A mild and efficient method for the synthesis of structurally diverse 1,2,3-triazolylidene palladium(II) diiodo complexes. Comparison of catalytic activities for Suzuki-Miyaura coupling

Article abstract of DOI:10.1016/j.jorganchem.2015.10.002

Synthesis of mononuclear and PEPPSI type palladium diiodo complexes of 1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylidene without the use of strong bases and silver salts and at ambient conditions using Pd(OAc)₂ is reported. Using stoichiometric amounts of bidendate ligands such as pyrazine, 4,4′-bipyridine and DABCO bridged binuclear palladium diiodo complexes were obtained in excellent yields. By simple variation of reagents and their stoichiometry, one can control the reactions towards the selective formation of mononuclear [(Tz)₂Pd(I)₂] complexes, iodo bridged binuclear complexes [(Tz)Pd(I)(μ-I)₂Pd(I)(Tz)], mononuclear PEPPSI type complexes [(Tz)Pd(I)₂(Py)], bridged binuclear PEPPSI type complexes [(Tz)Pd(I)₂-(bridge biPy)-(I)₂Pd(Tz)]. The catalytic activities of these three structurally different types of complexes are compared for Suzuki-Miyaura coupling reaction.

Full text of DOI:10.1016/j.jorganchem.2015.10.002

Journal of Organometallic Chemistry 799-800 (2015) 232e238
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
A mild and efficient method for the synthesis of structurally diverse
1,2,3-triazolylidene palladium(II) diiodo complexes. Comparison of
catalytic activities for SuzukieMiyaura coupling
Bemineni Sureshbabu, Venkatachalam Ramkumar, Sethuraman Sankararaman*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of mononuclear and PEPPSI type palladium diiodo complexes of 1,4-diphenyl-3-methyl-1,2,3-
triazol-5-ylidene without the use of strong bases and silver salts and at ambient conditions using
Pd(OAc)2 is reported. Using stoichiometric amounts of bidendate ligands such as pyrazine, 4,40-bipyr-
idine and DABCO bridged binuclear palladium diiodo complexes were obtained in excellent yields. By
simple variation of reagents and their stoichiometry, one can control the reactions towards the selective
Received 12 July 2015
Received in revised form
29 September 2015
Accepted 2 October 2015
Available online 9 October 2015
formation of mononuclear [(Tz)2Pd(I)2] complexes, iodo bridged binuclear complexes [(Tz)Pd(I)(m-
I)2Pd(I)(Tz)], mononuclear PEPPSI type complexes [(Tz)Pd(I)2(Py)], bridged binuclear PEPPSI type com-
plexes [(Tz)Pd(I)2-(bridge biPy)-(I)2Pd(Tz)]. The catalytic activities of these three structurally different
types of complexes are compared for SuzukieMiyaura coupling reaction.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
N-heterocyclic carbene
NHCePd complex
PEPPSI
Mesoionic NHC
Cross coupling
1. Introduction
activities of Tz-PdX2-Py (Tz ¼ 1,2,3-triazol-5-ylidene) type com-
plexes [10]. Instead of pyridine whenever bidendate ligands such as
N-Heterocyclic carbene (NHC) ligands have dominated the
scene in transition metal organometallic chemistry during the past
two decades due to their ease of synthesis, extraordinary stability
and functional group tolerance and excellent catalytic activities [1].
Among the transition metals palladium, iridium and rhodium have
led the development in view of their importance in homogeneous
catalysis, palladium in cross-coupling reactions [2] and iridium and
rhodium in CeH activation [3] and hydrogenation reactions [4].
Among the NHCs imidazol-2-ylidenes (normal NHC) [5] and 1,2,3-
triazol-5-ylidenes (mesoionic NHC) [6] are very popular. Palladium
complexes of both of these NHC ligands have served as pre-
catalysts in a wide range of coupling reactions with unprece-
dented catalytic activities [7]. One such complex is NHC-PdX2-Py
(X ¼ halogen, Py ¼ pyridine), commonly known as the PEPPSI
(Pyridine-Enhanced Precatalyst, Preparation, Stabilization and
Initiation) complex, introduced by Organ in 2006 [8]. Since then
tremendous progress has been made in the chemistry and catalysis
involving Im-PdX2-Py (Im ¼ imidazol-2-ylidene) type complexes
[9]. A few reports have appeared on the synthesis and catalytic
4,40-bipyridne (bipy), pyrazine (Pz) and 1,4-diazabicyclooctane
(DABCO) are used the corresponding bridged binuclear com-
plexes, Im-PdX2-bidendatePy-PdX2-Im, were formed [11]. Recently
we have reported a new method for the synthesis of normal and
mesoionic (NHC)2ePdI2 complexes [12]. This methodology does
not use strong bases [13] or silver salts [14] and more importantly
the reactions were carried out at ambient temperature [15]. Herein
we describe the utilization of this method for the synthesis of
mononuclear PEPPSI type complexes and bridged binuclear com-
plexes using 1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylidene as the
mesoionic NHC ligand. We also report an improved version of this
method wherein the reaction duration is considerably shortened. In
the improved version we used (n-Bu)4NI along with triazolium
iodide and Pd(OAc)2 in CH2Cl2. Under these conditions the re-
actions were accelerated and went to completion faster than re-
ported earlier and gave the desired palladium diiodo complexes in
excellent yields.
2. Results and discussion
2.1. Synthesis of NHCePd complexes
* Corresponding author.
E-mail address: sanka@iitm.ac.in (S. Sankararaman).
We have reported earlier that the reaction of triazolium iodide 1
http://dx.doi.org/10.1016/j.jorganchem.2015.10.002
0022-328X/© 2015 Elsevier B.V. All rights reserved.

B. Sureshbabu et al. / Journal of Organometallic Chemistry 799-800 (2015) 232e238
233
with Pd(OAc)2 in CH2Cl2 at room temperature yielded complex 2 in
near quantitative yields [12]. The reaction did not depend on the
amount of Pd(OAc)2, both 0.6 equivalents and 1.2 equivalents with
respect to 1 gave the same result. However, triazolium iodide
(1aec) as the iodide source, the reactions were very slow and took
several days to go to completion. In order to improve the rates of
these reactions we used (n-Bu)4NI along with the triazolium iodide
and Pd(OAc)2. Stoichiometric amounts of (n-Bu)4NI on reaction
with Pd(OAc)2 gave dark black colour solution presumably due to
the formation of palladate [(n-Bu4N)2PdI2(OAc)2] as an intermedi-
ate. Evidence for the formation of the palladate complex is derived
from 1H NMR studies and detailed mechanism of formation of the
NHCePd complex under base free conditions wherein acetate acts
as the base is reported earlier from our group [12]. Further addition
of triazolium salts (1aec) yielded the corresponding mono nuclear
palladium NHC complexes (2aec) in good yields (Scheme 1). Tri-
azolium iodide as the source of iodide ion the reactions took much
longer to attain completion (1a and 1b in 36 h and 1c in 48 h). The
reactions were considerably faster with (n-Bu)4NI. Complexes 2aec
were characterized by comparison of the spectroscopic data with
authentic samples prepared in the laboratory (SI) [12] 1H NMR
spectra of 2aeb (see SI) clearly indicated the presence of syn and
anti rotomers for complex 2 [16]. We then explored the possibility
of synthesizing (Tz)2PdCl2 derivatives using (n-Bu)4NCl instead of
the iodide salt. Triazolium salt 1a was treated with (n-Bu)4NCl in
CH2Cl2 and stirred at room temperature for 2 h then Pd(OAc)2 was
added and allowed to stir at rt for 4 d. But, the same mononuclear
palladium(II) diiodo compound 2a was isolated in 66% of yield
instead of the corresponding palladium(II) dichloro NHC complex.
We also investigated the reaction of 1a with 1.2 equivalents of
Pd(OAc)2 in the presence of excess KI which yielded very cleanly the
corresponding bridged binuclear complex 3 in 95% yield as a bright
yellow solid (Scheme 2). The 1H and 13C NMR spectra (in DMSO-d6)
of the product obtained in this reaction were identical to 3 reported
earlier by Albrecht [15].
Scheme 2. Synthesis of mononuclear and iodo bridged binuclear complexes.
Scheme 3. Initial attempt towards the synthesis of PEPPSI type complexes.
near quantitative yield. A simple one pot procedure has been
developed in which complex 3 was prepared in situ as shown in
Scheme 2 and further treated with 2.0 equivalents of pyridine to
obtain 5 without any contamination of complex 4 (Scheme 4).
