2,2′-Dihydroxy-3,3′-[(1E,1′E)-hydrazine-1,2-diylidenedimethylidyne]dibenzoic acid N,N-dimethylformamide disolvate

The title compound, C16H12N2O6·2C3H7NO, lies across a crystallographic inversion centre which is situated at the midpoint of the central N—N bond. The substitution at the C=N bond adopts a trans configuration and it is essentially coplanar with the benzene ring [N—C—C—C torsion angles = −173.9 (4) and 6.4 (6)°]. All torsion angles involving non-H atoms are close to 180°. Intramolecular O—H⋯O and weak C—H⋯O hydrogen bonds form S(6) and S(5) ring motifs, respectively, while intermolecular O—H⋯O and weak C—H⋯O hydrogen bonds connect the Schiff base molecule to solvent dimethylformamide molecules.

The title compound, C 16 H 12 N 2 O 6 Á2C 3 H 7 NO, lies across a crystallographic inversion centre which is situated at the midpoint of the central N-N bond. The substitution at the C N bond adopts a trans configuration and it is essentially coplanar with the benzene ring [N-C-C-C torsion angles = À173.9 (4) and 6.4 (6) ]. All torsion angles involving non-H atoms are close to 180 . Intramolecular O-HÁ Á ÁO and weak C-HÁ Á ÁO hydrogen bonds form S(6) and S(5) ring motifs, respectively, while intermolecular O-HÁ Á ÁO and weak C-HÁ Á ÁO hydrogen bonds connect the Schiff base molecule to solvent dimethylformamide molecules.
Schiff bases are one of most prevalent mixed-donor ligands in the field of coordination chemistry. Schiff bases have been used widely as ligands in the formation of transition metal complexes. There has been growing interest in Schiff base ligands, mainly because of their wide application in the field of biochemistry, synthesis, and catalysis (Pal et al., 2005;Hou et al., 2001;Ren et al., 2002).
Herein, we report the synthesis and crystal structure of the Schiff-base compound, (I). The molecule lies across a crystallographic inversion centre which is situated at the midpoint of the N-N (1.393 (6) Å) bond. The molecular structure of (I) is shown in Fig.1. All bond lengths are within in normal ranges (Allen et al., 1987). The N1-C8 [1.281 (5) Å] and N1-N1 i [1.393 (6) Å] (symmetry code: (i) -x+2, -y+2, -z+1) distances indicate these correspond to double and single bonds, respectively. The torsion angles indicate that the molecule is essentially planar with the C=N bond adjacent to the benzene rings adopting a trans configuration with resect to its substitution. Intramolecular O-H···O and C-H···O hydrogen bonds form S(6) and S(5) ring motifs, respectively (Bernstein et al., 1995). The Schiff-base molecule and solvent DMF molecules are connected by intermolecular hydrogen bonds ( Fig.1 and Table. 2). Some crystal structures which are closely related to the title compound have already been studied (Chattopadhyay et al.,2008;Cucos et al., 2006;Fu, 2007;Mijanuddina et al., 2004;Dreuw et al., 2005;Sreerama et al., 2007).

Experimental
Reagents and solvents used were of commercially available quality. To a stirred solution of 3-formylsalicylic acid (0.332 g, 2 mmol) in absolute methanol (10 ml) was added dropwise hydrazine hydrate (0.050 g, 1 mmol). After a few minutes, an orange precipitate appeared, which was isolated by filtration, washed with methanol, and dried in air. Crystals of (I) suitable for X-ray diffraction were obtained by recrystallized the crude product from DMF solution.

Refinement
H atoms were positioned geometrically and refined using a riding model, with C-H = 0.93-0.96 Å and with U iso (H) = 1.2U eq (C) or 1.5U eq (C) for methyl H atoms.

Figures
Fig . 1 Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness 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 threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) 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.