Reactions of nitrogen oxides with heme models. Low temperature spectral characterization of the unstable nitrato-nitrosyl complex FeIII(TPP) (ONO2)(NO)

Article abstract of DOI:10.1039/b302061d

Reaction of NO gas with low temperature films of the η²-nitrato model heme Fe^III(TPP)(O₂NO) (TPP = mesotetraphenylporphyrinato^2-) leads to formation of the previously unknown η¹-nitrato nitrosyl speci

Full text of DOI:10.1039/b302061d

Reactions of nitrogen oxides with heme models. Low temperature
spectral characterization of the unstable nitrato-nitrosyl complex
III
Fe (TPP)(ONO )(NO)†
2
ab
b
b
c
Tigran S. Kurtikyan,* Garik G. Martirosyan, Manya E. Hakobyan and Peter C. Ford*
a
Molecular Structure Research Centre NAS, 375014, Yerevan, Armenia. E-mail: tkurt@msrc.am
Armenian Research Institute of Applied Chemistry (ARIAC), 375005, Yerevan, Armenia
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106,
USA. E-mail: ford@chem.ucsb.edu
b
c
Received (in Cambridge, UK) 21st February 2003, Accepted 3rd June 2003
First published as an Advance Article on the web 13th June 2003
2
Reaction of NO gas with low temperature films of the h -
III
2
nitrato model heme Fe (TPP)(O NO) (TPP = meso-
tetraphenylporphyrinato2 ) leads to formation of the pre-
2
1
viously
unknown
Fe (TPP)(ONO
spectroscopy with isotopically substituted nitrogen oxides.
h -nitrato
nitrosyl
species
III
2
)(NO) as characterized by IR and optical
The interaction of the nitrogen oxides NO with metal-
x
loporphyrins, especially with heme models, has drawn con-
siderable attention owing to the potential relevance of such
reactions to mammalian biochemistry.1 Numerous ferro- and
,2
2
ferri-heme complexes with NO and NO have been charac-
3
terized, but nitrato complexes are quite rare. In this context, we
report the use of low temperature spectroscopy of sublimed
layers to characterize the previously unknown nitrato nitrosyl
2
complex Fe(TPP)(ONO )(NO) and to study the subsequent
reactions of this unstable species.
Low temperature sublimates of Fe(TPP) on KBr or CaF
2
2
substrate were prepared from Fe(TPP)(B) (B = pyridine or
piperidine) as previously described.4 Such M(TPP) layers
obtained by sublimation onto a low-temperature (77 K) surface
are sponge-like and have high microporosity. Potential re-
actants easily diffuse across these layers, and adducts thus
formed can be studied spectroscopically without solvent
interference.
Fig. 1 IR spectra of Fe(TPP) derivatives in sublimed layers at 80 K: (a)
5
Fe(TPP) (solid line), (b) Fe(TPP)(O NO) formed by exposure of Fe(TPP) to
2
NO
2
(P = 1 Torr) for 10 min at 293 K followed by exhaustive high vacuum
NO)(15NO) formed by supplying
NO (10 Torr) to (b) at 80 K, warming to 160 K and re-cooling (dotted
line).
pumping (dashed line), (c) Fe(TPP)(O
2
15
A Fe(TPP) sublimed layer was heated to room temperature
6
under dynamic vacuum, then NO
2
gas was introduced, and the
Feltham). The band at 548 cm that is shifted to 542 cm21
9
21
sample was cooled to 80 K to record the spectra. As shown
7
2
with 15NO can be assigned to the iron–nitrogen stretch n{Fe–
previously, this procedure leads to the formation of the h -
nitrato complex Fe(TPP)(O NO) (eqn. 1)
2
N(NO)}.
The IR spectrum of the related nitro-nitrosyl complex
Fe(TPP) + 2 NO ? Fe(TPP)(O
2
2
NO) + NO
(1)
Fe(TPP)(NO
2
)(NO) (in chloroform) was reported to display a
2
1
which was characterized by its known IR spectrum displaying
comparable band at 549 cm
with an isotopic shift of 6
2
21
21 10
coordinated NO
3
bands at 1531 and 1275 cm (Fig. 1, dashed
cm .
line). Gaseous 15NO was introduced and the system was
warmed to 160 K then cooled to 80 K. The resulting product
2
In Fe(TPP)(O NO) the nitrate ligand is bound in a slightly
asymmetric bidentate mode.11 The Fe(III) is d high-spin and
5
2
1
displayed new bands at 1863, 1505, 1266, 978 and 542 cm . If
instead, 14NO was used, these appeared at 1901, 1505, 1266,
2
1
15
9
78 and 548 cm (Figs. 1 and 2). Reaction of NO with a film
15
containing Fe(TPP)( NO
470, 1248, 963 and 542 cm (see Table 1 and ESI, Fig. S1†).
3
) gives the analogous bands at 1863,
2
1
1
The intensities of these bands correlate and clearly indicate the
formation of a new nitrato nitrosyl complex from the reaction of
2
NO with Fe(TPP)(O NO).
