Homocoupling of aryl halides promoted by an NiCl2/bpy/Mg system in DMF

Article abstract of DOI:10.1002/aoc.3168

Homocoupling of aryl halides (2 ArX → Ar-Ar) promoted by NiCl ₂/2,2-bipyridine (bpy)/Mg mixtures in DMF has been studied. Mixtures of NiCl₂, bpy and Mg in DMF promoted homocoupling of aryl halides such as phenyl bromide and p-tolyl bromide to give the coupling products in good (e.g. approximately 60-75%) yields, and the homocoupling products were easily isolated from the reaction mixtures. Application of this homocoupling to dibromo-aromatic compounds (Br-arylene-Br: 2,5-dibromopyridine, 2,7-dibromo-9.9-dioctylfluorene and 2,7-dibromo-9,10-dioctyl-9,10- dihydrophenanthrene) gave the corresponding π-conjugated polymers, -(arylene)_n-, in good yields. Organometallic processes for the homocoupling are discussed. Copyright

Full text of DOI:10.1002/aoc.3168

Full Paper
Received: 2 March 2014
Revised: 10 April 2014
Accepted: 27 April 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3168
Homocoupling of aryl halides promoted by an
NiCl /bpy/Mg system in DMF
2
Takakazu Yamamoto*
Homocoupling of aryl halides (2 ArX → Ar―Ar) promoted by NiCl /2,2′-bipyridine (bpy)/Mg mixtures in DMF has been studied.
2
2
Mixtures of NiCl , bpy and Mg in DMF promoted homocoupling of aryl halides such as phenyl bromide and p-tolyl bromide to give
the coupling products in good (e.g. approximately 60–75%) yields, and the homocoupling products were easily isolated from
the reaction mixtures. Application of this homocoupling to dibromo-aromatic compounds (Br–arylene–Br: 2,5-dibromopyridine,
2
,7-dibromo-9.9-dioctylfluorene and 2,7-dibromo-9,10-dioctyl-9,10-dihydrophenanthrene) gave the corresponding π-conjugated
polymers, –(arylene) –, in good yields. Organometallic processes for the homocoupling are discussed. Copyright © 2014 John
n
Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: homocoupling; nickel; aryl halide; magnesium; DMF
Introduction
Experimental
Zerovalent nickel complexes [Ni(0)L
m
] have been used for
Materials
homocoupling of aryl halides ArX (2 ArX + [Ni(0)L ] → Ar―Ar).
m
1–3]
Organic monohalides such as phenyl bromide and p-tolyl bromide,
[
Isolated Ni(0) complexes such as [Ni(cod)2]
(cod = 1,5-
2,7-dibromo-9,9-dioctylfluorene, 2,5-dibromopyridine, bis(1,5-
cyclooctadiene) and Ni(0) complexes generated by reduction of
cyclooctadiene)nickel(0) ([Ni(cod) ]), bpy, 1,2-dimethoxybenzene,
[4–7]
2
Ni(II) species with Zn
have been used as the homocoupling
Mg turnings, [Cu(acac) ] (acac = acetylacetonato), dry solvents,
2
(or condensing) agent, and the homocoupling reaction has been
poly(4-vinylpyridine) (P4VP, cross-linked) and a standard polymer
applied for dehalogenative polycondensation of dihaloaromatic
(poly(9,9-dioctylfluorene-2,7-diyl)) were purchased from Tokyo
compounds to give π-conjugated polymers (e,g. n X-arylene-X + n
Chemical Industry Co. Ltd, Sigma-Aldrich Co. LLC, Wako Pure
Chemical Industries Ltd or Kanto Chemical Co. Inc. Molecular
sieve-dried DMF was obtained by adding molecular sieve 4A to
commercially available dry DMF. Highly pure (purity = 99.95%) 2,7-
dibromo-9.9-dioctylfluorene was kindly donated by JFE Chemical
Corporation. Polyethylene glycol (PEG 4000 from Tokyo Chemical
Industry Co. Ltd; average molecular weight = 2600–3800) was
dissolved in dry DMF under heating and this solution was dried
with a 4 Å molecular sieve. The P4VP powder was dried and
degassed under vacuum. Water was removed from an aqueous
solution of polyethyleneimine (PEI, purchased from Wako Pure
Chemical Industries Ltd; average molecular weight = 1800) under
vacuum at 115°C (for 3 h). The obtained PEI solid gave N and O
contents of 29.74% and 4.05%, which corresponded to an N:O ratio
of about 8:1 (=29.74%/14:4.05%/16), suggesting a partly hydrated
[
8–12]
[
Ni(0)L ] → –(arylene) –; arylene = p-phenylene, etc.).
m n
π-Conjugated polymers have been the focus of numerous studies
[
13–16]
because of their attractive chemical and physical properties.
