Electrochemistry and spectroelectrochemistry of iron porphyrins in the presence of nitrite

Article abstract of DOI:10.1016/S0020-1693(00)00387-X

The reaction of nitrite with ferric and ferrous porphyrins was examined using visible, infrared and NMR spectroscopy. Solutions of either ferric or ferrous porphyrin were stable in the presence of nitrite, with only complexation reactions being observed. Under voltammetric conditions, though, a rapid reaction between nitrite and iron porphyrins was observed to form the nitrosyl complex, Fe(P)(NO), where P=porphyrin. The products of the reduction of ferric porphyrins in the presence of nitrite were confirmed by visible spectroelectrochemistry to be Fe(P)(NO) and [Fe(P)]₂O. Visible, NMR and infrared spectroscopy were used to rule out the formation of Fe(P)(NO) by the iron-catalyzed disproportionation of nitrite. A reaction between iron porphyrins and nitrite only occurred by the presence of both oxidation states (ferric/ferrous). The kinetics of the reaction were monitored by visible spectroscopy, and the reaction was found to be first-order with respect to Fe(OEP)(Cl) and Fe(OEP). The products were the same as those observed in the spectroelectrochemical experiment. The rate was not strongly dependent upon the concentration of nitrite, indicating that the coordinated, not the free nitrite, was the reaction species. The kinetics observed were consistent with a mixed oxidation state nitrite-bridged intermediate, which carried out the oxygen transfer reaction from nitrite to the iron porphyrin. The effect of nitrite coordination on the reaction rate was examined.

Full text of DOI:10.1016/S0020-1693(00)00387-X

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 314 (2001) 49–57
Electrochemistry and spectroelectrochemistry of iron porphyrins
in the presence of nitrite
Zhongcheng Wei, Michael D. Ryan *
Chemistry Department, Marquette Uni6ersity, PO Box 1881, Milwaukee, WI 53201, USA
Received 3 June 2000; accepted 17 November 2000
Abstract
The reaction of nitrite with ferric and ferrous porphyrins was examined using visible, infrared and NMR spectroscopy.
Solutions of either ferric or ferrous porphyrin were stable in the presence of nitrite, with only complexation reactions being
observed. Under voltammetric conditions, though, a rapid reaction between nitrite and iron porphyrins was observed to form the
nitrosyl complex, Fe(P)(NO), where P=porphyrin. The products of the reduction of ferric porphyrins in the presence of nitrite
were confirmed by visible spectroelectrochemistry to be Fe(P)(NO) and [Fe(P)]2O. Visible, NMR and infrared spectroscopy were
used to rule out the formation of Fe(P)(NO) by the iron-catalyzed disproportionation of nitrite. A reaction between iron
porphyrins and nitrite only occurred by the presence of both oxidation states (ferric/ferrous). The kinetics of the reaction were
monitored by visible spectroscopy, and the reaction was found to be first-order with respect to Fe(OEP)(Cl) and Fe(OEP). The
products were the same as those observed in the spectroelectrochemical experiment. The rate was not strongly dependent upon
the concentration of nitrite, indicating that the coordinated, not the free nitrite, was the reaction species. The kinetics observed
were consistent with a mixed oxidation state nitrite-bridged intermediate, which carried out the oxygen transfer reaction from
nitrite to the iron porphyrin. The effect of nitrite coordination on the reaction rate was examined. © 2001 Elsevier Science B.V.
All rights reserved.
Keywords: Iron porphyrins; Nitrite; Spectroelectrochemistry; Reaction kinetics; Nitrosyl
1. Introduction
phyrins generally form bis-nitro low-spin complexes
[1,3].
Over the past two decades, the reaction of nitrite
with iron porphyrins and heme-containing enzymes has
been the subject of considerable study. Nitrite is a very
versatile molecule that can be involved in a wide variety
of reactions, such as coordination, acid/base or oxygen
transfer chemistry. Because of this reactivity, nitrite can
be, in some cases, a necessary intermediate (in the
nitrogen cycle) or, in others, a toxic material. Although
nitrite can react with a large number of transition
metals, the primary focus of this work will involve iron
porphyrins, which are models for iron enzymes that
react with nitrite. Nitrite can coordinate with both the
ferric [1–3] and ferrous porphyrins [4]. Ferric por-
Fe(P)(X)+NO2− X Fe(P)(NO2)+X− K1
Fe(P)(NO2)+NO2− X Fe(P)(NO2)2− K2
If X−=halide, the mono-nitrite complex is only
present over a narrow concentration range [1]. Weaker
axial ligands such as NO3− or ClO4− can be readily
displaced by nitrite, and both the mono- and bis-com-
plexes have been observed.
