Angewandte
Chemie
NO2 (15NO2), obtained by oxidation of NO (15NO) with excess
not clear owing to uncertainty regarding stoichiometry (one
or two nitrites), mode of ligation (O or N bound), and
whether solvent is coordinated. However, the electronic
spectral similarities with the thin-film experiments strongly
suggest that the unstable species seen in the earlier solution
study[21] was indeed the five-coordinate nitrito complex
Fe(tpp)(h1-ONO) characterized here.
pure dioxygen, was dried over P2O5 and then was purified by
fractional distillation until a pure white solid was obtained. 15NO with
98.5% enrichment was purchased from the Institute of Isotopes,
Republic of Georgia.
Received: July 11, 2005
Revised: October 7, 2005
Continued addition of NO2 increments to the layered
nitrite complex Fe(por)(h1-ONO) at room temperature led to
the formation of the known nitrato complexes Fe(por)(h2-
O2NO). This is consistent with a previous study[9] that found
Published online: December 2, 2005
Keywords: heme · IR spectroscopy · iron · nitrogen oxides ·
.
porphyrinoids
that interaction of NO2 gas with a sublimed layer of Fe(tpp)
2
=
gave Fe(tpp)(h -O2NO). Notably, the high-frequency n(N O)
band at n˜ ꢀ 1530 cmÀ1 for this complex is disposed in the same
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=
spectral range as the n(N O) of O-coordinated nitrito ligand.
This coincidence is likely the reason that the intermediacy of
the nitrito complexes in such reactions was not detected in the
previous study.
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[8,22]
ligand has been proposed previously
as a reasonable
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interpretation of kinetic and spectral data for various trans-
formations that take place in the course of the NOx
interaction with heme models in solids and in solution. The
demonstration herein that such five-coordinate iron-porphy-
rin nitrito complexes are formed from the reaction of Fe(por)
and NO2 at room temperature confirms these suggestions.
In summary, the data reported herein indicate that the
first stage of the reaction of NO2 with the ferrous porphyrins
Fe(por) leads to the formation of the previously unknown
five-coordinate ferric nitrito complex Fe(por)(h1-ONO). The
mechanism of subsequent transformations upon the addition
of new NO2 increments, which leads to the eventual formation
of nitrato complexes, is now under detailed investigation.
Experimental Section
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The ferrous porphyrinates Fe(por) are very sensitive to oxygen and
are readily oxidized to FeIII derivatives. For this reason the more
stable hexacoordinate complexes, Fe(por)(B)2, with nitrogen bases
(por= TPP or the meso-tetra-p-tolylporphyrinato dianion; B is Py or
Pip) were used as the precursors. These were synthesized by a known
procedure.[23] The low-temperature sublimate was prepared[24] by
placing a Fe(por)(B)2 sample in a Knudsen cell and heating to about
470 K under high vacuum (P = 3 10À5 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(por) onto the KBr or CaF2 substrate occurred. In the latter case,
CaF2 plates were also used as the optical windows of the cryostat. The
metal–aryl porphyrins layers obtained in this manner by sublimation
onto a low-temperature (77 K) surface are spongelike[25a,b] and have
high microporosity.[25c] Potential reactants easily diffuse across these
layers, and adducts thus formed can be studied spectroscopically
without solvent interference.
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Sublimation was typically carried out over periods of 0.3–2.0 h to
build up layers of thickness sufficient for UV/Vis and IR spectral
studies. The Fe(por) sublimed layer was then heated to room
temperature under dynamic vacuum, and small increments of NO2
gas were introduced for a short-time period followed by a period of
pumping out. The FTIR and UV/Vis spectra were measured after
each NO2 introduction.
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Angew. Chem. Int. Ed. 2006, 45, 492 –496
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