Synthesis of mesoporous metal complex-silica materials and their use as solvent-free catalysts

Article abstract of DOI:10.1039/c0nj00509f

Incorporation of various Pd(ii) complexes into the framework of MSU-X mesoporous silica has been achieved by co-condensation using a facile solvent-free one-pot synthesis. The use of ligands with triethoxysilyl terminal groups permitted the synthesis of t

Full text of DOI:10.1039/c0nj00509f

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PAPER
Synthesis of mesoporous metal complex-silica materials and their use as
solvent-free catalystsw
Noemi Linares, Angel E. Sepulveda, Marıa C. Pacheco, Jesu
´ ´ ´
s R. Berenguer,*b
a
b
c
b
c
a
Elena Lalinde, Carmen Najera and Javier Garcia-Martinez*
´
Received (in Gainesville, FL, USA) 1st July 2010, Accepted 19th August 2010
DOI: 10.1039/c0nj00509f
Incorporation of various Pd(II) complexes into the framework of MSU-X mesoporous silica has
been achieved by co-condensation using a facile solvent-free one-pot synthesis. The use of ligands
with triethoxysilyl terminal groups permitted the synthesis of three different metallosilanes
precursors (metal complexes with ligands containing trialkoxysilane terminal groups), which allow
for the homogeneous in situ incorporation of metal complexes covalently bonded to the porous
support. Inorganic precursor tetraethylorthosilicate was used both as silica source and as solvent
for the synthesis of the complexes, avoiding the use of other organic co-solvents, making the
synthesis environmentally benign. The gentle synthesis conditions used such as neutral pH,
room temperature and mild ethanol extraction of the surfactant, allowed a cleaner route for
the immobilization of homogeneous Pd(II) catalysts in mesoporous silica, while protecting the
structural and chemical integrity of the metal complexes. For comparison purposes,
monomer complexes [trans-PdCl L ] (L = NH (CH ) Si(OEt) , 4-C H N–(CH ) Si(OEt ),
2
2
2
2 3
3
5
4
2 2
3
2 2 2 3
PPh (CH ) Si(OEt) ) were synthesized using the same aerobic reaction conditions to those use
for the co-condensation processes and fully characterized before their incorporation in the
mesoporous silica. The catalytic performance of these materials was tested for the
Suzuki-Miyaura reaction under solvent-free conditions. Efficient mixing of all the components
was accomplished by applying either magnetic stirring or ball milling. The good yields obtained,
even at room temperature, confirmed the catalytic activity of the metal complexes once
incorporated into the mesoporous silica framework. The possibility to work under solvent-free
conditions even with solid starting reactants, is a significant step forward in the Suzuki-Miyaura
coupling reaction because its benefits in terms of cost and impact of the environment.
homogeneous catalysts, such as recovery and recycling, and
2
limits the loss and agglomeration of metal.
Introduction
Current synthesis of metal complex-supported catalyst is
based on the heterogenization of successful homogeneous
1
catalysts. The process overcomes the main limitations of
Nanostructured silica materials are widely used as catalyst
supports because their large surface area, controllable
surface chemistry and porosity, excellent stability (chemical
3
–5
and thermal) and good accessibility.
Nowadays, different
a
Laboratorio de Nanotecnologı´a Molecular,
approaches are known to support homogeneous catalysts,
Departamento de Quı´mica Inorga´nica, Universidad de Alicante,
Carretera San Vicente s/n, E-03690, Alicante, Spain.
E-mail: j.garcia@ua.es; Web: www.ua.es/grupo/nanolab
6
from the simplest physisorption of the complexes to the
,7
covalent attachment of their ligands in nanostructured
8–13
b
Departamento de Quı´mica-Grupo de Sı´ntesis Quı ´m ica de La Rioja,
silica supports.
Surface functionalization of mesoporous
UA-CSIC, Universidad de La Rioja, E-26006, Logron˜o, Spain.
Web: www.unirioja.es/dptos/dq/grupos/materiales
Grupo de Procesos Catalı ´t icos en Sı ´n tesis Orga´nica,
Departamento de Quı´mica Orga´nica, Universidad de Alicante,
Carretera San Vicente s/n, E-03690, Alicante, Spain
silica materials by covalent bonding of organic species on
preformed silica (grafting) has become the most common
approach. At present, many complexes have been hetero-
genized by this method, grafting one or more ligands of the
complex on the support. Subsequently, these ligands are
reacted with the metal to finally obtain the desired
c
w Electronic supplementary information (ESI) available: Fig. S1
(
N
2
isotherms and TEM images of silica materials prepared with
EtOH in the synthesis), Fig. S2 (DRUV spectra of MSU-
PdCl (APTS) and MSU-PdCl (PPETS) materials), Fig. S3 (FT-IR
spectra of MSU-PdCl (APTS) and MSU-PdCl (PPETS) materials),
1
4–20
2
2
2
2
catalyst.
The main drawbacks of this approach include
2
2
2
2
long time refluxing conditions in non-polar solvents and the
presence of an excess of the organosilane, the poor dispersion
of the complexes on the support, the difficulty to prepare and
Fig. S4 (SEM and mapping images of the silica materials). CCDC
reference number 777001. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0nj00509f
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 225–234 225

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control certain structures, as well as the characterization of the
final complex obtained.
