Synthesis and catalytic property of iron pincer complexes generated by Csp3-H activation

Article abstract of DOI:10.1021/om500429r

When the diphosphinito PCP ligand (Ph₂P(C₆H ₄))₂CH₂ (1) was treated with Fe(PMe ₃)₄ and FeMe₂(PMe₃)₄, the C_sp3-H activation products [(Ph₂P(C ₆H₄))₂CH]Fe(H)(PMe₃)₂ (2) and [(Ph₂P(C₆H₄))(PhP(C₆H ₄)₂)CH]Fe(PMe₃)₂ (3) were obtained at room temperature. The generation of product 3 underwent one C _sp3-H and one C_sp2-H bond activation process. The new iron hydride complex 2 showed good activity in the catalytic hydrosilylation of aldehydes and ketones by using (EtO)₃SiH as the hydrogen source under mild conditions. Complexes 2 and 3 were characterized by spectroscopic methods and X-ray diffraction analysis.

Full text of DOI:10.1021/om500429r

Article
pubs.acs.org/Organometallics
Synthesis and Catalytic Property of Iron Pincer Complexes Generated
3
by Csp −H Activation
Hua Zhao, Hongjian Sun, and Xiaoyan Li*
School of Chemistry and Chemical Engineering, Key Laboratory of Special Functional Aggregated Materials, Ministry of Education,
Shandong University, Shanda Nanlu 27, 250199 Jinan, People’s Republic of China
S
* Supporting Information
ABSTRACT: When the diphosphinito PCP ligand (Ph2P-
(C6H4))2CH2 (1) was treated with Fe(PMe3)4 and Fe-
3
Me2(PMe3)4, the Csp −H activation products [(Ph2P-
(C6H4))2CH]Fe(H)(PMe3)2 (2) and [(Ph2P(C6H4))(PhP-
(C6H4)2)CH]Fe(PMe3)2 (3) were obtained at room
temperature. The generation of product 3 underwent one
3
2
Csp −H and one Csp −H bond activation process. The new iron
hydride complex 2 showed good activity in the catalytic
hydrosilylation of aldehydes and ketones by using (EtO)3SiH as
the hydrogen source under mild conditions. Complexes 2 and 3 were characterized by spectroscopic methods and X-ray
diffraction analysis.
n the past few years, the widespread existence of
hydrocarbons has caused activation studies on C−H bonds
iridium complex.9 In this paper, we report our recent progress
in stoichiometric C−H bond activation by using an iron-based
system under mild reaction conditions. The novel hydrido iron
pincer complex 2 prepared from the reaction of (Ph2P-
(C6H4))2CH2 (1) with Fe(PMe3)4 was fully characterized
I
to become one of the most important fields in organic synthesis
due to not only the inertness of the C−H bond but also the
potential economic benefits in synthetic applications.1 The C−
H bond acting as one “unfunctionalized” group, its cleavage
often occurs under harsh conditions. Early efforts in C−H
activation were successful by taking advantage of a directing
group via chelation. This could enhance the reactivity of the
C−H bond and result in its activation.2 Most work on C−H
3
,while the reaction of 1 with FeMe2(PMe3)4 via both Csp −H
2
and Csp −H bond activation delivered the pincer Fe(II)
complex 3. The hydrido pincer iron(II) complex 2 could
catalyze the hydrosilylation of aldehydes and ketones.
