Organonickel complexes encumbering bis-imidazolylidene carbene ligands: Synthesis, X-ray structure and catalytic insights on Buchwald-Hartwig amination reactions

Article abstract of DOI:10.1016/j.jorganchem.2016.12.029

New four coordinated homoleptic bis(diimidazolylidene)nickel(II) complexes (C1 & C2) were synthesized and characterized by elemental analysis, NMR (¹H and¹³C) as well as ESI-Mass spectrometry. The molecular structure of the complex C1 was identified by means of single-crystal X-ray diffraction analysis, which revealed that the complexes possess a distorted square planar geometry with chelating bis(diimidazolylidene) NHC ligands and two non coordinating bromide counter ions in tetradentate C₄fashion. A survey of their catalytic activity in Buchwald?Hartwig amination has been performed. The newly synthesized complexes also catalyzed the amination of aryl chlorides in the presence of KO^tBu. Various aryl chlorides and amines can react smoothly to give the corresponding aminated products in moderate to high yields. The scope of the reaction encompasses electronically varied aryl chlorides and nitrogen-containing heteroaryl chlorides, including pyridine and quinoline derivatives. Both secondary and primary amines are well tolerated under the optimal reaction conditions.

Full text of DOI:10.1016/j.jorganchem.2016.12.029

Journal of Organometallic Chemistry 831 (2017) 1e10
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Organonickel complexes encumbering bis-imidazolylidene carbene
ligands: Synthesis, X-ray structure and catalytic insights on Buchwald-
Hartwig amination reactions
,
*
Muthukumaran Nirmala a, Gandhi Saranya a, Periasamy Viswanathamurthi a
,
Roberta Bertani b, Paolo Sgarbossa b, Jan Grzegorz Malecki c
a Department of Chemistry, Periyar University, Salem 636 011, India
b Department of Industrial Engineering, University of Padova, Via Marzolo 9, 35131 Padova, Italy
c Department of Crystallography, Silesian University, Szkolna 9, 40-006 Katowice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
New four coordinated homoleptic bis(diimidazolylidene)nickel(II) complexes (C1 & C2) were synthe-
sized and characterized by elemental analysis, NMR (1H and 13C) as well as ESI-Mass spectrometry. The
molecular structure of the complex C1 was identified by means of single-crystal X-ray diffraction
analysis, which revealed that the complexes possess a distorted square planar geometry with chelating
bis(diimidazolylidene) NHC ligands and two non coordinating bromide counter ions in tetradentate C4
fashion. A survey of their catalytic activity in BuchwaldꢀHartwig amination has been performed. The
newly synthesized complexes also catalyzed the amination of aryl chlorides in the presence of KOtBu.
Various aryl chlorides and amines can react smoothly to give the corresponding aminated products in
moderate to high yields. The scope of the reaction encompasses electronically varied aryl chlorides and
nitrogen-containing heteroaryl chlorides, including pyridine and quinoline derivatives. Both secondary
and primary amines are well tolerated under the optimal reaction conditions.
Received 5 August 2016
Received in revised form
20 December 2016
Accepted 22 December 2016
Available online 30 December 2016
Keywords:
Diimidazolium NHC ligand
NieNHC complexes
X-ray diffraction
CeN coupling reaction
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
a reprisal. Driven by moribund natural reserves of precious metals
and their consequent tremendous price increases, the development
Aryl amines are very important compounds in organic synthesis
because of their potential applications in chemistry, pharmaceuti-
cals, materials sciences and polymers and thus are very important
intermediates in the chemical industry [1]. Transition metal cata-
lyzed CeN bond formation has become a prevailing and reliable
method for the synthesis of a variety of aromatic amines under mild
and convenient conditions [2]. Besides the abundant traditional
methods for the synthesis of such compounds [3], the CeN cross-
coupling reactions catalyzed by palladium have become one of
the most powerful methods due to their high efficiency of diverse
substrates under mild conditions [4]. Compared to palladium,
much less research has been devoted to the first-row counterpart
nickel. Nevertheless, nickel has proven to be a cost-effective and
viable alternative for most of the achievable transformations using
palladium [5]. Nickel's time in catalysis, however, has returned with
of catalytic reactions based on economical earth abundant mate-
rials has become a key strategic issue, as highlighted by a recent
report from the European Commission [6]. More importantly,
nickel presents a number of characteristics that makes it much
more than just “ersatz palladium” [7]. For instance, it is more
nucleophilic than palladium and thus favors the oxidative addition
of electrophiles less common than aryl halides, such as phenol
derivatives [8] and aryl fluorides [9]. Many Ni based systems have
been developed for CeC and CeN bond formation and other
coupling reactions [10]. Among the organic electrophiles involved,
aryl chlorides seem to be the most desirable ones due to their lower
cost, easy availability and higher stability compared to their iodide
and bromide counterparts in spite of their lower reactivity [11]. To
achieve efficient coupling of aryl chlorides, kinds of tertiary phos-
phine ligands, which are usually air and thermal-sensitive, have
been synthesized to date [12]. On the other hand, during the past
years, N-heterocyclic carbenes (NHCs) have been in the focus of
academia and industry in many fields of chemistry [9]. Rivalling
* Corresponding author.
E-mail address: viswanathamurthi72@gmail.com (P. Viswanathamurthi).
http://dx.doi.org/10.1016/j.jorganchem.2016.12.029
0022-328X/© 2016 Elsevier B.V. All rights reserved.

2
M. Nirmala et al. / Journal of Organometallic Chemistry 831 (2017) 1e10
more reputable phosphine ligands, these carbon-based donors
have become ubiquitous in organometallic chemistry and, building
on pioneer work by Herrmann in the late 90s, transformed ho-
mogeneous transition metal catalysis [13,14]. NHC's behave like
2. Results and discussion
2.1. Synthetic strategy
typical strong
s
-donor ligands with non-negligible
p
-acceptor
The diimidazolium NHC ligands (L1 & L2) were selected as
potential ligand platforms of different wingtip substituents on
imidazole ring, and were synthesized according to known methods
[23,24] by the reaction of corresponding substituted imidazole, and
an excess of dibromomethane. The reaction mixture was stirred at
130 ꢁC for 2 h then cooled to room temperature to give a white
precipitate of corresponding diimidazolium NHC ligands (L1 & L2)
in good yield (~83%) after washing with acetone (Scheme 1). The
new ligands L1 & L2 obtained are highly air and moisture stable.
They are not soluble in acetone, hexane, diethyl ether, but dissolves
readily in CHCl3, CH2Cl2, EtOH, MeOH, DMF and DMSO.
Nickel complexes (C1 & C2) with corresponding NHC ligands (L1
& L2) were typically prepared from the mixture of diimidazolium
NHC ligands (L1 & L2) and Ni(OAc)2.4H2O in ethanol in the pres-
ence of Et4NBr and Et3N at 80 ꢁC in air as depicted in Scheme 2 that
afforded the homoleptic [(diNHC)2Ni]2þ complexes with chelating
diNHC and two non coordinating bromide counterions as the only
isolated products in quantitative yield (~82%). The complexes were
insoluble in chloroform, acetone, hexane, and diethyl ether, but
dissolve readily in EtOH, MeOH, DMF and DMSO, which may be due
to higher degree of organization in the solid state. They are also
thermally stable and will not melt over 300 ꢁC. The compounds
were characterized by 1H, 13C NMR, ESI-MS and single-crystal X-ray
diffraction. Their purity was confirmed by elemental analysis. The
analytical data (C, H, N) of the compounds diimidazolium NHC li-
gands (L1 & L2) and their nickel complexes (C1 & C2) are in good
agreement with the proposed molecular formulae.
abilities in comparison to alkyl phosphines, although enhancing
their plead as ancillary ligands and a wide range of donor proper-
ties can be accessed within the broader ligand class and also they
show similar abilities to stabilize the various oxidation states and
coordinative unsaturated intermediates that appear in catalytic
reactions [15]. The steric uniqueness of NHC ligands further
distinguish them from their phosphine counterparts; the combi-
nation of shorter metaleligand bonds and flanking substituents
that are directed towards the bound metal, permit NHC ligands to
encroach deep into the metal coordination sphere [16]. In addition,
NHCs exhibit superior qualities regarding ligand dissociation and
degradative cleavage [17]. Both properties lead to higher complex
stability. Underpinning these hallmarks, the ability to tune the
electronic and steric environment of the metal coordination sphere
using NHC ligands have shown unprecedented catalytic activity
under homogeneous conditions in many important organic re-
actions [15]. As a result, the olefin metathesis, carbon-carbon and
carbon-heteroatom bond forming reactions have benefited greatly
from the development of NHCs complexes of ruthenium [18] and
palladium [19], respectively. However, the development of a gen-
eral, robust and operationally simple catalytic system of transition
metal catalyze coupling reactions has remained significantly
challenging.
