On the reactivity of the silylene PhC(NtBu)2SiNPh2 toward organic substrates

Article abstract of DOI:10.1002/zaac.201300138

The reactivity studies of stable silylenes with organo isothiocyanates are so far not reported. In this manuscript we describe for the first time the reaction of the stable silylene LSiNPh₂ (1) [L = PhC(NtBu) ₂] with PhNCS to yield t

Full text of DOI:10.1002/zaac.201300138

ARTICLE
DOI: 10.1002/zaac.201300138
On the Reactivity of the Silylene PhC(NtBu)2SiNPh2 toward Organic
Substrates
Ramachandran Azhakar,[a] Herbert W. Roesky,*[a] Hilke Wolf,[a] and Dietmar Stalke*[a]
Keywords: Silylenes; Heteroleptic silylenes; Silathione; Silacyclopropene; Cycloaddition
Abstract. The reactivity studies of stable silylenes with organo isothio- Treatment of 1 with 3,5-di-tert-butyl-o-benzoquinone yielded the
cyanates are so far not reported. In this manuscript we describe for the [1+4]-cycloaddition product 4. Compounds 2, 3, and 4 were charac-
first time the reaction of the stable silylene LSiNPh2 (1) [L =
PhC(NtBu)2] with PhNCS to yield the silathione LSi(S)NPh2 (2), with
terized by elemental analysis, multinuclear NMR spectroscopy, and
EI-MS spectrometry. The molecular structures of compounds 2, 3, and
elimination of PhNC. The reaction of PhCϵCPh with LSiNPh2 4 were established unequivocally by single-crystal X-ray structural
resulted in the [1+2]-cycloaddition product silacyclopropene 3. analysis.
similar to the reaction of LSitBu with PhCϵCPh.[16] This is in
Introduction
contrast to the reaction of LSiCl with PhCϵCPh which pro-
The compounds with low-valent silicon are highly reactive
ceeds in a 2:1 ratio to yield the four-membered Si2C2 ring
compound (disilacyclobutene).[17] The reaction of 1 with 3,5-
and have very short lifetimes at ambient temperature. For
many decades silylenes were considered as short-lived species
either in the gas phase, in solution, or trapped in frozen matri-
ces.[1,2] They feature both nucleophilic as well as electrophilic
reactive sites at the silicon atom which have a high lying lone
pair orbital and a low lying vacant pπ orbital.[3] In 1986 Jutzi
et al. published the remarkable ten-coordinate π complex,
di-tert-butyl-o-benzoquinone yielded the [1+4]-cycloaddition
product 4, with a pentacoordinate silicon atom. The latter re-
sult is similar to the reaction product of LSiCl with 3,5-di-tert-
butyl-o-benzoquinone which we reported earlier.[18]
(Me5C5)2Si with divalent silicon.[4] The isolation of the first Results and Discussion
stable N-heterocyclic silylene (NHSi) was reported by West et
Compounds 2, 3, and 4 were obtained by the reactions of
al. in 1994.[5] The NHSi of West is a silicon analogue of
Arduengo’s N-heterocyclic carbene (NHC).[6] The NHCs have
been studied widely and find an extensive variety of applica-
tions such as organocatalyst reagents and also as excellent σ-
donor ligands to stabilize compounds with unusual oxidation
states.[7–9] Many important reactivity studies of stable silylenes
toward a variety of substrates have been reported.[10–14] Re-
cently we synthesized a functionalized silylene LSiNPh2 (1)
[L = PhC(NtBu)2] by the facile metathesis reaction of LiNPh2
with LSiCl.[15] Reactions of stable silylenes with organo iso-
thiocyanates are not reported in the literature. Herein we de-
scribe the reaction of LSiNPh2 with PhNCS that resulted in
the silathione LSi(S)NPh2 (2), with elimination of PhNC. Fur-
ther we report on the reactivity of 1 with PhCϵCPh and 3,5-
di-tert-butyl-o-benzoquinone. Treatment of 1 with PhCϵCPh
yielded the [1+2]-cycloaddition product 3 which contains a
three-membered silacyclopropene ring. Compound 3 is formed
LSiNPh2 (1) with PhNCS (for 2), PhCϵCPh (for 3), and 3,5-
di-tert-butyl-o-benzoquinone (for 4) each in the molar ratio of
1:1 (Scheme 1). The resulting compounds 2–4 are soluble in
common organic solvents and they are stable, both in the solid
state as well as in solution without any decomposition under
* Prof. Dr. H. W. Roesky
E-mail: hroesky@gwdg.de
* Prof. Dr. D. Stalke
E-Mail: dstalke@chemie.uni-goettingen.de
[a] Institut für Anorganische Chemie
Universität Göttingen
Tammannstrasse 4
37077 Göttingen, Germany
Scheme 1. Synthesis of Compounds 2, 3, and 4.
