Si-H Bond Activation with Bullock's Cationic Tungsten(II) Catalyst: CO as Cooperating Ligand

Article abstract of DOI:10.1021/jacs.9b10304

An in-depth investigation of the reaction of tertiary hydrosilanes with [CpW(CO)₂(IMes)]⁺[B(C₆F₅)₄]^- reveals a fundamentally new Si-H bond activation mode. Unlike the originally proposed oxidative addition of the Si-H bond to the tungsten(II) center, there is strong experimental and NMR spectroscopic evidence for the involvement of one of the CO ligands of the cationic complex in the Si-H bond breaking event. The Si-H bond is heterolytically cleaved to form a tungsten(II) hydride and a silylium ion, which is stabilized by one of the CO ligands. This reactive hydrosilane adduct was eventually isolated and characterized by X-ray diffraction analysis. Quantum-chemical calculations support a cooperative mechanism, but a stepwise process consisting of oxidative addition and subsequent tungsten-to-oxygen silyl migration in the tungsten(IV) silyl hydride is also energetically feasible. However, our combined spectroscopic and computational analysis favors the cooperative pathway. The newly identified hydrosilane adduct is the key intermediate of Bullock's ionic carbonyl hydrosilylation.

Full text of DOI:10.1021/jacs.9b10304

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Article
Si-H Bond Activation with Bullock’s Cationic
Tungsten(II) Catalyst: CO as Cooperating Ligand
Julien Fuchs, Elisabeth Irran, Peter Hrobarik, Hendrik F. T. Klare, and Martin Oestreich
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b10304 • Publication Date (Web): 06 Nov 2019
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Si–H Bond Activation with Bullock’s Cationic Tungsten(II) Catalyst:
CO as Cooperating Ligand
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†
†
,‡
,†
,†
Julien Fuchs, Elisabeth Irran, Peter Hrobárik,* Hendrik F. T. Klare,* and Martin Oestreich*
†Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany
‡Department of Inorganic Chemistry, Faculty of Natural Sciences, Comenius University, 84215 Bratislava, Slovakia
+
]− reveals
ABSTRACT: An in-depth investigation of the reaction of tertiary hydrosilanes with [CpW(CO)
2
(IMes)] [B(C
6
F
)
5 4
a fundamentally new Si–H bond activation mode. Unlike the originally proposed oxidative addition of the Si–H bond to the
tungsten(II) center, there is strong experimental and NMR spectroscopic evidence for the involvement of one of the CO
ligands of the cationic complex in the Si–H bond breaking event. The Si–H bond is heterolytically cleaved to form a
tungsten(II) hydride and a silylium ion, which is stabilized by one of the CO ligands. This reactive hydrosilane adduct was
eventually isolated and characterized by X-ray diffraction analysis. Quantum-chemical calculations support a cooperative
mechanism but a stepwise process consisting of oxidative addition and subsequent tungsten-to-oxygen silyl migration in
the tungsten(IV) silyl hydride is also energetically feasible. However, our combined spectroscopic and computational
analysis favors the cooperative pathway. The newly identified hydrosilane adduct is the key intermediate of Bullock’s ionic
carbonyl hydrosilylation.
INTRODUCTION
The activation of hydrosilanes by transition-metal
1
complexes spans a wide range of bonding scenarios
leading to different mechanisms for carbonyl
2,3
hydrosilylation. Oxidative addition of the hydrosilane to
the metal center (I + II → III) is commonly seen with
neutral electron-rich transition metals. This redox process
4
is the initial step of the classic Ojima-type mechanism
(
Scheme 1, right cycle) and is followed by migratory
insertion of the coordinated carbonyl compound into the
M–Si bond (III + IV → V). Reductive elimination releases
the silyl ether and closes the catalytic cycle (V → VI + I).
Conversely, nonclassical σ interactions of hydrosilanes
aSi = triorganosilyl (R
3
Si).
1
2
with a metal center as in η - or η -complexes are usually
In 2003, Bullock and co-workers introduced a recyclable
observed when using electrophilic cationic transition
6
+
cationic tungsten(II) catalyst
1
for carbonyl
5
metals (VII + II → VIII), resulting in ionic outer-sphere
7
hydrosilylation. Rather atypical for cationic transition-
3
hydrosilylations (Scheme 1, left cycle). Here, silyl transfer
metal complexes, an oxidative addition of hydrosilane 2 to
to the carbonyl oxygen atom (VIII + IV → IX + X) and
subsequent transfer of the hydride to the silylcarboxonium
ion (IX + X → VI + VII) are the key bond-forming events,
and the metal does not change its oxidation state.
