Synthesis and structure of cobalt(II) iodide bearing a bulky N-heterocyclic carbene ligand, and catalytic activation of bromoalkanes

Article abstract of DOI:10.1016/j.jorganchem.2012.12.025

A divalent cobalt iodine complex bearing 1,3-bis(mesityl)imidazol-2-ylidene and pyridine ligands was synthesized and its structure was determined. The cobalt center has a typical d⁷-tetrahedral geometry, as expected. Catalytic application of this cobalt complex with bromoalkanes and Grignard reagents demonstrated high-yield formation of alkenes as a result of β-hydrogen elimination; in sharp contrast, the activation of alkyl halides was not successful using the larger N-heterocyclic carbene ligand, 1,3-bis(2,6-diisopropyl-phenyl)imidazol-2-ylidene. In the presence of styrene, Heck reaction proceeded with trans selectivity. The reaction of a substrate containing a bromobenzyl moiety yielded a homocoupling product.

Full text of DOI:10.1016/j.jorganchem.2012.12.025

Journal of Organometallic Chemistry 727 (2013) 44e49
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and structure of cobalt(II) iodide bearing a bulky
N-heterocyclic carbene ligand, and catalytic activation
of bromoalkanes
*
Kouki Matsubara , Aya Kumamoto, Hitomi Yamamoto, Yuji Koga, Satoshi Kawata
Department of Chemistry, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 August 2012
Received in revised form
A divalent cobalt iodine complex bearing 1,3-bis(mesityl)imidazol-2-ylidene and pyridine ligands was
synthesized and its structure was determined. The cobalt center has a typical d -tetrahedral geometry, as
expected. Catalytic application of this cobalt complex with bromoalkanes and Grignard reagents dem-
onstrated high-yield formation of alkenes as a result of b-hydrogen elimination; in sharp contrast, the
activation of alkyl halides was not successful using the larger N-heterocyclic carbene ligand, 1,3-bis(2,6-
diisopropyl-phenyl)imidazol-2-ylidene. In the presence of styrene, Heck reaction proceeded with trans
selectivity. The reaction of a substrate containing a bromobenzyl moiety yielded a homocoupling
product.
7
29 November 2012
Accepted 4 December 2012
Keywords:
Cobalt(II) iodide
N-Heterocyclic carbene
Ó 2012 Elsevier B.V. All rights reserved.
b-Elimination
Heck reaction
Homocoupling
Kumada coupling
1. Introduction
catalysts [6]. Several cobalt NHC complexes have been synthe-
sized and structurally determined [7]. However, only a few ex-
The first-row late transition metals such as iron, cobalt, nickel
and copper are now the most important transition metals used as
catalysts in the development of organic reactions [1], rather than
the catalytically active noble metals, such as ruthenium, rhodium,
and palladium [2]. The development of alternative processes for
cross-coupling reactions mediated by these first-row metal cata-
lysts has also attracted much attention [3]. Because of the difficulty
of identification as a result of their paramagnetic properties,
structurally defined cobalt complexes have rarely been used as
catalysts [4], in comparison with their nickel and iron counterparts.
Using well-defined complexes is essential to obtain information on
the reaction mechanism and/or discover novel reactions. Fruitful
results have been obtained in studies of cross-coupling reactions of
alkyl halides catalyzed by in situ-generated cobalt species [5]. It is
therefore important to develop active and useful cobalt complexes
for cross-coupling reactions of alkyl halides.
