Ionic iron(iii) complexes of bis(phenol)-functionalized imidazolium cations: Synthesis, structures and catalysis for aryl Grignard cross-coupling of alkyl halides

Article abstract of DOI:10.1039/c2dt30931a

A series of bis(phenol)-functionalized imidazolium salts, 1,3-bis(4,6-di-R¹-2-hydroxybenzyl)-2-R²-4,5-di-R ³-imidazolium chlorides H₃LⁿCl (R¹ = ^tBu, R² = R³ = H, H₃L ¹Cl, 1; R¹ = CH₃, R² = R³ = H, H₃L²Cl, 2; R¹ = ^tBu, R ² = H, R³ = Cl, H₃L³Cl, 3; R ¹ = ^tBu, R² = CH₃, R³ = H, H₃L⁴Cl, 4), were used to produce a novel series of ionic iron(iii) complexes [H₃Lⁿ][FeX₄] (n = 1, X = Cl, 5; n = 2, X = Cl, 6; n = 3, X = Cl, 7; n = 4, X = Cl, 8; n = 1, X = Br, 9; n = 3, X = Br, 10). All of the complexes were characterized by Raman spectroscopy and electrospray ionization mass spectrometry. Elemental analysis and X-ray crystallography were also used. All of the complexes were non-hygroscopic and air-stable, with five of them existing as solids (5, 7-10) and one as an oil (6) at room temperature. A preliminary catalytic study on the cross-coupling reactions of aryl Grignard reagents with primary and secondary alkyl halides bearing β-hydrogens, revealed that all of the ionic iron(iii) complexes exhibited good to excellent catalytic activity. Complexes 5, 6 and 8 exhibited optimal activity, whereas 7, 9 and 10 showed only moderate activity. Furthermore, by simply decanting the cross-coupling product in the ether layer, complexes 5 and 6 could be reused in at least seven successive runs without significant loss in catalytic activity.

Full text of DOI:10.1039/c2dt30931a

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A Table of contents entry
Color graphic:
MgBr
X
[Fe] (1 mol%)
ArMgX (1.2 equiv)
30 oC, Et2O (30 to 120 min)
X = Br, Cl
GC yields: 99-48%
OH
R1
R2
-
OH
FeX4
R1
N
N
R3
[Fe] =
R3
R1
R1
One sentence:
Iron-containing bis(phenol)-functionalized imidazolium salts as efficient precatalysts
for the coupling of aryl Grignard reagents with alkyl halides

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Proofs to: Prof. HongꢀMei Sun
College of Chemistry, Chemical
Engineering and Materials Science
Soochow University
Suzhou 215123, P. R. China
Fax: +(86)512ꢀ65880305
Tel: +(86)512ꢀ65880330
Eꢀmail: sunhm@suda.edu.cn
Ionic iron(III) complexes of bis(phenol)-functionalized
imidazolium cations: synthesis, structures and catalysis for aryl
Grignard cross-coupling of alkyl halides
Haiꢀning Deng, Yaꢀlin Xing, Congꢀliang Xia, Hongꢀmei Sun,* Qi Shen, Yong Zhang
The Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical
Engineering and Materials Science, Soochow University, Suzhou 215123, People’s Republic of
China
*corresponding author
eꢀmail address: sunhm@suda.edu.cn (H. M. Sun)
†CCDC reference numbers 862971 and 875587 for 5 and 9, respectively.
1

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Abstract:
A
series
of
bis(phenol)ꢀfunctionalized
imidazolium
salts,
1,3ꢀbis(4,6ꢀdiꢀR1ꢀ2ꢀhydroxybenzyl)ꢀ2ꢀR2ꢀ4,5ꢀdiꢀR2ꢀimidazolium chlorides H3LnCl (R1
= tBu, R2 = R3 = H, H3L1Cl, 1; R1 = CH3, R2 = R3 = H, H3L2Cl, 2; R1 = tBu, R2 = H,
t
R3 = Cl, H3L3Cl, 3; R1 = Bu, R2 = CH3, R3 = H, H3L4Cl, 4), were used to produce a
novel series of ionic iron(III) complexes [H3Ln][FeX4] (n = 1, X = Cl, 5; n = 2, X = Cl,
6; n = 3, X = Cl, 7; n = 4, X = Cl, 8; n = 1, X = Br, 9; n = 3, X = Br, 10). All of the
complexes were characterized by Raman spectroscopy and electrospray ionization
mass spectrometry. Elemental analysis and Xꢀray crystallography were also used. All
of the complexes were nonꢀhygroscopic and airꢀstable, with five of them existing as
solids (5, 7ꢀ10) and one as an oil (6) at room temperature. A preliminary catalytic
study on the crossꢀcoupling reactions of aryl Grignard reagents with primary and
secondary alkyl halides bearing βꢀhydrogens, revealed that all of the ionic iron(III)
complexes exhibited good to excellent catalytic activity. Complexes 5, 6 and 8
exhibited optimal activity, whereas 7, 9 and 10 showed only moderate activity.
Furthermore, by simply decanting the crossꢀcoupling product in the ether layer,
complexes 5 and 6 could be reused in at least seven successive runs without
significant loss in catalytic activity.