terms of high yield, selectivity and simplicity is specially
desirable. The solvent-free approximation is relatively simple
if at least one of the reagents is a liquid or a gas, but when both
of the substrates are solids their accessibility to the catalyst will
An alternative approach to overcome these challenges
involves, first, the synthesis of the desired metal complexes
with terminal trialkoxysilyl groups in their ligands. Once
characterized, the subsequent co-condensation of these
complexes with the silica precursor during the formation of
the mesoporous material yields a hybrid material in which the
metal complex forms part of the silica framework in a similar
way to the incorporation of organic groups in periodic
4
5
limit the speed and yield of the reaction. The use of
ball-milling is widely applied for grinding minerals into fine
particles and for the preparation and modification of
4
6
inorganic solids. In organic reactions, this technique can
significantly improve the mixing of the reactants when
4
7
performing reactions with solid starting materials. Mild
operation conditions, absence of solvents and easy work up
make ball mill chemistry a really interesting option for
2
1–23
mesoporous organosilica materials
behind this method is inspired by the use of bridged organo-
silane precursors (CH (CH O) Si–R–Si(O(CH CH to
(PMOs). The idea
4
8
3
2
)
n
3
2
)
n
3
)
3
catalytic reactions. Herein, we report the use of mesoporous
Pd complex-silica catalysts for the Suzuki-Miyaura reaction
using solid/liquid reactants under solvent-free conditions.
smartly and homogeneously introduce different functionalities
in mesostructured silicas. Recently, our group have used
this approach to carry out the direct incorporation of
functionalized Pd nanoparticles in a silica matrix by covalent
binding, using mercaptopropyltriethoxysilane as functionalizing
Experimental
2
4
agent.
General considerations
Despite the advantages that a priori this method presents,
such as simplicity, homogeneous distribution of the active site
and better characterization of the catalytic active complex, few
IR spectra were recorded on a Nicolet Nexus FT-IR Spectro-
meter. For mononuclear complexes 1–3, all the samples were
prepared and recorded as Nujol mulls between polyethylene
sheets and KBr pellets. All the mesoporous materials samples
were prepared as pure material pellets and KBr diluted pellets.
NMR spectra were recorded on a Bruker ARX 300 spectro-
meter; chemical shifts are reported in ppm relative to external
2
5–31
examples are found elsewhere.
The difficulty to preserve
the complexes during the synthesis of nanostructured
materials due to the synthetic conditions (high pH,
hydrothermal treatment and chemical surfactant extraction)
makes challenging the incorporation of metal complexes by
the co-condensation method. However, since the discovery of
a new strategy for the synthesis of mesoporous silica materials
1
13
1
31
1
TMS ( H and C{ H}) or H PO ( P{ H}), and coupling
3
4
constants are given in Hz. Diffuse reflectance UV-vis (DRUV)
data of pressed pure powder were recorded on a Shimazdu
UV-3600 spectrophotometer with a Harrick praying mantis
accessory, and recalculated following the Kubelka Munk
function. MALDI and ES (including exact mass experiments)
spectra were recorded on Microflex MALDI-TOF or
micrOTOF-Q Bruker spectrometers, respectively. Elemental
analyses of 1–3 were carried out with a Perkin-Elmer 2400
CHNS/O microanalyzer and the organic amounts in the
catalysts were measured with elemental combustion analyses
on a Carlo Erba CHNS–O EA1108 analyzer. Palladium
content was determined by ICP-AES on a Perkin Elmer
7300 DV spectrometer, with the samples dissolved in aqua
regia and the undissolved siliceous matter filtered off prior to
analysis. Transmission electron microscopy (TEM) studies
were carried out on a JEOL JEM-2010 microscope (200 kV,
0.14 nm of resolution). Samples for TEM studies were
prepared by dipping a sonicated suspension of the sample in
methanol on a carbon-coated copper grid. Scanning electron
microscopy (SEM) images of the samples and electron energy-
dispersive spectroscopy (EDS) were carried out using a
Hitachi S-3000N microscope and Bruker XFlash 3001 spectro-
meter; for the SEM images the samples were previously
0
0
32
using neutral surfactants (S N ) by Tanev and Pinnavaia in
994, a myriad of new mesostructures have been prepared
using neutral surfactants, neutral pH and facile solvent
1
3
3–38
extraction methods of the surfactants.
These milder
synthesis pathways permit the incorporation of complexes by
the co-condensation method.
2
8–30
Recently, Li and co-workers
have used this approach to
synthesize different mesoporous organometalsilicas containing
Au(I), Ru(II), Rh(I) or Pd(II) organometallicsilanes, which
exhibited similar activity and selectivity to the related homo-
geneous organometallic catalysts and much higher than the
corresponding immobilized catalyst obtained by grafting
method. Nevertheless, organic co-solvents and thermal treat-
ment is still required in the synthesis of these materials.
In the present work, we describe a facile aerobic, room
temperature, one-pot synthesis preparation of different
metalorganic/inorganic materials in which the use of a neutral
surfactant for the synthesis of mesoporous MSU-X type silica
has allowed the incorporation of various Pd(II) complexes
containing amine, pyridine or phosphine ligands into the silica
framework. Monomer complexes have been prepared under
the same reaction conditions and fully characterized before the
formation of the mesoporous materials by elemental analyses,
covered with gold. Porous texture was characterized by N
2
1
13
1
31
1
spectroscopic ( H,
C{ H} and
P{ H} NMR, FTIR,
adsorption at 77 K in an AUTOSORB-6 apparatus. The
samples were previously degassed for 5 h at 373 K at
DRUV) and spectrometric techniques.
ꢁ
5
49
The catalytic performance of these materials has been tested
in the Suzuki-Miyaura reaction under solvent-free conditions
both under magnetic stirring and ball-milling conditions. The
development of environmentally friendly solvent-free catalytic
5 ꢀ 10 bars. [PdCl
2 2
(PhCN) ] was prepared as reported
and the rest of the reagents were used as received without
further purification.