In recent years, some of the nickel(II)-based NHC complexes
exhibit higher catalytic activity than palladium-based NHC com-
plexes in various coupling reactions have led to more active cata-
lysts and mild reaction conditions [20]. Many groups have
expanded Ni-catalyzed CeN cross coupling to wide array of elec-
trophiles to achieve good results [21,27,28]. The first NHC com-
plexes of the type (Ph3P)2Ni(1-nap)Cl used in CeN coupling
reactions with excellent results were reported in 2007 by Yang [22].
All of these methods have their own merits; however, these
methods are associated with certain demerits like instability of
catalyst, high catalytic loading and requirement of severe condi-
tions etc. Hence, the development of stable catalytic system with
NHCeNi(II) catalysts those can be performed under mild conditions
has received much attention.
2.2. Spectroscopic description
IR spectra of free ligands were compared with those of the
corresponding complexes in order to confirm the coordination of
ligand to nickel metal. The ligand showed a strong band in the
region 1347e1395 cmꢀ1 due to nN-C-N. This band has been shifted to
higher frequency 1423e1456 cmꢀ1 in the metal complex, indicating
the coordination of ligand to metal through carbene carbon. A
strong vibration was observed at 1593ꢀ1577 cmꢀ1 in the spectra of
complexes corresponding to C]C stretching. In addition, vibrations
corresponding to the presence of CeC also appeared in the ex-
pected region.
Even though, many Ni based systems have been developed for
CeN bond formation reactions, the use of well-defined [Ni(NHC)]
precatalysts has been explored to a lesser extent despite the
interesting activity of such systems [14c]. Supplementing the
earlier commentaries and our previous success using dianionic
bis(aryloxyeNHC) as a supporting ligand in Ni-based catalysis [23],
we sought to investigate the use of a cis-chelating bidentate N-
heterocyclic carbenes ligated Ni(II)-complexes as precatalyst in
CeN bond forming reactions.
The 1H NMR spectra of the ligands (L1 & L2) and complexes (C1
& C2) showed the signals in the expected region (Fig. S1-S3, ESIy).
As expected the NHC hydrogen atom of the azolium salts gives rise
to a singlet at 9.70e9.54 ppm in the ligands. The generation of free
carbene and subsequent formation of the [NieNHC] complexes
were unambiguously confirmed by the absence of the 1H NMR
resonances of imidazolium (NCHN) protons. In addition, a singlet
130oC
2 h
N
N
CH2Br2
N
N
R
N
N
2Br
R
R
L1 = Me
L2 = Ph
Scheme 1. Synthesis of diimidazolium NHC ligands.

M. Nirmala et al. / Journal of Organometallic Chemistry 831 (2017) 1e10
3
2+
N
N
N
R
R
N
R
N
N
Ethanol, reflux 4 h
2Br
Ni
N
N
R
Ni(CH3COO)2.4H2O
2Br
N
R
N
R
L1 = Me
L2 = Ph
N
N
C1 = Me
C2 = Ph
Scheme 2. General synthesis of [NieNHC] complexes (C1 & C2).
appeared around 3.91e3.16 ppm for compounds L1 and C1 corre-
sponding to terminal eCH3 group protons. Furthermore, the
spectra of all the complexes showed a series of signals for aromatic
protons in the region 7.35e7.17 ppm. In addition, a clear singlet
appeared around 6.15e7.12 ppm for all the compounds (L1, L2, C1 &
C2) corresponding to the methylene bridge protons [25].
water molecule, presumably from the wet crystallization solvent,
incorporated in the asymmetry unit. As depicted in the figure, the
ligand L1 is coordinated in a bidentate fashion and showed a dis-
torted square planar geometry around the metal centre. The two
CeNieC planes defined by the two chelating diNHC ligands are
exactly coplanar with an average NieC bond length of 1.902(4) Å
and a CeNieC bite angle of C(1)eNi(1)ꢀC(7) ¼ 94.02(17) & C(1)
eNi(1)ꢀC(7) ¼ 85.98(17) for the fine and six membered chelate
rings. The CeC and CeN bond distances within the imidazoline-2-
ylidene based ring systems and the NieC bond distances are
The 13C NMR spectra show the expected signals in the appro-
priate regions (Fig. S4-S7, ESIy). The spectra of ligands show the
carbenic carbon signal in the region 138.0e137.85 ppm. For the
uncoordinated ligand the imieC appeared in the region
124.25e122.12 ppm. The disappearance of the 1H NMR signals of
NCHC, and the downfield 13C NMR signals of NCN carbene carbon at
ca. 173.46e172.01 ppm in the 1H and 13C NMR spectra of [NieNHC]
complexes are indicative of the Ni-Ccarbene bond formation [26]. It is
worth of noting that only one singlet was observed for the carbene
carbon in 13C NMR spectrum of this complex. The signals observed
in the region 136.29e123.26 ppm in the spectra of ligands and the
complexes are assigned to aromatic carbons. For the C1 complex,
the signal appeared at 36.99 ppm is attributed to the methyl carbon
attached to the ring nitrogen.
The ESI-Mass analysis of diimidazolium NHC ligands (L1 & L2)
showed that the base peaks at m/z 177.1 and 301.1, respectively,
were originated by the [M-2Br-H]þ ionic species. As for the com-
plexes (C1 & C2) abundant peaks at 491.0 and 775.1, respectively,
corresponding to the ionic species containing the ligands coordi-
nated to Ni, were observed (Fig. S8-S11, ESIy). The isotopic patterns
were in agreement with the simulated ones.
Table 1
Crystal data and structure refinement parameters for complex C1.
C1
CCDC number
Empirical formula
Formula weight
T (K)
1469201
18 H28Br2N8NiO2
607.01
295
C
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
0.71073
Triclinic
P-1
7.7788(8)
b (Å)
c (Å)
8.7807(10)
10.0922(12)
115.087(11)
94.562(9)
102.13(1)
599.19 (13)
1
1.682
4.175
306
a
b
g
(o)
(o)
(o)
Volume (Å3)
Z
Density (calculated) Mg mꢀ3
Absorption coefficient mmꢀ1
F(000)
2.3. X-ray crystal structure description of complex C1
Scan range for data collection (deg)
Index ranges
3.86 to 26.80
ꢀ9 < ¼ h < ¼ 10
ꢀ10 < ¼ k < ¼ 11
ꢀ13 < ¼ l < ¼ 13
5359/2791,0.0531
0.996
2791/0/152
1.018
R1 ¼ 0.0531
wR2 ¼ 0.1095
R1 ¼ 0.0905
wR2 ¼ 0.1306
Even though the analytical and spectral data gave some idea
about the molecular formulae of the complexes, they do not indi-
cate the exact coordination of bis(carbene) units in them. To gain
additional insight into the coordination chemistry and the struc-
tural parameters of the complexes, single crystals of one of the
complexes (C1) were grown by slow evaporation of the concen-
trated ethanolic solution of complex and characterized by X-ray
diffraction analysis. The crystal data and structure refinement pa-
rameters for complex 1 are collected in Table 1 and the selected
bond lengths and bond angels are depicted in Table 2. The ORTEP
view and packing arrangement of atoms in unit cell are given in
Figs. 1 and 2. The structure is solved in the space group P-1 with a
Reflections collected/unique, Rint
Completeness to thetamax
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2
s
(I)]a
R indices (all data)
a
Structures were refined on F2o: wR2 ¼ [∑[w(Fo2 ꢀ Fc2)2]/∑w(Fo2)2]1/2, where
wꢀ1 ¼ [∑(F2o) þ (aP)2 þ bP] and P ¼ [max(Fo2, 0) þ 2F2c]/3.

4
M. Nirmala et al. / Journal of Organometallic Chemistry 831 (2017) 1e10
Table 2
consistent with both contributions from
s and p-donation to the
Selected geometrical parameters for complex C1.
metal centre and -stabilization of the carbene onto the adjacent
p
nitrogens. These parameters indicate that the Ni atom takes dis-
torted square planar coordination geometry.
Interatomic distances (Å)
Ni(1)eC(1)
Ni(1)eC(1)
Ni(1)eC(7)
1.901(4)
1.901(4)
1.903(4)
1.903(4)
Ni(1)eC(7)
3. Catalytic studies
Bond angles(ꢁ)
C(1)eNi(1)ꢀC(1)
C(1)eNi(1)ꢀC(7)
C(1)eNi(1)ꢀC(7)
C(1)eNi(1)ꢀC(7)
C(1)eNi(1)ꢀC(7)
C(7)eNi(1)ꢀC(7)
N(1)eC(1)ꢀNi(1)
N(2)eC(1)ꢀNi(1)
N(3)eC(7)ꢀNi(1)
N(4)eC(7)ꢀNi(1)
180.0
3.1. Carbon-nitrogen bond formation
94.02(17)
85.98(17)
94.02(17)
85.98(17)
180.0
134.2(3)
121.8(3)
121.8(3)
134.2(3)
The newly synthesized NieNHC complexes were evaluated as
precatalysts for CeN bond formation reaction.
3.1.1. Evaluation of conditions for the NieNHC complex (C1)
catalyzed amination reaction
The optimization process was lead toward the determination of
the best reaction conditions to analyze the substrate scope. Initial
studies were performed using chlorobenzene with aniline as
building blocks, and it was found that satisfactory yields were ob-
tained with 1 mol % of C1 in the presence of KOtBu as a base and
dioxane as solvent and the outcome was depicted in Table 3. The
choice of bases was also crucial for the reaction. Of the various base
explored: NaOtBu, KOH, NaOH, K2CO3, Na2CO3, KHCO3 and KOtBu,
the catalyst was active only in the presence of KOtBu in 97% yield
(Table 3, entry 7). In the presence of other bases such as K2CO3
(Table 3, entry 4) and KHCO3 (Table 3, entry 6), almost no desired
product can be detected. To this end, we noticed that a strong base
was required to promote the reaction. In the presence of KOtBu,
some common solvents were tested and the moderate yield was
obtained in toluene (Table 3, entry 12). Out of different solvents
tested during the course of optimization, the solvent such as DMF,
CH3CN, DMSO and iPrOH were found to be ineffective (Table 3,
entry 9, 10, 11, 13). Solvents such as THF and Et2O could bring about
only little conversion. However, dioxane was found as the best
solvent for the C e N coupling reaction resulting in 97% isolated
yield at the temperature of 90 ꢁC (Table 3, entry 7). Moreover, in the
absence of catalyst and base the CeN coupling reaction was not
observed (Table 3, entry 15, 16, 17d). Since the temperature is also
crucial for the success of a coupling process, we have carried out the
Fig. 1. ORTEP representation of the X-ray crystal structure of [NieNHC] complex C1.
The two non coordinating bromide ions and water molecules have been omitted for
clarity. Thermal ellipsoids are shown at 30% probability.
Fig. 2. Packing diagram of complex C1.

M. Nirmala et al. / Journal of Organometallic Chemistry 831 (2017) 1e10
5
Table 3
Evaluation of conditions for model C e N coupling reaction using complex C1a.
Entry
Base
Solvent
Temp (ꢁC)
Yield (%)b
1
2
3
4
5
6
7
8
NaOtBu
KOH
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
THF
90 ꢁC
90 ꢁC
90 ꢁC
90 ꢁC
90 ꢁC
90 ꢁC
90ꢁC
21
17
36
n. r
52
n. r
97
42
n. r
n. r
n. r
76
n. r
43
n. r
n. r
n. r
n. r
n. r
62
NaOH
K2CO3
Na2CO3
KHCO3
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBuc
KOtBuc
e
65 ꢁC
153 ꢁC
85 ꢁC
190 ꢁC
110 ꢁC
83 ꢁC
35 ꢁC
90 ꢁC
90 ꢁC
90 ꢁC
90 ꢁC
90 ꢁC
90 ꢁC
9
DMF
10
11
12
13
14
15
16
17d
18
19
20
CH3CN
DMSO
Toluene
iPrOH
Et2O
Dioxane
DMF
Dioxane
DMF
DMSO
Toluene
Fig. 3. Effect of catalyst (mol %) with respect to time on amination of aryl chlorides.
the present homogeneous catalyst system, the amination reaction
has been extended to the copious aryl chlorides with anilines
consisting of diverse functional groups. Table 4 summarizes the
catalytic activity of C1 & C2 under the dioxane/KOtBu recipe for
this coupling reaction. Various anilines having both electron rich,
poor and sterically hindered groups underwent the reaction
smoothly and gave rise to good to excellent yields (Table 4, entry
1e16). The sterically-encumbered substrates seem to be a chal-
lenge in the previously reported methods, especially at low
catalyst loadings. However, to our delight the anilines bearing
sterically-hindered substituents such as 2-methyl aniline (Table 4,
entry 5), 2,4-dimethylaniline (Table 4, entry 8), 2,4,6-trimethyl
aniline (Table 4, entry 9), 2,6-dimethylaniline (Table 4, entry
10), 2,6-diisopropylaniline (Table 4, entry 11) are reacted well
with corresponding aryl chlorides to furnish excellent yield of
coupling products under optimized conditions. The sterically
hindered aryl chlorides such as 2-methylphenylchloride (Table 4,
entry 5), 3-chlorophenyldimethylamine (Table 4, entry 13), 2,6-
dimethylphenylchloride (Table 4, entry 12) are all compatible
for coupling reactions. The reaction of 2-chloroquinoline was
carried out with p-toluidine to obtain N-p-tolylquinolin-2-amine
(Table 4, entry 14) in 89e94% yield. In addition, unsubstituted
aryl chlorides afforded the products in higher yields (Table 4,
entry 1, 3, 4). The reaction of N-methylaniline with 4-
chloropyridylchloride could also be coupled, revealing the for-
mation of solely N-methyl-N-phenylpyridin-4-amine (Table 4,
entry 15) in 92e96% isolated yield after 4 h of reaction time. Also,
by changing the substitution on the nitrogen atom from methyl to
simple ethyl group provided the diarylamine in a modest yield
58e62% (Table 4, entry 16).
KOtBu
KOtBu
KOtBu
a
All reactions were carried out using chlorobenzene (1 mmol), aniline (1.3
mmol), base (1.3 mmol), catalyst (1 mol %), solvent (2 mL) at 90 ꢁC for 4 h.
b
Isolated yield after column chromatography.
No catalyst was used; n.r: no reaction.
The reaction was carried out using chlorobenzene (1 mmol), aniline (1.3 mmol),
c
d
without base, catalyst (1 mol %), solvent (2 ml) at 90 ꢁC for 12 h.
reaction in DMF, DMSO, and toluene at 90 ꢁC to nullify the advan-
tage of high temperature and to compare the conversion yield with
other solvents without the influence of temperature. No conversion
took place in DMF and DMSO at 90 ꢁC (Table 3, entry 18, 19) due to
lack of minimum thermal energy that should be supplied to cross
the energy barrier. In the case of toluene as the solvent (Table 3,
entry 20), the conversion yield diminished to 62% at 90 ꢁC.
3.1.2. Catalyst screening
We continued the CeN bond formation reaction optimization
process after finding the need for a base to activate the catalyst (C1)
in the amination reaction. The following step was done to study the
influence of the substituents and catalyst loading on the catalytic
activity of NieNHC complexes (C1 & C2). The results are shown in
Fig. 3. Catalyst screening in the model reaction revealed that all the
NieNHC complexes triggered the reaction. The results also indicate
that the higher catalyst loadings lead to higher yields. Conversely, a
decrease in the catalyst (mol %) led to diminish the yield of the
product considerably. Further, the NieNHC catalyst with methyl as
a wingtip substituent exhibit slightly higher activity than those
containing phenyl as a wingtip substituent. This behavior indicates
that the influence of steric effects is only slightly significant (even
no significance in some conversions) in the catalytic activity.
The optimization process was led toward the determination of
the best reaction conditions to evaluate the substrate scope. The
newly synthesized NieNHC complexes were effectively catalyzed
CeN bond formation reaction between aryl chlorides and a range of
amines with 1.3 mmol of KOtBu as a base and 1 mol % of catalyst in
presence of dioxane at 90 ꢁC for 4 h.
3.1.4. Amination of aryl chlorides with secondary amines
To expand the scope of the present homogeneous catalyst sys-
tem, the CeN coupling reaction has been extended to the copious
aryl chlorides consisting of diverse functional groups with sec-
ondary amines. Table 5 summarizes the catalytic activity of
NieNHC catalysts C1 & C2 under the dioxane/KOtBu recipe for this
coupling reaction. All the reactions proceed smoothly to give the
corresponding aminated products in good to high yield (Table 5,
entry 1e13). Electron poor aryl chlorides generally gave the ami-
nation product in good to excellent yields (Table 5, entry 5e10, 12).
Surprisingly, while pyrolidine, morpholine, piperidine, and piper-
azine were effectively coupled with chlorobenzene to give rise a
moderate to excellent product yield (Table 5, entry 1, 2, 4, 11).
3.1.3. Amination of aryl chlorides with aniline derivatives
Under the optimal conditions identified, to expand the scope of

6
M. Nirmala et al. / Journal of Organometallic Chemistry 831 (2017) 1e10
Table 4 (continued)
Table 4
[NieNHC] catalyzed coupling of aryl chlorides with aniline derivativesa.
Entry
Products
TOFc
C1
Yield (%)b
C2
24
C1
C2
13
25
98
96
TOFc
C1
Yield (%)b
14
22
24
89
94
Entry
1
Products
C2
24
C1
C2
24
97
92
15
16
24
15
23
16
96
58
92
62
2
23
23
92
93
a
3
4
25
25
24
24
98
99
97
96
All reactions carried out using aryl chlorides (1 mmol), anilines (1.3 mmol),
KOtBu (1.3 mmol), catalyst (1 mol %), and dioxane (2 mL) at 90 ꢁC for 4 h.
b
Isolated yield after column chromatography.
TOF ¼ TON/times.
c
Moreover, the reaction of 2-chloropyridine was carried out with
morpholine to obtain 4-(pyridin-2-yl)morpholine (Table 5, entry
13) in 87e91% yield.
5
6
7
25
24
23
24
23
24
98
96
93
94
92
94
3.1.5. Amination of aryl chlorides with primary amines
In order to ensure the generality of this finding, a variety of
primary aliphatic amines underwent this amination process in
good to excellent isolated yields using the above optimized proto-
col. The results of this study are summarized in Table 6. The range of
electron rich (Table 6, entry 2, 4) and electron neutral (Table 6,
entry 3) aryl chlorides coupled with cyclopentyl, cyclohexylamines
in high yields. To our pleasure, the sterically hindered aryl chlorides
such
as
2,6-dimethyl
phenylchloride
and
2,6-
diisopropylphenylchloride were coupled with primary aliphatic
amines provided the desired products in excellent yield (Table 6,
entry 5, 6, 7, 8). Reaction of sterically hindered chloroarene such as
(3-chloro-phenyldimethylamine) successfully furnished respective
coupled products with cyclohexylamine in 91e93% yield (Table 6,
entry 9). As nitrogen-containing heterocycles possessing amino
substituents often are of particular pharmaceutical interest.
Inspired by the above interesting result, we have extended the
scope of the present NieNHC catalytic system to the amination of
heteroaryl chlorides. However, the C e N coupling reactions be-
tween nitrogen containing heteroaryl electrophiles and primary
amines are exigent, presumably because of the coordinating
properties of both pyridines and primary amines. Such reactions
proceed smoothly to give the corresponding aminated products in
good yields using the previously developed conditions. An array of
heteroaryl chlorides that underwent this amination reaction shel-
tered a range of 2-pyridyl (Table 6, entry 11, 12, 16, 17), 3-pyridyl
(Table 6, entry 13, 14, 15, 18) chlorides containing electron
donating groups (OMe) and electron withdrawing groups (CN &
CF3). In general, these reactions afforded the corresponding heter-
oaryl alkylamines in good to excellent yields of 67e99%. Even the
sterically hindered dibenzylamine reacted with 3-chloropyridine to
give the coupled product in 62e67% yield (Table 6, entry 19). The
reaction of 2-chloropyridine was carried out with octylamine to
obtain N-octyl-3-aminopyridine (Table 6, entry 10) in 96e94%
yield. Interestingly (4-methoxyphenyl)methanamine reacted well
with 1-chloro-3-methoxybenzene and 1-chloro-3-methylbenzene
8
24
24
24
22
23
25
23
21
98
97
96
89
93
98
92
84
9
10
11
12
20
19
80
76

M. Nirmala et al. / Journal of Organometallic Chemistry 831 (2017) 1e10
7
Table 5
to furnish excellent yield of coupling products in 88e97% yields
under optimized conditions (Table 6, entry 20, 21).
[NieNHC] catalyzed coupling of aryl chlorides with secondary aminesa.
In addition to the limitations on the scope of the coupling of aryl
halides with amines, the mechanism of the coupling of aryl halides
with amines catalyzed by nickel complexes has not been studied.
Many papers cite the potential of Ni(I) intermediates [29], either as
part of a one-electron redox event or as part of a mechanism
involving Ni(I) and Ni(III) intermediates. In contrast, the mecha-
nism for the palladium-catalyzed amination of aryl electrophiles
has been studied in detail, including the kinetic behavior of the
catalytic reaction and stoichiometric reactions of isolated in-
termediates [30e33]. Such studies have not been conducted on
nickel-catalyzed couplings to form CeN bonds, but are particularly
important to determine how to create catalysts of nickel that are as
long-lived as those of palladium and that react with similarly broad
scope. The mechanism of the Buchwald-Hartwig aminations of aryl
chloride with primary alkyl amines, using a Ni precatalyst, has been
recently studied, in detail, by Hartwig and co-workers [34]. Based
on previous literature reports [35], it appears to us that both steric
and electronic effect combines to mediate the coupling process.
Initially, the electron donor properties of the carbene facilitate the
activation of aryl chlorides. The present protocol provides an
extremely convenient, highly efficient and less expensive alterna-
tive for the synthesis of aryl amines. Efforts are currently underway
in our laboratory to elucidate the mechanistic details of these CeN
bond forming reactions.
