A remarkable end-on activation of diazoalkane and cleavage of both C-Cl bonds of dichloromethane with a silylene to a single product with five-coordinate silicon atoms

Article abstract of DOI:10.1021/om201031n

The 1:1 reaction of benzamidinato-stabilized chlorosilylene PhC(NtBu) ₂SiCl (1) with CH(SiMe₃)N₂ resulted in the formation of colorless [PhC(NtBu)₂Si(Cl){N₂CH(SiMe ₃)}]₂ (2), which consists of a four-membered Si2N2 ring. Surprisingly, N2 elimination from the diazoalkane did not occur, but rather an end-on activation of the nitrogen was observed. For the mechanism, we propose the formation of a silaimine complex A as an intermediate, which is formed during the reaction and dimerized under [2 + 2] cycloaddition to 2. In contrast, treatment of 1 with dichloromethane afforded a 2:1 product, [{PhC(NtBu) ₂Si(Cl₂)}₂CH₂] (3), which is obviously formed by oxidative addition under cleavage of both C-Cl bonds and formation of two Si-Cl and two Si-C bonds. Both silicon atoms in 3 are five-coordinate. Compounds 2 and 3 were characterized by single-crystal X-ray studies, multinuclear NMR spectroscopy, and EI-mass spectrometry.

Full text of DOI:10.1021/om201031n

Article
pubs.acs.org/Organometallics
A Remarkable End-On Activation of Diazoalkane and Cleavage of
Both C−Cl Bonds of Dichloromethane with a Silylene to a Single
Product with Five-Coordinate Silicon Atoms†
Sakya S. Sen, Jakob Hey, Daniel Kratzert, Herbert W. Roesky,* and Dietmar Stalke*
Institut fur Anorganische Chemie, Georg-August-Universitat, Tammannstrasse 4, D-37077, Gottingen, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: The 1:1 reaction of benzamidinato-stabilized chlorosilylene PhC(NtBu)2SiCl (1) with CH(SiMe3)N2 resulted in
the formation of colorless [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2), which consists of a four-membered Si2N2 ring.
Surprisingly, N2 elimination from the diazoalkane did not occur, but rather an end-on activation of the nitrogen was observed.
For the mechanism, we propose the formation of a silaimine complex A as an intermediate, which is formed during the reaction
and dimerized under [2 + 2] cycloaddition to 2. In contrast, treatment of 1 with dichloromethane afforded a 2:1 product,
[{PhC(NtBu)2Si(Cl2)}2CH2] (3), which is obviously formed by oxidative addition under cleavage of both C−Cl bonds and
formation of two Si−Cl and two Si−C bonds. Both silicon atoms in 3 are five-coordinate. Compounds 2 and 3 were
characterized by single-crystal X-ray studies, multinuclear NMR spectroscopy, and EI-mass spectrometry.
Another very interesting class of compounds with which the
chemistry of silylenes so far is not well-studied are diazoalkanes.
The interaction of diazoalkanes and transition-metal compounds
leads to a variety of products, depending on the electronic
properties of the diazoalkanes and the formal charge on the
metal center. These reactions are extremely interesting given
the utility for the generation of alkylidene complexes that are
active for catalytic cyclopropanation of alkene metathesis.17 In
contrast, the reaction of diazoalkane with main group metals is
scarcely known in the literature.18 In many cases, N2 eli-
mination is not observed and stable diazoalkane complexes are
isolated.19 In 2004, we reported the synthesis of a diimi-
nylaluminum(III) compound L1Al(NCPh2)2 (L1 = HC-
{(CMe)(NAr)}2, Ar = 2,6-iPr2C6H3) by the treatment of
L1Al(I) with Ph2CN2.20 The analogous diiminylsilane was
obtained by Driess et al. from the reaction of L2Si(II) (L2 =
Ar−N−C(Me)CH−C(CH2)−N−Ar) with Ph2CN2.21
Very recently, we described a unprecedented end-on nitrogen
insertion of a diazo compound into a L1Ge(II)−H
bond.22 Moreover, we also showed that the reaction of a
N-heterocyclic germylene (L2Ge(II)) with CH(SiMe3)N2
afforded a diazogermylene L1GeC(N2)SiMe3, which slowly
rearranged to isonitrile-trimethylsilyl germanium(II) amide
INTRODUCTION
■
One of the research fields that has blossomed since the advent
of silylenes is their novel reaction chemistry. This is a highly
interdisciplinary enterprise that has gathered efforts from syn-
thetic chemists and theoreticians and leads to an exponentially
increasing number of publications.1 This emanates from the
understanding of chemical properties and bonding character-
istics of silylenes that contain an electrophilic Si(II) center.
