Rational design of ligand precursors to prepare abnormal (Mesoionic) and normal carbene complexes and zwitterionic CX-type palladacycles (X = C, N)

Article abstract of DOI:10.1002/ejic.201301465

The preparation of a series of ligand precursors that feature imidazo[1,2-a]pyridine and amido functional groups is reported. The reactions between the new ligand precursors, pyridine, and palladium acetate in DMF afforded monodentate Pd^II abno

Full text of DOI:10.1002/ejic.201301465

FULL PAPER
DOI:10.1002/ejic.201301465
Rational Design of Ligand Precursors to Prepare
Abnormal (Mesoionic) and Normal Carbene Complexes
and Zwitterionic CX-Type Palladacycles (X = C, N)
Shih-Ji Chen,[a] Yuan-Deng Lin,[a] Yen-Hsin Chiang,[a] and
Hon Man Lee*[a]
Keywords: Coordination chemistry / Carbenes / Palladium / Zwitterions / X-ray diffraction
The preparation of a series of ligand precursors that feature
imidazo[1,2-a]pyridine and amido functional groups is re-
ported. The reactions between the new ligand precursors,
atoms was obtained. The use of a similar ligand precursor
with an NH instead of an NMe group produced a zwitterionic
palladalactam complex. In contrast to the previous work in
pyridine, and palladium acetate in DMF afforded mono- the literature, owing to the rigidity of the fused heterocyclic
dentate PdII abnormal (mesoionic) carbene complexes. An
isomeric PdII normal carbene complex with a formally iden-
tical steric environment to that of the abnormal carbene
counterpart was also prepared. The isomeric pairs were char-
acterized by X-ray structural studies. Under different com-
plexation conditions, in which PdCl2 was employed as metal
precursor, a zwitterionic CC-type palladacycle formed by the
C–H activation at the ortho-N-phenyl and methylene carbon
ring, the palladalactam complex did not undergo intramo-
lecular proton transfer through the coordinated N atom.
Overall, we demonstrated that proper modification of the
structures of ligand precursors and complexation conditions
allowed us to obtain a full range of intriguing PdII complexes
including normal and abnormal carbene complexes, and
zwitterionic palladalactam and CC-type palladacycles by
means of A–H bond (A = N, C) activations.
intramolecular proton-transfer mechanism from 3a. Not-
ably, the ligand precursor 1a,b also consists of a C4 proton
on the heterocyclic ring, which could deprotonate to form
an abnormal carbene complex. Theoretical calculation,
however, showed that the formation of an abnormal carb-
ene complex derived from 1a was energetically unfavorable.
In this study, we redesigned ligand precursor 2; the major
difference from 1 is that it possesses an imidazo[1,2-a]pyr-
idine moiety instead of an imidazole moiety. Also, the C5
position on the imidazole ring is blocked with an aryl
group. Such modification in the structure results in a
change of coordination activity, including the successful
formation of abnormal carbene complexes. It is of great
interest to achieve a PdII abnormal carbene complex with
an amido functional group since PdII complexes with nor-
mal carbene analogues have been shown to be highly ef-
ficient in catalyzing classical and nonclassical C–C coupling
reactions.[8] For comparative purposes, we designed and
prepared 2Ј as well. It was a potential ligand precursor for
achieving an isomeric PdII normal carbene complex with a
formally identical steric environment around the metal cen-
ters to the abnormal and normal carbene complexes. Inter-
estingly, by varying the complexation conditions, a CC-type
palladacycle formed by C–H activation at the ortho-N-
phenyl and methylene carbon atoms was also obtained. In-
triguingly, we found that the aforementioned intramolecular
proton transfer was forbidden in the palladalactam complex
derived from the new ligand precursor. Also it is note-
Introduction
Since the isolation and crystallographic characterization
of the first free carbene by Arduengo et al.,[1] N-heterocy-
clic carbene (NHC) ligands have been extensively studied
and proven to be highly versatile in a wide range of catalytic
transformations.[2] Recently, it has been shown that imid-
azole-based NHC ligands could have an alternative binding
mode through the C(4/5) atom instead of the common C(2)
atom in the imidazolium ring.[3] Such “abnormal” (meso-
ionic) NHC ligands have been attracting attention,[4] be-
cause they are usually stronger electron donors than their
normal counterparts[3a,5] and thus exhibited better catalytic
activities in general,[5d,6] especially in reactions that involve
the activation of unreactive bonds.[5d,6a,6f] Previously, we
published the ligand precursor 1a,b (Scheme 1) that bears
amido and imidazole moieties, in which there were several
sites that could undergo C–H cleavage to generate bidentate
Pd complexes, including palladalactam 3a, the five- and six-
membered palladacycles 4a,b and the six-membered pallad-
acycle 5a, which resulted from N-phenyl metalation
(Scheme 2).[7] The formation of 4a involved an interesting
[a] Department of Chemistry,
National Changhua University of Education,
Changhua 50058, Taiwan, R.O.C.
E-mail: leehm@cc.ncue.edu.tw
http://chem.ncue.edu.tw/people/bio.php?PID=10
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301465.
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Scheme 1. Ligand precursors used in this work. Potential deprotonation sites are marked with asterisks.
Scheme 2. Range of PdII complexes derived from ligand precursors 1, 2, and 2Ј.
worthy to mention that the CC-type palladacycles and pal-
Results and Discussion
ladalactams are zwitterionic organometallates that feature
anionic palladium centers.[9] Whereas zwitterionic cationic
Preparation of Ligand Precursors
metal complexes are attracting growing interest for catalytic
applications and are common in the literature,[9a,9b] anionic
Ligand precursors 2a–d were prepared as shown in
metal complexes are relatively rare, and most of them have Scheme 3a. The intermediates 6a–d were prepared by the
been derived from phosphonium ylides or conjugated regioselective Pd-catalyzed direct C–H arylation between
equivalents.[9c] The imidazolium precursors in Scheme 1 imidazo[1,2-a]pyridine and aryl bromides.[10] Such a strat-
represent a new class of versatile ligand precursors for the egy allowed the easy tuning of the electronic property of
synthesis of zwitterionic anionic palladacycles. As also the ligands; in particular, the coupled aryl/phenyl rings are
shown in Scheme 2, since no non-zwitterionic connected the wingtip groups of the potentially formed abnormal
Lewis structure can be drawn by resonance of the π elec- carbene ligands.[11] Subsequent quaternization reactions of
trons, the CC-type palladacycles and palladalactam com- the imidazo[1,2-a]pyridine derivatives with 2-chloro-N-
plexes can be classified as “mesoionic” or “truly zwitter- methyl-N-phenylacetamide or 2-chloro-N-phenylacetamide
ionic”. The synthetic details and characterizations including led to the formation of 2a–d in pure form with 60–77%
single-crystal X-ray diffraction studies are presented.
