Air-stable, convenient to handle Pd based PEPPSI (pyridine enhanced precatalyst preparation, stabilization and initiation) themed precatalysts of N/O-functionalized N-heterocyclic carbenes and its utility in Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1039/b706607d

Several new air-stable, convenient to handle and easily synthesized Pd based PEPPSI (Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) type precatalysts supported over N/O-functionalized N-heterocyclic carbenes (NHC) namely, trans-[1-(benzyl)-3-(N-t-butylacetamido)imidazol-2- ylidene]Pd(pyridine)Cl₂ (2), trans-[1-(2-hydroxy-cyclohexyl)-3- (benzyl)imidazol-2-ylidene]Pd(pyridine)Cl₂ (3) and trans-[1-(o-methoxybenzyl)-3-(t-butyl)imidazol-2-ylidene]Pd(pyridine)Br ₂ (4), have been designed. Specifically, the Pd-NHC complexes, 2, 3 and 4, were conveniently synthesized from their respective imidazolium halide salts by the reaction with PdCl₂ in pyridine in presence of K ₂CO₃ as a base. A new imidazolium chloride salt, 1-(benzyl)-3-(N-t-butylacetamido)imidazolium chloride (1) was synthesized by the alkylation reaction of benzyl imidazole with N-t-butyl-2-chloroacetamide. The molecular structures of the imidazolium chloride salt, 1, and the Pd-NHC complexes, 2, 3 and 4, have been determined by X-ray diffraction studies. The density functional theory studies of the 2, 3 and 4 complexes were carried out to in order to gain insight about their structure, bonding and the electronic properties. The nature of the NHC-metal bond in these complexes was examined using Charge Decomposition Analysis (CDA), which revealed that the N-heterocyclic carbene ligands are effective σ-donors. In addition, the catalysis studies revealed that the Pd-NHC complexes, 2, 3 and 4, are effective catalysts for the Suzuki-Miyaura type C-C cross-coupling reactions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b706607d

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www.rsc.org/dalton | Dalton Transactions
Air-stable, convenient to handle Pd based PEPPSI (pyridine enhanced
precatalyst preparation, stabilization and initiation) themed precatalysts of
N/O-functionalized N-heterocyclic carbenes and its utility in
Suzuki–Miyaura cross-coupling reaction†‡
Lipika Ray,a Mobin M. Shaikhb and Prasenjit Ghosh*a
Received 1st May 2007, Accepted 6th July 2007
First published as an Advance Article on the web 2nd August 2007
DOI: 10.1039/b706607d
Several new air-stable, convenient to handle and easily synthesized Pd based PEPPSI (Pyridine
Enhanced Precatalyst Preparation, Stabilization and Initiation) type precatalysts supported over
N/O-functionalized N-heterocyclic carbenes (NHC) namely, trans-[1-(benzyl)-3-
(N-t-butylacetamido)imidazol-2-ylidene]Pd(pyridine)Cl2 (2), trans-[1-(2-hydroxy-cyclohexyl)-3-
(benzyl)imidazol-2-ylidene]Pd(pyridine)Cl2 (3) and trans-[1-(o-methoxybenzyl)-3-(t-butyl)imidazol-2-
ylidene]Pd(pyridine)Br2 (4), have been designed. Specifically, the Pd–NHC complexes, 2, 3 and 4, were
conveniently synthesized from their respective imidazolium halide salts by the reaction with PdCl2 in
pyridine in presence of K2CO3 as a base. A new imidazolium chloride salt, 1-(benzyl)-3-(N-t-butyl-
acetamido)imidazolium chloride (1) was synthesized by the alkylation reaction of benzyl imidazole
with N-t-butyl-2-chloroacetamide. The molecular structures of the imidazolium chloride salt, 1, and
the Pd–NHC complexes, 2, 3 and 4, have been determined by X-ray diffraction studies. The density
functional theory studies of the 2, 3 and 4 complexes were carried out to in order to gain insight about
their structure, bonding and the electronic properties. The nature of the NHC–metal bond in these
complexes was examined using Charge Decomposition Analysis (CDA), which revealed that the
N-heterocyclic carbene ligands are effective r-donors. In addition, the catalysis studies revealed that the
Pd–NHC complexes, 2, 3 and 4, are effective catalysts for the Suzuki–Miyaura type C–C
cross-coupling reactions.
Grubbs catalyst for olefin metathesis.9 Specifically, when loosely
Introduction
bound pyridine and its substituted derivatives, acting as the
With N-heterocyclic carbenes (NHCs) being extremely successful
“throwaway” ligands, replaced a more tightly bound tricyclo-
in homogeneous catalysis1 and with Pd emerging as an undis-
puted leader in the catalysis of many important C–C bond
hexylphosphane (PCy3) in the Grubbs second-generation catalyst,
=
[(H2IMes)(PCy3)(Cl)2Ru CHPh], higher activities were observed
forming reactions2 namely, the Hiyama,3 Kumada,4 Negishi,5
even for the more challenging acetonitrile Cross-Metathesis (CM)
Suzuki,6 and Stille7 reactions, rational catalyst designing involv-
ing Pd–NHC complexes has thus taken center stage and has
brought forth significant advancements in the area of catalyst
development in recent years. The focus has mainly been on
developing a universal catalyst that would be highly efficient,
robust, user-friendly and convenient to synthesize and could
be employed for various Pd-mediated cross-coupling reactions.2
The key to a rational catalyst designing exercise lies in the
successful realization of important fundamental concepts8 and
one such instance has been in the introduction of a “throwaway
ligand,” intended to give way to the incoming substrate, in the
reactions.9 Along a similar theme, PEPPSI (Pyridine Enhanced
Precatalyst Preparation, Stabilization and Initiation), a term
recently coined and demonstrated by Organ and coworkers,10,11
also resulted in highly active catalysts for various Pd-mediated
C–C cross-coupling reactions.