+
complex 1 was postulated to form tungsten(IV) silyl
+
hydride 3 (Scheme 2). This step is typically the entry into
an Ojima-type catalytic cycle but Bullock’s proposal
switches to an ionic outer-sphere mechanism where silyl
+
Scheme 1. Transition-Metal Catalyzed Carbonyl
Hydrosilylation: Ojima-Type (Right) and Ionic Outer-
transfer from 3 to the carbonyl oxygen atom of 4 is
followed by hydride transfer from the neutral
a
+
Sphere Mechanism (Left)
monohydride 5 to silylcarboxonium ion 6 to yield silyl
8
ether 7. This unique proposal prompted us to look into the
hydrosilylation of acetophenone (4) using Bullock’s
+
complex 1 .
Scheme 2. Proposed Catalytic Cycle for the Carbonyl
Hydrosilylation with Bullock’s Complex
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downfield shift of the resonance in the 29Si NMR
spectrum. This result is, to the best of our knowledge, the
first example of a heterolytic Si–H bond activation by a
cationic transition metal complex with participation of a
1
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11
1
2–14
CO ligand.
The C–O bond of the silylated CO ligand
+
[1.25 Å] in the solid-state structure of 8a is substantially
longer than the C–O bond reported for neutral
tungsten(II) monohydride 5 [1.17 Å] (without the silyl
6
+
group). In contrast, the W–CCOSi bond in 8a [1.79 Å] is
significantly shorter than the corresponding W–CCO bond
in 5 [1.94 Å] and close to the sum of the triple-bond
0
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5
covalent radii [1.75 Å], thus featuring a carbyne-like
character (W≡C–OSi) as also evident from the natural
localized molecular orbital (NLMO) analysis (cf. Figures
S18 and S19 in the Supporting Information). The W–H
+
bond in 8a [1.73 Å] is similar to the W–H bond in 5 [1.78
+
Å]. The average O–Si–C angle in 8a [103.7°] as well as the
O–Si bond length [1.80 Å] are comparable to the reported
aThe [B(C ]− counteranion is omitted for clarity. IMes = 1,3-
F )
6 5 4
dimesitylimidazol-2-ylidene, Mes = 2,4,6-trimethylphenyl.
values of the benzaldehyde-stabilized silylium ion
+
–
16
[C H
6
5
C(O)H·SiEt
3
] [CHB11
H
5
Br ] [104.25° and 1.78 Å]. The
6
RESULTS AND DISCUSSION
As the oxidative addition of hydrosilane 2 to complex 1+
was not secure, we began with an investigation of the
+
sum of all C–Si–C angles of 345.8° in 8a is larger than the
reported value for [C
+
–
6
H
5
C(O)H·SiEt
3
] [CHB11
H
5
Br ]
6
[342.5°], consistent with a highly electrophilic silicon atom
having a distorted trigonal spatial arrangement of the
substituents. This is also reflected by a natural population
analysis (NPA), showing that the positive charge is
hydrosilane-activation step. An equimolar mixture of
+
Et
3
SiH (2a) and 1 in deuterated chlorobenzene was
submitted to NMR spectroscopic analysis, revealing the
clean formation of a tungsten(II) hydride featuring a
hydride shift of δ –2.52 ppm with the corresponding
+
predominantly located at the silicon atom in 8a (cf. Figure
S17 in the Supporting Information).
1
1
tungsten satellites ( JW,H = 36 Hz) in the H NMR spectrum
Scheme 3. Synthesis and Molecular Structure of
(
Figure 1, left). Further characterization of this compound
+
–a
Hydrosilane Adduct 8a [B(C
6
F
5 4
) ]
13
disclosed two notable facts: (1) The C NMR chemical shifts
for the two CO ligands at δ 216.6 and 240.6 ppm were
significantly different, in contrast to those seen for
complex 1 at δ 247.1 and 246.2 ppm. (2) The resonance
signal in the Si NMR spectrum at δ 42.7 ppm showed no
+
9
29
tungsten satellites, indicating that the silyl group might
not be attached to the tungsten center (Figure 1, right).
However, we were not able to conclusively assign the
+
structure of 8a solely based on NMR spectroscopic
measurements.
