Palladium(II) complexes featuring bidentate pyridine-triazole ligands: Synthesis, structures, and catalytic activities for Suzuki-Miyaura coupling reactions

Article abstract of DOI:10.1016/j.jorganchem.2013.10.046

Preparation of the Pd(II) complexes containing 2-(4-R-1,2,3-triazol-1-yl) pyridine [R = C₆H₅ (1), NC₅H₄ (2), n-C₆H₁₃ (3)] were described. Crystal structures of 1 and 2 revealed a square planar geometry with bidentate ligand coordination to Pd using different N donor of the triazole ring. Catalytic studies indicated that 1-3 exhibited moderate to high activity for Suzuki-Miyaura coupling between aryl bromides and phenylboronic acid under mild and aerobic conditions.

Full text of DOI:10.1016/j.jorganchem.2013.10.046

Journal of Organometallic Chemistry 750 (2014) 35e40
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Palladium(II) complexes featuring bidentate pyridineetriazole
ligands: Synthesis, structures, and catalytic activities for Suzukie
Miyaura coupling reactions
Sudarat Jindabot a b, Kriengkamol Teerachanan c, Pech Thongkam a,b,
,
b,
a,
b
d
d
Supavadee Kiatisevi , Tossapol Khamnaen , Phairat Phiriyawirut ,
Sumate Charoenchaidet , Thanasat Sooksimuang , Palangpon Kongsaeree ,
Preeyanuch Sangtrirutnugul
Center for Catalysis, Department of Chemistry, Faculty of Science, Mahidol University, 272 Rama VI Rd., Bangkok 10400, Thailand
d
e
b
a,b,
*
a
b
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Rama VI Rd., Bangkok 10400,
Thailand
c
The Materials Science and Engineering Program, Faculty of Science, Mahidol University, 272 Rama VI Rd., Thung Phaya Thai, Ratchathewi, Bangkok 10400,
Thailand
d
Thai Polyethylene Co., Ltd., 10 I-1 Rd., Mapta Phut Industrial Estate, Muang District, Rayong 21150, Thailand
National Metal and Materials Technology Center, Thailand Science Park, Klong Luang, Pathumthani 12120, Thailand
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 September 2013
Received in revised form
Preparation of the Pd(II) complexes containing 2-(4-R-1,2,3-triazol-1-yl)pyridine [R ¼ C H (1), NC H
6
5
5 4
(
6
2), n-C H13 (3)] were described. Crystal structures of 1 and 2 revealed a square planar geometry with
bidentate ligand coordination to Pd using different N donor of the triazole ring. Catalytic studies indi-
cated that 1e3 exhibited moderate to high activity for SuzukieMiyaura coupling between aryl bromides
and phenylboronic acid under mild and aerobic conditions.
25 October 2013
Accepted 26 October 2013
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
PyridineeTriazole
Palladium
Suzuki reaction
1. Introduction
diketiminate [17], and pyridine-based compounds [18e23] as
catalyst supports. Rigidity and high stability towards air of the
Pd-catalyzed cross coupling is considered one of the most
important reactions in organic synthesis [1,2]. Among many types
of CeC cross-coupling reactions, SuzukieMiyaura coupling [3] is
most widely used because of its high catalytic efficiency,
commercially available starting materials, and low catalyst toxicity.
Most prominent Pd catalysts for SuzukieMiyaura coupling reaction
are supported by electron-rich phosphane [4e8] or N-heterocyclic
carbene (NHC) ligands [9e15]. Although these catalysts are highly
active, their sensitivity to oxygen requires an inert atmosphere to
carry out the cross-coupling reactions. More recent progress in the
area of Pd-catalyzed cross-coupling has shown an increase in the
nitrogen-based ligands lead to more stabilization of the resulting
catalysts. As a result, the cross-coupling reactions can usually be
performed at high temperatures and sometimes under aerobic
conditions [24,25]. Despite various examples of Pd catalysts for
Suzuki reaction, development of such catalysts to allow reduced
catalyst loading, mild reaction conditions, and wide substrate scope
is still of importance.
