Hydrolysis of imines. Part 2. Kinetics and mechanism of hydrolysis of N-salicylidene-2-aminopyridine in the presence and absence of copper(II) ion. A study of the catalytic effect of some mixed-ligand complexes of copper(II)

Abstract

The kinetics of hydrolysis of N-salicylidene-2-aminopyridine (HL) have been investigated in aqueous 5% MeOH (I= 0.1 mol dm^–3) in the presence and absence of Cu^II. The Schiff-base anion is found to undergo hydroxide-independent hydrolysis with k=(3.5 ± 0.2)× 10^–2 s^–1 at 35 °C, ΔH^‡= 59.7 ± 1.0 kJ mol^–1, ΔS^‡=–79 ± 3 J K^–1 mol^–1; the observed solvent deuterium isotope effect [k(H₂O)/k(D₂O)= 1.6 at 35 °C] on this path is consistent with intramolecular catalysis by the phenoxide ion. The possibility of the intramolecular general base-catalysed hydration of the imine linkage by pyridine ring nitrogen of the imine is investigated. The copper(II) chelate, [CuL]⁺ undergoes acid-catalysed hydrolysis of the imine linkage with k=(4.4 ± 0.3)× 10² dm³ mol^–1 s^–1 at 35 °C. The mixed-ligand complexes [Cu(bipy)L]⁺, [Cu(phen)L]⁺, and [Cu(im)₂L]⁺(bipy = 2, 2′-bipyridyl, phen = 1,10-phenanthroline, and im = imidazole) have also been found to be effective catalysts.