4,4′-Dichloro-2,2′-[2,2-dimethylpropane-1,3-diylbis(nitrilomethylidyne)]diphenol

The crystal of the title Schiff base compound, C19H20Cl2N2O2, contains of two crystallographically independent molecules with similar conformations. In each molecule, two intramolecular O—H⋯N bonds generate S(6) motifs. The N atoms are also in close proximity to two H atoms of the dimethylpropane groups, with H⋯N distances between 2.59 and 2.62 Å. The imine group is coplanar with the benzene ring. The dihedral angles between the benzene rings in the two independent molecules are 58.20 (12) and 47.95 (12)°. The structure displays short intermolecular Cl⋯Cl [3.3869 (11) Å] and Cl⋯O [3.175 (2)–3.204 (2) Å] interactions. The crystal structure is further stabilized by weak intermolecular C—H⋯O, C—H⋯π and π–π [centroid–centroid distances 3.6416 (13)–3.8705 (14) Å] interactions.


Comment
In the field of coordination chemistry, Schiff base is one of most prevalent versatile ligands. The Schiff base compounds have received much attention due to their important role in the development of coordination chemistry related to catalysis and enzymatic reaction, magnetism and supramolecular architectures (Casellato & Vigato 1977). In comparison to the Schiff base metal complexes, there is only a relatively small number of free Schiff base ligands which have been characterized structurally (Calligaris & Randaccio, 1987). Structures of Schiff bases derived from substituted benzaldehydes and closely related to the title compound have been reported (Li et al., 2005;Bomfim et al., 2005;Glidewell et al., 2005Glidewell et al., , 2006Sun et al., 2004).
In the title compound (I, Fig. 1), bond lengths (Allen et al., 1987) and angles are within the normal ranges and are comparable with the related bromo-substituted compound (Fun et al., 2008). The asymmetric unit of (I) consists of two crystallographically independent molecules A and B. The intramolecular O-H···N hydrogen bonds generate S(6) ring motifs.

Experimental
The synthetic method has been described earlier (Fun et al., 2008). Single crystals suitable for X-ray diffraction were obtained by evaporation of an ethanol solution at room temperature.

Refinement
The H atoms of the hydroxy groups were located from the difference Fourier map and refined freely. The rest of the hydrogen atoms were positioned geometrically and refined using a riding model, with C-H = 0.95-0.99 Å and with U iso (H)= 1.2-1.5U eq (C). The reflection (002) was omitted as its intensity was affected by the beam backstop.
supplementary materials sup-2 Figures Fig. 1. The molecular structure of (I), with atom labels and 50% probability ellipsoids for non-H atoms. Intramolecular interactions are shown as dashed lines.

Special details
Experimental. The low-temperature data was collected with the Oxford Cyrosystem Cobra low-temperature attachment.
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
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 > 2sigma(F 2 ) is used only for calculating R-factors(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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )