One-electron oxidized product of difluoroiron(iii) porphyrin: Is it iron(iv) porphyrin or iron(iii) porphyrin π-cation radical?

Article abstract of DOI:10.1039/c1dt10561b

The electronic structure of [Fe(TMP)F₂], which is formally a one-electron oxidation equivalent above [Fe^III(TMP)F ₂]^-, has been examined in solution by ¹H NMR, UV-Vis, and Moessbauer spectroscopy. In CD₂Cl₂-CD ₃OD solution at 193 K, the pyrrole-H and m-H signals appeared at 128.2 and 116.7 ppm, respectively. The UV-Vis spectrum showed broad absorption bands at 560-680 nm. The Moessbauer spectrum taken in frozen toluene-methanol solution exhibited a very broad single line from which the IS and QS values were determined by computer simulation to be 0.50 and 0.14 mm s^-1, respectively. On the basis of these results, it was concluded that the one-electron oxidized product of [Fe(TMP)F₂]^- should be formulated as the iron(iii) radical cation [Fe^III(TMP) F₂], not as iron(iv) porphyrin [Fe^IV(TMP)F₂] as previously suggested.

Full text of DOI:10.1039/c1dt10561b

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PAPER
One-electron oxidized product of difluoroiron(III) porphyrin: is it iron(IV)
porphyrin or iron(III) porphyrin p-cation radical?†
Akira Ikezaki,a,b Masashi Takahashic and Mikio Nakamura*a,b,d
Received 1st April 2011, Accepted 28th June 2011
DOI: 10.1039/c1dt10561b
The electronic structure of [Fe(TMP)F2], which is formally a one-electron oxidation equivalent above
[FeIII(TMP)F2]-, has been examined in solution by 1H NMR, UV-Vis, and Mo¨ssbauer spectroscopy. In
CD2Cl2–CD3OD solution at 193 K, the pyrrole-H and m-H signals appeared at 128.2 and 116.7 ppm,
respectively. The UV-Vis spectrum showed broad absorption bands at 560–680 nm. The Mo¨ssbauer
spectrum taken in frozen toluene–methanol solution exhibited a very broad single line from which the
IS and QS values were determined by computer simulation to be 0.50 and 0.14 mm s-1, respectively. On
the basis of these results, it was concluded that the one-electron oxidized product of [Fe(TMP)F2]-
should be formulated as the iron(III) radical cation [FeIII(TMP∑)F2], not as iron(IV) porphyrin
[FeIV(TMP)F2] as previously suggested.
electron oxidized species was unsuccessful due to the instability
Introduction
of the product.21,22 Density functional studies also predicted the
formation of [FeIV(TPP)F2] rather than [FeIII(TPP∑)F2].23 Recently,
High-valent iron porphyrin complexes are key reactive intermedi-
ates in heme enzymes such as Cytochrome P450 and peroxidase.1–3
Ghosh and Taylor reported the opposite result on the basis
Revealing the nature of the reactive intermediates has been an
of the more accurate and sophisticated CASPT2 calculations;
[FeIII(TPP∑)F2] is 0.31 eV more stable than [FeIV(TPP)F2].24 More
issue of great interest. Thus, extensive studies have been done
on the formation and characterization of various one- and two-
recently, Panchmatia et al. reached the same conclusion by using
electron oxidized products of iron(III) porphyrins.4,5 In principle,
one-electron oxidation of iron(III) porphyrin produces either
iron(IV) porphyrin or the iron(III) porphyrin radical cation.6–12
Actually however, iron(IV) porphyrins are rarely obtained except
for the complexes having an FeIV O unit.11,12 A unique example
is [FeIV(TMP)(OMe)2] reported by Groves et al. in 1985.13,14
The factors affecting the iron(IV) states have been discussed
theoretically by Ghosh et al.15,16 Finding another example of
an iron(IV) complex is quite important to reveal the general
conditions that stabilize the iron(IV) porphyrin relative to the
iron(III) porphyrin radical cation.17–19 Among various axial ligands,
fluoride should be the most plausible candidate because it is
classified as a strong field ligand as methoxide.20 In fact, the
oxidation potential of [FeIII(TPP)F2]- is quite unusual because the
first oxidation wave is observed at +0.68 V, which is ca. 0.4 V less
anodic than that of a large number of high-spin five-coordinate
[FeIII(TPP)X] compounds.21 However, the isolation of the one-
DFT+U and UB3LYP techniques.25 In this paper, we will describe
the electronic structure of [Fe(Por)F2], which corresponds to the
one-electron oxidized product of [FeIII(Por)F2]-, not only to answer
the controversial issue but also to seek for the general conditions
to obtain iron(IV) porphyrin complexes.