1,2,3-Triazole compounds have emerged as promising ligands in
catalysis [26e31]. The preparation involves the copper-catalyzed
azide-alkyne cycloaddition (CuAAC) or “click” reaction to afford
the 1,4-disubstituted-1,2,3-triazole compound in good yields [32e
34]. The aromatic, thermally robust triazole product is analogous
to pyridine but the substituents on the triazole ring can be more
readily adjusted depending on the organic azide and terminal
alkyne substrates. Based on facile synthesis and substituent
modification, the triazole-based ligands appear attractive as cata-
lyst supports. Recently, the pyridineetriazole compounds 2-((4-R-
use of nitrogen-based chelates such as porphyrin [16],
b-
*
Corresponding author. Center for Catalysis, Department of Chemistry, Faculty of
Science, Mahidol University, 272 Rama VI Rd., Bangkok 10400, Thailand.
E-mail address: preeyanuch.san@mahidol.ac.th (P. Sangtrirutnugul).
0
022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.10.046

36
S. Jindabot et al. / Journal of Organometallic Chemistry 750 (2014) 35e40
1
,2,3-triazol-1-yl)methyl)pyridine (R ¼ C
6
H
5
, n-C
3
H
7
) have been
8.52 (m, 1H, ArH), 8.25 (d J ¼ 8 Hz, 1H, ArH), 7.94 (d J ¼ 7 Hz, 3H,
ArH), 7.47 (t J ¼ 7 Hz, 2H, ArH), 7.37 (m, 2H, ArH).
investigated as ligands for Pd-catalyzed Suzuki reactions [18,19].
3
However, the corresponding Pd(II) catalysts [Pd(
,2,3-triazol-1-yl)methyl)pyridine)][X] (X ¼ BF
h
-C
3
H
5
)(2-((4-R-
2
An alternative synthesis involved the use of Cu(OTf) (0.15 g,
0
2
1
4
, ClO
4
) were air-
0.42 mmol) and Cu (5 cm sheet) as pre-catalysts under the same
sensitive, most likely as a result of the PdeC(allyl) bond. Along
the same line, a series of mono- and multi-nuclear palladium
complexes containing 2-pyridyl-1,2,3-triazole ligands have
recently been investigated as catalysts for CeC cross coupling re-
actions [35]. We were particularly interested in exploring the
bidentate pyridineetriazole ligands of the type 2-(4-R-1,2,3-
1
reaction conditions. The product L was obtained in 79% yield
(0.73 g, 3.28 mmol).
2.1.2. 2-[4-(2-Pyridyl)-1,2,3-triazol-1-yl]pyridine (L
Compound L was prepared in an identical manner to L
2-azidopyridine (1.00 g, 8.30 mmol), (CuOTf) $C (0.42 g,
0.83 mmol), and 2-ethynylpyridine (0.94 g, 9.10 mmol). Recrys-
tallization by slow evaporation of the benzene solution afforded L
2
)
2
1
using
2
6 6
H
triazol-1-yl)pyridine (R ¼ C
Miyaura cross-coupling reaction. It was expected that an increase in
ligand rigidity and conjugation combined with weaker electron-
donating property would result in the corresponding Pd(II) com-
plexes PdCl [2-(4-R-1,2,3-triazol-1-yl)pyridine] that were stable in
6 5 5 4 6
H , NC H , n-C H13) for Suzukie
2
H
1
p
as a crystalline brown solid in 65% yield (1.20 g, 5.40 mmol).
NMR (500 MHz, CDCl ):
3
d
9.17 (s, 1H, N]CH), 8.64 (d J ¼ 5 Hz, 1H,
2
pyH), 8.53 (d J ¼ 5 Hz, 1H, pyH), 8.23 (m, 2H, pyH), 7.93 (m, 1H,
pyH), 7.81 (dt J ¼ 8 Hz, 2 Hz, 1H, pyH), 7.36 (m, 1H, pyH), 7.26 (m,
air and more active as catalysts toward CeC cross-coupling
reactions.
13
1
3
1H, pyH). C{ H} NMR (125 MHz, CDCl ): d 150.2, 149.8, 149.3,
1
48.9, 148.8, 139.1, 136.9, 123.8, 123.1, 120.6, 119.6, 114.0 (aryl C). MS
þ
(ESI, positive) m/z for [C12
H
9
N
5
þ Na] , found: 246.10. Anal. Calcd.
2
. Experimental section
for C12
H
9
N
5
: C, 64.56; H, 4.06; N, 31.37. Found: C, 64.42; H, 3.86; N,
31.21.
