Palladium(II) complexes of N, N-diphenylacetamide based thio/selenoethers and flower shaped Pd16S7 and prismatic Pd17Se15 nano-particles tailored as catalysts for C-C and C-O coupling

Article abstract of DOI:10.1039/c7dt01279a

2-Bromo-N,N-diphenylacetamide (P1) prepared by reacting diphenylamine with α-bromoacetyl bromide, on treatment with Na₂S and Na₂Se generated in situ, has resulted in thio and selenoether ligands, L1 ((Ph₂NCOCH₂)₂S) and L2 ((Ph₂NCOCH₂)₂Se), respectively. Reacting these ligands with Na₂PdCl₄ in ethanol at room temperature resulted in their complexes [Pd(L1/L2)₂Cl₂] (C1/C2). P1, L1, L2, C1 and C2 were characterized by ¹H, ¹³C{¹H} and ⁷⁷Se{¹H} NMR spectroscopy, IR spectroscopy and High resolution mass spectrometry (HR-MS). Single crystal structures of all the five compounds were determined by X-ray diffraction. In both C1 and C2 the geometry of Pd is nearly square planar. Flower shaped Pd₁₆S₇ (size 26-50 nm) and prismatic Pd₁₇Se₁₅ NPs (size 20-55 nm) were synthesized by a single source precursor route (thermolysis at ～280 °C in trioctylphosphine) from air and moisture insensitive C1 and C2, respectively. These shapes of the two nanophases were unknown hitherto. They were characterized by powder X-ray diffraction (PXRD), SEM-EDX and HR-TEM and found to show good catalytic activity for Suzuki-Miyaura (Pd loading 0.5 mol%; yield up to 96%) and C-O (Pd loading 0.5 mol%; yield up to 96%) coupling reactions (at 100 °C) of aryl bromides with phenylboronic acid and phenol, respectively. The complexes C1 and C2 were found very efficient for Suzuki-Miyaura and C-O coupling as revealed by their optimum loading of 0.0001-0.01 and 0.1 mol% of Pd, respectively, for the two reactions. The reuse of the complexes or NPs as a catalyst is demonstrated.

Full text of DOI:10.1039/c7dt01279a

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Palladium(II) complexes of N,N-diphenylacetamide
based thio/selenoethers and flower shaped Pd S
16 7
Cite this: DOI: 10.1039/c7dt01279a
and prismatic Pd Se nano-particles tailored as
17
15
catalysts for C–C and C–O coupling†
Poornima Singh
and Ajai K. Singh
*
2-Bromo-N,N-diphenylacetamide (P1) prepared by reacting diphenylamine with α-bromoacetyl bromide,
on treatment with Na
2
S and Na
NCOCH
ethanol at room temperature resulted in their complexes [Pd(L1/L2)
2
Se generated in situ, has resulted in thio and selenoether ligands, L1
Se), respectively. Reacting these ligands with Na PdCl in
Cl ] (C1/C2). P1, L1, L2, C1 and C2
(
(Ph
2
NCOCH
2
)
2
S) and L2 ((Ph
2
2
)
2
2
4
2
2
1
13
1
77
1
were characterized by H, C{ H} and Se{ H} NMR spectroscopy, IR spectroscopy and High resolution
mass spectrometry (HR-MS). Single crystal structures of all the five compounds were determined by X-ray
diffraction. In both C1 and C2 the geometry of Pd is nearly square planar. Flower shaped Pd16
7
S (size
26–50 nm) and prismatic Pd17Se15 NPs (size 20–55 nm) were synthesized by a single source precursor
route (thermolysis at ∼280 °C in trioctylphosphine) from air and moisture insensitive C1 and C2, respect-
ively. These shapes of the two nanophases were unknown hitherto. They were characterized by powder
X-ray diffraction (PXRD), SEM-EDX and HR-TEM and found to show good catalytic activity for Suzuki–
Miyaura (Pd loading 0.5 mol%; yield up to 96%) and C–O (Pd loading 0.5 mol%; yield up to 96%) coupling
reactions (at 100 °C) of aryl bromides with phenylboronic acid and phenol, respectively. The complexes
C1 and C2 were found very efficient for Suzuki–Miyaura and C–O coupling as revealed by their
optimum loading of 0.0001–0.01 and 0.1 mol% of Pd, respectively, for the two reactions. The reuse of
the complexes or NPs as a catalyst is demonstrated.
Received 10th April 2017,
Accepted 5th July 2017
DOI: 10.1039/c7dt01279a
rsc.li/dalton
such tailored particles can have interesting applications in
optics, electronics and biomedical science. Consequently,
Introduction
1
3
Platinum group metal–chalcogen nanoparticles (NPs) are of chemical methods for the synthesis of a particular nano-phase
interest due to their potential for various applications in which may result in shapes unknown for them so far are worth
1
–4
5
catalysis,
organic molecular transformations,
DNA exploring.
The crystalline phases of palladium–sulfur NPs known so
graphic films, biological sensors, semiconducting electronic far are: Pd S, Pd S, Pd S , PdS, Pd S and PdS . However,
6
sequence recognition, recording films in optical discs, litho-
3
7
4
3
16
7
2.8
2
3
8
9
devices, hydrogen storage, information storage, and plasmo- palladium–selenium NPs exist in more crystalline phases. The
nics. The properties of NPs vary with their shape, size, com- common ones are: PdSe, Pd Se , Pd Se , Pd Se , Pd Se ,
position and crystallinity. The role of size and shape is very Pd2.5Se, Pd Se, Pd Se, Pd Se, Pd Se, Pd Se and PdSe . In a
1
0
1
7
15
34 11
7
4
7
2
1
1
14
3
7
4
4.5
8
2
important for the unique electronic, magnetic, catalytic, and typical method for their preparation two constituent elements
optical properties of such particles exploited in the booming are placed in an evacuated sealed tube and annealed at high
1
2
15
nano-industry. Therefore, the synthesis of the nano-particles temperature for many days. The mechanochemical method
1
6
with well-defined shape and size is of current importance, as using the reactants in the solid state is also used as an
alternative. The Pd S NPs used in H -assisted sulfidation
1
6
7
2
1
7
18
with H S, and lithography, are reported to be generally
spherical in shape. Flower shaped Pd16S NPs have not been
7
reported so far. Cubic, spherical and flower shaped
Pd Se NPs are known, but there is not any report on the
prismatic shape to the best of our knowledge. The Pd17Se15
2
1
9
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
2
0
21
20a
1
7
15
†
Electronic supplementary information (ESI) available: Single crystal data of P1,
L1, L2, C1 and C2; NMR, mass and PXRD of NPs. CCDC 1524520–1524524. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c7dt01279a
22
NPs have been explored for hydrogen evolution, counter
electrodes in dye sensitized solar cells, enhancement of
2
3
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2
4
20a
1
13
1
superconducting properties
and thin film formation.
are reported in ppm relative to TMS in H and C{ H} and
7
7
1
Some palladium(II) selenolates and thiolates on thermolysis in Me Se in the Se{ H} NMR spectra. IR spectra in the range of
2
an 1 : 1 mixture of oleic acid (OA) and 1-octadecene (ODE) give 4000–400 cm− were recorded on a Nicolet Protége 460 FT-IR
1
Pd17Se15 and a mixture of Pd16
S
7
(ca. 68%) and PdS (ca. 32%) spectrometer using KBr pellets. The elemental analyses were
2
5
NPs, respectively. There are very limited reports on the cata- carried out using a Perkin-Elmer 2400 Series II C, H, N analy-
2
1,26,27
lytic activity of Pd16
of Pd16 and Pd17Se15 NPs was noticed in Suzuki–Miyaura were performed by electron spray ionization (10 eV, 180 °C
coupling (SMC) reactions catalyzed by Pd-complexes of (N, S, source temperature) and using sodium formate as a calibrant
S
7
and Pd17Se15 NPs.
