Crystal structures of 5,5′-bis(hydroxymethyl)-3,3′-biisoxazole and 4,4′,5,5′-tetrakis(hydroxymethyl)-3,3′-biisoxazole

Crystal structure, packing, and FTIR characterization of 5,5′-dihydroxymethyl-3,3′-biisoxazole and 4,4′,5,5′-tetrahydroxymethyl-3,3′-biisoxazole are reported.


Chemical context
The five-membered, heterocyclic isoxazole moiety forms the basis for a number of medical and agricultural products, as well as energetic materials (Galenko et al., 2015;Sausa et al., 2017;Wingard et al., 2017a,b;Sysak & Obmiń ska-Mrukowicz, 2017). Its versatility stems from the electronegative oxygen and nitrogen atoms, which provide the ring nucleophilic activity, and its three carbon atoms, which afford the addition of a variety of functional groups. The title compounds 5,5 0bis(hydroxymethyl)-3,3 0 -biisoxazole (1) and 4,4 0 ,5,5 0 -tetrakis-(hydroxymethyl)-3,3 0 -biisoxazole (2) exhibit two isoxazole rings, each attached with one or two hydroxymethy groups. These compounds have been synthesized recently in our laboratory as useful precursors to a new class of energetic materials. The addition of nitric acid to the title compounds results in nitrate esterification, yielding the energetic materials biisoxazolebis(methylene dinitrate) (3) and biisoxazoletetrakis(methyl nitrate) (4), where a nitrate functional group replaces the hydrogen atom in the hydroxyl groups (Wingard et al., 2017a,b). These derivative compounds are potential energetic plasticizing ingredients in nitrocellulose or meltcastable formulations because the rings present Lewis-base behavior towards electrophilic nitrocellulose and the alkyl nitric esters afford miscibility and compatibility with conventional energetic plasticizers.

Supramolecular features
Intermolecular hydrogen bonding plays a key role in the stabilization of the crystal structures of the title compounds. Figs. 3 and 4 show the packing of (1) and (2), respectively, and Tables 1 and 2 list their hydrogen-bonding geometries. Compound (1) displays hydrogen bonding between the oxygen atoms O2, belonging to the hydroxy groups, and the N1 atoms of the isoxazole rings of adjacent molecules, generating a supramolecular framework parallel to (201) [O2Á Á ÁN1 i = 2.8461 (15) Å ; symmetry code: (i) x À 1 2 , Ày + 1 2 , z À 1 2 ]. In contrast, compound (2) forms a network of hydrogen bonds involving the hydroxy groups O2-H2A and O3-H3A of adjacent molecules, so that each OH group acts both as An overlay of the asymmetric units of compounds (1) and (2), depicted in red and green, respectively.
The crystal structure of (1) reveals a slip-stacked geometry of the rings in the b-axis direction, with centroid-to-centroid distances of 4.0652 (1) Å and plane-to-plane shifts of 2.256 (2) Å . In contrast, in compound (2) the rings are stacked along the a-axis direction, with centroid-to-centroid distances of 4.5379 (4) Å and plane-to-plane shifts of 2.683 (2) Å .  (1) and (2), respectively, with the hydrogen atoms in the OH groups replaced by NO 2 moieties. A superimposition of the respective isoxazole rings of compound (1) and (3) yields an r.m.s. deviation of 0.004 Å (Fig. 5A). In both molecules, the rings adopt a trans conformations; however, in (1) the O1 and O2 atoms are in a trans conformation with respect to the C1-C4 bond, whereas in (3) the corresponding O atoms are in a cis conformation. In (1), the plane encompassing the atoms O2, C4, and C1 forms a dihedral angle of 12.72 (1) with respect to the mean plane of the isoxazole ring, in contrast to a value of 66.8 (2) in (3) for the corresponding atoms. A similar comparison between (2) and (4) yields an r.m.s. deviation of 0.01 Å for the superimposition of the isoxazole rings, and dihedral angles of 53.78 (8) and 69.37 (7) for (2) (planes formed by the atoms O2/C4/C1 and O3/C5/C2, respectively) compared to those of 84.54 (14) and 84.81 (18) or 79.19 (15) and 82.32 (17) for (4) (Fig. 5B). The most striking supramolecular difference between the title compounds and (3) and (4) is that the former exhibit hydrogen bonding, which contributes to the stability of their crystal structure.

Refinement
Crystal data, data collection, structure solution and refinement details are summarized in Table 3. The hydrogen atoms for compound (1) were refined using a riding model with C-H = 0.93 or 0.98 Å and U iso (H) = 1.2U eq (C) and O-H = 0.74-0.85 Å and U iso (H) = 1.5U eq (O), whereas for compound (2) all the hydrogen atoms were refined independently including isotropic displacement parameters.   Overlays of the asymmetric units of (1) and (3) (A) and (2) and (4)

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.64217 (17)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.44 e Å −3 Δρ min = −0.22 e Å −3 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.