One-electron-mediated rearrangements of 2,3-disiladicarbene

Article abstract of DOI:10.1021/ja504821u

A disiladicarbene, (Cy-cAAC)₂Si₂ (2), was synthesized by reduction of Cy-cAAC:SiCl₄ adduct with KC₈. The dark-colored compound 2 is stable at room temperature for a year under an inert atmosphere. Moreover, it is stable up to 190°C and also can be characterized by electron ionization mass spectrometry. Theoretical and Raman studies reveal the existence of a Si=Si double bond with a partial double bond between each carbene carbon atom and silicon atom. Cyclic voltammetry suggests that 2 can quasi-reversibly accept an electron to produce a very reactive radical anion, 2^?-, as an intermediate species. Thus, reduction of 2 with potassium metal at room temperature led to the isolation of an isomeric neutral rearranged product and an anionic dimer of a potassium salt via the formation of 2^?-.

Full text of DOI:10.1021/ja504821u

Communication
pubs.acs.org/JACS
One-Electron-Mediated Rearrangements of 2,3-Disiladicarbene
Kartik Chandra Mondal,† Prinson P. Samuel,† Herbert W. Roesky,*,† Rinat R. Aysin,‡ Larissa A. Leites,‡
Sven Neudeck,† Jens Lubben,† Birger Dittrich,*,§ Nicole Holzmann,∥ Markus Hermann,∥
̈
and Gernot Frenking*,∥
†Institut fur Anorganische Chemie, Georg-August-Universitat, Tammannstrasse 4, 37077 Gottingen, Germany
̈
̈
̈
‡A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov str., 119991 Moscow, Russia
§Fachbereich Chemie, Universitat Hamburg, Martin-Luther-King-Platz 6, Raum AC 15c (Erdgeschoss), 20146 Hamburg, Germany
∥Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Strasse, 35032 Marburg, Germany
̈
̈
S
* Supporting Information
Robinson’s A with cAAC: might be expected as a biradical, and
ABSTRACT: A disiladicarbene, (Cy-cAAC)2Si2 (2), was
synthesized by reduction of Cy-cAAC:SiCl4 adduct with
KC8. The dark-colored compound 2 is stable at room
temperature for a year under an inert atmosphere.
Moreover, it is stable up to 190 °C and also can be
characterized by electron ionization mass spectrometry.
Theoretical and Raman studies reveal the existence of a
SiSi double bond with a partial double bond between
each carbene carbon atom and silicon atom. Cyclic
voltammetry suggests that 2 can quasi-reversibly accept
an electron to produce a very reactive radical anion, 2•−, as
an intermediate species. Thus, reduction of 2 with
potassium metal at room temperature led to the isolation
of an isomeric neutral rearranged product and an anionic
dimer of a potassium salt via the formation of 2•−.
the possible formation of a radical anion or a radical cation can be
studied from its redox chemistry. Herein we targeted the
synthesis of a cAAC: analogue of A, (Cy-cAAC)2Si2 (2), and its
cyclic voltammetry (CV) was investigated. CV shows that
formation of the radical anion 2•− from 2 is a quasi-reversible
process. Utilizing this finding, unprecedented one-electron
(from potassium metal)-mediated rearrangements of 2 are
reported via the formation of a very reactive radical anion, 2•−, as
an intermediate.
The Cy-cAAC: carbene readily forms the adduct Cy-cAAC:→
SiCl4 (1). Reduction of 1 with KC8 in a molar ratio of 1:4
produces (Cy-cAAC:)2Si2 (2) in 54% yield (Scheme 1), which is
Scheme 1. Synthesis of Compound 2 from 1 under KC8
Reduction
n 2008, Robinson et al.1 paved the way for N-heterocyclic
Icarbene (NHC:)-stabilized diatomics with the L:→Si(0)
Si(0)←:L compound (A) (L: = NHC: = :C[N(2,6-iPr2C6H3)-
CH]2), prepared by potassium graphite reduction of NHC:→
SiCl4. Since then, molecular species containing the elemental
units Ge2,2 Sn2,3 P2,4,5 As2,6 and B27 have been prepared in the
presence of various NHCs, which have an enormous impact on
the stabilization of compounds of zero-valence elements through
strong σ-donation. When compound A was reacted with BH3·
THF, cleavage of the silicon−silicon double bond was observed,
and two quite different “push−pull” stabilized products with
borane- and carbene-coordinated silylene moieties were
isolated.8 Perhaps due to its unusual stability, reactivity studies
of A are sparse.1,8
higher than that of NHC:SiSi:NHC (A, 21%).1 However, 2 is
not obtained when Cy-cAAC: is directly reacted with NHC:Si
Si:NHC in THF at ambient temperature. When 2 was treated
with 2 equiv of Cy-cAAC:, formation of (Cy-cAAC:)2Si12 was
not observed.
