Ionic Ni(ii) complexes containing a triphenylphosphine ligand and an imidazolium cation: Synthesis, structures and catalysis for aryl Grignard cross-coupling of aryl halides

Article abstract of DOI:10.1039/c3dt00086a

Novel ionic Ni(ii) complexes of general formula [R₂im] [Ni(PPh₃)Cl₃] (R₂im = 1,3-bis(2,6- diisopropylphenyl)imidazolium cation, IPrim, 1a; R₂im = 1,3-diisopropylimidazolium cation, ⁱPrim, 2a) were easily prepared in high yields by the direct reaction of Ni(PPh₃)₂Cl ₂ with 1 equiv. of N,N′-dihydrocarbylimidazolium salt, [R ₂im]Cl. Their bromide analogs [R₂im][Ni(PPh ₃)Br₃] (R₂im = IPrim, 1b; R₂im = ⁱPrim, 2b) were synthesized by the same reaction in the presence of excess NaBr. The reaction of Ni(DME)Cl₂ (DME = 1,2-dimethoxyethane) with 2 equiv. of [IPrim]Cl led to the formation of the complex [IPrim] ₂[NiCl₄] (3) in an almost quantitative yield. All these complexes were characterized by elemental analysis, electrospray ionization mass spectrometry (ESI-MS), and ¹H NMR spectroscopy; X-ray crystallography was performed for 1a, 1b, 2a, and 2b. A catalytic study of the cross-coupling reactions of aryl Grignard reagents with aryl halides revealed that complexes 1a and 1b possessed the highest activities. In comparison, complexes 2a, 2b, 3, and the related biscarbene Ni(ii) complex Ni(IPr) ₂Cl₂ [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2- ylidene] exhibited moderate activities; the least active complexes were Ni(PPh₃)₂Cl₂ and [NEt₄][Ni(PPh ₃)Cl₃].

Full text of DOI:10.1039/c3dt00086a

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Ionic Ni(II) complexes containing a triphenylphosphine
ligand and an imidazolium cation: synthesis, structures
and catalysis for aryl Grignard cross-coupling of aryl
halides†
Cite this: Dalton Trans., 2013, 42, 8437
Yan-Chao Xu, Jie Zhang, Hong-Mei Sun,* Qi Shen and Yong Zhang
Novel ionic Ni(II) complexes of general formula [R2im][Ni(PPh3)Cl3] (R2im = 1,3-bis(2,6-diisopropylphenyl)-
imidazolium cation, IPrim, 1a; R2im = 1,3-diisopropylimidazolium cation, iPrim, 2a) were easily prepared in
high yields by the direct reaction of Ni(PPh3)2Cl2 with 1 equiv. of N,N’-dihydrocarbylimidazolium salt,
i
[R2im]Cl. Their bromide analogs [R2im][Ni(PPh3)Br3] (R2im = IPrim, 1b; R2im = Prim, 2b) were synthesized
by the same reaction in the presence of excess NaBr. The reaction of Ni(DME)Cl2 (DME = 1,2-dimethoxy-
ethane) with 2 equiv. of [IPrim]Cl led to the formation of the complex [IPrim]2[NiCl4] (3) in an almost quan-
titative yield. All these complexes were characterized by elemental analysis, electrospray ionization mass
spectrometry (ESI-MS), and 1H NMR spectroscopy; X-ray crystallography was performed for 1a, 1b, 2a, and
2b. A catalytic study of the cross-coupling reactions of aryl Grignard reagents with aryl halides revealed
that complexes 1a and 1b possessed the highest activities. In comparison, complexes 2a, 2b, 3, and the
related biscarbene Ni(II) complex Ni(IPr)2Cl2 [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] exhibi-
ted moderate activities; the least active complexes were Ni(PPh3)2Cl2 and [NEt4][Ni(PPh3)Cl3].
Received 10th January 2013,
Accepted 3rd April 2013
DOI: 10.1039/c3dt00086a
www.rsc.org/dalton
complexes.5 Imidazolium salts have also been widely used in a
range of cross-coupling processes, because carbene ligands can
Introduction
Transition-metal-catalyzed cross-coupling reactions represent be easily generated by in situ dehydrohalogenation of the corres-
one of the most powerful and versatile tools for the construc- ponding imidazolium salts with various bases.3a,b,4a,6
tion of carbon–carbon bonds.1 The development of highly
Recently, research efforts have also been devoted to the syn-
efficient, yet robust and easy-to-access catalysts for applications thesis of mixed phosphine–NHC complexes of nickel in order
to cross-coupling reactions continues to attract much interest. to combine activity and robustness in the corresponding com-
In this context, although most studies have their emphasis on plexes. The synergic effect between these two types of ligands,
palladium-based systems, nickel-based catalysts have attracted which has been successfully applied in palladium-based
increasing attention, because nickel is much cheaper and systems,7 is mostly explained by the currently accepted view
easier to remove from the final products than palladium.2 that the labile phosphine ligand is capable of reversible dis-
Several nickel-based catalysts perform particularly well com- sociation from the metal center to generate in situ a vacant
pared with palladium systems in various reactions,3 for coordination site that enables easy complexation of substrates,
example, the Grignard cross-coupling reaction of aryl chlor- whereas the tightly bound NHC ligand is supposed to stabilize
ides.4 One of the main breakthroughs in this area has been the the active species as well as facilitate oxidative addition and/or
application of N-heterocyclic carbenes (NHCs) as an alternative reductive elimination during the catalytic coupling processes.
to traditional tertiary phosphine ligands to improve the stabi- In this context, bidentate phosphine–imidazolium precursors
lities and catalytic activities of the corresponding nickel or their NHC derivatives, as well as mixtures of monodentate
phosphines and monodentate NHCs, have been extensively
used in catalysis. For example, Poli et al. showed that zwitterio-
nic Ni(II) complexes containing a bidentate phosphine–imida-
zolium ligand (Ni-I) were very active catalysts for the cross-
coupling of aryl Grignard reagents with aryl chlorides,8 and
Lee et al. demonstrated that cationic Ni(II) complexes of biden-
tate phosphine–NHC ligands (Ni-II) were very effective in the
Suzuki cross-coupling reaction, and can even be superior to
The Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123,
People’s Republic of China. E-mail: sunhm@suda.edu.cn; Fax: +(86)512-65880305;
Tel: +(86)512-65880330
†CCDC 874238, 867537, 867539, and 867538 for 1a, 1b, 2a, and 2b, respectively.
For crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3dt00086a
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 8437–8445 | 8437

View Article Online
Paper
Dalton Transactions
their Pd(II) counterparts.3c However, these bidentate ligands
generally suffer from limitations such as multistep prepa-
ration, harsh conditions, and high costs.
Jamison et al. described a catalytic system, based on [Ni-
(cod)2]/P(OPh)3/IPr [cod = cyclooctadiene, IPr = 1,3-bis(2,6-di-
isopropylphenyl)imidazol-2-ylidene], capable of highly selec-
tive coupling of alkenes and aldehydes,9 and Sasaki et al.
reported that [BMim]2[NiCl4] (BMim = 1-n-butyl-3-methylimi-
dazolium cation) was active in the Suzuki cross-coupling reac-
tion of chloroarenes in the presence of 2 equiv. of PPh3.10 In
the latter case, a nickel carbenic species produced by pretreat-
ing [BMim]2[NiCl4] with K3PO4 was suggested to be the cataly-
tically active species. However, a large amount of catalyst (up
to 20 mol%)9 or an excess of PPh3 10 is usually needed in these
systems.
Notably, Matsubara et al. reported that a mixed Ni(II)
complex bearing both a phosphine and an NHC ligand, Ni-
(PPh3)(IPr)Cl2 (Ni-III), showed distinctly higher catalytic activity
in aryl Grignard cross-coupling, whereas the bis(NHC)
complex of Ni(IPr)2Cl2 and the bisphosphine complex of Ni-
(PPh3)2Cl2 showed much lower activities under the same con-
ditions.11a Additionally, Ni(PPh3)(IPr)Cl2 can be used as a very
efficient catalyst for other organic transformations, such as
α-arylation of acyclic ketones,11b amination of haloarenes,11b
and controlled polymerization of chlorothiophenes.11c A very
recent study by Matsubara et al. also showed that a monovalent
nickel complex, Ni(PPh3)(IPr)Cl, was more effective than its
biscarbene analog, Ni(IPr)2Cl, in the aryl Grignard cross-coup-
ling of aryl halides and amination of haloarenes.12 All these
results demonstrate that the combination of a monodentate
phosphine ligand and a monodentate NHC ligand impart
promising synergic effects to nickel-based catalysts. However,
the fact that Ni(PPh3)(NHC)Cl2 could not be prepared unless
the bulky IPr ligand was used might limit further investi-
gations of the synthesis and applications of such complexes.
We are therefore interested in exploring a versatile synthetic
method for constructing a Ni(II)-based catalytic system with
phosphine–NHC synergic effects.
Scheme 1 Typical mixed Ni(II) complexes.
in Scheme 1 (1), at 30 °C, the reaction of Ni(PPh3)2Cl2 with
1 equiv. of the N,N′-diarylimidazolium salt [R2im]Cl [R2im =
1,3-bis(2,6-diisopropylphenyl)imidazolium
cation,
IPrim]
occurred smoothly in tetrahydrofuran (THF) and gave the
desired complex 1a in 90% yield as bright blue crystals. In
addition, even when more than 1 equiv. of [IPrim]Cl was
added, only 1a was obtained, with recovery of some [IPrim]Cl.
To assess the potential of the new synthetic approach, a
similar reaction with another imidazolium salt was tested. It
was found that the substitution reaction of one PPh3 ligand in
Ni(PPh3)2Cl2 with the N,N′-dialkylimidazolium salt [R2im]Cl
(R2im
=
1,3-diisopropylimidazolium cation, iPrim) also
occurred smoothly and afforded the analogous complex 2a in
high yield. The latter reaction, however, proceeded at a slower
rate than that using [IPrim]Cl, and an elevated temperature
(60 °C) was required to obtain the high yield.
Notably, the exchange of the [Ni(PPh3)Cl3]− anion for the
[Ni(PPh3)Br3]− anion also proceeded smoothly by the addition
of sodium bromide (NaBr). As seen in Scheme 2 (2), the treat-
ment of Ni(PPh3)2Cl2 with 1 equiv. of [IPrim]Cl in the presence
of excess NaBr afforded the analogous complex 1b in 82%
yield as bright green crystals, and a similar reaction with
[iPrim]Cl produced the bromide 2b in 75% yield at an elevated
temperature (60 °C).
Complexes 1a and 1b are air stable, whereas complexes 2a
and 2b are somewhat sensitive to moisture (stable in dry air)
and decompose slowly to white solids in a moist atmosphere.14
Better solubility profiles were observed for these Ni(II) com-
plexes than for the corresponding N,N′-dihydrocarbylimida-
zolium salts [R2im]X, and all of them dissolved easily in THF.
The difference in solubility between the Ni(II) complex and the
corresponding [R2im]X facilitated effective purification of the
target product.
As a continuation of our research on the nickel chemistry of
NHCs,13 we report here the facile synthesis of a new type of
ionic Ni(II) complex, containing a triphenylphosphine ligand
and an imidazolium cation, [R2im][Ni(PPh3)X3] (R2im = 1,3-
dihydrocarbylimidazolium cation; X = Cl and Br), and their
catalytic potential in aryl Grignard cross-coupling of aryl
halides. Notably, the easily available [R2im][Ni(PPh3)X3] is very
effective, whereas the related Ni(II) complexes such as
[R2im]2[NiCl4], Ni(NHC)2X2, Ni(PPh3)Cl2 and [NEt4][Ni(PPh3)-
Cl3] showed obviously lower activities under the same
conditions.
Results and discussion
Synthesis and characterization of ionic Ni(II) complexes
The target ionic Ni(II) complexes 1a–2b were readily syn-
thesized using Ni(PPh3)2Cl2 as the starting material. As shown Scheme 2 Synthesis of ionic nickel(II) complexes 1a–2b.
8438 | Dalton Trans., 2013, 42, 8437–8445
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
Attempts to synthesize the corresponding neutral mixed of the paramagnetic properties of the metal ion.8 The 13P NMR
Ni(II) complexes bearing both a PPh3 ligand and an NHC spectra of 1a–2b exhibited no signal in CD3OCD3, indicating
ligand by dehydrohalogenation of 1a–2b with various bases, direct bonding of the PPh3 ligand to the nickel. The relatively
following procedures similar to those for their palladium ana- sharp resonances in the 1H NMR spectra of 1a–2b can be
logs7d,15 or zwitterionic Ni(II) complexes Ni-I,8 were investi- rationalized by the hypothesis that there is no strong coordi-
gated. Typically, the addition of a strong base such as nBuLi8 nation between the imidazolium cation and the nickel center,
or KOtBu11a to a THF solution of 1a induced a color change to which can be further confirmed by X-ray crystallography. The
dark red, indicating the formation of a nickel carbenic species, 1H NMR spectrum of 3 is also very similar to that of [IPrim]Cl,
very similar to the change observed in the dehydrohalogena- and only minor differences in the chemical shifts were
tion of Ni-I.8 However, our attempts to isolate the expected observed.
mixed Ni(II) complex, Ni(PPh3)(IPr)Cl2, were unsuccessful, and
Crystals suitable for X-ray diffraction studies were grown
the pure biscarbene Ni(II) complex of Ni(IPr)2Cl2 together with from a concentrated THF solution (1a and 2a) or a THF–
some amounts of PPh3 were isolated from the reaction solu- toluene solution (1b and 2b) of the obtained Ni(II) complexes
tion. This was mostly caused by the substitution reaction of at 0 °C. The crystallographic data and selected bond lengths
Ni(PPh3)(IPr)Cl2, which was generated by in situ dehydrohalo- and angles are listed in Tables 1 and 2, respectively. The mole-
genation of 1a, with an in situ generated IPr ligand during the cular structures of these complexes are shown in Fig. 1 (1a and
reaction procedure. Matsubara et al. demonstrated that Ni- 1b) and 2 (2a and 2b).
(PPh3)(IPr)Cl2 could be converted to Ni(IPr)2Cl2 in quantitative
yield by the addition of more than 1 equiv. of IPr ligand.11a
As shown in Fig. 1 and 2, each of the four molecular struc-
tures contains one imidazolium [R2im]+ cation and one [Ni-
Stirring a sample of Ni(DME)Cl2 (DME = 1,2-dimethoxy- (PPh3)X3]− anion. The structures of the [R2im]+ cations do not
ethane) with 2 equiv. of [IPrim]Cl in THF at 30 °C, an alterna- change significantly upon reaction of the imidazolium salts
tive procedure with mild conditions relative to those used in with Ni(PPh3)2Cl2, as the bond distances and angles within the
the previously reported methods,16 led to the formation of the imidazolium ring differ only slightly, and these parameters are
complex [IPrim]2[NiCl4] (3) in 95% yield as blue crystals comparable to those of previously reported imidazolium
(Scheme 3). The reported methods, which used anhydrous salts.8,17 For complexes 1a and 1b, each plane of the phenyl
or NiCl2·6H2O16d,f as nickel sources, usually pro- ring is oriented nearly perpendicular to the imidazolium ring,
16a–c
NiCl2
ceeded above 70 °C and needed a much longer reaction time. with dihedral angles ranging between 84.2 and 85.6° in 1a and
Complex 3 dissolved in THF, but was insoluble in toluene and between 85.2 and 86.0° in 1b, as well as at an angle of 23.2°
hexane. Similar to previously described imidazolium tetra- (1a) or 24.6° (1b) to the plane of the other phenyl ring in the
chloronickelate(II) complexes,16 complex 3 is hygroscopic, and system. In all four [Ni(PPh3)X3]− anions, each nickel atom is
was immediately converted to a green solid when water was coordinated by three halogen atoms and one phosphorus
added deliberately, but the blue color could be regenerated on atom in a distorted tetrahedral geometry, with Ni–Cl, Ni–Br,
heating under vacuum.
and Ni–P bond lengths in the ranges 2.209(2)–2.254(2) Å,
All the obtained complexes were characterized by elemental 2.308(2)–2.332(2) Å, and 2.308(2)–2.318(1) Å, respectively. The
analysis, electrospray ionization mass spectrometry (ESI-MS), mean values of the Ni–Cl bond lengths are 2.240 Å for 1a and
and 1H NMR spectroscopy, and by X-ray crystallography for 2.225 Å for 2a, whereas the mean values of the Ni–Br bond
complexes 1a–2b. The positive ion ESI-MS of 1a–3 were used to lengths are 2.364 Å and 2.349 Å for 1b and 2b, respectively.
establish the presence of the imidazolium cation, and in all These values are unexceptional and lie within the ranges
cases a peak with an intensity of almost 100% indicative of the found for other complexes containing [Ni(PR3)X3]− anions.8,18
imidazolium cation was observed. The 1H NMR spectra of Nevertheless, it is worth noting that complexes 1a–2b have very
complexes 1a–2b exhibited characteristic resonances similar to similar Ni–P bond lengths [2.318(1), 2.310(2), 2.314(1), and
those of the corresponding imidazolium salts, except that 2.308(2) Å in 1a, 1b, 2a, and 2b, respectively], even though they
slightly down-field chemical shifts, together with three broad bear either a different imidazolium cation or a different halide
and weak signals at ranges of δ = −4.11–2.69, 3.81–5.08, and atom. The angles at nickel are in the ranges 99.14(6)–
18.74–19.04 ppm, respectively, were observed. Such shift vari- 121.22(6)° for 1a, 99.60(6)–123.38(5)° for 1b, 102.44(5)–
ations and broadening are very similar to those observed for 120.68(6)° for 2a, and 102.60(6)–124.11(5)° for 2b. In the struc-
previously reported bidentate phosphine–imidazolium salts tures of both 1a and 1b, the X(3)–Ni–X(1) angle is significantly
and their derived zwitterionic Ni(II) complexes Ni-I, as a result wider [121.22(6)° for 1a and 123.38(5)° for 1b] than the other
two X–Ni–X angles. A similar trend, i.e., a much wider X(2)–Ni–
X(3) angle [120.68(6)° for 2a and 124.11(5)° for 2b], is observed
in the structures of 2a and 2b. Such a trend has been observed
in the structures of the previously reported zwitterionic Ni(II)
complexes Ni-I, and may be related to the C–H⋯X interaction
between the [R2im]+ cation and the [Ni(PR3)X3]− anion.8
Indeed, a number of hydrogen contacts less than 3 Å are
Scheme 3 Synthesis of complex 3.
