Controlled synthesis of nickel(ii) dihalides bearing two different or identical N-heterocyclic carbene ligands and the influence of carbene ligands on their structures and catalysis

Article abstract of DOI:10.1039/c1dt10077g

Ni(ii) dihalides bearing two different or identical NHC ligands have been prepared via a controlled indene elimination synthesis, and the former product provides a new route for the design of biscarbene Ni(ii)-based catalysts. The indene elimination reaction of the indenynickel(ii) complex (1-H-Ind)Ni(NHC)X (Ind = indenyl) with one equiv. of a distinct imidazolium salt at 100 °C afforded the first example of Ni(ii) dihalides bearing two different NHC ligands, i.e., Ni(iPr)(IPr)X₂ [iPr = 1,3-diisopropylimidazol-2- ylidene, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), X = Cl, 1; X = Br, 2] and Ni(iPr)(IMes)Br₂ [IMes = 1,3-bis(mesityl)imidazol-2- ylidene, 3]. Alternatively, complexes 1-3 can be synthesized using a bis-indenyl Ni(ii) complex (1-H-Ind)₂Ni as starting materials via a step-by-step indene elimination at different reaction temperatures. The direct reaction of (1-R-Ind)₂Ni (R = H or Me) with two equiv. of imidazolium salts at 100 °C afforded Ni(ii) dihalides bearing two identical NHC ligands, i.e., Ni(iPr)X₂ (X = Cl, 4; X = Br, 5) and Ni(IPr)Cl₂ (6). All of these complexes were characterized by elemental analysis, NMR spectroscopy and X-ray crystallography for complexes 1-5. The two identical or different NHC ligands in complexes 1-6 changed the coordination sphere of the nickel center from a typical square-planar geometry to a slightly tetrahedral array. A preliminary catalytic study on the cross-coupling reactions of aryl Grignard reagents with aryl halides revealed that complexes 1 and 2 possess the highest activity. In comparison, complexes 3 and 6 exhibited moderate activity and the least active complexes were 4 and 5.

Full text of DOI:10.1039/c1dt10077g

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 4697
www.rsc.org/dalton
PAPER
Controlled synthesis of nickel(II) dihalides bearing two different or identical
N-heterocyclic carbene ligands and the influence of carbene ligands on their
structures and catalysis†
Zhi-hong Liu, Yan-Chao Xu, Ling-Zhi Xie, Hong-Mei Sun,* Qi Shen and Yong Zhang
Received 14th January 2011, Accepted 16th February 2011
DOI: 10.1039/c1dt10077g
Ni(II) dihalides bearing two different or identical NHC ligands have been prepared via a controlled
indene elimination synthesis, and the former product provides a new route for the design of biscarbene
Ni(II)-based catalysts. The indene elimination reaction of the indenynickel(II) complex
(1-H-Ind)Ni(NHC)X (Ind = indenyl) with one equiv. of a distinct imidazolium salt at 100 ◦C afforded
the first example of Ni(II) dihalides bearing two different NHC ligands, i.e., Ni(iPr)(IPr)X2 [iPr =
1,3-diisopropylimidazol-2-ylidene, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), X = Cl, 1;
X = Br, 2] and Ni(iPr)(IMes)Br2 [IMes = 1,3-bis(mesityl)imidazol-2-ylidene, 3]. Alternatively, complexes
1–3 can be synthesized using a bis-indenyl Ni(II) complex (1-H-Ind)2Ni as starting materials via a
step-by-step indene elimination at different reaction temperatures. The direct reaction of (1-R-Ind)2Ni
(R = H or Me) with two equiv. of imidazolium salts at 100 ◦C afforded Ni(II) dihalides bearing two
identical NHC ligands, i.e., Ni(iPr)X2 (X = Cl, 4; X = Br, 5) and Ni(IPr)Cl2 (6). All of these complexes
were characterized by elemental analysis, NMR spectroscopy and X-ray crystallography for complexes
1–5. The two identical or different NHC ligands in complexes 1–6 changed the coordination sphere of
the nickel center from a typical square-planar geometry to a slightly tetrahedral array. A preliminary
catalytic study on the cross-coupling reactions of aryl Grignard reagents with aryl halides revealed that
complexes 1 and 2 possess the highest activity. In comparison, complexes 3 and 6 exhibited moderate
activity and the least active complexes were 4 and 5.
greatly from the development of NHCs complexes of ruthenium6
Introduction
and palladium,2 respectively.
During the past decade there has been increasing interest in the ap-
The replacement of palladium with nickel in catalytic reactions
plication of N-heterocyclic carbenes (NHCs) as ancillary ligands
has received increasing attention in recent years because nickel
for developing transition metal-based homogeneous catalysts,1–2
particularly as alternative to traditional tertiary phosphines.3
Exploring new, simple ways to synthesize NHC complexes has
also stimulated their application in a variety of catalytic processes.4
Besides their strong s-electron donation and tight coordination,
the representative advantages of NHCs as ancillary ligands
in catalysts design appears to be their tunable electronic and
steric properties.5 These advantages are achieved mostly through
modifications of the NHC moiety either at the nitrogen atom
or on the carbon backbone, which leads to the modulation of
the catalytic behaviors of their metal complexes. As a result, the
olefin metathesis and C–C cross-coupling reactions have benefited
is more cost-effective compared to palladium.2b–c In particular,
some of the Ni(II)-based NHC complexes exhibit higher catalytic
activity than palladium-based NHC complexes.7 However, in com-
parison with the intensively studied NHC-palladium complexes, a
relatively small number of well-defined NHC complexes involving
nickel have been reported.1e
Among these available well-defined NHC complexes of
nickel, biscarbene Ni(II) complexes of the formula Ni(NHC)2X2,
which were derived from widely used bisphosphine analogues
Ni(PR3)2X2, were reported by Herrmann et al. in 1997 as the
first example of structurally identified NHC complexes of Ni(II).8
Since then a variety of derivatives bearing two identical NHC
ligands, i.e., monodentate free carbenes,9–11 bidentate bridged
carbenes12 and functionalized carbenes,7b,13 have been synthesized
and structurally characterized. Studies on the catalytic activities
of these Ni(II) complexes reveal that a large proportion of these
complexes are effective catalysts for various C–C cross-coupling
reactions, such as Suzuki,7b,8,9a,13c–d,13f–g,13l Heck,13d Kumada,9e,12d,13h
Negishi13i and reductive Ullmann couplings.10b In addition, a
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 reference numbers 805189–805191 for 1–3, 636608–636607 for 4–
5, respectively. For crystallographic data in CIF or other electronic format
see DOI: 10.1039/c1dt10077g
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 4697–4706 | 4697
©

View Article Online
few of these complexes were applied as catalysts to the olefin
dimerization9b–c and norbornene polymerization reactions,13b and
as metal-templates for nonhalo complexes.10c Focusing on these
C–C cross-coupling reactions, the biscarbene Ni(II) complexes are
believed to be in situ reduced to active Ni(0) species, which catalyze
these reactions via a Ni(0) ↔ Ni(II) cycle.3b,7a,9a Accordingly, it
is commonly accepted that, in a typical C–C coupling catalytic
cycle, the strong s-electron donating ability of a NHC ligand aids
the initial reduction and the oxidative addition.14 Moreover, the
steric bulkiness of a NHC ligand can increases the rate of the
reductive elimination.7,9a,10b However, it is reasonable to consider
that a bulky NHC ligand may also hinder the coordination of
incoming substrates to the center metal and hence decrease the
activity of the center metal.9e,15 This prompted us to develop
the biscarbene Ni(II) complexes of the type Ni(NHC)(NHC)¢X2,
because such structural variation may serve as an alternative
strategy to modulate the coordination sphere of the center metal
by considering both the electric and steric environment of the
nickel center. No example of the target Ni(II) complex has been
reported and no other method described in the literature could be
used for synthesizing the Ni(NHC)(NHC)¢X2 species.
