Palladium complexes of abnormal N-heterocyclic carbenes as precatalysts for the much preferred Cu-free and amine-free Sonogashira coupling in air in a mixed-aqueous medium

Article abstract of DOI:10.1039/b913068c

A series of new PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed precatalysts of abnormal N-heterocyclic carbenes for the highly desirable Cu-free and amine-free Sonogashira coupling in air in a mixed-aqueous medium is reported. Specifically, the PEPPSI themed (NHC)PdI₂(pyridine) type precatalysts, 1b-4b, efficiently carried out the highly convenient Cu-free and amine-free Sonogashira coupling of aryl bromides and iodides with terminal acetylenes in air in a mixed aqueous medium. Complexes, 1b-4b, were synthesized by the direct reaction of the corresponding imidazo[1,2-a]pyridinium iodide salts, 1a-4a, with PdCl₂ in pyridine in the presence of K₂CO₃ as a base while the imidazo[1,2-a]pyridinium iodide salts, 1a-4a, were in turn synthesized by the alkylation reactions of the respective imidazo[1,2-a]pyridine derivatives with alkyl iodides. The density functional theory (DFT) studies revealed that these imidazol-3-ylidene[1,2-a]pyridine derived abnormal carbenes are strongly σ-donating and consequently significantly weaken the catalytically important labile trans pyridine ligand in 1b-4b.

Full text of DOI:10.1039/b913068c

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Palladium complexes of abnormal N-heterocyclic carbenes as precatalysts for
the much preferred Cu-free and amine-free Sonogashira coupling in air in a
mixed-aqueous medium†
Alex John,a Mobin M. Shaikhb and Prasenjit Ghosh*a
Received 1st July 2009, Accepted 24th September 2009
First published as an Advance Article on the web 29th October 2009
DOI: 10.1039/b913068c
A series of new PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation)
themed precatalysts of abnormal N-heterocyclic carbenes for the highly desirable Cu-free and
amine-free Sonogashira coupling in air in a mixed-aqueous medium is reported. Specifically, the
PEPPSI themed (NHC)PdI2(pyridine) type precatalysts, 1b–4b, efficiently carried out the highly
convenient Cu-free and amine-free Sonogashira coupling of aryl bromides and iodides with terminal
acetylenes in air in a mixed aqueous medium. Complexes, 1b–4b, were synthesized by the direct
reaction of the corresponding imidazo[1,2-a]pyridinium iodide salts, 1a–4a, with PdCl2 in pyridine in
the presence of K2CO3 as a base while the imidazo[1,2-a]pyridinium iodide salts, 1a–4a, were in turn
synthesized by the alkylation reactions of the respective imidazo[1,2-a]pyridine derivatives with alkyl
iodides. The density functional theory (DFT) studies revealed that these imidazol-3-ylidene[1,2-
a]pyridine derived abnormal carbenes are strongly s-donating and consequently significantly weaken
the catalytically important labile trans pyridine ligand in 1b–4b.
bulk quantities, especially as a basic medium for the cross-coupling
Introduction
reaction. Hence, a significant challenge faced at the moment lies
A direct and hence convenient access to the conjugated “enyne”
in developing the Sonogashira coupling under both Cu-free and
amine-free conditions.7 While the Cu-free condition would ensure
the coupling being tolerant to aerobic conditions, the amine-free
conditions would circumvent the toxicity issues associated with
the use of some of the amines.6
and “arylalkyne”skeletons ubiquitous inmanybiologically impor-
tant compounds1 as well as in engineered materials2 is provided
by the Sonogashira reaction.3 Being simplistic in approach, the
Sonogashira reaction offers straightforward coupling of the aryl
halides with terminal acetylenes catalyzed by palladium in the
presence of copper as a cocatalyst in a basic reaction medium.
However, despite its simplicity and new found popularity, several
issues persist that constrict the broad based utility of the reaction
like that of the requirement of stringent air and moisture free
conditions, as exposure to trace amounts of oxygen or an oxidizing
environment leads to the formation of unwanted homo-coupled
products by the so-called Glaser coupling.4 The air and moisture
sensitivity of the Sonogashira coupling arises from the formation
of a Cu–acetylide intermediate that is extremely sensitive to
aerobic conditions, potentially explosive and often yields un-
wanted Glaser-type homo-coupled products upon exposure to
air.5 Therefore, achieving Cu-free conditions that would provide
tolerance to aerobic conditions is a formidable challenge in
Sonogashira coupling reactions. However, an important limitation
for the Cu-free condition stems from the use of excess amines as
bases, some of which pose risk to human health6 when used in
Our interest lies in advancing the N-heterocyclic carbene
chemistry8 with emphasis on designing highly efficient robust
precatalysts for a variety of C–C bond forming reactions ranging
from the palladium mediated Suzuki–Miyaura,9 Sonogashira10
and Hiyama10b couplings to the nickel catalyzed base-free Michael
reaction.11 In this regard, we have recently reported a series of
palladium precatalysts of N/O-functionalized N-heterocyclic
carbenes for Sonogashira coupling under amine-free conditions
and observed that precatalysts with a more electron-rich metal
center show better activity.10c Continuing with our ongoing efforts
on the C–C cross-coupling reactions, we sought to undertake
the more challenging Sonogashira coupling under the highly
desirable Cu-free and amine-free conditions. In order to achieve
this objective, we were in search of a suitable strongly s-donating
ancillary ligand for designing our palladium precatalysts on the
basis of our earlier observation of electron-rich metal centers
displaying superior performances for the Suzuki–Miyaura9 and
Sonogashira coupling reactions.10c In this regard, the abnormal
N-heterocyclic carbenes (NHCs), particularly the imidazole
based ones, caught our attention owing to their greater s-do-
nating ability compared to their normal N-heterocyclic carbene
counterparts, which has been attributed to the carbene lone pair
being located adjacent to a single electronegative N atom in the
abnormal N-heterocyclic carbenes as opposed to being flanked by
two electronegative N atoms in case of the normal N-heterocyclic
carbenes.12 Further impetus for our study came from the fact
aDepartment of Chemistry, Indian Institute of Technology Bombay, Powai,
Mumbai 400 076, India. E-mail: pghosh@chem.iitb.ac.in; Fax: +91-22-
2572-3480; Tel: +91-22-2576-7178
bNational Single Crystal X-ray Diffraction Facility, Indian Institute of
Technology Bombay, Powai, Mumbai 400 076, India
† Electronic supplementary information (ESI) available: X-ray crystallo-
graphic data, B3LYP coordinates of the optimized geometries of 1b–4b,
computational data and the control experiment data. CCDC reference
numbers 687466, 688351, 698778 and 698779. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b913068c
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that very little is known about the catalytic exploits of abnormal
N-heterocyclic carbenes though normal N-heterocyclic carbenes
have been phenomenally successful in homogeneous catalysis.
Early indications of the promising catalytic potential of abnormal
N-heterocyclic carbenes are very evident in the handful of recent
reports that exist in the literature.13 For example, a palladium
complex bearing an abnormally bound dicarbene ligand
namely, {N,N-methylene-di-[(N¢-i-propyl-2-methyl)imidazol-4-
ylidene)]Pd(CH3CN)2}(BF4)2 exhibited a better performance for
the hydrogenation of cyclooctene than its normal dicarbene coun-
terpart namely, {N,N-methylene-di-[(N¢-i-propyl)imidazol-2-
ylidene)]Pd(CH3CN)2}(BF4)2.14 Similarly, another related abnor-
maldicarbene rhodium complexnamely, {N,N-methylene-di-[(N¢-
we have now decided to extend the study to abnormal carbenes
by setting out to design PEPPSI themed precatalysts stabilized by
imidazol-3-ylidene[1,2-a]pyridine derived abnormal carbenes.
