5-Amino-3-methyl-1-phenyl-1H-pyra- zole-4-carbaldehyde hemihydrate: a polarized electronic structure within hydrogen-bonded sheets of R 10 8 (34) rings Jairo Quiroga, a Jorge Trilleras, b Justo Cobo c and Christopher Glidewell d * a Departamento de Quı ´mica, Universidad de Valle, AA 25360 Cali, Colombia, b Departamento de Quı ´mica, Universidad del Atla ´ntico, Km 7 vı ´a antigua Puerto Colombia, Barranquilla, Colombia, c Departamento de Quı ´mica Inorga ´nica y Orga ´nica, Universidad de Jae ´n, 23071 Jae ´n, Spain, and d School of Chemistry, University of St Andrews, Fife KY16 9ST, Scotland Correspondence e-mail: cg@st-andrews.ac.uk Received 14 December 2009 Accepted 14 December 2009 Online 19 December 2009 In the title compound, C 11 H 11 N 3 O0.5H 2 O, the water molecule lies across a twofold rotation axis in the space group Pbcn. The bond distances in the organic component provide evidence for polarization of the electronic structure. The molecular components are linked into puckered sheets of R 10 8 (34) rings by a combination of O—HN and N—HO hydrogen bonds; adjacent sheets are weakly linked by an aromatic – stacking interaction. Comparisons are made with some fused- ring analogues. Comment The Vilsmeier–Haack formylation reaction allows access to a large variety of heterocyclic carbaldehyde derivatives, all of which permit a wide range of functional elaboration, acting in particular as extremely valuable intermediates for the synth- esis of new fused heterocyclic systems. Accordingly, we have now prepared the title compound, (I) (Ha ¨ ufel & Breitmaier, 1974), as a versatile precursor for the synthesis of fused pyrazole derivatives of potential pharmaceutical interest, and we report here its molecular and supramolecular structure, which we briefly compare with those of the fused analogues (II) (Trilleras et al. , 2008) and (III) (Low et al. , 2006) (see scheme). Compound (I) is a stoichiometric hemihydrate (Fig. 1) and the asymmetric unit consists of one molecule of the organic component in a general position with a water molecule lying across a twofold rotation axis. Within the selected asymmetric unit, the water molecule lies across the axis along ( 1 2 , y, 3 4 ) and the two molecular components are linked by an O—HN hydrogen bond (Fig. 1 and Table 2). The formyl group is almost coplanar with the pyrazole ring, as shown by the value of the C5—C4—C41—O41 torsion angle of 1.5 (5)  . The displacement of formyl atom O41 from the mean plane of the pyrazole ring is only 0.020 (2) A ˚ . This coplanarity may be a consequence of both the intramolecular N—HO hydrogen bond (Table 2), which forms an S(6) motif (Bernstein et al., 1995), and the electronic delocalization (see discussion below). However, the two rings are very far from being coplanar and their planes make a dihedral angle of 47.5 (2)  , possibly as a consequence of HH repulsion between the H atoms bonded to atoms C16 and N51. The bond distances (Table 1) within the organic component provide evidence for polarization of the electronic structure. The C5—N51 bond is short for its type [mean value (Allen et al., 1987) = 1.355 A ˚ , lower quartile value = 1.340 A ˚ ], as is the organic compounds Acta Cryst. (2010). C66, o47–o49 doi:10.1107/S0108270109053906 # 2010 International Union of Crystallography o47 Acta Crystallographica Section C Crystal Structure Communications ISSN 0108-2701 Figure 1 The independent components of compound (I), showing the atom- labelling scheme and the hydrogen bond (dashed line) linking the components within the selected asymmetric unit. Atom O1 lies on a twofold rotation axis. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radii.