Larock indole synthesis using palladium complexes immobilized onto mesoporous silica

Article abstract of DOI:10.1016/j.apcata.2010.08.046

Heterogeneous palladium catalysts were prepared by covalent immobilization of palladium (II) complexes of the general formula PdCl₂L₂ (L = P, CN) onto SBA-15 silica using a post-synthetic method. The state of the hybrid materials was characterized using a wide variety of molecular and solid-state techniques. In general, all the palladium modified solids exhibited highly ordered mesostructures while keeping the integrity of the parent molecular precursors. The catalytic performances of the materials were evaluated in the heteroannulation of 2-iodoaniline and 2-bromoaniline with triethyl(phenylethynyl)silane which showed high activities and selectivities with isolated yields in the range of 80-85%. Despite a decrease in the initial activity, quantitative conversion of iodoaniline to the expected product was observed over multiple recyclings, making these materials particularly efficient for such applications. The catalytic process was shown to be homogeneous in nature, the solid apparently serving as a reservoir for the metal between cycles.

Full text of DOI:10.1016/j.apcata.2010.08.046

Applied Catalysis A: General 388 (2010) 179–187
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Larock indole synthesis using palladium complexes immobilized onto
mesoporous silica
Nelly Bataila, Anissa Bendjerioub, Laurent Djakovitcha,∗∗, Véronique Dufaudb,∗
a Université de Lyon, CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, 2 avenue Albert Einstein, F-69626 Villeurbanne, France
b Université de Lyon, CNRS, UMR 5182, Laboratoire de Chimie, Ecole Normale Supérieure de Lyon, 46 Allée d’Italie, F-69364 Lyon Cedex 07, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 June 2010
Received in revised form 20 August 2010
Accepted 23 August 2010
Available online 23 September 2010
Heterogeneous palladium catalysts were prepared by covalent immobilization of palladium (II) com-
plexes of the general formula PdCl2L2 (L = P, CN) onto SBA-15 silica using a post-synthetic method.
The state of the hybrid materials was characterized using a wide variety of molecular and solid-
state techniques. In general, all the palladium modified solids exhibited highly ordered mesostructures
while keeping the integrity of the parent molecular precursors. The catalytic performances of the
materials were evaluated in the heteroannulation of 2-iodoaniline and 2-bromoaniline with tri-
ethyl(phenylethynyl)silane which showed high activities and selectivities with isolated yields in the
range of 80–85%. Despite a decrease in the initial activity, quantitative conversion of iodoaniline to the
expected product was observed over multiple recyclings, making these materials particularly efficient
for such applications. The catalytic process was shown to be homogeneous in nature, the solid apparently
serving as a “reservoir” for the metal between cycles.
Keywords:
Indoles
SBA-15
Palladium catalysts
Larock synthesis
Heterogenous catalysis
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
iodoanilines with internal alkynes known as the Larock indole
synthesis (Fig. 1) has emerged as one of the most powerful synthetic
The indole nucleus is an important substructure found in
numerous natural or synthetic alkaloids [1,2]. The diversity of the
structures encountered, as well as their biological and pharmaceu-
tical relevance, have motivated research aimed at the development
of new economical, efficient and selective synthetic strategies,
particularly for the synthesis of functional indole rings [3–6].
Classical methods include the Fischer indole synthesis from aryl
hydrazone, the Batcho–Leimgruber synthesis from o-nitrotoluenes
and dimethylformamide acetals, the Gassman synthesis from N-
haloanilines, the Madelung cyclization of N-acyl-o-toluidines and
the reductive cyclization of o-nitrobenzyl ketones. While these pro-
cedures have contributed to this important area, the synthesis of
functional indoles has proven challenging.
Recently, transition-metal catalyzed transformations, and par-
ticularly palladium catalyzed reactions, have been developed,
providing increased tolerance toward functional groups and gener-
ally leading to higher reaction yields [7–16]. Many of these methods
have proven to be most powerful and are currently applied in
the target- or the diversity-oriented synthesis of multi-functional
indoles. Recently, the palladium catalyzed heteroannulation of 2-
procedure to provide access to 2,3-substituted indoles [17–22].
However, these Larock procedures rely on the use of solu-
ble palladium catalysts (featuring often costly phosphine ligands),
leading to significant difficulties including the separation of the
catalytic material from the reaction mixture, lack of adequate cat-
alyst recycling methods and relatively high palladium and ligand
contamination of the products. The latter problems are intolerable
in the context of biological applications. Obviously, an analogous
catalytic heterogeneous method would eliminate any of these
drawbacks [23–26].
With the aim of resolving this issue we reported previously
the first successful use of heterogeneous palladium catalysts (Pd/C,
[Pd]/NaY) at low loadings (2 mol%) for indole syntheses under
ligand- and salt-free reaction conditions [27]. While the resulting
protocol featured reasonably priced reagents and solvents render-
ing the method competitive, practicable, and scalable it remained
limited due to relative lack of catalysts activity and recyclability.
In order to propose advanced protocols featuring highly active
and recyclable catalysts we undertook the development of hybrid
materials based on the SBA-15 mesoporous silica modified by
grafted palladium complexes bearing either phosphine or cyano
ligands. In this contribution, we describe the preparation and the
characterization of these new catalysts and their successful applica-
tion to the “salt-free” Larock indole syntheses. To fulfil economical
considerations, we used exclusively technical grade solvent for the
catalytic runs and demonstrated that the procedure tolerates very
∗
Corresponding author. Tel.: +33 472728857; fax: +33 472728860.
Corresponding author. Tel.: +33 472445381; fax: +33 472445399.
E-mail addresses: Laurent.Djakovitch@ircelyon.univ-lyon1.fr (L. Djakovitch),
∗∗
vdufaud@ens-lyon.fr (V. Dufaud).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.08.046

180
N. Batail et al. / Applied Catalysis A: General 388 (2010) 179–187
CP-MAS on 29Si and 13C. The spinning rate for both was 10 kHz and
samples were spun at the magic angle using ZrO2 rotors. The exper-
imental details for the 29Si and 13C CP-MAS NMR experiments were
as follows: contact time: 5 and 3 ms respectively, 90◦ 1H transmitter
pulse length: 3 ␮s, number of scans: 15,000–50,000 and repeti-
tion time: 4 s. 31P CP-MAS experiments were performed with a
2.5 mm double resonance Brucker MAS probe at a spinning rate
of 20 kHz. Contact time was set to 2 ms, repetition time to 10 ms
and the number of scans was fixed at 20,000 with a 90◦ 1H trans-
mitter pulse of 2.85 ␮s. High-resolution electron microscopy was
carried out with a JEM 2010 (Cs = 0.5 mm) microscope. The accel-
erating voltage was 200 kV with LaB 6 emission current, a point
resolution of 0.19 nm, and a useful limit of EDS LINK-ISIS. Energy-
dispersive X-ray microanalysis (EDX) was conducted using a probe
size of 25–100 nm to analyze grains of the phases. Samples were
sonically dispersed in ethanol and deposited on a holey carbon
copper grid before examination. XPS measurements were recorded
on a Kratos Axis Ultra DLD spectrometer, using a monochromated
Al K␣ X-ray source with a pass energy of 20 eV. The measure-
ments of the binding energies were referred to the characteristic
C 1s peak of the carbon fixed at the generally accepted value of
285.0 eV. Metal determinations were performed by ICP-AES (Activa
Jobin Yvon) spectroscopy from a solution obtained by treatment
of the solid catalyst with a mixture of HF, HNO3 and H2SO4 in a
Teflon reactor at 150 ◦C. Liquid NMR spectra were recorded on a
BRUKER AC-250 spectrometer. All chemical shifts were measured
relative to residual 1H or 13C NMR resonances in the deuter-
ated solvents: CDCl3, ı 7.26 ppm for 1H, 77.0 ppm for 13C; DMSO,
ı 2.50 ppm for 1H, 39.5 ppm for 13C. Data are reported as fol-
lows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet,
q = quartet, m = multiplet, br = broad). Melting points were deter-
mined in open capillary tubes and are uncorrected. Analytical thin
layer chromatography (TLC) was performed on Fluka Silica Gel 60
Fig. 1. Retro-synthetic approach toward the synthesis of indole nucleus following
the Larock palladium catalyzed methodology.
low catalyst loading (1 mol%) compared to those reported generally
for such syntheses under homogeneous conditions. Furthermore,
we evidenced the high recyclability of the catalytic materials.
2. Experimental section
All manipulations were conducted under a strict inert atmo-
sphere or vacuum conditions using Schlenk techniques including
the transfer of the catalysts to the reaction vessel. In the case of cat-
alysts synthesis, the solvents were dried using standard methods
´
˚
and stored over activated 4 A molecular sieves. Tetraethoxysi-
lane (TEOS) and poly(ethyleneoxide)-poly(propyleneoxide)-poly
(ethyleneoxide) block copolymer (Pluronic 123, Mw: 5000) were
purchased from Aldrich Chemical and used without further
purification. Palladium chloride, diphenylphosphine and dicyclo-
hexylphosphine were obtained from Fluka. 2-(diphenylphosphino)
ethyltriethoxysilane, 3-cyanopropyltriethoxysilane, 3-aminopro-
pyltriethoxysilane and 3-chloropropyltriethoxysilane were pur-
chased from ABCR and stored under nitrogen. 3-(dicyclohexyl-
phosphino)propyltriethoxysilane was cleanly obtained through a
one pot two-step synthesis by first reacting dicyclohexylphos-
phine with n-BuLi at 0 ◦C followed by addition to the reaction
mixture of 3-chloropropyltriethoxysilane at −78 ◦C. After stir-
ring at room temperature another 20 h the desired product was
purified by distillation under reduced pressure (73% yield, b.p.
150 ◦C (15 mm)) [35]. 3-{bis[(diphenylphosphanyl)methyl]amino}
propyltriethoxysilane was prepared in high yield by react-
ing 3-aminopropyltriethoxysilane with paraformaldehyde and
diphenylphosphine in refluxing toluene as described by Del
Zotto et al. [36]. Both bifunctional phosphines were char-
acterized by 31P, 1H and 13C NMR spectroscopy and the
results are in good accord with those reported in the litera-
ture. PdCl2(PhCN)2 and (3-cyanopropyltriethoxysilane) palladium
dichloride, PdCl2(NC(CH2)3Si(OEt)3)2 4, were prepared accord-
ing procedures previously described in the literature from PdCl2
and respectively benzonitrile and 3-cyanopropyltriethoxysilane at
80 ◦C during 2 days [37,38].
F254. GC analyses were performed on a HP 4890 chromatograph
equipped with a FID detector, a HP 6890 autosampler and a HP-5
column (cross-linked 5% phenyl-methylsiloxane, 30 m × 0.25 mm
i.d. × 0.25 ␮m film thickness) with nitrogen as carrier gas. GC–MS
analyses were obtained on a Shimadzu GC-MS-QP2010S equipped
with a Sulpelco SLB-5MS column (95% methylpolysiloxane + 5%
phenylpolysiloxane, 30 m × 0.25 mm × 0.25 ␮m) with Helium as
carrier gas. Conversions were determined by GC based on the rela-
tive area of GC-signals referred to an internal standard (biphenyl)
calibrated to the corresponding pure compounds. The experi-
mental error was estimated to be ꢁrel = 5%. Chemical yields
refer to pure isolated substances. Purification of products was
accomplished by flash chromatography performed at a pressure
slightly greater than atmospheric pressure using silica (Macherey-
Nagel Silica Gel 60, 230–400 mesh) with the indicated solvent
system.