The reaction of PCP pincer ligand (Ph2P(C6H4))2CH2 (1)
with Fe(PMe3)4 in THF at room temperature resulted in the
formation of hydrido pincer Fe(II) complex 2 via one oxidative
addition process after 12 h with the substitution of two PMe3
ligands by two Ph2P− groups (eq 1). The hydrido Fe(II)
2
bond activation has been focused on the activations of Csp −H
3
bonds while there have been fewer reports on Csp −H bond
activation because, in general, the former is much easier than
the latter.3
Because of the strongly chelating nature of PCP pincer
ligands, it is easy to carry out the activation of intramolecular
C−H bonds, and this has also attracted a substantial amount of
interest.4 The PCP pincer complexes formed by transition
metals could also be used as catalysts in homogeneous
catalysis.5
In early work, precious-metal (Rh, Ir, Pd, Pt etc.) complexes
3
were often chosen to conduct the study of Csp −H bond
complex 2 was isolated as red crystals from Et2O at 0 °C in a
yield of 65%. Complex 2 was stable at room temperature and
could be handled in air in the solid state without noticeable
decomposition even after 1 day.
activation. Reports on cheap and environmentally friendly iron
complexes are rare.3b,6 Our group had been focusing on the
3
Csp −H bond activation of pincer ligands supported by Fe, Co,
3
and Ni complexes. We explored the Csp −H activation of
In the infrared spectrum of 2, one typical stretching band for
(Ph2POCH2)2CH2 induced by iron and cobalt complexes
1
the Fe−H bond was observed at 1938 cm−1. The H NMR
7
3
under mild conditions. As a continuation of the study on Csp −
spectrum of complex 2 in C6D6 gave evidence for the hydrido
ligand at −12.42 ppm as a tdd peak due to coupling between
the hydrido hydrogen and the four coordinated phosphorus
H bond activation, we tried to change the molecular skeleton of
the pincer ligands. The diphosphinito PCP ligand (Ph2P-
(C6H4))2CH2 was introduced because it showed considerable
inertness to precious metals, even under severe conditions.8 By
changing the substituents on the phosphorus atom from aryl to
alkyl, Warren realized a double C−H activation by using an
Received: April 24, 2014
Published: July 1, 2014
© 2014 American Chemical Society
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Article
2
atoms with coupling constants J(PH) of 75.4, 43.5, and 17.4
Hz (Supporting Information, Figure S1). The proton signal of
Fe−CH appears at 4.67 ppm as one singlet. The signals of two
PMe3 ligands are situated at 0.92 and 0.88 ppm. In the 31P
NMR spectrum, two types of PMe3 groups and one kind of
diphenylphosphanyl group appear at 25.0, 38.0, and 96.0 ppm
with a relative integral ratio of 1 (dt):1 (dt):2 (bs). The doublet
of triplets (dt) signals were formed by the PPMe atom with two
3
Figure 2. Molecular structure of 3. Hydrogen atoms are omitted for
clarity.
identical P−PPh atoms and another PPMe atom. A broad singlet
2
3
(bs) was found for the two P−PPh atoms, although this peak
2
should be a triplet or quartet. The structure of 2 was confirmed
by X-ray diffraction structure analysis (Figure 1). In the
distorted-octahedral coordination sphere and makes up the
equatorial plane P1, P2, P9, and C21 along with C7 and P4
located at the axial positions with the angle C7−Fe1−P4 =
172.5(3)°. In comparison with complex 2, complex 3 has three
chelate rings. Two of them are five-membered rings formed by
the [PCP]-pincer ligand. The third is a four-membered chelate
ring formed by ortho metalation of one of the phenyls of the
Ph2P− group via Csp −H bond cleavage. These three chelate
rings are in three different equatorial planes and are almost
perpendicular to each other.
2
In comparison with many reactions catalyzed by noble
metals, less work on homogeneous catalysis with Fe, Co, and
Ni has been disclosed, although their compounds are
inexpensive. Currently, there are more and more reports on
the use of iron catalysts in a variety of organic synthetic
reactions.12 Hydrido iron complexes often act as important
intermediates in the catalytic cycle. Some catalytic systems of
hydrido iron complexes have been established in hydro-
genation13 and hydrosilylation processes.14 Milstein explored
iron-catalyzed reduction reactions using [PNP]-pincer iron
complexes.15
Figure 1. Molecular structure of 2. Most hydrogen atoms are omitted
for clarity.
molecular structure of 2, the iron atom is centered in a
distorted-octahedral geometry caused by the small hydrido
ligand. Therefore, the bond angle P4−Fe1−P7 (137.13(5)°)
strongly deviates from 180°. The angle P4−Fe1−P7 bends
toward the direction of the orientation of the hydrido ligand.
The axial H1−Fe1−P9 is almost perpendicular to the
equatorial plane formed by C19, P4, P7, and P10 . Both
Fe1−P9 (2.252(1) Å) and Fe1−P10 (2.28(2) Å) are
remarkably longer than both Fe1−P4 (2.176(1) Å) and Fe1−
P7 (2.184(1) Å) due to the strong trans influence of the
hydrido hydrogen (H1) and the carbanion (C19). The Fe1−
H1 (1.56(4) Å) and Fe1−C19 distances (2.170(4) Å) are
It was confirmed that iron hydride complex 2 could be used
in the reduction of aldehydes and ketones by using silane as the
hydrogen source under mild conditions with a catalyst loading
of 0.3−1 mol % (eq 3). The experiments showed that complex
within the normal ranges for Fe−H (1.51−1.69 Å)10 and Fe−
37,11
Csp
bonds. Since the C19 atom is sp3 hybridized, not only
are the two five-membered chelate rings not coplanar but also
each five-membered chelate ring is nonplanar. The four
chemical bonds linked to C19 (C19−Fe1, C19−H19, C19−
C15, and C19−C20) must meet the requirements of bond
angles for the sp3 hybridization of C19.
Through the reaction of 1 with FeMe2(PMe3)4 in THF at
room temperature, complex 3 was obtained after 12 h. 3 could
be crystallized from n-pentane in a yield of 45% (eq 2).
2 as catalyst was necessary for this reduction reaction of
benzaldehyde under the given conditions (Table 1). Interest-
ingly, Ph3SiH and Et3SiH were not suitable for this catalytic
system, while the reaction proceeded quantitatively with
(EtO)3SiH as a reducing agent.
The scope of the substrates was expanded to explore the
catalytic hydrosilylation reactions (Table 2). Different
aldehydes could be reduced to the corresponding alcohols
with 0.3−1 mol % of catalyst 2 at 50 °C in THF in variable
yields over different periods (Table 2). It can be seen from
Table 1. Catalytic Activity of 2 for Hydrosilylation of
a
PhCHO
b
entry
catalyst
silane
time (h)
conversion (%)
1
In the H NMR spectrum the signal of Fe−CH appears at
1
2
3
4
(EtO)3SiH
(EtO)3SiH
Ph3SiH
24
1
0
>99
0
4.95 ppm as one broad singlet. In the 31P NMR spectrum, three
types of signals appear at 91.2, 31.1, and 23.0 ppm with a
relative integral ratio of 1:2:1. From the molecular structure of
3 (Figure 2) established by an X-ray diffraction study it could
2
2
2
24
24
Et3SiH
0
3
a
be concluded that complex 3 was formed by one Csp −H and
Conditions: PhCHO (1.0 mmol), silane (2.0 mmol), complex 2
(0.003 mmol), THF (1 mL), 50 °C. Determined by GC with n-
dodecane as an internal standard.
b
2
one Csp −H bond activation process. The molecular structure
of complex 3 confirms that the central iron atom is situated in a
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Article
Table 2 that this catalytic system has a certain tolerance for
both the electron-withdrawing group(s) and electron-donating
group(s) on the phenyl ring. The substrates with the electron-
drawing substituents (Table 2, entries 3−8 and 10) were
reduced to the corresponding alcohols with a catalyst loading of
0.3 mol % in a few hours. The reduction of the substrates with
electron-donating groups was slow and needed a longer
reaction time and a higher catalyst loading to reach full
conversion (Table 2, entries 2 and 11). In comparison with
aldehydes, ketones are less reactive (Table 2). It was difficult to
reach full conversions, though we had attempted to optimize
the reaction conditions. For the ketones, there was no obvious
trend similar to that for aldehydes. The experimental results
indicate that the polarity of the carbonyl group has an effect on
the hydrosilylation reactions. With a decrease in the polarity of
carbonyl group, the reaction time increased and the reaction
rate was reduced.