N,N-Dimethylformamide (DMF) is often used as the solvent for
the homocoupling reaction and dehalogenative polycondensa-
tion using the isolated Ni(0) complexes. However, use of Ni(0)
complexes generated by reduction of Ni(II) species with Mg in
DMF has not received much attention. Mg is thought to react
with both the Ni(II) species and ArX, and the Grignard reagent
[
17,18]
formed by the reaction with ArX may be trapped by DMF;
this seems to be one of reasons for less attention.
If an NiCl /ligand/Mg system in DMF is usable for the C―C
2
coupling reaction of ArX, it will serve as a new convenient
homocoupling (or condensing) reagent. Because Mg has a stronger
reducing ability than Zn, the system might work as an advantageous
homocoupling reagent. In addition, results of the homocoupling
reactions might add new information to organometallic chemistry.
[
25]
structure of (CH
2
CH
2
NH.1/8H
2 n
O) .
2,7-Dibromo-9,10-dioctyl-9,10-
[
26]
dihydrophenanthrene was prepared as previously reported.
Previously we briefly reported that an NiCl
2
/bpy/Mg (bpy = 2,2′-
Measurements and General Procedures
bipyridine) system in DMF is actually usable for homocoupling of
1
[19]
IR and H NMR spectra were recorded on JASCO FT-IR-460 plus
and JEOL JNM-LA300 spectrometers, respectively. NMR yields of
ArX.
This homocoupling can be easily carried out and might
expand the scope of organometallic homocoupling of ArX. We
now report this homocoupling under various conditions, and discuss
organometallic aspects of this homocoupling. Application of this
homocoupling to dehalogenative polycondensation of dihaloaromatic
compounds to give π-conjugated polymers will also be reported.
Basic reactions of zero-valent nickel complexes with organic halides
* Correspondence to: Takakazu Yamamoto, Chemical Resources Laboratory,
Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503,
Japan. E-mail: takakazuyamamoto@kir.biglobe.ne.jp
Chemical Resources Laboratory, Tokyo Institute of Technology, Midori-ku,
Yokohama 226-8503, Japan
[
20–24]
have been the subject of many organometallic studies.
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.

T. Yamamoto
coupling products were obtained by adding a measured amount
of CHCl CHCl to the reaction product and comparing peak areas
of the product and CHCl CHCl
the peak area ratio between the CHCl
2
CHCl
2
peak and 4,4′-
2
2
dimethylbiphenyl peak indicated 75% yield of 4,4′-dimethylbiphenyl.
Peaks of TolBr were not observed. DMF in the remaining part
of the reaction mixture was removed under vacuum, and the
remaining solid was treated with diluted hydrochloric acid (4 M,
2
2
.
For example, a small part of the reaction mixture obtained by
the homocoupling of p-tolyl bromide (TolBr) was taken out, and
1
[30]
a measured amount of CHCl
spectrum of this mixture in CDCl
homocoupling product (Tol–Tol) and CHCl CHCl . The amount
2
CHCl
2
was added. The H NMR
15 ml). Extraction with hexane (×3) and removal of hexane by
3
showed peaks of the
natural evaporation yielded crystals of 4,4′-dimethylbipheny
1
(102 mg, 73% yield). IR and H NMR spectra indicated the isolation
2
2
of Tol–Tol was calculated from areas of a CHCl CHCl2 peak
of pure 4,4′-dimethylbiphenyl.
2
(
at around δ 6.0) and CH peak of Tol–Tol (at around δ 2.4). These
H NMR peaks are sometimes somewhat broadened, presumably
3
1
Polymerization
due to the presence of paramagnetic Ni(II) species and/or solid
particles in the crude reaction mixture. Figure S1a (supporting
Preparation of PFlu(9,9-Oct) and PH Ph(9,10-Oct)
2
1
information) shows an example of the somewhat broadened H
1
A mixture of NiCl
in dry DMF (40 ml) was stirred at 70°C for 10 min. Mg (600 mg,
5 mmol) was added, and the mixture was stirred at the temper-
2
(270 mg, 2.1 mmol) and bpy (650 mg, 4.2 mmol)
NMR spectrum. However, a sharper H NMR spectrum could be
obtained depending on the case, as shown in Fig. S1b. As
described below, extraction of formed Tol–Tol with hexane and
removal of hexane via natural evaporation gave crystals of Tol–
2
ature for several minutes. After addition of commercially available
2,7-dibromo-9.9-dioctylfluorene (Monomer-1, 1.17 g, ~2.1 mmol;
1
Tol, which gave rise to a reasonable H NMR spectrum, as shown
NiCl /Monomer-1 = ~1.0), the reaction mixture was stirred at
2
in Fig. S2.
