A new polymorph of 1-({[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]iminio}methyl)naphthalen-2-olate

The title compound, C15H17NO4, containing two molecules in the asymmetric unit is a polymorph of the crystal structure published by Martínez et al. [(2011). Eur. J. Org. Chem. pp. 3137-3145] which at 120 K is monoclinic with one molecule in the asymmetric unit. Both molecules in the title compound are in the trans form. In the crystal, N—H⋯O and O—H⋯O hydrogen bonds connect molecules, forming a two-dimensional network parallel to (001).

As a part of our studies on the synthesis and structural properties of Schiff bases with naphthaldehyde and methylamine, we have determined the structure of the title compound ( Fig. 1). Some examples of related structures already appear in the literature (Wang et al., 2011;Kennedy et al., 2013;Abu-Dief et al., 2015). The structure of the title compound contains two molecules in the asymmetric unit ( Fig. 1) in contrast to the polymorph in which there is a single molecule (Martínez et al., 2011). Both molecules in the title compound are in the trans form. In the crystal, N-H···O and O-H···O hydrogen bonds connect molecules forming a two-dimensional network parallel to (001) (Fig. 2).

S2. Synthesis and crystallization
An ethanol solution (10 mL) of tris(hydroxymethyl)aminomethane (tris, 0.1 mol, 0.1211g) was added to another ethanol (10mL) containing 2-hydroxy-1-naphthaldehyde (0.1 mol, 0.1728 g), Then the solution was refluxed for 2 h and cooled to room temperature. The mixture was filtered and dried under vacuum. The title compound was crystallized as block crystals from a solution of ethanol by slow evaporation.

S3. Refinement details
All H atoms were visible in differnce Fourier maps and the presence of those bonded bonded to N1 and N2 confirm the enolate form. Utimately, all H atoms were placed in calculated positions with C-H = 0.93-0.97Å, N-H = 0.86Å and O -H = 0.82Å and were included in the refinment in a riding-motion approximation with U iso (H)=1.2U eq (C,N) and

Figure 1
The molecular structure of the title compound with displacement ellipsoids drawn at the 30% probability level.

Figure 2
Part of the crystal structure with the hydrogen bonds drawn as dashed lines.   (7) 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.