Catalytic reactions were carried out under magnetic stirring
or mechanochemical ball mill conditions using a Mixer Mill
3
9–44
systems,
which, also, offer other synthetic advantages in
2
26 New J. Chem., 2011, 35, 225–234
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

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+
requires 737.1204; m/z (MALDI+) 681 (M ꢁ Cl, 100%);
Retsch MM 200 model, housing two Teflon lined stainless-
steel cups, each containing one Eppendorf plastic centrifuge
tube with three stainless steel balls, and sealed by a stainless-
steel lid fitted with a Teflon gasket. The reaction mixtures and
products were analyzed by a gas chromatograph (Agilent
ꢁ1
nmax(KBr)/cm 2974vs (C–H), 2926s (C–H), 2874s (C–H),
1615s (py), 1428vs (C–H), 1102vs, 1080vs, 817s, 785s, 669w,
443w (py); 324m (Pd–Cl), 266m (Pd–N); d (300 MHz; CDCl ;
H
3
SiMe ) 8.64 (4H,d) and 7.17 (4H,d) (J
3
= 6.3, C H , py),
6
H–H
4
6
3
890N) using a HP-5MS capillary column (fused silica,
0 m, 0.25 mm i.d.) and a flame ionization detector with
3.82 (12H, q, JH–H 7.0, OCH CH ), 2.74 (4H, m, CH CH Si),
2
3
2
2
2
1.23 (18H, t, JH–H 7.0, OCH
2
CH ), 0.90 (4H, m, CH
3
CH
2
Si);
4
decane as an internal standard.
d
C
(75.5 MHz; CDCl ; SiMe
3
3
) 157.0 (s, C , py), 152.6
s, C , py), 124.5 (s, C , py), 58.6 (s, OCH CH ), 28.4
s, CH CH Si), 18.3 (s, OCH CH ), 11.0 (s, CH
abs,max(solid state diffuse reflectance)/nm 238, 322, 390.
2
3
(
2
3
Synthesis of Pd(II) complexes
(
2
2
2
3
2
CH
2
Si);
Synthesis of [trans-PdCl
APTS) 1, 4-C N–(CH
suspensions of [PdCl (PhCN)
2
L
2
] (L = NH
Si(OEt (PETS) 2). Orange
] (0.10 g, 0.26 mmol) in 5 mL
2
(CH
2
)
3
Si(OEt)
3
l
(
H
5 4
2
)
2
3 2
)
2
2
Synthesis of [trans-PdCl (PPETS) ]
2
2
3
(PPETS
=
of tetraethoxysilane (TEOS) were treated with 0.52 mm of the
PPh (CH ) Si(OEt) ). Addition of 100 mL (0.28 mmol) of
2
2 2
3
corresponding amine or pyridine ligands (122 mL of APTS or
PPETS on an orange suspension of 53 mg (0.14 mmol) of
1
40 mL of PETS), and the mixtures were stirred for 15 min to
[PdCl (PhCN) ] in 10 mL of TEOS/EtOH (1 :1) yielded the
2
2
yield yellow solutions. Evaporation of the solutions to small
volume (B1–2 mL), addition of 15 mL of hexane and cooling
at ꢁ30 C for one hour caused the precipitation of complexes 1
precipitation of 3 as a yellow solid, which was filtered after
15 min of stirring (90 mg, 70%). (Found: C, 51.88; H, 5.90%.
C H Cl O P PdSi requires C, 51.64; H, 6.28%). m/z (ES+)
4
0
58
2
6
2
2
(
0.12 g, 76%) or 2 (0.14 g, 75%) as yellow solids.
951.1551 (M + Na). C H Cl NaO P PdSi requires
40 58 2 6 2 2
+
9
51.1556; m/z (MALDI+) 932 (M ꢁ 2H, 13%), 895
Data for 1. Anal. found: C, 34.55; H, 7.20; N, 4.46%.
+
ꢁ1
(
(
(
M
ꢁ Cl, 100%); nmax(KBr)/cm 3074m (C–H), 3060m
C
18
H
46Cl
2
N
2
O
6
PdSi
2
requires C, 34.87; H, 7.48; N, 4.52%.
NaO PdSi
2
+
C–H), 2972s (C–H), 2923s (C–H), 2890s,br (C–H), 1586w
ph), 1572w (ph), 1482s (C–H), 1437vs (P–C), 1102vs, 1071vs,
m/z (ES+) 641.1198 (M + Na). C18
2
H46Cl N
2
6
requires 641.1204; m/z (ES+) 643 (M + Na, 1%), 585
7
69s, 736vs, 693s, 490s, 480s; 470, 348m (Pd–Cl), 247m (Pd–P);
(300 MHz; CDCl ; SiMe ) 7.70, 7.36 (20H, m, Ph), 3.73
CH ), 2.49 (4H, m, PCH CH Si),
H–H 7.0, OCH CH ), 0.81 (4H, m,
+
+
ꢁ Cl, 7%), 385 (M ꢁ 2Cl ꢁ Si(OEt) , 100%);
(
M
3
d
H
3
3
ꢁ
1
nmax(KBr)/cm 3274s (N–H), 3231vs (N–H), 3146s, 2973vs
(
12H, q, JH–H 7.0, OCH
2
3
2
2
(
C–H), 2930vs (C–H), 2886vs (C–H), 1586s (N–H), 1469m
1
.15 (18H, t,
J
2
3
(
C–H), 1166vs, 1102vs, 1078vs, 814s, 793s, 770s, 483m (Pd–N),
PCH CH Si);d (75.5 MHz; CDCl ; SiMe ) 133.8 (t, o-C,
2
2
C
3
3
3
35m (Pd–Cl); d
H
(300 MHz; CDCl
Si–H 142.6, OCH CH ), 2.79 (2H, t) and 2.76
CH CH Si, both signals partially
overlap), 2.67 (4H, m, NH ), 1.73 (4H, m, JH–H 7.5,
NCH CH CH Si), 1.23 (18H, t, JH–H 7.0,
OCH CH ), 0.63 (4H, t, H–H 8, NCH
d (75.5 MHz; CDCl ; SiMe ) 58.4 (s, OCH CH ); 47.5
3
; SiMe
3
) 3.82 (12H, q,
2
JP–C 5.8, PPh ), 130.3 (s, p-C, PPh ), 130.1 (pst, i-C,
3
3
3
J
H–H 7.0,
J
2
3
1+3
3
JP–C 44.9, PPh ), 128.2 (t, m-C, JP–C 4.9, PPh ), 58.5
3
3
(
2H, t) (JH–H 6.5, NCH
2
2
2
1+3
(
s, OCH
s, OCH
PO ) 21.4 (s); labs,max(solid state diffuse reflectance)/nm
10, 242, 294sh, 343.
