Acetylacetonato-based pincer-type nickel(ii) complexes: Synthesis and catalysis in cross-couplings of aryl chlorides with aryl Grignard reagents

Article abstract of DOI:10.1039/c8dt01295d

In this work, three different types of acetylacetonato-based pincer-type nickel(ii) complexes (2) were prepared. Complex 2a possessed the tridentate ONN ligand, which was constructed by the condensation reaction of acetylacetone with N,N-diethylethylenediamine. Complex 2b contained the PPh₂ donor group in contrast to the NEt₂ group in 2a, i.e., an ONP ligand framework. Complex 2c was composed of the NNN ligand, which was prepared by the reaction of 4-((2,4,6-trimethylphenyl)amino)pent-3-en-2-one with N,N-diethylethylenediamine. In addition to X-ray diffraction analysis, these complexes were characterized spectroscopically. Their catalytic activity for a cross-coupling reaction of aryl halides with aryl Grignard reagents was also evaluated. Among these complexes, 2b acted as an effective catalyst for the cross-coupling reaction using aryl chlorides as electrophiles. The electronic properties of these Ni(ii) complexes were investigated by cyclic voltammetry and density functional theory calculations.

Full text of DOI:10.1039/c8dt01295d

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DOI: 10.1039/C8DT01295D
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Acetylacetonato-based pincer-type nickel(II) complexes:
Synthesis and catalysis in cross-couplings of aryl chlorides with
aryl Grignard reagents
Received 00th January 20xx,
Accepted 00th January 20xx
Erika Asano, a Yuki Hatayama, a Nobutaka Kurisu, a Atsufumi Ohtani, a Toru Hashimoto, a
DOI: 10.1039/x0xx00000x
Youji Kurihara, a Kazuyoshi Ueda, a Shinji Ishihara, b Hirotaka Nagao c and Yoshitaka Yamaguchi a,*
www.rsc.org/
In this work, three different types of acetylacetonato-based pincer-type nickel(II) complexes (2) were prepared. Complex
2a possessed the tridentate ONN ligand, which was constructed by the condensation reaction of acetylacetone with N,N-
diethylethylenediamine. Complex 2b contained the PPh2 donor group in contrast to the NEt2 group in 2a, i.e., an ONP
ligand framework. Complex 2c was comprised of the NNN ligand, which was prepared by the reaction of 4-((2,4,6-
trimethylphenyl)amino)pent-3-en-2-one with N,N-diethylethylenediamine. In addition to X-ray diffraction analysis, these
complexes were characterized spectroscopically. Their catalytic activity for a cross-coupling reaction of aryl halides with
aryl Grignard reagents was also evaluated. Among these complexes, 2b acted as an effective catalyst for the cross-coupling
reaction using aryl chlorides as electrophiles. The electronic properties of these Ni complexes were investigated by cyclic
voltammetry and density functional theory calculations.
several effective ligand systems for cross-coupling reactions
have been reported. In recent years, tridentate pincer-type
complexes have generated as lot of interest because the
Introduction
Nickel-catalyzed cross-coupling reactions of an organic halide
with a Grignard reagent were first described independently by
Kumada/Tamao and Corriu.1 Subsequently, such transition
metal-catalyzed cross-coupling reactions of organic halides
with organometallic reagents have developed into reliable and
versatile tools for modern synthetic organic chemistry.2 In
these reactions, organic chlorides are less frequently employed
as electrophiles in comparison with the corresponding
bromides and iodides. This is because of the poor reactivity of
the C–Cl bond. However, organic chlorides are useful
substrates because of their low cost and the wide diversity of
available compounds. Therefore, the development of highly
active catalysts for organic chlorides in cross-coupling
reactions has received significant attention.3 In order to
achieve the activation and smooth scission of the C–Cl bond in
an electrophile on the metal, ancillary ligands are often used
to provide an appropriate steric and electronic environment
around the metal center. Naturally, such ligands play an
important role in determining the catalytic properties. To date,
pincer-type ligand stabilizes the metal complexes and its
properties can be tuned to achieve the best reactivity of the
5
complex.4,
Consequently, extensive attention has been
focused on the combination of pincer-type ligands with Ni,5–14
which is one of the most attractive metals because of its
significantly low cost as compared to precious metals such as
palladium, rhodium, etc.
We have recently reported the synthesis of iron complexes
bearing the tridentate
β
-aminoketonato ligand,15 which was
easily prepared by the condensation reaction of acetylacetone
with a primary amine tethering an additional donor unit. These
iron complexes proved to be highly effective catalysts for the
cross-coupling reaction of alkyl halides and aryl Grignard
reagents16 and the atom-transfer radical polymerization
reaction of styrenes.17 In order to elucidate both the generality
and the availability of this type of ligands for constructing
pincer-type complexes, novel Ni(II) complexes bearing β-
aminoketonato- and -diketiminato-based tridentate ligands
β
were prepared. The cross-coupling reaction of aryl halides with
aryl Grignard reagents was also investigated using these Ni(II)
complexes as catalysts. It was found that the modifications of
the ligand framework had a significant influence on the
catalytic performance. In this paper, we have described the
synthesis and structures of the pincer-type Ni(II) complexes
and their application as catalysts for the cross-coupling
reactions. Furthermore, the electronic properties of the Ni(II)
complexes were estimated by cyclic voltammetry (CV) and
density functional theory (DFT) calculations.
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Results and discussion
Synthesis and characterization
The synthetic procedures for the acetylacetone-based
tridentate pro-ligands 1-H are shown in Scheme 1. The pro-
ligands 1a-H16 and 1b-H18 were prepared according to the
literature reported method of a condensation reaction of
acetylacetone with the appropriate primary amine tethering
an additional donating group such as NEt2 or PPh2 in the
presence of a catalytic amount of H2SO4 in toluene at reflux.
