Related literature {#sec1}
==================

For structural studies of 2-amino­pyridine, see: Chao *et al.* (1975[@bb4]). For recent mol­ecular co-crystals and salts of 2-amino­pyridine, see: Sivaramkumar *et al.* (2010[@bb13]); Chitra *et al.* (2008[@bb5]); Quah *et al.* (2008[@bb10]); Xie (2007[@bb15]); Li *et al.* (2006[@bb8], 2007[@bb7]); Yang & Qu (2006[@bb16]); Bis & Zaworotko (2005[@bb2]). For the use of 2-amino­pyridine in the synthesis of pharmaceuticals, see: O'Neil (2006[@bb9]). For our previous work on screening for molecular co-crystals and salts, see: He *et al.* (2009[@bb6]).

Experimental {#sec2}
============

 {#sec2.1}

### Crystal data {#sec2.1.1}

C~5~H~7~N~2~ ^+^·C~10~H~9~O~2~ ^−^*M* *~r~* = 256.30Triclinic,*a* = 8.6147 (17) Å*b* = 9.0555 (18) Å*c* = 9.2346 (18) Åα = 75.56 (3)°β = 87.72 (3)°γ = 72.79 (3)°*V* = 666.0 (2) Å^3^*Z* = 2Mo *K*α radiationμ = 0.09 mm^−1^*T* = 110 K0.33 × 0.33 × 0.22 mm

### Data collection {#sec2.1.2}

Rigaku Saturn CCD area-detector diffractometerAbsorption correction: multi-scan (Blessing, 1995[@bb3]) *T* ~min~ = 0.972, *T* ~max~ = 0.9819557 measured reflections3271 independent reflections3103 reflections with *I* \> 2σ(*I*)*R* ~int~ = 0.017

### Refinement {#sec2.1.3}

*R*\[*F* ^2^ \> 2σ(*F* ^2^)\] = 0.052*wR*(*F* ^2^) = 0.145*S* = 1.123271 reflections184 parametersH atoms treated by a mixture of independent and constrained refinementΔρ~max~ = 0.31 e Å^−3^Δρ~min~ = −0.24 e Å^−3^

 {#d5e562}

Data collection: *CrystalClear* (Rigaku, 2008[@bb11]); cell refinement: *CrystalClear*; data reduction: *CrystalClear*; program(s) used to solve structure: *SHELXS97* (Sheldrick, 2008[@bb12]); program(s) used to refine structure: *SHELXL97* (Sheldrick, 2008[@bb12]); molecular graphics: *X-SEED* (Barbour, 2001[@bb1]); software used to prepare material for publication: *SHELXTL* (Sheldrick, 2008[@bb12]) and *PLATON* (Spek, 2009[@bb14]).

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

Crystal structure: contains datablocks global, I. DOI: [10.1107/S1600536810049093/ng5075sup1.cif](http://dx.doi.org/10.1107/S1600536810049093/ng5075sup1.cif)

Structure factors: contains datablocks I. DOI: [10.1107/S1600536810049093/ng5075Isup2.hkl](http://dx.doi.org/10.1107/S1600536810049093/ng5075Isup2.hkl)

Additional supplementary materials: [crystallographic information](http://scripts.iucr.org/cgi-bin/sendsupfiles?ng5075&file=ng5075sup0.html&mime=text/html); [3D view](http://scripts.iucr.org/cgi-bin/sendcif?ng5075sup1&Qmime=cif); [checkCIF report](http://scripts.iucr.org/cgi-bin/paper?ng5075&checkcif=yes)

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: [NG5075](http://scripts.iucr.org/cgi-bin/sendsup?ng5075)).

This work was supported by the Science and Engineering Research Council of A\*STAR (Agency for Science, Technology and Research), Singapore.

Comment
=======

2-Aminopyridine is one of the three positional isomers of aminopyridine and is widely used as an intermediate in the synthesis of pharmaceuticals (O\'Neil, 2006). We have chosen 2-aminopyridine and 1-phenylcyclopropane-1-carboxylic acid for cocrystallization experiment as an extension work to our previous study on screening for molecular cocrystals and salts (He *et al.*, 2009).

The crystal structure of the title salt contains each one molecule of 2-aminopyridine and 1-phenylcyclopropane-1-carboxylic acid in the asymmetric unit (Fig. 2). Bis and Zaworotko revealed four types of two-point recognition possibilities in molecular complexes formed between 2-aminopyridine and carboxylic acids (Bis & Zaworotko, 2005). They have distinguished the type I--IV (Fig 1) synthons depending on whether the interacting complementary functional groups are the same or different. Type I involves formation of the carboxylic acid homosynthon; type II involves formation of the 2-aminopyridine homosynthon; type III and IV involve formation of 2-aminopyridine-carboxylic acid and 2-aminopyridinium-carboxylate supramolecular heterosynthons, respectively.

