An isolable iron(ii) bis(supersilyl) complex as an effective catalyst for reduction reactions

Article abstract of DOI:10.1039/c9dt00116f

An isolable 14-electron iron bis(supersilyl) complex, Fe[Si(SiMe₃)₃]₂(THF)₂, was successfully synthesized from the reaction of FeBr₂ with K[Si(SiMe₃)₃] and its structure was unambiguously determined by single-crystal X-ray diffraction analysis. The complex is coordinatively unsaturated and exhibits high catalytic activity toward the hydrosilylation of carbonyl compounds and the reductive silylation of dinitrogen.

Full text of DOI:10.1039/c9dt00116f

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DOI: 10.1039/C9DT00116F
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An Isolable Iron(II) Bis(supersilyl) Complex as an Effective Catalyst
for Reduction Reactions
Shogo Aratab and Yusuke Sunada*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An isolable 14-electron iron bis(supersilyl) complex,
The most widely used synthetic method to generate
Fe[Si(SiMe3)3]2(THF)2, was successfully synthesized from the organosilyl complexes is based on the oxidative addition of the
reaction of FeBr2 with K[Si(SiMe3)3] and its structure was “H–Si” group in hydrosilanes to low-valent transition-metal
unambiguously determined by single-crystal X-ray diffraction centers.2b,c Low-valent iron carbonyls have been effectively
analysis. The complex is coordinatively unsaturated and exhibits employed as precursors to afford a variety of iron complexes
high catalytic activity toward the hydrosilylation of carbonyl with organosilyl ligands.2b,c,5 However, this approach generally
compounds and the reductive silylation of dinitrogen.
allows only the preparation of coordinatively saturated
complexes, and the liberation of CO requires harsh reaction
conditions such as high temperatures or UV irradiation. The lack
of suitable isolable low-valent 3d metal precursors that do not
contain CO ligands has hitherto prevented the direct synthesis
of coordinatively unsaturated iron organosilyl complexes. A clue
to overcome this drawback was provided by Tilley and co-
workers, who described the synthesis of the anionic iron
bis(supersilyl) complex, Li(THF)3{FeCl[Si(SiMe3)3]2}, by reaction
of FeCl2 with the lithium salt of the supersilyl anion.6
Subsequent treatment with Me3SiOSO2CF3 afforded the neutral
Catalytic transformations mediated by transition-metal
complexes are some of the most important tools in modern
organometallic chemistry. Owing to the increasing demand for
environmentally benign organic syntheses, the development of
highly active base-metal catalysts has received much attention
in the last decade.1 The design and development of
coordinatively unsaturated base-metal complexes is the most
straightforward approach to such green chemistry. It is well
known that the introduction of organosilyl ligands to a metal
center effectively contributes to the formation of coordinatively
unsaturated complexes due to the strong -donating
properties and high trans influence of the organosilyl ligands.2
As representative examples, in-situ-generated coordinatively
unsaturated 14-electron iron species of the type
“Fe(R3Si)2(L)(L’)” show desirable catalytic performance in
reductive transformation reactions, i.e., hydrogenations or
hydrosilylations {(R3Si)2 = o-(SiMe2)2C6H4; L = L’ = CO or CNR}3 or
reductive silylations of dinitrogen (R = Me; L = THF, L’ = N2)4. If
the analogous 14-electron iron species can be isolated and used
in catalysis, more effective catalytic systems could be
potentially developed. However, to the best of our knowledge,
coordinatively
unsaturated
bis(supersilyl)
complex
Fe[Si(SiMe3)3](THF), which was examined by IR spectroscopy
and hydrolytic degradation studies. This prompted us to explore
a potentially facile synthetic route to bis(organosilyl) iron
complexes based on a salt metathesis with appropriate
modifications. In the present study, we describe a one-step
synthesis of the first crystallographically characterized 14-
electron bis(organosilyl) iron(II) complex Fe[Si(SiMe3)3]2(THF)2
(1) using the potassium salt of the supersilyl anion (Scheme 1).
The obtained iron complex displays good catalytic activity in
reduction reactions, including the hydrosilylation of carbonyl
compounds and the reductive silylation of dinitrogen. The facile
in situ generation of catalytically active iron species was
demonstrated by simply mixing the widely available and easy-
to-handle precursors of 1, which will undoubtedly facilitate the
development of further practical catalytic transformations.
