Reactions of nitrogen oxides with the five-coordinate Fe III(porphyrin) nitrito intermediate Fe(Por)(ONO) in sublimed solids

Article abstract of DOI:10.1021/ja067245h

Detailed experimental studies are described for reactions of several nitrogen oxides with iron porphyrin models for heme/N_xO_y systems. It is shown by FTIR and optical spectroscopy and by isotope labeling experiments that reaction of small increments of NO₂ with sublimed thin layers of the iron(II) complex Fe(Por) (For = meso-tetraphenylporphyrinato dianion, TPP, or meso-tetra-p-tolylporphyrinato dianion, TTP) leads to formation of the 5-coordinate nitrito complexes Fe(Por)(η¹-ONO) (1), which are fairly stable but very slowly decompose under vacuum giving mostly the corresponding nitrosyl complexes Fe(Por)(NO). Further reaction of 1 with new NO₂ increments leads to formation of the nitrato complex Fe(Por)(η²-O₂NO) (2). The interaction of NO with 1 at low temperature involves ligand addition to give the nitrito-nitrosyl complexes Fe(Por)(η¹-ONO)(NO) (3); however, these isomerize to the nitro-nitrosyl analogs Fe(Por)(η¹-NO₂)(NO) (4) upon warming. Experiments with labeled nitrogen oxides argue for an intramolecular isomerization ( flipping ) mechanism rather than one involving dissociation and rebinding of NO₂. The Fe(III) centers in the 6-coordinate species 3 and 4 are low spin in contrast to 1, which appears to be high-spin, although DFT computations of the porphinato models Fe(P)(nitrite) suggest that the doublet nitro species and the quartet and sextet nitrito complexes are all relatively close in energy. The nitro-nitrosyl complex 4 is stable under an NO atmosphere but decomposes under intense pumping to give a mixture of the ferrous nitrosyl complex Fe(Por)(NO) and the ferric nitrito complex Fe(Por)(η¹-ONO) indicating the competitive dissociation of NO and NO₂. Hence, loss of NO from 4 is accompanied with nitro → nitrito isomerization consistent with 1 being the more stable of the 5-coordinate NO₂ complexes of iron porphyrins.

Full text of DOI:10.1021/ja067245h

Published on Web 03/06/2007
Reactions of Nitrogen Oxides with the Five-Coordinate
FeIII(porphyrin) Nitrito Intermediate Fe(Por)(ONO) in Sublimed
Solids
Tigran S. Kurtikyan,*,†,‡ Astghik A. Hovhannisyan,† Manya E. Hakobyan,‡
James C. Patterson,§ Alexei Iretskii,§,| and Peter C. Ford*,§
Contribution from the Molecule Structure Research Centre (MSRC) NAS, 375014 YereVan,
Armenia, Research Institute of Applied Chemistry (ARIAC), 375005 YereVan, Armenia,
Department of Chemistry and Biochemistry, UniVersity of California at Santa Barbara, Santa
Barbara, California 93106-9510, and Department of Chemistry, Lake Superior State UniVersity,
Sault Sainte Marie, Michigan 49783
Received October 16, 2006; E-mail: kurto@netsys.am; ford@chem.ucsb.edu
Abstract: Detailed experimental studies are described for reactions of several nitrogen oxides with iron
porphyrin models for heme/NxOy systems. It is shown by FTIR and optical spectroscopy and by isotope
labeling experiments that reaction of small increments of NO2 with sublimed thin layers of the iron(II) complex
Fe(Por) (Por ) meso-tetraphenylporphyrinato dianion, TPP, or meso-tetra-p-tolylporphyrinato dianion, TTP)
leads to formation of the 5-coordinate nitrito complexes Fe(Por)(η1-ONO) (1), which are fairly stable but
very slowly decompose under vacuum giving mostly the corresponding nitrosyl complexes Fe(Por)(NO).
Further reaction of 1 with new NO2 increments leads to formation of the nitrato complex Fe(Por)(η2-O2NO)
(2). The interaction of NO with 1 at low temperature involves ligand addition to give the nitrito-nitrosyl
complexes Fe(Por)(η1-ONO)(NO) (3); however, these isomerize to the nitro-nitrosyl analogs Fe(Por)(η1-
NO2)(NO) (4) upon warming. Experiments with labeled nitrogen oxides argue for an intramolecular
isomerization (“flipping”) mechanism rather than one involving dissociation and rebinding of NO2. The Fe-
(III) centers in the 6-coordinate species 3 and 4 are low spin in contrast to 1, which appears to be high-
spin, although DFT computations of the porphinato models Fe(P)(nitrite) suggest that the doublet nitro
species and the quartet and sextet nitrito complexes are all relatively close in energy. The nitro-nitrosyl
complex 4 is stable under an NO atmosphere but decomposes under intense pumping to give a mixture
of the ferrous nitrosyl complex Fe(Por)(NO) and the ferric nitrito complex Fe(Por)(η1-ONO) indicating the
competitive dissociation of NO and NO2. Hence, loss of NO from 4 is accompanied with nitro f nitrito
isomerization consistent with 1 being the more stable of the 5-coordinate NO2 complexes of iron porphyrins.
Introduction
Possible endogenous NO2 sources are NO autoxidation in lipid
membranes5 and nitrite ion oxidation by peroxidase enzymes;6
Nitric oxide (nitrogen monoxide) has several well-established
roles in mammalian biology closely tied to its interactions with
metalloproteins, especially heme proteins.1 In this context, there
is a continuing interest in other physiological pathways involving
NO and other NxOy species, including nitrogen dioxide (NO2),
nitrite ion (NO2-), and nitrate (NO3-). NO2 is a strong oxidant
and nitrating species that initiates destructive pathways in living
systems,2 and several disease states have been suggested to be
connected with its exogenous and endogenous formation.3,4
in addition the NO reactions with superoxide ion lead to
peroxynitrite ion (ONOO-),7 the decay of which affords nitrate
ion and NO2 in competitive processes.8 Reaction of NO with
nitrate as mediated by a heme center also produces nitrogen
dioxide.9 With regard to NO2-, nitrite ion is ubiquitous in
mammalian tissues and fluids and is the largest reservoir of
accessible NO equivalents in the cardiovascular system.10 Recent
studies indicate that NO2- may be a vasodilator or, more likely,
† Molecule Structure Research Centre (MSRC) NAS.
‡ Research Institute of Applied Chemistry (ARIAC).
§ University of California at Santa Barbara.
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| Lake Superior State University.
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Diego, CA, 2000.
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9
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J. AM. CHEM. SOC. 2007, 129, 3576-3585
10.1021/ja067245h CCC: $37.00 © 2007 American Chemical Society

Five-Coordinate Intermediate Fe(Por)(ONO)
A R T I C L E S
forms the known vasodilator NO via nitrite reductase activity
of deoxyhemoglobin or deoxymyoglobin.10d,e
More recently, there have been several demonstrated nitrito
complexes of heme models.21-23 For example, a metastable
compound prepared21 by photolysis of Fe(TPP)(NO2)(NO) in
a low-temperature KBr pellet was concluded to be the nitrito-
nitrosyl complex Fe(TPP)(ONO)(NO) on the basis of the
observation of N-isotope sensitive bands in difference IR spectra
characteristic for the ν(NdO) and ν(N-O) stretching modes
of the Fe-O-NdO moiety and extensive DFT calculations.
