Synthesis and characterization of functionalized titania-supported Pd catalyst deriving from new orthopalladated complex of benzophenone imine: Catalytic activity in the copper-free Sonogashira cross-coupling reactions at low palladium loadings

Article abstract of DOI:10.1039/c6ra17914b

The present work describes the preparation of organically modified TiO₂-supported Pd catalyst originated from the new benzophenone imine-derived CN-palladacycle. The heterogeneous organic-inorganic hybrid catalyst system has been characterized by FT-IR, XRD, SEM, EDX, TEM and XPS techniques and exhibited good catalytic activity in the Sonogashira cross-coupling reactions of phenylacetylene with aryl halides under copper-, amine- and phosphine-free conditions, in conjunction with the ultra low catalyst Pd-loading. Significantly, the heterogeneous Pd catalyst allowed the reaction of phenylacetylene with aryl iodides to improve in excellent yields under very mild conditions using green solvent. Finally, the reusability of the supported Pd-complex was investigated by multiple reuses of the supported catalyst in subsequent Sonogashira cross-couplings.

Full text of DOI:10.1039/c6ra17914b

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Page 1 of 34
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Synthesis and characterization of functionalized titania-supported Pd
catalyst deriving from new orthopalladated complex of benzophenone
imine: catalytic activity in the copper-free Sonogashira cross-coupling
reactions at low palladium loadings
a
a
b
Kazem Karami,* Sedigheh Abedanzadeh, Pablo Hervés,
a
Department of Chemistry, Isfahan University of Technology, Isfahan 84156/83111, Iran. Eꢀmail: karami@cc.iut.ac.ir
Departamento de Quimica Fisica, Universidade de Vigo, 36310 Vigo, Spain. Eꢀmail: jherves@uvigo.es
b
Abstract
The present work describes the preparation of organically modified TiO ꢀsupported Pd catalyst
2
originated from the new bezophenone imineꢀderived CNꢀpalladacycle. The heterogeneous organicꢀ
inorganic hybrid catalyst system has been characterized by FTꢀIR, XRD, SEM, EDX, TEM and XPS
techniques and exhibited good catalytic activity in the Sonogashira crossꢀcoupling reactions of
phenylacetylene with aryl halides under copperꢀ, amineꢀ and phosphineꢀ free conditions, in
conjunction with the ultra low catalyst Pdꢀloading. Significantly, the heterogeneous Pd catalyst
allowed the reaction of phenylacetylene with aryl iodides to improve in excellent yields under very
mild conditions using green solvent. Finally, the reusability of the supported Pdꢀcomplex was
investigated by multiple reuses of the supported catalyst in subsequent Sonogashira crossꢀcouplings.
Keywords: palladacycle, TiO , Sonogashira, catalyst, heterogeneous
2

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1
. Introduction
Nitrogenꢀcontaining palladacycles have received growing interest due to their applications in many
areas including organic synthesis, optical resolution, design of new metallomesogens and antitumoral
drugs, asymmetric synthesis, intermolecular aromatic CꢀH bond activation, synthesis and reactivity of
1
ꢀ10
organometallic complexes with biologically important ligands and drug delivery.
They show
promising catalytic activities as a new family of homogeneous and heterogeneous palladium catalyst
1
1ꢀ15
precursors,
due to the fact that Nꢀdonor ligands strongly donate electrons to the metal center,
1
6, 17
stabilizing various oxidation states of metals and subsequently influencing their reactivity.
The Sonogashira coupling reaction of terminal alkynes with aryl halides is one of the most
important methods which has been widely applied for the synthesis of natural products,
1
8ꢀ23
pharmaceutical compounds and polymeric materials.
This reaction is generally catalyzed by
homogeneous Pd complexes in the presence of toxic phosphine ligands, using copper salts as coꢀ
2
4ꢀ29
catalysts and amine as a solvent or base, under inert condition which has so many drawbacks.
Consequently, there have been many efforts to introduce highly active and easily reusable catalysts
through convenient Cuꢀ, amine and phosphineꢀ free methods under environmentally friendly
conditions.
Supported palladium catalysts with high dispersion and narrow size distribution of palladium
particles would address the limited practical application of homogeneous palladium catalysts and
exhibit high activity, recyclability, low palladium loading and promising performances in the synthetic
and industrial chemistry. In this regard, a wide variety of solid materials as heterogeneous Pd supports
3
0
31
32
33
34
including active carbon, mesoporous silica, metal oxides, zeolites, magnetic materials,
3
5
36
37
hydrotalcites, hydroxyapatite, and organic–inorganic hybrid have been investigated. However,
many heterogeneous catalytic systems suffer from high palladium loading and/or low catalytic
38
activities due to the palladium agglomeration and catalyst deactivation.
In this study, we report the synthesis and characterizations of a nitrogenꢀcontaining
orthopalladated complex driving from bis[4ꢀ(dimethylamino)phenyl]methaniminium chloride
39ꢀ42
(
AuramineꢀO). In continuation of our interest in developing new supported catalysts,
herein we use

