Thermodynamic and kinetic studies on the complexation reactions of aqua(rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) copper (II)(blue) with neutral monodentate ligands

Abstract

In order to study the charge effects of the entering and leaving groups in substitution reactions of five-co-ordinate complexes, the equilibrium constants (K) of the complexation reactions of a trigonal-bipyramidal complex, aqua(rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane)-copper(II), [CuL(H₂O)]²⁺(blue isomer), with eight neutral monodentate ligands (L′), ammonia, methyl-, dimethyl-, trimethyl- and ethyl-amine, aniline, pyridine and piperidine, as well as the rate constants for the formation (k_f) and the aquation (k_aq) of [CuL(L′)]²⁺ have been determined in 0.20 mol dm^–3 NaClO₄ at 25.0°C. The formation rate constants are discussed in relation to a combination of the basicity, the softness and the size of the entering group. A plot of log k_f against log K gives a straight line with a slope of 0.98. The kinetic results are consistent with an associative-interchange, I_a, mechanism.