Catalytic activity of Pd dithiolate complexes with large-bite-angle diphosphines in Heck coupling reactions

Article abstract of DOI:10.1002/aoc.5405

Palladium(II) complexes of aryl dithiolates and wide-bite-angle diphosphines Xantphos and dppf have been developed as efficient catalysts in Suzuki and Suzuki carbonylation reactions. The catalytic activity of these highly stable, discrete and charged complexes was investigated in Heck coupling reactions of styrene and a variety of aryl bromides. Under optimized reaction conditions these palladium complexes showed excellent activity with high turnover number (6 × 10⁶) and high turnover frequency (4 × 10⁵ h^?1). The effect of bite angle of diphosphines on the catalytic activity of the complexes [Pd₂(P^∩P)₂(SC₁₂H₈S)]₂(OTf)₄ followed the trend P^∩P = Xantphos > dppf > dppe as the order of their bite angles. The catalyst could be reused, and after three cycles the formation of significant amount of Pd nanoparticles was noticed, which were characterized using powder X-ray diffraction, energy-dispersive X-ray analysis and transmission electron microscopy. The high catalytic activity has been attributed to the Pd nanoparticles.

Full text of DOI:10.1002/aoc.5405

Received: 30 August 2019
DOI: 10.1002/aoc.5405
Revised: 25 October 2019
Accepted: 6 November 2019
F U L L P A P E R
Catalytic activity of Pd dithiolate complexes with large-bite-
angle diphosphines in Heck coupling reactions
1
2
1,3
Pravin A. Mane | Suman Neogy | Sandip Dey
1
Chemistry Division, Bhabha Atomic
Palladium(II) complexes of aryl dithiolates and wide-bite-angle diphosphines
Xantphos and dppf have been developed as efficient catalysts in Suzuki and
Suzuki carbonylation reactions. The catalytic activity of these highly stable, dis-
crete and charged complexes was investigated in Heck coupling reactions of
styrene and a variety of aryl bromides. Under optimized reaction conditions
these palladium complexes showed excellent activity with high turnover num-
Research Centre, Mumbai, 400 085, India
2
Mechanical Metallargy Division, Bhabha
Atomic Research Centre, Mumbai,
400 085, India
3
Homi Bhabha National Institute,
Mumbai, 400 094, India
Correspondence
Sandip Dey, Chemistry Division, Bhabha
Atomic Research Centre, Mumbai
6
5
−1
ber (6 × 10 ) and high turnover frequency (4 × 10 h ). The effect of bite angle
\
of diphosphines on the catalytic activity of the complexes [Pd (P P)
2
2
\
400 085, India.
(
SC H S)] (OTf) followed the trend P P = Xantphos > dppf > dppe as the
12
8
2
4
Email: dsandip@barc.gov.in
order of their bite angles. The catalyst could be reused, and after three cycles
the formation of significant amount of Pd nanoparticles was noticed, which
were characterized using powder X-ray diffraction, energy-dispersive X-ray
analysis and transmission electron microscopy. The high catalytic activity has
been attributed to the Pd nanoparticles.
K E Y W O R D S
bite angle, dithiolate, Heck reaction, macrocyclic Pd complex, Xantphos
[
6–8]
1
| INTRODUCTION
Phosphines
and their modified forms played a crucial
role in the initial development of homogeneous Pd cata-
lysts until the 1990s. Later Pd complexes of N-based
Carbon–carbon bond forming reactions using Pd catalysts
has led to a revolution in the preparation of organic com-
pounds, biomolecules, pharmaceuticals and fine
[
9]
[10]
[8]
ligands,
imines
N-heterocyclic
and palladacycles
carbenes,
oximes,
[
11]
[3,8,11,12]
have emerged as
[1,2]
chemicals.
Multistep organic synthesis can be conve-
highly efficient catalysts in Suzuki and Heck C–C cross-
coupling reactions.
Each catalyst has its specific features and advantages.
