Synthesis, Characterization, Crystal Structures, and Catalytic C-C Coupling and Hydrosilylation Reactions of Palladium(II) Complexes Derived from CNC Pincer-Type N-Heterocyclic Carbenes

Article abstract of DOI:10.1002/ejic.201500393

A series of pyridine linker containing bis(benz)imidazolium salts serving as precursors to CNC pincer-type N-heterocyclic carbene (NHC) ligands were prepared by successive N-alkylation of the azole core. Binuclear NHC complexes of silver(I) were synthesized by in situ deprotonation of the corresponding (benz)imidazolium salt with silver(I) oxide in acetonitrile at elevated temperature in the dark. Subsequently, pincer-type palladium(II)-NHC complexes were prepared by transmetalation using silver(I)-NHC complexes in acetonitrile at elevated temperatures. All compounds were fully characterized by elemental analysis, FTIR spectroscopy, and ¹H and ¹³C NMR spectroscopy. X-ray diffraction studies on single crystals of three compounds confirmed the distorted square-planar geometry around the palladium(II) center. All complexes are monomeric in nature, and the bis-NHC ligand chelates in the CNC mode. The effective catalytic potentials of the palladium complexes were demonstrated by their high activities in aqueous-phase Suzuki-Miyaura and Heck-Mizoroki cross-coupling reactions of phenylboronic acid and n-butyl acrylate with a range of functionalized bromobenzene derivatives. The former coupling reaction afforded biphenyls in yields of 71-99%, whereas the latter reaction evidenced low yields of the n-butyl cinnamates. Further, all the palladium complexes were studied for their potential in a catalytic hydrosilylation reaction.

Full text of DOI:10.1002/ejic.201500393

FULL PAPER
DOI:10.1002/ejic.201500393
Synthesis, Characterization, Crystal Structures, and
Catalytic C–C Coupling and Hydrosilylation Reactions of
Palladium(II) Complexes Derived from CNC Pincer-Type
N-Heterocyclic Carbenes
Rosenani A. Haque,*[a] Patrick O. Asekunowo,[a]
Srinivasa Budagumpi,[b] and Linjun Shao[c]
Keywords: Homogeneous catalysis / Cross-coupling / Hydrosilylation / Palladium / Carbene ligands
A series of pyridine linker containing bis(benz)imidazolium
salts serving as precursors to CNC pincer-type N-heterocy- complexes are monomeric in nature, and the bis-NHC ligand
clic carbene (NHC) ligands were prepared by successive N- chelates in the CNC mode. The effective catalytic potentials
alkylation of the azole core. Binuclear NHC complexes of sil- of the palladium complexes were demonstrated by their high
square-planar geometry around the palladium(II) center. All
ver(I) were synthesized by in situ deprotonation of the corre-
sponding (benz)imidazolium salt with silver(I) oxide in aceto-
nitrile at elevated temperature in the dark. Subsequently,
pincer-type palladium(II)–NHC complexes were prepared by
transmetalation using silver(I)–NHC complexes in aceto-
nitrile at elevated temperatures. All compounds were fully
characterized by elemental analysis, FTIR spectroscopy, and
1H and 13C NMR spectroscopy. X-ray diffraction studies on
single crystals of three compounds confirmed the distorted
activities in aqueous-phase Suzuki–Miyaura and Heck–
Mizoroki cross-coupling reactions of phenylboronic acid and
n-butyl acrylate with a range of functionalized bromobenz-
ene derivatives. The former coupling reaction afforded bi-
phenyls in yields of 71–99%, whereas the latter reaction evi-
denced low yields of the n-butyl cinnamates. Further, all the
palladium complexes were studied for their potential in a
catalytic hydrosilylation reaction.
having both functionalized and unfunctionalized NHC li-
gands can be formed make them one of the most suitable
and popular classes of precatalysts next to metal–phosphine
complexes in various organic transformations.[6] Palladium
complexes of NHC ligands having donor moieties con-
structed from N/S/O-based heterocycles and containing he-
teroatoms such as nitrogen, oxygen, and sulfur have been
reported.[7–9]
Most research efforts have been based on mononuclear
palladium–NHC complexes of the general formula [(NHC)2-
PdCl2], in which the NHC is usually a monodentate entity
derived from either an imidazole or a benzimidazole back-
bone; functionalized multidonor NHC ligands used in the
formation of palladium complexes that can serve as a start-
ing point for the development of new classes of palladium
complexes have been less explored. Among them, tridentate
pincer-type NHC ligands have gained increasing popularity
owing to the robustness and thermal stability of the resulted
complexes,[10] which yield highly active yet stable precata-
lysts. The advantage of using pincer, and especially CNC
pincer-type, functionalized NHC ligands is the rapid cre-
ation of a coordinatively and electronically unsaturated pal-
ladium center, which is open for incoming substrate mo-
lecules. This further allows rapid recoordination and stabili-
zation of the active species, which eliminates the product to
Introduction
N-Heterocyclic carbene (NHC) complexes of late transi-
tion metals are interesting owing to their wide range of ap-
plications as structural mimics for several biological sys-
tems[1] and as catalysts in various types of organic transfor-
mations.[2] Among them, silver- and palladium-based NHC
complexes have been a hot topic over the last two decades
as a result of their potential applications as metal-based
drugs[3] and C–C coupling catalysts,[4] respectively. The key
advantages associated with the latter complexes include the
nature of the active metal centers, which can be tuned by
varying the oxidation state and strength of the NHC ligand
field. Furthermore, the nature of the palladium center in
two oxidation states provides interesting catalytic properties
in various transformation reactions.[5] These features and
the ease with which a wide variety of palladium complexes
[a] The School of Chemical Sciences, Universiti Sains Malaysia,
11800 USM, Penang, Malaysia
E-mail: rosenani@usm.my
[b] Centre for Nano and Material Sciences, Jain University, Jain
Global Campus,
Bangalore 562112, Karnataka, India
[c] Institute of Applied Chemistry, Shaoxing University, Shaoxing,
Zhejiang Province 312000, People’s Republic of China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500393.
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be formed. As a result, a library of palladium complexes the mixture in ice-cold water and extraction from chloro-
having CNC- and CSC-based pincer-type NHC ligands form. Owing to similar chemical properties, benzimidazole
have been reported.[11]
was also used as core moiety to prepare 1-allylbenzimid-
On the other hand, NHC–palladium catalysis is one of azole, 1-pentylbenzimidazole, and 1-hexylbenzimidazole,
the most significant chemical transformations in the field and these compounds indeed required reaction conditions
of synthetic organic chemistry for the formation of new C– similar to those described for the N-alkylimidazole deriva-
C and C–N bonds and for C–H activation.[12] A variety of tives.
CNC pincer-type NHC ligands have been used to tailor a
We recently reported the syntheses of new N-allyl-substi-
series of neutral palladium(II)–NHC complexes of the type tuted imidazolium and benzimidazolium salts (i.e., 1, 3, 6,
[NHC2–Pd–Cl2],[13,14] which are suitable for C–C coupling and 8) obtained by treating 2,6-bis(N-imidazole-1-ylmeth-
reactions. To the best of our knowledge, monocationic pal- yl)pyridine or 2,6-bis(N-benzimidazole-1-ylmethyl)pyridine
ladium(II) complexes of CNC pincer-type ligands having with allyl bromide in acetonitrile under refluxing conditions
hexafluorophosphate counterions have not yet been re- for 20 h followed by addition of KPF6 to yield the hexa-
ported or employed for C–C coupling purposes. Further, fluorophosphate salts. Subsequently, silver(I)–NHC com-
the electronic and steric changes of the CNC pincer-type plexes 11 and 13 were obtained by treating salts 6 and 8
NHC backbones were suggested to have a pivotal influence with Ag2O in acetonitrile at moderate temperatures for
on their catalytic efficacies.[15] To study these effects, we ex- 24 h.[16] Owing to the simplicity of this route – it requires
amined a series of new palladium(II) complexes of NHC two simple steps (successive N-alkylation) and the genera-
ligands having different heterocyclic cores, steric bulk, and tion of the free NHC is not mandatory – we then focused
electronic properties, and their C–C and C–Si bond-forma- our efforts to develop an array of pyridine-linked azolium
tion capacities were evaluated.
salts having bromide as well as hexafluorophosphate coun-
terions and their binuclear silver(I)–NHC and palladi-
um(II)–NHC complexes. To access this series of salts and
NHC complexes, simple deprotonation followed by met-
alation and transmetalation routes were used for the silver
and palladium complexes, respectively, as these synthetic
pathways do not require the use of a glove box or other
measures to be considered for air-sensitive compounds.
