Synthesis of anionic iron(II) complex bearing an n-heterocyclic carbene ligand and its catalysis for aryl grignard cross-coupling of alkyl halides

Article abstract of DOI:10.1021/om100482w

The reaction of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) with one equivalent of a novel imidazolium salt of iron(II), [FeBr ₃(C₄H₈O)](HIPr)·C₄H ₈O (1), afforded the anionic iron(II) comp

Full text of DOI:10.1021/om100482w

Organometallics 2010, 29, 4189–4192 4189
DOI: 10.1021/om100482w
Synthesis of Anionic Iron(II) Complex Bearing an N-Heterocyclic Carbene
Ligand and Its Catalysis for Aryl Grignard Cross-Coupling of Alkyl Halides
Huan-huan Gao, Chun-hui Yan, Xue-Ping Tao, Ying Xia, Hong-Mei Sun,*
Qi Shen, and 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
Received May 17, 2010
Summary: The reaction of 1,3-bis(2,6-diisopropylphenyl)imida-
zol-2-ylidene (IPr) with one equivalent of a novel imidazolium salt
of iron(II), [FeBr3(C4H8O)](HIPr) C4H8O (1), afforded the
outstanding catalytic activity for cross-coupling reactions and
related transformations.3 However, in comparison with inten-
sively studied palladium-based complexes, the NHC complexes
of iron remain scarely considered in this respect,1i even if the
development of iron-based catalysts is of great interest due to
iron being more cost-effective and environmentally benign
compared to palladium.4
3
anionic iron(II) complex bearing an N-heterocyclic carbene ligand
[Fe(IPr)Br3](HIPr) C7H8 (2), which shows extremely high acti-
3
vity in comparison with the other iron(II)-based precatalysts in the
cross-coupling reaction of 4-tolylmagnesium bromide with cyclo-
hexyl bromide.
In fact, iron remains one of the least studied late transition
metals with NHCs.1i To date, only a few kinds of well-
defined iron NHC complexes have been reported, which
include hexacarbene complexes,5 tetracarbene complexes,6
tricarbene complexes,7 biscarbene complexes,8 and piano-
stool monocarbene complexes that are co-ligated by cyclo-
pentadienyl and CO ligands.9 Among them, only two bis-
Introduction
During the past decade, N-heterocyclic carbenes (NHCs)
have received increasing attention as alternative ligands for
the development of late transition metal based homogeneous
catalysts,1 which is mostly due to their advantages over tradi-
tional phosphine ligands such as stronger σ-electron donation,
tighter coordination, and more steric bulkiness.2 In most cases,
the replacement of phosphine ligands with electron-rich NHC
ligands is of benefit to improve the stability and catalytic activity
of their complexes. As a result, a large number of well-defined
NHC complexes of Pd(II), especially singly ligated NHC com-
plexes such as [(NHC)Pd(allyl)Cl]3a and [(NHC)PdCl2-
(pyr)] (pyr = 3-chloropyridine),3b have been found to show
8a
carbene complexes, i.e., Fe(NHC)2X2 and Fe(CNC)Br2
[CNC = 2,6-bis(imidazolylidene)pyridine],8b have been
found to show good catalytic activity for atom transfer radical
polymerization of styrene and methyl methacrylate8a and the
cross-coupling reaction of 4-tolylmagnesium bromide with
cyclohexyl bromide.10 Therefore, further study on the breadth
of iron-based NHC complexes, in parallel with the growing
interest for iron catalysis, is highly desired.1i
As a continuation of our research on the iron11 and
nickel12 chemistry of NHCs, we herein report the synthesis
€
(5) Frankel, R.; Kernbach, U.; Bakola-Christianopoulou, M.; Plaia,
*To whom correspondence should be addressed. Fax: (86)512-
65880305. Tel: (86)512-65880330. E-mail: sunhm@suda.edu.cn.
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Coord. Chem. Rev. 2004, 248, 2239–2246. (d) Crudden, C. M.; Allen, D. P.
Coord. Chem. Rev. 2004, 248, 2247–2273. (e) Pugh, D.; Danopoulos, A. A.
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r
2010 American Chemical Society
Published on Web 08/26/2010
pubs.acs.org/Organometallics

4190 Organometallics, Vol. 29, No. 18, 2010
Gao et al.
