Synthesis and crystal structures of N,2,4,6-tetramethylanilinium trifluoromethanesulfonate and N-isopropylidene-N,2,4,6-tetramethylanilinium trifluoromethanesulfonate

Two aniline-based trifluoromethylsulfonate (trifluoromethanesulfonate) salts, N,2,4,6-tetramethylanilinium trifluoromethanesulfonate and N-isopropylidene-N,2,4,6-tetramethylanilinium trifluoromethanesulfonate, were synthesized and characterized by single-crystal X-ray diffraction.


Structural commentary
Both compounds 1 and 2 are ionic compounds based on cations of a 2,4,6-trimethylanilinium unit with functionalization of the amine group and the anion trifluoromethanesulfonate (i.e.triflate).For 1, a secondary ammonium ion results from bonds to a 2,4,6-trimethylphenyl ring, a methyl group, and two hydrogen atoms (Fig. 1).The hydrogen atoms of the ammonium nitrogen atom form hydrogen bonds with the oxygen atoms of neighboring triflate anions.For 2, the iminium ion consists of an isopropylidene group (nitrogen atom double bonded to a carbon atom attached to two methyl groups) with a 2,4,6-trimethylphenyl ring and a methyl group also attached to the nitrogen (Fig. 2).As there are no hydrogen atoms on the iminium nitrogen atom, the organic cation of 2 does not form any classical hydrogen bonds.The 2,4,6-trimethylphenyl groups in both 1 and 2, and the isopropylidene group in 2 are, as expected, nearly planar with r.m.s.deviations from planarity (including the nitrogen atom in the defined planes) of only 0.0263, 0.0111 and 0.0200 A ˚, respectively.In both compounds, the carbon functional groups of the nitrogen atom lie approximately perpendicular to the trimethylphenyl ring (Fig. 3).For 1, the angle between the calculated mean plane of the methyl group (defined as C1, N1 and C10) and the mean plane of the aniline ring (N1 and C1-C6) is 89.71 (9) � , while for 2, the angle between the mean

Figure 3
The organic functional groups bound to nitrogen atom are approximately orthogonal to the ring of the 2,4,6-trimethylphenyl group for both (a) 1 and (b) 2. For clarity, the hydrogen atoms have been omitted.See Fig. 1 for additional display details.

Supramolecular features
The dominant intermolecular forces in 1 consist of strong N-H� � �O hydrogen bonding (Table 2) and �-� stacking interactions (Table 3), while in 2 no classical hydrogen bonds are present and �-� interactions are highly slipped.Instead, interactions in 2 are governed by a series of weak C-H� � �O/F interactions (Table 4; listed H� � �O/F distances are up to 2.70 A ˚). Similar C-H� � �O interactions are also present in 1 but they are much less pronounced; the C-H� � �O distances and angles indicate that they are more likely dispersion (i.e.van der Waals) interactions rather than weak directional hydrogen bonds.For both compounds, C-H� � �F and C-H� � �� interactions are very weak and not well defined (Tables 2-4).
For 1, the principal directional interactions are the N-H� � �O hydrogen bonds.Both ammonium hydrogen atoms are hydrogen bonded to an oxygen atom of neighboring triflate anions (Fig. 4; Table 2).One triflate anion is located on either side of the ammonium nitrogen atom.The hydrogenbonding arrangement leads to a one-dimensional chain that extends in the ac plane and is propagated by the n-glide plane at (x, 0.75, z).In addition, the organic cations form dimers with the 2,4,6-trimethylphenyl rings arranged in a paralleldisplaced geometry where the 2,4,6-trimethylphenyl rings are offset relative to each other.The cations that make up the dimers are symmetry-related by inversion so that the ammonium groups are opposite of each other, likely to avoid Coulombic repulsions (Fig. 5).The distance between the calculated centroids of the benzene rings in each dimer is 3.9129 (8) A ˚, and the interplanar spacing and ring slippage are 3.5156 (5) and 1.718A ˚, respectively [determined with PLATON (Spek, 2020); Table 3].
In 2, the organic cations also form inversion-related dimers, but the rings are highly slipped (Fig. 5) with respect to each other and �-� interaction, if present at all, is limited to just the outermost atoms C4 and C5.The distance between the calculated centroids of the benzene rings in each dimer is 4.8937 (8) A ˚, and interplanar spacing and ring slippage are 3.3646 (5) and 3.553 A ˚, respectively (Table 3).In the absence

