Monomeric Pd(II) complexes with trans-chelated pyrazole ligands as effective pre-catalysts for Heck cross-coupling reaction under mild aerobic conditions

Article abstract of DOI:10.1016/j.jorganchem.2013.08.016

A new family of mononuclear palladium(II) complexes with bidentate trans-chelating pyrazole ligands [PdCl₂(L^R)] (R = H, 1; Et, 2; Ph, 3; CHPh₂, 4), where new ligand L^Et = 2-ethyl-1,3-bis-(3,5-dimethyl-pyrazol-1-

Full text of DOI:10.1016/j.jorganchem.2013.08.016

Journal of Organometallic Chemistry 745-746 (2013) 387e392
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Journal of Organometallic Chemistry
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Monomeric Pd(II) complexes with trans-chelated pyrazole ligands as
effective pre-catalysts for Heck cross-coupling reaction under mild
aerobic conditions
,
a
a
b
Chang-Chuan Chou a , Chia-Chi Yang , Hong-Bin Syu , Ting-Shen Kuo
*
a Center for General Education, Chang Gung University of Science and Technology, Tao-Yuan 333, Taiwan
b Instrumentation Center, Department of Chemistry, National Taiwan Normal University, Taipei 11650, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new family of mononuclear palladium(II) complexes with bidentate trans-chelating pyrazole ligands
[PdCl2(LR)] (R ¼ H, 1; Et, 2; Ph, 3; CHPh2, 4), where new ligand LEt ¼ 2-ethyl-1,3-bis-(3,5-dimethyl-
pyrazol-1-ylmethyl)-2,3-dihydro-1H-perimidine, and a new mononuclear palladium(II) complex with
monodentate trans-chelated pyrazole ligand [PdCl2(Ph-pz0)2] (5), where Ph-pz0 ¼ 3,5-dimethyl-1-
phenyl-1H-pyrazole, have been synthesized and characterized. Complexes 2e5 were also structurally
confirmed by single crystal X-ray diffraction analysis. Remarkably, under very mild conditions (65
e70 ꢀC), complex 4 appeared to be an active pre-catalyst for the Heck coupling reactions of iodobenzene
and t-butyl acrylate in MeOH. In the presence of co-catalyst [nNBu4]Br, however, complex 5 displayed the
highest catalytic activity for the Heck reactions of activated bromobenzene and t-butyl acrylate.
Ó 2013 Elsevier B.V. All rights reserved.
Received 12 June 2013
Received in revised form
1 August 2013
Accepted 4 August 2013
Keywords:
Palladium(II) complex
Trans-chelating ligand
Steric influence
Phosphine-free
Low temperature
Heck reaction
1. Introduction
traditional reaction temperatures (140 ꢀC) [30]. Furthermore, the
related dinuclear Pd(II) complexes have been effective pre-
Since its development in 1971, the palladium-catalyzed Heck
reaction has become one of the most powerful synthetic meth-
odologies for CeC bond formation [1e3]. Taking advantage of this
reaction, the olefination of aryl halides or vinyl halides can be
readily achieved and applied to the preparation of a wide variety of
organic compounds [4e8], and natural products [9]. However, the
toxic, air-sensitive and expensive phosphorus-containing ligands
are generally utilized for the generation of active Pd(II) catalysts
[10e16]. Thus, in recent year, extensive effort has been dedicated
to exploring the effective phosphine-free Pd(II) complexes bearing
N-heterocyclic carbene ligands [17e20], N2-donor ligands (such as
hydrazones [21,22], bispyridine [23e25], bisimidazole [26e28],
bisbenzoimidazole [29] or bispyrazole derivatives [30e32] etc.), or
even N4-tetradentate [33] as alternative catalysts. In terms of Pd(II)
complexes bearing trans-chelated bispyrazole ligands, the mono-
nuclear species have proven to be effective pre-catalysts under
catalysts under very mild reaction temperatures (80 ꢀC) [31]. Due
to the fact that these types of Pd(II) complexes are adequately
stable in air and moisture, there is considerable interest in deter-
mining whether the monomeric Pd(II) complexes with trans-
chelated pyrazole ligands could be pre-catalysts for the Heck re-
action under low-temperature (<80 ꢀC) [34,35] and aerobic con-
ditions. As part of our research to develop coordinatively
unsaturated copper(I) complexes with neutral pyrazole ligands
[36,37], the perimidine-based bispyrazole ligands LH, LPh and
LCHPh2 (Scheme 1), were exploited [38,39]. These ligands are
effective trans-chelators for copper(I) ions in the production of two
and three coordinate copper(I) complexes and were used here to
generate a new family of mononuclear palladium complexes with
bidentate trans-chelated pyrazole ligands, [Pd(LR)Cl2] (R ¼ H, 1; Et,
2; Ph, 3; CHPh2, 4; as shown in Scheme 1), to test their use as pre-
catalysts in Heck reactions, particularly under low-temperature
conditions. To shed some light on the trans-chelating effect of
the LR ligands, a new mononuclear palladium(II) complex with
monodentate trans-chelated pyrazole ligands [PdCl2(Ph-pz0)2] (5)
was also synthesized for comparison.
