Chemical context   {#sec1}
==================

Hydrogen bonding is the most common way of generating supra­molecular organic systems in crystal engineering and mol­ecular recognition.  Hydrogen-bonded systems generated from organic cations and anions are of special inter­est as they would be expected to form stronger hydrogen bonds than those in neutral mol­ecules (Reetz *et al.*,1994[@bb24]; Bertolasi *et al.*,  2001[@bb3]).  Squaric acid (H~2~C~4~O~4~, H~2~sq), has been of inter­est because of its cyclic structure and potential aromaticity and is used as a building block in crystal engineering due to the simplicity of the cyclic units.  It can be found in three forms: uncharged H~2~sq, the Hsq^−^ monoanion and the sq^2−^ dianion.  The mono- and dianions are often produced following deprotonation by amines (Lam & Mak, 2000[@bb21]; Mathew *et al.*,  2002[@bb23]).  The squarate derivatives are almost flat because of the π-conjugation of their C---C and C---O bonds, and therefore their four oxygen atoms behave as planar (*sp* ^2^) electron donors of one or two lone pairs of electrons.  Recently, we reported the synthesis and characterization of the same organo­ammonium squarate as the title compound but as a hydrate in the triclinic space group *P* (Korkmaz & Bulut, 2013[@bb17]).  We report here the unsolvated form of this salt, which crystallizes in the monoclinic space group *P*2~1~/*c*. 

Structural commentary   {#sec2}
=======================

2-(Amino­meth­yl) pyridine forms a salt with two squaric acid mol­ecules and each mol­ecule of the acid loses one proton.  One of these is transferred to the N atom of the pyridine ring, generating the 4-(amino­meth­yl)morpholinium mono-cation.  The other from the second acid mol­ecule is engaged in the formation of a homo-conjugated hydrogen squarate anion *via* a short, symmetric O5---H5*A*⋯O3 \[2.4583 (14) Å\] hydrogen bond (Fig.  1[▸](#fig1){ref-type="fig"}).  The electron density associated with this H atom is shared by the O5 and O3 atoms, indicating a large degree of ionic character (Gilli & Gilli, 2000[@bb10]).  Considering the range (2.38--2.50 Å) of Gilli's classification for such an inter­action, this hydrogen bonding can be referred to as negative charge-assisted hydrogen bonding \[(−) CAHB\] (Gilli & Gilli, 2009[@bb11]; Gilli *et al.*,  2001[@bb9] *;* Becke, 1993[@bb2], Lee *et al.*,  1988[@bb22]) and can be represented as \[-O⋯H⋯O-\]^−^.

N1/C1--C5, C7--C10 and C11--C14 are defined as rings 1, 2 and 3, respectively, with centroids *Cg*1, *Cg*2 and *Cg*3.  The dihedral angles between the mean plane of ring 1 and those of rings 2 and 3 are 18.818 (8) and 31.564 (6)°, respectively.  The dihedral angle between the two squarate anions is 29.19 (1)°.  The angles between the C---C bonds in the Hsq^−^ anions are close to 90°, with the oxygen atoms directed almost along the diagonals. 

The C---C distances in the planar squarate ring systems reflect partial double-bond character for C9---C10, C7---C10, C11---C12 and C11---C14 with distances of 1.4291 (17), 1.4357 (16), 1.4139 (17) and 1.4465 (18) Å, respectively.  In contrast C7---C8, C8---C9, C12---C13 and C13---C14 display more single-bond character with distances of 1.4886 (17), 1.4929 (17), 1.4802 (18) and 1.5141 (17) Å, respectively.  The Hsq^−^ ion has one C---O bond (C11---O5) at 1.3023 (17) Å, which is significantly longer than a normal single C---O bond.  This most likely reflects the involvement in the negative charge-assisted hydrogen bonding mentioned earlier.  At 1.3000 (15) Å, the C10---O4 bond is similarly extended.  The remaining C---O bonds in both rings display a similar pattern with one obvious C=O double bond in each ring \[C8=O2, 1.2268 (15) Å and C13=O7, 1.2141 (17) Å\] and the others of inter­mediate length in the range 1.2356 (16) to 1.2658 (15) Å, indicating some delocalization occurring in both rings. 