Low-angle X-ray powder diffraction (XRD) data were acquired
on
a Bruker D5005 diffractometer using Cu K␣ monochro-
˚
matic radiation (ꢀ = 1.054184 A). Nitrogen adsorption–desorption
isotherms at 77 K were measured using a Micromeritics ASAP
2010M physisorption analyzer. The samples were evacuated at
160 ◦C for 24 h before the measurements. Specific surface areas
were calculated following the BET procedure. Pore size distribution
was obtained using the BJH pore analysis applied to the desorption
branch of the nitrogen adsorption/desorption isotherm. Infrared
spectra were recorded from KBr pellets using a Mattson 3000
IRFT spectrometer. A Netzsch thermoanalyser STA 409PC was used
for simultaneous thermal analysis combining thermogravimetric
(TGA) and differential thermoanalysis (DTA) at a heating rate of
10 ◦C min−1 in air from 25 to 1000 ◦C. Solid-state NMR MAS and
CP-MAS experiments were performed on a Bruker DSX 400 spec-
trometer at spectral frequencies of 161.99, 79.49 and 100.63 MHz
for respectively 31P, 29Si and 13C nuclei. Chemical shifts were ref-
erenced to 85% aqueous H3PO4 for 31P NMR and to TMS for 29Si
and 13C. A 4 mm triple resonance Bruker MAS probe was used for
2.1. Synthesis of homogeneous organophosphine palladium
catalysts
A
series of phosphine-based palladium complexes of the
type, PdCl2L2, with L = mono- or diphosphine, was synthesized
by reacting the appropriate amount of phosphine ligand (1 or
2 equivalents respectively for di- and monophosphine) with
bis(benzonitrile)palladium dichloride at room temperature in
methylene chloride by adapting a procedure reported elsewhere
[35].
2.1.1. Bis {(diphenylphosphino)propyltriethoxysilane)palladium
dichloride, 1
Yield: 95% as a yellow solid; 31P { H} NMR (81 MHz, CDCl3,
1
25 ◦C) ı 21.4 (s, trans isomer), 31.7 (s, cis isomer); 13C NMR

N. Batail et al. / Applied Catalysis A: General 388 (2010) 179–187
181
(50 MHz, CDCl3, 25 ◦C) ı 4.1 (s, CH2Si), 18 (s, CH3CH2O), 18.9 (t,
2.3. Catalysis
1J(C,P) = 13.4 Hz, CH2P), 58.3 (s, CH3CH2O), 128 (t, 1J(C,P) = 5.1 Hz,
phenyl ring), 128.3 (t, 1J(C,P) = 5.1 Hz, phenyl ring), 130.2 (s, phenyl
ring), 133.7 (t, 1J(C,P) = 5.9 Hz, phenyl ring); elemental analysis
calcd (%) for C40H58Cl2O6P2PdSi2: Pd 11.44, P 6.66; found: Pd 11.7,
P 6.04.
2.3.1. Catalytic procedure
The heteroannulation of 2-iodoaniline with triethyl
(phenylethynyl)silane was chosen as a benchmark reaction to
evaluate the performance of the palladium hybrid materials. The
catalytic reaction was carried out under an argon atmosphere in
a two-necked flask fitted with argon inlet and a septum allowing
direct sampling of the reaction mixture under argon flow. In a
typical procedure, the hybrid catalyst (1 mol% of Pd based on ele-
mental analysis) was first evacuated at 40 ◦C during 4 h to remove
physisorbed water and then suspended in 4 mL DMF followed by
addition of 2-iodoaniline (1 mmol), triethyl(phenylethynyl)silane
(3 mmol) and Na2CO3 (3 mmol). The reactor was placed under
stirring in a preheated oil bath at 120 ◦C. The advancement of the
reaction was monitored by GC–MS. After cooling at room temper-
ature, the reaction mixture was filtered through a pad of celite,
which was washed with EtOAc (100 mL). The resulting organic
layer was then washed with Na2CO3 (2× 40 mL) and brine (40 mL)
and finally dried over Na2SO4. After removal of the solvent under
reduced pressure, the crude product was fully deprotected accord-
ing to the previously reported method [27] before being purified
by flash chromatography over silica (petroleum ether/methylene
chloride 9:1). 3-phenyl-1H-indole was obtained as a white solid
in 80–85% yield depending on the catalytic solid involved in the
reaction. Data obtained are in accordance with the literature.
mp 85–87 ◦C; ␯max (KBr): 3410, 3390, 3055, 3035, 2925, 1538,
2.1.2. Bis{(dicyclohexylphosphino)
propyltriethoxysilane)}palladium dichloride, 2
1
Yield: 85% as yellow crystals; 31P { H} NMR (81 MHz,
CDCl3, 25 ◦C)
ı ı
22 (s); 13C NMR (50 MHz, CDCl3, 25 ◦C)
12.7 (t, 3J(C,P) = 6.4 Hz, CH2Si), 18.25 (s, CH3CH2O), 19.04 (s,
CH2CH2CH2Si), 21.96 (t, 1J(C,P) = 10.2 Hz, CH2P), 26.35 (s, Cp), 27.13
(t, 3J(C,P) = 5.3 Hz, Cm), 27.44 (t, 3J(C,P) = 5.9 Hz, Cm), 28.7 (brs, Co),
29.3 (brs, Co), 32.8 (t, 1J(C,P) = 11.2 Hz, Ci), 58.3 (s, CH3CH2O); ele-
mental analysis calcd (%) for C42H86Cl2O6P2PdSi2: Pd 10.83, P 6.30;
found: Pd 9.9, P 6.33.
2.1.3. {Bis[(diphenylphosphanyl)methyl]amino}
propyltriethoxysilanepalladium dichloride, 3
1
Yield: 99% as greenish solid; 31P { H} NMR (81 MHz, CDCl3,
25 ◦C) ı 7 (s); 13C NMR (50 MHz, CDCl3, 25 ◦C) ı 7.6 (s, CH2Si), 18.2
(s, CH3CH2O), 18.6 (s, CH2CH2CH2Si), 56 and 57 (dd, 1J(C,P) = 2.3 Hz
and 3J(C,P) = 47 Hz, NCH2P), 58.4 (s, CH3CH2O), 65 (t, 3J(C,P) = 10 Hz,
NCH2CH2CH2), 128.2–134 (phenyl carbons); elemental analysis
calcd (%) for C35H45Cl2NO3P2PdSi: Pd 13.4, P 7.8; found: Pd 13, P
7.5.
