COMMUNICATION
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A new aspect of nickel-catalyzed Grignard cross-coupling reactions:
selective synthesis, structure, and catalytic behavior of a T-shape
three-coordinate nickel(I) chloride bearing a bulky NHC ligandw
a
a
b
a
Satoshi Miyazaki, Yuji Koga, Taisuke Matsumoto and Kouki Matsubara*
Received (in Cambridge, UK) 24th November 2009, Accepted 28th January 2010
First published as an Advance Article on the web 11th February 2010
DOI: 10.1039/b924716e
Novel T-shape three-coordinate nickel(I) chlorides bearing an
2
a nickel(0) complex, Ni(IPr) (1), with p-chlorotoluene at room
N-heterocyclic carbene ligand, NiCl(IPr)2 (IPr = 1,3-bis-
temperature (Scheme 2). This result strongly suggests that the
product selectivity reported by Kochi can be controlled by the
two bulky carbene ligands in 1. Apparently, these two carbene
ligands hamper the formation of oxidative addition adducts,
probably due to steric hindrance between the aryl moiety and
two IPr ligands. The use of aryl chlorides, such as 4-chloro-
anisole, 4-chlorobenzophenone and 2-chlorobenzaldehyde,
afforded the production of 2 (similar yields) and arenes
derived from an aryl radical in THF. The expected biaryl
(
2,6-diisopropylphenyl)imidazolin-2-ylidene), were isolated by a
reaction of Ni(0)(NHC) with aryl chlorides. This Ni(I) complex
2
was shown to act as a catalyst in a cross-coupling reaction of
aryl halides with phenylmagnesium chloride.
Oxidative addition of aryl halide to zerovalent nickel complexes
is an important reaction involved in significant cross-coupling
reactions, such as Suzuki–Miyaura and Kumada–Corriu reac-
1
0
tions, a-arylation, N-arylation, and Heck reactions. Kochi et al.
have previously investigated oxidative addition of aryl halides to
product as a result of radical coupling, 4,4 -dimethylbiphenyl,
1
was detected in the H NMR and GC-MS spectrum for
2
in detail. One electron transfer from nickel(0) to aryl
Ni(PEt)
4
the crude mixture as shown in Electronic Supplementary
Information (ESIw). Further reaction of 2 with chloroarenes
halide affords the production of nickel(I)-aryl-halide-anion
radicals, which are converted to ArNi(II)XL or Ni(I)XL and
6
b
did not yield [NiCl
(IPr) ] (4), but, when dissolved in
2 2
n
n
an aryl radical (Scheme 1). However, there is a paucity of
literature describing the corresponding nickel(I) halides: only
nickel(I) chlorides having two or three triphenylphosphine
CDCl , 4 was generated quantitatively. Complex 2 was quite
3
unstable in air, smoothly changing color from yellow to pale
red. Complex 2 was isolated by recrystallization at ꢀ30 1C
3
ligands have been presented. A recent study proposed Ni(I)-
4
catalyzed Negishi cross-coupling reaction of alkyl halide.
2
under an N atmosphere from hexane–THF (3/1) solution
(34% yield).z
Delocalization of the unpaired electron in the terpyridine
ligand was found to be the key to the stabilization of the
catalytic active species, illustrating the importance of the Ni(I)
chemistry in the catalytic cross-coupling reactions.
6 6
The dissolution of crystals of 2 in C D led to the formation
8
1
of a dimeric complex 3 and free IPr with 2, detected in the H
NMR (Scheme 2). However, with excess free IPr added to the
solution, 3 was not detected, indicating the existence of an
equilibrium between 2 and 3 + 2IPr. We confirmed this
equilibrium by addition of IPr to isolated 3 giving a mixture
containing 2 and free IPr (see ESIw).
Conversely, high catalytic activity has often been achieved
t
, P Bu
when bulky supporting ligands, such as PCy
3
3
, IPr and
5
SIPr, were used in these reactions. In our recent studies,
nickel complexes bearing bulky N-heterocyclic carbene (NHC)
ligand, 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (IPr),
were shown to perform efficient catalytic cross-coupling
The complex 2 is paramagnetic, S = 1/2 and g = 2.18 as
demonstrated by magnetic susceptibility measurements and
1
the presence of broad H resonances in the NMR spectrum.
6
reactions of haloarenes. Although several nickel(II) oxidative
Single crystals of 2 were obtained from the solution of either
hexane or THF–hexane and the structure was determined by
X-ray diffraction studies.y As shown in Fig. 1, both structures
showed rare, planar, 3-coordinate T-shape Ni(I) structures
(the sum of the angles around Ni is 360.0(3)). One hexane or
two THF molecules are included in each cell; however, the
solvent was not located in the coordination sphere of nickel.
addition products have been isolated using moderate-size
7
ligands, it remains unclear how an aryl halide adds to nickel(0)
species bearing bulky ligands and what the key intermediate is in
the catalytic reactions. Consequently this prompted us to deter-
mine the nickel oxidative adducts bearing a bulky ligand, IPr,
and to examine the catalytic activity of the isolated adducts.
A novel, monovalent, 15-electron 3-coordinate nickel chloride,
1
0
Neither Clꢁ ꢁ ꢁH hydrogen bonding nor Niꢁ ꢁ ꢁH anagostic
1
1
[
NiCl(IPr)
2
] (2), was selectively generated from the reaction of
interactions were observed at the methyl groups of the
NHC ligand. Although some 3-coordinate nickel(I) complexes
bearing diphosphine and diketiminate ligands have been
a
b
Department of Chemistry, Faculty of Science, Fukuoka University,
-19-1 Nanakuma, Fukuoka 814-0180, Japan.
E-mail: kmatsuba@fukuoka-u.ac.jp; Fax: +81-92-865-6030
Analytical Center, Institute for Advanced Material Study,
Kyushu University, 6-1 Kasuga, Fukuoka, Japan
8
w Electronic supplementary information (ESI) available: Experimental
details. CCDC 737686–737687. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b924716e
2
n
Scheme 1 Reaction of Ni(0)L with haloarenes.
1
932 | Chem. Commun., 2010, 46, 1932–1934
This journal is ꢂc The Royal Society of Chemistry 2010