4,4′-(Hexane-1,6-diyldioxy)dianiline

The complete molecule of the title compound, C18H24N2O2, is generated by a crystallographic inversion centre. The torsion angles in the hexamethylene chain are consistent with an antiperiplanar conformation, whereas the conformation of the O—CH2—CH2—CH2 unit is gauche. The three-dimensional crystal packing is stabilized by N—H⋯O and N—H⋯N hydrogen bonding.

The complete molecule of the title compound, C 18 H 24 N 2 O 2 , is generated by a crystallographic inversion centre. The torsion angles in the hexamethylene chain are consistent with an antiperiplanar conformation, whereas the conformation of the O-CH 2 -CH 2 -CH 2 unit is gauche. The three-dimensional crystal packing is stabilized by N-HÁ Á ÁO and N-HÁ Á ÁN hydrogen bonding.

Comment
Aromatic diamines are valuable building blocks for the preparation of high-performance polymers including polyamides, polyimides and polyureas (Mehdipour-Ataei, 2005). Therefore, these can be used to produce desired alterations in the chemical nature of macromolecular chains (Mehdipour-Ataei et al., 2007). Much research in recent years has focused on the design and synthesis of novel diamines in order to obtain suitable polymers. One of the popular approaches to achieve this goal is the introduction of flexible linkages such as an ether moiety (Shao et al., 2007) and/or methylene spacers (Yin et al., 1998) in the core structure of the diamines. These linkages increase the degree of freedom by reducing the segmental rotational barrier and inhibit close chain packing. The title compound, (I), in which flexible methylene spacers are present between the aryl ether moieties, is an outcome of efforts to modify the aromatic diamine monomers by flexible linkages in order to improve the processability and performance of the resulting polymers.
Molecules of (I) (Fig. 1) are located about a crystallographic centre of inversion. All torsion angles in the hexamethylene chain indicate an antiperiplanar conformation whereas the conformation of the O-CH 2 -CH 2 -CH 2 unit is gauche.
The crystal packing (Table 1) is stabilized by N-H···O and N-H···N hydrogen bonds which lead to a three-dimensional network.

Experimental
The title compound (I) was synthesized in two steps. In the first step, bis(4-nitrophenoxy)hexane was prepared by Williamson's reaction. A three-neck round bottom flask equipped with Dean-Stark trap, thermometer, magnetic stirrer and nitrogen inlet was charged with a suspension of 1,6-hexane diol (2.25 g; 19.1 mmol) and anhydrous potassium carbonate (5.3 g; 38.2 mmol) in a mixture of N,N'-dimethyl formamide (DMF) (60 ml) and toluene (20 ml), and refluxed (at 403-408 K) for 2 h for azeotropic removal of water. After cooling to 343-343 K, 1-fluoro-4-nitro benzene (4.05 ml; 38.2 mmol) was added and the mixture was again refluxed for 6 h. Subsequently, some toluene was distilled off and the resulting mixture was poured into 500 ml of chilled water after cooling to room temperature. The crude product was filtered as yellow solid, washed thoroughly with water and recrystallized from ethanol to afford bis(4-nitrophenoxy)hexane. In the second step, a two-neck flask was charged with 1,6-bis(4-nitrophenoxy)hexane (2.5 g; 6.94 mmol), hydrazine monohydrate (10 ml), ethanol (80 ml) and 0.1 g of 5% palladium on carbon (Pd-C). The mixture was refluxed for 18 h and then filtered to remove the Pd-C. The filtrate was concentrated on rotary evaporator to remove the solvent and the resulting crude solid was recrystallized from ethanol to afford colourless crystals suitable for X-ray analysis, which were stored in air-tight glass bottles for further studies. Yield 72%; m.p. 414 K. Elemental analysis. Found C,72.03,H,7.90,N,9.25. Calculated for C 18 H 24 N 2 O 2 : C, 71.97, H, 8.05, N, 9.33 and C-H(methylene) = 0.99 Å, and with U(H) = 1.2 U eq (C). The H atoms bonded to N were freely refined, see Table 1 for distances. Fig. 1. Perspective view of (I) showing the atom labelling scheme and displacement ellipsoids at the 50% probability level. H atoms are drawn as spheres of arbitrary radii. Unlabelled atoms are related by the symmetry operator 1 -x, -y, 1 -z.

Special details
Geometry. All e.s. 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 Rfactors(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.