Sulfonated 1,3-bis(4-pyridyl)propane

In the title compound, 4-[3-(3-sulfonatopyridin-1-ium-4-yl)propyl]pyridin-1-ium-3-sulfonate, C13H14N2O6S2, the molecule is zwitterionic, with the sulfonic acid proton transfered to the basic pyridine N atom. Also, the structure adopts a butterfly-like conformation with the sulfonate groups on opposite sides of the ‘wings’. The dihedral angle between the two pyridinium rings is 83.56 (7)°, and this results in the molecule having a chiral conformation and packing. There is strong intermolecular hydrogen bonding between the pyridinium H and sulfonate O atoms of adjoining molecules. In addition, there are weaker intermolecular C—H⋯O interactions.

In the title compound, 4-[3-(3-sulfonatopyridin-1-ium-4-yl)propyl]pyridin-1-ium-3-sulfonate, C 13 H 14 N 2 O 6 S 2 , the molecule is zwitterionic, with the sulfonic acid proton transfered to the basic pyridine N atom. Also, the structure adopts a butterfly-like conformation with the sulfonate groups on opposite sides of the 'wings'. The dihedral angle between the two pyridinium rings is 83.56 (7) , and this results in the molecule having a chiral conformation and packing. There is strong intermolecular hydrogen bonding between the pyridinium H and sulfonate O atoms of adjoining molecules. In addition, there are weaker intermolecular C-HÁ Á ÁO interactions.
RJB wishes to acknowledge the NSF-MRI program (grant CHE-0619278) for funds to purchase the diffractometer, and we thank the Office of Naval Research for financial support.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: FL2337).

Comment
From the titled compound C 13 H 14 N 2 O 6 S 2 we can see that the sulfonatation of 1,3-bis(4-pyridyl)propane occurs in the meta position under mercury catalysis, the same as when pyridine is sulfonated under similar conditions (McElvain & Goese, 1943). From recent studies, 1,3-bis(4-pyridyl)propane ligands have been shown (Chen et al., 2010;Correa et al., 2010;Sun et al., 2010;Zheng et al., 2010) to be very flexible, and this has been taken advantage of in supramolecular chemistry. Our interest was to link the sulfonated pyridines into a zwitterionic polymer. (Estrin & Entelis, 1974;Sundaram et al., 2010).
Polymer zwitterions are very advantageous in the sense that their distinct polar ends have high electric dipoles, and can readily reorient to an applied electric field if the chain is flexible.
In view of the interest in a flexible zwitterionic polymer backbone, the starting material, 1,3-bis(4-pyridyl)propane was sulfonated. The structure of this derivative is reported here. The structure shows that both pyridine rings have been sulfonated in the 3-position. As is to be expected for a moiety containing both acidic and basic substituents, the molecule is zwitterionic, with the sulfonic acid proton transfered to the basic pyridine N. The structure has adopted a "butterfly-like" conformation with the sulfonate groups on opposite sides of the "wings". The dihedral angle of 83.56 (7)° between the two pyridinium rings is shown in Figure 2. This has resulted in the molecule being chiral in the solid state even though it is not chiral in solution. There is strong intermolecular hydrogen bonding between the pyridinium H and sulfonate O atoms of adjoining molecules. In addition there are weaker intermolecular C-H···O interactions.

Experimental
The compound was prepared by adding a mercury catalyst (0.05 g) to 0.33 g of 1,3-bis(4-pyridyl)propane dissolved in 3.24 g of fuming sulfuric acid (oleum). The reaction mixture was placed in a quartz tube that was sealed under vacuum with a fill factor of 10/15.4 cm. The quartz tube was then placed into a pressurized hydrothermal vessel that was set in a furnace at a temperature of 245°C. The pressure vessel attained an internal temperature of 204°C, and the reaction continued for 5 days. After cooling, the quartz tube was removed from the chamber behind a blast shield in a fume hood, and was frozen with liquid nitrogen before opening. The liquid was then poured into an Erlenmeyer flask containing about 10 ml of triply distilled water and was allowed to stand. Crystals of the titled compound slowly appeared over a month at which point they were washed with alcohol and allowed to air dry on top of the oven at about 50°C. Approximately 0.15 g was isolated (25%).  Fig. 1. Diagram of C 13 H 14 N 2 O 6 S 2 illustrating the atom numbering scheme used. Thermal ellipsoids are at the 30% probability level.  as those based on F, and R-factors based on ALL data will be even larger.