2

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Introduction
During the past decade there has been an explosion of interest in ionic liquids (ILs)
with a wide range of applications because of their potential advantages over
conventional organic solvent, such as wide liquid range, negligible vapor pressure,
high thermal stability and good solubility.1 Furthermore, given that ILs are salts
generally composed of distinct organic cations with inorganic or organic anions, they
are capable of being tuned with relative ease by the selection of the appropriate cation
and anion combination,2 and thereby can be designed for taskꢀspecific purposes.3
While a great deal of interest in ILs has centered on their use as environmentally
friendly alternatives to volatile organic solvents,4 ILs are now being considered and
even employed as attractive alternatives to traditional ligands and/or ionic tags of
traditional ligands in the development of transition metalꢀbased catalysts.5ꢀ6 In
comparison with traditional ligand supported systems, such modified ionic transition
metal complexes (so called transition metal containing ILs, MetILs7) can invariably
offer their own distinct advantages in terms of stability, activity, low toxicity and
reusability, which can lead to improved catalytic performance and provide
greenꢀchemistry strategies for chemical synthesis.6 Of various types of ILs available,
imidazolium salts are the most popular one employed, mainly because their steric and
electronic properties can be wellꢀtailored to meet specific requirements by altering
either the Nꢀsubstituents or the ringꢀbackbone. Furthermore, functionalized
imidazolium cations have received considerable attention based on the notion that the
presence of an appropriate coordinating group on the fiveꢀmembered ring could
3

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immobilize metal catalysts effectively and thereby increase catalyst activity and
improve catalyst recycling.6a In this context, a range of different functional groups,
including phosphines,8 nitriles,9 pyridines,10 pyrazoles,11 amines,12 ethers,13 alcohols,14
14b, 18
esters,15 alkoxysilanes,16 acids,17 salens,12b,
salicylaldoximes,18b BINOLs,19
bis(oxazoline),20 thioethers18a, 21 and sulfonamides,22 have been designed into the
imidazolium cations. The resulting cations have exhibited great potential in several
catalytic systems, including rutheniumꢀbased systems for olefin metathesis
reactions13aꢀb and palladiumꢀbased systems for crossꢀcoupling reactions.10 In this
respect, it is of substantial interest to develop ionic transition metal complexes
containing functionalized imidazolium cations since it can offer new opportunities in
catalysis.
The development of ironꢀbased catalysts has received increasing attention with
renewed enthusiasm for a variety of crossꢀcoupling reactions in recent years, because
iron is more costꢀeffective and environmentally benign than palladium or nickel.23
Wellꢀdefined iron complexes, developed for the crossꢀcoupling reaction of aryl
Grignard reagents with primary or secondary alky halides bearing
represent one of the most successful examples of an ironꢀbased catalytic system in
this area, which is mostly owing to their ability to efficiently suppress ꢀH elimination
βꢀhydrogens,
β
and their potentiality from a mechanistic perspective.24 For example, wellꢀdefined iron
complexes, such as the lowꢀvalent iron(ꢀII) complex,24 iron(III) salen complexes,25
iron(II) or iron(III) imine complexes,26 and iron(III) amineꢀbis(phenolate)
complexes,27 have been used to good effect in this regard. A recent report on the
4

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utility of an easily prepared ionic iron(III) complex of imidazolium cation
[bmim][FeCl4] (bmim = 1ꢀbutylꢀ3ꢀmethylimidazolium cation)28 as a recyclable
catalyst for the aryl Grignard crossꢀcoupling of alkyl halides with superior activity is
worthy of particular note.28b The use of imidazolium chloride can modify the highly
29
hygroscopic FeCl3 into a nonꢀhygroscopic and airꢀstable IL, which provides a
practical strategy for the design of ironꢀbased catalyst with potential largeꢀscale
applications. The liquid state of this ionic iron(III) complex may, however, lead to
some problems with structural definition and handling. Moreover, although various
functionalized imidazolium cations have been widely used to construct transition
metal catalysts, as mentioned above, they have been hardly explored to any significant
extent as immobilizing reagents for generating ironꢀbased catalysts,30 even though the
presence of a functional group might facilitate their formation and improve their
reactivity.
In a continuation of our work focused on the development of functionalized
imidazolium salts31 and ionic iron complexes for application in carbonꢀcarbon bond
formation,32 we herein report the synthesis and corresponding structures of a series of
bis(phenol)ꢀfunctionalized imidazoliumꢀbased ionic iron(III) complexes and their
application as catalysts in the aryl Grignard crossꢀcoupling reaction of alky halides
bearing
βꢀhydrogens. Although bis(phenol)ꢀfunctionalized imidazolium salts have
been known for some time,33 the use of this class of imidazolium salts as
immobilizing reagents in the generation of metalꢀbased catalysts has, to the best of
our knowledge, not been reported in the literature.
5

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Results and Discussion
Synthesis and characterization of ionic iron(III) complexes
Three new imidazolium chlorides, H3L2Cl (2), H3L3Cl (3), and H3L3Cl (4), were
readily synthesized according to a published procedure that used for 131f (Scheme 1),
and characterized by elemental analysis and NMR spectroscopy. The most significant
1
features of the H NMR spectra of 2ꢀ3 were the signals assigned to the imidazolium
1
protons (C2ꢀH), appearing at 9.30 and 9.54 ppm, respectively.42f The H NMR
spectrum of 4 showed characteristic resonances similar to those of 1,31f except for the
disappearance of a C2ꢀH signal at 9ꢀ11 ppm and the presence of a singlet at 3.03 ppm,
caused by replacement of the C2ꢀH position with a methyl group.
