Six-coordinate nitrato complexes of iron(III) porphyrins

Article abstract of DOI:10.1021/ic702102j

The interaction of tetrahydrofuran (THF) with thin films of the nitrato complexes Fe^III(Por)(η²-O₂NO) [Por = meso-tetraphenylporphyrinato (TPP) and meso-tetratolylporphyrinato (TTP) dianion] at low temperature leads to the formation of the six-coordinate nitrato complex Fe(Por)(THF)(NO₃), which was characterized by IR and UV-visible spectroscopies. Formation of the THF adduct was accompanied by nitrate linkage isomerization from bidentate to monodentate coordination. The iron(III) center remains in a high spin state in contrast with the previously observed low-spin nitratonitrosyl complex Fe(TPP)(NO)(η¹-ONO ₂). Upon warming, THF dissociates to restore the initial five-coordinate bidentate nitrato complex.

Full text of DOI:10.1021/ic702102j

Inorg. Chem. 2008, 47, 787−789
Six-Coordinate Nitrato Complexes of Iron(III) Porphyrins
Gurgen M. Gulyan,† Tigran S. Kurtikyan,*,†,‡ and Peter C. Ford*,§
Molecule Structure Research Center NAS, 26 Azatutyan aV, YereVan, Armenia, Armenian Research
Institute of Applied Chemistry, 375053 YereVan, Armenia, and Department of Chemistry and
Biochemistry, UniVersity of California, Santa Barbara, California 93106-9510
Received October 25, 2007
The interaction of tetrahydrofuran (THF) with thin films of the nitrato
complexes FeIII(Por)( 2-O2NO) [Por
meso-tetraphenylporphyri-
nato (TPP) and meso-tetratolylporphyrinato (TTP) dianion] at low
temperature leads to the formation of the six-coordinate nitrato
complex Fe(Por)(THF)(NO3), which was characterized by IR and
decay of the peroxynitritoiron(III) intermediate formed by
the reaction of NO with oxyhemoglobin or oxymyoglobin.5
Thus, it is of interest to establish whether nitratoferriheme
models could be prepared with other Lewis base proximal
ligands. Reported here are spectroscopic studies demonstrat-
ing the formation of metastable six-coordinate complexes
upon the low-temperature reaction of tetrahydrofuran (THF)
with the five-coordinate precursors FeIII(Por)(NO3) in sub-
limed layered solids. We also describe experiments with NH3,
demonstrating that the nitrate ion can be reversibly displaced
by such Lewis bases in the layered solids.
η
)
UV−visible spectroscopies. Formation of the THF adduct was
accompanied by nitrate linkage isomerization from bidentate to
monodentate coordination. The iron(III) center remains in a high
spin state in contrast with the previously observed low-spin
nitratonitrosyl complex Fe(TPP)(NO)(η
1-ONO2). Upon warming,
The nitrato complex Fe(TPP)(η2-O2NO) (I) was prepared
by the reaction of excess NO2 gas with thin amorphous
FeII(TPP) layers (TPP ) meso-tetraphenylporphyrin dianion)
formed by sublimation onto a CaF2 or KBr surface as
previously described.6 When a low-temperature sample of I
under vacuum was exposed to THF vapors, the IR spectral
changes shown in Figure 1 were observed. The bands at
1527, 1273, and 967 cm-1 assigned to the η2- coordinated
nitrato ligand disappeared, and new bands emerged at 1491,
1280, and ∼1000 cm-1. Band assignments were substantiated
by isotopic labeling, with the latter bands having isotopic
analogues at 1457, 1258, and 986 cm-1 for the THF reaction
with Fe(TPP)(η2-O215NO) (Figure 2). Analogous results were
seen for Por ) TTP (Table 1and Figure S1 in the Supporting
Information; TTP ) meso-tetra-p-tolylporphyrin dianion).
