Synthesis and characterization of Di- and tetracarbene iron(II) complexes with chelating N-heterocyclic carbene ligands and their application in aryl Grignard-Alkyl halide cross-coupling

Article abstract of DOI:10.1021/om200870w

A series of new and known bis(imidazolium) chloride and bromide salts bridged by either a methylene group (1-8, 10a,b) or an ethylene group (9a,b) and bearing different N substituents (Me, Et, Bn, tBu, Mes) have been reacted with [Fe{N(SiMe₃)₂}₂]₂ to yield the four-coordinate iron(II) complexes [LFeX₂] (11-20; X = Cl, Br; L = chelating bis(imidazolylidene) ligand). Molecular structures of six of these complexes have been characterized by X-ray crystallography, and selected examples have been characterized by ¹H NMR and UV-vis spectroscopy, cyclic voltammetry, Moessbauer spectroscopy, and SQUID magnetometry. In all cases the iron(II) is found in a distorted-tetrahedral environment; it is in the high-spin state and shows large quadrupole splittings in the range 3.67-4.03 mm?s^-1 (δ = 0.73-0.81 mm?s^-1). Subtleties of the metric parameters depend on the bridging unit between the two imidazolylidene groups, the peripheral N substituents, and the coligand (Cl or Br). In case of rather small (Me, Et) or flexible (Bn) N substituents the dicarbene species [LFeX₂] are formed together with ferrous tetracarbene complexes [L₂FeX₂] (21-23), which are difficult to separate and could not be isolated in pure form. When the latter are dissolved in MeCN in the presence of residual [FeBr₂(solv) _x], however, they transform into the ionic complexes [L ₂Fe(MeCN)₂][FeBr₄] (24-26), which have been characterized by single-crystal X-ray diffraction. They feature low-spin iron(II) (Moessbauer parameters δ ≈ 0.15 mm s^-1, ΔE_Q ≈ 1.36 mm s^-1) and distorted-octahedral structures with the two MeCN ligands in a cis configuration. Selected examples of the new dicarbene complexes [LFeX₂] have been tested as catalysts for the standard cross-coupling reaction between p-tolylmagnesium bromide and bromo- or chlorocyclohexane. They show moderate activity that appears to be generally lower than for related complexes with two monodentate NHC ligands, but the activities clearly depend on the peripheral N substituents and the linker between the two imidazolylidene groups; the best results are obtained for complex 19, which features a long ethylene bridge and bulky Mes substituents, and hence the most shielded metal center.

Full text of DOI:10.1021/om200870w

Article
pubs.acs.org/Organometallics
Synthesis and Characterization of Di- and Tetracarbene Iron(II)
Complexes with Chelating N-Heterocyclic Carbene Ligands and Their
Application in Aryl Grignard−Alkyl Halide Cross-Coupling
Steffen Meyer, Claudia Manuela Orben, Serhiy Demeshko, Sebastian Dechert, and Franc Meyer*
̈
̈ ̈ ̈
Institut fur Anorganische Chemie, Georg-August-Universitat Gottingen, Tammannstrasse 4, D-37077 Gottingen, Germany
*
S Supporting Information
ABSTRACT: A series of new and known bis(imidazolium)
chloride and bromide salts bridged by either a methylene
group (1−8, 10a,b) or an ethylene group (9a,b) and bearing
different N substituents (Me, Et, Bn, tBu, Mes) have been
reacted with [Fe{N(SiMe ) } ] to yield the four-coordinate
3
2 2 2
iron(II) complexes [LFeX ] (11−20; X = Cl, Br; L = chelating
2
bis(imidazolylidene) ligand). Molecular structures of six of
these complexes have been characterized by X-ray crystallo-
1
graphy, and selected examples have been characterized by H
NMR and UV−vis spectroscopy, cyclic voltammetry, Mo
̈
ssba-
uer spectroscopy, and SQUID magnetometry. In all cases the iron(II) is found in a distorted-tetrahedral environment; it is in the
−1
−1
high-spin state and shows large quadrupole splittings in the range 3.67−4.03 mm·s (δ = 0.73−0.81 mm·s ). Subtleties of the
metric parameters depend on the bridging unit between the two imidazolylidene groups, the peripheral N substituents, and the
coligand (Cl or Br). In case of rather small (Me, Et) or flexible (Bn) N substituents the dicarbene species [LFeX ] are formed
2
together with ferrous tetracarbene complexes [L FeX ] (21−23), which are difficult to separate and could not be isolated in pure
2
2
form. When the latter are dissolved in MeCN in the presence of residual [FeBr (solv) ], however, they transform into the ionic
2
x
complexes [L Fe(MeCN) ][FeBr ] (24−26), which have been characterized by single-crystal X-ray diffraction. They feature low-
2
2
4
−1
−1
̈
spin iron(II) (Mossbauer parameters δ ≈ 0.15 mm s , ΔE ≈ 1.36 mm s ) and distorted-octahedral structures with the two
MeCN ligands in a cis configuration. Selected examples of the new dicarbene complexes [LFeX ] have been tested as catalysts for
Q
2
the standard cross-coupling reaction between p-tolylmagnesium bromide and bromo- or chlorocyclohexane. They show moderate
activity that appears to be generally lower than for related complexes with two monodentate NHC ligands, but the activities clearly
depend on the peripheral N substituents and the linker between the two imidazolylidene groups; the best results are obtained for
complex 19, which features a long ethylene bridge and bulky Mes substituents, and hence the most shielded metal center.
INTRODUCTION
More generally, bidentate methylene- or ethylene-bridged
bis(imidazolylidene) ligands are quite popular scaffolds for
transition-metal catalysts, especially in palladium chemistry.
Among the reasons are the simple preparation of the respective
bis(imidazolium) salts, the precursors of NHC ligands, and the
relatively high stability of the bis(imidazolylidene) species. The
synthesis of NHC complexes is usually performed in situ by
deprotonation of the imidazolium salts and subsequent
treatment with metal complexes (Scheme 1; route A), by direct
reaction of the imidazolium salt with metal compounds containing
■
N-heterocyclic carbenes (NHCs) have gained great popularity
as ligands in organometallic chemistry and are finding
14
1
,2
widespread use in homogeneous catalysis. They are usually
viewed as strongly σ-donating ligands with little or negligible π
back-bonding. Chemical and substitutional inertness is often
mentioned among their particular advantages, and this is
expected to be further amplified by incorporating the NHC
moiety into a chelating scaffold. Mainly in the past decade a
3
−5
variety of iron complexes with monodentate NHC ligands
and several NHC ligated iron clusters have been published.
6
basic ligands (route B), or by cyclization of metal-bound isocyanides
4,5
Some of the mononuclear iron NHC complexes as well as
species generated in situ from imidazolium salts and suitable
iron precursors showed high activities in homogeneous
3l,15
(route C).
Almost all iron NHC complexes reported so far have been
3,6
7
,8
prepared via route A, while palladium NHC complexes are
catalysis. However, only a few iron complexes with chelating
bis- or tris(imidazolylidene) ligands are known so far. These
comprise iron complexes with tridentate 2,6-bis-
16
often prepared according to route B by the use of Pd(OAc)2.
In contrast, Fe(OAc) has not yet been successfully used for
2
9
10
NHC complex synthesis, and our attempts have also failed.
(
imidazolylidene)pyridine, tris(imidazolylidene)borate, and
tris(imidazolylidene)triethylamine as well as methylene-
bridged bis(imidazolylidene)
been employed in model systems for [FeFe]-hydrogenases.
11
3d,12
ligands. The latter have also
Received: September 15, 2011
Published: November 17, 2011
13
©
2011 American Chemical Society
6692
dx.doi.org/10.1021/om200870w|Organometallics 2011, 30, 6692−6702

Organometallics
Article
Scheme 1. Typical Routes for the Synthesis of Transition-
Metal NHC Complexes
salts and [Fe{N(SiMe ) } ] . Depending on the ligand
3 2 2 2
substituents, either dicarbene iron complexes or mixtures of
dicarbene and tetracarbene iron complexes are formed. For
each type of complexes several representative examples have
been analyzed by X-ray diffraction. The catalytic activity of
selected dicarbene iron complexes is examined in a benchmark
aryl Grignard−alkyl halide cross-coupling reaction. During the
final stages of the preparation of this paper, similar dicarbene
18
complexes were reported by Zlatogorsky et al.
RESULTS AND DISCUSSION
■
Synthesis of Bis(imidazolium) Salts 1−10. The prep-
aration of bis(imidazolium) salts was performed in an ACS
glass autoclave by stirring the reactants for 1−3 days at 110 °C
(
DCM) or 130 °C (THF) in analogy to the procedure reported
19
by Scherg et al., although yields of the known salts 1−3, 5, 6a,
19−24
7
(
6
a and 8a−10a
Table 1). Reaction conditions and workup of the chloride salts
b−10b as well as the phenyl-substituted bromide salt 4 have
were in most cases lower via this route
been slightly modified compared to the literature procedure
(see the Experimental Section). Prior to further use the
bis(imidazolium) salts were evaporated to absolute dryness in
Some iron NHC complex syntheses have been reported using
[
Fe{N(SiMe ) } ] or [Cp*FeN(SiMe ) ] for imidazolylidene
3 2 2 2 3 2
3g,9b,e
−3
formation and complexation.
Since the only side product,
high vacuum (10 mbar) at 100−150 °C (i.e., below the
melting point). The absence of residual H O was checked by
hexamethyldisilazane, is rather unreactive and can be removed by
2
1
17
H NMR spectroscopy. This procedure is necessary, because
evaporation, and since [Fe{N(SiMe ) } ] is readily accessible,
3
2 2 2
most bis(imidazolium) salts are hygroscopic and [Fe{N-
we have chosen route B for the present study. The bis-chelate
NHC iron(II) dihalide complexes presented herein are related to
monodentate NHC iron(II) dihalide complexes initially reported
(
SiMe ) } ] , which is used in the following step, is highly
3 2 2 2
water and oxygen sensitive. Bis(imidazolium) salts 4, 6b, 7b
and 10b have not been reported before and have been fully
characterized. 4 was analyzed by X-ray diffraction, since no
molecular structure of any CHPh-bridged bis(imidazolium) salt
has been published yet. Colorless single crystals were obtained
3b
3n
by Grubbs et al. and recently investigated by Deng et al.