Entry
1
Products
TOFc
C1
Yield (%)b
C2
25
C1
C2
24
96
98
99
79
2
3
24
19
25
20
97
74
4
5
17
19
18
20
67
74
72
79
4. Conclusions
In summary, we have developed a practical protocol for the
preparation of two new nickel complexes bearing diimidazolium
NHC ligands with different wingtip substituents (C1 (R ¼ Me) and
C2 (R ¼ Ph)) in the imidazole ring. These complexes were fully
characterized by NMR, and single crystal X-ray diffraction studies. A
survey of their catalytic properties in BuchwaldꢀHartwig amina-
tion was presented. The reaction has a broad scope and could be
applied to a variety of electron poor aryl chlorides as well as
nitrogen-containing heteroaryl chlorides, including pyridine and
quinoline derivatives. Electron rich and sterically hindered aryl
chlorides could be coupled with various amines. Good to excellent
yields were obtained with the vast majority of substrates. The use of
the dioxane solvent system proved to be the most beneficial. The
nickel diimidazolium NHC complexes are usually decomposes in
the presence of trace amount of water. In our case, the exceptional
stability of our NHC complexes may be due to steric factor [36].
Efforts are currently underway in our research group to expand the
scope of this method to a library of different functionalized sub-
strates, to elucidate mechanistic principles underlying these CeN
bond formation reactions with even greater activity and to inves-
tigate the outstanding stability of our NHC complexes.
6
23
24
93
96
7
8
25
16
25
17
98
65
99
69
9
25
25
98
99
10
11
12
13
17
21
20
22
17
22
21
23
67
84
80
87
69
88
83
91
5. Experimental section
5.1. General considerations
Unless otherwise noted, all experiments with metal complex
and the NHC ligand were carried out without taking precautions to
exclude air and moisture. All catalytic reactions were carried out
under an atmosphere of dry Ar or N2 using standard Schlenk
techniques. All solvents were reagent grade or better. All reagents
were purchased from Aldrich chemical Co. and used as received
without further purification. Thin-layer chromatography (TLC) was
performed on Merck 1.05554 aluminum sheets precoated with
silica gel 60 F254, and the spots were visualized with UV light at
254 nm or under iodine. Column chromatography purifications
a
All reactions carried out using aryl chlorides (1 mmol), secondary amines
(1.3 mmol), KOtBu (1.3 mmol), catalyst (1 mol %), and dioxane (2 mL) at 90 ꢁC for 4 h.
b
Isolated yield after column chromatography.
TOF ¼ TON/time.
c

8
M. Nirmala et al. / Journal of Organometallic Chemistry 831 (2017) 1e10
Table 6 (continued)
Table 6
[NieNHC] catalyzed coupling of aryl chlorides with primary amines.a.
Entry
Products
TOFc
C1
Yield (%)b
Entry
1
Products
TOFc
C1
Yield (%)b
C2
17
C1
C2
C2
24
C1
C2
19
16
62
67
23
90
94
82
92
86
87
2
3
4
5
21
22
23
21
21
23
22
22
85
89
90
82
20
21
24
23
24
22
97
92
94
88
a
All reactions carried out using aryl chlorides (1 mmol), primary amines
(1.3 mmol), KOtBu (1.3 mmol), catalyst (1 mol %), and dioxane (2 mL) at 90 ꢁC for 4 h.
b
Isolated yield after column chromatography.
TOF ¼ TON/time.
6
7
20
22
21
22
79
86
82
89
c
were performed by Merck silica gel 60 (0.063e0.200 mm). Micro-
analyses of carbon, hydrogen and nitrogen were carried out using a
Vario EL III elemental analyzer. Infrared spectra of the ligands and
the metal complexes were recorded as KBr discs in the range of
4000ꢀ400 cmꢀ1 using a Nicolet Avatar model FT-IR spectropho-
tometer. 1H (300.13 MHz) and 13C (75.47 MHz) NMR spectra were
taken in DMSO-d6 or CDCl3 at room temperature with a Bruker
AV400 instrument with chemical shifts relative to tetramethylsi-
lane. ESI MS analyses were performed using a Finningan LCQ-Duo
ion-trap instrument, operating in positive ion mode (sheath gas
flow N2 30 au, source voltage 4.0 kV, capillary voltage 21 V, capil-
lary temperature 200 ꢁC). The He pressure inside the trap was kept
constant. The pressure, directly read by an ion gauge (in the
absence of the N2 stream), was 1.33 ꢂ 10ꢀ5 Torr. Sample solutions
were prepared by dissolving the compounds (3 mg) in CH3CN
(5 ml) or MeOH (5 mL) immediately before analysis. Melting points
were determined in open capillary tubes on a Technico micro
heating table and are uncorrected.
8
9
21
23
22
23
84
91
89
93
10
11
24
22
24
23
94
89
96
93
12
13
19
23
20
24
75
92
79
96
5.2. Synthesis of diimidazolium NHC ligands
The diazolium NHC ligands were prepared according to previous
literature reports [37,38]. Typically, mixture of dibromomethane
(5.0 mL, 1 mmol) and 1-substituted imidazole (2 mmol) was added
in a round bottom flask and stirred at 130 ꢁC for 2 h, and then stored
at room temperature. The resultant white precipitate were
collected and washed with acetone.
14
15
25
23
24
24
99
90
95
94
5.2.1. Compound L1 (R ¼ Me)
16
17
21
17
22
18
84
67
88
72
The synthetic procedure of this compound was the same as that
of above representative procedure, using 1-methylimidazole to give
a white solid L1. Yield: 80%. M.Pt: 123 ꢁC. Anal.Calc.for.C9H14N4Br2:
C, 31.98; H, 4.17; N, 16.57%. Found: C, 31.66; H, 4.38; N, 16.21%. IR
(KBr disks, cmꢀ1); 1570 (C]C), 1553 (NeCeN). 1H NMR
(300.13 MHz, DMSO-d6,
d): 9.54 (s, 2H, NCHN), 8.08 (s, 2H, CHimi),
7.82 (s, 2H, CHimi), 6.76 (s, 2H, NCH2N), 3.91 (s, 6H, CH3). 13C NMR
18
24
25
96
99
(75.47 MHz, DMSO-d6, d): 138.0 (NCHN), 124.25 (CHimi), 121.86
(CHimi), 57.76 (NCH2N), 36.23 (CH3). ESI-MS (rel.ab.%): [M-Br]þ, m/z
257 (22%), [M-2BreH]þ, m/z 177 (100%).

M. Nirmala et al. / Journal of Organometallic Chemistry 831 (2017) 1e10
9
5.2.2. Compound L2 (R ¼ Ph)
The absorption corrections were performed by the multi-scan
method. Corrections were made for Lorentz and polarization ef-
fects. The structures were solved by direct methods using the
program SHELXS [39]. Refinement and all further calculations were
carried out using SHELXL [39]. The H atoms were included in
calculated positions and treated as riding atoms using the SHELXL
default parameters. The non-hydrogen atoms were refined aniso-
tropically using weighted full-matrix least squares on F2. Atomic
scattering factors were incorporated into the computer programs.
The synthetic procedure of this compound was the same as that
of above representative procedure, using 1-phenylimidazole to give
a white solid L2. Yield: 83%. M.Pt: 119 ꢁC. Anal.Calc.for.C19H18N4Br2:
C, 49.38; H, 3.93; N, 12.12%. Found: C, 49.63; H, 3.54; N, 12.22%. IR
(KBr disks, cmꢀ1); 1558 (C]C), 1534 (NeCeN), 1437 (CeC). 1H NMR
(300.13 MHz, DMSO-d6,
7.88 (s, 2H, CHimi), 7.84e7.68 (m, 10H, ArH), 6.94 (s, 2H, NCH2N). 13
NMR (75.47 MHz, DMSO-d6, ): 137.85 (NCHN); 134.97, 130.84,
d): 10.37 (s, 2H, NCHN), 8.49 (s, 2H, CHimi),
C
d
130.76 & 123.56 (ArC), 122.46 (CHimi), 122.12 (CHimi), 58.98
(NCH2N).). ESI-MS (rel.ab.%): [M-2BreH]þ, m/z 301 (100%).
