Bis(pyridinium) naphthalene-1,5-disulfonate dihydrate

The asymmetric unit of the title organic salt, 2C5H6N+·C10H6O6S2 2−·2H2O, consists of a pyridinium cation, half a naphthalene-1,5-disulfonate dianion and a water molecule. The dianion has a crystallographically imposed centre of symmetry. In the crystal, N—H⋯O and O—H⋯O hydrogen bonds link cations, anions and water molecules into a three-dimensional network.

The asymmetric unit of the title organic salt, 2C 5 H 6 N + Á-C 10 H 6 O 6 S 2 2À Á2H 2 O, consists of a pyridinium cation, half a naphthalene-1,5-disulfonate dianion and a water molecule. The dianion has a crystallographically imposed centre of symmetry. In the crystal, N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds link cations, anions and water molecules into a threedimensional network.
Unfortunately,the dielectric constant of the title compound as a function of temperature indicates that the permittivity is basically temperature-independent, below the melting point (411k-412k) of the compound, we have found that title compound has no dielectric disuniform from 80 K to 405 K. Herein we descibe the crystal structure of this compound.
The asymmetric unit of the title compound ( Fig. 1) consists of a pyridinium cation, a half of a naphthalene-1,5-disulfonate anion and a free water molecule, the anion having crystallographically imposed centre of symmetry. The pyridinium and naphthalene rings are oriented to form a dihedral angle of 13.89 (6)°. In the crystal, cations, anions and water molecules are connected by N-H···O and O-H···O intermolecular hydrogen bonds into a three-dimensional structure ( Fig. 2; Table 1)

Experimental
The title compound was obtained by the addition of naphthalene-1,5-disulfonate acid (3.62 g, 0.01 mol) to a solution of pyridine (1.6 g, 0.02 mol) in methanol, in the stoichiometric ratio 1: 2. Good quality single crystals were obtained by slow evaporation of the solvent after six days (yield 52%).

Refinement
The water H atoms were located in a difference Fourier map and refined as riding, with the O-H distances restrained to 0.82 Å and with U iso (H) = 1.5U eq (O). All other H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H = 0.93 Å, N-H = 0.86 Å, and with U iso (H) = 1.2U eq (C, N).

Computing details
Data collection: CrystalClear (Rigaku, 2005); cell refinement: CrystalClear (Rigaku, 2005); data reduction: CrystalClear (Rigaku, 2005); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure:  The molecular structure of the title compound, with displacement ellipsoids drawn at the 30% probability level. Atoms labelled with suffix A are generated by the symmetry operator (2-x, -y, 1-z).  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 )
x y z U iso */U eq O4 0.4852 (