Easy access to silicon(0) and silicon(II) compounds

Article abstract of DOI:10.1021/ic400357p

Two different synthetic methodologies of silicon dihalide bridged biradicals of the general formula (Lⁿ?)₂SiX ₂ (n = 1, 2) have been developed. First, the metathesis reaction between NHC:SiX₂ and Lⁿ: (Lⁿ: = cyclic akyl(amino) carbene in a 1:3 molar ratio leads to the products 2 (n = 1, X = Cl), 4 (n = 2, X = Cl), 6 (n = 1, X = Br), and 7 (n = 2, X = Br). These reactions also produce coupled NHCs (3, 5) under C-C bond formation. The formation of the coupled NHCs (L^m = cyclic alkyl(amino) carbene substituted N-heterocyclic carbene; m = 3, n = 1 (3) and m = 4, n =2 (5)) is faster during the metathesis reaction between NHC:SiBr₂ and L ⁿ: when compared with that of NHC:SiCl₂. Second, the reaction of L¹:SiCl₄ (8) (L¹: =:C(CH ₂)(CMe₂)₂N-2,6-iPr₂C ₆H₃) with a non-nucleophilic base LiN(iPr)₂ in a 1:1 molar ratio shows an unprecedented methodology for the synthesis of the biradical (L¹?)₂SiCl₂ (2). The blue blocks of silicon dichloride bridged biradicals (2, 4) are stable for more than six months under an inert atmosphere and in air for one week. Compounds 2 and 4 melt in the temperature range of 185 to 195 C. The dibromide (6, 7) analogue is more prone to decomposition in the solution but comparatively more stable in the solid state than in the solution. Decomposition of the products has been observed in the UV-vis spectra. Moreover, compounds 2 and 4 were further converted to stable singlet biradicaloid dicarbene-coordinated (L ⁿ:)₂Si(0) (n = 1 (9), 2 (10)) under KC₈ reduction. Compounds 2 and 4 were also reduced to dehalogenated products 9 and 10, respectively when treated with RLi (R = Ph, Me, tBu). Cyclic voltametry measurements show that 10 can irreversibly undergo both one electron oxidation and reduction.

Full text of DOI:10.1021/ic400357p

Article
pubs.acs.org/IC
Easy Access to Silicon(0) and Silicon(II) Compounds
Kartik Chandra Mondal, Prinson P. Samuel, Mykyta Tretiakov, Amit Pratap Singh, Herbert W. Roesky,*
A. Claudia Stuckl, Benedikt Niepotter, Elena Carl, Hilke Wolf, Regine Herbst-Irmer, and Dietmar Stalke*
̈
̈
Institut fur Anorganische Chemie, Georg-August-Universitat, Tammannstraβe 4, 37077-Gottingen, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: Two different synthetic methodologies of silicon
dihalide bridged biradicals of the general formula (Ln•)2SiX2 (n = 1,
2) have been developed. First, the metathesis reaction between
NHC:SiX2 and Ln: (Ln: = cyclic akyl(amino) carbene in a 1:3 molar
ratio leads to the products 2 (n = 1, X = Cl), 4 (n = 2, X = Cl), 6 (n
= 1, X = Br), and 7 (n = 2, X = Br). These reactions also produce
coupled NHCs (3, 5) under C−C bond formation. The formation of
the coupled NHCs (Lm = cyclic alkyl(amino) carbene substituted N-
heterocyclic carbene; m = 3, n = 1 (3) and m = 4, n =2 (5)) is faster
during the metathesis reaction between NHC:SiBr2 and Ln: when
compared with that of NHC:SiCl2. Second, the reaction of L1:SiCl4 (8) (L1: =:C(CH2)(CMe2)2N-2,6-iPr2C6H3) with a non-
nucleophilic base LiN(iPr)2 in a 1:1 molar ratio shows an unprecedented methodology for the synthesis of the biradical
(L1•)2SiCl2 (2). The blue blocks of silicon dichloride bridged biradicals (2, 4) are stable for more than six months under an inert
atmosphere and in air for one week. Compounds 2 and 4 melt in the temperature range of 185 to 195 °C. The dibromide (6, 7)
analogue is more prone to decomposition in the solution but comparatively more stable in the solid state than in the solution.
Decomposition of the products has been observed in the UV−vis spectra. Moreover, compounds 2 and 4 were further converted
to stable singlet biradicaloid dicarbene-coordinated (Ln:)2Si(0) (n = 1 (9), 2 (10)) under KC8 reduction. Compounds 2 and 4
were also reduced to dehalogenated products 9 and 10, respectively when treated with RLi (R = Ph, Me, tBu). Cyclic voltametry
measurements show that 10 can irreversibly undergo both one electron oxidation and reduction.
