Palladacycles of thioethers catalyzing Suzuki-Miyaura C-C coupling: Generation and catalytic activity of nanoparticles

Article abstract of DOI:10.1021/om4001956

Tridentate thioether ligands, 2-HO-4-R-C₆H₃-(C ₆H₄)CHNH(CH₂)₃SPh [R = H (L1) or -OMe (L2)] react with Na₂PdCl₄, giving palladacycles [PdCl(C^-,N,S)] (1: (C^-,N,S) = L1-H; 2: (C^-,N,S) = L2-H). The ¹H and ¹³C{¹H} NMR spectra of ligands and their palladacycles have been found to be characteristic. Complexes 1 and 2 have also been characterized with HR-MS. The crystal structure of 2 has been solved. The Pd-S bond length is 2.428(2) A, and palladium has a nearly square planar geometry. During the course of catalysis of Suzuki-Miyaura C-C coupling using 1 and 2 as catalysts, unexpected formation of Pd ₁₆S₇ nanoparticles (NPs) has been observed with both complexes. This is the first time that such an observation has been made with palladacycles of thioethers used in this coupling reaction. The efficiency of 2 in carrying out the coupling is significantly lower than that of 1. Complex 2 has an additional -OMe group in the ligand structure, and the size of Pd ₁₆S₇ NPs formed from this complex are larger (6 nm) than those obtained from 1 (2 nm).

Full text of DOI:10.1021/om4001956

Article
pubs.acs.org/Organometallics
Palladacycles of Thioethers Catalyzing Suzuki−Miyaura C−C
Coupling: Generation and Catalytic Activity of Nanoparticles
Gyandshwar Kumar Rao, Arun Kumar, Satyendra Kumar, Umesh B. Dupare, and Ajai K. Singh*
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi-110016, India.
S
* Supporting Information
ABSTRACT: Tridentate thioether ligands, 2−HO−4−R−C6H3−
(C6H4)CHNH(CH2)3SPh [R = H (L1) or −OMe (L2)] react
with Na2PdCl4, giving palladacycles [PdCl(C−,N,S)] (1: (C−,N,S)
1
= L1−H; 2: (C−,N,S) = L2−H). The H and 13C{1H} NMR
spectra of ligands and their palladacycles have been found to be
characteristic. Complexes 1 and 2 have also been characterized
with HR-MS. The crystal structure of 2 has been solved. The Pd−S
bond length is 2.428(2) Å, and palladium has a nearly square
planar geometry. During the course of catalysis of Suzuki−Miyaura
C−C coupling using 1 and 2 as catalysts, unexpected formation of
Pd16S7 nanoparticles (NPs) has been observed with both
complexes. This is the first time that such an observation has been made with palladacycles of thioethers used in this coupling
reaction. The efficiency of 2 in carrying out the coupling is significantly lower than that of 1. Complex 2 has an additional −OMe
group in the ligand structure, and the size of Pd16S7 NPs formed from this complex are larger (6 nm) than those obtained from 1
(2 nm).
aryl chlorides, which are the cheapest and most readily available
among the aryl halides. The L1 and L2 synthesized as a part of
our research program on chalcogenated Schiff bases11 and their
reduced forms have been found worth studying to design
thioether palladacycles 1 and 2, which may show potential as
Suzuki catalysts.
INTRODUCTION
■
Transition metal-catalyzed C−C bond forming reactions are
powerful synthetic tools in organic chemistry.1,2 Palladium
species constitute a class of versatile and useful catalysts for
such organic transformations. The facile interchange between
Pd(0) and Pd(II) or Pd(II) and Pd(IV) and the tolerance of
palladium compounds to many functional groups present on
substrate are mainly responsible for their versatility.2 Not only
Suzuki−Miyaura coupling but other C−C bond forming
reactions such as ethylene oligo/polymerization, Heck and
Negishi coupling, etc., also rely on this facile interconversion of
oxidation states. The most important Suzuki−Miyaura catalysts
include complexes of Pd(II) with bulky and electron-rich
phosphines3,4 and carbenes,5 and palladacycles,2,6,7 owing to
their high efficiency and the ease with which they can be
modified.
In the last few decades interest in palladium complexes of
ligands containing a combination of various donor groups, such
as Suzuki−Miyaura catalysts, has grown significantly as the
distinct features of each donor atom can confer unique
properties to the complex.2,6−8 Sulfur has been incorporated
in framework of many ligands, and Pd(II) complexes of sulfated
Schiff bases,9 pincer type S ligands,7 and some other S-
containing ligands10 have emerged as a family of air-stable,
moisture-insensitive, and efficient catalysts. The sulfur-ligated
palladacycles are important among this family of Suzuki
catalysts, and in recent years an increase in research on them
has been noticed. They have shown good promise for the
coupling of aryl bromides and iodides, but most of
them7c−g,8,9a,c−e have not been reported to transform effectively
The in situ formation of Pd NPs has been reported in
Suzuki−Miyaura coupling reactions catalyzed with palladium
complexes of organosulfur ligands.7b These Pd NPs are
reported to be stabilized in the presence of additive n-
Bu4NBr.7g,11d However, in some cases the absence of this
additive gives a better yield.7g There is enough evidence that
suggests that Pd(0) species leached from the surface of such
NPs are the true catalysts during the course of reaction, and the
role of the ligand (including its architecture) is limited to
affecting size, dispersion, and the chemical nature of the NPs.
