Selective functionalization of a bis-silylene

Article abstract of DOI:10.1039/c3cc43199a

Functionalization of N-heterocyclic carbenes (NHCs) has an important influence on their stability, Lewis donor, and acceptor properties. In this study, we report on the selective functionalization of a four-membered N-heterocyclic bis-silylene (2,6-Ar₂C₆H ₃NSi:)₂ (1) (Ar = 2,4,6-iPr₃C₆H ₂) with mono-oxygen sources N₂O and Me₃NO. Treatment of 1 with N₂O results in the selective formation of mono-silylene (2,6-Ar₂C₆H₃NSi(OH) ₂)(2,6-Ar₂C₆H₃NSi:) (2) as a major product, along with a small amount of further oxidized product (2,6-Ar ₂C₆H₃NSi(OH)₂)₂ (3). Compound 2 is the first four-membered mono-silylene with a di-coordinate silicon atom.

Full text of DOI:10.1039/c3cc43199a

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Selective functionalization of a bis-silylene†
Rajendra S. Ghadwal,* Ramachandran Azhakar, Kevin Propper,
¨
Birger Dittrich and Michael John
Cite this: Chem. Commun., 2013,
49, 5987
Received 30th April 2013,
Accepted 15th May 2013
DOI: 10.1039/c3cc43199a
www.rsc.org/chemcomm
Functionalization of N-heterocyclic carbenes (NHCs) has an impor- with a formal SiQO bond,13 and silaheterocycles.8,14 Also,
tant influence on their stability, Lewis donor, and acceptor proper- functionalization of a NHC15 has been shown with IPrꢀSiCl2
ties. In this study, we report on the selective functionalization of a (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene).16
four-membered N-heterocyclic bis-silylene (2,6-Ar2C6H3NSi:)2 (1)
A few silicon compounds containing two silylene moieties in
(Ar = 2,4,6-iPr3C6H2) with mono-oxygen sources N2O and Me3NO. a single molecule have been prepared14 using Lewis-base
Treatment of 1 with N2O results in the selective formation of stabilization and are known as bis-silylenes. Like silylenes,
mono-silylene (2,6-Ar2C6H3NSi(OH)2)(2,6-Ar2C6H3NSi:) (2) as
a
bis-silylenes react with different organic substrates to form
major product, along with a small amount of further oxidized silicon compounds in which both Si(II) centers transform into
product (2,6-Ar2C6H3NSi(OH)2)2 (3). Compound 2 is the first four- Si(IV) centers.6–14 To the best of our knowledge, there is no
membered mono-silylene with a di-coordinate silicon atom.
report on the reaction of a bis-silylene, in which only one
silylene moiety is oxidized to a Si(IV) center, thereby maintain-
Organosilicon compounds are valuable precursors in organic ing the other silylene center. This is due to the high reactivity of
and organometallic synthesis, and materials science.1 The these compounds.
