Palladium complexes of 2-formylpyridine thiosemicarbazone and two related ligands: Synthesis, structure and, spectral and catalytic properties

Article abstract of DOI:10.1016/j.ica.2014.10.010

Thiosemicarbazones of 2-formylpyridine, 2-aceylpyridine and 2-benzoylpyridine, abbreviated in general as HL-R (where H depicts the acidic hydrogen and R the fragment (R = H, Me and Ph) linked to the imine-carbon) react with [Pd(PPh₃)₂Cl₂] in refluxing ethanol in the presence of triethylamine to afford complexes of the type [Pd(L-R)(PPh₃)]Cl (1, R = H; 2, R = Me, 3, R = Ph). Structures of the complexes 1 and 3 have been determined by X-ray crystallography, and the structure of complex 2 has been optimized by DFT. In all three the thiosemicarbazone ligand binds to the metal center as a monoanionic tridentate NNS-donor forming two adjacent five-membered chelate rings, the triphenylphosphine occupies the fourth coordination site on palladium. Complexes 1-3 display intense absorptions in the visible and ultraviolet regions, which have been analyzed by TDDFT calculations. All the complexes are found to efficiently catalyze Suzuki, Heck and Sonogashira type C-C cross-coupling, and C-N coupling reactions of aryl halides with primary and secondary amines. All the catalytic reactions are found to proceed under ligand-free condition.

Full text of DOI:10.1016/j.ica.2014.10.010

Inorganica Chimica Acta 425 (2015) 67–75
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Palladium complexes of 2-formylpyridine thiosemicarbazone
and two related ligands: Synthesis, structure and, spectral
and catalytic properties
a
b
a,
⇑
Piyali Paul , Ray J. Butcher , Samaresh Bhattacharya
a
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
Department of Chemistry, Howard University, Washington, DC 20059, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 June 2014
Received in revised form 8 October 2014
Accepted 9 October 2014
Available online 22 October 2014
Thiosemicarbazones of 2-formylpyridine, 2-aceylpyridine and 2-benzoylpyridine, abbreviated in general
as HL-R (where H depicts the acidic hydrogen and R the fragment (R = H, Me and Ph) linked to the imine-
carbon) react with [Pd(PPh
plexes of the type [Pd(L-R)(PPh
3
)
2
Cl
2
] in refluxing ethanol in the presence of triethylamine to afford com-
)]Cl (1, R = H; 2, R = Me, 3, R = Ph). Structures of the complexes 1 and 3
3
have been determined by X-ray crystallography, and the structure of complex 2 has been optimized by
DFT. In all three the thiosemicarbazone ligand binds to the metal center as a monoanionic tridentate
NNS-donor forming two adjacent five-membered chelate rings, the triphenylphosphine occupies the
fourth coordination site on palladium. Complexes 1–3 display intense absorptions in the visible and
ultraviolet regions, which have been analyzed by TDDFT calculations. All the complexes are found to effi-
ciently catalyze Suzuki, Heck and Sonogashira type C–C cross-coupling, and C–N coupling reactions of
aryl halides with primary and secondary amines. All the catalytic reactions are found to proceed under
ligand-free condition.
Keywords:
2
-Formylpyridine thiosemicarbazone
Palladium complexes
Syntheses
Structures
C–C and C–N coupling reactions
Ó 2014 Elsevier B.V. All rights reserved.
1
. Introduction
Thiosemicarbazone complexes of the transition metals have
the pyridine-nitrogen to the metal center, we have selected the
thiosemicarbazone of 2-formylpyridine, and two related ligands,
viz. thiosemicarbazones of 2-aceylpyridine and 2-benzoylpyridine,
for the present study. The chosen thiosemicarbazones are
abbreviated in general as HL-R, where H depicts the acidic hydro-
gen and R the fragment (R = H, Me and Ph) linked to the imine-car-
bon. The main aim of the present work has been to study the
interaction of the selected thiosemicarbazones (HL-R) with
received considerable attention, largely because of their bioinor-
ganic relevance [1]. However, we have been exploring the chemis-
try of transition metal complexes of the thiosemicarbazones,
primarily because of the variable binding mode displayed by these
ligands in their complexes, and the present work has emerged out
of this exploration [2]. In a recent study we have observed that
benzaldehyde thiosemicarbazone (HL), and four similar ligands,
3 2 2
[Pd(PPh ) Cl ] and see whether they bind to palladium in an
NNS-mode (I) without any rotation around the pre-existing imine
react with [Pd(PPh
3
)
2
Cl
2
] to yield a group of mixed-ligand com-
bond. The other, and equally important, objective has been to
3
plexes of type [Pd(PPh )(L)Cl] (Scheme 1), in which the thiosemi-
carbazones display a NS-mode of coordination [2a,b]. In view of
the structure of the uncoordinated thiosemicarbazone, which is
similar to that shown for HL [2b], this NS-mode of coordination
is a bit uncommon as it involves a change in geometry around
the pre-existing C@N bond. In order to examine whether
substitution of the phenyl ring in HL by a pyridyl ring, with the
pyridine-nitrogen at the ortho position with respect to the imine
group, can prevent this change in geometry via coordination of
Cl
PPh3
Pd
[Pd(PPh
3
)
2
Cl
2
]
H
-
PPh
3
,
-
HCl
N
H
NH
2
N
S
H
N
N
S
NH
2
[Pd(PPh
3
)(L)Cl]
HL
⇑
Corresponding author.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
Scheme 1.
http://dx.doi.org/10.1016/j.ica.2014.10.010
020-1693/Ó 2014 Elsevier B.V. All rights reserved.
