Monomeric three-coordinate N-heterocyclic carbene nickel(I) complexes: Synthesis, structures, and catalytic applications in cross-coupling reactions

Article abstract of DOI:10.1021/acs.organomet.6b00419

A series of three-coordinate monovalent nickel halide complexes bearing N-heterocyclic carbene (NHC) ligands, i.e., NiCl(IPr)(L) [L = pyridine, P(OPh)₃, bis(diphenylphosphino)butane (dppb), IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], NiX(IMes)(PPh₃) (X = Cl and Br, IMes = 1,3-bis(mesityl)imidazol-2-ylidene), were prepared. The complexes were identified using NMR spectroscopy, superconducting quantum interference device (SQUID), and X-ray crystallography. Additionally, ESR spectra of NiCl(IPr)(pyridine) were taken in toluene. These complexes had three-coordinate Y-shaped geometries in both the solid and solution states. The compounds containing IPr showed equilibrium between the monomeric and dimeric forms, with liberation of ligands. Addition of 1,2-bis(diphenylphosphino)ethane and 1,3-bis(diphenylphosphino)propane to the dinickel(I) IPr complex instead of dppb resulted in heterolytic cleavage to nickel(0) and nickel(II) species. Catalysis of Suzuki cross-coupling and Buchwald-Hartwig amination of aryl bromide using the complexes was investigated. The efficiencies in the amination of aryl bromide depended strongly on the additional donor ligands.

Full text of DOI:10.1021/acs.organomet.6b00419

Article
pubs.acs.org/Organometallics
Monomeric Three-Coordinate N‑Heterocyclic Carbene Nickel(I)
Complexes: Synthesis, Structures, and Catalytic Applications in
Cross-Coupling Reactions
Kouki Matsubara,*,† Yukino Fukahori,†,‡ Takahiro Inatomi,† Saeko Tazaki,† Yuji Yamada,† Yuji Koga,†
Shinji Kanegawa,§ and Toshikazu Nakamura‡
†Department of Chemistry, Fukuoka University, 8-19-1 Nanakuma, Fukuoka 814-0180, Japan
‡Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan
§Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
S
* Supporting Information
ABSTRACT: A series of three-coordinate monovalent nickel halide complexes bearing N-heterocyclic carbene (NHC) ligands,
i.e., NiCl(IPr)(L) [L = pyridine, P(OPh)3, bis(diphenylphosphino)butane (dppb), IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-
2-ylidene], NiX(IMes)(PPh3) (X = Cl and Br, IMes = 1,3-bis(mesityl)imidazol-2-ylidene), were prepared. The complexes were
identified using NMR spectroscopy, superconducting quantum interference device (SQUID), and X-ray crystallography.
Additionally, ESR spectra of NiCl(IPr)(pyridine) were taken in toluene. These complexes had three-coordinate Y-shaped
geometries in both the solid and solution states. The compounds containing IPr showed equilibrium between the monomeric
and dimeric forms, with liberation of ligands. Addition of 1,2-bis(diphenylphosphino)ethane and 1,3-bis(diphenylphosphino)-
propane to the dinickel(I) IPr complex instead of dppb resulted in heterolytic cleavage to nickel(0) and nickel(II) species.
Catalysis of Suzuki cross-coupling and Buchwald−Hartwig amination of aryl bromide using the complexes was investigated. The
efficiencies in the amination of aryl bromide depended strongly on the additional donor ligands.
the following reasons: (1) reduction to nickel(I) species is
easier than reduction to nickel(0) in catalytic cycles; (2) the
unpaired electron weakens the nickel−ligand bonding inter-
actions, resulting in easier elimination of ligand and/or the
product in catalysis than the diamagnetic nickel species.4
Therefore, the development of nickel(I)-catalyzed organic
reactions and new catalytic systems is important in both
organic and organometallic chemistry.
Examples of catalysts starting from well-defined nickel(I)
precursors have rarely been reported,4 although many examples
of monovalent nickel complexes are known.6 This is because
nickel(I) compounds are frequently thermally unstable and
disproportionate to form nickel(0) and nickel(II) compounds.
They are also extremely unstable in air and generally form
INTRODUCTION
■
Organonickel-catalyzed organic transformations have been
widely studied in recent decades, and many useful reactions
have been discovered.1 A general feature of nickel-mediated
reactions is oxidation of a zerovalent species to a nickel(II)
species, which is then reduced to complete the catalytic cycle.
