Synthesis and crystal structures of 3,6-di­hydroxy­picolinic acid and its labile inter­mediate dipotassium 3-hy­droxy-6-(sulfonato­oxy)pyridine-2-carboxyl­ate monohydrate

A two-step synthesis of 3,6-di­hydroxy­picolinic acid, its crystal structure and that of a labile inter­mediate, are described.

Abstract

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.

Chemical context  

3,6-Di­hydroxy­picolinic acid (3-hy­droxy-6-pyridone-2-carb­oxy­lic acid), C₆H₅NO₄, is an inter­mediate in the metabolism of picolinic acid by several microorganisms (Shukla & Kaul, 1973 ▸; Shukla et al., 1977 ▸; Qiu et al., 2019 ▸). It was isolated from culture media and partially characterized by Shukla & Kaul (1973 ▸; Shukla et al., 1977 ▸) whose work is misrepresented by Qiu et al. (2019 ▸) by stating that their work was only theoret­ical. It was synthesized by a six-step procedure from 3-hy­droxy­picolinic acid and characterized by mass spectrometry and NMR data (Qiu et al., 2019 ▸, with C. Shen).

We report here a two-step synthesis also starting with 3-hy­droxy­picolinic acid by an Elbs oxidation (Behrman, 1988 ▸; 2021 ▸) and crystal structures of both the inter­mediate sulfate ester (I) and of 3,6-di­hydroxy­picolinic acid (II). We considered two routes, as shown in Fig. 1 ▸. Both give the desired product but we chose the pathway from 3-hy­droxy­picolinic acid because in the first step, the dipotassium salt of the 6-sulfate ester precipitates from the mixture in an almost pure state.

Figure 1.

Two synthetic routes to 3,6-di­hydroxy­picolinic acid (II), showing tautomerism of the product.

This sulfate ester is extraordinarily sensitive to acid-catal­yzed hydrolysis; acidification at room temperature (RT) gives complete hydrolysis in a few minutes. The isomeric 6-oxo-picolinic acid-3-sulfate is much more stable although subject to anchimeric assistance (Benkovic, 1966 ▸); it is not completely hydrolyzed after 22 h, RT, pH 2. Nantka-Namirski & Rykowski (1972a ▸,b ▸) used boiling 20% sulfuric acid for four hours to effect the hydrolyses of the 5-sulfate esters of two di­hydroxy­nicotinic acids. For the rapid hydrolysis of pyridyl-4-sulfate see Jerfy & Roy (1970 ▸) and Goren & Kochansky (1973 ▸) for the effects of impurities. A reasonable representation of the mechanism of the hydrolysis for the 4-sulfate is shown in Fig. 2 ▸ a. The 2-sulfate should behave similarly (Fig. 2 ▸ b). Jerfy & Roy (1970 ▸) also showed that sulfation of 2-pyridone gives the sulfamate, so that it has not yet been possible to prepare the pyridyl-2-sulfate for comparative purposes. However, a route to 5-hy­droxy­pyridine-2-sulfate together with the isomeric 4- and 6-sulfate esters is known (Behrman & Pitt, 1958 ▸). Examination of these mixed esters by electrophoresis shows that all three are rapidly hydrolyzed under acid catalysis, as predicted by the model. The reaction between potassium per­oxydi­sulfate and 3-hy­droxy­picolinic acid was carried out as usual in KOH solution except that if there is excess per­oxy-di­sulfate, the sulfate ester and the peroxide precipitate from the reaction mixture together: to avoid this the persulfate was used as the limiting reagent. The ester was crystallized from water and 3,6-di­hydroxy­picolinic acid obtained by hydrolysis.

Figure 2.

(a) A reasonable representation for the mechanism of hydrolysis for the 4-sulfate. (b) The o-sulfate analog ought to behave similarly.

Structural commentary  

The asymmetric unit in I (Fig. 3 ▸) contains a single pyridine ring with a carboxyl­ate group at the 2-position, a hydroxyl group at the 3-position, and a sulfate group attached to the 6-position. Charge balance is provided by a pair of K⁺ cations. There is also a single water mol­ecule of crystallization present. The carboxyl­ate C—O distances are 1.2510 (16) and 1.2758 (16) Å for C7—O3 and C7—O4, respectively. The longer of these is part of an S(6) intra­molecular hydrogen-bonded ring with the hydroxyl group. In the sulfate group, the oxygen atom bound at the 6-position [C6—O2 = 1.3968 (14) Å] is longer [S1—O2 = 1.6343 (9) Å] than the other three S=O bonds [range 1.4361 (9) to 1.4486 (9) Å], as would be expected for this bonding arrangement.

Figure 3.

An ellipsoid (50% probability) plot of the asymmetric unit of I. The intra­molecular hydrogen bond is shown by the thick dashed line. Dotted lines indicate close contacts of the K⁺ cations.

In II (Fig. 4 ▸), the nitro­gen atom of the ring is protonated. Charge balance is provided by the deprotonated 2-carboxyl­ate group [C7—O3 = 1.262 (3), C7—O4 = 1.259 (3) Å], leading to a zwitterionic mol­ecule. The intra­molecular S(6) hydrogen-bonded ring from I is preserved in II. The 6-position of the ring is occupied by a second hydroxyl group. As discussed in more detail in section 6 (Refinement), there is a small minor disorder component [refined occupancy 4.7 (3)%] and probable inversion twinning.

Figure 4.

An ellipsoid (50% probability) plot of the asymmetric unit of II with the intra­molecular hydrogen bond shown as a thick dashed line.

Supra­molecular features  

In the packing of I, strong O_w—H⋯O (w = water) hydrogen bonds exist in which the water mol­ecule acts as a linker between c-glide-related anions. The water oxygen is also coordinated to the K⁺ cations, both of which are seven coord­inate. Around K1, coordination distances range from 2.7376 (9)–2.9102 (10) Å (for K⋯O) and 2.7869 (11) Å (K1⋯N). For K2, coordination distances range from 2.6769 (9) to 2.9525 (10) Å (all K⋯O). The coordination geometry about each K⁺ cation is very roughly penta­gonal bipyramidal, with K1 much more distorted than K2. As each K⁺ cation is coordinated to the water mol­ecule, the KO₆N and KO₇ polyhedra augment the extended chains that propagate parallel to c (Fig. 5 ▸, Table 1 ▸). In addition, there are weaker C—H⋯O contacts: pairs of C5—H5⋯O6ⁱⁱ inter­actions form (12) inversion-related dimers and C4—H4⋯O5ⁱ contacts link mol­ecules via c-glide symmetry (symmetry codes as per Table 1 ▸). This in turn joins c-glide-related pyridine rings into an extended π–π stack along the c-axis direction. Adjacent rings within the stacks are almost parallel; the dihedral angle being 0.38 (4)° with a centroid–centroid distance of 3.698 (1) Å. These columns of hydrogen-bonded and π-stacked mol­ecules are inter­linked by the aforementioned chains of K⁺ cations and water mol­ecules into a three-dimensional network (Fig. 5 ▸).

Figure 5.