Our interest lies in designing high performance Pd catalysts sta-
bilized by N/O-functionalized N-heterocyclic carbenes (NHCs)
for a variety of C–C bond forming reactions particularly, the
Suzuki–Miyaura cross-coupling reaction.12 The growing popular-
ity of the Suzuki–Miyaura cross-coupling reaction can be ascribed
to mild reaction conditions (ambient temperature ca. 80–100◦ C
in air), commercial availability of diverse boronic acids that are
also environmentally safer than many organometallic reagents,
the ease of handling and removal of boron by-products and
functional group tolerance.13 The extent of the importance of
the Suzuki–Miyaura cross-coupling reaction can be gauged from
the diverse range of its applications that span from materials
to pharmaceuticals to polymers to ligands in organometallic
chemistry to natural product synthesis.13 For example, many im-
portant natural products and pharmaceuticals possessing complex
aDepartment of Chemistry, Powai, Mumbai, 400 076
bNational Single Crystal X-ray Diffraction Facility, Indian Institute of Tech-
nology Bombay, Powai, Mumbai, 400 076. E-mail: pghosh@chem.iitb.ac.in;
Fax: +91 22 2572 3480
† CCDC reference numbers 623546 (1), 630834 (2), 627573 (3) and 623095
(4). For crystallographic data in CIF or other electronic format see DOI:
10.1039/b706607d
‡ Electronic supplementary information (ESI) available: B3LYP coordi-
nates for geometry optimized calculations and CDA of 2ꢀ, 3ꢀ and 4ꢀ. See
DOI: 10.1039/b706607d
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architectures like, kendamycin,14 myxovirescin,15 marinomycin
A,16 vancomycin17 etc. have been synthesized employing the
Suzuki–Miyaura cross-coupling reaction.
following a ring-opening reaction of the cyclohexene oxide
with imidazole and benzyl chloride reported earlier by us,22
the 1-(o-methoxybenzyl)-3-(t-butyl)imidazolium bromide12 and
1-(benzyl)-3-(N-t-butylacetamido)imidazolium chloride 1 were
synthesized by the direct alkylation reaction of the respective
imidazoles with the corresponding alkyl halides. Specifically, the
new imidazolium chloride salt, 1, was synthesized by the alkylation
reaction of benzyl imidazole with N-t-butyl-2-chloroacetamide in
70% yield. The formation of 1 was verified by the appearance of the
Our intention in using N/O-functionalized N-heterocyclic
carbenes for designing these precatalysts was in enhancing the
stability of these precatalysts as the incorporation of polar
groups often leads to greater stability of metal–NHC complexes
through a variety of intermolecular interactions,18 chelation
to the metal center,19 etc. For example, higher decomposition
temperatures (ca. > 180 ◦C) were observed for the two cationic Ag–
NHC complexes, [(1-i-propyl-3-{N-phenylacetamido}imidazol-2-
ylidene)2Ag]+Cl−18 and {[1-(2,4,6-trimethylphenyl)-3-(N-phenyl-
1
diagnostic NCHN peak at 10.1 ppm in the H NMR spectrum.
The two bridging methylene peaks of the benzyl and the N-t-
butylacetamido moieties appeared at 5.45 ppm and 5.31 ppm in the
1H NMR spectrum and at 53.0 ppm and 51.8 ppm in the 13C NMR
spectrum. The amide proton of the –CONH–moiety appeared
downfield shifted at 8.66 ppm in the 1H NMR spectrum while the
carbonyl band of the –CONH– moiety appeared at 1685 cm−1 in
the infrared spectrum.
+
acetamido)imidazol-2-ylidene]2Ag} Cl−,20 bearing N/O-func-
tionalized sidearms as N-substituents.
Here in this contribution, we report the syntheses and struc-
tural characterizations of a series of air-stable, convenient to
handle and easily accessible PEPPSI styled precatalysts, trans-
(NHC)Pd(pyridine)X2 (X = Cl, Br) (Fig. 1), 2, 3 and 4, stabilized
over N/O-functionalized N-heterocyclic carbene ligands and
containing a “throwaway” pyridine ligand. These 2, 3 and 4
precatalysts are effective for the Suzuki–Miyaura cross-coupling
reactions of phenylboronic acids and aryl halides (X = Br,
I). In addition, the synthesis and the structural characteri-
zation of a new imidazolium chloride salt, 1-(benzyl)-3-(N-t-
butylacetamido)imidazolium chloride 1 is also described.
The molecular structure of the imidazolium chloride salt, 1,
has been determined by X-ray diffraction studies (Table 1 and
˚
˚
Fig. 2). The N–C bond distances of 1.329(3) A and 1.330(3) A in
the imidazolium ring are consistent with its aromatic nature and
are in concurrence with the values reported for other structurally
characterized imidazolium halide salts. For example, in closely
related imidazolium halide salts the corresponding N–C distances
are, 1-(2-hydroxycyclohexyl)-3-(N-t-butylacetamido)imidazolium
chloride22 [1.326(6) A, 1.314(6) A], 1-i-propyl-3-(2-oxo-2-t-butyl
˚
˚
ethyl)imidazolium chloride23 [1.320(2) A, 1.323(2) A] and 1-(2,4,6-
trimethylphenyl)-3-(N-phenylacetamido)imidazolium chloride20
[1.324(3) A, 1.322(3) A]. Similarly, the ∠N–C–N angle of
108.60(17)◦ of the imidazolium ring in 1 is similar to that of other
related imidazolium halide salts.
˚
˚
˚
˚
Fig. 1 trans-(NHC)Pd(pyridine)X2 (X = Cl, Br) 2, 3 and 4.
Results and discussion
The N/O-functionalized imidazolium halide salts were syn-
thesized as outlined in Scheme 1. While the 1-(2-hydroxy-
cyclohexyl)-3-(benzyl)imidazolium chloride21 was synthesized by
Fig. 2 ORTEP of 1 with thermal ellipsoids drawn at 50% probability
◦
˚
˚
level. Selected bond lengths (A) and angles ( ): Selected bond lengths (A)
and angles (◦): N(1)–C(1) 1.329(3), N(2)–C(1) 1.330(3), N(1)–C(1)–N(2)
108.41(17).
The most important aspect of the imidazolium chloride 1 struc-
ture is the presence of [Cl · · · H–N] type H-bonding interaction
Scheme 1
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Table 1 X-Ray crystallographic data for 1, 2, 3 and 4
Compound
1
2
3
4
Lattice
Formula
Triclinic
C16H22ClN3O
307.82
Triclinic
C21H26Cl2N4OPd
527.76
Triclinic
C21H25Cl2N3OPd
512.74
Monoclinic
C20H25Br2N3OPd
589.65
Formula weight
Space group
P-1
P-1
P-1
P21/c
˚
a/A
6.0741(3)
10.996(3)
12.313(4)
82.75(2)
83.452(14)
81.642(13)
803.3(3)
2
150(2)
0.71073
1.273
0.241
25.00–3.35
2819
10.829(2)
11.2668(15)
11.429(2)
107.068(14)
111.852(19)
100.997(14)
1164.3(4)
2
150(2)
0.71073
1.505
1.045
32.50–3.05
4090
8.9556(10)
9.830(2)
14.552(2)
71.923(17)
75.819(10)
64.689(16)
1091.5(3)
2
150(2)
0.71073
1.560
1.111
32.17–3.09
3827
12.9252(3)
10.8313(3)
16.0613(4)
90.00
102.569(2)
90.00
2194.64(10)
4
150(2)
0.71073
1.785
4.501
32.19–3.20
3840
248
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
V/A
Z
3
˚
Temperature/K
˚
Radiation (k/A)
q(calcd.)/g cm−3
l(Mo-Ka), mm−1
h/◦
No. of data
No. of parameters
R1
wR2
GOF
197
0.0411
0.1049
1.207
269
0.0374
0.0963
1.009
253
0.0365
0.0834
1.088
0.0682
0.2190
1.169
between the Cl− anion and the amide proton of the –CONH–
˚
substituent of the imidazole ring. The [Cl · · · N] distance of 3.267 A
is comparable to the sum of the individual van der Waals radii
(Cl–N = 3.30 A).24 Quite interestingly, the Cl− anion was found to
˚
interact with the amide proton of the –CONH– substituent and
not with the acidic NCHN proton of the imidazole ring. In this
regard, it is worth mentioning that the H-bonding interaction of
the halide counter anion with the acidic proton of the functional
sidearm (HX– ; X = O, NR) as well as with that of the acidic
NCHN proton have been observed for various imidazolium halide
salts. Indeed, examples of imidazolium halide salts exhibiting all
22
˚
three [Cl · · · H–O] [d(Cl · · · O) = 3.091 A], [Cl · · · H–N] [d(Cl · · · N)
=
20
23
˚
˚
˚
3.244 A, 3.236 A] and [Cl · · · H–C] [d(Cl · · · C) = 3.373 A] types
of H-bonding interactions have been recently reported by us. It is
worth mentioning that the H-bonding interaction in imidazolium
halide salts has been extensively studied by NMR25 and X-ray
diffraction techniques.26
The 2, 3 and 4 precatalysts were conveniently synthesized
by direct reaction of the imidazolium halide salt with PdCl2
1
in pyridine (Scheme 2).11 The 13C{ H} NMR of 2, 3 and 4
showed the metal bound carbene NCN–Pd resonances appearing
at 157–153 ppm and are comparable to that observed in other
reported Pd–NHC complexes (175–145 ppm).27 The 2, 3 and 4
complexes have been structurally characterized and, as expected,
the geometries around the Pd centers were found to be square
planar (Table 1 and Fig. 3–5) with the Ccarbene–Pd–Cl angles in
Fig. 3 ORTEP of 2 with thermal ellipsoids drawn at 50% probability level.
◦
˚
Selected bond lengths (A) and angles ( ): N(1)–C(1) 1.356(4), N(2)–C(1)
1.337(4), Pd(1)–C(1) 1.957(3), Pd(1)–Cl(1) 2.3026(9), Pd(1)–N(4) 2.089(3),
C(1)–Pd(1)–N(4) 178.63(11), C(1)–Pd(1)–Cl(1) 87.62(9).
2 [C1–Pd1–Cl2 = 91.24(9)◦], 3 [C1–Pd1–Cl2 = 88.70(11)◦] and
4 [C1–Pd1–Br2 = 86.3(2)◦] being closer to ca. 90◦. While the
Ccarbene–Pd–Npyridine angles in 2 [C1–Pd1–N4 = 178.63(11)◦], 3 [C1–
Pd1–N3 = 179.66(14)◦] and 4 [C1–Pd1–N3 = 176.0(3)◦] are nearly
˚
linear. The Pd-Ccarbene bond distances [1.953(8)–1.967(4) A] in these
˚
˚
complexes, 2 [Pd1–C1 = 1.957(3) A], 3 [Pd1–C1 = 1.967(4) A] and
˚
4 [Pd1–C1 = 1.953(8) A], are comparable to that reported for
other Pd–NHC complexes.19,28,29 The two Pd–Cl bond distances
˚
˚
Scheme 2
in 2 [Pd1–Cl1 = 2.3026(9) A, Pd1–Cl2 = 2.2913(9) A] and 3
4548 | Dalton Trans., 2007, 4546–4555
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of both intermolecular as well as intramolecular H-bonding
interactions involving the functional sidearm substituents were
observed in these complexes e.g., 2 (intermolecular [Cl · · · H–
˚
N] interaction, d(Cl · · · N) = 3.306 A), 3 (intramolecular [Cl · · · H–
˚
O] interaction, d(Cl · · · O) = 3.316 A, intermolecular [Cl · · · H–C]
˚
interaction, d(Cl · · · C) = 3.695 A) and 4 (intermolecular [Br · · · H–
˚
C] interaction, d(Br · · · C) = 3.666 A, intermolecular [Br · · · H–C]
˚
interaction, d(Br · · · C) = 3.583 A).
The role of the metal bound pyridine is vital to the observed
high activities of the PEPPSI based precatalysts and thus merits
discussion. For the 2, 3 and 4 complexes, the Pd-bound pyridine
appeared as distinct resonances in comparison to the free pyridine
in the 1H NMR spectra of these complexes.31 However, the
definitive proof came from the X-ray diffraction studies that
showed the pyridine was, indeed, coordinated to Pd and was tilted
by ca. 39◦–46◦ relative to the plane containing the imidazolium
ring of the NHC ligand.32 Interestingly, the Pd–N (pyridine)
˚
bond distances [2.100(8)–2.089(3) A] in 2, 3 and 4 are closer to
that in a related complex, [1,3-bis-2,6-di-i-propylphenyl-imidazol-
11
˚
2-ylidene]PdCl2(3-chloropyridine) [2.137(2) A], bearing a sim-
ilar trans pyridine ligand, but are longer than that observed
˚
[2.017(4) A] in [{CNC}Pd(pyridine)][BF4]2, [CNC = (2,6-bis{[N-
Fig. 4 ORTEP of 3 with thermal ellipsoids drawn at 50% prob-
◦
methyl-Nꢀ-methylene]imidazol-2-ylidene}pyridine],33 in which the
metal bound pyridine was not trans to the NHC ligand. It is
worth noting that owing to the strong trans-effect34 of the N-
heterocyclic carbene ligand, the Pd–N (pyridine) bond distance
of the trans-pyridine is expected to be longer, thereby, resulting
in weaker binding of the pyridine and subsequent triggering of
the “throwaway” pyridine ligand dissociation in the initiation
step of the catalytic cycle.10,11 Indeed, when the precatalyst 4
˚
ability level. Selected bond lengths (A) and angles ( ): N(1)–C(1)
1.345(5), N(2)–C(1) 1.343(5), Pd(1)–C(1) 1.967(4), Pd(1)–Cl(1) 2.3071(10),
Pd(1)–N(3) 2.096(3), C(1)–Pd(1)–N(3) 179.66(14), C(1)–Pd(1)–Cl(1)
89.07(11).
◦
was heated at 85 C with 3 equivalents of phenylboronic acid in
the absence of aryl halides, simultaneous formations of pyridine
and biphenyl, as verified independently by gas chromatography,
were observed. Further support for the relatively weaker Pd–
pyridine bond came from the density functional theory studies
as the Pd–pyridine bond dissociation energy [De (Pd-pyridine)],
computed for the geometry optimized structures of 2, 3 and 4
respectively designated by 2ꢀ, (32.9 kcal mol−1), 3ꢀ (32.7 kcal mol−1),
and 4ꢀ (30.8 kcal mol−1) using B3LYP/SDD, 6-31G(d) level of
theory, were found to be significantly lower than the (NHC)–Pd
bond dissociation energy [De (Pd–Ccarbene)] in these complexes, 2ꢀ
(81.9 kcal mol−1), 3ꢀ (82.4 kcal mol−1) and 4ꢀ (77.6 kcal mol−1) and
thus the pyridine moiety in these palladium complexes are more
amenable to dissociation.
Detailed bonding studies on the geometry optimized structures
2ꢀ, 3ꢀ and 4ꢀ were performed using Charge Decomposition Analysis
(CDA) that involved breaking the molecule into two fragments, i.e.
N-heterocyclic carbene (NHC) (fragment 1) and Pd(pyridine)X2
(fragment 2) (Table 2). The optimized geometries were obtained
from the crystallographic co-ordinates using B3LYP/SDD, 6-
31G* level of theory for the complexes 2, 3 and 4. The elec-
Fig. 5 ORTEP of 4 with thermal ellips◦oids drawn at 50% probability level.
˚
Selected bond lengths (A) and angles ( ): N(1)–C(1) 1.359(11), N(2)–C(1)
1.341(12), Pd(1)–C(1) 1.953(9), Pd(1)–Br(1) 2.3993(13), Pd(1)–N(3)
2.100(8), C(1)–Pd(1)–N(3) 176.0(3), C(1)–Pd(1)–Br(1) 87.6(2).
tron donation from the carbene fragment to the metal center
r
[NHC −→ Pd(pyridine)X2] is denoted as d while the electron
˚
˚
[Pd1–Cl1 = 2.3071(10) A, Pd1–Cl2 = 2.3068(10) A] and the
back donation from the metal center to the carbene moiety
p
˚
two Pd–Br bond distances in 4 [Pd1–Br1 = 2.399(2) A, Pd1–
[NHC ←− Pd(pyridine)X2] is represented by b and the (d/b)
r
ratio gives a measure of the forward [NHC −→ Pd(pyridine)X2]
p
˚
Br2 = 2.393(2) A] are comparable to the sum of the individual
30
˚
˚
covalent radii (Pd–Cl = 2.273 A and Pd–Br = 2.423 A). Quite
interestingly, though no chelation of the functionalized sidearms
to Pd was observed in any of the 2, 3 and 4 complexes, a variety
and backward [NHC ←− Pd(pyridine)X2] donations occurring
in these complexes. Thus high (d/b) ratio in 2ꢀ (2.59), 3ꢀ (2.79)
and 4ꢀ (3.99) complexes are indicative of the predominantly
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r
p
Table 2 Charge decomposition analysis (CDA) results showing the [NHC
Pd(pyridine)X ] donation (d), the [NHC
Pd(pyridine)X ] donation
−−−→
←−−
2
2
(b) and the d/b ratio for the geometry optimized structures, 2ꢀ, 3ꢀ and 4ꢀ are shown
r
p
Complexes
[NHC
Pd(pyridine)X ]
[NHC
Pd(pyridine)X ]
d/b ratio
−−−→
←−−
2
2
2ꢀ
3ꢀ
4ꢀ
0.559
0.570
0.603
0.216
0.204
0.151
2.59
2.79
3.99
r-donating nature of the N-heterocyclic carbene ligands. In this
regard, it is worth noting that theoretical studies earlier carried
out by us18,23 on Ag–NHC complexes and also by others35 suggest
that N-heterocyclic carbenes as ligands, in general, are strong
r-donors with minimal p-accepting abilities and, thus would be
conducive to the stabilization of the Pd center after the pyridine
dissociation.10,11
In order to gain insight about the NHC–palladium bonding,
a detailed molecular orbital analysis was carried out using
the AOMix-CDA software,36 in which the orbital contributions
to the frontier molecular orbitals of the geometry optimized
(NHC)Pd(pyridine)X2 type complexes, 2ꢀ (X = Cl), 3ꢀ (X = Cl) and
4ꢀ (X = Br), from the individual NHC and the Pd(pyridine)X2 frag-
ments (X = Cl, Br) were considered. Interestingly, the molecular
orbitals signifying the NHC–palladium interactions were found
buried deep inside the surface i.e. in 2ꢀ (HOMO-16 and HOMO-
29, Fig. 6), 3ꢀ (HOMO-16 and HOMO-28, Fig. 7) and 4ꢀ (HOMO-
14 and HOMO-25, Fig. 8) and were consistent with the inert
character of the NHC–palladium bond that are often attributed to
the “spectator” nature of the NHC ligand in general.1a Specifically,
the contributions of the NHC and the Pd(pyridine)X2 fragments to
these molecular orbitals were, 2ꢀ [HOMO-16 {20% NHC fragment,
71% from Pd(pyridine)Cl2}, HOMO-29 {29% NHC fragment,
52% from Pd(pyridine)Cl2}] (Fig. 6), 3ꢀ [HOMO-16 {31% NHC
fragment, 58% from Pd(pyridine)Cl2}, HOMO-28 {29% NHC
fragment, 54% from Pd(pyridine)Cl2}] (Fig. 7) and 4ꢀ [HOMO-
14 {24% NHC fragment, 64% from Pd(pyridine)Br2}, HOMO-
Fig. 7 Orbital interaction diagram showing the major contributions of
the NHC–palladium bond in 3ꢀ.
