Reactivity of mono-meso-substituted iron(II) octaethylporphyrin complexes with hydrogen peroxide in the absence of dioxygen. Evidence for nucleophilic attack on the heme

Article abstract of DOI:10.1021/ja011545b

Treatment of the mono-meso-substituted iron(II) octaethylporphyrin complexes, (py)₂Fe^II(meso-NO₂-OEP), (py)₂Fe^II(meso-CN-OEP), (py)₂Fe^II(meso-HC(O)-OEP), (py)₂Fe^II(meso-Cl-OEP), (py)₂Fe^II(meso-OMe-OEP), (py)₂Fe^II(meso-Ph-OEP), and (py)₂Fe^II(meso-n-Bu-OEP), with hydrogen peroxide in pyridine-d₅ at -30°C in the strict absence of dioxygen has been monitored by ¹H NMR spectroscopy. The product oxophlorin complexes are stable as long as the samples are protected from exposure to dioxygen. Hydrogen peroxide reacts cleanly with mono-meso-substituted iron(II) porphyrins in pyridine solution under an inert atmosphere to form mixtures of three possible oxygenation products, (py)₂Fe(cis-meso-R-OEPO), (py)₂Fe(trans-meso-R-OEPO), and (py)₂Fe(OEPO). The yields of (py)₂Fe(OEPO), which results from replacemement of the unique meso substituent, as a function of the identity of the meso substituent decrease in the order NO₂ > HC(O) ～ CN ～ Cl > OMe > Ph, Bu, which suggests that the species responsible for attack on the porphyrin periphery, is nucleophilic in nature. A mechanism involving isoporphyrin formation through attack of hydroxide ion on a cationic iron porphyrin with an oxidized porphyrin ring is suggested. The identity of the unique meso functionality also affects the regiospecificity of substitution when the unique meso group is retained. Although random attack at the two different meso sites is expected to yield a cis/trans product ratio of 2, the observed ratios vary in the following order: cyano, 5.0; n-butyl, 4.9; chloro, 3.2; formyl, 2.6; methoxy, 1.9; phenyl 1.4.

Full text of DOI:10.1021/ja011545b

J. Am. Chem. Soc. 2001, 123, 11719-11727
11719
Reactivity of Mono-Meso-Substituted Iron(II) Octaethylporphyrin
Complexes with Hydrogen Peroxide in the Absence of Dioxygen.
Evidence for Nucleophilic Attack on the Heme
Heather Kalish,† Jason E. Camp,† Marcin Ste¸pien´,‡ Lechosław Latos-Graz3yn´ski,‡ and
Alan L. Balch*
Contribution from the Departments of Chemistry, UniVersity of California, DaVis, California 95616, and
UniVersity of Wrocław, Wrocław, Poland
ReceiVed June 25, 2001
Abstract: Treatment of the mono-meso-substituted iron(II) octaethylporphyrin complexes, (py)2FeII(meso-
NO2-OEP), (py)2FeII(meso-CN-OEP), (py)2FeII(meso-HC(O)-OEP), (py)2FeII(meso-Cl-OEP), (py)2FeII(meso-
OMe-OEP), (py)2FeII(meso-Ph-OEP), and (py)2FeII(meso-n-Bu-OEP), with hydrogen peroxide in pyridine-d5
1
at -30 °C in the strict absence of dioxygen has been monitored by H NMR spectroscopy. The product
oxophlorin complexes are stable as long as the samples are protected from exposure to dioxygen. Hydrogen
peroxide reacts cleanly with mono-meso-substituted iron(II) porphyrins in pyridine solution under an inert
atmosphere to form mixtures of three possible oxygenation products, (py)2Fe(cis-meso-R-OEPO), (py)2Fe-
(trans-meso-R-OEPO), and (py)2Fe(OEPO). The yields of (py)2Fe(OEPO), which results from replacemement
of the unique meso substituent, as a function of the identity of the meso substituent decrease in the order NO2
> HC(O) ∼ CN ∼ Cl > OMe > Ph, Bu, which suggests that the species responsible for attack on the porphyrin
periphery is nucleophilic in nature. A mechanism involving isoporphyrin formation through attack of hydroxide
ion on a cationic iron porphyrin with an oxidized porphyrin ring is suggested. The identity of the unique meso
functionality also affects the regiospecificity of substitution when the unique meso group is retained. Although
random attack at the two different meso sites is expected to yield a cis/trans product ratio of 2, the observed
ratios vary in the following order: cyano, 5.0; n-butyl, 4.9; chloro, 3.2; formyl, 2.6; methoxy, 1.9; phenyl 1.4.
Introduction
or ferryl hemoglobin, respectively.7,8 Further reaction of ferryl
hemoglobin is reported to result in heme degradation with the
formation of fluorescent heme degradation products.9,10 Usually,
heme degradation is catalyzed by the enzyme heme oxygenase,
which acts on heme as substrate and uses that heme to bind
and activate dioxygen for heme cleavage.11,12 This process
requires an external electron source, and hydrogen peroxide can
be employed as a substitute for dioxygen and two electrons.
With hydrogen peroxide as oxidant, the heme/heme oxygenase
complex is converted under anaerobic conditions13 into the
R-meso-hydroxyheme/heme oxygenase complex, a known in-
termediate in heme cleavage.14 Electrophilic attack by an FeIII-
O-O-H intermediate on the heme meso position has been
proposed as the mechanism for the initial step in porphyrin ring
opening.11
Hydrogen peroxide interacts with heme proteins and hemes
themselves in many significant ways. Peroxidases are heme
enzymes that utilize hydrogen peroxide to generate highly
oxidized hemes that are subsequently used to oxidize organic
substrates,1 while catalases are designed to destroy hydrogen
peroxide through disproportionation. In both of these types of
heme enzymes, the Fe(III) state reacts with hydrogen peroxide
and is oxidized by two electrons to produce ferryl intermediates
and heme radicals.2,3 Although cytochrome P450 is designed
to utilize dioxygen to oxidize substrates, it can also utilize
hydrogen peroxide to effect the same transformations via the
peroxide shunt.4 Again, two-electron oxidation of the heme
protein is involved, but reactive intermediates do not become
detectable in this process. Bovine cytochrome c oxidase also
produces an oxidized protein with a ferryl ion and a porphyrin
free radical as well as a second free radical assigned as a
tryptophan cation radical when it is treated with hydrogen
peroxide.5,6 Treatment of myoglobin or hemoglobin with
hydrogen peroxide results in the formation of ferrylmyoglobin
The reactions of hydrogen peroxide with simple iron por-
phyrins, particularly with meso-tetra(aryl)porphyrins, have been
(5) Rigby, S. E. J.; Ju¨nemann, S.; Rich, P. R.; Heathcote, P. Biochemistry
2000, 39, 5921.
