Peripheral mechanism of a carbonyl hydrosilylation
catalysed by an SiNSi iron pincer complex†
Toni T. Mets¨
anen,‡
a
Daniel Gallego,‡
a
Tibor Szilv´
asi,
b
Matthias Driess*
a
and Martin Oestreich*
a
Combined experimental and theoretical analysis of the carbonyl hydrosilylation catalysed by an iron(0)
pincer complex reveals an unprecedented mechanism of action. The iron(0) complex is in fact a
precatalyst that is converted into an iron(II) catalyst through oxidative addition of a hydrosilane. Neither
the hydrogen atom nor the silicon atom bound to the iron(II) centre are subsequently transferred onto
the carbonyl acceptor, instead remaining at the sterically inaccessible iron(II) atom throughout the
catalytic cycle. A series of labelling, crossover and competition experiments as well as the use of a
silicon-stereogenic hydrosilane as a stereochemical probe suggest that the iron(II) site is not directly
involved in the hydrosilylation. Strikingly, it is the silyl ligand attached to the iron(II) atom that acts as a
Lewis acid for carbonyl activation in this catalysis. The whole catalytic process occurs on the periphery
of the transition metal. Computation of the new peripheral as well as plausible alternative inner and
outer sphere mechanisms support the experimental findings.
Introduction
Iron-catalysed carbonyl hydrosilylation can be traced back to
seminal reports by Brunner
1
but these contributions were
decades ahead of their time. With few exceptions, catalyst
development was focused on complexes of rare transition
metals while limited progress had been made involving
abundant transition metals. The pressing demand for
sustainable processes nally shied iron catalysis into the
limelight,
2
and several iron-based catalysts for carbonyl
hydrosilylation with different ligand designs were introduced
in recent years.
3
The advent of these new catalysts immediately
poses the question whether the mechanisms are similar to
those established for rare transition-metal complexes or totally
unprecedented.
4
However, little detail is known about the
mechanisms of action of iron complexes in catalytic
hydrosilylation.
Mechanisms of transition-metal-catalysed hydrosilylations
exhibit a wide variety of modes of activation.
5
However, the
known mechanisms are characterised as either inner sphere
6
where both the substrate and the hydrosilane are directly in
contact with the metal or outer sphere
7,8
where only one of the
two is in contact with the metal centre. The proposed mecha-
nisms for iron-catalysed hydrosilylations range from inner
sphere mechanisms with s-bond-metathesis-type Si–H bond
cleavage at an iron–oxygen bond
3k,3l
to outer sphere mechanisms
with iron acting as a Lewis acid,
3h
either activating the hydro-
silane or the carbonyl group.
Driess and co-workers recently introduced silylenes as
s-donor ligands in iron-based catalysis, and iron(0) complexes 1
and 2(Fig. 1) were applied to carbonyl hydrosilylation.
9
Coop-
erativity between the iron(0) atom and the silicon(II) hydride in 1
was postulated to be relevant in the catalytic cycle.
9a
The SiNSi
iron(0) pincer complex 2was, in turn, believed to be a pre-
catalyst
9b,10
but a detailed mechanistic analysis remained chal-
lenging. We report here the disclosure of a unique mechanism
of a transition-metal-catalysed carbonyl hydrosilylation that
takes place neither inner nor outer sphere but on the periphery of
the metal centre without its direct involvement.
Fig. 1 Iron(0) complexes 1(ref. 9a) and 2(ref. 9b) introduced by Driess
and co-workers.
a
Institut f¨
ur Chemie, Technische Universit¨
at Berlin, Straße des 17. Juni 115, 10623
b
Department of Inorganic and Analytical Chemistry, Budapest University of Technology
and Economics, Szent Gell´
ert t´
er 4, 1111 Budapest, Hungary
†Electronic supplementary information (ESI) available: Experimental procedures
and computational details, characterisation, crystallographic and
quantum-chemical calculation data as well as NMR spectra. CCDC 1416378. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5sc02855h
‡These authors contributed equally.
