Generation and EPR Spectroscopy of the First Silenyl Radicals, R2C=Si.?R: Experiment and Theory

Article abstract of DOI:10.1002/anie.201901772

The first two persistent silenyl radicals (R₂C=Si^.?R), with a half-life (t_1/2) of about 30 min, were generated and characterized by electron paramagnetic resonance (EPR) spectroscopy. The large hyperfine coupling constants (hfccs) (a(²⁹Si_α)=137.5–148.0 G) indicate that the unpaired electron has substantial s character. DFT calculations, which are in good agreement with the experimentally observed hfccs, predict a strongly bent structure (?C=Si?R=134.7–140.7°). In contrast, the analogous vinyl radical, R₂C=C^.?R (t_1/2≈3 h), exhibits a small hfcc (a(¹³C_α)=26.6 G) and has a nearly linear geometry (?C=C?R=168.7°).

Full text of DOI:10.1002/anie.201901772

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Accepted Article
Title: Generation and Electron Paramagnetic Spectroscopy of the First
Silenyl Radicals (R2C=Si•-R). Experiment and Theory
Authors: Daniel Pinchuk, Yosi Kratish, Jomon Mathew, Lieby
Zborovsky, Dmitry Bravo-Zhivotovskii, Boris Tumanskii, and
Yitzhak Apeloig
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Generation and Electron Paramagnetic Spectroscopy of the First
•
Silenyl Radicals (R C=Si -R). Experiment and Theory
2
+
+
Daniel Pinchuk , Yosi Kratish , Jomon Mathew, Lieby Zborovsky, Dmitry Bravo-Zhivotovskii, Boris
*
*
Tumanskii and Yitzhak Apeloig
Dedicated to Professor Josef Michl on the occasion of his 80th birthday
Abstract: The generation and characterization by electron
4 3 2 2 2 2
CCl , Ph CCl, MX (ZnCl , HgCl , HgF )) to 1a or 1b yields the
paramagnetic resonance (EPR) spectroscopy of the first two
persistent silenyl radicals 2a and 2b, respectively (Eq. 1), which
were characterized by EPR spectroscopy (Figure 1). Silenyl
radicals 2a and 2b have a half-life time (t1/2) at 300 K of about 30
min. Interestingly, both 2a and 2b do not dimerize, due to steric
reasons. [10] Silenyl radical 2a decays to a complex mixture of
products, among them is the corresponding H-substituted silene
•
persistent silenyl radicals (R
2
C=Si -R) having a half-life time (t1/2) of
about 30 min is reported. The large hyperfine coupling constants
29
(

hfccs) (a( Si ) = 137.5-148.0 G) indicate a substantial s-character of
the unpaired electron. DFT calculations, which are in good agreement
with the experimentally observed hfccs, predict a strongly bent
structure (∡C=Si-R = 134.7°-140.7°). In contrast, the analogous vinyl
3
which is formed via hydrogen abstraction. When nBu SnH was
•
13
radical (R
2
C=C -R) (t1/2 ~ 3 h), exhibits a small hfcc (a( C

) = 26.6 G)
added to the reaction mixture (after the generation of radical 2a)
the H-substituted silene becomes the major product, but other
unidentified products are also observed. For this reason, a
cleaner method to generate the silenyl radicals was searched.
and has a nearly linear geometry (∡C=C-R = 168.7°).
•
Vinyl radicals, R
2
C=C -R, are common intermediates widely
[1]
used in organic synthesis.
extensively both experimentally [2] and theoretically. In contrast,
Consequently, they were studied
[3]
very little is known about the analogous group 14 congener
•
radicals, R
2
E=E` -R, (E, E`= C, Si, Ge) and they were observed
only in cryogenic matrix. [4] Disilenyl radicals were proposed as
intermediates in some reactions, [5] but spectroscopic evidence for
their presence was not provided. Silicon is the closest congener
of carbon, yet the fundamental properties of many silicon and
carbon compounds, in particular multiply-bonded compounds, are
Silyl mercury compounds are good precursors for
[
11]
generating cleanly silyl radicals.
silene 3 was therefore prepared by addition of tBuHgCl to 1a
Scheme 1, step a). Irradiation of a toluene solution of 3 (λ > 250
The mercury-substituted
[
6]
very different.
It is therefore of fundamental interest to
(
characterize new silicon compounds and silicon-centered reactive
intermediates and to compare their properties with their carbon
analogues, adding significantly to our basic understanding of
molecular geometry, chemical bonding, electronic structure, etc.
The recent synthesis of the first metal substituted silenes
(R
C=SiRLi) [7] and disilenes (R Si=SiRLi) [8] expand significantly
2 2
the repertoire of silicon reagents leading to the isolation of novel
doubly-bonded silicon compounds. [9]
nm, 30 min, r.t) within the EPR spectrometer cavity, produces an
intensive EPR signal of silenyl radical 2a, accompanied by
precipitation of mercury (Scheme 1, step b). Upon decay of radical
a two products are observed: H-substituted silene 4 and cyclic
,3-disilacyclopentane 5 (Scheme 1, step c). Interestingly, when
is irradiated for 2 h, cyclic 5 is the only product. [12]
2
1
3
Here, we report the generation and characterization by
electron paramagnetic resonance spectroscopy (EPR) supported
by the results of Density Functional Theory (DFT) calculations, of
•
Scheme 1. Synthesis and photolysis of mercury substituted silene 3.
the first two persistent silenyl radicals R
2
C=Si -R (2a and 2b). The
synthesis and calculated properties of the analogous persistent
•
We suggest that silenyl radical 2a decays by hydrogen
abstraction (probably from the toluene solvent), yielding silene 4
which under further irradiation undergoes intramolecular
cyclization yielding 5. Photochemical intramolecular cyclization of
silenes was previously reported. [ To support this proposal, 4
was prepared and its photochemistry was studied. Addition of
vinyl radical R
2
C=C -R, are also reported.
Recently, we reported the synthesis of two silenyl lithiums
a [7a] and 1b. [7b] Addition of a variety of reagents ((CO)
5
ReBr,
1
13]
Dr. D. Pinchuk, Dr. Y. Kratish, Dr. J. Mathew, Dr. L. Zborovsky, Dr. D. Bravo-
Zhivotovskii, Dr. B. Tumanskii and Prof. Y. Apeloig.
Schulich Faculty of Chemistry and the Lise Meitner-Minerva Center for
Computational Quantum Chemistry
Technion-Israel Institute of Technology
Haifa 32000 (Israel)
tBuOH to 1a yields silene 4 (Scheme 2, step a). The structure of
4
was confirmed by NMR and mass spectrometry. [
14a]
Silene 4
exhibits a characteristic 29Si NMR signal at 134 ppm, assigned to
a sp -silicon atom, in excellent agreement with the calculated
value of 135 ppm [ . In the 1H NMR spectrum the silenic
hydrogen (H)Si=C appears at 6.67 ppm (J(Si-H) = 173.73 Hz).
Similar NMR values were previously reported for hydrogen
substituted silenes. [ Silene 4 is stable at room temperature,
2
14b]
E-mail: tboris@technion.ac.il, apeloig@technion.ac.il
[
+]
D. Pinchuk and Y. Kratish contributed equally to this work.
15]
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Angewandte Chemie International Edition
10.1002/anie.201901772
COMMUNICATION
however, irradiation (λ > 250 nm, 2 h, r.t) of 4 produces
quantitatively cyclic 5 (Scheme 2, step b), supporting the
suggestion that 5 is not formed directly from radical 2a, but is
obtained from silene 4 which is the initial decay product of radical
Support for the interpretation of the EPR spectrum and
additional insight is provided by quantum-mechanical calculations.
The geometries and EPR parameters of 2a and 2b were
[
24]
calculated using Density Functional Theory (DFT)
at the
2a.
UPBE0 [25]/def2-TZVPP//UB97D [26]/6-311+G(d,p) level of theory.
[
22]
The calculated EPR hfccs for 2a are in good agreement with
the experimental hfccs (Table 1). The calculated spin density
Figure 2a) indicates a significant hyperconjugative interaction
between the singly occupied orbital of Si1 with the trans σ*(C1-
(