Results and discussion
1H NMR spectroscopy
To obtain the complex that is the one-electron oxidation equivalent
above [FeIII(TMP)F2]-, the methanol-d4 (CD3OD) solution of
tetrabutylammonium fluoride(Bu4NF) was added at 193 K to the
CD2Cl2 solution of [Fe(TMP∑)(ClO4)2] in an NMR sample tube as
shown in Scheme 1. The starting complex, [FeIII(TMP∑)(ClO4)2],
aDepartment of Chemistry, School of Medicine, Toho University, Ota-ku,
Tokyo, 143-8540, Japan. E-mail: mnakamu@med.toho-u.ac.jp
bResearch Centre for Materials with Integrated Properties, Toho University,
Funabashi, 274-8510, Japan
cDepartment of Chemistry, Faculty of Science, Toho University, Funabashi,
274-8510, Japan
dDivision of Chemistry, Graduate School of Science, Toho University,
Funabashi, 274-8510, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1dt10561b
Scheme 1 Formation of the complex that is the one-electron oxidation
equivalent above [FeIII(TMP)F2]-.
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should be partially converted to [FeIII(TMP∑)(CD3OD)2](ClO4)2
and/or [FeIII(TMP∑)(ClO4)(CD3OD)](ClO4) in CD2Cl2–CD3OD
solution. In fact, the pyrrole-H signal appeared at ca. 100 ppm at
195 K, which clearly indicates that the complex adopts the S = 5/2
spin state; [FeIII(TMP∑)(ClO4)2] shows the pyrrole-H signal at -22.5
ppm since the complex adopts the mixed S = 3/2, 5/2 spin state.6,13
The o-CH3, m-H, and p-CH3 signals were observed extremely
downfield at 41, 121, and 27 ppm, respectively, in CD2Cl2–CD3OD
solution, suggesting that the porphyrin has a half-occupied a2u
orbital.13
Fig. 1 shows the 1H NMR spectra taken after the addition of (a)
1.0, (b) 1.5, and (c) 2.0 equiv of Bu4NF. When 1.0 equiv of Bu4NF
was added, the starting complex was completely converted to the
new complex signified as A as shown in Fig. 1(a). The signal at
112.8 ppm was assigned to the pyrrole-H of A by the spectral
comparison with the pyrrole-d8 complex shown in Fig. 1(a-1).
The presence of the downfield shifted pyrrole-H signal indicates
that A adopts the high-spin (S = 5/2) state. The mesityl signals
shifted upfield but they still appeared at extremely downfield
positions; the o-CH3, m-H, and p-CH3 signals were observed at
31.4 (average), 89.3 (average), and 18.7 ppm, respectively. Thus,
the radical cationic state is maintained even after the addition of
1.0 equiv of Bu4NF. Close inspection of the spectrum revealed
that the o-CH3 gave two very broad signals as shown in Fig. 1(a-
2). The results suggest that A is the mono-F complex formulated
either by five-coordinate [FeIII(TMP∑)F]ClO4 or by six-coordinate
[FeIII(TMP∑)F(CH3OH)]ClO4 and/or [FeIII(TMP∑)F(ClO4)]. By
the further addition of Bu4NF, the signals of A were gradually
replaced by those of the new complex signified as B as shown in
Fig. 1(b). When 2.0 equiv of Bu4NF was added, the signals of A
were almost completely replaced by those of B as shown in Fig.
