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Equilibrium between Fe(IV) porphyrin and Fe(III) porphyrin radical cation: new insight into the electronic structure of high-valent iron porphyrin complexes

Authors:
Akira Ikezaki
Akira Ikezaki
Akira Ikezaki
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Masashi Takahashi at Toho University
Masashi Takahashi
Mikio Nakamura at Toho University
Mikio Nakamura
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UV-Vis, NMR, and Mössbauer studies have revealed that [Fe(TMP)(N(3))(2)], showing the Mössbauer parameters quite similar to those of the ferryl species of MauG, CytP450(BM3), Cyt P450(CAM), and CPO, exists as equilibrium mixtures of Fe(iv) porphyrin and Fe(iii) porphyrin radical cation.
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ISSN 1359-7345
Chemical Communications
www.rsc.org/chemcomm Volume 49 | Number 30 | 18 April 2013 | Pages 3061–3160
COMMUNICATION
Akira Ikezaki, Mikio Nakamura et al.
Equilibrium between Fe(
IV) porphyrin and Fe(III) porphyrin radical cation: new insight into the
electronic structure of high-valent iron porphyrin complexes
3098 Chem. Commun., 2013, 49, 3098--3100 This journal is
c
The Royal Society of Chemistry 2013
Cite this: Chem. Commun., 2013,
49, 3098
Equilibrium between Fe(IV) porphyrin and Fe(III)
porphyrin radical cation: new insight into the
electronic structure of high-valent iron porphyrin
complexes†
Akira Ikezaki,*
a
Masashi Takahashi
bc
and Mikio Nakamura*
bc
UV-Vis,NMR,andMo
¨
ssbauer studies have revealed that [Fe(TMP)(N
3
)
2
],
showing the Mo
¨
ssbauer parameters quite similar to those of the fe rryl
species of MauG, CytP450
BM3
,CytP450
CAM
, and CPO, exists as equili-
brium mixtur es of Fe(
IV) porphyrin and Fe(III) porphyrin radical cation.
It is now well established that there are two types of electronic
ground states in low-spin Fe(
III) porphyrins depending on which d
orbital has unpaired electron, i.e. (d
xy
)
2
(d
xz
,d
yz
)
3
(d
p
-type) and
(d
xz
,d
yz
)
4
(d
xy
)
1
(d
xy
-type).
1,2
One electron oxidation of Fe(III)
porphyrins usually produces Fe(
III)porphyrinradicalcations,
whereanelectronisremovedfromtheporphyrina
2u
HOMO i n
thecaseofmeso-substituted complexes.
3–5
Because the Fe(III)ion
has various spin-states and electron configurations, the Fe(
III)
porphyrin radical cations should have a wide variety of electronic
ground states. If the Fe(
III) ion is a low-spin ion, two types of
electronic ground states are expected, i.e. (d
xy
)
2
(d
xz
,d
yz
)
3
(a
2u
)
1
and
(d
xz
,d
yz
)
4
(d
xy
)
1
(a
2u
)
1
states.Wehaverecentlyreportedthat,while
[Fe
III
(TMP
)(HIm)
2
]
2+
(1)‡ adopts the (d
xy
)
2
(d
xz
,d
yz
)
3
(a
2u
)
1
ground
state with total spin S =1,[Fe
III
(TMP
)(
t
BuNC)
2
]
2+
exhibits the
diamagnetic S =0(d
xz
,d
yz
)
4
(d
xy
,a
2u
)
2
ground state because of the
strong interaction between half-occupied a
2u
and half-occupied
d
xy
orbitals probably in the ruffled porphyrin framework.
5–8
Furthermore, [Fe
III
(TMP
)(4,5-Cl
2
Im)
2
]
2+
has exhibited the
1
H NMR spectroscopic properties which are just between those
of 1 and [Fe
III
(TMP
)(
t
BuNC)
2
]
2+
.Thus,[Fe
III
(TMP
)(4,5-Cl
2
Im)
2
]
2+
exists as an equilibrium mixture of the two electron configura-
tional isomers as shown in eqn (1), which is a general description
to express the electronic structure of all the low-spin Fe(
III)
porphyrin radical cations.
5,6
The idea has led us to expect
(d
xy
)
2
(d
xz
,d
yz
)
3
(a
2u
)
1
$ (d
xz
,d
yz
)
4
(d
xy
,a
2u
)
2
(1)
that there should also be an equilibrium between two kinds of
one-electron oxidation products of Fe(
III) porphyrin, i.e. Fe(IV)
porphyrin and Fe(
III) porphyrin radical cation. Thus, some
complexes should exhibit the spectroscopic properties that
are just between those of pure Fe(
IV) porphyrin and Fe(III)
porphyrin radical cation. In this communication, we will report
a rare example showing the equilibrium mentioned above.
The complex in question is [Fe(TMP)(N
3
)
2
](2), which has
been prepared by the addition of the CH
3
OH solution of NaN
3
to the CH
2
Cl
2
solution of [Fe
III
(TMP
)(ClO
4
)
2
] at 193 K.
9
Fig. S1
of ESI† shows the UV-Vis spectra observed by the stepwise
addition of NaN
3
. The absence of the isosbestic point supports
the stepwise formation of 2. It should be noted that a very broad
band ascribed to the porphyrin radical is still observable
between 650 and 750 nm even after the addition of 20 equiv.
of N
3
, suggesting that 2 maintains some radical character.