4
Scheme 2. Synthesis and photolysis of hydrogen-substituted silene 4.
4
29
Si ) orbital, resulting in the larger a( Si (trans)) compared with
2
9
3
a( Si (cis)).
Silenyl radical 2a was studied by EPR spectroscopy (Figure
2
a is calculated to have a strongly bent structure at the
1
). Its spectrum is characterized by an unusually large hyperfine
2
1
1
radical center with a ∡Si Si C bond angle of 140.7° (Figure 2),
by 16.6° larger than in silenyl lithium 1a (124.1°) [7a]. The C =Si1
bond length in 2a is 1.744 Ǻ, by 0.018 Ǻ shorter than in 1a (1.762
coupling constants (hfccs) of the unpaired electron with the 29Si
,

1
29Si
and 29Si
nuclei; [16a] [a( Si ) = 137.5 G [16b], a( Si
29
29
) = 32.3 G;




2
9
29
a( Si(trans)) = 17.5 G] ( Si, I = ½, natural abundance 4.7%).
These hfccs are much larger than in analogous tri-coordinated
silyl radicals. [17] For example, the hfccs in [(tBu
Ǻ) [7a]. The Si =C1 bond in 2a is planar (∡Si C Si Si = 0.0)
1
4
1
1
2

[7a]
1
2
compared with 5.0 in 1a . The Si -Si distance of 2.396 Ǻ is by
.09 Ǻ shorter than in 1a (2.486 Ǻ)[7a]. Natural Bond Orbital (NBO)
2
MeSi)
2
HSi]-
0
•
•
(
tBu
2 2 2 3
MeSi) Si and (tBu MeSi) Si , having similar silyl substituents
[
27]
in 2a has a s(58%)p(42%) hybridization
analysis indicates that Si
with electron occupancy of (0.9 el.), consistent with the large

2
9
29
29
to 2a, are: [a( Si

 
) = 59.3 G, a( Si ) = 7.3 G, a( Si ) = 10.4 G],
) = 58.0 G, a( Si

) = 7.9 G], respectively [19] The
[
18]
29
29
and [a( Si

2
2
9
1
a( Si

) of 137.5 G. The data for radical 2b is similar to that for
9
very large a( Si

) points to a substantial s character of the
2a (Table 1).
unpaired electron orbital in silenyl radical 2a and therefore a
[
17]
strongly bent structure at the radical center.
The large -hfcc
29
•
[19]
in 2a (a( Si

) = 32.3 G), relative to (tBu
2
MeSi)
3
Si (7.9 G)
is
attributed to increased contribution of the direct coupling
mechanism in 2a relative to (tBu
29Si4
2
MeSi)
3
Si•.
[20]
The hfcc of
(trans) (atom numbering is given in Figure 1) is 17.5 G; in
contrast, coupling with 29Si3(cis) is not observed experimentally [21]
and according to calculations is 4.4 G (Table 1). The larger hfcc
of 29Si4(trans) than that of 29Si3(cis) supports the bent structure of 2a
and is a manifestation of the larger hyperconjugation interactions
with a substituent anti to the singly occupied orbital than that with
a syn-substituent. [2d, 17] The hyperfine coupling with the -H atoms
of the methyl group on Si2 [a(3H
) = 2.0 G] are much larger
) ~ 0.1-0.3 G]. [17]

Figure 2. Calculated spin density (hydrogen atoms are omitted for clarity). (a)
of radical 2a. (b) of radical 6.
•
compared to that in (R
3
Si)
3
Si radicals [a(3H