* Corresponding author. Tel.: þ886 3 2118999x5583; fax: þ886 3 2118866.
E-mail address: ccchou@mail.cgust.edu.tw (C.-C. Chou).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.08.016

388
C.-C. Chou et al. / Journal of Organometallic Chemistry 745-746 (2013) 387e392
and refined by full-matrix least-squares procedures using the
SHELXL-97 program [41]. All of the non-hydrogen atoms were
refined with anisotropic temperature factors. All hydrogen atoms
were located geometrically and refined in the riding mode. Addi-
tional crystallographic data as CIF files are available as Supporting
Information.
N
N
Ph
Cl
Pd
Cl
N
N
N
N
N
N
N
N
N
N
N
Cl
Pd
Cl
R
N
N
R
Ph
PdCl2(CH3CN)2 (6)
N
[PdCl2(Ph-pz′)2] (5)
2.4. Synthesis of 2-ethyl-2,3-dihydro-1H-perimidine: the precursor
of LEt
R
[ Pd(L )Cl2
]
R
( R = H, Et,
Ph, CHPh2
L
R = H (1), Et (2), Ph (3),
CHPh2 (4)
)
A
40 ml absolute EtOH (99.5%) solution of 1,8-diamino-
naphthalene (3.16 g, 20 mmol) and propionaldehyde (3.48 g,
60 mmol) was stirred at room temperature for 6 h. Then the solvent
and the excess propionaldehyde were removed under vacuum. The
resulting crude product was purified on a silica chromatography
column with CH2Cl2/hexane (1:1) as eluent to give 2.89 g (73%) of
Scheme 1. The ligands LR and Pd(II) complexes 1e6.
2. Experimental
oily product. 1H NMR (CD2Cl2, 500.13 MHz):
d 7.23e7.33 (m, 4H, CH
2.1. General information and materials
of naphthalene), 6.54 (d, 2H, CH of naphthalene, J ¼ 7.3 Hz), 4.44 (s,
All solvents, reagents and Ph-pz0 were obtained from commer-
cial sources and used as received without further purification. The
starting material PdCl2(CH3CN)2 6 was purchased and used as
received whereas sample PdCl2(COD) was prepared according to
literature procedures [40]. The ligands LH, LPh and LCHPh2 were
prepared as described previously [38,39].
br, 2H, NH), 4.35e4.37 (m, 1H, CH), 1.75e1.69 (m, 2H, CH2), 1.082 (t,
3H, CH3) ppm. 13C NMR (CD2Cl2, 125.77 MHz):
d 142.79 (naphtha-
lene), 135.46 (naphthalene), 127.42 (naphthalene), 117.55 (naph-
thalene), 114.17 (naphthalene), 106.01 (naphthalene), 66.31 (CH),
29.12 (CH2), 8.85 (CH3) ppm. Positive ESI-MS: 197.4 (Mþ ꢁ 1, 100%),
198.5 (Mþ, 12%).
2.5. Synthesis of [Pd(LH)Cl2] (1)
2.2. Physical measurements
The reaction of PdCl2(CH3CN)2 (0.26 g, 1.0 mmol) and LH (0.35 g,
1.1 mmol) was carried out in 10 ml CH2Cl2 for 4 h to give yellow
precipitate. After filtering, the product was washed by 5 ml CH2Cl2
and 10 ml Et2O for two times and dried in vacuo to yield 0.54 g
(96%). Anal. Calcd for C23H26N6PdCl2: C, 48.98; H, 4.61; N, 14.91.
Found: C, 48.33; H, 4.66; N, 14.72%.
1H NMR spectra were obtained on Bruker Avance 600.17 and
500.13 MHz NMR spectrometer. 13C NMR spectra were recorded on
Bruker Avance 125.77 MHz spectrometer. Mass spectra were ac-
quired on a Finnigan TSQ 700 spectrometer. Elemental analyses
were performed on
a HERAEUS CHNeOeS-Rapid elemental
analyzer by Instruments Center, National Chung Hsin University.