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2013, 639, (6), 934–938
934

On the Reactivity of the Silylene PhC(NtBu)2SiNPh2 toward Organic Substrates
an inert gas atmosphere. Compounds 2–4 were completely reside over the nitrogen atoms of compound 3 show a single
1
characterized by spectroscopic methods. The molecular struc- resonance at δ = 1.02 in its H NMR spectrum. In addition
tures of compounds 2–4 were established unequivocally by compound 3 exhibits its molecular ion in the mass spectrum
single-crystal X-ray structural analysis.
at m/z 605.
Compound 2 exhibits a single resonance at δ = –18.6 ppm
The molecular structure of compound 3 is shown in Fig-
in the 29Si NMR spectrum, which is comparable with that of ure 2. It crystallizes in the monoclinic space group C2/c. The
LSi(S)Cl (δ = –17.5 ppm).[19] The protons of the tBu groups structural index τ which defines the extent of deviation from
of compound 2 show a single resonance at δ = 1.12 ppm in trigonal bipyramidal to square pyramidal geometry (τ = 1 for
1
the H NMR spectrum. Further compound 2 displays its mo- perfect trigonal bipyramidal; τ = 0 for perfect square based
lecular ion in the mass spectrum at m/z 459.
pyramid[22]) is 0.40. The silicon coordination environment is
Silathione 2 crystallizes in the monoclinic space group P21/c derived from three nitrogen atoms and two carbon atoms. The
and the molecular structure is shown in Figure 1. The tentative bite angle (N1–Si1–N2) is 69.31(5)°. The angles of the newly
mechanism for the formation of 2 has been proposed in formed CSiC three-membered ring are C28–Si1–C29
Scheme 2. The reduction of isothiocyanates to isocyanides is 43.83(5)°, Si1–C28–C29 68.91(7)°, and C28–C29–Si1
reported in the literature.[20] The silicon atom is tetracoordi- 67.25(7)°, and the bond lengths are Si1–C28 1.8113(12) Å,
nate, and it adopts a distorted tetrahedral geometry. The coor- C28–C29 1.3602(16) Å and Si1–C29 1.8325(13) Å. These
dination environment is made up from three nitrogen atoms values are quite similar to those reported in the literature.[16]
and a sulfur atom. The N1–Si1–N2 bite angle between the sili- The Si1–N3 bond length was determined to be 1.7633(10) Å
con atom with the backbone ligand is 71.75(7)°. The bond and is comparable to the reported value of Si–N bond
length between the Si and the nitrogen atom of the NPh2 moi- lengths.[23]
ety is 1.7358(16) Å. The Si = S bond length is 1.9814(8) Å [in
LSi(S)Cl 2.079(6) Å].[19] The Si =
S bond length in
LSi(S)StBu was found to be 1.984(8) Å.[21]
Figure 2. Molecular structure of 3. The anisotropic displacement pa-
rameters are depicted at the 50% probability level. Selected bond
lengths /Å and bond angles /°: N1–Si1 1.8320(13), N2–Si1 1.9186(11),
N3–Si1 1.7633(10), C28–Si1 1.8113(12), C29–Si1 1.8325(13), C28–
C29 1.3602(16); N1–Si1–N2 69.31(5), N2–Si1–N3 98.95(5), N1–Si1–
N3 113.45(5), N3–Si1–C28 119.19(6), C29–Si1–N3 108.53(5), N2–
Si1–C28 111.55(5), N1–Si1–C28 126.07(5), C28–Si1–C29 43.83(5),
Si1–C28–C29 68.91(7), C28–C29–Si1 67.25(7).
Figure 1. Molecular structure of 2. The anisotropic displacement pa-
rameters are depicted at the 50% probability level. Selected bond
lengths /Å and bond angles /°: N1–Si1 1.8178(17), N2–Si1 1.8284(17),
N3–Si1 1.7358(16), S1–Si1 1.9814(8); N1–Si1–N2 71.75(7),
S1–Si1–N3 116.66(6), N1– Si1–N3 111.42(8).
