Promotion of reductive elimination reaction of diorgano(2,2′-bipyridyl)nickel(II) complexes by electron-accepting aromatic compounds, Lewis acids, and Bronsted acids

Article abstract of DOI:10.1246/bcsj.75.1997

Reductive elimination of R-R from dialkyl(2,2′-bipyridyl)nickel(II), [NiR₂(bpy)] 1 (R = CH₃ (1a), C₂H₅ (1b), n- C₃H₇ (1c)), caused by π-coordination of electron-accepting aromatic compounds and reductive elimination of Ar-Ar from [NiAr₂(bpy)] 2 (Ar = C₆F₅ (2a) and pyrazolyls (2b and 2c)) promoted by electron-accepting aromatic compounds, Lewis acids, and Bronsted acids have been investigated. ¹H-NMR and kinetic data indicate that π-coordination of the electron-accepting aromatic compound to [NiR₂(bpy)] leads to the reductive elimination of R-R. The rate of the reductive elimination obeys the second-order rate law, -d[1]/dt = k[1][electron-accepting aromatic compound]. Plots of log k vs Σσ_p of the electron-accepting aromatic compound give a line with a slope of 1.8. Bronsted acids cause reductive elimination of Ar-Ar from 2 selectively under several reaction conditions (e.g., 2a with CF₃COOH in air and 2b with HBr). The reductive elimination reaction of 2a caused by CF₃COOH obeys the second-order rate law, -d[2a]/dt = k′[2a][CF₃COOH], in air. The reaction of 2b with H₂SO₄ requires O₂, giving the rate equation, -d[2b]/dt = k″[2b]²[O₂]; k″ increases with [H₂SO₄], reaching a maximum value at a high [H₂SO₄]. UV-vis spectroscopy reveals the presence of the following equilibrium: 2b + H₂SO₄ ? 2b·H₂SO₄, and the equilibrium constant K_a is evaluated as K_a = [2b·H₂SO₄]/ ([2b][H₂SO₄]) = 47 M^-1 at 300.5 K. UV-vis data give information about the electronic states of 2 and the 2b-Bronsted acid adduct. Poly(6-hexylpyridine-2,5-diyl) with a higher molecular weight has been prepared according to the basic information.

Full text of DOI:10.1246/bcsj.75.1997

c. Jpn.
© 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 1997–2009 (2002) 1997
e Chemical
ciety of Ja-
n
e Chemical
ciety of Ja-
n
Promotion of Reductive Elimination Reaction of
Diorgano(2,2ꢀ-bipyridyl)nickel(Ⅱ) Complexes by Electron-Accepting
Aromatic Compounds, Lewis Acids, and Brønsted Acids.
Reductive Elimination Reaction of NiY2(bpy)
T. Yamamoto et al.
02
97
Takakazu Yamamoto,* Mahmut Abla, and Yasuharu Murakami
09
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503
(Received March 13, 2002)
SJA8
09-2673
063
Reductive elimination of R-R from dialkyl(2,2ꢀ-bipyridyl)nickel(Ⅱ), [NiR2(bpy)] 1 (R = CH3 (1a), C2H5 (1b), n-
C3H7 (1c)), caused by π-coordination of electron-accepting aromatic compounds and reductive elimination of Ar-Ar
from [NiAr2(bpy)] 2 (Ar = C6F5 (2a) and pyrazolyls (2b and 2c)) promoted by electron-accepting aromatic compounds,
Lewis acids, and Brønsted acids have been investigated. 1H-NMR and kinetic data indicate that π-coordination of the
electron-accepting aromatic compound to [NiR2(bpy)] leads to the reductive elimination of R-R. The rate of the reduc-
tive elimination obeys the second-order rate law, −d[1]/dt = k[1][electron-accepting aromatic compound]. Plots of log k
vs Σσp of the electron-accepting aromatic compound give a line with a slope of 1.8. Brønsted acids cause reductive elim-
ination of Ar-Ar from 2 selectively under several reaction conditions (e.g., 2a with CF3COOH in air and 2b with HBr).
The reductive elimination reaction of 2a caused by CF3COOH obeys the second-order rate law, −d[2a]/dt =
kꢀ[2a][CF3COOH], in air. The reaction of 2b with H2SO4 requires O2, giving the rate equation, −d[2b]/dt = kꢁ[2b]2[O2];
kꢁ increases with [H2SO4], reaching a maximum value at a high [H2SO4]. UV-vis spectroscopy reveals the presence of
the following equilibrium: 2b + H2SO4 ꢀ 2b•H2SO4, and the equilibrium constant Ka is evaluated as Ka = [2b•H2SO4]/
([2b][H2SO4]) = 47 M−1 at 300.5 K. UV-vis data give information about the electronic states of 2 and the 2b-Brønsted
acid adduct. Poly(6-hexylpyridine-2,5-diyl) with a higher molecular weight has been prepared according to the basic in-
formation.
02
02
.1
Reductive elimination is one of the most fundamental reac-
tions in organometallic chemistry.1–4 The reductive elimina-
tion of Y-Yꢀ from [Ni(Y)(Yꢀ)Ln] and [Pd(Y)(Yꢀ)Ln] (Y, Yꢀ =
organic group) is considered to be a key step in Ni- and Pd-
complex promoted organic synthesis1,5,6 and polymer synthe-
sis.7,8
Factors that control the reductive elimination from diorga-
nometallic complexes, [M(Y)(Yꢀ)Ln] (M = Ni, Pd; L = neutral
ligand such as 2,2ꢀ-bipyridyl and tertiary phosphines) have
long been investigated.1–3
age of metal-organic (M-Y) bonds to liberate Y-H com-
pounds;1,3c,9f,10 however, the present investigation reveals that it
can cause selective reductive elimination when the organic
group is the electron-accepting aryl group.
H+or HX
(2)
[NiAr2(bpy)] → Ar−Ar
Ar = aryl
Analysis of these reductive elimination reactions is expected to
improve our understanding of the nickel-complex-promoted
synthetic reactions. A part of these results have been published
in a communication form.9c–e
[M(Y)(Yꢀ)Ln] → Y-Yꢀ
(1)
Results and Discussion
However, there still remain unclarified factors concerning the
reductive elimination.
