A stable singlet biradicaloid siladicarbene: (L:)2Si

Article abstract of DOI:10.1002/anie.201208307

A trapped silicon atom: The compound (L:)₂Si with low-valent silicon was synthesized from its dichloride biradical precursor (L:) ₂SiCl₂ by reduction with KC₈. Theoretical analysis suggest that there are two donor-acceptor σ bonds L:→Si←:L. There is one σ lone-pair orbital at Si and one π orbital which features significant π-back-donation L:←Si→:L giving short Si-C bonds. Copyright

Full text of DOI:10.1002/anie.201208307

Angewandte
Chemie
DOI: 10.1002/anie.201208307
Low-Valent Silicon
A Stable Singlet Biradicaloid Siladicarbene: (LD)2Si**
Kartik Chandra Mondal, Herbert W. Roesky,* Martin C. Schwarzer, Gernot Frenking,*
Benedikt Niepçtter, Hilke Wolf, Regine Herbst-Irmer, and Dietmar Stalke*
Dedicated to Professor Cameron Jones on the occasion of his 50th birthday
taken as confirmation of the zero valent state of the silicon
Silicon, the congener of carbon, frequently shows different
chemistry than that of its sister element. It has been realized
that silicon prefers a positive charge if bonded to a more
electronegative atom. Compounds with silicon in lower
oxidation states are very important, since they can activate
small organic molecules which cannot be activated by
transition metals. In 2008 Robinson et al.[1] reported on the
adduct of two N-heterocyclic carbene (NHC) molecules with
disilicon A (Scheme 1). This unusual compound was prepared
atoms in A. Kira et al.[2,3] prepared a compound which was
= =
introduced as the first trisilaallene with a Si Si Si angle
between 135.46(5)8 and 136.49(6) and mentioned that com-
pounds of this type are highly flexible. The X-ray analysis
exhibits dynamic disorder even at low temperatures.[4] These
trisilaallenes have a bent structure and distinctly differ from
the carbon analogues. The differences were explained by
Frenking et al.[5,6]
However, in recent papers of Bertrand et al.,[7,8] Fꢁrstner
et al.,[9] and Frenking et al.[10–12] it was shown experimentally
and by DFT calculations that the allene type molecule B can
also be rationalized along a structure of type C in Scheme 2
Scheme 2. Schematic representation of an allene (B), a carbone (C),
a trisilaallene (D), and a silylone (E).
=
Scheme 1. Synthesis of NHCDSi SiDNHC (A). NHC=N-heterocyclic
carbene. Dip=2,6-diisopropylphenyl.
when the central C atom is bonded to the ligands L, which act
as donor ligands, such as phosphoranes or N-heterocyclic
carbenes (NHCs). Compounds C with the general formula
C(LD)2 and two electron lone pairs have been termed
carbones[10–12] whereas carbenes CR2 have only one lone
pair. When L is a carbene ligand in C(LD)2, the compound is
termed a carbodicarbene.[10,11] A related situation may be
envisaged for the silicon homologue of allene D. Theoretical
studies by Frenking et al.[5,6] suggested that the trisilaallene
that was synthesized by Kira et al.[3] should rather be
considered as a silylone Si(LD)2 E which explains why the
compound has a rather acute bonding angle Si-Si-Si and why
the substituents at the terminal silicon atoms are not
orthogonal to each other, whereas in the typical allene
structure they would be.
from the N-heterocyclic carbene and SiCl4 by reduction with
KC8. The silicon atoms of compound A exist in the formal
oxidation state zero and the structure was determined by
!
=
single-crystal X-ray analysis. The C!Si Si C backbone of
!
