Magneto-structural studies and thermal analysis of n-ethylpyridinium (n=2, 3, 4) tetrabromocuprate(II) complexes

Three n-ethylpyridinium salts of the tetrabromocuprate(II) anion [n = 2 (1), 3 (2) and 4 (3)] were prepared by air evaporation of an ethanolic solution of the aromatic amine base, HBr and CuBr2 in a molar ratio 2:2:1. The compounds were characterized by elemental analysis, IR, UV-Vis and ESR spectroscopies, thermal analysis and variable-temperature magnetic susceptibility measurements. The crystal structures of 2 and 3 were determined by means of single-crystal X-ray diffraction methods. The [CuBr4](2-) anion (with C-2 symmetry for 2) is connected to two organic cations through N-H...Br hydrogen bonds to form cation-anion-cation motifs which are held together by means of offset face-to-face interactions between the aromatic rings to give zig-zag one-dimensional chains in 2 and a dimeric entity in 3. The dimeric units of 3 are joined by means of weak Br...Br contacts to give one-dimensional chains as well. The X-band ESR spectra show quasi-axial signals which are clearly resolved operating at Q-band, with g(parallel to) much greater than g(perpendicular to) > 2.0, characteristic of a flattened [CuBr4](2) tetrahedral complex. The magnetic susceptibility measurements of powdered samples show that compounds behave as low-dimensional magnetic systems with weak antiferromagnetic exchange interactions. Thermal decomposition of the compounds yield copper(H) oxide and metallic Cu as stable final residues in synthetic air and dinitrogen atmospheres, respectively. (C) 2002 Elsevier Science Ltd. All rights reserved.