Fig. 8 Orbital interaction diagram showing the major contributions of
the NHC–palladium bond in 4ꢀ.
25 {39% NHC fragment, 35% from Pd(pyridine)Br2}] (Fig. 8).
However, detailed orbital interactions are more intricate due to
the contributions from various other fragment molecular orbitals
(FMO) in the palladium complexes. It is interesting to note that
Fig. 6 Orbital interaction diagram showing the major contributions of
the NHC–palladium bond in 2ꢀ.
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Table 3 Selected results of Suzuki–Miyaura cross-coupling reaction of
aryl halides (ArX, X = Br, I) catalyzed by 2, 3 and 4.a
donation of the carbene lone pair partially occurs to the Pd–
pyridine r* orbital as can be seen in Fig. 6, 7 and 8 thereby
weakening the Pd–pyridine bond.
A comparison of the solid state crystal structures of 2, 3 and 4
with that of the geometry optimized structures 2ꢀ, 3ꢀ and 4ꢀ revealed
good agreement between the X-ray and computed structures (See
Tables S4, S5 and S6‡).
Entry
1
Reagent
Product
2b
3b
4b
The Pd complexes 2, 3 and 4 efficiently catalyzed the Suzuki–
Miyaura cross-coupling (eqn (1)) of a wide variety of aryl halide
substrates with phenyl boronic acid at a low catalyst loading
(0.35 mol%). It is interesting to note that high modulations of
the yields (> 99–7%) of the cross-coupled products were seen,
with higher yields observed for aryl halides containing activating
substituents while lower conversions were observed for the ones
with less electron withdrawing substituents (Table 3). For example,
phenyl bromides having electron-withdrawing substituents like, –
CHO, –COMe and –NO2 moieties in the o/p-positions gave higher
product yields for 2 (52–79%), 3 (64–85%) and 4 (59–> 99%)
while those with electron-donating substituents like, –Me and
–OMe moieties in the o/p-positions, gave comparatively much-
lower product yields for 2 (37–48%), 3 (34–45%) and 4 (31–47%).
The yields observed for iodobenzene is, however, lower than the
activated bromo derivatives.
79
85
99
2
3
4
72
66
52
82
67
64
99
73
59
5
6
48
43
43
45
47
38
7
37
34
31
(1)
8
9
36
7
32
11
30
22
Quite interestingly, the commonly observed Csp2–Csp2 cross-
coupling (Table 3, entries 1–8) between the two Csp2 centers of
aryl halide and phenyl boronic acid can even be extended to a
relatively more challenging Csp3–Csp2 coupling (Table 3, entry 9) in
which a Csp3 carbon of benzyl bromide was coupled with the Csp2
carbon of phenyl boronic acid, albeit in lower yields.
Though all the three 2, 3 and 4 precatalysts displayed similar
trends, quantitative conversions were observed in the case of 4 for
the coupling of o- and p-bromobenzaldehydes. Hence, concentra-
tion dependence studies of the precatalyst 4 were carried out in
order to gauge the maximum turnover efficiency of the catalyst
(Table 4) and, indeed, high turnover numbers as high as 9700
at 8.6 × 10−3 mol% was observed for the o-bromobenzaldehyde
(Table 4). Interestingly, the pyridine tilt as defined by the torsion
angle between pyridine and the imidazolium ring32 is the lowest
for 4, which coincidently showed the highest conversion for o-
bromobenzaldehyde. However, detailed theoretical studies are
needed to elucidate the exact role of the catalyst structure in its
mode of action.
a Reaction conditions: 2.16 mmol of aryl halide (ArX, X = Br, I), 2.64 mmol
of phenyl boronic acid, 3.24 mmol of K2CO3, 7.5 × 10−3 mmol of catalyst,
30 mL of CH3CN, 12 h at 85 ◦C. b The yields (%) were determined by GC
using diethyleneglycol–di-n-butyl ether as an internal standard.
percatalysts11 successfully carried out Suzuki–Miyaura cross-
coupling of the more challenging aryl chlorides along with
the frequently encountered aryl bromide substrates but at
comparatively higher catalyst loadings (1–2 mol%) than the 2, 3
and 4 precatalysts (3.5 × 10−1–8.6 × 10−3 mol%) showed activity
toward the aryl bromide and iodide substrates.
Also worthwhile is the comparison of the activity of a trans-
(NHC)Pd(pyridine)X2 (X = halide) type PEPPSI precatalyst
with a similar trans-(NHC)2PdX2 precatalyst supported over the
same N-heterocyclic carbene (NHC) ligand. Interestingly, the
activity of 4, containing a N/O-functionalized N-heterocyclic
carbene and a “throwaway” pyridine ligand, was found to
be significantly lower than a trans-(NHC)2PdX2 (X = halide)
complex,12 containing two of the same N/O-functionalized N-
heterocyclic carbene ligand. For example, for the Suzuki–Miyaura
cross-coupling of o-bromobenzaldehyde with phenyl boronic acid,
the maximum TON (9700) (Table 4 entry 5) obtained for trans-[1-
(o-methoxybenzyl)-3-(t-butyl)imidazol-2-ylidene]Pd(pyridine)Br2
Important is the comparison of the precatalysts 2, 3 and 4
containing N/O-functionalized N-heterocyclic carbenes, with
the other related PEPPSI precatalysts, trans-[1,3-bis-2,6-di-
i-propylphenyl-imidazol-2-ylidene]PdCl2(3-chloropyridine),11
trans-[1,3-bis-2,6-di-ethylphenyl-imidazol-2-ylidene]PdCl2(3-
chloropyridine),11
and trans-[1,3-bis-2,4,6-tri-methylphenyl-
imidazol-2-ylidene]PdCl2(3-chloropyridine),11 containing non-
functionalized Arduengo type N-heterocyclic carbenes reported
by Organ and coworkers.10,11 Though the Organ’s PEPPSI
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Table 4 Selected results of Suzuki coupling of o/p-bromobenzaldehyde
with phenyl boronic acid catalyzed by 4a
FID. X-Ray diffraction data for 1, 2, 3 and 4 were collected on an
Oxford diffraction XCALIBUR-S instrument. The crystal data
collection and refinement parameters are summarized in Table 1.