=
ꢀ
A has Si Si C angles of 93.37(5)8 and long Si C bonds of
1.9271(15) ꢀ (Scheme 1). These bonding characteristics were
[*] Dr. K. C. Mondal, Prof. H. W. Roesky, B. Niepçtter, H. Wolf,
Dr. R. Herbst-Irmer, Prof. D. Stalke
Institut fꢀr Anorganische Chemie, Universitꢁt Gçttingen
Tammannstrasse 4, 37077 Gçttingen (Germany)
E-mail: hroesky@gwdg.de
dstalke@chemie.uni-goettingen
M. C. Schwarzer, Prof. G. Frenking
According to these interesting results we wanted to
prepare a compound with an alternating C-Si-C backbone,
in which the silicon is the central atom of an allene type unit.
As a precursor we selected (LD)2SiCl2 (1 in Scheme 3).[13] It
was characterized by X-ray structure analysis and X-band
EPR and found to exist as two polymorphs with paired and
two unpaired electrons located at the carbene carbon
atoms.[13] LD is a cyclic alkyl amino carbene (cAAC).[7,8] The
silicon atom in 1 adopts a distorted tetrahedral geometry. In
the course of the reaction it was reduced with two equivalents
of KC8 to yield compound 2, which contains a silicon atom
with only two cAAC LD carbene ligands coordinated to it. To
Fachbereich Chemie, Philipps-Universitꢁt Marburg
Hans-Meerweinstrasse, 35032 Marburg (Germany)
E-mail: frenking@chemie.uni-marburg.de
[**] H.W.R. thanks the Deutsche Forschungsgemeinschaft (DFG RO
224/60-I) for support. We are grateful to Prof. Dr. Marina Bennati,
Dr. Igor Tkach, Dr. Claudia Stꢀckl for EPR and Felix Klinke for UV/Vis
measurements. D.S. is grateful for funding from the DFG Priority
Programme 1178, the DNRF-funded Center for Materials Crystal-
lography (CMC) for support, and the Land Niedersachsen for
providing a fellowship in the Catalysis for Sustainable Synthesis
(CaSuS) Ph.D. program.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208307.
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Scheme 3. Synthetic route for the preparation of 2 from 1 under KC8
reduction.
Figure 1. One molecule of the molecular structure of compound 2. H
atoms and solvent molecule are omitted for clarity. Anisotropic
displacement parameters are depicted at the 50% probability level.
Selected experimental [calculated at M05-2X/SVP singlet; triplet] bond
lengths [ꢂ] and angles [8]: Si1–C1 1.8411(18) [1.849; 1.886], Si1–C21
1.8417(17) [1.850; 1.877], N1–C1 1.378(2) [1.367; 1.398], N2–C21
1.378(2) [1.368; 1.378]; C1-Si1-C21 117.70(8) [117.9; 115.8].
our knowledge there are no reports on a single silicon atom
coordinated by two carbene carbon atoms such as found in
2.[14] Synthesis of 2 is thus quite interesting (Scheme 3). A
related siladicarbene with NHC ligands in Si(LD)2 has been
theoretically postulated and calculated by Frenking et al.[5,6]
[(LD)2Si]·0.5(n-hexane) (2) was prepared from a 1:2 molar
ratio of (LD)2SiCl2 (1)[13] and KC8 in tetrahydrofuran. This
mixture was slowly warmed to room temperature and filtered.
The royal-blue filtrate was evaporated to dryness and
extracted with n-hexane. The concentrated dark blue solution
gave blue–black rods of compound 2 in a 95% yield, when it
was stored at 08C in a refrigerator.
The sum of angles around the carbene carbon atoms of 2
are on average 357.78 (358.18, 1) implying slight deviation
ꢀ
from trigonal-planar geometry. The carbene Si C bonds in 2
ꢀ
ꢀ
(Si1 C1 1.8411(18), Si1 C21 1.8417(17) ꢀ) are very close to
those in 1 (1.8455(16) and 1.8482(17) ꢀ)[13] which are
The crystals of 2 are stable in air for one day. Compound 2
melts at 194–1958C and decomposes at 202–2038C. It shows
UV absorption bands at 270, 327, 392, 570, and 611 nm which
distinctly shorter (ca. 0.14 to 0.1 ꢀ) than that in tricoordinate
[15]
ꢀ
ꢀ
NHCDSiCl2 (Si C 1.985(4) ꢀ).
