Palladium complexes of amido-functionalized N-heterocyclic carbenes as effective precatalysts for the Suzuki-Miyaura C-C cross-coupling reactions of aryl bromides and iodides

Article abstract of DOI:10.1016/j.jorganchem.2009.09.011

A series of air-stable, robust and highly active palladium based precatalysts of amido-functionalized N-heterocyclic carbenes for the Suzuki-Miyaura C-C cross-coupling reaction has been designed. In particular, the [1-R-3-{N-(benzylacetamido)imidazol-2-yl

Full text of DOI:10.1016/j.jorganchem.2009.09.011

Journal of Organometallic Chemistry 694 (2009) 4162–4169
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Journal of Organometallic Chemistry
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Palladium complexes of amido-functionalized N-heterocyclic carbenes
as effective precatalysts for the Suzuki–Miyaura C–C cross-coupling
reactions of aryl bromides and iodides
Sachin Kumar a, Mobin M. Shaikh b, Prasenjit Ghosh a,
*
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:
Received 4 July 2009
Received in revised form 22 August 2009
Accepted 3 September 2009
Available online 12 September 2009
A series of air-stable, robust and highly active palladium based precatalysts of amido-functionalized
N-heterocyclic carbenes for the Suzuki–Miyaura C–C cross-coupling reaction has been designed. In par-
ticular, the [1-R-3-{N-(benzylacetamido)imidazol-2-ylidene]2PdCl2 [R = i-Pr (1c) and CH2Ph (2c)] com-
plexes efficiently carried out the Suzuki–Miyaura coupling of the aryl bromide and iodide substrates
with phenyl boronic acid in good to excellent yields in air at 90 °C in 12 h. Quite interestingly, of these
palladium precatalysts, the i-propyl derivative (1c) exhibited superior activity as compared to the benzyl
derivative (2c). The density functional theory (DFT) studies carried out on the 1c and 2c complexes
Keywords:
Carbenes
Functionalized NHC
Suzuki–Miyaura coupling
Organometallic catalysis
Palladium–NHC complex
revealed the strong
-donation (d) to backward
of the Pd–NHC interaction in these complexes.
r
-donating nature of the NHC ligand as reflected in their high d/b ratio [i.e. forward
r
p
-donation (b)] of these complexes and, thus, point towards greater stability
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
the leaching of the catalysts [12–14]. The tight binding of the
NHC ligand to the metal is reflected in the high NHC–metal bond
dissociation energy eg. 74.8 kcal/mol in trans-[1-benzyl-3-(3,3-di-
methyl-2-oxobutyl)imidazol-2-ylidene]2PdBr2, [15] 75.1 kcal/mol
in cis-[1-benzyl-3-(N-t-butylacetamido)imidazol-2-ylidene]2PdCl2
[15] and 81.9 kcal/mol in trans-[1-(benzyl)-3-(N-t-butylacetami-
do)imidazol-2-ylidene]Pd(pyridine)Cl2 [16] etc. The additional
advantages of the N-heterocyclic carbenes arise from their easy
tunability and from their behavior as good ‘‘spectator” ligands,
which while influencing the catalysis, do not get consumed or
chemically react in the process.
In continuing with our efforts on designing new precatalysts for
a host of C–C bond forming reactions namely, the Hiyama [17],
Sonogashira [15,17,18], Suzuki–Miyaura [16,19] and the base-free
Michael reactions [20,21], we were in look out for new N-heterocy-
clic carbene based scaffolds for stabilizing palladium precatalysts
for the coupling reactions. More specifically, with regard to the
Suzuki–Miyaura coupling, we demonstrated the use of the trans-
(NHC)2PdX2 (X = halide) and the famed PEPPSI [22–27] themed
(NHC)PdX2(pyridine) type complexes, of which the former ones
showed ultra high activity [16,19]. Thus, the present efforts are
directed along the line of developing new trans-(NHC)2PdX2
(X = halide) type complexes for application in the Suzuki–Miyaura
cross-coupling reaction. In this connection, we additionally
emphasized on employing well-defined N-heterocyclic carbene
based precatalysts for the said study as opposed to performing it
As a convenient approach to accessing the ‘‘biaryl frameworks”,
so ubiquitous in many important bioactive molecules as well as in
value-added chemical feedstock, the Suzuki–Miyaura C–C cross-
coupling reaction, today, has emerged as a major work horse in syn-
thetic organic chemistry [1–7]. In particular, the famed reaction in-
volves highly efficient C–C cross-coupling of aryl halide with aryl
boronic acid in a catalytic fashion aided by palladium in presence
of a base. The extreme popularity of the Suzuki–Miyaura reaction
stems form its regio- and stereoselectivity, functional group toler-
ance, ready availability of the boron based starting materials, its
non-toxic nature and its air and moisture stability. Thus, because
of its utility as an important synthetic methodology, a significant
amount of research focus had been devoted to designing improved
catalysts for the Suzuki–Miyaura cross-coupling reaction. In this re-
gard it is noteworthy that though numerous catalysts of phosphine
based systems [8,9] have been reported for the coupling reaction,
the use of N-heterocyclic carbenes based ones is a relatively new
affair [10,11]. The N-heterocyclic carbene based precatalysts posses
distinct advantages that arise from the tight binding of the NHC
ligand to the metal, leading to a greater stability of the complex
under the catalysis conditions, and which in turn helps suppress
* Corresponding author. Tel.: +91 22 2576 7178; fax: +91 22 2572 3480.
