Mixed NHC/Phosphine Ni(II) Complexes: Synthesis and Their Applications as Versatile Catalysts for Selective Cross-Couplings of ArMgX with Aryl Chlorides, Fluorides, and Methyl Ethers

Article abstract of DOI:10.1021/acs.organomet.5b00874

New methods for the preparation of mixed NHC/phosphine Ni(II) complexes have been developed. It was shown that the quaternary ammonium cation in the easily available Ni(II) complexes [NEt₄][Ni(PPh₃)X₃] (X = Cl and Br) can act as a good leaving group in reactions of [NEt₄][Ni(PPh₃)X₃] with the bulky ItBu (ItBu = 1,3-ditertbutylimidazol-2-ylidene) or IPr [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] ligand, resulting in the corresponding mixed NHC/PPh₃ Ni(II) complexes Ni(PPh₃)(ItBu)X₂ (X = Cl, 1; X = Br, 2) or Ni(PPh₃)(IPr)Br₂ (3) in high yields. The PPh₃ ligand in these obtained mixed NHC/PPh₃ Ni(II) complexes can be easily substituted by a more electron-donating phosphine ligand, i.e., PCy₃, resulting in the corresponding mixed NHC/PCy₃ Ni(II) complexes Ni(PCy₃)(ItBu)Br₂ (4) and Ni(PCy₃)(IPr)Br₂ (5) in high yields. The crystal structures of these Ni(II) complexes have been characterized, which revealed a trans disposition of the NHC ligand to the phosphine ligand. The catalytic behaviors of them on varying the carbene ligand (ItBu vs IPr) as well as the phosphine ligand (PPh₃ vs PCy₃) were investigated in the cross-coupling of aryl Grignard reagents with a wide range of electrophiles. In addition to a significant synergic effect on their catalytic activities, high selectivity for the activation and transformation of C-Cl, C-F and C-O bonds was achieved based on the rational structural design. Complex 2 showed the highest catalytic activity for the cross-coupling of aryl chlorides and fluorides with aryl Grignard reagents, but exhibit little activity for the cross-coupling of aryl methyl ethers with aryl Grignard reagents. On contrast, complex 4 showed great potential for the aryl methyl ethers involved cross-coupling reactions, although its reactivity for the activation of the C-X bond is very poor. The difference in catalytic activity between 2 and 4 has been successfully employed to construct oligoarenes by selective cross-coupling reactions.

Full text of DOI:10.1021/acs.organomet.5b00874

Article
pubs.acs.org/Organometallics
Mixed NHC/Phosphine Ni(II) Complexes: Synthesis and Their
Applications as Versatile Catalysts for Selective Cross-Couplings of
ArMgX with Aryl Chlorides, Fluorides, and Methyl Ethers
Jie Zhang, Jin Xu, Yanchao Xu, Hongmei Sun,* Qi Shen, and Yong Zhang
The Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Suzhou 215123, People’s Republic of China
S
* Supporting Information
ABSTRACT: New methods for the preparation of mixed
NHC/phosphine Ni(II) complexes have been developed. It
was shown that the quaternary ammonium cation in the easily
available Ni(II) complexes [NEt4][Ni(PPh3)X3] (X = Cl and
Br) can act as a good leaving group in reactions of
[NEt4][Ni(PPh3)X3] with the bulky ItBu (ItBu = 1,3-
ditertbutylimidazol-2-ylidene) or IPr [IPr = 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene] ligand, resulting in the
corresponding mixed NHC/PPh3 Ni(II) complexes Ni(PPh3)-
(ItBu)X2 (X = Cl, 1; X = Br, 2) or Ni(PPh3)(IPr)Br2 (3) in
high yields. The PPh3 ligand in these obtained mixed NHC/
PPh3 Ni(II) complexes can be easily substituted by a more electron-donating phosphine ligand, i.e., PCy3, resulting in the
corresponding mixed NHC/PCy3 Ni(II) complexes Ni(PCy3)(ItBu)Br2 (4) and Ni(PCy3)(IPr)Br2 (5) in high yields. The
crystal structures of these Ni(II) complexes have been characterized, which revealed a trans disposition of the NHC ligand to the
phosphine ligand. The catalytic behaviors of them on varying the carbene ligand (ItBu vs IPr) as well as the phosphine ligand
(PPh3 vs PCy3) were investigated in the cross-coupling of aryl Grignard reagents with a wide range of electrophiles. In addition
to a significant synergic effect on their catalytic activities, high selectivity for the activation and transformation of C−Cl, C−F and
C−O bonds was achieved based on the rational structural design. Complex 2 showed the highest catalytic activity for the cross-
coupling of aryl chlorides and fluorides with aryl Grignard reagents, but exhibit little activity for the cross-coupling of aryl methyl
ethers with aryl Grignard reagents. On contrast, complex 4 showed great potential for the aryl methyl ethers involved cross-
coupling reactions, although its reactivity for the activation of the C−X bond is very poor. The difference in catalytic activity
between 2 and 4 has been successfully employed to construct oligoarenes by selective cross-coupling reactions.
