Selective formation of a coordinatively unsaturated metal complex at a surface: A SiO2-immobilized, three-coordinate ruthenium catalyst for alkene epoxidation

Article abstract of DOI:10.1002/anie.200700980

(Chemical Equation Presented) Unsaturated but air-stable: A three-coordinate Ru complex, 2, highly active for alkene epoxidation and recyclable in air, was prepared on SiO₂ by exploiting the exothermic reaction between O₂ and isobutyraldehyde (IBA) to eliminate a p-cymene ligand from a coordinatively saturated precursor 1 (Ru red, Cl dark blue, N green, S yellow, O blue, C gray, H white). In contrast, direct activation with O₂ alone was calculated to be endothermic.

Full text of DOI:10.1002/anie.200700980

Communications
DOI: 10.1002/anie.200700980
Immobilized Catalysts
Selective Formation of a Coordinatively Unsaturated Metal Complex at
a Surface: A SiO -Immobilized,Three-Coordinate Ruthenium Catalyst
2
for Alkene Epoxidation**
Mizuki Tada, Rudy Coquet, Jun Yoshida, Mutsuo Kinoshita, and Yasuhiro Iwasawa*
Dedicated to Süd-Chemie on the occasion of its 150th anniversary
Immobilization of metal complexes on support surfaces is still
the state of the art in the interdisciplinary field between
homogeneous and heterogeneous catalysis, irrespective of
long-term challenges of designing immobilized metal com-
plexes in a controllable manner, which is currently receiving
much attention in practice for a variety of catalytic chemical
The immobilized ruthenium complex was prepared in
three steps (Figure 1). N-Sulfonyl-1,2-ethylenediamine (A)
reacted with [Ru (p-cymene) Cl ] (B) to form monomeric
2
2
4
7]
[
ruthenium precursor complex C (see the Supporting Infor-
[
1]
syntheses. The potential of immobilized metal-complex
catalysts strongly depends on the nature of the support
surface, which can result in significant rate enhancement and
novel catalytic performance that analogous homogeneous
[
1,2]
complexes do not exhibit.
In homogeneous reactions catalyzed by metal complexes,
it is suggested that a coordinatively saturated metal complex
(
precursor) eliminates a ligand and is converted to an active
complex with an unsaturated metal center, which, because of
its low stability in solution, cannot be isolated and readily
[
3]
undergoes aggregation leading to loss of catalytic activity.
On the other hand, attachment of metal complexes on the
surface of oxide supports provides isolated single-site metal
[
1,4,5]
complexes.
The aim of this technique is not only the
simple surface immobilization of metal complexes, but also
the selective creation of unique active structures and reac-
[
2]
tivity on surfaces that are difficult to achieve in solution.
Here we report the synthesis of a novel three-coordinate
unsaturated Ru complex on an SiO surface by a new strategy
2
and the catalytic performance of this system. A monomeric p-
cymene-coordinated ruthenium N-sulfonyl-1,2-ethylenedia-
[
6]
mine complex was tethered on SiO , and we found that
2
exothermic reaction with O /isobutyraldehyde (IBA) pro-
2
moted stoichiometric elimination of a p-cymene ligand to
form an intermediate oxidant on a three-coordinate unsatu-
rated ruthenium complex at the surface, which is active for
alkene epoxidation.
[*] Dr. M. Tada, Dr. R. Coquet, J. Yoshida, M. Kinoshita,
Prof. Dr. Y. Iwasawa
Department of Chemistry
Graduate School of Science
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax: (+81)3-5800-6892
E-mail: iwasawa@chem.s.u-tokyo.ac.jp
[
**] XAFS measurements were performed at KEK-IMSS-PF with the
Figure 1. Preparation and structures of supported ruthenium complex
E and single-site unsaturated active ruthenium complex G on SiO2;
structures calculated by hybrid DFT are also illustrated. X in D–G is Si
or CH3.
approval of the PF-PAC (No. 2005G209).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
7220
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Angew. Chem. Int. Ed. 2007, 46, 7220 –7223

Angewandte
Chemie
mation). An SiO surface was modified with p-styryltrime-
Table 1: Catalytic activities of ruthenium catalysts for stilbene epoxida-
tion.
2
[
a]
thoxysilane, and ruthenium precursor complex C was teth-
ered to the functionalized surface D by coupling with the
styryl moiety to form immobilized ruthenium complex E with
a saturated metal center bearing a p-cymene ligand.
[
b]
Catalyst
Additives
Conv. [%]
Sel. [%]
[
[
[
Ru(NH ) ]Cl
IBA
IBA
IBA
IBA+ED
IBA
0
0
0
0
0
0
0
0
5
0
0
0
0
0
0
0
0
22
71
66
0
3
6
3
Ru(bpy)Cl2]
Ru (p-cymene) Cl ] (B)
The appearance of new peaks at d = ꢀ62and ꢀ54 ppm in
2
2
4
2
9
[c]
the Si solid-state NMR spectrum revealed that p-styryltri-
[Ru (p-cymene) Cl ] (B)
RuCl
2
2
4
methoxysilane reacted with SiOH groups on the SiO surface
3
2
[
c]
RuCl3
IBA+ED
none
and was tethered by formation of 30% ꢀSi(OSi) and 70%
3
complex C
ꢀ
Si(OSi) (OCH ) bonds (Figure 1, D). The n
625 cm disappeared after coupling of the styryl moieties of
C=C
band at
2
ꢀ
3
complex C
complex C
complex C
complex C
supported catalyst E
supported catalyst E
supported catalyst E
supported catalyst E
supported catalyst E
supported catalyst E
supported catalyst E
supported catalyst E
supported catalyst E
H O
2
1
2
1
PhIO
C and D, that is, precursor C was immobilized on the SiO2
surface by reaction with D. The loading of the immobilized
ruthenium complex was 2.7 wt% (Ru), which corresponds to
[d]
IBA
10
9
0
[
e]
IBA+SiO2
none
H O
2
0
0
0
.8 ruthenium complexes per square nanometer.
2
PhIO
CPBA
30
46
100
100
86
100
98
16
41
83
90
82
74
81
The structure of supported Ru complex E was charac-
[f]
terized by X-ray photoelectron spectroscopy (XPS), diffuse-
reflectance (DR) UV/Vis spectroscopy, and X-ray absorption
fine structure (XAFS). In UV/Vis spectra (Figure 2), similar
[
g]
IBA
IBA
IBA
IBA
IBA
[h]
[i]
[
[
j]
k]
[
a] CH Cl (5 mL), 298 K, 101.3 kPa O , Ru (0.01 mmol), Ru/stilbene/
2
2
2
additives=1:50:50, 72 h. [b] (produced epoxide [mol]/consumed stilbe-
ne [mol])”100. [c] ED=ethylenediamine, 0.01 mmol. [d] 24 h. [e] SiO2
(40 mg) was added. [f] m-chloroperbenzoic acid, 48 h. [g] 4 h. [h] Ru
0
.01 mmol, Ru/stilbene/IBA=1:1000:1000, CH Cl (12 mL). [i] Cyclo-
2
2
pentene epoxidation, 48 h. [j] Norbornene epoxidation, 48 h. [k] Cyclo-
octene epoxidation, 48 h.
H O , PhIO, and O /IBA. The ruthenium diamine complex C
2
2
2
in a homogeneous phase produced a small amount of stilbene
oxide with PhIO or O /IBA, but the reaction stopped after a
2
short time with formation of black precipitates. On the other
hand, immobilized ruthenium complex E exhibited good
catalytic performance with O /IBA: stilbene epoxidation was
2
completed with 83–90% epoxide selectivity, while PhIO and
m-chloroperbenzoic acid were poor oxidants with low con-
version (30 and 46%, respectively) and low epoxide selectiv-
ity (16 and 41%, respectively). Epoxidation of cyclopentene,
norbornene, and cyclooctene also proceeded smoothly on
immobilized Ru complex E (Table 1).
Figure 2. UV/Vis spectra of a) RuCl in ethanol, b) B in CH Cl , c) C in
3
2
2
CH Cl , d) supported ruthenium complex E, e) unsaturated Ru com-
2
2
plex G activated by O /IBA, and f) supported Ru catalyst after 100
2
cycles of stilbene epoxidation. a)–c) measured in transmission mode
(
(
vertical axis: absorption); d)–f) measured in diffuse-reflectance mode
vertical axis: reflectance).