The potassium salt of the supersilyl anion, KSi(SiMe3)3, was
obtained from the reaction of Si(SiMe3)4 and KOtBu according to
the report of Marschner.7 We found that the 14-electron
bis(supersilyl)iron(II) complex Fe[Si(SiMe3)3]2(THF)2 (1) was
readily obtained within 1 h from the reaction of FeBr2 with 2
crystallographically
characterized
14-electron
iron(II)
bis(organosilyl) complexes have not been reported so far.
a. Institute of Industrial Science, The University of Tokyo, 4-6-1, Komaba, Meguro-
ku, Tokyo 153-8505, Japan.
b. Department of Applied Chemistry, School of Engineering, The University of Tokyo,
4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, Japan.
Electronic Supplementary Information (ESI) available: Experimental,
crystallographic, and computational details, and crystal data for 1 and 2. CCDC
1884063 (1) and 1884064 (2). See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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N
equiv. of KSi(SiMe3)3 in THF at room temperature (Scheme 1).
Complex 1 was isolated in 89% yield as red purple crystals after
recrystallization from THF/pentane at –30 °C. An advantage of
this procedure is that 1 is easily prepared in one step without
the use of reagents that are difficult to handle such as
Me3SiOSO2CF3.
View Article Online
DOI: 10.1039/C9DT00116F
(Me3Si)3Si
(Me3Si)3Si
(Me3Si)3Si
(2 equiv.)
pyridine
pyridine
THF
THF
Fe
Fe
THF, r.t., 1 h
(Me3Si)3Si
2, quant
Scheme 2. Facile ligand exchange to afford 2.
(Me3Si)3Si
THF
KSi(SiMe3)3
(2 equiv.)
FeBr2
+
Fe
One interesting feature is that the two THF ligands in 1 can
be easily replaced by other auxiliary ligands. For instance, the
reaction of 1 with 2 equiv. of pyridine at room temperature
instantly furnishes Fe[Si(SiMe3)3]2(pyridine)2 (2) quantitatively
within 1 h (Figure S3 in the Supporting Information), and 2 was
isolated in 40% yield (Scheme 2). The formation of 2 was
confirmed by NMR spectroscopy, elemental analysis, and X-ray
diffraction measurements. In the 1H NMR spectrum of 2, three
broad resonances are observed at −13.02, 8.36, and 46.69 ppm,
which were assigned to the pyridine ligands coordinated to the
iron center, while the signal of the SiMe3 groups was observed
as a broad singlet at 6.56 ppm. The molecular structure of 2 was
unequivocally determined by X-ray diffraction analysis (Figure
S14 in the Supporting Information). These results clearly
indicate that the iron center in 1 can generate two vacant sites
via facile dissociation of the THF ligands.
THF, r.t., 1 h
THF
(Me3Si)3Si
1, 89% isolated yield
Scheme 1. Synthesis of 1.
The solid-state structure of 1, which represents the first
example of a crystallographically characterized 14-electron
iron(II) bis(organosilyl) complex, is shown in Figure 1. The iron
center in 1 is coordinated to two supersilyl and two THF ligands
and exhibits a distorted tetrahedral coordination geometry. The
Si(1)–Fe–Si(2) angle of 135.23(3)° is considerably wider than
that for a tetrahedral geometry, which should be attributed to
the steric repulsion between the two supersilyl ligands. The Fe–
Si bond distances (Fe–Si(1) = 2.5445(8) Å; Fe–Si(5) = 2.5445(8)
Å) are longer than those in the anionic bis(supersilyl) complex
reported by Tilley, (NEt4){FeCl[Si(SiMe3)3]2} (Fe–Si
=
The coordinatively unsaturated nature of 1 is reflected in its
good catalytic activity in reduction reactions, including the
hydrosilylation of carbonyl compounds, which is important for
the production of alcohols in the absence of hazardous reagents
such as LiAlH4. The development of iron-catalyzed
hydrosilylation reactions is currently a hot topic in synthetic
chemistry.10 For instance, Tilley et al. have reported a
coordinatively unsaturated iron bis(amide) complex,
Fe[N(SiMe3)2]2, with high catalytic activity toward the
hydrosilylation of aldehydes and ketones with Ph2SiH2 at room
temperature and a catalyst loading of 0.03–2.7 mol%.11
Turculet’s iron amide complex, which bears an N-
phosphinoamidinate ligand, also exhibits high catalytic activity
toward the reduction of carbonyl compounds with Ph2SiH2
under mild reaction conditions in the presence of 0.01–1 mol%
of the catalyst.12 Despite the large number of iron-catalyzed
hydrosilylation reactions developed, only a limited number of
reactions can be conducted using relatively low catalyst
loadings (<1 mol%) under mild reaction conditions, which
include short reaction times (within a few hours) and low
reaction temperatures such as room temperature. In other
words, the development of iron-catalyzed reactions that fulfill
these requirements remains challenging.
