Kn?lker-Type Iron Complexes Bearing an N-Heterocyclic Carbene Ligand: Synthesis, Characterization, and Catalytic Dehydration of Primary Amides

Article abstract of DOI:10.1021/acs.organomet.5b00553

A family of iron Kn?lker-type complexes bearing an N-heterocyclic carbene (NHC) ligand has been synthesized from the corresponding tricarbonyl(η⁴-cyclopentadienone)iron complex by reaction with the in situ generated NHC ligand, under UV irradiation (350 nm) at room temperature. The six complexes, obtained in good yields (64-84%) after purification on column chromatography, were fully characterized, including X-ray diffraction and electrochemical studies. The activity of these complexes in catalysis has been demonstrated for the dehydration of primary benzamides into benzonitriles under hydrosilylation conditions using inexpensive polymethylhydrosiloxane (PMHS) as the hydrosilane at 100 °C.

Full text of DOI:10.1021/acs.organomet.5b00553

Article
pubs.acs.org/Organometallics
Kno
̈
lker-Type Iron Complexes Bearing an N‑Heterocyclic Carbene
Ligand: Synthesis, Characterization, and Catalytic Dehydration of
Primary Amides
†
†
‡
‡
†
Saravanakumar Elangovan, Samuel Quintero-Duque, Vincent Dorcet, Thierry Roisnel, Lucie Norel,
Christophe Darcel,* and Jean-Baptiste Sortais*
,†
,†
†
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite
́
de Rennes 1, Team Organometallics: Materials and
Catalysis, Centre for Catalysis and Green Chemistry, and Centre de Diffractometrie X, Campus de Beaulieu, 263 av. du Gener
Leclerc, 35042 Rennes Cedex, France
‡
́
́
́
al
*
S Supporting Information
ABSTRACT: A family of iron Kno
an N-heterocyclic carbene (NHC) ligand has been synthesized
from the corresponding tricarbonyl(η -cyclopentadienone)iron
̈
lker-type complexes bearing
4
complex by reaction with the in situ generated NHC ligand,
under UV irradiation (350 nm) at room temperature. The six
complexes, obtained in good yields (64−84%) after purification
on column chromatography, were fully characterized, including
X-ray diffraction and electrochemical studies. The activity of these complexes in catalysis has been demonstrated for the
dehydration of primary benzamides into benzonitriles under hydrosilylation conditions using inexpensive polymethylhydrosilox-
ane (PMHS) as the hydrosilane at 100 °C.
INTRODUCTION
■
4
Since the discovery of the potential of tricarbonyl(η -
cyclopentadienone)iron complexes in catalysis by Casey in
1
2
007, especially in the reduction area, this family of complexes
2
has gained a renewal of interest over the last few years.
4
Tricarbonyl(η -cyclopentadienone)iron complexes were syn-
3
thesized in the 1950s and studied in depth by Kno
̈
lker in the
4
1
990s, including the synthesis of the (hydroxycyclopentadien-
4e
yl)iron hydride complex in 1999, which is the iron structural
5
̈
analogue of Shvo’s complex with ruthenium. Knolker-type
complexes are attractive due to their versatile applications in
redox catalysis: they have been successfully applied in
1
,6
6m,n,7c
hydrogenation, hydrogen transfer,
including asymmet-
6
h−j
7
8
ric versions,
oxidation, reductive amination, and more
Figure 1. Principal modifications of Kno
̈
lker-type complexes.
9
recently in hydrogen-borrowing reactions.
From the point of view of ligand design, two main strategies
̈
concerning the modification of the framework of the Knolker-
4
d,7c,d
type complexes have been followed, in order to improve the
catalytic activities or achieve novel reactivity. The first deals
with the modification of the substitution pattern of the
cyclopentadienone ring. Representative examples are given in
Figure 1, showing the modification of the nature of the
substituents at the 2- and 5-positions by substituting the TMS
one of the carbonyl ligands, by a nitrile (acetonitrile
or
6
a
6a
11
benzonitrile ), a pyridine, a phosphine, or a chiral
phosphoramidite. Thus, Knolker-type complexes can now
̈
be seen as an easily modulated organometallic platform that can
6h
be subjected to further modifications due to the growing impact
12
of iron complexes in catalysis. Surprisingly, Kno
̈
lker-type
6
k,8b
derivatives bearing an N-heterocyclic carbene (NHC) as a
groups with various silylated motifs
or with a phenyl
13
8
b
ligand have not been described to date. Hence, we have
explored the synthesis and catalytic activities of Knolker-type
complexes bearing various N-heterocyclic carbene ligands.
group. The nature of the cycle fused to the cyclo-
̈
pentadienone ring has also been modulated: five- or six-
4
b
membered rings including alkyl chains, heteroatoms (O, S,
8
6h,l,m
N ), and chiral versions.
Finally, the carbonyl moiety has
also been recently silylated. The second strategy to modulate
the structure of these complexes relies on the substitution of
10
Received: June 24, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.5b00553
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Article
RESULTS AND DISCUSSION
this complex was not stable and decomposed during the steps
of purification.
To synthesize unsaturated unsymmetrical NHC ligands, we
■
Preparation of the Complexes. Previously, two routes
have been developed to introduce a phosphine or a nitrile by
exchange with a CO ligand, either under thermal conditions in
́
have followed the recent procedure described by Basle and
15
4
a−c
Maudit to prepare the N-mesityl-N-cyclohexylimidazolium
salt. In a similar fashion, starting from (R)-phenylethylamine
and mesitylamine, a chiral dissymmetric imidazolium chloride
was prepared in moderate yield (28%). Both ligands have also
been successfully coordinated to the iron center following the
route described above (6 and 7; 65 and 76% yields,
respectively) (Figure 2).
refluxing Bu O or acetone in the presence of Me NO
or
2
3
4
d
under photochemical conditions using UV irradiation. To
introduce the N-heterocyclic carbene ligand in the coordination
sphere of the iron, we have chosen the photochemical pathway
on the basis of our experience in the photochemistry of iron
14
carbonyl complexes. As a model reaction, a solution of 1 and
of the free 1,3-dimesitylimidazol-2-ylidene carbene (IMes)
obtained by deprotonation with potassium hexamethyldisila-
zide (KHMDS) of the corresponding chloride salt, IMes·HCl)
was irradiated under UV (350 nm) in toluene for 20 h at room
temperature, leading to the expected ligand exchange of one
CO for the N-heterocylic carbene (Scheme 1). During the
Characterization of the Complexes. All of the complexes
(
1
13
1
have been fully characterized by H and C{ H} NMR, IR, X-
ray diffraction studies, cyclic voltammetry, HR-MS, and
elemental analysis.
NMR and IR Studies. The coordination of the N-heterocyclic
carbene has been ascertained by 13C{ H} NMR, where the
carbene carbon atoms appear between 181.5 and 187.8 ppm.
These chemical shifts are in agreement with those of the
1
Scheme 1. General Synthesis of the Complexes
16
carbene carbon of the analogous (NHC)Fe(CO) complexes.
4
In a similar manner, the carbons of the carbonyl groups appear
between 216.7 and 219.3 ppm, in accordance with the
1
6
(
NHC)Fe(CO) complexes. These signals are shifted to
4
+
low field in comparison to the CpFe(CO) (NHC) cationic
2
complexes, which display a chemical shift typically around
14,17
2
09−210 ppm.
This is in agreement with the poorer
donating ability of the neutral cyclopentadienone ligand in
comparison to the anionic cyclopentadienide ligand. In the case
of the chiral complex 7, the two resonances appear for the
diastereotopic CO ligands at 216.7 and 217.1 ppm.
The H NMR spectra of the complexes at room temperature
are straightforward: all of them show the presence of the
cyclopentadienone ring and the NHC ligand.
course of the reaction, the solution turned from brown to dark
deep yellow. The H NMR of the crude mixture indicated a
1
conversion of 90% of 1 to the expected complex 2 (10% of
unreacted 1 was also detected). Interestingly, the complex was
purified by column chromatography on silica gel in air, leading
to the isolation of 2 as a golden yellow solid in 78% yield. It
should also be pointed out that the reaction was performed on
a gram scale.
1
The stretching frequencies of the carbonyl ligands in IR are
reported in Table 1. In comparison to the unsubstituted
The same procedure was applied to the preparation of
complexes bearing various symmetrical unsaturated NHC
ligands bearing respectively a methyl (IMe), an isopropyl
Table 1. Summary of Relevant Spectroscopic and
Electrochemical Data
(
IiPr), or a cyclohexyl (ICy) substituent on each of the
E
1/2 (V)
p
TEP value
−1
c
complex νCO (cm−1) νCOav (cm−1
)
a
[ΔE (mV)]
b
(cm )
nitrogen atoms, leading to 3−5 in good yields (Figure 2). It is
noteworthy that the coordination of the saturated 1,3-dimesityl-
d
2
3
4
5
6
7
1978, 1913
1975, 1919
1975, 1924
1969, 1907
1980, 1919
1973, 1915
1945
1947
1949
1938
1949
1944
0.11 [−]
2050.5
2054.1
2051.5
2049.7
2050.8
0.16 [75]
0.12 [78]
0.10 [93]
0.04 [108]
4
,5-dihydroimidazol-2-ylidene (SIMes) N-heterocylic carbene
1
on 1 took place, as monitored by H NMR, but unfortunately
d
0.14 [91]
a
νCOav is the average value of the frequencies of the two CO stretching
b
bands (recorded in the solid state using ATR equipment). Conditions
unless stated otherwise: sample concentration 1 mM, Bu NPF (0.2
4
6
−
1
M) in CH Cl , v = 100 mV s , potentials reported in V vs [FeCp ]/
2
2
2
+
[
FeCp ] . The internal decamethylferrocene standard showed a wave
2
c
with ΔE = 60−70 mV. Tolman electronic parameters according to
p
d
−1
refs 15 and 18. Measured at 800 mV s .