E-mail address: pghosh@chem.iitb.ac.in (P. Ghosh).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.09.011

S. Kumar et al. / Journal of Organometallic Chemistry 694 (2009) 4162–4169
4163
under the more common ‘‘Ligand Assisted Catalysis” (LAC) condi-
tions is because the use of well-characterized catalysts display
shorter reaction times and also significantly reduces the unwanted
side reactions.
Here in this contribution, we report a series of robust and highly
efficient palladium complexes of amido-functionalized N-hetero-
cyclic carbenes namely, [1-R-3-{N-(benzylacetamido)imidazol-2-
ylidene]2PdCl2 [R = i-Pr (1c) and CH2Ph (2c)], for the Suzuki–Miya-
ura cross-coupling reactions of aryl bromides and iodides with
phenyl boronic acid in air (Fig. 1).
metal consistent with a d8 palladium(II) metal center (Figs. 2 and
3 and Supplementary material Table S1). The Pd–Ccarbene bond dis-
0
0
tances [2.029 (3) ÅA (1c) and 2.032 (6) ÅA (2c)] are compar0able to the
sum of the individual covalent radii of Pd and C (2.055 ÅA) [28] and
also to other related structurally characterized (NHC)2PdCl2 type
examples
namely,
trans-[1-benzyl-3-(3,3-dimethyl-2-oxobu-
0
tyl)imidazol-2-ylidene]2PdBr2 [2.018(9) ÅA] [15], cis-[1-benzyl-3-
0
(N-t-butylacetamido)imidazol-2-ylidene]2PdCl2 [1.998(2) ÅA] [15],
and trans-[1-benzyl-3-t-butylimidazol-2-ylidene]2PdCl2 [2.044(4)
0
0
ÅA] [29]. The Pd–Cl distances in 1c and 2c are 2.3167(8) ÅA and
0
2.309(2) ÅA, respectively. The other notable aspects of the 1c and
2c structures are the trans disposition of the of the N1 and N3 sub-
stituents of the imidazole rings, that make a coplanar arrangement
to each other.
Deeper understanding of the nature of the Pd–NHC interaction
could be obtained by the density functional theory studies. Specif-
ically, the respective geometry optimized structure of the palla-
dium complexes, 1c and 2c, were obtained by computing at the
B3LYP/SDD, 6-31G(d) level of theory using the atomic coordinates
adopted from the X–ray analysis (Supplementary material Tables
S2 and S3). Subsequent, single-point calculation was performed
on the geometry optimized structure to obtain insight on the elec-
tronic properties of the complexes.
Quite interestingly, the NBO analysis indicated that the Pd–
NHC bond is composed of an interaction of a Ccarbene sp2 orbital
with a sd orbital of the palladium center (Supplementary material
Table S9). Indeed, a careful look at the electronic configuration of
the palladium center in 1c and 2c relative to that in the PdCl2
species as well as of the Ccarbene center in 1c and 2c relative to
that in the free NHC ligand fragment reveal that electron dona-
tion occur from the free NHC ligand fragment on to the 5s orbital
of the palladium center in 1c and 2c (Supplementary material
Table S6). Corroborating the above mentioned observation, both
the Mulliken and Natural Charge analysis showed that as a result
2. Results and discussion
Two new amido-functionalized N-heterocyclic carbene ligands
were designed with the intent of stabilizing palladium complexes
for use as precatalysts for the Suzuki–Miyaura cross-coupling reac-
tion. In this regard the following new N-heterocyclic carbene pre-
cursors, 1-R-3-N-(benzylacetamido)imidazolium chloride salts
[R = i-Pr (1a) and CH2Ph (2a)], were synthesized by the direct reac-
tion of the N-benzyl-2-chloro-acetamide with the respective 1-R-
imidazole (R = i-Pr and CH2Ph) in 81–84% yield (Scheme 1). The
appearance of the downfield shifted peaks at ca. 9.97–10.05 ppm
in the 1H NMR and at ca. 132.6–135.5 ppm in 13C{1H} NMR, that
are characteristics of the NCHN resonances, indicated the clean for-
mation of the imidazolium chloride salts 1a and 2a.