Reductive Elimination and UV-Vis Data of Diorganon-
ickel(Ⅱ) Complex. Tables 1 and 2 summarize the results of
the reductive elimination of [Ni(alkyl)2(bpy)], [NiR2(bpy)] 1,
and [Ni(aryl)2(bpy)], [NiAr2(bpy)] 2, caused by various accel-
erating reagents. Diorgano(2,2ꢀ-bipyridyl)nickel(Ⅱ) complex-
es, [NiY2(bpy)], with the alkyl and aryl groups shown in Chart
1, are used in this study.
Structure of [NiAr2(bpy)]. The molecular structures of
complexes 2a, 2b, and 2c9c have been determined by X-ray
crystallography; they take a square planar structure around Ni,
as previously reported for 2c. The molecular structure of 2a is
shown in Figure 1 and that of 2b is given in the supporting
data.
A few papers have been published on the reductive elimina-
tion reaction of [Ni(alkyl)2Ln]2a–g and [Ni(alkyl)(aryl)Ln].2h
However, little attention has been paid to the reductive elimi-
nation reaction of [Ni(aryl)2Ln], presumably due to the pres-
ence of significantly fewer examples of isolated [Ni(aryl)2Ln].9
The diaryl nickel complex has been isolated mainly with elec-
tron-withdrawing aryl groups that stabilize the Ni-aryl bond.
In this paper, we report details of (i) the reductive elimina-
tion reaction of [Ni(alkyl)2(bpy)] (bpy = 2,2ꢀ-bipyridyl) accel-
erated by the coordination of electron-accepting aromatic com-
pounds such as m- and p-C6H4(CN)2 and (ii) the reductive
elimination reaction of [Ni(aryl)2(bpy)] induced by Lewis acid
and protic acids. The Brønsted acids usually induce the cleav-
Reductive Elimination from [NiR2(bpy)]. As shown in

1998 Bull. Chem. Soc. Jpn., 75, No. 9 (2002)
Reductive Elimination Reaction of NiY2(bpy)
Table 1. Results of the Reductive Elimination of [NiR2(bpy)] and [NiAr2(bpy)] (R = Alkyl, Ar =
Aryl) Promoted by Electron-Accepting Aromatic Compounds and Lewis Acids
Run R or Ar, complex
Accelerating
reagenta)
Solventb) Temp/°C
Yield of R-R
or Ar-Ar/%c)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Me (1a)
1a
1a
Et (1b)
1b
1b
1b
Pr (1c)
1c
1c
1c
m-C6H4(NO2)2
C6H5NO2
C6F5CF3
m-C6H4(NO2)2
p-C6H4(CN)2
o-C6H4(CN)2
C6H5CN
m-C6H4(NO2)2
o-C6H4(CN)2
C6H5CF3
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
70
95
93
84
87
93
96
81
100
100
97
93
90
89
0f
C6F6
1c
1c
1,3,5-C3H3N3
1,4-C4H4N2
acrylonitrile
THF
C6F5 (2a)
DMSO
or DMF
toluene
DMSO
toluene
toluene
CH3CN
CH3CN
CHCH3
CH3CN
CH3CN
15
16
17
18
19
20
21
22
23
2a
2a
2a
2a
acrylonitrile
maleic anhydrie
TCNE
TCNQ
C6F6
maleic anhydride
TCNE
TCNQ
reflux
100
reflux
105
50
50
50
50
r.t.
0
trace
0
100
3
66
44
94
89
C3N2(CH3)2Cl (2b)
2b
2b
2b
C3N2(CH3)COOMeCl (2c) TCNQ
Accelerating regent = Lweis acid
24
25
2b
2b
BF3•OEt2
RuCl3
CHCl3
CHCl3
30
30
28
82
d)
e)
a) 1,3,5-C3H3N3 = 1,3,5-triazine. 1,4-C4H4N2 = pyrazine. TCNE = tetracyanoethylene.
TCNQ = tetracyanoquinodimethane. An excess amount of the accelerating reagent (about 10
mol/1 mol of the Ni complex) was added. b) DMSO = dimethyl sulfoxide. DMF = N,N-dim-
ethylformamide. c) GLC yield. A trace amount of RH (methane) was detected in run 3. A
part of the gaseous product (C2H6 or butane) may be lost during the reaction, whereas the liq-
uid product (hexane) from 1c should remain in the reaction solution. d) Hydrated
(RuCl3•nH2O, n = ca. 3). e) In a dispersion system.
Chart 1. Organic groups in [NiY2(bpy)] used in this study.
Table 1, the reductive elimination of R-R from [NiR2(bpy)]
ination reaction of 1c. [NiR2(bpy)] is stable in usual aromatic
(1a–1c) is caused by the interaction with electron-accepting ar-
omatic compounds such as C6H5CN and C6F6, similar to that
caused by electron-accepting olefins.1,3a,b 1,3,5-triazine and
pyrazine (runs 12 and 13 in Table 1) are also regarded as π-
deficient aromatic compounds due to the presence of electron-
withdrawing imine nitrogens,11 and interaction with these het-
erocyclic aromatic compounds also induces the reductive elim-
solvents such as benzene and toluene.
Stability of and Preparative Conditions for [NiAr2(bpy)].
C6F5- is a typical electron-withdrawing group and the pyra-
zolyl groups in 2b and 2c are also regarded as electron-with-
drawing groups because they contain the imine nitrogen11 and
Cl. Complexes 2a–2c are prepared by the following reaction
route (Scheme 1) that involves the oxidative addition12 of aryl

T. Yamamoto et al.
Bull. Chem. Soc. Jpn., 75, No. 9 (2002) 1999
Table 2. Results of the Brønsted Acid-Promoted Reductive Elimination of Ar-Ar from [NiAr2(bpy)]
Run
Complex
HXa)
Atmosphere
Temp/°C
Yield/%b)
Ar-Ar
ArH
0
50
0
0
0
1
2
3
2a
2a
2a
2a
2a
CF3COOH(1.1M)
CF3COOH(neat)
CF3COOH(neat)
HCl(1.0 M)
air
N2
air
air
N2
r.t.
r.t.
r.t.
100
r.t.
