4-{[3-(4-Hydroxybenzylideneamino)-2,2-dimethylpropyl]iminiomethyl}phenolate dihydrate

The asymmetric unit of the title compound, C19H22N2O2·2H2O, comprises a zwitterionic form of the Schiff base compound and two water molecules of crystallization. Intermolecular N—H⋯O, C—H⋯O and O—H⋯N hydrogen bonds involving one of the water molecules in the asymmetric unit generate seven- and eight-membered rings, with R 2 1(7) and R 2 2(8) ring motifs, respectively. The dihedral angle beween the two aromatic rings is 86.5 (2)°. The imino and iminium groups are coplanar with the benzene rings to which they are attached, making dihedral angles (N—C—C—C) of −179.3 (5) and −179.2 (4)°, respectively. Validation software indicates the higher symmetry space group Pnma for this structure. However, this process ignores H atoms and the zwitterionic configuration of the main molecule breaks the higher symmetry. Solution in Pna21 provides a chemically sensible zwitterionic compound with improved residuals. In the crystal structure, molecules are linked together through intermolecular O—H⋯O, O—H⋯N, N—H⋯O and C—H⋯O interactions, forming a three-dimensional network. The crystal structure is further stabilized by intermolecular C—H⋯π interactions.

The asymmetric unit of the title compound, C 19 H 22 N 2 O 2 Á-2H 2 O, comprises a zwitterionic form of the Schiff base compound and two water molecules of crystallization. Intermolecular N-HÁ Á ÁO, C-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds involving one of the water molecules in the asymmetric unit generate seven-and eight-membered rings, with R 2 1 (7) and R 2 2 (8) ring motifs, respectively. The dihedral angle beween the two aromatic rings is 86.5 (2) . The imino and iminium groups are coplanar with the benzene rings to which they are attached, making dihedral angles (N-C-C-C) of À179.3 (5) and À179.2 (4) , respectively. Validation software indicates the higher symmetry space group Pnma for this structure. However, this process ignores H atoms and the zwitterionic configuration of the main molecule breaks the higher symmetry. Solution in Pna2 1 provides a chemically sensible zwitterionic compound with improved residuals. In the crystal structure, molecules are linked together through intermolecular O-HÁ Á ÁO, O-HÁ Á ÁN, N-HÁ Á ÁO and C-HÁ Á ÁO interactions, forming a three-dimensional network. The crystal structure is further stabilized by intermolecular C-HÁ Á Á interactions.

Comment
In the field of coordination chemistry, Schiff base compounds are among the most prevalent and versatile ligands. They have received much attention due to their important roles in the development of coordination chemistry related to catalysis and enzymatic reactions, magnetism, and supramolecular architectures. In comparison to the Schiff base metal complexes, only a relatively small number of free Schiff base ligands have been structurally characterized (Calligaris & Randaccio, 1987). Structures of Schiff bases derived from substituted benzaldehydes have been reported (Li et al., 2005;Bomfim et al., 2005;Glidewell et al., 2005Glidewell et al., , 2006Sun et al., 2004).
The asymmetric unit of the title compound, Fig. 1, comprises a zwitterionic Schiff base compound and two water molecules of crystallization. The zwitterion results from protonation of the imine N2 atom with the O2 hydroxy group deprotonated resulting in the formation of iminium and phenolate groups. Intermolecular N-H···O and C-H···O hydrogen bonds involving the O2W O atom as a bifurcated acceptor generate an R 2 1 (7) ring motif (Bernstein et al., 1995). Intermolecular  (Table 1).

Experimental
The synthetic method has been described earlier (Fun et al., 2008), except that 4-hydroxybenzaldehyde (2 mmol, 244 mg) and 2,2-dimethylpropane diamine (1 mmol, 102 mg) were used. Single crystals suitable for X-ray diffraction were obtained by evaporation of an ethanol solution at room temperature.

Refinement
The H atom of the hydroxy group was located from the difference Fourier map and refined freely. The H atoms of O2W were located from the difference Fourier map and constrained to refine with the carrier atom with U iso (H) = 1.5U eq (O) with distance restraint of 0.85 (1) Å. The H atoms of O1W were located from the difference Fourier map and constrained to refine with the carrier atom with U iso (H) = 1.5U eq (O). Other N-bound and O-bound H atoms were located from the difference Fourier map and refined freely, see Table 1. The rest of the H atoms were positioned geometrically and refined using a sup-2 riding model with C-H = 0.93-0.97 Å and U iso (H) = 1.2U eq (C). In the absence of sufficient anomalous scattering, 904 Friedel pairs were merged. Fig. 1. The molecular structure of the title compound with atom labels and 50% probability ellipsoids for non-H atoms. Intermolecular interactions involving the O2W water molecule are shown as dashed lines.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cyrosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
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.