Crystal structure and dimorphism of silicon tetraisocyanate Si(NCO) 4

Article abstract of DOI:10.1002/zaac.201100508

Silicon tetraisocyanate Si(NCO)₄ was obtained by reacting SiCl₄ and AgNCO in boiling toluene. The colourless liquid was analyzed by Raman and NMR spectroscopy. Structural studies on solid Si(NCO) ₄ (melting point: +26°C) have revealed it to exist in two polymorphic modifications. According to the results of single-crystal X-ray diffraction, at T = -173°C, α-Si(NCO)₄ exhibits triclinic symmetry (Pβar{1}$; a = 10.05(5), b = 10.50(2), c = 14.32(1) A, α = 91.62(1)°, β = 92, 32(1)°, γ = 99.68(1)°; V = 1488.56(3) A³; Z = 8). Above T = -33°C, a monoclinic phase evolves, β-Si(NCO)₄ (P2₁/c; a = 10.78(3), b = 7.11(1), c = 10.27(5) A, β = 99, 06(9)°; V = 777.39(1) A³; Z = 4). The charge distribution was studied for both polymorphs. In the solid state, Si(NCO)₄ is a tetrahedral molecule with the Si-N=C=O linkages bent at the nitrogen atoms. Copyright

Full text of DOI:10.1002/zaac.201100508

SHORT COMMUNICATION
DOI: 10.1002/zaac.201100508
Crystal Structure and Dimorphism of Silicon Tetraisocyanate Si(NCO)4
Carsten Ludwig Schmidt[a] and Martin Jansen*[a]
Keywords: Isocyanate; Silicon; Crystal structure; X-ray diffraction; Phase transitions; Polymorphs
Abstract. Silicon tetraisocyanate Si(NCO)4 was obtained by reacting 92,32(1)°, γ = 99.68(1)°; V = 1488.56(3) Å3; Z = 8). Above T =
SiCl4 and AgNCO in boiling toluene. The colourless liquid was ana-
lyzed by Raman and NMR spectroscopy. Structural studies on solid
–33 °C, a monoclinic phase evolves, β-Si(NCO)4 (P21/c; a = 10.78(3),
b = 7.11(1), c = 10.27(5) Å, β = 99,06(9)°; V = 777.39(1) Å3; Z = 4).
Si(NCO)4 (melting point: +26 °C) have revealed it to exist in two poly- The charge distribution was studied for both polymorphs. In the solid
morphic modifications. According to the results of single-crystal X- state, Si(NCO)4 is a tetrahedral molecule with the Si–N=C=O linkages
ray diffraction, at T = –173 °C, α-Si(NCO)4 exhibits triclinic symmetry
(P1; a = 10.05(5), b = 10.50(2), c = 14.32(1) Å, α = 91.62(1)°, β =
bent at the nitrogen atoms.
¯
pended in absolute toluene (distilled and stored over sodium wire) and
freshly distilled SiCl4 was dropped to this suspension whilst stirring.
An excess of 10% AgNCO was used. The suspension was stirred un-
der argon for 24 h under reflux. The color of the suspension changed
Introduction
Recently, we started evaluating molecular inorganic isocy-
anates as molecular precursors for the synthesis of new
(amorphous) ceramics.[1–4] For a proper understanding of the from colorless to violet-grey due to precipitation of AgCl. Heating to
reflux was found to be essential, most likely, because the high-tem-
perature polymorph of AgNCO, present under these conditions, is the
reactive species.[12] After filtering, the solvent of the resulting clear
solution was evaporated in vacuo and the slightly yellow liquid residue
was distilled at T = 186 °C at ambient pressure. The yield was quantita-
tive. Si(NCO)4, a moisture-sensitive colorless liquid was stored under
