Chemical Context   {#sec1}
==================

As a study in crystal engineering utilizing hydrogen bonding between disparate mol­ecules (Desiraju, 2003[@bb5]), we have been investigating the cocrystallization of various pyridine compounds with benzene carb­oxy­lic acids (Staun & Oliver, 2012[@bb15]). From previous work, 4-hy­droxy­pyridine undergoes hydrogen migration from the hy­droxy O to the pyridine N atom, yielding 4-pyridone (Tyl *et al.*, 2008[@bb16]). We were surprised to find that in the case of 4-hy­droxy­pyridin-1-ium 3,5-di­carb­oxy­benzoate, (**I**), an H atom is abstracted from one carb­oxy­lic acid group, yielding a pyridinium salt. This result allows for the hy­droxy O and pyridine N atom to both act as hydrogen-bond donors, rather than the donor/acceptor situation of the 4-pyridone species. These two mol­ecules have been incorporated as linker species in metal--organic frameworks (Guo *et al.*, 2011[@bb8]).

Structural Commentary   {#sec2}
=======================

The structure of (**I**) shows that the 4-hy­droxy­pyridine has abstracted an H atom from the benzene­tri­carb­oxy­lic acid, yielding a pyridinium cation and a carboxyl­ate anion (Fig. 1[▸](#fig1){ref-type="fig"}). Bond distances about the pyridine ring show some localization of the bonds: C1---C2 and C4---C5 are slightly shorter than the ideal aromatic distance \[1.367 (3) and 1.369 (3) Å, respectively, *cf.* 1.390 Å for an aromatic C---C bond\]. The N1---C1 and N1---C5 distances are typical for an aromatic N atom \[1.345 (3) and 1.348 (3) Å, respectively\]. The remaining bonds within the ring display typical aromatic distances \[C2---C3 = 1.405 (3) Å and C3---C4 = 1.402 (3) Å\]. The C3---O1 distance of 1.326 (2) Å is typical for a hy­droxy O atom bound to an aromatic ring. Bond angles within the pyridine ring are unexceptional.

Two of the three carb­oxy­lic acid groups show distinct single- and double-bond character \[C12---O3 = 1.305 (3) Å and C14---O7 = 1.332 (3) Å; C12---O2 = 1.224 (2) Å and C14---O6 = 1.204 (3) Å\]. The remaining carboxyl­ate group displays C---O bond distances that are similar to each other and indicate delocalization of the C---O bonds \[1.268 (3) and 1.249 (2) Å for C13---O4 and C4---O5, respectively\], supporting the proposed single negative charge on the benzene­tri­carb­oxy­lic acid mol­ecule. This is further supported by the presence of H atoms, located in a difference Fourier map, on atoms O3 and O7. Bond distances and angles within the benzene ring are as expected.

Supra­molecular Features   {#sec3}
==========================

The local inter­molecular contacts consist of the pyridinium cation forming a hydrogen bond from the hy­droxy group to the anionic carboxyl­ate group (O1⋯O5; see Table 1[▸](#table1){ref-type="table"} for detailed contacts) and from pyridine atom N1 to carboxyl­ate atom O4^i^ \[symmetry code: (i) −*x* + , *y* + , *z* − \]. Carb­oxy­lic acid atoms O3 and O7 are donors for hydrogen bonds to atoms O4^ii^ and O2^iii^, respectively \[symmetry codes: (ii) −*x*, −*y*, *z* − ; (iii) −*x*, −*y* + 1, *z* + \]. Since these hydrogen bonds extend over several mol­ecules, an extensive hydrogen-bonded network exists in this structure.

Pertinent features of this extended network are an (28) ring comprised of 3,5-di­carb­oxy­benzoate ions (Fig. 2[▸](#fig2){ref-type="fig"}) (Bernstein *et al.*, 1995[@bb1]). The carb­oxy­lic acid groups are involved in the hydrogen bonding within this ring. There is also an (44) ring of 3,5-di­carb­oxy­benzoate ions, that incorporate a different chain of carb­oxy­lic acid groups. These rings are bridged by the 4-hy­droxy­pyridinium cations resulting in the three-dimensional network. The hydrogen bonds within the structure are surprisingly strong, with O---H⋯O and N---H⋯O distances ranging from 2.533 (2) to 2.700 (2) Å (Table 1[▸](#table1){ref-type="table"}).

