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N2(SiCl3)4ЧA Tetrakissilylhydrazine as an Unexpected Product of the Reaction between N2 and SiCl4.

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Angewandte
Chemie
Communications
The reactivity of usually inert elemental nitrogen can be increased
significantly by microwave-induced plasma activation. An application
of this phenomenon is the reaction of SiCl4 with N2 under plasma
conditions to give the hydrazine derivative N2(SiCl3)4, as shown by M.
Binnewies and co-workers on the following pages.
Angew. Chem. Int. Ed. 2003, 42, 5955
DOI: 10.1002/anie.200352575
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5955
Communications
Plasma chemistry
N2(SiCl3)4—A Tetrakissilylhydrazine as an
Unexpected Product of the Reaction between N2
and SiCl4
Nils Schiefenhvel, Hans-Jrg Himmel, and
Michael Binnewies*
Dedicated to Professor Bernt Krebs
on the occasion of his 65th birthday
The formation of silicon–nitrogen compounds, such as Si3N4,
by the reaction of N2 with SiCl4 is not possible for
thermodynamic reasons. The Gibbs free energy DG0298 for
Equation (1) is calculated to be 1211.7 kJ mol1.
3 SiCl4 ðgÞ þ 2 N2 ðgÞ ! Si3 N4 ðsÞ þ 6 Cl2 ðgÞ
ð1Þ
The equilibrium for this reaction thus lies completely on
the side of the reactants. Si–N–Cl compounds, so-called
chlorosilazanes, can be considered as formal intermediate
products of this reaction. Thus similar states of equilibrium
can be expected. However, the reaction can be initiated if a
gas plasma is used for activation. The synthesis of N(SiCl3)3
(1) and Cl3SiN(SiCl2)2NSiCl3 (2) in a glow discharge and a
detailed characterization of these compounds has been
reported.[1, 2] Presumably the described reaction proceeds
because of the formation of nitrogen atoms under plasma
conditions. These nitrogen atoms have a relatively long
lifetime—up to several minutes under suitable conditions.[3]
The Gibbs free energy for Equation (2) is calculated to be
610.2 kJ mol1, and therefore the formation of Si3N4 should
occur readily. Thus the formation of chlorosilazanes should
also be thermodynamically allowed.
3 SiCl4 ðgÞ þ 4 NðgÞ ! Si3 N4 ðsÞ þ 6 Cl2 ðgÞ
Figure 1. Gas chromatogram of the plasma reaction products of N2
with SiCl4. The expansion (inset) shows the signal for 3.
ð2Þ
We also studied the reaction of SiCl4 with N2. Instead of a
glow discharge, we used microwaves to generate a plasma.[4]
We also found 1 and 2 as main products of the reaction under
these conditions. Furthermore, we observed another chlorosilazane, albeit in small amounts, which was identified by
means of GC–MS to be N2(SiCl3)4 (3; Figure 1).[5, 6] The only
reasonable structure for 3 is that of a silylhydrazine derivative
(Figure 2). As only small quantities of 3 could be obtained
from the plasma process, it is currently impossible to isolate
the compound and to determine its crystal structure.
Figure 2. Calculated molecular structure of 3 (N blue; Si gray; Cl
green).
The structure given in Figure 2 was obtained by quantum
chemical calculations with the TURBOMOLE program.[7]
The BP functional was used in combination with the SV(P)
and the TZVPP basis sets. For comparison, we also calculated
the structure of the hydrogen-substituted analogue N2(SiH3)4
(4; Figure 3), whose bond lengths and angles are known from
electron-diffraction studies.[8] The calculated data agree well
with experimental data (Table 1). The calculations support
our assumption that 3 can be described as tetrakis(trichlorosilyl)hydrazine (Figure 2). The NN bond length of 3 is nearly
identical to that in hydrazine. In contrast to hydrazine, the two
[*] Prof. Dr. M. Binnewies, Dr. N. Schiefenh*vel
Institut f-r Anorganische Chemie der Universit0t Hannover
Callinstrasse 9, 30 167 Hannover (Germany)
Fax: (+ 49) 511-762-19032
E-mail: binn@mbox.aca.uni-hannover.de
Dr. H.-J. Himmel
Institut f-r Anorganische Chemie der Universit0t Karlsruhe
Engesserstrasse 15, 76 128 Karlsruhe (Germany)
5956
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3. Calculated molecular structure of 4 (N blue; Si light gray;
H gray).
