Reactivity studies of heteroleptic silylenes with N2O

Article abstract of DOI:10.1021/om3008794

Reaction of heteroleptic silylenes LSiX (L = PhC(NtBu)₂; X = PPh₂ (1), NPh₂ (2), NMe₂ (3), OtBu (4)) with N₂O resulted in the oxidized dimeric product [LSi(X)(μ-O)] ₂ (X = PPh₂ (5), NPh₂ (6), NMe₂ (7), OtBu (8)), which contains a four-membered Si₂O₂ ring. Compounds 5-8 were characterized by spectroscopic and spectrometric techniques. The molecular structures of 5-8 were established by single-crystal X-ray structure analysis.

Full text of DOI:10.1021/om3008794

Article
pubs.acs.org/Organometallics
Reactivity Studies of Heteroleptic Silylenes with N2O
Ramachandran Azhakar, Kevin Propper, Birger Dittrich,* and Herbert W. Roesky*
̈
Institut fur Anorganische Chemie der Universitat Gottingen, Tammannstrasse 4, 37077 Gottingen, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: Reaction of heteroleptic silylenes LSiX (L = PhC(NtBu)2; X =
PPh2 (1), NPh2 (2), NMe2 (3), OtBu (4)) with N2O resulted in the oxidized
dimeric product [LSi(X)(μ-O)]2 (X = PPh2 (5), NPh2 (6), NMe2 (7), OtBu
(8)), which contains a four-membered Si2O2 ring. Compounds 5−8 were
characterized by spectroscopic and spectrometric techniques. The molecular
structures of 5−8 were established by single-crystal X-ray structure analysis.
widely used for carrying out catalytic oxidation reactions.
Treatment of LSiCl with N2O resulted in the formation of the
trimer [LSi(μ-O)Cl]3, which contains a Si3O3 six-membered
ring.20a However, the reaction of LSitBu with N2O yielded the
dimeric product with a Si2O2 four-membered ring.23a Driess et
al. reported the NHC-supported silanone upon reaction of N2O
with the adduct of NHC-silylene.24a In contrast to the reactivity
studies of N2O with silylenes, Severin et al. isolated a covalently
bonded nitrous oxide of NHC.24b In order to explore the
chemistry of the heteroleptic silylenes, we passed dry N2O gas
through a toluene solution of the heteroleptic silylene LSiX (X
= PPh2 (1), NPh2 (2), NMe2 (3), OtBu (4)).
INTRODUCTION
■
The taming of highly reactive species to isolate them as stable
species under laboratory conditions is a difficult task in
synthetic chemistry.1 Carbenes and silylenes R2E: (where R =
alkyl, aryl, H, or halogen and E = C or Si) are such highly
reactive species. They play a role of growing importance in
synthetic organic and organosilicon chemistry as well as in
material sciences.2−4 Initially silylenes had been recognized as
short-lived species in the gas phase, in solution, or trapped in
frozen matrices.5 They feature both nucleophilic and electro-
philic reactive sites at the silicon atom. Owing to the presence
of two nonbonding electrons in the HOMO and an empty p-
orbital as the LUMO, silylenes show Lewis acid as well as Lewis
base properties.6 The first remarkable divalent silicon ten-
coordinate π complex, (Me5C5)2Si, was reported by Jutzi and
co-workers in 1986.7 In 1994 West et al. isolated the first stable
N-heterocyclic silylene (NHSi).8 The NHSis are considered as
analogues of N-heterocyclic carbenes (NHCs), while the latter
have been studied widely and found diverse applications in
chemistry.9,10 They are used as excellent σ-donor ligands to
stabilize compounds with unusual oxidation states.11 Many
remarkable reactivity studies such as insertion, addition, metal
complexes, and Lewis acids of stable NHSis with a variety of
substrates have been reported.12−20 In 2006, we made a first
report on a base-stabilized monochlorosilylene LSiCl (L =
PhC(NtBu)2).21 Further, we stabilized dichlorosilylene as
NHC·SiCl2 (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-
2-ylidene or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene)
without using hazardous reducing agents such as alkali metals
or KC8. NHC·SiCl2 was obtained by a new synthetic procedure
comprising the reductive elimination of HCl from trichlor-
osilane in the presence of NHC under mild reaction
conditions.22 Recently we reported on a facile synthesis of
heteroleptic silylenes that were prepared by the metathesis
reaction of alkali metal amide, phosphide, alkoxide, or
organoalkyl reagents with amidinato ligand stabilized mono-
chlorosilylene LSiCl [L = PhC(NtBu)2] in toluene as solvent.23
Nitrous oxide is an effective donor of oxygen that has been
RESULTS AND DISSCUSSION
■
Compounds [LSi(X)(μ-O)]2 (X = PPh2 (5), NPh2 (6), NMe2
(7), OtBu (8)) were obtained by passing dry N2O gas through
a toluene solution of heteroleptic silylene LSiX (X = PPh2 (1),
NPh2 (2), NMe2 (3), OtBu (4)) for 15 minutes at room
temperature (Scheme 1). Compounds 5−8 are stable both in
the solid state and in solution for a long time without any
decomposition under an inert gas atmosphere.