−
1
av
Kno
̈
lker-type complex 1 (ν 2061, 2053, 1987 cm , ν
CO CO
8c
−1
2
033 cm ), or substituted Kno
̈
lker-type derivatives with a
benzonitrile (1971 cm ), a triphenylphosphine (1965
cm ), or an acetonitrile (1953.5 cm ) motif, the νCO
−
1
6a
−1
11a
−1 7c
av
values for all of the complexes appear at lower frequencies,
which demonstrate the strongly electron donating character of
the NHC ligand. In addition, the stretching frequencies were
Figure 2. NHC-substituted Kno
indicated under the structure).
̈
lker-type complexes (isolated yield
15,18
approximated to the reported TEP values for the NHCs.
B
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Electrochemical Studies. In order to understand more
specifically the electronic properties of the metal center, we
have also performed electrochemical studies of the NHC-
complexes 2−5, the values of the oxidation potentials correlate
well with the TEP values: the iron center is electron richer and
therefore easier to oxidize as the ligands are stronger donors (5
> 2 > 4 > 3). For the complexes bearing unsymmetrically
substituted carbene ligands (6 and 7), the correlation is less
obvious. Quite unexpectedly, complex 6 is more prone to
oxidation than the symmetrical conterparts 2 and 5, whereas
the E1/2 of 7 ranks, as expected, between those of 2 and 3.
X-ray Diffraction Studies. For each of the complexes,
suitable single crystals have been obtained by diffusion of
pentane in a concentrated solution of the complex in CH Cl .
Kno
̈
lker complexes. Cyclic voltammetry measurements have
been performed in dichloromethane (0.2 M Bu NPF ). The six
4
6
complexes showed an oxidation process at potentials between
+
0
.04 and 0.16 V vs [FeCp ]/[FeCp ] (Table 1 and Figures 3
2 2
2
2
The X-ray diffraction studies confirmed the molecular structure
of the complexes (Figures 5 and 6). In the solid state, the
piano-stool geometry of the precursor is maintained with angles
around the metal close to 90° (for example, in 2, the angles
OC−Fe−CO and OC−Fe−NHC are 86.58(9), 94.73(9), and
9
8.57(8)°). In the cases of the complexes bearing at least one
mesityl substituent on the imidazolidene motif, i.e. 2, 6, and 7,
an interesting geometrical feature is the proximity of one CO
ligand and the ipso carbon of the mesityl group and the
pronounced bent structure of the same carbonyl ligand (Figure
7
). Indeed, for the complexes 2, 6, and 7, the angles involving
the Fe−C−O atoms are 168.7(2), 170.0(3), and 170.6(5)°,
respectively, and the distances CO−C are 2.778(3),
ipso
Figure 3. Normalized CV traces of complexes 3, 5, and 6 in CH Cl .
2
2
2
.852(4), and 2.849(8) Å, respectively. Such features, i.e. a
Conditions: sample concentration 1 mM, 0.2 M Bu NPF , v = 100 mV
4
6
−
1
short distance d < 3.1 Å and an angle α < 175°, has been
described as the signature of a latent Cipso−CO interligand
interaction by Lugan on the basis of the studies of Fischer-type
s .
19
manganese carbene complexes. This noncovalent interaction
can be found in various NHC/carbonyl or NHC/alkylidene
complexes, including Fe−NHC and Ru−NHC complexes, and
its role has been identified in the case of the metathesis
19d
reaction.
Catalytic Applications. With these new complexes in
hand, we have explored their potential in catalytic reactions to
̈
enlarge the scope of applications of Knolker-type derivatives in
the reduction area. On the basis of our research interest in
hydrosilylation reactions with iron carbonyl catalysts such as
Fe(NHC)(CO) , Fe(CO) (cod), and Fe(CO) (trans-4-phe-
4
3
3
nylbuten-2-one) complexes, which were respectively applied for
16b
20
the hydrosilylation of esters
and carboxylic acids into
aldehydes, we have evaluated the activity of the new complexes
2
−6 in the dehydration of primary amides into nitrile
21
derivatives. To the best of our knowledge, only two examples
of the latter reaction catalyzed with iron have been reported by
21b
21c
Figure 4. Normalized CV traces of complexes 2 and 7 in CH Cl .
