Crystal structures of 2-bromo-1,1,1,3,3,3-hexamethyl-2-(trimethylsilyl)trisilane and 2-bromo-1,1,1,3,3,3-hexaisopropyl-2-(triisopropylsilyl)trisilane

The title compounds, I (HypSiBr), and II (TipSiBr), crystallized in the cubic space group Pa and the triclinic space group P , respectively. In both structures the central silicon atom is sterically hindered with bond angles that deviate from the expected 109.5° of a tetrahedron.


Structural commentary
The molecular structure of compound I (HypSiBr), is shown in Fig. 1, and selected geometrical parameters are given in Table 1. The asymmetric unit is composed of one trimethylsilyl group, with the central silicon atom Si1 and the bromine atom Br1 lying on a threefold rotation axis. This supersilylbromide crystallized in the cubic space group Pa3 with a central 4-coordinate silicon atom, Si1, that deviates slightly from an ideal tetrahedron due to the steric bulk of the attached trimethylsilyl (TMS) groups. The 4 descriptor for fourfold coordination around Si1 is 0.94 (where, for extreme forms, 4 = 0.00 for square-planar, 1.00 for tetrahedral and 0.85 for trigonal-pyramidal; Yang et al., 2007). Interestingly, the 4 descriptor for fourfold coordination around the TMS atom Si2 is 0.99, which demonstrates an ideal tetrahedral geometry around this silicon atom. The Si2-Si1-Si2 i,ii bond angle is 113.69 (2) while the Br1-Si1-Si2 bond angle is 104.83 (3) , indicating that the trimethylsilyl groups are forced away from one another. The Si1-Br1 bond length is 2.2990 (12) Å . As for Si2, the C-Si2-C bond angles range from 107.1 (2) to 110.55 (17) , while the C-Si2-Si1 bond angles range from 108.61 (10) to 110.16 (11) .
The asymmetric unit of compound II (TipSiBr), is shown in Fig. 2, and selected geometrical parameters are given in Table 1. This compound crystallized in the triclinic space group P1 with a central four-coordinate silicon atom, Si1, that deviates from the ideal tetrahedron as shown from its 4 descriptor for fourfold coordination of 0.90. The Br1-Si1-Si2/Si3/Si4 bond angles range from 98.44 (3) to 103.77 (3) , and the Si1-Br1 bond distance is 2.3185 (7) Å , which is longer than that of compound I [2.2990 (12) Å ]. The 4 descriptor values for atoms Si2, Si3 and Si4 (the silicon atoms of the triisopropylsilyl groups) are 0.96, 0.97 and 0.95, respectively, indicating that their coordination geometry is closest to an ideal tetrahedron.

Supramolecular features
There are no significant intermolecular contacts, other than weak van der Waals interactions, present in the crystals of compounds I or II. Compound II, however, contains four intramolecular C-HÁ Á ÁBr hydrogen bonds ( A view of the molecular structure of compound II, with atom labelling. Displacement ellipsoids are drawn at the 30% probability level, and all hydrogen atoms have been omitted for clarity.

Figure 1
A view of the molecular structure of compound I, with atom labelling. Displacement ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for clarity. Unlabelled atoms are related to the labelled atoms by threefold rotation symmetry [symmetry codes: (i) z, x, y; (ii) y, z, x]. from from 3.584 (3) to 3.726 (3) Å , and D-HÁ Á ÁA bond angles that range from 131 to 155 .

Database survey
The Cambridge Structural Database (CSD, version 5.39, February 2018;Groom et al., 2016) contains 1398 structures containing a Si 3 Si group. Of these, there are only 42 structures where the central silicon atom is bonded directly to a halogen.
Of particular interest to this work is the structure of tris-(trimethylsilyl)chlorosilane (III, HypSiCl) [CSD refcode QULWEA; Kuzora et al., 2009], the isotypic chloro derivative of compound I, and the structure of (iPr 3 Si) 3 SiH (IV, TipSiH), isotypic with compounds I and III. The analysis of IV by both X-ray and neutron diffraction has been described by Gaspar et al. (1999). Table 1 contains pertinent bond lengths and bond angles for compounds I, II, III (HypSiCl) and IV (TipSiH).
For compounds I and III the Si-X bond lengths follow the expected trend with the Si1-Cl bond length of QULWEA at 2.1248 (9) Å compared to the Si1-Br1 bond length of 2.2990 (12) Å for compound I. The Si1-Si2 bond length of the bromo derivative I reported here is 2.3477 (8) Å , which is slightly longer than the Si1-Si2 bond length of the chloro derivative at 2.3406 (6) Å . The central silicon atom of the chloro derivative appears less sterically hindered with an Si2-Si1-Cl1 bond angle of 105.508 (18) and Si2-Si1-Si2 i,ii bond angles of 113.126 , versus a smaller Si2-Si1-Br1 bond angle of 104.83 (3) and a larger Si2-Si1-Si2 i,ii bond angle of 113.69 (2) for compound I [symmetry codes: (i) z, x, y; (ii) y, z, x]. The protio derivative (HypSiH) is a liquid at room temperature, and the structure of the iodo derivative (HypSiI) has not been deposited in the CSD.
The X-ray data for compound IV (TipSiH) was not found in the CSD, but the journal article (Gaspar et al., 1999) contains all pertinent structural data to allow for a comparison with (iPr 3 Si) 3 SiBr, viz. compound II (TipSiBr). Like compounds I and III, compound IV crystallizes in the cubic space group Pa3, and the molecule possesses threefold rotation symmetry. The presence of a small hydrogen atom bonded to the central silicon atom Si1 allows the three ( i Pr 3 )Si-groups to push further away from one another, resulting in Si2-Si1-Si2 i,ii bond angles of 117.9 (1) and Si2 i,ii -Si1-H bond angles of 98.3 (1) [symmetry codes: (i) z, x, y; (ii) y, z, x]. In II, the corresponding Si-Si-Si bond angles range from 115.02 (4) to 116.59 (4) and the Si-Si-Br bond angles vary from 98.44 (3) to 103.77 (3) .

