Nickel complexes of nio-functionalized n-Heterocyclic carbenes as precatalysts for michael reactions in air at room temperature under the much preferred base-Free conditions

Article abstract of DOI:10.1002/ejic.200801060

A series of several new nickel precatalysts supported over N/O-functionalized N-heterocyclic carbenes (NHC) for the Michael reactions of B-dicarbonyl, B-keto ester, B-diester, and a-cyano ester compounds with a,B-unsaturated carbonyl compounds in air at ambient temperature under the much preferred base-free conditions are reported. Specifically, the nickel complexes, [1-(R1-aminocarbonylmethyl)-3-R2-imid-azol-2-ylidene]2Ni [R1 =2-C6H4(OMe); R2 = Me (1b), iPr (2b), CH2Ph (3b) and R1 = 2-CH2C4H3O; R2 =Me (4b), CH2Ph (5b)] carried out the highly convenient base-free Michael addition of the activated C-H compounds across a,B-unsaturated car- bonyl compounds in air at room temperature. The complexes 1b-5b were synthesized by the direct reaction of the respective imidazolium chloride salt with NiCl2-6H2Oin CH3CN in the presence of K2CO3 as a base. The exceptional stability of 1b-5b has been attributed to the deeply buried nickel-NHC CT-bonding molecular orbitals as evidenced from the density functional theory (DFT) studies. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009.

Full text of DOI:10.1002/ejic.200801060

FULL PAPER
DOI: 10.1002/ejic.200801060
Nickel Complexes of N/O-Functionalized N-Heterocyclic Carbenes as
Precatalysts for Michael Reactions in Air at Room Temperature Under the
Much Preferred Base-Free Conditions
Sriparna Ray,[a] Mobin M. Shaikh,[b] and Prasenjit Ghosh*[a]
Keywords: Nickel / N-heterocyclic carbene / Michael addition / Base-free / Density functional calculations
A series of several new nickel precatalysts supported over
N/O-functionalized N-heterocyclic carbenes (NHC) for the
Michael reactions of β-dicarbonyl, β-keto ester, β-diester, and
α-cyano ester compounds with α,β-unsaturated carbonyl
compounds in air at ambient temperature under the much
preferred base-free conditions are reported. Specifically, the
nickel complexes, [1-(R1-aminocarbonylmethyl)-3-R2-imid-
azol-2-ylidene]2Ni [R1 = 2-C6H4(OMe); R2 = Me (1b), iPr (2b),
CH2Ph (3b) and R1 = 2-CH2C4H3O; R2 = Me (4b), CH2Ph (5b)]
carried out the highly convenient base-free Michael addition
of the activated C–H compounds across α,β-unsaturated car-
bonyl compounds in air at room temperature. The complexes
1b–5b were synthesized by the direct reaction of the respec-
tive imidazolium chloride salt with NiCl2·6H2O in CH3CN in
the presence of K2CO3 as a base. The exceptional stability of
1b–5b has been attributed to the deeply buried nickel–NHC
σ-bonding molecular orbitals as evidenced from the density
functional theory (DFT) studies.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
free Michael reaction under neutral conditions assumes sig-
nificance as it offers the possibility of completely eliminat-
ing the unwanted side products of the classic base-catalyzed
Michael reaction.[8] It is worth noting that the transition-
metal-catalyzed Michael reactions indeed exhibit superior
chemoselectivity by virtue of its milder and also neutral re-
action conditions in comparison to the classic base-cata-
lyzed reaction.[9] Furthermore, compared to the base-cata-
lyzed Michael reactions, the transition-metal-catalyzed
Michael reactions enjoy certain additional advantages as
they often do not require inert or anhydrous conditions,[10]
can be performed in the absence of solvents,[11] or may even
provide near quantitative conversions at ambient tempera-
tures.[12] In light of the above facts we became interested in
developing the chemistry of base-free Michael reactions, in
particular by designing highly efficient transition-metal-
based catalysts.
Of late N-heterocyclic carbenes have been phenomenally
successful in homogeneous catalysis and in this regard we
have recently designed several N/O-functionalized N-het-
erocyclic carbene-based[13] catalysts for a host of important
transformations ranging from their use in producing biode-
gradable polymers by ring-opening polymerization (ROP)
of -lactides[14] to a variety of C–C cross-coupling reactions,
namely the Suzuki–Miyaura[15] and the Sonogashira coup-
ling reactions.[16] Apart from catalysis, we have also ex-
plored the utility of these N/O-functionalized N-heterocy-
clic carbene compounds in biomedical applications, par-
ticularly with regard to their anticancer and antimicrobial
properties.[17] The necessary impetus for our efforts towards
employing N-heterocyclic carbenes in base-free Michael re-
Introduction
Though widely accepted as a useful methodology for the
construction of C–C bonds, the Michael reaction, which
involves the addition of the β-dicarbonyl compounds with
acceptor-activated olefins, suffers from certain limitations
that arise from its strongly basic reaction conditions.[1] In
particular, the classic Michael reaction is often catalyzed
by a Brønsted base, which apart from yielding the desired
Michael addition products, also promotes a host of un-
wanted side reactions like, the aldol-cyclization, retro-
Claisen type decomposition, ester solvolyses, hetero Diels–
Alder dimerization, Knoevenagel reaction, etc. and as a
consequence of which, the yields of the preferred Michael
addition products are severely affected.[2] Hence, a major
challenge in this area lies in reducing or even completely
eliminating the formation of the undesired side products of
the classic base-catalyzed Michael reaction. Towards this
objective, several strategies that have been successfully em-
ployed involve the use of a weak Brønsted base[3] or those
that focus on achieving milder reaction conditions by em-
ploying basic zeolites,[4] alumina,[5] the phase-transfer catal-
ysis,[6] or solid-phase catalysis.[7] In this context, the base-
[a] Department of Chemistry, Indian Institute of Technology Bom-
bay,
Powai, Mumbai 400076, India
E-mail: pghosh@chem.iitb.ac.in
Fax: +91-22-2572-3480
[b] National Single Crystal X-ray Diffraction Facility, Indian Insti-
tute of Technology Bombay,
Powai, Mumbai 400076, India
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
1932
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Nickel Complexes of N/O-Functionalized N-Heterocyclic Carbenes
actions came from the fact that despite unprecedented suc- chloro-N-(2-furanylmethyl)acetamide in 65–76% yields
cesses seen by the N-heterocyclic carbenes in numerous (Scheme 1). The formations of 1a–5a were verified by the
catalytic reactions recently, their utility in the base-free appearance of the diagnostic imidazolium (NCHN) reso-
Michael reaction have largely remained unexplored.[18] nance in the highly downfield region of the 1H NMR (9.25–
Hence, we set out to design new N/O-functionalized N-het- 10.34 ppm) and 13C NMR (135.3–138.0 ppm) spectra.
erocyclic carbene-based precatalysts for the base-free
Michael reaction.
Here in this contribution, we report a series of highly
efficient nickel complexes, 1b–5b, (Figure 1) supported over
amido-functionalized N-heterocyclic carbene ligands, for
the base-free Michael reaction of β-dicarbonyl, β-keto ester,
β-diester, and α-cyano ester compounds with α,β-unsatu-
rated carbonyl compounds in air at ambient temperature.
The air and moisture stability of these precatalysts has been
attributed to deeply buried NHC–Ni σ-bonding orbitals as
seen from the density functional theory studies.
Scheme 1.
The nickel complexes 1b–5b were synthesized from the
corresponding imidazolium chloride salts 1a–5a by the
treatment with NiCl2·6H2O in CH3CN in the presence of
K2CO3 as a base in 61–66% yields (Scheme 2 and
Scheme 3). With regard to this it is worth mentioning that
there broadly exists two methods of preparation of the
nickel complexes of N-heterocyclic carbenes viz. (i) the one
involving the reaction of the N-heterocyclic carbene with
Ni0 precursors like Ni(COD)2[20] and (ii) the other involving
Figure 1. Nickel complexes of the amido-functionalized N-hetero-
cyclic carbene, 1b–5b.
