Cobalt(iii) complexes bearing bidentate, tridentate, and tetradentate N-heterocyclic carbenes: Synthesis, X-ray structures and catalytic activities

Article abstract of DOI:10.1039/b906242d

Cobalt(iii) complexes, [CoCl₂(L1)₂](PF₆) (1, L1 = N-methyl-N-(2-pyrimidinyl)imidazolylidene), [CoCl(L2) ₂](PF₆)₂ (2, L2 = N-picolyl-(2-pyrimidinyl) imidazolylidene), [Co(L3)₂](PF₆)₃ (3, L3 = bis(N-2-pyrimidylimidazolylidenyl)methane) and [CoCl₂(L3)](PF ₆) (4) have been prepared from the corresponding pyrimidine functionalized imidazolium salts [HL1](PF₆), [HL2](PF₆), and [H₂L3](PF₆)₂via in situ generated silver carbene complexes under mild conditions. These cobalt complexes were characterized by ¹H and ¹³C NMR spectroscopy and X-ray diffraction analysis. The cobalt complexes have been found to be good catalyst precursors for Kumada-Corriu cross-coupling reactions of aryl halides and Grignard reagents at room temperature. Complex 1 is more active under the mild conditions.

Full text of DOI:10.1039/b906242d

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N-Heterocyclic Carbenes
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Published in issue 35, 2009 of Dalton Transactions
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www.rsc.org/dalton | Dalton Transactions
Cobalt(III) complexes bearing bidentate, tridentate, and tetradentate
N-heterocyclic carbenes: synthesis, X-ray structures and catalytic activities†
Zhenxing Xi, Bin Liu, Chunxin Lu and Wanzhi Chen*
Received 31st March 2009, Accepted 27th May 2009
First published as an Advance Article on the web 6th July 2009
DOI: 10.1039/b906242d
Cobalt(III) complexes, [CoCl2(L1)2](PF6) (1, L1 = N-methyl-N-(2-pyrimidinyl)imidazolylidene),
[CoCl(L2)2](PF6)2 (2, L2 = N-picolyl-(2-pyrimidinyl)imidazolylidene), [Co(L3)2](PF6)3 (3, L3 =
bis(N-2-pyrimidylimidazolylidenyl)methane) and [CoCl2(L3)](PF6) (4) have been prepared from the
corresponding pyrimidine functionalized imidazolium salts [HL1](PF6), [HL2](PF6), and [H2L3](PF6)2
via in situ generated silver carbene complexes under mild conditions. These cobalt complexes were
characterized by 1H and 13C NMR spectroscopy and X-ray diffraction analysis. The cobalt complexes
have been found to be good catalyst precursors for Kumada–Corriu cross-coupling reactions of aryl
halides and Grignard reagents at room temperature. Complex 1 is more active under the mild
conditions.
complexes based on pyridine or pyrazole functionalized NHC lig-
ands via carbene transfer reactions and investigated their catalytic
Introduction
activities in Kumada–Corriu cross-couplings.15 As a continuation,
herein we describe the synthesis and structural characterization
of cobalt complexes containing pyrimidine functionalized NHC
ligands. These cobalt NHC complexes are efficient catalysts for
Kumada–Corriu cross-coupling reactions of aryl halides at room
temperature.
The catalytic organic processes based on the first row metals
have been attracting more and more attention because of their
low cost and low toxicity compared with heavy metals.1 Cobalt
complexes can exist in several oxidation states from -2 to 4 and
cobalt complexes may be square-planar, tetrahedral, triangular,
bipyramidal, or octahedral.2,3 These characters make cobalt a
promising catalyst for C–C and C–heteroatom formation re-
actions. For example, cobalt complexes have been reported to
catalyze aryl sulfur bond formation,4 Heck type reaction,5 and
cross coupling reactions via organomagnesium,6 organozinc7 and
organocopper reagents.8 Notably, transition metal catalysed cross-
coupling reactions have been one of most useful approaches
for the construction of C–C bonds, especially those involving
C(sp2) centres where typical SN2 substitutions can not operate.9 In
this context, the reaction between aryl halide and aryl Grignard
reagent (Kumada–Corriu cross-coupling) is one of most powerful
and versatile tools.10 However, cobalt catalyzed Kumada–Corriu
cross-coupling reactions between two C(sp2) centres have not been
known so far.
N-heterocyclic carbenes (NHCs) have found remarkable cat-
alytic activities as ligands in C–C coupling reactions because
they can stabilize metals in different oxidation states and sup-
port catalytically active intermediates that are coordinatively
unsaturated.11 A number of cobalt NHC complexes have been
prepared and characterized in recent years,12,13 and their catalytic
activities for intramolecular cyclotrimerization of triynes,14a se-
quential cyclization/cross-couplings of alkyl halides,14b and the
coupling of aryl bromides with alkyl Grignard reagents14c have
been reported. We have recently synthesized a series of nickel
Results and discussion
Synthesis of cobalt NHC complexes
It is worth noting that most of the structurally characterised
cobalt-NHC complexes were usually obtained from the reaction
of free NHC and cobalt salts,12a–e or reactions of cobalt amide
Co[N(SiMe3)2]2 and imidazolium salts.13b The transmetallation
reaction via a silver-carbene intermediate is more convenient
than the above-mentioned two procedures, which is employed
to prepare cobalt complexes, [Co(L1)2Cl2](PF6) (1, [HL1]+
=
N-methyl-N-(2-pyrimidinyl)imidazolium), [Co(L2)2Cl](PF6)2 (2,
[HL2]+ = N-picolyl-(2-pyrimidinyl)imidazolium), [Co(L3)2](PF6)3
(3, [H2L3]2+ = bis(N-2-pyrimidylimidazoliumyl)methane), and
[Co(L3)Cl2](PF6)3 (4). The synthetic routes for these cobalt-NHC
complexes are illustrated in Scheme 1. At room temperature,
addition of Co(PPh3)2Cl2 to the solution of the silver-NHC com-
plexes generated from [HL1](PF6), [HL2](PF6), and [H2L3](PF6)2
with Ag2O without isolation afforded the corresponding cobalt-
NHC complexes 1–4, respectively. All these reactions yielded
Co(III) complexes other than the expected Co(II) complexes due
to spontaneous oxidation in air. We found that complex 4 is not
stable in solution. It could change to 3 as confirmed by in situ
NMR spectrum.
