Structural and theoretical studies of the methoxycarbonylation of higher olefins catalysed by (Pyrazolyl-ethyl)pyridine palladium (II) complexes

Article abstract of DOI:10.1002/aoc.5175

Reactions of 2-[1-(3,5-dimethylpyrazol-1-yl)ethyl]pyridine (L1) and 2-[1-(3,5-diphenylpyrazol-1-yl)ethyl]pyridine (L2) with the [Pd (COD)Cl₂] or [Pd (COD)MeCl] produced palladium (II) complexes [Pd(L1)ClMe] (1), [Pd(L1)Cl₂] (C2), [Pd(L2)ClMe] (3), and [Pd(L2)Cl₂] (4) in quantitative yields. Solid state structures of complexes 1, 3 and 4 established the formation of mononuclear compounds, containing one bidentate ligand unit per metal atom, to give square planar complexes. All the other spectroscopic characterization data and elemental analyses were consistent with the observed structures. All the palladium (II) complexes 1–4 gave active catalysts in the methoxycarbonylation of 1-octenes. The catalysts demonstrated 100% chemoselectivities towards esters and favored the formation of linear isomers. Reaction conditions such as the type of phosphine derivative, acid promoter, solvent system, time, pressure and temperature have been investigated and shown to affect both the catalytic activity and regio-selectivity of the catalysts. Solid-angle modelling established the comparable steric contributions from the ligands, consistent with the similar regioselectivities of the resultant catalysts.

Full text of DOI:10.1002/aoc.5175

Received: 26 February 2019
DOI: 10.1002/aoc.5175
Revised: 6 July 2019
Accepted: 23 July 2019
F U L L P A P E R
Structural and theoretical studies of the
methoxycarbonylation of higher olefins catalysed by
(Pyrazolyl‐ethyl)pyridine palladium (II) complexes
Siyabonga Zulu | Mohd.G. Alam† | Stephen O. Ojwach
| Matthew P. Akerman
School of Chemistry and Physics,
Reactions of 2‐[1‐(3,5‐dimethylpyrazol‐1‐yl)ethyl]pyridine (L1) and 2‐[1‐(3,5‐
diphenylpyrazol‐1‐yl)ethyl]pyridine (L2) with the [Pd (COD)Cl2] or [Pd
(COD)MeCl] produced palladium (II) complexes [Pd(L1)ClMe] (1), [Pd(L1)
Cl2] (C2), [Pd(L2)ClMe] (3), and [Pd(L2)Cl2] (4) in quantitative yields. Solid
state structures of complexes 1, 3 and 4 established the formation of mononu-
clear compounds, containing one bidentate ligand unit per metal atom, to give
square planar complexes. All the other spectroscopic characterization data and
elemental analyses were consistent with the observed structures. All the
palladium (II) complexes 1–4 gave active catalysts in the methoxycarbonylation
of 1‐octenes. The catalysts demonstrated 100% chemoselectivities towards
esters and favored the formation of linear isomers. Reaction conditions such
as the type of phosphine derivative, acid promoter, solvent system, time, pres-
sure and temperature have been investigated and shown to affect both the
catalytic activity and regio‐selectivity of the catalysts. Solid‐angle modelling
established the comparable steric contributions from the ligands, consistent
with the similar regioselectivities of the resultant catalysts.
Pietermaritzburg Campus, University of
KwaZulu‐Natal, Private Bag X01,
Scottsville 3209, South Africa
Correspondence
Stephen O. Ojwach, School of Chemistry
and Physics, Pietermaritzburg Campus,
University of KwaZulu‐Natal, Private Bag
X01. Scottsville 3209, South Africa.
Email: ojwach@ukzn.ac.za
Present Address
†Department of Industrial Chemistry,
Addis Ababa Science and Technology
University, P. O. Box, 16417, Addis Ababa,
Ethiopia
Funding information
National Research Foundation South
Africa, Grant/Award Number:
CPRR98938; University of
KwaZulu‐Natal; NRF‐South Africa,
Grant/Award Number: CPRR‐98938;
NRF‐DST (South Africa) Centre of
Excellence
KEYWORDS
methoxycarbonylation, modelling, olefins, palladium, structures
1 | INTRODUCTION
such as Heck[6] and Suzuki‐Miyaura[7] coupling
reactions.
Transition metal catalyzed olefin transformation reac-
tions are among the key applications of transition metal
catalysts in industrial processes.[1] Examples include ole-
fin oligomerization,[2] polymerization[3] hydrogenation[4]
and oxidation[5] reactions among others. Among the
metal complexes, palladium systems represent a huge
proportion of the industrially used catalysts. This
attribute is due to the versatility of the palladium
complexes, in terms of catalytic activity, selectivity
and stability. There is no better illustration of
this versatility than in the use of palladium complexes
as catalysts in carbon–carbon coupling reactions
Another olefin transformation reaction where
palladium complexes have gained much promi-
nence is the methoxycarbonylation of olefins.[8]
Methoxycarbonylation of olefins is an attractive industrial
synthetic strategy due to the inherent production of useful
commodities such as pharmaceuticals, perfumes, surfac-
tants and fragrances.[1,9] Traditionally, phosphine‐based
palladium complexes have been used in these reactions
and still remain the most superior catalysts.[10,11] To date,
the main approach has been the use of in situ generated
palladium catalysts, by adding phosphine‐ligands to the
palladium metal salts. While these systems remain the
Appl Organometal Chem. 2019;e5175.
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ZULU ET AL.
catalysts of choice in methoxycarbonylation reactions,
they suffer from several drawbacks, such as high costs of
phosphine‐based catalysts, difficulty in their syntheses
and lack of understanding of the exact active species. Thus,
there has been a surge in the search for cheaper, well‐
defined and discrete metal complexes that could rival these
phosphine systems.
This has seen the emergence of N^P, N^O and N^N
donor palladium catalysts as suitable replacements of the
well‐established phosphine‐based catalysts. Notable exam-
ples include, the N^P palladium complexes, reported by
Aguirre and co‐workers, which showed promising results
with respect to region‐selectivities and catalytic activi-
ties.[12] In another piece of work, Chaudhari and
co‐workers used cationic N^O palladium systems which
also display remarkable catalytic performance.[13] Based
on the promising results we have obtained for the past
decade using pyrazolyl transition metal catalysts in various
olefin transformation reactions,[14] we opted to investigate
the ability of these types of complexes in the
methoxycarbonylation of higher olefins.