Bromide salts 2, 4, and 5 were synthesized by alkylation
of 2,6-bis(bromomethyl)pyridine with 1-(2Ј-methoxylethyl)-
1H-imidazole, 1-pentyl-1H-benzimidazole, and 1-hexyl-1H-
benzimidazole,[17] respectively, and they were obtained in
71–82% yield. The formation of corresponding hexafluoro-
phosphate salts 7, 9, and 10 was achieved by a salt me-
tathesis reaction performed with the use of KPF6 in meth-
anol (Scheme 1). Resulting hexafluorophosphate salts 7, 9,
and 10 were treated with Ag2O in acetonitrile at 50–60 °C
for 18 h in the dark to afford anticipated binuclear silver(I)–
NHC complexes 12, 14, and 15 in good to excellent yields
(80–86%) by precipitation upon the addition of diethyl
ether (Scheme 2). To enlarge the scope of these proligands,
we considered an alternative route starting from the brom-
ide counterparts. Bromide salts 2, 4, and 5 were stirred with
Ag2O in methanol at room temperature in the dark for 24 h,
and this was followed by salt metathesis with KPF6 again
to afford binuclear silver(I)–NHC complexes 12, 14, and 15
with only a slight decrease in the yields.
Results and Discussion
We first evaluated the reactivity of binuclear imidazole-
based silver complexes 11 and 12 toward palladium(II),
which were expected to form tetracoordinate, diamagnetic
square-planar CNC pincer-type complexes having the gene-
ral formula [NHC(CNC)–Pd–Cl]PF6; these complexes were
readily amenable to characterization by H and 13C NMR
1
spectroscopic techniques. In line with this analogy, benz-
imidazole-based CNC pincer-type palladium complexes 18–
20 were prepared from 13–15 in good yields. The catalytic
properties of palladium–NHC complexes 16–20 in two dif-
ferent C–C (Suzuki–Miyaura and Heck–Mizoroki) cross-
coupling reactions were assessed by monitoring their ability
to form the C–C coupled products. All palladium com-
plexes evinced good to excellent catalytic C–C coupling
properties in Suzuki–Miyaura cross-coupling reactions,
whereas they displayed weak activity in Heck–Mizoroki
cross-coupling reactions. Further, their potential in C–Si
bond formation was studied by using styrene and bis(trime-
thylsilyloxy)methylsilane. All synthesized compounds were
insoluble in organic solvents such as benzene, hexane, and
diethyl ether, whereas they were completely soluble in polar
solvents such as methanol, ethanol, DMF, and DMSO. Ele-
mental analysis data of the new compounds was in well
agreement with their structures.
Our first attempts to synthesize targeted palladium(II)–
NHC complexes 16–20 were performed by using azolium
salts as NHC proligands and [PdCl2(cod)] (cod = cyclooc-
tadiene), but owing to the inertness of these reactants at
room temperature and the formation of a black charlike
material at higher temperatures, we were not able to isolate
Synthesis of (Benz)imidazolium Salts, Binuclear Silver
Complexes, and CNC Pincer-type Palladium Complexes
Initially, 1-allylimidazole and 1-(2Ј-methoxyethyl)imid- any complex. Further, CNC pincer-type palladium(II)–
azole were prepared according to reported methods by N- NHC complexes 16–20 were synthesized from their respec-
alkylation of 1H-imidazole with allyl bromide and 1- tive silver(I)–NHC complexes 11–15 by adapting the trans-
bromo-2-methoxyethane, respectively, in DMSO at 40 °C metalation technique. In a typical transmetalation reaction,
under basic conditions; this was followed by decomposing the binuclear silver(I)–NHC complex was treated with
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Scheme 1. Syntheses of CNC pincer-type bis(carbene) proligands 1–10.
AgPF6 drives the transmetalation reaction to comple-
tion.[18] Following this method, air- and moisture-stable de-
rivatives can be formed, and the final isolation of the target
compound simply involves filtration followed by washing
with diethyl ether. On the other hand, we followed the
methodology developed by Nolan et al. for the synthesis of
palladium complex 17 without affecting the ether moiety.[19]
To tune the solubility as well as the catalytic properties of
the palladium complexes, the size of the N substituent was
increased.
NMR Spectroscopy Studies
The azolium salts and their silver and palladium NHC
complexes were characterized by NMR spectroscopy. The
1H NMR spectra of bromide salts 1–5 are rather similar to
those of corresponding hexafluorophosphate salts 6–10; the
resonance of the (benz)imidazolium C2 proton is observed
at 9.30 Ͻ δ Ͻ 10.05 ppm. Other aromatic, imidazole C4
and C5, benzimidazole C4–C7, and the central pyridine
ring resonances are observed in the range of 7.43 Ͻ δ Ͻ
8.10 ppm. Besides, resonances corresponding to the allyl,
2-methoxyethyl, n-pentyl, n-hexyl, and 2,6-pyridinemethyl
protons appear in the range of 0.9 Ͻ δ Ͻ 5.60 ppm.[16,20] In
the 13C NMR spectra, a characteristic signal is observed for
the C2 carbon atom in the range of 138.8 Ͻ δ Ͻ 145.8 ppm
for the (benz)imidazolium salts. The spectra also evidence
resonances for the aliphatic and aromatic carbon nuclei in
the range of 11.2 Ͻ δ Ͻ 51.5 and of 111.0 Ͻ δ Ͻ
132.1 ppm,[21] respectively.
Scheme 2. Syntheses of silver(I)–NHC complexes 11–15.
[PdCl2(cod)] for 12–18 h in acetonitrile at 50 °C in the dark
to afford the CNC pincer-type palladium(II)–NHC com-
plex in good yield (Scheme 3). The formation of insoluble
The 1H NMR spectra of binuclear silver–NHC com-
plexes 11–15 are fairly similar to those of corresponding
(benz)imidazolium salts 1–10 with slight differences in the
positions and intensities of the resonances. The signal of
the (benz)imidazolium C2 proton (9.30 Ͻ δ Ͻ 10.05 ppm),
however, completely disappears, which is indicative of the
successful formation of the silver–NHC complex.[22] In the
13C NMR spectra of the silver complexes, a considerable
downfield shift is observed for the resonance corresponding
to the carbene carbon nucleus, which moves from 138.8 Ͻ
δ Ͻ 145.8 ppm for the (benz)imidazolium salts to 180.6 Ͻ δ
Scheme 3. Syntheses of palladium(II) complexes of CNC pincer-
type NHC ligands.
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Ͻ 189.3 ppm; this is indicative of the formation of carbene Single-Crystal X-ray Diffraction Studies
carbon–silver bonding.[22] The H NMR spectra of palladi-
1
To obtain additional insight into the coordination and
supramolecular chemistry, and the structural parameters of
the compounds, suitable single crystals of palladium(II)
complexes 16, 19, and 20 were obtained for single-crystal
X-ray diffraction analysis. Crystals were grown by slow dif-
fusion of diethyl ether into an acetonitrile solution of the
respective palladium complex at room temperature. Crystal-
lographic data and structure refinement details are given in
the Supporting Information.
Palladium complex 16 crystallized in the monoclinic sys-
tem with the P21/c space group, and one molecule of the
CNC pincer-type palladium complex cation and one mol-
ecule of a hexafluorophosphate anion occupy the asymmet-
ric unit. The molecular structure of palladium complex 16
is shown in Figure 1. Selected bond lengths and bond
angles of the complex are given in Table 1. It is clear from
the structure that coordination occurs from the CNC donor
sites of the NHC ligand and that the fourth coordination
site is satisfied by a chlorido ligand; furthermore, a hexa-
fluorophosphate ion is found as the counteranion. The
three different coordinating sites of the CNC pincer-type
ligand – carbene carbon atoms, pyridine nitrogen atom, and
chloride – can be easily distinguished by comparing bond
lengths to the coordinated palladium center, as the C–Pd
and N–Pd bonds are significantly shorter than the Cl–Pd
bond, owing to the formation of chelates with pincer-type
ligands. The bond lengths between palladium and the carb-
ene carbon atoms and the pyridine nitrogen atom are
2.031(4) (Pd–C1), 2.022(4) (Pd–C13), and 2.065(3) Å (Pd–
um(II)–NHC complexes 16–20 are almost similar to those
of corresponding binuclear silver(I)–NHC complexes 11–
15, as no (benz)imidazolium C2 resonance (9.30 Ͻ δ Ͻ
10.05 ppm) is observed. In the 13C NMR spectra, the signal
corresponding to the carbene carbon nuclei is shifted to the
upfield region, and it moves from 180.6 Ͻ δ Ͻ 189.3 ppm
for the binuclear silver(I)–NHC complexes to 164.2 Ͻ δ Ͻ
175.5 ppm; this can be ascribed to the formation of a new
carbene carbon–palladium bond. This observation repre-
sents an upfield shift of the carbene carbon nuclei by about
16 ppm upon coordination to palladium, which is in line
with similar reported compounds.[23] Unlike the previously
reported 2,6-lutidinyl-based cationic CNC–NHC pincer-
type complexes of palladium(II), the 2- and 6-methylene
1H NMR signals are diastereotopic. Notably, a set of two
unique singlets in the range of 5.80 Ͻ δ Ͻ 6.25 ppm corre-
sponding to the 2- and 6-methylene protons of pyridine is
observed (a broad signal in the cases of complexes 16 and
17), whereas these resonances appear slightly upfield as a
single singlet in the corresponding silver(I)–NHC com-
plexes at about 5.25 Ͻ δ Ͻ 5.75 ppm. The observed cou-
pling constant (2JH,H = 15.5 Hz) for the palladium(II)–
NHC complexes is typical for geminal coupling of dia-
stereotopic protons.[24,25] Therefore, the complexes adopt a
twisted conformation, and the dynamic process involves in-
terconversion between the two possible conformations. This
also leads to a broad signal for the N-methylene protons of
the substituents in the form of a triplet at about δ = 5.15–
5.30 ppm in the case of 16 and 17, whereas this resonance
is split into two triplets at about δ = 4.65–4.95 ppm for pal-
ladium complexes 18 and 20.
