From silicon(II)-based dioxygen activation to adducts of elusive dioxasiliranes and sila-ureas stable at room temperature

Article abstract of DOI:10.1038/nchem.666

Dioxygen activation for the subsequent oxygenation of organic substrates that involves cheap and environmentally friendly chemical elements is at the cutting edge of chemical research. As silicon is a non-toxic and highly oxophilic element, the use of silylenes could be attractive for facile dioxygen activation to give dioxasiliranes with a SiO 2 -peroxo ring as versatile oxo-transfer reagents. However, the latter are elusive species, and have been generated and studied only in argon matrices at -233 °C. Recently, it was demonstrated that unstable silicon species can be isolated by applying the concept of donor-acceptor stabilization. We now report the first synthesis and crystallographic characterization of dioxasiliranes stabilized by N-heterocyclic carbenes that feature a three-membered SiO 2 -peroxide ring, isolable at room temperature. Unexpectedly, these can undergo internal oxygen transfer in toluene solution at ambient temperature to give a unique complex of cyclic sila-urea with C=O → Si=O interaction and the shortest Si=O double-bond distance reported to date.

Full text of DOI:10.1038/nchem.666

ARTICLES
PUBLISHED ONLINE: 30 MAY 2010 | DOI: 10.1038/NCHEM.666
From silicon(II)-based dioxygen activation to
adducts of elusive dioxasiliranes and sila-ureas
stable at room temperature
1
1
2
2
1
¨
Yun Xiong , Shenglai Yao , Robert Muller , Martin Kaupp and Matthias Driess
*
Dioxygen activation for the subsequent oxygenation of organic substrates that involves cheap and environmentally friendly
chemical elements is at the cutting edge of chemical research. As silicon is a non-toxic and highly oxophilic element, the
use of silylenes could be attractive for facile dioxygen activation to give dioxasiliranes with a SiO2–peroxo ring as versatile
oxo-transfer reagents. However, the latter are elusive species, and have been generated and studied only in argon matrices
at 2233 8C. Recently, it was demonstrated that unstable silicon species can be isolated by applying the concept of donor–
acceptor stabilization. We now report the first synthesis and crystallographic characterization of dioxasiliranes stabilized
by N-heterocyclic carbenes that feature a three-membered SiO2–peroxide ring, isolable at room temperature.
Unexpectedly, these can undergo internal oxygen transfer in toluene solution at ambient temperature to give a unique
complex of cyclic sila-urea with C5O ꢀ Si5O interaction and the shortest Si5O double-bond distance reported to date.
ctivation of dioxygen is important in nature and in industrial
Recently, the concept of donor–acceptor stabilization has proved
chemistry, as shown by the key role of oxygenation in bio- useful in the isolation of highly reactive main-group element com-
A
chemistry and in the production of value-added compounds. pounds. Striking examples include the isolation of disilicon (:Si¼Si:)
Although nature employs metalloenzymes that contain redox-active stabilized by N-heterocyclic carbene (NHC)22. In this context, we syn-
metal sites for dioxygen activation1, artificial chemical systems that thesized an isolable silaformamide–borane complex23, starting from a
involve electronically and coordinatively unsaturated centres2–6 or stable N-heterocyclic silylene24. Initially, we described the first iso-
metal nanoparticles7 as mediators were developed in academia and lation of a silanoic ester25. Very recently, we reported on NHC-
industrial laboratories and are able to convert dioxygen into much stabilized silanones (sila-ureas) that resulted from the facile oxygen-
more reactive superoxo (O22) and peroxo (O222) compounds for ation of the NHC-activated silylenes 1a and 1b with N2O (refs 26,27).