yields. The CH protons on the heteroaromatic rings in 2a–
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Scheme 3. Synthesis of ligand precursors.
d that deprotonate resonate at δ ≈ 8.8 ppm, which is similar ernization reaction with 2-chloro-N-methyl-N-phenylacet-
to those in the related ligand precursors based on imid- amide (Scheme 3b). The NCHN proton that deprotonates
azo[1,2-a]pyridine.[11] Compounds 2a–c, which bear N- resonates downfield at δ = 11.36 ppm relative to those pro-
methyl groups, are potential ligand precursors for the prep- tons with signals at δ ≈ 8.8 ppm in 2a–c.
aration of monodentate abnormal carbene complexes. For
purposes of comparison, it is of interest to obtain the iso-
Preparation of PdII Abnormal (Mesoionic) and Normal
meric normal carbene complex. Thus, we also synthesized
Carbene Complexes
the ligand precursor 2cЈ. The preparation was similar to
that of 2c. Instead of the imidazo[1,2-a]pyridine derivative
Since compound 2a–c contain several C–H sites that can
6c, the benzoimidazole derivative 6cЈ was used in the quat- undergo deprotonation, we explored different PdII precur-
Scheme 4. Synthesis of PdII abnormal and normal carbene complexes.
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sors and complexation conditions to obtain novel PdII com- the signal was at δ = 134.9 ppm. Relevant trans-PdII abnor-
plexes. In view of the fact that the preparation of normal mal carbene/pyridine complexes were reported by us pre-
carbene Pd complexes by reaction of Pd(OAc)2 with imid- viously.[11]
azolium salt was routinely employed, we stirred a mixture
The isomeric normal carbene complex 7cЈ was obtained
of the ligand precursors 2a–c, pyridine, and Pd(OAc)2 in by the reaction of the ligand precursor 2cЈ with
DMF at ambient temperature with subsequent addition of [PdCl2(cod)] (cod = cyclooctadiene) in the presence of base
LiCl to the solution, which gave the monodentate PdII ab- in pyridine (Scheme 4b). Similar trans-PdII carbene/pyridine
normal carbene complexes 7a–c in pure form (Scheme 4a). complexes have been reported by others[2i,12] and us.[13] It is
Successful formation of 7 was indicated by the absence of noteworthy that 7c and 7cЈ are an isomeric pair that share
a proton signal at δ ≈ 8.8 ppm in its NMR spectrum with a formally identical steric environment around their metal
respect to that of the ligand precursors. These complexes centers.[11] The carbene signal in the normal carbene com-
were stable in air but decomposed readily in dichlorometh- plex was observed at δ = 164.0 ppm, which is more down-
ane. To locate the carbene carbon signals in these com- field to that at δ = 135.8 ppm in the abnormal counterpart.
plexes, we performed a heteronuclear multiple bond corre-
The pyridine ligands in complexes 7a–c were easily sub-
lation (HMBC) experiment on 7b, thereby confirming that stituted by PPh3 to form complexes 8a–c, which exhibited
Figure 1. Molecular structures of 7c (left) and 7cЈ (right) with 50% probability ellipsoids for non-hydrogen atoms. Hydrogen atoms are
omitted for clarity. Only one of the independent molecules in the structure of 7cЈ is shown.
Figure 2. Molecular structures of 8a (left) and 8c (right) with 50% probability ellipsoids for non-hydrogen atoms. Hydrogen atoms are
omitted for clarity.
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Figure 3. Schematic diagram viewing along the N–Pd–C axis illustrating the interplanar angles of least-squares mean planes in 7c (left)
and 7cЈ (right).
phosphine resonances at δ ≈ 28 ppm. Notably, these com- of abnormal carbene complex 7c was slightly longer than
plexes exhibit very poor solubility in common organic sol- that of its normal carbene counterpart [1.963(3) versus
vent, which prevented us from obtaining proper 13C NMR 1.951(3) Å]. The Pd–C bond length in 7c is comparable to
spectra for 8a and 8c. Even though the solubility of 8b was those of related trans pyridine/abnormal carbene com-
sufficient to obtain an adequate spectrum, the carbene sig- plexes.[11] It is slightly shorter than the average bond length
nal was not observed, which was probably due to its coinci- of 1.99(3) Å (seen in 41 crystallographically determined Pd
dence with the phenyl signal at δ ≈ 134 ppm. Nevertheless, complexes with abnormal carbene ligands).[4d] An impor-
a cis disposition of the carbene and phosphine ligands in 8 tant structural difference between the two structures is the
was subsequently confirmed by X-ray diffraction studies. orientation of the heterocyclic ring with respect to the coor-
Examples of cis-PPh3/carbene PdII complexes are known in dination plane (which contains PdCl2CN atoms). The in-
the literature.[14]
terplanar angle between the least-squares mean plane of the
Crystals of 7c,cЈ and 8a,c were grown, and their struc- imidazo[1,2-a]pyridine ring and that of the coordination
tures were established by X-ray diffraction studies (Fig- plane in 7c is 71.5(5)°, whereas the benzoimidazole and the
ures 1 and 2, and Table S1 in the Supporting Information). coordination plane in 7cЈ are orthogonal to each other
Selected bond lengths and angles are provided in Tables 1 [89.2(5)°] (Figure 3).
and 2. The Pd atoms in these structures adopt a distorted
square-planar coordination geometry. The trans geometry
of 7c,cЈ was confirmed, whereas 8a,c adopted a cis geome-
try. The structural analyses revealed that the Pd–C distance
Synthesis of Zwitterionic CX-Type Palladacycles
(X = C, N)
By applying different complexation conditions and heat-
ing of a mixture of 2c with PdCl2 in pyridine in the presence
of K2CO3 as base, new dimeric Pd complex 9c was obtained
Table 1. Selected bond lengths [Å] and angles [°] of 7c and 7cЈ.
7c
7cЈ
(Scheme 5) instead of the abnormal carbene complex. This
complex is a zwitterionic CC-type palladacycle that exhibits
an anionic palladium center. It was formed by the C–H acti-
vation at the ortho-N-phenyl and methylene carbon atoms,
as indicated by the 1H NMR spectrum of the reaction mix-
ture displaying the characteristic methine proton resonance
at δ = 5.94 ppm, which is slightly downfield relative to the
original methylene signal of the ligand precursor at δ =
5.29 ppm. By changing the complexation conditions, an ar-
ray of factors might contribute to such a drastic change in
the outcome of the reaction. However, we believe that the
difference in the solubility of the palladium precursors
could play a crucial role. Previously, we successfully ob-
tained the monomeric pyridine adduct 5a by stirring the
corresponding ligand precursor with PdCl2 in pyridine at
80 °C with K2CO3 as base. However, in the case of 2c,
attempts to obtain an analogous monomeric adduct yielded
an impure sample. Presumably, such a monomeric pyridine
adduct was highly unstable, and partial decomposition that
involved decoordination of the pyridine ligand occurred.
Pleasingly, subsequent stirring of the crude solid in THF at
50 °C afforded the pyridine-free dimeric complex 9c in pure
form. Treatment of 9c with PPh3 in dichloromethane af-
Pd1–C13
Pd1–N3
Pd1–Cl1
Pd1–Cl2
C13–Pd1–Cl1
C13–Pd1–Cl2
N3–Pd1–Cl1
N3–Pd1–Cl2
C13–Pd1 –N3
Cl1–Pd1–Cl2
1.963(3)
2.131(3)
2.2999(9)
2.3052(10)
86.65(9)
89.85(9)
91.48(9)
92.12(9)
175.70(12)
176.22(3)
Pd1–C40
Pd1–N4
Pd1–C11
Pd1–C12
C40–Pd1–Cl1
C40–Pd1–Cl2
N4–Pd1–Cl1
N4–Pd1–Cl2
C40–Pd1 –N4
Cl1–Pd1–Cl2
1.951(3)
2.103(2)
2.3059(9)
2.3136(8)
88.31(9)
87.44(9)
92.36(7)
91.91(7)
177.21(10)
175.71(3)
Table 2. Selected bond lengths [Å] and angles [°] of 8a, 8c, and 10c.
8a
8c
10c
Pd1–C
1.984(9)
2.346(2)
2.363(2)
2.243(2)
92.3(2)
87.5(2)
87.19(9)
2.001(8)
2.353(3)
2.361(3)
2.252(3)
92.2(2)
Pd1–C14
Pd1–C19
Pd1–Cl1
Pd1–P1
2.090(9)
1.995(12)
2.400(3)
2.328(3)
Pd–Cl1
Pd–Cl2
Pd–P
C–Pd–P
C–Pd–Cl2
Cl1–Pd–P
C14–Pd1–C19 79.8(5)
87.8(2)
C19–Pd1–P1
Cl1–Pd1–P1
95.6(4)
88.62(11)
87.80(9)
92.25(9)
179.1(3)
Cl1–Pd–Cl2 92.97(8)
C–Pd –Cl1 178.6(2)
Cl1–Pd1–C14 97.5(3)
C19–Pd1–Cl1 166.3(4)
P–Pd–Cl2
178.47(8) 178.59(10) C14–Pd1–P1
171.6(4)
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Scheme 5. Formation of zwitterionic palladacycle by means of ortho-N-phenyl C–H activation.
forded the monomeric phosphine complex 10c, which exhi-
bits a phosphine resonance at δ ≈ 25 ppm. The presence of
the CH proton signals at δ ≈ 8.8 ppm firmly excludes the
possibility of 10c and 9c being abnormal carbene com-
plexes. The resonance of the coordinated methine carbon
atom in 10c is observed at δ = 54.0 ppm as a doublet with
a coupling constant of 105.7 Hz, thus indicating that the
phosphine ligand is in the trans disposition. The methine
proton signal appears at δ = 5.98 ppm, which is also slightly
downfield relative to the methylene signal of its ligand pre-
cursor at δ = 5.29 ppm. The deshielding of the coordinated
CH resonances in 9c and 10c suggests a possible acidic
character of the CH units. However, attempts to depro-
tonate these protons with a variety of bases resulted in the
decomposition of the compounds. These complexes are
Figure 4. Molecular structures of 10c with 35% probability ellip-
soids for non-hydrogen atoms. Hydrogen atoms are omitted for
clarity.
stable in air and poorly dissolve in halogenated solvents.
The poor solubility is attributable to the zwitterionic nature
of these complexes. The structure of 10c was successfully
established by the X-ray diffraction analysis (Figure 4,
Table 2). It features a Pd atom in distorted square-planar tively], thus indicating the higher trans influence of the co-
coordination geometry with the PPh3 ligand trans to the ordinated C atom.
methine carbon atom. The bite angle of the bidentate ligand
When using the ligand precursor 2d that bears an NH
is 79.8(5)°. The Pd–C bond trans to the P atom [2.090(9) Å] instead of an NMe group on the amido moiety, the out-
is longer than that trans to the Cl atom [1.995(12) Å], which come of the complexation reaction was very different from
is consistent with the higher trans influence of the phos- that of 2c (Scheme 6). Instead of the formation of a CC-
phine ligand. Also, the Pd–P bond [2.328(3) Å] is longer type palladacycle with N-phenyl metalation such as 10c or
than those of in 8a and 8c [2.243(2) and 2.252(3) Å, respec- an abnormal carbene complex such as 7c, the CN-type pal-
Scheme 6. Synthesis of zwitterionic palladalactam.
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ladacycle, palladalactam complex 11d, was obtained. This new ligand precursors 2a–c, 2cЈ, and 2d. In contrast to 1,
compound is similar to 3a derived from ligand precursor complexation of 2a–c with Pd(OAc)2 led to the formation
1a.[7] Similar to 9c and 10c, compound 11d is also a zwitter- of the abnormal carbene PdII complex. An isomeric normal
ionic organometallic compound that features an anionic carbene PdII complex with a steric environment formally
palladium ion. The methine signal is observed at δ = identical to that of the abnormal carbene counterpart was
4.92 ppm, which, unlike in the case of 9c and 10c, is upfield also obtained from the ligand precursor 2cЈ. Instead of an
to the methylene signal in the ligand precursor 2d. A substi- abnormal carbene complex, a change in reaction conditions
tution reaction of this complex with PPh3 in dichlorometh- led to the formation of the zwitterionic CC-type palladacy-
ane afforded the zwitterionic phosphine complex 12d, which cle 10c from the ligand precursor 2c. Using the ligand pre-
shows a phosphine signal at δ = 29.0 ppm, similar to that cursor 2d, the zwitterionic palladalactam complex 11d was
at δ = 28.4 pm of a related compound.[7] The methine 13C formed. Overall, we demonstrated that proper modification
signal is observed as a singlet at δ = 39.0 ppm, which is of the ligand precursors and complexation conditions al-
consistent with the cis disposition of the PPh3 ligand. The lowed us to obtain a full range of intriguing PdII complexes
corresponding proton signal appears at δ = 4.87 ppm, including abnormal and normal carbene complexes, zwit-
which is also upfield to the methylene signal in the ligand terionic palladalactam and CC-type palladacycles by means
precursor. It is noteworthy that the coordinated methine of A–H bond (A = N, C) activations. Investigation of the
proton signals in the palladalactum compounds 3a, 11d, catalytic properties of these new PdII complexes is ongoing
and 12d that feature four-membered chelate rings appear at in our laboratory. Intriguingly, the chiral character of 9c,
more upfield positions (δ ≈ 4.6–4.9 ppm) with respect to the 10c, 11d, and 12d owing to the formation of metal-an-
methylene signals of their ligand precursors, whereas the chored sp3-carbon atoms could be explored for the develop-
corresponding proton signals for the CC-type palladacycles ment of pure chiral complexes, which might be applicable
5a, 9c, and 10c, which possess six-membered rings, are in asymmetric catalysis.
downfield shifted at δ ≈ 5.6–6.0 ppm (see above), thereby
reflecting the importance of the chelate ring size on the
Experimental Section
chemical shifts of the coordinated methine proton reso-
nances. Interestingly, the coordinated N atom in 3a could
serve as an internal base to promote the hydrogen atom
transfer of the methyl proton to the amido N atom to form
the CC-type palladacycle 4a.[7] Hydrogen atom transfer
General Considerations: All manipulations were performed under
dry nitrogen by using standard Schlenk techniques. Solvents were
dried by standard procedures. Starting chemicals were purchased
from commercial sources and used as received. H, 13C{1H}, and
1
from an sp2-carbon atom was also possible, thereby afford- 31P{1H} NMR spectra were recorded at 300.13, 75.48, and
121.49 MHz, respectively, with a Bruker AV-300 spectrometer. Ele-
mental analyses were carried out with a Thermo Flash 2000 CHN-
O elemental analyzer. High-resolution mass spectroscopy (HRMS)
was performed with a Finnigan/Thermo Quest MAT mass spec-
trometer at National Chung Hsing University (Taiwan). Infrared
spectra were acquired with a Varian Cary 640 infrared spectropho-
tometer. Imidazo[1,2-a]pyridine was prepared according to a litera-
ture procedure.[15] Compounds 6a–d and 6cЈ were prepared accord-
ing to literature procedures.[11] Since imidazolium derivatives 2a–d
and 2cЈ are hygroscopic, HRMS was performed instead of elemen-
tal analyses. The purity of all the compounds was confirmed by
their NMR spectra (see the Supporting Information).
ing the six-membered palladacycle 4b (see Scheme 2). How-
ever, in the case of 11d, most likely on account of the rigid-
ity of the fused heterocyclic ring, such internal proton
transfer did not occur. Noteworthy, complex 11d is soluble
in common organic solvents. But like the aforementioned
phosphine complex, 12d exhibits only poor solubility in ha-
logenated solvents, which could also be related to its zwit-
terionic nature.
Infrared Spectroscopy
The C=O stretching frequencies of the ligands in the nor-
mal and abnormal carbene complexes at approximately
1669 cm–1 are very similar, whereas those C=O bonds in the
zwitterionic complexes stretch at lower frequencies and fall
in a wider range of 1602–1621 cm–1. Interestingly, the C=O
stretching frequencies correlate well with their 13C NMR
spectroscopic chemical shifts (see the Supporting Infor-
mation). For example, in 7b and 11d, the greater deshielding
of the C=O nucleus in the latter complex (δ = 172.4 ppm in
11d versus δ = 165.5 ppm in 7b) reflects the reduced elec-
tron density about the C=O bond and hence a lower
stretching frequency in 11d (1602 versus 1668 cm–1 in 7b).
Synthesis of 2a: A mixture of 3-(4-fluorophenyl)imidazo[1,2-a]pyr-
idine (2.00 g, 9.40 mmol) and 2-chloro-N-methyl-N-phenylacet-
amide (1.73 g, 9.40 mmol) in THF (30 mL) was placed in a Schlenk
flask. The mixture was heated under reflux for 48 h. After cooling,
the white solid was collected on a frit, washed with THF, and dried
under vacuum. Yield: 2.80 g (77%). 1H NMR ([D6]DMSO): δ =
3.24 (s, 3 H, NCH3), 5.26 (s, 2 H, NCH2), 7.48–7.82 (m, 10 H, Ar
H), 8.12 (t, JH,H = 9.0 Hz, 1 H, Ar H), 8.31–8.44 (m, 2 H, Ar H,
imi H), 8.80 (d, JH,H = 6.0 Hz, 1 H, NCH) ppm. 13C{1H} NMR
([D6]DMSO): δ = 37.9 (NCH3), 49.5 (NCH2), 113.0, 117.2 (d, JC,F
= 21.8 Hz, CH), 118.4, 121.4, (d, JC,F = 3.7 Hz), 125.1, 125.2,
127.4, 128.1, 128.9, 130.4, 132.3 (d, JC,F = 9.0 Hz), 134.3, 140.6,
141.9, 163.5 (d, JC,F = 248 Hz), 164.7 (C=O) ppm. HRMS (ESI):
calcd. for C22H19ClFN3O [M – Cl]+ 360.1512; found 360.1517.
