Photoinduced reversible structural transformation and selective oxidation catalysis of unsaturated ruthenium complexes supported on SiO2

Article abstract of DOI:10.1002/anie.200803122

(Chemical Equation Presented) Ru experienced? Two novel coordinatively unsaturated SiO₂-supported Ru complexes were prepared by photoinduced ligand elimination, accompanied by dissociative coordination of a surface OH group to the unsaturated Ru center by photoirradiation. Wavelength- and atmosphere-dependent photoinduced reversible interconversion occurs between the two Ru complexes. One of the complexes is catalytically active for the photooxidation of cycloalkanes with O₂.

Full text of DOI:10.1002/anie.200803122

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-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803122.
2
irradiation (l > 370 nm) under O (Figure 2e–h). Neither O
2
2
9
252
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Angewandte
Chemie
Scheme 1. Photoinduced synthesis of unsaturated SiO -supported Ru complexes C1 and C2, and atmosphere/wavelength-dependent photo-
2
induced interconversion between C1 and C2. Bond distances were determined by EXAFS.
Table 1: The amounts of free p-cymene detected by GC and chlorine
estimated by XPS, and XPS binding energies of Ru 3d5/2
Similar photoirradiation of a solution of precursor com-
plex A in cyclohexane did not bring about the elimination of
p-cymene (< 12% dissociation after 5 h). When
[
[b]
Catalyst
Free p-cymene
Cl [mol%]
Ru 3d5/2 binding
energy [eV]
[
a]
[c]
5,11
[
mol%]
1,3,5,7,9,11,14-heptacyclohexyltricyclo[7,3,3,1 ]heptasilox-
ane-3,7,14-triol (D), with SiꢀOH functional groups, was
B
C1
C2
0
102
99
99
282.0
282.2
282.2
[
d]
added into the homogeneous solution of A, the p-cymene
ligand readily photodissociated (96% dissociation after 3 h),
indicating that surface SiꢀOH groups positively contributed
60–85
87
[a] The solution after the photoirradiation was analyzed by GC.
to the photodissociation of the p-cymene ligand (Supporting
Percentages given with respect to Ru. [b] Estimated by the intensity
ratio of Cl 2p to Ru 3p3/2. Percentages given with respect to Ru.
Information 5). A similar effect of SiꢀOH groups on SiO on
2
[
5a]
[c] Determined by XPS. [d] Free p-cymene in the solution was further
immobilized Ru complexes was reported previously.
oxidized to many products, depending on the photoirradiation con-
ditions.
In density functional theory (DFT) calculations for C1
and C2 (Supporting Information 6), Si(OH)(OCH3)3 was
used as a model of a surface Si-OH group. Simple Ru–OHꢀ
Si(OCH ) coordination did not stabilize the unsaturated Ru
3
3
nor N2 adsorption were detected in the structural trans-
formations. These results indicate that the photoinduced
reversible structural transformation of the supported Ru
complex. However, the cleavage of SiꢀOH provides several
potential HꢀRuꢀO(Si) structures with different coordination
symmetries. Among these possible structures, we found two
stable configurations (C1 and C2) with square-pyramidal
conformations (Scheme 1), in which the RuꢀH bond was
complexes on SiO is ascribed to a change in the coordination
2
symmetry of the Ru center.
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9253