There has been some controversy as to the stability
of the nitrite–ferric porphyrin complexes. Finnegan et
al. [5] found that ferric porphyrins reacted slowly with
nitrite to lead to Fe(P)(NO) and Fe(P)(NO3). O’Shea et
al. [6] observed that ferric nitrite complexes were stable
under an argon atmosphere, and could carry out a wide
variety of oxygen transfer reactions if an appropriate
substrate was present:
(1)
(2)
* Corresponding author. Tel.: +1-414-288 7065; fax: +1-414-288
7066.
E-mail address: michael.ryan@mu.edu (M.D. Ryan).
PFe(NO2)+S“PFeNO+SO
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(00)00387-X

50
Z. Wei, M.D. Ryan / Inorganica Chimica Acta 314 (2001) 49–57
A later report attributed the instability of nitrite–ferric
porphyrin complexes to the generation of ozone from
trace dioxygen [7]. In addition to oxygen transfer reac-
tions, nitrite may also be a nucleophile. Fanning and
Keefer [8] found that nitrite will react readily with
methylene chloride to form organonitro compounds.
In the assimilatory and dissimilatory nitrite reduc-
tases, nitrite is first reduced to NO. The NO molecule
will dissociate in cytochrome cd1 nitrite reductases, but
is reduced to ammonia in assimilatory nitrite reduc-
tases. The reaction of nitrite with metmyoglobin [9] and
hemoglobin [10,11] has also been examined. For both
proteins, the reaction product was an NO-coordinated
protein.
2.2. Equipment
Cyclic voltammograms were obtained with a Cypress
CySy2Ra potentiostat. The reference electrode was a
0.1 M Ag/AgNO3 electrode in acetonitrile. The working
and auxiliary electrodes were platinum. An optically
transparent thin-layer electrochemical (OTTLE) cell
with a platinum-gauze working electrode was used for
the visible spectroelectrochemical studies [15]. The spec-
tra were obtained with a Hewlett-Packard 8452 diode
array spectrophotometer. The infrared spectra were
obtained with a Mattson 4020 Galaxy FT-IR. The
infrared spectroelectrochemical cell was a modified
Wilmad semi-permanent cell [16]. The Teflon spacer
In spite of these studies, the details of the reaction of
nitrite with ferric and ferrous porphyrins are not clear.
Many of the experiments by Finnegan et al. [5] were
carried out in methylene chloride, creating some ambi-
guities as to whether the reactions were due to nitrite or
organonitro compounds. Some of the differences may
also be due to equilibrium shifts caused by the presence
or absence of halide ions (e.g. chloride as the axial
ligand). Munro and Scheidt [12] have shown that the
mono-nitrite complex is more reactive than the bis-nitro
complex. The presence of this complex was minimized
in the work of Castro et al. [13] because they utilized
ferric porphyrins with chloride as the axial ligand
(Fe(P)(Cl)). In order to investigate these issues, visible
and infrared spectroelectrochemistry will be used in this
work in order to characterize as completely as possible
the intermediates that are formed in the reaction of
nitrite with ferric and/or ferrous porphyrins.
between two KBr windows was replaced by a
polyethylene spacer in which the working, reference and
auxiliary electrodes were melt-sealed. The working and
auxiliary electrodes were fabricated from 100-mesh
platinum gauze (Aldrich Chemical Co.), and a silver
wire (0.05 mm diameter, Johnson Matthey) was used as
a pseudo reference electrode.
2.3. Procedures
All the voltammetric solutions were deoxygenated by
deaerating the solution for 15 min with pre-purified
dinitrogen. The dinitrogen was pre-saturated with the
solvent in order to prevent evaporation. The spec-
troelectrochemical solutions were prepared in a glove
box, and the data were obtained after the current had
decayed to the background. The visible kinetics experi-
ments were carried out using an anaerobic cell (Wilmad
Glass Co., WG-28-G). The iron porphyrin solution was
placed in the cuvette, and the nitrite solution was
placed in the side arm. The reaction was carried out in
an argon atmosphere. After the starting spectrum was
recorded, the solutions were mixed, and the spectra
were obtained using a Hewlett-Packard 8452 diode
array spectrophotometer in the kinetics mode. The
solutions were prepared and the cell was filled in a
glove box.
2. Experimental
2.1. Chemicals
Octaethylporphyrin iron(III) chloride (Fe(OEP)Cl),
tetraphenylporphyrin iron(III) chloride (Fe(TPP)Cl),
bis(triphenylphosphoranylidene)ammonium
nitrite
(PNPNO2), sodium nitrite, bis(triphenylphosphoranyli-
dene)ammonium chloride (PNPCl), dimethyl for-
mamide, tetrahydrofuran (THF), and deuterated NMR
solvents were obtained from Aldrich Chemical Co.
Sodium nitrite (15N) was obtained from Isotec, and
tetrabutylammonium perchlorate (TBAP) from G.F.