1
Figure 1. Selected segments of the H NMR spectrum (500
29
MHz, C
6 5
D Cl, 300 K) (left) and Si NMR spectrum (99 MHz,
+
–
C
D
6 5
Cl, 300 K) (right) of hydrosilane adduct 8a [B(C F ) ] .
6 5 4
The successful crystallization of hydrosilane adduct 8a+
from neat hydrosilane 2a and X-ray diffraction analysis
aThermal ellipsoids are shown at the 50% probability level;
counteranion and hydrogen atoms except for W–H are
omitted for clarity. Selected interatomic distances (Å) and
1
0
then revealed its molecular structure (Scheme 3). The
+
formation of 8a is the result of heterolytic cleavage of the
angles (deg): W–H: 1.73, W–CCO: 1.98, O–CCO: 1.16, W–CCOSi
:
+
Si–H bond by complex 1 . A silylium ion stabilized by the
1
1
.79, O–CCOSi: 1.25, O–Si: 1.80, Si–O–CCO: 124.2°, av(O–Si–C):
03.7°, Σ(C–Si–C): 345.8°.
carbonyl oxygen atom is in accordance with the observed
10
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Notably, the solid-state structure of hydrosilane adduct 8a+
features cis configuration of the CO groups, while the NMR
agreement for the key H, 13C, and 29Si NMR signals was
1
1
2
3
4
5
6
7
8
9
+
found for the structure of hydrosilane adduct trans-8a (cf.
Table S3 in the Supporting Information). According to the
quantum-chemical calculations, 8a represents the
+
chemical shifts of 8a in solution better match the trans
–
1
+
isomer, which was computed to be 0.4 kcal mol
energetically less stable (cf. Table S3 in the Supporting
Information). Its preference in solution could be attributed
to a counteranion effect and/or specific solvent-solute
interactions not considered in the implicit solvent model.
energetic minimum among possible adducts of Bullock’s
+
tungsten(II) complex 1 with hydrosilane 2a, whereas the
1
+
corresponding  -complex 9a and tungsten(IV) silyl
+
–1
hydride 3a are higher in energy by 3.3 and 11.8 kcal mol ,
+
Dissolving crystals of hydrosilane adduct cis-8a in
respectively. Coordination of the hydrosilane to the
+
deuterated chlorobenzene immediately resulted in
isomerization to the trans isomer. Theoretical calculations
confirmed that the cis-trans isomerization is a facile
process, proceeding quickly at room temperature through
electrophilic unsaturated 16-valence-electron complex 1
–
1
1
1
1
1
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proceeds barrierless and is exergonic by 9.0 kcal mol (blue
pathway). The barrier of 29.3 kcal mol for the subsequent
–
1
heterolytic Si–H bond cleavage through transition state
+
–1
+
transition state TS-8a over a low barrier of 9.5 kcal mol
8a _TS-A is too high in view of the fact that the formation
+
(
Scheme 4 and Figure S16 in the Supporting Information).
of 8a is a fast reaction at room temperature. However, the
Scheme 4. Calculated relative free energies (ΔG
0, in
kcal mol ) for the cis-trans isomerization of
hydrosilane adduct 8a
barrier for a concerted ligand-assisted process through the
r
–
1
–
1
same transition state would be 20.3 kcal mol and thus a
conceivable pathway (dashed green pathway). This
activation energy is almost twice as high as in the
cooperative Si–H bond activation by cationic
+
a
1
7
ruthenium(II) thiolate complexes, but still corresponds to
a reaction half-time of about one to two minutes at room
temperature according to the Arrhenius equation. In
contrast, the reaction barrier for the backward reaction is
–
1
too high (> 30 kcal mol ), explaining the irreversibility of
the hydrosilane activation process (see below). An
+
alternative mechanism for the formation of adduct 8a
+
could involve formation of silyl hydride 3a through
+
oxidative addition of Et
3
SiH (2a) to complex 1 followed by
a 1,3-silyl shift from tungsten to the oxygen atom of the
bound carbonyl ligand (red pathway). Such a sequence was
also proposed for the formation of siloxycarbyne
complexes from neutral transition metal carbonyl
18
–1
complexes. The computed barriers of 5.9 kcal mol for
+
oxidative addition through transition state 3a _TS and 19.5
kcal mol for silyl migration through transition state
8a _TS-B indeed suggest that this route is energetically
–
1
+
possible. To distinguish between both mechanistic
scenarios, we performed low-temperature NMR
aResults obtained at the B3LYP-D3(BJ)/ECP/6-31+G** level of
theory using an SMD solvation model.
measurements. The Si–H bond heterolysis by complex 1
+
also occurs at –40 °C in deuterated chlorobenzene, yet
requiring longer reaction time (> 1 h) until complete
conversion of hydrosilane 2a. However, no intermediates
To gain deeper insight into the hydrosilane activation step,
we employed quantum-chemical calculations (Figure 2).
