Reaction of the silylene PhC(NtBu)2SitBu with 4,4′-Bis(dimethylamino)thiobenzophenone and treatment of the silylene PhC(NtBu)2SiC(SiMe3)3 with 3,5-Di-tert-butyl-o- benzoquinone

Article abstract of DOI:10.1002/ejic.201300085

The reaction of LSitBu [L = PhC(NtBu)₂] (1) with 4,4′-bis(dimethylamino)thiobenzophenone resulted in the [1+2]-cycloaddition product silathiacyclopropane 3, bearing a three-membered SiCS ring. The reaction of LSiC(SiMe₃)₃ (2) with 3,5-di-tert-butyl-o-benzoquinone leads to the [1+4]-cycloaddition product 4. In 3 the silicon atom is five- and in 4 it is four-coordinate. Compounds 3 and 4 were characterized by spectroscopic and spectrometric techniques. The molecular structures of 1, 3, and 4 were unequivocally established by single-crystal X-ray structure analysis. Copyright

Full text of DOI:10.1002/ejic.201300085

FULL PAPER
DOI:10.1002/ejic.201300085
Reaction of the Silylene PhC(NtBu)2SitBu with 4,4Ј-
Bis(dimethylamino)thiobenzophenone and Treatment of the
Silylene PhC(NtBu)2SiC(SiMe3)3 with 3,5-Di-tert-butyl-o-
benzoquinone
Ramachandran Azhakar,[a] Herbert W. Roesky,*[a]
Julian J. Holstein,[a] and Birger Dittrich*[b]
Dedicated to Professor Vladimir Bregadze on the occasion of his 75th birthday
Keywords: Silylenes / Silicon / Heterocycles / Cycloaddition
The reaction of LSitBu [L = PhC(NtBu)2] (1) with 4,4Ј-bis(di-
dition product 4. In 3 the silicon atom is five- and in 4 it is
methylamino)thiobenzophenone resulted in the [1+2]-cyclo- four-coordinate. Compounds 3 and 4 were characterized by
addition product silathiacyclopropane 3, bearing a three-
membered SiCS ring. The reaction of LSiC(SiMe3)3 (2) with
3,5-di-tert-butyl-o-benzoquinone leads to the [1+4]-cycload-
spectroscopic and spectrometric techniques. The molecular
structures of 1, 3, and 4 were unequivocally established by
single-crystal X-ray structure analysis.
Introduction
tron-donor ability for coordinating electron acceptors.[8]
The three-membered ring compounds containing a silicon
atom are fascinating because of their high strain and novel
bonding arrangement within the ring.[9] In addition, three-
membered rings with high coordinate silicon at the position
adjacent to the heteroatom have attracted synthetic chem-
ists as a result of their unique structure and reactivity.[9] In
the case of a thia-Brook rearrangement the transition state
has been proposed as a three-membered ring compound
possessing a five-coordinate silicon at the position adjacent
to the sulfur atom.[10] Ando et al. reported the first structur-
ally characterized silathiacyclopropane.[11] This compound
was obtained by the reaction of dimesitylsilylene with
1,1,3,3-tetramethyl-2-indanethione. Later Brook et al. ob-
tained the silathiacyclopropane by the reaction of silene
with elemental sulfur.[12] Recently we synthesized silathiacy-
clopropane from LSiCl [L = PhC(NtBu)2] with 4,4Ј-bis(di-
methylamino)thiobenzophenone.[13] In this manuscript, we
report on the reaction of LSitBu (1) with 4,4Ј-bis(dimethyl-
amino)thiobenzophenone, which resulted in the [1+2]-cy-
cloaddition product silathiacyclopropane 3. The latter con-
sists of a three-membered SiCS ring. The reaction of LSiC-
(SiMe3)3 (2) with 3,5-di-tert-butyl-o-benzoquinone leads to
the [1+4]-cycloaddition product 4, with a four-coordinate
silicon atom. This is quite different from the five-coordinate
silicon atom product formed by the reaction of LSiCl with
3,5-di-tert-butyl-o-benzoquinone, which we reported pre-
viously.[14] We presume that the formation of a four-coordi-
nate product 4 might be due to the presence of a bulkier
C(SiMe3)3 group on the silicon atom.