2.3. X-ray crystallography
2.6. Synthesis of [Pd(LEt)Cl2] (2)
Crystal data collection and processing parameters are given in
Table 1. Diffraction data were carried out on a Bruker Kappa CCD
A 20 ml CH3CN solution of 2-ethyl-2,3-dihydro-1H-perimidine
(1.98 g, 10 mmol) and (3,5-dimethylpyrazol-1-yl)methanol (2.52 g,
20 mmol) was stirred at 60 ꢀC for 20 h. The solvent was removed in
vacuo to afford an oily crude product of LEt 4.16 g. Positive ESI-MS:
diffractometer with graphite monochromated Mo-K
a
radiation
ꢀ
(l
¼ 0.7107 A). The structures 2e5 were solved by direct methods
Table 1
X-ray crystallographic data for complexes 2e5.
2
3
2(4),CH2Cl2
5
Empirical Formula
Formula weight
C25H30Cl2N6Pd
591.85
C29H30Cl2N6Pd
639.89
2(C36H36Cl2N6Pd)$CH2Cl2
1544.94
C22H24Cl2N4Pd
521.75
T (K)
200(2)
200(2)
200(2)
0.71073
200(2)
ꢀ
l
(A)
0.71073
Monoclinic
P21/c
11.2048(4)
14.8379(6)
14.9696(6)
90
101.0430(10)
90
2442.70(16)
4
0.71073
Monoclinic
P21/c
8.96440(10)
16.1103(3)
18.7128(4)
90
99.1690(10)
90
2667.96(8)
4
0.71073
Monoclinic
P21/c
8.3397(10)
22.250(3)
12.2786(14)
90
98.7770(10)
90
2251.8(5)
4
Crystal system
Space group
Monoclinic
C2/c
31.7086(5)
12.2286(2)
22.2761(4)
90
126.2840(10)
90
6962.7(2)
4
ꢀ
a (A)
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
3
ꢀ
V (A )
Z
Dcalc (Mg mꢁ3
)
1.609
1.005
1.593
0.927
1.474
0.799
1.539
1.077
m
(mmꢁ1
)
F(000)
range for data collection
1208
1304
3160
1056
q
1.85e25.02ꢀ
16339
1.68e25.03ꢀ
18528
2.27e25.03ꢀ
30468
3.48e25.12ꢀ
8819
Reflections collected
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit on F2
R1, wR2
)
4294 (0.0254)
4294/0/307
1.073
4706 (0.0596)
4706/0/343
1.155
6154 (0.1030)
6154/0/420
1.044
3982 (0.0274)
3982/0/262
1.022
0.0281, 0.0811
0.0412, 0.1152
0.0633, 0.1658
0.0338, 0.0817

C.-C. Chou et al. / Journal of Organometallic Chemistry 745-746 (2013) 387e392
389
437.1 ([M þ Na]þ, 100%). The reaction of PdCl2(MeCN)2 (0.65 g,
5 mmol) and the oily LEt (2.07 g, 1.17 mmol) was then performed in
30 ml CH2Cl2 at room temperature for 1 h. The resulting solution
was filtered and the filtrate was cooled to 0e5 ꢀC in ice bath. After
introducing Et2O to the cooling filtrate, yellow precipitate was
obtained. The solid was collected and purified on a silica chroma-
tography column with CH3OH/CH2Cl2 (1:20) as eluent to give 0.62 g
(21% yield), Rf ¼ 0.78, of complex 2. The crystals suitable for X-ray
structure determination were grown from CH2Cl2/Et2O. 1H NMR
complex 5. Single crystals were grown from CH2Cl2/hexane. 1H
NMR (500.13 MHz, CDCl3): 7.77e7.58(m, 10H, aromatic ring),
6.02 (s, 2H, CH), 2.21 (s, 6H, CH3), 2.09 (s, 6H, CH3) ppm. 13C NMR
d
d
(125.77 MHz, CDCl3):
d 150.59, 143.79, 137.82, 129.60, 129.08,
128.35, 107.66, 13.96, 12.25 ppm. Positive ESI-MS: 1067.1
([2M þ Na]þ, 75%). Anal. Calcd for C22H24Cl2N4Pd: C, 50.64; H, 4.64;
N, 10.74. Found: C, 50.83; H,4.92; N,10.64.
2.10. General procedure for the Heck reaction
(CD2Cl2, 600.17 MHz): d 7.28e7.19 (m, 4H, CH of naphthalene), 6.56
(t, 1H, CH), 6.34 (d, 2H, CH of naphthalene, J ¼ 7.2 Hz), 5.96 (s, 2H,
CH of pyrazole), 5.86 (d, 2H, CH2, J ¼ 13.2 Hz), 5.67 (d, 2H, CH2,
J ¼ 13.2 Hz), 2.78 (s, 6H, CH3), 2.56 (s, 6H, CH3), 1.93 (quintet, 2H,
The reaction vessel was charged with halobenzene (5 mmol),
triethylamine (10 mmol), t-butyl acrylate (6.25 mmol) and a cata-
lyst (0.5 mol%) in MeOH (10 ml). The reaction mixture was heated
and refluxed in a pre-heated oil bath (70 ꢀC) for 24 h. At the end of
the reaction, the reaction mixture was diluted with n-hexane and
washed with water. The combined organic portion was dried over
anhydrous MgSO4. After removal of the solvent, the desired product
was purified by column chromatography using ethyl acetate and
hexane mixtures to obtain the Heck product.
CH2),1.01 (t, 3H, CH3) ppm. 13C NMR (CD2Cl2,125.77 MHz):
d 152.46,
144.10, 138.15, 135.68, 127.01, 118.95, 117.48, 108.48, 104.29, 80.50,
63.04, 26.16, 16.20, 12.45, 9.75 ppm. Positive ESI-MS: 555.0 ([Me
Cle1]þ, 100%). Anal. Calcd for C25H30Cl2N6Pd: C, 50.73; H, 5.11; N,
14.20. Found: C, 50.43; H, 4.85; N, 13.87%.
2.7. Synthesis of [Pd(LPh)Cl2] (3)
3. Results and discussion
The reaction of PdCl2(CH3CN)2 (2.31 g, 5.0 mmol) and LPh (1.29 g,
5.0 mmol) was carried out in 40 ml CH2Cl2 for 4 h. The yellow so-
lution was filtered and the Et2O was added into the filtrate to give
yellow precipitate 2.59 g (81%) of complex 3. X-ray-quality crystals
were grown by Et2O diffusion into saturated CH2Cl2 solution. 1H
3.1. Preparation of the ligand and the complexes
The ligands LH, LEt, LPh and LCH2Ph were prepared according to
previous reports [38,39]. The Pd(II) complexes [PdCl2(LR)] were
synthesized by the treatment of [PdCl2(CH3CN)2] with the corre-
sponding ligand in CH2Cl2 at room temperature. Purification of the
ligands LEt and LCH2Ph was difficult and unnecessary because further
complexation of these crude ligands with [PdCl2(CH3CN)2] 6 affor-
ded pure Pd(II) complexes 2 and 4. The elemental analyses for
complexes 1e4 were consistent with the respective empirical for-
mula. The characterization of complex 1 in solution was difficult
because of low solubility in organic solvents. However, the
substituted Et, Ph or CHPh2 group at the 2-position of the peri-
midine visibly improved the solubility of these Pd-complexes. The
positive ionization spectra (ESI-MS) of complexes 2e4 gave peak
values of m/z as follows: 555.0 (100%), which was attributable to
{[PdCl(LEt)]-1}þ for 2; 604.5 (100%), which was attributable to
{[PdCl(LPh)]-1}þ for 3; and, 693.1 (100%), which was attributable to
{[PdCl(LCHPh2)]-1}þ for 4. The 1H and 13C NMR spectra of complexes
2e4 showed only a single set of peaks for each complex, indicating a
symmetrical structure for 2e4, which was in agreement with the
solid state structures. The Pd(II) complex [PdCl2(Ph-pz0)2] (5) was
synthesized by the treatment of [PdCl2(COD)] with two equivalent
of ligand Ph-pz0 in CH2Cl2. The elemental analysis of 5 was consis-
tent with the empirical formula. The positive ionization spectra (ESI-
MS) of complex 5 gave a peak with a value of m/z at 1067.1 (75%),
which was attributable to {2[PdCl2(Ph-pz0)2]þNa }þ. The 1H and 13C
NMR spectra of 5 were in agreement with the solid state structure.
NMR (CD2Cl2, 500.13 MHz): d 7.88 (s, 1H, CH), 7.29e7.18 (m, 9H, CH
of aromatic), 6.41 (d, 2H, CH of aromatic, J ¼ 7.5 Hz), 6.07 (d, 2H,
CH2, J ¼ 13.5 Hz), 6.00 (s, 2H, CH of pyrazole), 5.95 (d, 2H, CH2,
J ¼ 13.5 Hz), 2.82 (s, 6H, CH3), 2.60 (s, 6H, CH3). 13C NMR (DMSO-d6,
125.77 MHz): d 150.71, 144.19, 141.93, 137.86, 134.47, 128.60, 127.75,
126.54, 117.69, 116.51, 107.52, 103.99, 78.75, 63.11, 54.89, 15.44,
11.52 ppm. Positive ESI-MS: 603.4 ([MeCle1]þ, 100%). Anal. Calcd
for C29H30Cl2N6Pd: C, 54.43; H, 4.73; N, 13.13. Found: C, 54.54; H,
4.85; N, 12.81%.
2.8. Synthesis of [Pd(LCHPh2)Cl2] (4)
The reaction of PdCl2(CH3CN)2 (0.65 g, 2.5 mmol) and LCHPh2
(2.76 g, 5.0 mmol) was carried out in 30 ml CH2Cl2 for 1 h. The
yellow solution was filtered and the filtrate was cooled to 0e5 ꢀC in
ice bath. After introducing Et2O to the cooling filtrate, yellow pre-
cipitate was obtained. The solid was collected and purified on a
silica chromatography column with CH3OH/CH2Cl2 (1:20) as eluent
to give 1.21 g (21% yield) of complex 4. X-ray-quality crystals were
grown by Et2O diffusion into saturated CH2Cl2 solution. 1H NMR
(CD2Cl2, 600.17 MHz): d 7.42e7.30 (m, 14H, CH of aromatic), 7.22 (d,
1H, CH, J ¼ 10.8 Hz), 6.09e6.07 (t, 2H, CH of aromatic), 5.85 (s, 2H,
CH of pyrazolyl), 5.31 (d, 2H, CH2, J ¼ 4.2 Hz), 4.63 (d, 1H, CH,
J ¼ 10.8 Hz), 4.52 (d, 2H, CH2, J ¼ 13.8 Hz), 2.72 (s, 6H, CH3), 2.22 (s,
6H, CH3) ppm. 13C NMR (CDCl3, 125.77 MHz):
d 152.36, 144.00,
139.90, 137.45, 135.80, 129.67, 129.43, 128.06, 127.14, 119.11, 117.64,
108.30, 104.83, 82.24, 62.76, 54.94, 16.12, 11.97 ppm. Positive ESI-
MS: 693.1 ([MeCle1]þ, 100%). Anal. Calcd for {2[C36H36Cl2N6Pd]$
CH2Cl2}: C, 56.75; H, 4.83; N, 10.88. Found: C, 56.32; H, 4.42; N,
10.36%.
3.2. X-ray structures of the complexes
X-ray structure determinations were performed on crystals of
the complexes 2e5, as shown in Figs. 1e4. The crystal data and
structure refinements for 2e5 are summarized in Table 1. As
anticipated, the X-ray diffraction analyses of 2e4 show that two
pyrazolic nitrogen atoms were coordinated to the Pd(II) center in a
trans-chelated fashion forming a metallocycle ring of ten members.
Two chlorine atoms were located in a trans fashion as well. There
was a pseudo mirror plane in the complexes containing C11, C12,
C23, Pd1, Cl1, and Cl2 atoms. The dihedral angles between the
pyrazoles in Pd(II) complexes were 16.7(1)ꢀ for 2, 17.9(2)ꢀ for 3 and
15.1(3)ꢀ for 4. The N1ePd1eN6 and Cl2ePd1eCl1 angles were
176.90(10)ꢀ and 175.64(3)ꢀ for 2, 176.64(14)ꢀ and 177.37(5)ꢀ for 3,
2.9. Synthesis of [PdCl2(Ph-pz0)2] (5)
A 10 ml toluene of 3,5-dimethyl-1-phenyl-1H-pyrazole (0.5 g,
2.9 mmol) and PdCl2(COD) (0.41 g, 1.44 mmol) was stir at 100 ꢀC for
16 h. The solvent was removed under vacuum then addition of
CH2Cl2 to dissolve the solid. The resulting solution was filtered and
hexane was added to induce precipitate. The resulting yellow solid
was filtered and washed with hexane to yield 0.70 g (93.5%) of

390
C.-C. Chou et al. / Journal of Organometallic Chemistry 745-746 (2013) 387e392
Fig. 1. ORTEP plot of complex 2 with thermal ellipsoids at 30% probability. Selected
ꢀ
ꢀ
bond distances (A) and bond angles ( ): N(1)ePd(1) 2.017(3), N(6)ePd(1) 1.996(3),
Cl(1)ePd(1) 2.2949(9), Cl(2)ePd(1) 2.3160(8), N(1)ePd(1)eN(6) 176.90(10), N(1)e
Pd(1)eCl(1) 89.16(8), N(6)ePd(1)eCl(1) 88.84(7), N(1)ePd(1)eCl(2) 93.28(7), N(6)e
Pd(1)eCl(2) 88.56(7), Cl(1)ePd(1)eCl(2) 175.64(3).
