Structural elucidation of chiral (imino)pyridine/phosphine palladium(II) complexes and their applications as catalysts in methoxycarbonylation of styrene

Article abstract of DOI:10.1016/j.poly.2021.115243

Treatment of ligands (S)-1-phenyl-N-(1-(pyridin-2-yl)ethylidene)ethanamine (L1), (R)-1-phenyl-N-(1-(pyridin-2-yl)ethylidene)ethanamine (L2), (S)-1-phenyl-N-((pyridin-2-yl)methylene)ethanamine (L3), (R)-1-phenyl-N-((pyridin-2-yl)methylene)ethanamine (L4), (S)-N-(2-(diphenylphosphino)benzylidene)-1-phenylethanamine (L5), and (R)-N-(2-(diphenylphosphino)benzylidene)-1-phenylethanamine (L6) with [Pd(COD)Cl₂] afforded the respective palladium complexes [Pd(L1)Cl₂] (1), [Pd(L2)Cl₂] (2), [Pd(L3)Cl₂] (3), [Pd(L4)Cl₂] (4), [Pd(L5)Cl₂] (5) and [Pd(L6)Cl₂] (6) in high yields. Solid-state structures of the complexes established N^N and N^P bidentate coordination mode of the ligands to give distorted square planar geometries. Complexes 1–6 displayed moderate catalytic activities in the methoxycarbonylation of styrene, to give predominantly branched esters of up to 95%. NMR spectroscopy studies pointed to possible decomposition of the active species, via ligand dissociation.

Full text of DOI:10.1016/j.poly.2021.115243

Polyhedron 203 (2021) 115243
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Polyhedron
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Structural elucidation of chiral (imino)pyridine/phosphine palladium(II)
complexes and their applications as catalysts in methoxycarbonylation
of styrene
1
⇑
Nondumiso L. Ngcobo, Saphan O. Akiri, Aloice O. Ogweno , Stephen O. Ojwach
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Treatment of ligands (S)-1-phenyl-N-(1-(pyridin-2-yl)ethylidene)ethanamine (L1), (R)-1-phenyl-N-(1-
(pyridin-2-yl)ethylidene)ethanamine (L2), (S)-1-phenyl-N-((pyridin-2-yl)methylene)ethanamine (L3),
(R)-1-phenyl-N-((pyridin-2-yl)methylene)ethanamine (L4), (S)-N-(2-(diphenylphosphino)benzylidene)-
1-phenylethanamine (L5), and (R)-N-(2-(diphenylphosphino)benzylidene)-1-phenylethanamine (L6)
with [Pd(COD)Cl2] afforded the respective palladium complexes [Pd(L1)Cl2] (1), [Pd(L2)Cl2] (2), [Pd(L3)
Cl2] (3), [Pd(L4)Cl2] (4), [Pd(L5)Cl2] (5) and [Pd(L6)Cl2] (6) in high yields. Solid-state structures of the
complexes established N^N and N^P bidentate coordination mode of the ligands to give distorted square
planar geometries. Complexes 1–6 displayed moderate catalytic activities in the methoxycarbonylation
of styrene, to give predominantly branched esters of up to 95%. NMR spectroscopy studies pointed to pos-
sible decomposition of the active species, via ligand dissociation.
Received 18 March 2021
Accepted 24 April 2021
Available online 28 April 2021
Keywords:
Palladium
Structures
Methoxycarbonylation
Styrene
Decomposition studies
Ó 2021 Elsevier Ltd. All rights reserved.
1. Introduction
the potential to produce chiral esters as useful feedstock for the
production of a wide range of fine chemicals [19,20].
Chiral catalysts play a significant role in the synthesis of special-
ity and fine chemicals such as pharmaceuticals, agrochemicals, fra-
grances among others [1]. It is therefore not surprising that great
attention has been dedicated to the development of chiral transi-
tion metal catalysts for the production of these enantiomerically
enriched feedstock [2–5]. One of the most industrially relevant cat-
alytic reactions is the methoxycarbonylation of olefins, since it
allows the accessibility to a wide range of important products such
as pharmaceuticals, food flavours, solvents, cosmetics, detergents
and surfactants [6,7]. Moreover, asymmetric methoxycarbonyla-
tion promotes the production of significant chiral esters [8]. Even
To date, few examples that report palladium(II) catalyzed asym-
metric methoxycarbonylation reactions include enantioselective
methoxycarbonylation of styrene reported by Konrad et al. using
phanephos palladium(II) complexes. This system gives a percent-
age yields and ee% of up to 99% and 91% respectively [3]. In another
report, Wang et al. studied chiral palladium(II) complexes
anchored on dipyridylphosphine ligands in the asymmetric
methoxycarbonylation of styrene, showing conversions of 71%
with ee% of up to 88% [21]. More recently, Olivieri et al. applied aryl
a-diimine palladium(II) complexes in the asymmetric methoxycar-
bonylation of olefins to give percentage yields of up to 97% of
diastereoisomers/racemic mixtures [22]. We have recently
reported the use of N^O and N^N-donor palladium(II) complexes
in the symmetric methoxycarbonylation of higher olefins with
promising outcomes [23–29]. In this current contribution, we
report the design of chiral N^N and N^P-donor palladium(II) com-
plexes as potential catalysts in the asymmetric methoxycarbonyla-
tion of olefins. Thus, the syntheses and structural characterization
of chiral (imino)pyridine and (imino)phosphine palladium(II) com-
plexes and their ability to catalyse the asymmetric methoxycar-
bonylation of styrene were investigated. The effect of complex
structure/ligand design, reaction conditions, nature of substrate
and catalyst deactivation pathways have been studied and are
herein discussed.
though
a number of transition metal complexes have been
employed in methoxycarbonylation reactions, palladium(II)
based-catalysts have drawn a lot of attention due to their high cat-
alytic activities and regio-selectivity [9–18]. Despite significant
advances in the development of palladium(II) catalysts in
methoxycarbonylation reactions, there are limited reports on the
applications of chiral palladium(II) catalysts in the asymmetric
methoxycarbonylation of olefins. This synthetic methodology has
⇑
Corresponding author.
E-mail address: ojwach@ukzn.ac.za (S.O. Ojwach).
Current address: Department of Applied chemistry, Technical University of
1
Mombasa, Mombasa, Kenya.
https://doi.org/10.1016/j.poly.2021.115243
0277-5387/Ó 2021 Elsevier Ltd. All rights reserved.

N.L. Ngcobo, S.O. Akiri, A.O. Ogweno et al.
Polyhedron 203 (2021) 115243
2.2. Molecular structures of complexes 1, 2 and 4–6
2. Results and discussion
Single crystals suitable for X-ray analyses of complexes 1, 2 and
4 were grown by slow diffusion of hexane solvent in their dichlor-
omethane solutions at room temperature, whilst single crystals of
compounds 5 and 6 were grown by slow diffusion of tetrahydrofu-
ran and hexane solvent mixture in dichloromethane solution of the
complexes. Figs. 1 and 2 show the molecular structures of the com-
plexes, while Table 1 gives the crystallographic data collection and
structural refinement parameters. Tables 2 and 3 contain selected
bond parameters of the compounds.