1486, 1338 cm−1 1H NMR (250 MHz, CDCl3) ı 8.07 (brs, 1H), 8.00
;
(d, J = 7.4 Hz, 1H), 7.71 (dd, J = 8.2, 1.0 Hz, 2H), 7.49 (t, J = 7.4 Hz, 2H),
7.44–7.18 (m, 5H); 13C NMR (63 MHz, CDCl3) ı 136.6 (C), 135.5 (C),
128.7 (CH), 127.4 (CH), 125.9 (CH), 125.6 (C), 122.3 (CH), 121.8 (CH),
120.3 (CH), 119.7 (CH), 118.2 (C), 111.4 (CH).
2.2. Synthesis of palladium mesoporous silica materials
2.3.2. Leaching procedure
Leaching studies were performed using the hot-filtration tech-
nique. After 10–40 min reaction of a standard catalytic run, the
supernatant solution was filtered at 120 ◦C through a hot cannula
with a microglass Whatman filter (in order to remove all fine parti-
cles). Then, the inorganic base (3 equiv.) filtered by part was added
and the mixture treated for further 6–15 h under standard reaction
conditions. The reaction was monitored over the total period by GC
and the results compared to a standard catalytic reaction.
The covalent immobilization of palladium complexes was per-
formed using a post-synthesis procedure by directly reacting the
alkoxysilane moieties of the phosphine (or cyano) ligand with
surface silanols of a surfactant-free mesoporous oxide. Meso-
porous SBA-15 type silica was used as support and was prepared
by the acid catalyzed, non-ionic assembly pathway described
elsewhere [28,29]. The structure directing agent (Pluronic 123)
was removed quantitatively from the as-synthesized material by
calcination at 500 ◦C overnight under air as evidenced by TGA
analysis and infrared spectroscopy. Prior to the grafting reac-
tion, the surfactant-free mesoporous silica was rigorously dried
under vacuum at 160 ◦C. Palladium complexes 1–4 (1.6 mmol)
dissolved in 20 mL of deaerated toluene were then added to a
suspension of SBA-15 silica (2 g) in dry toluene. The reaction
mixture was first stirred at 25 ◦C for 6 h to allow the diffu-
sion of the molecular precursor into the channels of the pores,
then heated overnight at temperatures ranging from 50 to 80 ◦C
depending on the stability of the palladium precursor. The result-
ing solid was filtered under nitrogen, washed thoroughly with
small amount of toluene and finally dried at 30 ◦C under vac-
uum.
2.3.3. Recycling procedure (multi-run experiments)
In the recycling studies, the catalyst activity was examined two
times (at 2 and 14 h reaction time) by check-up of conversion. In a
typical experiment, fresh catalyst was used as for a standard cat-
alytic run. After 24 h reaction new amounts of reagents (1 mmol of
2-iodoaniline, 3 mmol of triethyl(phenylethynyl)silane and 3 mmol
of Na2CO3) were added. The volume of solvent was adjusted in
order to restore the concentrations of reagents to that of the initial
run. Immediately after addition, based on GC analysis, the con-
centration of the 2-iodoaniline was considered as 100% and the
concentration in 3-phenyl-2-(triethylsilyl)-1H-indole to 0%. The
reaction was followed by GC for another 24 h and the procedure
was repeated four times.
[1]@SBA-15: 13C CP-MAS NMR ı 7.9, 17.4, 21, 59, 129; 29Si CP-
MAS NMR ı −44.7, −51.6, −99.5, −106; 31P CP-MAS NMR ı 22.9,
34.4; elemental analysis (wt%): Pd 3.06, P 1.88, Cl 2.12.
3. Results and discussion
[2]@SBA-15: 13C CP-MAS NMR ı 12, 20.7, 27.2, 59.5; 29Si CP-
MAS NMR ı −36.3, −41, −91.5, −98.4, −104.6; 31P CP-MAS NMR ı
23.8; elemental analysis (wt%): Pd 1.2, P 0.80, Cl 0.8.
3.1. Preparation of palladium mesoporous silica materials
[3]@SBA-15: 13C CP-MAS NMR ı 8.2, 17.4, 25.8, 58.9, 129.4; 29Si
CP-MAS NMR ı −38.5, −41, −86.4, −99.2, −104.6; 31P CP-MAS NMR
ı 11.7; elemental analysis (wt%): Pd 3.8, P 2.30, Cl 2.69, N 0.65.
[4]@SBA-15: 29Si CP-MAS NMR ı −46.7, −56, −65.1, −88.9,
−98.1, −105.7; elemental analysis (wt%): Pd 1.75, N 0.42.
Heterogeneous catalyst synthesis was achieved through the
covalent immobilization of palladium-based complexes on the
surface of preformed mesoporous SBA-15 type silica. SBA-15 sil-
ica represents one of the most attractive host materials for the
immobilization of catalytically relevant species owing to its large

182
N. Batail et al. / Applied Catalysis A: General 388 (2010) 179–187
Scheme 1. .
˚
pore volume and diameter (up to 80 A) which allow for bet-
ter reactant and product diffusion, hence reducing mass transfer
limitations and allowing even large molecules to access the cat-
alytic sites. In addition, SBA-15 possesses thick walls providing
thermal and hydrothermal stability which may be a key factor
for recycling experiments [28,29]. The covalent link to the sil-
ica surface was introduced through the ligands coordinated to
the metal centre which implied the use of bifunctional molecules
bearing on one side polycondensable organosiloxane precursor
moiety and on the other end functional group such as phosphine
or nitrile able to bind the metal site. Four different palladium
(II) complexes of the general formula PdCl2L2 were considered
(Scheme 1). The simplest ligand tested was the commercial 2-
(diphenylphosphino)ethyltriethoxysilanewhich aftercoordination
led to a palladium complex 1 bearing two monodentate phosphine
linkers. A saturated analog of this ligand (cyclohexyl in place of
phenyl, complex 2) provides a ligand system which is both more
electron-rich and more sterically hindered. The chelating diphos-
phine (EtO)3SiCH2CH2CH2N(CH2PPh2)2, which should provide a
stronger metal–ligand bonding, and its corresponding palladium
complex 3 were prepared to gain insight into the effect of chela-
tion on catalyst performance and stability toward leaching. Finally,
a commercial, phosphine-free ligand (EtO)3SiCH2CH2CH2CN was
tested affording an ethoxysilane-modified version of PdCl2 (com-
plex 4), particularly interesting for applications under oxidizing
conditions which render phosphine-based systems unstable.