The palladium complexes [1-R-3-{N-(benzylacetamido)imida-
zol-2-ylidene]2PdCl2 [R = i-Pr (1c) and CH2Ph (2c)] were obtained
by following the transmetallion route form the silver complexes
{[1-R-3-{N-(benzylacetamido)imidazol-2-ylidene]2Ag}+Clꢀ [R = i-
Pr (1b) and CH2Ph (2b)] by treatment with (COD)PdCl2 (COD = cis,-
cis-1,5-cyclooctadiene) in 52–64% yield. The silver complexes 1b
and 2b were in turn synthesized from the respective imidazolium
chloride salt 1a and 2a by the reaction with Ag2O in 61–67% yield.
Of interest, are the diagnostic M–Ccarbene resonances that appeared
downfield shifted [M = Ag, 179.8 ppm (1b) and 182.2 ppm (2b);
M = Pd, 169.2 ppm and 169.3 ppm (1c) and 159.0 ppm and
156.8 ppm (2c)] in the 13C{1H} NMR spectrum.
of the
r-donation from the free NHC ligand fragment, the elec-
tron density at the palladium center increased from that in the
PdCl2 species to that in 1c and 2c (Supplementary material Tables
S4 and S5). Also, attesting to the good
NHC ligand, the d/b ratio [1.96 (1c) and 2.98 (2c)], which is a
measure of the relative extent of the NHC to metal -donation
(d) and the metal to NHC -back donation (b) and is computed
r-donating ability of the
Quite expectedly, the molecular structure of the 1c and 2c
complexes revealed monomeric square-planar geometry at the
r
p
using the Charge Decomposition Analysis (CDA) method, for the
1c and 2c complexes were found to be greater than unity (Sup-
plementary material Table S8).
A closer look at the Pd–NHC interaction could be obtained by
constructing the molecular orbital (MO) correlation diagram from
the individual fragment molecular orbitals (FMO) of the respective
free NHC ligand fragment and the PdCl2 species (Figs. 4 and 5 and
Supplementary material Figs. S1 and S2). Quite interestingly, the
molecular orbitals HOMOꢀ39 (1c) and HOMOꢀ45 (2c) represent
NH
R
O
Cl
Pd
N
N
N
the
r
-orbital interaction between the palladium center and the
-bonding
free NHC ligand fragment. Specifically, the Pd–NHC
r
N
orbital, HOMOꢀ39 (25% NHC, 31% PdCl2), in 1c showed an interac-
tion of the carbene lone pair (HOMOꢀ1 of the free NHC fragments)
with a metal based vacant LUMO (72% palladium with 28% s and
44% d character) of the PdCl2 fragment (Fig. 4). In the 2c complex,
Cl
O
R
HN
the Pd–NHC
r
-bonding orbital, HOMOꢀ45 (21% NHC, 29% PdCl2)
showed an interaction of the carbene lone pair (HOMOꢀ1 of the
free NHC fragments) with a metal based vacant LUMO (72% palla-
dium with 28% s and 44% d character) of the PdCl2 fragment (Fig. 5).
It is worth noting that these low lying Pd–NHC
r-molecular orbi-
tals signify stable interaction which are less vulnerable to the elec-
trophilic or nucleophilic attacks. In this regard it is noteworthy that
carbenes along with its metal complexes, as such, are highly reac-
tive compounds that readily undergo reactions with electrophiles
and nucleophiles.
R = i-Pr (1c), CH2Ph (2c)
Fig. 1. Palladium complexes of amido-functionalized N-heterocyclic carbenes.

4164
S. Kumar et al. / Journal of Organometallic Chemistry 694 (2009) 4162–4169
NH
NH
O
O
N
N
N
N
Cl
Cl
R
R
R = i-Pr (1a), CH2Ph (2a)
Ag2O
Cl
NH
NH
R
R
O
O
Cl
Pd
N
N
R
N
N
N
N
N
(COD)PdCl2
Ag
N
Cl
O
O
R
HN
HN
R = i-Pr (1c), CH2Ph (2c)
R = i-Pr (1b), CH2Ph (2b)
Scheme 1.
O1
N3
N2
N1
C1
Pd1
Cl1
N1
Pd1
N3
O1
Cl1
C1
N2
Fig. 3. ORTEP of 2c with thermal ellipsoids drawn at 50% probability level. Selected
bond length (Å) and bond angles (°): Pd1–C1 2.032(6), Pd1–Cl1 2.309(2), C1–Pd1–
C1 180.00(14), C1–Pd1–Cl1 90.6(2), C1–Pd1–Cl1_3 89.4(2), Cl1–Pd–Cl1 180.0, N1–
C1–N2 105.7(5).
