2-hydroxybenzyl)-aminomethyl}-phenol (H₂L)], in its deprotonated form, providing a N₂O₂ donor set. Crystallographic analyses reveal that the complexes have a similar diphenoxo-bridged structure. Each metal ion is terminally coordinated by 2,4-ditert-butyl-phenolate oxygen, a tertiary amine, and a pyridyl nitrogen. From each ligand, unsubstituted phenolate oxygen provides bridging coordination. Thus, each metal center assumes M^IIN₂O₃ coordination. Whereas the geometry around the metal ion in 1·0.5 1,4-dioxane, 2·2 diethyl ether and, 4·0.5 1,4-dioxane is distorted trigonal-bipyramidal, in 3 each copper(II) center is in a square-pyramidal environment. Temperature-dependent magnetic behavior has been investigated to reveal intramolecular antiferromagnetic exchange coupling for these compounds (-J = 6.1, 28.6, and 359 cm^-1 for 1·0.5 1,4-dioxane, 2, and 3, respectively). Spectroscopic properties of the complexes have also been investigated. When examined by cyclic voltammetry (CV), all four complexes undergo in CH₂Cl₂ two reversible ligand-based (2,4-ditert-butylphenolate unit) one-electron oxidations [E_½¹ = 0.50-0.58 and E_½² = 0.63-0.75 V vs SCE (saturated calomel electrode)]. The chemically/coulometrically generated two-electron oxidized form of 3 rearranges to a monomeric species with instantaneous abstraction of the hydrogen atom, and for 4.0.5 1,4-dioxane the dimeric unit remains intact, exhibiting an EPR spectrum characteristic of the presence of Zn^II-coordinated phenoxyl radical (UV-vis and EPR spectroscopy). To suggest the site of oxidation (metal or ligand-centered), in each case DFT calculations have been performed at the B3LYP level of theory.