Silylenes have a propensity toward oxidation reactions due to
the conversion from the formally divalent to the thermody-
namically stable tetravalent state. The past few years have wit-
nessed that silylenes are potent Lewis bases,2 can be oxidized by
chalcogens,3 can form adducts with boranes4 and carbenes,5
and can take part in cyclization reactions with alkynes.6 Our
formal entry in silylene chemistry started in 2006, following the
synthesis of the first monochlorosilylene (PhC(NtBu)2SiCl)
(1) by the reduction of the corresponding trichlorosilane
(PhC(NtBu)2SiCl3) with finely divided potassium.7 However,
due to yield constraints, we were not able to investigate its
chemistry. In 2010, an improved synthesis of 1 resulted in 90%
yield by using LiN(SiMe3)2 as a dehydrochlorinating agent,8
and its systematic reactions with homo-8 and heteroalkynes,9
ketone,10 diketone,11 isocyanate,12 carbodiimide,12 imine,13
diimine,12 COT,12 diazobenzene,14 borane,15 and metal carbonyls16
were carried out.
Received: October 25, 2011
Published: December 27, 2011
© 2011 American Chemical Society
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Article
L1GeN(SiMe3)NC.23 These striking results prompted us to
probe the reaction of 1 with diazoalkane because, with respect to the
other reported N-heterocyclic silylenes, 1 possesses a remarkably di-
fferent pattern of reactivity toward unsaturated organic substrates.8−16
Herein, we report the 1:1 reaction of CH(SiMe3)N2 with 1 and the
isolation of the four-membered Si2N2 ring compound 2.
RESULT AND DISCUSSION
■
Addition of 1 equiv of CH(SiMe3)N2 to a toluene solution of 1
at −40 °C, followed by recrystallization and storing at −32 °C
in a freezer, furnished [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2
1
(2) as colorless crystals (see Scheme 1). In the H NMR
Scheme 1. Preparation of
[PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2)
Figure 1. Crystal structure of 2·2C7H8. Hydrogen atoms and two
toluene molecules are not shown for clarity. Anisotropic displacement
parameters are depicted at the 50% probability level. Selected bond
lengths (Å) and bond angles (deg): Si(1)−N(3) 1.7350(16), Si(1)−
N(3)#1 1.8090(15), Si(1)−N(1) 1.8227(16), Si(1)−N(2)
1.9445(16), Si(1)−Cl(1) 2.1431(7); N(3)−Si(1)−N(3)#1 80.33(7),
N(3)−Si(1)−N(1) 123.62(8), N(3)#1−Si(1)−N(1) 106.90(7),
N(3)−Si(1)−N(2) 100.30(7), N(3)#1−Si(1)−N(2) 176.09(7),
N(1)−Si(1)−N(2) 69.50(7). Symmetry generator #: −x, y, z + 1/2.
monoclinic space group P21/n.24 A two-fold symmetry axis
passes through the centroid of the four-membered Si2N2 ring.
The core structure consists of a rhombic-shaped Si2N2 unit
featuring alternating Si and N atoms. The coordination
geometry around the Si atoms can be described by trigonal-
bipyramidal geometry. To explain the axial and equatorial
arrangement, we selected Si(1). N(1), Cl(1), and N(3) which
reside in the equatorial positions, whereas N(2) and N(3)#1
occupy the axial positions of the TBP geometry. The Si−Si
interatomic separation in the four-membered Si2N2 ring is
2.7089(10) Å, which indicates that there is no bond between
the two Si atoms. The Si−N bond lengths in 2 are 1.7350(16)
and 1.8227(16) Å for equatorial nitrogen atoms, while the axial
Si−N bond lengths are 1.8090(15) and 1.9445(16) Å.25 The
Si(1)−Cl(1) bond length in 2 is 2.1431(7) Å, which is very
close to that of 1 (2.156(1) Å).7 The average N−N bond
length in 2 is 1.38 Å, which is consistent with the N−N single
bond length.26
Reactions of silylenes with haloalkanes have been studied in
detail by different research groups owing to the possibility of a
direct synthesis of alkylhalosilanes. It has been observed that
CH2Cl2 reacts in one or the other way with different silylenes,27
and therefore, these provocative reactions need further at-
tention. Specially, it remains to be examined how CH2Cl2 reacts
with 1, which already contains a Si(II)−Cl bond. As shown in
Scheme 3, the reaction of 1 with excess CH2Cl2 at room
spectrum of 2, a broad resonance appears at δ = 1.35 ppm for
the 36 tBu protons, which is shifted downfield compared with
that of 1 (δ = 1.08 ppm).8 A sharp resonance at δ = 0.43 ppm
corresponds to 18 SiMe3 protons. The two alkene CH protons
appear at δ = 8.29 ppm. Two resonances are observed at δ =
−10 and −70.2 ppm in the 29Si NMR spectrum. The former
resonance corresponds to the SiMe3 moiety, whereas the signal
at δ = −70.2 ppm is attributed to the two five-coordinate Si
atoms. In the EI-mass spectrum, the molecular ion is observed
at m/z 816.4, although with low intensity. However, the most
abundant peak exhibited at m/z 801.4, which corresponds to
[2-Me]. The mechanism of the reaction seems to be obvious.