In 2006, the first example of a structurally characterized nitrito
complex of a heme protein was prepared by the soaking
preformed crystals of aquo horse heart metmyoglobin (ferric
form with the proximal coordination site occupied by a histidine
residue) in sodium nitrite solution. The crystal structure clearly
shows the NO2- ligand to be coordinated as the O-bound nitrito
isomer.23
Moreover, in a preliminary communication,22 we reported that
the reaction of incremental NO2 with the iron(II) complex Fe-
(Por) (Por ) meso-tetraphenylporphyrinato dianion, TPP, or
meso-tetra-p-tolylporphyrinato dianion, TTP) gives the “elusive”
5-coordinate iron(III) nitrito complexes Fe(Por)(η1-ONO) (1)
(eq 1).
The role of nitrate ion (NO3-) in the biology of metabolically
linked nitrogen oxides (NO, NO2-, NO3-, etc.) remains to be
fully elucidated. It is generally considered a relatively unreactive
final oxidation product in mammalian systems that is excreted
in the urine,11 although infantile methemoglobinemia (blue baby
syndrome) has been attributed to excessive ingested nitrate that
is reduced by intestinal bacteria.12 In addition, physiological
nitrate is known to concentrate in the saliva where it is reduced
by bacterial nitrate reductase enzymes to nitrite, ingestion of
which has been proposed to serve a positive effect in normal
healthy humans.13 Mammalian nitrate reductases are essentially
unknown, although it has recently been shown that nitrate can
be activated by NO and heme models to yield other highly
reactive NxOy species.9 Such processes are potentially relevant
to the chemical biology of the nitrogen oxides as well as to
schemes for using such species as mediators in catalytic partial
oxidation of various substrates.14 Clearly, there is continuing
incentive to elucidate the fundamental chemistry of these NxOy
related to their interactions with heme models.15
As an ambidentate ligand, there are several modes by which
nitrite ion may bind to metal centers,16 including N(nitro) and
O(nitrito) coordination as well as η2-O2N linkage isomer
coordination.17 Before 2004, however, the only well-character-
Fe(Por) + NO2 f FeIII(Por)(η1-ONO)
(1)
In this article, we describe full details of the interaction of
NO with this species. The nitrito-nitrosyl complexes Fe(Por)-
(NO)(ONO) have been obtained by low-temperature interaction
of NO with 1, and the mechanism of the nitrito f nitro
isomerization upon warming has been investigated by using
mixed isotope substituted species. The reaction of additional
NO2 increments with 1, leading eventually to formation of
nitrato complexes, is also described in detail. These investiga-
tions are supported by DFT calculations that show that nitro
and nitrito structures have similar energies and provide insight
into the mechanism of the ligand exchange reaction upon
supplying labeled NO2 to 1.
-
ized NO2 complexes of a synthetic ferrous or ferric porphy-
rins18 or of a heme protein19 were N-coordinated nitro com-
plexes. An exception would be the anion of [K(222)][Fe(TpivPP)-
(NO2)(NO)] prepared by the reaction of Fe(TpivPP) with
Kryptofix-222-solubilized KNO2 followed by the reaction with
NO. The product had two crystalline forms. In one, the
Fe(TpivPP)(NO2)(NO)- anion had the nitro nitrosyl structure,
and in the other, one of the two independent anions in the
asymmetric unit displayed disorder that was concluded to result
from the presence of both nitro and nitrito isomers in the same
crystal; this linkage isomerism was manifested by differing
spectroscopic properties of these crystals.20
Experimental Section
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Rodriguez, J. R.; Feelisch, M. Proc. Nat. Acad. Sci. U.S.A. 2004, 101,
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The ferrous porphyrinates Fe(Por) are very sensitive to oxygen and
are readily oxidized to Fe(III) derivatives. For this reason the more
stable hexacoordinate ferrous complexes Fe(Por)(B)2 with nitrogen bases
(B is Py or Pip) were synthesized24 and used as the Fe(Por) precursors.
The low-temperature sublimates were prepared25 by placing a Fe(Por)-
(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 axial ligands, as monitored by measurement
of the pressure at the cryostat outlet. Liquid nitrogen (LN2) was then
poured into the cryostat, and the Knudsen cell was heated to 520 K,
whereupon Fe(Por) sublimed onto the 77 K surface of the KBr or CaF2
substrate. In the latter case, CaF2 plates were also used as the optical
windows of the cryostat. The metallo-arylporphyrinato layers obtained
in this manner are spongelike and have high microporosity, similar to
(11) Lundberg, J. O.; Govoni, M. Free Radical Biol. Med. 2004, 37, 395-400.
(12) Knobeloch, L.; Salna, B.; Hogan, A.; Postle, J.; Anderson, H. EnViron.
Health Perspect. 2000, 108, 675-678.
(13) Bjoerne, H.; Govoni, M.; Toernberg, D. C.; Lundberg, J. O.; Weitzberg,
E. Crit. Care Med. 2005, 33, 1722-1727.
(14) Taqui, M. M.; Chatterjee, D.; Shirin, Z.; Bajaj, H. C.; Siddiqui, M. R. H.;
Venkatasubramanian, K.; Bhadbhade, M. M. J. Mol. Catal. 1992, 72, 271-
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Green, S.; Walsh, R.; Standard, J.; Mardyukov, A. N. Inorg. Chem. 2005,
44, 2215-2223.
(20) Nasri, H.; Ellison, M. K.; Chen, S.; Huynh, B. H.; Scheidt, W. R. J. Am.
Chem. Soc. 1997, 119, 6274-6283.
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151-165.
(21) Lee, J.; Kovalevsky, A. Y.; Novozhilova, I.; Bagley, K; Coppens, P.;
Richter-Addo, G. B. J. Am. Chem. Soc. 2004, 126, 7180-7181. (b)
Novozhilova, I. V.; Coppens, P.; Lee, J.; Richter-Addo, G. B.; Bagley, K.
A. J. Am. Chem. Soc. 2006, 128, 2093-2104.
(16) Hitchman, M. A.; Rowbottom, G. L. Coord. Chem. ReV. 1982, 42, 55-
132.
(17) The term “nitro” will be used to describe metal complexes of the nitrite
anion (formally) that are N-coordinated; the term “nitrito” will be used to
describe those which are O-coordinated.
(22) Kurtikyan, T. S.; Ford, P. C. Angew. Chem., Int. Ed. 2006, 45, 492-496.
(23) Copeland, D. M.; Soares, A. S.; West, A. H.; Richter-Addo, G. B. J. Inorg.
Biochem. 2006, 100, 1413-1425.
(18) Wyllie, G. R. A.; Scheidt, W. R. Chem. ReV. 2002, 102, 1067-1089.
(19) (a) Einsle, O.; Messerschmidt, A.; Huber, R.; Kroneck, P. M. J.; Neese, F.
J. Am. Chem. Soc. 2002, 124, 11737-11745. (b) Williams, P. A.; Fulop,
V.; Garman, E. F.; Saunders, N. F. W.; Ferguson, F. G.; Hajdu, J. Nature
1997, 389, 406-412. (c) Crane, B. R.; Siegel, L. M.; Getzoff, E. D.
Biochemistry 1997, 36, 12120-12137.
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1720-1722. (b) Bhatti, W.; Bhatti, M.; Imbler, P.; Lee, A.; Lorenzen, B.
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1982, 104, 3744-3745.
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J. AM. CHEM. SOC. VOL. 129, NO. 12, 2007 3577

A R T I C L E S
Kurtikyan et al.
Table 1. IR and UV-Visible Spectra of the Nitrito and Nitrato Complexes Fe(Por)(ONO) and Fe(Por)(O2NO).