1(c). The broad signal at 129.0 ppm was assigned to the pyrrole-
H on the basis of the spectral comparison with the pyrrole-d8
complex as shown in Fig. 1(c-1). Thus, the high-spin state of the
iron(III) ion is maintained during the addition of Bu4NF. The large
downfield shifts of the o-CH3, m-H, and p-CH3 together with the
single signals for the o-CH3 and m-H indicate that B should be best
formulated as high-spin (S = 5/2) six-coordinate [FeIII(TMP∑)F2]
where the radical spin is in the porphyrin a2u orbital.
The same reaction was carried out in CD2Cl2 solution con-
taining no CD3OD. By the addition of 1.0 equiv of Bu4NF, the
1H NMR spectrum completely changed to show the formation
of a new complex signified as A¢ as given in Fig. S1(a) of
the ESI.† Although the pyrrole signal of A¢ was observed at a
downfield position, i.e. 114.6 ppm, as in the case of A formed in
CD2Cl2–CD3OD solution, all the other signals appeared in the
opposite direction. For example, the o-CH3 signals were observed
at -8.6 and -11.7 ppm, m-H signals at -34.9, -37.8 ppm, and
1
the p-CH3 signal at -5.7 ppm. Thus, the H NMR spectrum of
A¢ resembles that of five-coordinate [FeIII(TPP∑)Cl]SbCl6,7 and
should be expressed as [FeIII(TMP∑)F]ClO4. The large upfield shift
of the m-H signals can then be explained in terms of the anti-
ferromagnetic coupling between the unpaired electrons in the a2u
and dz orbitals;7,26,27 this interaction is symmetry allowed in five-
2
coordinate complexes where the iron(III) ion is placed out of the
porphyrin plane.28,29 The result, in turn, indicates that the mono-F
complex A formed in CD2Cl2–CD3OD solution should be for-
mulated as high-spin six-coordinate [FeIII(TMP∑)F(CD3OD)]ClO4
and/or [FeIII(TMP∑)F(ClO4)] rather than [Fe(TMP∑)F]ClO4.
When 2.0 equiv of Bu4NF was added, [Fe(TMP∑)F]ClO4 was
completely converted to the new complex signified as B¢. Since
1
the H NMR spectrum of B¢ shown in Fig. S1(b)† is essentially
the same as that of B formed in CD2Cl2–CD3OD solution, B¢
should be formulated as [FeIII(TMP∑)F2]. We have noticed that
B is much more stable than B¢. That is, while the reduction of
1
B¢ took place during the H NMR measurement at 223 K, no
reduction was observed in the case of B even at 298 K. The result
suggests that methanol is playing an important role in stabilizing
[FeIII(TMP∑)F2]. Presumably, the hydrogen bonding with methanol
weakens the field strength of fluoride, which in turn weakens the
reducing ability of the coordinating fluoride ions. Table 1 lists
the chemical shifts of these complexes together with those of
analogous oxidized complexes.
In order to confirm if B is really the one-electron oxidation
equivalent above [FeIII(TMP)F2]-, the titration experiment was
carried out using Bu4NI as a reducing agent. As shown in Fig.
2(a) and (b), the addition of 0.5 equiv of Bu4NI converted
50% of B to high-spin [FeIII(TMP)F]. Complete conversion to
[FeIII(TMP)F] was achieved by the addition of 1.1 equiv of
Bu4NI as shown in Fig. 2(c). Thus, the titration result clearly
indicates that B is one-electron oxidation equivalent above the
iron(III) porphyrin. It should be noted that the one-electron re-
duction of [FeIII(TMP∑)F2] (B) produces [FeIII(TMP)F] rather than
Fig. 1 1H NMR spectral changes of [FeIII(TMP∑)(ClO4)2] observed when
(a) 1.0, (b) 1.5, and (c) 2.0 equiv. of (Bu4N)F was added at 193 K. (a-1) and
(c-1) are the downfield region of the pyrrole-d8 complex. Symbols o1, m1,
p1, and py1 correspond to the o-CH3, m-H, p-CH3, and pyrrole-H signals
of mono-F complex (A). Similarly, the symbols o2, m2, etc. correspond to
those of bis-F complex (B).