Cite this: Chem. Sci.,2015,6,7143
Received 3rd August 2015
Accepted 11th September 2015
DOI: 10.1039/c5sc02855h
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Results and discussion
Optimisation and scope
The SiNSi pincer complex 2(2.5 mol%) was found to catalyse the
hydrosilylation of various acetophenones 3a–3i with silane 4a at
elevated temperatures
11
(Table 1, entries 1–9; see the ESI†for
the optimisation of the reaction conditions). Both electron-
donating (entries 1 and 3) as well as -withdrawing (entry 6)
substituents at the aryl group were tolerated with the exception
of a Et
2
N group in the para position (entry 2). The reaction was,
however, sensitive toward steric hindrance. Substituents in the
ortho position signicantly lowered the yield (entries 7–9), and a
2,6-disubstituted substrate did not react (3j, entry 10). Benzo-
phenone (3k, entry 11) reacted readily while propiophenone and
isobutyrophenone (3l and 3m, entries 12 and 13) afforded 18
and 16% yield, respectively. Hydrosilylation of cyclopropyl
substituted ketone (3n, entry 14) proceeded efficiently
12
(see the
ESI†for full scope).
Isolation of the active catalyst
To gain insight into the mechanism, we investigated the reac-
tion between iron(0) complex 2and hydrosilanes 4a–c(2/
7a–c, Scheme 1). Heating at 70 C, a new set of distinct signals
appeared in the
1
H as well as in the
29
Si and
31
P NMR spectra.
The
1
H NMR spectrum clearly indicated the formation of an
iron hydride, and detailed NMR analysis revealed that the
hydride was likely to be trans to the apical phosphine ligand.
The silyl group was assigned by 2D NMR experiments to be in
the equatorial position trans to the pyridine ligand. We then
obtained single crystals of 7b (Si ¼Me
2
PhSi) suitable for X-ray
diffraction analysis, and that conrmed the molecular structure
deduced from the NMR analysis. The structure shows a dis-
torted octahedral iron(II) coordination environment. The
hydride was located tilted toward one of the silylene donor arms
deviated from the trans coordination to the Me
3
P ligand (P–Fe–
H1 170.20(5)), a situation similar to that of known iron(II)
hydride pincer complexes.
3h
The Fe–Si bond distances
2.1509(7)/2.1715(7) ˚
A for Fe–Si1/Si2 and 2.2986(8) for Fe–Si3 are
within the range of iron silylene and silyl complexes.
9a,13
The
t-Bu groups encage the iron hydride with a Si1–Fe–Si2 angle far
from the linearity, 144.54(3).
To validate whether the thus formed iron(II) complex 7is the
active catalyst, we measured the kinetic proles for the hydro-
silylation of 3a with hydrosilane 4a catalysed by 2or 7a
(Scheme 2). Conversion with 2was only 15% aer 1 h while the
reaction had reached 74% with 7a. The reaction with 7a
continued with signicantly higher rate reaching 86% at 4 h
compared to only 53% with 2.Aer 22 h, nearly full conversion
Table 1 Carbonyl hydrosilylation with precatalyst 2
a
Entry 3R
1
R
2
Yield of 5
b
(%)
13a X¼MeO Me >99 (5a)
23b X¼Et
2
NMe40(5b)
33c X¼Me Me 82 (5c)
43d X¼Br Me >99 (5d)
53e X¼HMe93(5e)
63f X¼CF
3
Me 95 (5f)
73g X¼MeO Me 70 (5g)
83h X¼Me Me 70 (5h)
93i X¼Cl Me 49 (5i)
10 3j Mes Me 0 (5j)
11 3k Ph Ph 60 (5k)
12 3l Ph Et 18 (5l)
13 3m Ph i-Pr 16 (5m)
14 3n c-Pr Me >99 (5n)
a
Reactions were performed on 0.10 mmol scale employing precatalyst 2
(2.5 mol%) and (EtO)
3
SiH (4a 1.5 equiv.).
b
Average yield from two runs
determined by GLC-MS analysis and
1
H NMR spectroscopy using
anisole as internal standard.