Thus in this contribution, we report a modified
(pyrazolylethyl)pyridine ligand design to the one we
recently used in the methoxycarbonylation of olefins.[15]
The motivation for the incorporation of a methyl group
in the methylene bridge was to introduce chirality at the
methylene carbon for possible application in asymmetric
methoxycarbonylation catalysis. However, as we previ-
ously discussed,[16] only racemic mixtures of the ligand
were obtained. In this current work, we therefore report
the syntheses of palladium (II) complexes of these ligands,
their structural characterization and applications as
catalysts in methoxycarbonylation of higher olefins. The
dependence of catalyst performance on ligand structure,
nature of acid promoter, type of phosphine stabilizer and
olefin substrate, under different reactions conditions, have
been investigated. Density Functional Theoretical studies
and solid angle modelling have also been performed to
offer insights into the experimental trends observed.
corresponding neutral complexes 1–4 as brown solids in
low to good yields (44%–84%) as shown in Scheme 1. All
1
the compounds were characterized using H NMR, 13C
NMR, FT‐IR, mass spectroscopy, elemental analysis, and
single crystal X‐ray crystallography for L2, complexes 1, 2
and 4. As previously reported for the nickel complexes[16]
the palladium complexes (1–4) were racemic mixtures
and hence no R/S enantiomers were resolved.
1
Comparison of the H NMR and 13C NMR spectra of
the ligands and the respective palladium complexes,
allowed us to establish successful complex formation.
For instance, the methylene methyl protons were
recorded at 1.56 ppm and 1.94 ppm in ligand L1 and its
corresponding complex 1, respectively (Figure S1). The
additional signal recorded up field at 0.95 ppm in
complex 1 was assigned to the Pd‐CH3 protons and
further established the isolation of the desired compound.
Corroborative evidence was also obtained from the 13C
NMR spectra where the methylene carbon was reported
at 59.1 ppm and 58.7 ppm in L1 and 2, respectively
(Figure S2). IR spectroscopy was also used to determine
the identities of complexes 1–4. Typical IR frequencies
for C=Npz and C=Npy were reported in the range of
2162 cm−1‐2164 cm−1 and 1605 cm−1‐2112 cm−1. A
downfield shift of the C=Npz peak from 1975 cm−1 to
2164 cm−1 for ligand L1 and its corresponding complex
4 respectively (Figure S3), established the coordination
of ligand to the palladium atom.[17,18] Mass spectral data
of the complexes were also used to deduce their identities.
For example, mass spectrum of complex 4 (Figure S4),
showed an m/z signal at 503.1116 amu, corresponding
to the molecular ion (Mw = 502.73 g/mol). The loss of
the two phenyl groups on the pyrazolyl motif gave a base
peak at m/z = 348.1805 amu. Elemental analyses data of
complexes 1–4 tallied with the presence of one ligand
unit per palladium metal atom as depicted in Scheme 1,
and also confirmed the purity of the bulk materials.
2.2 | Molecular structures of compounds
L2, 1, 3 and 4
2 | RESULTS AND DISCUSSION
The structures of the palladium (II) complexes 1, 2 and 4
as well as the free ligand L2 were elucidated using single
crystal X‐ray diffraction methods. The displacement
2.1 | Syntheses and spectroscopic
characterization of palladium complexes
Ligands
2‐[1‐(3,5‐dimethylpyrazol‐1‐yl)ethyl]pyridine
(L1) and 2‐[1‐(3,5‐diphenylpyrazol‐1‐yl)ethyl]pyridine
(L2) were prepared following our recently published
procedures.[16] Reactions of synthons 2‐[1‐(3,5‐
dimethylpyrazol‐1‐yl)ethyl]pyridine (L1) and 2‐[1‐(3,5‐
diphenylpyrazol‐1‐yl)ethyl]pyridine (L2) with the [Pd
(COD)Cl2] or [Pd (COD)ClMe] metal salts produced the
SCHEME 1 Syntheses of 2‐(pyrazolyl‐ethyl)‐pyridine palladium
(II) complexes 1–4.

ZULU ET AL.
2 of 12
ellipsoid plots for compounds L2, 1, 2 and 4 are shown in
Figure 1, while Tables 1 and 2 contain data collection and
crystallographic parameters and selected bond lengths
and bond angles of the compounds respectively. Com-
pound L2 crystallized in the monoclinic space group
P21/n with a single molecule in the asymmetric unit
and Z = 4. The ligand geometry in many respects remains
unchanged when chelated to the Pd (II) metal center. The
key difference is the relative geometry of the pyridyl ring.
This rotation is illustrated by the N1–C5–N3–N2 torsion
angle which measures 110.4(2)° in the ligand. Rotation
about the C5–C6 bond is therefore required for metal
ion chelation. Complex 1 crystallizes in a mixture of two
isomers in a 3:1 ratio and showed positional disorder in
relation to the chloride and methyl ligands. The isomer
with the chloride ligand cis to the pyridyl N atom is dom-
inant, with a site occupancy of 75%. The minor isomer
has the methyl ligand cis to the pyridyl N atom (site occu-
pancy 25%). Despite complex 1 crystallizing in a mixture
of isomers (3:1), only one set of the signals is observed
in the 1H NMR spectra (Figure S1), which can be
assigned to the two being thermodynamic isomers rather
than kinetic in nature.
Density functional theory (DFT) simulations were used
to gain a deeper understanding of the solid‐state structures
and relative energies of the different conformers of the
compounds 1 and 3. Least squares fits of the experimental
solid state structures and lowest energy DFT‐simulated
structures are shown in Figure S5. The similarity of the
structures as measured by the root‐mean‐square deviations
show that the structures are in good agreement. The
relative energies of the two conformers confirmed that
the dominant form in the solid state is lower in energy
(Figure S6). Indeed, the energy difference between the
major isomer (observed) and the minor isomer of 8.82 kJ/
mol (Figure S6) is large enough to explain the observation
of one set of signals in the 1H NMR spectra of complex 2.
The three metal complexes all show nominally square
planar coordination geometries, consistent with the group
congener platinum (II) and is a consequence of the d8
electronic configuration associated vacant dx2‐y2 orbitals.
Coordination of the bidentate pyrazole‐pyridyl ligand
yields a six‐membered coordination ring.
The data in Table 2 clearly illustrate the approximately
square planar coordination geometry of the metal ion with
only small deviations from ideality imposed by the ligand
geometry in some cases. The bond lengths and angles are
in good agreement with those previously reported for com-
parable asymmetric palladium (II) complexes.[19–22] The
most significant deviation from the ideal bond angle of
90° is the N–Pd–N bond angle which averages 86.65(12)°.
Coordination of the ligand to the metal forms a six‐
membered ring which allows the bond to approach the
ideal angle compared to the more common five‐membered
rings, which shows significantly more acute angles.[20] On
the hand, the N–Pd–Cl bond angles are slightly obtuse.
This angle is not limited by ligand geometry and the larger
bond angle would minimize the non‐bonded repulsion
between the ortho phenyl C–H and methyl hydrogen
atoms. The sp3 hybridized carbon atom joining the pyridyl
and pyridine rings leads to a usual chelate geometry.
