Calorimetric and Spectrophotometric Studies of Copper(II) Chloro Complexes in Dimethyl Sulfoxide

Abstract

Abstract Complex formation equilibria between copper(II) and chloride ions have been studied by calorimetric and spectrophotometric titrations in dimethyl sulfoxide (DMSO) containing 0.2 mol dm−3 (C2H5)4NClO4 or 1 mol dm−3 LiClO4 as a constant ionic medium at 25 °C. Both calorimetric and spectrophotometric data obtained were well explained in terms of formation of [CuCln](2−n)+ (n=1–4) in each solution, and their formation constants, enthalpies and entropies were determined. It was found that the complexation at each step was appreciably suppressed in the LiClO4 DMSO solution compared to that in the (C2H5)4NClO4 one, which was ascribed to the extensive formation of LiCl ion-pairs in the former solution. As well as in N,N-dimethylformamide (DMF), positive values of stepwise ΔHn° and ΔSn° (n=1–3) and negative ΔH4° values were observed in DMSO. However, the ΔSn° (n=1, 2, and 4) values in DMSO were appreciably smaller than the corresponding ones in DMF. The result was interpreted in terms of stronger intermolecular interactions in the bulk DMSO than those in DMF. The entropies of transfer of [CuCln](2−n)+ (n=0–4) from DMF to DMSO were significantly positive. This result also suggested that DMSO is a highly associated solvent. Electronic spectra of individual copper(II) chloro complexes were extracted from the spectra measured by varying molar ratios of chloride to copper(II) ions in solution. In contrast to the electronic spectra of Cu2+, [CuCl]+ and [CuCl2], the spectra of [CuCl3]− and [CuCl4]2− in DMSO were not appreciably different from those in DMF, indicating that solvation of these complexes are rather weak in both solvents.