Two related proton-transfer compounds, namely piperazine- 1,4-diium 4-oxo-4H-pyran-2,6-dicarboxylate monohydrate, C4H12N2 2+C7H2O6 2H2O or (pipzH2)(cdo)H2O, (I), and piperazine-1,4-diium bis(6-carboxy-4-oxo-4H-pyran-2-carboxylate), C4H12N2 2+2C7H3O6 or (pipzH2)(cdoH)2, (II), were obtained by the reaction of 4-oxo-4H-pyran-2,6-dicarboxylic acid (chelidonic acid, cdoH2) and piperazine (pipz). In (I), both carboxyl H atoms of chelidonic acid have been transferred to piperazine to form the piperazine-1,4-diium ion. The structure is a monohydrate. All potential N—H donors are involved in N—H O hydrogen bonds. The water molecule spans two anions via the 4-oxo group of the pyranose ring and a carboxylate O atom. The hydrogen-bonding motif is essentially two-dimensional. The structure is a pseudomerohedral twin. In the asymmetric unit of (II), the anion consists of monodeprotonated chelidonic acid, while the piperazine- 1,4-diium cation is located on an inversion centre. The single carboxyl H atom is disordered in two respects. Firstly, the disordered H atom is shared equally by both carboxylic acid groups. Secondly, the H atom is statistically disordered between two positions on either side of a centre of symmetry and is engaged in a very short hydrogen-bonding interaction; the relevant O O distances are 2.4549 (11) and 2.4395 (11) A ˚ , and the O—H O angles are 177 (6) and 177 (5), respectively. Further hydrogen bonding of the type N—H O places the (pipzH2)2+ cations in pockets formed by the chains of (cdoH) anions. In contrast with (I), the (pipzH2)2+ cations form hydrogen-bonding arrays that are perpendicular to the anions, yielding a three-dimensional hydrogen-bonding motif. The structures of both (I) and (II) also feature – stacking interactions between aromatic