(6) Fabian, M.; Palmer, G. Biochemistry 1995, 34, 13802.
(7) Balch, A. L.; La Mar, G. N.; Latos-Grayz˘yn´ski, L.; Renner, M. W.;
Trhanabal, V. J. Am. Chem. Soc. 1985, 107, 3003.
(8) Giulivi, C.; Davies, K. J. A. Methods Enzymol. 1994, 186, 490.
(9) Nagababu, E.; Rifkind, J. M. Biochemistry 2000, 39, 12503.
(10) Nagababu, E.; Rifkind, J. M. Biochem. Biophys. Res. Commun. 1998,
247, 592.
† University of California.
‡ University of Wrocław.
(1) Everse, J.; Everse, K. E.; Grisham, M. B. Peroxidases in Chemistry
and Biology; CRC Press: Boca Raton, FL, 1991; Vols. I and II.
(2) Weiss, R.; Gold, A.; Trautwein, A. X.; Ternier, J. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: New York, 2000; Vol. 4, p 65.
(11) Ortiz de Montellano, P. R.; Wilks, A. AdV. Inorg. Chem. 2001, 51,
359.
(3) Strukul, G., Ed. Catalytic Oxidations with Hydrogen Peroxide as
Oxidant; Kluwer Academic Publishers: Boston, 1992.
(4) Ortiz de Montellano, P. R., Ed. Cytochrome P450: Structure,
Mechanism and Biochemistry; Plenum Press: New York, 1995.
(12) Yoshida, T.; Migita, C. T. J. Inorg. Biochem. 2000, 82, 33.
(13) Liu, Y.; Moe¨nne-Loccoz, P.; Loehr, T. M.; Ortiz de Montellano, P.
R. J. Biol. Chem. 1997, 272, 6909.
(14) Balch, A. L. Coord. Chem. ReV. 2000, 200-202, 349.
10.1021/ja011545b CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/01/2001

11720 J. Am. Chem. Soc., Vol. 123, No. 47, 2001
Kalish et al.
Scheme 1
utilized to effect olefin epoxidation as well as hydrocarbon
hydroxylation.15 However, reactions of hydrogen peroxide with
iron porphyrins frequently are accompanied by bleaching and
catalyst destruction. Consequently, a number of porphyrins have
been synthesized with substituent patterns designed to reduce
their susceptibility to peripheral oxidation.16,17 The results
reported here investigate the earliest stages in the heme
modification by hydrogen peroxide in cases where most of the
porphyrin meso sites are not protected by the presence of aryl
substituents.
subject of considerable study19-22 and has been crystallized and
characterized by X-ray diffraction in this laboratory.14,23
Spectroscopically, (py)2Fe(OEPO) is readily identified on the
1
basis of its characteristic H NMR spectrum, which displays
marked upfield shifts for the two types of meso protons and
both upfield and downfield shifts for the methylene protons.
Addition of pyridine to the dimer, {FeIII(OEPO)}2, in the strict
absence of dioxygen is another convenient route to prepare
(py)2Fe(OEPO), as also shown in Scheme 1.24,25 The electronic
Recently we have shown that treatment of (py)2FeII(OEP) with
hydrogen peroxide in pyridine solution results in the formation
of the meso-oxygenated heme (py)2(FeOEPO) as shown in
Scheme 1.18 The air-sensitive (py)2(FeOEPO) has been the
(18) Kalish, H. R.; Latos-Grayz˘yn´ski, L.; Balch, A. L. J. Am. Chem.
Soc. 2000, 122, 12478.
(19) Sano, S.; Sugiura, Y.; Maeda, Y.; Ogawa, S.; Morishima, I. J. Am.
Chem. Soc. 1981, 103, 2888.
(20) Morishima, I.; Fujii, H.; Shiro, Y. J. Am. Chem. Soc. 1986, 108,
3858.
(15) Meunier, B.; Robert, A.; Pratviel, G.; Bernadou, J. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: New York, 2000; Vol. 4, p 119.
(16) Traylor, P. S.; Dolphin, D.; Traylor, T. G. J. Chem. Soc., Chem.
Commun. 1984, 279.
(21) Morishima, I.; Shiro, Y.; Hiroshi, F. Inorg. Chem. 1995, 34, 1528.
(22) Balch, A. L.; Noll, B. C.; Reid, S. M.; Zovinka, E. P. Inorg. Chem.
1993, 32, 2610.
(23) Balch, A. L.; Koerner, R.; Latos-Grayz˘yn´ski, L.; Noll, B. C. J. Am.
Chem. Soc. 1996, 118, 2760.
(17) Dolpin, D.; Traylor, T. G.; Xie, L. Y. Acc. Chem. Res. 1997, 30,
251.
(24) Balch, A. L.; Latos-Grayz˘yn´ski, L.; Noll, B. C.; Olmstead, M. M.;
Zovinka, E. P. Inorg. Chem. 1992, 31, 2248.

ReactiVity of Fe(II) OEP Complexes with Hydrogen Peroxide
J. Am. Chem. Soc., Vol. 123, No. 47, 2001 11721
structure of (py)2Fe(OEPO) can be described by a combination
of the three canonical structures shown as 1a, 1b, and 1c in
Scheme 1. While the crystallographic data on this species
indicate that the iron is in a high spin state,7 the assignment of
oxidation states to the iron and ligand in this complex is less
clear, particularly in solution where the magnetic properties
indicate that changes in the magnetic coupling between metal
and ligand occur. Coupled oxidation of (py)2FeII(OEP) in
pyridine solution also produces (py)2(FeOEPO) as a detectable
intermediate. The coupled oxidation process, in which heme
degradation is brought about by dioxygen in the presence of a
reducing agent (generally hydrazine or ascorbic acid), has been
widely employed as a model for biological heme catabolism.26-28
This coupled oxidation procedure can be utilized to oxidize both
iron porphyrins29,30 and heme proteins such as myoglobin.31,32
Here we report on the reaction of hydrogen peroxide with
simple mono-meso-substituted iron(II) octaethylporphyrins un-
der carefully controlled conditions.
