Palladium (II) complexes chelated by 1-substituted-4-pyridyl-1H-1,2,3-triazole ligands as catalyst precursors for selective ethylene dimerization

Article abstract of DOI:10.1002/aoc.5595

A series of neutral as well as cationic palladium methyl complexes bearing 1-substituted-4-pyridyl-1H-1,2,3-triazole ligands were prepared and fully characterized by a range of analytical techniques. Conventional and 2D NMR spectroscopy as well as single-crystal X-ray diffraction analysis unambiguously determined the molecular structure of the complexes. The neutral complexes activated by methylaluminoxane were found to be effective catalysts in the ethylene dimerization reaction. The catalyst performance of the in-situ-generated active species was compared with the discrete cationic complexes of the same ligand scaffold. Activities and selectivities for the two systems were remarkably similar, pointing to similarities in the nature of the active species. Both catalytic systems showed a strong correlation of activity and selectivity with the nature of the ligand scaffold. Highest activities were attained when electron-withdrawing groups were incorporated into the triazole ring, while increasing steric bulk in the ortho-position on the pyridyl ring of the ligand led to the almost exclusive dimerization of ethylene with selectivities up to 94% observed toward 1-butene.

Full text of DOI:10.1002/aoc.5595

Received: 23 August 2019
DOI: 10.1002/aoc.5595
Revised: 13 January 2020
Accepted: 17 January 2020
F U L L P A P E R
Palladium (II) complexes chelated by 1-substituted-
4-pyridyl-1H-1,2,3-triazole ligands as catalyst precursors for
selective ethylene dimerization
Mohammed Cassiem Joseph1
| Andrew John Swarts2
| Selwyn Frank Mapolie1
1DST-NRF Centre of Excellence in
Catalysis (c*change), Department of
Chemistry and Polymer Science,
Stellenbosch University, Private Bag
1, Matieland, 7601Stellenbosch,
South Africa
2Catalysis and Synthesis Research Group,
Focus Area for Chemical Resource
BeneficiationNorth-West University,
11 Hoffman Street, Potchefstroom, 2520,
South Africa
A series of neutral as well as cationic palladium methyl complexes bearing
1-substituted-4-pyridyl-1H-1,2,3-triazole ligands were prepared and fully char-
acterized by a range of analytical techniques. Conventional and 2D NMR spec-
troscopy as well as single-crystal X-ray diffraction analysis unambiguously
determined the molecular structure of the complexes. The neutral complexes
activated by methylaluminoxane were found to be effective catalysts in the eth-
ylene dimerization reaction. The catalyst performance of the in-situ-generated
active species was compared with the discrete cationic complexes of the same
ligand scaffold. Activities and selectivities for the two systems were remarkably
similar, pointing to similarities in the nature of the active species. Both cata-
lytic systems showed a strong correlation of activity and selectivity with the
nature of the ligand scaffold. Highest activities were attained when electron-
withdrawing groups were incorporated into the triazole ring, while increasing
steric bulk in the ortho-position on the pyridyl ring of the ligand led to the
almost exclusive dimerization of ethylene with selectivities up to 94% observed
toward 1-butene.
Correspondence
Selwyn Frank Mapolie, DST-NRF Centre
of Excellence in Catalysis (cchange),
Department of Chemistry and Polymer
Science, Stellenbosch University, Private
Bag 1, Matieland,7601. Stellenbosch,
South Africa.
Email: smapolie@sun.ac.za
Funding information
National Research Foundation
South Africa, Grant/Award Number:
CPRR160408161799
K E Y W O R D S
1,2,3-triazole, ethylene oligomers, palladium (II) complexes, palladium–methyl complexes,
selective ethylene dimerization
1 | INTRODUCTION
block the axial coordination sites of the metal which, in
turn, inhibits associative ligand exchange, hindering
The development of late transition metal catalysts con-
taining bidentate nitrogen donor ligands for ethylene
oligo- and polymerization has seen a huge amount of
research interest over the past two decades.[1–3] The
seminal work of Brookhart and co-workers on the use
of α-diimine ligands in the development of highly active
olefin oligo−/polymerization catalysts opened up a new
area of research (Figure 1a). Depending on the ligand
architecture, it has been demonstrated that it is possible
to produce high-molecular-weight polyethylene when
the steric bulk of the ortho-substituent on the aryl ring
was increased.[4] These ortho-substituents essentially
chain transfer and favoring polymerization to produce
high-molecular-weight polymers. Conversely, decreasing
the steric bulk of these ligands adjacent to the metal
center leads to the oligomerization of olefins.[5,6] Subse-
quent to these initial reports, attention has shifted to
the design of new ligands in an effort to find highly
active and more selective catalyst systems for ethylene
oligomerization.[1,7,8] For example, Ojwach and co-
workers recently developed a series of hemi-labile
(imino)pyridine palladium complexes (Figure 1b) as
catalyst precursors for ethylene oligomerization.[9] Acti-
vation of these complexes with methylaluminoxane
Appl Organometal Chem. 2020;e5595.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5595

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JOSEPH ET AL.
FIGURE 1 Oligo-/polymerization
catalysts with different ligand settings:
Brookhart's cationic α-diimine catalysts (a),
Ojwach's hemilabile (imino)pyridine
catalysts (b) and 1,2,3-triazole-pyridinato
catalysts (c, this work)
(MAO) produced ethylene dimerization catalysts with
high selectivity toward 1-butenes. Furthermore, these
authors showed that catalytic activities could be
controlled by varying the ligand architecture with the
incorporation of different pendant arms onto the ligand
backbone.
2 | RESULTS AND DISCUSSION
2.1 | Synthesis and characterization of
1-substituted-4-pyridyl-1H-1,2,3-triazole
ligands, L1–L8
In an attempt to explore other N^N chelates as
ligand scaffolds, we turned our attention to pyridyl-
triazole ligands. This class of ligands have been gaining
increasing significance owing to the fact that they can
be regarded as functionalized analogs of the well-
known 2,20-bipyridine scaffold. The main advantage of
the pyridyl-triazole ligands over the ubiquitous bipy
type ligands is the ease of functionalization which can
be obtained by using conventional Click chemistry. The
ease of functionalization allows for facile fine-tuning of
the electronic and steric properties of the ligand archi-
tecture. The Click-based synthetic approaches are less
time-intensive than the synthetic methodologies
required to functionalize bipy-based systems or even
the conventional pyridine–imine ligand scaffolds.
Herein we thus report on the synthesis and catalytic
evaluation of a series of 1,2,3-triazole–pyridinato palla-
dium complexes with different substituents on the pyr-
idyl and triazole ring. The general structure of the
system focussed on in this paper is illustrated in
Figure 1c. Another feature of the complexes studied is
the variation of the substituent in position 6 of the
pyridyl unit of the ligand scaffold. We envisaged that
variation of the type of the substituents in this position
could perhaps impact on the regio-and chemo-
selectivity of the oligomerization process.
A series of alkyl- and aryl-substituted 1,2,3-triazolyl-
pyridine ligands (L1–L8) were prepared by conventional
and microwave-assisted copper-catalyzed alkyne-azide
cycloaddition between the appropriate alkyl- or aryl azide
(A1–A5) and 2-ethynylpyridine (P2, Scheme 1). In the
case of ligands L6 and L7, the starting alkynyl reagent
was prepared via a Sonogashira coupling reaction
between
2-bromo-6-methylpyridine
and
trimethylsilylacetylene followed by deprotection in the
presence of KOH to afford 6-ethynyl-2-methylpyridine
(P3). Incorporating a phenyl group onto the pyridyl ring
firstly
required
a
Suzuki-coupling
between
2,6-dibromopyridine and phenylboronic acid, followed by
a Sonogashira coupling and subsequent deprotection in
the presence of KOH to afford 2-ethynyl-
6-phenylpyridine (P4), which is the starting reagent for
the preparation of ligand L8 (Scheme 2). The low yield of
P4 is attributed to the formation of the by-product,
2,6-diphenylpyridine, which is observed in significant
amounts during the Suzuki coupling. The ligands were
isolated, after column chromatography, as air- and
moisture-stable solids in moderate to excellent yields.
The ligands varied in colour from yellow to dark
red/brown. Several of the ligands except L3 and L6–L8
have been reported previously.[10–13] Ligands L1–L8 were
characterized by a variety of spectroscopic and analytical