A series of new abnormal imidazole based N-heterocyclic
carbene precursors namely the imidazo[1,2-a]pyridinium iodide
salts, 1a–4a, having different steric demands, were synthesized by
the direct alkylation reaction of the corresponding imidazo[1,2-
a]pyridines with alkyl iodides (Scheme 1). Quite interestingly, for
1a–4a, significant downfield shifts in the 1H NMR (0.88–1.12 ppm)
and in the 13C NMR (3.5–4.3 ppm) of the H-3 and C-3 resonances,
the site of future carbene formation in the metallation step, relative
to the starting imidazo[1,2-a]pyridine derivatives were observed in
accordance with the formation of the imidazo[1,2-a]pyridinium
iodide salts.
+
(n-butyl)-2-methyl)imidazol-4-ylidene)]RhI2(CH3CN)2} I-, effi-
ciently catalyzed the transfer hydrogenation of ketones using i-
PrOH as a dihydrogen source, while its normal dicarbene rhodium
counterpart namely, {N,N-methylene-di-[(N¢-(n-butyl)-imidazol-
+
2-ylidene)]RhI2(CH3CN)2} I- showed significantly subdued
activity.15 Also, a heteroleptic palladium complex namely, [N,N¢-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene][N,N¢-bis(2,4,6-tri-
methylphenyl)imidazol-4-ylidene]PdCl2 bearing both normally
and abnormally bound carbene moieties displayed enhanced
catalytic activities in the Suzuki–Miyaura as well as in the Heck
cross-coupling reactions compared to its homoleptic normally
bound bis-carbene counterpart, [N,N¢-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene]2PdCl2.16 Hence, in order to assess the true
potential of abnormal N-heterocyclic carbenes in catalysis,
particularly in the C–C cross-coupling reactions, we set
out to synthesize palladium based precatalysts of abnormal
N-heterocyclic carbenes for use in the coupling reactions.
Here, in this contribution we report a series of PEPPSI17 themed
(NHC)PdI2(pyridine) type precatalysts, 1b–4b, of abnormal
N-heterocyclic carbenes (Fig. 1) that efficiently carried out the
highly desirable Cu-free and amine-free Sonogashira coupling
in air in a mixed-aqueous medium. Furthermore, the density
functional theory studies indicate that the abnormal imidazole
based carbene ligands in 1b–4b bind strongly to the metal center
thereby weakening the binding of the metal bound labile pyridine
ligand in these complexes while facilitating the initiation of the
catalytic cycle.
Scheme 1 Synthesis of abnormal NHC ligand precursors, 1a–4a.
The reaction of 1a–4a with PdCl2 in pyridine in the
presence of K2CO3 yielded the much desired PEPPSI themed
(NHC)PdI2(pyridine) type precatalysts, 1b–4b (Scheme 2). The
characteristic palladium bound carbene (Pd–Ccarbene) resonances
appeared at 130.2–130.6 ppm in the respective 13C NMR spectra
and are in good agreement with similar resonances observed in
Fig. 1 PEPPSI themed precatalysts, 1b–4b, of abnormal NHC ligands.
Results and discussion
Expanding
on
our
efforts
on
PEPPSI
themed
(NHC)PdX2(pyridine) (X = halide) type precatalysts, which
have so far been concerned with the development of normal
N/O-functionalized N-heterocyclic carbene based systems,9c,10
Scheme 2 Synthesis of PEPPSI themed precatalysts, 1b–4b, of abnormal
NHC ligands.
10582 | Dalton Trans., 2009, 10581–10591
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Table 1 X-Ray crystallographic data for 1b–4b
Compound 1b 2b 3b
Monoclinic Monoclinic Triclinic
related palladium abnormal N-heterocyclic carbene complexes
namely,
{N,N-methylene-di-[(N¢-R-2-methyl)imidazol-4-ylid-
4b
ene)]PdI2 [R = Me (123.1 ppm), iPr (121.9 ppm) and 2,4,6-trime-
thylphenyl (123.5 ppm)]14 and {N,N-methylene-di-[(N¢-i-propyl-
2-methyl)imidazol-4-ylidene)]Pd(CH3CN)2}(BF4)2 (127.0 ppm).14
It is worth pointing out that the Pd–Ccarbene resonances in complexes
1b–4b were found to be significantly upfield (~20–50 ppm) in
comparison to PEPPSI themed complexes of the normal N-hete-
rocyclic carbene ligands like, trans-[1-benzyl-3-(N-t-butylacet-
amido)imidazol-2-ylidene]Pd(pyridine)Cl2 (153.3 ppm),9c trans-
[1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene]Pd(pyri-
dine)Cl2 (153.2 ppm)9c and trans-[1,3-bis(2,6-diisopropylphenyl)-
imidazolin-2-ylidene]PdCl2(pyridine) (186.3 ppm).10b
Lattice
Monoclinic
Formula
Formula
weight
C38H34Pd2N6I4 C20H19PdN3I2 C21H21PdN3I2 C21H21PdN3I2
1295.11
661.58
675.61
675.61
¯
P1
Space group Pn
P21/n
P21/n
˚
a/A
9.5091(13)
14.4540(18) 14.5426(3)
14.6883(14) 14.3889(3)
90.000
93.267(10)
90.000
2015.5(4)
2
10.1323(2)
9.4796(10)
10.0617(13)
12.5813(17)
80.090(11)
69.351(11)
74.071(10)
1076.0(2)
2
13.2678(14)
9.3838(11)
17.6375(19)
90.00
96.558(9)
90.00
2181.5(4)
4
150(2)
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
V/A
Z
T/K
90.00
92.569(2)
90.00
2118.07(7)
4
3
˚
Important to a PEPPSI themed (NHC)PdI2(pyridine) type
precatalyst is the palladium bound pyridine moiety, which func-
tions as a labile ligand by giving way to the incoming substrate,
150(2)
0.71073
150(2)
0.71073
150(2)
0.71073
Radiation
0.71073
˚
(l/A)
1
and appeared as distinct resonances in the H NMR spectrum
r(calcd)/
2.134
2.075
3.800
2.085
3.742
2.057
3.692
compared to that of the free pyridine.18 Indeed, the molecular
g cm-3
m(Mo
3.991
structures of 1b–4b revealed the presence of a palladium bound
Ka)/mm-1
˚
pyridine moiety with the Pd–Npyridine distances in 1b [2.133(13) A],
q max/◦
32.5835
32.4554
3718
32.4780
3767
33.1734
3822
˚
˚
˚
2b [2.100(2) A], 3b [2.100(7) A] and 4b [2.121(6) A] being slightly
No. of data 6084
No. of
parameters
R1
453
237
246
246
longer compared to the sum of the individual covalent radii of
19
˚
Pd and N (1.983 A) (Fig. 2 and Fig. S1–S3 and Table 1, ESI†).
