A solid chelating ligand: Periodic mesoporous organosilica containing 2,2′-bipyridine within the pore walls

Article abstract of DOI:10.1021/ja4131609

Synthesis of a solid chelating ligand for the formation of efficient heterogeneous catalysts is highly desired in the fields of organic transformation and solar energy conversion. Here, we report the surfactant-directed self-assembly of a novel periodic mesoporous organosilica (PMO) containing 2,2′-bipyridine (bpy) ligands within the framework (BPy-PMO) from a newly synthesized organosilane precursor [(i-PrO) ₃Si-C₁₀H₆N₂-Si(Oi-Pr)₃] without addition of any other silane precursors. BPy-PMO had a unique pore-wall structure in which bipyridine groups were densely and regularly packed and exposed on the surface. The high coordination ability to metals was also preserved. Various bipyridine-based metal complexes were prepared using BPy-PMO as a solid chelating ligand such as Ru(bpy)₂(BPy-PMO), Ir(ppy) ₂(BPy-PMO) (ppy = 2-phenylpyridine), Ir(cod)(OMe)(BPy-PMO) (cod = 1,5-cyclooctadiene), Re(CO)₃Cl(BPy-PMO), and Pd(OAc) ₂(BPy-PMO). BPy-PMO showed excellent ligand properties for heterogeneous Ir-catalyzed direct C-H borylation of arenes, resulting in superior activity, durability, and recyclability to the homogeneous analogous Ir catalyst. An efficient photocatalytic hydrogen evolution system was also constructed by integration of a Ru-complex as a photosensitizer and platinum as a catalyst on the pore surface of BPy-PMO without any electron relay molecules. These results demonstrate the great potential of BPy-PMO as a solid chelating ligand and a useful integration platform for construction of efficient molecular-based heterogeneous catalysis systems.

Full text of DOI:10.1021/ja4131609

Article
pubs.acs.org/JACS
A Solid Chelating Ligand: Periodic Mesoporous Organosilica
Containing 2,2′-Bipyridine within the Pore Walls
†,‡
†
,∇,◆
†,‡,∇
‡,§
†,‡
Minoru Waki,
Yoshifumi Maegawa,
Kenji Hara, Yasutomo Goto, Soichi Shirai,
†
,‡
†
†
⊥
∥
#
Yuri Yamada, Norihiro Mizoshita, Takao Tani, Wang-Jae Chun, Satoshi Muratsugu, Mizuki Tada,
§
,†,‡
Atsushi Fukuoka, and Shinji Inagaki*
†
‡
Toyota Central R&D Laboratories., Inc. and Japan Science and Technology Agency (JST)/ACT-C, Nagakute, Aichi 480-1192,
Japan
§
Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan
⊥
Graduate School of Arts and Sciences, International Christian University, Mitaka, Tokyo 181-8585, Japan
∥
#
Department of Chemistry, Graduate School of Science and Research Center for Materials Science, Nagoya University, Furo-cho,
Chikusa, Nagoya 464-8602, Japan
*
S Supporting Information
ABSTRACT: Synthesis of a solid chelating ligand for the
formation of efficient heterogeneous catalysts is highly desired
in the fields of organic transformation and solar energy
conversion. Here, we report the surfactant-directed self-
assembly of a novel periodic mesoporous organosilica
(
PMO) containing 2,2′-bipyridine (bpy) ligands within the
framework (BPy-PMO) from a newly synthesized organosilane
precursor [(i-PrO) Si−C H N −Si(Oi-Pr) ] without addition
3
10
6
2
3
of any other silane precursors. BPy-PMO had a unique pore-
wall structure in which bipyridine groups were densely and regularly packed and exposed on the surface. The high coordination
ability to metals was also preserved. Various bipyridine-based metal complexes were prepared using BPy-PMO as a solid
chelating ligand such as Ru(bpy) (BPy-PMO), Ir(ppy) (BPy-PMO) (ppy = 2-phenylpyridine), Ir(cod)(OMe)(BPy-PMO) (cod
2
2
=
1,5-cyclooctadiene), Re(CO) Cl(BPy-PMO), and Pd(OAc) (BPy-PMO). BPy-PMO showed excellent ligand properties for
3
2
heterogeneous Ir-catalyzed direct C−H borylation of arenes, resulting in superior activity, durability, and recyclability to the
homogeneous analogous Ir catalyst. An efficient photocatalytic hydrogen evolution system was also constructed by integration of
a Ru-complex as a photosensitizer and platinum as a catalyst on the pore surface of BPy-PMO without any electron relay
molecules. These results demonstrate the great potential of BPy-PMO as a solid chelating ligand and a useful integration platform
for construction of efficient molecular-based heterogeneous catalysis systems.
INTRODUCTION
efficiency in their narrow channels. Thus, the development of a
new support preserving the inherent functions of metal
complexes is still a challenge for realizing their practical
applications.
Ordered mesoporous materials, synthesized by a surfactant-
directed self-assembly approach, are ideal supports due to their
high surface areas and uniform mesopores for facile diffusion of
molecules and ions. Periodic mesoporous organosilicas
(PMOs), synthesized from organic-bridged silane precursors
■
A variety of functional systems based on metal complexes with
chelating ligands have been constructed for applications such as
1
catalysis for organic synthesis and photosensitizers for
2
photoreaction systems. For practical applications, immobiliza-
tion of metal complexes on solid supports is important for easy
recovery and recycle uses and allows easy integration of
different components for constructing highly functional
3
systems. There are some successful examples for heterogen-
{
R-[Si(X) ] , n ≥ 2, R = organic group, X = OMe, OEt, Oi-Pr,
3
n
ized metal complex showing comparable or superior catalytic
performance to the homogeneous analogue. However,
3
allyl}, are especially promising because functional organic
groups can be incorporated into the framework without
plugging the mesopore space, in contrast to mesoporous silica6s
functionalized with organic groups grafted on the pore surface.
