Download citation
Download citation
link to html
The title compounds were prepared through de­phenyl­ation of hexa­phenyl­disilane with acetyl bromide or acetyl iodide in the presence of the corresponding aluminium halide. Both sub­stances were purified via sublimation and, for the first time, single crystals of hexa­bromo­disilane, Si2Br6, and a new polymorph of hexa­iodo­disilane, Si2I6, could be isolated. Mol­ecules of Si2Br6 are located on a special position of site symmetry 2/m with a quarter of a mol­ecule in the asymmetric unit. Mol­ecules of Si2I6 are located on a special position of site symmetry \overline{3} with a sixth of a mol­ecule in the asymmetric unit. The bond lengths of Si2Br6 and Si2I6 are in the usual ranges and both mol­ecules adopt a staggered conformation. It is inter­esting to note that Si2Br6 and Si2I6 do not form isomorphous structures. Moreover, an ortho­rhom­bic polymorph of the present structure of Si2I6 is already known [Jansen & Friede (1996). Acta Cryst. C52, 1333-1334]. Although the title compounds feature such small and simple mol­ecules they show completely different crystal structures.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S2053229614022992/qs3044sup1.cif
Contains datablocks au119, au116, global

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S2053229614022992/qs3044au119sup2.hkl
Contains datablock au119

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S2053229614022992/qs3044au116sup3.hkl
Contains datablock au116

CCDC references: 1029944; 1029945

Introduction top

Si2Br6 and Si2I6 are higher homologues of SiBr4 and SiI4 containing an Si—Si single bond. The halogenated disilanes can be reduced to Si2H6. All three substances are potential precursors for the deposition of elemental silicon in order to manufacture silicon wafers or nanostructures. Although Si2Br6 has been known for more than 50 years, no structural information has yet been collected (Schumb & Heath, 1946). In contrast with Si2Cl6, which is available on a kilogramme scale via cleavage of perchlorinated polysilane with chlorine gas (Molnar et al., 2012), Si2Br6 and Si2I6 have to be accessed via different strategies. The exchange of aryl groups against halogen atoms is described for many different substrates (Hengge & Kovar, 1977, 1979). In order to synthesize the hexahalogenodisilanes, the educt for the de­aryl­ation is Si2Ph6. The latter can be obtained via Wurtz-type coupling of chloro­tri­phenyl­silane in good yields (Gilman & Dunn, 1951; Bernert et al., 2014). In the case of bromination of phenyl silanes, up to now these de­aryl­ation reactions have been carried out with gaseous HBr, mainly in the presence of aluminium bromide or in liquid HBr (Hassler et al., 1982; Hassler & Pöschl, 1990; Hassler & Köll, 1997). In this work, a new method for substituting phenyl rings with Br and I atoms to yield the corresponding compounds Si2Br6, (I), and Si2I6, (II), was established. Therefore, Si2Ph6 was reacted with six equivalents of acetyl bromide or iodide in the presence of six equivalents of the corresponding aluminium halide salt (Fig. 1). After purification via sublimation, single crystals could be obtained.

Experimental top

Synthesis and crystallization top

Synthesis of Si2Br6 top

Si2Ph6 was synthesized according to a literature procedure (Gilman & Dunn, 1951). Si2Ph6 (5 g, 9.6 mmol) and aluminium bromide (18 g, 67 mmol) were mixed and the mixture was suspended in 30 ml of n-hexane. Acetyl bromide (8.3 g, 5.5 ml, 67.5 mmol) was added (Fig. 1). After addition, the colour of the reaction mixture changed to yellow and quickly darkened. After stirring for 2 h, stirring was stopped and a phase separation between a colourless phase and a dark-brown phase occurred. After phase separation, the solvent was removed from the upper colourless phase under reduced pressure to yield a white solid. The solid was sublimed twice to yield analytically pure Si2Br6 as single crystals (yield 3.5 g, 6.6 mmol, 69%). 29Si NMR (60 MHz, C6D6, δ, p.p.m.): -35.9.

Synthesis of Si2I6 top

Si2Ph6 (3 g, 5.7 mmol) was dissolved in n-hexane (30 ml), and freshly distilled acetyl iodide (8.1 g, 4 ml, 48 mmol) and aluminium iodine (19.5 g, 48 mmol) were added. After addition, the colour of the reaction mixture changed to yellow and quickly darkened. After stirring for 2 h, stirring was stopped and a phase separation between a colourless phase and a dark-brown phase occurred. After phase separation, the solvent was removed from the upper colourless phase under reduced pressure to yield a white solid. The solid was sublimed twice to yield analytically pure Si2I6 as single crystals (yield 1.8 g, 1.3 mmol, 23%). 29Si NMR (60 MHz, C6D6, δ, p.p.m.): -146.0.

Refinement top

Crystal data, data collection and structure refinement details are summarized in Table 1. All atoms were refined anisotropically. The highest peak in the difference density map for (I) (2.44 e Å-3) is on a mirror plane (0.0857, 1/2, 0.2862) and 1.07 Å from Si1; the lowest peak (-2.63 e Å-3) is on a general position at (0.1622, 0.4304, 0.2595) and 0.96 Å from Br1. Likewise, in (II), the highest peak (1.46 e Å-3) is 0.83 Å away from I1 and the lowest peak (-1.57 e Å-3) is 0.86 Å away from I1. The crystal in (I) was a very thin plate, 0.03 × 0.16 × 0.18 mm, and therefore highly dependent upon the accuracy of the absorption correction.

Results and discussion top

Si2Br6, (I) (Fig. 2), crystallizes in the monoclinic space group C2/m with a quarter of a molecule in the asymmetric unit. The molecules are located on a special position of site symmetry 2/m. The conformation of the Br—Si—Si—Br torsion angles is ideally staggered. The Si—Br bond lengths are 2.182 (3) and 2.188 (5) Å, and the Si—Si bond length is 2.313 (9) Å. The molecules are packed such that the Si—Si vectors are co-parallel (Fig. 3). The shortest inter­molecular Si···Si distance is 5.307 (9) Å [symmetry operator for the second Si atom is (-x + 1, -y, -z)] and the shortest inter­molecular Br···Br contacts are slightly below 4 Å [Br1···Br2i = 3.9430 (16) Å, Br2···Br2ii = 3.9497 (17) Å and Br1···Br2iii = 3.965 (3) Å; symmetry codes: (i) x + 1/2, y - 1/2, z; (ii) x + 1/2, -y + 1/2, z; (iii) x + 1, y, z].

Si2I6, (II) (Fig. 4), crystallizes in the trigonal space group R3 with a sixth of a molecule in the asymmetric unit. The molecules are located on a special position of site symmetry 3. Like (I), the torsion angles around the Si—Si bond in (II) are also found to be ideally staggered. The Si—I bond length has a value of 2.4248 (8) Å and the Si—Si bond length is 2.333 (5) Å. The shortest inter­molecular Si···Si distance is 6.537 (4) Å [symmetry operator for the second Si atom is (-x + 2/3, -y + 1/3, -z + 1/3)] and the shortest inter­molecular I···I contacts are slightly above 4 Å [I1···I1i = 4.1340 (8) Å, I1···I1ii = 4.2425 (10) Å and I1···I1iii = 4.3092 (11) Å; symmetry codes: (i) -y + 1, x - y + 1, z; (ii) -y + 1, x - y, z; (iii) -x + 2/3, -y + 1/3, -z + 1/3].

An orthorhombic polymorph of Si2I6, (IIa), has been reported (Jansen & Friede, 1996). It crystallizes in the orthorhombic space group Pnma with the molecules located on a mirror plane. Thus, half a molecule occupies the asymmetric unit. The Si—Si bond has a length of 2.323 (4) Å and the Si—I bonds range from 2.424 (3) to 2.428 (3) Å. These values are in good agreement with those observed for (II). However, the packing of the two polymorphs is completely different. In (II), the Si—Si vectors are parallel to the c axis and therefore co-parallel, as in Si2Br6 (Fig. 5). As a result, the Si—Si vectors are exactly coparallel. In (IIa), on the other hand, only one half of the Si—Si vectors are mutually coparallel, whereas the other half are inclined by 31.5° (Fig. 6) with respect to the first half. In (IIa), the shortest inter­molecular Si···Si distance is 5.693 Å [Si1···Si2i; symmetry code: (i) x, y, z + 1]. This is significantly shorter than in (II), but only slightly longer than in (I).

On the other hand, the shortest inter­molecular I···I contacts in (IIa) [I1···I2i = 4.184 Å, I2···I2ii = 4.183 Å, I3···I1iii = 4.242 Å and I4···I1iv = 4.252 Å; symmetry codes: (i) -x + 1/2, -y + 1, z + 1/2; (ii) -x, -y + 1, -z - 1; (iii) x - 1/2, y, -z + 1/2; (iv) -x + 1/2, -y + 1, z - 1/2] are in the same range as in (II).

The only known crystal structure of hexa­chloro­disilane is a co-crystal of Si2Cl6 with bis­(cis-1,2-di­phenyl-1,2-bis­(tri­chloro­sil­oxy)ethyl­ene) (Yang & Verkade, 2002). The conformation of Si2Cl6 is staggered and the Si—Si bond length is 2.3158 (16) Å. This is inter­mediate between the results from (I) and (II), and closer to the observed Si—Si bond length in Si2Br6. The Si—Si bonds in Si2I6 are slightly longer.

A Cambridge Structural Database (CSD, Version 5.35 of 2013 plus two updates; Allen, 2002) search of a four-coordinate Si atom bonded by any bond to a one-coordinate Br ligand yielded 151 fragments, with a mean Si—Br bond length of 2.24 (4) Å. For the Si—I fragment, 77 hits were found with a mean Si—I bond length of 2.49 (5) Å. These values are slightly longer than those found in (I) and (II).

Concluding, it can be remarked that hexa­bromo­disilane and hexa­iodo­disilane do not form isomorphous structures. Moreover, hexa­iodo­disilane crystallizes with two polymorphic forms. The inter­molecular Si···Si distances in the orthorhombic polymorph of Si2I6 and in Si2Br6 are similar, but they are completely different in the two polymorphs of Si2I6. On the other hand, inter­molecular halogen···halogen contacts are more or less in the same range in the two polymorphs of Si2I6. As might be expected, the inter­molecular Br···Br contacts are slightly shorter than the I···I contacts. Although Si2Br6 and Si2I6 feature such small and simple molecules they show completely different crystal structures.

Related literature top

For related literature, see: Allen (2002); Bernert et al. (2014); Gilman & Dunn (1951); Hassler & Köll (1997); Hassler & Pöschl (1990); Hassler et al. (1982); Hengge & Kovar (1977, 1979); Jansen & Friede (1996); Molnar et al. (2012); Schumb & Heath (1946); Yang & Verkade (2002).

Computing details top

For both compounds, data collection: X-AREA (Stoe & Cie, 2001); cell refinement: X-AREA (Stoe & Cie, 2001); data reduction: X-AREA (Stoe & Cie, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XP (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and publCIF (Westrip, 2010).

Figures top
[Figure 1] Fig. 1. Reaction scheme for the synthesis of Si2X6 (X = Br, I).
[Figure 2] Fig. 2. A perspective view of (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (A) x + 1, y, z + 1; (B) -x + 1, -y, -z + 1; (C) -x + 1, y, -z + 1.]
[Figure 3] Fig. 3. A packing diagram for Si2Br6, viewed in the ac plane.
[Figure 4] Fig. 4. A perspective view of (II), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (A) -y, x - y, z; (B) -x + y, -x, z; (C) -x, -y, -z; (D) y, -x + y, -z; (E) x - y, x, -z.]
[Figure 5] Fig. 5. A packing diagram for (II), viewed in the ac plane.
[Figure 6] Fig. 6. A packing diagram for (IIa), viewed in the ac plane.
(au119) hexabromodisilane top
Crystal data top
Br6Si2F(000) = 476
Mr = 535.64Dx = 3.293 Mg m3
Monoclinic, C2/mMo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yCell parameters from 1784 reflections
a = 6.6327 (10) Åθ = 3.6–25.9°
b = 11.4385 (14) ŵ = 22.45 mm1
c = 7.5069 (12) ÅT = 173 K
β = 108.488 (12)°Plate, colourless
V = 540.14 (14) Å30.18 × 0.16 × 0.03 mm
Z = 2
Data collection top
Stoe IPDS II two-circle
diffractometer
499 independent reflections
Radiation source: fine-focus sealed tube388 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.127
ω scansθmax = 25.0°, θmin = 3.6°
Absorption correction: multi-scan
[MULABS (Spek, 2009; Blessing, 1995)]
h = 77
Tmin = 0.107, Tmax = 0.552k = 1313
2090 measured reflectionsl = 88
Refinement top
Refinement on F20 restraints
Least-squares matrix: fullPrimary atom site location: structure-invariant direct methods
R[F2 > 2σ(F2)] = 0.093Secondary atom site location: difference Fourier map
wR(F2) = 0.218 w = 1/[σ2(Fo2) + (0.1321P)2 + ]
where P = (Fo2 + 2Fc2)/3
S = 1.02(Δ/σ)max < 0.001
499 reflectionsΔρmax = 2.44 e Å3
22 parametersΔρmin = 2.63 e Å3
Crystal data top
Br6Si2V = 540.14 (14) Å3
Mr = 535.64Z = 2
Monoclinic, C2/mMo Kα radiation
a = 6.6327 (10) ŵ = 22.45 mm1
b = 11.4385 (14) ÅT = 173 K
c = 7.5069 (12) Å0.18 × 0.16 × 0.03 mm
β = 108.488 (12)°
Data collection top
Stoe IPDS II two-circle
diffractometer
499 independent reflections
Absorption correction: multi-scan
[MULABS (Spek, 2009; Blessing, 1995)]
388 reflections with I > 2σ(I)
Tmin = 0.107, Tmax = 0.552Rint = 0.127
2090 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.09322 parameters
wR(F2) = 0.2180 restraints
S = 1.02Δρmax = 2.44 e Å3
499 reflectionsΔρmin = 2.63 e Å3
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Br10.7416 (3)0.00000.2572 (3)0.0341 (8)
Br20.2646 (3)0.15623 (13)0.2299 (2)0.0354 (8)
Si10.4523 (8)0.00000.3378 (6)0.0178 (12)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Br10.0311 (16)0.0348 (14)0.0452 (14)0.0000.0246 (11)0.000
Br20.0401 (14)0.0190 (9)0.0493 (11)0.0110 (6)0.0173 (8)0.0085 (5)
Si10.021 (3)0.005 (2)0.029 (2)0.0000.0108 (19)0.000
Geometric parameters (Å, º) top
Br1—Si12.188 (5)Si1—Br2i2.182 (3)
Br2—Si12.182 (3)Si1—Si1ii2.313 (9)
Br2i—Si1—Br2110.0 (2)Br2i—Si1—Si1ii108.92 (18)
Br2i—Si1—Br1110.16 (14)Br2—Si1—Si1ii108.92 (18)
Br2—Si1—Br1110.16 (14)Br1—Si1—Si1ii108.7 (3)
Symmetry codes: (i) x, y, z; (ii) x+1, y, z+1.
(au116) hexaiododisilane top
Crystal data top
I6Si2Dx = 4.149 Mg m3
Mr = 817.58Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3Cell parameters from 1398 reflections
Hall symbol: -R 3θ = 3.4–27.7°
a = 7.1436 (9) ŵ = 14.36 mm1
c = 22.213 (3) ÅT = 173 K
V = 981.7 (1) Å3Block, colourless
Z = 30.22 × 0.22 × 0.21 mm
F(000) = 1038
Data collection top
Stoe IPDS II two-circle
diffractometer
500 independent reflections
Radiation source: fine-focus sealed tube438 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.053
ω scansθmax = 27.4°, θmin = 3.4°
Absorption correction: multi-scan
[MULABS (Spek, 2009; Blessing, 1995)]
h = 95
Tmin = 0.144, Tmax = 0.152k = 89
1409 measured reflectionsl = 2825
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.047 w = 1/[σ2(Fo2) + (0.0824P)2]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.123(Δ/σ)max < 0.001
S = 1.05Δρmax = 1.46 e Å3
500 reflectionsΔρmin = 1.57 e Å3
14 parametersExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.0055 (6)
Crystal data top
I6Si2Z = 3
Mr = 817.58Mo Kα radiation
Trigonal, R3µ = 14.36 mm1
a = 7.1436 (9) ÅT = 173 K
c = 22.213 (3) Å0.22 × 0.22 × 0.21 mm
V = 981.7 (1) Å3
Data collection top
Stoe IPDS II two-circle
diffractometer
500 independent reflections
Absorption correction: multi-scan
[MULABS (Spek, 2009; Blessing, 1995)]
438 reflections with I > 2σ(I)
Tmin = 0.144, Tmax = 0.152Rint = 0.053
1409 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.04714 parameters
wR(F2) = 0.1230 restraints
S = 1.05Δρmax = 1.46 e Å3
500 reflectionsΔρmin = 1.57 e Å3
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
I10.32029 (8)0.32622 (6)0.08578 (2)0.0392 (4)
Si10.00000.00000.05252 (10)0.0279 (7)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
I10.0343 (4)0.0342 (4)0.0434 (6)0.0127 (2)0.00568 (13)0.00505 (12)
Si10.0287 (9)0.0287 (9)0.0265 (12)0.0143 (4)0.0000.000
Geometric parameters (Å, º) top
I1—Si12.4248 (8)Si1—I1ii2.4247 (8)
Si1—Si1i2.333 (5)Si1—I1iii2.4248 (8)
Si1i—Si1—I1ii107.74 (5)Si1i—Si1—I1iii107.74 (5)
Si1i—Si1—I1107.74 (5)I1ii—Si1—I1iii111.15 (5)
I1ii—Si1—I1111.15 (5)I1—Si1—I1iii111.15 (5)
Symmetry codes: (i) x, y, z; (ii) y, xy, z; (iii) x+y, x, z.

Experimental details

(au119)(au116)
Crystal data
Chemical formulaBr6Si2I6Si2
Mr535.64817.58
Crystal system, space groupMonoclinic, C2/mTrigonal, R3
Temperature (K)173173
a, b, c (Å)6.6327 (10), 11.4385 (14), 7.5069 (12)7.1436 (9), 7.1436 (9), 22.213 (3)
α, β, γ (°)90, 108.488 (12), 9090, 90, 120
V3)540.14 (14)981.7 (1)
Z23
Radiation typeMo KαMo Kα
µ (mm1)22.4514.36
Crystal size (mm)0.18 × 0.16 × 0.030.22 × 0.22 × 0.21
Data collection
DiffractometerStoe IPDS II two-circle
diffractometer
Stoe IPDS II two-circle
diffractometer
Absorption correctionMulti-scan
[MULABS (Spek, 2009; Blessing, 1995)]
Multi-scan
[MULABS (Spek, 2009; Blessing, 1995)]
Tmin, Tmax0.107, 0.5520.144, 0.152
No. of measured, independent and
observed [I > 2σ(I)] reflections
2090, 499, 388 1409, 500, 438
Rint0.1270.053
(sin θ/λ)max1)0.5950.648
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.093, 0.218, 1.02 0.047, 0.123, 1.05
No. of reflections499500
No. of parameters2214
Δρmax, Δρmin (e Å3)2.44, 2.631.46, 1.57

Computer programs: X-AREA (Stoe & Cie, 2001), SHELXS97 (Sheldrick, 2008), XP (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008) and publCIF (Westrip, 2010).

 

Follow Acta Cryst. C
Sign up for e-alerts
Follow Acta Cryst. on Twitter
Follow us on facebook
Sign up for RSS feeds