However, zerovalent nickel species, unlike zerovalent palladium
species, are generally unstable; therefore the development of
efficient methods for reduction to nickel(0) is important in
developing efficient catalytic processes.2 On the other hand,
monovalent nickel species have recently been reported in useful
cross-coupling reactions,3 and well-defined nickel(I) complexes
have been shown to mediate cross-coupling reactions of aryl
and alkyl halides;4 for example, Vicic et al. reported that a
terpyridine-stabilized nickel(I) complex catalyzed the Negishi
coupling of alkyl halides.5 These monovalent nickel-catalyzed
processes have advantages over the usual nickel(0) catalysts for
Received: May 25, 2016
© XXXX American Chemical Society
A
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oxidized compounds. Bulky N-heterocyclic carbene (NHC)
ligands thermally stabilize nickel(I) species in catalysis.4 Among
such nickel(I) complexes, monovalent nickel chloride bearing
two IPr ligands, i.e., Ni(IPr)2Cl, where IPr is 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene, is in equilibrium with
dinuclear nickel(I) chloride, [Ni(IPr)]2(μ-Cl)2 (1), with
elimination of one IPr ligand.4a This prompted us to add
PPh3 as a two-electron-donor ligand to 1, prepared by another
method,7 resulting in efficient generation of a three-coordinate
Y-shaped monomeric nickel(I) complex, Ni(IPr)Cl(PPh3)
(2a).8 Complex 2a was more air-stable than 1, and it catalyzed
Kumada−Tamao−Corriu coupling and Buchwald−Hartwig
aminations of aryl bromides more efficiently than did 1 and
Ni(IPr)2Cl.8 PPh3 is easily eliminated from 2a to form an
unsaturated and highly active, monomeric two-coordinate
species, Ni(IPr)Cl, or the dimeric nickel(I) complex 1 because
of the strong trans effect of the NHC ligand,9 and the stable
resting state, with recoordination of PPh3, stabilizes the catalytic
system. Excess PPh3 did not deactivate the process and did not
compete with the substrates for coordination to nickel. Our
preliminary results showed that Ni(IPr)2Cl reduction with a
Grignard reagent readily forms a zerovalent complex Ni(IPr)2,
without elimination of IPr, whereas 2a eliminates phosphine in
conjunction with the Grignard reagent to form a dimeric
nickel(I) transmetalation product.8 The introduction of other
donor ligands, including other phosphines and pyridine
derivatives, to 2 could be used to control the activity and
stability of the monovalent nickel complex 2.
Scheme 1. Preparation of Mononickel(I) IPr Complexes
monovalent nickel complex 2 but mixtures containing
zerovalent nickel complexes Ni(dppe)2 (5a)12 or Ni(dppp)2
(5b).13 Divalent complex Ni(IPr)2Cl2 (6) was also detected as
the product in the crude reaction mixture (see the SI).
Zerovalent complexes 5a and 5b were isolated as crystals in
24% and 32% yield, respectively, although 6 could not be
isolated. It should be noted that when 1,4-bis-
(diphenylphosphino)butane (dppb), in which the methylene
chain is one carbon longer than dppp, was used as the ligand,
the dinickel(I) dppb complex [Ni(IPr)Cl]2(dppb) (2d) was
successfully obtained in 74% yield upon recrystallization
(Scheme 2). However, the complex 2d rearranged slowly in
Scheme 2. Reaction of 1 with Bisphosphines
A large variety of bulky NHC ligands have been designed and
built for use in catalysts.10 We focused on the structural
differences between IPr and 1,3-bis(mesityl)imidazol-2-ylidene
(IMes); the different steric effects around the metal centers
result in clear differences between their reactivities and
stabilities. Monovalent nickel species bearing an IPr or IMes
ligand differ significantly; for example, preparation of an IMes
analogue of the dimeric nickel(I) compound 1 is impossible.
This indicates that the monomeric IMes analogue of 2 and the
IPr complex 2 cannot be synthesized using the same method.
However, several nickel(I) NHC complexes have been
prepared from Ni(cod)2 and NiX2L2 (X = halogen, L = 1,2-
dimethoxyethane or triphenylphosphine) in the presence of an
NHC.7,11
In this study, we prepared a series of nickel(I) complexes
bearing IPr and IMes and various two-electron-donor ligands.
