Conversion of a singlet silylene to a stable biradical

Article abstract of DOI:10.1002/anie.201204487

Silicon becomes colored: Stable biradicals were prepared from an N-heterocyclic carbene stabilized SiCl₂ and a cyclic alkyl(amino)carbene, and characterized as two polymorphs. The deep-blue crystals of one polymorph are stable upon exposure to air for about a week, while the solution in THF decomposes rapidly when exposed to air. In a side reaction, the different carbene species react with each other under C-H activation and C-C bond formation in the presence of the biradical. Copyright

Full text of DOI:10.1002/anie.201204487

Angewandte
Chemie
DOI: 10.1002/anie.201204487
Biradicals
Conversion of a Singlet Silylene to a stable Biradical**
Kartik Chandra Mondal, Herbert W. Roesky,* Martin C. Schwarzer, Gernot Frenking,*
Igor Tkach,* Hilke Wolf, Daniel Kratzert, Regine Herbst-Irmer, Benedikt Niepçtter, and
Dietmar Stalke*
Dedicated to Professor Heribert Offermanns on the occasion of his 75th birthday
Radicals are probably the most important reactive species in
organic chemistry and in biology.[1] The best known stable
triplet biradical is the oxygen molecule, which contains two
radical centers. The low reactivity of this molecule is due to
the forbidden nature of the triplet–singlet spin transition.
Singlet biradicals of main group elements have been reported
by Niecke,[2] Power,[3] Lappert,[4] Bertrand,[5] and Sekiguchi.[6]
In most cases, these localized singlet biradicals have non-
carbon skeletons that are based on four-membered ring
motifs. In contrast, stable triplet biradicals of main group
elements are rare. Recently Sekiguchi et al.[7] reported
a triplet diradical with two silicon atoms prepared by
reduction of a meta-substituted bissilyl phenyl derivative
with KC8. Power[8] and Lee et al. reviewed persistent stable
free radicals of Group 14.[9] Grꢀtzmacher et al.[10] reported on
radicals of main-group elements, Rajca et al.[11] described
singlet–triplet bistability and Roques et al.[12] investigated the
exchange in silole-bridged biradicals. Neese et al.[13] studied
the interaction of radical pairs through bonds and through
space. Thus, biradicals remain one of the central topics to be
studied because of their high reactivity and exciting physical
properties. However, to the best of our knowledge, stable
biradicals prepared from singlet silylene precursors stabilized
by N-heterocyclic carbenes have not been reported. There-
fore, biradicals are a challenging central topic for future
investigations because of their high reactivity and most
promising properties.
Herein, we report on the reaction of LDSiCl2 (1),[14]
prepared from NHC (LD = N-heterocyclic carbene, DC[N(2,6-
iPr2C6H3)CH]2) and HSiCl3 (Scheme 1), with L1D (L1D = DC-
Scheme 1. Synthesis of L1D2SiCl2 (2) from compound 1 and ligand L1D.
(CH2)(CMe2)2N-2,6-iPr2C6H3)[15] in THF to give dark-blue
L1D2SiCl2 (2). The reaction is exothermic. Compound 2 could
be isolated as two polymorphs (I and II) depending on the
crystallization conditions. Reaction of LDSiCl2 (1) with L1D in
the molar ratios of 1:1 and 1:2 at room temperature led to the
isolation of polymorph II of 2 in 35% and 78% yield,
respectively, with the formation of NHC and L2D (3). However,
the formation of compound 3 was not observed when LD was
reacted directly with L1D, thus implying that 2 slowly reacts
with free LD to produce 3. This reaction was also observed
when one additional equivalent of LD was reacted with 2. The
stoichiometry of 3:1 given in Scheme 1 is important to reach
a high yield of polymorph II of 2 (91%), because side product
[*] Dr. K. C. Mondal, Prof. Dr. H. W. Roesky, H. Wolf, D. Kratzert,
Dr. R. Herbst-Irmer, B. Niepçtter, Prof. Dr. D. Stalke
Institut fꢀr Anorganische Chemie, Georg-August-Universitꢁt
Tammannstrasse 4, 37077 Gçttingen (Germany)
E-mail: hroesky@gwdg.de
dstalke@chemie.uni-goettingen.de
M. C. Schwarzer, Prof. Dr. G. Frenking
ꢀ
ꢀ
3 is obtained by C H activation and C C bond formation (for
structure determination of 3, see the Supporting Informa-
tion). After the separation of 2 by filtration, the mother liquor
is still blue, but slowly starts to change its color to blue green
and finally to light yellow after one week. Prism-like crystals,
which were obtained from this solution, were identified as 3.