DOI: 10.1002/anie.200352575
Angew. Chem. Int. Ed. 2003, 42, 5956 –5957
Angewandte
Chemie
nitrogen atoms of the C2-symmetrical compounds 3 and 4
adopt a trigonal planar geometry (Table 1).
Compound 3 is the first known tetrakis(silyl)hydrazine
with completely halogenated silyl groups. The classic methods
for the preparation of silylhydrazines are based on the
elimination of hydrogen chloride from hydrazine and dichlorosilanes or on reactions between lithium hydrazides and
halosilanes.[9–11] The tetrakis(silyl)hydrazine 4 was obtained
in the gas-phase reaction of hydrazine with excess SiH3I.[8, 12]
The first crystal structure of a tetrakis(silyl)hydrazine was
described by Schmidbauer and co-workers, who obtained
N2(PhH2Si)4 (5) in the reaction of hydrazine with PhH2SiCl.[13]
Table 1: Comparison of the bond lengths [pm] and bond angles [8] of 3,
4, and N2H4.
NN
NSi
NH
SiCl
SiH
Si-N-Si
H-N-H
Si-N-N
H-N-N
3[a]
4[a]
4[b]
H2NNH2[a]
145.0
177.3
–
204.9/205.4/
205.7[c]
–
123.94
–
118.03
–
147.2
176.1
–
–
145.7 1.6
173.1 0.4
–
–
144.2
–
102.0/102.4
149.3/149.6[d]
129.39
–
115.30
–
148.7 1.4
129.7 0.7
–
115.15 0.7
–
107.10
107.29
[a] Calculations carried out with the BP functional and the TZVPP basis
set. [b] Data from electron-diffraction studies.[8] [c] The SiCl bond
lengths in SiCl4 were calculated to be 205.0 pm. [d] The SiH bond
lengths SiH4 were calculated to be 149.3 pm.
According to our quantum chemical calculations, the
reaction enthalpy DH00(0) for reaction in Equation (3)
amounts to 180.9 kJ mol1 (BP/SV(P)) and 179.7 kJ mol1
(BP/TZVPP). As an almost constant molar heat capacity is
expected for the reaction, the reaction enthalpy can be
estimated to be about 180 kJ mol1.
N2 H4 ðgÞ þ 4 SiCl4 ðgÞ ! N2 ðSiCl3 Þ4 ðgÞ þ 4 HClðgÞ
ð3Þ
When the known thermochemical data[14] are considered,
the standard enthalpy of formation for 3 is calculated as
DH0298 = 1994 kJ mol1. As SiI4 and 3 have similar molecular
weights, the value of the standard entropies of formation of
both gaseous compounds should be almost the same:
S0298(N2(SiCl3)4) S0298(SiI4) = 417 J mol1 K1.
A matter of particular interest is the reaction pathway that
leads to the formation of 3. According to thermodynamic
data, 3 cannot be obtained by the reaction of N2 with SiCl4.
However, the reaction of N2 with SiCl3 radicals—whose
existence can be anticipated under plasma reaction conditions—may lead to 3. A formation pathway via N2, SiCl2, and
SiCl4 is also possible from a thermodynamic point of view.
Both pathways would lead to 3 with preservation of the NN
bond. However, we presume a formation mechanism for 3
that includes the cleavage of the NN bond of N2. The
Angew. Chem. Int. Ed. 2003, 42, 5956 –5957
occurrence of ·N(SiCl3)2 radicals as intermediates seems to be
reasonable. The latter pathway would be consistent with the
chemical characteristics of N2F4, which partially dissociates
into NF2 radicals at room temperature.