These compounds were characterized using spectroscopy,
superconducting quantum interference device (SQUID)
measurements, and X-ray crystallography. The catalytic
activities of the monomeric nickel(I) complexes in the Suzuki
coupling and Buchwald−Hartwig amination of aryl bromides
were compared.
the benzene-d6 solution in a day to form a mixture of the
zerovalent complex Ni(dppb)2 (5c), suggesting that dis-
14
proportionation occurred via formation of the dinickel(I)
intermediate 2 even in the reactions with dppe and dppp.
Aggregation of the paramagnetic nickel(I) centers as a result of
the short methylene linkages in dppe and dppp may enable
facile electron transfer from one of the nickel(I) atoms to the
other, generating nickel(0) and nickel(II) species, whereas the
longer chain in dppb makes electron transfer slower. If electron
transfer from nickel(I) to nickel(I) occurred through the
bonding interaction, the dimeric nickel(I) complex 1 would
easily heterolytically split into nickel(0) and nickel(II) species;
however, this never occurs. Through-space electron transfer
between the two nickel(I) centers closely linked by the
bidentate ligands may be necessary in the disproportionation.
This is a rare and significant example of the electron transfer
rate being controlled by the distance between two monovalent
nickel centers (Ni−P−(CH2)3−P−Ni and Ni−P−(CH2)4−P−
Ni).15 Complex 2d was not used as a catalyst in the following
catalytic studies, because the complex 5c can be formed in situ
under the catalytic conditions.
RESULTS AND DISCUSSION
■
Preparation of Monomeric Nickel(I) NHC Complexes.
The IPr complexes 2 were synthesized from dimeric complex 1
and two-electron-donor ligands (Scheme 1). The P(OPh)3
analogue Ni(IPr)Cl(P(OPh)3) (2b) was readily isolated in 88%
yield as red-orange crystals. Addition of pyridine to a solution
of 1 afforded Ni(IPr)Cl(pyridine) (2c), in which only one
molecule of pyridine was coordinated to nickel, in 69% yield
upon recrystallization.
Bisphosphines also reacted with 1. However, the addition of
1,2-bis(diphenylphosphino)ethane (dppe) or 1,3-bis-
(diphenylphosphino)propane (dppp) gave not the expected
As noted above, the smaller IMes ligand did not provide the
analogue of the dimeric nickel(I) IPr complex 1, because the
bulky ligand, i.e., IPr, is probably desirable to stabilize the
B
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coordinatively unsaturated dimer complex kinetically. The
corresponding monomeric IMes analogues of 2 were therefore
synthesized using Whittlesey’s method.11 Ni(cod)2,
NiX2(PPh3)2 (X = Cl and Br), and IMes/HCl were mixed in
the presence of KN(SiMe3)2 as a base, resulting in in situ
generation of the free carbene, at room temperature. The
reaction efficiently afforded the expected Ni(IMes)X(PPh3)
(3a: X = Cl; 3b: X = Br) in 84% and 49% yields by
recrystallization (Scheme 3). These IMes analogues 3 were very
unstable in air, but the bromide complex 3b was more stable
than the chloride counterpart 3a.
slightly higher than the theoretical value and also of those of the
other complexes (Figure 1a). The ESR spectrum of 2c in
Scheme 3. Preparation of Mononickel(I) IMes Complexes
1
Broad paramagnetic signals were observed in the H NMR
spectra of the monomeric nickel(I) compounds 2b−d, 3a, and
3b at around δ 1−14, similar to those for 2a (see the SI). As
previously reported, an unpaired electron is delocalized only on
1
the nickel d-orbital, providing sharp solvent signals in the H
NMR spectra.4a DFT calculations also supported these results,
as described below.
When crystals of 2a and 2b were dissolved in benzene-d6, the
dimeric compound 1 was generated in situ with liberation of
1
the free ligand and was detected in the H NMR spectrum.