0

6
8
P. Paul et al. / Inorganica Chimica Acta 425 (2015) 67–75
explore catalytic activity of the resulting complexes towards
coupling reactions of various types. It may be worth mentioning here
2.2.2. Complex 2
24 4
Yield: 62%. Anal. Calc. for C26H N PSClPd: C, 52.27; H, 4.02; N,
9
.38. Found: C, 52.23; H, 4.07; N, 9.36%. Mass spectral data (ESI,
R = H, Me, Ph
+
À1
2
À1
positive mode, CH
3
CN): m/z 561 for [2-Cl] .
H NMR (300 MHz, DMSO-d ): 2.42 (s, CH
t, 1H, J = 9.0), 7.59–7.68 (PPh ), 7.80 (d, 1H, J = 8.0), 7.94 (t, 1H,
K
M
: 148 cm M
.
N
N
M
1
6
3
), 5.69 (s, NH ), 7.16
2
(
3
N
H
NH
2
N
S
3
1
R
N
R
N
J = 8.5), 8.10 (d, 1H, J = 9.0).
43.03 ppm. IR (cm ): 1639, 1599, 1563, 1480, 1456, 1432, 1380,
6
P NMR (300 MHz, DMSO-d ):
À1
S
NH2
1
2
8
315, 1257, 1146, 1098, 1020, 998, 747, 721, 694, 531, 506.
HL-R
I
.2.3. Complex 3
that palladium complexes are extensively utilized as catalyst for
the synthesis of industrially useful organic molecules, particularly
via C–C and C–N coupling reactions [3,4]. Reactions of the chosen
Yield: 71%. Anal. Calc. for C31
26 4
H N PSClPd: C, 56.46; H, 3.95; N,
.50. Found: C, 56.51; H, 3.97; N, 8.46%. Mass spectral data (ESI,
+
À1
2
À1
positive mode, CH
3
CN): m/z 623 for [3-Cl] .
K
M
: 144 cm M
.
3 2 2
thiosemicarbazones with [Pd(PPh ) Cl ] have been found to afford
1
H NMR (300 MHz, CDCl
3
): 5.29 (s, NH
+ 2H ), 7.78 (t, 1H, J = 8.5), 7.93 (t, 1H, J = 8.2),
2
), 7.07 (d, 1H, J = 9.0),
a group of mixed-ligand complexes, and the chemistry of these
complexes is reported in this paper, with particular reference to
their formation, structure and, catalytic efficiency towards C–C
and C–N coupling reactions.
⁄
7
8
.37–7.69 (PPh
3
31
.07 (t, 1H, J = 9.0), 8.73 (d, 2H, J = 9.0), 8.87 (d, 1H, J = 9.0).
P
À1
NMR (300 MHz, CDCl
3
): 43.10 ppm. IR (cm ): 1616, 1596, 1545,
1
7
482, 1454, 1437, 1384, 1320, 1263, 1181, 1097, 1027, 998, 745,
22, 695, 533, 510.
2
. Experimental
2.3. Physical measurements
2.1. Materials
Microanalyses (C, H and N) were performed using a Heraeus
Palladium chloride was obtained from Arora Matthey, Kolkata,
Carlo Erba 1108 elemental analyzer. Mass spectra were recorded
with a Micromass LCT electrospray (Qtof Micro YA263) mass spec-
trometer. NMR spectra were recorded in CDCl or DMSO-d solu-
3 6
India. The [Pd(PPh
reported procedure [5]. 2-Formylpyridine, 2-acetylpyridine and
-benzoylpyridine were obtained from Merck (India), Spectrochem
India) and Sigma–Aldrich, respectively. The thiosemicarbazone
3 2 2
) Cl ] complex was prepared by following a
2
(
tion on a Bruker Avance DPX 300 NMR spectrometer. IR spectra
were obtained on a Perkin Elmer Spectrum Two IR spectrometer
with samples prepared as KBr pellets. Solution electrical conduc-
tivities were measured in acetonitrile solution using a Philips PR
ligands (HL-R; R = H, Me and Ph) were prepared by reacting equi-
molar amounts of thiosemicarbazide and the respective pyridine-
derivative in warm ethanol [6]. All other chemicals and solvents
were reagent grade commercial materials and were used as
received.
À3
8499 bridge with a solute concentration of 10 M. Electronic spec-
tra were recorded on a JASCO V-570 spectrophotometer. Geometry
optimization by density functional theory (DFT) method and elec-
tronic spectral analysis by TDDFT calculation were performed
using the GAUSSIAN 03 (B3LYP/SDD-6–31G) package [7]. GC–MS anal-
yses were performed using a Perkin Elmer CLARUS 680 instrument.
2
.2. Preparation of the complexes
The [Pd(L-R)(PPh )]Cl complexes (1, R = H; 2, R = Me; 3, R = Ph)
3
2.4. X-ray crystallography
were prepared by following a general procedure. Specific details
are given below for a particular complex.
Single crystals of complex 1 were obtained by slow evaporation
of solvents from a solution of the complex in 1:1 methanol–aceto-
nitrile. Single crystals of complex 3 were obtained by slow evapo-
ration of solvent from a solution of the complex in acetonitrile.