Figure 1. Crystal structure of 1 with thermal ellipsoids at the
30% probability level (only one disordered coordinating THF
molecule shown). Hydrogen atoms and the cocrystallized non-
coordinating THF molecule are omitted for clarity. Selected
Figure 2. Crystal structure of 2 2C7H8 with thermal ellipsoids
3
at the 30% probability level. Hydrogen atoms and three toluene
˚
molecules are omitted for clarity. Selected bond lengths (A) and
˚
bond lengths (A) and angles (deg): Fe(1)-Br(1) 2.4267(17),
angles (deg): Fe(1)-Br(1) 2.4356(9), Fe(1)-Br(2) 2.4600(12),
Fe(1)-C(1) 2.110(7); C(1)-Fe(1)-Br(1) 108.18(9), C(1)-Fe(1)-
Br(2) 116.11(18), Br(1)-Fe(1)-Br(2) 104.52(3), Br(1A)-Fe(1)-
Br(2) 104.52(3).
Fe(1)-Br(2) 2.4128(16), Fe(1)-Br(3) 2.4148(16), Fe(1)-O(1)
2.062(16), H(6)-Br(2) 2.92, H(7)-Br(1) 2.95; O(1)-Fe(1)-Br(1)
104.4(4), O(1)-Fe(1)-Br(2) 98.9(4), O(1)-Fe(1)-Br(3) 106.1(4),
Br(1)-Fe(1)-Br(2) 113.40(6), Br(1)-Fe(1)-Br(3) 115.21(6),
Br(2)-Fe(1)-Br(3) 116.33(6).
Scheme 2
Scheme 1
isolated from a concentrated toluene solution in ca. 55%
yield, which was identified to be [Fe(IPr)Br3](HIPr) C7H8
and molecular structure of an anionic mononuclear iron(II)
complex bearing one NHC ligand, [Fe(IPr)Br3](HIPr) C7H8
3
3
(2) by elemental analysis (Scheme 2). Recrystallization from
toluene afforded colorless crystals of [Fe(IPr)Br3](HIPr)
(2), which represents the first example of singly ligated NHC
complexes of iron(II). The preliminary results on its catalytic
activity for the cross-coupling of 4-tolylmagnesium with
cyclohexyl bromide and other alkyl halides bearing β-hydro-
gen are also described.
3
3C7H8 (2 2C7H8) suitable for X-ray structure determination
3
(Figure 2). Complex 2 is thermally stable, but is very sensitive
to air compared to complex 1.
Complexes 1 and 2 were structurally determined by X-ray
crystallography. The crystallographic and measurement
data are shown in Table 1. Their crystal structures are
depicted in Figures 1 and 2.
To date, complex 1 is the first structurally characterized
imidazolium salt of iron(II).13 As shown in Figure 1, the iron
center is coordinated by three Br atoms and one O atom from
the solvated THF molecule in a distorted tetrahedral geo-
metry with angles at iron in the range 98.9(4)-116.33(6)°.
The Fe-Br8b,14 and Fe-O15 bond lengths are very close to
other iron(II) complexes reported in the literature. Notably,
two Br atoms of the anion form obvious hydrogen bonding
Results and Discussion
Our approach toward the target iron(II) complex starts from
a novel N,N-diarylimidazolium salt of iron(II), 1. At room
temperature, anhydrous FeBr2 reacted slowly with one equiva-
lent of imidazolium salt HIPr Br in THF (Scheme 1). After the
3
mixture was stirred for 24 h, removal of the solvent in vacuo and
recrystallization from THF produces yellow crystals of
[FeBr3(C4H8O)](HIPr) C4H8O (1) in ca. 70% yield, as identi-
3
fied by elemental analysis and X-ray diffraction (Figure 1).
Complex 1 is sensitive to air at room temperature, and its color
changed gradually from yellow to red and to brown in air.
When one equivalent of IPr was added to the THF
solution of 1 at room temperature, the resulting solution
changed color from yellow to pale gray quickly, accom-
pained by the appearance of white precipitates. After stirring
for 6 h, the solution was filtered and evaporated to dryness.
The residue was extracted with toluene. A white powder was
(13) (a) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002,
^
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Macromolecules 2007, 40, 7441–7452.
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Chem. 1991, 30, 2888–2894.