A view of the packing in I, viewed down the c axis. Intra­molecular hydrogen bonds are shown as thick dashed lines, open dashed lines indicate strong inter­molecular hydrogen bonds, and thin dashed lines denote weaker C—H⋯O inter­actions. Dangling hydrogen bonds extend beyond the depth of the view.

Table 1. Hydrogen-bond geometry (Å, °) for I .

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1O⋯O4	0.843 (19)	1.760 (19)	2.5429 (13)	153.6 (17)
C4—H4⋯O5i	0.95	2.64	3.3682 (16)	133
C5—H5⋯O6ii	0.95	2.35	3.2943 (15)	175
O1W—H1W⋯O3iii	0.83 (2)	1.97 (2)	2.7852 (13)	165 (2)
O1W—H2W⋯O4iv	0.83 (2)	2.09 (2)	2.8910 (14)	161 (2)

Symmetry codes: (i) x, -y+{\script{1\over 2}}, z-{\script{1\over 2}}; (ii) -x+1, -y+1, -z+1; (iii) x+1, y, z; (iv) x+1, -y+{\script{1\over 2}}, z+{\script{1\over 2}}.

The most remarkable feature of the packing in II are extended di-periodic hydrogen-bonded networks lying parallel to the ac plane (Fig. 6 ▸, Table 2 ▸) that are constrained to b = 0.25 (and 0.75) as a consequence of the unusual space-group symmetry of type Abm2. Strong O2—H2O⋯O3ⁱ and N1—H1N⋯O4ⁱ hydrogen bonds link pairs of 2₁ screw-related mol­ecules into (8) motifs that extend to form ribbons parallel to c. Weaker C4—H4⋯O2ⁱⁱⁱ and C5—H5⋯O1ⁱⁱ (symmetry codes as per Table 2 ▸) inter­actions join 2₁ screw-related ribbons to form the aforementioned networks. Contacts between these planar networks are limited to weak van der Waals inter­actions.

Figure 6.

A view of the planar diperiodic network in crystalline II, viewed normal to the ac plane. Intra­molecular hydrogen bonds are drawn as thick dashed lines, strong inter­molecular hydrogen bonds (O—H⋯O and N—H⋯O) are drawn as double dashed lines. Weaker hydrogen bonds (C—H⋯O) are drawn as thin dashed lines.

Table 2. Hydrogen-bond geometry (Å, °) for II .

The D⋯A distance for C4—H4⋯O2ⁱⁱⁱ is rather long, but within the bounds noted by Desiraju & Steiner (1999 ▸).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1N⋯O4i	0.99	1.77	2.755 (3)	169
O1—H1O⋯O4	0.89	1.76	2.535 (3)	145
C5—H5⋯O1ii	0.95	2.34	3.269 (3)	166
C4—H4⋯O2iii	0.95	2.76	3.712 (4)	180
O2—H2O⋯O3i	0.94	1.57	2.459 (3)	156
O2—H2O⋯O4i	0.94	2.56	3.372 (3)	144

Symmetry codes: (i) -x+1, -y+{\script{1\over 2}}, z-{\script{1\over 2}}; (ii) -x, -y+{\script{1\over 2}}, z-{\script{1\over 2}}; (iii) -x, -y+{\script{1\over 2}}, z+{\script{1\over 2}}.

Database survey  

A search of the Cambridge Structure Database (CSD v5.42, Nov. 2020; Groom et al., 2016 ▸) on a fragment composed of 3-picolinic acid with ‘any non-H’ at the 6-position gave only seven hits. None of these have much in common with I or II, but the most similar were AGUMEV (ammonium 2,4,6-tri­carb­oxy­pyridine-3-olate monohydrate; Li et al., 2010 ▸) and MAFTEU (6-chloro-3-tri­fluoro­meth­oxy pyridine-2-carb­oxy­lic acid; Manteau et al., 2010 ▸) in that their main components consist of a single pyridine ring with a carb­oxy­lic acid group adjacent to the ring nitro­gen and an oxygen (O⁻ in AGUMEV; O—CF₃ in MAFTEU) at the 3-position.

Space group Abm2 is not common, with only 62 entries listed in v5.42 of the CSD. Excluding polymers, entries flagged with known errors, and those without deposited coordinates left only 47 hits, i.e. <0.005% of known structures. Of these, only 29 have R ₁ ≤ 5% and none form planar networks in a manner similar to II. By any measure, the crystal structure of II is unusual.

Synthesis and crystallization  

3-Hy­droxy­pyridine-2-carb­oxy­lic acid-6-sulfate dipotassium salt monohydrate (I): Potassium hydroxide (85%, 1 g, 15 mmol) was dissolved in 10 ml of water and cooled. 3-Hy­droxy­picolinic acid (0.82 g, 6 mmol) followed by potassium per­oxy­disulfate (0.9 g, 3.3 mmol) were added. The reaction mixture was stirred at RT for 24 h. The precipitate was filtered and dried by washing with acetone, yielding 0.46–0.49 g (∼44%) of the dipotassium salt of 3-hy­droxy­pyridine-2-carb­oxy­lic acid-6-sulfate monohydrate (I). Compound I migrates on paper electrophoresis at pH 7.5 with R_p = 2 as a fluorescent spot. R_p is the migration distance relative to picric acid at R_p = 1 (the starting material has R_p = 1.1). Crystals of I grow from aqueous solution when treated as follows: 0.16 g were suspended in 2.0 ml of water, dissolved by heating carefully to about 313 K, and then cooled slowly to RT. Further cooling to 278 K overnight gave 0.12 g of needles. Analysis (%) calculated for C₆H₅NO₈K₂S: C, 2l.88; H, 1.53; N, 4.25. Found: C, 21.97; H, 1.44; N, 4.25. IR(Nujol): 3485, 3310, 3096, 1701, 1684, 1624, 1574, 1250, 1059, 957, 860, 833, 768, 716, 638 cm⁻¹. NMR(D₂O, 600 MHz) δ 7.27 (d, J = 8.82 Hz), 7.45 (d, J = 8.82 Hz). UV (in water): λ_max, (nm), ɛ (l mol⁻¹ cm⁻¹): 307, 1120; 230, 1200; 205, 4180. Heating behavior: I begins to discolor at about 448 K and then gets darker without melting up to 523 K.

3,6-Di­hydroxy­picolinic acid (II): The crude sulfate (I, 150 mg), was suspended in 2 ml of water and then heated to about 313 K to dissolve it. HCl was then added to about pH 2. A heavy precipitate formed immediately. After cooling, the colorless solid was filtered and washed with cold water to yield 50–60 mg of the product (yield 67–80%), R_p = 1. The proton NMR spectrum of II agreed with that reported by Qiu et al. (2019 ▸) except that our spectrum was taken in D₂O, 600 MHz, δ 6.69 (d, J = 9.65 Hz) and 7.45 (d, J = 9.65 Hz). These are shifted because of the solvent difference from Qiu et al. (2019 ▸), but the couplings are the same, as is the difference between the two resonances (δ 0.76). 50 mg of II was crystallized from 11 ml of hot water under argon to form 35 mg of crystals. Analytical results show that the precipitate is very nearly as clean as the crystals. Analysis (%): calculated for C₆H₅NO₄: C, 46.46; H, 3.25; N, 9.03. Found (crystals), C, 46.22; H, 3.20; N, 9.07. (precipitate), C, 45.65; H, 3.16; N, 9.10. IR: Nujol, 3120, 1614, 1540, 1360, 1269, 1100, 845, 810, 760, 621 cm⁻¹. Later, larger crystals were obtained using 88% formic acid as solvent. Upon heating, II carbonizes above 473 K without melting. UV (in water): λ_max, (nm), ɛ (l mol⁻¹ cm⁻¹) at pH 7: 346, 5970; 243, 4350; 221.5, 9450. Shukla & Kaul (1973 ▸) reported the pH-dependence of the spectrum. [See also Qiu et al. (2019 ▸), but on p. S2, line 3, read ‘240’ for ‘360’.]