PMOs with a variety of organic groups in the frameworks have
already been synthesized, including organic groups having
immobilization on conventional supports such as silica gel
has often resulted in decreases in the performances of metal
complexes, such as catalytic activity and selectivity, because the
uniform environment of the homogeneous metal complexes is
disturbed by unfavorable interactions with the solid surface and
5
nonuniform fixation on the surface. Moreover, the diffusion
limitation of substrate and product molecules has been a
problem for porous supports due to a decrease in reaction
Received: December 27, 2013
Published: February 26, 2014
©
2014 American Chemical Society
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7
,8
metal ligand abilities. Recently, we reported the synthesis of
silylated with triisopropoxysilane to give 2-bromo-5-
(triisopropoxysilyl)pyridine (2), followed by Stille coupling of
2 with bis(tributyltin) to successfully afford 5,5′-bis(triisopro-
poxysilyl)-2,2′-bipyridine (3). Although methoxysilyl or ethox-
ysilyl groups have been widely employed as organosilane
precursors for the synthesis of PMOs and silica-based sol−gel
materials, they have low tolerance for many organic reactions
and are difficult to purify due to their high reactivity. In
contrast, the isopropoxysilyl group was stable enough to allow
several reactions using organometallic reagents and purification
9
crystal-like PMOs containing divinylpyridine and phenyl-
pyridine groups in the frameworks from 100% organosilane
1
0
precursors of (EtO) Si−C H −C H N−C H −Si(OEt) and
3
2
2
5
3
2
2
3
(
EtO) Si−C H −C H N−Si(OEt) , respectively. These pyr-
3
6
4
5
3
3
idine-based PMOs have a crystal-like pore wall structure in
which the pyridine moieties are densely and regularly arranged
in the pore walls to form well-defined pore surface structures.
The phenylpyridine-PMO was successfully utilized as a solid
chelating ligand for the formation of metal complexes
21
(
ruthenium and iridium) on the pore surface by postsynthetic
by silica gel chromatography. The use of the isopropoxysilyl
group led to easy synthesis of the bipyridine precursor directly
attached to two alkoxysilyl groups 3.
1
0,11
cyclometalation.
2
,2′-Bipyridine is the most widely used chelating ligand for
forming metal complexes in coordination and supramolecular
BPy-PMO was prepared by hydrolysis and polycondensation
from precursor 3 under basic conditions in the presence of
octadecyltrimethylammonium chloride (C TMACl) surfactant
1
2,13
chemistry.
Although there have been several reports on the
14 15
synthesis of mesoporous silicas or PMOs containing 2,2′-
bipyridine ligands, they possessed a nonuniform pore surface
structure due to the amorphous pore walls in which 2,2′-
bipyridine groups were randomly distributed on the pore
surface or in the walls. The amorphous pore walls resulted from
the use of 2,2′-bipyridine precursors attached by one or two
trialkoxysilyl groups via linkers such as alkyl chains, amides, or
thioethers. Thus, the formation of crystal-like pore wall
structures has been limited to PMOs synthesized from rigid
18
18
as a structure directing agent, followed by extraction of the
surfactant with an acidic ethanol solution (Scheme 1). Strongly
basic conditions (0.25 M) with aging at 95 °C caused cleavage
of the Si−C bond in the bipyridine precursor (Figure S1). The
Si−C bond cleavage was very sensitive to basicity and
temperature due to the localization of electrons at the nitrogen
atoms and the resulting deficiency of electrons in the pyridine
rings, which was predicted from theoretical studies on Si−C
cleavage in organosilane precursors during the condensation
16
17
bridging organics such as benzene, biphenyl, naphthalene,
1
9
9
10
22
divinylbenzene, divinylpyridine, and phenylpyridine at-
tached to two silyl groups but no linkers. We envisaged that a
new precursor with a 2,2′-bipyridine directly attached to two
silyl groups would be essential for the synthesis of crystal-like
bipyridine-PMO.
Here, we report the successful synthesis of the crystal-like
bipyridine-PMO (BPy-PMO) from 100% organosilane pre-
process. However, at weak basicity (0.005 M) and room
temperature, the hydrolysis of the isopropoxysilyl groups was
found to be slow, which was attributed to the low
nucleophilicity of hydroxide for attacking the bulky isopropoxy
group. Consequently, the synthesis was carried out under
moderately basic conditions of 0.035 M and 50 °C with a long
aging time (6 days), which successfully afforded highly ordered
BPy-PMO powder. The 1 C cross-polarization (CP) magic-
angle-spinning (MAS) NMR, 29Si MAS NMR and FT-IR
spectra confirmed the preservation of both the bipyridine
moieties and Si−C bonds during the synthesis and extraction
3
cursor [(i-PrO) Si−C H N −Si(Oi-Pr) ]. BPy-PMO had a
3
10
6
2
3
well-defined pore surface structure where 2,2′-bipyridines were
densely and regularly arranged and preserved the high
coordination ability to metals. A variety of bipyridine-based
metal complexes including iridium (Ir), ruthenium (Ru),
rhenium (Re), and palladium (Pd) were easily formed on the
pore surface. Furthermore, we discovered the excellent ligand
properties of BPy-PMO in heterogeneous Ir-catalyzed direct
13
processes (Figures 1a−c and S2). The C CP MAS NMR
1
3
spectrum displayed the five aromatic resonances similar to
C
NMR spectrum of precursor 3, which can be assigned to
29
carbons on the pyridine ring. The Si MAS NMR spectrum
20
2
3
C−H borylation of arenes with bis(pinacolato)diboron and in
heterogeneous photocatalytic hydrogen evolution from water
using a Ru-complex and platinum integrated on the pore
surface.
showed T [SiC(OSi) (OH)] and T [SiC(OSi) ] signals, but
2
3
n
almost no Q [Si(OSi) (OH) , n = 2−4] signals between
90 and −120 ppm, indicating almost complete preservation of
n
4−n
−
2
Si−C bonds (Figure 1c). The presence of T signal also shows
that a considerable amount of silanol groups exist in BPy-PMO.