direct oxygenation. Among non-metal peroxides, organic derivatives The remarkably high oxophilicity of the silicon(II) atoms in 1
of H2O2 (for example, organohydroperoxides or cyclic organoperox- prompted us to probe whether they can activate dioxygen to give
ides) are well established as selective oxidants8. In line with this, diox- the isolable dioxasilirane adducts 2a and 2b. We now report for
iranes with a cyclic CO2–peroxo moiety represent powerful oxidants the first time the synthesis and crystallographic characterization of
towards unsaturated organic substrates that have numerous appli- elusive dioxasiliranes stabilized in the form of the NHC complexes
cations9,10. The latter achievements prompted a great deal of interest 2a and 2b and isolable at ambient temperature. Additionally, we
in heavier main-group element analogues of dioxiranes with a three- show that 2a undergoes a striking internal oxygen transfer in solution
membered EO2 ring (E ¼ Si, Ge, Sn and Pb) as potential oxygen- to give the unique cyclourea ꢀ sila-urea adduct 3.
transfer reagents. However, exploration of such systems as reactive
intermediates turned out to be rather difficult because of their intrinsic Results and discussion
thermolability11–13. Accordingly, the silicon analogues of dioxiranes, In spite of the sterically encumbered environment around silicon,
that is, dioxasiliranes I (Fig. 1) have received particular attention the NHC-activated, highly nucleophilic silicon(II) atoms in
because of their role as transient species in several photochemical precursors 1a and 1b reacted smoothly with dioxygen, even at low
and thermal transformations14–18. Dioxasiliranes I seem particular temperatures, to afford the first dioxasilirane adducts 2a and 2b,
appealing as oxidants because they should be accessible easily by the
direct addition of dioxygen onto silylenes, the silicon analogues of car-
O
O
O
benes, owing to the enormous oxophilicity of silicon. In fact, a few
derivatives of I have been synthesized by dioxygen activation with tran-
sient silylenes, which occurs even at very low temperatures (below
2233 8C) in argon matrices18–21. However, their pronounced thermo-
lability means they are elusive and could be characterized only by
means of spectroscopy. In other words, because of its instability a diox-
asilirane has not yet been isolated as a pure compound. Also, these
species can undergo internal oxo transfer to give the corresponding
silanoic esters II (Fig. 1) which, in turn, are also transient species
R
R
O2
Si
Si
Si
R
R
R
OR
I
II
Figure 1 | Transient dioxasiliranes I and their rearrangement to silanoic
esters II. I was generated by side-on addition of dioxygen onto a silylene
precursor. II resulted from internal oxygen transfer (oxygen insertion) of
diorgano-substituted dioxasiliranes I into a Si–C bond (R ¼ organo
or halogen).
because of the intrinsic high reactivity of the Si¼O double bond19–21
.
1Institute of Chemistry: Metalorganics and Inorganic Materials, Technische Universita¨t Berlin, Strasse des 17. Juni 135, Sekr. C2, D-10623 Berlin, Germany,
2Institute of Physical and Theoretical Chemistry, Department of Chemistry and Pharmacy, Universita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg,
Germany. e-mail: matthias.driess@tu-berlin.de
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.666
ARTICLES
Allowing the hypothetical free dioxasilirane to relax to a tetrahedral
SiO2N2-type structure after removal of the NHC donor from 2 pro-
vided
a
computed relaxation energy of 230 kJ mol21 (see
N
N
Supplementary Fig. S7). This indicates a large influence of the
NHC ligand on the overall silicon-coordination environment.
Solutions of 2a in toluene can be stored for several days at
220 8C without significant change. At room temperature,
however, the compound underwent internal oxygen transfer to
give the unique adduct 3 (Fig. 2), in which one of the oxygen
atoms in the SiO2–peroxo moiety in 2a was transferred internally
into the Si1–C30 (NHC) bond. The conversion occurred almost
quantitatively for 2a in benzene at ambient temperature. Similarly,
2b is thermolabile in solution at room temperature, but it gave a
mixture of as yet unidentified products.