Scheme 1 Identification of the catalytically active iron(II) complexes 7
from iron(0) precatalyst 2and molecular structure of 7b. Hydrogen
atoms except for the iron hydride are omitted for clarity.
Scheme 2 Kinetic profiles of the iron(0) and iron(II) complexes 2and
7a in hydrosilylation.
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is obtained for both. The greater initial rate of the catalysis with
7a strongly supports the assignment of the iron(II) complex 7as
the active catalyst.
Stoichiometric experiments: hydride transfer
With the iron(II) hydrides 7in hand, we had a closer look at the
hydride transfer. Maintaining 7b and deuterium-labelled
hydrosilane 4b-d
1
in THF at 70 C resulted in slow H/D
exchange, visible both at the silicon and iron atoms (Scheme 3,
le). The stoichiometric reaction between iron(II) hydride 7b,
hydrosilane 4b-d
1
(>95% deuteration grade), and ketone 3e was
puzzling though (Scheme 3, right). Initially, the H : D ratio at
the methine position of silyl ether 8eb is nearly 50 : 50.
However, it quickly decreases to 36 : 64 at 25% conversion
within 6 hours and then gradually increases again, returning to
50 : 50 at full conversion aer a few days. Meanwhile, the cor-
responding reaction with partially deuterated 4b-d
1
(ca. 50%
deuteration grade) yielded 8eb with little deuterium incorpo-
ration at 19% conversion (H/D ¼90 : 10). That ratio subse-
quently decreases to 78 : 22 to reach equilibrium aer 24 hours.
These results reveal that even though the hydride at the silicon
atom in 4is exchanging with the iron-bound hydride in 7,
hydride transfer to the carbonyl carbon atom of 3most likely
occurs from the hydrosilane 4and not from complex 7.
14
Also,
the reaction with partially deuterated hydrosilane indicates that
the kinetic isotope effect (KIE) of the hydride transfer is
signicant. Precise value of the KIE could not be measured due
to competing H/D exchanges (vide infra).
The possible H/D exchange at the methine position of silyl
ether 8eb was veried using 8eb-d
1
(Scheme 4, top). Treatment
of 8eb-d
1
with equimolar amounts of the iron(II) hydride 7b
indeed led to H/D scrambling. Conversely, no erosion of the
enantiomeric purity was seen when subjecting enantiopure silyl
ether (S)-8eb to the typical protocol (precatalyst 2and hydro-
silane 4b generate catalyst 7b, Scheme 4, bottom). The cong-
urational stability of (S)-8eb suggests that the hydride transfer
itself is irreversible, and a concerted mechanism involving
frontside attack at the asymmetrically substituted carbon atom
is needed to explain the hydrogen atom exchange between the
catalyst and the product.
These unusual scramblings were then investigated by DFT
calculations.
15
Both were found to proceed via a silylene-assis-
ted concerted mechanism (9a
‡
for Si–H and 10a
‡
for C–H,
Scheme 5) where the hydride on the iron atom is rst shied to
the silicon atom of the adjacent donor-stabilised silylene ligand
forming a pentacoordinate silicon atom
16
while the Si–HorC–H
bond interact with the now accessible iron centre. Both transi-
tion states are paired with their corresponding isomer where
the second silylene ligand accepts the hydride. Attempts to
locate the transient intermediates between the two degenerate
conformations were not successful.
Aer the transition state, the silylene-bound hydrogen atom
migrates back to the silicon and carbon atom, respectively. The
activation barriers of the scrambling reactions (18.5 kcal mol
1
for Si–H and 20.1 kcal mol
1
for C–H) are energetically acces-
sible under the reaction conditions.
