Palladium complexes of the N-fused heterocycle derived abnormal N-heterocyclic carbenes for the much-preferred Cu-free and the amine-free Sonogashira coupling in air

Article abstract of DOI:10.1016/j.poly.2013.01.062

Abnormal N-heterocyclic carbenes of the N-fused heterocycles were employed in stabilizing a series of PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed complexes of the type (abnormal-NHC) PdI₂(pyridine). Specifically, imidazo[1,2-a]pyridine based abnormal complexes, trans-{(1-ethyl-2-(4-R-phenyl)-imidazol-3-ylidene[1,2-a]pyridine)} PdI₂(pyridine) [R = F (1b), Cl (2b), Br (3b) and Me (4b)] were synthesized from the reaction of corresponding imidazo[1,2-a]pyridinium iodide salts (1-4)a with PdCl₂ in pyridine in the presence of K ₂CO₃ as a base. The (1-4)b complexes carried out the Sonogashira couplings of the aryl bromide and iodide substrates with terminal alkynes in a mixed aqueous medium in air under the Cu-free and amine-free conditions. The density functional theory studies revealed that in the (1-4)b complexes, the C_carbene-Pd σ-bonding interaction was significantly stronger than the trans Pd-N_pyridine σ-bonding interaction.

Full text of DOI:10.1016/j.poly.2013.01.062

Polyhedron xxx (2013) xxx–xxx
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Palladium complexes of the N-fused heterocycle derived abnormal N-heterocyclic
carbenes for the much-preferred Cu-free and the amine-free Sonogashira coupling
in air
⇑
Alex John, Sudipta Modak, Mahesh Madasu, Madanakrishna Katari, Prasenjit Ghosh
Department of Chemistry, 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:
Available online xxxx
Abnormal N-heterocyclic carbenes of the N-fused heterocycles were employed in stabilizing a series of
PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed complexes of
the type (abnormal-NHC)PdI2(pyridine). Specifically, imidazo[1,2-a]pyridine based abnormal complexes,
trans-{(1-ethyl-2-(4-R-phenyl)-imidazol-3-ylidene[1,2-a]pyridine)}PdI2(pyridine) [R = F (1b), Cl (2b), Br
(3b) and Me (4b)] were synthesized from the reaction of corresponding imidazo[1,2-a]pyridinium iodide
salts (1–4)a with PdCl2 in pyridine in the presence of K2CO3 as a base. The (1–4)b complexes carried out
the Sonogashira couplings of the aryl bromide and iodide substrates with terminal alkynes in a mixed
aqueous medium in air under the Cu-free and amine-free conditions. The density functional theory stud-
Keywords:
Palladium
Abnormal N-heterocyclic carbene (NHC)
Sonogashira reaction
Cu-free
Amine-free
DFT studies
ies revealed that in the (1–4)b complexes, the Ccarbene–Pd
ger than the trans Pd–Npyridine -bonding interaction.
r-bonding interaction was significantly stron-
r
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
imidazo[1,2-a]pyridine derived abnormal N-heterocyclic carbene
complexes for utility in the C–C cross-coupling reaction [14]. Our
continued interest in the in vogue palladium mediated C–C
cross-coupling reactions [15] primarily arises from the fact that
these reactions provide a platform of different families of C–C bond
forming reactions that proceed via a common mechanistic path-
way. Hence, we theorized that rationally designing catalyst sys-
tems for one particular cross-coupling reactions, would possibly
also work good for other ones too by virtue of these reaction
undergoing a common mechanism. Since all of these cross-cou-
pling reactions involve an oxidative addition of aryl halide to a cat-
alytic metal center [16], we rationalized that electron rich catalysts
would more conveniently affect the same and hence we turned to
the abnormal N-heterocyclic carbene based systems, as these li-
gands are known to be more electron rich than the normal N-
heterocyclic carbenes. For the purpose, we focused on the now
popular ‘‘PEPPSI’’ (Pyridine Enhanced Precatalyst Preparation Sta-
bilization and Initiation) [17] complexes of imidazo[1,2-a]pyridine
derived abnormal N-heterocyclic carbene ligands in the Sonogashira
coupling reactions.
A key attribute of the N-heterocyclic carbenes (NHCs) [1],
responsible for its unprecedented success in the world of homoge-
neous catalysis [2], is its strong
make them good ‘‘spectator’’ ligands but also help prevent leaching
under catalysis conditions [3]. The variants of these ligands [4],
particularly the ones that are more electron rich and comparatively
r-donating nature that not only
better
r-donating, have attracted attention, and in the context of
which, the abnormal N-heterocyclic carbenes aroused interest late-
ly [5]. In the abnormal N-heterocyclic carbenes, the carbenic car-
bon centers are relatively more electron rich as a result of being
flanked by a single heteroatom as opposed to the two in the normal
N-heterocyclic carbene ligands [6]. Using IR studies, Crabtree [7]
recently demonstrated that the abnormal N-heterocyclic carbene
ligands are better
r-donors than the normal ones and not surpris-
ingly so, increasingly more reports are pouring in about the abnor-
mal N-heterocyclic carbenes outperforming their normal
counterparts in several catalytic transformations like in the alkene
hydrogenations [8], Suzuki–Miyuara and Heck couplings [9] and
the transfer hydrogenation reactions [10].
Expanding on our longstanding interest on the N-heterocyclic
carbenes [11] and their utility in chemical catalysis [12] and
biomedical applications [13], we sought to look into the
Sonogashira reaction is popular for its convenient straightfor-
ward access to the ‘‘arylyne’’ and ‘‘enyne’’ motifs ubiquitous in
numerous molecules of interest to the organic synthesis [18]. The
reaction involves a palladium mediated direct coupling of aryl or
vinyl halides with terminal alkynes in the presence of a base, usu-
ally amines, and copper as a cocatalyst. The traditional form of the
Sonogashira coupling requires stringent anaerobic conditions as
⇑
Corresponding author. Fax: +91 22 2572 3480.
E-mail address: pghosh@chem.iitb.ac.in (P. Ghosh).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.01.062
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A. John et al. / Polyhedron xxx (2013) xxx–xxx
the reaction proceeds via a formation of a copper acetylide species,
that is extremely air and moisture sensitive and leads to the Glaser
coupling under the aerobic or even slightly oxidizing environ-
ments. Hence, performing the coupling under the Cu-free condi-
tions represents a procedural improvement that paves way for
the coupling under more amenable aerobic conditions [19]. The
copper-free version of the Sonogashira reaction [20], that tradi-
tionally uses copper, was developed by Cassar [21] and Heck [22]
and is popularly known as the Heck alkynylation reaction
[23,24]. Furthermore, owing to environmental concerns usually
associated with the use of amines [25], developing both Cu- and
amine-free conditions for the Sonogashira coupling thus becomes
of prime importance [26]. In this regard, we have recently reported
the Cu-free and also the Cu- and amine-free Sonogashira couplings
using palladium complexes of the normal as well as abnormal
N-heterocyclic carbene ligands [27]. Continuing further with our
efforts on the imidazo[1,2-a]pyridine derived abnormal N-hetero-
cyclic carbene ligands, we set out to synthesize related systems
with different substituents at a distant phenyl ring of the ligand
in order to gauge their collective steric and electronic influences
on their catalytic activities.
