Photocatalytic aerobic oxidation by a bis-porphyrin-ruthenium(IV) μ-Oxo dimer: Observation of a putative porphyrin-ruthenium(V)-oxo intermediate

Article abstract of DOI:10.1021/ol1005938

The title complexes catalyze the aerobic oxidations of hydrocarbons using visible light and atmospheric oxygen as oxygen source in sequences employing photodisproportionation reactions. The putative oxidants, ruthenium(V)-oxo porphyrin species, can be detected and studied in real time via laser flash photolysis methods.

Full text of DOI:10.1021/ol1005938

ORGANIC
LETTERS
2010
Vol. 12, No. 10
2246-2249
Photocatalytic Aerobic Oxidation by a
Bis-porphyrin-Ruthenium(IV) µ-Oxo
Dimer: Observation of a Putative
Porphyrin-Ruthenium(V)-Oxo
Intermediate
Eric Vanover,† Yan Huang,† Libin Xu,‡,§ Martin Newcomb,*,‡ and Rui Zhang*,†
Department of Chemistry, Western Kentucky UniVersity, 1906 College Heights
BouleVard, Bowling Green, Kentucky 42101, and Department of Chemistry, UniVersity
of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607
rui.zhang@wku.edu; men@uic.edu
Received March 11, 2010
ABSTRACT
The title complexes catalyze the aerobic oxidations of hydrocarbons using visible light and atmospheric oxygen as oxygen source in sequences
employing photodisproportionation reactions. The putative oxidants, ruthenium(V)-oxo porphyrin species, can be detected and studied in
real time via laser flash photolysis methods.
Selective oxidation is a key technology for the synthesis
of high value chemicals in the pharmaceutical and
petrochemical industries, but oxidations are among the
most problematic processes to control.1 Many stoichio-
metric oxidants with heavy metals are expensive and/or
toxic and, thus, impractical. The ideal green catalytic
oxidation process would use molecular oxygen or hydro-
gen peroxide as the primary oxygen source, recyclable
catalysts in nontoxic solvents, and an inexpensive energy
source.2 In this context, we have an interest in photo-
chemical generation of highly reactive metal-oxo inter-
mediates which, upon oxidization of substrates, give low-
valent metal complexes that can be recycled for catalytic
oxidations. One example of a catalytic aerobic oxidation
driven by a photodisproportionation reaction employed a
diiron(III)-µ-oxo bisporphyrin complex3 and photocata-
lytic oxidations of hydrocarbons using molecular oxygen
as the oxygen source with no added reducing agent for
the oxygen have been developed.4 The low reactivity of
the formed iron(IV)-oxo transients and poor quantum
efficiency are serious obstacles that limit the use of iron
porphyrins as practical photocatalysts, but the catalytic
† Western Kentucky University.
(2) Punniyamurthy, T.; Velusamy, S.; Iqbal, J. Chem. ReV. 2005, 105,
2329–2363.
‡ University of Illinois at Chicago.
§ Current Address: Department of Chemistry, Vanderbilt University,
Nashville, Tennessee 37235.
(1) Ba¨ckvall, J. E. Modern Oxidation Methods; Wiley-VCH Verlag:
Weinheim, 2004.
(3) Peterson, M. W.; Rivers, D. S.; Richman, R. M. J. Am. Chem. Soc.
1985, 107, 2907–2915.
(4) Wasser, I. M.; Fry, H. C.; Hoertz, P. G.; Meyer, G. J.; Karlin, K. D.
Inorg. Chem. 2004, 43, 8272–8281.