However, formation and involvement of NPs of a palladium
sulfide phase in the catalytic Suzuki coupling reaction have
never been reported. Suzuki−Miyaura coupling carried out in
the presence of palladacycles 1 and 2 results in the formation of
nanosized particles of composition Pd16S7, which appear to play
a role in the catalysis of the coupling via generation of Pd(0)
species. The formation of the Pd16S7 phase in Suzuki coupling
has been noticed for the first time. The unexpectedly high
difference in activities of 1 and 2 has been observed. The
complex 2, having an additional −OMe (with respect to 1), has
been made a part of this study, as its single crystal structure
Received: March 11, 2013
Published: April 4, 2013
© 2013 American Chemical Society
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Article
through Celite. The solvent was removed with a rotary evaporator, and
determination is possible, which throws light on the structure of
analogous 1. The Pd16S7 NPs resulting from 2 are larger in size
than those obtained from 1. This may cause a slow release of
Pd(0) from NPs of 2 and consequently low catalytic efficiency.
These results are described in this paper.
ligands L1 and L2 were obtained as a light yellow liquid.
1
L1: Yield: 0.272 g (78%). H NMR (300 MHz, CDCl3): δ (ppm)
1.212 (s, 2H, NH + OH), 1.867−1.883 (m, 2H, H6), 2.741−2.971 (m,
4H, H5, H7), 4.873 (s, 1H, H8), 6.720 (t, J = 7.2 Hz, 1H, H13), 6.814−
6.868 (m, 2H, H11, H14), 7.118−7.315 (m, 11H, H1, H2, H3, H12, H16,
H17, H18). 13C{1H} NMR (75 MHz, CDCl3): δ (ppm) 28.58 (C6),
30.07 (C5), 46.23 (C7), 63.93 (C8), 115.93 (C11), 118.75 (C13), 125.58
(C9), 127.06 (C1), 127.43 (C17), 127.70 (C18), 127.92 (C12), 128.07
(C2), 128.30 (C14), 128.39 (C16), 129.06 (C3), 136.36 (C4), 142.81
(C15), 156.68 (C10).
EXPERIMENTAL SECTION
■
General Experiments. Thiophenol, NaBH4, 3-chloropropylamine
hydrochloride, 2-hydroxybenzophenone, 2,4-dihydroxybenzophenone,
sodium tetrachloropalladate, potassium carbonate, and aryl halides
were procured from Sigma-Aldrich (USA). Reagents (commercially
available from local sources) were used as received without further
purification. The progress of every coupling reaction was monitored by
NMR spectroscopy. Yields refer to isolated yields of compounds that
1
L2: Yield: 0.325 g (86%). H NMR (300 MHz, CDCl3): δ (ppm)
1.258 (s, 2H, NH + OH), 1.830−1.911 (m, 2H, H6), 2.749−2.959 (m,
4H, H5 + H7), 3.723 (s, 1H, H19), 4.822 (s, 1H, H8), 6.285 (dd, J = 2.1
Hz, 8.1 Hz, 1H, H13), 6.434 (d, J = 2.1 Hz, 1H, H11), 6.686 (d, J = 8.4
Hz, 1H, H14), 7.158−7.296 (m, 10H, H1, H2, H3, H16, H17, H18),
7.499−7.622 (m, 4H,). 13C{1H} NMR (75 MHz, CDCl3): δ (ppm)
28.82 (C6), 31.46 (C5), 46.64 (C7), 55.07 (C8), 67.20 (C19), 102.10
(C11), 105.26 (C13), 116.90 (C9), 126.11 (C1), 127.20 (C16), 127.73
(C18), 128.86 (C2), 128.96 (C17), 129.43 (C3), 129.60 (C14), 135.77
(C4), 141.79 (C15), 158.78 (C10), 160.26 (C12).
Synthesis of 1 and 2. The Na2PdCl4 (0.205 g, 0.7 mmol) was
dissolved in 5.0 mL of water. A solution of ligand L1 (0.175 g, 0.5
mmol)/L2 (0.191 g, 0.5 mmol) made in 10 mL of acetone was added
to it with vigorous stirring. The mixture was further stirred for 2 h. The
orange-red solution was extracted with chloroform. The chloroform
layer was washed with 100 mL of water, dried with anhydrous Na2SO4,
and evaporated to dryness on rotary evaporator to obtain 1 and 2 as an
orange-colored powder. The single crystals of 2 were grown from 8:2
mixtures of CHCl3 and hexane.