Addition of extra portions (2−5 equiv) of the ligand to the
solutions resulted in the disappearance of 1, indicating that
there is equilibrium in solution between the monomeric and
dimeric nickel(I) complexes in the presence of the donor
ligands. The relative ratios of 1:2a−c in equilibrium depended
on the ligand: pyridine (2c) > PPh3 (2a) > P(OPh)3 (2b).16
This trend is related to the π-acceptor abilities of these ligands,
as expected. In contrast to the IPr complexes 2, the diamagnetic
dimeric complexes [Ni(IMes)]2(μ-X)2 were not observed when
crystals of 3a and 3b were dissolved in benzene-d6. When 5
Figure 1. (a) χmolT versus temperature plots of 2c and (b) frozen
solution ESR spectrum of 2c at 10 K (−263 °C) in dilute toluene.
toluene at −263 °C showed three-axial anisotropy [gxx = 2.042,
gyy = 2.235, gzz = 2.452] (Figure 1b). The χmolT calculated from
these g values is 0.47, which is almost in agreement with that
from the SQUID results.
Figure 2 shows the crystal structures of 2b, 2c, 2d, 3a, and
3b. Representative bond lengths and angles around the nickel
atom, including those for 2a, are listed in Table 1. The sum of
the angles around the nickel atom was almost 360° in each
complex, indicating planar three-coordinate geometries. The
Ni−C(carbene) and Ni−Cl distances and the angles around the
nickel atom did not differ between 2 and 3, indicating that the
differences between the shapes and sizes of the NHC ligands
did not affect the structures around the nickel atom. On the
basis of the π-acceptor ability of the phosphorus ligand, the P−
Ni bond distance in 2b was 0.1 Å shorter than that in 2a. The
Cl−Ni−C(carbene) angle in 2c, which contains pyridine, is
142.1(1)°, which is much larger than those for the other
phosphine complexes, i.e., 131−134°. The Ni−N(pyridine)
bond distance was 2.200(3) Å, which is significantly longer than
the usual nickel−nitrogen σ-bond distances, i.e., ca. 2.0 Å.17
Delocalization of the unpaired electron in the Ni−N(pyridine)
σ*-orbital was negligible (2.5%) (calculated below); therefore
this elongation must be ascribed to other reasons. This weak
interaction may stretch the Cl−Ni−C(carbene) hinge.
1
equiv of pyridine was added to a solution of 2c, the H NMR
spectrum showed no sharp signals assignable to free pyridine,
suggesting fast equilibrium between coordination and elimi-
nation of pyridine molecules. Although we have no direct
information on the solution structure, we assume that
elimination of pyridine gives a 13-electron linear structure
without coordination of pyridine or formation of a
tetracoordinate 17-electron complex, Ni(IPr)Cl(pyridine)2,
with tetrahedral geometry around the nickel(I) center.
The spin states of the series of monomeric nickel(I)
complexes were investigated based on SQUID measurements
of 2b−d. It was previously reported that 2a has S = 1/2 (χmol
T
= 0.44 cm3 K mol−1 at −263 °C, where χmol is molar magnetic
susceptibility) and a three-coordinate Y-shaped 15-electron
structure.8 The spin quantum numbers of the other complexes
were also 1/2 [χmolT = 0.35 (2b), 0.52 (2c), 0.34 (2d, per
nickel atom) at −253 °C] (see the SI). Any magnetic
interaction between nickel(I) centers in the dinuclear complex
2d was not observed, even at −268 °C, because the distance
between the two nickel atoms, which was estimated from the
crystal structure (shown below), was too long (ca. 8.30 Å) for
Theoretical Studies. The distribution of the single-
electron-occupied molecular orbital (SOMO) was investigated
by performing single-point DFT calculations at the fixed
geometries given by the crystallographic coordinates of 2a−c
them to interact with each other. The theoretical value of χmol
T
is 0.375 cm3 K mol−1 when S = 1/2; therefore that of 2c is
C
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Figure 3. SOMOs (red and blue mesh; density isosurface value of 0.02
au) in (a) 2a, (b) 2b, (c) 2c, and (d) 3a obtained from single-point
DFT calculations at the B3LYP/6-31G(d,p) level using crystallo-
graphic coordinates (nickel, yellow; orange, phosphorus; purple,
chlorine; nitrogen, light blue; carbon, green).
and π-back-donation, compared with those in the Ni−P bond.
There were no conspicuous differences between the IPr
complex 2a and the corresponding IMes analogue 3a,
suggesting that different substituents on the NHC ligands do
not affect the electronic structures of these complexes.
Figure 2. ORTEP drawings of nickel(I) complexes: (a) 2b, (b) 2c, (c)
2d, (d) 3a, and (e) 3b (50% probability thermal ellipsoids). All
hydrogen atoms are omitted. Three THF molecules and four phenyl
rings on dppb in 2d and two THF molecules in 3a and 3b are also
omitted for clarity.
