Synthesis and catalytic activity of iron complexes with bidentate NHC ligands

Article abstract of DOI:10.1039/c3dt32551b

A family of well-defined Fe^II complexes of the type {BnN(N-CH₂(CH₂)_n-N′-tert-butyl-imidazole- 2-ylidene)₂}FeCl₂ (Bn = benzyl; n = 1 (1) or 2 (2)), {BnN(N-CH₂(CH₂)_n-N′-methylbenzimidazole- 2-ylidene)₂}FeCl₂ (n = 1 (3) or 2 (4)) and {BnN(N-CH ₂CH₂CH₂-N′-methylbenzimidazole-2-ylidene) ₂}FeBr₂ (5) has been synthesized. These complexes are rare examples of Fe species supported by bidentate NHC ligands. Complexes 2, 3, 4 and 5 were characterized by X-ray crystallography and in all cases a distorted tetrahedral geometry is observed around the Fe center. The magnetic data is consistent with the complexes containing non interacting high spin Fe ^II centers (S = 2) and indicates that a large zero-field splitting (D) is present. The new complexes are highly active pre-catalysts for the homo-coupling of Grignard reagents.

Full text of DOI:10.1039/c3dt32551b

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Synthesis and catalytic activity of iron complexes with
bidentate NHC ligands†
Cite this: Dalton Trans., 2013, 42, 7404
Jianguo Wu,a Wei Dai,a Joy H. Farnaby,a Nilay Hazari,*a Jennifer J. Le Roy,b
Valeriu Mereacre,c Muralee Murugesu,b Annie K. Powellc and Michael K. Takasea
A family of well-defined FeII complexes of the type {BnN(N-CH2(CH2)n-N’-tert-butyl-imidazole-2-ylidene)2}-
FeCl2 (Bn = benzyl; n = 1 (1) or 2 (2)), {BnN(N-CH2(CH2)n-N’-methylbenzimidazole-2-ylidene)2}FeCl2 (n =
1 (3) or 2 (4)) and {BnN(N-CH2CH2CH2-N’-methylbenzimidazole-2-ylidene)2}FeBr2 (5) has been synthesized.
These complexes are rare examples of Fe species supported by bidentate NHC ligands. Complexes 2, 3, 4
and 5 were characterized by X-ray crystallography and in all cases a distorted tetrahedral geometry is
observed around the Fe center. The magnetic data is consistent with the complexes containing non inter-
acting high spin FeII centers (S = 2) and indicates that a large zero-field splitting (D) is present. The new
complexes are highly active pre-catalysts for the homo-coupling of Grignard reagents.
Received 24th October 2012,
Accepted 6th February 2013
DOI: 10.1039/c3dt32551b
www.rsc.org/dalton
treated with FeX2 (X = halide) to form complexes of the type
Introduction
(NHC)2FeX2,10 a number of different Fe complexes featuring
In recent years there has been significant interest in the deve- monodentate NHC ligands have been prepared.11 NHC donors
lopment of Fe complexes as replacements for expensive have also been incorporated into multidentate frameworks.
precious metal catalysts in
a
wide variety of organic For example Fehlhammer,12 Meyer13 and Smith14 have syn-
transformations.1–5 Not only is Fe significantly cheaper than thesized a number of Fe complexes with tripodal tridentate
the precious metals, it is also non-toxic, abundant and NHC ligands, while Danopoulos and co-workers have prepared
environmentally friendly. To date the majority of Fe complexes Fe complexes with tridentate pincer ligands, incorporating two
developed for catalytic applications feature nitrogen or phos- NHC donors.15–17
phine donors1–5 and there is a paucity of systems which use
Surprisingly Fe complexes supported by simple bidentate
N-heterocyclic carbene ligands (NHCs) to stabilize the metal bis(NHC) ligands, which are not part of a multidentate frame-
center.6 In fact, in general the chemistry of Fe complexes work, are rare.6 In fact, to the best of our knowledge there are
supported by NHC ligands has not been extensively studied only two systems of this type18–20 and their catalytic activity
compared with the late transition metals.6
has yet to be probed in detail.20 In 2011 the groups of
The first Fe complexes supported by NHC ligands were Ingleson18 and Meyer20 independently prepared FeII complexes
reported in the 1970s,7–9 but it is only in the last fifteen years supported with bidentate NHC ligands linked with aliphatic
that a variety of different complexes have been synthesized. (a) or phenylene (b) bridges (Fig. 1). In the case of the phenyl-
Starting from Grubbs’ report that free NHC ligands could be ene linked system it was not possible to separate the Fe
dihalide species from homoleptic complexes featuring two
aThe Department of Chemistry, Yale University, P. O. Box 208107, New Haven,
Connecticut, 06520, USA
bThe Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa,
ON K1N6N5, Canada
cInstitute for Inorganic Chemistry, Karlsruhe Institute of Technology, Engesserstr. 15,
D-76131 Karlsruhe, Germany. E-mail: nilay.hazari@yale.edu
†Electronic supplementary information (ESI) available: Further information
about magnetic measurements and X-ray crystallography. CCDC 907140–907142
and 917785 (these data {BnN-(CH2CH2CH2-N-tert-butyl-imidazole-2-ylidene)2}
FeCl2 (2) (907140), {BnN-(CH2CH2-N-methylbenzimidazole-2-ylidene)2}FeCl2 (3)
(907141),
{BnN-(CH2CH2CH2-N-methylbenzimidazole-2-ylidene)2}FeCl2
(4)
(907142) and {BnN-(CH2CH2CH2-N-methylbenzimidazole-2-ylidene)2}FeBr2 (5)
(917785)). For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3dt32551b
Fig. 1 Representative examples of FeII complexes with bidentate NHC ligands
which have been prepared in the literature.18,20
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bidentate NHC ligands and species of this type were also An Evans’ method NMR experiment on 5, indicated a μeff of
observed as a by-product for aliphatic linked systems with 4.79 μB, consistent with the presence of a high spin tetrahedral
small alkyl groups as the substituent on the imidazole. S = 2 FeII center. Compounds 3 and 4 were sufficiently soluble
Recently, Zlatogorsky and Ingleson demonstrated that in some in benzene to determine the magnetic moment using the
cases dihalide species with aliphatic linked bidentate NHC Evans’ method (1 and 2 were too insoluble) and values of 4.77
ligands could be converted into Fe hydrides in low yield.19 The and 4.84 μB, respectively, were obtained. These values also
only report of catalytic activity using Fe complexes supported suggest an S = 2 ground state. All of the new complexes were
by bidentate NHC ligands is from Meyer.20 His group showed characterized by IR spectroscopy and elemental analysis and
that some complexes related to type a were pre-catalysts for the structures of 2, 3, 4 and 5 were elucidated by X-ray crystal-
Kumada type cross-coupling but only moderate activity was lography (Fig. 2–5). Selected structural parameters are given in
observed.