Here in the manuscript, we report a series of PEPPSI themed
complexes (1–4)b of imidazo[1,2-a]pyridine derived abnormal
N-heterocyclic carbene ligands bearing substituents with varying
electron donating capacities at a distant phenyl group (Fig. 1).
The Ccarbene–Pd and Pd–Npyridine interactions in these complexes
have been probed with density functional theory studies. Lastly,
the utility of these complexes have been examined in the Cu-free
and the amine-free Sonogashira coupling of aryl halides substrates
with terminal alkynes (Eq. (1))
ð1Þ
2. Results and discussions
Continuing with our ongoing studies on the PEPPSI (Pyridine
Enhanced Precatalyst Preparation Stabilization and Initiation)
complexes of imidazo[1,2-a]pyridine based abnormal N-heterocy-
clic carbene ligands of the type, (abnormal-NHC)PdI2(pyridine), in
the copper-free and amine-free Sonogashira coupling reaction
[14], a series of less demanding variants with systematically varied
substituents at a distant phenyl ring of the ligand were thus
synthesized. Specifically, the alkylation reaction of the 2-(4-R-phe-
nyl)-imidazo[1,2-a]pyridines (R = F, Cl, Br and Me) with ethyl
iodide yielded the abnormal N-heterocyclic carbene precursors,
1-ethyl-2-(4-R-phenyl)-imidazo[1,2-a]pyridinium iodide [R = F
(1a), Cl (2a), Br (3a) and Me (4a)], in 62–76% yields (Scheme 1).
The (1–4)a compounds were identified by a characteristic proton
peak of the 3-position of the imidazo[1,2-a]pyridine ring appearing
highly downfield shifted at 8.64–8.91 ppm as compared to the
7.75–7.87 ppm of the starting 2-(4-R-phenyl)-imidazo[1,2-a]
pyridine (R = F, Cl, Br, and Me) substrates. Further confirmation
was obtained from the High Resolution Mass Spectrometric studies
that showed the respective molecular ion peaks at m/z, 241.1141
(1a), 257.0839 (2a), 301.0325 (3a), and 237.1391 (4a) in good
agreement with the calculated theoretical values within acceptable
0.1–5.0 ppm range of deviation.
The PEPPSI complexes (1–4)b were subsequently obtained by
the metallation reactions of the imidazo[1,2-a]pyridinium salts,
(1–4)a, with PdCl2 in pyridine in 44–88% in the presence of
K2CO3 as a base. As expected, upon metallation, the disappearance
of the characteristic proton peak of the 3-position of the imi-
dazo[1,2-a]pyridine ring was observed in the 1H NMR spectrum
of the (1–4)b complexes.
Concurring with PEPPSI themed complexes [17], the molecular
structure of (1–4)b, as determined by the X-ray diffraction studies,
exhibit a square-planar palladium center bound to an abnormal
N-heterocyclic carbene ligand and
a so-called ‘‘throwaway’’
pyridine ligand in a trans fashion, while the remaining two sites
are occupied by two iodine atoms (Fig. 2 and see Supporting Infor-
mation Figs. S1–S3 and Table S1). The Ccarbene–Pd distance of
1.967(4)–1.983(10) Å and the Pd–Npyridine distance of 2.084(9)–
2.130(3) Å in (1–4)b closely match with the corresponding
1.968(3)–1.984(8) Å and 2.100(2)–2.133(13) Å distances in the
closely related analogs [14], thereby suggesting that remote
Fig. 1. N-fused heterocycle derived abnormal N-heterocyclic carbene complexes of
Pd.
Scheme 1.
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A. John et al. / Polyhedron xxx (2013) xxx–xxx
3
(ꢀ0.13) as opposed to that in the (NHC)PdI2 fragment (ꢀ0.05) upon
coordination from the pyridine moiety in the (1–4)b complex.
The Charge Decomposition Analysis (CDA) studies yielded valu-
able insight on the strengths of the Ccarbene–Pd and the Pd–Npyridine
interactions through the estimations of the individual Ccarbene–Pd
(83.3–84.2 kcal/mol) and the Pd–Npyridine (28.6–29.5 kcal/mol)
bond dissociation energy (De) values in the (1–4)b complexes com-
puted at the B3LYP/SDD, 6-31G(d) level of theory (see Supporting
Information Table S11). As expected of a PEPPSI themed complex,
the Ccarbene–Pd bonding interactions were observed to be signifi-
cantly stronger than the Pd–Npyridine bonding interactions in the
(1–4)b complexes and are in agreement with the strong trans influ-
ence of the abnormal N-heterocyclic carbene ligands [29].
The molecular orbital (MO) correlation diagram of a particular
bonding interaction, constructed from the interaction of the indi-
vidual fragment molecular orbitals (FMOs), provides a vivid picture
of the concerned interaction. Such a molecular orbital (MO) corre-
lation diagram obtained for the Ccarbene–Pd
r-bonding interaction
from the individual NHC and the PdI2(pyridine) fragment orbitals
(FMOs) depicts the electron donation occurring from the carbene
lone pair of the abnormal NHC ligand fragment to the r⁄-orbital
of the Pd–Npyridine bond of the PdI2(pyridine) fragment in the (1–
4)b complexes (Figs.
3 and 4 and Supporting Information
Figs. S8–S13). Analogous molecular orbital (MO) correlation dia-
gram in case of the Pd–Npyridine interaction too showed similar
electron donation occurring from the nitrogen lone pair of the pyr-
idine fragment to the r⁄-orbital of the Ccarbene–Pd bond of the
(NHC)PdI2 fragment.