The cations and anions form homogeneous π-stacked columns parallel to the *c* axis, that is, 4-pyridinium cations stacking with other cations and 3,5-di­carb­oxy­benzoate anions stacking with other anions. The centroid-to-centroid distances for both the pyridinium and the di­carb­oxy­benzoate inter­actions are 3.6206 (13) Å, *i.e.* the *c*-axis spacing. The centroid-to-perpendicular distances are 3.3629 (9) Å for the cation and 3.4372 (9) Å for the anion. Both measurements are within accepted π--π contact ranges (see Table 2[▸](#table2){ref-type="table"}; Spek, 2009[@bb14]).

Database Survey   {#sec4}
=================

A search of the Cambridge Structural Database (CSD, Version 5.36 plus 3 updates; Groom & Allen, 2014[@bb7]) for 4-hy­droxy­pyridine and benzene­tri­carb­oxy­lic acid gave only five hits. In the compound that is most closely related to the title compound, namely benzene-1,3,5-tri­carb­oxy­lic acid pyridin-4(1*H*)-one (Campos-Gaxiola *et al.*, 2014[@bb3]), there are three mol­ecules of 4-pyridone present in the asymmetric unit. Benzene­tri­carb­oxy­lic acid and a tetra­kis­\[(pyridin-4-yl­oxy)meth­yl\]methane moiety (incorporating a 4-hy­droxy­pyridine functionality) have been utilized in the devlopment of frameworks incorporating copper and cadmium (Guo *et al.*, 2011[@bb8]).

Synthesis and Crystallization   {#sec5}
===============================

To a solution of benzene-1,3,5-tri­carb­oxy­lic acid (0.035 g, 1.24 mmol) in MeOH (3 ml) in a 20 ml vial was added a solution of 4-hy­droxy­pyridine (0.0218 g, 1.77 mmol) in MeOH (3 ml). The mixture was shaken vigorously, covered with perforated Parafilm and allowed to evaporate slowly over a period of 5 d, yielding colorless rod-like crystals.

Refinement   {#sec6}
============

Crystal data, data collection and structure refinement details are summarized in Table 3[▸](#table3){ref-type="table"}. Carb­oxy­lic, hy­droxy, and pyridinium H atoms were initally located in a difference Fourier map. H atoms on the 4-hy­droxy­pyridinium cation were refined freely. H atoms on the carb­oxy­lic acid groups were included with refined coordinates and atomic displacement parameters tied to that of the O atom to which they are bonded. C---H hydrogens were included in idealized positions riding on the C atom to which they are bonded, with C---H distances constrained to 0.95 Å and *U* ~iso~(H) = 1.2 *U* ~eq~(C).

The compound is achiral, but crystallizes with a noncentrosymmetric, polar space group. The Flack *x* parameter refined to 0.20 (8), which suggests the possibility of a small amount of inversion twinnning (Parsons *et al.*, 2013[@bb11]), but the strength of the anomalous signal is very weak. We compared both a model twinned by inversion and the untwinned model, and there was no significant difference. We therefore elected to model the structure without inclusion of a twin component.

Supplementary Material
======================

Crystal structure: contains datablock(s) I, New_Global_Publ_Block. DOI: [10.1107/S2056989015011780/pk2555sup1.cif](http://dx.doi.org/10.1107/S2056989015011780/pk2555sup1.cif)

Structure factors: contains datablock(s) I. DOI: [10.1107/S2056989015011780/pk2555Isup2.hkl](http://dx.doi.org/10.1107/S2056989015011780/pk2555Isup2.hkl)

###### 

Click here for additional data file.