Results and Discussion
With an eye on exploring the utility of N-heterocyclic the direct reaction of the imidazolium salts with various
carbenes in Michael reactions, particularly under the much NiII precursors like, Ni(acac)2,[21] NiBr2(DME),[22]
preferred base-free conditions, we set out to design metal- Ni(PPh3)2X2 (X = Cl, Br),[23] and anhydrous NiCl2 in the
based precatalysts of N/O-functionalized N-heterocyclic presence of any base.[24] All of the 1b–5b complexes were
carbenes. Our choice of nickel for the study was based on found to be diamagnetic from the 1H NMR and 13C NMR
prior reports of the use of simple nickel precursors under spectra suggesting the square-planar nature of the metal
“ligand-assisted catalysis” (LAC) conditions, in which the center in these complexes. The 13C NMR spectra of 1b–
catalytically active species were generated in situ during the 5b showed the characteristic highly downfield shifted nickel
course of the Michael reaction.[10,19] It is worth mentioning bound carbene (Ni–Ccarbene) resonance at 168.7–169.8 ppm,
that classic Michael reactions exhibit certain limitations like in keeping with the formation of these complexes. Further-
the need for the use of a base, or requiring anhydrous reac- more, the 1H NMR spectrum of 1b–5b revealed the absence
tion conditions or high temperatures and even then on of the amido-NH resonance of the amido-functionalized
many occasions the reactions proceed in low yields. In light sidearm of the N-heterocyclic carbene ligand, and thereby
of these facts, we focused on designing well-defined, robust suggesting its deprotonation under the basic reaction condi-
nickel precatalysts supported over N/O-functionalized N- tions followed by chelation to the nickel center.
heterocyclic carbenes for their utility in Michael reactions
Indeed, in the molecular structures of 1b–5b, the amido-
under the highly desired base-free conditions. Additionally, functionalized N-heterocyclic carbene ligand was seen che-
we also explored the possibility of this catalysis occurring lating to the nickel center through its carbene-C and amido-
under amenable conditions like in air and at ambient tem- N donor atoms (Figure 2 and Figure 3 and Supporting In-
perature.
formation Figures S1–S3 and Table S1). More interestingly,
In this regard a series of new amido-functionalized N- the two amido-functionalized N-heterocyclic carbene li-
heterocyclic carbene precursors namely, 1-(R1-aminocar- gands in 1b–5b were found to bind to the metal center in a
bonylmethyl)-3-R2-imidazolium chloride [R1 = 2-C6H4- cis fashion with two amido-N atoms occupying two adja-
(OMe); R2 = Me (1a), iPr (2a), CH2Ph (3a) and R1 = 2- cent sites of the square-planar geometry around the nickel
CH2C4H3O; R2 = Me (4a), CH2Ph (5a)] were synthesized atom while the two carbene-C atoms reside on the remain-
from the respective substituted imidazoles by the direct re- ing two adjacent sites. The Ni–Ccarbene distances in 1b
action with 2-chloro-N-(2-methoxyphenyl)acetamide or 2- [1.742(2) Å, 1.959(3) Å], 2b [1.863(2) Å, 1.857(2) Å], 3b
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S. Ray, M. M. Shaikh, P. Ghosh
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Scheme 2.
Figure 3. ORTEP diagram of 4b. Selected bond lengths (Å) and
angles (°): Ni1–C1 1.852(4), Ni1–C12 1.859(4), Ni1–N3 1.921(3),
Ni1–N6 1.938(3), C1–Ni1–C12 93.68(16), C1–Ni1–N3 86.59(14),
C12–Ni1–N6 87.35(14), N3–Ni1–N6 92.56(13).
Scheme 3.
Despite striking similarities among the 1b–5b structures,
an important feature, which sets the isopropyl derivative 2b
apart from the rest, is the observation of a strong π–π inter-
action in the former (Figure 4). Specifically, the two aryl
rings in 2b were found stacked parallel to each other dis-
[1.865(4) Å, 1.867(5) Å], 4b [1.852(4) Å, 1.859(4) Å], and 5b
[1.851(4) Å, 1.861(4) Å] are comparable to that observed in
other structurally characterized nickel N-heterocyclic carbene
complexes namely, [1-benzyl-3-(phenylaminocarbonylmeth-
yl)imidazol-2-ylidene]2Ni [1.860(2) Å and 1.853(3) Å],[24] [1-
(4-fluorobenzyl)-3-(phenylaminocarbonylmethyl)imidazol-
2-ylidene]2Ni [1.858(3) Å],[24] [1-(4-methoxybenzyl)-3-(phenyl-
aminocarbonylmethyl)imidazol-2-ylidene]2Ni [1.864(3) Å
and 1.866(3) Å],[24] and [1,3-bis(phenylaminocarbonylmeth-
yl)imidazol-2-ylidene]2Ni [1.843(5) Å and 1.846(5) Å].[24]
The Ni–N (amido) distances are as follows, 1b
[1.8564(19) Å, 2.052(2) Å], 2b [1.962(2) Å, 1.9636(19) Å], 3b
[1.982(4) Å, 1.966(4) Å], 4b [1.921(3) Å, 1.938(3) Å], and 5b
[1.922(3) Å, 1.922(4) Å].
playing
a
centroid(C01)-to-centroid(C02) distance of
3.926 Å and thereby exhibiting an intramolecular offset or
slipped-type π–π interaction.[25]
Figure 4. Offset or slipped π–π interaction observed in 2b dis-
playing a centroid(C01)-to-centroid(C02) distance of 3.926 Å.
In order to get a better insight into the nature of the
NHC–Ni interaction, detailed density functional theory
(DFT) studies were undertaken. In particular, geometry op-
timized structures were computed at the B3LYP/
LANL2DZ, 6-31G(d) level of theory using the atomic coor-
dinates adopted from an X-ray analysis and subsequently
the single-point calculations along with the post-wave func-
tion analysis using the natural bond order (NBO) method
Figure 2. ORTEP diagram of 1b. Selected bond lengths (Å) and
angles (°): Ni1–C1 1.959(3), Ni1–C14 1.742(2), Ni1–N3 1.8564(19),
Ni1–N6 2.052(2), C1–Ni1–N3 86.49(9), C14–Ni1–N6 86.42(10),
C1–Ni1–C14 93.01(11), N3–Ni1–N6 94.07(9).
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Nickel Complexes of N/O-Functionalized N-Heterocyclic Carbenes
were done at the same level of theory for the detailed under- worth mentioning that contrary to the NHC–Ni interac-
standing of the electronic properties of these complexes. tion, the other metal–carbene interactions like those in Fi-
The geometric parameters observed from the X-ray struc- scher carbene and Schrock carbene complexes are highly
tures were in strong agreement with those obtained from susceptible to both electrophilic and nucleophilic attack.[26]
the optimized geometries of 1b–5b. (Supporting Infor-
mation, Tables S7–S11). The strong electron-donating abil-
ity of the anionic amido-functionalized N-heterocyclic car-
bene (NHC) ligand to the nickel center was evident from
both natural and Mulliken charge analyses that showed a
significant increase in the electron density at the nickel cen-
ter in 1b–5b as a consequence of binding of the free NHC
ligand fragment relative to the bare NiII ion (see Supporting
Information, Tables S12–S16). Indeed, scrutiny of the elec-
tronic configuration of the nickel center in 1b–5b revealed
Figure 5. Interaction of the filled carbene lone pairs and the amido
that electron donation from the carbene lone pair occurred
groups of the free NHC ligand fragments with the unfilled 4s or-
bital of nickel in 1b–5b.
onto the 3d and 4s orbitals of the nickel center in these
complexes (Supporting Information, Table S17). Further
scrutiny of the hybridization of the Ccarbene and Ni atoms
in the 1b–5b complexes showed that while the Ccarbene
atoms were sp2, the Ni atoms had sd character (Supporting
Information, Table S18).
More significantly, the 1b–5b complexes successfully cat-
alyzed the base-free Michael reaction of a wide variety of
substrates ranging from cyclic ethyl 2-oxocyclopentane car-
boxylate, ethyl 2-cyclohexanone carboxylate and 2-acetylcy-
clopentanone, to acyclic ethyl cyanoacetate and diethyl ma-
lonate compounds with α,β-unsaturated carbonyl com-
pounds in air at ambient temperature [Equation (1) and
Equation (2)]. It is quite interesting that significantly higher
conversions were observed for the cyclic five-membered
ethyl 2-oxocyclopentane carboxylate and 2-acetylcy-
clopentanone and the six-membered ethyl 2-cyclohexanone
carboxylate substrates than the acyclic ethyl cyanoacetate
and diethyl malonate substrates. Furthermore, for the acy-
clic substrates, the (2-methoxyphenyl)aminocarbonyl-
methyl-substituted 1b–3b precatalysts were found to be con-
sistently more active than the (2-furanylmethyl)aminocar-
bonylmethyl-substituted 4b and 5b precatalysts. It is worth
nothing that base-free conditions are very much desirable
for the Michael reactions since often these reactions are
performed in basic conditions, which also facilitate a variety
of unwanted side reactions, namely the aldol-cyclization,
retro-Claisen type decomposition, Knoevenagel reaction,
etc. Specifically the β-dicarbonyl compounds, when treated
with α,β-unsaturated carbonyl compounds in the presence
of 5 mol-% of the precatalysts 1b–5b in chloroform, under-
went a clean conversion to the Michael reaction products
Using charge decomposition analysis (CDA), an estimate
of the NHC-ligand-to-metal σ donation, denoted by d, and
the metal-to-NHC-ligand π back-donation, denoted by b,
occurring in 1b–5b was made (Supporting Information,
Table S19). The d/b ratio for 1b (10.77), 2b (11.11), 3b
(10.66), 4b (11.36), and 5b (11.10) showed that the N-het-
erocyclic carbene ligands were strongly electron donating in
nature. It is worth noting that the σ donation (d) and π
back-donation (b) values obtained in 1b–5b represent the
donations (forward σ and backward π) between the metal
center and the functionalized NHC-ligand fragment, which
is a composite of two interactions, a carbene–nickel interac-
tion along with an amido-N–nickel interaction. It is note-
worthy that owing to the amido-functionalized nature of
the NHC ligand, the chelation to the metal center through
the carbene-C and amido-N was seen to occur in these
complexes resulting in a composite NHC-ligand-to-metal
interaction.
A better understanding of the NHC–Ni interaction in
1b–5b could be obtained from the molecular orbital (MO)
correlation diagram constructed from the interaction of two
free NHC-ligand fragments with the free NiII ion in these
complexes. Of importance is the NHC–nickel σ interaction
that is comprised of the interaction of the carbene lone pair
with the nickel-based orbital, as depicted by the following
orbitals, 1b (HOMO-31), 2b (HOMO-30), 3b (HOMO-36),
4b (HOMO-25), and 5b (HOMO-33) (see Figures 5–6 and
Figures S4–S12 in the Supporting Information). A careful
look at these NHC–nickel σ-bonding molecular orbitals re-
veals that the carbene lone pairs along with the amido
groups of the free NHC ligand fragments interact with the
unfilled 4s orbital of the nickel center (see Figures 5–6 and
Figures S4–S12 in the Supporting Information). These
deeply buried NHC–nickel σ-bonding molecular orbitals
are the reason behind the inert nature of the NHC–nickel
interaction, which in turn contribute towards the excep-
tional stability of these complexes. With regard to this it is
(1)
(2)
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1935

S. Ray, M. M. Shaikh, P. Ghosh
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Figure 6. Simplified orbital interaction diagram showing the major contributions of the NHC–nickel bond in 1b.
in good to excellent yields at room temperature (Table 1 and tures, 80–100 °C, compared with room temperature as is the
Table 2). All of the control experiments carried out with case with our base-free Michael addition reaction by the
NiCl2·6H2O and Ni(acac)2 as well as the blank experiments precatalysts 1b–5b.
performed for each of the individual substrates showed no
A proposed mechanism for the base-free Michael reac-
or negligible formation of the desired Michael addition tion involves the reaction of the nickel catalyst 1b–5b with
products (Supporting Information, Tables S20 and S21). a representative β-dicarbonyl compound to give an interme-
Quite interestingly, for both the ethyl 2-oxocyclopentane diate A containing an anionic β-dicarbonyl moiety bound
carboxylate and 2-acetylcyclopentanone substrates the reac- to the metal and a non-chelated protonated amido moiety
tion yields were lowest for the bulkiest tert-butyl acrylate (Scheme 4). The intermediate A then reacts with the α,β-
presumably due to steric reasons.
unsaturated carbonyl compound to give the intermediate C,
The significance of the N-heterocyclic carbene-based 1b– having the anionic Michael addition product bound to the
5b precatalysts for the base-free Michael reaction becomes metal by a transition-state B. The last step involves proton-
prominent when considering that there exists only one prior ation and subsequent release of the Michael addition prod-
report of the use of N-heterocyclic carbene under organoca- uct concomitant with the deprotonation and chelation of
talytic conditions i.e. in the absence of any metal for the the amido moiety to give back the nickel precatalyst 1b–5b.
enantioselective intramolecular Michael reaction.[27]
The most impressive features of the 1b–5b precatalysts
are their ability to carry out the Michael reaction under
base-free conditions and in air at room temperature. Also
Conclusions
worth noting is that the 1b–5b complexes represent well-
In summary, a series of nickel precatalysts 1b–5b that
defined precatalysts for the base-free Michael reactions efficiently carried out Michael reactions of β-dicarbonyl, β-
since most of the known reports commonly use non-N-het- keto ester, β-diester, and α-cyano ester compounds with
erocyclic carbene ligands under LAC conditions, in which α,β-unsaturated carbonyl compounds in air and at ambient
the catalytically active species is generated in situ and hence temperature under the much preferred base-free conditions
the active species is poorly characterized.[28] It is quite inter- have been designed. The air and moisture stability of these
esting that the comparison with a few prior reports that precatalysts has been attributed to the deeply buried NHC–
exist of simple nickel precursors, like Ni(acac)2,[29] show Ni σ-bonding orbitals as seen from the density functional
that the reactions were carried out at much higher tempera- theory studies.
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Nickel Complexes of N/O-Functionalized N-Heterocyclic Carbenes
Table 1. Selected results of 1,4-Michael type activated C–H addition across α,β-unsaturated substrates catalyzed by 1b–5b.
[a] Reaction conditions: 0.50 mmol of cyclic β-dicarbonyl or β-keto ester compounds, 0.60 mmol of α,β-unsaturated substrates,
2.50ϫ10–2 mmol of catalyst 1b–5b and 2 mL of CHCl3 were taken for each run at room temperature for 17 h. [b] GC yields were recorded
using diethyleneglycol di-n-butyl ether as an internal standard.
Table 2. Selected results of 1,4-Michael type activated C–H addition across α,β-unsaturated substrates catalyzed by 1b–5b.
[a] Reaction conditions: 0.50 mmol of acyclic β-diester or α-cyano ester compounds, 0.60 mmol of α,β-unsaturated substrates,
2.50ϫ10–2 mmol of catalyst 1b–5b and 2 mL of CHCl3 were taken for each run at room temperature for 17 h. [b] GC yields using
diethyleneglycol di-n-butyl ether as an internal standard.
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1937

S. Ray, M. M. Shaikh, P. Ghosh
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Scheme 4.
3JHH = 8 Hz, 1 H, C6H4), 7.65 (s, 1 H, NCHCHN), 7.16 (s, 1 H,
NCHCHN), 7.07 (t, JHH = 8 Hz, 1 H, C6H4), 6.90–6.85 (m, 2 H,
Experimental Section
3
General Procedures: All manipulations were carried out using a
combination of a glovebox and standard Schlenk techniques. Sol-
vents were purified and degassed by standard procedures.
NiCl2·6H2O was purchased from SD-fine Chemicals (India) and 1-
methylimidazole was purchased from Spectrochem Pvt. Ltd.
(Mumbai, India) and were used without further purification. The
1-isopropylimidazole[30] and 1-benzylimidazole[31] were prepared
according to the literature procedures. 1H and 13C{1H} NMR spec-
C6H4), 5.68 (s, 2 H, CH2), 3.96 (s, 3 H, OCH3), 3.87 (s, 3 H,
CH3) ppm. 13C{1H} NMR ([D6]DMSO, 100 MHz, 25 °C): δ =
164.2 (C=O), 150.1 (C6H4), 138.0 (NCHN), 126.5 (C6H4), 125.5
(C6H4), 123.9 (NCHCHN), 123.2 (NCHCHN), 122.3 (C6H4),
120.4 (C6H4), 111.6 (C6H4), 55.9 (OCH3), 51.4 (CH2), 36.0
(CH ) ppm. IR (KBr pellet): ν = 3436 (m), 1693 (s), 1601 (m), 1540
˜
3
(s), 1491 (w), 1462 (m), 1259 (w), 1177 (m), 1117 (w), 1023 (m),
756 (s), 624 (w) cm–1. HRMS (ES): m/z 246.1248 [(NHC) + H]+,
calcd. 246.1243.
1
tra were recorded with a Varian 400 MHz NMR spectrometer. H
NMR peaks are labeled as singlet (s), doublet (d), triplet (t), mul-
tiplet (m), and septet (sept). Infrared spectra were recorded with a
Perkin–Elmer Spectrum One FT-IR spectrometer. Mass spectrome-
try measurements were done with a Micromass Q-Tof spectrometer.
GC spectra were obtained with a PerkinElmer Clarus 600 equipped
with a FID. GC–MS spectra were obtained with a Perkin–Elmer
Clarus 600 T equipped with an EI source. Elemental Analysis was
carried out with a Thermo Quest FLASH 1112 SERIES (CHNS)
Elemental Analyzer. X-ray diffraction data for 1b–5b were collected
with an Oxford Diffraction Excaliber-S diffractometer. The crystal
data collection and refinement parameters are summarized in
Table S1. The structures were solved by direct methods and stan-
dard difference map techniques, and were refined by full-matrix
least-squares procedures on F2 with SHELXTL (Version 6.10).[32]
Synthesis of {1-[(2-Methoxyphenyl)aminocarbonylmethyl]-3-methyl-
imidazol-2-ylidene}2Ni (1b): 1-[(2-Methoxyphenyl)aminocarbon-
ylmethyl]-3-methylimidazolium chloride (0.500 g, 1.78 mmol),
NiCl2·6H2O (0.420 g, 1.77 mmol), and K2CO3 (0.735, 5.32 mmol)
were added to CH3CN (ca. 30 mL). The reaction mixture was re-
fluxed for 24 h, after which it was filtered and the solvent removed.
The residue was extracted in CH2Cl2 (ca. 20 mL) and dried under
vacuum to obtain the product 1b as a yellow solid (0.297 g, 61%).
Single crystals for X-ray diffraction studies were grown in a mini-
1
mum volume of CH3CN (ca. 2 mL). H NMR (CDCl3, 400 MHz,
25 °C): δ = 7.04 (br., 1 H, NCHCHN), 6.84 (br., 2 H, C6H4), 6.76
(br., 1 H, NCHCHN), 6.54 (s, 1 H, C6H4), 6.42 (s, 1 H, C6H4),
2
2
5.78 (d, JHH = 14 Hz, 1 H, CH2), 4.38 (d, JHH = 14 Hz, 1 H,
CH2), 3.90 (s, 3 H, OCH3), 3.22 (s, 3 H, CH3) ppm. 13C{1H} NMR
(CDCl3, 100 MHz, 25 °C): δ = 169.3 (NCN), 167.6 (C=O), 152.8
(C6H4), 135.3 (C6H4), 130.7 (C6H4), 123.3 (NCHCHN), 122.1
(C6H4), 121.3 (C6H4), 119.7 (NCHCHN), 111.4 (C6H4), 57.5
CCDC-678648 (for 1b), CCDC-645977 (for 2b), CCDC-700839 (for
3b), CCDC-675231 (for 4b), and CCDC-657169 (for 5b) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
(OCH ), 56.6 (CH ), 35.6 (CH ) ppm. IR (KBr pellet): ν = 2938
˜
3
2
3
(w), 1670 (w), 1599 (m), 1574 (s), 1492 (m), 1370 (m), 1238 (s),
1177 (w), 1117 (w), 1027 (w), 748 (m) cm–1. HRMS (ES): m/z
547.1587 [(NHC)2Ni + H]+·, calcd. 547.1604. C26H28N6NiO4·
0.5CH2Cl2 (589.7): calcd. C 53.97, H 4.96, N 14.25; found C 53.81,
H 4.77, N 13.60.
Synthesis of 1-[(2-Methoxyphenyl)aminocarbonylmethyl]-3-methyl-
imidazolium Chloride (1a): 2-Chloro-N-(2-methoxyphenyl)acet-
amide (2.00 g, 10.0 mmol) and 1-methylimidazole (0.821 g,
10.0 mmol) were added to toluene (ca. 10 mL) and refluxed over-
night to obtain a light brown precipitate, which was isolated by
decanting off the solvent. The residue was washed with hot hexane
(ca. 15 mL) and dried under vacuum to obtain the product 1a as
Synthesis of 1-Isopropyl-3-[(2-methoxyphenyl)aminocarbonylmethyl]-
imidazolium Chloride (2a): 2-Chloro-N-(2-methoxyphenyl)acetamide
(2.00 g, 10.0 mmol) and 1-isopropylimidazole (1.11 g, 10.1 mmol)
were added to toluene (ca. 10 mL) and refluxed overnight to obtain
a light brown precipitate, which was isolated by decanting off the
1
a light brown powder (1.97 g, 70%). H NMR (CDCl3, 400 MHz,
25 °C): δ = 10.17 (s, 1 H, NCHN), 9.72 (s, 1 H, NH), 8.01 (d,
1938
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1932–1941

Nickel Complexes of N/O-Functionalized N-Heterocyclic Carbenes
solvent. The residue was washed with hot hexane (ca. 15 mL) and
carbonylmethyl]imidazolium
chloride
(0.358 g,
1.00 mmol),
dried under vacuum to obtain the product 2a as a light brown pow-
NiCl2·6H2O (0.237 g, 1.00 mmol), and K2CO3 (0.417, 3.00 mmol)
der (2.23 g, 72%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ = 10.22 (s, were added to CH3CN (ca. 30 mL). The reaction mixture was re-
1 H, NCHN), 9.77 (s, 1 H, NH), 7.98 (d, 3JHH = 7 Hz, 1 H, C6H4),
fluxed for 24 h, after which it was filtered and the solvent removed.
The residue was extracted in CH2Cl2 (ca. 20 mL) and dried under
vacuum to obtain the product 3b as a yellow solid (0.214 g, 61%).
Single crystals for X-ray diffraction studies were grown in a mini-
3
7.67 (s, 1 H, NCHCHN), 7.26 (s, 1 H, NCHCHN), 7.05 (t, JHH
=
3
3
7 Hz, 1 H, C6H4), 6.85 (t, JHH = 7 Hz, 1 H, C6H4), 6.83 (d, JHH
= 7 Hz, 1 H, C6H4), 5.72 (s, 2 H, CH2), 4.66 [sept, JHH = 7 Hz, 1
H, CH(CH3)2], 3.83 (s, 3 H, OCH3), 1.55 [d, JHH = 7 Hz, 6 H,
3
3
1
mum volume of CH3CN (ca. 2 mL). H NMR (CDCl3, 400 MHz,
CH(CH3)2] ppm. 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ =
162.8 (C=O), 149.3 (C6H4), 135.3 (NCHN), 125.6 (C6H4), 124.6
(C6H4), 123.0 (NCHCHN), 121.2 (C6H4), 119.5 (NCHCHN), 118.8
(C6H4), 110.2 (C6H4), 55.1 (OCH3), 52.2 (CH2), 51.2 [CH(CH3)2],
25 °C): δ = 7.33 (br., 5 H, C6H5), 7.03 (br., 1 H, NCHCHN), 7.02
(br., 1 H, NCHCHN), 6.82–6.77 (m, 2 H, C6H4), 6.49–6.47 (m, 2
H, C6H4), 5.78 (d, 2JHH = 14 Hz, 1 H, CH2), 4.95 (d, 2JHH = 14 Hz,
2
2
1 H, CH2), 4.41 (d, JHH = 14 Hz, 1 H, CH2), 4.35 (d, JHH
=
21.9 [CH(CH ) ] ppm. IR (KBr pellet): ν = 3124 (w), 2972 (m), 1694
14 Hz, 1 H, CH2), 3.78 (s, 3 H, OCH3) ppm. 13C{1H} NMR (CDCl3,
100 MHz, 25 °C): δ = 169.8 (NCN), 167.2 (C=O), 152.3 (C6H4),
134.7 (C6H5), 134.6 (C6H4), 130.2 (C6H5), 129.1 (C6H5), 128.7
(C6H4), 127.6 (C6H5), 122.8 (NCHCHN), 122.4 (C6H4), 120.1
(NCHCHN), 119.4 (C6H4), 109.9 (C6H4), 57.7 (OCH3), 55.8 (CH2),
˜
3 2
(s), 1601 (s), 1574 (s), 1491 (m), 1460 (m), 1370 (m), 1289 (m), 1239
(s), 1178 (w), 1115 (m), 1024 (m), 747 (s) cm–1. HRMS (ES): m/z
274.1563 [(NHC) + H]+, calcd. 274.1556.
Synthesis of {1-Isopropyl-3-[(2-methoxyphenyl)aminocarbonylmethyl]-
52.7 (CH ) ppm. IR (KBr pellet): ν = 2933 (w), 3368 (m), 1742 (w),
˜
2
imidazol-2-ylidene}2Ni
(2b):
1-Isopropyl-3-[(2-methoxyphenyl)-
1599 (s), 1566 (s), 1494 (m), 1455 (m), 1369 (m), 1240 (s), 1117 (w),
1024 (m), 742 (m) cm–1. HRMS (ES): m/z 699.2202 [(NHC)2Ni +
H]+·, calcd. 699.2230. C38H36N6NiO4·CH2Cl2 (784.36): calcd. C
59.72, H 4.88, N 10.71; found C 60.12, H 5.55, N 11.16.
aminocarbonylmethyl]imidazolium chloride (0.310 g, 1.00 mmol),
NiCl2·6H2O (0.237 g, 1.00 mmol), and K2CO3 (0.417, 3.02 mmol)
were added to CH3CN (ca. 30 mL). The reaction mixture was re-
fluxed for 24 h, after which it was filtered and the solvent removed.
The residue was extracted in CH2Cl2 (ca. 20 mL) and dried under
vacuum to obtain the product 2b as a yellow solid (0.190 g, 63%).
Single crystals for X-ray diffraction studies were grown in a mini-
Synthesis of 1-[(2-Furanylmethyl)aminocarbonylmethyl]-3-methyl-
imidazolium Chloride (4a): 2-Chloro-N-(2-furanylmethyl)acetamide
(1.74 g, 10.0 mmol) and 1-methylimidazole (0.821 g, 10.0 mmol)
were added to toluene (ca. 10 mL) and refluxed overnight to obtain
a light brown precipitate, which was isolated by decanting off the
solvent. The residue was washed with hot hexane (ca. 15 mL) and
dried under vacuum to obtain the product 4a as a light brown
1
mum volume of CH3CN (ca. 2 mL). H NMR (CDCl3, 400 MHz,
25 °C): δ = 7.09 (br., 1 H, NCHCHN), 6.79 (br., 1 H, NCHCHN),
6.78–6.77 (m, 2 H, C6H4), 6.44–6.42 (m, 1 H, C6H4), 6.39–6.37 (m,
2
2
1 H, C6H4), 5.89 (d, JHH = 14 Hz, 1 H, CH2), 4.47 (d, JHH
=
3
14 Hz, 1 H, CH2), 4.41 [sept, JHH = 7 Hz, 1 H, CH(CH3)2], 4.05
(s, 3 H, OCH3), 1.53 [d, 3JHH = 7 Hz, 3 H, CH(CH3)2], 0.91 [d, 3JHH
= 7 Hz, 3 H, CH(CH3)2] ppm. 13C{1H} NMR (CDCl3, 100 MHz,
25 °C): δ = 168.7 (NCN), 167.6 (C=O), 152.0 (C6H4), 135.1 (C6H4),
129.6 (NCHCHN), 122.7 (C6H4), 122.6 (C6H4), 119.6 (NCHCHN),
116.2 (C6H4), 109.7 (C6H4), 58.0 (OCH3), 55.6 (CH2), 51.7
[CH(CH3)2], 25.2 [CH(CH3)2], 21.6 [CH(CH3)2] ppm. IR (KBr pel-
1
powder (1.80 g, 71%). H NMR ([D6]DMSO, 400 MHz, 25 °C): δ
= 9.25 (br., 1 H, NCHN), 9.24 (br., 1 H, NH), 7.74 (br., 2 H,
NCHCHN), 7.58 (s, 1 H, C4H3O), 6.38 (s, 1 H, C4H3O), 6.31 (s, 1
3
H, C4H3O), 5.10 (s, 2 H, CH2), 4.30 (d, JHH = 6 Hz, 2 H, CH2),
3.89 (s, 3 H, CH3) ppm. 13C{1H} NMR ([D6]DMSO, 100 MHz,
25 °C): δ = 165.0 (C=O), 151.4 (C4H3O), 142.3 (C4H3O), 137.9
(NCHN), 123.6 (NCHCHN), 123.1 (NCHCHN), 110.5 (C4H3O),
107.3 (C4H3O), 50.5 (CH2), 35.8 (CH3), 35.7 (CH2) ppm. IR (KBr
let): ν = 3428 (s), 2981 (w), 1627 (s), 1541 (m), 1462 (m), 1380 (s),
˜
1257 (m), 1181 (m), 1116 (w), 1025 (m), 755 (s), 656 (w) cm–1.
C30H36N6NiO4·H2O (621.35): calcd. C 57.99, H 6.16, N 13.53; found
C 58.41, H 6.37, N 13.87.
pellet): ν = 3473 (s), 1683 (s), 1565 (m), 1428 (w), 1346 (w), 1265
˜
(m), 1178 (m), 1080 (w), 1017 (m), 753 (s), 622 (m) cm–1. HRMS
(ES): m/z 220.1082 [(NHC) + H]+, calcd. 220.1086.
Synthesis of 1-Benzyl-3-[(2-methoxyphenyl)aminocarbonylmethyl]imid-
azolium Chloride (3a): 2-Chloro-N-(2-methoxyphenyl)acetamide
(2.00 g, 10.0 mmol) and 1-benzylimidazole (1.58 g, 10.0 mmol) were
added to toluene (ca. 10 mL) and refluxed overnight to obtain a
light brown solid precipitate, which was isolated by decanting off the
solvent. The residue was washed with hot hexane (ca. 15 mL) and
dried under vacuum to obtain the product 3a as a light brown pow-
der (2.33 g, 65%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ = 10.34 (s,
1 H, NCHN), 9.86 (s, 1 H, NH), 7.99 (d, 3JHH = 8 Hz, 1 H, C6H4),
7.62 (br., 1 H, NCHCHN), 7.36 (br., 5 H, C6H5), 7.04 (br., 1 H,
NCHCHN), 7.02 (d, 3JHH = 8 Hz, 1 H, C6H4), 6.85 (t, 3JHH = 8 Hz,
Synthesis of {1-[(2-Furanylmethyl)aminocarbonylmethyl]-3-methyl-
imidazol-2-ylidene}2Ni (4b): 1-[(2-Furanylmethyl)aminocarbonyl-
methyl]-3-methylimidazolium chloride (0.500 g, 1.95 mmol),
NiCl2·6H2O (0.463 g, 1.95 mmol), and K2CO3 (0.810, 5.86 mmol)
were added to CH3CN (ca. 30 mL). The reaction mixture was re-
fluxed for 24 h, after which it was filtered and the solvent removed.
The residue was extracted in CH2Cl2 (ca. 20 mL) and dried under
vacuum to obtain the product 4b as a yellow solid (0.304 g, 63%).
Single crystals for X-ray diffraction studies were grown in a mini-
1
mum volume of CH3CN (ca. 2 mL). H NMR (CDCl3, 400 MHz,
25 °C): δ = 7.31 (s, 1 H, C4H3O), 6.93 (br., 1 H, NCHCHN), 6.66
(br., 1 H, NCHCHN), 6.32 (s, 1 H, C4H3O), 6.07 (s, 1 H, C4H3O),
3
1 H, C6H4), 6.83 (t, JHH = 8 Hz, 1 H, C6H4), 5.70 (s, 2 H, CH2),
5.41 (s, 2 H, CH2), 3.83 (s, 3 H, OCH3) ppm. 13C{1H} NMR
(CDCl3, 100 MHz, 25 °C): δ = 163.3 (C=O), 150.0 (C6H4), 138.2
(C6H5), 137.6 (NCHN), 132.6 (C6H4), 129.4 (C6H5), 128.7 (C6H5),
126.4 (C6H5), 125.3 (C6H4), 124.0 (NCHCHN), 121.8 (C6H4), 120.8
(C6H4), 120.4 (NCHCHN), 110.8 (C6H4), 55.8 (OCH3), 53.4 (CH2),
2
2
5.08 (d, JHH = 15 Hz, 1 H, CH2), 4.81 (d, JHH = 15 Hz, 1 H,
2
2
CH2), 4.27 (d, JHH = 15 Hz, 1 H, CH2), 3.62 (d, JHH = 15 Hz, 1
H, CH2), 2.93 (s, 3 H, CH3) ppm. 13C{1H} NMR (CDCl3,
100 MHz, 25 °C): δ = 169.3 (NCN), 168.6 (C=O), 156.7 (C4H3O),
140.6 (C4H3O), 122.0 (NCHCHN), 121.2 (NCHCHN), 110.2
(C4H3O), 105.2 (C4H3O), 56.2 (CH3), 41.5 (CH2), 35.4 (CH2) ppm.
52.2 (CH ) ppm. IR (KBr pellet): ν = 3395 (m), 3262 (m), 3063 (w),
˜
2
1661 (s), 1601 (m), 1537 (s), 1497 (w), 1462 (m), 1270 (m), 1155 (m),
1024 (m), 761 (m), 633 (m) cm–1. HRMS (ES): m/z 322.1554 [(NHC)
+ H]+, calcd. 322.1556.
Synthesis of {1-Benzyl-3-[(2-methoxyphenyl)aminocarbonylmethyl]- 809 (w), 744 (s) cm–1. C22H24N6NiO4·CH2Cl2 (580.09): calcd. C
imidazol-2-ylidene}2Ni (3b): 1-Benzyl-3-[(2-methoxyphenyl)amino- 47.62, H 4.52, N 14.49; found C 46.90, H 4.98, N 15.22.
IR (KBr pellet): ν = 3413 (s), 1672 (m), 1578 (s), 1468 (w), 1392
˜
(m), 1300 (w), 1240 (w), 1179 (w), 1146 (w), 1077 (w), 1006 (m),
Eur. J. Inorg. Chem. 2009, 1932–1941
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1939

S. Ray, M. M. Shaikh, P. Ghosh
FULL PAPER
Synthesis of 1-Benzyl-3-[(2-furanylmethyl)aminocarbonylmethyl]imid- (i) σ donation from the NHCǞNi fragment and
azolium Chloride (5a): 2-Chloro-N-(2-furanylmethyl)acetamide
(1.74 g, 10.0 mmol) and 1-benzylimidazole (1.58 g, 10.0 mmol)
were added to toluene (ca. 10 mL) and refluxed overnight to obtain
a light brown solid precipitate, which was isolated by decanting off
(ii) π back-donation from the NHCǟNi fragment
The CDA calculations were performed using the program
AOMix,[40] using the B3LYP/LANL2DZ, 6-31G(d) wave function.
Molecular orbital (MO) compositions and the overlap populations
the solvent. The residue was washed with hot hexane (ca. 15 mL)
were calculated using the AOMix program. The analysis of the MO
and dried under vacuum to obtain the product 5a as a light brown
1
compositions in terms of occupied and unoccupied fragment orbit-
als (OFOs and UFOs, respectively), construction of orbital interac-
tion diagrams, and the charge decomposition analysis (CDA) were
performed using the AOMix-CDA.[41]
powder (2.52 g, 76%). H NMR ([D6]DMSO, 400 MHz, 25 °C): δ
= 9.44 (br., 1 H, NCHN), 9.26 (br., 1 H, NH), 7.87 (br., 1 H,
NCHCHN), 7.77 (br., 1 H, NCHCHN), 7.59 (s, 1 H, C4H3O),
7.44–7.38 (m, 5 H, C6H5), 6.39 (s, 1 H, C4H3O), 6.31 (s, 1 H,
C4H3O), 5.52 (s, 2 H, CH2), 5.12 (s, 2 H, CH2), 4.31 (s, 2 H,
CH2) ppm. 13C{1H} NMR ([D6]DMSO, 100 MHz, 25 °C): δ =
164.8 (C=O), 151.4 (C4H3O), 142.1 (C4H3O), 137.4 (NCHN), 134.8
(C6H5), 128.8 (C6H5), 128.6 (C6H5), 128.2 (C6H5), 124.0
(NCHCHN), 121.8 (NCHCHN), 110.3 (C4H3O), 107.2 (C4H3O),
General Procedure for the Michael Reaction: In a typical run, per-
formed in air, a 25 mL vial was charged with a mixture of the β-
dicarbonyl, β-keto ester, β-diester, or α-cyano ester compound
(0.50 mmol) and diethyleneglycol di-n-butyl ether (0.50 mmol) (in-
ternal standard). The precatalyst 1b–5b (5 mol-%) (Table 1) and
then CHCl3 (2 mL) were added to the reaction mixture and stirred
at room temperature for 1 h. The α,β-unsaturated carbonyl com-
pound (0.60 mmol) was then added to the reaction mixture and
stirred for another 16 h at room temperature after which the solu-
tion was analyzed by quantitative GC analysis using diethyl-
eneglycol di-n-butyl ether as an internal standard.
51.7 (CH ), 50.6 (CH ), 35.7 (CH ) ppm. IR (KBr pellet ): ν = 3450
˜
2
2
2
(w), 3064 (w), 1685 (s), 1560 (m), 1456 (w), 1261 (m), 1161 (m),
1080 (w), 1021 (w), 804 (w), 714 (m), 625 (w) cm–1. HRMS (ES):
m/z 296.1393 [(NHC) + H]+, calcd. 296.1399.
Synthesis of {1-Benzyl-3-[(2-furanylmethyl)aminocarbonylmethyl]-
imidazol-2-ylidene}2Ni (5b): 1-Benzyl-3-[(2-furanylmethyl)amino-
carbonylmethyl]imidazolium chloride (0.332 g, 1.00 mmol),
NiCl2·6H2O (0.237 g, 1.00 mmol), and K2CO3 (0.417, 3.00 mmol)
were added to CH3CN (ca. 30 mL). The reaction mixture was re-
fluxed for 24 hours, after which it was filtered and the solvent re-
moved. The residue was extracted in CH2Cl2 (ca. 20 mL) and dried
under vacuum to obtain the product 5b as a yellow solid (0.214 g,
66%). Single crystals for X-ray diffraction studies were grown in a
minimum volume of CH3CN (ca. 2 mL). 1H NMR (CDCl3,
400 MHz, 25 °C): δ = 7.31 (br., 2 H, C6H5), 7.29 (s, 1 H, C4H3O),
7.18 (br., 1 H, NCHCHN), 6.96–6.94 (m, 3 H, C6H5), 6.50 (br., 1
H, NCHCHN), 6.27 (s, 1 H, C4H3O), 6.06 (s, 1 H, C4H3O), 5.08
Supporting Information (see also the footnote on the first page of
this article): X-ray crystallographic data, B3LYP coordinates of the
optimized geometries of 1b–5b, computational data, and control
experimental data.
Acknowledgments
We thank Department of Science and Technology (DST), New
Delhi, for financial support of this research. We are grateful to the
National Single Crystal X-ray Diffraction Facility and Sophisti-
cated Analytical Instrument Facility at IIT Bombay, India, for the
crystallographic and other characterization data. Computational
facilities from the IIT Bombay Computer Center are gratefully ac-
knowledged. S. R. thanks CSIR-UGC, New Delhi for a research
fellowship.
2
2
(d, JHH = 15 Hz, 1 H, CH2), 4.81 (d, JHH = 15 Hz, 1 H, CH2),
2
2
4.40 (d, JHH = 15 Hz, 1 H, CH2), 4.28 (d, JHH = 15 Hz, 1 H,
2
2
CH2), 4.16 (d, JHH = 15 Hz, 1 H, CH2), 3.64 (d, JHH = 15 Hz, 1
H, CH2) ppm. 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ = 169.3
(NCN), 169.1 (C=O), 156.4 (C4H3O), 140.7 (C4H3O), 134.4
(C6H5), 129.1 (C6H5), 128.6 (C6H5), 127.5 (C6H4), 122.7
(NCHCHN), 120.2 (NCHCHN), 110.4 (C4H3O), 105.9 (C4H3O),
[1] a) D. A. Oare, C. H. Heathcock, Top. Stereochem. 1989, 19,
227–407; b) J. H. Nelson, P. N. Howells, G. L. Landen, G. C.
DeLullo, R. A. Henry, Fundam. Res. Homogeneous Catal.
1979, 3, 921–939; c) E. D. Bergmann, D. Ginsburg, R. Pappo,
Org. React. 1959, 10, 179–555.
[2] a) J. Christoffers, G. Koripelly, A. Rosiak, M. Rössle, Synthesis
2007, 1279–1300; b) J. Comelles, M. Moreno-Mañas, A. Vallri-
bera, ARKIVOC 2005, 9, 207–238; c) J. Christoffers, Eur. J.
Org. Chem. 1998, 1259–1266.
[3] a) A. García-Raso, J. García-Raso, B. Campaner, R. Mestres,
J. V. Sinisterra, Synthesis 1982, 1037–1041; b) J. Boyer, R. J. P.
Corriu, R. Perz, C. Réyé, J. Chem. Soc., Chem. Commun. 1981,
122–123.
[4] a) K. Motokura, M. Tada, Y. Iwasawa, J. Am. Chem. Soc.
2007, 129, 9540–9541; b) P. D. Shinde, V. A. Mahajan, H. B.
Borate, V. H. Tillu, R. Bal, A. Chandwadkar, R. D. Wakharkar,
J. Mol. Catal. A 2004, 216, 115–119; c) R. Sreekumar, P. Rug-
mini, R. Padmakumar, Tetrahedron Lett. 1997, 38, 6557–6560.
[5] a) K. Motokura, M. Tomita, M. Tada, Y. Iwasawa, Chem. Eur.
J. 2008, 14, 4017–4027; b) S. Castellano, A. Bertamino, I. Go-
mez-Monterrey, M. Santoriello, P. Grieco, P. Campiglia, G.
Sbardella, E. Novellino, Tetrahedron Lett. 2008, 49, 583–585;
c) B. C. Ranu, S. Bhar, Tetrahedron 1992, 48, 1327–1332; d) P.
Laszlo, P. Pennetreau, Tetrahedron Lett. 1985, 26, 2645–2648.
[6] a) K. Maruoka, Org. Process Res. Dev. 2008, 12, 679–697; b)
E. Diez-Barra, A. de la Hoz, S. Merino, P. Sánchez-Verdú, Tet-
rahedron Lett. 1997, 38, 2359–2362.
56.5 (CH ), 52.8 (CH ), 42.0 (CH ) ppm. IR (KBr pellet): ν = 3378
˜
2
2
2
(m), 3125 (w), 2925 (w), 1595 (s), 1392 (w), 1328 (w), 1298 (w),
1238 (w), 1145 (w), 998 (w), 930 (w), 807 (w), 730 (w), 699 (w) cm–1.
C34H32N6NiO4·CH2Cl2 (732.28): calcd. C 57.41, H 4.68, N 11.48;
found C 57.05, H 5.36, N 11.00.
Computational Methods: Density functional theory calculations
were performed on the five nickel complexes 1b–5b using the
GAUSSIAN 03[33] suite of quantum chemical programs. The Becke
three parameter exchange functional in conjunction with the Lee–
Yang–Parr correlation functional (B3LYP) has been employed in
this study.[34,35] The LANL2DZ basis set was used for the Ni
atom,[36] while all other atoms were treated with the 6-31G(d) basis
set.[37] All stationary points are characterized as minima by evaluat-
ing Hessian indices on the respective potential energy surfaces. A
tight SCF convergence (10–8 a.u.) was used for all calculations.
Inspection of the metal–ligand donor–acceptor interactions was
carried out using the charge decomposition analysis (CDA).[38]
CDA is a valuable tool in analyzing the interactions between mo-
lecular fragments on a quantitative basis, with an emphasis on the
electron donation.[39] The orbital contributions in the NHC–Ni
complexes, 1b–5b, can be divided into two parts:
1940
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Nickel Complexes of N/O-Functionalized N-Heterocyclic Carbenes
[7]
[8]
[26] a) D. A. Valyaev, M. G. Peterleitner, L. I. Leont’eva, L. N. No-
vikova, O. V. Semeikin, V. N. Khrustalev, M. Y. Antipin, N. A.
Ustynyuk, B. W. Skelton, A. H. White, Organometallics 2003,
22, 5491–5497; b) M. A. Esteruelas, A. I. González, A. M.
López, E. Oñate, Organometallics 2003, 22, 414–425; c) H. G.
Raubenheimer, M. W. Esterhuysen, A. Timoshkin, Y. Chen, G.
Frenking, Organometallics 2002, 21, 3173–3181; d) J. Bar-
luenga, J. Flórez, F. J. Fañanás, J. Organomet. Chem. 2001, 624,
5–17; e) J. Barluenga, A. A. Trabanco, J. Flórez, S. García-
Granda, E. Martín, J. Am. Chem. Soc. 1996, 118, 13099–13100;
f) L. L. Padolik, J. C. Gallucci, A. Wojcicki, J. Am. Chem. Soc.
1993, 115, 9986–9996.
[27] E. M. Phillips, M. Wadamoto, A. Chan, K. A. Scheidt, Angew.
Chem. Int. Ed. 2007, 46, 3107–3110.
[28] a) P.-S. Lin, M. Jeganmohan, C.-H. Cheng, Chem. Asian J.
2007, 2, 1409–1416; b) K. Itoh, S. Kanemasa, J. Am. Chem.
Soc. 2002, 124, 13394–13395.
[29] a) J. Clariana, N. Gálvez, C. Marchi, M. Moreno-Manˇas, A.
Vallribera, E. Molins, Tetrahedron 1999, 55, 7331–7344; b) J. H.
Nelson, P. N. Howells, G. C. DeLullo, G. L. Landen, J. Org.
Chem. 1980, 45, 1246–1249.
[30] E. Mas-Marzá, E. Peris, I. Castro-Rodr´iguez, K. Meyer, Orga-
nometallics 2005, 24, 3158–3162.
[31] H. M. Lee, C. Y. Lu, C. Y. Chen, W. L. Chen, H. C. Lin, P. L.