The title salt features type IV heterosynthon, where the 2-aminopyridinium ion forms a heterosynthon with the carboxylate group of the 1-phenylcyclopropane-1-carboxylate *via* two N--H···O (N···O = 2.6255 (15) and 2.7903 (15) Å) hydrogen bonds. Two such heterosynthons related by an inversion center dimerizes to form a four-component supramolecular unit sustained by N--H···O (N···O = 2.8230 (16) Å) hydrogen bonding (Fig. 3). The four-component supramolecular unit is further stabilized by a C--H···π interaction involving the 2-C--H of the pyridine ring and centroid of the phenyl ring of the carboxylate: C···*Cg1* (1 - *x*,1 - *y*,1 - *z*) = 3.5464 (15) Å, where *Cg1* denotes the centroid of the ring C10--C15 of 1-phenylcyclopropane-1-carboxylate (Fig. 4). Two prominent C--H···O interactions that involve the 11-C---H and 12-C---H of the phenyl ring of the 1-phenylcyclopropane-1-carboxylate and O1 of the same molecule at (-*x* + 1, -*y* + 1, -*z* + 2) and (*x* + 1, *y*, *z*), respectively, stabilize the overall crystal structure.

Experimental {#experimental}
============

0.1883 g (2 mmol) of 2-aminopyridine (Acros Organic, 99+%)) and 0.3246 g (2 mmol) of 1-phenylcyclopropane-1-carboxylic acid (Sigma, 97%) and were dissolved into 3 ml of ethyl acetate (Fisher Scientific, HPLC). Solution was then filtered through a 0.22µm PTFE filter. Filtered solution was finally sealed with Parafilm^?^ and small holes were made to allow solvent to slowly evaporate. The block-shaped crystal (0.33 × 0.33 × 0.22 mm) suitable for single-crystal X-ray diffraction was collected after one day.

Refinement {#refinement}
==========

H atoms bonded to N and O atoms were located in a difference map and allowed to ride on their parent atoms in the refinement cycles. Other H atoms were positioned geometrically and refined using a riding model.

Figures
=======

![The molecular structures of 2-aminopyridinium ion and 1-phenylcyclopropane-1-carboxylate, with atom labels and 50% probability displacement ellipsoids for non-H atoms.](e-66-o3339-fig1){#Fap1}

![Supramolecular synthons: I. a carboxylic acid homosynthon; II. 2-aminopyridine homosynthon; III. 2-aminopyridine-carboxylic acid heterosynthon; IV. 2-aminopyridine-carboxylate heterosynthon.](e-66-o3339-fig2){#Fap2}

![A four-component supramolecular unit that features heterosynthon IV and a C--H···π interaction in the crystal structure of the title salt.](e-66-o3339-fig3){#Fap3}

![Part of the crystal structure of the title salt, showing the arrangement of the four-component supramolecular units which are stabilized by C--H···O interactions.](e-66-o3339-fig4){#Fap4}

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

  ---------------------------------- ---------------------------------------
  C~5~H~7~N~2~^+^·C~10~H~9~O~2~^−^   *Z* = 2
  *M~r~* = 256.30                    *F*(000) = 272
  Triclinic, *P*1                    *D*~x~ = 1.278 Mg m^−3^
  Hall symbol: -P 1                  Mo *K*α radiation, λ = 0.71073 Å
  *a* = 8.6147 (17) Å                Cell parameters from 1840 reflections
  *b* = 9.0555 (18) Å                θ = 2.3--30.9°
  *c* = 9.2346 (18) Å                µ = 0.09 mm^−1^
  α = 75.56 (3)°                     *T* = 110 K
  β = 87.72 (3)°                     Block, colorless
  γ = 72.79 (3)°                     0.33 × 0.33 × 0.22 mm
  *V* = 666.0 (2) Å^3^               
  ---------------------------------- ---------------------------------------

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

  ---------------------------------------------------- --------------------------------------
  Rigaku Saturn CCD area-detector diffractometer       3271 independent reflections
  Radiation source: fine-focus sealed tube             3103 reflections with *I* \> 2σ(*I*)
  graphite                                             *R*~int~ = 0.017
  ω scans                                              θ~max~ = 28.3°, θ~min~ = 2.3°
  Absorption correction: multi-scan (Blessing, 1995)   *h* = −11→11
  *T*~min~ = 0.972, *T*~max~ = 0.981                   *k* = −11→11
  9557 measured reflections                            *l* = −12→12
  ---------------------------------------------------- --------------------------------------