N
R
N
N
N
R
N
N
R'
R'
R'
R'
O
2a
R
O2
N
N
O
O
Toluene
Si
R
Si
R
Si
R
O
N
N
3
1a R' = i-Pr
1b R' = Me
2a R' = i-Pr
2b R' = Me
Figure 2 | Synthesis of 2a, 2b and 3. Compounds 2a and 2b were obtained
from dioxygen activation by NHC-supported silylene precursor 1a and 1b,
respectively. Compound 3 was obtained by internal oxygen transfer of 2a
into the C ꢀ Si dative bond. R ¼ 2,6-i-Pr2C6H3.
Compound 3, in the form of colourless crystals, precipitated
readily from the reaction solution at room temperature and was iso-
lated in 72% yield. Its composition was proved by elemental analysis,
1
respectively (Fig. 2). The conversion of the precursors was evident and the constitution is consistent with the H, 13C and 29Si NMR
by gradual decolouration of the yellow solutions at 260 8C. and infrared spectroscopic data (see Supplementary Information). The
1
Subsequent concentration of the reaction solution and cooling at 29Si{ H} NMR spectrum of 3 displayed a singlet at d ¼ 277.1 ppm,
220 8C furnished colourless crystals of the desired NHC-stabilized which is comparable to that observed for the related NHC-stabilized
dioxasiliranes 2a and 2b in 79% and 69% yield, respectively, and silanones (cyclic sila-urea derivatives)26,27. A single-crystal XRD
both were stable at room temperature.
analysis revealed that 3 represents a unique cyclourea ꢀ sila-urea
Their compositions were corroborated by mass spectrometry and adduct, in which the oxygen atom of the cyclourea coordinates to
elemental analyses, and the constitutions are consistent with the 1H the silicon centre of the sila-urea moiety (Fig. 4).
and 29Si NMR data and infrared spectroscopy (see Supplementary
The silicon centre in 3 adopts a strongly distorted tetrahedral
Information). As a result of the pentacoordination of the silicon coordination, reminiscent of the situation in the related NHC-
1
atom, the 29Si{ H} NMR spectra show high-field singlets at stabilized cyclic sila-ureas26,27. Thus, the sum of the bond angles
d ¼ 2131.9 (2a) and 2133.3 ppm (2b). Single-crystal X-ray diffrac- for the X3Si subunit (X ¼ O1, N1 and N2) of 339.48 is slightly
tion (XRD) analyses proved that 2a and 2b are isostructural (Fig. 3). larger than the corresponding values of NHC-stabilized sila-ureas
The O2 ligand is ‘side-on’ coordinated to the tetravalent silicon atom and the C30–O2–Si1 angle is 145.4(2)8. The Si1–O1 distance of
and lies almost within the plane of the C3N2Si ring. Additionally, the 1.532(2) Å in 3 is most notable, as it is the shortest Si–O distance
Si atom binds to the respective NHC ligand and achieves an unex- in a molecular Si¼O compound hitherto reported23,25–27. The rela-
pected square-pyramidal coordination. The Si1–C30 (NHC) dis- tively long Si1–O2 (1.727(2) Å) bond and the short O2–C30
tance of 1.963(3) Å in 2a is about 0.1 Å shorter than that in the distance (1.294(3) Å) are in accordance with a dative interaction
precursor 1a and indicates dative bonding from the NHC ligand. between the cyclourea and the cyclic sila-urea moieties. This is
The Si1–N1 and Si1–N2 distances (1.767(2) and 1.799(2) Å, supported well by quantum-chemical calculations of the model
respectively) in 2a are a little shorter than those in 1a (1.794(2)
and 1.820(2) Å, respectively). The O–O distances of 1.547(3) Å in
2a and 1.510(3) Å in 2b, respectively, are a little longer than that
C32
C31
in dimesityldioxirane (1.50 Å) (ref. 28) and than those observed
for related ‘side-on’ metal peroxo complexes (1.4–1.5 Å) (ref. 29),
but shorter than those calculated for ‘donor-free’ dioxasiliranes
R'
N3
(1.57–1.81 Å) (refs 11,12,18). Similarly, the Si–O distances in 2a
N4
R'
(1.682(2) and 1.693(2) Å) and 2b (1.667(2) and 1.692(2) Å) are
slightly longer than those calculated for dioxasiliranes with tetra-
coordinate silicon atoms (1.632–1.678 Å) (ref. 11). Obviously,
these differences originate from the higher coordination number
at silicon. In addition, we carried out quantum-chemical calcu-
lations (using density functional theory; see the Supplementary
Information for computational details) on different model com-
pounds. The computed energy of the fragmentation of 2 into diox-
asilirane and NHC fragments was 25 kJ mol21
(B3LYP/TZVPP//RI-BP86/TZVP), which underlines the dative
character of this C ꢀ Si bond.