Given that precious metal complexes with bidentate NHC
Table 1.
ligands often generate highly active catalysts for a number of
different organic transformations,21,22 we were interested in
expanding the scope of Fe complexes supported by these
ligands. Previously, we synthesized Pd, Rh and Ir species sup-
ported by bidentate NHC ligands with flexible alkyl linkers
containing a central amine.23,24 These complexes are active cata-
lysts for the Heck and Suzuki reactions, and transfer hydro-
genation. Here, we show that the same ligands can be used to
support rare examples of FeII complexes with bidentate NHC
ligands. We demonstrate that these FeII species are highly
active pre-catalysts for the homo-coupling of Grignard
reagents.
ð1Þ
ð2Þ
Results and discussion
Synthesis and characterization
The Fe complexes 1–4 were prepared in high yields through
the reaction of Fe{N(SiMe3)2}2 with the appropriate chloride
salt of the protonated free ligands in toluene at room tempera-
ture (eqn (1)). A related reaction has previously been used to
metallate bidentate NHC ligands onto Fe but in contrast to
other systems there was no evidence for the formation of com-
plexes with two bidentate carbene ligands or dimeric
species.20 In general, the reactions of common FeII precursors
such as FeCl2, FeBr2 and FeBr2(THF)2 with the free carbene
ligands (the free carbene was generated using a literature pro-
cedure)24 in both coordinating and non-coordinating solvents
failed to yield isolable Fe–NHC complexes. The only exception
was the synthesis of the FeII dibromide 5, through the reaction
of FeBr2 with the appropriate free carbene (prepared in situ)
(eqn (2)). It has been observed in the few literature reports of
Fe–NHC complexes that the choice of FeII precursor and
solvent and the method of the carbene delivery can lead to
very different results15 and this appears to be the case with our
ligand as well. Complexes 1–5 were paramagnetic but unlike
other Fe complexes supported by NHC ligands it was only
1
possible to obtain a well defined H NMR spectrum for 5. We
believe that this is due to a combination of the low solubility
of 1–4 in all common solvents (even at elevated temperature)
and the inherent broadening of paramagnetic NMR spectra.
The 1H NMR spectrum of 5 showed the expected number of
peaks with slight shifts of some resonances outside the
Fig. 2 Thermal ellipsoid plot of 2. Hydrogen atoms and solvent of crystalliza-
tion have been removed for clarity. The disorder in the benzyl group has also
been omitted. Selected bond lengths (Å) and angles (°): Fe(1)–C(1) 2.161(4),
Fe(1)–C(2) 2.165(4), Fe(1)–Cl(1) 2.3147(12), Fe(1)–Cl(2) 2.3523(11), C(1)–Fe(1)–C(2)
113.71(15), C(1)–Fe(1)–Cl(1) 126.20(12), C(1)–Fe(1)–Cl(2) 93.44(11), C(2)–Fe(1)–
normal diamagnetic sweep width and significant broadening. Cl(1) 102.35(11), C(2)–Fe(1)–Cl(2) 116.57(11), Cl(1)–Fe(1)–Cl(2) 104.91(4).
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Fig. 5 Thermal ellipsoid plot of 5. Hydrogen atoms and solvent of crystalliza-
tion have been removed for clarity. Only one of the two independent molecules
is shown. Selected bond lengths (Å) and angles (°): Fe(1)–C(1) 2.115(3), Fe(1)–C(2)
2.109(3), Fe(1)–Br(1) 2.3969(12), Fe(1)–Br(2) 2.4209(6), C(1)–Fe(1)–C(2)
101.90(13), C(1)–Fe(1)–Br(1) 118.58(9), C(1)–Fe(1)–Br(2) 104.96(9), C(2)–Fe(1)–
Br(1) 110.93(9), C(2)–Fe(1)–Br(2) 114.63(10), Br(1)–Fe(1)–Br(2) 106.06(2).