1 a
IR freq in cm-
ax ligand stretching bands
compds
spin-sensitive bandsc
UV−vis, λmax in nm
Fe(TTP)(ONO)
Fe(TPP)(ONO)
Fe(TTP)(O2NO)
Fe(TPP)(O2NO)
1528 (1496) m, ν(NdO)
1526 (1499) m, ν(NdO)
1529 (1495) m, ν(NdO)
1527 (1493) m, ν(NdO)
901 (878) w, ν(N-O)
904 (885) w, ν(N-O)
1271 (1251) s, νa(NO2)
1271 (1253) s, νa(NO2)
751 (747) w, δ(ONO)
750 (747) w,b δ(ONO)
966 (957) w, νs(NO2)
966 (956) w, νs(NO2)
1495 m, 1340 m 428 w
1339 m, 436 w
1495 m, 1340 m, 428 w
1340 m, 436 w
509, 574 sh, 621 sh, 693
508, 577 sh, 659, 689
515, 578 sh, 618 sh, 698
513, 579 sh, 658, 691
a In parentheses data for 15N-labeled compounds are given. b Masked by intense porphyrin bands and is seen only in difference spectra. c In intermediate
spin-state Fe(Por) these bands are disposed at 1346 and 464 cm-1 (Por ) TPP) and 1505, 1346, and 454 cm-1 (Por ) TTP).
bulk samples of these compounds.26 Potential reactants easily diffuse
across these layers, and adducts thus formed can be studied spectro-
scopically without solvent interference.
package (Wavefunction, Inc., Irvine, CA) at the B3LYP level using
the LACVP* without symmetry constraints. The optimized structures
represent the equilibrium geometries of the molecules in gas phase.
The calculations were performed on a Western Scientific dual Xeon
2.66 GHz CPU workstation. Analogous calculations were also carried
out using the Gaussian 03 suite of programs (Gaussian, Inc.; Pittsburgh,
PA) at the B3LYP level with the LANL2DZ basis set, but no significant
differences were seen.
Sublimation was typically carried out over periods of 0.3-2.0 h to
build up layers of thickness sufficient for UV-visible and IR spectral
studies. The Fe(Por) sublimed layer was then heated to room temper-
ature under dynamic vacuum, after which small increments of NO2
(15NO2, N18O2) gas were introduced for a short time period (∼30 s)
followed by a period of evacuating the apparatus. During this procedure
the red Fe(Por) film immediately turns brown indicating the formation
of Fe(Por)(ONO). After formation of this complex was confirmed by
FTIR measurements,27 the layered film was exposed to new, small
increments of NO2 (or the 15N- or 18O-labeled analogs) which were
introduced into cryostat and then were removed by evacuating the
system after 3-5 min. Reactions of the layers containing 1 with NO
were carried out by first cooling the substrate to LN2 temperature and
then introducing into the cryostat a known quantity of NO measured
by a mercury manometer. The layer was slowly warmed, and IR or
UV-visible spectra were measured at different substrate temperatures
determined by a thermocouple.
NO (15NO) 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). The IR spectra did not show the presence
of any N2O, N2O3, or H2O. NO2 (15NO2), obtained by oxidizing purified
NO (15NO) with excess pure dioxygen, was purified by fractional
distillation until a pure white solid was obtained. It was noted that
storage of NO2 in the glass bulb slowly led to formation of trace
quantities of NO.28 This was not a problem when dry O2 was added to
the NO2 storage bulb. This O2 was removed by a multiple freeze-
pump-thaw procedure just before introducing NO2 into the cryostat.
15NO with 98.5% enrichment was purchased from the Institute of
Isotopes, Tbilisi, Republic of Georgia. N18O2 with 50% enrichment was
purchased from “Icon” and was additionally enriched by maintaining
with excess of 18O2 (Stanford Isotope Laboratory, 95% enrichment) in
a glass bulb. According to the mass spectra, the N18O2 used in the
experiments was about 75% enriched by 18O isotope.
Results and Discussion
Interaction of Small NO2 Increments with Fe(Por). In a
recent communication,22 we described the cryostat reaction of
small increments of NO2 gas with sublimed layers of Fe(TTP)
at room temperature. These results will be briefly summarized
here. Introduction of NO2 led to the appearance of three new
IR bands at 1528, 901, and 751 cm-1 and immediate change of
the film color from red to brown. These bands displayed their
isotope counterparts at 1495, 878, and 747 cm-1 when 15NO2
was used and were assigned to ν(NdO), ν(N-O), and δ(ÃΝÃ)
of O-coordinated nitrito ligand in the 5-coordinate iron porphyrin
complex Fe(TTP)(η1-ONO) (Supporting information Figure S1).
For the analogous NO2 reaction with Fe(TPP), small deviations
from these values were seen for the first two bands (Table 1),
but the third was masked by an intense TPP band and could be
registered only in the difference spectra. Hence the reaction of
NO2 with Fe(II) porphyrins leads to formation of the O-bound
nitrito complex Fe(Por)(η1-ONO) (1) as represented in eq 1 and
not the N-bound nitro adduct Fe(Por)(η1-NO2).
The nitrito complexes were fairly stable at ambient temper-
ature but under vacuum decomposed within a few days to give
(principally) the nitrosyl analogs FeII(Por)(NO). Heating to 340
K under vacuum shortens the time required for conversion of 1
to Fe(TTP)(NO) to several hours. Some µ-oxo-dimer [Fe-
(TTP)]2O (manifested by characteristic IR bands at 875 and 893
cm-1 27b
and very minor quantities of the nitrato complex 2
)
were formed during this procedure.
FTIR spectra were measured by “Nexus” and UV-visible spectra
by “Helios γ” spectrophotometers of Thermo Nicolet Corp.
DFT calculations of the Fe(P)(ONO) and the Fe(P)(NO2) (P ) the
porphinato dianion) were performed with the Spartan’04 program
DFT calculations for the model complexes Fe(P)(ONO) and
Fe(P)(NO2) (P2- ) porphinato dianion) at the B3LYP level
using the LACVP* basis set using no symmetry constraints gave
the nitrito isomer as the lowest energy structure but with the
(26) (a) Byrn, M. P.; Curtis, C. J.; Hsiou, Y.; Khan, S. I.; Sawin, P. A.; Tendick,
S. K.; Terzis, A.; Strouse, C. E. J. Amer. Chem. Soc. 1993, 115, 9480-
9487. (b) Kurtikyan, T. S.; Gasparyan, A. V.; Martirosyan, G. G.;
Zhamkochyan, G. H. J. Appl. Spectrosc. 1995, 62, 62-66 (in Russian).
(27) The IR spin-sensitive band27b in the range of 1350 cm-1 is a good indicator
for monitoring the reaction. The band at 1346 cm-1 of intermediate-spin
Fe(TPP)27c shifts to 1340 cm-1 upon formation of 1 and is absent when
the reaction proceeds completely. On the other hand the νa(NO2) band of
nitrato complex at 1271 cm-1 serves as criteria that the nitrato complex is
not yet formed. (b) Oshio, H.; Ama, T.; Watanabe, T.; Kincaid, J.;
Nakamoto, K. Spectrochim. Acta 1984, 40A, 863-870. (c) Collman, J. P.;
Hoard, J. L.; Kim, N.; Lang, G.; Reed, C. A. J. Am. Chem. Soc. 1975, 97,
2676-2681.
4
intermediate quartet spin state [Fe(P)(ONO)]. The calculated
energy difference between this and the low-spin nitro isomer
2[Fe(P)(NO2)] is insignificant (<1 kJ mol-1) while the high-
spin nitrito complex 6[Fe(P)(ONO)] is somewhat higher energy
4
(6.3 kJ mol-1). The suggestion that [FeIII(P)(ONO)] is the
lowest energy species was unexpected given an indication on
the basis of spin-sensitive ring vibrations (see below) that 1 is
high-spin, i.e., a sextet. While provocative to think that
intermediate-spin or mixed S ) 3/2, 5/2 ferric porphyrinato
complexes may be playing a role,29 Ghosh30 has reported that
DFT computation of the analogous chloro complex Fe(P)Cl gave
(28) This effect is more noticeable at low pressures of NO2. Freezing the purified
NO2 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 NO, this
solid takes on the bluish color from the trace N2O3.