argon. For growing single-crystals, Si(NCO)4 was mixed with a small
amount of toluene and stored at T = –4 °C, for several days.
underlying mechanisms of transforming the molecular through
oligomeric precursor states towards the final solid-state materi-
als, knowing structural data of the initial molecular precursors
is an indispensable prerequisite. Although silicon tetraisocyan-
ate, Si(NCO)4, has been known already for decades[5] and has
been applied for the preparation of SiO2 thin films by CVD
processes,[6] there is still some dispute concerning details of
the (molecular) structure of Si(NCO)4. Structural studies on
crystalline material were impeded by the fact, that this molecu-
lar isocyanate is liquid at ambient conditions and highly sensi-
tive to moisture. Based on spectroscopic results, a double bond
was assigned to the silicon–nitrogen linkage.[7,8] This would
make the nitrogen atom formally isoelectronic with carbon. In
this view, the lone-pair of electrons on the nitrogen atom
Single-Crystal X-ray Diffraction
The collection of the X-ray diffraction intensities was performed on a
SMART-APEX CCD diffractometer (Bruker AXS Inc.) with graphite-
monochromated Mo-Kα radiation. Low temperature measurements at
173 °C were carried out using a 700 series cryostream cooler (Oxford
would interact with empty orbitals on silicon resulting in a Cryosystems). The reflection intensities were integrated with the
SAINT subprogram[13] contained in the SMART software package.[14]
linear Si–N=C=O connectivity. These interpretations have not
An empirical absorption correction (SADABS)[15] was applied. The
yet been confirmed, and more recent Raman studies even
crystal structures were solved by direct methods and refined by full-
challenged the so-far accepted tetrahedral constitution of
matrix least-squares with the SHELXTL software package.[16] Further
Si(NCO)4.[9,10] In order to settle these discrepancies, we per-
details of the crystal structure investigation(s) may be obtained
formed a structural study on solid Si(NCO)4.
from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-
Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail:
crysdata(at)fiz-karlsruhe.de; on quoting the appropriate CSD number
(423745 for α-Si(NCO)4 and 423746 for β-Si(NCO)4).
Experimental Section
Synthesis
Si(NCO)4 was prepared according to the procedure previously pub-
X-ray Powder Diffraction
lished.[11] In brief, dried AgNCO (from KOCN and AgNO3) was sus-
For the XRD analysis, the liquid was filled in a 0.3 mm lithium borate
* Prof. Dr. Dr. h.c. M. Jansen
Fax: +49 711 689 1502
glass capillary (Hilgenberg, glass no. 14), sealed and measured in
transmission geometry. Ambient and low-temperature powder X-ray
diffraction studies were performed on a STOE STADI P instrument
with a linear PSD detector and with Cu-Kα1 radiation (λ = 1.5406 Å)
obtained from a curved Johansson-type monochromator using Ge
E-Mail: M.Jansen@fkf.mpg.de
[a] Max-Planck Institut für Festkörperforschung
Heisenbergstrasse 1
70569 Stuttgart, Germany
Z. Anorg. Allg. Chem. 2012, 638, (2), 275–278
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
275

