Syntheses and crystal structures of 2-methyl-1,1,2,3,3-pentaphenyl-2-silapropane and 2-methyl-1,1,3,3-tetraphenyl-2-silapropan-2-ol

The structures of two sterically hindered silicon compounds feature intermolecular C—H⋯π interactions in the solid state. The silapropan-2-ol compound also features an intermolecular O—H⋯π interaction.

The sterically hindered silicon compound 2-methyl-1,1,2,3,3-pentaphenyl-2silapropane, C 33 H 30 Si (I), was prepared via the reaction of two equivalents of diphenylmethyllithium (benzhydryllithium) and dichloromethylphenylsilane. This bisbenzhydryl-substituted silicon compound was then reacted with trifluoromethanesulfonic acid, followed by hydrolysis with water to give the silanol 2-methyl-1,1,3,3-tetraphenyl-2-silapropan-2-ol, C 27 H 26 OSi (II). Key geometric features for I are the Si-C bond lengths that range from 1.867 (2) to 1.914 (2) Å and a 4 descriptor for fourfold coordination around the Si atom of 0.97 (indicating a nearly perfect tetrahedron). Key geometric features for compound II include Si-C bond lengths that range from 1.835 (4) to 1.905 (3) Å , a Si-O bond length of 1.665 (3) Å , and a 4 descriptor for fourfold coordination around the Si atom of 0.96. In compound II, there is an intramolecular C-HÁ Á ÁO hydrogen bond present. In the crystal of I, molecules are linked by two pairs of C-HÁ Á Á interactions, forming dimers that are linked into ribbons propagating along the b-axis direction. In the crystal of II, molecules are linked by C-HÁ Á Á and O-HÁ Á Á interactions that result in the formation of ribbons that run along the a-axis direction.

Chemical context
The benzhydryl substituent and its derivatives occur in many medicinal compounds, for example: diphenhydramine, modafinil and meclizine (Fig. 1). The addition of the benzhydryl group to a drug significantly increases its lipophilicity and the two aromatic rings add electron density and bulk. There is an active field looking at the switching of silicon for carbon to discover new medicinal compounds and there have been several recent publications and reviews in the area (Franz & Wilson, 2013;Geyer et al., 2015;Ramesh & Reddy, 2018;Tacke & Doerrich, 2016). It seemed to us that another option is to replace the sulfoxide group with a silanol, in which the silicon has a size that is similar to sulfur and the alcohol will occupy the space of the sulfoxide oxygen. The conversion of a The benzhydryl group in some representative medicinal compounds. phenylsilane to a silanol by the reaction with trifluoromethanesulfonic acid followed by hydrolysis has been used previously (Kira et al., 2007;Shainyan et al., 2017), and worked well for the introduction of the silanol in compound II, silanol 2-methyl-1,1,3,3-tetraphenyl-2-silapropan-2-ol.
The steric bulk of the benzhydryl group has been used to advantage in several silyl reagents. It has been reported that the benzhydryldimethylsilylgroup is readily synthesized and undergoes facile oxidation with hydrogen peroxide to form alcohols (Peng & Woerpel, 2001). Yoshida and coworkers have started with a tris(diphenylmethyl)silane and further substituted aromatic rings using electrophilic aromatic substitution to produce the sterically demanding TEDAMS group (Terao et al., 2010). Unno et al. (2006) have addressed the influence of bulky silyl groups on the ability of silanols to hydrogen bond, and found that (i-Pr 3 Si) 3 SiOH exists as a monomer while (t-BuMe 2 Si) 3 SiOH is a hydrogen-bonded dimer. Our observation that compound II is monomeric indicates that the silicon atom is very hindered by the presence of the two benzhydryl groups.