OH
R2
OH
OH
R2
Cl-
N
OH
R1
R1
R1
THF, reflux
R1
Cl
N
+
N
N
R3
R3
R1
R3
R1
R1
R3
R1
3 (H3L3Cl): R1 = tBu, R2 = H, R3 = Cl
4 (H3L4Cl): R1 = tBu, R2 = CH3, R3 = H
1 (H3L1Cl): R1 = tBu, R2 = H, R3 = H
2 (H3L2Cl): R1 = CH3, R2 = H, R3 = H
Scheme 1 Synthesis of the bis(phenol)ꢀfunctionalized imidazolium chlorides
All of the bis(phenol)ꢀfunctionalized imidazolium chlorides (1ꢀ4) were obtained as
airꢀstable nonꢀhygroscopic white powders. All of the imidazolium chloride dissolved
readily in chlorinated solvents, and compound 3, which contained
a
4,5ꢀdichloroimidazole unit, was the most soluble of the four and also exhibited good
solubility in THF. In contrast, the remaining three compounds (1, 2 and 4) were
almost insoluble in THF. It is worthy of note that the melting point of 3 was much
lower than that of 1, indicating that the incorporation of chlorine groups at the C4 and
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C5 positions of the imidazolium ring tended to lead to a decrease in the melting point
of the salt.34
OH
OH
-
FeCl4
N
tBu
tBu
yield: 90%
yield: 87%
FeCl3
THF
N
H3L1Cl
+
r.t., 3 h
FeCl3 6H2O
tBu
tBu
OH
[H3L1][FeCl4] (5)
OH
R2
-
FeCl4
R1
R1
THF
r.t., 3 h
N
N
H3LnCl
+
FeCl3
R3
R3
R1
R1
n = 2, [H3L2][FeCl4] (6): R1 = CH3, R2 = H, R3 = H
n = 3, [H3L3][FeCl4] (7): R1 = tBu, R2 = H, R3 = Cl
yield: 91%
yield: 80%
n = 4, [H3L4][FeCl4] (8): R1 = tBu, R2 = CH3, R3 = H yield: 83%
Scheme 2 Synthesis of ionic iron(III) complexes 5ꢀ8
The target ionic iron(III) complexes 5ꢀ8 were readily prepared by the reaction of
anhydrous FeCl3 with one equivalent of H3LnCl in THF at room temperature (Scheme
2). In an alternative procedure, for instance, complex 5 can be conveniently
synthesized by a similar reaction using the inexpensive FeCl3⋅6H2O as a starting
material.28 After workup, [H3L1][FeCl4] (5), [H3L2][FeCl4] (6), [H3L3][FeCl4] (7), and
[H3L4][FeCl4] (8) were isolated as yellowꢀgreen crystals (5, 7ꢀ8) or a yellowꢀgreen oil
(6) in high yields, respectively.
THF
H3LnCl +
[H3Ln][FeBr4]
FeBr3 + 3 NaBr
60 oC, 12 h
n = 1, [H3L1][FeBr4] (9) yield: 85%
n = 3, [H3L3][FeBr4] (10) yield: 80%
Scheme 3 Synthesis of ionic iron(III) complexes 9ꢀ10
It is worthy of note that the sodium bromide mediated exchange of the anion from
ꢀ
ꢀ
FeCl4 to FeBr4 proceeded smoothly (Scheme 3). For example, the treatment of FeBr3
with one equivalent of 1 and 3 in the presence of excess NaBr afforded the analogous
7

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complexes 9 and 10, respectively, as brownꢀred crystals in high yields. These
reactions, however, proceeded at slower rate than those presented in Scheme 2
because NaBr is poorly solvated in THF, and extend reaction times (12 h) and
elevated temperatures were required to push the reaction to completion.
Although the ferric chlorides were highly hygroscopic, complexes 5ꢀ10 were
nonꢀhygroscopic and airꢀstable, and five of them (5, 7ꢀ10) existed as solids at room
temperature, making them easier to handle. Better solubility profiles were observed
for the imidazoliumꢀbased ionic iron(III) complexes than in the corresponding
imidazolium salts, and all of complexes dissolved easily in THF. In addition,
complexes 7 and 10, which contained the 4,5ꢀdichloroimidazole unit, dissolved well
in toluene. The difference in solubility between the bis(phenol)ꢀfunctionalized
imidazolium salt and the corresponding iron(III) complex facilitated the effective
purification of the target product.
(Figures 1 and 2)
The formation of ionic iron(III) complexes 5ꢀ10 was supported by Raman
spectroscopy and electrospray ionization mass spectroscopy (ESIꢀMS). Elemental
analysis was also used for complexes 5, 7ꢀ10 and Xꢀray crystallography for
complexes 5 and 9. The metal halide anions existed as discrete [FeX4]− anions, which
were easily identified by Raman spectroscopy. It is clear in Figure 1 that the spectrum
of 5 shows one strong peak at 334 cm−1, whereas the spectrum of 9 shows only one
strong peak at 204 cmꢀ1. These data coincided very closely to the literature values for
the [FeCl4]− and [FeBr4]− species, respectively.35 For comparison, the observed bands
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between 500 and 1700 cm−1 were very similar to those of 1, indicating the presence of
the [H3L1]+ cation in 5 and 9. The Raman spectra for the synthesized ionic iron(III)
complexes are shown totally in Figure 2. A predominant feature of the spectra was the
clean [FeCl4]− peak at the range of 333ꢀ337 cmꢀ1 for the ionic iron(III) chlorides and
the clean [FeBr4]− peak at the range of 202ꢀ204 cmꢀ1 for the bromide analogues. The
positive ion ESIꢀMS spectra of 5ꢀ10 were used to establish the presence of the
bis(phenol)ꢀfunctionalized imidazolium cations, and in all cases a peak of high
relative intensity indicative of the parent cations was observed. Thus, these
characteristic results confirmed that complexes 5ꢀ10 consisted of [H3Ln][FeX4]. The
1H NMR spectra of the complexes, however, were less informative because they
exhibit broad, shifted peaks with paramagnetism.
Single crystals of 5 and 9 were obtained from cold THF/hexane solutions, and their
molecular structures were determined by Xꢀray crystallography. The crystallographic
and measurement data are listed in Table 1. Their molecular structures are depicted in
Figures 3 and 4, with key bond parameters provided in the captions. Half of the
structure of 5 was C2ꢀsymmetrically generated, as depicted (Figure 3).