Supporting information file. DOI: [10.1107/S2056989015011780/pk2555Isup3.cml](http://dx.doi.org/10.1107/S2056989015011780/pk2555Isup3.cml)

CCDC reference: [1407819](http://scripts.iucr.org/cgi-bin/cr.cgi?rm=csd&csdid=1407819)

Additional supporting information: [crystallographic information](http://scripts.iucr.org/cgi-bin/sendsupfiles?pk2555&file=pk2555sup0.html&mime=text/html); [3D view](http://scripts.iucr.org/cgi-bin/sendcif?pk2555sup1&Qmime=cif); [checkCIF report](http://scripts.iucr.org/cgi-bin/paper?pk2555&checkcif=yes)

We thank the University of Notre Dame for generous support of the micro-focus source used in this project. SLS thanks the Indiana Academy of Science for a Junior Research Grant and the Henderson lab for kind donation of materials and research space.

Crystal data {#tablewrapcrystaldatalong}
============

  ------------------------------- ---------------------------------------
  C~5~H~6~NO^+^·C~9~H~5~O~6~^−^   *D*~x~ = 1.563 Mg m^−3^
  *M~r~* = 305.24                 Cu *K*α radiation, λ = 1.54184 Å
  Orthorhombic, *Pna*2~1~         Cell parameters from 2373 reflections
  *a* = 29.3465 (10) Å            θ = 3.4--71.2°
  *b* = 12.2113 (5) Å             µ = 1.10 mm^−1^
  *c* = 3.6206 (2) Å              *T* = 122 K
  *V* = 1297.47 (10) Å^3^         Rod, colorless
  *Z* = 4                         0.11 × 0.06 × 0.06 mm
  *F*(000) = 632                  
  ------------------------------- ---------------------------------------

Data collection {#tablewrapdatacollectionlong}
===============

  ------------------------------------------------------------------- --------------------------------------
  Bruker APEXII diffractometer                                        2322 independent reflections
  Radiation source: Incoatec micro-focus                              2172 reflections with *I* \> 2σ(*I*)
  Detector resolution: 8.33 pixels mm^-1^                             *R*~int~ = 0.018
  combination of ω and φ--scans                                       θ~max~ = 71.2°, θ~min~ = 3.0°
  Absorption correction: numerical *SADABS* (Krause *et al.*, 2015)   *h* = −27→35
  *T*~min~ = 0.694, *T*~max~ = 0.753                                  *k* = −13→14
  6000 measured reflections                                           *l* = −4→4
  ------------------------------------------------------------------- --------------------------------------

Refinement {#tablewraprefinementdatalong}
==========

  ---------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------
  Refinement on *F*^2^                                             Secondary atom site location: difference Fourier map
  Least-squares matrix: full                                       Hydrogen site location: mixed
  *R*\[*F*^2^ \> 2σ(*F*^2^)\] = 0.028                              H atoms treated by a mixture of independent and constrained refinement
  *wR*(*F*^2^) = 0.071                                             *w* = 1/\[σ^2^(*F*~o~^2^) + (0.0441*P*)^2^ + 0.1815*P*\] where *P* = (*F*~o~^2^ + 2*F*~c~^2^)/3
  *S* = 1.05                                                       (Δ/σ)~max~ = 0.001
  2322 reflections                                                 Δρ~max~ = 0.15 e Å^−3^
  213 parameters                                                   Δρ~min~ = −0.19 e Å^−3^
  1 restraint                                                      Absolute structure: Flack *x* determined using 786 quotients \[(I+)-(I-)\]/\[(I+)+(I-)\] (Parsons *et al.*, 2013)
  Primary atom site location: structure-invariant direct methods   Absolute structure parameter: 0.20 (8)
  ---------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------

Special details {#specialdetails}
===============

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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å^2^) {#tablewrapcoords}
==================================================================================================