All above-mentioned stoichiometries follow this common
observation. Note, when the mother liquor is dried, the
resulting solid residue also turns to light-yellow after 3–4 days
(see the Supporting Information).
Fachbereich Chemie, Philipps-Universitꢁt Marburg
Hans-Meerwein-Str., 35032 Marburg (Germany)
E-mail: frenking@chemie.uni-marburg.de
Dr. I. Tkach
Max-Planck-Institut fꢀr biophysikalische Chemie
Am Faßberg 11, 37077 Gçttingen (Germany)
E-mail: igor.tkach@mpibpc.mpg.de
[**] H.W.R. thanks the Deutsche Forschungsgemeinschaft for financial
support (RO 224/60-1). D.S. thanks the DNRF funded “Centre of
Materials Crystallography” and the doctoral programme “Catalysis
for Sustainable Synthesis”, provided by the Land Niedersachsen.
We thank Prof. Marina Bennati for EPR analyses, Prof. F. Meyer and
Dr. S. Demeshko for magnetic investigations, and S. Neudeck for
UV/Vis measurements.
Compound 2 forms two polymorphs (I and II) of blue-
black block-shaped crystals which differ only in the size of the
unit cell. However, the geometry of 2 in both polymorphs is
nearly identical. Syntheses of both polymorphs are similar.
When the blue solution of the reaction is stored at 08C,
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204487.
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crystals of polymorph II are formed initially. After 2–3 days
the round bottom flask is carefully rotated so that the blue
mother liquor slowly drains to the opposite side, leaving the
crystals of polymorph II in their place. Several lighter-blue
crystals of polymorph I are obtained after one to two weeks
along with crystals of polymorph II.
The crystallographic data given in this manuscript were
recorded on a single crystal of 2 grown at 08C (polymorph I).
Compound 2 crystallizes in the monoclinic space group C2/c.
It contains a four-coordinate silicon atom (Figure 1). In
As a result of their small sizes, a complete mechanical
separation of polymorph I from polymorph II was limited.
The ratio of I/II varies with the crystallization protocol (see
the Supporting Information). Thus polymorph I is obtained
from the solution stored at 08C, while polymorph II can be
crystallized both at room temperature and at 08C. The main
difference between the two polymorphs is that polymorph II
exists as dark-blue blocks while polymorph I forms lighter-
blue blocks. However, the striking difference is their stability.
The crystals of polymorph II are stable upon exposure to air
for about one week, and the powder of polymorph II is stable
in air for 2–3 days. The crystals of polymorph I are stable in
the mother liquor for a few weeks at 08C, but in an inert
atmosphere they remain blue for 2–3 days and then decom-
pose to a colorless solid. Such decomposition is not encoun-
tered when a solid sample enriched with polymorph I is stored
at 0 to ꢀ328C in a refrigerator. In contrast, the blue-black
crystals of polymorph II do not decompose when stored at
room temperature under an inert atmosphere for 3–4 months.
The blue THF solutions of both polymorphs are extremely
unstable when exposed to air, and rapidly turn to light-yellow
solutions under formation of the L1DH+Clꢀ salt, which was
Figure 1. Molecular structure of polymorph I of 2. Ellipsoids of
selected atoms are set at 50% probability. Selected experimental
[calculated for the triplet state at M05-2x/SVP level] bond lengths [pm]
and angles [8]: Si1–Cl1 206.62(7) [210.9], Si1–Cl2 206.90(8) [210.9],
Si1–C1 184.55(16) [184.9], Si1–C21 184.82(17) [184.9], C1–N1
139.94(19) [139.2], N2–C21 139.5(2) [139.2]; Cl1-Si1-Cl2 105.51(3)
[102.2], C1-Si1-C21 122.99(7) [118.8], C-Si-Cl 106.31(5) ꢀ107.33(5)
[107.3].