There are several advantages of this method compared to the
existing ones. First of all the present method, unlike the earlier
reports, [9,10] does not involve the use of any strong base or silver
salts. The reactions required only stoichiometric amounts of pyri-
dine derivatives. The reactions were carried out at ambient condi-
tions. No side products were observed in these reactions and hence
the PEPPSI type complexes (5e8) were easily obtained in pure form
(Scheme 4).
Under similar conditions used for the synthesis of 3 as in
Scheme 2, we attempted the synthesis of the corresponding chloro
bridged complex from the reaction of triazolium salt 1a, Pd(OAc)2
(1.2 equiv) in the presence of excess KCl. To our surprise it gave the
corresponding acetate bridged palladocycle complex 9 in 68% yield
as greenish yellow solid (Scheme 5) [16] The 1H and 13C NMR
spectra of the product 9 obtained in this reaction were identical to
that of the literature [16a]. Although the role of KCl in the formation
In an attempt to synthesize PEPPSI type complexes initially we
investigated the reaction of triazolium iodide 1a with 1.0 equivalent
of Pd(OAc)2 in the presence of excess KI and pyridine in CH2Cl2 as
solvent at room temperature. The reaction invariably yielded the
PEPPSI complex 5 along with (Py)2PdI2(4) as a mixture in 2:1 ratio,
respectively (Scheme 3). Despite several attempts, the mixture
could neither be separated by fractional crystallization nor by col-
umn chromatography. Authentic sample of 4 was prepared as a
bright orange solid by the reaction of PdCl2(CH3CN)2 in CH2Cl2 in
the presence of KI and pyridine and it was identified by 1H and 13
C
NMR spectroscopic methods.
Hong [10c] has reported the synthesis of 1,2,3-triazol-5-ylidene
based PEPPSI type complexes from the corresponding chloro
bridged binuclear palladium complex by treatment with pyridine
in CH2Cl2 at room temperature. Hong's method involved the in-
termediate silver carbene complex. In the present study treatment
of complex 3 with 2.0 equivalents of pyridine gave complex 5 in
Scheme 1. Synthesis of mononuclear palladium NHC complexes.
Scheme 4. Synthesis of 1,2,3-triazol-5-ylidene based PEPPSI type complexes.

234
B. Sureshbabu et al. / Journal of Organometallic Chemistry 799-800 (2015) 232e238
single crystal XRD data (Fig. 1, Table 1). In all the complexes the
geometry around palladium is square planar and the ligands
occupy trans positions as expected. The PdeCcarbene bond length
in all the complexes is around 1.96(1) Å and the PdeN bond length
is around 2.10(1) Å, comparable to other triazolylidene-Pd PEPPSI
type complexes [10]. In complex 11 both the IePdeI units are
coplanar and this plane is perpendicular to the plane containing
both the triazolylidene rings.
2.3. Comparison of catalytic activities
Scheme 5. Synthesis of acetate bridged palladocycle complex 9.
Crudden [10a] has reported Heck coupling of aryl bromides and
aryl iodides with methyl acrylate using 1,2,3-triazolylidene based
mesoionic PEPPSI type complexes as catalysts. Hong [10c] has re-
ported SuzukieMiyaura coupling of 1-chloro-2,6-dimethylbenzene
with phenylboronic acid to give high yield of 2,6-dimethylbiphenyl
at room temperature using PEPPSI type complex of sterically hin-
dered 1,4-bis(2,6-diisopropylphenyl)-1,2,3-triazol-5-ylidene palla-
dium complex in ethanol using potassium t-butoxide as base at
room temperature. Albrecht [10b] has reported the coupling be-
tween m-bromo anisole and phenylboronic acid using a variety of
bases and solvents and best yield was reported when the reaction
was carried out in 2-propanol using tetra-n-butyl ammonium
fluoride at 50 ꢀC using 1,2,3-triazolylidene based mesoionic car-
bene complex. Organ [10f] has used sterically hindered PEPPSI
complexes to carry out Suzuki coupling of sterically demanding
substrates for the synthesis of biaryls and binaphthyls. Herein,
compounds 2a, 3, 5 and 11 were screened and used as pre-catalysts
for SuzukieMiyaura coupling reactions of aryl bromides with
phenylboronic acid. A mixture of Pd(OAc)2 and PPh3 was examined
for comparison of catalytic activity with that of the NHC complexes.
