Crystal structure of [2-(triethylammonio)ethyl][(2,4,6-triisopropylphenyl)sulfonyl]amide tetrahydrate

The zwitterionic title compound shows a major disorder of the triisopropylphenyl group over two equally occupied locations. An interesting feature is the uncommon hydrate structure, exhibiting a tape-like motif which can be classified as a transition of the one-dimensional T4(2)6(2) motif into the two-dimensional L4(6)5(7)6(8) motif.


Chemical context
The title compound was isolated as by-product while purifying the corresponding sulfonylaziridine via column chromatography using a solvent mixture containing triethylamine. Interestingly, the zwitterionic title compound was formed by the nucleophilic ring-opening of the aziridine. This is so far undocumented for tertiary amines but well known for primary or secondary amines (Hu, 2003). We assume that this ringopening reaction is reversible, since the aziridine was isolated in the absence of water. Possibly, the zwitterionic structure is stabilized by the water molecules and/or by crystallization, preventing the reverse reaction. Furthermore, the four incorporated solvent water molecules in the crystal structure form a tape-like hydrate structure comparable to some known hydrogen-bonding motifs (Infantes et al., 2003). This is discussed further in the Supramolecular features section.

Supramolecular features
Intermolecular interactions occur mostly through hydrogen bonding of the water molecules among themselves and with the zwitterionic compound (Table 1). Three of the four water molecules form an infinite tape of interconnected four-and six-membered rings known as the T4(2)6(2) motif. Each ring contains a centre of symmetry and the tape expands in the [100] direction. Interestingly, the border of the tape is lined with the zwitterionic compound and one additional water molecule, thus expanding the tape with five-and sixmembered rings involving the O4-O6-O3-O5-N1 and O4-O3-O5-O2-S1-N1 atoms, respectively ( Fig. 2 and Fig. 3). The structure is comparable to the L4(6)5(7)6(8) motif, building up two-dimensional sheets, which are limited here by the zwitterionic amide. In summary, the hydrate structure discussed herein represents a transition between a one-dimensional tape and a two-dimensional sheet.

Figure 1
The molecular structure and atom numbering for the title compound with displacement ellipsoids drawn at the 30% probability level. Atoms of the minor disorder component are drawn with grey-coloured C atoms.

Figure 3
Hydrate-structure motifs already known (left and right) (Infantes et al., 2003) and the structure reported here (centre).

Synthesis and crystallization
N-[(2,4,6-Triisopropylphenyl)sulfonyl]aziridine was synthesized from ethanolamine as described in the recent literature (Buckley et al., 2013). Crystals of the title compound were obtained after a test tube containing small amounts of the sulfonylaziridine dissolved in a mixture of diethyl ether, pentane and triethylamine was left to evaporate over a period of 3 d.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms not involved in hydrogen bonds were positioned geometrically and refined using a riding model, with U iso (H) = 1.5U eq (C) for terminal and 1.2U eq (C) for non-terminal H atoms, with C-H = 0.98 Å . H atoms involved in hydrogen bonds were located in a difference Fourier synthesis map and were freely refined. The disorder of the triisopropylphenyl group was refined by a free variable to an occupancy ratio of 0.502 (2):0.498 (2). To ensure the stability of the phenyl ring in the refinement, the standard FLAT restraint was applied to atoms C11-C19 and a DELU restraint to atoms C11, C12 and C16, in both of the disorder domains. In addition, atoms C11, C11 0 and C16 0 required an additional ISOR restraint with a reduced deviation (s = 0.001 and st = 0.002).