The structure and Hirshfeld surface analysis of the salt 3-methacrylamido-N,N,N-trimethylpropan-1-aminium 2-acrylamido-2-methylpropane-1-sulfonate

The molecular and crystal structure of the salt 3-methacrylamido-N,N,N-trimethylpropan-1-aminium 2-acrylamido-2-methylpropane-1-sulfonate, that crystallizes with two unique pairs of cations and anions in the asymmetric unit, is reported. Hirshfeld surface analysis of the asymmetric unit and of the two individual salts is also carried out.


Chemical context
We are currently interested in tough hydrogels with a built-in capacity for self-healing, as a means of improving their performance in practical applications (Goswami et al., 2017;Pushparajan et al., 2018). One approach involves the polymerization of ion-pair comonomers (IPC) typically based on sulfonate anions and quaternary ammonium cations (McAdam et al., 2019). The covalent cross-linking of mixed cationic and anionic monomers generates polyampholytes (Zurick & Bernards, 2014) with additional toughness and selfhealing ability due to electrostatic interactions between the oppositely charged functional groups present (Ihsan et al., 2016;Haag & Bernards, 2017). The title IPC salt was first reported in 1978 at the emergence of this field (Salamone et al., 1978). The original synthesis utilized ion-exchange chromatography (Salamone et al., 1980) but this preparative methodology has been superseded by the argentometric mixing approach (Li et al., 2010).

Structural commentary
The title compound (1) is a salt consisting of a 3-methacrylamido-N,N,N-trimethylpropan-1-aminium cation and a 2-acrylamido-2-methylpropane-1-sulfonate anion. The asym- ISSN 2056-9890 metric unit contains two unique pairs of cations and anions and the individual cation/anion pairs are shown in Figs. 1 and 2. In the numbering scheme the two salts are distinguished by leading 1 and 2 characters. A feature of both cation/anion pairs is the substantial number of intermolecular contacts, N-HÁ Á ÁO, C-HÁ Á ÁO and weaker C-HÁ Á ÁN hydrogen bonds, Table 1, linking the cations to the anions, with the O12 and O22 atoms acting as bifurcated acceptors enclosing R 1 2 (6) ring motifs in each case.

Figure 3
Intermolecular contacts in the asymmetric unit of (1).

Figure 1
Salt 1 of the title compound showing the atom numbering with ellipsoids drawn at the 50% probability level. N-HÁ Á ÁO, C-HÁ Á ÁO and C-HÁ Á ÁN hydrogen bonds are drawn as dashed grey, cyan and green lines, respectively.  (Macrae et al., 2008). For the cations the most significant variations occur around the amide unit and for one of the methyl groups of the trimethylamine substituent, Fig. 4. The anions are even more closely comparable with only small variations around the amide N atoms and the vinyl groups, Fig. 5. While the cations both adopt stretched arrangements, aided by the central propyl units, the anions are U-shaped with the acrylamide and sulfonate residues on opposite vertices of the U. The relative conformations of the C O and vinyl double bonds within the C115 and C215 acrylamide substituents of the anions are s-cis, as found in similar compounds (Goswami et al., 2017). The two methacrylamide residues of the cations are similarly arranged.

Supramolecular features
In the crystal, a series of N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, Table 1, form double chains of cations and anions along the a axis with adjacent double chains forming sheets in the ac plane, Fig. 6. These sheets are stacked along the b-axis direction by additional C-HÁ Á ÁO hydrogen bonds, Fig. 7.

Hirshfeld Analysis
Further details of the intermolecular architecture of this salt are available using Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) with surfaces and two-dimensional fingerprint plots generated by CrystalExplorer (Turner et al., 2017). Hirshfeld surfaces of the asymmetric unit of the structure which comprises salts 1 and 2, viewed for opposite faces are shown in Fig. 8(a) and 8(b). The red circles on the Hirshfeld surfaces correspond to the N-HÁ Á ÁO and some of the numerous C-HÁ Á ÁO contacts that play a significant role in stabilizing the packing in this structure. Fingerprint plots of the contacts on the Hirshfeld surface of the asymmetric unit of (1) are shown in Fig. 9. These comprise HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH, and HÁ Á ÁO/OÁ Á ÁH and the much weaker and less significant Sheets of the cations and anions of (1) in the ac plane. All hydrogen bonds are shown as dashed cyan lines.