Data collection, structure solution and refinement  

The crystals were mounted using polyisobutene oil on the tip of fine glass fibers, which were fastened in copper mounting pins with electrical solder. Crystals of I were placed directly into the cold gas stream of a liquid-N₂ based cryostat, while crystals of II were handled using methods developed for macromolecular cryocrystallography (Parkin & Hope, 1998b ▸). Diffraction data were collected with the crystals at 90 K. Crystal data, data collection and refinement details are summarized in Table 3 ▸. In II, a small minor disorder component was apparent in a difference map. It was modeled by reflection of the major component across a mirror plane perpendicular to its c axis, with coordinates related by the mirror plane and constrained by mapping its coordinates to the major component via SHELXL FVAR parameters. A test for twinning by inversion using the Flack parameter [x = 0.5 (3); Flack & Bernardinelli, 1999 ▸] was indeterminate. The Hooft (Hooft et al., 2008 ▸) and Parsons (Parsons et al., 2013 ▸) parameters [y = 0.63 (7); z = 0.62 (10), respectively], however, each gave a much stronger suggestion of twinning by inversion. Since the space group itself has a twofold parallel to its c axis, this results in a mirror operation (i.e. a twofold rotation combined with inversion). Thus, the twinning and disorder in II may effectively be treated by the same operation, not unlike that in uric acid dihydrate (Parkin & Hope, 1998a ▸; Parkin, 2000 ▸). To ensure satisfactory refinement of disorder, constraints (SHELXL command EADP) were used to equalize displacement parameters of superimposed groups. Full occupancy (and major component for II) hydrogen atoms were found in difference-Fourier maps. Carbon-bound hydrogen atoms were included using riding models with constrained distances set to 0.95 Å (Csp ²H). In I, the water H atoms were refined but subject to distance and angle–distance restraints, while the hydroxyl H atom was refined freely. In II, for O—H and N—H groups, riding models that allowed the bond distance to refine were used. U _iso(H) parameters were set to values of either 1.2U _eq or 1.5U _eq (OH only) of the attached atom. The structures were validated using PLATON and checkCIF (Spek, 2020 ▸).

Table 3. Experimental details.

	I	II
Crystal data
Chemical formula	2K+·C6H3NO7S2−·H2O	C6H5NO4
M r	329.37	155.11
Crystal system, space group	Monoclinic, P21/c	Orthorhombic, A b m2
Temperature (K)	90	90
a, b, c (Å)	13.3366 (4), 11.5467 (3), 7.3078 (2)	10.2045 (6), 6.1282 (4), 9.7293 (6)
α, β, γ (°)	90, 103.553 (1), 90	90, 90, 90
V (Å3)	1094.02 (5)	608.42 (7)
Z	4	4
Radiation type	Mo Kα	Cu Kα
μ (mm−1)	1.09	1.27
Crystal size (mm)	0.18 × 0.16 × 0.13	0.14 × 0.10 × 0.02
Data collection
Diffractometer	Bruker D8 Venture dual source	Bruker D8 Venture dual source
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 ▸)	Multi-scan (SADABS; Krause et al., 2015 ▸)
T min, T max	0.793, 0.862	0.799, 0.971
No. of measured, independent and observed [I > 2σ(I)] reflections	22347, 2505, 2423	4025, 700, 690
R int	0.036	0.025
(sin θ/λ)max (Å−1)	0.650	0.634
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.020, 0.056, 1.08	0.024, 0.068, 1.11
No. of reflections	2505	700
No. of parameters	174	74
No. of restraints	3	4
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)	0.40, −0.44	0.20, −0.25
Absolute structure	–	Refined as a perfect inversion twin
Absolute structure parameter	–	0.5

Computer programs: APEX3 (Bruker, 2016 ▸), SHELXT (Sheldrick, 2015a ▸), SHELXL2018/3 (Sheldrick, 2015b ▸), XP in SHELXTL (Sheldrick, 2008 ▸) and CIFFIX (Parkin, 2013 ▸).

Supplementary Material

CCDC references: 2082685, 2082684

Additional supporting information: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

Dipotassium 3-hydroxy-6-(sulfonatooxy)pyridine-2-carboxylate monohydrate (I). Crystal data

2K+·C6H3NO7S2−·H2O	F(000) = 664
Mr = 329.37	Dx = 2.000 Mg m−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 13.3366 (4) Å	Cell parameters from 9459 reflections
b = 11.5467 (3) Å	θ = 3.1–27.5°
c = 7.3078 (2) Å	µ = 1.09 mm−1
β = 103.553 (1)°	T = 90 K
V = 1094.02 (5) Å3	Block, colourless
Z = 4	0.18 × 0.16 × 0.13 mm

Dipotassium 3-hydroxy-6-(sulfonatooxy)pyridine-2-carboxylate monohydrate (I). Data collection

Bruker D8 Venture dual source diffractometer	2505 independent reflections
Radiation source: microsource	2423 reflections with I > 2σ(I)
Detector resolution: 7.41 pixels mm-1	Rint = 0.036
φ and ω scans	θmax = 27.5°, θmin = 1.6°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −17→17
Tmin = 0.793, Tmax = 0.862	k = −14→14
22347 measured reflections	l = −9→9

Dipotassium 3-hydroxy-6-(sulfonatooxy)pyridine-2-carboxylate monohydrate (I). Refinement

Refinement on F2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: mixed
R[F2 > 2σ(F2)] = 0.020	H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.056	w = 1/[σ2(Fo2) + (0.0254P)2 + 0.7213P] where P = (Fo2 + 2Fc2)/3
S = 1.08	(Δ/σ)max = 0.001
2505 reflections	Δρmax = 0.40 e Å−3
174 parameters	Δρmin = −0.44 e Å−3
3 restraints	Extinction correction: SHELXL2018/3 (Sheldrick 2015b), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0116 (12)

Dipotassium 3-hydroxy-6-(sulfonatooxy)pyridine-2-carboxylate monohydrate (I). Special details

Experimental. The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder. It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994; Parkin & Hope, 1998). Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals.

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement progress was checked using Platon (Spek, 2020) and by an R-tensor (Parkin, 2000). The final model was further checked with the IUCr utility checkCIF.