The Si C bonds in 2 are
shorter than typical Si Caryl single bonds (1.879 ꢀ)[16a] but
ꢀ
ꢀ
are comparable to those found for LD2SiCl2 (1; 321, 350, 400,
slightly longer than those in some recently prepared Si Csp3
[2,3]
and 570 nm). The trisilaallene (R2Si Si SiR2) of Kira et al.
systems (1.8174(14) ꢀ)[16b] and much longer than Si C double
= =
=
bonds (1.702–1.775 ꢀ).[16c] The Si C bonds in 2 are slightly
ꢀ
has similar thermal stability while the UV/Vis spectrum has
absorptions at 390 and 584 nm.[2] The 29Si NMR spectrum of 2
has a singlet at d = 66.71 ppm which is downfield of that of the
starting material (LD)2SiCl2 (d = 4.13 ppm).[13] The chemical
shift of the central silicon atom of trisilaallene[2,3] was
shorter than the calculated value for the silylone Si(NHC)2
(1.869 ꢀ).[5,6] The carbene C N distances in 2 are similar to
ꢀ
those found in 1 while the orientation of the carbene ligands
in 2 is completely different to that in 1. The average N-Si-N
angle in 2 is about 1708 which is considerably wider than in
1 (ca. 1328). Superposition of the two molecules in the
asymmetric unit is shown in Figure S3 in the Supporting
Information.
The X-band EPR spectrum of 2 showed no EPR
resonance. This suggests a singlet ground state of 2. However,
the dark blue color of the compound suggests a rather small
HOMO–LUMO gap which could introduce a biradicaloid
character.
reported at d = 157 ppm. This shift indicates that the 3D
[2]
= =
orientation of the electron density in 2 and in R2Si Si SiR2
is different. The broadening of three 13C NMR resonances
(d = 68.7, 57.7 and 48.5 ppm) was not observed for 2 unlike
1.[13] The 13C NMR spectrum of 1 shows broadening of three
resonances (d = 73, 67, and 58 ppm) of the five-membered
ring skeleton (the corresponding resonances of LD are d = 82.5,
57.7, 50.3 ppm).[8]
ꢀ
Compound 2 crystallizes in the triclinic space group P1.
The asymmetric unit contains two nearly identical molecules
of 2 and one n-hexane lattice solvent molecule. The molecular
structure of 2 consists of one two-coordinate silicon atom
(Figure 1) at the center coordinated to the carbene carbon
atoms of the two cAAC ligands (LD).[7,8] This silicon atom
adopts a bent geometry. The C-Si-C bond angles of the two
symmetry independent molecules are 117.18(8)8 (C41-Si2-
C61) and 117.70(8)8 (C1-Si1-C21), respectively, which are
We optimized the geometry of 2 in the singlet and triplet
states at the M05-2X/SVP level of theory.[17–19] The calculated
bond lengths and angles, given in Figure 1, show that the
values for the singlet state are in very good agreement with
the experiment and in better agreement than those of the
triplet state. The superposition of the X-ray structure with the
calculated singlet and triplet forms shows a nearly perfect
alignment between the singlet form and the experimental
structure while the calculated triplet exhibits a significantly
greater deviation from the experiment (Figure S1, Supporting
Information).
smaller by about 58 than those of the precursor 1 (122.99(7)8).
= =
!
The Si Si Si[2,3] and CD!C DC[11c] bond angles of allene type
molecules are larger, close to 1358.
2
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Calculations at various levels of theory using the M05-2X/
SVP optimized geometries predict that the triplet form is
between 17.2 kcalmolꢀ1 (M05-2X/TZVPP) and 18.5 kcal
molꢀ1 (B3LYP/TZVPP//M05-2X/SVP) higher in energy than
the singlet. Thus, the calculations agree with the analysis of
the EPR spectrum that 2 is not in a triplet state.
negligible contribution from the singly excited state which
indicates that the molecule has some biradicaloid character
and low electronic excitation energy. Calculations at TDDFT-
B3LYP/SVP//M05-2X/SVP level give the lowest excitation at
550 nm with an oscillator strength of 0.157 while the second
band is calculated at 543 nm with an oscillator strength of
0.066. This result is in reasonable agreement with the
experimentally observed bands at 611 and 570 nm.