100
0
95
100
92
4
5c)
HBr(1.3 M)
6
7
8
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
HCl(0.3 M)
HCl(0.3 M)
HCl(0.3 M)
HBr(0.3 M)
HBr(0.3 M)
HNO3(0.3 M)
HNO3(0.3 M)
H2SO4(0.3 M)
H2SO4(0.3 M)
H2SO4(0.3 M)
air
O2
N2
air
N2
air
O2
air
air
O2
r.t.
30
50
30
50
30
30
30
70
r.t.
29
72
0
100
100
83
90
69
69
90
67
23
100
0
0
15
5
31
31
10
9
10
11
12
13
14
15
16
17
2c
2c
HNO3(6.0 M)
CH3COOH
air
air
r.t.
r.t.
97d)
81
0
0
a) In DMSO-d6 except for runs 2, 3, and 16 (aqueous solution) and 17 (1:1 mixture of CH3OH
1
and CHCl3). b) Determined by H NMR or GLC unless otherwise noted. Reaction time =
about 10 h. c) Reaction was almost completed in 30 min. d) Isolated yield. Carried out with
aqueous solution under heterogeneous conditions.
halide ArX to an Ni(0) complex and disproportionation5e,10b of
[Ni(Ar)(X)(bpy)] (cf. Experimental section).
The use of usual ArX (e.g., phenyl halides and tolyl halides)
or moderately electron-withdrawing ArX such as NC(C6H4)Cl-
p in the reaction with Ni(0) complexes did not give the [NiAr2-
(bpy)]-type complex. In these cases, the reaction gave the cou-
pling product Ar-Ar5e,12f or stopped at the oxidative addition to
yield [Ni(Ar)(X)(bpy)].12d,f,13 [NiAr2(bpy)] was isolable only
for highly electron-withdrawing Ar groups as in 2a–2c.
It was reported that cis-[Ni(Me)(Ar)(dpe)]2h (dpe = 1,2-
bis(diphenylphosphino)ethane) underwent reductive elimina-
tion much more easily than cis-[NiMe2(dpe)], and attempts to
prepare cis-[NiAr2(bpy)] with non-electron-withdrawing Ar
groups were unsuccessful. These results suggest a strong incli-
nation of cis-[NiAr2(bpy)] to undergo the reductive elimina-
tion. The ease of the reductive elimination of Ar-Ar from
[NiAr2(bpy)] may be related to the presence of the π-orbital at
the Ni-C carbon. For example, the π-orbitals on C1 and C7 car-
bons of 2a (Fig. 1) seem to overlap each other and such over-
lapping may enhance the reductive elimination of Ar-Ar from
[NiAr2(bpy)].
Fig. 1. Molecular structure of [Ni(C6F5)2(bpy)] 2a. Selected
bond lengths (Å) and angles (deg): Ni–C1 1.903(4), Ni–C7
1.907(4), Ni–N1 1.938(3), Ni–N2 1.936(4), C1–Ni–C7
86.7(2), N1–Ni–N2 82.9(2).
When the Ar group is highly electron-withdrawing, the cis-
[NiAr2(bpy)]-type complexes are isolable, as exemplified by
2a–2c. The Ni–Ar bond is considered to be polarized as Niδ+
-
Arδ− and the reductive elimination of electrically neutral Ar-Ar
will require a partial shift of electrons from the Ar group to
Ni;14 this explains the higher stability of [NiAr2(bpy)] with the
more strongly electron-withdrawing Ar group.14
UV-Vis Data for Electronic State of [NiAr2(bpy)]. Fig-
ure 2 compares the UV-vis spectra of 1b, 2b, and 2c. UV-vis
data of 1a–1c have already been reported previously.2a
Scheme 1. Formation of NiAr2(bpy).

2000 Bull. Chem. Soc. Jpn., 75, No. 9 (2002)
Reductive Elimination Reaction of NiY2(bpy)
Fig. 3. Energy diagram of bpy, Ni complexes, and HCl. π
(HOMO) level of bpy is estimated from its IP.15 σ and σ*
levels of HCl are estimated from its IP16a and UV absorp-
tion peak.16b The 1s energy of H is shown for the electron
affinity of H+.
Fig. 2. UV-vis spectra of 1b (—–), 2b (≥≥), and 2c (- - -)
in THF. For A1, A2, A3, B1, and B2, see Fig. 3. The shoul-
der peak of 2b at about 29000 cm−1 becomes clearer in
DMSO.
the absorption peak, as well as electron affinity of H+,16 are
also illustrated in Fig. 3, the discussion of which will be given
later.
The A2 peak of 2b and the A3 peak of 2c shift depending on
the kind of solvent in which the complex is dissolved. As
shown in Fig. 4, plots of the energy of the absorption peaks A2
and A3 (Fig. 2) vs Kosower’s Z values17 give a linear positive
correlation, supporting the assumption that the absorption
band is assignable to a kind of CT band.
The stable complexes 2a–2c, however, undergo reductive
elimination when they interact with strong π-acids (e.g.,
TCNQ), Lewis acids, and Brønsted acids (data shown in runs
18–25 in Table 1 and data in Table 2).
Low valent Ni-bpy complexes such as 1b usually give rise
to two MLCT (Ni → bpy) bands, e.g., A1 and B1, as shown in
Figs. 2 and 3 with a molar absorption coefficient (ε) of 3–4 ×
103 M−1 cm−1. The A1 and B1 arrows in Fig. 3 represent as-
signments of the two MLCT bands of 1b, which has been con-
firmed from UV-vis data measured under various conditions.2a
2b shows an A2 peak at 21600 cm−1 (462 nm; cf. Fig. 2) in
THF, which is assigned to an Ni → π1* MLCT transition as in-
dicated by arrow A2 in Fig. 3. 2b shows a B2 shoulder peak at
about 29000 cm−1, and this absorption peak seems assignable
to another Ni → π2* MLCT transition (B2 in Fig. 3). The ener-
gy difference (29000−21600 = 7400 cm−1) between the A2
and B2 transitions roughly agrees with that (7300 cm−1) be-
tween π1* and π2* levels of bpy15 depicted in Fig. 3, support-
ing the assignment. 2c gives an absorption peak A3; however,
in this case, the corresponding B3 peak seems to be hidden un-
der a strong π–π* transition band of bpy.