2.488(6)/2.491(6) Å). These Fe–Si bonds are also significantly
longer than those found in previously reported neutral 18-
electron Fe(II) bis(organosilyl) complexes (2.25–2.5 Å)8,
presumably due to steric hindrance as well as the open-shell
spin state of 1.9 It should be noted that Tilley et al. have also
reported the synthesis of Fe[Si(SiMe3)3]2(THF) and
Fe[Si(SiMe3)3]2(DME), albeit that a crystallographic analysis was
not undertaken. Related manganese analogues such as
Mn[Si(SiMe3)3]2(L)2 [(L)2 = dmpe or (O=CPh2)2] have been
reported by Tilley and exhibit a tetrahedral coordination
geometry with Si–Mn–Si angles of ca. 133° and Mn–Si bond
distances of ca. 2.64 Å.6a
Si(1)
O(2)
O(1)
Fe
Si(5)
Complex 1 exhibits good catalytic activity for the reduction
of aldehydes and ketones with Ph2SiH2; all reactions were
completed within a few hours at room temperature in the
presence of 0.1–0.5 mmol% of the catalyst. For instance,
benzaldehyde underwent the hydrosilylation by Ph2SiH2 at
room temperature within 1 h in the presence of 0.5 mol% of 1.
The product was isolated as benzyl acetate in 71% yield after a
workup with aqueous [nBu4N]F and acetylation by acetyl
chloride (Table 1, entry 1). An aliphatic aldehyde was also
reduced under the same reaction conditions, affording 3-
Figure 1. Molecular structure of 1 with 50% probability ellipsoids.
Complex 1 is paramagnetic and its 1H NMR spectrum (C6D6;
room temperature) exhibits a broad signal at 7.27 ppm for
SiMe3 as well as resonances at 20.60 and 27.33 ppm for the
coordinated molecules of THF. The solution magnetic moment,
measured by the Evans’ method (eff = 5.36), is consistent with
an open-shell S = 2 ground state. Complex 1 is highly sensitive
toward air and moisture, but the results of the elemental
analysis are consistent with the theoretically expected values.
2 | J. Name., 2012, 00, 1-3
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phenyl-1-propanol in 86% yield after workup with aqueous underwent a smooth and selective hydrosilylation VoiefwthAertickleeOtonlninee
[nBu4N]F (entry 2).
moiety to furnish the alcohol with an intact ester group (entry
6). Both aliphatic and alicyclic ketones were converted into the
corresponding alcohols in good yield within a few hours using
0.1–0.5 mol% of 1 at room temperature (entries 7 and 8).
DOI: 10.1039/C9DT00116F
Table 1. Reduction of aldehydes or ketones catalyzed by 1.a
[nBu4N]F aq.
O
OH
cat: 1
Ph2SiH2
+
r.t., time
cat: 1
(0.5 mol%)
R'
R
O
(2 equiv.)
R'
R
Cat. Loading time Conv. Yield
98%
r.t.
entry
1
Substrate
(mol%)
(h)
(%)b
(%)c
O
O
O
+
53% (1 h), 76% (24 h)
Ph2SiH2
0.5
1
> 99
71g
H
FeBr2 (1 mol%) /
Si(SiMe3)4 (2 mol%) /
KOtBu (2 mol%)
(3 equiv.)