FTIR Spectroscopy Studies
The FTIR spectra of the azolium salts display two me-
dium-intensity sharp bands at about ν = 1605 and
˜
1570 cm–1, and they can be assigned to the stretching vi-
brations of the C=N modules of the imidazole and pyridine
rings, respectively.[26] Also, three medium-intensity broad
bands at approximately ν = 2860, 2930, and 1100 cm–1 can
˜
be ascribed to the stretching vibrations of the C–H and C–
N fragments, respectively.[27] Interestingly, in the spectra of
the silver complex, bands corresponding to the stretching
vibrations of the imidazole ring C=N and C–N modules
are shifted towards the lower energy side compared with
their salts, whereas the band for the pyridine ring C=N
bond is unaffected in all the cases, which indicates coordi-
nation of the carbene carbon atoms.[28,29] In the case of the
palladium(II) complexes, both the pyridine and azolium
ring C=N modules are shifted to lower energy relative to
the same modules of their respective salts, which is consis-
tent with successful palladation in which the carbene car-
bon atoms and the pyridine nitrogen atom are involved in
the coordination. This is further confirmed by the crystal
structures of similar complexes.[30]
Figure 1. X-ray diffraction structure of palladium(II) complex 16
with ball-and-stick model drawn at 50% probability. The hexafluo-
rophosphate anion is omitted for clarity.
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N3), whereas the palladium chlorido bond length is dination environment in a distorted square-planar geometry
2.2943(11) Å (Pd–Cl1). The metal center has a C2NCl coor- with an average bond length of 2.103 Å, which is in good
agreement with palladium complexes having CNC pincer-
type ligand architectures with two trans-positioned imid-
azolin-2-ylidene donor groups.[31,32] The palladium center is
unsymmetrically coordinated by C2NCl sites of the ligand
with differences between the C1–Pd–C13 and Cl–Pd–N3
bond angles of 173.72(18) and 178.84(10)° respectively,
which are almost linear. It should, however, be noted that
a bond angle at the palladium center between two adjacent
donor sites is almost 90° and the internal ring angles at the
carbene carbon atom of each imidazole ring is almost 104°;
this is in good agreement with the values reported for sim-
ilar complexes.[33] In the extended crystal structure of com-
plex 16, two types of hydrogen-bonding interactions, CH–
F [2.496(8) Å] and CH–Cl [2.621(5) Å], are observed.
Complexes 19 and 20 are structurally almost similar even
though different NHC ligands are used, as demonstrated
by the crystal structures depicted in Figures 2 and 3. The
bond lengths and bond angles of complexes 19 and 20 are
Table 1. Selected bond lengths and angles for 16.
Bond lengths [Å]
C1–Pd1
N3–Pd1
C13–Pd1
Cl1–Pd1
N1–C14
N1–C1
N1–C2
N2–C1
N2–C3
N2–C4
2.031(4)
2.065(3)
2.022(4)
2.2943(11)
1.470(6)
1.356(6)
1.384(7)
1.351(6)
1.390(7)
1.459(6)
C4–C5
C5–N3
N3–C9
C9–C10
C10–N4
C13–N4
N4–C11
C13–N5
N5–C12
N5–C17
1.501(6)
1.346(5)
1.353(5)
1.500(6)
1.463(5)
1.349(5)
1.382(5)
1.342(5)
1.387(5)
1.469(5)
Bond angles [°]
C1–Pd1–C13
N3–Pd1–Cl1
C1–Pd1–N3
N3–Pd1–C13
Cl1–Pd1–C13 93.92(12)
Cl1–Pd1–C1
C15–C14–N1
173.73(17)
178.84(10)
87.32(15)
86.76(15)
N1–C1–N2
N2–C4–C5
C5–N3–C9
C9–C10–N4
N4–C13–N5
N15–C17–C18
C6–C7–C8
104.6(4)
110.4(4)
120.1(4)
111.2(3)
104.7(3)
113.6(4)
119.3(4)
92.04(13)
111.2(5)
Figure 2. X-ray diffraction structures of palladium(II) complex 19
(top: unit A, bottom: unit B) with ball-and-stick model drawn at
50% probability. The hexafluorophosphate anion is omitted for
clarity.
Figure 3. X-ray diffraction structures of palladium(II) complex 20
(top: unit A, bottom: unit B) with ball-and-stick model drawn at
50% probability. The hexafluorophosphate anion is omitted for
clarity.
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almost similar, as verified by the pertinent bond lengths and Table 3. Selected bond lengths and angles for 20.
bond angles listed in Tables 2 and 3. Both the complexes, 19
Bond lengths [Å]
Unit A
and 20, crystallize with two crystallographically independent
structural units (A and B), but 19 crystallizes in the mono-
clinic system with the P21/c space group, whereas 20 crys-
Unit B
C1A–Pd1A
N3A–Pd1A
2.025(8)
2.039(6)
2.019(8)
C1B–Pd1B
N3B–Pd1B
C21B–Pd1B
2.034(8)
2.061(6)
2.006(8)
2.2906(18)
1.342(10)
1.353(10)
1.338(10)
1.384(10)
1.339(10)
1.360(9)
¯
tallizes in the triclinic system with the P1 space group. The C21A–Pd1A
Cl1A–Pd1A
2.2874(18) Cl1B–Pd1B
asymmetric units of the complexes are composed of two
N1A–C1A
crystallographically independent palladium complex cat-
N2A–C1A
1.348(9)
1.358(10)
1.370(10)
1.339(9)
1.334(10)
1.381(10)
N1B–C1B
N2B–C1B
N4B–C21B
N5B–C21B
N3B–C9B
N3B–C13B
ions and two hexafluorophosphate ions, in which three of
N4A–C21A
the carbon atoms of the pentyl chain and the associated
hydrogen atoms of unit B of complex 19 are disordered over
two sets, whereas unit A of complex 19 (Figure 2) and
units A and B (Figure 3) of complex 20 are well-ordered
structures. It is clear from the crystal structures that the
N5A–C21A
N3A–C9A
N3A–C13A
Bond angles [°]
Unit A
Unit B
NHC ligands act as neutral tridentate entities that are li- C1A–Pd1A–C21A 171.7(3)
N3A–Pd1A–Cl1A 179.33(16) N3B–Pd1B–Cl1B
C1A–Pd1A–N3A 86.5(3) C1B–Pd1B–N3B
C1B–Pd1B–C21B
172.6(3)
177.97(17)
86.8(3)
gated to the palladium center in a CNC pincer-type fash-
ion. The other coordination site is completed by a chlorido
ligand to form a distorted square-planar geometry about
the metal center. Small deviation from a perfect square-
planar arrangement is found with bond angles of 173.9(3)
and 178.86(18)° for N3A–Pd1A–Cl1A and C1A–Pd1A–
C21A, respectively. The Pd–Ccarbene bond lengths in the
complexes [2.014(8)–2.030(7) Å] fall in the range observed
previously for neutral[34] and cationic pincer-type[35] trans-
bis(benzimidazolin-2-ylidene)palladium complexes. The in-
ternal ring angles, N1A–C1A–N2A and N4A–C21A–N5A,
at the carbene carbon atoms in 19 are 107.0(6) and
106.5(7)°, respectively. Almost the same dihedral angles and
internal ring angles at the carbene carbon atoms are ob-
served in complex 20, and this is indicative of the formation
of the desired complex. In the extended crystal structures,
CH–F (2.623–2.641 Å) and CH–Cl (2.673–3.148 Å)
hydrogen-bonding interactions are observed by connections
of the palladium complex cations and the hexafluorophos-
phate anions.
N3A–Pd1A–C21A 85.8(3)
Cl1A–Pd1A–C21A 94.7(2)
N3B–Pd1B–C21B 86.0(3)
Cl1B–Pd1B–C21B 93.2(2)
Cl1B–Pd1B–C1B
N1B–C1B–N2B
C9B–N3B–C13B
N4B–C21B–N5B
Cl1A–Pd1A–C1A
N1A–C1A–N2A
C9A–N3A–C13A
N4A–C21A–N5A
93.1(2)
94.0(2)
105.9(6)
118.6(7)
106.0(6)
106.7(7)
120.2(6)
105.6(7)
Catalytic C–C Cross-Coupling Studies
Palladium(0/II) complexes of NHC ligands are state-of-
the-art candidates as efficient catalysts in various C–C
cross-coupling reactions.[4,36] The Suzuki–Miyaura and
Heck–Mizoroki cross-coupling reactions are known as
powerful methods for the synthesis of symmetric and asym-
metric biaryls, which are important skeletons in the struc-
tures of conductive polymers, liquid crystals, and biolo-
gically active compounds. By changing the N substituents
of pyridine-tethered imidazoles and benzimidazoles, the ste-
ric and electronic properties of the complexes can be altered
to provide catalysts with enhanced catalytic C–C cross-cou-
pling performances. In light of favorable reports[37] on pal-
ladium(II) complexes of pincer-type NHC ligands, we
studied the catalytic potential of palladium complexes 16–
20 as promising candidates for Suzuki–Miyaura and Heck–
Mizoroki cross-coupling reactions.
Table 2. Selected bond lengths and angles for 19.
Bond lengths [Å]
Unit A
Unit B
C1A–Pd1A
N3A–Pd1A
C21A–Pd1A
Cl1A–Pd1A
N1A–C1A
N2A–C1A
N4A–C21A
N5A–C21A
N3A–C9A
N3A–C13A
2.030(7)
2.053(6)
2.014(8)
2.290(2)
1.338(9)
1.361(9)
1.350(9)
1.361(9)
1.353(9)
1.358(9)
C1B–Pd1B
N3B–Pd1B
C21B–Pd1B
Cl1B–Pd1B
N1B–C1B
N2B–C1B
N4B–C21B
N5B–C21B
N3B–C9B
N3B–C13B
2.016(7)
2.053(6)
2.043(8)
2.298(2)
1.356(9)
1.358(9)
1.363(10)
1.353(10)
1.365(9)
1.336(9)
Aqueous-Phase Suzuki–Miyaura Cross-Coupling Reaction
New C–C bonds can be achieved by the addition of a
haloarene to phenylboronic acids in the presence of a cata-
lyst, and this is called the Suzuki–Miyaura coupling reac-
tion. The usefulness of this reaction makes it an interesting
tool for the preparation of biologically and industrially use-
ful compounds. The catalytic activities of all palladium
complexes (with a catalyst loading of 2.5 mol-%) were in-
vestigated in this coupling reaction at 80 °C and potassium
acetate was used as the base. All catalytic reactions were
performed in air and without any additives; the results are
summarized in Table 4. All of the palladium complexes
were found to be active catalysts for this reaction, although
oxygen-functionalized palladium complex 17 gave only a
very low yield of the desired product. Notably, complexes
Bond angles [°]
Unit A
Unit B
C1A–Pd1A–C21A 173.9(3)
C1B–Pd1B–C21B
173.5(3)
179.34(18)
87.1(3)
N3A–Pd1A–Cl1A 178.86(18) N3B–Pd1B–Cl1B
C1A–Pd1A–N3A 86.4(3) C1B–Pd1B–N3B
N3A–Pd1A–C21A 87.6(3)
Cl1A–Pd1A–C21A 93.4(2)
N3B–Pd1B–C21B 86.9(3)
Cl1B–Pd1B–C21B 93.8(2)
Cl1B–Pd1B–C1B
N1B–C1B–N2B
C9B–N3B–C13B
N4B–C21B–N5B
Cl1A–Pd1A–C1A
N1A–C1A–N2A
C9A–N3A–C13A
N4A–C21A–N5A
92.5(2)
92.2(2)
107.0(6)
118.4(7)
106.1(6)
106.9(6)
119.1(7)
107.9(7)
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Table 4. Suzuki–Miyaura cross-coupling reaction of bromobenzene and boronic acid.[a]
[a] Reaction conditions: Catalyst (2.5 mol-%), PhBr (0.6 mmol), phenylboronic acid (1.2 equiv.), CH3COOK (3.5 equiv.) DMF/H2O (2:1,
5 g), 80 °C, 3 h. [b] Determined by GC–MS analysis, as the average of two runs.
16 and 18–20 were active and gave the products in yields of methoxybromobenzene, and 4-nitrobromobenzene, were
85–99% in water and DMF mixture. Comparison of allyl- used to couple with phenylboronic acid by using highly
substituted imidazole-based complex 16 with benzimid- active catalysts 18–20 (Table 4, entries 6–14) at 1 mol-%
azole-based analogue complex 18 revealed that the latter concentration. The coupling of activated and deactivated
showed slightly higher performance. However, at this stage functionalized aryl bromides with phenylboronic acid was
it is difficult to conclude that the increased efficiency of performed at 100 °C by using DMF and water as the sol-
complex 18 is an electronic effect. Further, comparison of vent system and CH3COOK as the base, and this resulted
benzimidazole-based palladium complexes 18–20, which in the formation of a library of the corresponding cross-
have different N substituents, revealed that complex 18 had coupled products in good to excellent yields (89–71%) as
the highest activity and delivered the product in 99% yield, depicted in Table 4. Interestingly, palladium pincer complex
whereas analogue complex 20 displayed slightly lower ac- 18 efficiently catalyzed the coupling of 4-bromoacetophen-
tivity. It cannot be said with certainty at this stage whether one, 4-methoxybromobenzene, and 4-nitrobromobenzene in
this difference in activity is a steric effect, as modifications excellent yields (89–76%), regardless of activating or deacti-
of the NHC ligands of these complexes are small in terms vating substituents, and the reactions were mostly complete
of both steric bulk and donor strength. Finally, introduc- in 3 h. This complex displayed consistently higher activity
tion of an oxygen functionality into the complex might lead in all the cases for this type of C–C coupling reaction. These
to coordination sites that are less readily accessible to in- results demonstrate that complexes with a CNC pincer-type
coming substrates, which in turn decreases the catalytic per- ligand field are excellent for the catalysis of cross-coupling
formance of complex 17.
reactions. Similar activities were found with palladium com-
Further, by using these optimized conditions a set of plexes having pincer-type NHC and other NHC palladium
functionalized aryl bromides, 4-bromoacetophenone, 4- complexes.[38]
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Heck–Mizoroki Cross-Coupling Reaction
uents, 4-bromoacetophenone, 4-methoxybromobenzene,
and 4-nitrobromobenzene, and they were coupled with n-
butyl acrylate by using 1 mol-% of active catalyst 18 or 19
at 140 °C for 6 h. Coupling activity of the two complexes
was again found, but low yields of approximately 18–20%
were obtained. However, palladium complexes 18 and 19
were still the two most effective in the series, which is con-
sistent with the Suzuki–Miyaura cross-coupling results.
This observation implies that the alkyl functionality with
the benzimidazole cores tagged to the pyridine ring has
some influence on the stability of the active species. Further
work on varying the reaction conditions and the stoichio-
metry of the reactants to improve the catalytic performance
of the complexes is underway.
The catalytic performance of all of the palladium com-
plexes (with a catalyst loading of 2.5 mol-%) was investi-
gated in the Heck–Mizoroki cross-coupling reaction at
110 °C with the use of potassium acetate as the base. All
of the reactions were performed in air, and the results are
summarized in Table 5. We found that these palladium
complexes having CNC pincer-type NHC ligands contain-
ing different alkyl substituents on the N atoms of the
(benz)imidazole ring were less efficient in Heck–Mizoroki
cross-coupling reactions. At room temperature, the forma-
tion of n-butyl cinnamate from the coupling of n-butyl acry-
late with bromobenzene in the presence of a catalyst is a
slow reaction, the rate of which gradually increases as the
temperature rises. Unfortunately, although longer reaction
times and higher temperatures were employed to promote
the reaction, the yields were still very low.
Hydrosilylation Reaction
The fact that NHC complexes of group X metals are well
Further, the reaction scope was extended to a set of func- acknowledged to serve as efficient catalyst systems to bring
tionalized activating and deactivating aryl bromide substit- about C–N[39] and C–Si[40] coupling reactions of different
Table 5. Heck–Mizoroki cross-coupling reaction of bromobenzene with n-butyl acrylate.[a]
[a] Reaction conditions: Catalyst (2.5 mol-%), PhBr (0.6 mmol), n-butyl acrylate (2.0 equiv.), CH3COOK (3.5 equiv.), DMSO (5 g), 110 °C,
6 h. [b] Determined by GC–MS analysis, as the average of two runs.
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types of aryl halides/olefins with secondary amines and silyl transmetalation, whereas silver complexes were prepared by
compounds, respectively, led us to explore the catalytic treating the respective azolium salts with silver oxide
properties of the group of palladium complexes. The cata- through an in situ deprotonation method. The salts and the
lytic hydrosilylation reactions performed by using com- corresponding silver and palladium complexes were charac-
plexes 16–20 were examined by using styrene and bis(tri- terized by using a range of spectroscopic and analytical
methylsilyloxy)methylsilane as substrates under a nitrogen techniques. Palladium complexes 16, 19, and 20 were char-
atmosphere at 70 °C. The scope of these catalytic reactions acterized by single-crystal X-ray diffraction methods. The
is shown in Table 6.
monoanionic NHC ligand is coordinated to the palladium
center in a tridentate fashion by the pyridine nitrogen atom
and the two carbene carbon atoms to form two six-mem-
bered chelate rings. The overall structures of the palladium
complexes are the same, but the bond lengths and bond
angles between the metal centers and the donor moieties
are different. In all the cases, the palladium center has a
distorted square-planar geometry because of chelate con-
straints originating from the NHC ligand. The title com-
plexes are believed to be model systems as catalysts for C–
C coupling reactions in green solvents. All of the palladium
complexes (except for complex 17) displayed excellent ac-
tivity, and the products were isolated in excellent yields (71–
99%) in the Suzuki–Miyaura cross-coupling reaction of
functionalized and unfunctionalized bromobenzenes with
phenylboronic acid for the formation of biphenyls in water/
DMF. It is evident from the catalytic results that benzimid-
azole-based complex 18 displayed better activity than imid-
azole analogue complex 16. Further, the former complex
displayed activity superior to that of substituted benzimid-
azole-based complexes 19 and 20. Their application in the
Heck–Mizoroki cross-coupling of functionalized and un-
functionalized bromobenzenes with n-butyl acrylate pro-
vided low yields of the desired n-butyl cinnamates. Further
investigation into the cross-coupling reactions by altering
the reaction conditions and the base are underway. Further,
complex 16 displayed good catalytic efficiency in the hydro-
silylation of styrene, whereas electronically tuned complex
17 displayed low activity.
Table 6. Hydrosilylation reaction of styrene.[a]
Entry
Catalyst
Yield [%][b]
A/B
1
2
3
4
5
16
17
18
19
20
57
37
53
50
43
83:17
84:16
83:17
86:14
88:12
[a] Reaction conditions: Catalyst (1 mol-%), styrene (8.0 mmol),
bis(trimethylsilyloxy)methylsilane (8.8 mmol), 70 °C, 6 h. [b] Deter-
mined by GC–MS analysis.
Coupling of styrene with bis(trimethylsilyloxy)methylsil-
ane was attempted in the presence and absence of catalysts
16–20 at 70 °C for 6 h. Reactions performed in the absence
of the catalysts did not provide any product, whereas all
of the reactions performed in the presence of the catalysts
afforded the expected C–Si coupled products (i.e., A and
B) in moderate to good yields. Though all three palladium
complexes displayed analogous catalytic efficacy, the best
result was obtained in the case of complex 16 as the cata-
lyst. This might be due to the presence of a less hindered
palladium center, which might allow the substrate to react
to form the desired product. Efficiency of the catalyst sys-
tem was found to decrease with an increase in the steric
bulk around the metal center. In the case of complex 20,
product formation was lower (43%), which indicates that
Experimental
General Considerations: All solvents used were of reagent grade and
were used without further purification. N-Allyl-substituted imid-
the metal was too hindered to allow the substrates to react. azolium and benzimidazolium bromide/hexafluorophosphate salts
(i.e., 1/6 and 3/8) and the corresponding binuclear silver(I)–NHC
complexes (i.e., 11 and 13) were prepared according to our previous
report.[16] 2,6-Bis(bromomethyl)pyridine, 2-methoxyethyl bromide,
n-pentyl bromide, n-hexyl bromide, potassium hexafluorophos-
phate, imidazole, benzimidazole, potassium hydroxide, silver(I) ox-
ide, dichloro(1,5-cyclooctadiene)palladium(II), phenylboronic acid,
bromobenzene, n-butyl acrylate, potassium acetate, styrene, and bi-
s(trimethylsilyloxy)methylsilane were purchased from Sigma–Ald-
rich and were used without further purification. 1-Allyl-1H-imid-
azole, 1-(2Ј-methoxylethyl)-1H-imidazole, 1-allyl-1H-benzimid-
azole, 1-pentyl-1H-benzimidazole, and 1-hexyl-1H-benzimidazole
were prepared as described previously[17] with slight modifications.
1H NMR and 13C NMR spectra were obtained at room tempera-
ture with a Bruker 500 MHz Ascend spectrometer from solutions
in [D6]DMSO by using tetramethylsilane as an internal standard.
FTIR spectra of the compounds were recorded in potassium brom-
Interestingly, the ratio of the C–Si coupled products re-
mains almost the same for all the catalytic systems, and this
indicates that their regioselective nature is similar. These re-
sults are comparable with those reported recently for plati-
num–NHC complexes.[4] Complex 17 displayed the least
catalytic performance among the series with a 37% yield of
the desired product. This difference in activity can be as-
cribed to the electronic effect of the NHC substitution.
Conclusion
The syntheses and characterization of a series of palla-
dium complexes incorporating CNC pincer-type NHC li-
gands was undertaken. These complexes were accessed from
their respective binuclear silver(I)–NHC complexes through ide disks by using a Perkin–Elmer 2000 system spectrometer in the
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range of 4000 to 400 cm–1. Melting points were assessed by using a
Stuart Scientific SMP-1 (UK) instrument. All reported compounds
were analyzed for carbon, hydrogen, and nitrogen by CHN microa-
nalysis by using a Perkin–Elmer 2400 LS Series CHN/S analyzer.
For X-ray single-crystal structure analysis, a Bruker SMART
APEX2–2009 CCD area-detector diffractometer was used for the
data collection, SAINT Bruker-2009 was used for cell refinement,
SAINT was used for data reduction, and SHELXTL was used to
solve the structure. Calculations, structure refinement, molecular
graphics, and the material for publication were achieved by using
the SHELTXL and PLATON software packages. Structures were
solved by direct methods and were refined by full-matrix least-
squares against F2. GC–MS analyses were performed for catalytic
evaluations by using a HP6890 series apparatus having an HP5
column with an injector, and nitrogen was used as the carrier gas.
ppm. FTIR (KBr disk): ν = 2965, 3115 ν(C–H, aliphatic and aro-
˜
matic), 1607 ν(C=N, benzimidazole), 1576 ν(C=N, pyridine ring),
1068 ν(C–N, benzimidazole) cm–1. C31H39F12N5P2 (771.61): calcd.
C 48.3, H 5.1, N 9.1; found C 48.7, H 5.3, N 9.0.
2,6-Bis(3-n-hexylbenzimidazol-1-ylmethyl)pyridine Dibromide/Bis-
(hexafluorophosphate) (5/10): Salt 5/10 was synthesized in analogy
to salt 2/7 by treating 1-hexylbenzimidazole (0.820 g, 4.25 mmol)
with 2,6-bis(bromomethyl)pyridine (0.560 g, 2.13 mmol). The so-
obtained salt was recrystallized from a solution of acetonitrile/di-
ethyl ether to yield salt 10 in pure form, yield 0.71 g (82%), m.p.
