Crystal structure and Hirshfeld surface analysis of 4-aminopyridinium thiocyanate–4-aminopyridine (1/1)

In the crystal of the title compound, the 4-aminopyridine molecules, 4-aminopyridinium and thiocyanate ions are held together by N—H⋯S and N—H⋯N hydrogen bonds.


Chemical context
Processes based on metathesis reactions are a greener alternative for the synthesis of organic materials, avoiding hazardous pollution to the environment (Grubbs, 2003). A Nobel prize was awarded for the development of metathesis reactions used for the synthesis of organic molecules. Later, new pharmaceuticals and agrochemical materials were developed using this reaction.
In order to access important sulfur-containing compounds, organic thiocyanates play vital role as synthetic intermediates (Castanheiro et al., 2016). The versatile thiocyanate ion can join to the reaction centre of a suitable cation or neutral molecule through the S or N atom, resulting in the assembly of supramolecular compounds (Lee et al., 2017). For example, the crystal of 2-aminocyclohexan-1-aminium thiocyanate involves N-HÁ Á ÁS and N-HÁ Á ÁN interactions between the thiocyanate anion and the amine and aminium groups, leading to the formation of a two-dimensional network (Salem et al., 2012). 4-Aminopyridine has many biological applications, especially in treating neurological problems. For example, it acts as a potassium channel blocker (Schwid et al., 1997). With this background, the present work is carried out and the results are reported here.

Structural commentary
The asymmetric unit of the title compound is composed of one 4-aminopyridine molecule, one 4-aminopyridinium cation and one thiocyanate anion as shown in Fig. 1. The cation forms ISSN 2056-9890 hydrogen bonds with the neutral molecule and with the anion ( Table 1). The bond lengths and angles in neutral 4-aminopyridine are similar to those in a previous report (Anderson et al., 2005), but the bond angle at the pyridine N3 atom is increased to 119.47 (14) due to the hydrogen-bonding interaction. The thiocyanate anion is linear with an N5-C11-S bond of 177.85 (18) . All bond lengths and angles in the aminopyridinium cation are within the normal ranges (Fun et al., 2010).

Hirshfeld surface analysis
To quantify the intermolecular contacts in the title structure, the Hirshfeld surface and two-dimensional fingerprint plots were calculated using Crystal Explorer (Turner et al., 2017). The Hirshfeld surface mapped over d norm is depicted in Fig. 4, where the red regions make apparent hydrogen bonds in this structure. The intensity of the red color is higher for N1-H1AÁ Á ÁS, indicating the strongest interaction as compared to other red spots on the Hirshfeld surface. The fingerprint plots show that the largest contributions are from HÁ Á ÁH

Database survey
A search of the Cambridge Crystallographic Database (CSD, version 5.40, update of September 19; Groom et al., 2016) was undertaken for structures containing 4-aminopyridine and for thiocyanate ions in the salts with organic ammonium cations. The room-temperature structure of 4-aminopyridine was reported by Chao & Schempp (1977). Anderson et al. (2005) redetermined the structure at 150 K and reported that pyramidalization occurs at the amino N atom, with the N atom displaced from the plane of the three C/H/H atoms to which it is bonded. An N-HÁ Á ÁN(pyridine) interaction links the molecules in a head-to-tail manner, forming zigzag chains along the c-axis direction. This is in contrast to the structure of the title compound, where N-HÁ Á ÁN(pyridine) interactions link the molecules in a tail-to-tail manner. van Rooyan & Boeyens (1975) Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 3 2 ; y À 1 2 ; Àz þ 3 2 ; (ii) Àx þ 1 2 ; y À 1 2 ; Àz þ 1 2 ; (iii) x; y; z À 1.

Figure 2
Hydrogen bonds in the crystal of the title compound.

Figure 3
Crystal packing diagram of the title compound showing two interpenetrating 3D nets of hydrogen bonds presented as blue and purple dotted lines.

Figure 1
View of the asymmetric unit of the title compound.

Synthesis and crystallization
4-Aminopyridine and sodium thiocyanate were purchased from Merck. A solution of equimolar amounts of 4-aminopyridine and sodium thiocyanate in double-distilled water was stirred intensively for nearly 4 h, filtered with Whatman filter paper and allowed to evaporate at room temperature. Colourless needle-like crystals of the title compound were obtained after a period of seven days.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2 Hirshfeld surface plotted over d norm and two-dimensional fingerprint plots for the title compound.

Computing details
Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXT2018/2 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX publication routines (Farrugia, 2012). 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.