Magneto-structural studies and thermal analysis of the 4-aminopyridinium tetrabromocuprate(II) monohydrate

Dark violet crystals suitable for X-ray analysis of the complex (4-apyH)2[Cu-Br4] ∗d H2O, where 4-apyH+ is 4-aminopyridinium, were prepared by air evaporation of an acetonitrile solution of the aromatic amine base, HBr and CuBr2 in a molar ratio 2 : 2 : 1. The compound has been characterized by elemental analysis, IR, UV-vis and EPR spectroscopies, thermal analysis and magnetic susceptibility measurements. The crystal structure consists of flattened [CuBr4]2− tetrahedra of C2 symmetry, organoammonium cations and water of crystallization molecules which are held together by electrostatic forces, an extensive three-dimensional network of O H ∗d∗d∗d Br, N H ∗d∗d∗d Br and N H ∗d∗d∗dO hydrogen bonds and weak face-to-face stacking interactions between the π systems of the aromatic rings. The X-band EPR spectra show an isotropic signal (giso = 2.16) which is resolved in an axial symmetry operating at Q-band, with g‖ = 2.302 > g⊥ = 2.084, characteristic of a flattened [CuBr4]2− tetrahedral complex. The magnetic susceptibility measurements indicate the presence of weak ferromagnetic interactions below 30 K, presumably through hydrogen contacts which are established between the water molecule and two [CuBr4]2− anions related by a two-fold axis.