Space group revsion of the triclinic polymorph of salicylaldehyde azine

The structure of the title compound, C14H12N2O2 {systematic name: 2,2′-[hydrazinediylidenebis(methanylylidene)]diphenol}, has already been determined in the triclinic space group P with Z = 4 [El-Medani, Aboaly, Abdalla & Ramadan (2004 ▶). Spectrosc. Lett. 37, 619–632]. However, the correct space group should be P21/c with Z = 4. This structure is a new polymorph of the already known monoclinic polymorph of salicyladehyde azine, which crystallizes in space group P21/n with Z = 2. The benzene rings form a dihedral angle of 46.12 (9)°. Two intramolucular O—H⋯N hydrogen bonds occur.


Related literature
For the structure of salicylaldehyde azine in P1 with Z=4, see El-Medani et al. (2004). For the other monoclinic polymorph of salicyladehyde azine, see for example Xue et al. (1994).  Table 1 Hydrogen-bond geometry (Å , ). be P2 1 /c with Z=4. The authors have determined the unit-cell parameters correctly, however, they thought that the structure is triclinic with four molecules in the asymmetric unit, whereas the correct description should be in the monoclinic crystal systems with two half molecules in the asymmetric unit. This structure is a new polymorph of the already known monoclinic polymorph of salicyladehyde azine, which crystallizes in the space group P2 1 /n with Z=2.

Experimental
The title compound crystallizes with two half molecules in the asymmetric unit, both of which are located on a crystallographic centre of inversion. The molecules are essentially planar (r.m.s. deviation for all non H-atoms 0.021 and 0.018 Å, for the two molecules in the asymmetric unit). Bond lengths and angles are in the normal ranges.
In the already known monoclinic polymorph there is just one half molecule in the asymmetric unit which is located on a centre of inversion. The dihedral angle between symmetry equivalent molecules is 64.6°. The title compound, on the other hand, crystallizes with two half molecules in the asymmetric unit, which enclose a dihedral angle of 47.4°. The dihedral angle between symmetry equivalent molecules is 67.5°. Thus the difference between the two monoclinic polymorphs is the different mutual orientation of the molecules in the unit cell.

Experimental
Hydrazine hydrate, (1 mmol) dissolved in 5 ml ethanol was added dropwise to a solution of salicylaldehyde, (2.2 mmol) in 10 ml ethanol at room temperature with continuous stirring. The reaction mixture was reflux for 4 h and completion monitored by TLC. The reaction mixture was concentrated and resulted product was separated. Single crystal of the compound, suitable for X-ray crystallography, was grown by slow evaporation from an ethyl acetate-ethanol solution (2:1). as colourless crystals:

Refinement
The H atoms were initially located by difference Fourier synthesis. Subsequently, H atoms bonded to C atoms were refined using a riding model, with C-H = 0.95 Å and with U iso (H) = 1.2U eq (C). The H atoms bonded to O were freely refined.