During optimization of the synthetic conditions, slow addition
of the precursor solution was found to be crucial to prepare
well-ordered BPy-PMO. Thus, the precursor solution was
slowly added over a period of 3 h at a constant speed using a
syringe pump (Figure S5). Fast addition of the precursor
caused the aggregation of precursor molecules in aqueous
reaction media due to the slow hydrolysis rate of isopropox-
ysilyl groups, which resulted in the formation of disordered
material.
RESULTS AND DISCUSSION
■
Synthesis of Crystal-Like BPy-PMO from 100%
Organosilane Precursor. The bipyridine precursor was
prepared in two steps from a commercially available reagent.
As shown in Scheme 1, 2-bromo-5-iodopyridine (1) was
Scheme 1. Synthesis of 5,5′-Bis(triisopropoxysilyl)-2,2′-
bipyridine and Preparation of BPy-PMO through Hydrolysis
and Polycondensation under Basic Conditions
The XRD patterns of surfactant-free BPy-PMO showed low
angle reflections at 2θ = 1.96°, 3.40°, and 3.84° (d spacings of
4
.50, 2.60, and 2.30 nm, respectively) corresponding to a two-
dimensional hexagonal lattice (a = 5.2 nm), and four other
0
peaks at a medium scattering angle of 7−23° (d spacings of
1
.16, 0.58, and 0.39 nm), corresponding to a lamellar structure
with a molecular-scale periodicity of bipyridine and silica
moieties in the pore walls (Figure 1d). The broad peak at 2θ =
1
8−25° was due to disordered arrangement of atoms in the
4
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Figure 1. (a) 13C CP MAS NMR spectrum of BPy-PMO (above) and
Figure 2. (a) SEM and (b−d) TEM images and (e) structural model
of BPy-PMO. Silicon, yellow; oxygen, red; carbon, gray; nitrogen,
pink; hydrogen, white. Hydrogen on bipyridine is omitted.
1
3
C NMR spectrum of precursor 3 (below). (b) Numbering system of
bipyridine. (c) 29Si MAS NMR spectrum, (d) XRD patterns, and (e)
nitrogen adsorption/desorption isotherms for BPy-PMO.
freedom to rotate about the Si−C axis because rotary motion of
the bridging organic groups in crystal-like benzene- and
biphenyl-PMOs with similar packing structures has been
demonstrated by NMR spectroscopy. It is suggested that
the framework bipyridine groups preserved their coordination
ability for metal complex formation.
silica belt-like layers shown later, but not due to amorphous
regions or contamination of amorphous materials. The nitrogen
adsorption/desorption isotherms showed a type-IV isotherm,
typical of ordered mesoporous materials (Figure 1e). The
density functional theory pore diameter (dDFT), Brunauer−
23
Emmett−Teller surface area (S ) and pore volume were 3.8
Elemental analysis of carbon and nitrogen in BPy-PMO gave
a carbon/nitrogen ratio of 4.5, which is consistent with that
expected from the chemical formula (C H N ) of the
BET
2
−1
3 −1
nm, 739 m g , and 0.41 cm g , respectively. The pore wall
thickness was estimated to be 1.4 nm (a0 − dDFT),
corresponding to three layers of the bipyridine moieties in
the pore walls.
10
6
2
bipyridine bridges. The amount of bipyridine moieties in
BPy-PMO was estimated from the nitrogen content as 3.18
−1
Figure 2a−d shows the scanning and transmission electron
microscopy (SEM and TEM) images of BPy-PMO. The SEM
image showed particle morphology with diameters of 200−500
nm (Figure 2a). The TEM images showed one-dimensional
channels going through the particles, indicating that the
particles have a single crystal-like structure (Figure 2b). The
enlarged TEM images showed a hexagonal arrangement of
uniform pores (Figure 2c) and many lattice fringes with a basal
spacing of ca. 1.2 nm in the perpendicular direction to the
mesochannels (Figure 2d), which was in good agreement with
the result obtained by XRD. Figure 2e shows a structural model
of BPy-PMO. The bipyridine and silica belt-like layers in the
walls are arranged alternatively with a periodicity of 1.16 nm in
the channel direction whose periodicity was clearly observed by
XRD and TEM (Figures 1d and 2d). A molecular mechanics
simulation showed that the neighboring bipyridine−bipyridine
distance is approximately 0.44 nm, which is larger than the
usual π−π stacking distance of aromatic compounds, although
the molecular arrangement and intermolecular distance can
fluctuate due to the flexibility of the silica framework. The
loosely packed bipyridine groups could have a high degree of
mmol g . Thermogravimetric analysis showed that weight loss
due to decomposition of the BPy moieties occurred in the
temperature range of 500−600 °C with a maximum at 570 °C,
showing high thermal stability up to 500 °C due to the robust
siloxane bond network (Figure S4).
Metal Complex Formations using BPy-PMO as a
Ligand. A variety of bipyridine-based metal complexes can
be readily formed on the pore surface of BPy-PMO by simple
stirring of the BPy-PMO powder in a solution containing a
metal precursor (Figure 3).
Ru(bpy) (BPy-PMO) was formed by dispersion of BPy-
2
PMO powder in ethanol solution containing Ru(bpy) Cl ·
2
2
24
2H O precursor (Figure S9). The UV−vis diffuse reflectance
2
spectrum of the obtained powder showed a broad band at
around 450 nm due to metal-to-ligand charge transfer (MLCT)
and an intense peak at around 300 nm due to π−π* transition
of ligands, which was very similar to the absorption spectrum of
a homogeneous solution of Ru(bpy) Cl except for peak
3
2
25
broadening (Figure 4a). The peak broadening suggests a
slight difference in the electronic state of Ru(bpy) (BPy-PMO)
2
from its homogeneous analogue because one of the bipyridine
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Figure 3. (a) CG image of the direct Ru-complex formation on the pore surface of BPy-PMO. Green, ruthenium. The other colors correspond to
those in Figure 2. (b) Chemical structures of metal complexes prepared using BPy-PMO as a solid chelating ligand in this study.