  ------ -------------- --------------- ------------ -------------------- --
         *x*            *y*             *z*          *U*~iso~\*/*U*~eq~   
  O1     0.23997 (5)    0.04165 (12)    0.3072 (5)   0.0195 (4)           
  H1O    0.2112 (11)    0.062 (3)       0.405 (11)   0.057 (11)\*         
  N1     0.32385 (6)    0.30371 (16)    0.1767 (5)   0.0181 (4)           
  H1N    0.3450 (9)     0.365 (3)       0.166 (11)   0.048 (10)\*         
  C1     0.33796 (7)    0.2029 (2)      0.0796 (6)   0.0186 (5)           
  H1A    0.3679         0.1933          −0.0157      0.022\*              
  C2     0.30997 (6)    0.11410 (19)    0.1159 (7)   0.0169 (4)           
  H2A    0.3201         0.0433          0.0444       0.020\*              
  C3     0.26602 (6)    0.12932 (17)    0.2608 (6)   0.0145 (4)           
  C4     0.25177 (7)    0.23551 (18)    0.3521 (6)   0.0160 (4)           
  H4A    0.2218         0.2481          0.4424       0.019\*              
  C5     0.28155 (7)    0.32085 (18)    0.3095 (6)   0.0174 (4)           
  H5A    0.2723         0.3929          0.3741       0.021\*              
  O2     −0.06609 (4)   0.25817 (12)    0.5787 (5)   0.0182 (3)           
  O3     −0.04180 (5)   0.08670 (13)    0.5070 (5)   0.0215 (4)           
  H3O    −0.0703 (9)    0.067 (2)       0.417 (9)    0.032\*              
  O4     0.11707 (4)    −0.02696 (12)   0.7027 (5)   0.0196 (4)           
  O5     0.16718 (4)    0.10740 (13)    0.6111 (6)   0.0252 (4)           
  O6     0.11934 (5)    0.47723 (13)    1.1519 (5)   0.0220 (4)           
  O7     0.05337 (5)    0.53129 (13)    0.8992 (5)   0.0204 (4)           
  H7O    0.0611 (8)     0.598 (3)       0.987 (9)    0.031\*              
  C6     0.01288 (6)    0.21569 (17)    0.6846 (6)   0.0127 (4)           
  C7     0.04618 (7)    0.13490 (17)    0.6462 (6)   0.0128 (4)           
  H7A    0.0380         0.0638          0.5628       0.015\*              
  C8     0.09153 (6)    0.15874 (17)    0.7304 (6)   0.0133 (4)           
  C9     0.10330 (6)    0.26321 (18)    0.8480 (6)   0.0142 (4)           
  H9A    0.1341         0.2795          0.9056       0.017\*              
  C10    0.07019 (6)    0.34410 (18)    0.8820 (6)   0.0131 (4)           
  C11    0.02463 (6)    0.32021 (17)    0.8012 (6)   0.0134 (4)           
  H11A   0.0019         0.3751          0.8261       0.016\*              
  C12    −0.03557 (6)   0.18988 (17)    0.5860 (6)   0.0138 (4)           
  C13    0.12794 (6)    0.07323 (18)    0.6782 (6)   0.0150 (4)           
  C14    0.08419 (7)    0.45642 (18)    0.9953 (6)   0.0156 (5)           
  ------ -------------- --------------- ------------ -------------------- --

Atomic displacement parameters (Å^2^) {#tablewrapadps}
=====================================