Pro-ligand 1c-H was prepared by the reaction of 4-((2,4,6-
Figure 1. ORTEP drawing of 1b-H (30% probability of thermal
ellipsoids). All hydrogen atoms except for H1 heve been
omitted for clarity. Selected bond lengths (Å) and angles (°):
C1–C2, 1.507(4); C2–C3, 1.374(4); C3–C4, 1.408(4); C4–C5,
1.511(4); O1–C4, 1.241(3); N1–C2, 1.324(4); N1–C6, 1.454(3);
N1–H1, 0.90(3); N1···O1, 2.687(3); O1···H1, 1.94(4); N1–C2–C1,
117.7(2); N1–C2–C3, 122.3(2); C1–C2–C3, 120.0(3); C2–C3–C4,
124.5(3); O1–C4–C3, 123.2(2); O1–C4–C5, 118.5(3); C3–C4–C5,
118.3(3); C2–N1–C6, 127.4(2); C2–N1–H1, 112.3(15); C6–N1–
H1, 119.8(15); N1–H1···O1, 139(2).
trimethylphenyl)amino)pent-3-en-2-one
with
N,N-
diethylethylenediamine. The former compound was prepared
by the condensation of acetylacetone with 2,4,6-
trimethylaniline. Compound 1c-H was isolated as a brown
liquid in 72% yield. These compounds were characterized by
NMR spectroscopy. In the 1H NMR spectrum of 1-H, the
characteristic signals of the N–H proton atom were downfield
at ~ 10–11 ppm.
Single crystals of 1b-H were obtained by recrystallization
from toluene/hexane and analyzed by X-ray diffraction. The
ORTEP drawing of 1b-H is shown in Figure 1. The position of
the hydrogen atom (H1) bonded to the nitrogen atom (N1) was
determined from the difference Fourier maps and refined
isotropically. The N1···O1 and O1···H1 distances were found to
be 2.687(3) and 1.94(4) Å, respectively. These distances are in
good agreement with that of the reported N–H···O hydrogen
double bond lengths. Although, the O1–C4 bond showed a
slight double bond character, it is conceivable that the
π-
electrons on the N1–C2–C3–C4–O1 unit would be delocalized
in this skeleton.
Next, the preparation of nickel(II) complexes
2 using these
ligands was examined (Scheme 2). Treatment of the nickel(II)
complex precursor [NiCl2(2,4-lutidine)2] with the lithiated
tridentate ligand (1a-Li), which was prepared in situ by the
reaction of 1a-H with n-BuLi in THF, led to the formation of the
2a as a purple solid in 99% yield. A similar procedure was used
for the preparation of complex 2b, i.e., the reaction of the
lithiated ligand 1b-Li with [NiCl2(2,4-lutidine)2]. Complex 2b
was isolated as an orange solid in moderate yield (70%). We
then examined the one-pot reaction of pro-ligand 1b-H with
[NiCl2(2,4-lutidine)2] in the presence of NEt3 as a base. In this
case, 2b was obtained in 55% yield. The best yield of 2b (97%)
was achieved by reacting [NiCl2(PPh3)2] as the metal precursor
instead of [NiCl2(2,4-lutidine)2] with 1b-H in the presence of
NEt3. Complex 2c was isolated as a red solid in 40% yield from
the reaction of the lithiated tridentate ligand 1c-Li with
[NiCl2(2,4-lutidine)2]. In contrast, the one-pot reaction of 1c-H
with [NiCl2(2,4-lutidine)2] in the presence of NEt3 afforded
complex 2c in 92% yield. Elemental analysis, NMR spectra, and
X-ray diffraction studies confirmed the formation of the
bond19. In the
β-aminoketone skeleton, the N1–C2 bond
length (1.324(4) Å) is within the mean value of the N–C single
bond (1.48 Å) and the N–C double bond (1.24 Å). The O1–C4
bond length (1.241(3) Å) was close to the C–O double bond
length (1.23 Å). The C2–C3 (1.374(4) Å) and C3–C4 (1.408(4) Å)
bond lengths were within the mean value of the C–C single and
NEt2
H2N
NH
O
cat. H2SO4
toluene, reflux
NEt2
O
O
1a-H (87%)
PPh2
H2N
NH
O
desired pincer-type Ni(II) complexes 2a
– .
2c
, the low magnetic resonance
assignable to the N–H proton atom in pro-ligands 1-H was not
observed. This indicated that the ligand was deprotonated
and coordinated to the nickel center as a monoanionic fashion.
In complex 2a the methylene proton atoms of the
1
NH2
In the H NMR spectra of
2
cat. H2SO4
toluene, reflux
PPh2
cat. H2SO4
toluene
reflux
1
1b-H (58%)
,
NEt2
diethylamino moiety showed two sets of doublet of quartets
at 2.46 and 3.18 ppm with coupling constants of 13.2 and 7.2
Hz, respectively. The corresponding methylene proton atoms
in the pro-ligand 1a-H showed a quartet at 2.57 ppm with J =
7.3 Hz. The nonequivalence of these methylene proton atoms
in complex 2a indicated that the ligand was coordinated to the
Ni center in a tridentate fashion. Similar spectroscopic features
were observed in complex 2c, namely, two sets of doublet of
H2N
NH
N
O
HN
cat. H2SO4
toluene, reflux
NEt2
1c-H (72%)
Scheme 1. Preparation of pro-ligands 1-H
2 | J. Name., 2012, 00, 1-3
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O
1) n-BuLi
2) [NiCl2(2,4-lutidine)2]
N
N
N
Ni
Cl
1a-H
THF
NEt2
2a (99%)
O
1) [NiCl2(PPh3)2]
2) NEt3
Figure 2. ORTEP drawings of complexes 2a – 2c (30%
probability of thermal ellipsoids). All hydrogen atoms have
been omitted for clarity.