70°C for 18 h. The reaction mixture was poured into water (450 ml)
GC yields of the product were obtained using a Shimadzu GC
[
30]
and concentrated hydrochloric acid (50 ml) was added.
After
8
AT chromatograph, a Chromatopac data processor, a Silicone
stirring the mixture, the yellow precipitate was collected by
filtration, and the obtained powder was washed with water and
methanol. Drying under vacuum gave 800 mg (96%) of poly
OV-17 (5%) column and He carrier gas. A TCD detector was used.
For example, a small part of the reaction mixture obtained by the
homocoupling of PhBr was taken out and measured amounts of
naphthalene and 1,2-diphenylethane were added. The amount of
Ph―Ph formed was calculated from the GC peak areas of naph-
thalene, Ph―Ph and 1,2-diphenylethane (in order of retention
time) and relative sensitivity of these compounds. An example
of the GC curve is shown in Fig. S3.
The NMR and GC yields usually agreed with that calculated
based on the weight of the product. However, sometimes a lower
yield was obtained with the weighing method, compared with
the NMR and GC yields. Partial loss of the product during isolation
of the product (especially during drying the product under
vacuum) is thought to be the reason for the lower yield.
Size-exclusion chromatography (SEC) (or gel permeation chroma-
tography (GPC)) for poly(9,9-dioctylfluorene-2,7-diyl) (PFlu(9,9-Oct);
see below) and poly(9,10-dioctyl-9,10-dihydrophenanthrene-2,7-
(
9,9-dioctylfluorene-2,7-diyl), PFlu(9,9-Oct). SEC data showed
Mn (number average molecular weight), Mp (peak molecular
weight in the SEC trace), M (weight average molecular weight)
w
and M (z-average molecular weight) of 1000, 3000, 5700, and
z
18 000, respectively, with M /M of 5.7. Analogous polymerization
w n
using Monomer-1 (580 mg, 1.1 mmol), NiCl (270 mg, 2.1 mmol;
2
NiCl /Monomer-1 = 1.9), bpy (650 mg, 4.2 mmol) and Mg
2
(600 mg, 25 mmol) gave analogous results with 90% yield and
M , M , M and M of 1300, 3300, 7300 and 24 000, respectively
n
p
w
z
1
w n
with M /M of 5.6. The IR and H NMR spectra of the obtained
polymer essentially agreed with (or resembled) those of com-
mercially available standard poly(9,9-dioctylfluorene-2,7-diyl).
Use of highly pure Monomer-1 (see above) gave analogous
polymer.
Preparation of PH
e.g. by using NiCl (260 mg, 2.0 mmol), bpy (620 mg, 4.0 mmol),
Mg (600 mg, 25 mmol) and 2,7-dibromo-9,10-dioctyl-9,10-
dihydrophenanthrene (Monomer-2, 594 mg, 1.06 mmol; NiCl
Monomer-2 = 1.9). Yield 91%. SEC data showed M and M
of 1000 and 14 000, respectively, with M /M of 14. The
2
Ph(9,10-Oct) was carried out analogously
2
diyl) (PH Ph(9,10-Oct)) was carried out at Tosoh Analysis and
Research Center Co. Ltd; the eluent was chloroform and molecular
weights were estimated versus polystyrene standards. SEC of poly
(
2
2
/
(pyridine-2,5-diyl) (PPy) was carried out at Tosoh Analysis and
n
w
Research Center using a Tosoh HLC-8120GPC system with
1
[27]
w
n
H
hexafluoro-i-propanol as the eluent and poly(methyl methacrylate)
standards.
NMR spectrum of the product essentially agreed with (or
resembled) that of previously reported poly(9,10-dioctyl-9,10-
dihydrophenanthrene-2,7-diyl), PH Ph(9,10-Oct), which was pre-
2
The homocoupling reaction and polymerization were carried
out under N using standard Schlenk techniques.
2
[26]
2
pared using [Ni(cod) ].
Coupling Reaction
Preparation of PPy
This is a typical example (No. I-11 in Table 1). A mixture of NiCl2
This is a typical example. A mixture of NiCl (260 mg, 2.0 mmol),
2
(
(
260 mg, 2.0 mmol), bpy (620 mg, 4.0 mmol) and dry DMF
10 ml) was stirred for 5 min at 70°C. When Mg (110 mg, 4.5 mmol)
bpy (620 mg, 4.0 mmol) and dry DMF (10 ml) was stirred at
80°C for 10 min. The color of the mixture turned from yellow
to light green. When Mg (150 mg, 6.2 mmol) was added to this
mixture under stirring, the mixture turned deep green rapidly.