1: Yield: 0.158 g (65%); mp 153 °C (dec). Anal. Found: C, 53.81;
H, 4.46; N, 2.91. Calcd for C22H22ClNOPdS: C, 53.89; H, 4.52; N,
2.86. 1H NMR (300 MHz, DMSO-d6): δ (ppm) 2.182−2.285 (m, 2H,
H6), 2.882−2.905 (m, 4H, H5 + H7), 4.354 (s, 1H, H8), 6.275−6.382
(m, 1H), 6.814−7.505 (m, 9H), 7.747 (d, J = 7.2 Hz, 1H), 8.244−
8.345 (m, 2H). 13C{1H} NMR (75 MHz, DMSO-d6): δ (ppm) 28.70
(C6), 29.00 (C5), 51.48 (C7), 65.90 (C8), 114.06, 118.97, 126.96,
128.48, 128.64, 129.34, 129.57, 129.89, 120.25, 132.71, 139.20, 157.97,
160.99. HR-MS [M − Cl] (m/z) = 454.0456; calcd value for
C22H22NOPdS = 454.0459 (ppm error δ: 0.7).
2: Yield: 0.185 g (71%); mp 159 °C (d). Anal. Found: C, 53.08; H,
4.66; N, 2.67. Calcd for C23H24ClNO2PdS: C, 53.09; H, 4.65; N, 2.69.
1H NMR (300 MHz, CDCl3): δ (ppm) 1.958−2.976 (m, 1H), 2.171−
2.238 (m, 1H), 3.142−3.225 (m, 2H), 3.360−3.429 (m, 2H), 3.624 (s,
3H, OMe), 4.051 (s, 1H, H8), 6.214 (s, 1H), 6.366−6.421 (m, 1H),
7.155−7.460 (m, 8H), 8.101−8.128 (m, 1H), 8.180−8.200 (m, 1H).
13C{1H} NMR (75 MHz, CDCl3): δ (ppm) 26.24 (C6), 35.86 (C5),
1
have a purity of ≥95% [established by H NMR]. The products of
Suzuki reactions were authenticated by matching spectroscopic data of
the products obtained by us with those reported in the literature.
1H and 13C{1H} NMR spectra were recorded on a Bruker
Spectrospin DPX 300 NMR spectrometer at 300.13 and 75.47
MHz, respectively, with chemical shifts reported in ppm relative to the
residual deuterated solvent or the internal standard tetramethylsilane.
Carbon-13 DEPT NMR experiments were used routinely to determine
the number of hydrogen atoms linked to carbon atoms. Elemental
analyses were carried out with a Perkin-Elmer 2400 Series II C, H, N
analyzer. Melting points were determined in an electrically heated
apparatus by taking the sample in a glass capillary sealed at one end.
High-resolution mass spectral (HR-MS) measurements were per-
formed with electron spray ionization (10 eV, 180 °C source
temperature, and sodium formate as calibrent) on a Bruker Micro
TOF-Q II, taking the sample in CH3CN.
Suitable crystals of palladacycle 2 were obtained by slow
evaporation of its solution made in a chloroform−hexane (8:2)
mixture. X-ray diffraction data for crystals of 2 were collected on a
Bruker AXS SMART-APEX diffractometer equipped with a CCD area
detector (Kα = 0.71073 Å; graphite monochromator). 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 apparatus. The measured intensities were
reduced to F2 and corrected for absorption with SADABS. Structure
solution, refinement, and data output were carried out with the
SHELXTL package by direct methods. Non-hydrogen atoms were
refined anisotropically. All hydrogen atoms were included in idealized
positions, and a riding model was used for the refinement. Images were
created with the program Diamond.
TEM studies were 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 onto a porous carbon film supported on a copper
grid, and then drying in air. The nanostructural phase morphology of
the sample was observed by using a Carl Zeiss EVO5O scanning
electron microscope (SEM). Nanostructures observed in the SEM, for
its elemental composition, were analyzed by an EDX system model
Quan Tax 200, which is based on the SDD technology and provides an
energy resolution of 127 eV at Mn Kα. The samples were scanned in
different regions in order to minimize the error in the analysis made to
evaluate the morphological parameters. Samples were mounted on a
circular metallic sample holder with a sticky carbon tape.
Powder X-ray diffraction (PXRD) studies were carried out on a
Bruker D8 Advance diffractometer with Ni-filtered Cu Kα radiation
using a scan speed of 1 s and scan step of 0.05°. Thermogravimetric
analysis (TGA) was carried out using a Perkin-Elmer system in flowing
nitrogen atmosphere with a heating rate of 10 °C/min.
Synthesis of L1 and L2. The C6H5S−(CH2)3−NC(Ph)C6H4−
2−OH (0.347 g, 1 mmol)/C6H5S−(CH2)3−NC(Ph)−4−OMe−
C6H3−2−OH (0.377 g, 1 mmol) prepared by reported methods9e and
NaBH4 (0.380 g, 1.0 mmol) were stirred for 10 h in 100 mL of dry
ethanol. The solvent was removed on a rotary evaporator. The ligand
L1/L2 was dissolved into 20 mL of dry chloroform and filtered
53.17 (C7), 55.04 (C8), 70.81 (H19), 102.17, 103.46, 106.63, 117.82,
128.67, 129.07, 129.75, 130.71, 132.46, 135.45, 136.43, 140.13, 160.38,
165.38, 169.97. HR-MS [M − Cl] (m/z) = 484.0554; calcd value for
C23H24NO2PdS = 484.0565 (ppm error δ: 2.4).
General Procedure for Suzuki Reaction of Aryl Halides with
Phenylboronic Acid. An oven-dried flask was charged with aryl
halide (1.0 mmol), phenylboronic acid (1.3 mmol), K2CO3 (2.0
mmol), and DMF/H2O (4.0 mL). A solution of catalyst 1 or 2 in
DMF was then added via syringe. The flask was placed on an oil bath
at 100 °C under aerobic conditions, and the reaction mixture stirred
until maximum conversion of aryl halide to coupled product occurred.
The mixture was extracted with diethyl ether (100 mL). The extract
was washed with water (100 mL) and dried over anhydrous Na2SO4.
The solvent of the extract was removed with a rotary evaporator, and
the resulting residue purified by column chromatography on silica gel
using an ethylacetate/hexane mixture (5:95% to 15:85%) as eluent.
Isolation of Nanoparticles Formed from Palladacycles 1 and
2 in Suzuki−Miyaura C−C Coupling. A mixture of palladacycle 1/2
(0.50 mmol), phenylboronic acid (1.3 mmol), 1-bromo-4-nitro-
benzene (1 mmol), and K2CO3 (2 mmol) in a DMF (4 mL)/water
(4 mL) mixture was heated at 100 °C in an oil bath for 2.5 h and then
cooled to room temperature. The solvent was decanted off, and the
black residue was mixed with 10 mL of acetone and 30 mL of water.
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Scheme 1. Synthesis of Ligands and Their Pd(II) Complexes
Figure 1. ORTEP diagram of 2 with 30% probability ellipsoids. Selected bond lengths (Å): Pd(1)−C(23) 1.970(7); Pd(1)−N(1) 2.059(6); Pd(1)−
Cl(1) 2.319(2); Pd(1)−S(1) 2.428(2). Selected bond angles (deg): C(23)−Pd(1)−N(1) 81.5(3); C(23)−Pd(1)−Cl(1) 93.3(2); N(1)−Pd(1)−
Cl(1) 173.73(19); N(1)−Pd(1)−S(1) 98.7(2); Cl(1)−Pd(1)−S(1) 86.47(8), C(6)−S(1)−C(7) 101.1(4); C(6)−S(1)−Pd(1) 106.9(3).
The mixture was centrifuged. The black residue was further washed
with a mixture of acetone and water (3:1, v/v) and dried. The residue
thus obtained was separated and subjected to powder-XRD. It was
found to be amorphous in the case of both complexes. These powders
were annealed in an argon atmosphere at 380 °C for 4 h, which
resulted in crystalline Pd16S7 nanoparticles in the case of both
complexes.
Hg Poisoning Test. The Hg (Hg:Pd::500:1) was taken in a
reaction flask before the addition of coupling reactants. Thereafter 4-
bromoanisole (1.0 mmol), phenylboronic acid (1.3 mmol), and 1/2
(0.1 mol %) or nanoparticles (1 mol % of Pd) isolated during
decomposition of 1/2 as catalyst were added, and the reaction under
optimum conditions was carried out. The expected product 4-
methoxybiphenyl was not obtained in significant yield after 24 h.
PPh3 Poisoning Test. The coupling reaction of 4-bromoanisole
with phenylboronic acid using 1/2 (0.1 mol %) or nanoparticles (1
mol % of Pd) isolated from decomposition of 1/2 in Suzuki−Miyaura
coupling, as a catalyst, was carried out in the presence of PPh3 (10 mol
%) under optimum conditions. After 24 h the expected cross-coupled
product 4-methoxybiphenyl was not obtained.
Two-Phase Test. First, 4-bromobenzoic acid was heterogenized by
immobilizing it on silica gel using a standard procedure (details in the
Supporting Information).12 Thereafter, a mixture of the immobilized
species (0.20 g), phenylboronic acid (0.36 g, 3 mmol), 4-
bromoacetophenone (0.20 g, 1 mmol), and K2CO3 (0.56 g, 4
mmol) was heated at 90 °C for 12 h in a DMF/water (6/3 mL)
mixture. After cooling to room temperature, the resulting mixture was
filtered through a G-4 crucible and the residue was washed with 20 mL
of H2O followed by diethyl ether (50 mL). Water (50 mL) was added
to the mixture of filtrate and washings, which were collected together.
The resulting mixture was extracted with diethyl ether (50 mL). The
solvent of the extract was removed under vacuum, and the residue was
analyzed using H NMR spectroscopy. The hydrolysis of the solid
1
residue (obtained in a G-4 crucible after washing) was carried out with
KOH (1.68 g dissolved in 10 mL of EtOH + 5 mL of H2O) at 90 °C,
and after 3 days, the resulting solution was neutralized with aqueous
20% (v/v) HCl. The neutralized solution was extracted with
dichloromethane followed by ethyl acetate. The solvent of the mixture
of extracts was evaporated under vacuum, and the resulting residue was
1
analyzed with H NMR.
RESULTS AND DISCUSSION
■
L1, L2, 1, and 2, prepared as given in Scheme 1, are stable
under ambient conditions and may be stored for several
months. The composition of complexes of L1 and L2 formed
with Pd(II) does not change on varying the metal:ligand ratio
during their synthesis. The ligands have higher solubility in
CHCl3, CH2Cl2, CH3CN, CH3OH, C2H5OH, and acetone than
those of their complexes. Pd complex 1 has good solubility in
DMSO and DMF, whereas in CHCl3, CH2Cl2, and CH3CN it
is sparingly soluble. Complex 2 shows good solubility in
DMSO, DMF, and CHCl3 but is sparingly soluble in CH2Cl2
and CH3CN. These ligands and their diamagnetic complexes 1
and 2 have been characterized by H and 13C{1H} NMR
1
spectroscopy and HR-MS (see Supporting Information for
scans of original spectra). These data are consistent with the
structures depicted for them in Scheme 1. The 13C{1H} NMR
spectra of complexes 1 and 2 have a new quaternary carbon
signal between 160 and 161 ppm, showing palladation of L in
both cases. The structure of 2 has been elucidated by X-ray
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diffraction on its single crystals obtained by slow evaporation of
its solution made in a chloroform/hexane mixture (8:2).
However attempts to crystallize complex 1 were unsuccessful.
The crystal data and structure refinement for complex 2 are
given in Table S1. Table S2 contains selected bond length and
bond angles for complex 2. Figure 1 shows an ORTEP diagram
of 2 with some selected bond lengths and angles. There is a
distorted square planar geometry at Pd in complex 2 with S and
C(aryl) disposed trans to each other. The Schiff base9e
precursor of L1 binds in an (O−, N, S) mode with Pd(II).
The (C−, N, S) bonding mode of L1/L2 in 1 and 2 leaving the
OH group uncoordinated occurs because ortho-palladation
probably precedes Pd−O bond formation. The Pd−N bond
length of 2.060(6) Å in 2 is almost the same as the reported
value, 2.010(4) Å, for Pd(II) complexes of tridentate selenated
Schiff bases9e and is longer than the 1.986(6) Å reported9e for
[PdCl{(C6H5)(C6H4-2-O−)CN(CH2)3SePh}]. The Pd−C
distance [1.970(7) Å] in 2 is consistent with the value of
1.971(16) Å7d reported for the Pd−C bond of the palladium-
(II)-(S,C,S) pincer complex. The Pd−S bond distance
[2.428(2) Å] of 2 has been found to be longer than that of a
tridentate sulfated Schiff base−Pd(II) complex,9e whereas the
length of the Pd−Cl bond of 2 [2.319(2) Å] is consistent with
the value [2.3159(7) Å] reported for the same Pd(II) complex
of a tridentate sulfated Schiff base.9e In the light of the structure
of 2 and the presence of a quaternary carbon signal in the
13C{1H} NMR spectra of 1, the ligand L1 presumably
coordinates with Pd in 1 in a monoanionic tridentate (S, N,
C−) mode, forming one six-membered and one five-membered
chelate ring. The geometry around Pd in 1 is also presumably
square planar. The Suzuki reactions (Scheme 2) were carried
Table 1. Suzuki−Miyaura Coupling Reactions Catalyzed by
a
1
Ar–X + PhB(OH)2 → Ar–Ph (X; Cl, Br)
b
entry no.
aryl halide
mol % Pd yield (%)
TON
1
1-bromo-4-nitrobenzene
4-bromobenzonitrile
4-chlorobenzaldehyde
4-chlorobenzaldehyde
4-bromobenzaldehyde
2-bromobenzaldehyde
4-bromobenzoic acid
4-Bromotoluene
0.1
84
95
94
28
95
86
94
84
95
93
94
95
50
40
50
53
72
840
95 000
94
2
0.001
1
3
4
0.01
0.001
0.1
2800
95 000
860
5
6
7
0.1
940
8
0.001
0.1
84000
950
9
4-bromotoluene
10
11
12
13
14
15
16
17
4-chlorotoluene
0.05
0.001
0.5
1860
94 000
190
4-bromoanisole
4-bromoaniline
2-Bromopyridine
3-bromopyridine
0.001
0.001
0.1
50000
40 000
500
4-chloropyridine
4-bromopyridine
0.002
0.001
26 500
72 000
3-bromoquinoline
a
Reaction conditions: 1.0 equiv of aryl halide, 1.3 equiv of
phenylboronic acid, 2 equiv of K2CO3, aquous DMF as solvent,
b
reaction time 15 h, and temperature of bath 100 °C. Isolated yield
after column chromatography.
Table 2. Suzuki−Miyaura Coupling Reactions Catalyzed by
a
2
Ar–X + PhB(OH)2 → Ar–Ph (X; Cl, Br)
entry
no.
yield
b
aryl halide
mol % Pd t (h)
(%)
TON
Scheme 2. Suzuki−Miyaura Coupling Reaction
1
1-bromo-4-nitrobenzene
4-bromobenzonitrile
4-chlorobenzaldehyde
4-bromobenzaldehyde
bromobenzene
0.05
0.05
1.0
2
2
93
95
1860
1900
2
3
24
5
4
0.05
0.1
91
87
86
93
1820
870
172
93
5
5
6
4-bromotoluene
0.5
12
2
out in the presence of 1 and 2, and the results are given in
Tables 1 and 2. For the reaction between 4-bromobenzalde-
hyde and phenylboronic acid under aerobic conditions at 100
°C for 15 h (catalyzed with 1), the different bases and solvents
were studied, and the best results were obtained with K2CO3
and DMF/water. The catalytic efficiency of 1 is higher than that
of 2, which is moderately active, as 0.05 to 1 mol % are required
for significant conversions (Table 2). For example coupling
between 4-bromobenzaldehyde and phenylboronic acid in the
presence of 0.001 mol % of 1 and for a reaction time of 15 h at
100 °C, the product biphenyl-4-caboxyldehyde in 95% yield
with TON 95000 (Table 1, entry 5) was obtained. When
catalyst 2 was used for the same coupling under similar reaction
conditions, a higher catalyst loading (0.05 mol %) gave a TON
of ∼1820 (Table 2, entry 4). Further the results of Suzuki−
Miyaura coupling given in Tables 1 and 2 indicate that the
coupling of aryl chlorides can be successfully achieved when 1
is used as catalyst, even in the case of deactivated ones. For
example 4-chlorotoluene is converted to 4-methylbiphenyl in
93% yield (TON ∼1860) within 15 h using 0.05 mol % of 1 as
catalyst (Table 1, entry 10). The deactivated species 4-
bromoaniline has also been successfully coupled (yield
∼95%) using 0.5 mol % of 1 (Table 1, entry 12). The cross-
coupling reactions of heteroaryl bromides and chlorides have
also been successfully catalyzed with 1. The coupling of 3-
7
4-bromoanisole
1.0
8
4-chloroanisole
1.0
24
15
12
12
9
4-bromoaniline
1.0
87
86
90
87
172
180
10
11
3-bromopyridine
4-bromopyridine
0.5
0.5
a
Reaction conditions: 1.0 equiv of aryl halide, 1.3 equiv of
phenylboronic acid, 2 equiv of K2CO3, aquous DMF as solvent, and
temperature of bath 100 °C. Isolated yield after column
chromatography.
b
bromoquinoline with phenylboronic acid (catalyzed by 0.001
mol % of 1) gives 3-phenylquinoline in ∼72% yield (TON on
the order 72 000). Similarly Suzuki−Miyaura coupling of 4-
chloropyridine giving 4-phenylpyridine in 50% yield has
occurred in the presence of 0.1 mol % of 1. The palladacyclic
complex 2 does not show any significant catalytic activity
toward aryl chlorides even at its 1 mol % loading (Table 2,
entries 3 and 8). Complexes 1 and 2 differ in structure by one
−OMe group, which is present in the case of 2. Such a large
variation in catalytic efficiency due to just the −OMe group is
unexpected. Therefore an attempt is made to gain insight into
the catalytic process by characterizing and analyzing the role of,
black particles that appear in the reactions during the course of
catalysis. It has already been reported on the basis of various
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experimental results that sulfur-containing palladacycles decom-
pose to Pd(0), and an equilibrium between palladium NPs and
the actual active species [Pd(0)−Pd(II)]7b exists during the
catalytic reaction.7b In view of such reports and the appearance
of black particles during the course of the catalytic reactions, it
is possible that the complexes (1 and 2) are not true catalysts
but dispensers of real catalysts either directly or via some
intermediate nanoparticle formulation. Thus a reaction between
1-bromo-4-nitrobenzene and PhB(OH)2 under optimum
conditions catalyzed with 1 as well as 2 has been examined
in detail with the hope that it may lead to some understanding
of correlation (if any) between the ligand’s framework, the
catalytic activity, and the nature of the NPs. The black residues
formed during a representative reaction catalyzed with 1 and 2
were separated and characterized by HR-TEM, EDX, and
powder X-ray diffraction (which reveal their amorphous
nature). The HR-TEM (Figure 2) indicates that the black
spherical (size 6 nm) when originated from 1 and rice-shaped
(size 100 nm) in the case of 2 (Figure 4). It has already been
Figure 4. TEM image of palladium sulfide nanoparticles obtained from
palladacycles 1 and 2, respectively, after annealing at 380 °C.
reported that the phase Pd16S7 has a body-centered cubic
structure with 46 atoms in the cubic cell and is structurally
related to the γ-brass structure.13 The average formal oxidation
state for palladium is +0.875, the majority of Pd−S contacts are
∼2.4 or 2.8 Å, and the Pd−Pd bond exists; therefore some Pd
most probably exists in the (0) oxidation state.
To check whether the catalytic activity is also associated with
these NPs, the aggregated Pd16S7 NPs obtained from 1 and 2
were examined for Suzuki−Miyaura coupling reaction (Table
3) of 4-bromonitrobenzene, bromobenzene, and 4-bromoani-
Table 3. Suzuki−Miyaura Coupling Reactions Catalyzed by
a
Pd16S7 NPs
Figure 2. TEM image of nanoparticles obtained from complexes 1 and
2.
NPs obtained
NPs obtained
from 1
from 2
powder consists of highly uniform and monodisperse spherical
shaped NPs (size: ∼2 nm for 1 and 6 nm for 2) (Figure 3). The
entry
no.
yield
yield
b
b
aryl halide
1-bromo-4-
mol %
(%)
mol %
(%)
1
0.1
88
1.0
81
nitrobenzene
bromobenzene
4-bromoanisole
2
3
0.1
0.1
81
84
1.0
1.0
69
80
a
Reaction conditions: 1.0 equiv of aryl halide, 1.3 equiv of
phenylboronic acid, 2 equiv of K2CO3, aquous DMF as solvent,
b
reaction time 24 h, and temperature of bath 100 °C. Isolated yield
after column chromatography.
sole with PhB(OH)2 under optimum conditions. The reaction
was smooth in the presence of 0.1 mol % of NPs obtained from
1 and 1 mol % of NPs obtained from 2. The reaction for 24 h at
100 °C resulted in the products 4-nitrobiphenyl, biphenyl, and
4-methoxybiphenyl, respectively, in good yields (Table 3).
However these NPs failed to couple ArCl. Overall these results
support the proposition that the palladacycles 1 and 2 act as
dispensers of nanosized Pd16S7, which seems to play a role in
catalysis of Suzuki coupling. In the recent past various pathways
for the involvement of in situ generated Pd NPs in catalysis
have been suggested. The mechanism involving Pd atom escape
from the palladium nanoparticles by an oxidative addition
generating discrete Pd(II) species in solution has been
proposed for the Suzuki reaction.14 Alternatively, the Pd(II)
complexes can be formed by oxidative addition to Pd(0) atoms
that have already leached into the solution.15 Recently XAS (X-
ray absorption spectroscopy) has been utilized to track
quantitatively the local structure of monodispersed Pd
nanoparticles during a Suzuki coupling reaction. By following
Figure 3. Size distribution of NPs obtained from 1 and 2.
large variation in catalytic activities of 1 and 2 may be partly
ascribed to the difference in size of NPs generated in situ from
them. These NPs were annealed at 380 °C for 4 h in argon, and
the resulting crystalline forms were characterized by HR-TEM,
EDX, and powder X-ray diffraction. The EDX studies (Figures
S26, S28, S30, and S32; Supporting Information) suggest that
the Pd:S ratio in NPs obtained from 1 and 2 both before and
after annealing is almost the same. After annealing, formation of
crystalline Pd16S7 nanoparticles from both 1 and 2 is supported
by their powder X-ray diffraction patterns (Figures S17 and
S18; Supporting Information). The HR-TEM of crystalline
Pd16S7 NPs generated on annealing has suggested that they are
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Scheme 3. Two-Phase Test on Suzuki−Miyaura Coupling with Catalyst 1
the local coordination environment in an operando mode (real-
time measurement under working conditions), strong evidence
has been found indicating that their reactivity is indeed
heterogeneous in origin and associated with the presence of
stable surface defect atoms.16 It has been hypothesized that the
pathways involving soluble metal complexes and insoluble
metal particles may take place in the same reaction vessel, and
both can contribute to the product formation. Thus all the
approaches, boundary cases of purely homogeneous or purely
heterogeneous systems as well as “cocktail”-like mixtures of the
catalysts, are possible.17 In view of these reports, the possibility
of molecular catalysis cannot be ruled out despite positive Hg
poisoning18 and PPh3 tests18 carried out during catalysis by 1,
2, or Pd16S7 NPs. Further NPs may contribute in a
homogeneous as well as heterogeneous fashion. The possibility
of homogeneous catalysis may arise due to leaching of surface
atoms of heterogeneous nanoparticles of Pd16S7 in solution (to
form a soluble Pd(II) intermediate11 such as Ar−Pd−Br by
oxidative addition). A two-phase test (Scheme 3) was
performed to understand further catalytic processes;19,20 of
course it cannot conclusively indicate the colloidal or molecular
nature of the catalyst. This test (called a three-phase test when
the catalyst is a solid phase) was developed by Rebek and co-
workers.19 If the activity of the catalyst is fully heterogeneous in
nature, the supported aryl halide is not expected to be
converted to a coupled product. However, the homogeneous
activity of the catalyst due to leaching of Pd(0) in solution will
lead to the conversion of the supported substrate to product.
Further, the colloidal catalyst being in solution can always cause
some conversion. A two-phase test (shown in Scheme 3) was
conducted utilizing the reaction of a mixture of 4-
bromoacetophenone and immobilized 4-bromobenzoic acid
(as amide) with phenylboronic acid under optimum reaction
conditions. The soluble part was separated by filtration and
analyzed after workup with 1H NMR to determine the yield of
the cross-coupled product (4-acetylbiphenyl), which was found
to be ∼99%. The solid phase was hydrolyzed, and analysis of
An intuitive proposition can be made that secondary
interactions due to the −OMe group in 2 make somewhat
large molecular aggregates, which in turn in the reaction
medium result in bigger and less efficient NPs, whereas in the
case of 1 such a possibility is less and NPs formed are small and
more efficient. The high efficiency may be assumed to some
extent to be related to the higher surface area of NPs obtained
from 1. The high surface area facilitates the reaction in every
case whether reactivity is due to Pd atoms escaped from the rim
of NPs or is associated with the presence of stable surface
defect atoms.
CONCLUSION
■
The newly synthesized palladacycle 1 of organosulfur ligand L1
is an efficient precatalyst for Suzuki−Miyaura C−C coupling
reactions. The catalytic activities of both 1 and 2 appear to be
via Pd16S7 NPs, generated in situ unexpectedly during the
course of catalysis, and mainly the Pd species leached from
them contribute to catalysis of Suzuki coupling probably via a
homogeneous process. The overall catalytic process has been
indicated as “cocktail” type by the two-phase test. This is the
first time that NPs of a palladium−sulfur phase have been
found to be involved in Suzuki−Miyaura coupling catalyzed
with palladacycles of thioethers. The size of Pd16S7 NPs and the
catalytic efficiency of precursor Pd complexes vary significantly
upon incorporation of a small substituent, an −OMe group, in
the ligand’s backbone. These results are of importance for
tailoring new efficient catalysts for Suzuki−Miyaura C−C
coupling reactions.
ASSOCIATED CONTENT
* Supporting Information
■
S
The NMR spectral data of L1, L2, 1, and 2; HR-MS of 1 and 2;
structural and refinement data of 2 (CCDC no. 891159); SEM,
EDX, PXRD of NPs; and NMR data of Suzuki coupling
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
the resulting products (after workup) with H NMR indicates
that ∼58% of the immobilized 4-bromobenzoic acid (as amide)
was converted to the cross-coupled product (biphenyl-4-
carboxylic acid), whereas ∼42% remained unreacted. These
results indicate that there is a significant contribution of
homogeneous Pd species (molecular or colloidal) to catalysis.
Either the palladium species (responsible for homogeneous
catalysis) is the palladium atoms leached from the in situ
generated nanoparticles of Pd16S7, or the molecular complex or
both are contributing to C−C coupling.17 Thus, it appears that
in the case of 1 and 2 coupling is catalyzed with a contribution
of nanosized Pd species homogeneously as well as heteroge-
neously, and such a possibility has been recently described as a
“cocktail”-like mixture of the catalysts.17
AUTHOR INFORMATION
Corresponding Author
*E-mail: aksingh@chemistry.iitd.ac.in, ajai57@hotmail.com.
Fax: +91 11 26581102. Tel: +91 11 26591379.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Council of Scientific and Industrial Research and Department
of Science and Technology India supported the work through
the award of SRF/RA and projects [CSIR: 01(2421)10/EMR-
II; DST: SR/S1/IC-40/2010]. The authors thank Professor A.
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12552−12574. (b) Kumar, A.; Rao, G. K.; Saleem, F.; Singh, A. K.
Dalton Trans. 2012, 41, 11949−11977. (c) Rao, G. K.; Kumar, A.;
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XRD facility.
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