To date, structural data for imidazoliumꢀbased ionic iron(III) complexes of the type
[imidazolium cation][FeX4] have been only scarcely reported, even though the study
of metal complexation in ILs is of great interest to support the extensive applications
of MetILs that are currently under development.2 Mudring and coꢀworkers recently
reported the first example of a crystal structure determined following the in situ
crystal growth of an oily iron(III) complex, [C2mim][FeCl4] (C2mim =
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1ꢀethylꢀ3ꢀmethylimidazolium).36a
Structural
data
for
a
1,3ꢀdibutylbenzimidazoliumꢀbased ionic iron(III) complex, [(C4H9)2bim][FeCl4]
[(C4H9)2bim = 1,3ꢀdibutylbenzimidazolium], were published soon after.36b An
additional
example
also
came
from
the
crystal
structure
of
[BDMim]4[FeIICl4][FeIIICl4]2, which was published by Kölle and Dronskowski in
2004.36c
(Table 1)
(Figures 3 and 4)
As shown in Figures 1 and 2, each of the two molecular structures contains one
bis(phenol)ꢀfunctionalized imidazolium [H3L1]+ cation and one [FeX4]− anion, and
two additional THF molecules in 9. In 5 and 9, the bond distances and angles within
the imidazolium ring and side chain differed only slightly, corresponding well with
the related imidazolium salt [H3L1]Br reported by Kawaguchi.33b A difference was
observed, however, with respect to the orientation of the phenolic chain. In 5, each
plane of the phenolic ring was oriented almost perpendicular to the plane of the
imidazolium ring with a dihedral angle of 84.7°, and at an angle of 89.3° to the plane
of the other phenolic ring in the system. In 9, two phenolic rings were arranged in an
almost parallel fashion, and oriented nearly perpendicular to the plane of the
imidazolium ring with smaller dihedral angles in the range of 77.8 to 72.1°. Of note,
for 5, a hydrogen bond formed between the phenolic hydrogen atom and one of the
carbon atoms of the tertꢀbutyl group in the imidazolium cation, with a bond distance
in the range of 2.64(5) to 2.82(5) Å. In contrast, the more acidic hydrogen atom at the
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2ꢀH position of the imidazolium ring was not involved in the hydrogen bonding
network. In 9, two THF molecules were incorporated into the crystal structure during
the crystallization process and were hydrogen bonded to the two phenolic hydrogen
atoms through their own oxygen atoms.
The [FeCl4]− anion in 5 formed a slightly distorted tetrahedral geometry around the
iron center with FeꢀCl(1) and FeꢀCl(2) bond distances of 2.186(2) and 2.200(2) Å,
respectively, which were in agreement with values found in other imidazoliumꢀbased
ionic iron(III) complexes.36 The ClꢀFeꢀCl angles of [FeCl4]− were between 105.91(9)°
and 110.37(6)° with a mean value of 109. 45°, which was close to the ideal tetrahedral
angle at ꢀ50°C.36a Similarly, the [FeBr4]− anion in 9 also had a slightly distorted
tetrahedral geometry, because the six BrꢀFeꢀBr angles (107.05(7)°, 107.12(7)°,
107.62(7)°, 109.61(7)°, 110.54(8)°, 110.95(7)°) for [FeBr4]− were obviously different
from each other, although each of four FeꢀBr bond distances was almost the same
(2.307(2)ꢀ2.325(2) Å). These values were in the expected range as found for other
complexes containing [FeBr4]− anions.37
Although the hydrogen bonding interaction between the imidazolium cation and
halide anion is well established,7, 36 none of the cationꢀanion interatomic distances
found in 5 and 6 fell within the generally accepted range for typical hydrogen bonding
interactions.38 Thus, both of 5 and 9 could be described as a simple collection of ion
pairs. The unique pattern in 5 or 9 was most likely a consequence of the steric
bulkiness of the bis(phenol)ꢀfunctionalized imidazolium cation.32a
Catalysis of ionic iron(III) complexes
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For the initial screening of catalyst performance, the reaction outlined in eq. 1 was
chosen as a model system, as it is a prototypical crossꢀcoupling reaction of an aryl
29
Grignard reagents with alkyl halides bearing
β
ꢀhydrogens.25,
This series of
experiments was performed using both cyclohexyl bromide and chloride as starting
reagents to highlight any differences in activity. The results obtained are summarized
in Table 2.
[Cat] (1 mol%)
ArMgBr (1.2 equiv)
MgBr
X
(1)
30 oC, Et2O (30 to 120 min)
X = Br, Cl
(Table 2)
It is clear that all of the ionic iron(III) complexes provided good to excellent yields
of the desired coupling product using cyclohexyl bromide as a starting reagent,
whereas modest to good yields were obtained in the presence of cyclohexyl chloride
with prolonged reaction times. Interestingly, the reaction proceeded smoothly without
the requirement of an inert atmosphere, and there was no obvious difference between
the use of an air atmosphere or pure argon (entry 4). Complexes, 5 and 8 showed the
highest activity of all of the iron(III) complexes, providing almost quantitative yields
of the desired coupling products when cyclohexyl bromide was used as the starting
reagent (entries 4 and 15). Furthermore, the yields still reached 77 and 76% for 5 and
8, respectively, even when cyclohexyl bromide was replaced with the chloride
analogue (entries 6 and 17). The near equality in the activity of 5 and 8 indicated that
the imidazolium tag present in 5 was catalytically silent and inert under the reaction
conditions,6a otherwise a significant effect would be observed on the coupling
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reaction on changing the imidazolium cation at its C2 position. Under the same
conditions, a slightly lower activity was seen when the bulky tertꢀbutyl substituent on
the phenolic motif was replaced by a methyl substituent (entries 10ꢀ12), implying that
aryl substituent imposed a negligible effect. Iron(III) salen complexes displayed
similar performances, where paraꢀ and orthoꢀtertꢀbutyl modified analogues showed
essentially the same activity as systems containing only hydrogen atoms at the same
positions.25 In contrast, replacing the hydrogen atoms at the 4ꢀ and 5ꢀpositions of the
imidazole ring with chlorine atoms proved to be clearly unfavorable, with 7 showing
lower activity than 8 (entries 13 and 14). Replacement of the anion from [FeCl4]− to
[FeBr4]−was also detrimental to performance, with 9 and 10 providing lower yields
than those obtained with 5 and 7 (entries 18ꢀ21), respectively.
