An orthorhombic polymorph of 2-(1,3,5-dithiazinan-5-yl)ethanol or MEA-dithiazine

The use of the H2S scavenger monoethanolamine triazine in a natural gas treatment facility produced a new Pbca polymorph (form II) of 2-(1,3,5-dithiazinan-5-yl)ethanol featuring O—H⋯N hydrogen-bonded dimers rather than one-dimensional, helical O—H⋯O strands as reported previously for the I41/a (form I) polymorph.


Chemical context
Hydrogen sulfide is a corrosive and lethal gas that is commonly encountered during the production of hydrocarbons from subterranean reservoirs (Marriott et al., 2016). The highly toxic H 2 S gas must be removed from the hydrocarbon stream to ensure a commercially salable product into the pipeline and refinery distribution network (Kermani et al., 2006). One of the most widely applied economical and effective strategies is to scavenge the hydrogen sulfide through the use of scavenger chemicals such as hexahydro-1,3,5-tris(hydroxyethyl)-s-triazine, colloquially referred to as monoethanolamine triazine (MEA-triazine) (Taylor et al., 2017). The MEA-triazine is routinely administered at remote field locations using engineering controls that enhance the efficiency with which the hydrogen sulfide reacts with the MEAtriazine scavenger.
The reaction of hydrogen sulfide with the MEA-triazine (1) scavenger has been described, see Fig. 1 (Bakke et al., 2001;Wylde et al., 2020). Furthermore, while hydrogen sulfide can theoretically react with a third molecule to yield a trithiane molecule, this reaction does not proceed under most conditions. As a result, sparingly soluble solid dithiazine (3) molecules or an amorphous polymerized dithiazine material are the typical final products under aqueous reaction conditions (Taylor & Matherly, 2011;Taylor et al., 2013;Wang et al., 2020). ISSN 2056-9890 Engineering protocols are implemented into the pipeline flow systems so that the sparingly soluble solids from the spent scavenger are continuously removed. However, these operations are occasionally not completely effective in eliminating solids and a buildup of intractable residues in the system will result, thus impeding the flow path and complicating field operations. This was the case at one production facility, where the title compound (3) crystallized within the field treatment system.
The molecular and crystal structure of title compound 3 has been determined before. The first polymorph, form I, in space group I4 1 /a was described independently by Galvez-Ruiz et al. (2004) and Wang et al. (2020). Recently, a second polymorph, in space group Pbca, was disclosed as a CSD communication (Unruh, 2021). No further details, such as the origin of the material, the solvent or mode of crystallization or other information was stated with the deposited cif file and the molecular and crystal structures were not discussed.

Structural commentary
Single crystals formed in the field treatment system and were used for intensity data collection without recrystallization. Data were collected at both room temperature as well as 150 K. The polymorph obtained was that of form II, in space group Pbca, and the data are in good agreement with those recently disclosed by Unruh (2021).
Phase purity of the sample as harvested from the field treatment system was verified by powder X-ray diffraction analysis (supplementary Figures S1 and S2). No phase change was observed between 150 K and room temperature. The unitcell parameters obtained by Rietveld analysis of the structural model against the room-temperature powder pattern are a = 9.6596 (6), b = 8.9248 (5), c = 17.748 (1) Å and V = 1530.03 (15) Å 3 (Table S1A), matching the parameters from the room-temperature single-crystal data collection (Table 4). Powder data and unit-cell parameters from samples recrystallized from either isopropanol or an isopropanol-water mixture are indistinguishable from those of the field sample ( Fig. S1, Fig. S2, Table S1).
The molecular conformations and packing at low temperature and room temperature are essentially identical and unless otherwise noted, the following discussion will be based on the 150 K data set. The title compound (Fig. 2) crystallizes in the orthorhombic space group Pbca. Bond distances and angles are in the expected ranges and unexceptional. Key torsion angles are given in Table 1. The nitrogen atom is sp 3 hybridized, as expected for a tris(methylene) substituted amine [C-N-C angles range from 113.16 (5) to 114.49 (5) , the bond angle sum is 342.06 ], the dithiazine ring exhibits the expected chair conformation, and the ethanolamine unit is substituted onto the ring in an axial orientation. The title compound has this in common with the other structures featuring a MEA-dithiazine moiety: The previously reported polymorph (form I) in space group I4 1 /a and also the bromide and chloride salts of MEA-dithiazine, which are protonated at the amine and isomorphous (Bushmarinov et al., 2009;Galvez-Ruiz et al., 2008), also feature an sp 3 -hybridized amine N atom, as well as a chair conformation dithiazine ring axially substituted by the ethanolamine.
The most flexible fragment of the MEA-dithiazine molecule, and worth a closer investigation, is the ethanolamine chain. In the form II polymorph the N1-C4-C5-O1 chain adopts a gauche conformation, with a torsion angle of 65.09 (7) ( Table 2). In the form I polymorph the conformation is also gauche, with a torsion angle of À70.1 (3) (in the chosen enantiomer). In the bromide and chloride salts, this torsion angle is slightly smaller (55.1, 54.2 and À55.0 in the three published structures), but also gauche. The form I polymorph and the salts thus exhibit the same chair conformation and axial position of the ethanolamine group as well as a common gauche conformation of the ethanolamine chain as observed for form II of the title compound, indicating a clear preference of the MEA-dithiazine molecule towards this specific molecular conformation. The molecular structure of 2-(1,3,5-dithiazinan-5-yl)ethanol. Displacement ellipsoids are shown at the 50% probability level. Table 1 Selected torsion angles ( ) for the 150 K structure.
What does differ between the three structures involving MEA-dithiazine or its cation are the C-N-C-C torsion angles between the dithiazine ring and the ethyl group. In form II, they are À104.74 (6) (C2-N1-C4-C5) and 122.32 (6) (C3-N1-C4-C5). The equivalent torsion angles in form I are 78.9 (3) and À148.1 (3) , respectively, and those of the chloride salt are 175.8 and À55.2 (Bushmarinov et al., 2009). Fig. 3, an overlay of the molecules from the three crystalline motifs, exemplifies the similarities and differences. The cause for the differences between the two polymorphs and the salt can be found among the intermolecular and packing interactions, to be discussed below.