Catalytic Applications in Buchwald−Hartwig Amina-
tion and Suzuki−Miyaura Cross-Coupling Reactions.
The obtained monovalent nickel compounds were used in
catalytic cross-coupling reactions. Their catalytic activities were
compared by performing Buchwald−Hartwig aminations of
bromobenzophenone with diphenylamine, under reaction
conditions using 2a as the catalyst, similar to those previously
reported.8 Scheme 4 shows the reaction protocol and yields of
the triarylamine products. Excess amounts of the two-electron-
donor ligands [PPh3, P(OPh)3, and pyridine] were added to 5
mol % of 1 to generate the corresponding monomeric nickel(I)
complexes 2a, 2b, and 2c in situ. The substrates were then
added, and the mixture was stirred at 40 °C for 24 h in THF.
The products were isolated after silica gel column chromatog-
raphy. The ligand strongly affected the product yields, which
were 99% for PPh3, 39% for P(OPh)3, and 98% for pyridine.
The smallest ligand, i.e., pyridine, did not inhibit interactions of
and 3a with the B3LYP functional and 6-31G(d,p) basis set
(Figure 3). In complex 2a, the unpaired electron is localized
mainly on the nickel d-orbital (51.4%) and the chlorine−nickel
π*-orbital (11.9%). It also forms a phosphorus−nickel σ*-bond
(12.4%).4c The distribution in 2b was similar to that in 2a:
nickel d-orbital (47.8%), Ni−Cl π*-orbital (13.8%), and Ni−P
σ-orbital (12.3%). In contrast, in 2c, the unpaired electron is
located mainly on nickel (80.7%), and the values for the Ni−Cl
and Ni−N bonds are 4.5% and 2.5%, respectively. Generally,
the nonbonding orbital energy of pyridine is lower than that of
phosphorus; therefore the energy gap between nickel and
nitrogen is larger than that between nickel and phosphorus,
weakening the nickel−pyridine interactions, including donation
Table 1. Representative Bond Distances and Angles for 2a, 2b, 2c, 2d, 3a, and 3b
2a (Y = P)
2b (Y = P)
2c (Y = N)
2d (Y = P)
3a (X = Cl)
3b (X = Br)
Bond Lengths (Å)
1.909(4)
Ni(1)−C(1)
Ni(1)−X(1)
Ni(1)−Y(1)
1.930(3)
2.1786(9)
2.201(1)
1.928(3)
2.1545(9)
2.1170(6)
1.937(2), 1.939(2)
2.1973(7), 2.1994(7)
2.2042(6), 2.2006(6)
1.938(4)
2.188(1)
2.205(1)
1.949(3)
2.3178(6)
2.2051(9)
2.201(1)
2.200(3)
Bond Angles (deg)
142.1(1)
X(1)−Ni(1)−C(1)
Y(1)−Ni(1)−C(1)
X(1)−Ni(1)−Y(1)
134.2(1)
112.1(1)
113.31(4)
359.5(1)
134.29(6)
109.41(6)
116.30(3)
360.00(6)
132.29(6), 136.99(6)
112.79(6), 111.71(6)
114.37(3), 110.72(3)
359.45(6), 359.42(6)
132.8(1)
111.6(1)
115.31(5)
359.7(1)
131.03(9)
112.76(9)
115.95(3)
359.74(9)
107.2(1)
110.73(9)
∑
Ni
360.0(1)
D
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Scheme 4. Triarylamine Formation via Buchwald−Hartwig
Amination
CONCLUSION
■
In summary, we have developed synthetic methods for a series
of monovalent nickel complexes bearing mixed ligands, namely,
bulky NHCs and other two-electron-donor ligands. The
complex structures were determined using SQUID, ESR, and
X-ray crystallography. The chemical structures of the two-
electron-donor ligands affected the stabilities, SOMO distribu-
tions, and catalytic activities of the corresponding nickel(I)
complexes. Pyridine weakly coordinates to nickel(I) and is
easily liberated to form the dimeric complex, whereas P(OPh)3
strongly binds to nickel(I) and stabilizes the monomeric form.