Smith Chemical Co. Bis(triphenylphosphoranyli-
dene)ammonium nitrite (15N) was obtained from the
metathesis reaction of PNPCl and sodium nitrite-15N in
distilled water. The product precipitated out of solution
and was vacuum dried. Zinc amalgam was prepared by
a literature procedure [14]. Ferrous-OEP was obtained
by the reduction of Fe(OEP)(Cl) in THF by zinc
amalgam.
3. Results and discussion
3.1. Stability of ferric porphyrin–nitrite complexes
Nitrite in the presence of iron porphyrins has been
notorious for the ease by which it can undergo redox
reactions. In order to characterize its reactivity fully, it
is critical that conditions under which redox chemistry
can be avoided are identified and the spectral features
of important iron porphyrin species are correctly iden-
tified. The stability of solutions of Fe(OEP)(X) (where
X=NO3, Cl) in the presence of nitrite was examined
by visible, infrared and NMR spectroscopy. The chem-

Z. Wei, M.D. Ryan / Inorganica Chimica Acta 314 (2001) 49–57
51
ical processes that occurred when nitrite (as the PNP+
salt) was mixed with Fe(OEP)(X) were identified by
monitoring the spectral changes in an anaerobic cell, as
described in Section 2. At high concentrations of nitrite
(approximately 10 mM), the spectrum of the
Fe(OEP)(NO2)2− was obtained immediately upon mix-
ing. Over a period of 1000 s, no additional reactions
were observed. When X=NO3−, the spectrum of the
mono-nitrite complex (Fe(OEP)(NO2)) was obtained
when the concentrations of nitrite and Fe(OEP)(NO3)
were 0.10 mM (Table 1). Under these conditions, the
complex was slightly less stable, with a 4% absorbance
change observed over a period of 1000 s. If the reaction
was allowed to continue for 9 h, the spectrum for the
m-oxo complex ([Fe(OEP)]2O) was observed. There was
no evidence in the visible spectra for Fe(OEP)(NO), a
product often formed by an oxygen transfer reaction.
Further evidence for the stability of Fe(OEP)(X) in
the presence of nitrite was obtained by infrared spec-
troscopy. Equimolar mixtures of Fe(OEP)(NO3) or
Fe(OEP)(Cl) (1.0 mM) and nitrite (1.0 mM) were exam-
ined in THF. Once again there was no evidence for the
nitrosyl complex, even though it could have been ob-
served under these concentration conditions. Unfortu-
nately, the bands for the coordinated nitrite at this
concentration of iron porphyrin could not be observed
owing to their lower molar absorptivity and/or the
ligands such as perchlorate in the presence of a coordi-
nating solvent (Fe(OEP)(ClO4): l=35.5 [18,19]). When
the concentrations of Fe(OEP)(NO3) and NaNO2 were
both 1 mM, the high-spin spectrum disappeared and
was replaced by a low-spin ferric porphyrin spectrum.
If an oxygen transfer reaction occurred under the con-
ditions of this experiment to form Fe(OEP)(NO) and
Fe(OEP)(NO3), the latter complex should be observed
1
in the H NMR [5]. In addition, the experiments were
completed rapidly enough so that there was no evidence
for the m-oxo complex (CH2: 6.04 and 5.08 ppm [20]),
which was the long-term decomposition product ob-
served in our work by visible spectroscopy.
The stability of Fe(OEP)(Cl)/NO2− solutions is in
agreement with the results of Castro [7], who found
that mixtures of Fe(P)(Cl) and nitrite were quite stable
in the absence of dioxygen. The instability of stoichio-
metric ratios of Fe(P)(X) and nitrite (where X is an
easily displaceable anion) was similar to the work of
Finnegan et al. [5], who observed a slow decomposition
of the mono-nitrite complex. Unlike their work,
though, we were unable to detect the disproportiona-
tion of nitrite to ferrous-NO and nitrate. This differ-
ence may be due to formation of organonitro
compounds in methylene chloride [8]. The kinetic stud-
ies in this work confirmed the report by Munro and
Scheidt [12] that the mono-nitrite complex was more
reactive than the bis-complex.
solvent bands below 1500 cm−1
.
1
The H NMR spectra of 1 mM Fe(OEP)(NO3) in the
presence of 0, 1 and 10 mM nitrite in DMF-d7 were
also obtained. In the absence of nitrite, a high-spin
ferric porphyrin spectrum was obtained with a reso-
nance at 49.2 ppm, which is due to the methylene
protons. Generally, two resonances are observed for the
methylene protons for a five-coordinate complex, but a
single methylene resonance has been observed for weak
The changes in the visible spectra of Fe(OEP)(Cl) as
a function of nitrite concentration are shown in Fig. 1.
