One-pot, single-step, and gram-scale synthesis of mononuclear [(η6-arene)Ni(N-heterocyclic carbene)] complexes: Useful precursors of the Ni0-NHC unit

Article abstract of DOI:10.1021/om500088p

Recently, the combination of Ni(0) and N-heterocyclic carbenes (NHCs) has been paid special attention because of its high reactivity toward bond-forming reactions and the activation of unreactive bonds. Thus, it would be very worthwhile to provide a highly reactive, easily accessible, and versatile Ni⁰-NHC precursor. Herein we report a one-pot, single-step, and gram-scale method for the synthesis of [(η⁶-arene)Ni(NHC)] complexes from commercially available Ni(cod)₂ and NHC (or its salt) via hydrogenation of COD. The structure and bonding situations of [(η⁶-arene)Ni(NHC)]s were evaluated by NMR, X-ray, and DFT studies. An application to the synthesis of Ni⁰-NHC complexes via ligand exchange reactions is also demonstrated.

Full text of DOI:10.1021/om500088p

Article
pubs.acs.org/Organometallics
One-Pot, Single-Step, and Gram-Scale Synthesis of Mononuclear
[(η6‑arene)Ni(N-heterocyclic carbene)] Complexes: Useful Precursors
of the Ni0−NHC Unit
Yoichi Hoshimoto,†,‡ Yukari Hayashi,† Haruka Suzuki,† Masato Ohashi,† and Sensuke Ogoshi*,†,§
†Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
‡Frontier Research Base for Young Researchers, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
§JST, ACT-C, Suita, Osaka 565-0871, Japan
S
* Supporting Information
ABSTRACT: Recently, the combination of Ni(0) and N-heterocyclic carbenes (NHCs) has
been paid special attention because of its high reactivity toward bond-forming reactions and
the activation of unreactive bonds. Thus, it would be very worthwhile to provide a highly
reactive, easily accessible, and versatile Ni0−NHC precursor. Herein we report a one-pot,
single-step, and gram-scale method for the synthesis of [(η6-arene)Ni(NHC)] complexes
from commercially available Ni(cod)2 and NHC (or its salt) via hydrogenation of COD. The
structure and bonding situations of [(η6-arene)Ni(NHC)]s were evaluated by NMR, X-ray, and DFT studies. An application to
the synthesis of Ni0−NHC complexes via ligand exchange reactions is also demonstrated.
complexes is very intriguing, since the η6-coordinated arene
would be more labile and less noxious than olefins, carbon
monoxide, and other donor ligands. Therefore, developing a
practical method for the synthesis of ANCs would expand the
INTRODUCTION
■
Over the past couple of decades, a wide range of fields in
chemistry, including synthetic, organometallic, and inorganic,
have benefited from the development of the chemistry of N-
heterocyclic carbenes (NHCs). Because of their distinctive
electronic and steric properties, NHCs have been widely
employed as auxiliary ligands and as organocatalysts in the field
range of applications for them as catalyst precursors in organic
synthesis and as a source of Ni0−NHC units in organometallic
chemistry. Here we report the single-step, one-pot, and gram-
of organic synthesis.1 As a part of these pursuits, the
scale synthesis of a series of ANCs. We found that the
hydrogenation of a mixture of Ni(cod)2 and NHC in an arene
combination of palladium and NHCs has been well establish-
ed.1a−c,2 In contrast, the application of NHCs toward nickel-
medium at room temperature gave an ANC in nearly
quantitative yield (Scheme 1c). We also provide the first
crystallographic structure of a monomeric ANC with the solid
evidence of the η6 coordination mode of an arene ring. In
addition, the applications of ANCs as the source for a Ni0−
NHC unit are demonstrated.
catalyzed/-mediated reactions has been less developed,
although nickel is a more inexpensive and ubiquitous
element.1a−c,3,4 Thus, it would be worthwhile to provide a
readily accessible, highly reactive, and widely applicable Ni0−
NHC source from commercially available Ni(cod)2, where
COD is 1,5-cyclooctadiene.
A variety of Ni0−NHC complexes have been synthesized to
date, such as Ni(CO)3(NHC), bis(η2-alkene)Ni(NHC), (η2:η2-
diene)Ni(NHC), and [Ni(NHC)2].5 These complexes have
been used as a source of Ni0−NHC reactive species; however,
difficulties in preparation and ligand substitution may make
chemists hesitant to use them. In contrast to these well-known
Ni0−NHC complexes, less is known about (η6-arene)nickel(N-
heterocyclic carbene) (ANC) complexes.6−8 Two examples of
ANC complexes have been reported (Scheme 1). One is
[Ni(IPr)]2, in which one of the 2,6-iPr-C6H3 rings in the Ni0−
IPr moiety coordinates with the other Ni(0) atom to form a
dimeric structure.7 The complex was synthesized from
Ni(cod)2 and Ni(dme)Cl2 in 19% overall yields (Scheme 1a).