Fig. 3 Thermal ellipsoid plot of 3. Hydrogen atoms and solvent of crystalliza-
tion have been removed for clarity. The structure contains two fully disordered
molecules, only the major component is shown here. Selected bond lengths (Å)
and angles (°): Fe(1)–C(1) 2.113(8), Fe(1)–C(2) 2.110(8), Fe(1)–Cl(1) 2.308(7),
Fe(1)–Cl(2) 2.287(7), C(1)–Fe(1)–C(2) 104.1(6), C(1)–Fe(1)–Cl(1) 109.9(5), C(1)–
Fe(1)–Cl(2) 106.2(3), C(2)–Fe(1)–Cl(1) 112.6(5), C(2)–Fe(1)–Cl(2) 112.2(4), Cl(1)–
Fe(1)–Cl(2) 109.9(5). Distances and angles for minor component are given in the
ESI.†
Table 1 Structural data for 2–5
Complex
Fe–NHC (Å)
NHC–Fe–NHC (°)
Yaw anglea (°)
2
2.158(6), 2.166(4)
2.113(8), 2.110(8)
2.112(11), 2.123(11)
2.090(5), 2.125(5)
2.102(5), 2.122(5)
2.109(3), 2.115(3)
2.102(3), 2.117(4)
113.71(15)
104.1(6)
103.7(9)
108.39(18)
100.61(17)
101.90(13)
109.61(13)
5.7, 6.2
2.6, 3.3
3b
4c
5c
3.4, 4.5
3.1, 5.2
0.9, 6.3
3.3, 6.5
a Yaw angle is the difference between the two M–C–N angles divided
by 2. b Two fully disordered molecules present. c Two independent
molecules present in the asymmetric unit.
fashion, as the distance between the central amine of the
linker and Fe is greater than 4.5 Å. Similar bidentate binding
has been observed when these ligands coordinate to Pd.23,25,26
The Fe–NHC bond distances range from a minimum of 2.090(5)
Å in 4 to a maximum of 2.166(4) Å in 2. These distances are
consistent with values observed for related Fe–NHC systems
with both monodentate and bidentate NHC ligands20 and the
longer distances in the tert-butyl imidazole species are pre-
sumably to minimize steric clash between the tert-butyl groups
and the chloride ligands. Interestingly the NHC–Fe–NHC bond
angles in 2–5 are larger than those observed in most bis(mono-
denate) and all bidentate systems. The bite angles in 2–5 vary
from 100.61(17)° in 4 to 113.71(15)° in 2, whereas in related Fe
systems with a methylene group linking the two NHC donors
the angle is approximately 90°.18,20 In bis(monodenate) NHC
systems the NHC–Fe–NHC angle is around 100°.10,27 The large
linker length in 2–5 presumably causes the increased bite
angle and a similar effect has been observed in late transition
metal systems.22 The Yaw angles (a measure of in-plane distor-
tion) are also smaller in 2–5 than in other bidentate systems,
indicative of the greater flexibility of the linker. The larger Yaw
Fig. 4 Thermal ellipsoid plot of 4. Hydrogen atoms and solvent of crystalliza-
tion have been removed for clarity. Only one of the two independent molecules
is shown. Selected bond lengths (Å) and angles (°): Fe(1)–C(1) 2.090(5), Fe(1)–
C(2) 2.125(5), Fe(1)–Cl(1) 2.2746(12), Fe(1)–Cl(2) 2.2787(13), C(1)–Fe(1)–C(2)
108.39(18), C(1)–Fe(1)–Cl(1) 115.40(13), C(1)–Fe(1)–Cl(2) 105.09(13), C(2)–Fe(1)–
Cl(1) 106.03(12), C(2)–Fe(1)–Cl(2) 108.35(12), Cl(1)–Fe(1)–Cl(2) 113.36(5).
The solid state structures show that the geometry around Fe
is distorted tetrahedral, with only minor differences in the
bond lengths and angles around Fe depending on whether
there are four carbon or six carbon atoms in the linker. In fact,
the change from tert-butyl imidazole in 2 to 1-methylbenzimid-
azole in 4 appears to make a larger difference to the overall
structure. In all cases the ligand clearly binds in a bidentate
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angle for the tert-butyl complex 2 compared with the 1-methyl- are also consistent with the magnetic moments obtained in
benzimidazole complexes 3, 4 and 5 is consistent with the solution for compounds 3–5. For compounds 1–4, the χT
larger steric bulk of the tert-butyl group. The effect of changing product was near linear from 300 K down to 50 K, then rapidly
from an FeII dichloride to a FeII dibromide is negligible as the decreased below 50 K to reach minimum values of 1.1 (1),
structures of 4 and 5 are virtually identical. This suggests that 1.0 (2), 1.5 (3), and 2.1 (4) cm3 K mol−1 at 1.9 K. The low temp-
in these systems there is relatively little steric crowding around erature decrease can be attributed to factors such as weak
the Fe center as the complex can accommodate the larger intermolecular interactions (for example in 2 the closest inter-
bromide ligands without any changes to the Fe–NHC binding. metal distance is approximately 9.46 Å from X-ray crystallo-
Despite the flexibility of the linker, the solid state confor- graphy), thermal depopulation and/or magnetic anisotropy
mation of the linker is similar in 2–5. The only major differ- arising from mixing of the ground state with low lying excited
ence is that the nitrogen atom in the amine linker is inverted states. Fitting of the temperature dependence of χT data for
in 2 compared with 4 and 5. As a result the orientation of the 1–4 assuming simple zero-field splitting (ZFS) effects reveals
benzyl group is slightly different.
g = 1.93 (1), 1.93 (2), 2.05 (3), and 2.05 (4) and D = −18.32 (1),
The direct current (dc) magnetic susceptibility of complexes −22.2 (2), −5.71 (3), −5.71 (4) K. These large ZFS parameters
1–4 was investigated under an applied dc field of 0.1 T and in are consistent with no EPR transitions being observed using
the temperature range of 1.9–300 K. The χT vs. T plot for 1 is an X-band EPR spectrometer.