2
2
Beller and us, using respectively diethoxymethylsilane and
phenylsilane as the dehydrating reagents. In this study, we have
selected the inexpensive PMHS (polymethylhydrosiloxane) as
the dehydration reagent, complex 2 as the model precatalyst,
and benzamide as the model substrate to optimize the
conditions of the reaction (Scheme 2 and Table 2).
To our delight, benzamide can be converted to benzonitrile
in high yield using 5 mol % of catalyst 2 at 100 °C in the
presence of 5 equiv of PMHS in different solvents, such as
toluene, CPME (cyclopentyl methyl ether), and 1,4-dioxane
Conditions: sample concentration 1 mM, 0.2 M Bu NPF , v = 100 mV
4
6
−
1
s .
and 4). This process can be assigned to the Fe(II)/Fe(III)
redox couple. It occurs at a low potential due to the donating
effect of the carbene ligand (note: that the oxidation process of
an analogous complex in which the carbene ligand is replaced
by a phosphine ligand is found at 0.52 V; see Figure S1 in the
Supporting Information). In the case of complexes 3−6, the
monoelectronic wave is reversible (3 and 4) or quasi-reversible
(
Table 2, entries 1−3). The reaction did not proceed in THF,
(
5 and 6) with i /i ratios of 1. However, for complex 2, the
c a
dimethyl carbonate, or ethanol (see Table S1 in the Supporting
Information). It must be pointed out that decreasing the
temperature (entry 4), the catalyst loading (entry 5), or the
quantity of hydrosilanes (entries 6 and 7) has a deleterious
effect on the reaction. We have also checked the effect of UV
irradiation to promote the reaction at 35 °C, but the conversion
was very low (entry 8).
oxidation wave is chemically irreversible at low scan rates (i /i
c
a
−1
=
0.56 at 100 mV s ). This is an indication that the Fe(III)
species is not stable in that case. The oxidation potential E1/2
could be measured only at higher scan rates, at which the
chemical reversibility was obtained. A similar behavior is
−1
obtained for the complex 7 (i /i = 0.75 at 100 mV s ). For
c
a
C
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Article
Figure 5. ORTEP views of complexes 2−4 with thermal ellipsoids
drawn at the 50% probability level. One molecule of CH Cl was
2
2
omitted for clarity for complex 2. The fused ring of the cyclo-
pentadienone ligand of complex 4 is disordered over two positions,
only one of which is depicted.
Figure 6. ORTEP views of complexes 5−7, with thermal ellipsoids
drawn at the 50% probability level.
We then compared the efficiencies of the different catalysts:
at 100 °C in toluene, under the optimized conditions,
complexes 2−6 displayed similar activities, as full conversions
were obtained in each case (see Table S2 in the Supporting
2
2
Information). Interestingly, under such conditions, Kno
catalyst 1 was completely inactive (entry 9) and Fe(IMes)-
CO) led to a moderate yield of 57% (entry 10), which
̈
lker
(
4
demonstrates both the importance of the N-heterocyclic
carbene and the influence of the cyclopentadienone as the
ligand.
Figure 7. Noncovalent CO−Cipso interaction in the solid state.
Once the optimized conditions were found, the scope of the
dehydration of primary amides with PMHS catalyzed by 2 was
then explored, and the results are summarized in Table 3. It is
noteworthy that, except for the benzamide, the poor solubility
of other primary amides in toluene or CPME was problematic.
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Scheme 2. Dehydration of Benzamide into Benzonitrile
Catalyzed by Iron
Finally, the conjugated cinnamic amide (Table 3, entry 7) was
converted to the cinnamyl nitrile (65% conversion, 61% yield).
Interestingly, only 2% of the saturated product 3-phenyl-
propionamide, resulting from the reduction of the conjugated
CC bond, was detected.
CONCLUSION
■
Table 2. Optimization of the Parameters of the Iron-
Catalyzed Dehydration of Benzamides
a
In conclusion, we have synthesized and fully characterized a
new family of NHC-substituted iron Kno
̈
lker-type complexes
b
entry amt of PMHS (equiv)
solvent
toluene
temp (°C) yield (%)
by UV-promoted substitution of one CO ligand by the
corresponding NHC ligand. All of the complexes have been
obtained in good yields and fully characterized. Their potential
in catalysis has been demonstrated in the case of the
dehydration of primary benzamides into benzonitrile deriva-
tives using the inexpensive PMHS as the dehydrating reagent.