The monomeric complexes Ni(IPr)Cl(L) were not thermally
stable when diphosphines bridged with a three- or two-carbon
unit, e.g., dppp or dppe, were introduced, and mixtures of
zerovalent and divalent nickel complexes were obtained,
probably because of easy electron transfer and subsequent
disproportionation. However, a dppb complex of nickel(I) was
successfully isolated and characterized, although it was slowly
transformed into a mixture of nickel(0) and nickel(II) species.
The bulky NHCs IPr and IMes do not affect the electronic
structures of the monomeric nickel(I) complexes, but the
dimeric IPr complex [Ni(IPr)]2(μ-Cl)2 is much more stable
than its IMes analogue. Monovalent nickel complexes bearing
both IPr and IMes ligands are active in cross-coupling reactions
of aryl bromides. Buchwald−Hartwig aminations using the
mononickel(I) IPr complexes as catalysts proceeded efficiently
to yield triarylamines. The IPr complexes did not catalyze the
Suzuki cross-coupling reactions of aryl halides, although the
active IMes analogues did. These small differences derived from
the NHC ligand structures help precisely control catalytic
processes. Mechanistic studies of these cross-coupling reactions
using monovalent nickel complexes and the development of air-
stable nickel(I) catalyst precursors are now in progress.
the substrates with nickel. Pyridine can be easily eliminated
from 2c to generate the active form 1, whereas little P(OPh)3 is
eliminated from 2b, preventing generation of the active species.
Interestingly, PPh3 adequately controls the concentration of 1
and stabilizes the unstable active species to form the
monomeric complex as the resting state.
In our preliminary research, a series of IPr complexes of
nickel(I) were inactive in the Suzuki cross-coupling reactions of
aryl halides. In general, the Suzuki cross-coupling reaction does
not require steric hindrance derived from bulky ligands on
metal centers,18 in contrast to Buchwald−Hartwig amination,
which does require bulky ligands.19 In this case, the bulkiness of
the ligand is critical in inhibiting the reaction: the IMes
complex is catalytically active, but the IPr complex is not. Louie
et al. reported monovalent-nickel-catalyzed Suzuki coupling of
aryl halides using a bis(IMes) complex of nickel(I), [NiCl-
(IMes)2].4b Here, we used the analogous complex 3a,
containing PPh3 instead of IMes.
We used an activated aryl bromide, 4-bromobenzophenone,
in cross-coupling with phenylboronic acid in the presence of
the monovalent nickel chloride complex 3a (10 mol %) (Table
2). Base screening showed that using NaOtBu and KOtBu
Table 2. Suzuki−Miyaura Cross-Coupling Reactions of Aryl
Bromides with Phenylboronic Acid
EXPERIMENTAL SECTION
■
General Procedures. All experiments were performed in an inert
gas atmosphere using standard Schlenk techniques and a glovebox
(MBraun UNIlab), unless otherwise stated. THF, hexane, toluene, and
benzene-d6 were distilled from benzophenone ketyl and stored in a
nitrogen atmosphere with 4A molecular sieves. Chloroform-d was
distilled from CaH2 and stored in a nitrogen atmosphere. Other
reagents were used as received or distilled just before use if possible.
1H NMR spectra were obtained at room temperature using a Varian
Mercury Y Plus 400 MHz spectrometer. Chemical shifts (δ) were
recorded in parts per million from the solvent signal. GC-MS was
performed using an Agilent 6890N gas chromatograph coupled with a
JEOL JMS-GC mate II mass spectrometer. A 60 m InertCap 1 column
(0.25 mm i.d., 0.25 μm film thickness) was used, and the injection
temperature was 270 °C. Elemental analysis was performed using a
Yanaco CHN Corder MT-5 autosampler. CW X-band ESR spectra
were measured by a Bruker E500 ESR spectrometer. The measure-
ment temperature was controlled by an Oxford ESR900 cryostat and
an ITC503 temperature controller in the temperature range from 4 to
296 K. The g value was calibrated using a NMR teslameter. The
magnetic properties of the materials were investigated using a
Quantum Design MPMS-5S SQUID magnetometer. Column
chromatography of organic products was performed using silica gel
(Kanto Kagaku, silica gel 60 N, spherical, neutral). The NHC was
prepared from the imidazolium salts using the published method.20
The monovalent dinuclear nickel IPr complex 1 was prepared as
described in the literature.21
a
entry
R−
base
solvent
yield/%
1
2
3
4
5
6
7
8
PhCO−
PhCO−
PhCO−
PhCO−
PhCO−
PhCO−
PhCO−
CH3O−
NaOtBu
KOtBu
toluene
toluene
toluene
toluene
toluene
THF
54
65
66
79
88
68
72
45
K3PO4
K2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
CPME
toluene
a
The yields were determined with GC-MS. Calibration was carried out
using a standard sample of 4,4′-dimethoxybiphenyl.