Dipotassium 3-hydroxy-6-(sulfonatooxy)pyridine-2-carboxylate monohydrate (I). Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
K1	−0.11303 (2)	0.44196 (2)	0.29817 (4)	0.01068 (9)
K2	0.65851 (2)	0.37254 (2)	0.80722 (4)	0.01048 (9)
S1	0.39446 (2)	0.56402 (3)	0.68497 (4)	0.00841 (9)
O1	0.18689 (8)	0.05598 (7)	0.56350 (13)	0.01168 (19)
H1O	0.1278 (15)	0.0662 (15)	0.584 (3)	0.018*
O2	0.32641 (7)	0.49448 (8)	0.50302 (12)	0.00997 (18)
O3	0.01000 (7)	0.34563 (8)	0.61547 (13)	0.01185 (19)
O4	0.02411 (7)	0.15242 (8)	0.61413 (13)	0.01227 (19)
O5	0.32663 (7)	0.58023 (8)	0.81136 (13)	0.01295 (19)
O6	0.41500 (7)	0.66929 (8)	0.59303 (13)	0.01295 (19)
O7	0.48251 (7)	0.49241 (8)	0.76027 (14)	0.0149 (2)
N1	0.20276 (8)	0.37108 (9)	0.55903 (14)	0.0088 (2)
C2	0.16613 (9)	0.26318 (11)	0.57169 (17)	0.0088 (2)
C3	0.22393 (10)	0.16468 (11)	0.55284 (17)	0.0093 (2)
C4	0.32289 (10)	0.17884 (11)	0.52150 (17)	0.0105 (2)
H4	0.363874	0.113328	0.509029	0.013*
C5	0.35984 (9)	0.28931 (11)	0.50905 (17)	0.0104 (2)
H5	0.426568	0.302283	0.487955	0.013*
C6	0.29543 (10)	0.38126 (10)	0.52862 (17)	0.0089 (2)
C7	0.05895 (9)	0.25453 (11)	0.60404 (17)	0.0096 (2)
O1W	0.85262 (7)	0.30218 (9)	0.79832 (14)	0.0151 (2)
H1W	0.8909 (13)	0.3167 (17)	0.726 (2)	0.023*
H2W	0.8914 (13)	0.3145 (17)	0.903 (2)	0.023*

Dipotassium 3-hydroxy-6-(sulfonatooxy)pyridine-2-carboxylate monohydrate (I). Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
K1	0.00957 (14)	0.00968 (14)	0.01288 (14)	0.00117 (9)	0.00285 (10)	−0.00086 (9)
K2	0.00967 (14)	0.01126 (14)	0.01074 (14)	0.00044 (9)	0.00281 (10)	0.00146 (9)
S1	0.00738 (15)	0.00825 (15)	0.00966 (15)	−0.00042 (10)	0.00215 (11)	−0.00085 (10)
O1	0.0117 (5)	0.0075 (4)	0.0169 (5)	−0.0005 (3)	0.0054 (4)	0.0000 (3)
O2	0.0121 (4)	0.0089 (4)	0.0087 (4)	−0.0027 (3)	0.0019 (3)	−0.0002 (3)
O3	0.0096 (4)	0.0123 (4)	0.0142 (4)	0.0010 (3)	0.0038 (3)	0.0002 (3)
O4	0.0109 (4)	0.0112 (4)	0.0154 (4)	−0.0017 (3)	0.0044 (3)	0.0009 (3)
O5	0.0123 (4)	0.0164 (5)	0.0111 (4)	−0.0004 (4)	0.0048 (3)	−0.0019 (4)
O6	0.0147 (5)	0.0093 (4)	0.0160 (4)	−0.0028 (3)	0.0059 (4)	−0.0002 (3)
O7	0.0108 (4)	0.0139 (4)	0.0180 (5)	0.0033 (3)	−0.0005 (4)	−0.0017 (4)
N1	0.0096 (5)	0.0093 (5)	0.0072 (5)	−0.0002 (4)	0.0015 (4)	−0.0006 (4)
C2	0.0086 (5)	0.0101 (6)	0.0076 (5)	−0.0008 (4)	0.0018 (4)	0.0005 (4)
C3	0.0112 (6)	0.0082 (6)	0.0078 (5)	−0.0010 (4)	0.0008 (4)	0.0000 (4)
C4	0.0104 (6)	0.0109 (6)	0.0101 (6)	0.0021 (4)	0.0023 (4)	−0.0014 (4)
C5	0.0087 (5)	0.0132 (6)	0.0096 (5)	−0.0002 (5)	0.0026 (4)	−0.0014 (4)
C6	0.0108 (6)	0.0082 (5)	0.0070 (5)	−0.0021 (4)	0.0010 (4)	−0.0001 (4)
C7	0.0091 (6)	0.0129 (6)	0.0065 (5)	−0.0007 (4)	0.0011 (4)	0.0002 (4)
O1W	0.0113 (5)	0.0165 (5)	0.0181 (5)	0.0005 (4)	0.0044 (4)	0.0003 (4)

Dipotassium 3-hydroxy-6-(sulfonatooxy)pyridine-2-carboxylate monohydrate (I). Geometric parameters (Å, º)