The isotopic shifts in the bands summarized in Table 1
2
1
demonstrate that the 1901 and 548 cm bands for the product
1
4
14
of Fe(TPP)(O
2
NO) plus NO are associated with the nitrosyl
2
1
while those at 1505, 1266 and 978 cm are associated with the
2
1
15
nitrato ligand. The 1901 cm band with a N isotopic shift of
2
1
about 240 cm can be assigned to NO stretching vibration of
coordinated nitrosyl in analogy to linear FeNO units of other
{
FeNO}6 complexes8 (using the notation of Enemark and
†
Electronic supplementary information (ESI) available: IR and UV-Visible
Fig. 2 Low frequency IR spectra at T = 80 K of thin layers containing
spectra of A. See http://www.rsc.org/suppdata/cc/b3/b302061d/
2 2
Fe(TPP)(O NO) (solid line) and (NO)Fe(TPP)(ONO ) (dashed line).
1
706
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2
1
Table 1 IR spectral data (in cm ) of nitrosyl and nitrate groups for: I –
longer wavelengths and disposed at 436, 547 and 582 nm in A.
This is analogous to other mixed nitrogen oxide complexes of
Fe(III)(P) (P = porphyrin); for example, the spectrum of the
14
14
14
15
Fe(TPP)(O NO
2
)( NO); II – Fe(TPP)(O NO
2
)( NO); III – Fe(TPP)(O-
15
14
15
) (15NO)
2 2
NO )( NO); IV – Fe(TPP)(O NO
nitro nitrosyl complex Fe(TPP)(NO
33, 545 and 577 nm in chloroform solution.10
To our knowledge, the only six-coordinate nitrato Fe(P)
2
)(NO) displays maxima at
I
II
III
IV
Assignment
4
1
1
1
901 s
505 m
266 s
1862 s
1505 m
1266 s
978 w
1901 s
1470 m
1247 s
963 w
1863 s
1470 m
1248 s
963 w
n(NNO)
1
complex described previously is a trans h -nitrato aquo
n
a
2
(NO )
s 2
n (NO )
complex (Fe(P)(ONO )(H O)) (P not identified) in an un-
2
2
3
9
5
78 w
48 vw
n(N–O)
n{Fe–N(NO)}
published study reported in a review by Wyllie and Scheidt. It
seems likely that formation of six-coordinate complexes
containing the nitrato ligand will be accompanied by bidentate–
monodentate transition to alleviate nonbonded repulsions
between nitrate oxygens and porphyrin nitrogens. This process
should not require the large expenditure of energy as evidenced
from the existence of monodentate and bidentate binding in the
iron porphyrins Fe(OEP)(ONO ) and Fe(TPP)(O NO) differing
542 vw
548 vw
542 vw
reveals a large out-of-plane displacement (0.6 Å) toward the
2
lone axial ligand. The h -nitrate ligand would be expected to
show three IR active stretching modes for such a structure, a
high frequency n(NNO) stretch for the uncoordinated oxygen
plus symmetric and asymmetric modes for the coordinated NO
2
2
3
2
only in the nature of peripheral substituents.
2
1
fragment. However, only two bands at 1531 and 1275 cm of
The nitrato nitrosyl complex spectrally characterized here
decomposes under ambient conditions, and the nature of the
products formed depends on whether this occurs under an NO
atmosphere. In the absence of NO it returns to the nitrato
compatible intensities are seen (Fig. 1, dashed line)7 so one of
,11
the expected bands, perhaps the symmetric n
too weak to be observed or is masked by porphyrin absorptions.
Upon adding NO to Fe(TPP)(O NO), these bands shift to lower
frequencies by 26 and 9 cm , respectively (Fig. 1, dotted line).
More importantly, the relative intensities change, with the high
frequency band diminishing and the lower frequency one
becoming much stronger. In addition a new isotopically
s 2
(NO ) mode, is
2
complex Fe(TPP)(O NO), while under NO a series of chemical
2
2
1
transformations occur that is now under investigation.
In summary, the low temperature interaction of NO gas with
thin films of Fe(TPP)(O NO) leads to formation of a new six-
2
coordinate complex that is formulated as Fe(TPP)(ONO )(NO)
2
2
1
sensitive band appears at ~ 980 cm
.
based on the IR and UV-Vis data. This reaction is accompanied
These IR spectra changes can be interpreted in terms of the
bidentate–monodentate transition of nitrate coordination illus-
trated by Scheme 1. The highest frequency nitrato band now
by bidentate–monodentate isomerization of coordinated nitrate
and transition of Fe(III) ion from high-spin to low-spin. The
complex formed is stable at low temperatures but upon warming
undergoes further transformations to products apparently de-
pendent on the presence of an NO atmosphere.
Studies in Armenia were supported by the INTAS (Grant
#911) and at UCSB were supported by the US National Science
Foundation and the Petroleum Research Fund.
