A simplified two-step synthesis of 3,6-di­hydroxy­picolinic acid (3-hy­droxy-6-oxo-1,6-di­hydro­pyridine-2-carb­oxy­lic acid), C6H5NO4 (II), an inter­mediate in the metabolism of picolinic acid, is described. The crystal structure of II, along with that of a labile inter­mediate, dipotassium 3-hy­droxy-6-(sulfonato­oxy)pyridine-2-carboxyl­ate monohydrate, 2K+·C6H3NO7S2−·H2O (I), is also described. Compound I comprises a pyridine ring with carboxyl­ate, hydroxyl (connected by an intra­molecular O—H⋯O hydrogen bond), and sulfate groups at the 2-, 3-, and 6-positions, respectively, along with two potassium cations for charge balance and one water mol­ecule of crystallization. These components are connected into a three-dimensional network by O—H⋯O hydrogen bonds arising from the water mol­ecule, C—H⋯O inter­actions and π–π stacking of pyridine rings. In II, the ring nitro­gen atom is protonated, with charge balance provided by the carboxyl­ate group (i.e., a zwitterion). The intra­molecular O—H⋯O hydrogen bond observed in I is preserved in II. Crystals of II have unusual space-group symmetry of type Abm2 in which extended planar networks of O—H⋯O and N—H⋯O hydrogen-bonded mol­ecules form sheets lying parallel to the ac plane, constrained to b = 0.25 (and 0.75). The structure was refined as a 50:50 inversion twin. A minor disorder component was modeled by reflection of the major component across a mirror plane perpendicular to c.

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Published in Acta Crystallographica Section E, v. 77, part 6.

This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.

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Funding for this research was provided by: The OSU Emeritus Academy.

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Analytical data were provided by Robertson Microlit Laboratories.

Supplementary crystallographic information: 3D view

hb7975sup1.cif (927 kB)
Crystal structure: contains datablocks I, II, global. DOI: https://doi.org//10.1107/S2056989021004904/hb7975sup1.cif

hb7975Isup2.hkl (200 kB)
Structure factors: contains datablock I. DOI: https://doi.org//10.1107/S2056989021004904/hb7975Isup2.hkl

hb7975IIsup3.hkl (58 kB)
Structure factors: contains datablock II. DOI: https://doi.org//10.1107/S2056989021004904/hb7975IIsup3.hkl

hb7975Isup4.cml (3 kB)
Supporting information file. DOI: https://doi.org//10.1107/S2056989021004904/hb7975Isup4.cml

hb7975IIsup5.cml (2 kB)
Supporting information file. DOI: https://doi.org//10.1107/S2056989021004904/hb7975IIsup5.cml

hb7975.pptx (494 kB)
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