5,5′-Dimethoxy-2,2′-[2,2-dimethylpropane-1,3-diylbis(nitrilomethylidyne)]diphenol

The asymmetric unit of the title Schiff base compound, C21H26N2O4, consists of four crystallographically independent molecules, viz. A, B, C and D. The A and D, and the B and C molecules are related by a pseudo-inversion centre, and the remaining pairs of molecules differ in the orientations of one of the methoxy groups. In each independent molecule, intramolecular O—H⋯N hydrogen bonds generate two S(6) ring motifs. The dihedral angles between the benzene rings in molecules A, B, C and D are 65.86 (19), 50.41 (19), 68.59 (19) and 50.85 (19)°, respectively. In the crystal structure, molecules are linked by C—H⋯O hydrogen bonds, forming R 2 2(8) dimers. In addition, weak C—H⋯π interactions are observed.

The asymmetric unit of the title Schiff base compound, C 21 H 26 N 2 O 4 , consists of four crystallographically independent molecules, viz. A, B, C and D. The A and D, and the B and C molecules are related by a pseudo-inversion centre, and the remaining pairs of molecules differ in the orientations of one of the methoxy groups. In each independent molecule, intramolecular O-HÁ Á ÁN hydrogen bonds generate two S(6) ring motifs. The dihedral angles between the benzene rings in molecules A, B, C and D are 65.86 (19), 50.41 (19), 68.59 (19) and 50.85 (19) , respectively. In the crystal structure, molecules are linked by C-HÁ Á ÁO hydrogen bonds, forming R 2 2 (8) dimers. In addition, weak C-HÁ Á Á interactions are observed.

Comment
In the field of coordination chemistry, Schiff base is one of most prevalent versatile ligand. The Schiff base compounds have recieved much attention due to their important role in the development of coordination chemistry related to catalysis and enzymatic reaction, magnetism, and supramolecular architectures. In comparison with Schiff-base metal complexes, only a relatively small number of free Schiff base ligands have been characterized (Calligaris & Randaccio 1987). Crystal 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., 2006;Sun et al., 2004).
The asymmetric unit of the title compound ( Fig. 1), consists of four crystallographically independent molecules, A, B, C and D. Molecules in A/D and B/C pairs are related by pseudo-inversion centres at (0.497 0.123 0.332) and (0.486 0.627 0.331), respectively. The other pairs of molecules viz. A/B, C/D, A/C and B/D differ in the orientations of one of the methoxy groups (O4-C19). Bond lengths in the independent molecules are within normal ranges (Allen et al.,1987) and are comparable to those observed in a related structure (Fun et al., 2008). In each independent molecule, intramolecular O-H···N hydrogen bonds generate two S(6) ring motifs (Bernstein et al. 1995), with the imino group being coplanar with the benzene ring. The N atoms are also in close proximity to the H atoms of dimethylpropane groups of adjacent independent molecules, with H···N distances lying in the range 2.57-2.62 Å. The dihedral angles between the two benzene rings in molecules A, B, C and D are 66.0 (2)°, 50.5 (2)°, 68.5 (2)° and 50.9 (2)°, respectively.
In the crystal structure, symmetry related A/C and B/D pairs of molecules are linked by C-H···O hydrogen bonds forming R 2 2 (8) dimers (Fig. 2). The crystal structure is further stabilized by weak C-H···π interactions (Table 1).

Experimental
In a 50 ml round-bottomed flask, 4-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 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.

Refinement
H atoms of hydroxyl groups were constrained using a freely rotating O-H bond with a fixed distance of 0.84 Å and U iso (H)= 1.5 U eq (O). The remaining H atoms were positioned geometrically and refined using a riding model, with C-H = 0.95-0.99 Å and U iso (H)= 1.2 or 1.5 U eq (C). A rotating-group model was applied for the methoxy methyl groups.

Special details
Experimental. The low-temperature data was collected with the Oxford Cyrosystem Cobra low-temperature attachment. 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.