The reaction of 1 with CH(SiMe3)N2 initially afforded a
silaimine complex A through end-on oxidative addition of
CH(SiMe3)N2 at the silicon atom. However, this complex is
kinetically unstable due to the absence of any bulky substituents
at the α-nitrogen atom and consequently dimerizes in situ
under a [2 + 2] cycloaddition reaction to form [PhC(NtBu)2Si-
(Cl){N2CH(SiMe3)}]2 (2) with five-coordinate silicon atoms
(see Scheme 2).
Scheme 2. Mechanism for the Formation of
[PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2)
Scheme 3. Preparation of [{PhC(NtBu)2Si(Cl2)}2CH2] (3)
Single crystals suitable for X-ray diffraction studies were obtained
from a toluene solution of [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2
(2) at −32 °C in a freezer (see Figure 1). 2 crystallizes in the
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Article
temperature occurred smoothly to give the 2:1 oxidative
addition product 3 in 68% yield. No 1:1 oxidative addition
product was observed in the reaction mixture even though the
concentration of 1 was much lower than that of CH2Cl2.
Formation of similar 2:1 products has been observed in the
reactions of dialkyl silylene27b and bis[di(trimethylsilyl)methyl]-
stannylene with dichloromethane.28 However, the resulting
product contains four-coordinate silicon atoms,27b whereas 3
exhibits two five-coordinate silicon atoms. Species of this type
have not been reported so far. In the EI-mass spectrum of 3, no
molecular ion peak is observed, but it exhibits the most
abundant peak with the highest relative intensity at m/z 601,
which corresponds to [M+ − 2Cl]. In the 29Si NMR, a sharp
resonance exhibits at δ = −79.5 ppm, which can be attributed
to the two five-coordinate silicon atoms.
Figure 2. Molecular structure of [{PhC(NtBu)2Si(Cl2)}2CH2] (3).
Hydrogen atoms except at C(31) are not shown for clarity.
Anisotropic displacement parameters are depicted at 50% probability
level. Selected bond lengths [Å] and angles [deg]: N(1)−Si(1)
1.8155(8), N(2)−Si(1) 1.9414(8), Cl(1)−Si(1) 2.2172(4), Cl(2)−
Si(1) 2.1079(4), C(31)−Si(1) 1.8674(10); N(1)−Si(1)−C(31)
128.51(4), N(1)−Si(1)−N(2) 69.94(3), C(31)−Si(1)−N(2)
98.56(4), N(1)−Si(1)−Cl(2) 118.83(3), C(31)−Si(1)−Cl(2)
111.19(3), N(2)−Si(1)−Cl(2) 91.21(3), N(1)−Si(1)−Cl(1)
98.01(3), C(31)−Si(1)−Cl(1) 93.54(3), N(2)−Si(1)−Cl(1)
166.63(3), Cl(2)−Si(1)−Cl(1) 89.717(14), Si(1)−C(31)−Si(2)
128.29(5).
The mechanism of the reaction is still not clear, but it is
obvious that it cannot be explained by a radical mechanism
because a single dichloromethane molecule reacts with two
silylene molecules even in the presence of a large excess of
dichloromethane. As shown in Scheme 4, we can propose that
Scheme 4. Tentative Mechanism for the Formation of 3
CONCLUSION
■
We describe the reactions of base-stabilized chlorosilylene
PhC(NtBu)2SiCl with CH(SiMe3)N2 and CH2Cl2. The unique
feature of the former reaction is the end-on activation of CH-
(SiMe3)N2 under preservation of the N2 moiety in the product
[PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2) and formation of a
four-membered Si2N2 ring. In the latter reaction, 2 equiv of
silylene 1 react with CH2Cl2 to yield exclusively [{PhC(NtBu)2-
Si(Cl2)}2CH2] (3), which contains five-coordinate silicon atoms.
