6,6′-Dimethoxy-2,2′-[2,2-dimethylpropane-1,3-diylbis(nitrilomethylidyne)]diphenol

The title Schiff base compound, C21H26N2O4, exhibits two crystallographically independent molecules in the asymmetric unit with similar conformations. The imino groups are coplanar with the benzene rings; the maximum deviations of the N atoms from the planes comprising the benzene rings and the imino groups are −0.037 (4), 0.013 (4), −0.021 (5), and 0.008 (5) Å. The dihedral angles between the benzene rings in the two molecules are 53.64 (17) and 51.93 (17)°. Strong intramolecular O—H⋯N hydrogen bonds generate S(6) ring motifs. The N atoms are also in close proximity to the H atoms of the dimethylpropane groups, with H⋯N distances between 2.54 and 2.75 Å. The crystal structure is further stabilized by weak intermolecular C—H⋯O hydrogen bonds, weak intermolecular C—H⋯π interactions and π–π contacts involving the imine C atom and two C atoms from the adjacent benzene rings.


S1. Comment
Schiff bases are one of the most prevalent mixed-donor ligands in the field of coordination chemistry. They play an important role in the development of coordination chemistry related to catalysis and enzymatic reactions, magnetism, and supramolecular architectures (Calligaris & Randaccio, 1987). Structures of Schiff bases derived from substituted benzaldehydes and closely related to the title compound have been reported earlier (Li et al., 2005;Bomfim et al., 2005;Glidewell et al., 2006;Sun et al., 2004).
The molecule of the title compound ( Fig. 1), consists of two crystallographically independent molecules, A and B, with similar conformations. The bond lengths (Allen et al., 1987) and angles are within the normal ranges. The imino groups in both molecules are coplanar with the benzene rings they are attached to, and the dihedral angles between the benzene rings in molecules A and B are 53.64 (17) and 51.93 (17)°, respectively. Strong intramolecular O-H···N hydrogen bonds generate S(6) ring motifs (Bernstein et al., 1995). The nitrogen atoms are also in close proximity to the hydrogen atoms of the dimethylpropane groups with H···N distances between 2.54 and 2.75 Å ( Table 2). There is also a significant πstacking interaction between the planar sections associated with C1A-C6A-C7A-N1A and C5B-C6B-C7B-N1B (Table   1), and the crystal structure is further stabilized by weak intermolecular C-H···O hydrogen bonds and weak intermolecular C-H···π interactions (Cg1 and Cg2 are the centroids of the C12B-C17B and C12A-C17A benzene rings) ( Table 2).

S2. Experimental
In a 50 ml round-bottomed flask, 3-methoxy salicylaldehyde (2 mmol, 304 mg) was added into a 30 ml ethanolic solution of 2,2-dimethyl-1,3-propane diamine (1 mmol, 102 mg) and then the mixture was next refluxed for 1 h. The resulting yellow solid was filtered and washed with cold ethanol. Single crystals suitable for X-ray diffraction were obtained by slow evaporation of an ethanol solution at room temperature.

S3. Refinement
H atoms of the hydroxy groups were positioned by a freely rotating O-H bond and constrained with a fixed distance of 0.84 Å. The rest of the hydrogen atoms were positioned geometrically and refined using a riding model with C-H = 0.93-0.97 Å and U iso (H) = 1.2 or 1.5U eq (C). A rotating-group model was applied for the methyl hydrogen atoms of the methoxy groups. In the absence of sufficient anomalous scattering effects the Friedel pairs were merged prior to refinement.    The crystal structure of the title compound, viewed down the a axis.

6,6′-Dimethoxy-2,2′-[2,2-dimethylpropane-1,3-diylbis(nitrilomethylidyne)]diphenol
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.