Reactions of nitrogen oxides with heme models. Spectral and kinetic study of nitric oxide reactions with solid and solute FeIII(TPP)(NO 3)

Article abstract of DOI:10.1021/ja042237r

The reaction(s) of nitric oxide (nitrogen monoxide) gas with sublimed layers containing the nitrato iron(III) complex Fe^III(TPP) (η²-O₂NO) (1, TPP = meso-tetraphenyl porphyrinate ^2-) leads to formation of several iron porphyrin species that are ligated by various nitrogen oxides. The eventual products of these low-temperature solid-state reactions are the nitrosyl complex Fe(TPP)(NO), the nitro-nitrosyl complex Fe(TPP)(NO₂)(NO), and 1 itself, and the relative final quantities of these were functions of the NO partial pressure. It is particularly notable that isotope labeling experiments show that the nitrato product is not simply unreacted 1 but is the result of a series of transformations taking place in the layered material. Thus, the nitrato complex formed from solid Fe(TPP)(η²-O₂NO) maintained under a ¹⁵NO atmosphere was found to be the labeled analogue Fe(TPP)(η²-O₂¹⁵NO). The reactivities of the layered solids are compared to the behaviors of the same species in ambient temperature solutions. To interpret the reactions of the labeled nitrogen oxides, the potential exchange reactions between N₂O₃ and ¹⁵NO were examined, and complete isotope scrambling was observed between these species under the reaction conditions (T = 140 K). Overall it was concluded from isotope labeling experiments that the sequence of reactions is initiated by reaction of 1 with NO to give the nitrato nitrosyl complex Fe(TPP)(η¹-ONO₂)(NO) (2) as an intermediate. This is followed by a reaction in the presence of excess NO that is equivalent to the loss of the nitrate radical NO₃^. to give Fe(TPP)(NO) as another transient species. A plausible pathway involving NO attack on the coordinated nitrate of 2 resulting in the release of N₂O₄ concerted with electron transfer to the metal center is proposed.

Full text of DOI:10.1021/ja042237r

Published on Web 04/09/2005
Reactions of Nitrogen Oxides with Heme Models. Spectral and
Kinetic Study of Nitric Oxide Reactions with Solid and Solute
FeIII(TPP)(NO3)
Tigran S. Kurtikyan,*,†,‡ Gurgen M. Gulyan,† Garik G. Martirosyan,†
Mark D. Lim,§ and Peter C. Ford*,§
Contribution from the Research Institute of Applied Chemistry (ARIAC), 375005,
YereVan, Armenia, Molecule Structure Research Centre (MSRC) NAS, 375014, YereVan,
Armenia, and Department of Chemistry and Biochemistry, UniVersity of California,
Santa Barbara, California 93106
Received December 24, 2004; E-mail: tkurt@msrc.am; ford@chem.ucsb.edu
Abstract: The reaction(s) of nitric oxide (nitrogen monoxide) gas with sublimed layers containing the nitrato
iron(III) complex FeIII(TPP)(η2-O2NO) (1, TPP ) meso-tetraphenyl porphyrinate2-) leads to formation of
several iron porphyrin species that are ligated by various nitrogen oxides. The eventual products of these
low-temperature solid-state reactions are the nitrosyl complex Fe(TPP)(NO), the nitro-nitrosyl complex Fe-
(TPP)(NO2)(NO), and 1 itself, and the relative final quantities of these were functions of the NO partial
pressure. It is particularly notable that isotope labeling experiments show that the nitrato product is not
simply unreacted 1 but is the result of a series of transformations taking place in the layered material.
Thus, the nitrato complex formed from solid Fe(TPP)(η2-O2NO) maintained under a 15NO atmosphere was
found to be the labeled analogue Fe(TPP)(η2-O215NO). The reactivities of the layered solids are compared
to the behaviors of the same species in ambient temperature solutions. To interpret the reactions of the
labeled nitrogen oxides, the potential exchange reactions between N2O3 and 15NO were examined, and
complete isotope scrambling was observed between these species under the reaction conditions (T ) 140
K). Overall it was concluded from isotope labeling experiments that the sequence of reactions is initiated
by reaction of 1 with NO to give the nitrato nitrosyl complex Fe(TPP)(η1-ONO2)(NO) (2) as an intermediate.
This is followed by a reaction in the presence of excess NO that is equivalent to the loss of the nitrate
•
radical NO3 to give Fe(TPP)(NO) as another transient species. A plausible pathway involving NO attack
on the coordinated nitrate of 2 resulting in the release of N2O4 concerted with electron transfer to the metal
center is proposed.
Introduction
tions between various NOx complexes of the heme model
compound Fe(TPP) (TPP ) meso-tetraphenyl porphyrinate2-
)
Nitric oxide (NO) has been well-characterized as an important
regulatory molecule in mammalian biology. The interaction of
NO with ferro- and ferri-heme proteins plays a central role in
this bioregulatory activity,1 and the reactions of NO with various
iron porphyrin complexes have been investigated as models for
these processes.2 In addition to simple association reactions, NO
has been shown to participate in reversible redox reactions
involving biologically relevant metal centers. In model reactions
it is clear that metal complexes mediate both oxygen atom and
electron-transfer processes involving NO and various other
nitrogen oxide species (NOx). Such oxidative processes may
not only be significant in the biological chemistry of various
NOx but also have potential in the catalytic oxidations of various
organic substrates.3 It was in these contexts that the present study
was initiated, with the goal of elucidating the redox transforma-
using the bidentate nitrato complex Fe(TPP)(η2-O2NO) (1)4 as
the entry point. This compound can be prepared by reaction of
the oxo-bridged [Fe(TPP)]2O with HNO3,4 and it has also been
shown to be a product of the NO2 reactions with Fe(TPP) species
both in the solid state and in solutions.5,6
The present investigations were carried out both in solution
and in sublimed layers of the iron porphyrin complex using
infrared and electronic spectroscopic probes of the transforma-
tions. The low-temperature spectral investigations of the sub-
limed layers provide the opportunity to observe the stepwise
reactions that lead to the formation of the final products and to
(3) (a) Tovrog, B. S.; Mares, F. M.; Diamond, S. E. J. Am. Chem. Soc. 1980,
102, 6616-6618. (b) Andrews, M. A.; Chang, T.; Cheng, C.-W. Organo-
metallics 1985, 4, 268-274. (c) Castro, C. E. J. Org. Chem. 1996, 61,
6388-6395. (d) Goodwin, J., et al. Inorg. Chem. 2001, 40, 4217-4225.
(4) Phillippi, M. A.; Baenziger N.; Goff, H. M. Inorg. Chem. 1981, 20, 3904-
3911.
(5) Kurtikyan, T. S.; Stepanyan, T. G.; Akopyan, M. E. Russ. J. Coord. Chem.