C30
C5
N2
C4
O2
O1
C3
Si1
C2
N1
C1
A square-pyramidal configuration is rare for pentacoordinate
silicon compounds, so we wanted to learn whether the unusual
coordination geometry of silicon in 2 is caused by steric substituent
effects. Thus, the structure of the parent system in which all substi-
tuents were replaced by hydrogen atoms was also optimized fully.
Although the orientations of the NHC and dioxygen ligands
change somewhat, the overall coordination around silicon remains
square pyramidal to a large extent. This indicates that the origin
of the structural preference is electronic rather than steric, as is
the case for related pentacoordinated spirobicyclic silicates with
Figure 3 | Oak Ridge Thermal-Ellipsoid Plot (ORTEP) of the isostructural
molecules 2a (R′ 5 i-Pr) and 2b (R′ 5 Me) in the crystal. Thermal
ellipsoids are drawn at 50% probability level. H atoms are omitted for clarity.
Selected distances (Å) and angles (8) of 2a: Si1–O1, 1.682(2); Si1–O2,
1.693(2); Si1–N2, 1.767(2); Si1–N1, 1.799(2); Si1–C30, 1.963(3); O1–O2,
1.547(3); O1–Si1–O2, 54.6(1); N2–Si1–N1, 98.5(1); O1–Si1–C30, 103.9(1);
O2–Si1–C30, 101.2(1). Selected distances (Å) and angles (8) of 2b: Si1–O1,
1.667(2); Si1–O2, 1.692(2); Si1–N2, 1.781(2); Si1–N1, 1.777(2); Si1–C30,
1.933(3); O1–O2, 1.510(3); O1–Si1–O2, 53.4(1); N2–Si1–N1, 98.8(1);
O1–Si1–C30, 104.1(1); O2–Si1–C30, 99.1(1).
silicon bonded to one carbon and four oxygen atoms30,31
.
578
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.666
ARTICLES
C32
C31
N3
N4
O2
C30
O2
C5
C4
N2
Si
O1
Si1
C3
O1
C2
N1
C1
Figure 6 | ELF representation (ELF surface 5 0.80 isosurface) of the
Si–O1 versus C–O2 and C–O2 ꢀ Si bonding of the model compound 3′
(B3YLP/TZVPP//RI-BP86/TZVP). The polar Si–O1 bond and the even more
polar dative C–O2 ꢀ Si bond may be contrasted to the more covalent C–O2
bond. ELF areas outside the O1–Si–O3 moiety are removed for clarity.
Figure 4 | ORTEP of the molecular structure of 3 in the crystal. Thermal
ellipsoids are drawn at 50% probability level. H atoms are omitted for clarity.
Selected distances (Å) and angles (8): Si1–O1, 1.532(2); Si1–O2, 1.727(2);
Si1–N2, 1.732(2); Si1–N1, 1.744(2); O2–C30, 1.294(3); N1–C2, 1.413(3);
N2–C4, 1.410(3); C2–C3, 1.460(4); C3–C4, 1.341(4); C31–C32, 1.346(4);
O1–Si1–O2, 112.20(9); N2–Si1–N1, 102.7(1); C30–O2–Si1, 145.4(2).
bonding picture of 3 as a complex between a cyclic sila-urea and a
cyclourea donor, as depicted in the resonance structure 3A in Fig. 5.