2
CH
3
), 19.1 (pst,
2
JP–C 27.0, PCH CH
2
Si), 18.2
2
(
2
CH
3
), 4.3 (s, PCH
2
CH Si); d
2
P 3
(121.5 MHz; CDCl ;
4
2
2
2
JSi–H 125.9,
H
3
4
2
3
J
2
CH CH Si);
2
2
2
C
3
3
2
3
(
s, NCH CH CH Si); 24.9 (s, NCH CH CH Si); 18.3
2 2 2 2 2 2
2 2
Crystal structure determination of [trans-PdCl (PPETS) ] 3
(
s, OCH CH ); 7.3 (s, NCH CH CH Si). labs,max(solid state
2
3
2
2
2
Pale-yellow single crystals were obtained by slow evaporation
of a toluene–hexane solution of complex 3 at room tempera-
ture. X-Ray intensity data were collected with a NONIUS
kCCD area-detector diffractometer, using graphite-
diffuse reflectance)/nm 241, 341, 380.
Data for 2. Anal. found: C, 43.60; H, 5.85; N, 4.12%.
PdSi requires C, 43.61; H, 6.47; N, 3.91%.
m/z (ES+) 737.1198 (M + Na). C26 NaO PdSi
C
26
H46Cl
2
2
N O
6
2
˚
2
H46Cl N
2
6
2
monochromated Mo-Ka radiation (l 0.71073 A). Images were
Scheme 1 Schematic representation of the preparation of mesoporous metal complex-silica materials by surfactant directing co-condensation.
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 225–234 227

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ꢁ
1
processed using the DENZO and SCALEPACK suite of
2 2
Spectroscopic data for MSU_PdCl (PETS) . nmax(KBr)/cm
5
programs. The structures were solved by Patterson and
0
3437s,br, 2961m (C–H), 2926m (C–H), 2855m (C–H), 1634s,
1456w (C–H), 1082vs, 799s, 669w (py), 456vs, 278m (Pd–N);
labs,max(solid state diffuse reflectance)/nm 237, 338.
5
1
Fourier methods using the DIRDIF92 program, and the
5
absorption correction was performed using SORTAV. Rest
2
5
3
of computation was performed with the SHELXL-97. All
non-hydrogen atoms were refined anisotropically.
Spectroscopic data for MSU_PdCl
ꢁ
2
(PPETS)
2
. nmax(KBr)/
cm 3465s,br, 2960m (C–H), 2927m (C–H), 2856w (C–H),
1
1
2
2
639s, 1511w, 1455w (C–H), 1437w (P–C), 1085vs, 798s, 467s,
47m (Pd–P); labs,max(solid state diffuse reflectance)/nm 208,
41, 254sh, 294, 339sh with a tail to 400.
Crystal data. C H Cl O P PdSi , M = 930.28, triclinic,
2
4
0
58
2
6
2
˚
a = 9.2480(4), b = 11.4080(7), c = 11.7210(8) A, a =
3
˚
7
6.248(3), b = 71.031(4), g = 78.739(3)1, V = 1126.49(11) A ,
ꢀ
T = 120(1) K, space group P1, Z = 1, 13 908 reflections
Catalytic test
measured, 4607 unique (Rint = 0.0649). The final R [I > 2s(I)]
and wR (all data) were 0.0378 and 0.0839, respectively.
2
Catalyst materials were tested in the Suzuki coupling reaction
of phenylboronic acid with 1-bromonaphthalene and
0
CCDC reference number 777001.
4
2 3
-bromoacetophenone in the presence of K CO or KOH.
For crystallographic data in CIF or other electronic format
see DOI: 10.1039/c0nj00509f
Reactions were carried out in air at room temperature and
solvent-free conditions using magnetic stirring in the case of a
liquid substrate, such as 1-bromonaphthalene. In the case of
Synthesis of mesoporous metal complex-silica catalysts
0
the solid 4 -bromoacetophenone the reaction was performed
Catalysts were synthesized by co-condensation of the Pd(II)
complexes 1–3 and tetraethylorthosilicate (TEOS, Aldrich) as
silica sources, Triton X-100 (Alfa Aesar) was used as
in a ball-milling apparatus (mechanochemical agitation). For
comparison purposes, magnetic stirring and different solvents
were also tested.
structure-directing agent and ammonium fluoride (NH F,
4
In a typical reaction, a mixture of phenylboronic acid
(48.2 mg, 0.75 mmol), aryl bromide (0.25 mmol), base
Aldrich) as nucleophilic catalyst for the condensation of the
silica network (Scheme 1). A reported synthesis of MSU-X
2 3
(K CO , 0.5 mmol) and the catalyst (see Tables 3 and 4) was
5
4
type silica was judiciously adapted for the incorporation of
the Pd(II) complexes.
stirred for the time indicated in Table 3. When ball-milling was
used, the reaction mixture was introduced in an Eppendorf
plastic centrifuge tube (1.5 mL) with three stainless steel balls,
and sealed by a stainless-steel lid fitted with a Teflon gasket.
The reaction vessel was fixed on the vibration arms of the
ball-milling apparatus, along with three stainless balls of 5 mm
diameter. Then, the vibration was adjusted for the reaction
start, a milling cycle involving 30 min agitation periods
followed by a 5 min cooling pause was selected. This milling
cycle was then repeated until the reaction times showed in
Table 4. Once the reaction is stopped, 10 ml of hexane was
added to the mixture and the suspension was filtered off and
washed with hexane and ether (10 mL). The filtrates were
evaporated affording the corresponding biphenyls.
