Synthesis and properties of diphenoxo-bridged CoII, NiII, CuII, and ZnII complexes of a new tripodal ligand: generation and properties of MII-coordinated phenoxyl radical species.

Four dinuclear complexes of composition [MII2(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-di tert-butyl-6-{(2-pyridyl)ethyl-aminomethyl}-phenol (H2L)], in its deprotonated form, providing a N 2O 2 donor set. Crystallographic analyses reveal that the complexes have a similar diphenoxo-bridged structure. Each metal ion is terminally coordinated by 2,4-di tert-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 (II)N 2O 3 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-di tert-butylphenolate unit) one-electron oxidations [E1/2(1)=0.50-0.58 and E1/2(2)=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 ZnII-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.