Currently, Ni(II) complexes bearing two identical NHC ligands
are primarily synthesized by either the substitution of the phos-
phine ligand in Ni(PR3)2X2 by free carbenes8,9c,9e,12 or by in situ
deprotonation of the corresponding imidazolium salts with Ni(II)
acetate.7b,8,9b,9d,10,12b,13a,13c–d,13h Such Ni(II) complexes can also be
synthesized by transmetalation via silver complexes,13b,13g,13j,13l or by
the oxidative addition of a hydrocarbyl halide to a biscarbene Ni(0)
complex Ni(NHC)2.9a A common characteristic of these methods
is the simultaneous attachment of two NHC ligands onto one
center metal. Therefore, it appears impossible to treat the metal
with two different carbenes during these reported procedures.
afford biscarbene Ni(II) complexes bearing two different NHC
ligands (1–3). This new synthetic route features the attachment
of two NHC ligands onto one metal center via a controlled
step-by-step procedure. In addition, a facile one-pot synthesis of
Ni(NHC)2X2 (4–6) via the direct reaction of (1-R-Ind)2Ni (R =
H or Me) with two equiv. of imidazolium salts was explored,
which precludes the use of more vigorous reaction conditions
reported previously. In addition, these biscarbene Ni(II) complexes
were screened for their catalytic activities in the cross-coupling of
arylmagnesium halides with aryl halides.
Results and discussion
Synthesis of biscarbene Ni(II) complexes
The indene elimination from indenylnickel(II) NHC complexes
went smoothly at 100 ◦C and gave Ni(II) dihalides bearing two
different NHC ligands in good yields.
As shown in Scheme 1, the desired complex Ni(iPr)(IPr)Cl2
(1) [iPr = 1,3-diisopropylimidazol-2-ylidene, IPr = 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene] can be isolated from the
reaction of (1-H-Ind)Ni(iPr)Cl with one equiv. of 1,3-bis(2,6-
diisopropylphenyl)imidazolium chloride (IPr·HCl) in 55% yield
as bright red crystals. Alternatively, complex 1 was synthesized
by the reaction of (1-H-Ind)Ni(IPr)Cl with one equiv. of 1,3-
diiospropylimidazolium chloride (iPr·HCl) with almost the same
yield. Since (1-H-Ind)Ni(iPr)Cl16a is more easy-to-access than (1-
H-Ind)Ni(IPr)Cl,16c the former reaction route is more economical.
To assess the potential of the new synthetic approach, a similar
reaction with other Ni(II) precursors and the imidazolium salt
were tested. As seen in Scheme 2, treatment of (1-H-Ind)Ni(iPr)Br
with one equiv. of IPr·HCl in the presence of NaBr afforded
the analogue complex 2 in 60% yield, and a similar reaction
with 1,3-bis(mesityl)imidazolium bromide (IMes·HBr) yielded the
corresponding complex 3. Of note, the exchange of the anion from
Cl to Br worked smoothly by the addition of NaBr during the
procedure (Scheme 2).
(1)
Further examination revealed that complexes 1–3 could also
be conveniently synthesized by a step-by-step indene elimination
reaction using the bis-indenyl Ni(II) complex as starting mate-
rials. As shown in Scheme 3, the introduction of two different
NHC ligands to one nickel center was greatly affected by the
reaction temperature, whereas variation of the ratio of the Ni(II)
precursor and imidazolium salt had little effect. Nevertheless, it is
necessary to completely remove the unreacted (1-H-Ind)2Ni and
imidazolium salt from the first indene elimination system in order
We previously found that indene elimination through the reac-
tion of bis-indenyl Ni(II) complexes (1-R-Ind)2Ni (Ind = indenyl)
with the corresponding imidazolium salt at 45 ◦C represents
a convenient route for the synthesis of indenylnickel(II) NHC
complexes (eqn (1)).16 Similar protocols involving either Cp2Ni or
Cp*Ni(acac) lead to the corresponding Ni(II) NHC complexes via
cyclopendiene elimination.17 Herein we show that a further indene
elimination from indenylnickel(II) NHC complexes at 100 ◦C can
Scheme 1 Synthesis of complex 1 via indene elimination.
4698 | Dalton Trans., 2011, 40, 4697–4706 This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Scheme 2 Synthesis of complexes 2 and 3 via indene elimination.
to avoid the formation of the Ni(II) complex bearing identical
NHC ligands during the second indene elimination procedure.
Fortunately, (1-H-Ind)2Ni can be easily removed by washing the
crude product with hexane and the imidazolium salt can be easily
removed by extracting the obtained solid with toluene. Thus, this
new synthetic route from (1-H-Ind)2Ni features the attachment of
two different NHC ligands onto one metal center by a controlled
step-by-step procedure.
Accordingly, the direct reaction of (1-R-Ind)2Ni with two equiv.
of the imidazolium salt at 100 ◦C should be a straightforward
route for the synthesis of Ni(II) dihalides bearing identical NHC
ligands. As shown in Scheme 4, the novel biscarbene Ni(II) complex
Ni(iPr)2Cl2 (4), with a relatively less sterically bulky carbene ligand,
was easily isolated from the reaction of (1-H-Ind)2Ni with two
equiv. of iPr·HCl in good yield (56%). The same reaction with
(1-CH3-Ind)2Ni afforded 4 in a higher yield (67%). Following this
procedure, either the dibromide analogue Ni(iPr)2Br2 (5) or the
dichloride analogue Ni(IPr)2Cl2 (6) bearing the more sterically
bulky carbene ligand, could be obtained in yields of 65% or 60%,
respectively. This one-pot procedure from (1-R-Ind)2Ni requires
no extra step to isolate and purify the free NHC ligand, gives rise to
negligible amounts of by-products and allows the easy isolation of
the desired complex. Consequently, this procedure is an alternative
convenient method for the synthesis of Ni(II) complexes of the type
Ni(NHC)2X2.
solutions of them were stirred at 80 ◦C over a period of 6 h
under pure argon. All complexes dissolved readily in THF and
chlorinated solvents of which 1, 2 and 6, containing IPr as the
co-ligand, possess better solubility and dissolve well in toluene,
hexane and diethyl ether. In contrast, complexes 3, 4 and 5 were
barely soluble in toluene, and insoluble in hexane and diethyl
ether.
The formation of complexes 1–6 are supported by elemental
analysis and NMR spectroscopy. The data for Ni(IPr)2Cl2 (6) were
identical to those reported previously.9c,9e NMR spectroscopic
data of 1–3 indicated the presence of two different N-heterocyclic
carbene ligands (Table 1). For example, each 1H NMR spectrum
shows two singlet signals assigned to the protons at the 4 (H4) and
5 (H5) positions of the carbene rings at d 6.47 and 7.00 ppm for 1, d
6.51 and 7.01 ppm for 2, and d 6.55 and 6.93 ppm for 3. In each 13
C
NMR spectrum inequivalent Ccarbene resonances were observed as
two singlet signals at d 164.79 and 177.41 ppm for 1, d 163.68 and
178.23 ppm for 2, and d 162.67 and 174.70 ppm for 3. On the other
hand, in the NMR spectra of complexes 4–6, the 1H resonance due
to H4/H5 appeared as one singlet at d 6.81 ppm for 4,6.82 ppm
for 5 and 6.54 ppm for 6, whereas the singlet signal assigned to
the equivalent Ccarbene appeared at d 166.49 ppm for 4, 167.59 ppm
for 5 and 168.42 ppm for 6. The NMR data of complexes 1–6 are
in good agreement with other known biscarbene Ni(II) complexes
bearing two identical NHC ligands.8–10
In addition, since the above mentioned singlet signals in the 1H
NMR spectra of 1–3 could be further assigned by 1H-1H NOESY
experiments, the resonance due to H4/H5 of the iPr ligand of 1
and 2 had a slightly upfield shift of Dd 0.34 and 0.31 ppm when
compared to the corresponding signal for 4 and 5, respectively. The
signal assigned to the same position protons of the IPr ligand of 1
Characterization of biscarbene Ni(II) complexes
All described complexes were isolated as bright red crystals with
air-stability. Complexes 1–3 are thermally stable in solution since
no ligand exchange is observed even if the toluene or THF
Scheme 3 Synthesis of complexes 1–3 via a step-by-step indene elimination.
The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 4697–4706 | 4699
This journal is
©

View Article Online
Scheme 4 Synthesis of complexes 4–6 via a one-pot indene elimination.
Table 1 Selected 1H and 13C NMR chemical shifts in biscarbene Ni(II) complexesa
d (N(CH)2N)
d (NCH2N)
Complex
d (NCHMe2)
iPr
IPr/IMes
iPr
IPr/IMes
Ni(iPr)(IPr)Cl2 (1)
Ni(iPr)(IPr)Br2 (2)
Ni(iPr)(IMes)Br2 (3)
Ni(iPr)2Cl2 (4)
5.78
5.76
5.86
6.66
6.62
/
/
/
/
6.47
6.51
6.55
6.81
6.82
/
/
/
/
7.00
7.01
6.93
/
164.8
163.7
162.7
166.5
167.6
/
177.4
178.2
174.7
/
Ni(iPr)2Br2 (5)
/
/
Ni(IPr)2Cl2 (6)
6.54
6.54
6.58
6.63
168.4
168.4
170.4
168.9
b
Ni(IPr)2Cl2
b
Ni(IPr)2Br2
/
/
b
Ni(IMes)2Br2
a
d: ppm. b cited from ref. 9e.
and 2 was downfield from that of Dd 0.46 ppm for 6 and Dd 0.43
For complexes 4 and 5, since the nickel atom resides in each case
at special point of symmetry, so the distances and angles found
in each of them stemmed from symmetry restrictions and half of
the molecule of 4 or 5 shown in Fig. 3 is drawn with its nickel
atom on the crystallographic inversion center. The X-ray analysis
revealed that the crystalline states of 1–3 are monomeric, whereas
each of 4 and 5 consisted of two independent molecules in the
asymmetric unit and both were refined. Since both independent
molecules presented in the crystals of 4 and 5 showed essentially
the same structural parameters, only one of them is presented.
As seen from Fig. 1, 2 and 3, for all of these complexes, a trans
arrangement of the carbene ligands in the solid state was detected
with the sum of the four bond angles at the Ni equal to 360.8◦
for 1, 361.2◦ for 2 and 360.0◦ for 3–5. Two carbene ring planes
are oriented nearly perpendicular to the NiC2X2 coordination
plane (X = Cl, Br) with dihedral angles ranging from 70.3 to
89.8◦, as well as at angles close to 90◦ (ranging from 87.4 to
94.3◦) to the halide ligands. The Ni–Ccarbene bond lengths were
ppm for Ni(IPr)2Br2,9e respectively. Similar differences in pairwise
9e
d values among the spectral data of 3, 5 and Ni(IMes)2Br2 were
also observed (Table 1).
Furthermore, similar variations of pairwise d values were found
1
in the 13C{ H} NMR spectra11f of these complexes. For example,
the signal due to the Ccarbene of the iPr moiety in 1 is slightly upfield
from that of Dd 2.11 ppm for 4, whereas the signal due to the
Ccarbene of the IPr moiety in 1 showed a remarkable downfield shift
of Dd 8.99 ppm compared to the corresponding signals for 6.9e
The formation of complexes 1–5 was further confirmed by
X-ray crystallography. Crystals suitable for X-ray diffraction
studies were grown from cold toluene–hexane solutions of 1–5.
The crystallographic data and selected bond lengths and angles are
listed in Tables 2, 3 and 4, respectively. The molecular structures
of these complexes are shown in Fig. 1 (1 and 2), 2 (3) and 3 (4
and 5), respectively. Half of the structure of 3 depicted in Fig. 2 is
symmetry-generated with its crystallographically defined C2 axis
passing through the nickel center and two carbene carbon atoms.
˚
found to lie in a range of 1.888–1.929 A, whereas the Ni–Cl and
4700 | Dalton Trans., 2011, 40, 4697–4706
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Table 2 X-ray crystallographic data for 1–5
1
2
3
4
5
Formula
Formula weight
T/K
C36H52Cl2N4Ni
670.43
193(2)
Mo-Ka
Monoclinic
P21/n
C36H52Br2N4Ni
759.35
223(2)
Mo-Ka
Monoclinic
P21/n
C30H40Br2N4Ni
675.19
223(2)
Mo-Ka
Tetragonal
P43 21 2
C18H32Cl2N4Ni
434.09
193(2)
Mo-Ka
Monoclinic
P21/c
C18H32Br2N4Ni
523.01
193(2)
Mo-Ka
Monoclinic
P21/c
Radiation used
Crystal system
Space group
Unit cell dimensions
˚
a/A
12.5520(11)
13.6298(12)
21.938(2)
103.117(2)
3655.2(6)
4
1.218
0.706
1432
3.14–25.35
35003
12.663(2)
13.596(2)
22.415(4)
104.855(2)
3730.3(12)
4
1.352
2.691
1576
3.14–25.50
16211
11.533(4)
11.533(4)
22.823(7)
90 [0.12(4)]a
3036.0(17)
4
1.477
3.296
1384
3.07–25.50
5587
16.280(4)
8.7682(16)
16.734(4)
111.472(4)
2222.9(8)
4
1.297
1.121
920
3.39–27.48
24040
16.474(4)
8.907(2)
16.744(4)
110.525(5)
2301.0(10)
4
1.510
4.324
1064
3.14–27.48
25167
˚
b/A
˚
c/A
b (◦)
3
˚
V/A
Z
Dc/g cm-3
m/mm-1
F(000)
q range/◦
Reflection collected
Independent reflections, Rint
Goodness-of-fit on F2
R1, wR2 [I > 2s(I)]
R1, wR2 (all data)
6688, 0.0371
1.136
0.0429, 0.0835
0.0501, 0.0862
6862, 0.0290
1.034
0.0912, 0.2804
0.1018, 0.2927
2807, 0.0303
1.124
0.0868, 0.2533
0.0944, 0.2654
5082, 0.0365
1.182
0.0579, 0.1089
0.0710, 0.1138
5273, 0.0407
1.113
0.0668, 0.1804
0.0809, 0.1906
a Absolute structure parameter.
◦
˚
Table 3 Selected bond lengths (A), angles ( ) and structural parameters for 1–3
1
2
3
Ni1–Cl1
Ni1–Cl2
Ni1–C1
Ni1–C28
2.1831(7)
2.1875(6)
1.923(2)
1.907(2)
Ni1–Br1
Ni1–Br2
Ni1–C1
Ni1–C28
2.2577(13)
Ni1–Br1
Ni1–Br1A
Ni1–C1
2.2670(14)
2.2670(14)
1.929(12)
1.888(12)
2.2847(12)
1.912(6)
1.897(7)
Ni1–C12
C1–Ni1–C28
Cl1–Ni1–Cl2
C1–Ni1–Cl1
C1–Ni1–Cl2
C28–Ni1–Cl1
C28–Ni1–Cl2
173.10(9)
172.51(3)
94.30(6)
88.19(6)
89.15(7)
89.12(7)
C1–Ni1–C28
Br1–Ni1–Br2
C1–Ni1–Br1
C1–Ni1–Br2
C28–Ni1–Br1
C28–Ni1–Br2
171.2(3)
170.21(5)
88.78(17)
95.15(17)
87.8(2)
C1–Ni1–C12
Br1–Ni1–Br1 A
C1–Ni1–Br1
C1–Ni1–Br1A
C12–Ni1–Br1
C12–Ni1–Br1A
180.000(1)
174.85(10)
92.58(5)
92.58(5)
87.42(5)
87.42(5)
89.5(2)
Dihedral angle between the NiC2X2 coordination plane and the carbene ring
plane
NiC2Cl2/IPr
NiC2Cl2/iPr
iPr/IPr
78.0
79.8
24.9
NiC2Br2/IPr
NiC2Br2/iPr
iPr/IPr
75.9
89.5
19.0
NiC2Br2/IMes
NiC2Br2/iPr
iPr/IMes
70.3
86.1
23.6
˚
˚
Ni–Br bond lengths were 2.1831–2.1806 A and 2.3111–2.2577 A,
respectively. These structural data are in agreement with values
found for other biscarbene Ni(II) complexes of the type trans-
Ni(NHC)2X2.8–10
On the other hand, as anticipated, a more detailed study on
the structures of 1–5 reveals that there are obvious structural
differences among these complexes. For example, each molecular
structure of 4 and 5 consists of a linear nickel center, i.e., both
of the trans Ccarbene–Ni–Ccarbene and X–Ni–X angles in 4 or 5 are
equal to 180.0◦, with two carbene rings in a parallel conformation
(dihedral angle ca. 0.0◦ for 4 or 5). In addition, the two Ni–Ccarbene
bonds and the two Ni–X bonds presented in 4 or 5 are pairwise
equal in length. Thus, each of 4 and 5 exhibits a typical square-
planar trans geometry with the nickel atom as the inversion center,
as observed for analogous biscarbene Ni(II) complexes of the
type trans-Ni(NHC)2X2, which bearing identical symmetrical8–10
or unsymmetrical9b substituted carbene rings.
◦
˚
Table 4 Selected bond lengths (A), angles ( ) and structural parameters
for 4–5
4
5
Ni1–Cl1
Ni1–C1
2.1806(17)
1.919(3)
Ni1–Br1
Ni(1)–C(1)
2.3111(7)
1.918(5)
C1–Ni1–C1A
Cl1–Ni1–Cl1A
C1–Ni1–Cl1
180.0(2)
180.00(4)
89.62(9)
90.38(9)
C1–Ni–C1A
Br1–Ni1–Br1A
C1–Ni1–Br1
C1–Ni1–Br1A
180.0
180.0
89.82(14)
90.18(14)
C1–Ni1–Cl1A
Dihedral angle between NiC2X2 coordination plane
and the carbene ring plane
NiC2Cl2/iPr
iPr/iPr
89.8
0.0
NiC2Br2/iPr
iPr/iPr
79.2
0.0
In contrast, for complex 3, besides the trans Ccarbene–Ni–Ccarbene
angle being 180.0◦ and the two Ni–Br bonds with equal length,
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 4697–4706 | 4701
©

View Article Online
Fig. 1 Molecular structures of 1 and 2 with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
Fig. 3 Molecular structures of 4 and 5 with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
More notably, for complexes 1 and 2, besides some structural
characteristics, i.e., a rather bent trans Br–Ni–Br angle, a staggered
conformation of the two carbene rings and two different lengths
of the Ni–Ccarbene bonds, to the structures resemble those of 3.
However, further influences of the coordination of two different
NHC ligands onto one nickel center were observed as followed:
(1) the trans Ccarbene–Ni–Ccarbene angle was also bent slightly from
linearity to 173.10(9)◦ in 1 and to 171.2 (3)◦ in 2; (2) the
lengths of the two Ni–X bonds are slightly different from each
Fig. 2 Molecular structure of 3 with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
˚
other, i.e., 2.1831(7) and 2.1875(6) A for 1 and 2.2577(13) and
˚
2.2847(12) A for 2. As a result, the simultaneous coordination
some structural variations were observed. These include: (1) the
trans Br–Ni–Br angle is bent slightly from the ideal 180.0◦ to
174.85(10)◦; (2) the two carbene rings are no longer parallel to
each other and adopt a staggered conformation (dihedral angle
ca. 23.6◦); and (3) the two Ni–Ccarbene bonds are not equal in length.
A longer distance between Ni and Ccarbene of the IMes moiety (Ni–
of iPr and IPr ligands to one nickel atom in 1 or 2 changes the
coordination sphere of the nickel atom from a typical square-
planar geometry to a distorted one that appears to be a slightly
tetrahedral array. The more distorted geometry around the nickel
atom of 2 compared to that of 3 possibly be attributed to the larger
steric influence of the IPr ligand compared to the IMes ligand.9e
Interestingly, such structural characteristics are rather similar
to that of a monocarbene monophosphine complex of the type
Ni(PPh3)(IPr)X2 reported by Matsubara et al.9e For example, in
˚
C1 = 1.929(12) A) compared with that between the same nickel
˚
atom and the Ccarbene of the iPr moiety (Ni–C12 = 1.888(12) A) was
observed.
4702 | Dalton Trans., 2011, 40, 4697–4706
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
two independent molecules of Ni(PPh3)(IPr)Cl2, the trans Ccarbene
–
3 (entry 13) and 6 (entries 21–22) showed moderate activity. The
least reactive complexes were 4 and 5 which furnished only 8 (entry
16) and 14% (entry 19) yields of 4-methylbiphenyl, respectively. In
addition, the observation that complexes 3–5 showed relatively
lower activities in Et2O (entries 12, 15 and 18) compared to their
performances in THF (entries 13, 16 and 19) may be due to their
very poor solubility in Et2O. The yield for 4-bromotoluene is
generally higher than that for 4-bromoanisole, which is most likely
due to the later compound being more deactivated compared to
the former.18 Thus, these results obtained show clearly that the
present structural variations of these biscarbene Ni(II) complexes
can exert a significant influence on their catalytic activities. The
highest activity of 1 and 2 in the cross-coupling reaction possibly
be due to the steric bulkiness provided by the IPr ligand and
the electric donation of the iPr ligand. Alternatively, the different
activities among these biscarbene Ni(II) complexes suggest that a
more dissymmetric coordination sphere around the nickel center,
i.e., that found in 1 or 2, should benefit their higher catalytic
activities. This is in agreement with the notion that a dissymmetric
coordination environment around a center metal can permit the
facile activation of M(II) to M(0) (M = Ni7b and Pd19) and
hence strong reactivity of the complex. The fact that complex
6 showed higher activity than complex 4 supports the concept
that the sterically bulky NHC ligand, especially a ligand with
the sterics at the ortho position of the imidazolyl aryl groups,
can facilitate the reductive elimination to form the desired cross-
coupling product together with the regeneration of the catalytically
active species.10b,20
Ni–P angles of 173.8(1)◦ and 165.9(1)◦, the trans Cl–Ni–Cl angles
of 178.10(5) and 165.78(6)◦ as well as different lengths of the two
9e
˚
Ni–Cl bonds (i.e. 2.146(1) and 2.277(1) A) were observed.
Therefore, the present structural variations among the com-
plexes 1–6 reflect the attachment of two different NHC ligands
onto one nickel atom. The use of two different NHC ligands
represents an alternative strategy to efficiently modulate the
coordination environment around the center metal, mostly due
to the different electric and/or steric influences of the two NHC
ligands.
Catalysis of biscarbene Ni(II) complexes
The reactivity of complexes 1–6 was tested in the Kumada–Corriu
reaction to disclose any effect due to the coordination variation of a
center metal on catalysis. The cross-coupling of phenylmagnesium
bromide with 4-bromotoluene or 4-bromoanisole using 1 mol %
◦
of a catalyst at 45 C was chosen as standard test reactions (eqn
(2)). The results obtained are summarized in Table 5.
(2)
As can be seen from Table 5, there are significant differences
in catalytic activities among these bis(NHC) Ni(II) complexes
under the present conditions. Complexes 1 and 2 exhibit the
highest catalytic activity for both 4-bromotoluene (entries 1 and
7) and 4-bromoanisole (entries 4 and 9). For example, the cross-
coupling reaction of 4-bromotoluene with phenylmagnesium
bromide afforded a quantitative yield of 4-methylbiphenyl with
1 mol % of 1 or 2 in 1.5 h, and the yield still reached 92% for 1
(entry 3) or 95% for 2 (entry 8) even when the loading of 1 or 2 was
reduced to 0.5 mol % in 2 h, respectively. Moreover, complexes
Notably, complexes 1 and 2 can also catalyze the cross-coupling
reactions of other kinds of coupling partners under the present
conditions used. This includes the coupling of 4-bromoanisole
with phenylmagnesium chloride (entry 5: 1, 88%), the coupling of
4-chlorotoluene (entry 10: 2, 48%) or 4-chloroanisole (entry 11: 2,
Table 5 The cross-couplings of aryl Grignard reagents with aryl halides catalyzed by biscarbene Ni(II) complexesa
Entry
Catalyst
Solvent (volume ratio)
X
X¢
R
Yield (%)
1b
2c
1
1
1
1
1
1
2
2
2
2
2
3
3
3
4
4
4
5
5
5
6
6
6
Et2O
Et2O
Et2O
Et2O
Et2O–THF (2/3)
Et2O–THF (2/3)
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
THF
THF
Et2O
THF
THF
Br
Br
Br
Br
Br
Cl
Br
Br
Br
Cl
Cl
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
CH3
CH3
CH3
OCH3
OCH3
OCH3
CH3
CH3
OCH3
CH3
OCH3
CH3
CH3
OCH3
CH3
CH3
OCH3
CH3
CH3
OCH3
CH3
CH3
100
92
92
92
88
36
100
95
94
48
40
10
56
38
trace
8
trace
trace
14
6
68
3d
4
5
6
7b
8d
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Et2O
THF
THF
Et2O
THF
Et2O
69
53
OCH3
a Reaction conditions: Ni(II) complex (1.0 mol %), C6H5MgBr (1.2 mmol, 1.0 M in Et2O), C6H5MgCl (1.2 mmol, 1.0 M in THF), Aryl bromide (1.0 mmol),
total volume (2.0 mL), 45 ◦C (temperature of the oil bath), 2 h, GC yield. b 1.5 h. c C6H5MgBr (1.5 mmol, 1.0 M in Et2O). d Ni(II) complex (0.5 mol %).
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 4697–4706 | 4703
©

View Article Online
40%) with phenylmagnesium bromide, and even the coupling of
4-chloroanisole with phenylmagnesium chloride (entry 6: 1, 36%).
These results illustrate the potential of these new type of Ni(II)
complexes as suitable precatalysts for C–C coupling applications.
Synthesis
Ni(iPr)(IPr)Cl2 (1), procedure A. A Schlenk flask was charged
with (1-H-Ind)Ni(iPr)Cl (0.53 g, 1.47 mmol), IPr·HCl (0.62 g,
1.47 mmol), toluene (15 mL) and a stir bar. The resulting mixture
was stirred for 12 h at 100 ◦C, filtered, and evaporated to dryness.
The residue was extracted with hot toluene and crystallized
from concentrated toluene–hexane at 0 ◦C. The product was
precipitated as bright red crystals in yield of 55% (0.54 g). mp 227–
Conclusions
The present work provides a facile synthetic route for producing
novel biscarbene Ni(II) complexes of the type Ni(NHC)(NHC)¢X2
(1–3) via a controlled step-by-step indene elimination reaction
using available bis-indenyl Ni(II) complexes as starting materials.
In addition, the direct reaction of bis-indenyl Ni(II) complexes
with two equiv. of imidazolium salts at 100 ◦C afforded the
biscarbene Ni(II) complexes of the type Ni(NHC)2X2 (4–6). The
two identical or different NHC ligands in complexes 1–6 changed
the coordination sphere of the nickel center from a typical square-
planar geometry to a slightly tetrahedral array. Our studies also
demonstrate that the simultaneous coordination of two different
NHC ligands to one nickel center can be used as an alternative
strategy to significantly improve the catalytic activity of Ni(II)
complexes. This is because this structural variation can modulate
both the electric and steric environment of the nickel center
in a straightforward manner. The highest catalytic activity was
observed for complexes 1 or 2 as shown by the cross-coupling
reactions of aryl Grignard reagents with aryl halides. This high
activity is possibly related to the more dissymmetric coordination
sphere around the nickel center in 1 or 2. Since these new type Ni(II)
complexes are readily accessible by the reaction of (1-R-Ind)2 with
a variety of imidazolium salts under mild conditions, there is the
potential to further fine-tune their catalytic activity by surveying
well-matched pairs of NHC ligands. The detailed structure-
reactivity relationship study of biscarbene Ni(II) complexes as
well as the scope of these Ni(II) complexes in other C–C coupling
reactions are currently under investigation in our laboratory
◦
231 C (dec). Anal. Calcd for C36H52Cl2N4Ni: C, 64.89; H, 7.69;
1
N, 7.88. Found: C, 64.70; H, 7.80; N, 8.06. H NMR (CDCl3):
d 1.06 (d, 12H, Ar-CH(CH3)2), 1.21 (d, 12H, NCH(CH3)2), 1.37
(d, 12H, Ar-CH(CH3)2), 3.02 (m, 4H, Ar–CH(CH3)2), 5.78 (m,
2H, NCH(CH3)2), 6.47 (s, 2H, iPr-NCH CHN-iPr), 7.00 (s, 2H,
Ar-NCH CHN–Ar), 7.46 (d, 4H, m-Ar-H), 7.61 (t, 2H, p-Ar-
H). 13C NMR (CDCl3): 22.7 (Ar-CH(CH3)2), 23.3 (NCH(CH3)2),
26.8 (Ar-CH(CH3)2), 28.9 (Ar-CH(CH3)2), 51.7 (NCH(CH3)2),
116.1 (iPr–NCH CHN-iPr), 123.8, 123.9 (m-Ar-C and Ar–
NCH CHN–Ar), 129.8 (p-Ar-C), 136.4 (o-Ar-C), 148.0 (ipso-
Ar-C), 164.8 (iPr–NCN-iPr), 177.4 (Ar–NCN–Ar).