The chemistry of stable singlet silylenes has received con-
siderable attention[1] after the first report of the stable N-
heterocyclic silylene (NHSi) by West et al. in 1994.[2] The
presence of two non-bonding electrons in the HOMO and
a vacant p orbital as the LUMO shows that silylenes have
the tendency to function both as Lewis acids as well as
Lewis bases.[3] Because of the presence of an ambiphilic
character, many noteworthy reactivity studies with a variety
of substrates have been documented.[1,4–6] We reported on
a facile method for the synthesis of heteroleptic silylenes[7]
by the metathesis reaction of alkali metal amide, phosphide,
alkoxide, or organoalkyl reagent with amidinato ligand sta-
bilized monochlorosilylene LSiCl [L = PhC(NtBu)2].[7c,7d]
Silylenes are highly reactive species and provide an alterna-
tive route to inaccessible new organosilicon compounds,
which are difficult to prepare by conventional methods. In
general, three- and four-membered ring compounds with a
heteroatom play a substantial role in the development of
heterocyclic chemistry.[8] The high degree of strain in the
three-membered ring compounds results in various proper-
ties such as high reactivity in the ring cleavage and low elec-
[a] Institut für Anorganische Chemie der Universität Göttingen,
Tammannstrasse 4, 37077 Göttingen, Germany
E-mail: hroesky@gwdg.de
Homepage: http://www.uni-goettingen.de/en/46240.html
http://www.uni-goettingen.de/de/71797.html
[b] Institut für Anorganische und Angewandte Chemie, Universität
Hamburg
Martin-Luther-King-Platz 6, 20146 Hamburg, Germany
E-mail: birger.dittrich@chemie.uni-hamburg.de
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spectrum. The protons of the SitBu group resonate at δ =
1.40 ppm. The mass spectrum of compound 3 exhibits the
molecular ion at m/z = 600.
Results and Discussion
Compounds LSitBu [L = PhC(NtBu)2] (1) and LSiC-
(SiMe3)3 (2) were prepared as reported in the literature.[7c]
The molecular structure of 1 is shown in Figure 1. Com-
pound 1 crystallizes in the monoclinic space group Pbca.
The silicon atom is three coordinate and features a trigonal
pyramidal geometry, with the lone pair of electrons residing
on the apex. The coordination environment of the silicon is
made up of two nitrogen atoms of the amidinato ligand and
one tertiary carbon atom of the alkyl group. The Si(1)–
C(20) bond length in 1 is 1.9571(10) Å. This is longer than
that in compound [LSitBu(μ-O)]2 [1.9209 (13) Å] reported
in the literature.[7c] The bond length between the chelating
nitrogen atoms and the silicon are Si1–N1 1.8803(8) and
Si1–N3 1.8937(8) Å, respectively. The bite angle N1–Si1–
N3 is 69.10(3)°. The Si atom is shifted out of the plane
defined by a N1–C1–N3 angle of 0.3012(0.0002) Å.
The molecular structure of the [1+2]-cycloaddition prod-
uct 3 was unequivocally established by single-crystal X-ray
structural analysis. Compound 3 crystallizes in the mono-
clinic space group P21/n. The molecular structure is shown
in Figure 2. The three-membered ring of 3 comprises a car-
bon, sulfur, and silicon atom. The silicon atom is five-coor-
dinate, made up from two nitrogen atoms, a sulfur atom,
and two carbon atoms. The structural index τ, which de-
fines the extent of deviation from trigonal-bipyramidal to
square-pyramidal geometry (τ = 1 for perfect trigonal bipy-
ramidal; τ = 0 for perfect square-based pyramid), is 0.32.[15]
The N–Si–N bite angle at the silicon atom of the backbone
amidinate ligand is 68.22(14)°, whereas in 1 it is 69.10(3)°.