1
2
3
4
5
6
7
8
9
1
5
5
5
5
5
3
2
5
5
5
100
100
100
80
97
94
97
18
51
87
35
6
1,4-dioxane
CPME
CPME
CPME
CPME
CPME
CPME
toluene
toluene
c
100
100
100
UV
100
100
̈
These results open the way to apply Knolker-type complexes in
hydrosilylation reactions.
d
e
2
EXPERIMENTAL SECTION
■
f
0
57
General Methods. All reactions were carried out with oven-dried
glassware using standard Schlenk techniques under an inert
atmosphere of dry argon or in an argon-filled glovebox. Toluene,
THF, diethyl ether (Et O), and CH Cl were dried over a Braun MB-
SPS-800 solvent purification system and degassed by thaw−freeze
cycles. Technical grade pentane and diethyl ether were used for
column chromatography. Analytical TLC was performed on Merck
a
Typical conditions: catalyst 2 (5 mol %), benzamide (0.25 mmol),
toluene (2 mL), and PMHS (5 equiv) were added in that order under
an argon atmosphere, and the solution was heated to 100 °C for 24 h.
2
2
2
b
c
Determined by GC using dodecane as the internal standard. 3 mol %
d
e
of catalyst 2 was used. 350 nm, 35 °C. 1 as the catalyst.
f
(
IMes)Fe(CO) as the catalyst.
4
6
0F254 silica gel plates (0.25 mm thickness). Column chromatography
was performed on Acros Organics Ultrapure silica gel (mesh size 40−
0 μm, 60 Å). All reagents were obtained from commercial sources,
and liquid reagents were dried on molecular sieves and degassed prior
a
Table 3. Scope of Primary Amides to Nitriles
6
1
13
1
to use. H and C{ H} NMR spectra were recorded in CDCl or
3
C D at ambient temperature unless otherwise stated, on Bruker
6
6
AVANCE 500, AVANCE 400, and AVANCE 300 spectrometers at
13
5
00, 400, and 300 MHz, respectively. 1H and C spectra were
1
calibrated using the residual solvent signal as an internal standard ( H,
CDCl 7.26 ppm, C D 7.16 ppm; C, CDCl central peak is 77.00
13
3
6
6
3
ppm, C D central peak is 128.06 ppm). IR spectra were measured
6
6
with a Shimadzu IR-Affinity 1 instrument with ATR equipment. HR-
MS spectra and microanalyses were carried out by the corresponding
facilities at the CRMPO, Centre Reg
l’Ouest, University of Rennes 1.
́
ional de Mesures Physiques de
X-ray diffraction data were collected on an APEXII Bruker-AXS
diffractometer equipped with a CCD detector, using graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å) at T = 150(2)
K. The structure was solved by direct methods using the SIR97
2
3
program and then refined with full-matrix least-squares methods
2
24
25
based on F (SHELXL-97) with the aid of the WINGX program.
Electrochemical studies were carried out under argon using an Eco
Chemie Autolab PGSTAT 30 potentiostat; the working electrode was
a Pt disk, and the SCE electrode was used as the reference electrode.
The sample concentration was ∼1 mM (CH Cl , 0.2 M Bu NPF ),
2
2
4
6
and the solution was carefully deoxygenated by argon bubbling. The
applied potential was measured with decamethylferrocene as the
internal reference and calculated versus ferrocene with E°([FeCp ]/
2
a
Reaction conditions: catalyst 2 (5−8 mol %), amide (1 mmol), THF/
,4-dioxane (2:2 mL), and PMHS (5 equiv), 100 °C, under an argon
+
+
[
FeCp ] ) = +0.542 V vs [FeCp* ]/[FeCp* ] . UV irradiation was
2
2
2
1
performed in a Rayonet RPR-100 apparatus at 350 nm.
b
1
c
1
5,26
7c
atmosphere. The conversion was determined by H NMR. Isolated
yield. 2% of 3-phenylpropionitrile was detected.
The imidazolium salts NHC·HX
̈
and Knolker’s complex 1
d
were prepared according to the published procedures.
Synthesis of the Complexes. Synthesis of Complex 2:
Representative Procedure. To a suspension of 1,3-dimesitylimidazo-
lium chloride (1.20 g, 3.6 mmol, 1.5 equiv) in toluene (10 mL) was
added KHMDS (0.5 M in toluene, 7.9 mL, 3.9 mmol, 1.6 equiv)
dropwise at room temperature, and the solution was vigorously stirred
for 30 min. The resulting white suspension was directly filtered
through Celite into a Schlenk tube containing Knolker’s precursor 1
̈
(1.00 g, 2.3 mmol, 1 equiv) in toluene (10 mL). Then, the reaction
mixture was irradiated under UV (350 nm) at room temperature for
Therefore, the solvent of the reaction was switched to a mixture
of THF and dioxane (1/1, v/v). The dehydration proceeded
well for amides bearing an electron-donating group (Table 3,
entries 1−4), giving full conversions with good isolated yields
(
(
72−84%). Benzamides with electron-withdrawing groups
Table 3, entries 5 and 6) such as bromo and chloro were
also well tolerated, as no dehalogenation product was detected.