Figure 2 shows the shapes of the HOMO and HOMOꢀ1
of the singlet form of 2 which exhibit the typical features of
= =
In comparison, the theoretical analysis on R2Si Si
SiR2
[2,3]
suggested a zwitterionic nature of the corresponding
= =
energy-minimized conformation. The HOMO of R2Si Si
SiR2 is a p-type orbital and the HOMOꢀ1 is a s lone pair
orbital, these orbitals are very similar to the HOMO and
HOMOꢀ1 of 2.[2] Thus, the bonding situation in 2 and in
Kiraꢂs compound[2] is very much the same, therefore R2Si Si
= =
SiR2 should be classified as a siladisilylene instead of
a trisilaallene.[5b]
Keeping in mind the biradical electronic structure of
precursor 1 it is intriguing whether compound 2 can possibly
be better described as a silylene (i.e., SiII compound) which
carries two radical substituents in which the two unpaired
electrons give the p HOMO (Figure 2). Computational
evidence supports the assignment of 2 as a Si0 species,
however. We calculated Si(Me)2 as a model silylene and
compared it with 2. The equilibrium bond angle C-Si-C of
Si(Me)2 (91.18) is much more acute than in 2 (117.98). The
most striking difference between the two compounds comes
from the proton affinities. Compound 2 strongly binds two
protons at Si but Si(Me)2 binds only one proton at Si (PA(1) =
224.1 kcalmolꢀ1). A second proton binds to one methyl group
(PA(2) = 151.1 kcalmolꢀ1). NBO calculations show that the
electron density at Si in 2 (q = 0.55 e) is larger than in Si(Me)2
(q = 0.91). The positive partial charge at Si which agrees with
the electronegativities of silicon and carbon indicates that the
ꢀ
Figure 2. Plots of the HOMO (a) and HOMO 1 (b) of 2.
a silylone SiL2.[10–12] The HOMOꢀ1 is a s lone-pair orbital at
Si while the HOMO is a p-type orbital which has the largest
extension at Si but exhibits significant Si C p bonding. This
ꢀ
ꢀ
situation is in agreement with the calculated Si C bond
lengths for 2 (1.849 ꢀ and 1.850 ꢀ) which are slightly shorter
than in Si(NHC)2 (1.869 ꢀ).[10–12] The ligand cAAC in 2 is
a better p-acceptor than NHC,[8] because the divalent carbon
donor atom in the NHC ligand is attached to two nitrogen
atoms whereas there is only one nitrogen atom available for
N!C p donation in the cAAC ligand.
The NBO analysis of 2 which was carried out with the
option of multicenter bonding gives one s lone-pair orbital
and a three-center C-Si-C p orbital of which 40% is at Si and
30% is at each C atom. The best representation of the
bonding situation of the siladicarbene 2 is shown in Scheme 4.
!
Si!C p back-donation is larger than the Si C s donation.[25]
Finally, the largest coefficient of the p HOMO of 2 is at the
central Si atom which makes it more reasonable to discuss the
!
p orbital at the central moiety in terms of C Si!C donation
!
rather than C!Si C interactions. Therefore, we think that
the bonding situation in 2 is better described as silylone rather
than silylene.