(3)
Based on the assumption that the A and B peaks of com-
plexes 2a–2c can also be assigned to the MLCT transition, the
energy level of the highest energy orbital (occupied d orbital)
at Ni can be evaluated, as shown in Fig. 3. Due to the electron-
withdrawing nature of the Ar group, the energy of the orbital at
Ni of [NiAr2(bpy)] is lowered compared with that of [NiR2-
(bpy)].
In view of the energy level of the d-orbital of Ni, the elec-
tron-withdrawing ability of the Ar group is considered to in-
crease in the order: C3N2(CH3)2Cl (in 2b) < C3N2(CH3)-
(COOCH3)Cl (in 2c) < C6F5 (in 2a). Energy levels of σ and
σ* orbitals of HCl evaluated from the ionization potential and
The characterization of 3a–3c is given in the experimental
part. The molecular structure of 3c as determined by X-ray
crystallography is shown in the supporting data. In contrast to
the reaction of 2a–2c with Brønsted acid, reactions of 1a and
1b with Brønsted acids normally evolved RH, methane and
ethane, respectively. The reductive elimination of 1a–1c
caused by the electron-accepting aromatic compounds and that
of 2a–2c caused by Brønsted acids have been studied kineti-
cally and spectroscopically, and the obtained results are dis-
cussed in the following part.

T. Yamamoto et al.
Bull. Chem. Soc. Jpn., 75, No. 9 (2002) 2001
Fig. 4. Plots of the energy of the A2 peak of 2b (left) and A3 peak of 2c (right) vs Kosower’s Z value.17
Reductive Elimination of 1a-1c Caused by Electron-
Accepting Aromatic Compounds. [NiR2(bpy)] is stable in
solvents commonly used for UV-vis spectroscopy at room tem-
perature under N2. However, as described above, the reaction
of 1a–1c with electron-accepting aromatic compounds causes
reductive elimination with the liberation of R-R and changes in
the UV-vis spectrum.
When the aromatic compound has an Ar-CN or Ar-F bond,
prolonged reaction leads to oxidative addition of the Ar-CN18
or Ar-F bond to [Ni(bpy)]. This nickel(0) complex is consid-
ered to be formed according to the reductive elimination and to
be stabilized by coordination of solvent, however, it undergoes
the reaction with the Ar-CN and Ar-F compounds to yield the
corresponding oxidative addition products (e.g., [Ni(CN)-
(C6H4CN-p)(bpy)]).
Figure 5 shows changes of the UV-vis spectrum during the
reactions of 1b and 1c with C6H4(CF3)2-p, and C6F6, respec-
tively. The reaction proceeds with isosbestic points. Other
combinations of 1a–1c and electron accepting-aromatic com-
pounds give analogous spectroscopic changes.19 Kinetic and
spectroscopic studies of the reaction reveal the following fea-
tures of the reaction.
(i) The plot of log (Abs(t)−Abs(∞)) (Abs(t) = absorbance
at peak A (e.g., A1 for 1b) at time t; Abs(∞) = absorbance at
peak A at infinite time) shown in Figure 5 against time t gives a
straight line, indicating that the reaction follows the first-order
kinetics with respect to concentration of [NiR2(bpy)]. The
slope of the straight line increases linearly with the concentra-
tion of the electron-accepting aromatic compound, affording
the following rate equation.
Fig. 5. Upper graph (Ⅰ): reaction of [NiEt2(bpy)] 1b with
C6H4(CF3)2-p (2.2 M) at 25 °C in THF. Time/102 s: (a) 0;
(b) 350; (c) 760; (d) 1500; (e) 3200; (f) ∞. Lower graph
(Ⅱ): reaction of NiPr2(bpy) 1c with C6F6 (0.091 M) at 34.5
°C in THF. Time/102 s: (a) 0; (b) 12; (c) 24; (d) 36; (e) ∞.
−d[1]/dt = k [1][electron-accepting aromatic compound] (4)
(ii) As shown in Fig. 6, plots of the logarithm of the second-
order rate constant k obtained for 1b vs the sum of the Ham-
mett σp values of the substituents, Σσp, in the electron-accept-
ing aromatic compound reveals a trend that the accelerating ef-
fect on the reductive elimination increases with an increase in
the electron-accepting ability of the aromatic compound. Ex-
cept for C6H5NO2, a roughly linear correlation holds between k
and Σσp with a slope of about 1.8.
C6H5NO2 exceptionally strongly accelerates the reductive
elimination. This may be due to the direct attack of the posi-
tively charged Cδ+-NO2δ− carbon center at the nucleophilic Ni
in 1b rather than the electrophilic attack of the aromatic com-
pound through the benzene ring.

2002 Bull. Chem. Soc. Jpn., 75, No. 9 (2002)
Reductive Elimination Reaction of NiY2(bpy)
The enhancement effect of the electron-accepting olefin and
the aromatic compounds may be associated with the results of
synthetic C–C coupling reactions promoted by Ni, if the reduc-
tive elimination step is a rate determining step. For example,
the coupling reaction between RMgX and RꢀX, RMgX + RꢀX
→ R-Rꢀ, has been successfully carried out when olefinic halide
or aryl halide is used as RꢀX, whereas the coupling reaction
does not proceed well by using alkyl halides.5b In contrast,
there is no restriction on the kind of R group in RMgX, sug-
gesting that enhancement of the reductive elimination of R-Rꢀ
from a supposed intermediate species [Ni(R)(Rꢀ)Ln] by coordi-
nation of the electron-accepting olefinic halide or aryl halide is
essential for the catalytic reaction. For some other metal-pro-
moted synthetic reactions, however, such a difference between
olefinic or aryl halide and aliphatic halide may not be impor-
tant. Facile reductive elimination reaction of Ni-sp2-C com-
pound discussed above is also considered. Ni-catalyzed cou-
pling reactions are promoted in the presence of dienes,7f and it
has recently been reported that Ni-catalyzed cross coupling of
Grignard reagent proceeds with alkyl halides in the presence of
dienes5f which will cause back-donation from nickel. Effects
of electron-accepting olefin to promote Ni-catalyzed coupling
reactions were also reported.2e–g The reductive elimination of
Et-Et from 1b in the presence of phenyl chloride was previous-
ly reported.2c
Fig. 6. Plot of log k obtained with 1b at 25 °C vs Σδp of the
electron-accepting aromatic compound. In THF.