amples of catalysts for organic reactions, particularly for cross-
coupling reactions, have been reported [4]. In addition, cobalt
catalysts bearing bulky NHC ligands are unknown, although
Oshima and Yorimitsu et al. have reported efficient cobalt-
mediated activation of alkyl halides in the presence of bulky
NHC ligands as cocatalysts [8]. Our previous report showed that
the bulky NHC ligand, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene (IPr) coordinates to divalent cobalt halides to form un-
stable dinuclear cobalt(II)eIPr complexes, which were stabilized
by addition of pyridine to yield monomeric cobalt(II) complexes,
namely Co(IPr)X
2
(Py) (1a: X ¼ I, 1b: X ¼ Br, 1c: X ¼ Cl) (Scheme 1)
[9]. These compounds successfully catalyzed KumadaeTamaoe
Corriu cross-coupling reaction of aryl halides, but were not
suitable for catalytic activation of alkyl halides. We hypothesized
that cobalt complex bearing less bulky NHC ligand such as
1,3-bis(mesityl)imidazol-2-ylidene (IMes) instead of IPr could
perform catalytic activation of alkyl halides. Here we report the
synthesis, structure, and reactivity of a new IMesecobalt(II)
diiodide, which mediates catalytic activation of alkyl halides.
Interestingly, the products obtained depended on the conditions:
Recently, N-heterocyclic carbenes (NHCs) have become one of
the most valuable candidates designing active and stable metal
1-alkene or homocoupling products in the presence of a Grignard
reagent, and Heck reaction products in the presence of styrene
and a Grignard reagent.
*
Corresponding author.
E-mail address: kmatsuba@fukuoka-u.ac.jp (K. Matsubara).
0
022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.12.025

K. Matsubara et al. / Journal of Organometallic Chemistry 727 (2013) 44e49
45
Scheme 1. Synthesis of 1a in the previous report [9].
2
. Results and discussion
2.1. Synthesis and structure of cobalt(II) (IMes) diiodide (2)
In our previous study, IPr was added to THF solutions of cobal-
t(II) iodide, bromide, and chloride to form dicobalt IPr complexes
Co(IPr)X] (X ¼ I, Br, Cl) [9]. However, the complexes
ꢀX)
sometimes decomposed to a monomeric cobalt anion [Co(IPr)
[
2
(m
2
ꢀ
þ
X
3
] [IPrH] , probably because of the presence of a small amount of
water. Addition of pyridine to these dimers effectively stabilized
them and gave monomeric neutral complexes, Co(IPr)X (Py) (1a:
Fig. 1. UVevisible spectra for compounds 1a and 2 in toluene (0.50 mM) in the area
from 500 to 800 nm.
2
X ¼ I, 1b: X ¼ Br, 1c: X ¼ Cl), in good yields. Among the complexes,
the iodide 1a was the most active catalyst in the Kumada coupling
of aryl halides. Based on the above experimental results, the
monomeric cobalt iodide analog bearing the IMes ligand, Co(IMes)
pyridine, could be ruled out. Compound 2 was paramagnetic and
the spin-quantum number was 3/2, the same as in the case of the
corresponding IPr analogs [9].
2 2
I (Py) (2), was similarly synthesized by the reaction of CoI with
IMes in THF, followed by addition of pyridine (Scheme 2). The re-
action proceeded smoothly in THF at room temperature. Direct
addition of IMes and CoI to pyridine also gave 2 efficiently. Dimeric
2
species are probably generated in situ, as the corresponding IPr
complexes were formed previously [9].
The structure of compound 2 was confirmed by X-ray crys-
tallography, as shown in Fig. 2. The cobaltecarbon (C1), cobalte
nitrogen (N1), and cobalteiodine (I1 and I2) bond distances are
ꢀ
2.058(8), 2.050(6), and 2.595(1) and 2.591(1) A, respectively, which
are almost the same as those of the IPr analog 1a, 2.043(6),
ꢀ
The product 2 was air-unstable and recrystallized under an
2.066(3), 2.5869(7) and 2.6012(6) A. These bond lengths are in the
ꢁ
inert-gas atmosphere from THF and n-hexane at ꢀ30 C (26% yield
range of general single bond interactions. However, the C(1)e
ꢁ
ꢁ
by recrystallization). In elemental analysis of 2, the obtained
carbon, hydrogen and nitrogen contents of 44.68, 4.57 and 6.06%,
respectively, agreed with the theoretical values of 44.85, 4.20 and
Co(1)eN(3) bond angle in the tetrahedral geometry was 111.8(3) ,
that was smaller than the corresponding angle in 1a (115.4(1) ).
