Kinetics of Ligand Substitution in Bis(N-t-butylsalicylideneiminato)-copper(II): Aprotic Organic Solvents as Media

Article abstract of DOI:10.1039/DT9820000577

Stopped-flow spectrophotometry has been used to study the kinetics of ligand substitution in the title complex 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)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 + k_H2O^II2), and of contributions caused protic admixtures such as methanol, e.g.k_MeOH.The investigation of the dependence K₀ = f() 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 E-4-E-2S^-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 2000-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 patways induced by the solvent, by water, and by the incoming ligand is discussed.

Full text of DOI:10.1039/DT9820000577

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