It is worthy of note that the present crossꢀcoupling reaction could be performed
successfully at the mild temperature of 30 °C, and that the Grignard reagent could be
added in a single portion at room temperature without the requirement for slow
addition via a syringe pump. Furthermore, the catalytic activity of 5 was compared
with other related iron(III)ꢀbased catalysts reported in the literature. Normally, 2ꢀ5
mol% or higher loadings of the iron complexes and 1.5ꢀ2 equiv of Grignard reagent
were usually required to achieve satisfactory yields for reactions similar to those
shown in eq. 1.24ꢀ29 Therefore, the present results suggest that 5 and 8 might be among
the most efficient precatalysts for the crossꢀcoupling reaction of aryl Grignard
reagents with alkyl halides bearing βꢀhydrogens, which is mostly due to enhanced
stabilization of catalytically active iron species via the aryloxoꢀfunctionalized
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imidazolium cations generated in situ during the catalytic procedures.
(Table 3)
Substrate screening was subsequently carried out, and the results obtained are
summarized in Table 3. A comparison of the range of the Grignard reagents used in
the reaction with cyclohexyl bromide (entries 1ꢀ6) indicated that electronꢀrich or
electronꢀneutral aryl Grignard reagent reacted rapidly to give the desired coupling
products (entries 1ꢀ3), whereas reagents with electronꢀwithdrawing groups reacted
sluggishly (entry 5). It is worthy of note that an excellent isolated yield was also
obtained with 2ꢀmethylphenylmagnesium bromide (entry 2), although the introduction
of two orthoꢀsubstituents led to a complete loss in activity (entry 4), which is
consistent with observations from other iron(III)ꢀbased catalysts reported in the
literature.25, 29bꢀc To the best of our knowledge, there have been no other reports in the
literature describing the crossꢀcoupling of such sterically hindered aryl Grignard
reagent, i.e. 2,6ꢀdimethylphenylmagnesium bromide, with secondary alkyl halides.29c
Upon changing the Grignard reagent to phenylmagnesium chloride, the reaction
proceeded smoothly, although with a lower isolated yield of 72% (entry 6). A higher
isolated yield of the desired coupling product was typically obtained when cyclohexyl
bromide was employed rather than the corresponding chloride or iodide. This general
trend in the reactivity of cyclohexyl halides appeared to Br > I > Cl. This was the
same pattern as that previously reported for iron(III) salen complexes,25
[bmim][FeCl4],28b and FeCl3/phosphine systems.29c The current iron(III)ꢀcatalyzed
crossꢀcoupling was applicable to cycloheptyl bromide and cyclopentyl bromide under
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the same conditions described for cyclohexyl bromide, although the yields were lower
than those recorded for cyclohexyl bromide (entries 8 and 9). Acyclic secondary alkyl
bromides provided similar results to those of cyclohexyl bromide (entries 12 and 13),
whereas the corresponding primary bromides were usually less reactive (entries 11
and 14). Unfortunately, tertiary alkyl bromide gave none of the desired coupling
product (entry 15).
Recyclability of ionic iron(III) complexes
(Table 4)
Recent research has revealed that some types of imidazoliumꢀbased ionic metal
complexes can be reused several times without any significant loss in catalytic
activity.6a For example, 5 mol % of [bmim][FeCl4] was reused in the crossꢀcoupling
of 4ꢀfluorophenylmagnesium bromide and dodecyl bromide, with the isolated yield of
the desired coupling product decreasing only slightly from 86% in the first run to 76%
after the fifth run.28b In our work, having established that complexes 5, 6 and 8
showed optimal activity, we proceeded to study their recycling ability in the
crossꢀcoupling reaction of 4ꢀmethylphenylmagnesium bromide and cyclohexyl
bromide. Starting with 5 mol % of 5, the upper ether layer containing the desired
coupling product was decanted after the reaction was complete, whereas the catalyst
was left in the black sediment, which was washed twice with 0.5 mL of diethyl ether
and evaporated to dryness. Fresh cyclohexyl bromide and nꢀhexadecane (as an
internal standard) were mixed in a diethyl ether solvent and added to the remaining
catalyst. The Grignard reagent was then added and the reaction was found to proceed
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smoothly under same conditions. The data listed in Table 4 indicated that complex 5
could be reused in at least seven successive runs of the model reaction, with
cyclohexyl bromide being almost completed consumed. The isolated yield of desired
coupling product, however, gradually dropped from 89% in the first run to 72% after
the seventh run (entries 1ꢀ7). This may be attributed to catalyst leaching during the
decanting and extraction steps.28b Complex 6 showed similar recycling ability during
the same recycling process. The recycling experiments with 8, however, led to a
considerable decrease in the isolated yield from 89% in the first run to 40% after the
fourth run, indicating that the introduction of a nonꢀcoordinating group (i.e. methyl
group) at the C2 position of imidazolium cation facilitated catalyst leaching. Welton
also observed a similar phenomenon, when a Pd(II)ꢀbased catalyst could not be
efficiently trapped into 2ꢀmethylimidazolium salts, even in the presence of other
supporting
ligands.39
Nevertheless,
comparisons
to
nonfunctionalized
[bmim][FeCl4]28b demonstrate the superiority of the present systems with respect to
recyclability, which is mostly due to the weak interaction derived from the phenolic
group.