E
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2
0 h. After completion of the reaction, the initial brown solution
(CH ), 24.8 (CH ), 24.4 (CH ), 22.3 (CH ), 21.0 (p-CH ), 18.9 (o-
2
2
2
2
3
2
7
−1
turned dark yellow. The solvent was evaporated under vacuum. The
resulting residue was purified by column chromatography on silica gel.
A yellow fraction of the expected complex was collected (petroleum
ether/ethyl acetate 80/20) and concentrated under vacuum to give 2
CH ), 0.71 (SiMe ). IR (ATR, ν, cm ): 1980, 1919, 1575, 1548.
3 3
27
−1
IR (CH Cl , ν, cm ): 1979, 1919, 1570. Anal. Calcd for
2
2
C H N O FeSi : C, 63.61; H, 7.65; N, 4.25. Found: C, 64.09; H,
3
5
50
2
3
2
+
7.80; N, 3.89. HR-MS [ESI]: m/z [M + H] calcd for
56
(
1.25 g, 78%) as a golden yellow powder. Single crystals suitable for X-
ray diffraction studies were grown by layering a concentrated solution
of the complex in CH Cl with pentane.
C H N O FeSi 659.2782, found 659.2779 (0 ppm).
35
51
2
3
2
Synthesis of Complex 7. Following the representative procedure
2
2
using 1-mesityl-3-[(R)-1-phenylethyl]imidazolium chloride (235 mg,
0.32 mmol) and 1 (200 mg, 0.48 mmol), complex 7 was obtained as a
yellow powder (250 mg, 76%).
1
H NMR (400 MHz, CDCl ): δ 6.99 (s, 4 H, CHMes), 6.85 (s, 2 H,
3
CHNHC), 2.35 (s, 6 H, p-CH ), 2.19−2.17 (m, 4 H, CH ), 2.15 (s, 12
H, o-CH ), 1.34−1.31 (m, 2 H, CH ), 0.98−0.95 (m, 2 H, CH ), 0.05
3
2
1
3
3
3
2
2
H NMR (400 MHz, CDCl ) δ 7.47 (d, J = 7.8 Hz, 2H), 7.36 (d, J
3
13
1
(
1
1
(
(
s, 18 H, SiMe ). C{ H} NMR (100 MHz, CDCl ) δ 217.4 (2 CO),
3
3
=
7.5 Hz, 2H), 7.28 (m, 1H), 6.98 (s, 1H, CH ), 6.97 (s, 1H,
Mes
87.8 (NCN), 179.5 (CO), 139.6 (Co), 138.2 (Ci), 136.4 (Cp),
29.4 (Cm), 126.2 (CHNHC), 104.9 (CCSi), 67.5 (CSi), 24.6
3
3
CHMes), 6.91 (d, J = 1.8 Hz, 1H, CH ), 6.84 (d, J = 1.8 Hz, 1H,
NHC
3
CHNHC), 6.06 (q, J = 6.5 Hz, 1 H, CH), 2.34 (s, 3H, CH ), 2.40−
3Mes
CH ), 21.9 (CH ), 21.0 (p- CH ), 19.1 (o −CH ), 0.5 (SiMe ). IR
−1
2
2
3
3
3
2
.21 (m, 4H, CH ), 2.08 (s, 3H, CH3Mes), 2.05 (s, 3H, CH ), 1.94
2 3Mes
ATR, ν, cm ): 1978, 1913, 1577. Anal. Calcd for C H N O FeSi :
3
38
50
2
3
2
(
d, J = 6.5 Hz, 3H, CH ), 1.70−1.40 (m, 4H, CH ), 0.11 (s, 9H,
3
2
C, 65.69; H, 7.25; N, 4.03. Found: C, 65.24; H, 7.18; N, 3.92. HR-MS
13
1
+
56
SiMe
3
), 0.06 (s, 9H, SiMe ). C{ H} NMR (125 MHz, CDCl ) δ
3
3
[
ESI]: m/z [M + H] calcd for C H N O FeSi 695.2787, found
38 51 2 3 2
2
1
17.1 (CO), 216.7 (CO), 187.4 (NCN), 180.0 (CO), 142.2 (CqAr),
6
95.2782 (0 ppm).
Synthesis of Complex 3. Following the representative procedure
39.5 (C ), 138.2 (C ), 136.5 (C ), 136.2 (C ), 129.3
qMes
qMes
qMes
qMes
(
CHMes), 129.2 (CHMes), 128.3 (m-CH ), 127.3 (p-CH ), 127.2 (o-
Ar Ar
using 1,3-dimethylimidazolium hexafluorophosphate (87 mg, 0.35
mmol) and 1 (100 mg, 0.24 mmol), complex 3 was obtained as a
yellow powder (75 mg, 64%).