The other is (η6-toluene)Ni(IPr*) (TNI*), which was
synthesized via a simple and practical method from Ni(dme)Cl2
in 56% yield (Scheme 1b).8 The formation of TNI* was
confirmed by elementary analysis. The reactivity of these ANC
RESULTS AND DISCUSSION
■
During the course of the present research on Ni0−NHC
catalysis,9,10 we found the generation of (η6-benzene-d6)Ni-
(IPr) (BNI-d6) by mixing Ni(cod)2 and IPr in benzene-d6 at
room temperature (Scheme 2). After 31 h, the concomitant
formation of Ni(IPr)2 (9%) was also observed, while the major
product was BNI-d6 (72%). Neither BNI-d6 nor Ni(IPr)2 was
obtained as a single product, even when the reaction time was
prolonged for several days, which indicates that BNI-d6 and
Ni(IPr)2 exist in equilibrium with Ni(cod)2, IPr, and COD in
benzene-d6.11 In order to prepare BNI-d6 selectively, it is
important to remove COD before the formation of Ni(IPr)2.
However, an attempt to isolate BNI-d6 via the evaporation of
Received: January 24, 2014
Published: February 26, 2014
© 2014 American Chemical Society
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a
Scheme 1. Reported Synthetic Methods of ANCs
a
Abbreviations: COD, 1,5-cyclooctadiene; DME, dimethoxyethane; EA, elemental analysis; COA, cyclooctane.
a b
,
Scheme 2. Generation of BNI-d6
a
b
Conversion of IPr: 56% (5 min); 90% (31 h). Evaporation was conducted repeatedly within 10 min after mixing starting materials in C6H6, and
1
the yield of products was determined by H NMR in C6D6.
COD failed to give an inseparable mixture of Ni(cod)2, IPr,
BNI-d6, and Ni(IPr)2.
situ preparation of TNI was successfully achieved by the
reaction of Ni(cod)2 (0.080 mmol), IPr·HCl (1.1 equiv),
tBuOK (1.0 equiv), and H2 (1 atm) in toluene for 30 min,
which was confirmed by 1H NMR in C6D6 through
transformation into BNI-d6 (Scheme 3b). In this case, the
formation of cyclooctene (COE) was observed while BNI-d6
was formed as a single nickel complex in an arene medium.
Thus, this method would be suitable for the generation of
ANCs in situ prior to the Ni0−NHC catalysis or organonickel
complex synthesis. Under these reaction conditions, the
reduction of COD to COA was complete within 30 h. The
preparation of (η6-arene)Ni(IMes) failed under the present
hydrogenation conditions, and the formation of Ni(IMes)2 was
observed as a major product.
Next, we tried the hydrogenation of COD to cyclooctane
(COA), a compound that would no longer have the ability to
coordinate to Ni(0).12 The treatment of a toluene solution of
Ni(cod)2 and IPr with H2 (8 atm) at room temperature
resulted in the formation of (η6-toluene)Ni(IPr) (TNI) as a
single product via the hydrogenation of COD to COA. The
reaction mixture was filtered through a Celite followed by
concentration in vacuo to give TNI as a reddish brown solid in
97% isolated yield with >99% purity. This simple operation can
afford TNI on a 2.00 mmol scale (>1.00 g). (η6-Toluene)Ni-
(SIPr) (TNSI), (η6-toluene)Ni(IPr*) (TNI*), and their
benzene-coordinated complexes (BNI, BNSI, and BNI*)
were also prepared (Scheme 3a). The gram-scale preparation
of TNI and TNSI would be particularly noteworthy, since IPr
and SIPr are one of the most frequently employed NHCs to
date, and they are commercially available.1,4 Furthermore, an in
The molecular structure of BNI* was unambiguously
determined by X-ray crystallography, which clearly shows that
the coordination mode of the benzene ring is η6 (Figure 1).13
The distance between Ni and the η6-benzene ring is 1.588(4)
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not observed in the 1H NMR spectra measured in THF-d8 from
−60 to −5 °C.13 The coordination of the arene ring in all of the
complexes presented in Scheme 3, with the exception of BNI*,
was validated by 1H and 13C NMR in cyclohexane-d12 at 25 °C.
For example, resonances of the arene moiety of the coordinated
toluene in TNI were observed at δH 5.07 (1H) and 5.17−5.22
Scheme 3. Single-Step, One-Pot, Gram-Scale, and Selective
Synthesis of ANCs via Hydrogenation of COD: (a)
Preparation on 2.00 mmol Scale with Isolated Yields; (b) In
a b
,
Situ Preparation on 0.08 mmol Scale
1
(4H) and δC 88.5, 89.9, 90.7, and 101.6 in the H and 13C
NMR spectra, which were shifted upfield in comparison with
those of noncoordinated toluene (δH 7.1−7.3 and δC 125.8,
128.7, 129.4, 137.9). These results are consistent with the
results of the X-ray analysis.
