Crystal structures of three sterically congested disilanes

Three sterically congested silanes, namely 1,1,2,2-tetraisopropyl-1,2-diphenyldisilane, 1,1,2,2-tetrakis(2-bromopropan-2-yl)-1,2-diphenyldisilane and 1,2-di-tert-butyl-1,1,2,2-tetraphenyldisilane, show lengthening of the Si—Si and Si—C bonds as compared with disilanes with smaller substituents. The packing of the tetrakis(2-bromopropan-2-yl) compound is partly organized by attractive Br⋯Br interactions.


Figure 2
Perspective view of 2, with labeling scheme and 50% probability displacement ellipsoids. Only the major orientation of the disordered bromoisopropyl group is shown.

Figure 3
Perspective view of 3, with labeling scheme and 50% probability displacement ellipsoids. Only the major orientation of the disorder is shown [symmetry code: (i) 2 À x, Ày, Àz].

Figure 1
Perspective view of the two independent molecules of 1, with labeling scheme and 50% probability displacement ellipsoids.
Cg1 is the centroid of C1-C6 the ring.

Figure 4
Packing of 1, viewed along the b-axis direction.

Figure 5
Packing of 2, viewed along the a-axis direction, with the C-HÁ Á ÁBr hydrogen bonds (Table 1) shown as black dotted lines and BrÁ Á ÁBr interactions as brown dotted lines.

Figure 7
Compounds from the database survey.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. In compound 2, the bromoisopropyl group containing Br4 is rotationally disordered about the Si2-C16 axis in an 0.8812 (9):0.1188 (9) ratio. The two components of the disorder were refined with restraints that their geometries be comparable to one another and to those of the other three bromoisopropyl groups. Compound 3 exhibits 'whole molecule' disorder in a 0.9645 (7):0.0355 (7) ratio with the Si-Si bonds in the two components making an angle of ca 66 . The alternate location of the unique Si atom was obtained from a difference Fourier map and its inclusion in the structure-factor calculation allowed enough atoms of its phenyl groups to be located so that these could be completed and refined as rigid hexagons. Following this, the remaining atoms of the minor component could be located and they were refined with restraints that the geometry be comparable with that of the major component. In all three structures, the H atoms were included as riding contributions in idealized positions: C-H = 0.95-0.98 Å with U iso (H) = 1.5U eq (Cmethyl) and 1.2U eq (C) for other H atoms. For all compounds, data collection: APEX3 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg & Putz, 2012); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Special details
Experimental. The diffraction data were obtained from 3 sets of 400 frames, each of width 0.5° in ω, colllected at φ = 0.00, 90.00 and 180.00° and 2 sets of 800 frames, each of width 0.45° in φ, collected at ω = -30.00 and 210.00°. The scan time was 15 sec/frame. 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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. Hatoms attached to carbon were placed in calculated positions (C-H = 0.95 -1.00 Å). All were included as riding contributions with isotropic displacement parameters 1.2 -1.5 times those of the attached atoms.

Special details
Experimental. The diffraction data were obtained from 3 sets of 400 frames, each of width 0.5° in ω, colllected at φ = 0.00, 90.00 and 180.00° and 2 sets of 800 frames, each of width 0.45° in φ, collected at ω = -30.00 and 210.00°. The scan time was 20 sec/frame. 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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. Hatoms attached to carbon were placed in calculated positions (C-H = 0.95 -0.98 Å). All were included as riding contributions with isotropic displacement parameters 1.2 -1.5 times those of the attached atoms. The bromodimethyl group based on C16 is rotationally disordered over two nearly superimposable sites in an 88:12 ratio. The two components of the disorder were refined subject to restraints that their geometries be comparable.

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

Special details
Experimental. The diffraction data were obtained from 3 sets of 400 frames, each of width 0.5° in ω, colllected at φ = 0.00, 90.00 and 180.00° and 2 sets of 800 frames, each of width 0.45° in φ, collected at ω = -30.00 and 210.00°. The scan time was 20 sec/frame. 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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. Hatoms attached to carbon were placed in calculated positions (C-H = 0.95 -0.98 Å). All were included as riding contributions with isotropic displacement parameters 1.2 -1.5 times those of the attached atoms. The centrosymmetric molecule is disordered over two orientations about the center in a 96:4 ratio. The two components of the disorder were refined subject to restraints that their geometries be comparable. In addition, the phenyl ring of the minor component overlapping with one from the major component was refined as a rigid hexagon.