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Table 1 1H NMR chemical shifts of one-electron oxidized complexes
Complexesa
T/K
193
Solvb
Py
o-CH3
m
p-CH3
18.7
ESg
Ref.
[Fe(TMP∑)F(CH3OH)]X (A)
[Fe(TMP∑)F]X (A¢)
i
112.8
116.6
32.0
30.8
-8.6
-11.7
40.2
40.7
2.51
23.0
32.2
38.1
37.6c
34.4c
109.7d
2.4
89.5
89.1
-34.9
-37.8
116.7
112.2
6.63
58.2
90.9
113.2
-12.3
FeIII (S = 5/2), Por∑
FeIII(S = 5/2), Por∑, AF
This work
This work
193
ii
-5.7
[Fe(TMP∑)F2] (B)
193
193
193
193
195
193
299
i
128.2
123.3
4.71
-61.2
-22.5
100.3
66.1
23.2
21.2
2.15
7.1
19.7
24.2
29.5f
FeIII (S = 5/2), Por∑
FeIII (S = 5/2), Por∑
This work
This work
9
9
13
This work
7
[Fe(TMP∑)F2] (B¢)
[Fe(TMP∑)(tBuNC)2]X2
[Fe(TMP∑)(HIm)2]X2
[Fe(TMP∑)(ClO4)2]
[Fe(TMP∑)(CH3OH)2]X2
[Fe(TPP∑)Cl]X
ii
ii
ii
ii
iii
ii
FeIII (S = 1/2, dxy), Por∑, AF
FeIII (S = 1/2, dp), Por∑
FeIII (S = 5/2, 3/2), Por∑
FeIII (S = 5/2), Por∑
FeIII (S = 5/2), Por∑, AF
[Fe(TiPrP∑)X2]
[Fe(TMP)(OCH3)2]
298
195
ii
i
-64.1
-37.5
23.8e
7.72
FeIII (S = 3/2), Por∑
FeIV (S = 1)
10
13
2.86
a X = ClO4
.
b Solvents: (i) CD2Cl2–CD3OD, (ii) CD2Cl2, (iii) CD3OD. c o-H. d meso-CH(a). e meso-CH3. f p-H. g ES: Electronic structure, Por∑: porphyrin
-
radical cation, AF: antiferromagnetic coupling between iron(III) and porphyrin radical.
Fig. 3 Temperature dependence of the 1H NMR chemical shifts of B
taken in CD2Cl2–CD3OD solution.
to 0.00578); both pyrrole-H and m-H show a slight curvature at
higher temperature. Most of the lines exhibit sizable deviation
from the diamagnetic position at 1/T = 0 which is observed in
other iron(III) porphyrin radical cations.7
UV-vis spectroscopy
UV-vis spectroscopy is a useful method to determine the oxidation
site in the complex.26 Fig. 4 shows the UV-vis spectral changes
observed when Bu4NF was added to the CH2Cl2–CH3OH solution
of [FeIII(TMP∑)(ClO4)2]. The black, green, and red lines represent
the spectra measured at 193 K after the addition of 0.0, 1.0. and
3.0 equiv of Bu4NF, respectively. The spectra drawn by the green
and red lines, therefore, correspond to the complexes A and B,
respectively. It should be noted that both A and B exhibit the broad
absorption bands at 560–680 nm characteristic to the porphyrin
radical. In contrast, the absorption maxima of the six-coordinate
iron(IV) complex [FeIV(TMP)(OCH3)2] were reported to be 425.5,
546, and 575 nm. Thus, the UV-Vis spectra clearly indicate that
both A and B have a radical cationic porphyrin ring.