0.0469
0.1187
1.047
0.0175
0.0392
1.034
0.0607
0.1847
1.114
0.0549
0.1364
1.084
The Pd–Npyridine distances in these abnormal carbene complexes, 1b
wR2
˚
˚
˚
˚
[2.133(13) A], 2b [2.100(2) A], 3b [2.100(7) A] and 4b [2.121(6) A]
are comparable to what is observed in the other PEPPSI
themed analogs of normal imidazole based carbenes namely,
GOF
˚
in these abnormal carbene complexes, 1b [2.133(13) A], 2b
[2.100(2) A], 3b [2.100(7) A] and 4b [2.121(6) A] are comparable
to that observed in the benchmark PEPPSI themed precatalyst
trans-[1-benzyl-3-(N-t-butylacetamido)imidazol-2-ylidene]Pd-
9c
˚
˚
˚
˚
(pyridine)Cl2 [2.089(3) A],
trans-[1-(2-hydroxy-cyclohexyl)-
9c
˚
3-(benzyl)imidazol-2-ylidene]Pd(pyridine)Cl2 [2.096(3) A] and
trans-[1-(o-methoxybenzyl)-3-(t-butyl)imidazol-2-ylidene]Pd(py-
ridine)Br2 [2.100(8) A], and is similar to the recent observation
namely, trans-[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]-
PdCl2(3-chloropyridine) [2.137(2) A], which may be attributed
9c
17g
˚
˚
that M–Ccarbene bond distances are not very sensitive to small
changes in bond order.13a Similarly, the Pd–Npyridine distances
to the presence of the electron withdrawing chlorine moiety
on the pyridine ring. Quite expectedly, owing to the stronger
trans-influence of the carbene ligand, the Pd–Npyridine distances
in complexes 1b–4b were found to be longer than those in
similar palladium complexes containing non-NHC ligands as
20
˚
in, [2,6-bis(2¢-indolyl)pyridine]Pd(pyridine) [2.040(3) A] and
-
[bis(bis(2-pyridylmethyl)amine-N,N¢,N¢¢)]Pd(pyridine)](ClO4 )2
[2.037(3) A].21 Indeed, the estimated Pd–Npyridine bond dissociation
˚
energy De/(Pd–pyridine) computed at B3LYP/SDD, 6-31G(d)
level of theory (Tables S1–S4, ESI†) for 1b (29.2 kcal mol-1), 2b
(28.9 kcal mol-1), 3b (28.5 kcal mol-1) and 4b (28.7 kcal mol-1)
complexes (Table S5, ESI†) were found to be slightly weaker than
that estimated for the related normal imidazole based carbene
complexes like, trans-[1-benzyl-3-(N-t-butylacetamido)imidazol-
2-ylidene]Pd(pyridine)Cl2 (32.9 kcal mol-1),9c trans-[1-(2-hy-
droxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene]Pd(pyridine)Cl2
(32.7 kcal mol-1)9c and trans-[1-(o-methoxybenzyl)-3-(t-butyl)imi-
dazol-2-ylidene]Pd(pyridine)Br2 (30.8 kcal mol-1)9c at the same
level of theory. Thus the comparatively weaker as well as elongated
Pd–Npyridine bond in the abnormal carbene complexes, 1b–4b,
as suggested by the calculations in comparison to the normal
imidazole based carbene counterparts suggest greater s-donating
tendency of the abnormal carbenes.
The greater s-donating ability of the abnormal N-heterocyclic
carbenes is corroborated by recent experimental as well as
theoretical studies. For example, in cis-(normal/abnormal-
NHC)Ir(CO)2Cl, the complexes containing abnormal imidazole
◦
˚
Fig. 2 ORTEP of 1b. Selected bond lengths (A) and angles ( ): Pd1–C1
1.980(19), Pd1–N3 2.133(13), Pd1–I2 2.5959(15), Pd1–I1 2.6073(14),
C1–Pd1–N3 173.1(6), C1–Pd1–I2 87.1 (5), N3–Pd1–I2 92.4(3).
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based N-heterocyclic carbenes like (1-methyl-2,5-diphenyl-3-i-
propylimidazol-4-ylidene)Ir(CO)2Cl (nav (CO) = 2003 cm-1),12c
(1-benzyl-3-methyl-imidazol-2-ylidene[1,2-a]pyridine)Ir(CO)2Cl
(nav (CO) = 1999 cm-1)12a and [1-(4-methoxybenzyl)-3-methyl-
at B3LYP/SDD, 6-31G(d) level of theory for complexes 1b
(83.9 kcal mol-1), 2b (83.9 kcal mol-1), 3b (75.2 kcal mol-1) and
4b (75.1 kcal mol-1) (Table S5, ESI†) were also comparable with
that estimated for these related normal imidazole based carbene
complexes like, trans-[1-benzyl-3-(N-t-butylacetamido)imidazol-
2-ylidene]Pd(pyridine)Cl2 (81.9 kcal mol-1),9c trans-[1-(2-hy-
droxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene]Pd(pyridine)Cl2
(82.4 kcal mol-1)9c and trans-[1-(o-methoxybenzyl)-3-(t-butyl)imi-
dazol-2-ylidene]Pd(pyridine)Br2 (77.6 kcal mol-1)9c at the same
level of theory.
Further insights into the nature of the NHC–Pd interaction
could be obtained from the molecular orbital correlation diagram
constructed from the individual fragment molecular orbitals
(FMOs) of the free NHC ligand fragment with the PdI2(NC5H5)
fragment using AOMix software23 (Fig. 3 and 4, and Fig. S4–
S13, ESI†). In this regard, the NHC–Pd s-bonding interactions
[1b (HOMO-15, HOMO-21), 2b (HOMO-15, HOMO-21), 3b
(HOMO-15, HOMO-22) and 4b (HOMO-15, HOMO-22)] in-
volving the interaction of the abnormal carbene lone pair of the
NHC ligand fragment with the s*-antibonding orbital of the Pd–
Npyridine bond of the PdI2(NC5H5) fragment are important, thereby
giving credence to the notion of strong s-donating ability of these
abnormal carbene ligands leading to significant weakening and
consequently also lengthening of the Pd–Npyridine bond in the 1b–4b
complexes. Another interesting aspect is the low lying NHC–Pd
s-bonding molecular orbitals suggesting the inert nature of the
NHC–Pd interaction in these complexes. In this regard it is worth
noting that unlike the normal as well as abnormal NHC–metal
interaction, other metal–carbene interactions e.g. the Schrock or
the Fischer carbenes are highly reactive towards both electrophiles
as well as nucleophiles.
Quite significantly, all of the 1b–4b complexes efficiently carried
out the Sonogashira coupling of aryl bromides and iodides with
terminal acetylenes in air in a mixed-aqueous medium under the
highly desirable Cu-free and amine-free conditions (eqn (1) and
Table 2). Specifically, when a mixture of aryl iodide and terminal
acetylene in the presence of 2 mol% of the precatalysts 1b–4b was
heated at 90 ◦C in air in a DMF–H2O (3 : 1 v/v) mixed medium,
a clean conversion to the cross-coupled product was observed
in a relatively short period of 1 h. A slightly different reaction
condition was, however, employed in case of the aryl bromide
substrates, for which a higher precatalyst loading of 4 mol% and
a longer reaction time of 3 h were used. In order to establish the
practical efficacy of the cross-coupling reaction, isolated yields
for the all the substrates used in the catalysis were obtained for a
representative palladium 1b precatalyst (Table 2). Consistent with
the stronger nature of the aryl bromide bond compared to the
aryl iodide bond,24,25 the Sonogashira couplings were observed
for only the activated aryl bromide substrates, while the same
proceeded cleanly for a variety of aryl iodide substrates bearing
electron donating or electron withdrawing substituents. However,
the palladium 1b–4b precatalysts were found to be ineffective
in coupling aryl chloride substrates (p-ClC6H4X, where X =
NO2, CHO, CF3 and COPh) and only trace amounts ca. 5–6%
of the cross-coupled product could be detected for the highly
activated p-ClC6H4NO2 substrate (Table 2). The presence of a
weakly bound and labile pyridine moiety in these precatalysts, 1b–
4b, as suggested from the DFT studies was further substantiated
by the detection of free pyridine in the reaction mixture under the
imidazol-2-ylidene[1,2-a]pyridine]Ir(CO)2Cl
(nav
(CO)
=
2008 cm-1)12a displayed lower CO stretching frequencies
when compared to normal imidazole based N-heterocyclic
carbenes like (1,3-dibutylimidazol-2-ylidene)Ir(CO)2Cl (nav
(CO) = 2020 cm-1),22 thereby suggesting that the abnormal
imidazole based N-heterocyclic carbenes are more strongly
s-donating than their normal counterparts and hence, result
in lowering of the CO stretching frequency. Further support in
favor of a more electron-rich carbenic center in the abnormal
imidazole based N-heterocyclic carbenes, relative to that of its
normal counterpart comes from density functional theory (DFT)
studies. Specifically, both the Mulliken and natural charges
at the carbenic carbon for the respective abnormal imidazole
based N-heterocyclic carbene, 1,3-dimethylimidazol-4-ylidene,
and the normal analogue, 1,3-dimethylimidazol-2-ylidene, when
computed at the B3LYP/6-31G(d) level of theory, showed that
the carbenic carbon in the abnormal carbene is more electron-rich
than the normal one (Tables S6–S8, ESI†).
Further measure of the greater s-donating ability of the
abnormal carbene ligand in complexes 1b–4b comes from the
Charge Decomposition Analysis (CDA) that estimates the ex-
s
tent of the s-donation
by d, and the p-back donation
, designated
NHC ⎯⎯→PdI2(NC5H5 )
n
, des-
NHC←⎯⎯ PdI2(NC5H5 )
ignated by b, occurring in these complexes. It is worth noting
that a higher d/b ratio was observed in the abnormal car-
bene 1b (4.47), 2b (4.62), 3b (4.61) and 4b (4.65) complexes
(Table S9, ESI†) compared to what is seen in the related
normal imidazole based carbene complexes like, trans-[1-benzyl-3-
(N-t-butylacetamido)imidazol-2-ylidene]Pd(pyridine)Cl2 (2.59),9c
trans-[1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene]-
Pd(pyridine)Cl2 (2.79)9c and trans-[1-(o-methoxybenzyl)-3-(t-
butyl)imidazol-2-ylidene]Pd(pyridine)Br2 (3.99)9c also obtained at
the same level of theory and is in concurrence with the greater
s-donating ability of the abnormal carbene in complexes 1b–4b.
Both the Mulliken and natural charge analysis showed a significant
increase of electron density at the palladium center as a result
of coordination form the free abnormal carbene ligand fragment
in complexes 1b–4b (Tables S10–S13, ESI†). A scrutiny of the
electronic configuration of the palladium center in complexes 1b–
4b, reveals that the electron donation from the abnormal carbene
lone pair occurs on the 5s orbital of palladium (Tables S14–S15,
ESI†). Furthermore, the hybridization at the Ccarbene in complexes
1b–4b was found to be of sp2 type while the Pd center had sd
character (Table S16, ESI†).
Analogous to the Pd–Npyridine bond in complexes 1b–4b, the
˚
˚
Pd–Ccarbene bond distances in 1b [1.980(19) A], 2b [1.968(3) A],
˚
˚
3b [1.984(8) A] and 4b [1.976(8) A] (Fig. 2 and Fig. S1–
S3, ESI†), are also comparable to those observed for the
related normal imidazole based carbene complexes namely,
trans-[1-benzyl-3-(N-t-butylacetamido)imidazol-2-ylidene]Pd-
9c
˚
(pyridine)Cl2 [1.957(3) A],
trans-[1-(2-hydroxy-cyclohexyl)-
9c
˚
A]
3-(benzyl)imidazol-2-ylidene]Pd(pyridine)Cl2
[1.967(4)
and trans-[1-(o-methoxybenzyl)-3-(t-butyl)imidazol-2-ylidene]-
9c
˚
Pd(pyridine)Br2 [1.953(9) A]. Along the same line, the Pd–
Ccarbene bond dissociation energy De/(Pd–Ccarbene) also computed
10584 | Dalton Trans., 2009, 10581–10591
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Fig. 3 Simplified orbital interaction diagram showing major contributions to the NHC-palladium bonding orbital HOMO-15 in 1b.
Sonogashira conditions. Specifically, liberation of up to 42% of free
pyridine was observed for the precatalysts, 1b–4b, as confirmed by
GC and GCMS studies. The homogeneous nature of catalysis
by the precatalysts, 1b–4b, was confirmed by carrying out the
“classical Hg-drop test” which showed no effect on the catalysis
results.26
(2-hydroxycyclohexyl)-3-(benzyl)imidazol-2-ylidene]Pd(pyri-
9c
dine)Cl2
and trans-[1-benzyl-3-(3,3-dimethyl-2-oxobutyl)imi-
dazol-2-ylidene]Pd(pyridine)Br2,10c showed substantial reaction
enhancement of up to 64% for the aryl iodide substrates and up to
69% for the aryl bromide substrates in the case of the abnormal car-
bene precatalysts (1b–4b). Quite significantly, the catalysis results
achieved with the palladium precatalysts, 1b–4b, were found to be
comparable to those obtained with the benchmark PEPPSI precat-
alyst, trans-[1,3-bis(2,6-diisoproylphenyl)imidazolin-2-ylidene](3-
17g
chloropyridine)PdCl2
(Table S17, ESI†).
under analogous reaction conditions
(1)
Important is the comparison of the performances of these ab-
normal carbene 1b–4b precatalysts with other reported examples
of Sonogashira coupling. In this regard it is worth noting that
most of the reports on Sonogashira coupling are of phosphine
based catalysts27 while that of N-heterocyclic carbene based
ones are surprisingly rare.28 Furthermore, of these examples,
many are reported under “Ligand Assisted Catalysis” (LAC)
conditions using simple palladium precursors with only a handful
of well-defined structurally characterized examples known to
date. Though popularly used, the “Ligand Assisted Catalysis”
(LAC) suffers from many disadvantages as for instance from very
little insight available on the catalytically active species resulting
in poor mechanistic understanding to batch-to-batch variations
of the reaction yields to requiring the use of a large excess
of expensive ligands. Hence, from this perspective the use of
well defined precatalysts like that of 1b–4b becomes significant.
With regard to the other structurally characterized precatalysts
A comparison with the control experiments carried out with
PdCl2 not only upholds the superiority of the precatalysts,
1b–4b, over such a simple metal precursor as a source of
palladium but also highlights the so-called “ligand-effect” of these
abnormal carbene based precatalysts in the Sonogashira coupling.
Specifically, under identical reaction conditions the precatalysts,
1b–4b, showed significant reaction enhancement over PdCl2 of up
to 71% and 41% for the aryl iodide and the aryl bromide substrates
respectively (Table S17, ESI†).
