Sterically hindered (pyridyl)benzamidine palladium(II) complexes: Syntheses, structural studies, and applications as catalysts in the methoxycarbonylation of olefins

Article abstract of DOI:10.1002/aoc.6439

Reactions of ligands (E)-N′-(2,6-diisopropylphenyl)-N-(4-methylpyridin-2-yl)benzimidamide (L1), (E)-N′-(2,6-diisopropylphenyl)-N-(6-methylpyridin-2-yl)benzimidamide (L2), (E)-N′-(2,6-dimethylphenyl)-N-(6-methylpyridin-2-yl)benzimidamide (L3), (E)-N′-(2,6-dimethylphenyl)-N-(4-methylpyridin-2-yl)benzimidamide (L4), and (E)-N-(6-methylpyridin-2-yl)-N′-phenylbenzimidamide (L5) with [Pd(NCMe)₂Cl₂] furnished the corresponding palladium(II) precatalysts (Pd1–Pd5), in good yields. Molecular structures of Pd2 and Pd3 revealed that the ligands coordinate in a N^N bidentate mode to afford square planar compounds. Activation of the palladium(II) complexes with para-tolyl sulfonic acid (PTSA) afforded active catalysts in the methoxycarbonylation of a number of alkene. The resultant catalytic activities were controlled by the both the complex structure and alkene substrate. While aliphatic substrates favored the formation of linear esters (>70%), styrene substrate resulted in the formation of predominantly branched esters of up to 91%.

Full text of DOI:10.1002/aoc.6439

Received: 15 June 2021
DOI: 10.1002/aoc.6439
Revised: 6 August 2021
Accepted: 25 August 2021
R E S E A R C H A R T I C L E
Sterically hindered (pyridyl)benzamidine palladium(II)
complexes: Syntheses, structural studies, and applications
as catalysts in the methoxycarbonylation of olefins
Saphan O. Akiri
| Stephen O. Ojwach
School of Chemistry and Physics,
University of KwaZulu-Natal,
Pietermaritzburg, South Africa
Abstract
Reactions of ligands (E)-N0-(2,6-diisopropylphenyl)-N-(4-methylpyridin-2-yl)
benzimidamide (L1), (E)-N0-(2,6-diisopropylphenyl)-N-(6-methylpyridin-2-yl)
benzimidamide (L2), (E)-N0-(2,6-dimethylphenyl)-N-(6-methylpyridin-2-yl)
benzimidamide (L3), (E)-N0-(2,6-dimethylphenyl)-N-(4-methylpyridin-2-yl)
benzimidamide (L4), and (E)-N-(6-methylpyridin-2-yl)-N0-phenylbenzimidamide
(L5) with [Pd(NCMe)2Cl2] furnished the corresponding palladium(II)
precatalysts (Pd1–Pd5), in good yields. Molecular structures of Pd2 and Pd3
revealed that the ligands coordinate in a N^N bidentate mode to afford square
planar compounds. Activation of the palladium(II) complexes with para-tolyl
sulfonic acid (PTSA) afforded active catalysts in the methoxycarbonylation of a
number of alkene. The resultant catalytic activities were controlled by the both
the complex structure and alkene substrate. While aliphatic substrates favored
the formation of linear esters (>70%), styrene substrate resulted in the formation
of predominantly branched esters of up to 91%.
Correspondence
Stephen O. Ojwach, School of Chemistry
and Physics, University of KwaZulu-
Natal, Private Bag X01, Scottsville,
Pietermaritzburg 3209, South Africa.
Email: ojwach@ukzn.ac.za
Funding information
University of KwaZulu-Natal; DST-NRF
(South Africa) Centre of Excellence in
Catalysis
K E Y W O R D S
(pyridyl)benzamidine, alkenes, methoxycarbonylation, palladium(II), structures
1 | INTRODUCTION
catalytic process has employed mostly in situ generated
active catalysts from the reactions of various of
Value addition of olefins via transition metal catalysts con-
stitute some of the most versatile and industrially relevant
processes.[1–3] Some of these reactions include but not lim-
ited to carbonylation,[4] polymerization,[5–7] oxidation,[8]
Suzuki cross-coupling reactions,[9] hydrogenation,[10,11]
and metathesis.[12] Of the carbonylation reactions,
methoxycarbonylation of olefins has continued to gain
much traction as various industrial and domestic products
such as food additives, pesticides, fragrances, and solvents
can be accessed through this protocol.[13,14]
palladium(II) metal salts and the phosphine-based
ligands.[16–20] However, despite the success of these
phosphine-based palladium(II) catalysts, they are not
without any blemish. Key among the limitations is the
relatively high costs of their establishment, in addition to
their isolation and handling. Besides, insufficient knowl-
edge on the true nature of the active species and mecha-
nistic insights of the in situ catalysts limits rationale
catalyst design and development.
Pursuant to these limitations, significant efforts are
being channeled toward the invention of alternative donor
ligands, which are cheaper and amenable to synthetic
manipulations such as N^N,[21–27] N^P,[28,29] and
N^O[30,31] among others.[32] In addition, the design,
Traditionally, metal catalyzed methoxycarbonylation
reactions have been driven by phosphine-based
palladium(II)-based catalysts, as they exhibit superior
catalytic activity as well as selectivity.[13,15] Similarly, the
Appl Organomet Chem. 2021;e6439.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6439

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AKIRI AND OJWACH
isolation, and structural characterization of discrete
palladium(II) catalysts is attractive as it allows for better
understanding of the catalytic process in terms of kinetics,
mechanistic, and deactivation profiles. All these have the
benefit of the discovery of improved catalysts that can
effectively be used in methoxycarbonylation catalysis. In
our latest account, we delineate the synthesis and
structural characterization of palladium(II) complexes
anchored on structurally rigid (pyridyl)benzamidine ligand
networks. Moreover, methoxycarbonylation of various
alkenes using these sets of complexes are expounded.
Detailed understanding of the contribution of the phos-
phine additives and acid promoters in active species gener-
ation and the influence of the various alkene substrates
have been undertaken and are presented herein.
in moderate yields (69%–73%) (Scheme 1). Reactions of
(pyridyl)benzamidine synthons with [Pd(NCMe)2Cl2]
precursor afforded the corresponding palladium(II)
complexes (Pd1–Pd5) in high yields of 76%–87%
(Scheme 1).
1
The H NMR, 13C NMR, FT-IR spectroscopies, ele-
mental analyses, and mass spectrometry were employed
in characterizing the synthesized compounds and single
crystal analyses for compounds Pd2 and Pd3. 1H
NMR characterization tool was instrumental in inferring
the successful synthesis of both the ligands and
their respective palladium coordination compounds
(Figures S1–S10). For instance, the methyl-pyridyl pro-
tons in ligand L2 and Pd2 were recorded at 2.50 to
3.51 ppm, respectively (Figures S2 and S7). A more out-
standing feature was observed in the isopropyl proton sig-
nals in ligands L1 and L2. Whereas two sets of signals of
the CH protons were observed in ligands L1 and L2, only
one set of signals were recorded in their respective
palladium(II) complexes, Pd1 and Pd2. The two set of
signals in ligands L1 and L2 are accredited to the exis-
tence of isomeric E anti and E syn mixtures, due the
unhindered rotation in the ligands. Upon complexation,
this rotation is hindered due to the rigidity imposed by
2 | RESULTS AND DISCUSSION
2.1 | Synthesis of (pyridyl)benzamidine
ligands and their palladium(II) complexes
The (pyridyl)benzamidine ligands (L1–L5) were
prepared according to modified literature protocols[33,34]
SCHEME 1 Synthesis of pyridyl benzamidine ligands and their palladium(II) complexes

