Facile synthesis of dichlorosilane by metathesis reaction and dehydrogenation of dihydrogermane by a frustrated Lewis pair

Article abstract of DOI:10.1039/c002395g

The reaction of GeCl₂·dioxane with 1,4-diazabutadiene compounds [(HCNAr)₂, Ar = 2,6-iPr₂C₆H ₃, (1); Ar = 2,4,6-Me₃C₆H₂, (2)] leads to the formation of dichlorogermane derivatives [{N(2,6-iPr ₂C₆H₃)CH}₂]GeCl₂ (3) and [{N(2,4,6-Me₃C₆H₂)CH}₂]GeCl ₂ (4) respectively. The reaction of compound 2 with SiCl ₂·NHC (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) (5) results in the formation of dichlorosilane [{N(2,4,6-Me₃C ₆H₂)CH}₂]SiCl₂ (6) which is the precursor for the synthesis of N-heterocyclic silylene. Furthermore the reaction of dichlorogermane 3 with two equivalents of AlH₃·NMe ₃ in toluene leads to the dihydrogermane [{N(2,6-iPr ₂C₆H₃)CH}₂]GeH₂ (7). Interestingly this dihydrogermane (7) is dehydrogenated by the frustrated Lewis pair, N-heterocyclic carbene (1,3-di-tert-butylimidazol-2-ylidene) and tris(pentafluorophenyl)borane, to form the N-heterocyclic germylene [{N(2,6-iPr₂C₆H₃)CH}₂]Ge (8).

Full text of DOI:10.1039/c002395g

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PAPER
www.rsc.org/dalton | Dalton Transactions
Facile synthesis of dichlorosilane by metathesis reaction and dehydrogenation
of dihydrogermane by a frustrated Lewis pair†
Anukul Jana,a Gasˇper Tavcˇar,a Herbert W. Roesky*a and Carola Schulzkeb
Received 4th February 2010, Accepted 7th May 2010
First published as an Advance Article on the web 3rd June 2010
DOI: 10.1039/c002395g
=
The reaction of GeCl2·dioxane with 1,4-diazabutadiene compounds [(HC NAr)2, Ar = 2,6-iPr2C6H3,
(1); Ar = 2,4,6-Me3C6H2, (2)] leads to the formation of dichlorogermane derivatives
[{N(2,6-iPr2C6H3)CH}2]GeCl2 (3) and [{N(2,4,6-Me3C6H2)CH}2]GeCl2 (4) respectively. The reaction
of compound 2 with SiCl2·NHC (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) (5) results
in the formation of dichlorosilane [{N(2,4,6-Me3C6H2)CH}2]SiCl2 (6) which is the precursor for the
synthesis of N-heterocyclic silylene. Furthermore the reaction of dichlorogermane 3 with two
equivalents of AlH3·NMe3 in toluene leads to the dihydrogermane [{N(2,6-iPr2C6H3)CH}2]GeH2 (7).
Interestingly this dihydrogermane (7) is dehydrogenated by the frustrated Lewis pair, N-heterocyclic
carbene (1,3-di-tert-butylimidazol-2-ylidene) and tris(pentafluorophenyl)borane, to form the
N-heterocyclic germylene [{N(2,6-iPr2C6H3)CH}2]Ge (8).
mane derivative of 1,4-diazabutadiene is the precursor for the
Introduction
synthesis of germylene. Unfortunately, when the reduction of
dichlorogermane is carried out with a reducing agent this forms an
intractable mixture of products. Till now there are no well defined
reactions reported for the use of the dichlorogermane derivative
of 1,4-diazabutadiene. Herein we report a new method for the
synthesis of the dichlorosilane derivative of 1,4-diazabutadiene,
and the reaction of dichlorogermane with AlH3·NMe3,8 to form
dihydrogermane. Finally we also describe the dehydrogenation
of dihydrogermane using a frustrated Lewis pair (N-heterocyclic
carbene and borane) to form germylene.
In recent years the chemistry of group 14 elements has received
considerable attention, especially compounds with low valent
group 14 elements, due to their unique properties, and reactions.1
Most compounds with low valent group 14 elements are kinetically
stabilized by using a chelating ligand, a heterocyclic ligand, a bulky
alkyl, or a bulky aryl ligand.2 1,4-Diazabutadiene compounds
are mostly used as starting materials for the synthesis of N-
heterocyclic carbenes and their heavier analogues.3 Moreover they
are also used for the synthesis of N-heterocyclic carbene analogues
containing group 13 and 15 elements.4 In the case of group 14,
with carbon and silicon 1,4-diazabutadiene type ligands are much
more developed compared to those of the heavier congeners. There
are reports in the literature of the synthesis of dichlorosilane
and dichlorogermane using 1,4-diazabutadiene compounds.5,6 In
the case of dichlorogermane, a one pot synthesis using 1,4-
diazabutadiene with GeCl2·dioxane leads to the desired product.