There is a slight variation of the Si–C20 bond length of
1.934(5) Å in
3
when compared with that of
1
[1.9571(10) Å]. The angles of the newly formed SSiC three-
membered ring are S1–Si1–C4 [54.32(12)°], Si1–C4–S1
[71.03(14)°], and C4–S1–Si1 [54.64(13)°]. Correspondingly
the bond lengths within the three-membered ring are Si1–
S1 [2.2073(16) Å], S1–C4 [1.896(4) Å], and Si1–C4
[1.904(4) Å]. These values are comparable to those of the
SSiC three-membered ring reported in the literature.[11–13]
Figure 1. Molecular structure of 1. The anisotropic displacement
parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond lengths [Å] and bond
angles [°]: N1–Si1 1.8803(8), N3–Si1 1.8937(8), C20–Si1 1.9571(10);
N1–Si1–N3 69.10(3), N1–Si1–C20 102.37(4), N3–Si1–C20
102.44(4).
Equimolar amounts of LSitBu (1) and 4,4Ј-bis(dimethyl-
amino)thiobenzophenone lead to the [1+2]-cycloaddition
product 3 (Scheme 1). Compound 3 is soluble in benzene
and toluene. It is stable both in the solid state as well as in
solution for a long time without any decomposition under
an inert gas atmosphere.
Figure 2. Molecular structure of 3. The anisotropic displacement
parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond lengths [Å] and bond
angles [°]: N1–Si1 1.839(3), N3–Si1 2.001(3), C20–Si1 1.934(5),
Si1–S1 2.2073(16), S1–C4 1.896(4), Si1–C4 1.904(4); N1–Si1–N3
68.22(14), S1–Si1–C4 54.32(12), Si1–C4–S1 71.03(14), C4–S1–Si1
54.64(13), N1–Si1–C20 110.19(18), N3–Si1–C20 100.96(18), N3–
Si1–S1 149.30(12), N1–Si1–C4 130.10(18).
The reaction of equimolar amounts of LSiC(SiMe3)3 and
3,5-di-tert-butyl-o-benzoquinone afforded the [1+4]-cyclo-
addition product 4 (Scheme 2). Compound 4 is soluble in
common organic solvents. Like 3, compound 4 is stable
both in the solid state as well as in solution. Compound 4
exhibits a double resonance in its 29Si NMR spectrum [δ =
–0.96 C(SiMe3)3 and –16.98 (LSi) ppm]. The protons of the
Scheme 1. Synthesis of compound 3.
The 29Si NMR spectrum of 3 exhibits a single resonance tBu groups, which reside over the nitrogen atoms of com-
at δ = –86.09 ppm, which is shifted upfield compared with pound 4, display a single resonance at δ = 1.32 ppm in the
that of LSitBu (δ = 61.50 ppm).[7c] The protons of the tBu 1H NMR spectrum. We surmise that the single resonance
groups attached to the nitrogen atoms of compound 3 show might be from the coordination of both nitrogen atoms,
1
two resonances (δ = 1.10 and 1.28 ppm) in the H NMR from the amidinato ligand, to the silicon center in solution.
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The SiMe3 protons resonate at δ = 0.58 ppm. Further, com-
pound 4 shows its molecular ion in the mass spectrum at
m/z = 710.
Experimental Section
General: Syntheses were carried out under an inert atmosphere of
dinitrogen in oven-dried glassware using standard Schlenk tech-
niques. All other manipulations were accomplished in a dinitrogen
filled glove box. Solvents were purified by a MBRAUN solvent
purification system MB SPS-800. Compounds 1 and 2 were pre-
pared as reported in the literature.[7c] 1H NMR and 29Si NMR
spectra were recorded with a Bruker Avance DPX 300 or a Bruker
Avance DRX 500 spectrometer, using C6D6 as 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, University of
Göttingen.
Scheme 2. Synthesis of compound 4.