1
187–188 °C. H NMR (500 MHz, [D6]DMSO, 298 K): δ = 0.85 (t,
J = 7.0 Hz, 6 H, 2 CH3), 1.20 (m, 4 H, 2 CH2CH3), 1.30 (m, 4 H,
2 CH2CH2CH2), 1.75 (m, 4 H, 2 CH2CH2CH2CH2), 3.67 (m, 4 H,
2 NCH2CH2), 4.45 (t, J = 6.5 Hz, 4 H, 2 NCH2), 5.90 (s, 4 H, 2
CH2-pyridine), 7.48 (t, J = 8.5 Hz, 2 H, Ar-H), 7.65 (d, J = 8.5 Hz,
2 H, Ar-H), 7.66 (t, J = 7.5 Hz, 2 H, Ar-H), 7.94 (d, J = 8.5 Hz, 2
H, pyridine-H), 8.10 (t, J = 8.5 Hz, 1 H, pyridine-H), 10.05 (s, 2
H, 2 benzimidazolium H2Ј) ppm. 13C{1H} NMR (125 MHz, [D6]-
Syntheses of Azolium Salts
2,6-Bis[3-(2Ј-methoxyethyl)imidazol-1-ylmethyl]pyridine Dibromide/
Bis(hexafluorophosphate) (2/7): N-Methoxyethylimidazole (0.650 g,
5.15 mmol) was added dropwise to a stirring solution of 2,6-bis-
(bromomethyl)pyridine (0.680 g, 2.58 mmol) in acetonitrile
(30 mL) over a period of 1 h, and the mixture was heated at reflux
for the next 24 h. The solvent was removed under reduced pressure
to afford 2,6-bis[3-(2Ј-methoxyethyl)imidazol-1-ylmethyl]pyridine
dibromide (2). Dibromide salt 2 was treated with potassium hexa-
fluorophosphate (0.94 g, 5.15 mmol) in MeOH/H2O (1:1 v/v,
40 mL). The mixture was stirred at room temperature for 3 h and
allowed to stand overnight. Then, the solvent was removed under
reduced pressure, and the resultant white powder was washed with
distilled water (3ϫ 5 mL) to remove unreacted KPF6 and air dried.
So-obtained crude solid bis(hexafluorophosphate salt) 7 was
recrystallized from a solution of acetonitrile/methanol to yield salt
7 in pure form, yield 0.75 g (71%), m.p. 174–176 °C. 1H NMR
(500 MHz, [D6]DMSO, 298 K): δ = 3.30 (s, 6 H, 2 OCH3), 3.75 (t,
J = 5.0 Hz, 4 H, 2 CH2-OCH3), 4.37 (t, J = 5.0 Hz, 4 H, 2 NCH2),
5.55 (s, 4 H, 2 CH2-pyridine), 7.52 (d, J = 6.0 Hz, 2 H, 2 imid-
azolium H5Ј), 7.62 (d, J = 6.5 Hz, 2 H, 2 imidazolium H4Ј), 7.75
(d, J = 8.0 Hz, 2 H, pyridine-H), 7.97 (t, J = 7.5 Hz, 1 H, pyridine-
H), 9.30 (s, 2 H, 2 imidazolium H2Ј) ppm. 13C{1H} NMR
(125 MHz, [D6]DMSO, 298 K): δ = 50.2 (CH2), 51.5 (NCH2), 52.7
(CH2-pyridine), 69.4 (OCH3), 122.6 (imidazolium Ar-C), 125.4,
130.7 (pyridine Ar-C), 138.8 (imidazolium-C2) ppm. FTIR (KBr
DMSO, 298 K):
δ = 11.3 (CH3), 18.7 (CH2CH3), 23.4
(CH2CH2CH3), 33.7 (CH2CH2CH2CH3), 38.9 (NCH2CH2), 49.5
(NCH2), 52.6 (CH2-pyridine), 117.6, 123.9, 127.4 (benzimidazole
Ar-C), 129.0, 132.1, 132.8 (pyridine Ar-C), 142.2 (benzimidazolium
C2) ppm. FTIR (KBr disk): ν = 2971, 3120 ν(C–H, aliphatic and
˜
aromatic), 1609 ν(C=N, benzimidazole), 1596 ν(C=N, pyridine
ring), 1060 ν(C–N, benzimidazole) cm–1. C33H43F12N5P2 (799.66):
calcd. C 49.6, H 5.4, N 8.8; found C 49.1, H 5.5, N 8.9.
Syntheses of Binuclear Silver(I)–NHC Complexes
2,6-Bis[3-(2Ј-methoxyethyl)imidazol-1-ylmethyl]pyridinedisilver(I)
Bis(hexafluorophosphate) (12): A round-bottomed flask wrapped
with aluminum foil was charged with salt 7 (0.648 g, 1 mmol) and
acetonitrile (20 mL) and then Ag2O (0.230 g, 1 mmol) was added.
The mixture was stirred at 50–60 °C for 18 h. The mixture was then
filtered through a bed of Celite, and the solvent was concentrated
to 3 mL under reduced pressure. Addition of diethyl ether to the
solvent afforded a white solid, which was filtered and washed with
dioxane and diethyl ether. Recrystallization of the white powder
from methanol/diethyl ether yielded binuclear silver complex 12 as
a white crystalline solid in pure form, yield 0.55 g (80%), m.p. 245–
246 °C. 1H NMR (500 MHz, [D6]DMSO, 298 K): δ = 3.17 (s, 12
H, 4 OCH3) 3.69 (t, J = 5.25 Hz, 8 H, 4 CH2-OCH3), 4.35 (t, J =
4.75 Hz, 8 H, 4 NCH2), 5.11 (d, J = 5.0 Hz, 8 H, 4 NCH2), 5.67
(s, 8 H, 4 CH2-pyridine), 7.42 (s, 8 H, 4 imidazolium-H4, H5), 7.73
(d, J = 8.0 Hz, 4 H, pyridine-H), 7.80 (t, J = 8.0 Hz, 2 H, pyridine-
H) ppm. 13C{1H} NMR (125 MHz, [D6]DMSO, 298 K): δ = 53.2
(NCH2), 55.5 (CH2-pyridine), 68.3 (OCH3), 139.0 (imidazolium
C4Јand C5Ј), 144.0, 145.7, 156.7 (pyridine Ar-C), 180.6 (imid-
disk): ν = 2960, 3130 ν(C–H, aliphatic and aromatic), 1610 ν(C=N,
˜
imidazole), 1568 ν(C=N, pyridine ring), 1060 ν(C–N, imidazole),
922 ν(C–O) cm–1. C19H27F12N5O2P2 (647.38): calcd. C 35.3, H 4.2,
N 10.8; found C 35.5, H 4.6, N 10.9.
2,6-Bis(3-n-pentylbenzimidazol-1-ylmethyl)pyridine Dibromide/Bis-
(hexafluorophosphate) (4/9): Salt 4/9 was synthesized in analogy to
salt 2/7 by treating 1-pentylbenzimidazole (0.800 g, 4.25 mmol)
with 2,6-bis(bromomethyl)pyridine (0.560 g, 2.13 mmol). The so-
obtained salt was recrystallized from a solution of acetonitrile/di-
ethyl ether to yield salt 9 in pure form, yield 0.65 g (80%), m.p.
azolium C2-Ag) ppm. FTIR (KBr disk): ν = 2964, 3100 ν(C–H,
˜
aliphatic and aromatic), 1602 ν(C=N, imidazole), 1555 ν(C=N, pyr-
idine ring), 1050 ν(C–N, imidazole), 911 ν(C–O) cm– 1
.
C38H50Ag2F12N10O4P2 (1216.54): calcd. C 37.5, H 4.1, N 11.5;
found C 37.7, H 4.4, N 11.7.
1
182–184 °C. H NMR (500 MHz, [D6]DMSO, 298 K): δ = 0.90 (t,
2,6-Bis(3-n-pentylbenzimidazol-1-ylmethyl)pyridinedisilver(I) Bis-
J = 7.0 Hz, 6 H, 2 CH3), 1.15 (m, 4 H, 2 CH2CH3), 1.25 (m, 4 H, (hexafluorophosphate) (14): Synthesized in analogy to complex 12
2 CH2CH2CH2), 1.75 (m, 4 H, 2 NCH2CH2), 4.40 (t, J = 6.5 Hz, by treating benzimidazolium hexafluorophosphate salt 9 (0.773 g,
4 H, 2 NCH2), 5.81 (s, 4 H, 2 CH2-pyridine), 7.65 (t, J = 8.0 Hz,
2 H, Ar-H), 7.76 (d, J = 8.0 Hz, 2 H, Ar-H), 7.82 (t, J = 7.5 Hz, 2
H, Ar-H), 7.94 (d, J = 8.5 Hz, 2 H, pyridine-H), 8.10 (t, J = 8.5 Hz,
1 H, pyridine-H), 10.00 (s, 2 H, 2 benzimidazolium H2Ј) ppm.
13C{1H} NMR (125 MHz, [D6]DMSO, 298 K): δ = 11.3 (CH3),
1 mmol) with Ag2O (0.230 g, 1 mmol). The so-obtained crude com-
plex was recrystallized from a solution of methanol/diethyl ether to
yield complex 14 in pure form, yield 0.63 g (86%) M.p. 253–255 °C.