C. L. Schmidt, M. Jansen
SHORT COMMUNICATION
(111). The special cooling setup used in this study consisted of a cold
air blower with a AD51 Dry Air Unit (Oxford Cryosystems) allowing
to maintain constant temperatures at T Ͻ 0 °C. The peak profiles and
lattice parameters were determined by a LeBail-fit using the program
GSAS.[17]
Thermal Analysis
For DSC (DSC 404, Netzsch), the sample was heated in a corundum
crucible with a heating rate of 10 K·min–1 under argon, the cooling
rate was 2 K·min–1.
Figure 1. Le-Bail fit of the measured powder pattern of β-Si(NCO)4
at T = –30 °C: monoclinic, a = 10.75 Å, b = 7.24 Å, c = 10.42 Å, β =
99.72°.
Spectroscopy
For the Raman experiments the sample was filled in a 0.3 mm lithium
borate glass capillary (Hilgenberg, glass no. 14). The spectra were
recorded with an excitation line of 632.817 nm (Labram 010, single
grating) at acquisition times between 5 to 300 sec with a laser power
between 0.4 and 0.004 mW. Liquid NMR spectra of the samples dis-
solved in [D8]toluene were recorded using a Bruker Avance DPX-300
SB operating at 7.05 T. In the 13C spectra, one signal was recorded at
δ = 122.2 ppm.
check whether this discrepancy could be the result of a phase
transition, the single-crystal was warmed from T = –173 °C to
T = –33 °C and measured again. In fact, the symmetry changed
to monoclinic with the lattice constants a = 10.78(3) Å, b =
7.11(0) Å, c = 10.27(5) Å, β = 99.06(9)°, V = 777.39(1) Å3,
confirming those obtained from powder data.
The lattice parameters of the two modifications α-Si(NCO)4
(triclinic) and β-Si(NCO)4 (monoclinic) are similar and upon
phase transition, one axis is halved. The molecular structure of
Si(NCO)4 and both crystal structures are shown in Figure 2,
Figure 3 and Figure 4, further crystallographic information is
listed in Table 1 and Table 2.
Theory
Atomic charges calculations were performed using the program CRYS-
TAL06.[18] Calculations have been performed using DFT with the hy-
brid functional B3LYP. The basis sets used were [5s4d1d] for sili-
con,[19] [3s2p1d] for carbon[20] and [3s2p1d] for nitrogen.[21] Concern-
ing the oxygen atom, the inner [2s1p] exponents were taken as de-
scribed in.[22] For the sp-exponent, a range from 0.5 to 0.191 was used
and for the d-exponent a value of 0.8 was provided. Overall, the basis
set for oxygen was [4s3p1d]. The geometrical parameters used were
those obtained from the single-crystal studies.
Results and Discussion
Liquid Si(NCO)4 was analyzed at room temperature by Ra-
man spectroscopy. Bands were found at 1471 cm–1, 618 cm–1,
494 cm–1, 294 cm–1 and 251 cm–1, in full accordance with pre-
viously published data.[7] More recently, the Raman data was
analyzed assuming a molecular structure with silicon coordi-
nated tetrahedrally by four NCO ligands forming an isocyan-
ate.[9]
The melting point of Si(NCO)4 was determined as T =
+26 °C by DSC (data not shown). X-ray powder diffraction
studies have been performed starting with a frozen sample of
Si(NCO)4 in a capillary at T = –33 °C. All of the recorded
reflections were indexed based on a monoclinic unit cell (a =
10.75 Å, b = 7.24 Å, c = 10.42 Å, β = 99.72°). The observed
powder pattern and the resulting Le-Bail fit is shown in Fig-
ure 1.
Figure 2. Molecular structure of Si(NCO)4 in α-Si(NCO)4.
The Si–N distances found (1.64–1.69 Å) are shorter than the
Single crystals were transferred directly from the cold solu- values expected for a Si–N single bond of about 1.74 Å,
tion (toluene solution cooled with acetone/dry ice) into a pre- whereas for a double bond Si=N values close to 1.56 Å have
cooled inert oil. A crystal selected and picked in the oil matrix, been reported.[23–25]. All remaining distances are in the ex-
was cooled down to T = –173 °C and measured at this tem- pected ranges.
perature. The crystal structure was solved in the triclinic space
group P1. This structural data did not comply with the X-ray definitely bent at the nitrogen atom in contrast to previous
powder diffraction data obtained at T = –33 °C. In order to claims on the observation of a linear group of that kind.[8] The
According to our structural studies, the Si–N=C=O group is
¯
276
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© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2012, 275–278