Acknowledgment
5.3. Synthesis of diimidazolium nickel(II) complexes (C1 & C2)
The authors express their sincere thanks to Department of Sci-
ence and Technology, New Delhi, for financial support for this work
under the DST FAST TRACK Scheme (No. SR/FT/CS-66/2011). One of
the authors (MN) thanks DST-SERB for the award of fellowship.
To an ethanol (20 mL) solution of corresponding diimidazolium
salt (1.00 mmol) was added nickel(II) acetate tetrahydrate (44.8 mg,
0.183 mmol), Et4NBr (165 mg, 0.785 mmol), and Et3N (3 drops). The
mixture was stirred at 80 ꢁC for 4 h. The precipitated green solid of
[NieNHC] complex was collected and washed with small amount of
EtOH (6 ml).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2016.12.029.
5.3.1. Compound C1
Yield: 83%. M.P.: 289 ꢁC (decomp.). Anal.Calc.for.C18H24N8Br2Ni:
C, 37.60; H, 4.91; N, 19.49%. Found: C, 37.83; H, 4.65; N, 19.81%. IR
(KBr disks, cmꢀ1); 1575 (C]C), 1563 (NeCeN). 1H NMR
References
[1] (a) S. Suwanprasop, T. Nhujak, S. Roengsumran, A. Petsom, Ind. Eng. Chem.
Res. 43 (2004) 4973e4978;
(300.13 MHz, DMSO-d6,
6.15 (s, 4H, NCH2N), 3.16 (s, 12H, CH3). 13C NMR (75.47 MHz, DMSO-
d6, ): 173.46 (Ccarbene), 124.66 (CHimi), 122.27 (CHimi), 51.93
d): 7.32 (s, 4H, CHimi), 7.18 (s, 4H, CHimi),
(b) S.A. Lawrence (Ed.), Amines: Synthesis, Properties and Application, Cam-
bridge University Press, Cambridge (, 2004;
(c) D. O'Hagan, Nat. Prod. Rep. 17 (2000) 435e446;
d
(NCH2N), 36.99 (CH3). Single crystals suitable for X-ray determi-
nation was grown in ethanolic solution of C1 at room temperature.
ESI-MS (rel.ab.%): [M-Br-2H]þ, m/z 491 (45%); [C7H7N4Ni]þ, m/z 205
(100%).
(d) A. Belfield, G.R. Brown, A.J. Foubister, Tetrahedron 55 (1999)
11399e11428.
[2] (a) J. Barluenga, F. Aznar, C. Valdes, Angew. Chem. Int. Ed. 43 (2004) 343e345;
(b) L. Xu, D. Zhu, F. Wu, R. Wang, B. Wan, Tetrahedron 61 (2005) 6553e6560;
(c) R.A. Altman, B.P. Fors, S.L. Buchwald, Nat. Protoc. 2 (2007) 2881e2887;
(d) T. Ogata, J.F. Hartwig, J. Am. Chem. Soc. 130 (2008) 13848e13849;
(e) H. Zhao, R. Qiao, J. Org. Chem. 75 (2010) 3311e3316;
(f) J.D. Senra, L.C.S. Aguiara, A.B.C. Simas, Curr. Org. Synth. 8 (2011) 53e78.
[3] (a) M.B. Smith, J. March (Eds.), March's Advanced Organic Chemistry: Re-
actions, Mechanisms and Structure, fifth ed., John Wiley & Sons Inc., New
York, 2001;
5.3.2. Compound C2
Yield: 81%. M.pt: 277 ꢁC (decomp.). Anal.Calc.for.C38H32N8Br2Ni:
C, 55.71; H, 3.94; N, 13.68%. Found: C, 55.47; H, 3.69; N, 13.45%. IR
(KBr disks, cmꢀ1); 1562 (C]C), 1531 (NeCeN), 1440 (CeC). 1H NMR
(b) H. Heaney, Chem. Rev. 62 (1962) 81e97.
[4] For some recent selected reviews, please see: (a) J. Bariwal, E. Van der Eycken,
Chem. Soc. Rev. 42 (2013) 9283e9303;
(300.13 MHz, DMSO-d6,
7.35e7.33 (m, 10H, ArH), 7.26e7.17 (m, 10H, ArH), 7.12 (s, 4H,
NCH2N). 13C NMR (75.47 MHz, DMSO-d6,
): 172.01 (Ccarbene),
d): 7.38 (s, 4H, CHimi), 7.37 (s, 8H, CHimi),
(b) R.J. Lundgren, M. Stradiotto, Chem. Eur. J. 18 (2012) 9758e9769;
(c) D.S. Surry, S.L. Buchwald, Angew. Chem. Int. Ed. 47 (2008) 6338e6361;
(d) J.F. Hartwig, Acc. Chem. Res. 41 (2008) 1534e1544.
d
136.29, 130.55, 130.28, 129.17, 126.56 and 123.26 (ArC), 122.28
(CHimi), 121.98 (CHimi), 52.03 (NCH2N). ESI-MS (rel.ab.%):
[C38H39N8O2BrNiK]þ, m/z 815 (25%), [M-Br-2Hþ2H2O]þ, m/z 775
(50%); [C19H16N4Na]þ, m/z 323 (45%), [C19H18N4K]þ, m/z 341 (100%).
ꢀ
ꢀ
[5] (a) Y. Fort, C. Comoy, in: S. Diez-Gonzalez (Ed.), N-Heterocyclic Carbenes: from
Laboratory Curiosities to Efficient Synthetic Tools, Royal Society of Chemistry,
Cambridge, U.K, 2011, pp. 284e316;
(b) C.S.J. Cazin (Ed.), N-heterocyclic Carbenes in Transition Metal Catalysis and
Organocatalysis, Springer, Berlin (, 2010;
(c) S. Budagumpi, R.A. Haque, A.W. Salman, Coord. Chem. Rev. 256 (2012)
1787e1830.
5.4. Catalysis
[6] European Commission, Report on Critical Raw Materials for the EU, May 2014.
http://ec.europa.eu/enterprise/policies/raw-materials/files/docs/crm-report-
on-critical-rawmaterials_en.pdf (accessed 29 July 2015).
[7] For excellent overviews of recent advances in homogeneous nickel catalysis,
see: (a) S.Z. Tasker, E.A. Standley, T.F. Jamison, Nature 509 (2014) 299e309;
(b) V.P. Ananikov, ACS Catal. 5 (2015) 1964e1971.
5.4.1. General procedure for the Buchwald-Hartwig reaction
Under an N2 atmosphere, KOtBu (1.3 mmol), [NieNHC] catalyst
(1 mol %), dioxane (2 mL), amines (1.3 mmol) and aryl chlorides
(1.0 mmol) were successively added into a Schlenk tube. The
mixture was stirred vigorously at 90 ꢁC for 4 h. Then the solvent
was removed under reduced pressure and the residue was purified
by column chromatography on silica gel (eluent: PE/EA ¼ 15:1) to
give the pure products. The reported yields are the average of two
runs. The catalytic reactions have been given in Tables 3e6. The
resulting amines were identified by comparison of the 1H & 13C
NMR data with those previously reported (ESIy).
