Bifunctional nickel precatalysts of amido-functionalized N-heterocyclic carbenes for base-free Michael reaction under ambient conditions

Article abstract of DOI:10.1016/j.jorganchem.2011.09.007

A series of new bifunctional nickel precatalysts, [1-(R)-3-N- (benzylacetamido)imidazol-2-ylidene]₂ Ni [R = Me (1b), i-Pr (2b), and CH₂Ph (3b)], containing a Lewis acidic metal site and a Lewis basic amido-N site in a pendent ligand sidearm, have been successfully designed for base-free Michael addition reaction. Specifically, the nickel (1-3)b complexes catalyzed the highly desired base-free Michael addition reactions of representative cyclic 5-membered β-dicarbonyl and β-ketoester substrates with a variety of activated olefinic compounds in air at ambient temperature in good to excellent yield. The nickel (1-3)b complexes were synthesized from the reactions of the corresponding imidazolium chloride salts, (1-3)a, with NiCl₂?6H₂O in presence of K ₂CO₃ as a base in 55-73% yield. The density functional theory (DFT) studies performed on the nickel complexes suggested the presence of a strong Ni-NHC σ-interaction in these complexes.

Full text of DOI:10.1016/j.jorganchem.2011.09.007

Journal of Organometallic Chemistry 696 (2012) 4159e4165
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Bifunctional nickel precatalysts of amido-functionalized N-heterocyclic carbenes
for base-free Michael reaction under ambient conditionsq
,
Sachin Kumara, Anantha Narayanana, Mitta Nageswar Raoa, Mobin M. Shaikhb, Prasenjit Ghosha
*
a Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
b National Single Crystal X-ray Diffraction Facility, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new bifunctional nickel precatalysts, [1-(R)-3-N-(benzylacetamido)imidazol-2-ylidene]2 Ni
[R ¼ Me (1b), i-Pr (2b), and CH2Ph (3b)], containing a Lewis acidic metal site and a Lewis basic amido-N
site in a pendent ligand sidearm, have been successfully designed for base-free Michael addition reac-
tion. Specifically, the nickel (1e3)b complexes catalyzed the highly desired base-free Michael addition
Received 6 July 2011
Received in revised form
5 September 2011
Accepted 8 September 2011
reactions of representative cyclic 5-membered b-dicarbonyl and b-ketoester substrates with a variety of
activated olefinic compounds in air at ambient temperature in good to excellent yield. The nickel (1e3)
b complexes were synthesized from the reactions of the corresponding imidazolium chloride salts, (1e3)
a, with NiCl2ꢀ6H2O in presence of K2CO3 as a base in 55e73% yield. The density functional theory (DFT)
Keywords:
Carbene
Nickel
Bifunctional
Catalysis
studies performed on the nickel complexes suggested the presence of a strong NieNHC s-interaction in
these complexes.
Ó 2011 Elsevier B.V. All rights reserved.
Michael reaction
1. Introduction
interests for their applications as dyes and fine chemicals and for
their biological activity [16,17].
The Michael addition involves the addition of a nucleophile,
called a Michael Donor, to an activated electrophilic olefin, known
as a Michael acceptor [1e3]; the reaction, being atom economic, is
largely popular for its simplicity in constructing quaternary ster-
eocenters with great efficiency and enantioselectivity [4e9]. It has
thus evolved into a powerful carbonecarbon bond forming tool for
synthesizing enantio-enriched, highly functionalized and intricate
carbon skeletons in natural products, biologically active
compounds and heterocyclic compounds etc.
The widespread applicability of Michael addition arises from its
mild reaction conditions, high functional group tolerance and from
its versatility in bond constructions that extend beyond the
conventional CeC bonds to a variety of CeX (X ¼ N, O, P, S) bonds.
There exist several variations of the parent reaction mainly in the
forms of oxa-Michael [10], involving oxygen nucleophiles like
alcohols, phospha-Michael [11,12], involving phosphorus nucleo-
philes eg. triphenylphosphine, thia-Michael [13], involving thiol
nucleophiles and finally the aza-Michael [14,15], involving nitrogen
nucleophiles like amines. All of these variants rightfully occupy key
positions in the synthesis of many a heteroatom compounds of
An important prerequisite of Michael reaction is the use of
a Brønsted base, which many a times affects the reaction in terms of
yielding a host of unwanted side-products that cut into the overall
reaction yield [1e3]. Specifically, the presence of a base in the
reaction medium promotes a wide variety of side reactions like,
ester solvolyses, hetero DielseAlder dimerization, aldol cyclization,
retro-Claisen CeC bond cleavage etc. Hence, against this backdrop,
performing base-free Michael addition is of considerable interest to
both industry and academia alike. It is thus anticipated that the
absence of a base would suppress all of the undesired secondary
reactions that unfavorably affects the reaction yield. In this regard
two different strategies have been employed in achieving base-free
Michael addition, the one taking recourse to organocatalysis [18]
while the other employing transition metals [19e21]. A common
premise that runs beneath both these approaches involves the use
of bifunctional catalysts possessing both acidic and basic sites
[22e25]. The organocatalysis provides a better alternative to
traditional metal based catalysis by avoiding the use of metals that
usually have various toxicity issues associated with it.
With one of the ongoing theme of our research being in
exploring the biomedical [26,27] and catalytic [28e41] potentials of
N-heterocyclic carbene [42e47] complexes of transition metals, we
became interested in constructing a variety of CeX (X ¼ C,
heteroatom) bonds catalytically, including the base-free Michael
reactions using bifunctional catalysts [48,49]. We rationalized that
q
Dedicated to Professor Christian Bruneau on his 60th birthday.
* Corresponding author. Tel.: þ91 22 2576 7178; fax: þ91 22 2572 3480.