10.1021/ol1005938  2010 American Chemical Society
Published on Web 04/15/2010

efficiency of diiron(III)-µ-oxo bisporphyrin systems was
improved by Nocera and co-workers who employed
“Pacman” ligand designs with organic spacer hinges
serving to preorganize two iron centers in a cofacial
arrangement.5–8
reported methods.20 The hydroxyl axial ligands were
readily replaced with chloride anions to give a µ-oxo
complex formulated as [RuIV(TPP)Cl]2O (Supporting
Information, Figure S1). Two other dimer complexes,
[RuIV(4-CF3-TPP)OH]2O (1b) and [(RuIV(4-MeOTPP)
OH]2O (1c), were prepared in a similar manner. The
Among the high-valent metal-oxo species, metal(V)-oxo
complexes deserve special attention because they are highly
reactive, although rare and elusive. The known porphyrin
manganese(V)-oxo complexes showed higher reactivity than
well-studied iron(IV)-oxo porphyrin radical cation ana-
logues.9,10 A recent paper by Collins et al. reported spectro-
scopic evidence for an oxoiron(V) complex supported by a
tetraanionic ligand that showed unprecedented reactivity.11
Putative porphyrin- and corrole-iron(V)-oxo transients
produced by laser flash photolysis methods displayed the
appropriate high levels of reactivity expected for iron(V)-oxo
species,12,13 and we recently reported a photodisproportion-
ation of a bis-corrole-iron(IV) µ-oxo dimer that apparently
gave the same type of corrole-iron(V)-oxo transient in a
system that has potential for light-driven oxidation cataly-
sis.14
1
complexes were characterized by UV-visible, H NMR,
and IR spectra that matched those reported.20 When
dissolved in acetonitrile solution, complexes 1 were
thermally stable to hydrocarbons, but irradiation with light
(λmax ) 420 nm) of anaerobic solutions of complexes 1
in the presence of excess triphenylphosphine (50 mM)
resulted in changes in the absorption spectra with isos-
bestic points (Figure 1). Similar spectral changes were
Porphyrin-ruthenium(V)-oxo transients15,16 are attractive
candidates for oxidations, and these species are proposed
intermediates in very efficient catalytic processes,17 although
not yet observed directly; computational studies suggest that
they are stable with respect to ruthenium(IV)-oxo porphyrin
radical cations.18 In this study, we explore the applicability
of ruthenium porphyrins, which are known to display good
oxidative robustness,19 in a light-driven catalytic process.
We report that ruthenium(IV) µ-oxo bisporphyrin complexes
catalyze the aerobic oxidation of hydrocarbons using visible
light and atmospheric oxygen as oxygen source in sequences
employing photodisproportionation reactions to give putative
ruthenium(V)-oxo species as shown in Scheme 1.
Figure 1
.
UV-vis spectral change of 1a (6 × 10-6 M) in the
presence of excess Ph3P (50 mM) upon irradiation with a 300-W
visible lamp (λmax ) 420 nm) in anaerobic CH3CN solution. The
spectra were recorded at t ) 0, 2, 4, 6, and 10 min.
previously observed upon the photolyses of diiron(III)-
µ-oxo bisporphyrin complexes in the presence of Ph3P.21
According to previous studies by Collman and co-
workers,22 the product formed with λmax at 430 nm was
assigned as RuII(TPP)(PPh3)2. The spectral signature of
RuII(TPP)(PPh3)2 was further confirmed by production of
the same species in the known experiment of the RuVI-
(TPP)O2 with Ph3P.23
Scheme 1
(7) Rosenthal, J.; Pistorio, B. J.; Chng, L. L.; Nocera, D. G. J. Org.
Chem. 2005, 70, 1885–1888.
(8) Rosenthal, J.; Luckett, T. D.; Hodgkiss, J. M.; Nocera, D. G. J. Am.
Chem. Soc. 2006, 128, 6546–6547.
(9) Groves, J. T.; Lee, J.; Marla, S. S. J. Am. Chem. Soc. 1997, 119,
6269–6273.
(10) Zhang, R.; Horner, J. H.; Newcomb, M. J. Am. Chem. Soc. 2005,
127, 6573–6582.
(11) Tiago de Oliveira, F.; Chanda, A.; Banerjee, D.; Shan, X.; Mondal,
S.; Que, L., Jr.; Bominaar, E. L.; Muenck, E.; Collins, T. J. Science 2007,
315, 835–838.
(12) Harischandra, D. N.; Zhang, R.; Newcomb, M. J. Am. Chem. Soc.
2005, 127, 13776–13777.
(13) Pan, Z.; Zhang, R.; Fung, L. W. M.; Newcomb, M. Inorg. Chem.
2007, 46, 1517–1519.
The diruthenium(IV)-µ-oxo-bis[5,10,15,20-tetraphe-
nylporphyrin] precursor 1a was synthesized according to
(14) Harischandra, D. N.; Lowery, G.; Zhang, R.; Newcomb, M. Org.
Lett. 2009, 11, 2089–2092.
(15) Groves, J. T.; Bonchio, M.; Carofiglio, T.; Shalyaev, K. J. Am.
Chem. Soc. 1996, 118, 8961–8962.