EXPERIMENTAL SECTION
■
All reactions and handling of reagents were performed under an
atmosphere of dry nitrogen or argon using standard Schlenk tech-
niques or a glovebox where the O2 and H2O levels were usually kept
below 1 ppm. CH(SiMe3)N2 was purchased from Sigma-Aldrich and
used as received. Dichloromethane was dried over CaH2. Compound 1
was prepared according to literature methods.8 NMR spectra were
recorded on Bruker Avance 200, Bruker Avance 300, and Bruker
Avance 500 MHz NMR spectrometers. C6D6 was dried by stirring for
2 days over Na/K alloy, followed by distillation in vacuum and
degassed. EI-MS spectra were obtained with a Finnigan MAT 8230 or
a Varian MAT CH5 instrument (70 eV) by EI-MS methods. Elemental
the reaction proceeds through a conventional acid−base complex
intermediate, with a chlorine atom donating a lone pair of
electrons to silicon. The methylene carbon in the initial LSiCl−
dichloromethane complex (L = PhC(NtBu)2) is attacked by
another LSiCl to form a dichlorosilyl anion and a (chloromethyl)-
silyl cation stabilized by intramolecular complexation, followed
by a nucleophilic attack of the silyl anion to the chloromethyl
carbon to afford [{PhC(NtBu)2Si(Cl2)}2CH2] (3).27b,28,29
The molecular graph of [{PhC(NtBu)2Si(Cl2)}2CH2] (3) is
shown in Figure 2. Compound 3 crystallizes in the monoclinic
space group P21/c.24 Both Si atoms in 3 adopt a distorted trigonal-
bipyramidal geometry. To assign the axial and equatorial positions,
we selected Si(1). C(31), Cl(1), and N(1) reside at the equatorial
position of Si(1), whereas N(2) and Cl(1) occupy the axial
positions with a N(2)−Si(1)−Cl(1) bond angle of 166.63(3)°.
It is noticeable that the Si−N and Si−Cl bond lengths differ
significantly depending on whether equatorial or axial positions
are occupied (cf. Si−Nax = 1.9414(8) and 1.9935(8) Å, Si−Neq =
analyses were performed by the Analytisches Labor des Instituts fur
̈
Anorganische Chemie der Universitat Gottingen.
̈
̈
Synthesis of [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2). To the
toluene solution (20 mL) of 1 (0.29 g, 1.00 mmol) (trimethylsilyl)-
diazomethane (0.49 mL, 1.01 mmol, 2 M in n-hexane) was added at
−40 °C. The reaction mixture was slowly warmed to room
temperature and stirred for 4 h. The solvent was removed under
reduced pressure and the crude product was extracted with toluene
(10 mL). Concentration and storing at −32 °C in a freezer afforded
1
colorless crystals of 2 (0.48 g, 29.3%); mp: 160−165 °C. H NMR
(200 MHz, C6D6, 25 °C): δ 0.43 (s, 18H, TMS), 1.35 (br, 36 H, tBu),
6.83−7.45 (m, 10H, Ph), 8.29(s, 2H, CH(TMS)) ppm. 29Si{1H}
NMR (99.36 MHz, C6D6, 25 °C): δ −10, −70.2 ppm. EI-MS: m/z:
816.4 (4%) (M+), 801.4 (M+ − Me) (100%). For the elemental
analysis, the crystals of 2·2C7H8 were kept in a vacuum overnight to
remove two molecules of toluene. Elemental analysis for
C38H66Cl2N8Si4: calcd. C, 55.78; H, 8.13; N, 13.69; found, C, 55.16;
H, 8.12; N, 12.10.
1.8155(8) and 1.8118(8) Å; Si−Clax = 2.2172(4) Å, Si−Cleq
=
2.1079(4) Å). The two Si−C bond lengths are almost the same
(1.8674(10) and 1.8726(10) Å), which correspond to the Si−C
single bonds and are similar to those reported in the literature.25c,30
The geometry of C(31) is best described as heavily distorted
tetrahedral where the sum of the bond angles is 420.8°.
