Ligand substitution in bis(N-alkylsalicylideneiminato)copper(II) complexes: comparison of activation and transfer data obtained from solvent mixtures

Abstract

Stopped-flow spectrophotometry has been used to study the kinetics of ligand substitution in three bis(N-alkylsalicylideneiminato)copper(II) complexes [Cu(Rsaln)2](R = Et, Pri, or But) with N-ethylsalicylideneimine (H-Etsaln) or N-phenylsalicylideneimine (H-Phsaln) in solvent mixtures of methanol–butan-2-ol and methanol–2-methylbutan-2-ol. The rate of substitution follows a two-term rate law, rate =(ks+kligand[ligand])[complex], although for most systems studied the second-order contribution kligand[ligand] is negligibly small, so that rate =ks[complex]. The determination of ks, the so-called solvent path, in alcohol mixtures of different composition has led to δΔG‡. From the temperature dependence of ks, the activation parameters ΔH‡ and ΔS‡ have been obtained. On the basis of solubilities determined spectrophotometrically, the Gibbs free energy of transfer, δGtr, has been obtained for the various mixtures. An attempt is made to correlate the activation data δΔG‡ for the solvent path with the transfer data δGtr and this is discussed in terms of ground- and transition-state solvation. The significance of the different co-ordination geometry of the three complexes studied for the kinetics and for solvation is examined. It is shown that there is a correlation between Inks and Reichardt's solvent polarity parameter ET(30), which is of mechanistic importance.