In conclusion, we have prepared royal-blue, two coordi-
nate, singlet biradicaloid siladicarbene (LD)2Si, which is stable
under an inert gas atmosphere. This is the first example of
a silicon atom in the formal oxidation state of zero, stabilized
by two carbene molecules. The amorphous powder of (LD)2Si
is stable for an hour while its crystals are stable in air for about
a day. The preparation of such unique species provides the
roadmap for biradicaloids that show unexpected stability. The
calculated bond lengths and angles match fairly well with the
singlet state and are also in a good agreement with those of
the experimental data, and better than those of the triplet
state. Calculations at various levels of theory show that the
triplet form is 17.2 to 18.5 kcalmolꢀ1 higher in energy than the
singlet. This result has also been supported by X-band EPR
experiments. The ligand cAAC in 2 is a better p-acceptor than
NHC,[8] because the divalent carbon donor atom in the NHC
ligand is attached to two nitrogen atoms while there is only
one nitrogen atom available for the N!C p donation in the
Scheme 4. Schematic representation of the bonding situation in 2.
As a further probe for the classification of 2 as a silylone, we
calculated the first and the second proton affinities (PAs)
which have been suggested as indicator of the ylidone
character of EL2 compounds.[5] The theoretically predicted
values at RI-BP86/SVP are PA(1) = 272.2 kcalmolꢀ1 and
PA(2) = 186.7 kcalmolꢀ1. The very large value for the
second PA(2) clearly supports the assignment of 2 as a genuine
silylone.
We carried out CASSCF(2,2)/SVP calculations using the
M05-2X/SVP optimized geometry to see if 2 may have
a biradical character. The calculation gave coefficients of 0.96
for the closed-shell 2,0 configuration, ꢀ0.28 for the 1,1
configuration, and 0.0 for the 0,2 configuration. Thus,
compound 2 is a closed-shell singlet which has a non-
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ꢀ
cAAC ligand. This difference explains the slightly shorter Si
C bonds in 2 than in Si(NHC)2. The NBO analysis of 2 gives
one s lone-pair orbital and a three-center C-Si-C p orbital
with 40% at Si and 30% at each C. The bonding situation in 2
is shown in Scheme 4. Further calculations to see if 2 may
have a biradical character showed that compound 2 is
a closed-shell singlet which has a non-negligible contribution
from the singly excited state which indicates that the molecule
has a biradicaloid character and low electronic excitation
energy. Calculations at TDDFT-B3LYP/SVP//M05-2X/SVP
level produce the lowest excitation at 550 nm with an
oscillator strength of 0.157, while the second band is
calculated at 543 nm with an oscillator strength of 0.066.
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Experimental Section
2: THF (85 mL) was cooled to ꢀ788C and added to an 1:2 mixture of
(LD)2SiCl2 (1; 3 mmol, 2007 mg) and KC8 (6 mmol, 810 mg). Then the
mixture was warmed to room temperature in 15–20 min and stirred
for 14 h and filtered. The royal-blue filtrate was evacuated to dryness
and extracted with n-hexane (90 mL) and the resulting volume was
reduced to 20–30 mL under vacuum. The concentrated dark blue
solution produced blue–black rods of compound 2 in 95% yield when
it was stored at 08C in a refrigerator. The crystals of 2 are stable in air
for one day. M.p. 194–1958C, decomp.202–2038C; UV/Vis bands at
570 and 611 nm. 1H NMR (C6D6, 2988C, 500.133 MHz): d = 7.31–7.19
(d, 2Har, m-H), 7.14–7.06 (d, 2Har, m-H), 7.07–7.04 (m, 2Har, p-H),
3.17 (m, 2H, CHMe2), 2.68 (m, 2H, CHMe2), 1.91 (s, 2H, CH2), 1.86
(s, 2H, CH2), 1.75 (s, 6H, NCMe2), 1.74 (s, 6H, NCMe2), 1.50 (d, 6H,
CHMe2), 1.31 (d, 6H, CHMe2), 1.25 (d, 6H, CHMe2), 1.07 (d, 6H,
CHMe2), 1.0 (s, 6H, CMe2), 0.90 ppm (s, 6H, CMe2); 13C NMR: d =
210.9 (carbene), 136.2, 128.9, 125.4, 124.8, 68.7, 57.7, 48.5, 36.0, 31.9,
31.3, 30.1, 29.8, 29.4, 28.3, 27.9, 25.1 ppm; 29Si NMR: d = 66.71 ppm.
