A facile photosynthesis of trans-dioxoruthenium(vi) porphyrins

Article abstract of DOI:10.1039/c003094e

Photolysis of porphyrin-ruthenium(iv) dichlorate complexes with visible light results in homolytic cleavage of the O-Cl bond in the chlorates to form trans-dioxoruthenium(vi) species bearing either sterically encumbered or unencumbered porphyrin ligands.

Full text of DOI:10.1039/c003094e

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A facile photosynthesis of trans-dioxoruthenium(VI) porphyrinsw
Yan Huang, Eric Vanover and Rui Zhang*
Received 15th February 2010, Accepted 7th April 2010
First published as an Advance Article on the web 28th April 2010
DOI: 10.1039/c003094e
Photolysis of porphyrin-ruthenium(IV) dichlorate complexes
with visible light results in homolytic cleavage of the O–Cl bond
in the chlorates to form trans-dioxoruthenium(VI) species
bearing either sterically encumbered or unencumbered porphyrin
ligands.
ligand cleavage reactions for production of a variety of
high-valent transition metal-oxo derivatives.21 Unlike the
commonly used chemical oxidations, the photochemical
approach produces metal-oxo species essentially instantly,
and permits direct detection of metal-oxo species and kinetic
studies of their oxidations within much shorter timescales than
the fastest mixing experiments.22–25 In this study, we describe
our progress in the development of a new photochemical
method that led to generation of dioxoruthenium(VI) complexes
bearing sterically hindered, unhindered and chiral porphyrin
ligands (Scheme 1).
Organic ligand-complexed transition metal-oxo intermediates
have been proposed as the active oxidizing species in many
metal-catalyzed oxidation reactions that employ sacrificial
oxidants such as hydrogen peroxide, peroxy acids, and
iodosobenzene.1,2 Many transition metal catalysts have been
designed to mimic the predominant oxidation catalysts in
Nature, namely the cytochrome P450 enzymes.3,4 In some
cases, high-valent transition metal-oxo active intermediates
can be observed spectroscopically by rapid mixing techniques
or by the production of low-reactivity analogues;5,6 but in
many cases the active metal-oxo species does not accumulate
to detectable quantities, and the actual oxidizing species
remains speculative. In this regard, trans-dioxoruthenium(VI)
porphyrin complexes have received much attention, and
have been developed as a well-characterized model system
for heme-containing enzymes.7–15 This class of compounds
is neutral and readily dissolves in nonpolar organic solvents,
and yet is reactive toward useful organic oxidations
including alkene epoxidations and alkane hydroxylations.16
Furthermore, the reactive dioxoruthenium(VI) species can
be regenerated by aerobic oxidation, and therefore, they
can catalyze aerobic oxidation reactions in the absence of
co-reductants.10,17
We studied the three porphyrin systems shown in Scheme 1,
using abbreviations that follow those previously established.z
According to the method reported by Gross and co-workers,26
the dichlororuthenium(IV) complexes (1) were prepared by
heating corresponding RuII(Por)(CO) in refluxing CCl4. Each
of these dichlororuthenium(IV) complexes was known and
1
characterized by distinct H NMR (paramagnetically shifted
pyrrolic protons at d ꢀ55.8 (1a), ꢀ58.2 (1b) and ꢀ52.3 (1c)
ppm in CDCl3) in agreement with those reported.26,27
Exchange of the counterions in 1 with Ag(ClO3) gave the
corresponding dichlorate salts 2 as desired photo-labile
precursors that were subsequently used for photochemical
reactions (See Fig. S1-3 in supporting informationw for
1
UV-visible and H NMR spectra).
Irradiation of dichlorate complexes 2 in anaerobic CH3CN
with visible light from a tungsten lamp resulted in changes in
absorption spectra with isosbestic points at 596, 436, 412
and 336 nm (Fig. 1). In Fig. 1, 2a decayed, and a new
species 3a was produced, displaying a stronger Soret band
at 420 nm and a weaker Q band at 518 nm that is
characteristic of RuVI(TMP)O2. Accordingly, 3a was assigned
as RuVI(TMP)O2. The spectral signature of RuVI(TMP)O2
was further confirmed by 1H NMR and IR (see Fig. S4 and S5
The first isolation and characterization of the sterically
hindered trans-dioxoruthenium(VI) porphyrin complex
RuVI(TMP)O2 by oxidation of RuII(TMP)(CO) with m-CPBA
was reported by Groves and Quinn.7 Che and co-workers had
reported the syntheses of non-sterically encumbered trans-
dioxoruthenium(VI) porphyrins RuVI(TPP)O2 and RuVI(OEP)O2
in the presence of coordinating solvents (such as methanol).8
Later, chiral trans-dioxoruthenium(VI) complexes were prepared
in a similar manner and characterized with satisfactory
spectroscopic properties.18–20 Some dioxoruthenium(VI)
porphyrin complexes are even determined by the X-ray crystal
structures.10,16 Recently, we have developed photo-induced
Department of Chemistry, Western Kentucky University,
1906 College Heights Blvd., Bowling Green, Kentucky, USA.
E-mail: rui.zhang@wku.edu; Fax: +1-270-7455631;
Tel: +1-270-7453803
w Electronic supplementary information (ESI) available: UV-vis