Ni(iPr)(IPr)Cl2 (1), procedure B. A Schlenk flask was charged
with (1-H-Ind)Ni(IPr)Cl (0.72 g, 1.20 mmol), iPr·HCl (0.23 g,
1.20 mmol), toluene (15 mL) and a stir bar. The solution was stirred
for 12 h at 100 ◦C, filtered, and evaporated to dryness. The residue
was extracted with hot toluene and crystallized from concentrated
toluene–hexane at 0 ◦C. The product was precipitated as bright
red crystals in yield of 58% (0.47 g).
Ni(iPr)(IPr)Cl2 (1), procedure C. A Schlenk flask was charged
with iPr·HCl (0.38 g, 2.00 mmol), CH2Cl2 (4 mL) and a stir bar.
To this solution was added (1-H-Ind)2Ni (0.55 g, 1.91 mmol) in
15 mL of THF via syringe at room temperature. The solution
was stirred for 12 h at 45 ◦C, and evaporated to dryness. The
residue was washed with hexane, and extracted with toluene. To
this solution was added IPr·HCl (0.81 g, 1.91 mmol) and a stir
bar. The following preparation is similar to procedure A. After
workup, the product was obtained as bright red crystals in yield
of 48% (0.61 g).
Experimental Section
General procedures
Ni(iPr)(IPr)Br2 (2), procedure A. Following a procedure simi-
lar to the synthetic procedure A of 1, a Schlenk flask was charged
with (1-H-Ind)Ni(iPr)Br (0.41 g, 1.00 mmol), IPr·HCl (0.42 g,
1.00 mmol), NaBr (0.12 g, 1.20 mmol), toluene (15 mL) and a
stir bar. After workup, the product was precipitated as bright red
crystals in yield of 60% (0.46 g). mp 204–207 ◦C (dec). Anal. Calcd
for C36H52Br2N4Ni: C, 57.19; H, 6.65; N, 7.39. Found: C, 57.04; H,
6.81; N, 7.38. 1H NMR (CDCl3): d 1.05 (d, 12H, Ar-CH(CH3)2),
1.18 (d, 12H, NCH(CH3)2), 1.42 (d, 12H, Ar-CH(CH3)2), 3.17
(m, 4H, Ar–CH(CH3)2), 5.76 (m, 2H, NCH(CH3)2), 6.51 (s, 2H,
iPr-NCH CHN-iPr), 7.01 (s, 2H, Ar-NCH CHN–Ar), 7.44 (d,
4H, m-Ar-H), 7.58 (t, 2H, p-Ar-H). 13C NMR (CDCl3): d 23.2
(Ar-CH(CH3)2), 23.3 (NCH(CH3)2), 27.1 (Ar-CH(CH3)2), 29.3
(Ar-CH(CH3)2), 52.0 (NCH(CH3)2), 116.8 (iPr–NCH CHN-
iPr), 124.0, 124.8 (m-Ar-C and Ar–NCH CHN–Ar), 130.1 (p-
Ar-C), 136.7 (o-Ar-C), 148.0 (ipso-Ar-C), 163.7 (iPr–NCN-iPr),
178.2 (Ar–NCNAr).
All manipulations were performed under pure argon with rigorous
exclusion of air and moisture using standard Schlenk techniques.
Solvents were distilled from Na/benzophenone ketyl under pure
argon prior to use. (1-H-Ind)2Ni,21 1,3-diisopropylimidazolium
chloride (iPr·HCl),22 1,3-diisopropylimidazolium bromide
(iPr·HBr),22 1,3-bis(mesityl)imidazolium bromide (IMes·HBr),22
1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (IPr·HCl),23
(1-H-Ind)Ni(iPr)X (X = Cl or Br)16a and (1-H-Ind)Ni(IPr)Cl16c
were prepared by published methods. Organic reagents used for
cross-coupling reactions were purchased from Acros Organics,
Aldrich, and Alfa Aesar. Elemental analysis was performed
by direct combustion on a Carlo-Erba EA-1110 instrument.
NMR spectra were measured on a Varian Unity INOVA 400
or VNMRS 300 MHz spectrometer at 25 ◦C, the assignments
were confirmed by 1H-1H NOESY and HMBC experiments.
Gas chromatographic (GC) analysis was performed on a Varian
CP-3800 instruments equipped with an FID detector and a
capillary column AT.OV-101 (30 m ¥ 0.32 mm i.d., 0.10 mm
film). The oven program temperature was 80 ◦C (2 min)//
10 ◦C min-1//260 ◦C (2 min).
Ni(iPr)(IPr)Br2 (2), procedure B. Following a procedure sim-
ilar to the synthetic procedure C of 1, (1-H-Ind)2Ni (0.55 g,
1.91 mmol) in 15 mL of THF was added to a solution of iPr·HBr
4704 | Dalton Trans., 2011, 40, 4697–4706
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
(0.47 g, 2.00 mmol) in CH2Cl2 (4 mL) and stirred for 12 h at 45 ◦C.
After workup, IPr·HCl (0.81 g, 1.91 mmol) and NaBr (0.27 g,
2.29 mmol) were added. Yield: 0.76 g (52%).
26.0 (CH(CH3)2), 28.1 (CH(CH3)2), 123.8 (m-Ar-C and Ar–
NCH CHN–Ar), 129.1 (p-Ar-C), 136.8 (o-Ar-C), 146.7 (ipso-
Ar-C), 168.4 (Ar–NCN–Ar).
Ni(iPr)(IMes)Br2 (3), Procedure A. Following a procedure
similar to the synthetic procedure A of 1, a Schlenk flask was
charged with (1-H-Ind)Ni(iPr)Br (0.41 g, 1.00 mmol), IMes·HBr
(0.39 g, 1.00 mmol), toluene (15 mL) and a stir bar. After workup,
the product was precipitated as bright red crystals in yield of 63%
(0.42 g). mp 247–253 ◦C (dec). Anal. Calcd for C30H40Br2N4Ni: C,
53.41; H, 5.85; N, 8.22. Found: C, 53.37; H, 5.97; N, 8.30. 1H NMR
(CDCl3): d 1.29 (d, 12H, NCH(CH3)2), 2.38 (s, 12H, o-Ar–CH3),
2.46 (s, 6H, p-Ar–CH3), 5.88 (m, 2H, NCH(CH3)2), 6.55 (s, 2H,
iPr-NCH CHN-iPr), 6.93 (s, 2H, Ar-NCH CHN–Ar), 7.14
(d, 4H, m-Ar-H). 13C NMR (CDCl3): d 19.8 (NCH(CH3)2), 21.4
(Ar-CH(CH3)2), 23.1 (Ar-CH(CH3)2), 51.9 (NCH(CH3)2), 116.4
(iPr–NCH CHN-iPr), 123.1 (Ar–NCH CHN–Ar), 129.0 (m-
Ar-C), 136.5 (o-Ar-C), 137.3 (p-Ar-C), 138.6 (ipso-Ar-C), 162.7
(iPr–NCN-iPr), 174.7 (Ar–NCN–Ar).
X-ray structural determination
Suitable single crystals of 1, 2, 3, 4 and 5 were sealed in a thin-
walled glass capillary for X-ray structural analysis. Diffraction
data were collected on a Rigaku Mercury CCD area detector
at 193(2) K (for 1, 4 and 5) or at 223(2) K (for 2 and 3). The
structure was solved by direct methods and refined by full-matrix
least-squares procedures based on F2. All non-hydrogen atoms
were refined with anisotropic displacement coefficients. Hydrogen
atoms were treated as idealized contributions. The structures were
solved and refined using SHELXS-97 and SHELXL-97 programs,
respectively. Crystal data and collection and main refinement
parameters are given in Tables 2 and 3. Selected bond lengths
˚
(A) and angles (deg) for these complexes are given in Tables 4 and
5.