E-mail address: pghosh@chem.iitb.ac.in (P. Ghosh).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.09.007

4160
S. Kumar et al. / Journal of Organometallic Chemistry 696 (2012) 4159e4165
R
R
N
N
N
N
N
O
N
Ni
O
1b 3b
2b
R = Me ( ), i-Pr ( ), CH2Ph ( )
Fig. 1. Nickel complexes of amido-functionalized N-heterocyclic carbenes are shown.
a basic functionality attached to an N-heterocyclic carbene ligand
skeleton of the catalyst would make it bifunctional by virtue of its
simultaneous presence with an acidic metal center in the same
catalyst. The N-heterocyclic carbene, as such as a ligand, offers
conducive electronic and steric environments amenable to easy
tunability of the ligand topology, which is often achieved through
incorporation of a variety of electron withdrawing, electron
donating and sterically demanding functional groups. As far as the
choice of the catalyst was concerned, we intended to employ nickel
owing to a report of the use of Ni(acac)2 in the Michael addition
reaction [50] and because of our prior reports of the use of
bifunctional nickel N-heterocyclic carbene complexes for its base-
free version [48,49].
Fig. 2. ORTEP diagram of 1b with thermal ellipsoids drawn at 50% probability level.
K2CO3 as a base in 55e73% yield (Scheme 1). The diamagnetic
nature of the nickel (1e3)b complexes were evident from the 1H
and 13C{1H} NMR study, which showed the disappearance of
a characteristic (NCHN) (1e3)a ligand resonance in its 1H NMR
spectrum while the appearance of a new diagnostic NieCcarbene
peak at a highly downfield region at ca. 169.2e169.9 ppm in its
corresponding 13C{1H} NMR spectrum. More interestingly, the
methylene (CH2) moiety of the chelating N-heterocyclic carbene
ligand in the nickel (1e3)b complexes appeared to be diaster-
eotopic in nature as it exhibited two sets of doublets each dis-
playing two-bond geminal coupling (2JHH) of 14 Hz in the 1H NMR
spectrum. The N-heterocyclic carbene precursors, i.e. the (1e3)
a imidazolium chloride salts, were synthesized from the corre-
sponding 1-(R)imidazoles (R ¼ Me, i-Pr, CH2Ph) by the treatment
with N-benzyl-2-chloro-acetamide.
Here in this contribution, we report a series of bifunctional
nickel precatalysts, [1-(R)-3-N-(benzylacetamido)imidazol-2-
ylidene]2Ni [R ¼ Me (1b), i-Pr (2b), and CH2Ph (3b)] (Fig. 1) that
carried out the base-free Michael reaction of representative cyclic
5-membered
b-dicarbonyl and b-ketoester substrates with
a variety of activated olefinic compounds in air at ambient
temperature in good to excellent yield. The NieNHC interaction in
the nickel (1e3)b complexes have been studied using density
functional theory (DFT) studies.
The X-ray diffraction studies revealed that the molecular
structures of the nickel (1e3)b complexes are monomeric and
isostructural, with each exhibiting a square planar geometry at the
metal center, and which is in concurrence with the diamagnetic
nature of the (1e3)b complexes as ascertained earlier from the
NMR experiments (Fig. 2 and see supporting information Figs. S1
and S2 and Table S2). Another interesting features of the (1e3)
b structures is that the amido-functionalized N-heterocyclic car-
bene ligands were found chelated to the metal center in a cis-
disposition to each other.
2. Results and discussions
A series of bifunctional nickel (1e3)b catalysts, possessing
a Lewis acidic metal site and a Lewis basic amido-N site, built in
a functionalized sidearm of a chelating N-heterocyclic carbene
ligand, was specifically designed for carrying out base-free Michael
addition reaction under ambient conditions. In particular, amido-
functionalized N-heterocyclic carbenes namely, 1-(R)-3-N-(benzy-
lacetamido)imidazol-2-ylidines (R
¼
Me, i-Pr, CH2Ph) were
employed in stabilizing the targeted bifunctional (1e3)b catalysts
that were obtained from the reaction of the respective (1e3)a imi-
dazolium chloride salts and NiCl2ꢀ6H2O in CH3CN in presence of
The NieCcarbene bond distances in 1b [1.843(2) Å and 1.861(2) Å],
2b [1.860(3) Å and 1.859(3) Å] and 3b [1.841(4) Å and 1.858(4) Å], are
R
R
R
N
N
N
N
N
R
N
N
N
NiCl2 6H2O
Ni
N
Cl
Cl
N
O
O
O
NH
O
HN
R = Me (1a), i-Pr (2a), CH2Ph (3a)
R = Me (1b), i-Pr (2b), CH2Ph (3b)
Scheme 1.

S. Kumar et al. / Journal of Organometallic Chemistry 696 (2012) 4159e4165
4161
slightlyshorterthan thesum of the individual covalent radii ofnickel
and carbon (NieC ¼ 1.926 Å) atoms [51] but are comparable to other
reported nickel N-heterocyclic carbene type complexes namely, [1-
(benzyl)-3-{N-(C6H5NHCOCH2)}imidazol-2-ylidene]2Ni [1.860(2) Å
and 1.853(3) Å] [52], [(1-(benzyl)-3-{N-(2,6-di-i-propylphenylace-
tamido)}imidazol-2-ylidene)(1-(benzyl)-3-{N-(2,6-di-i-propylphe-
ion using AOMix program [53]. Important are the molecular
orbitals (MOs) depicting the NHCenickel -interactions involving
s
a filled carbene lone pair orbital with an empty metal based orbital
i.e. 1b (HOMO-29), 2b (HOMO-31) and 3b (HOMO-37) (Fig. 3 and
see supporting information Figs. S3 and S4). Specifically, the
NieNHC
s
-bonding orbital, HOMO-29 (78% NHC, 6% Ni2þ), in 1b
nylacetamidato)}imidazol-2-ylidene)]NieOH [1.871(2)
Å
and
showed an interaction of the carbene lone pair (HOMO-7 of the free
NHC fragments) with a metal based vacant LUMO (100% Ni) of the
Ni2þ fragment (Fig. 3). Along the same line, HOMO-31 (70% NHC, 5%
Ni2þ) in 2b exhibited an interaction of the carbene lone pair
(HOMO-7 of the free NHC fragments) with a metal based vacant
LUMO (100% Ni) of the Ni2þ fragment (See Supporting Information
Fig. S3) and the HOMO-37 (65% NHC, 5% Ni2þ) in 3b exhibited an
interaction of the carbene lone pair (HOMO-7 of the free NHC
fragments) with a metal based vacant LUMO (100% Ni) of the Ni2þ
1.873(2) Å] [48] and [1-N-(2-methylenefuran-acetamido)-3-R-imi-
dazol-2-ylidene]2Ni {R ¼ Me [1.852(4) Å, 1.859(4) Å], CH2Ph
[1.851(4) Å and 1.861(4) Å]} [49] complexes.
Along the similar line, the NieNamido bond distances in 1b
[1.9204(18) Å and 1.9254(18) Å], 2b [1.9204(18) Å and 1.9254(18) Å]
and 3b [1.934(4) Å and 1.921(4) Å] too were comparable to other
related (NHC)2Ni type complexes namely, [1-N-(2- methylenefuran-
acetamido)-3-R-imidazol-2-ylidene]2Ni {R ¼ Me [1.921 (3) Å and
1.938 (3) Å], CH2Ph[1.922(3) Å and1.922(4)Å]} [49]and [1-(R)-3-{N-
(phenylacetamido)}imidazol-2-ylidene]2Ni {R ¼ CH2Ph [1.939(2) Å
and 1.938(2) Å], 4-F-C6H4CH2 [1.937(2) Å]} [52] complexes. The
NieNamido bond distances were, however, found to be longer than
the sum of the individual covalent radii of nickel and nitrogen
(NieN ¼ 1.854 Å) atoms [51].
fragment (See Supporting Information Fig. S4). These
s-bonded
molecular orbitals in (1e3)b, being deeply seated, are expected to
be unaffected by the incoming electrophiles or nucleophiles,
thereby making these nickel complexes relatively inert as
compared to the so-called Fischer and Schrock carbene ones that
are prone to nucleophilic and electrophilic attacks [54e59].
Quite interestingly, all of the nickel (1e3)b complexes success-
fully catalyzed the base-free Michael addition reaction for the
In order to get an insight into the NHCemetal interaction in the
(1e3)b complexes, the detailed density functional theory (DFT)
studies were carried out. Specifically, the geometry optimized
structures of the nickel (1e3)b complexes were obtained at the
B3LYP level of theory using the LANL2DZ, 6-31G(d) basis sets form
the respective X-ray coordinates followed by single point calcula-
tion on the geometry optimized structures. Further insights into the
carbeneemetal bond came from the postewave function analysis
using the natural bond orbital (NBO) method, performed on the
(1e3)b complexes as well as on the corresponding ligands and the
metal ion fragments.
representative cyclic 5-membered
b-ketoester and b-dicarbonyl
substrates with a variety of activated olefinic compounds in air at
ambient temperature in good to excellent yields (Equation 1 and
Table 1). Specifically, the cyclic 5-membered b-dicarbonyl and b-
ketoester compounds when treated with the activated olefins in
presence of 5 mol % of the nickel (1e3)b precatalysts in CH3CN, the
Michael addition products were obtained at room temperature
after 16 h. Of the various 5-membered cyclic substrates, the
b-
ketoester compounds, i.e. the ethyl 2-oxocyclopentane carboxylate
The strong
s
edonating nature of the N-heterocyclic carbene
and the methyl 2-oxocyclopentane carboxylate substrates, pro-
(NHC) ligand in (1e3)b was evident from both natural and Mulliken
charge analyses of the computed structures. Specifically, significant
increase in the electron density at the nickel center, associated with
the complex formation, were observed in the nickel (1e3)
b complexes with subsequent reduction of the same on the free
NHC ligand fragments (See Supporting Information Tables S3eS5).
Furthermore, the Natural Bond Orbital (NBO) analysis revealed that
the NieNHC bonds consists of an interactions of the sp2 hybridized
carbene carbon orbital with the sd hybridized nickel orbital in the
nickel (1e3)b complexes (See Supporting Information Tables S6
and S7). Electronic configuration of the metal in (1e3)b revealed
that the electron donation from the carbene lone pair occurred on
to the 3d and 4s orbital of the nickel center (See Supporting
Information Tables S6 and S7).
ceeded in higher yields compared to the corresponding
bonyl substrate i.e. the 2-acetylcyclopentanone (Table 1), while for
both the cyclic 5-membered -ketoester and -dicarbonyl
compounds, the reaction yields were found to be poor for the
bulkiest t-butyl acrylate substrate due to the obvious steric reasons.
Lastly, the control runs, carried out with NiCl2ꢀ6H2O, and the blank
experiments, performed for each of the individual substrates,
showed no or negligible formation of the desired Michael addition
b-dicar-
b
b
The Charge Decomposition Analysis (CDA) were performed in
order to obtain an estimate of NHC ligand to metal
s-donation
denoted by d, and the metal to NHC ligand -back donation
p
denoted by b, occurring in the nickel (1e3)b complexes (See Sup-
porting Information Table S8). The high d/b ratio estimated for 1b
(11.13), 2b (11.16) and 3b (11.00) derivatives are consistent with the
strongly
with very little
donation (d) and
s
-donating nature of the N-heterocyclic carbene ligands
-back bonding ability. It is worth noting that the
-back donation (b) values obtained in these
p
s-
p
nickel (1e3)b complexes represent the interaction between the
metal center and the NHCeligand fragment, which is a composite
of a NieCcarbene and a NieNamido interaction, arising out of the
chelation of an amido-functionalized ligand sidearm to the metal
center.