Additionally, the large ionicity of the Si¼O bond in 3 and in
model compounds is clearly apparent from ELF plots (Fig. 6 and
compound L′(NHC¼O)Si¼O (3′), in which the 2,6-i-Pr2C6H3 Supplementary Information), and supports the role of the second
groups at nitrogen in the C3N2 ligand are replaced by methyl sub- main resonance form 3B, as shown in Fig. 5. ELF is a real-space
stituents and the organic groups in the cyclourea moiety by hydro- function closely related to electron pairing and allows us also to
gen atoms (see Supplementary Information). The bonding visualize successfully the bonding in strongly delocalized cases
characteristics of 3′ were analysed as for the model systems (see Supplementary Information references). Essentially, the ELF
L′Si¼O (4), (H2N)2Si¼O (5) and H2Si¼O (6) within the framework domain for Si–O1 bonding is merged with the large O1 lone-pair
of natural resonance theory (NRT; see Supplementary Information). type domain, consistent with a very polar bond. The ELF surface
Compared to the simplest Si¼O reference model 6, delocalization for the C¼O2 ꢀ Si donor interaction exhibits an even larger polar-
reduces the covalent Si–O bond order in the sequence 5, 4, 3′, as ization towards the O2 atom (Fig. 6; note, in comparison, the ELF
confirmed by the NRT bond orders (see Supplementary domain for the more covalent C¼O2 bond), consistent with the
Table S5). At the same time, the ionic contribution increases, general dative-bonding picture developed above. Single-point calcu-
which overall leads to relatively similar bond orders. The simpler lations (B3LYP/TZVPP//RI-BP86/TZVP) on 3′ provide a binding
Wiberg bond indices (see Supplementary Table S4) decrease, in energy of 56 kJ mol21 between the cyclourea and the sila-urea, again
agreement with the reduced covalency. In all cases, a partial mul- in agreement with a largely dative character of the C¼O ꢀ Si¼O
tiple-bonding character remains present for the Si¼O1 bond.
interaction.
In conclusion, the first isolable adducts 2a and 2b of transient
Furthermore, the NRT bond orders, the Wiberg bond indices
and plots of the electron localization function (ELF) (see Fig. 6 dioxasiliranes stable at ambient temperature were prepared
and Supplementary Figs S5 and S6) support the coordinative through facile oxygen activation with the nucleophilic NHC-
nature of the C¼O2 ꢀ Si interaction and thus the general activated silylenes 1a and 1b. Strikingly, the five-coordinate silicon
atoms in 2a and 2b adopt a square-pyramidal geometry, seldom
observed for pentacoordinated silicon compounds, and feature a
‘side-on’ coordinated peroxo ligand. Remarkably, 2a undergoes
internal oxygen transfer in toluene solution at ambient temperature
N
N
to afford solely the first cyclourea ꢀ cyclic sila-urea adduct 3 with a
unique C¼O ꢀ Si¼O dative interaction and the shortest Si¼O
distance reported to date. These results highlight the importance
of donor–acceptor stabilization for the isolation of elusive silicon–
oxygen species isolable even at room temperature. The facile
access to ‘bottleable’ dioxasiliranes 2 could pave the way to novel,
mild and aprotic oxygenation reagents in organic synthesis.
Respective investigations are in progress.
N
N
O
O
R
R
N
N
N
N
–
Si
O
Si
O
R
R
3A
3B
Figure 5 | Resonance structures 3A and 3B. These represent two of the
leading mesomeric forms in accordance with results from density functional
theory calculations and NRT. Structure 3A shows the dative C¼O ꢀ Si¼O
interaction and 3B depicts that the Si¼O moiety in 3 bears significant
ylide character. (R ¼ 2,6-i-Pr2C6H3).
