Klaus J. Wannowius

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

Stopped-flow spectrophotometry has been used to study the kinetics of ligand substitution in the title complex [Cu(Butsaln)2] with N-ethylsalicylideneimine in a variety of aprotic organic solvents. The rate of substitution follows a two-term rate law, rate =(k0+kligand[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 ks, of a water contribution (kH2OI[H2O]+kH2OII[H2O]2), and of contributions caused by protic admixtures such as methanol, e.g. kMeOH[MeOH]. The investigation of the dependence k0= f([H2O]) reveals that for toluene and carbon tetrachloride ks= 0. The ks 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 Reichardts ET(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 kligand, kMeOH, and especially kH2OI varies with the type of aprotic solvent, the reactivity of water, kH2OI 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.

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

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 Reichardts solvent polarity parameter ET(30), which is of mechanistic importance.

Kinetik des Metallaustausches bei [Bis(salicylaldiminato)]-kupfer(II)-Komplexen

The kinetics of isotopie copper exchange between various [bis-(N-phenylsalicylaldiminato)]copper(II) complexes (= CuL2) and mono(pyridine)copper(II) acetate (= CuAc2py) was studied in dichloromethane as solvent in the temperature range —20 to +20°C. The exchange follows the experimental rate law (5), which is simplified in certain cases due to &[(CuAc2py)2]1/2 < 1· The variation of substituents X on the salicylaldehyde ring and of substituents Y on the N-phenyl ring leads via Hammett plots to the conclusion that substituent effects become apparent as rate increasing or rate decreasing only in those cases, in which they cause an increase or decrease in electron density at the donor oxygen. Substituents Y in 2-position, and especially in 2.6-position, reduce the rate of exchange with increasing van der Waals radius of Y. The mechanistic implications of the results are discussed.