Crystal structures of 2,5-diazido-1,4-phenylene diacetate and 2,5-diazido-1,4-phenylene dibutyrate

2,5-Diazido-1,4-phenylene diacetate and dibutyrate are the first structurally characterized representatives with a trans-diazidophenylene entity. Both molecules possess inversion symmetry; however, the compounds crystallize in different crystal systems (triclinic versus monoclinic).


Chemical context
In recent years, copper(I)-catalysed cycloaddition of organic azides and alkynes towards 1,4-disubstituted triazoles attained immense interest in various fields of organic chemistry and became famous as the 'cream of the crop' of click chemistry (Moses & Moorhouse, 2007). In materials chemistry, this kind of reaction is often applied for the synthesis of functional polymers (Qin et al., 2010).
The title compounds, (I) and (II), were synthesized to investigate their applicability in such polymerizations, viz. AA-BB polymerizations with dialkynes. The synthetic accessibility of the two compounds from inexpensive starting materials is remarkable, making them suitable for large scale preparation. However, their electron-deficient character represents a challenge to the polymerization parameters. The crystal structures of (I) and (II) are reported herein.

Structural commentary
The molecular structures of (I) and (II) are displayed in Figs. 1 and 2, respectively. Both molecules possess inversion symmetry. Although the two molecules differ only in the ester moiety (acetate versus butyrate), the crystal symmetry is ISSN 1600-5368 different, i.e. triclinic for (I), with Z = 1, and monoclinic for (II), with Z = 2. The diazidophenylene moieties do not differ significantly from planarity, with a maximum deviation of 0.0216 (7) Å in (I) and 0.0330 (14) Å in (II), for the unsubstituted atom C3 in both cases. The azide groups, both in trans positions to each other, deviate slightly from a linear arrangement, with an N-N-N angle of 173.01 (9) for (I) and 172.59 (16)

Supramolecular features
There are no notable features in terms ofstacking interactions or hydrogen bonding in either structure. The crystal packing of (I) and (II) seems to be dominated mainly by van der Waals forces (Figs. 3 and 4, respectively).

Database survey
In the Cambridge Structural Database (Version 5.35, last update February 2014;Allen, 2002) no structures of compounds containing a trans-diazidophenylene entity are listed, making the two examples presented herein the only ones reported so far.

Synthesis and crystallization
Both target compounds were synthesized following a two-step protocol (Fig. 5), previously published for 2,5-diazido-1,4phenylene diacetate by Moore et al. (1969). In view of the light sensitivity of the intermediate compound 2,5-diazidobenzene-1,4-diol, all reactions were carried out under light protection.
Preparation of 2,5-diazidobenzene-1,4-diol: 1,4-benzoquinone (10.81 g, 100.0 mmol, 1.0 equivalent) was dissolved in glacial acetic acid (100 ml, 1.0 M) and cooled to 288 K using an ice-water bath. NaN 3 (14.3 g, 220 mol, 2.2 equivalents) was dissolved in water (44 ml, 5.0 M) and added to the cooled and stirred solution of 1,4-benzoquinone in one portion. Stirring was stopped after 15 min and the flask was sealed and stored at 278 K overnight for crystallization. Vacuum filtration afforded a light-yellow solid, which was washed three times with water and dried in vacuo overnight to afford 2,5-di-   The molecular structure of compound (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 80% probability level. Unlabelled atoms are generated by the symmetry code (Àx + 1, Ày, Àz).