Scheme 2), which in both cases were synthesized via route A.
(
Scheme 2. Dicarbene Iron(II) Dihalide Complexes Synthesized
by slow diffusion of Et O into a MeCN solution. The molecular
i
3b
2
by Grubbs et al. (X = Br, Cl; R = Pr) and Deng et al. (X = Cl;
structure of 4, which crystallizes in the monoclinic space group
P2 /n, is shown in Figure S3 (Supporting Information). The
1
i
3n
R = Me, Et, Pr)
angle formed by the two imidazolium rings is 72.9°; the rings
are bridged by C4 with an N1−C4−N3 angle of 106.9°. The
C1/5−N distances of the imidazolium rings lie within a range
of 1.32−1.34 Å with N−C−N angles of 107.7° (N3−C5−N4)
and 108.4° (N1−C1−N2). The protons H1, H4, and H5 form
rather short C−H···Br hydrogen bonds (2.57−2.82 Å) to the
counteranions; these distances are in the common range and
are in accordance with values reported for compounds with
We report the synthesis and characterization of several new
iron(II) bis-chelate NHC complexes that are accessible via in
situ carbene generation using the respective bis(imidazolium)
Table 1. Yields of Bis(imidazolium) Salts 1−10 (Left) and Iron(II) NHC Complexes 11−20 (Right)
salt
R
Y
X
found (lit.) yield/%
complex
R
Y
X
yield/%
1
1
2
9
9
0
1
2
3
4
5
Me
Me
Me
Me
Et
CH2
Br
Br
Br
Br
Br
Br
Cl
Br
Cl
Br
Cl
Br
Br
Cl
41 (83)
67 (90)
60 (70)
42
11
Me
Me
Me
Me
Et
CH2
Br
Br
Br
Br
Br
Br
Cl
Br
Cl
Br
Cl
Br
Br
Cl
(CH2)2
(CH2)3
CHPh
CH2
12
(CH2)2
(CH2)3
CHPh
CH2
84
80
13
14
2
2
1
2
78 (80)
64 (78)
59
15
65
79
44
83
90
82
86
68
85
63
6
6
7
7
8
8
9
1
1
a
b
a
b
a
b
Bn
Bn
CH2
16a
16b
17a
17b
18a
18b
19
Bn
Bn
CH2
CH2
CH2
t
19
t
Bu
CH2
44 (79)
35
Bu
CH2
tBu
CH2
tBu
CH2
1
2
2
9
3
4
Mes
Mes
Mes
Ad
CH2
84 (68)
53 (76)
68 (45)
Mes
Mes
Mes
Ad
CH2
CH2
CH2
(CH2)2
CH2
(CH2)2
CH2
1
9
0a
0b
61 (67)
59
20a
20b
Ad
CH2
Ad
CH2
6
693
dx.doi.org/10.1021/om200870w|Organometallics 2011, 30, 6692−6702

Organometallics
Article
Scheme 3. Preparation of Dicarbene Iron(II) Dihalide Complexes from Bis(imidazolium) Salts
similar C−H···Br interactions and with related bis-
were not carried out. Cyclic voltammetry was performed for
17a, 18a,b, and 19 in MeCN or DCM (Table 2). All these
25,26
(
imidazolium) salt derivatives.
Synthesis of Dicarbene Iron(II) Complexes 11−20. A
suspension of the respective bis(imidazolium) salt in Et O or
a
Table 2. Electrochemical Data for Selected Complexes
2
THF is treated with [Fe{N(SiMe ) } ] , and the ferrous NHC
Epa
Epc
3
2
2 2
complex
solvent
complex precipitates during the reaction (Scheme 3). Filtration
and rinsing affords the desired product as a white to brownish
powder, usually in around 80% yield. The yields of 11, 14, 15,
and 16b are derogated by tetracarbene complex byproduct
formation (see below); in the case of 11 no pure material could
be isolated. In some cases further purification was hampered by
the low solubility and limited stability in solution. The solid
complexes 12−15 are quite air sensitive, and complexes 16
and 17 decompose within a couple of minutes up to 1 h in air,
while complexes 18−20 with sterically demanding substituents
1
1
1
1
7a
8a
8b
9
+1.11
+1.06
+1.48
+1.07
−1.74
MeCN
DCM
MeCN
DCM
−0.73
−1.99
a
Peak potentials of irreversible processes in V vs NHE at 100 mV/s.
complexes show irreversible processes in both oxidation and
reduction. The first oxidation (anodic peak potential E ) is
a
p
observed at around +1.1 V vs NHE for several [LFeBr2]
complexes (the cyclic voltammogram of 17a is shown in Figure
S1 (Supporting Information) as an example), but at +1.48 V for
chloride complex 18b. Apparently the corresponding ferric
complexes are unstable or major structural changes occur upon
oxidation of the ferrous complexes. We also tried to synthesize
the corresponding dicarbene iron(III) species directly when
using 8a and [Fe{N(SiMe ) } ] instead of [Fe{N(SiMe ) } ] ,
but it was not possible to identify or isolate any dicarbene
iron(III) complex. Instead, reduction occurred and iron(II)
NHC complex 18a crystallized from the reaction mixture at
30 °C. These observations suggest that the potential
dicarbene iron(III) halide species are not accessible, in
accordance with electrochemical findings.
(
Mes and Ad) are air stable for several hours. In solution
all these NHC complexes quickly decompose in the presence
of air.
The solubility of the new dicarbene iron(II) dihalide
complexes in nonpolar solvents is generally poor, while polar
and particularly protic solvents induce decomposition. They are
insoluble in alkanes and toluene, sparingly soluble in DCM and
THF, and soluble but slowly decompose in MeCN, DMF, and
DMSO. Complexes with bis(imidazolylidene) ligands bearing
mesityl and benzyl substituents (16a,b, 18a,b, and 19)
represent exceptions, since they show better solubility in
3
2
3
3 2 2 2
−
DCM. Bulk crystallization from DCM, CHCl , THF, and
3
dioxane in most cases was not successful; only 19 recrystallizes
from DCM. Appending a phenyl group at the backbone
methylene bridge (14) slightly increases the solubility. A similar
effect was also observed for tetracarbene complexes 21
Mossbauer spectroscopy of complexes 12, 13, and 15−20 at
̈
−1
80 K showed isomer shifts of 0.73−0.81 mm s and large
quadrupole splittings in the range 3.67−4.03 mm s− (Table 3),
1
(
insoluble in DCM) and 22 (phenyl substituted, soluble in
Table 3. Mo
Complexes at 80 K
̈
ssbauer Parameters of Selected Dicarbene Iron(II)
DCM; see below). As a consequence, the purification and
characterization of almost all dicarbene iron(II) complexes 11−
δ (mm s−1
ΔEQ (mm s−1
Γ (mm s−1
)
complex
)
)
20 is limited by the poor solubility or stability in solution.
1
1
1
1
1
1
1
1
1
1
2
2
2
0.73
0.74
0.74
0.73
0.74
0.79
0.80
0.74
0.77
0.77
0.81
0.78
3.94
3.75
3.96
3.93
3.96
3.78
3.87
4.03
3.88
3.91
3.67
3.79
0.28
0.27
0.26
0.26
0.29
0.31
0.29
0.26
0.27
0.26
0.26
0.28
However, the purity of most complexes after filtration, rinsing,
and evaporation of the remaining solvents was quite good, as
3
5
indicated by elemental analysis (EA) and Mo
copy (MB, see below).
̈
ssbauer spectros-
6a
6b
7a
7b
8a
8b
9
Spectroscopic, Electrochemical, and Magnetic Char-
acterization of Complexes 12−20. From the series of new
dicarbene iron(II) dihalide complexes the molecular structures
of 14, 15, 16b, 17a, 18a, and 19 were determined by X-ray
diffraction. All complexes were characterized by EI-MS, MB,
and EA (except for 11, 14, and 17b, which contained significant
0a
0b
amounts of impurities). Those complexes soluble in DCM
1
(
17a, 18a,b, and 19) were additionally characterized by H
which is at the upper end of the 3−4 mm s− range typical for
high-spin iron(II) in a tetrahedral environment (S = 2).
This comparison refers to iron complexes with thiourea or
1
NMR and UV−vis spectroscopy.
1
26b,27
H NMR spectra of 17a in DMSO-d and 18a,b and 19 in
6
CD Cl showed broad signals in the range of −5 to 60 ppm.
2
2
1
Except for the H NMR spectrum of 18a, the number of signals
and their integrals did not match expectations, likely due to
paramagnetic broadening beyond detection for some signals.
thiolate ligands, since Mossbauer data of tetrahedral iron NHC
̈
10g
complexes are scarce. The high-spin state (S = 2) was further
corroborated by a SQUID measurement of 18a, giving μeff = 5.1
1
3
Paramagnetic signal broadening also prevented reasonable
C
μ (Figure 1). Mossbauer spectra of all iron complexes with
̈
B
NMR measurements; hence, C−H correlation experiments
bromide ligands show rough backgrounds due to a strong
6
694
dx.doi.org/10.1021/om200870w|Organometallics 2011, 30, 6692−6702

Organometallics
Article
̈
Figure 1. Mossbauer spectra of 18b at 80 K (left) and SQUID data for 18a at 0.5 T (right). The solid lines represent the calculated curve fits.
absorption of γ radiation by bromine. According to Mo
spectroscopy most of the new complexes did not contain any or
less than 5% iron impurities.
̈
ssbauer
in 17a, bearing bulky tert-butyl substitutents, are they slightly
longer (2.13 Å), presumably for steric reasons. In complexes with
methylene-bridged bis(imidazolylidene) ligands, the six-membered
chelate rings adopt a distorted boatlike conformation. X−Fe−X
(X = Cl, Br) and C−Fe−C angles are wider for X = Br than for
X = Cl due to the larger atom radius of bromide; the C−Fe−C
angle furthermore depends on the peripheral NHC substituents
Solid-State Structures. Single crystals of 14 were
obtained by cooling a DCM solution to −30 °C. Complex
15 was crystallized by slow evaporation of pentane into a DCM
solution at 8 °C. A few tiny single crystals of 17a were obtained
by crystallization from hot MeCN solution; this method is not
convenient for bulk recrystallization, however, since most of the
dicarbene iron complex decomposes. 18a crystallized from the
reaction mixture of 8a and Fe[N(SiMe ) ] in THF at −30 °C.
