Structure and Hirshfeld surface analysis of the salt N,N,N-trimethyl-1-(4-vinylphenyl)methanaminium 4-vinylbenzenesulfonate

The molecular and crystal structure of the salt N,N,N-trimethyl-1-(4-vinylphenyl)methanaminium 4-vinylbenzenesulfonate is reported. A Hirshfeld surface analysis of the salt and its individual components is also presented.


Chemical context
Hydrogels continue to be the subject of intense study, particularly with regard to biomedical applications and new technologies (Van Vlierberghe et al., 2011;Sun et al., 2015;Goswami et al., 2017;Pushparajan et al., 2018). Limiting development has been the poor mechanical strength of conventional hydrogel formulations. Numerous strategies, singly and in combination, have been utilized in efforts to improve toughness and stretchability, and the results have been extensively reviewed (Naficy et al., 2011;Peak et al., 2013;Zhao, 2014). Our current approach is to build in capacity for self-healing, and exploits polyampholytes (Zurick & Bernards, 2014), polymers formed from the covalent cross-linking of mixed cationic and anionic monomers. The title compound is one such set of ion-pair co-monomers, simply prepared from commercially available trimethylammonium cation and sulfonate anion salts. ISSN 2056-9890

Structural commentary
The asymmetric unit of the title salt, (I), comprises an N,N,Ntrimethyl-1-(4-vinylphenyl)methanaminium cation and a 4-vinylbenzenesulfonate anion, linked by a C14-H14BÁ Á ÁO3 hydrogen bond (Table 1) between a methyl group of the trimethylmethanaminium unit and a sulfonate oxygen, Fig. 1. The vinyl substituent on the benzene ring of the cation is disordered over two sites with a refined occupancy ratio of 0.542 (11):0.458 (11). In the cation, the C7/C13/N1 and C10/ C101/C102 planes of the methanaminium and major vinyl substituents on the benzene ring subtend angles of 86.6 (3) and 10.5 (9) , respectively, to the ring plane. In contrast, excluding the sulfonate O atoms, the S and ordered vinyl substituents lie close to the benzene ring plane in the anion with an r.m.s. deviation of 0.0753 Å from the S1/C1-C6/C41/ C42 plane.

Supramolecular features
Packing in this salt is dominated by an extensive number of C-HÁ Á ÁO hydrogen bonds,  Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Chains of cations and anions of (I) along the a axis. Hydrogen bonds are shown as cyan dotted lines [symmetry code: (i) x À 1, y, z].

Figure 1
The asymmetric unit of the title compound showing the atom numbering with ellipsoids drawn at the 50% probability level. The C-HÁ Á ÁO hydrogen bond linking the two components is drawn as a dotted black line. For clarity, only the major disorder component of the vinyl substituent on the benzene ring of the cation is shown.

Hirshfeld surface analysis
Further details of the intermolecular architecture of this salt were obtained using Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) with surfaces and two-dimensional fingerprint plots generated by CrystalExplorer (Turner et al., 2017).
Hirshfeld surfaces viewed for opposite faces of the complete salt are shown in Fig. 7. Both disorder components are included in these surface calculations. The red circles on the Hirshfeld surfaces correspond to the numerous C-HÁ Á ÁO contacts that play a significant role in stabilizing the packing in this structure. Fingerprint plots of the principal contacts on the Hirshfeld surface of the salt are shown in Fig. 8. These comprise HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH, and HÁ Á ÁO/OÁ Á ÁH contacts. The much less significant CÁ Á ÁC and HÁ Á ÁS/SÁ Á ÁH contributions are not shown in the figure but are detailed in Table 2.
It is also instructive to investigate the differences in contacts for the discrete cation and anion components of (I) by recording fingerprint plots of the cation and anion individually. All of the surface contributions for the cation and anion are also shown in Table 2 Zigzag chains of anions along c. Hydrogen bonds are shown as cyan dotted lines [symmetry code: (v) 3 2 À x, 1 À y, z À 1 2 ].

Figure 6
Overall packing for (I) viewed along the a-axis direction.

Figure 7
Hirshfeld surfaces of (1) viewed for opposite faces of the salt. displayed in Fig. 8. The most notable differences between the values for the salt and its components are that the HÁ Á ÁH van der Waals interactions increase significantly for the cation, while the anion shows considerable increases in the HÁ Á ÁO/ OÁ Á ÁH and HÁ Á ÁC/CÁ Á ÁH contacts. These differences reflect the fact that, whereas the contacts for the cations are limited to cation-anion interactions, the anions are also involved in distinct anion-anion contacts, vide supra. The CÁ Á ÁC and HÁ Á ÁS/SÁ Á ÁH contributions to all of the surfaces are very weak but are included in Table 2 for completeness.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms 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 vinyl substituent on the benzene ring of the cation is disordered over two sites (C101=C102 and C103=C104) with a refined occupancy ratio of 0.542 (11):0.458 (11).

Funding information
We     CrysAlis PRO (Rigaku OD, 2018); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b) and TITAN (Hunter & Simpson, 1999); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2014 (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. The vinyl substituent on the benzene ring of the cation is disordered over two sites with a refined occupancy ratio of 0.542 (11):0.458 (11).