A comparison of the performance of the PEPPSI themed
precatalysts of abnormal carbenes (1b–4b) with that of nor-
mal carbenes, made by performing the Sonogashira coupling
under analogous conditions with two representative PEPPSI
themed precatalysts of normal carbenes namely, trans-[1-
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Fig. 4 Simplified orbital interaction diagram showing major contributions to the NHC-palladium bonding orbital HOMO-21 in 1b.
of N-heterocyclic carbenes for the Sonogashira coupling, the
aqueous medium in a relatively short reaction time of 1–3 h, and
also in its ability to perform the Sonogashira coupling under highly
desirable Cu-free and amine-free conditions.
trans-[(3-pyrrolidincarbamoyl-1-methylimidazolin-2-ylidene)(1-
29
methylimidazole)]PdI2 was used for aryl bromides and iodides
substrates while [1,1¢-dimethyl-3,3¢-methylene-4-diimidazolin-
30
2,2¢-diylidene]PdI2 was used under Cu-free conditions for
the aryl bromide substrates. Additionally, the {[3-butyl-
1-(1,10-phenanthroline-2-yl)imidazol-2-ylidene]PdCl}(PF6)31 ex-
hibited moderate to good conversions for the Cu-free Sonogashira
cross-coupling reaction in the presence of 1–2 mol% of a co-ligand,
PPh3. Quite interestingly, along similar lines, in the presence
of PPh3 as a co-ligand, the {[3-(2,4-dimethyl-1,8-naphthyrid-7-
Conclusions
In summary, a series of robust PEPPSI themed precatalysts, 1b–
4b, of abnormal carbenes for the highly convenient Sonogashira
coupling of aryl bromides and iodides with terminal acetylenes
in air in a mixed-aqueous medium under much challenging Cu-
free and amine-free conditions have been synthesized. The density
functional theory studies showed that the abnormal N-heterocylic
carbene ligand in the palladium precatalysts, 1b–4b, are strongly
s-donating and sufficiently destabilize the trans labile pyridine
ligand by weakening and lengthening the Pd–Npyridine bond in
these complexes. Complexes 1b–4b represent the first examples
of the use of abnormal N-heterocyclic carbene precatalysts for
Sonogashira coupling under the much desired Cu-free and amine-
free conditions.
32
yl)-1-picolylimidazol-2-ylidene]Pd}(PF6)2 precatalyst effectively
carried out a one-pot sequential Heck and Sonogashira cross-
couplings of aryl dihalides. Recently, we have also reported
well-defined palladium based precatalysts supported over N/O-
functionalized N-heterocyclic carbenes namely, trans-[1-(benzyl)-
10c
3-(3,3-dimethyl-2-oxo-butyl)imidazol-2-ylidene]2PdBr2 and cis-
[1-(benzyl)-3-(N-t-butylacetamido)imidazol-2-ylidene]2PdCl2,10c
for the Sonogashira coupling of aryl iodides in air in a mixed
aqueous medium under the highly desirable amine-free conditions.
Significantly enough, to our knowledge, complexes 1b–4b, repre-
sent the first examples of the use of abnormal carbene complexes
for the Sonogashira coupling in air in mixed-aqueous medium
and that too under the more challenging Cu-free and amine-free
conditions.
Experimental
General procedures
The most striking attributes of these 1b–4b precatalysts are their
robust nature and high efficiency in coupling in air in a mixed
All manipulations were carried out using standard Schlenk
techniques. Solvents were purified and degassed by standard
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Table 2 Selected results for Sonogashira cross-coupling reaction of aryl halides (ArX; X = I, Br, Cl) catalyzed by 1b–4b
Yielda
Time/h 1b
>99 (85)d >99 >99 >99
Entry Reagent
1b
Reagent
Cross-coupled product
2b
3b
4b
1
1
1
1
1
1
1
3
3
3
3
2b
84 (83)d
>99 93
91
3b
>99 (85)d 84
94
57
93
4b
64 (61)d
80
74
5b
>99 (77)d >99 90
>99
>99
>99
76
6b
94 (87)d
80 (78)d
99 (86)d
85
76
86
81
7b
8c
>99 78
9c
>99 (83)d 72
76
>99
68
10c
11c
>99 (89)d >99 82
6
6
5
6
a The yields (%) were determined by GC using diethyleneglycol-di-n-butyl ether as an internal standard. b 1.00 mmol of aryl iodide, 2.00 mmol of acetylene,
2.00 mmol of Cs2CO3 and 2 mol% of catalyst 1b–4b in 10 mL of DMF–H2O (7 : 3 v/v) at 90 ◦C for 1 h. c 1.00 mmol of aryl bromide or chloride, 2.00 mmol
of acetylene, 2.00 mmol of Cs2CO3 and 4 mol% of catalyst 1b–4b in 10 mL of DMF–H2O (7 : 3 v/v) at 90 ◦C for 3 h. d Yields in the parentheses refer to
isolated yields.
procedures. 2-Amino-3-picoline was purchased from Sigma
Aldrich, Germany, 2-amino-5-picoline, 2-amino-4-picoline and
2-aminopyridine were purchased from Spectrochem Chemicals,
India and used without any further purification. 1H and 13C
Analysis was carried out on Thermo Quest FLASH 1112 SERIES
(CHNS) Elemental Analyzer.
Synthesis of 1-methyl-2-phenyl-imidazo[1,2-a]pyridinium
iodide (1a)
1
{ H} NMR spectra were recorded on a Varian 400 MHz
NMR spectrometer. 2-Phenyl-imidazo[1,2-a]pyridine, 8-methyl-
2-phenyl-imidazo[1,2-a]pyridine, 6-methyl-2-phenyl-imidazo[1,2-
a]pyridine and 7-methyl-2-phenyl-imidazo[1,2-a]pyridine were
synthesized by modification of procedures reported in literature.12a
1H NMR peaks are labelled as a singlet (s), doublet (d) or triplet
(t). Mass spectrometry measurements were done on a Micromass
Q-Tof spectrometer. GC spectra were obtained on a PerkinElmer
Clarus 600 equipped with a FID. GCMS spectra were obtained on
a PerkinElmer Clarus 600 T equipped with an EI source. Elemental
A mixture of 2-phenyl-imidazo[1,2-a]pyridine (1.94 g, 9.99 mmol)
and CH3I (5 mL, 80.3 mmol) in CH3CN (ca. 25 mL) was stirred
at reflux for 24 h. The reaction mixture was cooled to room
temperature and then concentrated under vacuum. The residue
obtained was washed with hot hexane (ca. 3 ¥ 15 mL) to obtain
product 1a as a brown solid (2.42 g, 72%). Product 1a was
recrystallized from CH2Cl2. 1H NMR (CDCl3, 400 MHz, 25 ◦C):
3
d 9.41 (d, 1H, JHH = 8 Hz, o-C7H5N2), 8.86 (s, 1H, C7H5N2),
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3
3
8.24 (d, 1H, JHH = 8 Hz, m-C7H5N2), 7.98 (t, 1H, JHH
=
(NCH3), 18.5 (CH3). IR (KBr pellet) data cm-1: 3124(w), 2923(w),
1640(m), 1585(w), 1518(s), 1415(w), 1295(w), 1090(w), 790(m),
742(m), 699(m). HRMS (ES): m/z 223.1236 (NHC-ligand)+ Calcd
223.1235. Anal. Calcd for C15H15IN2·0.5CH2Cl2: C, 47.41; H, 4.11;
N, 7.13. Found: C, 46.81; H, 4.18; N, 6.82.
3
8 Hz, p-C7H5N2), 7.62 (d, 2H, JHH = 8 Hz, o-C6H5), 7.57–
3
7.53 (m, 3H, m-C6H5 and m-C7H5N2), 7.44 (t, 1H, JHH = 8 Hz,
p-C6H5), 4.10 (s, 3H, NCH3). 13C NMR (CDCl3, 100 MHz, 25 ◦C):
139.6 (C8a-C7H5N2), 137.8 (C2-C7H5N2), 133.9 (ipso-C6H5), 130.9
(p-C6H5), 129.8 (m-C6H5), 129.3 (o-C6H5), 129.3 (C5-C7H5N2),
124.4 (C7-C7H5N2), 117.5 (C8-C7H5N2), 112.9 (C6-C7H5N2), 111.8
(C3-C7H5N2), 33.5 (NCH3). IR (KBr pellet) data cm-1: 3091(m),
3021(m), 1773(m), 1719(m), 1651(m), 1535(s), 1489(m), 1446(m),
1427(m), 1372(m), 1308(w), 1264(w), 1223(w), 1181(m), 1151(m),
1025(m), 793(m), 762(s), 733(m), 696(s), 544(m), 478(w). HRMS
(ES): m/z 209.1074 (NHC-ligand)+ Calcd 209.1079. Anal. Calcd
for C14H13IN2: C, 50.02; H, 3.90; N, 8.33. Found: C, 50.00; H, 3.81;
N, 8.31.
Synthesis of trans-[{(1,8-dimethyl-2-phenyl-imidazol-3-
ylidene[1,2-a]pyridine)}Pd(pyridine)I2] (2b)
A mixture of 1,8-dimethyl-2-phenyl-imidazo[1,2-a]pyridinium io-
dide (0.154 g, 0.440 mmol), PdCl2 (0.078 g, 0.441 mmol) and
K2CO3 (0.338 g, 2.45 mmol) in pyridine (ca. 4 mL) was stirred
at reflux for 16 h. The reaction mixture was cooled to room
temperature, diluted with CHCl3 (ca. 20 mL) and then washed
with aq. CuSO4 solution (ca. 50 mL). The organic layer was
separated, dried over anhydrous MgSO4 and then concentrated
under vacuum to yield product 2b as a brown solid (0.081 g,
Synthesis of trans-[{(1-methyl-2-phenyl-imidazol-3-ylidene[1,2-
a]pyridine)}Pd(pyridine)I2] (1b)
56%). 1H NMR (CDCl3, 400 MHz, 25 ◦C): d 9.32 (d, 1H, 3JHH
=
3
A mixture of 1-methyl-2-phenyl-imidazo[1,2-a]pyridinium iodide
(0.271 g, 0.806 mmol), PdCl2 (0.143 g, 0.808 mmol) and K2CO3
(0.653 g, 4.73 mmol) in pyridine (ca. 5 mL) was stirred at reflux
for 16 h. The reaction mixture was cooled to room temperature,
diluted with CHCl3 (ca. 20 mL) and then washed with aq. CuSO4
solution (ca. 50 mL). The organic layer was separated, dried
over anhydrous MgSO4 and then concentrated under vacuum
8 Hz, o-C7H3N2), 8.98 (d, 2H, JHH = 8 Hz, o-NC5H5), 7.95 (d,
3
3
2H, JHH = 8 Hz, o-C6H5), 7.66 (t, 1H, JHH = 8 Hz, p-NC5H5),
3
3
7.59 (t, 2H, JHH = 8 Hz, m-NC5H5), 7.51 (t, 1H, JHH = 8 Hz,
p-C6H5), 7.29–7.24 (m, 3H, p-C7H3N2 and m-C6H5), 7.09 (t, 1H,
3JHH = 8 Hz, m-C7H3N2), 3.99 (s, 3H, NCH3), 2.79 (s, 3H, CH3).
13C NMR (CDCl3, 100 MHz, 25 ◦C): 153.8 (o-NC5H5), 141.5 (C8a-
C7H3N2), 138.1 (C2-C7H3N2), 137.3 (p-NC5H5), 132.2 (ipso-C6H5),
131.7 (m-C6H5), 131.6 (C7-C7H3N2 and C8-C7H3N2), 130.4 (Pd-
C), 129.1 (p-C6H5), 128.5 (o-C6H5), 124.3 (m-NC5H5), 120.6 (C5-
C7H3N2), 114.2 (C6-C7H3N2), 34.9 (NCH3), 19.3 (CH3). IR (KBr
pellet) data cm-1: 2922(m), 1601(m), 1511(m), 1484(w), 1445(m),
1409(w), 1253(w), 1212(w), 1123(w), 1070(w), 1018(m), 798(w),
759(m), 736(w), 697(s). Anal. Calcd for C20H19I2N3Pd·C5H5N:
C, 40.54; H, 3.27; N, 7.56. Found: C, 41.17; H, 2.91;
N, 7.74.
1
to yield product 1b as a brown solid (0.215 g, 83%). H NMR
(CDCl3, 400 MHz, 25 ◦C): d 9.40 (d, 1H, JHH = 8 Hz,
3
3
o-C7H4N2), 9.00 (d, 2H, JHH = 8 Hz, o-NC5H5), 8.00 (d, 2H,
3JHH = 8 Hz, o-C6H5), 7.67 (t, 1H, JHH = 8 Hz, p-NC5H5),
3
3
3
7.59 (t, 2H, JHH = 8 Hz, m-NC5H5), 7.51 (t, 1H, JHH = 8 Hz,
3
p-C6H5), 7.37 (d, 1H, JHH = 8 Hz, m-C7H4N2), 7.28–7.24 (m,
4H, m/p-C7H4N2 and m-C6H5), 3.79 (s, 3H, NCH3). 13C NMR
(CDCl3, 100 MHz, 25 ◦C): 153.9 (o-NC5H5), 141.6 (C8a-C7H4N2),
137.3 (C2-C7H4N2), 137.3 (p-NC5H5), 133.7 (ipso-C6H5), 131.4 (m-
C6H5), 130.2 (Pd-C), 129.9 (C5-C7H4N2 and C7-C7H4N2), 129.2
(p-C6H5), 128.6 (o-C6H5), 124.3 (m-NC5H5), 114.4 (C8-C7H4N2),
108.7 (C6-C7H4N2), 32.1 (NCH3). IR (KBr pellet) data cm-1:
2923(m), 1773(m), 1719(m), 1601(m), 1527(s), 1478(m), 1446(m),
1216(w), 1152(w), 1070(w), 1022(m), 789(w), 756(s), 697(s). Anal.
Calcd for C19H17I2N3Pd·0.5C5H5N: C, 37.58; H, 2.86; N, 7.13.
Found: C, 37.94; H, 2.57; N, 6.93.
Synthesis of 1-ethyl-6-methyl-2-phenyl-imidazo[1,2-a]pyridinium
iodide (3a)
A mixture of 6-methyl-2-phenyl-imidazo[1,2-a]pyridine (0.703 g,
3.38 mmol) and CH3CH2I (2 mL, 25.0 mmol) in CH3CN (ca.