Corroborating our earlier observation [14], the (1–4)b com-
plexes efficiently carried out the Cu-free and the amine-free
Sonogashira coupling in the presence of Cs2CO3 as a base (Eq. (1)
and Table 1) and the conditions of which were arrived after a
detailed optimization study. The variation of stoichiometries
attempted for the representative substrates, iodobenzene and
phenylacetylene, as catalyzed by 2b, suggested that the optimum
yield was obtained at 2 equiv. of phenylacetylene with respect to
iodobenzene (see Supporting Information Table S12). The solvent
dependent studies showed that the good yields were observed in
a mixed media containing water (see Supporting Information
Table S13). The variation of catalyst concentration gave optimum
yield at 4 mol% of the catalyst loading (see Supporting Information
Table S14). Finally, the variation of different bases for the Sono-
gashira coupling gave the best results for the coupling reactions
using Cs2CO3 (see Supporting Information Table S15).
Fig. 2. Ortep of 1b. Selected bond lengths (Å) and angles (°): Pd1–C1 1.973(9), Pd1–
N3 2.115(7), Pd1–I2 2.6072(8), Pd1–I1 2.5936(8), C1–Pd1–N3 176.8(3), C1–Pd1–I1
88.8(2), N3–Pd1–I1 92.8(2), I1–Pd1–I2 172.57(3).
substitution at a distant 4-position of the phenyl ring of the abnor-
mal N-heterocyclic carbene ligand exert very little influence on its
geometrical parameters. In (1–4)b, while the Ccarbene–Pd distances
[1.967(4)–1.983(10) Å] compare well with the sum of the individ-
ual covalent radii of carbon and palladium (2.055 Å) [28], the Pd–
Npyridine distances [2.084(9)–2.130(3) Å] were significantly longer
than the sum of the individual covalent radii of nitrogen and palla-
dium (1.983 Å) [28]. This observation is in concurrence with the
‘‘throwaway’’ nature of the pyridine ligand in the PEPPSI themed
complexes arising out of the strong trans influence of the abnormal
N-heterocyclic carbene ligands [29].
The observed Ccarbene–Pd and Pd–Npyridine bond distances in (1–
4)b are direct manifestations of the individual interactions and
thus understanding the latter would provide valuable insights
about the former. Hence, the density functional theory studies
were undertaken on these (1–4)b complexes by computing the
electronic structures using the coordinates obtained from the
respective X-ray analysis. The post-wave function analysis using
Natural Bond Orbital method was performed for obtaining a de-
tailed understanding of the bonding interactions in the (1–4)b
complexes (see Supporting Information Figs. S4–S7 and Tables
Significantly enough, when a series of activated and non-
activated aryl halide substrates namely, 4-iodoacetophenone, iodo-
benzene, 4-iodotoluene, 4-iodoanisole, 4-bromoacetophenone, 4-
bromobenzaldehyde and 4-bromobenzonitrile were treated with
terminal alkynes namely, phenylacetylene and 4-methylphenyl-
acetylene, in the presence of the (1–4)b complexes, the Sonogash-
ira coupled ‘‘arylyne’’ products were obtained in moderate to
excellent yields (40–>99)%. Among the (1–4)b complexes, the
bromo derivative (3b) was the most active exhibiting (71–>99)%
conversion for all of the above-mentioned substrates, followed by
the chloro (2b) (50–>99)% and the methyl (4b) (48–>99)% deriva-
tives, which were mutually comparable and these were finally fol-
lowed by the fluoro derivative (1b) (40–>99)%. On the aryl halide
substrate front, the activated ones namely, 4-iodoacetophenone
substrate exhibited (>99)% conversions followed by the 4-bromo-
acetophenone showing (80–88)% conversions and the 4-bro-
mobenzonitrile showing (80–88)% conversions for all of the
(1–4)b precatalysts. No Sonogashira products were observed for
the coupling of aryl bromide and iodide substrates with a repre-
sentative alkynyl substrate namely, propargyl bromide, as cata-
lyzed by 2b (see Supporting Information Table S16). The aryl
chlorides did not yield any product under these reaction conditions
S2–S12). In particular, the Ccarbene–Pd
r-bonding interaction was
found to be significantly polar in character with donation of 68%
from the Ccarbene center and 32% from the Pd center in the (1–4)b
complexes (see Supporting Information Table S10). The Mulliken
charge analyses of the Ccarbene–Pd
r-bonding interaction reveal
that the NHC carbene-C becomes electron deficient (0.19) upon
binding to the metal center in (1–4)b as compared to the free
NHC ligand fragment (ꢀ0.12) concomitant with the metal center
becoming more electron rich (ꢀ0.13) relative to that in the PdI2
(pyridine) fragment (0.15) upon binding of the free-NHC ligand
fragment. Similarly, that of the Pd–Npyridine
r-bonding interaction
show that the metal center in (1–4)b becomes more electron rich
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A. John et al. / Polyhedron xxx (2013) xxx–xxx
0
HOMO
LUMO
-5
24%
10%
HOMO-5
5%
20%
33%
HOMO-15
HOMO-10
HOMO-12
-10
-15
Fig. 3. Simplified orbital interaction diagram showing major contribution to the NHC–palladium bonding orbital HOMO-15 in 1b.
-6
HOMO
LUMO
37%
4%
38%
HOMO-15
HOMO-12
HOMO-18
-9
15%
-12
Fig. 4. Simplified orbital interaction diagram showing major contribution to the Pd–NC5H5 bonding orbital HOMO-15 in 1b.
as observed for two representative activated aryl chloride sub-
strates namely, 4-chlorobenzaldehyde and 4-chlorobenzonitrile,
which catalyzed by 2b (see Supporting Information Table S17).
The non-activated aryl bromide substrates were found to be inef-
fective for the coupling reaction under these conditions (see Sup-
porting Information Table S18).
The evidence in favor of the homogeneity of the reaction was
obtained from the ‘‘Classical Hg-drop’’ test [30] performed on a
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A. John et al. / Polyhedron xxx (2013) xxx–xxx
5
Table 1
Selected results for Sonogashira cross-coupling reaction of aryl halides (ArX; X = I, Br) catalyzed by (1–4)b.
Entry
Reagent
Reagent
Cross-coupled product
Yielda
1b
2b
3b
4b
1
2
3
4
5
6
7
8
99
>99
99 (83)b
99
CH3CO
I
CH3CO
64
40
53
53
94
80
90
69
60
50
72
90
81
87
94 (89)b
76 (68)b
80 (68)b
71 (61)b
74 (61)b
88 (87)b
91 (78)b
93
48
54
91
50
81
81
I
I
CH3
I
H3C
H3CO
I
H3CO
CH3CO
OHC
NC
Br
CH3CO
OHC
NC
Br
Br
a
The yields (%) were determined by GC using diethyleneglycol-di-n-butylether as an internal standard. 1.00 mmol of aryl halides, 2.00 mmol of acetylene, 2.00 mmol of
Cs2CO3 and 4 mol% of catalyst (1–4)b in 10 mL of DMF:H2O (7:3 v/v) at 90 °C for 3 h.
b
Yields in the parentheses refer to isolated yields.
representative 1b precatalyst, that showed comparable yields with
or without the presence of mercury in the reaction mixture (see
Supporting Information Table S19). Significant amplifications of
the product yields of up to >99% were observed, when compared
to the blank runs, performed in the absence of any precatalyst,
and of up to 80% and 66% with respect to the control experiments
performed respectively with PdCl2 and (COD)PdCl2 under analo-
gous reaction conditions (see Supporting Information Table S19).