Recent low-temperature structural studies reveal symmetric
bidentate coordination mode of the nitrato ligand in Fe(TPP)-
(η2-O2NO).7a Because of unrecognized nitrate ion disorder,
slightly asymmetric bidentate coordination was suggested
earlier.7b The iron(III) center is in the high-spin electronic
state with a large out-of-plane displacement (∼0.6 Å) from
the porphyrin ring. The three IR-active stretching modes
THF dissociates to restore the initial five-coordinate bidentate nitrato
complex.
The biological roles of the nitrogen oxides (NOx) often
involve interactions with heme proteins; thus, there is
considerable interest in model studies of these reactions.1
For the ligands NO and NO2, a number of five- and six-
coordinate iron(II) and iron(III) porphyrinato complexes have
been characterized;2 however, examples of six-coordinate
nitrato (NO3) complexes are limited. Among these are trans-
Fe(Por)(H2O)(η1-ONO2), described in a review,2 the unstable
nitrosyl species Fe(Por)(NO)(η1-ONO2) spectrally character-
ized at low temperature,3 and the η1-nitrate adduct of chloro-
peroxidase (with cysteine in the proximal coordination site),
for which the crystal structure has been determined.4 In addi-
tion, a nitratoferriheme complex is a likely transient in the
* To whom correspondence should be addressed. E-mail: kurto@netsys.am
(T.S.K.), ford@chem.ucsb.edu (P.C.F.).
† Molecule Structure Research Center NAS.
‡ Armenian Research Institute of Applied Chemistry.
§ University of California.
(1) (a) Ignarro, L. Nitric Oxide: Biology and Pathobiology; Academic
Press: San Diego, 2000. (b) Kirsch, M.; Korth, H.-G.; Sustmann, R.;
de Groot, H. Biol. Chem. 2002, 383, 389-399. (c) Pfeiffer, S.; Mayer,
B.; Hemmens, B. Angew. Chem., Int. Ed. 1999, 38, 1714-1731. (d)
Bryan, N. S.; Rassaf, T.; Maloney, R. E.; Rodriguez, C. M.; Saijo, F.;
Rodriguez, J. R.; Feelisch, M. Proc. Natl. Acad. Sci., U.S.A. 2004,
101, 4308-4313.
(5) (a) Doyle, M. P.; Hoeskstra, J. W. J. Inorg. Biochem. 1981, 14, 351.
(b) Herold, S.; Exner, M.; Nauser, T. Biochemistry 2001, 40, 3385-
3395. (c) Ford, P. C.; Lorkovic´, I. M. Chem. ReV. 2002, 102, 993-
1017.
(2) Wyllie, G. R. A.; Scheidt, W. R. Chem. ReV. 2002, 102, 1067-1089.
(3) (a) Kurtikyan, T. S.; Martirosyan, G. G.; Hakobyan, M. E.; Ford, P.C.
Chem. Commun. 2003, 1706-1707. (b) Kurtikyan, T. S.; Hovhanni-
syan, A. A.; Hakobyan, M. E.; Patterson, J.; Iretskii, A.; Ford, P. C.
J. Am. Chem. Soc. 2007, 129, 3576-3585.
(4) Kuhnel, K.; Blankenfeldt, W.; Terner, J.; Schlichting, I. J. Biol. Chem.
2006, 281, 2390-2399.
(6) (a) Kurtikyan, T. S.; Stepanyan, T. G.; Akopyan, M. E. Russ. J. Coord.
Chem. 1999, 25, 721-725. (b) Experimental procedures used for the
preparation of these materials are described in the Supporting
Information.
(7) (a) Wyllie, G. R. A.; Munro, O. Q.; Schulz, Ch. E.; Scheidt, W. R.
Polyhedron 2007, 26, 4664-4672. (b) Philippi, M. A.; Baenziger, N.;
Goff, H. M. Inorg. Chem., 1981, 20, 3904 -3911.