ꢁ
The other angle, C(1)eCo(1)eI(av.) was 112.0(2) , which is also
ꢁ
6
.40%, respectively. The structure of 2 in solution was similar to that
smaller than that of 1a (115.9(1) ). The decrease of the angles in the
of 1a, indicated by the UVevisible absorption spectroscopy. The
absorption bands from 600 to 800 nm, 636, 675 and 713 nm,
in Fig. 1 were assigned as the ded* transition, whereas those of 1a
appeared at 644, 681 and 711 nm. The three separate bands are
probably the result of spineorbit coupling and the unsymmetric
structure. The spectra suggest that 2 has the same tetrahedral
d -geometry in solution as 1a [9]. Moreover, these bands did not
broaden at room temperature, suggesting that any equilibrium
reactions, such as formation of the dimer with liberation of
tetrahedral geometry of 2 could be caused by the smaller methyl
groups in IMes of 2 in comparison with the bulky isopropyl groups
in IPr of 1a, which could crowd the surroundings of the cobalt
center. A
pep stacking interaction was found in 2 between the
pyridine ring and one of the two mesityl rings; the average distance
ꢀ
between them was 3.5 A. This stacking interaction of the pyridine
7
ring was not observed in 1a because of steric hindrance caused by
the two isopropyl groups in IPr. These differences are easily seen in
the space-filling models of 1a and 2 shown in Fig. 3.
Fig. 2. ORTEP drawing of 2 (50% Probability of thermal ellipsoids). Representative
bond lengths and angles are as follows: Co(1)eC(1), 2.058(8); Co(1)eN(3), 2.050(6);
ꢀ
Co(1)eI(1), 2.595(1); Co(1)eI(2), 2.591(1) A; C(1)eCo(1)eI(1), 105.5(2); C(1)eCo(1)e
I(2), 118.5(2); C(1)eCo(1)eN(3), 111.8(3), N(3)eCo(1)eI(1), 105.9(2); N(3)eCo(1)eI(2),
ꢁ
Scheme 2. In situ preparation of the monomeric complex 2.
103.2(2) .

46
K. Matsubara et al. / Journal of Organometallic Chemistry 727 (2013) 44e49
Fig. 3. Space-filling models of 1a (left) and 2 (right) are depicted using the X-ray crystallographic results. The colors of the atoms are purple (I), red (Co), blue (N), gray (C) and white
H). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(
2
.2. Catalytic behavior of cobalt(II) (IMes) diiodide (2) with alkyl
the Heck reaction product (entries 3 and 4). The alkene products A
and B were not generated using a sterically hindered Grignard re-
agent, 2,6-dimethylphenylmagnesium bromide (entry 6), suggesting
halides
As noted in our previous report, the complex 1a performed the
that b-elimination of the octylcobalt species or styrene coordination
catalytic activity to aryl halides with Grignard reagents but not
good to alkyl halides [9]. We therefore first examined the reaction
of 4-bromobiphenyl or 1-bromooctane with phenylmagnesium
chloride in the presence of a catalytic amount of 2 (5 mol%). As
the Kumada-coupling reaction of aryl halides was successful using
might be suppressed by coordination of 2,6-dimethylphenyl group
after transmetallation. Phenylmagnesium chloride was the most
suitable Grignard reagent for the Heck reaction (entry 5). Product A is
a known compound, identified on the basis of the H NMR spec-
troscopy as a trans form, which is generally produced in transition
metal-catalyzed Heck reactions with styrene [12].