AKIRI AND OJWACH
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the chelating effect of the ligand in the complex coordi-
nation sphere.[35–37] 13C NMR spectra was also beneficial
in the structural elucidation of the formed ligands and
complexes (Figures S11–S20). To demonstrate, the
methyl-pyridine carbons in ligand L2 and its complex
corresponding Pd2 were observed at 24.0 and 28.1 ppm,
respectively (Figures S12 and S17).
2.2 | Molecular structures of complexes
Pd2 and Pd3
Single crystals of compounds Pd2 and Pd3 appropriate
for X-ray screening were grown through slow diffusion
of hexane solvent into corresponding concentrated ace-
tonitrile solutions at ambient temperature. Table 1 rep-
resents the crystallographic data and structural
refinement parameters, whereas the Pd2 and Pd3
molecular structures are given in Figures 1 and 2,
respectively. Complex Pd3 crystallized with one acetoni-
trile solvent molecule in its lattice. Both complexes Pd2
and Pd3 exhibited six-membered chelate rings, con-
taining one N^N bidentate ligand and two chloride
ligands to complete a four coordination environment
(Figures 1 and 2).
The average Pd–Cl bond lengths for complexes Pd2
and Pd3 of 2.29705 0.0025 and 2.30765 0.020 Å,
respectively, are comparable. The Pd–Nimine and Pd–Npy
bond lengths are 2.0426 and 2.0383 Å, respectively, for
complex Pd2; values comparable with 2.0200 and
2.0353 Å observed for complex Pd3, pointing to similar
electronic properties of the complexes. The average
In the IR spectral data, the ν(C═N) imine bond was
employed to confer both the formation and successful
coordination of the ligands to the palladium center
(Table S1 and Figures S21–S30). As a demonstration, the
ν(C═N) signals for L1 and the corresponding complex
Pd1 were was displayed at 1618 and 1602 cmꢀ1, respec-
tively (Figures S21 and S26). Besides, the ν(N–H)
stretching was observed in every ligand and
palladium(II) complex, indicating the absence of
deprotonation. Mass spectrometry was applied in sub-
stantiating the molecular masses of all the synthesized
compounds (Table S1). In general, all the compounds
showed m/z signals that relate to their individual molec-
ular ions (Figures S31–S40). For instance, in the mass
spectra of ligand L1 and complex Pd1, m/z peaks at
372.29 and 548.12 amu were recorded, corresponding to
their molar masses of 371.24 and 547.08 g molꢀ1
respectively (Figures S31 and S35).
,
Pd–Cl, Pd–Nimine
, and Pd–Npy of 2.30232 0.015,
2.0313 0.012, and 2.0368 0.009 Å for complexes Pd2
TABLE 1 Structure refinement and
Parameter
Pd2
Pd3
crystallographic data summary for
Empirical formula
C25 H29 Cl2 N3 Pd
548.81
C23 H24 Cl2 N4 Pd
533.76
complexes Pd2 and Pd3
Formula weight
Temperature
Wavelength
100.02 K
0.71073 Å
Monoclinic
P 21/n
100(2) K
0.71073 Å
Monoclinic
C 2/c
Crystal system
Space group
Unit cell dimensions
a (Å)
11.9925(6)
13.1943(6)
16.0911(8)
90ꢁ
21.0638(14) Å
16.2935(10) Å
13.3045(9) Å
90ꢁ
b (Å)
c (Å)
α (ꢁ)
β (ꢁ)
107.868(3)ꢁ
90ꢁ
105.638(3)ꢁ.
90ꢁ
γ (ꢁ)
Volume
2423.3(2) Å3
4397.1(5) Å3
Z
4
8
Density (calculated)
Absorption coefficient
F(000)
1.504 Mg/m3
1.004 mmꢀ1
1120
1.613 Mg/m3
1.105 mmꢀ1
2160
Crystal size
0.240 ꢂ 0.140 ꢂ 0.120 mm3 0.232 ꢂ 0.178 ꢂ 0.173 mm3
Theta range for data
collection
1.870 to 26.725ꢁ
1.603 to 26.779ꢁ

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AKIRI AND OJWACH
FIGURE 1 Molecular structure of Pd2
drawn at 50% probability ellipsoids. Selected
bond lengths (Å): Pd(1)–N(1), 2.0383 (16);
Pd(1)–N(2), 2.0426(16); Pd(1)–Cl(2), 2.2945(5);
Pd(1)–Cl(1), 2.2996(5); N(2)–C(5), 1.399(2); N
(2)–C(6), 1.372(2). Selected bond angles (ꢁ): N
(1)–Pd(1)–N(3), 87.74(6); N(1)–Pd(1)–Cl(2),
176.1 0(5); N(3)–Pd(1)–Cl(2), 92.81(5); N(1)–Pd
(1)–Cl(1), 92.37(5); N(3)–Pd(1)–Cl(1), 169.59(5);
Cl(2)–Pd(1)–Cl(1), 87.781(18)
FIGURE 2 Molecular structure of Pd3 with
atom numbering scheme. The displacement
ellipsoids of atoms are shown at the 50%
probability level: Selected bond lengths (Å)
Pd(1)–N(1), 2.0200(14); Pd(1)–N(3), 2.0353(14);
Pd(1)–Cl(2), 2.2878(5); Pd(1)–Cl(1), 2.3275(4); N
(2)–C(7), 1.375(2); N(2)–C(6), 1.401(2). Selected
bond angles (ꢁ): N(1)–Pd(1)–N(3), 86.32(6); N
(1)–Pd(1)–Cl(2), 179.24(4); N(3)–Pd(1)–Cl(2),
93.01(4); N(1)–Pd(1)–Cl(1), 91.72(4); N(3)–Pd
(1)–Cl(1), 169.36(4); Cl(2)–Pd(1)–Cl(1), 88.878
(16)
and Pd3 are statistically similar to the average lengths
of 2.2993 0.012 and 2.021 0.018 Å reported for
related palladium complexes.[36–38] Complex Pd3
exhibits bite angles for N(1)–Pd(1)–N(3) of 86.32(6) and
N(3)–Pd(1)–Cl(2) of 93.01(4), whereas complex Pd2 has
bite angles for N(1)–Pd(1)–N(3) of 87.74(6) and Cl(2)–Pd
(1)–Cl(1) of 92.81(5), confirming slightly distorted square
planar geometries in both compounds. A closer exami-
nation of the bite angles reveals that complex Pd2 expe-
riences slightly more distortion as compared with
complex Pd3, as the average deviation of the bite angles
from the typical 90ꢁ is 2.5 and 2.3, respectively.

AKIRI AND OJWACH
5 of 14
The calculated Tau parameters (τ) for complexes Pd2
and Pd3 of 0.14 and 0.08, respectively, further supported
this trend.
(Pd:PPh3 of 1:3) led to a slight drop in percentage yields
62%. The diminished catalytic performance at enhanced
amounts of PPh3 could be connected to the competition
between PPh3 and 1-hexene to coordinate to the palla-
dium active site.[41,42] Comparatively, no observable shift
in regioselectivity was noted with change in Pd:PPh3
ratios as in all cases, linear esters of 63%–66% were
obtained (Table 2, entries 2–4).
2.3 | Methoxycarbonylation catalyzed by
palladium(II) complexes Pd1–Pd5
2.3.1 | Preliminary investigations of
complexes Pd1–Pd5 in the
Considering the observed significant influence of
PPh3 and its inherence concentration, in controlling the
catalytic activities in the methoxycarbonylation reactions,
we sought to gain more insights on its role on the possi-
ble active species formation using in situ NMR spectros-
copy. Thus, Pd5:PPh3 ratios of 1:1 and 1:2 were used to
monitor the changes in the 31P NMR spectral data over a
period of 6 h (Figures 3 and S42). The spectra data
obtained at Pd5:PPh3 ratio of 1:1 displayed two peaks at
around 23 and 32 ppm, indicating possible [Pd(PPh3)Cl
(L5)]+ intermediate formation as shown in eq. 1. The two
peaks in the 31P NMR spectral data could be assigned to
PPh3 ligands in the [Pd(PPh3)Cl(L5)],+ cis and trans
to the pyridyl and imine nitrogen atoms. The active cata-
lyst [Pd(PPh3)Cl(L5)]+ is formed in both cases when Pd:
PPh3 ratios are 1:1 and 1:2. However, excess PPh3 is nec-
essary to shift the reactions toward the active species [Pd
(PPh3)Cl(L5)]+ as opposed to the inactive [Pd(L5)Cl2]
complex. The use of Pd5:PPh3 ratio of 1:2 resulted in an
additional peak at ꢀ5 ppm, assigned to the free PPh3
group. The higher catalytic activities observed at higher
Pd:PPh3 ratio of 2, compared with the ratio of 1:1, there-
fore avers that excess amounts of PPh3 is necessary to sta-
bilize the active species.
methoxycarbonylation of 1-hexene
Preliminary investigation of the propensity of complexes
Pd1–Pd5 to catalyze the methoxycarbonylation 1-hexene
was studied using ratios of 1:2:10: 200(the Pd:PPh3:PTSA:
hexene). The initial reaction temperature and CO of 90ꢁC
and 60 bar were employed respectively (Table S2). The
identification and quantification of the catalysis products
was accomplished using GC and GC-MS techniques
(Figure S41). Under these reaction conditions, complex
Pd5 was found to be the most active with affording per-
centage yields of 68% (Table S2, entry 5). Thus, complex
Pd5 was used for further optimization reactions and
investigations. We first probed the effect of Pd:PPh3 ratio
of 1:1, 1:2 and 1:3 (Table 2, entries 1–4). Expectedly, only
traces of products were observed in the absence of PPh3,
underpinning the importance of the PPh3 to stabilize the
generated active species.[39,40] This is also augmented
by decomposition of the catalyst and formation of
Pd(0) observed in the reaction chamber. Increasing the
amount of the PPh3 from one fold to two folds was mar-
ked with improved yields from 52% to 68% (Table 2,
entries 2 and 3). Notably, increasing the PPh3 quantity to
In attempts to further understand the role played by
the PTSA acid, we varied the Pd5:PTSA ratio from 1:10
to 1:40 (Table 2, entries 3–7). It was evident that the
amount of the acid promoter had profound influence on
the catalytic activity of complex Pd5. For example, per-
centage yields of 68% and 88% were recorded at Pd5:
PTSA ratios of 1:10 and 1:30, respectively (Table 2, entries
5–7). However, a further increase of the Pd5:PTSA ratio
to 1:40 saw a drop in percentage yields to 73%. Lower cat-
alytic activities at higher PTSA concentrations could be
attributed to ligand hydrolysis,[43] leading to catalyst
decomposition. To confirm this hydrolysis, 1H NMR
spectral data of the mixtures (Pd5:PTSA ratios of 1:30
and 1:40) were monitored over a 6 h using period
(Figure S42). While the spectrum acquired at Pd5:PTSA
ratio of 1:30 did not exhibit any observable changes, the
spectral data for the Pd5:PTSA ratio of 1:40 exhibited
new signals corresponding to the free ligands. This there-
fore established catalyst decomposition via ligand dissoci-
ation at higher PTSA concentrations, consistent with the
low catalytic activities observed.
TABLE 2 Optimization of the Pd:PPh3 and Pd:PTSA ratios
using complex Pd5
Entry Pd:PPh3 Pd:PTSA Yield (%)a B/L(%)b TONc
1d
0
1:10
1:10
1:10
1:10
1:20
1:30
1:40
trace
52
-
-
2
1:1
1:2
1:3
1:2
1:2
1:2
38/62
37/63
34/66
40/60
37/63
40/60
104
136
124
158
176
146
3
68
4
62
5
79
6
88
7
73
Note: Reaction conditions: Solvent: methanol/toluene (40 ml); Pd: hexene
ratio, 1:200; time, 24 h; temp: 90ꢁC; pressure: 60 bar.
a% yields and conversions obtained using an internal standard
(ethylbenzene) from GC.
bMolar ratio between branched and linear esters computed using
commercial sample (linear methyl heptanoate).
cTON = (mol. prod/mol. Pd).
dReaction carried out with no PPh3.

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AKIRI AND OJWACH
FIGURE 3 In situ 31P NMR spectral variation over time at different Pd:PPh3 ratios using complex Pd5
2.3.2 | Investigation of the impacts of
reaction conditions on methoxycarbonylation of
1-hexene
raising the catalyst systems' temperature from 90ꢁC to
100ꢁC was marked by a notable increase in yields from
84% to 94%, respectively. However, a higher temperature
of 105ꢁC was met with a sharp decline in the percentage
yields to 71% (Table 3, entries 3–5). Poor catalytic activi-
ties at elevated temperatures is correlated with thermal
decomposition of the catalyst, as deduced from the
presence of palladium(0), observed in the catalyst
reaction chamber at 100ꢁC.[46] It is worth mentioning the
increased amounts of branched esters at elevated temper-
atures. For instance, 34% and 43% of the branched esters
were produced at 90ꢁC and110ꢁC, respectively (Table 3,
entries 3 vs. 6). Such an observation has been made
before and associated with enhanced isomerization reac-
tions at higher temperatures.[47]
To qualitatively determine the optimum catalyst concen-
tration in the methoxycarbonylation reaction, we deter-
mined the TON values at Pd loadings of 0.25 mol%
(1:400), to1 mol% (1:100) using complex Pd5 (Table 3,
entries 1–4). While an increase in percentage yields was
observed at higher Pd loadings, there was a general
decline in the TON (Figure 4). For example, at 0.25 mol%
(1:400) and 1 mol% (1:100), percentage yields of 60% and
95% were reported, whereas TONs of 252 and 95 were
obtained, respectively (Table 3, entries 1 vs. 4). Lower
TONs at higher catalyst loading may results from catalyst
aggregation.[44,45] An optimum catalyst loading of 1:300
(0.33%) corresponding to a TON of 252 was thus
established (Table 3, entry 3).
The dependence of catalytic performance of complex
Pd5 on CO pressure was scrutinized through varying it
from 50 to 70 bar (Table 3, entries 5, 7, and 8). Expect-
edly, a higher pressure of 60 bar concomitantly led to
enhanced yields of 94%, in comparison with percentage
Next, we turned out attention to establishing the opti-
mum reaction pressure and temperature. We noted that

