Palladium(ii) thiocarboxamide complexes: Synthesis, characterisation and application to catalytic Suzuki coupling reactions

Article abstract of DOI:10.1039/c2dt12243j

A simple route to synthesise palladium(ii) complexes from the reaction of N-substituted pyridine-2-thiocarboxamide ligands and PdCl₂(PPh ₃)₂ has been developed. The new complexes are very soluble in common solvents and have been fully characterised (elemental analysis, FT-IR, ¹H, ³¹P, ¹³C-NMR), including an X-ray diffraction analysis. The molecular structures of all the complexes were determined and reveal the presence of square planar geometry around Pd with little distortion. The complexes were tested in the Suzuki coupling of electronically deactivated aryl and heteroaryl bromides and were found to have much greater activity, without using any promoting additives or phase transfer agent under aerobic conditions. Higher reaction rates are obtained by varying R substituents on the aromatic ring of pyridine-2-thiocarboxamide. The effect of other variables on the cross-coupling reaction, such as temperature, solvent and base, is also reported.

Full text of DOI:10.1039/c2dt12243j

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PAPER
Palladium(II) thiocarboxamide complexes: synthesis, characterisation and
application to catalytic Suzuki coupling reactions†
Elangovan Sindhuja,a Rengan Ramesh*a and Yu Liub
Received 22nd November 2011, Accepted 7th February 2012
DOI: 10.1039/c2dt12243j
A simple route to synthesise palladium(II) complexes from the reaction of N-substituted pyridine-2-
thiocarboxamide ligands and PdCl2(PPh3)2 has been developed. The new complexes are very soluble in
common solvents and have been fully characterised (elemental analysis, FT-IR, 1H, 31P, 13C-NMR),
including an X-ray diffraction analysis. The molecular structures of all the complexes were determined
and reveal the presence of square planar geometry around Pd with little distortion. The complexes were
tested in the Suzuki coupling of electronically deactivated aryl and heteroaryl bromides and were found to
have much greater activity, without using any promoting additives or phase transfer agent under aerobic
conditions. Higher reaction rates are obtained by varying R substituents on the aromatic ring of pyridine-
2-thiocarboxamide. The effect of other variables on the cross-coupling reaction, such as temperature,
solvent and base, is also reported.
significant process has been made toward activation of aryl
Introduction
halides through the use of various alkylphosphine ligands in the
coupling reaction.13 However, lately, the most developed and
The synthesis of biaryls has been commonly applied in a variety
studied catalysts for the formation of biaryls are palladacycles.14
of contexts, ranging from natural-products synthesis to materials
chemistry (polymers, dendrimers and nanostructured materials)
as well as in fine chemistry (agrochemicals, pharmaceuticals),
including large-scale production.1 In this regard, the procedures
familiarised by Suzuki,2 Stille,3 Ullmann,4 Negishi,5 Semmel-
hack,6 Kharasch,7 Meyers,8 and Lipshutz9 have become modern
tools in the chemist’s repertory. Among the fascinating attributes
of the Suzuki reaction are a wide availability due to low toxicity
of boronic acids, stability to the atmosphere, and the facile
removal of the boron-containing side products (Scheme 1). The
exemplary approach of performing this reaction is to employ aryl
halides and organometals containing magnesium, zinc or boron
in the presence of palladium catalysts. It is known that various
palladium10 and nickel-based11 catalysts and nanosized palla-
dium12 are also very effective with aryl halides. Recently
Palladium(II) thiosemicarbazone complexes are active catalysts
in carbon–carbon coupling reactions such as the Suzuki reaction
and the Heck reaction.15 Babak Karimi and co-workers have
reported an efficient and recyclable water-soluble NHC–Pd
polymer catalyst for the Suzuki coupling of aryl chlorides in
water at room temperature.16 Hyunmin Yi and co-workers have
reported a Suzuki coupling reaction of aryl iodides with aryl-
boronic acids using viral-templated palladium nanocatalysts.17
The use of a palladium–guanidine complex immobilised on
SBA-16 as a highly active and recyclable catalyst for Suzuki
coupling for various aryl bromides and arylboronic acids under
mild conditions has been described.18 Palladium(0) complex cat-
alysed regiocontrolled Sonogashira and Suzuki cross-coupling
reactions via a one-pot double-coupling approach have been
reported.19 In addition, a phosphine-free carbonylative cross-
coupling reaction of aryl iodides with arylboronic acids catalysed
by immobilisation of palladium in MCM-41 has been studied.20
A palladium N-heterocyclic carbene complex was used as an
efficient catalyst for the activation of aryl chlorides at room
temperature.21 A solid slide coated with a multilayered palla-
dium–pyridyl complex has been used as an efficient catalyst for
various aryl halides and arylboronic acids for Suzuki coupling.22
Further, Suzuki coupling reactions of aryl halides with
aSchool of Chemistry, Bharathidasan University, Tiruchirappalli-
620024, India. E-mail: ramesh_bdu@yahoo.com; Fax: +0091-431-
2407045/2407020; Tel: +91-431-2407053
bInstitute of High Energy Physics, Chinese Academy of Sciences, Beijing
100049, China
†Electronic supplementary information (ESI) available: 1H, 31P, 13C
-NMR data and spectra, IR and Elemental analysis data. CCDC 852894,
852895, 830763 and 830766. For ESI and crystallographic data in CIF
or other electronic format see 10.1039/c2dt12243j
Scheme 1 The Suzuki biaryl coupling reaction.
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arylboronic acids were successfully carried out with monodis-
persed Pd nanoparticles on diamine functionalised LDH.23 Fur-
thermore, Vivek Polshettiwar, Aziz Fihri and co-workers
reported Suzuki–Miyaura cross-coupling reactions of deactivated
aryl chloride in water with low catalyst loading, without using
any phase transfer reagent.24 Among different ligand systems of
palladium the most efficient catalytic systems are palladium–
phosphine complexes and palladacycles.
coupling have not been investigated. In this paper, we report the
synthesis and characterisation of new palladium(II) thiocarboxa-
mide complexes. The most interesting features of these catalysts
are: (i) the relative insensitivity to the presence of deactivating
groups on the aryl and heteroaryl bromide, (ii) the good stability
under an atmosphere of air and (iii) the good conversion attain-
able with a wide variety of substrates upon a suitable choice of
the reaction conditions. The evaluation of catalysts and optimis-
ation of the reaction for the coupling of aryl and heteroaryl bro-
mides with arylboronic acids have been carried out.
Nowadays, the most important challenge is the development
of a ubiquitous method for the cross-coupling of nitrogen con-
taining heterocycles.25 Nitrogen-based heterocycles are pervasive
in biologically active compounds, but particularly detrimental to
catalyst activity when palladium is used.26 The presence of such
groups, which are particularly widespread in medicinal chem-
istry,27 can lead to less reactivity in coupling reactions. In
addition, the Suzuki reaction is an efficient method in the area of
drug developments including nitrogen heterocycles as substrates.
In N-substituted pyridine-2-thiocarboxamide ligands, a variety
of amines are used to tune the steric and electronic properties of
the products. Such ligands have the potential to coordinate to
metal centres in different ways, such as deprotonation of the
ligand, which may occur prior to coordination, and because
thioamides have two tautomeric forms they may act as either a
monoanionic N,N (coordination of thioketo tautomer) or S,N
(coordination of the thiol form) bidentate ligand. Alternatively,
the neutral ligand may coordinate to the metal centre via the
pyridyl nitrogen and sulfur atoms. However, coordination via the
amide nitrogen and the sulfur (either as the neutral ligand or
after deprotonation) would give four-membered metallacycles. In
the case of the deprotonated ligand both tautomeric forms will
again be possible but in the case of the protonated ligand only
the thioketo isomer is formed (Scheme 2). Palladium(II) com-
plexes have been reported as pincer ligands for the three coordi-
nation modes,28 and gold(III) has been reported for the bidentate
coordination.29 Further, a number of Pd(II) complexes bearing
bidentate thiocarboxamides have been synthesised and character-
ised.30 Platinum(II) complexes also exist with the ligand, coordi-
nated via either the S,N or N,N binding modes.31
Results and discussion
Synthesis and characterisation of the complexes
The N-substituted pyridine-2-thiocarboxamide ligands 1a–d
were prepared as reported in the literature,32 by reaction of the
appropriate aniline with 2-methyl pyridine in the presence of
Na2S·9H2O and sulphur at reflux temperature. The new palla-
dium catalyst precursors of the type [Pd(Cl)(PPh3)(L)] were syn-
thesised by reacting pyridine-2-thiocarboxamide ligands 1a–d
with one equivalent of PdCl2(PPh3)2 in ethanol under reflux for
3 h (Scheme 3).
Coordination by sulfur and nitrogen induces changes in the
position and intensity of the bands and the proximity of the
phenyl ring bands (triphenylphosphine ligand) makes it difficult
to clearly assign the vibration modes in IR spectra. The disap-
pearance of the CvS band observed in the free ligand correlates
with the loss of double-bond character upon deprotonation of the
N–H group. This supports the lengthening of the C–S bond
which we observe in representative molecular structures. For
thiocarboxamides bonding in the thiolate form, the CvS band
generally shifts to lower energy in the region 820–790 cm−1
.