Department of Chemistry, Zhejiang University, Xixi Campus, Hangzhou,
310028, People’s Republic of China. E-mail: chenwzz@zju.edu.cn; Fax: +86-
571-88273314; Tel: +86-571-88273314
† Electronic supplementary information (ESI) available: Additional struc-
tural and NMR data. CCDC reference numbers 725129–725132. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b906242d
Cobalt-NHC complexes 1–3 were characterized by 1H and
13C NMR spectroscopy as well as elemental analysis. Complex
4 was isolated as a minor product in the reaction of [H2L3]2+,
Ag2O, and Co(PPh3)2Cl2. Attempts to prepare 4 by employment
of an excess of Co(PPh3)2Cl2 are not successful. This would yield
7008 | Dalton Trans., 2009, 7008–7014
This journal is
The Royal Society of Chemistry 2009
©

Fig. 2 Molecule structure of the cation [Co(L2)2Cl]2+ (2).
spectrum of 2 gives two doublets at d 5.28 and 4.48 ppm, whereas
1H NMR of 3 shows two doublets at d 6.73 and 6.40 ppm,
respectively, ascribed to CH2 moieties linking two heterocylic rings.
13C NMR of 1 exhibits a singlet at d 180.3 ppm, characteristic
of carbenic carbons. Unfortunately, we could not observe Co–C
resonance signals in 13C NMR spectra of 2 and 3. Actually,
few 13C NMR data on carbenic carbons of cobalt(III) complexes
containing NHC ligands has been described in the literature.13d
Structural description
The structures of complexes 1, 2, and 4 have been further
determined by single-crystal X-ray diffraction, and their detailed
crystallographic data were summarized in Table 1. X-Ray diffrac-
tion analysis showed that the cationic structure of complex 1
consists of one cobalt atom and two L1 ligands. The molecular
structure of 1 is shown in Fig. 1. As shown in Fig. 1, the central
Co(III) ion is hexa-coordinated by two pyrimidine, two NHC
carbons, and two chloride ions displaying octahedral geometry.
Two chlorides and two NHC rings are cis-positioned, and two
pyrimidine groups are arranged in a trans fashion. Both C–Co–C
and Cl–Co–Cl angles are close to 90◦. The two heteroaryl rings
of N-methyl-N-(2-pyrimidinyl)imidazolylidene were perfectly co-
planar with one chloride ion. The Co–C bond distances of ca.
Scheme 1 Synthesis of cobalt-NHC complexes 1–4.
unidentified mixtures with difficulty of isolation. Fortunately, we
were able to separate a few crystals of 4. Its characterization
1
was primarily based on X-ray single crystallography. H NMR
spectra of complexes 1–3 in DMSO-d6 showed the complete
disappearance of all the acidic H-imidazolium protons. In the
1H NMR spectrum of 1, two sets of resonance signals assigned to
N-methyl-N-(2-pyrimidinyl)imidazolylidene were observed. This
may suggest that there exist two isomers in the acetonitrile
solution and the ratio of two isomers is 1 : 1 determined by
NMR spectrum. However, only one isomer with two chlorides
cis-positioned was crystallized (See Fig. 1). On the contrary to
˚
1.91 A (Table 2) are shorter than those of the previously reported
Co(III)–NHC complexes.13 The Co–N bond distances are not
unusual compared to other cobalt(III) complexes.13,16
1
1, H NMR spectra of 2 and 3 show only one set of resonance
signals of N-picolyl-(2-pyrimidinyl)imidazolylidene and bis(N-2-
pyrimidylimidazolylidenyl)methane, although two ligands of 2 are
chemically inequivalent in its solid state (Fig. 2), illustrating that
rapid dissociation and association may occur in solution. The
two protons of methylene groups in 2 and 3 become inequivalent
because of restricted rotation upon complexation. 1H NMR
Cobalt complex 2 has also been characterized by single-crystal
X-ray diffraction. As illustrated in Fig. 2, the central cobalt atom
in 2 is hexa-coordinated by two NHC carbons, three N atoms of
pyrimidine or pyridine groups, and one chloride ion in octahedral
geometry. The two coordinated pyrimidine rings are nearly co-
planar withtheir neighbouring imidazolylidene rings with dihedral
angle of 6.8◦ and 12.6◦, respectively. The pyridine ring bonded to
cobalt atom is approximately co-planar with pyrimidine ring and
imidazolydidene ring of the same ligand, and is almost parallel
to the uncoordinated pyridine group with a dihedral angle of ca.
