Evidence of solubility of the acetylide ion C22-: Syntheses and crystal structures of K2C2·2NH 3, Rb2C2·2NH3, and Cs 2C2·7NH3

Article abstract of DOI:10.1002/anie.201206349

Most alkali- and alkaline-earth-metal acetylides of composition A ₂^IC₂ (A^I=Li-Cs) and (AE)IIC ₂ ((AE)II=Mg-Ba) have been known since the 19th century.[1,2] Very early crystal structure investigations had already revealed that they contA ^In C₂ ^2- dumbbells,[3, 4] but the complex phase behavior of these compounds was elucidated only very recently.[5] In the first half of the last century, CaC₂ in particular, which is accessible from CaO and coal, attracted a lot of interest as an industrial feedstock of acetylene by hydrolysis.[6] Owing to the limited petroleum resources, this reaction has found a revival within the last years.[7] Apart from hydrolysis however, reports on further reactions of alkali- and alkaline-earthmetal acetylides are quite rare, which is surely due in part to the fact that these compounds are insoluble in common solvents.[8] However, during the synthesis of ternary copper acetylides A^ICuC₂ with A^I=K-Cs, we observed that a suspension of A^I 2C₂ in liquid ammonia reacts with CuI at temperatures as low as -78°C within minutes.[9] To clarify whether this reaction can be interpreted as a (low) solubility of A ^I 2C₂ in liquid ammonia, we decided to investigate the system A^I 2C₂/ NH3(l) in more detA^Il. The versatility of liquid ammonia to stabilize unusual mA^In-group-element clusters has been shown by several impressive examples.[10] Herein we report the crystal structures of the ammoniates K2C₂·2NH3 (1), Rb2C₂·2NH3 (2), and Cs2C₂·7NH3 (3),[11a-c] which are the first examples of binary acetylide solvates. Additionally, the crystal structure of [Li(NH3)4]C₂H (4),[11d] which contA ^Ins the hydrogen acetylide anion C₂H-, is briefly presented.