The catalytic activities of the (1–4)b complexes are comparable
to that of the only other known example of Cu-fee and amine-fee
Sonogashira coupling reported earlier by us [14] using related ana-
logs, and the results suggest that the substitution at the 4-position
of the phenyl ring in the (1–4)b complexes does not have any bear-
ing on the overall yields of the coupling reaction. Also, important is
the comparison of the catalytic activity of the (1–4)b complexes
with the ones reported in the literature. For example, a PEPPSI
substrate [34], while another PEPPSI themed chiral N-heterocyclic
carbene complex displayed superior activity and a broad substrate
scope in the coupling of bromo and iodo substrates with terminal
alkynes [19a]. The other notable examples that exhibits activity
similar to that of the (1–4)b complexes for the Sonogashira cou-
pling of aryl bromo and iodo substrates include the N-heterocyclic
carbene based systems derived from the imidazole [35] and tria-
zole [36] rings. Apart from the palladium N-heterocyclic carbene
complexes, the Sonogashira coupling has also been extensively
studied for bulky phosphine based ligands mostly under ‘‘Ligand
Assisted Catalysis’’ (LAC) conditions [37].
3. Conclusions
In summary, a series of imidazo[1,2-a]pyridine derived PEPPSI
themed (1–4)b complexes of abnormal N-heterocyclic carbene li-
gands have been synthesised. The computational studies under-
taken suggest the presence of a stronger Ccarbene–Pd interaction
than the Pd–Npyridine interaction on the (1–4)b complexes in tune
with a stronger trans influence of these abnormal N-heterocyclic
carbene ligands that aptly make the pyridine bound to the palla-
dium center a good ‘‘throwaway’’ ligand. These complexes effec-
tively perform the Sonogashira coupling under the much desired
Cu-free and amine-free conditions, exhibiting comparable activi-
ties to that of their related analogs.
themed
(IPr⁄)Pd(py)Cl2
(IPr⁄ = 1,3-bis[2,6-bis[(4-tert-butyl-
phenyl)methyl]-4-methylphenyl]imidazol-2-ylidene) type com-
plex showed much lower activity in the coupling of iodobenzene
with phenyl acetylene in comparison to that observed for the
(1–4)b complexes [31]. Quite remarkably, related imidazole and
triazole based PEPPSI complexes namely, (1,2,4-trimethyltriazol-
yl-2,4-diylidene)[PdCl2(pyridine)]2, trans-{(1,3-dimethylimidazol-
ylidene)}PdCl2(pyridine)} and trans-{(1,4-dimethyltriazolylidene)}
PdCl2(pyridine), have been successfully used for tandem Sonogash-
ira/cyclic hydroalkoxylation reaction for synthesizing a series of
benzofuran derivatives [32]. Several tris-palladium tripodal N-het-
erocyclic carbene complexes namely, (timtebtBu){Pd(ICy)I2}3, (tim-
4. Experimental
tebtBu){Pd(PPh3)I2}3 and (timtebdipp){Pd(PPh3)I2}3 [timtebtBu
=
1,3,5-{tris(tertbutylimidazol-2-ylidene)methyl}-2,4,6-triethylben-
zene, ICy = 1,3-dicyclohexylimidazol-2-ylidene, timtebdipp = 1,3,5-
{tris({2,6 diisopropylphenyl}imidazol-2-ylidene)methyl}-2,4,6-tri-
ethylbenzene] however showed much inferior activity in the Sono-
gashira coupling of bromo and iodo substrates with phenyl
acetylene as compared to the (1–4)b complexes [33]. More inter-
estingly, another trimetallic silver-palladium based N-heterocyclic
4.1. General procedures
All manipulations were carried out using standard Schlenk
techniques. Solvents were purified and degassed by standard pro-
cedures. 2-Aminopyridine, 4-fluoroacetophenone, 4-chloroaceto-
phenone, 4-bromoacetophenone and 4-methylacetophenone
were purchased from Spectrochem Chemicals, India and used
without any further purification. 1H and 13C{1H}, 19F NMR spectra
were recorded on a Varian and Bruker 400 MHz NMR spectrome-
ter. 1H NMR peaks are labeled as singlet (s), doublet (d), triplet
(t), quartet (q), doublet of doublets (dd), doublet of triplets (dt),
carbene complex, [AgPd2(g
3-C3H5)2(L2)2](PF6), [L2 = 3-((N-mesity-
limidazolylidenyl)methyl)-5-methylpyrazolate)], showed compa-
rable efficiency to that of the (1–4)b complexes in the
Sonogashira coupling of aryl bromides with the phenyl acetylene
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A. John et al. / Polyhedron xxx (2013) xxx–xxx
triplet of doublets (td), triplets of triplets (tt), multiplet (m). The 2-
(4-fluoro-phenyl)-imidazo[1,2-a]pyridine [38], 2-(4-chloro-phe-
nyl)-imidazo[1,2-a]pyridine [39], 2-(4-bromo-phenyl)-imidazo
[1,2-a]pyridine [40] and 2-(4-methyl-phenyl)-imidazo[1,2-a]pyri-
dine [41] were synthesized by modification of procedures reported
in literature. Mass spectrometry measurements were done on a
Micromass Q-Tof spectrometer. X-ray diffraction data were col-
lected on a Bruker P4 diffractometer equipped with a SMART
CCD detector and crystal data collection and refinement parame-
ters are summarized in the Supporting Information Table S1. 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 [42]. GC spectra were
obtained on a PerkinElmer Clarus 600 equipped with a FID. All
GC–MS analysis were done by Agilent 7890A GC system connected
with 5975C inert XL EI/CI MSD (with triple axis detector). Elemen-
tal Analysis was carried out on Thermo Quest FLASH 1112 SERIES
(CHNS) Elemental Analyzer.
(CH2CH3). 19F NMR (CDCl3, 376 MHz, 25 °C): ꢀ111.7 (s). IR Data
(KBr pellet) cmꢀ1: 3448 (w), 2982 (w), 1638 (w), 1601 (m), 1525
(s), 1491 (s), 1466 (w), 1445 (s), 1353 (w), 1223 (s), 1197 (m),
1156 (m), 1067 (w), 1014 (w), 846 (m), 791 (m), 749 (s), 698
(m), 518 (w). HRMS (ES): m/z 551.9565 [MꢀI]+, calcd 551.9564.