The solid-state structures of compounds 1, 2 and 4 adopt mon-
oclinic crystal systems and P21 space groups. On the other hand,
compounds 5 and 6 crystallize in orthorhombic crystal systems
and P21 P21 P21 space groups. Complexes 5 and 6 crystallized with
severely distorted THF solvent molecules in their crystalline lattice.
The solid-state structures of all the complexes confirmed the for-
mation of mononuclear complexes, in which the ligands adopt
bidentate coordination modes to the palladium metal atoms. The
coordination environment around the palladium atoms thus con-
sist of one bidentate N^N and N^P ligands and two chloride ligands
to give square planar structures [33,37,38].
The average Pd–Cl bond distance of complexes 1, 2 and 4 of
2.296 Å 0.054 Å, is slightly longer than the average Pd–Cl bond
length of 2.284 0.022 Å obtained for 464 similar structures using
MOGUL structural data search [39]. Nevertheless, it is worth men-
tioning that the mean Pd–Cl bond distances lies within the range of
2.284–2.422 Å reported in literature for similar structures. Further-
more, the mean Pd–Npy (2.020 Å) and Pd–Nimine (2.034 Å) bond
lengths are statistically comparable to the Pd–Npy and Pd–Nimine
average bond distances of 2.039 0.029 Å (165 structures) and
2.015 0.039 Å (30 structures) respectively reported for similar
structures [31]. In general, the Pd-Cl bond lengths in complexes
1, 2 and 4, averaging 2.295 Å are in agreement with the Pd-Cl bond
length of 2.298(15) Å, averaged for 491 Pd complexes as reported
in the Cambridge Structural Database (CSD) [40]. The Cl(1)–Pd–Cl
(2) bond angles of complexes 1 (89.33°) and 2 (89.33°) and com-
plex 4 (91.00°) were of close proximity to the expected 90° for
square planar geometry. Thus, the spatial arrangement of the
bonded atoms around the palladium atoms in complexes 1, 2
and 4 can be best described as slightly distorted square planar
geometries.
Similarly, the molecular structures of complexes 5 and 6 con-
tain one N^P chelated ligand (L5 or L6) and two Clꢀ ligands to form
four coordinate compounds (Fig. 2). The average Pd–Cl bond dis-
tance of 2.331 0.025 Å for 5 and 6 is slightly longer than the aver-
age Pd–Cl bond length of 2.296 0.054 Å reported for complexes 1,
2 and 4. In addition, this average distance, is longer than the aver-
age bond length of 2.284 0.022 Å calculated for 464 similar struc-
tures from MOGUL data search [39]. The longer Pd-Cl bond
distances in the N^P-chelated complexes 5 and 6, suggest a stron-
ger trans-influence from the P atom in comparison to the N-donor
atom [43]. This hypothesis is supported by the longer Pd–Cl(2)
bond length of 2.378(2) Å, trans to the P atom, in comparison to
the Pd–Cl(1) bond distance of 2.284(3) Å, trans to the imine N-atom
in complex 5, consistent with previous reports [41,42].
2.1. Synthesis of the chiral imine ligands and their palladium(II)
complexes
The chiral (imino)pyridine ligands (S)-1-phenyl-N-(1-(pyridin-
2-yl)ethylidene)ethanamine (L1), (R)-1-phenyl-N-(1-(pyridin-2-
yl)ethylidene)ethanamine (L2), (S)-1-phenyl-N-((pyridin-2-yl)
methylene)ethanamine
(L3),
(R)-1-phenyl-N-((pyridin-2-yl)
methylene)ethanamine (L4), were prepared following published
literature methods as shown in Scheme 1 [30–32]. On the other
hand, the new (imino)phosphine ligands (S)-N-(2-(diphenylphos-
phino)benzylidene)-1-phenylethanamine (L5), and (R)-N-(2-
(diphenylphosphino)benzylidene)-1-phenylethanamine (L6) were
obtained in excellent yields (94–95%) by reactions of (S)/(R)-
methylbenzylamine with 2-(diphenylphosphino)benzaldehyde in
dichloromethane (Scheme 1). Treatments of synthons L1–L6 with
[Pd(COD)Cl2] produced the corresponding palladium(II) complexes
1–6 in high yields (Scheme 1).
All the new compounds were characterized by NMR and IR
spectroscopies, mass spectrometry, microanalysis and single crys-
tal X-ray crystallography for complexes 1, 2 and 4–6. In general,
the 1H NMR spectra of ligands L1–L6 gave signature peaks of the
chiral *CH protons as quartets between 4.61 ppm and 4.99 ppm
(Figs. S1–S6). Comparison of the 1H NMR spectra of the ligands
and their respective palladium(II) complexes (Figs. S7–S12),
allowed us to deduce successful complex formation. For instance,
the 1H NMR spectra of ligand L4 and its corresponding complex
4 showed signals of the chiral *CH protons at 4.71 ppm and
6.11 ppm respectively (Fig. S13). Similar trends were observed
for other ligands and their respective palladium(II) complexes, in
line with previous literature reports [33]. We also employed two
dimensional 1H–1H NMR (COSY and NOESY) to compare the
solution and solid-state structure of complexes
3 and 4
(Figs. S14–S17). The COSY spectrum of complex 3 indicates a cou-
pling interaction between the chiral *CH proton at 6.06 ppm and
the CH3 protons at 1.92 ppm. Similarly, the peak marked (III) indi-
cates a coupling interaction between the 3-py protons at 7.69 ppm
and the imine proton 9.35 ppm (Fig. S14). Thus, the complexes
exhibit the same solution and solid-state configurations (Fig. 1).
13C NMR spectroscopy was also used to deduce the identities of
the compounds (Figs. S18–S29). For example, while the imine car-
bon shifted from about 160.5 ppm to about 167.0 ppm upon com-
plexation, an up-field shift from 69.6 ppm to 64.7 ppm was
observed for the chiral carbon (Figs. S18–S29). In addition, the
N^P-donor compounds L5 and L6 and their respective palladium
(II) complexes 5 and 6 were characterized using 31P NMR spec-
troscopy. The 31P NMR signal of ligands L5 and L6 at about
ꢀ13.1 ppm (Figs. S30 and S31) were in good agreement with liter-
ature findings [32,34,35]. More importantly, the formation of the
respective complexes 5 and 6 (Figs. S32 and S33), was deduced
from the downfield shifts of the ligand signals from ꢀ13.1 ppm
to about 31.4 ppm in complexes 5 and 6 [23,36].
FT-IR spectroscopy was also employed in establishing success-
ful complex formation (Table S1 and Figs. S34–S45). For example,
the t(C@N)imine stretch of ligand L6 and its corresponding palla-
dium(II) complex 6, were observed at
1 1
634 cmꢀ1 and
The Cl(1)–Pd–Cl(2) bond angles of 90.02(10)° and 89.44(11)° for
compounds 5 and 6 respectively, portray slightly distorted square
planar geometries. Comparisons of the geometries of compounds 1,
2, and 4 vs complexes 5–6, reveal an interesting outcome. While
one, would expect complexes 5 and 6 to display greater distortions,
due to the more sterically demanding phenyl groups, comparable
distortions was observed. This could be rationalized from the
remote proximity of the phenyl groups from the palladium(II) cen-
tre, hence exerting little influence on the resultant geometries.