well developed and applied commonly in cross-coupling
reactions.3
INTRODUCTION
■
In recent years, N-heterocyclic carbenes (NHCs) have become
an intensively studied class of ligands for the development of
late-transition metal-based homogeneous catalysts.1 While one
main area of these studies has been directed toward the
application of NHCs as alternatives to traditional phosphine
ligands, another significant part of the investigations has
focused on the potential to make these two types of ligands
work together in order to construct highly active and/or
selective, yet robust catalysts.2 In this context, various mixed
NHC/phosphine Pd(II) complexes, for example, have been
successfully designed for a variety of cross-coupling reactions.3
Mechanistic investigations have generated the explanation for
the synergic effect between the two types of ligands that the
labile phosphine ligand is capable of reversibly dissociating from
the palladium to generate in situ a vacant coordination site
readily for substrate binding, whereas the tightly bound NHC
ligand is supposed to stay with the palladium to stabilize the
active species as well as facilitate the oxidative addition and/or
reductive elimination in the catalytic processes. On this basis,
the mixed NHC/phosphine complexes of Pd(II) have been
On the other hand, the replacement of palladium by nickel in
transition metal-catalyzed cross-coupling reactions has also
attracted increasing attention since it represents significant cost
savings.4 Mechanistic features of nickel-catalyzed cross-coupling
reactions favor elemental steps which are considerably similar
to those of palladium mediated ones. However, in comparison
with the intensively studied mixed NHC/phosphine Pd(II)
complexes, the related Ni(II) complexes are still less
explored.5−10
To date, only several kinds of well-defined mixed NHC/
phosphine Ni(II) complexes have been reported involving
monodentate NHC/phosphine Ni(II) complexes,6,8 bidentate
phosphine-functionalized imidazolium cations9 or derived
NHCs7 supported Ni(II) complexes and tridentate fluoroal-
koxy-functionalized NHC-based Ni(II) complex.10 Among
them, the monodentate NHC/phosphine Ni(II) complex, i.e.,
Received: October 21, 2015
© XXXX American Chemical Society
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Ni(PPh3)(IPr)Cl2 [IPr = 1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene], has become more appealing, not only
due to its superior activity in the Kumada cross-coupling
reaction as compared to those of Ni(IPr)2Cl2 and Ni-
(PPh3)2Cl2,6a but also due to its remarkable catalytic perform-
ances for α-arylation of acyclic ketones,6b amination of
haloarenes,6b and controlled cross-coupling of 2-substituted6c
or 3-substituted thiophenes.6d These results show that a
promising synergic effect can be achieved by the combining
of a monodentate phosphine ligand and a monodentate NHC
in nickel-catalyzed reactions. Furthermore, it is also reasonable
that, in the case of complexes of type Ni(PPh3)(IPr)X2, the use
of different pairs of mixed monodentate ligands could favor a
fine-tuning of catalytic performances in a more facile and
variable fashion, as those observed in palladium-based
catalysis.2a,3a However, the scope of such kind of Ni(II)
complexes remains extremely narrow. This limitation is due
principally to the fact that Ni(PPh3)(IPr)Cl2 and its bromide
analogue were occasionally isolated from the substitution
reaction of the phosphine ligands in Ni(PPh3)2X2 only with IPr
ligand,6a which once was a well-known preparative approach to
biscarbene Ni(II) complexes of the type Ni(NHC)2X2.11 Thus,
the development of a facile synthetic approach to such kind of
mixed NHC/phosphine Ni(II) complexes is highly desired.
As a continuation of our studies on the development of
nickel-based catalysis of NHCs,12 herein we describe the facile
synthesis of a series of mixed NHC/PR3 Ni(II) complexes 1−5.
Notably, these Ni(II) complexes, possessing different pairs of
NHC/phosphine ligands, can be used as efficient catalysts for
the selective cross-coupling of aryl Grignard reagents with a
wide range of electrophiles including aryl chlorides, aryl
fluorides and aryl methyl ethers.
obtained from the substitution reaction of Ni(PPh3)2Br2 with
IPr reported previously, whereas only a 21% yield was
reached.6a Of note, there is trace amount of biscarbene Ni(II)
complexes observed in these reactions, mostly due to the
difference in reactivity between the phosphine ligand and the
quaternary ammonium cation. The formation of complexes 1−
3 were supported by elemental analysis, NMR spectroscopy,
and X-ray diffraction studies. In C6D6, the NMR data of
complexes 1−3 are similar to those of Ni(PPh3)(IPr)Cl2
reported previously.6a The carbenic carbon resonances in 1−
3 were observed as doublets with large coupling constants
centered at 153.0 (JCP = 128.7 Hz), 156.1 (JCP = 124.0 Hz) and
171.115 (JCP = 131.4 Hz) ppm, respectively, suggesting the trans
position of the PPh3 ligand with respect to the NHC
ligand.6a,8,11 At room temperature, the 31P NMR resonances
due to the PPh3 ligand of 1−3 were observed as sharp singlet at
7.9, 14.0, and 19.5 ppm, respectively. Interestingly, the
resonances due to the carbenic carbon atom together with
the phosphorus atom in 2 are obviously shifted upfield with
respect to the corresponding signals observed for 3, which
might reflect the difference of electronic properties between the
ItBu ligand and the IPr ligand, such that the carbenic carbon
atom in 3 might be more electrophilic.3b,14a
Complexes 1−3 are air and moisture stable in solid state.
However, the color of their solutions changed in several
minutes upon expositing to open air, resulting in white solids.
The solubility of complex 3 is relatively better compared with
those of 1 and 2. Complex 3 is soluble in toluene, CHCl3 and
THF, whereas both 1 and 2 merely dissolve in CHCl3 and
THF.