The covalent grafting of palladium complexes 1–4 onto organic-
free SBA-15 silica was carried out in degassed toluene at
temperatures ranging from 50 to 80 ◦C depending on the stability
of the palladium precursor. The unreacted palladium complexes
were removed by thorough washing the solid with toluene and
methylene chloride leading to hybrid Pd materials referred to as
[X]@SBA-15 with X = 1–4. Several methods were used to character-
ize the molecular and bulk properties of the catalytic solids: X-ray
Fig. 2. X-ray powder diffraction patterns of SBA-15 mesoporous materials after
grafting of palladium precursors.
powder diffraction (XRD) at small and wide angles, transmission
electron microscopy (with EDX), elemental analysis, thermo-
gravimetric analysis, infrared spectroscopy, multi-nuclear NMR
spectroscopy and nitrogen sorptions. The physicochemical prop-
erties of the hybrid materials derived from the powder XRD and
sorption analyses are summarized in Table 1. Typical XRD pat-
terns of the hybrid materials are shown in Fig. 2. All the palladium
modified solids exhibited typical diffractograms characteristic of
hexagonally ordered mesophases with the presence of three well
resolved peaks in the 2ꢂ-range of 0.6◦–3◦ attributed respectively
to the (1 0 0), (1 1 0) and (2 0 0) reflexions. The presence of higher
order reflexions indicates that the chemical bonding procedure
Table 1
Physical and textural properties of palladium containing hybrid silica materials.
a
b
d
e
Catalyst
Pd (%wt
)
d100 (Å)
a0 (Å)
Wall thicknessc (Å)
Vp (cm3 g−1
)
Dp (Å)
SBET (m2 g−1
)
CBET
[1]@SBA-15
[2]@SBA-15
[3]@SBA-15
[4]@SBA-15
3.06
1.2
3.8
96
100
105
98
111
115
121
113
57
52
63
50
0.66
0.80
0.53
0.87
54
63
58
63
601
553
367
644
181
114
110
122
1.75
Different batches of bare SBA-15 silica were used to construct the various catalysts and their physical and textural properties are summarized in Supplementary Material
(Table S1).
a
d(100) spacing.
√
b
a0 = 2d(100)/ 3, hexagonal lattice parameter calculated from XRD.
Calculated by a0 – pore size.
Total pore volume at P/P0 = 0.980.
Pore size from desorption branch applying the BJH pore analysis.
c
d
e

N. Batail et al. / Applied Catalysis A: General 388 (2010) 179–187
183
Solid-state 29Si NMR spectroscopy provided further information
about the nature of the link to the surface, the silicon environ-
ment and the degree of functionalization. The 29Si CP-MAS NMR
spectra of SBA-15 mesoporous materials after grafting of palladium
precursors 1–4 (Fig. 4) displayed discernable peaks in two differ-
ent spectral regions: one region ranging from −95 to −110 ppm
characteristic of Q-type (Qn = Si–(OSi)n–(OH)4−n) silicates originat-
ing from the siliceous bulk material and another set of signals
appearing in the −40 to −80 ppm spectral region ascribed to T-
type (Tm = RSi–(OSi)m–(OH)3−m) silicates, that is, the silicon atoms
attached to the phosphine and/or cyano containing alkyl chain
which is a clear indication of covalent bonding. Note that multiple
peaks were obtained from each of the modified oxides, suggest-
ing different degrees of linkage of the organosiloxane group with
the silica surface. All four modified oxides exhibited principally T1
and T2 sites, that is, an anchoring of the organic species via one
or two Si–O–Si bonds. In the case of palladium complexes bearing
monophosphine ligands, [1]@SBA-15 and [2]@SBA-15, the absence
of T0 sites seems to suggest that both phosphine linkers were
involved, at least, in one covalent bond with the support resulting
in chelation through the silica surface.
Examination of the 13C CP-MAS NMR spectra of the palladium
modified solids along with the solution phase spectra of the cor-
responding molecular precursors 1–3 (Supplementary Material,
Figs. S1–S3) led to the conclusion that the structure of the organic
fragments (mainly the ligands coordinated to the metal centre)
in the grafted solids remained intact throughout the functional-
ization process. One should also note the occurrence, in all three
spectra, of two resonances corresponding to unreacted alkoxysi-
lane groups (ı = 17 and 58 ppm) indicating the presence of residual
ethoxy groups at the point of attachment with the surface, which
is also consistent with the presence of not fully condensed T1 and
T2 sites.
Fig. 3. Nitrogen adsorption/desorption isotherms (top) and pore size distribution
(bottom) of palladium functionalized SBA-15 silica materials.
Solid-state 31P NMR spectroscopy proved to be a useful tech-
nique to elucidate the structural integrity and coordination mode
of the phosphine linkers at the palladium centre in the supported
complexes. The 31P CP-MAS NMR spectra of the modified SBA-
15 silica materials are presented in Supplementary Material (Figs.
S4–S6) together with the liquid 31P NMR of their respective molec-
ular precursors 1–3. In all cases, the Pd–P bond seemed unaffected
by grafting: free ligands were not detected (negative ı expected
at higher magnetic field) and the 31P NMR chemical shifts present
in the spectra of the grafted materials correspond to coordinated
phosphine ligands (see Section 2 for numerical values). As expected
for heterogenized species, a relatively broad signal centred at 23.8
did not diminish the long-range structural ordering of the mate-
rials. d spacings ranging from 105 to 96 A were determined for the
˚
grafted samples which corresponds to unit-cell parameters varying
˚
between 121 and 111 A. An increase in the unit-cell parameters of
the hybrids compared to the parent silicas was generally observed
which could stem from a change in wall thickness and/or modifica-
tion of the pore size (vide infra) (Supplementary Material, Table S1).
Nitrogen adsorption–desorption measurements were used to
examine the textural properties of the hybrid materials. Typi-
cal isotherms and pore size distributions are depicted in Fig. 3.
All the materials exhibited type IV isotherms and H1 hysteresis
loops characteristic of mesoporous solids and SBA architectures.
A steep capillary condensation step appearing at relative pres-
sure of P/P0 = 0.6–0.8 (Fig. 3, top) indicated the presence of regular
mesopores in the samples which was confirmed by a narrow pore
size distribution in the mesopore range (Fig. 3, bottom). High BET
surface areas (367–644 m2 g−1) were obtained for all the sam-
ples upon functionalization with average pore diameters and pore
˚
volumes varying respectively between 54 and 63 A and 0.53 and
0.87 cm3 g−1. It is worth noting that significant decreases in spe-
cific surface areas, pore diameters and pore volumes compared
to parent SBA-15 silicas (Supplementary Material, Table S1) were
observed for almost all the solids after functionalization with the
exception of (3-cyanopropyltriethoxysilane) palladium dichloride
modified solid, [4]@SBA-15, for which only a slight decrease in
surface area was noticed (from 704 m2 g−1 for plain SBA-15 to
644 m2 g−1 after grafting). Also, it is interesting to mention that, in
the case of organophosphine palladium complexes modified mate-
rials, the decrease in pore size was accompanied by a significant
˚
˚
increase in the wall thickness (from 34–45 A to 52–63 A). Taken
together, these observations are consistent with the presence of
a significant amount of grafted palladium species attached to the
internal surface of the mesopores.
Fig. 4. CP-MAS 29Si NMR of SBA-15 mesoporous materials after grafting of palladium
precursors.

184
N. Batail et al. / Applied Catalysis A: General 388 (2010) 179–187
Scheme 2. .
and 11.7 ppm was observed for respectively [2]@SBA-15 (Fig. S5)
and [3]@SBA-15 (Fig. S6). In the case of [1]@SBA-15 (Fig. S4)
one may discern the presence of two resonances at ı = 22.9 and
34.4 ppm, which can be assigned to respectively the trans- and cis-
isomers of the grafted species (the liquid NMR spectrum of the
molecular precursor 1 showed two peaks at ı = 21.4 and 31.7 ppm).
This result clearly indicates that the monophosphine ligands have
survived the grafting reaction and are still coordinated by pairs to
the palladium central atom.
Quantitative determinations of metal and organic contents were
performed for all the hybrid materials by thermogravimetric anal-
ysis (TG/DTG) in air and elemental analysis. Generally, similar
thermal profiles were observed for all the hybrids, composed of
three weight loss regions (Supplementary Material, Fig. S7). A first
weight loss occurred at temperatures up to about 200 ◦C and was
assigned to the desorption of physisorbed water. A second weight
loss occurred at temperatures ranging from 200 to 600 ◦C presum-
ably arising from the decomposition of organic species. Depending
on the nature of the organic fragments, several desorption peaks
could be observed in this region with weight losses ranging from
4.6 to 16.6%wt (Supplementary Material, Table S2). Note that the
first decomposition peak occurs at relatively elevated tempera-
ture (∼280 ◦C) revealing the high thermal stability of the palladium
immobilized materials regardless the nature of the coordinated lig-
ands. The third weight loss region, which took place above 600 ◦C,
was ascribed to the release of water formed from the condensation
of silanols in the silica structure. In quantitative terms, one should
consider the organic loading of the surface in comparison to the
dry mineral weight of the material, in this case, the weight of the
material at the end of thermal treatment. Thus, the functional group
loading in the grafted palladium solids was found to be in the range
of 0.05–0.21 g/g of dry SiO2 as determined by TGA (Supplementary
Material, Table S2). Elemental analyses of palladium, phosphorus,
chlorine and nitrogen obtained for the palladium containing hybrid
materials are summarized in Table S3 (Supplementary Material).
In general, upon functionalization, palladium content was found
to vary between 1.2 and 3.8%wt in the hybrid solids. In the case
of materials functionalized with organophosphine palladium com-
plexes, a phosphorus/palladium molar ratio of 2.1–2.3 (expected
2) and a chlorine/palladium molar ratio of 2–2.1 (expected 2) were
found indicating that on average two phosphorus and two chlorine
atoms per palladium centre were left on the surface. This suggests
that the stoichiometry of the initial molecular precursors 1–3 was
not affected by the grafting procedure. In the case of [4]@SBA-15,
which contains weakly coordinative cyano ligands, the integrity
of the molecular precursor 4 was also maintained with a nitro-
gen/palladium molar ratio of 1.8 (expected 2).
Fig. 5. Catalytic activity of palladium mesoporous SBA-15 silica materials in the
heteroannulation of 2-iodoaniline with triethyl(phenylethynyl)silane.
More insights were obtained from kinetic studies (Fig. 5).
Except for the cyclohexylphosphine-based catalyst [2]@SBA-
15 all show clearly high activities for the studied reaction
as determined from initial rates: 3.8 mmol mmolPd−1 min−1
for the cyano catalyst [4]@SBA-15, 2.0 mmol mmolPd−1 min−1
for
the
monodentate
diphenylphosphine-based
catalyst
[1]@SBA-15 and 1.6 mmol mmolPd−1 min−1 for the chelated
diphosphino catalyst [3]@SBA-15. Interestingly, all these
materials show higher activities than the previously evalu-
ated catalysts; Pd/C (0.5 mmol mmolPd−1 min−1
) and Pd/NaY
(1.3 mmol mmolPd−1 min−1) (Supplementary Material, Fig. S8).
