A group of diphosphine-thiosemicarbazone complexes of palladium: Efficient precursors for catalytic C–C and C–N coupling reactions

Article abstract of DOI:10.1016/j.ica.2018.10.053

Reaction of 4-R-benzaldehyde thiosemicarbazone (denoted in general as HL-R; where H stands for the dissociable acidic proton and R (R = OCH₃, CH₃, H, Cl and NO₂) for the substituent) with [Pd(dppe)(EtOH)₂]²⁺, generated in situ via interaction of [Pd(dppe)Cl₂] (dppe = 1,2-bis(diphenylphosphino)ethane) with AgNO₃ in hot ethanol, in the presence of triethylamine affords a group of orange complexes of the type [Pd(dppe)(L-R)]NO₃. Structures of [Pd(dppe)Cl₂] and [Pd(dppe)(L-OCH₃)]NO₃ have been determined by X-ray crystallography. In the [Pd(dppe)(L-R)]NO₃ complexes, the thiosemicarbazone ligands are coordinated to the metal center as monoanionic bidentate N,S-donors forming five-membered chelate rings. The [Pd(dppe)(L-R)]NO₃ complexes show intense absorptions in the visible and ultraviolet regions, which have been analyzed by TDDFT calculations. All the [Pd(dppe)(L-R)]NO₃ complexes are found to efficiently catalyze Suzuki-type C–C and Buchwald-type C–N coupling reactions.

Full text of DOI:10.1016/j.ica.2018.10.053

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Research paper
A group of diphosphine-thiosemicarbazone complexes of palladium: Efficient
precursors for catalytic C-C and C-N coupling reactions
Kiran Thapa, Piyali Paul, Samaresh Bhattacharya
PII:
S0020-1693(18)31139-3
https://doi.org/10.1016/j.ica.2018.10.053
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Reference:
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Please cite this article as: K. Thapa, P. Paul, S. Bhattacharya, A group of diphosphine-thiosemicarbazone complexes
of palladium: Efficient precursors for catalytic C-C and C-N coupling reactions, Inorganica Chimica Acta (2018),
doi: https://doi.org/10.1016/j.ica.2018.10.053
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A group of diphosphine-thiosemicarbazone complexes of
palladium: Efficient precursors for catalytic C-C and C-N
coupling reactions
Kiran Thapa,‡ Piyali Paul, Samaresh Bhattacharya*
Department of Chemistry, Inorganic Chemistry Section,
Jadavpur University, Kolkata - 700 032, India
* Corresponding author. Tel./fax: +91 33 24146223.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
‡
Science Academies' Summer Research Fellow (2013) from the Department of
Chemistry, B. Borooah College, Guwahati, Assam 781007.
_______________________________________________________________________
ABSTRACT
Reaction of 4-R-benzaldehyde thiosemicarbazone (denoted in general as HL-R; where H
stands for the dissociable acidic proton and R (R = OCH3, CH3, H, Cl and NO2) for the
substituent) with [Pd(dppe)(EtOH)2]2+, generated in situ via interaction of [Pd(dppe)Cl2]
(dppe = 1,2-bis(diphenylphosphino)ethane) with AgNO3 in hot ethanol, in the presence of
triethylamine affords a group of orange complexes of the type [Pd(dppe)(L-R)]NO3.
Structures of [Pd(dppe)Cl2] and [Pd(dppe)(L-OCH3)]NO3 have been determined by X-ray
crystallography. In the [Pd(dppe)(L-R)]NO3 complexes, the thiosemicarbazone ligands
are coordinated to the metal center as monoanionic bidentate N,S-donors forming five-
membered chelate rings. The [Pd(dppe)(L-R)]NO3 complexes show intense absorptions in
the visible and ultraviolet regions, which have been analyzed by TDDFT calculations. All
the [Pd(dppe)(L-R)]NO3 complexes are found to efficiently catalyze Suzuki-type C-C and
Buchwald-type C-N coupling reactions.
_______________________________________________________________________
Keywords:
1,2-Bis(diphenylphosphino)ethane
Benzaldehyde thiosemicarbazone
Ag-assisted stepwise synthesis
Mixed-bis complexes of palladium
Catalytic C-C and C-N coupling reactions

2
1. Introduction
The present work has originated from our continued interest in thiosemicarbazone
complexes of transition metals in general [1-18], and those of palladium in particular [19-
27]. The chemistry of thiosemicarbazone complexes has been attracting considerable
current attention [28-46], largely because of the biological application of such complexes.
However, we have been particularly attracted to the ability of these ligands to display
variable binding mode in their complexes [1-27]. For example, we have recently observed
that 4-R-benzaldehyde thiosemicarbazones (denoted in general as HL-R, where H stands
R = OCH3, CH3,
R
H, Cl, NO2
Pd
H
N
H
NH2
S
N
S
H
N
N
NH2
R
R
HL-R
I
R
H
Pd
Pd
N
S
N
S
H
N
H
N
NH2
NH2
II
III
for the dissociable acidic proton) bind to palladium as a bidentate N,S-donor forming a
rather uncommon five-membered chelate ring (I) [25,26]. With respect to the structure of
the uncoordinated HL-R ligand, this mode of binding (I) was quite intriguing as it
involved a change in geometry around the pre-existing imine (C=N) bond. The same
thiosemicarbazones have also been observed to bind to palladium, under a different set of
experimental conditions, as monoanionic N,S-donors, again forming five-membered

