4-Cyanopyridinium bromide

In the title compound, C6H5N2 +·Br−, the pyridine N atom is protonated and involved in an intermolecular N—H⋯Br hydrogen bond which, together with weak C—H⋯N hydrogen bonds, results in the formation of a chain along the c axis. Weak intermolecular C—H⋯Br interactions between pyridine H atoms and Br− anions connect these chains into a network parallel to the bc plane.

In the title compound, C 6 H 5 N 2 + ÁBr À , the pyridine N atom is protonated and involved in an intermolecular N-HÁ Á ÁBr hydrogen bond which, together with weak C-HÁ Á ÁN hydrogen bonds, results in the formation of a chain along the c axis. Weak intermolecular C-HÁ Á ÁBr interactions between pyridine H atoms and Br À anions connect these chains into a network parallel to the bc plane.

Wen-Ni Zheng Comment
Simple organic salts containing strong intermolecular H-bonds have attracted attention as materials which display ferroelectric-paraelectric phase transitions (Fu et al., 2011a;Fu et al., 2011b). With the purpose of obtaining crystals of organic salts which might undergo such phase transitions, various organic molecules have been studied and a series of new materials have been elaborated (Dai & Chen 2011). Herewith we present the synthesis and crystal structure of the title compound, 4-cyanopyridinium bromide.
In the title compound ( Fig. 1), the bond lengths and angles have normal values. The asymmetric unit is composed of one 4-cyanopyridinium cation and one Branion. The protonated N atom is involved in a strong N-H···Br hydrogen bond (Table 1) which accompanying the C5-H5A···N2 H-bond generates a linear chain parallel to c-axis while weak C1-H1A···Br and C2-H2A···Br1 interactions serve to link the chains into a 3-dimensional layer structure ( Fig. 2 and Table   1).

Experimental
Isonicotinonitrile (20 mmol), aqueous HBr (5 mL, 2 mol/L) and ethanol (50 mL) were added to a 100mL flask. The mixture was stirred at 60° C for 2 h, and then the precipitate was filtrated out. Colourless crystals suitable for X-ray diffraction were obtained by slow evaporation of the filtrate.

Refinement
All H atoms attached to C atoms were situated into the idealized positions and treated as riding with C-H = 0.93 Å (aromatic) with U iso (H)=1.2U eq (C). The positional parameters of the H atom (N) were refined freely. And in the last stage of the refinement, they were restrained with the H-N = 0.90 (2)Å, with U iso (H)=1.2U eq (N).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.