observed in all the structures, and the shortest distance,
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 8437–8445 | 8439

View Article Online
Paper
Dalton Transactions
Table 1 X-ray crystallographic data for 1a–2b
1a
1b
2a
2b
Formula
C
45H52Cl3N2NiP
C45H52Br3N2NiP
950.30
223(2)
Mo-Kα
Orthorhombic
Pna21
C27H32Cl3N2NiP
580.58
293(2)
Mo-Kα
Monoclinic
P21/c
C27H32Br3N2NiP
713.96
223(2)
Mo-Kα
Monoclinic
P21/c
Formula weight
Temperature/K
Radiation used
Crystal system
Space group
816.92
223(2)
Mo-Kα
Orthorhombic
Pna21
Unit cell dimensions
a/Å
b/Å
c/Å
17.270(7)
14.224(5)
17.322(7)
90
4255(3)
4
17.4911(18)
14.3761(14)
17.6688(19)
90
4442.9(8)
4
15.958(4)
9.663(2)
19.083(4)
90.547(4)
2942.4(11)
4
16.0114(5)
9.7678(8)
19.4056(3)
91.043(11)
3034.5(3)
4
β/°
V/Å3
Z
Dc/g cm−3
1.275
0.715
1.421
3.202
1.311
1.004
1.563
4.660
μ/mm−1
F(000)
θ range/°
Reflection collected
Independent reflections, Rint
Goodness-of-fit on F2
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
1720
3.00–25.50
19 706
7031, 0.0427
1.114
0.0531, 0.1322
0.0592, 0.1487
1936
3.06–27.47
24 646
8189, 0.0494
1.092
0.0651, 0.1601
0.0828, 0.1732
1208
3.25–25.35
22 080
5339, 0.0841
0.936
0.0558, 0.1357
0.1044, 0.1651
1424
3.24–25.50
15 163
5628, 0.0448
1.087
0.0736, 0.1837
0.1069, 0.2047
(PPh3)Cl3]. The experiments were performed using 4-bromo-
anisole and its analogous chloride, respectively, as starting
reagents, in order to highlight activity differences. The results
obtained are summarized in Table 3.
Table 2 Selected bond lengths (Å) and angles (°) for 1a–2b
1a
1b
2a
2b
Ni(1)–X(1)
Ni(1)–X(2)
Ni(1)–X(3)
Ni(1)–P(1)
N(1)–C(19)
N(1)–C(20)
N(2)–C(19)
N(2)–C(21)
C(20)–C(21)
X(2)–Ni(1)–X(3)
X(2)–Ni(1)–X(1)
X(3)–Ni(1)–X(1)
X(2)–Ni(1)–P(1)
X(3)–Ni(1)–P(1)
X(1)–Ni(1)–P(1)
2.254(2)
2.226(2)
2.239(2)
2.318(1)
1.340(7)
1.389(6)
1.324(7)
1.379(6)
1.336(8)
115.72(8)
108.18(7)
121.22(6)
105.88(5)
99.14(6)
104.46(6)
2.392(1)
2.332(2)
2.368(1)
2.310(2)
1.336(9)
1.369(9)
1.318(9)
1.4009)
1.321(11)
118.81(8)
100.49(7)
123.38(5)
109.09(7)
99.60(6)
104.20(7)
2.226(1)
2.209(2)
2.240(2)
2.314(1)
1.329(6)
1.360(7)
1.323(6)
1.362(6)
1.334(7)
120.68(6)
110.05(7)
113.53(6)
104.68(5)
102.44(5)
103.13(5)
2.340(1)
2.346(1)
2.361(1)
2.308(2)
1.332(11)
1.373(11)
1.301(10)
1.378(11)
1.328(13)
124.11(5)
106.70(6)
111.56(5)
106.22(6)
102.60(6)
103.62(6)
ð1Þ
As can be seen from Table 3, there are significant differ-
ences among the catalytic activities of these Ni(II) complexes
under the present conditions. The catalytic activities of 1a and
1b are higher than the other Ni(II) complexes. For example, the
cross-coupling reaction of 4-bromoanisole with phenylmagne-
sium bromide afforded a quantitative yield of 4-methoxybiphe-
nyl with 0.5 mol% of 1a or 1b in 1 h at 30 °C (entries 1 and 3),
and for 1a the yield still reached 95% even when the reaction
time was reduced to 30 min. In comparison, complexes 2a, 2b,
3, and Ni(IPr)2Cl2 showed moderate activities, and respectively
furnished 64% (entry 5), 67% (entry 7), 80% (entry 9), and 75%
(entry 11) yields of 4-methoxybiphenyl. The least reactive com-
plexes were Ni(PPh3)2Cl2 and [NEt4][Ni(PPh3)Cl3], which
afforded the product in merely 26% (entry 13) and 33% (entry
15) yields, respectively. The apparent difference in catalytic
activity between 1a and [NEt4][Ni(PPh3)Cl3] indicates clearly
that the imidazolium cation plays an important role in the
activity of the present ionic Ni(II) complex, i.e. acts as a precur-
sor to an active Ni-NHC species. Using less reactive 4-chloro-
anisole as the starting reagent, the same trend in catalytic
2.52 Å, is found in 1a. In 1a and 1b, each [R2im]+ cation is
close to three halide atoms belonging to two different [Ni-
(PPh3)X3]− anions, whereas in 2a and 2b, five and four hydro-
gen contacts, which stem from four and three different [Ni-
(PPh3)X3]− anions, are observed, respectively. Such hydrogen
bonding interaction between the imidazolium cation and the
metal-containing anion has been well established and explains
to some degree their markedly higher melting points relative
to, for example, those of the corresponding imidazolium
salts.8,16,17
Catalysis by ionic Ni(II) complexes
To disclose any synergic effects resulting from the ionic Ni(II) activity was observed when the catalyst loading was increased
complexes of [R2im][Ni(PPh3)X3], the standard Kumada–Corriu to 1.5 mol%. Notably, the catalytic activity of 1a was found to
reaction, outlined in eqn (1), was chosen as a test reac- be very close to that of a mixed Ni(II) complex bearing both a
tion8,11a,19 for comparisons among [R2im][Ni(PPh3)X3] (1a–2b), phosphine and an NHC ligand, Ni(PPh3)(IPr)Cl2 (Ni-III), which
[IPrim]2[NiCl4] (3), Ni(IPr)2Cl2, Ni(PPh3)Cl2, and [NEt4][Ni- afforded a quantitative yield of 4-methylbiphenyl by using
8440 | Dalton Trans., 2013, 42, 8437–8445
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
Fig. 2 Molecular structures of 2a and 2b with thermal ellipsoids at the 15%
probability level. All hydrogens are omitted for clarity.
Fig. 1 Molecular structures of 1a and 1b with thermal ellipsoids at the 15%
probability level. All hydrogens are omitted for clarity.