Ligand syntheses were carried out under Ar using standard
Schlenk techniques or in a Braun drybox. Toluene (Fisher Scienti-
fic) was dried using PURE SOLV MD-5 solvent purification system
from Innovative Technology Inc. All other solvents were used as
2
.1.3. 2-(4-Hexyl-1,2,3-triazol-1-yl)pyridine (L
Compound L was prepared in an identical manner to L
azidopyridine (0.37 g, 3.08 mmol), (CuOTf) $C
.31 mmol), and 1-octyne (0.50 mL, 3.36 mmol). The product was
purified by column chromatography over silica gel using hexane/
ethyl acetate (7:3) as eluent to afford L as yellow oil in 54% yield
0.38 g, 1.65 mmol). H NMR (500 MHz, CDCl ):
8.40 (dd J ¼ 4 Hz,
Hz, 1H, pyH), 8.23 (s, 1H, N]CH), 8.10 (dt J ¼ 8 Hz, 1 Hz, 1H, pyH),
),1.66 (p
3
)
3
1
from 2-
(0.16 g,
2
6 6
H
received. (CuOTf)
2
$C
6 6
H (Aldrich) was used as received and stored
0
under Ar. Cu(OTf)
2
, ethylenediaminetetraacetic acid (EDTA), 1-
octyne, phenylacetylene, 2-ethynylpyridine, phenylboronic acid,
and aryl bromides were purchased from Aldrich and used without
3
1
(
1
3
d
further purification. 2-Azidopyridine [36] and Pd(cod)Cl
cod ¼ 1,5-cyclooctadiene) [37] were prepared according to the
2
(
7
.82 (m,1H, pyH), 7.24 (m,1H, pyH), 2.73 (t J ¼ 8 Hz, 2H, CH
2
literature.
13
1
J ¼ 8 Hz, 2H, CH
2
), 1.34e1.22 (m, 6H, CH
2 3
), 0.81 (m, 3H, CH ). C{ H}
1H (500.1 MHz), 13C{ H} (124.7 MHz) NMR spectra were ac-
quired on Bruker AV-500 spectrometer equipped with a 5 mm
proton/BBI probe. All NMR spectra were recorded at room tem-
perature and referenced to protic impurities in the deuterated
1
NMR (125 MHz, CDCl
3
): d 149.4, 148.9, 148.4, 139.0, 123.1, 118.0,
113.7 (aryl C), 31.6, 29.2, 28.8, 25.6, 22.5, 14.0 (hexyl C). MS (ESI,
þ
positive) m/z for [C13
C
2
H
18
N
4
þ Na] , found: 253.15. Anal. Calcd. for
13 18 4
H N : C, 67.80; H, 7.88; N, 24.33. Found: C, 68.07; H, 8.06; N,
1
13
1
solvent for H and solvent peaks for C{ H}. Multiplicities are re-
ported as follows: s (singlet), d (doublet), t (triplet), p (pentet), m
4.67.
(
multiplet), dd (doublet of doublets), or dt (doublet of triplets).
2
.1.4. PdCl
To a CH
was added L
stirred at room temperature. After 24 h, brown yellow pre-
cipitates were filtered and washed with 30 mL of CH Cl
2
(L
1
) (1)
solution (20 mL) of Pd(cod)Cl
(0.15 g, 0.67 mmol) and the reaction mixture was
Elemental analyses were conducted by Chemistry Department,
Mahidol University. The solutions obtained from the catalytic ex-
periments were analyzed by GLC on a 6850 Agilent Technologies
gas chromatograph. GCeMS spectra were recorded on an HP 5890
series II gas chromatograph interfaced to an HP 5971 quadrupole
mass detector.
2
Cl
2
2
(0.19 g, 0.67 mmol)
1
2
2
.
1
The complex 1 was obtained in 66% yield (0.18 g, 0.44 mmol). H
NMR (500 MHz, CD CN):
9.19 (s, 1H, N]CH), 9.13 (d J ¼ 5 Hz,
3
d
1
H, ArH), 8.39 (m, 1H, ArH), 8.01 (d J ¼ 8 Hz, 1H, ArH), 7.94 (d
2
2
.1. Synthesis and characterization
J ¼ 7 Hz, 2H, ArH), 7.74 (t J ¼ 7 Hz, 1H, ArH), 7.57 (t J ¼ 7 Hz, 2H,
ArH), 7.52 (t J ¼ 7 Hz, 1H, ArH). MS (ESI, positive) m/z for
þ
.1.1. 2-(4-Phenyl-1,2,3-triazol-1-yl)pyridine (L
1
)
[C13
H
10
N
4
PdCl
PdCl
2.48; N, 14.03.