PALLADIUM (II) COMPLEXES OF SUBSTITUTED-4-PYRIDYL-1H-1,2,3-TRIAZOLES
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SCHEME 1 Synthesis of 1-substituted-4-pyridyl-1H-1,2,3-triazole ligands (L1–L7) and their palladium methyl complexes (C1–C7)
techniques. Briefly, Fourier transform-infrared (FT-IR)
(DCM) afforded the 1,2,3-triazolyl-pyridinato palladium
spectroscopy displayed the pyridyl ring ν(C=N) absorp-
(II)methyl-chloride complexes, C1–C8. The complexes
were isolated as pale-yellow/off-white/amorphous solids
in moderate to excellent yields of 56–89% (Schemes 1 and
2). Complexes C2–C4 displayed solubility only in
dimethyl sulfoxide. The solubility of the complexes
improved significantly when the aryl substituent on the
triazole moiety was replaced by an octyl chain. In fact,
C1 and C6 are the only complexes that are soluble in
chlorinated solvents. The solubility of the aryl-substituted
complexes improved slightly when a trifluoromethyl sub-
stituent (complexes C5, C7 and C8) was introduced onto
the aryl ring. Characterization of these complexes by FT-
IR spectroscopy revealed a shift to higher wavenumbers
of the ν(C=N) absorption band of the pyridine unit which
1
tion band in the range 1593–1606 cm−1. H NMR spectra
of the compounds revealed the triazole proton as a singlet
and the imine proton of the pyridyl ring as a doublet in
the range δ 8.56–9.49 ppm and δ 8.59–8.66 ppm respec-
tively in DMSO-d6. The bulk purities of ligands L3 and
L6–L8 were confirmed by elemental analysis.
2.2 | Synthesis and characterization of
1,2,3-triazolyl-pyridinato-palladium methyl-
chloride complexes, C1–C8
The reaction of ligands L1–L8 with the methylpalladium
(II) precursor, [(COD)PdMeCl], in dichloromethane
SCHEME 2 Synthesis of 1-substituted-
4-phenylpyridyl-1H-1,2,3-triazole ligand (L8)
and the palladium methyl complex (C8)

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JOSEPH ET AL.
occurred in the range 1598–1615 cm−1, confirming coor-
dination to the metal.[14] The 1H NMR spectra of the
complexes (see Supporting Information) provided further
evidence of ligand coordination to palladium. Specifi-
cally, the signal of the proton in position 6 of the pyridine
ring was shifted downfield and observed in the range δ
8.68–8.79 ppm in DMSO-d6. The electronic properties of
these complexes are demonstrated by the relative chemi-
methyl ligands. These authors showed that for various
palladium methyl–chloride complexes, which contained
pyridyl based N^N chelates, upfield coordination-induced
shifts were indicative of the proton in position 6 of the
pyridyl ring being in close proximity to the methyl ligand
rather than the chloride. If the latter situation prevailed,
one would have expected that chloride ligand with its rel-
atively higher deshielding ability should show a strong
downfield coordination-induced shift. This is indeed
what we observed for our complexes, with the signal for
the pyridyl proton in position 6, becoming deshielded on
coordination to the palladium center. Thus, in conjunc-
tion with the 2D NOESY, we can conclude that in
solution, the isomer in which the Cl ligand is cis to the
pyridine nitrogen is the dominant species in solution.
This is unlike the situation for the unsymmetrical
α-diimine complexes reported by Guo et al.,[18,19] where
mostly mixtures of both cis and trans isomers were
detected. These authors found that, in cases where bulky
substituents were incorporated into the ligand frame-
work, the amount of the cis isomer present increases.
The molecular structures of complexes C1, C4, C5
and C7 were unambiguously determined by single-crystal
X-ray diffraction analysis. The crystallographic data,
which include selected bond lengths, bond angles and
torsion angles, are presented in Tables S2† and S3†,
respectively. The molecular structures of the complexes
show coordination of the ligand through the pyridyl-N
atom and the proximal-N atom of the triazole ring
(Figure 3). For all complexes, the geometry around the
palladium center corresponds to a slightly distorted
square planar arrangement with the largest extent of
distortion observed in the chelating N(1)–Pd(1)–N(2) bite
angles. In general, the Pd–N(py) bond distances are
longer than those involving the proximal-N atom of the
triazole ring. For instance, the Pd–N(1) bond distance of
complex C1 [2.1785 (19) Å] is remarkably longer than
Pd–N(2) owing to the trans influence exerted by the
Pd–Me fragment. Similar trends were observed for other
reported bidentate palladium methyl complexes.[20,21]
The Pd–N(Py) bond length of the complexes is influenced
by the substituent on the pyridyl-ring. Incorporating a
methyl group adjacent to the pyridyl-N atom of the
pyridine ring led to further elongation along the
Pd–N(1) bond for complex C7 (Figure 3). It is evident
from these structures that the complexes adopt a geome-
try in which the chloride ligand is cis relative to the pyr-
idyl nitrogen (isomer 2), correlating with the results
obtained from 2D NOESY. Complex C4 shows hydrogen
bonding interaction between the solvate and the hydroxy
moiety and in the case of C5, there is hydrogen bonding
between the solvate and the triazole proton. C7 shows no
H-bonding between itself and any solvent molecules.
1
cal shift of the signal for triazole proton in the H NMR
spectrum of the complexes (Figure S71†). As the
electron-donating ability of the substituent on the triazole
ring increases, the signal for triazole proton becomes
more shielded. This is due to the electron density within
the triazole ring increasing. Further characterization by
ESI–MS analysis revealed characteristic isotope clusters
corresponding to [M − Cl]+ and [M − Cl + MeCN]+ frag-
ments (see Supporting Information).[15,16] Lastly, elemen-
tal analysis confirmed the composition and bulk purity of
the complexes.
Owing to the asymmetrical nature of the neutral
complexes, the structural geometry of these complexes
was probed in more detail. The complexes can adopt two
possible isomers, i.e. isomer 1 (Figure 2a), in which the
Pd–Me bond is cis relative to the pyridyl ring, and isomer
2 (Figure 2b), where the Pd–Me bond is trans relative to
the pyridyl ring.
1H NMR spectroscopy provides evidence for the for-
1
mation of only one isomer in solution. H NMR spectra
of the all the complexes reveal only one isomer present in
solution. To determine the relative position of the methyl
ligand in the complexes, a 2D NOESY experiment was
performed using complex C1 in chloroform
(Figure S39†). The spectrum shows no evidence for any
interaction between the pyridyl-imine proton (H1) and
the protons of the methyl group, as evidenced by the
absence of a cross-peak in the NOESY spectrum. This
result suggests that isomer 2 in which the methyl is trans
to the pyridine nitrogen is the one present in solution. In
addition to relying on the NOESY experiment, we also
employed an approach reported by Elsevier and co-
workers[17] in which coordination induced chemical
shifts were used to deduce the relative position of the pro-
ton in position 6 of the pyridyl ring relative to the Cl or
FIGURE 2 The two possible isomers for palladium complex
C1: (a) isomer 1, Pd–Me bond cis relative to the pyridyl ring and
(b) isomer 2, Pd–Me bond trans relative to the pyridyl ring

PALLADIUM (II) COMPLEXES OF SUBSTITUTED-4-PYRIDYL-1H-1,2,3-TRIAZOLES
5 of 14
FIGURE 3 Molecular structure of complexes C1, C4, C5 and C7 with atomic numbering, drawn at 50% probability ellipsoids. The
second independent structure (in the case of C7) and hydrogen atoms are omitted for clarity (in the case of C1 and C7)
2.3 | Synthesis and characterization of
cationic 1,2,3-triazolyl-pyridinato-
palladium methyl complexes, C9–C11
Initially, during the application of complexes C1–C8 in
catalytic ethylene oligomerization, we generated the cat-
ionic active species in-situ using MAO as an activator.
FIGURE 4 Target cationic palladium methyl complex (left)
with the formation of the by-product, tris(pyridine)palladium
methyl complex (right)
Subsequent to this, we attempted to prepare discrete
single-site cationic complexes using NaBArF as a chlo-
4
ride abstractor. This, unfortunately, led to the formation
of unstable BArF − adducts when attempting to isolate
4
the complexes as Et2O and CH3CN solvates. We then
attempted to isolate the cationic species using pyridine as
a solvent. Although the pyridine solvate appeared rela-
tively more stable, the reaction unfortunately consistently
yielded a mixture of the target cationic complex as well
as a cationic by-product [Pd(pyridine)3CH3]+ (Figure 4).
The latter was always present in significantly large
amounts. Figure S74† (Supporting Information) shows a
1H NMR spectrum of the mixture containing the tris
(pyridine)methyl palladium by-product. The formation of
this species as a by-product has previously been detected
in the preparation of other cationic Pd–methyl
complexes.[16]
Several attempts to vary the reaction conditions with
the hope of enhancing the production of the target cat-
ionic species were fruitless. In addition, attempts to sepa-
rate the target complexes from the [Pd(pyridine)3CH3]+
by-product were also not successful. Thus, not being able
to isolate the target BArF − adducts in pure form forced
4
us to explore the use of an alternative anion as a counter-
ion to the target cationic species. In this regard, we inves-
−
tigated the use of PF6 as a counterion, and fortunately,

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JOSEPH ET AL.
this gave stable cationic complexes that we were able to
isolate in a pure form. The cationic complexes with PF6
−48.1(2), which reveals that the phenyl substituent and
pyridyl ring are perpendicular to each other, hence the
phenyl group could induce some sort of steric control
around the metal center.
−
as counterion, C9–C11, were prepared by the reaction of
the corresponding neutral complexes (C5, C7 and C8)
with a chloride abstractor, AgPF6, in the presence of ace-
tonitrile as the weakly coordinating solvent (Scheme 3).
These complexes were isolated in high yields (81–88%),
and they displayed solubility in acetonitrile and DMSO
and were partially soluble in chlorinated solvents. The
cationic PF6− complexes were characterized by a range of
analytical techniques. In the FT-IR spectra, the
ν(CN) stretching frequency is shifted to slightly higher
wavenumbers (in the range 1615–1618 cm−1). Further-
more, the appearance of an upfield singlet in the range
2.4 | Ethylene oligomerization reactions
Preparative-scale ethylene oligomerization was con-
ducted at room temperature employing neutral
complexes C1–C8 as pre-catalyst in the presence of a co-
catalyst, MAO (Scheme 4). In a typical experiment,
10 μmol of the complex was reacted with excess co-
catalyst in toluene and stirred at various pressures,
temperatures and reaction times.
1
1.50–1.75 ppm in the H NMR spectra of the complexes
can be assigned to the methyl protons of acetonitrile
coordinated to the Pd center. The molecular structure of
complex C11 was also unambiguously determined by
single-crystal X-ray diffraction analysis. Selected crystal-
lographic data for this complex are tabulated in
Tables S2† and S3† in the Supplementary Information.
The molecular structure of C11 shows a dissociated ion-
pair in which the separation of the cation and anion is
represented by a Pd–P distance of 5.625 Å. The coordina-
tion sphere of the metal is occupied by the chelating
pyridyl-triazole and the methyl ligands. The fourth coor-
dination site is occupied by a weakly coordinating aceto-
nitrile molecule which occurs cis to the pyridyl nitrogen
donor atom (Figure S72†). The geometry around the
metal center corresponds to a distorted square planar
arrangement, with the largest deviation observed in
the N(1)–Pd(1)–N(5) angle (102.14ꢀ) as a result of aceto-
nitrile coordination. For the cationic complex, the
Pd–N(1) bond distance was also remarkably longer than
Pd–N(2) owing to the trans influence of the methyl
group. Interesting to note was the relative orientation of
the phenyl substituent in the 6-position on the pyridyl
ring. The torsion angle C(2)–C(1)–C(15)–C(16) is
2.4.1 | Ethylene oligomerization
reactions employing neutral complexes
C1–C8 as pre-catalyst
Complexes C1–C8 were evaluated as pre-catalysts, and
the results are summarized in Table 1. The complexes,
upon activation with MAO, oligomerize ethylene with
the highest turnover frequency (TOF) of 5594 obtained
for complex C5 (Table 1, entry 5). The product distribu-
tion consists mainly of C4 products with small amounts
of C6 and C8 oligomers observed. The higher selectivity
toward C4 products can be attributed to the absence of
steric crowding around the metal center, a well-known
factor in controlling the product distribution.[4,6] Bro-
okhart and co-workers reported the application of cat-
ionic palladium phosphine-oxazoline complexes in
ethylene oligomerization.[22] Complexes containing steri-
cally bulky ligands in axial positions yielded a Schulz–
Flory distribution of longer-chain α-olefins with the
highest TOF value of 15760 h−1 obtained. In these exam-
ples reported by Brookhart, the ligands have sufficient
steric bulk to effectively block chain transfer to a signifi-
cant degree, thus leading to long-chain α-olefins. How-
ever, in the absence of axial steric bulk, only ethylene
dimerization was observed. This is indeed what we also
observe. It is therefore not surprising that in our case,
complexes C1–C5 show similar product distributions
owing to their similar steric characteristics around the
metal center. In addition, the major component of the C4
fraction consists largely of 2-butene isomers with the
larger fraction being trans-2-butene. This is largely due to
the fact that the originally formed 1-butene can readily
undergo isomerization under the reaction conditions
employed. This is not surprising as Pd complexes with
N^N chelating ligands systems are known to easily
undergo facile isomerization, especially if ligands are not
SCHEME 3 Synthesis of cationic 1,2,3-triazolyl-pyridinato-
palladium methyl complexes, C9–C11

PALLADIUM (II) COMPLEXES OF SUBSTITUTED-4-PYRIDYL-1H-1,2,3-TRIAZOLES
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SCHEME 4 Preparative-scale ethylene
oligomerization employing neutral complexes
TABLE 1 Preparative-scale ethylene oligomerization employing complexes C1–C8 as catalyst precursorsa
Product distribution and selectivity (%)c
TOFb
2857
5118
4404
4483
5594
2222
4682
4205
C4
80
82
79
81
81
89
92
93
C6
15
13
15
13
15
7
C8
5
1-C4:2-C4
45: 55
43: 57
43: 57
44: 56
45: 55
73: 27
71: 29
94: 6
d
Entry
Complex
C1
1
2
3
4
5
6
7
8
C2
5
C3
6
C4
6
C5
4
C6
4
C7
5
3
C8
6
1
aReaction conditions: catalyst loading, 10 μmol; pressure, 20 bar; temperature, 30ꢀC; time, 1 h; co-catalyst, MAO; Al:Pd, 1000:1; solvent, PhMe (40 ml). b
.
Turnover frequency (TOF) in units of molethylene mol Pd−1 h−1 c Determined by GC-FID. d The ratio of trans- to cis-2-butenes is approximately 2:1 in most
cases.
sterically demanding. In a recent paper by Albrecht and
co-workers, they also observed extensive 1-butene isom-
erization while performing ethylene dimerization using
pyridylidene-amide palladium complexes.[23]
bidentate NHC–pyridine ligands with varying steric char-
acteristics.[24] Selective dimerization of ethylene was
observed in the presence of the bulky 2,6-dimethylphenyl
substituent on the pyridyl-ring. However, when the steric
strain is reduced, a mixture of low molecular weight olig-
omerization products is isolated.
Incorporating a methyl group in the ortho-position on
the pyridyl ring led to a significant improvement in the
selectivity toward 1-butene. For example, ethylene dimer-
ization catalyzed by complexes C6 and C7 displayed a
26–28% increase in the amount of 1-butene produced
under the same reaction conditions. However, for these
catalysts lower activities were observed (Table 1, entries
6 and 7). Complex C8 in which the bulkier phenyl group
is incorporated into the ligand architecture displayed a
significant increase in selectivity with 94% of the product
mixture being 1-butene (Table 1, entry 8). It is evident
from these results that the steric bulk imparted by the
large phenyl group in the ortho-position slows down the
re-coordination and subsequent isomerization of
1-butene to yield 2-butenes (Scheme 5). This is also
supported by the observation that less trimerization and
tetramerization products were produced by those cata-
lysts possessing some degree of steric bulk in the position
ortho to the pyridine nitrogen. The fact that no polymer
or long-chain oligomer formation was observed indicates
that the steric hindrance exerted by the phenyl group is
not large enough to effectively block the axial positions of
the metal center, which allows β-H elimination to still
proceed in the presence of the phenyl group. In a similar
study, Albrecht and co-workers developed a series of pal-
ladium carbene complexes which contain different C^N
In the case of our complexes, the nature of the substit-
uent on the triazole ring, although not affecting
regio-selectivity, had a marked effect on the activity of
the complexes. Higher activities were achieved when aryl
groups were incorporated as a substituent on the triazole
ring. For instance, the TOF of the reaction increased
significantly when the octyl substituent on the triazole
ring was replaced with a phenyl group (Table 1, entry 1).
Increasing the electron-withdrawing nature of the aryl
ring by incorporating groups such CF3 as substituents led
to an increase in TOF from 5118 (C2) to 5594 (C5)
(Table 1, entries 2 and 5). On the other hand, electron-
donating groups on the aryl substituent led to a decrease
in the catalytic activities with TOF's of 4404 and 4483
being achieved for complexes C3 and C4 respectively
(Table 1, entries 2–4). The enhanced catalytic perfor-
mance achieved with electron-withdrawing substituents
is attributed to the fact that these substituents enhance
the overall electrophilicity of the metal center, hence pro-
moting monomer coordination. Jordan and co-workers
reported a series of cationic palladium alkyl complexes
ligated by bis (heterocycle)methane ligands.[25,26] They
showed that ethylene insertion rates were enhanced
when the electrophilicity of the metal is increased as a

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JOSEPH ET AL.
SCHEME 5 Proposed catalytic olefin dimerization, isomerization and oligomerization cycles for C1–C8
result of a decrease in the sigma donor ability of
the ligand.
butene fraction gradually increased with an increase in
the ethylene pressure from 10 to 50 bar. Although C8
was not the most active catalyst, the complex preferen-
tially dimerized ethylene with conversions of 86–88% and
selectivity of around 89% toward 1-butenes being
observed at lower ethylene pressures (Table 2, entries
3 and 6). It is evident from these results that the reaction
conditions had minimal effect on butene selectivity in the
case of C8 and that the bulky phenyl group incorporated
on the pyridine ring was actually responsible for the com-
plex being more selective. In this instance, the phenyl
group imposed steric congestion around the vacant site,
which in turn limited the re-coordination and isomeriza-
tion of 1-butene to produce internal butenes. Similarly,
the formation of longer-chain olefins was less favored
using complex C8 as catalyst (Scheme 5).
Next, we investigated the effect of MAO concentra-
tion on ethylene oligomerization by varying the co-
catalyst to catalyst ratio (Al:Pd) from 50:1 to 3000:1. Both
complexes C5 and C8 were catalytically inactive when an
Al:Pd ratio of 50:1 was employed (Table 2, entries 10 and
11). Increasing the Al:Pd ratio from 500:1 to 1000:1 led to
an improvement in the activity from 3095 to 5594 and
from 2063 to 4205 for C5 and C8 respectively (Table 1,
entries 5 and 8; Table 2, entries 12 and 13). Increasing
the Al:Pd ratio to 3000:1, however, led to a significant
loss in activity with TOF values of 2857 and 2460 being
observed for complexes C5 and C8 respectively (Table 2,
entries 14–15). These results highlight the importance of
careful control over the co-catalyst concentration, as cata-
lyst deactivation seems to occur at Al:Pd ratios in excess
of 1000:1, which is possibly due to an increase in
trimethylaluminum impurities in the catalytic reaction
mixture.[28,29]
2.4.2 | Effect of reaction conditions on
ethylene oligomerization employing
complexes C1, C5 and C8
Since these complexes were found to be active catalysts
in the oligomerization of ethylene, the catalytic behavior
of complexes C1, C5 and C8 was studied in more detail to
explore the impact of various reaction conditions
(Table 2). Increasing the ethylene pressures from 5 to
30 bar led to a dramatic increase in activity with a con-
comitant increase in selectivity toward butenes being
observed (Table 2, entries 1–8). For example, TOF's of
1071 and 6665 were obtained for C5 at 5 and 30 bar
respectively with a 24% selectivity improvement toward
butenes at the higher pressure. This is more likely due to
the higher monomer concentration at the elevated pres-
sure, which favors ethylene coordination to the metal
center, resulting in less tri- and tetra-merization being
observed. Interesting to note was the decrease in the
amount of internal butenes produced at higher pressures.
For instance, the percentage of internal butenes observed
decreased from 69 to 42% when the pressure was
increased from 5 to 30 bar, respectively (Table 2, entries
1 and 8). At higher pressures, the probability of 1-butene
coordinating to the metal was lower when compared to
ethylene; hence the isomerization process was retarded.
Similar observations were made by Smith and co-workers
in which the catalytic behavior for a series of palladium
complexes for ethylene oligomerization was examined.[27]
They found that the 1-butene component within the

PALLADIUM (II) COMPLEXES OF SUBSTITUTED-4-PYRIDYL-1H-1,2,3-TRIAZOLES
9 of 14
TABLE 2 Effect of reaction pressure and co-catalyst loading on ethylene oligomerization employing complexes C1, C5 and C8 as
catalyst precursorsa
Product distribution and selectivity (%)c
d
Entry
1
Complex
C1
Pressure (bar)
TOFb
793
C4
71
70
86
75
74
88
95
94
95
–
C6
20
21
10
18
20
8
C8
9
9
4
7
6
4
0
0
0
–
–
5
2
6
2
1-C4:2-C4
30: 70
31: 69
87: 13
36: 64
35: 65
89: 11
55: 45
58: 42
94: 6
5
2
C5
5
1071
913
3
C8
5
4
C1
10
10
10
30
30
30
20
20
20
20
20
20
1785
2936
2261
3571
6665
4523
–
5
C5
6
C8
7
C1
5
8
C5
6
9
C8
5
10 e
11 e
12 f
13 f
14 g
15 g
C5
–
–
C8
–
–
–
–
C5
3095
2063
2857
2460
82
90
81
91
13
8
43: 57
91: 9
C8
C5
13
7
45: 55
92: 8
C8
aReaction conditions: catalyst loading, 10 μmol; temperature, 30ꢀC; time, 1 h; co-catalyst, MAO; Al:Pd, 1000:1; solvent, PhMe (40 ml). b TOF in units of
molethylene mol Pd−1 h−1 c Determined by GC-FID. d The ratio of trans- to cis-2-butenes is approximately 2:1 in most cases. e Al:Pd = 50 (MAO used as
co-catalyst). f Al:Pd = 500 (MAO used as co-catalyst). g Al:Pd = 3000 (MAO used as co-catalyst).
.
In order to examine the neutral complexes in terms of
catalyst stability, a series of additional catalytic reactions
were conducted at various reaction times. A two-stage
reaction profile was observed (Figure 5a). Increasing the
reaction time from 0.5 to 1 h led to an initial increase in
the activity of the catalysts. However, extending the reac-
tion time to 2 h and beyond showed a decrease in activ-
ity. For example, increasing the reaction time to 1 h led
to an increase in TOF from 4444 to 5594 for complex C5;
however, lower TOFs were attained at 2 and 4 h respec-
tively. In the case of C1 and C8, the catalysts became
almost completely inactive after 4 h, at which TOFs of
436 and 952 were observed, respectively. It is clear from
these results that the catalyst systems show high initial
activity but then gradually deactivate after about
1 h. Therefore, we can conclude that the complexes are
stable within the first hour of the reaction after which it
undergoes some degree of catalyst decomposition.[9,30]
This is evidenced by the observation of small amounts of
Pd black in the reaction mixture. Extended reaction times
had minimal influence on the chemo- and regio-
selectivity of the complexes. In the case of the sterically
less demanding catalyst precursors, C1 and C5, there was
only a slight increase in octene formation over time
(Figure 5b and d). In addition, a slight increase in inter-
nal butene isomers was observed (Figure 5c and e). Both
of these observations are probably due to the fact that,
over time, the 1-butene formed can re-coordinate and
become involved in either isomerization or chain exten-
sion processes (Scheme 5). The subsequent chain growth
and/or isomerization of the initially formed 1-butene
product occurred to a lesser degree for C8 (Figure 5f and
g). The bulky ligand would be expected to hinder both
these processes as it impacts negatively on 1-butene coor-
dination, which is a prerequisite for both processes.
2.5 | Ethylene oligomerization
employing discrete cationic complexes C9–
C11
In addition to utilizing the neutral Pd–methyl-chloride
complexes as catalyst precursors to generate the cationic
active species in-situ, we also briefly evaluated the use of
discrete cationic Pd–Me complexes as catalysts. Thus, cat-
ionic complexes C9–C11 were employed in the catalytic
ethylene oligomerization reaction. The results of this
investigation are summarized in Table 3. These discrete
cationic complexes exhibit appreciable catalyst activity
with TOF values ranging from 4364 to 5554 with complex
C9 which has only a H-substituent in position 6 on the
pyridine, showing the highest activity. C11 with
the bulky phenyl substituent has the lowest activity.
In the case of the latter, initial coordination of the alkene
substrate is hindered by the presence of the relatively

10 of 14
JOSEPH ET AL.
FIGURE 5 (a) Time profile for complexes C1, C5 and C8 showing significant loss in activity after 1 h. Product distribution and
selectivity of (b and c) complex C1, (d and e) complex C5 and (f and g) complex C8 respectively over time
TABLE 3 Preparative-scale ethylene oligomerization employing cationic complexes C9–C11 as catalyst precursorsa
Product distribution and selectivity (%) c
d
Entry
Complex
C9
TOFb
5554
4761
4364
C4
83
90
92
C6
17
10
8
C8
–
1-C4:2-C4
45: 55
74: 26
93: 7
1
2
3
C10
–
C11
–
aReaction conditions: catalyst loading, 10 μmol; pressure, 20 bar; temperature, 30ꢀC; time, 1 h; solvent, PhMe (40 ml). b TOF in units of molethylene mol Pd−1
h−1 c Determined by GC-FID. d The ratio of trans- to cis-2-butenes is approximately 2:1 in most cases.
.
bulky phenyl group, slowing down the overall reaction
rate. The trend with regards to the catalyst activity for the
three discrete cationic complexes is similar to what was
observed for the neutral analogs of these complexes
(Table 1, entries 5–8). When comparing the activities of
the neutral catalyst precursors activated by MAO at the
optimum Al:Pd ratio of 1000:1 with those obtained using
their discrete cationic analogs, we note a remarkable cor-
relation between the two. This seems to indicate that the
nature of the active species is similar in both cases. The
discrete cationic complexes, however, outperform the
MAO-activated neutral complexes when Al:Pd ratios
either lower or higher than 1000:1 are employed. In
terms of regio-selectivity, complex C11 with the relatively
bulky substituent in position 6 on the pyridine ring
shows the highest production of 1-butene.
A
1-butene:2-butene ratio of 93:7 is observed in this case
(Table 3, entry 3). Once again, this is comparable with
what was observed for the neutral analog, C8 when acti-
vated with MAO using an Al:Pd ratio of 1000:1. These
results seem to indicate in the case of the neutral com-
plex that an optimum concentration of the cationic active
species is generated in-situ at an Al:Pd ratio of 1000:1.
Furthermore, the fact that similar catalytic activity

PALLADIUM (II) COMPLEXES OF SUBSTITUTED-4-PYRIDYL-1H-1,2,3-TRIAZOLES
11 of 14
performance is observed for both the MAO-activated
complexes and the discrete cationic complexes, seems to
indicate a fair degree of similarity in the active species for
the two types of systems employed. This is not unex-
pected since it is reasonably envisaged that the active spe-
cies generated from both C8 and C11 would have similar
ligand arrangements around the palladium center. The
only difference we would expect between the two cationic
intermediates would be the relative positions of the coun-
terions since there is a considerable difference in the size
of the two counterions. It would, however, appear that
the latter does not significantly impact the oligomeriza-
tion ability of the two catalytic systems as similar activi-
ties and selectivities are observed. Given the similarity in
the catalytic behavior observed using both the neutral
and the cationic catalyst precursors, it is also not unrea-
sonable to conclude that a similar mechanistic pathway
is operative for the two systems.
From the above discussion, it emerges that the reac-
tivity of these palladium triazole pyridine complexes
can be fine-tuned by careful selection of substituents on
both the pyridine and triazole rings. The location of the
substituents in these bifunctional ligand systems seems
to be an important factor to consider when designing
catalysts with the ligand system investigated in this
study.
Thus, from the preliminary results, it would appear
that substituents on the triazole ring component of the
ligand scaffold have a greater influence on the activity of
the catalyst system, while substituents in the pyridine
ring, especially those in position 6, have a significant
impact on the regioselectivity of the dimerization process.
The latter, however also influences activity, but this is
not as significant as its steric impact. These results
reported here give some useful insight into catalyst
design methodologies that could be adopted to improve
selectivity in these triazole pyridine systems. This is
currently forming the basis of ongoing research in our
laboratory, where ligands such as those reported here are
being designed for other catalytic applications.
C9–C11 were employed as catalysts, pointing to simi-
larities in the nature of the active species for both clas-
ses of complexes.
The catalyst activity of the complexes was essentially
dictated by the nature of the substituent on the triazole
component of the bifunctional ligand system. Electron-
withdrawing substituents such as CF3 enhance the elec-
trophilicity of the metal center, which promotes the initial
coordination of the alkene, resulting in higher activities.
Substituents in position 6 of the pyridine ring of the
ligand scaffold on the other hand influence the selectiv-
ity of the catalysts. Ethylene dimerization was favored
when the sterically bulky phenyl group was incorporated
in the 6-position on the pyridyl ring, as was the case for
complexes C8 and C11. For these complexes, a regio-
selectivity approaching 95% toward 1-butene was
observed. The results obtained demonstrate that both
catalyst activity and selectivity are sensitive to ligand
architecture and that both steric and electronic charac-
teristics play a role in this regard. In the systems investi-
gated here, these effects are however largely imparted by
different components of the pyridine–triazole ligand
scaffold. The initial results reported here provide valu-
able insights into the design of other potential alkene
oligomerization catalysts based on these pyridine–
triazole ligand scaffolds in which the constituent compo-
nents control different aspects of the catalyst's
performance.
4 | EXPERIMENTAL
4.1 | General
Unless otherwise stated, all reagents were acquired from
Sigma-Aldrich, Merck and Alfa-Aesar and used without
further purification. Solvents were obtained from Merck
and Kimix and purified using Pure Solv™ micro solvent
purifiers fitted with activated alumina columns. Selected
ligands were purified by flash chromatography using an
automated Isolera One Biotage flash chromatography
unit, equipped with a 200–450 nm variable UV detector.
Melting point determinations were recorded on a Stuart
Scientific SMP3 apparatus and are reported as
uncorrected. FT-IR analyses were performed on a
Thermo Nicolet AVATAR 330 instrument with a smart
ATR and recorded as neat samples. 1H, 13C and 31P NMR
spectra were recorded on Varian VNMRS 300 MHz and
Varian Unity Inova 400 MHz spectrometers. The chemi-
cal shifts for proton and carbon spectra are referenced
internally to the residual deuterated solvents and
referenced externally to tetramethyl silane. Chemical
shifts for 31P NMR spectra are referenced externally
3 | CONCLUSION
A series of neutral methyl-chloride–palladium com-
plexes containing pyridine–triazole ligands as well as a
series of cationic analogs in chloride displaced by
acetonitrile were successfully employed as catalysts in
ethylene oligomerization. The neutral complexes
C1–C5 activated with methylaluminoxane produced
mainly butenes, with small quantities of hexenes and
octenes also detected. Comparable catalytic results
were obtained when the discrete cationic complexes

