Palladium(II) thiosemicarbazone complex: Synthesis, structure and application to carbon-oxygen cross-coupling reaction

Article abstract of DOI:10.1016/j.inoche.2014.03.002

A simple route for the synthesis of square-planar palladium(II) 3-methyl-thiophene-2-aldehyde thiosemicarbazone complex is described. The composition of the complex has been established by elemental analysis and spectral methods. The molecular structure was confirmed by single crystal X-ray diffraction study. Further, the complex was used as a potential catalyst for the carbon-oxygen cross-coupling reaction of activating, non-activating and deactivating aryl iodides or aryl bromides with p-cresol.

Full text of DOI:10.1016/j.inoche.2014.03.002

Inorganic Chemistry Communications 44 (2014) 67–69
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Palladium(II) thiosemicarbazone complex: Synthesis, structure and
application to carbon–oxygen cross-coupling reaction
Pandimuni Kalpaga Suganthy , Rupesh Narayana Prabhu b,1, Venugopal Shanmugham Sridevi a,⁎
a
a
Department of Chemistry, Periyar E.V.R. College, Tiruchirappalli 620 023, Tamil Nadu, India
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple route for the synthesis of square-planar palladium(II) 3-methyl-thiophene-2-aldehyde thiosemicarbazone
complex is described. The composition of the complex has been established by elemental analysis and spectral
methods. The molecular structure was confirmed by single crystal X-ray diffraction study. Further, the complex
was used as a potential catalyst for the carbon–oxygen cross-coupling reaction of activating, non-activating and
deactivating aryl iodides or aryl bromides with p-cresol.
Received 13 February 2014
Accepted 7 March 2014
Available online 14 March 2014
Keywords:
©
2014 Elsevier B.V. All rights reserved.
Thiosemicarbazone ligand
Palladium(II) complex
Crystal structure
Carbon–oxygen cross-coupling
Thiosemicarbazones are an important class of Schiff base ligands
known for their selectivity and sensitivity towards various metal ions.
These ligands are known to coordinate to a metal center as bidentate li-
gands via the azomethine nitrogen either in the neutral thione form or
in the anionic thiolate form forming five membered chelate rings [1,2].
The ligand can also bind to the metal ion via the hydrazinic nitrogen
and the thiolate sulfur as monoanionic bidentate N,S donor forming a
four membered chelate ring [3].
of thiosemicarbazide with 3-methyl-thiophene-2-carboxaldehyde [15].
The new Pd(II) complex (1) was prepared by the following procedure:
a solution of the ligand (0.20 mmol) in CH
added to a warm ethanolic solution (20 mL) of [PdCl
2
Cl
2
(20 mL) was slowly
3 2
(PPh ) ]
2
(0.20 mmol). The reaction mixture was then kept at room temperature
for 3–4 days. The precipitated red solids were filtered, washed with cold
ethanol and diethyl ether to yield the complex in 79% yield (Scheme 1).
The complex is air stable in both the solid and the liquid states at
room temperature and is non-hygroscopic. The complex is readily solu-
ble in common organic solvents producing intense colored solutions.
The elemental analysis of the complex (Calc.: C, 49.84; H, 3.85; N, 6.97;
S, 10.64%. Found: C, 49.94; H, 3.83; N, 6.95; S, 10.66%) is in good agree-
ment with the general molecular formula proposed. The FT-IR spectrum
of the complex confirmed the coordination of the imine nitrogen (νC_N
The aryl ether unit is a common structural feature in numerous nat-
ural products, pharmaceuticals, fragrances, cosmetics and polymers
[
4–6]. Palladium complexes are promising catalysts for C\O cross-
coupling reactions. At low catalyst loading and lower reaction tempera-
ture, promising approaches for the C\O coupling reaction of various
aryl halides with oxygen nucleophiles have been reported by using pal-
ladium catalysts containing monophosphines [7], diphosphines [8],
biphenyl-based ligands [9], ferrocene-based ligands [10], phosphino-
pyrroles or phosphino-indoles [11] or bipyphos ligands [12]. The key
advantages of the palladium arylation of oxygen nucleophiles are the
mild conditions under which it is conducted, the high tolerance towards
various functional groups that is observed, and the commercial avail-
ability and stability of substrates to heat, oxygen, and water [13,14].
Herein we report, the application of the Pd(II) thiosemicarbazone
complex as catalyst in the C\O cross-coupling reaction of p-cresol
with aryl iodides and bromides containing different electronic effect.
The thiosemicarbazone ligand (HL) was prepared by the condensation
−
1
−1
1591 cm ) and thiol sulfur (νC_S 812 cm ) to the metal atom [16,17].
The electronic spectrum of the complex shows that a strong absorption
below 350 nm is assigned to ligand centered transition in addition to a
combination of charge transfer and d–d transitions around 399 nm to
1
470 nm [18,19]. The H NMR spectrum of 1 (Fig. S1, Supporting Material)
displayed multiplets in the region δ 7.8–6.9 ppm and has been assigned
to the aromatic protons of the coordinated PPh and thiosemicarbazone
3
ligand. The singlet due to azomethine proton (δ 8.7 ppm) in 1 is slightly
downfield compared to the free ligand (δ 8.2 ppm), suggesting
deshielding upon coordination to Pd(II) ion [20,21]. The singlet that
appeared for the N\NH\C_S proton of the free ligand at δ 11.3 ppm
is absent in 1, supporting enolization and coordination of the thiolate
sulfur to the Pd(II) ion. In addition, the methyl protons of the thiophene
⁎
Corresponding author. Tel.: +91 431 242 0079; fax: +91 431 2332010.
E-mail address: sridevi.evr@gmail.com (V.S. Sridevi).
Present address: School of Chemistry, University of Hyderabad, Hyderabad 500 046,
13
ring and the NH
2
protons are present in the expected region. The
C
1
NMR of 1 (Fig. S1, Supporting Material) shows resonances in the expect-
ed regions and revealed a downfield shift of the azomethine carbon
Andhra Pradesh, India.
http://dx.doi.org/10.1016/j.inoche.2014.03.002
1387-7003/© 2014 Elsevier B.V. All rights reserved.

68
P.K. Suganthy et al. / Inorganic Chemistry Communications 44 (2014) 67–69
Cl
Pd
H
H3C
H
H3C
PPh3
[
PdCl (PPh ) ]
2 3 2
N
N
N
S
HN
S
S
S
EtOH-CH Cl
2
2
NH2
NH2
1
Scheme 1. Synthesis of palladium(II) thiosemicarbazone complex, 1.
Fig. 1. ORTEP diagram of complex 1 at 30% probability.
relative to the free ligands indicating coordination of the azomethine ni-
trogen to the metal center. Also, the signal assigned to the thioketone
carbon, which moves upfield from δ 179 ppm in the free ligand to
δ172.6 ppm in 1, results from the reduced C\S bond order upon coordi-
nation [22].
electron-donating groups reacted with p-cresol. For a given substituent
(R), aryl iodides gave better conversions of the desired diaryl ethers
(entries 1–5) than the corresponding aryl bromides (entries 6–10). It is
also observed that aryl halides with electron-withdrawing groups such
as nitro (entries 1, 6) or acetyl (entries 2, 7) coupled effectively and
gave better conversions of the desired diaryl ethers than that of
unsubstituted (non-activated) aryl halides (entries 3, 8). The conversions
were significantly reduced when electron-donating groups such as
methyl (entries 4, 9) or methoxy (5, 10) were present in the aryl halide.
Further, controlled experiments indicated that no cross-coupling product
was observed in the absence of the catalyst or base for any of the sub-
strates examined in Table 1.
The ORTEP view of the complex is shown in Fig. 1 and the summary
of the data collection and refinement parameters is given in Table S1
(
Supporting Material). In the crystal structure, the ligand coordinates
to the Pd(II) ion via the azomethine nitrogen and the thiolate sulfur
forming one five membered chelate ring. One PPh group and one chlo-
3
ride ion also coordinate to the Pd(II) ion to form a ClNPS square plane.
The angles around Pd(II) ion are N(1)\Pd(1)\S(2) = 83.01(7)°,
S(2)\Pd(1)\P(1) = 93.57(3)°, P(1)\Pd(1)\Cl(1) = 89.41(3)° and
N(1)\Pd(1)\Cl(1) = 94.24(7)°, and bond lengths are 2.3353(9) Å for
Pd(1)\Cl(1), 2.2584(8) Å for Pd(1)\S(2), 2.076(2) Å for Pd(1)\N(1)
and 2.2526(8) Å for Pd(1)\P(1). The complex has a distorted square
planar geometry as reflected from the bond parameters around Pd(II)
ion [23].
Palladium complexes are becoming increasingly popular as catalyst in
bringing about carbon–carbon or carbon-hetero atom cross coupling re-
actions of different types, however, to the best of our knowledge, there is
no report on the use of Pd(II) thiosemicarbazone complex as catalyst for
the carbon–oxygen cross-coupling reaction. This has led us to explore
catalytic efficiency of complex 1 in carbon–oxygen cross-coupling reac-
tion (Table 1) of p-cresol with different activated (electron-withdrawing
groups) and deactivated (electron-donating groups) aryl iodides or aryl
Table 1
a
Pd(II) thiosemicarbazone catalyzed C\O coupling reactions of p-cresol with aryl halides.
Conversionb (%)
Entry no.
Aryl halide
1
2
3
4.
5.
6
7
8
9
.
.
.
1-Iodo-4-nitrobenzene
4-Iodo acetophenone
Iodo benzene
4-Iodo toluene
4-Iodo anisole
1-Bromo-4-nitrobenzene
4-Bromo acetophenone
Bromo benzene
4-Bromo toluene
4-Bromo anisole
94
90
84
76
66
90
86
80
72
62
.
.
.
.
10.
bromides in DMF–K
24]. The desired diaryl ethers were formed in moderate to excellent con-
versions when haloarenes bearing electron-withdrawing as well as
2
CO
3
at relatively low temperature (Scheme 2)
a
Reaction conditions: p-cresol (1.0 mmol), aryl halide (1.5 mmol), complex 1
CO (2.0 mmol), DMF (4 mL) at 80 °C for 12 h under N atmosphere.
Average of 2 runs, determined by GC–MS analysis.
[
(10 μmol), K
2
3
2
b

P.K. Suganthy et al. / Inorganic Chemistry Communications 44 (2014) 67–69
69
OH
X
Complex 1
DMF, K CO
O
+
2
3
H C
R
H C
3
R
3
o
80 C, 12 h
(
X = I or Br; R = NO , COCH , H, CH , OCH )
2
3
3
3
Scheme 2. Carbon–oxygen cross-coupling reaction.
[
[
6] B.M. Crowley, Y. Mori, C.C. McComas, D. Tang, D.L. Boger, J. Am. Chem. Soc. 126
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In conclusion, a new square-planar Pd(II) complex bearing
monoanionic bidentate thiosemicarbazone ligand was synthesized
and characterized by analytical and spectral methods. X-ray diffrac-
tion study of the complex confirms the N and S coordination mode
of the ligand and reveals the presence of a distorted square planar
geometry around the Pd(II) ion. The utility of the complex as an ex-
cellent pre-catalyst for the C\O cross-coupling reaction has been
highlighted by the coupling reaction of activating, neutral and
deactivating aryl iodides and bromides with p-cresol. The diaryl
ethers were obtained in moderate to excellent conversions.
(
[
[
[
10] N. Kataoka, Q. Shelby, J.P. Stambuli, J.F. Hartwig, J. Org. Chem. 67 (2002)
5553.
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Appendix A. Supplementary material
[
[
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Crystallographic data for the structural analysis have been deposited
with Cambridge crystallographic center, CCDC No. 891959. Copies of
this may be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallographic Data
[19] K.I. Goldberg, J. Valdes-Martínez, G. Espinosa-Pérez, L.J. Ackerman, D.X. West, Poly-
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[
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Smith, Polyhedron 28 (2009) 2839.
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336
1
0
33; or e-mail: deposit@ccdc.cam.ac.uk. Experimental procedures, H
[21] R.N. Prabhu, R. Ramesh, RSC Adv. 2 (2012) 4515.
[
[
1
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22] R.N. Prabhu, R. Ramesh, Tetrahedron Lett. 54 (2013) 1120.
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Veg, J. Organomet. Chem. 701 (2012) 17.
NMR, C NMR Crystal data and structure refinement for the complex.
Supplementary data to this article can be found online at http://dx.doi.
org/10.1016/j.inoche.2014.03.002.
[24] Typical procedure for the carbon–oxygen coupling reactions: In an oven-dried
round bottom flask under an atmosphere of nitrogen at room temperature were
2 3
placed p-cresol (1 mmol), complex 1 (10 μmol), K CO (2 mmol) and DMF (2
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mixture was cooled to room temperature, diluted with EtOAc (20 mL), washed
dil. HCl and water. The combined organic layer was filtered through a pad of Celite,
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dried over anhydrous Na
average of two runs).
2 4
SO and the crude products were analyzed by GC–MS
(
(2012) 1356.