1999, 25, 721-725.
(6) (a) Kurtikyan, T. S.; Martirosyan, G. G.; Lorkovic’, I. M.; Ford, P. C. J.
Am. Chem. Soc. 2002, 124, 10124-10129. (b) Lim, M. D.; Lorkovic, I.
M.; Wedeking, K.; Zanella, A. W.; Massick, S. M.; Ford, P. C. J. Am.
Chem. Soc. 2002, 124, 9737-9743.
† Research Institute of Applied Chemistry.
‡ Molecule Structure Research Centre NAS.
§ University of California.
(1) Ignarro, L. Nitric Oxide: Biology and Pathobiology; Academic Press: San
Diego, CA, 2000.
(2) Ford, P. C.; Lorkovic, I. M. Chem. ReV. 2002, 102, 993-1017.
9
6216
J. AM. CHEM. SOC. 2005, 127, 6216-6224
10.1021/ja042237r CCC: $30.25 © 2005 American Chemical Society

Reactions of Nitrogen Oxides with Heme Models
A R T I C L E S
characterize the intermediates that define these transformations.
The solution studies offer the opportunity to evaluate whether
such reactions follow analogous pathways in fluid media and
to observe the dynamics of these processes.
Experimental Section
Preparation of Fe(TPP)(η2-O2NO) Sublimates. Fe(II) porphyri-
nates are very sensitive to oxygen and readily transform to Fe(III)
derivatives. For this reason, the more stable Fe(TPP)(B)2 complexes
with nitrogen bases (B is pyridine or piperidine) were used as parent
compounds. These were synthesized by a known procedure.7 The low-
temperature sublimate was prepared8 by placing an Fe(TPP)(B)2 sample
in a Knudsen cell and heating to about 470 K under high vacuum (P
) 3 × 10-5 Torr). Evacuation for 3 h resulted in the complete
elimination of the coordinated axial ligands B, as monitored by
measurement of the pressure at the outlet of the cryostat. Liquid nitrogen
was then poured into the cryostat, and the Knudsen cell was heated to
520 K, at which temperature sublimation of Fe(TPP) onto the KBr
and CaF2 substrate occurred. In the latter case, CaF2 plates were also
used as the optical windows of the cryostat.9,10 The M(TPP) layers
obtained in this manner by sublimation onto a low-temperature (77 K)
surface are spongelike and have high microporosity.11 Potential reactants
easily diffuse across these layers, and adducts thus formed can be
studied spectroscopically without solvent interference.
Figure 1. Low-temperature (80 K) FTIR spectra of (a) a thin film of Fe-
(TPP)(η2-O2NO) (solid line), (b) after supplying 10 Torr NO to cryostat at
80 K followed by warming to 140 K, pumping briefly to remove excess
NO and recooling to 80 K (dashed line), (c) the same after interaction of
15NO with Fe(TPP)(η2-O215NO) (dotted line).
Sublimation was typically carried out over periods of 0.1-2.0 h to
build up layers of thickness convenient for UV-visible and IR spectral
studies. The Fe(TPP) sublimed layer was heated to room temperature
under dynamic vacuum, and then NO2 gas was introduced. As shown
previously,5 this procedure leads to the formation of a very clean
spectrum of the bidentate nitrato complex 1. These layers were then
cooled with liquid nitrogen, and NO was fed to the cryostat from a
vessel provided with a mercury manometer to measure the pressure of
NO. The layers were warmed to specified temperatures controlled by
thermocouple, and FTIR or electronic absorption spectra were measured.
Labeled and Unlabeled Nitric Oxide. NO gas was purified by
passage through KOH pellets and a cold trap (dry ice/acetone) to remove
higher nitrogen oxides and trace quantities of water. The purity was
checked by IR measurements of the layer obtained by slow deposition
of NO on the cooled substrate of the optical cryostat (80 K), which
did not show the characteristic IR bands of N2O, N2O3, or H2O, thus
indicating the absence of these species. (Freezing the purified NO with
the liquid nitrogen on the wall of the glass storage bulb serves as a
rapid procedure for checking purity, since in the presence of NO2, this
solid takes on the bluish color from the trace N2O3.) 15NO with 98.5%
enrichment was purchased from the Institute of Isotopes, Republic of
Georgia, and was purified by the same procedures. NO2 (15NO2),
obtained by oxidation of NO (15NO) with excess of pure dioxygen,
was dried over P2O5 and then was purified by fractional distillation
until a pure white solid was obtained.
quantitatively using vacuum line techniques directly into an airtight
flask that included a 1.0-cm quartz cuvette for monitoring reactions
via changes in their UV-visible spectra. A typical reaction would be
initiated by injection of the porphyrin solution into this reaction vessel,
shaken vigorously to provide gas/solution equilibration, and then
mounting the cuvette in the spectrophotometer. The electronic spectra
were acquired at 10-s intervals for the initial 200 s and at longer time
increments afterward.
Infrared spectra of reacting solutions were recorded using rapid
mixing techniques with a custom-made instrument. Two gastight
syringes were connected via PEEK tubing to a mixing block that
preceded the entrance to a 0.5-mm CaF2 IR cell. Before use, the
apparatus was evacuated overnight under vacuum. Purified NO was
transferred using vacuum line techniques into a sealed glass flask and
brought into the glovebox where degassed CH2Cl2 was injected into
the NO-containing flask. The equilibrated solution was then loaded
directly into one of the gastight syringes. The second syringe was filled
with a CH2Cl2 solution of 1. A mechanical device was used to engage
the two syringes simultaneously, causing the solutions to mix and flow
to an IR cell mounted in an FTIR spectrometer. The first acquisition
was taken while the syringe driver was running, and spectra were
acquired after this was disengaged and the exit valve was closed.
Instrumentation. FTIR spectra of porphyrin sublimates were
obtained on Nicolet Nexus FTIR and Specord M-80 spectrometers, and
electronic spectra were recorded on a Specord M-40 spectrophotometer.
Solution kinetics studies were carried out on an HP 8452 diode array
spectrophotometer or on a modernized BioRad model FTS-Pro FTIR
spectrophotometer.
Solution Studies. Solid Fe(TPP)(η2-O2NO) (1) was prepared fol-
lowing a published procedure4 and was stored in an inert atmosphere
glovebox. Solutions were prepared in the glovebox using distilled and
deoxygenated solvents and then loaded into gastight syringes for transfer
to the reaction cells. NO was purified, measured, and transferred
Results and Discussion
(7) Epstein, L. M.; Straub, D. K.; Maricondi, C. Inorg. Chem. 1967, 6, 1720-
1722.