The 29Si NMR spectrum of 5 exhibits two resonances of a
doublet centered at δ −88.33 and −88.35 ppm (JSiP = 33 Hz),
which is upfield shifted compared to that of 1 (δ 34.3, JSiP = 152
Hz), consistent with compounds containing five-coordinate
silicon.23b The 31P NMR spectrum of compound 5 shows a
broad resonance (δ −51.95 ppm), with two silicon satellites of
J
SiP = 33 Hz. The tBu protons for compound 5 in its 1H NMR
spectrum appear as a singlet (δ 0.92 ppm). Compounds 6−8
exhibit two resonances, δ −108.09 and −108.17 (6), −99.44
and −100.16 (7), and −113.39 and −113.99 (8) ppm,
respectively, in their 29Si NMR spectrum. The narrow
resonances in the 29Si NMR chemical shifts of compounds
6−8 might be due to the five-membered rings being in opposite
Received: September 15, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om3008794 | Organometallics XXXX, XXX, XXX−XXX

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Article
Scheme 1. Synthesis of Compounds 5−8
1
directions to each other. The H NMR spectra of 6−8 also
show two resonances of equal intensity for tBu protons, which
reside over the nitrogen atoms. Again these two resonances
exhibit very little difference, which might be due to the slightly
different geometrical orientation of the two silicon atoms in the
molecule. Compounds 5−7 each display their fragment ion
[M+ − X] in its mass spectrum at m/z 735 (X = PPh2 (5)), 718
(X = NPh2 (6)), and 594 (X = NMe2 (7)), respectively. The
mass spectrum of compound 8 displays its molecular ion peak
at m/z 696.
The molecular structures for compounds 5−8 are given in
Figures 1−4. In all cases the silicon atom is five-coordinate and
Figure 2. Molecular structure of 6. The anisotropic displacement
parameters are depicted at the 50% probability level. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and bond angles
(deg): N2−Si1 1.8356(10), N1−Si1 1.9514(11), N3−Si1 1.7663(10),
O1−Si1 1.6750(9), O1*−Si1 1.7131(9); N1−Si1−N2 68.96(4), N2−
Si1−N3 111.82(5), N1−Si1−N3 95.49(5), O1−Si1−O1* 84.73(4),
O1−Si1−N2 121.01(5), O1*−Si1−N2 100.27(4), O1−Si1−N1
91.27(4), N1−Si1−O1* 164.41(5), O1−Si1−N3 125.43(5), Si1−
O1−Si1* 95.27(4).
Å). Similarly, two types of Si−O bond lengths are present. The
shorter bond length is 1.6703(10) Å, and the longer one
measures 1.7205(10) Å. These shorter and longer bond lengths
are corresponding to the axial and equatorial positions. The
Si1−P1 bond distance in 5 is 2.2895(6) Å, which is shorter
compared to that of 1 (2.3111(6) Å).23b The Si−O−Si and O−
Si−O bond angles are 94.69(5)° and 85.32(5)°. The
coordination environments of 6 and 7 are derived from the
three nitrogen atoms and two oxygen atoms. Like 5, there are
two types of Si−N (amidinato ligand) and Si−O bond lengths
present in 6 and 7. The Si1−N3 bond distance is 1.7663(10) Å
in 6. The Si1−N3 and Si2−N4 bond distances in 7 are
1.7271(13) and 1.7259(13) Å,26 which are comparable to that
of 3 (1.724(2) Å).27 The Si−O−Si and O−Si−O bond angles
are 95.27(4)° and 84.73(4)° in 6. In 8, the coordination
environment of silicon is derived from three oxygen atoms and
two nitrogen atoms. The Si1−O3 and Si1−O4 bond distances
are 1.6445(10) and 1.6422(10) Å.
