Acetylene–ammonia–18-crown-6 (1/2/1)

The title compound, C2H2·C12H24O6·2NH3, was formed by co-crystallization of 18-crown-6 and acetylene in liquid ammonia. The 18-crown-6 molecule has threefold rotoinversion symmetry. The acteylene molecule lies on the threefold axis and the whole molecule is generated by an inversion center. The two ammonia molecules are also located on the threefold axis and are related by inversion symmetry. In the crystal, the ammonia molecules are located below and above the crown ether plane and are connected by intermolecular N—H⋯O hydrogen bonds. The acetylene molecules are additionally linked by weak C—H⋯N interactions into chains that propagate in the direction of the crystallographic c axis. The 18-crown-6 molecule [occupancy ratio 0.830 (4):0.170 (4)] is disordered and was refined using a split model.


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Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: NC2288).

Tobias Grassl, Markus Hamberger and Nikolaus Korber Comment
The crystal structure of the title compound was determined in the course of investigations regarding the reactivity of acetylene in liquid ammonia.
In the crystal structure the acetylene molecule shows moderate hydrogen bonding in axial direction to an ammonia molecule on each side with a H···N distance of 2.3422 (15) Å and a C-H···N angle of 180°. Two ammonia molecules are located below and above the crown ether plane, bound by hydrogen bonds to the oxygen atoms in the ring ( Fig. 1 and  Table 1).This arrangement leads to one-dimensional strands along the crystallographic c-axis, that are packed in a kind of hexagonal closest arrangement (Fig. 3). The formation of hydrogen bonds between acetylene and ammonia molecules as well as the interaction of ammonia molecules with the crown ether is essential to stabilize the fugitive acetylene molecule in the solid state as was shown previously by Boese et al. (Boese et al., 2009) in C 2 H 2 *NH 3 . Due to the absence of stronger intermolecular interactions the optimization of hydrogen bonds is the driving force for the axial stacking of the molecules along the crystallographic c-axis. This can also be observed in acetylene containing material such as co-crystallized C 2 H 2 *NH 3 (Boese et al., 2009) and co-crystals of acetylene and acetone/DMSO (Boese et al., 2003) or azacycles (Kirchner et al., 2004). Experimental 0.039 g(1.0 mmol) potassium and 0.264 g(1.00 mmol)18-crown-6 were placed under argon atmosphere in a baked-out reaction vessel and 30 ml of dry liquid ammonia were condensed. The mixture was stored at 236 K for one week to ensure that all substances were completely dissolved. Afterwards an excess of acetylene gas was fed into the solution until the colour changed from deep blue to colourless. Colourless crystals of the title compound were obtained after further storage at 236 K for nine month. Well soluble potassium hydrogen acetylide KC 2 H remained in solution.

Refinement
The O atom and one C atom of the crown ether are disordered and were refined using a split model with sof of 0.830 (4) and 0.170 (4). The C-H H atoms were positioned with idealized geometry and refined isotropic with U iso (H) = 1.2 U eq (C) using a riding model. The N-H H atom was located in difference map and refined in the riding mode approximation. structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg & Putz, H, 2011); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

Figure 1
Crystal structure of the title compound with labeling and displacement ellipsoids drawn at the 50% probability level.
Disordering is shown as full and open bonds. Symmetry codes: (i) 2/3 -x, 4/3 -y, 1/3 -z; (ii) -1/3 + y, 1/3 -x + y, 4/3 -z; (iii) 2/3 + x-y, 1/3 + x, 4/3 -z.    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 > σ(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (