Zinc(II)-phenoxyl radical complexes: Dependence on complexation properties of Zn-phenolate species

Zinc(II) complexes of N3O-donor tripodal ligands, 2,4-di(tert-butyl)-6-{[bis(2-pyridyl) methyl] aminomethyl} phenol (HtbuL), 2,4-di(tert-butyl)-6-{[(6-methyl-2-pyridyl) methyl]-(2-pyridyl) methylaminomethyl} phenol (HtbuLMepy), and 2,4-di(tert-butyl)-6-{[bis(6-methyl-2-pyridyl) methyl] aminomethyl} phenol (HtbuL(Mepy)(2)), [Zn(tbuL)Cl] center dot CH3OH (1), [Zn(tbuLMepy)Cl] (2), and [Zn(tbuL(Mepy)(2))Cl] (3), respectively, were prepared and structurally characterized by the X-ray diffraction method. All the complexes were found to have a mononuclear structure with a coordinated phenolate moiety, the geometry of the Zn(II) center being 5-coordinate trigonal-bipyramidal. The Zn(II) binding ability of the ligands with and without 6-methyl-2-pyridylmethyl moieties was evaluated for similar ligands, which lacked the t-butyl groups at the 2-and 4-positions of the phenol moiety, by the stability constants determined by potentiometric titration at 25 degrees C (I = 0.1 M (KNO3)). The stability of the complexes was found to be in the order L > LMepy > L(Mepy)(2), reflecting the steric hindrance of the 6-methyl group of the pyridine ring. Complexes 1, 2, and 3 were converted to the phenoxyl radicals upon oxidation with Ce(IV), giving a phenoxyl radical pi-pi* transition band at 394-407 nm. ESR and resonance Raman spectra established that the radical species had a Zn(II)-phenoxyl radical bond. The cyclic voltammograms showed similar quasi-reversible redox waves with E-1/2 = 0.68, 0.67, and 0.63 V (versus Ag/AgCl) for 1, 2, and 3, respectively, corresponding to the formation of the phenoxyl radical, which displayed a first-order decay. The half-lives, 58.6, 25.8, and 15.6 min at -40 degrees C for 1, 2, and 3, respectively, follow the order of the stability constants of the complexes, indicating that the metal(II) -phenoxyl radical stability is in close relationship with the complexation properties of the present series of N3O-donor ligands. (C) 2008 Elsevier B. V. All rights reserved.