Kinetics and mechanism of formation and dissociation of copper(II) and nickel(II) complexes of the Schiff base bis(acetylacetone)ethylenediimine in aqueous-organic solvent media

Abstract

In NH4NO3+NH4OH buffered 10% (v/v) dioxan-water media (pH 7.0–8.5), thePseudo-first-order rate constant for the formation of the title complexes M(baen),i.e. ML, conforms to the equation 1/kobs=1/k+1/(kKo.s · TL), where TL stands for the total ligand concentration in the solution, Ko.s is the equilibrium constant for the formation of an intermediate outer sphere complex and k is the rate constant for the formation of the complex ML from the intermediate. Under the experimental conditions the free ligand (pKa>14) exists virtually exclusively in the undissociated form (baenH2 or LH2) which is present mostly as a keto-amine in the internally hydrogen-bonded state. Although the observed formation-rate ratio kCu/kNi is of the order of 105, as expected for systems having “normal” behaviour, the individual rate constants are very low (at 25°C, kCu=50 s−1 and kNi=4.7×10−4s−1) due to the highly negative ΔS≠ values (−84.2±3.3 JK−1M−1 for CuL and −105.8±4.1 JK−1M−1 for NiL); the much slower rate of formation of the nickel(II) complex is due to higher ΔH≠ value (41.2±1.0 kJM−1 for CuL and 78.2±1.2 kJM−1 for NiL) and more negative ΔS≠ value compared to that of CuL. The Ko.s values are much higher than expected for simple outer-sphere association between [M(H2O)6] and LH2 and may be due to hydrogen bonding interaction. In acid media ([H+], 0.01–0.04 M) these complexes M(baen) dissociate very rapidly into the [M(H2O)6]2+ species and baenH2, followed by a much slower hydrolytic cleavage of the ligand into its components,viz. acetylacetone and ethylenediamine (protonated). For the dissociation of the complexes kobs=k1[H+]+k2[H+]2. The reactions have been studied in 10% (v/v) dioxan-water media and also ethanolwater media of varying ethanol content (10–25% v/v) and the results are in conformity with a solvent-assisted dissociativeinterchange mechanism involving the protonated complexes.