3
2 3
Complexes 16b and 19 were crystallized by slow evaporation
of pentane into a THF solution at 8 °C. Molecular
structures are shown in Figures 2−5 and Figures S4 and S5
Figure 3. ORTEP plot (50% probability thermal ellipsoids) of the
molecular structure of 16b. Hydrogen atoms have been omitted for
clarity. Selected bond lengths (Å) and angles (deg): Fe1−C1 =
2
2
.107(3), Fe1−C5 = 2.111(3), Fe1−Cl2 = 2.2631(9), Fe1−Cl1 =
.2769(8); C1−Fe1−C5 = 88.45(11), C1−Fe1−Cl2 = 114.38(8),
C5−Fe1−Cl2 = 114.61(9), C1−Fe1−Cl1 = 111.30(8), C5−Fe1−Cl1 =
115.35(8), Cl2−Fe1−Cl1 = 111.06(3).
Figure 2. ORTEP plot (50% probability thermal ellipsoids) of the
molecular structure of 14. Hydrogen atoms and solvent molecules
have been omitted for clarity. Selected bond lengths (Å) and angles
(
2
deg): Fe1−C1 = 2.093(4), Fe1−C5 = 2.093(4), Fe1−Br1 =
.4095(6), Fe1−Br2 = 2.4274(6); C1−Fe1−C5 = 89.18(14), C1−
Fe1−Br1 = 115.88(10), C5−Fe1−Br1 = 113.86(10), C1−Fe1−Br2 =
11.80(10), C5−Fe1−Br2 = 109.10(10), Br1−Fe1−Br2 = 114.34(2).
1
(
Supporting Information). Complex 17a crystallizes in the
orthorhombic space group P2 2 2 ; the other five complexes
1
1 1
crystallize in the monoclinic space groups P2 (18a), P2 /n (14,
Figure 4. ORTEP plot (50% probability thermal ellipsoids) of the
molecular structure of 17a. Hydrogen atoms have been omitted for
clarity. Selected bond lengths (Å) and angles (deg): Fe1−C5 =
1
1
1
5), and P2 /c (16b, 19). In all complexes the iron atom is
1
coordinated by the chelating NHC ligand and two halides in a
distorted-tetrahedral geometry. The average iron−carbon distances
of around 2.10 Å are in good agreement with Fe−C bond lengths
2
2
.126(6), Fe1−C1 = 2.128(6), Fe1−Br1 = 2.4490(9), Fe1−Br2 =
.4549(10); C5−Fe1−C1 = 95.6(2), C5−Fe1−Br1 = 113.57(14),
C1−Fe1−Br1 = 112.03(15), C5−Fe1−Br2 = 106.17(15), C1−Fe1−
Br2 = 105.15(15), Br1−Fe1−Br2 = 121.02(4).
3a,d
of other tetrahedral bis(imidazolylidene) iron complexes; only
6
695
dx.doi.org/10.1021/om200870w|Organometallics 2011, 30, 6692−6702

Organometallics
Article
complexes upon reaction with [Fe{N(SiMe ) } ] in THF, but
3
2 2 2
in lower amounts compared to those of 1, 4, and 5. ESI-MS
confirmed the formation of tetracarbene iron(II) complexes
when 11 and 14−16 were treated with MeCN. In the Mo
̈
ssbauer
spectrum of 24 (Figure 6) signals for both the tetracarbene
−1
dication (low-spin (ls) iron(II) with δ = 0.15 mm s , ΔE =
Q
−
1
−1
1
.36 mm s , Γ = 0.38 mm s ) and the tetrabromoferrate(II)
−1
anion (high-spin (hs) iron(II) with δ = 1.05 mm s , ΔE = 2.88
Q
−1
−1 28
29
mm s , Γ = 0.51 mm s ) are found. NMR measurements
were hampered by the paramagnetic tetrabromoferrate(II),
though the tetracarbene complex itself is diamagnetic. Attempts
to exchange the [FeBr4] for PF6− or BF4− have failed so far.
The molecular structures of 24−26 (Figure 7 and Figures S6
and S7 (Supporting Information)) were determined by X-ray
diffraction. In all cases yellow single crystals could be grown by
2−
Figure 5. ORTEP plot (50% probability thermal ellipsoids) of the
molecular structure of 19. Hydrogen atoms and the solvent molecule
have been omitted for clarity. Selected bond lengths (Å) and angles
slow diffusion of Et O into MeCN solutions of the complexes.
2
(
2
deg): Fe1−C1 = 2.100(6), Fe1−C6 = 2.108(5), Fe1−Br2 =
.4410(11), Fe1−Br1 = 2.4433(11); C1−Fe1−C6 = 103.7(2), C1−
Fe1−Br2 = 106.95(16), C6−Fe1−Br2 = 106.08(14), C1−Fe1−Br1 =
09.62(16), C6−Fe1−Br1 = 110.72(16), Br2−Fe1−Br1 = 118.66(4).
2
4 crystallizes in the monoclinic space group P2 /m, 25 in
1
̅
triclinic P1, and 26 in orthorhombic Ama2. All three complexes
exhibit very similar structural parameters. The iron(II) ion is
found in an octahedral environment, with the coordination
sphere composed of two bis(imidazolylidene) ligands and two
1
(
Table 4). In case of the ethylene bridge (19) the C−Fe−C angle
2
−
is found to be the largest in this series (103.7°).
acetonitrile molecules in a cis configuration. The [FeBr4]
Synthesis and Characterization of Tetracarbene Iron-
II) Complexes. When bis(imidazolium) salts that have small
counteranion is associated with the NHC complex cation by
two or more C−H···Br hydrogen bonds.
(
(
Me, Et) substituents are reacted with [Fe{N(SiMe ) } ] ,
Fe−C distances trans to MeCN ligands are in the range 1.92−
1.95 Å, while those trans to another NHC moiety are in the range
1.96−1.99 Å, reflecting the strong NHC trans influence. These
bond lengths are slightly longer than Fe−C distances (1.90−1.96
Å) reported for octahedral tetracarbene iron complexes with two
tridentate {CNC} donor ligands (bis(imidazolylidene)pyridine
3
2 2 2
octahedral tetracarbene iron(II) complexes are formed as a
byproduct in addition to the desired dicarbene iron(II)
complexes, giving the otherwise white suspension a distinct
color (21, red; 22, green; 23, yellow; Scheme 4). Due to the
similar solubilities of di- and tetracarbene complexes, their
separation and isolation proved difficult. Dicarbene 11 and
tetracarbene 21, for example, precipitate as a mixture during the
reaction, but neither of them was found to be soluble in any
solvent without either ligand exchange or decomposition;
therefore, they could not be separated and characterized. In the
case of 22 separation was achieved, although no analytically
pure bulk material could be obtained. Instead, the identity of
the related tetracarbene complexes 24−26 was elucidated by X-
ray diffraction of single crystals that grew upon slow diffusion of
9
a,b
iron(II) complexes).
Fe−C distances of 1.99−2.00 Å were
10a,b
previously found in hexacarbene iron(II) complexes.
In
comparison to Fe−C distances of the dicarbene iron(II)
complexes discussed above (2.09−2.13 Å), bond lengths in
24−26 are 0.11−0.18 Å shorter, since the iron(II) ion is in its
low-spin state and anionic ligands (halides) are absent. C−Fe−C
and C−Fe−N angles of trans-standing ligands are found in the
range 172.6−179.2°, thus indicating only slight distortion from
octahedral geometry for all three complexes 24−26.
Et O into MeCN solutions of the crude products 21−23.
Cross-Coupling Catalysis. Transition-metal-catalyzed
Grignard cross-coupling reactions have become a well-established
2
It was found that complexes 21−23 exchange both bromide
ligands upon treatment with acetonitrile, leading to the
tetracarbene iron(II) salts [L Fe(MeCN) ][FeBr ] (24−26).
3
0,31
C−C coupling method in recent years.
The Kumada−Curriu
coupling of aryl Grignard reagents with primary or secondary
alkyl halides is typically performed with Ni and Pd catalysts.
2
2
4
32,33
The tetrabromoferrate(II) anion is formed from the two
bromide ions and residual solvated FeBr . The behavior of 14
Due to economical and environmental aspects, however, iron-
2
in DCM solution suggests that this dicarbene complex may be in
dynamic equilibrium with the corresponding tetracarbene
complex 22 and [FeBr (solv) ] via bis(imidazolylidene) ligand
catalyzed cross-coupling reactions have become an attractive
34
alternative. Inspired by the work of Kochi et al., several groups
have reported iron-based catalysts that show good activities and
surmount significant β-hydride elimination product formation,
which is the main limitation if primary and especially secondary
2
x
transmetalation. The position of the equilibrium seems to
depend on solvent, concentration and temperature. In MeCN
the driving force supposably is irreversible [L Fe(MeCN) ]-
4,8
alkyl halides are used. In general, transition-metal complexes
2
2
[
FeBr ] salt formation. Bis(imidazolium) salts with benzyl
with NHC ligands have proven to be effective catalysts for C−C
4
1,2
residues (6a,b) are flexible enough to also form tetracarbene
coupling reactions. However, among the various iron(III) and
Table 4. Selected Bond Lengths and Angles of Several Dicarbene Iron(II) Dihalide Complexes
complex
X
ϕ(Fe−C) (Å)
ϕ(Fe−X) (Å)
C−Fe−C (deg)
X−Fe−X (deg)
N−CH −N (deg)
2
1
1
1
1
1
1
4
Br
Br
Cl
Br
Br
Br
2.093
2.098
2.109
2.127
2.113
2.104
2.418
2.423
2.270
2.452
2.407
2.442
89.18(14)
90.01(15)
88.45(11)
95.6(2)
114.34(2)
120.42(3)
111.06(3)
121.02(4)
117.56(2)
118.66(4)
111.4(3)
111.6(3)
111.7(2)
114.1(4)
110.6(2)
5
6b
7a
8a
9
87.42(12)
103.7(2)
6
696
dx.doi.org/10.1021/om200870w|Organometallics 2011, 30, 6692−6702

Organometallics
Article
Scheme 4. Formation of Tetracarbene Iron(II) Complexes
iron(II) catalysts tested in aryl Grignard−alkyl halide cross-
coupling reactions, only two well-defined systems with NHC
ligands have yet been reported: namely, the pyridyl bis(carbene)
All of the studied iron NHC complexes mediate the coupling
reaction (Table 5), albeit with clearly different catalytic
activities. In the case of complexes 17a and 20a, which are
8g
pincer complex by Bedford et al. and a monodentate NHC
insoluble in Et O, reactions were performed in THF instead,
2
4
complex by Gao et al. Both complexes, as well as some in situ
but this led to higher amounts of β-hydride elimination product
generated iron NHC catalysts, showed above 90% conversion to
the coupled product. These findings motivated preliminary
studies on the catalytic activity of selected examples of the new
dicarbene iron complexes reported herein that contain potentially
more rugged bidentate NHC chelate ligands.