◦
˚
9.5 . The two Co–C bond distances are 1.815(4) and 1.913(4) A
(Table 2). The difference of the two Co–C bond distances is
probably originated from the different coordination mode of two
ligands and different trans effect of chloride and nitrogen-donating
group.
Although we have repeatedly collected the diffraction data of 3,
its crystallographic data is not satisfactory due to poor quality
of single crystals and severe disorder of hexafluorophosphate
anions and solvent molecules. Fortunately the central cobalt ion
Fig. 1 Molecule structure of the cation [Co(L1)2Cl2]+ (1).
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7008–7014 | 7009
©

Table 1 Summary of the crystallographic data for 1, 2 and 4
[Co(L1)2Cl2]·(PF6), 1
[Co(L2)2Cl]·(PF6)2, 2
[Co(L3)Cl2]·(PF6), 4
Formula
C16H16Cl2CoF6N8P
595.17
Monoclinic
C26H22ClCo F12N10P2
858.86
Triclinic
¯
P1
C15H12Cl2CoF6N8P
579.13
Triclinic
¯
P1
FW/g mol-1
Crystal system
Space group
P21/c
˚
a/A
11.951(1)
14.151(2)
13.177(2)
11.071(1)
11.570(1)
13.366(1)
84.192(2)
78.440(1)
70.381(1)
1578.9(3)
2
1.807
0.840
8294
5477(0.0263)
0.942
7.385(1)
12.699 (2)
13.077(2)
63.430(1)
74.017(1)
82.368(2)
1054.3(2)
2
1.824
1.218
5449
3652(0.0268)
1.034
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
92.545(2)
3
˚
V/A
2226.3(5)
4
1.776
1.156
11 006
3900 (0.0309)
1.091
0.0368, 0.0879
0.0592, 0.1072
Z
D
calcd/Mg m-3
m/mm-1
Reflections collected
Reflections independent (Rint
GOF on F2
R1, wR2 (I > 2sI)
R1, wR2 (all data)
)
0.0496, 0.1137
0.0873, 0.1283
0.0494, 0.1188
0.0795, 0.1402
and ligand atoms can be reliably determined from the diffraction
Fourier map. The structure of 3 is composed of one cobalt and
two bis(N-2-pyrimidylimidazolylidenyl)methane ligands. Both lig-
ands are coordinated in tridentate fashion. The central cobalt
atom is also hexa-coordianted by two pyrimidine rings and
four imidazolylidene rings. One pyrimidine group of each L3 is
uncoordinated. Two of the four imidazolylidenes are arranged
trans to two pyrimidine groups, whereas the other two are trans
each other. The crystallographic data and molecular structure are
listed in supplementary materials and are not discussed here.
Compound 4 was obtained as a minor product in the prepara-
tion of 3. Its structure is shown in Fig. 3. the structural analysis
revealed that the cation contains one Co(III) ion, one L3 ligand,
and two chloride ions. The Co(III) ion is hexa-coordinated in
octahedral sphere by two carbene, two pyrimidine rings and
two chloride ions. The ligand is coordinated in tetradentate way
occupying the equatorial plane, whereas two chlorides are located
in the two axial positions. The non-hydrogen atoms of L3 are
almost perfectly co-planar, as indicated by small dihedral angles
(1.1–2.5◦) between the coordination plane defined by CoC2N2 and
the imidazolylidene and pyrimidine rings. The Cl–Co–Cl axis is
perpendicular to the plane defined by CoC2N2 as indicated by
Cl–Co–N and Cl–Co–C angles, which are all close to 90◦. The
Co–C bond distance for Co(III)-NHC complexes are found to
13
˚
range from 1.93 to 1.96 A. The Co–Cl and Co–N distances are
˚
ca. 2.22 and 2.06 A, respectively.
Catalytic properties
Transition metal catalyzed Kumada–Corriu coupling reaction
is a powerful method for preparation of biaryl derivatives.
Cobalt catalysed Kumada–Corriu coupling reactions of aryl
halides with Grignard reagents were examined by using 1–3 as
catalyst under mild conditions. The results were summarized in
Table 3. To evaluate the catalytic activities of complexes 1–3, we
firstly investigated the catalytic reaction of phenyl bromide with
p-tolylMgBr at room temperature. The results show that when
1 mol% 1 was used, the reaction gave a moderate yield of
4-methylbiphenyl (entry 1). When the amount of 1 was increased
to 2 mol%, the reaction could be completed within 2 h at room
temperature, and the desired biaryls were isolated in 96% yield
(entry 2). With the same catalyst loading, complexes 2 and 3
catalyzed the couplings affording the corresponding biphenyls in
75% and 56% (entries 3 and 4), respectively. These data illustrate
that complex 1 is more active than 2 and 3 under the same
conditions. This is probably because the bulkier substituents of
2 and 3 might inhibit the close approach of the substrate and the
metal centre.
˚
Co–C distances are 1.823(5) and 1.828(4) A (Table 2), which are
shorter than those of the reported Co(III)-NHC complexes.13 The
To explore the scope of the cross couplings, we ultilized
2–4 mol% of complex 1 as catalyst to study the reactions between
various aryl bromides, aryl chlorides and Grignard reagents. At
room temperature, catalyst 1 is efficient for heterocylic chlorides
such as 2-chloropyrimidine, and the desired product was isolated in
nearly quantitative yield (entry 7). Unfortunately, we found that
1 was less efficient for the coupling reaction of chlorobenzene,
and only a poor yield (21%) was obtained (entry 5). Our cobalt
catalyst is also applied to the coupling reactions of heterocyclic
bromide and vinyl bromide. For example, 2-bromopyridine and
1-(2-bromovinyl)benzene were easily coupled with p-tolylMgBr to
produce the corresponding products in excellent yields (entries
8 and 11). Aryl bromide bearing electron donating group such
Fig. 3 Molecule structure of the cation [Co(L3)Cl2]+ (4).