availability of new stable compounds with low-valent silicon
Reactions of mono- and bis-silylenes with N2O to form
atoms (e.g., multiple bonded silicon compounds, silylenes and Si(IV) compounds have been reported.7,8 Activation of small
related species), which are otherwise only studied as short-lived molecules by compounds containing low-valent main group
reactive species, facilitates the preparation of new compounds elements that mimic transition metals has attracted atten-
which are difficult to isolate by traditional methods.2,3 The first tion during the last few years.17 Reactions of N2O and com-
stable silene and disilene were reported in 1981 by Brook et al. pounds with low-valent main group elements are quite
and West et al., respectively.4 The West group later succeeded varied. Severin and co-workers have recently reported the
in isolating an N-heterocyclic silylene (NHSi),5 a compound formation of the NHCꢀN2O adduct by the reaction of NHC with
containing a di-valent di-coordinate silicon atom with a lone N2O, which at higher temperature resulted in the formation of a
pair of electrons. This seminal publication sparked further ketone.18
interest in low-valent silicon chemistry and since then a num-
Very recently, we have isolated the first dimeric silaisonitrile9
ber of silylene compounds have been prepared using different (2,6-Ar2C6H3NSi:)2 (1) (Ar = 2,4,6-iPr3C6H2) (Scheme 1), which is
stabilization concepts.6–8 Nevertheless, these stable silylenes stable at room temperature. Compound 1 features a four-
are quite reactive and prone to oxidative addition reactions and membered (Si2N2) ring with two di-valent di-coordinate silicon
lead to formation of Si(IV) compounds upon reaction with atoms, each bearing a lone pair of electrons. Thus, 1 is the first
organic substrates such as alkenes, alkynes, amines, imines, base-free stable bis-silylene. Theoretical studies suggest that 1
ketones, thioketones, alcohols, and azides.6–16 Some of the is a four p-electron antiaromate.9 Therefore, partial oxidation of
interesting silicon compounds that are made available using 1 to a mono-silylene should be favoured due to the formation
silylene precursors include dimeric silaisonitrile,9 silaoxirane,10 of a two p-electron aromate.14 Herein we report on such a
silacyclopropene,11 silaaziridine,12 silacarbonyl compounds species. Selective functionalization of a bis-silylene 1 with N2O
(Scheme 1) gives the mono-silylene (2,6-Ar2C6H3NSi(OH)2)-
¨
¨
¨
Institut fur Anorganische Chemie, Georg-August-Universitat Gottingen,
(2,6-Ar2C6H3NSi:) (2).
¨
Tammannstrasse 4, 37077 Gottingen, Germany. E-mail: rghadwal@uni-goettingen.de
Treatment of a toluene solution of 1 with N2O yielded 2 as a
major product (62%), whereas a further oxidized product, bis-
silanediol 3, was isolated as a minor product (8%). Reaction of
1 with Me3NO afforded only compound 3. Moreover, 2 reacts
† Electronic supplementary information (ESI) available: Experimental procedures
and X-ray crystallographic information of compounds 2 and 3. CCDC 882690 and
882691. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3cc43199a
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Fig. 1 ORTEP-representation of the molecular structure of 2; anisotropic dis-
placement parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted and only ipso-carbons of 2,4,6-triisopropylphenyl groups are
shown for clarity. Selected bond lengths [Å] and angles [1]: Si1–O1 1.6246(12),
Si1–O2 1.6273(11), Si1–N1 1.7322(10), Si1–N2 1.7332(3), Si2–N1 1.7705(10),
Si2–N2 1.7796(10); O1–Si1–O2 111.61(6), O1–Si1–N1 106.78(5), O1–Si1–N2
121.36(5), N1–Si1–N2 88.74(4), N1–Si2–N2 86.09(4).
Scheme 1 Synthesis of compounds 2 and 3.
further with Me3NO to form 3. Compounds 2 and 3 were
obtained as colorless crystals, soluble in common organic
solvents and stable under an inert atmosphere.
mono-silylene was observed from the 29Si NMR spectrum of 2,
The mechanism for the formation of 2 and 3 is at present which shows two resonances at d 111.16 and ꢁ62.42 ppm. The
unknown. Alkyne analogues of germanium and tin react with former resonance is characteristic of the silylene compounds5,7
N2O and yield respective metal(II) hydroxides,19,20 in which the with di-coordinate silicon atoms, whereas the latter is consistent
hydrogen abstraction mechanism by radical type intermediates with those observed for Si(IV) compounds with tetra-coordinate
was suggested. Stable four-membered (E2N2) (E = Si, Ge, or Sn) silicon atoms.7–14 The 29Si NMR spectrum of 3 exhibits reso-
cyclic biradical species are known.21,22 Their unusual stability is nance at d ꢁ68.26 ppm and is consistent with those of tetra-
explained by the presence of heteroatoms in the ring. To coordinate silicon compounds.7–14
ascertain the origin of hydrogen in products 2 and 3, reaction
The molecular structure of compound 2 is shown in Fig. 1.
of 1 with N2O in dry toluene-d8 was carried out. Surprisingly, Compound 2 crystallizes in the monoclinic space group P21/n.