With respect to the phosphine ligand, we selected PCy3 (Cy
= cyclohexyl) to broaden the scope of the mixed NHC/
phosphine Ni(II) complexes since recent reports showed that it
was an excellent ligand for the currently intensively explored
nickel-catalyzed cross-coupling of phenolic derivatives.16 As
shown in Scheme 1 (2), the desired Ni(II) complexes, i.e.,
Ni(PCy3)(ItBu)Br2 (4) and Ni(PCy3)(IPr)Br2 (5), were
readily prepared by the substitution reaction of the PPh3
ligand in 2 and 3 with PCy3, respectively. After workup pure
complexes were obtained as violet crystals in excellent yields
(89% and 91%, respectively). Similar protocol has been
previously reported by Lee et al. for the synthesis of the
mixed NHC/PR3 Pd(II) complexes of the type Pd(PCy3)
(NHC)X2, whereas the target Pd(II) complexes were obtained
in ca. 50% yields.3b,c The NMR spectra of 4 and 5 exhibit the
characteristic resonances similar to those of the corresponding
PPh3-based Ni(II) complexes, except the resonances assigned
to the PCy3 ligand. Notably, the 31P NMR resonance shifts
from 14.0 in 2 to 5.9 ppm in 4, meanwhile that shifts from 19.5
in 3 to 9.0 ppm in 5. Complexes 4 and 5 show the desirable air
and moisture stability and, their behaviors in solution are very
close to those of 2 and 3. Notably, the solubility of 4 and 5 is
obviously better than that of 2 and 3, such that dissolve readily
even in hexane.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. We selected the bulky
1,3-ditertbutylimidazol-2-ylidene (ItBu) to synthesize the new
target Ni(II) complex because the steric bulkiness of a NHC
ligand is generally of benefit to high catalytic activity,13 and the
use of ItBu as a bulky coligand is rare in Ni(II) complexes.14
As shown in Scheme 1 (1), the reaction of the readily
available [NEt4][Ni(PPh3)X3] (X = Cl and Br) with one
Scheme 1. Synthesis of the Mixed NHC/PR3 Ni(II)
Complexes
Structural Description. The formation of complexes 1−5
was further confirmed by X-ray crystallography. The molecular
structures of these complexes are shown in Figures 1 (1 and 2),
2 (3) and 3 (4 and 5), respectively. The selected bond lengths
and angles are listed in Table 1.
These data reveal that the structures of these complexes are
similar to each other, wherein the nickel atom adopts a slightly
distorted square planar geometry with a trans arrangement of
the NHC and phosphine ligand, similar to the only known
equivalent of in situ generated ItBu occurred quickly in THF at
room temperature, and afforded the desired Ni(II) complexes
Ni(PPh3)(ItBu)X2 (X = Cl, 1; X = Br, 2) as dark purple crystals
in high yields (85 and 80%, respectively). A similar reaction
with another NHC ligand, i.e., IPr, also occurred quickly, giving
complex 3 in 82% yield, which is much higher than that
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Figure 1. Molecular structures of 1 (left) and 2 (right) with thermal ellipsoids at the 30% probability level. Hydrogens are omitted for clarity.
previously observed with Pd(II) analogues,3d,e which is
consistent both with the stronger σ-donor ability and the
larger steric bulkiness of the PCy3 ligand relative to the PPh3
ligand. In terms of NHC ligand, complexes bearing IPr ligand
display a more distorted square coordination geometry with
Ccarbene−Ni−P angle smaller than the ideal 180° (compare 2
with 3, and 4 with 5). Notably, despite the large differences in
the electronic and steric properties of NHC ligands (ItBu vs
IPr) and phosphine ligands (PPh3 vs PCy3), no clear trend with
Ni-X bond length was observed, revealing that both electronic
effect and steric hindrance do have an influence.
Catalytic Behaviors. The Group 10 metal catalyzed
Kumada cross-coupling reaction is one of the most powerful
tools for the formation of C−C bonds.17 In this field, despite
the undoubted success of palladium-based systems, recent
reports indicate that some effective nickel-based catalysts also
particularly promote the cross-coupling of more challenging
substrates, such as the less reactive, yet the less expensive and
most widely available aryl chlorides,9c,13b,14c,18−20 the inert aryl
fluorides,13a,19−21 and aromatic ether derivatives.14c,22
To disclose any effect due to the coordination variation of a
center metal on catalysis, we initially tested the catalytic
performance of complexes 1−5 and related Ni(II) complexes in
the cross-coupling of phenylmagnesium bromide with phenyl
chloride or phenyl fluoride.
Figure 2. Molecular structure of 3 with thermal ellipsoids at the 30%
probability level. Hydrogens and isopropyl groups are omitted for
clarity.
example of Ni(PPh3)(IPr)Cl2.6a The Ni−Ccarbene bond lengths
were found to lie in a narrow range of 1.905−1.931 Å, whereas
the Ni−P bond lengths and Ni-X bond lengths were 2.249(2)-
2.308(2) Å and 2.190(2)-2.3346(6) Å, respectively. These
structural data are within the expected range for similar Ni(II)
complexes.6−8,10
As seen from Table 2, there are significant differences in
catalytic activities of these Ni(II) complexes under conditions
similar to those reported previously.6a For example, by using 4-
chlorotoluene as a substrate, complex 2 is the most effective
catalyst precursor, affording a 93% yield of 4-methylbiphenyl
On the other hand, as expected, a detailed structural analysis
reveals that there are some structural differences among these
complexes. For example, the complex bearing the PCy3 ligand
has a longer Ni−Ccarbene bond length and a longer Ni−P bond
length (compare 4 with 2, and 5 with 3), a similar trend
Figure 3. Molecular structures of 4 (left) and 5 (right) with thermal ellipsoids at the 30% probability level. Hydrogens and isopropyl groups are
omitted for clarity.
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Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1−5
1
2
3
4
5
Ni(1)−P(1)
Ni(1)−C(19)
Ni(1)−X(1)
Ni(1)−X(2)
C(19)−Ni(1)−P(1)
C(19)−Ni(1)−X(1)
C(19)−Ni(1)−X(2)
P(1)−Ni(1)−X(1)
P(1)−Ni(1)−X(2)
X(1)−Ni(1)−X(2)
2.249(2)
1.911(8)
2.190(2)
2.203(2)
176.7(3)
91.6(2)
2.250(1)
1.905(4)
2.335(1)
2.329(1)
175.59(12)
87.34(11)
89.77(11)
92.17(3)
87.34(11)
174.76(2)
2.242(2)
1.922(5)
2.298(1)
2.311(1)
170.25(15)
94.11(13)
88.23(13)
92.80(4)
87.14(4)
163.29(4)
2.308(2)
1.927(7)
2.334(1)
2.351(1)
178.6(2)
88.0(2)
2.292(1)
1.931(2)
2.312(1)
2.306(1)
169.07(6)
87.25(6)
91.48(6)
89.61(3)
94.08(2)
165.83(2)
87.3(2)
86.8(2)
89.14(8)
91.74(8)
174.83(10)
92.90(6)
92.39(6)
172.97(5)
number of ways such as facilitate the reductive elimination for
the formation of the desired biaryl products together with the
regeneration of the catalytic active species,13 and accelerate the
initial reduction step as well as the oxidative addition step
during the catalytic cycle.23 The Ni(II) bromides, i.e., 2 and 3,
exhibited catalytic activities higher than those of their chloride
partners, which are the same as those previously reported.24
However, the reason for the significant difference on catalytic
activity between the PPh3-based Ni(II) complexes and the
PCy3-based ones is unclear.
