Electronic effects on the structures and catalytic properties of (pyrazol-1-yl)phenylmethanone palladium(II) complexes

Article abstract of DOI:10.1016/j.jorganchem.2017.07.033

Four new (pyrazol-1-yl)phenylmethanone ligands and their palladium(II) complexes were synthesized and characterized by various analytical techniques. The electronic properties of substituents on the phenyl ring of the ligand have significant effects on th

Full text of DOI:10.1016/j.jorganchem.2017.07.033

Accepted Manuscript
Electronic effects on the structures and catalytic properties of (pyrazol-1-
yl)phenylmethanone palladium(II) complexes
Simphiwe M. Nelana, Kamlesh Kumar, Ilia A. Guzei, Tebello Mahamo, James Darkwa
PII:
S0022-328X(17)30462-X
10.1016/j.jorganchem.2017.07.033
JOM 20047
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 15 May 2017
Revised Date: 20 July 2017
Accepted Date: 26 July 2017
Please cite this article as: S.M. Nelana, K. Kumar, I.A. Guzei, T. Mahamo, J. Darkwa, Electronic effects
on the structures and catalytic properties of (pyrazol-1-yl)phenylmethanone palladium(II) complexes,
Journal of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.07.033.
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Graphical Abstract
We have examined the electronic effects of substituents in the phenyl ring of (pyrazol-1-
yl)phenylmethanone on the structure and the catalytic property of their palladium complexes.

