Communications
DOI: 10.1002/anie.200803122
Heterogeneous Catalysis
Photoinduced Reversible Structural Transformation and Selective
Oxidation Catalysis of Unsaturated Ruthenium Complexes Supported
on SiO **
2
Mizuki Tada, Yusaku Akatsuka, Yong Yang, Takehiko Sasaki, Mutsuo Kinoshita, Ken Motokura,
and Yasuhiro Iwasawa*
Dedicated to the Catalysis Society of Japan on the occasion of its 50th Anniversary
[
2]
The photoirradiation of a SiO -supported Ru complex was
porting Information 1). The local coordination structure of
2
found to promote the selective formation of two different,
B was similar to that of A. Ultraviolet irradiation (l >
novel unsaturated Ru structures on the surface, dependent on
275 nm) of B under N was found to cause the stoichiometric
2
an O or N atmosphere, differing in the orientation of an Ruꢀ elimination of a coordinated p-cymene ligand from the
2
2
H moiety. One of these structures, owing to an appropriate
supported Ru complex. 87% of free p-cymene was detected
RuꢀH conformation, catalyzed the selective photooxidation
in a solution after the photoirradiation of B under N for 2 h
(Table 1) affording the coordinatively unsaturated Ru com-
2
of cycloalkanes with O . The two surface-bound unsaturated
2
Ru complexes undergo reversible structural interconversion
by photoexcitation at different wavelengths under different
atmospheres.
plex C2. The elimination of p-cymene was also evidenced by
C solid-state magic-angle spinning (MAS) NMR spectros-
copy (Figure 1).
1
3
On heterogeneous catalyst surfaces, owing to the limited
accessibility of reactants, rate-enhancement and new catalytic
strategies can often be developed using novel, coordinatively
unsaturated metal structures, which are hard to isolate in
homogeneous solutions. Attachment of metal complexes onto
a surface results in their stabilization and prevents aggrega-
X-ray photoelectron spectroscopy (XPS) revealed a shift
in binding energy of Ru 3d , on elimination of p-cymene,
5
/2
from 282.0 eV for B, to 282.2 eV for C2, (Table 1 and
Supporting Information 2). The shift in the binding energy
of Ru 3d5/2 indicates that the surface Ru complex is positively
[
4]
charged by the photoirradiation. However, the similar ratio
of the XPS signal intensities for Cl 2p to Ru 3p3/2 in B and C2
indicates that the supported Ru complex C2 retains a Cl
ligand. A change was also evident in the Ru K-edge X-ray
absorption near-edge structure (XANES) spectroscopic
signal (see Supporting Information 3). Ru K-edge extended
X-ray absorption fine structure (EXAFS) spectroscopic
analysis revealed two coordinations, RuꢀO(N) and RuꢀCl,
[1]
tion and decomposition. Recently, we produced a novel
three-coordinate unsaturated ruthenium complex on a SiO2
surface by coupling with SiO -bound p-styryltrimethoxysi-
2
[
2]
lane. The unsaturated Ru complex was highly active for
selective alkene epoxidation using a mixture of isobutyralde-
hyde and O . However, the Ru complex was inactive for
2
selective oxidation of saturated hydrocarbons with O as a
2
sole oxidant, which may be more important from the view-
point of practical use as a catalyst.
with bond orders of 3.2 and 1.0 , and bond distances of (2.10 ꢁ
0.01) ꢀ and (2.38 ꢁ 0.01) ꢀ, respectively (see Supporting
Information 4), which confirms the retention of Cl, suggested
by XPS, and also indicates surface coordination to Ru by
A SiO -supported Ru complex (B) was prepared using a
2
[
3]
N-sulfonyl-1,2-ethylenediamine–Ru complex (A) and p-
1
3
styryltrimethoxysilane-functionalized SiO2 (Scheme 1, Sup-
oxygen, alongside the immobilization by silane coupling. C
solid-state NMR spectroscopy indicated that the organic
diamine ligand was retained during photoinduced p-cymene
elimination (Figure 1).
[
*] Dr. M. Tada, Y. Akatsuka, Dr. Y. Yang, M. Kinoshita, Dr. K. Motokura,
Prof. Dr. Y. Iwasawa
Department of Chemistry, Graduate School of Science
The University of Tokyo
Photoirradiation (l > 275 nm) of B under an O atmos-
2
phere also resulted in dissociation of a p-cymene ligand
(Table 1, Figure 1, Scheme 1) but afforded a different struc-
ture, C1, as evidenced by a very different UV/Vis spectrum to
that of C2 (Figure 2). The spectrum for C2, produced under
N shows two signals in the visible-light region, at 468 nm and
7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan).
Fax: (+81)3-5800-6892
E-mail: iwasawa@chem.s.u-tokyo.ac.jp
Dr. T. Sasaki
2
,
Department of Complexity Science and Engineering
Graduate School of Frontier Sciences, The University of Tokyo
5
6
96 nm (Figure 2d), whereas that for C1, produced under O2,
has one signal, at around 517 nm (Figure 2e). However, the
XPS Ru 3d5/2 signal (at 282.2 eV), solid-state NMR spectrum,
and Ru K-edge EXAFS spectrum of C1 were almost the same
as those of C2. Notably, C1 and C2 are interconverted
reversibly: C1 was converted into C2 by photoirradiation (l >
-1-5, Kashiwanoha, Kashiwa-shi, Chiba 277-8561 (Japan).
**] XAFS measurements were performed at KEK-IMSS-PF (No.
005G209 and 2008G154). The work was supported by a Grant-in-
[
2
Aid for Scientific Research on Priority Areas (No. 18065003) from
MEXT.
2
75 nm) under N , and C2 was converted into C1 by photo-
2
irradiation (l > 370 nm) under O (Figure 2e–h). Neither O
2
2
9
252
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 9252 –9255