Comparison of the C—H⋯O bonding in two crystalline phases of 1,4-dithiane 1,1,4,4-tetraoxide

The structures of two crystalline phases of 1,4-dithiane-1,1,4,4-tetraoxide have been determined and found to have similar local C—H⋯O hydrogen-bonding arrangements in spite of differences in the molecular packing.

The crystal structures of two crystalline phases of 1,4-dithiane 1,1,4,4-tetraoxide, C 4 H 8 O 4 S 2 , have been determined in order to examine the nature of possible intermolecular hydrogen bonds. Phase 1 is monoclinic, space group C2/m, with unit-cell dimensions of a = 9.073 (8), b = 7.077 (6), c = 5.597 (5) Å and = 105.89 (1) . The molecule adopts 2/m symmetry and all of the molecules are related by translation and thus have the same orientation. Phase 2 is also monoclinic but in space group P2 1 /n with unit-cell dimensions of a = 7.1305 (5), b = 5.7245 (4), c = 8.3760 (6) Å and = 91.138 (2) . In this phase, the molecule sits on an inversion center and the molecules within the unit cell adopt quite different orientations. In both phases, examination of the potential C-HÁ Á ÁO hydrogen bonds around each of the independent oxygen atoms (one axial and the other equatorial) shows the general OÁ Á ÁH patterns to be quite similar with each oxygen atom in contact with four neighboring H atoms, and each H atom contacting two neighboring O atoms. While none of the HÁ Á ÁO contacts is particularly short (all are greater than 2.5 Å ), each molecule has 32 such contacts that form an extensive intermolecular network. A 1 H NMR spectrum of the compound dissolved in DMSO shows a singlet of 8H at 3.677 which indicates that the C-H bonds are only moderately polarized by the single adjacent -SO 2 -moiety: strongly polarized C-H bonds have values in the 5-6 range [Li & Sammes (1983). J. Chem. Soc. Perkin Trans. 1, pp. 1303-1309]. The phase 1 crystal studied was non-merohedrally twinned.

Chemical context
Some years ago, multiple studies of C-HÁ Á ÁX (X = N, O) intramolecular hydrogen bonds were carried out on a series of 1,3-dithiane 1,1,3,3-tetraoxides which had various substituents at the 2 position located between the two SO 2 groups. The remaining C-H bond in the 2 position is strongly polarized given the electron-withdrawing properties of the two adjacent sulfone groups. The substituents bonded at the 2 position contained nitrogen or oxygen electron-pair donors which, with proper chain lengths, were able to form an intramolecular hydrogen bond to the polar hydrogen atom. ISSN 2056-9890 The chemistry and NMR/IR spectroscopic information of a wide variety of compounds were reported in a number of papers (see Li & Sammes, 1983, and references therein). Focusing on those compounds with significant shifts of the polar methine hydrogen in the 1 H-NMR spectra, their crystal structure determinations clearly demonstrated the formation of intramolecular hydrogen bonds (Harlow et al., 1984). Never explored, however, was the nature of the C-HÁ Á ÁO interactions likely to be found in the unsubstituted compound itself. As a matter of curiosity, we consequently decided to undertake the crystal structure determinations of the two possible (1,3-and 1,4-) dithiane tetraoxides and the unique 1,3,5-trithiane hexaoxide. All three of the compounds have unusually high melting/decomposition temperatures and we wanted to explore and compare the nature of the intermolmolecular C-HÁ Á ÁO interactions in this group of uncomplicated compounds. As a start on this project, we report herein the completion of the structures of two crystalline phases of 1,4-dithiane 1,1,4,4-tetraoxide, a compound which has no dipole moment, has the same 1:2 O:H ratio as water, and decomposes above 627 K.

Structural commentary
1,4-Dithiane 1,1,4,4-tetraoxide contains two crystalline phases as determined from an X-ray diffraction pattern of the assynthesized powder. When sublimed, crystals of both phases were also produced and it was only by chance that the two laboratories involved picked different phases. Fig. 1 compares the molecular ORTEP drawings of the molecules in the two phases. The molecule in phase 1 adopts 2/m symmetry while in phase 2 the molecule sits on a center of symmetry. The intramolecular bond distances and angles for the two phases are comparable.