ACCEPTED MANUSCRIPT
Electronic effects on the structures and catalytic properties of (pyrazol-1-
yl)phenylmethanone palladium(II) complexes
Simphiwe M. Nelana,a Kamlesh Kumar,*a Ilia A. Guzei,a,b Tebello Mahamo,a James
Darkwa*a
aDepartment of Chemistry, University of Johannesburg, P.O. Box 524, Auckland Park
Kingsway Campus, Auckland Park 2006, South Africa; e-mail address:
jdarkwa@gmail.com (J. Darkwa). Kksingh85@gmail.com (K. Kumar)
bDepartment of Chemistry, University of Wisconsin-Madison, 1101 University Avenue,
Madison, WI 53706, USA
Abstract
Four new (pyrazol-1-yl)phenylmethanone ligands and their palladium(II) complexes were
synthesized and characterized by various analytical techniques. The electronic properties of
substituents on the phenyl ring of the ligand have significant effects on the structures as well
as the catalytic properties of the complexes toward ethylene oligomerization and the
subsequent Friedel-Crafts alkylation of toluene by the ethylene oligomers. Electron-
withdrawing groups (ca. Cl and F) of the ligands contributed to the formation of binuclear
palladium complexes, whereas electron-donating groups (ca. H and CH3) resulted in the
formation of mononuclear complexes. These complexes on activation with EtAlCl2 form
active catalysts which dimerize ethylene to mainly butenes (C4) and subsequent Friedel-
Crafts alkylation of toluene by butenes to regioisomers of butyltoluenes. This is the first
example of pyrazolyl palladium(II) compounds that perform this tandem reaction of
oligomerization and Friedel-Crafts alkylation.
Keywords: Palladium catalysts; Electronic effect; ethylene dimerization; Friedel-Crafts
alkylation.
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1. Introduction
After the pioneering work by Brookhart and co-workers on nickel(II) and
palladium(II) α-diimmine complexes as polymerization catalysts [1,2], there has been an
increased interest in olefin oligomerization and polymerization reactions using late-
transition metal catalysts due to their great functional group tolerance [3-11]. Transition
metal complexes with nitrogen-donor ligands such as imines and pyridines have been better
explored due to the high electrophilic nature of their metal centres in these complexes.
Electrophilicity of the metal complexes is considered to be an important factor in the
oligomerization and polymerization reactions because the first step involves the interaction
of the olefin and the metal centre in a catalyst. Therefore, it is important to make the metal
centre strongly electrophilic in order to increase catalytic activity. Another essential factor in
ligand design for olefin transformation reactions is the steric environment around the metal
centre. It has been shown that complexes with less sterically demanding substituents favour
oligomerization [12-14] whereas metal complexes with sterically hindered ligands favour
polymerization [15].
Late transition metal complexes based on pyrazolyl ligands have recently received a
lot of attention due to their use in olefin transformation reactions [16-23]. This is because of
the weaker σ-donor nature of pyrazoles compared to imine and pyridine ligands, make the
pyrazolyl complexes more electrophilic. Furthermore, pyrazoles and their derivatives are
attractive ligands as their steric and electronic properties can be fine-tuned by an appropriate
choice of substituents on the pyrazole ring.
We have studied the influence of electron-withdrawing carbonyl groups located
between pyrazolyl and phenyl groups in pyrazolylphenylmethanone and
pyrazolylheterocyclemathanone ligands on the nucleophilicity of the pyrazolyl nitrogen
atoms and their bonding with the central metal in palladium complexes [24]. We observed
that the presence of carbonyl functional group appears to reduce the σ-donor ability of the
nitrogen atoms in pyrazolyl ligands and also the palladium complexes showed higher
activity for ethylene polymerization in comparison to the simple substituted pyrazole
complexes.
In the present work, we studied the influence of electron-donating and electron-
withdrawing groups present on the phenyl ring of (pyrazol-1-yl)phenylmethanone ligands
on the nucleophilicity of the pyrazole nitrogen atoms and their ability to bind with central
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metal of palladium complexes. We have also investigated the catalytic activity of
synthesized complexes as pre-catalysts for ethylene oligomerization and observed that these
palladium complexes could be used to oligomerize ethylene and the oligomers alkylating
toluene to alkyltoluenes.
2. Experimental section
2.1 Materials and physical measurements
All manipulations of air and moisture-sensitive compounds were performed under a
dry nitrogen atmosphere using standard Schlenk techniques. Solvents were analytical grade
and were dried and distilled prior to use. Toluene and dichloromethane were dried over
sodium-benzophenone and P2O5 respectively. Benzoyl chloride, 2-chlorobenzoyl chloride,
3,5-dimethylpyrazole, 2-flourobenzoyl chloride, o-toluoyl chloride and ethylaluminium
dichloride (EtAlCl2), 25 wt% (1.8 M) solution in toluene were purchased from Sigma-
Aldrich and used as received. Polymer grade ethylene was obtained from Afrox Ltd (South
Africa).
FT-IR spectra of compounds were recorded as nujol mulls or KBr pellets on a
Perkin-Elmer, Paragon 1000 PC FT-IR spectrometer. NMR spectra were recorded on a
13
Gemini Varian 3000 instrument (1H at 300 MHz, C{1H} at 50 MHz). Chemical shifts (δ)
are reported in ppm and referenced to the residual 1H (7.26 ppm) and 13C signals (77.0 ppm)
1
13
of deuterated chloroform (CDCl3) or dimethylsulfoxide (DMSO-d6) H (2.50 ppm) and C
(39.0 ppm) as internal reference. Microanalysis was performed at the University of Cape
Town, South Africa. Oligomeric products were analyzed on a Shimadzu GCMS-QP 2010
version 2; with an FID detector and a 30 m x 0.25 mm, 100% dimethoxypolysiloxane ZB1
column. The injector and detector temperatures were kept constant at 250 °C. The program
was set at the initial temperature of 40 °C and finishing temperature of 350 °C.
2.2 Synthesis of ligands
2.2.1 (3,5-Dimethylpyrazol-1-yl)phenylmethanone (L1)
Benzoyl chloride (1.40 g, 10.40 mmol) was added to a solution of 3,5-
dimethylpyrazole (1.00 g, 10.40 mmol) in toluene (55 mL). The reaction mixture was
refluxed for 24 h after addition of Et3N (1 mL). The reaction mixture was cooled to room
temperature and filtered to remove Et3NH+Cl− as by-product. Evaporation of solvent gave a
white residue, which was purified by column chromatography using silica gel with CH2Cl2:
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hexane (4:1). Analytically pure L1 was obtained as a white solid after evaporating the
eluent. Yield, 1.44 g, 69%. 1H NMR (CDCl3): δ 8.01 (d, 2H, phenyl, J = 14.4 Hz); 7.51 (m,
13
3H, phenyl); 6.07 (s, 1H, 4-pz); 2.35 (s, 3H, 5-CH3); 1.96 (s, 3H, 3-CH3). C{1H} NMR
(CDCl3): δ 167.6, 154.7, 144.5, 133.3, 126.3, 116.1, 110.2, 13.6, 13.3. Anal. Calcd. for
C12H12N2O: C, 71.98; H, 6.04; N, 13.99. Found: C, 72.04; H, 6.10; N, 13.76. IR (nujol
mull): ν(C=O) = 1698 cm-1.
2.2.2 (3,5-Dimethylpyrazol-1-yl)-o-toluoylmethanone (L2)
Compound L2 was prepared in a similar manner as L1 from o-toluoyl chloride (0.42
g, 2.27 mmol) and 3,5-dimethylpyrazole (0.26 g, 2.27 mmol). Product was obtained as a
1
colorless oil. Yield, 0.40 g, 83%. H NMR (CDCl3): δ 7.93 (m, 2H, phenyl); 7.73 (m, 2H,
phenyl); 6.19 (s, 1H, 4-pz); 2.22 (s, 3H, 5-CH3); 2.11 (s, 3H, 3-CH3); 1.93 (s, 3H, CH3,
phenyl). 13C{1H} NMR (CDCl3): δ 170.2, 152.6, 144.1, 133.7, 126.8, 115.6, 111.4, 14.1,
13.8, 13.5. Anal. Calcd. for C13H14N2O: C, 72.87; H, 6.59; N, 13.07. Found: C, 72.46; H,
6.15; N, 13.17. IR (nujol mull): ν(C=O) 1704 cm-1.
2.2.3 (2-Chlorophenyl)-(3,5-dimethylpyrazol-1-yl)methanone (L3)
Compound L3 was prepared in a similar manner as L1 from 2-chlorobenzoyl
chloride (1.82 g, 10.40 mmol) and 3,5-dimethylpyrazole (1.00 g, 10.40 mmol). Product was
1
obtained as a white solid. Yield, 1.88 g, 77%. H NMR (CDCl3): δ 7.39 (m, 4H, phenyl);
6.03 (s, 1H, 4-pz); 2.36 (s, 3H, 5-CH3); 2.16 (s, 3H, 3-CH3). 13C{1H} NMR (CDCl3): δ
170.2, 155.6, 147.5, 133.8, 126.4, 116.2, 112.6, 110.5, 13.4, 13.1. Anal. Calcd. for
C12H11ClN2O: C, 61.41; H, 4.72; N, 11.94. Found: C, 61.26; H, 4.66; N, 12.01. IR (nujol
mull): ν(C=O) = 1701 cm-1.
2.2.4. (2-Flourophenyl)-(3,5-dimethylpyrazol-1-yl)methanone (L4)
Compound L4 was prepared in a similar manner as L1 from 2-flourobenzoyl
chloride (1.65 g, 10.51 mmol) and 3,5-dimethylpyrazole (1.01 g, 10.51 mmol). Product was
1
obtained as a colorless oil. Yield, 1.95 g, 86%. H NMR (CDCl3): δ 7.30 (m, 2H, phenyl);
6.94 (m, 2H, phenyl); 6.11 (s, 1H, 4-pz); 2.37 (s, 3H, 5-CH3); 1.91 (s, 3H, 3-CH3). 13C{1H}
NMR: δ 165.3, 156.9, 144.3, 132.4, 130.1, 123.2, 115.7, 111.2, 13.6, 13.3. Anal. Calcd. for
4

