Synthesis of a recyclable and efficient Pd(ii) 4-(2-pyridyl)-1,2,3-triazole complex over a solid periodic mesoporous organosilica support by "click reactions" for the Stille coupling reaction

Article abstract of DOI:10.1039/c4ra06459c

A highly efficient, reusable B-PMO-TZ-Pd(ii) catalyst was synthesized by anchoring 4-(2-pyridyl)-4-(2-pyridyl)-1,2,3-triazole ligand over the surface of an organo-functionalized benzene support containing periodic mesoporous organosilica (B-PMO) via "click reaction" and the subsequent complexation with PdCl₂. B-PMO materials with uniform hexagonal arrangements were prepared using C₁₆ alkyl trimethyl ammonium bromide [CTAB] surfactant under basic conditions. The physiochemical properties of the functionalised catalyst were analysed by elemental analysis, ICP-OES, XRD, N₂ sorption, TGA & DTA, solid state ¹³C, ²⁹Si NMR spectra, FT-IR, XPS, UV-vis, SEM and TEM. XRD and N₂ sorption revealed the morphological and textural properties of the synthesized catalyst, confirming that ordered mesoporous channel structure was retained even after the multistep synthetic procedure. The (100), (110) and (200) reflections in B-PMO are evidence of its good structural stability and the existence of long range order. The TGA-DTA results reveal that the synthesized catalyst B-PMO-TZ-Pd(ii) was thermally stable, even at high temperature. The organic moieties anchored over the surface of B-PMO were demonstrated by solid state ¹³C NMR and FT-IR spectroscopy. Solid state ²⁹Si NMR spectroscopy provides information about the degree of functionalization of the surface silanol group. The electronic environment and oxidation state of Pd in B-PMO-TZ-Pd(ii) were monitored by XPS and UV-visible techniques. Moreover, the morphologies and topographic information of the synthesized B-PMO-TZ-Pd(ii) catalyst were confirmed by SEM and TEM analyses. The catalytic properties of the catalyst for the Stille coupling reaction were screened, and higher catalytic activities with high TONs were observed. The anchored solid B-PMO-TZ-Pd(ii) catalyst can be recycled efficiently and reused several (four) times without a major loss in reactivity. This journal is

Full text of DOI:10.1039/c4ra06459c

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PAPER
Synthesis of a recyclable and efficient Pd(II) 4-(2-
pyridyl)-1,2,3-triazole complex over a solid periodic
mesoporous organosilica support by “click
reactions” for the Stille coupling reaction†
Cite this: RSC Adv., 2014, 4, 43070
Priti Sharma and A. P. Singh*
A highly efficient, reusable B-PMO-TZ-Pd(II) catalyst was synthesized by anchoring 4-(2-pyridyl)-4-(2-
pyridyl)-1,2,3-triazole ligand over the surface of an organo-functionalized benzene support containing
periodic mesoporous organosilica (B-PMO) via “click reaction” and the subsequent complexation with
PdCl
ammonium bromide [CTAB] surfactant under basic conditions. The physiochemical properties of the
functionalised catalyst were analysed by elemental analysis, ICP-OES, XRD, N sorption, TGA & DTA, solid
state C, Si NMR spectra, FT-IR, XPS, UV-vis, SEM and TEM. XRD and N sorption revealed the
2
. B-PMO materials with uniform hexagonal arrangements were prepared using C16 alkyl trimethyl
2
13
29
2
morphological and textural properties of the synthesized catalyst, confirming that ordered mesoporous
channel structure was retained even after the multistep synthetic procedure. The (100), (110) and (200)
reflections in B-PMO are evidence of its good structural stability and the existence of long range order.
The TGA-DTA results reveal that the synthesized catalyst B-PMO-TZ-Pd(II) was thermally stable, even at
high temperature. The organic moieties anchored over the surface of B-PMO were demonstrated by
13
29
solid state C NMR and FT-IR spectroscopy. Solid state Si NMR spectroscopy provides information
about the degree of functionalization of the surface silanol group. The electronic environment and
oxidation state of Pd in B-PMO-TZ-Pd(II) were monitored by XPS and UV-visible techniques. Moreover,
the morphologies and topographic information of the synthesized B-PMO-TZ-Pd(II) catalyst were
confirmed by SEM and TEM analyses. The catalytic properties of the catalyst for the Stille coupling
reaction were screened, and higher catalytic activities with high TONs were observed. The anchored
solid B-PMO-TZ-Pd(II) catalyst can be recycled efficiently and reused several (four) times without a major
loss in reactivity.
