4-[(2-Hydroxybenzylidene)amino]-N-(5-methylisoxazol-3-yl)benzenesulfonamide: a monoclinic polymorph

The title compound, C17H15N3O4S, is a monoclinic polymorph with space group P21/c of the previously reported triclinic form in P [Subashini et al. (2009 ▶). J. Chem. Crystallogr. 39, 112–116]. In both polymorphs, intramolecular O—H⋯N hydrogen bonds and dimer formation via a pair of intermolecular N—H⋯N hydrogen bonds with an R 2 2(8) motif are observed. The two polymorphs differ in the next level of supramolecular organization involving C—H⋯O hydrogen bonds with varied packing and different conformations.


Experimental
Schiff bases derived from sulfonamide compounds also display enzymatic inhibition (Supuran et al., 1997). A polymorph, I, of the title compound has been reported earlier from our group (Subashini et al., 2009). In the present study we report a new dimorph, II, and its crystal structure.
The new dimorph crystallizes in monoclinic system (P2 1 /c) which contains only one molecule in the asymmetric unit ( Fig. 1). There is one N-H···N intermolecular hydrogen bond [N2···N3 = 2.899 (3) Å; Table 1], where the amido nitrogen acts as a donor and the nitrogen of the isoxazole acts as an acceptor. The donor and acceptor of one of the Schiff base molecule pair up with another inversely related molecule to form a dimer with graph set notation R 2 2 (8) (Fig 2). A similar type of homosynthon is also formed in the polymorph I. A recent statistical survey has shown that the absence of amino group in the sulfonamide compounds further encourages amido protons to hydrogen bond with an activated nitrogen (atom of a heterocycle that lies in conjugation with the amido nitrogen) of the heterocyclic ring to form an optimum sized R 2 2 (8) ring motif (Adsmond & Grant, 2001). This could be the reason why both the polymorphs form the same synthons thereby succumb to similar first order arrangement of molecules.
Polymorph II forms an intramolecular C-H···O bond with a graph set notation S(6) similar to its predecessor. It is only the weaker bonds in both the polymorphs that are significantly responsible for the differences in arrangement of molecules in the crystal. In both the polymorphs the homosynthon combines with each other through a couple of weak C-H···O bonds (C10-H10···O1 in polymorph II) forming a large ring motif with graph set notation R 4 4 (12) and R 4 4 (14) for polymorphs I and II respectively. These homosynthons stretch along a particular direction linked through weak C-H···O interaction forming a supramolecular chain (Fig. 3).
In addition to the packing differences there are changes in the conformations of the molecules which are obvious from their torsion angles. Though there are many possible conformations in which the molecule could exist, the steric and electrostatic constraints of sulfonamides have restricted them to torsion angles between 70-120° and 60-90° for ε1 and ε2 respectively [where ε1 and ε2 are the modulus of the torsion angles along C-S and S-N bonds respectively] (Kálmán et al., 1981). The angles ε1 and ε2 for polymorph II lie in the expected range. The bond lengths and the bond angles among the polymorphs are more or less close to one another but differ hugely in their torsion angles.
To conclude with, both dimorphs posses same intermolecular hydrogen bonds leading to formation of dimers through N-H···N hydrogen bonds, showing similar first level of organization. They differ only in the higher level supramolecular organization through C-H···O hydrogen bonds. A thorough inspection reveals that conformational changes in the molecule lead to the packing differences between the dimorphs.

supplementary materials sup-2 Experimental
The base of the title compound was synthesized by refluxing ethanolic solutions of 4-amino-N-(5-methyl-3-isoxazolyl) benzene sulfonamide (SMZ) (25 mg, Qualigens) with 2-hydroxy benzaldehyde (SA) (25 mg, LOBA Chemie) in an equimolar ratio. The mixture was refluxed for up to 6 h and then transferred to a beaker which was eventually allowed to evaporate slowly in the mother liquor. After a couple of days yellow crystals separated out. This base was characterized using singlecrystal XRD method and found to be a new polymorph.

Refinement
All hydrogen atoms were positioned geometrically and were refined using riding model.

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 e.s.d.'s are taken into account in the estimation of distances, angles and tor-

sion angles
Refinement. Refinement on F 2 for ALL reflections except those flagged by the user for potential systematic errors. Weighted Rfactors wR and all goodnesses 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 observed criterion of F 2 > σ(F 2 ) is used only for calculating -R-factor-obs 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.