(iii) As shown in Fig. 6, the k values observed for 1b at 25
°C are in a range of about 10−5–10−2 M−1 s−1. These k values
are considerably smaller than those (k = 10−4–10−1 M−1 s−1
)
observed for the similar reductive elimination of 1b caused by
electron-accepting olefins.2a The Eyring plot of the k value
gives a straight line. The kinetic parameters (∆G‡, ∆H‡, ∆S‡)
estimated from the slope and intercept of the line are listed in
the supporting data.
(iv) The k value increases in the order of 1a < 1b < 1c.
Plots of log k values obtained in reactions with C6F5CF3 vs
Taft’s σ* value20 of the R group in 1a–1c give a straight line
with a slope of −0.72. For the acceleration of the reductive
elimination of 1a–1c caused by an eletron-accepting olefin
(methyl acrylate), similar plots gave a straight line with a slope
of −2.5.2a
Reductive Elimination Promoted by Brønsted Acid.
Organotransition metal complexes [MYLn] usually undergo
protonolysis in the reaction with Brønsted acid to produce
YH.1,10
[MYLn] + H+ → YH
(6)
However, for the isolated [NiAr2(bpy)] with the electron-
withdrawing Ar group used in this study, the reaction of
[NiAr2(bpy)] with Brønsted acids HX leads to reductive elim-
ination to produce Ar-Ar.
The reductive elimination caused by the aromatic compound
seems to take place through the π-coordination of the aromatic
compound with Ni of [NiR2(bpy)].
HX
2a − 2c → 3a – 3c
(3ꢀ)
2a–2c were stable at 150 °C even in air, and thermolyses of 2b
and 2c started at 178 °C and 306 °C under N2, respectively.
2a, 2b, and 2c were stable in DMF and DMSO solution at 60
°C for 5 h. However, such stable nickel complexes undergo re-
ductive elimination in the presence of a Brønsted acid. For ex-
ample, the reactions of 2b and 2c with hydrochloric acid (6 M)
and nitric acid (6 M) in air gave 3b and 3c in 55 and 97% iso-
lated yields, respectively.
Table 2 summarizes the results of the reactions of [NiAr2-
(bpy)] with HX. As shown in Table 2, the ratio of the two
products, Ar-Ar and ArH, depends on the kind of HX and the
atmosphere to which the reaction is exposed. The data given
in Table 2 and other experimental results reveal the following
features of the reductive elimination promoted by the Brønsted
acid.
(5)
Variable temperature 1H-NMR spectroscopy indicates that the
position of the CH3 signal shifts slightly as the temperature is
lowered.9d In contrast, the peak position of the CH2 signal
shifts in the direction of the upper magnetic field.9d This shift
of the CH2 peak is reasonably accounted for by an anisotropic
up-field magnetic effect21 of the coordinated aromatic ring in 4
and a shift of the equilibrium (K in Eq. 5) in the direction of 4
at low temperature. On the other hand, the CH3 group will re-
ceive the magnetic effect to a lesser extent. π-Arene complex-
es of Ni have been reported.22,23
(i) Addition of CF3COOH to a solution of 2a in air leads to
quantitative reductive elimination (run 1 in Table 2). Figure 7
shows changes in the 19F-NMR spectrum on addition of
CF3COOH to a DMSO-d6 solution of 2a. The 19F-NMR data
reveal that C6F5-C6F5 is the sole product in the reaction of 2a
All these results are consistent with the enhancement of the
reductive elimination by coordination of the electron-accepting
aromatic compound.

T. Yamamoto et al.
Bull. Chem. Soc. Jpn., 75, No. 9 (2002) 2003
air or O2, both protonolysis and reductive elimination are pro-
moted, and in pure O2 the reductive elimination becomes pre-
dominant (run 7).
Brønsted acids may promote the reductive elimination by
interacting with the Ni center or the nitrogen in the bpy ligand.
However, the latter case seems unlikely since (a) the accelera-
tion effect of acid varies depending on the kind of acid as de-
scribed later, (b) the effect of O2 is difficult to explain on the
basis of the interaction with the nitrogen of bpy, and (c)
[NiCl2(bpy)] is isolated in about 50% yield after the reductive
elimination in the presence of HCl.
Acceleration of the reductive elimination by interaction of
the Brønsted acid with nitrogen in the pyrazolyl ligand of 2b is
less likely. There are two reasons: (a) the reductive elimination
products, bipyrazoles 3b and 3c (cf. Eq. 3), are not protonated
with diluted acids presumably due to the weak basicity of 3b
and 3c; and (b) 2a with the C6F5 ligand also undergoes the re-
ductive elimination promoted by the Brønsted acid.
(ii) HBr promotes the reductive elimination reaction of 2a
and 2b both in the presence and in the absence of O2 (runs 5, 9,
and 10). The specific effect of HBr may be related to its
unique redox behavior.24 For the HBr-promoted reductive
elimination, the evolution of H2 was confirmed by gas chroma-
tography. In the case of the HCl-promoted reductive elimina-
tion in air, H2 was not detected and the consumption of O2 was
observed instead.
In order to obtain further information regarding the reduc-
tive elimination, the following spectroscopic and kinetic stud-
ies on the reaction of Brønsted acids with 2a and 2b have been
carried out. Although further research seems to be necessitat-
ed to clarify detailed reaction mechanism, the following data
reveal several features of the reductive elimination.
Spectroscopic Data and Equilibrium. Since the reduc-
tive elimination caused by the Brønsted acid does not proceed
rapidly, the interaction of the Brønsted acid with [NiAr2(bpy)]
can be investigated by UV-vis spectroscopy.
Fig. 7. Changes in the 19F-NMR spectrum of 2a on addition
of CF3COOH (2.3 M) to a DMSO-d6 solution of 2a in air.
(a): before addition of CF3COOH; (b) 5 h after addition of
CF3COOH (2.3 M) at 40 °C; (c) after completion of the re-
action. The chemical shift δ is referred to CF3COOH. The
signals marked by an asterisk (*) are due to 2a, whereas
those marked by an open circle (ꢀ) are due to C6F5-C6F5.