formation of compound 2 was again observed (as evident from Compound 2 features a four-membered Si2N2 ring with di- and
similar NMR and IR spectral data), which was isolated in tetra-coordinate silicon atoms. As observed for compound 1, a
56% yield.23 No signal was observed in the deuterium NMR similar Si system with the Si2N2 core, compound 2 shows
2
spectrum of 2 for OD (D = H) groups, prepared in toluene-d8. dynamic disorder.9 Two main occupancies for the Si2N2 core
This excludes the involvement of radical type intermediates and occurred and were included in our model.24 The Si2–N1
hence hydrogen abstraction from the solvent molecules. There- (1.7705(10) Å) and Si2–N2 (1.7796(10) Å) bond lengths
fore, in the present study, we assume the initial formation of a are longer than the Si1–N1 (1.7322(10) Å) and Si1–N2
silaketone (20) as an intermediate in the reaction of 1 with N2O, (1.7332(3) Å) ones. Similarly, the N1–Si2–N2 (86.09(4)1) bond
(Scheme 1) which upon reaction with residual water23 would angle is smaller than that of N1–Si1–N2 (88.74(4)1). Compound
%
afford compound 2. Similarly, further oxidation of 2 with N2O 3 crystallizes in the triclinic space group P1. Each of the silicon
would yield 30, which upon reaction with residual water would atoms is tetra-coordinate and features a distorted tetrahedral
give 3.
geometry (Fig. 2). The Si2–O3 1.586(10) and Si2–O4 1.598(11)
Silylenes and unsaturated silicon compounds are unstable bond lengths are shorter than those of Si1–O1 1.627(2) and
in protic solvents and react immediately to form Si(IV) com- Si1–O2 1.627(2).
pounds. In view of this reactivity it seems unlikely to be able to
Invariom refinements were performed for 2 and 3, taking
isolate a silicon compound containing a silylene moiety into account non-spherical electron density in the aspherical
together with an O–H functional group within the same mole- scattering factors.24 These refinements lead to improvements
cule. The stability of silylene 2, which bears two hydroxyl in the figures of merit, the physical significance of the aniso-
groups on the Si(IV) silicon, may be explained by steric protec- tropic displacement parameters, and highlight un-modeled
tion of the silylene moiety by the bulky terphenyl groups. features in the residual electron density. The improvement of
Further reactivity studies of 2 with various substrates will be the structural model can be illustrated by comparing the
performed in the future to explore its properties.
residual electron density map from IAM (independent atom
The 1H and 13C NMR spectra of 2 and 3 show the resonances model) and invariom refinement.24 This becomes most obvious
for the terphenyl groups on the nitrogen atoms. Formation of for the phenyl ring as shown in Fig. 3.
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In summary we have presented a convenient method for
selective functionalization of a bis-silylene (1) with N2O. The
selective formation of mono-silylene (2) upon reaction of 1 with
N2O could be explained due to the formation of a two p-electron
aromate. However, reaction of 1 with Me3NO gave the further
oxidized product diaminosilanol (3). Compound 2 features a
silylene and two hydroxyl (OH) functionalities in a single
molecule and may serve as a model for kinetic stability.
R. S. G. is grateful to Prof. Dietmar Stalke for his generous
support. B. D. acknowledges funding from the Deutsche
Forschungsgemeinschaft. The authors thank Prof. Herbert W.
Roesky for important scientific discussion.
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2 (a) P. Jutzi and U. Schubert, Silicon Chemistry: From the Atom 23 All manipulations were carried out under strict anhydrous condi-
to Extended Systems, Wiley-VCH, Weinheim, 2003, pp. 1–494;
(b) N. Auner and J. Weis, Organosilicon Chemistry VI: From Molecules
tions. Solvents and reagents were dried and purified prior to use.
See ESI† for further details.
to Materials, Wiley-VCH, Weinheim, 2005, pp. 1–998; (c) M. Veith, 24 See ESI†.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 5987--5989 5989