When 2,5-dibromopyridine (Monomer-3, 145 mg, 0.61 mmol)
was added, the mixture turned blackish. After stirring for 7 h
at 80°C, the reaction mixture was poured into diluted aqueous
ammonia (~1.7 M, 400 ml). A yellow slurry was separated by
decantation and washed with an aqueous solution of a disodium
salt of ethylenediaminetetraacetic acid (EDTA.2Na) and water.
was added to this mixture under stirring, the mixture turned deep
green (characteristic of low-valent Ni–bpy complexes
rapidly. After 3 min p-tolyl bromide (TolBr, 263 mg, 1.54 mmol)
was added and the mixture was stirred for 8 h at 70°C. After
cooling to room temperature (r.t.) a small portion (0.32 g) of the
reaction mixture was removed and CHCl
added to this part. The H NMR spectrum of this sample in CDCl
clearly shows peaks of 4,4′-dimethylbiphenyl, and comparison of
[
28,29]
)
2
CHCl
2
(8.2 mg) was
1
3
wileyonlinelibrary.com/journal/aoc
Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2014)

Homocoupling of ArX by NiCl /bpy/Mg in DMF
2
a
2
Table 1. Results of C―C coupling of phenyl halides (PhX) and p-tolyl bromide (TolBr) using mixtures of NiCl , bpy, and Mg in DMF
b
c
d
No.
ArX
Ni/ArX
Ligand (bpy)/Ni
Mg /Ni
Temperature (°C)
Time (h)
Yield (%)
I-1
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhBr
PhCl
PhCl
PhF
1.0
1.3
1.9
2.0
0.2
0.2
2
1.5
2.3
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
90
90
70
70
70
90
90
100
100
70
70
70
55
50
50
8
13
13
12
8
74
74
68
71
58
60
I-2
2
I-3
1
5.0
I-4
1
10
I-5
2
4.9
I-6
2
24
11
7
e
I-7
FeCl
3
, 1.0
2
2.5
–
I-8
1.0
0.3
1.3
1.3
1.3
1.3
1.3
1.8
2.0
2.0
1.9
2.1
2.1
2.0
1.9
2
5.0
11
10
8
70
63
I-9
2
24
e
I-10
I-11
I-12
I-13
I-14
I-15
I-16
I-17
I-18
I-19
I-20
I-21
I-22
I-23
I-24
I-25
I-26
I-27
I-28
I-29
I-30
2
2.3
–
TolBr
TolBr
TolBr
TolBr
TolBr
TolBr
TolBr
TolBr
TolBr
TolBr
TolBr
TolBr
TolBr
TolBr
TolBr
TolBr
TolBr
PhBr
PhBr
TolBr
2
2.3
8
75
68
47
92
2 in DMAc
2.3
8
2 in DMSO
2.3
8
1
Zn, 2.0
3.1
7
f
0
7
(~10)
f
0
3.0
4
(~10)
e
0
Zn, 3.0
3.1
4
–
f
PEG, 5
7
(~10)
f
P4VP, 3
2.8
8
(~10)
f
cod, 5
2.5
7
(~10)
DMOB, 33
3.0
5
0
0
DMOB, 24
Zn, 3.0
3.0
5
[Cu(acac)
[Cu(acac)
2
2
], 2.0
], 2.0
0
5
0
0
Zn, 3.0
0
5
0
[Ni(cod)
[Ni(cod)
[Ni(cod)
[Ni(cod)
2
2
2
2
], 1.9
], 1.9
], 1.9
], 0.6
1
7
98
100
98
82
73
60
0
PEG, 5
0
0
7
0
7
g
h
h
0
26
24
24
[NiCl
[NiCl
2
(PPh
(PPh
3
)
)
2
2
], 1.0
], 1.0
PPh
PPh
3
, 2
, 2
Zn, 1.0
Zn, 1.0
2
3
3
a
Data using other reaction conditions are also shown.
b
2 2 3
Molar ratio. NiCl was used unless otherwise noted. [Ni(cod) ], bis(1,5-cyclooctadiene)nickel(0); PPh , triphenylphosphine.
c
Molar ratio. Bpy (2,2′-bipyridine) was used in DMF unless otherwise noted. DMAc, N,N-dimethylacetamide; DMSO, dimethylsulfoxide; PEG,
polyethylene glycol (molarity is based on O); P4VP, poly(4-vinylpyridine) (molarity is based on N).
d
Molar ratio. Mg was used unless otherwise noted. For I-7, I-23, I-24, Mg/(Fe or Cu) is shown.
e
f
Trace.