The coupling reaction of 4-bromoanisole with phenylboronic acid
was taken as model reaction and the reactions were carried out in
ethanol as solvent. The results are summarized in Table 2.
These results clearly show that the PEPPSI type complex 5 is
catalytically more reactive than complexes 2 and 3 (compare en-
tires 1, 3 and 6, Table 2). With complex 2a after 2.5 h only 40% yield
of 4-phenylanisole was obtained and the reminder was unreacted
4-bromoanisole and it took more than 12 h for the completion of
the reaction. In the presence of complex 3 (2 mol%) and 11 (2 mol%)
the reactions were complete only after 5 h and 6 h, respectively
(Table 2, entries 4 and 9). When PPh3 (2 mol%) was added along
with 3 the reaction was faster and had gone to completion within
2.5 h to give 94% yield of the product. Under these conditions the
bridged binuclear complex 3 most likely coordinates with PPh3 to
of complex 9 is not clear, its presence was necessary because in the
absence of KCl only complex 2a was obtained (Scheme 2).
Huynh [11a] has reported the conversion of a bromo bridged
binuclear palladium complex to the corresponding bipyridine
bridged binuclear complex [Im-Pd(Br)2-bipy-Pd(Br)2-Im] by treat-
ment with 4,4’-bipyridine in CH2Cl2 at room temperature. The
corresponding linearly bridged binuclear complexes of the type
[Tz-Pd(X2)-bipy-Pd(X2)-Tz] are not known till now. In the present
study when 0.6 equivalents of bidendate ligands such as 4,40-
bipyridine (bipy), pyrazine (Pz) and 1,4-diazabicyclooctane
(DABCO) were used instead of pyridine derivatives the corre-
sponding linearly bridged binuclear complexes 10e12, respectively,
were obtained in high yields (Scheme 4). Unlike in the case of
synthesis of PEPPSI type complexes 5e8 it was not necessary to add
Pd(OAc)2 and bidendate ligands sequentially. All the reagents can
be mixed simultaneously and bridged complexes 10e12 were the
sole products formed in these reactions (Scheme 6). It must be
emphasized that in all of these reactions presented in Schemes 1e6
presence of acetate is crucial as an in-built base. Hence the source of
palladium has to be Pd(OAc)2 base free synthetic method reported
herein. When PdCl2(CH3CN)2 was used as a source of palladium the
reactions did not occur. For instance there was no reaction when 1a
was treated with PdCl2(CH3CN)2 up to 20 h under otherwise
identical conditions as in Scheme 2. Similarly there was no reaction
when 1a was treated with PdCl2(CH3CN)2 in the presence of 1
equivalent of n-Bu4NI under otherwise identical conditions as in
Scheme 1.
2.2. Structural characterization
The structures of complexes 5, 8 and 11 were established by
Scheme 6. Synthesis of bridged binuclear complexes 10e12.

B. Sureshbabu et al. / Journal of Organometallic Chemistry 799-800 (2015) 232e238
235
Fig. 1. ORTEP diagram (50% probability) of the structure of complexes 5 (top), 8 (middle) and 11 (bottom). Hydrogen atoms are omitted for clarity.
give the corresponding mononuclear [(Tz)Pd(Cl)2(PPh3)] which in
turn forms the active catalyst. Using catalyst 5the reaction was
complete within 2.5 h and addition of PPh3 did not affect the
duration of the reaction significantly (Table 2, entries 6 and 7).