In order to gain insight into the bonding situation in BNI, a
DFT calculation was conducted.13 The highest occupied
molecular orbital (HOMO) of BNI consists of two degenerate
orbitals (−3.67 eV), which involve both contribution of the
back-donation from the electron-rich Ni(0) center to IPr
(Figure 2a, left) and the donation from IPr to the Ni(0) center
a
b
0.080 mmol scale. Determined by 1H NMR in C6D6 via
transformation into BNI-d6. Isolated yield is given in parentheses.
Figure 2. Selected calculated molecular orbitals of BNI: (a) HOMOs;
(b) HOMO-2 (left) and HOMO-3 (right).
(Figure 2a, right). The energy levels of HOMO-1, HOMO-2,
and HOMO-3 are −3.80, −4.35, and −4.37 eV, respectively.
Although HOMO-1 is antibonding, HOMO-2 and HOMO-3,
both of which would be almost degenerate, show the back-
donation from the Ni(0) to the π* orbital of benzene (Figure
2b). The LUMO was calculated to consist mainly of N-
diisopropylphenyl moieties on IPr (−0.05 eV; Figure S11,
Supporting Information).
The equilibrium constants of the exchange reaction between
the coordinated toluene in TNI and arenes were determined by
means of NMR in cyclohexane-d12, showing that a more
electron poor arene ring can strongly coordinate to the Ni0−IPr
unit, since such an arene can efficiently accept back-donation
from the electron-rich Ni(0) center (Figure S6, Supporting
Information).13,14
An interconversion between mononuclear TNI and binuclear
[Ni(IPr)]2 was examined (Scheme 4). The conversion of TNI
into [Ni(IPr)]2 took place in THF at room temperature, and
[Ni(IPr)]2 was isolated in 77% yield after 48 h. In addition,
[Ni(IPr)]2 was reversibly converted into TNI quantitatively in
toluene at 40 °C after 2 days.15 The preparation of bis(η2-
PhCHCH2)Ni(IPr),5k [Ni(dppe)(IPr)],5l [Ni(IPr)2],5n [(η2-
quinoline)Ni(IPr)]2 (1), and [Ni(IMes)(IPr)] (2) was also
demonstrated via a ligand substitution reaction with TNI.
Figure 1. Molecular structure of BNI* (ellipsoids set at 30%
probability). Hydrogen atoms are omitted for clarity. Selected bond
distances (Å): Ni−C6H6(centroid of the ring) 1.588(4), Ni−C1
2.123(8), Ni−C2 2.128(8), Ni−C3 2.15(1), Ni−C4 2.117(8), Ni−C5
2.120(6), Ni−C6 2.132(9), Ni−C7 1.836(6).
Å, which is almost the same value as that observed in (η6-
benzene)Ni(bis(di-tert-butylphosphino)methane) (1.606(5)
Å)6c and (η6-toluene)Ni(RH Si), (RH = 1,1,4,4-tetrakis-
2
2
(trimethylsilyl)butane-1,4-diyl substituent) (1.617(1) Å).6a
The angle C7−Ni−(centroid of the coordinated benzene
ring) is 177°, which also supports its η6 coordination. To the
best of our knowledge, this is the first crystallographic
identification of a monomeric ANC. This result supports the
proposed structure of TNI*, as reported by Hillhouse and
Cundari et al.8
In the 1H and 13C NMR spectra measured in THF-d8 at −60
°C, resonances of the coordinated benzene of BNI* were
observed at δH 5.26 and δC 89.9, respectively. These chemical
shifts were shifted upfield in comparison with those of
noncoordinated benzene (δH 7.92 and δC 129.0). Moreover,
they were comparable to those of the structurally well-defined
(η6-benzene)Ni(bis(di-tert-butylphosphino)methane) (δH 5.95
and δC 92.0 in THF-d8 at −60 °C).6c A significant change was
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a
Scheme 4. Preparation of Ni0−IPr Complexes from TNI
knowledge, this is the first example of a heteroleptic bis-NHC
Ni(0) complex.19
a
The reaction was carried out in n-pentane at room temperature, and
quantitative formation of the products was observed within 30 min
except for the interconversion between TNI and [Ni(IPr)]2.
These reactions were complete within 30 min in n-pentane at
room temperature to afford the products quantitatively and also
took place in arene mediums such as benzene and toluene. The
quantitative formation of 1 is especially noteworthy, because
the selective preparation of 1 from Ni(cod)2 is quite
challenging. In fact, 1 was generated as an inseparable
equilibrium mixture of 1, Ni(cod)2, IPr, quinoline, and COD
in the reaction of quinoline, Ni(cod)2, and IPr. The structure of
1 was unambiguously determined by NMR and X-ray analyses.
As shown in Figure 3, 1 forms a dimeric structure with a 16-
Figure 4. Molecular structure of 2 (ellipsoids set at 30% probability).
Hydrogen atoms are omitted for clarity. Selected bond distances (Å):
Ni−C1 1.815(8), Ni−C2 1.830(9). Selected bond angle (deg): C1−
Ni−C2 179.6(3).
CONCLUSION
■
In conclusion, a practical method for the synthesis of (η6-
arene)Ni(N-heterocyclic carbene) (ANC) complexes was
developed. ANCs having IPr and SIPr can be prepared on a
gram scale from commercially available Ni(cod)2 via simple,
single-step, and one-pot procedures. The key feature of the
present method is the hydrogenation of 1,5-cyclooctadiene.
The products were fully characterized by NMR, which would
support the coordination of arene to a Ni0−NHC unit.
Moreover, the first crystallographic structure of a monomeric
ANC is reported, providing solid evidence for the η6
coordination of the arene. A study of the equilibrium between
the coordinated toluene in TNI and the arenes in a ligand
exchange reaction showed that a (η6-arene)Ni(IPr) with a more
electron-poor arene would be thermodynamically favored. The
preparation of some Ni0−IPr complexes was demonstrated via
a ligand substitution reaction from TNI. In particular, the
selective preparation of [(η2-quinoline)Ni(IPr)]2, in which the
C7−C8 double bond coordinates to the nickel center, and
[Ni(IPr)(IMes)], a Ni(0) complex having two different NHCs,
was achieved for the first time. ANCs could be very useful
precursors of a Ni0−NHC unit in the field of organic and
organometallic chemistry. Thus, we hope the present work and
the methodology will lead to further developments in these
fields of chemistry.
Figure 3. Molecular structure of 1 (ellipsoids set at 30% probability).
Hydrogen atoms and iPr groups are omitted for clarity. Selected bond
distances (Å): Ni−N1 1.973(3), Ni−C7* 1.981(3), Ni−C8* 2.009(3),
Ni−C10 1.887(3), C7−C8 1.445(5).
electron nickel center in which the nitrogen atom of the
quinoline ligand coordinates to the Ni0−IPr fragment and one
of the CC bonds (C7−C8) coordinates to the other Ni. The
C7−C8 bond distance is elongated to 1.445(5) Å in
comparison with that of noncoordinated quinoline (1.36−
EXPERIMENTAL SECTION
General Considerations. All manipulations were conducted
■
under a nitrogen atmosphere by using standard Schlenk or drybox
1.38 Å).16 In addition, both the H and 13C NMR spectra of 1
1
1
techniques. H, 13C, 31P, and 19F NMR spectra were recorded on a
clearly indicate the η2 coordination of the C7−C8 double bond
to the Ni(0) center, as shown by the upfield shift of those
signals (δH 3.62 and 3.68, and δC 45.9 and 52.9, respectively).
These results indicate that the elongation of the C7−C8 bond
in quinoline is due to the strong back-donation from the Ni0−
IPr unit. A variety of coordination modes of quinoline to
transition metals have been reported to date;17 however, the
direct observation of the η2 coordination of its benzene moiety
is quite rare.18 The heteroleptic bis-NHC Ni(0) complex 2 was
also identified by X-ray analysis (Figure 4). To the best of our
JEOL AL-400, Bruker DPX 400, or Bruker AVANCE III spectrometer
at 25 °C unless otherwise stated. The chemical shifts in the H NMR
1
spectra were recorded relative to Me4Si or residual protonated solvent
(C6D5H (δ 7.16), C7D7H (δ 2.09), THF-d7 (δ 1.72), or C6D11H (δ
1.38)). The chemical shifts in the 13C spectra were recorded relative to
Me4Si or deuterated solvent (C6D6 (δ 128.06), C7D8 (δ 137.48),
THF-d8 (δ 67.21), or C6D12 (δ 26.43)). The chemical shifts in the 31
P
NMR spectra were recorded using 85% H3PO4 as external standard.
The chemical shifts in the 19F NMR spectra were recorded relative to
α,α,α-trifluorotoluene (δ −65.64). Elementary analyses were per-
formed at the Instrumental Analysis Center, Faculty of Engineering,
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Osaka University. For some compounds, accurate elemental analysis
was precluded by extreme air or thermal sensitivity and/or systematic
problems with elemental analysis of organometallic compounds. X-ray
crystal data were collected with a Rigaku RAXIS-RAPID imaging plate
diffractometer.