10.1021/ic702102j CCC: $40.75
Published on Web 12/29/2007
© 2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 3, 2008 787

COMMUNICATION
modes were observed upon the low-temperature interaction
of NO with Fe(TPP)(η2-O2NO) in layered solids to give
Fe(TPP)(NO)(η1-ONO2).3 This interpretation also gains sup-
port from experiments with the octaethylporphyrinato com-
plex Fe(OEP)(η1-ONO2), which had been shown to have
monodentate coordination.9 When this was prepared by the
reaction of NO2 with Fe(OEP) in sublimed layers, the Fourier
transform infrared (FTIR) spectrum revealed nitrate bands
at 1515 and 1276 cm-1 [νa(NO2), and νs(NO2), respectively],
with the latter much more intense than the former (Figure
S2 in the Supporting Information).
The same reaction with THF occurs also with Fe(OEP).
In this case, however, because of initial monodentate nature
of the coordinated nitrato ligand, FTIR spectral changes are
not so drastic compared with meso-aryl-substituted deriva-
tives. The high-frequency νa(NO2) band at 1515 cm-1 shifts
to 1489 cm-1 while the νs(NO2) band undergoes a minor
high-frequency shift and slightly gains in intensity (Figure
S3 in the Supporting Information). The decomposition
temperature and reversibility of the THF binding in this case
are very close to those for meso-aryl-substituted species.
Figure 3 illustrates the changes in the electronic absorption
spectra (Q-band region) as Fe(TPP) reacts with NO2 to give
(eventually) I, which was then was exposed to THF vapors
at 155 K to give Fe(TPP)(THF)(η1-ONO2). In each case, the
IR spectrum of the layered solid was recorded to confirm
the identity of the species under consideration. At T > 180
K, the complex began to dissociate THF, regenerating the
original spectrum.
Figure 1. Solid line: FTIR spectrum of Fe(TPP)(η2-O2NO) in the sublimed
solid at 180 K. Dashed line: Spectrum after introduction of THF into the
cryostat at 120 K, slow warming to 180 K, followed by intense pumping
to remove excess THF. Asterisks denote bands due to coordinated THF.
The porphyrin vibrations provide additional insight into
the electronic structure of the new complexes.10 A band in
the vicinity of 1350 cm-1 representing the porphyrin core
mode corresponding to ν(Ca-Cm) mixed with some
ν(Cm-phenyl) lies at higher frequencies for low-spin com-
plexes, which also demonstrate a low-energy porphyrin core
deformation mode at ∼450 cm-1. For the high-spin nitrato
complex I, these bands lie at 1341 and 436 cm-1. However,
for Fe(TPP)(THF)(η1-ONO2) and its TTP analogue, these
bands remain at the ranges characteristic of the high-spin
complexes (Table 1). In contrast, the nitrosylnitrato complex
Fe(TPP)(NO)(η1-ONO2) was found to be low-spin (Figure
S4 in the Supporting Information) because of the very strong
Fe-NO interaction.3
Missing from the FTIR spectra of the η1-nitrato complexes
described here is a band at 1385 cm-1 that has been re-
ported for IR spectra recorded in KBr pellets for several
metalloporphyrinatonitrato complexes, including Fe(OEP)-
(η1-ONO2).11 This band was also absent in the spectrum of
the thin layers of Fe(OEP)(η1-ONO2) solids prepared using
the cryostat technique (Figure S2 in the Supporting Informa-
Figure 2. FTIR spectra at 180 K of Fe(TPP)(THF)(η1-ONO2) (solid line)
and Fe(TPP)(THF)(η1-O15NO2) (dashed line) formed by low-temperature
(155 K) interactions of THF with layered Fe(TPP)(η2-O2NO) and Fe(TPP)-
(η1-O215NO), respectively.
expected for this structure would be a high-frequency
ν(NdO) for the uncoordinated oxygen and two (symmetric
and asymmetric) modes for the coordinated NO2 fragment,
as seen in Table 1. The addition of THF shifted the higher
frequency bands at 1527 and 1273 cm-1 by -36 and +7
cm-1, respectively, with noticeable changes of the intensity
ratio; the higher frequency band diminished, and the lower
frequency band increased. The same pattern was observed
for the 15N-enriched sample (Figure 2). In addition, a weak,
isotopically sensitive band appeared at ∼1000 cm-1 that
overlaps with an intense porphyrin band but is clearly seen
for the 15N-labeled compound. These changes can be
reasonably interpreted in terms of nitrate isomerization from
bidentate to monodentate coordination (Scheme 1).