12 of 14
JOSEPH ET AL.
relative to 85% phosphoric acid (H3PO4). Elemental ana-
lyses were performed on a Vario EL cube elemental ana-
lyzer instrument. ESI–MS analyses were recorded on
Waters API Quattro Micro and Waters API Q-TOF
Ultima instruments by direct injections. The precursor
[(COD)PdMeCl][17,31] and the aliphatic[32] and aromatic
azides[33] were prepared according to literature
procedures.
silica gel using DCM–MeOH (99:1) as the eluent. The
solvent was removed in vacuo which afforded ligand L3
as a dark-orange crystalline solid (0.85 g, 89%). FT-IR
(ATR, ν): 1600 cm−1 (C=N). 1H NMR (chloroform-d,
3
600 MHz): δ 8.60 (d, 1H, JH–H = 4.9 Hz, H1); δ 8.28 (d,
3
1H, JH–H = 7.9 Hz, H4); δ 8.23 (s, 1H, H7); δ 7.82 (td,
3
4
1H, JH–H = 7.7 Hz, JH–H = 1.8 Hz, H3); δ 7.33 (t, 1H,
3JH–H = 7.7 Hz, H11); δ 7.25 (dd, 1H, 3JH–H = 4.8 Hz, 4JH–
= 1.2 Hz, H2); δ 7.20 (d, 2H, JH–H = 7.7 Hz, H10) δ
3
H
2.06 (s, 6H, Me). 1H NMR (DMSO-d6, 300 MHz): δ
3
4
4.2 | NMR abbreviations
8.80 (s, 1H, H7); δ 8.63 (dq, 1H, JH–H = 4.8 Hz, JH–
= 1.0 Hz, H1); δ 8.15 (dt, 1H, JH–H = 7.9 Hz,
3
H
3
4
s = Singlet, d = doublet, t = triplet, dd = doublet of dou-
blets, td = triplet of doublets, dt = doublet of triplets,
dq = doublet of quartets, quin. = quintet, sept. = septet,
m = multiplet (denotes complex pattern for a single
proton resonance), comp. = complex (denotes complex
pattern of overlapping proton resonances). Superscripts
denote protons as per numbering scheme in Figure S1†
in the Supporting Information document.
4JH–H = 1.2 Hz, H4); δ 7.95 (td, 1H, JH–H = 7.6 Hz, JH–
= 1.8 Hz, H3); δ 7.45–7.36 (comp., 2H, H2,11); δ
H
7.34–7.28 (comp., 2H, H10); δ 1.99 (s, 6H, Me). 13C{1H}
NMR (chloroform-d, 151 MHz): δ 150.4 (CAr); δ 149.6
(CAr); δ 148.5 (CAr); δ 137.1 (CAr); δ 136.0 (CAr); δ 135.6
(CAr); δ 130.2 (CAr); δ 128.6 (CAr); δ 124.0 (CAr); δ
123.1 (CAr); δ 120.5 (CAr); δ 17.6 (CMe). HRMS (ESI+
mode, m/z): calcd for [M + H]+ 251.1297; found 251.1302.
Anal. calcd. (%) for C15H14N4ꢁ0.1CH2Cl2: C, 70.39; H,
5.55; N, 21.77; found: C, 70.83; H, 5.48; N, 21.71.
4.3 | Synthesis of ligands and complexes
4.3.2 | Synthesis of 1-octyl-4-pyridyl-1H-
1,2,3-triazole ligated palladium methyl
complex (C1)
The procedure for the preparation of ligand L3, neutral
complex C1 and cationic complex C9 is given below as
representative examples. Detailed characterization data
of all other ligands and complexes prepared are discussed
in the Supporting Information†.
To a stirred N2-degassed solution of (COD)PdMeCl
(0.26 g, 1.00 mmol) dissolved in DCM (3 ml) was added
ligand L1 (0.28 g, 1.1 mmol) dissolved in DCM (3 ml)
and the reaction was stirred for 2 h at room temperature.
After the allotted time, the solution was concentrated by
reducing the solvent volume in vacuo. The DCM solution
was layered with Et2O, resulting in the product precipi-
tating out of solution. The precipitate was filtered and
washed with Et2O which afforded the product as a light-
yellow powder (0.36 g, 87%). FT-IR (ATR, υ): 1614 cm−1
4.3.1 | Synthesis of
1-(2,6-dimethylphenyl)-4-pyridyl-1H-
1,2,3-triazole ligand (L3)
This method was adopted and modified according to a
literature procedure.[34] To a 10 ml microwave tube was
added Cu (PPh3)3Br (0.02 g, 0.02 mmol) followed by
Et3N (2 ml), 2-ethynylpyridine (0.38 ml, 3.8 mmol),
2-azido-1,3-dimethylbenzene (0.62 g, 4.2 mmol) and dry
THF (2 ml) after which the tube was sealed and placed
1
(C=N). H NMR (chloroform-d, 600 MHz): δ 8.68 (d, 1H,
3JH–H = 5.2 Hz, H1); δ 8.37 (s, 1H, H7); δ 7.80 (td, 1H,
4
3
3JH–H = 8.0 Hz, JH–H = 1.4 Hz, H3); δ 7.73 (d, 1H, JH–
= 8.0 Hz, H4); δ 7.27 (dd, 1H, JH–H = 5.3 Hz, JH–
3
4
H
in
the
microꢀwave
reactor
(time,
30
min,
H = 1.1 Hz, H2); δ 4.45 (t, 2H, 3JH–H = 7.6 Hz, H8); δ 1.96
3
temperature, 100 C and power, 150 W). After the allot-
ted time, the reaction mixture was allowed to cool to
room temperature and the solvent removed in vacuo to
afford a dark-brown residue. H2O (40 ml) was added,
and the product was extracted with DCM (3 × 20 ml).
The combined organic layers were washed with a 1 M
EDTA/NaOH solution (3 × 15 ml). After separation, the
DCM fraction was dried over MgSO4 and filtered by grav-
ity filtration. The solvent was removed in vacuo, and the
crude product was then column chromatographed on
(quin., 2H, JH–H = 7.1 Hz, H9); δ 1.39–1.19 (comp., 10H,
3
H
10–14); δ 1.11 (s, 3H, HPd–Me); δ 0.86 (t, 3H, JH–
= 6.9 Hz, H15). H NMR (DMSO-d6, 400 MHz): δ 9.17
1
H
(s, 1H, H7); δ 8.72 (d, 1H, JH–H = 5.3 Hz, H1); δ 8.16 (t,
3
1H, JH–H = 7.9 Hz, H3); δ 8.06 (d, 1H, JH–H = 8.1 Hz,
3
3
H4); δ 7.64 (t, 1H, JH–H = 6.5 Hz, H2); δ 4.52 (t, 2H, JH–
3
3
= 7.3 Hz, H8); δ 1.95–1.84 (comp., 2H, H9); δ 1.36–1.16
H
(comp., 10H, H10–14); δ 0.98 (s, 3H, HPd–Me); δ 0.84 (t, 3H,
3JH–H = 6.8 Hz, H15). 13C{1H} NMR (chloroform-d,
151 MHz): δ 148.3 (CAr); δ 148.2 (CAr); δ 146.6 (CAr); δ