Although the four atoms coordinated to the metal ion are
co‐planar, the ligand itself is distinctly dome‐shaped as a
consequence of the sp3 hybridized carbon (Figure S7).
The absence of classical hydrogen bond donors from the
three metal complexes precludes strong hydrogen bonding
interactions (Table S1). However, the chloride ligand
enables the formation of stabilizing intermolecular
C–H···Cl interactions (Figures S8‐S11).
2.3 | Methoxycarbonlyation of higher
olefins catalysed by complexes 1–4
Complexes
1–4
were
screened
in
the
methoxycarbonylation of olefins using 1‐octene as the
FIGURE 1 Displacement ellipsoid plot (50% probability surfaces) for the Pd (II) complexes (a) 1, (b) 2, (c) 4 and ligand (d) L2. The Pd (II)
complexes all show a comparable nominally square planar coordination geometry. All data were collected at 100 K. The hydrogen atoms as
well as the atoms of the minor component of complex (1) have been displayed with arbitrary radii

3 of 12
ZULU ET AL.
TABLE 1 Crystal data and structure refinement details for the Pd (II) complexes 1, 2, 4 and ligand L2
Crystal Data
1
2
4
L2
Chemical formula
Molar mass (g mol−1
C13H18ClN3Pd
358.15
C12H15Cl2N3Pd
378.57
C22H19Cl2N3Pd
502.70
C22H19N3
325.40
)
Crystal system, space group
Temperature (K)
a, b, c (Å)
Orthorhombic, P212121
100(2)
Triclinic, P‐1
100(2)
Triclinic, P‐1
100(2)
Monoclinic, P21/n
100(2)
7.254(1), 12.410(1),
15.2824(15)
8.185(5), 8.562(5),
11.245(5)
10.736(5), 10.936(5),
12.390(5)
8.348(2), 13.852(2),
15.587(3)
α, β, γ (0)
90, 90, 90
71.195(5), 87.246(5),
69.290(5)
114.985(5), 97.083(5),
104.375(5)
90, 103.257(4), 90
V (Å3)
1375.7(2)
695.8(7)
2
1233.4
2
1754.5(5)
4
Z
4
Radiation type
Mo Kα
μ (mm−1
)
1.53
1.70
0.98
0.07
Crystal size (mm)
Data Collection
Diffractometer
0.22 × 0.09 × 0.06
0.42 × 0.08 × 0.04
0.70 × 0.31 × 0.15
0.46 × 0.25 × 0.11
Bruker APEXII CCD diffractometer
Absorption correction
Tmin, Tmax
Multi‐scan, SADABS
0.669, 0.746
0.666, 0.745
0.657, 0.745
0.958, 0.998
No. of measured, independent
20525, 3395, 3368
22403, 2701, 2634
30530, 4845, 4716
14539, 3413, 3095
and observed [I > 2σ(I)] reflections
Rint
0.028
0.024
0.23
0.021
Refinement
R[ F 2 > 2σ( F 2)], wR( F 2), S
No. of reflections
No. of parameters
H‐atom treatment
0.017, 0.041, 1.13
0.014, 0.034, 1.04
0.021, 0.049, 1.06
0.037, 0.091, 1.07
3395
178
2701
166
4845
254
3413
227
H‐atom parameters constrained
0.54, −0.51 0.41, −0.37
Δρmax, Δρmin (e Å−3
)
0.85, −0.88
0.25, −0.19
model substrate at 90 °C, 60 bar of CO, PPh3 as the stabi-
lizer and HCl as the acid promoter. The [Pd]/[PPh3]/
[HCl]/[1‐octene] ratio of 1/2/10/200 in methanol/toluene
solvent mixture was employed (Scheme 2). The results
obtained for complexes 1–4 (Figure 2) showed that the
complexes form active catalysts giving percentage
conversion of 81%–91% within 24 hr. Generally,
all the complexes afforded 100% chemoselectivity
towards esters and regioselectivity of 51%–58% for the
linear esters. Typical GC and GC–MS spectra of the
methoxycarbonylation products are shown in Figures S12
and S13 respectively.
In terms of the effect of complex structure, there was a
slight impact of ligand architecture on the catalytic
activities. For instance, complexes 2 and 4, bearing methyl
(L1) and phenyl (L2) substituents on the pyrazolyl motif
afforded conversions of 91% and 84%, respectively. Simi-
larly, complexes 1 (L1) and 3 (L2) gave conversions of
86% and 82%, indicating that increased steric hindrance
around the palladium metal resulted in diminished cata-
lytic activities. The role of the phenyl substituents may be
two‐fold; steric encumbrance and increased electron dona-
tion leading to reduced electrophilicity of the metal center.
In both cases, substrate coordination to the metal center
would be hindered.[23] The Pd‐Cl/Me groups also showed
some marginal influence in the catalytic activities,
with complex
2
bearing Pd‐Cl showing higher
conversions(91%) compared to its analogous Pd‐Me
complex 1 (86%). This has been previously reported by
other researchers[24] and may be associated with the lower
stability of the Pd‐Me complexes, and/or reactive Pd‐Cl in
the formation of the Pd‐hydride active intermediate.[25]
With respect to regioselectivity, there was no signifi-
cant variation in the composition of either branched or
linear esters with the pyrazolyl substituent. One would
expect higher composition of the branched esters for

ZULU ET AL.
4 of 12
TABLE 2 Selected bond parameters for complexes 1, 2 and 4
complexes 1–4 may explain this lack of clear dependence
of regioselectivity on the pyrazolyl substituents. From the
solid state structures of complexes 1, 2 and 4, both
the methyl and phenyl groups are facing away from the
palladium atom, thus exerting little steric influence
around the palladium center. In an effort to relate the
degree of steric crowding by the ligands around the metal
ion with the catalytic activity of the metal chelates, the
steric shielding parameter, GMT (complex) was calculated
using the program Solid G (Figure 3).[27] The data in
Figure 3 shows that the more sterically bulky ligand
(L2), bearing phenyl substituents on the pyrazol ring does
not significantly increase the steric shielding of the metal
center. For instance, using the analogous complexes 1
and 3, ligands L1 (Me) and L2 (Ph) shields 70.64% and
73.87% of the Pd metal respectively. This relatively minor
difference could thus explain the similar regioselectivities
of the all the complexes.
Bond
Length (Å)
Bond
Angle (°)
1
Pd1–Cl1
Pd1–N1
Pd1–N2
Pd1–C13
2
2.276(2)
2.131(2)
2.059(2)
2.106(5)
Cl1–Pd1–N1
N1–Pd1–N2
N2–Pd1–C13
Cl1–Pd1–C13
93.79(7)
87.11(8)
90.80(10)
88.30(10)
Pd1–Cl1
Pd1–Cl2
Pd1–N1
Pd1–N2
4
2.290(1)
2.296(1)
2.029(2)
2.022(2)
Cl1–Pd1–N1
N1–Pd1–N2
N2–Pd1–Cl2
Cl2–Pd1–Cl1
90.92(4)
86.56(6)
92.79(4)
89.68(2)
Pd1–Cl1
Pd1–Cl2
Pd1–N1
Pd1–N2
2.285(1)
2.297(1)
2.023(2)
2.035(1)
Cl1–Pd1–N1
N1–Pd1–N2
N2–Pd1–Cl2
Cl2–Pd1–Cl1
90.26(5)
86.29(7)
92.57(5)
90.76(2)
2.4 | Influence of acid promoter and
phosphine additive
Due to the superior catalytic activity of complex 2 in
comparison to the other complexes, it was used for
further investigations. First to be studied was the effect
of acid promoters using inorganic, organic and Lewis
acids (Table 3, entries 1–6). The role of the acid
promoter in the methoxycarbonylaion reaction is
believed to be the generation of the Pd‐hydride, consid-
ered as the active species.[28,29] The most reactive acid
was HCl (91%), while the organic acids, methyl sulfonic
(MSA) and para‐tolylsulfonic (p‐TsOH) were inactive.
This trend is similar to those found in literature and
relates to the strength and coordinating abilities of the
acids.[10,30] With respect to H2SO4, rapid decomposition
of the active species due to its poor stabilizing
ability, may account for its relative lower activity
compared to HCl.[31]
SCHEME 2 Methoycarbonylation of 1‐octene catalyzed by
palladium complexes 1–4.
Surprisingly, an opposite trend was reported using the
Lewis acids EtAlCl2 and AlMe3, where the less Lewis
acidic AlMe3 (80%) was found to be more active
compared to the more acidic EtAlCl2 (72%). The reason
for this observation is unclear to us at this time, though
the ability to stabilize the active palladium species by
the more basic acid promoter (better electron‐donor)
may be hypothesised. The identity of the acid promoter
also influenced product composition. Most intriguing
was the complete switch of regioselectivity towards the
branched esters for EtAlCl2 (81%) compared to the other
acids (Table 3, entries 1–6). This points to the formation
of a different active species for EtAlCl2, which appears
to promote a 2,1 insertion of the 1‐octene substrate. No
FIGURE 2 Methoxycarbonylation of 1‐octene using complexes
1–4, time, 24 hr; temp, 90 °C; pressure, 60 bar; solvent, 50 ml
toluene and 50 ml methanol; Pd/PPh3/HCl/1‐octene = 1/2/10/200
complexes 1 and 3 bearing smaller methyl substitu-
ents.[26] To the contrary, complex 3 gave slightly higher
(49%) composition of branched esters compared to the
45% obtained for complex 4, containing the bulkier
phenyl group. A closer examination of the structures of

5 of 12
ZULU ET AL.
FIGURE 3 Illustration of the degree of
steric shielding (%) of the metal ion
surface, GMT (complex), for the palladium
complexes 1–4. The blue shadow
represents screening by the chelating
pyrazol‐pyridyl ligand; the green
represents the methyl ligand and the
purple the chloride ligands
compared to the less bulkier PPh3, which gave conver-
sions of 91%. This trend may be assigned to a hindered
substrate coordination to the palladium metal when the
bulky PCy3 phosphine is used.[33] In addition, we noted
that the more electron rich P (OMe)3 group resulted in
lower percentage conversion (56%) compared to PPh3
(91%). The lower reactivity reported for P (OMe)3 can be
understood to originate from reduced electrophilicity of
the metal center, thus limiting substrate coordination.[23]
The nature of the phosphine additive was also observed
to confer some influence on the regioselectivity of the
resultant catalysts. Consistent with previous findings,[34]
changing from PPh3 to P (OMe)3 was followed by a
drastic shift in the composition of the branched esters
from 41% to 82% (Table 3, entries 1 and 9). This trend
has been associated with minimal steric hindrance to give
the bulkier branched esters.[35]
TABLE 3 The effect of acid promoter and phosphine additive in
the methoxycarbonylation of 1‐octenea
Acid
promoter
Phosphine
additive
Conv. (%)b
l/b (%)b
Entry
1
2
3
4
5
6
7
8
9
HCl
PPh3
91
59/41
51/49
19/81
56/44
33/67
36/64
18/82
‐
H2SO4
EtAlCl2
AlMe3
HCl
PPh3
70
PPh3
72
PPh3
80
ddPPe
eP (Cy)3
P (OMe)3
PPh3
16
HCl
37
HCl
56
p‐TsOH
cMSA
trace
trace
PPh3
‐
aTemp, 90 °C; solvent system, 50 ml toluene and 50 ml methanol; [Pd]/
[PR3]/[Acid]/1‐octene]; 1/2/10/200; P(CO), 60 bar; time, 24 hr;
bConversion of olefin to esters and ratio between linear and branched esters
determined by GC.
cMethanesulfonic acid (MSA),
d1,2‐bis (diphenylphosphino)ethane,
etricyclohexylphosphine.
2.5 | Role of solvent in
methoxycarbonlylation of 1‐octene
The role played by the solvent system in regulating
the catalytic behavior of the complexes in
methoxycarbonylation of 1‐octene was probed using dif-
ferent solvent systems and complex 2 (Figure 4). The best
solvent system was the toluene/methanol mixture, which
recorded 91%, while cyclohexane/methanol (19%) solvent
combination gave the lowest conversion of 19%. The
reduced conversion in cyclohexane/methanol could
be apportioned to the low solubility of complex 2 in
cyclohexane. This was supported by comparative
activities witnessed in chlorobenzene and toluene
solvents. On the other hand, the negative effect of DMSO
solvent, which is more polar and expected to display
better solubility, may be assigned to its enhanced coordi-
nation ability to the metal center. This has the potential
to block olefin coordination, consistent with the observed
diminished catalytic activities.[36–38] On the contrary, a
change in the solvent system did not significantly
influence the regioselectivity of the catalyst, affording
between 54%–60% of the linear esters across the board.
This is reasonable as there is no known role of the solvent
in controlling the steric parameters of the active catalyst.
such report is can be found in literature, and further
studies would be necessary to understand this
phenomenon.
It has been widely reported that the use of different
phosphine derivatives to stabilize the active palladium
species significantly affect the catalytic performance of
palladium complexes in methoxycarbonylation reac-
tions.[32,33] Complex 2, was thus used to probe this feature
using both chelating and monodentate phosphine groups,
offering various electronic and steric properties (Table 3,
entries 1, 7–9). From the results, it was evident that
monodentate phosphines gave higher conversions com-
pared to the chelating phosphines. For example, conver-
sions of 91% and 16% were observed for PPh3 and dppe,
respectively (Table 3, entries 1 vs 7). This may be associ-
ated with competition between the chelated phosphine
ligand and the olefin substrate for the vacant active
site.[32] With respect to steric bulk, the relatively sterically
demanding PCy3 afforded lower conversions of 37%,

ZULU ET AL.
6 of 12
catalytic activities at lower temperatures. Reduction of
the CO pressure from 40 bar, 50 bar to 60 bar led to a
gradual decline in percentage conversion of complex 2
from 91%, 77% to 48% respectively (Table 4, entries 2,
4–5). This trend is not unusual and can be attributed
to the diminished insertion of CO at lower pressures.
However, there was no appreciable change in regiose-
lectivity of complex 2 with change in both reaction tem-
perature and pressure.
In order to establish the optimum catalyst concentra-
tion of complex 2, the 1‐octene/2 ratio was varied from
100 to 400 corresponding to 1 mol% to 0.25 mol%
respectively (Table 4, entries 1, 5–6). Decreasing the
1‐octene/2 ratio from 400 to 100 (increasing the catalyst
concentration) appeared to increase the percentage
conversions from 55% to 93%, respectively. However, this
interpretation is deceptive upon critical analyses of the
TOF values obtained. For instance, TOFs of 9.1 hr−1
and 3.9 hr−1 were observed at [1‐octene/[2] ratios of 400
(0.25 mol%) and 100 (1 mol%), respectively (Table 4,
entries 5 and 6). These results thus indicate that the
increase in percentage conversions was not in the same
order of magnitude with the increase in catalyst loadings.
It is therefore conceivable, that using lower catalyst
loadings of 0.25 mol% gives the best results qualitatively,
in good agreement with reports of Zolezzi et al.[39] In
terms of regioselectivity, we observed the formation of
more branched esters (40%) at higher catalyst loadings
of 1 mol% compared to 18% of the branched esters
reported at 0.25 mol% (Table 4, entries 5 and 6). This
could be attributed to enhanced isomerization reactions
at higher catalyst loadings.[35,38]
FIGURE 4 The effect of solvent system on percentage conversion
and regioselectivity on methoxycarbonylation of 1‐octene using
complex 2; 2/PPh3/HCl/1‐octene; 1/2/10/200, temp, 90 °C; P (CO),
60 bar, time, 24 hr
2.6 | Effect of reaction conditions and
olefin chain length
The next series of studies were carried out to investigate
how changes in the reaction temperature, pressure,
reaction time, catalyst loading and olefin chain length
would affect the catalytic ability of complex
2
(Table 4). First, we lowered the reaction temperature
from 90 °C to 60 °C and observed comparable conver-
sions of 91% and 89% respectively (Table 4, entries 1
and 2). This depicts thermal stability of complex 2 at
higher temperatures, and more importantly, good
Lastly, we studied the stability of complex 2 in the
methoxycarbonylation of 1‐octene by varying the reac-
tion times from 12 hr to 36 hr using [1‐octene]/[2]
molar ratio of 200 (Table 4, entries 1, 7–9). Maximum
TOF was obtained at shorter reaction times of 12 hr
(7.7 hr−1) and 24 hr (7.6 h−1), while a significant
decline at longer reaction time of 36 hr (5.3 hr−1) was
reported. The comparable TOF values at 12 hr and
24 hr indicate appreciable catalyst stability within this
period, while a drop in TOF within 36 hr is evident of
catalyst deactivation. Significantly, we noted a concomi-
tant increase in the composition of the branched esters
from 16% to 49% with increase in reaction time from
12 hr to 36 hr (Table 4, entries 7 and 9). This occur-
rence is not common in methoxycarbonylation reac-
tions, though isomerization via 2,1 insertions may be
implicated.[26]
TABLE 4 Effect of pressure, temperature, reaction time and cat-
alyst loading on the methoxycarbonylation of 1‐octene using com-
plex 2a
Time/ [2]/
PCO/ T/ hr
Entry bar oC
[octene]
TOF/
%Convb l/bb h−1
1
2
3
4
5
6
7
8
9
60
60
40
50
60
60
60
60
60
90 24
60 24
90 24
90 24
90 24
90 24
90 12
90 32
90 36
1/200
1/200
1/200
1/200
1/100
1/400
1/200
1/200
1/200
91
89
48
77
93
55
46
94
99
58/42 7.6
59/41 7.4
55/45 4.1
57/43 6.4
60/40 3.9
82/18 9.1
74/16 7.7
55/45 5.9
51/49 5.5
Further investigations to establish the influence of
olefin chain length on the catalytic performance of com-
plex 2 were carried out using styrene, 1‐octene, 1‐nonene,
and 1‐decene (Figure 5). From Figure 5, the catalytic
aTemp, 90 °C; solvent system, 50 ml toluene and 50 ml methanol; [1‐octene]/
[HCl]/[PPh3]/[Pd], 200/10/2/1; P(CO), 60 bar; time, 24 hr;
bDetermined by GC.

7 of 12
ZULU ET AL.
FIGURE
5
The effect of olefin substrate on percentage
SCHEME 3 Proposed activation and stabilization pathways of
complex 2 in the presence of HCl and triphenylphosphine, PPh3.
conversion and regio‐selectivity towards linear esters using
catalyst 2. Reaction conditions: temperature, 90 °C; 2/PPh3/HCl/
olefin, 1/2/10/200; Pressure (CO), 60 bar; time, 24 hr
activity of 2 was marginally influenced by the nature of
the substrate. As an illustration, on changing from
1‐hexene to 1‐decene, a slight drop in percentage
conversion from 93% to 78% was observed. This, some-
how, contradicts our previous reports, where a significant
decline in catalytic activity from 90% to 20% was observed
for 1‐hexene and 1‐decene, respectively[15,34,40] and
underscores the superiority of the current systems. The
higher reactivity of styrene substrate is normal and can
be linked to the generally more reactive benzylic
palladium (II) intermediate in comparison to the alkyl
palladium (II) species.[41] The identity of the substrate
was found to marginally influence the regioselectivity of
the ester products. For example, 41% and 48% of the
branched esters were obtained for 1‐hexene and 1‐decene,
respectively, consistent with increase in internal isomers
with olefin chain length.[26] More intriguing was the
drastic shift to 78% of the branched esters for styrene
substrate, indicating that a 2,1 insertion mode was
the preferred mode of styrene coordination to the
metal atom.
the CAM‐B3LYP hybrid exchange‐correlation functional
using the Coulomb‐attenuating method. The relative
energies of the complex [Pd(L2)Cl2] (2) and the
intermediates were calculated as shown in Figure 6. The
intermediate zerovalent complexes are significantly
different in structure as the Pd(0) atom has a d10 electron
configuration and therefore a strong preference for the
formation of linear palladium complexes. This is in con-
trast to the square planar coordination geometry of the
d8 Pd (II) ion. This difference in electron configuration
provides a likely explanation for why the [Pd (PPh3)2]
complex is the lowest energy zerovalent complex. The soft
metal ion will favor coordination to the soft P‐donor
ligands as opposed to the harder N‐donor ligand. The
bidentate nature of the N‐donor ligand will also inhibit
the formation of a linear complex and is hence higher
in energy, in contrast to the monodentate P‐donor
ligands.