Scheme 2
Results and Discussion
The reactions of the mono-meso-substituted iron(II) porphy-
rins, (py)2FeII(meso-NO2-OEP), (py)2FeII(meso-HC(O)-OEP),
(py)2FeII(meso-CN-OEP), (py)2FeII(meso-Cl-OEP), (py)2FeII-
(meso-OMe-OEP), (py)2FeII(meso-Ph-OEP), and (py)2FeII(meso-
n-Bu-OEP), with hydrogen peroxide were conducted at -30
°C in the absence of dioxygen. Red pyridine-d5 solutions of
these low-spin (S ) 0) complexes were prepared under a
dinitrogen atmosphere to exclude any oxidation by dioxygen
and cooled to -30 °C. Introduction of dioxygen-free hydrogen
peroxide (50% in water)/pyridine-d5 solution (1/60 v/v) into the
1
sample produced a color change to brownish green. The H
NMR spectra of the resulting solutions showed the presence of
new sets of resonances due to the new, paramagnetic products.
After reaction there was no evidence for the formation of any
precipitate, and so the spectral data are indicative of the entire
course of the reaction. Once formed at -30 °C, the resulting
samples are stable and can be warmed to room temperature and
above without further reaction as long as they are protected from
dioxygen.
play four equally intense methylene resonances, while (py)2Fe-
(cis-meso-R-OEPO) will produce eight equally intense meth-
ylene resonances.
1
Part A of Figure 1 shows H NMR spectra resulting from
the addition of hydrogen peroxide to a pyridine-d5 solution of
1
(py)2FeII(meso-Cl-OEP). Part B of this figure shows the H
As shown in Scheme 2, the reaction of a mono-meso-
substituted iron porphyrin with hydrogen peroxide can result
in the formation of three products, (py)2Fe(cis-meso-R-OEPO),
(py)2Fe(trans-meso-R-OEPO), and (py)2Fe(OEPO). Air-sensitive
(py)2Fe(OEPO), which forms if the meso substituent itself is
replaced in the reaction with hydrogen peroxide, is readily
NMR spectrum of (py)2Fe(OEPO) which was prepared by
dissolving {FeIII(OEPO)}2 in pyridine-d5 under an inert atmo-
sphere. The resonances in this spectrum have all been assigned
on the basis of integrated resonance intensity and 1D NOE
difference spectra as reported previously.18 Three regions of the
spectrum are of particular importance in analyzing the new data
reported here. The far upfield region contains the meso proton
resonances which are broadened because of their proximity to
the paramagnetic iron ion. The downfield region at ca. 30 ppm
consists of well-separated resonances due to the methylene
protons in position 2 (see Scheme 2), while the methylene
protons at position 1, which is closest to the oxygen atom of
the tetrapyrrole, produce a resonance at -8 ppm. All the other
resonances of (py)2Fe(OEPO) are crowded into the region
between 5 and -2 ppm and are less useful in identifying
components in complex product mixtures.
1
identified on the basis of its characteristic H NMR spectrum
(see trace B of Figure 1).14,18,20,21 The 1H NMR spectra of (py)2-
Fe(cis-meso-R-OEPO) and (py)2Fe(trans-meso-R-OEPO) can be
readily distinguished and identified since they differ in sym-
metry. For example, (py)2Fe(cis-meso-R-OEPO) will display two
equally intense meso resonances (corresponding to individual
protons), while (py)2Fe(trans-meso-R-OEPO) will display only
one meso proton resonance (corresponding to two meso
protons). Additionally, (py)2Fe(trans-meso-R-OEPO) will dis-
(25) Balch, A. L.; Noll, B. C.; Reid, S. M.; Zovinka, E. P. Inorg. Chem.
1993, 32, 2610.
Comparison of the spectra in traces A and B reveals that
(py)2Fe(OEPO) is a significant product of the reaction of
(py)2FeII(meso-Cl-OEP) with hydrogen peroxide. However, the
presence of additional resonances indicates that (py)2Fe(cis-
meso-Cl-OEPO) and (py)2Fe(trans-meso-Cl-OEPO) are formed
as well. Thus, in the far upfield meso region there are three
additional resonances. Examination of the integrated intensities
of these resonances indicates that the resonances at -125 and
(26) Warburg, O.; Negelein, E. Chem. Ber. 1930, 63, 1816.
(27) Lagarias, J. C. Biochim. Biophys. Acta 1982, 717, 12.
(28) St. Claire, T. N.; Balch, A. L. Inorg. Chem. 1999, 38, 684.
(29) Balch, A. L.; Latos-Grayz˘yn´ski, L.; Noll, B. C.; Olmstead, M. M.;
Szterenberg, L.; Safari, N. J. Am. Chem. Soc. 1993, 115, 1422.
(30) Balch, A. L.; Latos-Grayz˘yn´ski, L.; Noll, B. C.; Olmstead, M. M.;
Safari, N. J. Am. Chem. Soc. 1993, 115, 9056.
(31) O’Carra, P.; Colleran, E. FEBS Lett. 1969, 5, 295.
(32) Hildebrand, D. P.; Tang, H.; Luo, Y.; Hunter, C. L.; Smith, M.;
Brayer, G. D.; Mauk, A. G. J. Am. Chem. Soc. 1996, 118, 12909.

11722 J. Am. Chem. Soc., Vol. 123, No. 47, 2001
Kalish et al.
Figure 1. (A) The 500 MHz 1H NMR spectrum of a pyridine-d5
solution of (py)2FeII(meso-Cl-OEP) after treatment with hydrogen
peroxide at -30 °C and warming to 0 °C. Inset A′′ shows an expansion
of the downfield methylene region, while inset A′ shows the upfield
methylene region. (B) The 500 MHz 1H NMR spectrum of a pyridine-
d5 solution of (py)2Fe(OEPO). Inset B′ shows an expansion of the
spectrum from 5 to -10 ppm. Resonances of (py)2Fe(OEPO) are labeled
m and m′ for the meso proton resonances, 1-4 for the methylene
protons corresponding to the numbering given in Scheme 1, and 1*-
4* for the methyl protons as numbered in Scheme 1. Resonances of
(py)2Fe(cis-meso-Cl-OEPO) and (py)2Fe(trans-meso-Cl-OEPO) are
labeled C and T, respectively.