Ir(ppy) (BPy-PMO) was prepared by reaction with [Ir-
2
27
(
ppy) Cl]2 in CH Cl . The UV−vis diffuse reflectance
spectrum showed an absorption band of MLCT at 378 nm
with a long tail above 450 nm. The tail was attributed to a
2
2
2
direct spin-forbidden absorption from the singlet ground state
+
̂
̂
to triplet excited state as observed for [Ir(C N) (N N)] , where
(
2
̂
̂
C N) and (N N) indicate a cyclometalating ligand such as 2-
phenylpyridine and a neutral ligand such as 2,2′-bipyridine
27
(
Figure S12). The emission of Ir(ppy) (BPy-PMO) was also
2
observed at a similar wavelength for the homogeneous analogue
27b
(
Figure S13).
Ir(cod)(OMe)(BPy-PMO) was synthesized by treating an
end-capped BPy-PMO with [Ir(OMe)(cod)]2 precursor in
benzene (Figure S15). The end-capped BPy-PMO was
prepared by trimethylsilylation of surface silanol groups with
N-trimethylsilylimidazole for application to catalysis (see
Supporting Information, section 3). XANES spectrum and
EXAFS Fourier transform at Ir LIII edge of the PMO were in
good agreement with those of the homogeneous complex
[Ir(OMe)(cod)(bpy)] (Figure S16). The X-ray photoelectron
spectroscopy (XPS) spectra of the PMO displayed almost the
same binding energies of 61.3 eV for Ir 4f7/2 and 64.5 eV for Ir
4
f5/2 as the homogeneous analogue (Figure S17). These results
suggest successful formation of Ir(cod)(OMe)(BPy-PMO).
Figure 4. (a) UV−vis absorption, (b) photoluminescence, (c) Ru K-
edge XANES spectra, and (d) EXAFS Fourier transform (κ = 30−140
Re(CO) Cl(BPy-PMO) was prepared by treatment with
3
−1
28
nm ) of Ru(bpy) (BPy-PMO) (red line) and Ru(bpy) Cl (black
Re(CO) Cl in toluene. The UV−vis spectrum of Re(CO) Cl-
2
3
2
5
3
line).
(
BPy-PMO) showed an MLCT absorption band (around 390
nm) assignable to the coordination of Re to bpy ligand (Figure
2
6
S19). In the FT-IR spectrum of the Re complex, three
characteristic CO stretching bands were observed at 1893,
ligands was covalently fixed in the PMO framework by silicon
26
atoms at the 5,5′-positions. This was also suggested by the
slightly different emission band from that of the homogeneous
one (Figure 4b). A detailed investigation on this difference is
currently underway in our laboratory. The X-ray absorption fine
structure (XAFS) measurement was also carried out to
elucidate the structure of the Ru center in the pore walls
−1
1
920, and 2023 cm , which is in good agreement with those of
Re(bpy)(CO) Cl (Figure S20). The XAFS measurement also
suggested the formation of the desired Re complex in the PMO
material (Figure S21).
Pd(OAc)
29
3
(BPy-PMO) was obtained by reaction with
2
30
(
(
Figure 4c,d). The X-ray absorption near edge spectroscopy
XANES) spectrum and extended X-ray absorption fine
Pd(OAc) in THF. The UV−vis diffuse-reflectance spectrum
2
of the PMO showed the appearance of a shoulder band around
370 nm, suggesting that Pd(II) directly interacts with BPy
groups in the PMO framework without formation of Pd black
structure (EXAFS) Fourier transform at Ru K-edge showed
spectral characteristics corresponding to those of homogeneous
2+
Ru(bpy) Cl , suggesting successful formation of [Ru(bpy) ]
(Figure S23). The formation of Pd(OAc) (BPy-PMO) was also
3
2
3
2
complex on the pore surface of BPy-PMO (Figure S10).
confirmed by XPS investigation. As shown in Figure S24, the
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binding energy of Pd 3d5/2 in the PMO is 337.0 eV, indicating
the strong coordination of BPy groups in PMO to Pd(II)
species. The observed binding energy is lower than that of
Direct C−H borylation was carried out in the presence of
Ir(OMe)(cod)(BPy-PMO) (denoted by Ir-PMO in this
section, 0.75 mol % Ir) and bis(pinacolato)diboron (B pin )
2
2
3
37.5 eV for free Pd(OAc) , suggesting that the electron-
(0.33 mmol) in arenes (20 mmol) at 80 °C for 12 h. The
catalytic activity was compared to those of a homogeneous
iridium catalyst with a molecular bipyridine ligand and
heterogeneous iridium catalysts with bipyridine ligands grafted
on silica gel (L1), mesoporous silica (L2), and polystyrene
(L3). Ir-PMO catalyzed the direct C−H borylation of benzene
and gave the desired product in high yield of 94%, which was
higher than those obtained with a homogeneous catalyst (80%)
and grafted catalysts on silica gel (33%) and mesoporous silica
2
donating character of BPy induced a negative shift in binding
31
energy for the Pd species.
The loading amounts of metals on BPy-PMO were
determined by inductively coupled plasma (ICP)-atomic
emission spectrometry (AES) and are listed in Table 1. The
Table 1. Physical Parameters of Metal Complexes Prepared
Using BPy-PMO
(
63%). The grafted catalyst on polystyrene showed no catalytic
activity.
In order to elucidate the catalytic behavior, the reaction
loading of metal
mmol/g
metal/
bpy
S
dDFT
nm
BET
m /g
2
metal-BPy-PMO
Ru(bpy) (BPy-PMO)
0.20
0.26
0.16
0.071
0.095
0.056
530
700
520
3.2
3.1
3.5
2
kinetics were studied as shown in Figure 5a. The PMO catalyst
Ir(ppy) (BPy-PMO)
2
Ir(OMe)(cod)(BPy-
PMO)
Re(CO) Cl(BPy-
1.12
0.56
500
3.2
3
PMO)
Pd(OAc) (BPy-PMO)
0.74
0.28
580
740
3.4
3.8
2
BPy-PMO
―
―
−1
amounts of metals were 0.16−1.12 mmol g , which indicated
that 7−56% of bipyridine groups in BPy-PMO were
coordinated to metals. The large differences in the loading
amounts of metals may be explained by the bulkiness of free
ligands (Figure 3b) or the reactivity of BPy-PMO with each
metal complex precursor. However, the loading amounts can be
controlled to some extent by conditions such as concentration
of metal precursors in the treated solution. The XRD patterns
and nitrogen adsorption/desorption isotherms for BPy-PMOs
after the formation of metal complexes showed almost
complete preservation of the ordered mesoporous structure
and molecular scale periodicity, indicating high chemical
stability of BPy-PMO (see Supporting Information, section
Figure 5. (a) Reaction kinetic curves of C−H borylation of benzene
catalyzed by iridium catalysts based on BPy-PMO (black line), 2,2′-
bipyridine (red line), L1 (blue line), L2 (green line), and L3 (gray
line). (b) Reusability of recovered PMO catalyst, first cycle (original:
4
). In addition, the metal-BPy-PMOs still have large mesopores
9
4%), second cycle (91%), third cycle (88%), and fourth cycle (78%).
2
−1
of 3.1−3.8 nm and high surface areas of 500−740 m g
Table 1).
The above results clearly indicate the great potential of BPy-
(
retained its activity until the end of the reaction, while the
reaction rates of homogeneous and grafted iridium complexes
decreased within 2 h. The decreased reaction rates indicate
deactivation of the active center possibly due to aggregation of
the iridium species and/or undesired interaction between the
active center and solid surface. These results suggest that BPy-
PMO is an effective solid ligand for suppression of aggregation
and undesired interactions of the metal center due to the isolate
binding of metals on the well-defined surface. It is noteworthy
that the initial reaction rate for Ir-PMO is almost the same as
that of the homogeneous catalyst with the same amount of Ir.
This result suggests a very small diffusion limitation of
molecules (B pin , products, etc.) in the mesochannels due to
PMO as a chelating ligand for the formation of a variety of
solid-state bipyridine-based metal complexes with high surface
areas. The metal complex-BPy-PMOs have the potential to
show good catalytic performance similar to their homogeneous
analogues because of the well-defined catalytic centers on the
surface and efficient mass transportation due to the large
mesopores. Furthermore, the high stability of BPy-PMO could
allow easy separation and recycle use of the heterogeneous
metal catalysts.
Solid Ligand Properties for Direct C−H Borylation of
Arenes. Construction of isolated and well-defined catalytic
active centers onto solid supports is a subject of central
importance in the development of efficient heterogeneous
2
2
the large pore size. The grafted mesoporous silica showed a
slightly lower initial reaction rate than Ir-PMO, possibly due to
the effect of the protruded iridium complexes into the
mesochannels. The grafted silica gel showed an initial reaction
rate similar to that of Ir-PMO because of the large mean pore
diameter (6.3 nm) of the silica gel (Figure S26).
3
2
molecular catalysts. In homogeneous catalysis, the Ir-
bipyridine complex was found to show both high catalytic
activity and functional tolerance for direct C−H borylation of
arenes, one of the most important transformations for
33
preparing useful aryl boryl ester intermediates. To date,
heterogeneous molecular catalysts for C−H borylation have
The scope of this system was then examined using various
types of substrates (Table 2, entries 6−12). Ir-PMO catalyzed
C−H borylation of both electron-rich and -poor substituted
arenes. In the case of monosubstituted arenes, the reaction gave
regioisomeric mixtures of the meta- and para-borylation
34
been limited to the Ir-silica-supported phosphine complex
35
and polymeric Ir-bipyridine complex catalysts. Herein, BPy-
PMO was used as a solid chelating ligand for the heterogeneous
Ir-catalyst for direct C−H borylation of arenes.
4
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dx.doi.org/10.1021/ja4131609 | J. Am. Chem. Soc. 2014, 136, 4003−4011

Journal of the American Chemical Society
Article
3
6
Table 2. Ligand Effects in Ir-Catalyzed Direct C−H
mediator), and as a sacrificial reducing agent. In this work,
we have constructed a heterogeneous photocatalytic hydrogen
evolution assembly of BPy-PMO integrated with a [Ru-
Borylation of Arenes with B pin
2
2
(
bpy)3]2+ photosensitizer and platinum catalyst on the pore
surface without any electron mediators such as methylviologen
as shown in Figure 6a.
a
entry
R
ligand
yield (%)
1
2
3
4
5
6
7
8
9
H
H
BPy-PMO
2,2′-bipyridine
L1
94
80
33
63
0
H
H
L2
H
L3
b
CH3
OMe
BPy-PMO
BPy-PMO
BPy-PMO
BPy-PMO
BPy-PMO
BPy-PMO
BPy-PMO
85
c
88
d
CO Me
88
2
1,2-(CH3)2
1,2-Cl2
60
92
81
47
1
1
1
0
1
2
1,3-Cl2
1,3-(CF3)2
a
b
Isolated yield of aryl boronate based on boron atom in B pin . m:p =
3:37. m:p = 63:37. m:p = 56:44.
2
2
c
d
6
products (Table 2, entries 6−8). In contrast, regioselective
borylation was observed for disubstituted arenes. Both 1,2- and
1
,3-disubstituted arenes yielded the corresponding boronate
esters as single regioisomers due to steric hindrance of the
substituents (Table 2, entries 9−12). Similar behaviors were
32
Figure 6. (a) Schematic illustration of a hydrogen evolution system
based on Pt/Ru-PMO. Ru-complex is represented as a CPK model.
observed for the homogeneous system.
The PMO catalyst was easily recovered by simple filtration
and was available for recycle use (Figure 5b). The recovered
PMO catalyst retained good catalytic activity with slight loss of
product yield for at least three cycles. In contrast, the
homogeneous and the grafted catalysts showed almost no
catalytic activity for recycle use, indicating almost complete
deactivation during the first reaction cycle. These results
indicate good recyclability of the Ir-PMO catalyst.