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

  ------------------------------------- -------------------------------------------------------------------------------------------------
  Refinement on *F*^2^                  Primary atom site location: structure-invariant direct methods
  Least-squares matrix: full            Secondary atom site location: difference Fourier map
  *R*\[*F*^2^ \> 2σ(*F*^2^)\] = 0.052   Hydrogen site location: inferred from neighbouring sites
  *wR*(*F*^2^) = 0.145                  H atoms treated by a mixture of independent and constrained refinement
  *S* = 1.12                            *w* = 1/\[σ^2^(*F*~o~^2^) + (0.0812*P*)^2^ + 0.1565*P*\] where *P* = (*F*~o~^2^ + 2*F*~c~^2^)/3
  3271 reflections                      (Δ/σ)~max~ \< 0.001
  184 parameters                        Δρ~max~ = 0.31 e Å^−3^
  0 restraints                          Δρ~min~ = −0.24 e Å^−3^
  ------------------------------------- -------------------------------------------------------------------------------------------------

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.
  Refinement. Refinement of *F*^2^ against ALL reflections. The weighted *R*-factor *wR* and goodness of fit *S* are based on *F*^2^, conventional *R*-factors *R* are based on *F*, with *F* set to zero for negative *F*^2^. The threshold expression of *F*^2^ \> σ(*F*^2^) is used only for calculating *R*-factors(gt) *etc*. and is not relevant to the choice of reflections for refinement. *R*-factors based on *F*^2^ are statistically about twice as large as those based on *F*, and *R*- factors based on ALL data will be even larger.
  -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

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

  ----- --------------- -------------- -------------- -------------------- --
        *x*             *y*            *z*            *U*~iso~\*/*U*~eq~   
  O1    0.24208 (10)    0.61579 (11)   0.84154 (10)   0.0282 (2)           
  N1    0.14380 (12)    0.42572 (12)   0.71820 (11)   0.0242 (2)           
  O2    0.43518 (12)    0.64375 (12)   0.67744 (10)   0.0338 (2)           
  N2    0.35609 (13)    0.42026 (15)   0.55735 (13)   0.0309 (3)           
  C1    0.22273 (14)    0.37913 (14)   0.60017 (13)   0.0238 (2)           
  C6    0.41282 (13)    0.76759 (14)   0.87938 (13)   0.0234 (2)           
  C3    0.02113 (16)    0.25068 (15)   0.57653 (14)   0.0290 (3)           
  H3    −0.0231         0.1918         0.5263         0.035\*              
  C9    0.36026 (13)    0.66849 (14)   0.79228 (13)   0.0235 (2)           
  C11   0.71088 (14)    0.73844 (14)   0.88508 (13)   0.0246 (2)           
  H11   0.7259          0.6408         0.9589         0.029\*              
  C10   0.55559 (14)    0.82560 (14)   0.82490 (12)   0.0230 (2)           
  C12   0.84456 (14)    0.79240 (15)   0.83861 (13)   0.0268 (3)           
  H12   0.9499          0.7318         0.8806         0.032\*              
  C2    0.15868 (15)    0.28868 (14)   0.52668 (13)   0.0257 (3)           
  H2    0.2114          0.2548         0.4432         0.031\*              
  C5    0.00793 (15)    0.38539 (15)   0.76921 (13)   0.0269 (3)           
  H5    −0.0431         0.4190         0.8534         0.032\*              
  C14   0.67030 (17)    1.02220 (15)   0.66824 (15)   0.0319 (3)           
  H14   0.6560          1.1190         0.5934         0.038\*              
  C15   0.53697 (15)    0.96742 (15)   0.71553 (14)   0.0292 (3)           
  H15   0.4320          1.0277         0.6725         0.035\*              
  C4    −0.05647 (16)   0.29794 (16)   0.70228 (14)   0.0304 (3)           
  H4    −0.1509         0.2694         0.7390         0.036\*              
  C7    0.39043 (16)    0.72252 (17)   1.04712 (14)   0.0321 (3)           
  H7A   0.3474          0.6310         1.0878         0.038\*              
  H7B   0.4712          0.7340         1.1135         0.038\*              
  C13   0.82364 (15)    0.93486 (15)   0.73082 (14)   0.0288 (3)           
  H13   0.9145          0.9725         0.6999         0.035\*              
  C8    0.27730 (15)    0.87131 (17)   0.95326 (15)   0.0332 (3)           
  H8A   0.2883          0.9746         0.9617         0.040\*              
  H8B   0.1645          0.8716         0.9359         0.040\*              
  H6    0.407 (2)       0.393 (2)      0.475 (2)      0.038 (4)\*          
  H9    0.389 (2)       0.492 (2)      0.600 (2)      0.041 (5)\*          
  H1    0.185 (2)       0.496 (2)      0.762 (2)      0.044 (5)\*          
  ----- --------------- -------------- -------------- -------------------- --