Fig. 3. ORTEP plot of complex 4 with thermal ellipsoids drawn at 30% probability. The
CH2Cl2 molecules have been omitted for clarity. N(1)ePd(1) 2.013(6), N(6)ePd(1)
2.014(6), Cl(1)ePd(1) 2.2943(19), Cl(2)ePd(1) 2.3267(18), N(1)ePd(1)eN(6) 176.0(2),
N(1)ePd(1)eCl(1) 89.49(18), N(6)ePd(1)eCl(1) 88.51(17), N(1)ePd(1)eCl(2) 90.83(18),
N(6)ePd(1)eCl(2) 91.15(16), Cl(1)ePd(1)eCl(2) 179.57(7).
and 176.0(2)ꢀand 179.57(7)ꢀ for 4, respectively, showing a slight
distorted square planar geometry for the palladium(II) center. The
under aerobic conditions, with the exception of complex 1 due to
its poor solubility. The starting material PdCl2(MeCN)2 6 was
considered a pre-catalyst for the blank test. The results of the cat-
alytic CeC coupling reactions are listed in Tables 2 and 3. As in a
typical Heck reaction, the iodobenzene and t-butyl acrylate in DMF
were used as a test reaction for pre-catalysts 2e6. The catalytic
reactions were first carried out at a traditional reaction tempera-
ture (140 ꢀC), which gave a nearly quantitative yield of trans-cin-
namic acid t-butyl ester with a 0.1 mol % loading in 3 h for
complexes 2e6. With a lowering of the reaction temperature to
70 ꢀC and an increase in catalysts loading to 0.5 mol %, the desired
cinnamate was simultaneously decreased for all complexes 2e6,
with a yield of w40% for 24 h (entries 6e10). Apparently, complexes
2e6 show a similar effectiveness in the formation of desired trans t-
butyl cinnamate under identical conditions in DMF. When the Heck
ꢀ
average bond distances of PdeN (2.007e2.020 A) and PdeCl
ꢀ
(2.305e2.311 A) fall in the normal range for monomeric palladium
(II) compounds [14]. The Pd(II) ion in complex 5 was in a nearly
square-planar environment with the pyrazole ligand situated trans
to it. The dihedral angle between the pyrazoles in the Pd(II) com-
plex was only 2.3 (1)ꢀ, which was indicative of an almost co-planar
arrangement for these two pyrazolic rings. The pyrazole N-Ph
groups were on the opposite side of the Pd coordination plane. The
ꢀ
average bond distances of PdeN and PdeCl were 2.014 A and
ꢀ
2.299 A, respectively, which approximated those of complexes 2e4.
3.3. Catalytic Heck reaction
The palladium complexes 2e6 were examined as pre-catalysts
for the Heck reaction using phenyl halides and t-butyl acrylate
Fig. 2. ORTEP plot of complex 3 with thermal ellipsoids drawn at 30% probability.
N(1)ePd(1) 2.012(4), N(6)ePd(1) 2.028(4), Cl(1)ePd(1) 2.3216(12), Cl(2)ePd(1)
2.2957(13), N(1)ePd(1)eN(6) 176.64(14), N(1)ePd(1)eCl(1) 90.72(11), N(6)ePd(1)e
Cl(1) 92.13(11), N(1)ePd(1)eCl(2) 86.74(11), N(6)ePd(1)eCl(2) 90.39(11), Cl(1)e
Pd(1)eCl(2) 177.37(5).
Fig. 4. ORTEP plot of complex 5 with thermal ꢀellipsoids drawn at 30% probability.
ꢀ
Selective bond distances (A) and bond angles ( ): N(1)ePd(1) 2.004(3), N(3)ePd(1)
2.013(3), Cl(1)ePd(1) 2.2901(10), Cl(2)ePd(1) 2.3085(9), N(1)ePd(1)eN(3) 179.28(11),
N(1)ePd(1)eCl(1) 90.07(8), N(3)ePd(1)eCl(1) 89.22(8), N(1)ePd(1)eCl(2) 89.05(8),
N(3)ePd(1)eCl(2) 91.67(8), Cl(1)ePd(1)eCl(2) 178.98(4).