Interestingly, 2 also displays a high activity toward less
reactive aryl fluorides, i.e., 4-fluorotoluene, affording the desired
product in 92% yield after 5 h with a 2 mol % loading at room
temperature (entry 6), and the same trend in catalytic activity,
i.e., 2 ≥ 1 ≫ 3 ≥ Ni(PPh3)(IPr)Cl2, was observed in turn.
Despite the increasing attention on the activation of C−F bond
and related transformation, the catalytic activation of the inert
C−F bond remains challenging because of the inherent
strength of the C−F bond.13a With respect to the reported
nickel catalysts for the cross-coupling of aryl fluorides with aryl
Grignard reagents, 2 shows higher catalytic activity than
Ni(IPr)2,13a Ni(acac)2/phosphine oxide,19a,d Ni(acac)2/phos-
phites,19b Ni(PR2R′)2X2,19c NHC-based pincer Ni(II) complex-
es,19e and Ni(I) complexes of Ni(PPh3)(NHC)X,19f but lower
than Ni(acac)2/hydroxyphosphine system,20a,b bidentate NHC-
based Ni(II) complex,20c and P,N,N-pincer Ni(II) complex-
es.20d
We also investigated the scope of 2 with a representative set
of coupling partners. As shown in Table 3, a variety of aryl
chlorides, aryl fluorides and heteroaryl chlorides were capable
of cross-couplings with arylmagnesium bromides to give the
corresponding coupling products in good to excellent yields.
To our delight, the steric hindrance and functional group
compatibility are considerably well-tolerated by this system,
especially in the formation of di-ortho-substituted biaryls (8d,e,
8f and 8n), ketal-functionalized biaryls (8l−n), pyridine
derivatives (8o−r), and the ester-functionalized biaryl (8s).
Interestingly, with only 0.1 mol % of 2, the cross-coupling of 2-
chloroquinoline with 4-methoxyphenylmagnesium bromide was
completed in 2 h (97% isolated yield) at room temperature. For
this type of reaction, the catalytic activity of complex 2 is very
close to that of Ni(IPr)(allyl)Cl, which possesses of the highest
catalytic activity for such a Kumada reaction with hetero-
aromatic chlorides catalyzed by nickel-based catalysts.14c
Moreover, it is noteworthy that this protocol could also be
extended to the synthesis of triaryl compounds either from 1-
chloro-3-fluorobenzene via a sequential cross-coupling (8j) or
from 1,3-dichlorobenzene via a simultaneous process (8k).
Table 2. Cross-Coupling of 4-MeC6H4X with C6H5MgBr
Catalyzed by Ni(II) Complexes
a
entry
catalyst (mol %)
1 (1)
X
T (h)
yield (%)
1
Cl
Cl
F
3
97
78
85
2
1 (0.5)
5
3
1 (2)
5
b
4
2 (1)
Cl
Cl
F
3
99 (98)
5
2 (0.5)
5
93
92
77
72
9
6
2 (2)
5
7
3 (1)
Cl
F
5
8
3 (2)
10
5
9
4 (1)
Cl
Cl
Cl
F
10
11
12
13
14
5 (1)
5
8
Ni(PPh3)(IPr)Cl2 (1)
Ni(PPh3)(IPr)Cl2 (2)
Ni(PPh3)2Br2 (1)
Ni(IPr)2Br2 (1)
5
63
65
14
26
10
5
Cl
Cl
5
a
Reaction conditions: 4-MeC6H4X (1.0 mmol), C6H5MgBr (1.5
mmol, 1.0 M in THF) was added in one portion, THF (total volume:
2.0 mL), 25 °C, GC yield using n-hexadecane as internal standard,
b
average of two trials. Isolated yield.
even when its loading was reduced to 0.5 mol % (entry 5),
whereas its chloride analogue 1 exhibited an obviously lower
activity and afforded a 78% yield with the same loading (entry
2). Meanwhile, complex 3 showed moderate activity compared
with that of 2, affording the product in 77% yield at a 1 mol %
loading (entry 7). Also Ni(PPh3)(IPr)Cl2 showed moderate
activity catalytic activity (entry 11, 63% yield) compared with
that of the ItBu-derivative 1. In comparison, the catalytic
activity of Ni(PPh3)2Br2 (entry 13) or Ni(IPr)2Br2 (entry 14) is
much lower than that of complex 3, similar to the previously
reported pattern with Ni(PPh3)(IPr)Cl2.6a To our surprise, it is
found that the least reactive complexes were 4 and 5, which
afforded the product in merely 9% (entry 9) and 8% (entry 10)
yields, respectively. The differences in catalytic activity among
1−5 and related Ni(II) complexes clearly show that both of the
NHC ligand and the phosphine ligand can exert a significant
influence on the catalytic activity of the mixed NHC/PPh3
Ni(II) complexes, in which a significant synergic effect on their
catalytic activity can also be observed. The fact that the catalytic
activities of 1 and 2 are higher than those of Ni(PPh3)(IPr)Cl2
and 3, respectively, can possibly be ascribed to both the larger
steric bulkiness and the stronger electron-donating property of
the ItBu ligand compared to the IPr ligand, which may help in a
D
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Since PCy3 has been particularly used as the effective ligand
in the nickel-catalyzed cross-coupling of Grignard reagents with
aryl methyl ethers,16,21 we are interested in whether Ni(PCy3)
(NHC)X2 represents an alternative catalytic precursor for such
cross-coupling reaction. As far as we are aware, mixed NHC/
phosphine Ni(II) complexes have not been used as precatalysts
in this type of reaction. Hence, we tested the catalytic
performance of complexes 4, 5 and related Ni(II) complexes
in the cross-coupling between 2-methoxynaphthalene and 4-
MeC6H4MgBr, which is a prototype cross-coupling of aryl
Table 3. Cross-Coupling of Aryl Chlorides and Fluorides
with ArMgBr Catalyzed by 2
a
21c,e
methyl ethers with aryl Grignard reagents.14g,
As shown in Table 4, we encountered significant differences
in the catalytic behavior of these Ni(II) complexes, which were
Table 4. Cross-Coupling of 2-MeOC8H7 with 4-
a
MeC6H4MgBr Catalyzed by Ni(II) Complexes
entry
complex (mol %)
T (h)
yield (%)
1
2
3
4
5
6
7
8
1 (1)
10
10
10
10
10
10
10
10
2
2 (1)
3
3 (1)
10
b
4 (1)
99 (98)
5 (1)
90
82
86
21
Ni(PCy3)2Cl2 (1)
Ni(PCy3)2Br2 (1)
Ni(IPr)2Br2 (1)
a
Reaction conditions: 2-methoxynaphthalene (1.0 mmol), 4-
MeC6H4MgBr (1.5 mmol, 1.0 M in THF) was added in one portion,
THF (total volume: 2.0 mL), 25 °C, GC yield using n-hexadecane as
b
internal standard, average of two trials. Isolated yield.
