4-Carboxypiperidinium 1-carboxycyclobutane-1-carboxylate

The title salt, C6H12NO2+center dot C6H7O4- or ISO+center dot CBDC-, is an ionic ensemble assisted by hydrogen bonds. The amino acid moiety (ISO or piperidine-4-carboxylic acid) has a protonated ring N atom (ISO+ or 4-carboxypiperidinium), while the semi-protonated acid (CBDC- or 1-carboxycyclobutane-1-carboxylate) has the negative charge residing on one carboxylate group, leaving the other as a neutral -COOH group. The -+NH2- state of protonation allows the formation of a two-dimensional crystal packing consisting of zigzag layers stacked along a separated by van der Waals distances. The layers extend in the bc plane connected by a complex network of N-H center dot center dot center dot O and O-H center dot center dot center dot O hydrogen bonds. Wave-like ribbons, constructed from ISO+ and CBDC- units and described by the graph-set symbols C-3(3)(10) and R-3(3)(14), run alternately in opposite directions along c. Intercalated between the ribbons are ISO+ cations linked by hydrogen bonds, forming rings described by the graph-set symbols R-6(6)(30) and R-4(2)(18). A detailed analysis of the structures of the individual components and the intricate hydrogen-bond network of the crystal structure is given.