Ni(i)-Ni(iii) cycle in Buchwald-Hartwig amination of aryl bromide mediated by NHC-ligated Ni(i) complexes

Article abstract of DOI:10.1039/c8cy02427h

Intermediate amide complexes of NHC-ligated monovalent nickel in Buchwald-Hartwig amination of aryl halides were isolated and structurally characterized. The amide complexes reacted with aryl bromide to form a cross-coupling product. Lowerature observation of the oxidative addition product of the Ni(i) amide complex with aryl bromide indicated the presence of a Ni(iii) intermediate. The results showed that a well-defined mononuclear NHC-Ni(i) complex can act as a key intermediate in homogeneous catalysis.

Full text of DOI:10.1039/c8cy02427h

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DOI: 10.1039/C8CY02427H
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Ni(I)-Ni(III) Cycle in Buchwald-Hartwig Amination of Aryl Bromide
Mediated by NHC-ligated Ni(I) Complexes
Received 00th January 20xx,
Accepted 00th January 20xx
Takahiro Inatomi,a Yukino Fukahori,a Yuji Yamada,a Ryuta Ishikawa,a Shinji Kanegawa,b Yuji Kogaa
and Kouki Matsubara*a
DOI: 10.1039/x0xx00000x
Intermediate amide complexes of NHC-ligated monovalent nickel in Buchwald-Hartwig amination of aryl halides were
isolated and structurally characterized. The amide complexes reacted with aryl bromide to form a cross-coupled product.
Low-temperature observation of the oxidative addition product of Ni(I) amide complex with aryl bromide indicated the
presence of Ni(III) intermediate. The results showed that a well-defined mononuclear NHC-Ni(I) complex can act as a key
www.rsc.org/
intermediate
in
homogeneous
catalysis.
As shown in Chart 1, one of the possible pathways involving
Ni(I) intermediates is (1) an active Ni(0) intermediate couples
with Ni(II) into a stable Ni(I) dimer reaching a resting state
equilibrium,4 (2) monomeric Ni(I) forms in situ to become a
deactivated complex as an off-cycle product,5 and (3)
unsaturated Ni(I) dimer promotes the catalytic cycle as a key
intermediate.6 Alternatively, (4) an active mononuclear Ni(I)
intermediate in a catalytic cycle is also proposed using stable
Ni(I) complexes.7 However, it is quite difficult to
experimentally prove that a Ni(I) complex acts as a true active
intermediate in a catalytic cycle, because (5) it sometimes
disproportionates into Ni(0) and Ni(II), which can be the real
active species in catalysis.8 There are several theories that
support the Ni(I)-Ni(III) cycle but no clear experimental
evidence to the best of our knowledge.9
Introduction
Nickel-catalysed organic transformations involving active
nickel intermediates with odd-valence numbers have been
challenging targets in recent chemistry of catalysis.1 This has
become an intriguing unsolved problem, occurring with
chemical advances such as the search for catalytically active
species and analysis of reaction mechanisms by computational
chemistry. Although monovalent nickel complexes are
thermally stable and can be easily isolated under an inert-gas
atmosphere,2 direct involvement of Ni(I) in catalytic cycles has
not been observed for decades. Recently, however, several
reports have proposed catalytic pathways that involve
mononuclear Ni(I)-L and dinuclear L-Ni(I)-Ni(I)-L complexes,
where
L is monodentate or bidentate ligand, such as
phosphine and N-heterocyclic carbine (NHC).3-7 Depending on
the structure of the ligands or the type of reactions,
mononuclear Ni(I) is generated as an off-cycle product or can
act as a key intermediate in the cycle. If it is revealed that Ni(I)
complexes can act as a true active intermediate, development
of new types of reactions using nickel catalysts could be
possible. Furthermore, a Ni(I)-Ni(III) cycle can be considered to
be analogous to a Pd(0)-Pd(II) cycle, where various highly
efficient catalytic reactions have been developed. That is
because a Ni(0) complex is easily oxidised to the corresponding
Ni(II) and hardly reduced back to Ni(0), while a 2e-oxidation of
Ni(I) into Ni(III) can be relatively difficult but 2e-oxidised
intermediate Ni(III) can be easily reduced.3
Chart 1. Various pathways of the active intermediates in nickel-
catalysed systems involving monomeric Ni(0), Ni(I), and Ni(II)
complexes and Ni(I) dimer.
a. Department of Chemistry, Fukuoka University, 8-19-1 Nanakuma, Fukuoka 814-
0180, Japan.
b. Institute for Advanced Materials Chemistry and Engineering, Kyushu University,
744 Motooka, Fukuoka 819-0395, Japan.
Dinuclear L-Ni(I)-Ni(I)-L often forms a mononuclear active
catalyst in literature.10-12 Based on theoretical calculations,
Chirik et al. proposed the generation of a mononuclear Ni(I)
Electronic Supplementary Information (ESI) available: CCDC 1878020-1878026. For
ESI and crystallographic data CIF see DOI: 10.1039/x0xx00000x
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IPr, immediately afforded monomeric tetra-coordinate Ni(I) halides
DOI: 10.1039/C8CY02427H
Ni(IPr)X(Bipy) [2a (X = Cl) and 2b (X = Br)] (Scheme 1). Compounds
2a and 2b were successfully isolated as dark purple crystals upon
recrystallization in 95 and 93% yields, respectively. Although the
previously reported three-coordinated 15e Ni(I) complexes
hydride complex in catalytic hydrosilylation which can be
formed from a dinickel(I) hydride complex bearing redox-
active ligands as a result of a one-electron reduction of the
ligand.10 Hazari et al. showed that a Ni(I) dimer, [Ni(NHC)]2(μ-
Cl)(μ-η3-Cp), and its disproportionation product, monomeric
Ni(I) complex CpNi(IPr), where Cp is a η5-cyclopentadienyl, and
IPr is 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidne, were
catalytically active in the Suzuki-Miyaura cross coupling of 4-
chlorotoluene.11 They concluded that homolytic cleavage of
the Ni(I)-Ni(I) bond into the monomer complexes is necessary
to activate the dimer for catalysis. Our group also reported
that the Sigman's Ni(I) dimer, [Ni(IPr)]2(μ-Cl)2,12 cleaves its
Ni(I)-Ni(I) bond in the presence of phosphines and pyridine to
give mononuclear Ni(I) complexes.13 Both monomer and dimer
complexes can catalyse Kumada-Tamao-Corriu coupling and
Buchwald-Hartwig amination of aryl halides. However, even in
these reactions, it is unclear how the monomeric Ni(I)
complexes behave in catalysis, as the real intermediate such as
Ni(III) species, have not been detected or isolated
experimentally.