20 mL) was stirred at reflux for 16 h. The reaction mixture
was cooled to room temperature and then concentrated under
vacuum. The residue obtained was washed with hot hexane (ca.
2 ¥ 15 mL) to obtain product 3a as a light brown solid (0.691 g,
56%). Product 3a was recrystallized from CH2Cl2. 1H NMR
(CDCl3, 400 MHz, 25 ◦C): d 9.27 (s, 1H, o-C7H4N2), 8.71 (s,
Synthesis of 1,8-dimethyl-2-phenyl-imidazo[1,2-a]pyridinium
iodide (2a)
3
A mixture of 8-methyl-2-phenyl-imidazo[1,2-a]pyridine (2.19 g,
10.5 mmol) and CH3I (5 mL, 80.3 mmol) in CH3CN (ca. 25 mL)
was stirred at reflux for 24 h. The reaction mixture was cooled
to room temperature and then concentrated under vacuum. The
residue obtained was washed with hot hexane (ca. 3 ¥ 15 mL) to
obtain product 2a as a brown solid (3.30 g, 90%). Product 2a was
recrystallized from CH2Cl2. 1H NMR (CDCl3, 400 MHz, 25 ◦C): d
9.13 (d, 1H, 3JHH = 8 Hz, o-C7H4N2), 8.64 (s, 1H, C7H4N2), 7.63–
7.61 (m, 2H, m/p-C7H4N2), 7.56–7.53 (m, 4H, o/m-C6H5), 7.17 (t,
1H, 3JHH = 8 Hz, p-C6H5), 4.22 (s, 3H, NCH3), 2.97 (s, 3H, CH3).
13C NMR (DMSO-d6, 100 MHz, 25 ◦C): 139.3 (C8a-C7H4N2),
137.7 (C2-C7H4N2), 134.3 (ipso-C6H5), 130.5 (p-C6H5), 130.0 (m-
C6H5), 129.2 (o-C6H5), 127.0 (C5-C7H4N2), 125.2 (C7-C7H4N2),
123.4 (C8-C7H4N2), 117.2 (C6-C7H4N2), 112.7 (C3-C7H4N2), 35.0
1H, C7H4N2), 8.16 (d, 1H, JHH = 8 Hz, m-C7H4N2), 7.81 (d,
3
1H, JHH = 8 Hz, p-C7H4N2), 7.57 (m, 5H, C6H5), 4.53 (q,
3
2H, JHH = 7 Hz, NCH2CH3), 2.48 (s, 3H, CH3), 1.39 (t, 3H,
◦
3JHH = 7 Hz, NCH2CH3). 13C NMR (CDCl3, 100 MHz, 25 C):
137.7 (C8a-C7H4N2), 137.1 (C2-C7H4N2), 136.9 (ipso-C6H5), 130.9
(p-C6H5), 129.8 (m-C6H5), 129.4 (o-C6H5), 128.0 (C5-C7H4N2),
127.5 (C6-C7H4N2), 124.7 (C7-C7H4N2), 113.1 (C8-C7H4N2), 110.9
(C3-C7H4N2), 41.0 (NCH2CH3), 17.9 (CH3), 15.1 (NCH2CH3).
IR (KBr pellet) data cm-1: 3062(m), 1648(m), 1526(s), 1489(w),
1470(w), 1446(w), 1384(w), 1183(m), 1123(w), 1024(w), 799(m),
771(s), 737(w), 704(m). HRMS (ES): m/z 237.1397 (NHC-
ligand)+ Calcd 237.1392. Anal. Calcd for C16H17IN2·0.75CH2Cl2:
C, 47.01; H, 4.36; N, 6.55. Found: C, 47.08; H, 4.87;
N, 5.97.
10588 | Dalton Trans., 2009, 10581–10591
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at reflux for 16 h. The reaction mixture was cooled to room
temperature, diluted with CHCl3 (ca. 20 mL) and then washed
with aq. CuSO4 solution (ca. 50 mL). The organic layer was
separated, dried with anhydrous MgSO4 and then concentrated
under vacuum to yield product 4b as a brown solid (0.116 g,
Synthesis of trans-[{(1-ethyl-6-methyl-2-phenyl-imidazol-3-
ylidene[1,2-a]pyridine)}Pd(pyridine)I2] (3b)
A mixture of 1-ethyl-6-methyl-2-phenyl-imidazo[1,2-a]pyridinium
iodide (0.155 g, 0.426 mmol), PdCl2 (0.070 g, 0.395 mmol) and
K2CO3 (0.306 g, 2.21 mmol) in pyridine (ca. 5 mL) was stirred
at reflux for 16 h. The reaction mixture was cooled to room
temperature, diluted with CHCl3 (ca. 20 mL) and then washed
with aq. CuSO4 solution (ca. 50 mL). The organic layer was
separated, dried with anhydrous MgSO4 and then concentrated
under vacuum to yield the product 3b as a brown solid (0.098 g,
68%). 1H NMR (CDCl3, 400 MHz, 25 ◦C): d 9.12 (s, 1H,
78%). 1H NMR (CDCl3, 400 MHz, 25 ◦C): d 9.24 (d, 1H, 3JHH
=
3
8 Hz, o-C7H3N2), 8.98 (d, 2H, JHH = 8 Hz, o-NC5H5), 7.92
3
3
(d, 2H, JHH = 8 Hz, o-C6H5), 7.65 (t, 1H, JHH = 8 Hz, p-
3
NC5H5), 7.57 (t, 2H, JHH = 8 Hz, m-NC5H5), 7.51 (t, 1H,
3JHH = 8 Hz, p-C6H5), 7.24 (t, 2H, JHH = 8 Hz, m-C6H5), 7.13
3
3
(s, 1H, m-C7H3N2), 7.04 (d, 1H, JHH = 8 Hz, m-C7H3N2), 4.14
3
(q, 2H, JHH = 8 Hz, NCH2CH3), 2.58 (s, 3H, CH3), 1.33 (t,
3
o-C7H3N2), 9.00 (d, 2H, JHH = 8 Hz, o-NC5H5), 7.91 (d, 2H,
3H, JHH = 8 Hz, NCH2CH3). 13C NMR (CDCl3, 100 MHz,
3
3JHH = 8 Hz, o-C6H5), 7.66 (t, 1H, 3JHH = 8 Hz, p-NC5H5), 7.58
25 ◦C): 153.8 (o-NC5H5), 142.1 (C8a-C7H3N2), 141.0 (C2-C7H3N2),
137.2 (p-NC5H5), 135.9 (ipso-C6H5), 132.9 (C7-C7H3N2), 131.3
(m-C6H5), 130.5 (Pd-C), 129.0 (p-C6H5), 128.5 (o-C6H5), 124.2 (m-
NC5H5 and C5-C7H3N2), 116.7 (C8-C7H3N2), 107.5 (C6-C7H3N2),
39.9 (NCH2CH3), 21.9 (CH3), 15.2 (NCH2CH3). IR (KBr pellet)
data cm-1: 2964(m), 2923(m), 1646(w), 1601(w), 1525(m), 1445(m),
1353(w), 1261(m), 1205(w), 1024(s), 801(s), 749(m), 697(s). Anal.
Calcd for C21H21I2N3Pd: C, 37.33; H, 3.13; N, 6.22. Found: C,
37.03; H, 2.76; N, 6.25.