1H NMR (500 MHz, [D6]DMSO, 298 K): δ = 0.90 (t, J = 7.0 Hz,
12 H, 4 CH3), 1.15 (sext, 8 H, 4 CH2CH3), 1.25 (quint, 8 H, 4
18.5 (CH2CH3), 23.7 (CH2CH2CH2), 37.9 (NCH2CH2), 48.8 CH2CH2CH3), 1.75 (quint, 8 H, 4 NCH2CH2), 4.40 (t, J = 7.5 Hz,
(NCH2), 50.6 (CH2-pyridine), 115.7, 122.8, 127.6 (benzimidazole 8 H, 4 NCH2), 5.81 (s, 8 H, 4 CH2-pyridine), 7.43 (t, J = 7.5 Hz,
Ar-C), 122.5, 131.1 (pyridine Ar-C), 145.8 (benzimidazolium C2Ј)
4 H, Ar-H), 7.55 (d, J = 8.5 Hz, 4 H, Ar-H), 7.64 (t, J = 7.5 Hz, 4
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H, Ar-H), 7.90 (d, J = 8.5 Hz, 4 H, pyridine-H), 8.10 (t, J = 7.0 Hz,
complex was recrystallized from a solution of acetonitrile/diethyl
2 H, pyridine-H) ppm. 13C{1H} NMR (125 MHz, [D6]DMSO, ether to yield complex 17 as a pale yellow solid in pure form, yield
1
298 K): δ = 13.8 (CH3), 21.7 (CH2CH3), 28.3 (CH2CH2CH3), 40.0 45 mg (80%), m.p. 190–191 °C. H NMR (500 MHz, [D6]DMSO,
(NCH2CH2), 48.7 (NCH2), 51.3 (CH2-pyridine), 120.7, 127.0, 133.2 298 K): δ = 3.17 (s, 6 H, 2 OCH3), 3.69 (t, J = 5.25 Hz, 4 H, 2
(benzimidazole Ar-C), 118.2, 125.7, 134.2 (pyridine Ar-C), 189.3
CH2-OCH3), 5.17 (t, J = 4.75 Hz, 4 H, 2 NCH2), 5.65 (s, 4 H, 2
CH2-pyridine), 7.70 (d, J = 6.0 Hz, 4 H, 2 imidazolium H4,H5),
7.83 (d, J = 8.0 Hz, 2 H, pyridine-H), 7.90 (t, J = 7.0 Hz, 1 H,
pyridine-H) ppm. 13C{1H} NMR (125 MHz, [D6]DMSO, 298 K):
(benzimidazolium C2-Ag) ppm. FTIR (KBr disk): ν = 2968, 3120
˜
ν(C–H, aliphatic and aromatic), 1601 ν(C=N, benzimidazole), 1563
ν(C=N, pyridine ring), 1055 ν(C–N, benzimidazole) cm–1
.
C62H74Ag2F12N10P2 (1464.99): calcd. C 50.8, H 5.1, N 9.6; found δ = 24.5 (OCH3), 28.8 (CH2-OCH3), 53.2 (NCH2), 55.5 (CH2-pyr-
C 50.3, H 5.2, N 9.4.
idine), 123.4, 129.0 (imidazolium C4Ј and C5Ј), 126.7, 144.0, 156.7
(pyridine Ar-C), 172.1 (imidazolium C2-Pd) ppm. FTIR (KBr
2,6-Bis(3-n-hexylbenzimidazol-1-ylmethyl)pyridinedisilver(I) Bis-
(hexafluorophosphate) (15): Synthesized in analogy to complex 12
by treating benzimidazolium hexafluorophosphate salt 10 (0.800 g,
1 mmol) with Ag2O (0.230 g, 1 mmol). The so-obtained crude com-
plex was recrystallized from a solution of methanol/diethyl ether to
yield complex 15 in pure form, yield 0.6 g (82%), m.p. 261–262 °C.
1H NMR (500 MHz, [D6]DMSO, 298 K): δ = 0.85 (t, J = 7.0 Hz,
12 H, 4 CH3), 1.20 (sext, 8 H, 4 CH2CH3), 1.30 (quint, 8 H, 4
CH2CH2CH3), 1.75 (quint, 8 H, 4 CH2CH2CH2CH3), 3.45 (quint,
8 H, 4 NCH2CH2), 4.45 (t, J = 7.5 Hz, 8 H, 4 NCH2), 5.90 (s, 8
H, 4 CH2-pyridine), 7.42 (t, J = 7.5 Hz, 4 H, Ar-H), 7.55 (d, J =
7.5 Hz, 4 H, Ar-H), 7.67 (t, J = 7.5 Hz, 4 H, Ar-H), 7.90 (d, J =
8.5 Hz, 4 H, pyridine-H), 8.05 (t, J = 7.0 Hz, 2 H, pyridine-H)
ppm. 13C{1H} NMR (125 MHz, [D6]DMSO, 298 K): δ = 13.8
( C H 3 ) , 2 1 . 7 ( C H 2 C H 3 ) , 2 8 . 3 ( C H 2 C H 2 C H 3 ) , 3 1 . 4
(CH2CH2CH2CH3), 40.0 (NCH2CH2), 48.7 (NCH2), 50.6 (CH2-
pyridine), 118.5, 120.7, 127.0 (benzimidazole Ar-C), 121.3, 133.2,
135.2 (pyridine Ar-C), 189.3 (benzimidazolium C2-Ag) ppm. FTIR
disc): ν = 2930, 2849 ν(C–H, aliphatic and aromatic), 1596 ν(C=N,
˜
imidazole), 1570 ν(C=N, pyridine ring), 1100 ν(C–N, imidazole),
1300 ν(C–O–C), 1270 ν(C–O) cm–1. C19H25ClF6N5O2PPd (642.25):
calcd. C 35.5, H 3.9, N 10.9; found C 35.9, H 4.0, N 11.2.
2,6-Bis(3Ј-allybenzimidazol-1-ylmethyl)pyridinepalladium(II) Hexa-
fluorophosphate (18): Synthesized in analogy to complex 16 by tre-
ating binuclear silver complex 13 (75 mg, 0.05 mmol) with
PdCl2(cod) (30 mg, 0.11 mmol). The so-obtained crude palladium
complex was recrystallized from a solution of acetonitrile/diethyl
ether to yield complex 18 as a yellow solid in pure form, yield
44 mg (60%), m.p. 198.5–199 °C. 1H NMR (500 MHz, [D6]DMSO,
2
298 K): δ = 5.25 (d, J = 6.0 Hz, 4 H, 2 NCH2), 5.36 (dd, JHH
=
1.5 Hz, 3JHH = 10.5 Hz, 2 H, CH=HHcis), 5.50 (dd, 2JHH = 1.5 Hz,
3JHH = 17.0 Hz, 2 H, CH=CHHtrans), 5.95 (d, J = 15.5 Hz, 2 H,
CH2-pyridyl), 6.28 (d, J = 15.5 Hz, 2 H, CH2-pyridyl), 6.12–6.18
(m, 2 H, 2 CH), 7.45 (t, J = 7.5 Hz, 4 H, 2 Ar-H), 7.52 (t, J =
7.5 Hz, 4 H, 2 Ar-H), 7.74 (t, J = 7.5 Hz, 2 H, Ar-H), 8.15 (d, J =
8.5 Hz, 2 H, pyridine-H), 8.25 (t, J = 7.5 Hz, 1 H, pyridine-H)
ppm. 13C{1H} NMR (125 MHz, [D6]DMSO, 298 K): δ = 50.7
(NCH2), 52.9 (CH2-pyridine), 111.8 (CH=CH2 ), 112.14
(CH=CH2), 118.2, 122.3, 128.6, 133.0 (benzimidazole Ar-C), 121.2,
133.4, 154.9 (pyridine Ar-C), 175.5 (benzimidazolium C2-Pd) ppm.
(KBr disk): ν = 2965, 3133 ν(C–H, aliphatic and aromatic), 1599
˜
ν(C=N, benzimidazole), 1583 ν(C=N, pyridine ring), 1050 ν(C–N,
benzimidazole) cm–1. C66H82Ag2F12N10P2 (1521.10): calcd. C 52.1,
H 5.4, N 9.2; found C 52.3, H 5.6, N 9.7.