Materials. Benzene, toluene, hexane, pentane, cyclohexane,
benzene-d6, toluene-d8, THF-d8, and cyclohexane-d12 were distilled
from sodium benzophenone ketyl prior to use. All commercially
available reagents were distilled over CaH2 under reduced pressure
prior to use. N-Heterocyclic carbenes (NHCs) were furnished by the
known procedures.10
mmol), and benzene (20 mL), and the reaction was conducted at
room temperature for 16 h to give BNSI (1.06 g, 2.00 mmol, 100%) as
a reddish brown solid. 1H NMR (400 MHz, C6D12): δ 7.21 (t, J = 7.6
Hz, 2H, Ar-H), 7.11 (d, J = 7.6 Hz, 4H, Ar-H), 5.19 (s, 6H, η6-C6H6),
3.64 (s, 4H, NC2H4N), 3.12−3.05 (m, 4H, CH(CH3)2), 1.37 (d, J =
6.8 Hz, 12H, CH(CH3)2, partially overlapped with C6D11H), 1.18 (d, J
= 6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR (100 MHz, C6D12): δ
214.2, 148.2, 140.4, 128.0, 123.9, 90.2 (η6-C6H6), 52.5, 29.3, 25.3, 24.4.
Anal. Calcd for C33H44N2Ni: C, 75.15; H, 8.41; N, 5.31. Found: C,
74.61; H, 8.61; N, 5.69.
Synthesis of (η6-toluene)Ni(IPr*) (TNI*). A reaction tube was
charged with Ni(cod)2 (22.2 mg, 0.08 mmol), IPr* (73.2 mg, 0.08
mmol), and toluene (15 mL). The resulting solution was transferred
into an autoclave reactor, and then H2 was pressurized at 8 atm. After
the reaction mixture was stirred at room temperature for 12 h, the
resultant brown solution was quickly filtered, and all volatiles were
removed under reduced pressure to give TNI* (82.8 mg, 0.079 mmol,
99%) as a reddish brown solid. Identification of TNI* was conducted
by 1H NMR in C6D6 via transformation into BNI*-d6, which is
identical with that previously reported.8
General Procedures for Synthesis of [(η6-arene)Ni(N-hetero-
cyclic carbene)] (ANC) Complexes. Note: Employing well-dried and
-degassed solvents is essential to generate ANCs quantitatively.
Commercially available dehydrated solvents may cause a decrease of
purity of the products. A reaction tube was charged with Ni(cod)2 (2.0
mmol), NHC (2.0 mmol), and solvent (20 mL). The resulting
solution was transferred into an autoclave reactor, and then H2 was
pressurized quickly (recommended within 5 min) into the reactor at
5−8 atm. After the reaction mixture was stirred at room temperature
overnight (∼16 h), the resultant brown solution was filtered, and all
volatiles were removed under reduced pressure to give [(η6-
arene)Ni(NHC)] (ANC) as a reddish brown solid.
Synthesis of [(η6-benzene)Ni(IPr*)] (BNI*). A reaction tube was
charged with Ni(cod)2 (10.8 mg, 0.04 mmol), IPr* (36.2 mg, 0.04
mmol), and benzene (0.5 mL). The resulting solution was transferred
into an autoclave reactor, and then H2 was pressurized at 5 atm. After
the reaction mixture was stirred at room temperature for 3 h, the
resultant brown solution was quickly filtered, and all volatiles were
removed under reduced pressure to give BNI* (40.3 mg, 0.038 mmol,
99%) as a reddish brown solid. A single crystal suitable for X-ray
diffraction analysis was prepared by recrystallization from benzene/
Synthesis of (η6-toluene)Ni(IPr) (TNI). The general procedure was
followed with Ni(cod)2 (549 mg, 2.00 mmol), IPr (778 mg, 2.00
mmol), and toluene (20 mL), and the reaction was conducted at room
temperature for 16 h to give TNI (1.05 g, 1.94 mmol, 97%) as a
1
reddish brown solid. H NMR (400 MHz, C6D12): δ 7.27 (t, J = 7.4
Hz, 2H, Ar-H), 7.16 (d, J = 7.4 Hz, 4H, Ar-H), 6.58 (s, 2H, NC2H2N),
5.22−5.17 (m, 4H, η6-C6H5CH3), 5.07 (t, J = 5.4 Hz, 1H, η6-
C6H5CH3), 2.88−2.81 (m, 4H, CH(CH3)2), 1.53 (s, 3H, η6-
C6H5CH3), 1.31 (d, J = 6.8 Hz, 12 Hz, CH(CH3)2), 1.07 (d, J =
6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR (100 MHz, C6D12): δ 196.2,
147.0, 139.8, 128.6, 123.6, 119.6, 101.6 (η6-C6H5CH3), 90.7 (η6-
C6H5CH3), 89.9 (η6-C6H5CH3), 88.5 (η6-C6H5CH3), 29.1, 24.7, 23.7,
21.4 (η6-C6H5CH3). Anal. Calcd for C34H44N2Ni: C, 75.70; H, 8.22;
N, 5.19. Found: C, 75.27; H, 8.53; N, 5.51.