Figure 1. Molecular structure of 5. The anisotropic displacement
parameters are depicted at the 50% probability level. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and bond angles
(deg): N2−Si1 1.8248(12), N1−Si1 1.9824(12), P1−Si1 2.2895(6),
O1−Si1 1.6703(10), O1*−Si1 1.7205(10); N1−Si1−N2 68.62(5),
N2−Si1−P1 119.60(4), N1−Si1−P1 93.94(4), O1−Si1−O1*
85.32(5), O1−Si1−N2 121.53(5), O1*−Si1−N2 102.40(5), O1−
Si1−N1 92.19(5), N1−Si1−O1* 167.60(5), Si1−O1−Si1* 94.69(5).
arranged in a distorted trigonal bipyramidal geometry with a τ
value25 (τ = 1 for perfect trigonal bipyramidal; τ = 0 for perfect
square based pyramid) of 0.77 (5), 0.65 (6), {0.65 for both Si1
and Si2} (7), and {0.70 for Si1 and 0.67 for Si2} (8). The
coordination environment features two nitrogen atoms from
the supporting amidinato ligand, two oxygen atoms, and one
phosphorus atom for 5. The four-membered Si2O2 ring in 5 is
slightly distorted from planarity. There are two types of Si−N
(amidinato ligand) bond lengths present, one being shorter
(Si1−N2 1.8248(12) Å) and one longer (Si1−N1 1.9824(12)
CONCLUSION
■
In summary we report on the reactivity of heteroleptic silylene,
LSiX (X = PPh2 (1), NPh2 (2), NMe2 (3), OtBu (4)) with
N2O, which afforded the dimeric product [LSi(X)(μ-O)]2 (X =
PPh2 (5), NPh2 (6), NMe2 (7), OtBu (8)) containing a Si2O2
six-membered ring. This result is in contrast to the trimer
[LSi(Cl)(μ-O)]3 obtained from the reaction of LSiCl with
B
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Article
Figure 3. Molecular structure of 7. The anisotropic displacement parameters are depicted at the 50% probability level. Hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and bond angles (deg): N2−Si1 1.8370(12), N1−Si1 2.0020(13), N3−Si1 1.7271(13), O1−Si1 1.6688(11),
O2−Si1 1.7240(11), N5−Si2 1.8369(12), N6−Si2 2.0116(12), N4−Si2 1.7259(13), O1−Si2 1.7265(11), O2−Si2 1.6724(11); N1−Si1−N2
67.58(5), N2−Si1−N3 114.87(6), N1−Si1−N3 96.60(6), O1−Si1−O2 85.08(5), O1−Si1−N2 120.39(6), O2−Si1−N2 99.09(5), O1−Si1−N1
90.48(5), O1−Si1−N3 122.53(6), O2−Si1−N1 161.22(5), N5−Si2−N6 67.51(5), N4−Si2−N5 115.98(6), N4−Si2−N6 96.37(6), O1−Si2−O2
84.89(5), O2−Si2−N5 118.61(6), O1−Si2−N5 99.51(6), O1−Si2−N6 90.44(5), O1−Si2−N6 161.90(5), O2−Si2−N4 123.18(6), Si1−O1−Si2
95.03(6), Si2−O2−Si1 95.00(5).
Figure 4. Molecular structure of 8. The anisotropic displacement parameters are depicted at the 50% probability level. Hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and bond angles (deg): N2−Si1 1.8477(12), N1−Si1 1.9801(11), O3−Si1 1.6445(10), O1−Si1 1.6738(9),
O2−Si1 1.7149(9), N3−Si2 1.8428(11), N4−Si2 1.9723(11), O4−Si2 1.6422(10), O2−Si2 1.6744(9), O1−Si2 1.7198(9); N1−Si1−N2 68.09(5),
N2−Si1−O1 121.58(5), N1−Si1−O3 90.82(5), O1−Si1−O2 84.98(4), O1−Si1−N2 121.58(5), O2−Si1−N2 100.75(5), O1−Si1−N1 90.58(5),
O2−Si1−N1 163.37(5), N3−Si2−N4 68.04(5), N3−Si2−O1 99.33(5), N3−Si2−O2 122.77(5), O1−Si2−O2 84.80(4), O2−Si2−O4 122.25(5),
O1−Si2−N4 162.45(5), O2−Si2−N4 91.93(5), Si1−O1−Si2 95.01(5), Si2−O2−Si1 95.18(5).