2
þ
Na] , found: 422.92. Anal. Calcd. for
The preparation was slightly modified from the literature
38]. Under Ar, to a foil-wrapped 100 mL Schlenk flask equipped
with a stir bar, 2-azidopyridine (0.50 g, 4.16 mmol), and
CuOTf) $C (0.21 g, 0.42 mmol) were added toluene (20 mL),
C
13
H
10
N
4
2
: C, 39.08; H, 2.52; N, 14.02. Found: C, 38.97; H,
[
(
2
6
H
6
2.1.5. PdCl
To a stirring 80 mL CH
0.90 mmol) was added a 20 mL CH
2 2
(L ) (2)
followed by PhC^CH (0.50 mL, 4.58 mmol). The reaction mixture
was heated at 120 C for 24 h. After cooling to room temperature,
2
Cl
2
solution of Pd(cod)Cl (0.27 g,
2
Cl solution of L (0.20 g,
2
ꢀ
2
2
all volatiles were removed under reduced pressure. The
0.90 mmol) at room temperature. The solution was stirred for 4 h,
remaining solid was then diluted with CH
filtered. The brown filtrate was stirred in a 10% v/v NH
lution (30 mL) of EDTA (0.12 g, 0.42 mmol) for 2 h. The solution
was extracted with CH Cl
(3 ꢁ 30 mL), washed with saturated
aqueous solution of NaCl (30 mL), and dried in anhydrous
Na SO All volatiles were then removed under vacuum.
2
Cl
2
(30 mL) and
after which the reaction mixture was filtered and the precipitates
4
OH so-
were washed with 30 mL of CH
solid in 87% yield (0.31 g, 0.78 mmol). H NMR (500 MHz, DMSO-
):
2 2
Cl , resulting in a brown yellow
1
2
2
d
6
d
10.1 (s, 1H, N]CH), 9.02 (d J ¼ 6 Hz, 1H, pyH), 8.71 (d J ¼ 5 Hz,
1H, pyH), 8.42 (d J ¼ 8 Hz, 1H, pyH), 8.35 (t J ¼ 7 Hz, 1H, pyH), 8.24
(dt J ¼ 7 Hz, 2 Hz, 1H, pyH), 8.12 (d J ¼ 8 Hz, 1H, pyH), 7.73 (m, 2H,
2
4
.
1
3
1
Slow evaporation of the benzene solution at room temperature
produced a colorless crystalline solid in 63% yield (0.58 g,
6
pyH). C{ H} NMR (125 MHz, DMSO-d ): d 149.5,149.1,148.4,148.0,
147.4, 141.4, 140.8, 126.0, 125.9, 122.7, 122.5, 115.9 (aryl C). MS (ESI,
.61 mmol). 1H NMR (500 MHz, CDCl
):
d
8.81 (s, 1H, N]CH),
positive) m/z for [C12
H
9
N
5
PdCl
þ 2Na] , found: 446.99. Anal.
2þ
2
3
2

S. Jindabot et al. / Journal of Organometallic Chemistry 750 (2014) 35e40
37
Calcd. for C12
H, 2.13; N, 17.25.
H
9
N
5
PdCl
2
: C, 35.98; H, 2.26; N, 17.48. Found: C, 35.78;
Table 1
Crystallographic data.
1
2
2
.1.6. PdCl
Synthesis of complex 3 followed that of 1. Pd(cod)Cl
.29 mmol) and L (0.067 mg, 0.29 mmol) was stirred in 10 mL
Cl at room temperature for 4 d, after which the solvent was
2 3
(L ) (3)
Empirical formula
Formula weight
T (K)
C
13
H
10Cl
2
N
4
Pd
C
12 9 2 5
H Cl N Pd
2
(0.083 mg,
399.55
293(2)
Monoclinic
P2 /c
9.3309(4)
14.8679(6)
11.9702(6)
90.00
122.549(2)
90.00
400.54
293(2)
Monoclinic
P2 /c
8.0063(3)
18.6094(9)
9.3511(4)
90.00
103.547(2)
90.00
0
CH
3
Crystal system
Space group
2
2
1
1
removed in vacuo. The resulting yellow solid was washed with
diethyl ether (15 mL). Crystallization by a layer diffusion of DMF and
EtOH at room temperature afforded the complex 3 in 41% yield
ꢀ
a (A)
ꢀ
b (A)
c ( ꢀA )
1
ꢀ
(
0.048 mg, 0.12 mmol). H NMR (500 MHz, CD
3
CN):
d
9.06 (d
a
b
g
( )
ꢀ
( )
J ¼ 6 Hz,1H, pyH), 8.62 (s, 1H, N]CH), 8.33 (t J ¼ 8 Hz,1H, pyH), 7.93
d J ¼ 8 Hz, 1H, pyH), 7.68 (t J ¼ 7 Hz, 1H, pyH), 2.84 (t J ¼ 8 Hz, 2H,
trzCH ), 1.40e1.32 (m, 6H, CH ), 0.90 (m,
), 1.72 (p J ¼ 8 Hz, 2H, CH
H, CH ). C{ H} NMR (125 MHz, CD CN): 152.0, 150.1, 147.8,
ꢀ
( )
(
ꢀ
3
V (A )
1399.80(11)
4
1.906
1354.48(10)
4
1.964
2
2
2
Z
13
1
ꢂ3
3
3
3
d
r
calc (mg mm
)
ꢂ1
m
(mm
)
1.701
1.760
144.5, 126.6, 122.9, 114.5 (aryl C), 32.2, 29.4, 29.3, 26.4, 23.3, 14.4
þ
Crystal size (mm)
Color, habit
Reflections collected
Independent reflections (Rint
Data/restraints/parameters
Goodness of fit
0.20 ꢁ 0.18 ꢁ 0.15
Orange, cube
6325
3214 (0.0481)
3214/0/182
1.13
0.30 ꢁ 0.22 ꢁ 0.20
Orange, cube
6145
2668 (0.0561)
2668/0/181
1.04
(
4
hexyl C). MS (ESI, positive) m/z for [C13
30.98. Anal. Calcd. For C13 PdCl
Found: C, 38.32; H, 4.52; N, 14.03.
H
18
N
4
PdCl
2
þ Na] , found:
H
18
N
4
2
: C, 38.30; H, 4.45; N, 13.74.
)
2.2. X-ray analysis
Final R indices [I > 2
s
(I)]
R
1
¼ 0.0595
wR
¼ 0.1590
¼ 0.0823
wR
R
1
¼ 0.0311
wR
¼ 0.0809
¼ 0.0344
wR
2
2
The single crystal X-ray analyses of pyridine-tetrazole, com-
Final R indices (all data)
R
1
R
1
plexes 1 and 2 were carried out at the Mahidol crystallographic
facility. Diffraction measurements were made on a 4 K Bruker
2
¼ 0.1703
2
¼ 0.0831
Largest diff. peak/hole
1.056 and ꢂ1.268
0.929 and ꢂ0.614
SMART [39] CCD area detector diffractometer using graphite-
ꢀ
monochromated Mo K
a
radiation (
l
¼ 0.71073 A). Crystals were
mounted in paratone oil and held at room temperature during data
collection. Cell constants and an orientation matrix for data
collection were obtained from a least-square refinement using the
Table 2
Selected bond lengths (A).
ꢀ
ꢀ
ꢀ
measured positions of reflections in the range 0.998 <
q
<29.6 (for
(for 1), and
ꢀ
.998 < q < 26.7 (for 2). The frame data were integrated by the
1
2
Pdðpytrz PrÞCl a
i
2
ꢀ
ꢀ
pyridine-tetrazole), 0.407
<
q
<
28.7
PdeNpy
PdeNtrz
PdeCl1
PdeCl2
N2eN3
N3eN4
2.054(5)
2.014(5)
2.271(2)
2.274(2)
1.372(6)
1.274(7)
2.066(2)
1.999(2)
2.270(1)
2.275(1)
1.308(3)
1.342(3)
2.055(4)
2.007(4)
2.264(1)
2.285(1)
1.312(6)
1.349(5)
ꢀ
0
program SAINT [32] and corrected for Lorentz and polarization
effects. The structure was solved by the maXus crystallographic
software package [40], using direct methods (SIR97) [41] and
2
refined by full-matrix least-squares method on (Fobs
)
using the
a
Ref. [46].
SHELXL97 software package [42].
For pyridine-tetrazole. Crystals were grown from slow evapo-
2
ration of the 1:1 THF:Et O solution of pyridine-tetrazole at room
temperature.
Table 3
Selected bond angles ( ).
For 1. Orange crystals were grown by layering MeOH onto a DMF
solution of 1 at room temperature.
ꢀ
1
2
Pdðpytrz PrÞCl a
i
2
For 2. Orange crystals were grown by layering EtOH onto a DMF
solution of 2 at room temperature.