626 cmꢀ1 respectively (Figs. S39 and S45). In general, up-field
shifts of the imine carbon signals were recorded upon complex for-
mation, in good agreement with literature reports [36]. Mass spec-
tral data of complexes 5 and 6 for example, showed base peaks at
m/z
= 573 amu, corresponding to their molecular mass of
570.78 g/mol (Figs. S46–S57). Elemental analyses of complexes
1–6, were in good agreement with the proposed structures in
Scheme 1, and confirmed their bulk purity.
2

N.L. Ngcobo, S.O. Akiri, A.O. Ogweno et al.
Polyhedron 203 (2021) 115243
Scheme 1. Synthesis of chiral (imino)pyridine and (imino)phosphine ligands L1–L6 and their respective palladium(II) complexes 1–6.
Fig. 1. Solid state structures of the palladium(II) complexes 1, 2 and 4.
6
( )
5
( )
Fig. 2. Molecular structures of the chiral N^P donor palladium(II) complexes 5 and 6.
Another plausible explanation could be the formation of six-mem-
pressure, temperature of 90 °C and [Pd]:[PPh3]:[acid]:[styrene]
ratio of 1:2:10:200 and solvent system, methanol 50 mL and
toluene 50 mL. From the GC and GC–MS analyses, only methyl-3-
phenylpropanoate (linear product) and methyl-2-phenyl-
propanoate (branched product) were identified as the major ester
products (Figs. S58–S64) using ethylbenzene as the internal stan-
dard. From Table 4, low to moderate percentage yields (19%–
59%) were realized. With respect to regio-selectivity, the catalysts
favoured the formation of branched esters (83–87%).
bered chelate rings in compounds 5 and 6, in comparison to the
five-membered rings in complexes 1, 2, and 4. This has the net
effect of minimizing ring strain in compounds 5 and 6 [43].
2.3. Methoxycarbonylation of olefin using palladium(II) complexes 1–6
as catalysts
2.3.1. Evaluation of complexes 1–6 as catalysts in the
The catalytic activities of the complexes in the methoxycar-
bonylation of styrene were found to be dependent on the identity
of the complex structure/coordinated ligand. Interestingly, the
N^N donor palladium complexes (1–4) were more active than
the analogous N^P donor complexes 5 and 6. For instance, while
methoxycarbonylation of styrene
The complexes (1–6) were investigated as catalysts in the
asymmetric methoxycarbonylation of styrene, using para-tolylsul-
fonic acid (PTSA) as the acid promoter and PPh3 as additive
(Table 4). The following conditions were employed: 60 bar of CO
3

N.L. Ngcobo, S.O. Akiri, A.O. Ogweno et al.
Polyhedron 203 (2021) 115243
Table 1
Crystal data collection and refinement parameters for complexes 1, 2, 4 5 and 6.
Parameter
1
2
4
5
6
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C
15H16Cl2N2Pd
C15H16Cl2N2Pd
401.60
100 (2)
0.71073
Monoclinic
P21
7.1266 (5)
24.2585 (19)
9.0656 (6)
90.000
C14H14Cl2N2Pd
387.57
100 (2)
0.71073
Monoclinic
P21
14.4832 (8)
8.4803 (5)
18.3880 (10)
90.000
C117H114Cl10N4O2P4Pd
570.74
100 (2)
C59H57Cl7N2OP2Pd2
602.74
100 (2)
0.71073
Orthorhombic
P 21 21 21
9.1628 (7)
10.3591 (7)
29.968 (2)
90.000
401.60
100 (2)
0.71073
Monoclinic
P21
7.1677 (7)
24.265 (2)
9.0510 (8)
90.000
0.71073
Orthorhombic
P 21 21 21
9.2643 (5)
10.3531 (6)
29.5544 (16)
90.000
b (Å)
c (Å)
a
(°)
b (°)
105.495 (2)
90.000
1517.0 (7)
4
1.758
1.566
105.311 (3)
90.000
1511.64 (19)
4
1.765
1.571
103.684 (3)
90.000
2194.3 (2)
6
1.760
1.620
90.000
90.000
2834.7 (3)
4
1.337
90.000
90.000
2844.5 (4)
4
1.407
c
(°)
Volume (A3)
Z
Dcalcd (mg/m3)
Absorption coefficient (mmꢀ1
F(000)
)
0.913
1152
0.919
1216
800
800
1152
Theta range for data collection (°)
Reflections collected/unique
Completeness to theta (%)
Goodness-of-fit on F2
1.678–27.469
13,534
99.1
1.677–27.501
25,289
99.1
1.447–27.703
38,574
99.2
1.378–29.010
52,196
99.3
1.359–27.530
91,891
99.8
1.019
1.060
1.015
0.902
1.575
R indices (all data)
R1 = 0.0160
wR2 = 0.0382
R1 = 0.0165
wR2 = 0.0385
R1 = 0.0141
wR2 = 0.0323
R1 = 0.0261
wR2 = 0.0604
R1 = 0.0853
wR2 = 0.2396
Largest diff. peak and hole (e Åꢀ3
)
0.401 and ꢀ0.394
0.294 and ꢀ0.461
0.361 and ꢀ0.250
0.621 and ꢀ0.834
6.970 and ꢀ0.939
Table 2
Selected bond lengths and bond angles of the N^N complexes 1, 2 and 4.
The main thrust of this work was to develop chiral palladium(II)
catalysts for the asymmetric methoxycarbonylation of styrene to
produce chiral methyl-2-phenylpropanoate esters. Unfortunately,
all the complexes were non-enantioselective, as the branched
products gave only racemic mixtures (Figs. S59–S65). The highest
enantioselectivity achieved was 2.6% ee (Fig. S60), using complex
2 (Table 4, entry 2). This lack of enantioselectivity could be attrib-
uted to the location of the chiral centre, which is outside the palla-
dium(II) complex coordination sphere. We recently reported
similar low ee% in the transfer hydrogenation of ketones using
nickel(II) and iron(II) of these N^N-donor ligands [44].
Complex
1
2
4
Selected bond lengths(Å)
Pd(1)–N1
Pd(1)–N2
Pd(1)–Cl(1)
Pd(1)–Cl(2)
2.012(3)
2.031(30
2.295(9)
2.298(9)
2.013(3)
2.031(3)
2.295(9)
2.298(9)
2.029(2)
2.037(2)
2.290(7)
2.300(6)
Selected bond angles(°)
N
py–Pd–N1
80.10(1)
89.33(3)
175.64(8)
96.26(8)
173.56(8)
80.10(1)
89.33(3)
175.64(8)
96.26(8)
173.56(8)
80.27(8)
89.39(2)
176.48(6)
96.21(6)
174.39(6)
Cl(1)–Pd–Cl(2)
N1–Pd–Cl(1)
N(2)–Pd–Cl(1)
N2–Pd–Cl(2)
2.3.2. The effect of PPh3:Pd ratios and solvent system in the
methoxycarbonylation reactions of styrene using complex 4
Next, using the most active complex 4, we focused on the opti-
mization of the reaction conditions using styrene substrate. It has
been established that Pd:PPh3 ratio do have profound effects on
the catalytic performance of the palladium(II) complexes in the
methoxycarbonylation reactions of olefins [45]. We thus varied
the Pd:PPh3 ratio from 1:1 to 1:6 using complex 4 (Table 5, entries
1–4). An optimum Pd:PPh3 ratio of 1:2 (percentage yields of 63%)
was realized. The observed lower catalytic activities at lower Pd:
PPh3 ratio of 1:1 (23%) may be assigned to insufficient stabilization
of the active species. Indeed, in this experiment, traces of palla-
dium(0) black were observed in the reactor at the end of the reac-
tion, pointing to possible decomposition of the active species [45].