Compound 5. Quantity used: LSiPPh2, 1.00 g. Yield: 0.92 g, 89%.
Anal. (%) Calcd for C54H66N4O2P2Si2 (921.25): C, 70.40; H, 7.22; N,
6.08. Found: C, 70.31; H, 7.12; N, 6.01. 1H NMR (500 MHz, CD2Cl2,
25 °C): δ 0.92 (b, 36H, C(CH3)3), 7.20−7.96 (m, 30H, ArH) ppm.
31P{1H} NMR (202.46 MHz, CD2Cl2, 25 °C): δ −51.95 (b, JSiP = 33
Hz) ppm. 29Si{1H} NMR (99.36 MHz, CD2Cl2, 25 °C): δ −88.33 (d,
JSiP = 33 Hz), −88.35 (d, JSiP = 33 Hz) ppm. EI-MS: m/z 735 [M+ −
PPh2].
Compound 6. Quantity used: LSiNPh2, 1.05 g. Yield: 0.95 g,
87.3%. For elemental analysis, 6·toluene was treated under vacuum for
six hours to remove the toluene molecules. Anal. (%) Calcd for
C54H66N6O2Si2 (887.31): C, 73.09; H, 7.50; N, 9.47. Found: C, 73.06;
H, 7.42; N, 9.45. 1H NMR (500 MHz, CDCl3, 25 °C): δ 0.89 (s, 18H,
C(CH3)3), 0.92 (s, 18H, C(CH3)3), 6.96−7.60 (m, 30H, ArH) ppm.
29Si{1H} NMR (99.36 MHz, CDCl3, 25 °C): δ −108.09 (s), −108.17
(s) ppm. EI-MS: m/z 718 [M+ − NPh2].
N2O, containing a six-membered Si3O3 ring. The formation of
the dimers is obviously due to the presence of bulkier X = PPh2
(1), NPh2 (2), NMe2 (3), OtBu (4) groups on the silicon
atoms.
EXPERIMENTAL SECTION
■
Syntheses were carried out under an inert atmosphere of dinitrogen in
oven-dried glassware using standard Schlenk techniques. All other
manipulations were accomplished in a dinitrogen-filled glovebox.
Solvents were purified by an MBraun solvent purification system, MB
SPS-800. Compounds 1−4 were prepared as reported in the
literature.23b 1H and 29Si NMR spectra were recorded with a Bruker
Avance DPX 300 or a Bruker Avance DRX 500 spectrometer, using
CD2Cl2 and CDCl3 as solvents. Chemical shifts δ are given relative to
SiMe4. EI-MS spectra were obtained using a Finnigan MAT 8230
spectrometer. Elemental analyses were performed at the Institut fur
̈
Anorganische Chemie, Universitat Gottingen.
̈
̈
Compound 7. Quantity used: LSiNMe2, 1.10 g. Yield: 0.93 g, 80%.
Anal. (%) Calcd for C34H58N6O2Si2 (639.03): C, 63.90; H, 9.15; N,
13.15. Found: C, 63.83; H, 9.13; N, 13.03. 1H NMR (500 MHz,
CDCl3, 25 °C): δ 1.08 (s, 18H, C(CH3)3), 1.13 (s, 18H, C(CH3)3),
2.72 (s, 6H, N(CH3)2), 2.74 (s, 6H, N(CH3)2), 7.24−7.38 (m, 10H,
General Procedure for the Synthesis of [LSiX(μ-O)]2 (X =
PPh2 (5), NPh2 (6), NMe2 (7), OtBu (8)). Toluene (60 mL) was
added to a 100 mL Schlenk flask containing the respective heteroleptic
silylene compounds. Dry N2O gas was passed through this solution for
15 min.
C
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ArH) ppm. 29Si{1H} NMR (99.36 MHz, CDCl3, 25 °C): δ −99.44 (s),
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Compound 8. Quantity used: LSiOtBu, 1.00 g. Yield: 0.90 g, 86%.