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Crystal data for 2: LD2Si (LD = DC(CH2)(CMe2)2N-2,6-iPr2C6H3)
Mr = 642.09 gmolꢀ1
,
triclinic, space group P1, a = 9.145(2), b =
ꢀ
19.347(3), c = 24.566(3) ꢀ, a = 112.00(3), b = 98.33(2), g = 91.71(2)8,
V= 3970.6(12) ꢀ3, Z = 4, m(Mo-Ka) = 0.089 mmꢀ1
,
T= 101(2) K,
87462 reflections measured, 16274 unique reflections, Rint = 0.0629,
916 parameters refined, R1 (all data) = 0.0728, R1 [I > 2s(I)] =
0.0457, wR2 (all data) = 0.1108, wR2[I>2s(I)] = 0.0980, GOF =
1.020, largest diff. peak and hole 0.379 and ꢀ0.304 eꢀꢀ3
.
CCDC 896896 contains 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.
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Computational Details: Geometry optimizations of
2 were
carried out using the DFT functional BP86[17] and M05-2X[18] with
def2-SVP basis sets.[19] The RI-Approximation was applied whenever
possible.[20] The optimized geometry was verified as a minimum on the
potential-energy surface by calculation of the vibrational frequencies
analytically at the RI-BP86/SVP level of theory (AOFORCE).[21]
Improved energies were calculated at M05-2X and RI-B3LYP with
the larger basis sets def2-TZVPP[22] using M05-2X/SVP optimized
geometries. Calculations were carried out with the program package
Gaussian09[23] for M05-2X and TurboMole 6.3[24] for other func-
tionals.
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[19] A. Schꢃfer, H. Horn, R. Ahlrichs, J. Chem. Phys. 1992, 97, 2571 –
2577.
Received: October 16, 2012
[20] R. Ahlrichs, Phys. Chem. Chem. Phys. 2004, 6, 5119 – 5121.
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Chem. Phys. Lett. 2004, 384, 103 – 107.
Published online: && &&, &&&&
Keywords: biradicals · DFT calculation · low oxidation state ·
silicon · X-ray structure
[22] F. Weigend, M. Hꢃser, H. Patzelt, R. Ahlrichs, Chem. Phys. Lett.
1998, 294, 143 – 152.
.
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Chemie
[23] Gaussian09, RevisionC.1, M. J. Frisch et al. Gaussian, Inc.,
Wallingford CT, 2010. For full reference see Supporting Infor-
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[24] R. Ahlrichs, M. Bꢃr, M. Hꢃser, H. Horn, C. Kçlmel, Chem. Phys.
Lett. 1989, 162, 165 – 169.
suggests that the (BH)!C p back-donation is slightly larger than
!
the (BH) C s donation and where the borane (BH)(Me)2
carries a larger partial charge of q(BH) = 0.61 e: R. Kinjo, B.
Donnadieu, M. A. Celik, G. Frenking, G. Bertrand, Science 2011,
333, 610 – 613.
[25] A similar situation has been reported for the related
complex(BH)(cAAC)2 where the partial charge q(BH) = 0.05 e
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Angewandte
Communications
Communications
Low-Valent Silicon
A trapped silicon atom: The compound
(LD)2Si with low-valent silicon was syn-
thesized from its dichloride biradical
precursor (LD)2SiCl2 by reduction with KC8.
Theoretical analysis suggest that there are
K. C. Mondal, H. W. Roesky,*
M. C. Schwarzer, G. Frenking,*
B. Niepçtter, H. Wolf, R. Herbst-Irmer,
!
D. Stalke*
&&& — &&&
two donor–acceptor s bonds LD!Si DL.
There is one s lone-pair orbital at Si and
A Stable Singlet Biradicaloid
Siladicarbene: (LD)2Si
one p orbital which features significant p-
!
ꢀ
back-donation LD Si!DL giving short Si
C bonds.
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