(entries 1 and 2) and potassium phosphonate and carbonate
(entries 3 and 4) provided moderate to good yields (54−79%)
of the cross-coupling products. Cesium carbonate was the best
base and afforded the product in 88% yield (entry 5). Toluene
was a better solvent than THF and cyclopentyl methyl ether
(CPME) (entries 6 and 7). When the less active bromoanisole
was used as the substrate, the yield after 4 h was moderate
(45%; entry 8).4b
[NiCl(P(OPh)3)(IPr)] (2b). In a glovebox, [(μ-Cl)(IPr)Ni]2 (1; 0.03
mmol, 30 mg), P(OPh)3 (0.06 mmol, 15 μL), and THF (0.5 mL) were
added to a 5 mL screw-capped tube. After the compounds had
dissolved, hexane (1.5 mL) was slowly added and the solution was
E
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cooled to −30 °C. Colorless crystals of 2b were obtained after removal
X-ray Crystallography of 2b−d, 3a, and 3b. Single crystals of
2b−d and 3a,b for X-ray diffraction were grown at −30 °C from the
THF/hexane solutions. The data for 2b−d and 3a,b were obtained at
123 K using a Rigaku Saturn CCD diffractometer with a confocal
mirror and graphite-monochromated Mo Kα radiation (λ = 0.710 70
Å). Data reduction of the measured reflections was performed using
the software package CrystalStructure.22 The structures were solved by
direct methods (SIR2008)23 and refined by full-matrix least-squares
fitting based on F2, using the program SHELXL 97-2, PC version.24 All
non-hydrogen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms were located at ideal positions and
included in the refinement, but were restricted to riding on the atom
to which they were bonded. CCDC 1472756−1472760 contain the
supplementary crystallographic data for this paper. A copy of the data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/cgi-bin/
catreqcgi.
DFT Calculations. All DFT calculations were performed using the
Gaussian 09 package.25 The B3LYP functional was used, with a
standard split-valence basis set, 6-31G(d,p). The single-point
calculations used to obtain SOMOs of 2a, 2b, 2c, and 3a were
performed using crystallographic coordinates without geometry
optimization and with a tight self-consistent field convergence
criterion. All computations were performed using the computer
facilities at the Research Institute for Information Technology, Kyushu
University.
of the liquid and washing with a small amount of hexane (85 mg, 88%
1
yield). H NMR (C6D6): δ 1.62 (bs), 4.13 (bs), 6.94 (bs), 7.68 (bs),
8.09−8.31 (bs), 10.44 (bs). Anal. Calcd (%) for C45H51N2O3PClNi: C,
68.16; H, 6.48; N, 3.53. Found: C, 68.63; H, 6.59; N, 3.57.
[NiCl(pyridine)(IPr)] (2c). In a glovebox, [(μ-Cl)(IPr)Ni]2 (1;
0.03 mmol, 30 mg), pyridine (0.12 mmol, 9.6 μL), and THF (0.5 mL)
were added to a 5 mL screw-capped tube. After the compounds had
dissolved, hexane (1.5 mL) was slowly added and the solution was
cooled to −30 °C. Vermillion crystals of 2c were obtained after
removal of the liquid and washing with a small amount of hexane (24
1
mg, 69% yield). H NMR (C6D6): δ 1.18 (bs), 1.75 (bs), 2.48 (bs),
3.08 (bs), 6.37 (bs), 6.95 (bs), 10.7 (bs). Anal. Calcd (%) for
C32H41N3ClNi: C, 68.41; H, 7.36; N, 7.48. Found: C, 68.33; H, 7.32;
N, 7.38.
[NiCl(IPr)]2(dppb) (2d). In a glovebox, [(μ-Cl)(IPr)Ni]2 (1; 0.05
mmol, 48 mg), dppb (0.05 mmol, 21 mg), and THF (0.5 mL) were
added to a 5 mL screw-capped tube. After the compounds had
dissolved, hexane (1.5 mL) was slowly added and the solution was
cooled to −30 °C. Yellow crystals of 2d were obtained after removal of
the liquid and washing with a small amount of hexane (63 mg, 91%
yield). 1H NMR (C6D6): δ 1.75 (bs), 4.55 (bs), 8.52 (bs), 10.78 (bs).