On the other hand, diminished catalytic activities at higher concen-
trations of PPh3 (Pd:PPh3 ratio of 1:4 and 1:6) of 59% and 27%
respectively (Table 5, entries 3 and 4) were observed. This may
be assigned to the competition between the coordinated PPh3
ligands and incoming styrene substrate [46]. The role of the PPh3
additive in stabilization of the active species was evident in the
lack of catalytic activity of complex 5 in the absence of PPh3
(Table 5, entry 8). In terms, of regio-selectivity, no discernible
influence of the Pd/PPh3 ratios was observed, as in all cases, the
branched esters (74%–76%) were predominant (Table 5, entries
1–4).
Table 3
Selected bond lengths and bond angles of the N^P chelated complexes 5 and 6.
Complex
5
6
Selected bond distances (Å)
Pd(1)–P(1)
Pd(1)–N(1)
Pd(1)–Cl(1)
Pd(1)–Cl(2)
2.212(3)
2.056(8)
2.284(3)
2.378(2)
2.218(3)
2.043(10)
2.282(3)
2.380(3)
Selected bond angles (°)
P(1)–Pd–N(1)
86.6(2)
86.2(3)
P(1)–Pd–Cl(1)
P(1)–Pd–Cl(2)
N(1)–Pd–Cl(1)
92.91(10)
171.85(9)
174.4(2)
93.90(12)
172.61(11)
176.0(3)
the N^N–donor complex 4 displayed percentage yields of 63%, the
analogous N^P-donor complex 5 exhibited percentage yield of only
23% (Table 4, entries 4 and 5). In terms of regio-selectivity, no dis-
cerning influence of product distributions was observed as a func-
tion of catalyst structure. In all cases, regio-selectivity towards the
branched methy-2-phenyl-propoanate esters (83–87%) were
reported. This can be rationalized from the comparable steric
crowding around the palladium(II) atoms in all the complexes, as
deduced from their molecular structures (Figs. 1 and 2).
We then probed the influence of the solvent systems in attempts
to improve the catalytic activities using complex 4 at the optimum
4

N.L. Ngcobo, S.O. Akiri, A.O. Ogweno et al.
Polyhedron 203 (2021) 115243
Table 4
iment, we used complexes 3 and 5 and Pd/PPh3/PTSA ratios of
1:2:10, over a 48 h period (Figs. 3 and S66). The use of 1H NMR
spectroscopy enabled us to assess the possibility of ligand dissoci-
ation in complex 3. From the 1H NMR spectra (Fig. S66), the signa-
ture peak of methine *CH proton in complex 3 was recorded as a
quartet at 6.01 ppm. However, when complex 3 was subjected to
PTSA/PPh3 for a duration of 5 h, the signal was reported up-field
at 4.37 ppm, corresponding to the free ligand signal (Fig. S66). This
was indicative of ligand dissociation from the palladium(II) coordi-
nation sphere under the experimental conditions. Hence, the low
catalytic activities observed for complexes 1–6 in the methoxycar-
bonylation of styrene could be attributed to the decomposition of
the active species.
The methoxycarbonylation of styrene catalyzed by complexes 1–6.a
OCH3
O
OCH3
CO/MeOH
Catalyst
+
O
Entry
Catalyst
% Yieldb
B/Lc
ee%
1
2
3
4
5
6
1
2
3
4
5
6
59
54
61
63
23
19
84/16
87/13
85/15
85/15
84/16
83/17
1.8
2.6
0.4
2.4
1.4
1.8
Similarly, upon subjecting complex 5 to the same conditions,
31P NMR spectra exhibited five signals at 20 ppm, 23 ppm,
ꢀ5 ppm, ꢀ12 ppm and ꢀ14 ppm in comparison to one signal at
31.4 ppm in complex 5 (Fig. 3). The signals at ꢀ5 ppm and
ꢀ13 ppm correspond to the free PPh3 and ligand L5 respectively.
The absence of the original complex signal at 31.4 ppm implies that
complex 5 underwent partial hydrolysis to form the free ligand and
the starting materials as shown in Scheme 2. The partial decompo-
sition of complex 5 could therefore explain the low catalytic activ-
ities of complexes 5 and 6.
a
Reaction conditions: Pressure: 60 bar; time: 24 h; temp: 90 °C; [Pd]:[PPh3]
[acid]:[styrene] ratio; 1:2:10:200, acid: PTSA; Solvent: methanol 50 mL and toluene
50 mL; 0.5% mol palladium (0.08 mmol); styrene (1.8 mL, 16.0 mmol).
b
% of styrene converted to esters determined from GC using ethylbenzene as
internal standard.
c
Molar ratio between branched and linear ester.
Table 5
Effect of solvents and solvent systems on catalytic behaviour of complex 4 in the
methoxycarbonylation of styrene.a
To further understand the role of the PPh3 group in generation
of the active species, we also performed in situ 31P NMR studies
using complex 5 in the absence of PPh3. From Fig. 4, it was
observed that complex 5 undergoes a complete decomposition to
the respective ligand L5 (signal at ꢀ13 ppm) within 3 h. This is
consistent with extensive formation of palladium black observed
in the reactor in the reaction done without a PPh3 and lack of cat-
alytic activity reported (Table 5, entry 8). From Figs. 3 and 5, it is
evident that the dissociation of the ligand is partial in the presence
of PPh3, due to the signals corresponding to the coordinated ligand
and PPh3 and free PPh3 (spectra C). This corroborates the catalytic
activities reported upon addition of PPh3.
Entry
Pd:PPh3
Solvent system
%Yieldb
B/Lc,d
1
2
3
4
5
6
7
8
1:1
1:2
1:4
1:6
1:2
1:2
1:2
1:0
MeOH:Toluene (1:1)
MeOH:Toluene (1:1)
MeOH:Toluene (1:1)
MeOH:Toluene (1:1)
MeOH:Toluene (1:15)
MeOH:Toluene (15:1)
MeOH:THF (1:1)
23
62
59
27
25
54
30
Trace
76/24
76/14
74/26
75/25
95/5
41/59
95/5
–
MeOH:Toluene (1:1)
dNo PPh3 added.
a
Reaction conditions: Pressure: 60 bar; time: 24 h temp: 90 °C; [Pd]:[PPh3]
[acid]:[styrene] ratio; 1:2:10:200 acid: PTSA; 0.5% mol palladium (0.09 mmol);
styrene (2 mL, 17.4 mmol).
b
% of styrene converted to esters determined from GC using ethyl benzene as
internal standard.
