Bis(pyrazolyl)palladium(II) complexes as catalysts for Mizoroki–Heck cross-coupling reactions

Article abstract of DOI:10.1016/j.poly.2019.03.030

Recent progress in carbon–carbon cross-coupling reactions has resulted in the discovery of highly active catalysts for carrying out such transformations. However, due to the wide array of applications of the products from cross-coupling reactions, there is the need to design suitable catalysts that permit the practical and economical synthesis of the cross-coupled products. Palladium complexes with bulky and electron-donating ligands have served as excellent (pre)catalysts for the Mizoroki–Heck cross-coupling reaction. By using bulky pyrazole-based ligands, we have prepared palladium(II) complexes with controlled steric and electronic properties of the metal center. We have used these bulky bis(pyrazolyl)palladium(II) complexes as (pre)catalysts for the Mizoroki–Heck cross-coupling reaction. The (pre)catalysts displayed high activity and selectivity, giving high catalytic conversions at a low (pre)catalyst loading and short reaction times. A mercury poisoning test confirmed that the (pre)catalysts promoted the Mizoroki–Heck cross-coupling homogenously and do not decompose into palladium black during the reactions. The catalytic systems were also tolerant to the presence of functional groups, such as 4-CF₃, 4-CH₃, 4-CO₂Me and 4-CO₂Et, on the alkene substrates.

Full text of DOI:10.1016/j.poly.2019.03.030

Accepted Manuscript
Bis(pyrazolyl)palladium(II) complexes as catalysts for Mizoroki-Heck cross-
coupling reactions
Edward Ocansey, James Darkwa, Banothile C.E. Makhubela
PII:
S0277-5387(19)30203-7
https://doi.org/10.1016/j.poly.2019.03.030
POLY 13831
DOI:
Reference:
Polyhedron
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Revised Date:
Accepted Date:
9 February 2019
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17 March 2019
Please cite this article as: E. Ocansey, J. Darkwa, B.C.E. Makhubela, Bis(pyrazolyl)palladium(II) complexes as
catalysts for Mizoroki-Heck cross-coupling reactions, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.
2019.03.030
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Bis(pyrazolyl)palladium(II) complexes as catalysts for Mizoroki-Heck
cross-coupling reactions
Edward Ocansey, James Darkwa* and Banothile C.E. Makhubela*
Department of Chemical Science, University of Johannesburg, Kingsway Campus, Auckland
Park, 2006, South Africa.
Abstract
Recent progress in carbon-carbon cross-coupling reactions has resulted in the discovery of
highly active catalysts for carrying out such transformations. However, due to the wide array
of applications of the products from cross-coupling reactions, there is the need to design
suitable catalysts that permit the practical and economical synthesis of the cross-coupled
products. Palladium complexes with bulky and electron-donating ligands have served as
excellent (pre)catalysts for the Mizoroki-Heck cross-coupling reaction. By using bulky
pyrazole-based ligands, we have prepared palladium(II) complexes with controlled steric and
electronic properties of the metal centre. We have used these bulky bis(pyrazolyl)palladium(II)
complexes as (pre)catalysts for the Mizoroki-Heck cross-coupling reaction. The (pre)catalysts
displayed high activity and selectivity, giving high catalytic conversions at a low (pre)catalyst
loading and short reaction times. A mercury poisoning test confirmed that the (pre)catalysts
promoted the Mizoroki-Heck cross-coupling homogenously and do not decompose into
palladium black during the reactions. The catalytic systems were also tolerant to the presence
of functional groups, such as 4-CF3, 4-CH3, 4-CO2Me and 4-CO2Et, on the alkene substrates.
Keywords: Bulky-pyrazolyl ligands; palladium(II) complexes; cross-coupling; Mizoroki-
Heck; homogenous catalysts

1.0 Introduction
Transition metal catalysts have played a significant role in synthetic chemistry and the
discovery of cross-coupling reactions has led to the design of new organic compounds and
materials with a wide array of applications. These compounds and materials include drugs,
natural products, optoelectronic materials and polymers.1 One such example of a cross-
coupling reaction is the Mizoroki-Heck reaction, which allows coupling between organic
halides or pseudohalides and alkenes.2 This cross-coupling reaction is of immense value to
organic chemists as it allows the substitution of alkenes with high stereoselectivity for trans
coupled products.1 In spite of the impressive progress made in carbon-carbon cross-coupling
reactions, especially the Mizoroki-Heck reaction, there is still a high demand for both
economical and practical cross-coupling processes that use ultra-low catalyst loadings and give
high turnover numbers.2
Palladium is mostly employed as the metal of choice in the Mizoroki-Heck cross-coupling3–5
and other cross-coupling reactions.6–8 This is because of the advantages it has over its most
promising competitor, nickel.5,9–11 These advantages include high activity, negligible or no by-
product formation and no additive requirements. However, palladium is an expensive metal
and any palladium catalyst that can operate efficiently at an extremely low catalyst loading is
desirable. In order to develop such effective catalytic systems, palladium complexes with
ligands that contribute increased electron-donation and steric bulk have been introduced as
catalyst precursors. This has resulted in increased turnover numbers in Mizoroki-Heck cross-
coupling reactions.12 In addition, complexes that are air- and moisture-stable offer greater
stability and easy handling of the catalyst precursor when compared to traditional Mizoroki-
Heck cross-coupling catalytic systems - which make use of air- and moisture-sensitive ligands
such as alkyl phosphines.13 The use of bulky (non-phosphine based) ligands in preparing
palladium(II) complexes presents the opportunities to not only isolate air- and moisture-stable
catalyst precursors, but to also introduce (pre)catalysts with favourable stereo-electronic
properties.14-19 These stereo-electronic properties have been shown to result in high activity.14–
17
Pyrazolyl-based palladium(II) complexes are stable and efficient catalyst precursors for
Mizoroki-Heck cross-coupling reactions, at mild temperatures with fast reaction times.14 Such
pyrazolyl complexes, particularly those where the pyrazolyl ligands coordinate to the
palladium in a bis-coordination mode,15,22 have good potential to serve as alternatives to the
often unstable palladium phosphine and NHC-based (pre)catalysts. We herein report the

synthesis of bulky bis(pyrazolyl)palladium(II) complexes with electron-donating and electron-
withdrawing substituents on the pyrazolyl ring or on the benzyl ring, as stable catalyst
precursors for Mizoroki-Heck cross-coupling reactions.
1.1 Experimental
1.1.1 Materials and methods
All reactions were carried out in air unless otherwise stated. All solvents used were reagent
grade, purchased from Sigma Aldrich and dried under nitrogen before use. The NMR spectra
were obtained using a Bruker-400 MHz NMR spectrometer (1H at 400 MHz). The chemical
shifts are reported relative to the internal standard tetramethylsilane (δ 0.00 ppm) and
referenced to the residual proton and carbon signals at δ 7.24 and 77.0 ppm respectively of
CDCl3. Melting points were obtained using a Gallenkamp Digital Melting-point Apparatus 5A
6797. Elemental analysis was performed on a Thermos Scientific FLASH 2000 CHNS-O
Analyzer. Mass spectrometry was performed using a Waters Synapt G2 mass spectrometer
with both ESI positive and Cone Voltage 15 V. XRD spectra were obtained from a Bruker
APEX-II CCD Diffractometer. Analytical thin layer chromatography (TLC) was performed on
silica gel coated aluminium plates (0.2 mm). The developed plates were visualized with UV
light or under iodine staining. Silica gel column chromatography was performed using silica
gel 60 (70-230 mesh).
1.1.2 General experimental procedure- Synthesis of the pyrazolyl ligands 1-4
Synthesis of the previously reported pyrazolyl ligands 1-4 first involved the preparation of 3,5-
ditertbutylpyrazole and 3,5-diphenylpyrazole. In a typical procedure, 3,5-disubstituted
pyrazole (1 mmol) was reacted with methyl-4-(chloromethyl)benzoate (1mmol) or benzyl
bromide (1 mmol) in DMSO in the presence of NaH (4 mmol) at room temperature for 16 h.
Water was then added to the reaction mixture and the product extracted using EtOAc. The
crude product was then purified via column chromatography using a 1:3 v:v diethyl
ether:hexane mixture.18
(1) Yield: 184 mg, 50 %; Melting point: 123 oC. 1H NMR (100 MHz, CDCl3, 30 oC, δ, ppm):
7.94 (d, 2H, JHH = 8 Hz, Harom), 7.83 (d, JHH = 8 Hz, 2H, Harom), 7.45-7.30 (m, 8H, Harom), 7.18
13
(d, 2H, JHH = 8 Hz, Harom), 6.70 (s, 1H, Hpz), 5.45 (s, 2H, HCH2), 3.89 (s, 3H, HCH3). C{1H}
o
NMR (100 MHz, CDCl3, 30 C, δ, ppm): 166.77, 151.43, 145.67, 142.77, 133.32, 130.55,

130.37, 130.25, 129.99, 129.44, 128.85, 128.86, 127.81, 126.77, 125.75, 103.95, 53.11, 52.11.
Elemental analysis, Anal. calcd. for C24H20N2O2: C, 78.24; H, 5.47; N, 7.60%; Found: C, 78.33;
H, 5.59; N, 7.51%
(2) Yield: 170 mg, 52%; Melting point: 139 oC. 1H NMR (400 MHz, CDCl3, 30 oC, δ, ppm):
7.92 (d, 2H, JHH = 7.9 Hz, Harom), 6.93 (d, JHH = 8 Hz, 2H, Harom), 5.90 (s, 1H, Hpz), 5.49 (s,
1H, HCH2), 3.89 (s, 3H, HCH3), 1.28 (s, 9H, HtBu), 1.22 (s, 9H, HtBu). 13C{1H} NMR (100 MHz,
o
CDCl3, 30 C, δ, ppm): 166.87, 160.65, 151.94, 144.63, 129.75, 129.65, 128.81, 125.85,
125.68, 100.39, 54.00, 51.97, 31.21, 30.26. Elemental analysis, Anal. calcd. for C20H28N2O2:
C, 73.14; H, 8.59; N, 8.53%, Found: C, 73.29; H, 8.41; N, 8.33%
o
1
(3) Yield: 241 mg, 78%; Melting point: 95 C. H NMR (400 MHz, CDCl3, 30 °C, δ, ppm):
7.70 (d, 3JHH = 7.2 Hz, 2H, Harom), 7.38-7.23 (m, 11H, Harom), 7.09 (d, 3JHH = 6.8 Hz, 2H, Harom),
6.59 (s, 1H, Hpz), 5.33 (s, 2H, HCH2). 13C{1H} NMR (100 MHz, CDCl3, 30 oC, δ, ppm): 150.95,
145.48, 137.76, 133.49, 130.61, 128.80, 128.73, 128.49, 128.40, 127.50, 127.48, 126.91,
126.85, 126.66, 103.71, 53.21. Elemental analysis, Anal. calcd. for C22H18N2: C, 85.13; H,
5.85; N, 9.03 %, Found: C, 85.49; H, 5.75; N, 9.41 %.
o
(4) Yield: 207 mg, 77 %; Melting point: 61 C. 1H NMR (400 MHz, CDCl3, 30 oC, δ, ppm):
7.27 (t, 3JHH = 5.6 Hz, 3H, Harom), 6.89 (d, 3JHH = 7.2 Hz 2H, Harom), 5.89 (s, 1H, Hpz), 5.49 (s,
2H, HCH2), 1.29 (s, 9H, HtBu), 1.24 (s, 9H, HtBu). 13C{1H} NMR (100 MHz, CDCl3, 30 oC, δ,
ppm): 161.33, 151.19, 139.20, 128.1, 128.08 126.65, 126.51, 125.77, 100.05, 54.05, 31.81,
30.29. Elemental analysis, Anal. calcd. for C18H26N2: C, 79.95; H, 9.69; N, 10.36 %, Found C,
80.11; H, 9.64; N, 10.04 %.
Synthesis of 1a
To 541 mg (1.47 mmol) of methyl 4-((3,5-diphenyl-1 H-pyrazol-1-yl)methyl)benzoate in a
round bottom flask, 25.0 mL of methanol was added and stirred. 824 mg (14.7 mmol) of KOH
was added and the resulting mixture refluxed at 85 oC for 18 h. The solvent was then removed
by rotary evaporation, after which 25.0 mL of distilled water was added to the crude product
to form a slurry. 30.0 % HCl(aq) was then added dropwise to precipitate the product. The product
was then filtered and washed with 2 x 25.0 mL distilled water and air dried for 12 h. Appearance
white solid (Yield: 409 mg, 79.0%). Solubility: Soluble in chloroform, methanol, DCM and
diethyl ether; insoluble in water. Melting point range: 175-176 oC. FT-IR (vmax/cm-1): 1683.78
(C=O), 1609.40 (C=N). 1H NMR (400 MHz, CDCl3, 30 oC, δ, ppm): 7.99 (d, 2H, Harom), 7.86
(d, 2H, Harom), 7.44-7.29 (m, 8H, Harom), 7.16 (d, 2H Harom), 6.67 (s, 1H, Hpz), 5.42 (s, 2H, HCH2),
o
13C{1H} NMR (100 MHz, CDCl3, 30 C, δ, ppm): 171.06, 151.42, 145.76, 143.58, 133.07,
131.11, 131.00, 130.92, 130.79 130.54, 130.23, 130.11, 129.95, 129.87, 129.70, 104.00, 52.96.

HR-ESI-MS[M+H]+: 355.1401. Elemental analysis, Anal. calcd. for C23H18N2O2: C, 77.95; H,
5.12; N, 7.90%; Found C, 77.90; H, 5.23; N, 7.63%
1.1.3 General experimental procedure - Synthesis of the bis(pyrazolyl)palladium(II)
complexes 5-9
Synthesis of the previously reported [{bis(methyl 4-((3,5-diphenyl-1 H-pyrazol-1-
yl)methyl)benzoate)}-palladium (II) chloride] (5), [{bis(methyl 4-((3,5-ditertbutyl-1 H-
pyrazol-1-yl)methyl)benzoate)}-palladium (II) chloride] (6), [{bis(1-benzyl-3,5-diphenyl-1 H-
pyrazole)}-palladium(II) chloride] (7) and [{bis(1-benzyl-3,5-di-tert-butyl-1 H-pyrazole)}-
palladium (II) chloride (8) involved the reaction of the corresponding ligand (0.5 mmol) and
[PdCl2(MeCN)2] (0.25 mmol) in 10 ml DCM for 24 h. At the end of the reaction, the amount
of solvent was reduced in vacuo to about 2 ml, after which the product was precipitated out of
DCM by the dropwise addition of diethyl ether or hexane.18
1
(5) Yield: 180 mg, 79 %. Melting point: 276 oC (decomposed without melting). H NMR (400
MHz, CDCl3, 30 oC, δ, ppm): 8.17 (d, 2H, Harom), 7.94(d, 2H, Harom), 7.33-7.28 (m, 6H, Harom),
13
7.00 (d, 4H, Harom), 6.38 (s, Hpz, 1H), 6.15 (s, HCH2, 2H), 3.92 (s, HCH3, 3H); C{1H} NMR
o
(100 MHz, CDCl3, 30 C, δ, ppm): 168.05, 156.12, 149.81, 138.43, 131.85, 129.45, 129.20,
128.91, 128.89, 128.75, 128.65, 128.50, 128.39, 128.08, 127.98, 105.19, 57.15, 52.10.
Elemental analysis, Anal. calcd. for C48H40Cl2N4O4Pd: C, 63.06; H, 4.41, N, 6.13 % Found
C,63.21; H, 4.69; N, 6.39 %.
(6) Yield: 166 mg, 80 %; Melting point: 240 oC (decomposed without melting). 1H NMR (400
MHz, CDCl3, 30 °C, δ, ppm): 8.11 (d, JHH = 8 Hz, 2H, Harom), 8.09 (d, 2H, Harom), 7.24 (d, 2H,
Harom), 7.09 (d, 2H, Harom), 6.00 (m, 6H, HCH2, Hpz), 3.95 (s, 3H, HCH3), 3.84 (s, 3H, HCH3), 1.95
(s, HtBu, 6H), 1.90 (s, HtBu, 1H), 1.83 (s, HtBu, 6H), 1.62 (s, HtBu, 2H), 1.55 (s, HtBu, 3H), 1.29 (s,
HtBu, 1H), 1.24 (s, HtBu, 7H), 1.21 (s, HtBu, 7H), 1.15 (s, HtBu, 3H). 13C{1H} NMR (100 MHz,
o
CDCl3, 30 C, δ, ppm): 167.50, 167.29, 167.25, 167.15, 165.55, 160.99, 157.28, 153.41,
144.30, 141.99, 141.78, 130.79, 130.69, 130.64, 130.39, 130.29, 130.21, 130.15, 130.15,
129.77, 127.22, 126.95, 125.89, 105.46, 105.16, 56.64, 51.99, 33.17, 32.89, 32.39, 31.89,
31.84, 31.79, 31.70, 31.58, 31.49, 31.45, 30.60, 30.37, 29.95. Elemental analysis, Anal. calcd.
for C40H56Cl2N4O4Pd: C, 57.59; H, 6.77, N, 6.72 % Found C,57.33; H 6.45; 6.91 N %.
(7) Yield: 119 mg, 60 %: Melting point: 251 oC. 1H NMR (400 MHz, CDCl3, 30 °C, δ, ppm):
8.21 (d, JHH = 7.2 Hz, 2H, Harom), 7.30-7.13 (m, 9H, Harom), 6.99 (d, 2H, Harom), 6.95 (t, JHH
=