Supra­molecular features   {#sec3}
==========================

The two hydrogen squarate anions are linked in the asymmetric unit by a short hydrogen-bonding inter­action O5---H5*A*⋯O3 \[2.4583 (14) Å\] related to the proton-sharing inter­action discussed earlier.  This pair of anions is further linked to the 2-(aza­niumylmeth­yl)pyridinium dication by an N1---H1*A*⋯O1 hydrogen bond, Fig.  1[▸](#fig1){ref-type="fig"}, Table 1[▸](#table1){ref-type="table"}.  O5---H5*A*⋯O3, N2---H2*B*⋯O2^i^ and N2---H2*B*⋯O5^i^ hydrogen bonds form rings with an (7) graph-set motif while N2---H2*C*⋯O6^i^ and N2---H2*B*⋯O5^i^ hydrogen bonds combine to form (7) rings.  In addition, heteronuclear N2---H2*C*⋯O6^i^, and N2---H2*A*⋯O8^ii^ and homonuclear O4---H4*A*⋯O6^iii^ and O5---H5*A*⋯O3 hydrogen bonds generate a larger (14) ring motif (Fig.  2[▸](#fig2){ref-type="fig"}, Table 1[▸](#table1){ref-type="table"}).  The crystal packing also features unusual weak C7---O1⋯*Cg*2^ii^, C7---O1⋯*Cg*3^iv^ and C13---O7⋯*Cg*2^v^ inter­actions reinforced by π--π stacking inter­actions.  These latter contacts \[*Cg*1· · ·*Cg*3 = 2.5382 (9) Å, *Cg*2· ··*Cg*2^ii^ = 3.5997 (9) Å, *Cg*2· · ·*Cg*3^iv^ = 3.6406 (10) Å and *Cg*3·· ·*Cg*2^v^ = 3.6406 (10) Å; symmetry codes: (ii) 1 − *x*, 1 − *y*, 1 − *z*; (iv)  − *x*,  + *y*,  − *z*; (v)  − *x*, − + *y*,  − *z*\] also contribute to the stabilization of the crystal packing (Fig.  3[▸](#fig3){ref-type="fig"}). 

Database survey   {#sec4}
=================

A search of the Cambridge Structural Database (Version 5.38, update February 2017; Groom *et al.*,  2016[@bb12]) revealed the structures of various organo­ammonium squarates (Georgopoulos *et al.*,  2005[@bb8]; Wang & Stucky, 1974[@bb28]; Kanters *et al.*,  1991[@bb13]; Kolev *et al.*,  2000[@bb15]; Karle *et al.*,  1996[@bb14]; Angelova *et al.*,1996[@bb1]).  In the squarate anion form, the anions are generally linked to amines by N---H⋯O hydrogen bonds (Gilli *et al.*,  2001[@bb9]; Korkmaz *et al.*,  2011[@bb20]; Dega-Szafran *et al.*,  2012[@bb6]).  Structures of 2-(amoniometh­yl) pyridinium, di­hydrogen squarate and squaric acid derivatives are also known (Korkmaz *et al.*,  2011[@bb20]; Korkmaz & Bulut, 2013[@bb17], 2014[@bb18]).  Often, the supra­molecular architectures of similar structures have been investigated together with their spectroscopic properties, including their potential non-linear optical (NLO) behaviour (Bosshard *et al.*,  1995[@bb4]; Kolev *et al.*,  2008[@bb16]).  The literature also reveals that squarenes show photo-chemical, photo-conductive and NLO properties and that they can be used as electron acceptors in photo-sensitive devices (Korkmaz *et al.*,  2016[@bb19]). 

Synthesis and crystallization   {#sec5}
===============================

All chemical reagents were analytical grade commercial products.  The solvent was purified by conventional methods.  Squaric acid (H~2~Sq; 0,46 g, 4 mmol) and 2-(amino­meth­yl)pyridine (0,24 g; 2 mmol) were dissolved in water (25 cm^3^) to obtain a mixture in the molar ratio 2:1 and the solution was heated to 323 K in a temperature-controlled bath and stirred for one h. The reaction mixture was then slowly cooled to room temperature.  The crystals formed were filtered and washed with 10 cm^3^ of methanol, and dried in air. 

