A study of pseudo 1-D copper(I) halide systems exhibiting anomalous copper(II) character: Physical characterization of (paraquat)Cu₂X₄ (X = Cl, Br, I)

The salts (paraquat)Cu₂X₄ (X = Cl, Br, I) have been synthesized and their crystal structures determined. The chloride salt is tetragonal, space group P4₂/n, a = 16.352(2), c = 5.911(2) Å, V = 1580.7(6) ų, D_x = 1.87 g cm^-3, Z = 4 and R = 0.0546. The bromide analog is monoclinic, space group P2₁/n, a = 8.715(6), b = 11.103(6), c = 13.386(7) Å, β = 98.02(4)°, V = 1666(1) ų, D_x = 2.52 g cm^-3, Z = 4 and R = 0.0680. The iodide salt is tetragonal, space group P4₂2₁2, a = 12.416(2), c = 6.544(3) Å, V = 1008.8(5) ų, D_x = 2.70 g cm^-3, Z = 2 and R = 0.0995. Infinite chains of distorted, edge sharing, Cu(I)X₄ ^3- tetrahedra are common to all three structures. The bond distances and angles about the copper centers are consistent with known Cu(I)X₄ ^3- species. The paraquat cations form stacks parallel to the chain axis, and all three structures show weak N⋯X and C⋯X contacts. The chloride and iodide salts are nearly isostructural, and differential scanning calorimetry shows a phase transition in the chloride salt at 188 °C. The iodide salt is a pure copper(I) system. A temperature dependent magnetic susceptibility experiment shows the bromide salt to contain a 2% Cu₂Br₆ ^2- impurity. Two chloride compounds showing the same structure have been synthesized. One of these salts has 1% copper(II) character, while the other has 10% copper(II) character. EPR shows the 1% character to be a CuCl₂ · 2H₂O impurity. Magnetic susceptibility, EPR, powder diffraction and electrical conductivity results show that the 10% copper(II) character is incorporated into the [CuCl₂]_∞ chain structure, with an accompanying vacancy. Preliminary conductivity measurements show that the Cl and I salts are insulators, but hint at enhanced conductivity for the bromide analog.