In the absence of nitrite, the Soret band was observed
at 374 nm, with additional bands at 532 and 629 nm.
Upon addition of nitrite, the Soret band shifted to
389 nm, the 532 and 629 nm bands decreased, and a
new band appeared at 586 nm (Table 1). Several isos-
bestic points are observed in the spectra, indicating the
Table 1
Electronic spectra of nitrite complexes of iron porphyrins
Species
Solvent
Band maxima/nm (m/mM−1 cm−1
)
Reference
Fe(TPP)(Cl)
Fe(TPP)(NO2)
DMF
DMF
CH2Cl2
DMF
CH2Cl2
CH2Cl2
DMF
DMF
CH2Cl2
THF
CH2Cl2
THF
THF
DMF
THF
DMF
414 (95), 508 (11), 570 (4.0), 691 (2.7)
380 (49.7), 416 (95), 508 (12), 572 (3.87), 690
417 (120), 508 (9.6), 570sh, 650 (2.4), 687 (2.4)
413 (98), 506 (10.7), 568 (4.8), 607s, 690 (2.7)
412, 506, 579
424, 551, 651
425, 552
422 (141), 553 (9.6), 608sh, 690sh
424, 546, 576sh, 602sh
374 (95.3), 504 (12.5), 532 (13.4), 629 (8.17)
378 (99.6), 504 (8.7), 534 (9), 578 (2.6), 634 (4.5)
392 (174.5), 494 (14.0), 613 (7.45)
394 (102), 534 (11), 588sh, 620 (6.8)
391, 541, 564s, 588s
this work
[17]
[5]
this work
[3]
[3]
Fe(TpivPP)(NO2)
Fe(TpivPP)(NO2)2−
[3]
Fe(TPP)(NO2)2−
Fe(OEP)(Cl)
[1], this work
[1], this work
this work
[20]
this work
this work
this work
this work
this work
Fe(OEP)(NO3)
Fe(OEP)(NO2)
Fe(OEP)(NO2)2−
389 (96), 558 (11.9), 586 (10.7)
381, 502, 535, 588s, 627

52
Z. Wei, M.D. Ryan / Inorganica Chimica Acta 314 (2001) 49–57
tion of nitrite, the Soret band shifted to 422 nm and the
508 nm band shifted to 553 nm. The spectroscopic
changes were quite comparable to the spectra reported
by Nasri et al. [3] for Fe(TpivPP)(NO2)2− (424 and
551 nm) in methylene chloride (Table 1).
The spectra of Fe(TPP)(NO2) and Fe(TPP)(NO2)2−
can be obtained from analysis of the spectral changes as
a function of nitrite concentration. The concentration
of the mono-nitrite complex was enhanced in this set of
data because no additional chloride ions were added
(1 mM chloride was added to all the Fe(OEP)(Cl)/ni-
trite solutions above). By using an iterative process, the
values of K1 and K2 were varied, and the concentrations
of Fe(TPP)(Cl), Fe(TPP)(NO2), Fe(TPP)(NO2)2−, and
Cl− were calculated. From these concentration values,
the spectra of Fe(TPP)(Cl), Fe(TPP)(NO2) and
Fe(TPP)(NO2)2− can be calculated by a least-squares
fit. The values of K1 and K2 were adjusted to minimize
the errors. The spectral features are summarized in
Table 1. The spectra of Fe(TPP)(Cl) and Fe(TPP)(NO2)
in DMF are quite similar. In addition, there is substan-
tial agreement between the spectrum obtained by our
work and by Finnegan et al. [5]. Comparisons of the
calculated and experimental spectra for several concen-
trations are shown in Fig. 2 for K1=2 and K2=200
(i2=400).
Fig. 1. Visible spectra of 0.050 mM Fe(OEP)(Cl) in 0.10 M TBAP.
Solvent: THF. Dotted line: 0 mM (PNP)NO2. Thick solid line:
10 mM (PNP)NO2 with 0 mM (PNP)Cl. Thin solid lines: intermediate
concentrations of 2.0, 4.0, 6.0 and 10. mM (PNP)NO2 with 1 mM
(PNP)Cl.
The stability of Fe(II)(OEP) in the presence of nitrite
was also examined using the same anaerobic cell proce-
dure. The Fe(II)(OEP) and nitrite solutions (as the
PNP+ salt) were mixed, and the spectral changes were
monitored using a diode array spectrophotometer. No
spectral changes were observed upon mixing. This indi-
cates either that nitrite does not complex with
Fe(II)(OEP) or that the complex has the same high-spin
spectrum as Fe(II)(OEP). In any case, no redox reac-
tion was observed. It is important to note that extreme
care must be used to prevent leakage of dioxygen into
the cell. This leakage was probably the cause of the
small decrease in the Soret band (less than 5%) after
1000 s. If any dioxygen were to leak in, a redox reac-
tion, which will be discussed below, was observed. The
experiment was repeated with the picket-fence por-
phyrin (Fe(II)(TpivPP)), and the spectrum for the pre-
viously characterized low-spin ferrous-nitrite complex
was obtained [4].