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Ni(IPr)Cl(L), where
L was pyridine, triphenylphosphine, and
triphenylphosphite, were very unstable in the air and rapidly
oxidised even in the solid state, the complex 2a is rather stable, at
least in the solid state. The half-lives of the crystals were about 15
minutes at room temperature. The corresponding bromide
analogue 2b was more stable than the chloride analogue 2a. It
should be noted that the 1H NMR spectra for 2a and 2b did not
show any signals assigned as the starting materials 1a and 1b, even
though the analogous three-coordinate Ni(I) complexes readily
regenerate the dimeric nickel complexes 1a and 1b in equilibrium.13
This result indicated that chelating Bipy ligand in 2a and 2b can
stabilize the Ni(I) complexes, compared to monodentate pyridine
and phosphines in three-coordinate Ni(I) complexes.13
Generally, an unpaired electron on the monovalent nickel
promotes facile elimination of ligands and/or products, and
smooth single electron transfer (SET) enables facile activation
of substrates. Therefore, if coordinatively-unsaturated
mononuclear Ni(I) complexes can be appropriately used not as
a deactivator but as active key catalyst, it will be a very
important finding, potentially leading to development of new
catalytic transformations.
Scheme 1. Preparation of monomeric Ni(I) halides with 2,2’-
Recently, based on our and other groups’ studies using nickel
complexes bearing bulky NHCs, thermally stable nickel(I)
complexes have been defined and applied to several catalytic
processes.13,14 Electron-donating NHC ligands may destabilize
low-valent metal species such as Ni(0), probably by supressing
competing Ni(0)-catalysed systems. However, a recent study
showed that IPr-Ni(0) system can also catalyse Ni(0)-Ni(II)
systems in the amination of pyridyl chloride.15 Therefore, clear
evidence is needed to show that Ni(I) complexes can act as key
intermediates in such catalytic reactions.
Herein, monomeric IPr-Ni(I) complexes are found to be the
active species in the Buchwald-Hartwig amination of aryl
bromide. The ligand 2,2’-bipyridyl (Bipy) provided the air-
stable mononuclear tetrahedral IPr-Ni(I) complexes at least in
the solid state, for more than several minutes. Experimental
and theoretical studies also suggested that easy elimination of
Bipy forms a linear two-coordinate (13e) IPr-Ni(I) complexes as
the active species, one of which was isolated and well
determined. Furthermore, the paramagnetic intermediates
were successfully detected in a X-band EPR spectrum at low
temperature, upon oxidative addition of aryl bromide to the
IPr-Ni(I), indicating the existence of the Ni(I)-Ni(III) cycle in the
NHC-Ni-catalysed system for the first time experimentally.
bipyridyl.
These complexes were characterised by NMR spectroscopy,
ESI-TOF MS, SQUID, and elemental analysis. The 1H NMR
spectra of 2a and 2b showed characteristic broadened signals
due to paramagnetic nature of nickel (See ESI). The spin states
of the complexes were studied based on SQUID
measurements. The SQUID measurements supported the
existence of an unpaired electron on the nickel atom. In our
previous reports, the three-coordinate Ni(I) complexes had S =
1/2 (χmol/T = 0.34 - 0.52 cm3 K mol-1 at 20 K, where χmol is molar
magnetic susceptibility). In complexes 2a and 2b, the values
were 0.68 and 0.72 at 20 K, respectively, higher than those of
the three-coordinate complexes and the theoretical χmol/T
value, which is 0.375 cm3 K mol-1 when S = 1/2. The large
difference of the χmol/T values in 2a and 2b could be attributed
to the spin state of the unpaired electron in nickel, probably in
the degenerate electron orbitals, composed of the pseudo-
tetrahedral nickel orbitals. Additionally, the χmol/T values did
not depend on temperature T even at 10 K, suggesting that
other electronic structures, such as high-spin Ni(II) and a
bipyridyl anion radical, can be ruled out according to Curie’s
law.
Suitable crystals suitable for X-ray crystallography were
obtained from a THF/hexane solution at −30 ºC, enabling the
confirmation of the crystal structures of these complexes
(Figure 1). The representative bond distances and angles of 2a
and 2b are listed in Table 1. These complexes have distorted
tetrahedral geometry around the nickel atoms. The distances
Results and discussion
Preparation and Structures of Ni(I) Bipyridyl Complexes. Addition
of Bipy to a pale-yellow solution of di-Ni(I) halides [Ni(iPr)]2(μ-X)2
[1a (X = Cl) and 1b (X = Br)] bearing a bulky N-heterocyclic carbene,
2 | J. Name., 2012, 00, 1-3
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from electron donor Ni(I) to form an ion pair consisting of
DOI: 10.1039/C8CY02427H
anionic radical species and cationic Ni(II), which may lead to
inactivated nickel species. Therefore, nitrobenzene was added
to 2a in THF solution of at 40ºC for 24 h. The colour of the
solution changed to black, and the reaction was completely
inhibited as expected (see ESI).
of nickel-carbene bonds for 2a and 2b were 1.982(5) and
1.959(4) Å, respectively, similar to those of the analogous 15e
complexes of Ni(IPr)Cl(L).
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Figure 1. ORTEP drawings of complexes (a) 2a and (b) 2b with 50%
thermal ellipsoids. Hydrogen atoms and two co-crystallised THF
molecules were omitted for clarity.
Scheme 2. Buchwald-Hartwig amination of 4-chloro- and 4-bromo-
benzophenone with diphenylamine.
Table 1. Representative bond distances (Å) and angles (º) of 2a
and 2b.
