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 Bu^t) 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 =(k_s+k_ligand[ligand])[complex], although for most systems studied the second-order contribution k_ligand[ligand] is negligibly small, so that rate =k_s[complex]. The determination of k_s, the so-called solvent path, in alcohol mixtures of different composition has led to δΔG^‡. From the temperature dependence of k_s, the activation parameters ΔH^‡ and ΔS^‡ have been obtained. On the basis of solubilities determined spectrophotometrically, the Gibbs free energy of transfer, δG_tr, 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 δG_tr 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 Ink_s and Reichardt's solvent polarity parameter E_T(30), which is of mechanistic importance.