Surprisingly, the curve obtained for the cyclohexylphosphine-
based catalyst [2]@SBA-15 showed an inflexion point at ca.
20 min probably related to a modification in the nature of the
supported palladium species. While the initial rate arises only
0.3 mmol mmolPd−1 min−1, the activity reached after this point
0.9 mmol mmolPd−1 min−1. However, further studies were not per-
formed with this catalyst due to its lower activity in comparison
to that achieved with the other materials evaluated in the present
contribution.
When heterogeneous materials are used in liquid phase, ques-
tions regarding leaching and recycling should be addressed. These
issues were addressed for the two most promising catalysts,
namely [1]@SBA-15 and [4]@SBA-15. Leaching is almost often
observed when supported palladium catalysts are used in such
cross-coupling reactions [23,30]. Several methodologies are com-
monly used [23,31], the hot-filtration method, when applied with
care, provides quick and reliable information about leaching. The
procedure (see Section 2) was applied to [1]@SBA-15 after 20 min
reaction corresponding to ca. 40% conversion and after 10 min (33%
conversion) for [4]@SBA-15. Comparing a standard run (in the pres-
ence of solid material) to the hot-filtration run (Fig. 6) clearly
showed further product formation after removal of the heteroge-
neous catalyst, up to a conversion of 80% after 6 h for [1]@SBA-15
and 75% after 6.5 h for [4]@SBA-15. These data clearly indicate that
active palladium species are dissolved during the reaction. The
differences in performances observed between the runs with and
without the solid material can be related to: (1) the lack of con-
tinuous palladium source due to the removal of the material; (2)
3.2. Palladium catalyzed Larock indole synthesis
These catalytic materials, [1]–[4]@SBA-15, were applied to
Larock indole synthesis (Scheme 2) using optimized reaction con-
ditions previously described for Pd[(NH3)4]/NaY and Pd/C (i.e.
2-iodoaniline (1.0 mmol), alkyne (3.0 mmol), Na2CO3 (3 mmol),
[Pd] catalyst (1 mol%), DMF (4.0 mL), 120 ◦C) [27]. Initially, catalysts
[1]–[4]@SBA-15 were evaluated for the synthesis of 3-phenyl-2-
(triethylsilyl)-1H-indole over a period of 24 h; in all cases isolated
yields of 80–85% were achieved.

N. Batail et al. / Applied Catalysis A: General 388 (2010) 179–187
185
a strong deactivation that could not be fully related to mass
loss.
This phenomenon was further studied through multi-run exper-
iments, with analysis of each cycle performed both early and later
in the cycle. This study was carried out on the two most highly
performing catalysts, the cyano catalyst [4]@SBA-15 and the mon-
odentate diphenylphosphine-based catalyst [1]@SBA-15. In each
series, a first run was performed using fresh catalyst for 24 h.
Aliquots were removed after 2 and 14 h of reaction and analyzed by
GC. At this point, a new portion of the reagent mixture was added
(1 mmol of 2-iodoaniline, 3 mmol of triethyl(phenylethynyl)silane
and 3 mmol of Na2CO3) and the volume of solvent was adjusted in
order to restore the concentrations of reagents to that of the initial
run. The reaction was then allowed to proceed an additional 24 h,
aliquots being withdrawn and analyzed 2 and 14 h after the addi-
tion of reactants. The process was repeated to a total of five fixed
time reaction cycles.
Data obtained in the short run study (Fig. 7, left) confirm deac-
tivation of the catalysts from initially moderate (i.e. 30% for the
2nd run) to low conversions (i.e. 17–9% for the 3rd to 5th runs).
Differences between the two catalysts were observed: while the
activity of [4]@SBA-15 continuously decreased, that of [1]@SBA-
15 remained constant after the 3rd run (stabilized at ca. 14%). The
long run study (Fig. 7, right) shows that despite the lower initial
activity implied by the short run study, the catalyst system remains
viable over the five runs. That is, as one can observe in Fig. 7, quan-
titative conversion to the expected product is observed for every
run when the 14 h reaction time was used, making these materials
particularly efficient for such applications.
To further explore the origin of observed deactivation, used
catalysts were analyzed by X-ray diffractometry and electron
microscopy (including EDX). Specifically, for catalysts [1]@SBA-15
and [4]@SBA-15, data were obtained for fresh catalyst and catalyst
recovered after a single 14 h run. The latter catalysts were filtered
and washed with water and DMF to remove the salts (NaI and
NaHCO3) formed during the reaction.
Small angle XRD analysis for all samples indicated long-range
mesoscopic ordering, indicating that no wholesale collapse of the
mesostructure occurred over five runs (Figs. S9 and S10) even
though, in some cases, decrease in intensity of the d100 reflection
was observed (ascribed mainly to lower local ordered regions).
In the wide-angle XRD patterns (Fig. S11), none of the samples
displayed the characteristic diffraction pattern of crystalline pal-
ladium phase, indicating that no significant sintering occurred.
Electron microscopy was shown to be destructive for the cyano
palladium catalyst, [4]@SBA-15.1 In the TEM imagery as illustrated
for [1]@SBA-15 (Fig. 8), long-range ordering was also clearly evi-
dent for each of the three samples. The TEM/EDX data (Table 2) for
the fresh catalyst showed that palladium was uniformly localized
in well ordered phases of the solid and the ligand to metal ratio
(i.e. the P/Pd ratio) of the molecular precursor 1 was confirmed
(1.9 for an expected value of 2.0). This observation was confirmed
over a multiple sample areas of the surface of each of the materials
tested. Note that the elemental analysis of a milligram scale sample
of [1]@SBA-15 also showed a phosphorous/palladium ratio of 2.1.
