A long symmetric N⋯H⋯N hydrogen bond in bis(4-aminopyridinium)(1+) azide(1−): redetermination from the original data

The redetermined structure of the title molecular salt possesses one of the longest symmetric N⋯H⋯N hydrogen bonds known [N⋯N = 2.678 (3) Å].

, has been redetermined from the data published by Qian & Huang [Acta Cryst. (2010), E66, o3086; refcode WACMIY (Groom et al., 2016)]. The improvement of the present redetermination consists in a correction of the site-occupancy parameter of the bridging H atom between the pyridine rings, as well as of its position. The present study has shown that the bridging H atom (site symmetry 2) is involved in a symmetric NÁ Á ÁHÁ Á ÁN hydrogen bond, which is one of the longest ever observed [NÁ Á ÁN = 2.678 (3) Å ]. In addition, there are also present weaker N am -HÁ Á ÁN az hydrogen bonds (am = amine and az = azide) of moderate strength and -electron pyridineÁ Á Á-electron interactions in the structure. All the azide N atoms also lie on a twofold axis.

Chemical context
Structures that contain hydroxyl and secondary and primary amine groups are sometimes determined incorrectly because of an assumed geometry of these groups from which the applied constraints or restraints were inferred. In such cases, the correct geometry is missed as it is not verified by inspection of the difference electron-density maps. Thus, a considerable number of structures could have been determined more accurately -cf. Figs. 1 and 2 in Fá bry et al. (2014). The inclusion of such erroneous structures causes bias in crystallographic databases such as the Cambridge Structural Database (Groom et al., 2016).
In the course of recalculation of suspect structures that were retrieved from the Cambridge Structural Database (Groom et al., 2016), the structure determination of the title structure by Qian & Huang (2010) with the pertinent CSD refcode WACMIY became a candidate for a checking recalculation. The reason was that both the primary and secondary amine groups were constrained with distance constraints equal to 0.86 Å , with planar conformation and U iso (H) = 1.2U eq (N).
Inspection of the publication of the title structure by Qian & Huang (2010) has revealed that the bridging hydrogen atom H2a, lying between two symmetry-equivalent nitrogen atoms related by a crystallographic twofold axis, was modelled by two (undisordered) H atoms both with occupational parameters equal to 1: such a structural motif is impossible. The present article describes the redetermination of bis(4-aminopyridinium)(1+) azide(1À), which was reported by Qian & Huang (2010).

Structural commentary
The components of the title molecular salt are shown in Fig. 1. It is seen that the bridging hydrogen atom (H2a) interconnects symmmetry-related 4-aminopyridine molecules; the symmetry operation for atoms with the suffix 'a' is the same as symmetry code (i) in Table 1 and Fig. 2, viz. Àx + 1, y, Àz + 1 2 . The interplanar angle between the pyridine rings N2/C1-C5 and N2 i /C1 i -C5 i is 87.90 (7) . Table 1 lists the hydrogen bonds in the structure. The packing of the ions in the unit cell is shown in Fig. 2. Fig. 3 shows the difference electron-density map calculated without the bridging hydrogen atom H2a in the region N2Á Á Á(H2a)Á Á ÁN2 i . A well-defined, single peak in this map indicates that H2a is situated on a twofold axis, i.e. it is involved in a symmetric hydrogen bond while not being disordered. This hydrogen bond is the strongest hydrogen bond in the structure and is one of the family of long symmetric hydrogen bonds NÁ Á ÁHÁ Á ÁN as listed in Table 1 Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; y; Àz þ 1 2 ; (ii) x þ 1; y; z; (iii) x þ 1 2 ; y À 1 2 ; z.

Figure 1
View of the constituent molecules of the title structure after the improved refinement. The displacement ellipsoids are depicted at the 30% probability level (Spek, 2009).

Figure 3
A section of the difference electron-density map for the present redetermined title structure, which shows the build up of the electron density between the atoms N and N i [symmetry code: (i) Àx + 1, y, Àz + 1 2 ]. Positive and negative electron densities are indicated by continuous and dashed lines, respectively. The increment of electron density between the neighbouring contours is 0.02 e Å À3 (Petříček et al., 2014). of moderate strength (Gilli & Gilli, 2009). Atom H1a forms a link to the terminal azide nitrogen atom N3 while H1b bonds to the other terminal azide atom N5. The graph-set motif is described in the Supramolecular features section. In addition to the hydrogen-bonding interactions, there are also -electron ringÁ Á Á-electron pyridine interactions in the structure. The distance between the ring centroids N2/C1-C5 and N2 iv /C1 iv -C5 iv is 3.7145 (17) Å [symmetry code: (iv) Àx + 1, Ày + 1, Àz + 1].
Once again, the present redetermination emphasizes the importance of careful examination of the difference electrondensity maps during a structure determination.

Supramolecular features
In addition to the above-mentioned symmetric hydrogen bond N2Á Á ÁH2aÁ Á ÁN2 i [symmetry code: (i) Àx + 1, y, Àz + 1 2 ] for which the graph-set motif notation is missing (the donors act simultaneously as acceptors in the title structure; Etter et al., 1990) the principal graph-set motif in which the primary amine group as well the azide atoms are involved is R 6 4 (20). In a detail, the atoms involved in this graph-set motif are as follows (Fig. 2) The hydrogen bonds in this graph set motif are directed along the unit-cell parameter b.

Synthesis and crystallization
The preparation of the title compound was described by Qian & Huang et al. (2010) in the supporting information of their article.

Database survey
The structure determination by Qian & Huang (2010) has been included into the Cambridge Structural Database (Groom et al., 2016) under the refcode WACMIY. Table 3 lists the details regarding the crystal data, data collection and the refinement. The starting structural model was taken from the determination by Qian & Huang (2010). All hydrogen atoms were discernible in the difference electron-density map. The aryl hydrogen atoms were constrained by C aryl -H aryl = 0.93 Å and U iso (H aryl ) = 1.2U eq (C aryl ). The positional parameters of the primary amine hydrogen atoms were refined freely while their displacement parameters were constrained by U iso (H N2 ) = 1.2U eq (N2). The bridging hydrogen atom H2a involved in the symmetric hydrogen bond N2Á Á ÁH2aÁ Á ÁN2 i was refined freely. Refinements using JANA2006 and SHELXL (Sheldrick, 2008) with the threshold for observed diffractions I = 2(I) led to the same result of the bridging hydrogen atom being located on the twofold axis. Data collection: SMART (Bruker, 2000); cell refinement: SAINT (Bruker, 2000); data reduction: SAINT (Bruker, 2000); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: JANA2006 (Petříček et al., 2014); molecular graphics: PLATON (Spek, 2009), DIAMOND (Brandenburg & Putz, 2005) and JANA2006 (Petříček et al., 2014); software used to prepare material for publication: JANA2006 (Petříček et al., 2014). Special details Experimental. The structure was solved by direct methods (Bruker, 2000) and successive difference Fourier syntheses.