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a strategy to convert a homogeneous catalyst into a heterogeneous one through the stabilization of
active Pd sites onto the large surface area of inorganic support by an organic spacer to create organicꢀ
inorganic hybrid catalyst. A simple route is described to prepare a supported CNꢀpalladacycle catalyst
2
based on functionalized TiO as a very suitable inorganic support. The resulting ultra Pdꢀloaded
material was investigated as a catalyst for copperꢀ and amineꢀ free Sonogashira coupling reactions of
phenylacetylene with aryl halides which were carried out in the absence of phosphine ligands under
air. In the context of green chemistry, heterogeneous catalysis of Sonogashira coupling reactions of
phenylacetylene with aryl iodides carried out in ethanol as a green solvent.
2
. Results and discussions
2.1. Synthesis and characterization of chloro-bridged CN-palladacycle
Nitrogenꢀderived palladacycles, have attracted much attention as exciting catalyst precursors for cross
coupling reactions due to their accessibility, thermal stability, slow decomposition and high catalytic
1
1,43
activity.
New dinuclear CNꢀpalladacycle was synthesized through the reaction of palladium (II) acetate
with bis[4ꢀ(dimethylamino)phenyl]methaniminium chloride (AuramineꢀO) in 1:1 molar ratio. The
green orthopalladated chloroꢀbridged complex could be produced directly from the refluxing reaction
of Pd(OAc)
2
with AuramineꢀO ligand in toluene for one 24 hours (method A). However, during the
with AuramineꢀO ligand was carried out in THF, at
second method (B), the reaction of Pd(OAc)
2
room temperature for higher reaction time of 72 hours to afford the pure product in higher yield. The
dinuclear CNꢀpalladacycle was characterized by IR and NMR spectroscopies. The elemental analysis
result of the prepared compound was in good agreement with the calculated values.

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H C
CH3
N
3
N
H C
CH3
3
NH2+ Cl-
Auramine-O
Pd(OAc)2
(
i) or(ii)
-
HOAc
H C
CH3
N
3
N
CH3
H C
3
HN
Pd
Cl
Cl
Pd
NH
H C
CH3
3
N
N
CH3
Auramine-O Dimeric Palladacycle (ADP)
CH
3
Scheme 1 Synthetic method for new orthometallated compound. (i) toluene, ꢁ, 24 h ; (ii) THf, 72 h.
ꢀ1
As illustrated in Fig. 1, the FTꢀIR spectrum of AuramineꢀO ligand shows the peak at 1685 cm
4
4
ꢀ1
due to the C=N stretching vibration. This band appears at the lower energies (1606 cm ) in the
spectrum of synthesized dinuclear complex (Fig. 1b) indicating the coordination of the ligand to the
metal. As clearly shown in Fig. 1a, the iminium salt displays strong, broad NꢀH stretching absorptions
ꢀ1
in the 2250 to 3000 cm region, where overlap with CꢀH absorption occurs. The lower frequency
ꢀ1
region of FTꢀIR spectrum shows the characteristic bands at 674 and 416 cm related to the ν(PdꢀC)
45,46
and ν(PdꢀN) vibrations in the dinuclear complex, respectively (Fig 1b).
These vibrational bands
confirm the successful coordination of the CNꢀorthopalladation of AuramineꢀO ligand to the
palladium(II) center by covalent interaction.
1
In the H NMR spectrum, the signal related to the methyl protons of cyclometallated Auramineꢀ
1
O appears at 3.00 ppm. All of the H NMR signals for the aromatic protons of the complex are shifted
with respect to the parent iminium salt confirming the coordination of AuramineꢀO.

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2
Fig. 1 FTꢀIR spectra of: (a) AuramineꢀO ligand, (b) dinuclear palladium complex (ADP) (c) TiO , (d) fresh
TiO ꢀsupported Pd catalyst.
2
2
2.2. Synthesis and characterization of functionalized TiO - supported Pd catalyst
Organicꢀinorganic hybrid catalysts simultaneously have the advantage of easy separation and recovery
of the heterogeneous catalysts in addition to homogeneous reaction conditions. They are composed of
4
7,48
three components including support, organic spacer and an active center.

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In order to design heterogeneous catalysts, solid supports containing nitrogen as donor atoms
have been achieved special interest. Since Nꢀdonor ligands are air stable, less expensive and nontoxic,
they can be used to replace phosphine groups and coordinate to the various transition metal
complexes.
Due to the fact that inorganic supports like TiO
and resistance against solvent, chemical reagents and high temperature, a general rout has been
designed to support a CNꢀpalladacyclic complex onto the organically modifiedꢀTiO surface to
2
show advantages such as mechanical stability
4
9
2
prepare an efficient heterogeneous preꢀcatalyst. The synthetic procedure is illustrated in Scheme 2.
TiO usually shows pHꢀdependent surface charges in the aqueous solution because of the
2
2
existence of TiꢀOH on the surface. The absorption property of TiO greatly changes with environment
of different pH value. Considering that the catalytic properties of supported catalysts depend on their
preparation conditions, we tried to prepare the ligand functionalized TiO using the pHꢀdependent
2
5
0,51
procedure.
To obtain the TiO
2
ꢀsupported Pd catalyst, a mixture of chloroꢀbridged orthopalladated
were refluxed in acetone for 24 hours.
complex (ADP) and a functionalized TiO
2
Scheme 2 Synthetic method and proposed route for the synthesis of new TiO ꢀ supported catalyst. (i) dry
2
toluene, 12 h, reflux (ii) acetone, 24 h.

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2
The stabilization of CNꢀpalladacycle onto the functionalized TiO is due to donation of an
electron lone pairs on the nitrogen groups of 3ꢀaminopropyltrimethoxysilane (APTES) into the
unoccupied orbital of the palladium atoms in continuous of the bridge cleavage reaction of the dimeric
compound ADP (Scheme 2).
Fig. 1 shows the FTꢀIR spectra of AuramineꢀO ligand, AuramineꢀO dimeric palladacycle
(
ADP), TiO and fresh TiO ꢀsupported Pd catalyst. Corresponding to the FTꢀIR spectrum of TiO ꢀ
2 2 2
–
1
supported catalyst illustrated in Fig. 1d, the broad band centered at 500ꢀ600 cm is likely due to the
5
2
–1
vibration of the TiꢀO bonds in the TiO
2
lattice. The peak at 1627cm and the broad peak appearing
–
1
45,52ꢀ53
at 3451cm are assigned to vibrations of hydroxyl groups.
In modified sample these peaks
4
5, 54
overlap with broad bands of the stretching and deformation modes of NH
2
groups.
The stretching
ꢀ1
ꢀ1
vibrations of aliphatic groups are appeared at 2951 cm . The bands at 1124 and 1383 cm are
5
5
attributed to the characteristic absorptions of the SiꢀCH and SiꢀO groups, respectively. Also, the
2
−
1
presence of several bands with medium intensity in the 2839–3062 cm regions is allocated to CꢀH
stretching of groups (symmetric and asymmetric stretching).
The elemental analysis further proved the existence of C, H, and N atoms in the prepared TiO
supported Pd catalyst.
Powder Xꢀray diffraction (XRD) analysis for fresh catalyst exhibits the sharp reflections
belonging to the TiO nanoparticles in the anatase phase. Although no Pd species could be detected by
2
ꢀ
2
XRD pattern due to the low amount of Pd content or its high dispersion on the solid surface, the
presence of Pd species was clearly evidenced by Xꢀray photoelectron spectroscopy (XPS) verifying
2
the orthopalladetd complex ADP coated on the TiO .

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2
Fig. 2 Powder XRD pattern of TiO ꢀsupported Pd catalyst.
2
Fig. 3 shows high resolution XPS spectra of TiO ꢀsupported Pd catalyst. The Pd 3d spectrum in
Fig. 3a related to the fresh catalyst, shows the characteristic Pd 3d5/2 and 3d3/2 doublet peaks, with a
0
peak separation of 5.26 eV. The fitted 3d5/2 peaks are located near the expected positions for the Pd
2
+
56ꢀ58
56ꢀ57
and Pd species at around 335.3
and 337.6 eV , respectively. The fitting indicates that Pd in the
2
+
0
0
fresh catalyst is a mixture of Pd and Pd states. After the first run, Pd is observed in higher
2
+
0
proportions, which indicates that after the catalytic cycle Pd cations were transformed into Pd (Fig.
3b). According the Pd 3d spectrum of Pd catalyst after the forth catalytic cycle (Fig. 3c), presence of
0
the characteristic Pd 3d5/2 and 3d3/2 doublet peaks related to the Pd species confirming the conversion
2
+
0
2+
of the whole Pd ions to Pd . In fact, the percentage of Pd decreases from 53% for fresh catalyst to
6% from first run and approximately 0% for the fourth run as we can see in figure 3. According to
the XPS data of Pd catalyst before and after the recycle, the Si 2p spectrum shows only one
3
58
component, with the Si 2p peak position around 103.3 eV, assigned to SiꢀO bonds. The Ti 2p
spectrum shows the characteristic Ti 2p3/2 and 2p1/2 doublet peaks, with a peak separation of 5.54 eV.
2
The characteristic shakeꢀup peaks for TiO are also observed. The fitted 2p3/2 peak is located near the
5
8
expected positions for the TiO species at around 458.7 eV. The HRꢀXPS N 1s spectra of the fresh
2
5
8
Pd catalyst shows two components, one at around 400.0 eV attributed to CꢀN bonds and another
+
59
minor component at around 401.9 eV attributed to N ꢀH bonds. The N 1s fitted spectrum after the
58
first recycle shows one component at around 400.0 eV attributed to CꢀN bonds and after the forth

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recycle, the nitrogen presented a very low signal and the peak was not possible to fit. The Cl 2p
spectrum of the fresh Pd catalyst shows the characteristic Cl 2p3/2 and 2p1/2 doublet peaks, with a peak
separation of 1.6 eV. After the recycle, no chlorine signal is detected which indicate that after the
2
+
0
catalytic cycles Pd species were transformed into Pd and the ligand was removed from palladacycle.
6
0
2
As illustrated in the similar investigations the formation of TiO ꢀsupported Pd(0) nanoparticles
are possibly due to the donation of an electron lone pairs on the nitrogen groups into an unoccupied
orbital of the palladium nanoparticles. Moreover, the presence of ethanol as e solvent, possibly leads
to the reduction of Pd(II) to Pd(0).
XPS analysis showed that the palladium content of fresh catalyst was about 1.5 %. The
palladium content of catalyst after the reaction cycle was considerable, according to the XPS analysis.
Further information about homogeneous or heterogeneous nature of the catalyst was obtained by hot
filtration test which were in accordance with the results obtained from XPS analysis and confirmed
that using this catalyst reaction proceeds mostly under heterogeneous conditions.