Finally, a hot-filtration experiment was conducted to check
the catalytic activity of leached Ir species. After the reaction had
proceeded for 1 h with a yield of 54%, the PMO catalyst was
filtered by using a membrane filter (0.20 μm) under argon, and
the resulting transparent solution was allowed to react under
identical conditions for another 23 h (Figure S27). No
significant change in the product yield was observed, indicating
that the reaction completely stopped. ICP analysis showed that
the concentration of Ir species in the filtrate was under the
detection limit (1 ppm). These results clearly indicate that the
catalytic reaction occurred by the iridium complex fixed on
BPy-PMO without leaching of Ir species.
(b) Time-dependent hydrogen evolution curves of Pt/Ru-PMO (red
line) and homogeneous Ru(bpy) Cl and colloidal platinum particles
3
2
system (black line) under irradiation of visible light (>385 nm).
Ru(bpy) (BPy-PMO) (denoted by Ru-PMO in this section)
2
was loaded with Pt by stirring in an aqueous Pt(IV) salt
solution at 80 °C (Figure S29). The loading amount of Pt was
−1
1.10 mmol g , which was determined by ICP analysis. TEM
revealed the formation of many small nanoparticles with
diameters of 2−3 nm in the PMO particles (Figure S30). The
XRD pattern exhibited very broad peaks at d = 0.22 and 0.19
nm corresponding to (111) and (200) faces of the Pt crystal
37
(Figure S31a). These results indicate that Pt nanoparticles
were supported on Ru-PMO, including inside the mesochan-
nels, owing to the small sizes of the Pt nanoparticles.
The photocatalytic reaction was carried out using a Pyrex
reaction vessel connected to a glass-enclosed gas circulation and
evacuation system. Pt-loaded Ru-PMO was suspended in an
acetate buffer solution containing EDTA as a sacrificial
reducing agent in the vessel. Hydrogen was evolved
continuously and steadily under irradiation of visible light
(>385 nm). The amount of produced hydrogen reached 300
μmol after 24 h, corresponding to a Ru-based turnover number
of 184.
A Platform for Integrated Heterogeneous Photo-
catalytic Hydrogen Evolution System. BPy-PMO is also
expected to be a useful platform with which to integrate
different functional components for construction of efficient
2
+
solar energy conversion systems. [Ru(bpy)3] is a typical
photosensitizer which has been widely applied to homogeneous
and heterogeneous photocatalysis systems such as photo-
catalytic hydrogen evolution from water in the presence of
platinum (as a catalyst), methylviologen (as an electron
This indicates that Ru-PMO can act as a photosensitizer for
hydrogen evolution in combination with Pt nanoparticles
without any electron mediators. Direct electron transfer from
Ru complexes to Pt nanoparticles occurred in this system
4
008
dx.doi.org/10.1021/ja4131609 | J. Am. Chem. Soc. 2014, 136, 4003−4011

Journal of the American Chemical Society
Article
because both components were fixed on the pore surface and
were in constant contact with each other. As a control
experiment, homogeneous Ru(bpy) Cl and colloidal platinum
0.2). Pore-size distributions were calculated using the DFT method
(
DFT kernel: N at 77 K on silica, cylindrical pores, nonlinear density
2
functional theory (NLDFT) equilibrium model). Pore volumes were
estimated by the t-plot method. SEM and TEM observations were
performed using a Hitachi S-3600N and a Jeol JEM-EX2000 operating
at 5 kV and 200 kV, respectively. IR spectra were collected on a
Thermo Fisher Scientific Nicolet Avatar-360 FT-IR spectrometer
using an attenuated total reflection attachment. UV−vis absorption
and fluorescence emission spectra were obtained using Jasco V-670
and FP-6500 spectrometers, respectively. XPS was recorded with an
ULVAC Quantera SXM using Al Kα as the X-ray source. The charge
neutralization function was employed to compensate for the charge
built up on solid samples by X-ray irradiation. Binding energies were
referenced to In3d5 at 444.4 eV from indium oxide (In O ). XAFS
3
2
particles were used instead of Pt-loaded Ru-PMO with the
same Ru and Pt contents. The homogeneous system showed
almost no hydrogen evolution (Figure 6b). However, the
addition of methylviologen to the homogeneous system
brought about evolution of a considerable amount of hydrogen.
These results suggest that BPy-PMO acts a useful platform to
2
+
integrate [Ru(bpy)3] and Pt on the pore surface and
promotes efficient photoinduced electron transfer from the
photosensitizer to the catalyst without any electron relay
molecules.
2
5
measurements at Ru K-edge and Re L -edge were conducted by a
III
quick XAFS (QXAFS) method in transmission mode at BL01B1 of
SPring-8. X-rays stemming from synchrotron were monochromatized
by Si(311) at Ru or Si(111) Re double-crystal monochromator, and
the monochromator was quickly rotated within ∼6 min for the
CONCLUSIONS
■
A novel crystal-like PMO with a high density of bipyridine
groups in the framework was successfully synthesized from a
bipyridine-bridged organosilane precursor. BPy-PMO was
demonstrated to act as a versatile chelating ligand for the
formation of various bipyridine-based metal complexes on the
pore surface due to the high coordination ability of the
framework bipyridine ligands. BPy-PMO was successfully used
in the heterogeneous Ir-catalyzed direct C−H borylation of
arenes, which showed high activity similar to the corresponding
homogeneous Ir-complex and high durability due to the
suppression of Ir-complex aggregation on the pore surface. Ir-
BPy-PMO allowed for easy recovery and recycle use and very
low contamination of Ir in the final product. BPy-PMO was
also a useful platform for construction of a heterogeneous
measurement of a QXAFS spectrum. XAFS measurements at Ir L -
III
edge were conducted at 30 K by a step scan method in transmission
mode at BL-12C of KEK-PF using Si(111) double-crystal mono-
chromator. An ion chamber was used to detect I (intensity of incident
0
X-rays), and another one placed behind the samples was used to detect
I (intensity of transmitted X-rays). Background subtraction was
38
3
performed with Autobk and Spline smoothing algorithm. The k -
weighted EXAFS oscillations were Fourier transformed into R-space.
