Sodium p-toluenesulfinate tetrahydrate

The title compound, Na+·C7H7O2S−·4H2O, is the hydrate of the sodium salt of para-toluenesulfinic acid. The molecular geometry around the sulfur atom is tetrahedral with X–S–Y angles spanning a range of 102.23 (6)–110.04 (6)°. In the crystal, the water molecules connect the sodium cations into chains along the b axis via O—H⋯O hydrogen bonds. An intermolecular O—H⋯π interaction is also observed.


Comment
Multidentate ligands play a major role in the synthesis of coordination polymers and metal-organic framework compounds (MOFs). Especially derivatives of benzoic acid have found widespread use in this aspect and a variety of these coordination polymers have been characterized in solution and in the solid state. Owing to the desire to synthesize functionalized MOFs whose poresizes or even complete architectural set-ups might easily be influenced upon variation of external parameters such as pH value or the presence and concentration of molecules that might reside inside the pores of these MOFs, chelating ligands related to benzoic acid but with the ability to change their bonding behaviour are necessary. In this aspect, para-toluenesulfinic acid seemed of interest since it may act as neutral or anionic ligand, and even the sulfur atom may show donor action. In order to gather structural information to allow for the tailored synthesis of MOFs based on para-toluenesulfinic acid, we determined the crystal structure of its sodium salt. The crystal structure of the sodium salt of para-toluenesulfonic acid is apparent in the literature (Reinke & Rudershausen, 1999).
Taking into account the lone pair on the sulfur atom, the latter is present in a pseudo-tetrahedral molecular geometry. The X-S-Y angles span a range of 102.23 (6)-110.04 (6) °. The least-squares planes defined by the atoms of the aromatic system on the one hand and the SOO group on the other hand intersect at an angle of 64.47 (6) °. The sodium cation is coordinated by six water molecules (two of them symmetry-generated) of which two act as bridging ligands to the neighbouring sodium cation and thus foster the formation of a "sodium-acqua-polymer" chain along the crystallographic b axis (Fig. 2). The angles between two trans-orientated water molecules in the resultant [Na(H 2 O) 6 ] + octahedra were found adopting values between 163.96 (5) ° and 173.68 (4) °.
The crystal structure is dominated by hydrogen bonds. Except for one of the hydrogen atoms on one water molecule that is part of a O-H···π interaction, all of the water molecules take part in O-H···O hydrogen bonding. Each of the sulfinic acid group's O atoms acts as multifold acceptor. In terms of graph set analysis (Etter et al. (1990); Bernstein et al. (1995)), the description of the hydrogen bonding systems necessitates a DDDDDDDD descriptor on the unitary level. In total, the components of the crystal structure are connected to double layers perpendicular to the crystallographic a axis with the hydrophobic aromatic moieties forming the outer surfaces of these layers. π-Stacking is not a prominent feature of the crystal structure with the shortest distance between two aromatic systems found at 5.5359 (11) Å.

Experimental
The compound was obtained commercially (KEG). Crystals suitable for the X-ray diffraction study were obtained upon free evaporation of an aqueous solution thereof.

Refinement
Carbon-bound H-atoms were placed in calculated positions (C-H 0.95 Å) and were included in the refinement in the riding model approximation, with U(H) set to 1.2U eq (C). The H-atoms of the water molecules were located on a difference Fourier supplementary materials sup-2 map, and their O-H distances as well as their H-O-H angles were refined using DFIX instructions with one common free variable, with their U(H) set to 1.5U eq (O).