AKIRI AND OJWACH
7 of 14
TABLE 3 Optimization of reaction
conditions in 1-hexene
Entry
Pd:1-hexene
1:100
Temperature
Pressure
Yield (%)a
B/L (%)b
35/65
34/66
34/66
35/65
39/61
43/57
38/62
37/63
39/61
43/57
41/59
40/60
40/60
TONc
1
90
60
60
60
60
60
60
50
70
60
60
60
60
60
95
92
84
60
94
71
66
77
68
8
95
methoxycarbonylation using complex
Pd5
2
1:200
90
184
252
240
282
213
198
231
204
24
3
1:300
90
4
1:400
90
5
1:300
100
110
100
100
70
6
1:300
7
1:300
8
1:300
9
1:300
10d
11e
12f
13g
1:300
100
100
100
100
1:300
35
59
78
105
186
234
1:300
1:300
Note: Reaction conditions: Solvent, methanol/toluene (40 ml); Pd:PPh3:PTSA:hexene ratio; 1:2:30; time,
24 h.
a% yields and conversions obtained using an internal standard (ethylbenzene) from GC.
bMolar ratio between branched and linear esters computed using commercial sample (linear methyl
heptanoate).
cTON = (mol.prod/mol. Pd).
dReaction done for 2 h.
eReaction done for 6 h.
fReaction done for 12 h.
gReaction done for 18 h.
FIGURE 4 Graphical plots showing TON and TOF at different reaction times and catalyst loadings using complex Pd5. Reaction
conditions: (a) Solvent system, methanol/toluene (40 ml); [Pd]:[PPh3][PTSA] ratio; 1:2:30; time, 24 h, temp, 100ꢁC. (b) Solvent system,
methanol/toluene (40 ml); [Pd]:[PPh3][PTSA]:[hexene] ratio; 1:2:30:300; time, 24 h, temp, 100ꢁC
yields of 66%, observed at 50 bar. This is consistent with
rapid CO insertion at higher pressures and well
supported by previous literature reports.[48] Contrary to
the catalytic activity trends reported, no notable effects of
CO pressure on product regioselectivity was evident.
Lastly, the stability profile of complex Pd5 was ventilated
by comparisons of TOFs at reaction times from 2 to 24 h
(Figure 4). From Figure 4b, lower yields were observed at
a shorter reaction time. For instance, while percentage
yields of 8% and 35% were observed after 2 and 6 h,
respectively, increases to 59% and 78% at 12 and 18 h
were reported, respectively. Quantitatively, 24-h reaction
time could be regarded as the optimum reaction time as
maximum percentage yields of 94% was realized. How-
ever, qualitatively, shorter reaction times gave better
results. For instance, while a yield of only 35% was

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AKIRI AND OJWACH
observed at 6 h reaction, it coincided with the highest
turnover frequency of 17.5 hꢀ1, versus 11.9 hꢀ1 observed
at 24 h. This shows that while the catalytic activity of
complex Pd5 is appreciably retained over time, there is
some catalyst deactivation occurring with longer reaction
times.
complexes reported in literature. In terms of catalytic
activity, while some systems have reported slightly higher
activities, for instance palladium(II) system anchored on
the 2-(diphenylphosphinoamino) pyridine ligands (TON
of 340), others have reported similar catalytic activities.
For example, palladium(II) catalysts based on mixed
N^N^O(S) (TON of 264) in the methoxycarbonylation
reaction.[29,32] While the catalytic activities of the current
are comparable with our previous systems,[21,25,30,51]
complexes Pd1–Pd5 registered significant improvement
with respect to regioselectivity. For instance, while most
of our previous reports produced linear and branched
products in almost equal quantities, the current com-
plexes afforded up to 75% of the linear esters in the
methoxycarbonylation of 1-hexene.
Two sets of control experiments were undertaken in
efforts to establish the isolated palladium(II) complexes
and ligands roles in the methoxycarbonylation reactions
(Table 4, entries 6 and 7). First, we compared the behav-
ior of complex Pd5 with an in situ generated palladium
catalyst using [Pd(NCMe)2Cl2]/L5 system. A notable
reduction in catalytic activity (48%) was observed in the
catalytic set up with the in situ generated catalyst com-
pared with 94% attained by complex Pd5 (Table 4, entries
5 vs. 6). This affirms the crucial role by played by discrete
catalysts, consistent with previous findings.[52,53] The use
of [PdCl2(PPh3)2] salt in the methoxycarbonylation reac-
tions have also been reported.[54] We thus compared its
catalytic performance with our complexes under similar
conditions. Even the catalytic activity displayed by
[PdCl2(PPh3)2] of 72% was lower than 94% for
complex Pd5, it was comparable with complexes Pd1
and Pd2. With respect to regioselectivity, expectedly
[PdCl2(PPh3)2] produced 53% of the linear products, com-
pared with compositions of 65%–75% reported for the
more sterically hindered complexes Pd1–Pd5 (Table 4,
entries 1–5). These differences in catalytic behavior thus
confirm that the active species in complexes Pd1–Pd5
contain coordinated ligand and differs from
[PdCl2(PPh3)2]. Finally, we used Hg(0) drop test to eluci-
date whether the active species is purely homogeneous or
contain some active Pd(0) nanoparticles using complex
Pd5. The addition of five drops of Hg(0) marginally
reduced the percentage yields from 94% to 89% (Table 4,
entries 5 vs. 8). This points to largely homogeneous
nature of the active species using complex Pd5.[55]
2.3.3 | The influence of complex structure
Having established the best reaction conditions using
complex Pd5, the complex/ligand structure influence
was then studied by testing the other complexes Pd1–
Pd4 in 1-hexene methoxycarbonylation (Table 4, entries
1–5). From the data, it was apparent that the ligand struc-
ture controlled the behavior of the precatalyst in terms of
catalytic performance. For instance, complexes Pd5 and
Pd2, with unsubstituted and isopropyl substituted aryls
close to their coordination sphere afforded percentage
yields of 94% and 73%, respectively (Table 4, entries 2 vs.
5). The lower catalytic activities reported for the more ste-
rically hindered complex Pd2 is anticipated and is linked
to the limited access of the alkene substrate to the active
metal site.[49] In terms of selectivity, the more sterically
hindered complex Pd2 expectedly afforded more of the
sterically less demanding linear esters (75%) in compari-
son with 65% reported for the less bulky complex Pd5.[50]
Similar observations were made for the remaining com-
plexes. The catalytic performance observed for complexes
Pd1–Pd5 compares well with similar palladium
TABLE 4 Influence of complex structure in 1-hexene
methoxycarbonylation
Entry
Complex
Pd1
Pd2
Pd3
Pd4
Pd5
-
Yield (%)a
B/L (%)b
28/72
25/75
31/69
32/68
35/65
33/67
47/53
37/63
TONc
240
222
249
264
282
144
216
264
1
80
74
83
88
94
48
72
89
2
3
4
5
6d
7e
8f
-
Pd5
Note: Reaction conditions: Solvent, methanol/toluene (both 20 ml); Pd:PPh3:
PTSA:hexene ratio; 1:2:30:300; time, 24 h; temp, 100ꢁC.
a% yields and conversions obtained using an internal standard
(ethylbenzene) from GC.
bMolar ratio between branched and linear esters computed using
commercial sample (linear methyl heptanoate).
cTON = (mol.prod/mol. Pd).
2.3.4 | Investigation of the substrate scope
using complex Pd5
d[Pd(NCMe)2Cl2]/L5 system.
ePdCl2(PPh3)2] as a catalyst.
fHg poisoning investigation.
The alkene substrate was studied using terminal alkenes
of varied chain lengths, internal alkenes, and styrene at