Synthesis of 2b: The compound was prepared by a procedure sim-
ilar to that of 2a. A mixture of 3-(4-methoxyphenyl)imidazo[1,2-a]-
Conclusion
On the basis of our previous works on the ligand precur-
pyridine (2.00 g, 8.90 mmol) and 2-chloro-N-methyl-N-phenylacet-
1
sors 1, we designed and investigated the reactivity of the amide (1.63 g, 8.90 mmol) was used. Yield: 2.0 g (60%). H NMR
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([D6]DMSO): δ = 3.24 (s, 3 H, NCH3), 3.86 (s, 3 H, OCH3), 5.29
9.0 Hz), 135.9 (Pd–C), 137.8, 141.5, 142.4, 151.1, 162.9 (d, JC,F
=
(s, 2 H, NCH2), 7.20 (d, JH,H = 9.0 Hz, 2 H, Ar–H), 7.48–7.71 (m, 248 Hz), 165.5 (C=O) ppm. IR (KBr): ν = 1670 (C=O) cm–1.
˜
8 H, Ar H), 8.10 (t, JH,H = 9.0 Hz, 1 H, Ar H), 8.35 (d, JH,H
=
Synthesis of 7b: The preparation was similar to that of 7a. Pd-
(OAc)2 (0.305 g, 1.36 mmol), 2b (0.555 g, 1.36 mmol), pyridine
(0.109 g, 0.136 mmol), and LiCl (0.114 g, 2.72 mmol) were used. A
pale yellow solid was obtained. Yield: 0.270 g (55%). M.p. 263.2–
264.5 °C. C28H26Cl2N4O2Pd (627.85): calcd. C 53.56, H 4.17, N
9.0 Hz, 1 H, Ar H), 8.42 (s, 1 H, imi H), 8.77 (d, JH,H = 9.0 Hz, 1
H, NCH) ppm. 13C{1H} NMR ([D6]DMSO): δ = 37.9 (NCH3),
49.5 (NCH2), 55.9 (OCH3), 113.0, 115.5, 116.9, 118.3, 124.4, 126.1,
127.2, 128.1, 128.9, 130.4, 131.2, 134.0, 140.5, 141.9, 161.1
(COCH3), 164.7 (C=O) ppm. HRMS (ESI): calcd. for
C23H22ClN3O2 [M – Cl]+ 372.1712; found 372.1717.
1
8.92; found C 53.85, H 4.19, N 8.99. H NMR (CDCl3): δ = 3.32
(s, 3 H, NCH3), 3.85 (s, 3 H, OCH3), 5.62 (s, 2 H, NCH2), 6.69–
7.07 (m, 3 H, Ar H), 7.26–7.41 (m, 6 H, Ar H), 7.55–7.64 (m, 3 H,
Ar–H), 7.74 (t, JH,H = 9.0 Hz, 1 H, Ar H), 7.93 (d, JH,H = 9.0 Hz,
Synthesis of 2c: The compound was prepared by a procedure sim-
ilar to that of 2a. A mixture of 3-phenylimidazo[1,2-a]pyridine
(3.40 g, 17.0 mmol) and 2-chloro-N-methyl-N-phenylacetamide
(3.21 g, 17.0 mmol) were used. Yield: 2.40 g (70%). 1H NMR ([D6]-
2 H, Ar H), 8.19 (d, JH,H = 9.0 Hz, 1 H, Ar H), 8.92 (d, JH,H
=
6.0 Hz, 2 H, py H) ppm. 13C{1H} NMR (CDCl3): δ = 38.0 (NCH3),
DMSO): δ = 3.25 (s, 3 H, NCH3), 5.29 (s, 2 H, NCH2), 7.48–7.75 50.9 (NCH2), 55.3 (OCH3), 110.7, 114.5, 114.8, 120.9, 122.8, 124.1,
(m, 11 H, Ar H), 8.12 (t, JH,H = 9.0 Hz, 1 H, Ar H), 8.35 (d, JH,H 125.9, 127.4, 127.8, 128.7, 130.0, 132.1, 134.9 (Pd–C), 137.6, 141.3,
= 9.0 Hz, 1 H, Ar H), 8.50 (s, 1 H, imi H), 8.85 (d, JH,H = 6.0 Hz, 142.5, 151.2, 159.9, 165.6 (C=O) ppm. IR (KBr): ν = 1668 (C=O)
˜
1 H, NCH) ppm. 13C{1H} NMR ([D6]DMSO): δ = 37.9 (NCH3), cm–1.
49.5 (NCH2), 113.1, 118.4, 125.0, 126.1, 127.3, 128.2, 128.9, 129.5,
Synthesis of 7c: The preparation was similar to that of 7a. Pd-
130.1, 130.4, 130.8, 134.2, 140.7, 141.9, 164.7 (C=O) ppm. HRMS
(OAc)2 (0.208 g, 0.930 mmol), 2c (0.35 g, 0.93 mmol), pyridine
(0.075 mL, 0.930 mmol), and LiCl (0.312 g, 7.45 mmol) were used.
A pale yellow solid was obtained. Yield: 0.30 g (54%). M.p. 218.6–
(ESI): calcd. for C22H20ClN3O [M – Cl]+ 342.1606; found 342.1610.
Synthesis of 2d: The compound was prepared by a procedure sim-
ilar to that of 2a. A mixture of 3-phenylimidazoimidazo[1,2-a]pyr-
220.3 °C. C27H24Cl2N4OPd (597.82): calcd. C 54.24, H 4.05, N
1
idine (3.20 g, 16.9 mmol) and 2-chloro-N-phenylacetamide (2.61 g, 9.37; found C 53.76, H 3.92, N 8.90. H NMR (CDCl3): δ = 3.33
16.9 mmol) were used. Yield: 4.40 g (74%). 1H NMR ([D6]DMSO):
δ = 5.61 (s, 2 H, NCH2), 7.05 (t, JH,H = 6.0 Hz, 1 H, Ar H), 7.30
(t, JH,H = 9.0 Hz, 2 H, Ar H), 7.53–7.75 (m, 8 H, Ar H), 8.09 (t,
(s, 3 H, NCH3), 5.65 (s, 2 H, NCH2), 7.00 (t, JH,H = 6.0 Hz, 1 H,
Ar H), 7.29–7.77 (m, 13 H, Ar H), 8.05 (d, JH,H = 6.0 Hz, 2 H, Ar
H), 8.27 (d, JH,H = 6.0 Hz, 1 H, Ar H), 8.91 (d, JH,H = 6.0 Hz, 2
JH,H = 6.0 Hz, 1 H, NCCHCH), 8.39 (d, JH,H = 9.0 Hz, 1 H, Ar H, py H) ppm. 13C{1H} NMR (CDCl3): δ = 38.0 (NCH3), 51.0
H), 8.58 (s, 1 H, imi H), 8.83 (d, JH,H = 6.0 Hz, 1 H, NCH), 11.17 (NCH2), 110.7, 114.9, 122.9, 124.1, 125.0, 126.1, 127.6, 127.8,
(s, 1 H, NH) ppm. 13C{1H} NMR ([D6]DMSO): δ = 50.2 (NCH2), 128.7, 128.9, 129.0, 130.1, 130.6, 135.8 (Pd–C), 137.6, 141.5, 142.9,
112.5, 118.4, 119.6, 124.2, 124.9, 125.1, 126.5, 127.5, 129.3, 129.7,
130.1, 130.9, 134.6, 139.0, 140.6, 164.1 (C=O) ppm. HRMS (ESI):
calcd. for C21H18ClN3O [M – H]+ 362.1060; found 362.1063.
151.2, 165.6 (C=O) ppm. IR (KBr): ν = 1670 (C=O) cm–1.
˜
Synthesis of 7cЈ: In a 20 mL Schlenk flask, [PdCl2(cod)] (0.088 g,
0.310 mmol), 2cЈ (0.117 g, 0.310 mmol), and K2CO3 (0.171 g,
1.24 mmol) were dissolved in dry pyridine (10 mL) under nitrogen.
The solution was stirred at 60 °C for 12 h. The solvent was com-
pletely removed under vacuum. The residue was redissolved in
dichloromethane and washed with water. The organic phase was
dried with anhydrous MgSO4, and the solvents were evaporated to
dryness under vacuum to give a solid. Diethyl ether was added,
Synthesis of 2cЈ: A mixture of 1-phenyl-1H-benzo[d]imidazole
(0.870 g, 4.47 mmol) and 2-chloro-N-methyl-N-phenylacetamide
(0.822 g, 4.47 mmol) in THF (30 mL) was placed in a Schlenk
flask. The mixture was heated under reflux for 48 h. After cooling,
the white solid was collected on a frit, washed with THF, and dried
1
under vacuum. Yield: 0.500 g (30%). H NMR (CDCl3): δ = 3.32
(s, 3 H, NCH3), 5.73 (s, 2 H, NCH2), 7.25–7.79 (m, 14 H, Ar H), and the yellowish solid formed was collected on a frit and dried
11.29 (s, 1 H, NCHN) ppm. 13C{1H} NMR (CDCl3): δ = 38.0
(NCH3), 48.9 (NCH2), 112.9, 113.5, 124.0, 124.8, 127.0, 127.5,
127.6, 128.1, 129.2, 130.1, 130.7, 130.8, 131.8, 132.9, 141.0, 144.0,
under vacuum. Yield: 0.083 g (45%). M.p. 250.1–250.5 °C.
C27H24Cl2N4OPd (597.82): calcd. C 54.24, H 4.05, N 9.37; found
C 54.29, H 4.25, N 8.96. 1H NMR (CDCl3): δ = 3.35 (s, 3 H,
163.9 (C=O) ppm. HRMS (ESI): calcd. for C22H20ClN3O NCH3), 5.58 (s, 2 H, NCH2), 7.28–7.63 (m, 14 H, Ar H), 7.75 (t,
[M – Cl]+ 342.1606; found 342.1604.
JH,H = 9.0 Hz, 1 H, Ar H), 7.94 (d, JH,H = 9.0 Hz, 2 H Ar H), 8.75
(d, JH,H = 3.0 Hz, 2 H Ar H) ppm. 13C{1H} NMR (CDCl3): δ =
37.9 (NCH3), 49.8 (NCH2), 110.9, 111.6, 123.5, 123.8, 124.4, 127.7,
127.9, 128.7, 129.3, 129.5, 130.1, 135.2, 135.4, 136.9, 138.1, 142.6,
Synthesis of 7a: Dry DMF (10 mL) was added to a 20 mL Schlenk
flask that contained Pd(OAc)2 (0.226 g, 1.01 mmol), 2a (0.4 g,
1.01 mmol), and pyridine (0.081 mL, 1.01 mmol). The mixture was
stirred at ambient temperature for 12 h. The solvent was completely
151.1, 164.0 (Pd–C), 165.6 (C=O) ppm. IR (KBr): ν = 1670 (C=O)
˜
cm–1.
removed under vacuum, and
a solution of LiCl (0.336 g,
8.08 mmol) in anhydrous ethanol (10 mL) was added to the brown
residue, which was stirred vigorously for 30 min. The precipitate
was washed with diethyl ether, filtered through a frit, and dried
under vacuum. An air-stable yellow solid was obtained. Yield:
0.36 g (56%). M.p. 198.1–200.2 °C. C27H23Cl2FN4OPd (615.81):
Synthesis of 8a: In a 20 mL Schlenk flask, 7a (0.20 g, 0.324 mmol)
and PPh3 (0.0851 g, 0.324 mmol) in dichloromethane (10 mL) were
stirred at ambient temperature overnight. The mixture was filtered
through silica, and the solvent was completely removed under vac-
uum. The residue was washed with THF in which the side product
trans-[PdCl2(PPh3)3] dissolved. The off-white solid was filtered
1
calcd. C 52.66, H 3.76, N 9.10; found C 52.19, H 3.72, N 8.79. H
NMR (CDCl3): δ = 3.32 (s, 3 H, NCH3), 5.62 (s, 2 H, NCH2), 7.01 through a frit and dried under vacuum. Yield: 0.14 g (55%). M.p.