also quite different from those observed from Table 3.
Interestingly, complex 4, Ni(PCy3)(ItBu)Br2, effectively
promoted this type of reaction with a 1 mol % loading,
resulting in the desired cross-coupling product in almost
quantitative yield (entry 4), meanwhile its IPr-based analogue 5
afforded a slightly lower yield of 90% (entry 5) with the same
loading. On contrast, all of PPh3-based complexes, i.e., 1−3,
performed poorly under the same reaction conditions, affording
very low yields around 2−10% (entries 1−3). The bi-
sphosphine Ni(II) complex exhibited moderate activity (entries
6 and 7), whereas, in comparison, the biscarbene Ni(II)
complex exhibited a lower activity, and furnished 21% yield of
the desired cross-coupling product (entry 8). Thus, these
results discussed above show clearly that the present structural
variations of these mixed NHC/phosphine Ni(II) complexes
can also exert significant influences on their catalytic
selectivities. Notably, the replacement of the PPh3 ligand by
the PCy3 ligand allows an easy exchange of the activation of the
C(aryl)-X bond by a nickel center for the activation of the
C(aryl)-OMe bond by the corresponding nickel center.
a
Conditions: 2 (1 mol %), aryl halides (1.0 mmol), ArMgBr (1.5
mmol, 1.0 M in THF), THF, (total volume: 2.0 mL), 25 °C, isolated
yield. 2 (5 mol %). 2 (2 mol %). 60 °C. 2 (3 mol %), ArMgBr (3
mmol, 1.0 M in THF), total volume (4.0 mL). 2 (3 mol %). 2 (0.1
mol %). 2 (5 mol %), ZnCl2 (10 mol %), LiCl (1.2 mmol), NMP (1.5
mL), ArMgBr (2.0 mmol, 1.0 M in THF), THF (total volume: 4.5
mL), 1.3 equiv of 4-MeC6H4MgBr was used, after the reaction
mixtures were stirred for 2 h, additional 0.66 equiv of 4-MeC6H4MgBr
was added, and stirring was continued for 2 h.
b
c
d
e
f
g
h
In recent years phenolic derivatives have received increasing
attention as an attractive alternative to traditional aryl halides in
various cross-coupling reactions through C−O bond activa-
tion.16 Among the unconventional groups reacted in this way,
the activation of an inert C(aryl)−OMe bond is even more
attractive in terms of its easy availability and high atom
economy.16d,f However, the application of aryl methyl ethers in
the Kumada cross-coupling still remain limited,21 which mostly
be attributed to the higher bond dissociation energy (BDE) of
the C(aryl)−OMe bond compared with that of the
corresponding C(aryl)−Cl bond.16a
The catalytic performance of 4 was also compared with other
nickel-based catalysts reported in the literature. In general, 2−5
mol % loadings of Ni(II) complexes or a large excess of the
Grignard reagents are required to achieve satisfactory yields for
the reaction shown in Table 4.14c,21c,e Therefore, the present
results suggest that complex 4 might be among the most
efficient precatalysts for the cross-coupling reaction of aryl
E
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Organometallics
Article
methyl ethers with aryl Grignard reagents, possibly due to a
synergic effect between ItBu and PCy3.
We then briefly investigated the versatility of complex 4 with
other coupling partners. As seen from Table 5, substantial kinds
89% for 8a, and 91% for 10i) at a higher temperature (110 °C).
These results indicate a broad generality of the substrate scope.
Finally, the difference in catalytic activity between 2 and 4
has been successfully applied to the selective cross-coupling of
the chloride moiety or the methoxy group for the construction
of oligoarenes, which are important building blocks of advanced
materials, natural products and drug molecules.22 As shown in
Scheme 2, in the first cross-coupling procedure, treatment of
the naphthalene compound 11a with 2-MeC6H4MgBr and
C10H7MgBr in the presence of 2 (1 mol %) led to the selective
cross-coupling of the aryl chloride to afford biaryl products 12a
and 12b, respectively, whereas the C(aryl)-OMe bond
remaining intact. Next, with a 2 mol % loading of 4, 12a and
12b reacted smoothly with 4-MeC6H4MgBr and C6H5MgBr to
give naphthalene derivatives 13a and 13b, respectively.