O
0.5
0.5
1
1
> 99
> 99
86
87
2
3
H
Scheme 3. Deoxygenation of 4’-methoxyacetophenone catalyzed by
generated Fe catalyst.
1 or in-situ-
O
To our surprise, the reaction of 4’-methoxyacetophenone
with Ph2SiH2 proceeded in a deoxygenative manner to
quantitatively afford 4-ethylanisole as the sole product (Scheme
3). This reaction was completed within 1 h at room temperature
in the presence of 0.5 mol% of 1. The deoxygenation of ketones
is an important tool in organic synthesis, exemplified by the
Clemmensen and Wolff–Kishner reactions. However, they are
generally associated with the use of stoichiometric amounts of
toxic reagents and harsh reaction conditions. Iron-catalyzed
reactions with hydrosilanes as the deoxygenating reagent could
represent a convenient alternative to these conventional
methods, albeit that they have scarcely been investigated thus
far. A system composed of FeCl3 as the catalyst and
polymethylhydrosiloxane (PMHS) as the reducing reagent has
recently been investigated by Campagne and co-workers.13
However, this reaction requires harsh reaction conditions
including high catalyst loadings (10 mol%) and microwave
irradiation. To the best of our knowledge, the deoxygenation
reaction shown in Scheme 3 is the first iron-catalyzed reaction
that proceeds under very mild reaction conditions and low
catalyst loading.
O
O
4d
5
0.1
0.5
4
2
> 99
> 99
92
90
Ph
Br
O
O
6e
0.5
2
> 99
77
O
O
0.5
0.1
1
4
> 99
> 99
82
7
O
98h
8f
aAll reactions were carried out with carbonyl compounds (1 mmol), catalytic
b
amount of 1, and Ph2SiH2 (2.2 mmol) at room temperature. Conversion of the
starting materials was determined by 1H NMR in the presence of an internal
standard. cisolated yield. dreaction was performed with 3 mmol of 4-acetylbiphenyl
in dioxane (1.5 mL). ereaction was performed in dioxane (0.5 mL). reaction was
f
Complex 1 is highly active, but sensitive to air and moisture.
Therefore, it is of particular importance that the catalytically
active species can be generated by simply mixing the iron salt
and precursors of the supersilyl ligand. Namely, catalytic
amounts of FeBr2 (1 mol%), Si(SiMe3)4, and KOtBu in a 1:2:2 ratio
were mixed in tetrahydropyran, before this solution was added
to a mixture of 4’-methoxyacetophenone and Ph2SiH2 at room
temperature. Although the yield of the product was lower than
that obtained by the reaction catalyzed by isolated 1 (53% after
performed with 3 mmol of cyclohexanone. gisolated as acetylated product. hyield
was determined by 1H NMR by using hexamethylbenzene as an internal standard.
The hydrosilylation of ketones with Ph2SiH2 catalyzed by 1
also proceeded effectively at room temperature within 1–4 h.
For instance, complete consumption of acetophenone was
confirmed in the reaction with Ph2SiH2 in the presence of 0.5
mol% of 1 after 1 h; the subsequent workup with aqueous
[nBu4N]F afforded 2-phenylethyl alcohol in 87% isolated yield
(entry 3). In contrast, no reaction occurred when
monohydrosilanes such as PhMe2SiH or bifunctional
hydrosilane Me2(H)SiOSi(H)Me2 were used as the reducing
reagent. The high catalytic activity of 1 was demonstrated by
the reaction with 4-acetylbiphenyl with a lower catalyst loading;
the reaction with 0.1 mol% of 1 afforded the product in
quantitative yield with a catalyst turn over number (TON) of
1000 (entry 4). Reductive dehalogenation did not occur in the
reaction with p-bromoacetophenone (entry 5), furnishing the
corresponding alcohol in 90% yield. Methyl-4-acetylbenzoate
1
1 h; 76% after 24 h), a H NMR analysis of the crude product
confirmed the formation of the corresponding deoxygenated
product (Scheme 3). Product formation was not observed when
the reaction was conducted in the absence of FeBr2.‡ It should
be emphasized that all reagents used in the present catalyst
system can be handled under aerobic conditions and are
commercially available. These advantages render the present
catalyst system very attractive for iron-catalyzed reactions.§
Another reduction reaction catalyzed by 1 is the reductive
silylation of atmospheric dinitrogen. Recently, Nishibayashi and