PALLADIUM (II) COMPLEXES OF SUBSTITUTED-4-PYRIDYL-1H-1,2,3-TRIAZOLES
13 of 14
138.2 (CAr); δ 124.7 (CAr); δ 123.7 (CAr); δ 123.4 (CAr); δ
120.9 (CAr); δ 52.6 (Caliphatic); δ 31.8 (Caliphatic); δ 30.0
(Caliphatic); δ 29.1 (Caliphatic); δ 29.0 (Caliphatic); δ 26.5
(Caliphatic); δ 22.7 (Caliphatic); δ 14.2 (Caliphatic); δ −6.9 (CPd–
program POV-Ray.[43] CSD 1921497 for complex C1, CSD
1921498 for complex C4, CSD 1921499 for complex C5,
CSD 1921500 for complex C7 and CSD 1969487 for complex
C11 contain the supplementary crystallographic data for
this paper.
Me). HRMS (ESI+ mode, m/z): calcd for [M
−
Cl + MeCN]+ 420.1980, found 420.1394. Anal. calcd (%)
for C16H25ClN4Pd: C, 46.28; H, 6.07; N, 13.49; found: C,
46.04; H, 6.18; N, 13.57.
4.5 | Procedure for preparative ethylene
oligomerization
4.3.3 | Synthesis of [Pd(Me)(CH3CN)
(L5)][PF6] (C9)
A 250 ml Parr stainless-steel high-pressure autoclave was
charged with the required amount of toluene and co-
catalyst and sealed under an inert atmosphere in a
glovebox. The reactor was first brought to the set temper-
ature at which point a dispersion of the pre-catalyst in
toluene (total reaction volume: 40 ml) was added via
syringe under positive ethylene pressure. The reactor was
then pressurized with ethylene to the required pressure
after which stirring commenced (150 rpm). The ethylene
feed was maintained throughout the catalytic reaction,
and the volume of ethylene consumed was monitored
with a mass flow meter. After the allotted time, the reac-
tor was cooled to −78ꢀC; then the reactor was slowly
vented to release excess ethylene. The reaction mixture
was quenched with ethanol (5 ml) while maintaining the
temperature below −20ꢀC to minimize the loss of any vola-
tile products. A liquid sample was filtered through a syringe
filter and the filtrate analyzed by GC-FID using p-xylene as
internal standard. No long-chain oligomers or polymers
were produced during these reactions. Turnover frequencies
were calculated from the volume of ethylene consumed
during the reaction as a function of time.
To a stirred N2-degassed suspension of complex C5
(130 mg, 0.29 mmol) in MeCN (3 ml) was added AgPF6
(88 mg, 0.349 mmol) dissolved in MeCN (3 ml) and the
resulting solution was stirred for 3 h at room tempera-
ture. After the allotted time, the reaction was filtered over
Celite and concentrated to the minimal volume under
vacuum. Upon addition of Et2O, the product precipitated
out of solution as a white powder (153 mg, 88%). FT-IR
(ATR, υ): 1618 cm−1 (C=N). 1H NMR (acetonitrile-d3,
600 MHz): δ 9.32 (s, 1H, H7); δ 8.73 (d, 1H, JH–
3
3
4
= 5.2 Hz, H1); δ 8.34 (td, 1H, JH–H = 7.8 Hz, JH–
H
= 1.6 Hz, H3); δ 8.22 (d, 2H, JH–H = 8.5 Hz, H9,10); δ
3
H
8.18 (d, 1H, JH–H = 8.1 Hz, H4); δ 8.15 (d, 2H, JH–
3
3
= 8.5 Hz, H9,10); δ 7.84–7.78 (m, 1H, H2); δ 2.32 (s, 3H,
H
H
Pd–Me); δ 1.53 (br, 3H, HMeCN). 13C{1H} NMR (acetoni-
trile-d3, 151 MHz): δ 151.1 (CAr); 150.2 (CAr); 146.9 (CAr);
141.8 (CAr); 139.7 (CAr); 132.7 (CAr); 128.6 (CAr); 127.5
(CAr); 125.6 (CAr); 124.2 (CAr); 123.8 (CAr); 122.9 (CAr);
122.6 (C7), 1.8 (CMeCN), −2.9 (CPd–Me). HRMS (ESI+
mode, m/z): calcd for [M]+ 452.0314, found 452.0325;
calcd for [M − MeCN]+ 411.0049, found 411.0064. HRMS
(ESI− mode, m/z): calcd for [M]− 144.9642, found 144.9635.
ACKNOWLEDGEMENTS
The authors acknowledge the financial support by the
National Research Foundation of South Africa (grant
no. CPRR160408161799) and the c*change Centre of
Excellence in Catalysis, the Department of Science and
Technology, South Africa. The Division of Research
Development at Stellenbosch University is also thanked
for their financial support.
4.4 | X-ray crystal structure
determination
Single X-ray diffraction intensity data were collected on a
Bruker Apex DUO diffractometer with a CCD area detec-
tor[35] using graphite monochromated Mo-Kα radiation
(λ = 0.71073 Å). Data collection, reduction and refinement
were performed using SMART and SAINT software.[36]
Absorption corrections[37] and other systematic errors were
accounted for using SADABS.[38] All structures were solved
by Direct Methods and refined by full-matrix least-squares
on F2 using SHELXS-13 and SHELXL-16[39,40] within the X-
Seed graphical user interface.[41,42] All non-hydrogen atoms
were refined anisotropically, and hydrogen atoms were
placed using calculated positions and riding models. High-
resolution molecular diagrams were produced using the
CONFLICT OF INTEREST
There are no conflicts to declare.
ORCID
Mohammed Cassiem Joseph https://orcid.org/0000-
0003-1732-5592
Andrew John Swarts https://orcid.org/0000-0002-7942-
8557
Selwyn Frank Mapolie https://orcid.org/0000-0001-
6891-0412

14 of 14
JOSEPH ET AL.
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Joseph MC, Swarts AJ,
Mapolie SF. Palladium (II) complexes chelated by
1-substituted-4-pyridyl-1H-1,2,3-triazole ligands as
catalyst precursors for selective ethylene
dimerization. Appl Organometal Chem. 2020;e5595.
https://doi.org/10.1002/aoc.5595
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P. Fornasiero, E. Zangrando, B. Milani, M. Albrecht, Organo-
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