Sr[C2(COO)2]: the first anhydrous salt of acetylenedicarboxylic acid

Single crystals of Sr[C(2)(COO)(2)] were formed at the phase boundary of an aqueous silica gel that contained acetylenedicarboxylic acid and an aqueous solution of SrCl(2). The crystal structure (I4(1)/amd, Z=4) shows a diamond-like topology, with the strontium ions in an eightfold co-ordination by the oxygen atoms of six carboxylate ligands with two of them co-ordinating in a chelating-type bidentate mode. As each oxygen atom of the carboxylate groups is involved in both a chelating-type bidentate and unidentate co-ordination and, therefore, bridging two strontium atoms, a three-dimensional framework results. Sr[C(2)(COO)(2)] exhibits a surprising thermal stability. In air decomposition starts at about 720 K and in an argon atmosphere around 750 K. This decomposition is accompanied by a mass loss of approximately 12 %, which is probably due to the loss of one CO molecule. The resulting residue is amorphous to X-rays. A small negative thermal expansion is found between 30 and 280 K, whereas between room temperature and 573 K almost no thermal expansion is observed. Single-crystal investigations at different temperatures show a strong "guitar string" vibration of the oxygen atom perpendicular to the contacts to the strontium atoms; this could be responsible for the thermal expansion behaviour.