The results of the visible and other spectroscopic
studies clearly indicate that both the ferric and ferrous
oxidation states of iron porphyrins are stable in the
presence of nitrite. The only reactions that were ob-
served were complexation reactions between iron and
nitrite. If any complexation occurs between
Fe(II)(OEP) and nitrite, the product is a high-spin
complex, rather than the low-spin complex observed
with the picket-fence porphyrin.
Fig. 2. Visible spectra of 0.10 mM Fe(TPP)(Cl) in 0.10 mM TBAP.
Solvent: DMF. (A) 0.0 mM nitrite; (B) 0.50 mM nitrite; (C) 10 mM
nitrite. Dotted lines are the calculated spectra for K1=2 and K2=
200. Solid lines are the experimental spectra.
presence of only two species in equilibrium. The best fit
was obtained for the addition of two nitrite ions to the
complex, forming Fe(OEP)(NO2)−2 . From the variation
in the absorbance as a function of nitrite concentration,
a i2 value of 2797 could be calculated.
Fe(OEP)(Cl)+2NO2−XFe(OEP)(NO2)2−+Cl−
i2
There was no evidence in the visible spectra for the
mono-nitrite complex, Fe(OEP)(NO2), when 1.0 mM
chloride was present in the solution. The spectrum of
Fe(OEP)(NO2)2− was identical whether one started
with Fe(OEP)(Cl) or Fe(OEP)(NO3).
Similar experiments were also carried out using
Fe(TPP)(Cl), and were consistent with the formation of
the bis-nitro complex, Fe(TPP)(NO2)2−. In the absence
of nitrite, the Soret band was observed at 414 nm, with
additional bands at 508, 649 and 691 nm. Upon addi-

Z. Wei, M.D. Ryan / Inorganica Chimica Acta 314 (2001) 49–57
53
3.2. Cyclic 6oltammetry
All the voltammetric studies were carried out under
conditions where it has been shown that no significant
decomposition of the ferric species would occur.
The cyclic voltammetry of Fe(OEP)(Cl) was exam-
ined in the presence of nitrite in DMF and THF, and
was consistent with previous results (see fig. 3 of Ref.
[1]). In the absence of nitrite, two reduction waves were
observed for Fe(OEP)(Cl) in both solvents between
+0.3 and −1.9 V versus Ag/AgNO3, with peak poten-
tials at −0.88 (wave Ia) and −1.76 V (wave II). When
nitrite was added, a new wave was observed at
−1.51 V (wave Ib), and the first wave shifted slightly to
−0.84 V. The new wave (wave Ib) occurred at the same
potential as Fe(OEP)(NO). Wave II was essentially
unchanged. Although the potential of wave II was
unchanged, more than one redox process may be occur-
ring because the Ep value for [Fe(OEP)]2O is essentially
the same as the second wave of Fe(OEP)Cl. In addition
to shifting slightly in potential, the cathodic current for
wave Ia decreased substantially when nitrite was added.
The voltammetric data clearly indicate that a reac-
tion is occurring between iron porphyrin and nitrite, as
had been reported previously [1,5,7,21]. The voltam-
metric evidence strongly suggests the formation of
Fe(OEP)(NO). These results appear to contradict the
visible and infrared evidence, that both ferric and fer-
rous porphyrins are stable towards redox reactions in
the presence of nitrite. In order to investigate this
apparent contradiction, visible spectroelectrochemistry
and further kinetic experiments were carried out.
3.3. Visible spectroelectrochemistry
The reduction of Fe(OEP)(Cl) in DMF or THF in
the absence of nitrite gave rise to a typical Fe(II)(OEP)
spectrum (Soret band at 410 nm) at wave I and
Fe(I)(OEP)− spectrum at wave II (Fig. 3). When the
solution was electrolyzed at wave Ia in the presence of
nitrite, the Fe(II)(OEP) Soret band was attenuated
significantly, and only appeared as a small shoulder
(Fig. 4, curve A). In addition, a new band appeared at
557 nm, which was due to Fe(OEP)(NO). Using the
known spectra for the ferrous and ferric porphyrins, the
composition of the product solution at wave Ia was
calculated: 0.20 mM Fe(OEP), 0.10 mM Fe(OEP)(NO),
0.10 mM [Fe(OEP)]2O and no Fe(OEP)(Cl). The calcu-
lated spectrum for the final product is shown in Fig. 4
(curve B). It is important to note that the nitrosyl and
the m-oxo complex, formed by the reaction of nitrite of
iron porphyrins, were generated in a 1:1 mole ratio. In
addition, there was a significant amount of Fe(OEP)
remaining, even though excess nitrite was present. This
last result was consistent with the visible spectroscopic
studies. These results indicate that both iron oxidation
states must be present for the reaction to occur, sug-
gesting a mixed oxidation state bridged intermediate:
Fig. 3. Thin-layer visible spectroelectrochemistry of 0.50 mM
Fe(OEP)(Cl). Thin solid line: initial spectrum; thick solid line: spec-
trum at wave Ia; dashed line: spectrum at wave II. Solvent: DMF;
supporting electrolyte: 0.10 M TBAP; working electrode: platinum
gauze.