7010 | Dalton Trans., 2009, 7008–7014
This journal is
The Royal Society of Chemistry 2009
©

◦
˚
Table 2 Selected bond distances (A) and angles ( ) of 1, 2, and 4
1
2
4
˚
Bond distances/A
Co(1)–C(9)
Co(1)–C(1)
Co(1)–N(7)
Co(1)–N(3)
Co(1)–Cl(2)
Co(1)–Cl(1)
1.907(3)
1.915(3)
1.942(3)
1.950(3)
2.277(1)
2.297(1)
Co(1)–C(1)
Co(1)–C(14)
Co(1)–N(5)
Co(1)–N(3)
Co(1)–N(8)
Co(1)–Cl(1)
1.815(4)
1.913(4)
1.972(3)
1.976(3)
2.005(3)
2.253(1)
Co(1)–C(9)
Co(1)–C(2)
Co(1)–N(3)
Co(1)–N(7)
Co(1)–Cl(2)
Co(1)–Cl(1)
1.823(5)
1.828(4)
2.059(4)
2.077(4)
2.226(2)
2.231(1)
Bond angles/◦
C(9)–Co(1)–C(1)
C(9)–Co(1)–N(7)
C(1)–Co(1)–N(7)
C(9)–Co(1)–N(3)
C(1)–Co(1)–N(3)
N(7)–Co(1)–N(3)
C(9)–Co(1)–Cl(2)
C(1)–Co(1)–Cl(2)
N(7)–Co(1)–Cl(2)
N(3)–Co(1)–Cl(2)
C(9)–Co(1)–Cl(1)
C(1)–Co(1)–Cl(1)
N(7)–Co(1)–Cl(1)
N(3)–Co(1)–Cl(1)
Cl(2)–Co(1)–Cl(1)
91.91(14)
82.37(13)
95.37(12)
96.47(13)
82.02(13)
177.12(12)
176.42(10)
87.87(10)
94.09(9)
C(1)–Co(1)–C(14)
C(1)–Co(1)–N(5)
C(14)–Co(1)–N(5)
C(1)–Co(1)–N(3)
C(14)–Co(1)–N(3)
N(5)–Co(1)–N(3)
C(1)–Co(1)–N(8)
C(14)–Co(1)–N(8)
N(5)–Co(1)–N(8)
N(3)–Co(1)–N(8)
C(1)–Co(1)–Cl(1)
C(14)–Co(1)–Cl(1)
N(5)–Co(1)–Cl(1)
N(3)–Co(1)–Cl(1)
N(8)–Co(1)–Cl(1)
97.14(18)
89.88(17)
90.98(14)
80.60(17)
89.68(15)
170.46(15)
172.07(16)
81.60(15)
97.96(13)
91.55(14)
87.73(13)
174.99(13)
90.22(10)
89.95(10)
93.42(10)
C(9)–Co(1)–C(2)
C(9)–Co(1)–N(3)
C(2)–Co(1)–N(3)
C(9)–Co(1)–N(7)
C(2)–Co(1)–N(7)
N(3)–Co(1)–N(7)
C(9)–Co(1)–Cl(2)
C(2)–Co(1)–Cl(2)
N(3)–Co(1)–Cl(2)
N(7)–Co(1)–Cl(2)
C(9)–Co(1)–Cl(1)
C(2)–Co(1)–Cl(1)
N(3)–Co(1)–Cl(1)
N(7)–Co(1)–Cl(1)
Cl(2)–Co(1)–Cl(1)
86.8(2)
167.18(19)
80.45(18)
79.86(19)
166.64(19)
112.92(16)
89.75(16)
89.49(15)
89.15(12)
90.57(12)
89.88(16)
89.70(15)
91.04(12)
90.15(12)
179.12(6)
87.04(9)
85.78(10)
175.89(10)
87.70(8)
94.86(9)
94.65(4)
as OMe could give a moderate yield (entry 9). As expected, the
coupling reactions of aryl bromides bearing ortho substituents
such as 2-bromotoluene and 1-bromonaphthalene gave lower
conversions as a result of hindrance effect (entry 6 and 10).
As part of our study, the influence of Grignard reagents was also
studied. When o-tolylMgBr was employed, the substrates such
as 2-chloropyrimidine, 2-bromopyridine, and 1-(2-bromovinyl)-
benzene could be coupled to afford the corresponding product in
yields of 78%, 67%, and 81% (entries 13, 14, and 17), respectively.
Obviously, the yields are lower than the reactions of the same
substrates with p-tolylMgBr. Furthermore, the coupling of aryl
bromides bearing electron donating group and ortho substituents
such as 4-bromoanisole and 2-bromonaphthalene could proceed
in yields of ca. 50% (entries 15 and 16). The data revealed that our
cobalt catalyst is a bit sensitive to steric hindrance at the 2-position.
We can also synthesize terphenyls via one-pot double coupling of
aryl dihalides by using 4 mol% 1 as catalyst. For example, both
1,3- and 1,4-ditolylbenzene could be obtained in good yields from
1,3-dichlorobenzene and 1,4-dichlorobenzene (entries 19 and 20).