The relativistic DFT calculations revealed that neither
+
and no other hydrides than 8a were detected, suggesting
that a concerted cooperative Si–H bond activation mode
+
tungsten(IV) silyl hydride 3a formed by oxidative addition
+
through transition state 8a _TS-A is operative (dashed
1
+
of Et SiH (2a) nor  -complex 9a with end-on
3
green pathway). However, the oxidative addition–silyl
migration pathway cannot be completely ruled out at this
stage.
coordination of the hydrosilane, both previously
7,8
considered as the active species, match the experimental
fingerprint resonances given above. Instead, a very good
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Figure 2. Calculated reaction free energies (ΔG , in kcal mol ) for three different pathways of Si
(
–
H
bond activation of Et
3
SiH
+
2a) by cationic complex 1 . Results obtained at the B3LYP-D3(BJ)/ECP/6-31+G** level of theory using an SMD solvation model.
To explore the reactivity and role of hydrosilane adduct 8a+
in the hydrosilylation, we conducted catalytic and
stoichiometric control experiments. To probe its ability to
act as precatalyst in the hydrosilylation reaction, we added
catalytic amounts of 8a to an equimolar mixture of
hydrosilane 2a and ketone 4 (Scheme 5). Full conversion to
silyl ether 7a was observed.
Scheme 6. Probing the Reversibility of the
Hydrosilane Activation by a Stoichiometric Crossover
Experiment
+
Scheme 5. Catalytic Activity of Hydrosilane Adduct 8a+
aRatios determined by NMR spectroscopic analysis.
+
Surprisingly, the expected tungsten(II) deuteride 8 -d
1
was
2
not detected in the H NMR spectrum, implying that the
exchange of the silyl group proceeds without participation
of the hydride at the tungsten center. Two mechanistic
pictures can explain these results. Pathway A involves a σ-
We were also interested to experimentally verify whether
the activation of the hydrosilane is reversible or not.
Consistent with the computations (see above), a NOESY
NMR measurement showed no chemical exchange
between hydrosilane adduct 8a and free hydrosilane 2a.
However, mixing adduct 8b and deuterated hydrosilane
1
9
bond metathesis at the CO ligand through TS-I, while
pathway B proceeds by an S
deuterosilane 2a-d
N
2-type silyl transfer to
+
+
1
under formation of hydronium ion 10
+
and neutral tungsten(II) monohydride 5 through TS-II
Et
3
SiD (2a-d
1
) in a crossover experiment revealed exchange
20
(
Scheme 7, top). To distinguish between these pathways,
+
of the silyl group at the CO ligand in 8b along with
Si
21
silicon-stereogenic hydrosilane ( S)-2c with e.r. = 90:10
was employed as a stereochemical probe (Scheme 7,
formation of Me
2
PhSiD (2b-d ) (Scheme 6).
1
Si
bottom). Mixing enantioenriched ( S)-2c with catalytic
amounts of tungsten(II) complex 1 does not cause any loss
+
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of enantiomeric purity. This retention of the configuration
at the silicon atom suggests a σ-bond metathesis (TS-I) as
an S 2-type silyl transfer (TS-II) would lead to (at least)
partial racemization at the silicon atom.
1
2
3
4
5
6
7
8
9
N
1
7
Scheme 7. Probing Mechanistic Pathways for the Silyl
Group Exchange
1
1
1
1
1
1
1
1
1
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In summary, we were able to confirm an ionic outer-sphere
mechanism for the hydrosilylation with Bullock’s cationic
tungsten(II) catalyst. Strikingly, the CO ligand is
cooperating in this catalysis and engages in the Si–H bond
activation event, leading to the formation of a stable
carbonyl-oxygen-stabilized silylium ion and a tungsten(II)
hydride. In accordance with our earlier studies, this
analysis confirms that Bullock’s catalyst is now the third
example for a cationic transition-metal complex to follow
a two-silicon cycle in a carbonyl hydrosilylation.
