The oxamate anion: A flexible building block of hydrogen-bonded architectures for crystal engineering

In order to examine the possibilities of using the oxamate anion, [OOCCONH2](-), as a versatile building block of low-dimensional, hydrogen-bonded scaffolding, the preferential assembly patterns displayed by this anion were identified using X-ray single-crystal data for a sequence of simple oxamate salts. Several new compounds were synthesized, and the crystal structures of six materials are presented: piperidinium oxamate (1), 2-amino-5-nitropyridine 2-amino-5-nitropyridinium oxamate (2), imidazolium oxamate (3), (4-chlorobenzyl)ammonium oxamate (4), (3-chlorobenzyl)ammonium oxamate (5), and potassium oxamate monohydrate (6). The oxamate anion acts as a building block of either 1-D (ribbons or ladders) or 2-B (layers) motifs. These architectures (generated by NH ... O hydrogen bonds) are the result of the selectivity of complimentary hydrogen-bend sites of the anion. However, the type of network adopted is also influenced by cation-anion interactions, and the role of the physical-chemical characteristics of the cation in inducing a specific anionic motif, is discussed. The oxamate anion, as well as other deprotonated carboxylic amides, may represent useful additions to the existing collection of building blocks capable of generating extended hydrogen-bonded motifs for crystal engineering.