Salts of 4-[(benzylamino)carbonyl]-1-methylpyridinium and iodide anions with different cation:iodine stoichiometric ratios

The ability of 4-[(benzylamino)carbonyl]-1-methylpyridinium to form iodide salts with cation:iodine ratio different from equimolar was studied and a Hirshfeld surface analysis was performed to investigate the intermolecular interactions.

In a continuation of this work, we attempted to obtain a new polymorphic form of this compound using not only different solvents (ethanol, methanol, 2-propanol, etc.), but also non-standard methods of activating the crystallization process. To do this, experiments on recrystallization from water under an ultrasonic field effect were carried out. It should be noted that under normal conditions, 4-[(benzyl-

Structural commentary
The crystal structures of the salts under study consist of the same 4-[(benzylamino)carbonyl]-1-methylpyridinium cation (C 14 H 15 N 2 O + ) and different anions. There is one cation, one iodide anion and half of the neutral I 2 molecule in the asymmetric unit of compound I (Fig. 1, left). The neutral I 2 molecule is located in a special position in relation to the symmetry centre coinciding with the midpoint of the I-I bond. Thus, the cation:iodine atoms ratio is 1:2 in compound I. The asymmetric unit of compound II contains two cations (A and B), one triiodide anion (I 3 À ) and two halves of triiodide anions located on special positions in relation to the symmetry centre ( Fig. 1, right). The cation:iodine atoms ration is 1:3 in compound II.
The positive charge of the cation is localized at the quaternized nitrogen atom of the pyridine ring. This results in the N1-C6 and N1-C2 bond elongation (Table 1). The carbamide group is non-coplanar to the plane of the aromatic ring (as evidenced by the N2-C7-C4-C3 torsion angles; Table 1) as a result of steric repulsion between them [with short H2Á Á ÁH3 and H2Á Á ÁC3 contacts (as compared to the van der Waals radii sums; Zefirov, 1997) of 2.34 and 2.87 Å , respectively]. The cations in the two compounds under study differ in the conformation of the benzyl substituent. The phenyl fragment of the benzyl substituent is located in a Àsc position relatively to the C7-N2 bond in I or in a +sc position in molecule A and an ap position in molecule B of II (cf the C7-N2-C8-C9 torsion angles in Table 1). The aromatic ring is turned relative to the carbamide fragment (see the N2-C8-C9-C10 torsion angles).
In the structure of II, the A and B cations form stacking dimers as a result of the interaction of the aromatic systems of the pyridine and benzene rings [the distance between the planes of aromatic cycles is 3.45 (1) Å , slippage 1.119 Å ).

Hirshfeld surface analysis
Intermolecular interactions can be analyzed using Hirshfeld surface analysis and 2D fingerprint plots (Turner et al., 2017). The Hirshfeld surfaces were calculated for the cations found in two structures under study using a standard high surface resolution, mapped over d norm (Fig. 3). The red spots, corresponding to contacts that are shorter than the van der Waals radii sum of the closest atoms, are observed at the hydrogen atom of the amino group. At the carbonyl group, red spots are found only in the cations of II. The two-dimensional finger-print plots show that the hydrogen bonds in II are stronger (note the sharp spikes in Fig. 3).
To compare intermolecular interactions of different types in more quantitative way, their contributions to the total Hirshfeld surfaces were analysed (Fig. 4). The main contribution is provided by HÁ Á ÁH short contacts (44.9% for I, 45% for cation A and 36.8% for cation B in II). The contribution of the IÁ Á ÁH/ HÁ Á ÁI short contacts is also significant [17.3% in I, 21.7% (molecule A) and 25.5% (molecule B) in II], as is that of the CÁ Á ÁH/HÁ Á ÁC interactions [17.2% in I, 15.5% (molecule A) and 10.7% (molecule B) in II]. Surprisingly, the contributions of the OÁ Á ÁH/HÁ Á ÁO interactions are very similar in the two structures [9.7% in I, 9.5% (molecule A) and 9.6% (molecule B) in II] despite the stronger N-HÁ Á ÁO hydrogen bonds in the structure of II.