Formation of Fe(TPP)(η1-ONO2)(NO) (2) in Low-Tem-
perature Films. The nitrato nitrosyl complex 2 represents the
only characterized example of a six-coordinate nitrato ferro or
ferri-porphyrinato complex. This was described several years
ago in a preliminary communication12 and is described in greater
detail here. Figure 1 presents the FTIR spectra of the product
(8) Nakamoto, K.; Watanabe, T.; Ama, T.; Urban, M. W. J. Am. Chem. Soc.
1982, 104, 3744-3745.
(9) Experiments using CaF2 optics were performed to avoid additional bands
that appear in the 1350 cm-1 region due to interaction of NO2 gas with
salt windows.10
(10) Peters, S. J.; Ewing, G. E. J. Phys. Chem. 1996, 100, 14093-14102.
(11) (a) Byrn, M. P.; Curtis, C. J.; Hsiou, Y.; Khan, S. I.; Sawin, P. A.; Tendick,
S. K.; Terzis, A.; Strouse, C. E. J. Am. Chem. Soc. 1993, 115, 9480-
9497. (b) Byrn, M. P.; Curtis, C. J.; Khan, S. I.; Sawin, P. A.; Tsurumi,
R.; Strouse, C. E. J. Am. Chem. Soc. 1990, 112, 1865-1874. (c) Kurtikyan,
T. S.; Gasparyan, A. V.; Martirosyan, G. G.; Zhamkochyan, G. H. J. Appl.
Spectrosc. (Russ.) 1995, 62, 62-66.
(12) Kurtikyan, T. S.; Martirosyan, G. G.; Hakobyan, M. E.; Ford, P. C. Chem.
Commun. 2003, 1706-1707.
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J. AM. CHEM. SOC. VOL. 127, NO. 17, 2005 6217

A R T I C L E S
Kurtikyan et al.
Table 1. IR Spectral Data of Nitrosyl and Nitrate Groups for Fe(TPP)(ONO2)(NO) in cm-1
Fe(TPP)(ONO2)(NO)
Fe(TPP)(ONO2)(15NO)
Fe(TPP)(O15NO2)(NO)
Fe(TPP)(O15NO2)(15NO)
assignment
1899 s
1505 m
1265 s
969 w
605 vw
548 vw
1862 s
1505 m
1265 s
969 w
602 vw
542 vw
1900 s
1472 m
1246 s
954 w
605 vw
548 vw
1863 s
1472 m
1246 s
954 w
602 vw
542 vw
ν(NdO)
νa(NO2)
νs(NO2)
ν(N-O)
δ(Fe-N-O)
ν[Fe-N(NO)]
obtained after exposing a low-temperature (80 K) layer of solid
Fe(TPP)(η2-O2NO) to NO gas, warming to 140 K, pumping
out the excess NO, and then recooling. The transformation is
characterized by appearance of new bands at 1899 s, 1506 m,
1265 s, and 969 w cm-1 and disappearance of bands at 1528
and 1273 cm-1 attributed to bidentate-coordinated nitrate.
Additionally, a porphyrin band at 1341 cm-1 is shifted to 1350
cm-1, while new weak bands at 605, 548, and 464 cm-1 appear
at low frequency (Supporting Information Figure S1). The
intensities of the new bands correlate during their growth
(Supporting Information Figure S2) and can be confidently
assigned to the formation of 2 as depicted in eq 1.
Fe(TPP)(η2-O NO)
Fe(TPP)( η1-ONO )(NO)
+ NO h
(1)
2
2
(1)
(2)
Figure 2. (a) Low-temperature (80 K) UV-visible spectrum of a sublimed
layer of Fe(TPP)(η2-O2NO) (dashed line). (b) Optical spectrum after
exposing 1 to NO (5 Torr at 77 K warming to 140 K and recooling (solid
line).
When the same procedure is carried out with the isotopically
labeled materials 15NO and Fe(TPP)(η2-O215NO), the new bands
appear at the shifted frequencies 1863 s, 1472 m, 1246 s, 954
w, 602 vw, and 542 vw cm-1 (Figure 1, dotted line), but others
at 1350 and 464 cm-1 are not affected by the isotope labeling.
If instead the reaction is between 15NO gas and unlabeled 1,
the product spectrum displays bands at 1862 s, 1505 m, 1265
s, 969 w, 602 vw, and 542 vw cm-1 (Supporting Information
Figure S3), that is, only the new bands at 1862, 602, and 542
cm-1 are shifted relative to the product bands seen in the reaction
porphyrin complexes of mixed nitrogen oxides. For example,
the electronic spectrum of the nitro nitrosyl complex Fe(TPP)-
(NO2)(NO) (3) in chloroform solution displays a spectrum with
maxima at 433, 545, and 577 nm.14
The bidentate nitrate ligand of Fe(TPP)(η2-O2NO) (1) has
been shown to be bound in a slightly asymmetric mode with a
large out-of-plane displacement (0.6 Å) of the d5 high-spin Fe-
(III) center toward the lone axial ligand.4 For this structure, the
η2-nitrate ligand would be expected to show three IR-active
stretching modes, a high-frequency ν(NdO) stretch for the
uncoordinated oxygen, and symmetric and asymmetric stretching
modes for the coordinated NO2 fragment. Two bands of
compatible intensity seen for 1 at 1530 and 1275 cm-1 in Nujol
mull4 and in sublimed layers5 were assigned, respectively, to
ν(NdO) and to the asymmetric mode of the coordinated NO2
fragment.15 The low-intensity, but isotopically sensitive, FTIR
band near 965 cm-1 (Figure S3) can plausibly be assigned to
the symmetric stretching mode of the NO2 fragment of 1.15
Adding NO to samples of 1 shifts the high-frequency
vibrations of coordinated nitrate to lower frequencies with a
change in the relative intensities, the higher energy band
diminishing and the lower energy band becoming much stronger.
Additionally there is enhancement and shift of the weak
symmetric stretching mode of the NO2 fragment. These changes
can be interpreted in terms of the bidentate-to-monodentate
transition of the nitrate coordination illustrated by Scheme 1.
The highest-frequency nitrato band at 1505 cm-1 now represents
νa(NO2), while that at ∼1260 cm-1 is assigned to νs(NO2). For
comparison, the IR spectrum of the monodentate nitrato complex
Fe(OEP)(η1-ONO2) (OEP ) octaethylporphyrinato2-) displays
between unlabeled NO and 1 shown in Figure 1. When this
15
procedure was performed with unlabeled NO and Fe(TPP)(O2
-
NO), the IR spectrum displayed bands at 1900 s, 1472 m, 1246
s, 954w, 605 vw, and 548 vw cm-1 (Supporting Information
Figure S3) where only those at 1900, 605, and 548 cm-1 are
unchanged relative to the unlabeled reaction. Spectral data
obtained for these systems are summarized in Table 1. These
results indicate that, in the 80-140 K temperature interval, the
only observable reaction proceeding in the layer is addition of
NO to Fe(TPP)(η2-O2NO) (1) to give the six-coordinate complex
Fe(TPP)(η1-ONO2)(NO) (2).