Communications
located either in an equatorial site (singlet state, C1) or an
axial site (triplet state, C2).
UV/Vis spectra of these two structures calculated by DFT
successfully reproduced the observed spectra of C1 and C2.
The wavelength- and atmosphere-dependent reversible trans-
formation between C1 and C2 can be explained by the energy
diagram of the Ru species by the DFT calculations, where the
C2!C1 transformation may be assisted by O (Supporting
2
Information 7). It was also reported that Si–H species
contributed to formation of a Ru–H complex on a SiO
2
[
5b]
surface.
6
II
h -Arene ligands on Ru can be labilized by ultraviolet
[
6]
irradiation or heating. The possibility that p-cymene is
eliminated thermally in this reaction was examined by heating
B at 373 K for 3 h under O or N without irradiation. Neither
2
2
dissociation of p-cymene nor change in the UV/Vis spectrum
of B occurred in either case (Figure 2a–c). No structural
transformation between C1 and C2 occurred by thermal
reaction at 373 K either. The photoexcitation threshold
energies were 275 nm for C1 and 370 nm for C2, which were
attributed to Ru d + Cl p!Ru d + Ph p and Ru d!Ph p
transitions, respectively, as suggested by time-dependent
density functional theory (TDDFT) calculations.
The unsaturated Ru complex C1 was found to be catalyti-
cally active and reusable for the selective photooxidation of
cycloalkanes to the corresponding alcohols and ketones using
1
3
Figure 1. C liquid/solid-state NMR spectra for p-styryltrimethoxysi-
lane (*), N-sulfonyl-1,2-ethylenediamine–Ru complex A, the SiO -sup-
ported Ru complex B, and the unsaturated Ru complexes C1 and C2.
2
O . SiO , Ru metal, RuO , RuCl , [Ru (p-cymene) Cl ],and
2
2
2
3
2
2
4
the homogeneous Ru precursor complex A were all essen-
tially inactive for this process (Table 2). The complex C1
(
prepared under O ) produced cyclohexanol (turnover fre-
2
ꢀ
1
ꢀ1
quency (TOF) = 3.1 h ) and cyclohexanone (TOF = 2.8 h )
from cyclohexane without any induction period at room
temperature (Supporting Information 8). Cyclohexane oxi-
dation proceeded linearly for at least 6 h and turnover
number (TON) of the photooxidation reached 35 after 6 h
with 96% selectivity. Cyclohexane oxidation on C1 did not
proceed thermally at 353 K after 24 h.
Initially, neither B (with coordinated p-cymene) nor C2
(
prepared under N ) catalyzed photooxidation. However,
2
after an induction period of 2 h, similar catalytic activities to
that for C1 were obtained (Supporting Information 8).
Elimination of p-cymene from B under O was completed in
2
2
h, indicating that the induction period, in the B-catalyzed
reaction, corresponded to a structural change from B to C1
releasing p-cymene ligand. The induction period (2 h) for the
C2-catalyzed photooxidation is explained as the time taken
for phototransformation from C2 to C1 under an O2
atmosphere. After the induction periods, UV/Vis spectra of
the Ru species were similar to that of C1, in both cases. These
results suggest that C1 was the active species for selective
photooxidation using O2.
Formation of peroxide intermediate RuꢀOOH on the
Figure 2. Diffuse-reflectance UV/Vis spectra of the SiO -supported Ru
2
complexes and assignments of the peaks. a) Complex B; b) B heated
at 373 K for 2 h under N ; c) B heated at 373 K for 2 h under O ; d) C2
supported Ru complex, and in the solution phase, was
investigated by iodometry. After the photooxidation of
cyclohexane on the supported Ru catalyst, 20 mol% peroxide
2
2
(
(
formed by photoirradiation of B at l>275 nm for 1 h under N ; e) C1
2
formed by photoirradiation of B at l>275 nm for 1 h under O ;
2
(
with respect to Ru) was detected, whereas no free organic
peroxide was detected in the reaction solution. The peroxide
f) (after e) photoirradiation of C1 at l>275 nm for 2 h under N2
(
C1!C2); g) (after f) photoirradiation of C2(l>370 nm) for 1 h under
RuꢀOOH may be formed by the insertion of O into RuꢀH in
O (C1!C2!C1); h) (after g), photoirradiation of C1 at l>275 nm
2
2
for 2 h under N (C1!C2!C1!C2).
species C1 at an excited state (Scheme 2). The C2 structure is
2
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254
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Angewandte
Chemie
Table 2: Catalytic performance for photooxidation of cycloalkanes using
O2.
plexes, depending on the light wavelength and atmosphere.
[
a]
The structure C1, produced by photoirradiation under O , was
2
the active species for the photooxidation of cycloalkanes
using O2.
ꢀ
1
ꢀ1
Catalyst
none
n
TOF(R=O) [h
]
TOF (R-OH) [h ]
Experimental Section
[
3]