Compound 2b proposes the coordination of two PPh3
ligands and the bidentate N‐donor ligand to the Pd(0)
metal center to form a four‐coordinate complex. Numer-
ous attempts were made to simulate a plausible struc-
ture for a compound of this structure. These were all
found to be unstable and reverted to the stable [Pd
(PPh3)2] species and free ligand. This result is in agree-
ment with literature, which shows that this is an unsta-
ble coordination geometry for Pd(0). A search of the
Cambridge Structural Database (CSD) shows no
reported structures of a Pd(0) complex with two
triphenylphosphine ligands in a cis configuration and a
bidentate N‐donor ligand.[42] This alludes to the instabil-
ity of complexes of this nature. There is a single exam-
ple of a related complex with cis‐PPh3 ligands and cis‐
2.7 | Determination of the role of PPh3 by
DFT studies
In order to establish the role played by the PPh3 additive
in the generation and stabilization of the active species in
these methoxycarbonylation reactions, we simulated
pre‐catalyst 2, and its various adducts [Pd(0)(L1)] (2A),
[Pd(0)(L1)(PPh3)2] (2B), [Pd(II)(L1)HCl] (2C) and [Pd(0)
(L1)(PPh3)H] (2D). In addition, complex [Pd(0)(PPh3)2]
was computed to determine the possible displacement of
ligand L2 from the metal coordination sphere upon
addition of excess PPh3 ligand (Scheme 3). The
simulations were performed with Gaussian 09 W using
monodentate
N‐donor
ligands,[43]
where
the
monodentate nature of the coordinated ligands in this
case makes this scenario possible.

ZULU ET AL.
8 of 12
FIGURE 6 DFT simulation of the energies of the proposed intermediates for the activation and stabilization of the palladium complex (2)
in the presence of PPh3 and HCl
Thus, from the DFT calculated energies (Figure 6), it
is clear that complex [Pd(0)(L1)(PPh3)2] (2B) is energeti-
cally unfavorable, hence unlikely to be the active spe-
cies. While the [Pd(0)(PPh3)2], is very stable, this is
unlikely to be the active species due to the strong Pd‐
N bonds, thus the (pyrazolylmethyl)pyridine ligand
may be not easily displaced by the PPh3 ligand. Addi-
tionally, the observation of the dependence of catalytic
performance on complex structures/ligand motif, sug-
gests that the active species contains the ligands, consis-
tent with our recent NMR studies.[34] Thus from the
DFT calculations, the mono‐ligated PPh3 compound
[Pd(0)(L1)(PPh3)Cl] (2D), may explain the role of the
PPh3 in stabilizing the active Pd hydride species. Even
though the hydride compound [Pd(0)(L1)HCl] (2C),
appears more stable, the addition of excess PPh3 is
likely to displace the Cl ligand to form 2D. Indeed,
the dependence of catalytic activities on the nature of
the phosphine groups further supports the presence of
2D as the active intermediate, consistent with the DFT
results.
steric environment around the metal atom in the com-
plexes, consistent with the comparable regioselectivity
towards linear esters. The nature of the acid promoter,
phosphine additive and reaction conditions controlled
the catalytic performance of the complexes. While
increase in olefin chain length did not significantly alter
the catalytic activity, it did influence product distribution.
DFT studies show the possible formation of mono‐
phosphine stabilized palladium hydride species as the
active intermediate.
4 | EXPERIMENTAL SECTION
4.1 | General materials and
instrumentation
All manipulations were carried out under a nitrogen
atmosphere using standard Schlenk techniques. The sol-
vents used; absolute ethanol (≥96%), diethylether
(≥99%) and toluene (≥99.8%) were of analytical grade
and were dried over sodium and distilled prior to use.
While dichloromethane (≥99.8%) was dried and distilled
over P2O5 respectively. Reagents, 3,5‐dimethylpyrazole
(99% purity), 1,5‐cyclooctadiene (≥99%), 1,3‐diphenyl‐
1,3‐propanedione (98%), tetrabutylammonium bromide
(≥99%), sodium hydroxide (≥98%), sodium borohydride
(≥99%), 2‐acetylpyridine (≥99%), palladium (II) chloride
(≥99%) and thionyl chloride (≥99%) were obtained from
Sigma Aldrich and used as received. Ligands 2‐[1‐(3,5‐
dimethylpyrazol‐1‐yl)ethyl]pyridine (L1) and 2‐[1‐(3,5‐
diphenylpyrazol‐1‐yl)ethyl]pyridine (L2) were prepared
following our recently published procedures.[16] NMR
spectra were recorded on a Bruker Ultrashield 400
3 | CONCLUSIONS
In summary, four palladium (II) complexes supported by
(pyrazolyethyl)pyridine ligands have been synthesized
and structurally characterized. The complexes are mono-
metallic in which the coordination sphere consists one
bidentate pyrazolyl ligand and two auxiliary
chloride/methyl ligands to give distorted square planar
geometry. The complexes form active catalysts in the
methoxycarbonylation of higher olefins to afford mainly
linear esters. Solid angle modelling established similar

9 of 12
ZULU ET AL.
instrument (1H NMR 400 and 500 MHz, 13C NMR
100 MHz) in DMSO‐d6 and CDCl3 solutions at room
temperature. Chemical shifts are reported in δ (ppm)
and referenced to the residual CHCl3 in CDCl3. Cou-
pling constants are measured in Hertz (Hz). Elemental
analyses were performed using CHNS‐O Flash 2000
Thermoscientific analyser. Mass spectra were recorded
on an LC Premier micro‐mass spectrometer while infra-
red spectra were obtained using Spectrum 100 FT‐IR
spectrometer.
NMR (DMSO, δ, ppm): 2.45 (3H, s, pz‐CH3); 2.44 (3H,
3
s, pz‐CH3); 2.94 (d, 3H, JHH = 8.75, ‐CH‐CH3); 5.51 (s,
3
H, pz‐CH3); 6.17 (q, 1H, JHH = 7.90, CH3‐C‐H); 7.61
3
3
(dd, 1H, JHH = 7.4, 5‐py‐H); 8.12 (td, 1H, JHH = 7.74,
3
4‐py‐H); 7.92 (d, 1H, JHH = 7.48, 3‐py‐H); 8.88 (d, 1H,
3JHH = 7.94, 6‐py‐H). 13C NMR (DMSO, δ, ppm): 11.7
(pz‐CH3), 15.24 (pz‐CH3), 23.2 (CH‐CH3), 58.2 (CH3‐
CH), 108.2 (3‐pz‐C), 125.8 (5‐py‐C), 126.1 (3‐py‐C),
141.6 (4‐py‐C), 143.3 (2‐pz‐C), 152.1 (6‐py‐C), 154.16
(4‐pz‐C), 156.1 (2‐py‐C). MS (ESI): m/z (%) = 401.98
([M + Na]+, 100%). FT‐IR: νC=N(pz) = 2164 cm−1
,
νC=N(py) = 2112 cm−1. Anal. Calcd for C12H15Cl2N3Pd:
C, 38.07; H, 3.99; N, 11.10. Found: C, 38.15; H, 3.97;
N, 10.87.