C.-C. Chou et al. / Journal of Organometallic Chemistry 745-746 (2013) 387e392
391
Table 2
As shown in Table 2, under the same reaction conditions in DMF,
complexes 2e6 showed similar effectiveness. To differentiate their
catalytic performance, we tried the use of MeOH as a solvent.
Consequently, the catalytic activity of complex 6 became inactive
(only 7% yield Heck product, entry 15) whereas complex 4 became
very active (100% yield Heck product, entry 13), indicating a sig-
nificant solvent effect. Meanwhile, when the Heck coupling reac-
tion occurred in MeOH, the Pd black was noticed in 10 min for
complex 6, whereas the Pd black was noticed ca. 3.5 h for complex 4
and ca. 1.0 h for complexes 2 and 3, indicating that the bulky ligand
LCHPh2 enabled a greater degree of stability to complex 4. In fact, the
catalytic Heck reaction for complex 4 was above 80% yield (in 3.5 h).
Besides, the increment of Heck products for complexes 2 and 3 was
very limited once the Pd black was formed (ca.1.0 h). We thus
speculated that the Heck reactions were mainly catalyzed by Pd(II)
complex rather than Pd(0) nanoparticles.
Heck coupling of iodobenzene and t-butyl acrylate under aerobic condition.a
NEt3
I
+
CO2-t-Bu
R
CO2-t-Bu
DMF
Entry
Cat
Cat (mol %)
Time (h)
T (ꢀC)
Yieldb (%)
TON
1
2
3
4
5
6
7
8
9
10
2
3
4
5
6
2
3
4
5
6
0.1
0.1
0.1
0.1
0.1
0.5
0.5
0.5
0.5
0.5
3
3
3
3
3
24
24
24
24
24
140
140
140
140
140
70
70
70
70
70
98
97
96
99
97
43
36
41
40
35
980
970
960
990
970
86
72
82
80
70
A plausible mechanism with a trans-bidentate nitrogen ligand for
the Heck reaction was once reported by Kawano where three coor-
a
Reaction conditions: 5.0 mmol of iodobenzene, 6.25 mmol of t-butyl acrylate,
10 mmol of NEt3, 10 ml of DMF.
dinate species [PdArX(h
1-N,N)] in the catalytic cycle were proposed
b
Isolated yields.
[24]. In the present study, we inferred that the 2-substituent of
complexes 2e4 would have a greater opportunity to straightfor-
wardly affect the reactivity. For complex 4, the sufficient steric hin-
drance of the -CHPh2 group and perhaps an agnostic interaction
caused by an appropriate orientation of the -CHPh2 group [42] pro-
vided substantial help to stabilize the key three coordinate in-
termediates in the catalytic cycle and improve its catalytic activity.
With complexes 2 and 3, the insufficient steric hindrance of ligands
LEt and LPh failed to provide the adequate stability that LCHPh2 did,
which resulted in an earlier appearance of Pd black and a low yield.
Accordingly, we attributed the high catalytic activity of complex 4 to
reactions were performed in MeOH at 70 ꢀC (oil bath), the exclusive
formation of desired product can be differentiated among 2e6
(Table 3, entries 11e15). In particular, the yield of complex 4 gave a
quantitative yield (entry 13), as shown in Table 3, whereas the
blank test of 6 gave a low yield of 7% (entry 15), indicating the
substantial influence of the trans-chelating ligands e in particular,
LCHPh2. Because.of the high efficiency in producing the desired
cinnamate, complex 4 was used as a catalyst to test other electron-
rich and electron-deficient iodobenzenes. As a result, a quantitative
yield for electron-deficient iodobenzenes (entries 16e17) and a
poor yield for electron rich iodobenzenes (entries 18e19) were
obtained. Further lowering of the catalyst loading of 4 to 0.01 mol%
yielded a turnover number of 4200 (entry 21).
the steric effect of the supporting ligand LCHPh2
.
On the other hand, when using an activated aryl bromide such
as 4-nitro-bromobenzene, complexes 2e5 simultaneously lost their
activity. Their catalytic performance, however, continued to be
better than that of the most inefficient of pre-catalysts, which was 6
(entry 26). Interestingly, in the presence of co-catalyst [nBu4N]Br,
complex 5 gave a nearly quantitative yield (entry 30) after a 1.0 mol
% loading but the co-catalyst does not have striking influence on the
efficiency of the pre-catalysts 2e4 (entries 27e29). Furthermore,
compared with complex 6 (entry 31), a decreasing effect in catalytic
activity for complexes 2 and 3 (entries 27, 28) revealed a negative
effect of the trans-chelating bidentate ligand. For the high activity
of 5, a bromobenzene was also tested. Unfortunately, the yield was
very low at 4% (entry 32).