Structural commentary
The molecular structure of compound I is shown in Fig. 2. The Si-C bond lengths range from 1.867 (2) to 1.914 (2) Å , with the Si-C1 bond to the methyl group being the shortest. The 4 descriptor for fourfold coordination around Si1 is 0.97, indicating a nearly perfect tetrahedral geometry around this silicon atom (where 0 = square planar, 0.85 = trigonal pyramidal, and 1 = tetrahedral; Yang et al., 2007). The Si1-C1 bond and aromatic ring (C4-C9) are nearly co-planar with a C1-Si1-C4-C5 torsion angle of 12.2 (2) . The orientation of the benzhydryl group bonded to C2 is such that when the molecule is viewed down the C2-Si1 bond the methyl group (C1) is anti to H2 (torsion angle C1-Si1-C2-H2 is 169 ), with the aromatic rings gauche. For the benzhydryl group containing C3, the hydrogen atom H3 is gauche to the methyl group (C1) with a C1-Si1-C3-H3 torsion angle of 69 , with the aromatic ring (C22-C27) occupying the anti position.
The molecular structure of compound II is shown in Fig. 3. The Si-C bond lengths range from 1.835 (4) to 1.905 (3) Å , with an Si-O bond length of 1.665 (3) Å . The 4 descriptor for fourfold coordination around Si1 is 0.96, again indicating an almost perfect tetrahedral geometry around this silicon atom. The orientation of the C2 benzhydryl group is such that the hydrogen atom H2 is anti to the methyl group (C1) with a C1-Si1-C2-H2 torsion angle of À165 . For the benzhydryl group containing C3, the hydrogen atom H3 is again gauche to the methyl group (C1) with a C1-Si1-C3-H3 torsion angle of 55 , and the aromatic ring C22-C27 occupies the anti position. An intramolecular C-HÁ Á ÁO hydrogen bond is present between H27 and O1 with an HÁ Á ÁA distance of 2.55 Å (Table 2).

Supramolecular features
In the crystal of I, molecules are linked by two pairs of intermolecular C-HÁ Á Á interactions involving inversion- The molecular structure of compound I, with the atom-labeling scheme. Displacement ellipsoids are drawn at the 40% probability level. For clarity, the hydrogen atoms have been omitted.

Figure 3
The molecular structure of compound II, with the atom-labeling scheme. Displacement ellipsoids are drawn at the 40% probability level. For clarity, the C-bound hydrogen atoms have been omitted. related compounds ( Fig. 4 and Table 1). The result of these interactions is the formation of dimers that are linked to form ribbons along the b-axis direction (Fig. 5).
In the crystal of II, inversion-related molecules are linked by a pair of O-HÁ Á Á interactions, forming dimers (Table 2, Fig. 6). Similar interactions between aryl groups and OH groups in silanols have been reported previously (Al-Juaid et al., 1992). In the crystal of II, the dimers are linked by a pair of C-HÁ Á Á interactions (Table 2), so forming ribbons that propagate along the a-axis direction (Fig. 7).

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.40, May 2019; Groom et al., 2016) gave only one hit for a structure in which a silicon atom is bonded to two benzhydryl groups, viz. bis(diethylamino)bis(diphenylmethyl)silane (CSD refcode YEPTUI; Huppmann, et al., 1994). In The crystal packing of compound I, viewed along the a-axis, showing the supramolecular ribbons formed by intermolecular C-HÁ Á Á interactions (Table 1; shown as dashed purple lines). Only hydrogen atoms H20 and H24 are shown for clarity.

Figure 6
Intramolecular hydrogen bond (blue dotted lines) and intermolecular C-HÁ Á Á and O-HÁ Á Á interactions ( this compound, the silicon atom is also bonded to two diethylamino groups. There are four other structures in the CSD with a silicon atom bonded to one benzhydryl group and a different alkyl group (this count excludes organometallic compounds). These compounds include, tert-butyl  Hill & Hitchcock, 2002). This search revealed zero structures in the CSD that contained a silanol group where the silicon atom is bonded to a benzhydryl group. However, the related structures (triphenylmethyl)silanetriol acetone solvate (GAWVUW; Kim, et al., 2005) and (triphenylmethyl)silanetriol tetrahydrofuran solvate (BAVQOF; Yoo, et al., 2001) are both silanetriols that bear a trityl group (-CPh 3 ) coordinated to the central silicon atom.

Computing details
For both structures, data collection: APEX2 (Bruker, 2013); cell refinement: SAINT (Bruker, 2013); data reduction: SAINT (Bruker, 2013); 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;Bourhis et al., 2015); software used to prepare material for publication: CrystalMaker (Palmer, 2007). 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.