Stable Radicals from Commonly Used Precursors Trichlorosilane and Diphenylchlorophosphine

Article abstract of DOI:10.1021/jacs.5b02403

Intermediate species dichlorosilylene was generated in situ from trichlorosilane and inserted into the P-Cl bond of diphenylchlorophosphine (Ph₂P-Cl) to obtain Ph₂P-SiCl₃ (1). Monodechlorination of 1 by cyclic alkyl(amino)

Full text of DOI:10.1021/jacs.5b02403

Communication
pubs.acs.org/JACS
Stable Radicals from Commonly Used Precursors Trichlorosilane and
Diphenylchlorophosphine
Sudipta Roy,† A. Claudia Stuckl,† Serhiy Demeshko,† Birger Dittrich,*,‡ Jann Meyer,† Bholanath Maity,§
̈
Debasis Koley,*,§ Brigitte Schwederski,∥ Wolfgang Kaim,*,∥ and Herbert W. Roesky*,†
†Institut fur Anorganische Chemie, Georg-August-Universitat, 37077 Gottingen, Germany
̈
̈
̈
̈
‡Institut fur Anorganische und Angewandte Chemie, Universitat Hamburg, 20146 Hamburg, Germany
̈
§Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, West Bengal,
India
∥Universitat Stuttgart, Institut fur Anorganische Chemie, 70569 Stuttgart, Germany
̈
̈
S
* Supporting Information
was utilized as an elegant precursor for the syntheses of low-
coordinate silylenes [(NHC→SiCl)2 and (NHC→Si)2].7c
Analogously, NHC forms the NHC→PCl3 adduct with
trichlorophosphine (PCl3). NHC→PCl3 was completely dehalo-
genated to obtain the bisphosphinedene, (NHC→P)2.7d The
electronic structures often dramatically change7e in case of the
cyclic alkyl(amino) carbene (cAAC) analogues of the above-
mentioned compounds. The affinity of carbene towards Si and P
centers has only rarely been studied.8 Two decades ago, du Mont
et al. reported on the first synthesis route of phosphine-
substituted chlorosilane.9 Afterward, several other research
groups10 developed various synthesis routes for the preparation
of phosphasilenes. However, to the best of our knowledge there
are not yet any reports on the stable and isolable radicals of
phosphino-chlorosilanes.11 Eventually, we realized that the
chemistry of carbene/phosphino-chlorosilane is completely
different than that of carbene/SiCl4 and carbene/PCl3. Herein,
we report on the synthesis, characterization, and theoretical
calculations of a carbene-stabilized diphenylphosphino-dichlor-
osilane radical with the general formula Ph2P-Si(cAAC·)Cl2 (2,
Scheme 1; Me2-cAAC (2a) and Et2-cAAC (2b)).
ABSTRACT: Intermediate species dichlorosilylene was
generated in situ from trichlorosilane and inserted into the
P-Cl bond of diphenylchlorophosphine (Ph2P-Cl) to
obtain Ph2P-SiCl3 (1). Monodechlorination of 1 by cyclic
alkyl(amino) carbenes (cAACs)/KC8 in THF at low
temperature led to the formation of stable radicals Ph2P-
Si(cAAC·)Cl2 (2a,b). Compounds 2a,b were characterized
by X-ray single crystal diffraction, mass spectrometry and
studied by cyclic voltammetry and theoretical calculations.
Radical properties of 2 are confirmed by EPR measure-
ments that suggest the radical electron in 2 couples with
14N (I = 1), 35/37Cl (I = 3/2), and 31P (I = 1/2) nuclei
leading to multiple hyperfine lines. Hyperfine coupling
parameters computed from DFT calculations are in good
agreement with those of experimental values. Electronic
distributions obtained from the theoretical calculations
suggest that the radical electron mostly resides on the
carbene C of 2.
adicals and carbenes are very reactive intermediates1 that
Precursor Ph2P-SiCl3 (1) has been synthesized via the
insertion reaction of SiCl2 into the P-Cl bond of Ph2P-Cl.