NpyePdeNtrz
Cl1ePdeCl2
79.65(19)
90.97(5)
95.86(14)
93.46(14)
11.5(9)
79.89(10)
91.19(3)
93.89(7)
95.15(8)
27.2(4)
80.60(20)
91.01(5)
94.0(1)
94.4 (1)
52.3(2)
1
Both complexes 1 and 2 crystallize in the P2 /c space group. All
N
trzePdeCl1
pyePdeCl2
non-hydrogen atoms were refined anisotropically while the
hydrogen atoms were placed in calculated positions and not
refined. Crystallographic data of both 1 and 2 are listed in Table 1.
Selected bond lengths and angles are shown in Table 2.
N
b
u
a
Ref. [46].
b
The dihedral angle between the triazole and aryl substituent planes.
2.3. General procedures for SuzukieMiyaura cross-coupling
reactions
after purification by thin layer chromatography on silica gel using
hexane/ethyl acetate (9:1) as eluent. For 4-(NO )C Ph, hexane
2
6 4
H
The Pd catalyst stock solution (2.5 mM) was freshly prepared
prior to use by diluting the Pd catalyst (0.025 mmol) with DMF in a
0 mL volumetric flask. Then, the round bottom flask was succes-
sively charged with ArBr (1.00 mmol), PhB(OH) (1.10 mmol), K CO
1.00 mmol), H O (5 mL), DMF (5 mL), followed by addition of an
was used as eluent. The resulting biaryl products were character-
1
ized by H NMR spectroscopy and GCeMS methods.
1
2
2
3
(
2
2.4. Mercury poisoning tests
appropriate amount of the Pd catalyst solution. The reaction
mixture was stirred at room temperature for a given time, after
To a round bottom flask charged with Hg (0.10 g, 0.50 mmol,
which the product was extracted by 3 ꢁ 15 mL of CH
combined organic layers were washed with 50 mL of saturated
aqueous solution of NaCl, dried over anhydrous Na SO , and the
solvent was removed in vacuo. The isolated yields were determined
2
Cl
2
. The
Hg:Pd ¼ 500:1) was added 4-(NO
PhB(OH) (0.13 g, 1.10 mmol), K CO
of DMF:H O, followed by 0.0010 mmol of Pd catalyst (1 or 2). The
reaction mixture was stirred at room temperature and, after a given
2
)C
6 4
H Br (0.20 g, 1.00 mmol),
2
2
3
(0.14 g, 1.00 mmol) and 10 mL
2
4
2

38
S. Jindabot et al. / Journal of Organometallic Chemistry 750 (2014) 35e40
1 3
Scheme 1. Synthesis of L eL .
time, the coupling product was extracted by 3 ꢁ 15 mL of CH
The combined organic layers were washed with 50 mL of saturated
aqueous solution of NaCl, dried over anhydrous Na SO , and the
2 2
Cl .
Scheme 2. Synthesis of 1e3.
2
4
solvent was removed in vacuo. The isolated yields were determined
after purification by thin layer chromatography on silica gel using
hexane as eluent.
Interestingly, only the pyridyl-substituted Pd(II) complex 2 was
1
stable in DMSO-d
6
, as the H NMR spectra of 1 and 3 in DMSO-d
6
revealed the presence of free ligands as a major species, suggesting
ligand displacement possibly by DMSO molecules.
3
. Results and discussion
Single crystals of complexes 1 and 2 suitable for X-ray crystal-
lographic analyses were grown at room temperature from a solu-
tion mixture of DMF/MeOH and DMF/EtOH, respectively. For both
complexes, the Pd(II) centers possess a square planar geometry as
evidenced by the angle sum about Pd of 359.9 (1) and 360.1 (2).