Fig. 2 1H NMR spectral changes of B after the addition of (a) 0.0, (b)
0.5, and (c) 1.1 equiv of iodide at 193 K. The symbols o2, m2, p2, and py2
indicate the o, m, p, and pyrrole signals of [FeIII(TMP∑)F2] (B), while the
symbols m0 and py0 are m and pyrrole signals of [FeIII(TMP)F].
[FeIII(TMP)F2]-. This is understandable because [FeIII(TMP)F]
is converted to [FeIII(TMP)F2]- only in the presence of a large
excess of fluoride.30 This is in sharp contrast to the case of
the corresponding iron(III) radical cation [FeIII(TMP∑)(ClO4)2].
In the latter case, 2.1 equiv of fluoride is sufficient to convert
[FeIII(TMP∑)(CD3OD)2] to [FeIII(TMP∑)F2] (B).
Mo¨ssbauer spectroscopy
Fig. 3 shows the temperature dependence of the 1H NMR signals
of B. Plots of the chemical shifts against 1/T showed an adequately
good straight line within the range 298 to 173 K (1/T: 0.00336
Mo¨ssbauer spectroscopy is also a powerful method to elucidate
the oxidation and spin states of iron porphyrin complexes.31,32 Fig.
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P2 adopt the S = 5/2 state. Thus, they were assigned to either
[FeIII(TMP∑)(ClO4)(CH3OH)]+ or [FeIII(TMP∑)(CH3OH)2]2+. The
complex P3 given by the blue line accounts for 33% and consists
of a very broad doublet with a line width of 0.87 mm s-1. The
IS and QS values were determined to be 0.58 and 2.14 mm s-1,
respectively, though these values should contain a sizable amount
of error due to the signal broadness. The relatively large QS value
suggests that P3 is assigned to [FeIII(TMP∑)(ClO4)2], which adopts
1
the mixed S = 5/2, 3/2 spin state as revealed by the H NMR
spectrum. The presence of the S = 3/2 isomer should expand the
QS value, assuming that the exchange rate between the S = 5/2
and S = 3/2 spin isomers is fast on the Mo¨ssbauer timescale at
77 K; the QS values of the S = 3/2 complexes are in general larger
than 3.0 mm s-1.33,34,35
Fig. 4 UV-vis spectral changes observed after the addition of 0.0 (black),
1.0 (green), and 3.0 (red) equiv of Bu4NF to the CH2Cl2–CH3OH solution
of [FeIII(TMP∑)(ClO4)2] at 195 K.
Fig. 5(b) shows the Mo¨ssbauer spectrum obtained by the
addition of 1.0 equiv of Bu4NF. Obviously, two species are present
in the frozen solution. The minor species was assigned to the
unreacted [FeIII(TMP∑)(ClO4)2], since the IS and QS values, 0.60
and 2.21 mm s-1, respectively, are similar to those of P3 in Fig. 5(a).
The major component shows a typical paramagnetically relaxed
spectrum, which was analyzed using a Blume–Tjon model.36 The
obtained Mo¨ssbauer parameters were IS = 0.51, e2qQ (quadrupole
coupling constant) = 0.99 mm s-1, Hint (internal magnetic field) =
8.0 T, t (relaxation time) = 8.8 ns with fixed h (asymmetric
parameter) = 0. The positive value of e2qQ leads the QS value to be
0.50 mm s-1. This species was assigned to the mono-F complex (A),
[FeIII(TMP∑)F(ClO4)] or [FeIII(TMP∑)F(CH3OH)]+, as mentioned
in the 1H NMR section.
5 shows the Mo¨ssbauer spectra taken at 77 K after the addition
of (a) 0.0, (b) 1.0, and (c) 2.0 equiv of Bu4NF to the toluene–
methanol solution of 57Fe-enriched [FeIII(TMP∑)(ClO4)2] (95.5%
57Fe). Fig. 5(a) shows a broad unsymmetrical doublet, which
can be explained in terms of the presence of several complexes
with different axial ligands. As mentioned, they are supposed
to be [FeIII(TMP∑)(ClO4)2], [FeIII(TMP∑)(ClO4)(CH3OH)]+, and
[FeIII(TMP∑)(CH3OH)2]2+. In fact, the observed Mo¨ssbauer spec-
trum shown by the yellow line was reproduced by considering
the presence of the three kinds of complexes. The complex P1
drawn by the red line has the largest population, i.e. 53%, and
its IS and QS values were 0.54 and 1.06 mm s-1, respectively.