3
3
(t, 2H, JHH = 8 Hz, m-NC5H5), 7.51 (t, 1H, JHH = 8 Hz, p-
C6H5), 7.41 (d, 1H, 3JHH = 8 Hz, m-C7H3N2), 7.27–7.24 (m, 3H,
3
p-C6H5 and p-C7H3N2), 4.17 (q, 2H, JHH = 8 Hz, NCH2CH3),
2.53 (s, 3H, CH3), 1.34 (t, 3H, JHH = 8 Hz, NCH2CH3). 13C
3
NMR (CDCl3, 100 MHz, 25 ◦C): 153.8 (o-NC5H5), 139.6 (C8a-
C7H3N2), 138.0 (C2-C7H3N2), 137.2 (p-NC5H5), 136.4 (ipso-C6H5),
132.6 (C7-C7H3N2), 131.4 (m-C6H5), 130.6 (Pd-C), 129.0 (p-C6H5),
128.5 (o-C6H5), 128.0 (C6-C7H3N2), 125.0 (C5-C7H3N2), 124.2
(m-NC5H5), 108.0 (C8-C7H3N2), 40.1 (NCH2CH3), 18.4 (CH3),
15.2 (NCH2CH3). IR (KBr pellet) data cm-1: 3789(m), 2921(m),
1525(m), 1446(m), 1262(m), 1196(m), 798(m), 772(m), 697(s).
Anal. Calcd for C21H21I2N3Pd: C, 37.33; H, 3.13; N, 6.22. Found:
C, 37.96; H, 3.16; N, 5.99.
Computational methods
Density functional theory calculations were performed on the
palladium 1b–4b complexes including their fragments using
GAUSSIAN 0333 suite of quantum chemical programs. The Becke
three parameter exchange functional in conjunction with Lee–
Yang–Parr correlation functional (B3LYP) has been employed in
the study.34 The Stuttgart–Dresden effective core potential (ECP),
representing 19 core electrons along with the valence basis sets
(SDD) is used for palladium and iodine atoms.35 All other atoms
are treated at the 6-31G(d) basis set.36 All stationary points are
characterized as minima by evaluating Hessian indices on the
respective potential energy surfaces. Tight SCF convergence (10-8
a.u.) was used for all calculations. Natural Bond Orbital (NBO)
analysis was performed using the NBO 3.137 program implemented
in the GAUSSIAN 03 package.
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
molecular fragments on a quantitative basis, with an emphasis
on electron donation.39 The orbital contributions in the geometry
optimized palladium(II) (NHC)PdI2(pyridine) 1b–4b complexes
can be divided into three parts:
Synthesis of 1-ethyl-7-methyl-2-phenyl-imidazo[1,2-a]pyridinium
iodide (4a)
A mixture of 7-methyl-2-phenyl-imidazo[1,2-a]pyridine (0.504 g,
2.42 mmol) and CH3CH2I (3 mL, 37.5 mmol) in CH3CN (ca.
15 mL) was stirred at reflux for 18 h. The reaction mixture
was cooled to room temperature and then concentrated under
vacuum. The residue obtained was washed with hot hexane
(ca. 2 ¥ 15 mL) to obtain product 4a as a light brown solid
1
(0.512 g, 58%). Product 4a was recrystallized from CH2Cl2. H
NMR (CDCl3, 400 MHz, 25 ◦C): 9.39 (d, 1H, JHH = 8 Hz,
3
o-C7H4N2), 8.75 (s, 1H, C7H4N2), 8.21 (s, 1H, m-C7H4N2), 7.58–
3
7.54 (m, 5H, C6H5), 7.26 (d, 1H, JHH = 8 Hz, m-C7H4N2), 4.53
(q, 2H, 3JHH = 7 Hz, NCH2CH3), 2.68 (s, 3H, CH3), 1.40 (t, 3H,
◦
3JHH = 7 Hz, NCH2CH3). 13C NMR (CDCl3, 100 MHz, 25 C):
146.9 (C8a-C7H4N2), 139.2 (C2-C7H4N2), 136.7 (ipso-C6H5), 130.8
(p-C6H5), 129.5 (m-C6H5), 129.3 (o-C6H5), 128.9 (C7-C7H4N2),
124.7 (C5-C7H4N2), 119.9 (C8-C7H4N2), 112.8 (C6-C7H4N2), 110.1
(C3-C7H4N2), 40.8 (NCH2CH3), 22.0 (CH3), 15.0 (NCH2CH3).
IR (KBr pellet) data cm-1: 3065(m), 1647(m), 1516(m), 1489(w),
1471(w), 1446(w), 1382(m), 1336(w), 1284(w), 1191(m), 1112(w),
806(m), 770(m), 743(m), 699(m), 601(w), 506(w). HRMS (ES):
m/z 237.1396 (NHC-ligand)+ Calcd 237.1392. Anal. Calcd for
C16H17IN2·CH2Cl2: C, 45.46; H, 4.26; N, 6.24. Found: C, 46.14; H,
3.98; N, 5.75.
(i) s-donation from the [NHC → PdI2(pyridine)] fragment
(ii) p-back donation from [NHC ← PdI2(pyridine)] fragment
and
(iii) repulsive polarization (r)
The CDA calculations are performed using the AOMix,23 using
the B3LYP/SDD, 6-31G(d) wave function. Molecular orbital
(MO) compositions and the overlap populations were calculated
using the AOMix program. The analysis of the MO compositions
in terms of occupied and unoccupied fragment orbitals (OFOs
and UFOs, respectively), construction of the orbital interaction
diagrams, and the charge decomposition analysis (CDA), was
performed using the AOMix-CDA.40
Synthesis of trans-[{(1-ethyl-7-methyl-2-phenyl-imidazol-3-
ylidene[1,2-a]pyridine)}Pd(pyridine)I2] (4b)
A mixture of 1-ethyl-7-methyl-2-phenyl-imidazo[1,2-a]pyridinium
iodide (0.160 g, 0.439 mmol), PdCl2 (0.067 g, 0.378 mmol) and
K2CO3 (0.316 g, 2.29 mmol) in pyridine (ca. 5 mL) was stirred
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 10581–10591 | 10589
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General procedure for the Sonogashira cross-coupling reaction of
aryl iodides
the supplementary crystallographic data for the palladium 1b–4b
complexes respectively.†
In a typical catalysis run, performed in air, a 25 mL vial was
charged with a mixture of the aryl iodide, terminal alkyne, Cs2CO3
and diethyleneglycol-di-n-butyl ether (internal standard) in the
molar ratio of 1 : 2 : 2 : 1. Palladium complexes 1b–4b (2 mol%)
were added to the mixture followed by the solvent (DMF–H2O,
3 : 1 v/v, 10 mL) and the reaction mixture was heated at 90 ◦C for
1 h, after which an aliquot was filtered and the product analyzed
by gas chromatography using diethyleneglycol-di-n-butyl ether as
an internal standard.
Acknowledgements
We thank DST, New Delhi, for financial support of this research.
We are grateful to the National Single Crystal X-ray Diffraction
Facility and Sophisticated Analytical Instrument Facility at IIT
Bombay, India, for the crystallographic and other characterization
data. Computational facilities from the IIT Bombay Computer
Center are gratefully acknowledged. AJ thanks CSIR, New Delhi
for research fellowship.
General procedure for the Sonogashira cross-coupling reaction of
aryl bromides and chlorides
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9c
2-ylidene]Pd(pyridine)Cl2
and (31%) trans-[1-benzyl-3-(3,3-
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