Also noticeable is the marked upfield shifted g-factor (g = 2.001)
•
[17]
Table 1. Experimental and calculated
a
hfccs, geometries and the s/p character
relative to (R
3
Si)
3
Si radicals (g = 2.004-2.005).
Silenyl radical
Substitution of
of the singly occupied orbital in radicals 2a, 2b and 6.
[
22]
2b (Eq. 1) has a similar EPR spectrum to 2a.
29
s/p-
character
in %)
Si
or

∡
SiEE’ E=E‘ bond
29
13
29
29
Radical
Method
Si

C


Si(trans) Si(cis) C(3H )
the silyl group on C in 2a by a 1-adamantyl group in 2b results in
(
°) distance (Å)
13
(
C

2
9
a small increase in a( Si

) to 148.0 G, similarly to the effect
Experimental
-
-
-
137.5 32.3
-
17.5
15.6
-
-
2.0
2.2
1.8
2.2
2
a
b
•
[17]
Calculated 140.7
Experimental
Calculated 134.7
Experimental
Calculated 168.7
1.744
-
1.761
-
1.312
58/42
122.2 30.7 8.2
148.0 31.0
148.7 29.4 8.8
4.4
-
4.1
observed in (R
3
Si)
3
Si radicals.
-
-
56/44
-
-
2
-
b
-
26.6
21.1
9.4 15.4
9.6 14.6
46.5
43.1
46.5 0.11
39.4
6
b
3/97
0.7
a
UPBE0/def2-TZVPP//UB97D/6-311+G(d,p); b C(6H

)
Additional support for the proposed structure of silenyl
radical 2a is provided by high resolution mass spectrometry using
atmospheric pressure chemical ionization (APCI), in which ions
with the exact mass of 2a and its expected isotope distribution are
observed by using both positive-ion and negative-ion methods
Figure 3). [ Molecular ions of 2a may also be formed by a
28]
(
hydrogen atom loss from 4 or 5. To exclude this possibility, 4 and
were measured under the same conditions (positive-ion APCI),
and only the molecular ions of [4+H] and [5+H] are observed
with no traces of 2a ions, supporting the conclusion that the
observed 2a mass, results from the presence of radical 2a.
5
Figure 1. (a) EPR spectrum of silenyl radical 2a (300 K, toluene) [g = 2.001;
a(29Si ) = 137.5 G; a(29Si ) = 32.3 G; a(29Si(trans)) = 17.5 G]; (b) Simulated EPR
+
+


[
23]
spectrum of 2a ; (c) Expanded central quartet at g=2.001, [a(3H) = 2.0 G]; (d)
Simulation of the central quartet. [23]
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Angewandte Chemie International Edition
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COMMUNICATION
2
9
2,3
) values, for the trans-Si3

Interestingly, the calculated a( Si
(
2
43.1 G) and cis-Si (39.4 G) are different, indicating a slightly bent
structure while experimentally they are the same at 46.5 G. The
2
9
2,3
experimental equivalence of the a( Si ) results from fast
inversion around the radical center in line with the calculated
•
of only ~1 kcal·mol-1. [3f] The
inversion barrier for H
2
C=C -SiMe
3
calculated spin density distribution (Figure 2b) indicates
significant hyperconjugative interactions between the singly
1
2
2
2
3
occupied orbital on C with σ* (C -Si ) and σ* (C -Si ), resulting in
Figure 3. (a) Negative-ion APCI high resolution mass spectrum of 2a; (b)
Simulated mass spectrum of 2a.
2
9
relatively large a( Si
6