1
H NMR spectra shown in Fig. S2 of ESI† were obtained by the
addition of 2.0 equiv. of NaN
3
(CD
3
OD solution) to the CD
2
Cl
2
solution of [Fe
III
(TMP
)(ClO
4
)
2
] at 195 K. Two characteristic
signals are observed at 42.1 and +34.3 ppm assigned to the
pyrrole-H(py-H) and meta-H signals, respectively, on the basis
of the spectral comparison with the pyrrole-d
8
complex. The
large isotropic shifts of these signals clearly indicate that 2 is
expressed as the low-spin Fe(
III) porphyrin radical cation as 1.
4,5
Table 1 lists the
1
H NMR chemical shifts (CD
2
Cl
2
, 195 K) of one-
electron oxidation products of Fe(
III) porphyrin complexes.
Close inspection of the data in Table 1 reveals that the meta-H
signal shifts upfield from +58.2 to +34.3, and then to +7.7 ppm as the
axial ligand changes from HIm( 1)toN
3
(2), and then to CH
3
O
(3);
3 is well characterized as an Fe(
IV)complexwithS =1.
9
Thus, the
spin densitie s at the meso -C atoms decrease on going from 1 to 2,
and then to 3, which in turn indicates that the radical character
decreases in the same order. In contrast, the py-H signal shifts
downfield from 61.2 to 42.1 ppm as the axial HIm ligand
is replaced by N
3
. The result also supports the decrease in
radical c haracter; the downfield shift of the py-H signal is 22 ppm
at 217 K when radical cationic [Fe
III
(TDP
)(HIm)
2
]
2+
is reduced to
[Fe
III
(TDP)(HIm)
2
]
+
.
5
The downfield shift is, however, only 4.5 ppm,
when the axial N
3
ligand is replaced by CH
3
O
, which is in sharp
a
Department of Chemistry, School of Medicine, Toho University, Otaku, Tokyo 143-
8540, Japan. E-mail: ikezaki@med.toho-u.ac.jp
b
Department of Chemistry, Faculty of Science, Toho University, Funabashi, Chiba
274-8510, Japan. E-mail: mnakamu@med.toho-u.ac.jp
c
Research Center for Materials with Integrated Properties, Toho University,
Funabashi, 274-8510, Japan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cc40319j
Received 14th January 2013,
Accepted 11th February 2013
DOI: 10.1039/c3cc40319j
www.rsc.org/chemcomm
ChemComm
COMMUNICATION
This journal is
c
The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 3098--3100 3099
contrast to the case of the meta-Hsignal.Thisisbecausetheelectronic
structure of iron changes from (d
xy
)
2
(d
xz
,d
yz
)
3
in 1 to (d
xy
)
2
(d
xz
,d
yz
)
2
in
3.Because3 has two unpaired electrons in the d
p
orbitals, the py-H
signal appears quite upfield, 37.5 ppm, in spite of the absence of the
radical spin on the porphyrin ring. As mentioned, the chemical shift
of the meta-H signal of 2 is just between the corresponding signals of
1 and 3. The result suggests that 2 should best be expressed as an
equilibrium mixture of Fe(
III) porphyrin radical cation and Fe(IV)
porphyrin as shown by eqn (2). The equilibrium constant is supposed
to be close to 1 on the basis of the chemical shifts of the meta-H
signals in 1–3.The
1
H NMR result is consistent with the UV-Vis result
showing the presence of radical species.
(d
xy
)
2
(d
xz
,d
yz
)
3
(a
2u
)
1
$ (d
xy
)
2
(d
xz
,d
yz
)
2
(a
2u
)
2
(2)
In order to reveal the factors that can affect the equilibrium given
ineqn(2),wehavepreparedaseriesofpara-substituted complexes
[Fe(p-X-TPP)(N
3
)
2
], where X is OCH
3
,H,F,andCF
3
group. The
chemical shifts of these complexes determined at 193 K are also
listed in Table 1 together with the Hammett s
p
values. The plots of the
ortho-andmeta-H chemical shifts against Hammett s
p
values have
yielded a good linearity as shown in Fig. 1 with the slopes +28.8 and
17.1 ppm, respectively. This is because the a
2u
orbital is stabilized by
the electron withdrawing para-substituent, which in turn destabilizes
the radical cationic state, resulting in the increases in population
of the Fe(
IV) isomer. The chemical shifts of the meta-H and py-H
signals of [Fe(p-CF
3
-TPP)(N
3
)
2
]are16.5and40.1 ppm, respectively,
both of which are much closer to the corresponding signals of 3, i.e.
7.7 and 37.5 ppm, respectively, as compared with those of other
complexes. These results support that [Fe(p-CF
3
-TPP)(N
3
)
2
]exists
mainly as the Fe(
IV) isomer though the population of the Fe(III)
porphyrin radical cation is not negligibly small.
One can argue that the electron withdrawing meso-substituents
make the a
2u
orbital stabilize so that the HOMO could be changed
from the a
2u
to the a
1u
orbital. In such a case, the decrease in
isotropic shift observed for the meta-H signal can be explained in
terms of the increase in population of the a
1u
radical cation because
the a
1u
orbital has zero coefficient at the meso-C atoms.
10,11
However,
if [Fe(p-CF
3
-TPP)(N
3
)
2
]isactuallyexpressedasthea
1u
radical cation,
then the py-H signal should appear extremely upfield because the
a
1u
orbital has large coefficient on the b-pyrrole positions.
10,11
The
data in Table 1 indicate that the py-H signals of these complexes are
observed in a relatively narrow region, i.e.32.1 to 40.1 ppm.
