2-(4-Carb­oxy­piperidinium-1-yl)pyridine-3-carboxyl­ate

The title compound, C₁₂H₁₄N₂O₄, crystallizes as a zwitterion. A negative charge is delocalized in the deprotonated carboxyl group attached to the pyridine ring. The piperidine N atom accepts a proton and the ring is transformed into a piperidinium cation. There is an intra­molecular N—H...O hydrogen bond between the protonated NH and a carboxyl­ate O atom. In the crystal, an O—H...O hydrogen bond between the carboxyl group and the carboxyl­ate O atom of another mol­ecule generates a helix along the b axis.

N-(3-Hy­droxy­phen­yl)nicotinamide

In the title mol­ecule, C₁₂H₁₀N₂O₂, the benzene and pyridine rings form a dihedral angle of 5.01 (8)°. The amide group is twisted by 33.54 (7)° from the plane of the pyridine ring. In the crystal, mol­ecules are linked into centrosymmetric dimers via pairs of O—H...N hydrogen bonds. N—H...O hydrogen bonds further link dimers related into chains along the b axis.

2,2′,2′′,2′′′-(1,4-Phenyl­enedinitrilo)­tetra­acetic acid dihydrate

In the title compound, C₁₄H₁₆N₂O₈·2H₂O, the complete organic molecule is generated by crystallographic inversion symmetry. The dihedral angles between the aniline ring and the acetic acid groups are almost identical, viz. 82.61 (7) and 80.33 (7)°. In the crystal, O—H...O hydrogen bonds link the organic mol­ecules and water mol­ecules, forming zigzag chains the c axis. An intra­molecular O—H...O hydrogen bond is also observed.

3-(Dihy­droxy­bor­yl)anilinium 6-carb­oxy­pyridine-2-carboxyl­ate

In the anion of the title molecular salt, C₆H₉BNO₂⁺·C₇H₄NO₄⁻, the dihedral angles between the –COO²⁻ and –CO₂H groups and their attached ring are 4.02 (13) and 21.41 (10)°, respectively. The B atom in the cation adopts a syn–syn geometry and the dihedral angle between the –B(OH)₂ group and its attached ring is 11.06 (5)°. In the crystal, O—H...O, N—H...O and N—H...N hydrogen bonds link the components into a three-dimensional network.