Refinement   {#sec6}
============

Crystal data, data collection and structure refinement details are summarized in Table 2[▸](#table2){ref-type="table"}.  All C-bound hydrogen atoms were included in calculated positions with C---H = 0.93 Å (aromatic) and 0.97 Å (methyl­ene) and allowed to ride, with *U* ~iso~(H) = 1.2*U* ~eq~(C).  The NH~3~ group (N---H = 0.89 Å) was also allowed to ride in the refinement with *U* ~iso~(H) = 1.5*U* ~eq~(N).  The O-bound H atoms and N1-bound H atom were located in a difference-Fourier map and refined with *U* ~iso~(H) = 1.2*U* ~eq~(O) and *U* ~iso~(H) = 1.5*U* ~eq~(N). 

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

Crystal structure: contains datablock(s) I. DOI: [10.1107/S2056989017004376/sj5524sup1.cif](https://doi.org/10.1107/S2056989017004376/sj5524sup1.cif)

Structure factors: contains datablock(s) I. DOI: [10.1107/S2056989017004376/sj5524Isup2.hkl](https://doi.org/10.1107/S2056989017004376/sj5524Isup2.hkl)

###### 

Click here for additional data file. 

Supporting information file.  DOI: [10.1107/S2056989017004376/sj5524Isup3.cml](https://doi.org/10.1107/S2056989017004376/sj5524Isup3.cml)

CCDC reference: [1538918](http://scripts.iucr.org/cgi-bin/cr.cgi?rm=csd&csdid=1538918)

Additional supporting information: [crystallographic information](http://scripts.iucr.org/cgi-bin/sendsupfiles?sj5524&file=sj5524sup0.html&mime=text/html); [3D view](http://scripts.iucr.org/cgi-bin/sendcif?sj5524sup1&Qmime=cif); [checkCIF report](http://scripts.iucr.org/cgi-bin/paper?sj5524&checkcif=yes)

The authors are grateful to the Scientific and Technological Research Application and Research Center, Sinop University, Turkey, for the use of the diffractometer. 

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

  --------------------------------- ---------------------------------------
  C~6~H~10~N~2~^2+^·2C~4~HO~4~^−^   *F*(000) = 696
  *M~r~* = 336.26                   *D*~x~ = 1.586 Mg m^−3^
  Monoclinic, *P*2~1~/*n*           Mo *K*α radiation, λ = 0.71073 Å
  *a* = 7.4653 (7) Å                Cell parameters from 9896 reflections
  *b* = 15.4548 (14) Å              θ = 3.0--28.3°
  *c* = 12.2095 (12) Å              µ = 0.13 mm^−1^
  β = 90.073 (4)°                   *T* = 296 K
  *V* = 1408.7 (2) Å^3^             Block, bronze
  *Z* = 4                           0.17 × 0.13 × 0.11 mm
  --------------------------------- ---------------------------------------

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

  ---------------------------------------------------------- --------------------------------------
  Bruker APEXII CCD diffractometer                           3090 reflections with *I* \> 2σ(*I*)
  φ and ω scans                                              *R*~int~ = 0.042
  Absorption correction: multi-scan (SADABS; Bruker, 2013)   θ~max~ = 28.3°, θ~min~ = 3.0°
  *T*~min~ = 0.672, *T*~max~ = 0.746                         *h* = −9→9
  70681 measured reflections                                 *k* = −20→20
  3496 independent reflections                               *l* = −16→16
  ---------------------------------------------------------- --------------------------------------