The [1+4]-cycloaddition product 4 was confirmed by sin-
gle-crystal X-ray structural analysis. Compound 4 crys-
tallizes in the triclinic space group P1 and the molecular
structure is depicted in Figure 3. The silicon atom is four-
coordinate, made up of one nitrogen atom, two oxygen
atoms, and a carbon atom. The Si1–C20 bond length is
1.8842(17) Å.
Synthesis of 3: Toluene (60 mL) was added to a Schlenk flask
(100 mL) containing 1 (0.55 g, 1.74 mmol) and 4,4Ј-bis(dimethyl-
amino)thiobenzophenone (0.49 g, 1.72 mmol). The reaction mix-
ture was stirred at room temperature for 6 h. The solvent was re-
duced in vacuo (10 mL) and stored at –26 °C to obtain single crys-
tals of 3 in 1 w. Yield: 0.82 g, 79%. C36H52N4SSi (600.98): calcd.
¯
1
C 71.95, H 8.72, N 9.32; found C 71.87, H 8.66, N 9.31. H NMR
(500 MHz, C6D6, 25 °C): δ = 1.10 (s, 9 H, tBu), 1.28 (s, 9 H, tBu),
1.40 [s, 9 H, Si(CH3)3], 6.33–6.35 (m, ArH), 6.76–7.00 (m, ArH),
7.90–7.92 (m, ArH), 8.09–8.11 (m, ArH) ppm. 29Si{1H} NMR
(99.36 MHz, C6D6, 25 °C): δ = –86.09 (LSi) ppm. EI–MS: m/z (%)
= 600 [M+] (35), 568 [M+ – S] (100), 512 [M+ – 2N(CH3)2] (15), 455
[M+ – 2N(CH3)2 – C(CH3)3] (20), 398 [M+ – 2N(CH3)2 – 2C(CH3)3]
(15).
Synthesis of 4: Toluene (60 mL) was added to a Schlenk flask
(100 mL) containing 2 (0.52 g, 1.06 mmol) and 3,5-di-tert-butyl-o-
benzoquinone (0.29 g, 1.07 mmol). The reaction mixture was
stirred at room temperature for 6 h. The solvent was reduced in
vacuo (10 mL) and stored at 0 °C to obtain single crystals of 4 in
2 w. Yield: 0.58 g, 77%. C39H70N2O2Si4 (711.33): calcd. C 65.85, H
1
9.92, N 3.94; found C 65.79, H 9.83, N 3.86. H NMR (500 MHz,
C6D6, 25 °C): δ = 0.58 [s, 27 H, Si(CH3)3], 1.32 (s, 18 H, tBu), 1.57
(s, 18 H, tBu), 6.80–7.11 (m, ArH) ppm. 29Si{1H} NMR
Figure 3. Molecular structure of 4. The anisotropic displacement
parameters are depicted at the 50% probability level. Hydrogen
atoms and methyl carbon atoms of the tBu groups are omitted
for clarity. Selected bond lengths [Å] and bond angles [°]: N1–Si1
1.7593(14), O4–Si1 1.6781(11), O5–Si1 1.6988(11), C20–Si1
1.8842(17); O4–Si1–O5 94.63(5), O4–Si1–N1 102.95(6), O5–Si1–
N1 107.04(6), N1–Si1–C20 126.67(7), O4–Si1–C20 112.93(7), O5–
Si1–C20 107.86(7).
(99.36 MHz, C6D6, 25 °C):
δ = –0.96 [C(SiMe3)3], –16.98
(LSi) ppm. EI-MS: m/z (%) = 710 [M+] (35), 695 [M+ – CH3] (30),
654 [M+ – C(CH3)3] (100), 639 [M+ – C(CH3)3 – CH3] (70), 598
[M+ – 2C(CH3)3] (15), 583 [M+ – 2C(CH3)3 – CH3] (50).