2
Rplay important roles in numerous chemical reactions. The
majority of these two types of intermediate species2 are generally
produced in low concentrations with a short lifetime. Recently,
carbenes were suggested to be suitable ligands toward the
stabilization of unstable radical intermediates.3 The halogen
derivatives of main group elements are often chosen for the
generation of radicals via reductive dehalogenation.4 Chlor-
osilanes are in general important chemical species because of
their utilization in industry,5a and they are chemical text book
examples as well.5b HSiCl3 and monoalkyl-substituted chlor-
osilane (RSiH2Cl, R = tBu3Si) react with carbenes (N-
heterocyclic carbene, NHC) in a 2:1 molar ratio to produce
stable silylene, NHC→SiCl2, and NHC→SiHR, respectively,
under elimination of NHC·HCl.6 This synthesis approach
facilitates the development of compounds with low-valent Si.
The above-mentioned stable silylenes can be used to activate
small organic molecules.7a Generally, transition-metal-complex-
es7b are involved in such reactions. The response of
tetrachlorosilane (SiCl4) toward carbenes is little different.
They favor the formation of a stable adduct, NHC→SiCl4,7c that
Scheme 1. Synthesis Route of Ph2P-Si(cAAC·)Cl2 (2a,b)
Received: March 6, 2015
Published: March 27, 2015
© 2015 American Chemical Society
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DOI: 10.1021/jacs.5b02403
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Communication
Intermediate species SiCl2 was generated in situ12,13 (reacting
HSiCl3 with Et3N in toluene at a 1:1.1 molar ratio) below −80 °C
and reacted with Ph2PCl in 1:1 molar ratio to obtain 1 as a
colorless high-boiling liquid in 75% yield. Treatment of 1 with
cAAC in the presence of KC8 in 1:1:1 molar ratio in THF at
−105 °C to room temperature resulted in the formation of a
greenish-red solution. Red blocks/rods of 2 (cAAC = Me2-cAAC
(2a), Et2-cAAC (2b), Scheme 1) were isolated in 22−25% yield.
2a,b were synthesized by controlling the reaction temperature
and the molar ratio of the precursor and the reducing agent. A 1:1
molar mixture of cAAC/KC8 and one equivalent of 1 were placed
separately in two different flasks that were cooled using a liquid
nitrogen bath. Precooled THF (∼ −100 °C) was added to the
flask containing 1 via a canula. The solution of 1 was then passed
into the other flask containing cAAC and KC8 (∼ −105 °C). The
resultant reaction solution was stirred for 5 min at ∼ −105 °C by
frequent manual dipping of the flask into liquid nitrogen. At this
temperature, no color change was observed. The temperature of
the reaction solution was allowed to rise slowly, over 5−10 min;
during this time, the color of the slurry changed from colorless to
green. This is the crucial part of the synthesis protocol. The green
solution was vigorously stirred with occasional manual dipping of
the flask into the liquid nitrogen bath (six to eight times over 40−
45 min; each dipping for ∼15 s with manual shaking) to obtain an
intensely green solution. The temperature was slowly raised to
room temperature (over 30 min), and the resultant solution was
stirred (for 3 h at room temperature ) to produce a greenish-red
solution with deposition of black graphite that was separated by
filtration. The filtrate was dried under vacuum and extracted with
n-hexane. Dark red crystals of 2 are formed in 22−25% yield
when the concentrated n-hexane solution is stored in a
refrigerator (0 °C). Crystals of 2 are separated by filtration and
washed with a flash of n-hexane inside the glovebox and dried
under vacuum. 2a,b are further purified via recrystallization at
−32 °C from THF or toluene (2a) and n-hexane (2b, at 0 °C; see
Supporting Information). For comparison, when a similar
reaction was carried out with equivalent amount of NHC instead
of cAAC, NHC →SiCl2 was isolated.6a
Figure 1. Molecular structure of 2a. H atoms are omitted for clarity.
Selected experimental bond lengths, calculated at the M06-2X/def2-
SVP level of theory and given in angstrom, and angles given in degrees:
Si1-C1 1.826(3) [1.842], Si1-Cl1 2.0503(12) [2.078], Si1-Cl2
2.0790(11) [2.096], Si1-P1 2.2643(11) [2.285], C1-N1 1.392(4)
[1.382], C1-Si1-P1 116.56(10) [119.42], Cl1-Si1-Cl2 105.37(5)
[103.94], P1-Si1-Cl1 109.19(5) [112.02], P1-Si1-Cl2 99.97(4)
[97.07], C1-Si1-Cl1 111.62(11) [112.05], C1-Si1-Cl2 113.04(10)
[110.17], Si1-P1-C21 103.17(10) [105.71], Si1-P1-C27 99.40(10)
[98.45], C21-P1-C27 103.09(13) [105.64].