Interestingly, although homogeneous complex C and
immobilized catalyst E showed similar color changes under
the epoxidation conditions in the presence of IBA/O , their
2
peaks before and after immobilization (C and E) were
observed at 326 (Rud!Php) and 447 nm (Clp!Rud). The
presence of the d–p transition in the spectrum of complex E
indicates that it has a p-cymene ligand, and the Ru oxidation
state and local coordination resemble those of precursor C
catalytic performances were completely different (Table 1).
Immobilized ruthenium complex E was converted to active
unsaturated complex G under the catalytic reaction condi-
tions, while the homogeneous complex C aggregated to form
precipitates, which led to loss of the activity. Addition of SiO2
to the homogeneous phase of C did not alleviate the
deactivation; hence, chemical grafting on the surface is the
key to preventing aggregation of the active unsaturated
complexes in solution.
2
+
(
see the Supporting Information). Hence, the surface ruthe-
nium complex E has the same coordination environment as
precursor C (Figure 1).
Catalytic performance in stilbene epoxidation differed
greatly between the homogeneous ruthenium complexes and
the supported catalyst (Table 1). [Ru(NH ) ]Cl and [Ru-
In the presence of an excess of IBA/O , the nearly
2
stoichiometric amount of p-cymene (89% based on Ru)
detected in the reaction solution by GC indicated release of
the p-cymene ligand, while no release of p-cymene occurred
3
6
3
(
bpy)Cl ] (bpy = 2,2’-bipyridine) with both N-donor and
2
chloro ligands like E, [Ru (p-cymene) Cl ] (B), and RuCl
2
2
4
3
did not catalyze stilbene epoxidation with oxidants such as O2,
under N atmosphere. Hybrid DFT calculations showed that
2
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www.angewandte.org
7221

Communications
immobilized ruthenium complex E tends to react with IBA
and O with elimination of the p-cymene ligand and a gain of
catalytic stilbene epoxidation. The unsaturated ruthenium
complex cannot be produced by ligand elimination and
exchange, but can be generated in conjunction with an
exothermic reaction in the coordination sphere.
2
ꢀ
1
4
2kJmol for peroxide intermediate F (Figure 1). When an
excess of IBA and O was available, peroxide F was trans-
2
formed into unsaturated species G with weakly coordinated
with O (O=O distance: 0.130 nm), whereby two molecules of
2
the corresponding acid were produced in an exothermic step Experimental Section
ꢀ
1
(
356 kJmol ). Indeed, Ru K-edge EXAFS curve-fitting
N-Sulfonyl-1,2-ethylenediamine (A) and ruthenium precursor com-
[7]
plex C were synthesized by literature methods (see the Supporting
analysis was consistent three-coordinate unsaturated Ru
species G with two RuꢀN bonds (coordination number 2.2)
with a length of 0.207 nm and an RuꢀCl bond (coordination
Information). SiO (Aerosil 200, Degussa) was calcined at 773 K for
2
2h and refluxed in a solution of p-styryltrimethoxysilane in anhy-
drous toluene at 383 K for 18 h under N . The SiO was subjected to
2
2
number 1.3) with a length of 0.234 nm (see the Supporting
Information).
Soxhlet extraction and then dried under vacuum. The amount of
ꢀ2
grafted p-styryl moieties was 1 nm . The ruthenium precursor C
(135 mg) was treated with D (0.67 g) in anhydrous CHCl (10 mL) in
Ruthenium 3d XPS binding energies revealed that the
valence of the immobilized Ru complex increased from 2( E)
to 3 (G) on catalyst activation with O /IBA (Ru 3d binding
energies: C: 281.6, E: 281.5, and G: 282.0 eV). The oxygen
molecule in optimized structure G was negatively charged
3
the presence of 2,2’-azobis(isobutyronitrile) (AIBN, 1.1 mg) and then
refluxed at 333 K for 24 h under N2 atmosphere. The obtained
2
5/2
ruthenium complex on SiO , E, was subjected to Soxhlet extraction
2
and dried under vacuum.