AKIRI AND OJWACH
9 of 14
TABLE 5 Effects of substrate identity on
methoxycarbonylation using complex Pd5
TABLE 6 Methoxycarbonylation of styrene using palladium
complexes Pd1–Pd5
Entry
Substrate
1-hexene
Yield (%)a
B/L(%)b
31/69
81/19
91/09
43/57
48/52
54/46
86/14
60/40
67/33
TONc
282
213
297
249
204
162
153
126
96
Entry
Complex
Yield (%)a
B/L(%)b
82/18
79/21
88/12
86/14
91/09
TONc
1
2
3
4
5
6
7
8
9
94
71
99
83
68
54
51
42
32
1
2
3
4
5
Pd1
Pd2
Pd3
Pd4
Pd5
85
81
88
94
99
255
Trans-2-hexene
Styrene
243
264
1-heptene
1-octene
282
297
1-decene
Note: Reaction conditions: Solvent system, methanol/toluene (40 ml); [Pd]:
[PPh3][PTSA]:[hexene] ratio; 1:2:30:300; time, 24 h; temp, 100ꢁC.
a% yields and conversions obtained using an internal standard
(ethylbenzene) from GC.
bMolar ratio between branched and linear esters computed using
commercial sample (linear methyl heptanoate).
cTON = (mol.prod/mol. Pd).
Trans-4-octene
1-dodecene
Tetradecene
Note: Reaction conditions: Solvent, methanol/toluene (both 20 ml); Pd:PPh3:
PTSA: substrate ratio; 1:2:30:300; time, 24 h; temp, 100ꢁC.
a% yields and conversions obtained using an internal standard
(ethylbenzene) from GC.
bMolar ratio between branched and linear esters computed using
commercial sample.
cTON = (mol.prod/mol. Pd).
instance, 14% of the linear ester product formed in the
methoxycarbonylation trans-4-octene is due to its isom-
erization to form 1-octene.
The higher catalytic activities and regioselectivity
observed for complex Pd5 in the methoxycarbonylation
of styrene prompted us to investigate the other complexes
Pd1–Pd4 in the styrene reaction (Table 6). This is similar
to the trends observed in the reactions involving
1-hexene, with complex Pd5 being the most active (99%),
while complex Pd2 was the least active (81%). More
significantly, while linear esters were predominantly
obtained in the methoxycarbonylation of 1-hexene
(65%–75%), styrene substrate gave predominantly
branched esters within the range of 79% to 91% (Table 4
vs. Table 6). This points to possible role of electronic and
steric contributions in controlling the product
distributions.[59]
optimized reactions conditions of Pd:PPh3:substrate:
PTSA (1:2:300:30), at 100ꢁC and CO pressure of 60 bar
(Table 5). From the data, it was evident the olefin sub-
strate's identity affected the regioselectivity and catalytic
activity of complex Pd5. In general, terminal alkenes
afforded higher percentage yields than the internal
analogs, whereas styrene recorded the highest
catalytic activity. For instance, styrene, 1-hexene, and
1-tetradecence alkenes gave percentage yields of 99%,
94%, and 32%, respectively. Similarly, percentage yields
of 68% and 51% were obtained for 1-octene and trans-
4-octene, respectively (Table 5). These results are in good
agreements with previous reports and have been associ-
ated with steric and electronic contributions from vari-
ous alkenes.[22,25,30,51] The superior percentage yields
recorded for styrene (>99%) is normal and is attributed
to the resultant benzylic palladium(II) intermediate,
3 | CONCLUSIONS
This paper established the coordination behavior of five
palladium(II) complexes (Pd1–Pd5) anchored on rigid
(pyridyl)benzamidine ligands with different steric
requirements. The molecular structures of the palladium
compounds revealed distorted square planer geometries,
containing a single bidentate neutral ligand, and two
chloride ligands. All the complexes formed active species
in the methoxycarbonylation of various alkene substrates.
Both the catalytic activities and regioselectivities were
impacted by the steric and electronic dispositions of the
palladium(II) complexes and identity of the alkene sub-
strate. Sterically hindered complexes favored the forma-
tion of linear esters. Terminal olefins produced mostly
linear esters, whereas internal alkenes and styrene
formed predominantly branched esters. NMR kinetics
which is
a more active as compared to alkyl
palladium(II) species, in aliphatic alkenes.[56] The iden-
tity of the olefin substrate also controlled the type of the
ester products yielded. As expected, percentage composi-
tions of the branched esters of 39% and 81% for 1-hexene
and 2-hexene substrates were obtained respectively. Sim-
ilarly, 1-hexene and 1-tretradecene gave branched esters
of 39% and 67%, respectively (Table 5, entries 1 vs. 9).
Preference to branched ester products with increased
carbon chain length can be explained from higher
number of possible isomers.[57,58] The small quantities
of the linear ester products observed in the
methoxycarbonylation of internal alkenes may be associ-
ated with isomerization to give terminal alkenes. For

10 of 14
AKIRI AND OJWACH
established the stabilization role of PPh3 and catalyst
deactivation though ligand hydrolysis at high acid
concentrations.
eliminated under a reduced pressure to afford imidoyl
chloride as a yellow oil. Immediately, toluene (30.00 ml),
trimethylamine (0.80 ml, 5.50 mmol), and 2-amino-
4-methyl pyridine (0.54 g, 5.00 mmol) were introduced,
and the resultant mixture refluxed for 24 h under nitro-
gen atmosphere. The obtained crude substance was then
filtered and the solvent removed under vacuum to obtain
a dark brown oil, followed by recrystallization in metha-
nol to afford white crystalline compound L1.
Yield = 1.28 g (69%.) 1H NMR (400 MHz, CDCl3): δH
(ppm) (Isomer [E-anti and E-syn] ratio 1: 5), major iso-
4 | EXPERIMENTAL
4.1 | General materials and
instrumentation
The chemicals benzoyl chloride (97%), triethylamine
(99%), thionyl chloride (95%), 2-amino-4-methyl pyri-
dine, 2-amino-6-methyl pyridine, 2,6-diisopropylaniline
(90%), 2,6-dimethyl aniline (99%), aniline (>99.5%), and
PdCl2 were procured from Sigma-Aldrich and used in
synthesis with no additional purification. The solvents
acquired from Merck were of analytical grade, and vari-
ous procedures were followed to dry them prior to use.
Whereas DCM was dried by distilling over P2O5 and
subsequently storing in molecular sieves, drying of
methanol was done by heating over iodine-activated
magnesium metal. In addition, drying of toluene was
done over sodium wire and benzophenone. A Bruker
Ultrashield 400 (1H NMR 400 MHz, 13C NMR 100 MHz)
3
mer, 1.17 (d, 12H, JHH = 6.80 Hz, CH(CH3)2), 2.43 (s,
3H, CH3), 3.18 (m, 2H, CH(CH3)2), 7.02–7.44 (m, 11H,
pyridyl and phenyl protons). Minor isomer: 0.96 (d, 12H,
3JHH = 6.80 Hz CH(CH3)2), 2.43(s, 3H, CH3), 3.08 (m,
2H, CH(CH3)2), 7.02–7.44 (m, 11H, pyridyl and phenyl
protons). 13C NMR (100 MHz, CDCl3, δ ppm); major iso-
mer: 162.5 (N═C), 147.4 (2pyC), 144.3 (6PyC), 132.6
(4PyC), 132.2 (iPrC), 129.8 (2,6 iPrC), 128.8 (1ArC), 128.3
(4ArC), 127.1 (5PyC), 126.1 (3,5 ArC), 123.4 (3,5 ArC),
121.1 (3,5 iPrC), 119.6 (4 iPrC), 114.6 (3PyC), 29.8 (CH
(CH3)2), 22.8 (PyCH3), 21.6 (CH(CH3)2). Minor isomer:
162.5 (N═C), 147.4 (2pyC), 144.3 (6PyC), 132.6 (4PyC),
132.2 (1iPrC), 129.8 (2,6 iPrC), 128.8 (1ArC), 128.3(4ArC),
127.1 (5PyC), 126.1 (3,5 ArC), 123.4 (3,5 ArC), 121.1 (3,5
iPrC), 119.6 (4 iPrC), 114.6 (3PyC), 25.1 (CH(CH3)2), 22.8
(PyCH3), 21.4 (CH(CH3)2). ESI-MS: m/z (%)
372 [(M+ + H, 100%)]F. IR νmax/cmꢀ1: ν(C═N) = 1637.
1
spectrometer were used in recording H NMR and 13C
NMR spectra in deuterated chloroform at ambient tem-
perature. The residual carbon and proton peaks at 77.0
and 7.24 ppm, respectively, were used in referencing of
the chemical shift values (δ). In addition, the recording
of the IR spectra was done on a Perkin–Elmer Spectrum
100 in the 600–4000 cmꢀ1 range. While a Thermal Sci-
entific Flash 2000 was used for elemental analyses, LC
premier micromass was applied in carrying out the
analysis of mass spectra. Moreover, Varian QP2010 and
CP-3800 was used in GC–MS and GC analyses,
respectively.
4.2.2 | (E)-N0-(2,6-diisopropylphenyl)-N-
(6-methylpyridin-2-yl)benzimidamide (L2)
L2 was synthesized following the procedure for L1
using 2,6-diisopropylaniline (0.95 ml, 5.00 mmol), ben-
zoyl chloride (0.55 ml, 5.00 mmol), triethylamine
(0.80 ml. 5.50 mmol), thionyl chloride (1.00 ml,
14.00 mmol), and 2-amino-6-methyl pyridine (0.54 g,
5.00 mmol). The resulting yellow product was rec-
rystallized from ethanol to give light yellow crystals.
Yield = 1.36 g (73%). 1H NMR (400 MHz, CDCl3): δH
(ppm) (isomer [E-anti and E-syn] ratio 1: 1. 6), major
isomer, 1.19 (d, 12H, CH(CH3)2), 2.50 (s, 3H, CH3),
3.24 (m, 2H, CH(CH3)2), 6.96–7.71 (m, 11H, pyridyl
and phenyl protons). Minor isomer: 1.00 (d, 12H,
CH(CH3)2), 2.50 (s, 3H, CH3), 3.06 (m, 2H, CH(CH3)2),
7.02–7.44 (m, 11H, pyridyl and phenyl protons). 13C
NMR (100 MHz, CDCl3, δ ppm); major isomer: 154.2
(N═C), 144.6 (2pyC), 139.3 (6PyC), 138.7 (4PyC), 132.2
(iPrC), 129.6 (2,6 iPrC), 128.8 (1ArC), 128.1(4ArC),
127.1 (5PyC), 126.1 (3,5 ArC), 123.9 (3,5 ArC), 123.4
(3,5 iPrC), 119.4 (4 iPrC), 110.9 (3PyC), 28.8 (CH
(CH3)2), 24.0 (PyCH3), 22.8 (CH(CH3)2). Minor isomer:
4.2 | Synthesis and characterization of
(pyridyl)benzamidine ligands the
respective palladium(II) complexes
4.2.1 | (E)-N0-(2,6-diisopropylphenyl)-N-
(4-methylpyridin-2-yl)benzimidamide (L1)
To a stirred solution of 2,6-diisopropylaniline (0.95 ml,
5.00 mmol) in dichloromethane (30 ml), was added ben-
zoyl chloride (0.55 ml, 5.00 mmol) and triethylamine
(0.80 ml. 5.50 mmol). After 3 h, the solvent reduced
under vacuum after filtration to obtain a brown oil. An
excess of thionyl chloride (1.00 ml, 14.00 mmol) was then
introduced and the reaction left to proceed further 2 h at
80ꢁC. The unreacted thionyl chloride was then

AKIRI AND OJWACH
11 of 14
154.2 (N═C), 144.6 (2pyC), 139.3 (6PyC), 138.7 (4PyC),
132.2 (iPrC), 129.6 (2,6 iPrC), 128.8 (1ArC), 128.1
(4ArC), 127.1 (5PyC), 126.1 (3,5 ArC), 123.9 (3,5 ArC),
123.4(3,5 iPrC), 119.4 (4 iPrC), 110.9 (3PyC), 25.4 (CH
(CH3)2), 24.0 (PyCH3), 21.6.8 (CH(CH3)2). ESI-MS: m/z
(%) 372 [(M+, 100%)]. IR νmax/cmꢀ1: ν(C═N) = 1637.
(ArCH3), ESI-MS: m/z (%) 316 [(M + H, 100%)]. IR
νmax/cmꢀ1: ν(C═N) = 1647
4.2.5 | (E)-N-(6-methylpyridin-2-yl)-N0-
phenylbenzimidamide (L5)
L5 was synthesized following the procedure for L1 using
aniline (0.45 ml, 5.00 mmol), benzoyl chloride (0.55 ml,
5.00 mmol), triethylamine (0.80 ml, 5.50 mmol), thionyl
chloride (1.0 ml, 14.00 mmol), and 2-amino-6-methyl pyr-
idine (0.54 g, 5.00 mmol). Upon evaporating the solvent,
ligand L5 was obtained as brown oil. Yield = 0.91 g
4.2.3 | (E)-N0-(2,6-dimethylphenyl)-N-
(6-methylpyridin-2-yl)benzimidamide (L3)
Ligand L3 was synthesized by applying the same protocol
for L1 using 2,6-dimethylaniline (1.23 ml, 10.00 mmol),
benzoyl chloride (1.10 ml, 10.00 mmol), triethylamine
(1.60 ml. 11.00 mmol), thionyl chloride (2.00 ml,
28.00 mmol), and 2-amino-6-methyl pyridine (1.08 g,
10.00 mmol). The solvent was then removed under
1
(80%). H NMR (400 MHz, CDCl3): δH (ppm) 2.38 (s, 3H,
CH3), 7.18–7.91 (m, 13H, H, pyridyl and phenyl protons).
13C NMR (100 MHz, CDCl3, δ ppm); 166.3 (N═C), 151.3
(pyC), 146.1 (PyC), 137.9 (4PyC), 131.8 (ArC), 129.1
(ArC), 129.0 (ArC), 128.8(ArC), 128.2 (PyC), 127.0 (ArC),
125.3 (ArC), 124.6(ArC), 121.4 (ArC), 120.2 (PyC), 21.4
(PyCH3). ESI-MS: m/z (%) 288 [(M+ + H, 100%)]. IR
νmax/cmꢀ1: ν(C═N) = 1658.
reduced pressure to give L3 as
a brown solid.
Yield = 2.72 g (73%). 1H NMR (400 MHz, CDCl3): δH
(ppm): 2.21 (s, 6H, CH3-Ar), 2.58 (s, 3H, CH3-Py), 7.18
(m, 3H, H-Py), 7.38 (m, 3H, H-ArPy), 7.54 (t, 1H,
3
3JHH = 7.6 Hz, H-Ar), 7.61 (d, 2H, JHH = 7.6 Hz, H-Ar),
7.85 (t, 1H, 3JHH = 7.8 Hz, H-Ar), 8.63 (d, 1H,
3JHH = 8.2 Hz, H-Ar). 13C NMR (100 MHz, CDCl3, δ
ppm); 160.7 (N═C), 158.4 (2pyC), 152.9 (6PyC), 145.5
(ArC), 137.1 (4PyC), 136.8 (ArC), 130.2 (ArC), 129.9
(4ArC), 128.8 (ArC), 128.7 (ArC), 128.6 (5PyC), 128.5
(ArC), 127.9 (ArC), 125.4 (3PyC), 24.5 (PyCH3), 18.8
(ArCH3). ESI-MS: m/z (%) 316 [(M + H, 100%)]. IR
νmax/cmꢀ1: ν(C═N) = 1642.
4.2.6 | (E)-N0-(2,6-diisopropylphenyl)-N-
(4-methylpyridin-2-yl)benzimidamide
palladium(II) (Pd1)
A solution of [Pd(NCMe)2Cl2] (0.05 g, 0.19 mmol) in
DCM (10 ml) was mixed with a solution L1 (0.07 g,
0.19 mmol) in DCM (5 ml), affording a yellow solution.
Under stirring, the set up was allowed to run for 24 h at
ambient temperature. Upon the reduction of the solvent
under reduced pressure, and a recrystallization using
hexane (5 ml) complex Pd1 was afforded as a pure
bright yellow solid. Yield = 0.08 g (76%). 1H NMR
(400 MHz, CDCl3): δH (ppm) 0.01 (d, 3H,
4.2.4 | (E)-N0-(2,6-dimethylphenyl)-N-
(4-methylpyridin-2-yl)benzimidamide (L4)
Ligand L4 was synthesized following the synthetic route
for L1 using 2,6-dimethylaniline (1.23 ml, 10.00 mmol),
benzoyl chloride (1.10 ml, 10.00 mmol), triethylamine
(1.60 ml, 11.00 mmol), thionyl chloride (2.00 ml,
28.00 mmol), and 2-amino-4-methyl pyridine (1.08 g,
10.00 mmol). L4 was then revealed as a dark brown oil
upon solvent removal under reduced pressure.
Yield = 2.76 g (74%). 1H NMR (400 MHz, CDCl3): δH
(ppm): 2.21 (s, 6H, CH3-Ar), 2.58 (s, 3H, CH3-Py), 7.18
(m, 3H, H-Py), 7.38 (m, 3H, H-ArPy), 7.54 (t, 1H,
3
3JHH = 6.60 Hz, CH(CH3)2), 1.24 (d, 3H, JHH = 6.88 Hz
3
CH(CH3)2), 1.51 (d, 3H, JHH = 6.88 Hz CH(CH3)2), 2.07
(d, 3H, 3JHH = 6.88 Hz CH(CH3)2), 2.70 (s, 3H, CH3),
4.12 (m, 2H, CH(CH3)2), 6.76 (m, 1H, H-Py), 6.91 (m,
2H, H-Py), 7.04 (m, 6H, H-Ar,Py), 7.22 (t, 1H,
3JHH = 7.68 Hz, H-Ar), 7.24 (m, 1H, H-py). 13C NMR
(100 MHz, CDCl3, δ ppm) 147.1 (N═C), 146.1 (2pyC),
144.4 (6PyC), 140.7 (4PyC), 139.1 (1iPrC), 131.0 (2,6
iPrC), 130.7 (1ArC), 129.6 (4ArC), 129.4 (5PyC), 129.1
(3,5 ArC), 128.4 (3,5 ArC), 127.0(3,5 iPrC), 123.4 (4 iPrC),
129.0 (3PyC), 29.1(CH(CH3)2), 28.1 (PyCH3), 25.8 (CH
(CH3)2), 22.4(CH(CH3)2), 22.3(CH(CH3)2), 21.5 (CH
(CH3)2). ESI-MS: m/z (%) 548 [(M+ + H, 100%)]. IR
3
3JHH = 7.6 Hz, H-Ar), 7.61 (d, 2H, JHH = 7.6 Hz, H-Ar),
7.85 (t, 1H, 3JHH = 7.8 Hz, H-Ar), 8.63 (d, 1H,
3JHH = 8.2 Hz, H-Ar). 13C NMR (100 MHz, CDCl3, δ
ppm); 160.5 (N═C), 158.8 (2pyC), 153.9 (4PyC), 148.5
(ArC), 137.8 (6PyC), 136.1 (ArC), 130.5 (ArC), 129.3
(4ArC), 128.9 (ArC), 128.7 (ArC), 128.5 (5PyC), 128.2
(ArC), 127.9 (ArC), 125.8 (3PyC), 21.4 (PyCH3), 18.8
νmax/cmꢀ1
:
ν(C═N) = 1602. CHN. Anal. Calc. for
C25H29Cl2N3Pd: C, 54.71; H, 5.33; N, 7.66. Found: C,
54.60; H, 5.51; N, 7.51.

12 of 14
AKIRI AND OJWACH
4.2.7 | (E)-N0-(2,6-diisopropylphenyl)-N-
(6-methylpyridin-2-yl)benzimidamide
palladium(II) (Pd2)
(84%). 1H NMR (400 MHz, CDCl3): δH (ppm): 2.22 (s, 6H,
CH3-Ar), 2.36 (s, 3H, CH3-Py), 6.78 (d, 2H,
3
3JHH = 7.6 Hz, H-Ar,H-Py), 7.14 (t, 4H, JHH = 7.68 Hz,
H-Ar), 7.25 (m, 3H, H-Ar), 7.32 (m, 2H, H-Ar). 13C NMR
(100 MHz, CDCl3, δ ppm); 158.1 (2-pyC), 157.1 (N═C),
153.1 (4PyC), 146.1 (ArC), 144.3 (6-PyC), 140.7 (ArC),
130.9 (ArC), 130.7 (4-ArC), 129.6 (ArC), 129.4
(ArC), 129.1 (5-PyC), 128.4 (ArC), 126.9 (ArC), 123.4
Complex Pd2 was made by applying the synthetic proto-
col for complex Pd1 using ligand L5 (0.07 g, 0.19 mmol)
and [Pd(NCMe)2Cl2] (0.05 g, 0.19 mmol). Bright yellow
3
solid. Yield = 0.09 g (85%); 0.02 (d, 3H, JHH = 6.60 Hz,
3
CH(CH3)2), 1.23 (d, 3H, JHH = 6.88 Hz CH(CH3)2), 2.06
(3-PyC), 21.8 (PyCH3), 18.9 (ArCH3) IR νmax/cmꢀ1
ν(C═N) = 1610. CHN. Anal. Calc. for C21H21Cl2N3Pd: C,
:
(d, 3H, 3JHH = 6.88 Hz CH(CH3)2), 2.47 (d, 3H,
3JHH = 6.88 Hz CH(CH3)2), 3.51 (s, 3H, CH3), 2.40 (m,
2H, CH(CH3)2), 6.94 (m, 1H, H-Py), 7.08 (m, 2H, H-Py),
51.19; H, 4.30; N, 8.53. Found: C, 51.20; H, 4.52; N, 8.64.
3
7.27 (m, 6H, H-Ar,Py), 7.85 (t, 1H, JHH = 7.68 Hz, H-
Ar), 8.39 (m, 1H, H-py). 13C NMR (100 MHz, CDCl3, δ
ppm) 147.1 (N═C), 146.1 (2-pyC), 145.5 (6-PyC), 144.3
(4-PyC), 140.7 (1iPrC), 139.1 (2,6 iPrC), 132.0 (1ArC),
130.9(4-ArC), 129.6 (5-PyC), 129.4 (3,5-ArC), 126.9
(3,5-ArC), 124.3(3,5 iPrC), 123.4 (4 iPrC), 119.9 (3PyC),
29.1(CH(CH3)2), 28.1 (PyCH3), 25.8 (CH(CH3)2), 22.4
(CH(CH3)2), 22.3 (CH(CH3)2), 21.5 (CH(CH3)2). ESI-MS:
4.2.10 | (E)-N-(6-methylpyridin-2-yl)-N0-
phenylbenzimidamide palladium(II) (Pd5)
Complex Pd5 was made by applying the synthetic proto-
col for complex Pd1 using ligand L5 (0.05 g, 0.19 mmol)
and [Pd(NCMe)2Cl2] (0.05 g, 0.19 mmol). Brown solid.
Yield = 0.07 g (78%). 1H NMR (400 MHz, CDCl3): δH
(ppm) 2.47 (s, 3H, CH3), 7.50–8.04 (m, 13H, H, pyridyl
and phenyl protons). 13C NMR (100 MHz, CDCl3, δ
ppm); 166.3 (N═C), 147.2 (pyC), 143.4 (PyC), 137.9
(4-PyC), 130.8 (ArC), 128.4 (ArC), 128.2 (ArC), 127.3
(ArC), 126.9 (PyC), 126.0 (ArC), 125.4 (ArC), 123.8(ArC),
121.1 (ArC), 119.2 (PyC), 20.8 (PyCH3). ESI-MS: m/z (%)
417 [(M+ –CH3–Cl), 100%]. IR νmax/cmꢀ1: ν(C═N) = 1619.
CHN. Anal. Calc. for C19H17Cl2N3Pd: C, 49.11; H,
3.69; N, 9.04. Found: C, 49.33; H, 3.67; N, 8.92.
m/z (%) 548 [(M+ + H, 100%)]. IR νmax/cmꢀ1
(C═N) = 1604. CHN. Anal. Calc. for C25H29Cl2N3Pd: C,
:
ν
54.71; H, 5.33; N, 7.66. Found: C, 54.83; H, 5.35; N, 7.47.
4.2.8 | (E)-N0-(2,6-dimethylphenyl)-N-
(6-methylpyridin-2-yl)benzimidamide
palladium(II) (Pd3)
Complex Pd3 was made by applying the synthetic proto-
col for Pd1 using [Pd(NCMe)2Cl2] (0.05 g, 0.19 mmol)
and L3 (0.06 g, 0.19 mmol). Brown solid. Yield = 0.083 g
(87%). 1H NMR (400 MHz, CDCl3): δH (ppm): 2.24 (s, 6H,
CH3-Ar), 2.60 (s, 3H, CH3-Py), 7.10 (d, 3H,
4.3 | Methoxycarbonylation catalytic
reactions
3
3JHH = 7.6 Hz, H-Ar,H-Py), 7.21 (t, 2H, JHH = 7.68 Hz,
4.3.1 | Typical experimental protocol for
methoxycarbonylation and product analyses
H-ArPy), 7.54 (m, 2H, H-Ar), 7.62 (m, 1H, H-Ar), 7.76
(m, 2H, H-Ar), 7.95 (m, 1H, H-Ar). 13C NMR (100 MHz,
CDCl3, δ ppm); 158.8 (2pyC), 157.8 (6PyC), 155.5 (N═C),
147.5 (ArC), 137.1 (4PyC), 136.4(ArC), 130.2 (ArC), 129.9
(4ArC), 128.8 (ArC), 128.7 (ArC), 128.6 (5PyC), 128.5
(ArC), 127.9 (ArC), 125.3 (3PyC), 21.8 (PyCH3), 18.9
(ArCH3). IR νmax/cmꢀ1: ν(C═N) = 1607. CHN. Anal. Calc.
for C21H21Cl2N3Pd: C, 51.19; H, 4.30; N, 8.53. Found: C,
51.35; H, 4.11; N, 8.51.
All the methoxycarbonylation catalytic reactions were
carried out in a stainless steel autoclave Parr reactor
furnished with sampling valve, an in-built cooling sys-
tem, and a unit for controlling temperature. As an exam-
ple of the catalytic experiment, Pd1 (0.03 g, 0.05 mmol),
PTSA (0.09 g), 1-hexene (1.3 ml, 10.00 mmol), and PPh3
(0.03 g, 0.1 mmol), which translates to 0.5 mol%, were
mixed in a Schlenk tube. A methanol and toluene mix-
ture (each 20 ml) was then introduced. The resulting
solution was then transferred to the reactor and purged
with the carbon monoxide gas three times and the unit
adjusted to the desired pressure, temperature and stirring
speed. Upon the reaction's completion, the unit's temper-
ature was left to adjust to ambient temperature and the
surplus CO was taken off through an outlet. For analysis,
some amounts of the product were obtained and
4.2.9 | (E)-N0-(2,6-dimethylphenyl)-N-
(4-methylpyridin-2-yl)benzimidamide
palladium(II) (Pd4)
Complex Pd4 was made by applying the synthetic proto-
col for Pd1 using [Pd(NCMe)2Cl2] (0.05 g, 0.19 mmol)
and L4 (0.06 g, 0.19 mmol). Brown solid. Yield = 0.08 g

AKIRI AND OJWACH
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ACKNOWLEDGMENTS
The authors are grateful to the DST-NRF (South Africa)
Centre of Excellence in Catalysis (c*change) and
the University of KwaZulu-Natal for financial support
and Mr. Sizwe Zamisa for the crystal structure refine-
ment of complexes Pd2 and Pd3.
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AUTHOR CONTRIBUTIONS
Saphan Akiri: Conceptualization; data curation;
formal analysis; investigation; methodology. Stephen
O. Ojwach: Conceptualization; funding acquisition;
project administration; resources; supervision.
[27] N. L. Ngcobo, S. O. Akiri, A. O. Ogweno, S. O. Ojwach, Polyhe-
dron 2021, 203, 115243.
CONFLICT OF INTEREST
The authors declare no known conflicts of interests.
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DATA AVAILABILITY STATEMENT
The data that supports the findings of this study are
available in the supplementary material of this article.
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ORCID
Saphan O. Akiri https://orcid.org/0000-0001-9631-2196
Stephen O. Ojwach https://orcid.org/0000-0001-5309-
4926
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