Zwitterionic 1-{(E)-[(2-methylphenyl)iminiumyl]methyl}naphthalen-2-olate

The title Schiff base, C18H15NO, crystallizes in its zwitterionic form and an N—H⋯O hydrogen bond closes an S(6) ring. The dihedral angle between the aromatic ring systems is 36.91 (10)°. Weak aromatic π–π stacking occurs in the crystal [minimum centroid–centroid separation = 3.7771 (15) Å].


Comment
Schiff bases formed by condensation reactions of 2-hydroxy-1-aromatic aldehydes with various amines have been extensively studied (Deneva et al., 2013;Martınez et al., 2011). An interesting feature of these compounds is their faculty to display two possible tautomeric forms, the phenol-imine (OH) and the keto-amine (NH) forms. Depending on the tautomers, two types of intramolecular hydrogen bonds are observed in Schiff bases, O-H···N in phenol-imine and N-H···O in keto-amine forms. Another intermediate form of the Schiff base compounds is also known as zwitterion with an ionic intramolecular hydrogen bond N + -H···O -.
The molecular structure of (I) is illustrated in Fig. 1. The dihedral angle between the benzene ring and naphthalene ring is 33.7 (3)°. An intramolecular N-H···O hydrogen bond is found ( Table 1).
The C12-N13 bond 1.312 (3) Å and the C2-O1 bond 1.301 (3) Å of the title compound are the most important indicators of the tautomeric type. While the C2-O1 bond is a double bond for a keto-amine tautomer, this bond has a single bond character in the corresponding phenol-imine tautomer; in addition, the C12-N13 bond is also a double bond in the phenol-imine tautomer but is a single bond length in the keto-amine tautomer (Albayrak et al., 2010;Petek et al., 2007). However, in the title Schiff base, these bond distances have intermediate values between single and double bonds which are 1.362 Å and 1.222 Å respectively for C-O and 1.339 and 1.279 Å respectively for C-N bond distance (Allen et al., 1987). The shortened C2-O1 bond and the slightly longer C12-N13 bond provide structural evidence for the zwitterionic tautomeric form of the title compound.

Experimental
A mixture of a solution containing (3 mmol) of 2-hydroxy-1-naphthaldehyde and (3 mmol) of o-toluidine in 8 ml absolute ethanol. The mixture was stirred and heated under reflux for ca 5 h. The resulting solution was reduced under vacuum and cooled. A yellow solid was obtained; filtered off, washed with cold water and dried, the product was recrystallized from acetonitrile solvent as yellow rods.

Refinement
All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. All H atoms, attached to carbon atoms have been placed in calculated positions positions and refined as riding, with C-H = 0.95 (aromatic), 0.98 Å(methyl) and N-H = 0.88, respectively, and U iso (H) = 1.2 U eq (C,N) or 1.5 U eq (C methyl ). The absolute structure was indeterminate in the present experiment.  Molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.