The electronic absorption spectra recorded under identical
conditions and using CaF2 optics as used to record the FTIR
spectra with the same sample also demonstrated formation of
the new complex. The Soret band of 1 in the 80 K thin film
appears at 423 nm, about 10 nm red-shifted from that seen in
the ambient temperature solution spectrum of 1, but the Q-bands
at 511 and 572 nm are close to the reported solution values.4,13
Upon addition of NO gas, the optical spectrum (Figure 2, dashed
line) is significantly changed. The Soret and Q-bands of the
new species are shifted to higher wavelengths (436, 547 and
582 nm) as illustrated in Figure 2 (solid line). Notably, this
spectrum is similar to that of other hexacoordinate FeIII
(13) The UV-vis spectrum of the film sample dissolved in CH2Cl2 is quite
similar to reported solution data;4 hence the red shift of Soret band is
evidently a solid-state effect.
(14) Yoshimura, T. Inorg. Chim. Acta 1984, 83, 17-21.
(15) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination
Compounds, 3rd ed.; Wiley: New York, 1978; pp 244-247.
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Reactions of Nitrogen Oxides with Heme Models
A R T I C L E S
Scheme 1. Schematic Representation of the Reaction between
Fe(TPP)(NO3) and NO
a coordinated nitrate band at 1515 cm-1 (KBr pellet).16 Closer
disposition of the high-frequency bands is one criterion of
monodentate coordination in nitrate complexes.15 The band at
∼970 cm-1 can be assigned to the N-O vibration for the
coordinated oxygen.
The low-frequency weak bands at 605 (602) and 548 (542)
cm-1 sensitive to NO labeling should be assigned to metal-
nitrosyl vibrations. The former was tentatively assigned to δ-
(Fe-N-O), while the latter to ν(Fe-N) by analogy with
Fe(III)(tropocoronand)(nitrosyl) complex,17 in which deforma-
tion Fe-N-O mode of linearly bound NO is disposed at higher
frequencies and revealed smaller isotopic shift than Fe-N
stretching.
Figure 3. Formation of nitro nitrosyl complex Fe(TPP)(NO2)(NO) from
nitrato nitrosyl complex Fe(TPP)(η1-ONO2)(NO) during the course of
warming the sublimed layers from 145 to 180 K in the presence of NO (4
Torr).
Low-Temperature Reactions of Fe(TPP)(ONO2)(NO) with
NO. Transformations of solid Fe(TPP)(O2NO) in thin films
under a NO atmosphere can be divided into three temperature-
dependent processes. At very low temperatures (80-140 K),
the only process is the formation of nitrato nitrosyl complex 2.
Raising the temperature into the 140-200 K range leads to the
further reaction of NO with this species, as illustrated in Figure
3. Warming of the layered material, in which 2 has been formed,
leads to gradual intensity decreases in its characteristic bands
at 1899, 1506 1266, and 969 cm-1, while new bands at 1863,
1452, and 1296 cm-1 gain intensity. The latter indicate the
formation of the well-known nitro nitrosyl complex Fe(TPP)-
(NO2)(NO) (3).6,14
Further warming to room temperature is accompanied with
partial reformation of the nitrato complex 1 and the appearance
of the iron(II) nitrosyl complex Fe(TPP)(NO) (4). At this stage,
the reactions are sensitive to the partial pressure of NO, and
depending on PNO, different relative quantities of 1, 3, and 4
are formed. At relatively high PNO (a few decades of Torr), 3
is the main species seen in the solid layer, while at lower PNO
(a few Torr) all three are present in comparable amounts.
Continuous evacuation to remove the headspace gases shifts
the product mixture from Fe(TPP)(NO2)(NO) to give increased
yields of Fe(TPP)(O2NO) and Fe(TPP)(NO).6
Additional information regarding the nature of 2 can be drawn
from porphyrin vibrational modes that reveal systematic changes
depending on the spin and oxidation states of axial complexes
of Fe(TPP).18 The band in vicinity of 1350 cm-1 representing
a porphyrin core mode (ν(Ca-Cm) mixed with some ν(Cm-
phenyl)) lies at higher frequencies in low-spin complexes.
Similarly, a low-energy porphyrin core deformation mode occurs
in the range 450 cm-1. For the high-spin η2-nitrato complex 1,
these bands lie at 1341 and 436 cm-1. Upon coordination of
NO, they shift to 1350 and 464 cm-1 (see Figures 1 and S1),
indicating a change to the low-spin state. This result is consistent
with other low-spin, six-coordinate ferri-heme nitrosyl com-
plexes19 and supports the conclusion that, under these conditions,
NO reacts with Fe(TPP)(η2-O2NO) to give Fe(TPP)(η1-ONO2)-
(NO). While this bidentate-to-monodentate transition may
alleviate nonbonded repulsions between nitrate oxygens and
porphyrin nitrogens, it is likely to involve relatively moderate
energy expenditures given the existence of monodentate and
bidentate binding, respectively, in Fe(OEP)(ONO2) and Fe(TPP)-
(O2NO), which differ only in the nature of the peripheral
substituents. To our knowledge, the only other six-coordinate
nitrato Fe(P) complex known is an aqua complex (Fe(P)(η1-
ONO2)(H2O)) (P not identified) reported in a review by Wyllie
and Scheidt.19
Scheme 2 offers several scenarios as starting points for
discussing the reactions following exposing sublimed layers of
Fe(TPP)(O2NO) to NO gas. In each case, the first stage is
formation of the nitrato nitrosyl complex 2 as evidenced from
the low-temperature solid-state FTIR and by rapidly mixing
UV-visible measurements in solution (see below). Three
different pathways may be considered in evaluating the second
stage of the reaction leading to Fe(TPP)(NO2)(NO). Scenario
(i) suggests that this is initiated by â-bond cleavage of NO2
from 2 to give an oxo nitrosyl complex FeIV(TPP)(O)(NO),
scenario (ii) involves oxygen atom abstraction from the coor-
dinated nitrato ligand by excess NO to give directly a species
of the same stoichiometry (but perhaps a linkage isomer) as 3
and NO2, and scenario (iii) would be initiated by dissociation
Notably, ambient temperature solution studies described
below also indicate the formation of 2 as an intermediate in the
reaction of 1 with NO, although the former is unstable toward
decay to other products. This behavior is in contrast to the
greater stability of the analogous ruthenium complex Ru(TTP)-
(ONO2)(NO) (TTP ) meso-tetra(4-tolyl)porphyrinate dianion),
which was first prepared by the solution reaction of Ru(TTP)-
(NO)Cl with AgNO320 and subsequently by the reaction of Ru-
(TTP)(CO) with NO2/N2O4.21
(16) Ellison, M. K.; Shang, M.; Kim, J.; Scheidt, W. R. Acta Crystallogr., Sect.