Received 30th June 2014
Accepted 28th August 2014
DOI: 10.1039/c4ra06459c
www.rsc.org/advances
wide variety of organo-bridged hybrid mesoporous materials are
prepared by choosing a suitable organo siloxane precursor,
Introduction
Since the discovery of silica-based mesoporous materials, (R
0
O)
3
-SiRSi(OR
0
)
3
, where R ¼ –(CH
2 6 4
)x–, –C H –, or –CH^CH–.
extensive research has been devoted to the synthesis of siliceous The nature of the organic groups in the hybrid materials
1
and non-siliceous mesoporous materials. Furthermore, a new controls properties including ion exchange capacity and
class of organic–inorganic hybrid mesoporous materials inte- dielectric constant as well as mechanical properties such as
3
grating organic groups in the wall channel has been reported; in surface hydrophobicity and hydrothermal stability. The intro-
this new class of materials, the organic molecules reside on the duction of organic functionalities into the silica framework
pore surfaces, incorporating organic groups (like ethane, modies the physical and chemical properties of the meso-
ethene, benzene, biphenyl, etc.) into the framework as molec- porous materials, while their inorganic features make them
2
ularly bridged ligands. These unique hybrid materials are more thermally and structurally stable. In addition, the nature
known as periodic mesoporous organosilicas (PMO) and are of the organic groups integrated inside the frame wall channels
obtained by the hydrolysis and condensation reactions of sil- of the mesoporous materials can change the nature of the
sesquioxane precursors in the presence of surfactant species. A surfactant–silicate interactions and thereby the nal structure
4
of the developed materials.
The sustained development of homogeneous catalysts for
Catalysis Division, CSIR-National Chemical Laboratory, Pune 411008, India. E-mail:
ap.singh@ncl.res.in; Fax: +91 20 2590 2633; Tel: +91 20 2590 2497
ne chemical production has led to attempts to heterogenize
†
Electronic supplementary information (ESI) available: Details of Stille coupling these catalysts. At present, several methods for the hetero-
1
13
reaction product H,
C
NMR, details of experimental section. See DOI:
5
genization of homogeneous catalysts have been explored. The
1
0.1039/c4ra06459c
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immobilization of metal complexes onto the surfaces of solid 3-azidopropyltrimethoxy silane using the post graing method
18
supports is highly desirable in the development of reusable (Scheme 1A and B). Azide-functionalized hybrid mesoporous
catalysts. Recently, the Cu(I)-catalysed azide–alkyne “click benzene-PMO (B-PMO-N ) was reacted with 2-ethynylpyridine
3
chemistry” reaction has received considerable attention due to via the “click method” to give the 4-(2-pyridyl)-1,2,3-triazole
19
its simplicity, mild reaction conditions, quantitative conver- complex, which was further metalated by PdCl
2
.
The cata-
sions and high tolerance of functional groups; this method has lytic properties of the synthesised B-PMO-TZ-Pd(II) were
been shown to be one of the best tools for covalent attachment screened in the Stille C–C coupling reaction. The synthesized
between appropriately functionalized molecules and solid materials were systematically characterised by nitrogen sorp-
6
supports via 1,2,3-triazole linkages. The 1,3-dipolar cycloaddi- tion, cross polarization magic angle spinning (CP MAS) NMR
tion reactions between organic azides and terminal alkynes and infrared spectroscopies, elemental analysis, and thermog-
were rst systematically studied by “Huisgen” several decades ravimetric and differential thermogravimetric (TGA-DTA) anal-
7
ago. These reactions demonstrate high regioselectivity to give yses to gain complete structural and compositional
1
,4-linked triazoles [click triazole] with excellent yields, high information.
atom efficiencies and low by-products. Since the discovery of the The small-angle X-ray diffraction (XRD) patterns of the (a) as-
click reaction, it has been applied in diverse areas of chemistry synthesized B-PMO, (b) surfactant-extracted B-PMO, (c) B-PMO-
such as dendrimers and polymers, drug discovery, materials and (d) B-PMO-TZ-Pd(II) are shown in Fig. 1. The as-
N
3
8
science and bioconjugation. Triazole ligands have the potential synthesized B-PMO sample exhibits an intense peak at a 2q
to act as nitrogen-donor ligands since they have long been value of 2.1 and one broad peak at a 2q value of 4.2. Aer solvent
known to coordinate with a variety of transition metals. The extraction of as-synthesized B-PMO, the peaks were well
9
click method provides an extremely convenient route for the resolved, and an increase in peak intensity was observed; this
synthesis of (2-pyridyl)-substituted 1,2,3-triazoles, which could clearly demonstrates the removal of surfactant molecules from
0
10
be potentially used as 2,2 -bipyridine analogues.
