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 syn thesised from ethanolic solutions containing CuBr2, HBr and the aromatic ba se in a 1 : 2: 2 molar ratio. The compounds have been characterised by elem ental analysis, IR, UV/VIS and EPR spectroscopies, thermal analysis, variab le-temperature magnetic susceptibility measurements and single-crystal X-ra y diffraction. In all four crystal structures, the tetrabromocuprate(II) an ion is connected to two organic cations through N-H . . . Br hydrogen bonds to form cation-anion-cation molecular units, which are held together by me ans 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 b ond and the aromatic stacking interactions are strongly influenced by the p osition of the methyl group in the pyridinic ring of the cations. The delic ate balance between these non-covalent intermolecular forces, together with the electrostatic interactions, ultimately determines the structure and, a s a consequence, the physical properties (thermal stability, density and ma gnetic 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 transmi tted through the hydrogen bond and the pi -stacking interactions. Compound 1 shows a reversible phase transition at 65 degreesC with a thermal hystere sis of 4 K.