Reversible O-O bond cleavage and formation between Mn(IV)-peroxo and Mn(V)-oxo corroles

Article abstract of DOI:10.1021/ja1066465

Mn(IV)-peroxo and Mn(V)-oxo corroles were synthesized and characterized with various spectroscopic techniques. The intermediates were directly used in O-O bond cleavage and formation reactions. Upon addition of proton to the Mn(IV)-peroxo corrole, the for

Full text of DOI:10.1021/ja1066465

Published on Web 09/16/2010
Reversible O-O Bond Cleavage and Formation between Mn(IV)-Peroxo and
Mn(V)-Oxo Corroles
Sun Hee Kim,† Hyejin Park,† Mi Sook Seo,† Minoru Kubo,‡ Takashi Ogura,‡ Jan Klajn,§
Daniel T. Gryko,§ Joan Selverstone Valentine,†,| and Wonwoo Nam*,†
Department of Bioinspired Science, Ewha Womans UniVersity, Seoul 120-750, Korea, Picobiology Institute,
Graduate School of Life Science, UniVersity of Hyogo, Hyogo 678-1297, Japan, Institute of Organic Chemistry,
Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland, and Department of Chemistry and
Biochemistry, UCLA, Los Angeles, California 90095-1569
Received July 27, 2010; E-mail: wwnam@ewha.ac.kr
Scheme 1
Abstract: Mn(IV)-peroxo and Mn(V)-oxo corroles were synthe-
sized and characterized with various spectroscopic techniques.
The intermediates were directly used in O-O bond cleavage and
formation reactions. Upon addition of proton to the Mn(IV)-peroxo
corrole, the formation of the Mn(V)-oxo corrole was observed.
Interestingly, addition of base to the Mn(V)-oxo corrole afforded
the formation of the Mn(IV)-peroxo corrole. Thus, we were able
to report the first example of reversible O-O bond cleavage and
formation reactions using in situ generated Mn(IV)-peroxo and
Mn(V)-oxo corroles.
at 10 °C afforded the formation of a new intermediate, 2, with
absorption bands at 435 and 590 nm within 1 min (Scheme 1,
reaction A; Figure 1 and SI, Figure S2 for UV-vis spectral
changes). The intermediate was stable enough to be characterized
by various spectroscopic methods (∼10 min at 10 °C). The
electrospray ionization mass spectrum (ESI MS) of 2 exhibited
a prominent ion peak at a mass-to-charge ratio (m/z) of 1090.8,
which corresponds to the mass and isotope distribution pattern
of [Mn(TFMPC)(O2)(CH3CN)(CH3OH)]- (calculated m/z 1091.1;
Figure 2a). The ESI MS of 2, prepared with D2O2 in D2O,
showed the identical mass peak (SI, Figure S3), whereas 2,
prepared with H218O2 in H216O, showed a mass peak at m/z
1094.8, which corresponds to [Mn(TFMPC)(18O2)(CH3CN)-
(CH3OH)]- (Figure 2a, inset). The 4 mass unit increase upon
the substitution of 16O with 18O indicates that 2 contains an O2
unit derived from H2O2. The X-band EPR spectrum of 2
exhibited a broad signal at g ≈ 4 (Figure 2b and SI, Figure S4
for EPR spectra taken during reactions), which we assign to a
high-spin S ) 3/2 Mn(IV) with a strong zero-field splitting.6
Taken together, the spectroscopic data provide strong evidence
that 2, generated in the reaction of 1 and H2O2 in the presence
of base, is a high-spin Mn(IV) complex bearing an O2 ligand,
[Mn(IV)(TFMPC)(O2)]-.7
Dioxygen O-O bond-cleaving and bond-forming reactions occur
at transition metal centers in a number of metalloenzymes. For
example, heme and nonheme iron monooxygenases bind and
activate O2 to generate high-valent iron-oxo intermediates via O-O
bond cleavage of iron(III)-peroxo precursors, [Fe(III)-O2]+.1 The
O-O bond activation of iron-O2 adducts has for that reason been
intensively investigated using synthetic iron porphyrins to elucidate
mechanisms of the iron-oxo intermediate formation over the past
several decades (e.g., heterolysis vs homolysis).2
In the case of the O-O bond formation, the photosynthetic
conversion of water into dioxygen occurs at the oxygen-evolving
complex (OEC) in photosystem II (PS II); a manganese(V)-oxo
intermediate has been implicated as an active species for the O-O
bond formation.3 Very recently, Gao et al. reported an elegant result
that the reaction of a Mn(V)-oxo corrole and hydroxide affords O2
evolution; the O-O bond formation in the reaction was proposed
to occur via nucleophilic attack of hydroxide ion on a Mn(V)-oxo
moiety.4 Thus, O-O bond cleavage and O-O bond formation
reactions in metalloenzymes have been successfully mimicked by
biomimetic model studies. However, reversible O-O bond cleavage
and formation between metal-O2 and metal-oxo species has not been
successfully modeled.5 Herein we report the first example of a
reversible conversion between manganese-peroxo and -oxo corrole
complexes via O-O bond cleavage and formation processes (see
Scheme 1).
Addition of 1.2 equiv of H2O2 to a solution containing a
manganese(III) corrole, Mn(TFMPC) (1, 3 × 10-2 mM) (TFMPC
) 5,10,15-tris(3,5-trifluoromethylphenyl)corrolato trianion) (see
structure in Supporting Information (SI), Figure S1), and
tetramethylammonium hydroxide (TMAH, 20 equiv) in CH3CN
† Ewha Womans University.
‡ University of Hyogo.
§ Polish Academy of Sciences.
| UCLA.
Figure 1. UV-vis spectra of 2 (solid blue line), 3 (solid red line), after
addition of acid (20 equiv of HClO4) to 2 (dashed green line), and after
addition of base (20 equiv of TMAH) to 3 (dashed black line).
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C O M M U N I C A T I O N S
2 was converted to 3 by addition of benzoyl chloride (data not
shown). It has been well-documented that the reactions of metal-
peroxo species, including Mn(III)-peroxo porphyrins, with benzoyl
chloride lead to formation of their corresponding high-valent metal-
oxo species.11 However, the O-O bond cleavage mechanism of
the putative Mn(IV)-OOH intermediate is not yet clear at this
moment (see Scheme 2 for O-O bond homolysis vs heterolysis).
Nevertheless, to our knowledge, this is the first demonstration of
the analogous conversion of a Mn-peroxo corrole to a Mn-oxo
corrole upon protonation.
Scheme 2. Proposed Mechanisms of O-O Bond Cleavage and
Formation between Mn-Peroxo and -Oxo Corroles
Figure 2. (a) ESI MS spectrum of 2. Inset shows isotope distribution
patterns for 2-16O (left) and 2-18O (right). The mass peaks at m/z 1017.1
and 1048.1 correspond to [Mn(TFMPC)(CH3O)]- and [Mn(TFMPC)-
(CH3O)2]-, respectively. (b) EPR spectrum of 2. Inset shows a parallel mode
X-Band CW-EPR spectrum of the starting MnIII(TFMPC) complex. Also,
see SI, Figure S4.
We then synthesized a Mn(V)-oxo corrole complex, 3, by
reacting 1 with 2.0 equiv of iodosylbenzene (PhIO) in CH3CN at
10 °C (Scheme 1, reaction B; Figure 1 and SI, Figure S5 for
UV-vis spectral changes).8 The ESI MS of 3 exhibited a prominent
ion peak at m/z 1002.1, whose mass and isotope distribution pattern
corresponds to Mn(TFMPC)(O) (calculated m/z 1002.1; Figure 3a).
When 3 was prepared with isotopically labeled PhI18O in the
presence of H218O, a mass peak corresponding to Mn(TFMPC)(18O)
appeared at m/z 1004.2 (calculated m/z 1004.1; Figure 3a, inset). 3
is EPR silent (SI, Figure S4), suggesting a diamagnetic d2 (S ) 0)
species as has been reported for other Mn(V)-oxo corrole and
corrolazine complexes.8,9 The resonance Raman spectrum of 3,
measured in CH3CN at -40 °C with 442-nm laser excitation,
displays an isotope sensitive band at 957 cm-1, which shifts to 920
cm-1 upon introduction of 18O (Figure 3b). The observed isotopic
shift of -37 cm-1 with 18O substitution is in close agreement with
the value calculated for a Mn-O diatomic oscillator (-42 cm-1).
The calculated force constant for the 957 cm-1 mode by a simple
Hook’s law is 6.7 mdyne/Å, which is consistent with triply bonded
Mn-O bonds in Mn(V)-oxo complexes.8c,9,10 Taken together, the
spectroscopic data demonstrate that 3 is a Mn(V)-oxo corrole with
a Mn-O triple bond, MnV(O)(TFMPC).
In the O-O bond formation reaction, addition of base to the
Mn(V)-oxo complex, 3, produced the Mn(IV)-peroxo complex, 2,
quantitatively (Scheme 1, reaction D; Figure 1). The formation of
2 was further confirmed by taking EPR and ESI MS of the resulting
solution; a broad signal at g ≈ 4 was observed in the EPR spectrum
(SI, Figure S4). In ESI MS experiments, 2 prepared in the reaction
of Mn(V)16O and 16OH- contained an 16O16O group (SI, Figure
S6). Similarly, 2 prepared in the reaction of Mn(V)18O and 16OH-
contained an 18O16O group (SI, Figure S6). These results demon-
strate that the peroxo ligand in 2 was generated by the O-O bond
formation between the Mn-oxo moiety in 3 and the hydroxide ion,
followed by a deprotonation of a hydroperoxo ligand by another
hydroxide ion (Scheme 2).4,12 However, there is an electron missing
in the proposed mechanism, and this is probably due to a greater
stability of a Mn(IV)-peroxo corrole species under the reaction
conditions.
Remarkably, a reversible conversion between the Mn(IV)-peroxo
and Mn(V)-oxo corroles was observed upon addition of acid and
base (Scheme 1, reactions C and D); this reversible cycle could be
repeated several times without showing a significant decrease of
the absorption bands corresponding to the products. First, the
Mn(IV)-peroxo complex, 2, was converted to the Mn(V)-oxo
complex, 3, upon addition of HClO4 (Scheme 1, reaction C); the
full formation of 3 was confirmed by the UV-vis and EPR spectra
of the resulting solution (Figure 1 and SI, Figure S4). In addition,
In conclusion, we have synthesized and characterized new
Mn(IV)-peroxo and Mn(V)-oxo corroles and used them in O-O
bond cleavage and formation reactions. We have also shown the
first example of reversible O-O bond cleavage and formation
between high-valent metal-oxo and metal-peroxo species. Future
studies will be focused on understanding mechanistic aspects
involved in the O-O bond cleavage and formation processes.
Acknowledgment. The research was supported by KOSEF/
MEST of Korea through the CRI, WCU (R31-2008-000-10010-
0), and GRL (2010-00353) Programs (to W.N. and J.S.V.), Grant-
in-Aid for scientific research (C) (No. 21570171) and Priority Area
(No. 477) (KAKENHI) and GCOE program (Picobiology) by
MEXT, Japan (to T.O.), and Polish Ministry of Science and Higher
Education (Contract 50611) (to D.T.G.).
Supporting Information Available: Experimental details and
Figures S1-S6. This material is available free of charge via the Internet
at http://pubs.acs.org.
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