A triclinic polymorph of 4-cyanopyridinium hydrogen chloranilate

The asymmetric unit of the triclinic polymorph of the title compound (systematic name: 4-cyanopyridinium 2,5-dichloro-4-hydroxy-3,6-dioxocyclohexa-1,4-dien-1-olate), C6H5N2 +·C6HCl2O4 −, consists of two crystallographically independent cation–anion units, in each of which the cation and the anion are linked by an N—H⋯O hydrogen bond. In the units, the dihedral angles between the cation and anion rings are 78.43 (11) and 80.71 (11)°. In the crystal, each unit independently forms a chain through N—H⋯O and O—H⋯N hydrogen bonds; one chain runs along the c axis while the other runs along [011]. Weak C—H⋯O, C—H⋯N and C—H⋯Cl interactions are observed between the chains.

The asymmetric unit of the triclinic polymorph of the title compound consists of two crystallographically independent cation-anion units, in each of which the cation and the anion are held together by N-H···O hydrogen bond. The dihedral angle between the N1/C13-C17 pyridine ring and the C1-C6 of the acid ring is 78.43 (11)° in one unit, while the angle between the N3/C19-C23 and C7-C12 rings is 80.71 (11)° in the other unit.
In the crystal structure of the monoclinic polymorph, the acid molecule (A) and the base molecule (B) afford an  Table 1) hydrogen bonds runs along the c axis, while the other chain formed by N3-H3···O6 and O8-H8···N4 ii (symmetry code in Table 1) hydrogen bonds runs along the [011] direction.
No H atom disorder is observed in these hydrogen bonds. Between the chains, C-H···O, C-H···N and C-H···Cl interactions (Table 1) are observed.

Experimental
Single crystals were obtained by slow evaporation from an acetonitrile solution (130 ml) of chloranilic acid (0.60 g) and 4-cyanopyridine (0.30 g) at room temperature.

Refinement
C-bound H atoms were positioned geometrically (C-H = 0.95 Å) and refined as riding, with U iso (H) = 1.2U eq (C). The Oand N-bound H atoms were found in a difference Fourier map and refined freely. The refined distances are O-H = 0.80 (4) and 0.93 (5) Å, and N-H = 0.90 (4) and 0.95 (4) Å.  The asymmetric unit of the title compound, with the atom-labeling. Displacement ellipsoids of non-H atoms are drawn at the 50% probability level. The dashed lines indicate the N-H···O hydrogen bonds.

Special details
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.