Note
Organometallics, Vol. 29, No. 18, 2010 4191
Table 1. X-ray Crystallographic Data for 1 and 2
Table 2. Activities of Catalytic Systems Based on Iron(II)
Complexesa
1
2 2C7H8
3
precatalyst
loading (mol %)
empirical formula
fw
temperature (K)
λ(Mo Ka) (A)
cryst syst
C35H53Br3FeN2O2
829.37
223(2)
0.71075
monoclinic
P21/c
C75H97Br3FeN4
1350.15
223(2)
0.71075
monoclinic
C2/m
runb
precatalyst
yieldc
1
2
3
4
FeBr2
1
5
3
3
1
0.5
1
1
1
5
41
74
80
96
89
83
90
86
94h
˚
1 þ HIPr Br
3
space group
unit cell dimens
2
2
2
2
5
6d
7e
8f
9g
˚
a (A)
12.221(2)
32.359(5)
10.1880(17)
96.912(5)
3999.7(11)
4
1.377
3.403
1696
0.80 ꢀ 0.80 ꢀ 0.50
3.03-25.50
20 224
7417, 0.0692
1.126
25.818(4)
17.062(2)
20.125(3)
127.016(4)
7078.8(19)
4
1.267
1.949
2824
0.80 ꢀ 0.25 ꢀ 0.20
3.05-25.50
18 183
6762, 0.0389
1.070
˚
b (A)
˚
c (A)
2
Fe(CNC)Br2
β (deg)
3
V (A )
˚
a Cyclohexyl bromide (1.0 mmol), 4-MeC6H4MgBr (1.5 mmol), Et2O,
reflux, 30 min, Ar. b 4-MeC6H4MgBr was added in one portion at 0 °C,
the reaction solution was stirred for 2 min and then refluxed for 30 min.
c Yield of the isolated product after column chromatography, average of
2 trials. d Cyclopentyl bromide(1.0 mmol). e 2-Bromohexane (1.0 mmol).
f 1-Bromooctane. g Cyclohexyl bromide (2.0 mmol), 4-MeC6H4MgBr
(4.0 mmol), Et2O, reflux, 30 min. h Conversion.
Z
Dcalc (g cm-3
)
μ (mm-1
F(000)
cryst size (mm)
)
θ range (deg)
no. of reflns collected
no. of reflns unique, Rint
goodness-of-fit on F2
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
During the past few years, one of the most notable
successes in iron-based catalysis is that several well-defined
iron complexes have been developed for the cross-coupling
reaction of aryl Grignard reagents with primary or second-
ary alkyl halides bearing β-hydrogens, which is mostly due to
their ability to suppress the undesired β-hydrogen elimina-
tion efficiently as well as their potential from a mechanistic
point of view.19 To assess the catalytic activity of complex 2
and related iron(II)-based salts, the reaction shown in eq 1 was
chosen, as it is a prototype cross-coupling of aryl Grignard
reagents with alkyl halides bearing β-hydrogens.10,19
0.0917, 0.2160
0.1516, 0.2449
0.0780, 0.2052
0.1009, 0.2218
with the H6 and H7 protons of the imidazolium ring,
respectively.16 Thus, complex 1 can be described as a simple
collection of ion pairs, which is quite different from those
found in N,N0-dialkylimidazolium-based salts; that is, the
general formula [HNHC]2[MCl4] (M = Pd17 and Ni18) was
reported. This unique pattern in 1 is mostly attributed to the
steric bulkiness of the aryl-substituted imidazolium cation.
X-ray structure determination revealed that complex 2 is
an anionic mononuclear iron(II) complex. As seen from
Figure 2, the iron center is coordinated by a carbene carbon
atom and three Br atoms in a distorted tetrahedral geometry
with angles in the range 104.52(3)-116.11(18)°. The bond
˚
length of Fe-Ccarbene in 2 is 2.110(7) A, which falls within the
It is worth noting that the present cross-coupling reaction
can be performed successfully at elevated temperature, and
the Grignard reagent can be added in one portion at room
temperature without the requirement of slow addition via a
syringe pump. The preliminary results are listed in Table 2.
Complex 2 showed the highest activity. Almost a quantita-
tive yield of the coupling product was obtained at 1 mol %
loading, and the yield still reached 89% even when the
loading was reduced to 0.5 mol % (runs 4, 5). Meanwhile,
1 exhibited moderate activity and provided a coupling
product in 74% yield with a 3 mol % loading (run 2), and
FeBr2 was the least active one, affording the product merely
in 41% yield with a 5 mol % loading (run 1). The difference in
activity between 1 and FeBr2 may be attributed to the more
stable active species in 1 stabilized by imidazolium salts.13b,19c
Considering the suggestion that the NHC ligand could
be formed in situ via the deprotonation of an imidazolium
salt by Grignard reagents,1i,10 the same reaction with a mix-
range of previously reported values in iron NHC complexes,
i.e., shorter than those in Fe(CNC)Br2 (2.166(10) and
2.193(10) A),8b HB(tBuIm)3FeBr (tBuIm = 1-tert-butylimi-
˚
7a
˚
dazol, 2.123(3)-2.135(3) A), and trans-Fe(NHC)2Cl2
(2.1363(15) A),8a while longer than those in Fe(aryloxo-func-
˚
11a
˚
tionalized NHC)2 (2.076(4)-2.094(4) A),
Fe(alkenyloxo-
functionalized NHC)2 (2.023(7)-2.047(5) A),11b CpFe(CO)2-
˚
(NHC)I (1.980(5) A),9a [Fe(2,6-bis(NHC)pyridine)(MeCN)3]-
˚
[BPh4]2 (1.944(5) and 1.947(5) A),8b and [Fe(bis(pyridylimida-
˚
zolidene)methane)2](PF6)2 (1.801(6) A).8d A notable feature in
˚
this structure is that there is no discrete hydrogen bonding
interaction between the Br atoms of the anion and any proton
of the imidazolium ring, which is quite different from the
interaction between the anion and the cation present in 1.
˚
The Fe-Br bond lengths of 2.4356(9) and 2.4600(12) A in 2 are
significantly longer than those found in 1, which reflects a bulky
hindrance around the center metal exerted by the IPr ligand
and/or a strong σ-electron donation of the IPr ligand. Addi-
tionally, the carbene ring plane is oriented nearly perpendicular
to that occupied by three bromide ligands.
ture of 1 and HIPr Br was conducted. However, this reac-
3
tion gave the product in a yield almost the same as that
obtained by 1, indicating that it is not as easy as expected
to form a IPr ligand in situ under the present reaction
conditions (run 3). Alternatively, it is not easy to form a Fe-
Ccarbene bond in situ. The same situation was also recently
(16) Taylor, R.; Kennard, O. J. Am. Chem. Soc. 1982, 104, 5063–
5070.
(17) (a) Ortwerth, M. F.; Wyzlic, M. J.; Baughman, R. G. Acta
Crystallogr. 1998, C54, 1594–1596. (b) Yang, X.; Fei, Z.; Geldbach, T. J.;
Phillips, A. D.; Hartinger, C. G.; Li, Y.; Dyson, P. J. Organometallics 2008,
27, 3971–3977.
(18) Hitchcock, P. B.; Seddon, K. R.; Welton, T. J. Chem. Soc.,
Dalton Trans. 1993, 2639–2643.
€
(19) (a) Martin, R.; Furstner, A. Angew. Chem., Int. Ed. 2004, 43,
3955–3957. (b) Bedford, R. B.; Bruce, B. W.; Frost, R. M.; Goodby, J. W.;
Hird, M. Chem. Commun. 2004, 2822–2823. (c) Bica, K.; Gaertner, P. Org.
Lett. 2006, 8, 733–735. (d) Chowdhury, R. R.; Crane, A. K.; Fowler, C.;
Wong, P. K.; Kozak, C. M. Chem. Commun. 2008, 94–96.

4192 Organometallics, Vol. 29, No. 18, 2010
Gao et al.
reported by Nakamura et al..20 Thus, it is reasonable that
the high activity shown by 2 in the present cross-coupling
reaction is related to the presence of a Fe-Ccarbene bond in
complex 2. In addition, complex 2 also showed good cata-
lytic activity for the cross-coupling reaction of 4-tolylmagne-
sium bromide with other alkyl bromides, such as cyclopentyl
bromide, 2-bromohexane, and 1-bromooctane (runs 6-8).
Notably, the catalytic activity of 2 is found to be much
higher than that of a biscarbene complex of iron(II) Fe-
(CNC)Br2 reported by Bedford et al. (run 9).10 Normally,
2-5 mol % or higher loadings of iron complexes are usually
required to achieve satisfactory yields for the reaction shown
in eq 1.10,19 Therefore, the present results suggest that
complex 2 might be among the most efficient precatalysts
for the cross-coupling reaction of aryl Grignard reagents
with alkyl halides bearing β-hydrogens, possibly due to a
singly ligated NHC active species with high nucleophilicity.3
analysis. Diffraction data were collected on a Rigaku Mercury
CCD area detector at 193(2) K. The structures were solved
by direct methods and refined by full-matrix least-squares
procedures based on |F|2. All non-hydrogen atoms were refined
with anisotropic displacement coefficients. Hydrogen atoms were
treated as idealized contributions. The structures were solved and
refined using SHELXS-97 and SHELXL-97 programs, respec-
tively. Crystal data and collection and main refinement parameters
are given in Table 1.
Synthesis of [FeBr3(C4H8O)](HIPr) C4H8O (1). A Schlenk
3
flask was charged with bis(2,6-diisopropylphenyl)imidazolium
bromide (1.9062 g, 4.06 mmol), THF (20 mL), and a stirring bar.