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Synthesis of [{PhC(NtBu)2Si(Cl2)}2CH2](3). The chlorosilylene 1
(0.05 g, 0.17 mmol) was added to 5 mL of dichloromethane at room
temperature. The reaction mixture was allowed to stay at room
temperature overnight. Attempts to grow the crystals from the mother
liquor failed due to the high solubility of 3 in CH2Cl2. The solvent was
then removed under vacuum, and toluene (10 mL) was added to
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obtain colorless crystals of 3 (0.15 g, 68.22%); mp: 150−155 °C. H
NMR (200 MHz, C6D6, 25 °C): δ 1.27 (br, 36H, tBu), 1.4 (s, 2H,
CH2), 6.68−7.49 (m, 10 H, Ph) ppm. 29Si{1H} NMR (99.36 MHz,
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(SHELXL-97).34 All non-hydrogen atoms were refined with
anisotropic displacement parameters. The hydrogen atoms were
refined isotropically on calculated positions using a riding model
with their Uiso values constrained to equal to 1.5 times the Ueq of their
pivot atoms for terminal sp3 carbon atoms and 1.2 times for all other
carbon atoms. The positions of the hydrogen atoms at C(31) of 3
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(excluding structure factors) for the structures35 reported in this paper
have been deposited with the Cambridge Crystallographic Data
Centre. The crystal data are available from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
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ASSOCIATED CONTENT
* Supporting Information
■
(11) Tavca
Stalke, D. Organometallics 2010, 29, 3930−3935.
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The Supporting Information contains the CIF files for 2 and 3.
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http://pubs.acs.org.
̌
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AUTHOR INFORMATION
Corresponding Authors
■
(15) Jana, A.; Leusser, D.; Objartel, I.; Roesky, H. W.; Stalke, D.
Dalton Trans. 2011, 40, 5458−5463.
(16) (a) Tavcar, G.; Sen, S. S.; Azhakar, R.; Thorn, A.; Roesky, H. W.
̌
*Tel: +49551393001, +49551393000. Fax: +49551393373.
E-mail: hroesky@gwdg.de (H.W.R.), dstalke@chemie.uni-
goettingen.de (D.S.).
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ACKNOWLEDGMENTS
■
H.W.R. thanks the Deutsche Forschungsgemeinschaft
(DFGRO 224/55-3) for financial support. D.S. is grateful for
funding from the DFG Priority Programme 1178, the DNRF
funded Center for Materials Crystallography (CMC) for
support, and the Land Niedersachsen for providing J.H. and
D.K. with a fellowship in the Catalysis for Sustainable Synthesis
(CaSuS) Ph.D. program.
DEDICATION
■
†This paper is dedicated to Professor Alan H. Cowley.
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(35) Crystal data for 2·2C7H8:C52H82Cl2N8Si4; fw, 1002.51; T,
200(2) K; monoclinic; space group, P21/n; a = 12.6130(5) Å, b =
11.3070(5) Å, c = 21.1483(9) Å, β = 104.573(2)°; V = 2919.0(2) Å3;
Z = 2; ρcalcd = 1.141 mg/m3; μ = 0.233 mm−1; Mo Kα; F(000), 1080;
crystal size, 0.30 × 0.20 × 0.04 mm; θ range for data collection, 1.71−
27.12°; reflections collected/unique observed reflections [I > 2σ(I)],
35865/6388; [R(int), 0.0335]; data/restraints/parameters, 6388/0/
308; GOF on F2, 1.060; R1 [I > 2σ(I)] = 0.0423; wR2 (all data) =
0.1167; largest diff peak/hole, 0.319 and −0.308 e/Å3; CCDC No
843607. Crystal data for 3: C31H48Cl4N4Si2; fw, 674.71; T, 100(2) K;
monoclinic; space group, P21/c; a = 14.8427(7) Å, b = 13.6256(7) Å,
c = 17.5591(8) Å, β = 100.029(2)°; V = 3496.9(3) Å3; Z = 4; ρcalcd
=
1.382 mg/m3; μ = 0.434 mm−1; Mo Kα; F(000), 1432; crystal size,
0.20 × 0.20 × 0.18 mm; θ = 1.39−29.82°; reflections collected/unique
observed reflections [I > 2σ(I)], 98691/10034; [R(int), 0.0291]; data/
restraints/parameters, 10034/2/436; GOF on F2, 1.029; R1 [I >
2σ(I)] = 0.0251; wR2 (all data) = 0.0678; largest diff peak/hole =
0.417 and −0.192 e/Å3; CCDC No 843608.
439
dx.doi.org/10.1021/om201031n | Organometallics 2012, 31, 435−439