Reactions of co-ordinated imines. Part I. Kinetics and mechanism of hydrolysis of N-2-pyridylmethyleneaniline both in the presence and absence of copper(II) ions

Abstract

The hydrolysis of N-2-pyridylmethyleneaniline (L) has been studied at 25 °C and I= 0.1 mol dm^–3 over the pH range 6.6–13.5. The observed rate constants (K_obs.) at constant pH pass through a minimum at pH 10.5. The reaction is subject to specific acid and base catalysis. Rate constants have been obtained for the reactions (i) and HL⁺+ H₂O → Products, k= 1.26 × 10^–1 dm³ mol^–1 s^–1(i), L +[OH]^–→ Products, k= 2.17 × 10^–2 dm³ mol^–1 s^–1(ii)(ii) and for the kinetically indistinguishable processes (iii) and (iv) where k= 2.8 × 10^–4 s^–1. The effect of L + H₂O → Products (iii), HL⁺+[OH]^–→ Products (iv) copper (II) ions on the hydrolysis has been studied at 25 °C and I= 0.1 mol dm^–3, under conditions where the 1 : 1 complex [CuL]²⁺ is the predominant species in solution. Evidence for an equilibrium of the type [CuL(OH₂)₂]²⁺ [graphic omitted] [CuL(OH)(OH₂)]⁺+ H₊ for which pK_a is ca. 7.2 has been obtained from the kinetic measurements. Rate constants have been derived for the hydrolytic reactions (v)–(vii). Base hydrolysis of [CuL(OH₂)₂]² is some 4.4 ×[CuL(OH)₂]²⁺+ H₂O → Products, k= 3.15 × 10^–6 dm³ mol^–1 s^–1(v), [CuL(OH₂)₂]²⁺+[OH]^–→ Products, k= 9.72 × 10³ dm³ mol^–1 s^–1(vi), [CuL(OH)(OH₂)]⁺+[OH]^–→ Products, k= 2.75 × 10³ dm³ mol^–1 s^–1(vii) 10⁵ times faster than base hydrolysis of L at 25 °C and I= 0.1 mol dm^–3. Possible mechanisms for the reactions are discussed.