These observations may be attributed to the enolisation of –NH–
CvS and subsequent coordination through the deprotonated
sulphur.33 The IR spectra of the complexes did not display ν(S–
H) at 2585–2570 cm−1 suggesting the deprotonation of the thiol
proton prior to coordination. Moreover, a new band is appeared
around 1281–1267 cm−1 which corresponds to ν(C–S) for the
thiocarboxamide complexes. In addition, other characteristic
To the best of our knowledge, the use of palladium(II) com-
plexes containing thiocarboxamides as catalysts in Suzuki
Scheme 2 Possible coordination modes of N-substituted pyridine-2-thiocarboxamide ligands.
5352 | Dalton Trans., 2012, 41, 5351–5361
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Scheme 3 The formation of palladium catalyst precursors of the type [Pd(Cl)(PPh3)(L)]. Conditions: (i) [PdCl2(PPh3)2], ethanol, 80 °C, 3 h.
Fig. 1 The molecular structure of [Pd(Cl)(PPh3)(L1)], 2a.
Fig. 2 The molecular structure of [Pd(Cl)(PPh3)(L2)], 2b.
bands due to triphenylphosphine are also present around
1436 cm−1 in the spectra of all the complexes.34 A weak inten-
sity band is observed at 1093 cm−1 characteristic of the coordi-
nated pyridine.35 The 31P NMR spectra for all the complexes
2a–d, show a singlet in the range δ 30.59–31.03 ppm indicating
1
the presence of one PPh3 group in the complexes. The H NMR
spectra of the complexes reveal that the coordination of the palla-
dium atom to the pyridyl nitrogen causes a significant downfield
shift for the proton adjacent to the nitrogen (H-1), in the range δ
9.58–9.79 ppm. The amide proton is lost upon coordination and
is not observed in the spectra of the palladium(II) complexes. All
the complexes show a multiplet in the range δ 6.85–8.34 ppm
for the aromatic protons including triphenylphosphine and a
singlet around δ 2.10 ppm and δ 2.25 ppm in the complexes 2b
and 2c, respectively, for the methyl protons. In 13C NMR spectra
the signal assigned to the thioketone carbon, which moves
upfield from δ 190–180 ppm in the ligand to δ 149–161 ppm in
the palladium(II) complexes, results from the reduced C–S bond
order on coordination.
Fig. 3 The molecular structure of [Pd(Cl)(PPh3)(L3)], 2c.
X-ray molecular structures
bond distances are gathered in Table 1. The complex 2a crystal-
lises in the ‘pbca’ space group. The palladium centre adopts
approximately a square planar coordination geometry, which
The molecular structures of all the complex 2a–d were deter-
mined and are shown in Fig. 1–4, and selected bond angles and
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confirms that the ligands are coordinated to palladium as their
deprotonated thiol tautomer, through the sulfur and the pyridyl
nitrogen atom and the remaining two coordination sites are
occupied by chloride and triphenylphosphine ligands. The thio-
carboxamide ligand binds the metal center at N and S forming
one five membered chelate ring with bite angles of 85.17(7)° N
(1)–Pd(1)–S(1), 94.54(3)° S(1)–Pd(1)–P(1), 94.10(7)° N(1)–Pd
(1)–Cl(1) and 86.36(3)° P(1)–Pd(1)–Cl(1). The bond lengths of
Pd(1)–N(1) and Pd(1)–S(1) are 2.104(3) and 2.2633(8) Å,
respectively. These bond distances are very similar to those
observed in other palladium(II) complexes.36 Further, the Pd(1)–
P(1) bond length of 2.2494(8) Å is in agreement with other
structurally characterised palladium–phosphine complexes.37
Further, it was observed that the complexes 2b–d, adopt a
similar geometry as in the complex 2a, with slight changes in
bond angles and bond distances.
Catalytic studies
Inspection of the literature reveals that there is not a set rule that
a specific solvent and a certain base are used to attain the highest
efficiency of catalysts in Suzuki coupling reactions. We have per-
formed the Suzuki coupling of aryl and heteroaryl bromides
with arylboronic acids using all Pd(II) thiocarboxamide com-
plexes as catalysts. In order to optimise the reaction conditions,
we initially carried out the reaction of p-bromoanisole with phe-
nylboronic acid using complex 2a (0.1 mol%) as a test catalyst.
First the reaction was conducted without any base and no reac-
tion (NR) was observed. Then several bases were tested and a
Fig. 4 The molecular structure of [Pd(Cl)(PPh3)(L4)], 2d.
Table 1 Selected bond lengths (Å) and angles (°) for the complexes 2a–d
Complex
2a
2b
2c
2d
Pd(1)–N(1)
Pd(1)–S(1)
Pd(1)–P(1)
Pd(1)–Cl(1)
S(1)–C(6)
N(2)–C(6)
2.104(3)
2.2633(8)
2.2494(8)
2.3374(8)
1.759(3)
1.277(4)
2.1102(16)
2.2486(6)
2.2624(5)
2.3403(6)
1.756(2)
2.105(2)
2.2557(8)
2.2608(8)
2.3379(9)
1.754(3)
1.269(4)
2.100(5)
2.2491(15)
2.2368(17)
2.3304(15)
1.739(6)
1.277(3)
1.288(7)
N(1)–Pd(1)–S(1)
N(1)–Pd(1)–P(1)
S(1)–Pd(1)–P(1)
N(1)–Pd(1)–Cl(1)
S(1)–Pd(1)–Cl(1)
P(1)–Pd(1)–Cl(1)
C(5)–N(1)–Pd(1)
C(6)–S(1)–Pd(1)
85.17(7)
177.23(7)
94.54(3)
94.10(7)
176.52(3)
86.36(3)
118.6(2)
99.61(11)
84.75(5)
175.24(5)
92.09(2)
93.60(5)
174.15(3)
89.89(2)
84.66(7)
176.51(7)
92.73(3)
93.20(7)
175.45(4)
89.56(3)
118.8(2)
84.88(13)
176.75(13)
91.87(6)
95.52(13)
179.07(8)
87.73(6)
118.36(14)
101.15(8)
119.2(4)
100.74(19)
100.63(11)
Table 2 The effect of the base on Suzuki couplinga
Entry
Base
Yield (%)b
1
2
3
4
5
6
NaOH
KOH
Na2CO3
K2CO3
K3PO4
60
57
68
69
66
36
CH3COONa
a Conditions: 4-bromoanisole (2.0 mmol), phenylboronic acid (3.0 mmol), 0.1 mol% 2a, 150 °C, 5 h. b Isolated yields.
5354 | Dalton Trans., 2012, 41, 5351–5361
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Table 3 Effect of the solvent on Suzuki couplinga
Entry
Solvent
Yield (%)b
1
2
3
4
5
6
7
8
9
Toluene
p-Xylene
THF
Dioxane
DMAc
DMSO
DMF
85
80
10
42
36
13
69
72
76
NMP
Dioxane–H2O (95 : 5)
a Conditions: 4-bromoanisole (2.0 mmol), phenylboronic acid (3.0 mmol), 0.1 mol% 2a, K2CO3 (4.0 mmol), reflux, 5 h. b Isolated yields.
Table 4 The effect of the temperature and time on Suzuki couplinga
Entry
Temp (°C)
Time (h)
Yield (%)b
1
2
3
4
5
6
20
20
40
60
80
80
3
5
5
5
3
5
11
26
57
68
74
85
a Conditions: 4-bromoanisole (2.0 mmol), phenylboronic acid (3.0 mmol), 0.1 mol% 2a, K2CO3 (4.0 mmol), toluene (20 ml). b Isolated yields.
good to excellent yield of the coupling products was observed in
all cases (Table 2). The optimisation of bases, which was studied
with DMF as solvent, showed K2CO3, Na2CO3, and K3PO4 as
the inorganic bases to choose, although biphenyl was obtained
as a byproduct in some of the cases. Other bases such as NaOH,
KOH and weak inorganic base such as NaOAc were substan-
tially less effective, and weak organic bases such as TEA, piper-
idine and pyrrolidine failed to promote the reaction, probably
because of their ability to bind strongly to palladium. The use of
inorganic bases such as carbonates or phosphates allows us to
obtain much higher reaction rates, since the best activity is
achieved employing the lesser basic carbonates. It is worth
noting here that the best reaction rates are obtained using the
inexpensive sodium or potassium carbonates.
The solvent effect on the reaction rate of coupling has been
studied using a various solvents (Table 3). We investigated
apolar solvents such as toluene and p-xylene (entries 1 and 2)
with K2CO3 as base and in the presence of complex 2a (0.1 mol
%), both solvents gave high conversions of p-methoxybiphenyl
accompanied by considerable amounts of biphenyl. Next, we
examined different highly polar solvents and the catalytic
activity of the complex is found to be lower, possibly owing to
the coordinating properties of these solvents. Reactions con-
ducted were not efficient in THF and DMSO and poor yields
obtained when dioxane and DMAc were the solvents of choice
(entries 4 and 5). The reaction was more efficient in DMF (69%)
or NMP (72%) but still with significant amounts of biphenyl as
a byproduct (entries 7 and 8). The dioxane–H2O solvent system
with K2CO3, however, gave fairly good product yields.