Fig. 2. ORTEP of 1c with thermal ellipsoids drawn at 50% probability level. Selected
bond length (Å) and bond angles (°): Pd1–C1 2.029(3), Pd1–Cl1 2.3161(8), C1–Pd1–
C1 180.000(1), C1–Pd1–Cl1 90.41(8), C1–Pd1–Cl1_3 89.59(8), Cl1–Pd–Cl1
180.000(1), N1–C1–N2 104.4(2).
are similar to that estimated for other related palladium complexes
namely, trans-[4-benzyl-1-i-propyl-1,2,4-triazol-5-ylidene]PdBr2
(NC5H5) (56.3 kcal/mol) [17], trans-[4-benzyl-1-(t-butylaminocar-
bonylmethyl)-1,2,4-triazol-5-ylidene]PdBr2(NC5H5) (58.1 kcal/mol)
[17], trans-[1-benzyl-3-(3,3-dimethyl-2-oxobutyl)imidazol-2-yli-
dene]2PdBr2 (74.8 kcal/mol) [15], cis-[1-benzyl-3-(N-t-butylace-
Lastly, the strength of the Pd–NHC interaction could be gauged
by estimating the Pd–Ccarbene bond dissociation energy (De) at
B3LYP/SDD, 6-31G(d) level of theory and that gave a value of
68.3 kcal/mol (1c) and 69.0 kcal/mol (2c) for these palladium com-
plexes (Supplementary material Table S7). The bond dissociation
energy (De) values of 68.3 kcal/mol (1c) and 69.0 kcal/mol (2c)
tamido)imidazol-2-ylidene]2PdCl2
(75.1 kcal/mol)
[15]
and
trans-[1-(benzyl)-3-(N-t-butylacetamido)imidazol-2-ylidene]PdCl2-
(NC5H5) (81.9 kcal/mol) [16].

S. Kumar et al. / Journal of Organometallic Chemistry 694 (2009) 4162–4169
4165
N
0
NH
:
N
O
Cl
Pd
O
Cl
Pd
N
N
N
N
NH
Cl
Cl
O
HN
1c
33 %
HOMO-1
13 %
LUMO
HOMO-31
HOMO-33
19 %
-10
9 %
10 %
HOMO-39
HOMO-5
Fig. 4. Simplified orbital interaction diagram showing the major contribution of NHC–Pd bond in 1c.
N
N
NH
:
0
O
Cl
Pd
Cl
Pd
N
N
N
N
O
NH
Cl
Cl
O
HN
2c
34 %
HOMO-1
17 %
11 %
LUMO
HOMO-34
-10
10 %
11 %
HOMO-5
HOMO-45
HOMO-39
Fig. 5. Simplified orbital interaction diagram showing the major contribution of NHC–Pd bond in 2c.
Significantly enough, the palladium complexes 1c and 2c effi-
ciently carried out the Suzuki–Miyaura C–C cross-coupling reac-
tions of aryl bromides and iodides with phenyl boronic acid at
0.35 mol% of the catalyst loading in good to excellent yields in
air after 12 h at 90 °C (Table 1 and Eq. (1)). Notably, effective
conversions were observed for activated aryl bromides containing
electron withdrawing substrates while for the aryl iodides sub-
strates the same could be seen for both the electron withdrawing
as well as electron donating substrates. Quite interestingly, the pal-
ladium 1c precatalyst, containing a i-propyl N1-substituent, was
found to be more efficient that the 2c precatalyst containing a ben-
zyl N1-substituent presumably due to the greater steric demand of

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S. Kumar et al. / Journal of Organometallic Chemistry 694 (2009) 4162–4169
Table 1
Selected results of Suzuki–Miyaura cross coupling reaction catalyzed by 1c and 2c.
Entry
1
Reagenta
Reagenta
Product
Yieldb (%)
1c
2c
99
99
NC
(HO)2B
Br
NC
2
99
99
(HO)2B
Br
NO2
NO2
3
4
5
99
85
78
87
99
74
O2N
(HO)2B
(HO)2B
(HO)2B
Br
O2N
OHC
Br
OHC
Br
CHO
CHO
6
7
8
9
94
96
74
42
67
24
48
26
MeOC
MeOC
(HO)2B
(HO)2B
(HO)2B
(HO)2B
Br
MeOC
MeOC
I
I
MeO
I
MeO
a
Reaction conditions: 0.50 mmol of aryl halide, 0.60 mmol of phenyl boronic acid, 0.75 mmol of K2CO3, 0.35 mol% of 1c or 2c, diethyleneglycol-di-n-butyl ether (0.50 mmol,
internal standard) and 10 mL of CH3CN were taken for each run at 90 °C temperature (12 h).
b
GC yields.
the i-propyl group over the benzyl moiety. A comparison of the
catalysis results of 1c and 2c with blank as well as the control
experiments performed with PdCl2 clearly highlight the so-called
‘‘ligand influence” present in these palladium based 1c and 2c pre-
catalysts. Specifically, amplifications of up to 68% were observed
with respect to the control experiments in case of the catalysis
by 1c and 2c (Supplementary material Table S10). The homoge-
neous nature of the Suzuki–Miyaura coupling reaction was further
confirmed by performing the classical Hg(0) drop experiment
[30,31] for a representative precatalyst 2c that showed no signifi-
cant reduction in the reaction yield in presence or absence of mer-
cury under analogous conditions (Supplementary material Table
S11).
