Crystal structure of 3,3′-biisoxazole-5,5′-bis(methylene) dinitrate (BIDN)

The crystal structure and packing of the energetic compound 3,3′-bis-isoxazole-5,5′-bis-methylene dinitrate is reported. Major FTIR, Raman, UV absorption peaks, as well as experimental and calculated density are reported.


Chemical context
Isoxazole compounds have attracted much interest in recent years because of their potential usefulness in medicine, agriculture, and in the field of energetic materials (Galenko et al., 2015;Wingard et al., 2017). The title compound is an isoxazolebased energetic material that has been synthesized recently in our laboratory. It has potential use as a trinitrotoluene replacement in melt-castable and Composition B formulations, and as an energetic plasticizing ingredient in nitrocellulose-based propellant formulations. The compound is composed of two heterocyclic isoxazole rings, each bonded to an alkyl nitric ester group. The heterocyclic base has nonbonded electron lone pairs which can exhibit Lewis-base behavior towards electrophilic materials such as nitrocellulose, whereas the alkyl nitric esters provide miscibility and compatibility with commonly used energetic plasticizers.

Database survey
An open literature search, as well as a search of the Cambridge Structural Database (Groom et al., 2016) and the Crystallography Open Database (Gražulis et al., 2009) yielded many hits for bis-isoxazole-containing compounds and several on 3,3 0 and 5,5 0 bis-isoxazole-based compounds, the most pertinent studies relating to the title compound being the crystal structures of 3,3 0 -bisoxazole (Cannas & Marongiu, 1968;CCDC 1111317, BIOXZL) and 5,5 0 -diphenyl-3,3 0 -bisoxazole (van der Peet et al., 2013;CCDC 935274). In these compounds, the rings also adopt planar trans conformations, similar to that observed in the title compound.

Synthesis and crystallization
The synthesis of the title compound has been reported recently (Wingard et al., 2017). Briefly, a solution of sodium bicarbonate was added to a mixture of dichloroglyoxime Crystal packing viewed along the a axis. Dashed lines represent contacts between atoms N1Á Á ÁH2, N11Á Á ÁH4A, and C11Á Á ÁO4 (blue) and O41Á Á ÁH4B (red).

Figure 1
Molecular conformation and atom-numbering scheme. Non-labeled atoms are generated by inversion (Àx, 1 À x, 1 À z). Non-hydrogen atoms are shown as 50% probability displacement ellipsoids. methanol to produce the intermediate compound 5,5 0 -dihydroxymethyl-3,3 0 -bis-isoxazole (75% yield). Then, this compound (0.120 mol) was added portionwise over ten minutes to 90% nitric acid (150 ml) placed in a 250 ml roundbottom flask equipped with a stir bar, and cooled in an icewater bath. No exotherm was observed during the addition. The reaction mixture was stirred for four hours while the water-ice bath was warmed to room temperature. The reaction mixture was poured onto ice, resulting in the formation of a white precipitate, which was collected by Bü chner filtration and dried, giving the title compound (92% yield). Slow solvent evaporation of a solution in acetonitrile yielded suitable single crystals for the X-ray diffraction experiments at room temperature. Based on the cell dimensions and molecular weight, the calculated crystal density of 1.609 Mg m À3 at 297 K is in excellent agreement with the value of 1.585 Mg m À3 measured using a pycnometer at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The hydrogen atoms were refined using a riding model with C-H = 0.93 or 0.97 Å and U iso (H) = 1.2U eq (C).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.