C 1996, C52, 3040-3043.
•
of an NO3 radical (perhaps assisted by reaction with NO) to
give directly 4, which is then rapidly oxidized to 3 by reaction
with the NO2 resulting from reaction of NO3 with NO.25
(17) Franz, K. J.; Lippard, S. J. J. Am. Chem. Soc. 1999, 121, 10504-10512.
(18) Oshio, H.; Ama, T.; Watanabe, T.; Kincaid, J.; Nakamoto, K. Spectrochim.
Acta 1984, 40A, 863-870.
(19) Wyllie, G. R. A.; Scheidt, W. R. Chem. ReV. 2002, 102, 1067-1089.
(20) Bohle, D. S.; Hung, C.-H.; Smit, B. D. Inorg. Chem. 1998, 37, 5798-
5806.
(21) Kang, Y.; Zyryanov, G. V.; Rudkevich, D. M. Chem. Commun. 2003,
2470-2471.
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A R T I C L E S
Kurtikyan et al.
Scheme 2. Pathways for the Reactions of Fe(TPP)η2-O2NO) with
Excess NO
IR measurements with isotopically labeled NO were used to
lend insight into differentiating these pathways, with particular
focus on the early stages before subsequent reactions lead to
isotopic scrambling. For example, reaction of Fe(TPP)(O2NO)
with excess 15NO (system A) leads first to the formation of
Fe(TPP)(ONO2)(15NO), and different products are predicted for
the subsequent reactions according to the three scenarios.
Scenario (i) should give Fe(TPP)(15NO2)(15NO), scenario (ii)
should give Fe(TPP)(NO2)(15NO), and scenario (iii) should give
a mixture of Fe(TPP)(15NO2)(15NO) and Fe(TPP)(NO2)(15NO).
Since all three are accompanied by formation of 1 mol of NO2,
which in excess NO will give dinitrogen trioxide (N2O3), the
appearance of the latter in the low-temperature spectrum would
also be expected.
The results were as follows: Figure 4 (bottom) displays the
FTIR spectral changes observed when excess 15NO was added
to Fe(TPP)(O2NO) (system A) to give Fe(TPP)(ONO2)(15NO),
and then the mixture was gradually warmed. The further reaction
of Fe(TPP)(ONO2)(15NO) is manifested by diminishing intensi-
ties of bands at 1860 (not shown), 1506, and 1266 cm-1
accompanied with the appearance of new bands at ∼1830 (not
shown), 1452, 1422, 1295, and 1273 cm-1. The new bands at
1830, 1452, and 1295 cm-1 belong to the ν(15NO), νa(NO2),
and νs(NO2) absorptions of Fe(TPP)(NO2)(15NO), while those
at 1422 and 1273 cm-1 can be assigned to 15NO2 stretches of
the doubly labeled complex Fe(TPP)(15NO2)(15NO). In the latter
case, unambiguous assignment of the band at 1273 cm-1 is
complicated, since (15N)2O3 also shows absorptions in the same
region. IR bands at 1830, 1546, and 1277 cm-1 are reported
for solid (15N)2O3.22 The growth of the weak band at ∼1550
cm-1 confirms formation of (15N)2O3. These coincidences do
not challenge the formation of Fe(TPP)(15NO2)(15NO) at this
stage, since the band at 1422 cm-1 appears in a region relatively
free of interference from the spectra of other NOx species and
of porphyrin itself. However, the bands at 1273 cm-1 (Figure
Figure 4. Isotope effects on FTIR spectra changes during the course of
warming sublimed layers from 145 to 170 K in the presence of nitric oxide
(4 Torr). Bottom: 15NO + Fe(TPP)(NO3) (system A). Top: NO + Fe-
(TPP)(15NO3) (system B).
4, bottom) and in the vicinity of 1830 cm-1 may represent the
overlapping absorptions characteristic of Fe(TPP)(15NO2)(15NO)
and (15N)2O3.
Analogous data for addition of NO to Fe(TPP)(O215NO)
(system B) to give initially Fe(TPP)(O15NO2)(NO) are displayed
in Figure 4 (top). The same analysis shows the disappearance
of the latter to be accompanied by growth of bands at 1860
(not shown), 1452, 1422, 1296, and 1275 cm-1. This can be
interpreted in terms of the initial formation of two isoto-
pomers: Fe(TPP)(NO2)(NO) (bands at 1860, 1452, and 1296
cm-1) and Fe(TPP)(15NO2)(NO) (bands at 1860, 1422, and 1275
cm-1). Simultaneously, a weak band in the vicinity of 1590
cm-1 appears (due to N2O3) and enhances the intensity of the
porphyrin band disposed in this region. Other bands character-
istic of solid N2O3 appear at 1863 and 1297 cm-1 22
overlap with bands assigned to Fe(TPP)(NO2)(NO).
,
and they
Interpretation of Isotope Studies of the Low-Temperature
Reaction of 2 with NO. To a first approximation, these data
appear to be consistent with scenario (iii) and not with (i) or
(ii). For example, scenario (i) predicts that system A would give
Fe(TPP)(15NO2)(15NO) and singularly labeled dinitrogen trioxide
(O15NNO2) and system B to give Fe(TPP)(NO2)(NO) and
ON15NO2.23 In contrast, system A gave Fe(TPP)(NO2)(15NO),
Fe(TPP)(15NO2)(15NO), and (15N)2O3 while system B gave Fe-
(TPP)(NO2)(NO), Fe(TPP)(15NO2)(NO), and N2O3. Addition-
ally, no new IR bands were found in the region where ferryl
ν(FedO) stretches are usually disposed (750-860 cm-1).24
(22) ) Hisatsune, I. C.; Devlin, J. P.; Wada, Y. J. Chem. Phys. 1960, 30, 714-
(23) Here we consider only the more stable N-N bonded form of N2O3.
(24) Nakamoto, K. J. Mol. Struct. 1997, 408/409, 11-16.
719.
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6220 J. AM. CHEM. SOC. VOL. 127, NO. 17, 2005

Reactions of Nitrogen Oxides with Heme Models
A R T I C L E S
Scheme 3
The situation is more ambiguous with regard to scenario (ii),
which predicts system A formation of (15N)2O3 and Fe(TPP)-
(NO2)(15NO) (after isomerization of the initially formed nitrito
complex) and system B formation of Fe(TPP)(15NO2)(NO) and
N2O3. For each set of reaction conditions, products not predicted
by scenario (ii) were formed, Fe(TPP)(15NO2)(15NO) in system
A and Fe(TPP)(NO2)(15NO) in system B.
