Structural characterization, molecular dynamics, dielectric and spectroscopic properties of tetrakis(pyrazolium) bis(μ2-bromo-tetrabromobismuthate(III)) dihydrate, [C3N2H5]4[Bi2Br10]•2H2O

The structure of [C3N2H5](4)[Bi2Br10]center dot 2H(2)O, (PBB) was determined by single crystal X-ray diffraction at 100 K. It crystallizes in the monoclinic space group C2/m, with a = 12.992(4) angstrom, b = 16.326(5) angstrom, c = 8.255(3) angstrom, beta = 108.56 degrees(3), V = 1659.9(9) angstrom(3) and Z = 2. The structure consists of discrete binuclear [Bi2Br10](4-) anions, ordered pyrazolium cations and water molecules. The crystal packing is governed by strong N-H center dot center dot center dot O and weak O-H center dot center dot center dot Br hydrogen bonds. A sequence of structural phase transitions in PBB was established on the basis of differential scanning calorimetry and dilatometric studies. Two reversible first-order phase transitions were found: (I -> II) at 381/371 K (on heating/ cooling) and (II -> III) at 348/338 K. Dielectric response near both phase transitions is characteristic of crystals with the plastic-like phases. Over the phase III a low frequency dielectric relaxator is disclosed. The possible molecular motions in the PBB compound are characterized by the H-1 NMR studies. The infrared spectra of polycrystalline compound in the temperature range 300-380 K are reported for the region 4000-400 cm(-1). The observed spectral changes through the structural phase transition III -> II are attributed to an onset of motion both of the pyrazolium cations and water molecules. (C) 2007 Elsevier Masson SAS. All rights reserved.