Reversible complexation of ethylene by a silylene under ambient conditions

Article abstract of DOI:10.1021/ja411951y

Treatment of toluene solutions of the silylenes Si(SAr^Me6) ₂ (Ar^Me6 = C₆H₃-2,6(C ₆H₂-2,4,6-Me₃)₂, 1) or Si(SAr ^Pri4)₂ (Ar^Pri4 = C₆H ₃-2,6(C₆H₃-2,6-Prⁱ₂) ₂, 2) with excess ethylene gas affords the siliranes (Ar ^Me6S)₂SiCH₂CH₂ (3) or (Ar ^Pri4S)₂SiCH₂CH₂ (4). Silirane 4 evolves ethylene spontaneously at room temperature in toluene solution. A Van't Hoff analysis by variable-temperature ¹H NMR spectroscopy showed that ΔG_assn = -24.9(2.5) kJ mol^-1 for 4. A computational study of the reaction mechanism using a model silylene Si(SPh)₂ (Ph = C₆H₅) was in harmony with the Van't Hoff analysis, yielding ΔG_assn = -24 kJ mol^-1 and an activation energy ΔG? = 54 kJ mol^-1.

Full text of DOI:10.1021/ja411951y

Communication
pubs.acs.org/JACS
Reversible Complexation of Ethylene by a Silylene under Ambient
Conditions
Felicitas Lips,† James C. Fettinger,† Akseli Mansikkamaki,‡ Heikki M. Tuononen,*,‡ and Philip P. Power*,†
̈
†Department of Chemistry, University of California, One Shields Avenue, Davis, California 95616, United States
‡Department of Chemistry, University of Jyvaskyla, P.O. Box 35, FI-40014 Jyvaskyla, Finland
̈
̈
̈
̈
S
* Supporting Information
Scheme 1. Illustration of Reversible Reactions of Main-Group
Compounds with Ethylene near Room Temperature
ABSTRACT: Treatment of toluene solutions of the
silylenes Si(SArMe
)
(ArMe = C6H3-2,6(C6H2-2,4,6-
6
6
2
i
4
i
4
Me3)2, 1) or Si(SArPr
)
(ArPr = C6H3-2,6(C6H3-2,6-
2
Pri2)2, 2) with excess ethylene gas affords the siliranes
i
(ArMe S)2SiCH2CH2 (3) or (ArPr S)2SiCH2CH2 (4).
Silirane 4 evolves ethylene spontaneously at room
temperature in toluene solution. A Van’t Hoff analysis by
6
4
1
variable-temperature H NMR spectroscopy showed that
ΔGassn = −24.9(2.5) kJ mol−1 for 4. A computational study
of the reaction mechanism using a model silylene Si(SPh)2
(Ph = C6H5) was in harmony with the Van’t Hoff analysis,
yielding ΔGassn = −24 kJ mol−1 and an activation energy
ΔG⧧ = 54 kJ mol−1.
he reactions of silylenes with unsaturated carbon−carbon
Tbonded molecules have been studied extensively because of
their fundamental nature and their importance for the synthesis
of small silicon-containing rings.1 Several groups have shown that
transient silylenes generated either photochemically or via
reduction of precursor diorganosilicon(IV) dihalides afforded
silirane products in a stereospecific manner when reacted with
olefins, in agreement with the singlet ground state of the
silylene.2 The opposite reaction to regenerate the silylene with
olefin elimination can usually be observed only at higher
temperatures.3,4 In general, reversible reactions between olefins
and main-group molecules are rare. The only currently published
temperature, and show that the reaction is strongly affected by
the steric properties of the silylene and olefin.
Exposure of toluene solutions of 1 or 2 to ethylene gas under
anaerobic and anhydrous conditions results in a discharge of the
yellow color of 1 (within a few seconds) or 2 (within 1 min), and
the formation of the siliranes 3 and 4, as shown in Scheme 2.