(
IV), hydrodehalogenation product (V), and homocoupling
product (II) (entries 9 and 10).
Figure 7. ORTEP plot (40% probability thermal ellipsoids) of the
molecular structure of 24. Hydrogen atoms, solvent molecules, and the
2
−
[
FeBr4] counterion have been omitted for clarity. Selected bond
lengths (Å) and angles (deg): Fe1−C1 = 1.931(5), Fe1−C11 =
1
1
.938(5), Fe1−C15 = 1.965(5), Fe1−N5 = 1.972(5), Fe1−N6 =
.983(4), Fe1−C5 = 1.986(5); C1−Fe1−C11 = 90.4(2), C1−Fe1−
C15 = 91.5(2), C11−Fe1−C15 = 86.9(2), C1−Fe1−N5 = 176.4(2),
C11−Fe1−N5 = 90.52(19), C15−Fe1−N5 = 91.99(19), C1−Fe1−
N6 = 96.09(19), C11−Fe1−N6 = 172.54(19), C15−Fe1−N6 =
2
9
̈
Figure 6. Mossbauer spectrum of 24 at 80 K.
8
9.30(19), N5−Fe1−N6 = 83.22(17), C1−Fe1−C5 = 86.6(2), C11−
The cross-coupling reaction of p-tolylmagnesium bromide
with bromo- or chlorocyclohexane (Scheme 5) was chosen as a
test reaction because it is a common benchmark example of an
aryl Grignard−secondary alkyl halide coupling. The reaction
Fe1−C5 = 94.5(2), C15−Fe1−C5 = 177.6(2), N5−Fe1−C5 =
8
9.94(19), N6−Fe1−C5 = 89.6(2).
The highest activity was found for complex 19 with 75%
conversion at 5 mol % catalyst loading (entry 4). Reduction of
the loading to 1 mol % went along with a decreased conversion
8
d,g
may lead to up to five products (I−V).
By NMR only
heterocoupled product I and homocoupled product II could be
identified in the product mixtures. The formation of trace
amounts of byproducts III−V was detected by GC/MS of the
organic phase after workup; but only for runs in THF and runs
with chlorocyclohexane are the amounts of III−V non-
negligible. Initial screening of the reaction conditions (solvent,
temperature, duration) led to the same favourable procedure that is
commonly followed for most aryl Grignard−alkyl halide couplings
of 66%. In situ generation of 19 by the reaction of FeBr with
2
bis(imidazolium) salt 9 in the presence of p-tolylmagnesium
bromide surprisingly led to slightly higher conversion of 79%
(entry 3). Other dicarbene complexes as well as catalysts
generated in situ from bis(imidazolium) salts and FeBr were
2
tested, but none of them outperformed precatalyst 19. It should
be noted that 19 features the most shielded metal center of the
present series of dicarbene complexes, because of the long
4,8c,d
(
Et O, reflux, 0.5 h).
2
6
697
dx.doi.org/10.1021/om200870w|Organometallics 2011, 30, 6692−6702

Organometallics
Article
Scheme 5. Transition-Metal-Catalyzed Reaction between
Bromo- or Chlorocyclohexane and p-Tolylmagnesium
Bromide
by the Grignard reagent to give the active iron species. Reports
about the use of Fe/NHC cross-coupling catalysts are still
relatively rare, but some previously reported iron catalysts with
2
equiv of monodentate NHC ligands tested under similar
conditions (catalysts prepared in situ from FeBr and 2 equiv of
2
the respective imidazolium salt) showed somewhat higher
4
activities than the new complexes investigated here. Possibly
the constrained chelate arrangement of the present bis-
(
imidazolylidene) ligands is less favorable for adapting to the
different stereoelectronic requirements of iron in its various
redox states during the catalytic cycle. Higher activity was also
observed for the anionic [Fe(NHC)Br3]− precatalyst, which4
was ascribed to the pronounced nucleophilicity of that species.
It remains to be investigated, however, whether the NHC
ligands indeed remain bound to the active Fe species during
turnover, or whether other species (such as nanoparticulate Fe)
are formed.
ethylene linker between the two imidazolylidene groups and
the bulky Mes substituents. Formation of the homocoupled
product II was usually found in the range of 5−8%.
CONCLUSIONS
An efficient synthesis of bis-chelate NHC iron(II) dihalide
■
[
FeBr (thf) ] (entry 2) was used for comparison, giving 69%
2
2
conversion to I; simple FeBr was previously reported to yield
complexes [LFeX
2
] (X = Cl, Br) has been developed using
2
4
[
Fe{N(SiMe ) } ] as a basic precursor for in situ deprotona-
4
1% conversion under identical conditions. In addition to
3 2 2 2
tion of the respective bis(imidazolium) salts. Preferential
tetracarbene complex formation was observed if the ligand
bears small (Me, Et) or flexible (Bn) substituents. Complexes
bromocyclohexane, the less reactive chlorocyclohexane was also
used as substrate (entries 11−13). In this case the chloride-
ligated complex 18b did not give reasonable conversion (15%),
while catalyst 19 and the in situ generated analogue (9 + FeBr2)
both provided conversions of around 60%. However, in
[
L FeX ] (in the presence of residual [FeBr (solv) ]) transform
2 2 2 x
into [L Fe(MeCN) ][FeBr ] when dissolved in MeCN.
2
2
4
Molecular structures of several dicarbene (14, 15, 16b, 17a,
8a, 19) and tetracarbene iron(II) complexes (24−26) as well
1
Table 5. Iron-Catalyzed Coupling of Bromo- or Chloro-
as of the precursor bis(imidazolium) salt 4 have been
determined crystallographically. Complex 19 was found to be
the catalytically most active compound in a standard aryl
Grignard−alkyl halide cross-coupling reaction, but activities are
generally lower than previously reported activities of related
complexes with two monodentate NHC ligands; in the
previous work, however, precatalyst complexes usually had
not been isolated and fully characterized. The new dicarbene
iron(II) complexes reported here may be valuable starting
materials for the synthesis of iron clusters and may be tested in
further iron-mediated catalytic reactions. Work in that direction
is in progress.
a
cyclohexane with p-Tolylmagnesium Bromide
b
conversion to compd
(%)
cat. loading
(mol %)
entry
cat.
FeBr2
I
II
4
1
5
41
69
79
75
66
56
62
46
30
5
2
3
4
5
6
7
8
FeBr (thf)2
5
5
2
FeBr + 9
5 + 5
8
5
2
19
5
19
1
6
12
5
7
18a
16a
17a
20a
5
5
EXPERIMENTAL SECTION
5
6
■
c
9
5
15
9
General Considerations. Air-sensitive reactions were carried out
under dry argon using standard Schlenk techniques, or in a glovebox
(MBRAUN LabMaster) under a nitrogen atmosphere with less than
0.1 ppm of O and H O. Solvents were dried and degassed by standard
c
10
11
12
13
5
d
d
d
FeBr + 9
5 + 5
60
60
15
11
14
5
2
2
2
19
5
5
procedures before use. Bromocyclohexane and chlorocyclohexane
were degassed and dried over molecular sieves; all other chemicals
were purchased from commercial sources and used as received.
37
18b
a
Conditions: bromocyclohexane (1.0 mmol), p-toluolmagnesium
b
3
6
t
37
38
37
bromide (1.5 mmol), catalyst, Et O, reflux, 30 min. Conversion to
products I and II determined by H NMR (1,3,5-trimethoxybenzene
as internal standard), average of two trials. Solvent THF; no
2
Ethylimidazole (Et-Im),
Bu-Im, Bn-Im, Mes-Im, Ad-Im,
1
17
and [Fe{N(SiMe ) } ]
were prepared according to published
3
2 2 2
c
procedures. The bis(imidazolium) salts 1−3, 5, 6a, 7a, 8a, 9, and
conversion observed in Et O for these complexes, due to insolubility.
10a were synthesized following the general procedure for methylene-
2
d
19
Chlorocyclohexane used instead of bromocyclohexane.
bridged imidazolium salts reported by Scherg et al. Spectral data for
2
0
21
22
23
24
1
3
, 5, 6a, 8b, and 9 were found as previously reported. H and
1
3
1
contrast to reactions with bromocyclohexane, considerably
more homocoupled product II was generated.
C{ H} NMR spectra were recorded on a Bruker Avance DRX 500 or
Bruker Avance 300 spectrometer at 25 °C; the chemical shifts (δ) are
given in ppm relative to TMS. Mass spectrometry was performed with
a Finnigan MAT 8200 (EI), an Applied Biosystems API 2000 (ESI), or
a Bruker HCT ultra (ESI) instrument. UV−vis spectra were collected
with a Varian Cary 50 Bio instrument using Schlenk quartz cuvettes.