In contrast, the complex P2 drawn by the green line accounts
for only 12%. The IS and QS values were 0.20 and 0.60 mm
s-1, respectively. The small QS values suggest that both P1 and
In contrast to Fig. 5(a) and (b), Fig. 5(c) shows only one iron
site suggesting the formation of [FeIII(TMP∑)F2] (B). This is un-
derstandable because the solution contains 2.0 equiv of F- which
-
has much stronger field-strength than ClO4 and CH3OH. The
spectrum again showed a paramagnetically relaxed spectrum and
was analyzed using the same model. The Mo¨ssbauer parameters
were estimated to be IS = 0.50, e2qQ = 0.29 mm s-1, Hint = 8.0 T, t =
9.9 ns. The estimated QS value was thus 0.14 mm s-1. The small
QS value confirms that the complex also adopts the high-spin
state. Table 2 lists the Mo¨ssbauer parameters of all the complexes
observed during the titration processes together with the possible
structural formulae. It should be noted that the QS value decreased
from 1.06 (P1) to 0.50 (A) and then to 0.14 mm s-1 (B) as the weak
axial ligands such as perchlorate and/or methanol were replaced
by much stronger fluoride in a stepwise fashion. The result is
understandable because the increase in the s-donor strength of
the axial ligand should decrease the electric field gradient Vzz.
Interestingly the estimated QS value for A is close to the average
of those for P1 and B, suggesting that the Vzz value is varied by
the axial ligands additively.
Conclusion and perspective
The 1H NMR, UV-vis, and Mo¨ssbauer spectroscopy have
been used to elucidate the electronic structure of the complex
which is formally the one-electron oxidation equivalent above
[FeIII(TMP)F2]-. The electronic structure of [Fe(TMP)F2] turns
out to be quite different from that of [Fe(TMP)(OCH3)2] although
the field strength of F- is similar to that of OCH3 . As shown in Fig.
-
Fig. 5 Mo¨ssbauer spectra observed after the addition of (a) 0.0, (b)
1.0, and (c) 2.0 equiv of Bu4NF to the toluene–methanol solution of
[FeIII(TMP∑)(ClO4)2] (95.5% 57Fe) at 77 K.
6, the stable structure of the former complex is formulated as the
iron(III) porphyrin radical cation [FeIII(TMP∑)F2] while the latter is
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Table 2 Mo¨ssbauer parameters in frozen toluene–methanol solution at 77 K
F- added
0.0 equiv
Complexa
IS
QS
Area (%)
55
Structural formula
b
P1
0.54
1.06
III
i
+
⎛
⎞
[Fe (TMP )(ClO )(CH OH)] or
⎟
⎟
⎟
⎟
⎟
⎠
⎜
⎜
⎜
⎜
4
3
III
i
2+
⎜
[Fe (TMP )(CH OH) ]
⎝
P2
P3
0.20
0.58
0.60
2.14
12
33
3
2
[FeIII(TMP∑)(ClO4)2]
b
1.0 equiv
A
0.51
0.50
major
III
i
⎛
⎞
[Fe (TMP )F(ClO4 )] or
⎟
⎟
⎜
⎜
⎜
⎜
⎟
III
i
+⎟
⎟
⎜
⎝
[Fe (TMP )F(CH OH)]
⎠
3
P3
B
0.60
0.50
2.21
0.14
minor
100
[FeIII(TMP∑)(ClO4)2]
2.0 equiv
[FeIII(TMP∑)F2]
a P1, P2, P3 are the iron sites in the starting complexes. A and B are the mono-F and di-F complexes, respectively. See the text. b Structures are not
specified.