4-bromoanisole as starting material with a 0.5 mol% loading
in 30 min at 18 °C, and 39% yield of the desired product when
4-chloroanisole was used instead of 4-bromoanisole.11a The
catalytic activity of 1b was comparable to that of a zwitterionic
Ni(II) complex bearing a bidentate phosphine–imidazolium
salt ligand (Ni-I), reported by Poli et al.8b The present work
therefore clearly shows that the simple, easily accessible ionic
Ni(II) complexes of [R2im][Ni(PPh3)X3] can exert a significant
synergic effect on their catalytic activity. The high catalytic
activities of 1a and 1b are mainly the result of the stability pro-
vided by the in situ-generated bulky IPr ligand and the hemi-
lability of the PPh3 ligand on the nickel center.11 The fact that
the catalytic activities of 1a and 1b are higher than those of 2a
and 2b, respectively, could be ascribed more to a result of the
steric bulkiness provided by the in situ-generated NHC
ligand,4a which can facilitate reductive elimination to form the
desired cross-coupling product as well as to regenerate the cata-
lytically active species, than the acceleration of oxidative
addition brought about by the strong σ-donating ability of the
in situ-generated NHC ligand.4c
Table 3 The cross-couplings of C6H5MgBr with p-MeOC6H4X catalyzed by Ni(II)
complexesa
Entry
Catalyst (mol%)
X
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
1a (0.5)
1a (1.5)
1b (0.5)
1b (1.5)
2a (0.5)
2a (1.5)
2b (0.5)
2b (1.5)
Br
Cl
Br
Cl
Br
Cl
Br
Cl
Br
Cl
Br
Cl
Br
Cl
Br
Cl
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
100 (95b)
85
100
86
64
65
67
63
80
55
75
52
26
27
33
32
9
3 (0.5)
3 (1.5)
10
11
12
13
14
15
16
Ni(IPr)2Cl2 (0.5)
Ni(IPr)2Cl2 (1.5)
Ni(PPh3)2Cl2 (0.5)
Ni(PPh3)2Cl2 (1.5)
[NEt4][Ni(PPh3)Cl3] (0.5)
[NEt4][Ni(PPh3)Cl3] (1.5)
a Reaction conditions: C6H5MgBr (1.5 mmol, 1.0 M in THF), p-
MeOC6H5X (1.0 mmol), total volume (2.0 mL), 30 °C, GC yield. b 0.5 h.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 8437–8445 | 8441

View Article Online
Paper
Dalton Transactions
The performances of the catalytic systems with other coup- excellent yields. Notably, complex 1a displayed good activity
ling partners were investigated and are summarized in toward aryl chlorides, which is less reactive and yet the most
Table 4.
attractive substrate among the aryl halides from the stand-
As can be seen from Table 4, a host of aromatic bromides points of cost and availability.4,20 In particular, the electronic
and chlorides reacted smoothly with various arylmagnesium properties exert the expected influences, that is the electron-
halides to give the corresponding coupling products in good to poor arenes being the most active (entries 8–10, 22),
Table 4 The cross-couplings of aryl Grignard reagents with aryl halides catalyzed by 1aa
Entry
1
ArX
Ar′MgX
Product
Yield (%)
96 (100b)
2
3
4
89
72
72
5c
82
6d
7d
8e
85 (90b)
67
94 (99b)
9e
99b
98b
10e
11
100b
83
12c
13d
14d
15
81
65
60
16c
88
17
18
19
95 (100b)
72
68
8442 | Dalton Trans., 2013, 42, 8437–8445
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
Table 4 (Contd.)
Entry
20c
ArX
Ar′MgX
Product
Yield (%)
58
21d
22
83
42 (99b,c
52
)
23d
24d
25
∼0
85
a Conditions: 1a (0.5 mol%), aryl Grignard reagents (1.5 mmol, 1.0 M in THF), aryl bromide (1.0 mmol), total volume (2.0 mL), 30 °C, 1 h,
isolated yield. b GC yield. c Grignard reagents (1.2 mmol, 0.3 M in THF), total volume (4.8 mL). d 1a (1.5 mol%), 8 h. e 1a (1.0 mol%), 5 h.
meanwhile the steric hindrance is partially tolerated by the Ni(II) complexes in other carbon–carbon coupling reactions are
system as demonstrated by 2-chlorotoluene, but 2-chloro-m- currently under investigation in our laboratory.
xylene showed little activity (entries 6, 23, 24). This is similar to
the previously reported pattern with the Ni(acac)2/[IPrim]Cl cata-
lytic system.4 Therefore, these results suggest the great poten-
tial of the new type of Ni(II) complex of [R2im][Ni(PPh3)X3] as a
Experimental section
General procedures
suitable precatalyst for C–C cross-coupling applications.
All manipulations were performed under pure argon with rigo-
rous exclusion of air and moisture using standard Schlenk
techniques. Solvents were distilled from Na/benzophenone
Conclusions
ketyl under pure argon prior to use. Ni(PPh3)2Cl2,21 [IPrim]-
13g
Cl,22 [iPrim]Cl,23 Ni(IPr)2Cl2
and [NEt4][Ni(PPh3)Cl3]24 were
A new type of ionic Ni(II) complex, of general formula [R2im]-
[Ni(PPh3)X3] (1a–2b), was easily synthesized via the simple
reaction of Ni(PPh3)2Cl2 with 1 equiv. of N,N′-dihydrocarbyl-
imidazolium salt. It is of interest that the monophosphine
Ni(II) complexes of [IPrim][Ni(PPh3)X3] (X = Cl, 1a; X = Br, 1b)
showed much higher catalytic activities in the cross-coupling
of aryl Grignard reagents with aryl halides than did other
related Ni(II) complexes, i.e., [iPrim][Ni(PPh3)X3] (X = Cl, 2a;
X = Br, 2b), [IPrim]2[NiCl4] (3), Ni(IPr)2Cl2, Ni(PPh3)Cl2, and
[NEt4][Ni(PPh3)Cl3]. This apparently high catalytic activity is
possibly due to the stabilization effect of an in situ-generated
bulky IPr ligand and the hemilability of the PPh3 ligand on the
catalytically active species. Since a variety of bisphosphine
Ni(II) dihalides and imidazolium salts are available, we
reasoned that the construction of the ionic Ni(II) complex of
general formula [R2im][Ni(PR3)X3] can be used as an alterna-
tive strategy to significantly improve the catalytic activity of
Ni(II) complexes with phosphine/NHC synergic effects. A
detailed structure–reactivity relationship study of this new type
of Ni(II) complex, the isolation of the expected monophosphine
prepared by published methods. Organic reagents used for
cross-coupling reactions were purchased from Acros Organics,
Sigma-Aldrich and Alfa Aesar. Elemental analysis was per-
formed by direct combustion on a Carlo-Erba EA-1110 instru-
ment. NMR spectra were measured on a Varian Unity INOVA
400 or a VNMRS 300 MHz spectrometer at 25 °C. Electrospray
Ionization-Mass spectrum (ESI-MS) data were recorded on a
6220 Accurate-Mass TOF LC/MS instrument. The melt points
were determined on a Diamond DSC (Perkin Elmer) using
powder samples under an N2 atmosphere (50 mL min−1). The
system was heated from 50 °C to 350 °C at 20 °C min−1. Gas
chromatographic (GC) analysis was performed using a Varian
CP-3800 instrument equipped with an FID detector and a
capillary column AT.OV-101 (30 m × 0.32 mm i.d., 0.10 μm
film). The oven program temperature was 80 °C for 2 min,
increased to 280 °C at 10 °C min−1, and held for 2 min.
Synthesis
[IPrim][Ni(PPh3)Cl3] (1a). A Schlenk flask was charged
monocarbene complexes of nickel, and the scope of these with Ni(PPh3)2Cl2 (0.65 g, 1.00 mmol), [IPrim]Cl (0.43 g,
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 8437–8445 | 8443

View Article Online
Paper
Dalton Transactions
1.00 mmol), THF (15 mL) and a stir bar. The resulting mixture
[IPrim]2[NiCl4] (3). A Schlenk flask was charged with Ni-
was stirred for 2 h at 30 °C, filtered, and evaporated to dryness. (DME)Cl2 (0.22 g, 1.00 mmol), [DIPrim]Cl (0.86 g, 2.00 mmol),
The residue was washed with toluene and extracted with hot THF (15 mL) and a stir bar. The resulting mixture was stirred
THF and crystallized from concentrated THF at 0 °C. The for 2 h at 30 °C, filtered, then crystallized from concentrated
product was precipitated as bright blue crystals in a yield of THF at 0 °C. The product was precipitated as blue crystals in a
90% (0.74 g). M.p. 285.3 °C. Anal. calcd for C45H52Cl3N2NiP: C, yield of 95% (0.94 g). M.p. 252.0 °C. Anal. calcd for
66.16; H, 6.42; N, 3.43. Found: C, 66.22; H, 6.53; N, 3.43. 1H C54H74Cl4N4Ni: C. 66.20; H, 7.61; N, 5.72. Found: C, 66.33; H,
NMR (δ, 400 MHz, (CD3)2CO): −2.69 (br s, Ph-H), 1.41 (d, J = 7.62; N, 5.68. 1H NMR (δ, 400 MHz, (CD3)2CO): 1.23 (d, J =
8.0 Hz, 12H, CH(CH3)2), 1.43 (d, J = 4.0 Hz, 12H, CH(CH3)2), 6.8 Hz, 24H, CH(CH3)2), 1.39 (d, J = 6.8 Hz, 24H, CH(CH3)2),
2.98 (m, 4H, CH(CH3)2), 5.07 (br s, Ph-H), 7.63 (d, J = 8.0 Hz, 2.37 (m, 8H, CH(CH3)2), 7.38 (s, 4H, NCHCHN), 7.51 (d, J =
4H, m-Ar-H), 7.76–7.79 (m, 2H, p-Ar-H), 9.61 (s, 2H, NCHCHN), 7.6 Hz, 8H, m-Ar-H), 7.67–7.71 (m, 4H, p-Ar-H), 9.59 (s, 1H,
9.96 (s, 1H, NCHN), 18.83 (br s, Ph-H). MS (ESI+): m/z 389.29 NCHNAr). MS (ESI+): m/z 389.30 [C27H37N2]+.
[C27H37N2]+ (100%).
X-ray structural determination
[IPrim][Ni(PPh3)Br3] (1b). A Schlenk flask was charged with
Ni(PPh3)Cl2 (0.65 g, 1.00 mmol), [IPrim]Cl (0.43 g, 1.00 mmol),
NaBr (0.93 g, 9 mmol), THF (15 mL) and a stir bar. The result-
ing mixture was stirred for 2 h at 30 °C, filtered, and evapor-
ated to dryness. The residue was washed with toluene and
extracted with hot THF and crystallized from concentrated
THF–toluene at 0 °C. The product was precipitated as bright
green crystals in a yield of 82% (0.78 g). M.p. 294.8 °C. Anal.
calcd for C45H52Br3N2NiP: C, 56.88; H, 5.52; N, 2.95. Found: C,
56.84; H, 5.59; N, 2.95. 1H NMR (δ, 400 MHz, (CD3)2CO): −3.56
(br s, Ph-H), 1.33 (d, J = 8.0 Hz, 12H, CH(CH3)2), 1.38 (d, J =
Single crystals of 1a–2b for X-ray diffraction studies were
sealed in a thin-walled glass capillary. The data were collected
on a Rigaku Mercury CCD area detector at 223(2) K (for 1a, 1b,
and 2b) or at 293(2) K (for 2a). Structures were solved by direct
methods and refined by full-matrix least-squares procedures
based on F2 using SHELXS-97 and SHELXL-97 programs. All
non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were assigned to idealized positions and were included
in structure factor calculations.
8.0 Hz, 12H, CH(CH3)2), 2.75 (m, 4H, CH(CH3)2), 4.29 (br s, General procedure for the cross-coupling reactions
Ph-H), 7.61 (d, J = 8.0 Hz, 4H, m-Ar-H), 7.75–7.78 (m, 2H,
In atypical example, complex 1a (0.005 mmol), aryl bromide
(1.0 mmol), and THF (0.5 mL) were added to a Schlenk tube,
and the mixture was stirred at 0 °C for 2 minutes. To the solu-
tion was added a solution of PhMgBr (1.5 mL, 1.0 M in THF)
p-Ar-H), 8.96 (s, 2H, NCHCHN), 9.92 (s, 1H, NCHN), 19.04 (br s,
Ph-H). MS (ESI+): m/z 389.30 [C27H37N2]+ (100%).