shown in Fig. 6 (with others given in the ESI†). At room temp-
The field dependence of the magnetization and reduced
erature (300 K), the χT values are 2.91 (1), 2.93 (2), 3.04 (3) and magnetization of 1 are shown in Fig. 7a and 7b, while the cor-
3.12 (4) cm3 K mol−1, which is in good agreement with the responding data for 2–4 are given in the ESI.† The magnetiza-
theoretical value of 3.0 cm3 K mol−1 for non-interacting high tion of 1–4 under 0–7 T applied field displays a rapid
spin S = 2 FeII ions in a tetrahedral environment. These results increase from 0 T to 2 T followed by a gradual increase without
reaching saturation; M = 1.80 (1), 1.99 (2), 2.75 (3), and 3.38 (4)
μB at 1.9 K. The non saturation values are lower than the theo-
retically derived saturation value of 4.89 μB. A temperature
dependence of the magnetization for complexes 1–4 are seen
in the M vs. HT−1 plots. The curves between 1.9 K and 8 K do
not superimpose onto a single master curve, indicative of mag-
netic anisotropy and/or the presence of low-lying excited
states. Overall our magnetic data suggests that the change
from tert-butyl-imidazole to 1-methylbenzimidazole has a
larger effect on the magnetic properties of the complexes
than changing the linker length from four carbon atoms to
six carbon atoms. This also demonstrates that the magnetic
properties of the complexes can be changed by varying the
electronics of the ligands.
The Mössbauer spectrum of 4 was measured at 3.0 K. A well
defined quadrupole doublet was observed (Fig. 8) with an
isomer shift of 0.71 mm s−1 and quadrupole splitting of
Fig. 6 Temperature dependence of the χT product at 0.1 T for complex 1 (with
3.77 mm s−1, respectively. Although relatively small, the
χ being the molar susceptibility per molecule defined as M/H).
Fig. 7 (a) Field dependence of the magnetization and (b) reduced magnetization for 1 at 1.9, 3, 5, and 8 K.
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Table 2 Catalyst screen for homo-coupling of p-tolylmagnesium bromidea
Entry
Catalyst
Oxidant/additive
Yieldb (%)
1
2
3
4
5
6
7
8
No catalyst
FeCl2
FeBr2
FeBr2(THF)2
FeCl3
(IMes)2FeCl2
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
CHCl3
1,2-Dichloroethane
O2
4-Chlorobenzene
4-Bromobenzene
None
<2
83
80
85
79
63
99
98
99
99
99
87
84
65
56
60
67
99
53
41
82
89
33
92
c
Fig. 8 Mössbauer spectrum of a polycrystalline sample of 4 recorded at 3 K in
zero field. The solid line is the result of least-squares fitting of a doublet to the
data.
1
2
3
4
5
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
FeCl2 + free carbened
isomer shift values are consistent with other high-spin four-
coordinate FeII ions in a tetrahedral coordination environ-
ment28,29 and are virtually identical to those reported by Evans
and Ingleson for a tetrahedral FeII complex supported by a
bidentate NHC ligand with a methylene bridge.18 The large
quadrupole splitting is due to the high sensitivity of this para-
meter to small distortions from exact tetrahedral symmetry,
which has been demonstrated in previous studies of tetrahed-
rally coordinated FeII.30 In the presence of a tetrahedral ligand
FeCl3 + free carbened
1e
2e
3e
4e
4f
4f
4g
4f
4f
4h (in situ Grignard)
5
field environment the free ion D state splits into a doublet of
4
Drop of Hg
E symmetry (containing the dz2 and dx2−y2 orbitals) and a T2
triplet (containing the dxy, dxz and dyz orbitals) with the
doublet lying lowest in energy. The observation of a large
quadrupole splitting confirms that the lower doublet must be
split by a distortion of the ion from pure tetrahedral geometry,
which is supported by the solid state structure of 4 (vide supra).
a The conditions for the reaction were p-tolylmagnesium bromide
(0.36 mL of a 0.5 M solution in diethyl ether, 0.18 mmol), catalyst
(0.0036 mmol, 2 mol%) in 0.9 mL THF at room temperature. After
30 minutes the reaction was quenched by addition of wet 0.2 mL
CHCl3. b The yield of the reaction is the average from two runs
determined by 1H NMR spectroscopy after 30 minutes using 1,3,5-
trimethyoxybenzene as an internal standard. c IMes = 1,3-bis-(2,4,6-
trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene. The compound
(IMes)2FeCl2 was prepared according to a literature procedure.11i d The
free carbene ligand BnN(CH2CH2CH2-N-tert-butyl-imidazole-2-ylidene)2
prepared as described in ref. 24 was utilized. In these reactions the Fe
salt was stirred with the free carbene for 30 minutes and the Grignard
reagent was introduced. e 1 mol% of catalyst (0.0018 mmol) was used.
f 0.18 mmol of the additives were used. g Excess 1 atm O2 was used.
h The Grignard reagent was generated in situ. The conditions for the
reaction were 1-bromo-4-methylbenzene (30.8 mg, 0.18 mmol), Mg
turnings (8.7 mg, 0.36 mmol), catalyst (0.0036 mmol, 2 mol%) in
0.9 mL THF at room temperature. The catalyst was added after the
reaction mixture had been filtered to remove excess Mg turnings.
Catalysis
The homo-coupling of Grignard reagents is considered to be
an easy and efficient method for accessing a variety of di- or
poly-aromatic and conjugated olefinic species.31 Since the
initial report by Kharasch and Field in 1941 that FeCl3 could
catalyze homo-coupling reactions,32 it has been demonstrated
that a number of different Fe complexes can catalyze this
reaction.33–36 Furthermore, recently several groups studying
iron catalyzed Kumada reactions have observed the homo-
coupling of the Grignard reagent as a side reaction.37–41 In
general, either 1,2-dichloroethane, an aryl halide or O2 are
used as the oxidants for Fe catalyzed homo-coupling reactions,
although there are two reports describing the reaction in the
absence of an oxidant.42,43 Complexes 1–5 were all active pre-
catalysts for the homo-coupling of p-tolylmagnesium bromide
in a THF–diethyl ether mixture, with high yields of 4,4′-
dimethyl-1,1′-biphenyl observed (Table 2, entries 7–11). In all
cases, the yields were better than those obtained using
standard FeII precursors such as FeCl2, FeBr2 or FeBr2(THF)2
(entries 2–4), while no activity was observed when there is no
Fe source present (entry 1). A preformed complex does not
need to be utilized and premixing the free carbene,24 BnN-
(CH2CH2CH2-N-tert-butyl-imidazole-2-ylidene)2 with FeCl2, fol-
lowed by addition of a Grignard reagent, also resulted in
homo-coupling with only slightly reduced yield (entry 12). Per-
forming a similar reaction with BnN(CH2CH2CH2-N-tert-butyl-
imidazole-2-ylidene)2 and FeCl3 also gave a high yield of the
homo-coupling product (entry 13), demonstrating that either
an FeII or FeIII species could be used as the pre-catalyst.