Fe(II)(OEP)+Fe(III)(OEP)(NO2)2−
“(OEP)Fe(II)ꢀO2NꢀFe(III)(OEP)+NO2−
(OEP)Fe(II)ꢀO2NꢀFe(III)(OEP)
(3)
“Fe(OEP)(NO)+Fe(OEP)(O)
(4)
(5)
Fe(OEP)(O)+Fe(OEP)“[Fe(OEP)]2O
(the oxidation states of the cleavage products are pre-
sented for illustrative purposes; other oxidation states
can be written). The details of the coordination of the
bridged intermediate can only be speculated on at this
time; other coordination states could also be written.
When the potential was shifted to wave Ib, the Soret
band for Fe(II)(OEP) increased due to reduction of
[Fe(OEP)]2O. Authentic m-oxo complex yielded the
same spectroelectrochemical changes. In addition to the
Fig. 4. Thin-layer visible spectroelectrochemistry of 0.50 mM
Fe(OEP)(Cl) in the presence of 5.0 mM (PNP)NO2. Curves (A).
Potential is scanned through wave Ia. Dotted line: initial spectrum;
thin solid lines intermediate spectra; thick solid line: spectrum at wave
Ia. Curve (B). Calculated spectrum based on [Fe(OEP)]=0.20 mM;
[Fe(OEP)(NO)]=0.10 mM;
[(Fe(OEP)2)O]=0.10 mM.
Solvent:
DMF; supporting electrolyte: 0.10 M TBAP; working electrode: plat-
inum gauze.

54
Z. Wei, M.D. Ryan / Inorganica Chimica Acta 314 (2001) 49–57
for the m-oxo complex at 870 cm−1 could not be ob-
served due to solvent absorption.
3.5. Reaction kinetics
Owing to diffusion processes, the voltammetric or
spectroelectrochemical reduction of Fe(OEP)(X) will
lead to solutions that contain both oxidation (ferric/fer-
rous) states. In order to investigate the reaction of
nitrite with iron porphyrins under these conditions, the
kinetics of the reaction of nitrite with mixed oxidation-
state solutions were examined. When both oxidation
states were present, an immediate reaction was ob-
served when nitrite was added (Fig. 6). Using the
known spectra of Fe(OEP)(Cl) (in 5 mM nitrite),
Fe(OEP), Fe(OEP)(NO) and [Fe(OEP)]2O, it was possi-
ble to calculate the concentration of each species as a
function of time. The changes in concentration of reac-
tants and products obeyed the following reaction:
Fig. 5. Thin-layer visible spectroelectrochemistry of 0.50 mM
Fe(OEP)(Cl) in the presence of 5.0 mM (PNP)NO2. Dotted line:
spectrum at wave Ia; thin solid line: spectra at potentials between
wave Ia and wave Ib; thick solid line: spectrum at wave Ib; dashed
line: spectrum at wave II. Solvent: DMF; supporting electrolyte:
0.10 M TBAP; working electrode: platinum gauze.
3Fe(III)(P)(NO2)2− +2Fe(II)(P)
“Fe(II)(P)(NO)+[Fe(III)(P)]2O+NO2−
(6)
From the concentration of reactants as a function of
time, it is possible to determine the rate law and the
rate constant for the reaction.
If Eq. (3) is the rate-determining step, the rate law
should be first-order in Fe(OEP)(NO2)−2 and Fe(OEP),
and the following equation can be written:
rate=k[Fe(III)(OEP)(NO2)−][Fe(II)(OEP)]
(7)
Fig. 6. (A) Visible spectra obtained after mixing Fe(OEP)/
Fe(OEP)(Cl) with nitrite. [Fe(OEP)(Cl)]=[Fe(OEP)]=0.050 mM;
[NO2−]=10 mM; solvent: THF. Dotted solid line: 0 s; thin solid
lines: 200, 400, 600 and 800 s. Thick solid line: 1000 s.