Compound 2 melts in the range of 188−190 °C and was
further characterized by electron ionization mass spectrometry
(EI-MS, m/z (%) = 706.4 (100)) (see Supporting Information
(SI)). The UV−vis spectrum of 2 recorded in n-hexane solution
shows two absorption bands at 416 and 528 nm (see SI). The
magnetic susceptibility measurements of 2 confirmed a singlet
spin ground state (see SI). The 29Si NMR spectrum of 2 was
recorded in C6D6. It shows a single resonance at 249.1 ppm,
which is more downfield shifted than that of A (δ = 224.5 ppm).1
The 13C NMR spectrum of 2 exhibits a resonance at 234.7 ppm,
The carbene carbon atom of NHC: is bound to two σ-
withdrawing and two π-donating N atoms. When the cyclic
alkyl(amino) carbene (cAAC:) is used instead of the NHC:, one
π-donating and σ-withdrawing N atom is replaced by one σ-
donating quaternary C atom. Thus, the cAAC: becomes more
nucleophilic but also more electrophilic when compared with
NHC:.9 Several radical species of main-group elements, such as
PN•+, P2•+, phosphinyl radical cation, H−B•+, CO•+, and ketene
with biradical character, were stabilized with cAAC:.10
Both cAAC: and NHC: form stable adducts with SiCl4.11 In a
recent review,10 Bertrand et al. mentioned that an analogue of
Received: May 14, 2014
© XXXX American Chemical Society
A
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which is more upfield shifted than that for Cy-cAAC:, 309.4 ppm,
but more downfield shifted than that for (Me2-cAAC)2Si, 210.0
ppm.12
there are several bands of enhanced intensity just in the region
450−500 cm−1. One of them, most likely the strongest at 482
cm−1, can be assigned to νSiSi. This rather high frequency
confirms that this band can be attributed to the double-bond
stretch (νSiSi). Inspection of the electronic structure of 2 gives
clear evidence for the occurrence of a SiSi double bond. The
shape of the HOMO−1 of 2 (Figure 2) can unambiguously be
Compound 2 crystallizes in the space group P1 and does not
̅
possess a center of inversion like A. The asymmetric unit
contains a complete molecule of 2 (Figure 1). The CcAAC−Si
Figure 2. Plot of the HOMO−1 of 2.
identified with the SiSi π bond. The pattern of chemical
bonding was further elucidated by QTAIM (Quantum Theory of
Atoms in Molecules) calculations18 on 2. The molecular graphs
of 2 (Figure S6) exhibit all expected bonds and ring critical
points.
Figure 1. Molecular structure of compound 2. H atoms are omitted for
clarity. Selected experimental [calculated at BP86/TZVPP for the
singlet state] bond lengths [Å] and angles [°] (as averages of two
independent molecules): C1−Si1A 1.876(4) [1.871], C1C−Si2A
1.849(4) [1.869], Si1A−Si2A 2.254(3) [2.264]; C1−Si1A−Si2A
101.22(13) [106.0], C1C−Si2A−Si1A 105.14(13) [107.9].
Topological parameters of important (3,−1) bond critical
points (BCPs) are presented in Table S9. The relationship
between the electron density ρ(r) value and its Laplacian ∇2ρ(r)
for the SiSi bond shows that it is a covalent bond, and its high
ellipticity (ε = 0.36 for 2 and 0.38 for 2′) points to a significant
double bond character. The same is demonstrated by the
delocalization index (DI) value of δ(Si,Si), 1.25 (cf. DI = 1.33 for
H2SiSiH2.)19 In the deformation electron density maps, the
lone pairs on each Si atom are clearly seen; see projections in the
C−Si−Si−C plane (Figure S7a), in a perpendicular plane
(Figure S7b), and in the Si−C−N plane (Figure S7c).
The topological parameters of the Si−C BCPs (Table S9)
correspond to the intermediate character of these interactions
and are close to those in a (Cy-cAAC)2Si molecule. In particular,
for both molecules the DI values are significantly higher (av.