2
1
represents n
assigned to n
complex Fe(OEP)(ONO
displays an IR band for the coordinated nitrate at 1515 cm
KBr pellet)12 similar to the high frequency nitrato band seen
a
(NO
2
), while that in the vicinity of 1250 cm is
1
s
(NO ). The previously described h -nitrato
2
) (OEP = octaethylporphyrinato2
2
)
2
2
1
(
here. Closer disposition of the high frequency bands is
considered as a criterion of monodentate coordination in nitrate
Notes and references
1 (a) P. C. Ford and I. M. Lorkovic’, Chem. Rev., 2002, 102, 993; (b) L.
Cheng and G. B. Richter-Addo, in The Porphyrin Handbook, 4, eds. K.
M. Kadish, K. M. Smith and R. Guilard, Academic Press, New York,
complexes.13 The weak band at 980 cm can be assigned to the
21
N–O vibration for the O atom coordinated to the Fe(III) ion.13
Thus, as indicated by Scheme 1, we conclude that the product of
1
the reaction of NO with Fe(TPP)(O
2
NO) is the h -nitrato
2
000, ch. 33.
nitrosyl complex Fe(TPP)(ONO )(NO) (A).
2
2
(a) Nitric Oxide, Biology and Pathology, ed. L. J. Ignarro, Academic
Press, San Diego, 2000; (b) Nitric Oxide and Infection, ed. F. C. Fang,
Kluwer Academic/Plenum Publishers, New York, 1999.
Additional information regarding the nature of A can be
drawn from porphyrin vibrational modes that reveal regular
changes depending on the spin and oxidation state of axial
3 G. R. A. Wyllie and W. R. Scheidt, Chem. Rev., 2002, 102, 1067.
4 (a) T. S. Kurtikyan, G. G. Martirosyan, I. M. Lorkovic’ and P. C. Ford,
J. Am. Chem. Soc., 2002, 124, 1012; (b) K. Nakamoto, T. Watanabe, T.
Ama and M. W. Urban, J. Am. Chem. Soc., 1982, 104, 3744.
14
complexes of Fe(TPP). It has been found that the band in
2
1
vicinity of 1350 cm representing a porphyrin core mode
corresponding to n(C –C ) mixed with some n(C –phenyl) lies
a
m
m
5
(a) M. P. Byrn, C. J. Curtis, Y. Hsiou, S. I. Khan, P. A. Sawin, S. K.
Tendick, A. Terzis and C. E. Strouse, J. Am. Chem. Soc., 1993, 115,
at higher frequencies in low-spin complexes. The same
character demonstrates a low energy porphyrin core deforma-
9
480; (b) T. S. Kurtikyan, A. V. Gasparyan, G. G. Martirosyan and G.
2
1
2
tion mode in the range 450 cm . For the high spin h -nitrato
complex Fe(TPP)(O NO), these bands lie at 1342 and 436
cm . Upon additional coordination of NO these bands shift to
H. Zhamkochyan, J. Appl. Spectrosc., 1995, 62, 62 (Russ.) (c) T. S.
Kurtikyan and T. H. Stepanyan, Russ. Chem. Bull., 1998, 47, 695.
2
2
1
15
6
The NO
2
(
NO
2 2
) used in these studies was obtained by O oxidation of
2
1
15
1
351 and 464 cm (see Figs. 1 and 2 ) indicating a low-spin
state for the new complex. This result is consistent with other
-coordinate ferri-heme nitrosyl complexes in which the iron is
NO ( NO) and was purified by fractional distillation until a pure white
solid was obtained. The NO ( NO) used was purified by passage
through KOH pellets and a dry ice/acetone cooled trap to remove higher
15
6
x 2
NO and trace H O. The purity was checked by IR measurements of the
layer obtained by low rate deposition of NO on the cooled substrate of
an optical cryostat (77 K). The IR spectra did not show the presence of
located close to the center of the porphyrin plane and the
electronic state is low-spin.3
Electronic absorption spectra (ESI, Fig. S2†) confirm
formation of the new complex upon reaction of NO with low-
temperature films of Fe(TPP)(O NO). The bands of the nitrato
2
complex at 423, 511 and 572 nm are significantly shifted to
2 2 3 2
N O, N O or H O.
7
8
9
T. S. Kurtikyan, T. G. Stepanyan and M. E. Akopyan, Russ. J. Coord.
Chem., 1999, 25, 721.
M. K. Ellison and W. R. Scheidt, J. Am. Chem. Soc., 1999, 121,
1
510.
J. H. Enemark and R. D. Feltham, Coord. Chem. Rev., 1974, 13, 339.
1
1
0 T. Yoshimura, Inorg. Chim. Acta, 1984, 83, 17.
1 M. A. Phillippi, N. Baenziger and H. M. Goff, Inorg. Chem., 1981, 20,
3
904.
1
1
1
2 M. K. Ellison, M. Shang, J. Kim and W. R. Scheidt, Acta Crystallogr.,
Sect. C, 1996, 52, 304.
3 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordina-
tion Compounds, 3rd edn., Wiley, New York, 1978, p. 244.
4 H. Oshio, T. Ama, T. Watanabe, J. Kincaid and K. Nakamoto,
Spectrochim. Acta, 1984, 40A, 863.
Scheme 1
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