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Fig. 3 XPS spectra of high resolution Pd 3d of (a) fresh TiO
2
ꢀsupported Pd catalyst, (b) after the first run and (C)
after the forth run.

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To obtain the morphology and particle size of the catalyst, FEꢀSEM images of the catalyst were
carried out (Fig. 4). Furthermore, energyꢀdispersive Xꢀray spectroscopy (EDX) obtained from SEM
shows the presence of Pd atoms as well as Ti, Si, N, and O atoms in the structure of fresh TiO2ꢀ
supported Pd catalyst (Fig. 5).
Fig. 4 FEꢀSEM image of fresh TiO ꢀsupported Pd catalyst.
2
2
Fig. 5 EDX spectrum of the fresh TiO ꢀsupported Pd catalyst
According to the TEM images, the Pd nanoparticles are formed and well dispersed through the
supported inorganic surface. It was verified that synthesis of catalyst through the pH adsorption
method played an important role in the distribution of nanoparticles. The size distribution of fresh
2
TiO ꢀsupported Pd catalyst is shown in Fig. 6. The histogram could be fitted to a Gaussian function,
yielding a mean size of 2.24 nm based on the sizes of 90 particles (Fig. 6a). According o the TEM

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0
images after the first run, the average particle size of Pd in supported Pd catalyst was 2.61 nm. The
0
average particle size of Pd after the forth run was 3.31 nm.
Fig. 6 TEM images of (a) fresh TiO
2
ꢀsupported Pd catalyst, (b) after the first run and (C) after the forth run: The
histograms illustrate the size distribution of Pd catalyst.

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2.3. Sonogashira reactions catalyzed by Pd NPs
There are several disadvantages in the earlier reported methods for the Sonogashira coupling reactions
such as employing expensive, toxic and air/moisture sensitive phosphine ligands, long reaction times,
low yields, inert conditions and environmental pollution caused by the formation of side products and
6
1
use of large amount of amines as organic solvents or bases. In the copperꢀmediated Sonogashira
6
2
coupling reactions, the Glaserꢀtype oxidative dimerization of the terminal alkynes cannot be avoided
which produce undesired side products (diynes) that are generally difficult to separate. Consequently,
in our screening experiments, the realization of a Sonogashira reaction catalyzed by the low Pdꢀ
loading of TiO
2
ꢀsupported Pd catalyst in the absence of copper, amine and phosphine ligands was our
goal.
The effectiveness of supported Pd catalyst for the Sonogashira crossꢀcoupling reaction of
iodobenzene with phenylacetylene was initially investigated. First we investigated the effect of solvent
in the Sonogashira reaction of iodobenzene with phenylacetylene (Table 1, entry 1ꢀ7). The yield was
2
significantly decreased when H O was used as the solvent (Table 1, entry 5); this may be due to the
insolubility of the substrate and catalyst in water. However, application of 1: 1 aqueous EtOH
improves the yield to some extent (Table 1, entry 6). On the other hand inferior results were obtained
3
with solvents like THF, CH CN, toluene and DMF (Table 1, entries 1ꢀ4). As shown in Table 1 better
performance of the catalytic system was observed when EtOH was used and based on the above we
have considered EtOH for further optimization. A very low Pdꢀloading of 0.005 mol % was found to
be the optimum amount of catalyst. To investigate the effect of the base which plays a crucial role in
the overall performance of a catalytic system in a Sonogashira reaction, several inorganic and organic
bases were tested (Table 1, entry 7ꢀ10). K
we have considered it for further optimization. The preliminary investigation for reaction optimization
was performed with 0.005 mol% of Pd catalyst and two equivalent of K CO in EtOH (4 mL) at 85ºC
2 3
CO was found to be reactive in our catalytic system and
2
3
under aerobic condition. The results obtained are highlighted in Table 1. The turnover frequency of the
catalyst was defined to be 5880, which was regarded as a factor to show the high efficiency of the
catalyst.

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Table 1
To demonstrate the versatility of the catalytic system, we investigated the reaction using a
variety of aryl iodide with phenylacethylene under the optimized conditions. It was observed that aryl
iodides underwent cross coupling within a short reaction time at very low catalyst Pdꢀloading of 0.005
mol%, with excellent yields (Table 2). They may produce very high TOFs in such catalytic reactions.
Table 2
To study the performance of TiO ꢀsupported Pd catalyst for the Sonogashira crossꢀcoupling
2
reaction of aryl bromide with phenylacethylene, optimization of the reaction conditions with various
solvents and catalyst dosages were applied for bromobenzaldehyde in different range of time reactions
and temperatures (Table 3). Among the different reaction conditions, maximum conversion was
observed when we have considered the reaction in DMF (4 mL), in the presence of 0.02 mol% of Pd
catalyst at 110 ºC, under aerobic condition for 8 hours.
Table 3
The generality of the current system has been investigated with several electronically diverse
aryl bromide and phenylacetylene (Table 4, entries 1ꢀ8). Different functionality at the phenyl ring of
the aryl halide moiety affects the reaction conditions. When the reactions of activated aryl bromides
containing electron deficient phenyl rings with phenylacetylene were carried out in DMF at 110 ºC for
8
h, excellent yields of desired crossꢀcoupling products were observed . However, the electron
donating groups at the para position of aryl bromides couple smoothly with phenylacetylene to give
good to excellent yield of cross coupling products. Considering the steric effect, aryl bromides with
ortho substituents, provided the corresponding products in high yields (Table 4, entry 4).

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Table 4
Finally Sonogashira reactions of phenylacetylene and aryl chlorides were also investigated in
our present system, under the optimized condition used for the case of aryl bromides. To check the
effectiveness of our protocol, phenylacetylene was reacted with electron rich 4ꢀaminorobenzene and
electron deficient nitrochlorobenzene. Although aryl chlorides are not so reactive and are employed
63ꢀ65
less in palladiumꢀcatalyzed coupling reactions,
modest to good yield of desired cross coupling
products was observed in all cases (Table 4, entries 9ꢀ12).
A possible catalytic reaction cycle for the Sonogashira reaction has been shown in Scheme 3,
2
over the active Pdꢀsites present in the TiO ꢀ supported heterogeneous catalyst. At first, oxidative
addition of aryl halide to the Pd catalyst takes place, followed by transmetallation of alkyne and lastly,
reductive elimination of the final product.

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2
Scheme 3 Possible catalytic cycle of the heterogeneously catalyzed Cuꢀfree Sonogashira reaction over TiO ꢀ
supported Pd catalyst.
Aryl halides containing electronꢀwithdrawing groups improve the oxidative addition of CꢀX to
2
5
the Pd catalyst, as an activated halo derivatives. However, electronꢀrich aryl halides require harsher
experimental conditions to give acceptable results. Although, the mechanism of the Sonogashira
reactions has not yet been established clearly, we assume that the lower tendency of electronꢀrich aryl
6
6,67
halides towards metalloꢀoxidative insertions could be responsible for this diminished reactivity.
Although, there is no second metal in the copperꢀfree Sonogashira reaction, the second step is
conventionally called transmetallation. Several studies have attempted to understand how the alkyne is
transferred to the Pd catalyst through the transmetallation step. Amines generally used in the
Sonogashira reaction, appreciably deprotonated the alkyne in solution. But despite the fact that the

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Sonogashira reaction is occurred in the amineꢀfree condition in this study, we cannot have the
deprotonation of the alkyne in solution. Therefore, deprotonation is hypothesized to occur on the Pd
2
center. Initially, the alkyne coordinates to the Pd center in the η ꢀfashion, pulling electron density
away from the acetylene. This makes the terminal proton more acidic, enabling deprotonation by
6
8ꢀ70
weaker bases (Scheme 3).
Importantly, the electronic properties of the terminal alkyne have
considerable effects the on the deprotonation mechanism: electron withdrawing groups (EWG)
68
decrease and electron donating groups (EDG) increase the energy required for deprotonation. The
0
base initially forms the more reactive species of the type Pd L
2
in the oxidative addition step lead to
7
0
accelerate the overall reaction. Using of stronger bases, an excess amount of phenylacetylene and
7
1
more acidic phenylacetylenes considerably increase the rate by affecting the deprotonation step.
Furthermore, the reaction kinetics of Sonogashira reaction of phenylacetylene with iodobenzene
with 0.02 mol% catalyst was investigated and the result is shown in Fig. 7. A control experiment
indicated that the coupling reaction did not occur smoothly in the absence of a catalyst. The yield of
the product increased quickly with reaction time until it reached 96% at the 30 minutes.
Fig. 7 Kinetic profiles of the Pd catalyst in the Sonogashira reaction. (a) Normal reaction kinetics in the presence
of catalyst; (b) Reaction kinetics in the absence of catalyst. Reaction conditions: iodobenzene (1.0 mmol),
2 3
phenylacetylene (1.2 mmol), K CO (2.0 mmol), EtOH (4 ml), catalyst (0.02 mol% Pd), 85 °C, 30 min.

