Synthesis, characterization, and reactivities of manganese(V)-oxo porphyrin complexes

Article abstract of DOI:10.1021/ja066460v

The reactions of manganese(III) porphyrin complexes with terminal oxidants, such as m-chloroperbenzoic acid, iodosylarenes, and H₂O₂, produced high-valent manganese(V)-oxo porphyrins in the presence of base in organic solvents at room temperature. The manganese(V)-oxo porphyrins have been characterized with various spectroscopic techniques, including UV-vis, EPR, ¹H and ¹⁹F NMR, resonance Raman, and X-ray absorption spectroscopy. The combined spectroscopic results indicate that the manganese(V)-oxo porphyrins are diamagnetic low-spin (S = O) species with a longer, weaker Mn-O bond than in previously reported Mn(V)-oxo complexes of non-porphyrin ligands. This is indicative of double-bond character between the manganese(V) ion and the oxygen atom and may be attributed to the presence of a trans axial ligand. The [(Porp)Mn^V=O]⁺ species are stable in the presence of base at room temperature. The stability of the intermediates is dependent on base concentration. In the absence of base, (Porp)-Mn ^IV=O is generated instead of the [(Porp)Mn^V=O]⁺ species. The stability of the [(Porp)Mn^V=O]⁺ species also depends on the electronic nature of the porphyrin ligands: [(Porp)Mn ^V=O]⁺ complexes bearing electron-deficient porphyrin ligands are more stable than those bearing electron-rich porphyrins. Reactivity studies of manganese(V)-oxo porphyrins revealed that the intermediates are capable of oxygenating PPh₃ and thioanisoles, but not olefins and alkanes at room temperature. These results indicate that the oxidizing power of [(Porp)Mn^V=O]⁺ is low in the presence of base. However, when the [(Porp)Mn^V=O]⁺ complexes were associated with iodosylbenzene in the presence of olefins and alkanes, high yields of oxygenated products were obtained in the catalytic olefin epoxidation and alkane hydroxylation reactions. Mechanistic aspects, such as oxygen exchange between [(Porp)Mn^V=¹⁶O]⁺ and H₂ ¹⁸O, are also discussed.

Full text of DOI:10.1021/ja066460v

Published on Web 01/03/2007
Synthesis, Characterization, and Reactivities of
Manganese(V)-Oxo Porphyrin Complexes
Woon Ju Song,† Mi Sook Seo,† Serena DeBeer George,‡ Takehiro Ohta,|
Rita Song,† Min-Jung Kang,† Takehiko Tosha,| Teizo Kitagawa,|
Edward I. Solomon,§ and Wonwoo Nam*,†
Contribution from the Department of Chemistry, DiVision of Nano Sciences, and Center for
Biomimetic Systems, Ewha Womans UniVersity, Seoul 120-750, Korea, Stanford Synchrotron
Radiation Laboratory, SLAC, Stanford UniVersity, Stanford, California 94309, Okazaki Institute
for IntegratiVe Bioscience, National Institutes of Natural Sciences, Okazaki, Aichi 444-8787,
Japan, and Department of Chemistry, Stanford UniVersity, Stanford, California 94305
Received September 7, 2006; E-mail: wwnam@ewha.ac.kr
Abstract: The reactions of manganese(III) porphyrin complexes with terminal oxidants, such as m-
chloroperbenzoic acid, iodosylarenes, and H2O2, produced high-valent manganese(V)-oxo porphyrins in
the presence of base in organic solvents at room temperature. The manganese(V)-oxo porphyrins have
been characterized with various spectroscopic techniques, including UV-vis, EPR, 1H and 19F NMR,
resonance Raman, and X-ray absorption spectroscopy. The combined spectroscopic results indicate that
the manganese(V)-oxo porphyrins are diamagnetic low-spin (S ) 0) species with a longer, weaker Mn-O
bond than in previously reported Mn(V)-oxo complexes of non-porphyrin ligands. This is indicative of double-
bond character between the manganese(V) ion and the oxygen atom and may be attributed to the presence
of a trans axial ligand. The [(Porp)MnVdO]+ species are stable in the presence of base at room temperature.
The stability of the intermediates is dependent on base concentration. In the absence of base, (Porp)-
MnIVdO is generated instead of the [(Porp)MnVdO]+ species. The stability of the [(Porp)MnVdO]+ species
also depends on the electronic nature of the porphyrin ligands: [(Porp)MnVdO]+ complexes bearing electron-
deficient porphyrin ligands are more stable than those bearing electron-rich porphyrins. Reactivity studies
of manganese(V)-oxo porphyrins revealed that the intermediates are capable of oxygenating PPh3 and
thioanisoles, but not olefins and alkanes at room temperature. These results indicate that the oxidizing
power of [(Porp)MnVdO]+ is low in the presence of base. However, when the [(Porp)MnVdO]+ complexes
were associated with iodosylbenzene in the presence of olefins and alkanes, high yields of oxygenated
products were obtained in the catalytic olefin epoxidation and alkane hydroxylation reactions. Mechanistic
aspects, such as oxygen exchange between [(Porp)MnVd16O]+ and H218O, are also discussed.
[(Porp)+•FeIVdO]+, as an active oxidant in oxidation reactions,2
a number of iron(IV)-oxo porphyrin π-cation radicals have
been synthesized by the reaction of iron(III) porphyrins with
terminal oxidants such as m-chloroperbenzoic acid (m-CPBA)
and iodosylarenes (ArIO), characterized with various spectro-
scopic techniques, and studied in a variety of oxidation reactions,
including alkane hydroxylation and olefin epoxidation.3,4
Introduction
An important objective in understanding biological oxidation
by heme-containing monooxygenases is to elucidate the nature
of the reactive intermediates and the mechanism of oxygen atom
transfer from the intermediates to organic substrates.1 Since the
catalytic cycle of cytochrome P450 (CYP 450) is believed to
involve a high-valent iron(IV)-oxo porphyrin π-cation radical,
Synthetic manganese(III) porphyrins have also been exten-
sively studied as CYP 450 models in oxygen-atom-transfer
† Ewha Womans University.
‡ Stanford Synchrotron Radiation Laboratory.
| Okazaki Institute for Integrative Bioscience.
(3) (a) Nam, W. In ComprehensiVe Coordination Chemistry II: From Biology
to Nanotechnology; Que, L., Jr., Tolman, W. T., Eds.; Elsevier Ltd.:
Oxoford, 2004; Vol. 8, pp 281-307. (b) Fujii, H. Coord. Chem. ReV. 2002,
226, 51-60. (c) McLain, J. L.; Lee, J.; Groves, J. T. In Biomimetic
Oxidations Catalyzed by Transition Metal Complexes; Meunier, B., Ed.;
Imperial College Press: London, 2000; pp 91-169. (d) Watanabe, Y. In
The Porphyrin Handbook, Vol. 4; Kadish, K. M., Smith, K. M., Guilard,
R., Ed.; Academic: New York, 2000; Vol. 30, pp 97-117.
(4) (a) Groves, J. T.; Haushalter, R. C.; Nakamura, M.; Nemo, T. E.; Evans,
B. J. J. Am. Chem. Soc. 1981, 103, 2884-2886. (b) Gross, Z.; Nimri, S.
Inorg. Chem. 1994, 33, 1731-1732. (c) Gross, Z.; Nimri, S.; Barzilay,
C. M.; Simkhovich, L. J. Biol. Inorg. Chem. 1997, 2, 492-506. (d) Goh,
Y. M.; Nam, W. Inorg. Chem. 1999, 38, 914-920. (e) Song, W. J.; Ryu,
Y. O.; Song, R.; Nam, W. J. Biol. Inorg. Chem. 2005, 10, 294-304.
§ Stanford University.
(1) (a) Ortiz de Montellano, P. R. Cytochrome P450: Structure, Mechanism,
and Biochemistry, 3rd ed.; Kluwer Academic/Plenum Publishers: New York,
2005. (b) Denisov, I. G.; Makris, T. M.; Sligar, S. G.; Schlichting, I. Chem.
ReV. 2005, 105, 2253-2278. (c) Meunier, B.; de Visser, S. P.; Shaik, S.
Chem. ReV. 2004, 104, 3947-3980. (d) Groves, J. T. Proc. Natl. Acad.
Sci. U.S.A. 2003, 100, 3569-3574. (e) Ortiz de Montellano, P. R.; De Voss,
J. J. Nat. Prod. Rep. 2002, 19, 477-493.
(2) (a) Kellner, D. G.; Hung, S.-C.; Weiss, K. E.; Sligar, S. G. J. Biol. Chem.
2002, 277, 9641-9644. (b) Schlichting, I.; Berendzen, J.; Chu, K.; Stock,
A. M.; Maves, S. A.; Benson, D. E.; Sweet, R. M.; Ringe, D.; Petsko,
G. A.; Sligar, S. G. Science 2000, 287, 1615-1622. (c) Egawa, T.; Shimada,
H.; Ishimura, Y. Biochem. Biophys. Res. Commun. 1994, 201, 1464-1469.
9
1268
J. AM. CHEM. SOC. 2007, 129, 1268-1277
10.1021/ja066460v CCC: $37.00 © 2007 American Chemical Society

Manganese(V)−Oxo Porphyrin Complexes
A R T I C L E S
reactions.5 Although manganese porphyrins have shown promise
as versatile catalysts in oxidation reactions over the past two
decades, only recently have the key manganese(V)-oxo por-
phyrin intermediates been isolated, spectroscopically character-
ized, and studied in oxidation reactions.6-9 Groves and co-
workers reported the generation and characterization of the first
manganese(V)-oxo porphyrin complexes in aqueous solution
and the reactivities of these complexes in olefin epoxidation
and in the oxidation of bromide and nitrite ions.6 Subsequently,
Nam and co-workers demonstrated that a manganese(V)-oxo
porphyrin can be generated with a biologically relevant oxidant,
H2O2, in aqueous solution and that the formation of the
intermediate depends markedly on the pH of the reaction
solutions.7 Very recently, Newcomb and co-workers reported
the generation of manganese(V)-oxo complexes via laser flash
photolysis methods in organic solvents and the kinetic studies
of the intermediates in olefin epoxidation and alkane hydroxy-
lation.8 Naruta and co-workers synthesized a dinuclear MnVd
O porphyrin complex in the presence of base in organic solvents
that showed O2 evolution via O-O bond formation between
the manganese(V)-oxo moieties.9 In addition to the manganese-
(V)-oxo porphyrins, manganese(V)-oxo complexes bearing
non-porphyrinic macrocycles, such as corrole and corrolazine,
have been isolated and characterized.10-15 Interestingly, the
manganese(V)-oxo complexes of non-porphyrin ligands are
very stable at room temperature and are poor oxidants in
oxygen-atom-transfer reactions. Thus, insight into the chemical
properties of the long-sought manganese(V)-oxo intermediates
has been obtained by recent developments in isolating and
characterizing manganese(V)-oxo complexes of porphyrin and
non-porphyrin ligands. In this paper, we report the generation
and characterization of manganese(V)-oxo porphyrin com-
plexes that are stable at room temperature in the presence of
Figure 1. Structure of manganese(III) porphyrin complexes used in this
study.
base in organic solvents. The stability and reactivities of the
manganese(V)-oxo porphyrins have been investigated in detail
in oxygenation reactions under stoichiometric and catalytic
conditions.
Results and Discussion
Preparation and Characterization of Manganese(V)-Oxo
Porphyrin Complexes. Addition of m-chloroperbenzoic acid
(m-CPBA) to a reaction solution containing a manganese(III)
porphyrin chloride, Mn(TDCPP)Cl (see Figure 1 for the
structures of manganese porphyrin complexes used in this
study),16 and tetrabutylammonium hydroxide (TBAH) in a
solvent mixture of CH2Cl2 and CH3CN (1:1) at 25 °C resulted
in the immediate generation of a manganese(V)-oxo porphyrin
complex, [(TDCPP)MnVdO]+ (1a), with a strong and sharp
Soret band at 444 nm and a Q-band at 560 nm (Figure 2a; see
Supporting Information, Figure S1 for UV-vis spectra of other
manganese(V)-oxo porphyrins; Table 1 summarizes the UV-
vis absorption bands).6-9 The intermediate persisted for several
hours (t1/2 ≈ 40 min) at 35 °C. The formation of 1a was also
observed when Mn(TDCPP)Cl was treated with other oxidants,
such as iodosylarenes (i.e., PhIO and F5PhIO) and H2O2, under
identical conditions.
(5) (a) Meunier, B. Chem. ReV. 1992, 92, 1411-1456. (b) Mansuy, D. Coord.
Chem. ReV. 1993, 125, 129-141. (c) Groves, J. T. In Cytochrome P450:
Structure, Mechanism, and Biochemistry, 3rd ed.; Ortiz de Montellano,
P. R., Ed.; Kluwer Academic/Plenum Publishers: New York, 2005; pp
1-43.
(6) (a) Groves, J. T.; Lee, J.; Marla, S. S. J. Am. Chem. Soc. 1997, 119, 6269-
6273. (b) Jin, N.; Groves, J. G. J. Am. Chem. Soc. 1999, 121, 2923-2924.
(c) Jin, N.; Bourassa, J. L.; Tizio, S. C.; Groves, J. T. Angew. Chem., Int.
Ed. 2000, 39, 3849-3851.
(7) Nam, W.; Kim, I.; Lim, M. H.; Choi, H. J.; Lee, J. S.; Jang, H. G. Chem.
Eur. J. 2002, 8, 2067-2071.
(8) (a) Zhang, R.; Newcomb, M. J. Am. Chem. Soc. 2003, 125, 12418-12419.
(b) Zhang, R.; Horner, J. H.; Newcomb, M. J. Am. Chem. Soc. 2005, 127,
6573-6582.
(9) Shimazaki, Y.; Nagano, T.; Takesue, H.; Ye, B.-H.; Tani, F.; Naruta, Y.
Angew. Chem., Int. Ed. 2004, 43, 98-100.
(10) (a) Mandimutsira, B. S.; Ramdhanie, B.; Todd, R. C.; Wang, H.; Zareba,
A. A.; Czernuszewicz, R. S.; Goldberg, D. P. J. Am. Chem. Soc. 2002,
124, 15170-15171. (b) Wang, S. H.; Mandimutsira, B. S.; Todd, R.;
Ramdhanie, B.; Fox, J. P.; Goldberg, D. P. J. Am. Chem. Soc. 2004, 126,
18-19. (c) Lansky, D. E.; Mandimutsira, B.; Ramdhanie, B.; Clause´n, M.;
Penner-Hahn, J.; Zvyagin, S. A.; Telser, J.; Krzystek, J.; Zhan, R.; Ou, Z.;
Kadish, K. M.; Zakharov, L.; Rheingold, A. L.; Goldberg, D. P. Inorg.
Chem. 2005, 44, 4485-4498.
The manganese(V)-oxo porphyrin complex 1a was further
characterized by EPR, 1H and 19F NMR, resonance Raman, and
X-ray absorption spectroscopy. The X-band EPR of 1a shows
1
no signal (data not shown).6b,7,9 The H NMR spectrum of 1a
(11) (a) Golubkov, G.; Bendix, J.; Gray, H. B.; Mahammed, A.; Goldberg, I.;
Dibilio, A. J.; Gross, Z. Angew. Chem., Int. Ed. 2001, 40, 2132-2134. (b)
Gross, Z.; Golubkov, G.; Simkhovich, L. Angew. Chem., Int. Ed. 2000,
39, 4045-4047.
(12) Liu, H.-Y.; Lai, T.-S.; Yeung, L.-L.; Chang, C. K. Org. Lett. 2003, 5, 617-
620.
in CD3CN displays sharp 1H resonances in the normal aromatic
region (δ 7-9 ppm) (Figure 2b). The â-pyrrole proton appears
at 8.7 ppm as a sharp singlet, and the meta and para protons of
the phenyl group appear at 7.9 and 8.1 ppm with an integral
(13) Zhang, R.; Harischandra, D. N.; Newcomb, M. Chem. Eur. J. 2005, 11,
5713-5720.
1
ratio of 2:1, respectively. Similarly, the H NMR spectrum of
(14) (a) Miller, C. G.; Gordon-Wylie, S. W.; Horwitz, C. P.; Strazisar, S. A.;
Peraino, D. K.; Clark, G. R.; Weintraub, S. T.; Collins, T. J. J. Am. Chem.
Soc. 1998, 120, 11540-11541. (b) Workman, J. M.; Powell, R. D.; Procyk,
A. D.; Collins, T. J.; Bocian, D. F. Inorg. Chem. 1992, 31, 1548-1550.
(c) Collins, T. J.; Powell, R. D.; Slebodnick, C.; Uffelman, E. S. J. Am.
Chem. Soc. 1990, 112, 899-901. (d) Collins, T. J.; Gordon-Wylie, S. W.
J. Am. Chem. Soc. 1989, 111, 4511-4513.
[(TDFPP)MnVdO]+ exhibits sharp signals for the â-pyrrole
(16) Abbreviations: TDCPP, meso-tetrakis(2,6-dichlorophenyl)porphinato
dianion; TDFPP, meso-tetrakis(2,6-difluorophenyl)porphinato dianion;
TPFPP, meso-tetrakis(pentafluorophenyl)porphinato dianion; TMP, meso-
tetramesitylporphinato dianion; TDMPP, meso-tetrakis(2,6-dimethylphenyl)-
porphinato dianion; TM-2-PyP, meso-tetrakis(N-methyl-2-pyridyl)porphi-
nato dianion.
(15) MacDonnell, F. M.; Fackler, N. L. P.; Stern, C.; O’Halloran, T. V. J. Am.
Chem. Soc. 1994, 116, 7431-7432.
9
J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1269

A R T I C L E S
Song et al.
Figure 3. (a) Resonance Raman spectra of 1a-16O (blue line), 1a-18O (red
line), and the difference between 1a-16O and 1a-18O (black line). (b)
Resonance Raman spectra of [(TM-2-PyP)MnVd16O]5+ (red line), [(TM-
2-PyP)MnVd18O]5+ (blue line), and the difference between [(TM-2-PyP)-
MnVd16O]5+ and [(TM-2-PyP)MnVd18O]5+ (black line). See Experimental
Section for detailed reaction procedures.
Figure 2. (a) UV-vis spectra of Mn(TDCPP)Cl (0.15 mM) (red dashed
line), Mn(TDCPP)Cl (0.15 mM) in the presence of TBAH (3 mM) (blue
dotted line), and 1a (0.15 mM) (black solid line). Inset shows a magnification
of the Q-band region of the manganese porphyrins (1.5 mM). (b) 1H NMR
spectrum of 1a. 1a was prepared by reacting Mn(TDCPP)Cl (1.5 mM) with
H2O2 (3 mM) in the presence of TBAH (30 mM) in CD3CN at ambient
temperature. Chemical shifts (in ppm) were referenced to TMS (δ ) 0.0
ppm).
character between the manganese(V) ion and the oxygen atom
(vide infra). Resonance Raman spectra were also obtained for
[(TDFPP)MnVdO]+ and [(TPFPP)MnVdO]+. These data ex-
hibit νMndO frequencies of 755 and 753 cm-1, respectively,
indicating that the MnVdO stretching frequency is not sensitive
to the porphyrin ligand environment.17 It is worth noting that
the observed MnVdO stretching frequency of 1a is lower than
Table 1. UV-Vis Absorption Bands and Rate Constants for the
Natural Decay of [(Porp)MnVdO]+ Complexes
absorption bands
Soret band
(nm), log
Q-bands
rate of natural
1
a
(Porp)MnVd
O
ꢀ
(nm), log
ꢀ
decay, kobs (s-
)
[(TPFPP)MnVdO]+
[(TDFPP)MnVdO]+
[(TDCPP)MnVdO]+ (1a)
[(TDMPP)MnVdO]+
434, 5.1(3)
436, 5.1(6)
444, 5.1(6)
438, 5.1(5)
553, 4.2(3)
552, 4.2(6)
560, 4.1(4)
562, 4.0(6)
599, 3.8(3)
561, 4.0(4)
598, 3.7(4)
1.3(2) × 10-4
1.9(3) × 10-4
4.6(4) × 10-4
8.5(6) × 10-4
that of dinuclear (OH)Mn(V)dO species (νMndO ) 791 cm-1 9
)
but similar to that of MnIV(TMP)(O) (νMndO ) 754 cm-1).18 In
addition, when we prepared the manganese(V)-oxo porphyrin
complex, [(TM-2-PyP)MnVdO]5+ 16
in aqueous solution as
,
[(TMP)MnVdO]+
441, 5.1(3)
1.6(2) × 10-3
reported by Groves and co-workers6b,c and measured the
resonance Raman band of the Mn-O moiety, we observed a
strong, isotope-sensitive band at 729 cm-1, which shifted to 696
cm-1 when the manganese(V)-oxo porphyrin was generated
in H218O (Figure 3b; see Supporting Information, Figure S3 for
the UV-vis spectrum). A weak, isotope-sensitive band at 502
cm-1, which shifted to 475 cm-1 upon introduction of 18O, was
also detected in the resonance Raman spectrum (Figure 3b).
The bands at 729 and 502 cm-1 are assigned to ν(MnVdO)
and ν(MnVsOH), respectively.9 In contrast to the manganese-
(V)-oxo porphyrins, manganese(V)-oxo complexes bearing
non-porphyrinic macrocycles exhibit a Mn-O stretching band
at a higher frequency (e.g., ∼980 cm-1) in organic solvents,
which indicates triple-bond character between the Mn(V) ion
and the oxo group.10a,14b,c The triple-bond character of Mn-O
moieties in five-coordinate non-porphyrinic manganese(V)-oxo
complexes has been confirmed by short Mn-O bond lengths
a First-order rate constants, kobs, were determined by monitoring absor-
bance changes of Q-bands of [(Porp)MnVdO]+ (1.5 mM) in a 0.1-cm UV
cell at 35 °C.
proton at 8.9 ppm and the meta and para protons of the phenyl
group at 7.7 and 8.1 ppm with an integral ratio of 2:1,
respectively (Supporting Information, Figure S2a). Further, the
[(TDFPP)MnVdO]+ complex shows a sharp 19F NMR peak at
-109 ppm for the phenyl fluorine (Supporting Information,
Figure S2b).11b,12 Such well-resolved signals in the 1H and 19
F
NMR spectra demonstrate that 1a and other manganese(V)-
oxo complexes are diamagnetic low-spin d2 species.6b,10b,11b,12
Resonance Raman Analysis. The resonance Raman spec-
trum of 1a, measured in CH3CN at ambient temperature with
442-nm laser excitation, exhibited an isotope-sensitive band at
759 cm-1, which shifted to 724 cm-1 when 1a-18O was
generated with H218O2 or upon addition of H218O to the solution
of 1a-16O (vide infra) (Figure 3a).9 The observed isotopic shift
by 35 cm-1 upon 18O substitution is in good agreement with
the value calculated (∆νcalc ) -34 cm-1) from the Mn-O
diatomic harmonic oscillator. Further, the observed Mn-O
frequency at ∼759 cm-1 is consistent with double-bond
(17) It has been shown in iron porphyrin systems that the νFedO frequencies of
high-valent iron(IV)-oxo porphyrin complexes are not sensitive to the
overall oxidation state (i.e., ∆νFedO of [(Porp)FeIVdO] and [(Porp)+FeIVd
O]+ < 10 cm-1) and the electronic nature of porphyrin ligands of the iron-
oxo species: Kitagawa, T.; Mizutani, Y. Coord. Chem. ReV. 1994, 135/
136, 685-735.
(18) Czernuszewicz, R. S.; Su, Y. O.; Stern, M. K.; Macor, K. A.; Kim, D.;
Groves, J. T.; Spiro, T. G. J. Am. Chem. Soc. 1988, 110, 4158-4165.
9
1270 J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007

Manganese(V)−Oxo Porphyrin Complexes
A R T I C L E S
Figure 4. Comparison of the normalized Mn K-edge XAS data for the
MnIII(TDCPP)(OH) (blue), MnIV(TDCPP)(O) (purple), and [MnV(TDCPP)-
(O)]+ (red) complexes.
Figure 6. (a) Comparison of the EXAFS data (solid lines) and the fits to
the data (dashed lines) for MnIII(TDCPP)(OH) (blue), MnIV(TDCPP)(O)
(purple), and [MnV(TDCPP)(O)]+ (red). (b) The corresponding non-phase-
Figure 5. Comparison of the normalized Mn K-edge XAS data of the [MnV-
(TDCPP)(O)]+ complex (red) to a five-coordinate [MnV(HMPAB)(O)]-
complex (blue).
shift-corrected Fourier transforms of the MnIII(TDCPP)(OH) (blue), MnIV
(TDCPP)(O) (purple), and [MnV(TDCPP)(O)]+ (red) complexes.
-
(∼1.56 Å).10c,14a,c,d,15 However, we have observed in the present
study that the Mn-O stretching Raman bands of manganese-
(V)-oxo porphyrins are in the range of 760-790 cm-1 in
organic solvents. This indicates a longer, weaker Mn-O bond,
which suggests double-bond character.9 The longer, weaker
Mn-O bond (relative to known five-coordinate Mn(V)-oxo
complexes) has been further confirmed by X-ray absorption
spectroscopic studies, which suggest that the weakening of the
Mn-O bond is due to the presence of a sixth ligand trans to
the Mn-oxo bond (vide infra).
X-ray Absorption Spectroscopy (XAS)/Extended X-ray
Absorption Fine Structure (EXAFS) Results. A comparison
of the normalized Mn K-edge XAS data for the Mn(III)-, Mn-
(IV)-, and Mn(V)-TDCPP (1a) complexes is shown in Figure
4. The rising edge energy clearly increases across this series
(increasing by ∼2 eV upon oxidation of Mn(III) to Mn(IV),
and by an additional ∼1 eV on going to Mn(V)), consistent
with the increasing effective nuclear charge on the manganese.
Figure 5 shows a comparison of the normalized Mn K-edge
XAS data for 1a to those of the previously reported Na[MnV-
(HMPAB)(O)] (HMPAB ) 1,2-bis(2-hydroxy-2-methylpro-
panamido)benzene) complex.19 The rising edge positions are
essentially identical, which thus confirms a Mn(V) oxidation
state in the manganese porphyrin complex. However, the pre-
edges (at ∼6542 eV) of these two complexes are very different.
The dramatic decrease in intensity requires an increased
coordination number in 1a compared to the five-coordinate Na-
[MnV(HMPAB)(O)] complex.19 The presence of a strong trans
axial ligand would weaken the Mn(V)-oxo bond (thus lowering
4pz mixing) and would also shift the Mn more into the equatorial
plane (lowering 4px,py mixing), resulting in a more centrosym-
metric Mn center. All of these factors contribute to a reduced
electric dipole contribution and a weaker pre-edge intensity in
1a,20 compared to the previously reported five-coordinate Mn-
(V)-oxo complex.
The k3-weighted EXAFS data and fits for the Mn(III)-, Mn-
(IV)-, and Mn(V)-TDCPP complexes are shown in Figure
6a. A comparison of the corresponding Fourier transforms
(k ) 2-11 Å-1) is shown in Figure 6b. There are clear changes
in the overall beat pattern of the EXAFS data and in the Fourier
transforms upon oxidation. The best fits to the data are
summarized in Table 2. For the Mn(III) complex, the EXAFS
are best fit by inclusion of five Mn-N/O interactions at 2.01
Å, with additional outer-shell contributions from the porphyrin.
Attempts to add a sixth ligand resulted in a slightly poorer fit
(error increased from 0.45 to 0.54). Fits were also attempted in
(19) Weng, T.-C.; Hsieh, W.-Y.; Uffelman, E. S.; Gordon-Wylie, S. W.; Collins,
T. J.; Pecoraro, V. L.; Penner-Hahn, J. E. J. Am. Chem. Soc. 2004, 126,
8070-8071.
(20) Westre, T. E.; Kennepohl, P.; DeWitt, J. G.; Hedman, B.; Hodgson, K. O.;
Solomon, E. I. J. Am. Chem. Soc. 1997, 119, 6297-6314.
9
J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1271

A R T I C L E S
Song et al.
Table 2. EXAFS Fit Results
Mn(III)
R (Å)
Mn(IV)
R (Å)
Mn(V)
R (Å)
σ
2 (Å2)
σ
2 (Å2)
σ
2 (Å2)
5 Mn-N/O
2.01
0.0036
6 Mn-N/O
1.99
0.0044
2 Mn-O
4 Mn-N
8 Mn-C
16 Mn-C-N
4 Mn-C
16 Mn-C-N
∆E0
1.68
2.04
3.06
3.18
3.45
4.38
-0.56
0.60
0.0051
0.0023
0.0056
0.0080
0.0043
0.0068
8 Mn-C
16 Mn-C-N
4 Mn-C
16 Mn-C-N
∆E0
3.04
3.14
3.44
4.32
-4.61
0.45
0.0036
0.0100
0.0013
0.0079
8 Mn-C
16 Mn-C-N
4 Mn-C
16 Mn-C-N
∆E0
3.01
3.15
3.41
4.31
-0.92
0.44
0.0036
0.0073
0.0034
0.0071
errora
errora
errora
a Error is given by ∑[(øobsd - øcalcd)2 k 6]/∑[(øobsd2 k 6].
which the first shell was split into shorter and longer compo-
nents; however, this resulted in the two distances coalescing to
the same value. The EXAFS results are consistent with the Mn-
(III) complex having an axial hydroxide ligand.
ties with various spectroscopic techniques. The manganese(V)-
oxo porphyrins are diamagnetic low-spin (S ) 0) species, as
characterized by EPR and 1H and 19F NMR spectroscopies. The
Mn-O stretching frequency and the Mn-O bond length of 1a,
determined by resonance Raman and X-ray absorption spec-
troscopy, respectively, are both indicative of a longer, weaker
bond than has been observed in previously reported Mn(V)-
oxo complexes bearing non-porphyrinic ligands. This suggests
double-bond character between the manganese(V) ion and the
oxygen atom in porphyrin systems, as indicated by the weak
Mn K-pre-edge and the EXAFS data. The weaker Mn-O bond
may be attributed to a trans effect due to the presence of a sixth
ligand, whereas the previously reported non-porphyrin Mn(V)-
oxo complexes are all five-coordinate. More detailed investiga-
tions, including DFT calculations, are underway to understand
how the ligands of manganese(V)-oxo complexes influence
the manganese(V)-oxo bond orders.
Effect of Base on the Stability of Manganese(V)-Oxo
Porphyrins. It has been reported previously that the formation
of manganese(V)-oxo porphyrins is markedly influenced by
the pH of reaction solutions; the intermediates are generated at
high pH values in aqueous solution6,7 or in the presence of base
in organic solvents.9 We therefore investigated the base effect
on the generation of manganese(V)-oxo porphyrins by carrying
out reactions with Mn(TDCPP)Cl and m-CPBA in the presence
of different amounts of TBAH. When Mn(TDCPP)Cl was
reacted with m-CPBA in the absence of TBAH in a solvent
mixture of CH2Cl2 and CH3CN (1:1) at 25 °C, (TDCPP)MnIVd
O (2a) was generated (see Supporting Information, Figure S4
for the UV-vis and EPR spectra of 2a).6,7,22 In the presence of
10 equiv of TBAH, 1a was formed but quickly disappeared,
and the decay of 1a became slower with the increase of TBAH
concentration (Supporting Information, Figure S5). Moreover,
when 10 equiv of HClO4 was added to a solution of 1a which
was generated in the presence of 20 equiv of TBAH, the
intermediate immediately reverted back to the starting [MnIII-
(TDCPP)]+ complex. These results demonstrate that the role
of base is to stabilize manganese(V)-oxo porphyrins. Although
we do not know the exact role of base in increasing the stability
of manganese(V)-oxo species, the reactivity of manganese-
(V)-oxo complexes may be controlled by binding the hydroxide
ion as an axial ligand (i.e., axial ligand effect).23 The presence
of a sixth trans axial ligand in the Mn(V) complex is supported
by the XAS results (vide supra).
The Mn(IV) data are best fit by inclusion of six Mn-N/O
interactions at 1.99 Å, with additional outer-shell contributions
from the porphyrin ring. A fit that included only five Mn-N/O
interactions gave an essentially identical error value; however,
the increased coordination is supported by the decreased pre-
edge intensity relative to the Mn(III) complex. The first shell
could not be split into two components. A plausible trans axial
ligand bound to a Mn(IV) ion is m-chlorobenzoate (m-CBA),
derived from m-CPBA, which has been used to generate the
Mn(IV)-oxo porphyrin complex (vide infra).
The [MnV(TDCPP)(O)]+ complex (1a) is best fit by two
Mn-O interactions at 1.68 Å and four Mn-N interactions at
2.04 Å, with additional outer-shell contributions from the
porphyrin. Inclusion of only a single Mn-O at 1.68 Å results
in a significant increase in the error (from 0.60 to 0.99). The
larger Debye-Waller value on this component (as compared
to the four Mn-N interactions at 2.04 Å) may suggest that the
two trans axial Mn-O interactions are at slightly different
distances; however, their separation is beyond the resolution
limits of the data. The absence of the 1.68 Å Mn-O component
gives an error of 3.60 and a significant low-frequency compo-
nent in the residual. No similar short component can be fit in
the Mn(III) or Mn(IV) complexes.
The short Mn-O distances for the [MnV(TDCPP)(O)]+
complex (1a) are 0.13 Å longer than the Mn-O distances in
the five-coordinate Na[MnV(O)(HMPAB)]19 and other non-
porphyrinic Mn(V)-oxo complexes,10c,14a,c,d,15 which may be
attributed to a trans effect due to the presence of the sixth ligand.
Qualitatively, the increase in distance is consistent with the
change in Mn-O stretching frequency from the resonance
Raman data. However, quantitatively, Badger’s rule would
predict an even larger change in distance (1.80 Å based on the
change in frequency) than is observed.21 Density functional
theory (DFT) calculations on 1a, both with and without a trans
axial ligand, are currently in progress to understand the
correlation between the EXAFS-derived distances and the
experimental frequencies.
In summary, we have prepared manganese(V)-oxo porphy-
rins in organic solvents and characterized their physical proper-
(21) (a) Badger, R. M. J. Chem. Phys. 1934, 2, 128-131. (b) Herschbach,
D. R.; Laurie,V. W. J. Chem. Phys. 1961, 35, 458-463.
(22) (a) Groves, J. T.; Stern, M. K. J. Am. Chem. Soc. 1988, 110, 8628-8638.
(b) Arasasingham, R. D.; He, G.-X.; Bruice, T. C. J. Am. Chem. Soc. 1993,
115, 7985-7991. (c) Ayougou, K. Bill, E.; Charnock, J. M.; Garner,
C. D.; Mandon, D.; Trautwein, A. X.; Weiss, R.; Winkler, H. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 343-346.
(23) (a) Meunier, B.; Guilmet, E.; De, Carvalho, M.-E.; Poilblanc, R. J. Am.
Chem. Soc. 1984, 106, 6668-6676. (b) Battioni, P.; Renaud, J. P.; Bartoli,
J. F.; Reina-Artiles, M.; Fort, M.; Mansuy, D. J. Am. Chem. Soc. 1988,
110, 8462-8470. (c) Collman, J. P.; Tanaka, H.; Hembre, R. T.; Brauman,
J. I. J. Am. Chem. Soc. 1990, 112, 3689-3690.
9
1272 J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007

Manganese(V)−Oxo Porphyrin Complexes
A R T I C L E S
Scheme 1. Oxygen Exchange between Mn(V)-Oxo Species and
H218O
bearing an electron-rich porphyrin ligand is more stable than
one bearing an electron-deficient porphyrin.4d,24 Further, the
stability order of manganese(V)-oxo porphyrins in the presence
of base is opposite to the reactivity and stability order of [(Porp)-
MnVdO]+ complexes observed in the absence of base but
similar to that of (Porp)MnIVdO species.8b In the latter case,
the inverted stability order of manganese(IV)-oxo porphyrins
was correlated with ease of the disproportionation of (Porp)-
MnIVdO to [MnIII(Porp)]+ and [(Porp)MnVdO]+ species.8b,13
We then investigated the reactivity of 1a in oxygen-atom-
transfer reactions with various substrates, such as triphenylphos-
phine (PPh3), thioanisole (C6H5SCH3), cyclooctene, and cy-
clooctane at 35 °C. Upon addition of 20 equiv of PPh3 to the
solution of 1a at 35 °C, the intermediate reverted back to the
starting [MnIII(TDCPP)]+ complex immediately, and product
analysis of the resulting solution revealed that Ph3PO was
produced quantitatively. With thioanisole, 1a reverted back to
the starting complex, showing isosbestic points at 496 and 568
nm (Figure 7a). Pseudo-first-order fitting of the kinetic data
allowed us to determine the kobs value to be 7.5 × 10-3 s-1 at
35 °C. The pseudo-first-order rate constants increased propor-
tionally with thioanisole concentration, giving a second-order
rate constant of 2.6(8) × 10-2 M-1 s-1 (Figure 7b). When
pseudo-first-order rate constants were determined with various
para-substituted thioanisoles and plotted against σp, a good
correlation was observed, with Hammett F value of -0.65
(Figure 7c). The negative F value indicates the electrophilic
character of the oxo group of 1a in oxygen-atom-transfer
reactions.25 Further, by determining rate constants from 283 to
308 K, we were able to calculate activation parameters of ∆Hq
) 7(2) kcal mol-1 and ∆Sq ) -44(2) cal mol-1 K-1 for the
oxidation of p-NH2-thioanisole by 1a (Figure 7d).
When the reactivity of 1a was examined in olefin epoxidation
and alkane hydroxylation, the rate of the disappearance of 1a
was not affected by the addition of cyclooctene and cyclooctane
to reaction solutions. Further, product analyses revealed that
no oxygenated products were generated in these reactions,
indicating that 1a is not capable of oxygenating olefins and
alkanes under these conditions. These results are of interest since
it has been generally believed that manganese(V)-oxo porphy-
rin complexes are highly reactive and the sole reactive species
in the catalytic oxygenation of olefins and alkanes by manga-
nese(III) porphyrins and terminal oxidants.5 The low reactivity
of 1a observed in the present study may be ascribed to the
binding of an anionic axial ligand (i.e., OH-) that would serve
Figure 7. (a) UV-vis spectral changes of 1a (1.5 mM) upon addition of
100 equiv of thioanisole at 35 °C. Inset shows absorbance traces monitored
at 560 nm. (b) Plot of kobs against thioanisole concentration to determine a
second-order rate constant. (c) Hammett plot of log krel against σp of
thioanisoles in the reactions of 1a (1.5 mM) and para-X-substituted
thioanisoles (10 equiv to 1a) at 35 °C. (d) Plot of first-order-rate constants
against 1/T to determine activation parameters.
Reactivities of Manganese(V)-Oxo Porphyrins. We have
shown above that manganese(V)-oxo porphyrins are highly
stable in the presence of base; 1a decays slowly with a rate
constant of 4.6(4) × 10-4 s-1 at 35 °C (Table 1). This rate was
not dependent on the concentration of [(Porp)MnVdO]+ species.
However, the decay rate of manganese(V)-oxo porphyrins was
dependent on the nature of the porphyrin ligands; electron-rich
manganese porphyrins decayed faster than electron-deficient
manganese porphyrins (Table 1 lists kobs values for the decay
of manganese(V)-oxo porphyrins).6b It is of interest to note
that the stability order of manganese(V)-oxo porphyrins is
opposite to that of iron(IV)-oxo porphyrin π-cation radicals.
In iron porphyrins, an iron(IV)-oxo porphyrin π-cation radical
(24) Dolphin, D.; Traylor, T. G.; Xie, L. Y. Acc. Chem. Res. 1997, 30, 251-
259.
(25) (a) McPherson, L. D.; Drees, M.; Khan, S. I.; Strassner, T.; Abu-Omar,
M. M. Inorg. Chem. 2004, 43, 4036-4050. (b) Abu-Omar, M. M. Chem.
Commun. 2003, 2102-2111. (c) Sivasubramanian, V. K.; Ganesan, M.;
Rajagopal, S.; Ramaraj, R. J. Org. Chem. 2002, 67, 1506-1514.
(26) (a) Meunier, B.; Bernadou, J. Struct. Bonding. 2000, 97, 1-35. (b)
Bernadou, J.; Meunier, B. Chem. Commun. 1998, 2167-2173. (c) Bernadou,
J.; Fabiano, A.-S.; Robert, A.; Meunier, B. J. Am. Chem. Soc. 1994, 116,
9375-9376.
9
J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1273

A R T I C L E S
Song et al.
Table 3. Catalytic Oxygenation of Hydrocarbons by 1a and PhIOa
entry
substrate
product
yield (%)b,c
1
cyclohexene
cyclohexene oxide
cyclohexenol
24 ( 4
2 ( 1
cyclohexenone
cyclooctene oxide
cis-stilbene oxide
trans-stilbene oxide
benzaldehyde
trans-stilbene oxide
benzaldehyde
cyclohexanol
3 ( 1
2
3
cyclooctene
cis-stilbene
36 ( 4
27 ( 4
3 ( 1
2 ( 1
4
5
6
trans-stilbene
cyclohexane
cyclooctane
4 ( 1
2 ( 1
10 ( 3
9 ( 3
cyclohexanone
cyclooctanol
cyclooctanone
16 ( 4
13 ( 3
a Reactions were run at least in triplicate, and the data reported represent
the average of these reactions. See Experimental Section for detailed reaction
conditions. b Yields were determined on the basis of the amounts of PhIO
added. c No formation of oxygenated products was detected in the absence
of the manganese catalyst.
Figure 8. Plots of 18O (%) in Ph3PO against (a) incubation time and (b)
concentration of H218O for the oxygen exchange between 1a and H218O.
Reaction conditions: (a) 1a (1.5 mM) was incubated with H218O (1.4 M,
6 µL of 95% 18O-enriched) in the presence of TBAH (30 mM) in a solvent
mixture (0.4 mL) of CH2Cl2 and CH3CN (1:1) at 25 °C. (b) Incubation
time was 15 min with different amounts of H218O.
Figure 9. Plausible intermediates involved in the oxygenation of hydro-
carbons by [(Porp)MnVdO]+ and PhIO.
and alcohols were produced in the olefin epoxidation and alkane
hydroxylation, respectively (Table 3). In the epoxidation of
cyclohexene, cyclohexene oxide was produced predominantly,
with small amounts of allylic oxidation products such as
cyclohexenol and cyclohexenone (entry 1). In cis-stilbene
epoxidation, cis-stilbene oxide was the major product, with the
formation of small amounts of trans-stilbene oxide and ben-
zaldehyde (entry 3). This result indicates that the olefin
epoxidation is stereospecific, as reported in the epoxidation of
cis-stilbene by Mn(TDCPP)Cl and PhIO.28 Also, as observed
in the iron porphyrin-catalyzed epoxidation of trans-stilbene by
PhIO,29 only a small amount of trans-stilbene oxide was
produced in the epoxidation of trans-stilbene (entry 4). In the
hydroxylation of alkanes by 1a and PhIO, equimolar amounts
of alcohol and ketone products were produced (entries 5
and 6).
Since we have demonstrated above that 1a does not oxygenate
olefins and alkanes, the observation of the formation of
oxygenated products in the catalytic olefin epoxidation and
alkane hydroxylation by 1a and PhIO implies the generation of
an active oxidant that is different from 1a. Similar to the present
results, it has been reported previously that manganese(V)-
oxo complexes of corrolazine and corrole ligands are inactive,
but the Mn(V)-oxo complexes produce oxygenated products
in the oxidation of olefins and sulfides by PhIO.10a,11b Figure 9
depicts plausible oxidants that may be involved in the oxygen-
ation reactions by 1a and PhIO. Those are (Porp)MnV(O)(OIPh)
(3), [(Porp)MnVIdO]2+ (4), [(Porp)MnVdO]+ with a high-spin
Mn(V) state (5), and [(Porp)MnVdO(X)]+ bearing a different
axial ligand from 1a (6) (e.g., H2O). Structure 3, a PhIO-
manganese(V)-oxo adduct, has been frequently suggested as
an active oxidant in metal complex-catalyzed oxidation reactions
by PhIO, such as in the oxidation of sulfides by (corrolazine)-
to decrease the electrophilicity of the Mn-oxo complex toward
organic substrates (vide infra).
We have also investigated the oxygen exchange between the
oxo group of 1a and H218O by incubating 1a in the presence of
H218O and then adding PPh3 to the resulting solution.26,27 The
degree of 18O exchange was then determined by analyzing 16
O
and 18O percentages in Ph3PO product (Scheme 1). Figure 8a
shows that the amounts of 18O found in the Ph3PO product
increased with the incubation time of 1a (see Supporting
Information, Figure S6 for the analysis of 18O % in Ph3PO).
Figure 8b shows that the amounts of 18O incorporated into the
product increased proportionally with the amounts of H218O in
reaction solutions. The present results provide direct evidence
that manganese(V)-oxo porphyrins exchange their oxygen atom
with H218O.6a,7 Interestingly, we found that the rate of oxygen
exchange between manganese(V)-oxo porphyrins and H218O
was extremely slow under the reaction conditions; the calculated
kobs value of 4.9(2) × 10-2 s-1 determined in the presence of
base in organic solvents at 25 °C is much slower than the
estimated rate (kexchange ≈ 103 s-1) of oxo-aqua interchange in
an manganese(V)-oxo porphyrin in aqueous solution.6a
Catalytic Oxygenation of Olefins and Alkanes by Man-
ganese(V)-Oxo Porphyrins and PhIO. We have used man-
ganese(V)-oxo porphyrins as catalysts in olefin epoxidation
and alkane hydroxylation by PhIO. We first generated 1a by
reacting Mn(TDCPP)Cl with m-CPBA in the presence of 20
equiv of TBAH. Organic substrates were then added to the
reaction solution, followed by solid PhIO (20 equiv). While the
UV-vis spectrum of 1a was retained during the reaction,
product analysis of the reaction mixture revealed that epoxides
(27) (a) Lee, K. A.; Nam, W. J. Am. Chem. Soc. 1997, 119, 1916-1922. (b)
Seo, M. S.; In, J.-H.; Kim, S. O.; Oh, N. Y.; Hong, J.; Kim, J.; Que, L.,
Jr.; Nam, W. Angew. Chem., Int. Ed. 2004, 43, 2417-2420. (c) Song,
W. J.; Sun, Y. J.; Choi, S. K.; Nam, W. Chem. Eur. J. 2006, 12, 130-137.
(28) Park, S.-E.; Song, W. J.; Ryu, Y. O.; Lim, M. H.; Song, R.; Kim, K. M.;
Nam, W. J. Inorg. Biochem. 2005, 99, 424-431.
(29) Groves, J. T.; Nemo, T. E. J. Am. Chem. Soc. 1983, 105, 5786-5791.
9
1274 J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007

Manganese(V)−Oxo Porphyrin Complexes
A R T I C L E S
Mn(V)dO and PhIO.10b,30 Structure 4, a manganese(VI)-oxo
porphyrin complex, has not yet been identified, but Golubkov
and Gross recently reported the characterization of a (nitrido)-
Mn(VI) corrole complex.31 A high-spin Mn(V)-oxo complex
(5) has been proposed as an active oxidant on the basis of DFT
calculations on a (salen)Mn(V)-oxo intermediate in Jacobsen-
Katsuki epoxidation reactions.32 According to the DFT calcula-
tions, manganese(V)-oxo complexes having different spin states
(e.g., singlet, triplet, and quintet) show markedly different
reactivities in oxidation reactions. Further, Shaik and co-workers
conducted DFT calculations with oxoiron(IV) porphyrin π-cat-
ion radicals and proposed that the reactivity of the oxoiron(IV)
porphyrins is significantly affected by the spin states of the
intermediates (i.e., a low-spin doublet state and a high-spin
quartet state).1c,33 Finally, since the low reactivity of 1a may
be due to the binding of hydroxide as an axial ligand,
replacement of the axial hydroxide ligand upon addition of PhIO
in the catalytic oxygenation reactions could generate an
intermediate with high reactivity. Indeed, it has been well
documented that the presence of neutral nitrogen bases (e.g.,
imidazoles) in manganese porphyrin-catalyzed reactions in-
creases product yields dramatically.34 Therefore, structure 6 may
be a Mn(V)-oxo porphyrin complex bearing a different axial
ligand (e.g., H2O). At the present time, none of the proposed
species in Figure 9 has been identified, and intensive mechanistic
studies are needed to elucidate the exact nature of active oxi-
dant(s) in manganese complex-catalyzed oxidation reactions.
reactions;35 the intermediates are capable of oxygenating PPh3
and thioanisoles but not olefins and alkanes. Moreover, the rate
of oxygen exchange between the manganese(V)-oxo species
and H218O in the presence of base in organic solvents was found
to be very slow. These results are in contrast to previous
suggestions that manganese(V)-oxo porphyrins are invariably
highly reactive in oxygenation reactions and that the intermedi-
ates exchange their oxygen with H218O at a fast rate. We have
also reported that manganese(V)-oxo porphyrins associated
with terminal oxidants, such as PhIO, afforded high product
yields in the oxygenation of olefins and alkanes. Future studies
will focus on elucidating the effect(s) of base on the chemical
properties of manganese(V)-oxo species and the nature of
oxygenating intermediate(s) generated in the reactions of
manganese(V)-oxo porphyrins and terminal oxidants.
Experimental Section
Materials. Dichloromethane (anhydrous) and acetonitrile (anhy-
drous) were obtained from Aldrich Chemical Co. and purified by
distillation over CaH2 prior to use. All reagents purchased from Aldrich
were the best available purity and used without further purification
unless otherwise indicated. m-CPBA was purified by washing with
phosphate buffer (pH 7.4) followed by water and then dried under
reduced pressure. Iodosylarenes were prepared according to published
procedures.36 The purities of the oxidants were determined by iodo-
metric titration.37 H218O (95% 18O-enriched) and H218O2 (90% 18O-
enriched, 2% H218O2 in water) were purchased from ICON Services
Inc. (Summit, NJ). Mn(TDCPP)Cl, Mn(TDFPP)Cl, Mn(TPFPP)Cl, Mn-
(TMP)Cl, and Mn(TM-2-PyP)Cl5 were obtained from Mid-Century
Chemicals (Posen, IL). Mn(TDMPP)Cl was synthesized according to
published procedures.38
Conclusion
Manganese(V)-oxo species have been frequently invoked
as reactive species in the catalytic oxygenation of hydrocarbons
by manganese(III) porphyrins and terminal oxidants.5 In the
present study, we have prepared manganese(V)-oxo porphyrins
that are highly stable at room temperature in the presence of
base in organic solvents. The manganese(V)-oxo porphyrins
were characterized with various spectroscopic techniques and
found to be diamagnetic low-spin (S ) 0) species with a longer,
weaker Mn-O bond than that found in previously characterized
Mn(V)-oxo complexes. This is suggestive of double-bond
character between the manganese(V) ion and the oxygen atom
and originates from the presence of a sixth trans axial ligand.
The stability of the manganese(V)-oxo species was found to
depend on the concentration of base and the electronic nature
of porphyrin ligands, but not on the concentration of the
manganese(V)-oxo species. The low-spin manganese(V)-oxo
porphyrins showed a low reactivity in oxygen-atom-transfer
Instrumentation. UV-vis spectra were recorded on a Hewlett-
Packard 8453 spectrophotometer equipped with a circulating water bath.
EPR spectra were obtained on a JEOL JES-FA200 spectrometer at 4
K. 1H NMR spectra were measured with a Bruker DPX-250 spectrom-
eter, and chemical shifts were reported as δ values from standard solvent
peaks. 19F NMR spectrum was measured with a Varian Unity-Inova
500 MHz spectrometer. Product analyses for the oxidation of PPh3 and
the epoxidation of cis- and trans-stilbenes were performed on a
DIONEX Summit Pump Series P580 equipped with a variable-
wavelength UV-200 detector (HPLC). Products were separated on a
Waters Symmetry C18 reverse-phase column (4.6 × 250 mm), and
detection was made at 215 and 254 nm. Product analyses for the
oxidation of sulfides, the epoxidation of cyclohexene and cyclooctene,
and the hydroxylation of alkanes were performed on an Agilent
Technologies 6890N gas chromatograph equipped with a flame
ionization detector (GC) and a Hewlett-Packard 5890 II Plus gas
chromatograph interfaced with a Hewlett-Packard model 5989B mass
spectrometer (GC-MS). LC-ESI MS spectra for the determination of
18O percentage in Ph3PO in isotopically labeled H218O experiments were
collected on a Finnigan Surveyor Integrated HPLC system (PDA
detector and LC pump) connected with a Thermo Finnigan (San Jose,
CA) LCQ Advantage MAX quadrupole ion trap instrument. The
separation of product was achieved by on-column injection to a Hypersil
GOLD column (5 µm, 4.6 × 250 mm) using MeOH:H2O (3:1) as eluent
(30) (a) Nam, W.; Ryu, Y. O.; Song, W. J. J. Biol. Inorg. Chem. 2004, 9, 654-
660 and references therein. (b) Mahammed, A.; Gross, Z. J. Am. Chem.
Soc. 2005, 127, 2883-2887. (c) Collman, J. P.; Zeng, L.; Brauman, J. I.
Inorg. Chem. 2004, 43, 2672-2679. (d) Collman, J. P.; Zeng, L.; Decre´au,
R. A. Chem. Commun. 2003, 2974-2975. (e) Nam, W.; Jin, S. W.; Lim,
M. H.; Ryu, J. Y.; Kim, C. Inorg. Chem. 2002, 41, 3647-3652.
(31) Golubkov, G.; Gross, Z. J. Am. Chem. Soc. 2005, 127, 3258-3259.
(32) (a) Strassner, T.; Houk, K. N. Org. Lett. 1999, 1, 419-421. (b) Cavallo,
L.; Jacobsen, H. Eur. J. Inorg. Chem. 2003, 892-902. (c) Abashkin,
Y. G.; Burt, S. K. Org. Lett. 2004, 6, 59-62.
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2005, 105, 2279-2328. (b) Shaik, S.; de Visser, S. P.; Kumar, D. J. Biol.
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Chang, C. K. Chem. Commun. 2003, 620-621. (b) Collman, J. P.; Chien,
A. S.; Eberspacher, T. A.; Zhong, M.; Brauman, J. I. Inorg. Chem. 2000,
39, 4625-4629. (c) Battioni, P.; Renaud, J. P.; Bartoli, J. F.; Reina-Artiles,
M.; Fort, M.; Mansuy, D. J. Am. Chem. Soc. 1988, 110, 8462-8470.
(d) Meunier, B.; Guilmet, E.; De, Carvalho, M.-E.; Poilblanc, R. J. Am.
Chem. Soc. 1984, 106, 6668-6676.
(35) It has been suggested from DFT calculations that the reactivity of a low-
spin Mn(V)-oxo porphyrin complex is low: de Visser, S. P.; Ogliaro, F.;
Gross, Z.; Shaik, S. Chem. Eur. J. 2001, 7, 4954-4960.
(36) Saltzman, H.; Sharefkin, J. G. Organic Syntheses; Wiley: New York, 1973;
Collect. Vol. V, p 658.
(37) Lucas, H. J.; Kennedy, E. R.; Formo, M. W. Organic Syntheses; Wiley:
New York, 1955; Collect. Vol. III, p 483.
(38) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828-836.
9
J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1275

A R T I C L E S
Song et al.
at a flow rate of 1 mL/min, at a spray voltage 4.7 kV and a capillary
temperature at 220 °C.
of the data was achieved by subtracting the spline and normalizing the
post-edge region to 1. The resultant EXAFS was k
the impact of high-k data.
3-weighted to enhance
Preparation of Manganese(V)- and Manganese(IV)-Oxo Com-
plexes. In general, 1 (0.15 mM) was prepared by adding 2 equiv of
m-CPBA (0.3 mM, diluted in 50 µL of CH3CN) into a 0.1-cm UV
cuvette containing a manganese(III) porphyrin chloride (0.15 mM) and
TBAH (3.0 mM) in a solvent mixture (0.5 mL) of CH3CN and CH2Cl2
(1:1) at 25 °C. 2 was generated by reacting Mn(TDCPP)Cl with 2 equiv
of m-CPBA in a solvent mixture (0.5 mL) of CH3CN and CH2Cl2
(1:1) at 25 °C. The formation of 1 and 2 was monitored by a UV-vis
spectrophotometer, and the resulting solution was used immediately
for further studies.
Theoretical EXAFS signals ø(k) were calculated using FEFF (version
7.0)40 and fit to the data using EXAFSPAK.41 The non-structural
parameter E0 was also allowed to vary but was restricted to a common
value for every component in a given fit. The structural parameters
varied during the refinements were the bond distance (R) and the bond
variance (σ2). σ2 is related to the Debye-Waller factor, which is a
measure of thermal vibration and static disorder of the absorbers/
scatters. Coordination numbers were systematically varied in the course
of the analysis, but they were not allowed to vary within a given fit.
Single scattering paths and the corresponding multiple scattering paths
were linked during the course of refinements.
Reactions of [(TDCPP)MnVdO]+ (1a) with Organic Substrates.
All reactions were run in a 0.1-cm UV cuvette by monitoring UV-vis
spectral changes of reaction solutions. 1a was prepared by reacting
MnIII(TDCPP)Cl (1.5 mM) with 2 equiv of m-CPBA (3 mM) in the
presence of TBAH (30 mM) in a solvent mixture (0.4 mL) of CH2-
Cl2/CH3CN (1:1) at 35 °C. Substrate (0.15 M) was then added to the
solution of 1a. After the completion of the reaction, the reaction mixture
was directly analyzed by HPLC, GC, and/or GC-MS. Product yields
were determined by comparison against standard curves prepared with
known authentic samples.
Resonance Raman Measurements. Samples for resonance Raman
were prepared as follows: 1a-16O was prepared by reacting Mn-
(TDCPP)Cl (0.4 mM) with m-CPBA (0.8 mM) in the presence of
TBAH (8 mM) in CH3CN (0.5 mL) at 10 °C, whereas 1a-18O was
prepared by reacting Mn(TDCPP)Cl (0.4 mM) with H218O2 (0.8 mM)
or exchanging the oxygen atom of 1a-16O with H218O (15 µL of 95%
18O-enriched). The [(TM-2-PyP)MnVd16O]5+ and [(TM-2-PyP)MnVd
18O]5+ intermediates were prepared by reacting [MnIII(TM-2-PyP)]5+
(0.4 mM) with PhI16O and PhI18O (0.8 mM, diluted in 10 µL of CH3-
OH) in the presence of TBAH (8 mM) in H216O (0.5 mL) and H218O
(0.5 mL), respectively, at 10 °C. The absence of Mn(IV)-oxo species
was confirmed by taking the UV-vis spectra of resonance Raman
samples. The samples were transferred in a quartz spinning cell pre-
cooled at 10 °C. Resonance Raman spectra were obtained using a liquid-
nitrogen-cooled CCD detector (model LN/CCD-1100-PB, Roper Sci-
entific) attached to a 1-m single polychromator (model MC-100DG,
Ritsu Oyo Kogaku). An excitation wavelength of 441.6 nm was
provided by a He-Cd laser (model CD4805R, Kinmon Electric), with
4 mW power at the samples. All measurements were carried out with
a quartz spinning cell (1000 rpm) at ∼10 °C. Raman shifts were
calibrated with indene, and the accuracy of the peak positions of the
The labeled water (H218O) experiments for oxygen exchange between
1a and H218O were carried out as follows: 1a was prepared as described
above. Appropriate amounts of H218O (95% 18O-enriched) were then
added to the solution of 1a, followed by incubation of the resulting
solution for the given time. After addition of PPh3 (0.15 M) to the
solution of 1a, the 18O percentage in the Ph3PO product was determined
by analyzing reaction solutions with LC-ESI MS, and the 16O and 18
O
compositions in the Ph3PO product were analyzed by the relative
abundances of m/z ) 279.2 for [Ph3P16O + H+]+ and m/z ) 281.2 for
[Ph3P18O + H+]+.
Raman bands was (1 cm-1
.
Catalytic Oxygenation of Organic Substrates by 1a and PhIO.
1a was prepared by reacting MnIII(TDCPP)Cl (1.5 mM) with 2 equiv
of m-CPBA (3 mM) in the presence of TBAH (30 mM) in a solvent
mixture (0.5 mL) of CH2Cl2/CH3CN (1:1) at 25 °C. Substrate (0.15
M, diluted in CH2Cl2/CH3CN (1:1)) was added to the reaction solution,
followed by the addition of solid PhIO (20 equiv to the catalyst). For
comparison, control experiments were carried out in the absence of
the manganese catalyst in the oxidation of cyclohexene and cyclohex-
ane. After the solid PhIO disappeared completely (ca. 2 h), the resulting
solution was directly analyzed by GC, HPLC, and/or GC-MS. Product
yields were determined by comparison against standard curves prepared
with known authentic samples.
XAS Data Collection and Analysis. XAS data for MnIII(TDCPP)-
(OH), MnIV(TDCPP)(O), and [MnV(TDCPP)(O)]+ (1a) were recorded
at the Stanford Synchrotron Radiation Laboratory (SSRL) on focused
beam line 9-3, under ring conditions of 3 GeV and 80-100 mA. A
Si(220) monochromator (fully tuned) was used for energy selection. A
Rh-coated mirror (set to a cutoff of 10 keV) was used for harmonic
rejection. All XAS samples (∼2 mM) were measured as solutions in
CH3CN. Samples were loaded into 2-mm Delrin XAS cells with Kapton
windows and then frozen immediately in liquid nitrogen prior to XAS
measurements. During XAS measurements, samples were maintained
at a constant temperature of 10 K by an Oxford Instruments CF1208
continuous-flow liquid-helium cryostat.
Data were measured in fluorescence mode using a Canberra Ge 30-
element array detector. XAS data were measured to k ) 11 Å-1 due to
contributions from both Fe contamination and diffraction (resulting from
a poor glass). Internal energy calibration was performed by simultaneous
measurement of the absorption of a Mn foil placed between the second
and third ionization chambers. The first inflection point of the Mn foil
was assigned to 6539.0 eV. Samples were monitored for photoreduction
throughout the course of data collection. Only those scans which showed
no evidence of photoreduction were used in the final average. The data
represent eight, six, and six scan averages for the MnIII(TDCPP)(OH),
MnIV(TDCPP)(O), and [MnV(TDCPP)(O)]+ complexes, respectively.
Acknowledgment. This research was supported by the
Ministry of Science and Technology of Korea through Creative
Research Initiative Program (to W.N.), the Korea Research
Foundation (KRF-2005-217-C00006 to M.S.S.), the BK 21
Program (to M.-J.K.), Grant-in-Aid from the Ministry of
Education, Culture, Sports, Science and Technology, Japan
(14001004 to T.K.), and a JSPS Research Fellowship for Young
Scientists (to T.O. and T.T.). SSRL operations are funded by
the U.S. Department of Energy, Office of Basic Energy
Sciences. The Structural Molecular Biology program is sup-
ported by the National Institutes of Health, National Center for
Research Resources, Biomedical Technology Program, and by
The averaged data were processed as described previously39 by fitting
a second-order polynomial to the post-edge region and subtracting this
background from the entire spectrum. A three-region cubic spline was
used to model the smooth background above the edge. Normalization
(40) (a) Rehr, J. J.; Mustre de Leon, J.; Zabinsky, S. I.; Albers, R. C. J. Am.
Chem. Soc. 1991, 113, 5135-5140. (b) Mustre de Leon, J.; Rehr, J. J.;
Zabinsky, S. I.; Albers, R. C. Phys. ReV. B 1991, 44, 4146-4156.
(41) George, G. N. EXAFSPAK & EDG_FIT, 2000, Stanford Synchrotron
Radiation Laboratory, Stanford Linear Accelerator Center, Stanford
University, Stanford, CA 94309.
(39) DeWitt, J. G.; Bentsen, J. G.; Rosenzweig, A. C.; Hedman, B.; Green, J.;
Pilkington, S.; Papaefthymiou, G. C.; Dalton, H.; Hodgson, K. O.; Lippard,
S. J. J. Am. Chem. Soc. 1991, 113, 9219-9235.
9
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Manganese(V)−Oxo Porphyrin Complexes
A R T I C L E S
the Department of Energy, Office of Biological and Environ-
mental Research. We thank Prof. James E. Penner-Hahn
(University of Michigan) for providing the Mn K-edge data of
Na[MnV(HMPAB)(O)].
of (TM-2-PyP)MnVdO (Figure S3), UV-vis and EPR spectra
of (TDCPP)MnIVdO (2a) (Figure S4), time traces for the natural
decay of 1a in the presence of different amounts of base (Figure
S5), and LC-ESI MS of PPh3O obtained in isotope labeling
experiment (Figure S6). This material is available free of charge
via the Internet at http://pubs.acs.org.
Supporting Information Available: UV-vis spectra of
1
[(Porp)MnVdO]+ complexes (Figure S1), H and 19F NMR
spectra of [(TDFPP)MnVdO]+ (Figure S2), UV-vis spectrum
JA066460V
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J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1277