K1—O4i	2.7376 (9)	K2—K2x	4.6216 (3)
K1—O3	2.7428 (10)	S1—O7	1.4361 (9)
K1—O5ii	2.7843 (10)	S1—O6	1.4457 (9)
K1—N1ii	2.7869 (11)	S1—O5	1.4486 (9)
K1—O3ii	2.8099 (10)	S1—O2	1.6343 (9)
K1—O1Wiii	2.856 (1)	O1—C3	1.3577 (15)
K1—O1iv	2.9102 (10)	O1—H1O	0.843 (19)
K1—S1ii	3.7848 (4)	O2—C6	1.3968 (14)
K1—K1ii	3.9268 (5)	O3—C7	1.2510 (16)
K1—K2v	4.2041 (4)	O4—C7	1.2758 (16)
K1—K2iii	4.7508 (4)	N1—C6	1.3112 (16)
K2—O7	2.6769 (9)	N1—C2	1.3492 (16)
K2—O6vi	2.7084 (10)	C2—C3	1.3988 (17)
K2—O1W	2.7286 (10)	C2—C7	1.5057 (17)
K2—O2vii	2.7842 (9)	C3—C4	1.4010 (17)
K2—O5viii	2.8012 (9)	C4—C5	1.3781 (18)
K2—O6vii	2.8993 (10)	C4—H4	0.9500
K2—O1ix	2.9525 (10)	C5—C6	1.3940 (17)
K2—S1vii	3.5751 (4)	C5—H5	0.9500
K2—S1vi	3.6348 (4)	O1W—H1W	0.832 (15)
K2—K2i	4.6216 (3)	O1W—H2W	0.831 (15)
O4i—K1—O3	85.28 (3)	O5viii—K2—K1xi	41.02 (2)
O4i—K1—O5ii	125.34 (3)	O6vii—K2—K1xi	154.00 (2)
O3—K1—O5ii	125.08 (3)	O1ix—K2—K1xi	43.784 (19)
O4i—K1—N1ii	152.53 (3)	S1vii—K2—K1xi	138.793 (10)
O3—K1—N1ii	102.82 (3)	S1vi—K2—K1xi	106.147 (8)
O5ii—K1—N1ii	71.26 (3)	O7—K2—K2i	112.19 (2)
O4i—K1—O3ii	96.31 (3)	O6vi—K2—K2i	75.48 (2)
O3—K1—O3ii	90.00 (3)	O1W—K2—K2i	67.86 (2)
O5ii—K1—O3ii	123.94 (3)	O2vii—K2—K2i	71.370 (18)
N1ii—K1—O3ii	58.00 (3)	O5viii—K2—K2i	153.22 (2)
O4i—K1—O1Wiii	74.51 (3)	O6vii—K2—K2i	33.178 (19)
O3—K1—O1Wiii	70.34 (3)	O1ix—K2—K2i	124.254 (19)
O5ii—K1—O1Wiii	75.87 (3)	S1vii—K2—K2i	50.703 (5)
N1ii—K1—O1Wiii	132.96 (3)	S1vi—K2—K2i	56.517 (8)
O3ii—K1—O1Wiii	158.67 (3)	K1xi—K2—K2i	131.084 (6)
O4i—K1—O1iv	81.77 (3)	O7—K2—K2x	104.80 (2)
O3—K1—O1iv	163.61 (3)	O6vi—K2—K2x	35.86 (2)
O5ii—K1—O1iv	71.02 (3)	O1W—K2—K2x	90.71 (2)
N1ii—K1—O1iv	84.60 (3)	O2vii—K2—K2x	174.19 (2)
O3ii—K1—O1iv	81.53 (3)	O5viii—K2—K2x	49.13 (2)
O1Wiii—K1—O1iv	115.26 (3)	O6vii—K2—K2x	129.27 (2)
O4i—K1—S1ii	142.24 (2)	O1ix—K2—K2x	108.403 (19)
O3—K1—S1ii	111.50 (2)	S1vii—K2—K2x	152.157 (10)
O5ii—K1—S1ii	18.571 (19)	S1vi—K2—K2x	49.568 (8)
N1ii—K1—S1ii	59.06 (2)	K1xi—K2—K2x	64.932 (5)
O3ii—K1—S1ii	116.43 (2)	K2i—K2—K2x	104.485 (11)
O1Wiii—K1—S1ii	79.72 (2)	O7—S1—O6	115.80 (6)
O1iv—K1—S1ii	84.87 (2)	O7—S1—O5	114.01 (6)
O4i—K1—K1ii	91.22 (2)	O6—S1—O5	113.72 (6)
O3—K1—K1ii	45.69 (2)	O7—S1—O2	106.09 (5)
O5ii—K1—K1ii	143.23 (2)	O6—S1—O2	99.47 (5)
N1ii—K1—K1ii	77.04 (2)	O5—S1—O2	105.75 (5)
O3ii—K1—K1ii	44.305 (19)	O7—S1—K2vii	117.03 (4)
O1Wiii—K1—K1ii	115.57 (2)	O6—S1—K2vii	51.28 (4)
O1iv—K1—K1ii	124.38 (2)	O5—S1—K2vii	127.54 (4)
S1ii—K1—K1ii	125.061 (11)	O2—S1—K2vii	48.78 (3)
O4i—K1—K2v	86.50 (2)	O7—S1—K2ix	134.65 (4)
O3—K1—K2v	144.54 (2)	O6—S1—K2ix	40.71 (4)
O5ii—K1—K2v	41.328 (19)	O5—S1—K2ix	73.18 (4)
N1ii—K1—K2v	100.27 (2)	O2—S1—K2ix	114.92 (3)
O3ii—K1—K2v	125.21 (2)	K2vii—S1—K2ix	79.729 (8)
O1Wiii—K1—K2v	74.21 (2)	O7—S1—K1ii	134.83 (4)
O1iv—K1—K2v	44.587 (19)	O6—S1—K1ii	109.13 (4)
S1ii—K1—K2v	59.892 (7)	O5—S1—K1ii	37.75 (4)
K1ii—K1—K2v	168.952 (12)	O2—S1—K1ii	69.61 (3)
O4i—K1—K2iii	101.95 (2)	K2vii—S1—K1ii	94.176 (9)
O3—K1—K2iii	86.38 (2)	K2ix—S1—K1ii	79.600 (8)
O5ii—K1—K2iii	46.95 (2)	C3—O1—K1xii	114.60 (7)
N1ii—K1—K2iii	104.70 (2)	C3—O1—K2vi	116.45 (7)
O3ii—K1—K2iii	161.00 (2)	K1xii—O1—K2vi	91.63 (3)
O1Wiii—K1—K2iii	30.91 (2)	C3—O1—H1O	104.4 (12)
O1iv—K1—K2iii	106.07 (2)	K1xii—O1—H1O	95.2 (12)
S1ii—K1—K2iii	48.809 (6)	K2vi—O1—H1O	131.0 (12)
K1ii—K1—K2iii	129.234 (11)	C6—O2—S1	118.44 (7)
K2v—K1—K2iii	61.786 (6)	C6—O2—K2vii	134.86 (7)
O7—K2—O6vi	96.93 (3)	S1—O2—K2vii	105.02 (4)
O7—K2—O1W	163.59 (3)	C7—O3—K1	120.18 (8)
O6vi—K2—O1W	98.84 (3)	C7—O3—K1ii	120.90 (8)
O7—K2—O2vii	80.73 (3)	K1—O3—K1ii	90.00 (3)
O6vi—K2—O2vii	142.83 (3)	C7—O4—K1x	133.68 (8)
O1W—K2—O2vii	83.96 (3)	S1—O5—K1ii	123.68 (5)
O7—K2—O5viii	83.24 (3)	S1—O5—K2viii	138.63 (5)
O6vi—K2—O5viii	81.17 (3)	K1ii—O5—K2viii	97.65 (3)
O1W—K2—O5viii	103.61 (3)	S1—O6—K2ix	118.92 (5)
O2vii—K2—O5viii	134.52 (3)	S1—O6—K2vii	105.82 (5)
O7—K2—O6vii	82.98 (3)	K2ix—O6—K2vii	110.96 (3)
O6vi—K2—O6vii	93.96 (2)	S1—O7—K2	163.84 (6)
O1W—K2—O6vii	91.42 (3)	C6—N1—C2	117.72 (11)
O2vii—K2—O6vii	48.87 (3)	C6—N1—K1ii	119.90 (8)
O5viii—K2—O6vii	164.72 (3)	C2—N1—K1ii	119.70 (8)
O7—K2—O1ix	101.29 (3)	N1—C2—C3	121.82 (11)
O6vi—K2—O1ix	143.63 (3)	N1—C2—C7	116.38 (11)
O1W—K2—O1ix	68.03 (3)	C3—C2—C7	121.79 (11)
O2vii—K2—O1ix	71.74 (3)	O1—C3—C2	121.99 (11)
O5viii—K2—O1ix	70.16 (3)	O1—C3—C4	119.11 (11)
O6vii—K2—O1ix	119.21 (3)	C2—C3—C4	118.90 (11)
O7—K2—S1vii	78.73 (2)	C5—C4—C3	118.94 (11)
O6vi—K2—S1vii	116.72 (2)	C5—C4—H4	120.5
O1W—K2—S1vii	90.11 (2)	C3—C4—H4	120.5
O2vii—K2—S1vii	26.201 (18)	C4—C5—C6	117.37 (11)
O5viii—K2—S1vii	155.84 (2)	C4—C5—H5	121.3
O6vii—K2—S1vii	22.895 (18)	C6—C5—H5	121.3
O1ix—K2—S1vii	97.67 (2)	N1—C6—C5	125.24 (11)
O7—K2—S1vi	109.94 (2)	N1—C6—O2	115.26 (11)
O6vi—K2—S1vi	20.38 (2)	C5—C6—O2	119.36 (11)
O1W—K2—S1vi	84.12 (2)	O3—C7—O4	124.78 (11)
O2vii—K2—S1vi	127.21 (2)	O3—C7—C2	118.96 (11)
O5viii—K2—S1vi	98.27 (2)	O4—C7—C2	116.25 (11)
O6vii—K2—S1vi	80.302 (19)	K2—O1W—K1xiii	116.56 (4)
O1ix—K2—S1vi	145.27 (2)	K2—O1W—H1W	132.9 (13)
S1vii—K2—S1vi	102.902 (10)	K1xiii—O1W—H1W	94.4 (14)
O7—K2—K1xi	116.69 (2)	K2—O1W—H2W	108.4 (13)
O6vi—K2—K1xi	99.86 (2)	K1xiii—O1W—H2W	96.0 (14)
O1W—K2—K1xi	64.86 (2)	H1W—O1W—H2W	102.0 (15)
O2vii—K2—K1xi	114.43 (2)
O7—S1—O2—C6	55.28 (10)	K1ii—S1—O7—K2	142.06 (18)
O6—S1—O2—C6	175.77 (9)	C6—N1—C2—C3	−0.18 (17)
O5—S1—O2—C6	−66.14 (10)	K1ii—N1—C2—C3	−161.62 (9)
K2vii—S1—O2—C6	167.38 (11)	C6—N1—C2—C7	−179.2 (1)
K2ix—S1—O2—C6	−144.57 (8)	K1ii—N1—C2—C7	19.36 (14)
K1ii—S1—O2—C6	−77.22 (8)	K1xii—O1—C3—C2	101.95 (11)
O7—S1—O2—K2vii	−112.10 (5)	K2vi—O1—C3—C2	−152.82 (9)
O6—S1—O2—K2vii	8.39 (5)	K1xii—O1—C3—C4	−77.87 (12)
O5—S1—O2—K2vii	126.48 (5)	K2vi—O1—C3—C4	27.36 (14)
K2ix—S1—O2—K2vii	48.05 (4)	N1—C2—C3—O1	−179.28 (11)
K1ii—S1—O2—K2vii	115.40 (3)	C7—C2—C3—O1	−0.31 (18)
O7—S1—O5—K1ii	−133.28 (6)	N1—C2—C3—C4	0.54 (18)
O6—S1—O5—K1ii	90.99 (7)	C7—C2—C3—C4	179.51 (11)
O2—S1—O5—K1ii	−17.12 (7)	O1—C3—C4—C5	179.41 (11)
K2vii—S1—O5—K1ii	32.58 (8)	C2—C3—C4—C5	−0.41 (18)
K2ix—S1—O5—K1ii	94.73 (5)	C3—C4—C5—C6	−0.03 (18)
O7—S1—O5—K2viii	43.85 (10)	C2—N1—C6—C5	−0.32 (18)
O6—S1—O5—K2viii	−91.88 (9)	K1ii—N1—C6—C5	161.09 (9)
O2—S1—O5—K2viii	160.01 (7)	C2—N1—C6—O2	175.49 (10)
K2vii—S1—O5—K2viii	−150.29 (5)	K1ii—N1—C6—O2	−23.10 (13)
K2ix—S1—O5—K2viii	−88.14 (7)	C4—C5—C6—N1	0.42 (19)
K1ii—S1—O5—K2viii	177.13 (12)	C4—C5—C6—O2	−175.23 (11)
O7—S1—O6—K2ix	−129.41 (6)	S1—O2—C6—N1	94.96 (11)
O5—S1—O6—K2ix	5.50 (8)	K2vii—O2—C6—N1	−102.36 (12)
O2—S1—O6—K2ix	117.46 (5)	S1—O2—C6—C5	−88.96 (12)
K2vii—S1—O6—K2ix	125.55 (7)	K2vii—O2—C6—C5	73.72 (14)
K1ii—S1—O6—K2ix	45.88 (6)	K1—O3—C7—O4	−88.64 (13)
O7—S1—O6—K2vii	105.04 (6)	K1ii—O3—C7—O4	160.99 (9)
O5—S1—O6—K2vii	−120.05 (5)	K1—O3—C7—C2	89.91 (11)
O2—S1—O6—K2vii	−8.09 (5)	K1ii—O3—C7—C2	−20.46 (14)
K2ix—S1—O6—K2vii	−125.55 (7)	K1x—O4—C7—O3	−30.63 (18)
K1ii—S1—O6—K2vii	−79.67 (4)	K1x—O4—C7—C2	150.78 (8)
O6—S1—O7—K2	−44.2 (2)	N1—C2—C7—O3	0.67 (17)
O5—S1—O7—K2	−179.00 (19)	C3—C2—C7—O3	−178.36 (11)
O2—S1—O7—K2	65.0 (2)	N1—C2—C7—O4	179.34 (10)
K2vii—S1—O7—K2	13.6 (2)	C3—C2—C7—O4	0.31 (17)
K2ix—S1—O7—K2	−89.3 (2)