Supramolecular features
Packing diagrams (Fig. 2) reveal that the packing for the two forms is quite different. In phase 1, all of the molecules are related by simple translational symmetries and thus all the molecules have the same orientation. In phase 2, the molecules have two different orientations in a somewhat herringbone fashion. Thus, one might expect any C-HÁ Á ÁO contacts to be quite different for the two phases but, in fact, they are very similar. Figs. 3 and 4 compare the environments of O1 (equatorial oxygen atom) and O2 (axial oxygen atom). Packing diagrams for crystalline phase 1 as viewed nearly along the c axis and phase 2 as viewed nearly along the a-axis. In phase 1, all the molecules are related by translation and thus have the same orientation. In phase 2, the molecules have two different orientations.

Figure 4
Environment of the axial oxygen atom, O2, in phases 1 and 2. In this case, the environments are still similar, but less so than for equatorial O1.

Figure 3
Environment of the equatorial oxygen atom, O1, in phases 1 and 2. Although the packing of the molecules is quite different, the arrangement of the C-HÁ Á ÁO contacts in both phases is seen to be very similar.
In all cases, each oxygen atom is in contact with four hydrogen atoms arranged in a distorted square. Probably for steric reasons, the distortion is less for the equatorial oxygen atom than for the axial oxygen atom.
Each oxygen atom in both phases 'sees' four hydrogen atoms while each hydrogen atom 'sees' two oxygen atoms. This bifurcation of the hydrogen contacts means that none of the HÁ Á ÁO distances is particularly short. It should also be pointed out that each methylene group has only one neighboring sulfone group, which would limit the polarization of the C-H bonds compared to our previous studies where the C-H bond of interest sat between two sulfone groups. Thus, very short C-HÁ Á ÁO bonds were not expected. The exact details of the C-HÁ Á ÁO contacts are given in Table 1. Thus, while there are no really short C-HÁ Á ÁO contacts (none less than 2.50 Å ), every donor and every acceptor plays a role in forming a extensive network of contacts in which each molecule has a total of 32 interactions with its neighbors.
The shortest C-HÁ Á ÁO contacts tend to be between the equatorial oxygen atoms, O1, and the equatorial hydrogen atoms labeled with the suffix B. These also come with C-HÁ Á ÁO angles that are closest to being linear, 148 to 160 . Presumably the difference between axial and equatorial HÁ Á ÁO contacts is mostly due to steric effects, the equatorial atoms being more accessible. The shorter contacts can undoubtedly be classified as true C-HÁ Á ÁO hydrogen bonds using, as a guide, the seminal study of weak hydrogen bonds by Desiraju & Steiner (1999). The remaining bonds are probably better described as mostly electrostatic in nature. However, as Desiraju & Steiner point out, there are no hard limits for determining what may, and may not, be a true hydrogen bond.

Synthesis and crystallization
Following literature procedures (Schultz et al., 1963), a 100 mL round-bottom flask was charged with 1,4-thiane (Sigma-Aldrich; 1.005 g, 8.4 mmol) in 25 mL glacial acetic acid. To this were added 10 mL 30% hydrogen peroxide solution (excess) in 25 mL of glacial acetic acid. The solution was heated to 323 K for 12 h under stirring over an oil bath. The white solid that formed was filtered and washed with water (3 Â 25 mL) and diethyl ether (3 Â 25 mL) (yield: 1.325 g, 86%). Crystals suitable for structural analysis were grown by sublimation of the solid. NMR data were recorded on a Bruker Avance 400 MHz with d 6 -DMSO as solvent, referenced to residue proteo-DMSO. TGA/DSC data showed decomposition occurring from 627 to 739 K.

1,4-Dithiane 1,1,4,4-tetraoxide (14-disulphone-phase1)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.32 e Å −3 Δρ min = −0.32 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. Refinement. Refined as a 2-component twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq S1 0.65320 (6)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.39 e Å −3 Δρ min = −0.44 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.