spectra of 1a and 2a, 1H NMR spectra of 1a–3a, IR spectrum of 3a,
and time-resolved spectra for generation of 3b and 3c. See DOI:
10.1039/c003094e
Scheme 1
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This work was supported by the Petroleum Research Fund
(PRF 48764-GB4) and Kentucky EPSCoR NSF (REG 2008)
grants.
Notes and references
z Abbreviations used in this study: TMP = 5,10,15,20-tetramesityl-
porphyrin dianion (a); TPP = 5,10,15,20-tetraphenylporphyrin dianion
(b); D4-Por*
=
5,10,15,20-tetrakis[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-
octahydro-1,4:5,8-dimethanoanthracen-9-yl]porphyrin dianion (c).
All commercial reagents were of the best available purity and were
used as supplied unless otherwise specified. HPLC grade acetonitrile
(99.93%) was distilled over P2O5 prior to use. 5,10,15,20-Tetra-
mesitylporphyrin (TMPH2),31 tetraphenylporphyrin (TPPH2)32 and
33
D4-Por*H2 free ligands and their ruthenium(II) carbonyl complexes
Fig. 1 Time-resolved UV-visible spectra of 2a (8 ꢂ 10ꢀ6 M)
upon irradiation with visible light in anaerobic CH3CN solution
at 22 1C. Spectra were recorded at t = 0, 6, 14, 20, 26 and
50 min.
RuII(Por)(CO) were prepared by literature methods.16 RuIV(Por)Cl2
(1) was prepared by refluxing RuII(Por)(CO) complexes in CCl4.
Formation of 1b required overnight refluxing and no m-oxo dimer
[Ru(TPP)Cl]2O was observed. Treatment of compounds 1 (typically
5–10 mg) with ca. 2-fold excess of Ag(ClO3) in anaerobic CH3CN gave
the dichlorate complexes 2, characterized by UV-vis and 1H NMR.
When the solution of 2 with concentration in the range of (5–12) ꢂ
10ꢀ6 M was irradiated with visible light from a tungsten lamp at
ambient temperature, the formation of 3 was complete in ca. 50 min,
monitored by UV-visible spectroscopy. After purification by passing
through a short Al2O3 dry column, the dioxo complexes (3) were
isolated with >95% yields as dark red-purple crystalline solids, which
gave satisfactory spectroscopic characterization spectra.
in supporting informationw). Control experiments showed
that no dioxo species was formed in the absence of light.
Thus, the photolysis reactions of the dichlorate complexes 2
result in homolytic cleavage of the O–Cl bond in the
two chlorate counterions simultaneously to produce neutral
dioxoruthenium(VI) species 3 via two one-electron photo-
oxidation pathways. The photochemical reactions of chlorates
2 are analogous to the photochemical cleavages of porphyrin-
manganese(III) chlorates,24 corrole-manganese(IV) chlorates28
and corrole-iron(IV) chlorates,23 which give neutral porphyrin-
manganese(IV)-oxo, corrole-manganese(V)-oxo and corrole-
iron(V)-oxo derivatives by homolytic cleavage of the O–Cl
bond in the chlorate.
1 B. Meunier, Chem. Rev., 1992, 92, 1411.
2 R. A. Sheldon, Metalloporphyrins in Catalytic Oxidations, Marcel
Dekker, New York, 1994.
3 I. G. Denisov, T. M. Makris, S. G. Sligar and I. Schlichting, Chem.
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4 P. R. Ortiz de Montellano, in Cytochrome P450 Structure,
Mechanism, and Biochemistry, Kluwer Academic/ Plenum,
New York, 2005.
5 B. Meunier, in Metal-Oxo and Metal-Peroxo Species in Catalytic
Oxidations, Springer–Verlag, Berlin, 2000.
In a fashion similar to that described for the generation of
3a, the sterically unhindered RuVI(TPP)O2 (3b) and chiral
RuVI(D4-Por*)O2 (3c) were also generated (See Fig. S6 and
S7 in supporting informationw), demonstrating the generality
of this photochemical method. These complexes (3) were
isolated and characterized by UV-visible, 1H NMR and
IR spectra that matched those reported.7–8,10 The use of
other solvents such as CH2Cl2 gave similar results. Product
degradation was observed when higher-energy UV light
(lmax = 350 nm) was used instead of the visible light.
Photolysis of MnIII(Por)(NO3) complexes was reported to give
MnIV(Por)(O) species by homolytic cleavage of an O–N
bond,29 similar to those of chlorate complexes.24 However,
we found that photolyses of ruthenium(IV) dinitrate complexes
did not afford the dioxo complexes 3 even with prolonged
irradiation and higher-energy light (lmax = 350 nm).
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In conclusion, we report here a new preparation of trans-
dioxoruthenium(VI) porphyrin complexes by an extremely
easy photochemical approach. We have demonstrated the
generality of the methodology in sterically encumbered and
unencumbered systems under the same conditions, bypassing
the ligand limitation observed in chemical methods. We
believe these promising results will stimulate the further
exploration of photo-synthetic methodology to produce other
high-valent metal-oxo complexes. Given that porphyrin-
ruthenium(V)-oxo transients are more attractive candidates
for oxidations,30 the extension of this method for generation
of the putative ruthenium(V)-oxo species is underway in our
laboratory.
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This journal is The Royal Society of Chemistry 2010
3778 | Chem. Commun., 2010, 46, 3776–3778