Synthesis, characterization and structure of a new zinc(II) complex containing the hexadentate N, N′, N, N′-bis[(2-hydroxy-3,5-di- tert-butylbenzyl)(2-pyridylmethyl)]-ethylenediamine ligand: Generation of phenoxyl radical species

Abstract

This work summarizes the results of our studies on the structural, spectral and redox properties of a mononuclear zinc(II) complex with the new H 2L ligand (H 2L = N, N′, N, N′-bis[(2-hydroxy-3,5-di- tert-butylbenzyl)(2-pyridylmethyl)]-ethylene diamine). The crystal structure of the complex [Zn II(HL)] · ClO 4 ( 1) was determined by X-ray crystallographic analysis. The structure of this complex consists of a discrete mononuclear cation [Zn II(HL)] +, in a strongly distorted geometry with a slight tendency toward a distorted square pyramidal geometry, as reflected by the structural index parameter τ of 0.44. The zinc(II) cation is coordinated to one oxygen and four nitrogen atoms: the pyridine nitrogen atoms (N22 and N32), tertiary amine nitrogen atoms (N1 and N4) and phenolate oxygen atom (O10). 1H and 13C NMR spectral data show a rigid solution structure for 1 in agreement with X-ray structure. Potentiometric studies of complex 1 were also performed and revealed three titratable protons which are attributed to the protonation/deprotonation of two phenol groups (p[ K] a1 = 4.04 and p[ K] a3 = 11.34) and dissociation of a metal-bound water molecule (p[ K] a2 = 7.8). The phenolate groups in complex 1 are suitably protected by bulky substituents ( tert-butyl) in the ortho- and para-positions, which through electrochemical oxidation generate a one-electron oxidized phenoxyl species in solution. This radical species was characterized by UV–Vis, EPR and electrochemical studies. The Zn(II)-phenoxyl radical species is of bioinorganic relevance, since its spectroscopic, redox and reactivity properties can be used to establish the role of phenoxyl radicals in biological and catalytical systems.