Symmetry codes: (i) x, −y+1/2, z−1/2; (ii) −x, −y+1, −z+1; (iii) x−1, −y+1/2, z−1/2; (iv) −x, y+1/2, −z+1/2; (v) x−1, y, z−1; (vi) −x+1, y−1/2, −z+3/2; (vii) −x+1, −y+1, −z+1; (viii) −x+1, −y+1, −z+2; (ix) −x+1, y+1/2, −z+3/2; (x) x, −y+1/2, z+1/2; (xi) x+1, y, z+1; (xii) −x, y−1/2, −z+1/2; (xiii) x+1, −y+1/2, z+1/2.

Dipotassium 3-hydroxy-6-(sulfonatooxy)pyridine-2-carboxylate monohydrate (I). Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1O···O4	0.843 (19)	1.760 (19)	2.5429 (13)	153.6 (17)
C4—H4···O5i	0.95	2.64	3.3682 (16)	133
C5—H5···O6vii	0.95	2.35	3.2943 (15)	175
O1W—H1W···O3xiv	0.83 (2)	1.97 (2)	2.7852 (13)	165 (2)
O1W—H2W···O4xiii	0.83 (2)	2.09 (2)	2.8910 (14)	161 (2)

Symmetry codes: (i) x, −y+1/2, z−1/2; (vii) −x+1, −y+1, −z+1; (xiii) x+1, −y+1/2, z+1/2; (xiv) x+1, y, z.