1
13
8 Hz, JHH = 7.6 Hz, 2H, Harom), 6.31 (s, Hpz, 1H), 6.10 (s, HCH2, 2H). C{1H} NMR (100
MHz, CDCl3, 30 oC, δ, ppm): 155.55, 149.19, 137.39, 131.08, 129.16, 129.05, 129.01, 128.85,
128.79, 128.60, 128.55, 128.51, 128.40, 128.39, 110.20, 56.51. Elemental analysis, Anal.
calcd. for C44H36Cl2N4Pd: C, 66.22; H, 4.55, N, 7.02 % Found C, 66.56; H 4.41; 6.78 N %.
(8) Yield: 112 mg, 63 %: Melting point: 222 oC (decomposed without melting). 1H NMR (100
MHz, CDCl3, 30 oC, δ, ppm): 7.41-7.22 (m, 6H, Harom), 7.05 (d, 4H, Harom), 6.05 (m, 6H, Hpz,
HCH2), 1.96 (s, HtBu, 7H), 1.93 (s, HtBu, 2H), 1.84 (s, HtBu, 9H), 1.69 (s, HtBu, 2H), 1.25 (d, JHH
= 14.4 Hz, HtBu, 7H), 1.20 (d, JHH = 6 Hz, HtBu, 7H), 1.17 (s, HtBu, 2H), 0.86 (s, HtBu, 2H).
o
13C{1H} NMR (100 MHz, CDCl3, 30 C, δ, ppm): 165.14, 164.18, 157.01, 156.91, 135.77,
135.60, 129.11, 128.81, 128.69, 128.30, 128.11, 127.91, 127.19, 126.85, 125.80, 105.30,
105.27, 105.01 55.87, 55.73, 33.01, 32.91, 30.66, 30.34. Elemental analysis, Anal. calcd. for
C36H52Cl2N4Pd: C, 60.21; H, 7.30, N, 7.80% Found C, 60.39; H, 7.55; N, 7.61%.
Synthesis of 9
In a Schlenk tube, 100 mg of 4-((3,5-diphenyl-1 H-pyrazol-1-yl)methyl)benzoic acid (0.28
mmol) was added to 36.7 mg (0.14 mmol) of [PdCl2(MeCN)2] using 5.00 mL of DCM as the
solvent. The reaction mixture was then stirred for 24 h under argon at room temperature. The
solvent was reduced in vacuo until the crude product began to precipitate. Hexane (5.00 mL)
was then added dropwise until the product completely precipitated from the solution. The
precipitate was filtered off and washed with 2 x 5.00 mL hexane and dried under vacuum for
6 h in order to obtain the product. Appearance: yellow powder, (Yield = 81.0 mg, 65.0 %).
Solubility: Partially soluble in chloroform, xylene and toluene, insoluble in methanol, water,
hexane and diethyl ether. Melting point: 246 °C (decomposed without melting). FT-IR
1
(vmax/cm-1): 3379.74 (O-H), 1687.04 (C=O), 1609.16 (C=N). H NMR (400 MHz, D2O+KOH,
30 °C, δ, ppm): 8.75 (d, 2H, Harom), 7.62-7.57 (m, 4H, Harom), 7.49-7.48 (m, 2H, Harom), 7.31-
7.28 (m, 4H, Harom), 7.13 (d, 2H, Harom), 6.61 (s, 1H, Hpz), 6.42 (s, 2H, HCH ). 13C{1H} NMR
2
(100 MHz, CDCl3, 30 °C, δ, ppm): 158.15, 155.29, 149.78, 137.89, 132.81, 129.41, 129.22,
128.99, 128.73, 128.54, 128.41, 128.37, 128.25, 128.21, 127.81, 106.54, 57.92. HR-ESI-
MS[M]+: 885.1085. Elemental analysis, Anal. calcd. for C46H36Cl2N4O4Pd: C, 62.35; H, 4.09,
N, 6.32 % Found C, 62.18; H 4.31; 6.52 N %.

1.1.4 General experimental description of the catalysis
The general procedure involved charging a Radley Carousel 12 Plus Reaction Station™ 24 x
150 mm quick-thread glass reaction tube with the required amounts of aryl halide, alkene, base,
internal standard (n-decane), catalyst and solvent. The reaction was then allowed to proceed at
the specified temperature for the specified time. To calculate the conversion, the GC was first
calibrated using standard samples of the aryl halide, solvent, n-decane and trans-stilbene and
cis-stilbene in order to accurately determine the retention times of these compounds. A sample
of the reaction mixture in the reaction vessel was taken prior to the start of the reaction and
analyzed by GC to determine the ratios of the various components. Aryl halide conversion to
biphenyl was calculated by analyzing the retention peak areas of aryl halide with reference to
the internal standard n-decane before and after the reaction. GC analyses were performed using
a Scion 456-GC with a 30 m x 0.25 mm cyanopropylphenylmethylpolysiloxane phase column
set to an initial temperature of 80 °C and then increased to 250 °C at 10 °C/min.
1.1.5 X-ray crystallography of 6 and 8
Single crystal diffraction studies were carried out using Quazar multi-layer optics
monochromated Mo Kα radiation (k = 0.71073 Å) on a APEX II diffractometer (for 6) and
Bruker D8 Venture kappa geometry diffractometer (for 8) with duo Is sources, a Photon 100
CMOS detector and APEX II control software.19 The X-ray diffraction measurements were
performed at 100 K (for 6) and 150(2) K (for 8). Data reduction was performed using
SAINT+,19 and the intensities were corrected for absorption using SADABS.19 The structures
were solved by direct methods using SHELXT,20 using the SHELXL-2014/721 program. The
non-hydrogen atoms were refined anisotropically. All H atoms were placed in geometrically
idealised positions and constrained to ride on their parent atoms.

1.2 Results and discussion
1.2.1 Synthesis of the bis(pyrazolyl)palladium(II) complexes
Ligands 1-4 readily formed complexes with [PdCl2(MeCN)2] in DCM at room temperature
over a 24 h reaction time and the complexes (5-9) were easily isolated after precipitation using
diethyl ether or hexane (Scheme 1). The square planar palladium(II) complexes were obtained
in good yields from this reaction procedure, yielding air- and moisture-stable compounds.
Complexes 5-9 can be kept on a benchtop for 18 months without decomposition and have high
thermal stability (decompose without melting above 200 °C). Characteristic downfield shifts
1
in the positions of the CH2 protons, from ~ δ 5.5 ppm to around δ 6.0 ppm, in the H NMR
spectra of the complexes served as an indication of successful complexation of the ligands to
1
the palladium ions, see Figures SI-1 and SI-4. All other H NMR peaks were seen in the
expected regions in the spectra of 5-9. For example, in complex 9, the aromatic proton signals
resonate in the region δ 7.13–8.75 ppm, while the CH2 and aromatic pyrazolyl proton signals
were seen at δ 6.42 and 6.61 ppm, respectively.
Scheme 1: Synthesis of the pyrazolyl complexes.