Few 1,3-biradicals of cyclobutane type were synthesized,
isolated, and characterized.6 However, they suffer from low
yield and stability. We have recently developed a unique
synthetic route for air stable 1,3-biradical (L1·)2SiCl2 (2)7a
(Scheme 1) and 1,3-biradicaloid (L1:)2Si (9)7b (Scheme 2) (L1:
=:C(CH2)(CMe2)2N-2,6-iPr2C6H3). Compound 2 was synthe-
sized by reacting NHC:SiCl2 (1a) (NHC = N-heterocyclic
carbene8a) with a cyclic alkyl(amino) carbene8b (cAAC: = Ln:, n
= 1 and 2) in a 1:3 molar ratio, and 9 was prepared by reducing
2 with 2 equiv of KC8. However, the overall syntheses of 2 and
9 involve precursor NHC:SiX2 (1) and as a result of that more
1. INTRODUCTION
Radicals1 and biradicals2 are very reactive chemical species
which can act as important intermediates in many chemical and
biological processes. They are known1a as persistent and stable
isolable highly reactive species with odd-electron centers which
are stabilized by either thermodynamic or kinetic control.1,2
There are several known stable carbon and silicon centered
biradicals.2b Among them, carbon centered 1,3-biradicals are
sometimes proposed as very important reactive intermediates in
some chemical reactions.3 Unfortunately, isolation and
chemical characterization of such species are most often
restricted because of their short lifetime and high reactivity.3,4
Carbon centered 1,3-biradicals were generated under photolysis
of azoalkanes below 10 K and were characterized by electron
paramagnetic resonance (EPR) spectroscopy in a frozen
matrix.5 These 1,3-biradicals with short lifetime convert into
other products depending upon the electronic ground state
(singlet or triplet).5e The interactions between the radical
centers were investigated by ab initio calculations which
revealed a complex interplay between through-bond and
through-space communication.5e The triplet ground state is
possible when both effects are similar in magnitude while the
singlet ground state can be more stable when one of the effects
is significantly dominant over the other.5e Further theoretical
investigations5f,g on SiX2 (X = H, Me, F, OR, etc.) bridged
cyclopentane-1,3-diyls predicted to possess a persistent singlet
ground state under conditions without using a trapping agent.5g
Scheme 1. General Metathesis Route for Compounds 2−7
Received: February 12, 2013
Published: March 25, 2013
© 2013 American Chemical Society
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of THF to the mixture of Ln: and NHC:SiBr2 (1b) resulted in a
similar blue solution and immediate crystalline blue products 6
and 7 are formed but quickly changed to green and then to
light yellow solutions. The properties are different when
compared to those of 2 and 4. This shows that compounds 6
and 7 are not as stable as 2 and 4. We tried to recrystallize the
blue microcrystalline powder of 6 from THF/n-hexane to form
single crystals for X-ray diffraction. However, decomposition of
6 was always encountered in solution although it was isolated as
a microcrystalline solid which is stable in an inert atmosphere in
the solid state. Furthermore, the NMR and UV−vis spectra
were recorded before 6 decomposes. Compound 7 is
moderately stable, and large blue blocks of 7 were obtained
in 6% yield. The reaction is performed in 3.25:1 molar ratio in
THF and immediately after the reaction, the solvent was
removed under vacuum and extracted with n-hexane from
which dark greenish-blue blocks of 7 were formed within 2−3
h. These crystals are stable in solution for a day, but they slowly
disappear after one to two days. Meanwhile colorless big blocks
of 5 appear which could be manually separated because of their
appreciable size and difference in color than those of 7. Blue
blocks of 7 are stable in air for about 5 min, and they also
decompose to a colorless unidentified solid in an inert
atmosphere after 2−3 days.
SiX2 (X = H, Me, F, OR) bridged cyclopentane-1,3-diyls are
theoretically predicted to possess a persistent singlet ground
state.5g Our experimental results on bis-cyclic alkyl(amino)
carbene based SiX2 1,3-biradicals (X = Cl (2, 4) and Br (6, 7))
showed that the stability of these biradicals depends on the
nature of X on the silicon center. More electronegative chloride
substituents on the silicon center provide better stability to the
biradical state than does its congener bromide. At this point we
wanted to generalize whether the most electronegative fluorine
1,3-biradical analogue is the most stable one. Thus, we chose 2
and 4 as precursor and treated them with Me3SnF to replace
chlorine by fluorine atoms. In general, the color of the reaction
solution changed from dark blue to intense violet which
indicated the progress of the reaction. Unfortunately our efforts
to isolate the silicon difluoride bridged 1,3-biradical remained
elusive. A decomposition was encountered both at room
temperature and even at 0 °C to −32 °C. Finally a colorless
solid was obtained after 2 weeks. Our observation is in line to
some extent with theoretical prediction.5g
Scheme 2. Syntheses of Compounds 9 and 10
laborious and time-consuming work is needed. In this context it
is worth mentioning that compounds with low valent silicon
were generally synthesized under reduction of siliconhalides
with KC8 by Kira,9a Robinson,9b Filippou,9c and others.9d We
developed the synthetic procedures of compounds with low
valent silicon under NHC9d,e or lithium bis(trimethylsilyl)-
amide9e mediated elimination of HCl from the silicon center.
Also sodium and potassium metals can be utilized for the
reduction, but they frequently suffer from low yield of the
products.10 Herein we report on some unprecedented synthetic
methodologies for biradicals (2, 6, 7, 4) and biradicaloids (9,
10).
2. RESULTS AND DISCUSSION
We have shown that the reaction of 2 equiv of NHC with 1
equiv of HSiCl3 produces NHC:SiCl2 1a under the
precipitation of the NHC·HCl salt.9d We have examined
several reactions of 1.11 It has been shown that cAAC (Ln:) is
more nucleophilic and as well as electrophilic when compared
with that of NHC because of the replacement of one σ-
electron-withdrawing and π-donating N-R group of NHC by a
non π-donating and σ-electron-donating carbon.12 cAAC
possesses a singlet spin ground state, and the energy gap
between the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) is smaller
when compared with that of NHC. This might play a crucial
role between their reactivities. To extend our ongoing
chemistry we have reacted a colorless solution of Ln: (n = 1,
2) with the light yellow solution of 1a. The color of the solution
immediately turned to a dark blue solution of 1,3-biradical
(Ln·)2SiCl2 (n = 1 (2) and 2 (4)). Coupled NHCs (3 (n = 1)
and 5 (n = 2)) are formed as side products under C−H bond
activation and C−C bond formation. Compounds 3 and 5 are
not formed by direct reaction between cAACs and NHCs
(Scheme 1). Compounds 2 and 4 are always formed
independently of the molar ratio used (1:1 or 2:1 or 3:1) of
cAAC and NHC:SiCl2. However, the yield depends on the
ratio. The yield of 4 is 46% and 81% if the ratio of cAAC and
NHC:SiCl2 is 2:1 and 2.7:1, respectively. Compounds 2 and 4
can be crystallized both from n-hexane and tetrahydrofuran
(THF). In the solid state 2 does not contain any lattice solvent
molecules while 4 crystallizes either as 4·n-hexane or as 4·THF
depending on the solvent used during the crystallization.