Yoshizawa have reported that iron species such as Fe(CO)5 and
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Cp’2Fe (Cp’ = C5H2(SiMe3)3) mediate this reaction to afford 25
and 35 equiv. of N(SiMe3)3, respectively.4 An interesting feature
of this reaction is that theoretical calculations suggest that the
bis(organosilyl) iron(II) complex “Fe(SiMe3)2(THF)” acts as the
key intermediate, although experimental evidence to support
that hypothesis has not been provided. In the present work, we
discovered that 1 catalyzes the reductive silylation of dinitrogen
using excess SiMe3Cl and KC8 at room temperature under
atmospheric pressure of N2. Complex 1 (0.2 mol%) generated
22.9 equiv. of N(SiMe3)3 per Fe atom in THF, under concomitant
formation of byproducts including Me3Si-SiMe3 and mono- and
bis-silylated ring-opened THF, i.e., Me3SiO(CH2)3CH2R (R = H or
SiMe3). In contrast, the TON slightly increased to 25.9 when the
reaction was conducted in 1,2-dimethoxyethane (DME), and the
formation of the aforementioned byproducts was suppressed
(Scheme 4). A reaction was not observed in toluene, despite the
high solubility of 1 in toluene. Although the detailed mechanism
for this reaction is still unclear, our results seem to support the
notion that a coordinatively unsaturated bis(organosilyl)iron
“Fe(SiMe3)2(THF)” complex can effectively mediate the
reductive silylation of dinitrogen.
Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/C9DT00116F
Notes and references
‡ We found that in-situ-generated active species based on this method can also
catalyze the hydrosilylation of acetophenone or cyclohexanone with Ph2SiH2.
However, this reaction also proceeds with the same rate in the absence of FeBr2; in
other words, KOtBu may be the true catalyst for the hydrosilylation of aldehydes and
ketones with Ph2SiH2. Details on this reaction will be reported in the near future.
§ In Tilley’s catalytic system, the reduction of ketones with Ph2SiH2 also occurs when
a 1:2 mixture of FeBr2 and NaN(SiMe3)2 is used as the catalyst, albeit that the
addition of air- and moisture sensitive NaN(SiMe3)2 is required as an activator.
1
(a) B. Plietker, In Iron Catalysis in Organic Chemistry, Ed.; B.
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cat 1 (0.2 mol%)
2 N(SiMe3)3
N2
(1 atm)
+ KC8 + Me3SiCl
solvent
r.t., 24 h
22.9 equiv. (in THF)
25.9 equiv. (in DME)
Not detected (in toluene)
Scheme 4. Reductive silylation of atmospheric dinitrogen catalyzed by 1.
In conclusion, we have synthesized the first
crystallographically
bis(organosilyl) complex, Fe[Si(SiMe3)3]2(THF)2, by
characterized
14-electron
iron
salt
4
5
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a
metathesis reaction between FeBr2 and K[Si(SiMe3)3] in THF.
The high catalytic activity of 1, resulting from its coordinative
unsaturation, was demonstrated for the hydrosilylation of
aldehydes and ketones with Ph2SiH2, as well as by the reductive
silylation of dinitrogen. It should be emphasized that the
hydrosilylation reactions proceeded effectively with relatively
low catalyst loadings (0.1–0.5 mol%) and were all completed
within 1–4 h at room temperature. Of particular interest is that
the reaction of 4’-methoxyacetophenone with Ph2SiH2
catalyzed by 1 proceeds in a deoxygenative manner. The
catalytically active species for this reaction can be easily
generated in situ by simply mixing commercially available and
easy-to-handle precursors. The present study thus presents a
simple and promising strategy to generate coordinatively
unsaturated base-metal organosilyl catalysts that may open
new ways to construct well-designed and highly reactive base-
metal catalysts. Research in this direction is currently in
progress in our laboratories.
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We warmly thank JSPS KAKENHI Grant No.18H04240 in
Precisely Designed Catalysts with Customized Scaffolding, Grant
in Aid for Scientific Research (B) (No. 16H04120) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan, Inamori Foundation, Iketani Science and Technology
Foundation, and Shitagau Noguchi Foundation.
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