Structure of Jahn–Teller distorted solvated copper(ii) ions in solution, and in solids with apparently regular octahedral coordination geometry

Abstract

Regular octahedral coordination has been reported for some copper(II) complexes in the solid state on the basis of crystallographic studies, e.g. hexaaquacopper(II) bromate, [Cu(OH₂)₆](BrO₃)₂, hexaaquacopper(II) hexafluorosilicate, [Cu(OH₂)₆]SiF₆, and hexakis(pyridine-1-oxide)copper(II) perchlorate, [Cu(ONC₅H₅)₆](ClO₄)₂. These results are not consistent with the elongated octahedral configuration expected from the Jahn–Teller theorem for the d⁹ copper(II) ion nor, in some cases, with results from electron spin resonance studies. The present lattice-independent EXAFS study confirms that the local structure in the copper(II) complexes mentioned above is, in all cases, consistent with a Jahn–Teller induced elongation. Mean equatorial and axial Cu–O bond distances of 1.96(1) and 2.32(2) Å, and 1.95(1) and 2.27(3) Å, were obtained for the hexaaquacopper(II) ions in the bromate and hexafluorosilicate salts, respectively. For the hexakis(pyridine-1-oxide)copper(II) perchlorate only the equatorial mean Cu–O bond distance of 1.96(1) Å could be observed. Evidently, there is orientational disorder of the tetragonally elongated octahedral complexes resulting in too high crystallographic space group symmetry and copper sites in apparently regular coordination geometry. For the hydrated copper(II) ion in aqueous solution, five- and six-coordinated models with different geometries have been evaluated by means of EXAFS and large angle X-ray diffraction (LAXS) data. The combined results are consistent with a Jahn–Teller elongated octahedral configuration with Cu–O_eq 1.95(1) Å, Cu–O_ax 2.29(3) Å, and a distinct second hydration sphere with about eight water molecules and a mean Cu⋯O_II distance of 4.17(3) Å. In dimethylsulfoxide solution EXAFS and LAXS methods show the solvated copper(II) ions to have mean equatorial and axial Cu–O bond distances of 1.96(1) and 2.24(2) Å, respectively. As a model compound for the EXAFS studies, the crystal structure of hexakis(dimethylsulfoxide)copper(II) perchlorate dimethylsulfoxide (1/2), [Cu(OS(CH₃)₂)₆](ClO₄)₂·2(CH₃)₂SO, was determined.