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]6Sb4Br18·2H2O: α modification – tetragonal form and β modification – triclinic form, have been synthesized and structurally characterized at 100 K. The α-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 β-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]6Sb4Br18·2H2O (α 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 α-polymorph were studied in a wide temperature range by means of a proton magnetic resonance (1H NMR) technique. Two minima of the spin–lattice relaxation time (T1) 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.

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