Bis[2-(2,4-dinitrobenzyl)pyridinium] biphenyl-4,4′-disulfonate trihydrate

In the structure of the title salt, 2C12H10N3O4 +·C12H8O6S2 2−·3H2O, determined at 173 K, the biphenyl-4,4′-disulfonate dianions lie across crystallographic inversion centres with the sulfonate groups interacting head-to-head through centrosymmetric cyclic bis(water)-bridged hydrogen-bonding associations [graph set R 4 4(11)], forming chains. The 2-(2,4-dinitrobenzyl)pyridinium cations are linked to these chains through pyridinium–water N—H⋯O hydrogen bonds and a two-dimensional network is formed through water bridges between sulfonate and 2-nitro O atoms, while the structure also has weak cation–anion π–π aromatic ring interactions [minimum ring centroid separation = 3.8441 (13) Å].


Comment
The Lewis base 2-(2,4-dinitrobenzyl)pyridine (DNBP) has been a compound of considerable interest for more than 40 years because of its unusual photochromic characteristics. Irradiation of the colourless crystals with light of wavelength 400nm or less results in the formation of a deep blue coloration in a reversible tautomeric reaction. The structure of the colourless form has been determined (Seff & Trueblood, 1968;Scherl et al., 1996), while in another determination (Naumov et al., 2002), the structures of both forms were determined, confirming the presence of two-photon excitation giving nitro-assisted proton transfer (NAPT) involving an oxygen of the o-nitro substituent group. The effect is not present in the p-nitro-substituted isomer. Although the structure of the chloride salt of DNBP is known (Naumov et al., 2005), no other examples of analogous compounds are present in the CSD.
Of a number of reactions of DNBP with aromatic carboxylic and sulfonic acids in 50% ethanol-water, we found that only one, biphenyl-4,4'-disulfonic acid (BPDS) gave crystals of suitable quality for X-ray analysis, the title compound 2(C 12 H 10 N 3 O 4 + ) C 12 H 8 O 6 S 2 2-. 3H 2 O (I), the structure of which is reported here. The structures of 1:2 proton-transfer compounds of BPDS are also not prevalent, e.g. with β-alanine (Liao et al., 2001), but the bis(guanidinium) salt is notable as a co-host structure for cooperative guest recognition in clathrate formation with numerous aromatic monocyclic and polycyclic hydrocarbons Holman & Ward, 2000).
With compound (I) (Fig. 1), the BPDS dianions lie across crystallographic inversion centres with the sulfonate groups interacting head-to-head through centrosymmetric cyclic bis(water)-bridged hydrogen-bonding associations [graph set R 4 4 (11) (Etter et al., 1990)], forming one-dimensional chain structures (Fig 2). The cations are linked to these chains through pyridinium N + -H···O water hydrogen bonds (Table 1). The second water molecule (O2W) which has only 50% occupancy, forms a O sulfonate ···H-O-H···O o-nitro hydrogen bond, bridging the chains down the b axial direction, giving a two-dimensional network structure. There are also weak cation-anion π-π aromatic ring interactions present [minimum ring centroid

Experimental
The title compound was synthesized by heating together under reflux for 10 minutes, 1 mmol quantities of 2-(2,4dinitrobenzyl)pyridine with biphenyl-4,4'-disulfonic acid in 50 ml of 50% ethanol-water. After concentration to ca. 30 ml, supplementary materials sup-2 partial room temperature evaporation of the hot-filtered solution gave colourless blade-shaped flat prisms (m.p. 413 K) from which a block section was cleaved for the X-ray analysis.

Refinement
Hydrogen atoms involved in hydrogen-bonding interactions were located by difference methods and their positional and isotropic displacement parameters were refined. Other H atoms were included in the refinement at calculated positions [C-H = 0.93 Å (aromatic) and 0.97 Å (aliphatic) and with U iso (H) = 1.2U eq (C)], and treated as riding. One of the water molecules was found to have partial occupancy which was refined to 0.50 (1) and subsequently set invariant. Fig. 1. Molecular configuration and atom naming scheme for the DNBP cation, the BPDS dianion and the two water molecules of solvation [O1W , O2W, with the latter having SOF = 0.5 (1)], in the asymmetric unit of (I). The dianion lies across an inversion centre [symmetry code (iii) -x + 2, -y + 1, -z +1] and displacement ellipsoids are drawn at the 50% probability level.

Special details
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell esds are taken into account in the estimation of distances, angles and tor-

sion angles
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 Rfactors(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.