When Pd(OAc)2and PPh3 were used, reaction took much longer
(14 h) for the completion (Table 2, entry 13). The palladium NHC

236
B. Sureshbabu et al. / Journal of Organometallic Chemistry 799-800 (2015) 232e238
Table 1
pallado cycle complex 9 was formed in the presence of KCl. Syn-
Crystallographic data of compounds 5, 8 and 11.
thesis and structural characterization of mononuclear PEPPSI type
palladium complexes [(Tz)Pd(I)2(Py)] (5e8) and bridged binuclear
palladium complexes [(Tz)Pd(I)2(Bridging bidendatePy)Pd(I)2(Tz)]
(10e12) of 1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylideneare re-
ported. The method of synthesis involves mild conditions using
only stoichiometric amounts of pyridine ligands. The methodology
presented herein avoids the use of strong bases to generate the NHC
and devoid of silver carbene complexes which normally results in
stoichiometric amount of silver waste. Finally, in comparison with
other NHC complexes (2, 3 and 11) the PEPPSI type complex 5 was
found to be catalytically more reactive for SuzukieMiyaura
coupling of aryl bromides at ambient conditions. Aryl chlorides
failed undergo SuzukieMiyaura coupling under identical
conditions.
Parameters
5
8
11
Formula
Formula weight
C20H18I2N4Pd
674.58
0.71073
Monoclinic
P21/c
12.6404 (5)
9.8042 (4)
17.7388 (7)
90
99.932 (2)
90
C20H17ClI2N4Pd
709.03
0.71073
Orthorhombic
Pbcn
17.398 (4)
16.028 (4)
16.715 (4)
90
C34 H30I4N8Pd2
1271.06
0.71073
Orthorhombic
Pnnm
10.2717 (5)
12.9221 (6)
15.6387 (8)
90
Radiation
l
(Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
aꢀ
bꢀ
90
90
90
90
gꢀ
V (Å3)
2165.41 (15)
296 (2)
4
4661.1 (2)
293 (2)
8
2075.75 (18)
293 (2)
2
T (K)
Z
Reflections/unique/
3805/
4718/
1903/
Rint
m
0.039
3.72
0.039
3.57
0.082
3.87
mmꢁ1
4. Experimental section
F(000)
range
1272
2.38 to 28.33
1.052
R1 ¼ 0.0397
wR2 ¼ 0.0894
R1 ¼ 0.0464
wR2 ¼ 0.0950
2672
2.1 to 26.05
1.048
R1 ¼ 0.0272
wR2 ¼ 0.0549
R1 ¼ 0.0485
wR2 ¼ 0.0654
1188
2.4 to 26.7
1.170
R1 ¼ 0.0392
wR2 ¼ 0.0980
R1 ¼ 0.0489
wR2 ¼ 0.1046
q
All the reactions were performed under nitrogen atmosphere
and in distilled dichloromethane. Pd(OAc)2(Aldrich) was used as
received. 1,2,3-triazolium salts 1a [16], 1b [17], 1c [18] were pre-
pared according to literature procedure.
Goodness-of-fit on F2
Final R indices
R indices
(all data)
4.1. Instrumentation
complexes 2a, 3,
5 and 11showed higher catalytic activity
Infrared (IR) spectra were recorded on a JASCO 4100 FT-IR
spectrometer. 1H NMR spectra were measured on Bruker AVANCE
400 MHz and 500 MHz spectrometers. Chemical shifts were re-
ported in ppm from tetramethylsilane as internal standard.13C NMR
spectra were recorded on Bruker 100 MHz and 125 MHz spec-
trometers with complete proton decoupling. Chemical shifts were
reported in ppm using residual solvent peaks as internal standard.
High-resolution mass spectra (HRMS) were performed on Micro-
mass ESI Q-TOF micro mass spectrometer equipped with a Harvard
apparatus syringe pump. X-ray crystallographic data were collected
compared to Pd(OAc)2 and PPh3. The results in Table 2 show that
compound 5 is the most efficient catalyst among the ones tested for
the SuzukieMiyaura coupling of aryl bromides with phenyl boronic
acid. When 1 mol% of 5 was employed in the SuzukieMiyaura re-
actions, both electron-rich and electron-deficient aryl bromides
gave moderate to good yields of the corresponding biphenyl de-
rivatives (Table 3). Complex 5 was inactive towards the coupling of
aryl chlorides unless activated such as 2-chloropyridine (Table 3,
entries 7e10).
on
a
Bruker-AXS Kappa CCD-Diffractometer with graphite-
radiation (
3. Conclusions
monochromator Mo K
a
l
¼ 0.71073 Å). The structures
were solved by direct methods (SHELXS-97) and refined by full-
matrix least squares techniques against F2 (SHELXL-97).