1
1
a as the catalyst [9], the compound 2 also mediated the
coupling of 4-bromobiphenyl with phenylmagnesium chloride to
form terphenyl in an 89% yield (90% yield using 1a). However,
in contrast to the case of 1a, 1-octene and 2-octene (total 73%,
Interestingly, the ratios of 1-bromooctane and styrene were
important for the predominant formation of the Heck reaction
product. When similar ratios of 1-bromooctane and styrene were
used, the product ratios of A and B were comparable (Table 2, en-
tries 1 and 2). However, a higher concentration of styrene than that
of 1-bromooctane resulted in predominant formation of A (yields
containing 18% of 2-octene) generated as
b-elimination and/or
subsequent olefin isomerization products, after complete con-
sumption of 1-bromooctane in only 10 min at room temperature
(Scheme 3). The Kumada coupling product, n-octylbenzene
up to 50%) (entries 3 and 4) [13]. As previously commented, the b-
was obtained in only a 15% yield. Oshima and Yorimitsu achieved
the catalytic formation of 1-alkenes and 2-alkenes from 2-
elimination process completed in several minutes at room tem-
perature, but it took several hours in the presence of styrene. This
indicates that coordination of styrene to the octylcobalt species
bromoalkanes mediated by CoCl
SiCH MgCl, after screening of the free ligand (precursors) and
Grignard reagents [10]. IMes/HCl and phenylmagnesium bromide
with CoCl did not provide sufficiently good results, with forma-
2 2
and IMes/HCl with Me Ph-
2
competes with intramolecular
the catalytic cycle.
b-elimination to form 1-octene in
2
It might be possible to suppress
b-elimination to achieve an
tion of only 8% and 9% of 1- and 2-alkenes, respectively, and re-
covery of 2-bromoalkane (25%), in sharp contrast to our result,
using 1-bromoalkane with the cobalt iodide complex 2.
efficient Heck reaction by using alkyl halides with no
b
-hydrogen
atoms, such as benzyl halides and carbonylmethyl halides. We
therefore examined the reaction of 9-bromofluorene with styrene
in the presence of phenylmagnesium chloride. The Heck reaction
did not proceed, but a catalytic homocoupling reaction occurred to
form bifluorene quantitatively in 5 h at room temperature. In the
absence of styrene, bifluorene was also formed in the same manner
(Scheme 4). Several studies of metal-catalyzed reductive homo-
coupling reactions of benzyl halides have been reported [14].
However, most of the reactions required high reaction tempera-
tures and/or high catalyst loadings. For example, the reaction of
We then attempted the Heck reaction of an alkyl halide with
styrene in the presence of 2 and a Grignard reagent. Oshima et al.
achieved a similar Heck reaction of an alkyl halide using cobalt
chloride bearing a bisphosphine ligand [11]. However, the reaction
using cobalteNHC complex has not been reported. As shown in
Table 1, several Grignard reagents were used in the reaction. Other
reducing agents, such as sodium tert-butoxide, resulted in no reac-
tion. The starting 1-bromooctane was completely consumed when
trimethylsilylmethylmagnesium chloride and methylmagnesium
bromide were used (entries 1 and 2). However, the amounts of 1-
octene (B) and other by-products, such as hexadecene and hex-
adecane, were high compared with that of the Heck reaction product
ꢁ
benzyl halides with iron powder at 80 C in the presence of
a copper chloride catalyst formed bibenzyl [14a], and benzyl
chloride coupled in the presence of Pd/C and sodium formate
ꢁ
at 120 C [14b]. Manganese(II) chloride (10 mol%) catalyzed
1
-phenyl-1-decene (A). Similarly, using ethylmagnesium bromide
homocoupling of benzyl bromide with Mg at room temperature
[14c]. Neither a cobalt reagent as the catalyst nor a Grignard
and isopropylmagnesium chloride did not provide adequate yields of
Scheme 3. Reaction of n-octyl bromide with phenylmagnesium bromide in the presence of 2.