Figure 2. The far upfield meso region of the 500 MHz 1H NMR spectra
obtained from the reaction of hydrogen peroxide with (A) (py)2FeII-
(meso-NO2-OEP), (B) (py)2FeII(meso-OMe-OEP), (C) (py)2FeII(meso-
CN-OEP), (D) (py)2FeII(meso-HC(O)-OEP), (E) (py)2FeII(meso-Ph-
OEP), and (F) (py)2FeII(meso-n-Bu-OEP) in pyridine-d5. Addition of
hydrogen peroxide was conducted at -30 °C, and the samples were
warmed to 0 °C, where the spectra were recorded. The inset to trace C
shows the -40 to -110 ppm region of the spectrum, where the
resonances of the cis and trans isomers are found. The inset to trace D
shows the -10 to -90 ppm region of the spectrum, where the
resonances of the cis and trans isomers are found. Resonances of
(py)2Fe(OEPO) are labeled m and m′, while those of the cis and trans
isomers are labeled C and T, respectively.
-195 ppm have equal intensity, and these resonances are
assigned to (py)2Fe(cis-meso-Cl-OEPO). The remaining reso-
nance at -140 ppm is assigned to (py)2Fe(trans-meso-Cl-
OEPO), which is expected to produce only a single resonance
from the two equivalent meso protons. Similarly, in the
downfield methylene region (from 20 to 30 ppm) there are three
resonances in addition to the resonance of (py)2Fe(OEPO). Inset
A′′ of Figure 1 shows an expansion of this region. The two
equally intense resonances at 22 and 23.5 ppm are assigned to
the methylene groups of (py)2Fe(cis-meso-Cl-OEPO), while the
remaining resonance at 23 ppm is assigned to the methylene
protons of (py)2Fe(trans-meso-Cl-OEPO). Finally, in the 0 to
-10 ppm region, which is shown in expanded form in inset A′
of Figure 1, there are also four resonances: two equally intense
resonances from (py)2Fe(cis-meso-Cl-OEPO), a unique reso-
nance from (py)2Fe(trans-meso-Cl-OEPO), and the clearly
identifiable resonance from (py)2Fe(OEPO). Careful examina-
tion of the integrated intensities of the resonances in each region
reveals that the compounds (py)2Fe(OEPO), (py)2Fe(cis-meso-
Cl-OEPO), and (py)2Fe(trans-meso-Cl-OEPO) are formed in
ratios of 1:2.7:1.
Similar reactions of hydrogen peroxide with other mono-
meso-substituted iron(II) octaethylporphyrins have been con-
ducted. Figure 2 shows portions of the 1H NMR spectra in the
far upfield region that result from treating samples of (py)2FeII-
(meso-NO2-OEP) (trace A), (py)2FeII(meso-MeO-OEP) (trace
B), (py)2FeII(meso-CN-OEP) (trace C), (py)2FeII(meso-HC(O)-
OEP) (trace D), (py)2FeII(meso-Ph-OEP) (trace E), and (py)2FeII-
(meso-n-Bu-OEP) (trace F) with hydrogen peroxide. As can be
seen in trace A, for (py)2FeII(meso-NO2-OEP) the spectrum
consists only of resonances due to (py)2Fe(OEPO); there is no
evidence for the formation of the potential products (py)2FeII-
(cis-meso-NO2-OEP) and (py)2FeII(trans-meso-NO2-OEP). In
traces B, C, and D, there are five resonances in the upfield
region, and consequently all three possible products form from
these three mono-meso-substituted porphyrins. However, for
(py)2FeII(meso-Ph-OEP) and (py)2FeII(meso-n-Bu-OEP), as can
be seen in traces E and F, respectively, there is no evidence for
the formation of (py)2Fe(OEPO). However, in both cases
resonances from the corresponding cis and trans isomers of the
substituted oxygenated hemes are observed. The assignment of
resonances in this region has been made through careful
integration of the resonance intensities at several temperatures
whenever there was a difficult assignment. For example, in trace
B the upfield resonance labeled C is so broad that it does not
appear to have the same intensity as its downfield counterpart.
However, consideration of the line width together with exami-
nation of the spectrum at higher temperatures indicates that the
assignment is correct.
1
Figure 3 shows the downfield methylene region of the H
NMR spectra of samples of (py)2FeII(meso-MeO-OEP) (trace
A), (py)2FeII(meso-CN-OEP) (trace B), (py)2FeII(meso-HC(O)-
OEP) (trace C), (py)2FeII(meso-Ph-OEP) (trace D), and (py)2FeII-
(meso-n-Bu-OEP) (trace E) after treatment with hydrogen
peroxide. As expected from the data shown in Figure 2, the
methylene resonance of (py)2Fe(OEPO) is present in traces A,
B, and C but absent in traces D and E. In traces B, C, and D,

ReactiVity of Fe(II) OEP Complexes with Hydrogen Peroxide
J. Am. Chem. Soc., Vol. 123, No. 47, 2001 11723
1
Figure 4. Temperature-dependent 500 MHz H NMR spectra of a
sample of (py)2FeII(meso-OMe-OEP) in pyridine-d5 after treatment with
hydrogen peroxide. Resonances are labeled in accord with those used
in Figure 3.
of local crowding on functional group mobility have been seen
in other octaethylporphyrins with unique meso substituents.33
Traces D and E of Figure 3 also reveal additional resonances
due to the unique meso substituents. In trace D for (py)2Fe-
(meso-Ph-OEPO), two resonances at 14 and 17 ppm are assigned
to the meta protons of the phenyl substituents of the cis and
trans isomers, respectively. The relative intensities of these
resonances is the same as the relative amounts of the cis and
trans isomers given by the resonances in the 20-30 ppm region.
The inset to trace E for (py)2Fe(meso-Bu-OEPO) shows the
downfield region where the resonances of the R-methylene
protons of the n-butyl group are found. In this case the
assignment is based on the assumption that the R-methylene
protons will experience the greatest hyperfine shift for any of
the n-butyl protons as a result of the large spin density at the
meso position. Since the resonances shown in this inset are the
only resonances found outside the 0-10 ppm region that are
otherwise unassigned, they have been assigned to the R-meth-
ylene protons of the n-butyl substituent. The other resonances
of the n-butyl group have not been detected and probably are
found in the crowded region from 0 to 10 ppm. Likewise, for
(py)2Fe(meso-MeO-OEPO) we have not located the methoxy
methyl resonance, which is also probably located in the crowded
0-10 ppm region.