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).
Entry
4 (mol%)
Yieldb(%)
TON
Synthesis of 1-(benzyl)-3-(N-t-butylacetamido)imidazolium
chloride 1. A mixture of benzyl imidazole (1.06 g, 6.71 mmol)
and N-t-butyl-2-chloroacetamide (1.00 g, 6.71 mmol) was
dissolved in toluene (ca. 40 mL) and the reaction mixture was
refluxed at 110 ◦C for 12 h when a sticky solid separated out. The
solid was isolated by decanting off the solvent and washed with
hot hexane (3 × ca.10 mL) to obtain the product 1 as a light yellow
1
2
3
4
5
3.5 × 10−1
8.6 × 10−2
3.5 × 10−2
1.7 × 10−2
8.6 × 10−3
>99
>99
>99
>99
83
288
1160
2900
5500
9700
◦
1
solid (1.45 g, 70%). H NMR (CDCl3, 400 MHz, 25 C): d 10.1
(s, 1H, NCHN), 8.66 (br, 1H, NH), 7.59 (br, 1H, NCHCHN),
7.41–7.39 (m, 5H, C6H4), 7.09 (br, 1H, NCHCHN), 5.45 (s, 2H,
1
Entry
4 (mmol)
Yieldb(%)
TON
CH2), 5.31 (s, 2H, CH2), 1.37 (s, 9H, C(CH3)3). 13C{ H} NMR
◦
(CDCl3, 100 MHz, 25 C): d 163.5 (CO), 136.8 (NCHN), 132.6
6
7
8
3.5 × 10−1
8.6 × 10−2
3.5 × 10−2
>99
>99
58
288
1160
1700
(ipso-C6H5), 129.1 (o-C6H5), 129.1 (m-C6H5), 128.6 (p-C6H5),
123.6 (NCHCHN), 120.9 (NCHCHN), 53.0 (CH2), 51.8 (CH2),
51.7 (C(CH3)3), 28.2 (C(CH3)3). IR data (KBr) cm−1 3441 (m),
3212 (s), 3047 (m), 3011 (m), 2969 (m), 2928 (w), 2905 (w), 1685
(s), 1544 (s), 1497 (w), 1456 (m), 1396 (m), 1367 (m), 1283 (m),
1225 (m), 1162 (s), 1097 (w), 1032 (w), 1018 (w), 950 (w), 920 (w),
872 (w), 829 (w), 796 (w), 776 (w), 729 (m), 713 (s), 674 (w), 621
(m), 578 (m), 458 (w). Anal. Calcd. for C16H22ClN3O: C, 62.43;
H, 7.20; N, 13.65. Found: C, 61.76; H, 8.02; N, 13.96%.
a Reaction conditions: 2.16 mmol of aryl halide, 2.64 mmol of phenyl
boronic acid, 3.24 mmol of K2CO3, complex 4, 30 mL of CH3CN, 12 h at
85 ◦C. b Determined by GC using diethyleneglycol–di-n-butyl ether as an
internal standard.
(4) is significantly lower (TON = 49 700, after 12 h at 85 ◦C) than its
bis-NHC analog, trans-[1-(o-methoxybenzyl)-3-(t-butyl)imidazol-
Synthesis of trans-[1-(benzyl)-3-(N-t-butylacetamido)imidazol-
2-ylidene]Pd(pyridine)Cl2 2. A mixture of 1-(benzyl)-3-(N-t-
butylacetamido)imidazolium chloride 1 (0.167 g, 0.543 mmol),
PdCl2 (0.105 g, 0.592 mmol) and K2CO3 (0.375 g, 2.71 mmol)
were refluxed in pyridine (ca. 5 mL) for 16 h. The reaction
mixture was filtered and the solvent was removed under vacuum.
Then the residue was washed with aqueous CuSO4 solution and
the aqueous layer was extracted with dichloromethane (ca. 3 ×
10 mL). Then the organic layer was collected and the solvent was
removed under vacuum to obtain the product 2 as a yellow solid
(0.116 g, 40%). 1H NMR (CDCl3, 400 MHz, 25 ◦C): d 8.93 (d, 2H,
3JHH = 8 Hz, o-NC5H5), 7.74 (t, 1H, 3JHH = 8 Hz, p-NC5H5), 7.40
12
2-ylidene]2PdCl2 also obtained under the same conditions (after
12 h at 85 ◦C). The greater activity of trans-[1-(o-methoxy-
benzyl)-3-(t-butyl)imidazol-2-ylidene]2PdCl2 may be due to the
formation of a more electron-rich active species, trans-[1-(o-
methoxybenzyl)-3-(t-butyl)imidazol-2-ylidene]2Pd0, containing
two electron-donating NHC ligands, as opposed to the
active species, trans-[1-(o-methoxybenzyl)-3-(t-butyl)imidazol-2-
ylidene]Pd0 (solvent),10,11 formed from the precatalyst 4 containing
only one NHC ligand, and thereby greatly enhancing the oxidative
addition of the aryl halide, often the rate-determining step in the
catalytic cycle, in the case of the former.37
3
(t, 2H, JHH = 8 Hz, m-NC5H5), 7.31 (m, 5H, C6H5), 7.04
(br, 1H, NCHCHN), 6.71 (br, 1H, NCHCHN), 5.77 (s, 2H,
Experimental
1
CH2), 5.15 (s, 2H, CH2), 1.27 (s, 9H, C(CH3)3). 13C{ H} NMR
(CDCl3, 100 MHz, 25 ◦C): d 165.8 (CO), 153.3 (NCN–Pd),
151.1 (o-NC5H5), 138.5 (ipso-C6H5), 138.2 (p-NC5H5), 128.9 (o-
C6H5), 128.8 (m-C6H5), 128.6 (p-C6H5), 124.6 (m-NC5H5), 122.5
(NCHCHN), 122.1 (NCHCHN), 55.6 (CH2), 54.7 (CH2), 51.9
(C(CH3)3), 28.3 (C(CH3)3). IR data (KBr) cm−1 3416 (s), 3306
(m), 3130 (m), 1682 (m), 1638 (m), 1618 (m), 1543 (w), 1450 (m),
1400 (s), 1262 (w), 1246 (w), 1165 (w), 1071 (w), 1017 (w), 804
(w), 761 (w), 722 (w), 691 (m), 617 (w), 481 (w). Anal. Calcd. for
C21H27Cl2N4OPd·1/2(CH2Cl2): C, 45.20; H, 4.94; N, 9.81. Found:
C, 45.59; H, 5.42; N, 8.99%.