This method was developed by West et al.6a and later followed
by Jones and his coworkers.6b Dichlorosilane was prepared by
the reaction of the dilithium salt of 1,4-diazabutadiene and SiCl4
or by the direct reaction of 1,4-diazabutadiene with HSiCl3 in
the presence of a base such as diaza-[1,3]-bicyclo-[5.4.0]-undecane
(DBU). Consequently we sought to change the methodology for
the synthesis of the dichlorosilane derivative by using SiCl2·NHC
(NHC = 1,3-bis(2,6–diisopropylphenyl)imidazol-2-ylidene) in-
stead. The latter was recently reported by our group.7
Results and discussion
The reaction of GeCl2·dioxane either with N,N-di(2,6-
di-isopropylphenyl)-1,2-diiminoethane (1) or N,N-di(2,4,6-tri-
methylphenyl)-1,2-diiminoethane (2) results in the formation
of dichlorogermanes [{N(2,6-iPr2C6H3)CH}2]GeCl2 (3)5b and
[{N(2,4,6-Me3C6H2)CH}2]GeCl2 (4) respectively in good yield
(Scheme 1).
Dichlorosilane derivatives are the precursors for the synthesis
of N-heterocyclic silylene by reductive elimination of chloride.
According to the retro synthetic point of view, the dichloroger-
Scheme 1 Synthesis of dichlorogermanes 3 and 4.
aInstitut fu¨r Anorganische Chemie, Universita¨t Go¨ttingen, Tammannstrasse
4, 37077, Go¨ttingen, Germany. E-mail: hroesky@gwdg.de; Fax: +49-551-
393373
Compounds 3 and 4 are stable in an inert atmosphere at ambient
temperature without any decomposition and are both very soluble
in common organic solvents, such as n-hexane, n-pentane, toluene,
diethyl ether, and THF. Compounds 3 and 4 are characterized by
bSchool of Chemistry, Trinity College Dublin, Dublin 2, Ireland
† CCDC reference number 745452 for 4. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/c002395g
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1H NMR spectroscopy and EI mass spectrometry. Furthermore,
compound 4 was characterized by single crystal X-ray structural
analysis (Fig. 1).
Scheme 3 Synthesis of dihydrogermane 7.
1
7 was characterized by H and 13C NMR spectroscopy and EI
mass spectrometry. In the 1H NMR the Ge–H resonance arises at
d 6.59 ppm, which is downfield shifted when compared with that
of previously reported digermene ArGe(H)Ge(H)Ar (Ar = 2,6-
(2,6-iPr2C6H3)2C6H3) (d 5.87 ppm)15 and is shifted upfield when
comparedwiththat of LGeH (L = CH{(CMe)2(2,6-iPr2C6H3N)2})
(d 8.08 ppm).9a The backbone C–H resonance arises at d 5.70 ppm,
which is shifted upfield when compared with that of the starting
compound 3 (d 5.86 ppm). Unfortunately we were not able to
prove this result by a X-ray crystal structural analysis of 7 due to
the lack of suitable crystals.
Fig. 1 Molecular structure of 4. Anisotropic displacement parameters are
depicted at the 50% probability level and all the hydrogen atoms are omitted
◦
˚
for clarity; selected bond lengths [A] and angles [ ]: Ge1–Cl1 2.1448(8),
Ge1–N1 1.802(2), N1–C10 1.412(4), C10–C10A 1.326(6); N1–Ge1–N1A
91.79(15), N1–Ge1–Cl1 118.77(8), Cl1–Ge1–Cl1A 99.68(5).
In the solid state compound 4 exists as a monomer. The five-
membered C2N2Ge ring is planar with a distorted tetrahedral
˚
germanium center. The C10-C10A (1.326(6) A) and C10-N1
˚
(1.412(4) A) bond lengths in 4 indicate that the electrons of the five-
The reduction of compound 3 with potassium does not lead
to the germylene derivative by reductive elimination of chloride.
Therefore we proposed that the preparation of the germylene
derivative may be possible from compound 7 with the elimination
of two hydrogen atoms.
membered ring are localized exhibiting double and single bonds.