A better understanding of the NHCeNi interaction in (1e3)
b could be obtained from the molecular orbital (MO) correlation
diagram constructed from the individual fragment molecular
orbitals (FMOs) of the free NHC ligand fragments with the free Ni(II)
Fig. 3. Simplified orbital interaction diagram showing the major contributions of the
NHCeNi bond in 1b.

4162
S. Kumar et al. / Journal of Organometallic Chemistry 696 (2012) 4159e4165
Table 1
Selected results of 1,4-Michael type activated CeH addition across a,b-unsaturated substrates catalyzed by (1e3)b.
Entry
1
Reagenta
Reagenta
Product
1b yieldb (%)
2b yieldb (%)
3b yieldb (%)
O
O
O
88
83
80
68
81
65
Me
Me
Me
Me
COO
e
COOM
O
O
O
O
e
M
O
O
O
O
Me
Et
O
O
2
3
4
83
82
90
81
93
95
COO
COO
COO
Me
COO
O
Et
O
O
O
Me
COO
O
Ot-Bu
O
Ot-Bu
OMe
OEt
COOMe
O
OMe
OEt
O
O
O
5
6
7
8
94
99
86
73
96
95
87
35
84
99
77
75
O
O
O
COOEt
O
COOEt
O
Ot-Bu
Ot-Bu
COOEt
O
O
O
O
COOEt
CN
9
87
59
58
37
78
74
COOEt
O
10
COMe
O
e
M
O
O
O
OMe
11
74
49
73
65
76
71
O
COMe
O
Et
O
OEt
12
13
O
O
COMe
O
Ot-Bu
O
O
Ot-Bu
21
29
13
25
18
40
COMe
CN
14
COMe
a
Reaction conditions: 0.50 mmol of dicarbonyl compound, 0.75 mmol of
a
,
b
-unsaturated substrate, 2.50 ꢁ l0ꢂ2 mmol (5 mol %) of precatalyst (1e3)b, diethyleneglycol-di-
n-butyl ether (0.50 mmol; internal standard) and 3 mL of CH3CN at room temperature for 16 h.
b
GC yields were recorded using diethyleneglycol di-n-butyl ether as an internal standard.

S. Kumar et al. / Journal of Organometallic Chemistry 696 (2012) 4159e4165
4163
products, thereby upholding the high catalytic attribute of the
a result of the protonation from the
b
-dicarbonyl substrate
nickel (1e3)b complexes (see supporting information Table S1).
bound (Scheme 2). The intermediate A then reacts with an
activated olefin substrate giving another intermediate C, via
a transition state B in which the anionic Michael addition
product is formed. The last step involves protonation of the
anionic Michael addition followed by the release of the product
with the concomitant deprotonation and chelation of the amido
moiety to generate back the nickel precatalyst. The most
impressive feature of the proposed mechanism is the bifunc-
tional nature of the nickel (1e3)b precatalysts that successfully
carry out base-free Michael reaction under ambient conditions.
These bifunctional nickel (1e3)b precatalysts contained Lewis
acidic metal site while the amido-N site of the functional
sidearm serves as the Lewis basic site and which together in
tandem perform the desired catalysis.
O
O
COR1
1 3 b
( - )
COR1
+
R2
room temperature
R2
R1 = Me, OMe, OEt
R2 = CN, CO2Me, CO2Et, CO2t-Bu
Equation 1.
Important is the comparison of the catalytic activity of the nickel
(1e3)b precatalysts with the ones reported in the literature. Though
Michael additions have been extensively studied for long, the base-
free examples employing transition metals are relatively scarce
[1e3,19e21], and the ones involving well-defined nickel complexes
of N-heterocyclic carbenes remain only a handful, all of which have
emerged from our prior studies [48,49]. The activity of the nickel
(1e3)b precatalysts were found comparable to the other related
well-defined nickel based N-heterocyclic carbene precatalysts but
were found to be superior to a Ni(acac)2 mediated base-free Michael
reaction, that required elevated temperature [50,60].
A proposed mechanism for the base-free Michael reaction
initiates with the reaction of the nickel (1e3)b precatalysts with
a cyclic dicarbonyl compound to yield an intermediate (A) that
contain an anionic b-dicarbonyl moiety bound to the metal and
a non-chelated protonated amido moiety, which would form as
3. Conclusion
In summary, a series of new well-defined bifunctional nickel
(1e3)b precatalysts supported over amido-functionalized
N-heterocyclic carbene (NHC) ligands efficiently carried out base-
free Michael addition reaction of
a representative cyclic
5-membered -dicarbonyl and -ketoester compounds with a wide
b
b
range of activated olefins in air at ambient temperature. The density
functional theory (DFT) studies of the (1e3)b complexes pointed
toward greater a stability of the NieNHC interaction as the
molecular orbitals representing the
s-interaction in these
complexes were found to be low lying and deeply buried under-
neath the surface valence orbitals.
O
COR1
N
O
R
N
R
R
R
N
N
N
N
N
N
R1 = Me, OMe, OEt
O
HN
N
N
N
Ni
Ni
O
O
O
O
R1
A
)
(
R = Me (1b), i-Pr (2b), CH2Ph (3b)
R2
R2
R1
R2 = CN, CO2Me, CO2Et, CO2t-Bu
O
O
‡
N
O
N
R
R
N
N
R
R
N
N
N
O
O
HN
O
HN
R2
N
N
Ni
Ni
N
O
O
R1
R1
O
R2
O
(B)
C
(
)
Scheme 2.

4164
S. Kumar et al. / Journal of Organometallic Chemistry 696 (2012) 4159e4165
4. Experimental section
C
26H28NiN6O2ꢀ2.5H2O: C, 55.7; H, 5.9; N, 15.0. Found: C, 56.4; H,
6.0; N, 14.2.