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

  ------------------------------------- -------------------------------------------------------------------------------------------------
  Refinement on *F*^2^                  0 restraints
  Least-squares matrix: full            Hydrogen site location: mixed
  *R*\[*F*^2^ \> 2σ(*F*^2^)\] = 0.042   H-atom parameters not refined
  *wR*(*F*^2^) = 0.118                  *w* = 1/\[σ^2^(*F*~o~^2^) + (0.0556*P*)^2^ + 0.5691*P*\] where *P* = (*F*~o~^2^ + 2*F*~c~^2^)/3
  *S* = 1.11                            (Δ/σ)~max~ \< 0.001
  3496 reflections                      Δρ~max~ = 0.26 e Å^−3^
  227 parameters                        Δρ~min~ = −0.28 e Å^−3^
  ------------------------------------- -------------------------------------------------------------------------------------------------

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

  -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  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. 
  -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

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

  ----- -------------- -------------- -------------- -------------------- --
        *x*            *y*            *z*            *U*~iso~\*/*U*~eq~   
  C1    0.3472 (2)     0.59963 (10)   0.10814 (12)   0.0377 (3)           
  H1    0.376471       0.541181       0.111071       0.045\*              
  C2    0.2856 (2)     0.63543 (13)   0.01196 (13)   0.0479 (4)           
  H2    0.270245       0.601292       −0.050069      0.058\*              
  C3    0.2473 (2)     0.72223 (14)   0.00899 (14)   0.0507 (4)           
  H3    0.210063       0.747750       −0.056155      0.061\*              
  C4    0.2640 (2)     0.77205 (11)   0.10321 (14)   0.0442 (4)           
  H4    0.236500       0.830733       0.101700       0.053\*              
  C5    0.32152 (18)   0.73381 (8)    0.19877 (12)   0.0305 (3)           
  C6    0.3378 (2)     0.78078 (10)   0.30627 (14)   0.0419 (4)           
  H6A   0.424710       0.751293       0.352098       0.050\*              
  H6B   0.381247       0.839005       0.293104       0.050\*              
  C7    0.61186 (16)   0.53092 (8)    0.38741 (10)   0.0245 (2)           
  C8    0.59463 (17)   0.44771 (8)    0.32689 (10)   0.0273 (3)           
  C9    0.71303 (18)   0.40741 (8)    0.41129 (10)   0.0278 (3)           
  C10   0.72538 (16)   0.48889 (8)    0.46610 (10)   0.0247 (2)           
  C11   0.64141 (18)   0.16148 (8)    0.29601 (10)   0.0289 (3)           
  C12   0.54027 (17)   0.10155 (8)    0.23404 (10)   0.0271 (3)           
  C13   0.57553 (18)   0.03307 (9)    0.31604 (10)   0.0293 (3)           
  C14   0.67917 (17)   0.09946 (8)    0.38172 (10)   0.0272 (3)           
  N1    0.36494 (15)   0.64919 (7)    0.19743 (9)    0.0288 (2)           
  N2    0.16375 (17)   0.78523 (8)    0.36470 (10)   0.0346 (3)           
  H2A   0.177821       0.813962       0.427290       0.052\*              
  H2C   0.125260       0.731888       0.378838       0.052\*              
  H2B   0.083844       0.812501       0.322984       0.052\*              
  O1    0.54865 (14)   0.60498 (6)    0.37564 (8)    0.0328 (2)           
  O2    0.51318 (15)   0.42458 (7)    0.24462 (8)    0.0387 (3)           
  O3    0.77376 (16)   0.33276 (6)    0.42957 (9)    0.0409 (3)           
  O4    0.80146 (14)   0.51691 (6)    0.55513 (8)    0.0331 (2)           
  O5    0.68219 (19)   0.24159 (7)    0.27431 (9)    0.0457 (3)           
  O6    0.45452 (15)   0.10825 (7)    0.14494 (8)    0.0380 (3)           
  O7    0.54053 (18)   −0.04333 (7)   0.32469 (10)   0.0514 (3)           
  O8    0.76167 (16)   0.09899 (7)    0.46951 (8)    0.0400 (3)           
  H1A   0.419 (3)      0.6260 (13)    0.2598 (17)    0.048\*              
  H5A   0.725 (3)      0.2770 (14)    0.3380 (18)    0.060\*              
  H4A   0.868 (3)      0.4687 (15)    0.5894 (17)    0.060\*              
  ----- -------------- -------------- -------------- -------------------- --