GC/MS spectra were measured on a Thermo Finnigan Trace GC
using EI-MS. Melting points were determined in glass capillary tubes
on a Stanford Research Systems OptiMelt MPA 100 device; values are
Different views still exist about the mechanism of Fe-
8g,31
catalyzed cross-coupling reactions.
Mechanistic proposals
comprise oxidative addition/reductive elimination cycles with
Fe shuttling between the Fe(0)/Fe(II) or Fe(−II)/Fe(0)
3
5
8a,b
states, as well as radical-based scenarios.
In all cases the
actual catalytic cycle is likely preceded by precatalyst reduction
6
698
dx.doi.org/10.1021/om200870w|Organometallics 2011, 30, 6692−6702

Organometallics
Article
uncorrected. Mo
source in a Rh matrix using an alternating constant-acceleration Wissel
Mossbauer spectrometer operated in the transmission mode and
equipped with a Janis closed-cycle helium cryostat. Isomer shifts δ are
̈
ssbauer (MB) spectra were recorded with a 57Co
dec. MS (ESI , MeOH): m/z 335 [M − Br] , 253 [M − 2Br] , 171
+
+
+
+
[M − 2Br − CH − Im] .
3
̈
Synthesis of 1,1-Bis(3-benzylimidazolium-1-yl)methane Dichlor-
ide (6b). A solution of 1-benzylimidazole (3.00 g, 19.0 mmol) in
dichloromethane (50 mL) was stirred in a 100 mL ACE glass
autoclave at 110 °C for 3 days. The precipitate was filtered off and
washed with DCM (3 × 5 mL). Removal of the solvent under reduced
pressure at 150 °C yielded the bis(imidazolium) salt (2.26 g, 5.61
−
1
given in mm s relative to iron metal at ambient temperature. The
quadrupole splitting ΔE and full width at half-maximum Γ are given
Q
−
1
in mm s . Simulation of the experimental data was performed with
3
9
the Mfit program. Magnetic susceptibility measurements were
carried out on a Quantum-Design MPMS-XL-5 SQUID magneto-
meter equipped with a 5 T magnet in the range 2−300 K. The
powdered samples were contained in a gel bucket and fixed in a
nonmagnetic sample holder. Each raw data file for the measured
magnetic moment was corrected for the diamagnetic contribution of
the sample holder and the sample. Simulation of the experimental
mmol, 59%) as a hygroscopic white powder.
1
H NMR (300 MHz, DMSO-d ): δ 9.97 (s, 2H, NCHN), 8.25 (s,
6
2H, NCHCH), 7.91 (s, 2H, NCHCH), 7.41−7.45 (m, 10H, Ph), 6.82
1
3
1
(s, 2H, NCH
DMSO-d ): δ 138.8 (NCHN), 134.4 (NCH
(Ph), 130.1 (Ph), 124.8 (NCHCH), 123.8 (NCHCH), 60.1
N), 5.50 (s, 4H, CH ). C{ H} NMR (75 MHz,
2
2
Ph
C ), 130.7 (Ph), 130.5
2
6
40
+
+
magnetic data was performed with the julX program. Elemental
analyses were performed by the analytical laboratory of the Institute of
(NCH
329 [M − 2Cl − H] , 239 [M − 2Cl − Bn] . Mp: 260 °C dec. Anal.
Calcd for C21 : C, 62.85; H, 5.53; N, 13.96. Found: C, 62.08;
H, 5.40; N, 14.10.
2
N), 54.7 (CH+ Ph). MS (ESI , MeOH): m/z 365 [M − Cl] ,
2
+
Inorganic Chemistry at the Georg-August-University Go
an Elementar Vario EL III instrument.
̈
ttingen using
H22Cl N
2 4
X-ray Diffraction. X-ray data were collected on a STOE IPDS II
diffractometer (graphite-monochromated Mo Kα radiation, λ = 0.710
3 Å) by use of ω scans at −140 °C (Tables S1 and S2, Supporting
Synthesis of 1,1-Bis(3-tert-butylimidazolium-1-yl)methane Di-
chloride (7b). A solution of tert-butylimidazole (3.00 g, 24.2 mmol)
in dichloromethane (50 mL) was stirred in a 100 mL ACE glass
autoclave at 110 °C for 3 days. All volatiles were removed under
reduced pressure, and the residue was dissolved in boiling MeCN (50
7
Information). The structures were solved by direct methods and
refined on F2 using all reflections with SHELX-97. Most non-
hydrogen atoms were refined anisotropically. Most hydrogen atoms
were placed in calculated positions and assigned to an isotropic
41
mL). Into the warm MeCN solution was dropped Et O until
2
precepitation started. After 3 h at room temperature the mixture was
filtered and the filtrate stored at −20 °C overnight. The product
precipitated as a white solid. After filtration, a second crop was
2
displacement parameter of 0.08 Å . The positional and isotropic
thermal parameters of the hydrogen atoms H1, H4, and H5 in 4 were
refined freely. One of the [FeBr4]2 counterions and one Et O solvent
−
obtained by addition of Et O to the mother liquor and subsequent
2
2
cooling. Filtration and removal of the solvents under reduced pressure
molecule in 24 were found to be disordered and therefore refined with
half-occupancy. DFIX restraints (dC−C = 1.51 Å, dC−O = 1.42 Å) were
applied to model the disorder of the solvent molecule. All carbon and
oxygen atoms of the disordered part were refined isotropically. The
disorder of the two MeCN molecules in 25 was modeled by using
SAME restraints (occupancy factors 0.39(3)/0.61(3) and 0.57(3)/
at 120 °C yielded the bis(imidazolium) salt (1.41 g, 4.24 mmol, 35%)
as a hygroscopic white powder.
1
H NMR (300 MHz, DMSO-d ): δ 10.45 (pseudo-t, 2H, J = 1.6 Hz,
6
NCHN), 8.52 (pseudo-t, 2H, J = 1.9 Hz, NCHCH), 8.17 (pseudo-t,
2H, J = 1.9 Hz, NCHCH), 6.83 (s, 2H, NCH N), 1.60 (s, 18H, CH ).
): δ 136.7 (NCHN), 122.6
(NCHCH), 120.6 (NCHCH), 60.1 (NCH N), 57.2 (C ), 28.8 (CH ).
MS (ESI , MeOH): m/z 297 [M − Cl] , 261 [M − 2Cl − H] , 205
2
3
1
3
1
0
.43(3)). All atoms of the disordered parts were refined isotropically.
C{ H} NMR (75 MHz, DMSO-d
6
All CH Cl solvent molecules in 14 are disordered and were refined
2
q
3
2
2
+
+
+
using DFIX restraints (dC−Cl = 1.79 Å, dCl···Cl = 2.9 Å) and EADP
t
+
t
+
constraints. One CH Cl was found to be disordered about a center of
[M − 2Cl − Bu] , 149 [M − 2Cl − 2 Bu + H] . Mp: ∼140 °C dec.
Anal. Calcd for C15 : C, 54.05; H, 7.86; N, 16.81. Found C,
52.90; H, 7.90; N, 16.80.
2
2
inversion and was refined with a fixed occupancy of 0.5. The other
solvent molecule was found to be disordered about two positions
H26Cl N
2 4
(
occupancy factors: 0.490(3)/0.510(3)). The disordered parts of two
Synthesis of 1,1-Bis(3-adamantylimidazolium-1-yl)methane Di-
THF solvent molecules in 18a were refined isotropically. The absolute
structure parameters for 17a (0.021(16)) and 26 (0.020(10)) were
determined with SHELXL-97 according to Flack. Face-indexed
absorption corrections were performed numerically with the program
chloride (10b). Synthesis was performed according to the general
procedure described above.
Yield: 3.61 g (7.38 mmol, 59%). H NMR (300 MHz, DMSO-d
42
1
): δ
6
10.34 (br s, 2H, NCHN), 8.41 (br s, 2H, NCHCH), 8.18 (br s, 2H,
43
Ad
X-RED.
NCHCH), 6.80 (br s, 2H, NCH N), 2.21 (br s, 6H, CH ), 2.06 (br s,
2
Ad
Ad
13
1
General Procedure for the Synthesis of Methylene-Bridged
Bis(imidazolium) Chlorides. A solution of the N-substituted
imidazole (25.0 mmol) in dichloromethane (50 mL) was stirred in a
12H, CH
DMSO-d
59.8 (NCH2N), 57.5 (NC ), 41.3 (NCCH
2
), 1.71 (br s, 12H, CH
2
). C{ H} NMR (75 MHz,
): δ 136.1 (NCHN), 122.6 (NCHCH), 119.8 (NCHCH),
), 34.8 (CH ), 28.8
2
(CH ). Mp: >300 °C. MS (ESI , MeOH): m/z 453 [M − Cl] , 417
6
Ad
Ad
Ad
2
Ad
+
+
1
00 mL ACE glass autoclave at 110 °C for 3 days. All volatiles were
removed under reduced pressure. The residue was refluxed in MeCN
50 mL) for some minutes and cooled to room temperature. The
+ + +
[M − 2Cl − H] , 283 [M − 2Cl − Ad] , 135 [Ad] .
(
General Procedure for the Synthesis of Dicarbene Iron(II)
Dihalide Complexes. A suspension of the corresponding bis-
(imidazolium) salt (1.00 mmol) in THF (10 mL) was treated with
precipitate was filtered off and washed with MeCN (2 × 5 mL).
Evaporation of the solvent under high vacuum at 100−150 °C (below
the melting point) yielded the bis(imidazolium) salt as a hygroscopic
[Fe{N(SiMe
temperature overnight. The precipitate was filtered off, washed with
THF (2 × 3 mL) and Et O (2 × 3 mL), and then dried in vacuo. The
) } ] (500 mg, 0.67 mmol) and stirred at room
3 2 2 2
19
white powder. The synthesis was adapted from Scherg et al.
Synthesis of Phenylbis(3-methylimidazolium-1-yl)methane Di-
bromide (4). A solution of 1-methylimidazole (5.00 g, 60.9 mmol, 2.2
equiv) and α,α′-dibromotoluene (6.92 g, 27.7 mmol) was dissolved in
MeCN (60 mL). The reaction mixture was heated to reflux for 2 days.