Supramolecular features
Different intermolecular interactions, especially hydrogen bonds, are the core reason for the formation of the two MEAdithiazine polymorphs (form I and II). In the previously described crystal structure (form I), the molecules are connected through O-HÁ Á ÁO hydrogen bonds involving the hydroxyl O atoms as both hydrogen-bond donor and acceptor. A series of these interactions lead to the formation of onedimensional, helical spirals in which adjacent molecules are related to each other via the 4 1 screw axis of the I4 1 /a space group (Figs. 4 and 5). The graph-set motif of the infinite strands is C 1 1 (2). The helical chains are further supported by weak C-HÁ Á ÁS interactions involving the methylene group adjacent to the OH group and one of the sulfur atoms of a molecule related by three turns of the screw axis [C5Á Á ÁS2 i = 3.533 (3) Å ; symmetry code: (i) = À 3 4 + y, 3/4 -x, 3/4 -z; Fig. 5]. No other significant interactions involving H atoms are observed, and individual helical spirals run parallel and antiparallel to each other with no directional interactions between them. One other important aspect of the form-I polymorph is that the amine N atom does not act as a hydrogen-bond acceptor, neither towards the O-H group nor any of the methylene CH 2 hydrogen atoms.
In the form II structure described here, the amine N atom behaves differently and acts as the primary hydrogen-bond acceptor (Tables 2 and 3 An overlay of the two polymorphs of MEA-dithiazine (red: form II; blue form I) and of the cation of the chloride salt (green). Overlays are based on a least-squares fit of the dithiazine ring atoms.

Figure 4
Hydrogen-bonding interactions in the form I polymorph. Molecules are linked into infinite chains that extend along the c-axis direction. Hydrogen-bonded molecules are symmetry-related to each other via the 4 1 -screw axis of the I4 1 /a space group. Coordinates from Wang et al. (2020). Displacement ellipsoids are shown at the 50% probability level. H atoms not involved in hydrogen-bonding are omitted for clarity. of R 2 2 (10). (Table 3, Figs. 6 and 7). No O-HÁ Á ÁO interactions are observed in the form II polymorph.
Contrary to the form I polymorph, where the spiral chains propagate in the c-axis direction with no significant directional interactions between parallel strands, the primary building units in the form II structure are connected to each other via C-HÁ Á ÁO hydrogen bonds and weak C-HÁ Á ÁS interactions. The C-HÁ Á ÁO interactions are notably short for this kind of hydrogen bond [HÁ Á ÁO = 2.42 Å , CÁ Á ÁO = 3.3359 (9)Å ]. The C-HÁ Á ÁO interactions connect the hydrogen-bonded dimers into infinite layers lying perpendicular to (001).
The C-HÁ Á ÁS interaction is much weaker and connects parallel layers with each other. The layers, which are slightly corrugated, also interdigitate with each other, yielding a densely packed and rigid three-dimensional arrangement. This is reflected in the density of the crystals in form II, which is (at 150 K) 1.468 g cm À3 . The form I structure, with its lack of strong interactions between hydrogen-bonded spirals, is substantially less densely packed (1.407 g cm À3 at 173 K, or 4.3% less dense than form II). This points towards form II likely being the thermodynamically more stable polymorph. This is also supported by the melting temperatures of the two forms, where form I has a reported melting onset of 314 K in comparison to 316 K measured for form II ( Figure S1, supporting information). The difference in melting point between the two forms is small ($2K); this could be coincidental or they might point towards a phase transformation of one of the two forms upon heating, with only one form being present once the melting point temperature is reached. For form II, a differential scanning calorimetry (DSC) investigation did show any indication of a phase change ( Figure S2, supporting information). However, a thorough investigation of both polymorphs utilizing DSC would be necessary to determine the relative stabilities (Yu, 1995).

