1,1′-{(Hexane-1,6-diyl)bis[(azaniumylylidene)methanylylidene]}bis(naphthalen-2-olate)

The whole molecule of the title Schiff base compound, C28H28N2O2, is generated by inversion symmetry. It is formed from two units of ortho-hydroxynaphthaldehyde bridged with 1,6-diaminohexane. The N atoms are protonated and, thus, the structure is a bis-zwitterionic compound in the solid state. The zwitterion shows strong intramolecular N—H⋯O hydrogen bonds between the iminium N and the naphthalenolate O atoms.


S1.1. Refinement
The iminium H atom was located from a difference Fourier map and refined isotropically. C-bound H atoms were included in calculated positions and treated as riding atoms: C-H = 0.95 Å (CH) or 0.99 Å (CH 2 ) with Uiso(H) = 1.2Ueq(C-H aromatics ). .

S2. Results and discussion
The synthesis of the Schiff base is similar to those described in the literature (Ramos Silva et al., 2009;Li et al., 2007;Zhu et al., 2006;Sampath Kumar et al., 2010;Bhattacharjee et al., 2012). These ligands are also currently applied in coordination chemistry for the synthesis of Schiff base complexes of transition metals (Ouari et al., 2015;Ouari et al., 2010;Ourari et al., 2008;Ourari et al., 2006). Compounds of the type of the title molecule possess diverse biological properties such as anti-anxiety, anti-depressant (Zayed et al., 2015) and anti-tumor activities as well as antibacterial and fungicidal properties (Abou-Hussein et al., 2014;Sadeek et al., 2013). We report here the synthesis of title compound and its crystal structure.
A perspective view of the title molecule, which has crystallographically-imposed centrosymmetry, is shown in Fig. 1.

Figure 1
The molecular geometry of the title compound with displacement ellipsoids drawn at the 50% probability level. H atoms are represented as small spheres of arbitrary radii. Hydrogen bonds are shown as dashed lines. Only the non-H atoms of the asymmetric unit are labelled.

Figure 2
Crystal packing of the title compound viewed along the c axis.