Stoichiometric experiments: phosphine dissociation
To probe the potential lability of the phosphine, we performed
phosphine crossover experiments. Slow exchange of the
phosphine ligand with (CD
3
)
3
P(6-d
9
) was observed (Scheme 6,
Scheme 3 H/D scrambling at the silicon and iron atoms and identi-
fication of the hydride source.
Scheme 4 H/D scrambling at the methine carbon atom.
Scheme 5 Silylene-assisted H/D scrambling at the hydrosilane silicon
atom (9a
‡
, left) and the methine carbon atom (10a
‡
, right); Gibbs free
energies given in parentheses in kcal mol
1
;Si ¼Si(OEt)
3
.
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top le). When 7b was subjected to repeated reux/freeze–
pump–thaw cycles, we detected a new iron hydride species that
was tentatively assigned as the expected phosphine-dissoci-
ated iron silyl hydride 11b-d
6
with C
6
D
6
as an h
2
-ligand
(Scheme 6, top right). The generation of this new iron
compound was accompanied by formation of disilane 12b and,
with longer reaction times, decomposition into a complex
mixture.Theroleofthedisilane12b in the formation of the
complex 11b remains unexplained. Unfortunately, attempts to
isolate 11b-d
6
were unsuccessful. Its generation in the pres-
ence of acetophenone (3e) did not lead to the formation of the
silyl ether 8eb (see the ESI†for details), providing further
evidence against the role of 11 as an intermediate in the
catalytic reaction. In fact, the dissociation of the Me
3
P(6)is
signicantly slower in the presence of ketone 3e.Theforma-
tion 11 was also investigated computationally (Scheme 6,
bottom). Phosphine dissociation from 7a gives energetically
highly unfavoured intermediate cis-14a
17
that readily coordi-
nates benzene to form adduct 11a.
It must be noted here that catalysis with Guan's related
iron(II) POCOP-pincer complex is thwarted by additional Me
3
P
(6), indicating dissociation of one of the phosphine ligands as
part of the catalytic cycle.
3h
When we added 25 mol% of Me
3
P(6,
10 equiv./catalyst) to the reaction mixture, the reaction was
unaffected (Scheme 7, cf. Table 1, entry 5).
As is to be expected from the above observations, the hydride
complex 7b did not produce any silyl ether 8eb when reacted
stoichiometrically with ketone 3e in the absence of a hydro-
silane (Scheme 8, top le). What was fascinating though is that
the silyl ligand in 7b also remains untouched throughout the
catalysis: 7b derived from Me
2
PhSiH (4b) catalyses the hydro-
silylation of 3e with MePh
2
SiH (4c) with hardly any incorpora-
tion of the Me
2
PhSi moiety into the product; silyl ether 8ec
rather than 8eb is formed almost exclusively (Scheme 8, right).
This crossover experiment unequivocally proves that iron(II)
complexes 7are the actual catalysts, originating from oxidative
addition of hydrosilanes 3to the iron(0) complex 2;2is a pre-
catalyst. During the crossover experiment no changes in the
characteristic signals of complex 7b in the
1
H and
31
P NMR
spectra were detected. However, when the assumed inability of
7b and 4c to exchange their silyl groups was examined with
another control experiment (Scheme 8, bottom le), we
observed slow exchange with ca. 36% conversion of 7b to 7c
aer 24 h. The Me
2
PhSi/MePh
2
Si scrambling was accompanied
with formation of phosphine-dissociated, benzene-stabilised
compounds 11b and 11c. Only traces of Me
2
PhSiH (4b) were
observed, indicating that the exchange (7b to 7c) is in fact a side
product of the decomposition rather than simple scrambling of
the silyl groups.
Hydrosilylation with a silicon-stereogenic hydrosilane
With sufficient knowledge of the active catalyst, we decided to
analyse the stereochemical course at the silicon atom of the
reacting hydrosilane (Scheme 9).