Complexes 5, 6 and 9 all show distinctive signals in the range δ 160 to 170 ppm in their 13C{1H}
NMR spectra, which correspond to the carbonyl carbon atoms present in these compounds. In
addition, 5 and 6 give rise to peaks at around δ 50 ppm, which are assigned to the methyl groups
of the esters. Generally for 5, 6 and 9, there are downfield shifts in the positions of the
methylene (CH2) carbon peak from around δ 53.09 ppm (Figure SI-1) in the corresponding free
ligands to around δ 57 ppm, for the complexes. Similarly, the methylene (CH2) carbon signal
for 7 and 8 shifted from around δ 53 ppm in the free ligands to around δ 56 ppm in the
complexes (Figures SI-2 and SI-5). Furthermore, the mass spectra of these complexes generally
gave [M]+ molecular ions, which fragmented by sequential loss of the chloride ligands, as
shown in Figure SI-6.
1.2.2 Molecular structures of 6 and 8
The molecular structures of 6 and 8 were obtained using single-crystal X-ray diffraction, which
confirmed the coordination of the pyrazolyl ligands to the palladium centrse in a trans-fashion.
The crystal data and structure refinement parameters for these complexes are presented in Table
1. Slow evaporation at room temperature of separate DCM solutions of 6 and 8 yielded yellow
crystals suitable for XRD analysis. Complex 6 crystallized in the P21/n space group, while
complex 8 crystallized in the P21/c space group. The crystal structures of complexes 6 (Figure
1 (A)) and 8 (Figure 1 (B)) indicated the geometry around the palladium centre to be square
planar. While the bond lengths around the metal centres of 6 and 8 fall in the usual ranges, it is
interesting to comment on the differences in the bond angles. Both 6 and 8 possess bond angles,
around the palladium centre that deviate from ideal square planar bond angles. Complex 6 has
N(2)-Pd(1)-Cl(1) and N(2)-Pd(1)-Cl(1i) bond angles of 85.78(4) and 94.22(4)o, whilst complex
8 has N(2)-Pd(1)-Cl(1) and N(2)-Pd(1)-Cl(1i) bond angles of 88.00(9)o and 92.00(9)o
respectively. These deviations in bond angles may be the result of the steric bulk of the tertiary
butyl substituents on the pyrazolyl rings which force the complexes out of their regular square
planar geometry.

t
Figure 1: Molecular structures of 6 (A) and 8 (B). Hydrogen atoms and distorted Bu group (B) are
omitted for clarity.
Table 1: Crystal data for 6 and 8.
Identification code
Complex 6
C40H56N4O4Cl2Pd
834.24
Complex 8
C36H52Cl2N4Pd
718.11
Empirical formula
Formula weight
Temperature/K
100(2)
149.99
Crystal system
monoclinic
P21/n
monoclinic
P21/c
Space group
a/Å
13.0757(12)
11.2095(10)
13.9982(13)
90
10.8967(6)
15.9169(9)
11.4233(7)
90
b/Å
c/Å
α/°
β/°
93.366(2)
90
115.890(2)
90
γ/°
Volume/Å3
2048.2(3)
2
1782.42(18)
2
Z
ρcalc/g cm-3
1.3526
1.338
μ/mm-1
0.627
0.700
F(000)
870.9
752.0
Crystal size/mm3
0.317 × 0.2 × 0.2
Mo Kα (λ = 0.71073)
4.14 to 56.56
0.23 × 0.213 × 0.198
MoKα (λ = 0.71073)
4.718 to 53.218
Radiation
2Θ range for data collection/°

Index ranges
-17 ≤ h ≤ 17, -14 ≤ k ≤ 14, - -13 ≤ h ≤ 13, -20 ≤ k ≤ 19, -
18 ≤ l ≤ 15
14 ≤ l ≤ 14
Reflections collected
45343
46305
Independent reflections
5060 [Rint = 0.0386, Rsigma = 3639 [Rint = 0.0565, Rsigma =
0.0200]
0.0329]
Data/restraints/parameters
Goodness-of-fit on F2
5060/0/239
3639/0/253
1.026
1.203
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole / e Å-3
R1 = 0.0254, wR2 = 0.0604
R1 = 0.0300, wR2 = 0.0628
0.87/-0.44
R1 = 0.0528, wR2 = 0.0861
R1 = 0.0725, wR2 = 0.0914
0.53/-0.62
Pd-Npz and Pd-Cl bond lengths around the palladium centre for 6 and 8 were within the reported
ranges for similar bis(pyrazolyl) palladium complexes in the literature.22–24 Selected bond
lengths and angles for 6 and 8 are given in Table 2.
Table 2: Selected bond lengths and angles for complexes 6 and 8.
Bond angles (˚) / lengths (Å) Complex 6
Complex 8
2.2963(10)
2.051(3)
1.370(4)
88.00(8)
180.00
Pd(1)-Cl(1)
2.3094(4)
2.0520(12)
1.3766(17)
85.78(4)
Pd(1)-N(2)
N(2)-N(1)
N(2)-Pd(1)-Cl(1)
N(2)-Pd(1)-N(2)i
180.00
1.3 Catalytic studies
1.3.1 Evaluation of (pre)catalysts 5-8 in Mizoroki-Heck cross-coupling reactions
In order to examine the efficiency of the bis(pyrazolyl)palladium(II) complexes 5-8 in the
Mizoroki-Heck cross-coupling reaction, optimization was carried out using styrene and
iodobenzene as model substrates. The reaction was studied under various conditions, such as
different temperatures, base, catalyst loading and reaction times in an effort to find the most
suitable conditions.

1.3.2 Optimization of the reaction conditions for the Mizoroki-Heck cross-coupling
reaction
In determining the optimum reaction temperature, the cross-coupling reactions were carried at
80 to 140 oC (Table SI-1). Increasing the temperature from 80 to 120 oC resulted in a gradual
increase in the iodobenzene conversion to cis- and trans-stilbene (~ 6:1 ratio of trans:cis-
stilbene) for all four (pre)catalysts 5-8, from around 20 to 70 %. At 140 oC, 100 % conversion
of iodobenzene to stilbene was observed for all four (pre)catalysts. However, reducing the
temperature to 130 oC still resulted in 100 %, indicating that 130 oC was the optimum reaction
temperature. Changing the solvent from DMF to either 1,4-dioxane or toluene in an effort to
improve selectivity was unsuccessful as the (pre)catalysts decomposed during the catalytic
reaction. This suggested that the solvent (DMF) may be playing a crucial stabilizing role of the
active Pd0 species in solution. In addition, the size and electronic effects of the substituents
(tertiary butyl, phenyl or ester) on the (pre)catalysts did not significantly affect the selectivity,
but rather the stability of the product drives the selectivity, since trans-stilbene is more stable
than cis-stilbene.25 Changing the base from triethylamine to pyridine resulted in no
iodobenzene conversion (Table SI-2, Entries 13-16). Since pyridine is a weaker base as
compared to triethylamine, there may possibly be a reduced “mopping-up” of HI (a reaction
by-product) in the reaction step that leads to regeneration of the active Pd(0) species in the
catalytic cycle.7 Using KOH resulted in complete conversion to both cis and trans-stilbene in
a ratio of around ~ 7:3. Triethylamine was used as the base for the subsequent reactions because
of the ease of analysis of the reaction mixtures as it formed a completely homogenous mixture
at the end of the reaction, in contrast to the use of KOH which formed a solid-like mass,
requiring excessive dilution with DMF prior to GC analysis. However, either base can serve as
a viable alternative for possible large-scale applications. Interestingly, using K2CO3, which is
weaker than KOH, also resulted in no conversion. However, this poor conversion could also
be as a result of the poor solubility of the base in DMF.
With an optimum temperature of 130 °C and using triethylamine as base, decreasing the
catalyst loading from 0.5 mol % to 0.0375 mol % did not significantly reduce the conversion
for all four (pre)catalysts (Table SI-3). This resulted in an appreciable increase in the turnover
number from 200 (0.5 mol%) to around 2600 (0.0375 mol%) as a result of the lower catalyst
to substrate ratio. de Vries et al26 have reported that a higher catalyst loading (1.2 mol%)
resulted in low activity in Mizoroki-Heck cross-coupling reactions using [Pd(OAc)2] at 135

°C, and that this was due to formation of more unreactive palladium black at such higher
catalyst loadings.26 The yield of the cross-coupled products was the highest when the catalyst
loading was in the range 0.02 and 0.08 mol%. However, a decrease of catalyst loading to 0.0094
mol% saw a significant production of the cross-coupled product with a TON as high as 5858,
albeit with lowered aryl halide conversions of around 50 % (Table 3). These TONs are
extremely high when 5-8 are compared to similar bis-pyrazolyl palladium complexes for the
Mizoroki-Heck cross-coupling reaction, for which optimum TONs were around 990.22
Table 3: Optimum TON for Mizoroki-Heck cross-coupling reactions using 5-8.
Entry
Cat. (loading
mol%)
Conv.
(%)
52
TON TOF (h-1)
Selectivity (%)
trans-stilbene cis-stilbene
5 (0.0094)
6 (0.0094)
7 (0.0094)
8 (0.0094)
5605
5671
5858
4891
1401
1418
1464
1223
89
89
89
89
11
11
11
11
1
2
3
4
53
54
45
Reactions carried out in DMF (1.2 mL) with 1.0 eq of iodobenzene, 1.1 eq of styrene, 2.0 eq of Et3N and varied
Pd catalyst loading at 130o C and 600 rpm for 4h. Using n-decane as an internal standard. Cat. = catalyst, Conv.
= conversion.
A further decrease in catalyst loading to 0.00625 mol % resulted in a decreased conversion
from ~50 to ~20% for all four (pre)catalysts (Table SI-3). Thus, 0.0375 mol% was chosen as
the optimum catalyst loading. These catalysts are highly active when compared to similar
trans-[PdCl2(L)2] (pre)catalysts used for the Mizoroki-Heck cross-coupling reaction that have
been reported in the literature22,23 because of the higher TONs and TOFs under similar reaction
conditions.
The reaction time was also varied from 0.5 h to 4 h in order to determine the minimum time
required for the four catalytic systems to catalyze the Mizoroki-Heck cross-coupling reaction.
Table SI-4 shows the results obtained. In thirty minutes, appreciable turnover numbers (around
1800) were achieved for all four (pre)catalysts 5-8. (Pre)catalyst 8 had an exceptionally short
induction period, converting 74 % of the substrates to mainly trans-stilbene in just 30 min
(Table SI-4, Entry 4). (Pre)catalyst 5, on the other hand, had a relatively longer induction

period, converting only 72 % of the substrates to trans-stilbene in an hour (Table SI-4, Entry
5). In addition, the (pre)catalysts with tertiary butyl substituent, 6 and 8, had shorter induction
periods as compared to those with a phenyl substituent, 5 and 7. This suggests that steric bulk
on the precursor bis(pyrazolyl) palladium(II) complex might be more important for Mizoroki-
Heck cross-coupling reactions, as compared to electronic effects. Optimum conversions to cis-
and trans-stilbene for all (pre)catalysts were achieved at ~ 2 h.
1.3.3 Mercury poisoning test and catalyst recyclability for the Mizoroki-Heck cross-
coupling reaction
The homogeneity of the active species derived from (pre)catalysts 5-8 in the Mizoroki-Heck
cross-coupling reactions in DMF was evaluated using a mercury drop test. The cross-coupling
reactions using 5, 6, 7 or 8 were performed at the established optimum conditions for the
temperature and reaction time, with the appropriate amounts of catalyst loadings and base in
the presence of excess mercury. At the end of the reactions, the conversions were similar to
those obtained under the same conditions without metallic mercury being added to the
reactions. (Table SI-5). This suggests that the four catalysts promote the Mizoroki-Heck
reactions as homogenous catalysts in solution and no palladium black could have been formed
in situ to subsequently catalyse the reactions. This is true since the metallic mercury would
have scavenged all the colloidal particles and consequently inhibited the reactions from
proceeding (Table SI-5). The active catalyst is likely soluble Pd0 species, stabilized by the
solvent or the pyrazolyl ligands or both, since reactions in non-coordinating solvents gave poor
yields.26
(Pre)catalyst 9, which has water-solubilising carboxylic acid groups, was also found to promote
the reaction homogenously. This catalyst was then evaluated for possible recyclability in
aqueous-biphasic media, wherein the (pre)catalyst was added to an alkaline aqueous solution
(KOH in water) in order to render it water-soluble. Both starting materials and the base were
dissolved in toluene and added to the alkaline solution of the (pre)catalyst 9. This resulted in
immediate decomposition of the (pre)catalyst into a black unidentified species and no
conversion was observed. When DMF was used instead of toluene, no black species was
formed. However, this also resulted in no conversion, effectively showing that the catalysts are
not recyclable (Table SI-5).

1.3.4 Substrate variation for (pre)catalysts 5-8 for the Mizoroki-Heck cross-coupling
reaction
The (pre)catalysts 5-8 were evaluated for their ability to cross-couple substrates that possess
various substituents for the Mizoroki-Heck cross-coupling reaction. A series of
aliphatic/aromatic alkenes with electron-withdrawing groups and electron-donating groups
were reacted with a variety of aryl halides that had electron-withdrawing or electron-donating
groups in the presence of (pre)catalysts 5-8.
Using a catalyst loading of 0.0375 mol%, a temperature of 130 °C, triethylamine as the base, a
reaction time of 2 h and DMF as the solvent, 4-iodotoluene was effectively coupled with
styrene in the presence of the four (pre)catalysts (Table 4, entries 1-4). Under these conditions,
around 90 % conversion was obtained for all four precatalysts. The selectivity for the trans
products observed for all four (pre)catalysts for this reaction were also similar to that observed
when iodobenzene was coupled with styrene instead of 4-iodotoulene (Table SI-4, entries 13-
16). However, the conversions obtained for the Mizoroki-Heck cross-coupling of iodobenzene
and styrene were higher than those obtained for 4-iodotoluene and styrene in the presence of
the four (pre)catalysts. This reduction in conversion is possibly due to the electron-donating
effect of the CH3 group present on the aryl halide. This would result in an increased aromatic
C-I bond dissociation energy, leading to a lowered rate of oxidative addition. Similar
observations have been reported in the literature.27,28
Although the catalysts were unable to catalyse the coupling of bromobenzene when 2-
bromobenzonitrile was used as a coupling partner with styrene, conversion of the substrates
was achieved (Table 4, entries 5-8). This indicates the presence of the electron-withdrawing
cyano group served to sufficiently activate the halide-carbon bond, resulting in the formation
of the coupled product. However, for these cross-coupling substrates, important differences
were observed between the pre-catalysts 5-8, complex 6 being the most active. In this case,
there is a deviation from the norm, where catalysts with bulky and electron donating groups
are expected to be more active for the cross-coupling reaction. As observed earlier, DMF plays
a role for the Mizoroki-Heck cross-coupling reaction. For the less activated aryl halide, the
bond dissociation energy between the halide and the aromatic sp2 carbon atom is higher. This
means that the rate of oxidative addition is expected to be slower for this substrate, thus giving

the opportunity for forming more solvent-Pd adducts. Such solvent participation may impact
on the observed activity and has been reported in the literature.29,30 When using the sterically
hindered 2-iodoanisole as a reactant with styrene, significant aryl halide conversion was
observed despite the presence of the relatively bulky methoxy group at the ortho position
(Table 4, entries 21-24). However, these observed conversions were slightly lower when
iodobenzene was used instead of 2-iodoanisole. This indicates that steric bulk near the Ar-X
bond does not significantly affect the aryl halide conversion as compared to electronic effects.
Table 4: Variation of aryl halide for Mizoroki-Heck cross-coupling with styrene using (pre)catalysts 5-
8
Entry Cat.
X
Y
Z
Conversion
(%)
89
TON TOF (hr-1) Selectivity (%)
I
I
4-CH3
4-CH3
4-CH3
4-CH3
2-CN
2-CN
2-CN
2-CN
H
H
H
2370
2424
2182
2416
114
1185
1212
1091
1208
38
89
88
89
87
98
97
98
98
97
81
86
95
95
94
98
93
93
85
1
2
5
6
7
8
5
6
7
8
5
6
7
8
5
6
7
8
5
6
91
I
H
82
3
I
H
91
4
Br
Br
Br
Br
I
H
a57
a99
a71
a49
90
5
H
199
66
6
H
143
48
7
H
98
33
8
4-tBu
4-tBu
4-tBu
4-tBu
4-Cl
4-Cl
4-Cl
4-Cl
4-CF3
4-CF3
2398
2526
2469
2389
2538
2565
2605
2666
200
1199
1263
1234
1194
1269
1283
1302
889
67
9
I
H
95
10
11
12
13
14
15
16
17
18
I
H
93
I
H
90
I
H
95
I
H
96
I
H
98
I
H
99
I
H
a100
a100
I
H
200
67