PdCl2(NC5H5), [16] trans-[1-(2-hydroxy-cyclohexyl)-3-(benzyl)
imidazol-2-ylidene]PdCl2(NC5H5) [16] and trans-[1-(o-methoxy-
benzyl)-3-(t-butyl)imidazol-2-ylidene]PdBr2(NC5H5), [16] com-
plexes carried out Suzuki–Miyaura coupling of aryl bromide and
iodide substrates with phenyl boronic acid at 85 °C exhibiting a
conversion of 30–99%. However, the trans-(NHC)2PdX2 (X = halide)
type complexes namely, [1-(o-methoxybenzyl)-3-(t-butyl)imida-
zol-2-ylidene]2PdCl2, containing two N-heterocyclic carbene
ligands, exhibited ultra high turn over numbers for Suzuki–Miya-
ura cross-coupling of aryl bromides with phenyl boronic acid at
85 °C. In this backdrop, the complexes 1c and 2c also show consid-
erable activity by virtue of belonging to the class of more electron
rich trans-(NHC)2PdX2 (X = halide) type catalysts.
B(OH)2
X
3. Conclusions
1c or 2c
+
In summary, a series of air-stable, robust and highly efficient
precatalysts 1c and 2c supported over new amido-functionalized
N-heterocyclic carbene ligands for the Suzuki–Miyaura C–C
cross-coupling reaction of aryl bromides and iodides with phenyl
boronic acids have been designed. More interestingly, the sterically
demanding i-propyl derivative 1c exhibited superior activity com-
pared to the benzyl derivative 2c. The density functional theory
studies of the 1c and 2c complexes indicated the greater stability
of the Pd–NHC interaction as the molecular orbitals representing
ð1Þ
R
R
X= Br, I
R= CN, NO2, CHO, COMe, Me, OMe
Important is the comparison of reactivity of 1c and 2c
complexes with the other related N-heterocyclic carbene based
analogs. For example, the PEPPSI themed precatalysts namely,
the trans-[1-(benzyl)-3-(N-t-butylacetamido)imidazol-2-ylidene]

S. Kumar et al. / Journal of Organometallic Chemistry 694 (2009) 4162–4169
4167
these
r
-interactions in these palladium complexes were found to
116.8 (NCHCHN), 53.9 (CH2), 53.9 (CH(CH3)2), 42.9 (CH2), 23.6
(CH(CH3)2). IR (KBr pellet cmꢀ1): 1676 (
CO). Anal. Calc. for
be low lying and consequently are less susceptible to nucleophilic
or electrophilic attacks.
m
C30H38ClAgN6O2ꢂ0.5CH2Cl2: C, 52.30; H, 5.61; N, 12.00. Found: C,
52.60; H, 6.36; N, 11.99%.
4. Experimental
4.4. Synthesis of [1-(i-propyl)-3-{N-(benzylacetamido)imidazol-2-
ylidene]2PdCl2 (1c)
4.1. General procedures
A mixture of {[1-(i-propyl)-3-{N-(benzylacetamido)imidazol-2-
ylidene}]2Ag}+Clꢀ (0.304 g, 0.462 mmol) and (COD)PdCl2 (0.132 g,
0.462 mmol) was dissolved in CH2Cl2 (ca. 40 mL) and was stirred
at room temperature for 6 h, at which the point the formation of
an off-white AgCl precipitate was observed. The reaction mixture
was filtered, and the solvent was removed under vacuum to get
the crude product. The crude product was purified by column chro-
matography on silica gel, using a CHCl3/CH3OH mixture (50:1) as
an eluent to obtain the product 1c as a yellow solid (0.167 g,
52%). 1H NMR (CDCl3, 400 MHz, 25 °C): d 7.50 (s, 1H, NCHCHN),
7.42 (s, 1H, NCHCHN), 7.24–7.21 (m, 9H, C6H5), 7.04 (d, 1H,
3JHH = 8 Hz, o-C6H5), 6.98 (s, 2H, NCHCHN), 6.97 (br, 2H, NH), 5.59
All manipulations were carried out using a combination of a
glovebox and standard Schlenk techniques. Solvents were purified
and degassed by standard procedures. 1-i-propylimidazole [32], 1-
benzylimidazole [33] and (COD)PdCl2 [34] were prepared accord-
ing to the 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–El-
mer Spectrum One FT-IR spectrometer. Mass spectrometry mea-
surements were done on
a Micromass Q-Tof spectrometer.
Elemental Analysis was carried out on Thermo Quest FLASH 1112
SERIES (CHNS) Elemental Analyzer. X-ray diffraction data for com-
pounds 1c and 2c were collected on an Oxford Diffraction Excalib-
er-S diffractometer and crystal data collection and refinement
parameters are summarized in Table S1. The structures were
solved using direct methods and standard difference map tech-
niques, and were refined by full-matrix least-squares procedures
on F2 with SHELXTL (Version 6.10) [35,36]. GC spectra were obtained
on a Perkin–Elmer Clarus 600 equipped with a FID. GCMS spectra
were obtained on a Perkin–Elmer Clarus 600 T equipped with an
EI source.