Using the cumulative formation constant i2 and the
mass balance equation, Eq. (7) can be written in terms
of the total concentration of the ferric porphyrin,
[Fe(III)(OEP)] (sum of all the coordination forms of the
ferric porphyrin: Fe(III)(OEP)(Cl), Fe(III)(OEP)(NO2),
Fe(III)(OEP)(NO2)2−):
reduction of the m-oxo complex, a new band appeared
at 538 nm (Fig. 5), which is due to Fe(OEP)(NO)−.
Further reduction at wave II (Fig. 5) gave rise to a new
band at 685 nm (Fe(I)(OEP)−), and the 538 nm band of
Fe(OEP)(NO)− was unchanged. The Soret bands for
Fe(OEP)(NO)− and Fe(I)(OEP)− are quite similar,
hence only one Soret band was observed in Fig. 5.
Because of the overlap of wave Ib and wave II, reduc-
tion of the m-oxo complex will occur along with
Fe(OEP)(NO).
−
[Fe(III)(OEP)NO2)2
]
− 2
i2[NO2
]
=[Fe(III)(OEP)]
=h%[Fe(III)(OEP)]
2
i2[NO2−]2+[Cl−]
(8)
If [NO2−]ꢀ[Fe(III)(OEP)], then the fraction of the
ferric porphyrin species in the bis-nitro form will be
constant for a given concentration of nitrite (experi-
ment). In addition, when the ferric porphyrin concen-
tration is about 0.1 mM, over 90% of the ferric
porphyrin is in the bis-nitro form for concentrations of
nitrite greater than 5 mM. Eq. (7) can then be rewritten
as:
3.4. Infrared spectroelectrochemistry
It has previously been shown in this work that solu-
tions of Fe(OEP)(Cl) and nitrite do not lead to the
formation of Fe(OEP)(NO) at potentials significantly
positive of wave Ia. Electrolysis of Fe(OEP)(Cl) in the
presence of nitrite at a potential between waves Ia and
Ib led to the appearance of a band for Fe(OEP)(NO) in
the infrared spectrum at 1672 cm−1. When 15NO2− was
used, the band shifted to 1641 cm−1, which was identi-
cal to the band observed for Fe(OEP)(15NO). The band
rate=k%[Fe(III)(OEP)][Fe(II)(OEP)]
(9)
where k%=kh%. The following relationship can be writ-
2
ten between [Fe(III)(OEP)] and [Fe(II)(OEP)], based on
the stoichiometry of the reaction:

Z. Wei, M.D. Ryan / Inorganica Chimica Acta 314 (2001) 49–57
55
A plot of the left-hand sides of these two equations as
a function of t should yield straight lines with slopes
equal to 2k%MFe or 2k% respectively. The data for vari-
ous concentrations of iron porphyrin and nitrite are
shown in Fig. 7. The rate constants are summarized in
Table 2. The agreement between the experimental data
and the theoretical rate law confirms the assumption
that
the
reaction
is
first-order
in
both
Fe(III)(OEP)(NO2)2− and Fe(II)(OEP).
From Table 2, we can see that the observed rate
constant k% was not strongly dependent upon the con-
centration of nitrite. This indicates that it is the coordi-
nated nitrite that reacts with the ferric/ferrous
porphyrins, not the free nitrite in solution. From the
value of i2, the fraction of the ferric porphyrin in the
Fig. 7. Variation in the concentration of Fe(OEP)(Cl)/Fe(OEP) as a
function of time. Solid symbols: left y-scale. Open symbols: right
y-scale. Open symbols: [Fe(OEP)(Cl)]=[Fe(OEP)]=0.050 mM; (2)
[NO2−]=0.050 mM; (ꢀ) [NO2−]=0.50 mM; (ꢁ) [NO2−]=
5.0 mM. Solid symbols: ( ) [Fe(OEP)(Cl)]=0.050 mM; [Fe(OEP)]=
0.025 mM; [NO2−]=5.0 mM; (ꢂ) [Fe(OEP)(Cl)]=0.025 mM;
[Fe(OEP]=0.050 mM; [NO2−]=5.0 mM.
bis-nitrite form (h%) can be calculated, and the results
2
are summarized in Table 2. Although the value of k%
does decrease with the concentration of nitrite, the
dependence is much weaker than expected based on the
equilibrium concentration of Fe(OEP)(NO2)2−. Munro
and Scheidt [12] have shown that the mono-nitrite
complex was particularly reactive. Even though its
equilibrium concentration is not significant under our
conditions (which makes it possible to deconvolute the
experimental spectra by ignoring the presence of the
mono-nitro complex), it may be an important pathway
if its rate constant (k1) is larger than that for the
bis-nitrite complex (k2). The observed rate constant is
therefore the combination of the two reactions:
[Fe(III)(OEP)]=MFe+0.5[Fe(II)(OEP)]
where
M
Fe=MFe(III)(OEP),i−0.5MFe(II)(OEP),i
,
and
MFe(III)(OEP),i and MFe(II)(OEP),i are the initial concentra-
tions of the ferric and ferrous porphyrin, respectively.