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3578 J. AM. CHEM. SOC. VOL. 129, NO. 12, 2007

Five-Coordinate Intermediate Fe(Por)(ONO)
A R T I C L E S
Figure 1. Computed structures using DFT at the B3LYP level using the
LACVP* basis set of the lowest energy nitro and nitrito isomers (2[Fe(P)-
(NO2)] and 4[Fe(P)(ONO)], respectively) of the NO2 adduct to a iron(II)
porphinato complex Fe(P).
a quartet but ab initio computation gave a sextet lowest energy
state. Thus, there is computational ambiguity30 whether 1 is a
quartet, a sextet, or a mixed state.
Optimized structures for 2[FeIII(P)(NO2)] and 4[Fe(P)(ONO)]
are shown in Figure 1, and calculations for all spin states are
summarized in the Supporting Information. The optimized nitro
isomer has a distorted square-pyramidal geometry that correlates
with the rhombic EPR spectrum recorded for a transient picket
fence porphyrinato complex proposed to be Fe(TPivPP)(NO2).31a
Figure 2. FTIR spectral changes for Fe(TTP)(ONO) in sublimed layers
as the result of stepwise addition of new NO2 increments (PNO ≈ 0.5 Torr,
2
2-3 min exposure at 293 K) and evacuation after each introduction.
2
The porphine plane of [FeIII(P)(NO2)] is slightly ruffled with
a 7.2° angle between the planes defined by the oppositely located
sets of two pyrrole nitrogens and the meso-C atom between
them, and the Fe atom is 0.20 Å out of the plane defined by
the porphyrin nitrogens. Similar results were obtained for
another DFT/B3LYP computation32 of Fe(P)(NO2); however,
in that work only nitro isomers were considered and the
calculations were done under C2V symmetry constraint. The
4
optimized structure of [Fe(P)(ONO)] shows a less distorted
porphine (the angles between planes defined above are 0.7 and
1.5°), with the Fe out of the porphine plane by 0.26 Å.
These computations suggest that the nitrito and nitro linkage
isomers Fe(Por)(ONO) and Fe(Por)(NO2) should have similar
energies, although our experimental studies show only the
former as the initial product of NO2 reaction with Fe(TPP) or
Fe(TTP) in layered solids. One rationalization would be that
the more distorted nitrito complex 2[FeIII(P)(NO2)] may be
destabilized by the meso substituents of TPP and TTP or by
interactions in the polarizable condensed phase.
For the picket fence porphyrinato complexes,18 structural31b
and computational32 analyses of the 6-coordinate complex Fe-
(TPivPP)(NO2)(py) suggest weak hydrogen bonding between
O-atoms of nitro group and picket fence NH groups. Such
H-bonding may be expected to be present for 5-coordinate nitro-
complex Fe(TpivPP)(NO2) as well and to favor N-coordinated
isomer in that case.
Further Reaction of Fe(TTP)(ONO) with NO2. FTIR
spectral changes observed upon addition of new NO2 increments
to layers containing 1 are represented in the Figure 2. It is seen
that this procedure was accompanied by gradual disappearance
of the bands at 901 and 751 cm-1 while new bands at 1271
and 966 cm-1 grew in intensity. Additionally, the band in the
vicinity of 1530 cm-1 gained very significantly in intensity. In
experiments with the labeled 15NO2, these bands appeared at
1251, 957, and 1495 cm-1, respectively shifted to lower
Figure 3. FTIR spectra of thin layers of Fe(TTP) (solid line), Fe(TTP)-
(η2-O2NO) (dashed line), and Fe(TTP)(η2-O215NO) (dotted line).
frequencies (Figure 3), confirming that they are characteristic
of the coordinated nitrate. The X-ray crystal structure of Fe-
(TPP)(O2NO) has been determined,33 and this compound shows
an asymmetrically bidentate nitrato ligand with the high-spin
Fe(III) displaced 0.6 Å from the porphyrin plane. Such a η2-
nitrato ligand would be expected to show three IR-active
stretching modes, a ν(NdO) stretch for the uncoordinated
oxygen plus symmetric and asymmetric modes νs(NO2) and νa-
(NO2) for the coordinated NO2 fragment.34 The two higher
frequency bands (ν(NdO) and νa(NO2)) had been identified
previously;35a the present work also identifies the lower
frequency νs(NO2). These data are summarized in Table 1. The
recently synthesized ruthenium complex Ru(TTP)(NO)(NO3)
displays similar bands at 1269, 950, and 1515 cm-1 for the
nitrate ligand.36
(33) Phillippi, M. A.; Baenziger, N.; Goff, H. M. Inorg. Chem. 1981, 20, 3904
-3911.
(34) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination
Compounds, 3rd ed.; Wiley: New York, 1978; pp 244-247.
(35) (a) Kurtikyan, T. S.; Stepanyan, T. G.; Akopyan, M. E. Russ. J. Coord.
Chem. 1999, 25, 721-725. (b) Kurtikyan, T. S.; Stepanyan, T. H.;
Martirosyan, G. G.; Kazaryan, R. K.; Madakyan, V. N. Russ. J. Coord.
Chem. 2000, 26, 345-348. (c) Kurtikyan, T. S.; Stepanyan, T. G.;
Gasparyan, A. V.; Zhamkochyan, G. H. Russ. Chem. Bull. 1998, 47, 695-
698.
(29) Weiss, R.; Gold, A.; Terner, J. Chem. ReV. 2006, 106, 2550-2579 and
references therein.
(30) Ghosh, A. J. Biol. Inorg. Chem. 2006, 11, 712-724 and references therein.
(31) (a) Munro, O. Q.; Scheidt, W. R. Inorg. Chem. 1998, 37, 2308-2316. (b)
Cheng, L.; Powell, D. R.; Khan, M. A.; Richter-Addo, G. B. Chem.
Commun. 2000, 2301-2302.
(36) Kang, Y.; Zyryanov, G. V.; Rudkevich, D. M. Chem. Commun. 2003,
2470-2471.
(32) Conradie, J.; Ghosh, A. Inorg. Chem. 2006, 45, 4902-4909.
9
J. AM. CHEM. SOC. VOL. 129, NO. 12, 2007 3579

A R T I C L E S
Kurtikyan et al.
Thus, as demonstrated earlier,35a the final product of the NO2
reaction with Fe(TPP)(ONO) is the η2-nitrato species Fe(TPP)-
(η2-O2NO) (eq 2). While similar reactivity is seen with
manganese(II) analogs Mn(TPP),35b this behavior contrasts with
cobalt(II) complex Co(TPP), which gave only the nitro complex
Co(TPP)(NO2) under analogous experimental conditions.35c
Fe(TPP)(ONO) + NO2 f Fe(TPP)(η2-O2NO) + NO
(2)
Comparison of the IR spectra of Fe(TPP)(η1-ONO) and Fe-
(TPP)(η2-Ã2NO) (Table 1) shows that the ν(NdO) bands occur
at nearly the same frequency for the two complexes, although
the band for the latter is more intense. A similar coincidence of
the high-frequency stretching modes is seen for the ruthenium
nitrito and nitrato complexes Ru(TPP)(NO)(ONO)37 and Ru-
(TTP)(NO)(NO3),36 for which the values of 1515 and 1516 cm-1
were respectively reported.