Anal. Calc. for C20H18FI2N3Pdꢁ0.5NC5H5: C, 37.58; H, 2.87; N, 6.82.
Found: C, 36.89; H, 2.45; N, 7.23%.
4.1.3. Synthesis of 1-ethyl-2-(4-chlorophenyl)-imidazo[1,2-
a]pyridinium iodide (2a)
A mixture of 2-(4-chlorophenyl)imidazo[1,2-a]pyridine (1.19 g,
5.20 mmol) and ethyl iodide (4.89 g, 31.3 mmol) was refluxed in
acetonitrile (ca. 25 mL) for 18 h, after which, the reaction mixture
was cooled to room temperature and the solvent was removed un-
der vacuum. The residue was further purified by dissolving in min-
imum amount of chloroform (ca. 5 mL) and precipitating out the
product by the addition of diethyl ether (ca. 30 mL). The precipitate
was isolated by filtration, vacuum dried and recrystallized from
hot methanol to obtain the product 2a as a yellow crystalline solid
(1.52 g, 76%). 1H NMR (CDCl3, 400 MHz, 25 °C): d 9.47 (d, 1H,
4.1.1. Synthesis of 1-ethyl-2-(4-fluorophenyl)-imidazo[1,2-
a]pyridinium iodide (1a)
3
3JHH = 7 Hz, C7H5N2), 8.91 (s, 1H, C7H5N2), 8.13 (d, 1H, JHH = 9 Hz,
A mixture of 2-(4-fluorophenyl)imidazo[1,2-a]pyridine (1.01 g,
4.76 mmol) and ethyl iodide (7.43 g, 47.6 mmol) was refluxed in
acetonitrile (ca. 25 mL) for 18 h, after which, the reaction mixture
was cooled to room temperature and the solvent was removed un-
der vacuum. Subsequent recrystallization from CH2Cl2 gave the
product 1a as a brown solid (1.31 g, 76%). 1H NMR (DMSO-d6,
3
3
C7H5N2), 8.00 (t, 1H, JHH = 8 Hz, C7H5N2), 7.59 (d, 2H, JHH
=
3
3
8 Hz, C6H4), 7.55 (d, 2H, JHH= 7 Hz, C6H4), 7.47 (t, 1H, JHH = 8 Hz,
C7H5N2), 4.54 (q, 2H, 3JHH = 7 Hz, NCH2CH3), 1.41 (t, 3H, 3JHH = 7 Hz,
NCH2CH3). 1H NMR (DMSO-d6, 400 MHz, 25 °C):
d 8.98 (d,
3
3
1H, JHH = 7 Hz, C7H5N2), 8.62 (s, 1H, C7H5N2), 8.37 (d, 1H, JHH
=
3
9 Hz, C7H5N2), 8.11 (t, 1H, JHH = 8 Hz, C7H5N2), 7.74 (s, 4H, C6H4),
3
400 MHz, 25 °C): d 9.02 (d, JHH = 7 Hz, 1H, C7H5N2), 8.64 (s, 1H,
7.63 (t, 1H, 3JHH = 7 Hz, C7H5N2), 4.44 (q, 2H, 3JHH = 7 Hz, NCH2CH3),
3
3
C7H5N2), 8.39 (d, 1H, JHH = 9 Hz, C7H5N2), 8.10 (t, 1H, JHH = 8 Hz,
1.26 (t, 3H, JHH = 7 Hz, NCH2CH3). 13C{1H} NMR (DMSO-d6,
3
3
C7H5N2), 7.77 (m, 2H, C6H4), 7.63 (t, 1H, JHH = 7 Hz, C7H5N2), 7.50
100 MHz, 25 °C): 138.9 (C7H5N2), 135.5 (C7H5N2), 135.3 (C6H4),
133.8 (C6H4), 131.6 (C6H4), 129.3 (C7H5N2), 129.3 (C6H4), 124.1
(C7H5N2), 117.6 (C7H5N2), 113.6 (C7H5N2), 111.5 (C7H5N2), 40.1
(NCH2CH3), 14.5 (NCH2CH3). HRMS (ES): m/z 257.0839 [NHC+H]+,
calcd 257.0846. IR Data (KBr pellet) cmꢀ1: 3448 (w), 3106 (w),
3020 (m), 1646 (m), 1586 (m), 1524 (s), 1484 (m), 1451
(m), 1404 (m), 1348 (m), 1311 (w), 1207 (w), 1174 (m), 1091
(m), 1009 (m), 850 (m), 818 (m), 765 (s), 566 (w), 499 (m). Anal.
Calc. for C15H14ClIN2: C, 46.84; H, 3.67; N, 7.28. Found: C, 46.21;
H, 3.30; N, 7.13%.
3
(m, 2H, C6H4), 4.43 (q, 2H, JHH = 7 Hz, NCH2CH3), 1.26 (t, 3H,
3JHH = 7 Hz, NCH2CH3). 13C{1H} NMR (DMSO-d6, 100 MHz, 25 °C):
1
163.3 (d, JCF = 247 Hz, C6H4), 138.9 (C7H5N2), 135.5 (C7H5N2),
133.8 (C7H5N2), 132.3 (d, JCF = 9 Hz, C6H4), 129.2 (C7H5N2), 121.7
3
4
2
(d, JCF = 3 Hz, C6H4), 117.6 (C7H5N2), 116.4 (d, JCF = 22 Hz, C6H4),
113.5 (C7H5N2), 111.4 (C7H5N2), 40.1 (NCH2CH3), 14.5 (NCH2CH3).
19F NMR (CDCl3, 376 MHz, 25 °C): -107.5 (d, J = 4 Hz). IR Data
(KBr pellet) cmꢀ1: 3440 (w), 3104 (w), 3028 (w), 1646 (m), 1586
(m), 1527 (m), 1498 (s), 1469 (w), 1450 (w), 1410 (w), 1382 (w),
1354 (w), 1311(w), 1227 (m), 1166 (m), 1098 (m), 1011 (w), 845
(m), 806 (w), 780 (m), 761 (s), 574 (w), 537 (w), 510 (w). HRMS
(ES): m/z 241.1141 [NHC+H]+, calcd 241.1141. Anal. Calc. for
C15H14FIN2ꢁ0.5CH2Cl2: C, 45.33; H, 3.68; N, 6.82. Found: C, 44.97;
H, 3.17; N, 7.65%.
4.1.4. Synthesis of trans-{(1-ethyl-2-(4-chlorophenyl)-imidazol-3-
ylidene[1,2-a]pyridine)}PdI2(pyridine) (2b)
A mixture of 2-(4-chlorophenyl)-1-ethylimidazo[1,2-a]pyridini-
um iodide (0.304 g, 0.790 mmol), PdCl2 (0.141 g, 0.796 mmol) and
K2CO3 (0.558 g, 4.03 mmol) was refluxed in pyridine (ca. 5 mL) for
16 h. The reaction mixture was cooled to room temperature, di-
luted with CHCl3 (ca. 40 mL) and subsequently washed with satu-
rated aqueous CuSO4 solution (ca. 2 ꢂ 50 mL). The organic layer
was separated and dried over anhydrous MgSO4 and filtered. The
filtrate was concentrated under vacuum to give the product 2b
as a brown solid (0.242 g, 88%). 1H NMR (CDCl3, 400 MHz, 25 °C):
9.36 (d, 1H, 3JHH = 7 Hz, C7H4N2), 8.96 (dt, 2H, 3JHH = 6 Hz, 4JHH = 2 -
4.1.2. Synthesis of trans-{(1-ethyl-2-(4-fluorophenyl)-imidazol-3-
ylidene[1,2-a]pyridine)}PdI2(pyridine) (1b)
A mixture of 2-(4-fluorophenyl)-1-ethylimidazo[1,2-a]pyridini-
um iodide (0.306 g, 0.830 mmol), PdCl2 (0.146 g, 0.820 mmol) and
K2CO3 (0.691 g, 4.99 mmol) was refluxed in pyridine (ca. 5 mL) for
16 h. The reaction mixture was cooled to room temperature, di-
luted with CHCl3 (ca. 40 mL) and subsequently washed with satu-
rated aqueous CuSO4 solution (ca. 2 ꢂ 50 mL). The organic layer
was separated, dried over anhydrous MgSO4 and then filtered.
The filtrate was concentrated under vacuum to give the product
1b as a brown solid (0.199 g, 70%). 1H NMR (CDCl3, 400 MHz,
3
3
Hz, o-NC5H5), 7.87 (d, 2H, JHH = 8 Hz, C6H4), 7.66 (tt, 1H, JHH = 8 -
Hz, 4JHH = 2 Hz, p-NC5H5), 7.61–7.54 (m, 3H, C7H4N2, C6H4), 7.38 (d,
1H, 3JHH = 9 Hz, C7H4N2), 7.27–7.21 (m, 3H, C7H4N2, m- NC5H5), 4.15
3
3
3
3
25 °C): 9.34 (d, 1H, JHH = 7 Hz, C7H4N2), 8.95 (d, 2H, JHH = 6 Hz,
(q, 2H, JHH = 7 Hz, CH2CH3), 1.32 (t, 3H, JHH = 7 Hz, CH2CH3).
13C{1H} NMR (CDCl3, 100 MHz, 25 °C): 153.8 (NC5H5), 140.8
(C7H4N2), 137.4 (NC5H5), 135.5 (C7H4N2), 133.8 (C6H4), 132.7
(C6H4), 130.2 (C7H4N2), 129.0 (C6H4), 128.9 (C6H4), 125.0 (C-Pd),
124.4 (NC5H5), 114.5 (C7H4N2), 111.2 (C7H4N2), 108.7 (C7H4N2),
40.3 (CH2), 15.3 (CH3). IR Data (KBr pellet) cmꢀ1: 3444 (w), 2983
(w), 1637 (w), 1601 (m), 1524 (s), 1479 (s), 1446 (s), 1398 (w),
1353 (w), 1197 (m), 1156 (m), 1094 (m), 1068 (m), 1013 (m),
935 (w), 838 (m), 798 (w), 748 (s), 728 (m), 696 (m), 508 (w).
HRMS (ES): m/z 567.9287 [MꢀI]+, calcd 567.9269. Anal. Calc. for
3
o-NC5H5), 7.89 (br, 2H, C6H4), 7.65 (t, 1H, JHH = 7 Hz, C7H4N2),
7.57 (t, 1H, JHH = 8 Hz, C7H4N2), 7.37 (d, 1H, JHH = 9 Hz, C7H4N2),
3
3
3
7.29–7.19 (m, 5H, m-NC5H5, C6H4), 4.13 (q, 2H, JHH = 7 Hz, CH2
3
CH3), 1.31 (t, 3H, JHH = 7 Hz, CH2CH3). 13C{1H} NMR (CDCl3,
1
100 MHz, 25 °C): 163.2 (d, JCF = 248 Hz, C6H4F), 153.7 (o-NC5H5),
140.6 (C7H4N2), 137.3 (p-NC5H5), 135.5 (C7H4N2), 133.6 (C7H4N2),
3
4
133.3 (d, JCF = 8 Hz, C6H4F), 129.9 (C-Pd), 126.3 (d, JCF = 3 Hz,
2
C6H4F), 124.2 (m-NC5H5), 115.7 (d, JCF = 21 Hz, C6H4F), 114.3
(C7H4N2), 110.6 (C7H4N2), 108.5 (C7H4N2), 40.0 (CH2CH3), 15.2
Please cite this article in press as: A. John et al., Polyhedron (2013), http://dx.doi.org/10.1016/j.poly.2013.01.062

A. John et al. / Polyhedron xxx (2013) xxx–xxx
7
3
C20H18ClI2N3Pdꢁ0.33NC5H5: C, 36.02; H, 2.74; N, 6.46. Found: C,
1H NMR (CDCl3, 400 MHz, 25 °C): d 9.53 (d, 1H, JHH = 7 Hz,
3
35.49; H, 1.98; N, 6.36%.
C7H5N2), 8.82 (s, 1H, C7H5N2), 8.33 (d, 1H, JHH = 9 Hz, C7H5N2),
3
8.02 (t, 1H, JHH = 8 Hz, C7H5N2), 7.46 (m, 3H, C7H5N2
&
3
3
4.1.5. Synthesis of 1-ethyl-2-(4-bromophenyl)-imidazo[1,2-
a]pyridinium iodide (3a)
C6H4), 7.36 (d, 2H, JHH = 8 Hz, C6H4), 4.57 (q, 2H, JHH = 7 Hz,
3
NCH2 CH3), 2.45 (s, 3H, CH3), 1.41 (t, 3H, JHH = 7 Hz, NCH2CH3).