Conclusions
A family of easily synthesized and easyꢀtoꢀhandle ionic iron(III) complexes of
bis(phenol)ꢀfunctionalized imidazolium cations (5ꢀ10) have been developed as
excellent catalysts for the crossꢀcoupling of aryl Grignard reagents with primary and
secondary alkyl halides bearing
βꢀhydrogens under mild conditions. Moreover,
complexes 5 and 6 could be reused seven times without any significant loss in
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catalytic activity. This work has therefore demonstrated that the modification of
imidazole ring with phenolic groups represents a promising pathway towards the
design and synthesis of highly active and recyclable catalytic systems. The facile
synthesis of the ionic iron(III) complexes, their readily tailored properties and their
ease of handling makes them highly attractive for use in a wide range of catalytic
reactions. Further studies evaluating the full scope of this new type of catalysts are
currently being pursued.
Experimental Section
General procedures
All manipulations were performed under pure argon with rigorous exclusion of air
and moisture using standard Schlenk techniques. Solvents were distilled from
Na/benzophenone ketyl under pure argon prior to use. Anhydrous FeCl3, FeBr3,
FeCl3⋅6H2O and organic reagents used for the crossꢀcoupling reactions were
purchased from Acros Organics, SigmaꢀAldrich and Alfa Aesar. ArMgX was diluted
prior to use. Nꢀ(4,6ꢀDiꢀtertꢀbutylꢀ2ꢀhydroxybenzyl)ꢀ4,5ꢀdichloroimidazole,33b
Nꢀ(4,6ꢀdiꢀtertꢀbutylꢀ2ꢀhydroxybenzyl)ꢀ2ꢀmethylimidazole,33b
Nꢀ(4,6ꢀdimethylꢀ2ꢀhydroxybenzyl)imidazole,33b
and
1,3ꢀbis(4,6ꢀdiꢀtertꢀbutylꢀ2ꢀhydroxybenzyl)imidazolium chloride (H3L1Cl, 1)31f were
prepared by the published methods. Elemental analyses were performed by direct
combustion on a CarloꢀErba EAꢀ1110 instrument. NMR spectra were measured on a
Varian Unity INOVA 400 or VNMRS 300 MHz spectrometer at 25 °C. The Raman
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spectra were recorded on a LabRAM HR800 spectrometer. Electrospray
IonizationꢀMass spectrum (ESIꢀMS) data were recorded on a 6220 AccurateꢀMass
TOF LC/MS instrument. Gas chromatographic (GC) analysis was performed on a
Varian CPꢀ3800 instrument equipped with a FID detector and a capillary column
AT.OVꢀ101 (30 m × 0.32 mm i.d., 0.10 ꢁm film). The oven temperature was held at
80 °C for 2 min, increased to 280 °C at 10 °C/min, and held for 2 min.
Synthesis
H3L2Cl (2). A flask was charged with Nꢀ(4,6ꢀdiꢀmethylꢀ2ꢀhydroxybenzyl)imidazole
(4.06 g, 20 mmol) and THF (20 mL), and then
a
solution of
2ꢀchloromethylꢀ4,6ꢀmethylphenol (4.25 g, 25 mmol) in THF (25 mL) was slowly
added at reflux temperature. After stirring for 12 h, the solution was evaporated to
dryness to give the crude product. The product was washed with THF, and dried in
vacuo, which yielded white solid of H3L2Cl (3.37 g, 45%). M.p. 176ꢀ178 °C. Anal.
Calcd for C21H25N2O2Cl: C, 67.64; H, 6.76; N, 7.51. Found: C, 67.19; H, 6.89; N,
7.38. 1H NMR (CDCl3): δ 9.30 (s, 1H, NCHN), 8.57 (s, 2H, OH), 7.02 (d, J = 1.2 Hz,
2H, NCHC), 6.95 (s, 2H, PhꢀH), 6.84 (s, 2H, PhꢀH), 5.25 (s, 4H, PhCH2N), 2.37 (s,
6H, CH3), 2.21 (s, 6H, CH3). 13C NMR (CDCl3): δ 11.22 (CH3), 15.22 (CH3), 41.92
(CH2), 118.70 (C), 120.59 (C), 122.57 (C), 122.96 (C), 124.34 (C), 126.73 (C), 132.28
(C), 145.82 (C).
H3L3Cl (3). Following a procedure similar to the synthetic procedure of 2, a flask
was charged with Nꢀ(4,6ꢀdiꢀtertꢀbutylꢀ2ꢀhydroxybenzyl)ꢀ4,5ꢀdichloroimidazole (10.62
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g, 30.00 mmol) and THF (20 mL), and then
a
solution of
2ꢀchloromethylꢀ4,6ꢀdiꢀtertꢀbutylphenol (8.85 g, 35.00 mmol) in THF (25 mL) was
slowly added at reflux temperature. After workup, the product was precipitated as
white solid in 70% yield (12.40 g). M.p. 164ꢀ167 °C. Anal. Calcd for C33H47N2O2Cl3:
C, 64.96; H, 7.77; N, 4.59. Found: C, 64.87; H, 7.69; N, 4.58. 1H NMR (CDCl3): δ
9.54 (s, 1H, NCHN), 7.88 (s, 2H, OH), 7.42 (s, 1 H, PhꢀH), 7.36 (s, 1H, PhꢀH), 7.09 (s,
1 H, PhꢀH), 7.07 (s, 1 H, PhꢀH), 5.49 (s, 2H, PhCH2N), 5.06 (s, 2H, PhCH2N), 1.44 (s,
9H, C(CH3)3), 1.40 (s, 9H, C(CH3)3), 1.27 (s, 9H, C(CH3)3), 1.25 (s, 9H, C(CH3)3).