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C12H11FN2O: C, 66.05; H, 5.08; N, 12.84. Found: C, 66.16; H, 5.13; N, 13.00. IR (nujol
mull): ν(C=O) = 1703 cm-1.
2.3 Synthesis of palladium(II) complexes
2.3.1 Bis-{(3,5-dimethylpyrazol-1-yl)phenylmethanone)}palladium(II) dichloride (1)
Ligand L1 (0.55 g, 0.26 mmol) was added to a solution of [PdCl2(CH3CN)2] (0.36 g,
0.13 mmol) in dichloromethane (30 mL). The resulting yellow-orange solution was stirred at
room temperature for 24 h. The solution was then concentrated to 15 mL and an equal
amount of hexane was added. The solution was kept at -4 °C for 3 days to yield a yellow
solid, which was isolated by filtration to give titled compound. Yield, 0.34 g, 67%. 1H NMR
(CDCl3): δ 7.94 (d, 4H, phenyl, J = 5.6 Hz); 7.70 (d, 6H, phenyl, J = 5.6 Hz ); 6.01 (s, 2H,
4-pz); 2.35 (s, 6H, 5-CH3); 1.96 (s, 6H, 3-CH3). 13C{1H} NMR: δ 169.4, 157.4, 137.6,
133.3, 131.5, 128.4, 118.6, 106.5, 13.8, 13.5. Anal. Calcd. for C24H24Cl2N4O2Pd: C, 49.89;
H, 4.19; N, 9.70. Found: C, 49.62; H, 4.21; N, 9.55. IR (nujol mull): ν(C=O) 1702 cm-1.
2.3.2 Bis-{(3,5-dimethylpyrazol-1-yl)-o-toluoylmethanone}palladium(II) dichloride (2)
Complex 2 was prepared in a similar manner as complex 1 from L2 (0.10 g, 0.47
mmol) and [PdCl2(CH3CN)2] (0.06 g, 0.23 mmol). A yellow solid product was obtained.
1
Yield, 0.08 g, 59%. H NMR (CDCl3): δ 7.69 (s, 4H, phenyl); 7.25 (m, 4H, phenyl); 5.97
(s, 2H, 4-pz); 2.62 (s, 6H, 5-CH3); 2.55 (s, 6H, 3-CH3); 1.96 (s, 6H, o-toluoyl). 13C{1H}
NMR (CDCl3): δ 170.8, 157.2, 142.7, 136.6, 132.3, 126.1, 116.1, 112.2, 14.6, 13.3, 13.0.
Anal. Calcd. for C26H28Cl2N4O2Pd: C, 51.54; H, 4.65; N, 9.25. Found: C, 51.21; H, 4.62; N,
9.03. IR (nujol mull): ν(C=O) 1704 cm-1.
2.3.3
Bis-{(2-chlorophenyl)-(3,5-dimethylpyrazol-1-yl)methanone}dipalladium(II)
tetrachloride (3)
Complex 3 was prepared in a similar manner as complex 1 from L3 (0.20 g, 0.85
mmol) and [PdCl2(CH3CN)2] (0.111 g, 0.43 mmol). Red crystals suitable for X-ray analysis
were obtained. Yield, 0.097 g, 63%. 1H NMR (DMSO-d6): δ 7.69 (m, 4H, phenyl); 7.24 (m,
4H, phenyl); 6.24 (s, 2H, 4-Pz); 2.35 (s, 6H, 5-CH3); 1.96 (s, 6H, 3-CH3). 13C{1H} NMR: δ
166.9, 144.0, 131.6, 129.9, 127.0, 112.2, 13.9, 13.6. Anal. Calcd. for C24H22Cl6N4O2Pd2: C,
5

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34.98; H, 2.70; N, 6.80. Found: C, 35.21; H, 2.69; N, 6.67. IR (nujol mull): ν(C=O) 1704
cm-1.
2.3.4
Bis-{(2-flourophenyl)-(3,5-dimethylpyrazol-1-yl)methanone}dipalladium(II)
tetrachloride (4)
Compound 4 was prepared in a similar manner as complex 1 from L4 (0.170 g, 0.78
mmol) and [PdCl2(CH3CN)2] (0.10 g, 0.39 mmol). Red crystals suitable for X-ray analyses
were obtained. Yield, 0.078 g, 58%. 1H NMR (DMSO-d6): δ 7.61 (m, 4H, phenyl); 7.21 (m,
13
4H, phenyl); 6.29 (s, 2H, 4-Pz); 2.57 (s, 6H, 5-CH3); 2.01 (s, 6H, 3-CH3). C{1H} NMR
(DMSO-d6): δ 165.4, 156.9, 143.9, 132.4, 130.2, 123.2, 115.6, 115.2, 111.2, 13.6, 13.3.
Anal. Calcd. for C24H22Cl4F2N4O2Pd2: C, 36.44; H, 2.80; N, 7.08. Found: C, 36.50; H, 2.73;
N, 6.96. IR (nujol mull): ν(C=O) 1703 cm-1.
2.4. X-ray crystallography
Single crystals suitable for X-ray analyses were obtained from slow evaporation of
solvent from the mixture of hexane and dichloromethane for complexes 1 and 3, whereas
suitable crystal of complex 4 was obtained by slow diffusion of hexane into the chloroform
solution. X-ray crystal data collection was performed on a Bruker CCD-1000 diffractometer
with Mo Kα (λ = 0.71073Å) radiation and the diffractometer to crystal distance of 4.0 cm.
The reflections were successfully indexed by an automated indexing routine built in the
APEXII program suite. The data were collected using the full sphere data collection routine
to survey the reciprocal space to the extent of a full sphere to a resolution of 0.75 Å. Data
were harvested by collecting 2982 frames at intervals of 0.5° scans in ω and φ with exposure
times of 10 s per frame [25]. The data integration and reduction were processed with SAINT
software. A multi-scan absorption correction was applied to the collected reflections with
SADABS using XPREP. Structures were solved by the direct method using the program
SHELXS-97 and were refined on F2 by the full-matrix least-squares technique using the
SHELXL-97 program package. All non-hydrogen atoms were refined with anisotropic
displacement coefficients. The F atom in complex 4 is disordered in a 90:10 ratio over two
ortho positions and therefore restraints were applied to the disordered atoms. All hydrogen
atoms were included in the structure factor calculations at idealized positions and were
allowed to ride on the neighboring atoms with relative isotropic displacement coefficients
[26].
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2.5 General procedure for ethylene oligomerization
The ethylene oligomerization reactions were carried out in a 450 mL high pressure
reactor. The reactor was heated for 1 h at 120 °C under the vacuum and cooled under
nitrogen gas. After that, the appropriate amount of catalyst in toluene was transferred into
the reactor. The reactor was flushed with nitrogen for another 10 min and 30 mL of toluene
was transferred by cannula into the reactor and heated to 60 °C. In a Schlenk tube, toluene
(30 mL) was degassed and an appropriate amount of ethylaluminium dichloride (EtAlCl2)
was added using a syringe and then transferred via cannula into the reactor. After the
addition of the EtAlCl2 solution, the reactor was charged with ethylene (45 bar) and reactor
temperature was maintained at 50 °C. The reaction was stopped after 1 h by discontinuing
the ethylene feed to the reactor and cooling the reactor to below -10 °C. After releasing the
excess ethylene from the reactor, the reaction mixture was quenched with 10% HCl solution
in methanol and the products analyzed by GC and GC–MS. A mixture of products was
obtained after evaporating the volatiles on a rotatory evaporator.
3. Results and discussion
3.1 Syntheses of ligands (L1-L4)
Compounds L1-L4 were prepared by reacting ortho substituted benzoyl chloride
with one equivalent of 3,5-dimethylpyrazole in toluene (Scheme 1). The use of Et3N as a
base to abstract the acidic N-H proton in the pyrazole ring, led to the immediate
precipitation of Et3NH+Cl−. Reaction mixtures were, however, refluxed for 24 h to ensure
reaction completion, after which the Et3NH+Cl− was filtered off and the solvent was
removed on rotary evaporator to give the crude products. The crude products were further
purified by column chromatography on silica gel using a mixture of dichloromethane and
hexane (4:1 v/v) as eluent. Compounds L1–L4 are very stable in air and can be stored at
room temperature. There was no degradation observed even when these compounds were
kept for extended periods of time at room temperature. Compounds L1-L4 show good
solubility in most polar solvents such as dichloromethane, diethyl ether, chloroform and in
aromatic solvents such as toluene but are insoluble in hexane. All compounds were
characterized by NMR spectroscopy, IR spectroscopy and elemental analysis.
Insert Scheme 1
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3.2 Characterization of ligands (L1-L4)
3.2.1 NMR spectroscopic studies
1
The influence of the carbonyl group is quite clear in the H NMR spectra of these
compounds. There is a downfield shift in the singlet peak assignable to the proton at
position 4 of the pyrazolyl ring. This singlet resonates at ~5.85 ppm in the unsubstituted 3,5-
dimethylpyrazole. However, this appeared at ~6.12 ppm in (pyrazol-1-yl)phenylmethanone
ligands. The methyl groups in the pyrazolyl ring of L1-L4 appeared as two different singlets
1
in H NMR spectra. These resonances were in the range 2.22-2.35 ppm for the protons of
the methyl group at position 5, whereas the protons of the methyl group at position 3
appeared in the range 1.91-2.16 ppm. This observation is contrary to the one singlet peak
1
observed for methyls of 3,5-dimethylpyrazole in the H NMR spectrum because protons of
both methyls are magnetically equivalent.
In the 13C{1H} NMR spectra of ligands L1-L4, signal between 166 and 170 ppm
was assigned for the carbonyl carbon. These observations are consistent with the results
reported by Darkwa and co-workers for benzenedicarbonyl and benzenetricarbonyl linker
pyrazolyl compounds [24]. They reported the carbonyl groups have a significant influence
on the carbons in the benzene ring and pyrazolyl rings. In the compounds L1-L4 the
influence of the carbonyl group on the benzene was not significant.
3.2.2 FT-IR spectroscopic studies
Compounds L1-L4 were also characterized using FT-IR spectroscopy. The absence
of N-H stretching frequency of pyrazole ring in the 3180-3490 cm-1 region of the IR spectra
was the confirmation that protons of pyrazoles have been substituted. It was also observed
in the IR spectra of all the ligands that there was sharp peak between 1690 and 1705 cm-1
assignable to C=O stretching frequency. A similar observation for the C=O stretching
frequency between 1680 and 1716 cm-1 was also reported by Mohlala et al. for 2,6-
pyridinylcarbonylpyrazolyl compounds [27].
3.3 Synthesis and characterization of palladium(II) complexes (1-4)
The reaction of compounds L1-L4 with [PdCl2(CH3CN)2] in a 2:1 molar ratio
resulted in the formation of complexes 1-4 (Scheme 2). Complexes 1 and 2 were isolated as
yellow solids, whereas the other two palladium complexes (3 and 4) were isolated as red
solids. These palladium complexes were characterized by NMR, IR spectroscopy and
8

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elemental analysis; and 1, 3.CH2Cl2, and 4.2CHCl3 by single-crystal X-ray crystallography.
Complexes 3 and 4 with ligands L3 and L4 respectively that have electron-withdrawing
groups were found to be sparingly soluble in CDCl3; as a result their NMR spectra were
recorded in DMSO-d6. The 1H NMR spectra of 3 and 4 showed ~0.5 ppm downfield shift of
signal assignable to the proton at position 4 of the pyrazolyl in the palladium complexes
compared to the free ligands for most of the complexes. Additionally, there was a slight
chemical shift in the peaks assignable to the methyl protons at positions 3 and 5 of pyrazolyl
ring. This shows that these protons are not significantly affected by the presence of the
metal.
Insert Scheme 2
Ligands L1 and L2, which have electron-donating groups (H, CH3) in the ortho
position of the phenyl ring, resulted in formation of mononuclear palladium complexes (1
and 2). On the other hand, ligands L3 and L4, which have electron-withdrawing groups (Cl,
F) in the ortho position of the phenyl ring, resulted in the formation of binuclear palladium
complexes (3 and 4). The electron-donating or electron-withdrawing nature of the
substituent on the phenyl ring of the ligand, therefore determines the type of complex
formed. In essence, electron-withdrawing groups in the ligands reduce the donor ability of
the pyrazole and thus the electron density on the palladium; hence bridging chlorides are
required to stabilize the palladium centres.
We have reported that simple substituted pyrazoles, such as 3,5-dimethylpyrazole
[28], 3,5-ditertiarybutylpyrazole [28] and thiophenecarbonylpyrazolyl [29] form
mononuclear palladium complexes. However, bis(pyrazol-1-ylcarbonyl)benzene ligands
[24] form binuclear palladium complexes with bridging chlorides, similar to what we
observe for the complexes 3 and 4. Our initial reasoning was that bis(pyrazol-1-
ylcarbonyl)benzene ligands form binuclear complexes of type [{3,5-pzCO-1,3-
C6H4}Pd2Cl2(µ-Cl2)], because of the weak donor ability of pyrazole nitrogen in these
ligands. However, 1,2,4,5-tetrakis(3,5-dimethylpyrazolylmethyl)benzene forms [{(3,5-
Me2pzCH2)2C6H2}PdCl2(µ-Cl2)]2 [30] and bis-1,2-(3,5-dimethylpyrazolyl)benzene forms
[{(3,5-Me2pzCH2)2C6H4}2{PdCl2}2] [30] suggesting the distance between pyrazolyl units
could be a factor in determining how these ligands coordinate, hence form different types of
complexes. It, however, appears that for L3 and L4, the electron-withdrawing ability of the
substituent on the phenyl ring affects the donor properties of the pyrazolyl nitrogen and
9

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make them a poor donor ligand. This results the binuclear palladium complexes 3 and 4 in
which the palladium centres are stabilized by two bridging chlorides.
3.4 Molecular structures of complexes 1, 3 and 4
The molecular structures of complexes 1, 3 and 4 were determined by single crystal
X-ray crystallography. Details of crystal data and structure refinement parameters are
compiled in Table 1. Molecular structures with numbering Schemes are shown in Figures 1-
3 and selected bond lengths and bond angles for these complexes are given in the figure
captions.
Figures 1-3 show compound 1 as a mononuclear palladium complex whereas 3 and 4
as discrete chloro-bridged binuclear palladium complexes. All these complexes occupy
crystallographic centre of inversion and each of the palladium centres possesses a distorted
square planar coordination geometry. In complex 1, the palladium coordination sites are
occupied by two nitrogen atoms of pyrazole ligands and two chloride atoms. In contrast, in
complexes 3 and 4, two palladium atoms are bridged by two chloro groups and each
palladium centre is bound to three chloro atoms (two bridging and one terminal chloride)
and one nitrogen atom from pyrazole ligand.
The asymmetric unit of complex 1 contains two molecules with a small difference in
Pd–Cl and Pd–N bond distances. These distances for molecule one are 2.3019(7) and
2.029(2) Å, whereas for the second molecule these are 2.3074(7) and 2.023(2) Å
respectively. Expectedly, the distances to the terminal chloride ligands in complexes 3 and 4
are shorter than distances to the bridging chlorides: in complex 3, the Pd–Cl(terminal) bond
distance of 2.2688(15) Å is shorter than the average Pd–Cl(bridging) bond distance
2.319(19) Å; in complex 4 the Pd–Cl(terminal) and average Pd–Cl(bridging) bond distances
are 2.2749(4) and 2.314(15) Å respectively. These bond lengths are similar to the
respective Pd–Cl bond lengths in related palladium(II) complexes reported by Guzei et al.
[24] and Mohlala et al. [27]. The Pd–N bond distances in 3 and 4 are 2.006(4) and 2.015(2)
Å respectively. The L2Pd(µ2–Cl)2–PdL2 cores in complexes 3 and 4 are essentially planar
with Pd–Pd separations of 3.339 and 3.362 Å respectively. It is interesting to note that in the
crystal structures of bridged binuclear complexes the phenyl rings tilt toward the L2Pd(µ2–
Cl)2–PdL2 core with distances 3.695 and 3.667 Å, respectively.
Insert Figures 1-3
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Insert Table 1
3.5 Ethylene oligomerization reaction and Friedel-Crafts alkylation of toluene
We investigated all palladium complexes for their ability to catalyze the
oligomerization
of
ethylene
using
methylaluminoxane
(MAO),
modified
methylaluminoxane (MMAO), Et2AlCl and EtAlCl2 as co-catalysts. There were no
significant reactions observed for all complexes when MAO, MMAO and Et2AlCl were
used as co-catalysts. Only EtAlCl2 as a co-catalyst was able to activate the palladium
complexes to produce active catalysts which dimerized ethylene to mainly butenes and
subsequently producing butyltoluenes via Friedel-Crafts alkylation of the toluene that was
used as solvent in the ethylene reactions (Table 2 and Scheme 3). The major oligomerization
product is the dimerization of ethylene to butenes as a primary product. This then undergo
Friedel-Crafts alkylation to toluene to regio-isomers of mono- and di-butyltoluenes, plus
minor ethyl- and tributyltoluenes (Scheme 3). The formation of alkyltoluenes has been
reported recently for several nickel catalyzed olefins oligomerization reactions in toluene
from this group [18,21-23,31] and also from different groups [32,33].
Insert Scheme 3
Insert Table 2
Our previous studies with pyrazolyl-palladium complexes showed that these
complexes to be practically inactive for ethylene oligomerization reactions [22]. The
observed catalytic activity for the pyrazolyl palladium complexes in this study is
surprisingly high. We attribute the activity to the presence of the carbonyl linkers and the
substituents on the phenyl rings in the pyrazolyl ligands give the palladium atoms in the
catalysts the right electrophilic behavior to make these palladium complexes catalytically
active.
The catalytic data also showed the palladium complexes with electron-withdrawing
groups (Cl and F) in the benzene ring are more active as compared to the palladium
complexes with electron-donating groups (H and CH3) and activity is almost two fold higher
in the case of palladium complexes 3 and 4. This can be ascribed to the presence of electron-
withdrawing group which improves the electrophilic behavior of metal centre and in turn
increases the possibility of metal-ethylene interaction during the catalytic reaction.
11