) values (Table 1). According to calculations,
has a slightly bent structure, ∡C =C -Si = 168.7°, r(C =C ) =
1.312 Ǻ and it is slightly twisted with a Si C C Si torsion angle of
2
1
1
1
2
1
1
2
3
Comparison of silenyl radical 2a with the analogous vinyl
•

radical 6 is of interest. Vinyl radical (tBuMe
6), was generated by irradiation of (tBuMe
presence of tBuMe SiC≡CSiMe
tBu. [30] 6 is persistent at 300 K;
1/2 is ~3 h), significantly longer than that of (Me Si)
C=C•-
); t1/2 ~1 min at 273 K. [2c, 2d] The higher kinetic stability of 6
2
Si)
2
C=C -(SiMe
2
tBu)
16.8 . NBO analysis indicates that the unpaired electron resides
(0.8 el. occupancy) in an almost pure 2p orbital (s(3%)p(97%)),
2 2
Si)
Hg [29] in the
(
13
1
2
2
consistent with the observed small hfcc a( C ) of 26.6 G. The

(
(
t
3
2
significant difference in the bending angle at the radical center
between silenyl radical 2a and vinyl radical 6 is mainly attributed
to the extension mismatch between the radii of the 3s and the 3p
orbitals of Si, reducing the degree of s-p hybridization in 2a [31]
In conclusion, we report the generation of a new class of
SiMe
3
results from the larger tBuMe
decay reactions.
2
Si substituents which slow down
The EPR spectrum of vinyl radical 6 (Figure 4), is
13
1
characterized by g = 2.0021 and the following hfccs: a( C

) =
) = 46.5 G,
a(9H) = 0.056 G, a(6H) = 0.11 G, a(18H) = 0.22 G, a(12H) = 0.45
G (Table 1). The relatively small hfcc with 13
of 26.6 G, indicates
persistent (t1/2(300 K) ~ 30 min) silenyl radicals, 2a and 2b. The
1
3
2
29
1
29 2,3
29
2
6.6 G, a( C

) = 15.4 G, a( Si

) = 9.4 G, a(2 Si


large EPR hfccs, a( Si ) = 137.5 and 148.0 G, for 2a and 2b,
respectively, point to a significant s-character of the singly-
occupied orbital, in agreement with the radicals strongly bent
C

a small s-character of the orbital in which the unpaired electron
resides in agreement with the calculated C=C-Si of 168.7.
structures (∡C=Si-R = 134.7°-140.7°). In contrast, the analogous
13

vinyl radical 6 has a small hfcc (a( C ) = 26.6 G) and a nearly
Furthermore, the identical hfccs value with the two -silicon atoms, linear geometry (∡C=C-Si = 168.7°). The comparison of 2a vs 6
indicates a nearly linear structure or fast inversion around the
radical center. Similar hfccs were previously obtained for the
provides one more example for the different structural rules for
silicon compounds, resulting from the 3s-3p orbitals mismatch,
compared with analogous carbon compounds. [6]
•
). [2c, 2d] In contrast,
short-lived vinyl radical (Me
3
Si)
2
C=C -(SiMe
3
•
the strongly bent vinyl radical H
2
C=C -H has a much larger hfcc
of 13C
= 107.5 G. [2c] Thus, silyl substitution induces linearization
Supporting Information

