Crystal structures and Hirshfeld surface analysis of 2-(adamantan-1-yl)-5-(4-fluorophenyl)-1,3,4-oxadiazole and 2-(adamantan-1-yl)-5-(4-chlorophenyl)-1,3,4-oxadiazole

The title adamantane-oxadiazole hybrid compounds are built up from an adamantane unit and a halogenophenyl ring, [X = F (I), Cl (II)], in position 5 on the central 1,3,4-oxadiazole unit.

The crystal structures of the title adamantane-oxadiazole hybrid compounds, C 18 H 19 FN 2 O (I) and C 18 H 19 ClN 2 O (II), are built up from an adamantane unit and a halogenophenyl ring, [X = F (I), Cl (II)], in position 5 on the central 1,3,4oxadiazole unit. The molecular structures are very similar, only the relative orientation of the halogenophenyl ring in comparison with the central fivemembered ring differs slightly. In the crystals of both compounds, molecules are linked by pairs of C-HÁ Á ÁN hydrogen bonds, forming inversion dimers with R 2 2 (12) ring motifs. In (I) the dimers are connected by C-HÁ Á ÁF interactions, forming slabs lying parallel to the bc plane. In (II), the dimers are linked by C-HÁ Á Á and offsetinteractions [interplanar distance = 3.4039 (9) Å ], forming layers parallel to (101).
As seen in Fig. 3, the molecular structures of compounds (I) and (II) are very similar. The largest difference is highlighted by the structural overlay plot, and comes from the relative orientation of the halogenophenyl group with respect to the oxadiazole ring. In compound (II), the rings are almost coplanar with their mean planes being inclined to each other by 9.5 (1) , while in compound (I) the equivalent dihedral angle is 20.8 (2) .

Supramolecular features
In the crystals of both compounds, molecules are linked by pairs of C-HÁ Á ÁN hydrogen bonds, forming inversion dimers with R 2 2 (12) ring motifs (Tables 1 and 2, respectively). In the crystal of (I), the dimers are connected by C-HÁ Á ÁF interactions, forming slabs lying parallel to the bc plane ( Fig. 4 and Table 1). In the crystal of (II), the dimers are linked by C-HÁ Á Á and offsetinteractions, forming layers lying parallel to the (101) plane; see Fig. 5 and Table 2. The offsetinteractions involve inversion-related 4-chlorophenyl rings (C1-C6) with an intercentroid distance of 3.687 (1) Å , an interplanar distance of 3.404 (1) Å , and an offset of 1.418 Å . In Fig. 5  Molecular structure of compound (II), with the atom labelling and displacement ellipsoids drawn at the 50% probability level.

Figure 3
View of the structural overlay of compounds (I) and (II). Compound (I) is drawn according to element type, while compound (II) is drawn in pale green.

Figure 1
Molecular structure of compound (I), with the atom labelling and displacement ellipsoids drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ) for (I).

Hirshfeld surface analysis
The Hirshfeld surfaces for (I) and (II) mapped over d norm were calculated using CrystalExplorer 17 (Turner et al., 2017) with the default setting of arbitrary units range. The characteristic bright-red spots near atoms H3, H18A, N1 and F1 (Fig. 6) confirm the previously mentioned C3-H3Á Á ÁN1 i and C18-H18AÁ Á ÁF1 ii [symmetry codes: (i) Àx + 1, Ày + 1, Àz + 1; (ii) Àx + 1, y À 1 2 , Àz + 3 2 ] interatomic contacts in the crystal packing of (I). As expected, the same bright-red spots are observed near atoms H3 and N1 on the Hirshfeld surface of (II); see Fig. 7. The Hirshfeld surface mapped over the shapeindex property elegantly illustrates thestacking and the C-HÁ Á Á interactions observed in the crystal packing of (II). A view of the Hirshfeld surface mapped over d norm for compound (I) over the range À0.138 to 1.364 arbitrary units.

Figure 7
A view of the Hirshfeld surface mapped over d norm for compound (II) over the range À0.203 to 1.273 arbitrary units.

Figure 4
A view along the b axis of the crystal packing of compound (I). The hydrogen-bonding interactions (see Table 1) are shown as dashed lines. For clarity, only hydrogen atoms H3 and H18A have been included.

Figure 5
A view along the b axis of the crystal packing of compound (II), showing the C-HÁ Á ÁN hydrogen bonds and the C-HÁ Á Á interactions (see Table 2) as dashed lines. The offsetinteractions are indicated by double-headed red arrows. For clarity, only hydrogen atoms H3 and H12A have been included.

Figure 8
Two views, (a) and (b), of the Hirshfeld surface mapped over the shapeindex property for compound (II).
Two views are presented in Fig. 8. Thestacking between inversion-related 4-chlorophenyl rings (C1-C6) is indicated by the appearance of small blue regions surrounding a bright-red triangle within the six-membered ring (Fig. 8a), while the C12-H12AÁ Á Á(C1-C6) iii interaction [symmetry code: (iii) x + 1 2 , Ày + 1 2 , z + 1 2 ] appears as a large red region within the ring (Fig. 8b). The reduced cell of SOSXIJ indicates that it is isotypic with compound (II). Compound LAPVOP resides on a mirror plane, while compound SIKKAA crystallizes with two independent molecules in the asymmetric unit. The geometrical parameters of the oxadiazole rings are similar to those reported above for the title compounds. The 4-substituted phenyl rings are inclined to the oxadiazole ring by 0.0 in LAPVOP (as it lies in a mirror plane), 3.01 and 3.31 in the two independent molecules of SIKKAA and 10.44 in SOSXIJ. In the title compounds the corresponding dihedral angle is 20.8 (2) for compound (I) and 9.5 (1) for compound (II).

Synthesis and crystallization
Compounds (I) and (II) were synthesized via condensation of adamantane-1-carboxylic acid with 4-fluorobenzohydrazide, or 4-chlorobenzohydrazide in the presence of phosphorus oxychloride, as described previously (Kadi et al., 2007). Colourless plate-like crystals of compound (I) and colourless needle-like crystals of compound (II) were obtained by slow evaporation of CHCl 3 :EtOH (1:1 v:v) solutions at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3  calculated positions and treated as riding atoms: C-H = 0.95-1.00 Å with U iso = 1.2U eq (C). program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009). 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.