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

  ----- ------------ ------------- ------------ ------------- ------------- -------------
        *U*^11^      *U*^22^       *U*^33^      *U*^12^       *U*^13^       *U*^23^
  C1    0.0412 (7)   0.0344 (7)    0.0375 (8)   0.0016 (6)    0.0005 (6)    −0.0072 (6)
  C2    0.0440 (8)   0.0679 (11)   0.0320 (7)   −0.0028 (8)   −0.0042 (6)   −0.0111 (7)
  C3    0.0442 (9)   0.0732 (12)   0.0346 (8)   0.0039 (8)    −0.0084 (6)   0.0176 (8)
  C4    0.0458 (8)   0.0362 (8)    0.0505 (9)   0.0057 (6)    −0.0058 (7)   0.0159 (7)
  C5    0.0293 (6)   0.0253 (6)    0.0370 (7)   −0.0015 (5)   −0.0047 (5)   0.0011 (5)
  C6    0.0383 (7)   0.0333 (7)    0.0540 (9)   −0.0031 (6)   −0.0126 (7)   −0.0137 (6)
  C7    0.0285 (6)   0.0245 (6)    0.0207 (5)   −0.0021 (4)   −0.0029 (4)   0.0028 (4)
  C8    0.0327 (6)   0.0254 (6)    0.0237 (6)   −0.0010 (5)   −0.0045 (5)   0.0012 (5)
  C9    0.0344 (6)   0.0230 (6)    0.0259 (6)   −0.0008 (5)   −0.0073 (5)   −0.0009 (4)
  C10   0.0297 (6)   0.0222 (5)    0.0223 (5)   −0.0007 (4)   −0.0045 (4)   0.0009 (4)
  C11   0.0364 (6)   0.0242 (6)    0.0261 (6)   0.0020 (5)    −0.0054 (5)   −0.0052 (5)
  C12   0.0307 (6)   0.0257 (6)    0.0248 (6)   0.0040 (5)    −0.0059 (5)   −0.0052 (5)
  C13   0.0322 (6)   0.0280 (6)    0.0276 (6)   −0.0013 (5)   −0.0061 (5)   −0.0020 (5)
  C14   0.0297 (6)   0.0287 (6)    0.0234 (6)   0.0000 (5)    −0.0039 (5)   −0.0035 (5)
  N1    0.0310 (5)   0.0265 (5)    0.0287 (5)   0.0017 (4)    −0.0043 (4)   0.0019 (4)
  N2    0.0473 (7)   0.0276 (6)    0.0288 (6)   0.0021 (5)    −0.0120 (5)   −0.0032 (4)
  O1    0.0431 (5)   0.0240 (4)    0.0312 (5)   0.0049 (4)    −0.0081 (4)   0.0028 (4)
  O2    0.0505 (6)   0.0340 (5)    0.0315 (5)   −0.0006 (4)   −0.0189 (4)   −0.0030 (4)
  O3    0.0594 (7)   0.0227 (5)    0.0405 (6)   0.0071 (4)    −0.0227 (5)   −0.0051 (4)
  O4    0.0452 (5)   0.0257 (5)    0.0282 (5)   0.0014 (4)    −0.0150 (4)   −0.0028 (4)
  O5    0.0797 (9)   0.0227 (5)    0.0346 (5)   −0.0061 (5)   −0.0174 (5)   −0.0021 (4)
  O6    0.0520 (6)   0.0309 (5)    0.0310 (5)   0.0043 (4)    −0.0204 (4)   −0.0050 (4)
  O7    0.0700 (8)   0.0309 (6)    0.0532 (7)   −0.0154 (5)   −0.0214 (6)   0.0067 (5)
  O8    0.0525 (6)   0.0386 (6)    0.0290 (5)   −0.0023 (5)   −0.0171 (4)   −0.0034 (4)
  ----- ------------ ------------- ------------ ------------- ------------- -------------

