Palladium(ii)-1-phenylthio-2-arylchalcogenoethane complexes: Palladium phosphide nano-peanut and ribbon formation controlled by chalcogen and Suzuki coupling activation

Article abstract of DOI:10.1039/c4dt03471f

The ligands PhSCH₂CH₂EAr (L1-L3; E = S, Se or Te) and their Pd-complexes [PdLCl₂] (1-3) have been synthesized and authenticated with their ¹H, ¹³C{¹H}, ⁷⁷Se{¹H} and ¹²⁵Te{¹H} NMR spectra. Single crystal structures of 1 and 3 reveal the geometry of donor atoms around palladium as nearly square planar. Thermolysis of all three complexes in trioctylphosphine (TOP) at 350, 320 and 280 °C, respectively, results in a single phase of crystalline PdP₂. The morphology of the phase varies with 'E' to some extent. The nanopeanuts (size ～30 and ～35 nm) were formed with 1 and 2 as precursor complexes. On using complex 3 as a precursor nanoribbons are formed. The preferential growth in the (202) plane in the case of all the three precursor complexes has been rationalized in terms of texture coefficient and average crystallite size. All three complexes and PdP₂ NPs have been explored for Suzuki-Miyaura coupling of several aryl halides. Complexes 1 and 2 show good catalytic activity but complex 3 does not. The activity appears to result due to in situ generated palladium containing nanoparticles (NPs) in the case of 1 and 2. The formation of inactive large Pd aggregates in the case of 3 appears to be responsible for the difference. The PdP₂ NPs have been found to show good catalytic activity and recyclability up to six reaction cycles. The results of the three phase test suggest the involvement of both homogeneous and heterogeneous pathways in the activation of Suzuki coupling. DFT based free energy calculations are consistent with the results of catalysis via Pd(0) protected with the ligand. This palladium may also be released from in situ generated NPs. In the case of 3, negligible reactivity may be due to non-release of Pd. This journal is

Full text of DOI:10.1039/c4dt03471f

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Palladium(II)-1-phenylthio-2-arylchalcogenoethane
complexes: palladium phosphide nano-peanut and
ribbon formation controlled by chalcogen and
Suzuki coupling activation†
Cite this: Dalton Trans., 2015, 44,
6600
Gyandshwar Kumar Rao, Arun Kumar, Fariha Saleem, Mahabir P. Singh,
Satyendra Kumar, Bharat Kumar, Goutam Mukherjee and Ajai K. Singh*
The ligands PhSCH2CH2EAr (L1–L3; E = S, Se or Te) and their Pd-complexes [PdLCl2] (1–3) have been
synthesized and authenticated with their 1H, 13C{1H}, 77Se{1H} and 125Te{1H} NMR spectra. Single crystal
structures of 1 and 3 reveal the geometry of donor atoms around palladium as nearly square planar. Thermo-
lysis of all three complexes in trioctylphosphine (TOP) at 350, 320 and 280 °C, respectively, results in a
single phase of crystalline PdP2. The morphology of the phase varies with ‘E’ to some extent. The nano-
peanuts (size ∼30 and ∼35 nm) were formed with 1 and 2 as precursor complexes. On using complex 3
as a precursor nanoribbons are formed. The preferential growth in the (202) plane in the case of all the
three precursor complexes has been rationalized in terms of texture coefficient and average crystallite
size. All three complexes and PdP2 NPs have been explored for Suzuki–Miyaura coupling of several aryl
halides. Complexes 1 and 2 show good catalytic activity but complex 3 does not. The activity appears to
result due to in situ generated palladium containing nanoparticles (NPs) in the case of 1 and 2. The for-
mation of inactive large Pd aggregates in the case of 3 appears to be responsible for the difference. The
PdP2 NPs have been found to show good catalytic activity and recyclability up to six reaction cycles. The
results of the three phase test suggest the involvement of both homogeneous and heterogeneous path-
ways in the activation of Suzuki coupling. DFT based free energy calculations are consistent with the
results of catalysis via Pd(0) protected with the ligand. This palladium may also be released from in situ
generated NPs. In the case of 3, negligible reactivity may be due to non-release of Pd.
Received 12th November 2014,
Accepted 26th February 2015
DOI: 10.1039/c4dt03471f
www.rsc.org/dalton
Their high efficiency for Suzuki coupling is evidenced by the
Introduction
examples available for conversion of aryl chlorides8,9 with
The material properties of Pd containing nanostructures and good TON values. The in situ generation of Pd containing
palladium based catalysts are very promising.1–3 Palladium(II) NPs8–13 or quantum dots14 in the course of catalysis of Suzuki
complexes of chalcogen ligands as well as palladium nano- coupling with complexes of Pd(II) with organochalcogen
structures stabilized with chalcogen species have gained con- ligands is a unique aspect. It has been realized that the effect
siderable attention for catalytic activation of various chemical of the ligand on the efficiency of the catalyst most likely
transformations4–6 due to their efficiency, less moisture and occurs via the size and dispersion of in situ generated NPs
air sensitivity. Catalytic applications of such nanoparticles probably the true catalytic species.11 Several palladium(II) com-
(NPs) like complexes include Suzuki C–C coupling.6c,7–14 plexes of bi-, tri- and tetra-dentate organochalcogen ligands
have been found to be suitable for the synthesis of palladium
chalcogenide nanoparticles. The shape, size and composition
of these NPs depend on the ligand and reaction conditions.
The material properties of palladium chalcogenides and picti-
nides have made syntheses of precursor palladium complexes
very important.3,15 The chalcogen analogues of ethylene
diamine appear to be a handy choice for designing catalytically
active Pd complexes. Such ligands have found an important
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi-110016,
India. E-mail: aksingh@chemistry.iitd.ac.in, ajai57@hotmail.com;
Fax: +91 11 26581102; Tel: +91 11 26591379
†Electronic supplementary information (ESI) available: Single crystal data of
1 and 3; secondary interactions of complex 3, FESEM, SEM, EDX and PXRD of
NPs. CCDC 1014070 and 1014071. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4dt03471f
6600 | Dalton Trans., 2015, 44, 6600–6612
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aryl bromides and thiophenol were procured from local
sources. Reagents were used as received without further purifi-
cation. The progress of coupling reactions was monitored by
NMR spectroscopy. Yields refer to isolated yields of com-
pounds which have purity ≥95% [established by 1H NMR]. The
products of Suzuki reactions were authenticated by matching
spectroscopic data of the products obtained with those
reported in the literature.
1H, 13C{1H}, 77Se{1H} and 125Te{1H} NMR spectra were
recorded on a Bruker Spectrospin DPX-300 NMR spectrometer
at 300.00, 75.00, 57.24 and 94.69 MHz respectively. The chemi-
cal shifts are reported in ppm relative to the internal stan-
dards. Carbon-13 DEPT NMR experiments were used routinely
to determine the number of hydrogen atoms linked to carbon
atoms. The C, H and N analyses of samples dried in vacuo were
carried out with a Perkin-Elmer 2400 Series II C, H and N ana-
lyser. Melting points were determined in electrically heated
apparatus by taking the sample in a glass capillary sealed at
one end.