Ni
Cl
1b-H
THF
PPh2
2b (97%)
1b-H). In the 13C{1H} NMR spectrum of 2b, two doublet signals
corresponding to the OCCH3 carbon atoms in the
β-
N
1) [NiCl2(2,4-lutidine)2]
2) NEt3
aminoketonato unit were observed at 178.8 ppm (J = 2.7 Hz)
and 25.2 ppm (J = 6.4 Hz). These observations indicated that
1b was coordinated to the Ni center in a tridentate fashion and
the phosphorus atom was positioned trans to the oxygen atom
Ni
Cl
1c-H
THF
NEt2
2c (92%)
of the
The structures of the nickel(II) complexes 2a
were determined by X-ray analysis. The ORTEP drawings of 2a
β-aminoketonato unit.
,
2b and 2c
Scheme 2. Preparation of nickel compexes 2
,
2b and 2c are shown in Figure 2. The selected bond lengths
and angles for these complexes are listed in Table 1. These
complexes have a distorted square planar geometry around
the central metal, in which the ligand coordinates in a
tridentate pincer-type fashion. The angles for the ligands in the
mutually cis position bound to the Ni center were in the range
86.08(10)–95.48(8)° and the sum of the angles around the Ni
quartets corresponding to the methylene proton atoms in the
diethylamino group at 2.40 and 3.25 ppm with J =12.8 and 7.2
Hz, respectively. Complex 2b was characterized by comparing
1
1
the H NMR and 31P-decoupled H NMR (1H{31P} NMR) spectra
(see Experimental Section). In the 31P{1H} NMR spectrum of 2b
,
a singlet was observed at 37.5 ppm, which suggests that the
PPh2 moiety was coordinated to the Ni center (–20.8 ppm for
Table 1. Selected bond lengths (Å) and angles (°) of complexes 2a
,
2b and 2c
.
2a
2b
2c
Ni1-Cl1
Ni1-N1
Ni1-O1
Ni1-N2
N1-C2
O1-C4
C1-C2
C2-C3
C3-C4
C4-C5
2.2399(6)
1.8696(18)
1.8395(18)
1.980(2)
1.316(3)
1.296(3)
1.511(4)
1.408(3)
1.366(4)
1.506(4)
Ni1-Cl1
Ni1-N1
Ni1-O1
Ni1-P1
N1-C2
O1-C4
C1-C2
C2-C3
C3-C4
C4-C5
2.1732(7)
1.896(2)
1.8665(18)
2.1292(6)
1.321(3)
1.282(3)
1.517(4)
1.403(4)
1.369(4)
1.505(4)
Ni1-Cl1
Ni1-N1
Ni1-N2
Ni1-N3
N1-C2
N2-C4
C1-C2
C2-C3
C3-C4
C4-C5
2.2154(8)
1.860(2)
1.891(3)
1.994(3)
1.321(4)
1.339(4)
1.504(5)
1.384(5)
1.381(5)
1.527(5)
Cl1-Ni1-N1
O1-Ni1-N2
N1-Ni1-O1
N1-Ni1-N2
Cl1-Ni1-O1
Cl1-Ni1-N2
Ni1-N1-C2
Ni1-N1-C6
C2-N1-C6
C2-C3-C4
177.20(7)
178.36(9)
93.68(9)
Cl1-Ni1-N1
O1-Ni1-P1
N1-Ni1-O1
N1-Ni1-P1
Cl1-Ni1-O1
Cl1-Ni1-P1
Ni1-N1-C2
Ni1-N1-C6
C2-N1-C6
C2-C3-C4
174.50(6)
174.16(5)
95.48(8)
Cl1-Ni1-N1
N2-Ni1-N3
N1-Ni1-N2
N1-Ni1-N3
Cl1-Ni1-N2
Cl1-Ni1-N3
Ni-N1-C2
163.43(6)
169.29(9)
93.27(11)
86.08(10)
93.20(8)
90.34(7)
127.1(2)
114.36(16)
117.8(3)
125.8(3)
125.1(2)
87.11(9)
88.10(6)
87.40(6)
88.80(6)
91.74(6)
87.92(3)
127.51(17)
112.80(16)
119.49(19)
123.9(2)
123.64(18)
118.64(15)
117.4(2)
Ni-N1-C6
C2-N1-C6
C2-C3-C4
125.9(2)
Ni1-O1-C4
126.49(16)
Ni1-O1-C4
126.94(17)
Ni1-N2-C4
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center in complexes 2a and 2b was almost 360° (359.93° for 2a was close to the Ni1–N1 bond length in complexes
2.
and 360.30° for 2b). On the other hand, in complex 2c, the sum Therefore, it is conceivable that the monoanionic conjugated
of the angles around the Ni center was 362.89°. Furthermore, ligand system makes a considerable contribution to the
a notable feature was observed in the angles for the ligands in formation of the stable pincer-type Ni(II) complexes.
the mutually trans position bound to the Ni center. The Cl1–
Ni1–N1 angles decreased in the order 2a (177.20(7)°) > 2b Cross-coupling reaction catalyzed by Ni(II) complexes
(174.50(6)°) > 2c (163.43(6)°). The other mutually trans ligand
The pincer-type Ni(II) complexes (2a–2c) were examined as
angles showed similar features (O1–Ni1–N2 = 178.36(9)° for 2a
O1–Ni1–P1 = 174.16(5)° for 2b, and N2–Ni1–N3 = 169.29(9)°
for 2c). This trend may be explained by the presence of the
sterically bulky substituent on the tridentate ligand. Therefore,
it was assumed that the Ni center in complex 2c was sterically
,
catalysts in the cross-coupling reaction of 4-halotoluene with
phenylmagnesium bromide. The results of the optimization
experiments are summarized in Table 2.