4.1. General procedures
4.4. Synthesis of [1-(i-propyl)-3-N-(benzylacetamido)imidazol-
All manipulations were carried out using a combination of
a glovebox and standard Schlenk techniques. Solvents were puri-
fied and degassed by standard procedures. NiCl2ꢀ6H2O and 1-
methylimidazole were purchased from SD-fine Chemicals (India)
and Spectrochem (India) and were used without any further puri-
fication. The N-benzyl-2-chloro-acetamide [61], 2a [32] and 3a [32]
were prepared according to reported literature procedures. 1H and
13C{1H} NMR spectra were recorded on a Varian 400 MHz NMR
spectrometer. 1H NMR peaks are labeled as singlet (s), doublet (d),
triplet (t), and septet (sept). Infrared spectra were recorded on
a Perkin Elmer Spectrum One FT-IR spectrometer. Mass spectrom-
etry measurements were done on a Micromass Q-Tof spectrometer.
GC spectra were obtained on a Perkin Elmer Clarus 600 equipped
with a FID. GCeMS spectra were obtained on a Perkin Elmer Clarus
600 T equipped with EI source. Elemental Analysis was carried out
on Thermo Quest FLASH 1112 SERIES (CHNS) Elemental Analyzer. X-
ray diffraction data for (1e3)b were collected on an Oxford
Diffraction Excaliber-S diffractometer and crystal data collection
and refinement parameters are summarized in Supporting infor-
mation Table S2. The structures were solved using direct methods
and standard difference map techniques, and were refined by full-
matrix least-squares procedures on F2 with SHELXTL (Version 6.10)
[62,63].
2-ylidene]2Ni (2b)
A mixture of 1-(i-propyl)-3-N-(benzylacetamido)imidazolium
chloride (2a) (1.13 g, 3.86 mmol), NiCl2ꢀ6H2O (0.459 g, 1.93 mmol)
and K2CO3 (2.13 g,15.4 mmol) was refluxed in CH3CN (ca. 40 mL) for
24 h. The reaction mixture was filtered and the solvent was
removed under vacuum to give the product 2b as an yellow solid
(0.607 g, 55%). 1H NMR (CDCl3, 400 MHz, 25 ꢃC):
d 7.22 (t, 2H,
3JHH ¼ 8 Hz, m-C6H5), 7.15 (br,1H, p-C6H5), 7.11 (d, 2H, 3JHH ¼ 8 Hz, o-
C6H5), 6.98 (br, 1H, NCHCHN), 6.66 (br, 1H, NCHCHN), 4.90 (d, 1H,
2JHH ¼ 14 Hz, CH2), 4.85 (d, 1H, JHH ¼ 14 Hz, CH2), 4.29 (d, 1H,
2
2JHH ¼ 14 Hz, CH2), 3.54 (d, 1H, JHH ¼ 14 Hz, CH2), 3.33 (sept, 1H,
2
3JHH ¼ 7 Hz, CH(CH3)2), 1.03 (d, 3H, 3JHH ¼ 7 Hz, CH(CH3)2), 0.86 (d,
3H, 3JHH ¼ 7 Hz, CH(CH3)2). 13C{1H} NMR (CDCl3, 100 MHz, 25 ꢃC):
d
169.2 (NieNCN), 168.2 (CO), 143.8 (ipso-C6H5), 128.4 (m-C6H5),
128.0 (o-C6H5), 126.0 (p-C6H5), 122.5 (NCHCHN), 116.1 (NCHCHN),
56.7 (CH2), 51.3 (CH(CH3)2), 48.9 (CH2), 24.8 (CH(CH3)2), 21.1
(CH(CH3)2). IR (KBr Pellet): 1680 (nCO) cmꢂ1. Anal. Calcd. for
C30H36NiN6O2ꢀH2O: C, 61.1; H, 6.5; N, 14.3. Found: C, 60.9; H, 5.9; N,
15.2.
4.5. Synthesis of [1-(benzyl)-3-N-(benzylacetamido)imidazol-
2-ylidene]2Ni (3b)
4.2. Synthesis of 1-(methyl)-3-N-(benzylacetamido)imidazolium
A mixture of 1-(benzyl)-3-N-(benzylacetamido)imidazolium
chloride (1a)
chloride (3a) (1.00 g, 2.93 mmol), NiCl2ꢀ6H2O (0.348 g, 1.46 mmol)
and K2CO3 (1.62 g, 11.7 mmol) was refluxed in CH3CN (ca. 40 mL) for
24 h. The reaction mixture was filtered and the solvent was
removed under vacuum to give the product 3b as an yellow solid
A mixture of N-benzyl-2-chloro-acetamide (1.03 g, 5.61 mmol)
and 1-methylimidazole (0.565 g, 6.88 mmol) was dissolved in
toluene (ca. 10 mL) and refluxed at 110 ꢃC for 12 h during which
precipitation occurred. The precipitate was isolated by decanting
off the solvent and was washed with hot hexane (3 ꢁ ca. 15 mL) to
obtain the product 1a as an off-white solid (0.719 g, 48%). 1H NMR
(0.714 g, 73%). 1H NMR (CDCl3, 400 MHz, 25 ꢃC):
d 7.32 (t, 1H, p-
C6H5), 7.29 (br, 1H, p-C6H5), 7.26 (br, 2H, o-C6H5), 7.20 (br, 4H, m-
C6H5), 6.91 (br, 1H, NCHCHN), 6.86 (d, 2H, 3JHH ¼ 7 Hz, o-C6H5), 6.46
3
(br, 1H, NCHCHN), 4.91 (d, 1H, JHH ¼ 14 Hz, CH2), 4.40 (d, 1H,
3
(CDCl3, 400 MHz, 25 ꢃC):
d
9.66 (s, 1H, NCHN), 9.59 (s, 1H, NH), 7.53
3JHH ¼ 8 Hz, CH2), 4.37 (d, 1H, JHH ¼ 8 Hz, CH2), 4.14 (d, 1H,
3
3
(s, 1H, NCHCHN), 7.31 (d, 2H, JHH ¼ 7 Hz, o-C6H5), 7.24 (t, 2H,
3JHH ¼ 7 Hz, m-C6H5), 7.18 (br, 2H, p-C6H5 & NCHCHN), 5.30 (s, 2H,
CH2), 4.37 (d, 2H, 3JNH ¼ 6 Hz, CH2), 3.86 (s, 3H, CH3). 13C{1H} NMR
3JHH ¼ 14 Hz, CH2), 3.96 (d, 1H, JHH ¼ 14 Hz, CH2), 3.70 (d, 1H,
3JHH ¼ 14 Hz, CH2). 13C{1H} NMR (CDCl3, 100 MHz, 25 ꢃC):
d 169.9
(Ni-NCN), 169.3 (CO), 144.1 (ipso-C6H5), 134.1 (ipso-C6H5), 129.3 (m-
C6H5), 128.9 (m-C6H5), 128.6 (o-C6H5), 128.2 (o-C6H5), 126.1 (p-
C6H5),127.0 (p-C6H5),122.7 (NCHCHN),120.2 (NCHCHN), 56.7 (CH2),
53.1 (CH2), 49.5 (CH2). IR (KBr Pellet): 1676 (nCO) cmꢂ1. Anal. Calcd.