In a typical synthesis 0.58 g (0.9 mmol) of Triton X-100 was
magnetically stirred in 30 g of distilled water until a clear
solution was obtained. Then, 20 mg of PdCl
2
(PhCN)
2
(
0.05 mmol) was suspended in 1.9 g of TEOS (9 mmol,
1
wt% nominal Pd : SiO ) and the corresponding amount of
2
ligand was added. Orbital agitation was kept until total
dissolution of the formed complex and the mixture was added
dropwise to the solution of surfactant, being the two phases
obtained vigorously stirred. To induce the silica precipitation
3
.4 ml of a solution 0.05 M of ammonium fluoride was added.
The mixture was reacted at room temperature during 24 h
under vigorous stirring. The obtained solid was washed with
water, ethanol and acetone in succession, filtered off, and air
dried at 40 1C. Finally, the surfactant was removed by ethanol
extraction (0.2 g catalyst/50 ml ethanol) at room temperature
Results and discussion
Pd(II) complexes
for 12 h. The samples are labelled as MSU_PdCl
2 2
(APTS) ,
MSU_PdCl (PETS) and MSU_PdCl (PPETS) for the
2
2
2
2
Monomer complexes [trans-PdCl
APTS 1, 4-C N–(CH Si(OEt)
PPETS 3) were obtained by treatment of [PdCl (PhCN) ] with
2
L
2
] (L = NH
2
(CH
2
)
3
Si(OEt)
3
samples prepared with the Pd(II)-amine, Pd(II)-pyridine and
Pd(II)-phosphine complexes, respectively.
5
H
4
2
)
2
3
PETS 2, PPh
2
(CH
2
)
2
Si(OEt)
3
2
2
Pd concentration in the silica materials was varied between
the corresponding ligands in a 1 : 2 M/L ratio. In all the cases,
the reactions were carried out in similar aerobic conditions to
those employed for the synthesis of the catalysts (room
temperature, TEOS as solvent for 1 and 2, TEOS/EtOH 1 : 1
mixture for 3, see Experimental), thus avoiding the use of any
other organic co-solvent, not only in the synthesis of the
mononuclear complexes, but also in the preparation of the
hybrid mesoporous materials. Complexes 1–3 have been
characterized by elemental analyses and spectroscopic
0
.5–2.5 wt% nominal Pd : SiO
2
, case of the pyridine complex,
, case of the amine and
and 0.5–1.5 wt% nominal Pd : SiO
2
phosphine complexes. Different amounts of ethanol can be
added as co-solvent to increase the solubility of complexes in
the inorganic precursor until a clear solution is obtained,
volume ratios between 0 : 1 and 1 : 1 (v : v) EtOH : TEOS have
been tested giving similar materials (see Fig. S1 in the ESIw).
ꢁ1
1
13
1
31
1
2 2
Spectroscopic data for MSU_PdCl (APTS) . nmax(KBr)/cm
( H, C{ H} and P{ H} NMR, FTIR, DRUV) and spectro-
metric means (see Experimental). Solid-state Diffuse
Reflectance UV-vis spectra of complexes 1–3 display a typical
intraligand broad high energy band (237–243 nm) and low
3
1
404s,br, 2955m (C–H), 2926m (C–H), 2876w (C–H),
634s, 1513m (N–H), 1458m (C–H), 1082vs,br, 800s, 459vs;
labs,max(solid state diffuse reflectance)/nm 243, 290, 400.
2
28 New J. Chem., 2011, 35, 225–234
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2 2
Fig. 1 Molecular structure of [trans-PdCl (PPETS) ] 3 with 50%
Fig. 2 DRUV spectra of pressed pure powders of the MSU-
possibility ellipsoids. Selected bond distances and angles: Pd–Cl
˚
PdCl
2
(PETS)
2
material (dotted line) and the monomer complex
] 2 (solid line).
˚
2
.3020(7) A, Pd–P 2.3212(7) A, Cl–Pd–P 90.93(3)1.
[
trans-PdCl (PETS)
2
2
energy features (341, 380 nm 1; 322, 390 nm 2; 294, 343 nm 3)
most likely associated to metal perturbed intraligand transi-
tions. As expected for the trans-configuration of the
(see Fig. 2 and S2 in ESIw) show similar features to that
observed for the Pd(II) complexes 1–3, being practically
identical in the case of the high-energy absorption at
ca. 240 nm. Also, as can be seen in Fig. 3 for the pyridine
material (see also Fig. S2 in ESIw), the FTIR spectra of both,
complex and material, show several absorptions in the high
ꢁ1
complexes, only one n(Pd–Cl) (335 1, 324 2, 348 cm 3) and
ꢁ
1
ꢁ1
3)
n(Pd–N) (483 1, 266 cm
stretching absorption are observed in their FTIR spectra;
2) or n(Pd–P) (247 cm
5
5
while their NMR spectra confirm the presence of only one set
of signals corresponding to the amine, pyridine or phosphine
ligands.
ꢁ
1
energy region (2965–2840 cm ) characteristic of the n(C–H)
2
9
stretching vibration of the aromatic rings and the CH
ꢁ1
In this zone the absorptions observed at 3274 and 3231 cm
units.