CH ), 125.0 (CHNHC), 121.8 (CHNHC), 104.6 (CCSi), 99.7 (C
Ar
CSi), 70.6 (CSi), 69.5 (CSi), 57.7 (CH), 25.0 (CH ), 23.8 (CH ), 22.2
2
2
1
(CH ), 22.0 (CH ), 21.9 (CH ), 21.0(CH ), 19.1(CH3Mes), 18.4
H NMR (500 MHz, CDCl ): δ 6.98 (s, 2 H, CHNHC), 3.91 (s, 6 H,
2
2
3
3Mes
3
2
7
−1
(
CH3Mes), 0.5 (SiMe ), 0.4 (SiMe ). IR (ATR, ν, cm ): 1973, 1915,
1585, 1552. IR (CH Cl , ν, cm ): 1980, 1921, 1574. Anal. Calcd for
MeNHC), 2.45 (m, 4 H, CH ), 1.85−1.78 (m, 4 H, CH ), 0.17 (s, 18 H,
3
3
2
2
2
7
−1
SiMe3). 1 C{ H} NMR (125 MHz, CDCl ) δ 217.2 (2 CO), 185.0
3
1
2
2
3
C H N O FeSi : C, 65.28; H, 7.11; N, 4.11. Found: C, 65.39; H,
(
3
NCN), 176.8 (CO), 123.6 (CH ), 103.7 (CCSi), 70.9 (CSi),
37 48
2
3
2
NHC
+
9.9 (Me ), 24.6 (CH ), 22.6 (CH ), 0.3 (SiMe ). IR (ATR, ν,
7.04; N, 4.07. HR-MS [ESI]: m/z [M + Na] calcd for
NHC
2
2
3
−1
56
cm ): 1975, 1919, 1548. Anal. Calcd for C H N O FeSi : C, 54.31;
H, 7.04; N, 5.76. Found: C, 54.29; H, 7.03; N, 5.58. HR-MS [ESI]: m/
z [M + H] calcd for C H N O FeSi 487.1530, found 487.1533 (1
ppm).
Synthesis of Complex 4. Following the representative procedure
using 1,3-diisopropylimidazolium hexafluorophosphate (107 mg, 0.35
mmol) and 1 (100 mg, 0.24 mmol), complex 4 was obtained as a
C
37
H
48
N
2
O
3
NaSi
2
Fe 703.2450, found 703.2467 (2 ppm).
22
34
2
3
2
Typical Procedure for the Iron-Catalyzed Dehydration of
+
56
Primary Benzamide to Nitriles. A Schlenk tube containing a
stirring bar was loaded with the primary amide (1.0 mmol), followed
by the addition of the iron complex (0.05−0.08 mmol), the solvent
(dry toluene or 1,4-dioxane/THF, 4 mL), and finally PMHS (5.0
mmol). The mixture was stirred for 24 h at 100 °C. After completion
of the reaction, the mixture was cooled to room temperature. The
latter was diluted with ethyl acetate (5 mL) and concentrated under
reduced pressure (850 mbar, 40 °C). The residue was purified by silica
gel column chromatography, giving the corresponding nitrile.
22
35
2
3
2
yellow powder (110 mg, 84%).
1
H NMR (500 MHz, C D ): δ 6.53 (s, 2 H, CHNHC), 5.13 (sept, 3J
6
6
=
6.4 Hz, 2 H, CH ), 2.59−2.54 (m, 2H, CH ), 2.45−2.37 (m, 2 H,
Ipr
2
CH ), 1.83−1.79 (m, 2 H, CH ), 1.57−1.53 (m, 2 H, CH ), 1.24 (d,
2
2
2
3
13
1
1
2 H, J = 6.4 Hz, CH ), 0.32 (s, 18 H, SiMe ). C{ H} NMR (125
3Ipr 3
MHz, C D ) δ 219.3 (2 CO), 183.1 (NCN), 177.7 (CO), 119.0
6
6
ASSOCIATED CONTENT
(
(
1
CH ), 103.4 (CCSi), 72.6 (CSi), 51.8 (CH ), 25.5 (CH ), 24.7
■
NHC
Ipr
2
−1
CH3Ipr), 22.8 (CH ), 0.5 (SiMe ). IR (ATR, ν, cm ): 1975, 1924,
2
3
*
S
Supporting Information
583. Anal. Calcd for C H N O FeSi : C, 57.55; H, 7.80; N, 5.16.
2
6
42
2
3
2
+
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00553. Complete details of the X-ray analyses reported
herein have also been deposited at the Cambridge Crystallo-
graphic Data Center (CCDC 1414335 (2), 1414336 (3),
Found: C, 57.48; H, 7.85; N, 4.96. HR-MS [ESI]: m/z [M + H] calcd
for C H N O FeSi 543.2156 found 543.2158 (0 ppm).