Similar to that of 3, the [1+4]-cycloaddition product 4 reso-
nates at δ = –99.9 ppm in the 29Si NMR spectrum. The protons
of the tBu groups from the amidinato moiety display a double
1
resonance of equal intensity in the H NMR spectrum (δ =
1.10, 1.17 ppm). The protons of the tBu groups on the phenyl
ring resonate at δ = 1.36 and 1.47 ppm. Compound 4 displays
its molecular ion in the mass spectrum at m/z 647.
The [1+4]-cycloaddition product 4 was confirmed by single-
crystal X-ray structural analysis. Compound 4 crystallizes in
Scheme 2. Proposed mechanism for the formation of 2.
The [1+2]-cycloaddition product 3 displays a single reso- the monoclinic space group P21/n and the molecular structure
nance at δ = –141.1 ppm in the 29Si NMR spectrum (for 1, is depicted in Figure 3. The silicon atom is pentacoordinate,
δ = –20.5 ppm).[15] Like 2, the protons of the tBu groups that with a τ value of 0.71, made up of three nitrogen atoms, and
Z. Anorg. Allg. Chem. 2013, 934–938
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
935

R. Azhakar, H. W. Roesky, H. Wolf, D. Stalke
ARTICLE
solvent was reduced in vacuo (15 mL) and stored at –26 °C in a freezer
to obtain crystals of 2 in four days (0.40 g, 82%). Mp 215–217 °C
(decomp). Elemental analysis (%) calcd. for C27H33N3SSi (459.72): C,
two oxygen atoms. The Si1–N3 bond length was found to be
1.737(2) Å. The bond lengths of Si1–O1 and Si1–O2 are
1.6962(17) Å and 1.7321(17) Å, respectively.[18]
1
70.54; H, 7.24; N, 9.14. Found: C, 70.48; H, 7.22; N, 9.05. H NMR
(500 MHz, C6D6, 25 °C): δ = 1.12 (s, 18 H, C(CH3)3), 6.83–7.05 (m,
ArH), 7.15–7.18 (m, ArH), 7.59–7.62 (m, ArH) ppm. 13C NMR
(125.75 MHz, C6D6, 25 °C): δ = 30.83, 55.00, 124.08, 127.49, 127.64,
127.90, 127.94, 128.35, 128.83, 130.38, 130.54, 149.09, 175.21 ppm;
29Si{1H} NMR (99.36 MHz, C6D6, 25 °C): δ = –18.6 ppm. EI-MS:
m/z 459 [M+].
Synthesis of 3: Toluene (60 mL) was added to a 100 mL Schlenk flask
containing LSiNPh2 (1) (0.51 g, 1.19 mmol) and PhCϵCPh (0.22 g,
1.23 mmol) at room temperature. The reaction mixture was stirred for
14 h. Then the solution was filtered and the solvent of the filtrate was
reduced in vacuo to 25 mL. The concentrate was stored at –27 °C in
a freezer for a week to obtain single crystals of 3 (0.60 g, 83%). Mp
125–127 °C (decomp). Elemental analysis (%) calcd. for C41H43N3Si
(605.89): C, 81.28; H, 7.15; N, 6.94. Found: C, 81.26; H, 7.11; N,
6.87. 1H NMR (500 MHz, C6D6, 25 °C): δ = 1.02 (s, 18 H,
C(CH3)3), 6.95–7.52 (m, ArH) ppm. 13C NMR (125.75 MHz, C6D6,
25 °C): δ = 31.10, 53.54, 122.86, 125.64, 125.70, 127.44, 127.92,
128.11, 128.30, 128.40, 128.51, 128.87, 128.90, 129.28, 137.84,
139.99, 152.24, 174.77 ppm; 29Si{1H} NMR (99.36 MHz, C6D6,
25 °C): δ = –141.1 ppm. EI-MS: m/z 605 [M+].
Figure 3. Molecular structure of 4. The anisotropic displacement pa-
rameters are depicted at the 50% probability level. Selected bond
lengths /Å and bond angles /°: N1–Si1 1.937(2), N2–Si1 1.8066(19),
N3–Si1 1.737(2), O1–Si1 1.6962(17), O2–Si1 1.7321(17); N1–Si1–
N2 69.46(9), N1–Si1–N3 98.09(9), N2–Si1–N3 116.64(9), N3–Si1–
O2 96.57(9), O1–Si1–N3 121.88(9), O2–Si1–O1 90.18(8).