Certain porphyrin vibrational modes of Fe(TPP) complexes,
for example, bands at ∼1350 cm-1 (ν(Ca-Cm) mixed with ν-
(Cm-phenyl)) and at ∼450 cm-1 (δ(Pyr rotation))38 are sensitive
to the spin and oxidation state of the metal center.27b For the
intermediate-spin state Fe(TPP),27c these bands lie at 1346 and
464 cm-1. Upon coordination with NO2, they shift to 1341 and
434 cm-1 consistent with a change to high-spin for the new
complex (Table 1). For Fe(TTP), the analogous bands appear
initially at 1346 and 455 cm-1 and correspondingly shift upon
both nitrito and nitrato complex formation to 1340 and 428
cm-1. In this case, there is an additional band disposed at 1505
cm-1 that undergoes a shift to 1495 cm-1 upon the formation
of 1 and 2 that may also be indicative for an intermediate-spin
f high-spin transition. Complete band assignments for the TTP
derivatives on the basis of normal coordinate calculations have
not been made, although it is reasonable to suggest that this
band should belong to a complex mode consisting of ν(Câ-
Câ) + νa(Ca-Cm) + νs(Ca + Cm) that also appears in the IR
spectrum of Fe(TPP)Cl.38
Figure 4. Optical spectra of Fe(TTP) in a sublimed layer on a CaF2 plate
(solid line) and of the products formed after sequential addition of NO2
increments. The spectra were recorded after the FTIR spectra of the same
sample demonstrated first the formation of nitrito Fe(TTP)(ONO) (dashed
line) and then of nitrato Fe(TTP)(η2-O2NO) complexes (dotted line).
Scheme 1. Prospective Mechanism for Formation of 2 by the Oxo
Transfer from Free NO2 to 1
Both products displayed spectra characteristic of conversion
from the initial ferrous state to the ferric state, but the optical
spectra of 1 and 2 are quite similar (Table 1).
It is clear from the FTIR data that the reaction of NO2 with
ferrous porphyrinato complexes proceeds via two distinct stages.
During the first stage, seen for short reaction times with a very
DFT calculations (B3LYP/LACVP*) for the porphinato
complex Fe(P)(NO3) gave the quartet state as the lowest energy
low PNO , simple NO2 coordination to give the 5-coordinate
2
species, but the sextet was very close (∆E ) +0.3 kJ mol-1
)
nitrito complex 1 occurs. Subsequent reaction at higher PNO
2
while the doublet was much higher energy (∆E ) +27.2 kJ
mol-1). The optimized quartet structure was effectively mono-
dentate (Fe-O values of 1.97, 3.15, and 4.03 Å) while the sextet
was symmetrically bidentate (Fe-O values of 2.17, 2.17, and
3.80 Å) (Supporting Information Table S3). The latter is
consistent with the published structure of Fe(TPP)(η2-O2NO),
which is bidentate although unsymmetrical with bond lengths
of 2.019(4) and 2.323(8) Å.33 Nonetheless, the similar energies
of the monodentate and bidentate bonding modes is notable in
the context that the both have been reported for crystal structures
of Fe(Por)(NO3) complexes, depending on the nature of the
porphyrin ligand.18
Figure 4 illustrates changes in the UV-visible spectra upon
reaction of Fe(TTP) in sublimed layers with NO2 increments at
293 K to give first 1 and then 2. The optical spectra were
recorded after it was shown from the FTIR spectral changes
that the specified transformation had occurred in that sample.
leads to the slower formation of the nitrato species 2, presumably
with the concomitant formation of NO as illustrated in eq 2.
The prospective formation of NO draws support from the
observation that when, at the conclusion of this procedure, the
gaseous contents were frozen onto the cryostat wall with LN2,
a bluish solid was observed. This is a good qualitative indication
of the presence of N2O3 formed by reaction of NO with excess
NO2. Although NO is known to react with 2 to give the adduct
Fe(Por)(NO)(NO3), the latter is relatively unstable39 and is not
formed in spectrally registered quantities under the conditions
of this experiment.
One can envision two different scenarios for the second stage
of this transformation. One would be oxygen atom transfer from
the incoming NO2 to the N of the nitrito ligand (Scheme 1).
An alternative pathway would involve bond formation between
the N of the incoming NO2 and the coordinated O of the nitrito
ligand followed by NO release from that ligand (Scheme 2).
Both would give the monodentate nitrato complex as an
(37) Kurtikyan, T. S.; Martirosyan, G. G.; Lorkovic, I. M.; Ford, P. C. J. Am.
Chem. Soc. 2002, 124, 10124-10129.
(38) Paulat, F.; Praneeth, V. K. K.; Nather, C.; Lenhert, N. Inorg. Chem. 2006,
45, 2835-2856.
(39) Kurtikyan, T. S.; Martirosyan, G. G.; Hakobyan, M. E., Ford, P. C. Chem.
Commun. 2003, 1706-1707.
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3580 J. AM. CHEM. SOC. VOL. 129, NO. 12, 2007

Five-Coordinate Intermediate Fe(Por)(ONO)
A R T I C L E S
Scheme 2. Prospective Mechanism for Formation of 2 by the NO2
Attack at the Coordinated O Atom of Fe(Por)(ONO)
Scheme 3. Possible Mechanism for Exchange of
Fe(Por)(18ON18O) with Added NO2
intermediate. In principle, these two mechanisms could be
differentiated by isotope labeling experiments. For the first, the
reaction of unlabeled 1 with 15NO2 would give unlabeled 2.
For the second, the nitrogen of the nitrate ligand found in the
product would have originated with the gaseous 15NO2 giving
the labeled product Fe(Por)(η2-O215NO).
Figure 5. FTIR spectra of Fe(TTP)(η1-ONO) (lowest curve) after sequential
exposure under increments of 15NO2 gas (0.3 Torr, 3 min at 293 K) and
evacuation after each introduction. The bands at 1271 and 1251 cm-1 belong
to νa(NO2) and νa(15NO2) of Fe(TTP)(η2-O2NO) and Fe(TTP) (η2-O2
NO) correspondingly.
-
15
Table S4), although the triplet dinitrito analog 3[Fe(P)(ONO)2]
is less than 5 kJ mol-1 higher energy and several mixed nitrito
nitro spin states are within 10 kJ mol-1 of the lowest energy
structure. If one takes into account the calculated energies of
However, such analysis was complicated by the discovery
that concomitant to the second stage of the reaction exchange
between the added and coordinated nitrogen dioxide also occurs.
For example, when a sample of Fe(TTP)(18ON18O) prepared
from Fe(TTP) and N18O2 was exposed to a small increment of
unlabeled NO2, there was a shift in the δ(ONO) consistent with
such exchange (Supporting Information Figure S2). This process
can be envisioned to occur by reaction of the free NO2 at the
open coordination site of 1 to give a transient dinitrito
intermediate Fe(Por)(ONO)2 or one of its isomers (Scheme 3).
Such a pathway is analogous to the exchange of free and
coordinated NO demonstrated for Fe(Por)(NO) complexes using
isotopic labeling methods.40 For example, Fe(TPPyP) (TPPyP
) pyridyltriphenylporphyrinato dianion) in sublimed layers
reacts with NO to form the 5-coordinate Fe(TPPyP)(NO) and
the 6-coordinate trans-Fe(TPPyP)(L)(NO). In this case L is a
pyridyl from an adjacent Fe(TPPyP).41 Notably, isotopic
exchange with labeled NO occurred only with Fe(TPPyP)(NO),
and these data were interpreted in terms of reaction of the latter
with NO to give a trans-dinitrosyl intermediate. Analogous
trans-Fe(Por)(NO)2 species have been observed in low-temper-
ature solutions and characterized spectroscopically and theoreti-
cally.42
3
4[Fe(P)(ONO)], NO2 (-205.072 18 au), and [Fe(P)(NO2)2],
formation of the dinitro complex is estimated to be favorable
(-17 kJ mol-1), although the negative ∆S of this process would
make the equilibrium constant small. Nonetheless, these data
suggest that the reaction pathway shown in Scheme 3 is an
energetically reasonable mechanism for the NO2 exchange
processes.