1
Isolation by extraction was difficult. H NMR data indicated about 10% yield.
g
Data from references [1] and [2].
h
Data from reference [4]. Reaction conditions are from a typical example in this report.
The precipitate was collected by filtration and dried under
vacuum at 60°C to obtain poly(pyridine-2,5-diyl) PPy (40 mg,
Results and Discussion
Coupling of Phenyl Bromide and p-Tolyl Bromide and
8
5% yield). SEC data showed M , M and M of 11 000, 18 000
n p w
Related Reactions
and 25 000, respectively with M /M of 2.2. Analogous polymeri-
w
n
zation was carried out using a mixture obtained by the reaction
Table 1 shows results of coupling of phenyl halides (PhX) and
p-tolyl bromide (TolBr) using the Mg-reduced Ni complex in DMF
under various conditions.
of NiCl (1.63 g, 12.6 mmol), bpy (3.93 g, 25.2 mmol) and Mg
2
(243 mg, 10 mmol) in DMF (45 ml). Raising the temperature from
r.t. to 70°C gave a deep-green mixture. After the UV–visible spec-
trum of this deep-green reaction mixture was measured, the
remaining part of the green reaction mixture was used for
polymerization of Monomer-3 (710 mg, 3.0 mmol) at 70°C for
DMF
ArX
NiCl2-bpy + Mg
[Ni(0)Lm
]
(1)
Ar-Ar
1
1 h. This polymerization gave PPy in 34% yield, and SEC data
of this polymer showed M , M and M of 3500, 6300 and 7100,
respectively, with M /M of 2.1. The molecular weight of PPy
seemed to depend on delicate polymerization conditions. The IR
n
p
w
The reaction of yellow anhydrous NiCl
2
with bpy in DMF at 70°C
[
19]
w
n
for about 5–10 min gave a light-green suspension, which sug-
[
32,33]
gested the formation of [NiCl
2
(bpy)].
Addition of Mg to the
[
31]
spectrum supported the formation of PPy.
Appl. Organometal. Chem. (2014)
mixture gave a deep-green solution rapidly, indicating the
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

T. Yamamoto
occurrence of facile reduction of [NiCl
valent Ni complexes such as zero-valent [Ni(bpy)(solvent)
2
(bpy)] by Mg to give low-
Heating a DMF solution of [Ni(cod)
2
] at 70°C also gave a
[
28,29]
m
].
blackish suspension, suggesting the (partial) formation of
metallic Ni by liberation of cod. [Ni(cod) ] solutions are stable
however, they often give blackish
The deep-green DMF solution was not homogeneous, and some
solids were observed in the suspension. A part of this suspension
was removed and diluted with DMF. The UV–visible spectrum of
this diluted DMF suspension showed UV–visible peaks at 428 nm
2
[
35,36]
in the presence of cod;
slurry when stored without cod.
In contrast to cases of the NiCl –Mg and NiCl –Zn
2 2
systems, the blackish suspension formed from [Ni(cod) ]
2
ꢀ
1
ꢀ1
(
23 400 cm ) and 600 nm (16 700 cm ), and the energy differ-
ꢀ
1
ence (6700 cm ) between the two absorption peaks suggests that
promoted the C―C coupling reaction effectively even
without bpy (Nos. I-26 and 28). The difference in reactivity
between [Ni(cod) ] and Mg- or Zn-reduced NiCl might be
[
24,28,34]
the two peaks are assigned to nickel-to-bpy MLCT bands.
The reduction of the NiCl -bpy mixture with Mg in DMF proceeded
2
2
2
much faster than that of the NiCl -bpy mixture in DMF with Zn, as
due to the difference in the state of the formed metallic
2
judged from the velocity of the change of the color from light green
Ni, although, in the case of [Ni(cod) ], a main part of this
2
(
color of Ni(II)–bpy complex) to deep green (color of Ni(0)–bpy
complex might remain even after the formation of the
blackish suspension and promote the effective coupling
reaction.
complex). This is reasonable because Mg is a stronger reducing
agent than Zn.
The addition of PhBr or TolBr to the deep-green solution led to
smooth homocoupling of these ArX. Ar―Ar was obtained in
good yields. These results support the following assumption
about the relative velocity of basic reactions: (i) the reduction of
For [Ni(cod) ], cod may work as an effective stabilizer for
the zerovalent state of nickel. However, the addition of cod
2
to the NiCl –Mg system did not give good results for the
2
C―C coupling reaction (No. I-20).