The high-frequency band at 1491 cm-1 is now assigned
to νa(NO2) and that near 1280 cm-1 to νs(NO2). The smaller
separation of the high-frequency bands is characteristic of
monodentate nitrate coordination.8 The weak band at ∼1000
cm-1 can be assigned to the N-O bond for the oxygen atom
bound to iron(III). Similar changes for the nitrate stretching
(9) Ellison, M. K.; Shang, M.; Kim, J.; Scheidt, W. R. Acta Crystalogr.,
Sect. C 1996, C52, 3040-3045.
(10) Oshio, H.; Ama, T.; Watanabe, T.; Kincaid, J.; Nakamoto, K.
Spectrochim. Acta 1984, 40A, 863-870.
(11) (a) Suslick, K. S.; Watson, R. A. Inorg. Chem. 1991, 30, 912-919.
(b) Okubo, Y.; Okamura, A.; Imanishi, K.; Tachibana, J.; Umakoshi,
K.; Sasaki, Y.; Imamura, T. Bull. Chem. Soc. Jpn. 1999, 72, 2241-
2246. (c) Munro, O. Q.; Scheidt, W. R. Inorg. Chem. 1998, 37, 2308-
2316.
(8) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 3rd ed.; Wiley: New York, 1978; p 244.
788 Inorganic Chemistry, Vol. 47, No. 3, 2008

COMMUNICATION
Table 1. FTIR and UV-Visible Data for Fe(Por)(η2-O2NO) and Fe(Por)(THF)(η1-ONO2)a
spin-sensitive
bands, cm-1
compound
ν(NdO), cm-1
ν(N-O), cm-1
∼997b (986)
νa(NO2), cm-1
νs(NO2), cm-1
Q region, λmax (nm)
Fe(TPP)(η2-O2NO)
1527 (1489)
1273 (1253)
1491 (1457)
1273 (1254)
1486 (1454)
1505 (1470)
967 (958)
1280 (1258)
966 (957)
1277 (1258)
1266 (1248)
1341; 436
1334; 434
1341/1334; 428
1334; 427
1351; 464
512; 579 (sh); 654; 691
496; 529; 573 (sh); 650; 693
515; 573 (sh); 698
498; 534; 575 (sh); 612; 701
508; 547; 582
Fe(TPP)(THF)(η1-ONO2)
Fe(TTP)(η2 -O2NO)
1529 (1494)
Fe(TTP)(THF)(η1-ONO2)
Fe(TPP)(NO)(η1-ONO2)c
∼995b (984)
978 (963)
a Data in parentheses are for 15N-labeled compounds. b Masked by the intense porphyrin band. c Reference 3a.
reversible process described by eq 1. The absence of a similar
isotopically sensitive band in the spectra of products formed
by the reaction of I with THF (Figures 1 and 2) indicates
that analogous nitrate displacement does not occur with this
weaker base.
Fe(TPP)(η2-O2NO) + 2NH3 h Fe(TPP)(NH3)2+ + NO3
-
(1)
In summary, the low-temperature interaction of THF with
thin layers of Fe(Por)(η2-O2NO) results in the formation of
six-coordinate nitrato complexes of iron porphyrins display-
ing IR bands in the vicinity of 1500, 1280, and 1000 cm-1.