Scheme 2. Equilibrium Reactions for Compounds 1 and 2
with Ethylene in Toluene at Ambient Temperature To Form
the Siliranes 3 and 4
examples (Scheme 1) involve reversible cyclization of ethylene or
i
Pri4
norbornadiene with the distannynes ArPr SnSnAr
and
i
4
i
i
Pri8
ArPr SnSnAr
(ArPr = C6H3-2,6(C6H3-2,6-Pri2)2; ArPr
=
8
4
8
C6H-2,6(C6H2-2,4,6-Pri3)2-3,5-Pri2)5 to generate the 1,4-
distannabicyclo[2.2.0]butanes, as exemplified in Scheme 1 (eq
1), or the reversible binding of ethylene by the silylene−
phosphine complex (also described as a phosphonium silaylide),
as shown in Scheme 1 (eq 2), to give the corresponding silirane
product with a penta-coordinate silicon atom.6 The stability of
the product was found to be strongly related to the nucleophilic
character of the phosphine ligand, and irreversible binding was
observed when PR2 = PPh2. Herein, we report on the first
examples of reversible ethylene binding by stable, two-coordinate
1
The products were characterized by H, 13C{1H}, and 29Si
NMR spectroscopy (3 and 4) and by X-ray crystallography (4).
The 29Si NMR spectra of 3 and 4 in d8-toluene reveal an upfield
signal at −44.08 (3) and −42.22 ppm (4) that splits into a quintet
pattern with a 1:4:6:4:1 intensity ratio owing to coupling to the
29
1
hydrogens of the bound ethylene. The J Si− H coupling constant
Me6
silylenes. We demonstrate that Si(SArMe )2 (Ar
= C6H3-
6
i
4
i
2,6(C6H2-2,4,6-Me3)2, 1)7 and Si(SArPr
)
(ArPr = C6H3-
4
Received: November 22, 2013
Published: December 23, 2013
2
2,6(C6H3-2,6-Pri2)2, 2)8 react reversibly with ethylene at room
© 2013 American Chemical Society
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Journal of the American Chemical Society
Communication
is 5.62 (3) and 5.89 Hz (4). The 29Si NMR spectrum of 4 also
displays a signal at +270.7 ppm, due to free silylene 2,8 owing to
the existence of a dissociation equilibrium in solution. The H
(probably for steric reasons), even when they were refluxed
together in toluene.
1
The mechanism for the reaction of 1 and 2 with ethylene
(Scheme 2) was studied computationally (PBE1PBE/TZVP
Pri4
NMR spectrum of 4 in d8-toluene solutions displays signals at
5.25 ppm due to uncomplexed ethylene and at 0.64 (3) and 0.4
ppm (4) due to bound ethylene in addition to the signals of the
arylthiolate substituents. A Van’t Hoff analysis of the association
of ethylene with 2 using variable-temperature 1H NMR
spectroscopy afforded ΔHassn = −83.61(8.4) kJ mol−1 and
ΔGassn = −24.9(2.5) kJ mol−1 at 300 K, which is somewhat more
favorable than the ca. −3 kJ mol−1 reported for the reaction in
Scheme 1 (eq 2) with the phosphine group A.6 A Van’t Hoff
analysis of the association of ethylene with 1 using an ethylene-
saturated solution up to 343 K has not been possible to date due
to the solubility characteristics of 1 and 3.
level)10 using a model system in which the bulky ArMe and Ar
6
substituents were replaced with phenyl (Ph = C6H5) to reduce
computational costs; our previous studies on related systems
have shown that this simplification is well justified.11 The
optimized geometries of Si(SPh)2 (6) and (PhS)2SiCH2CH2 (7)
reproduce well most of the experimental structural parameters of
1 and 2. For example, the calculated Si−S distances are 2.160 and
2.149 Å for 6 and 7, respectively, whereas the Si−C bonds in 7 are
each 1.846 Å long, and therefore very similar to those in the
experimentally characterized siliranes (see above). The calcu-
lated C−Si−C angle in 7 (50.4°) is a perfect match with the
experimental data for 4, but the theoretically predicted S−Si−S
angle in 7 (105.2°) differs significantly from the crystallo-
graphically determined structure, mainly because the simplified
model system experiences low steric repulsion and dispersion
effects between its phenyl groups.8
The structural details of 4 were determined by X-ray
crystallography and are illustrated in Figure 1. It can be seen
The transition state for the association of ethylene with 6 (6-
TS) was located computationally, and it shows that the olefin
approaches the silylene in an asymmetric fashion from one side of
the S−Si−S plane (Si−C distances of 2.309 and 2.630 Å). The
activation energy (ΔG⧧) calculated for this process is 54 kJ
mol−1, whereas the calculated ΔHassn and ΔGassn for the
formation of 7 are −68 and −24 kJ mol−1, respectively, in very
good agreement with the numbers obtained from the Van’t Hoff
analysis.