Similarly, 2,6-ditolylpyridine could also be achieved in excellent
yield (entry 18).
may occur to release the target product and regenerate Co(I)
species (Scheme 2). We believe that NHC ligands can stabilize the
Co(I) active catalytic centre and facilitate the oxidative addition of
aryl halide due to their strong electron donating ability.
Scheme 2 Proposed mechanism for cobalt catalyzed cross coupling of
aryl halides.
Mechanism consideration
Conclusions
To fully understand the mechanism of the cobalt(III) catalyzed
C(sp2)–C(sp2) coupling reaction further study is required. Co(III)
can be reduced to Co(0), Co(I), and Co(II) species, and it is not
easy to distinguish which is the real active species at present. We
tentatively assume that Co(I) species is formed in the presence
of Grignard reagent. The coupling may start with the oxidative
addition of aryl halide to the Co(I) center to form an Ar’Co(III)
intermediate. Subsequent nucleophilic substitution of a halide
would give the diaryl Co(III) species. Finally, reductive elimination
In summary, we have synthesized a few cobalt(III) complexes
supported by pyrimidine functionalized NHC ligands via simple
carbene transfer reactions under mild conditions. These cobalt
complexes especially 1 exhibits good activities in Kumada–Corriu
cross-coupling reactions of aryl halides at room temperature.
These simple ligands and low cost of cobalt salt make the cobalt-
NHC complexes a complement for Ni and Pd catalysts and
a promising area for future development of non-noble metal
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7008–7014 | 7011
©

Table 3 Cross-coupling of aryl halides with Grignard reagents catalysed by 1–3a
Entry
Cat (mol%)
Ar
X
R
Product
Yieldb (%)
1
1 (1)
Phenyl
Br
4-Me
45
2
3
4
5
6
1 (2)
2 (2)
3 (2)
1 (2)
1 (2)
Phenyl
Phenyl
Phenyl
Phenyl
Br
Br
Br
Cl
Br
4-Me
4-Me
4-Me
4-Me
4-Me
2a
2a
2a
2a
96
75
56
21
56
2-MeC6H4
7
8
1 (2)
1 (2)
2-Pyrimidinyl
2-Pyridyl
Cl
Br
4-Me
4-Me
97
92
9
1 (2)
1 (2)
4-MeOC6H4
1-Naphthyl
Br
Br
4-Me
4-Me
71
62
10
11
1 (2)
Bromostyrene
Br
4-Me
93
12
13
1 (2)
1 (2)
Phenyl
2-Pyrimidinyl
Br
Cl
2-Me
2-Me
2p
57
78
14
15
16
1 (2)
1 (2)
1 (2)
2-Pyridyl
Br
Br
Br
2-Me
2-Me
2-Me
67
53
45
4-MeOC6H4
1-Naphthyl
17
18
1 (2)
1 (4)
Bromostyrene
2,6-Pyridyl
Br
Br
2-Me
4-Me
81
93
7012 | Dalton Trans., 2009, 7008–7014
This journal is
The Royal Society of Chemistry 2009
©

Table 3 (Contd.)
Entry
Cat (mol%)
Ar
X
R
Product
Yieldb (%)
19
20
1 (4)
1 (4)
1,4-Phenyl
1,3-Phenyl
Br
Br
4-Me
4-Me
83
82
Reaction conditions: aryl dihalide 1.0 mmol, Grignard reagent 2.4 mmol, cobalt catalyst 4 mol%.a Reaction conditions: aryl halide 1.0 mmol, Grignard
reagent 1.2 mmol, cobalt catalyst 2 mol%, THF, 3 mL, 2 h, room temperature. b Isolated yields.
catalysts. The potential of the cobalt-NHC catalysts in other
organic transformations is under investigation.
Synthesis of [Co(L2)2Cl](PF6)2 (2)
The compound was obtained as a yellow solid using the same
procedure as for 1 by using [HL2](PF6) (382 mg, 1 mmol), Ag2O
(116 mg, 0.5 mmol), and Co(PPh3)2Cl2 (327 mg, 0.5 mmol).
Yield: 206 mg, 48%. Crystals suitable for X-ray diffraction
study were grown at room temperature by slow diffusion of
ethyl ether into an acetonitrile solution of 2. Anal. calcd for
C26H25ClCoF12N10P2O1.5([Co(L2)2Cl](PF6)2·1.5H2O): C 35.25, H
2.84, N 15.81. Found: C 35.11, H 2.47, N 15.74. 1H NMR
(400 MHz, DMSO-d6): 9.30 (t, J = 2.4 Hz, 2 H, Hpyridine), 8.18
(t, J = 2.4 Hz, 2 H, Hpyridine), 8.04 (d, J = 4.0 Hz, 2 H, CHimidazole),
7.98 (d, J = 4.0 Hz, 2 H, CHimidazole), 7.80 (t, J = 7.6 Hz, 2 H,
5-C4H3N2), 7.74 (m, 4 H, 6-C4H3N2 + Hpyridine), 7.3 (t, J = 5.6 Hz,
2 H, Hpyridine), 7.19 (d, J = 7.6 Hz, 2 H, 4-C4H3N2), 5.28 (d, J =
16.8 Hz, 2 H, CH2), 4.48 (d, J = 16.8 Hz, 2 H, CH2) ppm. 13C
NMR (100 MHz, DMSO-d6): d 163.6, 160.5, 156.1, 150.3, 149.8,
138.0, 131.0, 124.1, 122.8, 122.3, 121.4, 54.1 ppm.