The electron-donating property of chalcogen elements
has been explored in the last decade, and as a result a
niently avoided by applying C–C bond forming reactions
performed under mild reaction conditions. In particular,
the Suzuki and Heck cross-coupling reactions are the
most widely used arylating protocols for the preparation
of biaryls and aryl-substituted olefins. A wide variety of
compounds including coordination complexes, Pd on
[
12–15]
handful of palladium complexes with S-based
Se-based
and
ligands have been recognized as
phosphine-free catalysts. Sulfur ligands have now been
[
16–18]
3
4
solid carriers or polymers and Pd nanoparticles
[3–5]
[19]
(PdNPs)
have been reported as homogeneous and
established as catalyst promoters,
rather than catalyst
as an old misconception. Carbene,
[
20]
[13,14]
heterogeneous catalysts for C–C coupling reactions. It is
well established that reactions involving soluble Pd com-
plexes are much faster and more efficient than those
involving the compounds in heterogeneous form.
poison
[
15]
[12]
cyclometalated
and pincer-type
Pd(II) complexes
derived from S-based ligands are well-known highly
active catalysts in C–C coupling reactions. In fact, with
Appl Organometal Chem. 2019;e5405.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 9
https://doi.org/10.1002/aoc.5405

2
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MANE ET AL.
their features of convenient synthesis, easier handling
and enhanced stability owing to the stronger Pd–S bond,
the palladium thiolate complexes can be more robust cat-
alysts than complexes derived from carbene-, amine-,
thioether- and selenium-based ligands. Several
design of co-ligands, square planar Pd(II) complexes can
be constructed as soluble discrete molecules with thiolate
ligands. We have developed supramolecular palladium
dithiolate complexes with capping ligand dppe, namely
[Pd (dppe) {SX(C H ) XS}] (OTf) (n = 1, 2; m = 2, 4;
2
2
6
4 n
m
2m
[21]
Pd(II) catalysts made from carbene–thiolate
and
X = none, CH ; dppe = 1,2-bis(diphenylphosphino)eth-
2
[22–24]
[25]
hybrid thiolate (NS,
PS ) ligands have been suc-
ane), as discrete molecules. These complexes showed
[
30]
[31]
cessfully applied in C–C coupling reactions. Nevertheless,
examples of C–C coupling reactions catalyzed by mac-
rocycles or supramolecular coordination complexes of Pd
are rare. Although a library of Pd and Pt supramolecular
coordination complexes have been prepared by self-
assembly processes, only a handful of examples exist in
terms of molecular recognition, sensing or catalysts in
excellent catalytic activity in Suzuki
and Heck
C–C
coupling reactions. Recently, these dithiolate complexes
have been modified by replacing dppe with wide-bite-
angle phosphines
(diphenylphosphino)xanthenes) and dppf (1,1 -bis(diph-
enylphosphino)ferrocene)
Xantphos
(9,9-dimethyl-4,5-bis
0
[
32]
(Scheme 1). The wide bite
angle and flexibility range of such bisphosphines make
[26]
Diels–Alder
reactions. A few self-assembled structures
the Pd complexes highly active catalysts in carbon–car-
[
33]
[34]
[35]
of the type Pd L (m = 4 or 6, n = 4 or 8; L = hydra-
bon and carbon–heteroatom (N
and O ) bond for-
m
n
[27]
[28]
zine
or oxadiazole
Pd(II) complexes
moiety) and macrocyclic
ming reactions.
[29]
have been reported as catalysts in
Macrocyclic Pd complexes of Xantphos exhibited excel-
lent activity with high turnover number (TON) in Suzuki
Suzuki and C–C coupling reactions.
[
32]
Moreover, complexes in macrocyclic form gain extra
C–C coupling reactions
compared to the dppe ana-
stability, which is an essential requirement for reactions
under harsh conditions especially in Heck reactions.
logues; moreover they also showed good activity in
[
29]
[36,37]
carbonylative Suzuki–Miyaura reactions.
