A Calorimetric Study on the Formation of [CuCln](2−n)+ (n=1–4) in Acetonitrile–N,N-Dimethylformamide Mixtures

Abstract

Abstract Complex formation equilibria between copper(II) and chloride ions have been studied by calorimetry in various acetonitrile(AN)–N,N-dimethylformamide(DMF) mixtures containing 0.2 mol dm−3 (C2H5)4NClO4 as a constant ionic medium at 25 °C. Calorimetric titration curves in each AN–DMF mixture examined were well explained in terms of formation of [CuCln](2−n)+ (n=1–4), and their formation constants, enthalpies, and entropies were determined. The overall enthalpy ΔHβ4° in any mixture, even in the acetonitrile-rich 0.025 mole fraction DMF mixture, was practically the same as that (19.2 kJ mol−1) in pure DMF, but largely different from that (−54.5 kJ mol−1) in pure AN. The result indicated that copper(II) ion was markedly preferentially solvated with DMF molecules in AN–DMF mixtures. On the other hand, the overall entropy value ΔSβ4° in any AN–DMF mixture was significantly larger than the values in both pure solvents. As to thermodynamic quantities of stepwise formation of [CuCln](2−n)+ (n=1–4), a remarkable change was found at the second step with varying solvent compositions. The stepwise enthalpy and entropy values of formation of [CuCl2] became more positive as the DMF content of the mixture decreased. Enthalpies of solution of anhydrous CuCl2 crystals were also measured in the same AN–DMF mixtures. The enthalpy ΔH\oversetos(CuCl2) of solution of the neutral [CuCl2] complex largely changed depending on the solvent composition, while ΔH\oversetos(Cu2+, 2Cl−), which is the sum of enthalpies of solution of relevant ions, remained practically unchanged. The result for the [CuCl2] complex indicated that the complex was not strongly preferentially solvated with DMF molecules in contrast to copper(II) ion in the AN–DMF mixtures.