Palladium(ii)-(E,N,E) pincer ligand (E = S/Se/Te) complex catalyzed Suzuki coupling reactions in water via in situ generated palladium quantum dots

Article abstract of DOI:10.1039/c3dt51658j

The (E,N,E) pincer ligands (ArECH₂CH₂)₂NH (L1/L2: Ar = Ph, E = S/Se; L3: Ar = CH₃O-p-C₆H ₄, E = Te) synthesized by reaction of PhS^-/PhSe ^-/CH₃O-p-C₆

Full text of DOI:10.1039/c3dt51658j

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Palladium(II)-(E,N,E) pincer ligand (E = S/Se/Te)
complex catalyzed Suzuki coupling reactions in water
Cite this: Dalton Trans., 2013, 42, 16939
via in situ generated palladium quantum dots†
Satyendra Kumar, Gyandshwar K. Rao, Arun Kumar, Mahabir P. Singh and
Ajai Kumar Singh*
The (E,N,E) pincer ligands (ArECH2CH2)2NH (L1/L2: Ar = Ph, E = S/Se; L3: Ar = CH3O-p-C6H4, E = Te)
synthesized by reaction of PhS−/PhSe−/CH3O-p-C6H4Te− with bis(2-chloroethyl)amine react with
Na2PdCl4 in aqueous ethanol, resulting in nearly square planar diamagnetic complexes [Pd(L)Cl]Cl (1–3),
where L = L1–L3. All the ligands (L1–L3) and their complexes (1–3) have been characterised with 1H,
13C{1H}, 77Se{1H} and 125Te{1H} NMR spectra and high resolution mass spectrometry. The single crystal
structures (determined with X-ray diffraction) of 2 and 3 have been solved (Pd–Se: 2.4104(5)/2.4222(6) Å;
Pd–Te: 2.560(2)/2.588(2) Å). The conversions for Suzuki–Miyaura coupling (SMC) of various aryl bromides
with phenylboronic and 4-formyl/acetyl phenylboronic acid in water using 2–3 mol% of each of the
complexes 1–3 have been found good. Complexes 1 and 2 show better catalytic activity than 3, as
higher yields were observed with them in a relatively short time. The coupling reactions appear to be
catalyzed with Pd(0) nanoparticles (NPs) generated in situ in the course of reaction. The NPs have been
isolated and HRTEM studies on them have revealed their size as ∼1–3 nm. The SEM-EDX indicates their
protection with organochalcogen fragments. Addition of TBAB was essential in some cases to get good
yield of cross coupled product. The isolated NPs show catalytic activity for SMC independently. The yields
of cross coupled product were excellent when NPs were reused. The two phase test suggests a relatively
low contribution of homogeneous Pd species in catalysis.
Received 21st June 2013,
Accepted 11th September 2013
DOI: 10.1039/c3dt51658j
www.rsc.org/dalton
and organic solvent have been generally used in Pd catalyzed
SMC. The pure water as a reaction medium has been used
Introduction
Palladium catalyzed Suzuki Miyaura C–C coupling (SMC) reac- in much less number of reports. Use of water as solvent
tion is among the very important organic transformations.1 now-a-days appeals to chemists5 as it is green, cheapest among
The biaryl derivatives prepared via this coupling play promis- solvents and readily available. Although organic solvents have
ing roles in the production of pharmaceutical ingredients, many attractive features, they still suffer from serious draw-
agrochemicals and natural products2 for industrial purposes. backs such as inflammable nature, low heat capacity and
For this coupling bulky and electron-rich phosphine–Pd3 or difficulty in their quantitative recovery.6
carbene–Pd complexes3b,4 are the most active catalysts. Such
The Pd(II) complexes of chalcogenated Schiff bases7–13 and
coupling reactions are performed in very large number of some other S or Se containing ligands11–18 have emerged as
cases under an inert atmosphere because many of the known a family of phosphine free, air and moisture insensitive
catalytic species are sensitive to oxygen or moisture.4 Therefore efficient catalysts. However with most of them SMC reactions
such reactions giving high yield under aerobic conditions con- have been carried out in organic solvents. The use of water as
tinue to be an important challenge. Further, mixtures of water a solvent in the case of catalysis by such kind of complexes is
rare.19–22 Among the advantages6 of using water as solvent in
SMC reactions one may include the ease in separation of
product and catalyst, high solubility of inorganic bases in
water and stability of aqueous solution of phenylboronic acid.
The clubbing of advantages of using Pd(II)-complexes of organo-
chalcogen ligands in catalyzing SMC with those of using
water as a solvent system is worth exploring and has been
carried out in this paper. For Pd based SMC in pure water, the
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: Spectral data of L1–L3
and 1–3; single crystal data of 2 and 3 (CCDC 945010 and 945011), SEM, EDX
and TGA of NPs. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt51658j
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tridentate ligands L1–L3. Palladium complexes of these
ligands are easily formed on the reaction of Na2PdCl4 with L1,
L2 or L3, as the donor groups present in them are much stron-
ger in comparison to Cl. The complexes were isolated by fil-
tration. As the complexes are ionic, their substantial amount
goes to filtrate. The filtrate on slow evaporation has resulted in
orange crystals of complexes 2 and 3. The ligands L1–L3 were
found soluble in common organic solvents. Their air stable
complexes are sparingly soluble in CH2Cl2, CH3CN and CHCl3
but soluble in DMSO and DMF. The 1H and 13C{1H} NMR
spectra of L1–L3 and their complexes are characteristic. On
complexation ECH2 (E = S/Se/Te) and NCH2 signals in 1H NMR
spectra have been found to appear as multiplets and
deshielded by ∼0.1–0.3 and 0.1–0.6 ppm respectively with
respect to those of corresponding free ligands. Further the
magnitude of deshielding for ECH2/NCH2 protons is highest
in case of 1 and lowest in case of 3. The signals of SCH2 and
NCH2 in 13C{1H} NMR spectra of 1 show shielding of 3.3 and
7.4 ppm respectively with respect to those of free L1.
The signal of ECH2 (E = Se/Te) in 13C{1H} NMR spectra
shows deshielding of 14.0 and 4.1 ppm (with respect to that of
corresponding free ligand) in case of 2 and 3 respectively. In
13C{1H} NMR spectra of L2 and L3, the signals of –NCH2 group
appear at 47.7 and 49.4 ppm respectively. On complexation
these signals are deshielded by 8.2 and 4.5 ppm respectively.
These observations imply the co-ordination of N and E with
palladium(II) in case of at least 2 and 3. The signals in 77Se{1H}
and 125Te{1H} NMR spectra of L2 and L3 appear at 284.9 and
414.1 ppm respectively (Fig. S9 and S12 in ESI†). However, 77Se-
{1H} and 125Te{1H} NMR spectra of 2 and 3 could not be
recorded due to instability of their solutions in DMSO. 1H
NMR spectra of complexes 1, 2 and 3 were recorded in D2O.
These complexes have been further treated with K2CO3 in D2O
at 100 °C for 0.5 h. No sign of their decomposition was
noticed in case of 1 and 2 but 3 decomposed immediately. The
single crystal structures of 2 and 3 have been established with
X-ray diffraction. However, attempts to crystallize complex 1
were unsuccessful. The crystal data and details of structure
refinement for complexes 2 and 3 are given in Table S1 of ESI.†
Tables S2 and S3 in ESI† contain selected bond lengths and
angles for complexes 2 and 3 respectively. Fig. 1 and 2 show
ORTEP diagram of 2 and 3 with few bond lengths and angles.
There is a distorted square planar geometry around Pd in both
the complexes. The Pd–N bond lengths, 2.046(3) and 2.07(1) Å
of 2 and 3 respectively are close to each other and greater than
the value, 1.985(3) Å reported for palladium complex of (Se,N,
Se) pincer.17 These Pd–N bond lengths are almost similar to
the value, 2.010(4) Å reported for Pd(II) complex of a tridentate
selenated Schiff base.7 The Pd–Se bond distances [2.4104(5)
and 2.4222(6) Å] of 2 have been found to be somewhat longer
than the value 2.3891(7) Å reported for palladium complex of
(Se,N,Se) pincer.17 The Pd–Cl bond lengths of 2 and 3 are
Scheme 1 Synthesis of ligands and complexes.
catalytic species used are Pd(OAc)2 with or without additives23
including ligands and palladium species immobilized24 on
various supports. Some discrete and well characterized Pd-
complexes25,26 of carbenes and other ligands have also been
explored for catalysis of this coupling reaction in water. The
pincer ligands are considered very important for designing Pd
catalysts for SMC because combination of donor groups may
be easily varied to control the steric and electronic properties
of palladium in their complexes.27 Palladium(II)-complexes of
(N,C,N), (P,N,P), (S,C,S), (S,N,S), (Se,N,Se), (P,C,P), (P,Si,P) and
other unsymmetrical pincer ligands have been reported as
efficient catalysts for SMC.27–30 However, only a limited
number of such complexes have been reported to show cata-
lytic activity in water.19–22,25 It was therefore thought worth-
while to explore the Pd-complexes of tridentate (E,N,E) pincer
ligands L1–L3 (E = S, Se, Te) for this purpose in pure water
(Scheme 1). These three complexes reported here efficiently
catalyze SMC of various aryl bromides. The catalysis occurs via
Pd NPs protected with organochalcogen fragments. To under-
stand these NPs including their role in catalysis is among the
major objectives of this work. The two phase test in the
present case has revealed cocktail like catalysis,14 but mostly
heterogeneous. The NPs formed in the course of reaction on
isolation from reaction mixture show catalytic activity for SMC
independently and they do not become inactive after a reaction
as they can be reused.
Results and discussion
Syntheses and structures
The syntheses of ligands L1–L3 and their palladium complexes 2.297(1) and 2.300(4) Å respectively and consistent with the
1–3 are summarized in Scheme 1. The reactions of bis(2-chloro- value, 2.3159(7) Å reported for the Pd(II) complex of a triden-
ethyl)amine hydrochloride with nucleophiles PhS−/PhSe−/ tate sulphated Schiff base.7 The Pd–Te bond lengths 2.560(2)
CH3O-p-C6H4Te− in EtOH under inert atmosphere result in and 2.588(2) Å in 3 are longer than the reported values
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Fig. 1 ORTEP diagram of 2 with 40% probability ellipsoids. H, water and chlor-
ide ion have been omitted for clarity. Bond lengths (Å): Pd(1)–Se(1)/Pd(1)–Se(2)
2.4104(5)/2.422(6), Pd(1)–N(1) 2.046(3); Pd(1)–Cl(1) 2.297(1). Bond angles (°):
N(1)–Pd(1)–Cl(1) 178.8(1); N(1)–Pd(1)–Se(1) 87.8(1); Cl(1)–Pd(1)–Se(1) 92.36(3);
N(1)–Pd(1)–Se(2) 87.3(1); Cl(1)–Pd(1)–Se(2) 92.42(3); Se(1)–Pd(1)–Se(2) 174.04(2).
Fig. 3 Intermolecular N–H⋯Cl and O–H⋯Cl interactions in complex 2.
Fig. 4 Intermolecular N–H⋯O, C–H⋯O and N–H⋯Cl interactions in complex 3.
Fig. 2 ORTEP diagram of 3 with 40% probability ellipsoids. H, water and chlor-
ide ion have been omitted for clarity. Bond lengths (Å): Pd(2)–N(2D) 2.07(13);
Pd(2)–Cl(2) 2.288(4); Pd(2)–Te(3)/Pd(2)–Te(4) 2.560(2)/2.588(2). Bond angles (°):
N(2D)–Pd(2)–Cl(2) 178.8(4); N(2D)–Pd(2)–Te(3) 89.6(4); Cl(2)–Pd(2)–Te(3)
90.0(1); N(2D)–Pd(2)–Te(4) 87.4(4); Cl(2)–Pd(2)–Te(4) 92.96(1), Te(3)–Pd(2)–Te(4)
176.35(6).
Table 1 Suzuki–Miyaura coupling catalyzed with 1 and 2a
R-4-C6H4-X + PhB(OH)2 → R-4-C6H4-Ph
Complex 1
Complex 2
Entry no.
Aryl halide
Yieldb
Yieldc
2.526(7)/2.518(9) Å (for Pd(II) complex of meso-(4-MeOC6H4Te)2-
CH2))31a and 2.517(1) Å (for a palladium complex of Me-4-
C6H4-TeCH2CH2C6H4N).31b Both the complexes crystallise with
one molecule of water in their crystal lattice. The N–H⋯Cl,
C–H⋯Cl and O–H⋯Cl secondary interactions in the crystal of 2
result in the formation of a three dimensional structure shown
in Fig. 3, where as N–H⋯Cl, N–H⋯O, O–H⋯O and C–H⋯Cl
interactions shown in Fig. 4 result in the formation of three
dimensional network in the case of 3. The non-covalent inter-
actions are more extensively shown in ESI (Fig. S2 and S3†).
1
4-Bromobenzaldehyde
4-Bromobenzaldehyde
1-Bromo-4-nitrobenzene
1-Bromobenzonitrile
4-Bromoacetophenone
4-Bromobenzoic acid
4-Bromoanisole
4-Bromoanisole
4-Bromotoluene
4-Bromotoluene
4-Bromopyridine
96
91
84
94
60
93
10
76
6
∼100
90
∼100
∼100
83
2d
3
4
5
6d
7
∼100
8
8e
9
72
5
10e
11
12
75
50
—
70
—
4-Chlorobenzaldehyde
—
a Reaction
conditions:
aryl/heteroaryl
halide
(1.0
mmol),
phenylboronic acid (1.5 mmol), K2CO3 (2.0 mmol), water (3 mL),
reaction time 12 h and temperature of bath 100 °C. 2 mol% of complex
1/2. b Isolated yield after column chromatography. c NMR % yield.
d Reaction time 4 h. e 1 mmol of TBAB was added.
Catalysis of Suzuki–Miyaura coupling
The complexes 1–3 have been explored for SMC. The solvent
and base always have a significant role in SMC.5,32 Conse-
quently different solvents have been found suitable for
different complexes used as catalysts. However, water has been SMC in green solvent water.19–22 In the present study SMC of
used as a co-solvent with many organic solvent viz. DMF, 4-bromobenzaldehyde with phenylboronic acid was used to
EtOH, dioxane, and THF.5,6 We have optimised with the optimize reaction conditions in water using K2CO3 as a base.
present complexes the reaction conditions in water. There are Complex 1 (2 mol%) catalyzes the reaction at 100 °C in pres-
few reports to our knowledge which have described the use of ence of K2CO3 and the yield of cross coupled product after
palladium complexes of organochalcogen ligands to catalyze 12 h has been found as ∼96% (Table 1: entry 1). However cross
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Table 2 Suzuki–Miyaura coupling of various arylboronic acids catalyzed with 1 and 2a
R-4-C6H4-X + ArB(OH)2 → R-4-C6H4-Ar
Complex 1
Complex 2
Entry no.
Arylboronic acid
Arylbromide
Yieldb
Yieldc
1
2
3
4
5
6
4-Formylphenylboronic acid
4-Acetylphenylboronic acid
4-Formylphenylboronic acid
4-Acetylphenylboronic acid
4-Formylphenylboronic acid
4-Acetylphenylboronic acid
4-Bromo benzaldehyde
1-Bromo-4-nitrobenzene
4-Bromo benzoic acid
95
82
93
89
96
95
∼100
92
95
∼100
94
91
a Reaction conditions: aryl bromide (1.0 mmol), arylboronic acid (1.2 mmol), K2CO3 (2.0 mmol), water (3 mL), reaction time 12 h, temperature of
bath 100 °C and 3 mol% of 1/2. b Isolated yield after column chromatography. c NMR % yield.
coupled product was obtained in 71% yield only when the but dispensers of real catalysts, as it has already been pro-
reaction was carried out for 12 h at 70 °C with 1 mol% catalyst posed that a palladacycle of reduced sulphated Schiff base
loading in the presence of K2CO3. When catalyst 2 was used decomposes to release Pd16S7 species when catalysing SMC
for the same coupling under similar reaction conditions the and the [Pd(0)–Pd(II)] cycle is involved in the catalytic pro-
conversion was ∼100%. Under optimum reaction conditions cess.14,33a,b In situ formation of Pd NPs has also been observed
(see footnote of Table 1) the 3 catalyzes the coupling of 4-bromo- in many cases during the catalytic process.33c–f Thus reactions
benzoic acid efficiently (yield of product biphenyl-4- of 4-bromobenzoic acid with PhB(OH)2 in water catalyzed with
carboxylic acid ∼100%). However 3 does not show significant 1–3 under optimum conditions have been examined in detail
activity for other aryl bromides even at its 2 mol% loading. The to understand the nature of black particles generated in situ
Te containing complex has been found less catalytically active during catalytic process. This reaction is selected because each
probably because Te is large in size and does not increase of the three complexes shows good activity for this coupling
much electronic charge density on Pd. It may cause steric (yield of cross coupled product ∼100%) with the formation of
effect also on Pd. Earlier10a also a complex of Pd(II) with Te- black particles. These black particles were washed to remove
ligands has been found much less efficient catalyst for C–C soluble impurities (see Experimental) and dissolved in chloro-
coupling than their S/Se analogues.14,16 The results of SMC form which does not leave any residue.
given in Tables 1 and 2 indicate that the coupling of several
The HR-TEM, SEM, SEM-EDX and powder X-ray diffraction
activated aryl bromides can be successfully achieved when 1 have been used to characterize these black particles after their
and 2 are used as catalyst without tetrabutylammonium separation. Powder X-ray diffraction shows their amorphous
bromide (TBAB) additive. However for deactivated aryl bro- nature. The HRTEM (Fig. 5) indicates that the black particles are
mides (4-bromotoluene and 4-bromoanisole) the coupled pro- uniform spherical shaped NPs (size of most of them ∼1–2 nm)
ducts were obtained in <10% yield (Table 1: entries 7 and 9). (Fig. 5 and 7). The SEM-EDX has revealed that NPs contain Pd
On addition of TBAB to the reaction mixture in these cases, as well as chalcogen and approximate Pd : E (E = S/Se/Te) ratios
the yields of cross coupled products were found considerably (in atom%) in NPs generated from 1–3 are ∼42 : 58, 45 : 55 and
increased (Table 1: entries 8 and 10). Possibly TBAB acts as a 46 : 54 respectively. In the UV-Vis spectra of NPs obtained from
phase transfer catalyst or stabilizes Pd(0) species generated complexes 1–3 in CHCl3, a peak around 257–260 nm (Fig. 6a)
during the coupling process. This observation indirectly was observed.33g Their fluorescence spectra in CHCl3 show a
supports the in situ generation of Pd NPs. However none of the strong emission at λ = 341, 337 and 340 nm respectively (Fig. 6b)
catalysts (1–3) shows any significant activity towards ArCl for NPs obtained from 1–3, as expected for quantum dots. The
(Table 1: entry 12).
NPs obtained from 1–3 were examined for SMC reaction
The activities of complexes 1 and 2 were also studied (Table 3) of 4-bromobenzoic acid with PhB(OH)2 to ascertain
(Table 2) towards the coupling of aryl bromides with two other whether the catalytic activity is associated with them or not. The
arylboronic acids. Both the catalysts have been found to show reaction was smooth in presence of the NPs (Pd content ∼2 mol%)
(Table 2) good activity for the coupling of 4-formyl/acetyl-phenyl- obtained from either of 1–3. The reactions for 5 h at 100 °C,
boronic acid with activated aryl bromides at a catalyst loading resulted in the cross coupled products in excellent yields for
of 3 mol%.
NPs obtained from 1–2 (Table 3). The recyclability of NPs was
The complex 3 does not show any significant catalytic studied by reusing them. It has been noticed that the catalytic
activity for the coupling of aryl bromides with these arylboro- activity remains same for second reaction cycle (yield ≥ 96%) in
nic acid derivatives even at its 3 mol% loading. The 1–3 and case of 1 and 2. However it diminishes significantly for second
Pd(II) complexes of other organochalcogen ligands (of course reaction cycle in case of NPs obtained from complex 3 as the
scantly reported) are competitive in their efficiencies.19,21,22
coupled product was obtained in 79% yield even after 12 h of
In the course of coupling reactions black particles appear. reaction time (Table 3). These results support the involvement
It is possible that the present complexes are not true catalysts of Pd NPs in catalysis of SMC with complexes 1–3. The time
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Table 3 Suzuki–Miyaura coupling catalyzed with NPs obtained from com-
plexes 1 to 3a
HOOC-4-C6H4-Br + PhB(OH)2 → HOOC-4-C6H4-Ph
Yieldb
NPs
NPs
NPs
Aryl halide
Cycle
from 1c
from 2c
from 3d
4-Bromobenzoic acid
1
2
∼100
∼100
∼100
96
∼100
79
a Reaction conditions: 4-bromobenzoic acid (1.0 mmol), phenylboronic
acid (1.5 mmol), K2CO3 (2.0 mmol), water (3 mL) and temperature of
bath 100 °C. b NMR % yield. Amount of NPs equiv. not more than
2 mol% of palladium. c Reaction time 5 h. d Reaction time 12 h.
profile of reaction of 4-bromobenzoic acid with phenylboronic
acid catalyzed with complex 1 and NPs obtained from it has
1
been monitored with H NMR spectroscopy and shown in ESI
(Fig. S1†). The difference between the first 2 h time profile of
isolated NPs and their precursor complex may be due to aggre-
gation of NPs in the course of isolation. The in situ generated
NPs are more active as their surfaces being fresh are very active.
It is also possible that in the early stages of catalysis discrete Pd(0)
is partially responsible for catalysis. The nucleation of these
species results in Pd NPs engulfed by organochalcogen frag-
ments. However, the role of formation of the present nanoparti-
cles in catalysis cannot be denied. The nature of real catalyst
and catalytic process was further investigated. The mercury poi-
soning and triphenylphosphine test34 in the presence of 2 mol%
of catalyst 1 were carried out on a representative coupling
reaction of 4-bromobenzoic acid with phenylboronic acid as
described in the Experimental section. On continuing reaction
in the presence of mercury at 100 °C only 56% conversion of
substrate into cross coupled product i.e. 4-phenylbenzoic acid
could be reached after 12 h.
Fig. 5 HRTEM images for NPs obtained from complexes 1, 2 and 3 respectively
(scale bar 10, 10 and 20 nm).
In triphenylphosphine poisoning test the biaryl product
was obtained in 68% yield when reaction was carried out for
optimum time and under optimized reaction conditions.
These results suggest that catalytic process is partially poi-
soned by both Hg and PPh3. This is on expected lines as Pd
NPs appear to be protected by organochalcogen species as
evident from the SEM-EDX results. The weight losses (%) in
TGA up to 500 °C were ∼50, 35 and 57% (Fig. S31–S33 in ESI†)
respectively in case of NPs isolated from complexes 1, 2 and 3.
The presence of organochalcogen skeletons most likely results
in these weight losses. Presuming palladium to chalcogen
ratio approximately 1 : 1.
Fig. 6 UV-Vis and photoluminescence spectrum of NPs obtained from complex
1–3.
The associated organic fragments with Pd(0) NPs obtained
from 1, 2 and 3 may be speculated as NH2CH2CH2SPh (calcu-
lated wt loss 46%), NH2CH2CH2SePh (calculated wt loss 39%)
and [NH2CH2CH2TeAr + NH2CH2CH2Ar] (calculated wt loss
56%). The possibility of the proposed fragment appears to be
logical in view of extrusion reactions of chalcogens. After TGA
residue left appears to be PdE (E = S, Se or Te). Due to large
size of tellurium and its more metallic nature the presence two
fragments is not exceptional.
Fig. 7 Size distribution of NPs obtained from 1–3 from Suzuki–Miyaura
coupling.
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and ratio of cross coupled product : unreacted one was 22 : 78.
This suggests that very low amount of catalytically active Pd
leaches from the in situ generated Pd nanoparticles and is
responsible for partial homogeneous catalysis.
A major
portion of catalysis occurs in a heterogeneous manner as large
portion of immobilized 4-bromobenzoic acid remains
unreacted. Thus, it appears that the coupling catalyzed with
1–3 is via Pd quantum dots and largely heterogeneous with
some contribution of homogeneous pathway. Recently such a
possibility has been described as “cocktail” of homogeneous
and heterogeneous catalytic processes.14,37–39
Experimental
Chemicals and reagents
Diphenyl diselenide, thiophenol, tellurium powder, bis(2-
chloroethyl)amine hydrochloride, sodium tetrachloropalla-
date, potassium carbonate, aryl halides, NaBH4 and aryl
boronic acids were procured from Sigma-Aldrich (USA) and
used without any further purification. Bis(4-methoxyphenyl)-
ditelluride was prepared by the reported method.40,41 The pro-
gress of the catalytic reactions was monitored by 1H NMR spectro-
scopy. The products obtained in SMC were authenticated by
matching their NMR spectroscopic data with the literature
values.
Scheme 2 Two phase test.
As these two poisoning tests could not fully affirm the
nature of catalytic process and the possibility of involvement
of surface atoms of nanoparticles of Pd in oxidative addition
to form soluble Pd(II) intermediate11,12 such as Ar–Pd–Br
exists, a two phase test (Scheme 2) was performed to under-
stand the nature of the reaction (heterogeneous vs. homo-
geneous) further.35,36 This test (called a three phase test when
the catalyst is in solid phase), is considered more definitive for
Physical measurements
establishing the nature of catalytically active metal species and The 1H, 13C{1H}, 77Se{1H} and 125Te{1H} NMR spectra have
settling the issue of homogeneous vs. heterogeneous.35 If the been recorded on a Bruker Spectrospin DPX-300 NMR spectro-
catalyst behaves in a heterogeneous fashion, the conversion of meter at 300.13, 75.47, 57.24 and 94.69 MHz respectively, and
immobilized aryl bromide into cross-coupled product is not chemical shift are reported in ppm relative to the normal stan-
expected. When Pd is released (i.e., catalysis is at least partially dards. Perkin Elmer Bio Lambda 20 spectrophotometer has
homogeneous), the immobilized substrate is also converted to been used to record UV-Vis spectra. Fluorescence spectra were
cross coupled product. The addition of a soluble aryl halide to recorded on Horiba Scientific Fluoromax-4 spectrofluorimeter.
the reaction mixture ensures the presence of a catalytic process All reactions were carried out in oven dried round bottom flask
and the active species in the reaction mixture. To carry out two under ambient condition. To create inert atmosphere commer-
phase test 4-bromobenzoic acid was immobilized as amide on cially available nitrogen gas was used after passing through
silica and its stability established first. For this purpose it was the traps containing solutions of alkaline anthraquinone,
treated with 0.8 M K2CO3 at 100 °C in 15 mL of ethanol–water sodium dithionite, alkaline pyrogallol, concentrated H2SO4,
mixture (2 : 1) for 12 h. The reaction mixture was acidified and KOH pellets. Melting point of ligands and complexes were
(20% v/v aq. HCl) and then extracted with ethyl acetate fol- determined by taking the sample in a glass capillary sealed at
lowed by dichloromethane. The residue obtained after evapor- one end, with an apparatus equipped with electric heating and
ating off the solvent from extracts mixed together was reported as such. High-resolution mass spectral (HR-MS)
1
subjected to H NMR analysis. The expected 4-bromobenzoic, measurements were made with a Bruker Micro TOF-Q II
if there was any hydrolysis of silica phase, was not detected.
machine using electron spray ionisation (10 eV, 180 °C source
In a two phase test (Scheme 2) an immobilized 4-bromo- temperature and sodium formate as calibrant) and dissolving
benzoic acid and 4-bromoacetophenone were reacted with phenyl- the sample in CH3CN. The diffraction data of single crystals of
boronic acid under optimum reaction conditions (see 2 and 3 were collected on a Bruker AXS SMART Apex CCD diffracto-
Experimental section for details). The soluble part was separ- meter using Mo-Kα (0.71073 Å) radiations at 195(2) and 298
ated by filtration and analyzed after workup with 1H NMR. The (2) K respectively. SADABS42 software was used for absorp-
yield of the cross-coupled product (4-acetylbiphenyl) has been tion correction wherever needed and SHELXTL for space
found to be 80% (0.156 g). The solid phase was hydrolysed, group, structure determination, and refinements.43 Non-hydro-
and resulting products after work up were analysed with 1H gen atoms were refined anisotropically. All hydrogen atoms
NMR. The immobilized 4-bromobenzoic acid, was converted to were included in idealized positions, and a riding model was
the cross-coupled product (biphenyl-4-carboxylic acid) partially used for the refinement. The least-squares refinement cycles
16944 | Dalton Trans., 2013, 42, 16939–16948
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on F2 were performed until the model converged. Images of 25 °C, Me2Te): 414.1. HR-MS [M + H] (m/z) = 545.9926; calcd
the crystals were drawn with Diamond program. TEM (Technai value for C18H24NO2Te2 = 545.9928 (error δ: −0.3 ppm).
G2 20 operated at 200 kV) was used to study nature of nanopar-
Synthesis of Pd-complexes 1–3
ticles. Samples for recording TEM were prepared by dispersing
the powdered solid in ethanol by an ultrasonic treatment, One out of ligands L1–L3 (1 mmol) was dissolved in ethanol
dropping the slurry onto a porous carbon film supported on a and stirred for 30 minutes at room temperature. Aqueous solu-
copper grid and then drying in air. The phase morphologies of tion of Na2PdCl4 (0.294 g, 1 mmol, in 3 mL of water) was
nanoparticles were observed with a Carl Zeiss EVO5O scanning added drop wise to the ligand solution. The reaction mixture
electron microscope (SEM) associated with an EDX system was stirred for 6 h at room temperature. The orange yellow pre-
model QuanTax 200, which is based on the SDD technology cipitate was filtered with a filter paper, washed with ethanol
and provides an energy resolution of 127 eV at Mn-Kα. The and dried in air. The yellow coloured single crystals of com-
samples for SEM were mounted on a circular metallic sample plexes 2 and 3 were grown by subjecting filtrate to slow
holder with a sticky carbon tape and scanned in different evaporation.
regions in order to minimize the error in the analysis made for
Complex 1: yellow solid, yield: (0.435 g, 93%); m.p. 214 °C
evaluating the morphological parameters. Size distribution (d). 1H NMR (300 MHz, DMSO-d6, 25 °C, TMS); δ (ppm):
has been determined by analyzing one hundred and ten 3.10–3.27 (m, 4H), 3.36–3.42 (m, 2H), 3.59–3.66 (m, 2H),
random NPs appearing in TEM image with the help of Irfan 7.54–7.58 (m, 6H), 8.03–8.08 (m, 4H). 13C{1H} NMR (75 MHz,
View software.
DMSO-d6, 25 °C, TMS): δ (ppm): 30.7 (C2), 53.2 (C1), 129.8
(C5), 130.1 (C3), 130.3 (C6), 131.7 (C4). HR-MS [M − Cl] (m/z) =
429.9678; calcd value for C16H19ClNPdS2 = 429.9679 (error δ:
Synthesis of L1–L344–46
Bis(2-chloroethyl)amine hydrochloride (0.704 g, 4 mmol) dis- 0.1 ppm).
solved in ethanol (10 mL) was added dropwise to a solution of
Complex 2: yellow solid, yield: (0.495 g, 88%); m.p. 170 °C
PhSNa/PhSeNa/CH3O-p-C6H4TeNa (8 mmol) generated in situ (d). 1H NMR (300 MHz, DMSO-d6, 25 °C, TMS); δ (ppm):
by the reaction of NaOH with thiophenol or NaBH4 reduction 3.12–3.250 (m, 4H), 3.51–3.71 (m, 4H), 7.53–7.57 (m, 6H),
of
diphenyldiselenide/bis(4-methoxyphenyl)ditelluride
at 8.09–8.15 (m, 4H). 13C{1H} NMR (75 MHz, DMSO-d6, 25 °C,
60 °C under nitrogen atmosphere. The reaction mixture was TMS): δ (ppm): 36.0 (C2), 55.9 (C1), 126.9 (C3), 129.4 (C6),
further heated at 60 °C for 8 h and then allowed to cool at 130.2 (C5), 132.8 (C4). HR-MS [M − Cl] (m/z) = 525.8575; calcd
room temperature. Solvent of the reaction mixture was reduced value for C16H19ClNPdSe2 = 525.8576 (error δ: 0.1 ppm).
to 10 mL on rotary evaporator and mixed with 50 mL of water.
Complex 3: yellow solid, yield: (0.657 g, 91%); m.p. 134 °C
The organic phase was extracted with chloroform (100 mL), (d). 1H NMR (300 MHz, DMSO-d6, 25 °C, TMS); δ (ppm):
washed three times with water (40 mL) and dried over anhy- 2.84–3.32 (bs, 8H), 3.80 (s, 6H), 7.02 (d, J = 8.4 Hz, 4H),
drous sodium sulphate. The solvent was evaporated off under 8.04–8.07 (m, 4H). 13C{1H} NMR (75 MHz, DMSO-d6, 25 °C,
reduced pressure on a rotary evaporator to obtain the product TMS): δ (ppm): 16.1 (C2), 55.4 (C7), 59.3 (C1), 105.2 (C3), 116.3
L1, L2 or L3. The present procedure is simpler and/or gives (C4), 138.6 (C5), 161.3 (C6). HR-MS [M − Cl] (m/z) = 685.8670;
better yield in comparison to earlier reported ones.44–46
calcd value for C18H23ClNO2PdTe2 = 685.8554 (error δ:
L1: yellow viscous oil, yield: (1.221 g, 85%). 1H NMR −2.4 ppm).
(300 MHz, CDCl3, 25 °C, TMS); δ (ppm): 2.81 (t, J = 6.9 Hz, 4H,
General procedure for Suzuki–Miyaura coupling of aryl
bromides with arylboronic acid
H2), 3.03 (t, J = 6.3 Hz, 4H, H1), 7.15–7.36 (m, 10H, H4, H5, H6).
13C{1H} NMR (75 MHz, CDCl3, 25 °C, TMS): δ (ppm): 34.0 (C2),
60.6 (C1), 126.2 (C6), 128.9 (C5), 129.7 (C4), 135.4 (C3). HR-MS Aryl bromide (1 mmol), aryl boronic acid (1.5 mmol), K2CO3
[M + H] (m/z) = 290.1025; calcd value for C16H20NS2 = 290.1032 (2 mmol), water (3 mL) and a catalyst out of 1–3 (2 mol%) was
(error δ: 2.3 ppm).
charged in a round bottom flask, which was placed on an oil
1
L2: light yellow solid, yield: (1.713 g, 89%). m.p. 106 °C. H bath under aerobic condition at 100 °C and stirred. The pro-
NMR (300 MHz, CDCl3, 25 °C, TMS); δ (ppm): 3.08–3.12 (m, gress of reaction was monitored with 1H NMR spectroscopy.
4H, H2), 3.21–3.26 (m, 4H, H1), 7.24–7.27 (m, 6H, H5, H6), When maximum conversion of ArBr into product occurred, the
7.49–7.53 (m, 4H, H4). 13C{1H} NMR (75 MHz, CDCl3, 25 °C, reaction mixture was cooled to room temperature. It was
TMS): δ (ppm): 22.0 (C2), 47.7 (C1), 127.6 (C3), 127.7 (C6), mixed with water (20 mL) and extracted with diethyl ether (3 ×
129.9 (C5), 133.1 (C4). 77Se{1H} NMR (57 MHz, CDCl3, 25 °C, 10 mL). For benzoic acid derivatives the reaction mixture was
Me2Se): δ (ppm) 284.9. HR-MS [M + H] (m/z) = 385.9924; calcd mixed with 20 mL of 20% HCl and extracted with ethyl acetate
value for C16H20NSe2 = 385.9920 (error δ: 0.9 ppm).
(3 × 10 mL). Organic phase was dried over anhydrous Na2SO4.
1
L3: White solid, Yield: (2.511 g, 92%); m.p. 84 °C. H NMR Its solvent was removed on rotary evaporator and the product
(300 MHz, CDCl3, 25 °C, TMS); δ (ppm): 2.94 (s, 8H, H1, H2), was subjected to 1H and 13C{1H} NMR studies. In case of
3.78 (s, 6H, OMe), 6.76 (d, J = 8.4 Hz, 2H, H4), 7.69 (d, J = 8.4 impure product, further purification by column chromato-
Hz, 2H, H5). 13C{1H} NMR (75 MHz, CDCl3, 25 °C, TMS): δ graphy on silica gel using ethyl acetate and hexane mixture
(ppm): 10.4 (C2), 49.4 (C1), 55.0 (OMe), 100.3 (C3), 115.0 (C4), as eluent was carried out and thereafter subjected to NMR
140.8 (C5), 159.6 (C6). 125Te{1H} NMR (94.69 MHz, CDCl3, studies.
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Isolation of nanoparticles formed from complex 1–3 in
Suzuki–Miyaura C–C coupling
washings were collected together and mixed with water
(50 mL). The resulting mixture was extracted with 50 mL of
diethyl ether. The solvent from the organic extract was evapor-
ated off on a rotary evaporator and the white residue thus
A
complex out of 1–3 (2 mol%), 4-bromobenzoic acid
(1 mmol), phenylboronic acid (1.5 mmol), K2CO3 (2 mmol)
and water (3 mL) were heated in a round bottom flask at
100 °C for 5 h. The solvent of the reaction mixture was de-
canted off while hot. The black residue present in the flask was
washed with a mixture of ethyl acetate (10 mL) and water
(30 mL). It was then dispersed in chloroform (20 mL). The
solvent was evaporated off to get black nanoparticles.
1
obtained was identified with H NMR. The residue separated
on G-4 crucible was hydrolysed with KOH (1.68 g dissolved in
5 mL of water + 10 mL of EtOH) at 90 °C for 3 days. Using
aqueous 20% (v/v) HCl solution the hydrolysed reaction
mixture was neutralised. It was extracted with dichloro-
methane (30 mL), followed by ethyl acetate (40 mL). The
organic extracts were combined together. Its solvent was evap-
1
orated off and the resulting residue was subjected to H NMR
Procedure for catalysis of SMC with recyclable NPs
to identify the products.
The independent catalytic activity of NPs isolated from SMC
was tested for the same coupling reaction separately, using
procedure optimised for complexes and taking in the place of
complex, NPs obtained from 1–3 (equivalent to ∼2 mol% of
Pd) as catalyst and heating the reaction mixture for 5 h. There-
after the mixture was mixed with 20 mL of 20% HCl and
extracted with ethyl acetate (3 × 10 mL). The organic phase was
dried over anhydrous Na2SO4. Its solvent was removed on
rotary evaporator to obtain products as white solid which were
Conclusions
Palladium complexes of type [Pd(L)Cl]Cl (1–3) (where pincer
ligand L = L1–L3 = (ArECH2CH2)2NH; (E = S/Se/Te)) have been
synthesized and characterized by multinuclear NMR spectro-
scopy and mass spectrometry. The complexes 2 and 3 have
been characterised with X-ray diffraction on their single crys-
tals. These complexes crystallize with one molecule of water in
their crystal lattice. The complexes 1–3 have been found to
efficiently catalyze Suzuki–Miyaura C–C coupling reactions of
various aryl bromides with three arylboronic acids in water.
Their 2–3 mol% is sufficient to give good yield of cross
coupled product. The activities of complexes 1 and 2 have
been found better than that of 3. The catalysis of Suzuki–
Miyaura coupling appears to occur via in situ generated Pd
quantum dots (size 1–3 nm) protected with organochalcogen
fragments as evident by SEM-EDX and TEM studies. These
NPs have been isolated and show catalytic activity for SMC
independently in each case. The results of two phase test
suggest that there is a relatively higher contribution of hetero-
geneous pathway to the catalysis.
1
characterized as usual with H and 13C{1H} NMR. The conver-
sion of 4-bromobenzoic acid to the product was almost quanti-
tative (Table 3). The NPs were recovered and again tested for
the same coupling reaction. They were found to catalyze the
coupling reaction again with good efficiency. The lowest yield
was 79% after the second catalytic reaction cycles.
Hg poisoning test
The coupling reaction of 4-bromobenzoic acid with phenyl-
boronic acid in the presence of 2 mol% of 1 under optimal
reaction condition was carried out. The progress of reaction
1
was monitored with H NMR spectroscopy. When 36% conver-
sion (10 minutes) took place, 400 equivalents of Hg (for 1 equi-
valent of Pd) were added to the reaction mixture. After
continuing reaction further for 12 h at 100 °C only 56% conver-
sion could be reached.
Acknowledgements
PPh3 poisoning test
Council of Scientific and Industrial Research, and Department
of Science and Technology India supported the work through
projects [CSIR: 01(2421)10/EMR-II; DST: SR/S1/IC-40/2010]. SK
and MPS thank to UGC for fellowship. GKR and AK thank
CSIR for fellowship. Authors thank Professor A. K. Ganguli of
IIT Delhi for providing HR-TEM facility.
Under optimal reaction conditions the 4-bromobenzoic acid
(1.0 mmol) was reacted with phenylboronic acid (1.2 mmol) in
the presence of PPh3 (4 mol%) for 0.5 h. Thereafter catalyst 1
(2 mol%) was added and reaction monitored with 1H NMR
spectroscopy. After 12 h of the reaction, cross coupled product
was obtained in 68% yield.
Notes and references
Two phase test
The 4-bromobenzoic acid-immobilized silica (0.20 g) prepared
by reported procedure,47 phenylboronic acid (0.36 g, 3 mmol),
4-bromoacetophenone (0.20 g, 1 mmol), K2CO3 (0.56 g,
4 mmol) and 2 mol% of catalyst 1 (best among the three com-
plexes) were heated at 100 °C for 12 h in water (5 mL). The
reaction mixture was cooled to room temperature and filtered
through G-4 crucible. The residue was washed with 50 mL of
water and then 30 mL of diethyl ether. The filtrate and
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