Synthesis of 2-Bromo-5-(triisopropoxysilyl)pyridine (2). A
5
00 mL three-neck flask connected to a condenser was charged with a
stir bar, 2-bromo-5-iodopyridine (8.0 g, 28.2 mmol), [Rh(cod)-
CH CN) ]BF (0.11 g, 0.28 mmol), and n-Bu NI (12.5 g, 33.8
(
3
2
4
4
mmol). Dry DMF (150 mL), dry Et N (24 mL), and triisopropox-
3
ysilane (8.7 g, 42.3 mmol) were added. The reaction mixture was
stirred at 100 °C for 24 h. After cooling to room temperature, the
reaction mixture was evaporated, extracted with diethyl ether, and
filtered, and then the solvent was completely evaporated. The residue
was purified by silica gel column chromatography (eluent: hexane/
molecular photocatalyst system. The integration of Ru(bpy)32+
photosensitizer and Pt catalyst on the pore surface exhibited
catalytic hydrogen evolution from water without any electron
mediators. These results demonstrate the great potential of
BPy-PMO as a solid chelating ligand and an integration
platform for a variety of molecular-based heterogeneous
catalysis systems such as organic transformations and solar
energy conversion.
CH
Cl
= 1:1 to 1:4) to give 2 (7.0 g, 69%) as a light yellow oil. IR
2
2
(neat): υmax 2974, 2933, 2895, 1562, 1539, 1446, 1383, 1371, 1286,
−1 1
1
174, 1117, 1074, 1043, 1016, 891, 872, 827, 762 cm ; H NMR (400
MHz, CDCl ) δ 1.18 (d, J = 6.2 Hz, 18H), 4.25 (septet, J = 6.2 Hz,
3
3
=
H), 7.44 (d, J = 7.8 Hz, 1H), 7.74 (dd, J = 1.8, 7.8 Hz, 1H), 8.53 (d, J
1.8 Hz, 1H); 13C NMR (100 MHz, CDCl ) δ 25.4, 65.9, 127.5,
3
EXPERIMENTAL SECTION
■
127.6, 144.3, 144.7, 155.7. FI-HRMS m/z calcd for C14
(M ): 361.0709; found: 361.0710.
H24BrNO
3
Si
+
Materials and Methods. All reactions for organic synthesis were
carried out under argon using standard high-vacuum and Schlenk-line
techniques. Unless otherwise noted, all chemicals, including dry
solvents, were purchased from commercial suppliers (Sigma-Aldrich,
Tokyo Chemical Industry Co., Ltd., and Wako Pure Chemical
Industries Ltd.) and used without further purification.
Mass spectra were recorded on a Micromass GCT Premier mass
spectrometer (FI: field ionization) and Micromass Q-TOF mass
spectrometer (ESI: electrospray ionization). H, C, and F NMR
spectra were obtained using a Jeol ECX-400 spectrometer operating at
00 MHz, 100 Hz, and 376 MHz, respectively. Chemical shifts are
Synthesis of 5,5′-Bis(triisopropoxysilyl)-2,2′-bipyridine (3).
A 1 L three-neck flask connected to a condenser was charged with a
stir bar, 2 (10.0 g, 27.6 mmol), bis(tributyltin) (8.0 g, 13.8 mmol), and
dry m-xylene (200 mL). Pd(PPh ) (0.5 g, 0.45 mmol) was added, and
3 4
reaction mixture was stirred at 130 °C for 40 h. The resulting mixture
was cooled down to room temperature and evaporated to remove
solvent. The residue was purified by silica gel column chromatography
1
13
19
(eluent: CH Cl to EtOAc/hexane = 1:10). The obtained orange-
2 2
yellow solid (5.0 g) was dissolved in acetonitrile (50 mL) and
extracted with hexane (500 mL). The hexane solution was treated with
activated carbon (3.0 g) for 10 min, filtered, and evaporated to give 3
(4.56 g, 59%) as a light yellow solid. IR (neat): υmax 2972, 2929, 2885,
1579, 1524, 1454, 1381, 1369, 1352, 1286, 1223, 1173, 1113, 1028,
4
reported in δ parts per million referenced to tetramethylsilane (TMS)
or residual protonated solvent as an internal standard. Si dipolar
29
13
decoupling (DD) and C cross-polarization (CP) MAS NMR
measurements were respectively performed at 79.49 and 100.6 MHz
at a sample spinning frequency of 4 or 6 kHz using a Bruker Avance
−1 1
1016, 889, 872, 843, 756, 737 cm ; H NMR (400 MHz, CDCl ) δ
3
1.24 (d, J = 6.0 Hz, 6H), 4.31 (septet, J = 6.2 Hz, 1H), 8.09 (dd, J =
13
4
00 spectrometer with a 7 mm zirconia rotor. For the 29Si MAS NMR
1.6, 8.0 Hz, 2H), 8.40 (d, J = 7.3 Hz, 2H), 8.92 (s, 2H); C NMR
measurements, the repetition delay was 60 s, and the pulse width was
.5 μs. For the 13C CP-MAS NMR measurements, the repetition delay
was 5 s, the contact time was 1.75 ms, and the pulse width was 4.5 μs
(100 MHz, CDCl
HRMS m/z calcd for C H N O Si (M ): 564.3051; found:
564.3041.