Before it was mentioned that SiX2 (X = H, Me, F, OR, etc.)
bridged carbon centered 1,3-biradicals were theoretically
modeled, but they remained experimentally difficult to prepare
until recently.7a However, to study the stability of silicon
dibromide bridged dicarbene (1,3-biradical), Ln: was reacted
with NHC:SiBr2 (1b)9c in a 3:1 molar ratio to obtain
(Ln·)2SiBr2 [n = 1 (6) and n = 2 (7)] (Scheme 1). Addition
Next we investigated the effect of electron donating
substituents (aryl or alkyl) on the stability of the 1,3-biradical
(Ln·)2SiX2 (X = Ph, Me, tBu). To substitute both the chlorine
atoms of (Ln·)2SiCl2 (2/4), the latter was reacted with 2 equiv
t
of RLi (R = Ph, Me, and Bu) in THF. Surprisingly, for each
reaction dehalogenated biradicaloid dicarbene-coordinated
(Ln:)2Si(0) (9/10) was obtained (Scheme 2). The maximum
yield of 72% was recorded for 4 when the most basic tBuLi was
employed. The yields of 58% and 35% were experimentally
recorded for MeLi and PhLi. The reaction of 4 with 2 equiv of
PhLi produced compound 10 and LiCl and Ph-Ph as
byproducts. We assumed that the reaction proceeds via a
radical pathway. This was confirmed by elemental and
spectroscopy analysis. We did not isolate phenylchloride as
one of the byproducts (see Supporting Information).
We have previously reported that (L1·)2SiCl2 (2) can be
reduced to (L1:)2Si (9) in 95% yield when treated with KC8 in
THF within 2−3 h.7b Similarly (L2:)2Si (10) was synthesized in
85% yield (Scheme 2). To examine the role of the solvent the
reduction of 2/4 to 9/10 with KC8 was performed in n-hexane
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instead of THF. The reaction was found to be extremely slow
and did not proceed to completion even after one week.
Unreacted KC8 was recovered, suggesting the more polar THF
to be a superior solvent over non polar n-hexane for the
reduction.
formation of the two electron sharing C−Si single bonds over
two C→Si coordinate bonds. Based on our experience biradical
4 could not be prepared following the similar procedure as
shown in Scheme 4. When the reaction was carried out with
another equivalent of LiN(iPr)2 the formation of L1:2Si (9) was
not observed. Compound 2 was smoothly reduced with RLi
(Ph, Me, tBu) to 9.
Previously, our theoretical investigations7b showed that
compound 9 possesses a biradicaloid character because of the
low HOMO−LUMO energy gap. The HOMO-1 is a lone pair
orbital at the silicon atom where the HOMO is a π-type obital
which has the largest extension at the silicon but exhibits
significant Si−C π-bonding. The latter distributes 30% on each
carbon center and the rest of 40% on the silicon. The NBO
value of 0.55 e of silicon was determined from a high level
theoretical simulation that suggests a formal oxidation state of
zero of the silicon atom in 9 is more likely rather than a Si(II).
This value is in good agreement with the difference in the
electronegativities of carbon and silicon. On the basis of the
structural evidence and theoretical findings, resonance
structures 9′/10′ of 9/10 can be suggested (Scheme 3).
3. STABILITY AND DECOMPOSITION STUDIES OF 4, 9,
AND 10
Blue plates of 4 retain their blue color for a week in air like 2.
But a powder sample of 4 is stable in air for 2 to 3 days and
afterwards hydrolyzes to the corresponding L2:H+Cl− salt and
SiO2. On exposure to air compounds 9/10 are less stable when
compared with their precursor 2/4. The dark blue plates of 9/
10 retain their blue color for 5 to 6 h and afterwards slowly
transform into colorless solids. The time of complete
disappearance of the blue color was recorded to be about 24
h for the crystals and 1 h for the powder samples of 9/10.
1H and 13C NMR spectra showed that 9/10 undergo aerial
oxidation at the carbene carbon centers to their corresponding
N-aryl amide derivatives 11/12 along with the formation of
SiO2. When N2O gas was passed through the THF solution of
9 for 1 h, compound 11 was obtained in 93% yield (Scheme 5).
a
Scheme 3. Resonance Structures of 9 and 10
Scheme 5. Aerial Decomposition of 9 and 10
a
The schematic representation drawn in the left formula shows the
results of a 3-center two-electron π bond by DFT calculations.7
As we have mentioned before, syntheses of the important
compounds 2/4 and 9/10 involve NHC:SiCl2 (1) as a
precursor of SiCl2 and thus alternative synthetic pathways are
extremely demanding because of time and labor economic
reasons. It is well established that Ln: is more nucleophilic and
electrophilic in nature, and LiN(iPr)2 (LDA) is known as a non
nucleophilic base. In this context we reacted the previously
isolated L1:SiCl4 (8) with LDA. During the addition of THF to
the 1:1 mixture of L1:SiCl4 and LiN(iPr)2 an immediate
formation of a dark blue solution of (L1·)2SiCl2 (2) was
observed (Scheme 4). The reaction was completed within 1 h.
4. SPECTROSCOPIC ANALYSIS
The 13C NMR spectra of the carbene carbon atoms of cAACs
Ln: exhibit resonance at 304.2 ppm (n = 1) and 309.4 ppm (n =
2).8b Similar resonances for the 1,3-biradicals 4, 6, and 7 were
not observed because of the presence of one unpaired electron
on each carbene carbon atom. A resonance at δ = 210.8 ppm
was obtained for 10 which is similar to that of 9.7b The 13C
NMR spectra of 11/12 show resonances at δ = 179.1 ppm (11)
and 178.7 ppm (12) which are corroborated with CO.