Therefore, we have considered that [Fe(p-CF
3
-TPP)(N
3
)
2
]existsnotas
the low-spin Fe(
III)witha
1u
porphyrin radical cation but mainly as
the Fe(
IV)porphyrin.
The above-mentioned hypothesis has further been confirmed by
the Mo
¨
ssbauer spectroscopy.
12–15
Fig. 2 shows the Mo
¨
ssbauer
spectra of
57
Fe(95%) enriched 1 and 2 takeninfrozenmethanol
solution at 77 K. In each spectrum, there are at least two minor
components considered to be the decomposed and/or reduced
products. Table 2 lists the Mo
¨
ssbauer parameters together with
those of 3.
9
While the isomer shift (d per mm s
1
)andquadrupole
Table 1 NMR chemical shifts and electronic structures of oxidized complexes
Complexes
1
H NMR chemical shifts
a
s
p
b
Electronic structure
c
Ref.o-CH
3
(o-H) m-H p-CH
3
(p-H) py-H
[Fe(TMP)(
t
BuNC)
2
]
2+
2.5 6.6 2.2 4.7 — A 6
[Fe(TMP)(4,5-Cl
2
Im)
2
]
2+ d
24.8
e
4.0
e
25.6
e
—A$ B6
[Fe(TMP)(HIm)
2
]
2+
(1) 23.0 58.2 7.1 61.2 — B 6
[Fe(TMP)(N
3
)
2
](2) 11.6 34.3 5.4 42.1 — B $ C This work
[Fe(p-X-TPP)(N
3
)
2
]
f
X=CH
3
O(22.9) 30.4 — 36.3 0.28 B $ C This work
X=H (15.4) 25.9 (7.5) 32.1 0.00 B $ C This work
X=F (10.6) 21.5 — 34.8 0.06 B $ C This work
X=CF
3
(0.4) 16.5 — 40.1 0.53 B $ C This work
[Fe(TMP)(CH
3
O)
2
](3)
g
2.4 7.7 2.9 37.5 — C 9
a
Chemical shifts (d p pm) determined in CD
2
Cl
2
at 193 K.
b
Hammett s
p
values.
c
Electronic structure of isomer: A, (d
xz
,d
yz
)
4
(d
xy
,a
2u
)
2
;B,(d
xy
)
2
(d
xz
,d
yz
)
3
(a
2u
)
1
;
and C, (d
xy
)
2
(d
xz
,d
yz
)
2
(a
2u
)
2
.
d
Each signal splits into several signals due to the slow rotation about the Fe–N
axial
bond.
e
Data at 213 K.
f
p-X-TPP is a
dianion of meso-tetrakis(p-substituted phenyl)porphyrin.
g
Data at 195 K.
Fig. 1 Plots of the chemical shifts against Hammett s
p
values in a series of
[Fe(p-X-TPP)(N
3
)
2
]: (a) ortho-H and (b) meta-H.
Fig. 2 Mo
¨
ssbauer spectra of
57
Fe(95%) enriched (a) 1 and (b) 2 taken in frozen
methanol solution at 77 K.
Communication ChemComm
3100 Chem. Commun., 2013, 49, 3098--3100 This journal is
c
The Royal Society of Chemistry 2013
splitting (DE
q
per mm s
1
)valuesof1 are in the region of low-spin
d
p
-type Fe(III) complexes, those of 2 are outside of this region as
showninFig.S3ofESI.†Thus,theobservedd and DE
q
values of 2,
0.12 and 2.27 mm s
1
, respectively, suggest that 2 exists as an
equilibrium mixture of Fe(
III)porphyrinradicalandFe(IV)porphyrin.
It is interesting to compare the Mo
¨
ssbauer parameters of 2
with those of heme proteins adopting the Fe(
IV) oxidation state.
MauG is a naturally occurring diheme protein that catalyzes the
final step in the biosynthesis of the protein-derived tryptophan
tryptophylquinone (TTQ) prosthetic group of methylamine
dehydrogenase (MADH).
16,17
The reactive intermediates of
MauG consist of two distinct Fe(
IV) species, heme 1 and heme
2.
18
Heme 1 is considered to be the Fe
IV
QO species with the d
and DE
q
values to be 0.06 and 1.70 mm s
1
, respectively. Heme
2, having a His–Tyr axial ligation, is quite unusual as a c-type
heme protein.
19
It shows the d and DE
q
values to be 0.17 and
2.54 mm s
1
, respectively, which are larger than the corres-
ponding values of any other Fe(
IV) species reported previously.
18
A quantum chemical calculation has revealed that the coordina-
tion structure should be Fe
IV
(His)(Tyr
).
20
Chloroperoxidase compound II exists as two distinct ferryl
species. The major species has d and DE
q
values to be 0.10 and
2.06 mm s
1
, respectively, and is considered to have a protonated
ferryl group, i.e. Fe
IV
(Cys
)(OH
).
21,22
The minor species showing
a much smaller DE
q
value, 1.59 mm s
1
, has tentatively been
assigned as Fe
IV
QO(Cys
). The Mo
¨
ssbauer parameters of the
compound II in cytochrome P450
BM3
and P450
CAM
have been
reported as follows: 0.13 (d) and 2.16 (DE
q
)mms
1
for th e former,
and 0.14 (d)and2.06(DE
q
)mms
1
for the latter at physiological
pH.
22
These intermediates are also known to have a protonated
ferryl unit, i.e.,Fe
IV
(Cys
)(HO
).