[8] (a) B.M. Rosen, K.W. Quasdorf, D.A. Wilson, N. Zhang, A.-M. Resmerita,
N.K. Garg, V. Percec, Chem. Rev. 111 (2011) 1346e1416;
(b) B.-J. Li, D.-G. Yu, C.-L. Sun, Z.-J. Shi, Chem. - Eur. J. 17 (2011) 1728e1759.
€
€
[9] (a) V.P.W. Bohm, C.W.K. Gstottmayr, T. Weskamp, W.A. Herrmann, Angew.
Chem. Int. Ed. 40 (2001) 3387e3389;
(b) T. Schaub, M. Backes, U. Radius, J. Am. Chem. Soc. 128 (2006)
15964e15965;
(c) M. Tobisu, T. Xu, T. Shimasaki, N. Chatani, J. Am. Chem. Soc. 133 (2011)
19505e19511.
[10] (a) S. Gu, P. Ni, W. Chen, Chin. J. Catal. 31 (2010) 875e886;
(b) V.V. Grushin, H. Alper, Chem. Rev. 94 (1994) 1047e1062;
(c) J. Yamaguchi, K. Muto, K. Itami, Eur. J. Org. Chem. (2013) 19e30;
(d) E. Negishi, Acc. Chem. Res. 15 (1982) 340e348;
5.5. X-ray crystallography
(e) F.-S. Han, Chem. Soc. Rev. 42 (2013) 5270e5298;
Crystal of complex C1 was mounted on glass fibers used for data
collection. Crystal data were collected at 295 K using a Gemini An
Ultra Oxford Diffraction automatic diffractometer. Graphite mon-
(f) J. Cornella, C. Zarate, R. Martin, Chem. Soc. Rev. 43 (2014) 8081e8097;
(g) M. Tobisu, N. Chatani, Acc. Chem. Res. 48 (2015) 1717e1726;
(h) V. Ritleng, M. Henrion, M.J. Chetcuti, ACS Catal. 6 (2016) 890e906;
(i) M. Henrion, V. Ritleng, M.J. Chetcuti, ACS Catal. 5 (2015) 1283e1302;
ochromated Mo-K
a
radiation (
l
¼ 0.71073 Å) was used throughout.

10
M. Nirmala et al. / Journal of Organometallic Chemistry 831 (2017) 1e10
(j) M. Marin, R.J. Rama, M.C. Nicasio, Chem. Rec. 16 (2016) 1819e1832.
[11] (a) R.B. Bedford, C.S.J. Cazin, D. Holder, Coord. Chem. Rev. 248 (2004)
2283e2321;
(e) T. Shimasaki, M. Tobisu, N. Chatani, Angew. Chem. Int. Ed. 49 (2010)
2929e2932;
(f) L. Ackermann, R. Sandmann, W. Song, Org. Lett. 13 (2011) 1784e1786;
(g) L. Ackermann, W. Song, R. Sandmann, J. Organomet. Chem. 695 (2011)
195e201;
(h) T. Mesganaw, A.L. Silberstein, S.D. Ramgren, N.F.F. Nathel, X. Hong, P. Liu,
N.K. Garg, Chem. Sci. 2 (2011) 1766e1771;
(i) S.D. Ramgren, A.L. Silberstein, Y. Yang, N.K. Garg, Angew. Chem. Int. Ed. 50
(2011) 2171e2173;
(b) A.F. Littke, G.C. Fu, Angew. Chem. Int. Ed. 41 (2002), 4176e1211;
(c) V.V. Grushin, H. Alper, in: S. Murai (Ed.), Activation of Unreactive Bonds
and Organic Synthesis, Springer, Berlin, 1999, pp. 193e226;
(d) V.V. Grushin, H. Alper, Chem. Rev. 94 (1994) 1047e1062.
[12] For some representative examples, please see: (a) S.M. Wong, O.Y. Yuen,
P.Y. Choy, F.Y. Kwong, Coord. Chem. Rev. 293e294 (2015) 158e186;
(b) S.M. Wong, C.M. So, F.Y. Kwong, Synlett 23 (2012) 1132e1153;
(c) K.H. Chung, C.M. So, S.M. Wong, C.H. Luk, Z.-Y. Zhou, C.P. Lau, F.Y. Kwong,
Synlett 23 (2012) 1181e1186;
(j) M. Tobisu, A. Yasutome, K. Yamakawa, T. Shimasaki, N. Chatani, Tetrahe-
dron 68 (2012) 5157e5161;
(k) J. Miller, J. Dankwardt, J. Penney, Synthesis (2003) 1643e1648.
(d) S. Rodriguez, B. Qu, N. Haddad, D.C. Reeves, W.-J. Tang, H. Lee,
D. Krishnamurthy, C.H. Senanayake, Adv. Synth. Catal. 353 (2011) 533e537;
(e) R.J. Lundgren, A. Sappong-Kumankumah, M. Stradiotto, Chem. Eur. J. 16
(2010) 1983e1991;
(f) G.-S. Chen, W.H. Lam, W.S. Fok, H.W. Lee, F.Y. Kwong, Chem. Asian J. 2
(2007) 306e313;
[22] Concurrent with the advent of Ni(II)ꢀ(
s-aryl) complexes as precatalysts, the
groups of Nolan, Matsubara, and Nicasio have made excellent use of pre-
ligated, air-stable NiꢀNHC complexes for use in CꢀN cross-coupling and other
Ni-catalyzed cross-coupling reactions.See the following references: (a)
ꢀ
R.A. Kelly, N.M. Scott, S. Díez-Gonzalez, E.D. Stevens, S.P. Nolan, Organome-
tallics 24 (2005) 3442e3447;
(g) L. Ackermann, J.H. Spatz, C.J. Gschrei, R. Born, A. Althammer, Angew. Chem.
Int. Ed. 45 (2006) 7627e7630.
(b) K. Matsubara, K. Ueno, Y. Koga, K.J. Hara, Org. Chem. 72 (2007)
5069e5076;
[13] (a) M.N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 510 (2014)
485e496;
(c) M.J. Iglesias, A. Prieto, M.C. Nicasio, Adv. Synth. Catal. 352 (2010)
1949e1954;
ꢀ
(b) D. Martin, M. Melaimi, M. Soleilhavoup, G. Bertrand, Organometallics 30
(2011) 5304e5313;
(d) M.J. Iglesias, J.F. Blandez, M.R. Fructos, A. Prieto, E. Alvarez, T.R. Belderrain,
M.C. Nicasio, Organometallics 31 (2012) 6312e6316;
(e) A.R. Martin, Y. Makida, S. Meiries, A.M.Z. Slawin, S.P. Nolan, Organome-
tallics 32 (2013) 6265e6270;
(c) F.E. Hahn, M.C. Jahnke, Angew. Chem. Int. Ed. 47 (2008) 3122e3172.
[14] (a) J.D. Egbert, C.S.J. Cazin, S.P. Nolan, Catal. Sci. Technol. 3 (2013) 912e926;
(b) S.P. Nolan, Acc. Chem. Res. 44 (2011) 91e100;
(f) C. Chen, L.-M. Yang, J. Org. Chem. 72 (2007) 6324e6327;
(g) X.-H. Fan, G. Li, L.-M. Yang, J. Organomet. Chem. 696 (2011) 2482e2484.
[23] M. Nirmala, G. Prakash, R. Ramachandran, P. Viswanathamurthi, J.G. Malecki,
W.G. Linert, J. Mol. Catal. A Chem. 397 (2015) 56e67.
[24] T.A.P. Paulose, S.-C. Wu, J.A. Olson, T. Chau, N. Theaker, M. Hassler, J.W. Qquail,
S.R. Foley, Dalton Trans. 41 (2012) 251e260.
[25] T. Yagyu, K. Yano, T. Kimata, K. Jitsukawa, Organometallics 28 (2009)
2342e2344.