  ----- ------------- ------------- ------------- ------------- ------------- --------------
        *U*^11^       *U*^22^       *U*^33^       *U*^12^       *U*^13^       *U*^23^
  O1    0.0143 (7)    0.0151 (8)    0.0291 (9)    −0.0021 (6)   0.0044 (7)    −0.0012 (7)
  N1    0.0152 (8)    0.0205 (10)   0.0185 (9)    −0.0067 (8)   −0.0004 (7)   0.0002 (8)
  C1    0.0122 (9)    0.0270 (12)   0.0166 (11)   −0.0006 (9)   0.0006 (8)    0.0004 (10)
  C2    0.0142 (9)    0.0208 (11)   0.0157 (10)   0.0018 (8)    0.0008 (8)    −0.0017 (9)
  C3    0.0133 (9)    0.0166 (11)   0.0135 (10)   −0.0014 (8)   −0.0015 (8)   −0.0005 (9)
  C4    0.0132 (9)    0.0180 (11)   0.0170 (10)   0.0010 (8)    0.0016 (8)    −0.0006 (10)
  C5    0.0190 (10)   0.0171 (11)   0.0161 (11)   −0.0012 (8)   0.0000 (8)    −0.0007 (9)
  O2    0.0116 (6)    0.0130 (7)    0.0299 (9)    0.0009 (5)    −0.0012 (6)   0.0000 (7)
  O3    0.0124 (7)    0.0138 (8)    0.0382 (11)   −0.0007 (6)   −0.0073 (7)   −0.0059 (7)
  O4    0.0122 (6)    0.0134 (7)    0.0331 (9)    0.0017 (6)    0.0034 (6)    0.0022 (7)
  O5    0.0120 (7)    0.0220 (9)    0.0417 (11)   −0.0023 (6)   0.0085 (7)    −0.0032 (8)
  O6    0.0179 (7)    0.0204 (8)    0.0277 (9)    −0.0050 (6)   −0.0047 (7)   −0.0032 (7)
  O7    0.0179 (8)    0.0126 (8)    0.0308 (10)   0.0003 (6)    −0.0019 (7)   −0.0057 (7)
  C6    0.0125 (8)    0.0126 (10)   0.0130 (9)    0.0008 (8)    0.0013 (7)    0.0010 (8)
  C7    0.0142 (8)    0.0112 (10)   0.0129 (10)   −0.0014 (7)   0.0007 (7)    0.0018 (8)
  C8    0.0128 (9)    0.0143 (10)   0.0129 (10)   −0.0005 (8)   0.0009 (8)    0.0022 (9)
  C9    0.0111 (9)    0.0166 (11)   0.0150 (10)   −0.0017 (8)   −0.0008 (8)   0.0002 (9)
  C10   0.0139 (9)    0.0136 (10)   0.0117 (10)   −0.0017 (8)   0.0003 (8)    0.0002 (8)
  C11   0.0139 (9)    0.0138 (10)   0.0127 (10)   0.0024 (8)    0.0004 (8)    0.0006 (8)
  C12   0.0152 (9)    0.0125 (10)   0.0137 (10)   0.0000 (8)    −0.0002 (8)   0.0009 (9)
  C13   0.0129 (9)    0.0159 (11)   0.0164 (10)   0.0001 (8)    0.0000 (8)    −0.0007 (9)
  C14   0.0139 (9)    0.0168 (11)   0.0162 (11)   −0.0012 (8)   0.0026 (9)    −0.0015 (9)
  ----- ------------- ------------- ------------- ------------- ------------- --------------

Geometric parameters (Å, º) {#tablewrapgeomlong}
===========================

  --------------------- -------------- ---------------------- --------------
  O1---C3               1.326 (2)      O5---C13               1.249 (2)
  O1---H1O              0.95 (3)       O6---C14               1.204 (3)
  N1---C1               1.345 (3)      O7---C14               1.332 (3)
  N1---C5               1.348 (3)      O7---H7O               0.90 (3)
  N1---H1N              0.98 (3)       C6---C11               1.388 (3)
  C1---C2               1.367 (3)      C6---C7                1.396 (3)
  C1---H1A              0.9500         C6---C12               1.499 (3)
  C2---C3               1.405 (3)      C7---C8                1.396 (3)
  C2---H2A              0.9500         C7---H7A               0.9500
  C3---C4               1.402 (3)      C8---C9                1.389 (3)
  C4---C5               1.369 (3)      C8---C13               1.506 (3)
  C4---H4A              0.9500         C9---C10               1.391 (3)
  C5---H5A              0.9500         C9---H9A               0.9500
  O2---C12              1.224 (2)      C10---C11              1.399 (3)
  O3---C12              1.305 (3)      C10---C14              1.489 (3)
  O3---H3O              0.93 (3)       C11---H11A             0.9500
  O4---C13              1.268 (3)                             
                                                              
  C3---O1---H1O         111 (2)        C6---C7---C8           119.90 (19)
  C1---N1---C5          121.26 (19)    C6---C7---H7A          120.1
  C1---N1---H1N         119.9 (18)     C8---C7---H7A          120.1
  C5---N1---H1N         118.7 (18)     C9---C8---C7           119.70 (18)
  N1---C1---C2          121.04 (19)    C9---C8---C13          119.95 (17)
  N1---C1---H1A         119.5          C7---C8---C13          120.29 (19)
  C2---C1---H1A         119.5          C8---C9---C10          120.39 (18)
  C1---C2---C3          118.9 (2)      C8---C9---H9A          119.8
  C1---C2---H2A         120.6          C10---C9---H9A         119.8
  C3---C2---H2A         120.6          C9---C10---C11         120.1 (2)
  O1---C3---C4          123.01 (18)    C9---C10---C14         119.06 (17)
  O1---C3---C2          118.02 (19)    C11---C10---C14        120.86 (18)
  C4---C3---C2          118.97 (19)    C6---C11---C10         119.51 (18)
  C5---C4---C3          119.15 (19)    C6---C11---H11A        120.2
  C5---C4---H4A         120.4          C10---C11---H11A       120.2
  C3---C4---H4A         120.4          O2---C12---O3          123.40 (18)
  N1---C5---C4          120.7 (2)      O2---C12---C6          123.77 (19)
  N1---C5---H5A         119.7          O3---C12---C6          112.83 (17)
  C4---C5---H5A         119.7          O5---C13---O4          124.62 (19)
  C12---O3---H3O        117.1 (18)     O5---C13---C8          116.56 (19)
  C14---O7---H7O        110.8 (17)     O4---C13---C8          118.82 (17)
  C11---C6---C7         120.42 (17)    O6---C14---O7          124.0 (2)
  C11---C6---C12        120.09 (17)    O6---C14---C10         124.07 (19)
  C7---C6---C12         119.45 (18)    O7---C14---C10         111.89 (17)
                                                              
  C5---N1---C1---C2     −0.7 (3)       C7---C6---C11---C10    −0.3 (3)
  N1---C1---C2---C3     −0.7 (3)       C12---C6---C11---C10   −178.11 (19)
  C1---C2---C3---O1     −177.5 (2)     C9---C10---C11---C6    −0.5 (3)
  C1---C2---C3---C4     2.1 (3)        C14---C10---C11---C6   177.9 (2)
  O1---C3---C4---C5     177.3 (2)      C11---C6---C12---O2    4.8 (4)
  C2---C3---C4---C5     −2.2 (3)       C7---C6---C12---O2     −173.0 (2)
  C1---N1---C5---C4     0.5 (3)        C11---C6---C12---O3    −175.4 (2)
  C3---C4---C5---N1     0.9 (3)        C7---C6---C12---O3     6.7 (3)
  C11---C6---C7---C8    0.9 (3)        C9---C8---C13---O5     −25.5 (3)
  C12---C6---C7---C8    178.7 (2)      C7---C8---C13---O5     151.8 (2)
  C6---C7---C8---C9     −0.7 (3)       C9---C8---C13---O4     154.9 (2)
  C6---C7---C8---C13    −178.04 (19)   C7---C8---C13---O4     −27.8 (3)
  C7---C8---C9---C10    −0.1 (3)       C9---C10---C14---O6    −20.3 (3)
  C13---C8---C9---C10   177.3 (2)      C11---C10---C14---O6   161.3 (2)
  C8---C9---C10---C11   0.7 (3)        C9---C10---C14---O7    159.0 (2)
  C8---C9---C10---C14   −177.74 (19)   C11---C10---C14---O7   −19.4 (3)
  --------------------- -------------- ---------------------- --------------

Hydrogen-bond geometry (Å, º) {#tablewraphbondslong}
=============================

  ---------------------- ---------- ---------- ----------- ---------------
  *D*---H···*A*          *D*---H    H···*A*    *D*···*A*   *D*---H···*A*
  O1---H1*O*···O5        0.95 (3)   1.59 (4)   2.533 (2)   171 (3)
  N1---H1*N*···O4^i^     0.98 (3)   1.73 (3)   2.700 (2)   173 (4)
  O3---H3*O*···O4^ii^    0.93 (3)   1.65 (3)   2.574 (2)   172 (3)
  O7---H7*O*···O2^iii^   0.90 (3)   1.79 (3)   2.678 (2)   166 (3)
  ---------------------- ---------- ---------- ----------- ---------------