A
mixture of
2-(4-bromophenyl)imidazo[1,2-a]pyridine
13C{1H} NMR (CDCl3, 100 MHz, 25 °C): 141.1 (C7H5N2), 138.5
(C7H5N2), 137.1 (C6H4), 133.8 (C6H4), 129.7 (C6H4), 129.5
(C7H5N2), 129.2 (C6H4), 121.3 (C7H5N2), 117.2 (C7H5N2), 112.9
(C7H5N2), 111.4 (C7H5N2), 40.8 (NCH2CH3), 21.1 (CH3), 14.9 (NCH2-
CH3). IR Data (KBr pellet) cmꢀ1: 3289 (w), 3087 (m), 3043 (m),
1644 (m), 1585 (m), 1525 (m), 1505 (m), 1467 (m), 1450
(m), 1376 (m), 1207 (w), 1181 (m), 1163 (m), 1114 (w), 1019
(m), 819 (m), 783 (m), 756 (s), 679 (w), 540 (w), 494 (w). HRMS
(ES): m/z 237.1391 [NHC+H]+, calcd 237.1392. Anal. Calc. for C16-
H17IN2ꢁ0.5CH3CN: C, 53.07; H, 4.85; N, 9.10. Found: C, 53.28; H,
5.19; N, 9.23%.
(0.772 g, 3.62 mmol) and ethyl iodide (3.01 g, 19.2 mmol) was re-
fluxed in acetonitrile (ca. 25 mL) for 18 h, after which, the reaction
mixture was cooled to room temperature and the solvent was re-
moved under vacuum. The residue was washed with diethyl ether
(ca. 20 mL) and recrystallized from hot methanol to obtain the
product 3a as yellow crystalline solid (0.757 g, 62%). 1H NMR
3
(DMSO-d6, 400 MHz, 25 °C): d 9.01 (d, 1H, JHH = 7 Hz, C7H5N2),
3
8.66 (s, 1H, C7H5N2), 8.39 (d, 1H, JHH = 9 Hz, C7H5N2), 8.11 (t, 1H,
3JHH = 8 Hz, C7H5N2), 7.86 (d, 2H, JHH = 8 Hz, C6H4), 7.66 (d,
3
3
3
2H, JHH = 8 Hz, C6H4), 7.63 (t, 1H, JHH = 6 Hz, C7H5N2), 4.45 (q,
3
3
2H, JHH = 7 Hz, NCH2CH3), 1.26 (t, 3H, JHH = 7 Hz, NCH2CH3).
13C{1H} NMR (DMSO-d6, 100 MHz, 25 °C): 139.0 (C7H5N2), 135.4
(C7H5N2), 133.9 (C6H4), 132.3 (C6H4), 131.8 (C6H4), 129.3
(C7H5N2), 124.5 (C6H4), 124.5 (C7H5N2), 117.7 (C7H5N2), 113.6
(C7H5N2), 111.5 (C7H5N2), 40.1 (NCH2CH3), 14.5 (NCH2CH3). IR Data
(KBr pellet) cmꢀ1: 3448 (w), 3109 (m), 3020 (m), 1646 (s), 1606
(m), 1563 (w), 1524 (s), 1484 (s), 1468 (s), 1402 (m), 1347 (m),
1312 (m), 1280 (w), 1206 (m), 1173 (m), 1158 (m), 1105 (m),
1069 (m), 1006 (m), 847 (m), 817 (m), 764 (s), 549 (w), 497 (m).
HRMS (ES): m/z 301.0325 [NHC+H]+, calcd 301.0340. Anal. Calc.
for C15H14IBrN2: C, 41.99; H, 3.29; N, 6.53. Found: C, 41.04; H,
3.18; N, 6.43%.
4.1.8. Synthesis of trans-{(1-ethyl-2-(4-tolyl)-imidazol-3-ylidene[1,2-
a]pyridine)}PdI2(pyridine) (4b)
A mixture of 2-(p-tolyl)-1-ethylimidazo[1,2-a]pyridinium io-
dide (0.319 g, 0.870 mmol), PdCl2 (0.154 g, 0.870 mmol) and
K2CO3 (0.566 g, 4.09 mmol) was refluxed in pyridine (ca. 5 mL)
for 16 h. The reaction mixture was cooled to room temperature, di-
luted with CHCl3 (ca. 40 mL) and subsequently washed with satu-
rated aqueous CuSO4 solution (ca. 2 ꢂ 50 mL). The organic layer
was separated and dried over anhydrous MgSO4 and filtered. The
filtrate was concentrated under vacuum to give the product 4b
as a brown solid (0.244 g, 83%). 1H NMR (CDCl3, 400 MHz, 25 °C):
3
3
d 9.37 (d, 1H, JHH = 8 Hz, C7H4N2), 8.98 (d, 2H, JHH = 6 Hz, o-
3
3
4.1.6. Synthesis of trans-{(1-ethyl-2-(4-bromophenyl)-imidazol-3-
ylidene[1,2-a]pyridine)}PdI2(pyridine) (3b)
NC5H5), 7.81 (d, 2H, JHH = 8 Hz, C6H4), 7.65 (t, 1H, JHH = 8 Hz, p-
NC5H5), 7.56 (t, 1H, JHH = 8 Hz, C7H4N2), 7.39–7.35 (m, 4H, C6H4,
& m-NC5H5), 7.26–7.19 (m, 2H, C7H4N2), 4.17 (q, 2H, JHH = 8 Hz,
NCH2CH3), 2.47 (s, 3H, CH3), 1.35 (t, 3H, JHH = 8 Hz, NCH2CH3).
3
3
A mixture of 2-(4-bromo-phenyl)-1-ethylimidazo[1,2-a]pyridi-
nium iodide (1.04 g, 2.42 mmol), PdCl2 (0.216 g, 1.21 mmol) and
K2CO3 (0.837 g, 6.05 mmol) was refluxed in pyridine (ca. 5 mL)
for 16 h. The reaction mixture was cooled to room temperature, di-
luted with CHCl3 (ca. 40 mL) and subsequently washed with satu-
rated aqueous CuSO4 solution (2 ꢂ 50 mL). The organic layer was
separated and dried over anhydrous Na2SO4 and filtered. The fil-
trate was concentrated under vacuum and further purified by col-
umn chromatography using silica gel as a stationary phase and
eluting with EtOAc:petroleum ether (v/v 1: 1) to give the product
3b as a brown solid (0.389 g, 44%). 1H NMR (CDCl3, 400 MHz,
3
13C{1H} NMR (CDCl3, 100 MHz, 25 °C): d 153.9 (o-NC5H5), 153.1
(C7H3N2), 140.7 (C7H3N2), 139.1 (C6H4), 137.3 (p-NC5H5), 136.8
(C6H4), 133.7 (C7H3N2), 131.3 (C6H4), 129.6 (C6H4), 129.4
(C7H3N2), 127.4 (C-Pd), 124.3 (m-NC5H5), 114.3 (C7H3N2), 108.6
(C7H3N2), 40.2 (NCH2CH3), 20.7 (CH3), 15.3 (NCH2CH3). IR Data
(KBr pellet) cmꢀ1: 1638 (m), 1601 (m), 1523 (s), 1493 (m), 1446
(s), 1354 (m), 1214 (m), 1196 (m), 1149 (m), 1070 (m), 1019 (m),
826 (m), 758 (s), 695 (m), 501 (m). HRMS (ES): m/z 468.9375
[MꢀIꢀNC5H5]+, calcd 468.9393. Anal. Calc. for C21H21I2N3-
Pdꢁ0.33NC5H5: C, 38.78; H, 3.25; N, 6.65. Found: C, 38.38; H,
3.17; N, 6.29%.
