Diphenoxido-Bridged CoII and ZnII Complexes of Tripodal N2O2 Ligands: Stabilisation of MII-Coordinated Phenoxyl Radical Species

Three new tripodal ligands with an N2O2 donor set, namely 2-tert-butyl-6-({(2-hydroxybenzyl)[2-(2-pyridyl)ethyl]amino}methyl)-4-methylphenol (H2L1), 2-tert-butyl-6-({(2-hydroxybenzyl)[2-(2-pyridyl)ethyl]amino}methyl)-4-methoxyphenol (H2L2) and 2-tert-butyl-6-({[2-(dimethylamino)ethyl](2-hydroxybenzyl)amino}methyl)-4-methoxyphenol (H2L3) have been synthesised. Treatment of the ligands with Co(CH3CO2)(2)center dot 4H(2)O or [Zn(H2O)(6)][ClO4](2) in the presence of Et3N provides the corresponding Co-II and Zn-II complexes of composition [M-2(II)(L-1)(2)] [M = Co (1) (single-crystals are a solvate with the composition [Co-2(II)(L-1)(2)]center dot 2CHCl(3), i.e. 1 center dot 2CHCl(3)); M = Zn (2)], [M-2(II)(L-2)(2)] [M = Co (3); Zn (4)] and [Co-2(II)(L-3)(2)] (5). Crystallographic analyses reveal that the complexes have closely similar diphenoxido-bridged structures. Each metal centre assumes (MN2O3)-N-II coordination. The geometry around each metal ion in 1 center dot 2CHCl(3) (tau = 0.76), 2 (tau = 0.77), 3 (tau = 0.74), 4 (tau = 0.76) and 5 [one Co-II (tau = 0.49) and the other Co-II (tau = 0.63)] is intermediate between ideal square-pyramidal (tau = 0) and trigonal-bipyramidal (tau = 1). Temperature-dependent magnetic studies reveal weak intramolecular antiferromagnetic exchange couplings for all the three Co-II complexes (-J = 1.84, 1.32 and 5.70 cm(-1) for 1, 3 and 5, respectively). Spectroscopic properties of the complexes have also been investigated. Cyclic voltammetric (CV) measurements of 1 and 2 show an irreversible oxidative response at E-pa (anodic peak potential) in the range 0.60-0.75 V relative to the SCE (saturated calomel electrode), whereas two successive quasi-reversible oxidative responses can be observed for 3-5 at E-1/2 values in the range 0.40-0.53 V relative to the SCE. Oxidative responses are due to the formation of M-II-coordinated phenoxyl radical species. The metal-coordinated phenoxyl radical species, generated by two-electron coulometric oxidation of 3-5, were characterised by CV and by adsorption and EPR spectroscopy. The stability of such species was determined by measuring the decay constant (absorption spectroscopy), which reveals that the phenoxyl radical species of 5 is more stable than that of 3 and 4. EPR spectroscopic studies (120 K) of coulometrically generated two-electron oxidised species of 4 in CH2Cl2 (containing 0.1 M TBAP) at 298 K reveal a combination of an isotropic S = 1/2 signal at g = 2 [(phenolato)(phenoxyl radical)-coordinated dizinc(II) complex] and a spin-triplet resonance (S = 1) that gives rise to the symmetric split-line pattern [bis(phenoxyl radical)cordinated dizinc(II) complex]. To pinpoint the site of oxidation (metal- or ligand-centred) in each case, DFT calculations were performed at the B3LYP level of theory.