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

  ----- ------------ ------------ ------------ ------------- ------------ -------------
        *U*^11^      *U*^22^      *U*^33^      *U*^12^       *U*^13^      *U*^23^
  O1    0.0256 (4)   0.0351 (5)   0.0315 (5)   −0.0151 (3)   0.0083 (3)   −0.0157 (4)
  N1    0.0263 (5)   0.0268 (5)   0.0226 (5)   −0.0103 (4)   0.0047 (4)   −0.0094 (4)
  O2    0.0381 (5)   0.0433 (5)   0.0329 (5)   −0.0232 (4)   0.0150 (4)   −0.0210 (4)
  N2    0.0292 (5)   0.0413 (6)   0.0326 (6)   −0.0188 (5)   0.0110 (4)   −0.0193 (5)
  C1    0.0240 (5)   0.0242 (5)   0.0238 (5)   −0.0080 (4)   0.0027 (4)   −0.0063 (4)
  C6    0.0212 (5)   0.0272 (6)   0.0249 (5)   −0.0082 (4)   0.0047 (4)   −0.0113 (4)
  C3    0.0351 (6)   0.0295 (6)   0.0286 (6)   −0.0172 (5)   0.0054 (5)   −0.0097 (5)
  C9    0.0213 (5)   0.0257 (5)   0.0256 (5)   −0.0078 (4)   0.0033 (4)   −0.0093 (4)
  C11   0.0257 (6)   0.0256 (5)   0.0247 (5)   −0.0100 (4)   0.0033 (4)   −0.0078 (4)
  C10   0.0238 (5)   0.0249 (5)   0.0244 (5)   −0.0094 (4)   0.0043 (4)   −0.0113 (4)
  C12   0.0242 (5)   0.0316 (6)   0.0292 (6)   −0.0113 (4)   0.0030 (4)   −0.0123 (5)
  C2    0.0302 (6)   0.0265 (6)   0.0234 (5)   −0.0111 (4)   0.0047 (4)   −0.0092 (4)
  C5    0.0292 (6)   0.0286 (6)   0.0245 (5)   −0.0105 (5)   0.0073 (4)   −0.0082 (4)
  C14   0.0403 (7)   0.0248 (6)   0.0318 (6)   −0.0137 (5)   0.0059 (5)   −0.0051 (5)
  C15   0.0287 (6)   0.0260 (6)   0.0321 (6)   −0.0074 (5)   0.0009 (5)   −0.0065 (5)
  C4    0.0323 (6)   0.0324 (6)   0.0314 (6)   −0.0169 (5)   0.0089 (5)   −0.0090 (5)
  C7    0.0325 (6)   0.0468 (8)   0.0260 (6)   −0.0199 (6)   0.0090 (5)   −0.0167 (5)
  C13   0.0326 (6)   0.0318 (6)   0.0308 (6)   −0.0182 (5)   0.0092 (5)   −0.0144 (5)
  C8    0.0276 (6)   0.0372 (7)   0.0429 (7)   −0.0110 (5)   0.0115 (5)   −0.0242 (6)
  ----- ------------ ------------ ------------ ------------- ------------ -------------

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

  ----------------- ------------- ----------------- -------------
  O1---C9           1.2702 (14)   C11---H11         0.9500
  N1---C1           1.3524 (15)   C10---C15         1.3927 (17)
  N1---C5           1.3597 (15)   C12---C13         1.3870 (18)
  N1---H1           0.983 (19)    C12---H12         0.9500
  O2---C9           1.2530 (15)   C2---H2           0.9500
  N2---C1           1.3261 (15)   C5---C4           1.3595 (18)
  N2---H6           0.913 (18)    C5---H5           0.9500
  N2---H9           0.949 (19)    C14---C13         1.385 (2)
  C1---C2           1.4178 (17)   C14---C15         1.3947 (17)
  C6---C10          1.4977 (15)   C14---H14         0.9500
  C6---C9           1.5122 (16)   C15---H15         0.9500
  C6---C7           1.5201 (17)   C4---H4           0.9500
  C6---C8           1.5224 (17)   C7---C8           1.491 (2)
  C3---C2           1.3614 (16)   C7---H7A          0.9900
  C3---C4           1.4125 (18)   C7---H7B          0.9900
  C3---H3           0.9500        C13---H13         0.9500
  C11---C10         1.3918 (17)   C8---H8A          0.9900
  C11---C12         1.3922 (16)   C8---H8B          0.9900
                                                    