the meso-channels of the B-PMO. Three well-resolved peaks
The palladium-catalysed cross-coupling reactions of aryl were observed and indexed as the (100), (110), and (200)
20
halides or halide equivalents with various nucleophiles have reections of p6mm hexagonal symmetry. In the case of
been shown to be a highly effective and practical method for the 3-azidopropyltrimethoxysilane (3-Az-PTMS)-immobilized B-PMO,
formation of C–C bonds. These coupling reactions make use of B-PMO-N3 and B-PMO-TZ-Pd(II) (Fig. 1c and d), the two-
1
1
a variety of trans-metalating agents such as organoboron,
dimensional hexagonal pore structures are preserved. However,
12
13
14
organomagnesium, organosilicon, organostannane and a continuous decrease in peak intensity was observed aer each
organozinc reagents. Among these, the Stille reaction involves modication. This decrease in peak intensity implies that the
the coupling of an organotin compound with an sp -hybridized azide modication along with the Pd complex immobilization
15
2
organic halide catalysed by palladium. The popularity of the might disturb the order of the mesoporous structure to a certain
Stille reaction is typically attributed to the stability in air and degree.
functional group tolerance of stannanes as well as their chemo-
The removal of surfactant groups from the pore channels of
selectivity and broad scope in terms of reaction. This trans- the hybrid materials, the presence of benzene fragments in the
formation has become a useful synthetic tool for C–C bond framework, and further modication via triazole ligand (click
formation and can also be extended to numerous organic reaction) on the mesoporous framework were analysed by FT-
1
6,17
electrophiles.
Accordingly, in this report, we gra Pd complexes of 4-(2- B-PMO, surfactant-extracted B-PMO, B-PMO-N
pyridyl)-1,2,3-triazole, a type of “click reaction” product, and its TZ-Pd(II). In the as-synthesized B-PMO, strong bands are present
IR. Fig. 2a–d shows the infrared spectra of the as-synthesized
3
, and B-PMO-
ꢀ1
derivatives into B-PMO phases and investigate their catalytic at 2921, 2851 and 1471 cm ; the rst two bands correspond to
properties in the Stille C–C coupling reaction. The immediate the C–H stretching vibration of the surfactant C16-TMABr. The
goals of our study were (i) to evaluate the heterogenization band at 1471 cm is due to the C–H bending vibration. The
method of the 4-(2-pyridyl)-1,2,3-triazole Pd(II) complex on the absence of peaks at 2921 cm and 2851 cm in surfactant-
organo-hybrid mesoporous B-PMO support, (ii) to characterize extracted B-PMO (Fig. 2b) suggests the complete removal of
the heterogenized B-PMO-TZ-Pd(II) by various physiochemical the template during the solvent extraction process. The pres-
ꢀ
1
ꢀ
1
ꢀ1
21
techniques, (iii) to measure the catalytic properties of the ence of the aromatic C–H stretching vibration at 3000 and 3064
ꢀ
1
complexes on the Stille C–C coupling reaction, and (iv) to cm in all samples suggests the retention of organic fragments
determine the stabilities of the catalysts as well as their recy- (–C –) in the framework positions (BTEB). The spectra of all
6
H
4
ꢀ
1
cling properties.
four samples show strong bands at 812 and 690 cm , corre-
ꢀ1
sponding to aromatic C–H bending, along with at 1656 cm
corresponding to the aromatic C]C bending of 1,2-bis-
triethoxysilyl)benzene (BTEB) fragments. The asymmetric and
,
Results and discussion
(
The hybrid mesoporous catalyst B-PMO-TZ-Pd(II) was obtained symmetric stretching vibrations of the Si–O bands, which relate
22
by the procedure outlined in Scheme 1. First, 3-chloropropyl- to the presence of residual silanol (Si–OH) groups, were
ꢀ
1
ꢀ1
trimethoxy silane was converted into 3-azidopropyltrimethoxy observed at 1160 cm and 911 cm , respectively. In addition,
ꢀ
1
ꢀ1
silane followed by the surface modication of benzene con- the presence of intense bands at 1050 cm and at 690 cm
taining periodic mesoporous organosilica (B-PMO) with assigned to the C–Si aromatic stretching vibrations proves that
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Scheme 1 Schematic diagram of (A) synthesis of 3-azidopropyltrimethoxysilane, (B) functionalization of azide over benzene PMO, (C) B-PMO
modification via 4-(2-pyridyl)-1,2,3-triazole ligand, and (D) Pd metalation over B-PMO-TZ.
Fig. 2 FT-IR spectrum of (a) as-synthesized B-PMO, (b) surfactant-
extracted B-PMO, (c) B-PMO-N
Fig. 1 Small angle XRD patterns of (a) as-synthesized B-PMO, (b) AZPTMS.
surfactant-extracted B-PMO, (c) B-PMO-N , and (d) B-PMO-TZ-Pd(II).
Inset shows the enlarged image of higher order diffraction.