A similar reaction was performed with 2 and SiCl2·NHC
(NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) (5),7
in toluene as solvent at room temperature (Scheme 2). After 30 min
1
the H NMR spectrum indicates the formation of dichlorosilane
Very recently we have demonstrated the dehydrogenation of
LGeH (L = CH{(CMe)2(2,6-iPr2C6H3N)2}) using a frustrated
Lewis pair (N-heterocyclic carbene and borane).16 In this com-
pound there are hydrogen atoms of two different polarities
(hydridic and protic) with the consequence that the N-heterocyclic
carbene abstracts protic and borane abstracts hydridic hydrogen
under the formation of imidazolium borate. However in com-
pound 7 there is only one type of hydrogen atom of one polarity
at the germanium atom. In the literature dehydrogenation of
alkanes is reported using transition metal catalysts.17 Recently,
computational studies on the possibility of conversion of alcohols
to ketones and aldehydes by phosphinoboranes were reported.18
It is also worth mentioning that Tamm et al. described the self-
dehydrogenation of a frustrated carbene borane pair.19
[{N(2,4,6-Me3C6H2)CH}2]SiCl2 (6) in almost quantitative yield,
matching with that of a previously reported sample.5b In the course
of the reaction there is formation of free carbene (NHC), which
was removed from the reaction solution by adding Me3N·HCl
to form the imidazolium salt. The latter was filtered off from
the solution and compound 6 was isolated from the remaining
filtrate. In contrast to the previous report5b no dilithiated starting
material of 2 has to be used. This is the first metathesis reaction of
a base stabilized dichlorosilylene. It is also worth mentioning that
a similar kind of reaction with SnCl2 does not occur.
Consequently compound
7
was reacted with equimolar
amounts of 1,3-ditert-butylimidazol-2-ylidene20 and B(C6F5)3
(Scheme 4). After 30 min the 1H NMR spectrum of this solution
indicates the complete disappearance of the Ge-H, and the reso-
nance of the backbone C–H of compound 7 shifts to d 6.74 ppm.
Therefore we can conclude the formation of N-heterocyclic
germylene [{N(2,6-iPr2C6H3)CH}2]Ge (8) with 1H NMR data
matching a previously reported sample.6b
21
Scheme 2 Synthesis of dichlorosilane 6.
In recent years we became interested in the synthesis of hydride,9
fluoride,10 hydroxide11 and amide12 compounds with low valent
group 14 elements. Therefore compound 3 was chosen as a precur-
sor for the synthesis of dihydrogermane. There are some reports
on the synthesis and reactions of hydrogermane compounds in
the literature.13 Recently Rivard et al. reported the Lewis acid base
stabilized germanium(II) dihydride, NHC·GeH2·BH3 (NHC = 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene).14 The reaction of 3
with AlH3·NMe3 in toluene leads to the dihydrogermane [{N(2,6-
iPr2C6H3)CH}2]GeH2 (7) in a good yield (Scheme 3). Compound
Scheme 4 Synthesis of germylene 8.
6218 | Dalton Trans., 2010, 39, 6217–6220
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Table 1 Crystallographic data of compound 4
Experimental section
Empirical formula
C20H24Cl2GeN2
General considerations
CCDC-No.
T/K
745452
140(2) K
Orthorhombic
Pbcn
17.174(3)
7.356(2)
15.804(3)
90
90
90
1996.5(7)
4
1.450
1.807
896
All manipulations were performed under a dry and oxygen free
atmosphere (N2) using standard Schlenk techniques or inside a
MBraun MB 150-GI glovebox maintained at or below 1 ppm
of O2 and H2O. Solvents were purified with the MBraun solvent
drying system. The starting materials 1,22 2,22 5,7 and 1,3-ditert-
butylimidazol-2-ylidene20 were prepared using literature proce-
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
1
dures. Other chemicals were purchased and used as received. H
and 13C NMR spectra were recorded on a Bruker Avance DRX
200 MHz instrument and referenced to the deuterated solvent.
Elemental analyses were performed by the Analytisches Labor
des Instituts fu¨r Anorganische Chemie der Universita¨t Go¨ttingen.
EI-MS were measured on a Finnigan Mat 8230 or a Varian MAT
CH5 instrument. Melting points were measured in sealed glass
tubes with a Bu¨chi melting point B 540 instrument.