Hydrogen atoms were inserted from geometry consideration using
the HFIX option of the program. For thin layer chromatography
We have prepared mononuclear [(Tz)2PdI2] complexes (2aec) in
good yields in short duration using (n-Bu)4NI with triazolium io-
dide (1aec), Pd(OAc)2 in CH2Cl2. Surprisingly, acetate bridged
Table 2
SuzukieMiyaura reactions of 4-bromoanisole with phenylboronic acid in the presence of NHC-Palladium catalysts.a
Entry
Pre-catalyst
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
2a
2a
3
3
2.5
12
2.5
5
2.5
2.5
2.5
2.5
6
2.5
6
2.5
14
40
90
54
94
94
95
95
45
93
95
94
Trace
84
3 þ PPh3
5
5 þ PPh3
11
11
11 þ PPh3
11 þ PPh3
Pd(OAc)2 þ PPh3
Pd(OAc)2 þ PPh3
9
10
11
12
13
a
Reaction conditions: 4-bromoanisole (1 equiv), phenylboronic acid (1.2 equiv), palladium complex 2a (1 mol%), 5 (1 mol%), 3, 11 (0.5 mol%), Pd(OAc)2 (1 mol%), t-BuOK
(1.5 equiv) in EtOH at rt.
b
Isolated yields.

B. Sureshbabu et al. / Journal of Organometallic Chemistry 799-800 (2015) 232e238
237
Table 3
7.36 (t, 1H, J ¼ 8 Hz), 6.89e6.92 (t, 2H, J ¼ 8 Hz), 4.06 (s, 3H), 2.80 (s,
3H), 2.79 (s, 3H). 13C NMR (100 MHz, CDCl3, 25 ꢀC):
27.3, 27.4, 37.6,
SuzukieMiyaura reactions of aryl halides with phenylboronic acid using 5 as cata-
d
lyst.a
122.4, 122.5, 126.3, 127.3, 128.7, 128.9, 130.2, 130.2, 131.1, 137.8,
139.6, 144.5, 159.4, 159.8; IR (KBr, cmꢁ1): 3050, 2977, 2917, 2356,
1624, 1472, 1318, 1164,793, 698; HRMS: [MþꢁI] m/z calcd for
C
22H22N4PdI 574.9924, found 574.9933.
Complex 7: 168 mg, 89%; mp.230 ꢀC.1H NMR (400 MHz, CDCl3,
Entry
Aryl halide (R)
Time (h)
Yield (%)b
25 ꢀC):
d
8.58e8.60 (m, 2H), 8.44e8.47 (m, 2H), 8.0e8.02 (m, 2H),
7.56e7.65 (m, 6H), 6.99e7.01 (d, 2H, J ¼ 8.0 Hz), 4.06 (s, 3H), 2.29 (s,
3H); 13C NMR (100 MHz, CDCl3, 25 ꢀC):
21.1, 37.8, 125.3, 125.7,
1
2
3
4
5
6
7
8
9
10
4-bromoanisole
2-bromoanisole
2.5
2.5
3.5
8
3
6
95
92
92
89
99c
93
38
0
d
4-bromobenzoic acid
2-bromobenzoic acid
1,4-dibromobenzene
1,3,5-tribromobenzene
2-chloropyridine
4-chlorotoluene
4-chlorobenzaldehyde
4-trifluoromethyl-chlorobenzene
127.5, 128.8, 129.0, 130.2, 130.8, 139.9, 144.4, 149.2, 153.2; IR (KBr,
cmꢁ1): 3050, 2927, 2850, 1620, 1494, 1333, 1066, 810, 761. HRMS:
[MþꢁI] m/zcalcd for C21H20N4PdI 560.9767, found 560.9791.
Complex 8: 178 mg, 91%; mp.240e245 ꢀC.1H NMR (400 MHz,
8
CDCl3, 25 ꢀC):
d
8.79 (d, 1H, J ¼ 4 Hz), 8.71 (d, 1H, J ¼ 4 Hz), 8.41 (d,
24
24
24
0
0
2H, J ¼ 8 Hz), 7.98 (d, 2H, J ¼ 8 Hz), 7.57e7.66 (m, 7H), 7.13e7.17 (m,
13
1H), 4.06 (s, 3H); C NMR (100 MHz, CDCl3, 25 ꢀC):
d 37.8, 124.6,
a
Reaction conditions: Aryl halide (1 equiv), phenylboronic acid (1.2 equiv),
125.7, 127.3, 128.9, 129.0, 130.3, 130.7, 132.1, 134.1, 137.5, 138.5, 139.7,
144.3, 151.9, 152.8; IR (KBr, cmꢁ1): 3078, 2921, 2850, 2366, 1596,
1462, 1066, 761, 691; HRMS: [MþꢁI] m/zcalcd for C20H17ClN4PdI
580.9221, found 580.9226.