In order to evaluate the catalytic activity of three nickel(II)
complexes, the cross-coupling reaction was conducted by
using 1.0 mmol of 4-chlorotoluene with 1.5 equivalents of
phenylmagnesium bromide and 1 mol% of the nickel(II)
complex in THF at 25 °C for 24 h (entries 1–3). It was found
that these complexes exhibited catalytic activity in the cross-
more crowded than in 2a and 2b
.
In all Ni(II) complexes, the C2–C3 and C3–C4 bond lengths
were in almost the same ranges of 1.384(5)–1.408(3) Å and
1.366(4)–1.381(5) Å, respectively. The N1–C2 bond lengths in
all complexes and the N2–C4 bond length for 2c were within
the mean value range of the N–C single and double bonds.
Furthermore, the O1–C4 bond lengths in 2a and 2b were
within the mean value range of the O–C single and double
coupling reaction and that complex 2b, which has the
aminoketonato skeleton with a PPh2 moiety as the third donor,
gave the best result. Complex 2b afforded 4-phenyltoluene (
in 83% yield along with the formation of homo-coupled
products, biphenyl ( , 12%) and 4,4-dimethylbiphenyl ( , 5%).
β-
3)
bonds. This implied that the π-electrons were delocalized on
the N1–C2–C3–C4–O1 and N1–C2–C3–C4–N2 frameworks. The
4
5
To investigate the influence of the halide in as electrophile, the
catalytic reactions were performed with 4-bromotoluene and
conjugated nitrogen atom N1 and the Ni center bond lengths
(1.8696(18) Å for 2a, 1.896(2) Å for 2b, and 1.860(2) Å for 2c
)
4-iodotoluene in the presence of a catalytic amount of 2b
.
were substantially shorter than the amino nitrogen atom N2 or
N3 and Ni bond lengths (Ni1–N2 = 1.980(2) Å for 2a and Ni1–
N3 = 1.994(3) Å for 2c). Furthermore, the bond length of the
other conjugated nitrogen atom N2 and Ni1 (1.891(3) Å) in 2c
Similar results were obtained in the case of 4-bromotoluene
(entry 4). On the other hand, with 4-iodotoluene, the yield of
Table 2. Optimization of the cross-coupling reaction catalyzed by Ni(II) complexes 2a
Entry
Ni catalyst
(mol%)
2a (1)
2b (1)
2c (1)
X
PhMgBr
(equiv.)
1.5
solvent
T
3
4
5
(p-TolX)
(°C)
25
25
25
25
25
25
25
25
25
25
50
0
(%)b
(%)b
(%)b
1
Cl
Cl
Cl
Br
I
THF
47
23
18
2
1.5
THF
83
48
80
52
19
19
48
73
69
84
74
82
84
91
12
25
13
37
7
5
18
6
3
1.5
THF
4
2b (1)
2b (1)
2b (1)
2b (1)
2b (1)
2b (1)
2b (1)
2b (1)
2b (1)
2b (1)
2b (1)
2b (2.5)
1.5
THF
5
1.5
THF
5
6
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
1.5
DME
Et2O
CPME
1,4-dioxane
toluene
THF
2
7
1.5
13
15
11
23
17
9
1
8
1.5
4
9
1.5
4
10
11
12
13
14
15
1.5
14
9
1.5
1.5
THF
4
2.0
THF
25
25
25
14
10
14
5
2.5
THF
7
1.5
THF
6
a
b
The reaction was carried out a 1.0 mmol scale of 4-halotoluene. Phenylmagnesium bromide was added at once. The yields
were determined by GLC analysis using octadecane as an internal standard.
4 | J. Name., 2012, 00, 1-3
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the cross-coupled product
homo-coupled product
3
decreased to 52% and that of the these results, it was concluded that THF was a suitable solvent
derived from the nucleophile for this reaction. The reactions were conducted at 50 °C and 0
4
increased to 37% (entry 5). In this case, 4-iodotoluene would °C. However, the yield of
3 was not improved at higher or
act as not only an electrophile but also an oxidizing agent for lower temperatures (entries 11, 12). Regarding the optimum
the nickel in the catalytic cycle. Therefore, it caused the amounts of the Grignard reagent and the catalyst 2b, it was
decrease in the cross-coupled product and the increase in the found that 1.5 equivalents of the Grignard reagent and 2.5
homo-coupled product. These results show that complex 2b mol% of the catalyst led to good yields of
3 (entries 13–15).
effectively catalyzed the cross-coupling reaction of aryl To demonstrate the efficiency of complex 2b as a catalyst
chloride as an electrophile. To further investigate the influence for the biaryl cross-coupling reaction, we investigated the
of the solvent, the reaction was conducted in different scope of the reaction by using different aryl chlorides and
solvents. In ethereal solvents such as 1,2-dimethoxyethane arylmagnesium bromides under the optimized reaction
(DME), Et2O, and cyclopentyl methyl ether (CPME), poor conditions. The results are summarized in Table 3. The aryl
results were obtained (entries 6–8). In 1,4-dioxane, product
was obtained in 73% yield (entry 9). Toluene was also an withdrawing group on the aromatic ring, afforded the desired
effective solvent and the yield of was 69% (entry 10). From
3
halide 4-chlorobenzotrifluoride, which has an electron-
3
Table 3. Results of cross-coupling reaction catalyzed by 2ba
2b (2.5 mol%)
Ar1 Cl
Ar2 MgBr
+
Ar1 Ar2
THF, T °C, Time
Entry
Ar1–Cl
Ar2–MgBr
T
(°C)
Time
Ar1–Ar2
Yield (%)b
85c
1
2
25
25
24 h
5 min
86 c
F3C
Cl
Cl
MgBr
F3C
3
4
25
25
24 h
88 c
35 c
MeO
MgBr
MgBr
MeO
5 min
Me
Me
Cl
5
25
24 h
87
Me
Me
6
7
25
24 h
24 h
67
81
Cl
Cl
MgBr
MgBr
reflux
8
9
Me
Me
25
25
24 h
24 h
Me
91
91
Me
Cl
MgBr
Me
Me
10
11
25
24 h
24 h
60
84
Cl
Cl
MgBr
MgBr
reflux
Me
Me
Me
12
13
25
72 h
72 h
5 c
16 c
Me
Me
reflux
Me
Me
Me
Me
14
15
25
72 h
72 h
2 c
42 c
Cl
Me
Me
MgBr
reflux
Me
a
The reaction was carried out with chloroarene (1.0 mmol) and arylmagnesium bromide (1.5 mmol) in the presence of the
b
catalyst 2b (0.025 mmol). Arylmagnesium bromide was added at once. The yield was determined by GLC analysis using
c
octadecane as an internal standard. The yield was determined by 1H NMR analysis using pyrazine as an internal standard.