Table 5. Cross-Coupling of Methoxyarenes with ArMgBr
a
Catalyzed by 4
CONCLUSION
■
The present work provides facile synthetic methods for the
synthesis of novel mixed NHC/phosphine Ni(II) complexes of
the type Ni(PR3)(NHC)X2 (1−5) via controlled ligand
substitution reactions using easily available Ni(II) complexes
[NEt4][Ni(PPh3)X3] as the starting materials. Under mild
conditions, these complexes have great potential as versatile
catalysts for the selective cross-coupling of aryl Grignard
reagents with a wide range of electrophiles including aryl
chlorides, aryl fluorides and aryl methyl ethers. Among them,
complex 2 showed the highest catalytic activity and great
potential in the cross-coupling reactions of aryl Grignard
reagents with aryl chlorides and fluorides, while the catalytic
activity of complex 4 is largely superior to those of 1−3 and 5
in the aryl Grignards cross-coupling of aryl methyl ethers. The
significant difference in catalytic activity between 2 and 4 has
been successfully applied in the synthesis of oligoarenes by the
selective cross-coupling reaction. Since a variety of NHC
ligands as well as phosphine ligands are available, this work
confirmed that the installation of a robust NHC ligand and a
labile phosphine ligand on one nickel atom can be used as an
excellent strategy for building highly active and/or selective, yet
robust catalysts of nickel. The research on further fine-tuning
their catalytic activity by surveying well-matched pairs of NHC/
PR3 ligands, the scope of these Ni(II) complexes in other cross-
coupling reactions, and related mechanism study are currently
under investigation in our laboratory.
a
Conditions: 4 (1 mol %), aryl ethers (1.0 mmol), ArMgBr (1.5
mmol, 1.0 M in THF), THF, (total volume: 2.0 mL), 25 °C, isolated
yield. 4 (2.5 mol %). 4 (5 mol %). 60 °C. 4 (2 mol %). 4 (3 mol
%). Toluene was used instead of THF, 110 °C.
b
c
d
e
f
g
of substrates including 1- or 2-methoxynaphthalene, 4-
methoxybiphenyl and 4-methylanisole are applicable. For
example, both 1-methoxynaphthalene and 2-methoxynaphtha-
lene reacted smoothly with a variety of aryl Grignard reagents
with yields varying from 89 to 98% (10b−10g), wherein
hindered Grignard reagents such as 2-MeC6H4MgBr and 2,4,6-
Me3C6H2MgBr participated also in the reaction to give the
desired products in excellent to good yields (98% for 10b, 89%
for 10c) by increasing reaction temperature to 60 °C and/or by
using 5 mol % loading of 4. For the less reactive biphenyl
methyl ethers and the least reactive phenyl methyl ethers,14c
cross-coupling reactions achieved completely (92% for 10h,
Scheme 2. Selective Cross-Coupling Strategies for Synthesis of Oligoarenes
F
DOI: 10.1021/acs.organomet.5b00874
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Hz, 2H, p-C6H3: NHC), 7.59−7.66 (m, 6H, P(C6H5)3). 13C NMR (75
MHz, C6D6) δ = 171.1 (d, JPC = 131.4 Hz, NCN), 147.3 (ipso-C6H3:
NHC), 136.3 (ortho-C6H3: NHC), 135.1 (ortho-C6H5: PPh3), 132.5
(d, JPC = 39.4 Hz, ipso-C6H5: PPh3), 130.1 (para-C6H5: PPh3), 129.1
(meta-C6H5: NHC), 127.2 (meta-C6H5: PPh3), 125.0 (para-C6H3:
NHC), 124.0 (CHCH), 29.0 (C(CH3)), 26.4 (CH3), 23.1 (CH3).
31P NMR (121 MHz, C6D6) δ = 19.5. Anal. Calcd for
C45H51Br2N2NiP: C, 62.17; H, 5.91; N, 3.22. Found: C, 62.13; H,
5.88; N, 3.17.
EXPERIMENTAL SECTION
■
General Experimental Procedures. All manipulations were
performed under pure argon with rigorous exclusion of air and
moisture using standard Schlenk techniques. Solvents were distilled
from Na/benzophenone ketyl under pure argon prior to use.
[NEt4][Ni(PPh3)Cl3],25 [NEt4][Ni(PPh3)Br3],25 Ni(PPh3)2Cl2,26 Ni-
(IPr)2Cl2,12d Ni(PCy3)2Cl2,27b Ni(PCy3)2Br2,27 1,3-ditertbutylimida-
zol-2-ylidene28 and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene29
were prepared by published methods. The synthesis of Ni(PPh3)-
(IPr)Cl2 has been reported previously,6a it could also be prepared
following the method similar to the synthetic procedure of 1 (see
Supporting Information for details). Organic reagents used for cross-
coupling reactions were purchased from Acros Organics, Sigma-
Aldrich and Alfa Aesar. Elemental analysis was performed by direct
combustion on a Carlo-Erba EA-1110 instrument. NMR spectra were
measured on a Varian Unity INOVA 400 or VNMRS 300 MHz
spectrometer at 25 °C. The melt points were determined on a
Diamond DSC (PerkinElmer) using powder samples under N2
atmosphere (50 mL/min). The system was heated from 50 to 350
°C at 20 °C/min. Gas chromatographic (GC) analysis was performed
on a Varian CP-3800 instruments equipped with an FID detector and
a capillary column AT.OV-101 (30 m × 0.32 mm i.d., 0.10 μm film).
High resolution mass spectra were obtained using GCT-TOF
instrument with ESI or CI source.