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2.4. Comparison with the other studies
To further evaluate the catalytic performance of catalyst, a comparison was made with several
literatureꢀreported supported catalysts (Table 5). The reactions of iodobenzene with phenylacetylene
were taken as the standard reactions and the results are listed in Table 5. It can be clearly seen that for
the Sonogashira reaction of iodobenzene with phenylacetylene, the current catalyst is superior to
others in terms of catalytic efficiency. Moreover, this catalyst also proved to be the most efficacious in
activity for the Sonogashira reaction, as reflected from its high TOF value. It should be noted that the
supportedꢀPd catalyzed Sonogashira reactions of iodobenzene with phenylacetylene were carried out
in EtOH in the absence of organic solvents, which makes this catalytic capacity more impressive. Thus
this catalytic system provides a simple and green method for the Sonogashira reaction of aryl iodides.
7
2ꢀ77
Compared to previous research,
this catalytic system requires a very small amount of catalyst Pdꢀ
loading and can be run under relatively mild reaction conditions.
Table 5
2.5. Recyclability
The reusability of the catalysts is a very important theme and makes them useful for commercial
applications. After the catalytic reactions, the recovery and reusability of TiO ꢀsupported Pd catalyst
2
were initially investigated in the Sonogashira reaction of iodobenzene with phenylacethylene under the
same condition. Before running another cycle, the solid was separated by simple filtration and washed
with acetone, water and then dried. The catalyst is reusable and used for four runs. The results are
shown in Fig. 8.

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Fig. 8 A recyclability test for catalyst Sonogashira crossꢀcoupling reaction of iodobenzene with
phenylacethylene (reaction condition are the same as given in Table 2).
3
. Experimental
3.1. General
1
H NMR spectra were recorded at room temperature on a 400 MHz Bruker spectrometer. Chemical
shifts (δ) are reported relative to internal TMS. C, H and N elemental analysis were performed using a
PE 2400 series analyzer. Fourier transform infrared (FTꢀIR) spectra were obtained in KBr pellets with
a Jasco FT/IR 680 plus instrument. Scanning electron microscopy (SEM) studies were carried out at
1
5 kV using a HITACHI Sꢀ4160 instrument (Japan). Energy dispersive Xꢀray analysis (EDX) results
were obtained at 20 kV using Philips (Model XLꢀ30) instrument. Transmission electron micrographs
TEM) were obtained with a JEOL JEM 1010 microscope operating at an accelerating voltage of 100
(
kV. The Xꢀray powder diffraction (XRD) pattern was recorded using a Scintag Xꢀray diffractometer
with a 1.54 °A (Cu Ka) Xꢀray radiation source. The XPS experiments were performed in a SPECS
Sage HR 100 spectrometer with a nonꢀmonochromatic Xꢀray source (Magnesium Kα line of
1253.6 eV energy and a power applied of 250 W), placed perpendicular to the analyzer axis and
calibrated using the 3d5/2 line of Ag with a full width at half maximum (FWHM) of 1.1 eV. An
electron flood gun was used during measurements to neutralize charging effects. The selected
resolution for the spectra of the different elements was 15 eV of Pass Energy and 0.15 eV/step. All
ꢀ8
Measurements were made in an ultra high vacuum (UHV) chamber at pressure around 8·10 mbar. In

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the fittings GaussianꢀLorentzian functions were used (after a Shirley background correction).
Analytical TLC was performed using Merck TLC Silica gel 60 F254 glass plates. Gas
chromatographic (GC) analyses were performed using an Agilent Technologies 6890N chromatograph
equipped with a flame ionization detector (FID) and an HB 50+ column (length = 30 m, inner diameter
320 ꢂm, and film thickness = 0.25 ꢂm). The temperature program for the GC analysis was from 70 to
200 °C at 20 °C/min, held at 200 °C for 0 min, heated from 200 to 280 °C at 10 °C/min and held at
280 °C for 1 min. The inlet and detector temperatures were set at 260 °C and 280 °C respectively.
Products were identified by comparison with authentic samples. All the reactants and solvents were
used as commercially available chemicals without any purification.
3
.2. Preparation of Auramine-O dimeric palladacycle (ADP)
Method A: bis[4ꢀ(dimethylamino)phenyl]methaniminium chloride (AuramineꢀO) (67.5 mg, 0.2
mmol) was added to the solution of Pd(OAc) (50 mg, 0.2 mmol) in toluene and the resulting mixture
2
was refluxed for 24 hours. The resulting solution was evaporated and the green solid was
recrystallized with cold 1:5 CH Cl / nꢀhexane and cold Et O to give darkꢀgreen chloroꢀbridge dimeric
2
2
2
complex.
Method B: bis[4ꢀ(dimethylamino)phenyl]methaniminium chloride (AuramineꢀO) (67.5 mg, 0.2
mmol) was added to the solution of Pd(OAc) (50 mg, 0.2 mmol) in THF and the resulting mixture
2
was stirred for 72 hours. The solvent was then evaporated and the solid residue treated with 1:5
2 2 2
CH Cl / nꢀhexane and cold Et O to give darkꢀgreen chloroꢀbridge dimeric complex.
Yield (62ꢀ76 %); Anal. Calc. for C H N Cl Pd : C, 51.03; H, 3.02; N, 10.50; Found. C, 51.87; H,
34
24
6
2
2
ꢀ1
1
2
6
.98; N, 10.28; IR (KBr, cm ): ν (PdꢀN) = 416, ν (PdꢀC) = 674, ν (C=N) = 1606; H NMR (DMSOꢀd ,
ppm): δ = 3.00 (d, 24H, aliphatic), 6.38 (d, 2H, aromatic), 6.81 (d, 4H, aromatic), 7.01 (d, 2H,
aromatic), 7.41 (d, 4H, aromatic), 7.47 (s, 1H, aromatic), 8.94 (s, 1H, aromatic).