The strong and broad peak centered at about δ = −50 ppm
is due to Teflon in the NMR instrument.
Figure 8 shows changes in the UV-vis spectrum of 2b on ad-
dition of H2SO4. As indicated in Fig. 8, the spectrum changes
with clear isosbestic points at 315, 335, 345, and 418 nm, sug-
gesting that there exists the following simple equilibrium:
Ka
with CF3COOH. In the case of the reaction of 2a with
CF3COOH, in the absence of O2 or air, the hydrolysis of 2a by
CF3COOH to give C6HF5 (run 2 in Table 2) proceeds very
slowly. Type and velocity of reactions of 2a with CF3COOH
and air:
(7)
2b + HX
2b•HX
5
If such an equilibrium is assumed, the equilibrium constant
Ka is given by the following Eq. 8:
CF3COOH + air
CF3COOH
air
Ka = [5]/{[2b][HX]} = (x−y)/{(y−y )[HX]},
(8)
∞
(→ C6F5-C6F5) >>
(→ C6HF5) >>
at moderate velocity
very low velocity
stable
where x is the absorbance of the initial solution containing 2b
at 450 nm, and y represents the absorbance after the addition of
the Brønsted acid, as shown in Fig. 8; y is assumed to become
HCOOH also promotes the reductive elimination in air, as
proved by 19F-NMR spectroscopy but at a considerably lower
reaction rate than CF3COOH. The importance of O2 in the re-
ductive elimination is also observed in the reaction of 2a (run
4) and 2b with HCl (runs 6–8). 2b shows considerable stabili-
ty in an aqueous solution of 0.3 M HCl under N2 at room tem-
perature. On raising the temperature to 50 °C under N2, proto-
nolysis of 2b takes place (run 8). However, in the presence of
y
when the completely shifted to the right. From Eq. 8, Eq. 9
∞
is derived
(x−y)/{y[HX]} = Ka−(y Ka)/y,
(9)
∞
and Fig. 9 exhibits plots of the (x-y)/{y[HX]} value vs 1/y for
the data shown in Fig. 8.

2004 Bull. Chem. Soc. Jpn., 75, No. 9 (2002)
Reductive Elimination Reaction of NiY2(bpy)
mol−1, respectively. Tolman reported25 a similar equilibrium
for the formation of an adduct between [Ni[P(OEt3)4]] and
H2SO4 in CH3OH and formulated the equilibrium as
K1
−
[NiL4 ] + H+
[HNiL4]+HSO4
(10)
For the proposed equilibrium, Tolman obtained a K1 value of
33 M−1, which is comparable to the Ka values summarized in
Table 3.
The equilibrium for the present system may also be formu-
lated (instead of Eq. 7) as:
Ka
(11)
2b + H+
[2b•H]+
However, we favor Eq. 7, since X in HX affects the conditions
of the reductive elimination, as will be described later, and the
dissociation constant of HX in organic solvents is small.26
According to the adduct formation with Brønsted acid (or
H+), the A2 absorption peak of 2b (cf. Figs. 2, 3, 4, and 8) at
440 nm in DMSO shifts by about 25 nm (or by 0.17 eV) to the
shorter wavelength region, as shown in Fig. 8. This shift is
reasonably explained by the lowering of the d orbital energy
due to the coordination of 2b with the Brønsted acid (or H+),
which will cause partial electron migration from Ni to the σ*
orbital of HX (cf. Fig. 3). Other Brønsted acids also lead to
similar shifts of the A2 peak of 2b to the shorter wavelength re-
gion. However, the addition of H2SO4 to a DMSO solution of
2a did not cause such an obvious shift of the UV-vis peak, re-
vealing that 2a had only a weak interaction with H2SO4 due to
the lower d orbital of 2a (Fig. 3), which would bring about less
extensive electron migration from Ni to the σ* orbital of the
acid (or to H+).
Fig. 8. Changes of the UV-vis spectrum of 2b on addition of
H2SO4 at 300.5 K under air. In DMSO. [2b] = 1.4 × 10−3
M. [H2SO4]/10−3 M: (a) 0, (b) 3.13, (c) 6.25, (d) 9.38, (e)
12.5, (f) 25.0, and (g) 37.5.
Kinetics of the Reductive Elimination. Some reactions
with Brønsted acids have been analyzed kinetically.
2a–CF3COOH. The UV-vis spectrum of 2a in tetrahydro-
furan (THF) in air did not show any observable change with
time. However, when CF3COOH was added to the THF solu-
tion, the UV-vis spectrum showed a decrease of 2a with time,
in accordance with the 19F NMR data shown in Fig. 7. The de-
cay curve of 2a obeys the pseudo first-order rate law, and the
pseudo-first-order rate constant increases linearly with [CF3-
COOH], resulting in the following rate equation:
Fig. 9. Plots of the (x−y)/{y[H2SO4]} value vs the 1/y value
for the UV-vis spectra shown in Fig. 9. In DMSO.
From the intercept and the slope of the straight line in Fig. 9,
Ka and y are evaluated as 47 M−1 and 0.12, respectively. Ta-
∞
ble 3 summarizes Ka values obtained with the Brønsted acids.
The van’t Hoff plot of the Ka value obtained with H2SO4 vs 1/T
gives ∆H° and ∆S° values of −10.1 kJ mol−1 and −2.4 J K−1
d[2a]/dt = kꢀ[2a][CF3COOH]
(12)
The kꢀ value at 40 °C is 3.2 × 10−5 M−1 s−1
.
2b-H2SO4. Figure 10 exhibits changes in the UV-vis spec-
trum during the reaction of 2b with H2SO4 in DMSO under air.
Kinetic results obtained from the UV-vis spectrum indicate
that the reaction obeys the second-order rate law with respect
to [2b]. The pseudo second-order rate constant increases lin-
early with O2 pressure in the atmosphere with which [O2] in
the solution is equilibrated:
Table 3. Ka Values for the Formation of the Adduct between
2b and Protic Acids
No.