For 1, Pd coordination to the pyridyl nitrogen N1 and the medial
nitrogen N3 of the triazole ring was observed (Fig. 2). On contrary,
3
.1. Pyridineetriazole ligands and their Pd(II) complexes
The 2-(4-R-1,2,3-triazol-1-yl)pyridine compounds [R ¼ C
6 5
H
1 5 4 2 6 3
(L ), NC H (L ), n-C H13 (L )] were prepared following the litera-
ture [38]. A reaction between 2-azidopyridine and one equiv of a
terminal alkyne catalyzed by [Cu(OTf)]
more air stable mixture of Cu (OTf)
ꢀ
ꢀ
2
$C
6
H
6
or, alternatively, the
II
0
ꢀ
2
and Cu in toluene at 120 C
2
according to the X-ray crystallographic analyses, ligand L favors Pd
afforded the corresponding pyridineetriazole products L
1
3
eL in
binding at the pyridyl nitrogen N1 and the proximal nitrogen N2 or
the 2-(1H-1,2,3-triazol-4-yl)pyridine chelate pocket as shown in
Fig. 3. Although crystal structures of the related Pd(II) complexes
moderate yields (54e79%; Scheme 1). It should be noted that the 2-
azidopyridine substrate is expected to exist as an equilibrium be-
tween closed form (tetrazole) and open form (azide) in solution (Eq
(
1)) [43]. We found that, in solid state, only tetrazole form was
obtained (Fig. 1). The presence of the tetrazole form of 2-
azidopyridine may explain the harsh reaction conditions and
I
reactive Cu catalyst generally required for formation of the pyri-
1 3
dineetriazole products L eL [36,38,44,45].
N
N
N
N3
N
N
azide
tetrazole
The corresponding Pd(II) complexes 1e3 were prepared in
moderate to high yields (41e87%) as brown yellow solids from re-
actions between an equimolar ratio of Pd(cod)Cl
tive ligands L eL in CH Cl at room temperature (Scheme 2). Due
to relatively low solubility of 1e3 in CH Cl and CHCl , their NMR
spectra were acquired in either CD CN or DMSO-d solution.
2
and the respec-
1
3
2
2
2
2
3
Fig. 2. Thermal ellipsoid diagram of 1 at the 30% probability level.
3
6
Fig. 1. Thermal ellipsoid diagram of pyridine-tetrazole at the 30% probability level.
Fig. 3. Thermal ellipsoid diagram of 2 at the 30% probability level.

S. Jindabot et al. / Journal of Organometallic Chemistry 750 (2014) 35e40
39
supported by bidentate pyridineetriazole ligands have been pre-
viously reported [18,19,46,47], to the best of our knowledge, 1
represents the first crystal structure of the five-membered palla-
dacylic ring with 2-(4-phenyl-1,2,3-triazol-1-yl)pyridine binding.
Similar to previous findings, the PdeNtrz(triazole) bond distances
are, in general, slightly shorter than those of the PdeNpy(pyridine)
bonds [18,19], suggesting stronger coordinating ability of the tri-
azole nitrogen. The bite angles are relatively small but comparable
better solubility of 1 in DMF compared to other solvents studied.
Under the same conditions, complexes 2 and 3 exhibited only
moderate catalytic efficiency (entries 7 and 10). Lowering the
catalyst loadings to 0.01 mol% resulted in significant reduction in
product yields for catalysts 1 and 2 (entries 5, 6, 8, and 9).
3.3. Substrate scope
2
to the related known structure of PdCl [2-(1-(4-isopropylphenyl)-
On the basis of different catalyst structures and efficiencies,
i
Pr
1
,2,3-triazol-4-yl)pyridine] ðPdðpytrz ÞCl Þ [46], as shown in
2
complexes 1 and 2 were chosen for further studies. A coupling
between PhB(OH) and different para-substituted aryl bromides,
using 0.1 mol% of 1 or 2 in 1:1 DMF:H O at room temperature
Table 3. Furthermore, the dihedral angles between the triazole ring
2
and aryl substituent planes appear much smaller for 1 and 2
2
ꢀ ꢀ
11.5(9) and 27.2(4) , respectively) compared to that of
(
afforded the corresponding biaryl products in moderate to high
yields (entries 1e4, Table 5.)
iPr
ꢀ
Pdðpytrz ÞCl (52.3(2) ). Based on the observed bond distances,
2
the PdeNtrz bond distance of 1, of which Ntrz is the medial nitrogen,
is the longest compared to the other two structures, where the Pd
center binds to the proximal nitrogen of the triazole ring. We reason
that the medial nitrogen surrounded by two electron-withdrawing
nitrogen atoms experiences stronger inductive effect and, as a
result, is a weaker donor compared to the proximal nitrogen, which
connects to nitrogen and a carbon atom. This hypothesis is in
agreement with our experimental observations. For example,
compared to preparations of the Pd(II) complex 2, formation of 1 is
much slower, requiring 24 h at room temperature whereas, for 2,
only 4 h is needed to reach 87% product yield under the same re-
action conditions. In addition, unlike complex 1, the Pd(II) complex 2
with proximal triazole nitrogen coordination were stable in DMSO,
showing no ligand-DMSO displacement. Another evidence of
stronger binding of the proximal triazole nitrogen to Pd(II) is based
on the crystal structure of 2. Of two possible chelating sites, Pd(II)
exclusively binds at the 2-(1H-1,2,3-triazol-4-yl)pyridine pocket.