56
3
26
43
2
2
Synthesis of Complex 5. Following the representative procedure
using 1,3-dicyclohexylimidazolium tetrafluoroborate (229 mg, 0.72
mmol) and 1 (200 mg, 0.48 mmol), complex 5 was obtained as a
yellow powder (210 mg, 70%).
1
414337 (4), 1414338 (5), 1414339 (6), 1414340 (7)). These
1
H NMR (500 MHz, CDCl ): δ 7.08 (s, 2 H, CH ), 4.81−4.75
3
NHC
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, by emailing data_request@ccdc.cam.ac.uk ,or
by contacting the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CD2 1EZ, U.K.; fax + 44 1223
336033.
(
(
1
(
(
1
m, 2 H, CH ), 2.48−2.35 (m, 4 H, CH ), 2.06−1.20 (m, 24 H), 0.11
s, 18 H, SiMe ). C{ H} NMR (125 MHz, CDCl ) δ 217.4 (2 CO),
Cy
2
13
1
3
3
81.5 (NCN), 178.3 (CO), 119.6 (CH ), 104.1 (CCSi), 68.5
NHC
CSi), 58.9 (CH ), 35.5 (CH ), 25.3 (CH ), 25.2 (CH ), 24.0
Cy
2
2Cy
2Cy
−1
CH ), 22.6 (CH ), 0.01 (SiMe ). IR (ATR, ν, cm ): 1969, 1907,
585. Anal. Calcd for C H N O FeSi : C, 61.72; H, 8.09; N, 4.50.
Found: C, 61.98; H, 8.32; N, 4.45. HR-MS [ESI]: m/z [M + H] calcd
for C H N O FeSi 623.2782, found 623.2781 (0 ppm).
2
2Cy
3
32
50
2
3
2
+
NMR spectra and characterization data for the complexes
56
3
and the products of catalysis (PDF)
32
51
2
2
Synthesis of Complex 6. Following the representative procedure
using 3-cyclohexyl-1-mesitylimidazolium chloride (220 mg, 0.72
mmol) and 1 (200 mg, 0.48 mmol), complex 6 was obtained as a
Crystallographic data for 2−7 (CIF)
yellow powder (203 mg, 65%).
AUTHOR INFORMATION
1
■
H NMR (400 MHz, CDCl ): δ 7.30 (s, 1 H, CHNHC), 6.95 (s, 2 H,
3
CHMes), 6.86 (s, 1 H, CHNHC), 4.79−4.73 (m, 1 H, CH ), 2.33−2.25
Corresponding Authors
*E-mail for C.D.: christophe.darcel@univ-rennes1.fr.
*E-mail for J.-B.S.: jean-baptiste.sortais@univ-rennes1.fr.
Cy
(
m, 8 H), 1.97 (s, 6H, o-CH ), 1.85−1.56 (m, 9 H), 1.28−1.21 (m, 4
3
13
1
H); 0.14 (s, 18 H, SiMe ). C{ H} NMR (125 MHz, CDCl ) δ 217.6
3
3
(
1
2 CO), 186.9 (NCN), 177.5 (CO), 139.3 (CqMes), 137.9 (CqMes),
Notes
36.3 (C ), 129.2 (CH ), 125.1 (CH ), 120.5 (CHNHC), 102.9
qMes
Mes
NHC
(
CCSi), 70.4 (CSi), 58.3 (CH ), 35.2 (CH ), 29.7 (CH ), 25.4
The authors declare no competing financial interest.
Cy
2
2
F
DOI: 10.1021/acs.organomet.5b00553
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(
13) Selected reviews on iron-NHC in catalysis: (a) Riener, K.;
ACKNOWLEDGMENTS
■
Haslinger, S.; Raba, A.; Ho
̈
gerl, M. P.; Cokoja, M.; Herrmann, W. A.;
ier, D.; Sortais,
C.D. and J.-B.S. thank the CNRS, the University of Rennes 1,
Kuhn, F. E. Chem. Rev. 2014, 114, 5215−5272. (b) Bez
̈
́
the Reg
Rennes 1 (grant to S.Q.-D.)
́
ion Bretagne (Ph.D. grant to S.E.), and the Fondation
J.-B.; Darcel, C. Adv. Synth. Catal. 2013, 355, 19−33. (c) Ingleson, M.
J.; Layfield, R. A. Chem. Commun. 2012, 48, 3579−3589.
(14) (a) Buchgraber, P.; Toupet, L.; Guerchais, V. Organometallics
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G
DOI: 10.1021/acs.organomet.5b00553
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(
27) In the solid state, two stretching bands for the CO of the
ketone are present, probably due to the presence of two rotamers. In
solution, only one band was observed.
H
DOI: 10.1021/acs.organomet.5b00553
Organometallics XXXX, XXX, XXX−XXX