Ni(iPr)(IMes)Br2 (3), procedure B. Following a procedure
similar to the synthetic procedure C of 1, (1-H-Ind)2Ni (0.55 g,
1.91 mmol) in 15 mL of THF was added to a solution of iPr·HBr
(0.47 g, 2.00 mmol) in CH2Cl2 (4 mL) and stirred for 12 h at 45 ◦C.
After workup, IMes·HBr (0.73 g, 1.91 mmol) was added. Yield:
0.75 g (58%).
General procedure for the cross-coupling reactions
In a typical example, complex 1 (0.01 mmol), aryl bromide
(1.0 mmol), and THF (0.8 mL) were added to a Schlenk tube,
and the mixture was stirred at room temperature for 2 min. To
the solution was added a solution of PhMgBr (1.2 mL, 1.0 M
◦
in Et2O) at 45 C, which was stirred for 2 h. Then, the reaction
Synthesis of Ni(iPr)Cl2 (4), procedure A. A Schlenk flask was
charged with (1-H-Ind)2Ni (0.29 g, 1.00 mmol), iPr·HCl (0.38 g,
2.00 mmol), toluene (15 mL) and a stir bar. The solution was
stirred for 12 h at 100 ◦C, filtered, and evaporated to dryness.
The residue was quickly washed with cold hexane, extracted with
hot toluene and crystallized from concentrated toluene–hexane
at 0 ◦C. The product was precipitated as orange red crystals in
yield of 56% (0.24 g). mp 217–221 ◦C (dec). Anal. Calcd for
C18H32Cl2N4Ni: C, 49.81; H, 7.41; N, 12.40. Found: C, 49.72; H,
7.45; N, 12.54. 1H NMR (CDCl3): d 1.71 (d, 24H, NCH(CH3)2),
was quenched by the addition of diluted hydrochloric acid (1 M,
0.5 mL). The mixture was extracted with acetic ether (3 ¥ 4 mL).
The yield of desired product is determined by GC analysis, using
n-hexadecane as internal standard substance.
Acknowledgements
We are grateful to the NSF of China (20772089) and the Key
Laboratory of Organic Chemistry of Jiangsu Province for financial
support.
6.66 (m, 4H, NCH(CH3)2), 6.81 (s, 4H, iPr-NCH CHN-iPr). 13
C
NMR (CDCl3): d 23.8 (NCH(CH3)2), 52.2 (NCH(CH3)2), 116.7
(iPr–NCH CHN-iPr), 166.5 (iPr–NCN-iPr).
Notes and references
Ni(iPr)Cl2 (4), procedure B. Following a procedure similar to
the synthetic procedure A of 4, a Schlenk flask was charged with
(1-Me-Ind)2Ni (0.32 g, 1.00 mmol), iPr·HCl (0.38 g, 2.00 mmol),
toluene (15 mL) and a stir bar. After workup, the product was
precipitated as bright red crystals in yield of 67% (0.29 g).
Complexes 5 and 6 were prepared similarly to 4, procedure B.
1 For see recent reviews: (a) N-heterocyclic Carbenes in Transition Metal
Catalysis, ed. F. Glorius, Springer-Verlag: Berlin, Germany, 2007; (b) D.
Pugh and A. A. Danopoulos, Coord. Chem. Rev., 2007, 251, 610; (c) L.
H. Gade and S. Bellemin-Laponnaz, Coord. Chem. Rev., 2007, 251,
718; (d) J. A. Mata, M. Poyatos and E. Peris, Coord. Chem. Rev., 2007,
251, 841; (e) S. D´ıez-Gonza´lez, N. Marion and S. P. Nolan, Chem. Rev.,
2009, 109, 3612; (f) M. Poyatos, J. A. Mata and E. Peris, Chem. Rev.,
2009, 109, 3677.
2 (a) E. A. B. Kantchev, C. J. O’Brien and M. J. Organ, Angew. Chem.,
Int. Ed., 2007, 46, 2768; (b) H. Clavier and S. P. Nolan, Annu. Rep.
Prog. Chem., Sect. B, 2007, 103, 193; (c) F. Boeda and S. P. Nolan,
Annu. Rep. Prog. Chem., Sect. B, 2008, 104, 184; (d) N. Marion and S.
Ni(iPr)Br2 (5). Yield: 0.34 g, 65%. Mp 194–196 ◦C (dec).
Anal. Calcd for C18H32Br2N4Ni: C, 41.34; H, 6.17; N, 10.71.
Found: C, 41.25; H, 6.50; N, 10.60. 1H NMR (CDCl3): d 1.68 (d,
i
24H, NCH(CH3)2), 6.62 (m, 4H, NCH(CH3)2), 6.82 (s, 4H, Pr-
P. Nolan, Acc. Chem. Res., 2008, 41, 1440; (e) S. W
Acc. Chem. Res., 2008, 41, 1523.
u¨rtz and F. Glorius,
NCH CHN-iPr). 13C NMR (CDCl3): d 23.5 (NCH(CH3)2), 52.4
(NCH(CH3)2), 117.0 (iPr–NCH CHN-iPr), 167.6 (iPr–NCN-
iPr).
3 (a) D. Bourissou, O. Guerret, F. P. Gabbai and G. Bertrand, Chem.
Rev., 2000, 100, 39; (b) W. A. Herrmann, Angew. Chem., Int. Ed.,
2002, 41, 1290; (c) F. E. Hahn, Angew. Chem., Int. Ed., 2006, 45, 1348;
(d) N. M. Scott, H. Clavier, P. Mahjoor, E. D. Stevens and S. P. Nolan,
Organometallics, 2008, 27, 3181.
4 For see selected reviews: (a) P. L. Arnold and S. Pearson, Coord. Chem.
Rev., 2007, 251, 596; (b) I. J. B. Lin and C. S. Vasam, Coord. Chem. Rev.,
2007, 251, 642; (c) O. Ku¨hl, Chem. Soc. Rev., 2007, 36, 592; (d) F. E.
Hahn and M. C. Jahnke, Angew. Chem., Int. Ed., 2008, 47, 3122; (e) P.
de Fre´mont, N. Marion and S. P. Nolan, Coord. Chem. Rev., 2009, 253,
862.
Ni(IPr)Cl2 (6). Yield: 0.54 g, 60%. Mp 254–257 ◦C (dec).
Anal. Calcd for C54H72Cl2N4Ni: C, 71.53; H, 8.00; N, 6.18.
1
Found: C, 71.35; H, 8.11; N, 6.13. H NMR (CDCl3): d 0.83
(d, 24H, CH(CH3)2), 0.95 (d, 24H, CH(CH3)2), 2.77 (m, 8H,
CH(CH3)2), 6.54 (s, 4H, Ar-NCH CHN–Ar), 7.15 (d, 8H, m-Ar-
H), 7.43 (t, 4H, p-Ar-H). 13C NMR (CDCl3): d 22.7 (CH(CH3)2),
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 4697–4706 | 4705
©

View Article Online
5 (a) L. Cavallo, A. Correa, C. Costabile and H. Jacobsen, J. Organomet.
Chem., 2005, 690, 5407; (b) S. D´ıez-Gonza´lez and S. P. Nolan, Coord.
Chem. Rev., 2007, 251, 874; (c) H. Jacobsen, A. Correa, A. Poater, C.
Costabile and L. Cavallo, Coord. Chem. Rev., 2009, 253, 687.
6 (a) T. M. Trnka and R. H. Grubbs, Acc. Chem. Res., 2001, 34, 18; (b) H.
Clavier, K. Grela, A. Kirschning, M. Mauduit and S. P. Nolan, Angew.