Figure 3. The downfield methylene region of the 500 MHz 1H NMR
spectra obtained from the reaction of hydrogen peroxide with (A)
(py)2FeII(meso-OMe-OEP), (B) (py)2FeII(meso-CN-OEP), (C) (py)2FeII-
(meso-HC(O)-OEP), (D) (py)2FeII(meso-Ph-OEP), and (E) (py)2FeII-
(meso-n-Bu-OEP) in pyridine-d5. Addition of hydrogen peroxide was
conducted at -30 °C, and the samples were warmed to 0 °C, where
the spectra were recorded. The inset to trace B shows the spectrum at
25 °C, where the resonances of the cis and trans isomers are fully
resolved. The inset to trace E shows the R-methylene resonance of the
n-Bu group. Resonances of (py)2Fe(OEPO) are labeled 2, while those
of the cis and trans isomers are labeled C and T, respectively.
three other resonances appear in this window. These three
resonances result from the presence of the cis and trans
isomers: (py)2Fe(cis-meso-CN-OEPO) and (py)2Fe(trans-meso-
CN-OEPO); (py)2Fe(cis-meso-HC(O)-OEPO) and (py)2Fe(trans-
meso-HC(O)-OEPO); and (py)2Fe(cis-meso-Ph-OEPO) and
(py)2Fe(trans-meso-Ph-OEPO).
The spectrum obtained from oxidation of (py)2FeII(meso-
MeO-OEP) with hydrogen peroxide shown in trace A of Figure
3 consists of six resonances, not the expected three, along with
the resonance from (py)2Fe(OEPO). The two equally intense
resonances at ca. 24 ppm are assigned to the presence of (py)2Fe-
(trans-meso-MeO-OEPO), while the other four resonances are
assigned to (py)2Fe(cis-meso-MeO-OEPO). Although the peak
heights are unequal for these four resonances, careful integration
of these resonances indicates that the four lines do have equal
intensity. The doubling of the number of resonances for each
of these two isomers results from restricted rotation of the meso-
MeO group about the O-C(meso) bond. Crowding of the
methoxy group between the adjacent pyrrole ethylene groups
forces this group to an out-of plane location that renders the
two sides of the porphyrin plane inequivalent. Similar effects
Figure 4 shows the effects of variation in the temperature on
the spectrum of the sample obtained by hydrogen peroxide
oxidation of (py)2FeII(meso-MeO-OEP). At 0 °C the four
resonances of (py)2Fe(cis-meso-MeO-OEPO) and the two
resonances of (py)2Fe(trans-meso-MeO-OEPO) are clearly
distinguished. Upon warming of the sample to 40 °C, the two
methylene resonances of (py)2Fe(trans-meso-MeO-OEPO) coa-
lesce. The four methylene resonances of (py)2Fe(cis-meso-MeO-
OEPO) also broaden and partially coalesce as the sample is
(33) Balch, A. L.; Latos-Grayz˘yn´ski, L.; Noll, B. C.; Olmstead, M. M.;
Zovinka, E. P. Inorg. Chem. 1992, 31, 2248.

11724 J. Am. Chem. Soc., Vol. 123, No. 47, 2001
Kalish et al.
Table 1. Percent Product Yields
genation products, as shown in Scheme 2. These complexes
are stable in pyridine solution as long as the solutions are
protected from reaction with dioxygen, which transforms the
oxygenated hemes into substituted biliverdins and verdohemes,
as described previously for (py)2Fe(OEPO).28
(py)2Fe-
(py)2Fe-
(py)2Fe- (cis-meso-R- (trans-meso-R-
(OEPO)
OEPO)
OEPO)
(py)2FeII(meso-NO2-OEP)
(py)2FeII(meso-HC(O)-OEP)
(py)2FeII(meso-CN-OEP)
(py)2FeII(meso-Cl-OEP)
(py)2FeII(meso-MeO-OEP)
(py)2FeII(meso-Ph-OEP)
(py)2FeII(meso-n-Bu-OEP)
100
28
22
21.5
12
0
0
54
65
57
57
59
83
0
17
13
21.5
31
41
17
As can be seen in Table 1, the product ratios obtained in the
heme/hydrogen peroxide reactions strongly depend of the
identities of the meso substituents. Thus, with a nitro group at
one of the meso positions, the reaction with hydrogen peroxide
results in the complete removal of the nitro group. However,
with a phenyl or n-butyl group at a meso position, there is no
loss of the unique substituent, and reactivity occurs exclusively
at the cis and trans meso C-H groups. Thus, the yields of
(py)2Fe(OEPO) as a function of the identity of the meso
substituent decrease in the order NO2 > HC(O) ∼ CN ∼ Cl >
OMe > Ph, Bu. A similar trend is seen in the Swain F parameter
(derived from Hammett σ values): NO2 (1.00) > CN (0.90) >
Cl (0.72) > OMe (0.54) > Ph (0.25) > Me (0.01).34 These
data suggest that the species responsible for attack on the
porphyrin periphery is nucleophilic in nature.
Scheme 3 sets out a proposed mechanism for the transforma-
tion of (py)2Fe(meso-R-OEP) into (py)2Fe(OEPO). Conversion
of (py)2Fe(meso-R-OEP) eventually into intermediate B involves
a two-electron oxidation, and there is ample precedent for
hydrogen peroxide producing a two-electron oxidation of a
heme. This process may be accomplished initially by Fenton
type oxidation35 to produce intermediate A, hydroxide ion, and
hydroxyl radical. The latter subsequently serves as the oxidant
to convert intermediate A into intermediate B. Hydroxyl radical
is known to be a powerful one-electron oxidant.36 Intermediate
B may exist as the Fe(IV) complex as shown in the chart, or it
may exist as the equivalent Fe(III)/porphyrin radical complex.
Examples of both forms are known, e.g., Fe(IV), (MeO)2FeIV-
(meso-tetra(mesityl)porphrin),37 and Fe(III)/porphyrin radical,
(O3ClO)2FeIII(OEP•).38
0
warmed. Upon cooling, these changes are reversed. The changes
are consistent with a dynamic process in which rotation of the
methoxy group about the O-C(meso) bond renders the two
sides of the porphyrin plane spectroscopically equivalent. As a
result, the dynamic process causes diasteriotopic methylene
protons of ethyl groups to become equivalent in a pairwise
fashion. Upon warming of the sample, the methylene resonance
of (py)2Fe(OEPO) undergoes a marked downfield shift. This
anti-Curie behavior has been observed before20,21 and is
consistent with the presence of magnetically coupled spins on
the ligand and metal. The resonances of (py)2Fe(cis-meso-MeO-
OEPO) and (py)2Fe(trans-meso-MeO-OEPO) also undergo non-
Curie shifts that are less pronounced over this temperature range,
in addition to the changes that result from rotation about the
O-C(meso) bond.
