Synthesis of a cyclometalated 1,3,5-triphenylpyrazole palladium dimer and its activity towards cross coupling reactions

Article abstract of DOI:10.1039/c4dt03908d

A phosphine free 1,3,5-triphenylpyrazole acetate-bridged palladacycle was prepared from simple commercially available starting materials. The activity of the palladacycle in the Mizoroki-Heck and Suzuki-Miyaura cross-coupling reactions was evaluated. The palladacycle precatalyst shows a wide substrate scope, both in Mizoroki-Heck as well as in Suzuki-Miyaura cross-coupling reactions using low catalyst loadings viz., 0.2 mol% and 0.1 mol% respectively.

Full text of DOI:10.1039/c4dt03908d

Dalton
Transactions
PAPER
Synthesis of a cyclometalated 1,3,5-
triphenylpyrazole palladium dimer and its
activity towards cross coupling reactions†
Cite this: Dalton Trans., 2015, 44,
5805
Ramesh Mamidala, Vanga Mukundam, Kunchala Dhanunjayarao and
Krishnan Venkatasubbaiah*
A phosphine free 1,3,5-triphenylpyrazole acetate-bridged palladacycle was prepared from simple com-
mercially available starting materials. The activity of the palladacycle in the Mizoroki–Heck and Suzuki–
Miyaura cross-coupling reactions was evaluated. The palladacycle precatalyst shows a wide substrate
scope, both in Mizoroki–Heck as well as in Suzuki–Miyaura cross-coupling reactions using low catalyst
loadings viz., 0.2 mol% and 0.1 mol% respectively.
Received 18th December 2014,
Accepted 10th February 2015
DOI: 10.1039/c4dt03908d
www.rsc.org/dalton
reactions. The importance of CN palladacycles was expanded
significantly by Buchwald,40 Gladysz,36 Najera,37 Nowotny34
Introduction
The palladium catalyzed Suzuki–Miyaura and Mizoroki–Heck and others. All these palladacycles showed high thermal, air
reactions are the most important C–C bond forming processes and moisture stability and high activity over conventional
and play important roles in modern organic synthesis.1–7 Pd(0) catalysts, which inspired us to investigate the synthesis
Normally, 1 to 5 mol% of palladium catalyst along with phos- of new and simple palladacycles. Although several reports exist
phine ligands are utilized to perform these cross-coupling on the synthesis and characterization of pyrazole (CN)
reactions.8–10 As organophosphines are air-sensitive, expensive palladacycles,58–60 surprisingly so far there is only one report
and poisonous, several research groups have shown interest in for the Heck reaction.44 Inspired and motivated by the impor-
developing phosphine free ligand systems,11–22 to promote the tance of C–C bond forming reactions using palladacycles, we
cross-coupling reactions. Palladacyles23–31 are an interesting synthesized a 1,3,5-triphenylpyrazole palladium dimer. The
group of organometallic compounds that contain one or two synthesis, characterization and activity studies towards
intramolecularly stabilized neutral donor atoms and one palla- Suzuki–Miyaura and Mizoroki–Heck reactions are presented
dium–carbon bond. They have gained considerable attention here.
owing to their stability towards air and moisture and also their
potential applications in materials science, catalysis, organic
synthesis, biological and medicinal chemistry.
Results and discussion
The first report on an application of a cyclopalladated tri-
o-tolyl-phosphine complex for cross-coupling reactions by
Herrmann and co-workers32 raised high expectations for the
palladacycles. Since then several groups have shown interest in
synthesizing CN,33–46 CS,47,48 and CP32,49–53 palladacycle cata-
lysts or precatalysts and pincer54–57 palladium complexes. The
CN palladacycles35–53 gained attention since the pioneering
work of Milstein and co-workers,33 who introduced a wide
variety of imine-based palladacycles for the cross-coupling
1,3,5-Triphenylpyrazole was synthesized from commercially
available starting materials 1,3-diphenyl-1,3-propanedione and
phenylhydrazine. The reaction of Pd(OAc)2 with 1,3,5-triphe-
nylpyrazole in glacial acetic acid afforded the dimeric pallada-
cycle in 59% yield (Scheme 1). The formation of 1,3,5-
triphenylpyrazole as well as the palladacycle was confirmed by
1
standard techniques like H, 13C NMR spectroscopy as well as
School of Chemical Sciences, National Institute of Science Education and Research
(NISER), Institute of Physics Campus, Bhubaneswar-751005, Orissa, India.
E-mail: krishv@niser.ac.in; Fax: (+) 091-674-2302436
†Electronic supplementary information (ESI) available: General experimental
details, catalyst optimization data and data for the products. CCDC 1040304. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4dt03908d
Scheme 1 Synthesis of a 1,3,5-triphenylpyrazole palladium dimer.
Dalton Trans., 2015, 44, 5805–5809 | 5805
This journal is © The Royal Society of Chemistry 2015

Paper
Dalton Transactions
Table 1 Crystal data and structure refinement parameters for the palla-
dacycle 1
Compound 1·CH2Cl2
Empirical formula
MW
C47H38Cl2N4O4Pd2
1006.51
T/K
Wavelength/Å
Crystal system
Space group
296(2)
0.71073 Å
Monoclinic
C2/c
a/Å
b/Å
c/Å
α/°
18.0741(4)
16.5018(4)
15.3290(3)
90
β/°
114.5910(10)
90
4157.28(16)
4
1.608
1.044
γ/°
V/Å3
Z
ρcalc/g cm−3
μ (MoKα)/mm−1
F(000)
2024
Crystal size/mm
θ range/°
Limiting indices
0.35 × 0.22 × 0.20
1.92–30.57
−25 ≤ h ≤ 25
−23 ≤ k ≤ 23
−21 ≤ l ≤ 20
34 962
Reflns collected
Fig. 1 Molecular structure of
a
1,3,5-triphenylpyrazole-palladium
Independent reflns
6333 [R(int) = 0.0382]
Absorption correction
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]a
Semi-empirical from equivalents dimer. Hydrogen atoms are omitted for clarity. Selected bond lengths
Full-matrix least square on F2
(Å) and bond angles (°): Pd1–C11 1.952(2), Pd1–N1 2.0175(17), Pd1–O1
6333/0/269
1.109
2.0440(15), Pd1–O2 2.1443(15), Pd1–Pd1* 2.9365(3), C11–Pd1–N1
80.86(8), C11–Pd1–O1 91.10(8), N1–Pd1–O1 171.39(7), C11–Pd1–O2
175.94(7), N1–Pd1–O2 98.25(7), O1–Pd1–O2 98(7), C11–Pd1–Pd1*
99.40(6), N1–Pd1–Pd1* 100.02(5), O1–Pd1–Pd1* 84.26(5), O2–Pd1–
Pd1* 76.82(4).
R1 = 0.0307
wR2 = 0.0788
R1 = 0.0428
wR2 = 0.0910
0.647 and −0.569
R indices (all data)a
Peakmax/holemin (e Å−3
)
a R1 = ∑kFo| − |Fck/∑|Fo|; wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2
.