23
CONCLUSION
Based on our experimental findings and computations we
suggest a revised catalytic cycle for the ionic outer-sphere
hydrosilylation with tungsten(II) complex 1 (Scheme 8).
The initial step is the irreversible activation of the Si–H
bond in hydrosilane 2 by 1 with involvement of one of the
+
ASSOCIATED CONTENT
Supporting Information
+
CO ligands. The resulting electrophilic silicon compound
+
The Supporting Information is available free of charge on the
ACS-Publications website at http://pubs.acs.org.
Experimental details, spectroscopic, crystallographic and
computational data (PDF)
8
transfers its silyl group S 2-like to the carbonyl oxygen
N
atom of acetophenone (4) in a reversible, energetically
downhill reaction. Additionally, 8 can undergo a silyl
group exchange at the CO ligand with hydrosilane 2 by a
σ-bond metathesis (TS-I). The silylcarboxonium ion 6 can
alternatively form pentacoordinate silicon compound 11
with another molecule of acetophenone (4). This explains
+
X-ray crystallographic data (CIF)
Cartesian coordinates of the selected DFT optimized
structures (XYZ)
+
+
racemization at the silicon atom using silicon-stereogenic
AUTHOR INFORMATION
Corresponding Authors
Si
hydrosilane ( R)-2c for the hydrosilylation of 4 (not
shown; see the Supporting Information for details).
Monohydride 5 is able to undergo hydride transfer to the
silylcarboxonium ion 6 but needs the assistance of
another hydrosilane molecule 2 to release silyl ether 7 with
peter.hrobarik@uniba.sk;
hendrik.klare@tu-berlin.de;
martin.oestreich@tu-berlin.de
+
+
22
regeneration of adduct 8 .
ORCID
Scheme 8. Revised Catalytic Cycle for the Ionic Outer-
Sphere Hydrosilylation with Bullock’s Complex
Julien Fuchs: 0000-0003-0816-6497
Elisabeth Irran: 0000-0001-6098-1996
Peter Hrobárik: 0000-0002-6444-8555
Hendrik F. T. Klare: 0000-0003-3748-6609
Martin Oestreich: 0000-0002-1487-9218
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
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This research was supported by the Deutsche
(8) This reaction pathway was further supported by theoretical
calculations: Fang, S.; Chen, H.; Wang, W.; Wei, H. Mechanistic
insights into the catalytic carbonyl hydrosilylation by cationic
1
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5
6
Forschungsgemeinschaft (Oe 249/20‐1) and the Fonds der
Chemischen Industrie (predoctoral fellowship to J.F., 2016–
2018). M.O. is indebted to the Einstein Foundation (Berlin) for
an endowed professorship. P.H. acknowledges financial
support from the Slovak Grant Agencies VEGA (Grant Nos.
+
[
CpM(CO)
2
(IMes)] (M = Mo, W) complexes: the intermediacy of
1
η -H(Si) metal complexes. New J. Chem. 2018, 42, 4923–4932.
9) Due to the insolubility of complex 1 in common non-
coordinating solvents, the C NMR resonance signals for the CO
+
(
13
1
/0507/17 and 1/0712/18) and APVV (Grant No. APVV-17-0324)
+ 6
8
ligands were measured for the THF-d adduct of 1 .
as well as European Union’s Horizon 2020 research and
innovation program under the Marie Skłodowska-Curie Grant
No. 752285. We also thank Dr. Sebastian Kemper (TU Berlin)
for expert advice with the NMR measurements.
(10) Due to disorder in the crystal, another structure of
+
hydrosilane adduct cis-8a with different interatomic distances
and angles for the silylated CO ligand was found. In Scheme 3,
duplicated atoms are omitted for clarity. Selected interatomic
distances (Å) and angles (deg) for the second version: W–H: 1.73,
W–CCO: 1.98, O–CCO: 1.16, W–CCOSi: 1.79, O–CCOSi: 1.36, O–Si: 1.74,
Si–O–CCO: 136.5°, av(O–Si–C): 104.1°, Σ(C–Si–C): 343°.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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reaction of monohydride 5 with silylcarboxonium ion 6a ,
+
1
2
3
4
5
6
7
8
9
forming quantitative amounts of silyl ether 7a only in presence of
another hydrosilane molecule 2a (see the Supporting Information
for details). This finding is in line with our previous
23
investigations. However, we were not able to uncover the exact
role of the second hydrosilane molecule neither experimentally
nor by quantum-chemical calculations.
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