Kinetic and equilibrium studies of the acid dissociation of the copper(II) and nickel(II) complexes of 5,7-dioxo-1,4,8,11-tetra-azacyclotetradecane

Abstract

The acid catalysed dissociation of the copper(II) and nickel(II) complexes of 5,7-dioxo-1,4,8,11-tetra-azacyclo-tetradecane (dioxocyclam = LH2) has been studied using nitric acid solutions over a range of temperatures at I = 1.0 mol dm si-3. The kinetic data for the copper complex can be fitted to the rate expression k obs = k K 2[H+]/(1 + K 2 [H+] with K = 24.7s−1 and K 2 = 65dm3mol−1 at 25° C. The analogous constants for the nickel(II) complex are K = 3.3s−1 and K 2 = 45dm3mol−1. The acid dissociation can be rationalized in terms of the kinetic scheme $$\begin{gathered} ML + H^ + MLH^ + \hfill \\ MLH^ + + H^ + MLH_2^{2 + } \hfill \\ MLH_2^{2 + } \xrightarrow{k}M^{2 + } + LH_2 \hfill \\ \end{gathered}$$ involving rapid pre-equilibrium protonation and rate-determining cleavage of the CuII-N(amide) bond. For this scheme, K obs = k K, where K 1 K 2[H+]2/(1 + K 1[H+] + K 1 K 2[H+]2), obs is the observed first-order rate constant at constant hydrogen ion concentration. At the acidities employed in the stopped flow measurements, the complex MLH+ is completely formed giving the simplified rate expression experimentally observed. The temperature dependence of the rate and equilibrium constants were determined and the appropriate thermodynamic parameters obtained. Using previously published kinetic data obtained in the pH 4.7–5.3 range it has been possible to derive the values K 1 = 6.6 × 103 dm3 mol−1 at 20° C (copper complex) and K 1 = 9.3 × 102 dm3 mol−1 at 25° C (nickel complex). The mechanism of the reaction is discussed in terms of protonation of the amide oxygen atom prior to cleavage of the metal-N (amide) bond in the rate-determining step. The interaction of copper (II) and nickel(II) with dioxocyclam was also studied by potentiometric and spectrophotometric methods and the relevant formation constants determined.