Supported ruthenium complex E was activated in CH Cl with an
2
2
ꢀ
(
O ). A new peak appeared in the visible region of the DR
2
equivalent amount of IBA relative to Ru under an atmospheric
UV/Vis spectrum, at 608 nm for G (Figure 2), which was
pressure of O . After a few minutes, activation was complete, the
2
confirmed by time-dependent hybrid DFT.
reaction solution was filtered, and the obtained sample was dried
under vacuum. These catalysts were characterized by solid-state
NMR (Chemagnetics CMX-300), FTIR (JASCO FT/IR-4200ST),
XPS (Rigaku XPS-7000), DR UV/Vis (JASCO V-550-DS), and
XAFS (KEK-PF, BL-10B and NW10A).
ꢀ1
The energy of structure G is 66 kJmol higher than that
of structure E (Figure 1), that is, p-cymene elimination from E
to form G does not proceed with O2 alone. p-Cymene
elimination does not occur with IBA either. Notably, ligand
elimination can proceed by taking advantage of the exother-
Hybrid DFT calculations were performed with the Gaussian03
[8]
package, and time-dependent DFTwas applied to calculate UV/Vis
[
9]
mic reaction of IBA with O on the energy-gaining route E!
transitions. The PBE0 hybrid functional was employed together
2
[
10]
F!G, whereby the reactant IBA also has a role as initiator of
active-structure formation. Furthermore, unsaturated Ru
complex G can react with IBA to reversibly form peroxide
with the DGDZVP basis set. PBE0 is a nonempirical hybrid GGA
functional and combines the lack of any adjustable parameter with
[
11]
remarkable results for a number of physicochemical observables.
Choosing DGDZVP allows the use of a relatively large and accurate
basis set for all atomic species including Ru, without the need for any
pseudopotential. The UV/Vis spectra were generated by using the
ꢀ1
intermediate F with release of 107 kJmol (Figure 1). In the
catalytic epoxidation, the amounts of stilbene oxide and
isobutyric acid were equal, that is, intermediate F reacted
selectively with stilbene, not with IBA, to produce stilbene
oxide under the catalytic reaction conditions.
[
12]
SWizard program, revision 4.2 with the Gaussian model, and the
ꢀ1
half-bandwidths were taken to be equal to 3000 cm . The VModes
program, rev. A7.2f was employed to calculate the contribution
percentage from selected atomic orbitals to molecular orbitals and
thus to assign UV/Vis transitions.
After 1000 cycles of stilbene epoxidation, characterization
of the ruthenium complex showed retention of the three-
coordinate unsaturated structure (see the Supporting Infor-
mation), and unsaturated Ru complex G was active in the
epoxidation reaction. It is noteworthy that three-coordinate
ruthenium complex G is quite stable under the reaction
conditions and also in air, in spite of its unsaturated structure.
This remarkable stability and durability make immobilized
catalyst G recyclable in catalytic reactions with retention of
Alkene epoxidation was performed at 298 K in CH Cl under an
2
2
atmospheric pressure of O . The reactants and products were
2
analyzed by GC (Shimadzu GC-14A) and GC-MS (Shimadzu GC-
2010) at appropriate intervals. The supported catalyst was recycled by
filtration and addition of the reactants and IBA. The loading of the
ruthenium complex on the surface was confirmed by XRF (Seiko
SEA-2120L).
Received: March 6, 2007
Revised: April 25, 2007
1
00% conversion and selectivity exceeding 80% (see the
Supporting Information). There was no decrease in the
ruthenium loading of the supported catalyst and no leaching
of the ruthenium complex into the solution phase after three
recycling processes of stilbene epoxidation (100 catalytic
cycles for each run).
Low stability of an unsaturated metal complex leading to
aggregation or decomposition in solution prevents exploita-
tion of metal-complex catalysis. Immobilization of metal
complexes to isolate the unsaturated metal centers on solid
surfaces is promising, not only for dramatic improvement of
catalytic activity, but also for a better understanding of key
issues in catalysis by unsaturated metal-complex structures.
We have presented a new way to create a three-coordinate Ru
Published online: June 15, 2007
Keywords: epoxidation · heterogeneous catalysis ·
.
immobilization · ruthenium · surface chemistry
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7223