Medically and pharmaceutically relevant compounds 1-phenyl-
pyrrolidin-2-one and 4,5-cis-pyrrolidinone show resonances at δ
= 174.2 and ∼176 ppm, respectively.13a,b The 29Si NMR
measurements of compounds 4, 6, and 7 are silent, while the
corresponding resonances of 9 and 10 were observed at δ =
66.71 ppm7b and 71.15 ppm. The UV−vis spectra of
compounds 2, 4, 6, 7, 9, and 10 were recorded in n-hexane
(see Supporting Information). The maxima of absorption bands
of 2, 4, 6, and 7 range from 565 to 585 nm while the
corresponding maxima of 9 and 10 were observed in the range
of 565 to 620 nm.
Scheme 4. Alternative Synthetic Route for Compound 2
The evaporation of the solvent followed by extraction of 2 with
n-hexane affords 2 as blue blocks in 48% yield upon slow
evaporation of the solvent under vacuum. LDA usually acts as a
base or a nucleophile but not as a reducing agent. To the best
of our knowledge such a chemical route for the reductive
synthesis of 1,3-biradical by LDA is unprecedented in silicon
chemistry.
5. CYCLIC VOLTAMMETRY
After the reaction was completed, we isolated the volatile
SiCl4 utilizing a cooling trap under vacuum. Based on our
observations the following reaction sequence given in Scheme 4
can be proposed. The driving force for this reaction is the
Since siladicarbene (9/10) contains four electrons on the
silicon atom, it was intriguing to investigate whether a
dichloromethane solution of 10 undergoes any reversible
electronic oxidation/reduction. Consequently a solution of 10
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was placed in the cyclic voltammeter in an inert atmosphere.
The cyclic voltammetric measurements show that 10 can
irreversibly undergo both one electron oxidation (+2.0 V) and
reduction (−0.65 V) (see Supporting Information).
6. ELECTRONIC GROUND STATE
We have demonstrated that the most stable electronic ground
state of 1,3-biradical 2 is a singlet (S = 0, polymorph II).7a
Magnetic and EPR measurements on 4 showed that 4 also
possesses a singlet electronic ground state (see Supporting
Information). Compound 10 is EPR silent confirming a singlet
ground state like that of biradicaloid 9.
7. CRYSTAL STRUCTURE COMPARISON
Figure 2. Superimposed cAACs of 2 (blue) and a free carbene
Compound 27a crystallizes in the monoclinic space group C2/c
(red).16.
(Figure 1). The central silicon atom exhibits a distorted
Figure 1. Molecular structure of compound 2.
tetrahedral coordination geometry (C1−Si1−C21 122.99(7)°).
The Si−Ccarbene bonds (1.8455(16)/1.8482(17) Å) in 2 are
significantly shorter than in the tricoordinated 1 (1.985(4)
Å).9d They are also shorter than a Si−Caryl bond (1.879 Å)14
but longer than a SiC double bond (1.702−1.775 Å).15
In comparison to the N−Ccarbene bonds in the similar free
carbene16 the bonds in 2 are slightly elongated (Table 1). This
implies that the double bond character of the N−C bond is less
pronounced and is further substantiated by the sum of angles of
355.5° and 355.3° in 2 compared to 360.0° and 359.9° in the
free carbene (Figure 2).
Figure 3. Molecular structures of compounds (a) 4·0.5 n-hexane and
(b) 7·0.5 n-hexane.
distances in 4·0.5 n-hexane/7·0.5 n-hexane are 1.854(2)/
1.8585(18) Å and 1.843(2)/1.8497(18) Å.7a Similarly Si−Cl/
Si−Br bond distances of 4/7 are 2.0701(8)/2.2405(6) Å and
2.0663(8)/2.2375(6) Å. These values are in good agreement
with the data reported in the literature.7a,9c The only significant
The isostructural compounds 4 (Figure 3a) and 7 (Figure
3b) crystallize both in the monoclinic space group P21/n with
half a molecule of n-hexane in the asymmetric unit as lattice
solvent (see Supporting Information). The Si−Ccarbene bond
a
Table 1. Selected Bond Lengths of Compounds 2, 4, 7, 9, 10, L2 BH, and the Free Carbenes (L1: and L2:)
2
2
4·0.5 S
7·0.5 S
9/2nd molecule
10
L1:16
L2:16
L22BH17
N1−C1 [Å]
1.3994(19)
1.400(2)
1.399(2)
1.378(2)/
1.374(2)
1.3819(16)
1.312
1.315
1.377
N2−C21/C24 [Å]
C1−N1−C4 [deg]
C21/24−N2−C24/C27 [deg]
1.395(2)
1.403(2)
112.04(15)
111.9(2)
1.398(2)
1.378(2)/
1.378(2)
1.3726(16)
116.04(10)
115.25(10)
1.390
111.81(12)
112.14(12)
112.17(14) 115.89(13)/
115.99(13)
118.74
118.41
116.39
115.63
111.87(14) 115.97(13)/
115.87(13)
sum of angles at N1 [deg]
355.5
355.3
19.8
355.7
354.8
19.3
355.6
354.7
19.6
359.9/359.9
360.0/359.9
2.3/2.9
359.9
359.9
2.9
360.0
1.8
359.9
2.1
359.1
359.1
5.0
sum of angles at N2 [deg]
angle between C5 and C1−N1−C4
angle between C25/C28 and C21/C24−N1−C24/27
20.3
21.2
21.6
1.6/2.2
3.0
5.3
a
S = n-hexane. The tables of bond lengths and bond angles of compounds 4, 5, 7, and 10 are given in the Supporting Information.
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difference to 2 is a slightly bigger C21/C24−Si1−C1−N1
torsion angle of 106.71(18) (4) and 107.03(16) (7),
respectively, compared to 2 (96.12(14)° (Figure 4).