Table 2. Buchwald-Hartwig amination of 4-bromobenzophenone
with diarylamines mediated by 2a and 2b.a
2a
2b
Bond Distances (Ǻ)
Ni(1)-C(1)
Ni(1)-Cl(1)
Ni(1)-N(3)
Ni(1)-N(4)
1.982(5)
Ni(1)-C(1)
Ni(1)-Br(1)
Ni(1)-N(3)
Ni(1)-N(4)
1.959(4)
2.4450(8)
1.968(4)
1.972(4)
2.3399(17)
1.984(4)
1.995(4)
Bond Angles (°)
C(1)-Ni(1)-Cl(1)
N(3)-Ni(1)-Cl(1)
N(4)-Ni(1)-Cl(1)
112.99(13)
102.94(13)
112.75(12)
C(1)-Ni(1)-Br(1)
N(3)-Ni(1)-Br(1)
N(4)-Ni(1)-Br(1)
111.46(13)
108.07(11)
106.81(10)
Buchwald-Hartwig Amination of 4-Halobenzophenone Using 2a
and 2b. The well-defined complex 2a catalysed the Buchwald-
Hartwig amination of 4-chloro- and 4-bromo-benzophenone
with diphenylamine even at 40ºC to yield 4-N,N-
diphenylamino-benzophenone in 18 and 95% yields,
respectively (Scheme 2). The results encouraged us to search
the scope of amines as listed in Table 2 for the reaction with 4-
bromobenzophenone mediated by 2a and also 2b under the
similar conditions. Most of the triarylamines were obtained in
good to excellent yields, 58‒96%. Electron-donating and
withdrawing substituents on the diarylamines did not affect
the yields of the products. However, sterically demanding
groups, such as a naphthyl group deactivated the process. The
reaction with carbazole gave several by-products in contrast to
the other reactions, reducing the yield of the amination
product to 58%. The reaction with indole, (4-
cyanophenyl)phenylamine, and (4-nitrophenyl)phenylamine
resulted in recovery of the starting materials. In these
unsuccessful reactions, the colour of the solution of the
reaction mixture was different from those providing the
desired products. For instance, the successful conditions gave
dark green solutions of nickel, whereas the addition of the
unsuccessful substrates provided yellowish brown or reddish
brown solutions, suggesting that the nickel complexes were
deactivated in the presence of these substrates. Electron-
withdrawing groups on aryl halides can accept an electron
a All yields were determined after column chromatography.
Isolation of intermediary compounds. According to our previous
reports
studying
catalytic
amination
of
4-
bromobenzophenone, the catalytic activity of 2a is comparable
to those of the three-coordinate Ni(I) complexes, NiCl(IPr)L (L =
PPh3, pyridine).13 The two-coordinate Ni(I) complex, Ni(IPr)Cl,
is proposed as the key intermediate in the catalytic cycle,
because the ligands L in the three-coordinate analogues can be
removed easily. However, coordination of Bipy to Ni(I) is
stronger than the phosphorus ligands or pyridine, as noted
above. Therefore, we are interested in how these efficient
reactions occur using 2a as the catalyst.
Addition of diphenylamine to the dark-purple solution of 2a in
the presence of sodium tert-butoxide at ambient temperature
immediately gave a dark-blue solution, containing Ni(I) amide
complex Ni(IPr)(NPh2)(bipy) (3a) (Scheme 3). The complex 3a
was successfully isolated upon recrystallization in 24% yield.
Similarly, 4,4’-dimethyldiphenylamine also gave the analogous
complex 3b in 12% yield upon recrystallization (Scheme 3).
Interestingly, although indole did not provide its amination
product with 2a, the transmetallation product 3c was obtained
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Figure 2. ORTEP drawings of complexes (a) 3a and (b) 4 with 50%
in low yield, 5% and thus was not fully characterised, although
the structure was successfully determined by X-ray
crystallography.
thermal ellipsoids. Hydrogen atoms and solvent moleVciuewleAsrt(ictolelOunelninee
DOI: 10.1039/C8CY02427H
in the crystals of 4) were omitted for clarity.
One of the most remarkable results in this study is that the
two-coordinate Ni(I) amide complex Ni(IPr)(NPh2) (4), which
may be one of the true active species in amination, was
successfully isolated (83% upon recrystallization) and
characterized. Compound 4 was obtained from the rapid
reaction of 1a with diphenylamine in the presence of sodium
tert-butoxide at room temperature (Scheme 4). Compound 4
was stable under an inert atmosphere in solution. The similar
monoamino Ni(I) complexes have also been previously
reported.16
Reactions of the intermediates. Isolation of Ni(I) amido
complexes 3a and 4 prompted us to conduct stoichiometric
reactions of them with 4-bromobenzophenone at room
temperature. Addition of aryl bromide to a THF solution of 3a
or 4 immediately changed colour and the H NMR spectra of
the crude mixtures showed the efficient generation of a cross-
1
coupling
product,
4-N,N-diphenylaminobenzophenone
(Scheme 5). In particular, the diamagnetic complex 1b, which
is one of the starting compounds in this catalysis, was clearly
detected in the crude product mixture by 1H NMR
spectroscopy (See ESI). The triarylamine product was isolated
after silica gel column chromatography in 72 and 70% yields
from 3a and 4, respectively. Additionally, the complex 3a was
revealed to be active in the catalytic amination of 4-
bromobenzophenone to yield the desired product in 47%
yield, strongly suggesting that complex 3a exists in the
catalytic cycle (Scheme 6). We considered that complex 3a can
exist at the resting state, and elimination of Bipy forms 4 as
the active species. That is, the equilibria of elimination and
coordination of the Bipy ligand between 3 and 4, and 2 and 1
occurs in activation and stabilization of these Ni(I) complexes.