3
, (d) B-PMO-TZ-Pd(II), and (e) 3-
3
23
stretching vibration of organic azide (N
the organic bridge was not cleaved during solvent extraction or peak in B-PMO-TZ-Pd(II) demonstrates that the azidopropyl
further modication processes (Fig. 2b–d). B-PMO-N shows a group successfully reacted with 2-ethynylpyridine via click
sharp absorbance at 2106 cm , which is a characteristic reaction.
3
). The absence of this
3
ꢀ
1
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13
29
Fig. 3 Solid state C CP/MAS NMR spectra of (a) as-synthesized B- Fig. 4 Solid state Si CP/MAS NMR spectra of (a) as-synthesized B-
PMO, (b) surfactant-extracted B-PMO, (c) B-PMO-N
TZ-Pd(II). Inset: B-PMO-TZ-Pd(II).
3
, and (d) B-PMO- PMO, (b) surfactant-extracted B-PMO, (c) B-PMO-N
3
, and (d) B-PMO-
TZ-Pd(II).
1
3
3
Fig. 3 shows the C CP MAS NMR spectra of the (a) as- complex; a corresponding increase in the T sites, which show
synthesized B-PMO, (b) surfactant-extracted B-PMO, (c) B- more intense anchoring of the 3-azidopropyltrimethoxysilane
1
PMO-N
3
, and (d) B-PMO-TZ-Pd(II). All the samples show an (3-Az-PTMS), in addition to a small T peak are also visible. The
2
intense peak at 135 ppm along with sidebands (denoted with silicon resonances of partially condensed T [(SiO)
2
(OH)–Si–C]
Si–C] sites are highly intense
the BTEB precursor. In addition, the appearance of intense in comparison to the T [(SiO)(OH) –Si–C] site. This indicates
peaks at 32 and 67 ppm in as-synthesized B-PMO spectrum the presence of organic moieties inside the framework and a
Fig. 3a) correspond to the carbon atoms of surfactant mole- high degree of condensation of the silanol groups.
3
stars) corresponding to the framework benzene carbon atom of and complete condensed T [(SiO)
3
1
2
(
1
3
cules. These peaks are absent in the C NMR spectrum of the
Thermal analyses (TGA and DTA) of the (a) as-synthesized B-
surfactant-extracted B-PMO (Fig. 3b), indicating that the PMO, (b) surfactant-extracted B-PMO, (c) B-PMO-TZ, and (d) B-
surfactant groups were removed from the pores of the hybrid PMO-TZ-Pd(II) were carried out in air to investigate the thermal
22
sample during the ethanol extraction procedures. In the B- stabilities of the synthesized hybrid materials; the results are
PMO-N3 spectrum, three peaks at 12, 24, and 56 ppm were shown in Fig. 5. TGA plots of all the synthesized and modied B-
ꢁ
observed corresponding to the carbon (C ) attached to Si, the PMO samples show approximately 5% weight loss below 120 C
1
middle carbon (C ) and the carbon (C ) attached to the azide caused by the desorption of physisorbed water molecules. The
2
3
23
ꢁ
ꢁ
(
1
N
3
), respectively. The appearance of smaller peaks at 125, 136, ꢂ12 weight% loss observed between 200 C and 370 C for the
49 and 166 ppm clearly demonstrate the functionalization of as-synthesized B-PMO corresponds to the removal of trapped
the 4-(2-pyridyl)-1,2,3-triazole Pd(II) complex on B-PMO. These surfactant within closed pores [Fig. 5A(a) and B(a)]. In contrast,
NMR results demonstrate the proper functionalisation of the 4- the absence of TGA and DTA weight losses in the surfactant-
ꢁ
ꢁ
(
2-pyridyl)-1,2,3-traizole Pd(II) complex on the mesoporous B- extracted B-PMO between 200 C and 370 C [Fig. 5A(b) and
PMO.
B(b)] indicates the complete removal of surfactant from B-PMO.
2
9
Solid state Si CP/MAS NMR spectra of the (a) as-synthesized The matrix decomposition, i.e., the loss of benzene [1,2-
B-PMO, (b) surfactant-extracted B-PMO, (c) B-PMO-N , and (d)
3
B-PMO-TZ-Pd(II) are shown in Fig. 4a–d. The spectrum of the
surfactant-extracted B-PMO sample shows intense peaks at ꢀ57
2
ppm, corresponding to T sites [R–Si–(OSi)
2
–OH], and ꢀ68 ppm,
3
corresponding to T sites [R–Si–(OSi)
Si(OSi)
3
]. The absence of Q units
[Q
n
n
(OH)4ꢀn] below ꢀ100 ppm conrms that there is
negligible hydrolytic Si–C bond cleavage, and that the bridge-
bonded organic group is intact in the silica framework under
the synthetic conditions. Aer solvent extraction and modi-
n
cation, only T sites are present in all the prepared samples,
indicating that the solvent extraction treatments removed the
24
organic surfactant groups from the wall positions. The orga-
nosilica samples (Fig. 4b–d) also show a decreased percentage
of T sites aer graing with the 4-(2-pyridyl)-1,2,3-triazole Pd(II) surfactant-extracted B-PMO, (c) B-PMO-TZ, and (d) B-PMO-TZ-Pd(II).