23
The data in Table 2 indicate
that the Mo
¨
ssbauer parameters of these intermediates are quite
close to the corresponding values of 2. Thus, it might be reason-
able to assume that the reaction intermediates of these heme
proteins, which commonly have a Fe
IV
(Cys
)(HO
) structure also
exist as an equilibrium either between Fe(
IV)porphyrinandFe(III)
porphyrin radical cation or between Fe(
IV)porphyrinandFe(III)
porphyrin with radical center on the protein. In fact, Li and
co-workers have pointed out the possibility of the equilibrium
between MauG based diheme and protein radical cation.
18
In conclusion, we have found on the basis of the UV-Vis,
1
H NMR, and Mo
¨
ssbauer spectroscopy that [Fe(TMP)(N
3
)
2
]existsas
an equilibrium mixture of Fe(
IV) porphyrin and Fe(III)porphyrin
radical cation, and that the population of the isomers can be
controlledbythenatureofaxialligands and peripheral substituents.
The synthesis and characte rization of a wide variety of high-va lent
complexes having HO a s one of the axial ligands such as
[Fe(Por)(X)(HO
)]
+
and [Fe(Por)(Y
)(HO
)] are quite important since
they can be good models for the reactive intermediates involved in
the catalytic cycles of cytochrome P450 and peroxidase. Such a study
is now in progress in this group.
This work was supported by Grant-in-Aid for Scientific
Research (No. 21750175 to A.I. and 22550157 to M.N.) from
MEXT, Japan. This work was partly supported by the Research
Center for Materials with Integrated Properties, Toho University.
A.I. is grateful for Toho University Joint Research Fund.
Notes and references
‡ Abbreviations: TMP, TDP, and p-CF
3
-TPP are the dianions of
5,10,15,20-tetramesitylporphyrin, 5,10,15,20-tetradurylporphyrin, and
5,10,15,20-tetrakis(4-trifluoromethylphenyl)porphyrin, respectively. Por is
thedianionofporphyriningeneral.HIm,imidazole;4,5-Cl
2
Im, 4,5-dichloro-
imidazole;
t
BuNC, tert-butylisocyanide.
1 F. A. Walker, in The Porphyrin Handbook, ed. K. M. Kadish,
K. M. Smith and R. Guilard, Academic Press, San Diego, 2000,
vol. 5, ch. 36.
2 M. Nakamura, Coord. Chem. Rev., 2006, 250, 2271.
3 M. A. Phillippi, E. T. Shimomura and H. M. Goff, Inorg. Chem., 1981,
20, 1322.
4 H. M. Goff and M. A. Phillippi, J. Am. Chem. Soc., 1983, 105, 7567.
5 A. Ikezaki, Y. Ohgo and M. Nakamura, Coord. Chem. Rev., 2009,
253, 2056.
6 A. Ikezaki, H. Tukada and M. Nakamura, Chem. Commun., 2008, 2257.
7 A. C. Chamberlin, A. Ikezaki, M. Nakamura and A. Ghosh, J. Phys.
Chem. B, 2011, 115, 3642.
8 J. Conradie and A. Ghosh, J. Phys. Chem. B, 2003, 107, 6486.
9 J. T. Groves, R. Quinn, T. J. McMurry, M. Nakamura, G. Lang and
B. Boso, J. Am. Chem. Soc., 1985, 107, 354.
10 H. Fujii, J. Am. Chem. Soc., 1993, 115, 4641.
11 H. Fujii, T. Yoshimura and H. Kamada, Inorg. Chem., 1997, 36, 6142.
12 P. G. Debrunner, Mo
¨
ssbauer Spectroscopy of Fe Porphyrins,inIron
Porphyrin Part III: Physical Bioinorganic Chemistry, ed. A. B. P. Lever
and H. B. Gray, VCH, New York, 1989, vol. 4, pp. 139–234.
13 P. Gu
¨
tlich, E. Eckhard and A. X. Trautwein, Mo
¨
ssbauer Spectroscopy
and Transition Metal Chemistry, Springer-Verlag, Berlin, 2011, p. 568.
14 E. V. Kudrik, P. Afanasiev, L. X. Alvarez, P. Dubourdeaux,
M. Cle
´
mancey, J.-M. Latour, G. Blondin, D. Bouchu, F. Albrieux,
S. E. Nefedov and A. B. Sorokin, Nat. Chem., 2012, 4, 1024.
15 The isomer shift values are given relative to a-iron foil at room
temperature.
16 Y. Wang, M. E. Graichen, A. Liu, A. R. Pearson, C. M. Wilmot and
V. L. Davidson, Biochemistry, 2003, 42, 7318.
17 C. M. Wilmot and E. T. Yukl, Dalton Trans., 2013, 42, 3127.
18 X. Li, R. Fu, S. Lee, C. Krebs, V. L. Davidson and A. Liu, Proc. Natl.
Acad. Sci. U. S. A.
, 2008, 105, 8597.
19 L. M. R. Jensen, R. Sanishvili, V. L. Davidson and C. M. Wilmot,
Science, 2010, 327, 1392.
20 Y.Ling,V.L.DavidsonandY.Zhang,J. Phys. Chem. Lett., 2010, 1, 2936.
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J. Am. Chem. Soc., 2006, 128, 11471.
Table 2 Mo
¨
ssbauer parameters and coordination structures of high-valent
hemes
Complexes
d
(mm s
1
)
DE
q
(mm s
1
)
Coordinating
atoms Ref.
[Fe(TMP)(HIm)
2
]
2+
(1)
a
0.15 1.87 N, N This work
[Fe(TMP)(N
3
)
2
](2)
a
0.12 2.27 N
,N
This work
[Fe(TMP)(CH
3
O)
2
](3)
b
0.025 2.104 O
,O
9
[Fe(TMP)(CH
3
O)
2
](3)
c
0.022 2.117 O
,O
9
Reaction intermediates of MauG
d
Heme 1, MauG 0.06 1.70 N, O
2
18
Heme 2, MauG 0.17 2.54 N, O
18
Ferryl form of CPO
d
0.10 2.06 S
,O
22
0.11 1.59 S
,O
2
22
Ferryl form of P450
BM3
d
0.13 2.16 S
,O
23
Ferryl form of P450
CAM
d
0.14 2.06 S
,O
23
a
Frozen methanol solution at 77 K.
b
Frozen toluene–methanol
solution at 77 K.
c
Frozen toluene–methanol solution at 4.2 K.
d
Frozen
water solution at 4.2 K.
ChemComm Communication
... Oxidation of am etalloporphyrin occurs either at the coordinated metal center or at the porphyrin macrocycle. [3,7,8] However,inmost of the cases,oxidation produces porphyrin p-cation radicals at the lower potentials.I nt he disilver(II) porphyrin dimer reported herein, the Ag II center is oxidized to Ag III upon stepwise oxidations and displays Ag III ···Ag III interactions in the 2e À -oxidized complex, leading to interesting photophysical properties. ...
... Ag···Ag separation has also decreased to 4.692 from adistance of 5.824 observed in the unoxidized complex syn-1.AMPS of 3.39 is observed for the complex syn-1·I 3 along with al ateral shift of 3.24 ,w hich suggests stronger intermacrocyclic interactions. However,both the Ag II centers are oxidized to Ag III in the 2e À -oxidized complexes syn-1·(PF 6 ) 2 and syn-1·(SbF 6 ) 2 ,which is clearly reflected in their reduced Ag À N por distances of 2.031(5) and 2.032 (8) ,r espectively,a sc ompared to the unoxidized complex syn-1.A lthough no Ag III porphyrin structure has been reported so far, such reduction of AgÀ N por distance from Ag II to Ag III is clearly visible in the complex with N-confused porphyrin. [13] Unlike in other 2e Àoxidized complexes of the ethane-bridged porphyrin dimer reported earlier that stabilize only the anti form of the complex, two porphyrin cores in syn-1·(X) 2 are not only in syn conformation but the rings are even more cofacial. ...
Silver(III)***Silver(III) Interaction Stabilizes the Syn-form in a Porphyrin Dimer Upon Oxidation
Article
The interaction between two Ag(II)porphyrins, connected covalently through a highly flexible ethane bridge, in the metalloporphyrin dimer has been investigated upon stepwise oxidations. X-ray structure determination of one and two-electron oxidized complexes has clearly revealed only metal-centered oxidation that results in short Ag-N (porphyrin) distance with large distortion in the porphyrin macrocycle. The 2e-oxidized complex exhibits significant metallophilic interaction in the form of a close Ag(III)***Ag(III) contact that brings two porphyrin ring more cofacial with syn-conformation which would otherwise stabilize in an anti-form. The interaction also leads to an intense emission peak at 546 nm at 77 K in the photoluminescence study.
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An iron chlorophyll derivative for enhanced degradation of bisphenol A: New insight into the generation mechanism of high-valent iron oxo species
Article
  • Aug 2022
  • CHEM ENG J
The pressure to environmental concern from toxic precursors of metalloporphyrin synthesis and the inadequate understanding of metalloporphyrin with β-site substituents on activation of peroxides have impeded the full potential of metalloporphyrin for the oxidation of recalcitrant organic compounds. Here, we report a “green” iron(III) chlorophyll derived from naturally abundant chlorophyll. It shows remarkable degradation efficiency of bisphenol A in the presence of hydrogen peroxide, with a second-order rate constant of 87.3 ± 1.1 L·mmol⁻¹·min⁻¹, which is 7-fold faster than the degradation rate of the Fenton oxidation system. It also demonstrates a wide pH tolerance range of 4-12 and is barely affected by concomitant inorganic anions or natural organic matter. Different from the generation of oxygen-centered radicals, including •OH and •O2⁻, in the Fenton system, a high-valent iron (Ⅳ) -oxo chlorin π-cation radical is formed with an atypical pathway of homolytic O−O bond cleavage via a proton-coupled electron transfer process and is responsible for the degradation of bisphenol A, which is confirmed with spectroscopic studies and theoretical analysis. This abundant “green” catalyst could be beneficial for wastewater treatment by addressing the limitation of a narrow acidic pH range and the susceptibility to scavenging of nonselective radicals in conventional Fenton reactions.
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Spin States in Iron Porphyrins
Chapter
  • Jun 2022
Studies on the electronic and magnetic structures of iron(II), iron(III), and iron(IV) porphyrin complexes and their radical cations are important for understanding aspects of the biological processes of heme proteins, including hemoglobin, myoglobin, cytochrome P450, catalase, and peroxidase. This chapter describes the main factors that stabilize the oxidation states, spin states, and electron configurations of these complexes. Special emphasis has been placed on 1 H NMR spectroscopy as a method of revealing the spin–spin interactions in iron porphyrin radical cations.
