Crystal structure and Hirshfeld surface analysis of N-{[diphenyl(vinyl)silyl]methyl}-2-methylpropan-2-ammonium chloride

In the title hydrochloride salt, the cation shows an unusually long Si—C bond length. In the crystal, the cations and anions are linked by N—H⋯Cl hydrogen bonds to generate [001] chains.

silyl]methyl}-2-methylpropan-2-amine, C 19 H 25 NSi, is a newly synthesized secondary aminomethylsilane that can be used, for example, to study carbolithiation reactions of vinylsilanes. Because the neutral compound did not crystallize well, the hydrochloride salt, C 19 H 26 NSi + ÁCl À , was formed, in which the two chloride ions in the asymmetric unit have crystallographic 1 site symmetry. An unusually long Si-C bond of 1.9117 (10) Å is observed in the cation, which may be ascribed to electronic effects due to the N + species. In the crystal, the cations and anions are linked by N-HÁ Á ÁCl hydrogen bonds to generate [001] chains. To further investigate the intermolecular interactions, a Hirshfeld surface analysis was performed, which showed that HÁ Á ÁH, CÁ Á ÁH/ HÁ Á ÁC and HÁ Á ÁCl/ClÁ Á ÁH contacts contribute 70.4, 20.0 and 8.3%, respectively.

Chemical context
There are only a few secondary (aminomethyl)silanes known to date because the synthesis is not feasible due to the high energy requirement and reaction time. With the assistance of a Finkelstein reaction, the iodomethylsilane can be synthesized to enhance the reactivity and shorten the reaction time (Finkelstein et al., 1910;Abele & Strohmann, 1997). However, it was possible to synthesize the (aminomethyl)diphenylvinylsilane 1 in an efficient way, starting from a (chloromethyl)silane. Because the (aminomethyl)silane 1 did not crystallize well, the hydrochloride salt 2 was formed to characterize the compound via X-ray diffraction. For example, the newly synthesized (aminomethyl)vinylsilane 1, C 19 H 2 5NSi, can be used for investigations of a carbolithiation reaction of the silane's vinyl group via lithiumalkyls. The received product can be used for the synthesis of functionalized alcohols by a Tamao oxidation (Tamao et al., 1983). The molecular structure is defined by an unusually long Si-C bond, which thus favors the cleavage of this bond. Usually, the aminomethyl sidearm contains two or three nitrogen centers and is essential for the feasibility of the reaction. It helps to break down the lithiumalkyl aggregates by forming a dative bond and also precoordinates the lithium ions, so they are in proximity to the vinyl group of the silane. Our own studies have shown that this stabilizes the transition state of the reaction, hence the activation energy of the deprotonation of the vinyl group is minimized and the reaction can be done under low temperatures and kinetic control, to prevent side reactions such as the -deprotonation or polymerization (Unkelbach & Strohmann, 2009). This new (aminomethyl)silane 1 contains only one nitrogen center in the sidearm and undergoes the carboli-thiation by a new mechanism for vinylsilanes. This mechanism is known from stilbenes, where two lithium cations stabilize the negative charge at the anionic carbon atom. With the use of chiral ligands, the reaction can be performed under stereogenic control (Tricotet et al., 2009). This opens a new field for interesting research in organosilicon chemistry.

Structural commentary
Compound 2 crystallized in a few minutes from an aqueous 1 M HCl solution of 1 at room temperature as a hydrochloride salt, C 19 H 26 NSi + ÁCl À , in the form of colorless needles in the centrosymmetric space group P2 1 /n. The molecular structure is illustrated in Fig. 1. Both chloride ions are located on special positions with 1 site symmetry.
The Si1-C15 bond length in the cation is 1.9117 (10) Å , which is slightly longer than the average for an Si-C bond and the Si1-C15-N1 bond angle is 116.21 (7) . The Si-C bond lengths are normally in the range of 1.857 to 1.905 Å for Csp 3 -SiX 3 compounds (Allen et al., 1987). The extended bond length may be ascribed to the cationic nitrogen atom in the -position to the silicon atom. It increases the electronegativity, which enhances the electron-withdrawing effect of the substituted -aminofunctionality. This enhances the p-character of the Si1-C15 bond, which leads to an elongated bond (Bent, 1961). The extended Si1-C15-N1 bond angle is due to the steric demand of the tert-butyl group. Some further examples are given in the Database survey section (Kirchoff et al., 2022). The angle between the C3-C8 and C9-C14 phenyl groups in 2 is 89.63 (2) , which is caused by the steric repulsion of the aromatic hydrogen atoms. The Si1-C1 bond length is 1.8577 (11) Å and C1-C2 is 1.3293 (16) Å ; the latter is positioned at the end of the default range of Csp 2 -Csp 2 bonds, which lie between 1.299 and 1.328 Å .
The cationic nitrogen center features a slightly disordered tetrahedral geometry. The angle between the hydrogen atoms is 107.2 (13) (H1A-N1-H1B), the angles between the C atoms and the H atoms are 107.8 (10) (H1B-N1-C15) and 109.6 (9) (H1A-N1-C15). Between the carbon atoms, the angle is 117.10 (7) (C15-N1-C16). All angles vary slightly from the ideal tetrahedron angles of 109.5 : the large C-N-C angle results from the bigger space requirement of the carbon atoms in comparison to the H atoms. The sum of angles around the nitrogen atom is 441.7 , so the overall structure is distorted tetrahedral. The bond length between N1 and C15 is 1.4928 (12) Å and it is 1.5330 (13) Å between N1 and C16. In the literature, Csp 3 -N bond lengths are in the range of 1.4816 to 1.5034 Å , so the N1-C16 bond is slightly extended.