Motivated
Another reason for thiolate ligands being less explored
lies in the fact that the resulting complexes are often poly-
meric or insoluble in nature, so they could not be applied
as homogeneous catalysts. However by appropriate
by the above considerations, we decided to evaluate the
catalytic activity of our Xantphos-capped Pd dithiolate
complexes in Heck coupling reactions of styrene and aryl
halides. We initially optimized the reaction conditions,
SCHEME 1 Diphosphine-capped
palladium thiolate complexes used as
a
catalysts in Heck reactions. Bite angle
and flexibility range in degrees are
b
[32]
shown in parentheses. Mane et al

CATALYSIS OF PD THIOLATE COMPLEXES OF DIPHOSPHINES IN HECK REACTIONS
3 of 9
followed by evaluation of coupling reactions of various aryl
halides and performance at low loading, and comparison
of the activity with that of traditional Pd salts. The effect of
bite angles of three diphosphines on the catalytic activity
and the reaction mixture was stirred until maximum
conversion of aryl halide to product occurred. The reaction
mixture was cooled to room temperature, diluted with
water (10 ml) and neutralized by dropwise addition of
dilute HCl (aq). The mixture was extracted with ethyl
acetate (3 × 20 ml), washed with water (2 × 15 ml) and dried
over anhydrous Na SO . The solvent of the extract was
\
\
of [Pd (P P) (SC H S)] (OTf) (P P = dppe, Xantphos,
2
2
12
8
2
4
dppf) was also assessed. Reuse of the catalysts, the forma-
tion and characterization of PdNPs and their active partici-
pation in the catalytic process are discussed here.
2
4
removed with a rotary evaporator, and the resulting
1
residue was analyzed using H NMR spectroscopy.
2
2
| EXPERIMENTAL
.1 | Procedure for Heck coupling
2.2 | Catalyst recycling and separation of
PdNPs
reaction
An oven-dried flask was charged with aryl halide
Similar to the procedure described above, 4-bromoace-
tophenone (1.0 mmol), styrene (1.5 mmol), catalyst
2 (0.1 mmol), DMA (3 ml) and NaOAc (2.0 mmol) in
water (1 ml) were used. Keeping other reaction condi-
tions as described above, the mixture was stirred for 16 h
(1.0 mmol), styrene (1.5 mmol), palladium complex
2
(0.1 mmol, equivalent to 0.4 mol% of Pd), DMA (3 ml)
and NaOAc (2.0 mmol) in water (1 ml). The flask was
ꢀ
placed in an oil bath at 120 C under a nitrogen atmosphere,
a
TABLE 1 Optimization of reaction parameters
ꢀ
b
Entry
Solvent
Base
Temp. ( C)
Time (h)
Yield (%)
Effect of solvent
1
DMA
NaOAc
NaOAc
NaOAc
NaOAc
120
120
120
120
16
16
16
16
100
86
2
DMF
3
Methanol
Dioxane
No product
15
4
Effect of base
5
DMA
DMA
DMA
DMA
DMA
NaOAc
120
120
120
120
120
16
16
16
16
16
100
81
6
Na
Et
KOH
Bu NOH
2
CO
3
7
3
N
53
8
35
9
4
21
Effect of temperature
1
1
1
1
0
1
2
3
DMA
DMA
DMA
DMA
NaOAc
NaOAc
NaOAc
NaOAc
120
110
100
80
16
16
16
16
100
79
64
38
Effect of time
1
1
1
4
5
6
DMA
DMA
DMA
NaOAc
NaOAc
NaOAc
120
120
120
16
12
8
100
77
51
a
1
b
Reaction conditions: aryl bromide (1.0 mmol), styrene (1.5 mmol), base (2.0 mmol) in water, DMA (3 ml), catalyst 2, 0.4 mol% of Pd used. Determined using
H NMR spectroscopy.

4
of 9
MANE ET AL.
\
0
[Pd (P P) (4,4 -SC H S)] (OTf) (P P = Xantphos (2),
2 2 12 8 2 4
\
and processed analogously. After the reaction, ethyl ace-
tate (3 × 15 ml) was added to the product mixture
resulting in the formation of two layers. The upper layer
of ethyl acetate, containing the product, was removed to
give 99% yield. The lower aqueous layer containing the
catalyst 2 was then used for another reaction, with
amounts of reactants accordingly being further added.