hand, we also examined the reaction between bromobenzene
and styrene. The reaction proceeds smoothly and gave essen-
tially a quantitative yield of the desired product (99%), which
is the best catalytic efficiency of our palladacycle under these
reaction conditions. As our catalyst is analogous to the catalyst
reported by Grigg and co-workers,44 we briefly studied the
efficacy of our palladacycle (1) in the Heck reaction (iodoben-
zene and methyl acrylate) using one of the lowest catalyst
loading given in Grigg’s work. It was found that the turnover
number (TON) of our catalyst reached up to 1.88 × 106 with
86% isolated yield (Table S3,† entry 1), under identical con-
ditions used by Grigg and co-workers.44 As expected, a
decrease in TONs was observed when the temperature of the
reaction medium reduced from 110 °C to 90 °C (Table S3,†
entry 3). However at the same catalyst loading employing
K3PO4 as a base (reaction conditions used in this work)
resulted in TON of 1.92 × 106 and 96% isolated yield
(Table S3,† entry 2). The observed high catalytic efficiency may
be (a) due to the electronic effect of two phenyl rings present
in our ligand and/or (b) due to NMP/K3PO4. It should be noted
that NMP was reported as a good stabilizing agent for Pd-
colloids in ligand free Heck reactions.62 In some instances63
the reactions were performed around 120 °C to obtain moder-
ate yields. It was also evident from the literature, Pd-cycles act
as a source of Pd(0) which can be released at a slower rate64
than that of the catalytic reaction. To explore the scope of the
high resolution mass spectrometry. We further analyzed the
palladacycle using X-ray crystallography (Table 1, Fig. 1). As
shown in Fig. 1, the palladacycle consists of two palladium
atoms bridged by two acetate ligands and each Pd-centre has a
chelating one nitrogen atom of the pyrazole and one cyclome-
talated carbon atom of the phenyl ring. The palladium adopts
a nearly square planar geometry. The Pd–Pd distance was
found to be 2.9365(3) Å which is consistent with other reported
acetate bridged palladacycles.58,61 The dinuclear complex
adopts a U-shaped or closed-book conformation where the two
pyrazole phenyl moieties stack on top of each other.
First we evaluated the activity of the palladacycle towards
the Mizoroki–Heck reaction. The coupling between bromoben-
zene and methyl acrylate was chosen as the model reaction. No
detectable conversion was observed in the absence of an
additive, K2CO3 as a base and DMF as a solvent at 110 °C
(Table S1,† entry 1); however, with increase in the temperature
to 140 °C we observed ca. 22% of the desired product. To
further improve the efficiency of the product formation, we
varied catalyst loading, temperature, bases and solvents (ESI†).
The use of NMP as a solvent at 0.2 mol% precatalyst loading
gave 81% yield of the desired trans-methyl cinnamate
(Table S1,† entry 15). With these optimized conditions in
5806 | Dalton Trans., 2015, 44, 5805–5809
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
Table 2 Palladacycle (1) catalyzed Heck reactions of aryl halides with
terminal olefins
Table 3 Palladacycle (1) catalyzed Suzuki reactions of aryl halides with
phenylboronic acid
Entry
Ar–X
R
Yielda (%)
TONc
Entry
Ar–X
Yielda (%)
TONc
1
2
3
4
5
6
7
8
C6H5Br
COOMe
Ph
COOMe
Ph
COOMe
Ph
COOMe
Ph
COOMe
Ph
COOMe
Ph
COOMe
Ph
COOMe
Ph
81(82)d
98
96
98
64
78
84
72
47
404
496
480
486
317
390
423
360
231
184
467
473
497
496
421
419
154
346
—
1
2
3
4
5
6
7
8
4-OMe-C6H4Br
2-OMe-C6H4Br
4-Ph-C6H4Br
99(99)d
98
991
977
794
937
928
992
992
—
378
379
108
39
4-OMe-C6H4Br
2-OMe-C6H4Br
4-Ph-C6H4Br
4-CN-C6H4Br
1-CH3-3-CH3-C6H3Br
C6H5I
81(82)d
94
4-CN-C6H4Br
1-CH3-3-CH3-C6H3Br
4-CHO-C6H4Br
4-CH3-C6H4I
4-COCH3-C6H4Cl
4-COCH3-C6H4Cl
4-CF3-C6H4Cl
93
99
99(99)d
Trace
38b
9
9
10
11
12
13
14
15
16
17
18
19
20
37
93
95
10
11
12
41b(40)d
11b
4-CH3-C6H4Cl
4-OCH3-C6H4Cl
4b
99b
99b (99)d
86b
84b
31
a Isolated yield after chromatography. b Reactions were performed at
100 °C for 6 h. c Turnover number (TON) based on the number of
moles of the isolated product. d Reactions were performed using
6 mmol of aryl halide.
4-Me-C6H4I
4-CF3-C6H4Cl
4-COMe-C6H4Cl
COOMe
Ph
COOMe
Ph
69
Trace
54(53)d
269
a Isolated yield after chromatography. b TBAB was not used. c Turnover
number (TON) based on the number of moles of the isolated product.
d Reactions were performed using 6 mmol of aryl halide.
not be converted effectively (Table 3, entry 8); however, 4 to
41% yields were achieved at elevated temperature (100 °C). We
have not made any attempts to determine whether unreacted
(1) or other palladium containing species could be detected at
the end of the catalytic reactions. However, a blank run was
carried out after the catalytic run to test whether a residual
catalyst is present in the system or not. As expected no product
formation was observed (Table S3,† entry 4).
reactions catalyzed by our palladacycle precatalyst under the
above mentioned reaction conditions, we examined the cross-
coupling of a variety of aryl bromides with methyl acrylate
and/or styrene. The results are presented in Table 2. To our
delight, the Heck arylation of aryl bromides proceeded
smoothly under these conditions yielding the corresponding
products in moderate to good yields; however, activated
4-bromobenzonitrile converted to the desired product with low
yields (Table 2, entries 9 and 10). Apart from aryl bromides,
Experimental section
Synthesis of 1,3,5-triphenyl pyrazole
aryl iodides could also be coupled under the same reaction 1,3-Diphenylpropane-1,3-dione (5.00 g, 22.3 mmol) and phenyl
conditions (Table 2, entries 13–16). Unfortunately our pre- hydrazine (2.42 mL, 24.5 mmol) were taken in a 100 mL round
catalyst is not active enough to handle aryl chlorides; bottom flask. Then 15 mL of methanol and 15 mL of acetic
however, activated aryl chloride gave the desired product in acid were added to the flask and the reaction mixture was
low yields.
refluxed for 12 h. To the reaction mixture, saturated sodium
We next investigated whether our palladacycle could also carbonate solution was added and the compound was
facilitate the Suzuki–Miyaura reactions. The reactions were extracted using dichloromethane. The solvent was removed
carried out under relatively milder conditions in the presence under vacuum and the residue was purified by column chrom-
of water and the results are presented in Table 3. In a typical atography (hexane–ethyl acetate as an eluent). Yield: 5.71 g
reaction, 1 equivalent of aryl bromide was reacted with 1.2 (86%). 1H NMR (400 MHz, CDCl3) δ = 7.96 (d, J = 8 Hz, 2H,
equivalents of phenyl boronic acid in the presence of 0.1 mol ArH), 7.29–7.47 (m, 13H, ArH), 6.83 (s, 1H, 4-pyrazole-H). 13C
% of the palladacycle, 1.2 equivalent of K3PO4 at 50 °C for 4 h. NMR (100 MHz, CDCl3) δ = 151.91, 144.74, 139.76, 132.68,
Both electron-rich as well as electron-poor aryl bromides and 130.44, 129.09, 128.89, 128.82, 128.65, 128.59, 128.36, 127.81,
aryl iodides could be converted to the desired products in 126.10, 125.57, 105.32. HR-MS (ESI): calcd for C21H16N2
moderate to high yields (Table 3, entries 1–7). Later we sub- ([M + H]+): 297.1386, found: 297.1373. Elemental analysis
jected our palladacycle precatalyst to cross-coupling between calcd (%) for C21H16N2: C 85.11, H 5.44, N 9.45; found:
phenyl boronic acid and aryl chlorides. Not surprisingly, C 84.95, H 5.29, N 9.40. IR (cm−1): 3061 (m), 3047 (m),
under the above-mentioned conditions aryl chlorides could 1595 (m), 1494 (s), 1362 (m), 1065 (m), 764 (s), 693 (s).
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 5805–5809 | 5807

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Synthesis of palladacycle [{Pd(TPPz)(µ-AcO)}2] (1)
Conclusion
Palladium acetate (1.00 g, 4.4 mmol) and 1,3,5-triphenylpyra-
zole (TPPz) (1.45 g, 4.8 mmol) were suspended in glacial acetic
acid (40 mL) and the mixture was heated in an oil bath
(100 °C, 2 h). The reaction mixture (hot condition) was filtered
through celite to remove palladium black and the solution was
concentrated under vacuum to 3 mL, layered with 15 mL
acetone and stored at 5 °C for 24 h. The yellow product was iso-
lated by filtration, washed with acetone and air dried. Yield:
1.21 g (59%). The compound was recrystallized from a mixture
In summary, we have synthesized and studied 1,3,5-triphenyl-
pyrazole palladacycle catalyst in Suzuki–Miyaura and
1
Mizoroki–Heck cross-coupling reactions. The palladacycle pre-
catalyst showed appreciable catalytic activity for the Mizoroki–
Heck reaction. Moreover, it showed good catalytic activity
towards Suzuki–Miyaura reactions under mild reaction con-
ditions. The synthetic accessibility from readily available start-
ing materials and possible structural versatility makes the
pyrazole palladacycle complexes promising catalysts or precata-
lysts for further development and possible applications in
other catalytic organic transformations. Further studies on this
line are ongoing in our group and will be reported in the
future.
1
of dichloromethane and hexanes to obtain single crystals. H
NMR (400 MHz, C6D6) δ = 7.67 (d, J = 8 Hz, 4H, ArH), 7.42 (d,
J = 8 Hz, 4H, ArH), 7.23–7.07 (m, 10H, ArH), 6.98–6.89 (m, 6H,
ArH), 6.72 (t, J = 8 Hz, 2H, ArH), 6.50 (t, J = 8 Hz, 2H, ArH),
6.40 (d, J = 8 Hz, 2H, ArH), 5.57 (s, 2H, ArH), 1.49 (s, 6H, Me).
13C NMR (100 MHz, C6D6) δ = 180.29, 153.70, 143.08, 141.13,
134.55, 133.18, 129.92, 129.63, 129.48, 129.21, 128.76, 128.19,
127.90, 123.44, 122.71, 111.88, 109.49, 22.70. HR-MS (ESI):
calcd for C44H34N4O2Pd2 ([M − OAc]+): 863.0690, found:
863.0614. Elemental analysis calcd (%) for C46H38N4O4Pd2:
C 59.82, H 4.15, N 6.07; found: C 59.65, H 3.99, N 6.27. IR
(cm−1): 3052 (m), 2928 (m), 1575 (s), 1412 (s), 1342 (m), 1002
(m), 758 (s), 696 (s). The crystallized material was used to
perform the reactions.
Acknowledgements
KV thanks the Department of Science and Technology (DST),
New Delhi for financial support (SR/S2/RJN-49/2011 and SR/
S1/IC-58/2012). RM, VM, and KD thank CSIR, New Delhi for
research fellowship. We also thank the reviewers for their
helpful suggestions.
General procedure for Mizoroki–Heck reactions
Notes and references
A Schlenk tube was charged with aryl halide (3.18 mmol),
potassium phosphate (6.36 mmol), tetra-n-butyl ammonium
bromide (0.31 mmol) and the precatalyst (6.4 × 10−3 mmol).
The tube was connected to a vacuum line under argon and
purged three times. N-Methyl-2-pyrrolidone (4 mL) and olefin
(6.36 mmol) were added. The reaction mixture was stirred at
160 °C for 30 h. The reaction mixture was cooled to room
temperature, diluted with ethyl acetate (10 mL) and filtered
through celite (1 cm height) silicagel (100–200 mesh, 1 cm
height). The filtration process was repeated with fresh ethyl
acetate (2 × 10 mL). The combined organic solution was
washed with water and brine solution. The organic phase was
dried over anhydrous sodium sulphate. After removal of the
solvent, the residue was subjected to column chromatography
on silica gel using an ethyl acetate and hexane mixture to
afford the Mizoroki–Heck product in high purity.
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