Database survey
A search of the Cambridge Structural Database (Version 5.42, update of November 2020; Groom et al., 2016) revealed the structure of the AmI salt with an equimolar cation:iodine atoms ratio (refcode BEBFIA; Drebushchak et al., 2017). A comparison of the cation conformations showed its flexibility resulting from rotation about the N-Csp 3 and Csp 3 -Car bonds. Hydrogen bond formation in structure I (on the left) and II (on the right).

Figure 3
Hirshfeld surfaces mapped over d norm. (at the top) and two-dimensional fingerprint plots (at the bottom) of cation in structure I and II.

Figure 4
Relative contributions of the strongest intermolecular interactions (in %) to the total Hirshfeld surface of cation in two iodide salts.

Synthesis and crystallization
Benzylamide isonicotinic acid (124 g, 0.585 mol) and 270 mL of 90% ethanol were loaded into a glass flask. The obtained solution was heated to a temperature of 313-314 K, and then methyl iodide (91g, 0.641 mol) was added dropwise. The reaction was stirred at a temperature of 313-314 K for 1 h, heated to boiling and boiled for 1 h. The reaction spontaneously cooled to a temperature of 313 K, then to a temperature of 283-288 K in a cooling water bath, and was stirred for 1.5 h at this temperature. The reaction mixture was filtered and the precipitate rinsed on the filter twice with 60 mL of cooled 96% ethanol. The product was dried at 313 K for 12 h. Yield: 145.5 g of crude 4-[(benzylamino)carbonyl]-1methylpyridinium iodide (88%); yellow crystals. 145.5 g of crude 4-[(benzylamino)carbonyl]-1-methylpyridinium iodide were dissolved in 450 mL of water under ultrasonic activation. The reaction was heated to boiling temperature, stirred at boiling for 30 min and filtered. The obtained solution was cooled slowly and evaporated for three weeks. The rod-shaped crystals of I and block-shaped crystals of II crystallized almost simultaneously.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Despite the presence of iodine atoms, crystals of salt II diffracted poorly due to their small size. All of the hydrogen atoms were located in difference-Fourier maps. Then, hydrogen atoms were refined as riding (AFIX 33 and 137 commands) with C-H = 0.96 Å , U iso (H) = 1.5U eq (C) for methyl groups (AFIX 43) and C ar -H = 0.93 Å , U iso (H) = 1.2U eq (C) for aromatic rings (AFIX 23) and Csp 2 -H = 0.97 Å , U iso (H) = 1.2U eq (C) for the methylene fragment.

Powder diffraction characterization
A powder diffraction pattern of salt II was registered using a Siemens D500 powder diffractometer (Cu K radiation, Bragg-Brentano geometry, curved graphite monochromator on the counter arm, 4 < 2 < 60 , D2 = 0.02 , time per step of 2 s). The Rietveld refinement of the obtained pattern (Fig. 5  Final Rietveld plots for II (on the left). Observed data points are indicated by red circles, the best-fit profile (black upper trace) and the difference pattern (blue lower trace) are shown as solid lines. The vertical green bars correspond to the Bragg positions of peaks. The calculated powder pattern for I is shown on the right. For both structures, data collection: CrysAlis PRO (Rigaku OD, 2018); cell refinement: CrysAlis PRO (Rigaku OD, 2018); data reduction: CrysAlis PRO (Rigaku OD, 2018); program(s) used to solve structure: SHELXT2014/5 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2016/6 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2020); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

4-[(Benzylamino)carbonyl]-1-methylpyridinium iodide-iodine (2/1) (I)
Crystal data 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.

4-[(Benzylamino)carbonyl]-1-methylpyridinium triiodide (II)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.70 e Å −3 Δρ min = −0.77 e Å −3 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (