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

Dielectric properties and phase transitions of the piperazinium tetrafluoroborate ([NH2(CH2)4NH][BF4][NH2(CH2)4NH][BF4], abbreviated as PFB) crystal are related to the one-dimensional arrangement of the cations linked by the bistable NH+⋯NNH+⋯N hydrogen bonds and molecular motions of the [BF4]−[BF4]− units. The crystal structure of [NH2(CH2)4NH][BF4][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 421/411 K (heating/cooling), at 386/372 K and at 364/349 K. 1H and 19F NMR measurements indicate the reorientational motions of [BF4]−[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][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 qualityFigure optionsDownload as PowerPoint slideHighlights
► 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][NH2(CH2)4NH][BF4] crystal is piezoelectric and polar at room temperature.
► Mechanism of phase transitions was proposed.