(t, JH,H = 6.0 Hz, 1 H, Ar H), 7.19–7.75 (m, 12 H, Ar H), 7.99– 234.6–235.4 °C. C40H33Cl2FN3OPPd (799.00): calcd. C 60.13, H
1
8.04 (m, 2 H, Ar H), 8.16 (d, JH,H = 9.0 Hz, 1 H, Ar H), 8.90 (d, 4.16, N 5.26; found C 60.33, H 4.50, N 5.24. H NMR (CDCl3): δ
JH,H = 6.0 Hz, 2 H, py H) ppm. 13C{1H} NMR (CDCl3): δ = 38.0
(NCH3), 50.9 (NCH2), 110.8, 115.1, 116.1 (d, JC,F = 21.8 Hz),
= 3.08 (s, 3 H, NCH3), 5.16–5.60 (m, 2 H, NCH2), 7.05–7.72 (m,
27 H, Ar–H), 8.00 (d, JH,H = 9.0 Hz, 1 H, Ar H) ppm. 31P{1H}
122.6, 124.2, 124.8, 125.0, 127.7, 127.8, 130.0, 132.6 (d, JC,F
=
NMR (CDCl ): δ = 28.3 ppm. IR (KBr): ν = 1670 (C=O) cm–1.
˜
3
Eur. J. Inorg. Chem. 2014, 1492–1501
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPER
Synthesis of 8b: The preparation was similar to that of 8a. Complex
7b (0.200 g, 0.318 mmol) and PPh3 (0.0835 g, 0.318 mmol) were
used. An air-stable yellow solid was obtained. Yield: 0.15 g (53%).
The solution was stirred at ambient temperature for 12 h. The sol-
vent was completely removed under vacuum. The residue was redis-
solved in dichloromethane and washed with water. The organic
M.p. 235.0–236.3 °C. C41H36Cl2N3O2PPd (811.03): calcd. C 60.72, phase was dried with anhydrous MgSO4, and the solvents were
H 4.47, N 5.18; found C 60.24, H 3.99, N 4.83. 1H NMR (CDCl3):
δ = 3.05 (s, 3 H, NCH3), 3.86 (s, 3 H, OCH3), 5.23–5.41 (m, 2 H,
NCH2), 6.87–7.93 (m, 28 H, Ar H) ppm. 13C{1H} NMR (CDCl3):
evaporated to dryness under vacuum to give a solid. Diethyl ether
was added, and the yellowish solid that had formed was collected
on a frit, washed with more diethyl ether and methanol, and dried
δ = 37.8 (NCH3), 51.1 (NCH2), 55.5 (OCH3), 110.2, 114.0, 115.2, under vacuum. A yellowish solid was obtained. Yield: 0.26 g (45%).
120.0, 122.6, 125.8 (d, JP,C = 6.8 Hz), 127.6 (d, JP,C = 10.5 Hz),
127.9, 128.1, 128.8, 129.8, 130.2, 130.5, 130.8, 131.6, 134.2 (d, JP,C
= 11.3 Hz), 140.8, 141.9, 149.1, 165.0 (C=O) ppm; the Pd–C signal
was not observed. 31P{1H} NMR (CDCl3): δ = 28.4 ppm. IR
M.p. 211.6–212.1 °C. C26H21ClN4OPd (547.33): calcd. C 57.05, H
3.87, N 10.24; found C 57.55, H 4.23, N 9.89. H NMR (CDCl3):
δ = 4.92 (s, 1 H, PdCH), 6.89 (t, JH,H = 9.0 Hz, 1 H, Ar H), 7.05
(t, JH,H = 9.0 Hz, 2 H, Ar H), 7.16–7.25 (m, 6 H, Ar H), 7.55–7.77
1
(KBr): ν = 1668 (C=O) cm–1.
(m, 6 H, Ar H), 8.29–8.42 (m, 3 H, Ar H, imi H), 8.57 (d, JH,H =
˜
3.0 Hz, 2 H, py H) ppm. 13C{1H} NMR (CDCl3): δ = 31.0
(CHCO), 114.6, 116.9, 122.6, 124.2, 124.4, 124.7, 124.9, 125.4,
128.3, 129.3, 129.7, 130.6, 131.1, 137.3, 139.2, 144.4, 151.8, 172.4
Synthesis of 8c: The preparation was similar to that of 8a. Complex
7c (0.200 g, 0.334 mmol) and PPh3 (0.0870 g, 0.334 mmol) were
used. An air-stable yellow solid was obtained. Yield: 0.11 g (67%).
M.p. 228.7–229.5 °C. C40H34Cl2N3OPPd (781.01): calcd. C 61.51,
H 4.39, N 5.38; found C 61.90, H 3.96, N 5.17. 1H NMR (CDCl3):
δ = 3.07 (s, 3 H, NCH3), 5.19–5.60 (m, 2 H, NCH2), 6.92–7.89 (m,
27 H, Ar H), 8.32 (br. s, 1 H, Ar H) ppm. 31P{1H} NMR
([D ]DMSO): δ = 28.4 ppm. IR (KBr): ν = 1670 (C=O) cm–1.
(C=O) ppm. IR (KBr): ν = 1602 (C=O) cm–1.
˜
Synthesis of 12d: The preparation was similar to that of 8a. Com-
plex 11d (0.160 g, 0.292 mmol) and PPh3 (0.0760 g, 0.292 mmol) in
dichloromethane (10 mL) were used. An air-stable yellow solid was
˜
6
obtained.
Yield:
0.1 g
(47%).
M.p.
263.8–264.6 °C.
C39H31ClN3OPPd (730.52): calcd. C 64.12, H 4.28, N 5.75; found
C 63.92, H 4.55, N 5.08. H NMR ([D7]DMF): δ = 4.87 (d, JH,H
Synthesis of 9c: PdCl2 (0.0600 g, 0.344 mmol), 2c (0.13 g,
0.344 mmol), and K2CO3 (0.190 g, 1.38 mmol) were added to a
20 mL Schlenk flask and dissolved in dry pyridine (10 mL) under
nitrogen. The solution was stirred at 80 °C overnight. The solvent
was then completely removed under vacuum. The residue was re-
dissolved in dichloromethane and washed with water. The organic
phase was dried with anhydrous MgSO4, and the solvents were
evaporated to dryness under vacuum to give a solid. This solid was
redissolved in THF, and the solution was stirred at 50 °C for 3 h.