3-Hydroxy-6-oxo-1,6-dihydropyridine-2-carboxylic acid (II). Crystal data

C6H5NO4	Dx = 1.693 Mg m−3
Mr = 155.11	Cu Kα radiation, λ = 1.54178 Å
Orthorhombic, Abm2	Cell parameters from 3820 reflections
a = 10.2045 (6) Å	θ = 4.3–78.5°
b = 6.1282 (4) Å	µ = 1.27 mm−1
c = 9.7293 (6) Å	T = 90 K
V = 608.42 (7) Å3	Plate, colourless
Z = 4	0.14 × 0.10 × 0.02 mm
F(000) = 320

3-Hydroxy-6-oxo-1,6-dihydropyridine-2-carboxylic acid (II). Data collection

Bruker D8 Venture dual source diffractometer	700 independent reflections
Radiation source: microsource	690 reflections with I > 2σ(I)
Detector resolution: 7.41 pixels mm-1	Rint = 0.025
φ and ω scans	θmax = 77.8°, θmin = 4.3°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −11→12
Tmin = 0.799, Tmax = 0.971	k = −7→7
4025 measured reflections	l = −11→12

3-Hydroxy-6-oxo-1,6-dihydropyridine-2-carboxylic acid (II). Refinement

Refinement on F2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.024	H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.068	w = 1/[σ2(Fo2) + (0.050P)2 + 0.0512P] where P = (Fo2 + 2Fc2)/3
S = 1.11	(Δ/σ)max = 0.024
700 reflections	Δρmax = 0.20 e Å−3
74 parameters	Δρmin = −0.25 e Å−3
4 restraints	Absolute structure: Refined as a perfect inversion twin
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.5

3-Hydroxy-6-oxo-1,6-dihydropyridine-2-carboxylic acid (II). Special details

Experimental. The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder. It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994; Parkin & Hope, 1998). Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals.

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refined as a two-component perfect inversion twin.

3-Hydroxy-6-oxo-1,6-dihydropyridine-2-carboxylic acid (II). Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
N1	0.3327 (2)	0.250000	0.44103 (19)	0.0146 (4)	0.953 (3)
H1N	0.421 (3)	0.25000 (2)	0.3976 (15)	0.017*	0.953 (3)
C2	0.3260 (2)	0.250000	0.5827 (2)	0.0131 (5)	0.953 (3)
C3	0.2033 (3)	0.250000	0.6458 (3)	0.0148 (5)	0.953 (3)
O1	0.19022 (19)	0.250000	0.7831 (2)	0.0188 (4)	0.953 (3)
H1O	0.269 (4)	0.25000 (2)	0.8219 (19)	0.028*	0.953 (3)
C4	0.0919 (3)	0.250000	0.5626 (3)	0.0194 (6)	0.953 (3)
H4	0.007769	0.250000	0.604343	0.023*	0.953 (3)
C5	0.1011 (2)	0.250000	0.4226 (3)	0.0191 (5)	0.953 (3)
H5	0.024166	0.250000	0.367711	0.023*	0.953 (3)
C6	0.2280 (2)	0.250000	0.3592 (3)	0.0154 (5)	0.953 (3)
O2	0.2369 (2)	0.250000	0.22580 (18)	0.0186 (4)	0.953 (3)
H2O	0.326 (3)	0.25000 (2)	0.200 (1)	0.028*	0.953 (3)
C7	0.4488 (2)	0.250000	0.6616 (2)	0.0158 (5)	0.953 (3)
O3	0.55590 (18)	0.250000	0.59684 (19)	0.0182 (4)	0.953 (3)
O4	0.43850 (17)	0.250000	0.7906 (2)	0.0224 (5)	0.953 (3)
N1'	0.3327 (2)	0.250000	0.55897 (19)	0.0131 (5)	0.047 (3)
H1'N	0.421 (3)	0.25000 (2)	0.6024 (16)	0.016*	0.047 (3)
C2'	0.3260 (2)	0.250000	0.4173 (2)	0.0146 (4)	0.047 (3)
C3'	0.2033 (3)	0.250000	0.3542 (3)	0.0154 (5)	0.047 (3)
O1'	0.19022 (19)	0.250000	0.2169 (2)	0.0186 (4)	0.047 (3)
H1'O	0.269 (4)	0.25000 (2)	0.178 (2)	0.028*	0.047 (3)
C4'	0.0919 (3)	0.250000	0.4374 (3)	0.0191 (5)	0.047 (3)
H4'	0.007769	0.250000	0.395657	0.023*	0.047 (3)
C5'	0.1011 (2)	0.250000	0.5774 (3)	0.0194 (6)	0.047 (3)
H5'	0.024166	0.250000	0.632289	0.023*	0.047 (3)
C6'	0.2280 (2)	0.250000	0.6408 (3)	0.0148 (5)	0.047 (3)
O2'	0.2369 (2)	0.250000	0.77420 (18)	0.0188 (4)	0.047 (3)
H2'O	0.326 (4)	0.25000 (2)	0.8001 (11)	0.028*	0.047 (3)
C7'	0.4488 (2)	0.250000	0.3384 (2)	0.0158 (5)	0.047 (3)
O3'	0.55590 (18)	0.250000	0.40316 (19)	0.0182 (4)	0.047 (3)
O4'	0.43850 (17)	0.250000	0.2094 (2)	0.0224 (5)	0.047 (3)

3-Hydroxy-6-oxo-1,6-dihydropyridine-2-carboxylic acid (II). Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
N1	0.0142 (9)	0.0190 (8)	0.0105 (11)	0.000	0.0000 (9)	0.000
C2	0.0127 (12)	0.0155 (9)	0.0112 (10)	0.000	−0.0016 (9)	0.000
C3	0.0151 (10)	0.0180 (11)	0.0113 (11)	0.000	0.0040 (8)	0.000
O1	0.0167 (9)	0.0274 (9)	0.0123 (8)	0.000	0.0041 (7)	0.000
C4	0.0159 (12)	0.0222 (10)	0.0202 (13)	0.000	0.0025 (10)	0.000
C5	0.0150 (11)	0.0259 (10)	0.0166 (10)	0.000	−0.0017 (9)	0.000
C6	0.0150 (12)	0.017 (1)	0.0141 (12)	0.000	−0.0031 (9)	0.000
O2	0.0163 (9)	0.0295 (8)	0.0101 (8)	0.000	0.0003 (6)	0.000
C7	0.0167 (10)	0.0196 (12)	0.0112 (14)	0.000	−0.0002 (9)	0.000
O3	0.0141 (8)	0.0268 (7)	0.0137 (8)	0.000	−0.0008 (6)	0.000
O4	0.0148 (12)	0.0368 (11)	0.0157 (9)	0.000	−0.0014 (7)	0.000
N1'	0.0127 (12)	0.0155 (9)	0.0112 (10)	0.000	−0.0016 (9)	0.000
C2'	0.0142 (9)	0.0190 (8)	0.0105 (11)	0.000	0.0000 (9)	0.000
C3'	0.0150 (12)	0.017 (1)	0.0141 (12)	0.000	−0.0031 (9)	0.000
O1'	0.0163 (9)	0.0295 (8)	0.0101 (8)	0.000	0.0003 (6)	0.000
C4'	0.0150 (11)	0.0259 (10)	0.0166 (10)	0.000	−0.0017 (9)	0.000
C5'	0.0159 (12)	0.0222 (10)	0.0202 (13)	0.000	0.0025 (10)	0.000
C6'	0.0151 (10)	0.0180 (11)	0.0113 (11)	0.000	0.0040 (8)	0.000
O2'	0.0167 (9)	0.0274 (9)	0.0123 (8)	0.000	0.0041 (7)	0.000
C7'	0.0167 (10)	0.0196 (12)	0.0112 (14)	0.000	−0.0002 (9)	0.000
O3'	0.0141 (8)	0.0268 (7)	0.0137 (8)	0.000	−0.0008 (6)	0.000
O4'	0.0148 (12)	0.0368 (11)	0.0157 (9)	0.000	−0.0014 (7)	0.000