Workup was performed as described in the general procedure. Yield:
2
desired complex was obtained as a white or brownish powder.
Complex 12. Yield: 340 mg (0.84 mmol, 84%). MS (EI): m/z 406
+
+
−1
[M] , 325 [M − Br] . MB (mm s ): δ = 0.73, ΔE = 3.94, Γ = 0.28
Q
(contains some impurity according to Mo
for C H Br FeN : C, 29.59; H, 3.48; N, 13.80. Found C, 29.92; H,
̈
ssbauer data). Anal. Calcd
4
.80 g (11.6 mmol, 42%). Colorless single crystals suitable for X-ray
1
0
14
2
4
diffraction were grown by slow diffusion of Et O into a MeCN
3.65; N, 13.85.
Complex 13. Yield: 336 mg (0.80 mmol, 80%). MS (EI): m/z 420
2
solution.
1
+
+
+
−1
H NMR (300 MHz, DMSO-d ): δ 9.44 (s, 2H, NCHN), 8.61 (s,
[M] , 339 [M − Br] , 257 [M − Br − C H N ] . MB (mm s ): δ =
6
4
5
2
1
H, CHPh), 8.02 (pseudo-t, 2H, J = 1.9 Hz, NCHCH), 7.93 (pseudo-
0.74, ΔE = 3.75, Γ = 0.27. Anal. Calcd for C H Br FeN : C, 31.46;
Q
11 16
2
4
t, 2H, J = 1.9 Hz, NCHCH), 7.44 (m, 2H, Ph), 7.59 (m, 3H, Ph), 3.90
H, 3.84; N, 13.34. Found C, 32.57; H, 4.06; N, 12.68.
1
3
1
(
(
(
s, 6H, CH3). C{ H} NMR (75 MHz, DMSO-d ): δ 137.9
Complex 15. Yield: 272 mg (0.65 mmol, 65%) as a brownish
powder. Colorless single crystals suitable for X-ray diffraction were
grown by slow evaporation of pentane into a CH Cl solution at 8 °C.
6
NCHN), 131.1 (Ph), 131.0 (Ph), 129.6 (Ph), 127.6 (Ph), 125.1
NCHCH), 121.1 (NCHCH), 71.4 (CHPh), 36.4 (CH ). Mp: 212 °C
3
2
2
6
699
dx.doi.org/10.1021/om200870w|Organometallics 2011, 30, 6692−6702

Organometallics
Article
+
+
+
+
+
−1
MS (EI): m/z 420 [M] , 339 [M − Br] , 231 [M − 2Br − Et] , 175
C H N ] , 135 [Ad] . MB (mm s ): δ = 0.78, ΔE = 3.79, Γ = 0.28.
3
3
2
Q
+
−1
[
M − FeBr − Et] . MB (mm s ): δ = 0.74, ΔE = 3.96, Γ = 0.26.
Anal. Calcd for C H Br FeN : C, 31.46; H, 3.84; N, 13.34. Found:
Anal. Calcd for C H Br FeN : C, 51.29; H, 5.74; N, 8.86. Found: C,
51.17; H, 5.94; N, 8.55.
2
Q
27 36 2 4
11
16
2
4
C, 31.28; H, 3.71; N, 13.43.
Complex 16a. The reaction was carried out in Et O, and the
Complex 20b. The reaction was carried out in 40 mL of THF per
1.0 mmol of bis(imidazolium) salt, and the precipitate was washed
2
precipitate was washed with Et O (3 × 5 mL) only. Yield: 430 mg
with THF (3 × 5 mL) and Et O (3 × 5 mL). Yield: 542 mg (0.63
2
2
+
+
+
(
0.79 mmol, 79%) as a brownish powder. MS (EI): m/z 522 [M] ,
mmol, 63%). MS (EI): m/z 542 [M] , 507 [M − Cl] , 202 [Ad +
+
+
+
+ + −1
4
[
64 [M − Br] , 237 [M − 2Br − Bn − Fe] , 158 [Bn + C H N ] , 91
C H N ] , 135 [Ad] . MB (mm s ): δ = 0.81, ΔE = 3.67, Γ = 0.26.
3
3
2
3
3
2
Q
+
−1
Bn] . MB (mm s ): δ = 0.73, ΔE = 3.93, Γ = 0.26. Anal. Calcd for
C H Br FeN : C, 46.36; H, 3.71; N, 10.30. Found C, 45.64; H, 3.66;
Anal. Calcd for C H Cl FeN : C, 59.68; H, 6.68; N, 10.31. Found:
Q
27 36 2 4
C, 59.40; H, 6.96; N, 10.04.
21
20
2
4
N, 10.39.
Complex 24. A suspension of the bis(imidazolium) salt 1 (676 mg,
Complex 16b. The reaction was carried out in Et O, and the
2.00 mmol) in MeCN (15 mL) was treated with [Fe{N(SiMe ) } ]
2
3 2 2 2
precipitate was washed with Et O (3 × 5 mL) only. Yield: 200 mg
(942 mg, 1.25 mmol) and stirred overnight at room temperature. The
2
(
0.44 mmol, 44%) as a brownish powder. Colorless single crystals
precipitate was filtered off and washed with THF (5 mL) and Et O
2
suitable for X-ray diffraction were grown by slow diffusion of pentane
(5 mL). The desired complex was obtained as a yellow powder (745
mg, 0.86 mmol, 86%) upon drying under vacuum. Yellow single
+
into a THF solution at 8 °C. Mp: 98 °C dec. MS (EI): m/z 455 [M] ,
+
+
+
+
4
(
20 [M − Cl] , 275 [M − 2Bn] , 158 [Bn + C H N ] , 91 [Bn] . MB
crystals suitable for X-ray diffraction were grown by slow diffusion of
3
3
2
−
1
+
mm s ): δ = 0.74, ΔE = 3.96, Γ = 0.29 (contains some impurity
Et O into a solution of the product in MeCN. MS (ESI , MeCN):
Q
2
+
according to Mo
5.41; H, 4.43; N, 12.31. Found: C, 55.17; H, 4.37; N, 12.26.
Complex 17a. Yield: 353 mg (0.83 mmol, 83%). Tiny colorless
̈
ssbauer data). Anal. Calcd for C H Cl FeN : C,
m/z 702.8 [M − FeBr − MeCN − H] , 487.1 [M − FeBr −
21
20
2
4
3
+
2+
−1
2+
5
2MeCN] , 204 [L
ΔE
Calcd for C22
29.61; H, 3.43; N, 15.78.
2
Fe] . MB (mm s ): [L
2
Fe(MeCN)
2
]
δ = 0.15,
2
−
= 1.36, Γ = 0.38; [FeBr = 2.88, Γ = 0.51. Anal.
Fe N10: C, 30.52; H, 3.49; N, 16.18. Found: C,
2
]
δ = 1.05, ΔE
Q
4
Q
single crystals suitable for X-ray diffraction were obtained by
H
30Br
4
crystallization from hot acetonitrile, although most of the complex
1
decomposes by this method. H NMR (300 MHz, DMSO-d ): δ 55.73
Complexes 14 and 22. A suspension of the bis(imidazolium) salt
6
(
(
br), 34.87 (br), 27.75 (br), 23.65 (br), 17.86 (br), 9.02 (br), 6.17
4 (1.00 g, 2.41 mmol) in THF (15 mL) was treated with [Fe{N-
(SiMe ) } ] (1.00 g, 1.33 mmol) and stirred at room temperature
3 2 2 2
+
+
br), 0.90 (br). MS (EI): m/z 476 [M] , 395 [M − Br] , 339 [M −
t
+
t
+
−1
Bu − Br] , 283 [M − 2 Bu − Br + H] . MB (mm s ): δ = 0.79,
overnight. The precipitate was filtered off and washed with THF (2 ×
10 mL). The green tetracarbene complex was separated from the
colorless dicarbene complex by rinsing with DCM (3 × 20 mL). The
filtrate was concentrated to 3 mL and filtered again. Evaporation of the
solvent from the filtrate afforded 22 as a green solid (305 mg, 0.42
mmol, 35%). Yellow single crystals of 25 suitable for X-ray diffraction
−1
−1
ΔE = 3.78, Γ = 0.31. UV−vis (DCM solution): λ (nm) (ε (M cm )):
3
1
Q
max
33 (∼1600). Anal. Calcd for C H Br FeN : C, 37.85; H, 5.08; N,
15
24
2
4
1.77. Found: C, 37.69; H, 5.26; N, 11.65.
Complex 17b. The reaction was carried out in Et O, and the
2
precipitate was washed with Et O (3 × 5 mL) only. Yield: 213 mg
2
+
+
(
0.55 mmol, 55%). MS (EI): m/z 386 [M] , 351 [M − Cl] , 330 [M −
were obtained by slow diffusion of Et O into a MeCN solution of 22.
2
t
+
t
+
+
−1
The residue from the second filtration was washed with Et O and
Bu] , 295 [M − Cl − Bu] , 148 [(C H N ) CH ] . MB (mm s ): δ =
0
2
3
3
2
2
2
dried in vacuo, affording 14 as a greenish powder (260 mg, 0.43 mmol,
.80, ΔE = 3.87, Γ = 0.29. Anal. Calcd for C H Cl FeN : C, 46.54;
Q
15 24
2
4
1
8%). Colorless single crystals suitable for X-ray diffraction were
H, 6.25; N, 14.47. Found C, 45.15; H, 6.04; N, 13.85.
grown from CH Cl solution by cooling to −30 °C for 1 week.
Analytical data for crystalline material of 14 are as follows. MS (EI):
Complex 18a. Colorless single crystals suitable for X-ray
2
2
diffraction were obtained from a reaction mixture containing 8a and
+
+
+
m/z 468 [M] , 387 [M − Br] , 307 [M − 2Br] , 252 [M − 2Br −
[
8
2
Fe{N(SiMe ) } ] in THF at −30 °C. Yield: 490 mg (0.82 mmol,
3
2 3
+
−1
−1
1
Fe] . UV−vis (DCM solution): λ
(nm) (ε (M cm )): 712
2%). H NMR (300 MHz, DMSO-d ): δ 59.02 (br, 2H), 21.93 (br,
max
6
(
∼500), 360 (∼2000). Anal. Calcd for C H Br FeN : C, 38.50; H,
H), 9.02 (2H), 5.65 (br, 4H) 3.03 (br, 6H), −4.82 (br, 12H). MS
15 16
2
4
+
+
−1
3.45; N, 11.97. Found: C, 39.00; H, 3.90; N, 11.44.