Figure 4. Superimposed molecular structure of compounds 2 (red)
and 4 (blue).
When compounds 2, 4, and 7 are compared, the angle
between the two cAAC as well as the geometry at the nitrogen
atoms is completely different. The N1−C1−Si1−C21/C24
torsion angle in 9 is 163.12(11)° and in 10 167.15(9)° while
the torsion angle in 2 is much smaller (96.13(14)°). The
geometry at the nitrogen atom in the cAAC in 9 and 10 is
nearly planar. The sum of angles is in good agreement with the
sum in the similarly built free carbenes (Table 1). This leads to
a deviation of the C25/C28 atom of just 1.6 to 3.0° from the
C21/C24−N1−C24/C27 plane. In 2 this is 20.3°. Following
this the nitrogen carbene carbon bonds are no longer elongated
like in 2. They are more similar to the shorter bonds in the free
carbenes or in L2 BH17 for which Bertrand et al. assumed a
2
donor−acceptor bond. Thus the bonding situation in 9 and 10
has to be considered also as a donor−acceptor bond between
the cAACs and a silicon atom of formal oxidation state zero. A
complete list of the crystallographic data and all bond lengths
and angles is given in the Supporting Information.
Compound 97bcrystallizes in the triclinic space group P1 with
̅
two molecules and one n-hexane lattice solvent molecule in the
asymmetric unit (Figure 5a). Compound 10 also crystallizes in
Figure 5. Molecular structures of (a) 9 and (b) 10. (c) Superposition
of the molecular structures of 9 (blue) and 10 (red).
the triclinic space group P1 but with only one molecule in the
̅
asymmetric unit and without any lattice solvent (Figure 5b).
However, the molecular structures of both are similar (Figure
5c).
SiX2 (X = Cl (2, 4), Br(6, 7)) was reacted with Ln: to produce
(Ln·)SiX2. Yields of compound 2 (91%) and 4 (85%) are very
high when compared with those of 6 (20%) and 7 (6%). The
chlorine substituents on the silicon atom induced high stability
when compared with that of the two other congeners bromine
(6, 7) and fluorine. The blue blocks of biradicals 2 and 4 are
stable in air for 1 week. The powder samples are stable for 2−3
days and hydrolyze to the Ln:H+Cl− salt. Compound 2 was also
synthesized reacting the L1:SiCl4 adduct with LiN(iPr)2 in
moderate yield. Ln· of compounds 2 and 4 can activate a C−H
bond and undergo C−C coupling reaction to produce the
coupled Ln-NHC (3,5) carbenes at room temperature.
Dehalogenated singlet siladicarbene biradicaloids (Ln:)2Si (9,
10) were obtained when the singlet biradical (Ln•)2SiCl2 (2, 4)
was reacted with RLi (R = Ph, Me, tBu). The highest yield of
72% was obtained when tBuLi was used. Compounds 2 and 4
were also conventionally reduced to 9 and 10 when reacted
with 2 equiv of KC8. Cyclic voltammetry measurements show
that 10 can irreversibly undergo both one electron oxidation
and reduction. Compounds 9 and 10 were oxidized to N-aryl
amides 11 and 12, respectively when N2O gas was passed
The Si−Ccarbene bond distances of 9 (C1−Si1 1.8411(18) Å,
C21−Si1 1.8417(17) Å, C41−Si2 1.8471(17) Å, and C61−Si2
1.8482(17) Å) and 10 (C1−Si1 1.8407(13) Å and C24−Si1
1.8531(14) Å) are quite similar to the bond lengths found in 2
(C1−Si1 1.8455(16) Å and C21−Si1 1.848(2) Å), but slightly
shorter than the calculated bond length for the (NHC)2Si
(1.869 Å).18a,b This could be explained by the fact that the
cAAC is a much better π-acceptor than the NHC. The C−Si−
C angles are 117.70(8)° and 117.18(8)° for 9 and 118.16(6)°
for 10. Thus the C−Si−C backbone is significantly bent, which
indicates that 9 and 10 should not be rationalized as allene type
structures. This is also supported by the angle between the five-
membered rings (26.3° and 26.7°) that clearly differs from 90°,
expected for that type of structure. Theoretical calculations on
R2CSiCR2 showed that 2-silaallenes have a linear structure
in contrast to 9 and 10.18b,c
8. CONCLUSION
We have developed novel methodologies for the synthesis of
silicondihalide bridged dicarbene biradicals. The base stabilized
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dark blue solution which was stirred for 12 h. The dark blue crystalline
precipitate of 4 was separated by filtration. The filtrate was dried and
extracted with n-hexane (60 mL). The volume of n-hexane solution
was reduced to 10 mL and stored at 0 °C in a refrigerator to form
colorless blocks of 5 in 50% yield after 1 week. Elemental analysis
found % (calcd) for C50H71N3; C 84.15 (84.09), H 10.20 (10.02), N
5.75 (5.88). 1H NMR (298 °C, THF-d8, δ ppm, 200.13 MHz): 7.37−
6.99 (m, 9H, Har), 4.20 (s, 0.5H, H), 3.70 (s, 0.5H, H), 3.97 (m, 2H,
CHMe2), 3.10 (m, 2H, CHMe2), 2.95 (m, 2H, CHMe2), 2.40−1.72
(m, 4H, cyclohexyl), 1.54−0.75 (m, 51H); EI MS: 713.
through the solution of 9 and 10. Compounds 9 and 10
exposed to air also produced the same products 11 and 12,
respectively.
9. EXPERIMENTAL SECTION
All reactions and handling of reagents were performed under an
atmosphere of dry nitrogen or argon using standard Schlenk
techniques or a glovebox where the O2 and H2O levels were usually
kept below 1 ppm. Ligand Ln: and NHC:SiCl2 were prepared
according to literature methods.8a,b,9a LiN(iPr)2, SiCl4, MeLi, PhLi,
and tBuLi were purchased from Sigma Aldrich and used as received.