13C NMR (CDCl3): δ 30.19 (CCH3), 31.43 (CCH3), 34.26 (CCH3), 35.04 (CCH3),
50.55 (CH2), 119.06 (C), 120.42 (C), 125.87 (C), 126.21 (C), 137.06 (C), 140.27 (C),
143.41 (C), 152.32 (C).
H3L4Cl (4). Following a procedure similar to the synthetic procedure of 2, a flask
was charged with Nꢀ(4,6ꢀdiꢀtertꢀbutylꢀ2ꢀhydroxybenzyl)ꢀ2ꢀmethylimidazole (1.51 g,
5.00
mmol)
and
THF
(30
mL),
and
then
a
solution
of
2ꢀchloromethylꢀ4,6ꢀdiꢀtertꢀbutylphenol (1.78 g, 7.00 mmol) in THF (10 mL) was
slowly added at reflux temperature. After workup, the product was precipitated as
white solid in 86% yield (2.28 g). M.p. 252ꢀ254 °C. Anal. Calcd for C34H51N2O2Cl: C,
73.55; H, 9.26; N, 5.05. Found: C, 73.45; H, 9.25; N, 4.98. 1H NMR (CDCl3): δ 7.75
(s, 2H, OH), 7.39 (s, 2H, PhꢀH), 7.07 (s, 2H, PhꢀH), 6.74 (d, J = 1.6 Hz, 2 H, NCHC),
5.23 (s, 4H, PhCH2N), 3.03 (s, 3H, CH3), 1.39 (s, 18H, C(CH3)3), 1.28 (s, 18H,
C(CH3)3). 13C NMR (DMSOꢀd): δ 10.32 (CH3), 30.23 (CCH3), 31.73 (CCH3), 34.36
(CCH3), 35.19 (CCH3), 48.28 (CH2), 121.62 (C), 123.51 (C), 124.25 (C), 124.63 (C),
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139.19 (C), 142.76 (C), 144.59 (C), 151.57 (C).
[H3L1][FeCl4] (5), procedure A. A Schlenk flask was charged with H3L1Cl (1.08 g,
2.00 mmol), THF (10 mL) and a stirring bar. To this suspension, FeCl3 (0.32 g, 2.00
mmol) in 10 mL of THF was added. The reaction mixture was stirred for 3 h at room
temperature, filtered and evaporated to dryness. The residue was recrystallized from
THF and hexane to yield yellowꢀgreen crystals (1.26 g, 90%). M.p. 212–215 °C (dec).
Anal. Calcd for C33H49N2O2FeCl4: C, 56.35; H, 7.02; N, 3.98. Found: C, 56.48; H,
7.08; N, 3.95. MS (ESI+): m/z 505.3788 [C33H49N2O2]+ (100%). Raman spectrum:
334 cmꢀ1 ([FeCl4]−).
[H3L1][FeCl4] (5), procedure B. A Schlenk flask was charged with H3L1Cl (1.08 g,
2.00 mmol), THF (10 mL) and a stirring bar. To this suspension, FeCl3ꢂ6H2O (0.54 g,
2.00 mmol) in 10 mL of THF was added. The following preparation is similar to
procedure A. After workup, the product was precipitated as yellowꢀgreen crystals in
87% yield (1.22 g). Anal. Calcd for C33H49N2O2FeCl4: C, 56.35; H, 7.02; N, 3.98.
Found: C, 56.43; H, 7.10; N, 3.97.
[H3L2][FeCl4] (6). Following a procedure similar to the synthetic procedure A of 5,
a Schlenk flask was charged with H3L2Cl (0.38 g, 1.00 mmol), THF (10 mL) and a
stirring bar. To this suspension, FeCl3 (0.16 g, 1.00 mmol) in 10 mL of THF was
added. After workup, the product was precipitated as yellowꢀgreen oil in 91% yield
(0.49 g). M.p.117–119 °C (dec). MS (ESI+): m/z 337.1927 [C21H25N2O2]+ (100%).
Raman spectrum: 335 cmꢀ1 ([FeCl4]−).
[H3L3][FeCl4] (7). Following a procedure similar to the synthetic procedure A of 5,
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a Schlenk flask was charged with H3L3Cl (1.22 g, 2.00 mmol), THF (10 mL) and a
stirring bar. To this suspension, FeCl3 (0.32 g, 2.00 mmol) in 10 mL of THF was
added. After workup, the product was precipitated as yellowꢀgreen crystals in 80%
yield (1.23 g). M.p. 171–173 °C (dec). Anal. Calcd for C33H47N2O2FeCl6: C, 51.32; H,
6.13; N, 3.63. Found: C, 50.89; H, 5.91; N, 3.88. MS (ESI+): m/z 573.3036
[C33H47N2O2Cl2]+ (100%). Raman spectrum: 337 cmꢀ1 ([FeCl4]−).