However, the yield for the mixture containing FeCl3 and the
free carbene was only slightly higher than that obtained by
simply using FeCl3 (entry 5). The yield for homo-coupling
using our bidentate systems was also higher than that
11i
obtained using (IMes)2FeCl2
(IMes = 1,3-bis-(2,4,6-trimethyl-
phenyl)-1,3-dihydro-2H-imidazol-2-ylidene) (entry 6), which
contains two monodentate NHC ligands. Overall, these results
strongly suggest that our bidentate ligand is assisting in cata-
lysis although the effect is relatively small.
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Table 3 Fe catalyzed homo-coupling of Grignard reagentsa
Complexes 1–4 all gave similar activity after 30 minutes at a
loading of 2 mol%, but at lower loading (1 mol%) there were
slight differences in performance (entries 14–17). At this stage
the exact reasons why certain ligands give slightly better
activity are unclear and there is no obvious trend based on
electronics or sterics. In our experiments, we believe that the
CHCl3 used to quench the reactions is the oxidant. This is sup-
ported by a high yield of the homo-coupling product being
obtained in an experiment in which CHCl3 was added to the
reaction mixture and not used as the quenching reagent
(entry 18). In contrast, a greatly reduced yield was observed
when the reaction was performed without an oxidant and then
worked up without the addition of CHCl3. Interestingly, the
addition of other oxidants resulted in inhibition of the reac-
tion (entries 19–22). Both O2 and 1,2-dichloroethane lower the
yield by between 40–50%, while a smaller decrease was seen
when either 4-chlorobenzene or 4-bromobenzene were added.
It should be noted that no cross-coupling products from a
Kumada type reaction are observed when 4-chlorobenzene or
4-bromobenzene were added. This means that our systems
are selective for homo-couplings in the presence of an aryl
chloride, whereas the catalysts used by Bedford and Nakamura
for the Kumada coupling,37–39 give high selectivity for cross-
coupling and only small amounts of the homo-coupling
product. It is not understood what causes this change in
selectivity.
Entry
Substrates
Yieldb (%)
1
2
3
Phenylmagnesium bromide
o-Methoxyphenylmagnesium bromide
m-Tolylmagnesium bromide
82
63
86
65
84
0
45
68
77
0
4
5
p-Methoxyphenylmagnesium bromide
p-Tolylmagnesium bromide
6
7
8
p-Trifluoromethylphenylmagnesium bromide
1-Naphthylmagnesium bromide
Benzylmagnesium bromide
9
10
11
p-Methylbenzylmagnesium bromide
(Phenylethynyl)magnesium bromide
Phenylmagnesium chloride
51
a The conditions for the reaction were substrate (0.54 mmol), catalyst
(6 mg, 0.0108 mmol, 2 mol%) in 3 mL THF at room temperature. After
30 minutes the reaction was quenched by addition of wet 0.2 mL
CHCl3. b Isolated yield.
decrease in yield compared with the corresponding Mg
bromide salt (entry 1). This is the first time that chloride salts
have been utilized for Fe catalyzed homo-coupling.
Performing the catalysis when the Grignard reagent was
generated in situ (entry 23), from Mg turnings and 1-bromo-
4-methylbenzene, resulted in a lower yield of the homo-
coupling product. In this reaction the excess Mg turnings were
removed by filtration prior to the addition of the Fe catalyst,
so there was no possibility of a reaction between 4 and Mg,
although an independent control experiment confirmed that 4
does not react with Mg. Although the reaction mixtures appear
homogeneous, the possibility of heterogeneous iron particles
forming and catalyzing the reaction cannot be discounted.
A control reaction performed with a drop of Hg added to the
reaction mixture (entry 24) resulted in a very small decrease in
yield, suggesting that the reaction is homogeneous.44 However,
the Hg test is not always definitive and it has not been conclus-
ively established that the homo-coupling observed in this work
Conclusions
We have synthesized and characterized a number of distorted
tetrahedral S = 2 FeII complexes supported by a bidentate NHC
ligand containing an amine linker. These complexes are some
of the most active Fe pre-catalysts for the homo-coupling of
Grignard reagents and in one case can even couple a Mg chlor-
ide salt. In future work we will look to further explore the reacti-
vity of these unusual Fe complexes with small molecules and
understand the mechanism by which homo-coupling occurs.
Experimental details
General methods
is homogeneous. Furthermore, it was not possible to isolate a Experiments were performed under a dinitrogen atmosphere
well defined Fe complex from the reaction mixture after in an M-Braun dry box or using standard Schlenk techniques.
quenching with CHCl3.