KC8 was prepared by reacting K metal with graphite in argon
atmosphere at 200 °C. Solvents were purified with the M-Braun
solvent drying system. Solution NMR spectra were recorded on
Bruker Avance 200, Bruker Avance 300, and Bruker Avance 500 MHz
NMR spectrometers. Deuterated NMR solvent 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 analyses were performed by the Analytisches Labor des
Synthesis of (L1·)2SiBr2 (6): Compound 6 was synthesized
following the procedure of 4. A molar mixture (4:1) of L1: (1.3 g, 4
mmol) and L:SiBr2 (1b)9c (0.576 g, 1 mmol) was placed into a 50 mL
round-bottom flask. After addition of THF (10 mL) at room
temperature to the colorless mixture, an immediate color change to
blue was observed. The blue product 6 was separated by immediate
filtration in 20% yield. Elemental analysis found % (calcd) for
C40H62Br2N2Si, 6; C 63.15 (63.31), H 8.30 (8.23), N 3.65 (3.69). Mp
1
180−182 °C, decomp. point 190 °C, UV λab = 583 nm. H (298 °C,
THF-d8, δ ppm, 500.130 MHz): 7.12 (m, 2H, m-Har), 7.09 (m, 2H, m-
Har), 7.04 (m, 2H, p-Har), 3.59 (m, 2H, CHMe2), 3.48 (m, 2H,
CHMe2), 1.93 (s, 6H, NCMe2), 1.87 (s, 4H, CH2), 1.77 (s, 6H,
NCMe2), 1.57 (s, 6H, CMe2), 1.55 (d, 6H, CHMe2, J = 6.66 Hz,), 1.47
(d, 6H, CHMe2, J = 6.66 Hz), 1.30 (d, 6H, CHMe2, J = 6.53 Hz), 1.14
(d, 6H, CHMe2, J = 6.65 Hz,), 0.79 (s, 6H, CMe2); 13C (δ ppm):
143.3, 127.3, 125.7, 124.2, 78.5, 67.4, 58.6, 29.1, 28.5, 27.7, 27.4, 26.8,
26.3, 25.3, 24.8. 29Si NMR (δ ppm); no resonance was observed.
Synthesis of (L2:)2SiBr2 (7): Compound 7 was synthesized
following a similar procedure to that used for 6. A molar mixture
(3.25:1) of L2: (1.056 g, 3.25 mmol) and L:SiBr2 (1b) (0.576 g, 1
mmol) was placed into a 50 mL round-bottom flask. After addition of
THF (10 mL) at room temperature to the colorless mixture, an
immediate color change to dark green-blue was observed. The mixture
was immediately dried under vacuum and extracted with n-hexane (20
mL) and filtered. The green-blue filtrate produced greenish blue plates
of 5 in 6% yield. The large dark green-blue plates of 7 were manually
separated inside the glovebox. Elemental analysis found % (calcd) for
C46H70Br2N2Si; C 65.76 (65.85), H 8.48 (8.41), N 3.45 (3.34). Mp
Instituts fur Anorganische Chemie der Universitat Gottingen. Melting
̈
̈
̈
points were measured in sealed glass tubes on a Buchi B-540 melting
̈
point apparatus. See reference 7a for the synthesis of (L1·)2SiCl2 (2)
and L3: (3).
Alternative synthetic method of (L1·)2SiCl2 (2): A molar mixture
(1:1) of L1:SiCl4 (8) (455 mg, 1 mmol) and LiN(iPr)2 (107 mg, 1
mmol) was placed into a 50 mL round-bottom flask. After addition of
THF (50 mL) at room temperature to the colorless mixture, an
immediate color change to dark blue was observed. Stirring continued
for 1 h, and THF was evaporated applying a vacuum. The resulting
blue solid was extracted with n-hexane (30 mL) to obtain a dark blue
solution which was stored at 0 °C in a refrigerator to form blue blocks
1
of 2 in 48% yield (based on carbene) after 2−3 days. H NMR (298
°C, THF-d8, δ ppm, 500.133 MHz): 7.12 (m, 2H, m-Har), 7.09 (m,
2H, m-Har), 7.04 (m, 2H, p-Har), 3.59 (m, 2H, CHMe2), 3.43 (m, 2H,
CHMe2), 1.89 (s, 6H, NCMe2), 1.85 (d, 2H, CH2, J = 12.38 Hz), 1.69
(s, 6H, NCMe2), 1.65 (d, 2H, CH2, J = 12.40 Hz), 1.57 (s, 6H, CMe2),
1.50 (d, 6H, CHMe2, J = 6.66 Hz), 1.39 (d, 6H, CHMe2, J = 6.67 Hz),
1.29 (d, 6H, CHMe2, J = 6.62 Hz), 1.14 (d, 6H, CHMe2, J = 6.80 Hz),
0.85 (s, 6H, CMe2); 13C NMR (δ ppm): 210.0 (C), 143.3, 127.5,
127.1, 125.5, 124.1, 73.5, 67.7, 58.4, 30.2, 29.1, 28.5, 27.7, 27.3, 26.3,
26.0, 25.4, 25.1.