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product in 85% yield after 24 h (entry 1). Furthermore, the Table 4. Results of cross-coupling reaction of dihalobenzene
reaction was complete within 5 min (entry 2). In the case of 4- with PhMgBra
X
2b (2.5 mol%)
Ph
chloroanisole, which has an electron-donating substituent, the
product was obtained in 88% yield after 24 h (entry 3).
However, after 5 min, the yield of the product was only 35%
(entry 4). 3-Chlorotoluene was effectively converted to 3-
phenyltoluene (87%, entry 5). In the case of 2-chlorotoluene,
the coupled product, 2-phenyltoluene, was obtained in 67%
yield under the optimized conditions (25 °C, 24 h, entry 6).
Under THF-refluxing conditions, the yield of 2-phenyltoluene
increased to 81% (entry 7). Next, we examined the reaction of
chlorobenzene with tolylmagnesium bromides. In the case of
X
Ph MgBr
Ph
+
THF, 25 °C
Time
Entry
1
dihalobenzene
Time
Yield (%)b
80 (88c)
5 min
2
3
24 h
24 h
67
11
both
p- and m-tolylmagnesium bromide, coupling products
were formed in 91% yield (entries 8 and 9). On treating with
o
-
tolylmagnesium bromide at 25 °C, 2-phenyltoluene was
formed in 60% yield (entry 10). The product yield increased to
84% under refluxing conditions (entry 11). 2-Chloro-1,3,5-
trimethylbenzene (mesityl chloride), as a sterically congested
substrate, was examined in the reaction with
phenylmagnesium bromide. When the reaction was conducted
at 25 °C for 72 h, only a small amount of the desired product
was obtained (5%, entry 12). The product yield did not
increase to any appreciable extent (16%, entry 13) even after
reflux for 72 h. In the reaction of chlorobenzene with 2-
mesitylmagnesium bromide at 25 °C for 72 h, the yield of the
product was not improved (2%, entry 14). On the other hand,
the product was formed in 42% yield under refluxing
conditions (entry 15).
4
5
5 min
24 h
52
62
a
The reaction was carried out with dihalobenzene (1.0 mmol)
and PhMgBr (2.5 mmol) in the presence of the catalyst 2b
(0.025 mmol). PhMgBr was added at once. b Isolated yield.
c
The yield was determined by GLC analysis using octadecane
as an internal standard.
the case of complex 2a, a similar oxidation potential was seen
at 0.64 V. On the other hand, complex 2c showed a lower
potential (0.26 V) than complexes 2a and 2b. These oxidation
potentials are reasonable values for the pincer-type Ni(II)
complexes as compared to literature values, which were
independently reported by Tonzetich12 and Zargarian.14a Based
on the oxidation potentials of these complexes, it is
conceivable that the electron density accumulated at the Ni
Next, we examined the reaction of dihalobenzene with 2.5
equivalents of phenylmagnesium bromide and 2.5 mol% of
complex 2b. The results are summarized in Table 4. In the case
of 1,4-dichlorobenzene, p-terphenyl was formed within 5 min
in 88% yield, as determined by gas-liquid chromatography
(GLC) analysis, and the product was isolated in 80% yield (entry
center increases in the order 2a
≈ 2b < 2c. We assumed that
1). In the case of 1,3-dichlorobenzene,
m-terphenyl was
the highest occupied molecular orbital (HOMO) levels of these
complexes would also increase in this order. Therefore, the
electronic properties of the series of Ni(II) complexes were
investigated theoretically.
isolated in 67% yield (entry 2), whereas the yield of
o
-
terphenyl decreased to 11% using 1,2-dichlorobenzene (entry
3). In contrast, the reaction with 1,4-dibromobenzene as an
electrophile gave p-terphenyl in 52% yield after 5 min and 62%
Computational study was carried out using Gaussian 09 at
the B3LYP level with LANL2DZ basis set for the Ni atom and 6-
311++G(d,p) for the other atoms. The LANL2DZ pseudo-
potential was used for the Ni center. Geometry optimizations
yield after 24 h (entries 4 and 5). These results clearly show
that complex 2b can activate the C–Cl bond, and more
effectively than the C–Br bond in the cross-coupling reaction.
of Ni(II) complexes 2a
optimized molecular structures are shown in Figure S7 (ESI).