Scientific Instruments Inc., and the 57Co(Rh) source was kept at
room temperature. The data were analyzed on a Mo¨ss Winn 3.0i
XP Program. The isomer shift is given relative to R-iron foil at
room temperature.
Sample preparation
[FeIII(TMP)(ClO4)] was prepared by the addition of AgClO4 to
Fig. 6 Electronic structures of the complexes that are one-electron oxida-
tion equivalents above (a) [FeIII(TMP)F2]- and (b) [FeIII(TMP)(OMe)2]-.
the CH2Cl2 solution of [FeIII(TMP)Cl] by the method reported
by Reed et al.26 Oxidation of [FeIII(TMP)(ClO4)] was performed
using Fe(ClO4)3 as the oxidant according to the procedure reported
by Groves et al.13 57Fe-enriched [FeIII(TMP)Cl] (95.5% 57Fe) was
prepared by metallating (TMP)H2 with 0.8 equiv 57FeCl2 in
refluxed DMF solution.
expressed as the iron(IV) porphyrin [FeIV(TMP)(OCH3)2]. It should
be pointed out here that the formation of [FeIII(TMP∑)F2] does
not necessarily indicate that [FeIII(TMP∑)F2] is the initial product
upon oxidation of [FeIII(TMP)F2]-. It could be possible that the
initially formed iron(IV) complex is immediately converted to
the thermodynamically more stable iron(III) radical cation.37 One
possible way to prepare iron(IV) porphyrin is to use much stronger
anionic bases such as metal amides. However, such a strong base
should easily reduce the iron(IV) ion as we have experienced the
reduction of [Fe(TMP∑)F2] to [Fe(TMP)F] by the coordinating F-
ion in CH2Cl2 solution. Thus, a fine tuning of the field-strength
of anionic ligands by using suitable solvent is quite important for
the synthesis of rare six-coordinate iron(IV) porphyrins without an
FeIV O unit. Such a study is now in progress in this laboratory.
Titration experiment
The 1H NMR sample tube containing a CD2Cl2 solution of
[FeIII(TMP∑)(ClO4)2] was placed in a liquid nitrogen bath, to which
a CD3OD solution of Bu4NF was added. The sample tube was
then immersed in a dry-ice–acetone bath and shaken for several
1
minutes. The H NMR spectra were recorded at each time after
the addition of a certain amount of Bu4NF.
A Mo¨ssbauer sample was similarly prepared. A flask containing
the toluene solution of [FeIII(TMP∑)(ClO4)2] was placed in a liquid
nitrogen bath, to which a CH3OH solution of Bu4NF was added.
The flask was then placed in the dry-ice–acetone bath and shaken
for several minutes to prepare the fluoride complexes. The solution
containing the fluoride complexes was transferred to a Mo¨ssbauer
cell placed in a liquid nitrogen bath with a syringe already cooled
by dry ice.
Experimental
Spectral measurements
UV-Vis spectra were recorded for the CH2Cl2–CH3OH solutions
at 195 K on a Shimadzu MultiSpec-1500 spectrophotometer
equipped with a UNISOKU liquid nitrogen cryostat, CoolSpec
UV USP-203-A. 1H NMR spectra were recorded for the CD2Cl2
solution in the presence and absence of CD3OD on a JEOL
Acknowledgements
1
LA300 spectrometer operating at 300.4 MHz for H. Chemical
shifts were referenced to the residual peak of dichloromethane
(d = 5.32 ppm). Iron-57 Mo¨ssbauer spectra were measured on
a Wissel Mo¨ssbauer spectrometer system (MR-260S, MA-260A,
DFG-500, and CMCA-550). The samples were kept in a Heli-
Tran LT-3 gas-flow cryostat from Advanced Research System
Inc. equipped with a 9620 digital temperature controller from
This work was supported by a Grant-in-Aid for Scientific Research
(No. 21750175 to A.I. and No. 22550157 to M.N.) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan. This work was also supported by the Project Research
Grant (No. 22-7 to A.I.) of the School of Medicine, Toho
University.
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9163–9168 | 9167
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