[iPrim][Ni(PPh3)Cl3] (2a). A Schlenk flask was charged with
Ni(PPh3)Cl2 (0.65 g, 1.00 mmol), [iPrim]Cl (0.19 g, 1.00 mmol),
at 0 °C with stirring. The color of the resulting solution turned
THF (15 mL) and a stir bar. The resulting mixture was stirred
to dark red immediately. Then, the resulting solution was
for 2 h at 60 °C, filtered, and evaporated to dryness. The
stirred for 1 h in an oil bath at 30 °C. After the reaction
residue was washed with toluene and extracted with hot THF
mixture was quenched by addition of diluted hydrochloric acid
and crystallized from concentrated THF at 0 °C. The product
(1 M, 0.5 mL), the mixture was extracted with acetic ether (3 ×
was precipitated as bright blue crystals in a yield of 85%
3 mL) and dried over MgSO4. The yield of the desired product
is determined by GC analysis, using n-hexadecane as an
internal standard substance.
Purification of the crude mixture by flash column chromato-
graphy [silica gel, 5.0% ethyl acetate–pet ether (60–90 °C)] gave
(0.49 g). M.p. 172.5 °C. Anal. calcd for C27H32Cl3N2NiP: C,
55.86; H, 5.56; N, 4.83. Found: C, 55.77; H, 5.56; N, 4.96. 1H
NMR (δ, 400 MHz, (CD3)2CO): −2.81 (br s Ph-H), 2.96 (d, J =
32 Hz, 12H, CH(CH3)2), 5.08 (br s, Ph-H), 7.61–7.81 (m, 2H,
CH(CH3)2), 8.91 (s, 2H, NCHCHN), 12.41 (s, 1H, NCHN), 18.74
the isolated yield of the desired coupling product. The identity
(br s, Ph-H). MS (ESI+): m/z 153.14 [C9H17N2]+ (100%).
of the product was confirmed by 1H NMR spectroscopy and TLC.
[iPrim][Ni(PPh3)Br3] (2b). A Schlenk flask was charged with
Ni(PPh3)Cl2 (0.65 g, 1.00 mmol), [iPrim]Cl (0.19 g, 1.00 mmol),
NaBr (0.93 g, 9 mmol), THF (15 mL) and a stir bar. The
resulting mixture was stirred for 2 h at 60 °C, filtered, and
Acknowledgements
evaporated to dryness. The residue was washed with toluene
and extracted with THF and crystallized from concentrated
THF–toluene at 0 °C. The product was precipitated as bright
green crystals in a yield of 75% (0.54 g). M.p. 154.6 °C (dec).
Anal. calcd for C27H32Br3N2NiP: C, 45.42; H, 4.52; N, 3.92.
Found: C, 45.33; H, 4.60; N, 3.89. 1H NMR (δ, 400 MHz,
(CD3)2CO): −4.11 (br s, Ph-H), 2.61 (d, J = 76 Hz, 12H,
NCH(CH3)2), 3.81 (br s, Ph-H), 7.48 (m, 2H, NCH(CH3)2),
8.68 (s, 1H, NCHCHN), 8.86 (s, 1H, NCHCHN), 12.56
(s, 1H, NCHN), 18.80 (br s, Ph-H). MS (ESI+): m/z 153.14
[C9H17N2]+ (100%).
We are grateful to the National Natural Science Foundation of
China (Grants 20772089 and 21172164), the Key Laboratory of
Organic Chemistry of Jiangsu Province, and the Priority Aca-
demic Program Development of Jiangsu Higher Education
Institutions (PAPD) for financial support.
Notes and references
1 (a) Metal-Catalyzed Cross-Coupling Reactions, ed. A. de
Meijere and F. Diederich, Wiley-VCH, Weinheim, 2nd edn,
8444 | Dalton Trans., 2013, 42, 8437–8445
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
2004; (b) J. P. Corbet and G. Mignani, Chem. Rev., 2006,
106, 2651.
2 (a) C. E. Tucker and J. G. de Vries, Top. Catal., 2002, 19,
111; (b) B. M. Rosen, K. W. Quasdorf, D. A. Wilson,
N. Zhang, A.-M. Resmerita, N. K. Garg and V. Percec, Chem.
Rev., 2011, 111, 1346.
3 (a) S. B. Blakey and D. W. C. Macmillan, J. Am. Chem. Soc.,
2003, 125, 6046; (b) J. Liu and M. J. Robins, Org. Lett., 2004,
6, 3421; (c) C.-C. Lee, W.-C. Ke, K.-T. Chan, C.-L. Lai,
C.-H. Hu and H. M. Lee, Chem.–Eur. J., 2007, 13, 582.
Y. Zhang, Organometallics, 2005, 24, 5925; (c) W. F. Li,
H. M. Sun, Z. G. Wang, M. Z. Chen, Q. Shen and Y. Zhang,
J. Organomet. Chem., 2005, 690, 6277; (d) H. M. Sun,
D. M. Hu, Y. S. Wang, Q. Shen and Y. Zhang, J. Organomet.
Chem., 2007, 692, 903; (e) W. F. Li, H. M. Sun, M. Z. Chen,
Q. Shen and Y. Zhang, J. Organomet. Chem., 2008, 693,
2047–2051; (f) L. Z. Xie, H. M. Sun, D. M. Hu, Z. H. Liu,
Q. Shen and Y. Zhang, Polyhedron, 2009, 28, 2585;
(g) Z. H. Liu, Y. C. Xu, L. Z. Xie, H. M. Sun, Q. Shen and
Y. Zhang, Dalton Trans., 2011, 40, 4697.
4 (a) V. P. W. Böhm, T. Weskamp, C. W. K. Gstöttmayr and 14 X. Wang, S. Liu, L. H. Weng and G. X. Jin, J. Organomet.
W. A. Herrmann, Angew. Chem., Int. Ed., 2000, 39, 1602; Chem., 2005, 690, 2934.
(b) Z. Xi, B. Liu and W. Chen, J. Org. Chem., 2008, 73, 3954; 15 C. J. Mathews, P. J. Smith, T. Welton, A. J. P. White and
(c) X. Zhang, B. Liu, A. Liu, W. Xie and W. Chen, Organo-
metallics, 2009, 28, 1336.
5 (a) S. Díez-González, N. Marion and S. P. Nolan, Chem.
Rev., 2009, 109, 3612; (b) M. Poyatos, J. A. Mata and
E. Peris, Chem. Rev., 2009, 109, 3677.
6 (a) B. Liu, Q. Xia and W. Chen, Angew. Chem., Int. Ed., 2009,
48, 5513; (b) B. Liu, X. Liu, C. Chen, C. Chen and W. Chen,
Organometallics, 2012, 31, 282, and the references cited.