1
hexane at −35 °C. H NMR (400 MHz, THF-d8, −60 °C; Figure S3,
Supporting Information): δ 7.41−7.35 (m, 16H, Ar-H), 7.26 (t, J = 6.2
Hz, 4H, Ar-H), 7.08−7.01 (m, 16H, Ar-H), 6.82 (d, J = 7.2 Hz, 8H,
Ar-H), 5.40 (s, 4H, CHPh2), 5.26 (s, 6H, η6-C6H6), 4.96 (s, 2H,
NC2H2N), 2.29 (s, 6H, CH3). 13C{1H} NMR (100 MHz, THF-d8,
−60 °C): δ 194.2, 145.6, 144.8, 142.5, 138.7, 138.1, 131.2, 130.1,
129.7, 128.9, 128.8, 127.1, 126.8, 120.1, 89.9 (η6-C6H6), 52.2, 21.8.
Anal. Calcd for C75H62N2Ni: C, 85.79; H, 5.95; N, 2.67. Found: C,
85.42; H, 6.03; N, 3.05. Details on the results of X-ray analysis are
found in Figure S9 (Supporting Information).
Synthesis of (η6-benzene)Ni(IPr) (BNI). The general procedure was
followed with Ni(cod)2 (551 mg, 2.00 mmol), IPr (779 mg, 2.00
mmol), and benzene (20 mL), and the reaction was conducted at
room temperature for 16 h to give BNI (1.01 g, 1.92 mmol, 96%) as a
reddish brown solid. 1H NMR (400 MHz, C6D12; Figure S1a,
Supporting Information): δ 7.29 (t, J = 7.6 Hz, 2H, Ar-H), 7.16 (d, J =
7.6 Hz, 4H, Ar-H), 6.59 (s, 2H, NC2H2N), 5.22 (s, 6H, η6-C6H5),
2.84−2.77 (m, 4H, CH(CH3)2), 1.30 (d, J = 6.8 Hz, 12 Hz,
CH(CH3)2), 1.07 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR
(100 MHz, C6D12; Figure S1b, Supporting Information): δ 195.9,
147.1, 139.6, 128.8, 123.6, 119.6, 89.1 (η6-C6H6), 29.2, 24.7, 23.7. Anal.
Calcd for C33H42N2Ni: C, 75.44; H, 8.06; N, 5.33. Found: C, 75.25; H,
8.17; N, 5.90. The 1H NMR spectrum of BNI-d6 (400 MHz, C6D6) is
also shown in Figure S2 (Supporting Information). The quantitative
preparation of BNI was also possible by the reaction of TNI with
benzene (excess).
Synthesis of TNI via in Situ Generation of IPr from IPr·HCl. A
reaction tube was charged with IPr·HCl (37.4 mg, 0.088 mmol),
tBuOK (9.0 mg, 0.080 mmool), and toluene (10 mL), and then the
reaction mixture was stirred at room temperature for 10 min. Then,
Ni(cod)2 (22.0 mg, 0.080 mmol) was added to the reaction mixture.
The resulting solution was transferred into an autoclave reactor, and
the reaction mixture was stirred at room temperature for 30 min under
an H2 atmosphere (1 atm). The resultant brown solution was filtered,
and all volatiles were removed under reduced pressure. At this
moment, the quantitative formation of BNI-d6 and the concomitant
formation of cyclooctene were confirmed by 1H NMR in C6D6.
Azeotropic removal of COE several dozen times with toluene gave
TNI (41.8 mg, 0.077 mmol, 96%) as a reddish brown solid.