[
NiCl (bpy)] by Mg proceeds rapidly; (ii) the homocoupling
2
reaction of PhBr and TolBr promoted by the formed zero-valent
Ni complex also proceeds rapidly; and (iii) the formation of the
Grignard reagent of PhBr and TolBr and/or the trapping reaction
(f) As described above, Mg-reduced Ni species without bpy did
not promote the C―C coupling reaction effectively. Bpy is
thought to stabilize the Ni(0) species by coordination. Di- or
polyamines such as ethylenediamine and diethers (O―O)
such as dimethoxyethane can coordinate with Ni. However,
addition of these types of ligands (see Experimental section
and Fig. 1) did not assist the C―C coupling effectively
(Nos. I-18, 19, 21 and 22).
[
17,18]
of the Grignard reagent by DMF
is a much slower process:
the rate of reduction of Ni(II) with Mg and the rate of
homocoupling of ArBr by the formed Ni(0) complex are
indeed much larger than the rate of reaction of ArBr with Mg
and the rate of trapping of the Grignard reagent by DMF:
ArBr = PhBr or TolBr.
o-Dimethoxybenzene (DMOB), polyethylene glycol (PEG)
and poly(4-vinylpyridine) (P4VP) may not stabilize the zero-
valent Ni, and/or the Ni(0) species coordinated with these
ligand may not be active for the C―C coupling. In the case
The results shown in Table I reveal the following features (a)–(g)
of the homocoupling reaction. Table 1 includes results reported
[1,2]
[4]
by Semmelhack et al.
and Kende et al. (see Nos. I-28–30) for
comparison. PhBr and TolBr gave analogous results to each other;
however, the isolation of the product by extraction seemed to be
easier when TolBr was used.
of [Ni(cod)
reaction (No. I-27). However, addition of polyethyleneimine
(PEI) to the [Ni(cod) ] system did not give good results for
the formation of Tol–Tol from TolBr.
[Ni(cod) ] is a superior condensing reagent for homocoupling
of ArX, giving Ar―Ar in higher yields under wider reaction
2
], addition of PEG did not disturb the C―C coupling
2
(a) The amount of added bpy (1 mol or 2 mol of bpy/1 mol of NiCl
2
)
2
did not affect the yield (Nos. I-1–4). However, the reaction did
not seem to proceed well in the absence of bpy (see Nos. I–15
and 16 and (e) below).
[
1–3]
conditions.
2
Based on this, [Ni(cod) ] is thought to be a
better coupling agent than the Ni(0) species generated by
reduction of Ni(II) species with Mg and Zn. However, the
Ni(II)–Mg and Ni(II)–Zn systems are easily constructed and
might be more useful in practical purposes.
(b) PhCl also gave the coupling product. However, PhF is not
active in the coupling reaction (Nos. I-8–10).
(
c) Use of 0.2 mol NiCl per PhBr also gave the coupling product in
2
good yield (Nos. I-5 and 6). The initially formed Ni(0) complex is
thought to be consumed by the reaction with PhBr to give
Ph–Ph and [NiBr L ] (L = ligand such as bpy), and [NiBr L ]
(g) Because metallic Cu is used in Ullmann coupling of
[
37]
ArX,
Mg-reduced metallic Cu might be expected to
promote the C―C coupling. However, the [Cu(acac) ]–Mg
2
m
2 m
2
thus formed is thought to be reduced by Mg to Ni(0) complexes
to make the catalytic use of Ni possible.
system did not cause the C―C coupling (No. I-23). Use of
Zn as the reducing agent gave analogous results (No. I-24).
Iron forms various complexes with bpy. However,
Mg-reduced Fe-bpy complexes did not promote the C―C
coupling either (No. I-7).
(
d) The C―C coupling reaction also proceeded well in N,N-
dimethylacetamide (DMAc) (No. I-12). Colon and Kelsey used
DMAc for the C―C coupling reaction of aryl halides by the Ni
[
5]
(
II)–L–Zn systems (L = bpy or PPh (triphenylphosphine)).
3
DMSO was also usable as the solvent, although the yield of
the product was lower (No. I-13).
(e) As described above, without bpy the C―C coupling did
not proceed effectively (Nos. I-15 and 16). In these cases,
the reaction of NiCl
pension, suggesting the formation of metallic Ni. The reac-
tion of NiCl with Zn also gave a blackish suspension (with a
slower reaction rate), and this NiCl –Zn system did not
2
with Mg rapidly gave a blackish sus-
2
2
promote the C―C coupling effectively either, when bpy was
absent (No. I-17).
Figure 1. Examples of ligands for Ni.