I
I
I
I
I
I
H
4-CF3
4-CF3
H
a100
a100
b94
b86
b94
200
200
188
172
188
198
67
67
47
43
47
50
96
88
90
91
92
88
19
20
21
22
23
24
7
8
5
6
7
8
H
2-OCH3
2-OCH3
2-OCH3
2-OCH3
H
H
H
b99
Reactions carried out in DMF (1.2 mL) with 8.8 mmol of arylhalide, 9.68 mmol of alkene, 17.6 mmol of Et3N and 3.3×10-3
mmol Pd catalyst (loading = 0.0375 mol%) at 130 oC and 600 rpm for 2 h. (a) Reactions carried out in DMF (1.2 mL) with
0.66 mmol of arylhalide, 0.726 mmol of alkene, 1.32 mmol of Et3N and 3.3×10-3 mmol Pd catalyst (loading = 0.5 mol%) at
130 oC and 600 rpm for 3 h; (b) = 4h. Using n-decane as the internal standard. Selectivity is reported relative to the trans-
isomer. Average error estimate (+/-); 5 (0.3211), 6 (0.2877), 7 (0.1992), 8 (0.2756)
Coupling iodobenzene with 4-trifluoromethylstyrene, 4-tert-butyl styrene or 4-chlorostyrene
(Table 4, entries 9-20) was also carried out to investigate the influence of electron-donating
and electron-withdrawing substituents on styrene in the cross-coupling reactions. At a catalyst
loading of 0.0375 mol% and a reaction time of 2 h, all four (pre)catalysts could effectively
couple 4-tert-butylstyrene, 4-chlorostyrene or 4-chloromethylstyrene with iodobenzene, giving
up to 100 % conversions. However, comparing the conversions obtained for iodobenzene, there
was an indication that the electronic effects contributed by the presence of both electron-
withdrawing and electron-donating groups on the aromatic ring of styrene. When an electron-
donating group was present on the aromatic alkene, the conversions were slightly reduced as
compared to when electron-withdrawing groups were present on the aromatic alkene. In
addition, selectivity towards the trans-isomers was similar for both types of substrates. Such
observations have also been reported in the literature.31,32
Replacing the aromatic alkene with aliphatic alkenes did not affect the aryl halide conversions
significantly. At a catalyst loading of 0.0375 mol%, a temperature of 130 oC, with triethylamine
as the base, a reaction time of 2 h and DMF as the solvent, iodobenzene was coupled with
methyl acrylate and ethyl acrylate effectively (Table 5, entries 1-8). These coupling reactions
yielded quantitative conversion of the aryl halide to the corresponding coupled product (trans-
cinnamate). In addition, the observed selectivity towards the coupled product (trans-isomer)
was also approximately 100 % for all reactions. This showed no significant difference between
the choice of alkyl group used as the alkoxy substituents for the aliphatic alkenes. As with the
case in coupling 2-iodoanisole with styrene in the presence of 5-8, changing the alkene from

an aromatic alkene to an aliphatic alkene only resulted in slight losses of the aryl halide
conversion, irrespective of the presence of the o-methoxy substituent present in 2-iodo anisole.
Table 5: Variation of alkenes for the Mizoroki-Heck cross-coupling for (pre)catalysts 5-8.
Entry Cat.
R
X
Conversion TON TOF (hr-1) Selectivity
(%)
100
100
100
99
(%)
>98
>99
>99
>98
>99
87
CO2Me
CO2Me
CO2Me
CO2Me
CO2Et
CO2Et
CO2Et
CO2Et
CO2Me
CO2Me
CO2Me
CO2Me
CO2Et
CO2Et
CO2Et
CO2Et
H
H
2667
2667
2667
2662
2667
2667
2667
2667
200
1333
1333
1333
1331
1333
1333
1333
1333
50
1
2
5
6
7
8
5
6
7
8
5
6
7
8
5
6
7
8
H
3
H
4
H
100
100
100
100
a100
a99
a98
a99
a100
a99
a98
5
H
6
H
>99
>99
87
7
H
8
2-OCH3
2-OCH3
2-OCH3
2-OCH3
2-OCH3
2-OCH3
2-OCH3
2-OCH3
9
198
50
97
10
11
12
13
14
15
16
196
49
94
198
50
93
200
50
94
198
50
94
196
49
96
a96
192
48
>99
Reactions carried out in DMF (1.2 mL) with 8.8 mmol of iodobenzene, 9.68 mmol of alkene, 17.6 mmol of Et3N and 3.3×10-
3 mmol Pd catalyst (loading = 0.0375 mol%) at 130 oC and 600 rpm for 2 h. (a) Reactions carried out in DMF (1.2 mL) with
0.66 mmol of arylhalide, 0.726 mmol of alkene, 1.32 mmol of Et3N and 3.3×10-3 mmol Pd catalyst (loading = 0.5 mol%) at
130 oC and 600 rpm for 4h; Using n-decane as an internal standard. Selectivity is reported relative to the trans-isomer. Average
error estimate (+/-); 5 (0.1176), 6 (0.1432), 7 (0.1288), 8 (0.1331).
1.4 Summary and conclusions

Five bis(pyrazolyl)palladium(II) complexes (5-9) were prepared and evaluated for their
efficacy in catalyzing the Mizoroki-Heck cross-coupling reaction. These catalysts were found
to be effective in promoting the cross-coupling reactions and similar aryl halide conversions
were obtained for all four (pre)catalysts when iodobenzene was coupled with styrene. These
results were observed irrespective of the presence of electron-donating and electron-
withdrawing groups present in the structures of the evaluated (pre)catalysts. Quantitative
iodobenzene conversion to trans-stilbene could be obtained for (pre)catalysts 5-8 in the
presence triethylamine as a base (in 2 h), even at catalyst loadings as low as 0.0375 mol %,
where turnover numbers of 2560 were demonstrated. (Pre)catalysts 5-9 promoted the
Mizoroki-Heck reactions by means of soluble palladium(0) species and not palladium black.
This was supported by mercury drop experiments, which resulted in no inhibition of the
catalytic activity. Conversions were slightly reduced when an electron-donating group was
present on the aryl halide, while using activated aryl bromides resulted in the cross-coupling
reaction taking place. On the other hand, the presence of electron-withdrawing groups on
aromatic or aliphatic alkene substrates resulted in slightly increased conversions.
1.5 Conflicts of interest
There are no conflicts to declare.
1.6 Appendix A. Supplementary data
Electronic supporting information (ESI) is available free of charge. CCDC 1555881 and
1833891 contain the supplementary crystallographic data for complexes 6 and 8, respectively.
These
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk
1.7 Acknowledgements
We acknowledge the National Research Foundation of South Africa (NRF) (Grant Numbers:
105557, 95517 and 99269), Sasol (University Collaboration Programme) and the University of
Johannesburg Centre for Synthesis and Catalysis for financial support. We would also like to
acknowledge Mr. Gershon Amenuvor and Mr. Novisi Oklu for their help in the solving the
crystal structures.

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Bis(pyrazolyl)palladium(II) complexes as catalysts for Mizoroki-Heck
cross-coupling reactions
Edward Ocansey, James Darkwa* and Banothile C.E. Makhubela*
Department of Chemical Science, University of Johannesburg, Kingsway Campus, Auckland
Park, 2006, South Africa.
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Bis(pyrazolyl)palladium(II) complexes with a slightly distorted square planar geometry around
the palladium centre as a result of steric encumbrance are presented.
High TONs in Mizoroki-Heck cross-coupling reactions were achieved using these
bis(pyrazolyl)palladium(II) complexes as (pre)catalysts.
Good functional group tolerance is demonstrated.