) are Selected geometrical parameters of these complexes are
catalytically active in the Kumada-Tamao-Corriu (KTC) reaction. summarized in Table S6 (ESI). The geometrical parameters of
Among these complexes, 2b possesses the -aminoketonato the Ni(II) complexes determined by DFT were found to be in
, 2b, and 2c were successful. The
Catalytic performance of 2 for the KTC reaction
As mentioned above, the three nickel(II) complexes (
2
β
framework with the PPh2 unit as the third donor and acts as an good agreement with those obtained from X-ray analysis,
effective pre-catalyst for this reaction. In order to elucidate the although the calculations predicted slightly longer bond
catalytic performance of these complexes, the electronic lengths.
properties of complexes
2
were estimated by CV. The cyclic
The plots of the HOMO and LUMO orbitals of complexes
2c are illustrated in Figure S8 (ESI). These complexes had
voltammograms of these complexes were measured in a 2a
–
CH2Cl2 solution. All complexes exhibited irreversible oxidation similar HOMO and LUMO orbitals. The HOMO of 2a (–5.68 eV)
waves, which are shown in Figures S5 and S6 (Electronic was similar to that of 2b (–5.61 eV) in energy, while the energy
Supplementary Information, ESI). The CV of complex 2b level of 2c (–5.23 eV) was obviously higher than those of 2a
showed a one-electron oxidation wave at 0.61 V vs. Fc/Fc+. In
and 2b. Furthermore, in these Ni(II) complexes, there was a
6 | J. Name., 2012, 00, 1-3
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ARTICLE
good relationship between the HOMO energies and oxidation
potentials determined by cyclic voltammetry. In these Ni(II)
complexes, the HOMO orbitals are mainly located on the six-
membered ring consisting of Ni and the conjugated ligand
framework. Therefore, it was assumed that the conjugated
ligand had an electronic influence on the Ni center and that
the Ni(II) complex 2c was more electron rich as comparted to
2a and 2b. In general, it is known that in an electron rich metal
complex, the oxidative addition reaction of an electrophile to
the metal takes place quite easily.20 Therefore, it was expected
that complex 2c would exhibit a high catalytic performance in
the cross-coupling reactions. However, as mentioned above,
the catalytic performance of complex 2b was superior to that
of 2a and 2c. Although, further investigation of the influence of
electronic factors on the catalytic activity is necessary, the
steric environment around the Ni center should be considered
as the most dominant factor affecting the catalytic
performance. The X-ray diffraction study of the series of Ni(II)
complexes revealed that complex 2c was more distorted from
the ideal square planar geometry around the Ni center
compared to complexes 2a and 2b (vide supra). This distortion
arises from the steric bulkiness of the ligand skeleton.
Therefore, in complex 2c, the interaction of the substrate with
the Ni center is encumbered and leads to poor catalytic activity.
Compared to complex 2b, complex 2a also shows lower
activity. These complexes show similar electronic features, as
estimated by CV and DFT. The low performance of 2a might be
attributed to the different “third donor”, i.e., the phosphorus
or nitrogen donor atoms. Phosphorus as the third donor in
complex 2b would elicit a higher performance in the cross-
coupling reaction. Further investigations of the influence of
the ligand framework on the catalytic performance are
currently underway.
Experimental
General procedures
All manipulations involving air- and moisture-sensitive
organometallic compounds were performed under an
atmosphere of nitrogen, which was dried with SICAPENT
(Merck Co., Inc.), using standard Schlenk tube or high vacuum
techniques. All solvents were distilled over appropriate drying
agents prior to use. 2-(Diphenylphosphino)ethylamine,21 1a-
H 16
,
1b-H 18
,
4-((2,4,6-trimethylphenyl)amino)pent-3-en-2-
one,22 [NiCl2(2,4-lutidine)2],23 and [NiCl2(PPh3)2]24 were
prepared according to literature reported procedures. The
other reagents employed in this work were commercially
available and used without further purification. 1H, 1H{31P},
13C{1H}, and 31P{1H} NMR spectra were recorded on BRUKER
DRX-300, DRX-500, or JEOL ECX-400 spectrometers at ambient
temperature. The H, H{31P}, and 13C{1H} NMR chemical shifts
were recorded in ppm relative to Me4Si as an internal
standard. The 31P{1H} NMR chemical shifts were recorded in
ppm relative to H3PO4 as an external standard. All coupling
constants were recorded in Hz. Multiplicity is indicated by s
(singlet), d (doublet), t (triplet), q (quartet), dt (doublet of
triplets), dq (doublet of quartets) and m (multiplet). Thin layer
chromatography was performed using Merck silica gel 60F-254
plates and examined under UV (254 nm) irradiation. Column
chromatography was performed using Silica Gel 60N
(spherical, neutral, 63–210 µm, Kanto Chemical Co., Inc.). High-
resolution mass spectra (HRMS) were recorded using fast atom
bombardment (FAB) ionization with a JEOL JMS-700 mass
spectrometer. Elemental analyses were performed on a Vario
EL elemental analyzer. GLC were recorded on a Shimadzu GC-
17A gas chromatograph using a ULBON HR-1 capillary column
(0.25 ID × 25 m, Shinwa Chemical Industries Ltd.).
1
1
Preparation of 1c-H
4-((2,4,6-Trimethylphenyl)amino)pent-3-en-2-one (1240 mg,
Conclusions
5.71 mmol),
N,N-diethylethylenediamine (992 mg, 1.20 mL,
8.54 mmol), and toluene (60 mL) were put in a round-bottom
flask. A few drops of H2SO4 and molecular sieves 4Å (ca. 10 g)
were added to the reaction mixture. The mixture was refluxed
for 48 h and then cooled to room temperature. After
neutralization with aq. KOH, the reaction mixture was
extracted with CH2Cl2. The combined organic extracts were
dried with Na2SO4, filtered, and concentrated in vacuo to
In this work, we investigated the synthesis of pincer-type Ni(II)
complexes and their catalytic performance for the KTC
reaction. β-Aminoketonato and β-diketiminato frameworks
tethering the nitrogen or phosphorus groups as the third
donor to the metal center led to the desired pincer-type Ni(II)
complexes
2. The Ni(II) complexes exhibited catalytic activity
for the KTC reaction; complex 2b bearing the
β
framework with the diphenylphosphino group as the third
-aminoketonato
1
obtain 1c-H as a brown liquid (1300 mg, 4.12 mmol, 72%). H
NMR (
NCCH3), 2.00 (s, 9H,
2.26 (s, 3H, NCCH3), 2.51 (q,
2H, NCH2CH2N), 3.30 (m, 2H, NCH2CH2N), 4.62 (s, 1H, CH), 6.84
(s, 2H, N-
-(CH3)3C6H2), 10.69 (broad s, 1H, NH). 13C{1H} NMR
, CDCl3): 11.9 (s, NCH2CH3), 18.3 (s, N- -( H3)3C6H2), 19.4 (s,
N- -( H3)3C6H2), 20.7 (s, NC H3), 21.1 (s, NC H3), 41.9 (s,
NCH2CH2N), 47.5 (s, N H2CH3), 53.8 (s, NCH2CH2N), 93.1 (s,
NC CN), 127.6 (s, N- -(CH3)3C6H2), 128.2 (s, N- -(CH3)3C6H2),
130.8 (s, N- -(CH3)3C6H2), 147.2 (s, N- -(CH3)3C6H2), 155.3 (s,
CCN), 166.0 (s, NCC
N). HRMS (FAB+) [M+H]+ Calc. for
δ
, CDCl3): 0.97 (t,
-(CH3)C6H2
= 7.3 Hz, 4H, NCH2CH3), 2.54 (m,
J
= 7.3 Hz, 6H, NCH2CH3), 1.59 (s, 3H,
donor showed remarkable catalytic performance. The
o
+ p
-(CH3)C6H2, overlapped),
combination of
β-aminoketonato and/or β-diketiminato
J
frameworks with the third donor enabled the fine tuning of
the electronic and steric factors around the metal center.