palladium complex 5 (1 mol%), t-BuOK (1.5 equiv) in EtOH at rt.
b
Isolated yields.
p-terphenyl is the product.
c
(TLC) analysis, E-Merck pre-coated TLC plates (silica gel 60 F254
grade, 0.25 mm) were used.
4.2.3. Synthesis of complex 9
A mixture of 1,2,3-triazolim salt iodide 1a (100 mg, 0.28 mmol)
and KCl (81 mg,1.1 mmol) in CH2Cl2 stirred at room temperature for
1 h. Then Pd(OAc)2 (67 mg, 0.30 mmol) was added. The reaction
mixture stirred at room temperature for 15 h. The mixture was
filtered and evaporates the solvent to give crude product. The crude
product dissolved in acetone and evaporates the solvent to give
greenish yellow product. Complex 9 has been reported from our
laboratory [16] and its characterization data are given in the SI.
4.2. Synthesis and characterization
4.2.1. General procedure for the preparation of mononuclear
palladium(II) diiodo NHC complexes (2aec)
A mixture of Pd(OAc)2 (0.6 equiv) and (n-Bu)4NI (1 equiv) in
dichloromethane (20 mL) was stirred at room temperature under
N2 atmosphere for 2 h to give a dark brown to black colour solution.
1,2,3-Triazolium salt iodide (1aec) (100 mg, 1 equiv) was added to
the mixture and stirring was continued for 8e12 h depending upon
the substrate to give orange or light yellow solution. Evaporation of
solvent gave a gummy product. The product was purified by col-
umn chromatography on silica gel using CH2Cl2: MeOH (9:1) as
eluent. Complexes 2aec were reported from our laboratory [12]
and their characterization data are given in the SI.
4.2.4. General procedure for the synthesis of bridged binuclear
complexes
A mixture of triazolium iodide 1a (100 mg, 0.27 mmol),
Pd(OAc)2 (67 mg, 0.3 mmol), KI (91 mg, 0.55 mmol), bridging ligand
(0.16 mmol, 0.6 equiv.) in dichloromethane gave a dark brown so-
lution which was stirred at room temperature for16 h (11 and 12)
and 3 d (10). During this period the dark brown colour changed to
dark orange. The reaction mixture was filtered and solid was
washed with CH2Cl2. The filtrate was evaporated to give the crude
product which was purified by washing with hexane and ether to
give the desired complexes as yellow solids.
4.2.2. General procedure for the synthesis of PEPPSI type complexes
A mixture of triazolium iodide 1a (100 mg, 0.27 mmol),
Pd(OAc)2 (67 mg, 0.3 mmol), KI (180 mg, 1.1 mmol) in dichloro-
methane gave a dark brown solution which was allowed to stir at
room temperature for 16 h. The reaction mixture was filtered and
solvent was evaporated to give binuclear complex 3 in 95% of yield.
In a one pot synthesis pyridine derivative (0.26 mmol, 2 equiv.) in
CH2Cl2 was directly added dropwise to the above reaction mixture.
Upon addition of pyridine derivatives the colour of the reaction
mixture changed from dark brown to dark yellow instantaneously.
The reaction mixture was stirred at room temperature for 12 h (5, 7,
and 8) and 2 d (6), respectively. Evaporation of the solvent gave the
crude product which was purified by washing with hexane and
ether to give PEPPSI type complexes as dark orange or yellow
crystalline solids.