Synthesis of 4: Hexane (60 mL) was added to a 100 mL Schlenk flask
containing LSiNPh2 (1) (0.35 g, 0.82 mmol) and 3,5-di-tert-butyl-o-
benzoquinone (0.18 g, 0.82 mmol) at room temperature. The reaction
mixture was stirred for 15 h. Then the mixture was filtered and the
solvent of the filtrate was reduced in vacuo to 20 mL. The concentrate
was stored at –27 °C in a freezer for 3 days to obtain the colorless
crystals of 4 (0.44 g, 83%). Mp 180–182 °C (decomp). Elemental
analysis (%) calcd. for C41H53N3O2Si (647.96): C, 76.00; H, 8.24; N,
Conclusions
In conclusion we have reported for the first time the reactiv-
ity of NHSi toward an organothiocyante. The reaction of the
silylene LSiNPh2 with PhNCS resulted in the silathione 2, with
elimination of PhNC. Treatment of LSiNPh2 with PhCϵCPh
yielded the [1+2]-cycloaddition product silacyclopropene 3, a
three-membered ring compound. The reaction of LSiNPh2 with
3,5-di-tert-butyl-o-benzoquinone resulted in [1+4]-cycload-
dition product 4. Both 3 and 4 contain pentacoordinate silicon
atoms.
1
6.48. Found: C, 75.92; H, 8.22; N, 6.42. H NMR (500 MHz, C6D6,
25 °C): δ = 1.10 (s, 9 H, C(CH3)3), 1.17 (s, 9 H, C(CH3)3), 1.36 (s, 9
H, C(CH3)3), 1.47 (s, 9 H, C(CH3)3), 6.83–7.26 (m, ArH) ppm. 13C
NMR (125.75 MHz, C6D6, 25 °C): δ = 30.32, 30.94, 31.62, 32.15,
34.45, 34.73, 54.19, 54.36, 107.67, 113.66, 122.92, 127.32, 127.54,
127.91, 128.11, 128.30, 128.38, 128.79, 129.89, 132.51, 133.03,
141.09, 145.47, 148.80, 151.39, 173.55 ppm; 29Si{1H} NMR
(99.36 MHz, C6D6, 25 °C): δ = –99.9 ppm. EI-MS: m/z 647 [M+].
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 glove box. Sol-
vents were purified by MBRAUN solvent purification system MB
Crystal Structure Determination
Suitable single crystals for X-ray structural analysis of 2, 3, and 4 were
mounted at low temperature in inert oil under an argon atmosphere by
applying the X-Temp 2 device.[24] The diffraction data for 2 was col-
lected at 100 K with a Bruker D8 three circle diffractometer equipped
with a SMART APEX II CCD detector and a microfocus source with
INCOATEC Quazar mirror optics (λ = 0.71073 Å).[25] The diffraction
data for 3 and 4 were collected at 100 K on a Bruker D8 three circle
diffractometer equipped with a SMART APEX II CCD detector and a
rotating anode with INCOATEC Quazar mirror optics (λ = 0.71073 Å).
The data were integrated with SAINT[26] and an empirical absorption
correction with SADABS[27] was applied. The structures were solved
by direct methods (SHELXS-2012) and refined against all data by full-
matrix least-squares methods on F2 (SHELXL-2012).[28] All non-hy-
drogen-atoms were refined with anisotropic displacement parameters.
SPS-800. All chemicals were purchased from Aldrich and used without
[15]
further purification. LSiNPh2
(1) was prepared as reported in the
literature. 1H, 13C, and 29Si NMR spectra were recorded with a Bruker
Avance DRX 500 spectrometer, using C6D6 as a solvent. 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 für Anorganische Chemie, Universität
Göttingen. For elemental analysis compounds were kept under vacuum
for six hours to remove the solvent molecules. Melting points were
measured in a sealed glass tube on a Büchi B-540 melting point
apparatus.
Synthesis of 2: To a toluene (30 mL) solution of LSiNPh2 (1) (0.46 g, The hydrogen atoms were refined isotropically on calculated positions
1.07 mmol) in a Schlenk flask (100 mL) was added PhNCS (0.16 g,
1.18 mmol) in toluene (20 mL) drop by drop at room temperature. The
using a riding model with their Uiso values constrained to 1.5 Ueq of
their pivot atoms for terminal sp3 carbon atoms and 1.2 times for all
reaction mixture was stirred at room temperature for 12 h. Then the other carbon atoms.
936 www.zaac.wiley-vch.de
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 934–938

On the Reactivity of the Silylene PhC(NtBu)2SiNPh2 toward Organic Substrates
Table 1. Crystal and structure refinement parameters for compounds 2, 3, and 4.