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4. Conclusion
A series of palladium complexes with different benzenecarbonylpyrazolyl ligands
are synthesized and fully characterized by various analytical techniques. The electron-
withdrawing groups (Cl and F) at phenyl ring of the ligand reduced electron donor ability of
pyrazole nitrogen and therefore, resulted binuclear complexes which are stabilized through
bridging chlorides. All palladium complexes upon activation with EtAlCl2 form active
catalyst that oligomerizes ethylene to mainly butenes and subsequently alkylated toluene to
butyltoluenes via a Friedel-Crafts alkylation process. The results of the oligomerization
studies demonstrated that fine-tuning of electrophilicity of the metal centre should lead to
high active pre-catalysts for ethylene oligomerization.
Acknowledgements
Financial support for this project from the National Research Foundation (NRF), South
Africa, the DST-NRF Centre of Excellence in Catalysis (c*change), South Africa and the
University of Johannesburg is gratefully acknowledged. K. K. and T. M. thank University of
Johannesburg for postdoctoral fellowships.
Supplementary data
1
Supplementary Figure S1 represent a typical H NMR spectrum of butyltoluenes.
Figures S2-S6 show GC chromatograms and GC-MS spectra of alkyltoluenes. CCDC
1503129-1503131 contain the supplementary crystallographic data for this paper.
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Tables
Table 1: Crystal data and structure refinement for complexes 1, 3 and 4
Complex
1
3.CH2Cl2
4.2CHCl3
Empirical formula
C24H24Cl2N4O2Pd
C25H24Cl8N4O2Pd2
C26H24Cl10F2N4O2Pd2
Formula weight
Temperature (K)
577.77
100(2)
908.88
100(2)
1029.79
100(2)
Wavelength (Å)
Crystal system
Cu K
triclinic
α
(λ
= 1.54178)
Mo K
Monoclinic
α
(
λ
= 0.71073)
Mo Kα (λ = 0.71073)
Monoclinic
ꢀ
Space group
a (Å)
b (Å)
C2/c
P21/n
P
1
7.8246(3)
8.6738(3)
17.2599(6)
85.386(2)
83.709(2)
85.744(2)
21.252(5)
11.321(3)
13.619(3)
90
101.766(6)
90
12.0644(5)
12.9322(6)
12.2263(5)
90
109.080(1)
90
c (Å)
α
β
γ
(°)
(°)
(°)
Volume (Å3)
1158.05(7)
3207.8(12)
1802.74(13)
Z
2
4
2
Density (Mg/m3)
Crystal size (mm3)
1.657
1.882
1.897
0.44 x 0.39 x 0.33
0.583 × 0.125 × 0.066 0.45 x 0.26 x 0.11
Reflections
collected
16309
15019
14546
Independent reflections 4022
3863
3690
Final
[I>2σ(I)]
R indices (all data)
R
indices R1 = 0.0262, wR2 =
0.0611
R1 = 0.0473, wR2 = R1 = 0.0188, wR2 =
0.0932 0.0484
R1 = 0.0924, wR2 = R1 = 0.0211, wR2 =
R1 = 0.0347, wR2 =
0.0655
1.028
0.1079
0.0496
GOOF (F2)
0.997
1.032
Largest diff. peak and 0.54 and -0.42
0.89 and -0.56
0.448 and -0.387
3
-
hole (eÅ )
Table 2: Ethylene oligomerization catalyzed by palladium complexes
Entry Catalyst Time Pressure Temperature Yield
TON
(°C)
(×105g mol-1 h-1)
(h)
(bar)
(g)
1
2
3
4
1
1
1
1
45
45
45
45
50
50
50
50
6.1
7.8
14.5
13.2
1.2
1.6
3.9
3.4
1
2
3
4
Reaction conditions: [Pd] = 14 µmol, Al:Ni ratio = 500 and Toluene = 60 mL