18
Catalyst 7b promoted the
reaction between highly enantioenriched hydrosilane (
Si
S)-4d
(e.r. > 95 : 5) and ketone 3e but conversion was slow as expected
from the data obtained with achiral triorganosilane 4b (see
Table S1, entry 12 in the ESI†). Aer 6 days, we were able to
Scheme 6 Phosphine scrambling and dissociation. Double-ended
arrows in 11b-d
6
show
1
H,
29
Si HMQC NMR correlation. Gibbs free
energies given in parentheses in kcal mol
1
[Si ¼SiMe
2
Ph].
Scheme 7 Effect of excess Me
3
P on the catalytic activity. Scheme 8 Stoichiometric control and crossover experiments.
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isolate the silyl ether (
Si
R)-8ed in 31% yield; diastereoselection
was poor. The enantiomeric ratio of unreacted (
Si
S)-4d was
found to be unaffected. Subsequent reductive cleavage of the
Si–O bond in (
Si
R)-8ed (known to proceed with stereoretention
at silicon atom
19
) liberated (
Si
S)-4d with overall retention of the
stereochemistry at the silicon atom (e.r. > 95 : 5). Hence, the
hydrosilylation step involves frontside attack at the silicon
atom, and that makes a mechanism involving Lewis-acid acti-
vation of the hydrosilane unlikely.
18
DFT calculation of conventional inner and outer sphere
mechanisms
15
Based on the combined ndings, we propose that the active
iron(II) catalyst 7is generated from the iron(0) precatalyst 2by
oxidative addition of hydrosilane 4to the zero-valent iron atom
(Scheme 1). As expected due to the steric congestion around the
iron(II) centre in 7a (grey box), we did not locate any structure
resulting from direct insertion of the ketone C]O group into the
iron hydride 7a or the silylene ligands (not shown). Instead, we
were able to nd a minimum structure for the phosphine-
dissociated complex cis-14a (Scheme 10, le). In agreement with
the experiments, cis-14a is however signicantly higher in energy
(29.2 kcal mol
1
relative to 7a). The intermediate cis-14a readily
coordinates THF forming the adduct 17a. This intermediate is
however a resting state if not a “mechanistic dead-end”. Ketone
coordination to the iron centre of cis-14a gives intermediate 19oa
with activated carbonyl group. The catalytic cycle is closed by an
outer sphere concerted hydrosilane addition 20oa
‡
to the ketone
with an activation barrier of 33.7 kcal mol
1
.
Isomerisation of cis-14a to trans-14a was found to be strongly
disfavoured, but again, ketone coordination to trans-14a low-
ered the energy signicantly (Scheme 10, right). The following
hydride transfer from 23oa passes through 24oa
‡
(30.9
kcal mol
1
)toafford the iron alkoxide 25oa. The silylated
alcohol is released by an inner sphere silylation through 26oa
‡
with an energy barrier of 34.9 kcal mol
1
. An alternative inner
sphere mechanism could be a reductive elimination from the
intermediate 25oa. However, the energy barrier for the transi-
tion state 27oa
‡
was found to be high, and the resulting iron(0)
complex 28 is energetically disfavoured. Recoordination of
phosphine 6gives iron(0) complex 29 which oxidatively adds to
a silane 4a to form 7a.
Peripheral mechanism: support from DFT calculations
In addition to the high energy barriers, neither outer nor inner
sphere mechanisms give satisfactory ts to the experimental
evidence. Hence, we looked for an adduct of 7a and 3o wherein
Scheme 9 Silicon-stereogenic hydrosilane as a stereochemical
probe.
Scheme 10 Alternative mechanisms. Gibbs free energies given in parentheses in kcal mol
1
[Si ¼Si(OEt)
3
].
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the silyl group would act as a Lewis acid (Scheme 11).