However, because ligand elimination equilibrium between 2
with Bipy and 1 cannot be observed, as noted above, we have
to demonstrate whether elimination of this generally robust
chelating ligand occurs from 2a or 2b even at room
temperature.17 Interestingly, as a result, it can be rapidly
released from 2b at room temperature in the presence of
another ligand. The UV-visible spectrum was monitored during
the addition of 2,2’-biquinoline (Biq) to a THF solution of 2b
(Scheme 7). Because Biq can coordinate more strongly than
Bipy does, due to its rigid structure derived from expanded π-
conjugation, Biq could irreversibly replace Bipy on the Ni(I)
complex. Therefore, we added Biq to complex 2b and
observed UV spectral changes in the solution at room
temperature. Since UV absorption of the complexes varied
greatly by substituting the ligand, the spectral changes were
clearly observed. As expected, the substitution of Bipy ligand
in 2b with Biq occurred immediately, resulting in a colour
change from dark purple to dark blue. The spectra of the
resulting solution containing 2b and Biq agreed well with the
isolated Biq complex 5 (Figure 3). Thus, complexes 2 and 3 are
considered to have elimination equilibria in solution to form
catalytically active complexes and free Bipy in situ. Such facile
elimination of Bipy is thought to be rare.17 Unfortunately,
because the exchange reaction was too rapid to monitor the
time-dependent ratio of these compounds, there was no
experimental evidence that Bipy was dissociatively eliminated.
However, the low dissociation energy barrier when 3a is
fragmented to form 4 and Bipy is supported by DFT calculation
(ΔE0 = +11.33 and ΔG0 = -4.67 kcal/mol at 298 K) (See ESI). In
contrast, ligand exchange for the NHC ligand was much slower
than for Bipy when free IMes was added to a solution of 2b.
This is also supported by a previous study where a slower
Scheme 3. Stoichiometric reaction of 2a with diarylamines.
Scheme 4. Stoichiometric reaction of 1a with diphenylamine.
The paramagnetic complexes 3 and 4 were characterised by 1H
NMR, UV-Vis, SQUID, and elemental analysis. Finally, the
structures were confirmed by X-ray crystallography. SQUID for
3a and 3b showed similar magnetic susceptibilities to those of
2a and 2b: χmolT = 0.70 and 0.73 at 100 K, respectively,
corresponding to S = 1/2. The crystal structures of 3a and 4
were determined by X-ray crystallography (Figure 2). Nickel,
nitrogen, and carbene carbon atoms are arranged in a
tetrahedral position in 3a, which is similar to 2a and 2b. In
compound 4: the angle of the three atoms was almost linear
with an angle of 169.11(11)º, whereas that of
bis(trimethylsilyl)amido Ni(I) was 163.2(2)º in previous
reports.16
(a)
(b)
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1
ligand exchange occurs for IPr in mononuclear Ni(I) bisIPr observed in the H NMR spectrum as an equilibrium mixture
View Article Online
DOI: 10.1039/C8CY02427H
complexes than with triphenylphosphine.6a
with 1a in C6D6.13 Therefore, we suppose that due to the weak
coordination ability of diphenylamine onto the Ni(I) centre, the
corresponding
three-coordinate
complex,
Ni(IPr)(Br)(diphenylamine), is less stable than the dinuclear
Ni(I) bromide.
Detection of oxidative addition intermediate by EPR. Getting
mechanistic insights into the oxidative addition of aryl halide to the
Ni(I) centre is a most interesting and challenging target. This is
because a Ni(III) intermediate has never been directly observed in
previous mechanistic studies of catalytic cycles, although some
reports have shown the structures of Ni(III) complexes and their
reductive elimination processes.3 Unfortunately, several attempts
to isolate the intermediate complex at −30ºC was unsuccessful, as
this compound was thermally unstable at higher temperatures,
leading to fast decomposition into a mixture composed of 1b and
triarylamine in a few minutes at around 20ºC. The 1H NMR spectrum
of the mixture after heating to room temperature showed only the
existence of these expected side products (see ESI). Therefore, we
attempted direct observation of the Ni(III) intermediate using EPR
spectroscopy at low temperature. After addition of 1 equiv of 4-
bromoanisole to the two-coordinate Ni(I) complex 4 in ether at
−40ºC, removal of the solvent under reduced pressure gave a dark
green crystalline solid. Then, the residue was dissolved in THF in a
sample tube and rapidly immersed into liquid N2 just before the EPR
measurement. As shown in Figure 4, signals due to new compounds
appeared in the EPR spectrum of the reaction mixture. In contrast,
no obvious signals were detected when the crystalline solid of
starting complex 4 was used as the sample. Whittlesey et al.
reported that an unquenched orbital angular momentum can be a
reason why there are no signals in the X- and Q-band when the EPR
spectrum of the two-coordinate bis-NHC Ni(I) complex were
measured, even though the S value of the complex was 1/2.18 The
observed signals suggested presence of a mixture of Ni(I) and Ni(III)
complexes, where the spin quantum number S = 1/2, based on
computer simulations and DFT calculations (see ESI). The Ni(III)
intermediate is likely formed after oxidative addition of 4-
bromoanisole to Ni(I) amide. Additionally, some three-coordinated
Ni(I) amide complex was also possible to exist as one of the
products detected in the EPR spectrum. The signals due to one of
the compounds, which could be a Ni(III) intermediate, were
assigned as rhombic distortion with gx = 2.125, gy = 2.104 and gz =
2.009, whereas the others were rhombic with gx = 2.555, gy = 2.308
and gz = 2.094. There are several examples of well-defined Ni(III)
complexes having square planar or square pyramidal geometry
around the nickel centre in literatures, showing average g values of
ca. 2.13-2.17,19 which are close to the value of gav = 2.08 for the
proposed Ni(III) intermediate. The integral ratio of the signals from
these Ni(I) and Ni(III) complexes was calculated to be ca. 3 : 2.20 An
alternative pathway can also be proposed from the Ni(I) complexes
where disproportionation into diamagnetic Ni(0) and Ni(II)
Scheme 5. Stoichiometric reaction of 3a and
bromobenzophenone.
4 with 4-
Scheme 6. Catalytic reaction of diphenylamine with 4-
bromobenzophenone using 3a as catalyst.
Scheme 7. Ligand exchange reactions of 2b with 2,2’-biquinoline
(Biq) at room temperature.
Figure 3. UV spectra for compound 2b before (black) and after
addition of Biq (gray), and for isolated Biq complex 5 (pale gray).