Received: August 6, 2003 [Z52575]
Published Online: November 24, 2003
.
Keywords: hydrazines · nitrogen · plasma chemistry · silazanes
[1] A. Pflugmacher, H. Dahmen, Z. Anorg. Allg. Chem. 1957, 290,
184 – 190.
[2] U. Wannagat, R. Flindt, D. J. Brauer, H. BHrger, F. DIrrenbach,
Z. Anorg. Allg. Chem. 1989, 572, 33 – 46.
[3] N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 1st
ed., Pergamon, Oxford, 1989.
[4] The apparatus contains needle valves to regulate the gas flow of
the reactants (a high-precision needle valve for N2 (Linde 4.0), a
precision needle valve (Young) for SiCl4 (Riedel 99 %, purified
by fractional condensation in vacuo)), a quartz tube (1 =
20 mm), and cold traps for condensation of the products. The
microwaves were produced in a microwave generator (Electromedical Supplies, Mikrotron 100, Mark 3, 2450 MHz) and
applied to the reaction zone through a hollow conductor. The
reaction is carried out in a dynamic vacuum, with p(N2) =
p(SiCl4) = 100 Pa.
[5] GC–MS: gas chromatograph Varian Star 3400 CX, mass spectrometer Finnigan MAT SSQ7000; ionization: EI (70 eV), CI
(reactant gas: methane 10 mbar). The fragments were identified
on the basis of the m/z values and the isotope pattern. Only the
m/z values of the most intense peak of each group are given
(relative intensity in parenthesis: MS (EI): [M+] 566 (15),
[M+SiCl4] 396 (100), [M+SiCl5] 361 (4), [M+NSi2Cl8] 212
(20), [SiCl3+] 133 (30); MS (CI): [M++H] 567 (87),
[M+Cl+CH4] 547 (15), [M+Cl] 531 (100).
[6] A colorless liquid product (1 mL) was obtained after a reaction
period of 20 h. Condensation in vacuo led to the separation of
90 % of the more volatile products 1 and 2. The residue was
analyzed by means of GC–MS.
[7] R. Ahlrichs, M. BNr, M. HNser, H. Horn, C. KIlmel, Chem. Phys.
Lett. 1989, 162, 165; K. Eichkorn, O. Treutler, H. Ohm, M. HNser,
R. Ahlrichs, Chem. Phys. Lett. 1995, 240, 283; K. Eichkorn, O.
Treutler, H. Ohm, M. HNser, R. Ahlrichs, Chem. Phys. Lett. 1995,
242, 652; K. Eichkorn, F. Weigend, O. Treutler, R. Ahlrichs,
Theor. Chem. Acc. 1997, 97, 119; F. Weigend, M. HNser, Theor.
Chem. Acc. 1997, 97, 331; F. Weigend, M. HNser, H. Patzelt, R.
Ahlrichs, Chem. Phys. Lett. 1998, 294, 143.
[8] C. Glidewell, D. W. H. Rankin, A. G. Robiette, G. M. Sheldrick,
J. Chem. Soc. A 1970, 318.
[9] U. Klingebiel, S. Schmatz, E. Gellermann, C. Drost, M.
Noltemeyer, Monatsh. Chem. 2001, 132, 1105.
[10] E. Gellermann, U. Klingebiel, T. Pape, F. Dall’Antonia, T. R.
Schneider, S. Schmatz, Z. Anorg. Allg. Chem. 2001, 627, 2581.
[11] K. Bode, U. Klingebiel, Adv. Organomet. Chem. 1996, 40, 1.
[12] B. J. Aylett, J. Inorg. Nucl. Chem. 1956, 2, 325.
[13] N. W. Mitzel, P. Bissinger, H. Schmidbaur, Chem. Ber. 1993, 126,
345.
[14] M. Binnewies, E. Milke, Thermochemical Data of Elements and
Compounds, 2nd ed., Wiley-VCH, Weinheim, New York, 2002.
www.angewandte.org
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5957
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