Consistent with the activities observed with the model reaction,
catalyst 1 is more efficient than 2 and gave higher product yields
under similar reaction conditions (Table 5). Additionally, for cata-
lyst 1, higher product yields were obtained with the activated aryl
bromides containing electron-withdrawing groups such as OMe
2
and NO (i.e., inductive effect), whereas the catalyst 2 was only
moderately active with no apparent trend with various para-
substituted aryl bromide substrates. We also found that, the more
sterically hindered ortho-substituted aryl bromides including 2-
(
Me)C
6
H
4
Br and 2,4,6-(Me)
3
C
6
H
2
Br yielded no coupling product
ꢀ
from both catalysts 1 and 2 even at 100 C after 24 h. When the
heteroaryl bromide such as 2-bromopyridine was used, low prod-
uct yields of 21% and 4% were obtained in the presence of 1 mol% of
ꢀ
1
and 2 at 100 C after 24 h, respectively (entry 5).
It should be mentioned that, over the course of the reactions,
formation of palladium black was observed with catalysts 1 and 3
while, with 2, the reaction remained homogeneous. This result
indicates better ligand-stabilized Pd(0) species of 2. Noted that
previous work involving other triazole-containing Pd(II) catalysts
2
Attempts to add another equivalent of Pd(cod)Cl to the DMF solu-
tion of 2 resulted in no further palladium coordination.
Table 5
3.2. Catalytic tests for SuzukieMiyaura cross-coupling reactions
Substrate scope for Pd-catalyzed SuzukieMiyaura coupling reactions.
To evaluate the catalytic activities of 1e3 toward Suzukie
Miyaura cross-coupling reactions, 4-(MeO)C
6 4 2
H Br and PhB(OH)
in the presence of K CO base were used as model substrates. The
2
3
effect of solvent was first investigated with the phenyl-substituted
catalyst 1 (entries 1e4, Table 4). With 0.5 mol% of 1, the best
catalytic activity was obtained from using a 1:1 ratio of DMF:H
which gave the coupling product in 95% yield. This is likely due to
2
O,
AreBra
Entry
1
2
Time (h) % yieldb
Time (h) % yieldb
Table 4
1
2
3
2
2
2
58
62
85
12
12
12
38
36
59
6 4 2
SuzukieMiyaura coupling of 4-(MeO)C H Br and PhB(OH) .
Entry
Catalyst
Solventa
CH CN:H
Time (h)
Yield (%)b
TON
20
104
132
190
850
80
130
590
1800
156
1
2
3
4
5
6
7
8
9
1
1
1
1
3
2
O
O
2
2
2
2
2
2
4
12
24
4
10
52
66
95
85
8
65
59
18
78
MeOH:H
EtOH:H
2
4
2
99 (54) d 12
26 (25)d
2
O
DMF:H
DMF:H
DMF:H
DMF:H
DMF:H
DMF:H
DMF:H
2
O
c
1
2
O
2
O
2
O
2
O
2
O
2
O
d
1
5c
24
21e
24
4e
2
c
2
d
2
a
General conditions: 0.1 mol% Pd catalyst, 1.00 mmol of ArBr, 1.10 mmol of
, 1.00 mmol of K CO in DMF:H O (1:1, 10 mL).
10
3
PhB(OH)
2
2
3
2
a
b
c
b
The ratio of solvent:H
Isolated yield after thin layer chromatography.
2
O ¼ 1:1.
Isolated yield after thin layer chromatography.
1.0 mol% Pd catalyst.
c
d
e
0
0
.1 mol% Pd catalyst.
.01 mol% Pd catalyst.
In the presence of excess Hg (Hg:Pd ¼ 500:1).
d
6 6
% yields were determined by GC with C Me as an internal standard.

40
S. Jindabot et al. / Journal of Organometallic Chemistry 750 (2014) 35e40
for Suzuki cross coupling also reported observation of palladium
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beginning of the reactions are responsible for catalyzing the
SuzukieMiyaura reactions [48e51]. To test whether the in situ
generated Pd(0) aggregates are the actual catalysts for our systems,
mercury poisoning experiments were carried out with catalysts 1
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Appendix A. Supplementary material
(
CCDC 961185, 961186, 961187 contain the supplementary crys-
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