Structural polymorphism in new organic-inorganic hybrid: Pyrazolium bromoantimonates(III) [C3N2H5]6Sb4Br18•2H2O (tetragonal and triclinic forms). Thermal, dielectric and proton magnetic resonance (1H NMR) studies on the tetragonal form

Two polymorphic forms of pyrazolium bromoantimonates(III) - [C3N2H5](6)Sb4Br18 center dot 2H(2)O: alpha modification - tetragonal form and P modification - triclinic form, have been synthesized and structurally characterized at 100 K. The alpha-polymorph consists of a unique three-dimensional (3D) anionic substructure built up of cyclic tetramers and discrete chains linked to each other, three nonequivalent pyrazolium cations and water molecules. The beta-polymorph is characterized by discrete cyclic anionic tetramers [Sb4Br18](-6), three nonequivalent cations and water molecules. The physical properties have been studied for the [C3N2H5](6)Sb4Br18 center dot 2H(2)O (alpha form). The differential scanning calorimetry (DSC) revealed a complex sequence of phase transitions above room temperature. The low-frequency dielectric relaxation process was found to appear at low temperatures, which was assigned to the dynamics of dipolar pyrazolium cations. The molecular motions of the pyrazolium cations in the alpha-polymorph were studied in a wide temperature range by means of a proton magnetic resonance (H-1 NMR) technique. Two minima of the spin-lattice relaxation time (T-1) are disclosed: at low temperatures a wide one attributed to a small-angle-libration of cations and motion of water molecules, whereas the high temperature one assigned to the reorientation of cation about its pseudo-fivefold axis. (c) 2008 Elsevier Masson SAS. All rights reserved.