A kinetic study of the reactivity in the cleavage of p-nitrophenyl acetate of a series of 2-pyridineoximes, 1-4, and their complexes with Ni(II), Zn(II) and Cu(II) is reported. Complexation of the oximic ligands leads to a remarkable increase in the acidity of the oximic group, following the order Cu(II) > Ni(II) > Zn(II). The oximates of free Ligands and their metal complexes, being alpha-nucleophiles, are quite effective in promoting the cleavage of PNPA. However, the reactivity, as defined by the second-order rate constants, is not predictably related to the acidity of the oximic function. The corresponding Bronsted plot shows a linear behaviour up to a pKa value of ca. 8, above which the reactivity of the uncomplexed oximic functions levels off to a limiting value. In the case of the metal ion complexes, a large effect of the Ligand structure on the reactivity was observed. As a consequence, complexes of appropriate ligands and metal ions show a reactivity that exceeds the apparent limiting one of the oximates. The origin of such effects and their possible implications in the development of new reactivators of phosphorylated acetylcholinesterase are discussed.

Activation of oximic nucleophiles by coordination of transition metal ions

MANCIN, FABRIZIO;TONELLATO, UMBERTO
2000

Abstract

A kinetic study of the reactivity in the cleavage of p-nitrophenyl acetate of a series of 2-pyridineoximes, 1-4, and their complexes with Ni(II), Zn(II) and Cu(II) is reported. Complexation of the oximic ligands leads to a remarkable increase in the acidity of the oximic group, following the order Cu(II) > Ni(II) > Zn(II). The oximates of free Ligands and their metal complexes, being alpha-nucleophiles, are quite effective in promoting the cleavage of PNPA. However, the reactivity, as defined by the second-order rate constants, is not predictably related to the acidity of the oximic function. The corresponding Bronsted plot shows a linear behaviour up to a pKa value of ca. 8, above which the reactivity of the uncomplexed oximic functions levels off to a limiting value. In the case of the metal ion complexes, a large effect of the Ligand structure on the reactivity was observed. As a consequence, complexes of appropriate ligands and metal ions show a reactivity that exceeds the apparent limiting one of the oximates. The origin of such effects and their possible implications in the development of new reactivators of phosphorylated acetylcholinesterase are discussed.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11577/1353717
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