Figure 4
Intermolecular hydrogen bonding in 1 between the ammonium hydrogen atoms and the trifluoromethanesulfonate oxygen atoms.The hydrogen bonding results in a one-dimensional chain that extends in the ac plane.
For clarity, only the atoms involved in the hydrogen bonding are labeled.See Fig. 1 for additional display details. [Symmetry code: Table 1 Angle Between the Mean Plane of the Organic Functional Groups and the Mean Plane of the Aniline Ring.
Angles were determined with SHELXL (for 1 and 2; Sheldrick, 2015b) or Mercury (for comparison compounds; Macrae et al., 2020) 4).Most important are the hydrogen-bond-like interactions that involve the iminium methyl group (C13) being hydrogen bonded to oxygen atoms of three different triflate anions (Fig. 6).This methyl group is directly bonded to the nitrogen atom and carries the largest partial positive charge, inducing formation of charge-assisted bonds that are unusually short for C-H� � �O interactions with C� � �O distances of 3.1723 (17), 3.3789 (18), and 3.3789 (18) A (Desiraju & Steiner, 2001).The isopropylidene methyl group

Figure 5
The organic cations of (a) 1 and (b) 2 form dimers that are related by a crystallographic inversion center and have intermolecular �-� interactions in a parallel-displaced geometry (black dotted lines).See Fig. 1 for additional display details.   (C11) also does exhibit another unusually short C-H� � �O bond [C� � �O distance of 3.477 (2) A ˚] (Fig. 6).Each [C 13 H 20 N + ] cation is hydrogen bonded to three triflate anions, and each triflate anion is hydrogen bonded to three organic cations, thereby generating a two-dimensional network of C-H� � �O interactions with layers extending perpendicular to the b-axis direction.The resulting layers interact with each other solely via dispersion interactions.
Both the centroid distance between the benzene rings and the ring slippage distance are longer for 2 than 1.However, the values for 2 are more aligned with the distances for 1,3,5trimethylbenzene, i.e. mesitylene.In the crystal structure of deuterated-1,3,5-trimethylbenzene (SOPLAL01; Ibberson et al., 2007) the molecules also form long �-� interactions with a parallel-displaced geometry, and the distance between the calculated centroids of neighboring benzene rings is 4.634 A ẘith a ring slippage of 2.850 A ˚(Table 3).Moreover, the longer distances of 2 are comparable to the centroid and ring slippage distances for a series of 2,4,6-trimethylanilinium cations with various counter-anions (Table 3).

