Acetyl-ene-ammonia-18-crown-6 (1/2/1).

The title compound, C(2)H(2)·C(12)H(24)O(6)·2NH(3), was formed by co-crystallization of 18-crown-6 and acetyl-ene in liquid ammonia. The 18-crown-6 mol-ecule has threefold rotoinversion symmetry. The acteylene mol-ecule lies on the threefold axis and the whole mol-ecule is generated by an inversion center. The two ammonia mol-ecules are also located on the threefold axis and are related by inversion symmetry. In the crystal, the ammonia mol-ecules are located below and above the crown ether plane and are connected by inter-molecular N-H⋯O hydrogen bonds. The acetyl-ene mol-ecules are additionally linked by weak C-H⋯N inter-actions into chains that propagate in the direction of the crystallographic c axis. The 18-crown-6 mol-ecule [occupancy ratio 0.830 (4):0.170 (4)] is disordered and was refined using a split model.

Evidence of solubility of the acetylide ion C2(2-): syntheses and crystal structures of K2C2⋠2 NH3, Rb2C2⋠2 NH3, and Cs2C2⋠7 NH3.

Carbon anions in solution: C(2)(2-) dumbbells are well-known in solid-state compounds. The crystallization of the title compounds now shows that acetylide ions are existent in solution and therefore chemistry with small dissolved carbon anions may be within reach.