2
3
0
It must be noted that Li and co-workers
have
ꢁ1
recently reported the preparation of periodic meso-
porous organometalsilicas with the incorporation of
(n(N–H)) for complex 1 or at 3074 and 3060 cm (n(C–H)Ph
for complex 3 are obscured by a broad band centred at
)
ꢁ1
[
cis-PdCl {PPh (CH ) Si(OEt) }] , which is, in fact, an isomer
2
ca. 3400 cm (n(O–H)) in the spectra of the corresponding
metal complex-mesoporous silica materials. A weak absorp-
2
2 2
3 2
of the phosphine derivative 3. Nevertheless, the mutually trans
disposition of the phosphine ligands in 3 has been undoubtedly
confirmed, not only spectroscopically, but also with a single-
crystal X-ray diffraction study, as shown in Fig. 1. We were
also able to grow several pale-yellow crystals of complex
ꢁ1
tion at ca. 1455 cm , which could be assigned to d(C–H)
5
5
deformation vibration,
is also observed for the three
materials. The spectra contain also other absorptions
characteristic of each of the ligands employed. Thus,
ꢁ
(APTS) display one peak at 1513 cm , which
2 2
1
[
trans-PdCl
2
(APTS)
2
] 1, which were subjected to single-crystal
MSU_PdCl
can be attributed to the N–H degenerate deformation
(H–N–H) of the NH fragments, while MSU_PdCl (PETS)
shows a weak absorption at 669 cm (d(Py)) corresponding to
X-ray diffraction studies. The crystals were not of sufficient
quality systematically, although the trans nature of the
derivative was also unequivocally confirmed.
d
a
2
2
2
ꢁ
1
5
5
an in-plane ring deformation of the pyridine rings.
MSU-PdCl
2
(PPETS)
ꢁ1
2
spectrum also displays a weak signal
Mesoporous metal complex-silica catalysts
at 1437 cm
characteristic of the n(P–C) stretching
2
9
The preparation of the hybrid mesoporous metal complex-
silica catalyst is illustrated in Scheme 1. Aerobic solutions of
the mononuclear derivatives 1–3 in TEOS (or TEOS : EtOH
for 3) were added to water solutions of the Triton X-100
surfactant. Ammonium fluoride was added to the mixture
allowing TEOS and the silane bridged-palladium(II) complexes
co-condensate for 24 h at room temperature. Finally, the
surfactant was removed by treatment of the solids obtained
with EtOH for 12 h at room temperature to give the meso-
vibration. All this vibration modes can be also observed,
appearing at very close wavenumbers, in the FTIR spectra of
the monomer Pd(II) complexes (see Fig. 3 and S3 in the ESIw).
By contrast, the n(Pd–Cl) observed for the three complexes
ꢁ
1
(ca. 340 cm ) and the n(Pd–N) of the amino derivative 1
(483 cm ) are covered by the strong absorption peak of Si–O
ꢁ
1
ꢁ
1
bonds (ca. 460 cm ) in the silica materials. Nevertheless, for
MSU_PdCl (PETS) and MSU_PdCl (PPETS) one weak
feature is observed in the low energy zone at 278 and
2
2
2
2
ꢁ
1
porous palladium complex-silica materials MSU_PdCl
L
2 2
247 cm , respectively, which can be ascribed to the
(
L = APTS, PETS, PPETS), as observed in their FTIR
n(Pd–N) or n(Pd–P) stretching vibrations, and correlate well
with those observed for the monomer derivatives (266 cm
ꢁ1
spectra, which show no characteristic signals from Triton
X-100.
ꢁ1
55
for 2 and 247 cm for 3).
The incorporation of the Pd(II) complexes into the meso-
porous silica support can be inferred from the FTIR and
DRUV spectra of the materials. Thus, the DRUV spectra
of pressed pure powders of the extracted silica materials
The Pd content was determined by ICP giving a 0.75 wt%
loading of Pd, case of pyridine ligand, 0.70 wt% case of
phosphine ligand and 0.65 wt% case of amine ligand. The
presence of organic amounts in the inorganic silica network of
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New J. Chem., 2011, 35, 225–234 229

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Fig. 3 FT-IR spectra of the MSU-PdCl
2
(PETS)
2
silica material (up) and the monomer complex [trans-PdCl
2
(PETS)
2
] 2 (down). For clarity, two
magnifications of the marked areas in the spectrum are shown in the left.
Table 1 Metal and organic incorporation in the mesoporous silica materials estimated by ICP-AES, elemental analysis and TGA
a
b
b
b
c
b
d
Pd
(wt%)
N
(wt%)
%C
(wt%)
%H
(wt%)
N : Pd
(molar ratio)
%Organic content
(EA)
%Organic content
(TGA)
Samples
e
0.65 (1.0 )
e
0.32 (0.27 )
MSU_PdCl
MSU_PdCl
MSU_PdCl
2
2
2
(APTS)
(PETS)
(PPETS)
2
4.00
3.40
3.50
1.50
1.26
1.30
3.7
2.8
—
6%
5%
5%
8%
8%
7%
e
0.75 (1.0 )
e
0.31 (0.27 )
—
2
e
0.70 (1.0 )
2
a
b
Calculated by ICP-AES analysis of the filtrate after treatment of the samples with aqua regia. Determined from elemental analysis.
c
d
Calculated from ICP-AES and elemental analysis data. Calculated from TGA after elimination of hydration water. Theoretical values.
e
the mesoporous materials was analyzed by elemental analysis
and TGA. The C, H and N content, and the calculated
percentage in weight of organic species are given in Table 1.
The experimental amount of N in the materials is similar to the
theoretical one, suggesting the total incorporation of the
ligands during the synthesis and extraction procedure. On
the other hand, the N/Pd experimental molar ratios calculated
thermogravimetric curves for extracted materials is similar to
the results obtained with the CHNS elemental analysis
(see Table 1), confirming the presence of the organic ligands
in the materials. DTA curves of the mesoporous metal
complex-silica materials present an exothermic peak around
250–270 1C indicating the decomposing temperatures of the
complexes and the similar hydrothermal stability for all
of them.
from these data for samples MSU_PdCl
2 2
(APTS) and
MSU_PdCl (PETS) are over 2, which is the theoretical ratio
2
2
Nitrogen adsorption/desorption isotherms of the meso-
porous silica materials and their corresponding pore size
distributions are shown in Fig. 4 and 5. For comparison
purposes, the isotherm of a conventional (complex-free)
MSU-X type silica is also included. All materials show type
IV isotherms with a first distinctive nitrogen uptakes at
from the starting silane bridged-Pd(II) complexes.
