4-Carb­oxy­piperidinium 1-carb­oxy­cyclo­butane-1-carboxyl­ate

The title salt, C₆H₁₂NO₂⁺·C₆H₇O₄^- or ISO⁺·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...O and O-H...O hydrogen bonds. Wave-like ribbons, constructed from ISO⁺ and CBDC^- units and described by the graph-set symbols C₃³(10) and R₃³(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 R₆⁶(30) and R₄²(18). A detailed analysis of the structures of the individual components and the intricate hydrogen-bond network of the crystal structure is given.