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Novel Lanthanide(III) Porphyrin-Based Metal–Organic Frameworks: Structure, Gas Adsorption, and Magnetic Properties
Article
Full-text available
  • Sep 2021
The present work focuses on the hydrothermal synthesis and properties of porous coordination polymers of metal–porphyrin framework (MPF) type, namely, {[Pr4(H2TPPS)3]·11H2O}n (UPJS-10), {[Eu/Sm(H2TPPS)]·H3O⁺·16H2O}n (UPJS-11), and {[Ce4(H2TPPS)3]·11H2O}n (UPJS-12) (H2TPPS = 4,4′,4″,4‴-(porphyrin-5,10,15,20-tetrayl)tetrakisbenzenesulfonate(4-)). The compounds were characterized using several analytical techniques: infrared spectroscopy, thermogravimetric measurements, elemental analysis, gas adsorption measurements, and single-crystal structure analysis (SXRD). The results of SXRD revealed a three-dimensional open porous framework containing crossing cavities propagating along all crystallographic axes. Coordination of H2TPPS4– ligands with Ln(III) ions leads to the formation of 1D polymeric chains propagating along the c crystallographic axis. Argon sorption measurements at −186 °C show that the activated MPFs have apparent BET surface areas of 260 m² g–1 (UPJS-10) and 230 m² g–1 (UPJS-12). Carbon dioxide adsorption isotherms at 0 °C show adsorption capacities up to 1 bar of 9.8 wt % for UPJS-10 and 8.6 wt % for UPJS-12. At a temperature of 20 °C, the respective CO2 adsorption capacities decreased to 6.95 and 5.99 wt %, respectively. The magnetic properties of UPJS-10 are characterized by the presence of a close-lying nonmagnetic ground singlet and excited doublet states in the electronic spectrum of Pr(III) ions. A much larger energy difference was suggested between the two lowest Kramers doublets of Ce(III) ions in UPJS-12. Finally, the analysis of X-band EPR spectra revealed the presence of radical spins, which were tentatively assigned to be originating from the porphyrin ligands.
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Stabilisation of tri-valent ions with vacant coordination site at a corrole-metal interface
Article
  • Aug 2020
By exploiting an established on-surface metallation strategy, we address the ability of the corrolic macrocycle to stabilise transition metal ions in high-valent (III) oxidation states in metal-supported molecular layers. This...
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Intermacrocyclic Interactions upon Stepwise Oxidations in a Monometallic Porphyrin Dimer: Ring versus Metal‐Center Oxidations and Effect of Counter Anions
Article
Full-text available
The effect of intermacrocyclic interactions was studied by controlled and stepwise oxidations of a monometallic silver(II) porphyrin dimer that contains a highly flexible ethane bridge. Monometallic dimers are unique systems and behave differently from their dimetallic analogues on the basis of their available sites for storing oxidizing equivalents. UV‐visible spectrometry, ¹H NMR spectroscopy, XPS and single crystal X‐ray diffraction studies clearly suggest the removal of the first electron from the metal center. The removal of the second electron occurred from the ring center to form a π‐cation radical and, thereby, form a very unique mixed‐valent species. However, unlike in all other ethane‐bridged metalloporphyrin dimers reported earlier, the 2e‐oxidized species showed quite unusual structures depending on the nature of counter ions. Ions, such as SbF6, SbCl6 and PF6, are engaged in strong interactions with the porphyrin π‐cation radical and causes substantial structural changes, including large deformation of the ring. The solid‐state structure remains intact in solution as well. The observations are further supported by DFT calculations.
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Constitution and Conductivity of Metalloporphyrin Tapes
Article
Full-text available
Metalloporphyrin tapes form in a solvent‐free oxidative chemical vapor deposition process on glass substrates. The metal center (M = NiII, CuII, ZnII, CoII, PdII, FeIIICl, 2H) in the 5,15‐disubstituted porphyrin monomer affects the initial C–C coupling step and consequently the formation of triply or doubly linked porphyrin tapes as well as the interchain interaction in the tape as shown by optical spectroscopy, high resolution mass spectrometry and X‐ray photoelectron spectroscopy. Optical spectroscopy and conductive atomic force microscopy reveal that these factors influence the near‐infrared absorbance and the electrical conductivity of the films. Consequently, the metal ion of the metalloporphyrin allows tuning of the macroscopic properties of the thin films composed of metalloporphyrin tapes.
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Piezo-promoted the generation of reactive oxygen species and the photodegradation of organic pollutants
Article
  • Aug 2019
Composite of porphyrins and piezoelectric materials is a promising method to overcome the limitation of photocatalytic response of composite catalysts, inhibit photogenerated electron-hole recombination and enhance photocatalytic degradation performance. Here, the Fe-S electronic channel is formed by the combination of MoS2 and iron porphyrin, which enhances the electron transfer performance of iron porphyrin to MoS2 semiconductor. At the same time, two-dimensional MoS2 surface with piezoelectric properties forms an electric field, which further enhances charge separation and piezoelectric catalytic performance. The photoexcitation of porphyrin and the piezoelectric excitation of molybdenum sulfide cooperate with each other under the simultaneous action of light and ultrasound. Oxygen radicals and hydroxyl radicals are enhanced, and the catalytic degradation performance is further enhanced. By strengthening the interaction between porphyrins and piezoelectric materials, especially bonding, a good and stable catalyst for pollutant degradation and purification was prepared.