1
The downfield methylene portion of the H NMR spectrum
resulting from addition of hydrogen peroxide to (py)2FeII(meso-
n-Bu-OEP) seen in trace E of Figure 3 also shows four equally
intense resonances that result from (py)2FeII(cis-meso-n-Bu-
OEP), and two equally intense resonances due to (py)2FeII(trans-
meso-n-Bu-OEP). The additional resonances arise from restricted
rotation of the n-Bu group about the C-C(meso) bond, as seen
for (py)2Fe(cis-meso-MeO-OEPO) and (py)2Fe(trans-meso-
MeO-OEPO). Increasing the sample temperature results in
broadening and coalescence of these resonances in a fashion
which is similar to the behavior seen in Figure 4.
The transformation of intermediate B into intermediate C is
accomplished by nucleophilic attack of hydroxide ion on the
heme. This reaction produces the isoporphyrin-containing
intermediate C. There is ample precedent for nucleophilic
additions to cationic metalloporphyrins39 (particularly iron(III)
tetraphenylporphyrin cation radicals)40-42 or for nucleophilic
additions to porphyrin radicals.43-45 The addition of hydroxide
ion to a cationic gold(III) tetraphenylporphyrin to produce a
stable isoporphyrin complex analogous to intermediate C has
particular significance in this regard.46 However, as noted above,
Integration of the relative intensities of the various resonances
in the spectra reported here allows the yields of the individual
products to be calculated. The relevant data are presented in
Table 1. The results given in this table have been replicated
three times for each different porphyrin.
The reactivity of iron porphyrins with hydrogen peroxide is
oxidation state dependent. The reactivity of iron(III) porphyrins
with hydrogen peroxide has also been explored under similar
conditions. Treatment of ClFeIII(OEP) or ClFeIII(meso-NO2-
OEP) with hydrogen peroxide in pyridine-d5 solution does not
produce a detectable change in the 1H NMR spectrum of either
complex. Additionally, 1H NMR studies reveal that addition of
potassium hydroxide to a pyridine solution of ClFeIII(OEP) or
ClFeIII(meso-NO2-OEP) does not form (py)2Fe(OEPO) but
produces new species with 1H NMR spectra that are indicative
of high-spin species which are presumed to be HOFeIII(OEP)
or HOFeIII(meso-NO2-OEP). As noted earlier in our study of
the reaction of (py)2FeII(OEP) with hydrogen peroxide,18 there
is no evidence for the formation of ferryl, (FedO)2+, intermedi-
ates in the reactions of hydrogen peroxide with (py)2FeII(meso-
R-OEP).
(34) Swain, C. G.; Unger, S. H.; Rosenquist, N. R.; Swain, M. S. J. Am.
Chem. Soc. 1983, 105, 492.
(35) Walling, C. Acc. Chem. Res. 1975, 8, 125.
(36) Kochi, J. A. In Free Radicals; Kochi, J. K., Ed.; John Wiley and
Sons: New York, NY, 1973; p 673.
(37) Groves, J. T.; Quinn, R.; McMurry, J. T.; Nakamura, M.; Lang,
G.; Boso, B. J. Am. Chem. Soc. 1985, 107, 354.
(38) Scheidt, W. R.; Song, H.; Haller, K. J.; Safo, M. K.; Orosz, R. D.;
Reed, C. A.; Debrunner, P. G.; Schultz, C. E. Inorg. Chem. 1992, 31, 941.
(39) Vicente, M. G. H. In The Porphyrin Handbook; Kadish, K. M.,
Smith, K. M., Guillard, R., Eds.; Academic Press: San Diego, 2000; p
149.
(40) Maek, A.; Latos-Grayz˘yn´ski, L.; Bartczak, T. J.; Zadlo, A. Inorg.
Chem. 1991, 30, 3222.
(41) Chmielewski, P.; Latos-Grayz˘yn´ski, L.; Rachlewicz, K. Magn.
Reson. Chem. 1993, 31, S47.
(42) Rachlewicz, K.; Latos-Grayz˘yn´ski, L. Inorg. Chem. 1995, 34, 718.
(43) Smith, K. M.; Barnett, G. H.; Evans, B.; Martynenko, Z. J. Am.
Chem. Soc. 1979, 101, 5953.
Conclusions
(44) Dolphin, D.; Halko, D. J.; Johnson, E. C.; Rousseau, K. In Porphyrin
Chemistry AdVances; Longo, F. R., Ed.; Ann Arbor Science Publisher,
Inc.: Ann Arbor, MI, 1979; p 119.
(45) Shine, H. J.; Padilla, A. G.; Wu, S. M. J. Org. Chem. 1979, 23,
4069.
Mechanism of Reaction. The results described above
demonstrate that hydrogen peroxide reacts cleanly with mono-
meso-substituted iron(II) porphyrins in pyridine solution under
an inert atmosphere to form mixtures of three possible oxy-

ReactiVity of Fe(II) OEP Complexes with Hydrogen Peroxide
J. Am. Chem. Soc., Vol. 123, No. 47, 2001 11725
Scheme 3
hydroxide ion does not transform iron(III) porphyrins into
(py)2Fe(OEPO). Hence, we propose that the more highly
oxidized intermediate B rather than A is attacked by hydroxide
ion. Isoporphyrins readily revert into porphyrins through the
loss of one of the substituents on the modified meso carbon.47
Thus, the transformation from C into the observed product,
(py)2Fe(OEPO), also has precedent. Pyridine reacts with meso-
tetraphenylporphyrin iron(III) radical cations at the â-pyrrole
sites to form stable substitution products and at the meso carbon
atoms to produce less stable isoporphyrins and porpho-
dimethenes.48 Since there are ethyl groups in the â-pyrrole sites
in the porphyrins studied here, these sites are protected from
attack by hydroxide or pyridine, and initial addition is directed
to the unprotected meso sites. However, pyridine is known to
add to the π-cation radicals of magnesium(II) porphyrins to yield
meso-pyridinium porphyrin salts.49,50 In the mechanism proposed
in Scheme 3, pyridine may add reversibly to the meso sites of
intermediate B, but the overall reaction may be driven to the
observed products by the addition of hydroxide, which becomes
irreversible through loss of a proton on the hydroxyl group.