(
EI): m/z 600 [M] , 519 [M − Br] . Mp: 155 °C dec. MB (mm s ):
General Procedure for Cross-Coupling Reactions. A dried
Schlenk tube was charged with catalyst, bromo- or chlorocyclohexane
δ = 0.74, ΔE = 4.03, Γ = 0.26. UV−vis (DCM solution): λ (nm)
(
Q
max
−
1
−1
ε (M cm )): 347 (∼1600). Anal. Calcd for C H Br FeN : C,
0.03; H, 4.70; N, 9.34. Found: C, 50.10; H, 4.70; N, 9.26.
Complex 18b. Yield: 439 mg (0.86 mmol, 86%). H NMR (500
25
28
2
4
(
1.00 mmol), and Et O (3 mL) in a glovebox. Under argon p-
5
2
1
tolylmagnesium bromide solution (3.00 mL, 1.50 mmol, 0.5 M in
Et O) was added at room temperature. The reaction mixture was
MHz, CD Cl ): δ 57.78 (br), 20.49 (br), 5.16 (br), 2.50 (br), −4.08
(
0
2
2
2
+
+
−1
stirred at 45 °C oil bath temperature for 30 min. The reaction mixture
br). MS (EI): m/z 510 [M] , 475 [M − Cl] . MB (mm s ): δ =
.77, ΔE = 3.88, Γ = 0.27. UV−vis (DCM solution): λ (nm) (ε
was quenched with H O (5 mL) and extracted with CH Cl (3 × 10
2
2
2
Q
max
−
1
−1
mL). The organic phase was dried with MgSO . All volatiles were
(
M
cm )): 345 (∼1500). Anal. Calcd for C H Cl FeN : C, 58.73;
4
25
28
2
4
removed in vacuo, and 1,3,5-trimethoxybenzene (0.25 mmol) was
H, 5.52; N, 10.96. Found: C, 58.64; H, 5.65; N, 10.90.
1
added to the remaining oil/solid as an internal standard for H NMR
Complex 19. A suspension of bis(imidazolium) salt 9 (1.00 g, 1.78
conversion determination. Identification of the products and side
products was performed by GC/MS of the organic phase after workup.
mmol) in Et O (20 mL) was treated with [Fe{N(SiMe ) } ] (880
mg, 1.17 mmol) and stirred overnight at room temperature. The
2
3 2 2 2
precipitate was filtered off and washed with Et O (3 × 3 mL);
2
afterward the solid was stirred in CH Cl for 10 min, filtered, and
2
2
ASSOCIATED CONTENT
■
washed with Et O (2 × 3 mL). Complex 19 was obtained as a white
2
*
S
Supporting Information
ssbauer spectrum of 19, CV spectrum of
7a, and X-ray structures of 18a, 25, and 26 and CIF files
giving crystallographic data for 4, 14, 15, 16b, 17a, 18a, 19, and
powder (742 mg, 1.21 mmol, 68%) upon removal of the solvents in
vacuo. Colorless single crystals suitable for X-ray diffraction were
Figures giving the Mo
̈
1
grown by slow evaporation of pentane into a THF solution at 8 °C. H
1
NMR (300 MHz, CD Cl ): δ 42.72 (br), 34.73 (br), 23.71 (br), 8.40
2
2
+
+
(
br), 4.76 (br), 0.08 (br). MS (EI): m/z 614 [M] , 533 [M − Br] .
2
4−26. This material is available free of charge via the Internet
at http://pubs.acs.org.
−1
Mp: 160 °C dec. UV−vis (DCM solution): λ
(nm) (ε (M
max
−
1
−1
cm )): 335 (∼1800). MB (mm s ): δ = 0.77, ΔE = 3.91, Γ = 0.26.
Q
Anal. Calcd for C H Br FeN : C, 50.84; H, 4.92; N, 9.12. Found: C,
26
30
2
4
5
0.42; H, 4.96; N, 9.00.
Complex 20a. The reaction was carried out in 40 mL of THF per
AUTHOR INFORMATION
■
Corresponding Author
1
.0 mmol of bis(imidazolium) salt, and the precipitate was washed
with THF (3 × 5 mL) and Et O (3 × 5 mL). Yield: 537 mg (0.85
mmol, 85%). MS (EI): m/z 632 [M] , 551 [M − Br] , 202 [Ad +
*Tel: +49 551 393012. Fax: +49 551 393063. E-mail: franc.
meyer@chemie.uni-goettingen.de.
2
+
+
6
700
dx.doi.org/10.1021/om200870w|Organometallics 2011, 30, 6692−6702

Organometallics
Article
(
c) Danopoulos, A. A.; Wright, J. A.; Motherwell, W. B. Chem.
ACKNOWLEDGMENTS
■
Commun. 2005, 784. (d) Pugh, D.; Wells, N. J.; Evans, D. J.;
Danopoulos, A. A. Dalton Trans. 2009, 7189. (e) Danopoulos, A. A.;
Pugh, D.; Smith, H.; Saßmannshausen, J. Chem. Eur. J. 2009, 15, 5491.
10) (a) Kernbach, U.; Ramm, M.; Luger, P.; Fehlhammer, W. P.
Angew. Chem., Int. Ed. 1996, 35, 310. (b) Frankel, R.; Kernbach, U.;
Bakola-Christianopoulou, M.; Plaia, U.; Suter, M.; Ponikwar, W.; Noth,
H.; Moinet, C.; Fehlhammer, W. P. J. Organomet. Chem. 2001, 617−
6
2
Financial support by the DFG (International Research Training
Group IGRK 1422 “Metal Sites in Biomolecules: Structures,
Regulation and Mechanisms”; see www.biometals.eu) is
gratefully acknowledged.
(
̈
̈
REFERENCES
1) (a) Bourissou, D.; Guerret, O.; Gabbai,
Rev. 2000, 100, 39. (b) Weskamp, T.; Bo
■
18, 530. (c) Nieto, I.; Cervantes-Lee, F.; Smith, J. M. Chem. Commun.
005, 3811. (d) Scepaniak, J. J.; Fulton, M. D.; Bontchev, R. P.; Duesler,
(
̈
F. P.; Bertrand, G. Chem.
̈
E. N.; Kirk, M. L.; Smith, J. M. J. Am. Chem. Soc. 2008, 130, 10515.
e) Nieto, I.; Ding, F.; Bontchev, R. P.; Wang, H.; Smith, J. M. J. Am.
J. Organomet. Chem. 2000, 600, 12. (c) Herrmann, W. A. Angew. Chem.,
Int. Ed. 2002, 41, 1290. (d) Hahn, F. E. Angew. Chem., Int. Ed. 2006, 45,
(
Chem. Soc. 2008, 130, 2716. (f) Scepaniak, J. J.; Young,
J. A.; Bontchev, R. P.; Smith, J. M. Angew. Chem., Int. Ed. 2009, 48,
́
́
348. (e) Diez-Gonzalez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009,
1
1
3
09, 3612. (f) Jahnke, M. C.; Hahn, F. E. Top. Organomet. Chem. 2010,
3
158. (g) Scepaniak, J. J.; Harris, T. D.; Vogel, C. S.; Sutter, J.; Meyer, K.;
Smith, J. M. J. Am. Chem. Soc. 2011, 133, 3824.
11) Vogel, C. S.; Heinemann, F. W.; Sutter, J.; Anthon, C.; Meyer,
K. Angew. Chem., Int. Ed. 2008, 47, 2681.
0, 95. (g) Fortman, G. C.; Nolan, S. P. Chem. Soc. Rev. 2011, 40, 5151.
(
2) N-Heterocyclic Carbenes in Transition Metal Catalysis and
Organocatalysis; Cazin, C. S. J., Ed.; Springer: Amsterdam, 2010.
3) (a) Hitchcock, P. B.; Lappert, M. F.; Thomas, S. A.; Thorne, A. J.
J. Organomet. Chem. 1986, 315, 27. (b) Louie, J; Grubbs, R. H. Chem.
Commun. 2000, 1479. (c) Buchgraber, P.; Toupet, L.; Guerchais, V.
Organometallics 2003, 22, 5144. (d) Mercs, L.; Labat, G.; Neels, A.;
Ehlers, A.; Albrecht, M. Organometallics 2006, 25, 5648. (e) Chen,
M.-Z.; Sun, H.-M.; Li, W.-F.; Wang, Z.-G.; Shen, Q.; Zhang, Y.
J. Organomet. Chem. 2006, 691, 2489. (f) Llewellyn, S. A.; Green,
M. L. H.; Green, J. C.; Cowley, A. R. Dalton Trans. 2006, 2535.
g) Ohki, Y.; Hatanaka, T.; Tatsumi, K. J. Am. Chem. Soc. 2008, 130,
7174. (h) Mercs, L.; Neels, A.; Albrecht, M. Dalton Trans. 2008,
570. (i) Kaufhold, O.; Hahn, F. E.; Pape, T.; Hepp, A. J. Organomet.
Chem. 2008, 693, 3435. (j) Li, B.; Liu, T.; Popescu, C. V.; Bilko, A.;
Darensbourg, M. Y. Inorg. Chem. 2009, 48, 11283. (k) Kandepi,
V. V. K. M.; Cardoso, J. M. S.; Peris, E.; Royo, B. Organometallics
(
(
(
(
12) Liu, B.; Xia, Q.; Chen, W. Angew. Chem., Int. Ed. 2009, 48, 5513.
13) Morvan, D.; Capon., J.-F.; Gloaguen, F.; Le Goff, A.; Marchivie,
M.; Michaud, F.; Schollhammer, P.; Talarmin, J.; Yaouanc, J.-J.
Organometallics 2007, 26, 2042.