[26] W. Li, H. Sun, H. Chen, Z. Wang, D. Hu, Q. Shen, Y. Zhang, Organometallics 24
(2005) 5925e5928.
ꢀ
(c) S. Díez-Gonzalez, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009)
3612e3676;
(d) C. Samojłowicz, M. Bieniek, K. Grela, Chem. Rev. 109 (2009) 3708e3742;
(e) G.C. Fortman, S.P. Nolan, Chem. Soc. Rev. 40 (2011) 5151e5169;
(f) E.A.B. Kantchev, C.J. O'Brien, M.G. Organ, Angew. Chem. Int. Ed. 46 (2007)
2768e2813.
[15] (a) Recent reviews of NHC ligands and their applications in catalysis include:
Special issues 5e6 of Coord, Chem. Rev. 251 (2007) 595e896;
(b) O. Kuhl, Chem. Soc. Rev. 36 (2007) 592;
(c) V. Cesar, S. Bellemin- Laponnaz, L. Gade, Chem. Soc. Rev. 33 (2004) 619;
(d) W.A. Herrmann, Angew. Chem. Int. Ed. 41 (2002) 1290;
(e) D. Bourissou, O. Guerret, F.P. Gabbai, G. Bertrand, Chem. Rev. 100 (2000)
39;
[27] (a) J.P. Wolfe, S. Wagaw, J.-F. Marcoux, S.L. Buchwald, Acc. Chem. Res. 31
(1998) 805e818;
(b) J.F. Hartwig, Angew. Chem. Int. Ed. 37 (1998) 2046e2067;
(c) A.R. Muci, S.L. Buchwald, Top. Curr. Chem. 219 (2002) 131e209;
(d) M. Kienle, S.R. Dubbaka, K. Brade, P. Knochel, Eur. J. Org. Chem. 20 (2007)
4166e4176;
(f) N.M. Scott, S.P. Nolan, Eur. J. Inorg. Chem. (2005) 1815;
(g) R. Tonner, G. Heydenrych, G. Frenking, Chem.
e Asian J. 2 (2007)
1555e1567.
(e) D.S. Surry, S.L. Buchwald, Angew. Chem. Int. Ed. 47 (2008) 6338e6361;
(f) J.F. Hartwig, Acc. Chem. Res. 41 (2008) 1534e1544.
[28] B. Gradel, E. Brenner, R. Schneider, Y. Fort, Tetrahedron Lett. 42 (2001)
5689e5692.
€
[16] T. Droge, F. Glorius, Angew. Chem. Int. Ed. 49 (2010) 6940e6952.
[17] (a) S.H. Hong, A. Chlenov, M.W. Day, R.H. Grubbs, Angew. Chem. Int. Ed. 46
(2007) 5148e5151;
(b) B.R. Galan, M. Gembicky, P.M. Dominiak, J.B. Keister, S.T. Diver, J. Am,
Chem. Soc. 127 (2005) 15702e15703;
(c) J.A. Cabeza, I. del Río, D. Miguel, M.G. Sanchez-Vega, Chem. Commun.
(2005) 3956e3958;
(d) S. Caddick, F.G.N. Cloke, P.B. Hitchcock, A.K. de Lewis, Angew. Chem. Int.
Ed. 43 (2004) 5824e5827;
(e) R.F. Jazzar, S.A. Macgregor, M.F. Mahon, S.P. Richards, M.K. Whittlesey,
J. Am. Chem. Soc. 124 (2002) 4944e4945.
[29] T.T. Tsou, J.K. Kochi, J. Am. Chem. Soc. 101 (1979) 6319e6332.
[30] M.S. Driver, J.F. Hartwig, J. Am. Chem. Soc. 117 (1995) 4708e4709.
[31] R.A. Widenhoefer, S.L. Buchwald, Organometallics 15 (1996) 2755e2763.
[32] M.S. Driver, J.F. Hartwig, J. Am. Chem. Soc. 119 (1997) 8232e8245.
[33] S. Shekhar, P. Ryberg, J.F. Hartwig, J.S. Mathew, D.G. Blackmond, E.R. Strieter,
S.L. Buchwald, J. Am. Chem. Soc. 128 (2006) 3584e3591.
[34] S. Ge, R.A. Green, J.F. Hartwig, J. Am. Chem. Soc. 136 (2014) 1617e1627.
[35] (a) J. Haung, G. Grass, S.P. Nolan, Org. Lett. 1 (1999) 1307e1309;
(b) J.P. Wolfe, S. Wagaw, J.-F. Marcoux, S.L. Buchwald, Acc. Chem. Res. 31
(1998) 805e818;
[18] (a) T.M. Trnka, R.H. Grubbs, Acc. Chem. Res. 34 (2001) 18e29;
(b) H. Clavier, K. Grela, A. Kirschning, M. Mauduit, S.P. Nolan, Angew. Chem.
Int. Ed. 46 (2007) 6786e6801.
(c) J.F. Hartwig, Acc. Chem. Res. 31 (1998) 852e860;
[19] (a) E.A.B. Kantchev, C.J. O'Brien, M.J. Organ, Angew. Chem. Int. Ed. 46 (2007)
2768e2813;
(d) J.F. Hartwig, Angew. Chem. Int. Ed. Engl. 37 (1998) 2046e2067;
(e) J.F. Hartwig, Synlett (1997) 329e340;
(b) H. Clavier, S.P. Nolan, Annu. Rep. Prog. Chem. Sect. B 103 (2007) 193e222;
(c) F. Boeda, S.P. Nolan, Annu. Rep. Prog. Chem. Sect. B 104 (2008) 184e210;
(d) N. Marion, S.P. Nolan, Acc. Chem. Res. 41 (2008) 1440e1449;
(e) S. Wurtz, F. Glorius, Acc. Chem. Res. 41 (2008) 1523e1533.
[20] (a) V.P.W. Bohm, T. Weskamp, C.W.K. Gstottmayr, W.A. Hermann, Angew.
Chem. Int. Ed. 39 (2000) 1602e1604;
(f) J.P. Wolfe, S.L. Buckwald, Angew. Chem. Int. Ed. Engl. 38 (1999)
2413e2416;
(g) X. Bei, T. Uno, J. Norris, H.W. Turner, W.H. Weinburg, A.S. Guram,
J.L. Petersen, Organometallics 18 (1999) 1840e1853;
(h) X. Bei, A.S. Guram, H.W. Turner, W.H. Weinburg, Tetrahedron Lett. 40
(1999) 1237e1240.
(b) C.-H. Lee, W.-C. Ke, K.-T. Chan, C.-L. Lai, C.-H. Hu, H.M. Lee, Chem. e Eur. J.
13 (2007) 582e591.
[21] (a) C.-Y. Gao, L.-M. Yang, J. Org. Chem. 73 (2008) 1624e1627;
(b) J.-H. Huang, L.-M. Yang, Org. Lett. 13 (2011) 3750e3753;
(c) L. Hie, S.D. Ramgren, T. Mesganaw, N.K. Garg, Org. Lett. 14 (2012)
4182e4185;
[36] A.V. Astakhov, O.V. Khazipov, E.S. Degtyareva, V.N. Khrustalev,
V.M. Chernyshev, V.P. Ananikov, Organometallics 34 (2015) 5759e5766.
[37] T.A.P. Paulose, S.-C. Wu, J.A. Olson, T. Chau, N. Theaker, M. Hassler, J.W. Quail,
S.R. Foley, Dalton Trans. 41 (2012) 251e260.
[38] W.A. Herrmann, J. Schwarz, M.G. Gardiner, M. Spiegler, J. Organomet. Chem.
575 (1999) 80e86.
(d) M. Tobisu, T. Shimasaki, N. Chatani, Chem. Lett. 38 (2009) 710e711;
[39] G.M. Sheldrick, Acta Crystallogr. Sect. A 64 (2008) 112e122.