A yellow solid formed. It was collected on a frit, washed with di-
ethyl ether, and dried under vacuum. Yield: 0.0636 g (90% based
on 2c). M.p. 226.8–227.4 °C. C44H36Cl2N6O2Pd2 (964.51): calcd. C
54.79, H 3.76, N 8.71; found C 54.57, H 4.05, N 8.30. 1H NMR
([D6]DMSO): 3.27 (s, 6 H, NCH3), 5.94 (s, 2 H, PdCH), 6.88–7.00
(m, 6 H, Ar H), 7.36–8.01 (m, 18 H, Ar H), 8.49 (s, 2 H, imi H),
8.69 (d, d, JH,H = 3.0 Hz, 2 H, NCH) ppm. 13C{1H} NMR
([D6]DMSO): δ = 33.3 (NCH3), 47.6 (CHCO), 115.5, 116.7, 117.1,
123.2, 124.3, 124.7, 125.9, 126.0, 126.6, 129.2, 130.0, 130.2, 130.5,
1
= 3.0 Hz, 1 H, PdCH), 7.04 (t, JH,H = 9.0 Hz, 1 H, Ar H), 7.30–
7.91 (m, 23 H, Ar H), 7.94 (t, JH,H = 9.0 Hz, 1 H, Ar H), 8.17–
8.20 (m, 2 H Ar H), 8.44 (d, JH,H = 9.0 Hz, 2 H, Ar H), 8.64 (m,
1 H, imi H), 8.86 (d, JH,H = 9.0 Hz, 1 H, Ar H) ppm. 13C{1H}
NMR ([D7]DMF): δ = 39.0 (PdCH), 113.1, 117.4, 121.0, 123.9,
125.3 (d, JP,C = 15.0 Hz), 125.4, 126.3, 127.6, 128.2 (d, JP,C
9.0 Hz), 129.1, 129.7, 130.2, 130.4, 132.2, 132.8, 134.6 (d, JP,C
=
=
12.0 Hz), 139.9, 148.2, 173.0 (C=O) ppm. 31P{1H} NMR (CDCl3):
δ = 29.0 ppm. IR (KBr): ν = 1614 (C=O) cm–1.
˜
X-ray Diffraction Studies: Samples were collected at 150(2) K with
a Bruker APEX II equipped with a CCD area detector and a
graphite monochromator utilizing Mo-Kα radiation (λ
=
0.71073 Å). The unit-cell parameters were obtained by least-
squares refinement. Data collection and reduction were performed
by using the Bruker APEX2 and SAINT software.[16] Absorption
corrections were performed by using the SADABS program.[17] All
the structures were solved by direct methods and refined by full-
matrix least-squares methods against F2 with the SHELXTL soft-
ware package.[18] All non-hydrogen atoms were refined anisotropi-
cally. All hydrogen atoms were fixed at calculated positions and
refined with the use of a riding model. In the structure of 7cЈ, there
are two independent molecules in the asymmetric unit, and only
one of them was used in the structural discussion. Crystallographic
data are listed in Table 1. CCDC-935046 (7c), -935045 (7cЈ),
-935047 (8a), -935048 (3c), and -935044 (10c) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
137.8, 138.9, 144.6, 170.0 (C=O) ppm. IR (KBr): ν = 1621 (C=O)
˜
cm–1.
Synthesis of 10c: In
a 20 mL Schlenk flask, 9c (0.500 g,
0.518 mmol) and PPh3 (0.270 g, 1.04 mmol) in dichloromethane
(10 mL) were stirred at ambient temperature overnight. An off-
white solid was obtained. Yield: 0.550 g (71%). M.p. 209.3–
209.6 °C. C40H33ClN3OPPd (744.55): calcd. C 64.53, H 4.47, N
1
5.64; found C 64.02, H 4.25, N 5.32. H NMR (CDCl3): δ = 3.40
(s, 3 H, NCH3), 5.98 (d, JH,H = 12.0 Hz, 1 H, PdCH), 6.26–6.31
(m, 1 H, Ar H), 6.60–6.64 (m, 1 H, Ar H), 6.73–6.77 (m, 2 H, Ar
H), 7.05 (t, JH,H = 6.0 Hz, 1 H, Ar H), 7.19–7.31 (m, 8 H, Ar H),
7.44–7.74 (m, 14 H, Ar H), 8.26 (d, JH,H = 6.0 Hz, 1 H, Ar H),
8.63 (s, 1 H, imi H) ppm. 13C{1H} NMR ([D6]DMSO): δ = 33.3
(NCH3), 54.0 (d, JP,C = 106 Hz, CHCO), 113.9, 115.6, 116.2, 122.3,
122.8, 124.1, 124.3, 125.9, 127.8 (d, JP,C = 10.5 Hz), 128.5, 129.3,
129.4, 129.8, 132.9 (d, JP,C = 36.9 Hz), 133.7 (d, JP,C = 19.6 Hz),
134.8 (d, JP,C = 12.0 Hz), 137.2, 139.4 (d, JP,C = 10.5 Hz), 144.0,
146.4 (d, JP,C = 7.5 Hz), 171. 8 (d, JP,C = 5.2 Hz, CO) ppm. 31P{1H}
Supporting Information (see footnote on the first page of this arti-
cle): Crystallographic data for all the structures and NMR spectra
for products.
Acknowledgments
NMR ([D ]DMSO): δ = 25.2 ppm. IR (KBr): ν = 1619 (C=O)
˜
6
cm–1.
We are grateful to the National Science Council of Taiwan for fin-
ancial support of this work. We also thank the National Center for
Synthesis of 11d: In a 20 mL Schlenk flask, PdCl2 (0.200 g,
1.18 mmol), 2d (0.430 g, 1.18 mmol), and K2CO3 (0.652 g, High-Performance Computing of Taiwan for computing time and
4.72 mmol) were dissolved in dry pyridine (10 mL) under nitrogen. financial support of the Conquest software.
Eur. J. Inorg. Chem. 2014, 1492–1501
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPER
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Received: November 19, 2013
Published Online: February 24, 2014
Eur. J. Inorg. Chem. 2014, 1492–1501
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