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Nitric Oxide Dioxygenase Activity of a Nitrosyl Complex of Cobalt(II) Porphyrinate in the Presence of Hydrogen Peroxide via Putative Peroxynitrite Intermediate
Article
  • Jan 2019
The reaction of a cobalt porphyrin complex, [(F8TPP)Co], 1 {F8TPP = 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrinate dianion} in dichloromethane with nitric oxide (NO) led to the nitrosyl complex, [(F8TPP)Co(NO)], 2. Spectroscopic studies and structural characterization revealed it as a bent nitrosyl of {CoNO}⁸ description. It was stable in the presence of dioxygen. However, it reacts with H2O2 in acetonitrile (or THF) solution at -40 °C (or -80 °C) to result in the corresponding Co(III)-nitrate complex, [(F8TPP)Co(NO3)], 3. The reaction presumably proceeds via the formation of a Co-peroxynitrite intermediate. X-Band electron paramagnetic resonance and electrospray ionization-mass spectroscopic studies suggest the intermediate formation of the [(porphyrin)Co(III)-O•] radical, which in turn supports the generation of the corresponding Co(IV)-oxo species during the reaction. This is in accord with the homolytic cleavage of the O-O bond in heme-peroxynitrite proposed in the nitric oxide dioxygenases activity. In addition, the characteristic peroxynitrite-induced phenol ring reaction was also observed.
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Silver(III)***Silver(III) Interaction Stabilizes the Syn-form in a Porphyrin Dimer Upon Oxidation
Article
  • May 2017
  • Angew Chem
The interaction between two AgII porphyrins, connected covalently through a highly flexible ethane bridge, in a metalloporphyrin dimer has been investigated upon stepwise oxidation. X-ray structure determination of one and two-electron oxidized complexes has clearly revealed only metal-centered oxidation that results in short Ag−N (porphyrin) distance with large distortion in the porphyrin macrocycle. The 2e-oxidized complex exhibits significant metallophilic interaction in the form of a close AgIII⋅⋅⋅AgIII contact that brings two porphyrin rings more cofacial with syn-conformation, which would otherwise stabilize in an anti-form. The interaction also leads to an intense emission peak at 546 nm at 77 K in the photoluminescence study.
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An N-bridged high-valent diiron-oxo species on a porphyrin platform that can oxidize methane
Article
Full-text available
  • Oct 2012
  • NAT CHEM
High-valent oxo-metal complexes are involved in key biochemical processes of selective oxidation and removal of xenobiotics. The catalytic properties of cytochrome P-450 and soluble methane monooxygenase enzymes are associated with oxo species on mononuclear iron haem and diiron non-haem platforms, respectively. Bio-inspired chemical systems that can reproduce the fascinating ability of these enzymes to oxidize the strongest C-H bonds are the focus of intense scrutiny. In this context, the development of highly oxidizing diiron macrocyclic catalysts requires a structural determination of the elusive active species and elucidation of the reaction mechanism. Here we report the preparation of an Fe(IV)(µ-nitrido)Fe(IV) = O tetraphenylporphyrin cation radical species at -90 °C, characterized by ultraviolet-visible, electron paramagnetic resonance and Mössbauer spectroscopies and by electrospray ionization mass spectrometry. This species exhibits a very high activity for oxygen-atom transfer towards alkanes, including methane. These findings provide a foundation on which to develop efficient and clean oxidation processes, in particular transformations of the strongest C-H bonds.
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Do the One-Electron Oxidized Derivatives of Some Six-Coordinate Low-Spin Iron(III) Porphyrins Feature Strong Metal−Ligand Ferromagnetic Coupling?
Article
Full-text available
  • Jun 2003
It has been recognized for some time that the high-valent complex [Fe(TPP)(1-MeIm)2]2+ (TPP = meso-tetraphenylporphyrin, 1-MeIm = 1-methylimidazole) exhibits an S = 1 ground state and has been described as a low-spin Fe(III) porphyrin π-cation radical with strong metal−porphyrin ferromagnetic coupling. Because strong metal−ligand ferromagnetic coupling is quite unusual, a DFT investigation of two truncated and symmetrized models of this complex, namely, [Fe(P)(Py)2]2+ and [Fe(P)(ImH)2]2+, was carried out. The results indicate that the ground-state electronic configurations may be described as low-spin Fe(IV)-like, with varying degrees of metal-to-porphyrin spin delocalization, which provides a natural explanation for the experimentalists' somewhat unusual description of the electronic structure of [Fe(TPP)(1-MeIm)2]2+.
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MauG: A di-heme enzyme required for methylamine dehydrogenase maturation
Article
  • Oct 2012
  • DALTON T
Methylamine dehydrogenase (MADH) requires the cofactor tryptophan tryptophylquinone (TTQ) for activity. TTQ is a posttranslational modification that results from an 8-electron oxidation of two specific tryptophans in the MADH β-subunit. The final 6-electron oxidation is catalyzed by an unusual c-type di-heme enzyme, MauG. The di-ferric enzyme can react with H(2)O(2), but atypically for c-type hemes the di-ferrous enzyme can react with O(2) as well. In both cases, an unprecedented bis-Fe(iv) redox state is formed, composed of a ferryl heme (Fe(iv)[double bond, length as m-dash]O) with the second heme as Fe(iv) stabilized by His-Tyr axial ligation. Bis-Fe(iv) MauG acts as a potent 2-electron oxidant. Catalysis is long-range and requires a hole hopping electron transfer mechanism. This review highlights the current knowledge and focus of research into this fascinating system.