for C38H36NiN6O2ꢀ2H2O: C, 64.9; H, 5.7; N, 12.0. Found: C, 65.6; H,
5.1; N, 12.4.
(CDCl3, 100 MHz, 25 ꢃC):
d 165.1 (CO), 138.3 (ipso-C6H5), 138.0
(NCHN), 128.7 (m-C6H5), 127.9 (o-C6H5), 127.3 (p-C6H5), 123.9
(NCHCHN), 122.6 (NCHCHN), 51.7 (CH2), 43.6 (CH2), 31.1 (CH3). IR
(KBr Pellet): 1686
(nCO) . HRMS (ES): m/z 230.1306
cmꢂ1
[(NHC) þ H]þ, calcd. 230.1293.
4.3. Synthesis of [1-(methyl)-3-N-(benzylacetamido)imidazol-
4.6. Computational methods
2-ylidene]2Ni (1b)
Density functional theory calculations were performed on the
nickel (1e3)b complexes using GAUSSIAN 03[ [64] suit of quantum
chemical programs. The Becke three parameter exchange func-
tional in conjunction with Lee-Yang-Parr correlation functional
(B3LYP) has been employed in the study [65,66]. The LANL2DZ
valence basis set was used for nickel atom [67e70], while all other
atoms are treated at the 6-31G(d) basis set [71]. All stationary
points are characterized as minima by evaluating Hessian indices
on the respective potential energy surfaces. A tight SCF conver-
gence criteria (10ꢂ8 a.u.) was used for all calculations. Natural Bond
Orbital (NBO) analysis was performed using the NBO 3.1 [72] Pro-
gramme implemented in the GAUSSIAN 03 package.
A mixture of 1-(methyl)-3-N-(benzylacetamido)imidazolium
chloride (1a) (0.307 g, 1.16 mmol), NiCl2ꢀ6H2O (0.138 g,
0.579 mmol) and K2CO3 (0.639 g, 4.62 mmol) was refluxed in
CH3CN (ca. 40 mL) for 24 h. The reaction mixture was filtered and
the solvent was removed under vacuum to give the product 1b as
an yellow solid (0.174 g, 58%). 1H NMR (CDCl3, 400 MHz, 25 ꢃC):
d
7.29 (t, 2H, 3JHH ¼ 7 Hz, m-C6H5), 7.22 (t, 1H, 3JHH ¼ 7 Hz, p-C6H5),
7.11 (d, 2H, 3JHH ¼ 7 Hz, o-C6H5), 6.91 (br, 1H, NCHCHN), 6.64 (br, 1H,
NCHCHN), 4.84 (d, 1H, 2JHH ¼ 14 Hz, CH2), 4.12 (d, 1H, 2JHH ¼ 14 Hz,
CH2), 4.05 (d, 1H, 2JHH ¼ 14 Hz, CH2), 3.64 (d, 1H, 2JHH ¼ 14 Hz, CH2),
2.81 (s, 3H, CH3). 13C{1H} NMR (CDCl3, 100 MHz, 25 ꢃC):
d 169.9
(NieNCN), 169.2 (CO), 144.5 (ipso-C6H5), 128.8 (m-C6H5), 128.1 (o-
C6H5), 126.2 (p-C6H5), 122.4 (NCHCHN), 121.1 (NCHCHN), 56.6 (CH2),
49.4 (CH2), 35.7 (CH3). IR (KBr Pellet): 1678 (nCO) cmꢂ1. HRMS (ES):
m/z 515.1697 {[NHC]2Ni þ H}þ, calcd. 515.1705. Anal. Calcd. for
The metaleligand donoreacceptor interactions were inspected
by using the charge decomposition analysis (CDA) [73] which is
a valuable tool for analyzing the interactions between molecular
fragments on a quantitative basis, with an emphasis on the electron

S. Kumar et al. / Journal of Organometallic Chemistry 696 (2012) 4159e4165
4165
[21] S. Hoz, Acc. Chem. Res. 26 (1993) 69e74.
[22] J.F. Bai, X.Y. Xu, Q.C. Huang, L. Peng, L.X. Wang, Tetrahedron Lett. 51 (2010)
2803e2805.
donation [74,75]. The orbital contributions in the nickel (1e3)
b complexes can be divided into three parts:
[23] M. Shibasaki, S. Matsunaga, Chem. Soc. Rev. 35 (2006) 269e279.
[24] S. Kanemasa, K. Ito, Eur. J. Org. Chem. (2004) 4741e4753.
[25] T. Okino, Y. Hoashi, Y. Takemoto, J. Am. Chem. Soc. 125 (2003) 12672e12673.
[26] S. Ray, J. Asthana, J.M. Tanski, M.M. Shaikh, D. Panda, P. Ghosh, J. Organomet.
Chem. 694 (2009) 2328e2335.