[H3L4][FeCl4] (8). Following a procedure similar to the synthetic procedure A of 5,
a Schlenk flask was charged with H3L4Cl (1.11 g, 2.00 mmol), THF (10 mL) and a
stirring bar. To this suspension, FeCl3 (0.32 g, 2.00 mmol) in 10 mL of THF was
added. After workup, the product was precipitated as yellowꢀgreen crystals in 83%
yield (1.18 g). M.p. 143–146 °C (dec). Anal. Calcd for C34H51N2O2FeCl4: C, 56.92; H,
7.17; N, 3.90. Found: C, 56.84; H, 7.06; N, 3.57. MS (ESI+): m/z 519.3963
[C34H51N2O2]+ (100%). Raman spectrum: 333 cmꢀ1 ([FeCl4]−).
[H3L1][FeBr4] (9). A Schlenk flask was charged with H3L1Cl (1.08 g, 2.00 mmol),
NaBr (0.31 g, 3.00 mmol), THF (10 mL) and a stirring bar. To this suspension, FeBr3
(0.59 g, 2.00 mmol) in 10 mL of THF was added. The reaction mixture was stirred for
12 h at 60 C, filtered and evaporated to dryness. The residue was recrystallized from
°
THF and hexane to yield brownꢀred crystals (1.50 g, 85%). M.p. 218–220 °C (dec).
Anal. Calcd for C33H49N2O2FeBr4(C4H8O): C, 46.62; H, 6.03; N, 2.94. Found: C,
46.30; H, 5.90; N, 2.80. MS (ESI+): m/z 505.3806 [C33H49N2O2]+ (100%). Raman
spectrum: 204 cmꢀ1 ([FeBr4]−).
[H3L3][FeBr4] (10). Following a procedure similar to the synthetic procedure of 9,
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a Schlenk flask was charged with H3L3Cl (1.22 g, 2.00 mmol), NaBr (0.31 g, 3.00
mmol), THF (10 mL) and a stirring bar. To this suspension, FeBr3 (0.59 g, 2.00 mmol)
in 10 mL of THF was added. After workup, the product was precipitated as brownꢀred
crystals in 80% yield (1.52 g). M.p. 185–187 °C (dec). Anal. Calcd for
C33H47N2O2FeCl2Br4: C, 41.72; H, 4.99; N, 2.95. Found: C, 41.96; H, 4.95; N, 3.12.
MS (ESI+): m/z 573.3017 [C33H47N2O2Cl2]+ (100%). Raman spectrum: 202 cmꢀ1
([FeBr4]−).
X-ray structural determination
Suitable single crystals of 5 and 9 were sealed in a thinꢀwalled glass capillary for
Xꢀray structural analysis. Diffraction data were collected on a Rigaku Mercury CCD
area detector at 223(2) K. The structures were solved by direct methods and refined
by fullꢀmatrix leastꢀsquares procedures based on F2. All nonꢀhydrogen atoms were
refined with anisotropic displacement coefficients. Hydrogen atoms were treated as
idealized contributions. The structures were solved and refined using SHELXSꢀ97
and SHELXLꢀ97 programs, respectively. Crystal data and collection and main
refinement parameters are given in Table 1. Selected bond lengths (Å) and angles (deg)
of these complexes are given in Figures 3 and 4, respectively.
General procedure for the cross-coupling reactions
A Schlenk tube was charged with an iron(III)ꢀbased catalyst (0.01 mmol), alkyl
halides (1.00 mmol), nꢀhexadecane (0.10 mmol) as internal standard, diethyl ether
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(1.15 mL) and a stirring bar. The mixture was stirred at 0 °C for 2 min.
4ꢀMethylphenylmagnesium bromide (1.35 mL, 1.20 mmol) was added to this solution
at 0 °C. The resulting solution turned black immediately and was then stirred for 30
min in an oil bath at 30 °C. After the reaction was quenched by addition of saturated
(
ammonium chloride solution, the mixture was extracted with diethyl ether 3 × 2 mL)
and dried over MgSO4. The GC yield of the desired product is determined by GC
analysis, using nꢀhexadecane as an internal standard. GC analysis was done directly
from the reaction as well as from the crude product and proved to be identical.
Purification of the crude mixture by flash column chromatography using petroleum
ether (60ꢀ90 °C) as eluent gave the isolated yield of desired coupling product. The
identity of the product was confirmed by 1H NMR spectroscopy and TLC.
Procedure for the recycling of iron(III) complexes catalyzed cross-coupling of
Grignard reagents with alkyl halides
A Schlenk tube was charged with an iron(III)ꢀbased catalyst (0.05 mmol), cyclohexyl
bromide (1.00 mmol), nꢀhexadecane (0.10 mmol) as internal standard, diethyl ether
(1.15 mL) and a stirring bar. The mixture was stirred at 0 °C for 2 min.
4ꢀMethylphenylmagnesium bromide (1.35 mL, 1.20 mmol) was added to this solution
at 0 °C. The resulting solution turned black immediately. Stirring was continued for
30 min in an oil bath at 30 °C and the reaction was left to settle down. The upper ether
layer containing the product was decanted carefully and the remaining catalyst layer
was washed twice with 0.5 ml of diethyl ether each. The combined organic layers
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were subjected directly to GC analysis, and then were concentrated to flash column
chromatography as shown above.
The remaining catalyst layer was evaporated to dryness, then fresh cyclohexyl
bromide (1.00 mmol), nꢀhexadecane (0.10 mmol) and diethyl ether (1.15 mL) were
added again to the catalyst. Subsequent addition of 4ꢀmethylphenylmagnesium
bromide (1.35 mL, 1.20 mmol) at 0 °C started the reaction and the reaction procedure
was repeated like described above.
Acknowledgement. We are grateful to the National Natural Science Foundation of
China (Grant 21172164), the Key Laboratory of Organic Chemistry of Jiangsu
Province, and the Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD) for financial support.
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Notes and references
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Kirchner, Ionic Liquids, Springer, Heidelberg, Germany, 2010.
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