Synthesis and crystallization
Compound I: Tris(trimethylsilyl)silane (2.0 g, 8.0 mmol) was added to an oven-dried nitrogen-flushed 250 ml Schlenk flask with a stir-bar. Bromobutane (2.0 g, 14.6 mmol) was filtered through a plug of silica gel in a Pasteur pipette and was transferred into the Schlenk flask. AIBN [2,2-azobis(2methylpropionitrile); 20 mg] was then added to the flask, and the reaction was heated to 333 K using an oil bath and then  Table 1 Selected bond lengths (Å ), bond angles ( ) and the fourfold coordination descriptor, 4 , a for compounds I (HypSiBr), II (TipSiBr), III (HypSiCl) and IV (TipSiH).

Figure 3
Intramolecular C-HÁ Á ÁBr hydrogen bonds (blue dashed lines; see Table 2) present in compound II. For clarity, only the hydrogen atoms involved in a hydrogen bonding are shown.
heating was stopped. After stirring the reaction overnight at room temperature, GC-MS analysis of a sample indicated incomplete reaction and more AIBN (11 mg) was added to the flask. The reaction was heated once more to 333 K for 1 h. Analysis by GC-MS now indicated that the reaction was complete. The flask was placed in a freezer at 243 K and colourless block-like crystals of I formed overnight. Removal of the solvent in vacuo yielded 2.2 g (85%). 1 H NMR (300 MHz, chloroform-d) 0.24 (s, 27H); 13 C NMR (75 MHz, chloroform-d) À0.51 ppm; GC-MS: 11.24 min, m/z = 328, base peak: 73.
Compound II: Tris(triisopropylsilyl)silane (110 mg, 0.22 mmol) was dissolved in freshly distilled benzene (10 ml) along with NBS (45 mg) and AIBN (2 mg, initiator). The mixture was heated using an oil bath at 333 K for 30 min, when GC-MS analysis indicated that no reaction had occurred. At this point the solution was heated with a heat gun until the reaction mixture turned slightly yellow. The yellow colour dissipated in less than 1 min. Analysis of the reaction mixture by 1 H NMR indicated that only 60% of the starting material had been consumed. An additional amount of NBS (Nbromosuccinimide; 20 mg) was added to the reaction flask, and the solution was again heated with a heat gun. The product was isolated by removing the solvent in vacuo and extracting the product from the crude reaction mixture with pentane. The pentane solution was filtered through glass wool, concentrated and weighed (135 mg). Analysis of the product with 1 H NMR indicated this was 90% pure. The product was further purified by dissolving this solid in 1 ml pentane, cooling to 195 K and isolating the colourless needle-like crystals of II by removing the solvent with a syringe, washing with pentane and drying in vacuo (yield 62 mg, 55%). 1 H NMR (300 MHz, C 6 D 6 ) 1.34 (d, J = 7.3 Hz, 54H), 1.66 (heptet, J = 7.4 Hz, 9H); 13 C NMR (75 MHz, chloroform-d) 16.4, 21.6; HRMS for C 17 H 63 BrSi 4 calculated 535.2642 (M À C 3 H 7 ), found 535.2641.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Br1 0.74011 (2) 0.74011 (2) Bromo-1,1,1,3,3,3-hexaisopropyl-2-(triisopropylsilyl) where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.36 e Å −3 Δρ min = −0.28 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. Refinement. All H atoms were positioned geometrically and refined using a riding model with C-H = 0.95-0.99 Å and wAith U iso (H) = 1.2 (1.5 for methyl groups) times U eq (C).

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