A monoclinic polymorph of (1E,5E)-1,5-bis(2-hydroxybenzylidene)thiocarbonohydrazide

The title compound, C15H14N4O2S, is a derivative of thioureadihydrazide. In contrast to the previously reported polymorph (orthorhombic, space group Pbca, Z = 8), the current study revealed monoclinic symmetry (space group P21/n, Z = 4). The molecule shows non-crystallographic C 2 as well as approximate C s symmetry. Intramolecular bifurcated O—H⋯(N,S) hydrogen bonds, are present. In the crystal, intermolecular N—H⋯S hydrogen bonds and C—H⋯π contacts connect the molecules into undulating chains along the b axis. The shortest centroid–centroid distance between two aromatic systems is 4.5285 (12) Å.

The title compound, C 15 H 14 N 4 O 2 S, is a derivative of thioureadihydrazide. In contrast to the previously reported polymorph (orthorhombic, space group Pbca, Z = 8), the current study revealed monoclinic symmetry (space group P2 1 /n, Z = 4). The molecule shows non-crystallographic C 2 as well as approximate C s symmetry. Intramolecular bifurcated O-HÁ Á Á(N,S) hydrogen bonds, are present. In the crystal, intermolecular N-HÁ Á ÁS hydrogen bonds and C-HÁ Á Á contacts connect the molecules into undulating chains along the b axis. The shortest centroid-centroid distance between two aromatic systems is 4.5285 (12) Å .

Related literature
For the crystal structure of the orthorhombic polymorph of the title compound reported without three-dimensional coordinates, see: Yanping et al. (1999). For the crystal structure of a methylated derivative of the title compound, see: Affan et al. (2010). For graph-set analysis of hydrogen bonds, see: Etter et al. (1990); Bernstein et al. (1995). Structures containing similar C=S distances were retrieved from the Cambridge Structural Database (Allen, 2002). For chelate ligands in coordination chemistry, see: Gade (1998 Table 1 Hydrogen-bond geometry (Å , ).

Comment
Chelate ligands have found widespread use in coordination chemistry due to the enhanced thermodynamic stability of resultant coordination compounds in relation to coordination compounds exclusively applying comparable monodentate ligands (Gade, 1998). Combining different donor atoms, a molecular set-up to accomodate a large variety of metal centers of variable Lewis acidity is at hand. In this aspect, the title compound seemed particularily interesting due to its use as strictly neutral or -depending on the pH value -as anionic or cationic ligand. In addition, due to the set-up of its donor atoms, a multitude of differently-sized chelate ligands can be formed. The presence of a thioketo group as well as amino groups, hydroxyl groups and imine-type nitrogen atoms further enhances the versatility of the title compound's ligating abilities. In our continuous interest in elucidating the rules influencing the formation of coordination compounds with different set-ups of NOS-donor atoms, we determined the crystal structure of the title compound to enable comparative studies with geometric parameters in envisioned coordination compounds. Although the compound has been reported to crystallize in the orthorhombic space In the crystal structure, intra-as well as intermolecular hydrogen bonds are apparent. While the intramolecular hydrogen bond -stemming from the hydroxyl group -shows bifurcation between the sulfur atom as well as the imine-type nitrogen atom, the intermolecular hydrogen bonds exclusively have the sulfur atom as acceptor. The presence of the sulfur-supported hydrogen bond is complemented by the results of IR spectroscopy that show the presence of three bands in the region for hydrogen bonds between oxygen, nitrogen and sulfur. In addition, C-H···π contacts can be observed that involve hydrogen atoms on the aromatic system. In terms of graph-set analysis (Etter et al., 1990;Bernstein et al., 1995), the descriptor for the bifurcated hydrogen bond is S(6)S(9) on the unitary level while the amino-group-supported hydrogen bonds necessitate a C 1 1 (4)C 1 1 (4) descriptor on the same level. A binary descriptor of R 1 2 (6) emphasizes the "chelation" of the sulfur atom by the two secondary amino groups. In total, the molecules are connected to waved, zigzag-type chains along the crystallographic b axis. The shortest intercentroid distance between two π-systems was measured at 4.5285 (12) Å and involves both aromatic moieties (Fig. 2).

Experimental
The compound was prepared upon reacting thiocarbohydrazide (0.50 mmol) with ortho-hydroxybenzaldehyde (1.00 mmol) in refluxing ethanol (15 ml) under nitrogen in analogy to a published procedure (Yanping et al., 1999). Crystals suitable for the X-ray diffraction study were obtained upon slow evaporation of the reaction mixture.

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 atom of the hydroxyl groups were allowed to rotate with a fixed angle around their respective C-O bonds to best fit the experimental electron density (HFIX 147 in the SHELX program suite (Sheldrick, 2008)). The H atoms of the amine groups were located on a difference Fourier map and refined with individual displacement parameters. Fig. 1. The molecular structure of the title compound, with atom labels and anisotropic displacement ellipsoids (drawn at 50% probability level). tacts (red dashed lines). Symmetry operators: i -x + 1, y -1/2, -z + 1/2; ii -x + 1, y + 1/2, -z + 1/ 2.