K. Matsubara et al. / Journal of Organometallic Chemistry 727 (2013) 44e49
47
Table 1
Heck reaction of 1-bromooctane with styrene catalyzed by 2.
Cat: 2 (5 mol%)
3 2 7
H C(H C)
RMgBr (2 equiv)
A
n-Oct Br
+
THF, rt, 14 h
(CH ) CH
2 5 3
n-Oct Br
B
C
Entry
RMgX
Yield (%)a
Scheme 4. Homo-coupling reaction of 9-bromofluorene.
A
B
C
1
2
3
4
5
6
Me
3
SiCH
3
MgCl
30
21
30
12
46
0
27
15
22
0
12
0
0
0
43
37
16
68
MeMgBr
EtMgBr
The activation process of alkyl halides using the cobalt catalysts
may be substantially different from that of aryl halides. In our hy-
pothesis, strong electron donation from the NHC ligand to the co-
i
PrMgCl
PhMgCl
2,6-Me PhMgBr
2
3
balt atom might destabilize the cobalt-carbon (sp ) bonding
interaction, which would make other reactions, such as b-elimi-
nation, alkene insertion, or homolytic elimination, faster than the
reductiveeelimination process.
a
The yields were determined by 1H NMR spectroscopy and GCeMS spectra using
a standard sample, mesitylene.
reagent as the reducing agent have been used in the previous
studies. This is the first example of cobalt-catalyzed homocoupling
of 9-bromofluorene using a Grignard reagent as the reducing
agent to achieve efficient formation of bifluorene at ambient
temperature.
3
. Conclusion
Structurally defined cobalt compounds active for catalytic
organic transformations have great potential and possibilities in
catalysis using first-row transition-metals. We successfully synthe-
sized and characterized divalent cobalteiodine complex bearing
7
2.3. Mechanistic insights in the reactions of alkyl halides
IMes and pyridine ligands. The cobalt center had a typical d -tetra-
hedral geometry, as expected. This cobalt complex performed cat-
alytic activation of bromoalkanes to provide highyields of alkenes in
the presence of PhMgCl. In the presence of styrene, a trans-selective
Heck reaction also occurred. The reaction of 1-bromofluorene con-
taining a bromobenzyl moiety yielded a homocoupling product
efficiently under mild conditions. These results show that the cata-
lytic performance of activation of alkyl halides of the IMesecobalt
complex is superior to that of the IPr analog. Further development
of the catalyst system by increasing the range of substrates and re-
actions, and monitoring of the active cobalt species in the catalytic
cycle are now in progress.
Because cobalt complexes can have various oxidation states, it is
difficult to discuss the mechanism in terms of the oxidation state of
cobalt without direct detection of the intermediates. We found that
the cobalt solution changed color from dark blueegreen to brown,
which is generally the color of a monovalent species rather than
a divalent species. After quenching, the color reverted to dark bluee
green. Chen et al. also proposed a monovalent cobalt species as the
intermediate in the catalytic cycle of the Kumada cross-coupling
reaction [4a]. Other important points are as follows: (1) styrene
coordination competes with
b-elimination of octylcobalt species;
(
2) the yield of the Kumada-coupling product, octylbenzene, was
low. Detailed studies, such as detection of intermediates and ki-
netics, are required for further discussion. The homocoupling
product was obtained from 9-bromofluorene. No other products,
such as the Kumada coupling or Heck reaction product, were
obtained, even in the presence of phenylmagnesium chloride and
styrene. We think that generation of benzyl radicals, which
are much more stable than primary alkyl radicals, from alkylcobalt
species is the most probable process. Reduction of cobalt
bromide species with the Grignard reagent could make the
catalytic process possible. We do not have any information on why
the cross-coupling reaction of the alkyl halide with the Grignard
reagents is not predominant in any of these cases, in contrast to the
successful Kumada coupling of aryl halides mediated by the cobalt
IPr halides.