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

  -------------------- -------------- ----------------------- --------------
  C1---N1              1.3388 (18)    C9---O3                 1.2593 (16)
  C1---C2              1.377 (2)      C9---C10                1.4291 (17)
  C1---H1              0.9300         C10---O4                1.3000 (15)
  C2---C3              1.372 (3)      C11---O5                1.3023 (17)
  C2---H2              0.9300         C11---C12               1.4139 (17)
  C3---C4              1.390 (3)      C11---C14               1.4465 (18)
  C3---H3              0.9300         C12---O6                1.2658 (15)
  C4---C5              1.376 (2)      C12---C13               1.4802 (18)
  C4---H4              0.9300         C13---O7                1.2141 (17)
  C5---N1              1.3475 (17)    C13---C14               1.5141 (17)
  C5---C6              1.505 (2)      C14---O8                1.2356 (16)
  C6---N2              1.485 (2)      N1---H1A                0.93 (2)
  C6---H6A             0.9700         N2---H2A                0.8900
  C6---H6B             0.9700         N2---H2C                0.8900
  C7---O1              1.2462 (15)    N2---H2B                0.8900
  C7---C10             1.4357 (16)    O3---H5A                1.46 (2)
  C7---C8              1.4886 (17)    O4---H4A                0.99 (2)
  C8---O2              1.2268 (15)    O5---H5A                1.00 (2)
  C8---C9              1.4929 (17)                            
                                                              
  N1---C1---C2         119.82 (15)    C10---C9---C8           89.62 (10)
  N1---C1---H1         120.1          O4---C10---C9           135.48 (12)
  C2---C1---H1         120.1          O4---C10---C7           131.74 (12)
  C3---C2---C1         119.01 (15)    C9---C10---C7           92.73 (10)
  C3---C2---H2         120.5          O5---C11---C12          129.70 (12)
  C1---C2---H2         120.5          O5---C11---C14          137.03 (12)
  C2---C3---C4         120.09 (15)    C12---C11---C14         93.24 (11)
  C2---C3---H3         120.0          O6---C12---C11          132.49 (13)
  C4---C3---H3         120.0          O6---C12---C13          136.82 (12)
  C5---C4---C3         119.43 (15)    C11---C12---C13         90.68 (10)
  C5---C4---H4         120.3          O7---C13---C12          135.73 (12)
  C3---C4---H4         120.3          O7---C13---C14          136.29 (13)
  N1---C5---C4         118.78 (14)    C12---C13---C14         87.94 (10)
  N1---C5---C6         117.37 (12)    O8---C14---C11          136.71 (12)
  C4---C5---C6         123.85 (14)    O8---C14---C13          135.18 (12)
  N2---C6---C5         111.82 (11)    C11---C14---C13         88.10 (10)
  N2---C6---H6A        109.3          C1---N1---C5            122.79 (12)
  C5---C6---H6A        109.3          C1---N1---H1A           119.1 (12)
  N2---C6---H6B        109.3          C5---N1---H1A           117.8 (12)
  C5---C6---H6B        109.3          C6---N2---H2A           109.5
  H6A---C6---H6B       107.9          C6---N2---H2C           109.5
  O1---C7---C10        135.70 (12)    H2A---N2---H2C          109.5
  O1---C7---C8         134.73 (11)    C6---N2---H2B           109.5
  C10---C7---C8        89.54 (10)     H2A---N2---H2B          109.5
  O2---C8---C7         134.48 (12)    H2C---N2---H2B          109.5
  O2---C8---C9         137.39 (12)    C9---O3---H5A           108.5 (9)
  C7---C8---C9         88.12 (9)      C10---O4---H4A          108.8 (12)
  O3---C9---C10        134.53 (12)    C11---O5---H5A          115.9 (12)
  O3---C9---C8         135.83 (12)                            
                                                              