Parameters
2
3·toluene
4
Empirical formula
Formula Weight
Crystal system
C27H33N3SSi
459.71
monoclinic
C41H43N3Si·C7H8
698.00
monoclinic
C41H53N3O2Si
647.95
monoclinic
Space group
P21/c
C2/c
P21/n
Unit cell dimensions
a = 15.190(5) Å
b = 10.883(4) Å
c = 16.458(3) Å
β = 110.96(2)°
2540.7(13) Å3, 4
1.202 g·cm–3
a = 31.623(4) Å
b = 16.340(7) Å
c = 19.088(9) Å
β = 125.97(2)°
7982(5) Å3, 8
a = 10.012(4) Å
b = 26.226(8) Å
c = 14.407(5) Å
β = 90.14(2)°
Volume, Z
3783(2) Å3, 4
Density (calculated)
Absorption coefficient
F (000)
1.162 g·cm–3
1.138 g·cm–3
0.194 mm–1
0.095 mm–1
2992
0.099 mm–1
984
1400
Crystal size
θ range for data collection
Limiting indices
0.2 ϫ 0.2 ϫ 0.2
1.44 to 26.37°.
–18 Յ h Յ 18;
–13 Յ k Յ 13;
–19 Յ l Յ 20
25948
5201 (Rint = 0.0479)
99.9% (θ = 26.37°)
Full- matrix least - squares on F2
5201 / 0 / 295
1.060
0.2 ϫ 0.1 ϫ 0.05
1.479 to 28.312°
–42 Յ h Յ 42;
–21 Յ k Յ 21;
–25 Յ l Յ 25
0.15 ϫ 0.15 ϫ 0.08
1.553 to 22.485°
–10 Յ h Յ 10;
–28 Յ k Յ 28;
–15 Յ l Յ 15
Reflections collected
Independent reflections
Completeness to θ
122463
51825
9932 (Rint = 0.0432)
100.0% (θ = 25.242°)
Full- matrix least - squares on F2
9932 / 135 / 596
1.038
R1 = 0.0392, wR2 = 0.0985
R1 = 0.0483, wR2 = 0.1035
0.336 and –0.307 e·Å–3
4936 (Rint = 0.0657)
100.0% (θ = 20.706°)
Full- matrix least - squares on F2
4936 / 0 / 436
1.089
R1 = 0.0442, wR2 = 0.0920
R1 = 0.0648, wR2 = 0.0987
0.199 and –0.281 e·Å–3
Refinement method
Data/ restraints/ parameters
Goodness of fit on F2
Final R indices [IϾ2σ (I)]
R indices (all data)
R1 = 0.0426, wR2 = 0.1001
R1 = 0.0588, wR2 = 0.1036
0.327 and –0.354 e·Å–3
Largest diff. peak and hole
Wang, C. Präsang, M. Asay, E. Irran, M. Driess, J. Am. Chem.
Soc. 2011, 133, 2868–2871; e) R. S. Ghadwal, R. Azhakar, H. W.
Roesky, K. Pröpper, B. Dittrich, C. Goedecke, G. Frenking,
Chem. Commun. 2012, 48, 8186–8188.
Disordered solvent molecules and disorder of a phenyl group in 3 were
refined using distance restraints.
Crystal and structure refinement parameters for compounds 2, 3, and
4 (Table 1). CIF files for 2 (CCDC-924135), 3 (CCDC-924136), and
4 (CCDC-924137). These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif.
[4] P. Jutzi, D. Kanne, C. Krüger, Angew. Chem. 1986, 98, 163–164;
Angew. Chem. Int. Ed. Engl. 1986, 25, 164.
[5] M. Denk, R. Lennon, R. Hayashi, R. West, A. V. Belyakov, H. P.
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Acknowledgments
[7] a) N-Heterocyclic Carbenes in Synthesis (Ed.: S. P. Nolan),
Wiley-VCH, Weinheim, 2006; b) N-Heterocyclic Carbenes in
Transition Metal Catalysis (Ed.: F. Glorius), Springer-Verlag,
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We are thankful to the Deutsche Forschungsgemeinschaft and Pro-
hama, Ludwigshafen, for supporting this work. R. A. is thankful to the
Alexander von Humboldt Stiftung for a research fellowship. D. S. and
H· W. are grateful to the Danish National Research Foundation
(DNRF93) funded Center for Materials Crystallography (CMC) for
partial support.
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Received: March 4, 2013
Published Online: May 8, 2013
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