(5) Chang, C. J.; Baker, E. A.; Pistorio, B. J.; Deng, Y.; Loh, Z.-H.;
Miller, S. E.; Carpenter, S. D.; Nocera, D. G. Inorg. Chem. 2002, 41, 3102–
3109.
(16) Wang, C.; Shalyaev, K. V.; Bonchio, M.; Carofiglio, T.; Groves,
J. T. Inorg. Chem. 2006, 45, 4769–4782.
(6) Pistorio, B. J.; Chang, C. J.; Nocera, D. G. J. Am. Chem. Soc. 2002,
124, 7884–7885.
(17) Ohtake, H.; Higuchi, T.; Hirobe, M. Heterocycles 1995, 40, 867–
903.
Org. Lett., Vol. 12, No. 10, 2010
2247

A series of ruthenium(IV)-µ-oxo bisporphyrins was
evaluated in the aerobic oxidation of cis-cyclooctene (Table
1). The reactions were carried out with 0.5 µmol of catalyst
gave reduced activity compared to [RuIV(TPP)OH]2O. The
substituent in the porphyrin ligand gave a noticeable effect on
the catalytic activity (entries 1, 8, and 10), with the most
electron-demanding system, namely [RuIV(4-CF3-TPP)OH]2O,
being the most efficient catalyst.
In a fashion similar to that described for cis-cyclooctene
epoxidation, the photocatalytic oxidations of a variety of
organic substrates were examined. Table 2 lists the oxidized
Table 1. Aerobic Photocatalytic Oxidation of cis-Cyclooctene
with Diruthenium(IV) µ-Oxo Porphyrinsa
entry
1
catalyst
solvent t/day
TONb, c
[RuIV(TPP)OH]2O 1a
CH3CN
1
2
3
1
1
1
1
1
1
1
1
1
220 ( 16
460 ( 40
640 ( 65
110 ( 18
140 ( 6
190 ( 10
340 ( 8
300 ( 35
70 ( 5
Table 2. Turnover Numbers for Alkenes and Benzylic C-H
Oxidations Using 1b as the Photocatalysta
2
3
4
CHCl3
C6H6
entry
substrate
product
TONb
norbornene oxidec
2-cyclohexenol
2-cyclohexenone
cyclohexe oxide
triphenylmethanol
diphenylmethanol
benzophenone
1-phenylethanol
acetophenone
200 ( 40
160 ( 14
350 ( 31
30 ( 3
1120 ( 48
820 ( 102
140 ( 18
380 ( 41
180 ( 19
2900 ( 140
3300 ( 240
3900 ( 280
THF
1
2
norbornene
cyclohexene
5d
6e
7
CH3CN
CH3CN
CH3CN
[RuIV(TPP)Cl]2O
8
[RuIV(4-CF3-TPP)OH]2O 1b CH3CN
CH3CN
250 ( 21
340 ( 9
190 ( 23
3
4
triphenylmethane
diphenylmethane
9e
10
[RuIV(4-MeOTPP)OH]2O 1c CH3CN
a The reaction was carried out in a Rayonet reactor, with 0.5 µmol of
catalyst in 5 mL of solvent containing 4 mmol of cis-cyclooctene. Oxygen-
saturated solutions were irradiated with visible light (λmax ) 420 nm) or
otherwise noted. Products were analyzed on an HP 5890 GC with a DB-5
capillary column employing an internal standard. b TON represents the total
number of moles of product produced per mole of catalyst. All reactions
were run three times, and the data reported are the averages with standard
deviation (1σ). c The major product was cis-cyclooctene oxide, detected in
>95% yield. d UV-vis light (λmax ) 350 nm). e 5 mg of anthracene was
added.
5
ethylbenzene
6d
7
xanthene
1-phenylethanol
9-xanthenol
9-xanthone
acetophenone
9-xanthone
8e
a Typically with 0.25 µmol of 1b in 5 mL of CH3CN containing 2-4
mmol of substrate and 5 mg of anthracene. b Determined for a 24 h
photolysis (λmax ) 420 nm). The values reported are the averages of 2-3
runs (1σ deviation). c >90% exo isomer. d One minor product was detected
by GC but not identified. e 48% product yield.
in 5 mL of oxygen-saturated solution containing 4 mmol of
cis-cyclooctene. After 24 h of photolysis with visible light
(λmax ) 420 nm), cis-cyclooctene oxide was obtained as the
only identifiable oxidation product (>95% by GC) with ca.