1
183−185 °C, decomp. point 192 °C, UV λab = 583 nm. H NMR
(THF-d8, 200 MHz, 298 K) 7.22−7.08 (m, 6H, Har), 3.07 (m, 2H,
CHMe2), 2.81 (m, 2H, CHMe2), 2.40−1.80 (m, 4H, CH2 (cyclo-
hexane)), 1.75−1.57 (m, 2H, CH2(cyclohexane)), 1.67 (d, 2H, CH2, J
= 12.34 Hz), 1.42−1.20 (m, 8H, CH2(cyclohexane)), 1.21 (d, 2H,
CH2, J = 12.20 Hz), 1.23−1.27 (m, 6H, CH2(cyclohexane)), 1.28 (d,
6H, CHMe2, J = 6.66 Hz,), 1.20 (d, 6H, CHMe2, J = 6.64 Hz), 1.17 (s,
6H, NCMe2), 1.13 (d, 6H, CHMe2, J = 6.66 Hz), 1.09 (d, 6H,
CHMe2, J = 6.54 Hz,), 0.82 (s, 6H, NCMe2); 13C NMR (δ ppm):
141.2, 125.5, 124.9, 124.2, 66.3, 51.2, 45.0, 31.8, 30.5, 29.5, 28.1, 26.8,
26.5, 26.2, 25.9, 25.6, 25.1, 24.8 and 22.6; 29Si NMR (δ ppm): no
resonance signal was recorded.
Synthesis of 4: Compound 4 was synthesized following the
previously mentioned procedure of 2.7a A 3:1 mixture of 1a (1.630
g, 3.34 mmol) and L2: (3.25 g, 10 mmol) was placed in a 100 mL
round-bottom flask. THF (20 mL) was added at room temperature.
The color of the solution immediately turned to dark blue. On stirring
the solution for 10 min dark blue crystalline precipitate of 4 was
obtained, which was separated by filtration. The yield of the 4 is 85%.
Compound 4 was further purified by recrystallization from n-hexane.
Single crystals of 4 were formed from THF or n-hexane. The yield of 4
decreased to 81% and 46% when the reaction was performed in 2.7:1
and 2:1 molar ratio, respectively. Elemental analysis found % (calcd)
for C46H70Cl2N2Si; C 73.58 (73.66), H 9.30 (9.40), N 3.78 (3.73). Mp
Synthesis of L1:SiCl4 (8): It was prepared following a similar
procedure9b using THF as a solvent.
Synthesis of (L1:)2Si (9): See reference 7a for the synthesis.
Synthesis of (L2:)2Si 10: The 1:2 mixture of (L2·)2SiCl2 (4) (585
mg, 0.78 mmol) and KC8 (112 mg, 1.57 mmol) was placed into a 100
mL round-bottom flask. THF (50 mL) was cooled to −78 °C and
added to the mixture which was stirred for 6 h at room temperature
and filtered. The dark blue filtrate was dried under vacuum to obtain a
solid mass which was extracted with n-hexane (60 mL). The volume of
the final solution was reduced to 5−7 mL and stored at 0 °C in a
refrigerator to form dark blue plates of 10 in 85% yield after 3−4 days.
1
189−191 °C, decom. point 195 °C, UV λab = 582 nm. H (C6D6,
500.133 MHz, δ ppm): 7.15−7.05 (m, 6H, Har), 3.75 (m, 2H,
CHMe2), 3.56 (m, 2H, CHMe2), 2.52−2.44 (m, 4H, CH2 (cyclo-
hexane)), 2.11−2.09 (m, 2H, CH2(cyclohexane)), 2.01−1.99 (d, 2H,
CH2, J = 12.44 Hz), 1.86−1.72 (m, 8H, CH2(cyclohexane)), 1.67−
1.65(d, 2H, CH2,
J = 11.98 Hz), 1.60−1.43 (m, 6H,
1
CH2(cyclohexane)), 1.51 (d, 6H, CHMe2, J = 6.67 Hz), 1.40 (d,
6H, CHMe2, J = 7.39 Hz), 1.37 (s, 6H, NCMe2), 1.35 (d, 6H,
CHMe2), 1.17 (d, 6H, CHMe2, J = 6.43 Hz), 0.78 (s, 6H, NCMe2);
13C NMR (δ ppm): 143.8, 126.8, 125.3, 124.4, 67.7, 51.0, 44.0, 32.0,
30.5, 29.3, 27.9, 26.9, 26.7, 26.3, 25.9, 25.7, 25.0, 24.9 and 23.0; 29Si
NMR (δ ppm): no resonance signal was recorded.
Mp (range) 194−195 °C, decomp. point 215 °C. H NMR (C6D6,
500.130 MHz, δ ppm): 7.21−7.07 (m, 6H, Har), 3.18 (m, 2H,
CHMe2), 2.93−2.85 (m, 2H, CH2(cyclohexane)), 2.66 (m, 2H,
CHMe2), 2.55−2.50 (m, 2H, CH2(cyclohexane)), 2.19−2.16 (d, 2H,
CH2, J = 12.57 Hz), 1.88−1.86 (d, 2H, CH2, J = 12.66 Hz), 1.81−1.70
(m, 10H, CH2(cyclohexane)), 1.65−1.55 (m, 2H, CH2(cyclohexane)),
1.49 (d, 6H, CHMe2, J = 6.41 Hz), 1.40−1.32 (m, 4H,
CH2(cyclohexane)), 1.31 (d, 6H, CHMe2, J = 6.67 Hz), 1.22 (d,
6H, CHMe2, J = 6.64 Hz), 1.16 (s, 6H, NCMe2), 1.01 (d, 12H,
Synthesis of L4: (5): A 3:1 molar mixture of 1 (1.630, 3.34 mmol)
and L2: (3.25g, 10 mmol) was placed in a 100 mL round-bottom flask.
THF (20 mL) was added at room temperature to form an immediate
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NCMe2 plus CHMe2). 13C NMR (δ ppm): 210.8, 148.7, 148.4, 136.2,
125.4, 124.9, 68.7, 53.4, 50.0, 44.0, 35.6, 30.2, 30.1, 29.5, 28.3, 27.9,
27.5, 25.3, 25.2, 24.4, 24.3, 23.6, 22.9; 29Si NMR (δ ppm): 71.15.
