Nitrogen Bases with Iron-Porphyrin Nitrito Complexes
was then heated to 520 K, whereupon Fe(Por) sublimed onto the
77 K surface of the KBr or CaF2 substrate (cooled by liquid
nitrogen, LN2) to give sponge-like metallo-arylporphyrinato layers
with high microporosity.14
is obtained for 5-coordinate species formed by the interaction
of small NO2 increments with thin layers of iron(II)-
porphyrinato complexes Fe(Por).5 The crystal structures for
-
NO2 complexes of several ferri-heme NiR proteins dem-
onstrate nitro coordination.1a,b,f However, the recently re-
ported structure of the 6-coordinate complex prepared by
soaking preformed horse heart metmyoglobin crystals in
aqueous NaNO2 is the oxygen-bound nitrito isomer, perhaps
stabilized by hydrogen bonding between a nitrito oxygen and
the distal His64 residue.6
Once Fe(Por) layers of thickness sufficient for UV-vis and IR
spectral studies were formed (0.3-2.0 h), they were heated to room
temperature under a dynamic vacuum. Small increments of NO2
(15NO2) gas were then introduced for ∼ 30 s, after which the
apparatus was evacuated. During this procedure, the red Fe(Por)
film turns brown, indicating the formation of Fe(Por)(ONO), and
this was confirmed by FTIR measurements.5 Measured quantities
of the nitrogen bases were then introduced into cryostat, and FTIR
or optical spectra of the layers were measured over the course of
time. Alternatively, the layered film of Fe(Por)(ONO) was cooled
by LN2, and small portions of pyridine, 1-Me-imidazole, or
ammonia were introduced into the cryostat. The film was slowly
warmed, and IR or UV-vis spectra (using CaF2 windows) were
measured at different substrate temperatures determined by a
thermocouple.
Nitrite complexes of iron(III)-porphyrins are difficult to
obtain because of the reactive nature of coordinated nitrite
and reactions between coordinated and free nitrite, leading,
inter alia, to the formation of nitrosyl complexes.7 This
instability has been addressed by using picket fence por-
phyrinato (TpivPP) complexes, in which the coordinated
nitrite would be located in the pocket formed by four
pivalamido residues.4b Such a complex with a protected
ligand binding site is the bis(nitrite) species [K(18-C-6)-
(H2O)][Fe(TpivPP)(NO2)2], which was synthesized and
structurally characterized by Scheidt and co-workers.8 This
is a precursor for the synthesis of mixed-ligand nitrite ion
complexes with N- and S- donor ligands9,10 by displacing
the nitrite group disposed on the open side of porphyrin and
leaving an N-coordinated nitro ligand in the protected site.
Structural11 and computational3d analyses of the 6-coordinate
pyridine-nitro complex Fe(TPiv)(py)(NO2) suggest weak
hydrogen bonding between oxygen atoms of the nitro group
and the picket fence NH groups.
The NO2 (15NO2) was obtained by oxidizing pure NO (15NO)15
with excess pure dioxygen, and this was further 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.
The FTIR and UV-vis spectra were respectively recorded using
Nexus and Helios γ spectrophotometers of the Thermo Nicolet
Corporation.
Results and Discussion
The iron(III)-nitrito complexes Fe(Por)(ONO) were ob-
tained by the interaction of NO2 gas with sublimed layers
of FeII(Por) (Por ) meso-tetraphenylporphyrinato dianion,
TPP, or meso-tetra-p-tolylporphyrinato dianion, TTP) as
described elsewhere.5a Reactants easily diffuse across these
layers, and this procedure allows spectroscopic studies of
the adducts without solvent interference. Exposure of the Fe-
(Por)(ONO) to low pressures (∼0.1 Torr) of Py or 1-MeIm
(1-methylimidazole) at room temperature leads to the
quick changes in the FTIR spectra that indicate reaction with
these nitrogen bases B. For B ) NH3, much higher pressures
(20 Torr) and the presence of ammonia in the gas phase
were needed to prepare the 6-coordinate complexes. The
FTIR spectra showed that the bands at 1527, 903, and 750
cm-1 characteristic of Fe(TTP)(ONO) diminished in intensity
upon the addition of B, whereas the new bands grew in
the vicinity of 1400, 1300, and 800 cm-1. This is
illustrated in Figure 1 for the reaction of Fe(TTP)(ONO) with
1-MeIm.
As it was mentioned above, the interaction of incremental
NO2 gas with iron(II)-porphyrins in sublimed solids leads
to the formation of 5-coordinate iron(III)-nitrito complexes
Fe(Por)(η1-ONO) (1).5 Can these species serve as a synthetic
entry for obtaining 6-coordinate mixed-ligand complexes
with trans nitrogen donors, and, if so, what type of coordina-
tion (nitro or nitrito) will be realized? This study has been
undertaken to address these questions.
Experimental Section
Low-temperature sublimates of the ferrous porphyrinates Fe-
(Por)12 were prepared as described previously5 by heating the
hexacoordinate Fe(Por)(B)2 complexes (B is pyridine (Py) or
piperidine)13 in a Knudsen cell at ∼470 K under a high vacuum (P
) 3 × 10-5 Torr) to eliminate the axial ligands. The Knudsen cell
(5) (a) Kurtikyan, T. S.; Ford, P. C. Angew. Chem., Int. Ed. 2006, 45,
492-496. (b) Kurtikyan, T. S.; Hovhannisyan, A. A.; Hakobyan, M.
E., Patterson, J. C.; Iretskii, A.; Ford, P. C. J. Am. Chem. Soc. 2007,
129. 3576-3585.
These bands lie in the spectral ranges where the νa(NO2),
νs(NO2), and δ(NO2) frequencies of coordinated nitro groups
(6) Copeland, D. M.; Soares, A. S.; West, A. H.; Richter-Addo, G. B. J.
Inorg. Biochem. 2006, 100, 1413-1425.
(7) Finnegan, M. G.; Lappin, A. G.; Scheidt, W. R. Inorg. Chem. 1990,
29, 181-185.
(8) Nasri, R.; Goodwin, J. A.; Scheidt, W. R. Inorg. Chem. 1990, 29,
185-191.
(13) (a) Epstein, L. M.; Straub, D. K.; Maricondi, C. Inorg. Chem. 1967,
6, 1720-1722. (b) Bhatti, W.; Bhatti, M.; Imbler, P.; Lee, A.;
Lorenzen, B. J. Pharm. Sci. 1972, 61, 307-309.
(14) (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,
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G. G. and Zhamkochyan, G. H. J. Appl. Spectrosc. 1995, 62, 62-66.
(15) (a) NO(15NO) was purified according to the procedure described in
detail in ref 15b. (b) Martirosyan, G. G.; Azizyan, A. S.; Kurtikyan,
T. S.; Ford, P. C. Inorg. Chem. 2006, 45, 4079-4087.
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(10) Nasri, H.; Haller, K. J.; Wang, Y.; Huynh, B. H.; Scheidt, W. R. Inorg.
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(11) Cheng, L.; Powell D. R.; Khan, M. A.; Richter-Addo, G. B. Chem.
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Inorganic Chemistry, Vol. 46, No. 17, 2007 7025