Selected crystal data and data collection parameters are given in
Table 1. Data were collected on a Bruker SMART CCD diffractome-
2
.2.1. Complex 1
-Formylpyridine thiosemicarbazone (26 mg, 0.14 mmol) was
dissolved in warm ethanol (30 mL) and triethylamine (14 mg,
2
ter using graphite monochromated Mo
K
a
radiation
0
3 2 2
.14 mmol) was added to it, followed by [Pd(PPh ) Cl ] (100 mg,
(k = 0.71073 Å). X-ray data reduction, structure solution and
0
.14 mmol). The mixture was then refluxed for 5 h to yield a yel-
refinement were done using SHELXS-97 and SHELXL-97 programs [8].
The structures were solved by the direct methods. In the structure
of complex 3, there was severely disordered solvent present on
crystallographic symmetry elements which could not be modeled.
lowish–brown solution. The solvent was evaporated and the solid
mass, thus obtained, was subjected to purification by thin layer
chromatography on a silica plate. With 1:3 acetonitrile-benzene
as the eluant, an orangish–yellow band separated, which was
extracted with acetonitrile. Evaporation of the acetonitrile extract
gave complex 1 as an orangish–yellow crystalline solid. Yield:
This was removed using the SQUEEZE routine from PLATON
.
2.5. Application as catalysts
6
22 4
8%. Anal. Calc. for C25H N PSClPd: C, 51.47; H, 3.77; N, 9.61.
Found: C, 51.53; H, 3.72; N, 9.65%. Mass spectral data (ESI, positive
+
À1
2
À1 1
2
.5.1. General procedure for the Suzuki coupling reactions
mode, CH
3
CN): m/z 547 for [1-Cl] .
K
M
: 145 cm M
.
H NMR
1
In a typical run, an oven-dried 10 mL round bottom flask was
charged with known mole percent of catalyst, Na CO
1.7 mmol), phenylboronic acid (1.2 mmol) and aryl halide
1 mmol) with the appropriate solvents (4 mL). The flask was
(
300 MHz, CDCl
3
)
: 5.73 (s, NH
2
), 7.19 (t, 1H, J = 9.0), 7.38 (s, 1H),
a
2
3
7
1
1
1
.58–7.72 (PPh
3
), 7.87 (d, 1H, J = 8.0), 8.06 (t, 1H, J = 8.5), 8.16 (d,
3
1
À1
(
(
H, J = 9.0).
P NMR (300 MHz, CDCl
3
): 43.04 ppm. IR (cm ):
646, 1602, 1562, 1480, 1458, 1434, 1375, 1318, 1255, 1179, 1098,
020, 997, 752, 722, 697, 532, 508.
placed in a preheated oil bath at required temp. After the specified
time the flask was removed from the oil bath and water (20 mL)
added, followed by extraction with ether (4 Â 10 mL). The com-
bined organic layers were washed with water (3 Â 10 mL), dried
1
Chemical shifts are given in ppm and multiplicity of the signals along with the
associated coupling constants (J in Hz) are given in parentheses. Overlapping signals
are marked with an asterisk.
2 4
over anhydrous Na SO , and filtered. Solvent was removed under

P. Paul et al. / Inorganica Chimica Acta 425 (2015) 67–75
69
Table 1
extraction with ether (4 Â 10 mL) was done. The combined organic
Crystallographic data for the complex 1 and complex 3.
layers were washed with water (3 10 mL), dried over anhydrous
Complex
1ÁH
2
O
3Á3.75H
2
O
2 4
Na SO , and filtered. Solvent was removed under vacuum. The res-
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
25
H
24
4
N OPSClPd
C
124
H
134
N
16
O
15
P
4
S
Cl
4 4
Pd
4
idue was dissolved in hexane and analyzed by GC–MS using Elite-5
columns, which are fused silica capillary columns coated with 5%
diphenyl and 95% dimethyl polysiloxane.
601.36
triclinic
P1ꢀ
2907.98
monoclinic
C2/c
9.1746(2)
9.4020(2)
15.5484(3)
107.030(1)
95.493(1)
95.559(1)
1265.46(5)
2
32.977(3)
b (Å)
c (Å)
15.1566(13)
31.115(4)
90
117.871(2)
90
13748(2)
4
1.405
3
. Results and discussion
a
(°)
b (°)
(°)
3.1. Synthesis and characterization
c
3
V (Å )
The reaction between 2-formylpyridine thiosemicarbazone (HL-
H) and [Pd(PPh ) Cl ] proceeded smoothly in refluxing ethanol in
3 2 2
Z
À3
D
calc (g cm
)
1.578
the presence of triethylamine to afford an orangish-yellow com-
plex 1 in a decent yield. Preliminary characterization (microanaly-
sis, mass, IR, NMR, etc.) on complex 1 indicated the presence of a
thiosemicarbazone and a triphenylphosphine in the coordination
sphere. In order to unambiguously characterize this complex, and
particularly to ascertain coordination mode of the thiosemicarbazone
ligand in it, its structure was determined by X-ray crystallography.