These pincer ligand systems can be easily prepared by the
condensation reaction of acetylacetone with amines and their
coordination to various transition metals is expected to
produce highly active metal catalysts. Further investigations on
the mechanistic aspects, the coupling reactions of various
organometallic reagents with organic electrophiles, and the
application of these ligand systems to other metals are
currently underway.
m
(
δ
o C
p
C
C
C
C
C
m
o
p
ε
N
C
C
m/z
C20H34N3: 316.2753; Found 316.2750.
Preparation of 2a
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A solution of [NiCl2(2,4-lutidine)2] (1240 mg, 3.61 mmol) in THF CDCl3): 1.23 (s, 3H, NCCH3), 1.67 (t,
(15 mL) was prepared and cooled to –78 °C. To this solution, a 1.92 (s, 3H, NCCH3), 2.21 (s, 3H, N-
THF solution of the lithiated ligand (1a-Li), which was prepared Hz, 2H, NCH2CH2N), 2.46 (dq, = 12.8, 7.2 Hz, 2H, NCH2CH3),
by the reaction of 1a-H (718 mg, 3.62 mmol) with 2.49 (s, 6H, N- -(CH3)3C6H2), 3.25 (dq, = 12.8, 7.2 Hz, 2H,
butyllithium (1.40 mL of the 2.67 M hexane solution, 3.74 NCH2CH3), 3.39 (t, = 6.4 Hz, 2H, NCH2CH2N), 4.56 (s, 1H, CH).
mmol) at –78 °C, was added. The reaction mixture was allowed 6.75 (s, 2H, N- , CDCl3): 11.2 (s,
-(CH3)3C6H2). 13C{1H} NMR (
to warm to room temperature. After 18 h, the volatiles were NCH2CH3), 19.3 (s, N- -( H3)3C6H2), 21.0 (s, N- -( H3)3C6H2),
removed under reduced pressure. The residual solid was 22.8 (s, NC H3), 23.4 (s, NC H3), 49.9 (s, N H2CH2N), 51.3 (s,
extracted with CH2Cl2 (30 mL) and the volatiles were removed H2CH3), 52.1 (s, N H2CH2N), 99.3 (s, NC CN), 127.8 (s, N-
under reduced pressure. The resulting purple solid was (CH3)3C6H2), 132.7 (s, N- -(CH3)3C6H2), 133.2 (s, N- -(CH3)3C6H2),
washed with hexane and dried in vacuo to yield 2a (1040 mg, 149.1 (s, N- -(CH3)3C6H2), 157.7 (s, N CCN), 158.0 (s, N CCN).
J
= 7.2 Hz, 6H, NCH2CH3),
p
-(CH3)3C6H2), 2.22 (t, J = 6.4
J
n
-
o
J
J
m
δ
o
C
p C
C
C
C
N
C
C
C
m-
o
p
ε
C
C
3.57 mmol, 99%). Analytically pure sample of 2a was obtained Typical procedure for the cross-coupling reaction (Table 2,
by recrystallization from CH2Cl2/hexane. Anal. Calc. for entry 15): GLC analysis
C11H21ClN2NiO: C, 45.33; H, 7.26; N, 9.61%. Found: C, 45.35; H, Complex 2b (10.2 mg, 0.025mmol), octadecane (145.8 mg,
7.33; N, 9.53%. 1H NMR (
δ
, CDCl3): 1.75 (s, 3H, CH3), 1.78 (t,
7.2 Hz, 6H, NCH2CH3), 1.90 (s, 3H, CH3), 2.14 (t, = 6.4 Hz, 2H, mL, 1.03 mmol) were placed in
NCH2CH2N), 2.46 (dq, = 13.2, 7.2 Hz, 2H, NCH2CH3), 3.06 (t, Phenylmagnesium bromide (1.5 mL of the 1.0 M THF solution,
6.4 Hz, 2H, NCH2CH2N), 3.18 (dq, = 13.2, 7.2 Hz, 2H, NCH2CH3), 1.5 mmol) was added at once to the reaction mixture. After
4.92 (s, 1H, CH). 13C{1H} NMR (
, CDCl3): 11.1 (s, NCH2CH3), 21.2 stirring for 24 h at 25 °C, 1 M hydrochloric acid (5 mL) was
(s, CH3), 24.0 (s, CH3), 50.2 (s, NCH2CH2N), 52.0 (s, NCH2CH2N + added to quench the reaction. The products were extracted
J
=
0.57 mmol), THF (5 mL), and 4-chlorotoluene (130.1 mg, 0.122
J
a
Schlenk tube.