3
˚
V/A
Z
Dc/g cm-3
m/mm-1
F(000)
q range [◦]
2.37 to 26.99
2102/0/154
17399/2102 (Rint = 0.0980)
0.0403, 0.0641
0.0697, 0.0702
1.100
Data/restraints/parameters
Reflections collected/unique
R1, wR2 [I > 2s(I)]a
R1, wR2 (all data)a
GoF
Synthesis of [{N(2,6-iPr2C6H3)CH}2]GeCl2 (3). Compound 1
(0.37 g, 1.0 mmol) was dissolved in diethyl ether (10 mL) and added
to a suspension of GeCl2·dioxane (0.23 g, 1.0 mmol) in diethyl
ether (10 mL) at -78 ◦C. The resulting suspension was warmed to
room temperature and stirred for 2 h. The suspension was filtered
and the filtrate was concentrated to about 10 mL. After storing
this concentrated solution overnight at -30 ◦C in a freezer, 3 was
-3
˚
Dr(max), Dr(min)/e A
0.373, -0.575
a R1 = RꢀFo|-|Fcꢀ/R|Fo|. wR2 = [Rw(Fo -Fc2)2/Rw(Fo2)2]0.5
.
2
(125 MHz, C6D6): d 148.49, 140.98, 126.92, 124.18 (Ar-C), 119.83
(NCH), 28.43 (CH(CH3)2), 24.87 (CH(CH3)2), 24.26 (CH(CH3)2)
ppm. EI-MS: m/z (%) 450 (100) [M+ - 2H].
1
formed as red-orange crystals. Yield: 0.375 g (72%). H NMR
(200 MHz, C6D6): d 7.08–7.18 (m, 6H, Ar-H), 5.86 (s, 2H, NCH),
3.61 (sept, 4H, CH(CH3)2), 1.46–1.09 (br, 24H, CH3) ppm.6b
Synthesis of [{N(2,6-iPr2C6H3)CH}2]Ge (8). A solution of
1,3-di-tert-butylimidazol-2-ylidene (0.18 g, 1.0 mmol) in toluene
(10 mL) was added drop by drop to a stirred solution of 7 (0.45 g,
1.0 mmol) in toluene (10 mL) at room temperature. Then a solution
of B(C6F5)3 (0.51 g, 1.0 mmol) in toluene (10 mL) was added
drop by drop to the reaction mixture. After another 30 min the
solvent was removed in vacuum and the residue was extracted
with n-hexane (25 mL) to give compound 8. Yield: 0.32 g (72%).
1H NMR (200 MHz, C6D6): d 6.98–7.24 (m, 6H, Ar-H), 6.74 (s,
2H, NCH), 3.24 (sept, 4H, CH(CH3)2), 1.25 (d, 12H, CH(CH3)2),
1.21 (d, 12H, CH(CH3)2) ppm.6b
Synthesis of [{N(2,4,6-Me3C6H2)CH}2]GeCl2 (4). Compound
4 was prepared in a similar way to compound 3 using compound
2 (0.29 g, 1.0 mmol) and GeCl2·dioxane (0.23 g, 1.0 mmol) as
◦
1
starting materials. Yield: 0.280 g (64%); mp 164 C. H NMR
(200 MHz, C6D6): d 6.75 (s, 4H, Ar-H), 5.75 (s, 2H, NCH), 2.39
(s, 12H, o-CH3), 2.12 (s, 6H, p-CH3) ppm. EI-MS: m/z (%) 436
(100) [M]+. Anal. calcd for C20H24Cl2GeN2 (435.90): C, 55.10; H,
5.55; N, 6.43. Found C, 55.19; H, 5.75; N, 6.43.
Synthesis of [{N(2,4,6-Me3C6H2)CH}2]SiCl2 (6). Compound
5 (0.49 g, 1.0 mmol) was dissolved in toluene (35 mL) and added
to a solution of 2 (0.29 g, 1.0 mmol) in toluene (15 mL) at
room temperature. After 30 min the resulting reaction solution
was transferred to another Schlenk flask containing Me3N·HCl
(0.48 g, 5.00 mmol) and stirred overnight. After removal of all
the volatiles in vacuum, the resulting residue was extracted with n-
hexane (30 mL) to yield compound 6. Yield: 0.21 g (55%). 1H NMR
(200 MHz, C6D6): d 6.75 (s, 4H, Ar-H), 5.50 (s, 2H, NCH), 2.42
(s, 12H, o-CH3), 2.07 (s, 6H, p-CH3) ppm.5b
Crystallographic details for compound 4. A suitable crystal of
4 was mounted on a glass fiber and data was collected on an IPDS
II Stoe image-plate diffractometer (graphite monochromated Mo-
˚
Ka radiation, l 0.71073 A) at 133(2) K. The data was integrated
with X-Area. The structure was solved by Direct Methods
(SHELXS-97)23 and refined by full-matrix least square methods
against F2 (SHELXL-97).23 All non-hydrogen atoms were refined
with anisotropic displacement parameters. The hydrogen atoms
were refined isotropically on calculated positions using a riding
model. Crystallographic data of 4 are presented in Table 1.