Suitable single crystals of complexes 1 and 3 were obtained
by slow evaporation of their solutions prepared in a chloro-
form–hexane (9 : 1) mixture. X-ray diffraction data for single
crystals of 1 and 3 were collected on a Bruker AXS SMART Apex
CCD diffractometer using Mo Kα radiation (0.710 73 Å) at
298(2) K. Frames were collected at T = 298 K by ω, φ, and
2θ-rotations with a full quadrant data collection strategy (four
domains each with 600 frames) at 10 s per frame with SMART.
The measured intensities were reduced to F2 and corrected for
absorption with SADABS.35 Structure solution, refinement and
data output were carried out with the SHELXTL package by
direct methods.36 Non-hydrogen atoms were refined anisotro-
pically. All hydrogen atoms were included in idealized posi-
tions and a riding model was used for refinement. Images
were created with the program Diamond.
High-resolution TEM characterization was carried out with
a Technai G2 20 electron microscope operated at 200 kV. The
specimens for TEM were prepared by dispersing the powder in
chloroform by ultrasonic treatment, dropping the slurry onto a
porous carbon film supported on a copper grid, and then
drying it in air. The phase morphology of the nano-structured
sample was studied by using a Carl Zeiss EVO5O Scanning
Electron Microscope (SEM) and Field Emission Scanning Elec-
tron Microscope (FESEM). Elemental compositions of nano-
particles on the SEM were analyzed using an EDX system
based on the SDD technology (model: Quan Tax 200 with an
energy resolution of 127 eV at Mn Kα or Quanta 200 FEG with
the Oxford-EDS system IE250X-MAX80). The samples were
scanned in different regions in order to minimize the error in
the analysis performed to evaluate the morphological para-
meters. Samples were mounted on a circular metallic sample
holder with sticky carbon tape. Powder X-ray diffraction
(PXRD) studies were carried out with Ni-filtered CuKα radi-
Scheme 1 Synthesis of ligands (L1–L3) and complexes (1–3).
role in designing catalysts for allylic functionalization of
terminal olefins, oxidative Heck vinylation and C–H
oxidation.16–18 It was therefore thought worthwhile to explore
PhSCH2CH2EAr (Scheme 1; E = S, Se or Te) for designing Pd
containing NPs and complexes as catalysts. The S, Se and Te
donor atoms of the chelating ligands differing in the elec-
tronic effects may influence reaction selectivities via the trans
effect.6 The Pd complexes of these ligands can be used as pre-
cursors for one pot synthesis of palladium chalcogenide NPs.
The use of palladium complexes of organochalcogen ligands
in the synthesis of palladium phosphide NPs is uncommon.
Thermolysis in trioctylphosphine (TOP) of Pd(II) complexes of
chalcogen ligands around 195–300 °C has resulted in pure
Pd-chalcogenide NPs.19–22 The NPs of phosphides23 of tran-
sition metals constitute an important class, due to immense
interest in their properties such as magnetism,24 optics25 and
energy storage.26–29 However such NPs have been studied for
a limited number of catalytic processes.30–32 Hydrotreating
reactions are the ones, which represent a huge proportion of
the work done on metal phosphides as catalysts in the last
decade.23,33 Herein, we report the effect of chalcogen donor
atoms on the efficiency of catalytic Suzuki–Miyaura coupling
(SMC) and the morphology of the PdP2 nanostructure. The
efficiency of SMC appears to be affected via NPs formed in
situ in the course of the catalytic process. Palladium(0)
required for SMC may be released from these NPs. The
growth of phosphides occurring in a preferential orientation
has been accounted by the texture coefficient and average
crystallite size. The DFT based free energy calculations are
performed to understand the difference in catalytic activities
of complexes. The negligible reactivity in the case of 3 may be
due to the instability of Pd(0) species formed in SMC. The
presence of tellurium, being more metallic than Se/S, may be
the possible reason.
Experimental section
Diphenyldiselenide, chloroethyl phenylsulfide, NaBH4 and ation using a scan speed of 1 s and a scan step of 0.05°.
sodium tetrachloropalladate were procured from Sigma- Thermogravimetric analysis (TGA) was carried out using a
Aldrich (USA). Bis(4-methoxyphenyl)ditelluride was syn- Perkin-Elmer system under a flowing nitrogen atmosphere
thesized using a known literature procedure.34 All other bases, with a heating rate of 10 °C min−1
.
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All the calculations were carried out using the GAUSSIAN-09 135.0 (C4). 77Se{1H} NMR (57 MHz, CDCl3, 25 °C, Me2Se):
program.37 The geometry of compounds 1, 2 and 3 was opti- δ (ppm): 321.6.
mized at the B3LYP38 level using a SDD basis set with the
L3: Light yellow solid; Yield: 0.60 g (80%); m.p. 39 °C (d).
effective core potential for palladium and chalcogens (S, Se, Anal. Found: C, 48.42; H, 4.35%. Calc. for C15H16OSTe:
1
Te) and the 6-311G* basis set for the rest of the elements. X-ray C, 48.44; H, 4.34%. NMR: H (300 MHz, CDCl3, 25 °C, TMS):
coordinates of intermediates and oxidative addition products δ (ppm): 2.93–3.00 (m, 2H, H6), 3.23–3.30 (m, 2H, H5), 3.80
were not available. Therefore the structures given in Scheme 3 (s, 3H, OMe), 6.76 (d, J = 8.7 Hz, 2H, H8), 7.16–7.24 (m, 5H, H1,
were modelled by using Gauss view software38 starting from H2, H3), 7.67 (d, J = 8.4 Hz, 2H, H7). 13C{1H} NMR (75 MHz,
the initial X-ray coordinates of the corresponding complexes CDCl3, 25 °C, TMS): δ (ppm): 6.9 (C6), 35.8 (C5), 55.2 (OMe),
(1, 2 and 3). Oxidative addition of PhBr to the intermediate 99.9 (C7), 115.2 (C8), 126.2 (C1), 128.9 (C2), 129.6 (C3), 135.1
structures of compounds 2 and 3 (2; E = S, Se and 3; E = S, Te) (C4), 141.4 (C9), 159.9 (C10). 125Te{1H} NMR (94.69 MHz,
may give two different products. However, energetically more CDCl3, 25 °C, Me2Te): δ (ppm): 511.2.
stable oxidative addition products are considered for thermo-
dynamic parameter calculations.
Synthesis of complexes 1–3
The procedure reported earlier for the synthesis of 139a was
modified and used to prepare all the three complexes. Na2PdCl4
(0.15 g, 0.5 mmol) was dissolved in 40 mL of water. The solu-
tion of ligand L1/L2/L3 (0.12 g/0.15 g/0.19 g, 0.5 mmol) pre-
pared in 60 mL of acetone was added to it with vigorous
stirring. The mixture was further stirred for 12 h. The orange
red solution was extracted with chloroform (3 × 100 mL). The
chloroform layer was washed with 100 mL of water, dried over
anhydrous Na2SO4 and filtered. The solvent was reduced up to
5 mL on a rotary evaporator and 200 mL of hexane was added to
this concentrate. The precipitate thus obtained was filtered
under vacuum to obtain 1/2/3 as orange coloured powder.
1 [PdL1Cl2]: Yield: 0.17 g (79%); m.p. 276 °C (d). Anal.
Found: C, 39.64; H, 3.33%. Calc. for C14H14Cl2PdS2: C, 39.68;
Synthesis of L1
Thiophenol (0.22 g, 2 mmol) was added to sodium hydroxide
(0.16 g, 4 mmol) refluxed in 50 mL of 10% aqueous ethanol
for 1 h. 2-Chloroethylphenylsulfide (0.34 g, 2.0 mmol) dis-
solved in 5 mL of ethanol was added to it and the reaction
mixture was refluxed further for 5 h. The resulting solution
was cooled and poured into cold water (30 mL). L1 was
extracted with chloroform (100 mL). The extract was washed
with water (2 × 50 mL) and dried over anhydrous sodium sul-
phate. The solvent was evaporated off under reduced pressure
on a rotary evaporator to obtain L1 as a colourless solid. Yield:
0.41 g (83%); m.p. 69 °C (d). Anal. Found: C, 68.27; H, 5.71.
Calc. for C14H14S2: C, 68.25; H, 5.73%. NMR: 1H (300 MHz,
CDCl3, 25 °C, TMS): δ (ppm): 3.16 (s, 4H, H5, H6), 7.24–7.37
(m, 10H, H1, H2, H3, H8, H9, H10). 13C{1H} NMR (75 MHz,
CDCl3, 25 °C, TMS): δ (ppm): 32.8 (C5 & C6), 126.1 (C1 & C10),
128.7 (C2 & C9), 129.5 (C3 & C8), 134.8 (C4 & C7).
1
H, 3.33%. NMR: H (300 MHz, CDCl3, 25 °C, TMS): δ (ppm):
3.10–3.22 (m, 2H, H5), 3.59–3.76 (m, 2H, H6), 7.41–7.63 (m,
6H, H1, H2, H9, H10), 7.92–8.16 (m, 2H, H3, H8). 13C{1H} NMR
(75 MHz, CDCl3, 25 °C, TMS): δ (ppm): 41.0 (C6), 43.4 (C5),
129.0 (C2 & C9), 130.1 (C1 & C10), 130.7 (C3 & C8), 131.8 (C4 & C7).
2 [PdL2Cl2]: Yield: 0.20 g (83%); m.p. 239 °C (d). Anal.
Synthesis of L2 and L3
Found: C, 35.71; H, 2.99%. Calc. for C14H14Cl2PdSSe: C, 35.73;
1
Diphenyldiselenide (0.32 g, 1 mmol)/bis(4-methoxyphenyl) H, 3.00%. NMR: H (300 MHz, CDCl3, 25 °C, TMS): δ (ppm):
ditelluride (0.47 g, 1 mmol) dissolved in 30 mL of EtOH was 3.36–3.68 (m, 3H, H6, H5), 4.12–4.13 (m, 1H, H5), 7.42–7.67 (m,
stirred under a nitrogen atmosphere and treated with a satu- 6H, H1, H2, H9, H10), 7.94–7.10 (m, 4H, H3, H8). 13C{1H} NMR
rated solution of sodium borohydride made in 3 mL of (75 MHz, CDCl3, 25 °C, TMS): δ (ppm): 43.8 (C6), 45.6 (C5),
aqueous NaOH (10%) dropwise till it became colourless due to 126.8 (C1), 128.9 (C2), 129.2 (C9), 129.6 (C10), 130.0 (C7), 131.0 (C3),
the formation of PhSeNa/4-MeO-C6H4TeNa. 2-Chloroethyl- 131.3 (C8), 131.6 (C4). 77Se{1H} NMR (57 MHz, CDCl3, 25 °C,
phenylsulfide (0.34 g, 2.0 mmol) dissolved in 10 mL of ethanol Me2Se): δ (ppm): 543.8.
was added to the colourless solution with constant stirring
3 [PdL3Cl2]: Yield: 0.21 g (75%); m.p. 138 °C (d). Anal.
and the mixture was stirred further for 6 h. It was poured Found: C, 32.81; H, 2.94%. Calc. for C15H16Cl2OPdSTe:
1
into cold water (30 mL). L2 or L3 was extracted with chloroform C, 32.80; H, 2.94%. NMR: H (300 MHz, CDCl3, 25 °C, TMS):
(2 × 50 mL). The organic layer was washed with water δ (ppm): 2.83–2.91 (m, 1H, H6), 3.40–3.51 (m, 2H, H6 + H5),
(2 × 50 mL) and dried over anhydrous sodium sulphate. The 3.63–3.65 (m, 1H, H5), 3.74 (s, 3H, OMe), 6.94 (d, J = 7.2 Hz,
solvent was evaporated off under reduced pressure on a rotary 2H, H8), 7.35–7.50 (m, 3H, H1, H2, H3), 7.74–7.93 (m, 4H, H3,
evaporator to obtain L2 or L3 as a solid.
H7). 13C{1H} NMR (75 MHz, CDCl3, 25 °C, TMS): δ (ppm): 30.2
L2: White solid; Yield: 0.51 g (87%); m.p. 45 °C (d). Anal. (C6), 41.5 (C5), 56.8 (OMe), 108.6 (C7), 117.2 (C8), 129.0 (C1),
Found: C, 57.33; H, 4.78. Calc. for C14H14SSe: C, 57.33; H, 130.8 (C2), 131.7 (C3), 137.8 (C4), 139.0 (C9), 161.3 (C10). 125Te{1H}
4.81%. NMR: 1H (300 MHz, CDCl3, 25 °C, TMS): δ (ppm): NMR (94.69 MHz, CDCl3, 25 °C, Me2Te): δ (ppm): 623.7.
3.11–3.14 (m, 2H, H6), 3.15–3.20 (m, 2H, H5), 7.16–7.30 (m,
Synthesis of palladium phosphide nano-phases
10H, H1, H2, H3, H8, H9, H10). 13C{1H} NMR (75 MHz, CDCl3,
25 °C, TMS): δ (ppm): 26.4 (C6), 34.4 (C5), 126.4 (C1), 127.2 A mixture of tri-n-octylphosphine (TOP: 5.0 mL; 11.0 mmol)
(C10), 128.9 (C2), 129.0 (C9), 130.0 (C3), 131.1 (C8), 131.4 (C7), and 1.0 mmol of one of the three complexes 1–3 (1: 0.42 g, 2:
6602 | Dalton Trans., 2015, 44, 6600–6612
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0.47 g, 3: 0.55 g,) was heated to ∼350/320/280 °C respectively Recyclability
under a nitrogen atmosphere for 2 h with continuous stirring
After completion of the cross-coupling of phenylboronic acid
with 4-bromobenzaldehyde/4-bromoanisole in the presence of
nano-peanuts obtained from complex 1, the reaction mixture
was centrifuged. The solvent was decanted and black residue
was washed with 2 × 20 mL diethyl ether and dried. It was
reused for a fresh catalytic cycle of Suzuki cross-coupling.
in a three neck flask. The colour of the mixture changed from
yellowish to brown-black within 10 min. The reaction mixture
was cooled to room temperature and 20 mL of dry acetone was
added into the flask to obtain black particles which were sep-
arated by centrifugation. The particles were further washed
with 2 × 20 mL of dry acetone and 3 × 20 mL dry ethanol. They
were centrifuged and dried in vacuo.
Hg poisoning test39b
Hg (Hg–Pd::500 : 1) was taken in a reaction flask before the
addition of various reactants. Thereafter, 4-bromoanisole
(1.0 mmol), phenylboronic acid (1.2 mmol) and 1 (1 mol
%)/nanoparticles (5 mol% of Pd) isolated during catalysis with
1/PdP2 NPs (4 mol% of Pd) synthesized from complex 1 were
added and reaction under optimum conditions was carried
out. The expected product 4-methoxybiphenyl was not
obtained in significant yield after 24 h.
General procedure for the Suzuki reaction catalyzed by 1–3
An oven-dried flask was charged with an aryl bromide
(1.0 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2.0 mmol)
and DMF–H2O (3.0 ml/1.0 ml). A solution of catalyst 1/2/3
(suitable mol% of Pd) in DMF was then added via a syringe.
The flask was placed on an oil bath maintained at 100 °C
under aerobic conditions and the reaction mixture was stirred
until maximum conversion of the aryl bromide to the product PPh3 poisoning test
occurred. The reaction mixture was cooled and poured to
The coupling reaction of 4-bromoanisole (1.0 mmol) with phe-
50 mL of water. The resulting aqueous mixture was extracted
with diethyl ether (2 × 50 mL). The organic layer was washed
with water (2 × 50 mL) and dried over anhydrous Na2SO4. The
solvent of the extract was removed with a rotary evaporator and
the resulting residue was purified by column chromatography
on silica gel using a chloroform and hexane mixture as the
eluent.
nylboronic acid (1.2 mmol) using 1 (1 mol%)/nanoparticles
(5 mol% of Pd) isolated from 1 in Suzuki–Miyaura coupling/
PdP2 NPs (4 mol% of Pd) synthesized using complex 1 as the
catalyst was carried out in the presence of PPh3 (1 mmol)
under optimum conditions. After 24 h the expected cross
coupled product 4-methoxybiphenyl was not obtained.
Three-phase test
A mixture of 4-bromobenzoic acid immobilized silica (0.20 g)
prepared by a standard procedure,39c phenylboronic acid
(0.36 g, 3 mmol), 4-bromoacetophenone (0.20 g, 1 mmol), and
K2CO3 (0.56 g, 4 mmol) was heated at 100 °C for 12 h in DMF
+ water (6 + 3 mL) in the presence of 1 mol% of 1. The result-
ing mixture was cooled and filtered through a G-4 crucible.
The residue was washed with 20 mL of H2O and then with
diethyl ether (50 mL). The filtrate and washings were collected
together and mixed with 50 mL of water. The resulting mixture
was extracted with diethyl ether (50 mL). The solvent of the
extract was evaporated off on a rotary evaporator, and the
residue was subjected to 1H NMR. The solid residue filtered
earlier was hydrolyzed with KOH (1.68 g dissolved in 10 mL of
EtOH + 5 mL of H2O) at 90 °C for 3 days. The resulting solu-
tion was neutralized with 20% (v/v) aqueous HCl, and extracted
with dichloromethane and then with ethyl acetate. The solvent
of the combined extract was evaporated off, and the resulting
residue was analyzed by 1H NMR.
Isolation of nanoparticles formed from complexes 1–3 in
Suzuki coupling
A mixture of complex 1/2/3 (0.50 mmol), phenylboronic acid
(1.2 mmol), 4-bromobenzaldehyde (1 mmol) and K2CO3
(2 mmol) in DMF (4 mL) + water (4 mL) was heated at 100 °C
on an oil bath for 2 h and then cooled to room temperature.
The resulting blackish suspension was mixed with 10 mL of
acetone. The mixture was centrifuged. The black residue was
washed with a mixture of acetone and water (3 : 1, v/v) three
times, separated, dried and subjected to PXRD.
General procedure for the Suzuki reaction catalyzed by PdP2
nanophases
An oven-dried flask was charged with an aryl bromide
(1.0 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2.0 mmol)
and DMF–H2O (3.0 ml/1.0 ml). The PdP2 nanophase (4 mol%)
as a catalyst was then added while stirring. The flask was
placed on an oil bath maintained at 100 °C under aerobic con-
ditions and the reaction mixture was stirred until maximum
conversion of the aryl bromide to the product occurred. The
Results and discussion
mixture was cooled and poured into 50 mL of water. The The syntheses of ligands L1–L3 and their Pd(II) complexes 1–3
resulting aqueous mixture was extracted with diethyl ether (2 × are summarized in Scheme 1. L1 and L2 are white solids. L3
20 mL). The organic layer was washed with water (20 mL) and also a solid is light yellow in colour. All ligands are soluble in
dried over anhydrous Na2SO4. The solvent of the extract was chloroform, dichloromethane and acetonitrile. The synthesis
removed with a rotary evaporator and the residue was sub- of their complexes was carried out at low concentrations, to
jected to an NMR study.
avoid otherwise possible polymerization. 1–3 have been found
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to be soluble in DMF and DMSO, sparingly soluble in chloro-
form and acetonitrile and insoluble in ethanol, methanol,
hexane and diethyl ether. The 1 : 1 (M–L) stoichiometry of the
three complexes was supported by elemental analyses.
NMR spectra
The 1H, 13C{1H}, 77Se{1H} and 125Te{1H} NMR spectra of L1–L3
and their complexes (1–3) were recorded in CDCl3 and DMSO-
d6 respectively (Fig. S1–S19†). The lone signal in the 77Se{1H}
NMR spectrum of L2 appearing at 321.6 ppm is shielded by a
signal at ∼139 ppm with respect to that of diphenyldiselenide.
However, a single signal in the 125Te{1H} NMR spectrum of L3
observed at 511.2 ppm is deshielded by a signal at ∼54 ppm
with respect to that of (4-CH3OC6H4Te)2 used as a precursor in
the synthesis of L3. In proton NMR spectra of complexes 1–3,
CH2E (H6) signals (∼3.36–3.76 ppm) are deshielded by
∼0.3–0.6 ppm with respect to those of the corresponding free
ligand (3.1–3.2 ppm). In 13C{1H} NMR spectra, signals of C5
(CH2S) show deshielding of 10.6, 11.2 and 5.7 ppm, respect-
ively, for 1, 2 and 3, with respect to those of the corresponding
free ligand. The signal of the C6 carbon atom (ECH2 group) in
13C{1H} NMR spectra of L1, L2 and L3 appears deshielded on
complexation by 8.2–23.3 ppm. In contrast, in 13C{1H} NMR
spectra of all the three complexes, the signals of C4 and C7
carbon atoms show shielding (3.0–8.7 ppm).
Fig. 2 Molecular structure of 3. Selected bond lengths (Å): Pd(1)–S(1)
2.260(19); Pd(1)–Cl(1) 2.330(19); Pd(1)–Cl(2) 2.375(19); Pd(1)–Te(1) 2.504
(8). Selected bond angles (°): S(1)–Pd(1)–Cl(1) 175.48(8); S(1)–Pd(1)–Cl(2)
90.05(7); Cl(1)–Pd(1)–Cl(2) 93.70(8); S(1)–Pd(1)–Te(1) 89.97(5); Cl(1)–
Pd(1)–Te(1) 86.19(6); Cl(2)–Pd(1)–Te(1) 177.90(5).
both complexes. A five membered chelate ring with S and E is
formed. The two Pd–S as well as Pd–Cl bond lengths of 1 are
consistent with each other. The Pd–S bond lengths in 1 and 3
[2.2851(14) and 2.260(19) Å respectively] are shorter than the
value 2.329(1) Å reported for trans-[Pd(PhSCH2SPh)2C12]40 and
longer than 2.2564(15) Å reported for the Pd(II) complex of 4-
Me-C5H3N-2-CH2SMe.41 The Pd–Cl bond lengths (2.3296(13) Å)
are identical for 1. Their values are 2.3300(19) and 2.3746(19)
Å for 3. All the present three values for Pd–Cl are longer than
2.292(1) Å reported for [Pd(PhSCH2SPh)2C12].40 However, they
are consistent with 2.3268 Å reported for the Pd(II) complex of
4-Me-C5H3N-2-CH2SMe.41 The Pd–Te bond length 2.5042(8) Å
of 3 is shorter than the values 2.526(7) and 2.518(9) Å reported
for the meso complex of Pd(II) with (4-MeO-C6H4Te)2CH2.42 The
Cl⋯H (aromatic) and Cl⋯H (CH2) non-covalent interactions in
the crystal of 1 result in the formation of three dimensional
sheet structures shown in Fig. 3. In the crystal of 3 the Cl⋯H
(aromatic), Cl⋯H (–OCH3) and Cl⋯Te interactions result in a
three dimensional network as shown in Fig. 4 and S20 (ESI†).
In the 77Se{1H} NMR spectrum of 2, the lone signal appears
at 543.8 ppm and is deshielded by 222.2 ppm with respect to
that of the free ligand. Furthermore, in the 125Te{1H} NMR
spectrum of 3, the single signal appearing at 623.7 ppm is
deshielded by 112.5 ppm with respect to that of the free
ligand. The deshielding in various signals is consistent with
the co-ordination of Pd(II) to S and E.
Crystal structures
The single crystal structures of 1 and 3 have been determined
by X-ray diffraction. Their molecular structures are shown in
Fig. 1 and 2, respectively, with 40 and 30% probability ellip-
soids. Their crystal data, structure refinement parameters,
bond lengths and bond angles are given in Tables S1–S3 in
ESI.† There is a distorted square planar geometry around Pd in
Catalytic activation of Suzuki–Miyaura C–C coupling
Complexes 1–3 have been explored for Suzuki–Miyaura coup-
ling. For preliminary investigations on catalysis with 1 of
Suzuki–Miyaura coupling, 4-bromobenzaldehyde was selected
as a substrate. It was reacted with PhB(OH)2 in an N,N-di-
methylformamide (DMF)–water mixture (3 : 1; v/v) in the pres-
ence of 1 as a catalyst (0.002 mol%). After 12 h of reaction at
100 °C, the yield of the product biphenyl-4-carboxaldehyde was
46% (Table 1; entry 5), when K2CO3 was used as a base. On
using bases NaOAc and Na2CO3 in place of K2CO3 the yield of
the coupled product was reduced (26 and 35% respectively;
Table 1). In place of the DMF–water mixture the use of DMSO
and DMF considerably reduced the yield of the product
(Table 1; entries 3 and 4). The cross coupled product was not
obtained when toluene or THF was used as a solvent (Table 1;
Fig. 1 Molecular structure of 1. Selected bond lengths (Å): Pd(1)–Cl(1)
2.330(13); Pd(1)–S(1) 2.285(14); S(1)–C(7) 1.833(5). Selected bond angles
(°): S(1)–Pd(1)–S(1) 89.11(8); S(1)–Pd(1)–Cl(1) 177.22(4); S(1)–Pd(1)–Cl(1)
88.52(6); Cl(1)–Pd(1)–Cl(1) 93.89(7).
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Fig. 3 Molecular packing of 1 showing Cl⋯H (aromatic) and Cl⋯H (CH2) interactions.
Fig. 4 Molecular packing of 3 showing Cl⋯H (aromatic) and Cl⋯H (–OCH3) interactions.
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Table 1 Optimization of reaction conditionsa
obtained in 95 and 77% yield respectively when 1 and 2 were
used as catalysts. However, complex 3 does not show signifi-
cant catalytic activity towards the Suzuki coupling reactions
even at its 1 mol% loading (Table 2).
Entry no.
Solvent
Base
Yieldb
1
2
3
4
5
6
7
8
Toluene
THF
DMSO
DMF
DMF + water
DMF + water
DMF + water
DMF + water
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
CH3COONa
NEt3
—
—
29
21
46
35
26
—
Preparation and characterization of PdP2 nanostructures
The complexes 1, 2 and 3 were subjected to TGA (Fig. S21–S23
in ESI†) and weight loss was found to be maximum at 350, 320
and 280 °C respectively. Thermolysis of 1, 2 and 3 in trioctyl-
phosphine was carried out at these three temperatures respect-
ively. The single phase of crystalline palladium phosphide
(PdP2) was formed in thermolysis of each of the three com-
plexes (1–3). No formation of any palladium-chalcogenide
phase was noticed. It is surprising in the background of recent
reports in which formation of palladium chalcogenide phases
has occurred by thermolysis of Pd(II) complexes of organosul-
fur/selenium ligands between 195 and 300 °C in TOP.19–22 It is
possible that the chalcogen ligand is exchanged with TOP and
it is the thermolysis of the TOP complex which results in PdP2.
31P NMR has been used to understand the prior ligand
exchange at room temperature with all the three complexes.
Each of the three complexes (10 mg) was treated separately
with TOP (50 mg) in DMSO-d6 and 31P NMR spectra were
recorded after ten minutes of addition. No significant differ-
ence was observed in any of the three spectra. The PdP2 NPs
have been synthesized at temperatures higher than the
decomposition temperatures of complexes. Probably formation
of TOP complexes at room temperature does not occur. The
formation of a pure PdP2 phase appears due to high reaction
temperatures used in the present case. However, the role of a
stronger donor ability of phosphorus relative to any chalcogen
cannot be denied in PdP2 formation. The formation of the pal-
ladium chalcogenide phases in TOP has been reported at
temperatures relatively lower20,43 than the present ones. For
a Reaction conditions: 1.0 mmol of 4-bromobenzaldehyde, 1.2 mmol of
phenylboronic acid and 2 mmol of a base, 0.002 mol% of 1, the
temperature of the bath 100 °C. Reaction time 12 h. b NMR % yield.
Table 2 Suzuki coupling reactions catalyzed by 1–3a
Yieldb (%)
Entry no.
Arylbromide
Mol%
1
2
3
1
2
3
1-Bromo-4-nitrobenzene
4-Bromobenzonitrile
4-Bromobenzaldehyde
0.1
0.1
93
90
78
89
39
93
95e
18
93e
91
95
81
91
33
94
77d
33
90
<5
—
<5
—
—
—
—
—
—
0.02c,d
0.02
0.002d
0.1
4
5
Bromobenzene
4-Bromotoluene
1
0.1d
1
6
4-Bromoanisole
a Reaction conditions: 1.0 equiv. of aryl bromide, 1.2 equiv. of
phenylboronic acid, and 2 equiv. of a base (K2CO3), aqueous DMF as
the solvent, the temperature of the bath 100 °C, reaction time 1 h.
b Isolated yield. c Reaction temperature 50 °C. d Reaction time 12 h.
e Reaction time 4 h.
entries 1 and 2). Thus catalytic activation with 1–3 in Suzuki– example the formation of palladium telluride has been
Miyaura coupling between phenylboronic acid and a variety of reported by thermolysis of Pd-complexes of organotellurium
activated and deactivated aryl bromides was explored in a ligands around 220 °C. In the present case, thermolysis of 3
DMF–water mixture (3 : 1; v/v) in the presence of K2CO3 resulting in the formation of PdP2 rather than palladium tell-
(Table 2).
uride may be attributed to the higher temperature of the order
The catalytic efficiencies of both 1 and 2 appear promising 280 °C used, and the greater donor ability of P relative to S or
as their concentrations 0.02–1.0 mol% have been found to be Te. The TEM studies have revealed the formation of peanut
sufficient for good conversion. 4-Bromobenzaldehyde is con- shaped nanostructures of average longest dimensions 30 and
verted to biphenyl-4-carboxaldehyde in 89 and 91% yield in 35 nm when complexes 1 and 2 were used respectively
1 h using 0.02 mol% of 1 and 2 respectively as the catalyst (Fig. 5a–c). The FESEM, SEM images and SEM-EDX of these
(Table 2; entry 3). The conversion in Suzuki–Miyaura coupling NPs are given in ESI (Fig. S33–S41†). However, nanoribbons
catalyzed with 1 and 2 is dependent on the reaction tempera- were formed by thermolysis of complex 3 (Fig. 5e), probably
ture and time (Table 2, entry 3). In coupling of 4-bromobenz- due to directional growth of the initially formed NPs. TEM was
aldehyde with phenylboronic acid at 50 °C for 12 h using used to follow growth of nanoribbons. The TEM image
0.02 mol% of 1 and 2 as the catalyst the yield of the cross- (Fig. 5d) recorded after 20 min suggests that the nucleation of
coupled product was obtained as 78 and 81% respectively peanut shape NPs starts at a very early stage leading to the for-
(Table 2, entry 3). At 100 °C, the yield for the same coupling in mation of nanoribbons later.
1 h was found to be 89 and 91% respectively. The deactivated
Powder XRD patterns of PdP2 nanostructures obtained from
species 4-bromoanisole (Table 2, entry 6) has also been complexes 1–3 are given in ESI (Fig. S24–S26 in ESI†). The
successfully coupled (yield; 93 and 90% with 1 and 2 respect- d-spacings corresponding to PXRD lines are 3.985, 2.931,
ively). However, in the coupling of 4-bromotoluene (Table 2, 2.887, 2.722, 2.561, 2.069, 2.016, 1.877, 1.837, 1.799, 1.654 and
entry 5), the cross coupled product 4-methylbiphenyl was 1.586 Å which correspond to (110), (211), (202), (200), (021),
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Fig. 6 Texture coefficient and average crystallite size of PdP2 nano-
structures obtained from complexes 1–3.
into account. The nearly unity value of C(hkl) of a facet rules
out preferential orientation of the (hkl) plane. PXRD data of
PdP2 NPs obtained from complexes 1–3 and Io(hkl) of standard
PXRD given in JCPDS file 13-0214 of PdP2 have been used to
calculate the texture coefficient. Its high value in a particular
plane is considered as an indicator for preferential growth in
that plane. The average crystallite size has to be high for the
plane for preferential growth. The calculated values of C(hkl)
and average crystallite sizes of various facets of the present
PdP2 nanoparticles are shown in Fig. 6. The average crystallite
size of nanostructures has been found to be in the ranges of
16–38, 10–36 and 29–221 nm for PdP2 NPs obtained from com-
plexes 1, 2 and 3 respectively.
The crystallite size in the plane (202) has been found to be
higher than those in other planes for all the three complexes.
However, its magnitude was found to be abnormally high in
the case of PdP2 nanostructures obtained from 3. The values
of C(hkl) for the (202) plane of these three nanostructures
obtained from 1, 2 and 3 were found to be 1.7, 1.73 and 4.3
respectively. This corroborates with the preferential orientation
of the plane (202) for growth of the three nanostructures46
during thermolysis of 1–3. Thus preferential growth along the
(202) plane indicated by crystallite size and C(hkl) values
probably results in nanopeanuts or nanoribbons, i.e. nano-
structures of anisotropic nature.
Fig. 5 TEM images of PdP2 nanophases obtained from (a) complex 1
(scale bar 50 nm), (b) complex 2 (scale bar 20 nm), (c) HRTEM of the
PdP2 nanophase obtained from complex 2, (d) complex 3 (20 minutes)
(scale bar 20 nm), (e) complex 3 (scale bar 100 nm), (f) NPs obtained
after the sixth reaction cycle (scale bar 20 nm).
(312), (311), (113), (130), (112), (132) and (221) planes, match-
ing with that of JCPDS file 13-0214. In PXRD patterns
(Fig. S24–S26†) of the three NPs lines of (202) planes were
found to be unusually intense, compared to those reported for
the PdP2 phase,20 but the intensity (Fig. S26†) was maximum
for NPs obtained from complex 3 relative to 1/2. Most likely for
growth the (202) plane is preferred which probably results in
nanopeanuts or nanoribbons. This preferential orientation of
a crystal plane has been rationalized in terms of texture coeffi-
cient44 which has been calculated by Hall’s method, using eqn
(1) 45 and the XRD powder pattern of PdP2 nanoparticles
corresponding to various planes. The average crystallite size is
calculated using the Scherrer equation.
Catalysis of Suzuki coupling with PdP2 nanostructures
The PdP2 nanophases obtained from complexes 1–3 were used
to catalyze Suzuki coupling reactions and the results are given
in Table 3. In the reaction between 4-bromobenzaldehyde and
phenylboronic acid under optimum conditions at 100 °C for
1 h in the presence of PdP2 NPs (4 mol% of Pd), the cross
coupled product was obtained in almost 100% yield (Table 3,
entry 1). The catalytic efficiency of PdP2 nanophases obtained
from complexes 1 and 2 for coupling of 4-bromoanisole was
found to be higher than that of NPs obtained from 3 under
similar reaction conditions including catalyst loading (Table 3,
entry 6).
ꢀ
X
IðhklÞi
1
n
IðhklÞn
CðhklÞ ¼
n
ð1Þ
IoðhklÞi
IoðhklÞn
where C(hkl) is the texture coefficient of the facet {hkl} and
The results of Suzuki–Miyaura coupling given in Table 4
other terms are I(hkl) = the intensity of (hkl) reflection of the indicate the reusability of PdP2 nanophases in coupling of aryl
sample under analysis; Io(hkl) = the intensity of (hkl) reflection bromides. The recyclability (for 6 cycles) of the nanostructures
of a bulk material sample; n: the number of reflections taken obtained from complex
2 was studied for coupling of
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images indicate that the black phase obtained in the case of 1
and 2 consists of highly uniform and monodispersed NPs of
spherical shape (average size: ∼7 nm for 1 and 5 nm for 2)
(Fig. 7). However in the case of 3, the formation of large aggre-
gates was observed. The high catalytic activity of 1/2 may be
rationalized at least partly in terms of the size of particles
generated in situ from their precursor complexes. The inactivity
of 3 probably stems from the failure in the formation of small
size particles. Using SEM-EDX the Pd–S ratio in NPs generated
from 1 was found to be 59 : 41. In the case of NPs from 2, the
Pd–Se–S ratio was found to be 39 : 45 : 16. On the other hand
the Pd–Te–S ratio in NPs obtained from complex 3 was
observed as 63 : 29 : 8 (Fig. S30–S32 in ESI†). The higher Pd in
the case of NPs obtained from complex 3 may be due to the
easy loss of the free ligand or its fragments (Te weak donor)
and the formation of bigger aggregates during the isolation
process of NPs. The nanophases obtained from complexes 1–3
were subjected to PXRD before and after annealing them at
300 °C under the flow of Ar. In both the cases no peak
appeared in PXRD. Thus formation of any palladium chalco-
genide nanophase cannot be established. Probably it happens
due to stabilization of the NPs (most probably of Pd) with the
ligand or its fragments.
Table 3 Suzuki coupling reactions catalyzed by PdP2 nanophases
obtained from 1–3a
Yieldb (%)
Entry no.
Aryl bromide
1
2
3
1
2
3
4
5
6c
4-Bromobenzaldehyde
4-Bromobenzoic acid
Bromobenzene
4-Bromotoluene
4-Bromoanisole
>99
97
89
76
73
95
>99
96
92
77
81
94
>99
89
74
29
17
25
4-Bromoanisole
a Reaction conditions: 1.0 equiv. of aryl bromide, 1.2 equiv. of
phenylboronic acid and 2 equiv. of a base (K2CO3), aqueous DMF–
water as a solvent, the temperature of the bath 100 °C, catalyst: 4 mol%
Pd, reaction time 1 h. b NMR % yield. c Reaction time 6 h.
4-bromoanisole and 4-bromobenzaldehyde. For coupling of
4-bromobenzaldehyde (Table 4; entry 1) no significant loss in
the activity of the catalyst was observed up to six cycles. The
conversion gradually decreased after the third cycle in the case
of 4-bromoanisole (Table 4; entry 2). However, lowering of con-
version may be reversed for the 4th, 5th and 6th reaction
cycles by increasing the reaction time. The TEM images of
nanostructures (obtained from complex 2) after the sixth reac-
tion cycle (Fig. 5f) suggest that they are uniformly dispersed
which is probably responsible for their sustained activity.
The association of the catalytic activity with these black NPs
has been verified. The catalysis of Suzuki–Miyaura coupling
reactions (Table 5) of 4-bromobenzaldehyde and 4-bromo-
anisole with PhB(OH)2 with these black NPs obtained from 1–3
Insight of catalysis of Suzuki coupling with complexes 1–3
The difference of one donor group (i.e. S/Se/Te), in complexes
1–3, has a very significant influence on the catalytic efficiency.
It is very high for 1/2 relative to that of 3. To understand the
reason for such an unexpectedly large variation due to one
donor site, the black particles which appeared at the begin-
ning of catalytic reactions with 1–3 were analyzed. This was
envisaged on the basis of reports that Pd-complexes are dis-
pensers of true catalysts.8–11,47 For example, decomposition of
sulfur containing palladacycles to Pd based NPs and an equili-
brium between these NPs and the actual active species Pd(0)47
coexist in the catalytic process47 based on the [Pd(0)–Pd(II)] key
step. Thus black particles formed in the reaction of 4-bromo-
benzaldehde with PhB(OH)2 catalyzed with 1–3 under
optimum conditions have been studied. Their SEM (Fig. S27–
Fig. 7 TEM image of nanoparticles obtained from complexes 1–3
S29 in ESI†), EDX (Fig. S30–S32 in ESI†) and HR-TEM (Fig. 7) respectively.
Table 4 Suzuki coupling reactions catalyzed by the PdP2 nanophase obtained from complex 1a
Reaction cycle
Entry no.
Aryl bromide
1
2
3
4
5
6
1b
2b
4-Bromo-benzaldehyde
4-Bromoanisole
>99
>99
>99
>99
>99
97 (>99)
>99
92 (>99)
>99
85 (97)
>99
79 (94)
a Reaction conditions: 1.0 equiv. of aryl bromide, 1.2 equiv. of phenylboronic acid, and 2 equiv. of K2CO3, aqueous DMF + water as the solvent,
the temperature of the bath 100 °C, reaction time 1 h, catalyst: 4 mol% of the nanophase obtained from 2, in parentheses the yield after 6 h.
b NMR % yield.
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Table 5 Suzuki coupling reactions catalyzed by NPs obtained from
1–3a
Yieldb (%)
Entry no.
Aryl bromide
1
2
3
1
2
4-Bromobenzaldehyde
4-Bromoanisole
94
78
89
59
—
—
a Reaction conditions: 1.0 equiv. of aryl bromide, 1.2 equiv. of
phenylboronic acid, and 2 equiv. of a base (K2CO3), aqueous DMF as
the solvent and the temperature of the bath 100 °C. 10 mg of NPs were
used. Reaction time 12 h. b NMR % yield.
Scheme 2 Three-phase test on Suzuki–Miyaura coupling with catalyst
1, Pd NPs obtained from 1 and PdP2 NPs.
catalytic reaction of an immobilized aryl halide (supported on
the silica surface) in the presence of a molecular aryl halide
with phenylboronic acid. A similar test has been developed by
Davies and co-workers,52 utilizing the resin support. 4-Bromo-
benzoic acid, immobilized on silica (as an amide), and 4-bromo-
acetophenone were reacted with phenylboronic acid in the
presence of 1 (2 mol%), Pd NPs obtained from 1 (10 mg) or
PdP2 nanopeanuts obtained from 1 (4 mol%) under optimum
reaction conditions. After 12 h, free aryl bromide was con-
verted to ∼100% in the presence of 1, Pd NPs or PdP2 nano-
peanuts obtained from it. The immobilized aryl bromide was
hydrolyzed and the products were subjected to an NMR study.
It was observed that, of the immobilized 4-bromobenzoic acid,
22, 76 and 62% were converted to the cross-coupled product
respectively in the presence of 1, Pd NPs obtained from 1 and
PdP2 nanopeanuts obtained from 1. In identical but separate
catalytic experiments with complex 1, Pd NPs obtained from
complex 1 and PdP2 NPs synthesized from 1, free aryl halide
(4-bromoacetophenone) was not added. The immobilized aryl
bromide on hydrolysis with KOH for three days gave products
which were subjected to an NMR study. The yield of the
coupled product in the absence of free aryl bromide was found
to be 0, 45 and 53%, respectively, which is reduced by 22, 31
and 9% respectively in the case of 1, Pd NPs obtained from 1
and PdP2 NPs given by 1. The significant decrease in the yield
of the cross coupled product in the case of 1 or Pd NPs
obtained from complex 1 suggests that probably bleeding of
Pd is easy in these cases. The free ArBr captures the bled
palladium to form ArPdBr which enters into a conventional
solution-phase reaction. However, with PdP2 NPs the possi-
bility of leaching of Pd is low which in turn does not affect the
yield of the product obtained from silica supported ArBr.
These results support the involvement of both the homo-
geneous and heterogeneous catalytic process in all the three
present cases.
under optimum conditions was explored. The reactions were
smooth in the presence of 10 mg of NPs obtained from 1 and
2. After carrying out reaction for 12 h at 100 °C, the cross
coupled products biphenyl-4-carboxaldehyde and 4-methoxybi-
phenyl respectively resulted in good yield (Table 5; entries 1
and 2). On the other hand 10 mg of the black coloured phase
obtained from the complex 3 failed to catalyze coupling of
these substrates. The complex 3 shows a sign of decompo-
sition at room temperature and on addition of a base, whereas
the complexes 1 and 2 do not.
Addition of tetrabutylammonium bromide (TBAB) (1 mmol)
to the coupling reaction of 4-bromobenzaldehyde catalyzed
with 3 (1 mol% Pd) does not show any significant formation of
the coupled product. However the yield of the cross coupled
product significantly increased from 39 to 93% for the same
substrate after addition of TBAB (1 mmol) to its coupling reac-
tion catalyzed with complex 1 (0.002 mol%). This supports the
important role of NPs in the catalytic process. The very fast
decomposition of complex 3 leads to the formation of palla-
dium black inactive for Suzuki coupling. Thus the role of the
ligand in controlling the nature and release of NPs appears to
be strengthened. The pathway for the involvement of in situ
generated Pd NPs in catalysis may be homogeneous48,49 or hetero-
geneous.50 Homogeneous mode involves Pd atom escape
from the palladium nanoparticles by an oxidative addition
generating discrete Pd(II) species in solution.49 Alternatively
Pd(II) complexes can be formed by oxidative addition to Pd(0)
atoms that have already leached into the solution.48
Nature of catalysis with Pd(II) complexes and PdP2
nanoparticles
To understand the nature of real catalytic species the Hg poi-
soning test and the PPh3 poisoning test have been carried out
on the representative reaction of 4-bromoanisole with phenyl-
boronic acid using 1 (1 mol%) or nanoparticles (5 mol%)
including PdP2 (4 mol%) formed by it. After 12 h of reaction the
DFT calculations
cross coupled product was not obtained in significant yield. The results of the three phase test suggest that there is a cock-
These tests suggest the involvement of a heterogeneous catalytic tail of homogeneous and heterogeneous pathways in the case
process. As these two tests are not sufficient to conclude about of catalysis with complexes 1/2, and the contribution of hetero-
the nature of the real catalytic process (i.e. homogeneous vs. het- geneous mode seems to be quite higher than that of the homo-
erogeneous), a three phase test was carried out. This test geneous one. However any of these two modes does not seem
(Scheme 2), developed by Rebek and co-workers,51 involves the to be operational in the case of complex 3 as it is catalytically
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inactive. The contribution of heterogeneous mode may be con- favourable trans oxidative addition products. The change in
sidered negligible due to in situ formation of large aggregates. free energy for this path was calculated and found to be −27.0,
However, the absence of the possibility of homogeneous mode −26.0 and −24.0 kcal mol−1 for complexes 1–3 respectively.
of operation cannot be explained on the basis of the size of
These values are consistent with the fact that in Suzuki
in situ formed structures. Thus, to understand the reason for coupling catalyzed with the present palladium(II) complexes,
this reactivity pattern, HOMO–LUMO energy gaps of all three the formation of active Pd(0) occurs as shown in Scheme 3.
complexes (1–3) were computed. The large energy gap between This palladium may also be released from NPs containing Pd
HOMO and LUMO is expected to give high stability and conse- or discrete Pd(0) protected with the ligand. The overall reactiv-
quently the lower reactivity of the catalyst. The HOMO–LUMO ity of a catalyst results from balancing between competitive
energy gaps for complexes 1–3 have been found to be almost inactivation and oxidative addition. In the case of complex 3,
similar. Therefore, any conclusion on the basis of the study of the possibility of oxidative addition is the lowest relative to
the HOMO–LUMO energy gap seems to be difficult.
1 and 2. Also the formation of small NPs has not been
observed with complex 3. Probably the Pd(0) complex is not
stabilized enough in the case of the Te-ligand due to its higher
metallic nature than Se. This in turn leads to aggregates of pal-
ladium and possibly is the main reason for the inactivity of
complex 3. These DFT calculations are in the gas phase.
However, they appear to be consistent with the possibility of
inactivity in the case of 3. In no way they provide any infor-
mation regarding the mechanism of generation and involve-
ment of palladium NPs in catalytic reaction in solution and
throw light on the homogeneous vs. heterogeneous catalysis
with the complexes as revealed by the three phase test.
Free energy based model
The experimental results suggest that the catalytic activity of
complex 3 is less than that of other two complexes. The free
energy change for a simplified pathway of oxidative addition of
the aryl halide to the catalyst (Scheme 3) for a gas phase reac-
tion has been computed. It has been presumed that in Suzuki
coupling catalyzed with the palladium(II) complexes, the for-
mation of active Pd(0) (NPs and/or discrete Pd(0) protected
with the ligand) occurs in a sequence (Scheme 3). The electron
rich bivalent palladium species {[Ar-S-CH2-CH2-E-Ar]Pd(0),
where E = S/Se/Te} can be stabilized by relieving the electron
density to a suitable ligand orbital by back-donation. The
species {[Ar-S-CH2-CH2-E-Ar]Pd(0)} can be dissociated into
palladium and free ligand. Furthermore, the nucleation of dis-
crete palladium may result in the formation of nanoparticles.
The possibility of dissociation of such an electron rich low oxi-
dation state of palladium is higher in the case of complex 3
due to the lowest π-accepting character of Te. The active Pd(0)
goes to the first step of the catalytic cycle of this coupling
involving oxidative addition (the rate limiting step in Suzuki
coupling) of the aryl halide to the palladium centre. The other
steps take place readily with or without the help from ancillary
ligands. There is always a competition between the oxidative
addition and the processes of Pd deactivation (due to for-
mation of Pd NPs which may further grow into large inactive
metal particles represented as palladium black). In the oxi-
dative addition, there are two possibilities for the addition of
bromobenzene to the Pd in the present case, i.e. bromine may
be cis or trans to the E (E = Se/Te) group. However, the thermo-
dynamic parameter has been reported here for energetically
Conclusions
Three Pd(II) complexes of the type [PdCl2L] [L = L1 to L3;
bidentate ligands of the (S, E) (E = S, Se, Te) type] have been
synthesized and characterized by multinuclear NMR spec-
troscopy. The single crystal structures of complexes 1 and 3
were determined. 1–3 have been explored for Suzuki–Miyaura
coupling reactions. The Pd(II) complexes with ligands of (S, S)
and (S, Se) types (1 and 2) show similar catalytic activity for the
Suzuki–Miyaura coupling reaction. Complex 3, i.e. the Te ana-
logue of 1 and 2, does not show significant catalytic activity for
this coupling. The reason for this observation has been ration-
alized. Furthermore, these complexes have been utilized for
the selective synthesis of crystalline PdP2 nano-peanuts and
ribbons. The formation of nanostructures was found to be
dependent on the nature of co-ordinating atoms of ligands as
complexes 1 and 2 form peanut shaped nanostructures
whereas complex 3 gives rise to nanoribbons. The difference in
morphologies has been explained on the basis of a pheno-
menon related to directional growth of structures during their
synthesis. Texture coefficients have also been calculated to
understand directional growth which is in the (202) plane. The
PdP2 nanostructures have also shown excellent catalytic activity
and recyclability towards Suzuki coupling reactions. DFT
based free energy calculations are consistent with catalysis
with Pd(0) protected with ligands or their fragments.
Acknowledgements
The Council of Scientific and Industrial Research, and the
Department of Science and Technology India supported the
Scheme 3 Possible path of Suzuki–Miyaura coupling with complexes
1–3.
6610 | Dalton Trans., 2015, 44, 6600–6612
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work through the projects [CSIR: 01(2421)10/EMR-II; DST: SR/ 13 A. Kumar, G. K. Rao, S. Kumar and A. K. Singh, Organo-
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6612 | Dalton Trans., 2015, 44, 6600–6612
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