Crystal structure and Hirshfeld surface analysis of (S)-N-methyl-1-phenylethan-1-aminium chloride

Ammonium chlorides, such as the title compound (S)-N-methyl-1-phenylethan-1-amnonium chloride (1), usually represent by-products in amination processes of chlorosilanes. Hirshfeld surface analysis and Hirshfeld atom refinement (HAR) were performed to obtain detailed information about the crystal packing and the exact position of the hydrogen atoms of compound 1.

The title compound C 9 H 14 N + ÁCl À , (1), can be synthesized starting from (S)-Nmethyl-1-phenylethan-1-amine (2). Compound 2 upon addition of HClÁEt 2 O leads to crystallization of compound 1 as colorless blocks. The configuration of compound 1 is stable as well as preserved in space group P2 1 2 1 2 1 . Ammonium chlorides, like the title compound, are often observed as undesirable byproducts in aminosilylation of chlorosilanes. Additionally, these by-products are usually soluble in selected organic solvents, which require difficult separation steps. Therefore, detailed studies on structural features and intermolecular interactions performed by Hirshfeld atom refinement (HAR) using NoSpherA2 [Kleemiss et al. (2021). Chem. Sci. 12, 1675Sci. 12, -1692 and Hirshfeld surface analysis were used to address structural issues on that separation problem.

Chemical context
Chiral amines represent a central role in synthetic chemistry, finding more and more applications in asymmetric syntheses (Liu et al., 2020). In addition to asymmetric inductions on double bonds of organic molecules, they also serve as amination reagents of chlorosilanes (Wannagat & Klemke, 1979;Veith, 1987). Next to methoxysilanes, those chlorosilanes are the most important starting compounds for the synthesis of aminosilanes (Bauer & Strohmann, 2012). The title compound (S)-N-methyl-1-phenylethan-1-aminium chloride (1), represents the ammonium chloride salt of (S)-N-methyl-1-phenylethan-1-amine (2), which is often used as a chiral auxiliary in reagent inductions on prochiral silicon centers (Bauer & Strohmann, 2014). Compound 2 and its derivatives are characterized by well-known methods of enantiomeric resolution (Ingersoll, 1937;Baltzly & Russell, 1953). The synthesis of Si-N-functionalized silanes starting from chlorosilanes in combination with amines is also very well known (Sakaba et al., 2015;Zibula et al., 2020). However, the formation of the undesirable ammonium chloride is often observed as a byproduct, which is also soluble in small amounts of selected organic solvents. The corresponding reaction is shown in the scheme below.
Compound 1 was crystallized for the first time and may be used to analyze supramolecular interactions, in particular those which could be directly related to the aforementioned separation problem. To describe the positions of the hydrogen atoms as accurately as possible, all hydrogen atoms were refined anisotropically by NoSpherA2 (Kleemiss et al., 2021).