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2014)

Homocoupling of ArX by NiCl /bpy/Mg in DMF
2
Coupling of Other Organic Halides
The oxidative addition (equation (3)) of RX to zero-valent nickel
[3,20–24,39]
complexes is well known.
Disproportionation (equation (4))
(bpy)] and [NiX (bpy)] has been re-
and isolation of [Ni(aryl) (bpy)] from [Ni(cod) ]–bpy–
aryl halide reaction systems has also been reported.
described below, isolation of cis-[Ni(aryl) ] complexes such as
Ni(aryl) (bpy)] is possible only for selected aryl groups.
The reductive elimination (equation (5)) of R―R from diorganonickel
complexes such as [Ni(R) (bpy)] and [Ni(R) (diphosphine)] is also
Table 2 shows results of the C―C coupling of p-bromoacetophenone,
benzyl bromide and 2-phenyl-1-bromoethane. For comparison,
related results reported in the literature are also shown.
of [NiCl(R)(bpy)] to [Ni(R)
ported,
2
2
[33]
2
2
[24,40]
As
As shown in No. II-1, CH
withdrawing acetyl group also underwent the C―C coupling reaction.
Similar results were reported for the coupling of CH COC Br-p
and NCC Br-p promoted by isolated Ni–cod complexes,
and the reported results are shown in Nos. II-4 and 5. PhCH
3 6 4
COC H Br-p with a moderately electron-
2 m
L
[
2
3
6 4
H
[
1,2,24]
6 4
H
2
2
2
Br
Br with an sp C―Br bond also underwent the
C―C coupling reaction (Nos. II-2 and 3).
[24,28,41]
known,
and it is proposed that the reductive elimination
from [Ni(aryl) (bpy)] is assisted by π-electron interaction
3
and PhCH
2
CH
2
[24,42,43]
2
between the two aryl groups attached to Ni (see Fig. 2).
Such Ni complexes were isolated when the aryl group is highly
On the Mechanism of the C―C Coupling Reaction and Effects
of Substituents
[24]
[40]
electron-withdrawing (e.g., ―C6F5
and chloropyrazolyl ).
When the two aryl groups in [Ni(aryl) (bpy)] are not highly elec-
2
tron-withdrawing, this Ni complex seems to undergo reductive
elimination easily.
As described above, the reaction of NiCl with bpy gave a light-
2
[
32,33]
green product, the color being characteristic of [NiCl (bpy)].
2
The Ni―C bond in [Ni(R) L ] is thought to be polarized as
Reduction of this light-green product with Mg smoothly gave a
deep-green product, and this color change supported the forma-
tion of low-valent Ni–bpy complex(es) such as [Ni(bpy)(solvent)]
2 m
δ+
δꢀ
Ni ―C due to the difference in electronegativity between Ni
and C. On the other hand, the reductive elimination releases
the neutral molecule, R―R. Consequently, the reductive elimina-
[28,29,38]
(see equation (2)):
[
44]
tion is thought to involve electron migration from C to Ni:
NiCl2 þ bpy þ Mg→½NiðbpyÞðDMFÞꢁ
(2)
This Ni(0) complex is thought to react with organic halide (RX)
to form [NiX(R)(bpy)] by oxidative addition of RX (equation (3)).
Disproportionation of [NiX(R)(bpy)] seems to occur to give [Ni(R)
This electron migration is thought to be difficult when the aryl
group is a strongly electron-withdrawing group, and the [Ni(aryl)2
2(bpy)] (equation (4)), and reductive elimination on [Ni(R) (bpy)]
2
(
bpy)] can be isolated in this case (e.g. isolated from [Ni(cod) ]/
2
gives the C―C coupling product, R―R (equation (5)):
[
24,40]
bpy/ArX systems
).
Attempts to isolate cis-[Ni(aryl) L ] (e.g. from the [Ni(cod) ]/bpy/
ArX system) has not been successful when the aryl group is not a
strongly electron group. In these cases the coupling products
½
NiðbpyÞðDMFÞꢁ þ RX→½NiXðRÞðbpyÞꢁ
(3)
(4)
(5)
2 m
2
Â
Ã
2
½NiXðRÞðbpyÞꢁ→ NiðRÞ ðbpyÞ þ ½NiX2ðbpyÞꢁ
2
2 m
Ar―Ar were obtained, instead of the isolation of cis-[Ni(aryl) L ].