(Under standard glovebox conditions purging was not per-
The substrate scope for homo-coupling was explored using formed between uses of pentane, hexane, diethyl ether,
compound 4 as the pre-catalyst (Table 3). The reaction is toler- benzene and toluene; thus when any of these solvents were
ant to substitution in the ortho, meta and para position of the used, traces of all these solvents were in the atmosphere and
aromatic ring (entries 2–5). There is a slight decrease in yield could be found intermixed in the solvent bottles.) Moisture-
when the electron donating methoxy group is present in the and air-sensitive liquids were transferred by stainless steel
para position (entry 4), while no reaction is observed with the cannula on a Schlenk line or in a dry box. The solvents for air-
electron withdrawing CF3 group in the para position (entry 6). and moisture-sensitive reactions were dried by passage
Successful coupling of sp3 hybridized carbon atoms in through a column of activated alumina followed by storage
benzylic systems was achieved (entries 8 and 9) but no reaction under dinitrogen. All commercial chemicals were used as
was observed using
a sp hybridized alkynyl substrate received except where noted. Phenylmagnesium bromide
(entry 10). Interestingly the reaction can also be performed and p-tolylmagnesium bromide were purchased from Aldrich.
using a Mg chloride salt (entry 11) with a relatively small 1-Naphthylmagnesium bromide, 2-methoxyphenylmagnesium
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Table 4 Crystal and refinement data for complexes 2, 3, 4 and 5
2
3
4
5
Empirical formula
Formula weight
Temperature (K)
a (Å)
b (Å)
c (Å)
C
37.85H53.01Cl2FeN5
C29H33Cl2FeN5O0.50
586.35
93(2)
12.9406(3)
9.3355(2)
22.9686(16)
90.00
91.132(6)
90.00
C32H36Cl2FeN5
617.41
93(2)
9.3137(2)
29.3333(5)
22.4626(16)
90.00
98.700(7)
90.00
C65H74Br4Fe2N10
1426.68
150(2)
679.56
93(2)
9.5506(2)
14.1282(3)
14.2614(10)
85.624(6)
87.635(6)
86.017(6)
1912.84(15)
2
9.434(4)
30.1020(7)
23.0220(16)
90.00
101.188(15)
90.00
6414(3)
4
α (°)
β (°)
γ (°)
Volume (Å3)
Z
2774.2(2)
4
6066.2(5)
8
Crystal system
Triclinic
P1
1.224
3.11 to 65.82
4.665
Monoclinic
P21/n
1.404
3.85 to 65.09
6.362
Monoclinic
P21/c
1.352
6.51 to 55.99
5.835
Monoclinic
P21/c
1.478
2.99 to 26.37
2.992
ˉ
Space group
dcalc (mg m−3
θ range (°)
)
μ (mm−1
)
Abs. correction
Semi-empirical from
equivalents
1.075
Semi-empirical from
equivalents
1.144
Semi-empirical from
equivalents
1.079
Semi-empirical from
equivalents
1.071
GOF
R1,a wR2
0.0832, 0.2388
0.0832, 0.1768
0.0627, 0.1627
0.0429, 0.0904
b
[I > 2σ(I)]
a R1 = Σ||Fo| − |Fc||/Σ|Fo|. b wR2 = [Σ[w(Fo2 − Fc2)2]/Σ[w(Fo2)2]1/2
.
bromide and 4-methoxyphenylmagnesium bromide were pur- MPMS-XL7 operating between 1.9 and 300 K for dc-applied
chased from Acros. The other Grignard reagents were prepared fields ranging from −7 to 7 T. The magnetization data was col-
in dry diethyl ether from the corresponding commercially lected at 100 K to check for ferromagnetic impurities which
available organic bromide by the standard method.45 Anhy- were found to be absent in (1–4). Diamagnetic corrections
drous iron(II) chloride and iron(III) chloride were purchased were applied for the sample holder and the core diamagnetism
from Alfa Aesar. Anhydrous iron(II) bromide was purchased from the sample (estimated with Pascal constants). Further
from Acros. NMR spectra were recorded on Bruker AMX-400, data for compounds 2–4 is provided in the ESI.†
-500 spectrometers at ambient probe temperatures. Robertson
Microlit Laboratories, Inc. performed the elemental analyses
(inert atmosphere). IR spectra were measured using a diamond
smart orbit ATR on a Nicolet 6700 FT-IR instrument. The
Mössbauer spectrum was acquired using a conventional
spectrometer in constant-acceleration mode equipped with a
57Co source (3.7 GBq) in a rhodium matrix. Isomer shifts are
given relative to α-Fe at room temperature. The Mössbauer
spectral absorbers contained 45 mg cm−2 of finely powdered
compound 4. The sample was inserted inside an Oxford Instru-
ments Mössbauer-Spectromag 4000 Cryostat. Mössbauer
spectra were evaluated using the NORMOS package program.
Literature procedures were used to prepare the following com-
pounds: 1,3-bis(2,6-diisopropylphenyl)-imidazolium chloride
(IMesHCl),46 Fe{N(SiMe3)2}2,47 [BnN(CH2CH2-N-tert-butyl-imid-
X-ray crystallography
Low-temperature diffraction data (ω-scans) were collected on
a
Rigaku MicroMax-007HF diffractometer coupled to a
Saturn994+ CCD detector with Cu Kα (λ = 1.54178 Å) or on a
Rigaku R-AXIS RAPID diffractometer coupled to a R-AXIS
RAPID imaging plate detector with Mo Kα radiation (λ =
0.71073 Å). All structures were solved by direct methods using
SHELXS49 and refined against F2 on all data by full-matrix
least squares with SHELXL-9750 using established refinement
techniques.51 All non-hydrogen atoms were refined anisotropi-
cally. All hydrogen atoms were included into the model at geo-
metrically calculated positions and refined using a riding
model. The isotropic displacement parameters of all hydrogen
atoms were fixed to 1.2 times the U value of the atoms they are
linked to (1.5 times for methyl groups). All disorders were
refined with the help of similarity restraints on the 1,2- and
1,3-distances and displacement parameters as well as rigid
bond restraints for anisotropic displacement parameters.