Microscopy of the used catalysts showed no gross changes in
the structure of the mesoporous silica networks, and notably, no
formation of large palladium particles. Analysis of the used sam-
Fig. 6. Residual activity of [1]@SBA-15 (top) and [4]@SBA-15 (bottom) after hot
filtration (dotted line) versus standard catalytic run (solid line).
deactivation of the dissolved palladium species to less active col-
loids or palladium black; or (3) to the removal of the contribution
of a heterogeneous catalysis to the overall conversion (which we
consider as an unlikely or very minor factor) [23].
For each of the leaching experiments, the transparent, colourless
solution was analyzed by AAS and found to contain palladium. The
values ranged from 14 to 32 ppm are reported in Table S4 of the Sup-
plementary Material. It should be noted that these concentrations
are below the acceptable standards under European regulations
for residual palladium content administered by oral and parental
routes in drug testing, and thus the products could be used with-
out further purifications (in regard to palladium content), a clear
advantage for such a heterogeneous system [32].
1
The TEM images of [4]@SBA-15, including the fresh catalyst, showed the
expected hexagonal channels of the SBA-15 with the presence of large palladium
particles and aggregates. This result, particularly for the fresh catalyst, is incon-
sistent with elemental analyses. We undertook an XPS study in order to ascertain
palladium oxidation state in the fresh material. One was able to observe visually the
solid sample turning from white to black at the point of contact with the XPS beam.
Recycling of the catalyst [1]@SBA-15 and [4]@SBA-15 was ini-
tially performed by separating the solid material from the reaction
mixture by filtration to afford a used catalyst that was recycled
after washing in a further run. The results thus obtained showed

186
N. Batail et al. / Applied Catalysis A: General 388 (2010) 179–187
Fig. 7. Recyclability of [1]@SBA-15 and [4]@SBA-15 for the heteroannulation of 2-iodoaniline with triethyl(phenylethynyl)silane examined at 2 h (left) and 14 h (right).
ples (Table 2), however, showed progressive loss of palladium upon
recycling, with no palladium detected in the three sites sampled
after the fifth run of the catalysts.
All results clearly indicate palladium loss upon recycling pro-
cedures. However, from our “in situ” recycling experiments we
demonstrated that the material still features good activity when
not removed from the reaction mixture. This suggests that for
some reason the leached palladium did not efficiently redeposit
on the support, which may be attributed to several factors. One
hypothesis would be that the material surface is not adapted to
accommodating or stabilizing the palladium after leaching, thus
preventing effective redeposition. Given the nature of the support
Fig. 8. TEM micrographs in the direction perpendicular to the pore axis of [1]@SBA-15: (a) fresh catalyst; (b) after the 1st cycle; and (c) after the 5th cycle.

N. Batail et al. / Applied Catalysis A: General 388 (2010) 179–187
187
Table 2
studies of the catalytic phenomena showed it to be homogeneous
in nature, the solid apparently serving as “continuous source” of
palladium between cycles, that is, as a catalyst reservoir.
Current work concentrates on optimization of the palladium
hybrid materials toward higher recycling capacities and on the
extension of the Larock indole syntheses to a variety of bromoani-
lines.
TEM/EDX analysis on [1]@SBA-15 samples (fresh catalyst and single run).
Catalyst
EDX samplea
Atom counts
P/Pd
Si
P
Pdb
fresh [1]@SBA-15
1
2
3
100
100
100
100
4.13
4.57
4.77
4.49
2.25
2.68
2.33
2.42
1.83
1.7
2.04
1.85
Average
Acknowledgements
[1]@SBA-15 after one 14-h run
1
2
3
100
100
100
100
3.59
3.23
3.28
3.37
0.90
0.80
0.76
0.82
3.98
4.03
4.31
4.1
The authors gratefully acknowledge the Région Rhône-Alpes
Programme CIBLE-2007 (Contract number 07 016376 01/02/03)
and the National Agency of Research (No. ANR-07-BLAN-0167-
01/02) for funding. NB thanks Région Rhône-Alpes for a grant. AB
thanks ANR for felowship. The authors kindly acknowledge the
“Technical Center” of IRCELYON for analyses and helpful discus-
sions (M. Aouine and L. Burel for TEM, P. Mascunan and N. Cristin
for AAS analyses, P. Delichère and S. Prakash for XPS, F. Bosselet and
G. Bergeret for XRD and C. Lorentz for solid-state NMR).
Average
a
Measurements taken at different locations on the sample.
From the milligram scale microanalysis, one would expect 1.72 atoms Pd per
b
100 atoms Si.
and its large surface area, we can very probably discard this hypoth-
esis. Alternatively, it could be that the palladium redeposition is
prevented by the large amount of salts present in the solution
(about 450 mg of sodium salts are produced in each run, or approx-
imately 2.3 g after 5 runs, compared to 35–65 mg of Pd-catalyst).
These salts probably crystallize upon cooling (and probably during
the reaction) with two concomitant effects: (a) salt crystalliza-
tion encapsulates leached palladium which was then eliminated
when the solid was washed with water and DMF prior to XRD and
TEM studies; (b) salt crystallization can occur at the pore mouth of
the SBA-15 material preventing effective palladium redeposition
by reducing the accessibility to the large internal support surface
area.
To date, we cannot with certitude discard one or the other
hypothesis; however we strongly support the second one as this
was previously observed with other microporous supports like
NaY zeolites that has already proven its capacity to accommodate
leached palladium toward efficient recycling in common cross-
coupling reactions [9].
As the main purpose of this work was to propose catalytic
systems with overall high performance, especially relative to
homogeneous state of the art, we have not prioritized the study of
this process, but specific experiments toward better understand-
ing are currently under way in order to optimize catalyst design
toward higher recycling.
Given the efficiency of these materials, we undertook other
studies on the much more challenging Larock synthesis from bro-
moaniline for which the sole examples described in homogenous
media involved the use of costly hindered phosphine or urea ligands
[33,34]. In initial experiments we performed the first “salt-free”
heterogeneously palladium catalyzed Larock reaction between the
2-bromoaniline with the triethyl(phenylethynyl)silane that gave
for all materials described in this contribution the expected indole
in high isolated yields (up to 85%).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2010.08.046.
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