The identity of the unique meso functionality also affects the
regiospecificity of substitution in those cases where that unique
meso group is retained. Thus, while random attack at the two
different meso sites is expected to yield a cis/trans product ratio
of 2, the observed ratios vary in the following order: cyano,
5.0; n-butyl, 4.9; chloro, 3.2; formyl, 2.6; methoxy, 1.9; phenyl
1.4. Thus, the iron porphyrins with cyano and n-butyl substit-
uents, with their very different electron-withdrawing and -donat-
ing characteristics, produce the greatest preference for cis
substitution. Several factors may be involved in causing these
variations in attack at the two different meso positions. The
introduction of a unique meso substituent will alter the nature
of the porphyrin π-orbitals. The modified versions of the frontier
3a2u-like and the 4e-like orbitals, which have large amplitudes
at the porphyrin meso positions, provide means of electronic
communication between these seemingly remote meso sites.
Even steric factors may be involved. The environment of the
meso sites in hemes has been recognized to be sterically
constricted.51 Introduction of a unique meso substituent will limit
the conformational flexibility of the adjacent ethyl substituents
in these modified porphyrins, and these constraints can be felt
throughout the macrocycle. For example, the unique meso
substituent in high-spin, five-coordinate ClFe(meso-R-OEP)
Fe(III) has been found to warp the porphyrin surface away from
the symmetrical dome shape found for ClFeIII(OEP).52 In the
cases studied so far, each substituent causes a distinctively
(46) Segawa, H.; Azumi, R.; Shimadzu, T. J. Am. Chem. Soc. 1992, 114,
7564.
(47) Dolphin, D.; Felton, R. H.; Borg, D. C.; Fajer, J. J. Am. Chem.
Soc. 1970, 73, 1970.
(48) Rachlewicz, K.; Latos-Grayz˘yn´ski, L. Inorg. Chem. 1996, 35, 1136.
(49) Barnett, G. H.; Evans, B.; Smith, K. M.; Besecke, S.; Fuhrhop, J.-
H. Tetrahedron Lett. 1976, 4009.
(50) Fuhrhop, H.-J.; Wanja, U.; Bunzel, M. Liebigs Ann. Chem. 1984,
426.
(51) Woodward, R. B. Angew. Chem. 1960, 72, 651.

11726 J. Am. Chem. Soc., Vol. 123, No. 47, 2001
Kalish et al.
Table 2. Chemical Shifts (in ppm) for Meso Proton Resonances at
0 °C
different change in the structure. Consequently, it is not possible
at this stage to make predictions about the nature of the distortion
caused by these new meso substituents to either the electronic
or the geometric structures of the modified porphyrin macro-
cycles.
cis isomer cis isomer trans isomer
Hm
Hm′
Hm
(py)2Fe(meso-OMe-OEPO)
(py)2Fe(meso-n-Bu-OEPO)
(py)2Fe(meso-Ph-OEPO)
(py)2Fe(OEPO)
(py)2Fe(meso-Cl-OEPO)
(py)2Fe(meso-CN-OEPO)
(py)2Fe(meso-HC(O)-OEPO)
-152
-146
-130
-127a
-125
-61
-226
-202
-191
-178b
-195
-104
-79
-168
-129
-140
The observations described here for the reaction of mono-
meso-substituted iron(II) porphyrins with hydrogen peroxide
contrast with the behavior observed for heme oxygenase. In that
reaction, the mechanism proposed for the initial phase of heme
degradation is postulated to involve electrophilic attack by an
FeIII-O-O-H species on a meso carbon of the heme.11 It is
probable that the difference in these two reactions results from
independent means of attack on the heme. In the present work
with the strongly ligating environment provided by pyridine
solution, the formation of an FeIII-O-O-H species is inhibited.
However, the mechanism set out in Scheme 3 provides an
alternate means of accomplishing the same result, heme
oxygenation.
Mono-meso-substituted iron(II) porphyrins have also been
used as substrates for heme oxygenase. Their behavior in this
reaction contrasts with the observations made here for the
reactions with hydrogen peroxide. Remarkably, R-meso-methyl-
protoheme reacts with dioxygen and heme oxygenase to yield
R-biliverdin.53,54 The R-methyl substituent does not protect the
attached meso carbon atom from oxidation as the butyl group
does in the reactions studied in this paper. However, the fate of
the meso carbon and its attached methyl group is not known.
In contrast, the reaction of meso-formyl-substituted hemes with
heme oxygenase yields biliverdins that retain appended formyl
groups.55 Thus, R-meso-formyl-mesoheme does not yield
R-biliverdin when treated with heme oxygenase. The presence
of a formyl group inhibits oxidation at the adjacent meso site.
In contrast, in the reaction of (py)2FeII(meso-HC(O)-OEP) with
hydrogen peroxide, as shown in Table 1, there is considerable
loss of the formyl substituent and the formation of (py)2Fe-
(OEPO).
The coupled oxidation of iron porphyrins in pyridine has long
been used as a model for heme oxidase.56 In this environment,
oxidation of protoheme by coupled oxidation gives a mixture
of the four possible isomeric biliverdins.57 However, the results
described here suggest that reactions in pyridine, which presents
a strongly ligating environment, can occur through an indepen-
dent mechanism as presented in Scheme 3 for the reaction with
hydrogen peroxide. At this point there is no model that allows
the oxidation of heme to be probed in a less strongly ligating
environment that might more effectively model the protein
environment in heme oxygenase to examine the intrinsic
specificity of heme attack.
Electronic Structure of the Oxygenated Intermediates. The
patterns of hyperfine shifts for the newly formed intermediates,
(py)2Fe(cis-meso-R-OEPO) and (py)2Fe(trans-meso-OEPO),
resemble that of (py)2Fe(OEPO), itself. Thus, all compounds
show meso resonances with upfield shifts, methylene resonances
with both upfield and downfield shifts, and methyl resonances
in the 5-0 ppm region. The meso proton resonances show the
-140
-47
-17
-41
a Since there are no isomers, this is the Hm resonance. b Since there
are no isomers, this is the Hm′ resonance
greatest variation in shifts. The single meso resonance is found
at -17 ppm for (py)2Fe(trans-meso-HC(O)-OEPO) at 0 °C,
while the corresponding resonance occurs at -168 ppm for
(py)2Fe(trans-meso-MeO-OEPO) (again at 0 °C). Since all new
intermediates observed in this work display spectral patterns
similar to that of (py)2Fe(OEPO), all involve the set of electronic
distributions seen in Scheme 1, with the meso substituent
offering a perturbation on the varying contributions of each
resonance form: A, B, and C. The hyperfine shift of the meso
proton resonances is dominated by the location of the individual
proton relative to the oxygen atom on the porphyrin periphery.