(i)
(ii)
s
-donation from the 2[NHC]ꢂ / Ni2þ fragments.
p
-back donation from 2[NHC]ꢂ ) Ni2þ fragments and
(iii) A repulsive interaction between the occupied MOs of these
two fragments.
[27] S. Ray, R. Mohan, J.K. Singh, M.K. Samantaray, M.M. Shaikh, D. Panda, P. Ghosh,
J. Am. Chem. Soc. 129 (2007) 15042e15053.
[28] M.K. Samantaray, C. Dash, M.M. Shaikh, K. Pang, R.J. Butcher, P. Ghosh, Inorg.
Chem. 50 (2011) 1840e1848.
[29] C. Dash, M.M. Shaikh, R.J. Butcher, P. Ghosh, Inorg. Chem. 49 (2010) 4972e4983.
[30] C. Dash, M.M. Shaikh, R.J. Butcher, P. Ghosh, Dalton Trans. 39 (2010)
2515e2524.
[31] A. John, M.M. Shaikh, P. Ghosh, Dalton Trans. (2009) 10581e10591.
[32] S. Kumar, M.M. Shaikh, P. Ghosh, J. Organomet. Chem. 694 (2009) 4162e4169.
[33] M.K. Samantaray, M.M. Shaikh, P. Ghosh, J. Organomet. Chem. 694 (2009)
3477e3486.
[34] C. Dash, M.M. Shaikh, P. Ghosh, Eur. J. Inorg. Chem. (2009) 1608e1618.
[35] L. Ray, S. Barman, M.M. Shaikh, P. Ghosh, Chem. Eur. J. 14 (2008) 6646e6655.
[36] L. Ray, M.M. Shaikh, P. Ghosh, Dalton Trans (2007) 4546e4555.
[37] L. Ray, V. Katiyar, S. Barman, M.J. Raihan, H. Nanavati, M.M. Shaikh, P. Ghosh,
J. Organomet. Chem. 692 (2007) 4259e4269.
The CDA calculations were performed using the AOMix [76]
program with the B3LYP/LANL2DZ, 6-31G(d) wave function.
Molecular orbital (MO) compositions and the overlap populations
were calculated using the AOMix program. Analysis of the MO
compositions in terms of occupied and unoccupied fragment
orbitals (OFOs and UFOs, respectively), construction of orbital
interaction diagrams, the charge decomposition analysis (CDA)
were performed using the AOMix-CDA [77].
4.7. General procedure for the base-free Michael reaction
[38] M.K. Samantaray, V. Katiyar, K. Pang, H. Nanavati, P. Ghosh, J. Organomet.
Chem. 692 (2007) 1672e1682.
[39] L. Ray, M.M. Shaikh, P. Ghosh, Organometallics 26 (2007) 958e964.
[40] L. Ray, V. Katiyar, M. Raihan, H. Nanavati, M.M. Shaikh, P. Ghosh, Eur. J. Inorg.
Chem. (2006) 3724e3730.
[41] M.K. Samantaray, V. Katiyar, D. Roy, K. Pang, H. Nanavati, R. Stephen,
R.B. Sunoj, P. Ghosh, Eur. J. Inorg. Chem. (2006) 2975e2984.
[42] A. John, P. Ghosh, Dalton Trans. 39 (2010) 7183e7206.
[43] C. Dash, M.M. Shaikh, P. Ghosh, J. Chem. Sci. 123 (2011) 97e106.
[44] M.K. Samantaray, K. Pang, M.M. Shaikh, P. Ghosh, Dalton Trans. (2008)
4893e4902.
In a typical run, performed in air, a 25 mL vial was charged with
a
mixture of the
b
-dicarbonyl or
b-ketoester compound
(0.50 mmol), activated ethylene (0.75 mmol), diethyleneglycol-di-
n-butyl ether (0.50 mmol) (internal standard) and precatalyst,
(1e3)b, (5 mol %) in CH3CN (3 mL). The reaction mixture was stirred
for 16 h at room temperature after which the product was quan-
tified by GC and GCMS analysis with regard to diethyleneglycol-di-
n-butyl ether used as an internal standard.
[45] M.K. Samantaray, K. Pang, M.M. Shaikh, P. Ghosh, Inorg. Chem. 47 (2008)
4153e4165.
[46] L. Ray, M.M. Shaikh, P. Ghosh, Inorg. Chem. 47 (2008) 230e240.
[47] M.K. Samantaray, D. Roy, A. Patra, R. Stephen, M. Saikh, R.B. Sunoj, P. Ghosh,
J. Organomet. Chem. 691 (2006) 3797e3805.
[48] M.K. Samantaray, M.M. Shaikh, P. Ghosh, Organometallics 28 (2009)
2267e2275.
[49] L. Ray, M.M. Shaikh, P. Ghosh, Eur. J. Inorg. Chem. (2009) 1932e1941.
[50] J.H. Nelson, P.N. Howells, G.C. DeLullo, G.L. Landen, J. Org. Chem. 45 (1980)
1246e1249.
[51] L. Pauling, The Nature of The Chemical Bond, third ed.., vol. 256, Cornell
University Press, Ithaca, NY, 1960, pp. 224e225.
Acknowledgments
We thank BRNS, Mumbai, for financial support of this research.
We are grateful to the National Single Crystal X-ray Diffraction
Facility and Sophisticated Analytical Instrument Facility at IIT
Bombay, India, for the crystallographic and other characterization
data. Computational facilities from the IIT Bombay Computer
Center are gratefully acknowledged. SK and MNR thank CSIR, New
Delhi, for research fellowship.
[52] C.Y. Liao, K.T. Chan, Y.C. Chang, C.Y. Chen, C.Y. Tu, C.H. Hu, H.M. Lee, Organ-
ometallics 26 (2007) 5826e5833.
[53] S.I. Gorelsky, AOMix: Program for Molecular Orbital Analysis. York University,
Toronto, Canada, 1997.http://www.sg-chem.net (accessed 21.06.2011).