at the radical center, similar to its planarization effect in tri-
coordinate silyl radicals.
Experimental procedures, EPR spectrum of 2b, synthesis and NMR data
of 3, 4, 5, DFT optimized geometries and energies of 2a, 2b, 3, 4, 5 and 6,
calculated NMR values of 3, 4 and 5, and dimerization energies for silenyl
radicals are presented in the Supporting Information.
Acknowledgements
This research was supported by the Israel Science Foundation
(
ISF) and the Minerva Foundation in Munich. B.T. and D.B.-Z,
are grateful to the Ministry of Immigrant Absorption, State of
Israel, for a Kamea fellowship.
Keywords: Silenyl radicals • Vinyl radicals • Silyl radicals • EPR
•
Metal substituted silenes
[
1]
a) W. H. Guo, H. Y. Zhao, Z. J. Luo, S. Zhang, X. Zhang, ACS Catalysis
2019, 9, 38-43; b) K. de la Vega-Hernández, E. Romain, A. Coffinet, K.
Bijouard, G. Gontard, F. Chemla, F. Ferreira, O. Jackowski, A. Perez-
Luna, J. Am. Chem. Soc. 2018, 140, 17632-17642; c) N. Chalotra, A.
Ahmed, M. A. Rizvi, Z. Hussain, Q. N. Ahmed, B. A. Shah, J. Org. Chem.
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S. Coutros, G. d. P. Gomes, I. V. Alabugin, J. Org. Chem. 2015, 80,
Figure 4. (a) EPR spectrum of vinyl radical 6 (300 K, hexane); (b) EPR spectrum
of 6 at high gain; (c) Expanded EPR spectrum of 6 at high resolution [g = 2.0021;
a(9H)=0.056 G, a(6H)=0.11 G, a(18H)=0.22 G, a(12H)=0.45 G]; (d) Simulated
EPR spectrum (the experimental hfccs were used for the simulation). [23]
11706-11717; e) K. Pati, A. M. Hughes, H. Phan, I. V. Alabugin, Chem.
Eur. J. 2014, 20, 390-393; f) D. P. Estes, J. R. Norton, S. Jockusch, W.
Sattler, J. Am. Chem. Soc. 2012, 134, 15512-15518; g) N. F. O’Rourke,
K. A. Davies, J. E. Wulff, J. Org. Chem. 2012, 77, 8634-8647; h) F. Dénès,
The calculated EPR hfccs values of 6 (Table 1) are in good
[
19]
agreement with the experimental values (Table 1).
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Angewandte Chemie International Edition
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COMMUNICATION
F. Beaufils, P. Renaud, Synlett, 2008, 16, 2389-2399; i) S. Z. Zard,
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have a stronger support for the structure of 5 we calculated several
diastereomers and compared their calculated NMR with the experimental
values. We found that the most stable diastereomer had the best
agreement with the experimental values. For NMR spectra and the
calculated NMR data for 5 see supporting information
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[14] a) For the NMR and mass spectrometry data see supporting information.
b) The NMR values of silene 4 were calculated at the HCTH407/def2-
TZVPP//B97D/6-311+G(d,p) level.
Chem. Soc. Perkin Trans. 2, 1999, 2305-2313; e) C. Galli, A. Guarnieri,
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[16] a) The common nomenclature for radicals is such that the atom on which
the unpaired electron is centered, is named  and the more distant atoms
are (), (), etc.; b) The 29Si hfcc slightly depends on the solvent and
increases to a(29Si) = 139.1 G in hexane at 300 K.
[17] a) B. Tumanskii, M. Karni, Y. Apeloig, in Organosilicon Compounds (Ed.:
V. Y. Lee), Academic Press, 2017, 231-294; b) B. Tumanskii, M. Karni,
Y. Apeloig, in Encyclopedia of Radicals in Chemistry, Biology and
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4
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According to DFT calculations (see Supporting Information) the
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This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201901772
COMMUNICATION
COMMUNICATION
Daniel Pinchuk, Yosi Kratish, Jomon
Mathew, Lieby Zborovsky, Dmitry Bravo-
Zhivotovskii, Boris Tumanskii and
Yitzhak Apeloig.
Generation and Electron
Paramagnetic Spectroscopy of the
•
2
The first two persistent (t1/2 ~ 30 min) silenyl radicals (R C=Si -R) were generated
from lithium and mercury substituted silenes and are characterized by EPR
spectroscopy and density functional theory (DFT) calculations. According to the
hyperfine coupling constants (hfccs) and the calculations, silenyl radicals have a
strongly bent structure (<SiSi=C = 140.7°), in contrast to the nearly linear R
The spin is mostly located in a Si(sp) orbital.
•
First Silenyl Radicals (R
2
C=Si -R).
Experiment and Theory
•
2
C=C -R.
This article is protected by copyright. All rights reserved.