As outlined in greater detail in Supporting Information
Scheme S1, scenario iii predicts the formation of Fe(TPP)-
(15NO2)(15NO), Fe(TPP)(NO2)(15NO), and both doubly and
singly labeled dinitrogen trioxide for the conditions of system
A, and Fe(TPP)(15NO2)(NO), Fe(TPP)(NO2)(NO), and both
singly labeled and unlabeled dinitrogen trioxide for the condi-
tions of system B. The experimental results obtained are in
agreement with scenario iii with the exception that only one
type of dinitrogen trioxide was observed under each set of
conditions, namely (15N)2O3 for system A and N2O3 for system
B. A possible explanation of this inconsistency would be isotopic
scrambling of the monolabeled species O15NNO2 and ON15NO2
by reaction with the excess nitric oxide present (15NO in system
A and NO in system B). This would give dinitrogen trioxide
with the same nitrogen labeling as the added nitric oxide (15NO
and NO, respectively). This process should be facilitated by
the formation of dinitrogen trioxide species isomers at very low
temperatures.26 Consistent with this point of view is the absence
of published data describing the mixed isotope species as
O15NNO2 or ON15NO2.
•
The fundamental problem with scenario iii is that the NO3
radical is a high energy species,28 and although its formation
from 2 should be facilitated by the high thermodynamic stability
of 4, the first step in this sequence would appear to be
substantially uphill. However, an alternative step by which the
loss of an •NO3 equivalent might be effected would be the direct
reaction of the excess NO with the coordinated nitrate ligand.
In effect, this is equivalent to the reductive nitrosation of a
coordinated ligand identified recently with a copper(II) com-
plex29 in the context that the coordinated anion is nitrosated
concomitant with the reduction of the metal center (in this case
the reduced metal species is 4) as illustrated in eq 2. With X-
) nitrate and LmMn+ ) Fe(TPP)(NO), the concerted transfer
of a NO3 equivalent to NO to give some form of N2O4 and the
very stable 4 as intermediates would give the same product
distributions as scenario iii while avoiding the formation of the
very high energy nitrate radical.
[LmM-X:]n+ + NO f [LmM]n+ + X-NO
(2)
The possible exchange reaction between NO and N2O3 was
examined under closely related experimental conditions using
amorphous Ni(TPP) layers as adsorbent.27 Partly oxidized NO
was deposited on the Ni(TPP) layer at 80 K and warmed to
120 K, and then the system was evacuated to expel any free
NO and then cooled back to 80 K. This procedure led to the
formation of N2O3 which was manifested by IR bands at 1842,
1593, and 1292 cm-1 and a small amount of N2O4 in the layer
Hence, we believe that the most plausible mechanism for
formation of Fe(TPP)(NO2)(NO) (3) from 2 involves the
pathway represented in Scheme 3. Reaction of NO2 with the
nitrosyl complex 4 would rapidly give 3.6 Notably, the reaction
of NO with coordinated nitrate to give an ONONO2 species is
the microscopic reversal of a likely first step in the formation
of Ru(TTP)(ONO2)(NO) by the reaction of N2O4 with Ru(TTP)-
(CO).21
Higher Temperature Reactions of Fe(TPP)(NOx) Films
with NO. For system A, further warming of the layer does not
significantly change the intensity of the 1296 cm-1 band of Fe-
(TPP)(NO2)(15NO), but the νs(15NO2) and νa(15NO2) bands (1275
and 1422 cm-1, respectively) corresponding to the fully labeled
nitro nitrosyl complex Fe(TPP)(15NO2)(15NO) grow more
intense. An analogous pattern is observed for system B (excess
NO) where the intensity of the νs(15NO2) band for Fe(TPP)-
(15NO2)(NO) (1275 cm-1) grows only to a certain level while
intensities of the νs(NO2) and νa(NO2) bands of Fe(TPP)(NO2)-
(NO) (1296 and 1452 cm-1, respectively) continue to increase.
At the end of this stage of reaction, the layers consist mostly of
Fe(TPP)(15NO2)(15NO) and Fe(TPP)(NO2)(NO) for experiments
with excess 15NO and NO, correspondingly. This process can
be rationalized according to the hypothetical pathway presented
in Scheme 4. Other mechanisms involving oxidation of coor-
dinated NO by NO2, decomposition of a putative dinitro
with weak IR bands at 1734 and 1254 cm-1 22 15
NO was then
.
introduced into the cryostat, and the FTIR spectra of this system
were measured at various temperatures upon slow warming. This
procedure showed that isotope exchange reaction occurs at
temperatures as low as 140 K. Thus, under the experimental
conditions used in the NO reaction with 2 to give 3, the
suggested isotopic exchange reaction should indeed lead to
formation of dinitrogen trioxide species dominated by the
isotopic composition of the excess nitric oxide.
Hence, of the three scenarios described by Scheme 2, the
most plausible appears to be scenario iii, which is represented
as initiating by elimination of a •NO3 radical (however, see the
discussion below). Beside the experimental results given above,
there is additional, indirect evidence consistent with this
scenario. During the course of the reaction of Fe(TPP)(ONO2)-
(NO) (2) with NO to give Fe(TPP)(NO2)(NO) (3), a weak band
in the vicinity of 1675 cm-1 initially grows and then diminishes
in intensity (Figure 3). This can be interpreted in terms of
formation and decay of Fe(TPP)(NO) (4) as an intermediate
•
resulting from NO3 elimination.
(28) (a) The electron affinity of the NO3 radical has been reported27b to be 375
kJ mol-1. (b) Lias, S. G.; Barmes, J. E.; Holmes, J. L.; Levin, R. D.; Mallard,
W. G. J. Phys. Chem. Ref. Data 1988, 17 (Suppl. 1), 1-86 as summarized
on p 43 in Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry,
4th ed.; Harper-Collins College Publishers: New York, 1993.
(29) (a) Tsuge, K.; DeRosa, F.; Lim, M. D.; Ford, P. C. J. Am. Chem. Soc.
2004, 126, 6564-6565. (b) Ford, P. C.; Fernandez, B. O.; Lim, M. D.
Chem. ReV., published online Feb 18, 2005. http://dx.doi.org/10.1021/
CR0307289.
(25) Wayne, R. P., et al. Atmos. EnViron. 1991, 25A, 1-203.
(26) Fateley, W. G.; Bent, H. A.; Crawford, B. J. Chem. Phys. 1959, 31, 204-
217.
(27) (a) At low temperatures, sublimed layers of Ni(TPP) react with neither
NO nor NO2, while at room temperature NO2 oxidizes porphyrin ring with
formation of Ni(TPP)•+(NO2)- ion pair.26b (b) Martirosyan, G. G.; Kur-
tikyan, T. S. Zh. Prikl. Khim. (Russ.) 1998, 71, 1595-1598.
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A R T I C L E S
Kurtikyan et al.