Trans ligation is accompanied by the bidentate-to-monoden-
tate linkage isomerization of the nitrato ligand, but the
complex remains in the high spin state. This behavior
contrasts with the analogous reaction with NO that gives a
low-spin product. The known six-coordinate nitrato com-
plexes of iron porphyrins all display monodentate nitrate
coordination perhaps as the result of nonbonded repulsions
between the oxygen atoms of bidentate nitrate and the
nitrogen atoms of the porphyrin core. Given that the five-
coordinate Fe(Por)(NO3) complexes display both modes of
coordination, e.g., Fe(OEP)(η1-ONO2)9 and Fe(TPP)(η2-
O2NO),7 the energy difference between the η1 and η2 isomers
must be small, as is also shown by density functional theory
calculations.3b The complexes spectrally characterized in the
present study and formulated as Fe(Por)(THF)(η1-ONO2) are
stable only at temperatures lower than 180 K. At higher
temperatures, they dissociate the weakly bound THF ligand,
and at room temperature, the initial nitrato complexes
Fe(Por)(η2-O2NO) are completely restored.
Figure 3. Visible spectra of the Fe(TPP) sublimed layer (solid line), after
interaction with NO2 to form the nitrato complex Fe(TPP)(η2-O2NO) (dashed
line), and after further reaction with THF by warming of the layer from
120 to 155 K (dotted line) (all on a CaF2 substrate).
Scheme 1
tion). It has been found that grinding solid Mn(TPP)(η1-
ONO2) with KBr leads to anion exchange to give Mn(TPP)Br
and KNO3 with concomitant growth of the 1385 cm-1 band
at the expense of those at 1473 and 1286 cm-1,12 and a
similar exchange of weakly bound anionic ligands during
KBr pellet preparation has been reported.13 For KBr pellets
with Fe(OEP)(NO3), the effect is dependent on the grinding
time as well (Figure S5 in the Supporting Information). Thus,
we conclude that the 1385 cm-1 band commonly seen in
Acknowledgment. Financial support from ISTC (Project
A-484) is gratefully acknowledged. P.C.F. also thanks the
U.S. National Science Foundation.
-
such systems is likely due to free NO3 formed by solid-
phase ion exchange with the KBr medium.
Supporting Information Available: Reference 6b describing
experimental details, Figure S1a,b demonstrating FTIR and UV-
visible spectra of Fe(TTP)(η2-O2NO) and Fe(TTP)(THF)(η1-ONO2),
respectively, Figure S2 representing FTIR spectra of Fe(OEP) and
Fe(OEP)(η1-ONO2), Figure S3 demonstrating FTIR spectra of
Fe(OEP)(η1-ONO2) and Fe(OEP)(THF)(η1-ONO2), Figure S4
demonstrating low-frequency FTIR spectra of FeTPP(η2-O2NO),
Fe(TPP)(NO)(η1-ONO2), and Fe(TPP)(THF)(η1-ONO2), Figure S5
representing FTIR spectra of Fe(OEP)(η1-ONO2) in a thin layer
and as KBr pellets with different extents of grinding, and Figure
S6 representing FTIR spectral changes observed in the sublimed
layer of Fe(TPP)(η2-O2NO) upon low-temperature interaction with
NH3. This material is available free of charge via the Internet at
http://pubs.acs.org.
The lability of the nitrate ligand is emphasized by the
reaction of Fe(TPP)(η2-O2NO) sublimed layers with am-
monia. Exposure of I to excess NH3 at 180 K led to the
disappearance of the nitrate bands at 1527, 1273, and 967
cm-1 and the appearance of a strong absorbance at 1370 cm-1
(Figure S6 in the Supporting Information). The latter can be
attributed to the intense ν3(E) vibration of free NO3- in the
matrix. This band has its isotopic counterpart at 1335 cm-1
in the experiments where NH3 interacts with layered solid
containing Fe(TPP)(η2-O215NO). Vigorous pumping on the
system led to regeneration of I, so the likely scenario is the
(12) Kurtikyan, T. S.; Stepanyan, T. H.; Martirosyan, G. G.; Kazaryan, R.
K.; Madakyan, V. N. Russ. J. Coord. Chem. 2000, 26, 345-348.
(13) Williamson, M. M.; Hill, C. L. Inorg. Chem. 1986, 25, 4648-4671.
IC702102J
Inorganic Chemistry, Vol. 47, No. 3, 2008 789