Crystal structure and characterization of the novel NH+⋯N hydrogen bonded polar crystal [NH2(CH2)4NH][BF4]

• Published in: Journal of solid state chemistry Vol. 187; pp. 35 - 44
• Main Authors: Wojtaś, M., Ga¸gor, A., Czupiński, O., Medycki, W., Jakubas, R.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 01.03.2012; Elsevier
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder; Cross-disciplinary physics: materials science; rheology; Dielectric, piezoelectric, ferroelectric and antiferroelectric materials; Dielectrics, piezoelectrics, and ferroelectrics and their properties; Diffusion in solids; Exact sciences and technology; Hybrid material; Materials science; NLO; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Self-diffusion in metals, semimetals, and alloys; Single-protonated piperazinium cation; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; Transport properties of condensed matter (nonelectronic); NLO; Hybrid material; Single-protonated piperazinium cation; Molecular motion; Space groups; Fluoroborates; Phase transitions; Hydrogen bonds; Nuclear magnetic resonance; Crystal structure; Differential scanning calorimetry; Temperature dependence; Piperazines; Cooling; Phase transformations; Polar crystals; Dielectric properties; Pyroelectricity; Monoclinic lattices; Heat treatments; Superionic conductors; Piezoelectric materials; Electrical conductivity measurement; Molecular reorientation; Self-diffusion
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2011.12.020

Dielectric properties and phase transitions of the piperazinium tetrafluoroborate ([NH2(CH2)4NH][BF4], abbreviated as PFB) crystal are related to the one-dimensional arrangement of the cations linked by the bistable NH+⋯N hydrogen bonds and molecular motions of the [BF4]− units. The crystal structure of [NH2(CH2)4NH][BF4] is monoclinic at room temperature with the polar space group Pn. The polar/acentric properties of the room temperature phase IV have been confirmed by the piezoelectric and pyroelectric measurements. DSC measurements show that the compound undergoes three first-order structural phase transitions: at 421/411K (heating/cooling), at 386/372K and at 364/349K. 1H and 19F NMR measurements indicate the reorientational motions of [BF4]− anions and piperazinium(+) cations as well as the proton motion in the hydrogen-bonded chains of piperazine along the [001] direction. Over the phase I the isotropic reorientational motions or even self-diffusion of the cations and anions are expected. The conductivity measurements in the vicinity of the II–I PT indicate a superionic phase over the phase I. It must be emphasized that the titled compound represents the first organic–inorganic simple salt containing the single-protonated piperazinium cation which was studied by means of the wide variety of experimental techniques. A survey of Cambridge Structural Database (CSD version 5.32 (November 2010) & updates (May 2011)) for structure containing the piperazinium cations yields 248 compounds with the doubly protonated piperazinium(2+) cations and only eight compounds with the singly protonated piperazinium(+) cations. Among these structures only one is the hybrid organic–inorganic material. This is piperazinium nitrate characterized structurally. The crystal packing of [NH2(CH2)4NH][BF4], phase IV. The dashed lines stand for the hydrogen bonds. The hydrogen bonds to BF4 groups are not included for the picture quality [Display omitted] ► The second organic–inorganic salt containing single-protonated piperazinium cation. ► The first such a crystal not only structurally characterized. ► The phase I revealed the features characteristic of super ionic phase. ► The [NH2(CH2)4NH][BF4] crystal is piezoelectric and polar at room temperature. ► Mechanism of phase transitions was proposed.