Conversion of TNI into (η2-PhCHCH2)2Ni(IPr). To a solution of
TNI (21.6 mg, 0.04 mmol) in n-pentane (5.0 mL) was added styrene
(9.4 μL, 0.08 mmol) at room temperature. After the reaction mixture
was stirred for 30 min, all volatiles were removed under reduced
pressure to give (η2-PhCHCH2)2Ni(IPr) as a dark green solid (28.4
mg, 0.04 mmol, 100%). Spectroscopic data of bis(η2-PhCH
CH2)Ni(IPr) were identical with those previously reported.5k
Conversion into [Ni(dppe)(IPr)]. To a solution of TNI (21.6 mg,
0.04 mmol) in n-pentane (5.0 mL) was added 1,3-bis-
(diphenylphosphino)ethane (DPPE) (15.9 mg, 0.04 mmol) at room
temperature. After the reaction mixture was stirred for 5 min, all
volatiles were removed under reduced pressure. The residue was
washed with cold n-pentane to give Ni(dppe)(IPr) as a reddish brown
solid (34.3 mg, 0.04 mmol, 100%). Spectroscopic data of Ni(dppe)-
(IPr) were identical with those previously reported.5l
Synthesis of (η6-toluene)Ni(SIPr) (TNSI). The general procedure
was followed with Ni(cod)2 (550 mg, 2.00 mmol), IPr (780 mg, 2.00
mmol), and toluene (20 mL), and the reaction was conducted at room
temperature for 16 h to give TNSI (1.07 g, 1.98 mmol, 99%) as a
1
reddish brown solid. H NMR (400 MHz, C6D12): δ 7.20 (t, J = 7.2
Hz, 2H, Ar-H), 7.11 (d, J = 7.2 Hz, 4H, Ar-H), 5.18−5.12 (m, 4H, η6-
C6H5CH3), 5.05 (t, J = 5.4 Hz, 1H, η6-C6H5CH3), 3.64 (s, 4H,
NC2H4N), 3.13−3.08 (m, 4H, CH(CH3)2), 1.49 (s, 3H, η6-
C6H5CH3), 1.18 (d, J = 6.8 Hz, 12H, CH(CH3)2). The resonances
i
of Pr-Hs (12H) are obscured by C6D11H (δ 1.38). 13C{1H} NMR
(100 MHz, C6D12): δ 214.2, 148.2, 140.6, 127.9, 124.0, 103.4 (η6-
C6H5CH3), 91.7 (η6-C6H5CH3), 91.2 (η6-C6H5CH3), 89.5 (η6-
C6H5CH3), 52.5, 29.3, 25.3, 24.4, 21.2 (η6-C6H5CH3). Anal. Calcd
for C34H46N2Ni: C, 75.42; H, 8.56; N, 5.17. Found: C, 74.95; H, 8.73;
N, 5.65.
Conversion of TNI into [(η2-quinolone)Ni(IPr)]2 (1). To a solution
of TNI (21.6 mg, 0.04 mmol) in n-pentane (5.0 mL) was added
quinolone (4.9 μL, 0.04 mmol) at room temperature. After the
Synthesis of (η6-benzene)Ni(SIPr) (BNSI). The general procedure
was followed with Ni(cod)2 (550 mg, 2.00 mmol), SIPr (781 mg, 2.00
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Organometallics
Article
reaction mixture was stirred for 30 min, all volatiles were removed
under reduced pressure to give 1 as a dark brown solid (45.8 mg, 0.04
mmol, 100%). A single crystal suitable for X-ray diffraction analysis
was prepared by recrystallization from C6H12 at room temperature. 1H
NMR (400 MHz, C6D6; Figure S4a, Supporting Information): δ 8.06
(d, J = 5.6 Hz, 2H, quinoline-C2H), 7.33−7.27 (m, 8H, Ar-H), 7.11
(d, J = 7.0 Hz, 4H, Ar-H), 6.75 (d, J = 6.8 Hz, 2H, quinoline-C4H),
6.48 (s, 4H, NC2H2N), 6.13 (dd, J = 5.6, 6.8 Hz, 2H, quinoline-C3H),
5.85 (d, J = 8.4 Hz, 2H, quinoline-C5H), 4.74 (dd, J = 5.4, 8.4 Hz, 2H,
quinoline-C6H), 3.68 (dd, J = 5.4, 6.0 Hz, 2H, quinoline-C7H), 3.61
(d, J = 6.0 Hz, 2H, quinoline-C8H), 3.39−3.32 (m, 4H, CH(CH3)2),
2.81−2.75 (m, 4H, CH(CH3)2), 1.63 (d, J = 6.8 Hz, 12H,
CH(CH3)2), 1.15 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.06 (d, J = 6.8
Hz, 12H, CH(CH3)2), 1.00 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13C{1H}
NMR (100 MHz, C6D6,; Figure S4b, Supporting Information): δ
199.8, 156.6 (C10), 147.1, 146.2, 145.2 (C2), 138.5, 129.2, 127.2 (C9),
124.1, 123.8, 123.7 (C4), 122.6, 113.5 (C5), 112.7 (C3), 52.9 (C8),
45.9 (C7), 28.9, 28.8, 26.4, 25.9, 23.4, 22.1. One Ar-C (C6) is obscured
by C6D6. Details on the results of X-ray analysis are found in Figure S9
(Supporting Information).