Next, we attempted to detect the intermediate of the reaction
of 1b with amine in the presence or absence of base. However,
detection of such intermediate compounds was unsuccessful
using NMR and EPR spectroscopy under any conditions. In our
previous study, a pyridine molecule coordinated to a IPr-Ni(I) complexes occurs as demonstrated in literatures.8 Cárdenas, et al.
chloride
to
form
a
three-coordinate
complex,
recently proposed an alternative radical oxidative addition pathway
on the basis of DFT evidence, in which two molecules of Ni(I) react
Ni(IPr)(Cl)(pyridine), was thermally stable in the solid state and
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with one alkyl halide to form Ni(II) alkyl and Ni(II) halide,
independently.21 However, because we cannot find the
corresponding Ni(0) and/or Ni(II) IPr complexes in the reaction
mixture of 4-bromoanisole with 4a or 3a, as noted above, these
possibilities are unlikely, at least under these conditions.
cycle. NHC ligands are generally stronger σ-VdieownAortricsle Othnlainne
phosphorus ligands, resulting in the stabilization of higher-
valent metal centers such as Ni(I) and Ni(III), and acting as
catalysts without disproportionation to Ni(0) and Ni(II) species.
DOI: 10.1039/C8CY02427H
Figure 5. Proposed reaction mechanism of Ni(I)-catalysed
Buchwald-Hartwig amination of aryl halides.
Conclusions
Figure 4. X-band EPR spectrum for the reaction mixture of 4 with 1
equiv of 4-bromoanisole (THF glass, at −178ºC) (black), which can be
reproduced by computer simulation of the proposed compounds:
mononuclear Ni(I) intermediate (red, rhombic: gx = 2.555, gy = 2.308
and gz = 2.094) and Ni(III) product (blue, rhombic: gx = 2.125, gy =
2.104 and gz = 2.009).
In summary, we have uncovered one of the catalytic cycles
involved in the Buchwald-Hartwig amination of aryl bromides
mediated by Ni(I) NHC complexes. Although a catalytic system
between Ni(I) and Ni(III) has been discussed as a possible
pathway in many previous reports, experimental studies
determining such intermediate key compounds have been
poorly defined. Our findings in this study showed that
mononuclear Ni(I) and Ni(III) NHC complexes are
experimentally and theoretically possible as intermediates in
the catalytic cross-coupling reaction. A bidentate ligand, 2,2'-
bipyridyl, can stabilize Ni(I) NHC complexes, which efficiently
mediate amination of aryl bromides with several amines
attempted. Several stoichiometric chemical reactions revealed
the presence of a two-coordinate 13e IPr-Ni(I) intermediate.
The Ni(III) intermediate was thermally unstable, but its
existence was strongly suggested by EPR measurements,
followed by the thermal formation of cross-coupling product
and diamagnetic dinuclear Ni(I) halide complex. 2,2'-Bipyridyl
was able to behave as a hemi-labile ligand to the Ni(I) centre,
leading the highly reactive intermediates to stable resting
states. Alternatively, the NHC ligand appears to bind to the
nickel centre in a stable manner, conserving the mononuclear
Ni(I) intermediates to some extent. We are now in the process
of studying further this unusual and highly active catalytic
process by conducting thorough experiments and discussions
which include kinetic analysis.
Proposed catalytic cycle. Based on the above aspects, we
proposed
a possible catalytic cycle for Ni(I)-catalysed
Buchwald-Hartwig amination of an aryl halide (Figure 5). From
two-coordinate mononuclear Ni(I) chloride, transmetallation
occurs with an amine and a base. Proton abstraction from the
amine can proceed before and/or after coordination to the Ni
centre with the base. Because amide complex 4 has a
monomeric structure, active mononickel intermediates are
favourable throughout this catalytic cycle. Complex 4 reacts
with the aryl halide to form the Ni(III) oxidative addition
product, which can easily eliminate triarylamine, resulting in a
reduction to the starting Ni(I) halide. Reversible coordination
of Bipy to two-coordinate Ni(IPr)X and Ni(IPr)NAr2 (4) forms
stabilised tetrahedral 17e Ni(I) complexes as resting states,
and thermal elimination of these complexes can regenerate
the active intermediates. The existence of the resting states
can decrease the concentration of these highly active
intermediates to avoid catalyst deactivation pathways at the
late stage of the reaction. Transmetallation of Ni(I) species
involved in the catalytic cross-coupling cycle has also been
proposed as a mechanism of Ni(I)-catalysed cross-coupling
reactions in literature.9 However, the other possible pathway,
via oxidative addition on Ni(I) halide and subsequent
transmetallation, may still be possible and should be studied in
the future.
Experimental
Experimental details
General. All experiments were carried out under an inert gas
atmosphere using standard Schlenk techniques and glovebox
(MBraun UniLab) as otherwise noted. THF, toluene, hexane,
benzene-d6 were distilled from benzophenone ketyl and stored
under a nitrogen atmosphere. Volatile organic reagents used for
Strong evidence in the literature indicates that the Ni(0)/Ni(II)
cycle is dominant even when Ni(I) phosphine complexes are
used.8 There is no direct evidence, but we believe that the
electron-donating ability of the ligand may alter the catalytic
6 | J. Name., 2012, 00, 1-3
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Catalysis Science & Technology
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Journal Name
ARTICLE
coupling reactions were distilled just before use. Other reagents mL Schlenk tube was charged with 2a (31.9 mg, 0.05 mmol), 2,2'-
View Article Online
were used as received. Bis(4-methoxyphenyl)amine,22a N-(p-tolyl)- bipyridyl (15.6 mg, 0.10 mmol), aromatic amine (0.60 mmol),
DOI: 10.1039/C8CY02427H
[1,1'-biphenyl]-4-amine,22b
tolyl)naphthalen-1-amine,22c
4-fluoro-N-phenylaniline,22a
N-(p- NaOtBu (71.1 mg, 0.75 mmol), 4-bromobenzophenone (130.6 mg,
1,3-bis(2,6- 0.50 mmol), and THF (0.2 mL). The reaction mixture was stirred at
and
diisopropylphenyl)imidazol-2-ylidene (IPr)22d were prepared 40-80°C for 24-48 h. After addition of water, the organic layer was
according to the literature methods. Nickel dimers : [Ni(IPr)(μ-X)]2 extracted with CH2Cl2 for three times. The combined organic layer
(X = Cl, Br) (1a, 1b) were prepared according to the literature was washed with brine and dried by Na2SO4, filtered and
methods.12 Column chromatography of organic products was concentrated in vacuo. The residue was purified with flash column
carried out using slica gel (Kanto Kagaku, silica gel 60N (spherical, chromatography eluted with dichloromethane/hexane (1/3) to
neutral)). The 1H NMR spectra were taken with a Bruker Avance- obtain corresponding triarylamines.
