The co-crystal N,N′-bis[(pyridin-1-ium-2-yl)methyl]ethanedithioamide bis(2,6-dinitrobenzoate)–2,6-dinitrobenzoic acid (1/4)

The asymmetric unit of title co-crystal, C14H16N4S2 2+·2C7H3N2O6 −·4C7H4N2O6, comprises a centrosymmetric dipyridinium dication, a 2,6-dinitrobenzoate anion and two independent 2,6-dinitrobenzoic acid molecules. The pyridinium rings are each approximately perpendicular to the central dithioamide unit [dihedral angle = 80.67 (12)°]. The carboxylate/carboxylic acid groups are approximately perpendicular to the benzene ring to which they are attached [dihedral angles = 78.85 (16), 81.46 (19) and 71.28 (15)°]. By contrast, the major twist exhibited by a nitro group is manifested in a dihedral angle of 32.66 (17)°. The most prominent feature of the crystal packing is linear supramolecular chains along [1-10], featuring O—H⋯O(carboxylate) and pyridinium-N—H⋯O hydrogen bonds. These are consolidated into a three-dimensional architecture by thioamide–nitro N—H⋯O, C—H⋯O and π–π [inter-centroid distance = 3.524 (2) Å] interactions. One of the nitro O atoms was refined over two sites; the major site was 0.65 (7) occupied.

We gratefully thank the Ministry of Higher Education (Malaysia) for funding structural studies through the High-Impact Research scheme (UM.C/HIR-MOHE/SC/03).
The asymmetric unit of (I) comprises half of a 2-({[(pyridin-1-ium-2-ylmethyl)carbamoyl]formamido}methyl)pyridin-1-ium dication, disposed about a centre of inversion, a 2,6-dinitrobenzoate anion and two molecules of 2,6-dinitrobenzoic acid, Fig. 1. The pyridin-1-ium rings lie to either side of the central dithioamide chromophore and adopt an almost perpendicular orientation forming a dihedral angle of 80.67 (12)°. In the anion, the carboxylate is inclined to the benzene ring to which it is attached forming a dihedral angle of 78.85 (16)°. A similar situation pertains in the neutral 2,6dinitrobenzoic acid molecules where the comparable dihedral angles are 81.46 (19) and 71.28 (15)°. By contrast, while all nitro groups are twisted out of the plane of the benzene ring to which they are attached, the greatest twist is seen in the O12-N6-C20-C15 torsion angle of -32.7 (4)°.
The deprotonated carboxylate O13,O14 group is pivotal in the crystal packing, as each oxygen atom accepts a hydrogen bond from an adjacent molecule of 2,6-dinitrobenzoic acid, Table 1. As well, the O14 atom accepts a hydrogen bond from the pyridinium residue. A supramolecular chain results, base vector [1 -1 0], as shown in Fig. 2. Chains are linked into a three-dimensional architecture by amide-N-H···O, C-H···O and π-π [inter-centroid distance between centrosymmetrically related C8-C13 rings = 3.524 (2) Å; symmetry operation = 1 -x, -y, 1 -z] contacts. Fig. 3 shows the unit-cell contents viewed down the axis of the chain.

Refinement
C-bound H-atoms were placed in calculated positions (C-H = 0.95-0.99 Å) and were included in the refinement in the riding model approximation with U iso (H) set to 1.2U eq (C). The O-and N-bound H-atoms were located in a difference Fourier map and were refined with a distance restraints of O-H = 0.84±0.01 Å and N-H = 0.88±0.01 Å, and with U iso (H) = 1.2U eq (N) and 1.5U eq (O). The maximum and minimum residual electron density peaks of 1.01 and 0.50 e Å -3 , respectively, were located 1.25 Å and 0.79 Å from the O6 atom. One of the nitro-O atoms was refined over two sites; the major site was present 0.65 (7).

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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.