Synthesis of BPy-PMO. A 300 mL one-neck flask was charged
with a stir bar, octadecyltrimethylammonium chloride (C TMACl:
2.43 g, 6.98 mmol), 6 N NaOH solution (0.77 mL), and distilled water
(132 mL). The mixture was heated at 50 °C. A solution of precursor 3
(2.97 g, 5.26 mmol) in ethanol (5 mL) was directly added to the
surfactant solution through the pipet at a constant speed (2.0 mL/h
3
) δ 25.5, 65.8, 120.4, 128.6, 143.7, 1548, 157.0. FI-
+
4
2
8
48
2
6
2
1
(
H 90° pulse). Chemical shifts were referenced to TMS and glycine
29
13
for Si and C NMR, respectively. XRD profiles were recorded on a
Rigaku RINT-TTR diffractometer using Cu Kα radiation (50 kV, 300
mV). Nitrogen adsorption and desorption isotherms were measured
using a Quantachrome Nova3000e sorptometer. BET surface areas
1
8
were calculated from the linear sections of BET plots (P/P = 0.1−
0
4
009
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Journal of the American Chemical Society
Article
2
for initial 1 h, then 2.5−3.0 mL/h) using a syringe-pump system under
vigorous stirring at 50 °C (Figure S1). The white suspension was then
sonicated for 2 h with stirring condition at 50 °C. The mixture was
stirred for another 3 days and continued heating at 50 °C for 3 days
under static conditions. Then, the resulted precipitation was filtered
and washed with distilled water, affording as-made bipyridine-bridged
PMO. The as-made sample (2.39 g) was added into the distilled water
mg, Pt content: 0.10 mmol/g). SBET = 570 m /g, V = 0.19 cc/g, d
= 2.7 nm.
p
DFT
Evaluation of Photocatalytic Hydrogen Evolution. The
photocatalysis test was carried out using a glass-enclosed gas-
circulation and gas-evacuation system. The vessel was loaded with
Pt/Ru(bpy) (BPy-PMO) (10 mg, 110 μmol Pt, 1.63 μmol Ru)
2
suspended in acetate buffer solution (25 mM, pH 4.2, 40 mL)
containing EDTA (1.0 mM), followed by irradiation of visible light
(>385 nm) from above using high-power xenon lamp. Producing
hydrogen was detected by gas chromatography. Amount of hydrogen
was plotted against irradiation time.
(
433 mL) in the presence of C TMACl (8.70 g) again to increase
18
condensation degree of organosilica framework. The mixture was
heated at 95 °C for 24 h under static conditions. After filtration, as-
made sample (1.56 g) with thermal treatment was extracted with acidic
ethanol solution (a mixture of 2 N HCl solution (7.2 mL) and ethanol
(
260 mL)) at room temperature for 12 h to give BPy-PMO as a white
ASSOCIATED CONTENT
Supporting Information
■
powder (1.39 g, 3.18 mmol BPy/g).
*
S
Synthesis of Ru(bpy) (BPy-PMO). A 200 mL three-neck flask
2
Detailed experimental procedure and Figures S1−S31. This
material is available free of charge via the Internet at http://
pubs.acs.org.
connected to a condenser was charged with a stir bar, BPy-PMO (300
mg), and Ru(bpy) Cl ·2H O (300 mg, 0.57 mmol). After addition of
2
2
2
dry ethanol (60 mL), the reaction mixture was stirred at 90 °C for 24
h. The resulting precipitate was filtered and washed with CH Cl to
2
2
give Ru(bpy) (BPy-PMO) as an orange powder (319.9 mg, Ru
content: 0.20 mmol/g).
2
AUTHOR INFORMATION
■
Corresponding Author
inagaki@mosk.tytlabs.co.jp
Synthesis of Ir(ppy) (BPy-PMO). A 50 mL two-neck flask
2
connected to a condenser was charged with a stir bar, BPy-PMO
(
100 mg), and [Ir(ppy) Cl] (176 mg, 1.88 mmol). After addition of
2 2
Present Address
Graduate School of Medicine, Dentistry and Pharmaceutical
Sciences, Okayama University, Tsushima-Naka, Kita-ku,
Okayama 700−8530, Japan
dry CH Cl (33 mL), the reaction mixture was stirred at 45 °C for 15
h. The resulting precipitate was filtered and washed with CH Cl to
give Ir(ppy) (BPy-PMO) as a light-orange powder (122.5 mg, Ir
content: 0.26 mmol/g).
◆
2
2
2
2
2
Synthesis of Ir(OMe)(cod)(BPy-PMO). A 50 mL Schlenk tube
was charged with a stir bar and end-capped BPy-PMO (77.5 mg, 0.226
Author Contributions
These authors contributed equally.
∇
mmol). A solution of [Ir(OMe)(cod)] (5.0 mg, 0.015 mmol Ir) in dry
2
Notes
benzene (20 mL) was added at room temperature. The reaction
mixture was stirred at room temperature for 12 h. The resulting
suspension was filtered and then washed with dry benzene. The
material was dried over under reduced pressure to give Ir(OMe)-
cod)(BPy-PMO) as a light-gray powder (80 mg, Ir content: 0.156
mmol Ir/g).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(
The authors thank Dr. Tetsu Ohsuna (Toyota Central R&D
Laboratories., Inc.) for TEM observations and Ms. Naoko
Takahashi (Toyota Central R&D Laboratories., Inc.) for XPS
measurement. The authors also thank Mr. Syuji Kajiya and Mr.
Kenichi Yagi (Toyota Central R&D Laboratories., Inc.) for FI-
HRMS and ESI-HRMS measurement. XAFS measurements
were performed at SPring-8 (2012A1014) and Photon Factory,
KEK (2013G222).
Synthesis of Re(CO) Cl(BPy-PMO). A 100 mL two-neck flask
3
connected to a condenser was charged with a stir bar, BPy-PMO (100
mg), and Re(CO) Cl (137 mg, 0.38 mmol). After addition of dry
5
toluene (50 mL), the reaction mixture was stirred at 110 °C for 24 h.
The resulting precipitate was filtered and washed with toluene to give
Re(CO) Cl(BPy-PMO) as a yellow-orange powder (149.1 mg, Re
3
content: 1.12 mmol/g).
Synthesis of Pd(OAc) (BPy-PMO). A 20 mL Schlenk-tube was
2
charged with a stir bar, BPy-PMO (100 mg), and Pd(OAc) (21 mg,
2
REFERENCES
■
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