General procedures
All manipulations were carried out using a combination of
glovebox and standard Schlenk techniques. Solvents were
purified and degassed by standard procedures. The synthetic
procedures of 1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazolium
chloride21
and
1-(o-methoxybenzyl)-3-(t-butyl)imidazolium
bromide12 were reported by us. Benzyl imidazole38 and N-t-butyl-
2-chloroacetamide39 were synthesized according to literature
procedures. 1H and 13C{ H} NMR spectra were recorded in
1
CDCl3 on a Varian 400 MHz NMR spectrometer. 1H NMR peaks
are labeled as singlet (s), doublet (d) and multiplet (m). Infrared
spectra were recorded on a Perkin Elmer Spectrum One FT-IR
spectrometer. Mass spectrometry measurements were performed
on a Micromass Q-Tof spectrometer. GC spectra were measured
on a Shimadzu gas chromatograph GC-15A equipped with a
Synthesis of trans-[1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-
2-ylidene]Pd(pyridine)Cl2 3.
A
mixture of 1-(2-hydroxy-
cyclohexyl)-3-(benzyl)imidazolium chloride (0.440 g, 1.51 mmol),
PdCl2 (0.294 g, 1.66 mmol) and K2CO3 (1.04 g, 7.54 mmol) were
refluxed in pyridine (ca. 5 mL) for 16 h. The reaction mixture
4552 | Dalton Trans., 2007, 4546–4555
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was filtered and the solvent was removed under vacuum. Then
the residue was washed with aqueous CuSO4 solution and the
aqueous layer was extracted with dichloromethane (ca. 3 ×
10 mL). Then the organic layer was collected and the solvent
was removed under vacuum to obtain the product 3 as a yellow
crystalline solid (0.364 g, 47%). 1H NMR (CDCl3, 400 MHz,
effective core potential (ECP), representing 19 core electrons,
along with valence basis sets (SDD) is used for palladium.43 All
other atoms are treated with 6-31G(d) basis set.44 All stationary
points are characterized as minima by evaluating Hessian indices
on the respective potential energy surfaces.
Inspection of the metal–ligand donor–acceptor interactions was
carried out using the charge decomposition analysis (CDA).45
CDA is a valuable tool in analyzing the interactions between
molecular fragments on a quantitative basis, with an emphasis on
the electron donation.46 The orbital contributions in the geometry
optimized NHC-Pd(pyridine)X2 type complexes, 2ꢀ (X = Cl), 3ꢀ
25 ◦C): d 9.03 (d, 2H, JHH = 8 Hz, o-NC5H5), 7.79 (t, 1H,
3
3
3JHH = 8 Hz, p-NC5H5), 7.47 (t, 2H, JHH = 8 Hz, m-NC5H5),
7.40–7.34 (m, 5H, C6H5), 6.95 (br, 1H, NCHCHN), 6.76 (br, 1H,
NCHCHN), 5.89 (d, 1H, 2JHH = 15 Hz, CH2), 5.82 (d, 1H, 2JHH
=
15 Hz, CH2), 5.09–5.03 (m, 1H, C6H10), 3.30–3.27 (m, 1H, C6H10),
2.32–2.24 (m, 2H, C6H10), 1.92–1.85 (m, 3H, C6H10), 1.63–1.53
(X = Cl) and 4ꢀ (X = Br), can be divided into three parts:
r
(m, 3H, C6H10). 13C{ H} NMR (CDCl3, 100 MHz, 25 ◦C): d
(i) r-donation from the [NHC −→ Pd(pyridine)X2] fragment
1
p
153.2 (NCN-Pd), 151.2 (o-NC5H5), 147.9 (ipso-C6H5), 138.1
(p-NC5H5), 135.0 (o-C6H5), 129.0 (m-C6H5), 128.9 (p-C6H5),
124.5 (m-NC5H5), 122.0 (NCHCHN), 119.7 (NCHCHN), 72.7
(C6H10), 66.9 (C6H10), 54.9 (CH2), 34.9 (C6H10), 32.9 (C6H10),
25.0 (C6H10), 24.3 (C6H10). IR Data (KBr) cm1 3118 (m), 2937
(w), 2849 (w), 1619 (m), 1605 (m), 1559 (w), 1448 (m), 1400 (s),
1278 (w), 1241 (w), 1215 (w), 1163 (w), 1069 (m), 1047 (w), 1032
(w), 955 (w), 868 (w), 761 (w), 732 (w), 691 (m), 615 (w), 472 (w).
Anal. Calcd. for C21H26Cl2N3OPd: C, 49.19; H, 4.91; N, 8.19.
Found: C, 48.83; H, 4.60; N, 8.04%.