It was known that the additive [nBu4N]Br increases the polarity of
the solvent and stabilizes the Pd nanoparticles [43]. We found that
[nBu4N]Br was a very effective co-catalyst for complex 5 (entry 31),
suggesting that Pd(0) nanoparticles could be formed during the
course of the reaction. Indeed, for complex 5, it was observed that
after the Pd black was formed (ca. 1.0 h, 1e2% Heck product), the
catalytic reaction began and the yield was steadily increased to ca.
97% in 24 h. Evidently, ligand 5 played some role in stabilizing the
Pd(0) nanoparticles. For complexes 2e4, the yields were only
improved slightly in 24 h after the Pd black was formed (ca. 0.5 h for
2, 1.0 h for 3, and 1.5 h for 4). We thus assumed that the trans-
chelating ligands are not quite beneficial for the stabilization of Pd
nanoparticles as a result of their relative low yield for complexes 2e4.
Table 3
Heck coupling of aryl halides and t-butyl acrylate under aerobic condition.a
NEt3
R
X
+
CO2-t-Bu
R
MeOH
CO2-t-Bu
Entry Cat Cat (mol %) Time (h) T (ꢀC) Aryl halides
Yieldb (%) TON
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27c
28c
29c
30c
31c
32c
2
3
4
5
6
4
4
4
4
4
4
2
3
4
5
6
2
3
4
5
6
6
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.1
0.01
0.5
0.5
0.5
0.5
0.5
1.0
1.0
1.0
1.0
1.0
1.0
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
PhI
PhI
PhI
PhI
PhI
35
30
100
32
70
60
200
64
7
14
4-COOMe-C6H4I 100
200
200
78
4-CNeC6H4I
4-MeeC6H4I
4-OMeeC6H4I
PhI
100
39
23
100
42
14
20
31
23
7
26
28
46
97
43
4
46
1000
4200
28
40
62
46
14
26
28
46
97
43
4
PhI
4-NO2eC6H4Br
4-NO2eC6H4Br
4-NO2eC6H4Br
4-NO2eC6H4Br
4-NO2eC6H4Br
4-NO2eC6H4Br
4-NO2eC6H4Br
4-NO2eC6H4Br
4-NO2eC6H4Br
4-NO2eC6H4Br
PhBr
4. Conclusions
A family of new structurally characterized Pd(II) complexes with
trans-bidentate pyrazole ligands [PdCl2(LR)] (R ¼ H, Et, Ph, CHPh2)
and with trans-monodentate pyrazole ligand [PdCl2(Ph-Pz0)2] were
successfully synthesized and their catalytic activity in the Heck
coupling reactions were investigated. Herein we have found that, in
a
Reaction conditions: 5.0 mmol of aryl halide, 6.25 mmol of t-butyl acrylate,
10 mmol of NEt3, 10 ml MeOH.
b
Isolated yields.
Additive: [nBu4N]Br (1 mol% relative to aryl bromides).
c

392
C.-C. Chou et al. / Journal of Organometallic Chemistry 745-746 (2013) 387e392
the absence of co-catalyst [nBu4N]Br, complex [PdCl2(LCHPh2)] is an
effective pre-catalyst for Heck reactions with high TONs of 4200
and provides even milder reaction conditions (65e70 ꢀC) than
previous reported bispyrazole-Pd(II) catalysts. Also, the results
reveal an intriguing steric-influence of the 2-substituents in the
linker of the ancillary trans-chelator LR. Besides, in the presence of
co-catalyst [nBu4N]Br, Pd(II) complex with a monodentate pyrazole
ligand, [PdCl2(Ph-Pz0)2], exhibited the highest catalytic activity
with respect to activated bromobenzene and t-butyl acrylate,
implying the availability of the monodentate pryazolyl-derived
ligand in the Heck reaction. Further investigations toward other
coupling reactions are in progress.
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Acknowledgments
We thank the National Science Council of the Republic of China
for a grant (NSC 100-2113-M-255-001-MY3) to support this
research. We also thank Lucas Hung-Chieh Chao for the preparation
of complex 3.
Appendix A. Supplementary data
CCDC 941007, 941008, 941009 and 941010 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Appendix B. Supplementary data
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Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.08.016.
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