The occurrence of catalyst deactivation is further supported by
the issues of the studies of the temperature influence on the cata-
lysis. Catalyst deactivation has been previously noticed in
Suzuki coupling by several authors and, in particular, Bedford
has pointed out the importance of the catalyst longevity in order
to achieve high substrate conversions and product yields.38,39
The use of mild reaction conditions is essential with substrates
bearing thermally unstable functional groups, and therefore the
development of systems active at low temperatures is of great rel-
evance. Accordingly, we have investigated the influence of the
temperature on the catalytic activity of complex 2a. As seen in
Table 4, when the reaction was carried out at 20 °C in toluene,
the yield of product was 26%, at 60 °C the yield of product was
improved to 68%, and what is more important, only traces of
1
biphenyl were detected by H NMR analysis of the crude reac-
tion mixture. A temperature of 80 °C was required to reach a
reasonable reaction speed (entry 5), although 74% yield was
obtained at 3 h, reaction went to completion (85% yield) within
5 h.
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Table 5 The effect of a low loading of catalyst on Suzuki couplinga
Entry
Mol% Pd
Yield (%)b
TON (mol product/mol cat)
1
2
3
4
5
10−1
10−2
10−3
10−4
10−5
85
83
78
69
12
850
8300
78 000
690 000
1 200 000
a Conditions: 4-bromoanisole (2.0 mmol), phenylboronic acid (3.0 mmol), complex 2a, K2CO3 (4.0 mmol), toluene (20 ml), 80 °C, 5 h. b Isolated
yields.
Table 6 Effect of the catalyst on Suzuki couplinga
Entry
Complex
Yield (%)b
1
2
3
4
2a
2b
2c
2d
85
87
88
92
a Conditions: 4-bromoanisole (2.0 mmol), phenylboronic acid (3.0 mmol), 0.1 mol%, K2CO3 (4.0 mmol), toluene (20 ml), 80 °C, 5 h. b Isolated
yields.
Low catalyst loading tests were performed in order to find out
the efficiency of the catalyst. In order to optimise the reaction
conditions, different catalyst : substrate (C : S) ratios were tested
in toluene and K2CO3 and the results are summarised in Table 5.
The coupling reaction carried out between 4-bromoanisole and
phenylboronic acid using a C : S ratio of 1 : 1000 proceeded to
completion with an 85% yield (entry 1). When increasing the
C : S ratio to 1 : 10 000 and 1 : 100 000, the reaction still pro-
ceeds smoothly, accompanied by a drop in yield. Interestingly,
these reactions could also be conducted with an ultra-low
loading of catalyst as low as 0.00001 mol%, with high turnover
numbers. Thus, it was concluded that a catalyst : substrate ratio
of 1 : 1000 is the best compromise between the optimum reaction
rate and S : C ratio in toluene.
We further examined the effect on performance of changing
the R substituents on the thiocarboxamide coordinated to palla-
dium. To this purpose, we have studied the catalytic activities of
all complexes 2a–d in the coupling reaction using a C : S ratio of
1 : 1000 and it was observed that the complex 2d showed
increased yields of the cross coupled products (Table 6). The
substitution of a pyridyl group at the terminal nitrogen of the
coordinated ligand and the planarity of complex 2d may be
responsible for its observed excellent catalytic activity over the
other three complexes. Using the complex 2d, Suzuki coupling
of a series of arylbromides with different arylboronic acids has
been carried out under optimised conditions (Table 7). In the
case of p-methoxyboronic acid the conversion to biaryls was
96% (entry 4), which takes place at a faster rate than that of
p-methylboronic acid (93%). The presence of the electron with-
drawing (Cl) substituent on boronic acids has a significant effect
on the conversion to their corresponding biaryls. Similar obser-
vations are also made in the case of triaryls. The p-bromoanisole
gave high conversions compared to other arylbromides.
Pyridines are the most common heterocyclic motif found in
pharmaceutically active compounds.40 Thus, preparative
methods for pyridine derivatives remain an essential research
topic in organic synthesis. Despite efforts by numerous groups,
pyridine-derived bromides have been proved to be a particularly
difficult class of substrate for the Suzuki reaction.41 The cross
coupling of heteroaryl bromides using the same complex 2d has
been performed and the results are summarised (Table 8). Inter-
estingly, these reactions could also be conducted with low
loading of catalyst (0.1 mol%). Heteroaryl bromides afforded
almost quantitative yields of products. Substituted-2-bromopyri-
dine underwent a smooth reaction with phenylboronic acid, pro-
viding a useful way for the synthesis of aryl-substituted nitrogen
heterocycles. These results indicate that with these α-substituted
heteroaryl bromides, a possible interaction between the hetero-
element and the palladium complex has a deactivation effect on
the rate of the reaction.
To monitor the recyclability of this catalytic system similar
reaction conditions with 0.1 mol% catalyst were employed. We
have observed some Pd blacks after each run and the catalyst
was completely deactivated after the fourth run. For weak bases
such as NaOAc, NEt3 deactivation occurred more slowly than
for strong bases. The first reaction afforded the corresponding
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Table 7 Suzuki coupling of aryl bromides with aryl boronic acidsa
Entry
1
Aryl bromide
Arylboronic acid (R′)
Product
Yield (%)b
92
H
2
3
4
5
6
7
8
9
Cl
87
93
96
81
78
86
94
78c
Me
OMe
H
Cl
Me
OMe
H
10
11
12
Cl
74c
83c
93c
Me
OMe
a Conditions: aryl bromide (2.0 mmol), arylboronic acid (3.0 mmol), 0.1 mol% 2d, K2CO3 (4.0 mmol), toluene (20 ml), 80 °C, 5 h. b Isolated yields.
c Double Suzuki cross coupling (aryl bromide (2.0 mmol), arylboronic acid (6.0 mmol), 0.2 mol% 2d, K2CO3 (8.0 mmol) were used).
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Table 8 Suzuki coupling of heteroaryl bromides with aryl boronic acidsa
Entry
1
Heteroaryl bromide
Arylboronic acid (R′)
Product
Yield (%)b
82
H
2
3
4
5
Cl
79
88
94
78
Me
OMe
H
6
7
8
9
Cl
74
86
95
73c
Me
OMe
H
10
11
12
Cl
71c
82c
90c
Me
OMe
a Conditions: heteroaryl bromide (2.0 mmol), arylboronic acid (3.0 mmol), 0.1 mol% 2d, K2CO3 (4.0 mmol), toluene (20 ml), 80 °C, 5 h. b Isolated
yields. c Double Suzuki cross coupling (heteroaryl bromide (2.0 mmol), arylboronic acid (6.0 mmol), 0.2 mol% 2d, K2CO3 (8.0 mmol) were used).
5358 | Dalton Trans., 2012, 41, 5351–5361
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coupling product in 92% yield, the product yield for the second
cycle was nearly the same (86%), however in the third cycle it
was 72% and it was reduced to 49% which was obtained after
the fourth run although more time was taken (8–10 h for each
run).
4.30; S, 4.92%. IR (KBr) ν = 1562(m), 1281(s) cm−1. NMR
ˉ
(CDCl3): δH (400 MHz) 9.58 (d, 1H, C-1), 7.65–7.25 (m, 23H,
Ar, PPh3) ppm. δC (100 MHz) 160.02, 149.62, 146.94, 138.81,
134.76, 134.65, 132.16, 132.06, 131.98, 131.16, 129.56, 129.30,
128.74, 128.59, 128.46, 128.25, 128.14, 126.42, 126.03, 124.72,
123.64, 121.44 ppm. δP (160 MHz) 30.76 ppm.
[Pd(Cl)(κ2-S,N-C6H4CSvN-(2-MePh)(PPh3)], 2b. Yield 74%.
M.p. 201 °C (with decomposition). Found: C, 58.81; H, 4.17; N,
4.46; S, 5.05. Calc. for C31H26ClN2PPdS: C, 58.96; H, 4.15; N,
Conclusions
The present work describes a simple and convenient method to
synthesise a series of palladium(II) thiocarboxamide complexes
incorporating triphenylphosphine. Analytical, spectral and X-ray
diffraction studies reveal that the ligand coordinated to palladium
via pyridine N and thiol S. All the complexes 2a–d were structu-
rally characterised by X-ray crystallography, which witnessed a
square planar geometry. We have developed efficient Pd(II) cata-
lysts for Suzuki coupling reactions under ambient conditions to
couple challenging substrates like deactivated aryl and heteroaryl
bromides that provides a moderate to good yield of desired pro-
ducts. Reactions also work with a low loading of catalyst under
aerobic atmosphere for mono and double Suzuki cross coupling.
Our current investigations are focused on the feasibility of elec-
tronic modification around phosphine–palladium catalysts,
through phosphorus substituents, with the view to increasing
their activity.
4.44; S, 5.08%. IR (KBr) ν = 1545(m), 1275(s) cm−1. NMR
ˉ
(CDCl3): δH (400 MHz) 9.58 (t, 1H, C-1), 7.93–6.86 (m, 23H,
Ar, PPh3), 2.10 (s, 3H, CH3) ppm. δC (100 MHz) 160.36,
149.57, 148.34, 138.83, 134.75, 134.64, 134.14, 130.10, 129.40,
129.34, 128.25, 128.13, 126.08, 125.52, 123.98, 123.39, 119.31,
19.99 ppm. δP (160 MHz) 31.03 ppm.
[Pd(Cl)(κ2-S,N-C6H4CSvN-(4-MePh)(PPh3)], 2c. Yield 91%.
M.p. 195 °C (with decomposition). Found: C, 59.10; H, 4.18; N,
4.42; S, 5.09. Calc. for C31H26ClN2PPdS: C, 58.96; H, 4.15; N,
4.44; S, 5.08%. IR (KBr) ν = 1549(m), 1267(s) cm−1. NMR
ˉ
(CDCl3): δH (400 MHz) 9.79 (t, 1H, C-1), 7.89–6.99 (m, 23H,
Ar, PPh3), 2.25 (s, 3H, CH3) ppm. δC (100 MHz) 159.29,
148.52, 147.26, 137.55, 133.69, 133.58, 130.08, 130.05, 129.03,
128.33, 128.29, 127.77, 127.18, 127.07, 125.00, 124.46, 122.92,
122.33, 118.25, 20.92 ppm. δP (160 MHz) 30.92 ppm.
Experimental
[Pd(Cl)(κ2-S,N-C6H4CSvN-(4-Py)(PPh3)], 2d. Yield 72%. M.
p. 206 °C (with decomposition). Found: C, 56.17; H, 3.72; N,
6.76; S, 5.18. Calc. for C29H23ClN3PPdS: C, 56.32; H, 3.75; N,
General
6.79; S, 5.18%. IR (KBr) ν = 1553(m), 1277(s) cm−1. NMR
All reactions were carried out under an atmosphere of air. C, H,
N and S analyses were carried out with a Vario EL III CHNS
elemental analyser. IR spectra were recorded on a Perkin–Elmer
ˉ
(CDCl3): δH (400 MHz) 9.59 (t, 1H, C-1), 8.34–6.85 (m, 23H,
Ar, PPh3) ppm. δC (100 MHz) 149.64, 138.61, 134.74, 134.69,
134.63, 134.58, 132.06, 131.13, 131.01, 130.98, 129.46, 128.91,
128.23, 128.17, 128.11, 128.05, 126.17, 126.02, 123.23,
121.42 ppm. δP (160 MHz) 30.59 ppm.
597 spectrophotometer, using KBr pellets. H NMR, 13C NMR
1
and 31P NMR were conducted on a high resolution Bruker
Avance 400 spectrometer, in CDCl3 as solvent. Melting points
were performed with an electrical instrument and are
uncorrected.
The ligands 1a–d,32 were prepared according to literature
methods. All other chemicals were used as received. Solvents
were dried and freshly distilled prior to use. Toluene was distilled
under nitrogen with Na-benzophenone. Column chromatography
was performed on neutral silica gel.
Catalysis
General method for the Suzuki coupling of aryl and hetero-
aryl bromides with arylboronic acid (Tables 7 and 8). To a
mixture of arylbromide (2.0 mmol), arylboronic acid (3.0 mmol)
and K2CO3 (4.0 mmol) in toluene (20 mL) was added the cata-
lyst (0.1 mol%) as a toluene solution (1.00 mL) made up to the
correct concentration by multiple volumetric dilutions of a stock
solution. The resultant mixture was then heated at 80 °C for 5 h.
At ambient temperature, H2O (10 ml) was added and the organic
layer was extracted with EtOAc (3 × 50 mL). The combined
organic layers were concentrated in vacuo and the remaining
residue was purified by Column chromatography (n-hexane–
EtOAc: 200 : 1) to yield a colorless solid.
Syntheses
General method for the synthesis of the palladium complexes.
A heated solution (50 °C) of appropriate ligands (1 equiv.) in
ethanol (20–30 mL) was stirred under an air atmosphere. Upon
dissolution of the pyridine-2-thiocarboxamide in ethanol,
PdCl2(PPh3)2 (1 equiv.) was added and the resultant mixture was
then heated to reflux temperature for 3 h, then allowed to cool to
room temperature. The orange precipitate was filtered, washed
with ethanol and dried in vacuo to give moderate to good yields.
All the complexes were highly soluble in MeOH, CH2Cl2,
CHCl3 and acetone.
Recycling of the catalyst
To a mixture of 4-bromoanisole (2.0 mmol) and phenylboronic
acid (3.0 mmol) was added catalyst (0.1 mol%) in toluene
(20 mL) at 80 °C in the presence of K2CO3 (4.0 mmol). Each
time, after completion of the reaction, the catalyst was recovered
by centrifugation and then washed thoroughly with toluene
[Pd(Cl)(κ2-S,N-C6H4CSvN-(2-ClPh)(PPh3)], 2a. Yield 66%.
M.p. 197 °C (with decomposition). Found: C, 55.09; H, 3.54; N,
4.28; S, 4.90. Calc. for C30H23Cl2N2PPdS: C, 55.27; H, 3.56; N,
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 5351–5361 | 5359

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Table 9 Crystal data and structure refinement parameters for complexes 2a–d
Complex
2a
2b
2c
2d
Empirical formula
Formula weight
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C30H23Cl2N2PPdS
651.83
0.71073
Orthorhombic
Pbca
9.0407(2)
14.913(3)
40.8653(7)
90
C31H26ClN2PPdS
631.42
0.71073
Monoclinic
C2/c
25.1402(9)
10.2928(3)
23.0874(9)
90
C31H26ClN2PPdS
631.42
0.71073
Monoclinic
C2/c
24.5145(4)
10.6055(1)
23.0862(4)
90
C29H23ClN3PPdS
618.38
0.71073
Orthorhombic
P2(1)2(1)2(1)
14.205(5)
9.867(5)
19.274(5)
90
β (°)
90
90
108.072(3)
90
5679.4(3)
8
1.477
0.900
2560
0.11 × 0.10 × 0.08
1.70 to 26.00
109.559(1)
90
5655.81(14)
8
1.483
0.904
2560
0.07 × 0.06 × 0.05
1.76 to 25.00
90
90
γ (°)
V (Å3)
5509.62(19)
8
1.572
1.025
2624
0.11 × 0.09 × 0.04
1.00 to 25.00
2701.5(18)
4
1.520
0.946
3120
0.06 × 0.06 × 0.05
1.78 to 29.64
Z
Dcalcd (Mg m−3
)
μ /(mm−1
F(000)
)
Crystal size (mm3)
Theta range for data
collection(°)
Index ranges
−10 < = h < = 10,
−17 < = k < = 17,
−48 < = l < = 48
47 858
−30 < = h < = 30,
−12 < = k < = 12,
−28 < = l < = 28
49 049
−29 < = h < = 29,
−12 < = k < = 12,
−27 < = l < = 27
43 972
−19 < = h < = 19,
−13 < = k < = 13,
−26 < = l < = 25
43 829
Reflections collected
Independent reflection
Max and min transmission
Refinement method
4845
5594
4988
7251
0.9602, 0.8956
Full-matrix least-squares
on F2
0.9315, 0.9075
Full-matrix least-squares
on F2
0.9465, 0.9262
Full-matrix least-squares
on F2
0.9883, 0.9860
Full-matrix least-squares
on F2
Data/restraints/parameters
4845/0/334
1.216
5594/0/335
1.055
4988/0/335
1.106
7251/0/325
0.912
Goodness-of-fit on F2
Final R indices [I > 2σ(I)] R1, 0.0296, 0.0624
0.029, 0.0557
0.0306, 0.0698
0.0477, 0.1023
wR2
R indices(all data)R1, wR2
R(int)
Flack parameter
0.0355, 0.0691
0.032
—
0.315 and −0.301
0.029, 0.0606
0.0314
—
0.343 and −0.222
0.0402, 0.075
0.0387
—
0.321 and −0.342
0.0950, 0.1238
0.202
−0.06(4)
0.746 and −1.178
Largest diff. peak and hole
(e Å−3
)
followed by copious amounts of water to remove the base
present in the used catalyst and finally by dichloromethane. The
recovered catalyst was dried under vacuum at 105–115 °C over-
night. This used catalyst was re-employed in four successive
cycles under identical conditions. The combined organic layers
were concentrated in vacuo and the remaining residue was
purified by column chromatography (n-hexane–EtOAc: 200 : 1)
to yield a colorless solid.
using SADABS software. Crystal data for the structures are
given in Table 9.
Acknowledgements
One of the authors (E. S.) thanks University Grants Commission
(UGC), New Delhi, for the award of UGC-BSR RFSMS. We
thank DST-India (FIST programme) for the use of Bruker
Avance DPX 400 MHz NMR and Bruker SMART APEX II dif-
fractometer at the School of Chemistry, Bharathidasan
University.
X-Ray structure determinations
Single crystals of all the complexes were grown by slow evapor-
ation of dichloromethane–ethanol mixture at room temperature.
A single crystal of suitable size was covered with Paratone oil,
mounted on the top of a glass fiber, and transferred to a Bruker
SMART APEX II single crystal X-ray diffractometer using
monochromated Mo-Kα radiation (kI = 0.71073 Å). Data were
collected at 293 K. The structure was refined by full matrix least-
squares method on F2 with SHELXL-97. Non-hydrogen atoms
were refined with anisotropy thermal parameters. All hydrogen
atoms were geometrically fixed and allowed to refine using a
riding model. Frame integration and data reduction were per-
formed using the Bruker SAINT-Plus (Version 7.06a) software.
The multi-scan absorption corrections were applied to the data
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