Anal. (%) Calcd for C38H64N4O4Si2 (697.11): C, 65.47; H, 9.25; N,
8.04. Found: C, 65.43; H, 9.22; N, 8.03. 1H NMR (500 MHz, CDCl3,
25 °C): δ 1.11 (s, 18H, C(CH3)3), 1.14 (s, 18H, C(CH3)3), 1.42 (s,
9H, OC(CH3)3), 1.46 (s, 9H, OC(CH3)3), 7.22−7.35 (m, 10H, ArH)
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−113.99 (s) ppm. EI-MS: m/z: 696 [M+], 639 [M+ − C(CH3)3], 623
[M+ − OC(CH3)3].
Crystal Structure Determination. Suitable single crystals for X-
ray structural analysis of 5−8 were obtained by storing their
corresponding toluene solutions at 0 °C for 24−48 h in the
refrigerator (Table S1). Crystals were taken out of the mother liquor
under an argon atmosphere using NVH oil. Diffraction data were
collected at 100 K on a Bruker three-circle diffractometer equipped
with a SMART 6000 CCD area detector and a Cu Kα rotating anode.
Due to good crystal quality, all four data sets were collected to the
edge of the Ewald sphere with high completeness and high
multiplicity. Raw data were integrated with SAINT,28 and an empirical
absorption correction with SADABS29 was applied. The structures
were solved by direct methods (SHELXS-97) and refined against F2 by
full-matrix least-squares methods using all data (SHELXL-2012).30
SHELXLE31 was used as refinement GUI. All non-hydrogen atoms
were refined with anisotropic displacement parameters. Hydrogen
atoms were refined unconstrained with displacement parameters
constrained to 1.2 or 1.5 of the Uiso of their parent atom. Compounds
5 and 8 showed rotational disorder in their tert-butyl groups.
Disordered groups were assigned to different part numbers to
suppress intermolecular bonds, and H atoms of these groups were
calculated and constrained to their parent atom sites. Compound 6
contains one toluene molecule with a rotating methyl group.
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ASSOCIATED CONTENT
* Supporting Information
CIF files for 5 (CCDC 900209), 6 (CCDC 900210), 7 (CCDC
900211), and 8 (CCDC 900212) and a table giving crystal data
and details of the structure solution and refinement for 5−8.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
(10) (a) Chuprakov, S.; Malik, J. A.; Zibinsky, M.; Fokin, V. V. J. Am.
Chem. Soc. 2011, 133, 10352−10355. (b) Dzik, W. I.; Zhang, P.; de
Bruin, B. Inorg. Chem. 2011, 50, 9896−9903. (c) Goedecke, C.;
Leibold, M.; Siemeling, U.; Frenking, G. J. Am. Chem. Soc. 2011, 133,
3357−3569. (d) Martin, D.; Melaimi, M.; Soleilhavoup, M.; Bertrand,
G. Organometallics 2011, 30, 5304−5313.
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12337.
AUTHOR INFORMATION
Corresponding Author
*E-mail: bdittri@gwdg.de; hroesky@gwdg.de.
Notes
■
(12) Yao, S.; van Wullen, C.; Sun, X.-Y.; Driess, M. Angew. Chem.
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2008, 120, 3294−3297; Angew. Chem., Int. Ed. 2008, 47, 3250−3253.
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̈
Soc. 2010, 132, 3038−3046. (b) Jana, A.; Schulzke, C.; Roesky, H. W.
J. Am. Chem. Soc. 2009, 131, 4600−4601. (c) Jana, A.; Schulzke, C.;
Roesky, H. W.; Samuel, P. P. Organometallics 2009, 28, 6574−6577.
(d) Azhakar, R.; Ghadwal, R. S.; Roesky, H. W.; Wolf, H.; Stalke, D.
Organometallics 2012, 31, 5506−5510.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Deutsche Forschungsgemeinschaft (DFG) for
supporting this work. R.A. is grateful to the Alexander von
Humboldt Stiftung for a research fellowship.
(14) (a) Yao, S.; Brym, M.; van Wullen, C.; Driess, M. Angew. Chem.
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2007, 119, 4237−4240; Angew. Chem., Int. Ed. 2007, 46, 4159−4162.
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