Kinetics of ligand substitution in bis(N-t-butylsalicylideneiminato)copper(II): aprotic organic solvents as media

Abstract

Stopped-flow spectrophotometry has been used to study the kinetics of ligand substitution in the title complex [Cu(Bu^tsaln)₂] with N-ethylsalicylideneimine in a variety of aprotic organic solvents. The rate of substitution follows a two-term rate law, rate =(k₀+k_ligand[ligand])[complex], with the substitution of the first ligand being rate controlling. It is shown that the ligand-independent rate term k₀ represents the sum of the genuine solvent contribution k_s, of a water contribution (k_H2O^I[H₂O]+k_H2O^II[H₂O]²), and of contributions caused by protic admixtures such as methanol, e.g. k_MeOH[MeOH]. The investigation of the dependence k₀= f([H₂O]) reveals that for toluene and carbon tetrachloride k_s= 0. The k_s values obtained for the aprotic solvents dimethylformamide, dimethylacetamide, tetramethylurea, dimethyl sulphoxide, acetonitrile, nitromethane, and pyridine lie in the range 10^–4–10^–2 s^–1. Their correlation with solvent parameters such as dielectric constant, donor or acceptor number, fluidity, or Reichardt's E_T(30) value is unsatisfactory, whereas the activation energy for the k₀ path correlates reasonably well with the heat of vaporization of the solvents. The size of k_ligand, k_MeOH, and especially k_H2O^I varies with the type of aprotic solvent, the reactivity of water, k_H2O^I being 2 000-fold greater in toluene than in dimethylformamide. Admixtures of 2,4-dimethylpentan-3-ol and 3-ethylpentan-3-ol to carbon tetrachloride act as water scavengers and suppress the water contribution to k₀. The mechanism of the various substitution pathways induced by the solvent, by water, and by the incoming ligand is discussed.