Crystal packing and physical properties of pyridinium tetrabromocuprate(II) complexes assembled via hydrogen bonds and aromatic stacking interactions

The compounds (pyH)(2)[CuBr4] (1) (pyH = pyridinium) and (n-MepyH)(2)[CuBr4] [n-MepyH = n-methyl-pyridinium; n = 2 (2), 3 (3) and 4 (4)] have been synthesised from ethanolic solutions containing CuBr2, HBr and the aromatic base in a 1 : 2: 2 molar ratio. The compounds have been characterised by elemental analysis, IR, UV/VIS and EPR spectroscopies, thermal analysis, variable-temperature magnetic susceptibility measurements and single-crystal X-ray diffraction. In all four crystal structures, the tetrabromocuprate(II) anion is connected to two organic cations through N-H . . . Br hydrogen bonds to form cation-anion-cation molecular units, which are held together by means of offset face-to-face interactions between the aromatic rings to give one-dimensional chains (1, 2 and 4) or a dimeric entity (3). The hydrogen bond and the aromatic stacking interactions are strongly influenced by the position of the methyl group in the pyridinic ring of the cations. The delicate balance between these non-covalent intermolecular forces, together with the electrostatic interactions, ultimately determines the structure and, as a consequence, the physical properties (thermal stability, density and magnetic behaviour) of the compounds. Magnetic susceptibility measurements of powdered samples show that the compounds exhibit weak antiferro- (1, 2, 4) or ferromagnetic (3) magnetic couplings between the copper centres transmitted through the hydrogen bond and the pi -stacking interactions. Compound 1 shows a reversible phase transition at 65 degreesC with a thermal hysteresis of 4 K.