Syntheses of Palladium(II)–NHC Complexes
FTIR (KBr disc): ν = 2860, 2924 ν(C–H, aliphatic and aromatic),
˜
2,6-Bis(3-allyimidazol-1-ylmethyl)pyridinepalladium(II) Hexafluoro-
phosphate (16): An acetonitrile (25 mL) solution of binuclear silver
complex 11 (58 mg, 0.05 mmol) and PdCl2(cod) (30 mg,
0.11 mmol) was stirred at 50 °C for 12 h in the dark. The reaction
was monitored by TLC, and upon completion of the reaction, the
mixture was filtered through a pad of Celite to remove precipitated
silver hexafluorophosphate, and the volatiles were removed under
vacuum. The so-obtained crude product was washed several times
with diethyl ether and recrystallized from an acetonitrile/hexane
mixture and dried under vacuum to yield CNC pincer-type palla-
dium complex 16 in pure form, yield 50 mg (79%), m.p. 181 °C. 1H
NMR (500 MHz, [D6]DMSO, 298 K): δ = 5.11 (d, J = 5.0 Hz, 4
1589 ν(C=N, benzimidazole), 1573 ν(C=N, pyridine ring), 1100
ν(C–N, benzimidazole) cm–1. C27H25ClF6N5PPd (706.34): calcd. C
45.9, H 3.6, N 9.9; found C 46.2, H 3.7, N 10.2.
2,6-Bis(3Ј-pentylbenzimidazol-1-ylmethyl)pyridinepalladium(II)
Hexafluorophosphate (19): Synthesized in analogy to complex 16
by treating binuclear silver complex 14 (73 mg, 0.06 mmol) with
PdCl2(cod) (33 mg, 0.12 mmol). The so-obtained crude palladium
complex was recrystallized from a solution of acetonitrile/hexane
to yield complex 19 as a pale yellow solid in pure form, yield 44 mg
1
(80%), m.p. 194 °C. H NMR (500 MHz, [D6]DMSO, 298 K): δ =
0.75 (t, J = 7.0 Hz, 6 H, 2 CH3), 1.25 (sext, 4 H, 2 CH2CH3), 1.35
( q u i n t , 4 H , 2 C H 2 C H 2 C H 3 ) , 1 . 9 5 ( q u i n t , 4 H , 2
CH2CH2CH2CH3), 4.95 (t, J = 6.5 Hz, 4 H, 2 NCH2), 5.85 (d, J
= 15.5 Hz, 2 H, CH2-pyridyl), 6.25 (d, J = 15.5 Hz, 2 H, CH2-
pyridyl), 7.48 (t, J = 7.0 Hz, 4 H, 2 Ar-H), 7.95 (d, J = 8.5 Hz, 4
H, 2 Ar-H), 8.10 (t, J = 7.5 Hz, 2 H, Ar-H), 8.15 (d, J = 8.5 Hz, 2
H, pyridine-H), 8.28 (t, J = 8.5 Hz, 1 H, pyridine-H) ppm. 13C{1H}
NMR (125 MHz, [D6]DMSO, 298 K): δ = 13.8 (CH3), 21.7
(CH2CH3), 28.3 (CH2CH2CH3), 40.0 (NCH2CH2), 48.7 (NCH2),
50.3 (NCH2-pyridine), 120.7, 127.0, 132.5 (benzimidazole Ar-C),
118.5, 123.6, 133.4, 138.5 (pyridine Ar-C), 175.0 (benzimidazolium
2
3
H, 2 NCH2), 5.16 (dd, JHH = 1.2 Hz, JHH = 11.0 Hz, 2 H,
2
3
CH=HHcis), 5.21 (dd, JHH = 1.2 Hz, JHH = 17.0 Hz, 2 H,
CH=CHHtrans), 5.67 (s, 4 H, 2 CH2-pyridine), 6.00–6.13 (m, 2 H,
2 CH), 7.85–7.90 (d, J = 6.0 Hz, 4 H, 2 imidazolium H4,H5), 7.92
(d, J = 8.0 Hz, 2 H, pyridine-H), 8.15 (t, J = 7.0 Hz, 1 H, pyridine-
H) ppm. 13C{1H} NMR (125 MHz, [D6]DMSO, 298 K): δ = 53.2
(NCH2), 55.5 (CH2-pyridine), 111.6 (CH=CH2), 122.9 (CH=CH2),
125.8, 129.0 (imidazolium C4Ј and C5Ј), 139.4, 143.0, 155.7 (pyr-
idine Ar-C), 171.6 (imidazolium C2-Pd) ppm. FTIR (KBr disc): ν
˜
= 3105, 2959 ν(C–H, aliphatic and aromatic), 1595 ν(C=N, benz-
imidazole), 1578 ν(C=N, pyridine ring), 1101 ν(C–N, imidazole)
cm–1. C19H21ClF6N5PPd (606.22): calcd. C 37.6, H 3.5, N 11.6;
found C 37.8, H 3.5, N 11.8.
C2-Pd) ppm. FTIR (KBr disc): ν = 2865, 2939 ν(C–H, aliphatic
˜
and aromatic), 1586 ν(C=N, benzimidazole), 1565 ν(C=N, pyridine
ring), 1105 ν(C–N, benzimidazole) cm–1. C31H37ClF6N5PPd
(766.48): calcd. C 48.6, H 4.9, N 9.1; found C 48.7, H 5.1, N 9.4.
2,6-Bis(3Ј-methoxyethylimidazol-1-ylmethyl)pyridinepalladium(II)
Hexafluorophosphate (17): Synthesized in analogy to complex 16 2,6-Bis(3Ј-hexylbenzimidazol-1-ylmethyl)pyridinepalladium(II)
by treating binuclear silver complex 12 (75 mg, 0.06 mmol) with
PdCl2(cod) (35 mg, 0.12 mmol). The so-obtained crude palladium
Hexafluorophosphate (20): Synthesized in analogy to complex 16
by treating binuclear silver complex 15 (74 mg, 0.06 mmol) with
Eur. J. Inorg. Chem. 2015, 3169–3181
3179
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPER
PdCl2(cod) (33 mg, 0.12 mmol). The so-obtained crude palladium
complex was recrystallized from a solution of acetonitrile/hexane
to yield complex 20 as a pale yellow solid in pure form, yield 44 mg
for the products by using GC (decane was used as the internal
standard).
CCDC-984746 (for 16), -984747 (for 19), and -984748 (for 20) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
1
(60%), m.p. 199 °C. H NMR (500 MHz, [D6]DMSO, 298 K): δ =
0.79 (t, J = 7.0 Hz, 6 H, 2 CH3), 1.25 (sext, 4 H, 2 CH2CH3), 1.35
( q u i n t , 4 H , 2 C H 2 C H 2 C H 3 ) , 1 . 9 5 ( q u i n t , 4 H , 2
CH2CH2CH2CH3), 3.67 (quint, 4 H, 2 NCH2CH2), 4.95 (t, J =
7.0 Hz, 4 H, 2 NCH2), 5.85 (d, J = 15.5 Hz, 2 H, CH2-pyridyl),
6.25 (d, J = 15.5 Hz, 2 H, CH2-pyridyl), 7.48 (t, J = 8.5 Hz, 4 H,
2 Ar-H), 7.78 (d, J = 8.5 Hz, 4 H, 2 Ar-H), 7.94 (t, J = 7.0 Hz, 4
H, 2 Ar-H), 8.05 (d, J = 8.5 Hz, 2 H, pyridine-H), 8.25 (t, J =
8.5 Hz, 1 H, pyridine-H) ppm. 13C{1H} NMR (125 MHz, [D6]
DMSO, 298 K): δ = 13. 8 (CH3 ), 21. 7 (CH2 CH3 ), 28.3
(CH2CH2CH3), 32.4 (CH2CH2CH2CH3), 38.6 (NCH2CH2), 40.0
(NCH2), 48.7 (NCH2-pyridine), 120.7, 127.0, 131.2 (benzimidazole
Ar-C), 118.7, 121.6, 133.9, 138.3 (pyridine Ar-C), 175.0 (benzimid-
Supporting Information (see footnote on the first page of this arti-
cle): Crystal data and structure refinement details for palladium
complexes 15, 19, and 20.
Acknowledgments
R. A. H. thanks the Universiti Sains Malaysia for the Research
University Grant 1001/PKIMIA/811217. P. O. A. thanks Institute
of Postgraduate Studies, Universiti Sains Malaysia for financial
support (grant GA: P-KD0004/12(R)).
azolium C2-Pd) ppm. FTIR (KBr disc): ν = 2863, 2930 ν(C–H,
˜
aliphatic and aromatic), 1595 ν(C=N, benzimidazole), 1570 ν(C=N,
p y r i d i n e r i n g ) , 1 1 0 8 ν ( C – N, b e n z i m i d a z o l e ) c m – 1
.
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Catalytic C–C Cross-Coupling Studies
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1
NMR spectroscopy and mass spectrometry. Data for biphenyl: H
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Received: April 25, 2015
Published Online: June 12, 2015
Eur. J. Inorg. Chem. 2015, 3169–3181
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