Experimental
All the chemicals were obtained from commercial suppliers and
used without further purification. [HL1](PF6),17 [HL2](PF6),17
18
[H2L3](PF6)2,17 and Co(PPh3)2Cl2 were prepared according to
the known procedure. Elemental analyses were performed on a
Flash EA1112 instrument. 1H and 13C NMR spectra were recorded
on Bruker Avance-400 (400 MHz) spectrometer. Chemical shifts
(d) are expressed in ppm downfield to TMS at d = 0 ppm and
coupling constants (J) are expressed in Hz.
Synthesis of [Co(L1)2Cl2](PF6) (1)
A solution of [HL1](PF6) (306 mg, 1 mmol) in 10 mL CH3CN was
treated with Ag2O (348 mg, 1.5 mmol). The mixture was allowed
to react at room temperature for 10 h, and then it was filtered
to remove a small amount of unreacted Ag2O. The filtrate was
treated with Co(PPh3)2Cl2 (327 mg, 0.5 mmol). After it was stirred
for 10 h at room temperature, the solution was filtered. The filtrate
was concentrated to ca. 5 mL. Addition of 20 mL of diethyl ether
gave a yellow solid. Yield: 187 mg, 63%. Crystals suitable for X-
ray diffraction study were grown at room temperature by slow
diffusion of ethyl ether into an acetonitrile solution of 1. Anal.
calcd for C17H18.5Cl2CoF6N8PO0.25 ([Co(L1)2Cl2](PF6).0.25Et2O):
Synthesis of [Co(L3)2](PF6)3 (3)
The compound was obtained as a yellow solid using the same
procedure as for 1 by using [H2L3](PF6)2 (596 mg, 1 mmol), Ag2O
(232 mg, 1 mmol), and Co(PPh3)2Cl2 (327 mg, 0.5 mmol). Yield:
248 mg, 45%. Crystals suitable for X-ray diffraction study were
grown at room temperature by slow diffusion of ethyl ether into
an acetonitrile solution. Anal. calcd for C31.6H28CoF18N16P3O0.4
([Co(L3)2](PF6)3·0.4Et2O): C 33.53, H 2.49, N 19.80. Found: C
1
33.16, H 2.31, N 19.54. H NMR (400 MHz, DMSO-d6): 9.00
(dd, J = 2.0, 4.8 Hz, 2 H, 6-C4H3N2), 8.89 (d, J = 4.8 Hz, 4 H,
4-C4H3N2), 8.77 (d, J = 2.4 Hz, 2 H, CHimidazole), 8.17 (d, J =
2.4 Hz, 2 H, CHimidazole), 8.02 (d, J = 2.0 Hz, 2 H, CHimidazole), 7.82
(d, J = 2.0 Hz, 2 H, CHimidazole), 7.81 (t, J = 4.8 Hz, 4 H, 5-C4H3N2),
7.53 (d, J = 5.6 Hz, 2 H, 6-C4H3N2), 7.39 (t, J = 5.6 Hz, 2 H,
5-C4H3N2), 6.73 (d, J = 14.4 Hz, 2 H, CH2), 6.40 (d, J = 14.4 Hz,
2 H, CH2) ppm. 13C NMR (100 MHz, DMSO-d6): d 163.0, 162.7,
160.5, 157.7, 136.8, 130.2, 129.3, 125.5, 122.8, 121.1, 119.3, 118.8,
36.2 ppm.
1
C 33.27, H 3.04, N 18.26. Found: C 32.82, H 2.81, N 17.86. H
NMR (400 MHz, DMSO-d6): 10.00 (dd, J = 2.4, 4.8 Hz, 1 H,
6-C4H3N2), 9.68 (dd, J = 1.6, 6.0 Hz, 2 H, 6-C4H3N2), 9.39 (dd,
J = 2.4, 4.8 Hz, 1 H, 6-C4H3N2), 9.21 (2d, J = 1.6, 4.8 Hz, 2 H,
4-C4H3N2), 8.91 (dd, J = 1.6, 4.8 Hz, 1 H, 6-C4H3N2), 8.58 (d,
J = 2.4 Hz, 1 H, CHimidazole), 8.52 (d, J = 1.6 Hz, 1 H, CHimidazole),
8.44 (d, J = 2.4 Hz, 2 H, CHimidazole), 8.10 (t, J = 4.8 Hz, 1 H, 5-
C4H3N2), 8.03 (d, J = 2.4 Hz, 1 H, CHimidazole), 7.84 (t, J = 4.8 Hz,
2 H, 5-C4H3N2), 7.73 (m, 4 H, 4-C4H3N2 + CHimidazole), 7.28 (t,
J = 5.2 Hz, 1 H, 5-C4H3N2), 4.26 (s, 3 H), 3.29 (s, 3 H), 2.93 (s, 6
H) ppm. 13C NMR (100 MHz, DMSO-d6): d 180.4 (Co–C), 163.3,
161.7, 160.5, 156.1, 156.0, 146.5, 128.3, 126.8, 125.9, 123.2, 122.0,
121.5, 63.4 ppm.
General procedure for the cross-coupling reaction
A Schlenk tube was charged with cobalt complexes (0.01–
0.04 mmol), anhydrous THF (3 mL), and aryl halide (1.0 mmol).
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7008–7014 | 7013
©

To the solution was added a solution of Grignard reagent (1.2–
2.4 mL, 1.0 M in THF) at room temperature with stirring. After
it was stirred for 2 h, the reaction was ceased by addition of water.