III400 Y plus 400 MHz spectrometer at room temperature. Chemical Stoichiometric reaction of Ni(I) complex 2a with diarylamines. In a
shifts (δ) were recorded in ppm from the solvent signal. The glovebox, complex 2a (100 mg, 0.16 mmol), diarylamine (0.16
magnetic properties of the materials were investigated using a mmol), NaOtBu (34.0 mg, 0.35 mmol) and THF (2 mL) were placed in
Quantum Design MPMS-5S superconducting quantum interference a 20 mL Schlenk tube. The suspension was stirred for 1 hour at
device (SQUID) magnetometer. The elemental analysis was carried room temperature to give a dark purple solution. The volatiles were
out with YANACO CHN Corder JM-11, AUTO-SAMPLER, using removed in vacuo, the residual solid was extracted with THF and
aluminum pan, where the samples were held in a glovebox. The UV- evaporated to dryness. The residue was washed with hexane to give
Vis measurements were taken with a PerkinElmer Lambda 35 3a or 3b as a black solid.
UV/Vis Spectrometer using either a 10 mm quartz cell. The X-band Complex 3a. The title compound was isolated as a black solid in 24%
EPR spectra were collected with a Bruker EMX Plus spectrometer yield. 1H NMR (C6D6): δ 0.81-1.59 (bs), 6.17 (bs), 6.77 (bs). Elemental
equipped with a continuous flow N2 cryostat. EPR simulation was analysis calcd (%) for C49H54N5Ni·0.5Et2O : C, 75.74; H, 7.35; N, 8.66;
conducted using a PHI program.23
found: C, 75.74; H, 7.38; N, 8.85.
[NiCl(bpy)(IPr)] (2a). In a glovebox, [Ni(IPr)(μ-Cl)]2 (1a) (30 mg, Complex 3b. The title compound was isolated as a black solid in 12%
0.030 mmol), 2,2'-bipyridyl (bpy) (9.5 mg, 0.061 mmol), and THF yield. Elemental analysis calcd (%) for C51H58N5Ni·0.8THF : C, 75.90;
(1.0 mL) were added to a 5 mL screw-capped tube. After the H, 7.58; N, 8.15. Found: C, 76.30; H, 7.44; N, 7.78.
compounds were dissolved, hexane (1.5 mL) was slowly added to Stoichiometric reaction of Ni(I) complex 1a with diarylamines. In a
the solution and cooled to -30°C. Dark purple crystals of 2a were glovebox, complex 1a (50.0 mg, 0.052 mmol), diarylamine (35.1 mg,
obtained, after removal of the liquid and washing with small 0.207 mmol), NaOtBu (24.0 mg, 0.249 mmol) and toluene (2 mL)
1
amount of cold hexane (60 mg, 0.094 mmol, 95% yield). H NMR were placed in a 20 mL Schlenk tube. The suspension was stirred for
(C6D6): δ 0.81~1.59 (bs), 6.17 (bs), 6.77 (bs). Elemental analysis 10 minutes at room temperature to give a dark blue solution. The
calcd (%) for C37H44ClN4Ni: C, 69.55; H, 6.94; N, 8.77; found: C, volatiles were removed in vacuo, the residual solid was extracted
69.18; H, 6.97; N, 8.44.
with toluene and evaporated to dryness. The residue was washed
1
[NiBr(bpy)(IPr)] (2b). In a glovebox, [Ni(IPr)(μ-Br)]2 (1b) (110 mg, with hexane to give 4 as a purple solid (53.2 mg, 83 %). H NMR
0.104 mmol), 2,2'-bipyridyl (bpy) (40.7 mg, 0.261 mmol), and THF (C6D6): δ 0.81-1.59 (bs), 6.17 (bs), 6.77 (bs). Elemental analysis
(15 mL) were added to a 25 mL screw-capped tube. After the calcd. (%) for C39H46N3Ni/Et2O: C, 74.89; H, 8.19; N, 6.09; found: C,
compounds were dissolved, hexane (23 mL) was slowly added to 74.63; H, 8.24; N, 6.54.
the solution and cooled to -30°C. Dark purple crystals of 2b were Stoichiometric reaction of Ni(I) amide complexes (3a, 4) with 4-
obtained, after removal of the liquid and washing with cold hexane bromobenzophene.
1
(132 mg, 0.193 mmol, 93% yield). H NMR (C6D6): δ 1.39 (bs), 6.32 Using complex 3a as a substrate
(bs),
C37H44BrN4Ni·2THF·1bpy: C, 67.15; H, 6.97; N, 8.54; found: C, 67.25; 0.065 mmol), bpy (51 mg, 0.33 mmol), 4-bromobenzophenone (17
H, 7.18; N, 8.45. mg, 0.065 mmol), and THF (1.0 mL). The reaction mixture was
6.77
(bs).
Elemental
analysis
calcd
(%)
for In a glovebox, a 20 mL Schlenk tube was charged with 3 (50 mg,
[NiBr(biq)(IPr)] (5). In a glovebox, [Ni(IPr)(μ-Br)]2 (1b) (80 mg, 0.076 stirred at 40°C for 24 h. After addition of water, the organic layer
mmol), 2,2’-biquinoline (biq) (42.8 mg, 0.167 mmol, 2.2 eq.), and was extracted with CH2Cl2 for three times. The combined organic
THF (1.0 mL) were added to a 5 mL screw-capped tube. After the layer was washed with brine and dried by Na2SO4, filtered and
compounds were dissolved, hexane (2.0 mL) was slowly added to concentrated in vacuo. The residue was purified with flash column
the solution and cooled to -30°C. Dark blue crystals of 5 were chromatography eluted with dichloromethane/hexane (1/3) to
obtained, after removal of the liquid and washing with small obtain the corresponding triarylamine (16.3 mg, 0.047 mmol, 72 %
amount of cold hexane (95.1 mg, 80% yield). 1H NMR (C6D6): δ yield).
