Crystal structure and Hirshfeld surface analysis of 1,3-diethynyladamantane

The title compound exhibits exceptionally weak intermolecular C—H⋯π hydrogen bonding of the ethynyl groups, with the corresponding H⋯π separations [2.91 (2) and 3.12 (2) Å] exceeding normal vdW distances. This bonding compliments distal contacts of the CH (aliphatic)⋯π type [H⋯π = 3.12 (2)–3.14 (2) Å] to sustain supramolecular layers.


Chemical context
Terminal alkynes provide self-complementary hydrogen-bond donor and acceptor functionality to sustain weak C-HÁ Á Á interactions (Nishio, 2004). The latter dominate the crystal structure of acetylene (McMullan et al., 1992). In the case of polyfunctional species, the significance of such C-HÁ Á Á interactions is rather low, since only 13.3% of related structures exhibit this kind of bonding (Allen et al., 2013). This may be associated with the specific geometry demands that concern an orthogonal orientation of the donor and acceptor alkyne groups. It is not surprising that examples for C-HÁ Á Á-driven self-assembly of terminal diynes are particularly rare. These examples are restricted to a few structures of hydrocarbons lacking stronger supramolecular interactions. Most of the literature precedents, such as 1,4-diethynylbenzene (Weiss et al., 1997), 1,4-diethynylcubane (Eaton et al., 1994) and ,!octa-and decadiynes (Bond, 2002) feature collinear orientations of the ethynyl groups within the molecules, which are beneficial for the generation of the simplest of supramolecular patterns. In the case of angular diynes, the demands of dense molecular packing may be less compatible with highly directional orthogonal interactions of C CH (donor) and C CH (acceptor) groups. One can anticipate the essential distortion and weakening (if not elimination at all) of the C-HÁ Á Á bonding.
In this context, we have examined the angular compound 1,3-diethynyladamantane and report its crystal structure ISSN 2056-9890 herein. The crystal packing of 1,3-disubstituted adamantanes also recently attracted attention in the context of polymorphism and the formation of plastic phases (Negrier et al., 2020).

Figure 2
Fragment of the title crystal structure showing two zigzag chains (marked in blue and grey) running along the b-axis direction in the crystal, with a set of shortest C-HÁ Á Á contacts indicated by dashed lines [symmetry codes:

Figure 1
The molecular structure of the title compound, showing the atom-labeling scheme. Displacement ellipsoids are drawn at the 40% probability level and the H atoms are shown as small spheres of arbitrary radii.

Hirshfeld analysis
The supramolecular interactions in the title structure have been further investigated and visualized by Hirshfeld surface analysis (Spackman & Byrom, 1997;McKinnon et al., 2004;Hirshfeld, 1977) performed with CrystalExplorer17 (Turner et al., 2017). The Hirshfeld surface of the molecule, mapped over d norm in the color range 0.0957 to 1.3378 a.u., indicates only a set of normal vdW contacts (white regions) corresponding to the closest interactions (Fig. 4). The two-dimensional fingerprint plot is appreciably reminiscent of the one for adamantane itself (Spackman & McKinnon, 2002), but accompanied by two additional diffuse features appearing as wings at the top left and bottom right of the plot (Fig. 5). These wings correspond to a series of CÁ Á ÁH/HÁ Á ÁC contacts. Nevertheless, HÁ Á ÁH contacts (the shortest ones are at the d e = d i = 1.2 Å level) are by far the major contributors (74.9%) to the entire surface, while the fraction of CÁ Á ÁH/HÁ Á ÁC contacts accounts for only 24.6%. The latter value may be compared with contributions of 40.0 and 32.4% calculated for ,!-octaand decadiynes (Bond, 2002) and this significant suppression of the CÁ Á ÁH/HÁ Á ÁC contacts is in line with the very weak C-HÁ Á Á bonding in the title structure, as described above. There are no stacking interactions of the ethynyl groups: the contribution of the CÁ Á ÁC contacts to the entire surface does not exceed 0.5%. Packing of the C-HÁ Á Á-bonded chains with the formation of layers, which are parallel to the bc plane. The blue color identifies a single chain that is marked in a similar manner in Fig. 2, and dashed lines indicate C-HÁ Á Á contacts within the layer and methyleneÁ Á Á contacts between adjacent layers. [Symmetry codes: (i) x, À 1 2 À y, 1 2 + z; (ii) Àx, À 1 2 + y, À0.5 À z; (iv) 1 À x, 1 2 + y, 1 2 À z.]

Figure 4
The Hirshfeld surface of the title compound mapped over d norm in the color range 0.0957 to 1.3378 a.u. showing the shortest HÁ Á Á contact with the normalized C-H distance.

Synthesis and crystallization
The title compound was synthesized in a three-step reaction sequence starting with selective dibromination of adamantane (Degtyarenko et al., 2014). The reaction product was crystallized from methanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in   (Sheldrick, 2015), DIAMOND (Brandenburg, 1999) and WinGX (Farrugia, 2012 ; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2018/1 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (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.