For full details see the Supporting Information. In brief, complex A
2
2
2
2
2
2
2
2
0.1
0.1
0
0
0
0
0
0
0
0.9
3.3
0
[
2]
[
b]
was treated with a SiO -supported p-styryl moiety in anhydrous
SiO
Ru
RuO2
2
2
[
c]
CHCl in the presence of azobisisobutyronitrile (AIBN). The reaction
3
mixture was heated at reflux for 24 h under N . Soxhlet extraction and
trace
0.17
0.16
0.18
0.10
1.1
[
c]
c]
2
[
subsequent drying under reduced pressure afforded the SiO2-
RuCl3
[
c,d]
supported p-cymene–Ru complex B (Supporting Information 1).
[
Ru (p-cymene) C ]
2
2
l4
[
c,d]
Selective elimination of the coordinated p-cymene ligand was carried
out by photoirradiation using a high-pressure 400 W Hg–Xe lamp
with an optical short-wavelength cut-off filter. In a quartz reactor, B
was dipped in water at 293 K and an absolute solvent (dichloro-
homogeneous A
[
e,f]
B with p-cymene
[
g]
3
.3
[
e,h]
B at 353 K
2
2
2
2
0
0
0
[
e,i]
methane or cyclohexane), and O , N , or Ar at atmospheric pressure
C1
0
0
2 2
[
e,h]
were admitted to the reactor. The stoichiometric elimination of p-
cymene was detected by flame ionization detector gas chromatog-
raphy (FID–GC) and GC–MS. The structures were characterized by
solid-state NMR, FTIR, XPS, diffuse reflectance UV/Vis, and
EXAFS spectroscopy.
C1 at 353 K
[
e,f]
e,f]
C1
3.1
3.2
3.1
0.9
2.8
1.8
trace
3.1
1
[
g]
2
.8
1.2
.9
[
C2
2
[
g]
2
[
3
e]
Catalytic photooxidation of alkanes was carried out in a neat
solution of each reactant under O atmosphere at 293 K. The quartz
C1 on Al O
2
1
3
4
8
1.7
0.14
6.1
1.4
0.12
2
[
[
[
[
e]
e]
e]
e]
C1
C1
C1
C1
2
reactor with a stirrer was irradiated by the 400 W Hg–Xe lamp
through a cut-off filter absorbing light with a wavelength less than
275 nm and a water layer absorbing in the infrared region. The
products (alkane, alcohol, and ketone) were analyzed at appropriate
intervals by FID-GC, GC–MS, and thermal conductivity detector
trace
[
a] Alkane (10.0 mL), O (101.3 kPa). 400 W Hg–Xe lamp (l>275 nm).
2
TOFs were estimated between 0–5 h. [b] 25 mg. [c] Ru (5.8 mmol).
d] Decomposed. [e] Catalyst (25 mg) (5.8 mmol Ru). [f] TOF for initial
h. [g] TOF for 3–5 h. [h] Heated at 373 K without photoirradiation.
i] Without photoirradiation.
(
TCD)-GC (O and CO ).
2 2
[
1
[
Received: June 30, 2008
Revised: August 19, 2008
Published online: October 22, 2008
geometrically inactive for Ru-OOH formation. The catalytic
reaction was first-order with respect to photon intensity for
both cyclohexanol and cyclohexanone (Supporting Informa-
tion 9). Hence, initial oxygen insertion is likely to be rate-
determining for the photooxidation.
In summary, we have succeeded in preparing novel
unsaturated Ru complexes (C1, C2) immobilized at double
sites on a SiO2 surface by the photoinduced selective
elimination of a p-cymene ligand in B, accompanied by
dissociative coordination of a surface OH group to the
unsaturated Ru center at the surface as a result of UV/Vis
light irradiation. We also detected reversible structural trans-
formation between the photoexcited supported Ru com-
Keywords: heterogeneous catalysis · immobilization ·
photooxidation · ruthenium · UV/Vis spectroscopy
.
[
1] a) J. M. Thomas, R. Raja, D. W. Lewis, Angew. Chem. 2005, 117,
6614; Angew. Chem. Int. Ed. 2005, 44, 6456; b) M. Tada, Y.
Iwasawa, Chem. Commun. 2006, 2833; c) J. M. Notestein, A.
Katz, Chem. Eur. J. 2006, 12, 3954; d) P. McMorn, G. J. Hutchings,
Chem. Soc. Rev. 2004, 33, 108; e) R. A. Sheldon, M. Wallau,
I. W. C. E. Arends, U. Schuchardt, Acc. Chem. Res. 1998, 31, 485;
f) A. Corma, H. Garcia, Adv. Synth. Catal. 2006, 348, 1391; g) C.
Copꢁret, J. M. Basset, Adv. Synth. Catal. 2007, 349, 78; h) J. C.
Fierro-Gonzalez, S. Kuba, Y. L. Hao, B. C. Gates, J. Phys. Chem.
B 2006, 110, 13326.
[
2] M. Tada, R. Coquet, J. Yoshida, M. Kinoshita, Y. Iwasawa,
Angew. Chem. 2007, 119, 7358; Angew. Chem. Int. Ed. 2007, 46,
7220.
[
3] K. J. Haack, S. Hashiguchi, A. Fujii, T. Ikariya, R. Noyori, Angew.
Chem. 1997, 109, 297; Angew. Chem. Int. Ed. Engl. 1997, 36, 285.
4] a) K. S. Kim, N. Winograd, J. Catal. 1974, 35, 66; b) P. G.
Gassman, C. H. Winter, J. Am. Chem. Soc. 1988, 110, 6130;
c) R. E. Shepherd, A. Proctor, W. W. Henderson, T. K. Myser,
Inorg. Chem. 1987, 26, 2440.
[
[
5] a) A. W. Kaplan, R. G. Bergman, Organometallics 1998, 17, 5072;
b) R. Berthoud, A. Baudouin, B. Fenet, W. Lukens, K. Pelzer,
J. M. Basset, J. P. Candy, C. Coperet, Chem. Eur. J. 2008, 14, 3523.
6] M. A. Bennett, A. K Smith, J. Chem. Soc. Dalton Trans. 1974, 233.
[
Scheme 2. Formation of catalytically active Ru–peroxide intermediate.
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9255