3
3
25 °C): 9.37 (d, 1H, JHH = 7 Hz, C7H4N2), 8.98 (d, 2H, JHH = 6 Hz,
3
3
o-NC5H5), 7.82 (d, 2H, JHH = 8 Hz, C6H4), 7.71 (d, 2H, JHH
=
8 Hz, C6H4), 7.67 (t, 1H, 3JHH = 8 Hz, p-NC5H5), 7.61 (t, 1H, 3JHH = 8 -
3
Hz, C7H4N2), 7.32 (d, 1H, JHH = 9 Hz, C7H4N2), 7.33–7.23 (m, 3H,
4.2. Computational methods
3
C7H4N2 & m-NC5H5), 4.17 (q, 2H, JHH = 7 Hz, CH2), 1.35 (t, 3H,
3JHH = 7 Hz, CH3). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): 153.9
(NC5H5), 140.9 (C7H4N2), 137.4 (NC5H5), 135.5 (C7H4N2), 133.9
(C6H4), 133.0 (C6H4), 131.9 (C6H4), 130.2 (C7H4N2), 129.4 (C6H4),
124.4 (NC5H5), 123.8 (C-Pd), 114.5 (C7H4N2), 111.3 (C7H4N2),
Density functional theory (DFT) calculations were performed on
the metal complexes (1–4)b using GAUSSIAN 09 [43] suite of quantum
chemical programs. The Becke three parameter exchange func-
tional in conjunction with Lee–Yang–Parr correlation functional
(B3LYP) has been employed in the study [44]. The polarized basis
set 6-31G(d) [45] was used to describe bromine, chlorine, fluorine,
carbon, nitrogen and hydrogen atoms. The Stuttgart–Dresden
effective core potential (ECP) along with valence basis sets (SDD)
was used for the palladium [46] and iodine atoms. Natural bond
orbital (NBO) analysis [47] was performed using NBO 3.1 program
implemented in the GAUSSIAN 09 package. Frequency calculations
were performed for all the optimized structures to characterize
the stationary points as minima.
108.6 (C7H4N2), 40.3 (CH2), 15.3 (CH3). IR Data (KBr pellet) cmꢀ1
:
3440 (m), 2962 (w), 2923 (m), 2852 (w), 1635 (w), 1599 (w),
1523 (m), 1465 (w), 1446 (s) 1259 (s), 1193 (w), 1155 (w), 1100
(s), 1073 (s), 1021 (s), 801 (s), 746 (m), 697 (w), 502 (w). HRMS
(ES): m/z 532.8350 [MꢀIꢀNC5H5]+, calcd 532.8342. Anal. Calc. for
C
20H18BrI2N3Pdꢁ0.5NC5H5: C, 34.64; H, 2.65; N, 6.28. Found: C,
34.21; H, 2.70; N, 6.30%.
4.1.7. Synthesis of 1-ethyl-2-(4-tolyl)-imidazo[1,2-a]pyridinium iodide
(4a)
The metal-ligand donor–acceptor interactions namely, (i)
(NHC)–PdI2(NC5H5) and (ii) (NHC)PdI2-NC5H5 were inspected by
using the Charge Decomposition Analysis (CDA) [48], which is a
valuable tool for analyzing the interactions between molecular
fragments on a quantitative basis with an emphasis on the electron
donation. [49]. The donor–acceptor interaction representing the
A
mixture of
2-p-tolylimidazo[1,2-a]pyridine (3.37 g,
16.2 mmol) and ethyl iodide (16.8 g, 107 mmol) was refluxed in
acetonitrile (ca. 25 mL) for 18 h, after which, the solvent was re-
moved under vacuum. The residue was washed with diethyl ether
(ca. 20 mL) to obtain the product 4a as a brown solid (4.02 g, 68%).
Please cite this article in press as: A. John et al., Polyhedron (2013), http://dx.doi.org/10.1016/j.poly.2013.01.062

8
A. John et al. / Polyhedron xxx (2013) xxx–xxx
(NHC)–PdI2(NC5H5) in (1–4)b were examined by using this tech-
nique. The orbital interaction between a donor NHC and an acceptor
fragment PdI2(NC5H5) in (1–4)b can be divided into three parts:
Center are gratefully acknowledged. S.M. thanks CSIR, New Delhi
and M.K. thanks BRNS, Mumbai for research fellowship.
Appendix A. Supplementary data
(i)
r
-donation as given by NHC ? PdI2(NC5H5) and designated
by (d),
-back donation as given by NHC PdI2(NC5H5) and desig-
nated by (b) and
CCDC 765841, 756064, 753977, and 756063 contain the supple-
mentary crystallographic data for 1b, 2b, 3b and 4b. These data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336
033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data
(1H NMR, 13C{1H} NMR, 19F NMR, IR, HRMS, elemental analysis,
CIF file giving X-ray crystallographic data of (1–4)b, the B3LYP
coordinates of the optimized geometries of (1–4)b, computational
data and the control experiment data) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.poly.2013.01.062.
(ii)
p
(iii) A repulsive interaction (r) between the occupied FMOs of
these two fragments.
Similarly, the donor–acceptor interaction representing the
(NHC)PdI2–NC5H5 in (1–4)b, were also examined by using this
technique. The orbital interaction between a donor C5H5N and an
acceptor fragment (NHC)PdI2 in (1–4)b can also be divided into
three parts:
(iv)
(v)
r
-donation as given by C5H5N ? (NHC)PdI2 and designated
by (d),
-back donation as given by C5H5N (NHC)PdI2 and desig-
nated by (b) and
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Acknowledgements
We thank Department of Science and Technology (Grant No:
SR/S1/IC-15/2011), New Delhi, for financial support of this re-
search. We are grateful to the National Single Crystal X-ray Diffrac-
tion Facility and Sophisticated Analytical Instrument Facility at IIT
Bombay, India, for the crystallographic and other characterization
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