Fig. 5 (A) TGA and (B) DTA plots of (a) as-synthesized B-PMO, (b)
2
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bis(triethoxysilyl)benzene (BTEB)] fragments bonded in the stabilization of the PdCl
2
complex on the modied B-PMO
mesopore frame wall channels observed in the range of 600–650 surfaces.
ꢁ
C for all samples (Fig. 5A and B), demonstrates that the present
The nitrogen adsorption–desorption results for B-PMO and
mesoporous host materials (B-PMO) are thermally stable up to a B-PMO-TZ-Pd(II) along with their corresponding pore size
ꢁ
25
temperature of 600 C. The DTA plot of B-PMO-TZ [Fig. 5B(c)] distribution curves are plotted in Fig. 7. The surface area,
ꢁ
shows two distinct exothermic peaks between 223–273 C and average pore diameter (D
3
p
) and pore volume (V
p
) observed for
ꢁ
18–338 C due to the propyltrimethoxysilane linker group and the B-PMO and B-PMO-TZ-Pd(II) samples are summarized in
the 1,2,3-triozole ring, respectively, along with the benzene Table 1. All samples show type-IV adsorption–desorption
ꢁ
fragment exothermic peak at 611 C [Fig. 5A(c)]. B-PMO-TZ- isotherms with the nitrogen uptake step at P/P
0
¼ 0.3–0.4,
ꢁ
ꢁ
Pd(II) also exhibits two broad peaks between 250 C and 400 C according to the IUPAC classication of mesopores.
and between 450 C and 600 C; the former weight loss may be
due to the decomposition of the 1,2,3-triaozole Pd complex volume were 796 m g , 38 A, and 0.49 cm g , respectively,
occluded in the mesoporous benzene-PMO, while the latter for B-PMO and 412 cm g , 33 A, and 0.21 cm g , respectively,
weight loss might correspond to the decomposition of matrix for B-PMO-TZ-Pd(II). The decrease in the total mesoporous pore
ꢁ
ꢁ
The total surface area, average pore diameter and pore
2
ꢀ1
3
ꢀ1
˚
3
ꢀ1
3
ꢀ1
˚
benzene moieties.
The UV-vis spectra of the (a) as-synthesized B-PMO, (b) metal complex immobilization on the B-PMO surface is indic-
surfactant-extracted B-PMO, (c) B-PMO-N , (d) B-PMO-TZ, and ative of the graing of complexes inside the channels of mes-
e) B-PMO-TZ-Pd(II) are shown in Fig. 6. The spectra of all B- oporous B-PMO. From Table 1, it is clear that even though
volume (58%), surface area (49%) and pore diameter (14%) aer
3
(
PMO samples show two typical sharp absorption peaks at 220 silylation procedures changed the textural properties of the
and 275 nm corresponding to the p–p* interaction between the mesoporous material, the decrease is more prominent aer
phenylene groups present in the B-PMO network. Aer the complex immobilization since the bulkier organic moieties
surfactant extraction process, an increase in absorption inten- inside the pore channels occupy a large area of the void space.
sity of the 220 and 275 nm peaks was observed in B-PMO, which Due to the anchoring of the 4-(2-pyridyl)-1,2,3-triazole Pd(II)
may be due to an increase in p electron interaction in the complex, the capillary condensation steps of B-PMO-TZ-Pd(II)
0
phenylene ring without surfactant molecules. In the case of B- result in reduced P/P values. This shi to slightly lower
PMO-TZ, the p–p* transition between 275 and 288 nm is well partial pressures, which indicates a possible reduction in pore
26
matched with the reported value. The palladium complex B- size and a partial distortion in pore arrangement, is consistent
PMO-TZ-Pd(II) shows broader absorption bands between 275 with the XRD results. The inection position in N sorption is
2
and 288 nm (Fig. 6e) than B-PMO-TZ (Fig. 6d). The B-PMO-TZ- known to depend on the diameter of the mesopores, and
Pd(II) shows two broad bands at around 379 nm and 424 nm, the sharpness usually indicates the uniformity of the
which are tentatively assigned to the metal to ligand charge mesopores due to the capillary condensation of N
transfer transition (MLCT); according to the literature, these mesopores.
2
within the
+
2
two bands can be assigned to the MLCT and the Pd d–d
The morphology and structural ordering of the hybrid cata-
27
transition, respectively. The broad absorption at up to 600 nm lysts were investigated by SEM and TEM. The morphologies of
in the spectrum of B-PMO-TZ-Pd(II) may be due to the strong the surfactant-extracted B-PMO and B-PMO-TZ-Pd(II) are shown
in Fig. 8A and B, respectively. B-PMO shows uniform arrays of
Fig. 6 DRS-UV-visible spectra of (a) as-synthesized B-PMO, (b)
surfactant-extracted B-PMO, (c) B-PMO-N
PMO-TZ-Pd(II).
3
, (d) B-PMO-TZ, and (e) B- Fig. 7 Nitrogen adsorption–desorption isotherms and BJH desorption
pore size distribution (inset) for (a) B-PMO and (b) B-PMO-TZ-Pd(II).
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Table 1 Textural properties of surfactant-extracted B-PMO & B-PMO-TZ-Pd(II) samples
b
Loading of Pd (wt%)
BET surface area
Average pore
Pore volume
(V , cm g )
p
a
2
ꢀ1
3
ꢀ1
˚
Sample
N (wt%)
Input
Output
(m g
)
P
diameter (D ) (A)
B-PMO
B-PMO-TZ-Pd(II)
796
412
38
33
0.49
0.21
1.5
4
2.2
a
b
Calculated based on the elemental (nitrogen) analysis value. Input is based on the amount of Pd during the synthetic reaction; output is based on
ICP-OES analysis.
X-ray photoelectron spectroscopy (XPS) is a powerful tool to
investigate the electronic properties of species formed on the
surfaces of the materials. The electronic environment, oxida-
tion state and multiplicity inuence the binding energy of the
core electron of the metal. The synthesised heterogeneous
catalyst B-PMO-TZ-Pd(II) was characterized by XPS. The accu-
racy of the measured binding energy (BE) was ꢃ0.3 eV. The
spectrum in Fig. 10, which displays the Pd binding energy in
B-PMO-TZ-Pd(II), exhibits two well-resolved peaks centred at
3
3
42 and 336.2 eV; these peaks are assigned to Pd 3d3/2 and Pd
5/2, respectively. The observed binding energy peaks indi-
Fig. 8 SEM images of (A) surfactant-extracted B-PMO and (B) B-
PMO-TZ-Pd(II).
d
cate that palladium is in a +2 oxidation state in the synthe-
sized B-PMO-TZ-Pd(II). According to literature, the pure
PdCl metal salt binding energies for the Pd 3d3/2 and Pd 3d5/2
2
8
2
meso-channels and clear molecular-scale periodicity in the SEM
images. The images of B-PMO-TZ-Pd(II) demonstrate that the
large molecular system is more dense than B-PMO aer the
anchoring of the 4-(2-pyridyl)-1,2,3-triazole Pd(II) complex over
the mesoporous surface.
The transmission electron microscopy (TEM) images of B-
PMO and B-PMO-TZ-Pd(II) are shown in Fig. 9A and B, respec-
tively. These images provide structural evidence that the mate-
rials organized into ordered arrays of two-dimensional
hexagonal mesopores. The highly uniform mesopores are
clearly evident from the TEM image of B-PMO (Fig. 9A), and the
formation of the uniformly distributed 4-(2-pyridyl)-1,2,3-
triazole Pd(II) complex inside the meso-channels is observed
in the TEM image of B-PMO-TZ-Pd(II) (dispersed dark spots in
Fig. 9B). These results clearly indicate that the mesopores in B-
PMO were retained even aer repeated treatments to anchor the
2
9
orbitals appear at 342.8 and 337.6 eV, respectively. In
comparison to the literature values, the Pd binding energies of
the synthesized B-MO-TZ-Pd(II) are shied towards lower
values (342 eV for Pd 3d3/2 and 336.2 eV for Pd 3d5/2). The
remarkable decrease in the Pd binding energy in B-PMO-TZ-
Pd(II) implies that there is a strong coordination interaction
between the Pd metal species and the 4-(2-pyridyl)-1,2,3-
triazole ligand. These results are in agreement with the UV-
vis observations.
4-(2-pyridyl)-1,2,3-triazole Pd(II) complex over B-PMO.
Fig. 9 TEM images of (A) surfactant-extracted B-PMO and (B) B-
PMO-TZ-Pd(II).
Fig. 10 XPS spectrum of B-PMO-TZ-Pd(II).
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Stille coupling reaction
(CsF); a biphenyl yield of 95% along with a turnover number
(
TON) of 239 were obtained.
