Crystal structure and Hirshfeld surface analysis of 1-[(benzyldimethylsilyl)methyl]-1-ethylpiperidin-1-ium ethanesulfonate

α-Aminosilanes are distinguished by a long Si—C bond, which was confirmed in the title compound. Additionally, the supramolecular interactions were determined by Hirshfeld surface analysis to investigate the influence of these contacts on the crystal packing.

The title molecular salt, C 17 H 30 NSi + ÁC 2 H 5 O 4 S À , belongs to the class of a-aminosilanes and was synthesized by the alkylation of 1-[(benzyldimethylsilyl)methyl]piperidine using diethyl sulfate. This achiral salt crystallizes in the chiral space group P2 1 . One of the Si-C bonds in the cation is unusually long [1.9075 (12) Å ], which correlates with the adjacent quaternary N + atom and was verified by quantum chemical calculations. In the crystal, the components are linked by weak C-HÁ Á ÁO hydrogen bonds: a Hirshfeld surface analysis was performed to further investigate these intermolecular interactions and their effects on the crystal packing.

Chemical context
Selective bond transformations on silicon compounds for the cleavage of Si-C bonds are of high interest in silicon chemistry (Denmark et al., 2007;Denmark & Liu, 2010). Compared to C-C bonds, analogous Si-C bonds can be cleaved heterolytically using strong nucleophiles (Tomooka et al., 2000;Li & Hu, 2007). However, the selectivity of such reactions is limited to specific silanes. In particular, -aminofunctionalized silanes are well suited for these processes, as shown by our previous studies . Our group has focused on using lithium organyls as strong nucleophiles to perform these Si-C transformations on highly substituted silanes (Bauer & Strohmann, 2014). In particular, derivatives of -piperdinobenzylsilanes have been intensively studied by our group (Strohmann et al., 2004;Otte et al., 2017). When strong nucleophiles are used, deprotonation in the benzyl position competes with the selective Si-C bond cleavage of the benzyl group. For this purpose, the -aminofunctionality seems to play a key role, which could be responsible for the activation of the subsequent Si-C bond cleavage. In addition, the positively charged ammonium group leads to an increased electronegativity, which enhances the electron-withdrawing effect of the substituted -aminofunctionality. Consequently, the -character of the Si-C bond is more pronounced, leading to an elongation of the bond. Thus, a selective cleavage of the amino functionality due to the elongated Si-C bond is also conceivable (Bent, 1960(Bent, , 1961Otte et al., 2017).
Several derivates of these -piperdinobenzylsilanes have been synthesized by our research group: 1-[(benzyldimethylsilyl)methyl]-1-ethylpiperidin-1-ium ethanesulfonate (1), the title compound, represents a compound that could lead to an extension of the aforementioned Si-C bond to the nitrogen ISSN 2056-9890 atom via the quaternary ammonium cation. Structural studies concerning this type of compound should better elucidate the reactivity as well as selectivity of Si-C cleavages of the benzyl-substituted -aminosilanes.

Structural commentary
Compound 1 crystallized from an n-pentane solution at 243 K in the form of colorless blocks with monoclinic (P2 1 ) symmetry. The chiral space group indicates that the achiral compound in the elementary cell is packed chirally; the Flack absolute structure parameter amounts to À0.005 (6) (Flack, 1983). The molecular structure of 1 is illustrated in Fig. 1. The Si-C bonds span the range 1.862 (2) to 1.908 (1) Å , as shown in Table 1. These values for the bond lengths are consistent with those in the literature, except for the long Si1-C10 bond length, which is related to the -aminosilane functionality (Allen et al., 1987). This observed elongation of the bond can be explained by the very electropositive feature of carbon atom C10. In addition, the ethylated ammonium cation pushes even more electron density from C10 toward the amino functionality. There are only a few known species with such a long Si-C bond, which in turn may play a crucial role in the reactivity of -amino-substituted silanes. Quantum chemical calculations at the M062X/6-31+G(d) level confirm the experimentally observed long Si-C bond. The calculated structure of compound 1 is shown in Fig. 2.
The silicon center in 1 features a tetrahedral geometry, which is significantly distorted, as shown by the smallest angle of 98.35 (5) (C7-Si1-C10) and the largest angle of 114.32 (7) (C8-Si1-C10). This geometric distortion has been observed in many complex substituted silicon compounds and depends on the substituents (Otte et al., 2017). However, the distortion is large for compound 1 compared to most known silanes (Krupp et al., 2020).