Alternative methods for the synthesis of 10.7b
To a solution of 4 (150 mg, 0.2 mmol) in THF (20 mL), tert-butyl
lithium in pentane (1.7 M, 0.29 mL, 0.5 mmol) was added slowly at
−78 °C. The solution was slowly warmed to room temperature and
stirred again for 6 h. THF was removed under vacuum, and the
product was extracted with n-hexane and stored at −18 °C to obtain
10 as a crystalline product in 72%.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data (CIF), UV−visible spectra, magnetic
properties, EPR spectra, cyclic voltammetry. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
10 was synthesized in 58% yield following the similar procedure as
mentioned above utilizing 4 (150 mg, 0.2 mmol) in THF (20 mL),
methyl lithium in diethyl ether (1.6 M, 0.31 mL, 0.5 mmol).
10 was synthesized in 35% yield following the similar procedure as
mentioned above utilizing 4 (150 mg, 0.2 mmol) in THF (20 mL),
phenyl lithium in dibutyl ether (1.9 M, 0.26 mL, 0.5 mmol).
Decomposition of 9 and 10: The dark blue powders of compounds
9 and 10 undergo oxidation to the corresponding colorless cyclic N-
aryl amides (11 and 12) and SiO2 on exposure to air for 1 h.
Compounds 11 and 12 are extracted with n-hexane and characterized
by NMR spectroscopy. The SiO2 was separated by filtration.
Synthesis of 11: Compound 9 (0.2 mmol, 120 mg) was placed into
a 50 mL round-bottom flash, and THF (30 mL) was added to obtain a
blue solution. N2O gas was bubbled through the solution for 1.5 h to
obtain a colorless solution which was dried under vacuum and
extracted with n-hexane (30 mL) and concentrated to 1−2 mL. The
solution was stored at 0 °C in a refrigerator for 1 week to obtain
needles of 11 in 93% yield: Mp 118−119 °C; C, H, and N found %
(calcd) for C20H31NO, 80.65 (79.86), H 10.12 (10.36), N 4.51 (4.65);
IR (KBr, cm−1) 3040, 2964, 2924, 2868, 1681 (CO), 1460, 1388,
1365, 1261, 1095, 1061, 801; 1H NMR (C6D6, 300.130 MHz, δ ppm):
7.20−7.12 (m, 3H, Har), 3.11 (m, 2H, CHMe2), 1.72 (s, 2H, CH2),
1.29 (s, 6H, NCMe2), 1.26 (d, 6H, CHMe2, J = 6.78 Hz), 1.22 (d, 6H,
CHMe2, J = 5.55 Hz), 0.97 (s, 6H, CMe2) ; 13C NMR (δ ppm): 179.1,
149.2, 131.7, 128.8, 124.1, 59.6, 54.6, 50.6, 39.9, 29.5, 29.2, 27.9, 26.4,
22.6; EI MS 301.
Compound 12 was synthesized following a similar procedure like
that of 11. Yield 85%. Mp 122−123 °C; C, H, and N found % (calcd)
for C23H35NO, C 79.91 (80.88), H 10.10 (10.33), N 4.05 (4.10); IR
(KBr, cm−1) 3035, 2966, 2920, 2829, 2868, 1682 (CO), 1461, 1387,
1364, 1262, 1095, 1062, 800; 1H NMR (C6D6, 300.132 MHz, δ ppm):
7.25−7.19 (m, 3H, Har), 3.11 (m, 2H, CHMe2), 1.75 (s, 2H, CH2)
2.10−1.95 (m, 2H, cyhexane), 1.65−1.56 (m, 4H, cyclohexane), 1.55−
139 (m, 4H, cyclohexane), 1.25 (d, 6H, CHMe2, J = 9.16 Hz), 1.20 (d,
6H, CHMe2, J = 6.27 Hz), 0.99 (s, 6H, NCMe2); 13C NMR (δ ppm):
178.7, 149.2, 131.5, 128.8, 124.6, 124.1, 62.1, 60.0, 53.5, 47.1, 38.8,
29.6, 29.01, 28.2, 27.8, 22.6; EI MS 341.
Crystal Structure Determination. Suitable single crystals for X-
ray structural analysis of compounds 4, 5, 7, and 10 were mounted at
low temperature in inert oil under argon atmosphere by applying the
X-Temp2 device.19 The data were collected on a Bruker D8 three
circle diffractometer equipped with a SMART APEX II CCD detector
and an INCOATEC Mo microfocus source with INCOATEC Quazar
mirror optics (7) and a Bruker TXS Mo rotating anode with
INCOATEC Helios mirror optics (4, 10).20 The data were integrated
with SAINT,21 and a semiempirical absorption correction with
SADABS22 was applied. The structure was solved by direct methods
(SHELXS-97) and refined against all data by full-matrix least-squares
methods on F2 (SHELXL-2012).23 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 1.5 Ueq of their pivot atoms
for terminal sp3 carbon atoms and 1.2 times for all other carbon atoms.
Compound 4 showed disorder of one of the cAACs substituents. This
disorder was also observed in 4•THF (4a). 7 showed a disordered
hexane lattice solvent molecule. The disordered residues and
molecules were resolved and modeled successfully using distance
restraints and restraints for the anisotropic displacement parameters.
For details of the crystal structure determination of (L1·)2SiCl2 (2) and
L3: (3) see reference 7a and for (L1·)2Si (9) see reference 7b.
*E-mail: hroesky@gwdg.de (H.W.R.), dstalke@chemie.uni-
goettingen.de (D.S.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by the Deutsche Forschungsgemein-
schaft (RO 224/60-1). We thank Prof. F. Meyer and Dr. S.
Demeshko for magnetic and CV investigations and T. Bayer, K.
Dalle ,and F. Klinke for UV−visible measurements. This work
is dedicated to Professor Oliver Reiser on the occasion of his
50th birthday.
ABBREVIATIONS
■
NHC, N-heterocyclic carbene; cAAC:, cyclic alkyl(amino)
carbene; LDA, lithium diisopropylamide
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