cAACs are well-known for possessing lower-lying LUMO.3,8c In
situ formation of a carbene radical anion intermediate (cAAC.−)
is favorable by −9.6 kcal mol−1. cAAC.−K+ reacts with 1 to
produce 2 with the formation of KCl. This step of the reaction is
favorable by −32.8 kcal mol−1. Moreover, the formation of the
radical at the carbene carbon is advantageous by −37.8 kcal
mol−1 in comparison to that at the phosphorus atom (Scheme 1).
2a,b are soluble in n-hexane, THF, and toluene and are stable
at 0 °C for several months. Crystals of 2a,b are stable at room
temperature for several months in an inert atmosphere, and
decompose to [(cAAC)PPh2]+Cl− after several days on exposure
to air. 2a,b melt in the range of 157−158 °C (2a) and 122−123
°C (2b), and have been further characterized by mass
spectrometry (Supporting Information). UV-vis spectra of 2a,b
were recorded in THF which show broad absorption bands at
430 and 435 nm, respectively (Supporting Information).
2a,b are characterized by X-ray single crystal diffraction (for
2b, see Supporting Information). 2a crystallizes in the
monoclinic space group P21/c. The central Si atom adopts a
near-tetrahedral coordination geometry and is bound to one P,
one carbene C, and two Cl atoms (Figure 1). The Si-CcAAC bond
distance in 2a is 1.826(3) Å, which is longer (by ∼0.12 Å) than
that of the CcAAC → Si coordinate13 bond but close to that of the
Si-C single bond,14g−i suggesting a covalent σ-sharing bond
between Si and carbene C atoms. The C-N bond distance is
1.392(4) Å, which is close to those values found in carbene-
centered radicals.14 N1 is located 0.12 Å above the C1C4C9
plane. The carbene C atom is almost (0.06 Å above) in the
C2Si1N1 plane. All these bond parameters suggest that the
radical electron resides on the carbene C atom (Schemes 1 and
2). One Cl atom (Cl2) is oriented nearly parallel to the C2C1N1
plane (angle between C2C1N1 and C1Si1Cl2 = 66.85°). The
C1Si1P1 and C2C1N1 planes exhibit an angle of 48.55°, whereas
Cl1 is almost located in the C2C1N1 plane (7.82°). Selected
bond lengths and bond angles are given in the caption of Figure
1.
[(Me2-cAAC)PPh2]+Cl− (Supporting Information) was ob-
tained as a major product when 1 was directly reacted with Me2-
cAAC in a 1:1 molar ratio, suggesting that Me2-cAAC prefers to
bind at the P center with the elimination of SiCl2. When 1 was
treated with KC8 in THF at room temperature, unreacted 1 is
recovered even after 2 days, indicating that the formation of
Ph2P-Si(·)Cl2 is not facile. On the basis of the experimental
evidence, a possible mechanism for the formation of 2a is
proposed in Scheme 2, and the energetics are calculated for 2a.
a
Scheme 2. Proposed Mechanism for the Formation of 2
DFT calculations of 2a,b were carried out for the doublet
states at the UM06-2X/SVP level of theory (for details, see
Computational Details in Supporting Information) to illustrate
the electronic structures and bonding. The optimized structure
a
ΔEe (M06-2X/SVP level) given in kcal mol−1.
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Figure 3. Experimental and simulated EPR spectra of 2a at 298 K.
calculated values are in good agreement with the simulated/
experimental results (vide supra). It is not surprising that the P1
atom shows maximum hyperfine coupling constant (20.54 G in
2a) because of its parallel alignment with the pz orbital of the C1
atom. The calculated proton affinities of 2a,b when one proton
binds to the C1 center are 184.1 and 219.2 kcal/mol,
respectively.
X-band EPR resonance spectra of 2a,b were recorded in
toluene (Figure 3, for 2b see Supporting Information). 2a
exhibits a partially resolved EPR spectrum that could be
simulated with data according to the DFT-calculated hyperfine
coupling: a(31P) 15.6 G (calcd 20.5 G), a(14N) 6.5 G (calcd 4.2
G), a(35Cl) 4.1 G (calcd 3.1 G).