The formation zer. High-resolution mass spectral (HR-MS) measurements
S
7
−
−
3
C ) and (N, Se, C ) ligands, respectively. They were isolated on a Bruker MIcroTOF-Q II, taking samples in CH CN. The
2
6
and found active for SMC after annealing. The Pd17Se15 NPs X-ray diffraction data on single crystals of P1, L1, L2, C1 and
generated by a single source precursor method and grafted on C2 were collected on a Bruker AXS SMART Apex CCD diffract-
graphene oxide were found efficient for C–O coupling (Pd ometer using Mo-Kα (0.71073 Å) radiation at 298(2) K. Frames
2
1
loading: 1 mol%). For the efficiency of a metal based catalyst were collected by ω, φ, and 2θ-rotations using a full quadrant
its designing is very important. The organic ligand may have a data collection strategy (four domains each with 600 frames) at
direct bearing on catalytic efficiency of its complex with metal. 10 s per frame using SMART. The measured intensities were
2
The NPs obtained by thermolysis of a metal complex may have reduced to F and corrected for absorption using SADABS.
the influence of the precursor complex or its ligand. The Structure solution and refinement were carried out using the
investigations on these aspects of a monodentate R S or R Se SHELXTL package by direct methods. Non-hydrogen atoms
2
7
2
2
ligand, in which R has a functionality, have not been reported to were refined anisotropically. Powder X-ray diffraction (PXRD)
the best of our knowledge. Thus the aim of the present study is studies were made using a Bruker D8 advance diffractometer,
to design Pd catalysts (in molecular as well as in nano-form) using Ni-filtered Cu-Kα radiation (a scan speed of 1 s and scan
using 2,2′-thiobis(N,N-diphenylacetamide) (L1) and 2,2′-selenobis step of 0.02°). HR-TEM studies were made using an instrument
(
[
N,N-diphenyl-acetamide) (L2). These ligands form complexes, TECNAI G2 20 operated at 200 kV. The samples for HR-TEM
Pd(L1/L2) Cl ] (C1/C2) used as a single source precursor (SSP) were prepared by dispersing the NPs in ethanol by ultrasonic
for generating flower shaped Pd16 and prismatic Pd17Se15 NPs. treatment. A few drops of the dispersed sample were placed on
These phases are known but not in these two shapes. Thus a porous carbon film supported on a copper grid and dried in
NPs in the flower shape and Pd17Se15 in the prismatic air. The Bruker-AXS Energy Dispersive X-Ray (EDX) system
2
2
S
7
Pd16S
7
shape have been prepared for the first time. For catalysis of SMC (Model QuanTax 200) was used for SEM-EDX studies.
and C–O cross coupling reactions, C1 and C2 are more efficient
than the nano-phases derived from them. C1 and C2 can catalyze
SMC even at a Pd loading of the order 0.0001 mol%.
Synthesis of 2-bromo-N,N-diphenylacetamide (P1)
2
9
It is prepared earlier by carrying out a reaction between bro-
moacetyl bromide and diphenylamine in the presence NEt for
3
16 h. However, we are able to obtain it in a good yield in 1 h in
the absence of NEt . Our workup of the reaction is also some-
what different. The detailed procedure is as follows.
Bromoacetyl bromide (0.808 g, 4.0 mmol) solution in 10 mL
of dichloromethane was added to a solution of diphenylamine
3
Experimental section
Chemicals and reagents
Diphenylamine, bromoacetyl bromide, sulfur and selenium
powder, tri-n-octylphosphine (TOP), phenol, potassium car-
bonate, cesium carbonate, sodium acetate, sodium t-butoxide,
phenylboronic acid, 4-bromobenzaldehyde, 4-bromotoluene,
(
0.338 g, 2.0 mmol) made in 20 mL CH Cl and stirred at 0 °C.
2 2
The resulting mixture was stirred further for 1 h, washed with
dilute HCl (3 × 20 mL) and extracted with dichloromethane
1
-bromo-4-nitrobenzene,
4-bromobenzonitrile,
4-bromo-
(25 mL). The organic layer was washed with water (3 × 25 mL)
anisole, 4-bromoacetophenone and 4-bromobenzene were pro-
cured from Sigma-Aldrich (USA) and used without further puri-
fication. Chloroform, dichloromethane, diethylether, aceto-
nitrile, acetone, ethanol, toluene, dimethyl formamide and
dimethyl sulfoxide of Analytical Reagent (AR) grade were dried
and dried over anhydrous sodium sulfate. Its solvent was evapor-
ated under reduced pressure on a rotary evaporator. The result-
ing white solid was crystallized with a CHCl –diethylether (3 : 1)
3
mixture to afford colorless crystals of P1. M.p.: 108 °C. Anal.
calc. for C14
H12BrNO: C, 57.95; H, 4.17; N, 4.83. Found: C, 57.88;
2
8
1
by using standard procedures and distilled before use. The
H, 4.21; N, 4.79%. H NMR (CDCl , 25 °C, TMS); (δ, ppm): 3.83
3
1
1
3
1
progress of the catalytic reaction was monitored by H NMR
(2 H, s, CH
2
), 7.32 (10 H, s, Ar). C{ H} NMR (CDCl
3
, 25 °C,
spectroscopy. The products obtained in the two coupling reac-
TMS); (δ, ppm): 28.2 (CH
2
), 121.8, 126.2, 128.5, 129.2, 142.1
1
+
tions were isolated and authenticated by comparing their H
(ArC), 166.5 (CO). HR-MS [M + Na] (m/z) 311.9995; calc. value
2
1,26
NMR spectral data with the earlier reported ones.
for C14H12BrNNaO; 311.9983 (δ: 3.8 ppm). Earlier reported
29a
characterization data partially match with the present one.
Physical measurements
1
13
1
77
1
Synthesis of 2,2′-thiobis(N,N-diphenylacetamide) (L1)
The H, C{ H} and Se{ H} NMR spectra were recorded at
3
3
0
00.13, 75.47 and 57.24 MHz, respectively. using a Bruker It was also reported earlier but the sulfur source was com-
Spectrospin DPX 300 NMR spectrometer. The chemical shifts mercial Na S and the reaction was carried out in water. The
2
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1
3
1
work up was different from the present one and characteriz- (10 H, m, aryl). C{ H} NMR (CDCl
3
, 25 °C, TMS); (δ, ppm):
ation was limited to the UV spectrum. The present procedure 39.6, 41.9 (CH S), 126.2, 126.5, 128.5, 129.0, 130.1, 130.3,
2
−
1
is as follows.
141.0, 142.0 (aryl C), 166.0, 166.4 (CO). IR (KBr, νmax/cm ):
Sulfur powder (0.064 g, 2.0 mmol) was refluxed in 25 mL of 1668 (s; νC–O).
EtOH with stirring under a dry N atmosphere. Solid sodium
borohydride (0.1513 g, 4.0 mmol) was added to it, so that the
solution became yellowish green in color due to the formation
2
Synthesis of trans-bis[2,2′-selenobis(N,N-diphenylacetamide)]
palladium(II) chloride [PdCl (L2) ] (C2)
2
2
of Na S. 2-Bromo-N,N-diphenylacetamide (P1) (1.160 g, It was synthesized by using the procedure mentioned above for
2
4
mmol) dissolved in 10 mL of CHCl
the Na S solution generated in situ and the mixture was Yield: 0.49
refluxed for 4 h. Thereafter, the mixture was stirred overnight C H Cl N O PdSe : C, 57.18; H, 4.11; N, 4.76. Found: C,
3
was added dropwise to C1. Single crystals of C2 were also grown like those of C1.
2
g
(83%). M.p.: 162 °C. Anal. calc. for
5
6
48
2
4
4
2
1
at room temperature, poured into cold water (25 mL) and 57.16; H, 4.07; N, 4.80%. H NMR (CDCl
extracted with chloroform (3 × 25 mL). The extract was washed ppm): 3.95 (2 H, s, CH Se), 7.21–7.37 (10 H, m, aryl). C{ H}
with water (3 × 40 mL) and dried over anhydrous sodium NMR (CDCl , 25 °C, TMS); (δ, ppm): 35.0 (CH Se), 126.5, 128.6,
3
, 25 °C, TMS); (δ,
1
3
1
2
3
2
1
77
3
sulfate and concentrated up to 5 mL by removing the solvent 129.0, 130.0, 142.2 (aryl C), 166.8 (CO). Se{ H} NMR (CDCl ,
−1
under reduced pressure on a rotary evaporator. Diethyl ether 25 °C, Me
1 mL) was added to the concentrate to afford L1 as a white
solid. Single crystals of L1 were grown by slow evaporation of
its solution made in a dichloromethane–hexane mixture A mixture of tri-n-octylphosphine (TOP) (2.5 mL, 5.5 mmol)
10 : 1). Yield: 0.74 g (81%). M.p.: 124 °C. Anal. calc. for and complex C1 (0.541 g, 0.5 mmol) or C2 (0.588 g, 0.5 mmol)
S: C, 74.31; H, 5.35; N, 6.19. Found: C, 74.34; H, was heated at ∼280 °C for 2 h with continuous stirring in a
2
Se): δ 300.62 ppm. IR (KBr, νmax/cm ): 1660 (s; νC–O).
(
Synthesis of palladium–sulfur/selenium nanoparticles
(
28 24 2 2
C H N O
1
5
3
.31; N, 6.18%. H NMR (CDCl , 25 °C, TMS); (δ, ppm): 3.44 (2 three necked flask under a nitrogen atmosphere. The color of
1
3
1
H, s, CH S), 7.33 (10 H, s, aryl). C{ H} NMR (CDCl , 25 °C, the mixture changed from yellow to grayish-black within
2
3
TMS); (δ, ppm): 35.0 (CH
2
S), 126.5, 129.0, 129.8, 142.5 (aryl C), 10 min and the mixture finally became a black suspension
−
1
1
69.1 (CO). IR (KBr, νmax/cm ): 1679 (s; νC–O). HR-MS [M + after 2 h. It was allowed to cool down to room temperature and
+
Na] (m/z) 475.145083; calc. value for C H N NaO S; 20 mL of dry acetone was added. The black NPs were separated
2
8
24
2
2
4
75.145070 (δ: 0.4 ppm).
by centrifugation, washed with dry acetone (3 × 20 mL) and
dried in vacuo. The bulk analyses of these NPs are given in ESI
Tables S1 and S2.†
Synthesis of 2,2′-selenobis(N,N-diphenylacetamide) (L2)
The above mentioned method for L1 was followed except that
selenium powder (0.158 g, 2.0 mmol) was replaced with sulfur.
Single crystals of L2 were grown by slow evaporation of its solu-
General method for the Suzuki–Miyaura coupling (SMC)
reaction
tion made in a dichloromethane–hexane mixture (10 : 1). Aryl bromide (1.0 mmol), phenylboronic acid (1.5 mmol),
Yield: 0.78 g (78%). M.p.: 136 °C. Anal. calc. for C28 Se: CO (2.0 mmol), DMF + H O (3.0 + 1.0 mL) and NPs (equi-
C, 67.33; H, 4.84; N, 5.61. Found: C, 67.38; H, 4.81; N, 5.59%. valent to 0.5 mol% of Pd) or complex (0.0001 to 0.01 mol% of
H
24
N
2
O
2
K
2
3
2
1
H NMR (CDCl
.36 (10 H, s, aryl). C{ H} NMR (CDCl
3
, 25 °C, TMS); (δ, ppm): 3.48 (2 H, s, CH
2
Se), Pd) were placed in an oven-dried round bottom flask and the
1
3
1
7
3
, 25 °C, TMS); (δ, mixture was stirred at 100 °C under aerobic conditions. The
1
ppm): 25.4 (CH Se), 125.7, 127.4, 128.1, 129.1, 141.9 (aryl C), progress of the reaction was monitored by TLC/ H NMR spec-
2
7
7
1
1
69.1 (CO).
Se{ H} NMR (CDCl
3
,
2
25 °C, Me Se): troscopy. When the maximum conversion of ArBr into the
δ 237.58 ppm. IR (KBr, νmax/cm− ): 1657 (s; νC–O). HR-MS product took place, the mixture was allowed to cool down to
1
+
[M + Na] (m/z) 523.0877; calc. value for C H N NaO Se; room temperature. It was extracted with diethyl ether (2 ×
2
8
24
2
2
5
23.0897 (δ: 3.8 ppm).
20 mL). The extract was washed thoroughly with water and
dried over anhydrous Na SO . The solvent from the extract was
completely evaporated on a rotary evaporator. The coupled
2
4
Synthesis of trans-bis[2,2′-thiobis(N,N-diphenylacetamide)]
palladium(II) chloride [PdCl (L1) ] (C1)
2
2
product as a residue was purified by column chromatography
1
A solution of Na
2
[PdCl
4
] (0.147 g, 0.5 mmol) made in an on silica gel and authenticated by comparing its H NMR spec-
ethanol–water mixture (5 : 1) was added to a solution of L1 trum (Fig. S29–S35 in the ESI†) with the report in the
2
6
(
0.453 g, 1 mmol) prepared in 20 mL of ethanol with vigorous literature.
stirring at room temperature for 4 h. The resulting yellow pre-
cipitate of C1 was filtered and recrystallized from
: 1 mixture of acetonitrile and methanol. Single crystals of C1 A round bottom flask was charged with aryl bromide
were grown by slow evaporation of its solution made in a (1.0 mmol), phenol (1.1 mmol), K CO (2.0 mmol), DMSO
: 1 mixture of acetonitrile and methanol. Yield: 0.42 g (78%). (4.0 mL) and NPs (equivalent to 0.5 mol% of Pd) or complex
M.p.: 146 °C. Anal. calc. for C56 PdS : C, 62.14; H, (0.1 mol% of Pd) and the mixture was stirred at 100 °C under
.47; N, 5.18. Found: C, 62.16; H, 4.43; N, 5.20%. H NMR aerobic conditions. On observing maximum conversion of
General method for C–O coupling reaction
a
1
2
3
1
H
48Cl
2
N
4
O
4
2
1
4
(
1
CDCl , 25 °C, TMS); (δ, ppm): 3.91 (2 H, s, CH S), 7.07–7.18 ArBr into the coupled product by TLC/ H NMR spectroscopy,
3
2
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the reaction mixture was cooled down to room temperature. It
was extracted with ethylacetate (2 × 20 mL), washed with water
and dried over anhydrous Na SO . The solvent from the extract
2 4
was completely evaporated on a rotary evaporator to obtain the
coupled product as a residue, which was purified by column
1
chromatography on silica gel and subjected to H NMR spec-
tral analysis (Fig. S36–S41 in the ESI†) as described for Suzuki–
2
1
Miyaura coupling.
Hg poisoning test
Hg (Hg : Pd :: 400 : 1) was taken in a reaction flask before the
addition of reactants. Thereafter, 4-bromobenzaldehyde,
phenylboronic acid or phenol and Pd S /Pd Se NPs (0.5 mol%
16
7
17 15
of Pd) or C1/C2 (0.01 mol%) were added and the reaction was
carried out under optimum conditions. With the NPs the yield of
the coupled product was 7/9% and 4/6%, respectively, for SMC
and C–O coupling, implying that the reaction was highly hetero-
geneous. 95/91% and 89/70% conversions were observed with
C1/C2 for SMC and C–O coupling reactions, respectively. Thus
catalysis by C1 and C2 is highly homogeneous.
7
Scheme 1 Synthesis of L1, L2, C1, C2, Pd16S and Pd17Se15 NPs.
2 2
NMR spectra of C1 and C2, a singlet of CH S/CH Se observed
PPh poisoning test
3
at δ 3.91/3.95 ppm was found deshielded by ∼0.5 ppm com-
pared to the corresponding signal of the ligand L1/L2. This
The coupling reaction of 4-bromobenzaldehyde with phenyl-
boronic acid or phenol using Pd S /Pd Se NPs (0.5 mol% of
1
supports the coordination of S/Se with Pd. In the H NMR
16
7
17 15
spectrum of C1/C2 the signals observed at δ 1.1 and 4.2 ppm
are due to weakly coordinated ethanol, and they have charac-
teristic patterns as well.
Pd) or C1/C2 (0.01 mol%) as a catalyst was carried out in the
presence of PPh (10 mol%) under optimum conditions. The
3
amount of the coupled product was insignificant in both the
coupling reactions catalyzed by NPs. The conversion with C1/C2
was 93/90% and 90/69% for SMC and C–O coupling reactions,
respectively. Thus the catalytic process is highly heterogeneous
with NPs and homogeneous with the two molecular complexes.
1
3
1
In the C{ H} NMR spectrum of L1, the signal of CH
δ 34.95 ppm) is deshielded in comparison with that of the pre-
cursor, Ph NCOCH Br (δ 28.02 ppm) while in the spectrum of
L2, the signal of CH Se (appearing at δ 25.42 ppm with Se
2
S
(
2
2
2
satellites at 25.0 and 25.9 ppm) is shielded compared to those
of P1 and L1. In the spectra of complexes C1 and C2 the signal
of methylene carbon was observed at δ 39.59, 41.89 and
Results and discussion
35.00 ppm, respectively, and deshielded compared to those of
The syntheses of ligands L1 and L2 and their Pd(II) complexes the corresponding ligands due to their ligation with Pd via S
1
3
C1 and C2 are summarized in Scheme 1. The thermolysis of or Se. In the C NMR spectrum of L1 there are four signals for
complexes to NPs is also summarized in this scheme.
the Ph rings whereas for L2 there are five signals. It may be
C1 and C2 gave NPs of different shapes when thermolyzed attributed to the presence of the amide Ph N(+)vC–O(−)
2
under identical conditions. Probably this was resulted due to resonant form, which makes the two Ph rings non-equivalent
1
3
the difference in the Pd–S and Pd–Se bond strengths, non- and the signals broad. The C NMR spectra of C1 and C2 also
covalent interactions in the crystals of C1 and C2 and sizes of have multiple signals due to their inequivalent Ph groups. The
1
3
S and Se.
carbonyl group appears in the C NMR spectra of L1 and L2
L1 and L2 are soluble in chloroform, dichloromethane and at δ 169.10 and 169.05 ppm, respectively, deshielded a bit com-
acetonitrile. In DMF and DMSO the solubility of complexes C1 pared to that of P1 (δ 166.50 ppm). In the spectra of complexes
and C2 was good but in chloroform, dichloromethane and C1 and C2 this carbonyl group appears at δ 166.03, 166.41 and
acetonitrile it was only moderate. They were sparingly soluble 166.82 ppm, respectively, shielded a bit compared to those of
in ethanol, methanol and THF.
their free ligands. Both ligand L2 and its complex C2 show a
single signal in their Se{ H} NMR spectrum, indicating the
presence of only one Se containing species in their solutions.
7
7
1
NMR spectra
1
13
1
77
1
The H, C{ H} and Se{ H} NMR spectra of P1, L1, L2, C1 The signal in the spectrum of L2 (δ 237.6 ppm) is deshielded
and C2 recorded in CDCl
and characteristic patterns were upon the formation of C2 (δ 300.4 ppm) due to the coordi-
3
found (ESI Fig. S1–S15†). In the proton NMR spectra of L1 and nation of L2 with Pd via Se and quaternization of the Se
L2 methylene protons appear as a singlet at δ 3.44 and centre. In the mass spectra of precursors P1, L1, and L2 (ESI,
3
.48 ppm, respectively, shielded by ∼0.4 ppm compared to Fig. S3, S6 and S12†), the peaks appearing at m/z 311.9995,
1
+
those of the precursor Ph NCOCH Br, P1 (δ 3.86 ppm). In H 475.145083 and 523.0877, respectively, are due to [M + Na] .
2
2
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Crystal structures
Single crystal structures of P1, L1, L2, C1 and C2 were deter-
mined by X-ray diffraction. Their crystal data, structure refine-
ment parameters, selected bond lengths/angles and intera-
tomic distances of non-covalent interactions are given in the
ESI (Tables S3–S5†). The supramolecular patterns due to sec-
ondary interactions are also shown in the ESI (Fig. S16–S21†).
The ORTEP diagrams (50% probability of ellipsoids) of P1, L1,
L2, C1 and C2 captioned with the geometrical parameters rele-
vant to the primary geometry around S, Se and Pd atoms are
shown in Fig. 1–5. H atoms are omitted for clarity.
Fig. 4 Molecular structure of C1. Selected bond lengths (Å): Pd(1)–S(1)
In L1 and L2 the geometry around the sulfur and selenium
atoms is ‘V’ shaped. The C(14)–S(1)–C(14) and C(14)–Se(1)–
C(15) bond angles, 102.1(3) and 97.0(2)°, respectively, are
2
.324(2); Pd(1)–Cl(1) 2.289(2); S(1)–C(14) 1.819(6); S(1)–C(15) 1.830(5).
Selected bond angles (°): S(1)–Pd(1)–S(1) 180.00; Cl(1)–Pd(1)–Cl(1)
80.00; S(1)–Pd(1)–Cl(1) 85.21(6); S(1)–Pd(1)–Cl(1) 94.79(6); C(14)–S(1)–
1
C(15) 98.43(27). Solvent molecule is omitted for clarity.
Fig. 1 Molecular structure of P1. Selected bond lengths (Å): C(13)–N(1)
1
(
1
.376(5); C(13)–O(1) 1.215(6); C(14)–Br(1) 1.941(4). Selected bond angles
°): C(6)–N(1)–C(7) 116.1(3); C(6)–N(1)–C(13) 121.2(3); C(7)–N(1)–C(13)
22.6(3).
Fig. 5 Molecular structure of C2. Selected bond lengths (Å): Pd(1)–
Se(1) 2.4150(6); Pd(1)–Cl(1) 2.2736(15); Se(1)–C(14) 1.955(5); Se(1)–C(15)
1.975(6). Selected bond angles (°): Se(1)–Pd(1)–Se(1) 180.00; Cl(1)–
Pd(1)–Cl(1) 180.00; Se(1)–Pd(1)–Cl(1) 87.18(5); Se(1)–Pd(1)–Cl(1) 92.82(5);
C(14)–Se(1)–C(15) 97.62(23). Solvent molecule is omitted for clarity.
3
1
consistent with the reported values. The coordination geo-
metry around palladium in both C1 and C2 is distorted square
planar. Pd(II) in C1 and C2 is coordinated with two pairs of
donor atoms. One is comprised of S/Se (trans to each other)
atoms and the other of trans Cl. In C1 Pd–Cl and Pd–S are
Fig. 2 Molecular structure of L1. Selected bond lengths (Å): S(1)–C(14)
2.2892(15) and 2.3235(14) Å respectively. These values are con-
#
1
1
1
.781(5); S(1)–O(1) 3.027(3). Selected bond angles (°): C(14)–S(1)–C(14)
02.1(3).
sistent with those reported earlier (Pd–S: 2.316(1)–2.325(1) Å;
Pd–Cl: 2.2859(8)–2.303(1) Å) for square planar trans-PdCl L
2
2
3
2
type complexes (L = S(n-Bu)(p-MePh), S(Me)
2
2
, SMePh, SPh ,
and S(n-Bu)(p-t-BuPh)). The Pd–S bond length of C1 is larger
than those reported for Pd complexes of bidentate (S, N) and
3
1
(
S, S) ligands i.e. [2.2727(14)–2.285(14) Å]. The Pd–Cl bond
distances in the present complexes are somewhat smaller than
the values reported [2.2968(14)–2.330(13) Å] for complexes of
Pd with bidentate ligands in which two Cl atoms are cis to
3
1
each other. These differences are in expected lines in view of
the difference in the trans influences experienced by the bonds
in C1 and C2 and the chelates of (S, N) and (S, S) ligands.
The Pd–Se and Pd–Cl bond distances, 2.4151(7) and 2.2736(15)
Å, respectively, of C2 are consistent with the values reported
Fig. 3 Molecular structure of L2. Selected bond lengths (Å): Se(1)–C(14)
.950(3); Se(1)–C(15) 1.959(5). Selected bond angles (°): C(14)–Se(1)–
C(15) 97.0(2).
1
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earlier for Pd–Se and Pd–Cl bond lengths [2.4200(13)–2.439(2)
and 2.263(6)–2.2989(12) Å, respectively] of trans-[PdCl L ]
2
2
3
2
systems
(L
=
(C
SiCH
4
H
3
S)SeMe, (SePh
2
), (C
6
H
2
Me -2,4,6)
-2,4,6)SeCH COMe
3
i
SeCH COMe, (Me
2
3
2
SeMe), (C
6
2
H Pr
3
2
and SeEt ). The Pd–Se bond length of C2 is larger than those
2
observed in the case of complexes of (Se, N) and (Se, Se)
3
1
ligands, [2.3693(8)–2.3991(15) Å], as expected on the basis of
the difference in the trans influence. The Pd–Cl bond distances
of C2 are shorter than some of its values reported earlier
31
[2.2799(15)–2.318(3) Å] for the complexes of bidentate ligands.
7
Fig. 7 PXRD patterns of (a) Pd16S and (b) Pd17Se15 NPs.
Characterization of Pd16
The molecular complexes C1 and C2 were used as a single
source precursor to synthesize Pd16 and Pd17Se15 NPs,
respectively, in a one-step process. The thermolysis of mixtures
of the complex C1/C2 with tri-n-octylphosphine (TOP) (TOP:
C1/C2 ratio = 11 : 1) at ∼280 °C (Scheme 1) under a dry nitro-
gen atmosphere resulted in these NPs. They were characterized
by powder X-ray diffraction (PXRD), SEM-EDX, high-resolution
transmission electron microscopy (HR-TEM).
7
S and Pd17Se15 NPs
shape of Pd16
by HR-TEM (Fig. 8). The Pd16
and the Pd Se ones prismatic. The average sizes (Fig. 9) of
S
7
and Pd17Se15 nanoparticles were determined
S
7
NPs were found flower shaped
S
7
17 15
Pd16
S
7
and Pd17Se15 NPs are ∼31–40 and ∼36–45 nm,
respectively.
7
The SEM patterns of Pd16S and Pd17Se15 nanoparticles are
shown in Fig. 6 and SEM-EDX in Fig. S22 and S23 (ESI†). The
Pd and S peaks are exhibited in SEM-EDX of NPs obtained
from C1 and of Pd and Se in the case of NPs derived from C2.
The ratio of Pd and S was ∼16 : 7 in the first case and of Pd
and Se was ∼17 : 15 in the second case. The PXRD pattern of
the present NPs (Fig. 7) matches with that of the cubic phase
of Pd S given in JCPDS 65-3155 with the lattice parameter a =
1
6 7
2
5,26a
8
.954 as well as with earlier reports.
Similarly, the PXRD
of the present Pd17Se15 nano-phase (Fig. 7) is consistent with
the one reported in JCPDS 73-1424 with lattice parameters of
2
1,26b
a = 10.60 in the literature.
The sharp nature of the PXRD peaks of the present NPs
indicates their high crystallinity and large size. The size and
7
Fig. 8 TEM Images of (a) & (b) flower shaped Pd16S NPs (scale bar
50 nm). (c) & (d) Prismatic Pd17Se15 NPs (scale bar 50 nm).
Fig. 6 SEM Images (a) and (b) of Pd16
S
7
NPs; (c) and (d) of Pd17Se15 NPs.
7
Fig. 9 Size distribution of (a) Pd16S and (b) Pd17Se15 NPs.
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Catalytic applications
obtained in 3 h using a DMF and water mixture (3 : 1) as a
solvent, 2.0 mmol of K CO as a base and Pd S /Pd Se NPs
2
3
16
7
17 15
Suzuki–Miyaura coupling (SMC) with flower shaped Pd16S
7
equivalent to 0.5 mol% of Pd as a catalyst. The other combi-
nations of solvents and bases gave low conversion. Not much
change in the yield was observed on increasing the mol% of
Pd (ESI Table S6,† entry 11).
and prismatic Pd17Se15 NPs. Palladium-catalyzed cross-coup-
ling reactions constitute an important methodology for the for-
3
3
mation of carbon–carbon or carbon–heteroatom bonds.
3
4
Among them, Suzuki–Miyaura coupling is important for
carbon–carbon bond formation. It involves coupling of organo-
boronates with aryl halides under basic conditions in various
organic solvents and their mixtures with water. In pure water
The time profile of coupling of 4-bromobenzaldehyde
(
1 mmol) with phenylboronic acid (1.5 mmol) is shown in
Fig. 10. The % conversion of reactant to product in the case of
7
Pd16S NPs increases from 35 to 61, when the reaction time is
3
5
also SMC is known. Applications of present Pd S and
1
6 7
increased from 0.5 to 1 h. In 3 h it becomes 90% and ∼100%
in 6 h but in the case of Pd17Se15 NPs the conversion is found
to increase from 39 to 96% with the reaction time from 0.5 to
Pd17Se15 NPs as a catalyst for Suzuki–Miyaura coupling were
studied. The coupling between 4-bromobenzaldehyde and
phenyl-boronic acid was used for the optimization of reaction
3
h and becomes ∼100% in 6 h. As the conversion was ≥90%
conditions (see ESI Table S6†) for catalysis with Pd16
7
S /
in 3 h, for exploring the scope of these NPs the reaction time
used was generally 3 h.
Pd Se NPs. DMF, EtOH, toluene and water were employed
1
7
15
as solvents and K CO , Cs CO and CH ONa as bases. At
2
3
2
3
3
The scope of Pd S /Pd Se NPs as a catalyst in the
1
6
7
17 15
1
00 °C (under aerobic conditions) good conversion was
Suzuki–Miyaura coupling reaction was investigated using
various aryl bromides (Table 1). The very poor conversion/no
conversion was the observation in the coupling of ArCl. The
results given in Table 1 reflect that Suzuki–Miyaura coupling
of aryl bromides can be successfully achieved when Pd16
7
S and
Pd Se NPs are used as catalysts. In the case of ArBr species
1
7
15
having electron-withdrawing groups a good yield (Table 1,
entries 1, 2, 4 and 6) results in less time. Aryl bromides having
electron-donating groups (Table 1, entries 3, 5 and 7) show
less reactivity, for example 4-bromoanisole gives 68% yield of
4
-methoxybiphenyl in 3 h using 0.5 mol% of Pd17Se15 NPs as a
catalyst. On increasing the reaction time from 3 h to 12 h,
-methoxybiphenyl was isolated only in 79% yield. Pd17Se15
NPs show better catalytic activity compared to Pd16 NPs. The
good yield in short time with a catalyst loading equivalent to
Fig. 10 Time profile of SMC of 4-bromobenzaldehyde with phenyl- 0.5 mol% of Pd makes the present NPs attractive. For the pro-
4
S
7
boronic acid.
posed mechanism of SMC with these NPs see ESI Fig S25.†
Table 1 Suzuki–Miyaura coupling reactions catalyzed by Pd16
S
7
and Pd17Se15 NPs
a
a
Pd16
S
7
NPs
Pd17Se15 NPs
b
b
Entry no.
1
Aryl bromide
Mol%
0.5
T (h)
Yield (%)
TON/TOF
180/60
Yield (%)
TON/TOF
192/64
3
90
96
88
78
93
68
95
73
2
3
4
5
6
7
0.5
0.5
0.5
0.5
0.5
0.5
3
3
3
3
3
3
83
70
85
62
91
65
166/55
140/47
170/57
124/41
182/61
130/43
176/59
156/52
186/62
136/45
190/63
146/49
a
Reaction conditions: Aryl bromide (1.0 mmol), phenylboronic acid (1.5 mmol), K
Isolated yield (%).
2
CO
3
(2.0 mmol), solvent: DMF : H
2
O (3 : 1 mL), temp. 100 °C.
b
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Earlier, catalysis of Suzuki C–C coupling reactions with catalytic performance compared to Pd16
Dalton Transactions
S
7
NPs. The perform-
2
6
Pd S and Pd Se NPs required 15–16 h for maximum con- ance of Pd Se NPs for C–O coupling has been found a bit
1
6
7
17 15
17 15
version, implying that the present procedure for generating lower compared to the performance shown for Suzuki–Miyaura
these NPs imparts them better catalytic activity. The present coupling. The time profile of C–O coupling is shown in Fig. 11.
Pd Se /Pd S NPs show better catalytic performance for SMC 80–85% conversion occurs in 3 h with both Pd16S7 and
1
7
15
16 7
than some Pd NP based catalysts. For the Cu
nanocomposite
0
1
4
Pd@AFGNS Pd17Se15 NPs. It improves marginally on carrying out the reac-
the required loading is equivalent to tion for more time.
.66 mol% of Pd and the time in which 1-bromobenzene or The results given in Table 2 reflect that the C–O coupling of
-iodobenzene reacts with phenylboronic acid to give a good aryl bromides can be successfully achieved using Pd16 or
Pd17Se15 NPs as a catalyst. The graphene oxide supported
NPs as a catalyst (Pd loading Pd Se NPs were found efficient for C–O coupling reactions
3
6a
S
7
yield is 6 h.
On using Pd@CQD@Fe O
3
6b
3
4
17 15
2
1
equivalent to 0.2 mol%) the SMC optimum reaction time is from when their loading was equivalent to 1.0 mol% Pd. The
.5 to 36 h. 4-Phenylbenzaldehyde was obtained by SMC in 92% optimum loading of the present Pd16 /Pd17Se15 NPs for the
as a catalyst C–O coupling reaction is its half only. Pd (dba) in the pres-
0
S
7
36c
yield in 4 h using Pd-NPs/P. a. kurdica gum
0.1 mol% Pd loading). In our case 90–96% of the product was ence of BrettPhos (2-(dicyclohexyl phosphino)3,6-dimethoxy-
obtained in 3 h, of course with a higher Pd loading. The catalysis 2′,4′,6′-triisopropyl-1,1′-biphenyl) catalyzes C–O coupling but
2
3
(
0
36d
with Pd /SBA-15
Pd loading: 0.05 mol%) and is better than the present NPs.
C–O coupling with flower shaped Pd16 and prismatic
Pd Se nanoparticles. The reaction between 4-bromobenzal-
takes only 1 h to obtain 90% yield in SMC the optimum catalyst loading is equivalent to 0.5–1.0 mol% Pd
(
S
7
1
7
15
dehyde and phenol was used for optimizing the catalysis of
C–O coupling with Pd16 /Pd17Se15 NPs (see ESI Table S7 for
optimization and Fig. 26† for mechanism). DMF, EtOH and
DMSO were employed as solvents and K CO , Cs CO and
NaO Bu as bases in the presence of Pd16 /Pd17Se15 NPs
loading equivalent to 0.5 mol% of Pd) as a catalyst at ∼100 °C
under aerobic conditions. The conversion was the best when
DMSO was used as a solvent and K CO as a base (Table S7†).
The scope of Pd S /Pd Se NPs as a catalyst for C–O coup-
S
7
2
3
2
3
t
S
7
(
2
3
1
6
7
17 15
ling under optimum conditions was investigated using various
aryl bromides (Table 2). The bromides having electron-donat-
ing groups (Table 2, entries 3, 5 and 7) show less reactivity
compared to those having electron-withdrawing groups
Fig. 11 Time profile of C–O coupling reaction of 4-bromobenzalde-
(
Table 2, entries 1, 2, 4 and 6). The Pd17Se15 NPs show better hyde with phenol.
Table 2 C–O coupling reactions catalyzed by Pd16
S
7
and Pd17Se15 NPs
a
a
Pd16
S
7
NPs
Pd17Se15 NPs
b
b
Entry no.
1
Aryl bromide
Mol%
T (h)
Yield (%)
TON/TOF
160/53
Yield (%)
TON/TOF
170/57
0.5
0.5
0.5
0.5
0.5
0.5
0.5
3
80
85
96
75
82
60
90
70
2
3
4
5
6
7
3
3
3
3
3
3
92
69
76
58
88
64
184/61
138/46
152/51
116/39
176/59
128/43
192/64
150/50
164/55
120/40
180/60
140/47
a
b
Reaction conditions: Aryl bromide (1.0 mmol), phenol (1.1 mmol), K
CO
2 3
(2.0 mmol), solvent: DMSO (4 mL), temp. 100 °C. Isolated yield (%).
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in the presence of Cs
2
CO
3
as a base. The reaction time is in phenylboronic acid in the presence of K
2
CO
3
base was carried
3
8a
the range of 1.5–24 h for good conversion.
C–O coupling with 2 mol% of Pd(OAc) in the presence of the (90/96% with Pd16
di-tert-butyl phosphine ligand gives good yield in 45 h using centrifugation, washed with acetone (2 × 25 mL), dried and
The catalysis of out using the optimized procedure. After complete conversion
2
7
S /Pd17Se15), the catalyst was recovered by
3
7b
toluene as a solvent and Cs CO as a base.
Palladium NPs reused with a fresh lot of reactants. On carrying out the coup-
explored for the C–O coupling reactions are not in our knowl- ling reaction under optimum conditions for 3 h the conversion
edge. Thus the present Pd16 and Pd17Se15 NP based pro- to the coupled product was found to be 78 and 82% for Pd16
cedure is competitive or better than the many previously and Pd Se NPs, respectively. In the third cycle executed in a
2
3
S
7
S
7
17
15
3
7,38
reported protocols.
similar way the conversion dropped to 65 and 70%, respectively
Fig. 12), and in the fourth cycle it was found to be further
poorer (Fig. 12). The TEM image after the fourth cycle shown in
The recyclability of Pd S and Pd Se NPs was checked for Fig. S24 (ESI†) indicates agglomeration of the catalysts and thus
(
Recyclability of catalyst
1
6
7
17 15
the SMC reaction. The reaction of 4-bromobenzaldehyde with a decrease in the efficiency and further recyclability.
Suzuki–Miyaura coupling reactions catalyzed by C1 and C2.
The complexes C1 and C2 were studied as a catalyst for SMC.
Upon reaction between 4-bromobenzaldehyde and phenylboro-
nic acid, DMF + H O (3 : 1) was found to be the best solvent
2
2 3
and K CO the best base (Table S8†). The conversion of
4-bromobenzaldehyde to the product 4-phenylbenzaldehyde
was ∼100% at 100 °C in 2 h when 0.001 mol% of C1 was used
as a catalyst. 95% conversion was achieved in 3 h on using C2
as a catalyst. In the case of C1, 45% yield was obtained in 2 h
when the catalyst loading was 0.0001 mol% and the yield
increased to 68% when the reaction was carried out for 12 h.
The Suzuki–Miyaura coupling of phenylboronic acid with a
variety of aryl bromides was examined using C1/C2 as a cata-
lyst and the results are shown in Table 3. The results reflect
that in the case of aryl bromides bearing electron withdrawing
7
Fig. 12 Recycling of Pd16S /Pd17Se15 NPs.
Table 3 Suzuki–Miyaura coupling reactions catalyzed by C1 and C2
a
a
Catalyst C1
Catalyst C2
b
b
Entry no.
1
Aryl bromide
Mol%
0.1
T (h)
Yield %
TON/TOF
T (h)
Yield %
TON/TOF
3
3
3
4
3
3
1
2
2
2
12
2
2
2
2
3
3
2
2
2
100
100
100
45
1.0 × 10 /1.0 × 10
1
3
3
3
12
3
3
3
3
3
3
3
3
3
100
100
98
51
72
100
100
98
70
80
75
36
90
82
1.0 × 10 /1.0 × 10
4
4
3
4
0
0
0
0
.01
1.0 × 10 /5.0 × 10
1.0 × 10 /3.33 × 10
5
4
.001
.0001
.0001
1.0 × 10 /5.0 × 10
9.8 × 10 /3.27 × 10
5
5
5
5
4
4.5 × 10 /2.25 × 10
5.1 × 10 /1.7 × 10
5
4
2
3
4
5
68
6.8 × 10 /5.7 × 10
7.2 × 10 /6.0 × 10
3
3
2
3
4
2
3
0.1
100
100
100
75
95
90
48
95
88
1.0 × 10 /5.0 × 10
1.0 × 10 /3.33 × 10
4
4
0
0
0
.01
.001
.0001
1.0 × 10 /5.0 × 10
1.0 × 10 /3.33 × 10
5
4
1.0 × 10 /5.0 × 10
9.8 × 10 /3.27 × 10
5
5
2
5
5
7.5 × 10 /3.75 × 10
7.0 × 10 /2.3 × 10
2
2
2
0.1
9.5 × 10 /3.17 × 10
8.0 × 10 /2.67 × 10
3
3
3
3
3
3
2
0
0
.01
.005
9.0 × 10 /3.0 × 10
7.5 × 10 /2.5 × 10
3
3
9.6 × 10 /4.8 × 10
7.2 × 10 /2.4 × 10
2
2
2
4
5
6
7
8
0.1
9.5 × 10 /4.75 × 10
9.0 × 10 /3.0 × 10
3
3
3
3
2
0.01
8.8 × 10 /4.4 × 10
8.2 × 10 /2.73 × 10
2
2
2
0.1
2
2
98
90
9.8 × 10 /4.9 × 10
3
3
68
60
6.8 × 10 /2.27 × 10
3
3
3
3
0.01
9.0 × 10 /4.5 × 10
6.0 × 10 /20 × 10
3
2
3
2
4
0.1
2
2
100
98
1.0 × 10 /5.0 × 10
3
3
100
90
1.0 × 10 /3.33 × 10
3
3
3
3
0.01
9.8 × 10 /4.9 × 10
9.0 × 10 /3.0 × 10
4
4
3
0.01
0.5
100
1.0 × 10 /2.0 × 10
0.5
98
9.8 × 10 /1.96 × 10
3
3
3
3
0.01
2
90
9.0 × 10 /4.5 × 10
3
75
7.5 × 10 /2.5 × 10
a
Reaction conditions: Aryl bromide (1.0 mmol), phenylboronic acid (1.5 mmol), K
CO
2 3
(2.0 mmol), solvent: DMF : H
2
O (3 mL : 1 mL),
b
temp. 100 °C. Isolated yield (%).
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groups the yield of SMC is good in less time as compared to 1 mol% and for a good yield 2 to 24 h of reaction time is
2
6a
−
those having electron donating groups. However, attempts to needed.
For palladium complexes of the (N, Se, C ) ligand
catalyze SMC of ArCl with the present catalysts did not the optimum catalyst loading is 0.001 to 0.1 mol% and the
2
6b
succeed. The results given in Table 3 suggest that C1 performs reaction time is 10 to 39 h.
The palladium complexes of
as a catalyst somewhat better than C2. Further complexes bidentate ligands of (S, S), (S, Se) and (S, Te) types show good
appear to be more catalytically active than NPs derived from performance for SMC in 2 to 4 h, using 0.002 to 1 mol%
2
6c
them. With 0.001 mol% loading of C1 and C2 as a catalyst up loading of the catalyst.
Palladium pincer complexes of
to ∼100% yield was achieved while with 0.0001 mol% loading, N-heterocyclic carbenes execute SMC in water in 3 h but with a
2
6d
moderate yields were achieved (Table 3, entries 1 and 2). C1 catalyst loading of 0.005 mol%. The optimum loading of C1
and C2 as a catalyst at a loading of 0.0001 to 0.001 mol% of Pd and C2 is lower than all these species and a good yield results
2
6
show better performance as compared to the earlier reports,
i.e. with a Pd loading of 0.001 mol% the reaction time was gen-
in 2–3 h only.
The time profile of SMC of 4-bromobenzaldehyde with phe-
erally more than 12 h whereas it was 2 h for C1 and 3 h for C2. nylboronic acid was carried out under the optimum conditions
C1 and C2 appear to be better than palladacycles known for as shown in Fig. 13. The yield becomes 72% after 20 min,
−
their performance for SMC of ArBr. Palladacycles of the (C , N, increases to 94% in 100 min, and becomes ∼100% after 2 h.
S) ligand catalyze SMC when the Pd loading is 0.001 to To know whether the catalyst remains alive after one cycle of
coupling reaction, a fresh lot of reactants, 4-bromobenzalde-
hyde (1 mol%), phenylboronic acid (1.5 mol%) and 2.0 mmol
of K CO were added and the reaction was carried out under
2 3
optimum conditions for 2 h. The addition of fresh reactants
was repeated after completion of the coupling and the results
shown in Fig. S27 of the ESI,† suggest that a significant
amount of catalyst remains alive even after five cycles. Hg and
triphenylphosphine poisoning tests were carried out. In both
the poisoning tests the cross coupled product was obtained in
good yields (C1/C2; 95/91% for Hg and 93/90% for PPh3)
which implies that catalysis is homogeneous.
C–O coupling reactions with catalysts, C1 and C2. The cata-
lysis with complexes C1 and C2 of C–O coupling reactions was
studied (Table 4). C1 appears to perform better than C2.
Fig. 13 Time profile of SMC of 4-bromobenzaldehyde (1 mmol) cata- During optimization (Table S9 of the ESI†), it was found that
lyzed by C1.
the best yield of the C–O coupling product was obtained in
Table 4 C–O coupling reactions catalyzed by C1 and C2
a
a
Catalyst C1
Catalyst C2
b
b
Entry no.
1
Aryl bromide
Mol%
0.1
T (h)
Yield %
TON/TOF
950/317
T (h)
Yield %
TON/TOF
720/240
3
95
98
78
75
63
85
98
70
3
72
85
70
68
55
78
95
68
2
3
4
5
6
7
8
0.1
0.1
0.1
0.1
0.1
0.1
0.1
3
980/327
780/260
750/250
630/210
850/283
980/1960
700/233
3
850/283
700/233
680/226
550/183
780/260
950/1900
680/226
3
3
3
3
3
3
3
3
0.5
3
0.5
3
a
b 1
2 3
Reaction conditions: Aryl bromide (1.0 mmol), phenol (1.1 mmol), K CO (2.0 mmol), DMSO (4 mL), temp. 100 °C. H NMR(%) yield.
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single crystal structures of L1 and L2 and their palladium com-
plexes C1 and C2 were determined. Pd S and Pd Se nano-
1
6
7
17 15
particles were synthesized from air and moisture insensitive
single source precursors, C1 and C2 by their thermolysis,
respectively. The Pd S nanoparticles were flower shaped and
1
6 7
Pd17Se15 ones prismatic (size ∼26–50 and ∼20–55 nm, respect-
ively). These NPs show good catalytic activity for the Suzuki–
Miyaura/C–O coupling between aryl bromides and phenylboro-
7
nic acid/phenol at 100 °C. For both Pd16S and Pd17Se15 NPs, a
loading equivalent to 0.5 mol% Pd was sufficient for C–O as
well as Suzuki–Miyaura coupling. The NPs are recyclable up to
four runs for SMC. In the fourth run the yield was 38 and 45%
Fig. 14 Time profile C–O coupling of 4-bromobenzaldehyde (1 mmol)
catalyzed by C1.
for Pd16
and C–O coupling. The optimum loadings are from 0.0001 and
.1 mol%. One lot of 0.001 mol% of C1 as a SMC catalyst con-
DMSO using K CO as a base. 4-Phenoxybenzaldehyde and verts six lots of 4-bromobenzaldehyde (1 mmol). Eight lots of
7
S and Pd17Se15, respectively. C1 and C2 catalyze SMC
0
2
3
1
-nitro-4-phenoxybenzene (Table 4, entries 1 and 2) show con- 1-bromo-4-nitrobenzene (1 mmol) undergo C–O coupling with
versions ∼95 and 98%, respectively in 3 h when C1 is used as one lot of 0.1 mol% C1.
a catalyst. The conversions are 72 and 85% in the case of cata-
lyst C2. The electronic effects of substituents on the benzene
ring are similar to those noticed in the case of SMC. The time Conflict of interest
profile of the C–O coupling reaction between 4-bromobenzal-
There are no conflicts of interest to declare.
dehyde and phenol is shown in Fig. 14. The conversion was
7% after 0.5 h and increased to 72% on carrying out the reac-
5
tion for 1.5 h. It became ∼100% after 3 h. For 1-bromo-4-nitro-
benzene (Table 4, entry 2) 100% conversion was obtained with
C1 and 85% in the case of C2. The coupling of 4-iodoacetophe-
none was very fast as 4-acetylbiphenyl was obtained in 100%
yield in 0.5 h. However, 4-chloroacetophenone did not
undergo coupling in the presence of C1 or C2 as a catalyst. For
the catalyst alive test after one cycle of the coupling reaction, a
fresh lot of reactants, 1-bromo-4-nitrobenzene (1 mol%),
phenol (1.1 mol%) and 2.0 mmol of K CO were added and
Acknowledgements
The Science and Engineering Research Board, Department of
Science and Technology, Government of India, New Delhi, is
gratefully acknowledged for financial assistance. PS is thankful
to the DST, New Delhi for the Start-Up Research Grant for
Young Scientists (SB/FT/CS-001/2013).
2
3
the reaction was carried out under optimum conditions for
Notes and references
3
h. The addition of fresh reactants was repeated after com-
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ESI,† suggest that a significant amount of catalyst remains
alive even after seven cycles.
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