1
characterized by H NMR spectroscopy. The conversion of
polymorph I to polymorph II or vice versa could thus far not
be unambiguously realized.
contrast to the known triplet biradical,[7] compound 2 has only
one silicon atom, the coordination of which is completely
different. The silicon dichloride unit is coordinated by two
cyclic alkyl(amino)carbene (L1D) ligands to adopt a distorted
tetrahedral geometry around the Si atom. The C1-Si1-C21
bond angle is widened to 122.99(7)8 because of steric reasons,
while the C-Si-Cl bond angles are in the normal range of
Both polymorphs were studied by X-ray structural
analysis and showed only small differences within three
times the estimated standard uncertainties (see Supporting
Information). Polymorph II of 2 melts in the range of 179–
1818C and decomposes at 185–1868C. The UV absorption at
569 nm may be compared with that of the tert-butylnitroxides
(450 nm)[12] or that of the bissilyl phenyl biradical (555 nm).[7]
The powder sample enriched with polymorph I melts at 167–
1688C and decomposes at 172–1738C.
ꢀ
106.31(5) to 107.33(5)8. The Si Ccarbene bond distances in 2 are
ꢀ
ꢀ
184.55(16) (Si C1) and 184.82(17) pm (Si1 C21), respec-
tively, and distinctly shorter (by about 14 pm) than that in
[14]
ꢀ
three-coordinate LDSiCl2 (1; Si C = 198.5(4) pm).
ꢀ
ꢀ
The Si C bonds in 2 are shorter than typical Si Caryl single
The 13C NMR spectra of both polymorph II and the
sample enriched with polymorph I of 2 are identical. They
show broadening of three resonances (d = 73.5, 67.7,
58.4 ppm) of the five-membered-ring skeleton (the corre-
sponding resonances of L1D are d = 82.5, 57.7, 50.3 ppm). The
resonance at d = 210 ppm corresponds to the carbene carbon
atom of L1D, which is shifted more upfield compared with that
of the free L1D (d = 304.2 ppm).[15] Moreover, this resonance
sometimes appears with low intensity (see the Supporting
Information). In the 29Si NMR spectrum of 2 a resonance was
observed at d = 4.13 ppm, but only when a diluted solution
was used. Notably, unlike in the triplet state, compound 2
contains two localized[10] unpaired electrons. The resonance is
shifted to high field, when compared with that of the starting
material 1 (d = 19.06 ppm).[14] These characteristics obviously
show that the intermolecular forces of 2 are responsible for
quenching the NMR resonance, thus indicating the radical
nature of this compound.
bonds (187.9 pm),[16] but slightly longer than Si C bonds
ꢀ
(181.74(14) pm) of 3,6-bis[bis(di-tertbutylmethylsilyl)silylide-
ne]cyclohexa-1,4-diene,[7] and much longer than Si C double
ꢀ
bonds (170.2–177.5 pm).[17] The Cl1-Si1-Cl2 bond angle in 2
(105.51(3)8) is larger than that of 1 ((97.25(6)8) and can be
explained by the stereochemically inactive lone pair. The
ꢀ
carbene C N bond distances (C1-N1 and N2-C21) in 2 are
139.94(19) and 139.5(2) pm, respectively, and thus larger than
those found in the similar free carbenes (131.2 and
131.5 pm),[18,19] but only slightly larger than those in the
similar boron compound (137.72(13) pm).[20]
The N-Ccarbene-C bond angles (N1-C1-C2 = 108.69(12)8
and N2-C21-C22 = 109.10(13)8) are bigger than those
observed in the similar carbenes (106.388, 106.758) as
a result of the bigger adamantyl or methyl(isopropyl)cyclo-
hexyl substituents compared to the dimethyl group. The sum
of the angles at the N atom in 2 (355.58) differs clearly from
3608 in the similar carbenes with planar environment (360.08,
359.98) and the similar boron compound (359.18). This infers
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ꢀ
that the carbene C N bond in 2 has less double-bond
geometry of the triplet state of 2 using unrestricted DFT
character, and the delocalization of the lone-pair electrons
on the nitrogen atom is less pronounced. The superposition of
both the polymorphs, packing diagram, bond distances and
angles are given in the Supporting Information.
calculations is in good agreement with the experimental data
(Figure 1). Optimization of the singlet state of 2 gave
a structure that is 7.5 kcalmolꢀ1 higher in energy than the
triplet state and in which the bond lengths exhibit larger
deviations from the experimental data than that of the triplet
(see the Supporting Information). Moreover, calculations
with the functional B3LYP gave a smaller gap (2.2 kcal
molꢀ1), but still in favor of the triplet. Theoretical studies on
rotamers with all other conformations showed structures of
higher energy.