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3
.3. Preparation and characterization of the TiO
TiO was activated by refluxing in concentrated hydrochloric acid (6 M) for 24 h and then washed
thoroughly with the deionized water and dried before undergoing chemical surface modification.
Refluxing the activated TiO (10 g) with 1.5 mmol 3ꢀaminopropyltrimethoxysilane in dry toluene for
8 h. The solid materials were filtered off and washed with hot toluene and then dried in oven at 110
2
- supported Pd catalyst
2
2
1
°
C overnight to give the surface bound amine group. The catalyst was prepared by stirring a mixture of
(4 g) and ADP (0.52 mmol, 0.117 g) in dry acetone (100 ml) at room
surface bound ligand L@TiO
2
temperature for 24 h. After stirring, the resulting greenish solid was filtered, washed with large volume
º
of acetone, ethanol and ether. It was then dried in an oven at 95 C overnight to furnish the
corresponding catalyst.
3
.4. General experimental procedure for Sonogashira reaction with aryl iodides
A round bottomed flask equipped with a condenser was charged with aryl iodides (1.0 mmol),
phenylacetylene (1.2 mmol), K CO (2.0 mmol) and Pd catalyst (0.005 mol% of Pd) in EtOH (4 ml).
2
3
Typical procedure for Sonogashira crossꢀcoupling reaction was placed in an oil bath and the mixture
was stirred and heated at 85 °C for 30 minutes.
3.5. General experimental procedure for Sonogashira reaction with aryl bromides and
chlorides
A round bottomed flask equipped with a condenser was charged with aryl aryl bromides or chlorides
(
2 3
1.0 mmol), phenylacetylene (1.2 mmol), K CO (2.0 mmol) and Pd catalyst (0.02 mol% of Pd) in
DMF (4 ml). Typical procedure for Sonogashira crossꢀcoupling reaction was placed in an oil bath and
the mixture was stirred and heated at 110 °C for 8h.

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3.6. General procedure for recycling reactions
When the corresponding Sonogashira reaction according to the procedure described in previous
sections was finished, the catalyst was washed with acetone and water. It was dried under air and
reused without any pretreatment for repeating cycles.
3.7. Characterization of the products of Sonogashira cross-coupling reactions
1
1
,2-diphenylethyne: H NMR (400 MHz, CDCl ): δ 7.59ꢀ7.62 (m, 2H), 7.47ꢀ7.50 (m, 1H), 7.40ꢀ7.45
3
1
3
78
(
m, 2H). C NMR (100 MHz, CDCl
3
): δ 133.3, 132.5, 121.6, 129.1, 82.4.
1
1
7
1
1
7
-amino-4-(phenylethynyl)benzene: H NMR (400 MHz, CDCl
3
, ppm): δ = 7.59ꢀ7.68 (m, 2H), 7.38ꢀ
13
.29 (m, 5H), 6.70 (m, 2H), 3.54 (s, 2H). C NMR (100 MHz, CDCl , ppm): δ = 149.36, 133.91,
3
7
8
32.22, 129.22, 127.70, 122.81, 112.22, 112.91, 89.29, 87.32.
1
-methyl-3-(phenylethynyl)benzene: H NMR (400 MHz, CDCl
.45ꢀ7.54 (m, 5H), 7.29 (t, 1H, J = 7.5 Hz), 7.17 (d, 1H, J = 7.3 Hz), 2.42 (s, 3H). C NMR (100
, ppm): δ = 138.21, 134.11, 131.36, 129.22, 128.54, 128.52, 128.37, 128.0, 123.11,
3
, ppm): δ = 7.60ꢀ7.67 (m, 2H),
1
3
MHz, CDCl
3
7
8
1
1
1
4
1
1
22.45, 87.63, 86.19, 22.36.
1
-nitro-2-(phenylethynyl)benzene: H NMR (400 MHz, CDCl
3
, ppm): δ = 8.11 (m, 1H), 7.70 (m,
13
3
H), 7.68 (m, 2H), 7.52 (m, 1H), 7.41 (m, 1H), 7.35 (m, 3H). C NMR (100 MHz, CDCl , ppm): δ =
78
6.59, 133.97, 132.80, 132.11, 128.98, 128.64, 128.12, 125.44, 122.42, 119.11, 99.22, 85.21.
1
-nitro-3-phenylethynyl-benzene: H NMR (400 MHz, CDCl
3
, ppm): δ = 8.31 (dd, 1H), 8.11 (m,
13
H), 7.76 (ddd, 1H), 7.76 (ddd, 1H), 7.46ꢀ7.50 (m, 3H), 7.31ꢀ7.32 (m, 3H). C NMR (100 MHz,
79
CDCl , ppm): δ = 148.2, 137.2, 131.8, 129.4, 129.1, 128.5, 126.4, 125.2, 122.9, 122.2, 91.9, 86.9.
3
1
1
7
=
-nitro-4-(phenylethynyl)benzene: H NMR (400 MHz, CDCl
3
, ppm): δ = 8.23 (d, J = 8.8 Hz, 2H),
1
3
3
.61 (d, J = 8.8 Hz, 2H), 7.47ꢀ7.57 (m, 2H), 7.24ꢀ7.36 (m, 3H). C NMR (100 MHz, CDCl , ppm): δ
7
8
144.2, 132.8, 132.3, 131.3, 128.9, 128.2, 126.4, 125.1, 93.3, 87.3.
1
4
2
1
-(phenylethynyl)benzaldehyde: H NMR (400 MHz, CDCl
3
, ppm): δ = 9.96 (s, 1H), 7.75ꢀ7.84 (m,
13
3
H), 7.56ꢀ7.66 (m, 2H), 7.45ꢀ7.52 (m, 2H), 7.30ꢀ7.37 (m, 3H). C NMR (100 MHz, CDCl , ppm): δ =
78
93.3, 138.2, 133.5, 132.4, 130.6, 129.4, 128.8, 127.3, 121.2, 93.3, 88.4.