  N1---C1---C2---C3    1.5 (2)        O1---C7---C10---C9      177.87 (16)
  C1---C2---C3---C4    −2.5 (3)       C8---C7---C10---C9      −0.11 (10)
  C2---C3---C4---C5    0.9 (3)        O5---C11---C12---O6     −4.0 (3)
  C3---C4---C5---N1    1.8 (2)        C14---C11---C12---O6    177.80 (15)
  C3---C4---C5---C6    −177.64 (15)   O5---C11---C12---C13    176.91 (15)
  N1---C5---C6---N2    −96.31 (15)    C14---C11---C12---C13   −1.33 (11)
  C4---C5---C6---N2    83.15 (19)     O6---C12---C13---O7     4.0 (3)
  O1---C7---C8---O2    0.6 (3)        C11---C12---C13---O7    −176.90 (19)
  C10---C7---C8---O2   178.65 (16)    O6---C12---C13---C14    −177.79 (17)
  O1---C7---C8---C9    −177.91 (15)   C11---C12---C13---C14   1.27 (10)
  C10---C7---C8---C9   0.10 (10)      O5---C11---C14---O8     2.2 (3)
  O2---C8---C9---O3    −0.3 (3)       C12---C11---C14---O8    −179.74 (17)
  C7---C8---C9---O3    178.15 (17)    O5---C11---C14---C13    −176.71 (18)
  O2---C8---C9---C10   −178.57 (17)   C12---C11---C14---C13   1.30 (10)
  C7---C8---C9---C10   −0.11 (10)     O7---C13---C14---O8     −2.1 (3)
  O3---C9---C10---O4   −0.8 (3)       C12---C13---C14---O8    179.77 (16)
  C8---C9---C10---O4   177.53 (16)    O7---C13---C14---C11    176.91 (19)
  O3---C9---C10---C7   −178.19 (17)   C12---C13---C14---C11   −1.24 (10)
  C8---C9---C10---C7   0.11 (10)      C2---C1---N1---C5       1.3 (2)
  O1---C7---C10---O4   0.3 (3)        C4---C5---N1---C1       −3.0 (2)
  C8---C7---C10---O4   −177.69 (15)   C6---C5---N1---C1       176.53 (13)
  -------------------- -------------- ----------------------- --------------

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

Cg2 and Cg3 are the centroids of the C7--C10 and C11--C14 rings, respectively. 

  ---------------------- ---------- ---------- ------------- ---------------
  *D*---H···*A*          *D*---H    H···*A*    *D*···*A*     *D*---H···*A*
  N1---H1*A*···O1        0.93 (2)   1.74 (2)   2.6598 (15)   166.7 (19)
  N2---H2*B*···O2^i^     0.89       2.05       2.8565 (16)   150
  N2---H2*B*···O5^i^     0.89       2.56       3.1613 (17)   126
  N2---H2*C*···O6^i^     0.89       2.02       2.8765 (16)   161
  N2---H2*A*···O8^ii^    0.89       1.90       2.7580 (15)   162
  O4---H4*A*···O6^iii^   0.99 (2)   1.51 (2)   2.4993 (14)   175 (2)
  O5---H5*A*···O3        1.00 (2)   1.46 (2)   2.4583 (14)   175 (2)
  C7---O1···*Cg*2^ii^    1.25 (1)   3.38 (1)   3.3226 (14)   77 (1)
  C7---O1···*Cg*3^iv^    1.25 (1)   3.39 (1)   3.3297 (14)   76 (1)
  C13---O7···*Cg*2^v^    1.21 (1)   3.52 (1)   3.4188 (15)   75 (1)
  ---------------------- ---------- ---------- ------------- ---------------

Symmetry codes: (i) −*x*+1/2, *y*+1/2, −*z*+1/2; (ii) −*x*+1, −*y*+1, −*z*+1; (iii) *x*+1/2, −*y*+1/2, *z*+1/2; (iv) −*x*+3/2, *y*+1/2, −*z*+1/2; (v) −*x*+3/2, *y*−1/2, −*z*+1/2. 

