Calorimetric and Spectrophotometric Studies on Formation of Copper(II) Chloride Complexes in N,N-Dimethylformamide

Abstract

Abstract Formation of copper(II) chloride complexes has been studied by calorimetric and spectrophotometric titrations in N,N-dimethylformamide (DMF) containing different ionic media, 0.1 and 0.2 mol dm−3(C2H5)4NClO4 and 1 mol dm−3 LiClO4 at 25 °C. In all the DMF solutions examined, calorimetric and spectrophotometric titration curves were well explained in terms of formation of a series of four mononuclear complexes, [CuCln](2−n)+(n=1–4), and formation constants, enthalpies and entropies of formation of the complexes were determined. Formation constants were determined by both calorimetry and spectrophotometry and most of the values obtained by the different methods were in good agreement each other. Calorimetric titration curves obtained in 0.1 and 0.2 mol dm−3 (C2H5)4NClO4 ionic media were practically the same, but they significantly differed from those obtained in 1 mol dm−3 LiClO4 ionic medium, the result indicating that the formation of copper(II) chloride complexes was more favored in the former ionic media than in the latter. Stepwise enthalpies of formation were positive for [CuCl]+, [CuCl2], and [CuCl3]−, but negative for [CuCl4]2− in the (C2H5)4NClO4 DMF solutions, while the values in the 1 mol dm−3 LiClO4 DMF solution were more negative by 6–8 kJ mol−1 than the respective values in the (C2H5)4NClO4 solutions. A significant difference was also found for stepwise entropies of formation of copper(II) chloride complexes between the ionic media. The ionic medium effect on the formation of the complexes found was ascribed to ion pairing of LiCl in the 1 mol dm−3 LiClO4 DMF solution, ion pairing of (C2H5)4NCl being insignificant in the (C2H5)4NClO4 DMF solutions used. A largely negative ΔH4° and a small ΔS4° values in the (C2H5)4NClO4 DMF solutions suggested that solvation of the [CUCl3]− complex was not strong, and thus, release of solvent molecules from the [CUCl3]− complex did not play an important role for the formation of [CuCl4]2−. Therefore, the enthalpy and entropy of formation of [CuCl4]2−. were mainly attributable to those of desolvation of chloride ions and of bond formation between Cl− and [CuCl3]−. The [CuCl4]2− complex may have no DMF molecules in the primary coordination sphere of the central metal ion within the complex. Electronic spectra of individual copper(II) chloride complexes were determined from spectra at various concentrations of chloride ions by using the formation constants obtained.