The structure (Fig. 1) shows that 2-formylpyridine thiosemicarba-
zone is coordinated to palladium, via dissociation of the acidic pro-
ton, as a monoanionic tridentate NNS-donor forming two adjacent
five-membered chelate rings (I, R = H). A triphenylphosphine is
also coordinated to the metal center. To balance the uni-positive
charge of this tetra-coordinated palladium complex, a chloride
ion exists outside the coordination sphere, which was also clearly
found in the crystal structure. The structure determination also
reveals that coordination by the pyridine-nitrogen has been effec-
tive, as anticipated, in preventing the rotation around the pre-
existing imine bond, observed with benzaldehyde thiosemicarba-
zone [2a,b]. It is relevant to mention here that such NNS-mode of
binding by this thiosemicarbazone ligands has precedence in the
literature [9]. It may also be mentioned here that a palladium com-
plex containing a dianionic NNS-coordinated thiosemicarbazone
and PPh is also reported in the literature [10]. The N SP coordina-
k (Å)
F(000)
Crystal size (mm)
0.71073
608
0.71073
5944
0.37 Â 0.24 Â 0.15 0.35 Â 0.25 Â 0.12
T (K)
298
173
À1
l
(mm
)
1.010
0.0224
0.0582
1.062
0.763
0.0745
0.1537
0.956
a
R
1
b
wR
2
Goodness-of-fit (GOF) on
2c
F
a
b
C
R
1
= ||F
o
| À |F
c o
||/|F |.
2
2
2
2
2
1/2
wR = [[w(F
2
o
À F
c
) ]/[w(F
o
) ]]
.
2
2
)2]/(M À N)]1/2, where M is the number of
Goodness-of-fit (GOF) = [[w(F
o
À F
c
reflections and N is the number of parameters refined.
vacuum. The residue was dissolved in hexane and analyzed by GC–
MS using Elite-5 columns, which are fused silica capillary columns
coated with 5% diphenyl and 95% dimethyl polysiloxane.
2
.5.2. General procedure for the Heck coupling reactions
In a typical run, an oven-dried 10 mL round bottom flask was
charged with a known mole percent of catalyst, Cs CO (1.7 mmol),
2 3
n-butyl acrylate (1.2 mmol) and aryl halide (1 mmol) with polyeth-
ylene glycol (4 mL). The flask was placed in a preheated oil bath at
50 °C. After the specified time the flask was removed from the oil
bath and water (20 mL) added, followed by extraction with ether
3
2
1
tion sphere around palladium is distorted considerably from ideal
square planar geometry, as manifested in the bond parameters
around the metal center (Table 2). The Pd–N, Pd–S and Pd–P dis-
tances are found to be normal, as observed in structurally charac-
terized complexes of palladium containing these bonds [2a,b,9a].
To check the generality of the observed NNS-mode of coordina-
tion displayed by 2-formylpyridine thiosemicarbazone, reaction of
[Pd(PPh ) Cl ] was also carried out with the other two selected
(
(
4 Â 10 mL). The combined organic layers were washed with water
SO , and filtered. Solvent
3 Â 10 mL), dried over anhydrous Na
2
4
was removed under vacuum. The residue was dissolved in hexane
and analyzed by GC–MS using Elite-5 columns, which are fused sil-
ica capillary columns coated with 5% diphenyl and 95% dimethyl
polysiloxane.
3
2
2
ligands, viz. 2-aceylpyridine thiosemicarbazone (HL-Me) and 2-
benzoylpyridine thiosemicarbazone (HL-Ph), under similar exper-
imental conditions as before, which afforded orangish-yellow com-
2
.5.3. General procedure for Sonogashira coupling reactions
To slurry of aryl halide (1 mmol), cuprous iodide (10 mol%) and
palladium catalyst (a known mol%) in an appropriate solvent
4 mL), phenylacetylene (1.2 mmol) and NaOH (1.7 mmol) was
(
added and heated at required temp. After completion of the reac-
tion (monitored by TLC), the flask was removed from the oil bath
and water (20 mL) added, followed by extraction with ether
(
(
4 Â 10 mL). The combined organic layers were washed with water
3 Â 10 mL), dried over anhydrous Na
2
SO , and filtered. Solvent
4
was removed under vacuum. The residue was dissolved in hexane
and analyzed by GC–MS using Elite-5 columns, which are fused sil-
ica capillary columns coated with 5% diphenyl and 95% dimethyl
polysiloxane.
2.5.4. General procedure for C–N coupling reactions
In a typical run, an oven-dried 10 mL round bottom flask was
t
charged with
a
known mole percent of catalyst, NaO –Bu
(
1.3 mmol), aryl amine (1.2 mmol) and aryl halide (1 mmol) with
the appropriate solvent(s) (4 mL). The flask was placed in a pre-
heated oil bath at required temp. After the specified time the flask
was removed from the oil bath, water (20 mL) was added, and
Fig. 1. View of the crystal structure of complex 1. The chloride ion outside the
coordination sphere is not shown.

7
0
P. Paul et al. / Inorganica Chimica Acta 425 (2015) 67–75
Table 2
3.2. Spectral studies
Selected bond lengths (Å) and bond angels (°) for the complexes 1 and 3.
1H NMR spectra of complexes 1–3 show broad signals within
.37–7.72 ppm for the coordinated PPh ligands, and a distinct
signal at around 5.6 ppm for the NH fragment. In complex 1, a sig-
Complex 1
7
3
Bond lengths (Å)
2
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–P(1)
Pd(1)–S(1)
2.1035(16)
2.0176(17)
2.2780(5)
2.2432(6)
C(6)–N(2)
N(2)–N(3)
N(3)–C(7)
C(7)–N(4)
C(7)–S(1)
1.287(3)
1.357(3)
1.320(3)
1.322(3)
1.773(2)
nal for the azomethine proton in the coordinated thiosemicarba-
zone appears at 7.38 ppm. In complex 2, signal for the methyl
group is observed at 2.42 ppm. In all the three complexes most
of the aromatic proton signals are clearly observed in the expected
region, while a few could not be detected due to their overlap with
Bond angles (°)
P(1)–Pd(1)–N(2)
S(1)–Pd(1)–N(1)
177.59(5)
163.51(5)
N(1)–Pd(1)–N(2)
S(1)–Pd(1)–N(2)
80.49(6)
83.20(5)
31
other signals in the same region. P NMR spectra of complexes 1–
3
show a single resonance within 43.03–43.10 ppm due to the
coordinated PPh ligand.
3
Complex 3
Infrared spectra of complexes 1–3 show many bands of differ-
ent intensities in the 400–4000 cm region. No attempt has been
Bond lengths (Å)
À1
Pd(1)–N(1A)
Pd(1)–N(2A)
Pd(1)–P(1A)
Pd(1)–S(1A)
2.079(4)
2.008(4)
2.2538(16)
2.2370(17)
C(6A)–N(2A)
N(2A)–N(3A)
N(3A)–C(13A)
C(13A)–N(4A)
C(13A)–S(1A)
1.321(6)
1.363(6)
1.319(7)
1.336(7)
1.764(6)
made to assign each individual band to a specific vibration. How-
ever, three strong bands have been observed near 532, 695, 748
À1
and 1098 cm in all the three complexes indicating the presence
3
of coordinated PPh ligands. Besides, comparison with the spec-
Bond angles (°)
trum of [Pd(PPh Cl
3
)
2
2
] shows the presence of several new bands
À1
P(1A)–Pd(1)–N(2A)
S(1A)–Pd(1)–N(1A)
176.48(12)
164.64(14)
N(1A)–Pd(1)–N(2A)
S(1A)–Pd(1)–N(2A)
80.25(18)
84.50(13)
(e.g. near 1646–1616, 1430, 1316 and 1179–1146 cm ) in the
spectrum of complexes 1–3, which are attributable to the coordi-
nated thiosemicarbazone ligand.
Complexes 1 and 3 are found to be readily soluble in polar
organic solvents like methanol, ethanol, acetonitrile, dichloro-
methane, chloroform, etc., producing intense yellow solutions.
Complex 2, however, is insoluble in dichloromethane and chloro-
form, but is soluble in the other polar organic solvents. Conduc-
tance measurement in acetonitrile solution shows that these
complexes behave as 1:1 electrolytes as expected (see Section 2).
Electronic spectra of the complexes were recorded in acetonitrile
solutions. Spectral data are presented in Table 3. Each complex
showed intense absorptions in the visible and ultraviolet regions.
To have an insight into the nature of these absorptions, TDDFT cal-
culations were performed on all three palladium complexes using
the Gaussian 03 package [7]. Phenyl rings of the triphenylphos-
phines in complexes 1–3 were replaced by hydrogens for simplify-
ing the calculation. The results of the TDDFT calculations for
complex 1 are presented in Table 4, and those for complexes 2
and 3 are deposited in Table S2 and S3 (Supplementary material)
respectively. Compositions of few frontier orbitals of complexes
plexes 2 and 3, respectively, in good yield. Preliminary character-
izations on these complexes indicated that they also have compo-
sition similar to that of complex 1. Structural characterization of
complex 2 by X-ray crystallography was not possible, as single
crystals of this complex could not be grown. However, its structure
was geometrically optimized through DFT calculations [7]. The
optimized structure of the complex cation is shown in Fig. S1 (Sup-
plementary material) and some computed bond parameters are
presented in Table S1 (Supplementary material). The optimized
structure of complex 2 is found to compare well with the crystal
structure of complex 1. Structure of complex 3, however, could
be determined by X-ray crystallography, and the structure
(Fig. 2) shows the same NNS-mode of binding (I, R = Ph) by the thi-
osemicarbazone ligand. The metrical parameters (Table 2) of this
structure are also comparable to those observed for complex 1.
From the structural data it is evident that the pyridine-nitrogen
in 2-formylpyridine thiosemicarbazone, and the other two related
ligands, has been very effective in enabling these ligands to bind to
palladium in the NNS-mode (I) and thus preventing the change in
geometry around the pre-existing imine bond during their
complexation.
1–3 are given in Table 5. Contour plots of some selected molecular
orbitals for complex 1 are shown in Fig. 3 and those of all the orbi-
tals, which are involved with the observed electronic spectral tran-
sitions, for all the three complexes are deposited as Figs. S2–S4
(Supplementary material). The results obtained are found to be
qualitatively similar for all three complexes, and hence only the
case of complex 1 is discussed here. The lowest energy absorption
at 457 nm is attributable to a combination of HOMO ? LUMO and
HOMO-1 ? LUMO transitions, and based on the nature of the par-
ticipating orbitals (Table 5; Fig. 3) the electronic excitation is
assignable to a mixture of MLCT, LMCT and LLCT transitions,
involving primarily the thiosemicarbazone ligand, with much less
contribution from the metal center. The next absorption at
4
16 nm is found to be due to a mixture of MLCT, LMCT, LLCT and
ILCT transitions, again with thiosemicarbazone ligand as the major
Table 3
Electronic spectral data.
a
À1
À1
Complex
kmax (nm) (
e
M
cm )
Complex 1 457b (2300), 416 (2600), 355b (9500), 336 (9800), 295 (15000)
b
b
Complex 2 455 (2100), 415 (2400), 353 (7300), 335 (8500), 297 (14200)
Complex 3 460b (2000), 416 (2300), 358b (7100), 339 (7500), 299 (12000)
a
Fig. 2. View of the crystal structure of complex 3. The chloride ion outside the
coordination sphere is not shown.
In acetonitrile solution.
Shoulder.
b

P. Paul et al. / Inorganica Chimica Acta 425 (2015) 67–75
71
Table 4
Main calculated transitions for complex 1 with composition in terms of molecular orbital contribution of the transition, excitation energies, and oscillator strength in acetonitrile.
a
Excited state
1
Composition
CI
E (eV)
Oscillator strength (f)
k
theo (nm)
Assignments
kexp (nm)
H À 1 ? L
À0.17848
0.65730
À0.17959
0.64458
0.16285
0.27799
0.56789
0.11252
0.11187
0.11087
0.46419
0.40906
0.12491
À0.11608
À0.13638
À0.19888
À0.19363
0.54077
À0.13805
0.19573
2.7141
0.0417
456.81
429.62
360.56
MLCT/LMCT/LLCT
MLCT/LMCT/LLCT
457
416
355
H ? L
2
3
H À 1 ? L + 1
2.8859
3.4387
0.0021
0.0124
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT
H ? L + 1
H À 3 ? L + 1
H À 4 ? L + 1
H À 4 ? L + 1
H À 1 ? L + 1
H À 4 ? L
4
5
3.6557
4.3077
0.2276
0.0311
339.15
387.82
336
295
H À 4 ? L + 1
H À 1 ? L
H À 1 ? L + 1
H ? L + 1
H ? L + 2
H À 10 ? L + 1
H À 9 ? L + 1
H À 5 ? L + 1
H À 13 ? L + 1
H À 1 ? L + 1
H ? L + 2
a
3 3
PPh was replaced by PH in the calculation.
Table 5
Compositions of selected molecular orbitals of the complexes.
a
Complex
Fragments
% Contribution of fragments to
H À 5
H À 4
H À 3
H À 2
H À 1
HOMO (H)
LUMO (L)
L + 1
L + 2
Complex 1
Pd
L–H
PH
10.1
73.4
16.5
23.6
70.3
5.7
25.6
60.5
13.9
43.1
56.9
0
19.9
83.1
0
18.5
81.5
0
8.7
91.3
0
26.4
58.2
15.4
7.3
92.7
0
3
Complex 2
Complex 3
Pd
L–CH
PH
22.2
59.3
18.5
27.4
66.4
6.2
30.2
54.7
15.1
54.8
45.2
0
19.4
80.6
0
18.1
81.9
0
25.4
74.6
0
25.3
47.8
26.9
10.6
89.4
0
3
3
Pd
L–Ph
PH
30.9
69.1
0
24.1
75.9
0
31.3
68.7
0
26.7
73.3
0
18.7
81.3
0
18.3
81.7
0
16.8
83.2
0
22.5
56.6
20.9
11.4
88.6
0
3
a
PPh
3
was replaced by PH
3
in the calculation.
contributor and with relatively less contributions from the phos-
phine and palladium. The remaining three absorptions at 355,
Table 6, where only the results obtained with complex 3 as the cat-
alyst are presented. Several cross-coupling reactions were performed
by varying both the aryl halide and the arylboronic acid. Coupling of
3
36 and 295 nm are also qualitatively similar to that at 416 nm,
with varying degree of contributions from the three fragments.
1 1 3
phenylboronic acid with three 4-R -phenyl iodides (R = CH CO,
CHO and CN) were tried, all of which afforded the expected C-C cou-
5
3
3
.3. Catalysis
pled products in excellent yields with high (ꢀ10 ) turnover numbers
(
entries 1–3). It may be mentioned here that such high turnover
numbers are relatively less common [12]. Coupling of para-iodoace-
tophenone with three 4-R -phenylboronic acids (R = OCH , CH and
.3.1. C–C cross-coupling reactions
The fact, that palladium complexes are well known to serve as
2
2
3
3
efficient catalysts in bringing about C–C cross-coupling reactions
of different types [11], led us to explore such catalytic properties
in the present group of complexes. The catalytic activity of these
complexes was examined for C–C cross-coupling reactions of three
types, viz. Suzuki, Heck and Sonogashira reactions.
Initially all the three complexes were tested as catalyst in the
Suzuki coupling of phenylboronic acid and p-iodoacetophenone
to yield the biphenyl product. After extensive optimization, it
Cl) also took place with similar efficiency (entries 4–6). A similar
trend was observed with the aryl bromides (entries 7–12), however,
with a higher catalyst loading (0.01 mol%). The attempted C–C cou-
pling involving aryl chlorides also proceeded smoothly, but with
much higher catalyst loading (0.1 mol%) and slightly harsh reaction
condition (entries 13–18). It is worth mentioning here that forma-
tion of new molecules through C–Cl bond activation is industrially
very important due to easy availability of the relatively inexpensive
aryl chlorides [13]. Attempts to bring about similar C–C coupling via
C–F bond activation of aryl fluoride, however, was unsuccessful
(entry 19).
Encouraged by the facile Suzuki coupling reactions catalyzed by
complex 3, we further investigated the catalytic activity of the
same complex in Heck reaction of different aryl halides with butyl
acrylate. Heck reactions of four para-substituted phenyl iodides
and butyl acrylate were found to proceed well in 1:1 ethanol-tolu-
ene with 0.05 mol% catalyst to afford the coupled products in fairly
2 3
was found that 0.001 mol% catalyst, 1.7 eqv. Na CO as base, etha-
nol as solvent, 75 °C reaction temperature, and 9 h reaction time,
furnished an excellent (>99%) yield of the desired C–C coupled
2
product. Though all the three complexes were found to show com-
parable catalytic efficiency, the best result was obtained with com-
plex 3 as the catalyst.2 The scope of the reaction is shown in
2
Yields of the products obtained with complex 1, complex 2 and complex 3 as
catalyst are 93%, 96% and >99% respectively.

7
2
P. Paul et al. / Inorganica Chimica Acta 425 (2015) 67–75
good yields (entries 1–4, Table 7). For similar reactions with para-
substituted phenyl bromides catalyst loading needed to be doubled
entries 5–8). However, for para-substituted aryl chlorides, much
H (HOMO)
L (LUMO)
(
higher catalyst loading (1 mol%), higher temperature and a differ-
ent solvent medium (polyethylene glycol) were required to obtain
fairly good yields (entries 9–12).
Finally we scrutinized the catalytic efficiency of complex 3 in
Sonogashira coupling reaction between aryl halides and phenyl
acetylene. The yields of Sonogashira coupling reactions were fairly
good (Table 8). As before, the coupling involving aryl iodides was
most facile (entries 1–4), and the same involving aryl bromides
and chlorides was successively more difficult (entries 5–8 and
entries 9–12).
H-1
L+1
3.3.2. C–N coupling reactions
Encouraged by the facile C–C coupling reactions, we were
inclined to try our catalyst for C–N coupling reaction as well. We
began our study with the coupling of iodobenzene and para-substi-
tuted anilines, and as anticipated, the C–N coupling reactions also
proceeded smoothly with 0.01 mol% catalyst affording the desired
products in good yields (entries 1 and 2, Table 9). Similar smooth
coupling was also observed for bromobenzene, but with 0.1 mol%
catalyst and longer reaction time (entries 3 and 4). When chloro-
benzene was used, higher catalyst loading (1.0 mol%), longer reac-
tion time and a different solvent, viz. dioxane, were required to
achieve reasonably good yield (entries 5 and 6). We also attempted
C–N coupling using fluorobenzene, but the targeted reaction could
not be brought about (entry 7). Besides primary amines, the scope
for arylation of secondary amines was next investigated using the
same catalyst. Two secondary amines, viz. morpholine and piperi-
dine, were selected for this purpose. The targeted reactions could
be achieved with three types of halobenzenes (viz. iodobenzene,
bromobenzene and chlorobenzene), with gradually increasing
difficulty and decreasing yields (Table 10). Such smooth C–N cou-
pling reactions with good yields are of significant contemporary
interest [14].
H-2
L+2
Fig. 3. Contour plots of selected molecular orbitals of complex 1 (PPh
3
was replaced
by PH in the calculation).
3
Table 6
Suzuki cross-coupling of aryl halides with phenylboronic acid.a
catalyst
R
1
X
+
2
(HO) B
R
2
R
1
R
2
Na CO
2 3
solvent
Entry
R
1
R
2
X
Solvent
Temp. (°C)
Time (h)
Amt. of cat. (mol%)
Yieldb (%)
TONc
1
2
3
4
5
6
7
8
9
COCH
CHO
CN
3
H
H
H
OCH
CH
Cl
H
H
H
I
I
I
I
I
I
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
F
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol:toluene (1:1)
ethanol:toluene (1:1)
ethanol:toluene (1:1)
ethanol:toluene (1:1)
ethanol:toluene (1:1)
ethanol:toluene (1:1)
polyethylene glycol
75
75
75
75
75
75
75
75
75
75
75
75
95
95
95
95
95
95
110
9
0.001
0.001
0.001
0.001
0.001
0.001
0.01
0.01
0.01
0.01
0.01
0.01
0.1
0.1
0.1
0.1
0.1
0.1
1
>99
93
100000
93000
100000
100000
100000
100000
10000
10000
10000
10000
10000
10000
880
850
810
930
950
920
0
11
12
10
10
12
10
13
16
13
15
18
12
15
20
18
16
18
24
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
88
85
81
93
95
92
0
COCH
COCH
COCH
COCH
CHO
3
3
3
3
3
3
3
3
CN
10
11
12
13
14
15
16
17
18
19
COCH
COCH
COCH
COCH
CHO
3
3
3
3
OCH
CH
Cl
H
H
H
OCH
CH
Cl
3
CN
COCH
COCH
COCH
COCH
3
3
3
3
3
H
a
b
c
2 3
Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.2 mmol), Na CO (1.7 mmol), Pd catalyst, solvent (4 mL).
Product detected by GC–MS and yield determined by GC–MS on the basis of residual aryl halide.
TON = turnover number ((mol of product)/(mol of catalyst)).

P. Paul et al. / Inorganica Chimica Acta 425 (2015) 67–75
73
Table 7
a
Heck cross-coupling of aryl halides with butyl acrylate.
catalyst
R
1
X
+
CO
2
Bu
R
1
Cs
2 3
CO
2
CO Bu
solvent
Entry
R
1
X
Solvent
Temp. (°C)
Time (h)
Amt. of cat. (mol%)
Yieldb (%)
TONc
1
2
3
4
5
6
7
8
9
H
I
I
I
I
Br
Br
Br
Br
Cl
Cl
Cl
Cl
ethanol-toluene (1:1)
ethanol-toluene (1:1)
ethanol-toluene (1:1)
ethanol-toluene (1:1)
ethanol-toluene (1:1)
ethanol-toluene (1:1)
ethanol-toluene (1:1)
ethanol-toluene (1:1)
polyethylene glycol
polyethylene glycol
polyethylene glycol
polyethylene glycol
110
110
110
110
110
110
110
110
150
150
150
150
12
12
16
18
14
14
18
20
20
24
28
28
0.05
0.05
0.05
0.05
0.1
0.1
0.1
0.1
1.0
85
83
76
65
72
74
67
59
66
62
58
50
1700
1660
1520
1300
720
740
670
590
66
COCH
CHO
CN
3
3
3
H
COCH
CHO
CN
H
1
1
1
0
1
2
COCH
CHO
CN
1.0
1.0
1.0
62
58
50
a
b
c
2 3
Reaction conditions: aryl halide (1.0 mmol), butyl acrylate (1.2 mmol), Cs CO (1.7 mmol), Pd catalyst, solvent (4 mL).
Product detected by GC–MS and yield determined by GC–MS on the basis of residual aryl halide.
TON = turnover number ((mol of product)/(mol of catalyst)).
Table 8
a
Sonogashira cross-coupling of aryl halides with phenylacetylene.
CuI
catalyst
R
X
+
H
Ph
R
Ph
NaOH
solvent
Entry
R
1
X
Solvent
Temp. (°C)
Time (h)
Amt. of cat. (mol%)
Yieldb (%)
TONc
1
2
3
4
5
6
7
8
9
H
I
I
I
I
Br
Br
Br
Br
Cl
Cl
Cl
Cl
ethanol-toluene (1:1)
ethanol-toluene (1:1)
ethanol-toluene (1:1)
ethanol-toluene (1:1)
ethanol-toluene (1:1)
ethanol-toluene (1:1)
ethanol-toluene (1:1)
ethanol-toluene (1:1)
polyethylene glycol
polyethylene glycol
polyethylene glycol
polyethylene glycol
110
110
110
110
110
110
110
110
150
150
150
150
12
12
14
18
16
12
17
20
24
24
24
24
0.01
0.01
0.01
0.01
0.1
0.1
0.1
0.1
1.0
>99
>99
>99
89
>99
98
81
77
66
>99
72
10000
10000
10000
8900
1000
980
810
770
66
100
COCH
CHO
CN
3
3
3
H
COCH
CHO
CN
H
1
1
1
0
1
2
COCH
CHO
CN
1.0
1.0
1.0
72
66
66
a
b
c
Reaction conditions: aryl halide (1.0 mmol), phenylacetylene (1.2 mmol), NaOH (1.7 mmol), Pd catalyst, CuI (10 mol%), solvent (4 mL).
Product detected by GC–MS and yield determined by GC–MS on the basis of residual aryl halide.
TON = turnover number ((mol of product)/(mol of catalyst)).
Table 9
a
C–N coupling reaction of aryl halides with primary amines.
H
N
catalyst
X
+
H
2
N
R
R
t
NaO- Bu
solvent
Entry
R
X
Solvent
Temp. (°C)
Time (h)
Amt. of cat. (mol%)
Yieldb (%)
TONc
1
2
3
4
5
6
7
H
OCH
H
OCH
H
OCH
H
I
I
toluene
toluene
toluene
toluene
dioxane
dioxane
dioxane
95
95
95
95
110
110
110
10
9
0.01
0.01
0.1
0.1
1.0
100
100
100
100
75
10000
10000
1000
1000
75
3
3
3
Br
Br
Cl
Cl
F
14
15
16
18
24
1.0
1.0
79
0
79
0
a
b
c
t
Reaction conditions: aryl halide (1.0 mmol), amines (1.0 mmol), NaO -Bu (1.7 mmol), Pd catalyst, toluene (4 mL).
Product detected by GC–MS and yield determined by GC–MS on the basis of residual aryl halide.
TON = turnover number ((mol of product)/(mol of catalyst)).

7
4
P. Paul et al. / Inorganica Chimica Acta 425 (2015) 67–75
Table 10
C–N cross-coupling reaction of aryl halides with secondary amines.a
catalyst
X
+ HN(R)R'
N(R)R'
t
NaO- Bu
dioxane
Entry
1
X
Amine
Temp. (°C)
Time (h)
12
Amt. of cat. (mol%)
0.01
Yieldb (%)
69
TONc
6900
I
110
110
110
110
130
130
HN
O
O
O
2
3
4
5
6
I
14
18
18
24
24
0.01
0.1
0.1
1
65
59
62
56
50
6500
590
620
56
HN
HN
HN
HN
HN
Br
Br
Cl
Cl
1
50
a
b
c
t
Reaction conditions: aryl halide (1.0 mmol), amines (1.0 mmol), NaO -Bu (1.7 mmol), Pd catalyst, toluene (4 mL).
Product detected by GC–MS and yield determined by GC–MS on the basis of residual aryl halide.
TON = turnover number ((mol of product)/(mol of catalyst)).
The present study thus demonstrates that complexes 1–3 can
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