Anal. Calcd (%) for C45H51N2O3PClNi: C, 70.76; H, 7.24; N, 4.03.
Found: C, 70.06; H, 7.33; N, 3.77.
[NiCl(PPh3)(IMes)] (3a).
A solution of 1,3-bis(2,4,6-
trimethylphenyl)imidazolium chloride [IMes/HCl] (428 mg, 1.26
mmol) and KN(SiMe3)2 (253 mg, 1.26 mmol) in THF (20 mL) was
stirred for 3 h. The mixture was filtered through Celite and added to a
mixture of Ni(cod)2 (158 mg, 0.57 mmol) and Ni(PPh3)Cl2 (374 mg,
0.57 mmol). The mixture was stirred at room temperature overnight
to afford a dark yellow solution. The yellow solution was filtered
through Celite, and the solvent was removed under vacuum. The
yellow solid was recrystallized from toluene/hexane. Yield: 634 mg
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00419.
■
S
Synthetic procedures, 1H NMR spectra, elemental
analysis, and SQUID for nickel complexes 2b−d and
3a,b (PDF)
1
(84%). H NMR (C6D6): δ 1.44 (bs), 3.24 (bs), 4.46 (bs), 8.26 (bs),
10.99 (bs). Anal. Calcd (%) for C39H39ClN2NiP: C, 70.88; H, 5.95; N,
4.24. Found: C, 70.52; H, 5.97; N, 4.31.
[NiBr(PPh3)(IMes)] (3b).
A solution of 1,3-bis(2,4,6-
AUTHOR INFORMATION
Corresponding Author
*E-mail (K. Matsubara): kmatsuba@fukuoka-u.ac.jp.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript. Y.Y. conducted theoretical calculations, S.K.
measured SQUID, and T.N. measured temperature-controlled
ESR of the nickel(I) complexes. The other authors contributed
equally.
trimethylphenyl)imidazolium chloride [IMes/HCl] (427 mg, 1.26
mmol) and KN(SiMe3)2 (256 mg, 1.26 mmol) in THF (20 mL) was
stirred for 3 h. The mixture was filtered through Celite and added to a
mixture of Ni(cod)2 (174 mg, 0.63 mmol) and Ni(PPh3)2Br2 (461 mg,
0.63 mmol). The mixture was stirred at room temperature overnight
to afford a dark yellow solution. The yellow solution was filtered
through Celite, and the solvent was removed under vacuum. The
yellow solid was recrystallized from THF/hexane. Yield: 440 mg
(49%). 1H NMR (C6D6): δ 1.51 (bs), 3.20 (bs), 4.35 (bs), 8.23 (d, J =
8.6 Hz, 4H), 10.94 (d, J = 8.3 Hz, 4H). Anal. Calcd (%) for
C47H56BrN2NiO2P: C, 66.37; H, 6.64; N, 3.29; Found: C, 66.53; H,
6.41; N 3.30.
Triarylamine Synthesis via Buchwald−Hartwig Amination.
In a typical example, 1 (24.1 mg, 25 μmol) and ligand (300 μmol), di-
p-(tolyl)amine (118.4 mg, 0.6 mmol), NaOtBu (71.1 mg, 0.74 mmol),
4-bromobenzophenone (130.6 mg, 0.5 mmol), and THF (0.2 mL)
were mixed and stirred at 40 °C for 24 h. After addition of water, the
organic layer was extracted three times with CH2Cl2. The combined
organic layers were washed with saturated NaCl(aq), dried with
Na2SO4, filtered, and concentrated in vacuo. The residue was purified
by short column chromatography, with ethyl acetate/hexane (1:10) as
the eluent, to give [4-[bis(4-methylphenyl)amino]phenyl]-
phenylmethanone as a yellow oil. Yields: PPh3 (186.9 mg, 99%),
pyridine (184.9 mg, 98%), P(OPh)3 (73.6 mg, 39%).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Japan Society for the
Promotion of Science (Grant-in-Aid for Scientific Research
22550104 and 25410123).
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DOI: 10.1021/acs.organomet.6b00419
Organometallics XXXX, XXX, XXX−XXX