J
J =
J
δ
NCH2CH3, overlapped), 99.6 (s, CH), 164.6 (s,
CH3).
Preparation of 2b
C
CH3), 176.8 (s, with Et2O and the yields of the products were determined by
GLC analysis using octadecane as an internal standard.
C
Compound 1b-H (191 mg, 0.61 mmol), [NiCl2(PPh3)2] (405 mg,
0.62 mmol), and THF (20 mL) were mixed in a Schlenk tube.
After stirring the reaction mixture for 1 h, NEt3 (0.10 mL, 73 mg,
0.72 mmol) was added to the mixture and the solution was
stirred for another 4 h at room temperature. Subsequently,
the solution was filtered through a Celite pad and the filtrate
was evaporated to dryness in vacuo. The residual solid was
washed with Et2O and dried in vacuo to yield 2b as an orange
solid (237 mg, 0.59 mmol, 97%). An analytically pure sample of
2b was obtained by recrystallization from CH2Cl2/hexane. Anal.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
The authors are grateful to Prof. Masatoshi Asami (Yokohama
Natl. Univ.) for fruitful discussions. We thank Dr. Takahiro
Iwamoto and Prof. Masaharu Nakamura (Kyoto Univ.) for their
kind help in the HRMS measurements, Dr. Tomoyo Misawa
(Sophia Univ.) for her kind help in the CV measurements, and
Mr. Kei Funatsu (Yokohama Natl. Univ.) for experimental
assistance. This work was supported by JSPS KAKENHI Grant
Numbers 25410063 and JP17K05805. This work was also
supported by the Collaborative Research Program of Institute
for Chemical Research, Kyoto University (grant number 2017-
20). The computations were performed using the Research
Center for Computational Science, Okazaki, Japan.
Calc. for C19H21ClNNiOP: C, 56.42; H, 5.23; N, 3.46%. Found: C,
1
56.14; H, 5.27; N, 3.37%. H NMR (
δ, C6D6): 1.27 (dt,
J
= 10.9,
6.8 Hz, 2H, NCH2CH2P), 1.35 (s, 3H, NCCH3), 1.99 (s, 3H, OCCH3),
2.41 (dt, = 25.2, 6.8 Hz, 2H, NCH2CH2P), 4.92 (s, 1H, CH), 6.98-
7.08 (m, 6H, P- -C6H5 + P- -C6H5), 7.93-8.01 (m, 4H, P- -C6H5).
1H{31P} NMR (
, C6D6): 1.27 (t, = 6.7 Hz, 2H, NCH2CH2P), 1.35
(s, 3H, NCCH3), 1.99 (s, 3H, OCCH3), 2.42 (t, = 6.7 Hz, 2H,
NCH2CH2P), 4.92 (s, 1H, CH), 6.99-7.09 (m, 6H, P- -C6H5 + P-
C6H5), 7.97 (d, = 7.0 Hz, 4H, P- , C6D6):
-C6H5). 13C{1H} NMR (
23.1 (s, NC H3), 25.2 (d, JPC = 6.4 Hz, OC H3), 30.6 (d, JPC
25.7 Hz, NCH2CH2P), 52.0 (d, JPC = 8.2 Hz, N H2CH2P), 99.8 (s,
J
p
m
o
δ
J
J
p
m-
J
o
δ
1
4
C
C
=
2
C
Notes and references
3
1
OC
52.2 Hz, P-
2JPC = 9.2 Hz, P-
CCN). 31P{1H} NMR (
Preparation of 2c
C
CN), 128.7 (d, JPC = 11.0 Hz, P-
m
-C6H5), 129.6 (d, JPC
-C6H5), 131.1 (d, JPC = 2.7 Hz, P- -C6H5), 133.7 (d,
-C6H5), 165.1 (s, OCC
N), 178.8 (d, 3JPC = 2.7 Hz,
, C6D6): 37.5.
=
1
(a) K. Tamao, K. Sumitani and M. Kumada, J. Am. Chem. Soc.,
1972, 94, 4374–4376; (b) R. J. P. Corriu and j. P. Masse, J.
Chem. Soc., Chem. Commun., 1972, 144.
4
ε
p
o
C
OC
δ
2
Review articles: (a) Cross-Coupling Reactions: A Practical
Guide, ed. N. Miyaura, Springer-Verlag, Berlin, Heidelberg,
2002; (b) Metal-Catalyzed Cross-Coupling Reactions, 2nd
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Fu, Angew. Chem. Int. Ed. 2002, 41, 4176–4211; (d) A. C.
Frisch and M. Beller, Angew. Chem. Int. Ed., 2005, 44, 674–
This complex was prepared from 1c-H (302 mg, 0.96 mmol),
[NiCl2(2,4-lutidine)2] (225 mg, 0.65 mmol), and NEt3 (0.10 mL,
73 mg, 0.72 mmol) with THF (10 mL) as the solvent in the same
manner as that described for 2b. Complex 2c was isolated as a
red solid (246 mg, 0.60 mmol, 92%). An analytically pure
sample of 2c was obtained by recrystallization from
Et2O/hexane. Anal. Calc. for C20H32ClN3Ni: C, 58.79; H, 7.89; N,
688; (e) J.-P. Corbet and G. Mignani, Chem. Rev., 2006, 106
2651–2710; (f) J. Terao and N. Kambe, Acc. Chem. Res., 2008,
41, 1545–1554; (g) X. Hu, Chem. Sci., 2011, , 1867–1886; (h)
R. Jana, T. P. Pathak and M. S. Sigman, Chem. Rev., 2011,
111, 1417–1492.
,
2
10.28%. Found: C, 58.48; H, 7.97; N, 10.12%. 1H NMR (
δ,
8 | J. Name., 2012, 00, 1-3
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Graphical Abstract
Synthesis of acetylacetoneto-based pincer-type nickel(II) complexes and their catalytic activity were
investigated.