Details of the crystal and refinement data for complexes 2, 3, 4
and 5 are given in Table 4 and the ESI.†
azolium)2]·2[Cl],23
lium)2]·2[Cl],24
[BnN(CH2CH2CH2-N-tert-butyl-imidazo-
{BnN(CH2CH2-N-methylbenzimidazolium)2}·
2[Cl],23 {BnN(CH2CH2CH2-N-methylbenzimidazolium)2}·2[Cl],24
FeBr2(THF)248 and (IMes)2FeCl2.11i
Magnetic measurements
A magnetic analysis was performed on crushed polycrystalline
samples of 1–4, wrapped in a polyethylene membrane sealed
in a glove box to prevent any sample degradation. The direct
Synthesis and characterization of new compounds
{BnN(CH2CH2-N-tert-butyl-imidazole-2-ylidene)2}FeCl2 (1). Fe-
current (dc) magnetic susceptibility measurements were {N(SiMe3)2}2 (141 mg, 0.38 mmol) was added to a suspension
obtained using a Quantum Design SQUID magnetometer of [BnN(CH2CH2-N-tert-butyl-imidazolium)2]·2[Cl] (180 mg,
7410 | Dalton Trans., 2013, 42, 7404–7413
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0.38 mmol) in 20 mL toluene. The mixture was stirred at
{BnN(CH2CH2CH2-N-methylbenzimidazole-2-ylidene)2}FeCl2
room temperature for 50 hours, after which stirring was (4). Fe{N(SiMe3)2}2 (94 mg, 0.25 mmol) was added to a suspen-
stopped, and the precipitate was allowed to settle for several sion of {BnN(CH2CH2CH2-N-methylbenzimidazolium)2}·
hours. The solution was carefully decanted from the precipi- 2[Cl] (131 mg, 0.25 mmol) in 12 mL toluene. The mixture was
tate, which was washed with toluene and pentane and dried stirred at room temperature for 16 hours, after which stirring
under vacuum to give 1 as a white powder. Yield: 191 mg was stopped, and the precipitate was allowed to settle for
(96%).
several hours. The solution was carefully decanted from the
IR (diamond tip, cm−1): 3459 (m), 3385 (br), 3264 (w), 3123 (m), precipitate, which was washed with toluene and pentane and
3092 (s), 3048 (s), 2978 (m), 2887 (w), 2829 (m), 2802 (m), dried under vacuum to give 4 as a pale yellow powder. Single
2355 (w, br), 1637 (w), 1565 (s), 1555 (s), 1448 (m), 1411 (w), crystals for X-ray analysis were grown from a benzene/pentane
1374 (s), 1313 (w), 1294 (w), 1257 (w), 1236 (m), 1204 (s), 1151 (s), solution at room temperature. Yield: 136 mg (93%).
1129 (s), 1116 (w), 1060 (w), 1053 (m), 1029 (w), 1002 (w),
IR (diamond tip, cm−1): 3371 (br), 3060 (w), 3021 (w),
984 (m), 938 (w), 925 (w). Anal. calcd (found) for 2948 (s), 2883 (w), 2843 (m), 2807 (m), 2116 (m), 1608 (w),
C25H37Cl2N5Fe: C, 56.19 (55.36); H, 6.98 (6.66); N, 13.11 1568 (s), 1485 (s), 1460 (s), 1454 (s), 1438 (s), 1393 (s), 1381 (s),
(12.19).
1368 (s), 1349 (s), 1313 (w), 1293 (w), 1277 (w), 1265 (w),
{BnN(CH2CH2CH2-N-tert-butyl-imidazole-2-ylidene)2}FeCl2 1254 (w), 1233 (w), 1210 (s), 1188 (m), 1170 (w), 1140 (m),
(2). Fe{N(SiMe3)2}2 (69 mg, 0.18 mmol) was added to a sus- 1126 (w), 1094 (w), 1061 (s), 1038 (m), 1026 (m), 1013 (m),
pension of [BnN(CH2CH2CH2-N-tert-butyl-imidazolium)2]·2[Cl] 1004 (w), 967 (w), 935 (w), 910 (w), 897 (w), 881 (w), 860 (w),
(93 mg, 0.18 mmol) in 12 mL toluene. The mixture was stirred 832 (w), 817 (w). Magnetic susceptibility (C6D6): 4.84 μB. Anal.
at room temperature for 50 hours, after which stirring was calcd (found) for C29H33Cl2N5Fe·THF: C, 60.93 (60.61); H, 6.37
stopped, and the precipitate was allowed to settle for several (6.24); N, 10.77 (10.55). The molecule was recrystallized from
hours. The solution was carefully decanted from the precipi- THF for purification.
tate, which was washed with toluene and pentane and dried
{BnN(CH2CH2CH2-N-methylbenzimidazole-2-ylidene)2}
under vacuum to give 2 as a white powder. Single crystals for FeBr2 (5). KOtBu (68 mg, 0.6 mmol) was added to a suspen-
X-ray analysis were grown from a benzene/pentane solution at sion of {BnN(CH2CH2CH2-N-methylbenzimidazolium)2}·2[Cl]
room temperature. Yield: 91 mg (88%).