220 turnovers (abbreviated as TON representing moles of
product/mol of catalyst) of catalyst 1a (entry 1). The trend
in the TONs roughly paralleled irradiation times. Control
experiments showed that no epoxide was formed in the
absence of either the catalyst or light. The results cannot be
ascribed to the chemistry of singlet oxygen, which is
characterized by efficient “ene” reactions of alkenes.24 The
use of other solvents instead of CH3CN, or air as oxygen
source (data not shown), resulted in reduced TONs (entries
2-4). Catalyst degradation was a problem with higher-energy
light, but the use of UV irradiation increased catalytic activity
(entry 5). It is interesting to note that the catalytic activity
was enhanced by adding small amounts of anthracene (entries
6 and 9). Quite surprisingly, the axial ligand on the metal
had a significant effect, and the [RuIV(TPP)Cl]2O (entry 7)
products and corresponding TONs using the [RuIV(4-CF3-
TPP)OH]2O (1b) as the photocatalyst. The trend in the TONs
roughly parallels the substrate reactivity, and significant
activity was observed with up to 3900 TON. After 24 h
photolysis, norbornene was oxidized to norbornene oxide
(exo mainly) with 200 TON (entry 1). Cyclohexene, in
contrast, is susceptible to the allylic oxidation that gave
primarily 2-cyclohexenone and 2-cyclohexenol along with
minor epoxide (entry 2). This product distribution is no
different than those typically obtained from cofacial iron
porphyrin photocatalysts.7 Activated hydrocarbons including
triphenylmethane, diphenylmethane, ethylbenzene, and xan-
thenes were oxidized to the corresponding alcohols and/or
ketones from overoxidation with total TONs ranging from
560 to 2900 (entries 3-6). Noticeably, the oxidation of
secondary benzylic alcohols gave the highest catalytic
activities (entries 7 and 8). Competitive catalytic oxidation
of ethylbenzene and ethlybenzene-d10 revealed a kinetic
isotope effect (KIE) of kH/kD ) 4.8 ( 0.2 at 298 K, similar
to the KIE reported for the reaction of ethylbenzene with an
electron-deficient iron(IV)-oxo porphyrin radical cation
species.25 The observed KIE is larger than those observed
in autoxidation processes,26 suggesting a nonradical mech-
anism that involves the intermediacy of ruthenium(V)-oxo
species as postulated in Scheme 1.
(18) Ogliaro, F.; de Visser, S. P.; Groves, J. T.; Shaik, S. Angew. Chem.,
Int. Ed. 2001, 40, 2874–2878.
(19) Che, C.-M.; Huang, J.-S. Chem. Commun. 2009, 3996–4015.
(20) Sugimoto, H.; Higashi, T.; Mori, M.; Nagano, M.; Yoshida, Z.;
Ogoshi, H. Bull. Chem. Soc. Jpn. 1982, 55, 822–828.
(21) Richman, R. M.; Peterson, M. J. Am. Chem. Soc. 1982, 104, 5795–
5796.
(22) Collman, J. P.; Barnes, C. E.; Brothers, P. J.; Collins, T. J.; Ozawa,
T.; Gallucci, J. C.; Ibers, J. A. J. Am. Chem. Soc. 1984, 106, 5151–5163.
(23) Che, C.-M.; Zhang, J.-L.; Zhang, R.; Huang, J.-S.; Lai, T.-S.; Tsui,
W.-M.; Zhou, X.-G.; Zhou, Z.-Y.; Zhu, N.; Chang, C. K. Chem.sEur. J.
2005, 11, 7040–7053.
(25) Pan, Z.; Zhang, R.; Newcomb, M. J. Inorg. Biochem. 2006, 100,
524–532.
(26) Sawyer, D. T.; Kang, C.; Llobet, A.; Redman, C. J. Am. Chem.
Soc. 1993, 115, 5817–5818.
(24) Kearns, D. R. Chem. ReV. 1971, 71, 395–427.
