Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S2053229614022992/qs3044sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S2053229614022992/qs3044au119sup2.hkl | |
Structure factor file (CIF format) https://doi.org/10.1107/S2053229614022992/qs3044au116sup3.hkl |
CCDC references: 1029944; 1029945
Si_{2}Br_{6} and Si_{2}I_{6} are higher homologues of SiBr_{4} and SiI_{4} containing an Si—Si single bond. The halogenated disilanes can be reduced to Si_{2}H_{6}. All three substances are potential precursors for the deposition of elemental silicon in order to manufacture silicon wafers or nanostructures. Although Si_{2}Br_{6} has been known for more than 50 years, no structural information has yet been collected (Schumb & Heath, 1946). In contrast with Si_{2}Cl_{6}, which is available on a kilogramme scale via cleavage of perchlorinated polysilane with chlorine gas (Molnar et al., 2012), Si_{2}Br_{6} and Si_{2}I_{6} 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 Si_{2}Ph_{6}. 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 Si_{2}Br_{6}, (I), and Si_{2}I_{6}, (II), was established. Therefore, Si_{2}Ph_{6} 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.
Si_{2}Ph_{6} was synthesized according to a literature procedure (Gilman & Dunn, 1951). Si_{2}Ph_{6} (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 Si_{2}Br_{6} as single crystals (yield 3.5 g, 6.6 mmol, 69%). ^{29}Si NMR (60 MHz, C_{6}D_{6}, δ, p.p.m.): -35.9.
Si_{2}Ph_{6} (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 Si_{2}I_{6} as single crystals (yield 1.8 g, 1.3 mmol, 23%). ^{29}Si NMR (60 MHz, C_{6}D_{6}, δ, p.p.m.): -146.0.
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.
Si_{2}Br_{6}, (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···Br2^{i} = 3.9430 (16) Å, Br2···Br2^{ii} = 3.9497 (17) Å and Br1···Br2^{iii} = 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].
Si_{2}I_{6}, (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···I1^{i} = 4.1340 (8) Å, I1···I1^{ii} = 4.2425 (10) Å and I1···I1^{iii} = 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 Si_{2}I_{6}, (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 Si_{2}Br_{6} (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···Si2^{i}; 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···I2^{i} = 4.184 Å, I2···I2^{ii} = 4.183 Å, I3···I1^{iii} = 4.242 Å and I4···I1^{iv} = 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 Si_{2}Cl_{6} with bis(cis-1,2-diphenyl-1,2-bis(trichlorosiloxy)ethylene) (Yang & Verkade, 2002). The conformation of Si_{2}Cl_{6} 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 Si_{2}Br_{6}. The Si—Si bonds in Si_{2}I_{6} 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 Si_{2}I_{6} and in Si_{2}Br_{6} are similar, but they are completely different in the two polymorphs of Si_{2}I_{6}. On the other hand, intermolecular halogen···halogen contacts are more or less in the same range in the two polymorphs of Si_{2}I_{6}. As might be expected, the intermolecular Br···Br contacts are slightly shorter than the I···I contacts. Although Si_{2}Br_{6} and Si_{2}I_{6} feature such small and simple molecules they show completely different crystal structures.
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).
Br_{6}Si_{2} | F(000) = 476 |
M_{r} = 535.64 | D_{x} = 3.293 Mg m^{−}^{3} |
Monoclinic, C2/m | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2y | Cell parameters from 1784 reflections |