Figure 2. Computed Mulliken spin density (top) of 2a,b at the UM06-
2X/TZVP//UM06-2X/SVP level of theory. Laplacian distribution
[∇2ρ(r)] in N1-C1-Si1 (bottom left) and C1-Si1-P1 plane (bottom
right) of 2a. Solid lines indicate the areas of charge concentration
(∇2ρ(r) < 0), whereas dotted lines represent charge depletion (∇2ρ(r) >
0). The range of contours of the Laplacian are −8 × 102 to +8 × 102.
Solid lines connecting atomic nuclei (black) are the bond paths, and
those lines separating the atomic basins (purple) indicate the zero-flux
surface crossing the molecular plane.
shows strong resemblance to the X-ray crystal structure, as seen
from the alignment and superposition of the structure of 2a
(Figure S9). NBO analysis of 2a entails that the C1 atom is
connected with N1, Si1, and C2 via single bonds with
occupancies of 1.953, 1.957, and 1.963 e, respectively. The C1
atom contributed ∼73% electron density to the C1-Si1 bond,
indicating polar character. This result is supported by the
properties of the bond critical point (BCP) obtained from AIM
(atoms in molecules) calculations. The electron density, ρ(r), at
the BCP of C1-N1 (0.294), C1-C2 (0.238), and C1-Si1 (0.119)
bonds along with the respective Laplacian (∇2ρ(r); −0.693,
−0.547, and +0.316, respectively) indicates covalent interaction
in former two bonds and closed-shell interaction in the last one.
However, the calculated ellipticity for C1-Si1 (εBCP = 0.208) is
much lower than that of a C→Si donor-acceptor-type bond (εBCP
= 0.56) reported previously,14a indicating its covalent nature.
NBO studies also suggest that C1 and P1 atoms contain a LP
(lone pair) with an occupancy of 0.866 (single electron, along pz
orbital; considering N1-C1-Si1-C2 lying on xy plane) and 1.882
e, respectively. This is further supported by results obtained from
the Laplacian of (3,−3) critical point, called valence-shell charge
concentrations (VSCC), shown in Figure 2 (bottom). From the
calculated Mulliken spin density plot for 2a,b shown in Figure 2
(top), it is revealed that the unpaired electron is mostly located
on the carbene C (ca. 76% in 2a and 75% in 2b, Figure S10), with
a comparatively lower contribution from the N1 atom (18% in 2a
and 20% in 2b) present in the cAAC fragment. Similar results can
be perceived by visualizing the SOMO, which is mostly
contributed by the C1, Si1, and N1 atoms (Figure S11). It is
interesting to note that the unpaired electron exhibits some finite
occupancy over Cl2 and P1 centers (Cl2 = ∼1% and P1 = ∼3% in
both 2a,b). Furthermore, we have calculated the EPR hyperfine
coupling constants for 2a,b at UB3LYP/TZVP level of theory.
The calculated hyperfine coupling constant values (Aiso) for N1,
P1, and Cl2 centers are 4.2, 20.5, and 3.1 G, respectively, of 2a
(for other atoms of 2a, see Table S6; for 2b, see Table S7). The
The spin density is mostly concentrated at trivalent C1 (ca.
75%); the adjacent nitrogen center receives about 19% spin
density. Accordingly, the g factors for both 2a,b (2.0027 and
2.0024, respectively) are very close to that of the free electron
(2.0023). The heteroatom 31P and one of the Cl atoms in beta
position exhibit detectable hyperfine coupling despite rather
small spin densities (<5%). A 29Si satellite coupling (4.7% nat.
abundance, I = 1/2) could be observed for 2b at about 10 G
(calcd 13.4 G; 3.6% spin density). 2b contains an apparently
mobile ethyl substituent of low symmetry that results in strongly
temperature-dependent EPR spectra between 183 and 340 K
(Supporting Information) with variable line widths. Although the
total spectral width corresponds to that of 2a, a simulation could
thus not be achieved within the accessible temperature range.
Temperature-dependent magnetic susceptibility was meas-
ured from 295 to 2 K. The observed χMT product of 2a at room
temperature is 0.32 cm3 mol−1 K, which is slightly lower than the
expected 0.375 cm3 mol−1 K for one unpaired electron (with S =
1/2), and stays almost constant down to 20 K. Below 20 K, χMT
decreases to 0.23 cm3 mol−1 K, which is likely due to
intermolecular antiferromagnetic coupling (Supporting Infor-
mation).