The mixture was extracted with ethyl acetate (3 ¥ 5 mL). The
combined organic layer was dried by MgSO4. The filtrate was
concentrated by rotary evaporation and the crude product was
purified by column chromatography on silica gel to afford the
desired product.
5 (a) Y. Ikeda, T. Nakamura, T. Yorimitsu and K. Oshima, J. Am. Chem.
Soc., 2002, 124, 6514; (b) W. Affo, H. Ohmiya, T. Fujioka, Y. Ikeda, T.
Nakamura, H. Yorimitsu, K. Oshima, Y. Imamura, T. Mizuta and K.
Miyoshi, J. Am. Chem. Soc., 2006, 128, 8086; (c) P. Shukla, Y. Hsu and
C. C. Cheng, J. Org. Chem., 2006, 71, 655; (d) M. Amatore, C. Gosmini
and J. Perichon, J. Org. Chem., 2006, 71, 6130.
6 (a) T. Tsuji, H. Yorimitsu and K. Oshima, Angew. Chem., Int. Ed.,
2002, 41, 4137; (b) E. Shirakawa, T. Sato, Y. Imazaki, T. Kimura and
T. Hayashi, Chem. Commun., 2007, 4513; (c) H. Ohmiya, T. Tsuji, H.
Yrimitsu and K. Oshima, Chem.–Eur. J., 2004, 10, 5640; (d) H. Ohmiya,
H. Yorimitsu and K. Oshima, J. Am. Chem. Soc., 2006, 128, 1886; (e) G.
Cahiez, C. Chaboche, C. Duplais and A. Moyeux, Org. Lett., 2009, 11,
277.
X-Ray structural determination
7 (a) I. Kazmierski, M. Bastienne, C. Gosmini, J.-M. Paris and J.
Perichon, J. Org. Chem., 2004, 69, 936; (b) P. Gomes, C. Gosmini and
J. Perichon, Synlett, 2002, 1673; (c) G. Dunet and P. Knochel, Synlett,
2007, 1383; (d) P. Gomes, C. Gosmini and J. Perichon, Org. Lett., 2003,
5, 1043.
8 (a) T. J. Korn and P. Knochel, Angew. Chem., Int. Ed., 2005, 44, 2947;
(b) T. J. Korn, M. A. Schade, S. Wirth and P. Knochel, Org. Lett., 2006,
8, 725.
9 (a) F. Diederich, P. J. Stang, Metal-Catalyzed Cross-Coupling Reactions,
Wiley-VCH, Weinheim, Germany, 2004; (b) A. de Meijere and F.
Diederich, in Metal-Catalyzed Cross-Coupling Reactions, Wiley-VCH,
Weinheim, 2nd edn, 2004; (c) M. Beller and C. Bolm, in Transition
Metals for Organic Synthesis, Wiley-VCH, Weinheim, 2nd edn, 2004;
(d) B. C. G. Soderberg, Coord. Chem. Rev., 2008, 252, 57.
10 (a) M. Kumada, Pure Appl. Chem., 1980, 52, 699; (b) H. Shinokubo
and K. Oshima, Eur. J. Org. Chem., 2004, 2081; (c) H. Ila, O. Baron,
A. J. Wagner and P. Knochel, Chem. Commun., 2006, 583.
11 (a) W. A. Hermann, Angew. Chem., Int. Ed., 2002, 41, 1290; (b) E.
Kantchev, C. O’Brien and M. Organ, Angew. Chem., Int. Ed., 2007, 46,
2768; (c) O. Kaufhold and F. E. Hahn, Angew. Chem., Int. Ed., 2008,
47, 4057; (d) F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed.,
2008, 47, 3122.
12 (a) R. E. Cowley, R. P. Bontchev, E. N. Duesler and J. M. Smith, Inorg.
Chem., 2006, 45, 9771; (b) X. Hu, I. Castro-Rodriguez and K. Meyer,
J. Am. Chem. Soc., 2004, 126, 13464; (c) R. W. Simms, M. J. Drewitt and
M. C. Baird, Organometallics, 2002, 21, 2958; (d) D. S. McGuinness,
V. C. Gibson and J. W. Steed, Organometallics, 2004, 23, 6288; (e) H.
Rensburg, R. Tooze, D. Foster and A. Slawin, Inorg. Chem., 2004, 43,
2468; (f) H. Rensburg, R. P. Tooze, D. F. Foster and S. Otto, Inorg.
Chem., 2007, 46, 1963; (g) G. Erker, R. Lecht, J. F. Petersen and H.
Bonnemann, Organometallics, 1987, 6, 1962; (h) D. W. Macomber and
R. D. Rogers, Organometallics, 1985, 4, 1485.
Single-crystal X-ray diffraction data were collected at 298(2) K
on a Siemens Smart/CCD area-detector diffractometer with a
˚
MoKa radiation (l = 0.71073 A) by using an w - 2q scan
mode. Unit-cell dimensions were obtained with least-squares
refinement. Data collection and reduction were performed using
the SMART and SAINT software.19 The structures were solved
by direct methods, and the non-hydrogen atoms were subjected to
anisotropic refinement by full-matrix least squares on F2 using the
SHELXTXL package.20 Hydrogen atom positions for all of the
structures were calculated and allowed to ride on their respective
˚
C atoms with C–H distances of 0.93–0.97 A and Uiso(H)) =
-1.2–1.5Ueq(C). Hydrogen atoms bound to water molecules were
located in the Fourier difference map, and their distances were
fixed. Further details of the structural analysis are summarized in
Table 1.