Figure 7
Overall packing of the title compound viewed along the b-axis direction.

Figure 5
An overlay of the two unique anions of (1), r.m.s. deviation 0.0228 Å . Table 3 Percentage contributions of the interatomic contacts to the Hirshfeld surface of the asymmetric unit of (1).

Contacts
Included surface Table 3. The surfaces of the two discrete salt components of the structure can also be examined individually. Fig. 10(a) and 10(b) for salt 1 and Fig. 11(a) and 11(b) for salt 2 show the Hirshfeld surfaces of the individual salts 1 and 2, for opposite faces in each case. An immediate observation, strongly supported by the surface area data found in the fingerprint plots, vide infra, is that the surface contacts in the two discrete Hirshfeld surfaces for opposite faces of the asymmetric unit of (1) mapped over d norm in the range À0.5027 to 1.6303 a.u.

Figure 10
Hirshfeld surfaces for opposite faces of salt 1 mapped over d norm in the range À0.4919 to 1.6314 a.u.

Table 4
Percentage contributions of the interatomic contacts to the Hirshfeld surface of the individual salts of (1). salts are reasonably similar to one another. Such similarities are also signalled by the closely comparable metrical data for the two salts and the results of the overlay experiments on the pairs of cations and anions discussed earlier.
It is also instructive to investigate the differences in contacts for the discrete cation and anion components of both salts by recording fingerprint plots for the two salts together with those of the discrete cations and anions. All of the surface contributions for the individual salts and their component cations and anions are shown in Table 4, with fingerprint plots for these contacts displayed in Fig. 12 for salt 1 and Fig. 13 for salt 2. The fingerprint plots for the two salts are closely analogous as indeed are the percentage contribution figures in Table 4, further highlighting their similarities. The most notable differences between the values for the salt and its components are that the HÁ Á ÁH van der Waals interactions are significantly greater for the cations in comparison to the anions, while the anion shows considerable increases in the HÁ Á ÁO/OÁ Á ÁH contacts reflecting the prominent role of the sulfonate O atoms in hydrogen bond formation. The HÁ Á ÁN/ NÁ Á ÁH contributions to all of the surfaces are very weak but are included for completeness.  (Guo et al. 2005). However, these results show that both components of this salt are unusual with no hits for any structures of related methylacrylamido cations nor acrylamidosulfonate anions. Indeed, the only structure showing even a moderately close relationship to either of the molecules reported here is N,N,N 0 ,N 0tetramethyl-N 00 -[3-(trimethylazaniumyl)propyl]guanidinium bis(tetraphenylborate) acetone solvate (Tiritiris, 2013) that contains the Me 3 N + (CH 2 ) 3 NH-fragment.

Synthesis and crystallization
The title compound was prepared via an argentometric mixing approach ( Hirshfeld surfaces for opposite faces of salt 2 mapped over d norm in the range À0.5029 to 1.6274 a.u.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. N-H hydrogen atoms were located in a difference-Fourier map and their coordinates were refined with U iso (H) = 1.2U eq (N). All H atoms bound to carbon were refined using a riding model with d(C-H) = 0.95 Å and U iso (H) = 1.2U eq (C) for aromatic and vinyl H atoms, d(C-H) = 0.99 Å and U iso (H) = 1.2U eq (C) for methylene and d(C-H) = 0.98 Å and U iso (H) = 1.5U eq (C) for methyl H atoms. The crystal studied was refined as a twocomponent inversion twin with a 0.58 (4):0.42 (4) domain ratio. Two reflections with F o >>> F c were omitted from the final refinement cycles.     (Macrae et al., 2008); software used to prepare material for publication: SHELXL2018 (Sheldrick, 2015b), enCIFer (Allen et al., 2004), PLATON (Spek, 2009), publCIF (Westrip 2010) and WinGX (Farrugia, 2012).

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 inversion twin. Two reflections with Fo >>> Fc were omitted from the final refinement cycles.