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1
1
2
1
1
2
-(4-phenylethynyl-phenyl)-ethanone: H NMR (400 MHz, DMSO, ppm): δ = 8.0 (d, 2H), 7.7 (d,
13
3
H), 7.6 (m, 2H), 7.5 (m, 3H), 2.6 (s, 3H). C NMR (100 MHz, CDCl , ppm): δ = 197.5, 136.3, 131.7,
79
31.6, 128.8, 128.5, 128.2, 122.6, 92.7, 88.6, 26.6.
1
-methoxy-4-(phenylethynyl)benzene: H NMR (400 MHz, CDCl
3
, ppm): δ = 7.42 (2H, m), 7.38 (d,
1
3
H, J = 8.8 Hz), 7.25 (m, 3H), 6.79 (d, 2H, J = 8.8 Hz), 3.73 (s, 3H). C NMR (100 MHz, CDCl
3
,
79
ppm): δ = 159.60, 133.0, 131.4, 128.30, 127.90, 123.60, 115.40, 114.0, 89.30, 88.0, 55.30.
1
2
-(phenylethynyl)pyridine: H NMR (400 MHz, CDCl
3
, ppm): δ = 8.55 (d, 1H, J = 5 Hz), 7.58ꢀ7.63
(
m, 1H), 7.51ꢀ 7.55 (m, 2H), 7.46 (d, 1H, J = 7 Hz), 7.26ꢀ7.35 ( m, 2H), 7.20 (s, 1H), 7.12ꢀ7.17 (m,
79
1H).
4
. Conclusion
A general approach has been designed to synthesize a new dinuclear chloroꢀbridged CNꢀpalladacycle
derived from bis[4ꢀ(dimethylamino)phenyl]methaniminium chloride (AuramineꢀO). In the next step,
2
we have successfully developed a new heterogeneous TiO ꢀsupported palladium catalyst originated
from the CNꢀpalladacycle precursor which is fully characterized with FTꢀIR, XRD, SEM, EDX, TEM
and XPS techniques. This hybrid catalyst demonstrated high catalytic activities in the copperꢀ, amine
and phosphineꢀfree Sonogashira coupling reactions of phenylacetylene with aryl halides (Cl, Br, I) in
the presence of very low catalyst Pdꢀ loading. This class of heterogeneous Pd catalyst allows the
reaction of phenylacethylene and aryl iodides to promote with remarkable yields under
environmentally green condition.
Acknowledgements
This work was financially supported by the Isfahan University of Technology. We gratefully
acknowledge the funding support received for this project from the Iranian Nano Technology Initiative
Council.

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Page 29 of 34
RSC Advances
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DOI: 10.1039/C6RA17914B
Tables
Table 1. Optimization of the reaction condition for the Sonogashira crossꢀcoupling reaction of iodobenzene with
phenylacetylene.
catalyst
I
+
Base-Solvent
Temperature-Time
a
Entry
[Pd](mol%)
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.02
Solvent
Base
Time
Temperature (°C)
Conversion (%)
1
2
3
4
5
6
7
8
9
DMF
K CO
6 h
85
85
60
41
2
3
CH CN
K CO
6 h
3
2
3
toluene
THF
K CO
4 h
85
trace
trace
trace
70
2
3
K CO
4 h
85
2
3
H O
K CO
4 h
85
2
2
3
Ethanol:H O
K CO
4 h
85
2
2
3
(
1:1)
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
K CO
30 min
85
99
2
3
Na CO
6 h
6h
85
43
2
3
K PO .3H O
85
90
3
4
2
1
1
1
0
1
2
NaOAc
4 h
85
10
K CO
6h
R. T.
85
trace
99
2
3
K CO
15 min
2
3
a
Reactions were monitored using GC.