4
. Experimental section
4.1. General considerations
All experiments were carried out under an inert gas atmo-
sphere using standard Schlenk techniques and glove box (MBraun
UniLab) unless otherwise noted. THF, toluene, hexane, benzene-
d
6
were distilled from benzophenone ketyl and stored under
a nitrogen atmosphere. Organic reagents used for coupling re-
actions were distilled just before use. Other reagents were used
as received. N-Heterocyclic carbene (IMes) was prepared ac-
cording to the published methods [15]. Column chromatography
of organic products was carried out using silica gel (Kanto
1
Kagaku, silica gel 60 N (spherical, neutral)). The H NMR spectra
were taken with a VARIAN Mercury Y plus 400 MHz spectrometer
Table 2
at room temperature. Chemical shifts (d) were recorded in
Changing the ratios of 1-bromooctane and styrene for dominant formation of the
Heck reaction product.
ppm from the solvent signal. The magnetic properties of the
materials were investigated using a Quantum Design MPMS-5S
superconducting quantum interference device (SQUID) magne-
tometer. The UVevisible spectra were taken with a JASCO V-550
spectrometer in toluene using a quartz cell (1.0 cm). The ele-
mental analysis was carried out with YANACO CHN Corder MT-5,
AUTO-SAMPLER, using thin aluminum foil, where the samples
were held in a glove box. GCeMS spectra were recorded on a JEOL
Entry
Oct-Br:Styrene
Yield (%)a
A
B
C
1
2
3
4
1.0:0.9
1.0:1.3
1.0:2.0
1.0:2.7
30
40
49
50
27
38
40
27
48
11
0
0
a
The yields were determined by 1H NMR spectroscopy and GCeMS spectra using
JMS-GCmateII (EI-MS), equipped with Agilent 6890
chromatography.
N gas
a standard sample, mesitylene.

48
K. Matsubara et al. / Journal of Organometallic Chemistry 727 (2013) 44e49
4.2. Synthesis of 2
Appendix A. Supplementary material
Anhydrous cobalt(II) iodide (62.5 mg, 0.20 mmol) was dissolved
in THF (1.0 mL). Then, IPr (609 mg, 0.20 mmol) and pyridine (16 L,
.20 mmol) was added. After addition of 2 mL of THF, the solution
CCDC-887524 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
m
0
was stirred for 2 h. The solvent was removed under reduced
pressure. The residual solid was dissolved in small amount of tol-
uene, filtered through celite, and added to a tube, where n-hexane
was added slowly onto the solution. Dark-green crystals of 2 were
Appendix B. Supplementary data
ꢁ
obtained during cooling the tube at ꢀ30 C (36 mg, 26%). Anal.
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.jorganchem.2012.12.025.
29 3 2
Calcd for C26H N CoI : C, 44.85; H, 4.20; N, 6.40. Found: C, 44.68;
H, 4.57; N, 6.06.
4
of 2
.3. Reaction of alkyl halide with Grignard reagent in the presence
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L, 0.30 mmol), sty-
(
added. After dissolved in ether (0.50 mL), 2 (7.0 mg, 0.01 mmol) and
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added to the solution. After stirring at room temperature for the
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(
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4
.5. Single crystal X-ray diffraction study of 2
[
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A single crystal of 2 suitable for a X-ray diffraction study was
ꢁ
grown at ꢀ30 C from the toluene/hexane solution. The data at
1
23 K was collected on a Rigaku Saturn CCD diffractometer, using
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(
non-hydrogen atoms were refined with anisotropic displacement
parameters. All H atoms were located at ideal positions and were
included in the refinement, but were restricted to ride on the atom
to which they were bonded.
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1
6
[
[
Acknowledgments
1
(
This study was financially supported by the Japan Society for the
Promotion of Science (Grant-In-Aid for Scientific Research
131 (2009) 11949e11963.
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2
2550104) and a fund from the Central Research Institute of
[
Fukuoka University (No. 117106).
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49
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[
[
1
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octane in 12% yield.
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