Conversion of TNI into Ni(IMes)(IPr) (2). To a solution of TNI
(21.6 mg, 0.04 mmol) in n-pentane (5.0 mL) was added IMes (12.2
mg, 0.04 mmol) at room temperature. After the reaction mixture was
stirred for 30 min, all volatiles were removed under reduced pressure
to give 2 as a deep blue solid (31.7 mg, 0.04 mmol, 100%). A single
crystal suitable for X-ray diffraction analysis was prepared by
recrystallization from toluene/hexane at −35 °C. 1H NMR (400
MHz, C6D6): δ 7.33 (t, J = 7.6 Hz, 2H, IPr-Ar-H), 7.13 (d, J = 7.6 Hz,
4H, IPr-Ar-H), 6.72 (s, 4H, IMes-Ar-H), 6.22 (s, 2H, IPr-NC2H2N),
5.95 (s, 2H, IMes-NC2H2N), 3.05−2.98 (m, 4H, IPr-CH(CH3)2), 2.30
(s, 6H, IMes-CH3), 2.02 (s, 12H, IMes-CH3), 1.35 (d, J = 6.8 Hz, 12H,
IPr-CH(CH3)2), 1.17 (d, J = 6.8 Hz, 12H, IPr-CH(CH3)2). 13C{1H}
NMR (100 MHz, C6D6): δ 194.9, 191.8, 145.8, 139.0, 138.6, 135.9,
135.1, 129.1, 127.9, 123.4, 119.6, 118.5, 28.8, 24.6, 23.7, 21.3, 18.5.
Anal. Calcd for C48H60N4Ni: C, 76.69; H, 8.05; N, 7.45. Found: C,
76.41; H, 8.08; N, 7.50. Details on the results of X-ray analysis are
found in Figure S9 (Supporting Information).
Global Young Researchers, Osaka University, on the Program
of MEXT.
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give [Ni(IPr)]2 (13.1 mg, 0.015 mmol, 77%). Spectroscopic data of
[Ni(IPr)]2 were identical with those previously reported.7
Conversion of [Ni(IPr)]2 into TNI. A solution of [Ni(IPr)]2 (18.5
mg, 0.021 mmol) in toluene (8.0 mL) was stirred for 48 h (2 days) at
room temperature. The quantitative formation of TNI was confirmed
by 1H NMR in C6D6 via transformation into BNI-d6. After all volatiles
were removed under reduced pressure, TNI (18.3 mg, 0.034 mmol,
83%) was obtained.
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F.; Fructos, M. R.; Prieto, A.; Alvarez, E.; Belderrain, T. R.; Nicasio, M.
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ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and CIF files giving detailed experimental
conditions for determination of the equilibrium constant of
arene exchange reactions, DFT calculation studies, character-
ization data, and crystallographic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
(6) For reported η6-arene Ni(0) complexes, see: (a) Watanabe, C.;
Inagawa, Y.; Iwamoto, T.; Kira, M. Dalton Trans. 2010, 39, 9414.
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.O.: ogoshi@chem.eng.osaka-u.ac.jp.
Notes
■
(b) Meltzer, A.; Prasang, C.; Milsmann, C.; Driess, M. Angew. Chem.,
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Int. Ed. 2009, 48, 3170. (c) Nickel, T.; Goddard, R.; Kruger, C.;
̈
̈
Porschke, K.-R. Angew. Chem., Int. Ed. 1994, 33, 879. See also the
following reports on η2-arene Ni(0) complexes: (d) Hatnean, J. A.;
Beck, R.; Borrelli, J. D.; Johnson, S. A. Organometallics 2010, 29, 6077.
The authors declare no competing financial interest.
(e) Bach, I.; Porschke, K.-R.; Goddard, R.; Kopiske, C.; Kruger, C.;
̈
̈
́
Rufinska, A.; Seevogel, K. Organometallics 1996, 15, 4959.
ACKNOWLEDGMENTS
■
(7) Lee, C. H.; Laitar, D. S.; Mueller, P.; Sadighi, J. P. J. Am. Chem.
Soc. 2007, 129, 13802.
This work was supported by Grants-in-Aid for Scientific
Research (A) (21245028) from MEXT and by ACT-C from
the JST. Y.H. acknowledges the Frontier Research Base for
(8) Laskowski, C. A.; Miller, A. J. M.; Hillhouse, G. L.; Cundari, T. R.
J. Am. Chem. Soc. 2011, 133, 771.
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(10) The NHCs employed in this paper are as follows: IPr, 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene; SIPr, 1,3-bis(2,6-diiso-
propylphenyl)imidazolin-2-ylidene; IMes, 1,3-bis(2,4,6-trimethylphen-
yl)imidazol-2-ylidene; IPr*, 1,3-bis(2,6-bis(diphenylmethyl)-4-methyl-
phenyl)imidazol-2-ylidene. The preparation methods of these NHCs
are found in: (a) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.
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Spek, A. L.; Tinant, B.; Reek, J. N. H.; Marko,
39, 1444.
́
I. E. Dalton Trans. 2010,
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equilibrium constant of arene exchange reactions.
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1282
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