Ethane-1,2-diammonium dibromide: a redetermination at 100 K

In the redetermined [for the previous study, see Søtofte (1976 ▶). Acta Chem. Scand. Ser. A, 30, 309–311] crystal structure of the title compound, C2H10N2 2+·2Br−, the H atoms have been located and the hydrogen-bonding scheme is described. The ethane-1,2-diammonium cation lies over a crystallographic inversion centre and straddles a crystallographic mirror plane with the C and N atoms in special positions. In the crystal, the cations and anions are linked by N—H⋯Br and N—H⋯(Br,Br) hydrogen bonds, which generate various ring and chain motifs including an R 10 5(32) loop.

In the redetermined [for the previous study, see Søtofte (1976). Acta Chem. Scand. Ser. A, 30, 309-311] crystal structure of the title compound, C 2 H 10 N 2 2+ Á2Br À , the H atoms have been located and the hydrogen-bonding scheme is described. The ethane-1,2-diammonium cation lies over a crystallographic inversion centre and straddles a crystallographic mirror plane with the C and N atoms in special positions. In the crystal, the cations and anions are linked by N-HÁ Á ÁBr and N-HÁ Á Á(Br,Br) hydrogen bonds, which generate various ring and chain motifs including an R 10 5 (32) loop.

Comment
As part of our ongoing study of the structural characteristics of organic-inorganic layered diammonium salts, the crystal structure of ethane-1,2-diammonium dibromide, (I), was determined. A search of the Cambridge Structural Database (Version 5.31, May 2010 release; Allen, 2002) revealed that the crystal structure of (I) had been previously determined 34 years ago (Søtofte, 1976) at room temperature. The information in the CSD CIF file however appears incomplete and the author also states that the contributions from the hydrogen atoms in the structure was ignored. Here we report the redetermined structure of the title compound at 100 K. All the H atom positions were clearly visible in the difference Fourier map and they were independently refined with ADP's constrained to values of 1.2 and 1.5 times the isotropic U values of the C and N atoms on which they ride. We also show packing arrangements, hydrogen bonding interactions, hydrogen bonding motifs as well as calculated torsion angles ( Table 3) that were previously not reported.
The ethane-1,2-diammonium cation lies over a centre of inversion and also straddles a mirror plane. The asymmetric unit contains one bromide anion and half of the ethane-1,2-diammonium cation (Figure 1). A close-up view of the hydrogen bonding interactions can be viewed in Figure 3. The hydrogen bond distances and angles for (I) can be found in Table 2. The hydrogen bonding network is three-dimensional and particularly complex, consisting of a variety of ring and chain motifs (identified using graphs sets in Mercury (Macrae et al., 2006). Because of the complexity and number of different motifs identified, we focus on one particularly interesting hydrogen-bonding ring motif in the structure that appears to be in the shape of a T (Figure 4.) and it was chosen to best describe the highest level hydrogen bonding motif evident in the crystal structure. Figure 4 shows a view of five diammonium cations and five bromide anions (viewed down the c axis) that are hydrogen bonded together to form a large, 32-membered T-shaped ring motif with graph set notation R 5 10 (32). Other ring motifs are evident -eight ring motifs and four chain motifs were identified from Mercury (Macrae et al., 2006) but are not depicted here.

Experimental
Compound (I) was prepared by adding 1,2-diamino-ethane (0.50 g, 2.25 mmol) to 47% hydrobromic acid (HBr, 2 ml, 37.07 mmol, Merck) in a sample vial. The mixture was then refluxed at 363 K for 2 h. The solution was cooled at 2 K h -1 to room temperature. Colourless blocks of (I) were collected.
supplementary materials sup-2 Refinement H atoms were clearly visible from the difference Fourier map. They were independently refined with the constraints U iso (H) = 1.2Ueq(C) and 1.5Ueq(N). For (I), the highest peak in the final difference map is 0.80Å from Br1 and the deepest hole is 0.98Å from C1. Fig. 1. : Molecular structure of (I), with displacement ellipsoids drawn at the 50% probability level. Atoms labelled with (i) are at the symmetry position (1 -x, y, 1 -z)