Ni(PCy3)(ItBu)Br2 (4). A Schlenk flask was charged with Ni(PPh3)-
(ItBu)Br2 (2) (0.68 g, 1.0 mmol), THF (10 mL) and a stir bar. To this
solution was added PCy3 (0.28 g, 1.0 mmol) in THF (10 mL) at room
temperature. The mixture was stirred for 30 min and evaporated to
dryness in vacuo. The residue was washed with hexane (3 × 10 mL),
extracted with toluene (3 × 10 mL), and crystallized from
concentrated toluene/hexane solution at 0 °C. The product was
precipitated as violet crystals in a yield of 89% (0.60 g), mp 189 °C;
1H NMR (400 MHz, C6D6) δ = 1.38−1.25 (m, 9H, P(C6H11)3), 1.73−
1.70 (m, 3H, P(C6H11)3), 2.00−1.83 (m, 12H, P(C6H11)3), 2.22−2.19
(m, 6H, P(C6H11)3), 2.50−2.47 (m, 21H, P(C6H11)3 + C(CH3)3),
6.47 (s, 2H, CHCH). 13C NMR (100 MHz, C6D6) δ = 156.7 (d, JPC
= 110.5 Hz, NCN), 119.9 (d, JPC = 3.6 Hz, CHCH), 58.9
(C(CH3)), 32.8 (CH3), 31.4 (d, JPC = 17.3 Hz, P(C6H11)3), 29.5
(P(C6H11)3), 27.9 (d, JPC = 9.7 Hz, P(C6H11)3), 26.9 (P(C6H11)3); 31
P
NMR (162 MHz, C6D6) δ = 5.9. Anal. Calcd for C29H53Br2N2NiP: C,
51.28; H, 7.87; N, 4.12. Found: C, 51.19; H, 7.79; N, 4.18.
Ni(PPh3)(ItBu)Cl2 (1). A Schlenk flask was charged with [NEt4][Ni-
(PPh3)Cl3] (0.56 g, 1.00 mmol), THF (10 mL) and a stir bar. To this
solution was added 1,3-ditertbutylimidazol-2-ylidene (0.18 g, 1.00
mmol) in THF (10 mL) at room temperature. The color of the
mixture changed into violet immediately. The solution was then stirred
for 1 h at room temperature. Volatiles were removed in vacuo. The
residue was washed with hexane (3 × 10 mL), extracted with THF (3
× 10 mL), and crystallized from concentrated THF solution at 0 °C.
The product was precipitated as dark purple crystals in a yield of 85%
Ni(PCy3)(IPr)Br2 (5). Following a procedure similar to the synthetic
procedure of 4, a THF (10 mL) solution of PCy3 (0.28 g, 1.0 mmol)
was added to a THF (10 mL) solution of Ni(IPr) (PPh3)Br2 (3) (0.87
g, 1.0 mmol) at room temperature. The mixture was stirred for 30 min
and evaporated to dryness in vacuo. The residue was washed with
hexane (3 × 10 mL), extracted with toluene (3 × 10 mL) and
crystallized from concentrated toluene/hexane solution at 0 °C. The
product was precipitated as violet crystals in a yield of 91% (0.81 g),
mp 220 °C; 1H NMR (400 MHz, C6D6) δ = 1.04 (d, J = 6.8 Hz, 12H,
CH(CH3)2), 1.23−1.08 (m, 9H, P(C6H11)3), 1.65−1.60 (m, 27H,
CH(CH3)2+ P(C6H11)3), 1.88−1.85 (m, 6H, P(C6H11)3), 2.16−2.11
(m, 3H, P(C6H11)3), 3.53−3.47 (m, 4H, CH(CH3)2), 6.61 (s, 2H,
CHCH), 7.41−7.34 (m, 6H, C6H3). 13C NMR (100 MHz, C6D6) δ
= 170.3 (d, JPC = 118.4 Hz, NCN), 147.6 (ipso-C6H3), 136.3 (ortho-
C6H3), 124.8 (d, JPC = 3.5 Hz, para-C6H3), 123.8 (CHCH), 32.7 (d,
JPC = 18.1 Hz, P(C6H11)3), 29.9 (P(C6H11)3), 29.0 (C(CH3)), 27.8 (d,
JPC = 9.9 Hz, P(C6H11)3), 26.7 (P(C6H11)3), 26.6 (CH3), 22.9 (CH3).
31P NMR (162 MHz, C6D6) δ = 9.0. Anal. Calcd for C45H69Br2N2NiP:
C, 60.90; H, 7.84; N, 3.16. Found: C. 60.98; H, 7.79; N, 3.20.
X-ray Structural Determination. Single crystals of 1−5 for X-ray
diffraction studies were sealed in a thin-walled glass capillary. The data
were collected on a Rigaku Mercury CCD area detector at 223(2) K
(for 1, 3 and 5) or at 293(2) K (for 2 and 4). Structures were solved
by direct methods and refined by full-matrix least-squares procedures
based on F2 using SHELXS-97 and SHELXL-97 programs. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
assigned to idealized positions and were included in structure factor
calculations.
1
(0.48 g), mp 187 °C; H NMR (400 MHz, C6D6) δ = 2.39 (s, 18H,
C(CH3)3), 6.43 (s, 2H, CHCH), 7.05−7.12 (m, 9H, C6H5), 8.03−
8.07 (m, 6H, C6H5). 13C NMR (75 MHz, C6D6) δ = 153.0 (d, JPC
128.7 Hz, NCN), 134.9 (d, JPC = 9.9 Hz, ortho-C6H5), 132.3 (d, JPC
=
=
38.5 Hz, ipso-C6H5), 129.7 (para-C6H5), 128.2 (meta-C6H5), 119.4 (d,
JPC = 3.8 Hz, CHCH), 59.2 (C(CH3)), 32.6 (CH3). 31P NMR (121
MHz, C6D6) δ = 7.9. Anal. Calcd for C29H35Cl2N2NiP: C, 60.87; H,
6.17; N, 4.90. Found: C, 60.75; H, 6.16; N, 4.88.
Ni(PPh3)(ItBu)Br2 (2). Following a procedure similar to the synthetic
procedure of 1, a THF (10 mL) solution of 1,3-ditertbutylimidazol-2-
ylidene (0.18 g, 1.00 mmol) was added to a THF (10 mL) suspension
of [NEt4][Ni(PPh3)Br3] (0.69 g, 1.00 mmol) at room temperature.