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

![Labeling scheme for (**I**). Displacement ellipsoids are depicted at the 50% probability level. The inter-ion hydrogen bond is shown as a dashed red line.](e-71-00861-fig1){#fig1}

![A view of (**I**) approximately along the crystallographic *c* axis. Color code: blue represents the (28) ring, purple the (44) ring, and green the bridging 4-hy­droxy­pyridinium cations.](e-71-00861-fig2){#fig2}

###### Hydrogen-bond geometry (, )

  *D*H*A*        *D*H      H*A*      *D* *A*    *D*H*A*
  -------------- --------- --------- ---------- ---------
  O1H1OO5        0.95(3)   1.59(4)   2.533(2)   171(3)
  N1H1NO4^i^     0.98(3)   1.73(3)   2.700(2)   173(4)
  O3H3OO4^ii^    0.93(3)   1.65(3)   2.574(2)   172(3)
  O7H7OO2^iii^   0.90(3)   1.79(3)   2.678(2)   166(3)

Symmetry codes: (i) ; (ii) ; (iii) .

###### -stacking interactions within (I)

  Interaction     *Cg* *Cg* ()   *Cg*perp ()
  --------------- -------------- -------------
  *Cg*1*Cg*1^i^   3.6206(13)     3.4373(9)
  *Cg*2*Cg*2^i^   3.6206(13)     3.3627(9)

*Cg*1 is the centroid of the 3,5-dicarboxybenzoate ring, *Cg*2 is the centroid of the 4-hydroxypyridinium ring \[symmetry code: (i) *x*, *y*, 1+*z*\], *Cg* *Cg* is the centroid-to-centroid distance, and *Cg*perp is the distance to the plane perpendicular to the ring centroid.

###### Experimental details

  ------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------
  Crystal data                                                              
  Chemical formula                                                          C~5~H~6~NO^+^C~9~H~5~O~6~
  *M* ~r~                                                                   305.24
  Crystal system, space group                                               Orthorhombic, *P* *n* *a*2~1~
  Temperature (K)                                                           122
  *a*, *b*, *c* ()                                                          29.3465(10), 12.2113(5), 3.6206(2)
  *V* (^3^)                                                                 1297.47(10)
  *Z*                                                                       4
  Radiation type                                                            Cu *K*
  (mm^1^)                                                                   1.10
  Crystal size (mm)                                                         0.11 0.06 0.06
                                                                            
  Data collection                                                           
  Diffractometer                                                            Bruker APEXII
  Absorption correction                                                     Numerical (*SADABS*; Krause *et al.*, 2015[@bb9])
  *T* ~min~, *T* ~max~                                                      0.694, 0.753
  No. of measured, independent and observed \[*I* \> 2(*I*)\] reflections   6000, 2322, 2172
  *R* ~int~                                                                 0.018
  (sin /)~max~ (^1^)                                                        0.614
                                                                            
  Refinement                                                                
  *R*\[*F* ^2^ \> 2(*F* ^2^)\], *wR*(*F* ^2^), *S*                          0.028, 0.071, 1.05
  No. of reflections                                                        2322
  No. of parameters                                                         213
  No. of restraints                                                         1
  H-atom treatment                                                          H atoms treated by a mixture of independent and constrained refinement
  ~max~, ~min~ (e ^3^)                                                      0.15, 0.19
  Absolute structure                                                        Flack *x* determined using 786 quotients \[(*I* ^+^)(*I* )\]/\[(*I* ^+^)+(*I* )\] (Parsons *et al.*, 2013[@bb11])
  Absolute structure parameter                                              0.20(8)
  ------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------

Computer programs: *APEX2* and *SAINT* (Bruker 2012[@bb2]), *SHELXS97* (Sheldrick, 2008[@bb12]), *SHELXL2014* (Sheldrick, 2015[@bb13]), *OLEX2* (Dolomanov *et al.*, 2009[@bb6]), *Mercury* (Macrae *et al.*, 2008[@bb10]), *POVRay* (Cason, 2003[@bb4]), *publCIF* (Westrip, 2010[@bb17]) and *PLATON* (Spek, 2009[@bb14]).