14.34 (bs), 11.54 (bs), 6.55 (bs), 5.99 (bs), 1.72 (bs), -0.25 (bs). Using complex 4 as a substrate
Elemental analysis calcd (%) for C45H48BrN4Ni: C, 68.98; H, 6.18; N, The experiment was similarly conducted to the above method,
7.15; found: C, 68.24; H, 6.32; N, 6.95.
General procedure for amination
using 4 (30 mg, 0.049 mmol), 4-bromobenzophenone (13 mg, 0.049
4- mmol), and THF (0.2 mL) to obtain the corresponding triarylamine
reaction
of
bromobenzophenone with aromatic amines. In a glovebox, a 20 (12.0 mg, 0.034 mmol, 70 % yield).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Page 8 of 11
ARTICLE
Journal Name
Catalytic reaction of diphenylamine with 4-bromobenzophenone 7.3 Hz, 2H, Benzoyl), 7.82 (d, J = 7.7 Hz, 1H, Naphthyl), 7.73 (d, J =
View Article Online
DOI: 10.1039/C8CY02427H
using 3a as catalysts. The experiment was similarly conducted to 8.2 Hz, 2H, tolyl), 7.65 (d, J = 9.5 Hz, 2H, Benzoyl), 7.53-7.47 (m, 3H),
the above catalytic method with 2a, using 3a (38.6 mg, 0.050 7.44-7.37 (m, 4H), 7.15 (d, J = 8.6 Hz, 2H, tolyl), 7.10 (d, J = 8.2 Hz ,
mmol), 4-bromobenzophenone (130.6 mg, 0.50 mmol), 2H, tolyl), 6.81 (d, J = 8.6 Hz, 2H, Benzoyl), 2.31 (s, 3H, tolyl). 13C
diphenylamine (101.5 mg, 0.60 mmol), bpy (46.9 mg, 0.30 mmol), NMR (CDCl3): δ 195.4 (C=O), 152.7 (Benzoyl), 143.9 (tolyl), 142.3
NaOtBu (71.1 mg, 0.74 mmol) and THF (0.2 mL) to obtain the (Naphthyl), 138.7 (Benzoyl), 135.3 (tolyl), 134.3 (Naphthyl), 132.2
corresponding triarylamine (82.8 mg, 0.237 mmol, 47 % yield).
Product details of amines.
(4-(bis(4-methoxyphenyl)amino)phenyl)(phenyl)methanone
(Benzoyl), 131.5 (Benzoyl), 131.0 (Benzoyl), 130.2 (Benzoyl), 129.6
(tolyl), 128.6 (Naphthyl), 128.4 (Naphthyl), 128.1 (tolyl), 127.4
(Naphthyl), 127.3 (Naphthyl), 126.8 (Naphthyl), 126.4 (Naphthyl),
This reaction was conducted with bis(4-methoxyphenyl)amine and 124.8 (Benzoyl), 123.9 (tolyl), 117.1 (Benzoyl), 20.9 (-CH3). Elemental
the reaction condition was 40°C/24 h. The title compound was analysis calcd (%) for C30H23NO·0.3H2O: C, 85.89; H, 5.69; N, 3.34;
1
isolated as a yellow oil in 91% yield. H NMR (CDCl3): δ 7.74 (d, J = found: C, 86.00; H, 5.55; N, 3.32.
6.4 Hz, 2H, Benzoyl), 7.67 (d, J = 9.0 Hz, 2H, Benzoyl), 7.53 (t, J = 6.4 (4-((3-fluorophenyl)amino)phenyl)(phenyl)methanone
Hz, 1H, Benzoyl), 7.44 (t, J = 7.2 Hz, 2H, Benzoyl), 7.14 (d, J = 9.0 Hz, This reaction was conducted with aniline and the reaction condition
4H, Anisyl), 6.88-6.84 (m, 6H), 3.81 (s, 6H, -OMe). 13C NMR (CDCl3): was 60°C/48 h. The title compound was isolated as a pale yellow
δ 195.1 (C=O), 157.1 (Anisyl), 152.8 (Benzoyl), 139.3 (Anisyl), 138.8 solid in 94% yield. 1H NMR (CDCl3): δ 7.78 (m, 4H), 7.56 (t, J = 7.5 Hz,
(Benzoyl), 132.1 (Benzoyl), 131.5 (Benzoyl), 129.6 (Benzoyl), 128.1 1H, Benzoyl), 7.48 (t, J = 7.4 Hz, 2H Benzoyl), 7.27 (m, 1H), 7.07 (d, J
(Benzoyl), 127.9 (Anisyl), 116.7 (Benzoyl), 115.0 (Anisyl), 55.5 (- = 7.7 Hz, 2H, Benzoyl), 6.93 (m, 2H), 6.74 (m, 1H) 6.22 (brs, 1H, -NH).
OMe). Elemental analysis calcd (%) for C27H23NO3: C, 79.20; H, 5.66; 19F NMR (CDCl3): δ -111.54. 13C NMR (CDCl3): δ 195.2 (C=O), 164.8
N, 3.42; found: C, 78.96; H, 5.82; N, 3.26.
(4-((4-fluorophenyl)(phenyl)amino)phenyl)(phenyl)methanone
(Ph, 1JC-F = 243.7 Hz), 147.0 (Benzoyl), 142.8 (Ph, 3JC-F = 10.7 Hz),
138.5 (Benzoyl), 132.6 (Benzoyl), 131.8 (Benzoyl), 130.7 (Ph, 3JC-F
=
This reaction was conducted with 4-fluoro-N-phenylaniline and the 9.7 Hz), 129.6 (Benzoyl), 128.2 (Benzoyl), 115.3 (Benzoyl), 115.2 (Ph,
reaction condition was 80°C/48 h. The title compound was isolated 4JC-F = 2.8 Hz), 109.6 (Ph, 2JC-F = 24.4 Hz), 106.8 (Ph, 2JC-F = 21.1 Hz).
1
as a yellow oil in 91% yield. H NMR (CDCl3): δ 7.77 (d, J = 7.0 Hz, Elemental analysis calcd (%) for C19H14FNO: C, 78.33; H, 4.84; N,
2H, Benzoyl), 7.72 (d, J = 8.8 Hz, 2H, Benzoyl), 7.55 (t, J = 7.4 Hz, 1H, 4.81; found: C, 78.08; H, 4.81; N, 4.75.