After 16 h, the first reaction cycle resulted in 92% yield;
further repeating a similar process, the second and third
reaction cycles yielded 75% and 62% of the product. After
the third cycle, the aqueous part was centrifuged and
decanted to afford a black residue. The residue was
washed with water (2 × 20 ml) followed by 5 ml of each
of ethanol and acetone and dried in vacuum. Powder X-
ray diffraction (PXRD) analysis of the residue confirmed
the formation of PdNPs.
dppf (3), dppe (4)) were prepared and characterized
according to methods recently reported by us
[30,32]
(Scheme 1). We initially optimized the reaction condi-
tions with respect to the parameters solvent, base, tem-
perature and time employing the coupling reaction of
4-bromoacetophenone and styrene as a model reaction.
The experiments on screening of commonly used solvents
were carried out first using catalyst 2. DMA gave quanti-
tative yield followed by 86% yield in dimethylformamide
ꢀ
(DMF) at 120 C within 16 h, in the presence of NaOAc
as base (Table 1, entries 1 and 2). The reaction in dioxane
yielded much less of the product, whereas no coupling
product was found in case of methanol used as solvent.
Interestingly, the same catalysts recently applied in the
Suzuki–Miyaura reaction gave excellent yields of product
biaryls in refluxing methanol, in fact the yields being
higher than those obtained using DMA, DMF and diox-
ꢀ
3
| RESULTS AND DISCUSSION
ane as solvent at higher temperatures (≥95 C). So DMA
was chosen as solvent for the present reaction medium
and various inorganic and organic bases were investi-
gated. The obtained results revealed that NaOAc is the
The catalysts [Pd(Xantphos)(1,4-SC H SH)] (OTf) (I)
6
4
2
2
and [Pd (Xantphos) (1,4-SC H S)] (OTf) (II) (1) and
2
2
6
4
2
4
a
TABLE 2 Reaction between aryl halide and styrene
b
Entry
Ar–X
Complex
“Pd” (mol%)
Time (h)
16
Yield (%)
80
TON
200
178
168
220
250
248
240
233
218
230
215
248
245
180
163
143
195
8
c
1
2
3
4
5
6
7
8
9
4-CH
4-CH
4-CH
3
C
C
6
6
H
H
4
4
Br
Br
1
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
2
c
3
3
2
16
71
c
OC
6
H
4
Br
1
24
67
4-H
2
NC
6
H
4
Br
2
2
16
88
4-CH
4-CH
4-CH
3
3
3
COC
6
H
4
Br
16
100
99
c
COOC
COOC
6
6
H
4
Br
Br
1
16
c
H
4
2
16
96
4-O
4-O
2
2
NC
NC
6
6
H
4
4
Br
Br
1
2
1
2
2
2
2
16
93
H
16
87
10
11
12
13
14
15
16
17
18
4-NCC
4-NCC
6
6
H
H
4
4
Br
Br
16
92
16
86
4-OHCC
2-OHCC
6
6
H
H
4
4
Br
Br
16
99
16
98
4-FC
6
H
4
Br
16
72
c
2-C
2-C
4-C
4
4
9
H
H
H
3
3
6
SBr
SBr
NBr
1
16
65
c
2
16
57
c
2
16
24
b
78
15
1
d
4-CH
3
COC
6
H
4
Cl
2
a
c
Reaction conditions: aryl halide (1.0 mmol), styrene (1.5 mmol), NaOAc (2.0 mmol), DMA (3 ml), water (1 ml). Determined using H NMR spectroscopy.
d
TBAB (3.0 mmol) used. TBAB (5.0 mmol) used.

CATALYSIS OF PD THIOLATE COMPLEXES OF DIPHOSPHINES IN HECK REACTIONS
5 of 9
best choice among other bases Na CO , KOH, Et N and
(Table 2, entries 1–4). The presence of electron-
withdrawing groups such as acetophenone, acetyl, nitro,
cyano, aldehyde and fluoro groups in the aryl bromide
showed improvement and gave the coupling products in
excellent to quantitative yields in the range 72–100%
(Table 2, entries 5, 8–14) without adding
tetrabutylammonium bromide (TBAB). Furthermore,
heteroaryl bromides having S or N in the aryl group also
successfully reacted giving the corresponding coupling
products (Table 2, entries 15–17). However, the reactions
of aryl bromides with bulkier groups and aryl chlorides
showed lower yields of the products.
2
3
3
Bu NOH (entries 5–9). Similar experiments by varying
4
the temperature and time were carried out using DMA as
solvent and NaOAc as base. The yields were improved
with increasing temperature and time, which were found
ꢀ
optimized at 120 C and 16 h, respectively (entries 10–16).
Having the optimized reaction conditions in hand,
the coupling reactions of styrene with a variety of aryl
bromides were investigated using 0.4 mol% of Pd of cata-
lysts 1 and 2. Both catalyst systems were tolerant to a
wide range of functional groups, such as NH , OMe,
2
COMe, COOMe, NO , CN and CHO. Aryl bromides con-
2
taining electron-donating groups, such as methyl, met-
hoxy and amine, at para position of benzene ring
afforded 80%, 71%, 67% and 88% of the desired products
Our next interest was to see the effect of lowering the
catalyst loading on performance. The experiments with
low loading of catalyst were carried out using the
a
TABLE 3 Effect of catalyst loading on Heck reaction
b
TOF (h−1
16
Entry
“Pd” (mol%)
0.4
Yield (%)
100
TON
)
1
2
3
4
5
250
0.01
92
9 200
575
c
0.001
80
80 000
690 000
5 800 000
5 000
c
0.0001
0.00001
69
43 125
362 500
c
58
a
b
1
Reaction conditions: aryl bromide (1.0 mmol), styrene (1.5 mmol), NaOAc (2.0 mmol) in water, DMA (3 ml), complex 2. Determined using H NMR
c
spectroscopy. TBAB (1.0 mmol) used.
\
a
2 2 8 2 4
TABLE 4 Effect of bite angle of phosphine on catalysis using [Pd (P P) (SC12H S)] (OTf)
\
ꢀ
b
Entry
Ar–Br
P P (complex)
Xantphos (2)
dppf (3)
Bite angle ( )
Yield (%)
c
1
2
3
4
5
6
7
8
9
4-CH
4-CH
4-CH
4-CH
4-CH
4-CH
3
3
3
3
3
3
C
C
C
6
6
6
H
H
H
4
4
4
Br
Br
Br
104.6
71
68
c
99.6
d
e
dppe (4)
84.7
64
COOC
COOC
COOC
6
6
6
H
H
H
4
4
4
Br
Br
Br
Xantphos (2)
dppf (3)
96
92
e
dppe (4)
90
2-C
2-C
2-C
4
4
4
H
H
H
3
3
3
SBr
SBr
SBr
Xantphos (2)
dppf (3)
57
48
e
dppe (4)
43
a
Reaction conditions: aryl bromide (1.0 mmol), styrene (1.5 mmol), NaOAc (2.0 mmol) in water, DMA (3 ml), TBAB (3.0 mmol) used, 0.4 mol% of Pd used.
b
1
c
[32] d
[30] e
[31]
Determined using H NMR spectroscopy. Mane et al.
Vivekananda et al.
Mane et al.

6
of 9
MANE ET AL.
coupling reaction of 4-bromoacetophenone and styrene
in the presence of tetranuclear complex 2. Upon decreas-
ing the concentration of 2 to 0.01 mol% of Pd, an excel-
lent yield of 92% of the product was obtained (Table 3,
entry 2). On reducing the loading further to 0.0001 mol%
of Pd, still a significant yield of 69% was found, and the
they exclusively form tetranuclear Pd complexes with the
4,4 -biphenylenedithiolate ligand both in solid and solu-
0
tion states. Accordingly, complexes 2 and 3 were pre-
pared and the catalytic activity was evaluated in the Heck
coupling reactions of three different aryl bromides
(Table 4) in identical reaction conditions. We have earlier
investigated the same Heck coupling reactions using the
5
TON is calculated as 7 × 10 . The reaction using
[31]
0
.00001 mol% of Pd of the catalyst showed 58% yield of
analogous complex 4 with dppe ligand as catalyst.
The
the product, with the corresponding TON increased to ca
obtained results indicate that irrespective of aryl bro-
mides, whether containing electron-donating group
(entries 1–3), electron-withdrawing group (entries 4–6) or
hetero atom (entries 7–9), the trend of catalytic activity of
the complexes follows the order 2 > 3 > 4. The catalytic
activity of the present complexes was compared with that
of commercially available Pd complexes such as
Pd(OAc) , PdCl (PPh ) , PdCl (Xantphos) and PdCl
6
6
4
× 10 (Table 3, entry 5) turnover frequency (TOF) of ca
× 10 h . The reusability of the catalyst was investi-
5
−1
gated using 4-bromoacetophenone and Pd complex 2 with
a loading of 0.4 mol% of Pd (Table S1). After the comple-
tion of zero cycle of the reaction with styrene, the solvent
ethyl acetate was added to extract the coupling product
(
E)-4-acetylstilbene in the organic phase. Then fresh reac-
2
2
3 2
2
2
tants in required quantities were added to the aqueous
phase containing the Pd catalyst for the first cycle of reac-
tion, which yielded 92% of product. In a similar manner
the second and third cycles yielded the same product in
(Table S2). Under the same reaction conditions, complex
1 showed higher activity than these Pd complexes. The
present results for C–C coupling reactions support the lit-
erature findings that Pd complexes of wider bite angle of
bisphosphine accelerate the rate of catalysis reaction. For
example, in case of allylic alkylation, the rate of catalytic
reaction was observed to be faster for Pd–dppf complex
7
5% and 62% yield, respectively. These results showed
that the recovered catalyst still retained high catalytic
activity.
A comparison of the catalytic activity of the
Xantphos-capped Pd complexes revealed that complex
[
38]
than the analogous Pd–dppe complex.
Moreover, we investigated the state of the Pd complex
after the catalysis reaction. Recently, we reported the
Suzuki reaction catalyzed by the present complex 2. After
six cycles under reflux in methanol, the formation of
PdNPs was noticed using PXRD. Similar intense peaks
1
is slightly more active than complex 2, as evident from
Table 2 irrespective of aryl bromide. 4-Tolyl bromide con-
taining electron-donating group gave 80% and 71% yield
(
2
1
entries
1
and 2) or the heteroaryl bromide
-bromothiophene yielded 65% and 57% (entries 15 and
6) when catalyzed by 1 and 2, respectively. These results
ꢀ
have been found at 2θ ~ 39.9 in the PXRD pattern of the
agree well with the catalytic activities of the same com-
plexes in Suzuki–Miyaura C–C coupling reactions
[30]
reported by us.
The less stable form of I in comparison
to II in 1 is mainly responsible for the difference in activ-
ity compared with 2. Catalyst 1 consists of dinuclear com-
plex I and tetranuclear complex II in nearly equal ratio
31
as evidenced by P NMR spectra in solution. Catalyst
exclusively exists in tetranuclear form II for which the
2
−
1
stability was estimated as −872 kcal mol using density
functional theory calculation. In the case of 1, the stabil-
ity of form II (−870 kcal mol ) was found to be about
−1
35 kcal mol− higher than I (−435 kcal mol ).
1
−1 [32]
The
4
less stable form I will generate more catalytic active spe-
cies than II, hence 1 showed slightly higher activity. The
thermogravimetric analysis of the complexes indicates
that the catalysts are stable enough in the solid state, the
ꢀ
[36]
decomposition starting after 305 C for catalyst 1,
after 320 C for catalysts 2
and
ꢀ
[36]
and 3 (Figures S1–S3). Next
was examined the effect of bite angle with varied flexibil-
ity range of the bisphosphines on the catalytic activity
under the present optimized reaction conditions. The
phosphines Xantphos, dppf and dppe were chosen, as
FIGURE 1 PXRD patterns of (a) complex 2 before catalysis,
(b) compound after zero cycle of catalysis reaction of complex 2 and
(c) compound after three cycles of catalysis reaction of complex 2

CATALYSIS OF PD THIOLATE COMPLEXES OF DIPHOSPHINES IN HECK REACTIONS
7 of 9
black residue obtained after zero cycle of the present
Heck reaction catalyzed by complex 2. Similarly, the pat-
tern of the residue obtained after three cycles showed the
ꢀ
ꢀ
broad peaks become more intense at 2θ ~ 39.9 , 46.5 ,
6
ꢀ
ꢀ
ꢀ
8.2 , 82.1 and 86.7 (Figure 1 and Figure S4). These
reflection peaks are attributed to the (111), (200), (220),
311) and (222) planes of face-centered cubic structure of
(
PdNPs (JCPDS file no. 46-1043). The average size of the
particles was calculated as 6 nm from the Debye–Scherrer
formula considering the broadening of the strongest
peak, which is less than observed from transmission elec-
tron microscopy (TEM) imaging. The TEM image in
Figure 2a shows the irregular shape of the PdNPs
obtained after catalysis reaction. The well-defined, sharp
diffraction rings in the selected area electron diffraction
pattern of Figure 2b, corresponding to the planes (111),
(200), (220) and (311), corroborated the nanocrystalline
nature of the particles as inferred from the PXRD results.
The lattice fringes with an inter-fringe distance of about
2
.22 Å closely matched the inter-planar distance of (111)
plane in the face-centered cubic structure of Pd
Figure 2c). The histograms of size distribution, counted
(
from 700 particles, are shown in Figure S5. The average
crystallite size of PdNPs is in the range 5–22 nm. The
energy-dispersive X-ray (EDAX) spectra show the pres-
ence of Pd and S as major and minor constituent ele-
ments present in a 2.5:1 ratio of atom% (Figure S6). The S
may be present as capping group of thiolate ligand; how-
ever, other elements such as P, F and Fe from the com-
plex were absent.
The present PXRD peaks are more prominent than
the peaks observed for the previously reported Suzuki
reaction, due to the formation of substantial amount of
PdNPs which is attributed to the harsher condition
(i.e. higher temperature and longer duration) used in the
Heck reaction. During the catalytic process the dithiolate
complexes of Pd(II) generate PdNPs, which serve as a res-
ervoir of catalytically active species. The thiolate ligand
act as a good stabilizer of the PdNPs to inhibit their
agglomeration to form inactive Pd black. The mercury
[39]
poisoning test
(Hg:catalyst 2 = 400:1) showed com-
plete inhibition of the catalytic activity, thus suggesting
the participation of heterogeneous PdNPs in the catalytic
cycle. Others have reported that thiol-capped PdNPs
[
40]
showed good activity in C–C coupling
tions.
and Heck reac-
There is sufficient evidence that supports that
heterogeneous Pd catalysts function as homogeneous cat-
[41]
[4,5]
alysts in Heck reactions.
So, the reaction mechanism
is more likely progress via Pd(0)/Pt(II) catalytic
[42,43]
cycle
which is purely homogeneous, whatever the
PdNP source or Pd atoms return to PdNPs after the catal-
ysis. The crystalline nature and reasonably narrow size
distribution suggest that the nanoparticles possibly form
FIGURE 2 PdNPs formed after three catalysis cycles of Pd
complex 2: (a) High-magnification TEM image; (b) selected area
[4]
electron diffraction pattern; (c) high-resolution TEM image

8
of 9
MANE ET AL.
via a particular type of metal cluster during the reaction.
Although the nanoparticles may contribute significantly
to the activity, comparing the PXRD peaks (Figure 1) par-
ticularly after zero cycle, the cocktail of species such as
molecular complex and metal clusters may also be pre-
sent in the reaction mixture with the nanoparticles, as
ORCID
Sandip Dey https://orcid.org/0000-0001-9198-8808
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[31] P. A. Mane, S. Dey, K. V. Vivekananda, Tetrahedron Lett. 2017,
P.A.M. is grateful to BARC-Mumbai University Collabo-
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How to cite this article: Mane PA, Neogy S,
Dey S. Catalytic activity of Pd dithiolate complexes
with large-bite-angle diphosphines in Heck
coupling reactions. Appl Organometal Chem. 2019;
e5405. https://doi.org/10.1002/aoc.5405
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