Crystal structure of (1S,4S)-2,5-diazoniabicyclo[2.2.1]heptane dibromide

The molecular structure of the 2,5-diazabicyclo[2.2.1]heptane parent ring has been characterized for the first time. The asymmetric unit contains two crystallographically independent cages of 2,5-diazabicyclo[2.2.1]heptane, each cage being protonated at the two nitrogen sites. The overall charge balance is maintained by four crystallogrphically independent bromide ions. In the crystal, the components of the structure are linked via a complex three-dimensional network of N—H⋯Br hydrogen bonds.

The cage of 2,5-diazabicyclo[2.2.1]heptane is frequently employed in synthetic chemistry as a rigid bicyclic counterpart of the piperazine ring. The 2,5-diazabicyclo[2.2.1]heptane scaffold is incorporated into a variety of compounds having pharmacological and catalytic applications. The unsubstituted parent ring of the system, 2,5-diazabicyclo[2.2.1]heptane itself, has not been structurally characterized. We herein report on the molecular structure of the parent ring in (1S,4S)-2,5-diazoniabicyclo[2.2.1]heptane dibromide, C 5 H 12 N 2 2+ Á2Br À . The asymmetric unit contains two crystallographically independent cages of 2,5diazabicyclo[2.2.1]heptane. Each cage is protonated at the two nitrogen sites. The overall charge balance is maintained by four crystallographically independent bromide ions. In the crystal, the components of the structure are linked via a complex three-dimensional network of N-HÁ Á ÁBr hydrogen bonds.

Structural commentary
The asymmetric unit of 1 contains two structurally independent cages of 2,5-diazabicyclo[2.2.1]heptane (Fig. 2) (Kiely et al., 1991;Beinat et al., 2013;. It is worth noting that the bicyclic bridged structure of 2,5-diazabicyclo[2.2.1]heptane determines the boat conformation of its cage (Fig. 1). Contrary to that, the piperazine ring is flexible and can adopt four different conformations: chair, boat, twist-boat and half-boat, the former being the energetically most favourable (SenGupta et al., 2014). A comparison of the hexagonal rings of 2,5-diazabicyclo[2.2.1]heptane and the chair conformer of piperazine ( Fig. 2) shows that the interatomic distances between the opposing nitrogen atoms are remarkably close. The latter feature can be important because the nitrogen sites are known to be pharmacophores frequently determining the biochemical activity of piperazine derivatives (Patel & Park, 2013). Therefore, the implication of the 2,5-diazabicyclo[2.2.1]heptane scaffold as a piperazine analogue in screening libraries looks quite reasonable from the structural point of view.

Figure 1
Two views of the diprotonated 2,5-diazabicyclo[2.2.1]heptane parent ring in 1 (in one of the two independent molecules in the asymmetric unit). The atomic numbering scheme is according to IUPAC notation. Displacement ellipsoids are drawn at the 30% probability level. Hydrogen atoms are depicted as fixed-size spheres of arbitrary radius. The bromide counter-ions have been omitted for clarity. Groom et al., 2016). Jordis et al. (1999)

(1S,4S)-2,5-Diazoniabicyclo[2.2.1]heptane dibromide
Crystal data 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )