Crystal structure and Hirshfeld surface analysis of 5-methyl-1,2,4-triazolo[1,5-a]pyrimidine

The nine-membered ring system of the title compound is essentially planar. In the crystal, molecules are linked via C—HTrz⋯NTrz and C—HPyrm⋯NTrz (Trz = triazole and Pyrm = pyrimidine) hydrogen bonds together with weaker C—HPyrm⋯NPyrm hydrogen bonds to form layers parallel to (02). The layers are further connected by π–π-stacking interactions between the nine-membered ring system, forming oblique stacks along the a-axis direction.


Chemical context
In recent years, much attention has been paid to the development of new methods for the synthesis and investigation of biological and pharmacological properties of [1,2,4]triazolo[1,5-a]pyrimidine derivatives (Chebanov et al., 2010;Lahmidi et al., 2016aLahmidi et al., ,b, 2018Sedash et al., 2012). Thus, these compounds have also received successful applications for the preparation of new poly-condensed heterocycles (Beck et al., 2011). Among the various classes of nitrogen-containing heterocyclic compounds such as triazolopyrimidine derivatives display a broad spectrum of biological activities, including anti-inflammatory (Ashour et al., 2013), anticancer (Hoffmann et al., 2017) and antibacterial (Mabkhot et al., 2016) activities. In a continuation of our research on the elaboration of new methods for the synthesis of various heterocyclic systems, we investigated the reaction of bis(2chloroethyl)amine hydrochloride with ethyl 2-(5-methyl-1-1,2,4-triazolo[1,5-a]pyrimidin-7-yl)acetate under phasetransfer catalysis conditions using tetra-n-butyl ammoniumbromide (TBAB) as catalyst and potassium carbonate as base to afford the title compound, 5-methyl-1,2,4-triazolo[1,5-a]pyrimidine, (I). We report herein its molecular and crystal structures along with the results of a Hirshfeld surface analysis.

Structural commentary
In the title compound ( Fig. 1), the nine-membered ring is planar to within 0.004 (1) Å (for atom C5), and the r.m.s. deviation of the fitted atoms is 0.009 Å . Methyl atom C6 is displaced by 0.032 (1) Å from the ring system.

Hirshfeld surface analysis
In order to visualize the intermolecular interactions in the crystal of the title compound, a Hirshfeld surface (HS) analysis (Hirshfeld, 1977;Spackman & Jayatilaka, 2009) was carried out using CrystalExplorer17.5 (Turner et al., 2017). In the HS plotted over d norm (Fig. 4) The packing viewed along the a-axis direction giving a plan view of the layers. C-HÁ Á ÁN hydrogen bonds are shown as black dashed lines and the orange dots mark thestacking interactions.

Figure 3
Packing seen along the b-axis direction giving a side view of the layers. Hydrogen bonds are depicted as in Fig. 2 and the -stacking interactions are shown as orange dashed lines.

Figure 4
View of the three-dimensional Hirshfeld surface of the title compound plotted over d norm in the range À0.1566 to 1.0057 a.u.
radii, and the red and blue colours indicate distances shorter (in close contact) or longer (distinct contact), respectively, than the van der Waals radii (Venkatesan et al., 2016). The bright-red spots appearing near N2 and hydrogen atoms H2, H3 and H4 indicate their roles as the respective donors and/or acceptors in the dominant C-HÁ Á ÁN hydrogen bonds; they also appear as blue and red regions corresponding to positive and negative potentials on the HS mapped over electrostatic potential (Spackman et al., 2008;Jayatilaka et al., 2005) as shown in Fig. 5. The blue regions indicate positive electrostatic potential (hydrogen-bond donors), while the red regions indicate negative electrostatic potential (hydrogen-bond acceptors). The shape-index of the HS is a tool to visualizestacking by the presence of adjacent red and blue triangles; if there are no adjacent red and/or blue triangles, then there are nointeractions. Fig. 6 clearly suggest that there areinteractions present in the crystal structure of (I).
The overall two-dimensional fingerprint plot, Fig. 7(a), and those delineated into HÁ Á ÁN/NÁ Á ÁH, HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH, NÁ Á ÁC/CÁ Á ÁN, NÁ Á ÁN and CÁ Á ÁC contacts (McKinnon et al., 2007) are illustrated in Fig. 7(b)-(g), respectively, together with their relative contributions to the Hirshfeld surface. The most important interaction is HÁ Á ÁN/NÁ Á ÁH, contributing 40.1% to the overall crystal packing, which is reflected in Fig. 7(b) as a pair of characteristic wings with the tips at d e + d i = 2.40 Å arising from the C-HÁ Á ÁN hydrogen bonds (Table 1) as well as from the HÁ Á ÁN/NÁ Á ÁH contacts (Table 3) View of the three-dimensional Hirshfeld surface of the title compound plotted over electrostatic potential energy in the range À0.0500 to 0.0500 a.u. using the STO-3 G basis set at the Hartree-Fock level of theory. Hydrogen-bond donors and acceptors are shown as blue and red regions around the atoms corresponding to positive and negative potentials, respectively.

Figure 6
Hirshfeld surface of the title compound plotted over shape-index.  Table 2 Selected interatomic distances (Å ).
thin and thick pair of wings with the tips at d e + d i $2.23 Å in Fig. 7(c), arise from the short interatomic HÁ Á ÁH contacts, which make a 35.3% contribution to the HS and are seen as widely scattered points of high density arising from the large hydrogen content of the molecule. In the absence of C-HÁ Á Á interactions, the pair of wings in the fingerprint plot delineated into HÁ Á ÁC/CÁ Á ÁH contacts (9.5% contribution to the HS) have a nearly symmetrical distribution of points, Fig. 7 (Cornelissen et al., 1989), but to the best of our knowledge, the molecule itself has not previously been structurally characterized.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were located in a difference Fourier map and were freely refined.

Funding information
The support of NSF-MRI grant No. 1228232 for the purchase of the diffractometer and Tulane University for support of the Tulane Crystallography Laboratory are gratefully acknowledged. TH is grateful to the Hacettepe University Scientific Research Project Unit (grant No. 013 D04 602 004).  program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg & Putz, 2012); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.  (7) −0.0043 (5)