Figure 6
Hydrogen-bonding interactions in the form II polymorph. Molecules are linked into centrosymmetric dimers connected via pairs of O-HÁ Á ÁN hydrogen bonds. Displacement ellipsoids are shown at the 50% probability level. >90 wt% dithiazine were dissolved in isopropyl alcohol, filtered, and dithiazine crystals were obtained from isopropanol-water mixtures by cooling to 278 K, and recrystallized twice to obtain large translucent crystals suitable for single-crystal structure analysis. All further analysis, including the measurement of the solubility of dithiazine in natural gas mixtures and variable pressure, was performed on the recrystallized samples. The original material from the field site was not further analyzed to verify that recrystallized and raw material were of the same kind, i.e., no powder XRD data of the original material were recorded, and no Rietveld analysis of the pattern was performed. The substantial difference in structure and density observed for the two polymorphs does indicate that they would also differ in other physicochemical properties. A noteworthy difference described herein is that crystalline material directly from the production facility and from laboratory recrystallization experiments were analyzed, and both resulted in the form II polymorph. bonds with a C 2 2 (10) graph-set motif. Three crystalline organic salts of 2-(1,3,5-dithiazinan-5-yl)ethanol have also been reported, including one with bromide and two with chloride counter-ions (HOSKIK and HOSKOQ; Bushmarinov et al., 2009;HOSKOQ01;Galvez-Ruiz et al., 2008). All three salts are isostructural, possessing a protonated nitrogen atom and charge-assisted O-HÁ Á ÁBr/Cl and charge-assisted N-HÁ Á ÁBr/Cl hydrogen bonds and R 2 4 (14) graph sets. In all nine structures, the dithiazine ring adopts a chair conformation.

Methods
Powder diffraction data were collected in focusing mode on a Panalytical Empyrean X-ray diffractometer equipped with Bragg-Brentano HD optics, a sealed-tube copper X-ray source ( = 1.54178 Å ), Soller slits on both the incident and receiving optics sides, and a PixCel3D Medipix detector. Samples were hand ground for 20 minutes using an agate mortar and pestle and packed in metal sample cups with a sample area 16 mm wide and 2 mm deep. 1/4 anti-scatter slits and 1/16 divergence slits as well as a 4 mm mask were chosen based on sample area and starting angle. Data were collected between 5 and 90 in 2 using Data Collector software (PANalytical, 2019). Rietveld refinements were performed against the models of the single-crystal-structure data sets using the HighScore software (PANalytical, 2018). Refinement of preferred orientation was included using a spherical harmonics model. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) data were measured using a TA Instruments Q20 DSC with heating rates of 5 C min À1 . The sample was run in an alumina open pan from 25 to 100 C. The DSC and TGA traces are given in the Supporting Information (Fig. S3).

Synthesis and crystallization
The commercially supplied MEA-triazine 1 solution, 47% active components in aqueous methanol (Innospec, Inc.), is stored in an external 650 gal polyethylene tank and is dosed into two inclined static mixers with a piston-style positive displacement pump. A combined dose of 16 gal d À1 is administered to the two static mixers. Each mixer contains bead media that create a tortuous pathway that facilitates efficient mixing and provides the contact time for the hydrogen sulfide gases to react. One inclined static mixer services the well gases removed from the wellhead recovery line and the second services the excess gas from the stock-tank vapor-recovery system. The gaseous mixture of natural gas and hydrogen sulfide pass through the static mixer that contains the liquid MEA-triazine 1 solution. The MEA-triazine 1 solution is administered at a rate to keep the hydrogen sulfide below a targeted threshold and to ensure the dithiazine product 3 is transported through the static mixer at a sufficient rate to alleviate any plugging from reactionary solids.
The crystalline solids created through this process were collected and characterized by single-crystal and powder X-ray analysis as well as differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The X-ray powder pattern was successfully indexed suggesting a single, pure bulk phase. Thermal analysis revealed negligible weight loss through 100 C by TGA, with a melt endotherm onset at 43 C (574 J g À1 ) and a peak temperature at 45 C.
Additionally, the crystalline material from the field equipment was recrystallized from aqueous isopropanol. Approximately 100 mg of crystalline material was dissolved with gentle heating in approximately 10 ml of neat IPA or 50:50 IPA:water. The samples were allowed to cool to room temperature and slowly evaporate. After 24 h, a crystalline material resulted, which was filtered, dried and characterized by powder X-ray diffraction (Fig. S1, S2D-S2I, Tables S1B, S1C).

Refinement
Single-crystal X-ray diffraction data, data collection and structure refinement details are summarized in Table 4. A common structural model was refined against the data collected at 150 K and at room temperature (no phase change was observed). The C-bound H atoms were positioned geometrically and constrained to ride on their parent atoms with C-H bond distances of 0.99 Å (at 150 K) and 0.97 Å (at room temperature). The positions of the hydroxyl H atoms were refined. U iso (H) values were set to 1.2U eq (C) or 1.5U eq (O). Acta Cryst. (2022). E78, 191-197 research communications

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.