2.4. Conclusions
c
Molar ratio between branched and linear ester.
d
No PPh3 added.
In summary, four (imino)pyridine ligands (L1–L4) and two
(imino)phosphine ligands (L5 and L6) were prepared and fully
characterized by NMR techniques, mass spectroscopy and infrared
spectroscopy. Reactions of ligands L1–L6 with [Pd(COD)Cl2]
afforded the corresponding palladium(II) complexes 1–6 in high
yields. The solid-state structures of complexes 1–6, showed that
the (imino)pyridine and (imino)phosphine ligands coordinate to
the palladium(II) in a bidentate fashion via N^N-donor and N^P-
donor atoms respectively to give distorted square-planar geome-
try. The complexes demonstrate low to moderate catalytic activi-
ties in the methoxycarbonylation of styrene. High regio-
selectivity towards branched ester products were exhibited, with
low ee%. Decomposition of the active species via ligand dissocia-
tion was established from NMR studies.
Pd/PPh3 ratio of 1/2 (Table 5, entries 5–7). Our results appear to sug-
gest that both the identity of the solvents and the respective ratios
influence the catalytic behaviour of complex 4. For example, the
best solvent system was methanol:toluene (1:1). This trend has
been observed by other researchers in the methoxycarbonylation
of olefins [25,53]. With respect to changing the nature of the sol-
vent, the use of methanol:THF (1:1) only gave lower percentage
yields of 30%. This observation may be attributed to the higher
polarity of THF, rendering the solvent medium more polar, thus
reducing substrate solubility in the reaction medium [47]. A notable
observation was the significant reduction of branched esters from
95% and 41% when the methanol:toluene ratio was changed from
1:15 to 15:1 respectively (Table 5, entries 5 and 6). Lower methanol
concentrations decrease the polarity of the reaction mixture, and
has been shown to favour the formation of branched esters due to
decreased Pd-ligand dissociation [48–51].
3. Experimental section
3.1. General materials and methods
2.3.3. NMR studies of decomposition pathways of the active species
In comparison to similar N^N and N^P catalysts, the current
complexes 1–6 displayed lower catalytic activities under compara-
ble conditions [10,11,52,53]. This prompted us to carry out some
mechanistic studies, with the view of understanding the driving
forces behind the observed low catalytic activities. To achieve this,
in-situ 1H NMR and 31P NMR spectroscopy experiments of the reac-
tion mixtures were performed to understand the possible nature of
the active species and their respective stability. In a typical exper-
All solvents were purchased from Sigma Aldrich and were dried
following relevant methods follows: Methanol was dried over 3 Å
molecular sieves overnight followed by distillation [54], toluene
was dried over sodium wire and benzophenone while acetone,
diethyl ether was dried via distillation using calcium hydride
[54], dichloromethane was dried over P2O5 and distilled prior to
use [55]. The chemicals, chloroform (CDCl3) alpha (S)-
a-Methyl-
benzylamine (ꢁ99.5%), (R)- -methylbenzylamine (ꢁ99.5%), 2-
a
5

N.L. Ngcobo, S.O. Akiri, A.O. Ogweno et al.
Polyhedron 203 (2021) 115243
Fig. 3. Overlaid 31P NMR spectra for in situ experiment of complex 5 mixed with Pd/PPh3PTSA (1:10:2 ratio) in CDCl3 over 5 h period and 2-(diphenylphosphine)
benzaldehyde (SM).
Scheme 2. Proposed deactivation pathway of complex 5 in the presence of PTSA.
pyridinecarboxaldehyde (ꢁ99%), 2-acetylpyridine (98%) 2-
(Diphenylphosphine)benzaldehyde, 2-methoxyethanamine, N1,
N1-diethylethane-1,2-diamine 2-hydroxybenzaldehyde and palla-
dium acetate were purchased from Sigma Aldrich and were used
without further purification. Starting material [Pd(COD)Cl2] and
was synthesized following literature method [56]. The chiral
(imino)pyridine ligands (S)-1-phenyl-N-(1-(pyridin-2-yl)ethyli-
dene)ethanamine (L1), (R)-1-phenyl-N-(1-(pyridin-2-yl)ethyli-
dene)ethanamine (L2), (S)-1-phenyl-N-((pyridin-2-yl)methylene)
ethanamine (L3), (R)-1-phenyl-N-((pyridin-2-yl)methylene)etha-
namine (L4), were prepared following published literature meth-
ods [30–32]. Detail spectroscopic data are provided as
Supplementary materials. Fourier transform infrared (FT-IR)
spectra were recorded with a PerkinElmer spectrophotometer in
the range 650–4000 cmꢀ1. All 1H NMR (400 MHz) and 31P NMR
(162 MHz) spectra were recorded at 25 °C with deuterated
CDCl3 a Bruker NMR spectrometer. 13C NMR spectra were obtained
a Varian Mercury 100 MHz on a Bruker spectrometer equipped
with Bruker magnet (9.395 T) at 25 °C. The chemical shift in
nuclear magnetic spectroscopies were all recorded in ppm
relative to CDCl3
d
1H: 7.26 ppm and d 13C: 77.6 ppm. Elemental
analyses were performed on Thermal Scientific Flash 2000 and
mass spectra were recorded on LC Premier micro-mass
Spectrometer.
6

N.L. Ngcobo, S.O. Akiri, A.O. Ogweno et al.
Polyhedron 203 (2021) 115243
Fig. 4. Overlaid 31P NMR spectra for in situ experiment of complex 5 and PTSA (Pd/PTSA ratio of 1:10) in the absence of PPh3 stabilizer in CDCl3 over a 5 h period. The data
shows complete decomposition of complex 5 to the respective ligand within 3 h.
Fig. 5. Overlaid 31P NMR spectra for in situ experiment of complex 5 comparing the peaks for Pd/PTSA (1:10) and Pd/PTSA/PPh3 in CDCl3 at 3 h. A (complex 5), B (ligand L5), C
(complex 5/PTSA/PPh3), D (complex 5/PTSA).
3.2. Synthesis of (imino)phosphine ligands and their palladium(II)
complexes
3.2.2. (R)-N-(2-(diphenylphosphino)benzylidene)-1-
phenylethanamine (L6)
Ligand L6 was synthesized following the same procedure used
in the synthesis of ligand L5 using 2-(diphenylphosphine)ben-
zaldehyde (0.25 g, 0.86 mmol) and (R)-methylbenzylamine
(1.05 g, 0.86 mmol). Yield = 0.32 g (94%). 1H NMR (400 MHz, CDCl3,
3.2.1. Synthesis of (S)-N-(2-(diphenylphosphino)benzylidene)-1-
phenylethanamine (L5)
A
mixture of 2-(diphenylphosphine)benzaldehyde (0.20 g,
3
3
0.69 mmol), (S)-methylbenzylamine (0.84 g, 0.69 mmol) in CH2Cl2
(20 mL), under nitrogen atmosphere, was stirred for 3 days. A yel-
low oily product was obtained after removal of solvent by vac-
uum, L5 Yellow oil. Yield = 0.26 g (95%).1H NMR (400 MHz,
CDCl3, d ppm): 1.39 (d, 3H, 3JHH = 7 Hz, CCH3); 4.45 (q, 1H, 3JHH = 7-
Hz, NCH); 6.91 (m, 1H, Haromatic); 7.37–7.21 (m, 17H, Haromatic);
d ppm): 1.42 (d, 3H, JHH = 7 Hz, CCH3); 4.49 (q, 1H, JHH = 7 Hz,
NCH); 6.94 (m, 1H, Haromatic); 7.37–7.21 (m, 17H, Haromatic); 8.08
3
(m, 1H, He); 8.99 (d, 1H, JHH = 5 Hz, Himine). 13C NMR (400 MHz,
CDCl3, d ppm): 24.50 (CH3), 69.77 (C-CH3), 126.73 (C-Ar), 128.12
(2C-Ar), 128.16 (2C-Ar), 128.39 (C-Ar), 128.65 (C-Ar), 128.73 (C-
Ar), 128.90 (C-Ar), 130.22 (C-Ar), 133.25 (C-Ar), 133.73 (C-Ar),
134.06 (C-Ar), 134.12 (C-Ar), 134.26 (C-Ar), 134.32 (C-Ar), 136.82
(C-Ar), 137.63 (C-Ar), 139.58 (C-Ar), 139.74 (C-Ar), 144.87 (C-Ar),
158.20 (C-Ar), 158.40 (C@N). 31P NMR (400 MHz, CDCl3, d, ppm):
3
8.08 (m, 1H, Haromatic) 8.99 (d, 1H, JHH = 5 Hz, Himine). 13C NMR
(400 MHz, CDCl3, d ppm): 24.50 (CH3), 69.77 (C-CH3), 126.73
(C-Ar), 128.12 (2C-Ar), 128.16 (2C-Ar), 128.39 (C-Ar), 128.65 (C-
Ar), 128.73 (C-Ar), 128.90 (C-Ar), 130.22 (C-Ar), 133.25 (C-Ar),
133.73 (C-Ar), 134.06 (C-Ar), 134.12 (C-Ar), 134.26 (C-Ar),
134.32 (C-Ar), 136.82 (C-Ar), 137.63 (C-Ar), 139.58 (C-Ar),
139.74 (C-Ar), 144.87 (C-Ar), 158.20 (C-Ar), 158.40 (C@N). 31P
NMR (400 MHz, CDCl3, d ppm): ꢀ13.0 (s). MS (ESI+) Calcd for
ꢀ13.0 (s). MS (ESI+) Calcd. for
(100%) = 393.172; Found: m/z (M+H2O, 100%) = 409.4708; m/
z = (M, 28%) = 393.50. FT-IR: t(C@N)pyridine = 1634 cmꢀ1
C14H14N2 (M+H): m/z
.
3.2.3. [{(S)-1-phenyl-N-(1-(pyridin-2-yl)ethylidene)ethanamine}
PdCl2] (1)
To a solution of [Pd(COD)Cl2] (0.10 g, 0.39 mmol) in CH2Cl2
(5 mL) was added to a solution of ligand L1 (0.09 g, 0.39 mmol)
C
27H24NP (M+H): m/z (100%) = 393.16; Found: m/z = (M+H2O,
100%) 409.4708; m/z (M, 30%) 393.50. T-IR: t(C@N)
= 1634 cmꢀ1
=
=
=
.
imine
7

N.L. Ngcobo, S.O. Akiri, A.O. Ogweno et al.
Polyhedron 203 (2021) 115243
3
in CH2Cl2 (5 mL) and the mixture stirred for 24 h. The solution was
concentrated in vacuo and Et2O (10 mL) added to precipitate an
orange solid. Recrystallization of the crude product by slow diffu-
sion of hexane in solution of dichloromethane containing the com-
pound gave single crystals suitable for X-ray analyses.
Yield = 0.14 g (82%). 1H NMR (400 MHz, CDCl3, d ppm) 1.86 (d,
3H, 3JHH = 7.2 Hz, NCCH3); 2.17 (s, 3H, CCH3); 6.74 (q, 1H, 3JHH = 7.2-
Hz, NCH); 7.32 (d, 1H, 3JHH = 7.2 Hz, 4Ph); 7.38 (t, 2H, 3JHH = 7.6 Hz,
2,6Ph); 7.45 (d, 2H, 3JHH = 7.6 Hz, 3,5Ph); 7.78 (t, 2H, 3JHH = 8.4 Hz,
NCH); 7.42 (t, 1H, JHH = 4.0 Hz 4Ph) 7.48–7.70 (m, 4H, 2,3,5,6Ph);
3
7.78 (dt, 2H; 8.09, 3,5Py) 7.80 (t, 1H, JHH = 7.6 Hz, 4Py); 8.09 (s,
1H, Himine), 9.42 (d, 1H, 3JHH = 5.2 Hz, 6Py). 13C NMR (400 MHz, CDCl3,
d ppm): 24.58 (CH3-C), 69.55 (C-CH3), 121.46 (C-Py), 124.69 (C-Ar),
126.73 (C-Py), 127.02 (2C-Ar), 128.51 (2C-Ar), 136.48 (C-Py),
144.60 (C-Ar), 149.34 (C-Py), 154.82 (C-Py), 160.46 (C@N). MS (ESI
+): calcd. m/z (100%) = 385.96, Found: m/z (M+Na) = 408.86. FT-IR:
t
C
(C@N)pyridine = 1445 cmꢀ1 t(C@N)imine = 1594 cmꢀ1. Anal. Calcd. for
;
14H14N2Cl2Pd: C, 43.50; H, 3.39; N, 7.25. Found: 43.86, 3.64, 7.37.
3
3
4,6Py); 8.15 (t, 1H, JHH = 7.6 Hz, 3Py); 9.50 (d, 1H, JHH = 5.6 Hz,
6Py). 13C NMR (400 MHz, CDCl3, d ppm): 19.39 (CH3-CN), 26.60
(CH3-C), 61.75 (C-Ph), 125.21 (C-Py), 126.22 (C-Ar), 127.61 (C-Py),
128.26 (C-Ar), 128.31 (C-Ar), 128.58 (C-Ar), 129.04 (C-Ar), 133.10
(C-Py), 137.19 (C-Ar), 141.07 (C-Py), 151.49 (C-Py), 158.28 (CN-
CH3). MS (ESI+): calcd. m/z (100%) = 399.97; Found: m/z (M
3.2.7. [{(S)-N-(2-(diphenylphosphino)benzylidene)-1-
phenylethanamine} PdCl2] (5)
To a solution of [Pd(COD)Cl2] (0.05 g, 0.19 mmol) with dichlor-
omethane (5 mL) was added to ligand L5 (0.07 g, 0.19 mmol) in
CH2Cl2 (5 mL) and the mixture stirred for 24 h. The solution was
concentrated in vacuo and diethylether (10 mL) added to precipi-
tate a yellow solid. Recrystallization of the crude product by slow
diffusion of hexane in solution of dichloromethane containing
the compound gave single crystals suitable for X-ray analyses.
Complex 6 Yield = 0.06 g (81%). 1H NMR (400 MHz, CDCl3, d
+Na) = 424.37. FT-IR: t ; t(C@N)imine = 1593-
(C@N)pyridine = 1438 cmꢀ1
cmꢀ1. Anal. Calcd. for C15H16N2Cl2Pd: C 44.97, H 3.77, N 6.99.
Found: C, 44.89; H 3.84; N, 6.84.
3.2.4. [{(R)-1-phenyl-N-(1-(pyridin-2-yl)ethylidene)ethanamine}
PdCl2] (2)
3
3
ppm):1.57 (d, 3H, JHH = 7.0 Hz, CCH3); 6.80 (q, 1H, JHH = 6.6 Hz,
NCH); 6.95 (t, 1H, 3JHH = 7.7 Hz, Haromatic); 7.12 (d, 2H, 3JHH = 6.8 Hz,
Haromatic); 7.31 (m, 4H, Haromatic); 7.37 (d, 5H, Haromatic); 7.60 (s, 1H,
Complex 2 was synthesized following the procedure described
for compound 1 using [Pd(COD)Cl2] (0.10 g, 0.39 mmol) and L2
(0.09 g, 0.39 mmol) were used. Yield = 0.14 g (89%). Recrystalliza-
tion of complex 2 by slow evaporation of CH2Cl2 solution afforded
single crystals suitable for X-ray analyses 1H NMR (400 MHz,
H
aromatic); 7.69 (m, 1H, Haromatic); 7.73 (s, 1H, Himine). 13C NMR
(400 MHz, CDCl3, d ppm): 26.60 (CH3), 69.87 (C-CH3), 116.61 (C-
Ar), 121.55 (C-Ar), 122.04 (C-Ar), 124.01 (C-Ar), 124.62 (C-Ar),
126.10 (C-Ar), 126.67 (C-Ar), 128.31 (C-Ar), 128.91 (C-Ar), 129.02
(C-Ar), 129.14 (C-Ar), 132.19 (C-Ar), 132.61 (C-Ar), 133.10 (C-Ar),
133.85 (C-Ar), 134.38 (2C-Ar), 134.49 (C-Ar), 135.51 (C-Ar),
135.59 (C-Ar), 137.19 (C-Ar), 137.66 (C-Ar), 139.01 (C-Ar), 162.03
(C-Ar), 162.11 (C@N). MS (ESI+): calcd. m/z (100%) = 569.01, Found:
m/z (M+H) = 573.0. 31P NMR (400 MHz, CDCl3, d ppm): 31.4 (s). FT-
3
CDCl3, d ppm): 1.87 (d, 3H, JHH = 7.2 Hz, NCCH3); 2.16 (s, 3H,
3
3
CCH3); 6.73 (q, 1H, JHH = 7.2 Hz, NCH); 7.32 (d, 1H, JHH = 7.2 Hz,
3
3
4Ph); 7.39 (t, 2H, JHH = 7.6 Hz, 2,6Ph); 7.45 (d, 2H, JHH = 7.6 Hz,
3,5Ph); 7.72 (d, 2H, 3JHH = 8.4 Hz, 4,5Py); 8.16 (t, 1H, 3JHH = 7.6 Hz,
3Py); 9.52 (d, 1H, JHH = 5.6 Hz, 6Py). 13C NMR (400 MHz, CDCl3, d
3
ppm): 19.39 (CH3-CN), 26.60 (CH3-C), 61.75 (C-Ph), 125.21 (C-Py),
126.22 (C-Ar), 127.61 (C-Py), 128.26 (C-Ar), 128.31 (C-Ar), 128.58
(C-Ar), 129.04 (C-Ar), 133.10 (C-Py), 137.19 (C-Ar), 141.07 (C-Py),
151.49 (C-Py), 158.28 (CN-CH3). MS (ESI+): calcd. m/z
(100%) = 399.97; Found: m/z = 400.0. FT-IR: t(C@N)pyridine = 1445-
IR:
t
(C@N)imine = 1626 cmꢀ1. Anal. Calcd. for C27H24NCl2PPdꢂCH2Cl2:
C, 55.73; H, 4.19; N, 2.39. Found: C, 55.69; H, 4.07; N, 2.08.
3.2.8. [{(R)-N-(2-(diphenylphosphino)benzylidene)-1-
phenylethanamine} PdCl2] (6)
cmꢀ1; 1601
t
(C@N)imine = 1601 cmꢀ1. Anal. Calcd. for C15H16N2Cl2Pd:
C, 44.67; H, 3.77; N, 6.99. Found: 44.77, 3.73, 7.27.
Complexes 6 was synthesized following the procedure
described for compound 5 using Pd(COD)Cl2] (0.15 g, 0.51 mmol)
3.2.5. [{(S)-1-phenyl-N-((pyridin-2-yl)methylene)ethanamine}PdCl2
(3)
and L6 (0.2 g, 0.51 mmol). Complex 7 Yield = 0.22 g (77%). 1H
3
NMR (400 MHz, CDCl3, d ppm): 1.57 (d, 3H, JHH = 7.0 Hz, CCH3);
Complex 3 was synthesized following the procedure described
for compound 1 using [Pd(COD)Cl2] (0.10 g, 0.39 mmol) and L3
(0.08 g, 0.39 mmol). Yield = 0.12 g (80%). Recrystallization of com-
plex 3 by slow diffusion of hexane in a CH2Cl2 solution afforded
single crystals suitable for X-ray analyses 1H NMR (400 MHz,
6.80 (q, 1H, 3JHH = 6.8 Hz, NCH); 6.95 (t, 1H, 3JHH = 7.8 Hz, Haromatic);
7.18 (d, 2H, 3JHH = 6.8 Hz, Haromatic); 7.31 (m, 5H, Haromatic); 7.47 (d,
5H, Haromatic); 7.54 (m, 4H, Haromatic); 7.60 (m, 1H, He); 7.69 (m, 1H,
H
aromatic). 7.74 (s, 1H, Himine). 13C NMR (400 MHz, CDCl3, d ppm):
26.60 (CH3), 69.87 (C-CH3), 116.61 (C-Ar), 121.55 (C-Ar), 122.04
(C-Ar), 124.01 (C-Ar), 124.62 (C-Ar), 126.10 (C-Ar), 126.67 (C-Ar),
128.31 (C-Ar), 128.91 (C-Ar), 129.02 (C-Ar), 129.14 (C-Ar), 132.19
(C-Ar), 132.61 (C-Ar), 133.10 (C-Ar), 133.85 (C-Ar), 134.38 (2C-
Ar), 134.49 (C-Ar), 135.51 (C-Ar), 135.59 (C-Ar), 137.19 (C-Ar),
137.66 (C-Ar), 139.01 (C-Ar), 162.03 (C-Ar), 162.11 (C@N). 31P
NMR (400 MHz, CDCl3, d ppm): 31.4 (s). MS (ESI+): calcd. m/z
3
3
3
CDCl3, d ppm): 1.89 (d, 3H, JHH = 6.8 Hz, CCH3); 6.09 (q, 1H,
-
-
3
JHH = 7.2 Hz, NCH); 7.44 (t, 1H, JHH = 4.0 Hz, 4Ph); 7.47 (d, 4H,
3
JHH = 4.4 Hz, 2,3,5,6Ph); 7.67 (t, 1H, JHH = 6.8 Hz, 5Py); 7.76 (d,
1H, 3JHH = 7.6 Hz, 3Py); 7.88 (s, 1H, 4Py); 8.11 (t, 1H, 3JHH = 7.6 Hz,
H
imine); 9.35 (d, 1H, 3JHH = 5.2 Hz, 6Py). 13C NMR (400 MHz, CDCl3, d
ppm): 21.0 (CH3-C), 64.74 (C-CH3), 128.56 (3C-Ar, C-Py), 129.32
(2C-Ar), 138.15 (C-Py), 140.45 (C-Ar), 151.14 (C-Py), 155.54 (C-
Py), 167.04 (C@N). MS (ESI+): calcd. m/z (100%) = 385.96, Found:
(100%)
= 569.01, Found: m/z (M+H) = 573.0. FT-IR: t(C@N)
= 1625 cmꢀ. 1 Anal. Calcd. for C27H24NCl2PPdꢂCH2Cl2: C, 54.82;
imine
m/z = 386.42. FT-IR: t(C@N)pyridine = 1445 cmꢀ1
;
t(C@N)imine = 1595-
H, 4.15; N, 2.34. Found: C, 54.96; H, 4.18; N, 2.12.
cmꢀ1. Anal. Calcd. for C14H14N2Cl2Pd: C, 43.50; H, 3.39; N, 7.25.
Found: 43.68, 3.54, 7.77.
3.3. X-ray crystallography analyses of thee complexes
3.2.6. [{(R)-1-phenyl-N-((pyridin-2-yl)methylene)ethanamine}PdCl2]
(4)
The X-ray data were recorded on a Bruker Apex Duo equipped
with an Oxford Instruments Cryojet operating at 100(2) K and an
Incoatec micro-source operating at 30 W power. The data were col-
Complex 2 was synthesized following the procedure described
for compound 1 using [Pd(COD)Cl2] (0.10 g, 0.39 mmol) and L4
(0.08 g, 0.39 mmol). Yield = 0.11 g (75%). Recrystallization of com-
plex 4 was done by slow evaporation of CH2Cl2 solution afforded sin-
gle crystals suitable for X-ray analyses. 1H NMR (400 MHz, CDCl3, d
lected with Mo Ka (k = 0.71073 Å) radiation at a crystal-to-detector
distance of 50 mm. The following conditions were used for the data
collection: omega and phi scans with exposures taken at 30 W X-
ray power and 0.50° frame widths using APEX2 [57]. The data were
reduced with the Programme SAINT [58] using outlier rejection,
3
3
ppm): 1.90 (d, 3H, JHH = 6.8 Hz, CCH3); 6.12 (q, 1H, JHH = 7.2 Hz,
8

N.L. Ngcobo, S.O. Akiri, A.O. Ogweno et al.
Polyhedron 203 (2021) 115243
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were located in the difference density map and refined isotopically.
3.4. General procedure for the methoxycarbonylation reactions
The catalytic methoxycarbonylation reactions were performed
in a stainless-steel autoclave equipped with a temperature control
unit and a sample valve. In an experiment the Pd complex, PPh3,
PTSA and styrene (1:2:10:200) were dissolved in a mixture of
methanol and toluene. The reactor was evacuated, and the catalytic
solution was introduced to the reactor via a cannula. The reactor
was purged three times with CO, and then set at the required pres-
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500 rpm. At the end of the reaction time, the reaction was cooled,
excess CO was vented off and samples drawn for GC analysis to
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CRediT authorship contribution statement
Nondumiso L. Ngcobo: Investigation, Validation, Software.
Saphan O. Akiri: Investigation, Validation, Software. Aloice O.
Ogweno: Investigation, Validation, Software. Stephen O. Ojwach:
Supervision, Resources, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
[19] M. Beller, Catalytic Carbonylation Reactions, Springer, 2006.
[20] M. Beller, J. Seayad, A. Tillack, H. Jiao, Catalytic Markovnikov and anti-
Markovnikov functionalization of alkenes and alkynes: recent developments
and trends, Angew. Chem. Int. Ed. 43 (2004) 3368–3398.
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Enantioselective bis-alkoxycarbonylation of styrene catalyzed by novel chiral
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196 (2003) 171–178.
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Acknowledgements
Bursaries to NL Ngcobo and SO Akiri provided by DST-NRF
(South Africa) Centre of Excellence in Catalysis (c*change,
OLE10.3-UKZN project). Postdoctoral research fellowship to AO
Ogweno received from The National Research Foundation (CPRR-
93898). Financial support to SO Ojwach received from the Univer-
sity of KwaZulu-Natal.
of 1,2-disubstituted olefins catalyzed by aryl
catalysts, Adv. Synth. Catal. 360 (2018) 3507–3517.
[23] T.A. Tshabalala, S.O. Ojwach, M.A. Akerman, Palladium complexes of
(benzoimidazol-2-ylmethyl) amine ligands as catalysts for
a-diimine palladium (II)
methoxycarbonylation of olefins, J. Mol. Catal. A: Chem. 406 (2015) 178–184.
[24] T.A.Tshabalala, S.O. Ojwach,Tuningtheregioselectivityof(benzimidazolylmethyl)
amine palladium (II) complexes in the methoxycarbonylation of hexenes and
octenes, Transit. Met. Chem. 43 (2018) 339–346.
[25] S.O. Akiri, S.O. Ojwach, Methoxycarbonylation of olefins catalysed by
homogeneous palladium (II) complexes of (phenoxy) imine ligands bearing
alkoxy silane groups, Inorg. Chim. Acta 489 (2019) 236–243.
[26] S.O. Akiri, S.O. Ojwach, Synthesis of MCM-41 immobilized (Phenoxy) imine
palladium (II) complexes as recyclable catalysts in the methoxycarbonylation
of 1-hexene, Catalysts 9 (2019) 143.
[27] S. Zulu, M.G. Alam, S.O. Ojwach, M.P. Akerman, Structural and theoretical
studies of the methoxycarbonylation of higher olefins catalysed by (pyrazolyl-
ethyl) pyridine palladium (II) complexes, Appl. Organomet. Chem. 33 (2019)
e5175.
[28] Z. Zulu, G.S. Nyamato, T.A. Tshabalala, S.O. Ojwach, Palladium (II) complexes of
(pyridyl) imine ligands as catalysts for the methoxycarbonylation of olefins,
Inorg. Chim. Acta 501 (2020) 119270.
[29] S.O. Akiri, S.O. Ojwach, Structural studies and applications of water soluble
(phenoxy) imine palladium (II) complexes as catalysts in biphasic
methoxycarbonylation of 1-hexene, J. Organomet. Chem. 121812 (2021).
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Appendix A. Supplementary data
Supplementary materials contain additional synthetic proto-
cols, spectroscopic and analytical data for the ligands and com-
plexes; NMR and IR spectroscopic and mass spectral data of the
compounds, GC chromatograms and GC-MS of the methoxycar-
bonylation products and X-ray crystallography files. The CCDC data
entries for the structures are CCDC: 2066900, 2066903, 2066930,
2066932 and 2066935 for compounds 1, 2, 4, 5 and 6. Supplemen-
tary data to this article can be found online at https://doi.org/10.
1016/j.poly.2021.115243.
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10