  C1---N1---C5      121.81 (10)   C3---C2---C1      119.58 (11)
  C1---N1---H1      117.1 (11)    C3---C2---H2      120.2
  C5---N1---H1      121.0 (11)    C1---C2---H2      120.2
  C1---N2---H6      118.9 (11)    C4---C5---N1      121.41 (11)
  C1---N2---H9      120.8 (11)    C4---C5---H5      119.3
  H6---N2---H9      119.5 (15)    N1---C5---H5      119.3
  N2---C1---N1      118.89 (11)   C13---C14---C15   119.75 (12)
  N2---C1---C2      122.65 (11)   C13---C14---H14   120.1
  N1---C1---C2      118.46 (11)   C15---C14---H14   120.1
  C10---C6---C9     117.44 (9)    C10---C15---C14   120.94 (12)
  C10---C6---C7     118.19 (10)   C10---C15---H15   119.5
  C9---C6---C7      115.04 (10)   C14---C15---H15   119.5
  C10---C6---C8     118.76 (10)   C5---C4---C3      118.07 (11)
  C9---C6---C8      115.60 (10)   C5---C4---H4      121.0
  C7---C6---C8      58.71 (9)     C3---C4---H4      121.0
  C2---C3---C4      120.64 (11)   C8---C7---C6      60.72 (9)
  C2---C3---H3      119.7         C8---C7---H7A     117.7
  C4---C3---H3      119.7         C6---C7---H7A     117.7
  O2---C9---O1      124.47 (11)   C8---C7---H7B     117.7
  O2---C9---C6      118.39 (10)   C6---C7---H7B     117.7
  O1---C9---C6      117.13 (10)   H7A---C7---H7B    114.8
  C10---C11---C12   120.92 (11)   C14---C13---C12   120.03 (11)
  C10---C11---H11   119.5         C14---C13---H13   120.0
  C12---C11---H11   119.5         C12---C13---H13   120.0
  C11---C10---C15   118.48 (11)   C7---C8---C6      60.57 (8)
  C11---C10---C6    120.06 (10)   C7---C8---H8A     117.7
  C15---C10---C6    121.46 (11)   C6---C8---H8A     117.7
  C13---C12---C11   119.87 (11)   C7---C8---H8B     117.7
  C13---C12---H12   120.1         C6---C8---H8B     117.7
  C11---C12---H12   120.1         H8A---C8---H8B    114.8
  ----------------- ------------- ----------------- -------------

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

  -------------------------------------------
  Cg1 is the centroid of the C10--C15 ring.
  -------------------------------------------

  --------------------- ------------ ------------ ------------- ---------------
  *D*---H···*A*         *D*---H      H···*A*      *D*···*A*     *D*---H···*A*
  N1---H1···O1          0.983 (19)   1.64 (2)     2.6255 (15)   176.2 (17)
  N2---H6···O2^i^       0.913 (18)   1.938 (18)   2.8230 (16)   162.6 (16)
  N2---H9···O2          0.949 (19)   1.844 (19)   2.7903 (15)   175.2 (16)
  C11---H11···O1^ii^    0.95         2.53         3.479 (2)     178
  C12---H12···O1^iii^   0.95         2.47         3.3212 (18)   149
  C2---H2···Cg1^i^      0.95         2.62         3.5464 (15)   166
  --------------------- ------------ ------------ ------------- ---------------

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

###### Hydrogen-bond geometry (Å, °)

*Cg*1 is the centroid of the C10--C15 ring.

  *D*---H⋯*A*         *D*---H      H⋯*A*        *D*⋯*A*       *D*---H⋯*A*
  ------------------- ------------ ------------ ------------- -------------
  N1---H1⋯O1          0.983 (19)   1.64 (2)     2.6255 (15)   176.2 (17)
  N2---H6⋯O2^i^       0.913 (18)   1.938 (18)   2.8230 (16)   162.6 (16)
  N2---H9⋯O2          0.949 (19)   1.844 (19)   2.7903 (15)   175.2 (16)
  C11---H11⋯O1^ii^    0.95         2.53         3.479 (2)     178
  C12---H12⋯O1^iii^   0.95         2.47         3.3212 (18)   149
  C2---H2⋯*Cg*1^i^    0.95         2.62         3.5464 (15)   166

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