0.85) than that for an ordinary Si−C bond in H3Si−CH3 (0.55)
and significantly lower than that for the double SiC bond in
H2SiCH2 (1.17).19 This means that, although the Si−C bonds
in 2 are somewhat weaker than in (Cy-cAAC)2Si, both types of
the Si−C bonds have a similar, partial double bond character;
namely, they involve C→Si donation of σ-type and partial C←Si
back-donation of π-type.
bond lengths commonly range from 1.849(4) to 1.876(4) Å. The
Si−C distance found for 2 (1.887(4) Å) is shorter than that in A
(1.921(15) Å)1 but is a little longer (∼0.04 Å) than those of the
previously reported biradical, (Me2-cAAC•)2SiCl2 (1.8455(16)
and 1.8482(17) Å),13 although it is very close to those of typical
Si−C (aryl) single bonds (∼1.879 Å).14
The C−Si bond lengths of 2 are longer than SiC double
bonds (170.2−177.5 pm),16 in particular that in trans-2,3-disila-
1,3-butadiene (1.727 Å).15 The experimental SiSi bond
distance of 2 (2.254(3) Å) is close to that of
NHC:SiSi:NHC in A (2.2294(11) Å).1 The C−N bond
length in 2 is 1.352(3) Å, which is close to those of 1.37−1.39 Å
in the biradicaloid (Me2-cAAC)2Si12 and biradical
(Me2-cAAC•)2SiCl2,12,13 suggesting significant back-donation
from the silicon atoms to each carbene carbon atom. For 2, the
C−Si−Si angles range from 101.22(13) to 105.14(13)°, ∼10°
wider than those of A (93.37(5)°).1 The lone pairs of electrons
on the two N atoms of NHC: in A are almost perpendicular to
the π-orbital of the SiSi double bond, while those of 2 are in
conjugation with the SiSi unit.
The CV of 2 was investigated, as suggested by Bertrand et al.10
CV of a THF solution of 2, containing 0.1 M [n-Bu4N]PF6 as an
electrolyte, shows two oxidations and three reductions (see SI for
complete CV). The quasi-reversible reduction at E1/2 = −1.40 V
versus Cp*2Fe/Cp*2Fe+ indicates the formation of a radical
anion, 2•− (Figure 3).
The purple color of the THF solution turned to red when 2
was reacted with metallic potassium at room temperature for 16
h. A similar transformation of color was observed during CV
measurements. However, the formation of a pale yellow solution
was easily observed around the surface of the piece of potassium
when the reaction solution was examined with a close look at a
static solution. Pale yellow rods of 3 were formed when the
concentrated THF solution was stored at −32 °C in a freezer. X-
ray single-crystal diffraction revealed a rearranged structure,
which is shown in Scheme 2 and Figure 4. Compound 3 can be
considered as an isomer of 2.
Compound 2 was further studied by theoretical calculations,
and the results were corroborated with experimental Raman data
(see SI). Despite a very complicated vibrational spectrum, we
hoped to identify the νSiSi vibration by its enhanced Raman
intensity because the polarization of a Si atom is greater than that
of a C atom. Normal coordinate analysis results show that the
calculated SiSi stretching mode for L′2Si2 (2′, L′ =
:C(CH2)(CMe2)2NMe) has a wavenumber of 475 cm−1 and is
well localized, its contribution to PED reaching 68%. This is in
contrast to the data of “normal” disilenes of the type C2SiSiC2,
where there is no normal mode characteristic to the SiSi bond,
because the SiSi and Si−C internal stretching coordinates are
heavily mixed and the degree of mixing depends on the particular
substituents.17 Their in-phase combination results in a Raman-
active normal mode with wavenumber in the range of 460−550
cm−1. In the experimental Raman spectrum of 2 (Figure S2),
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After the fractional crystallization of 3, the flask is rotated and
the red mother liquor is again stored at −32 °C in the freezer to
form small red blocks of 4, which crystallize in the monoclinic
space group P21/c. X-ray single-crystal diffraction (Scheme 2,
Figure 4) revealed that 4 can be considered as a dimer of the
monoanionic potassium salt, suggesting the radical anion 2•−
underwent elimination of a 2-propyl radical to produce an anion
together with the potassium cation, which dimerizes through a
silylone bridge to form 4. Each potassium atom accepts π-
chelation from two nearest aromatic rings.
A mechanism was proposed which is shown in Scheme 3. CV
suggests the formation of the radical anion 2•− (Figure 3) via a
Figure 3. Cyclic voltammogram of a THF solution of 2, containing 0.1
M [n-Bu4N]PF6 as electrolyte.