Methods
General considerations. All experiments and manipulations were carried out under
dry nitrogen using standard Schlenk techniques or in an MBraun drybox that
contained an inert atmosphere of purified nitrogen. Solvents were deoxygenated and
dried by standard methods, saturated with purified nitrogen and freshly distilled
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.666
ARTICLES
prior to use. The precursor complexes 1a (ref. 27) and 1b (ref. 26) were prepared
according to literature procedures. The 1H, 13C and 29Si NMR spectra were recorded
on Bruker ARX200 and AV 400 spectrometers. High-resolution electrospray
ionization mass spectra (ESI-MS) were measured on a Thermo Scientific LTQ
orbitrap XL. ESI-MS analyses were carried out with a Finnigan-MAT 955
instrument. The infrared spectra were taken on a Nicolet Magna 750 spectrometer
with nitrogen-gas purge. Elemental analyses were performed on a FlashEA 1112
CHNS Analyser.
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glass capillary in perfluorinated oil and measured in a cold nitrogen flow. The data
for compounds 2a, 2b and 3 were collected on an Oxford Diffraction Xcalibur S
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riding model. In 2b the toluene solvent molecule was disordered and refined with
restraints for the anisotropic displacement parameters. In 3 one of the isopropyl
groups was disordered and refined with restraints for the anisotropic displacement
parameters. CCDC 731581 (2a), 750427 (2b) and 731579 (3) contain the
supplementary crystallographic data for this paper.
Computational methods. For our calculations different models of compounds 2
and 3 were used. Crystal structure data were taken as a starting point for full-
structure optimizations of 2 and 3 without restrictions. For models 2′ and 3′ all
residues of the NHC fragment were replaced by hydrogen atoms and the aryl groups
of the dioxasilirane or cyclic sila-urea moiety, respectively, were substituted by
methyl groups (see Supplementary Fig. S4). Only the substituted groups and all
hydrogen atoms were optimized subsequently, with all other coordinates fixed at the
crystal structure data. To be able to compare the specific bonding characteristics of 2
and 3 with related species of the sila-urea type, the parent sila-urea fragment 4 (and
model 4′, in which the 2,6-i-Pr2C6H3 substituents at nitrogen were replaced by
methyl groups) and the much simpler model systems silaformaldehyde H2Si¼O (6)
(studied recently by Gusel’nikov and co-workers33) and sila-urea (H2N)2Si¼O (5)
(also studied extensively by Epping et al.34) were taken into account as reference
structures. These were optimized fully. For all optimizations, we used the BP86
functional in conjunction with the resolution-of-identity and a TZVP basis set.
Further information on subsequent single-point calculations for energetics and
wave-function analyses is given in the Supplementary Information.
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See also the Supplementary Information for detailed experimental conditions
and procedures, and analytical data for the novel compounds 2a, 2b and 3.
Received 19 October 2009; accepted 9 April 2010;
published online 30 May 2010
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This research was supported by the Deutsche Forschungsgemeinschaft (DR226/17-1) and
the Cluster of Excellence ‘Unifying Concepts in Catalysis’ sponsored by the Deutsche
Forschungsgemeinschaft and administered by the Technische Universita¨t Berlin. We thank
A. Company for experimental assistance. Work in Wu¨rzburg was supported by Deutsche
Forschungsgemeinschaft within priority programme SPP1178 ‘Experimental Electron
Density as the Key to Understand Chemical Interactions’ (KA1187/9-2).
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Author contributions
M.D. conceived and designed the concepts and experiments. Y.X. and S.Y. carried out the
experiments. S.Y. collected and solved the XRD data. R.M. and M.K. designed and
carried out the quantum-chemical work. M.D. and M.K. co-wrote the manuscript.
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The authors declare no competing financial interests. Supplementary information and
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chemical compound information accompany this paper at www.nature.com/
naturechemistry. Reprints and permission information is available online at http://npg.nature.
com/reprintsandpermissions/. Correspondence and requests for materials should be
addressed to M.D.
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