These differences could be attributed to the fact that the
monomer complexes formation has a yield of 70–75%
(
see Experimental), as well as to some lost of metal during
the incorporation procedure. As a result, the weakest ligand
for the Pd(II), the primary amine (APTS), leads to the lowest
amount of Pd loaded in the hybrid materials with identical
amounts of starting Pd(II). TGA and DTA analyses
0
P/P = 0.5 due to the capillary condensation of nitrogen
inside the mesopores. Based on the isotherms, textural para-
meters were calculated and listed in Table 2, being the average
pore diameter around 3.5 nm in all cases, typical in these
templated materials according to the size of the surfactant
(
not shown here) allow to establish the amount of organic
fragments incorporated in the mesoporous silica materials and
their hydrothermal stability. In all cases, and after elimination
of hydration water, the organic content calculated from the
5
6
used in their synthesis. The incorporation of the metal
complexes into the MSU-X type silica materials entails the
2
30 New J. Chem., 2011, 35, 225–234
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Fig. 4 (a) N
MSU_PdCl (APTS)
and MSU_PdCl (PPETS)
2
adsorption/desorption isotherms at 77 K for catalysts
(diamonds), MSU_PdCl
2
(PETS)
2
(triangles)
and for a
Fig. 5 (a) N
MSU_PdCl
(triangles) and 2.5 wt% (diamonds) nominal content of Pd :SiO
2
adsorption/desorption isotherms at 77 K for catalysts
2
2
(circles) with 1 wt% Pd :SiO
2
2 2
(PETS) prepared with 0.5 wt% (circles), 1
wt%
and
2
2
complex-free MSU type silica (squares). (b) Their corresponding pore
size distributions calculated from the adsorption branch using the BJH
method.
2
for a complex-free MSU type silica (squares). (b) Their corresponding
pore size distributions calculated from the adsorption branch using the
BJH method.
apparition of a second adsorption process at P/P0 > 0.8,
which is characteristic of textural interparticle meso/
macroporosity. The bimodal porosity of the catalyst, with
small (3.5 nm) mesopores and large interparticle mesopores/
macropores, can be observed in both, isotherms and pore size
distribution (see Fig. 4). In any case, materials prepared by
(Fig. 6). The bimodal porosity of the samples was also
confirmed by TEM analysis which show both small intra-
particle mesopores and large interparticle meso/macropores.
The combination of templated mesoporosity and interparticle
meso/macroporosity in these materials is expected to increase
the accessibility of bulky molecules to the interior of these very
open structures, which have very interesting applications
in catalysis. Moreover, TEM micrographs do not show
Pd(0) nanoparticles indicating that Pd(II) is atomically
dispersed and no agglomeration occurs during the synthesis
and extraction procedure. TEM micrographs and DRUV
spectra of mesoporous metal complex-silica materials before
and after surfactant extraction confirm, respectively, the
preservation of the mesostructure and chemical integrity of
the metal complexes after the removal of the surfactant.
SEM micrographs of mesoporous Pd(II) complexes-silica
materials (see Fig. S4 in the ESIw) show an irregular morphology
formed by nanosized particles, being the materials highly
homogeneous. This is consistent with the nanosized particles
observed by TEM. EDS analyses and chemical mapping
confirm both the presence and homogenous distribution of
Pd in of all the samples.
2 2
co-condensation of PdCl (APTS) present less sharp nitrogen-
uptake steps in the small mesoporosity region with higher
second adsorption steps, this trend is consistent with an
5
6
increase of the initial solution pH due to the ligand used
for the synthesis (primary amine).
The incorporation of the metal complexes causes a slight
decrease in the average pore size compared to the complex-free
material; as it could be expected. Interestingly, BET surface
area is not affected for the incorporation of the complexes;
being in some cases even higher than in the complex-free
material (see Table 2 and Fig. 5), probably due to the
complementary interparticular mesoporosity.
The morphology of the mesoporous catalysts was investigated
by transmission electron microscopy (TEM) and scanning
electron microscopy (SEM). The catalysts exhibit non-ordered
wormhole-like structures, typical of MSU-type materials
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Table 2 Textural parameters of mesoporous metal-complex silica materials with different Pd(II) contents
Surface
area/m g
Mesopore
volume/cm g
Interparticle
volume/cm g
Total pore
volume/cm g
Mesopore
size/nm
Large mesopore
size/nm
2
ꢁ1a
3
ꢁ1b
3
ꢁ1b
3
ꢁ1b
c
c
Samples
MSU (blank)
720
670
0.65
0.65
—
1.40
0.65
2.05
3.8
3.4
—
30
MSU_PdCl
MSU_PdCl
2
(APTS)
(PETS)
2
2
2
0
1
2
.5 wt% nominal Pd : SiO
wt% nominal Pd : SiO
.5 wt% nominal Pd : SiO
MSU_PdCl (PPETS)
2
2
715
750
670
800
0.50
0.65
0.50
0.65
1.90
1.75
2.05
1.85
2.40
2.40
2.55
2.50
3.4
3.4
3.4
3.2
*
80
*
2
2
2
*
a
Out of range of nitrogen adsorption technique. The BET surface area was estimated by using multipoint BET method using the adsorption data
*
b
in the relative pressure (P/P
the BJH method, the volume was measured at the plateau of the cumulative adsorption pore volume plot (approximately 8 nm). The total pore
volume was measured at P/P = 0.99. The pore volume of the large mesopore, interparticle volume, was estimated subtracting both
0
) range of 0.05–0.30. Mesopore volume was calculated from the adsorption branch of the nitrogen isotherm using
0
c
values. Average mesopore and large mesopore sizes were estimated from the adsorption branch of the nitrogen isotherm using the BJH method.
water afforded less than 2% conversion and the formation
of Pd black was observed. However, under solvent-free
conditions the corresponding cross-coupling took place with
4
1–60% of conversion after 5 h. Similar results were obtained
using MSU_PdCl (PPETS) as a catalyst. The possibility to
2
2
work under solvent-free conditions significantly reduces the
operating costs and the environmental impact of the process.