![ A view of the asymmetric unit of (I)[](#scheme1){ref-type="chem"}, showing the atom-numbering scheme and 30% probability displacement ellipsoids.  Dashed lines indicate hydrogen bonds.](e-73-00586-fig1){#fig1}

![ A view of the N---H⋯O and O---H⋯O inter­actions in the crystal of the title compound (hydrogen bonds are shown as dashed lines; see Table 1[▸](#table1){ref-type="table"} for numerical details).](e-73-00586-fig2){#fig2}

![ A packing diagram showing the C---O⋯π and π--π stacking inter­actions.  \[Symmetry codes: (ii) −*x* + 1, −*y* + 1, −*z* + 1; (iv) −*x* + , *y* + , −*z* + ; (v) −*x* + , *y* − , −*z* + .\] H atoms not involved in the inter­actions have been omitted for clarity.](e-73-00586-fig3){#fig3}

###### Hydrogen-bond geometry (Å, °)

*Cg*2 and *Cg*3 are the centroids of the C7--C10 and C11--C14 rings, respectively. 

  *D*---H⋯*A*          *D*---H    H⋯*A*      *D*⋯*A*       *D*---H⋯*A*
  -------------------- ---------- ---------- ------------- -------------
  N1---H1*A*⋯O1        0.93 (2)   1.74 (2)   2.6598 (15)   166.7 (19)
  N2---H2*B*⋯O2^i^     0.89       2.05       2.8565 (16)   150
  N2---H2*B*⋯O5^i^     0.89       2.56       3.1613 (17)   126
  N2---H2*C*⋯O6^i^     0.89       2.02       2.8765 (16)   161
  N2---H2*A*⋯O8^ii^    0.89       1.90       2.7580 (15)   162
  O4---H4*A*⋯O6^iii^   0.99 (2)   1.51 (2)   2.4993 (14)   175 (2)
  O5---H5*A*⋯O3        1.00 (2)   1.46 (2)   2.4583 (14)   175 (2)
  C7---O1⋯*Cg*2^ii^    1.25 (1)   3.38 (1)   3.3226 (14)   77 (1)
  C7---O1⋯*Cg*3^iv^    1.25 (1)   3.39 (1)   3.3297 (14)   76 (1)
  C13---O7⋯*Cg*2^v^    1.21 (1)   3.52 (1)   3.4188 (15)   75 (1)

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

###### Experimental details

  -------------------------------------------------------------------------- -------------------------------------------
  Crystal data                                                               
  Chemical formula                                                           C~6~H~10~N~2~ ^2+^·2C~4~HO~4~ ^−^
  *M* ~r~                                                                    336.26
  Crystal system, space group                                                Monoclinic, *P*2~1~/*n*
  Temperature (K)                                                            296
  *a*, *b*, *c* (Å)                                                          7.4653 (7), 15.4548 (14), 12.2095 (12)
  β (°)                                                                      90.073 (4)
  *V* (Å^3^)                                                                 1408.7 (2)
  *Z*                                                                        4
  Radiation type                                                             Mo *K*α
  μ (mm^−1^)                                                                 0.13
  Crystal size (mm)                                                          0.17 × 0.13 × 0.11
                                                                             
  Data collection                                                            
  Diffractometer                                                             Bruker APEXII CCD
  Absorption correction                                                      Multi-scan (*SADABS*; Bruker, 2013[@bb5])
  *T* ~min~, *T* ~max~                                                       0.672, 0.746
  No.  of measured, independent and observed \[*I* \> 2σ(*I*)\] reflections   70681, 3496, 3090
  *R* ~int~                                                                  0.042
  (sin θ/λ)~max~ (Å^−1^)                                                     0.668
                                                                             
  Refinement                                                                 
  *R*\[*F* ^2^ \> 2σ(*F* ^2^)\], *wR*(*F* ^2^), *S*                          0.042, 0.118, 1.11
  No.  of reflections                                                         3496
  No.  of parameters                                                          227
  H-atom treatment                                                           H-atom parameters not refined
  Δρ~max~, Δρ~min~ (e Å^−3^)                                                 0.26, −0.28
  -------------------------------------------------------------------------- -------------------------------------------

Computer programs: *APEX2* and *SAINT* (Bruker, 2013[@bb5]), *SHELXT2014* (Sheldrick, 2008[@bb25]), *SHELXL2016* (Sheldrick, 2015[@bb26]), *ORTEP-3 for Windows* and *WinGX* (Farrugia, 2012[@bb7]) and *PLATON* (Spek, 2009[@bb27]).