Chem., Int. Ed., 2007, 46, 6786.
7 (a) V. P. W. Bo¨hm, T. Weskamp, C. W. K. Gsto¨ttmayr and W. A.
Hermann, Angew. Chem., Int. Ed., 2000, 39, 1602; (b) C.-H. Lee, W.-C.
Ke, K.-T. Chan, C.-L. Lai, C.-H. Hu and H. M. Lee, Chem.–Eur. J.,
2007, 13, 582.
Organometallics, 2004, 23, 6002; (c) P. L. Chiu, C. L. Lai, C. F.
Chang, C. H. Hu and H. M. Lee, Organometallics, 2005, 24, 6169;
(d) K. Inamoto, J. Kuroda, K. Hiroya, Y. Noda, M. Watanabe and T.
Sakamoto, Organometallics, 2006, 25, 3095; (e) E. F. Hahn, B. Heidrich,
A. Hepp and T. Pape, J. Organomet. Chem., 2007, 692, 4630; (f) C. Y.
Liao, K. T. Chan, Y. C. Chang, C. Y. Chen, C. Y. Tu, C. H. Hu and H.
M. Lee, Organometallics, 2007, 26, 5826; (g) Z. Xi, X. Zhang, W. Chen,
S. Fu and D. Wang, Organometallics, 2007, 26, 6636; (h) W. H. Yang,
C. S. Lee, S. Pal, Y. N. Chen, W. S. Hwang, I. J. B. Lin and J. C. Wang,
J. Organomet. Chem., 2008, 693, 3729; (i) Z. X. Xi, B. Liu and W. Z.
Chen, J. Org. Chem., 2008, 73, 3954; (j) Z. X. Xi, Y. B. Zhou and W. Z.
Chen, J. Org. Chem., 2008, 73, 8497; (k) X. Zhang, B. Liu, A. Liu, W.
Xie and W. Chen, Organometallics, 2009, 28, 1336; (l) K. Inamoto, J.
Kuroda, E. Kwon, K. Hiroya and T. Doi, J. Organomet. Chem., 2009,
694, 389; (m) S. Ray, J. Asthana, J. M. Tanski, M. M. Shaikh, D. Panda
and P. Ghosh, J. Organomet. Chem., 2009, 694, 2328.
8 W. A. Herrmann, G. Gerstberger and M. Spiegler, Organometallics,
1997, 16, 2209.
9 (a) D. S. McGuinness, K. J. Cavell, B. W. Skelton and A. H. White,
Organometallics, 1999, 18, 1596; (b) D. S. McGuinness, W. Mueller, P.
Wasserschied, K. J. Cavell, B. W. Skelton, A. H. White and U. Englert,
Organometallics, 2002, 21, 175; (c) A. L. MacKinnon and M. C. Baird,
J. Organomet. Chem., 2003, 683, 114; (d) Q. X. Liu, F. B. Xu, Q.
S. Li, H. B. Song and Z. Z. Zhang, Organometallics, 2004, 23, 610;
(e) K. Matsubara, K. Ueno and Y. Shibata, Organometallics, 2006, 25,
3422.
10 (a) H. V. Huynh, C. Holtgrewe, T. Pape, L. L. Koh and E. Hahn,
Organometallics, 2006, 25, 245; (b) H. V. Huynh, L. R. Wong and P. S.
Nr, Organometallics, 2008, 27, 2231; (c) R. Jothibasu and H. V. Huynh,
Organometallics, 2009, 28, 2505.
14 N. Hadei, E. A. B. Kantchev, C. J. O’Brien and M. G. Organ, Org. Lett.,
2005, 7, 1991.
15 G. Altenhoff, R. Goddard, C. W. Lehmann and F. Glorius, Angew.
Chem., Int. Ed., 2003, 42, 3690.
16 (a) H. M. Sun, Q. Shao, D. M. Hu, W. F. Li, Q. Shen and Y. Zhang,
Organometallics, 2005, 24, 331; (b) D. M. Hu, H. M. Sun, Y. S. Wang, Q.
Shen and F. Xue, Chinese J. Struct. Chem., 2006, 25, 1255; (c) L. Z. Xie,
H. M. Sun, D. M. Hu, Z. H. Liu, Q. Shen and Y. Zhang, Polyhedron,
2009, 28, 2585.
11 For biscarbene Ni(0) complexes, see: (a) A. J. Arduengo III, S. F.
Gamper, J. C. Calabress and F. Davidson, J. Am. Chem. Soc., 1994, 116,
4391; (b) P. L. Arnold, F. G. N. Cloke, T. Geldbach and P. B. Hitchcock,
Organometallics, 1999, 18, 3228; (c) S. Caddick, F. G. N. Cloke, P.
B. Hitchcock and A. K. de K. Lewis, Angew. Chem., Int. Ed., 2004,
43, 5824; (d) T. Schaub, M. Backes and U. Radius, Organometallics,
2006, 25, 4196; (e) K. Matsubara, S. Miyazaki, Y. Koga, Y. Nibu, T.
Hashimura and T. Matsumoto, Organometallics, 2008, 27, 6020; (f) A.
A. Danopoulos and D. Pugh, Dalton Trans., 2008, 30.
12 (a) R. E. Douthwaite, D. Hau¨ssinger, M. L. H. Green, P. J. Silcock, P. T.
Gomes, A. M. Martins and A. A. Danopoulos, Organometallics, 1999,
18, 4584; (b) D. S. Clyne, J. Jin, J. C. Galluci and T. V. RajanBabu, Org.
Lett., 2000, 2, 1125; (c) R. E. Douthwaite, M. L. H. Green, P. J. Silcock
and P. T. Gomes, Organometallics, 2001, 20, 2611; (d) J. Berding, M.
Lutz, A. L. Spek and E. Bouwman, Organometallics, 2009, 28, 1845.
13 (a) M. V. Baker, B. W. Skelton, A. H. White and C. C. Williams,
Organometallics, 2002, 21, 2674; (b) X. Wang, S. Liu and G. X. Jin,
17 (a) C. D. Abernethy, A. H. Cowley and R. A. Jones, J. Organomet.
Chem., 2000, 596, 3; (b) R. A. Kelly III, N. M. Scott, S. D´ıez-Gonza´lez,
E. D. Stevens and S. P. Nolan, Organometallics, 2005, 24, 3442;
(c) V. Ritleng, C. Barth, E. Brenner, S. Milosevic and M. J. Chetcuti,
Organometallics, 2008, 27, 4223.
18 S. K. Schneider, C. F. Rentzsch, A. Kru¨ger, H. G. Raubenheimer and
W. A. Herrmann, J. Mol. Catal. A: Chem., 2007, 265, 50.
19 For a family of [Pd(ally)(Ipr)Cl] complex, Nolan et al. Have shown that
the substitution of the ally moiety increased the dissymmetry of the ally
ligand around the Pd center, which allows facile activation of Pd(II) to
Pd(0): N. Marion, O. Navarro, J. Mei, E. D. Stevens, N. M. Scott and
S. P. Nolan, J. Am. Chem. Soc., 2006, 128, 4101.
20 J. K. Huang and S. P. Nolan, J. Am. Chem. Soc., 1999, 121, 9889.
21 F. H. Ko¨hler, Chem. Ber., 1974, 107, 570.
22 A. J. Arduengo III, US Pat., 5 077 414, 1991.
23 L. Jafarpour, E. D. Stevens and S. P. Nolan, J. Organomet. Chem., 2000,
606, 49.
4706 | Dalton Trans., 2011, 40, 4697–4706
This journal is
The Royal Society of Chemistry 2011
©