Scheme 4. Possible Mechanism of Isotope Exchange for Fe(TPP)(NO2)(15NO) + 15NO
complex, and re-formation of isotopically exchanged 3 cannot
be ruled out. Isotope mixing could also occur just by ligand
exchange.
in system B can be assigned, respectively, to formation of Fe-
(TPP)(15NO)(O15NO2) and Fe(TPP)(ONO2)(NO). Further warm-
ing of the layer is accompanied by decreases of these bands
and increased intensities of new ones at 1253 and 1273 cm-1
corresponding to the appearance of Fe(TPP)(15NO3) and Fe-
(TPP)(NO3). The well-defined isosbestic points seen in Figure
5 indicate that these spectral changes are coupled to the
transformation of Fe(TPP)(O15NO2)(15NO) to Fe(TPP)(15NO3)
(in A) and Fe(TPP)(ONO2)(NO) to Fe(TPP)(NO3) (in B). Oddly,
under these conditions, the processes described before appear
to be reversed; that is, 3 is oxidized to the nitrato analogue 2,
which loses NO to regenerate 1.
In contrast to the well-defined temperature interval that
divides formation of the nitrato nitrosyl complex 2 from the
subsequent transformation of 2 to the nitro nitrosyl complex 3,
there are no clear temperature boundaries between the latter
processes and subsequent steps in these solid films leading to
the partial formation of Fe(TPP)(NO3) and Fe(TPP)(NO). In
the presence of NO, all three species, namely, 1, 3, and 4, are
formed in the layer in relative quantities depending on PNO
.
Exhaustive evacuation of NO from the cryostat depleted 3 and
increased quantities of 1 and 3.
A plausible pathway for the oxidation of the coordinated nitro
ligand would involve linkage isomerization of 3 to the nitrito
analogue Fe(TPP)(ONO)(NO) (5) prior to oxygen transfer from
one of the NOx species present, probably NO2. There is only
moderate spectral data regarding 5, which was recently observed
by irradiation of a KBr pellet containing Fe(TPP)(NO2)(NO)
at low-temperature conditions.30 In difference spectra, isotope-
sensitive bands at 1507 and 934 cm-1 were assigned to ν(Nd
O) and ν(N-O) of coordinated nitrito ligand in six-coordinate
nitrito nitrosyl complex Fe(TPP)(ONO)(NO). Similar isotope-
sensitive bands at 1528 and 908 cm-1 were recorded in the
course of low-temperature studies of the reaction between small
quantities of NO2 with sublimed layers of Fe(TPP) and were
assigned to the same stretching modes as those in five-coordinate
nitrito complex Fe(TPP)(ONO).31 Close examination of the
spectra presented in Figure 3 shows that the broad band in the
vicinity of 970 cm-1 has a low frequency shoulder, suggesting
the presence of an additional weak band with a maximum in
the vicinity of 940 cm-1. We believe that this isotope-sensitive
band belongs to 5, although bands in the vicinity of 1900 and
1500 cm-1 are obscured by those of Fe(TPP)(ONO2)(NO) (2).
Solution-Phase Reactivity Studies at Ambient Tempera-
ture. We have also briefly investigated the solution-phase
reactions of Fe(TPP)(O2NO) (1) with NO at ambient temper-
ature, and these gave spectral changes consistent with the
chemical processes found in the solid films. The transformations
after rapidly mixing solutions of 1 in toluene or dichloromethane
and NO were followed by UV-vis and IR spectral measure-
ments. Similar spectral changes were seen in both solvents.
Figure 6 displays the changes in the electronic spectrum
featuring the disappearance of 1 (λSoret ) 414 nm, λQ ) 510
nm) and the generation of a species with an electronic spectrum
similar to that of Fe(TPP)(NO2)(NO) (3) (λSoret ) 432 nm). Over
a period of minutes the solution underwent further spectral
changes to give a λSoret value at 406 nm, consistent with the
formation of Fe(TPP)(NO) (4). No simple [NO] and [N2O3]
Isotopic experiments with systems A and B help elucidate
reformation of Fe(TPP)(O2NO) during this stage (Figure 5). The
bands that appear at 1245 cm-1 in system A and at 1265 cm-1
(30) Lee, J.; Kovalevsky, A. Y.; Novozhilova, I.; Bagley, K.; Coppens, P.;
Richter-Addo, G. B. J. Am. Chem. Soc. 2004, 126, 7180-7181.
(31) Kurtikyan, T. S.; Gulyan, G. M.; Ford, P. C. Book of Abstracts; XVI All-
Russian Symposium on Modern Chemical Physics, Tuapse, Russia, Sept.
20-Oct. 1, 2004; p 179.
Figure 5. FTIR spectra changes resulting from warming sublimed layers
from 200 to 240 K in the presence of nitric oxide (4 Torr). Bottom: 15NO
+ Fe(TPP)(NO3) (system A). Top: NO + Fe(TPP)(15NO3) (system B).
9
6222 J. AM. CHEM. SOC. VOL. 127, NO. 17, 2005

Reactions of Nitrogen Oxides with Heme Models
A R T I C L E S
Figure 6. UV-vis absorption spectra recorded every 80.3 s after introduc-
tion of 1.6 mM NO to toluene solution containing 20 µM 1 (A, λSoret
)
414 nm, λQband ) 510 nm). This is interpreted in terms of the conversion
of Fe(TPP)(O2NO) to a new species (B, λSoret ) 432 nm), probably Fe-
(TPP)(NO2)(NO), followed by subsequent reaction to give Fe(TPP)(NO)
(C, λSoret ) 406 nm). Inset: time dependence of the absorbance changes at
432 nm (B).
Figure 8. Infrared spectral changes (degassed 0.5-mm CaF2 cell) during
the rapid mixing (deadtime ∼1 s) of 30 µM Fe(TPP)(NO3) with either 4.7
mM 14N16O (dark, solid line) or 4.7 mM 15N18O (light, dashed) in
dichloromethane. Reaction with 14N16O resulted in the formation of two
new species with absorption peaks matching the νNO characterized for Fe-
(TPP)(ONO2)(NO) (A, νNO ) 1909 cm-1) and Fe(TPP)(NO2)(NO) (B, νNO
) 1885 cm-1). Reaction with 15N18O resulted in spectra that were interpreted
as a mixture of Fe(TPP)(ONO2)(14N16O) (A, νNO ) 1909 cm-1), Fe(TPP)-
(NO2)(14N16O) (B, νNO ) 1885 cm-1), Fe(TPP)(ONO2)(15N16O) (C, νNO
1873 cm-1), and Fe(TPP)(ONO2)(15N18O) (D, νNO ) 1829 cm-1).