The SQUID magnetic susceptibility measurements on
a freshly prepared sample of polymorph II showed that the
crystals obtained from the reactions in the molar ratios of 1:2
or 1:3 (1 to L1D) produced 0.45% per mole of paramagnetic
polymorph I. The cT product (0.011 cm3 Kmolꢀ1) remains
almost constant in the temperature range 300-2 K. The
theoretically calculated value is 0.75 cm3 Kmolꢀ1 with two
uncoupled S = 1/2 and g = 2.0.[21,22] This result confirms that
polymorph II of 2 has diamagnetic spin ground state with
a closed shell and two spins that are opposite to each other.
An explanation is given in the part about magnetism in the
Supporting Information. However, antiferromagnetic inter-
actions in polymorph II of 2 are stronger than those found in
the biradical bis(tert-butylnitroxide).[12] The radical centers in
the latter are connected with each other by a molecular
bridge.
Calculation of the spin-density distribution of the triplet
state of 2 showed that the unpaired electrons are mainly
located at the carbon donor atoms of the ligands L1 and to
a minor extent at the nitrogen atoms (Figure 2). The spin
The slight change in the torsion angle can perturb the spin
ground state.[23] This could be the situation for polymorph I of
2. The cT product of the sample enriched with polymorph I of
2 is 0.05 cm3 Kmolꢀ1 at room temperature and is thus far
lower than the value calculated for two uncoupled S = 1/2
spins. The cT products increase very slightly from 300–113 K
to reach 0.08 cm3 Kmolꢀ1 at 113 K. The cT product reaches
0.128 cm3 Kmolꢀ1 at 83 K and remains constant until 10 K and
slightly decreases to 0.123 cm3 Kmolꢀ1 at 2 K. The slight
increase of the cT product at low temperature suggests very
weak ferromagnetic interactions. Fitting the experimental
data on 2 enriched with polymorph I below 83 K gave
a paramagnetic contribution of 16.4% per mole (see the
Supporting Information). This result suggests slight ferro-
magnetic spin polarization and/or coupling in 2 enriched with
polymorph I. As seen from the comparison of the two crystal
structures, there are slight deviations between the two
structures of polymorph I and II, which could explain the
differences in their stabilities and reactivities.
Figure 2. Calculated (M05-2x/SVP level) spin density of 2.
density at silicon is negligible, which is a very important result.
The bonding situation in 2 and the preference for a triplet
state can thus be explained as follows. The C!Si bond in the
NHC complex LDSiCl2 (1) comes from the donation of the
lone-pair orbital of the carbene into the vacant acceptor
ꢀ
orbital of SiCl2 (Figure 3a). In contrast, the C Si bonds in 2
X-band EPR spectra of both polymorphs were studied
(see the Supporting Information). The EPR signal in poly-
morph II of 2 is expected to originate from a small percentage
of polymorph I as a minor component. EPR resonance of
a solid sample of polymorph I is split at low power. A diluted
solution of polymorph I in C6D6 shows six hyperfine lines (see
the Supporting Information). The signal broadens on cooling
to 200 K.[12] The EPR intensities of both the solution and the
solid phase plotted against Tꢀ1 follow straight lines where the
signal intensity increases with decreasing temperature, thus
indicating very weak interactions between two spins in
polymorph I of 2. Since polymorph II is diamagnetic in the
temperature range of 300–2 K, it is EPR silent. Thus the only
source of the EPR signal is polymorph I of 2. For such
paramagnetic compounds, EPR intensity generally increases
with decrease of temperature, because the thermal molecular
motion is lower on cooling (see the Supporting Information).
We carried out DFT calculations at M05-2x/SVP level in
order to analyze the electronic structure of 2. The optimized
are electron-sharing bonds between the triplet states of SiCl2
and the ligands L1D where the unpaired electrons in the singly
occupied s orbitals of the carbene carbon atoms couple with
the unpaired electrons of SiCl2 (Figure 3b). One unpaired
electron remains at each of the carbene carbon atoms of the
ligand L1D, which couples with the lone-pair orbital of nitro-
gen. This explains why there is some spin density at the
nitrogen atom as well.
Figure 3. a) Donor–acceptor bonding in LDSiCl2 (1) and b) electron-
sharing bond in (L1D)2SiCl2 (2).
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ꢀ
Electron-sharing bonds C Si are stronger and shorter
than donor–acceptor bonds C!Si, which rationalizes why the
lower in energy than the triplet. A similar result was obtained
when the PBE0 functional was employed. Here the triplet
state was calculated with the SVP basis set to be 4.5 kcalmolꢀ1
lower in energy than the closed-shell singlet state (4.1 kcal
molꢀ1 with the larger TZVPP basis set) and the singlet
biradical state is 3.1 kcalmolꢀ1 more stable than the triplet
state (3.2 kcalmolꢀ1 with the larger TZVPP basis set). The
optimized geometries at the latter DFT level deviate only
slightly from the M05-2X results. The coordinates of the
optimized structures are given in the Supporting Information.