Scheme 3. Proposed Reaction Pathway for the Formation of 3
a
from 2•−
Scheme 2. Synthesis of Compounds 3 and 4
a
Relative free energies ΔG at BP86/SVP are given in kcal/mol.
quasi-reversible electron-transfer process between 2 and metallic
potassium. Since 2•− is not stable enough, as indicated by CV, it
undergoes bond activation followed by rearrangements. The
reaction proceeds in a cascade20 fashion via the intermediates I1,
I2, and I3 through cleavage of C−N bond of 2•−, followed by
migration of a 2,6-diisopropylphenyl (Dip) group from the
nitrogen atom of a five-membered carbene ring, causing C−N
bond breaking, CN bond formation, and then C(sp3)−H
bond splitting across the silicon−silicon double bond. On the
other end of the Cy-cAACSi unit, opening of the five-
membered carbene ring via cleavage of the Calkyl−N bond and
migration of the tertiary alkyl part to the nearest silicon atom take
place in a radical pathway. Finally, the electron is removed from
I3 at the end of the cycle to produce the neutral compound 3.
Alternatively, 2•− can undergo fragmentation via the
elimination of a 2-propyl radical from one isopropyl group
(Scheme 4), with the formation of a Si−Caromatic bond to form a
five-membered biheteroaromatic ring, which can be considered
as an N-substituted 3-silylindole. On the left side, the carbene−
silicon bond in 4 converted into a double bond16 (Si1B−C1B =
1.7815(18) Å), while on the right side, the negatively charged
silicon atom is bound to one silicon atom and a carbene carbon
atom and acts as a bridge between two potassium ions (Figure 4)
since the negative charge is centered on this silicon atom (Si1B).
Calculations at BP86/SVP of the free anions suggest that the
reaction 2− → 0.5 4− + 0.5 Me2CHCHMe2 is exergonic by −59.6
kcal/mol.
Figure 4. Molecular structures of compounds 3 (left) and 4 (right).
Hydrogen atoms (C−H) are removed for clarity. Selected experimental
[calculated at BP86/SVP; 4 was calculated as monomer without
K+(THF)] bond lengths [Å] and angles [°]: For 3, Si1B−C1B
1.9268(15) [1.960], Si1B−C3B 1.9011(15) [1.937], Si1B−C22A
1.9209(15) [1.952], Si1B−Si1A 2.3342(5) [2.367], C1B−Si1B−C3B
89.50 (6) [90.4], C22A−Si1B−Si1A 92.63(5) [91.9], C5B−C1B−N2B
117.53(13) [117.7]; Si1A−C1A 1.8987(15) [1.925], Si1A−C13A
1.8797(15) [1.909], C5A−C1A−N2A 116.30(13) [115.6], C1A−
N2A−C3A 110.22(13) [111.0]. For4, Si1B−C1B 1.7815(18) [1.800],
Si1B−C14B 1.8370(18) [1.867], Si1B−Si1A 2.3400(6) [2.373], C1B−
N2B 1.413(2) [1.420], C13B−N2B 1.407(2) [1.409], C13B−C14B
1.446(2) [1.459], Si1B−K1 3.7811(6), C15B−K1 3.1079(16), C1B−
Si1B−C14B 88.38(8) [87.7], N2B−C1B−C5B 111.15(14) [113.7],
C1B−Si1B−Si1A 134.88(6) [130.6], C14B−Si1B−Si1A, 126.11(6)
[132.5], C1B−Si1B−K1, 119.62(6), N2B−C1B−Si1B, 113.13(12)
[113.7]; Si1A−K1 3.5543(6), Si1A−K1′ 3.3907(6), Si1A−C1A
1.8199(17) [1.841], C1A−N2A 1.408(2) [1.424], K1−K1′ 4.424.
Compound 3 crystallizes in the triclinic space group P1. X-ray
̅
single-crystal diffraction (Figure 4) revealed that 3 underwent
multiple rearrangements when compared with 2. Both of the
silicon atoms of 3 adopt a distorted tetrahedral geometry. The
Si1B center is bound to three carbons (C1B, C3B, C22A) and
one silicon (Si1A) atom. Furthermore, Si1A is bound to two
carbons (C1A, C13A), one hydrogen (H1), and one silicon
(Si1B) atom. All four Si−C/H bonds are electron sharing σ
bonds. Bond parameters are given in the caption of Figure 4. It is
worth mentioning that both silicon atoms of 3 can be considered
as chiral centers, where Si1B sits in the center of the two five-
membered spiro rings.