The bonding interactions within 6-TS were analyzed with the
ETS-NOCV approach that combines the extended transition
state method for energy decomposition analysis with the theory
of natural orbitals of chemical valence.12 The results show that
the transition state can be described as a synergic donor−
acceptor interaction between 6 and ethylene wherein the
HOMO of the silylene (silicon−sulfur bond lone-pair combina-
tion) interacts with the LUMO (π*-orbital) of ethylene and vice
versa (Figure 2, left). The effect of these interactions on the
deformation density (the density of 6-TS minus the sum of
separate densities for 6 and ethylene) shows clearly the transfer
of electrons (from green to yellow) to generate the Si−C
bonding interaction (Figure 2, middle). It is of interest to note
that, when the geometry of 6-TS changes to that of the complex
7, the key orbital interactions are effectively “reversed” in the
process: for 7, the HOMO is a bonding combination between the
LUMOs of ethylene and 6, whereas the HOMO-7 orbital
involves an interaction between the HOMOs of the correspond-
ing molecular fragments (Figure 2, right).
The reversible reactions of 1 and 2 with ethylene are the first
such examples for a stable two-coordinate group 14 element low-
valent species and, apart from the aforementioned distannynes
and silylene−phosphine complex, are the only instances of
reversible complexation of olefins at room temperature by a
stable main-group element compound. However, a reversible
reaction with an unsaturated C−C multiple-bonded compound
has been observed for the reaction of the stannylene :Sn{CH-
Figure 1. Thermal ellipsoid plot (50%) of one of the two
crystallographically independent molecules of 4. Hydrogen atoms and
cocrystallized toluene molecules are not shown. Selected bond distances
(Å) and angles (deg): C1−C2, 1.569(2); C1−Si1, 1.8399(17); C2−Si1,
1.8332(16); Si1−S1, 2.1181(5); Si1−S2, 2.1167(5); S1−C3,
1.7911(15); S1−C33, 1.7953(15); C1−Si1−C2, 50.58(7); C1−C2−
Si1, 64.94(8); C2−C1−Si1, 64.49(8); S1−Si1−S2, 116.49(2).
that the silicon atom is four-coordinate, being complexed to two
sulfur and two carbon atoms with very irregular silicon
coordination geometry (cf. C−Si−C = 50.75(9)°; S−Si−S =
117.84(3)°). The S−Si−S angle is >30° wider than the
85.08(5) ° observed in the precursor silylene 2.8 The Si−S
distance (2.1179(7) Å in 4) is slightly shorter than those
(2.158(3) Å in 1 and 2.137(1) Å in 2) in the free silylenes,7,8 and
the Si−C bond length, 1.829(2) Å, is similar to those in other
siliranes.2c,6,9 The C−C bond length within the silirane ring
(1.560(3) Å) is consistent with single C−C bonding and is
similar to those in previously reported silirane structures.6,9 The
unremarkable bond lengths at silicon observed in 4 thus provide
little hint of its tendency to dissociate ethylene. Silylene 1 also
reacts readily with excess norbornadiene (NBD) to generate a
silirane 5 with a SiS2C2 core geometry similar to that of 3, in
which one of the double bonds of NBD has reacted with the
silicon atom in 1 (see Supporting Information). This reaction,
however, is irreversible up to 162 °C, at which temperature 5
decomposes. No reaction was observed between 2 and NBD
(SiMe3)2}213 with the strained cyclic acetylene SCH2CMe2C
CCMe2CH2, which affords the first example of a stannacyclo-
propene.14 In this case, the strained cis-bent structure of the
alkyne moiety promotes the reactivity as well as the reversibility
of the overall reaction.
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Journal of the American Chemical Society
Communication
Research Fellowship. We thank Dr. B. Rekken for performing
initial reactions of 2 with ethylene.
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ASSOCIATED CONTENT
* Supporting Information
Full experimental, spectroscopic, and computational details as
well as CIFs for compounds 4 and 5. This material is available
free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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Corresponding Authors
heikki.m.tuononen@jyu.fi
pppower@ucdavis.edu
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Dedicated to Prof. Michael F. Lappert on the occasions of his
85th birthday and the 64th anniversary of his first scientific
publication. We are grateful to the U.S. Department of Energy
(DE-FG02-07ER46475, P.P.P.), the Academy of Finland
(H.M.T.), and the Technology Industries of Finland Centennial
Foundation (A.M.) for support of this work. F.L. thanks the
Alexander von Humboldt Foundation for a Feodor Lynen
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