organic papers Acta Cryst. (2005). E61, o1007–o1009 doi:10.1107/S1600536805007373 Ray J. Butcher et al.  C 20 H 14 ClN 3 O 3 0.5H 2 O o1007 Acta Crystallographica Section E Structure Reports Online ISSN 1600-5368 A quasi-planar polyaromatic compound containing an azo and a Schiff base linkage Ray J. Butcher, a * Tushar S. Basu Baul, b Keisham Surjit Singh b and Frank E. Smith c a Department of Chemistry, Howard University, 525 College Street NW, Washington, DC 20059, USA, b Department of Chemistry, North- Eastern Hill University, NEHU Permanent Campus, Umshing, Shillong 793 022, India, and c Weill Cornell Medical College in Qatar, PO Box 24144, Doha, Qatar Correspondence e-mail: butcher@harker.nrl.navy.mil Key indicators Single-crystal X-ray study T = 293 K Mean (C–C) = 0.007 A ˚ R factor = 0.076 wR factor = 0.216 Data-to-parameter ratio = 13.3 For details of how these key indicators were automatically derived from the article, see http://journals.iucr.org/e. # 2005 International Union of Crystallography Printed in Great Britain – all rights reserved The title compound, 2-{(E)-3-[(E)-4-chlorophenylimino]-4-hy- droxyphenyldiazenyl}benzoic acid hemihydrate, C 20 H 14 Cl- N 3 O 3 0.5H 2 O, forms sheets, parallel to the (100) plane, that are linked by weak aromatic C—HO interactions. These sheets are in turn linked by strong hydrogen bonds to interstitial water molecules. Comment Aromatic monoamines react with 2-[(E)-2-(3-formyl-4- hydroxyphenyl)-1-diazenyl]benzoic acid to form products, such as the title compound, (I), which contain both azo and Schiff base (i.e. imino) linkages. Recently, a few tributyltin(IV) complexes containing similar azo and Schiff base components were obtained which exhibit interesting biological properties (Basu Baul et al., 2004). Their structures were determined using spectroscopic techniques and single-crystal X-ray crys- tallography. A similar azo–Schiff base organic compound, namely N-p-methoxybenzylidene-p-phenylazoaniline, is a mesogen and exhibits a nematic liquid-crystal phase; its X-ray structure has also been determined recently (Revannasiddaiah et al., 1997). In addition, structures of two similar molecules containing meta azo and imine substituents, also exhibiting liquid crystalline properties, have recently been determined (Baumeister et al., 1990; Cariati et al., 2004). The potential usefulness of such systems has prompted us to determine the structure of the title compound, (I). The title compound (Fig. 1) crystallizes in the monoclinic space group C2/c with Z = 8. Interstitial water molecules (one for each two of the title molecules) are located on four twofold axes that pass through the unit cell. The three-ring system assumes an extended conformation, with both outer rings slightly twisted with respect to the central aromatic ring. These quasi-planar molecules form sheets, parallel to the (100) plane (see Fig. 2), that are linked by weak aromatic C—HO interactions. Adjacent sheets are linked by strong hydrogen bonds to interstitial water molecules (see Fig. 3). Bond lengths and angles within the molecule are unremarkable and similar to those found in related molecules (Baumeister et al., 1990; Cariati et al., 2004). Received 16 December 2004 Accepted 8 March 2005 Online 18 March 2005