Crystal Structure and Dimorphism of Silicon Tetraisocyanate Si(NCO)4
Table 1. Crystal data (single-crystal X-ray diffraction) and structure
refinement for both polymorphs of Si(NCO)4.
α-Si(NCO)4
β-Si(NCO)4
¯
Space group
P1 (triclinic)
P21 /c (mono-
clinic)
Cell parameters /Å;°
a = 10.05(6)
b = 10.50(6)
c = 14.32(8)
α = 91.62(0)
β = 92.31(6)
γ = 99.68(3)
1488.6(1)
8
a = 10.75(5)
b = 7.24(4)
c = 10.42(6)
β = 99.72(9)
Cell volume /Å3
Z
799.0(9)
4
T /K
173
196.17
1.75
250
196.17
1.63
Formula weight /g·mol–1
ρcalc /g·cm–3
Radiation source/λ /Å
0.71073
0.71073
0.282
Absorption coefficient /mm–1 0.303
F (000)
784
392
θ-range /°
Index range
1.42 to 21.01
–10 Յ h Յ 10
–10 Յ k Յ 10
–14 Յ l Յ 14
8232
8232
470/0
1.152
–0.387/0.401
R1 = 0.0847
wR2 = 0.2027
R1 = 0.1354
wR2 = 0.2386
1.92 to 14.85
–7 Յ h Յ 7
–5 Յ k Յ 5
–7 Յ l Յ 7
1246
301
118/0
0.814
–0.095/0.087
R1 = 0.0355
wR2 = 0.0859
R1 = 0.0481
wR2 = 0.0983
Reflections collected
Independent reflections
Parameters/Restrains
GooF
Figure 3. Crystal structure and packing of α-Si(NCO)4 in the solid
state (at T = –173 °C) along [001].
min/max e-density /e·Å–3
Final R indices I Ͼ 2σ(I)
Final R indices (all data)
Table 2. Bond lengths /Å and angles /° for both polymorphs of
Si(NCO)4.
α-Si(NCO)4
β-Si(NCO)4
Si–N
C–N
C–O
1.64(7)–1.68(8)
1.17(1)–1.21(1)
1.16(1)–1.21(1)
1.65(2)–1.67(2)
1.10(2)–1.16(2)
1.12(2)–1.13(2)
N–Si–N
N–C–O
C–N–Si
106.3(4)–112.5(4)
174.9(9)–178.8(1)
150.1(8)–169.8(8)
106.0(8)–112.3(1)
178.7(4)–179.2(2)
157.4(2)–179.6(1)
+0.95, O: –0.49. Thus, silicon seems to bear a rather high posi-
tive charge compared to other compounds studied in a similar
way.[26] According to these results, the previously assumed
substantial charge transfer from nitrogen to silicon, which
would be associated to a linear Si–N–C configuration, cannot
be confirmed.
As a summary, we conclude that silicon tetraisocyante
Si(NCO)4 can be prepared in a very pure state as has been
monitored by several experimental techniques. In neither of
Figure 4. Crystal structure and packing of β-Si(NCO)4 in the solid
state (at T = –33 °C) along [010].
bending angles are found to range from 149° to 170° in α- the solid-state polymorphs studied, indications for a linear
Si(NCO)4. Si–N=C=O arrangement was found. However, Si–N distances
In order to shed further light on this issue, we have per- as observed turned out to be smaller than known for silicon-
formed a charge distribution analysis on solid Si(NCO)4. The nitrogen single bonds.
total energy calculation for both polymorphs have revealed that
Concerning chemical reactivity, nucleophiles (as H2O or
α-Si(NCO)4 is about 12.3 eV more stable than the high-tem- NH3) will most likely attack at the Si–N single bond and not
perature form β-Si(NCO)4. The calculated atomic charges for at the isocyanate function. Thus the respective reaction paths
α-Si(NCO)4 are Si: +1.81, N: –0.83, C: +0.87, O: –0.50, quite will be quite different to these encountered in the case of or-
similar to those for β-Si(NCO)4 with Si: +1.78, N: –0.89, C: ganic isocyanates.
Z. Anorg. Allg. Chem. 2012, 275–278
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
277

C. L. Schmidt, M. Jansen
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SHORT COMMUNICATION
The 3D molecular arrangement is reminiscent of a bcc pack-
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[13] SAINT 6.45, Bruker AXS Inc., Madison, WI, 2003.
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Acknowledgement
We thank A. Schulz for measuring the Raman data, K. Doll for support
and discussion with the charge distribution analysis, and J. Nuss for
carrying out the single crystal diffraction measurements. Support by
the Fonds der Chemischen Industrie (FCI) and the German National
Merit Foundation (scholarships for C.L. Schmidt) is gratefully
acknowledged.
[19] P. DЈArco, G. Sandrone, R. Dovesi, R. Orlando, V. R. Saunders,
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Published Online: January 5, 2012
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Z. Anorg. Allg. Chem. 2012, 275–278