To this suspension solution was added FeBr2 (0.8770 g, 4.06
mmol) in 60 mL of THF. The reaction mixture was stirred for
24 h at room temperature, filtered, and evaporated to dryness.
The residue was extracted with THF, and recrystallization from
concentrated THF yielded yellow crystals (2.36 g, 70%) suitable
for elemental analysis and X-ray diffraction determination. Mp:
95-97 °C. Anal. Calcd for C27H37Br3FeN2(C4H8O)2: C, 50.69; H,
6.44; N, 3.38. Found: C, 50.29; H, 6.87; N, 3.08.
Conclusions
Synthesis of [Fe(IPr)Br3](HIPr) C7H8 (2). A Schlenk flask
3
was charged with 1 (1.9072 g, 2.30 mmol), THF (15 mL), and a
stirring bar. To this solution was added IPr (0.8878 g, 2.30
mmol) in 15 mL of THF. The resulting solution changed color
from yellow to pale gray quickly, accompanied by the appear-
ance of white precipitates. After stirring for 6 h at room
temperature, the suspension solution was filtered and evapo-
rated to dryness. The residue was extracted with toluene. The
product 2 was obtained as a white powder from concentrated
toluene in ca. 55% yield (1.47 g), which was suitable for ele-
mental analysis. Mp: 119-121 °C. Anal. Calcd for C54H73Br3-
FeN4(C7H8): C, 62.84; H, 7.00; N, 4.81. Found: C, 62.94; H,
7.02; N, 4.46.
A novel anionic iron(II) complex bearing a bulky NHC
ligand, [Fe(IPr)Br3](HIPr) C7H8 (2), has been easily synthe-
3
sized for the first time by the reaction of a new imidazolium
salt of iron(II), [FeBr3(C4H8O)](HIPr) C4H8O (1), with
3
an equivalent of carbene ligand IPr. Complex 2 has been
found to show potential application as a precatalyst in the
cross-coupling reaction of aryl Grignard reagents with alkyl
halides bearing β-hydrogens under mild reaction conditions.
Further studies on the scope of this reaction and the applica-
tion of this new strategy in the synthesis of NHC complexes
of other transition metals are ongoing in our laboratory.
General Procedure for the Cross-Coupling of 4-Tolylmagne-
sium Bromide with Alkyl Bromides. A Schlenk tube was charged
with iron(II)-based precatalyst (0.05 mmol), alkyl bromide (1.00
mmol), diethyl ether (0.8 mL), and a stirring bar. The mixture
was stirred at 0 °C for 2 min. 4-Methylphenylmagnesium
bromide (1.50 mmol, 0.89 M solution in diethyl ether) was
added to this solution at 0 °C. The resulting solution changed
color to black and was then stirred for 30 min at reflux (oil bath
temperature was ca. 45 °C). After the reaction was quenched by
adding dilute hydrochloric acid (1 M, 0.5 mL), the mixture was
extracted with diethyl ether and dried with MgSO4. The solvents
were removed under reduced pressure, and the residual mixture
was separated by column chromatography by eluting with
petroleum ether (60-90 °C) to give the desired coupling pro-
Experimental Section
General Considerations. 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. Organic
reagents used for cross-coupling reactions were purchased from
Aldrich, and phenylmagnesium bromide was diluted prior to
use. Anhydrous FeBr2 was isolated from the reaction of HBr
(40%) with excess iron powder in Ar as a yellowish-brown
powder. 1,3-Bis(2,6-diisopropylphenyl)imidazolium bromide
(HIPr Br)21 and IPr22 were prepared by published methods.
3
1
Elemental analysis was performed by direct combustion on a
Carlo-Erba EA-1110 instrument. NMR spectra were measured
on a Unity Inova-400 spectrometer at 25 °C.
duct. The identity of the product was confirmed by H NMR
spectroscopy and TLC.
Structure Determination. For 1 or 2, a suitable crystal was
mounted in a thin-walled glass capillary for X-ray structural
Acknowledgment. We are grateful to the National
Natural Science Foundation of China (Grant 20772089)
for the financial support.
(20) Hatakeyama, T.; Hashimoto, S.; Ishizuka, K; Nakamura, M.
J. Am. Chem. Soc. 2009, 131, 11949–11963.
(21) Arduengo, A. J., III. U.S. Patent 5 077 414, 1991.
(22) Arduengo, A. J., III; Dias, H. V. R.; Harlow, R. L.; Kline, M.
J. Am. Chem. Soc. 1992, 114, 5530–5534.
Supporting Information Available: Crystallographic data, in
CIF format, for the structure analyses of 1 and 2. This material is
available free of charge via the Internet at http://pubs.acs.org.