Combining these equations, we can write:
d[Fe(II)(OEP)]
rate= −
2 dt
=k%(MFe+0.5[Fe(II)(OEP)])[Fe(II)(OEP)]
Fe(OEP)(NO2)+Fe(OEP)
Integration of this equation yields:
“(OEP)FeꢀNO2ꢀFe(OEP) k1
Fe(OEP)(NO2)2− +Fe(OEP)
MFe(III)(OEP),i
[Fe(III)(OEP)]
[Fe(II)(OEP)]
“(OEP)FeꢀNO2ꢀFe(OEP)+NO2− k2
"0.5MFe(II)(OEP),i
:
ln
M
=ln Fe(III)(OEP),i+2k%MFet
Including both pathways for the formation of the
bridged complex, and assuming that the rate of ligand
exchange is faster than the redox reaction, the observed
rate constant is equal to:
MFe(II)(OEP),i
MFe(III)(OEP),i
1
=0.5MFe(II)(OEP),i
:
k%=h%k1+h%k2
1
2
[Fe(II)(OEP)]
1
where h% is the fraction of the ferric porphyrin in the
1
=
+2k%t
MFe(II)(OEP),i
mono-nitro complex:
Table 2
Kinetics of the reaction of Fe(OEP)(Cl) with nitrite in THF
[NO2 −]/mM
M
Fe(III)(OEP),i/mM
MFe(II)(OEP),i/mM
Intercept
k
obs/M−1 s−1
h2
k
calc/M−1 s−1 a
Experiment
Calculated
0.050
0.50
5.0
5.0
5.0
0.050
0.050
0.050
0.050
0.025
0.050
0.050
0.050
0.025
0.050
0
0
0
0.69
1.0
0.0
0.0
0.0
0.71
1.0
3.2
6.2
7.5
8.0
5.3
0.036
0.31
0.94
0.94
0.94
3.1
7.1
7.6
7.6
7.6
a K1=0.013, K2=2000, k1=30 M−1 s−1 and k2=6 M−1 s−1
.

56
Z. Wei, M.D. Ryan / Inorganica Chimica Acta 314 (2001) 49–57
4. Conclusions
Nitrite is quite stable in the presence of either ferric
or ferrous porphyrins. The only reactions observed
were complexation reactions. The stability of these
complexes is quite dependent upon the complete exclu-
sion of dioxygen. When there is a mixture of oxidation
states, the solution has the right combination of Lewis
acids and bases to coordinate nitrite, and facilitate the
oxygen transfer reaction. The details of the cleavage
process still need experimental confirmation, but the
most likely process is the transfer of oxygen from the
ferric-coordinated nitrite to the ferrous species (path-
way B). 1H NMR evidence showed that the ferric
species was coordinated with nitrite to form a strong
low-spin complex for either the mono- or bis-nitro
species, whereas the ferrous complex is weak in the
absence of additional interactions provided by the
picket-fence porphyrin. Cleavage by pathway B would
also readily form the observed products in high yield.
Steric hindrance and/or ferrous coordination (as in the
picket-fence porphyrins), both of which would make
the bridged intermediate less favorable, was shown to
reduce significantly the rate of the redox reaction of
nitrite with iron porphyrins. Further studies investigat-
ing the importance of these factors are in progress.
Scheme 1.
[Fe(III)(OEP)(NO2)]
−
K1[NO2
[Cl−]+K1[NO2−]+i[NO2
]
=MFe(III)(OEP)
− 2
]
=h%MFe(III)(OEP)
1
and h% has been previously defined in Eq. (7) (because
the concentration of the mono-nitrite complex is always
small, K1 can be ignored in the calculation of h%, and
the cumulative formation constant i2 used instead).
2
2
Without K1, it is not possible to calculate h% exactly.
1
The value of K1, though, can be estimated to be 0.012
from the known value of K2 (2200) in DMF [1], because
i2=K1K2. With these values of the equilibrium con-
stant, the rate constant as a function of nitrite concen-
tration was calculated for k1=30 M−1 s−1 and
k2=6 M−1 s−1 (Table 2). It is not clear why the
mono-nitrite complex is more reactive than the bis-ni-
trite complex. When this experiment was repeated using
the picket-fence porphyrin, no reaction was observed
over the same time frame. While one side of the por-
phyrin is still exposed in this complex, the ferrous
species is complexed with nitrite under these conditions,
which would significantly reduce its ability to form a
bridged mixed oxidation-state intermediate.
Acknowledgements
We would like to acknowledge the Schmitt Founda-
tion for fellowship support. Partial support of this
research was provided by the National Institutes of
Health Grant GM 55917.
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