a = 6.6327 (10) Å | θ = 3.6–25.9° |
b = 11.4385 (14) Å | µ = 22.45 mm^{−}^{1} |
c = 7.5069 (12) Å | T = 173 K |
β = 108.488 (12)° | Plate, colourless |
V = 540.14 (14) Å^{3} | 0.18 × 0.16 × 0.03 mm |
Z = 2 |
Stoe IPDS II two-circle diffractometer | 499 independent reflections |
Radiation source: fine-focus sealed tube | 388 reflections with I > 2σ(I) |
Graphite monochromator | R_{int} = 0.127 |
ω scans | θ_{max} = 25.0°, θ_{min} = 3.6° |
Absorption correction: multi-scan [MULABS (Spek, 2009; Blessing, 1995)] | h = −7→7 |
T_{min} = 0.107, T_{max} = 0.552 | k = −13→13 |
2090 measured reflections | l = −8→8 |
Refinement on F^{2} | 0 restraints |
Least-squares matrix: full | Primary atom site location: structure-invariant direct methods |
R[F^{2} > 2σ(F^{2})] = 0.093 | Secondary atom site location: difference Fourier map |
wR(F^{2}) = 0.218 | w = 1/[σ^{2}(F_{o}^{2}) + (0.1321P)^{2} + ] where P = (F_{o}^{2} + 2F_{c}^{2})/3 |
S = 1.02 | (Δ/σ)_{max} < 0.001 |
499 reflections | Δρ_{max} = 2.44 e Å^{−}^{3} |
22 parameters | Δρ_{min} = −2.63 e Å^{−}^{3} |
Br_{6}Si_{2} | V = 540.14 (14) Å^{3} |
M_{r} = 535.64 | Z = 2 |
Monoclinic, C2/m | Mo Kα radiation |
a = 6.6327 (10) Å | µ = 22.45 mm^{−}^{1} |
b = 11.4385 (14) Å | T = 173 K |
c = 7.5069 (12) Å | 0.18 × 0.16 × 0.03 mm |
β = 108.488 (12)° |
Stoe IPDS II two-circle diffractometer | 499 independent reflections |
Absorption correction: multi-scan [MULABS (Spek, 2009; Blessing, 1995)] | 388 reflections with I > 2σ(I) |
T_{min} = 0.107, T_{max} = 0.552 | R_{int} = 0.127 |
2090 measured reflections |
R[F^{2} > 2σ(F^{2})] = 0.093 | 22 parameters |
wR(F^{2}) = 0.218 | 0 restraints |
S = 1.02 | Δρ_{max} = 2.44 e Å^{−}^{3} |
499 reflections | Δρ_{min} = −2.63 e Å^{−}^{3} |
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 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} > σ(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. |
x | y | z | U_{iso}*/U_{eq} | ||
Br1 | 0.7416 (3) | 0.0000 | 0.2572 (3) | 0.0341 (8) | |
Br2 | 0.2646 (3) | 0.15623 (13) | 0.2299 (2) | 0.0354 (8) | |
Si1 | 0.4523 (8) | 0.0000 | 0.3378 (6) | 0.0178 (12) |
U^{11} | U^{22} | U^{33} | U^{12} | U^{13} | U^{23} | |
Br1 | 0.0311 (16) | 0.0348 (14) | 0.0452 (14) | 0.000 | 0.0246 (11) | 0.000 |
Br2 | 0.0401 (14) | 0.0190 (9) | 0.0493 (11) | 0.0110 (6) | 0.0173 (8) | 0.0085 (5) |
Si1 | 0.021 (3) | 0.005 (2) | 0.029 (2) | 0.000 | 0.0108 (19) | 0.000 |
Br1—Si1 | 2.188 (5) | Si1—Br2^{i} | 2.182 (3) |
Br2—Si1 | 2.182 (3) | Si1—Si1^{ii} | 2.313 (9) |
Br2^{i}—Si1—Br2 | 110.0 (2) | Br2^{i}—Si1—Si1^{ii} | 108.92 (18) |
Br2^{i}—Si1—Br1 | 110.16 (14) | Br2—Si1—Si1^{ii} | 108.92 (18) |
Br2—Si1—Br1 | 110.16 (14) | Br1—Si1—Si1^{ii} | 108.7 (3) |
Symmetry codes: (i) x, −y, z; (ii) −x+1, −y, −z+1. |
I_{6}Si_{2} | D_{x} = 4.149 Mg m^{−}^{3} |
M_{r} = 817.58 | Mo Kα radiation, λ = 0.71073 Å |
Trigonal, R3 | Cell parameters from 1398 reflections |
Hall symbol: -R 3 | θ = 3.4–27.7° |
a = 7.1436 (9) Å | µ = 14.36 mm^{−}^{1} |
c = 22.213 (3) Å | T = 173 K |
V = 981.7 (1) Å^{3} | Block, colourless |
Z = 3 | 0.22 × 0.22 × 0.21 mm |
F(000) = 1038 |
Stoe IPDS II two-circle diffractometer | 500 independent reflections |
Radiation source: fine-focus sealed tube | 438 reflections with I > 2σ(I) |
Graphite monochromator | R_{int} = 0.053 |
ω scans | θ_{max} = 27.4°, θ_{min} = 3.4° |
Absorption correction: multi-scan [MULABS (Spek, 2009; Blessing, 1995)] | h = −9→5 |