3
3
(sept, 1H, JHH = 7 Hz, CH(CH3)2), 5.50 (sept, 1H, JHH = 7 Hz,
CH(CH3)2), 5.25 (s, 2H, CH2), 5.07 (s, 2H, CH2), 4.41 (d, 2H,
3
3JHH = 6 Hz, CH2), 4.39 (d, 2H, JHH = 6 Hz, CH2), 1.61 (d, 6H,
3
3JHH = 7 Hz, CH(CH3)2), 1.51 (d, 6H, JHH = 7 Hz, CH(CH3)2). 13C{1H}
NMR (CDCl3, 100 MHz, 25 °C): d 169.3 (Pd–NCN), 169.2 (Pd–
NCN), 167.1 (CO), 167.0 (CO), 138.2 (ipso-C6H5), 138.1 (ipso-C6H5),
128.5 (m-C6H5), 128.4 (m-C6H5), 127.8 (o-C6H5), 127.7 (o-C6H5),
127.2 (p-C6H5), 127.2 (p-C6H5), 121.9 (NCHCHN), 121.7 (NCHCHN),
118.1 (NCHCHN), 117.6 (NCHCHN), 54.4 (CH2), 54.0 (CH2), 52.8
(CH(CH3)2), 52.5 (CH(CH3)2), 43.5 (CH2), 43.4 (CH2), 23.3
(CH(CH3)2), 23.2 (CH(CH3)2). IR (KBr pellet cmꢀ1): 1683 (
mCO). Anal.
4.2. Synthesis of 1-(i-propyl)-3-N-(benzylacetamido)imidazolium
chloride (1a)
Calc. for C30H38PdCl2N6O2ꢂCH2Cl2: C, 47.92; H, 5.19; N, 10.82.
Found: C, 47.91; H, 5.87; N, 11.28%.
A mixture of N-benzyl-2-chloro-acetamide (3.20 g, 17.5 mmol)
and 1-i-propylimidazole (2.51 g, 22.7 mmol) was dissolved in tolu-
ene (ca. 10 mL), and refluxed at 110 °C for 12 h, at which point a so-
lid separated out. The solid was isolated by decanting off the
solvent and washed with hot hexane (3 ꢁ ca. 15 mL) to obtain
the product 1a as an off-white solid (4.31 g, 84%). 1H NMR (CDCl3,
400 MHz, 25 °C): d 10.05 (s, 1H, NCHN), 9.73 (s, 1H, NH), 7.57 (br,
4.5. Synthesis of 1-(benzyl)-3-N-(benzylacetamido)imidazolium
chloride (2a)
A mixture of N-benzyl-2-chloro-acetamide (3.15 g, 17.2 mmol)
and 1-benzylimidazole (2.72 g, 17.2 mmol) was dissolved in tolu-
ene (ca. 10 mL), and refluxed at 110 °C for 12 h, at which point a so-
lid separated out. The solid was isolated by decanting off the
solvent and washed with hot hexane (3 ꢁ ca. 15 mL) to obtain
the product 2a as an off-white solid (4.75 g, 81%). 1H NMR (CDCl3,
400 MHz, 25 °C): d 9.97 (s, 1H, NCHN), 9.73 (br, 1H, NH), 7.50 (s,
1H, NCHCHN), 7.36 (br, 2H, o-C6H5), 7.34 (br, 1H, o-C6H5), 7.31
3
1H, NCHCHN), 7.34 (d, 2H, JHH = 8 Hz, o-C6H5), 7.26 (t, 2H,
3JHH = 8 Hz, m-C6H5), 7.21 (m, 2H, p-C6H5 & NCHCHN), 5.37 (s, 2H,
CH2), 4.65 (sept, 1H, 3JHH = 7 Hz, CH(CH3)2), 4.40 (d, 2H, 3JHH = 6 Hz,
3
CH2), 1.58 (d, 6H, JHH = 7 Hz, CH(CH3)2). 13C{1H} NMR (CDCl3,
3
100 MHz, 25 °C): d 165.2 (CO), 138.0 (ipso-C6H5), 135.5 (NCHN),
128.3 (m-C6H5), 127.4 (o-C6H5), 126.9 (p-C6H5), 123.8 (NCHCHN),
119.4 (NCHCHN), 52.9 (CH2), 51.2 (CH(CH3)2), 43.1 (CH2), 22.6
(br, 1H, o-C6H5), 7.29 (br, 1H, m-C6H5), 7.20 (t, 3H, JHH = 7 Hz, m-
3
C6H5), 7.13 (d, 2H, JHH = 7 Hz, p-C6H5), 7.05 (s, 1H, NCHCHN),
3
5.36 (s, 2H, CH2), 5.32 (s, 2H, CH2), 4.36 (d, 2H, JHH = 6 Hz, CH2).
(CH(CH3)2). IR (KBr pellet cmꢀ1): 1684 (
258.1614 [(NHCꢀH)]+, calcd 258.1606.
mCO). HRMS (ES): m/z
13C{1H} NMR (CDCl3, 100 MHz, 25 °C): d 164.9 (CO), 138.2 (ipso-
C6H5), 137.6 (ipso-C6H5), 132.6 (NCHN), 129.8 (m-C6H5), 129.7
(m-C6H5), 128.8 (o-C6H5), 128.6 (o-C6H5), 127.9 (p-C6H5), 127.3
(p-C6H5), 124.0 (NCHCHN), 120.9 (NCHCHN), 53.6 (CH2), 51.8
4.3. Synthesis of {[1-(i-propyl)-3-N-(benzylacetamido)imidazol-2-
ylidene]2Ag}+Clꢀ (1b)
(CH2), 43.6 (CH2). IR (KBr pellet cmꢀ1): 1689 (
z 306.1620 [(NHCꢀH)]+, calcd 306.1606.
mCO). HRMS (ES): m/
A mixture of 1-(i-propyl)-3-N-(benzylacetamido)imidazolium
chloride (4.37 g, 14.9 mmol) and Ag2O (1.72 g, 7.45 mmol) in
dichloromethane (ca. 40 mL) was stirred at room temperature for
4 h. The reaction mixture was filtered, and the solvent was re-
moved under vacuum to give the product 1b as a white solid
(3.28 g, 67%). 1H NMR (CDCl3, 400 MHz, 25 °C): d 9.12 (br, 2H,
4.6. Synthesis of {[1-(benzyl)-3-N-(benzylacetamido)imidazol-2-
ylidene]2Ag}+Clꢀ (2b)
A
mixture of 1-(benzyl)-3-N-(benzylacetamido)imidazolium
chloride (4.37 g, 12.8 mmol) and Ag2O (1.48 g, 6.41 mmol) in
dichloromethane (ca. 40 mL) was stirred at room temperature for
4 h. The reaction mixture was filtered, and the solvent was re-
moved under vacuum to give the product 2b as a white solid
(2.89 g, 61%). 1H NMR (CDCl3, 400 MHz, 25 °C): d 9.36 (br, 2H,
NH), 7.30 (br, 2H, NCHCHN), 7.28 (br, 4H, o-C6H5), 7.27–7.25 (br,
4H, m-C6H5), 7.17 (br, 2H, o-C6H5), 7.15 (br, 4H, m-C6H5), 7.07–
3
NH), 7.31 (t, 4H, JHH = 3 Hz, m-C6H5), 7.28 (br, 4H, o-C6H5), 7.19
(br, 2H, p-C6H5), 7.17 (br, 2H, NCHCHN), 7.02 (br, 2H, NCHCHN),
3
5.00 (s, 4H, CH2), 4.64 (sept, 2H, JHH = 7 Hz, CH(CH3)2), 4.38 (s,
4H, CH2), 1.48 (d, 12H, 3JHH = 7 Hz, CH(CH3)2). 13C{1H} NMR (CDCl3,
100 MHz, 25 °C): d 179.8 (Ag–NCN), 166.8 (CO), 138.1 (ipso-C6H5),
128.2 (m-C6H5), 127.5 (o-C6H5), 126.8 (p-C6H5), 123.0 (NCHCHN),

4168
S. Kumar et al. / Journal of Organometallic Chemistry 694 (2009) 4162–4169
3
7.05 (br, 4H, p-C6H5), 6.87 (br, 2H, JHH = 2 Hz, NCHCHN), 5.07 (s,
(i)
(ii)
r-donation from the [NHC ? PdCl2] fragment,
4H, CH2), 5.04 (s, 4H, CH2), 4.36 (d, 4H, JHH = 6 Hz, CH2). 13C{1H}
p
-back donation from [NHC PdCl2] fragment and
3
NMR (CDCl3, 100 MHz, 25 °C): d 182.2 (Ag–NCN), 167.0 (CO),
138.5 (ipso-C6H5), 137.5 (ipso-C6H5), 129.1 (m-C6H5), 128.6 (m-
C6H5), 128.5 (o-C6H5), 127.9 (o-C6H5), 127.5 (p-C6H5), 127.1 (p-
C6H5), 123.6 (NCHCHN), 120.8 (NCHCHN), 55.5 (CH2), 54.1 (CH2),
(iii) Repulsive polarization (r).
The CDA calculations are performed using the program AOMIX [47],
using the B3LYP/SDD, 6-31G(d) wave function. Molecular orbital
(MO) compositions and the overlap populations were calculated
using the AOMIX program [48]. The 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) was performed using
the AOMix-CDA [49].