2248
Org. Lett., Vol. 12, No. 10, 2010

To probe the nature of the transient oxidizing species, we
conducted laser flash photolysis (LFP) studies similar to those
previously described for iron and manganese complexes.27
As shown in Figure 2A, the photodisproportionation of the
PP)(OH)] and that of 3a as RuIII(TPP)OH. It is noteworthy
that the photolysis efficiency could be enhanced by adding
benzophenone or anthracene, which presumably acts as a
photosensitizer. In the presence of anthracene (10 mM), the
photolysis of 1a had a quantum yield of 1.1× 10-3 (see the
Supporting Information for details), which is 10 times greater
than that reported for the photolysis of a bis-porphyrin
diiron(III) µ-oxo complex.3 In a slower subsequent phase
of the reaction (Figure S2 in the Supporting Information),
exposing photoproduct 3a to oxygen led to regeneration of
the dimetal(IV)-µ-oxo complex 1a through an autoxidation
pathway.22
Kinetic studies were accomplished by generating transient
2a in the presence of organic substrates at varying high
concentrations under pseudo-first-order conditions. The
kinetics are described by eq 1, where kobs is the observed
pseudo-first-order rate constant, k0 is the background rate
constant for decay in the absence of substrate, kox is the
second-order rate constant, and [substrate] is the concentra-
tion of substrate.
kobs ) k0 + kox[substrate]
(1)
The results from reactions of 2a formed by photolysis of
1a are shown graphically in Figure 2C, where plots of kobs
versus the concentrations of cyclohexene and cis-cyclooctene
were linear, and the rate constants (kox) for their reactions
with 2a are 1.8 × 103 and 1.3 × 103 M-1 s-1, respectively.
Second-order rate constants with other substrates are col-
lected in Table S1 (Supporting Information). Remarkably,
the observed rate constants for reactions of the putative
ruthenium(V)-oxo species 2a with alkenes are 5-6 orders
of magnitude greater than those for similar reactions of the
well-characterized trans-dioxoruthenium(VI) porphyrins.23
In summary, we have demonstrated that the ruthe-
nium(IV)-µ-oxo bisporphyrins catalyzed efficient aerobic
oxidation of alkenes and activated hydrocarbons using visible
light and atmospheric oxygen. The observed photocatalytic
oxidation is ascribed to a photodisproportionation mechanism
to afford a putative porphyrin-ruthenium(V)-oxo species
that can be directly observed and kinetically studied by laser
flash photolysis methods. Further studies to characterize the
observed transients more fully and to define synthetic
applications are ongoing in our laboratory.
Figure 2. (A) Kinetic trace at λmax (390 nm) showing a rapid decay
followed by a slow growing process after laser pulse. (B) Time-
resolved difference spectrum at 0.2, 1, 2, 5, 8, and 10 ms, following
355 nm irradiation of [RuIV(TPP)OH]2O (1a) in the presence of
benzophenone (10 mM) in CH3CN at 25 °C; difference spectrum
) spectrum (t) - spectrum (final ) 50 ms). In this representation,
decaying peaks have positive absorbances, whereas growing peaks
have negative absorbances. (C) Observed pseudo-first-order rate
constants for reactions of 2a with cyclohexene (circles) and cis-
cyclooctene (triangles) in CH3CN.
bisporphyrin diruthenium(IV)-µ-oxo complexes exhibited
a fast decay followed by a slow growth that were observed
in different time scales (Figure 2A). Figure 2B shows a time-
resolved difference spectrum for a 50 ms time scale, where
the only observable transients are those that increase or
decrease in concentration on the short time scale, 2a and 3a
in this case. Irradiation of the complex [RuIV(TPP)OH]2O
1a with 355 nm laser light at ambient temperature in CH3CN
solution instantly produced a highly reactive transient 2a
displaying a strong Soret band at 390 nm, which rapidly
decayed to form a compound (3a) with Soret band at 410
nm and Q-band at 530 nm (Figure 2B). The spectrum of 3a
was essentially identical to that of RuIII(OMe)TPP, which
was independently prepared from a reported method.15
Accordingly, we assigned the structure of 2a as [RuV(O)(T-
Acknowledgment. This work was supported by PRF
(48764-GB4) and Kentucky EPSCoR (REG 2008) grants to
R.Z. and by NIH (GM48722) and NSF (CHE-0601857)
grants to M.N. We thank Dr. Eric Conte for assistance with
the GC analysis.
Supporting Information Available: Figures S1-2 and
Table S1 and experimental methods. This material is
available free of charge via the Internet at http://pubs.acs.org.
(27) Zhang, R.; Newcomb, M. Acc. Chem. Res. 2008, 41, 468–477.
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