7 For selected recent examples, see: (a) N. Tsoureas,
A. A. Danopoulos, A. A. D. Tulloch and M. E. Light, Organo-
metallics, 2003, 22, 4750; (b) A. E. Wang, J. Zhong, J. H. Xie,
K. Li and Q. L. Zhou, Adv. Synth. Catal., 2004, 346, 595;
(c) G. D. Frey, J. Schütz, E. Herdtweck and W. A. Herrmann,
D. J. Williams, Organometallics, 2001, 20, 3848.
16 (a) P. B. Hitchcock, K. R. Seddon and T. Welton, J. Chem.
Soc., Dalton Trans., 1993, 2639; (b) C. M. Zhong, T. Sasaki,
A. Jimbo-Kobayashi, E. Fujiwara, A. Kobayashi, M. Tada
and Y. Iwasawa, Bull. Chem. Soc. Jpn., 2007, 80, 2365;
(c) X. J. Wen, L. P. Yu, D. H. Wang, X. B. Jin and G. Z. Chen,
Green Chem., 2008, 10, 296; (d) M. B. Meredith,
C. H. McMillen, J. T. Goodman and T. P. Hanusa, Poly-
hedron, 2009, 28, 2355; (e) T. Peppel and M. Köckerling,
Z. Anorg. Allg. Chem., 2010, 636, 2439; (f) M. C. A. Kuhn,
A. A. M. Lapis, G. Machado, T. Roisnel, J.-F. Carpentier,
B. A. D. Neto and O. L. Casagrande Jr., Inorg. Chim. Acta,
2011, 370, 505.
Organometallics, 2005, 24, 4416; (d) A. T. Normand and 17 (a) Z. Fei, D. Zhao, D. Pieraccini, W. H. Ang, T. J. Geldbach,
K. J. Cavell, Eur. J. Inorg. Chem., 2008, 2781; (e) N. Toselli,
D. Martin and G. Buono, Org. Lett., 2008, 10, 1453;
(f) C. Y. Liao, K. T. Chan, C. Y. Tu, Y. W. Chang, C. H. Hu
and H. M. Lee, Chem.–Eur. J., 2009, 15, 405; (g) C. C. Ho,
R. Scopelliti, C. Chiappe and P. J. Dyson, Organometallics,
2007, 26, 1588; (b) X. Yang, Z. Fei, T. J. Geldbach,
A. D. Phillips, C. G. Hartinger, Y. Li and P. J. Dyson, Organo-
metallics, 2008, 27, 3971.
S. Chatterjee, T. L. Wu, K. T. Chan, Y. W. Chang, 18 (a) E. C. Alyea, A. Costin, G. Ferguson, G. T. Fey, R. G. Goel
T. H. Hsiao and H. M. Lee, Organometallics, 2009, 28, 2837;
(h) A. Dumrath, X. Wu, H. Neumann, A. Spannenberg,
R. Jackstell and M. Beller, Angew. Chem., Int. Ed., 2010, 49,
8988; (i) N. Debono, A. Labande, E. Manoury, J. Daran and
R. Poli, Organometallics, 2010, 29, 1879.
8 (a) J. Wolf, A. Labande, J.-C. Daran and R. Poli, J. Organo-
met. Chem., 2006, 691, 433; (b) J. Wolf, A. Labande,
M. Natella, J.-C. Daran and R. Poli, J. Mol. Catal. A: Chem.,
2006, 259, 205.
and R. J. Restivo, J. Chem. Soc., Dalton Trans., 1975, 1294;
(b) L. R. Hanton and P. R. Raithby, Acta Crystallogr., Sect. B:
Struct. Crystallogr. Cryst. Chem., 1980, 36, 2417; (c) E. C.
Alyea, G. Ferguson, M. Parvez and J. E. Davies, Cryst. Struct.
Commun., 1982, 11, 1713; (d) M. Brenndörfer, H. A. Brune,
T. Debaerdemaeker and R. Hemmer, Z. Naturforsch., B:
Anorg. Chem. Org. Chem., 1985, 40, 357; (e) M. C. Smith,
S. C. Davies, D. L. Hughes and D. J. Evans, Acta Crystallogr.,
Sect. E: Struct. Rep. Online, 2001, 57, m509; (f) P. V. Rao,
S. Bhaduri, J. Jiang, D. Hong and R. H. Holm, J. Am. Chem.
Soc., 2005, 127, 1933.
9 C.-Y. Ho and T. F. Jamison, Angew. Chem., Int. Ed., 2007, 46,
782.
10 C. M. Zhong, T. Sasaki, M. Tada and Y. Iwasawa, J. Catal., 19 J. Berding, T. F. van Dijkman, M. Lutz, A. L. Spek and
2006, 242, 357. E. Bouwman, Dalton Trans., 2009, 6948.
11 (a) K. Matsubara, K. Ueno and Y. Shibata, Organometallics, 20 (a) L. G. Xie and Z. X. Wang, Chem.–Eur. J., 2010, 16, 10332;
2006, 25, 3422; (b) K. Matsubara, K. Ueno, Y. Koga and
K. Hara, J. Org. Chem., 2007, 72, 5069; (c) S. Tamba, K. Shono,
A. Sugie and A. Mori, J. Am. Chem. Soc., 2011, 133, 9700.
(b) M. J. Iglesias, A. Prieto and M. C. Nicasio, Org. Lett.,
2012, 14, 4318, and the references cited.
21 L. Venanzi, J. Chem. Soc., 1958, 719.
12 S. Nagao, T. Matsumoto, Y. Koga and K. Matsubara, Chem. 22 L. Jafarpour, E. D. Stevens and S. P. Nolan, J. Organomet.
Lett., 2011, 1036. Chem., 2000, 606, 49.
13 (a) H. M. Sun, Q. Shao, D. M. Hu, W. F. Li, Q. Shen and 23 A. J. Arduengo III, US Pat., 5 077 414, 1991.
Y. Zhang, Organometallics, 2005, 24, 331; (b) W. F. Li, H. M. 24 M. C. Smith, S. C. Davies, D. L. Hughes and D. J. Evans,
Sun, M. Z. Chen, Z. G. Wang, D. M. Hu, Q. Shen and
Acta Crystallogr., Sect. E: Struct. Rep. Online, 2001, 57, m509.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 8437–8445 | 8445