Supramolecular features
In the extended structure of 2 ( Fig. 2), the cations and anions are linked by N-HÁ Á ÁCl hydrogen bonds (Table 1)  The molecular structure of 2 showing 50% displacement ellipsoids. Hydrogen bonds are indicated by dotted lines.
To further analyze the supramolecular packing interactions, a Hirshfeld surface analysis was performed (Spackman & Jayatilaka, 2009). The Hirshfeld surface of the cation mapped over d norm in the range from À0.54 to 1.49 arbitrary units, generated by CrystalExplorer2021 (Spackman et al., 2021;Turner et al., 2017), is shown in Fig. 4. The fingerprint plots are illustrated in Fig. 5 and were also generated by Crystal-Explorer2021. Particularly noticeable on the Hirshfeld surface are the short N-HÁ Á ÁCl contacts, which are shown in red on the potential surface, see Fig. 4. Although they represent the smallest fraction of interactions (8.3%), they presumably have the greatest effect on the crystal structure. The HÁ Á ÁH contacts (70.4%) are the biggest fraction, but play a minor role in terms of the crystal packing. Analysis of the hydrogen-bonding network leads to the result that H1 can be assigned the graphset symbols D 1 1 (2) and D 1 2 (3), which means that the hydrogen bond extends from N1-H1AÁ Á ÁCl1 to another H1A-N1 grouping of a neighboring molecule. H2 can also be assigned D 1 1 (2) and D 1 2 (3) (Etter et al., 1990). Here, the hydrogen bond extends from N1-H1BÁ Á ÁCl2 to another H1B-N1 group of a neighboring molecule. These hydrogen bonds may be the reason why 2 crystallizes well compared to the neutral molecule 1. The Hirshfeld surface of compound 2 generated by CrystalExplorer21.   Figure 3 The crystal packing of compound 2 showing the C-HÁ Á ÁCl contacts.

Database survey
There are examples of crystallographically characterized structures with motifs like those in compound 2. In LAGLUE, the N-HÁ Á ÁCl hydrogen bond has a slightly longer NÁ Á ÁCl separation (3.124 Å ) than compound 2. The Si-C bond is shorter [1.905 (2) Å ] and the Si-C-N bond angle is comparable [115.86 (14) ]. The lengths between the carbon atoms and the cationic nitrogen center are similar to the corresponding bond lengths in 2 [1.498 (3) and 1.494 (3) Å ].
In WOLSEY, the Si-C distance of 1.871 (4) Å is shorter than in 2 but the Si-C-N bond angle is similar [114.9 (2) ] and the C-N bond is a bit extended [1.505 (6) Å ]. This could be caused by the ring strain of the aziridine ring and the electron-withdrawing effect of the (nitrophenyl)sulfonyl group located at the nitrogen center. In addition, the Si-Csp 2 bond length is 1.859 (7) Å , which is only slightly longer that the value for 2.
Finally, in AGILIL, the Si-C bond length is slightly shorter [1.907 (7) Å ] and the Si-C-N bond angle is slightly extended [120.8 (4) ], which is caused by the cyclic structure of the compound. The C-N distance is equal [1.498 (8) Å ] and the cyclic N-C bond lengths marginally shorter [1.509 (8) and 1.516 (8) Å ], again due to the cyclic structure.
The structures of WAVXAW and DAFKUT contain a similar structure motive (Si-C-N + ) to 2. In WAVXAW and DAFKUT, the Si-C bond lengths are 1.9074 (11) and 1.907 (3) Å , respectively, comparable to the value in 2. These extended bond lengths are due to the same electronic effects already described.

Synthesis and crystallization
The reaction scheme for the synthesis of compound 2 is shown in the scheme. A 1 M aqueous solution of HCl (0.11 mmol, 11 mL) was added to N-{[diphenyl(vinyl)silyl]methyl}-2methylpropan-2-amine (1) (0.10 mmol, 0.03 g) at room temperature. The product (2) was formed after five minutes as colorless needles.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms except H1A and H1B were positioned geometrically (C-H = 0.95-1.00 Å ) and refined using a riding model, with U iso (H) = 1.2U eq (C) for CH 2 and CH hydrogen atoms and U iso (H) = 1.5U eq (C) for CH 3 hydrogen atoms.
Acta Cryst. (2022). E78 research communications  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.