(14) For examples, see: (a) Herrmann, W. A.; Reisinger, C.-P.;
Spiegler, M. J. Organomet. Chem. 1998, 557, 93. (b) Hahn, F. E.; Foth,
M. J. Organomet. Chem. 1999, 585, 241. (c) Taige, M. A.; Zeller, A.;
Ahrens, S.; Goutal, S.; Herdtweck, E.; Strassner, T. J. Organomet. Chem.
(
1
5
2
007, 692, 1519.
15) (a) Hahn, F. E.; Tamm, M. J. Chem. Soc., Chem. Commun. 1993,
(
8
5
42. (b) Hahn, F. E.; Tamm, M. J. Chem. Soc., Chem. Commun. 1995,
96. (c) Hahn, F. E.; Tamm, M. Organometallics 1995, 14, 2597.
(
16) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem.,
Int. Ed. 2007, 46, 2768.
17) Andersen, R. A.; Faegri, K.; Green, J. C.; Haaland, A.; Lappert,
M. F.; Leung, W.-P.; Rypdal, K. Inorg. Chem. 1988, 27, 1782.
18) Zlatogorsky, S.; Muryn, C. A.; Tuna, F.; Evans, D. J.; Ingleson,
M. J. Organometallics 2011, 30, 4974.
19) Scherg, T; Schneider, S. K.; Frey, G. D.; Schwarz, J.; Herdtweck,
E.; Herrmann, W. A. Synlett 2006, 18, 2894.
20) Nachtigall, F. M.; Corilo, Y. E.; Cassol, C. C.; Ebeling, G.;
Morgon, N. H.; Dupont, J.; Eberlin, M. N. Angew. Chem. 2008, 120,
57.
21) Okuyama, K.; Sugiyama, J.; Nagahata, R.; Asai, M.; Ueda, M.;
Takeuchi, K. J. Mol. Catal. A: Chem. 2003, 203, 21.
22) Noujeim, N.; Leclercq, L.; Schmitzer, A. R. J. Org. Chem. 2008,
3, 3784.
2
010, 29, 2777. (l) Flores-Figueroa, A; Pape, T.; Weigand, J. J.; Hahn,
(
F. E. Eur. J. Inorg. Chem. 2010, 19, 2907. (m) Hatanaka, T.; Ohki, Y.;
Tatsumi, K. Chem. Asian J. 2010, 5, 1657. (n) Xiang, L.; Xiao, J.; Deng,
L. Organometallics 2011, 30, 2018.
4) Gao, H.; Yan, C.; Tao, X.-P.; Xia, Y.; Sun, H.-M.; Shen, Q.;
Zhang, Y. Organometallics 2010, 29, 4189.
5) Wang, Y.-S.; Sun, H.-M.; Tao, X.-P.; Shen, Q.; Zhang, Y. Chin.
Sci. Bull. 2007, 52, 3193.
6) For example, see: (a) Nehete, U. N.; Anantharaman, G.;
Chandrasekhar, V.; Murugavel, R.; Walawalkar, M. G.; Roesky, H. W.;
Vidovic, D.; Magull, J.; Samwer, K.; Sass, B. Angew. Chem., Int. Ed.
004, 43, 3832. (b) Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc.
007, 129, 7008. (c) Deng, L.; Holm, R. H. J. Am. Chem. Soc. 2008,
30, 9878. (d) Lavallo, V.; Grubbs, R. H. Science 2009, 326, 559.
(
(
(
(
(
(
1
(
2
2
1
(
7
(
(
23) Ahrens, S.; Strassner, T. Inorg. Chim. Acta 2006, 359, 4789.
24) Lee, H. M.; Lu, C. Y.; Chen, C. Y.; Chen, W. L.; Lin, H. C.;
(
7) For examples of polymerization reactions, see: (a) Hatakeyama, T.;
Hashimoto, S.; Ishizuka, K.; Nakamura, M. J. Am. Chem. Soc. 2009, 131,
1949. (b) Rosa, J. N.; Reddy, R. S.; Candeias, N. R.; Cal, P. M. S. D.;
Gois, P. M. P. Org. Lett. 2010, 12, 2686.
8) For examples of C−C cross-coupling reactions, see: (a) Martin,
R.; Furstner, A. Angew. Chem., Int. Ed. 2004, 43, 3955. (b) Nakamura,
M.; Matsuo, K.; Ito, S.; Nakamura, E. J. Am. Chem. Soc. 2004, 126,
686. (c) Nagano, T.; Hayashi, T. Org. Lett. 2004, 6, 1297.
d) Bedford, R. B.; Bruce, D. W.; Frost, R. M.; Goodby, J. W.;
Chiu, P. L.; Cheng, P. Y. Tetrahedron 2004, 60, 5807.
1
(25) Steiner, T. Acta Crystallogr. 1998, B54, 456.
(
26) (a) Kreisel, K. A.; Yap, G. P. A.; Theopold, K. H.
(
Organometallics 2006, 25, 4670. (b) Meyer, S.; Demeshko, S.; Dechert,
S.; Meyer, F. Inorg. Chim. Acta 2010, 363, 3088.
(27) (a) Fackler, J. P.; Moyer, T.; Costamagna, J. A.; Latorre, R.;
Granifo, J. Inorg. Chem. 1987, 26, 836. (b) Sproules, S.; Wieghardt, K.
Coord. Chem. Rev. 2010, 254, 1358.
̈
3
(
Hird, M. Chem. Commun. 2004, 2822. (e) Bedford, R. B.; Bruce,
D. W.; Frost, R. M.; Hird, M. Chem. Commun. 2005, 4161. (f) Bedford,
R. B.; Betham, M.; Bruce, D. W.; Davis, S. A.; Frost, R. M.; Hird, M.
Chem. Commun. 2006, 1398. (g) Bedford, R. B.; Betham, M.; Bruce,
D. W.; Danopoulos, A. A.; Frost, R. M.; Hird, M. J. Org. Chem. 2006,
(28) Edwards, P. R.; Johnson, C. E.; Williams, R. J. P. J. Chem. Phys.
1967, 47, 2074.
(29) Low signal-to-noise ratio is due to γ absorption by bromide.
The area ratio of 55:45, which deviates from the theoretically expected
ratio of 50:50, can probably be explained by higher Lamb−Mossbauer
̈
factors for ls than for hs iron(II) ions and deviation of the absorption
peaks from ideal Lorentz function. Also, some small amount of the
tetracarbene dication with one or two nonferrous anions cannot be
fully excluded.
(30) Metal-Catalyzed Cross-Coupling Reactions; de Meijere, A.,
Diederich, F., Eds.; Wiley-VCH: Weinheim, Germany, 2004.
(31) Jana, R.; Pathak, T. P.; Sigman, M. S. Chem. Rev. 2011, 111,
1417.
7
(
9
1, 1104. (h) Bica, K.; Gaertner, P. Org. Lett. 2006, 8, 733.
i) Hatakeyama, T.; Nakamura, M. J. Am. Chem. Soc. 2007, 129,
844. (j) Furstner, A.; Martin, R.; Krause, H.; Seidel, G.; Goddard, R.;
̈
Lehmann, C. W. J. Am. Chem. Soc. 2008, 130, 8773. (k) Chowdhury,
R. R.; Crane, A. K.; Fowler, C.; Kwong, P.; Kozak, C. M. Chem.
Commun. 2008, 94. (l) Qian, X.; Dawe, L. N.; Kozak, C. M. Dalton
Trans. 2011, 40, 933.
(
9) (a) McGuinness, D. S.; Gibson, V. C.; Steed, J. W.
Organometallics 2004, 23, 6288. (b) Danopoulos, A. A.; Tsoureas,
N.; Wright, J. A.; Light, M. E. Organometallics 2004, 23, 166.
(32) Frisch, A. C.; Beller, M. Angew. Chem., Int. Ed. 2005, 44, 674.
6
701
dx.doi.org/10.1021/om200870w|Organometallics 2011, 30, 6692−6702

Organometallics
Article
(
(
33) Netherton, M. R.; Fu, G. C. Adv. Synth. Catal. 2004, 346, 1525.
34) (a) Tamura, M.; Kochi, J. K. J. Am. Chem. Soc. 1971, 93, 1487.
(
b) Tamura, M.; Kochi, J. K. Synthesis 1971, 6, 303. (c) Kochi, J. K.
Acc. Chem. Res. 1974, 7, 351. (d) Neumann, S. M.; Kochi, J. K. J. Org.
Chem. 1975, 40, 599. (e) Smith, R. S.; Kochi, J. K. J. Org. Chem. 1976,
4
1, 502. (f) Kochi, J. K. J. Organomet. Chem. 2002, 653, 11.
35) (a) Furstner, A.; Leitner, A.; Mendez, M.; Krause, H. J. Am.
Chem. Soc. 2002, 124, 13856. (b) Bogdanovic,
Angew. Chem., Int. Ed. 2000, 39, 4610.
36) Raynal, M.; Cazin, C. S. J.; Vallee
Braunstein, P. Dalton Trans. 2009, 3824.
(
̈
́
́
B.; Schwickardi, M.
(
́
, C.; Olivier-Bourbigou, H.;
̈
(
37) Scheele, U. J. Entwicklung von Ubergangsmetallkomplexen mit
neuartigen Pyrazol-NHC und Pyridazin-NHC-Hybridliganden; Disserta-
tion, Cuvillier Verlag: Gottingen, Germany, 2008.
38) Hayat, S.; Rahman, A.; Choudhary, M. I.; Khan, K. M.;
Schuhmann, W.; Bayer, E. Tetrahedron 2001, 57, 9951.
39) Bill, E. Mfit; Max-Planck Institute for Bioinorganic Chemistry,
Mulheim/Ruhr, Germany.
40) Bill, E. julX; Max-Planck Institute for Bioinorganic Chemistry,
Mulheim/Ruhr, Germany.
̈
(
(
̈
(
̈
(
(
(
41) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
42) Flack, H. D. Acta Crystallogr. 1983, A39, 876.
43) X-RED; STOE & CIE GmbH, Darmstadt, Germany, 2002.
6
702
dx.doi.org/10.1021/om200870w|Organometallics 2011, 30, 6692−6702