In this case, ball-millling is less advantageous than magnetic
stirring, leading to a conversion lower than 20%.
0
The coupling reaction of a solid reactant, namely 4 -bromo-
acetophenone, with phenylboronic acid requires the use of
mechanochemical agitation to obtain a quantitative conversion
(
see Table 4). On the other hand, in the case of magnetic stirring
less than 8% conversion was observed. By optimization of the
reaction conditions conversion values up to 65% and 78% were
obtained for Pd(II)_phosphine and Pd(II)_pyridine based silica
materials, respectively. Remarkably, even with the diffusional
problems associated to nanostructured silica materials in solid
reactions, the use of mechanochemical agitation and Pd(II)
complex-silica materials yields high levels of conversion for
the coupling product of the Suzuki-Miyaura reaction.
Fig. 6 Representative TEM images of the MSU_PdCl
material prepared from pyridine ligand and 0.75 wt% loading of Pd
a) before and (b) after surfactant extraction; (c) and (d) show the
images of MSU_PdCl (APTS) and MSU_PdCl (PPETS) materials,
after surfactant
2 2
(PETS)
Reproducibility in solid state reactions is always an
important goal, mainly due to the heterogeneity of the
mixture. At this regard, different attempts of the same reaction
were carried out obtaining a better reproducibility for the
material prepared with the Pd(II)-pyridine complex.
(
2
2
2
2
respectively, prepared with 1 wt% nominal Pd : SiO
extraction. The scale bars represent 20 nm.
2
Catalytic activity
Conclusions
Mesoporous Pd(II) complexes-silica materials MSU_PdCl (PETS)
2
2
and MSU_PdCl (PPETS)
2
2
were tested for the Suzuki coupling
Different mesoporous Pd(II) complex-silica materials were
prepared, using an environmentally friendly method, by
co-condensation of three metal complexes containing
trialkoxysilane terminal groups with TEOS in the presence
of a structure directing agent, namely Triton X-100. This
synthesis strategy does not need organic co-solvents, anaerobic
or anhydrous environments, being carried out at mild condi-
tions, which preserve the integrity of the metal complexes in
the silica framework. Moreover, the synthesis of the mono-
nuclear Pd(II) complexes directly in TEOS, in the same
reaction conditions than those used for the co-condensation
processes, assures the correct characterization of the silane
bridged-Pd(II) entities incorporated to the hybrid mesoporous
material, avoiding, at the same time, the use of any additional
reaction of phenylboronic acid with a liquid aryl bromide such
as 1-bromonaphthalene (Table 3). Reactions were performed
at room temperature using both different solvents and solvent-
free conditions under magnetic stirring. In the case of the
0
solid 4 -bromoacetophenone (Table 4), reactions were
performed in a ball mill apparatus to improve the mixture of
the solid reactants at room temperature under solvent-free
conditions.
Table 3 shows the catalytic activity of the materials in the
coupling reaction of phenylboronic acid with 1-bromo-
naphthalene using 0.1 mol% catalyst loading and K
2
CO
3
or
as
KOH as bases. First attempts using MSU_PdCl (PETS)
2
2
catalyst in different solvents such as toluene, acetone and
2
32 New J. Chem., 2011, 35, 225–234
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Table 3 Catalytic activity of Pd(II) complexes-MSU materials in the coupling reaction of phenylboronic acid with 1-bromonaphthalene at RT
under different conditions
a,b
Catalyst
PhB(OH)
2
(eq.)
Base
Solvent
Agitation conditions
t/h
Conv. (%)
a
MSU_PdCl (PETS)
2
2
1.5
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
3
3
3
Toluene
Acetone
Magnetic
Magnetic
Magnetic
Magnetic
38
38
38
5
2
a
2
a
H
2
O
1
41
a
No
No
No
No
No
a
2
5
2
5
2
5
2
6
6
6
6
6
42
60
47
56
49
42
47
18
a
KOH
CO
KOH
CO
Magnetic
b
a
MSU_PdCl (PPETS)
2
2
1.5
K
2
3
3
Magnetic
a
a
Magnetic
b
a
1
.3
K
2
Mechano-chemical
a
b
Conversion determined by GC. Isolated yield.
0
Table 4 Catalytic activity of Pd(II) complexes-MSU materials in the coupling reaction of phenylboronic acid with 4 -bromoacetophenone at RT
under different conditions
a
Catalyst
Catalyst loading (mol% Pd)
0.1
PhB(OH)
2
(eq.)
K
2
CO
3
(eq.)
Agitation conditions
t/h
Conv. (%)
MSU_PdCl (PETS)
2
2
1.5
1.5
2
2
Magnetic
Mechano-chemical
16
1.5
8
0
.5
50
75
78
58
—
42
65
4
6
.5
0.5
0.1
2
1.5
2
3
2
3
2
1.5
16
1.5
7
MSU_PdCl (PPETS)
2
2
Magnetic
Mechano-chemical
0
0
.25
.25
1.3
a
Conversion determined by GC.
organic co-solvent (as toluene or tetrahydrofuran). The
surfactant can be easily removed by ethanol extraction at
room temperature without damage for the complexes,
producing materials with Pd(II) complexes homogeneously
and highly dispersed in a mesoporous support. The chemical
integrity of the metal complexes after surfactant extraction
was confirmed by spectroscopic techniques and the use of
these materials as catalysts for the Suzuki-Miyaura coupling
reaction of phenylboronic acid with various aryl bromides.
respectively. A. Carrillo and E. Serrano are kindly thanked for
their invaluable help during the project. Department of
Applied Physics of the University of Alicante, and especially
Dr Carlos Untiedt, for their continuous support.
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In the case of 4 -bromoacetophenone, conversion values
3
4
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6
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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011