Chiu, P. Y. Cheng, Tetrahedron 2004, 60, 5807–5825.
[32] a) G. M. Sheldrick, SHELXL-97, Program for refinement of
crystal structures; University of Gottingen, Germany, 1997; b)
G. M. Sheldrick, SHELXS-97, Structure solving program, Uni-
versity of Göttingen, Germany, 1997.
a) G. Sundararajan, N. Prabagaran, Org. Lett. 2001, 3, 389–
392; b) D. J. Macquarrie, Chem. Commun. 1997, 601–602.
a) Y. Qian, S. Xiao, L. Liu, Y. Wang, Tetrahedron: Asymmetry
2008, 19, 1515–1518; b) A. Kumar, Akanksha, Tetrahedron 2007,
63, 11086–11092; c) A. Lubineau, J. Augé, Tetrahedron Lett.
1992, 33, 8073–8074; d) R. Antonioletti, F. Bonadies, E. S. Mon-
teagudo, A. Scettri, Tetrahedron Lett. 1991, 32, 5373–5374.
a) D. A. Evans, D. Seidel, J. Am. Chem. Soc. 2005, 127, 9958–
9959; b) D. A. Evans, R. J. Thomson, F. Franco, J. Am. Chem.
Soc. 2005, 127, 10816–10817; c) B. Corain, M. Basato, J. Mol.
Catal. 1993, 81, 133–155; d) K. Irie, K. Miyazu, K. Watanabe,
Chem. Lett. 1980, 3, 353–354.
J. Christoffers, U. Rößler, T. Werner, Eur. J. Org. Chem. 2000,
701–705.
a) H. Firouzabadi, N. Iranpoor, M. Jafarpour, A. Ghaderi, J.
Mol. Catal. A 2006, 252, 150–155; b) G. Bartoli, M. Bosco,
M. C. Bellucci, E. Marcantoni, L. Sambri, E. Torregiani, Eur.
J. Org. Chem. 1999, 617–620; c) H. Kotsuki, K. Arimura, T.
Ohishi, R. Maruzasa, J. Org. Chem. 1999, 64, 3770–3773.
a) J. Christoffers, A. Mann, Chem. Eur. J. 2001, 7, 1014–1027;
b) J. Christoffers, Eur. J. Org. Chem. 1998, 759–761.
a) M. K. Samantaray, K. Pang, M. M. Shaikh, P. Ghosh, Dal-
ton Trans. 2008, 4893–4902; b) M. K. Samantaray, K. Pang,
M. M. Shaikh, P. Ghosh, Inorg. Chem. 2008, 47, 4153–4165; c)
L. Ray, M. M. Shaikh, P. Ghosh, Inorg. Chem. 2008, 47, 230–
240; d) M. K. Samantaray, D. Roy, A. Patra, R. Stephen, M.
Saikh, R. B. Sunoj, P. Ghosh, J. Organomet. Chem. 2006, 691,
3797–3805.
[9]
[10]
[11]
[12]
[13]
[33] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman Jr, J. A. Montgomery, T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tom-
asi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratch-
ian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gom-
perts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C.
Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A.
Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dap-
prich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q.
Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayak-
kara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez, J. A. Pople, GAUSSIAN 03, Re-
vision C.02, Gaussian, Inc., Wallingford CT, 2004.
[34] A. D. Becke, Phys. Rev. A 1988, 38, 3098–3100.
[35] C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785–789.
[36] a) E. B. Kadossov, K. J. Gaskell, M. A. Langell, J. Comput.
Chem. 2007, 28, 1240–1251; b) Q.-Z. Han, Y.-H. Zhao, H. Wen,
Data Sci. J. 2007, 6, S837–S846; c) D. C. Graham, K. J. Cavell,
B. F. Yates, Dalton Trans. 2007, 4650–4658; d) K. Cochran, G.
Forde, G. A. Hill, L. Gorb, J. Leszczynski, Struct. Chem. 2002,
13, 133–140.
[14]
[15]
a) M. K. Samantaray, V. Katiyar, K. Pang, H. Nanavati, P.
Ghosh, J. Organomet. Chem. 2007, 692, 1672–1682; b) L. Ray,
V. Katiyar, S. Barman, M. J. Raihan, H. Nanavati, M. M.
Shaikh, P. Ghosh, J. Organomet. Chem. 2007, 692, 4259–4269;
c) L. Ray, V. Katiyar, M. J. Raihan, H. Nanavati, M. M.
Shaikh, P. Ghosh, Eur. J. Inorg. Chem. 2006, 3724–3730; d)
M. K. Samantaray, V. Katiyar, K. Pang, D. Roy, H. Nanavati,
R. Stephen, R. B. Sunoj, P. Ghosh, Eur. J. Inorg. Chem. 2006,
2975–2984.
a) L. Ray, M. M. Shaikh, P. Ghosh, Dalton Trans. 2007, 4546–
4555; b) L. Ray, M. M. Shaikh, P. Ghosh, Organometallics
2007, 26, 958–964.
[16] L. Ray, S. Barman, M. M. Shaikh, P. Ghosh, Chem. Eur. J.
2008, 14, 6646–6655.
[17] S. Ray, R. Mohan, J. K. Singh, M. K. Samantaray, M. M.
Shaikh, D. Panda, P. Ghosh, J. Am. Chem. Soc. 2007, 129,
15042–15053.
[18] a) S. Díez-González, S. P. Nolan, Acc. Chem. Res. 2008, 41,
349–358; b) J. A. Mata, M. Poyatos, E. Peris, Coord. Chem.
Rev. 2007, 251, 841–859; c) S. T. Diver, Coord. Chem. Rev. 2007,
251, 671–701; d) H. M. Lee, C.-C. Lee, P.-Y. Cheng, Curr. Org.
Chem. 2007, 11, 1491–1524; e) U. Christmann, R. Vilar, Angew.
Chem. Int. Ed. 2005, 44, 366–374.
[19] H. Brunner, C. Krumey, J. Mol. Catal. A 1999, 142, 7–15.
[20] a) N. D. Clement, K. J. Cavell, L. Ooi, Organometallics 2006,
25, 4155–4165; b) T. Schaub, M. Backes, U. Radius, Organome-
tallics 2006, 25, 4196–4206.
[37] W. J. Hehre, R. Ditchfield, J. A. Pople, J. Chem. Phys. 1972, 56,
[21] a) H. V. Huynh, L. R. Wong, P. S. Ng, Organometallics 2008,
27, 2231–2237; b) K. Inamoto, J. Kuroda, K. Hiroya, Y. Noda,
M. Watanabe, T. Sakamoto, Organometallics 2006, 25, 3095–
3098.
2257–2261.
[38] S. Dapprich, G. Frenking, J. Phys. Chem. 1995, 99, 9352–9362.
[39] a) S. F. Vyboishchikov, G. Frenking, Chem. Eur. J. 1998, 4,
1439–1448; b) G. Frenking, U. Pidun, J. Chem. Soc., Dalton
Trans. 1997, 1653–1662.
[40] S. I. Gorelsky, AOMix: Program for Molecular Orbital Analy-
sis, York University, Toronto, Canada, 1997; http://www.sg-
chem.net/.
[22] D. Pugh, A. Boyle, A. A. Danopoulos, Dalton Trans. 2008,
1087–1094.
[23] K. Matsubara, K. Ueno, Y. Shibata, Organometallics 2006, 25,
3422–3427.
[24] C.-Y. Liao, K.-T. Chan, Y.-C. Chang, C.-Y. Chen, C.-Y. Tu, C.-
H. Hu, H. M. Lee, Organometallics 2007, 26, 5826–5833.
[25] C. Janiak, J. Chem. Soc., Dalton Trans. 2000, 3885–3896.
[41] S. I. Gorelsky, S. Ghosh, E. I. Solomon, J. Am. Chem. Soc.
2006, 128, 278–290.
Received: October 30, 2008
Published Online: March 9, 2009
Eur. J. Inorg. Chem. 2009, 1932–1941
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1941