3
chelate ring (II), but without any drastic change in geometry around the pre-existing
imine bond [22]. However, the phenyl ring, which shares almost the same plane with the
rest of the molecule in the uncoordinated ligand, is found to be significantly twisted
(through >20°) away from the plane containing the palladium coordinated -N-N=C-S-
fragment in chelate II. This twist was found to be particularly useful for an effective
agostic interaction between the 16-electron distorted square-planar palladium(II) and the
ortho C-H fragment. Further manipulations were successful in activating this ortho C-H
bond and thus inducing a C,N,S-mode of coordination (III) [22].
Apart from observing the three interestingly different modes of binding of the
benzaldehyde thiosemicarbazones in their palladium complexes (I, II and III), the other
interesting aspect of these complexes is their remarkable catalytic properties, which have
been manifested in facile C-C and C-N coupling reactions of different types [22,24-26].
Presence of a soft phosphorus-donor ligand, viz. triphenylphosphine (PPh3), along with a
N,S-coordinated thiosemicarbazone in a group of mixed-ligand complexes of type [Pd(L-
R)(PPh3)Cl] was found to be very effective in augmenting their catalytic efficiency
towards several C-C cross-coupling reactions of three different types [26]. The present
study was aimed at the incorporation of a second phosphorus donor, along with the
thiosemicarbazone, in such catalyst precursors, and thereby augmentation of their
catalytic efficiency. This paper describes the results obtained from this study, with special
reference to synthesis of a new family of palladium complexes having a PdNSP2 core,
their structures and, their catalytic efficiency towards Suzuki-type C-C and Buchwald-
type C-N coupling reactions.

4
2. Experimental
2.1. Materials
Palladium chloride, obtained from Arora Matthey, Kolkata, India, was converted
to Na2[PdCl4] by following a reported procedure [47]. 1,2-Bis(diphenylphosphino)ethane
(dppe) was obtained from Aldrich. [Pd(dppe)Cl2] was prepared by the reaction of
Na2[PdCl4] with dppe in ethanol [48]. The 4-R-benzaldehydes (R = OCH3, CH3, H, Cl
and NO2) and thiosemicarbazide were procured from E-Merck India Pvt. Ltd. The 4-R-
benzaldehyde thiosemicarbazones were prepared by reacting equimolar amounts of
thiosemicarbazide and the respective para-substituted benzaldehyde in hot 1:1 ethanol-
water mixture under stirring condition. All other chemicals and solvents were reagent
grade commercial materials, and were used as received.
2.2. Synthesis of complexes
The [Pd(dppe)(L-R)]NO3 (R = OCH3, CH3, H, Cl and NO2) complexes were
prepared by following a general procedure. Specific details are given below for a
particular complex.
2.2.1. [Pd(dppe)(L-OCH3)]NO3
[Pd(dppe)Cl2] (100 mg, 0.17 mmol) was taken in hot ethanol (40 ml) and AgNO3
(60 mg, 0.35 mmol) was added to it. The solution was stirred for 15 min, and then the
precipitated AgCl was removed by filtration. To the filtrate were added 4-
methoxybenzaldehyde thiosemicarbazone (35 mg, 0.17 mmol) and triethylamine (17 mg,
0.17 mmol). The solution was then heated at reflux for 1 h, whereby a brownish-orange
solution was obtained. Evaporation of the filtrate gave a dark solid, which was subjected
to purification by thin layer chromatography on a silica plate. With 1:2 acetonitrile-
benzene as the eluant, an orange band separated, which was extracted with acetonitrile.
Upon evaporation of the acetonitrile extract [Pd(dppe)(L-OCH3)]NO3 was obtained as a
crystalline orange solid. Yield: 73%. Anal. Calc. for C35H30N4O4P2SPd: C, 54.52; H, 3.89;
N, 7.27. Found: C, 54.83; H, 3.87; N, 7.29%. 1H NMR (300 MHz, CDCl3) [49]: 2.55-2.89
(2CH2) 3.69 (s, OCH3), 5.63 (s, NH2), 6.72 (d, J = 9.0 Hz, 2H), 6.78 (s, azomethine),
7.44-7.75 (2PPh2 + 2H)*. IR (cm−1): 1599, 1581, 1502, 1483, 1436, 1384, 1337, 1309,

5
1267, 1170, 1104, 1029, 998, 884, 857, 842, 821, 749, 719, 707, 692, 657, 595, 573, 531,
484.
2.2.2. [Pd(dppe)(L-CH3)]NO3
Yield: 72%. Anal. Calc. for C35H30N4O3P2SPd: C, 55.67; H, 3.98; N, 7.42. Found:
C, 55.71; H, 3.93; N, 7.47. 1H NMR (300 MHz, CDCl3): 2.32 (s, CH3), 2.64-2.95 (2CH2),
5.32 (s, NH2), 7.07 (d, J = 9.0 Hz, 2H), 7.31 (s, azomethine), 7.39-7.82 (2PPh2 + 2H)*. IR
(cm−1): 1603, 1584, 1501, 1485, 1435, 1384, 1335, 1311, 1271, 1180, 1102, 1025, 997,
887, 854, 819, 749, 717, 705, 690, 655, 531, 482.
2.2.3. [Pd(dppe)(L-H)]NO3
Yield: 75%. Anal. Calc. for C34H28N4O3P2SPd: C, 55.11; H, 3.78; N, 7.56. Found:
C, 55.20; H, 3.88; N, 7.53. 1H NMR (300 MHz, CDCl3): 2.59-2.94 (2CH2) 5.53 (s, NH2),
6.94 (s, azomethine), 7.31 (d, J = 9.0 Hz, 2H), 7.45-7.55 (2PPh2 + 2H)*. IR (cm−1): 1602,
1583, 1504, 1484, 1435, 1384, 1334, 1310, 1272, 1185, 1103, 1036, 998, 887, 850, 840,
824, 750, 717, 705, 691, 656, 532, 483.
2.2.4. [Pd(dppe)(L-Cl)]NO3
Yield: 79%. Anal. Calc. for C34H27N4O3P2SClPd: C, 52.65; H, 3.48; N, 7.23;
1
found: C, 52.62; H, 3.53; N, 7.21. H NMR (300 MHz, CDCl3): 2.57-2.93 (2CH2), 5.76
(s, NH2), 6.81 (s, azomethine), 7.19 (d, J = 8.7 Hz, 2H), 7.44-7.99 (2PPh2 + 2H)*. IR
(cm−1): 1604, 1584, 1502, 1485, 1436, 1384, 1336, 1312, 1269, 1170, 1105, 1035, 997,
879, 849, 839, 819, 749, 717, 705, 691, 656, 531, 483.
2.2.5. [Pd(dppe)(L-NO2)]NO3
Yield: 77%. Anal. Calc. for C34H27N5O5P2SPd: C, 51.95; H, 3.44; N, 8.91; found:
1
C, 52.03; H, 3.38; N, 8.97. H NMR (300 MHz, CDCl3) ppm: 2.57-2.97 (4H, CH2),
6.26 (s, NH2), 6.86 (d, J = 9.0 Hz, 2H), 7.28 (s, azomethine), 7.50-8.21 (2PPh2 + 2H)*. IR
(cm−1): 1609, 1584, 1501, 1484, 1435, 1384, 1336, 1311, 1267, 1171, 1104, 1035, 998,
878, 850, 837, 820, 750, 716, 705, 690, 655, 531,484.

6
2.3. Physical measurements
Microanalyses (C, H, N) were performed using a Heraeus Carlo Erba 1108
elemental analyzer. IR spectra were obtained on a Perkin Elmer Spectrum Two IR
spectrometer with samples prepared as KBr pellets. Electronic spectra were recorded on a
JASCO V-570 spectrophotometer. 1H NMR spectra were recorded in CDCl3 solutions on
a Bruker Avance DPX 300 NMR spectrometer using TMS as the internal standard.
Geometry optimization by density functional theory (DFT) method and electronic spectral
analysis by TDDFT calculation were performed using the Gaussian 03 (B3LYP/SDD-6-
31G) package [50]. GC-MS analyses were performed using a Perkin Elmer CLARUS 680
instrument.
2.4. X-Ray crystallography
Single crystals of [Pd(dppe)Cl2] were obtained by slow evaporation of solvent
from a solution of the complex in acetonitrile. Single crystals of [Pd(dppe)(L-OCH3)]NO3
were grown by slow evaporation of solvents from a solution of the complex in 1:1
acetonitrile-ethanol. Selected crystal data and data collection parameters for [Pd(dppe)(L-
OCH3)]NO3 are given in Table 1, and those for [Pd(dppe)Cl2] are deposited in Table S1
(Supplementary material). Data on both the crystals were collected on a Bruker SMART
CCD diffractometer. X-ray data reduction and, structure solution and refinement were
done using the SHELXS-97 and SHELXL-97 packages [51]. The structures were solved
by the direct methods.
2.5. Application as catalysts
2.5.1. General procedure for Suzuki-type C-C coupling reactions
In a typical run, an oven-dried 10 ml round bottom flask was charged with a
known mole percent of catalyst and base, phenylboronic acid (1.2 mmol) and aryl halide
(1.0 mmol) with the appropriate solvent(s) (4 ml). The flask was 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  10 ml). The combined
organic layers were washed with water (3  10 ml), dried over anhydrous Na2SO4, and
filtered. Solvent was removed under vacuum. The residue was dissolved in hexane and
analyzed by GC-MS.

7
2.5.2. General procedure for Buchwald-type C-N coupling reactions
In a typical run, an oven-dried 10 mL round bottom flask was charged with a
known mole percent of catalyst, NaOtBu (1.3 mmol), amine (1.2 mmol) and aryl halide (1
mmol) with the appropriate solvent(s) (4 ml). The flask was placed in a preheated oil bath
at required temp. After the specified time the flask was removed from the oil bath, water
(20 mL) was added, and extraction with ether (4  10 ml) was done. The combined
organic layers were washed with water (3  10 ml), dried over anhydrous Na2SO4, and
filtered. Solvent was removed under vacuum. The residue was dissolved in acetonitrile
and analyzed by GC-MS.
3. Results and discussion
3.1. Synthesis and characterization
As delineated in the introduction, the initial goal of the present study was to
synthesize a group of mixed-ligand benzaldehyde thiosemicarbazone complexes of
palladium having a NSP2 coordination sphere around the metal center. Given the usual
structure of the Pd(L-R) chelate (I), the two phosphorus donors are required to occupy
mutually cis positions. Our earlier work on the interaction of HL-R with [Pd(PPh3)2Cl2]
did not afford such a [Pd(PPh3)2(L-R)]+ species [22], probably due to trans disposition of
the two triphenylphosphines in the starting compound. In order to have a rigid cis-PdP2
core in the starting compound, so that it may be retained in the final product, we have
selected [Pd(dppe)Cl2] (dppe = 1,2-bis(diphenylphosphino)ethane) as the palladium
starting material for the present study. For incorporation of a thiosemicarbazone by
displacing the two mutually cis chlorides from [Pd(dppe)Cl2], we adopted the step-wise
synthetic route as shown in Scheme 1. The two coordinated chlorides were first taken out
Scheme 1
from [Pd(dppe)Cl2] by its interaction with Ag+ in hot ethanol, and reaction of 4-R-
benzaldehyde thiosemicarbazone with the in situ generated [Pd(dppe)(EtOH)2]2+ in the
presence of triethylamine afforded a family of orange complexes of the targeted type, viz.
[Pd(dppe)(L-R)]+, which were isolated in good yields as their nitrate salts. Preliminary
(microanalytical and spectroscopic) characterizations on these complexes were found to

8
be consistent with their compositions. In order to find out coordination mode of the 4-R-
benzaldehyde thiosemicarbazones in these complexes, structure of a selected member of
this family, viz. [Pd(dppe)(L-OCH3)]NO3, was determined by X-ray crystallography.
Crystal structure of the parent [Pd(dppe)Cl2] complex was also determined [48,52], for
comparing the changes in structural features, if any, caused upon displacement of the two
coordinated chlorides by a deprotonated thiosemicarbazone. The structure of
[Pd(dppe)(L-OCH3)]+ is shown in Fig. 1 and some selected bond parameters are presented
Fig. 1
in Table 2. The structure of [Pd(dppe)Cl2] and some relevant structural data are deposited
as Fig. S1 and Table S2 (Supplementary material) respectively. The structure of
[Pd(dppe)(L-OCH3)]+ shows that the thiosemicarbazone is coordinated to palladium as a
monoanionic bidentate N,S-donor forming a five-membered chelate ring (I, R = OCH3),
and dppe has remained chelated to the metal center. To balance the unipositive charge of
this tetra-coordinated palladium complex, a nitrate ion exists outside the coordination
sphere, which was found as a disordered anion in the crystal structure. Palladium is thus
nested in an NSP2 core in this complex, which is distorted significantly from the ideal
square-planar geometry, as manifested in the bond parameters around the metal center
(Table 2). The Pd-N, Pd-S and other bond distances within the Pd(L-OCH3) fragment are
found to be usual, as observed in structurally characterized palladium complexes
containing a similar fragment [24-26, 53-56]. Structural features in the Pd(dppe) fragment
of [Pd(dppe)(L-OCH3)]+ are found to be mostly similar to those of [Pd(dppe)Cl2].
However, the Pd-P bond trans to the coordinated sulfur in [Pd(dppe)(L-OCH3)]+ is found
to be slightly longer than the other Pd-P bond that is trans to the imine-nitrogen, while in
[Pd(dppe)Cl2] both the Pd-P lengths are expectedly similar. The dissimilarity in the two
Pd-P lengths in [Pd(dppe)(L-OCH3)]+ is attributable to the difference in trans-effect of the
thiolate sulfur and imine-nitrogen [57]. The P1-Pd1-P2 angle is found to be slightly
shorter in the thiosemicarbazone complex than that in the precursor complex, presumably
due to steric effect imposed by the coordinated thiosemicarbazone ligand. As all the
[Pd(dppe)(L-R)]NO3 complexes were synthesized similarly and they show similar
properties (vide infra), the other four [Pd(dppe)(L-R)]NO3 (R = CH3, H, Cl and NO2)
complexes are assumed to have similar structures as their R = OCH3 analogue. It is worth

9
mentioning here that mixed-bis palladium complexes of type [Pd(dppe)(L-L′)]+, where L-
L′ depicts a monoanaionic bidentate ligand, are limited in number [58-63]. And none of
these reported examples was synthesized in a step-wise fashion via Ag+-assisted
displacement of chlorides from [Pd(dppe)Cl2].
3.2. Spectral properties
1H NMR spectra of the [Pd(dppe)(L-R)]+ complexes, recorded in CDCl3 solutions,
show signals characteristic of both ligands. For the dppe ligand, two closely spaced
doublets are observed within 2.55-2.97 ppm for the methylene protons and broad signals
for the phenyl protons appear within 7.39-8.21 ppm. From the coordinated
thiosemicarbazone, signal for the NH2 protons appears as a distinct singlet within 5.32-
6.26 ppm, and another singlet is observed within 6.78-7.31 ppm for the azomethine
proton. Signals for the OCH3 and CH3 groups, in the [Pd(dppe)(L-OCH3)]+ and
[Pd(dppe)(L-CH3)]+ complexes, are observed at 3.69 and 2.32 ppm respectively. While
many of the expected signals from the aryl rings of the coordinated thiosemicarbazone
could be clearly identified, some could not be due to overlap problem.
Infrared spectra of the [Pd(dppe)(L-R)]NO3 complexes show many bands of
varying intensities within 4000-450 cm-1. A sharp and strong band observed near 1382
cm-1 in all five complexes indicates the presence of the nitrate ion. Three strong bands
observed near 531, 691 and 750 cm-1 in each [Pd(dppe)(L-R)]NO3 complex, and also in
[Pd(dppe)Cl2], are attributed to the coordinated dppe ligand. Comparison with the
spectrum of [Pd(dppe)Cl2] shows that several new bands are present in the spectrum of
each [Pd(dppe)(L-R)]NO3 complex (e.g. near 1603, 1584, 1502, 1485, 1335, 1269 and
1
1035 cm-1), which are attributable to the coordinated thiosemicarbazone ligand. The H
NMR and IR spectral data of the [Pd(dppe)(L-R)]NO3 complexes are therefore consistent
with their compositions.
The [Pd(dppe)(L-R)]NO3 complexes are soluble in common organic solvents like
methanol, ethanol, acetone, acetonitrile, dichloromethane, chloroform, etc., producing
intense orange solutions. Electronic spectra of the complexes were recorded in
dichloromethane solutions. Spectral data are presented in Table 3. Each complex shows

10
several intense absorptions spanning the visible and ultraviolet regions. To have an insight
into the nature of these absorptions, TDDFT calculations were performed on all five
palladium complexes using the Gaussian 03 package [50], and the results are found to be
similar for all the complexes. The main calculated transitions for [Pd(dppe)(L-OCH3)]+
and compositions of the molecular orbitals associated with these transitions are presented
respectively in Table 4 and Table 5, and contour plots of the same molecular orbitals are
shown in Fig. 2. Similar data and diagrams for the other four [Pd(dppe)(L-R)]+ (R = CH3,
Fig. 2
H, Cl and NO2) complexes are deposited in Table S3-S10 and Fig. S2-S5 (Supplementary
material). The results for [Pd(dppe)(L-OCH3)]+ are discussed here as a representative
case. The lowest energy absorption at 414 nm is attributable primarily to a HOMO →
LUMO transition, with much less HOMO → LUMO+1 and HOMO-1 → LUMO
character, and based on the nature of the participating orbitals the electronic excitation is
assignable to a combination of MLCT, LMCT, LLCT and ILCT transitions. The next two
absorptions at 356 and 308 nm are also found to be of similar nature. The absorption at
276 nm is largely due to HOMO-5 → LUMO+1 and HOMO-2 → LUMO+1 transitions,
and as both HOMO-5 and HOMO-2 have large contribution from dppe, and LUMO+1 is
delocalized mostly over the thiosemicarbazone ligand, this particular transition has
dominant dppe-to-thiosemicarbazone charge-transfer character.
3.3. Catalytic activity
Complexes of palladium(II), with the metal center coordinated to soft donors, are
attractive catalyst-precursors for different types of C-C and C-heteroatom coupling
reactions, particularly owing to the relative ease of in situ generation and stabilization of
palladium(0). In the present group of [Pd(dppe)(L-R)]+ complexes palladium has a NSP2
coordination sphere, and in view of the recognized soft nature of P- and S-donor sites, as
well as of the imine-nitrogen, these complexes appear to have the potential to serve as
good catalyst precursors. Catalytic activity of this group of mixed-bis palladium
complexes was first examined towards Suzuki-type C-C coupling reactions between
phenylboronic acid and p-iodoacetophenone using [Pd(dppe)(L-OCH3)]NO3 as the
catalyst precursor. Table S11 (Supplementary material) provides information on the

11
impact of various reaction parameters on the efficiency of this process. After several trials
for optimization, it was observed that the best result is obtained with 0.0001 mol% of
catalyst, Cs2CO3 as base, 95°C temperature and 3 h reaction time (entry 1, Table S11).
Under these optimized conditions the other four [Pd(dppe)(L-R)]NO3 (R ≠ OCH3)
complexes were found to exhibit similar catalytic efficiency (entries 10-13, Table S11).
However, the starting [Pd(dppe)Cl2] complex, when tried as the catalyst precursor, was
found to be significantly less active (entry 14, Table S11), indicating that presence of the
N,S-coordinated thiosemicarbazone in [Pd(dppe)(L-R)]+ has augmented the catalytic
efficiency. We have also compared the catalytic activity of [Pd(dppe)(L-OCH3)]+ (entry
1) with that of [Pd(L-OCH3)(PPh3)Cl], a complex reported earlier by us [26], and the
latter was found to be much less efficient (entry 15, Table S11), testifying that the
substitution of a triphenylphosphine and a chloride by a chelating dppe ligand has
genuinely been effective in enhancing the catalytic activity.
The scope of the reaction is shown in Table 6. As all the [Pd(dppe)(L-R)]NO3
complexes have shown comparable catalytic efficiency, the results obtained with
[Pd(dppe)(L-OCH3)]NO3 are only highlighted here. Coupling between p-substitited
phenyl iodides and p-substituted boronic acids was first attempted, and in each case the
targeted biaryl was obtained in good yield (entries 1-5). The observed turnover numbers
are remarkably high (~106) and such high turnover numbers are known only for few other
palladium-based catalytic systems [64-68]. Similar coupling of p-substituted boronic
acids with p-substituted phenylbromides was found to be slightly difficult, as manifested
primarily in the higher catalyst loading required to achieve comparable yield (entries 6-
10). A similar trend is observed on moving from aryl bomides to aryl chlorides (entries
11-15), where higher-boiling solvent, stronger base, longer reaction time and much higher
catalyst loading were needed to achieve good yields. Though the observed turnover
numbers for reactions involving the aryl chlorides are relatively low (~103), such facile C-
Cl bond activation is noteworthy as it is industrially very much welcome due to easy
availability of the relatively inexpensive aryl chlorides [69-73]. Finally, C-C coupling
reaction via the rather difficult C-F bond activation was explored (entries 16-20), and the
targeted biaryls were obtained only from reactions with phenylboronic acid (entries 16-
18), but in much lesser yields and under significantly more drastic experimental

12
conditions compared to the previous cases. However, it is interesting to note that the
[Pd(dppe)(L-R)]NO3 complexes can successfully catalyze C-C coupling between aryl
fluoride and phenylboronic acid, which is scarce in the literature [74-77].
The observed ease of the Suzuki coupling reactions, with particular reference to
the mild reaction conditions, prompted us to explore catalytic activity of these mixed-
ligand palladium complexes in Buchwald-type C-N coupling reactions of aryl halides
with primary and secondary amines. We began our study with the coupling of p-
iodoacetophenone and para-methoxy aniline. After several trials for optimization, we
observed that the best result is obtained with 0.01 mol% of catalyst, dioxane as solvent,
100°C temperature and 5 h reaction time (entry 1 in Table 7). The yields of the coupled
products decreased slightly when p-iodobenzaldehyde or p-iodobenzonitrile was used
instead of p-iodoacetophenone (entries 2 and 3). Similar decrease in yield was also
observed with decrease in electron-donating character of the p-substituent in the aryl
amine fragment (entries 4 and 5). As done in case of the C-C coupling reactions, here also
we tried the starting [Pd(dppe)Cl2] complex and the previously reported [Pd(L-
OCH3)(PPh3)Cl] complex [26] as catalyst precursor, and both were found to be much less
active (entry 6 and 7) than [Pd(dppe)(L-OCH3)]+ (entry 1). With aryl bromides as
substrates, higher catalyst loading (0.1 mol%), longer reaction time and a higher-boiling
solvent were necessary to achieve comparable yield (entries 8 – 12). With aryl chlorides
the yield of the coupled products were poor, even with much higher catalyst loading (1.0
mol%) and longer reaction time (entries 13 – 17). No C-N coupling could be achieved
with the aryl fluorides. The scope for arylation of secondary amines was investigated
using morpholine and piperidine. Coupling of these amines with the same set of aryl
halides was scrutinized using the same complex, viz. [Pd(dppe)(L-OCH3)]NO3, as
catalyst. The targeted reactions could be achieved with different types of para-substituted
halobenzenes, with gradually increasing difficulty and decreasing yields on moving from
iodo- to bromo- to chloro-aryls (Table 8). Such smooth C-N coupling reactions with good
yields are of significant contemporary interest [78-82].
The [Pd(dppe)(L-R)]NO3 complexes are thus found to be efficient catalysts for
both C-C and C-N coupling reactions. Apart from the relatively mild reaction conditions,

13
good yields and high turnover numbers, the other noticeable aspect of the observed
catalysis is that no additional ligand was necessary for the cross-coupling reactions, which
is always convenient but relatively less common [83-87]. This indicates that the dppe and
thiosemicarbazone ligands, coordinated to palladium in the catalyst precursor, do not
dissociate completely during in situ generation of the catalytically active palladium(0)
species, and thereby provide stability to the reduced metal center. Formation of such
species has been encountered in our recent studies [88].
4. Conclusions
The present study shows that [Pd(dppe)Cl2] can be utilized as a useful starting
material for the synthesis of mixed-bis complexes of palladium via Ag+-assisted removal
of the coordinated chlorides, as manifested in the facile synthesis of a group of
thiosemicarbazone complexes of type [Pd(dppe)(L-R)]+. This Ag+-assisted route has been
found to be very efficient, particularly in terms of the short reaction time and high yield of
the mixed-bis complexes of palladium. This study also demonstrates that the [Pd(dppe)(L-
R)]+ complexes are good catalyst precursors for efficient C-C and C-N coupling reactions
under relatively mild and external ligand-free condition. Presence of the dppe ligand, in
combination with a de-protonated thiosemicarbazone, has been found to be very effective
in enhancing the catalytic efficiency of the palladium center.
Acknowledgements
Financial assistance received from the Department of Science and Technology,
Government of West Bengal, Kolkata [Grant No. 746(Sanc.)/ST/P/S&T/2G-4/2013], the
Council of Scientific and Industrial Research, New Delhi [Grant No. 01(2788)/14/EMR-
II] and the University Grants Commission, New Delhi [Grant No. F.19-122/2014(BSR)],
is gratefully acknowledged. Kiran Thapa thanks the Science Academies' Summer
Research Fellowship Programme for his fellowship [Registration No-CHES1652].
Appendix A. Supplementary material
CCDC 1008791 (for [Pd(dppe)Cl2]) and 1008792 (for [[Pd(dppe)(L-OCH3)]NO3])
contain the supplementary crystallographic data for this paper. Crystal structure of
[Pd(dppe)Cl2] (Fig. S1), crystallographic data for [Pd(dppe)Cl2] (Table S1), bond
distances (Å) and bond angles (°) for [Pd(dppe)Cl2] (Table S2), results of TDDFT
calculations on the [Pd(dppe)(L-R)]+ (R = CH3, H, Cl and NO2) complexes (Table S3 –

14
S10); contour plots of some molecular orbitals of the [Pd(dppe)(L-R)]+ (R = CH3, H, Cl
and NO2) complexes (Fig. S2 – S5); Screening of experimental conditions for C-C
coupling reactions (Table S11).
.
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**********

18
Table 1
Crystallographic data for [Pd(dppe)(L-OCH3)]+.
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C35H34N3OP2SPd·NO3
775.09
Orthorhombic
P212121
14.3272(6)
15.4619(6)
17.4002(7)
3854.6(3)
4
b (Å)
c (Å)
V (Å3)
Z
Dcalcd.(mg m-3)
1.336
F(000)
1584
λ (Å)
Crystal size (mm3)
0.71073
0.19  0.25  0.33
296
T (K)
µ (mm-1)
0.658
Collected reflections
Rint
64854
0.040
Independent reflections
8911
a
R1
0.0421
b
wR2
0.1380
GOFc
0.59
aR1 =  Fo-  Fc / Fo 
2
2 2
bwR2 = [ [ w( Fo - Fc2)2] /  [w( Fo ) ] ]1/2
2
cGOF = [[w(Fo -Fc2)2]/(M-N)]1/2, where M is the number of reflections and N is the
number of parameters refined.

19
Table 2
Selected bond lengths (Å) and bond angles (°) for [Pd(dppe)(L-OCH3)]+.
Bond lengths (Å)
Pd(1)-P(1)
Pd(1)-P(2)
Pd(1)-S(1)
Pd(1)-N(1)
P(1)-C(1)
C(1)-C(2)
P(2)-C(2)
2.2947(11)
2.2484(12)
2.2840(13)
2.113(3)
C(27)-S(1)
C(27)-N(3)
C(27)-N(2)
N(2)-N(1)
N(1)-C(28)
1.735(5)
1.338(7)
1.310(6)
1.381(5)
1.285(6)
1.831(5)
1.510(7)
1.832(5)
Bond angles (°)
S(1)-Pd(1)-P(1) 175.72(5)
P(2)-Pd(1)-N(1) 174.21(9)
P(1)-Pd(1)-P(2) 84.53(4)
S(1)-Pd(1)-N(1) 82.67(10)
Table 3
Electronic spectral data of the complexes in dichloromethane solution.
max (nm) ( (M-1cm-1))
Complex

[Pd(dppe)(L-OCH3)]NO3 414 (1750), 356 (2580), 308a (2140),
276 (11400)
[Pd(dppe)(L-CH3)]NO3
[Pd(dppe)(L-H)]NO3
[Pd(dppe)(L-Cl)]NO3
[Pd(dppe)(L-NO2)]NO3
415 (1720), 356 (2770), 307a (2100),
273 (12000)
413 (1800), 355 (3100), 308a (2110),
278 (13000)
413 (1700), 356 (2870), 309a (2140),
276 (11000)
418 (1940), 358 (2900), 312a (2160),
281 (11800)
a Shoulder.

20
Table 4
Main calculated transitions for [Pd(dppe)(L-OCH3)]+ with composition in terms of
molecular orbital contribution of the transition, excitation energies, and oscillator strength
in acetonitrile.
Tran- Nature of
sition transition
No.
CI
E (eV) Oscillator λtheo(nm)
Assignments
strength (λexp(nm))
(f)
H-1 → L
H → L
H → L+1
H-1 → L
H-1 → L+1 0.12242
H → L+1
H-2→ L
H-1→ L+1
H → L+1
0.17946 3.0076
0.65926
0.12200
0.0069
0.4954
0.3642
412.23
(414)
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT
MLCT/LMCT/LLCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT
1
2
3
-0.33142 3.5148
352.75
(356)
0.56393
0.11187 4.0651
0.11087
305.00
(308)
-0.46419
MLCT/LMCT/LLCT
H-8→ L+1 -0.11880 4.5075
0.0150
275.06
(276)
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT/ILCT
MLCT/LMCT/LLCT
4
H-5→ L+1
H-3→ L+1
H-2→ L
0.47797
0.17448
-0.16594
0.30597
H-2→ L+1
H-1 → L+3 -0.14898
Table 5
Compositions of selected molecular orbitals of the [Pd(dppe)(L-OCH3)]+ complex.
Fragments
Contribution (%) of fragments to
H-8 H-5 H-3 H-2 H-1 HOMO LUMO L+1 L+3
(H)
(L)
Pd
24.9 27.5 36.9 29.5 21.6
34.8 3.6 52.8 23.1 74.3
17.7
31.6
21.9 18.2
L-OCH3
dppe
82.3
0
29.3
40.1
68.3
9.8
5.2
40.3 68.9 10.3 47.4
4.1
76.6

21
Table 6
Suzuki cross coupling of aryl halides with phenylboronic acid.a
[Pd(dppe)(L-OCH3)]NO3
R1
X
(HO)2B
R2
R1
R2
+
base, solvent
Entry R1
X
R2
Solvent
Base
Temp. Time Amt of cat
Yieldb TONc
(%)
(˚C)
(h)
(mol%)
1
2
COCH3
I
I
I
I
I
-H
-H
-H
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-
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
95
3
0.0001
98
92
90
93
87
100
97
95
86
79
980000
920000
900000
930000
870000
100000
97000
CHO
95
95
95
95
95
95
95
95
95
5
5
5
5
6
7
7
8
8
0.0001
0.0001
0.0001
0.0001
0.001
0.001
0.001
0.001
0.001
3
CN
4
COCH3
COCH3
-OCH3
-CH3
-H
5
6
COCH3 Br
7
CHO
CN
Br
Br
-H
8
-H
95000
toluene (1:1)
Ethanol-
toluene (1:1)
Ethanol-
toluene (1:1)
Toluene
9
COCH3 Br
COCH3 Br
COCH3 Cl
CHO
CN
COCH3 Cl
COCH3 Cl
COCH3
-OCH3
-CH3
86000
10
79000
11
12
13
14
15
16
-H
-H
-H
-OCH3
-CH3
-H
NaOH
NaOH
NaOH
NaOH
NaOH
105
105
105
105
105
130
10
12
12
12
12
24
0.05
0.05
0.05
0.1
0.1
1.0
90
86
81
77
74
66
1800
1720
1620
770
740
66
Cl
Cl
Toluene
Toluene
Toluene
Toluene
F
F
F
F
F
Polyethylene NaOtBu
glycol
17
18
19
20
CHO
-H
-H
Polyethylene NaOtBu
glycol
130
130
130
130
24
24
24
24
1.0
1.0
1.0
1.0
41
34
-
41
34
-
CN
Polyethylene NaOtBu
glycol
COCH3
COCH3
-OCH3 Polyethylene NaOtBu
glycol
-CH3
Polyethylene NaOtBu
glycol
-
-
a Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.2 mmol),
Cs2CO3 (2.5 mmol) or NaOH (1.7 mmol) or NaOtBu (1.3 mmol), Pd catalyst,
Solvent (4 ml).
b Determined by GC-MS on the basis of residual aryl halide (no other product derived
from the aryl halide was found).
c TON = turnover number [(mol of product)/(mol of catalyst)].

22
Table 7
C-N cross-coupling reaction of aryl halides with primary amines.a
catalystb
X
H2N
N
H
+
t
NaO Bu, solvent
R1
R2
R1
R2
Entry
R1
X
R2
Solvent
Temp. Time Amt of cat.
Yieldc
TONd
(˚C)
(h)
5
(mol%)
0.01
(%)
95
1
2
COCH3
CHO
I
I
OCH3
OCH3
OCH3
CH3
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
100
9500
9100
8400
8500
8300
4400
5900
900
100
100
100
100
100
100
150
5
6
0.01
0.01
0.01
0.01
0.01
0.01
0.1
91
84
85
83
44
59
90
3
CN
I
4
COCH3
COCH3
COCH3
COCH3
COCH3
I
5
5
I
H
7
6e
7f
8
I
OCH3
OCH3
5
I
5
Br
OCH3 Polyethylene
glycol
10
9
CHO
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
OCH3 Polyethylene
glycol
OCH3 Polyethylene
glycol
150
150
150
150
150
150
150
150
150
10
12
10
12
24
24
24
24
24
0.1
0.1
0.1
0.1
1
82
73
83
82
33
12
15
29
23
820
730
830
820
33
10
11
12
13
14
15
16
17
CN
COCH3
COCH3
COCH3
CHO
CH3
Polyethylene
glycol
H
Polyethylene
glycol
OCH3 Polyethylene
glycol
OCH3 Polyethylene
glycol
OCH3 Polyethylene
glycol
1
12
CN
1
15
COCH3
COCH3
CH3
Polyethylene
glycol
1
29
H
Polyethylene
glycol
1
23
a Reaction conditions: aryl halide (1.0 mmol), amines (1.0 mmol), NaOtBu (1.7 mmol),
Pd catalyst, polyetheleneglycol (4 mL).
b Catalyst = [Pd(dppe)(L-OCH3)]NO3
c Determined by GC-MS on the basis of residual aryl halide(no other product derived
from the aryl halide was found).
d TON = turnover number [(mol of product)/(mol of catalyst)].
e Catalyst = [Pd(dppe)Cl2]
f Catalyst = [Pd(L-OCH3)(PPh3)Cl]

23
Table 8
C-N cross-coupling reaction of aryl halides with secondary amines.a
catalystb
R1
X
HN(R)R'
R1
N(R)R'
+
t
NaO Bu, solvent
Entry
R1
X
Amines
Solvent
Temp. Time Amt of
Yieldc
TONd
(˚C)
(h)
cat.
(%)
(mol%)
1
2
COCH3
CHO
I
I
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
100
6
0.01
0.01
0.01
0.01
0.01
0.01
0.1
96
9600
8600
7400
9500
8100
7600
650
HN
HN
HN
HN
HN
HN
HN
HN
HN
HN
O
O
O
100
100
100
100
100
150
150
150
150
6
8
86
74
95
81
76
65
67
43
47
3
CN
I
4
COCH3
CHO
I
6
5
I
6
6
CN
I
8
7
COCH3
COCH3
COCH3
COCH3
Br
Br
Cl
Cl
Polyethylene
glycol
12
12
24
24
O
O
8
Polyethylene
glycol
0.1
670
9
Polyethylene
glycol
1
43
10
Polyethylene
glycol
1
47
a Reaction conditions: aryl halide (1.0 mmol), amines (1.0 mmol), NaOtBu (1.7 mmol),
Pd catalyst, solvent (4 mL).
b Catalyst = [Pd(dppe)(L-OCH3)]NO3
c Determined by GC-MS on the basis of residual aryl halide (no other product derived
from the aryl halide was found).
d TON = turnover number [(mol of product)/(mol of catalyst)].

24
2+
+
Ph Ph
P
Ph Ph
P
Ph Ph
P
H
H
Cl
Cl
O
O
Et
S
NH2
AgNO3
HL-R
-
Pd
Pd
Pd
NO3
N
ethanol
2AgCl
ethanol
NEt3
P
P
Et
P
N
-
Ph Ph
Ph Ph
Ph Ph
H
R
Scheme 1

25
Fig. 1. Crystal structure of the [Pd(dppe)(L-OCH3)]+ complex.
Hydrogen atoms are omitted for clarity.

26
H (HOMO)
L (LUMO)
H-1
H-2
L+1
L+3
H-3
H-5
H-8
Fig. 2. Contour plots of molecular orbitals of [Pd(dppe)(L-OCH3)]+, which are
associated with the observed electronic spectral transitions.

27
Manuscript No.:
Manuscript title:
ICA_2018_1026
A group of diphosphine-thiosemicarbazone complexes of
palladium: Efficient precursors for catalytic C-C and C-N coupling
reactions
Authors:
Kiran Thapa, Piyali Paul, Samaresh Bhattacharya*
Highlights
 4-R-benzaldehyde thiosemicarbazone ligands (HL-R) are chosen.
 The thiosemicarbazone ligands undergo Ag+-assisted reaction with [Pd(dppe)Cl2].
 Mixed-ligand complexes of type [Pd(dppe)(L-R)]+ are formed.
 The palladium complexes efficiently catalyze C-C and C-N coupling reactions.

28
Manuscript No.:
Manuscript title:
ICA_2018_1026
A group of diphosphine-thiosemicarbazone complexes of
palladium: Efficient precursors for catalytic C-C and C-N coupling
reactions
Authors:
Kiran Thapa, Piyali Paul, Samaresh Bhattacharya*
Graphical Abstract
Pictogram:
Ph Ph
P
Cl
AgNO3
HL-R
Pd
ethanol
P
ethanol
NEt3
Cl
2AgCl
-
Ph Ph
+
[Pd(dppe)(L-OCH3)]
Synopsis:
4-R-Benzaldehyde thiosemicarbazone ligands (HL-R) undergo Ag+-assisted reaction with
[Pd(dppe)Cl2] to afford mixed-ligand complexes of type [Pd(dppe)(L-R)]+. These
complexes have shown efficient catalytic efficiency towards Suzuki-type C-C and
Buchwald-type C-N coupling reactions.