To examine the scope of the catalytic properties of the
The Stille coupling reaction is an important reaction for the
synthesis of unsymmetrical and symmetrical biaryls. One of the
reasons for the popularity of the Stille reaction in modern
organic synthesis is the fact that triarylorganotin species are
readily available, quite stable in the presence of air and mois-
ture, and tolerate many functional groups. These properties are
mostly due to the low polarity of the Sn–C bond. The original
Stille coupling reaction requires a highly dry organic solvent, an
inert atmosphere, prolonged reaction time and a phosphine-
catalyst for the Stille coupling reaction, a variety of aryl halides
were reacted with phenyltributyltin in the presence of a catalytic
amount of B-PMO-TZ-Pd(II) (30 mg) at 120 C in the presence of
ꢁ
CsF base and DMSO solvent (Table 2). As shown in Table 2, the
Stille coupling reaction of phenyltributyltin with various aryl
halides proceeds smoothly under mild reaction conditions
giving the corresponding coupled products in high yields (75–
99%; Table 2, entries 1–10). The reaction of the mono-
substituted aryl halides chlorobenzene, bromobenzene and
iodobenzene with phenyltributyltin gave biphenyl yields of
35%, 75%, and 99%, respectively, and corresponding TONs of
30,31
containing catalyst.
The catalytic properties of the synthe-
sized B-PMO-TZ-Pd(II) catalyst were investigated by reacting
iodobenzene with phenyltributyltin in dimethylsulfoxide
ꢁ
(
DMSO) at 120 C in the presence of the base cesium uoride
8
4.6, 181.4, and 239.4 (Table 2, entries 1, 2, 3). The low reactivity
a
Table 2 Reactivity of the B-PMO-TZ-Pd(II) catalyst for the Stille coupling reaction
S. no.
Aryl halide
Product
Time (h)
Yield (%)
TON
84.6
181
1
2
24
10
35
75
3
4
5
5
99
95
239
229
5
8
95
229
6
7
8
4
12
5
99
96
99
239
232
239
9
9
89
215
a
Reaction conditions: iodobenzene (1 mmol), phenyltributyltin (1.15 mmol), base cesium uoride (CsF; 3 mmol), DMSO solvent (3 mL),
ꢁ
1
13
heterogeneous B-PMO-TZ-Pd(II) catalyst (30 mg), temperature 120 C. Isolated yields (product) were analysed by GCMS and H, C CP MAS
NMR (see ESI). TON (turnover number) ¼ moles of product converted per mole of Pd.
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of chlorides is usually attributed to the strength of the C–Cl phenyltributyltin was carried out in the presence of DMSO (3
ꢁ
bond (bond dissociation energies for Ph–X where X ¼ Cl: 96 kcal mL) at 120 C, giving a 99% yield of biphenyl within 2 h.
ꢀ
1
ꢀ1
ꢀ1
mol ; Br: 81 kcal mol ; I: 65 kcal mol ). From Table 2, it is Generally, the Stille coupling reaction generates Bu SnI
3
evident that the reactivity of aryl chlorides and bromides with precipitates, which may be deposited on the pore walls of
an arylstannane (phenyltributyltin) was lower than that of aryl BPMO-TZ-Pd(II) and cause a decrease in the catalytic activity. In
iodides, requiring comparatively longer reaction times for the present system, the Bu
3
SnI did not slow the reaction,
completion of the reaction. presumably due to the short and highly connected channels in
Furthermore, electron rich and electron poor aryl halides the meso-structures of the organosilica B-PMO-TZ-Pd(II) mate-
react smoothly with phenyltributyltin in similar reaction condi- rial, which provides good reactant and product transport. Aer
tions. The electron poor aryl halides 4-chloronitrobenzene, 4- the reaction, the catalyst was recovered by centrifugation,
bromonitrobenzene and 4-iodonitrobenzene reacted with phe- washed with DMSO and then dichloromethane, sonicated in
nyltributyltin to give the coupled product 4-nitrobiphenyl in acetone and nally dried under vacuum overnight. The recov-
yields of 95%, 95%, and 99%, respectively, with TON values of ered B-PMO-TZ-Pd(II) was re-used for the same reaction, and the
229.7, 229.7, and 239.4, respectively (Table 2, entries 4, 5, 6). same procedure was repeated four times; the catalyst retained a
Since the rst step of the Stille mechanism oxidative addition of similar level of activity, indicating the stability of the catalyst
an aryl halide to a Pd metal centre results in a Pd(II) transition (Fig. 11). The present study indicates that the catalyst can be
state, oxidative addition is oen the rate-determining step in the recycled a number of times without losing a signicant amount
catalytic cycle. The relative reactivity of aryl halides towards the of its activity.
Pd metal centre decreases in the order: I > Br [ C1. Aryl halides
To test if metal was leached from the solid catalyst during
activated by their proximity to electron withdrawing groups are reaction, a hot ltration test was performed. In this process,
more reactive in the oxidative addition than those with donating the Stille coupling reaction mixture was collected by ltration
ꢁ
groups, allowing the use of electron decient halides such as 4- at the reaction temperature (120 C) aer a reaction time of 1 h,
chloronitrobenzene in the cross-coupling reaction. Further, aryl which gave 58% conversions of iodobenzene. The residual
iodides are more reactive than bromides and chlorides; the activity of the supernatant solution was studied. Aer ltration
substituent effect in the aryl iodides appeared to be less signif- of the B-PMO-TZ-Pd(II) catalyst from the reaction mixture at the
ꢁ
icant than in the aryl chlorides and bromides.
elevated reaction temperature (120 C; in order to avoid
The electron rich aryl halides show moderate to excellent possible re-coordination or precipitation of soluble palladium
reactivity (89–99% yield) for the formation of the corresponding upon cooling), the coupling reactions did not proceed further.
products (4-methylbiphenyl, 4-methoxybiphenyl) in the Stille These hot ltration test results suggest that Pd was not leached
coupling reaction under similar reaction conditions. The reaction from the solid catalyst during the coupling reactions (Fig. S1,
of the electron rich para-substituted aryl halides 4-bromotoluene, ESI†). These results conrm that the palladium catalyst
4-iodotoluene and 4-bromoanisole with phenyltributyltin affor- remains on the support, even at elevated temperatures during
ded 96%, 99%, and 89% yields aer 12 h, 5 h and 9 h of reaction the reaction.
time, respectively (Table 2, entries 7, 8, 9). The coupling reaction
of chloronaphthalene with phenyltributyltin was investigated
under similar reaction conditions, but no desired coupling
product was obtained, even aer 24 h of reaction time.
Further, the hydrophobic nature of the support along with
the hydrothermal stability and well-ordered mesoporous
structure of the framework walls of B-PMO makes the PMO
support more advantageous than the classical nano-structured
silica support. Due to the presence of evenly-distributed
organic moieties (benzene) in dened positions and the cova-
lent anchoring of the metal complex, B-PMO-TZ-Pd(II) is robust
enough to withstand the harsh conditions of the catalytic
32
reactions. It also provides better access for the substrate
molecules into the well-dened active sites in the hydrophobic
pore channels of the PMO supports. Moreover, the hetero-
genized B-PMO-TZ-Pd(II) catalyst offers further advantages such
as the easy separation of catalyst and product as well as the
possible reuse of the catalysts.
Heterogeneity and recycling studies of the catalyst B-PMO-TZ-
Pd(II)
Fig. 11 Recycling of the B-PMO-TZ-Pd(II) catalyst. Reaction condi-
tions: iodobenzene (1 mmol), phenyltributyltin (1.15 mmol), base (CsF;
To investigate the recyclability of the optimized catalyst B-PMO-
TZ-Pd(II), the Stille coupling of iodobenzene and catalyst (30 mg), temperature 120 C.
3
mmol), DMSO solvent (3 mL), heterogeneous B-PMO-TZ-Pd(II)
ꢁ
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General procedure for Stille coupling reactions
Conclusion
To investigate the catalytic properties of B-PMO-TZ-Pd(II), a
Stille coupling reaction was carried out in a 25 mL two neck
round bottom ask. In a typical reaction, aryl halide (1 mmol),
phenyltributyltin (1.15 mmol), cesium uoride (CsF; 3 mmol),
In summary, a highly stable and recyclable B-PMO-TZ-Pd(II)
catalyst has been synthesized by anchoring 4-(2-pyridyl)-
1
,2,3-triazole over the inner surface of organo-
functionalized B-PMO and the subsequent complexation
with PdCl . The physiochemical properties of the function-
dimethylsulfoxide (DMSO; 3.5 mL) and B-PMO-TZ-Pd(II) catalyst
2
ꢁ
(
30 mg) were stirred under argon at 120 C. The reaction
alized B-PMO-TZ-Pd(II) catalyst were analysed by elemental
mixture was analysed by GC at measured time intervals. The
products were separated by column chromatography and ana-
analysis, ICP-OES, XRD, N sorption, TGA & DTA, solid state
2
1
3
29
C, Si NMR, FT-IR, XPS, UV-vis, SEM and TEM. XRD and
2
sorption analyses revealed the textural properties of the
1
13
lysed by GCMS, H, and C NMR (see ESI†).
N
synthesised catalyst and conrmed that the metal complex
was rmly attached to the B-PMO support. The (100), (110)
and (200) reections in the small angle XRD patterns of B-
PMO provided evidence for the existence of long range
order with structural stability. The thermal stabilities of the
synthesized B-PMO and B-PMO-TZ-Pd(II) were revealed by
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