)
at 1829 cm-1 (Figure 8). This is in agreement with the formation
of Fe(TPP)(ONO2)(15N18O) and the assignment of the isotope-
sensitive absorption bands at 1909 and 1829 cm-1 as the nitrosyl
stretch(es) of the nitrato nitrosyl complex 2. The other species
formed under these conditions (ν(NO) ) 1886 cm-1 14
) has been
characterized as Fe(TPP)(NO2)(NO). Unfortunately, the bands
expected for coordinated NO2 or ONO (1200-1550 cm-1) were
obscured by solvent overtones. At longer times, the absorption
bands characteristic of N2O3 (ν(NO) ) 1830 cm-1, ν(NO2) )
1600 cm-1 22
) decreased simultaneous with the growth of a new
Figure 7. IR spectral changes (degassed 0.5-mm CaF2 cell) after rapid
mixing (deadtime ∼1 s) of dichloromethane solutions containing 30 µM
Fe(TPP)(NO3) and 4.7 mM NO. The initial spectrum is taken during
continuous flow through the cell, and the following spectra were ac-
cumulated approximately 1, 2, and 4 min after the flow was stopped. The
quick formation and transformation of Fe(TPP)(ONO2)(NO) (A, νNO ) 1909
cm-1) was followed by the formation and decay of Fe(TPP)(NO2)(NO)
band at 1684 cm-1 characteristic of the ν(NO) of Fe(TPP)(NO).
Summary
We have described here redox transformations of NOx ligands
coordinated to the heme model Fe(TPP) in the solid state as
layered films and in aprotic media. The entry point to these
investigations was the η2-nitrato complex Fe(TPP)(η2-O2NO)
(1), which undergoes a rapid NO addition step to form Fe(TPP)-
(η1-ONO2)(NO) (2) (Scheme 1). In the presence of excess NO,
the latter species undergoes further reaction in a process that
would appear to be the result of a “simple” oxygen atom transfer
from 2 to give the nitro nitrosyl complex Fe(TPP)(NO2)(NO)
(3). However, reactivity studies in low-temperature films using
IR detection and isotope labeling techniques argue against this
transformation occurring by oxygen atom transfer from the η1-
coordinated nitrate of 2 to NO to give (initially) the nitrito
nitrosyl complex Fe(TPP)(ONO)(NO) (5) and NO2 (scenario ii
of Scheme 2). Similar arguments based on these data can be
mounted against a mechanism proceeding via â-bond cleavage
of the FeO-N bond of the coordinated nitrate of 2 to give a
ferryl complex and NO2 (scenario i). Neither of these scenarios
can explain the isotopic distributions of products when the
reactions are initiated with labeled NO or labeled 2, unless there
is significant isotopic exchange between the coordinated nitro
group and the free NO.
(B, νNO ) 1886 cm-1). N2O3 was also observed to decrease (C, νNO
)
-1
Inset: Eventual formation of Fe(TPP)(NO)
1600 cm ).
1830 cm-1, νNO2
)
(D, νNO ) 1684 cm-1), initial spectrum taken 4 min after stopping the flow
with the following spectra taken every 10 min.
dependencies on the rates of reaction were apparent, but it was
noted that the presence of N2O3 stabilized 3.
Addition of PPh3 (as an N2O3 sink) to the reaction mixture,
or scrupulous purification of NO prior to mixing, led to faster
transformation of 3 to 1.
A custom-made rapid flow apparatus was used in conjunction
with IR measurements to help reveal the identity of the
intermediates. As shown in Figure 7, rapid mixing of dichlo-
romethane solutions of NO and Fe(TPP)(O2NO) led to the
formation of a species that displayed IR band maxima at 1909
and 1886 cm-1. At longer times, the band at 1909 cm-1 was
found to decay, leaving the species with a band at 1886 cm-1
as the dominant product. On the basis of the analogous reactions
in the solid state, the band centered at 1909 cm-1 was assigned
as the ν(NO) of Fe(TPP)(ONO2)(NO) (2). Rapid mixing of
dichloromethane solutions of isotopically labeled 15N18O and
Fe(TPP)(O2NO) resulted in formation of a product with a band
9
J. AM. CHEM. SOC. VOL. 127, NO. 17, 2005 6223

A R T I C L E S
Kurtikyan et al.
A third pathway (scenario iii) involves Fe-O bond homolysis
of 2 to give Fe(TPP)(NO) (4) and the NO3 radical, followed
the nitrate ion, usually considered relatively innocuous in
mammalian systems, can be activated by NO and hemes to form
other reactive NOx intermediates.
•
by rapid trapping of this radical by excess NO to give N2O4,
which then reacts with 4 to give 3. While this would indeed
explain the product isotopic distributions given the demonstrated
exchange between N2O3 and NO under these conditions, the
Acknowledgment. Financial support from CRDF (Grant No.
AC2-2520-TB-03), the U.S. Department of Energy (DE-FG02-
04ER15506), and a Visiting Scholarship for T.S.K. at UCSB
from Petroleum Research Fund is gratefully acknowledged.
•
high energy of the NO3 radical would make the first step in
scenario iii highly endoergic. A more palatable alternative giving
the same isotopic distributions is represented above in Scheme
Supporting Information Available: Scheme S1 showing
transition of Fe(TPP)(ONO2)(NO) to Fe(TPP)(NO2)(NO) by
mechanism of NO3 radical elimination for mixed isotope species.
Complete citations for refs 3d and 25. Figure S1 demonstrating
IR spectra in the range 600-400 cm-1 of thin layers containing
Fe(TPP)(O2NO) and Fe(TPP)(ONO2)(NO), Figure S2 showing
spectral changes describing formation of (NO)Fe(TPP)(O15NO2)
in the course of Fe(TPP)(O215NO) interaction with NO, and
Figure S3 showing FTIR spectra of Fe(TPP)(ONO2)(15NO) and
Fe(TPP)(O15NO2)(NO) isotopomers. This material is available
free of charge via the Internet at http://pubs.acs.org.
•
3. In this model, loss of the NO3 radical is effected by attack
of NO at the oxygen of the coordinated nitrate concerted with
electron transfer to the metal center, leading to rapid dissociation
of the transient N2O4 ligand and the formation of 4. An analogy
can be drawn between this pathway and the recently documented
reductive nitrosylation of a copper(II)-coordinated amine that
has been suggested to occur via such an inner sphere electron
transfer process.29
It is notable that the various scenarios for NO interaction with
the nitrato complex 1 lead to the intermediacy of highly reactive
species such as NO2, N2O3, and N2O4 that contribute to oxidative
and nitrosative stress and can be damaging to cellular species.32
Hence, the present study shows that under certain conditions,
JA042237R
(32) Kirsch, M.; Korth, H.-G.; Sustmann, R.; de Groot, H. Biol. Chem. 2002,
383, 389-399.
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6224 J. AM. CHEM. SOC. VOL. 127, NO. 17, 2005