(105 mg, 0.2 mmol) in 10 ml THF. The mixture was stirred for
IR (diamond tip, cm−1): 3350 (br), 3133 (w), 3118 (m), 3081 (m), three hours. The volatiles were removed under vacuum. The
3058 (w), 3025 (w), 2980 (s), 2932 (m), 2870 (w), 2825 (w), resulting residue was extracted using toluene (2 × 6 mL) and
2812 (m), 2735 (w), 2110 (w, br), 1581 (w), 1564 (w), 1544 (m), filtered through celite. To this toluene solution, FeBr2 (39 mg,
1492 (w), 1461 (m), 1452 (m), 1438 (w), 1405 (w), 1379 (m), 0.18 mmol) was added. The suspension was stirred for
1370 (m), 1353 (w), 1339 (w), 1313 (w), 1285 (w), 1261 (w), 21 hours. The mixture was filtrated through celite and the fil-
1223 (s), 1202 (s), 1160 (w), 1130 (s), 1087 (w), 1075 (m), 1035 (w), trate collected. The volume of the filtrate was reduced to
996 (w), 915 (w). Anal. calcd (found) for C27H41Cl2N5Fe: C, approximately 1 mL and 5 mL pentane was added. The solu-
57.65 (56.73); H, 7.35 (7.25); N, 12.45 (12.03).
tion was carefully decanted from the precipitate, which was
{BnN(CH2CH2-N-methylbenzimidazole-2-ylidene)2}FeCl2 (3). Fe- washed with pentane and dried under vacuum to give 5 as a
{N(SiMe3)2}2 (440 mg, 1.17 mmol) was added to a suspension yellow powder. Single crystals for X-ray analysis were grown
of {BnN(CH2CH2-N-methylbenzimidazolium)2}·2[Cl] (580 mg, from a toluene/pentane solution at room temperature. Yield:
1.17 mmol) in 30 mL toluene. The mixture was stirred at room 65 mg (52%).
temperature for 50 hours, after which stirring was stopped,
1H NMR (400 MHz, C6D6): 10.50 (3H), 9.47 (2H), 8.83 (2H),
and the precipitate was allowed to settle for several hours. The 7.79 (2H), 6.04 (1H), 5.67 (2H), 3.37 (6H), 2.76 (4H), 2.12 (4H),
solution was carefully decanted from the product, which was 1.84 (4H), 0.96 (2H). IR (diamond tip, cm−1): 3461 (br), 3058
washed with toluene and pentane and dried under vacuum to (m), 3025 (m), 2940 (s), 2813 (s), 2723 (w), 2661 (w), 2323 (s),
give 3 as an off-white powder. Single crystals for X-ray analysis 2117 (s), 1992 (m), 1700 (s), 1619 (m), 1602 (m), 1569 (m),
were grown from a THF/pentane solution at room temperature. 1498 (s), 1481 (s), 1452 (s), 1432 (w), 1388 (w), 1363 (s),
Yield: 555 mg (86%).
1346 (w), 1292 (w), 1245 (m), 1207 (w), 1186 (m), 1176 (w),
IR (diamond tip, cm−1): 3378 (br), 3144 (w), 3080 (m), 1137 (m), 1126 (m), 1089 (w), 1072 (m), 1043 (w), 1025 (w),
3057 (m), 3026 (m), 2098 (w), 2951 (m), 2913 (w), 2894 (w), 1014 (s), 971 (br), 912 (w), 883 (w), 844 (m). Magnetic sus-
2845 (w), 2812 (m), 2799 (w), 2766 (w), 2120 (w), 1570 (s), ceptibility (C6D6): 4.84 μB. Anal. calcd (found) for
1488 (m), 1462 (s), 1447 (s), 1429 (m), 1413 (w), 1393 (w), C29H33Br2N5Fe: C, 52.20 (53.28); H, 4.98 (5.23); N, 10.50
1380 (m), 1364 (w), 1352 (m), 1298 (w), 1280 (m), 1262 (m), (10.27).
1208 (s), 1175 (w), 1134 (s), 1095 (m), 1070 (m), 1047 (m),
1021 (s), 1008 (m), 956 (m), 929 (m), 911 (m), 862 (w), 841 (m),
General procedure for homo-coupling reactions in Table 2
816 (w). Magnetic susceptibility (C6D6): 4.77 μB. Anal. calcd To a solution of iron catalyst (0.0036 mmol) in 0.9 mL THF and
(found) for C27H29Cl2N5Fe·THF: C, 59.92 (60.56); H, 5.84 additive (0.18 mmol, if necessary), p-tolylmagnesium bromide
(5.54); N, 11.27 (11.54). The molecule was recrystallized from (0.36 mL, 0.18 mmol, 0.5 M in diethyl ether) was added at room
THF for purification.
temperature. After 30 minutes the reaction was quenched by
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addition of wet 0.2 mL CHCl3. The volatiles were removed
under vacuum and internal standard trimethoxybenzene
(11.7 mg, 0.07 mmol) was added. A H NMR spectrum of the
(g) D. Bézier, F. Jiang, T. Roisnel, J.-B. Sortais and
C. Darcel, Eur. J. Inorg. Chem., 2012, 18, 1333;
(h) A. A. Danopoulos, P. Braunstein, M. Wesolek,
K. Y. Monakhov, P. Rabu and V. Robert, Organometallics,
2012, 31, 4102; (i) T. Hashimoto, S. Urban, R. Hoshino,
Y. Ohki, K. Tatsumi and F. Glorius, Organometallics, 2012,
31, 4474.
1
mixture was recorded in CDCl3 to calculate the yield.
General procedure for homo-coupling reactions in Table 3
The substrate (0.54 mmol) in THF or diethyl ether was added
to a stirred solution of catalyst 4 (6 mg, 0.0108 mmol) in 3 mL
THF. The mixture was stirred at room temperature for
30 minutes and then 1 mL wet CHCl3 was added. The solution
was filtered through silica gel and the resulting mixture was
evaporated to dryness. The products were then separated by
column chromatography on silica gel (250–400 mesh) with
either pentane or pentane–diethyl ether (50 : 1) as the eluent.
The homo-coupling products were identified by comparison of
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the H NMR spectra with those previously reported in the lit-
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17 A. A. Danopoulos, D. Pugh, H. Smith and
J. Sassmannshausen, Chem.–Eur. J., 2009, 15, 5491.
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Acknowledgements
We thank Louise Guard for assistance with X-ray
crystallography.
20 S. Meyer, C. M. Orben, S. Demeshko, S. Dechert and
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