Finally, we optimized the geometry of 2 in the singlet state
at the CASSCF(2,2)/SVP level. The optimized wave function
showed that the compound is an open-shell singlet species.
The coefficients for the three singlet components are 0.80 (2/
0), ꢀ0.60 (1/1), 0.0 (0.2). The conclusion of the calculations is
that compound 2 is a biradical, in which the species with two
unpaired electrons with opposite spin (closed-shell singlet
state) is slightly lower in energy than the triplet state.
ꢀ
C Si bonds in 2 are significantly shorter than in 1. The driving
ꢀ
force for the formation of the C Si electron-sharing bonds in
2, which requires the formal excitation of the fragments SiCl2
and L1 from the singlet state to the excited triplet state, is the
ꢀ1
ꢀ
much higher energy of the C Si bond (113.6 kcalmol per
bond) compared with the C!Si donor–acceptor bond in
L1DSiCl2 (42.5 kcalmolꢀ1) [Equations (1)–(3)].
2 ðTÞ ! SiCl2 ðTÞ þ 2 L1D ðTÞ
L1DSiCl2 ðSÞ ! SiCl2 ðSÞ þ L1D ðSÞ
2 ðTÞ ! SiCl2 ðSÞ þ 2 L1D ðSÞ
þ 227:2 kcal molꢀ1
þ 42:5 kcal molꢀ1
þ 67:3 kcal molꢀ1
ð1Þ
ð2Þ
ð3Þ
The calculated bond-dissociation energy (BDE) of 2
(227.2 kcalmolꢀ1) is sufficiently high to compensate for the
singlet–triplet excitation energies of SiCl2 (60.1 kcalmolꢀ1)
and 2L1D (2 ꢁ 49.9 kcalmolꢀ1). The net energy gain of
67.3 kcalmolꢀ1 exceeds the BDE of L1DSiCl2 (42.5 kcalmolꢀ1),
which means that 2 is thermodynamically stable toward
formation of the former singlet complex. In contrast, the
ligand LD has two nitrogen-donor atoms attached to the
carbene center (Scheme 1, Figure 3), thus resulting in a much
higher singlet–triplet gap (88.9 kcalmolꢀ1) than in L1D. This
explains why a triplet state (LD)2SiCl2 is not formed.
While the calculations show that the triplet state of 2 is
lower in energy than the closed-shell singlet state, they do not
preclude that a biradical with two electrons having opposite
spin may even be more stable than the triplet species, which
has two electrons with the same spin. The bonding model with
two electron-sharing bonds shown in Figure 3b would still be
valid, but the unpaired electrons at the ligands would have
opposite spins. The experimental findings suggest that the
reaction leads to two biradical species of 2, and that the main
component is a singlet biradical, while the minor component
is an unpaired biradical. Note that the spin density of the
triplet state (Figure 2) shows that the unpaired electrons are
located at the different L1D ligands. Therefore, we carried out
further extensive calculations, which support the experimen-
tal findings.
First, we optimized the broken-symmetry singlet biradical
state of 2 at the UM05-2X/SVP level using Gaussian09,[31] and
found that the singlet state is 2.6 kcalmolꢀ1 lower in energy
than the triplet (3.2 kcalmolꢀ1 in single-point energy calcu-
lations using the TZVPP basis set). Note that the dissociation
of the singlet biradical form of 2 into the singlet fragments is
endoenergetic by 70.5 kcalmolꢀ1. The optimized geometry of
the singlet biradical is only slightly different from the
geometry of the triplet (see the Supporting Information).
We also used different functionals in order to check if the
results are an artefact of the theoretical level. Geometry
optimizations of 2 at the B3LYP/SVP level predict that the
triplet state is 2.6 kcalmolꢀ1 (2.0 kcalmolꢀ1 with the larger
TZVPP basis set) lower in energy than the closed-shell
singlet. Further calculations of the biradical singlet state using
the spin-flip procedure in Turbomole gave a structure that is
3.3 kcalmolꢀ1 (3.5 kcalmolꢀ1 with the larger TZVPP basis set)
In conclusion, we have for the first time chemically
converted an NHC-based singlet silylene with a cyclic alkyl-
(amino)carbene to a stable biradical 2, and substituted NHC 3
ꢀ
ꢀ
under C H bond activation and C C bond formation at room
temperature. Compound 2 can be obtained in two poly-
morphic forms (I and II) which were investigated by magnetic
susceptibility measurements and EPR spectroscopy. The
magnetic susceptibility measurements confirm the diamag-
netic closed-shell electronic configuration of polymorph II of
2 with a very small amount (0.45%) of polymorph I as
paramagnetic contribution. Magnetic susceptibility showed
that compound 2 can be enriched with 16.4% of polymorph I,
which is weakly coupled unpaired biradicals and thus shows
an EPR resonance.