8,18,20
Coordination of the ketone to the silyl group via low energy
transition state 30oa
‡
(8.1 kcal mol
1
) led to intermediate 31oa
(2.8 kcal mol
1
) with a pentacoordinate silicon atom, and the
C]Odoublebondbeingsignicantly elongated compared to
its equilibrium distance from 1.211 to 1.251 ˚
A, indicating
activation.
21
Lewis pair formation is followed by coordination
of hydrosilane 4a to the carbonyl group in 31oa,andthe
hydrosilylation event releases 8oa through transition state
32oa
‡
with retention at the silicon atom. In accordance with
our labelling experiments (cf. Scheme 3), this is the rate-
determining step (14.3 kcal mol
1
). To further validate this,
we conducted a competition experiment between electron-rich
3a and electron-decient 3f (Scheme 12). The para substitu-
tion in 3exerts a pronounced electronic effect, and
F
3
C-substituted 3f was consumed signicantly faster than
MeO-substituted 3a. This reactivity pattern is not unprece-
dented, and it has been seen previously in the activation of
carbonyl compounds with silicon-based Lewis acids.
22
The
carbonyl carbon atom in 3f (X ¼CF
3
) is more positively
polarised accelerating the hydride transfer, than that of
donor-substituted 3a (X ¼OMe).Thereactivityisalsoin
agreement with the proposed KIE based on the control reac-
tions with deuterated silane 4b-d
1
(Scheme 3).
Conclusions
The value of the present study is that it demonstrates, to our
knowledge for the rst time, an unusual case where the tran-
sition metal of a catalyst complex is not directly involved in the
catalytic process. Activation of both substrate and reagent as
well as the bond-forming and -breaking events happen in the
ligand sphere, i.e., on the periphery of the transition metal.
Coincidentally, the mechanism becomes outer sphere at
silicon.
22
The more conventional inner or outer sphere mecha-
nisms do not apply to this unique catalyst.
Acknowledgements
This research was supported by the Cluster of Excellence UniCat
of the Deutsche Forschungsgemeinscha(EXC 314/2). T.S. is
grateful to The New Sz´
echenyi Plan TAMOP-4.2.2/B-10/1-2010-
0009. M.O. is indebted to the Einstein Foundation Berlin for an
endowed professorship.
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Scheme 12 Probing the electronic effect in a competition experiment.
Scheme 11 Peripheral mechanism. Gibbs free energies given in
parentheses in kcal mol
1
[Si ¼Si(OEt)
3
].
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15 All calculations were performed at uB97X-D/6–31G(d)[Fe:cc-
pVTZ] level of theory. Iron(II) complex 7a, triethoxysilane
(4a), and acetone (3o) were used as model substrates. See
the ESI†for full details.
16 The calculated structures show in fact partial opening of the
NHSi rings indicating formal oxidative addition taking place
on the silicon(II) centre. For a review on non-innocent
ligands, see: O. R. Luca and R. H. Crabtree, Chem. Soc.
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17 The complex cis-14a was calculated to be diamagnetic with
vacant centre on the iron. The singlet-triplet gap was
calculated to be only 5.1 kcal mol
1
.
18 (a) S. Rendler and M. Oestreich, Angew. Chem., Int. Ed., 2008,
47, 5997–6000; (b) Ref. 8h.
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20 During the preparation of this manuscript a neutral silicon
Lewis acid catalysis was reported: A. L. Liberman-Martin,
R. G. Bergman and T. D. Tilley, J. Am. Chem. Soc., 2015,
137, 5328–5331.
21 The adduct 31 was not observed by the
1
Hor
31
P NMR
spectroscopy, even at lower temperatures. Also attempts to
perform a Gutmann–Beckett analysis with Et
3
PO led to
only slight broadening of the
31
P NMR resonance of the
phosphine oxide.
22 (a)K.M
¨
uther and M. Oestreich, Chem. Commun., 2011, 47,
334–336; (b) Ref. 20.
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