Another point to take into account when examining isotope
labeling experiments to test possible mechanisms for the NO2
oxidation of 1 is that free NO once formed will undergo rapid
isotopic exchange with the free NO2 present.43
Figure 5 illustrates the FTIR spectral changes at frequencies
corresponding to the νa(NO2) band of the coordinated nitrate
when small increments of 15NO2 were added to unlabeled Fe-
(TTP)(η1-ONO) and allowed to react for a short period. After
each 15NO2 addition, the system was evacuated to remove any
nitric oxide and other gases. This spectral range is free from
any bands of the nitrito complex and, thus, is convenient for
such analysis. Interaction of small amounts of 15NO2 with Fe-
(Por)(ONO) leads principally to formation of Fe(Por)(O215NO);
however, all spectra show that some Fe(Por)(O2NO) is also
formed during this process. On first impression, these results
would appear to favor Scheme 2, since that mechanism predicts
that the nitrato nitrogen would originate with the free nitrogen
dioxide. The formation of some unlabeled product might be
explained by the simultaneous operation of both mechanisms.
However, the aforementioned isotope exchange between free
The intermediate species that would be formed by the reaction
of NO2 with the model compound 4[Fe(P)(ONO)] (in a gas phase
analog to Scheme 3 for the porphinato analogs) was examined
by DFT calculations. The lowest energy linkage isomer of this
(nominally) Fe(IV) complex was found to be the triplet spin
3
state dinitro complex [Fe(P)(NO2)2] (Supporting Information
(40) O’Shea, S. K.; Wang, W.; Wade, R. S.; Castro, C. E. J. Org. Chem. 1996,
61, 6388-6395.
(41) Kurtikyan, T. S.; Ogden, J. S.; Kazaryan, R. K.; Madakyan, V. N. J.
Porphyrins Phthalocyanines 2003, 7, 623-629.
(43) Such isotope-scrambling takes place in the amorphous layer of Ni(TPP)
even at very low temperatures9 and should proceed readily under the
conditions of the present experiments; it is presumably facilitated by
formation of dinitrogen trioxide species isomers.43b (b) Fateley, W. G.; Bent,
H. A.; Crawford, B. J. Chem. Phys. 1959, 31, 204-217.
(42) (a) Lorkovic, I. M.; Ford, P. C. J. Am. Chem. Soc. 2000, 122, 6516-6517.
(b) Patterson, J. C.; Lorkovic, I. M.; Ford, P. C. Inorg. Chem. 2003, 42,
4902-4908. (c) See also the following: Conradie, J.; Wondimagegn, T.;
Ghosh, A. J. Am. Chem. Soc. 2003, 125, 4968-4969.
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A R T I C L E S
Kurtikyan et al.
Table 2. IR and UV-Visible Data for 6-Coordinate Nitrito-Nitrosyl and Nitro-Nitrosyl Complexes of Iron(III) Porphyrins in Amorphous Thin
Layers
1 a
IR freq, cm-
compds
ax ligand stretching bands
spin-sensitive bands
UV−vis, λmax, nm
Fe(TPP)(ONO)(NO)
Fe(TTP)(ONO)(NO)
1887 (1850) s, ν(NtO); 1496 (1471) m, ν(NdO); 938 (920) m, ν(N-O)
1886 (1850) s, ν(NtO); 1497 (1464) m, ν(NdO);
1351 m, 462 w
1507 m, 1350 m, 452 w
548, 584
546, 580, 618 sh
935 (917) m, ν(N-O), 594 (585) vw, ν{Fe-N(NtO)}
1863 (1824) s, ν(NtO); 1455 (1425) m, νa(NO2); 1295 (1274) vs, νs(NO2)
1860 (1824) s, ν(NtO); 1456 (1424) m, νa(NO2); 1293 (1274) vs, νs(NO2);
553 (545) vw, ν{Fe-N(NtO)}
Fe(TPP)(NO2)(NO)
Fe(TTP)(NO2)(NO)
1350 m, 461 w
1507 m, 1349 m, 451 w
547, 582
547, 579, 617 sh
a In parentheses data for 15N-labeled compounds are given.
Fe(Por)(ONO) + NO h Fe(Por)(ONO)(NO)
(3)
nitrogen dioxide and 1 is significant when but minor quantities
of 2 are yet formed, so it appears that this exchange is faster
than the reaction leading to formation of 2 and there is
considerable scrambling of the isotopic labeling. Nonetheless,
formation of unlabeled 2 at very early stages of the reaction
suggests that the Scheme 1 is clearly in operation, at least in
competition with Scheme 2.
Upon further warming of the layered Fe(TPP)(ONO)(NO)
sample from 130 to 220 K, the FTIR spectral changes displayed
in Figure 6 are observed. New bands at 1860, ∼1452, and 1294
cm-1 appear and grow at the expense of Fe(TTP)(ONO)(NO)
bands at 1886, 1496, and 934 cm-1. The former bands have
their 15N-isotope counterparts at 1825, 1426, and 1274 cm-1
when layered Fe(TPP)(O15NO)(15NO) is warmed (Supporting
information Figure S5). The resulting spectrum is that of the
nitro-nitrosyl complex Fe(TTP)(NO2)(NO), a type of structure
reported by Yoshimura and characterized spectroscopically and
structurally for several iron porphyrins.18,44 Hence warming Fe-
(Por)(ONO)(NO) results in the nitrito f nitro isomerization of
coordinated NO2 (eq 4). This process proceeds both in the
presence of excess NO as well as when NO was evacuated at
130 K before warming the Fe(Por)(ONO)(NO) layer. The spin-
sensitive bands remain unchanged during this process consistent
with a low-spin nature of both 6-coordinate complexes.
Rudkevich et al.36 reported that reaction of Ru(TTP)(CO) with
2 equiv of NO2 in CH2Cl2 gives the nitrato-nitrosyl complex
Ru(TTP)(NO3)(NO). As a possible mechanism they proposed
N2O4 disproportionation to nitrosonium nitrate NO+NO3
-
-
followed by NO+ displacement of the carbonyl and then NO3
coordination at the trans axial site. Such a pathway is unlikely
in the present case owing to the low PNO and the absence of a
2
solvent to support ionic intermediates. On the other hand, a
mechanism such as described here for Fe(Por) can be envisioned
for the ruthenium analogs. For example, NO2 coordination to
Ru(TTP)(CO) would give Ru(TTP)(CO)(NO2), and the resulting
Ru center should be quite labile to CO dissociation to give Ru-
(TTP)(NO2) (linkage isomer unspecified). If the latter is present
as the nitrito isomer, then reaction with a second NO2 would
give Ru(TTP)(NO3) plus NO, which could combine to give Ru-
(TTP)(NO)(NO3).
Fe(TPP)(ONO)(NO) f Fe(TTP)(NO2)(NO)
(4)
This linkage isomerization was also probed using optical
spectroscopy; however, the difference between the UV-visible
spectrum of the nitrito-nitrosyl complex and that of the nitro-
nitrosyl analog is too small (Figure 7) to be used to differentiate
between these two forms (Table 2). This may explain why the
oft-postulated nitrito-nitrosyl isomer had not been characterized
previously by optical spectroscopy.9,45
It is worth noting that interaction of NO with sublimed layers
of 1 at 170 K leads to the formation of 3 and 4 in comparable
quantities.