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Imidazole complexes of low-spin iron(III) porphyrin π-cation radical species. Models for the compound I π-cation radical state of peroxidases
Article
  • Dec 1983
Addition of imidazole to methylene chloride solutions of the previously characterized iron(III) porphyrin π-cation radicals permits generation of new oxidized imidazole complexes. These transient paramagnetic bis(imidazole) complexes have been characterized in solution by low-temperature proton NMR measurements. Oxidized iron tetraphenylporphyrin derivatives exhibit large, alternating upfield and downfield phenyl proton chemical shifts, which are reminiscent of the parent iron(III) porphyrin radical. A far upfield pyrrole proton NMR signal resembles that of the low-spin iron(III) complex in both shift direction and line width. The oxidized imidazole complexes are thus best described as low-spin iron(III) porphyrin π-cation radicals. Magnetic moment estimates of 2.8 and 3.3 μB for a tetraphenylporphyrin derivative and octaethylporphyrin, respectively, indicate that metal and radical spins do not strongly couple. The proton NMR spectrum of the oxidized bis(imidazole)iron(III) etioporphyrin complex resembles that of horseradish peroxidase compound I in that a relatively sharp ring methyl signal is found in a far downfield position.
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Effects of the electron-withdrawing power of substituents on the electronic structure and reactivity in oxoiron(IV) porphyrin ??-cation radical complexes
Article
  • Jun 1993
The effects of the electron-withdrawing power of the substituents bound to a porphyrin ring on the electronic structures and the reactivities of oxoiron(IV) porphyrin pi-cation radical complexes were studied by using 2,7,12,17-tetramethyl-3,8,13,18-tetraarylporphyrins (aryl = (1) mesityl, (2) 2-chloro-6-methylphenyl, (3) 2,6-dichlorophenyl, or (4) 2,4,6-trichlorophenyl) and tetrakis-5,10,15,20-tetraarylporphyrins (aryl = (5) mesityl, (6) 2-chloro-6-methylphenyl, (7) 2,6-dichlorophenyl, or (8) 2,4,6-trichlorophenyl). The electronic structures of oxoiron(IV) porphyrin pi-cation radicals were investigated by low-temperature UV-visible absorption spectroscopy and proton NMR measurements. The absorption spectra of oxoiron(IV) porphyrin pi-cation radicals of compounds 1-4 changed with an increase of the electron-withdrawing power of ring substituents, while those of compounds 5-8 did not. Proton NMR measurements demonstrated that oxoiron(IV) porphyrin pi-cation radicals of compounds 1-4 have an a1u radical character and that those of compounds 5-8 are better described as an a2u radical species. The reactivities of oxygen atoms of oxoiron(IV) porphyrin pi-cation radicals were examined by competitive epoxidation of cyclohexene by two oxoiron(IV) porphyrin pi-cation radicals with different radical orbital occupancies or oxidation potentials. The oxygen atom with the higher oxidation potential was more reactive than that with the lower oxidation potential. Furthermore, the oxygen atom with the a1u radical state was almost as reactive as that with the a2u radical state. The results indicate that the reactivity of the oxygen atom of the oxoiron(IV) porphyrin pi-cation radical depends on its oxidation potential and is not affected by the a1u/a2u orbital occupancy.
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Preparation and Characterization of a Dialkoxyiron( IV) Porphyrin
Article
  • Jan 1985
(Perchlorato)(tetramesitylporphyrinato)iron(III) [Fe(TMP)(ClO4)] was oxidized with ferric perchlorate to generate the corresponding iron(III) porphyrin cation radical, Fe(TMP)(ClO4)2 (1). The 1H NMR and visible spectra of 1 were similar to those of other bis(perchlorato)iron(III) porphyrin cation radicals. The reaction of 1 with 1 equiv of potassium iodide regenerated Fe(TMP)(ClO4). The treatment of 1 with 2 equiv of sodium methoxide in methanol produced a new species, 2, which was shown to be an iron(IV) complex. The β-pyrrole protons of 2 were found at very high field, -37.5 ppm relative to TMS at -78°C in CD2Cl2. All other resonances were observed near the expected diamagnetic positions. The temperature dependence of the β-pyrrole resonance displayed the characteristic Curie behavior of an isolated paramagnet. The sharp singlet observed for the meta hydrogens of 2 suggested a structure with D4h symmetry. The coordination of methoxide in 2 was inferred from the requirement of 2 equiv of alkoxide to form them and the broadened 1H-methyl resonances, respectively. The oxidation of (TMP)FeIII(OCH3) with 1.2 equiv of iodosylmesitylene in methanol produced 2 and 0.7 equiv of free iodomesitylene. The Mössbauer spectra of 2 under conditions of variable temperature and variable magnetic field indicated a non-Kramers system with an isomer shift (δ) of -0.025 and a quadrupole splitting (ΔEQ) of 2.10. Magnetic susceptibility measurements on 2 gave a value of μβ = 2.9, close to the spin-only value for two unpaired electrons. No EPR signals were detected for 2. Taken together the data support an S = 1 iron(IV) porphyrin dimethoxide structure [FeIV(TMP)(OCH3)2] for 2.
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NMR studies on the electronic structure of one-electron oxidized complexes of iron(III) porphyrinates
Article
  • Aug 2009
  • COORDIN CHEM REV
This review describes the relationship between the NMR chemical shifts and electronic structures of one-electron oxidized products of iron(III) porphyrins such as iron(III) porphyrin radical cations or iron(IV) porphyrins. In the case of the former complexes, the iron(III) ions are classified into four types; (i) high-spin (S = 5/2), (ii) mixed high- and intermediate-spin (S = 5/2, 3/2), (iii) low-spin with the (dxy)2(dxz, dyz)3 ground state, and (iv) low-spin with the (dxz, dyz)4(dxy)1 ground state. In the case of the latter complexes, they mostly have FeIVO bonds. There is only one iron(IV) complex that has no FeIVO bond. The complexes classified as above exhibit unique NMR chemical shifts. Thus, the NMR spectroscopy serves a quite useful tool to determine the fine electronic structures of the one-electron oxidized iron(III) porphyrin complexes.
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