Electronic properties and crystal structures of the copper-doped zinc(II) bis(pyridine-3-sulphonate) hydrate system: a fluxional CuN2O4 chromophore

Abstract

The crystal structures of zinc(II) bis(pyridine-3-sulphonate) tetrahydrate, (1), copper(II) bis(pyridine-3-sulphonate) dihydrate, (2), and a 50% copper(II)-doped zinc(II) bis(pyridine-3-sulphonate) tetrahydrate, (3)(as an average structure), have been determined by X-ray crystallographic methods. All three complexes crystallise in the monoclinic space group P2₁/n, with: for (1), a= 8.60(5), b= 12.74(5), c= 7.55(5)Å, β= 97.0°, and Z= 2; for (2), a= 7.71(5), b= 11.00(5), c= 8.32(5)Å, β= 97.0(5)°, and Z= 2. In all three complexes a centrosymmetric rhombic octahedral MN₂O₄ chromophore is present, which involves a compression in (1) but an elongation in (2) and (3). The polycrystalline e.s.r. and electronic reflectance spectra are reported over the range 5–100% copper(II) doping and show significant variations with concentration. The polycrystalline e.s.r. spectra of 5% copper-doped (1) are reported over a temperature range down to 91 K and show marked changes in the g₂ and g₃ factors but not in g₁. The single-crystal e.s.r. spectra of this system at 298 and 91 K demonstrate that while the g₂ and g₃ vary with temperature, their directions are temperature invariant. The temperature-variable e.s.r. spectra are interpreted in terms of a fluxional CuN₂O₄ chromophere in 5% copper-doped (1) and are compared with the corresponding copper-doped K₂[Zn(OH₂)₆][SO₄]₂ system. The electronic properties of complex (2) and >50% copper-doped (1) suggest that the CuN₂O₄ chromophore has an elongated rhombic octahedral stereochemistry which is temperature independent.