However, when the aryl group has two substituents at the o-
positions, as in the case of a mesityl group, the cis-[Ni(aryl) L ]
Â
Ã
NiðRÞ ðbpyÞ →R ꢀ R þ ½NðbpyÞðsolventÞꢁ
2
2 m
[
24,34,45,46]
complex is isolable.
at the o-positions seems to hinder the approach of C
cis-[Ni(aryl) ] complex (see Fig. 2). This difficulty for the reductive
The presence of the two substituents
Total C―C coupling reaction of RX by [Ni(0)L ] is expressed as
m
1
and C in the
2
follows:
2 m
L
½
Nið0ÞLmꢁ þ 2 RX→½NiðIIÞX2Lmꢁ þ R―R
(6)
elimination from the cis-[Ni(o-disubstituted Ar)
2
L
m
] might be
Table 2. Results of C―C coupling of other organic bromides in DMF
a
b
c
No.
RX
Ni/RX
Bpy /Ni
Mg/Ni
Temp. (°C)
Time (h)
Yield (%)
28
II-1
II-2
II-3
CH
3
COC
6
H
4
Br-p
1.3
1.3
1.3
2
2
2.3
70
70
70
45
r.t.
40
54
50
7
13
13
36
g
PhCH
PhCH
2
Br
CH
2.3
80
2
2
Br
Br-p
2
2.3
21
93
78
0
d
II-4
II-5
II-6
II-7
II-8
CH
NCC
NC
2,6-Me
Aryl halides with two o-substituents
3
COC
Cl-p
Br-p
Br
6
H
4
Ni(cod)
2
, 0.6
0
0
e
d
d
f
6
H
4
Ni(cod)(bpy)
(1)
0
0
O
2
6
H
4
Ni(cod)
Ni(cod)
2
, 0.6
, 0.6
0
0
17
2
C
6
H
3
2
0
9.5
24
0
NiCl
2
(PPh
3
)
2
, 1.0
(PPh
3
, 2)
Zn, 1.0
0
a
2
,6-Me
2
C
6
H
3
Br, 2-bromo-m-xylene.
, triphenylphosphine.
Bpy is used unless otherwise noted.
b
PPh
3
c
d
Data from reference [1]. Reaction conditions are from a typical example in this report.
e
Data from reference [24].
f
Data from reference [4]. Reaction conditions are from a typical example in this report.
g
GC yield. Isolation yield was lower (~40%) because of partial loss of the product during drying of the product under vacuum.
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

T. Yamamoto
Figure 2. Postulated π-electron interaction between the two aryl groups.
X-ray crystallographic data of isolated [Ni(C (bpy)] indicates an essen-
6 5 2
F )
tially perpendicular arrangement of the two aryl groups, which is thought
to bring about the π-electron interaction shown above.
related to reported unsuccessful C―C coupling of o-disubstituted
aryl halides by Ni(0) complexes (see II-7 and -8 in Table 2).
When an excess amount of Mg was added (No. I-4 in Table 1;
Figure 3. Application of C―C coupling to the synthesis of π-conjugated
polymers. Monomer-1 = 2,7-dibromo-9.9-dioctylfluorene; Monomer-2 =
2
2
,7-dibromo-9,10-dioctyl-9,10-dihydrophenanthrene; Monomer-3
,5-dibromopyridine (see Experimental section).
=
Mg/NiCl = 10), the deep-green product, which was usually formed
2
in the reduction of the NiCl –bpy mixture, further changed to deep
2
reddish-brown in about 10–20 min at 70°C. Because the reduction
of transition metal–bpy complexes often gives anionic species such
can be applicable to the preparation of π-conjugated –(arylene)
n
–.
ꢀ
[47]
as [Ru(bpy) ] , the formation of anionic Ni–bpy complexes such
The present findings might contribute to a better understanding
of organometallic homocoupling of aryl halides and to development
of new C―C coupling reactions.
3
as Mg[Ni(bpy) ] is suggested. The C―C coupling of PhBr also
2
proceeded smoothly in the presence of excess Mg (No. I-4), and
occurrence of reactions such as Mg[Ni(bpy) ] + 4 ArX → 2 Ar-Ar +
2
[
NiX (bpy)] + MgBr is suggested.
Acknowledgments
2
2
The author thanks Professor H. Fukumoto of Ibaraki University
and Professor T. Koizumi, Professor Y. Taniguchi, Professor T. Kojima,
Dr A. Maeda, and Mrs S. Shiozaki of Tokyo Institute of Technology
for helpful discussion and experimental support.
Application to Synthesis of π-Conjugated Polymers
Usability of the NiCl –bpy–Mg mixture in DMF for the synthesis of
2
π-conjugated polymers has been examined. As shown in Fig. 3,
three kinds of Br–arylene–Br monomers were used for the prepa-
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Appl. Organometal. Chem. (2014)
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