Protic acids
Temp/K
Ka/M−1
1
2
3
4
5
6
7
8
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
HCl
N2
N2
N2
N2
Air
N2
N2
Air
300.0
306.0
308.0
316.5
300.5
313.0
301.2
300.5
43.7
39.5
37.3
35.4
46.5
38.4
21.6
41.0
−d[2b]/dt = kꢁ[2b]2[O2]
(13)
The third-order rate constant kꢁ is evaluated as 2.2 M−2 s−1 at
306 K by using the Henry coefficient of 12 mM bar−1 for usual
organic solvents.27
HBr
HNO3

T. Yamamoto et al.
Bull. Chem. Soc. Jpn., 75, No. 9 (2002) 2005
Fig. 10. Changes in the UV-vis spectrum during the reaction
of 2b (4.7 × 10−5 M) with H2SO4 (1.2 × 10−2 M) in
DMSO at 300 K under air. Reaction time: (1) 0 h; (2) 2 h;
(3) 4 h; (4) 6 h; (5) 8 h; (6) 10 h; (7) 12 h; (8) ∞ . Curve
(0) represents the UV-vis spectrum of 2b before addition
of H2SO4 (cf. Fig. 8).
Fig. 11. Dependence of the initial concentration of H2SO4 on
the pseudo-second-order rate constant kꢁ for the reaction of
complex 2b with H2SO4 under O2 at (a) 33.0 °C and (b)
43.0 °C. In DMSO.
Under O2, the kꢁ value varies with [H2SO4], as depicted in
Fig. 11. The saturating trend at high [H2SO4] (or Michaelis–
Menten type profile) may be a reflection of the adduct creating
equilibrium between 2b and H2SO4 as discussed above (Eqs.
7–11).
These kinetic results indicate that the activation of Ni–C
bonds in 2b takes place by coordination of both H2SO4 and O2
and suggest the formation of a 2:2:1 adduct of 2b, H2SO4, and
O2 such as that shown in Scheme 2. Water is considered to be
formed in the reaction.
Although the details are not yet clear, the reductive elimina-
tion of 3b from 2b caused by the interaction with HBr is con-
sidered to proceed via electron transfer assisted by several HBr
molecules.
2b-HCl under N2. As described above, this reaction gives
the hydrolysis product of 2b, 3-chloro-1,4-dimethylpyrazole
(Eq. 15). Kinetic study on this reaction by UV-vis spectrosco-
py reveals the following features of the reaction .
Such µ-peroxo complexes of transition metals are known.28
2b-HBr. As described above, this reaction proceeds even
under N2 to liberate 3b. Following the reaction by UV-visible
spectroscopy under N2 gave the following kinetic results.
(i) Plots of [2b]1/2 against time t give a straight line, reveal-
ing that the reaction obeys the following rate equation:
(15)
(i) The reaction obeys the pseudo first-order rate law with
respect to [2b], and the pseudo first-order rate constant linearly
increases with [HCl] to give the second-order rate equation:
−d[2b]/dt = kobs [2b]1/2
(14)
R = kꢀꢀꢀ [2b][HCl]
(16)
(ii) Plots of ln kobs vs ln [HBr] gives a straight line with a
slope of about 4, as depicted in Fig. 12.
where the kꢀꢀꢀ value at 313.0 K is 4.3 × 10−3 M−1 s−1
.
Scheme 2.

2006 Bull. Chem. Soc. Jpn., 75, No. 9 (2002)
Reductive Elimination Reaction of NiY2(bpy)
sults described above, treatment of the reaction mixture with
H2SO4 and CF3COOH in air will give a polymer with a higher
molecular weight; the Complex a is considered to give (Ar)a+b
(Ar)a, and (Ar)b in the work-up with HX, and treatment with
H2SO4 and CF3COOH will increase the proportion of (Ar)a+b
,
.
We have divided the reaction mixture obtained from
[Ni(cod)2], bpy, and 2,5-dibromo-6-hexylpyridine into two
portions, treated the two portions with HCl under N2 and
H2SO4 in air, respectively, and compared the molecular weights
of the polymers. The polymerization was carried out with 1.5
mol of [Ni(cod)2] per 1 mol of 2,5-dibromo-6-hexylpyridine;
the yield of the polymer was 80% (Scheme 3).
GPC analysis reveals that the number average molecular
weight, Mn, and weight average molecular weight, Mw, in-
crease in the order of polymer-1 (Mn = 2100; Mw = 3600) <
polymer-2 (Mn = 6400; Mw = 31000), as expected from the
above described results. Gel permeation chromatograms of
the polymers are exhibited in Fig. 13. The use of HBr, HCl,
CF3COOH in a similar polymerization of 2,5-dibromopyri-
dine, however, did not cause a significant difference in the
molecular weight of the obtained polymer, indicating that such
effect of the acid depends on the polymerization system.
Fig. 12. Dependence of kobs (in M1/2 s−1) on the initial con-
centration of KBr (in M) for the reaction of 2b with HBr
under N2. In DMSO.
(ii) Temperature dependence of the kꢀꢀꢀ value gives the fol-
lowing kinetic parameters at 313 K: Ea = 105 kJ mol−1; ∆H‡
= 103 kJ mol−1; ∆S‡ = 37 J mol−1 K−1; ∆G = 91 kJ mol−1
.
Conclusion
(iii) Use of DCl affords an isotope effect, kꢀꢀꢀ(HCl)/kꢀꢀꢀ(DCl),
of 2.8, which is within the a range of values for the primary
isotope effect, suggesting that direct interaction of H of HCl
(or H+) with the Ni center takes place in the reaction.
The reductive elimination of R-R from [NiR2(bpy)] is pro-
moted by coordination with electron-accepting aromatic com-
pounds, whereas the reductive elimination of Ar-Ar from
[NiAr2(bpy)] is caused by interaction with Brønsted acids. For
the latter reductive elimination, an assisting effect of O2 has
been revealed. These results support the view that interaction
with electron-accepting molecules, in general, activates the
Ni–C bonds in diorgano(2,2ꢀ-bipyridyl)nickel(Ⅱ) complexes.
The obtained information contributes to a better understanding
of Ni-promoted synthetic reactions.