The redox properties of 2a were investigated by cyclic
voltammetry measurements (Figure 4; for 2b, see the Supporting
Information) that show a one-electron quasi-reversible process at
E1/2 = −0.96 V against Cp*2Fe/Cp*2Fe+, suggesting the
formation of anion 2a− (Scheme 3). The calculated one-electron
ionization energy and electron affinity of 2a are 5.3 eV (122.4
kcal/mol; 2a+) and 1.1 eV (26.3 kcal/mol; 2a−), respectively.
In summary, we have developed an elegant synthesis route for
the preparation of cyclic alkyl(amino) carbene-stabilized
diphenylphosphinodichlorosilane radicals (2a,b). These are the
first examples of phosphinochlorosilane-containing stable
radicals. Crystals of 2a,b are stable in air for 20 min and for
months in an inert atmosphere at room temperature . They are
isolated as red blocks (2a) or rods (2b) and characterized by X-
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ASSOCIATED CONTENT
* Supporting Information
Experimental and theoretical details and characterization data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Authors
*birger.dittrich@chemie.uni-hamburg.de
*koley@iiserkol.ac.in
*kaim@iac.uni-stuttgart.de
*hroesky@gwdg.de
Notes
(14) (a) Niepotter, B.; Herbst-Irmer, R.; Kratzert, D.; Samuel, P. P.;
̈
The authors declare no competing financial interests.
Mondal, K. C.; Roesky, H. W.; Jerabek, P.; Frenking, G.; Stalke, D.
Angew. Chem., Int. Ed. 2014, 53, 2766; Angew. Chem. 2014, 126, 2806.
(b) Kinjo, R.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2010,
49, 5930; Angew. Chem. 2010, 122, 6066. (c) Back, O.; Celik, M. A.;
Frenking, G.; Melaimi, M.; Donnadieu, B.; Bertrand, G. J. Am. Chem. Soc.
2010, 132, 10262. (d) Kinjo, R.; Donnadieu, B.; Celik, M. A.; Frenking,
G.; Bertrand, G. Science 2011, 333, 610. (e) Lavallo, V.; Canac, Y.;
Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Angew. Chem., Int. Ed.
2006, 45, 3488; Angew. Chem. 2006, 118, 3568. (f) Martin, D.; Moore,
C. E.; Rheingold, A. L.; Bertrand, G. Angew. Chem., Int. Ed. 2013, 52,
7014; Angew. Chem. 2013, 125, 7152. (g) Mondal, K. C.; Roesky, H. W.;
Schwarzer, M. C.; Frenking, G.; Tkach, I.; Wolf, H.; Kratzert, D.; Herbst-
ACKNOWLEDGMENTS
■
H.W.R. thanks the Deutsche Forschungsgemeinschaft (DFG RO
224/60-I) for financial support and Prof. S. Schneider for acess to
UV-vis measurement. Parts of this research were carried out at
the light source Petra III at DESY, a member of the Helmholtz
Association (HGF). We would like to thank A. Meents for
assistance in using beamline PX11. We thank Wacker Chemie
AG for a gift of HSiCl3. B.M. thanks CSIR for SRF fellowship,
and D.K. thanks IISER-Kolkata for a start-up grant and CSIR for
project fund (01(2770)/13/EMR-II). We are thankful to K.C.
Mondal for his valuable advices. This paper is dedicated to
Manfred Scheer on the occasion of his 60th birthday.
Irmer, R.; Niepotter, B.; Stalke, D. Angew. Chem., Int. Ed. 2013, 52, 1801;
̈
Angew. Chem. 2013, 125, 1845. (h) Mondal, K. C.; Roesky, H. W.;
Stuckl, A. C.; Ihret, F.; Kaim, W.; Dittrich, B.; Maity, B.; Koley, D. Angew.
̈
Chem., Int. Ed. 2013, 52, 11804; Angew. Chem. 2013, 125, 12020.
(i) Mondal, K. C.; Dittrich, B.; Maity, B.; Koley, D.; Roesky, H. W. J. Am.
Chem. Soc. 2014, 136, 9568.
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