Complex 10: 165 mg, 86%; 1H NMR (400 MHz, CDCl3, 25 ꢀC):
d
8.91 (d, 4H, J ¼ 8 Hz), 8.42 (d, 4H, J ¼ 8 Hz), 7.98e8.0 (m, 4H),
7.57e7.65 (m, 12H), 7.32 (d, 4H), 4.07 (s, 6H). 13C NMR (100 MHz,
CDCl3, 25 ꢀC):
d 37.8, 122.0, 125.7, 127.3, 128.9, 129.0, 130.3, 130.8,
134.8, 139.8, 144.4, 145.7, 154.7; IR (KBr, cmꢁ1): 3057, 2927, 2846,
2356, 1603, 1494, 1329, 1269, 1178, 1073, 1020,765, 694; HRMS:
[Mþ1] m/zcalcd for C40H35N8Pd2I4 1346.7233, found 1346.7232.
Complex 11: 160 mg, 92%; 1H NMR (400 MHz, CDCl3, 25 ꢀC):
d
8.84 (s, 4H), 8.31 (d, 4H, J ¼ 8 Hz), 7.90 (d, 4H, J ¼ 8 Hz), 7.56e7.62
(m, 12H), 4.04 (s, 6H); 13C NMR (100 MHz, CDCl3, 25 ꢀC):
d
37.9,
125.7, 127.0, 128.9, 129.1, 130.4, 130.7, 139.5, 144.2, 149.2; IR (KBr,
cmꢁ1): 3050, 2921, 2846, 1596, 1476, 1455, 1420, 1333, 1185, 1069,
1020,771, 694; HRMS: [MþNa] m/zcalcd for C34H30N8Pd2I4Na
1292.6740, found 1292.6735.
Complex 5: 170 mg, 92%, mp.225 ꢀC.1H NMR (500 MHz, CDCl3,
25 ꢀC):
d
8.76e8.78 (m, 2H), 8.44e8.46 (m, 2H), 8.0e8.02 (m, 2H),
7.56e7.65 (m, 7H), 7.17e7.20 (t, 2H), 4.06 (s, 3H); 13C NMR
Complex 12: 162 mg, 92%; 1H NMR (400 MHz, CDCl3, 25 ꢀC):
(125 MHz, CDCl3, 25 ꢀC):
d
37.8, 124.3, 125.7, 127.4, 128.8, 129.0,
d
8.32 (d, 4H, J ¼ 4 Hz), 7.89 (d, 4H, J ¼ 8 Hz), 7.54e7.57 (m, 12H),
130.2, 130.8, 135.5, 137.3, 139.8, 144.4, 153.9; IR (KBr, cmꢁ1): 3072,
3.99 (s, 6H), 3.30 (s, 12H); 13C NMR (100 MHz, CDCl3, 25 ꢀC):
d
29.8,
2976, 2840, 1615, 1550, 1490, 790, 692; HRMS: [MþꢁI] m/z calcd for
37.8, 54.1, 125.5, 127.2, 127.7, 128.7, 128.8, 130.2, 130.2, 130.6, 139.6,
143.7; IR (KBr, cmꢁ1): 3057, 2927, 2850, 1592, 1490, 1464, 1329,
1269, 1182, 793, 758; HRMS: [Mþ1] m/zcalcd for C36H39N8Pd2I4
1302.7546, found 1302.7543.
C
20H18N4 PdI 546.9611, found 546.9630.
Complex 6: 159 mg, 82%; mp.255 ꢀC.1H NMR (400 MHz, CDCl3,
25 ꢀC):
d 8.31e8.34 (m, 2H), 7.97e8.0 (m, 2H), 7.56e7.64 (m, 6H),

238
B. Sureshbabu et al. / Journal of Organometallic Chemistry 799-800 (2015) 232e238
[9] a) C. Valente, S. Calimsiz, H.K. Hoi, D. Mallik, M. Sayah, M.G. Organ, Angew.
Acknowledgements
Chem. Int. Ed. 51 (2012) 3314e3332;
b) M.G. Organ, G.A. Chass, D.C. Fang, A.C. Hopkinson, C. Valente, Synthesis 17
(2008) 2776e2797;
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Inorg. Chem. (2014) 3600e3607;
BS thanks CSIR, New Delhi for a fellowship and SS thanks CSIR
and DST, New Delhi, for research grants and the Department of
Chemistry, IIT Madras for infrastructure.
e) Y.-C. Lin, H.-H. Hsueh, S. Kanne, L.-K. Chang, F.-C. Liu, J.J.B. Lin, Organo-
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(2012) 11354e11357;
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2015.10.002.
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