T_{min} = 0.144, T_{max} = 0.152 | k = −8→9 |
1409 measured reflections | l = −28→25 |
Refinement on F^{2} | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F^{2} > 2σ(F^{2})] = 0.047 | w = 1/[σ^{2}(F_{o}^{2}) + (0.0824P)^{2}] where P = (F_{o}^{2} + 2F_{c}^{2})/3 |
wR(F^{2}) = 0.123 | (Δ/σ)_{max} < 0.001 |
S = 1.05 | Δρ_{max} = 1.46 e Å^{−}^{3} |
500 reflections | Δρ_{min} = −1.57 e Å^{−}^{3} |
14 parameters | Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^{*}=kFc[1+0.001xFc^{2}λ^{3}/sin(2θ)]^{-1/4} |
0 restraints | Extinction coefficient: 0.0055 (6) |
I_{6}Si_{2} | Z = 3 |
M_{r} = 817.58 | Mo Kα radiation |
Trigonal, R3 | µ = 14.36 mm^{−}^{1} |
a = 7.1436 (9) Å | T = 173 K |
c = 22.213 (3) Å | 0.22 × 0.22 × 0.21 mm |
V = 981.7 (1) Å^{3} |
Stoe IPDS II two-circle diffractometer | 500 independent reflections |
Absorption correction: multi-scan [MULABS (Spek, 2009; Blessing, 1995)] | 438 reflections with I > 2σ(I) |
T_{min} = 0.144, T_{max} = 0.152 | R_{int} = 0.053 |
1409 measured reflections |
R[F^{2} > 2σ(F^{2})] = 0.047 | 14 parameters |
wR(F^{2}) = 0.123 | 0 restraints |
S = 1.05 | Δρ_{max} = 1.46 e Å^{−}^{3} |
500 reflections | Δρ_{min} = −1.57 e Å^{−}^{3} |
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 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} > σ(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. |
x | y | z | U_{iso}*/U_{eq} | ||
I1 | 0.32029 (8) | 0.32622 (6) | 0.08578 (2) | 0.0392 (4) | |
Si1 | 0.0000 | 0.0000 | 0.05252 (10) | 0.0279 (7) |
U^{11} | U^{22} | U^{33} | U^{12} | U^{13} | U^{23} | |
I1 | 0.0343 (4) | 0.0342 (4) | 0.0434 (6) | 0.0127 (2) | −0.00568 (13) | −0.00505 (12) |
Si1 | 0.0287 (9) | 0.0287 (9) | 0.0265 (12) | 0.0143 (4) | 0.000 | 0.000 |
I1—Si1 | 2.4248 (8) | Si1—I1^{ii} | 2.4247 (8) |
Si1—Si1^{i} | 2.333 (5) | Si1—I1^{iii} | 2.4248 (8) |
Si1^{i}—Si1—I1^{ii} | 107.74 (5) | Si1^{i}—Si1—I1^{iii} | 107.74 (5) |
Si1^{i}—Si1—I1 | 107.74 (5) | I1^{ii}—Si1—I1^{iii} | 111.15 (5) |
I1^{ii}—Si1—I1 | 111.15 (5) | I1—Si1—I1^{iii} | 111.15 (5) |
Symmetry codes: (i) −x, −y, −z; (ii) −y, x−y, z; (iii) −x+y, −x, z. |
Experimental details
(au119) | (au116) | |
Crystal data | ||
Chemical formula | Br_{6}Si_{2} | I_{6}Si_{2} |
M_{r} | 535.64 | 817.58 |
Crystal system, space group | Monoclinic, C2/m | Trigonal, R3 |
Temperature (K) | 173 | 173 |
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), 90 | 90, 90, 120 |
V (Å^{3}) | 540.14 (14) | 981.7 (1) |
Z | 2 | 3 |
Radiation type | Mo Kα | Mo Kα |
µ (mm^{−}^{1}) | 22.45 | 14.36 |
Crystal size (mm) | 0.18 × 0.16 × 0.03 | 0.22 × 0.22 × 0.21 |
Data collection | ||
Diffractometer | Stoe IPDS II two-circle diffractometer | Stoe IPDS II two-circle diffractometer |
Absorption correction | Multi-scan [MULABS (Spek, 2009; Blessing, 1995)] | Multi-scan [MULABS (Spek, 2009; Blessing, 1995)] |
T_{min}, T_{max} | 0.107, 0.552 | 0.144, 0.152 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 2090, 499, 388 | 1409, 500, 438 |
R_{int} | 0.127 | 0.053 |
(sin θ/λ)_{max} (Å^{−}^{1}) | 0.595 | 0.648 |
Refinement | ||
R[F^{2} > 2σ(F^{2})], wR(F^{2}), S | 0.093, 0.218, 1.02 | 0.047, 0.123, 1.05 |
No. of reflections | 499 | 500 |
No. of parameters | 22 | 14 |
Δρ_{max}, Δρ_{min} (e Å^{−}^{3}) | 2.44, −2.63 | 1.46, −1.57 |
Computer programs: X-AREA (Stoe & Cie, 2001), SHELXS97 (Sheldrick, 2008), XP (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008) and publCIF (Westrip, 2010).