Bis(4-fluoroanilinium) sulfate

In the crystal of the title molecular salt, 2C6H7FN+·SO4 2−, the cations and anions are linked by N—H⋯O and C—H⋯O hydrogen bonds into sheets parallel to the ab plane. The crystal studied was found to be a racemic twin with a 0.50 (10):0.50 (10) domain ratio.

In the crystal of the title molecular salt, 2C 6 H 7 FN + ÁSO 4 2À , the cations and anions are linked by N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds into sheets parallel to the ab plane. The crystal studied was found to be a racemic twin with a 0.50 (10): 0.50 (10) domain ratio.

Comment
Amine salts have attracted much attention as phase transition dielectric materials for their application in memory storage (Fu et al. 2007;Fu et al. 2009). Hydrogen bonding is one of the most versatile non-covalent forces in supramolecular chemistry and crystal engineering (Zimmerman & Corbin, 2000;Brunsveld et al., 2001;Desiraju, 2002). Therefore, in the past decades assessment of discrete hydrogen bonding patterns has received great attention (Steiner, 2002;Desiraju & Steiner, 1999;Boutobba et al., 2010) because of their widespread occurrence in biological systems.
The asymmetric unit of the title compound (Fig 1), contains two crystallographically independent 4-fluoroanilinium cations and a sulfate anion. The bond lengths (Allen et al., 1987) and angles are within normal ranges and comparable to those in a closely related crystal structure (Boutobba et al., 2010).

Experimental
To a solution of 4-fluoroaniline (10 mmol) in absolute ethanol was added sulfuric acid (5 drops) and the mixture refluxed for 4 h. After cooling the mixture to room temperature, a white solid appeared. This crude product was recrystallized from dimethylformamide to afford the desired product. M.p: 151-153°C.

Refinement
N-bound H atoms were located from a difference Fourier map, fixed at their found location and refined using a riding model with U iso (H) = 1.5U eq (N) [N-H = 0.8198 to 0.9875 Å]. The remaining H atoms were positioned geometrically [C-H = 0.93 Å] and refined using a riding model with U iso (H) = 1.2 U eq (C). The studied crystal is an inversion twin, the refined ratio of twin components being 0.50 (10): 0.50 (10). Fig. 1. The molecular structure of the title compound with atom labels with 30% probability displacement ellipsoids. Hydrogen atoms are shown as spheres of arbitrary radius.  as those based on F, and R-factors based on ALL data will be even larger.