43.4 (CH2). IR (KBr pellet cmꢀ1): 1659 (
mCO). Anal. Calc. for
C38H38AgN6O2ClꢂCH2Cl2: C 55.83; H, 4.81; N, 10.02. Found: C,
55.93; H, 4.47; N, 9.94%.
4.7. Synthesis of [1-(benzyl)-3-N-(benzylacetamido)imidazol
-2-ylidene]2PdCl2 (2c)
4.9. General procedure for the Suzuki–Miyaura reaction
A mixture of {[1-(benzyl)-3-{N-(benzylacetamido)imidazol-2-
ylidene}]2Ag}+Clꢀ (0.471 g, 0.625 mmol) and (COD)PdCl2 (0.178 g,
0.625 mmol) was dissolved in CH2Cl2 (ca. 40 mL) and was stirred
at room temperature for 6 h, at which the point the formation of
an off-white AgCl precipitate was observed. The reaction mixture
was filtered, and the solvent was removed under vacuum to get
the crude product. The crude product was purified by column chro-
matography on silica gel, using a CHCl3/CH3OH mixture (50:1) as
an eluent to obtain the product 2c as a yellow solid (0.315 g,
64%). 1H NMR (CDCl3, 400 MHz, 25 °C): d 8.11 (br, 2H, NH), 7.48-
7.38 (m, 10H, o/m-C6H5), 7.25-7.19 (m, 10H, m/p-C6H5), 7.02 (br,
1H, NCHCHN), 6.97 (br, 1H, NCHCHN), 6.82-6.77 (br, 1H, NCHCHN),
6.76–6.74 (br, 1H, NCHCHN), 5.84 (s, 1H, CH2), 5.60 (d, 2H,
3JHH = 6 Hz, CH2), 5.36 (s, 1H, CH2), 5.30 (s, 2H, CH2), 5.12 (s, 1H,
In a typical run, performed in air, a 25 mL vial was charged with
a mixture of the aryl halide compound (0.50 mmol), phenyl boro-
nic acid (0.60 mmol), K2CO3 (0.75 mmol) diethyleneglycol-di-n-bu-
tyl ether (0.50 mmol) (internal standard) in a molar ratio of
1:1.2:1.5 and precatalysts 1c or 2c (0.35 mol%) in CH3CN (10 mL).
The reaction mixture was refluxed for an appropriate period of
time, after which it was filtered and the product was analyzed by
GC and GCMS analysis using diethyleneglycol-di-n-butyl ether as
an internal standard.
Acknowledgements
We thank CSIR, New Delhi, for financial support of this research.
We are grateful to the National Single Crystal X-ray Diffraction
Facility and the Sophisticated Analytical Instrument Facility at IIT
Bombay, India, for the crystallographic data and other character-
ization data. Computational facilities from the IIT Bombay Com-
puter Center are gratefully acknowledged. S.K. thanks CSIR, New
Delhi, for research fellowship.
3
3
CH2), 5.07 (d, 2H, JHH = 6 Hz, CH2), 4.44 (d, 1H, JHH = 6 Hz, CH2),
3
3
4.41 (d, 1H, JHH = 6 Hz, CH2), 4.30 (d, 1H, JHH = 6 Hz, CH2).
13C{1H} NMR (DMSO-d6, 100 MHz, 25 °C): d 166.2 (CO), 166.1
(CO), 159.0 (Pd–NCN), 156.8 (Pd–NCN), 138.4 (ipso-C6H5), 138.2
(ipso-C6H5), 135.6 (ipso-C6H5), 135.4 (ipso-C6H5), 128.4 (m-C6H5),
128.3 (m-C6H5), 128.3(m-C6H5), 128.2(m-C6H5), 128.2(m-C6H5),
128.1 (m-C6H5), 128.0 (m-C6H5), 127.8 (m-C6H5), 127.7 (o-C6H5),
127.6 (o-C6H5), 127.5 (o-C6H5), 127.4 (o-C6H5), 127.4 (o-C6H5),
127.3 (o-C6H5), 126.7 (o-C6H5), 126.6 (o-C6H5), 125.1 (p-C6H5),
124.8 (p-C6H5), 124.2 (p-C6H5), 122.6 (p-C6H5), 121.9 (NCHCHN),
121.4 (NCHCHN), 121.2 (NCHCHN), 120.2 (NCHCHN), 53.2 (CH2),
52.6 (CH2), 52.2 (CH2), 52.6 (CH2), 42.5 (CH2), 42.3 (CH2). IR (KBr
Appendix A. Supplementary material
CCDC 679084 and 707910 contain the supplementary crystallo-
graphic data for 1c and 2c. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2009.09.011.
pellet cmꢀ1): 1635 (
CH2Cl2: C 55.68; H, 4.73; N, 10.12. Found: C, 55.87; H, 4.79; N,
m
CO). Anal. Calc. for C38H38PdN6O2Cl2ꢂ0.5
10.35%.
4.8. Computational methods
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