After workup, the residue was washed with hexane (3 × 10 mL),
extracted with THF (3 × 10 mL), and crystallized from concentrated
THF solution at 0 °C. The product was precipitated as dark purple
1
crystals in a yield of 80% (0.53 g), mp 195 °C; H NMR (400 MHz,
C6D6) δ = 2.32 (s, 18H, C(CH3)3), 6.45 (s, 2H, CHCH), 7.04−7.14
(m, 9H, C6H5), 8.03−8.08 (m, 6H, C6H5). 13C NMR (75 MHz, C6D6)
δ = 156.1 (d, JPC = 124.0 Hz, NCN), 134.9 (d, JPC = 9.6 Hz, ortho-
C6H5), 133.1 (d, JPC = 40.2 Hz, ipso-C6H5), 129.6 (para-C6H5), 128.3
(meta-C6H5), 120.4 (d, JPC = 4.0 Hz, CHCH), 59.2 (C(CH3)), 32.5
(CH3). 31P NMR (121 MHz, C6D6) δ = 15.0. Anal. Calcd for
C29H35Br2N2NiP: C, 52.69; H, 5.34; N, 4.24. Found: C, 52.73; H,
5.39; N, 4.33.
General Procedure for the Cross-Coupling of Ar′X with
ArMgBr. In a typical example, complex 2 (0.01 mmol), Ar′X (1.0
mmol), and THF (0.5 mL) were added to a Schlenk tube, and the
mixture was stirred at 0 °C for 2 min. To this stirred mixture was
added ArMgBr solution (1.5 mL, 1.0 M in THF) at 0 °C by syringe.
The color of the resulting mixture turned to brownish yellow
immediately. Then, this mixture was stirred for 5 h in an oil bath at 25
°C. Diluted hydrochloric acid (1 M, 0.5 mL) was added. The resulting
mixture was extracted with acetic ether (3 × 3 mL), dried over
anhydrate MgSO4, filtered, and concentrated. The residue was purified
by column chromatography [silica gel (230−400 mesh), 5.0% ethyl
acetate/pet ether]. The calibrated yield of desired product against n-
hexadecane as the internal standard was obtained using GC
spectroscopic analysis.
Ni(PPh3)(IPr)Br2 (3). Following a procedure similar to the synthetic
procedure of 1, a THF (10 mL) solution of 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene (0.39 g, 1.00 mmol) was added
to a THF (10 mL) suspension of [NEt4][Ni(PPh3)Br3] (0.69 g, 1.00
mmol) at room temperature. After workup, the residue was washed
with hexane (3 × 10 mL), extracted with toluene (3 × 10 mL), and
crystallized from concentrated toluene/hexane solution at 0 °C. The
product was precipitated as dark purple crystals in a yield of 82% (0.71
1
g), mp 166 °C; H NMR (400 MHz, C6D6) δ = 1.04 (d, J = 6.8 Hz,
12H, CH(CH3)2), 1.50 (d, J = 6.8 Hz, 12H, CH(CH3)2), 3.44−3.51
(m, 4H, CH(CH3)2), 6.63 (s, 2H, CHCH), 6.96−6.97 (m, 9H,
P(C6H5)3), 7.39 (d, J = 7.6 Hz, 4H, m-C6H3: NHC), 7.46 (t, J = 7.2
The identity of the product was confirmed by 1H NMR
spectroscopy and TLC.
G
DOI: 10.1021/acs.organomet.5b00874
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
General Procedure for the Cross-Coupling of Ar′OMe with
ArMgBr. In a typical example, complex 4 (0.02 mmol), Ar′OMe (1.0
mmol), and THF (0.5 mL) were added to a Schlenk tube, and the
mixture was stirred at 0 °C for 2 min. To this stirred mixture was
added ArMgBr (1.5 mL, 1.0 M in THF) by syringe. The color of the
resulting mixture turned to brownish yellow immediately. Then, the
resulting mixture was stirred for 10 h in an oil bath at 25 °C. Diluted
hydrochloric acid (1 M, 0.5 mL) was added. The resulting mixture was
extracted with acetic ether (3 × 3 mL), dried over anhydrate MgSO4,
filtered, and concentrated. The residue was purified by column
chromatography [silica gel (230−400 mesh), 5.0% ethyl acetate/pet
ether]. The calibrated yield of desired product against n-hexadecane as
the internal standard was obtained using GC spectroscopic analysis.
The identity of the product was confirmed by 1H NMR
spectroscopy and TLC.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00874.
■
S
̆
(10) Arduengo, A. J., III; Dolphin, J. S.; Gurau, G.; Marshall, W. J.
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Synthesis of Ni(PPh3)(IPr)Cl2, characterization of
complexes 1−5, and characterization of cross-coupling
products.
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(PDF)
Crystallographic information files (CIF) of complexes
1−5. (ZIP)
Accession Codes
CCDC reference numbers: 1 (CCDC 971433), 2 (CCDC
971434), 3 (CCDC 971435), 4 (CCDC 1014572), and 5
(CCDC 983587).
(13) For Ni(II)-based examples, see: (a) Bohm, V. P. W.;
̈
Gstottmayr, C. W. K.; Weskamp, T.; Herrmann, W. A. Angew.
̈
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(15) Matsubara et al. showed that Ni(PPh3)(IPr)Br2 could be
prepared via the reaction of Ni(PPh3)2Br2 with IPr in 21% yield.
However, according to their results, the corresponding signal due to
the carbene carbon was invisible in the room-temperature 13C NMR
spectrum, see ref 6a.
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Chem. Res. 2010, 43, 1486−1495. (b) Mesganaw, T.; Garg, N. K. Org.
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(17) (a) Cross-Coupling Reactions: A Practical Guide; Miyaura, N., Ed.;
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Germany, 2004.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sunhm@suda.edu.cn.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (Grants 21472134 and 21172164), the Key Laboratory
of Organic Chemistry of Jiangsu Province, and the Priority
Academic Program Development of Jiangsu Higher Education
Institutions (PAPD) for financial support. The authors declare
no competing financial interests.
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