(ii) p-back donation from [NHC ←− Pd(pyridine)X2] fragment
and
(iii) repulsive polarization (r)
The CDA calculations are performed using the program
AOMix,36 using the B3LYP/SDD, 6-31G* wave function. Molec-
ular orbital (MO) compositions and the overlap populations were
calculated using the AOMix program.36,47 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.48
Synthesis of trans-[1-(o-methoxybenzyl)-3-(t-butyl)imidazol-2-
ylidene]Pd(pyridine)Br2 4. A mixture of 1-(o-methoxybenzyl)-
3-(t-butyl)imidazolium bromide (0.381 g, 1.17 mmol), PdCl2
(0.229 g, 1.29 mmol) and K2CO3 (0.809 g, 5.85 mmol) were refluxed
in pyridine (ca. 5 mL) for 16 h. The reaction mixture was filtered
and the solvent was removed under vacuum. Then the residue was
washed with aqueous CuSO4 solution and the aqueous layer was
extracted with ethyl acetate (ca. 3 × 10 mL). Then the organic
layer was collected and the solvent was removed under vacuum to
obtain the product 4 as a yellow◦crystalline solid (0.291 g, 42%).
1H NMR (CDCl3, 400 MHz, 25 C): d 9.07 (d, 2H, 3JHH = 8 Hz,
General procedure for the Suzuki coupling reaction
In a typical run, a round bottom flask was charged with a mixture
of aryl halides (ArX, X = Br, I), phenylboronic acid and K2CO3
and diethyleneglycol–di-n-butyl ether (internal standard) in a
molar ratio of 1 : 1.2 : 1.5: 1 and to this mixture was added
precatalysts 2, 3 or 4 at varying mol% amounts (Tables 3 and
4). Acetonitrile (30 mL) was added to the reaction mixture and
refluxed for an appropriate period of time after which it was
filtered and the product was analyzed by gas chromatography
using diethyleneglycol–di-n-butyl ether as an internal standard.
o-NC5H5), 7.74 (t, 1H, 3JHH = 8 Hz, p-NC5H5), 7.61 (t, 2H, 3JHH
=
8 Hz, m-NC5H5), 7.34 (d, 1H, 3JHH = 8 Hz, o-C6H4), 7.29 (br, 1H,
NCHCHN), 7.02 (br, 1H, NCHCHN), 6.95 (t, 1H, 3JHH = 8 Hz,
m-C6H4), 6.89 (d, 1H, 3JHH = 8 Hz, m-C6H4), 6.78 (t, 1H, 3JHH
=
Conclusion
2
8 Hz, p-C6H4), 6.09 (d, 1H, JHH = 15 Hz, CH2), 6.03 (d, 1H,
2JHH = 15 Hz, CH2), 3.89 (s, 3H, OCH3), 2.10 (s, 9H, C(CH3)3).
In summary, a series of air stable, user-friendly and conveniently
synthesized Pd precatalysts 2, 3 and 4 trans-(NHC)Pd(pyridine)X2
(X = Cl, Br), stabilized over N/O-functionalized N-heterocyclic
carbenes and containing “throwaway” pyridine ligand have been
synthesized and structurally characterized. The weakly bound
nature of the pyridine moiety was further supported by density
functional theory studies as the estimated Pd–pyridine bond
dissociation energy [De (Pd–pyridine)] in the geometry optimized
complexes 2ꢀ (32.9 kcal mol−1), 3ꢀ (32.7 kcal mol−1) and 4ꢀ (30.8 kcal
mol−1) were found to be significantly lower than the (NHC)–Pd
bond dissociation energy [De (Pd–Ccarbene)] in the same 2ꢀ (81.9 kcal
mol−1), 3ꢀ (82.4 kcal mol−1) and 4ꢀ (77.6 kcal mol−1) complexes.
Furthermore, the NHCs were found to behave as strong r-donor
ligands with very little component of p-back-bonding from the
metal center to NHC in the palladium complexes. The catalysis
studies revealed that the 2, 3 and 4 complexes are excellent
precatalysts for Suzuki–Miyaura cross-coupling reactions at low
catalyst loadings (0.35 mol%) and exhibit not only the commonly
observed Csp2–Csp2 coupling but also the more challenging Csp3–Csp2
cross-coupling reactions.
13C{ H} NMR (CDCl3, 100 MHz, 25 ◦C): d 157.4 (NCN-Pd),
1
152.6 (o-NC5H5), 137.7 (p-NC5H5), 137.6 (OC6H4), 131.8 (ipso-
C6H4), 129.8 (o-C6H4), 124.4 (p-C6H4), 124.3 (m-C6H4), 120.9
(NCHCHN), 120.8 (m-NC5H5), 120.3 (NCHCHN), 110.4 (m-
C6H4), 58.6 (CH2), 55.3 (OCH3), 50.6 (C(CH3)3), 32.2 (C(CH3)3).
IR data (KBr) cm−1 3416 (s), 3137 (m), 2974 (m), 1603 (s), 1496
(m), 1463 (m), 1445 (m), 1400 (s), 1372 (m), 1348 (w), 1283 (w),
1253 (s), 1154 (w), 1118 (m), 1064 (w), 1047 (m), 1027 (s), 913 (w),
819 (w), 754 (s), 730 (m), 700 (m), 687 (s), 623 (m). Anal. Calcd. for
C20H26Br2N3OPd·1/2(NC5H5): C, 42.95; H, 4.41; N, 7.79. Found:
C, 43.50; H, 4.43; N, 8.54%.
Computational methods
The density functional theory calculations were performed on the
2, 3 and 4 complexes using GAUSSIAN 0340 suite of quantum
chemical programs. The Becke three parameter exchange func-
tional in conjunction with Lee–Yang–Parr correlation functional
(B3LYP) have been employed in this study.41,42 Stuttgart-Dresden
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Dalton Trans., 2007, 4546–4555 | 4553
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23 M. K. Samantaray, D. Roy, A. Patra, R. Stephen, M. Saikh, R. B. Sunoj
and P. Ghosh, J. Organomet. Chem., 2006, 691, 3797–3805.
24 A. Bondi, J. Phys. Chem., 1964, 68, 441–451.
Acknowledgements
We thank the Department of Science and Technology for financial
support of this research. We are grateful to the National Single
Crystal X-ray Diffraction Facility at IIT Bombay, India, for the
crystallographic results. P. G. thanks Professor R. B. Sunoj, IIT
Bombay, for helpful discussions. L. R. thanks IIT Bombay, India,
for a research fellowship. Computational facilities from the IIT
Bombay computer center are gratefully acknowledged.
25 J.-F. Huang, P.-Y. Chen, I.-W. Sun and S. P. Wang, Inorg. Chim. Acta,
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