Communications
À1
103)) prepared under similar experimental conditions (Fig-
pound 1, the n(O O) stretching is 2 cm lower and n(Fe O)
is 5 cmÀ1 higher than that of the imidazole-free side-on peroxo
[(tmp)FeIII(O22À)]. This result may be an indication that
À
À
ure S1 in the Supporting Information). The EPR spectrum of
complex 1 (Figure 3a) has a typical rhombic marker signal at
g ꢀ 4.2, which is similar to that of [(tmp)FeIII(O22À)] and is
consistent with a h2-peroxo–heme species. Complex 1 was
then further characterized by rR spectroscopy. The rR spectra
of complex 1 show two groups of isotope shifts (Figure 2).
À
binding of the axial imidazole ligand to iron weakens the O
O bond by increasing the p*-orbital electron density. The Fe
À
O bond is strengthened by back-donation from the oxygen to
the iron atom and by electron donation of the imidazole
ligand to the oxygen p* orbital through the trans effect. The
implication of association of the imidazole with the iron
porphyrin is further supported by the following observation:
in the presence of the coordinating solvent DMSO, the side-
on peroxo compound [(dmso)(tmp)FeIII(O22À)] exhibited
dramatically increased nucleophilicity relative to the parent
compound [(tmp)FeIII(O22À)], as a result of the axial associ-
One band appears at 807 cmÀ1 and shifts to 758 cmÀ1 upon 18
O
À
substitution. This band is assignable to the n(O O) stretching
vibration of a peroxo species. The other band at 475 cmÀ1
18
(16O) and 455 cmÀ1 ( O) is assigned to the n(Fe O) stretching
À
À
vibration. The relatively lower frequency of n(Fe O) is
similar to that of the nonheme h2-peroxo complexes.[15] For
À
À
comparison, the n(O O) and n(Fe O) stretching vibrations
of the side-on compound [(tmp)FeIII(O22À)] appear at
809 cmÀ1 and 470 cmÀ1, respectively (Figure S2 in the Sup-
porting Information). These results suggest that, for com-
ation of DMSO to the iron porphyrin.[12c–d] The rR spectrum
of [(dmso)(tmp)FeIII(O2 )] has n(O O) and n(Fe O) bands
at 807 cmÀ1 and 476 cmÀ1, respectively (Figure S3 in the
Supporting Information). These bands are essentially the
2À
À
À
À
same as those of complex 1. The observed n(O O) value is
comparable to that of similar side-on heme peroxide obtained
by IR spectroscopy.[12f] These results may suggest that
complex 1 is a seven-coordinate (7c) side-on peroxide species.
To the best of our knowledge, this is the first reliable rR
evidence for this type of side-on heme peroxides.[12f]
The most striking difference between complex 1 and
[(tmp)FeIII(O22À)], (7c versus 6c side-on peroxo), comes from
their reactivity toward protonation. Addition of methanol
(400 equivalents) to a solution of complex 1 in MeCN/THF
(20:80) at À658C afforded a new species, complex 2. The
electronic absorption spectrum of complex 1 underwent
distinct spectral changes upon addition of methanol, with a
shift in the Soret band from 440 nm to 428 nm and a split in
the Q bands to 535, 562, and 609 nm (Figure 1). The UV/Vis
features are similar to those of the previously reported
hydroperoxo–heme model compounds such as [(tmp)FeIII-
( OH)( OOH)] (lmax = 428, 563, 601 nm).[13a] The EPR
experiments further confirmed this transformation. Protona-
tion of complex 1 caused the disappearance of the signal at
g ꢀ 4.2 and a new set of signals appeared at g = 2.31, 2.19, and
1.95 (Figure 3b). This result clearly indicates the signature of
a low-spin ferric heme species in a strong field with small
g dispersion. This set of g values corresponds very well with
those of the previously reported hydroperoxo–heme model
À
À
Figure 2. Resonance Raman spectra of complex 1 containing a) 16
O
and b) 18O. c) Difference spectrum of (a)À(b). lex =441.6 nm, 77 K.
III
[13a]
À
compound [(Im)(tmp)Fe ( OOH)] (g = 2.32, 2.19, 1.94),
and is similar to the g values of the end-on ferric hydro-
peroxo–heme intermediate generated in enzymes using
cryoradiolytic methods. For example, the g values of the
ferric hydroperoxo–heme intermediate of hemoglobin are
2.31, 2.18, and 1.94.[6] The formation of MeOH/MeOÀ-bound
low-spin ferric species is ruled out by these observed g values,
as can be seen from the EPR spectrum of the room-
temperature decomposed product, which shows a wide span
of g values (Figure 3c). On the other hand, for the six-
coordinate (6c) side-on peroxide [(tmp)FeIII(O22À)], protona-
tion under similar experimental conditions produces no new
intermediates and leads directly to decomposition of the
peroxide (Figure S4 in the Supporting Information). Thus,
protonation of a 7c side-on peroxide switches the closed form
to an end-on hydroperoxide. These results indicate that axial
Figure 3. EPR spectra of a) complex 1, b) 2, and c) the room-temper-
ature decomposition product of 2, 77 K. The signal labeled with an
asterisk originates from residual O2 in the solution.
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 9262 –9267