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Schemes and Figures
Scheme 1: Synthesis of (pyrazol-1-yl)phenylmethanone compounds
Scheme 2: Synthesis of (pyrazol-1-yl)phenylmethanone)palladium(II) complexes (1-4)

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CH3
CH3
Pre-catalyst
EtAlCl2, Toluene
Butenes
+
+
C4H9
C2H5
B
A
CH3
CH3
CH3
+
+
C2H5
C4H9
C4H9
C2H5
C4H9
C4H9
C4H9
C
E
D
Scheme 3: Products formed in the pyrazolyl palladium catalyzed ethylene oligomerization
reactions
Figure 1: Molecular structure of complex 1 (50% probability ellipsoids). One molecule of
complex 1 has been omitted for clarity. Selected bond lengths (Å) and angles (°): for molecule
1; Pd1A–Cl1A = 2.3019(7), Pd1A–N1A = 2.029(2), N1A–N2A = 1.380(3), O1A–C6A =
1.202(3), N1A–Pd1A–Cl1A = 91.09(7), N1A–Pd1A–Cl1A′ = 88.92(7), N2A–N1A–Pd1A =
125.42(17), C2A–N1A–Pd1A = 127.94(18), N1A–Pd1A–N1A′ = 180.0, Cl1A–Pd1A–Cl1A′ =
180.0. For molecule 2: Pd1B–Cl1B = 2.3047(7), Pd1B–N1B = 2.023(2), N1B–N2B =

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1.376(3), O1B–C6B = 1.207(3), N1B–Pd1B–Cl1B = 90.49(7), N1B–Pd1B–Cl1B′ = 89.51(7),
N2B–N1B–Pd1B = 125.32(17), C2B–N1B–Pd1B = 127.42(18), N1B–Pd1B–N1B′ = 180.0,
Cl1B–Pd1B–Cl1B′ = 180.0.
Figure 2: Molecular structure of complex 3 (30% probability ellipsoids). The dichloromethane
molecule has been omitted for clarity. Selected bond lengths (Å) and angles (°): Pd1–Cl3 =
2.2688(15), Pd1–Cl2 = 2.3049(13), Pd1–Cl2A = 2.3323(15), Pd1–N1 = 2.006(4), N1–N2 =
1.371(5), O1–C6 = 1.206(6), Cl1–C12 = 1.741(5), Cl3–Pd1–Cl2A = 178.61(5), Cl3–Pd1–Cl2
= 90.77(5), Cl2–Pd1–Cl2A = 87.88(5), N1–Pd1–Cl3 = 90.44(13), N1–Pd1–Cl2 = 178.68(13),
N1–Pd1–Cl2A = 90.91(13), Pd1–Cl2–Pd1A = 92.12 (5), N2–N1–Pd1 = 123.7(3), C4–N1–Pd1
= 128.2(3).

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Figure 3: Molecular structure of complex 4 (50% probability ellipsoids). The chloroform
molecule has been omitted for clarity. Selected bond lengths (Å) and angles (°): Pd1–Cl1 =
2.2749(4), Pd1–Cl2 = 2.3028(5), Pd1–Cl2A = 2.3242(4), Pd1–N1 = 2.015(2), N1–N2 =
1.373(2), O1–C6 = 1.200(2), F1–C8 = 1.355(2), Cl1–Pd1–Cl2 = 91.38(2), Cl1–Pd1–N1 =
89.62(4), Cl1–Pd1–Cl2A = 178.08(2), N1–Pd1–Cl2 = 178.96(4), Cl2–Pd1–Cl2A = 86.79(2),
N1–Pd1–Cl2A = 92.21(2), Pd1–Cl2–Pd1A = 93.21(2), N2–N1–Pd1 = 122.5(1), C2–N1–Pd1 =
127.7(1).

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Highlights
•
•
•
•
Palladium(II) complexes and catalysis
Electronic nature of ligands determines structure of complexes
Ethylene dimerization and subsequent Friedel-Crafts alkylation of toluene.
Regioisomers of butyltoluenes