The isomerization of Fe(TPP)(ONO)(NO) back to Fe(TPP)-
(NO2)(NO) has been described by Lee et al.,21 who examined
the photolysis of the latter in solid KBr matrices. One photo-
product is Fe(Por)(ONO)(NO), and this undergoes a slow return
to the initial nitro complex. DFT computations by those workers
(for the porphinato analogs) demonstrated the lowest energy
nitrito nitrosyl complexes to be ∼20 kJ mol-1 higher energy
than the six-coordinate nitro-nitrosyl complexes. Similar studies
of the optimized structures of these isomers in one of our
laboratories using the 6-31g basis set led to the same conclu-
sion.46
Reaction of NO with 1. Introduction of NO into the cryostat
containing sublimed layers of Fe(Por)(ONO) at room temper-
ature leads to rapid formation of the known nitro-nitrosyl
complexes Fe(Por)(NO2)(NO).44 However, as we reported in
the preliminary communication,22 another pattern is observed
when this was done at LN2 temperature. At 80 K there was no
apparent reaction but slow warming of the layer to 140 K led
to the appearance of new FTIR bands at 1886, 1496, and 934
cm-1 at the expense of the Fe(TTP)(ONO) bands at 1528, 901,
and 751 cm-1 (Supporting Information Figure S3). The former
bands have their isotope counterparts at 1850, ∼1464, and 917
cm-1 when 15NO interacts with layers containing Fe(TTP)(O15-
NO) (Figure S4). Concomitantly, the color of the layer changes
from brown to red-purple. These observations and spectral
comparison to the metastable species obtained upon photolysis
of a low-temperature KBr pellet containing Fe(TPP)(NO)-
(NO2)21 led to the conclusion that the low-temperature addition
of NO to Fe(Por)(ONO) gives the nitrito-nitrosyl complex Fe-
(Por)(ONO)(NO) (eq 3). Infrared frequencies of nitrito-nitrosyl
complexes are in good agreement with those calculated and
reported in literature21b with frequencies of coordinated nitrito
group at about 1500 cm-1 [ν(NdO)] and 930 cm-1 [ν(N-O)].
Mixed Isotope Experiments: Mechanism of Nitrito-Nitro
Isomerization. Potential mechanisms for this linkage isomer-
ization are represented by Scheme 4a,b. The former would
(45) Lim, M. D.; Lorkovic, I. M.; Wedeking, K.; Zanella, A. W.; Works, C. F.;
Massick, S. M.; Ford, P. C. J. Am. Chem. Soc. 2002, 124, 9737-9743.
(46) Patterson, J. C. Ph.D. Dissertation, University of California, Santa Barbara,
CA, 2004.
(44) Yoshimura, T. Inorg. Chim. Acta 1984, 83, 17-21. (b) Ellison, M. K.;
Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 1999, 38, 100-108.
9
3582 J. AM. CHEM. SOC. VOL. 129, NO. 12, 2007

Five-Coordinate Intermediate Fe(Por)(ONO)
A R T I C L E S
Figure 6. FTIR spectral changes demonstrating nitrito-nitro isomerization
upon warming of thin layers containing Fe(TTP)(ONO)(NO) from 140 to
220 K.
Figure 8. (a) FTIR spectra demonstrating isomerization of 3 to 4 upon
warming the layer containing Fe(TTP)(ONO)(15NO) under 2.5 Torr 15NO
from 160 to 200 K. (b) After further warming to 260 K.
Figure 7. UV-visible spectra of Fe(TTP)(ONO) (solid line) after
low-temperature interaction (130 K) with NO and formation of nitrosyl-
nitrito complex (NO)Fe(TTP)(ONO) (dashed line) and its isomerization
to nitrosyl-nitro complex upon warming (NO)Fe(TTP)(NO2) (dotted
line).
15NO2) via the formation of N2O3.9 In that case, the linkage
isomerization of the selectively labeled compound Fe(Por)-
(ONO)(15NO) would give largely the doubly labeled product
Fe(Por)(15NO2)(15NO). In contrast, the intramolecular mecha-
nism (Scheme 4b) would give primarily the half-labeled product
Fe(Por)(NO2)(15NO).
Scheme 4. Possible Mechanisms of Nitrito-Nitro Isomerization
upon Transition from 3 to 4
The methodology described here can be used to prepare
nitrito-nitrosyl complexes with specifically labeled nitrogen
oxides. For example, Fe(Por)(ONO)(15NO) was obtained by the
low-temperature reaction of 15NO with Fe(Por)(ONO) layers,
while Fe(Por)(O15NO)(NO) was obtained by interaction of NO
with Fe(Por)(O15NO) (Figures S6a and S6b, respectively). The
frequencies of the individual bands characteristic of the stretch-
ing modes for the coordinated nitrogen oxides in these mixed
labeled species are the same as those found for the respective
vibrational modes in the corresponding isotopically uniform
samples.47 This observation indicates that kinematic coupling
between the axial ligands is weak.
Figure 8a displays FTIR spectral changes seen when the
layered material containing Fe(TTP)(ONO)(15NO) in the pres-
ence of excess 15NO (pressure) was warmed to temperatures
where nitrito f nitro isomerization starts to occur. The
frequency range shown is that where the intense νs(NO2) stretch
involve NO2 dissociation from 3 to give the nitrosyl complex
Fe(Por)(NO) plus free NO2 followed by intermolecular reaction
at the NO2 nitrogen atom to form 4. An alternative would be
an intramolecular concerted migration (“flipping”) of nitrogen
dioxide from the O- to the N-coordinated isomer (Scheme 4b).
It should be possible to differentiate such pathways by isotopic
labeling experiments. In the presence of excess 15NO, any NO2
released in a dissociative mechanism (Scheme 4a) would
undergo rapid isotopic exchange (e.g., 15NO + NO2 h NO +
(47) Stretching modes of nitrogen oxide groups in Fe(TTP)(ONO)(15NO) are
disposed at 1851, 1496, and 934 cm-1 while those of Fe(TTP)(O15NO)-
(NO) are at 1888, 1469, and 920 cm-1. Compare with the data of Table 2
for isotopically uniform species. The same pattern was observed for the
corresponding nitro-nitrosyl complexes.
9
J. AM. CHEM. SOC. VOL. 129, NO. 12, 2007 3583

A R T I C L E S
Kurtikyan et al.
Scheme 5. Decomposition of 4 upon Warming from 220 to 280 K
under Intense High-Vacuum Pumping
toward dissociation both of NO and of NO2, competitively,
under these conditions (Scheme 5). Loss of NO would give the
5-coordinate nitro complex Fe(TTP)(NO2), which isomerizes
to more stable nitrito form 1, while loss of NO2 would give the
stable nitrosyl complex Fe(TTP)(NO). Notably, Lim et al.45
showed in solution experiments that Fe(TPP)(NO)(NO2) dis-
sociates NO to form Fe(TPP)(NO2) and proposed that this is
followed by linkage isomerization to Fe(TPP)(ONO).