Acknowledgements
We gratefully acknowledge the financial support of NSFC
(20872129), the PhD. Programs Foundation of Ministry of
Education of China (200803350011), Zhejiang University K. P.
Chao’s High Technology Development Foundation.
Notes and references
13 (a) U. Plaia, H. Stolzenberg and W. P. Fehlhammer, J. Am. Chem.
Soc., 1985, 107, 2172; (b) A. A. Danopoulos, J. A. Wright, W. B.
Motherwell and S. Ellwood, Organometallics, 2004, 23, 4807; (c) X.
Hu and K. Meyer, J. Am. Chem. Soc., 2004, 126, 16322; (d) R. Frankel,
U. Kernbach, M. Bakola-Christianopoulou, U. Plaia, M. Suter, W.
Ponikwar, H. Noth, C. Moinet and W. P. Fehlhammer, J. Organomet.
Chem., 2001, 617–618, 530.
14 (a) N. Saino, D. Kogure and S. Okamoto, Org. Lett., 2005, 7, 3065;
(b) H. Someya, H. Ohmiya, H. Yorimitsu and K. Oshima, Org. Lett.,
2007, 9, 1565; (c) H. Hamaguchi, M. Uemura, H. Yasui, H. Yorimitsu
and K. Oshima, Chem. Lett., 2008, 37, 1178.
15 (a) Z. Xi, X. Zhang, W. Chen, S. Fu and D. Wang, Organometallics,
2007, 26, 6636; (b) Z. Xi, B. Liu and W. Chen, J. Org. Chem., 2008, 73,
3954; (c) Z. Xi, Y. Zhou and W. Chen, J. Org. Chem., 2008, 73, 8497;
(d) Y. Zhou, Z. Xi and W. Chen, Organometallics, 2008, 27, 5911.
16 (a) K. Okamoto and S. Fukumuzi, J. Am. Chem. Soc., 2003, 125, 12416;
(b) H. Kim, W. Kim, A. Lough, B. Kim and J. Chin, J. Am. Chem. Soc.,
2005, 127, 16776; (c) Y. Shimazaki, R. Kabe, S. Huth, F. Tani, Y. Naruta
and O. Yamauchi, Inorg. Chem., 2007, 46, 6755.
1 (a) A. Correa, O. G. Mancheno and C. Bolm, Chem. Soc. Rev., 2008,
37, 1108; (b) A. Furstner and R. Martin, Chem. Lett., 2005, 34, 624;
(c) B. D. Sherry and A. Furstner, Acc. Chem. Res., 2008, 41, 1500;
(d) H. Plenio, Angew. Chem., Int. Ed., 2008, 47, 2; (e) M. Meldal and
C. W. Tornoe, Chem. Rev., 2008, 108, 2952; (f) S. Reymond and J.
Cossy, Chem. Rev., 2008, 108, 5359; (g) G. Cahiez, C. Duplais and J.
Buendia, Chem. Rev., 2009, 109, 1434; (h) C. Gosmini, J. Begouin and
A. Moncomble, Chem. Commun., 2008, 3221.
2 (a) J. Matachek and R. Angellci, Organometallics, 1982, 1, 1541; (b) R.
Guilard, F. Burdet, J. Barbe, C. Gros, E. Espinosa, J. Shao, Z. Ou, R.
Zhan and K. Kadishi, Inorg. Chem., 2005, 44, 3972; (c) K. Alley, R.
Bircher, O. Waldmann, S. Ochsenbein, H. Guldel, B. Moubaraki, K.
Murray, F. Fernandez-Alonso, B. Abrahams and C. Boskovic, Inorg.
Chem., 2006, 45, 8950; (d) R. Klingler, M. Chen, J. Rathke and K.
Kramarz, Organometallics, 2007, 26, 352.
3 (a) C. Benelli and D. Datteschi, Inorg. Chem., 1982, 21, 1788; (b) D.
Corwin Jr, R. Fikar and S. Koch, Inorg. Chem., 1987, 26, 3079; (c) A.
Palacios, P. Alemany and S. Alvarez, Inorg. Chem., 1999, 38, 707;
(d) M. Ingleson, M. Pink, H. Fan and K. Caulton, Inorg. Chem., 2007,
46, 10321; (e) C. Bianchini, G. Giambastiani, A. Meli and A. Toti,
Organometallics, 2007, 26, 1303; (f) Y. Zhou, F. Meng, J. Zhang, M.
Zeng and H. Liang, Cryst. Growth Des., 2009, 9, 1402; (g) M. Whited,
N. Mankad, Y. Lee, P. Oblad and J. Peters, Inorg. Chem., 2009, 48,
2507.
17 J. Ye, W. Chen and D. Wang, Dalton Trans., 2008, 4015.
18 J. Chatt, J. Chem. Soc., 1961, 285.
19 SMART-CCD Software, version 4.05; Siemens Analytical X-ray In-
struments, Madison, WI, 1996.
20 G. M. Sheldrick, SHELXS-,97 and SHELXL-97, Program for
X-ray Crystal Structure Refinement; University of Go¨tingen, Go¨tingen,
Germany 1997.
4 Y. Wong, T. T. Jayanth and C. C. Cheng, Org. Lett., 2006, 8,
5613.
7014 | Dalton Trans., 2009, 7008–7014
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The Royal Society of Chemistry 2009
©