29Si NMR spectra of 3 and 4 show resonances at −5.0, −58.2
and −50.0, −56.5 ppm, respectively, which are far upfield shifted
when compared with those of 2 (249.1 ppm). Similarly, 13C
resonances of 3 are at 196.3, 186.6 ppm, which are also upfield
shifted with respect to those of 2 (234.7 ppm, see Table S11).
C
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Scheme 4. Proposed Reaction Pathway for the Formation of 4
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In conclusion, we have shown that disiladicarbene with the
general formula (Cy-cAAC:)2Si2 (2) can be synthesized via
reduction of the adduct (Cy-cAAC:)SiCl4 (1) with KC8. 2 is
stable at room temperature for a year under an inert atmosphere
and stable up to 190 °C. It is studied by Raman spectroscopy, and
the nature of the CcAAC-Si and SiSi bonds is corroborated with
theoretical calculation. CV suggests that 2 can quasi-reversibly
accept an electron to produce a very reactive radical anion, 2•−, as
an intermediate species. Consequently, reduction of 2 with
potassium metal led to the isolation of the isomeric rearranged
neutral product 3 and a dimer of potassium salt 4.
The electron-mediated transformation of 2 into 3 via the
formation 2•− followed by unusual rearrangements proceeds in a
cascade fashion. The energetics were studied by theoretical
calculations which also suggest an alternative pathway to explain
the formation of 4. The rearrangements are energetically
favorable. Until now, only the ring expansions of NHC-
containing compounds (Si,21 Be,22a B,22b Zn,22c) were reported.
To the best of our knowledge, this type of rearrangement has no
precedence in silicon chemistry until today.
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ASSOCIATED CONTENT
* Supporting Information
(17) (a) Leites, L. A.; Bukalov, S. S.; Garbuzova, I. A.; West, R.;
Mangette, J. J. Organomet. Chem. 1997, 536−537, 425−432.
(b) Garbuzova, I. A.; Leites, L. A.; Bukalov, S. S. J. Mol. Struct. 1997,
410, 467−470. (c) Leites, L. A.; Bukalov, S. S.; Mangette, J. E.;
Schmedake, T. A.; West, R. Mendeleev Commun. 1998, 8, 43−44.
■
S
Synthesis, UV and CV data, magnetic susceptibility, Raman
spectrum, table of crystallographic data, and theoretical details.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(d) Schmedake, T. A.; Haaf, M.; Apeloig, Y.; Muller, T.; Bukalov, S.;
̈
West, R. J. Am. Chem. Soc. 1999, 121, 9479−9480.
(18) Bader, R. F. W. Atoms in Molecules. A Quantum Theory; Clarendon
Press: Oxford, 1990.
AUTHOR INFORMATION
Corresponding Authors
hroesky@gwdg.de
■
(19) Chesnut, D. B. Chem. Phys. 2001, 271, 9−16.
(20) Rendler, S.; MacMillan, D. W. C. J. Am. Chem. Soc. 2010, 132,
5027−5029.
birger.dittrich@chemie.uni-hamburg.de
frenking@chemie-uni-marburg.de
Notes
(21) (a) Schmidt, D.; Berthel, J. H. J.; Pietsch, S.; Radius, U. Angew.
Chem. 2012, 124, 9011−9015; Angew. Chem., Int. Ed. 2012, 51, 8881−
8885. (b) Momeni, M. R.; Rivard, E.; Brown, A. Organometallics 2013,
32, 6201−6208.
The authors declare no competing financial interest.
(22) (a) Arrowsmith, M.; Hill, M. S.; Kociok-Kohn, G.; MacDougall,
̈
ACKNOWLEDGMENTS
■
D. J.; Mahon, M. F. Angew. Chem. 2012, 124, 2140−2142; Angew. Chem.,
Int. Ed. 2012, 51, 2098−2100. (b) Al-Rafia, S. M. I.; McDonald, R.;
Ferguson, M. J.; Rivard, E. Chem.Eur. J. 2012, 18, 13810−13820.
(c) Bose, S. K.; Fucke, K.; Liu, L.; Steel, P. G.; Marder, T. B. Angew.
Chem., Int. Ed. 2014, 53, 1799−1803; Angew. Chem. 2014, 126, 1829−
1834.
Dedicated to Prof. Konrad Seppelt on the occasion of his 70th
birthday. H.W.R. thanks the Deutsche Forschungsgemeinschaft
(DFG RO 224/60-I) for financial support. We would like to
thank Dr. Alke Meents for beamline P11.
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