Table 2. Catalytic Hydrosilylation of Aldehydes and
Ketones
a
In previous work on the iron-catalyzed hydrosilylation of
aldehydes and ketones, the best results were reported by Tilley
with 0.01−2.7 mol % of Fe{N(SiMe3)2}216 and Ph2SiH2 as the
hydrogen source. In the hydrosilylation of benzaldehyde, the
catalyst loading was 2.7 mol %. In comparison with the catalytic
exploration by [POCOP]-pincer iron hydride using (EtO)3SiH
as the hydrogen source,17 complex 2 in this work has better
catalytic activity for the hydrosilylation of benzaldehyde at
similar reaction temperatures because only 0.3% catalyst
loading was needed.
In summary, the pincer iron(II) hydrido complex 2 was
synthesized by the reaction of the [PCP] ligand (Ph2P-
3
(C6H4))2CH2 (1) with Fe(PMe3)4. Complex 3 as one Csp −H/
2
Csp −H bond activation product was obtained by the reaction
of 1 with FeMe2(PMe3)4. The molecular structures of
complexes 2 and 3 were determined by X-ray diffraction.
Importantly, complex 2 showed good activity in the catalytic
hydrosilylation of aldehydes and ketones with (EtO)3SiH as a
hydrogen source under mild conditions.
EXPERIMENTAL SECTION
■
General Procedures and Materials. All air-sensitive and volatile
materials were handled by using standard vacuum techniques. Solvents
were dried according to known procedures and freshly distilled prior
to use. Literature methods were applied for the preparation of
(Ph2P(C6H4))2CH2,8 Fe(PMe3)4,18a and FeMe2(PMe3)4.18b Infrared
spectra (4000−400 cm−1) were recorded on an FT-IR instrument
from Nujol mulls between KBr disks. NMR data were obtained on a
NMR spectrometer at 300 MHz. Single-crystal X-ray diffraction data
were collected using a CCD diffractometer with graphite-monochro-
mated Mo Kα (λ = 0.71073 Å) radiation. GC was carried out with n-
dodecane as an internal standard. All of the aldehydes and ketones
were freshly distilled or recrystallized.
Typical Procedure for the Syntheses of 2 and 3. Fe(PMe3)4
(396 mg, 1.1 mmol) or FeMe2(PMe3)4 (432 mg, 1.1 mmol) in 30 mL
of THF was added to a solution of 1 (563 mg, 1.0 mmol) in 40 mL of
THF. The mixture was stirred at room temperature for 12 h, and then
the solvent was removed by vacuum. The solid residue was extracted
with diethyl ether or n-pentane. 2 as red crystals was obtained from
diethyl ether at 0 °C. 3 as red crystals was obtained from n-pentane at
0 °C. Analytical data can be found in the Supporting Information.
General Procedure for the Catalytic Hydrosilylation. The
general procedure was same as in our earlier work,19 with different
catalyst loadings for some substrates.
a
Conditions: substrate (1.0 mmol), (EtO)3SiH (2.0 mmol), complex
b
2, THF (1 mL), 50 °C. GC yield with n-dodecane as an internal
standard.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures and CIF files giving crystallographic data and
details of the X-ray study for 2 and 3, H NMR peaks of the
hydrido hydrogen in 2 (Figure S1), analytical data and spectra
for 2 and 3, and NMR data for alcohols. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for X.L.: xli63@sdu.edu.cn.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the support by the NSF of China
(No. 21372143). We also thank Prof. Dieter Fenske and Dr.
Olaf Fuhr (Karlsruhe Nano-Micro Facility (KNMF), KIT) for
their kind assistance with the X-ray diffraction analysis.
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