Crystal structure of 1,1′-{(pentane-1,5-diyl)bis[(azaniumylylidene)methanylylidene]}bis(naphthalen-2-olate)

The whole molecule of the title Schiff base is generated by twofold rotational symmetry. It crystallizes as a bis-zwitterion, and there are two intramolecular N—H⋯O hydrogen bonds present. In the crystal, molecules are linked by pairs of C—H⋯O hydrogen bonds forming ribbons propagating along [001].

The microwave-assisted synthesis method, in solvent or solvent-free, is efficient and rapid. It gives cleaner reactions, is ease to use, gives higher yields and is a more economical synthetic process for the preparation of Schiff base compounds compared to conventional methods. It has been used to enhance the yield and reduce the time of certain reactions: for example, a one-step synthesis of D-A-D chromophores as active materials for organic solar cells (Jeux et al., 2015), or the synthesis of a series of acyclic Schiff basechromium(III) complexes .
In a continuation of our work on Schiff base ligands, we report herein on the crystal structure of the title compound, synthesized using two methods, viz. microwave irradiation and ISSN 2056-9890 conventional, by condensing o-hydroxynaphthaldehyde and 1,5-diaminopentane.

Database survey
Recently, our group reported the crystal structures of three new Schiff bases synthesized using conventional or ultrasonic irradiation methods by reacting primary amines and o-hydroxynaphthaldehyde (Ouari et al., 2015a,b,c). They too crystallize as bis-zwitterionic compounds with strong intramolecular N-HÁ Á ÁO hydrogen bonds forming S(6) ring motifs.
Method 2: Conventional synthesis The title Schiff base was prepared by condensation between 1,5-diaminopentane (51 mg, 0.5 mmol) and 2-hydroxy-1naphthaldehyde (172 mg, 1 mmol) in methanol (10 ml). The mixture was refluxed and stirred under a nitrogen atmosphere for 3 h. The precipitate obtained was filtered, washed with methanol and diethyl ether and dried in vacuum overnight. Yellow single crystals of the title compound were obtained by slow evaporation of a solution in methanol (yield 71%; m.p.: 438-440 K).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The iminium H atom was located from a difference Fourier map and freely refined. C-bound H atoms were included in calculated positions and treated as riding atoms: C-H = 0.95 À 0.99 Å with U iso (H) = 1.2U eq (C). Atom C14 lies on the twofold rotation axis and the H atoms were placed using instruction HFIX 23 (Sheldrick, 2015); the The molecular structure of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The intramolecular hydrogen bonds are shown as dashed lines (see Table 1). The unlabelled atoms are related to the labelled atoms by twofold rotational symmetry (atom C14 lies on the twofold axis; symmetry code: Àx, y, Àz + 1 2 ). Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Crystal packing of the title compound viewed along the b axis. The hydrogen bonds are shown as dashed lines (see Table 1