3-Hydroxy-6-oxo-1,6-dihydropyridine-2-carboxylic acid (II). Geometric parameters (Å, º)

N1—C6	1.333 (3)	N1'—C6'	1.333 (3)
N1—C2	1.381 (3)	N1'—C2'	1.381 (3)
N1—H1N	0.99 (3)	N1'—H1'N	0.99 (4)
C2—C3	1.394 (3)	C2'—C3'	1.394 (3)
C2—C7	1.470 (3)	C2'—C7'	1.470 (3)
C3—O1	1.342 (3)	C3'—O1'	1.342 (3)
C3—C4	1.395 (4)	C3'—C4'	1.395 (4)
O1—H1O	0.89 (4)	O1'—H1'O	0.89 (4)
C4—C5	1.365 (3)	C4'—C5'	1.365 (3)
C4—H4	0.9500	C4'—H4'	0.9500
C5—C6	1.434 (3)	C5'—C6'	1.434 (3)
C5—H5	0.9500	C5'—H5'	0.9500
C6—O2	1.301 (3)	C6'—O2'	1.301 (3)
O2—H2O	0.94 (4)	O2'—H2'O	0.94 (4)
C7—O4	1.259 (3)	C7'—O4'	1.259 (3)
C7—O3	1.262 (3)	C7'—O3'	1.262 (3)
C6—N1—C2	123.81 (19)	C6'—N1'—C2'	123.81 (19)
C6—N1—H1N	118.1	C6'—N1'—H1'N	118.1
C2—N1—H1N	118.1	C2'—N1'—H1'N	118.1
N1—C2—C3	119.0 (2)	N1'—C2'—C3'	119.0 (2)
N1—C2—C7	118.6 (2)	N1'—C2'—C7'	118.6 (2)
C3—C2—C7	122.41 (19)	C3'—C2'—C7'	122.41 (19)
O1—C3—C2	121.8 (2)	O1'—C3'—C2'	121.8 (2)
O1—C3—C4	119.8 (2)	O1'—C3'—C4'	119.8 (2)
C2—C3—C4	118.4 (2)	C2'—C3'—C4'	118.4 (2)
C3—O1—H1O	109.5	C3'—O1'—H1'O	109.5
C5—C4—C3	121.5 (2)	C5'—C4'—C3'	121.5 (2)
C5—C4—H4	119.2	C5'—C4'—H4'	119.2
C3—C4—H4	119.2	C3'—C4'—H4'	119.2
C4—C5—C6	119.5 (2)	C4'—C5'—C6'	119.5 (2)
C4—C5—H5	120.3	C4'—C5'—H5'	120.3
C6—C5—H5	120.3	C6'—C5'—H5'	120.3
O2—C6—N1	122.7 (2)	O2'—C6'—N1'	122.7 (2)
O2—C6—C5	119.5 (2)	O2'—C6'—C5'	119.5 (2)
N1—C6—C5	117.8 (2)	N1'—C6'—C5'	117.8 (2)
C6—O2—H2O	109.5	C6'—O2'—H2'O	109.5
O4—C7—O3	124.8 (2)	O4'—C7'—O3'	124.8 (2)
O4—C7—C2	116.7 (2)	O4'—C7'—C2'	116.7 (2)
O3—C7—C2	118.53 (19)	O3'—C7'—C2'	118.53 (19)
C6—N1—C2—C3	0.000 (1)	C6'—N1'—C2'—C3'	0.000 (1)
C6—N1—C2—C7	180.000 (1)	C6'—N1'—C2'—C7'	180.000 (1)
N1—C2—C3—O1	180.000 (1)	N1'—C2'—C3'—O1'	180.000 (1)
C7—C2—C3—O1	0.000 (1)	C7'—C2'—C3'—O1'	0.000 (1)
N1—C2—C3—C4	0.000 (1)	N1'—C2'—C3'—C4'	0.000 (1)
C7—C2—C3—C4	180.000 (1)	C7'—C2'—C3'—C4'	180.000 (1)
O1—C3—C4—C5	180.000 (1)	O1'—C3'—C4'—C5'	180.000 (1)
C2—C3—C4—C5	0.000 (1)	C2'—C3'—C4'—C5'	0.000 (1)
C3—C4—C5—C6	0.000 (1)	C3'—C4'—C5'—C6'	0.000 (1)
C2—N1—C6—O2	180.000 (1)	C2'—N1'—C6'—O2'	180.000 (1)
C2—N1—C6—C5	0.000 (1)	C2'—N1'—C6'—C5'	0.000 (1)
C4—C5—C6—O2	180.000 (1)	C4'—C5'—C6'—O2'	180.000 (1)
C4—C5—C6—N1	0.000 (1)	C4'—C5'—C6'—N1'	0.000 (1)
N1—C2—C7—O4	180.000 (1)	N1'—C2'—C7'—O4'	180.000 (1)
C3—C2—C7—O4	0.000 (1)	C3'—C2'—C7'—O4'	0.000 (1)
N1—C2—C7—O3	0.000 (1)	N1'—C2'—C7'—O3'	0.000 (1)
C3—C2—C7—O3	180.000 (1)	C3'—C2'—C7'—O3'	180.000 (1)

3-Hydroxy-6-oxo-1,6-dihydropyridine-2-carboxylic acid (II). Hydrogen-bond geometry (Å, º)

The D···A distance for C4—H4···O2ⁱⁱⁱ is rather long, but within the bounds noted by Desiraju & Steiner (1999).

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N···O4i	0.99	1.77	2.755 (3)	169
O1—H1O···O4	0.89	1.76	2.535 (3)	145
C5—H5···O1ii	0.95	2.34	3.269 (3)	166
C4—H4···O2iii	0.95	2.76	3.712 (4)	180
O2—H2O···O3i	0.94	1.57	2.459 (3)	156
O2—H2O···O4i	0.94	2.56	3.372 (3)	144

Symmetry codes: (i) −x+1, −y+1/2, z−1/2; (ii) −x, −y+1/2, z−1/2; (iii) −x, −y+1/2, z+1/2.

Funding Statement

This work was funded by OSU Emeritus Academy grant .