4.2 | Syntheses of pyrazolyl palladium (I)
complexes 1–4
4.2.1 | [{2‐(3,5‐dimethylpyrazol‐1‐yl)
ethylpyridine}PdClMe] (1)
4.2.3 | [{2‐(3,5‐diphenylpyrazol‐1‐yl)
ethylpyridine}PdClMe] (3)
To a solution of Pd (COD)MeCl (0.10 g, 4.96 mmol)
in diethylether (15 ml) was added a solution of 2‐[1‐
(3,5 ;dimethylpyrazol‐1‐yl)ethyl]pyridine, L1, (0.13 g,
4.96 mmol) in diethyl ether (10 ml) to form a light yel-
low precipitate. The resultant mixture was stirred for
24 hr and filtered to give a light yellow solid. Recrystal-
lization of the crude product from a mixture of CH2Cl2:
hexane (2:1) gave single crystals suitable for X‐ray anal-
ysis. Yield: 0.65 g (58%).1HNMR (CDC13, δ, ppm): 0.95
(s, 3H, Pd‐CH3), 2.34 (s, 3H, pz‐CH3); 2.68 (s, 3H,
Complex 3 was prepared following the procedure
described for
1 using [Pd (COD)MeCl] (0.10 g,
4.96 mmol) and L2 (0.14 g, 4.96 mmol). Recrystalliza-
tion of the crude product by slow diffusion of hexane
into a CH2Cl2 solution gave 3 as an analytically pure
compound. Yield: 0.18 g (53%).1H‐NMR (CDC13,δ,
3
ppm): 2.20 (s, 3H, Pd‐CH3); 2.99 (d, 3H, JHH
8.8,
=
3
CH‐CH3); 5.77 (q, 1H, JHH
6.6, CH3‐CH); 6.54 (s,
1H, pz‐CH); 7.46 (dd, 1H, JHH = 7.8, 5‐py‐CH), 7.13
(d, 1H, JHH = 7.7, 3‐py‐CH); 7.46 (m, 2H, Ph); 7.77
=
3
3
3
pz‐CH3); 1.94 (d, 3H, JHH = 8.9, ‐CH‐CH3); 5.67 (q,
3
3
1H, JHH = 6.7, CH3‐C‐H); 6.92 (s, 1H, pz‐H); 7.29 (d,
(m, 4H, Ph), 7.79 (td, 1H, JHH = 7.7, 4‐py‐CH); 8.27
1H, JHH = 7.8, 3‐py‐H); 7.32 (dd, 1H, JHH = 7.8, 5‐
(m, 4H, Ph); 9.25 (d, 1H, JHH = 7.6, 6‐py‐CH), 13C
3
3
3
3
py‐H); 7.79 (td, 1H, JHH
7.9, 4‐py‐H); 8.52 (d, 1H,
NMR (CDC13, δ, ppm): 13.7 (Pd‐CH3), 23.2 (CH‐CH3),
59.7 (CH3‐CH), 107.5 (3‐pz‐CH), 122.9 (5‐py‐CH), 124.9
(3‐py‐CH), 128.2 (2‐Ph‐CH), 128.42 (2‐Ph‐C), 128.73
(6‐ph‐C), 128.86 (6‐Ph‐CH), 129.07 (3‐Ph‐CH), 129.12
(3‐Ph‐CH), 129.30 (5‐Ph‐CH), 129.38 (5‐Ph‐CH), 129.64
(4‐Ph‐CH), 130.1 (4‐ph‐C), 134.1 (1‐Ph‐C), 135.1 (4‐py‐
CH), 146.2 (6‐py‐C‐N), 155.2 (4‐pz‐C), 156.3 (6‐py‐
C=N). MS (ESI): m/z (%) =483 (M+, 100%). FT‐IR:
νC=N(pz) = 2162 cm−1, νC=N(py) = 1606 cm−1. Anal.
Calcd for C23H22ClN3Pd: C, 57.28; H, 4.60; N, 8.71.
Found: C, 57.55; H, 4.43; N, 8.80.
=
3JHH = 8.0, 6‐py‐H). 13C‐NMR (CDC13, δ, ppm): δ 11.9
(pz‐CH3), 15.1 (pz‐CH3), 23.0 (Pd‐CH3), 25.2 (CH‐CH3),
58.7 (CH3‐CH), 107.40 (4‐pz‐C), 122.82 (5‐py‐C), 124.7
(3‐py‐C), 138.4 (4‐py‐C), 140.2 (2‐pz‐C), 151.2 (6‐py‐C),
152.2 (5‐pz‐C), 155.5 (2‐py‐C). MS (ESI): m/
z
(%)
=
308.05 [M
+
Na]+, 100%). FT‐IR:
νC=N(pz) = 2162 cm−1, νC=N(py) = 1980 cm−1. Anal.
Calcd for C13H18ClN3Pd: C, 43.59; H, 5.07; N, 11.73.
Found: C, 43.76; H, 5.28; N, 11.44.
4.2.2 | [{2‐(3,5‐dimethylpyrazol‐1‐yl)
ethylpyridine}PdCl2] (2)
4.2.4 | [{2‐(3,5‐Diphenylpyrazol‐1‐yl)
ethylpyridine}PdCl2] (4)
To a solution of [Pd (COD)Cl2] (0.10 g, 4.96 mmol) in
dichloromethane (10 ml) was added a solution of L1
(0.13 g, 4.96 mmol) in dichloromethane (10 ml) and
the resultant orange solution was stirred for 24 hr. After
the reaction period, the solution was concentrated to
about 5 ml and layered with hexane (5 ml). The mixture
was then kept at 4 °C to afford yellow single crystals
suitable for X‐ray analysis. Yield: 0.55 g (48%). 1H
Complex 4 was prepared following the procedure
described for 2 using [Pd (COD)Cl2] (0.10 g, 4.96 mmol)
and L2 (0.14 g, 4.96 mmol). Recrystallization from
hexane/CH2Cl2 solvent system at 4 °C produced orange
single crystals suitable for X‐ray analysis. Yield: 0.15 g
3
(44%).1H NMR (DMSO, δ, ppm): 3.29 (d, 3H, JHH =8.6,
3
CH‐CH3); 5.73 (q, 1H, JHH
6.4, CH3‐CH); 6.56
=

ZULU ET AL.
10 of 12
(s, H, pz‐CH); 7.27 (d, 1H, 3JHH = 7.6, 3‐py‐CH); 7.43 (dd,
1H, JHH = 7.8, 5‐py‐CH); 7.48 (m, 2H, Ph), 7.64 (td, 1H,
LanL2DZ (which makes use of effective core potentials)
basis set for the palladium (II) ion.[53–55] The X‐ray
coordinates of the metal chelates 1, 2 and 4 were used
for input structures. Normal geometry convergence
criteria were applied and no symmetry constraints
imposed. The input files were prepared using GaussView
5.0; the same program was used to analyze the output
files. Structural overlays were performed using
Mercury 3.10.[42]
3
3JHH
7.7, 4‐py‐CH), 7.62 (m, 4H, PH), 8.27 (m, 4H,
=
Ph); 9.35 (d, H, JHH = 7.6, 6‐py‐CH). 13C NMR (DMSO,
δ, ppm): 20.2 (CH‐CH3), 52.2 (CH3‐CH), 108.1 (3‐pz‐
CH), 124.1 (5‐py‐CH), 125.7 (3‐py‐CH), 127.6 (2‐Ph‐CH),
128.3 (2‐Ph‐CH), 128.4 (Ph‐CH), 130.61 (Ph‐CH), 130.9
(Ph‐CH), 134.0 (4‐py‐CH), 144.9 (2‐pz‐C‐CH3), 146.9
(6‐py‐CH), 154.79 (4‐pz‐CH), 155.3 (2‐py‐CN). MS (ESI):
3
m/z (%) =503 (M+, 10%). FT‐IR: νC=N(pz) = 2164 cm−1
,
νC=N(py) = 1605 cm−1. Anal. Calcd for C22H19Cl2N3Pd:
C, 52.56; H, 3.81; N, 8.36. Found: C, 52.38; H, 3.99;
N, 8.19.
4.5 | Methoxycarbonylation catalysis
procedure
The methoxycarbonylation experiments using complexes
1–4 as catalysts were performed in a high pressure Parr
reactor equipped with a temperature control unit,
cooling system, internal stirrer and a sampling valve.
In a typical experiment, complex 2 (0.012 g, 0.06 mmol),
PPh3 (0.03 g, 0.12 mmol), HCl (0.02 mL, 0.60 mmol)
and 1‐octene (2.00 mL, 12.00 mmol) corresponding to
[Pd]/[PPh3]/[HCl]/[1‐octene] ratio of 1:2:10:200 were
dissolved in a mixture of methanol (50 ml) and toluene
(50 ml). The reactor was then evacuated and the cata-
lytic solution was introduced to the reactor via a can-
nula. The reactor was purged three times with CO,
then heated at the required temperature. The desired
pressure was then set and the reaction stirred at
500 rpm for the entire duration of the experiment. At
the end of the reaction time, the reaction was cooled,
excess CO vented off and the samples drawn for GC
analysis to determine the percentage conversion of the
alkene substrate to esters. A typical sample for GC anal-
yses was prepared by passing it through a microfilter
into a GC vial. GC–MS analyses were run under the fol-
lowing standard chromatography conditions: −25 m
CPSil 19 capillary column, 1.2 mm film thickness,
Helium carrier column gas 5 psi, injector temperature
250 °C, oven program 50 °C for 4 mins rising to
200 °C at 20 °C/min and holding at 200 °C. The
percentage conversions were determined by comparing
the peak areas of the reactants and total products,
assuming 100% mass balance. The identities of the ester
products were assigned using standard authentic sam-
ples and mass spectral data.
4.3 | X‐ray crystallography
The Pd (II) complexes were all isolated as yellow
needle‐like crystals. X‐ray data were recorded on a
Bruker Apex Duo equipped with an Oxford Instruments
Cryojet operating at 100(2)
K and an Incoatec
microsource operating at 30 W power. Crystal and struc-
ture refinement data are given in Table 1. The data
were collected with Mo Kα (λ = 0.71073 Å) radiation
at a crystal‐to‐detector distance of 50 mm. Data were
collected using omega and phi scans with exposures
taken at 30 W X‐ray power and 0.50° frame widths
using APEX2.[44] The data were reduced with the
program SAINT[44] using outlier rejection, scan speed
scaling, as well as standard Lorentz and polarization
correction factors. A SADABS semi‐empirical multi‐scan
absorption correction[44] was applied to the data. Direct
methods, SHELX‐2016[45] and WinGX,[46] were used to
solve the data. All non‐hydrogen atoms were located
in the difference density map and refined anisotropically
with SHELX‐2016.[45] All hydrogen atoms were included
as idealized contributors in the least squares process,
with C‐Haromatic distances of 0.93 Å and Uiso = 1.2
Ueq, C‐Hmethine distances of 1.00 Å and Uiso = 1.2
Ueq and C–Hmethyl distances of 0.98 Å and Uiso = 1.5
Ueq. Platon SQUEEZE[47] was used to remove disor-
dered solvent from the lattice of complex 4. This process
left solvent accessible voids of 205.57 Å3; 16.7% of the
unit cell volume.
4.4 | Density functional theory
calculations
SUPPLEMENTARY INFORMATION
Density functional theory (DFT) simulations were per-
formed using Gaussian 09W[48] using the PBE hybrid
functional.[49,50] A split basis set was applied in the simu-
lations, this split basis set specified the 6‐311G level of
theory[51,52] for the C, H, Cl and N atoms and the
Supplementary materials contain additional NMR and IR
spectroscopic spectral data, mass spectra of the palladium
complexes and X‐ray crystallography files. The CCDC
data entries for the structures are CCDC: 1884240,

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1884241, 1884242 and 1884243 for compounds L2, 1, 2
and 4, respectively.
[22] K. J. Kilpin, J. D. Crowley, Polyhedron 2010, 29, 3111.
[23] L. K. Johnson, C. M. Killian, M. Brookhart, J. Am. Chem. Soc.
1995, 117, 6414.
ACKNOWLEDGMENTS
[24] K. Kumar, J. Darkwa, Dalton Trans. 2015, 44, 20714.
The authors are grateful to the NRF‐DST (South Africa)
Centre of Excellence in Catalysis (c*change, OLE‐10.3‐
UKZN,) National Research Foundation ‐ South Africa
(CPRR‐98938) and the University of KwaZulu‐Natal for
financial support.
[25] V. Goudy, A. Jaoul, M. Cordier, C. Clavaguéra, G. Nocton,
J. Am. Chem. Soc. 2017, 139, 10633.
[26] A. V. Smarun, W. Shahreel, S. Pramono, S. Y. Koo, L. Y. Tan,
R. Ganguly, D. Vidović, J. Organomet. Chem. 2017, 834, 1.
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[28] C. Zúñiga, S. A. Moya, P. J. C. L. Aguirre, Catal. Lett. 2009, 130,
ORCID
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