Synthesis of [{N(2,6-iPr2C6H3)CH}2]GeH2 (7). To a solution
◦
of 3 (0.52 g, 1.0 mmol) in toluene (25 mL) at -10 C was added
AlH3·NMe3 (2 mL, 1.0 M in toluene). The mixture was warmed
to room temperature and stirred for another 1 h. During this
time, the color of the solution changed from red to colorless.
Afterwards all volatiles were removed in vacuum and the residue
was extracted with n-hexane (30 mL). After complete removal of
n-hexane compound 7 remained. Yield: 0.38 g (84%); mp 135 ◦C.
1H NMR (500 MHz, C6D6): d 7.10–7.19 (m, 6H, Ar-H), 6.58 (s,
2H, GeH), 5.69 (s, 2H, NCH), 3.62 (sept, 4H, CH(CH3)2), 1.26
(d, 12H, CH(CH3)2), 1.22 (d, 12H, CH(CH3)2) ppm. 13C NMR
Conclusion
In this contribution we demonstrate that the reactivity of
N-heterocyclic carbene stabilized dichlorosilylene (SiCl2·NHC;
NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) can
mimic that of GeCl2·dioxane when treated with 1,4-diazabutadiene
=
((HC NAr)2, Ar = 2,4,6-Me3C6H2). Moreover the synthe-
sis of dihydrogermane is reported from the corresponding
chloro derivative by reaction with AlH3·NMe3. Subsequently its
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dehydrogenation by the use of
(N-heterocyclic carbene and borane) to form N-heterocyclic
germylene is accomplished.
a
frustrated Lewis pair
D. Stalke, J. Am. Chem. Soc., 2009, 131, 1288–1293; (c) A. Jana, S. S.
Sen, H. W. Roesky, C. Schulzke, S. Dutta and S. K. Pati, Angew. Chem.,
2009, 121, 4310–4312; A. Jana, S. S. Sen, H. W. Roesky, C. Schulzke,
S. Dutta and S. K. Pati, Angew. Chem., Int. Ed., 2009, 48, 4246–4248;
(d) A. Jana, H. W. Roesky, C. Schulzke and A. Do¨ring, Angew. Chem.,
2009, 121, 1126–1129; A. Jana, H. W. Roesky, C. Schulzke and A.
Do¨ring, Angew. Chem., Int. Ed., 2009, 48, 1106–1109; (e) A. Jana,
H. W. Roesky and C. Schulzke, Inorg. Chem., 2009, 48, 9543–9548.
10 (a) Y. Ding, H. Hao, H. W. Roesky, M. Noltemeyer and H.-G. Schmidt,
Organometallics, 2001, 20, 4806–4811; (b) A. Jana, H. W. Roesky, C.
Schulzke, A. Do¨ring, T. Beck, A. Pal and R. Herbst-Irmer, Inorg.
Chem., 2009, 48, 193–197.
Acknowledgements
This work was supported by the Deutsche Forschungsgemein-
schaft and the Alexander von Humboldt Foundation.
11 (a) L. W. Pineda, V. Jancik, H. W. Roesky, D. Neculai and A. M.
Neculai, Angew. Chem., 2004, 116, 1443–1445; L. W. Pineda, V. Jancik,
H. W. Roesky, D. Neculai and A. M. Neculai, Angew. Chem., Int. Ed.,
2004, 43, 1419–1421; (b) A. Jana, B. Nekoueishahraki, H. W. Roesky
and C. Schulzke, Organometallics, 2009, 28, 3763–3766; (c) A. Jana,
H. W. Roesky and C. Schulzke, Dalton Trans., 2010, 39, 132–138; (d) A.
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12 A. Jana, I. Objartel, H. W. Roesky and D. Stalke, Inorg. Chem., 2009,
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13 (a) M. Brynda, M. Geoffroy and G. Bernardinelli, Chem. Com-
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15 G. H. Spikes, J. C. Fettinger and P. P. Power, J. Am. Chem. Soc., 2005,
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