(NH4)2(C4O4), an anhydrous salt of acetylenedicarboxylic acid

Single crystals of anhydrous ammonium acetylenedicarboxylate ((NH4)2(C4O4), 1) were isolated from a precipitate crystallizing from an aqueous ammonia solution containing acetylenedicarboxylic acid. The crystal structure of 1 (C2/mC2/m, Z=2Z=2) consists of ordered ammonium cations, which are connected via NH⋯O hydrogen bonds (NO: 2.826 and 2.947 Å) to four oxygen atoms from four symmetry related acetylenedicarboxylates. The coordination by the bifunctional carboxylates leads to a three-dimensional framework structure. Coupled TGA/DTA/MS measurements show that 1 decomposes completely at about 450 K releasing NH3, CO2, and C2H2. No order–disorder transition occurs in the temperature range between 20 K and the decomposition temperature. In contrast to Sr(C4O4), the other known anhydrous acetylenedicarboxylate, no negative thermal expansion is found below room temperature.

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