1,4-Bis(4-aminophenoxy)benzene

The title compound, C18H16N2O2, is a precusor for the synthesis of polyimides. The molecule is located on a crystallographic inversion center and the terminal aminophenoxy rings are almost perpendicular to the central benzene ring with a dihedral angle of 85.40 (4)°. The molecular conformation is stabilized by N—H⋯O and N—H⋯N intermolecular hydrogen-bonding interactions.

The title compound, C 18 H 16 N 2 O 2 , is a precusor for the synthesis of polyimides. The molecule is located on a crystallographic inversion center and the terminal aminophenoxy rings are almost perpendicular to the central benzene ring with a dihedral angle of 85.40 (4) . The molecular conformation is stabilized by N-HÁ Á ÁO and N-HÁ Á ÁN intermolecular hydrogen-bonding interactions.

Related literature
For related literature on polyimides and their solubility, see: Yang et al. (2002). For examples of chemical-and heat-resistant polyimides, see: Butt et al. (2005). Choi et al. (2001) discuss polyimides with various length (n-alkoxy)phenyloxy side branches. For polyimides with improved properties such as processing from the melt or from solution, see: Eastmond & Paprotny (1996). For different structural modifications of the polymer backbone to reduce the chain interaction and their effect on chain packing and glass transition temperature, see: Yan et al. (2005) Table 1 Hydrogen-bond geometry (Å , ). (17 Symmetry codes: (i) x; Ày þ 1 2 ; z À 1 2 ; (ii) x þ 1; y; z.

Comment
Ether containing aromatic diamines are useful monomers for the preparation of soluble polyimides (Yang et al., 2002) which form a group of incredibly strong and astoundingly heat and chemical resistant polymers (Butt et al., 2005). Many efforts have been made to improve their processability while maintaining their excellent thermal and mechanical properties (Choi et al., 2001). Incorporation of flexible groups such as ether linkages were found successful in altering the crystallinity and the intermolecular interactions and to increase the solubility (Eastmond et al., 1996). Different structural modifications of the polymer backbone have been studied to reduce the chain interaction, such as the introduction of flexible links, such as -O-and CH 2 , to the main chain. This alteration disrupts the conjugation and increases the chain flexibility, which affects the chain packing but not the glass transition temperature (Yan et al., 2005 and references therein).
The title compound crystallizes in the monoclinic space group P2 1 /c and the molecule is located on a crystallographic inversion center. The bond lengths and bond angles are in normal ranges. The terminal aminophenoxy rings are almost perpendicular to the central benzene ring with a dihedral angle of 85.40 (4) °.
The compound exhibits a zigzag like packing pattern (figure 2). The hydrogen atoms of the amino groups are engaged in two types of intermolecular hydrogen bonding interactions. The first one with the nitrogen atoms of other amino groups and the second with the oxygen atoms of the phenoxy groups (Table 1). Intermolecular C-H···π contacts between adjacent phenoxy groups are also present (Fig. 3). The C-C distance of C6-H6···C6 iii (iii = x, 0.5 − y, 1/2 + z) is 2.74 (2) Å and C-H···C angle is 175 (1) °, while the dihedral angle between the two phenoxy mean planes is 88.82 (3) °. Additional contacts take place between the central phenyl group and the adjacent terminal phenoxy group with a C-C distance for C9-H9···C2 iv (iv = 1 + x, y, z) of 2.180 (1) %A.

Experimental
The title compound was synthesized in two steps. In the first step a mixture of 2.00 g (0.0180 mol) of hydroquinone, 5.00 g (0.0360 mol) of anhydrous K 2 CO 3 and 3.81 ml (0.036 mol) of 4-fluoronitrobenzene in a two neck round bottom flask having 70 ml of dimethyl acetamide was heated at 373 K for 20 h under a nitrogen atmosphere. The colour of the solution changes from yellow to dark brown as the reaction proceeds. After cooling to room temperature, the reaction mixture was poured in 800 ml of water to precipitate a yellow solid which was washed thoroughly with water and then separated by filtration. In the second step a 250 ml two neck flask was charged with 1.00 g (2.84 mmol) of the yellow solid, 10 ml of hydrazine monohydrate, 80 ml of ethanol and 0.06 g of 5% palladium on carbon (Pd/C). The mixture was refluxed for 16 h and then filtered to remove Pd/C. The solvent was evaporated and the resulting crude solid was recrystallized from ethanol to afford crystals suitable for X-ray analysis (yield:85%, m.p.: 455 K).
supplementary materials sup-2 Refinement All hydrogen atoms were located from the difference Fourier map and refined isotropically. Fig. 1. The molecular structure of the compound, with atom labels and 50% probability displacement ellipsoids for non-H atoms. The non-labled atoms have the symmetry operator (−x,-y + 1,-z + 1).    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 > 2sigma(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.