Benzoyl), 7.46 (t, J = 6.8 Hz, 2H, Ph), 7.33 (t, J = 7.9 Hz, 2H, Ph), 7.19- Computational details
7.12 (m, 5H), 7.04 (t, J = 8.6 Hz, 2H, Ph), 6.98 (d, J = 8.8 Hz , 2H, All the calculations were performed using the GAUSSIAN 09
Benzoyl). 19F NMR (CDCl3): δ -116.93. 13C NMR (CDCl3): δ 195.1 package.24 The geometry optimization was performed using the
(C=O), 161.1 (Ph, 1JC-F = 243.6 Hz), 151.9 (Benzoyl), 146.4 (Ph), 142.5 functionals of B3LYP, B3LYP-D3BJ, and BP86 without symmetry
(Ph, 4JC-F = 3.1 Hz), 138.4 (Benzoyl), 132.0 (Benzoyl), 131.7 (Benzoyl), restriction. The standard 6-31G(d) or 6-31G(d,p) basis sets for all
129.7 (Benzoyl), 129.6 (Ph), 129.5 (Benzoyl), 128.1 (Benzoyl), 128.0 atomes were employed. Furthermore, the nature of stationary
(Ph, 3JC-F = 8.2 Hz), 125.6 (Benzoyl), 124.7 (Ph), 119.0 (Ph), 116.7 points was checked by the vibrational frequency analysis at the
(Ph, 2JC-F = 22.5 Hz). Elemental analysis calcd (%) for C25H18FNO: C, same level. The dissociaton energy of 3a complex to 4 with bpy was
81.72; H, 4.94; N, 3.81; found: C, 81.70; H, 5.01; N, 3.78.
(4-(naphthalen-1-yl(phenyl)amino)phenyl)(phenyl)methanone
corrected for zero point energy (ZPE) and basis set superposition
error (BSSE). The relative Gibbs free energies were estimated at the
This reaction was conducted with N-phenylnaphthalen-1-amine and condition of 298.15 K and 1 atm. All the computation was carried
the reaction condition was 80°C/48 h. The title compound was out using the computer facilities at Research Institute for
1
isolated as a yellow oil in 79% yield. H NMR (CDCl3): δ 7.90 (t, J = Information Technology, Kyushu University.
8.0 Hz, 2H, Benzoyl), 7.81 (d, J = 8.4 Hz, 1H, Napthyl), 7.73 (d, J = 7.2 X-ray crystallography
Hz, 2H, Benzoyl), 7.67 (d, J = 8.7 Hz, 2H, Benzoyl), 7.50-7.36 (m, 7H), Single crystals of 2a-2b, 3a-3c, 4 and 5 for X-ray diffraction were
7.28-7.21 (m, 4H), 7.06 (t, J = 7.0 Hz, 1H, Benzoyl), 6.88 (d, J = 9.0 Hz grown at −30°C from the toluene/hexane (for 4) and the
, 2H, Benzoyl). 13C NMR (CDCl3): δ 195.1 (C=O), 152.3 (Benzoyl), THF/hexane (for the other compounds) solutions. All the data were
146.6 (Ph), 142.1 (Naphthyl), 138.5 (Benzoyl), 135.2 (Naphthyl), obtained at 123 K using a Rigaku Saturn CCD diffractometer with a
132.1 (Benzoyl), 131.9 (Benzoyl), 131.5 (Benzoyl), 130.9 (Benzoyl), confocal mirror and graphite-monochromated Mo Kα radiation (λ =
129.5 (Ph), 129.4 (Naphthyl), 128.8 (Naphthyl), 128.5(Ph), 128.0 0.71070 Å). Data reduction of the measured reflections was
(Naphthyl), 127.5(Ph), 127.4 (Naphthyl), 126.8 (Naphthyl), 126.4 performed using the software package CrystalStructure.25 The
(Naphthyl), 126.3(Ph), 124.3 (Benzoyl), 124.1 (Naphthyl), 123.7 structures were solved by direct methods (SIR2008)26 and refined
(Naphthyl), 117.7 (Benzoyl). Elemental analysis calcd (%) for by full-matrix least-squares fitting based on F2, using the program
C29H21NO·0.5H2O: C, 85.27; H, 5.43; N, 3.43; found: C, 85.20; H, SHELXL 97-2, PC version.27 All non-hydrogen atoms were refined
5.35; N, 3.14.
with anisotropic displacement parameters. All hydrogen atoms
were located at ideal positions and included in the refinement, but
(4-(naphthalen-1-yl(p-tolyl)amino)phenyl)(phenyl)methanone
This reaction was conducted with N-(p-tolyl)naphthalen-1-amine were restricted to riding on the atom to which they were bonded.
and the reaction condition was 80°C/48 h. The title compound was CCDC
isolated as a yellow oil in 72% yield. H NMR (CDCl3): δ 7.91 (t, J = crystallographic data for this paper. A copy of the data can be
1878020-1878026
contains
the
supplementary
1
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Conflicts of interest
The authors declare no conflict of interest.
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Acknowledgements
This work was supported by the Japan Society for the
Promotion of Science (Grant-in-Aid for Scientific Research
22550104 and 25410123).
7
Author Contributions
These authors contributed equally: YF made Ni complexes. TI
conducted experiments for catalysis and its mechanistic study. TI
also managed the whole study with KM. YY conducted theoretical
calculations. RI conducted EPR measurements. SK did SQUID
measurements. YK measured ESI-TOF MS and did elemental
analysis.
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DOI: 10.1039/C8CY02427H
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DOI: 10.1039/C8CY02427H
Table of Contents Entry
Ni(I)-Ni(III) Cycle in Buchwald-Hartwig Amination of Aryl Bromide Mediated by
NHC-ligated Ni(I) Complexes
Takahiro Inatomi,a Yukino Fukahori,a Yuji Yamada,a Ryuta Ishikawa,a Shinji Kanegawa,b
Yuji Kogaa and Kouki Matsubara*a
NHC-ligated Ni(I) intermediates in Buchwald-Hartwig amination of aryl halides
were isolated and determined. The presence of Ni(III) intermediate was also
indicated at low temperature.