Database survey
A survey of the Cambridge Structural Database (CSD version 5.45, update November 2023; Groom et al., 2016) for secondary ammonium cations with a 2,4,6-trimethylphenyl group, as in 1, yielded five entries (EDUWAD, HIBFOO, HIBFUU, HIBGAB and QARJUQ).Two of the cations have two 2,4,6-trimethylphenyl groups bound to the ammonium nitrogen atom but with different counter-anions, pentafluorobenzenesulfonate (HIBFOO; Sakakura et al., 2007) or 4-methylbenzenesulfonate (HIBGAB; Sakakura et al., 2007).A related secondary ammonium cation binds to one 2,4,6trimethylphenyl group and one 2,6-diphenylphenyl group and the counter-anion is a pentafluorobenzenesulfonate (HIBFUU; Sakakura et al., 2007).The last two entries also only contain one 2,4,6-trimethylphenyl group on the ammonium nitrogen atom.In one structure (EDUWAD; Ikhile & Bala, 2012), an ethyl-2-formamido-2,4,6-trimethylbenzene group is bound to the ammonium nitrogen atom and chloride serves as the counter-anion.The other structure (QARJUQ; Latham et al., 2012) is a zwitterion with a 3-methylbutan-2-ylcarbamoylbenzenesulfonate acting as the second group bound to the ammonium nitrogen atom.Lastly, a biphenyl system with two secondary ammonium nitrogen atoms (CATZEG; Li et al., 2022) is similar to 1.As in 1, the 1,1 0 biphenyl system has an ammonium nitrogen attached to the carbon atom in the 4 and 4 0 positions of the benzene rings and on each benzene ring two methyl groups are located on carbon atoms adjacent (3,3 0 and 5,5 0 positions, respectively) to the carbon atom with the ammonium nitrogen atom.In addition, the ammonium nitrogen atoms bind to a methyl group as in 1.A comparison of the angle between the mean planes of the functional groups and of the aniline ring reveal that the angles generally do not approach 90 � as in 1 (Table 1; the angles were measured between mean planes defined in a similar manner as for 1 in the Structural commentary section).The angles range from ca 50 to 77 � for the five structures with a 2,4,6-trimethylphenyl group attached to the ammonium nitrogen atom.For these structures, the bulkiness of the groups opposite the 2,4,6-trimethylphenyl groups may prevent the angle being close to 90 � .In 1, the group opposite to the 2,4,6-trimethylphenyl group is a smaller methyl group.For the 1,1 0 -biphenyl system (CATZEG) the angle (ca 85 � ) is closer to 90 � likely due to the two phenyl rings nearly lying in the same plane and the presence of a smaller methyl group.
A survey for compounds containing an iminium nitrogen atom with a 2,4,6-trimethylphenyl ring and with a doublebonded carbon atom bound to two additional carbon groups yielded only one entry (JIFFAI; Kremla ´c ˇek et al., 2018).Like 2, the iminium nitrogen atom is bound to a methyl group and a 2,4,6-trimethylphenyl group and the counter-anion is triflate.Unlike 2, substitution on the carbon atom of the iminium double bond consists of a methyl group and a bulky 3-methyl-2-(2,4,6-trimethylphenyl)-2H-indazol-7-yl group.Comparison of the equivalent angle between the mean planes of the functional groups and the aniline ring to that of 2 reveals that the angle (ca 83 � ) deviates more from 90 � than that of 2 (Table 1).In a related structure (RAVBIC; Chen et al., 2017), substitution on the carbon atom of the iminium double bond consists of a hydrogen atom and a {2-[(hydroxyl)(methoxy) methylidene]-4-methoxy-4-oxo}{[tris(pentafluorophenyl)] boron} group.In addition, the iminium nitrogen is bound to a methyl group.Also, this compound is a zwitterion instead of a triflate salt with the borate providing the negative charge.In regard to the angle between the mean planes of the functional groups and the aniline ring, there is an even larger deviation (ca 80 � ), likely due to the bulky {2-[(hydroxyl)(methoxy) methylidene]-4-methoxy-4-oxo}{[tris(pentafluorophenyl)] boron} group (Table 1).

Physical Methods
The 1 H and 13 C chemical shifts were reported in ppm (�) and referenced to CD 3 CN.All NMR spectra were recorded at 299.7 K on a Magritek Spinsolve 80 (Malvern, PA USA).Proton and carbon spectra were operated at 80.98 MHz and 20.36 MHz, respectively, with a field strength of 1.88 Tesla.Spectra were processed using MNova software Ver.14.3.3(Mestrelab Research, Escondido, CA USA).Infrared spectroscopy was performed using a Nicolet iS5 FTIR spectrometer (Thermo Electron North America LLC) outfitted with a diamond crystal ATR accessory and Omnic software Omnic version 9.2.98.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. Hydrogen atoms were placed in calculated positions and refined as riding on their carrier atoms with C-H distances of 0.95 A ˚for sp 2 carbon atoms, 0.98 A ˚for methyl carbon atoms, and 0.91 A ˚for ammonium nitrogen atoms.Methyl hydrogen atoms were allowed to rotate but not to tip to best fit the experimental electron density.The U iso values for hydrogen atoms were set to a multiple of the value of the carrying carbon atom or nitrogen atom (1.2 times for sp 2 -hybridized carbon atoms and the nitrogen atom or 1.5 times for methyl carbon atoms).

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.

Table 5
Experimental details.
Computer programs: APEX4 and SAINT