[54] D.A. Valyaev, M.G. Peterleitner, L.I. Leont’eva, L.N. Novikova, O.V. Semeikin,
V.N. Khrustalev, M.Y. Antipin, N.A. Ustynyuk, B.W. Skelton, A.H. White,
Organometallics 22 (2003) 5491e5497.
Appendix. Supplementary material
Supplementary material associated with this article can be found,
in the online version, at doi:10.1016/j.jorganchem.2011.09.007.
[55] M.A. Esteruelas, A.I. González, A.M. López, E. Oñate, Organometallics 22 (2003)
414e425.
[56] H.G. Raubenheimer, M.W. Esterhuysen, A. Timoshkin, Y. Chen, G. Frenking,
Organometallics 21 (2002) 3173e3181.
References
[57] J. Barluenga, J. Flórez, F.J. Fañanás, J. Organomet. Chem. 624 (2001) 5e17.
[58] J. Barluenga, A.A. Trabanco, J. Flórez, S. García-Granda, E. Martín, J. Am. Chem.
Soc. 118 (1996) 13099e13100.
[1] D.A. Oare, C.H. Heathcock, Stereochem 19 (1989) 227e407.
[2] J.H. Nelson, P.N. Howells, G.L. Landen, G.C. DeLullo, R.A. Henry, Fundam. Res.
Homogeneous Catal. 3 (1979) 921e939.
[3] J. Clayden, W. Greeves, S. Warren, P. Wothers, Organic Chemistry, seventh ed.
Oxford University Press, New York, 2007.
[59] L.L. Padolik, J.C. Gallucci, A. Wojcicki, J. Am. Chem. Soc. 115 (1993) 9986e9996.
ꢀ
[60] J. Clariana, N. Gálvez, C. Marchi, M. Moreno-Manas, A. Vallribera, E. Molins,
Tetrahedron 55 (1999) 7331e7344.
[61] G. Galliani, B. Rindone, F. Saliu, Tetrahedron Lett. 50 (2009) 5123e5125.
[62] G.M. Sheldrick, SHELXL-97, Program for Refinement of Crystal Structures.
University of Gottingen, Germany, 1997.
[63] G.M. Sheldrick, SHELXS-97, Structure Solving Program. University of Gottin-
gen, Germany, 1997.
[4] B.C. Hong, P. Kotame, C.W. Tsai, J.H. Liao, Org. Lett. 12 (2010) 776e779.
[5] E.M. Phillips, M. Wadamoto, A. Chan, K.A. Scheidt, Angew. Chem. Int. Ed. 46
(2007) 3107e3110.
[6] J.D. Angelo, D. Desmaéle, F. Dumas, A. Guingant, Tetrahedron Asymmetry 3
(1992) 459e505.
[64] M.J. Frisch, et al., Gaussian 03, Revision C.02. Gaussian, Inc., Wallingford CT, 2004.
[65] A.D. Becke, Phys. Rev. A 38 (1988) 3098e3100.
[7] Z.B. Li, S.P. Luo, Y. Guo, A.B. Xia, D.Q. Xu, Org. Biomol. Chem. 8 (2010)
2505e2508.
[66] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B. 37 (1988) 785e789.
[67] E.B. Kadossov, K.J. Gaskell, M.A. Langell, J. Comput. Chem. 28 (2007)
1240e1251.
[68] Q.Z. Han, Y.H. Zhao, H. Wen, Data Sci. J. 6 (2007) S837eS846.
[69] D.C. Graham, K.J. Cavell, B.F. Yates, Dalton Trans. (2007) 4650e4658.
[70] K. Cochran, G. Forde, G.A. Hill, L. Gorb, J. Leszczynski, J. Struct. Chem. 13 (2002)
133e140.
[8] J.A. Ma, D. Cahard, Angew. Chem. Int. Ed. 43 (2004) 4566e4583.
[9] N. Krause, Angew. Chem. Int. Ed. 37 (1998) 283e285.
[10] C.F. Nising, S. Bräse, Chem. Soc. Rev. (2008) 1218e1228.
[11] F. Lemasson, H.J. Gais, G. Raabe, Tetrahedron Lett. 48 (2007) 8752e8756.
[12] D. Enders, A. Saint-Dizier, M.I. Lannou, Eur. J. Org. Chem. (2006) 29e49.
[13] B.D. Mather, K. Viswanathan, K.M. Miller, T.E. Long, Prog. Polym. Sci. 31 (2006)
487e531.
[71] W.J. Hehre, R. Ditchfield, J.A. Pople, J. Chem. Phys. 56 (1972) 2257e2261.
[72] A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88 (1998) 899e926.
[73] S. Dapprich, G. Frenking, J. Phys. Chem. 99 (1995) 9352e9362.
[74] S.F. Vyboishchikov, G. Frenking, Chem. Eur. J. 4 (1998) 1439e1448.
[75] G. Frenking, U. Pidun, J. Chem. Soc. Dalton Trans. (1997) 1653e1662.
[76] S.I. Gorelsky, A.B.P. Lever, J. Organomet. Chem. 635 (2001) 187e196.
[77] S.I. Gorelsky, S. Ghosh, E.I. Solomon, J. Am. Chem. Soc. 128 (2006) 278e290.
[14] D. Enders, C. Wang, J.X. Liebich, Chem. Eur. J. 15 (2009) 11058e11076.
[15] P.R. Krishna, A. Sreeshailam, R. Srinivas, Tetrahedron 65 (2009) 9657e9672.
[16] V. Sriramurthy, O. Kwon, Org. Lett. 12 (2010) 1084e1087.
[17] Q. Li, C.H. Ding, X.H. Hou, L.H. Dai, Org. Lett. 12 (2010) 1080e1083.
[18] T. Boddaert, Y. Coquerel, J. Rodriguez, Chem. Eur. J. 17 (2011) 2266e2271.
[19] U. Kazmaier, Angew. Chem. Int. Ed 48 (2009) 5790e5792.
[20] J. Christoffers, Eur. J. Org. Chem. (1998) 1259e1266.