Figure 9. FTIR spectral changes demonstrating decomposition of nitro-
nitrosyl complex Fe(TPP)(NO2)(NO) upon warming its thin layers under
intense evacuation: 220 K (solid line); 250 K (dashed line); 280 K (dotted
line).
of the coordinated nitro group of 4 is disposed. These data show
that the 1294 cm-1 band corresponding to the isotopic species
Fe(TTP)(NO2)(15NO) grows faster and eventually is much
stronger than the lower frequency band at 1274 cm-1. The latter
corresponds to the νs(15NO2) of Fe(TTP)(15NO2)(15NO) but may
also be intensified by overlap with a similar vibration of 15N2O3,
a small amount of which may be formed as evidenced by the
appearance of a weak band at 1560 cm-1 belonging to the νa(15-
NO2) of this species.48 These data clearly show isomerization
of Fe(TTP)(ONO)(15NO) results mostly in formation of Fe-
(TTP)(NO2)(15NO), and we interpret this result as indicating
that the intramolecular isomerization (“flipping”) pathway is
the predominant one operating under these conditions. However,
the observation of a lesser amount of Fe(TTP)(15NO2)(15NO)
as well as of some 15N2O3 implies that NO2 dissociation is
also occurring during the course this study, although this may
reflect, at least partially, secondary reactions of the product (see
below).
In a different experiment, excess NO was removed from the
layered 4 at 220 K, and then the material was allowed to warm
to room temperature without further pumping. Under these
conditions, the main products were Fe(Por)NO and Fe(Por)-
(NO3). This result now can be easily understood taking into
account the oxidation of nitrito ligand by free NO2 (eq 2).
Although this reaction is relatively slow, failure to actively
remove the dissociated NO2 results in subsequent oxidation of
1 to 2 (eq 2). Here it is worth reexamining an earlier
experiment,45 in which concentrated solution of Fe(TPP)(NO2)-
(NO) was rapidly evacuated so that the solvent and all volatile
NOx species were removed. The solid residue was redissolved
in benzene where it displayed in NMR a resonance of 76.55
ppm, which was suggested to be the â-pyrrole of Fe(TPP)(NO2).
Over the course of 1 h, this peak disappeared and gave way to
a new one at 78.46 ppm thought to be Fe(TPP)(O2NO). It now
appears likely that the initial signal observed was that of Fe-
(TPP)(ONO).
When the sample primarily consisting largely of Fe(TTP)-
(NO2)(15NO) (the final curve in Figure 8a) was further heated
to 260 K under excess 15NO, the spectral changes (Figure 8b)
show that Fe(TTP)(15NO)(NO2) decreases while the fully labeled
Fe(TTP)(15NO)(15NO2) increases. Therefore, the nitro nitrosyl
complex 4 is labile to NO2 exchange, and such lability may be
responsible for at least some of the exchange seen in Figure
8a.
Decomposition of Fe(TTP)(NO2)(NO) at Higher T. When
layered 4 was warmed from 220 to 280 K under high vacuum
with active pumping, the FTIR spectral changes demonstrated
in Figure 9 were observed. Bands characteristic of 4 at 1860,
∼1452, and 1295 cm-1 completely disappeared and bands at
1680, 1528, 901, and 751 cm-1 gained in intensity. The band
at 1680 cm-1 certainly belongs to the nitrosyl complex Fe(TTP)-
(NO), while three others are assigned to stretching modes of
nitrito ligand in Fe(TTP)(ONO) (see above). The nitrato
complex manifested by a band at 1271 cm-1 is formed in
negligibly small quantities. Thus, the nitro-nitrosyl 4 is labile
Why Does Fe(Por)(NO2)(NO) Need Excess NO for Stabil-
ity? On the basis of present and previous studies, one may
postulate an answer for this question. Since NO and NO2
competitively dissociate from 4 to give Fe(Por)(ONO) and Fe-
(Por)(NO), respectively, the NO2 released reacts with the nitrito
complex 1 to give the nitrato complex 2. In the absence of excess
NO, the accumulation of 1 and NO2 favors this reaction.
However, the presence of excess NO adds new parameters. One
is the role of N2O3, which is formed by the reaction of NO
with NO2 under such conditions and is known from solution
studies to oxidize Fe(Por)(NO) back to 4.45 In addition excess
NO reacts with Fe(Por)(NO3) via a sequence of steps described
earlier9 and illustrated in Scheme 6 to re-form 4. Thus, under
such conditions, the nitro-nitrosyl complex 4 appears to be the
thermodynamic sink.
Summary. The interaction of NO2 gas with the Fe(II)
porphyrinato complex Fe(Por) proceeds via two distinct stages.
The first is the formation of the 5-coordinate Fe(III) nitrito
intermediate Fe(Por)(η1-ONO) (1); the second is the reaction
of 1 with new NO2 increments to form, eventually, the Fe(III)
nitrato complex Fe(Por)(η2-O2NO) (2). The mechanism of the
second stage of reaction was probed by using differently labeled
(48) The frequencies of two other vibrations (nitrosyl stretching and νs(NO2))
of 4 and dinitrogen trioxide (N2O3) are close to each other and are not
diagnostic for the low-temperature experiments where both these species
are present.
9
3584 J. AM. CHEM. SOC. VOL. 129, NO. 12, 2007

Five-Coordinate Intermediate Fe(Por)(ONO)
A R T I C L E S
Scheme 6. Dynamic Processes Taking Place in the Systems Containing 4 under Excess NOa
a In parentheses, the references cited describe corresponding processes or compounds.
NO2 in the first and second stages of reaction. However isotope
exchange reactions that also occur in the layer prevented an
unambiguous conclusion regarding this mechanism. Interaction
of the nitrito intermediate 1 with NO at low T resulted in
formation of the nitrito-nitrosyl complex 3 that had been
previously detected21 by the differential FTIR spectra as the
result of photolysis of low-T KBr pellets containing the nitro-
nitrosyl complex 4. Complex 3 isomerizes to the more stable
nitro-nitrosyl form 4 upon warming. Both 3 and 4 are low-
spin in contrast to the high- or intermediate-spin parent complex
1. By using isotope labeled compounds, it was shown that the
3 to 4 isomerization proceeds predominantly via intramolecular
“flipping” of the NO2 ligand rather than by dissociation followed
by reassociation with the formation of new Fe-N(NO2) bond.
Warming of 4 under conditions of continuous high-vacuum
pumping results in formation of the low-spin nitrosyl complex
Fe(Por)(NO) and high-spin 1, demonstrating competitive dis-
sociation of NO and NO2 from the initial nitro-nitrosyl complex
4. These and earlier results also provide an explanation why
excess NO is needed to stabilize the nitro-nitrosyl complex 4.
The upshot of these studies is that systems illustrate the multiple
interconversions of the various nitrogen oxide species mediated
by the heme model compounds and presumably by heme
proteins.
Acknowledgment. Financial support from ISTC (Project No.
A-484) is gratefully acknowledged. P.C.F. also thanks the U.S.
Department of Energy and the U.S. National Science Foundation
for support.
Supporting Information Available: Complete citation of ref
14b, Figure S1, demonstrating FTIR spectra of FeTPP and
FeTPP(ONO), Figure S2, showing FTIR spectral changes
describing isotope exchange reaction Fe(TTP)(18ON18O) f Fe-
(TTP)(ONO), Figure S3, showing FTIR spectral changes in Fe-
(TTP)(ONO) sublimed layer in course of low-temperature
interaction with NO, Figure S4, demonstrating FTIR spectra of
Fe(TTP)(NO)(ONO) and Fe(TTP)(15NO)(O15NO), Figure S5,
showing FTIR spectra of Fe(TTP)(NO2)(NO) and Fe(TTP)-
(15NO2)(15NO) obtained by warming the corresponding nitrito-
nitrosyl species, Figure S6a, showing FTIR spectral changes
demonstrating formation of Fe(TTP)(ONO)(15NO) upon low-
temperature interaction of 15NO gas with Fe(TTP)(ONO), Figure
S6b, showing formation of Fe(TTP)(O15NO)(NO) upon interac-
tion of NO gas with Fe(TTP)(O15NO), and Tables S1-4,
providing additional information regarding DFT computational
results. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA067245H
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J. AM. CHEM. SOC. VOL. 129, NO. 12, 2007 3585