Crystal Structure Determination: Crystals were taken out of the
mother liquor under an argon atmosphere using NVH oil. Diffrac-
tion data were collected at 100 K with a Bruker three-circle dif-
fractometer equipped with a SMART 6000 CCD area detector and
a Cu-Kα rotating anode. The data sets of 3 and 4 were collected
to the edge of the Ewald sphere with high completeness and high
multiplicity. Raw data were integrated with SAINT[17] and an em-
pirical absorption correction with SADABS[18] 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).[19] SHELXLE[20] was used as refinement GUI. All
non-hydrogen atoms were refined with anisotropic displacement
parameters. Hydrogen atoms were constrained to ride on their par-
ent atom with displacement parameters constrained to 1.2 or 1.5
of the Uiso of their parent atom. (Table 1).
The bond lengths of Si1–O4 and Si1–O5 are
1.6781(11) Å and 1.6988(11) Å, respectively. The bond
length between the silicon and nitrogen atom of the amidin-
ato ligand is 1.7593(14) Å, which is comparable to the
Si–N bond length reported in the literature.[16]
Conclusions
In summary we report on the reactivity of the monoal-
kyl-substituted silylene LSitBu with 4,4Ј-bis(dimethylam-
ino)thiobenzophenone, which afforded the CSiS three-
membered ring compound silathiacyclopropane 3 with a
five-coordinate silicon atom. The reaction of LSiC-
(SiMe3)3 with 3,5-di-tert-butyl-o-benzoquinone leads to the
[1+4]-cycloaddition product 4 exhibiting a four-coordinate
silicon atom.
CCDC-914774 (for 1), -914773 (for 3), and -914775 (for 4) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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FULL PAPER
Table 1. Crystal and Structure Refinement Parameters for Complexes 1, 3, and 4.
1
3
4
Empirical formula
Mr [gmol–1]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C19H32N2Si
316.56
orthorhombic
Pbca
11.5227(2)
18.2842(4)
18.4286(4)
C36H52N4SSi
600.97
monoclinic
P21/n
9.6497(3)
12.2223(4)
29.1719(11)
C39H70N2O2Si4
711.33
triclinic
¯
P1
10.5289(3)
11.5585(3)
19.1660(5)
77.4900(10)°
77.2730(10)
72.2970(10)
2139.14(10)
2
α [°]
β [°]
90.894(2)
γ [°]
V [Å3]
3882.60(14)
8
1.083
3440.2(2)
4
1.160
1.384
Z
ρcalcd. [gcm–3]
1.104
1.532
μ [mm–1]
1.040
F (000)
1392
1304
780
Crystal size [mm]
θ Range for data collection
Limiting indices
0.30ϫ0.20ϫ0.10
4.80 to 73.67°
–14ՅhՅ14;
–22ՅkՅ22;
–22ՅlՅ22
124304
0.09ϫ0.07ϫ0.02
3.03 to 66.58°
–9ՅhՅ11;
–13ՅkՅ11;
–30ՅlՅ31
44704
0.20ϫ0.20ϫ0.15
2.39 to 72.13°
–12ՅhՅ12;
–14ՅkՅ14;
–23ՅlՅ23
68660
Reflections collected
Independent reflections
Completeness to θ
Refinement method
3915 (Rint = 0.0296)
99.9% (θ = 73.67°)
5675 (Rint = 0.1381)
93.1% (θ = 66.58°)
8190 (Rint = 0.0284)
97.3% (θ = 72.13°)
Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2
Data/restraints/parameters
Goodness of fit on F2
Final R indices [IϾ2σ(I)]
R indices (all data)
3915/0/327
1.057
R1 = 0.0317, wR2 = 0.0838
R1 = 0.0317, wR2 = 0.0838
0.375 and –0.218
5675/360/392
1.038
R1 = 0.0740, wR2 = 0.1400
R1 = 0.1291, wR2 = 0.1652
0.455 and –0.407
8190/375/476
1.074
R1 = 0.0424, wR2 = 0.1107
R1 = 0.0432, wR2 = 0.1142
0.615 and –0.365
Largest diff. peak and hole [eÅ–3]
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Acknowledgments
The authors thank the Deutsche Forschungsgemeinschaft (DFG)
for supporting this work. R. A. is grateful to the Alexander von
Humboldt Stiftung for a research fellowship.
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