Structural commentary
Compound 1 crystallized from diethyl ether at room temperature in the shape of colorless blocks with orthorhombic (P2 1 2 1 2 1 ) symmetry. The absolute configuration of the chiral ammonium chloride 1 in the measured crystal can be assigned with the (S)-configuration using the Cahn-Ingold-Prelog (CIP) prioritization (Cahn et al., 1966); the Flack parameter amounts to À0.03 (3) (Flack, 1983). The molecular structure of 1 is illustrated in Fig. 1. All hydrogen atoms except H1b were refined using NoSpherA2 (Kleemiss et al., 2021). No particularly large ellipsoids are observed here, whereas hydrogen atom H1b is highly deformed and distorted in anisotropic treatment. The substantial contribution from deformation of the electron density involving the chloride ion, which also includes polarization, may be responsible for the observed ellipsoidal shape of the hydrogen atom H1b. Thus it is difficult to deconvolute the effect of thermal motion in this interaction and to model the same satisfactorily. Therefore, the hydrogen atom H1b was isotropically modeled for following analyses as shown in Fig. 1(a).
In the literature, known Csp 3 -N bond lengths are in a range of 1.4816 (4) Å (N1-C3) and 1.5034 (4) Å (N1-C3), which are typical for most structurally analyzed ammonium salts (Allen et al., 1987). To discuss the bond distances in the solid-state structure, quantum chemical calculations were performed at the level M062X/6-31+G(d), which gave comparable results. The molecular structure of compound 1 in the gas phase is shown in Fig. 1(b). All conformations were taken from the solid-state structure at the start of the optimization. The result of the calculation provides smaller Csp 3 -N bond lengths than from the solid-state structure in principle. The calculated bond lengths are 1.4762 Å (N1-C3) and 1.4946 Å (N1-C2).
Hydrogen-bond lengths as well as associated angles are shown in Table 1. The calculated hydrogen bond of 1.7051 Å (N1-H1bÁ Á ÁCl1) was not described sufficiently with the addition of the used potential and basis set. Therefore, a large deviation can be observed from the analyzed distance compared to the crystal structure. Further analyses concerning supramolecular interactions are discussed in detail in the next section.
The stereogenic carbon center features a tetrahedral geometry, which is slightly distorted as shown by the angle of 107.44 (2) (C1-C2-N1). However, the geometric distortion of a tetrahedral carbon center has been observed in many compounds with different substituents (Xu et al., 2000).

Supramolecular features
The crystal packing along the a-axis of compound 1 is shown in Fig. 2. To analyze supramolecular packing interactions in more detail, Hirshfeld surface analyses were performed. The Hirshfeld surface mapped over d norm in the range from À0.5483 to 1.5337 arbitrary units generated by Crystal-Explorer2021 (Spackman et al., 2021;Turner et al., 2017) is shown in Fig. 3. Fingerprint plots, which are illustrated in Fig. 4, were also generated by CrystalExplorer2021. First, the crystal structure was analyzed to clarify for the influence of hydrogen bonds. Particularly noticeable on the Hirshfeld surface are C-HÁ Á ÁCl contacts, which are shown in red on the potential surface in Fig. 3.
Figure 1 terms of crystal packing. In contrast, ClÁ Á ÁH/HÁ Á ÁCl contacts in particular, which represent the smallest fraction of interactions (15.1%), however represent the most intense contacts on the surface (Fig. 4). Hydrogen bonds with a length up to 2.200 Å are shown in Table 1. The analysis of the hydrogenbonding network leads to the result that all hydrogen bridges can be assigned to one graph-set motif. Both hydrogen bonds in Table 1 can be assigned D 1 1 (2) (Etter et al., 1990). In addition to the influence of C-HÁ Á ÁCl contacts, the influence of possible -interactions was analyzed by Crystal-Explorer2021. As can be seen in Fig. 2, compound 1 forms one-dimensional chains along the a-axis direction in the crystal structure. These can be attributed to the strong C-HÁ Á ÁCl interactions already mentioned, as well as additional C-HÁ Á Á interactions, which are illustrated in Fig. 2. Consequently, these -interactions could contribute a significant part to the crystal packing structure. However, C-HÁ Á Á contacts are only weakly visible on the Hirshfeld surface.  et al., 2004). A comparison with the last two structures mentioned shows that compound 1 is characterized by particularly short C-HÁ Á ÁCl (2.0-2.1 Å ) hydrogen bonds. The smallest observed hydrogen bond of the two literature known compounds amounts to 2.2 Å . These longer distances could be due to the more sterically demanding substituents, which are less pronounced in compound 1. Moreover, in terms of the crystal packing, the compounds do not exhibit one-dimensional chains, as C-HÁ Á Á contacts were not observed. This unique feature of (S)-N-methyl-1-phenylethan-1-aminium chloride (1), which does not appear in the literature compounds, could again be attributed to the steric crowding of the other compounds.

Figure 2
A view along the a-axis direction of the crystal packing of compound 1. Selected hydrogen-bond lengths (in Å ) are indicated.

Figure 3
Hirshfeld surface of compound 1 generated by CrystalExplorer21.