Effects on Polymer Synthesis. Various electron-accept-
ing π-conjugated polymers have been prepared according to
the Ni(0)-complex (typically a mixture of [Ni(cod)2] and bpy)
promoted dehalogenation polycondensation7f of dihaloaromat-
ic compounds; such polymerization is considered to proceed
via the reductive elimination from an [Ni(aryl)2(bpy)]-type
propagating species (Eq. 17).29 Polypyridine29 and polyanthra-
quinone30 are typical electron-accepting π-conjugated poly-
mers and behave as n-type conducting materials when one
constructs electronic devices.31 The (Ar)a, (Ar)b and (Ar)c
groups in complexes a and b may be combined with other Ni
species, since the obtained polymers29,30 did not contain halo-
gen and were H-terminated.
Experimental
Materials. Electron-withdrawing aromatic compounds,
Lewis acids, Brønsted acids, C6F5-C6F5, and C6HF5 were used as
purchased. [Ni(cod)2],32 1a–1c,2a and 2b–2c9c were prepared as
previously reported. 2a was prepared from [Ni(cod)2] (2.4 g, 8.7
mmol), cod (0.6 mL), bpy (1.4 g, 8.7 mmol), and C6F6 (4.3 g, 23
mmol) in THF (50 mL). After 24 h at room temperature, hexane
was added to the reaction mixture to obtain a precipitate. Work-
up, including extraction of [Ni(C6F5)2(bpy)] with toluene, gave 2.2
1
g (4.0 mmol) of [Ni(C6F5)2(bpy)]. H NMR(DMSO-d6): δ 8.54 (d,
2H), 8.25 (t, 2H), 7.54 (t, 2H), 7.47 (d, 2H). 19F NMR (DMSO-d6)
ppm from external CF3COOH: −38.56 (d, J = 28Hz), 82.83 (t, J
= 21 Hz), 85.47 (t, J = 20 Hz or dd). The IR spectrum agreed
with that of 2a prepared via a different route.9d The crystal for X-
ray crystallography was obtained by recrystallization from ace-
tone at −20 °C and was formulated as [Ni(C6F5)2(bpy)(acetone)].
In Fig. 1, acetone is omitted for simplification.
(17)
3b was prepared by treating a chloroform (20 mL) solution of
2b (30 mg) with an aqueous solution of HNO3 (6 M, 10 mL). Pu-
rification by column chromatography (SiO2, CH3COOC2H5) gave
a white powder of 3b in 57% yield. Mp 87–88 °C. 1H NMR
(DMSO-d6) δ 1.83 (s, 6H, CH3), 3.59 (s, 6H, CH3). HRMS:
Found: m/z 258.0367; Calcd for C10H12Cl2N4: M = 258.0439. 3c
1
After the polymerization, the reaction mixture is usually
treated with aqueous HCl. However, in view of the present re-
was prepared analogously in 97% yield. H NMR (CDCl3) δ 3.65
(s, 6H, CH3), 3.72 (s, 6H, CH3). HRMS: Found 346.0175; Calcd

T. Yamamoto et al.
Bull. Chem. Soc. Jpn., 75, No. 9 (2002) 2007
Scheme 3.
X-ray Crystal Structure Analysis of 2a.
2a: C25H14N2Ni-
F10O, Mr = 607.08, 0.4 × 0.5 × 0.5 mm, Monoclinic, P21/c (No.
1), a = 7.420(10) Å, b = 23.79(3) Å, c = 13.64 (1) Å, β = 93.67
(10), V = 2403(5) Å3, R = 0.047, Rw = 0.047, Z = 4, Dcalcd
=
1.678 Mg m−3, F(000) = 1216.00, µ = 9.08 cm−1, 2θmax = 55°,
No. observations (I > 3.00σ (I)) = 3595, No. of variable 352.
Calculations were carried out by using the program package teX-
san on a VAX-Ⅱ computer. Atomic scattering factors were taken
from the literature.33 A full-matrix least-squares refinement was
used for non-hydrogen atoms with anisotropic thermal parame-
ters. Hydrogen atoms were located by assuming ideal positions
and were included in the structure calculation without further re-
finement of the parameters. Crystallographic data have been de-
posited at the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
and copies can be obtained on request, free of charge, by quoting
the publication citation and the deposition numbers CCDC-
186401. The data are also deposited as Document No. 75036 at
the Office of the Editor of Bull. Chem. Soc. Jpn.
We are grateful to Dr. C.-H. Lee and Dr. B.-L. Lee of our
laboratory and Dr. M. Takeuchi of Showa Denko K. K. for ex-
perimental support. This work was supported by a Grant-in-
Aid from the Ministry of Education, Culture, Sports, Science
and Technology, as well as by a grant from CREST.
Supporting Data:
Fig. 13. Gel permeation chromatograms of poly(6-hexylpy-
ridine), P6HexPy, (vs polystyrene standards). For (a)
P6HexPy obtained after treatment of the polymerization
mixture with HCl (1 M) under N2 (polymer-1) and (b)
P6HexPy obtained after treatment of the polymerization
mixture with H2SO4 (1 M) in air (polymer-2).
Tables of the second-order rate constant, k (cf. Eq. 4), and ki-
netic parameters for the reductive elimination of R-R from 1
(Table S1), X-ray crystallographical data of 2b and 3c (Table S2),
and molecular structure of 2b (Fig. S1), and molecular structure of
3c (Fig. S2) are deposited as Document No. 75036 at the Office of
the Editor of Bull. Chem. Soc. Jpn. and are also available from the
author upon request.
for C12H12Cl2N4O4: M = 346.0236. The molecular structure of 3b
is given in the supporting data.
References
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acid (12 M, 1 mL) in THF under N2. After two days at room tem-
perature, the product was obtained in 45% yield by purification
with HPLC. 1H NMR(CDCl3) δ 2.00 (s, 3H, CH3), 3.79 (s, 3H,
CH3), 7.12 (s, 1H, pyrazol-H). The product was volatile. EI-MS
m/z 130 (M+); peak ratio (4:1) of the peaks at 130 (35Cl) and 132
(37Cl) agreed. 3-Chloro-4-methoxycarbonyl-1-methylpyrazole,
the hydrolysis product of 2c, was kindly donated by Nissan Chem-
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HFIP-806M columns and hexafluoro-i-propanol (eluent).
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3
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5
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1
tal and H-NMR analyses roughly agreed with those of [NiAr2-
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