Supramolecular features
The crystal packing along the b-axis of compound 1 is illustrated in Fig. 3. Further studies of the packing in the solid state were aimed at finding hydrogen bonds of compound 1 as well as discussing the intensities of those hydrogen bonds. These studies were performed using Hirshfeld surface analysis. The Hirshfeld surface mapped over d norm in the range from À0.072 to 1.201 arbitrary units as well as the related fingerprints plots generated by CrystalExplorer2021 (Spackman et al., 2021, Turner et al., 2017 are illustrated in Fig. 4. With a share of 71.4%, most of the interactions relate to weak van der Waals HÁ Á ÁH contacts, which should play a minor role for the packing of the crystal. In contrast, the role of OÁ Á ÁH/HÁ Á ÁO contacts should be predominant in the crystal arrangement in the unit cell, as shown by the significant red spots on the Hirshfeld surface. Numerous hydrogen bonds of the ethyl sulfate group to the ammonium cation are visible on the surface. The contribution of these contacts amounts to 16.6%. CÁ Á ÁH/HÁ Á ÁC contacts as well as HÁ Á ÁH contacts do not Table 1 Selected bond lengths (Å ).    show as intense spots on the Hirshfeld surface and should not be considered as relevant as the OÁ Á ÁH/HÁ Á ÁO contacts for the crystal packing. All hydrogen bonds up to a distance of 3.4 Å as well as an angle of at least 155 are listed in Table 2. According to Perlstein (2001), all hydrogen bonds listed in Table 2 have a weak to moderately strong character, which can be explained in particular by the non-linear angles of 156 (7) (C7-H7BÁ Á ÁO2 ii ) to 167 (2) (C3-H3Á Á ÁO2 i ). The shortest hydrogen-bond length is 3.1815 (16) Å and is the strongest supramolecular interaction with an angle of 162.8 (17) (C17-H17AÁ Á ÁO4). Analysis of the hydrogen-bonding network shows that all the hydrogen bonds shown in Table 2 can be assigned to one graph-set motif [D 1 1 (2); Etter et al., 1990] and all of these bonds are linearly connected to two different atoms.    Table 2 Hydrogen-bond geometry (Å , ). (3) 3.2990 (17) 167 (2)  Symmetry codes: (i) Àx; y À 1 2 ; Àz; (ii) x À 1; y; z.

Figure 3
A view along the b-axis direction of the crystal packing of compound 1.

Synthesis and crystallization
The reaction scheme for the synthesis of 1 is illustrated in Fig.  5: 1-[(benzyldimethylsilyl)methyl]piperidine (2) (0.81 mmol) was dissolved in acetone (3 ml) and diethyl sulfate (0.81 mmol) was added dropwise to the solution. The reaction mixture was stirred and heated for 6 h at 329 K. Afterwards the reaction was quenched by the addition of a mixture of H 2 O (2 ml) and NH 3 (2 ml). The aqueous phase was extracted three times with CH 2 Cl 2 and the combined organic phases were dried over Na 2 SO 4 . After the removal of volatile compounds, the raw product was dissolved in npentane (1 ml) and stored at 243 K. The title salt (1)     program(s) used to solve structure: SHELXS (Sheldrick, 2008); program(s) used to refine structure: SHELXL (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: CrystalExplorer21 (Spackman et al., 2021;Turner et al., 2017), publCIF (Westrip, 2010), Mercury (Macrae et al., 2020), GaussView 6.016 (Frisch et al., 2016), Gaussian 09 Revision A.02 (Frisch et al., 2016), SCHAKAL99 (Keller, 1999),   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.