Moreover, quantum chemical calculations of the L1D2SiCl2
molecule of polymorph I suggest that in 2 electron-sharing
bonds rather than conventional donor–acceptor C!Si bonds
are present between the carbon atoms of the carbene and the
silicon atom. Polymorph I is an unpaired biradical. Further
theoretical calculations at different levels showed that close-
shell singlet biradical L1D2SiCl2 (2) is the most stable species
(polymorph II). Compound 2 features an unprecedented
bonding situation that has so far not been reported between
a carbene and an acceptor. The bonding model may be used to
design further biradical species LxD2EX2. Additionally the easy
access, high yield, and relative high stability may lead to broad
utilization of this biradical.
Experimental Section
Synthesis of polymorph II of 2: A 50 mL round-bottom flask was
charged with a mixture of L1D (6 mmol) and LDSiCl2 (1, 2 mmol) in
a molar ratio of 3:1. After addition of THF (10 mL) at room
temperature to the colorless mixture, an immediate color change to
dark blue was observed. Stirring was continued for two to five
minutes and microcrystalline blue-black crystals of polymorph II of 2
were formed. Dark-blue-black blocks of 2 (91% yield) were grown
from the filtrate after storing the solution for one week in a freezer.
13C NMR: d = 210.0 ppm (carbene carbon atom, see the Supporting
Information); 29Si NMR: d = 4.13 ppm; melting point 179–1818C,
decomposition point 185–1868C, UV: lab = 569 nm.
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Suitable single crystals for X-ray structural analysis of 2 and 3
were mounted at low temperature in inert oil under argon atmos-
phere by applying the X-Temp2 device.[24] 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 (2) and a Bruker TXSMo
rotating anode with INCOATEC Helios mirror optics (3).[25] The data
were integrated with SAINT,[26] and a semi-empirical absorption
correction with SADABS[27] was applied. The structure was solved by
direct methods (SHELXS-97) and refined against all data by the full-
matrix least-squares methods on F2 (SHELXL-97).[28] All nonhydro-
gen 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 carbon atoms for terminal sp3 carbon atoms and
1.2 times for all other carbon atoms.
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2: M = 669.91 gmolꢀ1
, monoclinic, space group C2/c, a =
3435.2(10), b = 984.4(3), c = 2294.7(7) pm, b = 101.02(2)8, V=
7.62(1) nm3, Z = 8, m(Mo-Ka) = 0.231 mmꢀ1, T= 173(2) K, 132336
reflections measured, 8123 unique reflections, Rint = 0.0659, 422
parameters refined, R1 (all data) = 0.0449, R1 [I > 2s(I)] = 0.0374,
wR2 (all data) = 0.0990, wR2 [I > 2s(I)] = 0.0949, GOF = 1.104,
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largest diff. peak and hole 0.562·103 and ꢀ0.270 ꢁ 103 enmꢀ3
.
3: M = 679.04 gmolꢀ1
, monoclinic, space group P21/c, a =
1307.8(2), b = 1482.2(2), c = 2186.6(3) pm, b = 104.810(10)8, V=
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T= 111(2) K,
101802 reflections measured, 8403 unique reflections, Rint = 0.0781,
485 parameters refined, R1 (all data) = 0.0606, R1 [I > 2s(I)] =
0.0418, wR2 (all data) = 0.1064, wR2[I>2s(I)] = 0.0958, GOF =
1.023, largest diff. peak and hole 0.287 ꢁ 103 and ꢀ0.274 ꢁ 103 enmꢀ3
.
CCDC 885692 (2) and 885693 (3) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Geometry optimizations of the molecules have been carried out
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geometries were verified as minima on the potential energy surfaces
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Received: June 9, 2012
Revised: October 10, 2012
Published online: December 20, 2012
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Keywords: biradicals · cyclic alkyl(amino)carbenes ·
EPR spectroscopy · N-heterocyclic carbenes · singlet silylenes
.
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