Syntheses and Crystal Structures of Alkaline Earth Metal Hydrogen Acetylides AE(C ₂H) ₂ with AE = Ca, Sr, Ba

Abstract

Crystalline Ba(C ₂H) ₂ was obtained by the reaction of elemental barium dissolved in liquid ammonia, forming a blue electride, and acetylene (C ₂H ₂) injected into the reaction vial with the electride solution. From the colorless precipitate that was obtained after evaporation of the ammonia, the crystal structure of Ba(C ₂H) ₂ was solved and refined using synchrotron powder diffraction data. It crystallizes in the trigonal space group P3̅ m1 (no. 164) with Z = 1, all HC ₂ ^– anions are aligned along [001]. It is the first crystal structure of an alkaline earth metal hydrogen acetylide published up to now, showing a close similarity with the brucite (Mg(OH) ₂) and CdI ₂-type structures. For Sr(C ₂H) ₂, a product with a significantly reduced crystallinity was obtained, but its powder diffraction pattern makes an isotypic crystal structure very likely. IR/Raman spectroscopic investigations as well as GC analysis of the gases released upon hydrolysis unambiguously confirm the existence of HC ₂ ^– anions in these compounds. Similar results were obtained for Ca(C ₂H) ₂, however this compound is almost completely amorphous. Upon heating Ba(C ₂H) ₂ with additional elemental barium to 200 °C in vacuum, highly crystalline BaC ₂ ( I4/ mmm, Z = 2) was obtained.

Abstract

Ba(C ₂H) ₂ was synthesized by reaction of barium with acetylene (C ₂H ₂) in liquid ammonia. From the resulting crystalline powder, its crystal structure was solved and refined. Ba(C ₂H) ₂ crystallizes in the space group P3̅ m1 (no. 164) with Z = 1, all HC ₂ ^– anions are aligned along [001]. The structure is related to the CdI ₂ and the brucite-type structures (Mg(OH) ₂) and represents the first crystal structure of an alkaline earth metal hydrogen acetylide up to now.