Consideration of the spectrum of (py)2Fe(OEPO) suggests that
the protons trans to the oxygen atom (those labeled Hm′ in
Scheme 3) are expected to have chemical shifts that are upfield
of the resonances from the protons (Hm) that are cis to the
oxygen atom. Thus, for each cis isomer the downfield meso
resonance is assigned to Hm, and the upfield resonance is
assigned to Hm′. Inspection of the data in Figure 3 and the
chemical shifts for the meso resonances of these intermediates
in Table 2 reveals that the chemical shift for the meso proton
of the trans isomer is similar to that of one of the meso protons
of the cis isomer and far downfield from the other meso proton
resonance of the cis isomer. This is consistent with the fact that
the trans isomer possesses only meso protons (Hm) which are
cis to the oxygen atom. Finally, as can be seen in Table 2, the
meso proton resonances generally show a shift downfield as
the electron-withdrawing nature of the unique meso substituent
increases. However, (py)2Fe(trans-meso-MeO-OEPO) does not
follow this trend. The electron-withdrawing substituents in
(py)2Fe(trans-meso-NC-OEPO) and (py)2Fe(trans-meso-HC(O)-
OEPO) produce marked downfield shifts of the meso proton
resonances.
1
Additionally, the H NMR spectra of (py)2Fe(cis-meso-R-
OEPO) and (py)2Fe(trans-meso-R-OEPO) show non-Curie
behavior under variable temperature conditions. For example,
in addition to the effects of molecular rotation seen in Figure
4, the methylene resonances of all three species present show
decidedly non-Curie shifts upon warming. This non-Curie
behavior must result from variation in the coupling interactions
between the metal- and ligand-based spins in these intermediates
as previously discussed for (py)2Fe(OEPO).14,18,20,21,23
Experimental Section
(52) Kalish, H.; Camp, J. E.; Stepien´, M.; Latos-Grayz˘yn´ski, L.;
Olmstead, M. M.; Balch, A. L., manuscript in preparation.
(53) Torpey, J.; Ortiz de Montellano, P. R. J. Biol. Chem. 1996, 271,
26067.
(54) Torpey, J.; Lee, D. A.; Smith, K. M.; Ortiz de Montellano, P. R. J.
Am. Chem. Soc. 1996, 118, 9172.
Materials. Octaethylporphyrin and iron(III) octaethylporphyrin
chloride were purchased from Mid Century. Hydrogen peroxide, 50
wt % in water, was purchased from Aldrich. The mono-meso-substituted
porphyrins, meso-NO2-H2OEP,58 meso-CN-H2OEP,59 meso-Cl-H2OEP,60
(55) Torpey, J.; Ortiz de Montellano, P. R. J. Biol. Chem. 1997, 272,
22008.
(56) Warburg, O.; Negelein, E. Chem. Ber. 1930, 63, 1816.
(57) Bonnett, R.; McDonagh, A. F. J. Chem. Soc., Perkin Trans. 1 1973,
881.
(58) Bonnett, R.; Stephenson, G. F. J. Org. Chem. 1965, 30, 2791.
(59) Johnson, A. W.; Oldfield, D. J. Chem. Soc. (C) 1966, 794.
(60) Bonnett, R.; Gale, A. D.; Stephenson, G. F. J. Chem. Soc. (C) 1966,
1600.

ReactiVity of Fe(II) OEP Complexes with Hydrogen Peroxide
J. Am. Chem. Soc., Vol. 123, No. 47, 2001 11727
meso-MeO-H2OEP,61 meso-Ph-H2OEP,62 meso-n-Bu-H2OEP,60 and
meso-n-HC(O)-H2OEP,63 were prepared as described previously. Iron
was inserted by a standard procedure to yield the five-coordinate chloro
complexes, ClFeIII(meso-R-OEP).64 The bis(pyridine)iron(II) porphyrins
were subsequently prepared by reduction of the appropriate iron(III)
complex with zinc amalgam in pyridine solution in a controlled
atmosphere drybox under purified dinitrogen. Although generally this
reduction went smoothly to produce the corresponding iron(II) por-
phyrin, the reduction of ClFeIII(meso-HC(O)-OEP) must be stopped
after 5 min and the sample separated from the reducing agent. Longer
reaction times result in further reactions to produce (py)2FeIII(meso-
HOCH2-OEP) and other unidentified products.65
(3-4 mmol) was placed in an NMR tube and sealed with a rubber
septum and Parafilm. This solution was cooled to -30 °C in an ethanol/
liquid nitrogen bath. Dioxygen was removed from hydrogen peroxide
samples by five freeze/pump/thaw cycles. A pyridine-d5 solution of
hydrogen peroxide (60:1) was added via microsyringe to the porphyrin
solution at -30 °C to give hydrogen peroxide concentrations in the
range of 4-6 mmol. The progress of the reaction was followed by 1H
NMR spectroscopy.
Instrumentation. 1H NMR spectra were recorded on a Bruker
Avance 500 FT spectrometer (1H frequency is 500.13 MHz). The
spectra were recorded over a 100-kHz bandwidth with 64K data points
and a 5-µs 90° pulse. For a typical spectrum, between 500 and 1000
transients were accumulated with a 50-ms delay time. The residual 1H
resonances in the solvent, pyridine- d5, were used as a secondary
reference.
Reactions with H2O2. In a controlled nitrogen atmosphere box, a
dioxygen-free pyridine-d5 solution of the bis(pyridine)iron(II) porphyrin
(61) Barnett, G. H.; Hudson, M. F.; McCombie, S. W.; Smith, K. M. J.
Chem. Soc. Perkin Trans. 1 1973, 691.
(62) Kalisch, W. W.; Senge, M. O. Angew. Chem., Int. Ed. 1998, 37,
1107.
(63) Inhoffen, H. H.; Fuhrop, J.-H.; Voit, H.; Brockman, H. Ann. Chem.
1966, 695, 133.
(64) Adler, A. D.; Longo, F. R.; Kampas, F. J. Inorg. Nucl. Chem. 1970,
32, 2443.
Acknowledgment. We thank the NIH (Grant GM-26226,
A.L.B.) and the Foundation for Polish Science (L.L.G.) for
financial support and the NSF (Grant OSTI 97-24412) for partial
funding of the 500 MHz NMR spectrometer.
(65) Kalish, H. R. Ph.D. Thesis, Univeristy of California, Davis, 2001.
JA011545B