The first unambiguous determination of a nitrosyl-to-nitrite conversion in an iron nitrosyl porphyrin

Article abstract of DOI:10.1039/b006775j

The picket fence porphyrin complex [Fe(tpivpp)(NO)] undergoes complete conversion to the isolable N-bound nitrite derivative [Fe(tpivpp)(NO₂)(py)] in air in the presence of pyridine: reconversion to the starting nitrosyl complex is achieved using triphenylphosphine as reductant.

Full text of DOI:10.1039/b006775j

The first unambiguous determination of a nitrosyl-to-nitrite conversion in an
iron nitrosyl porphyrin
Lin Cheng,a Douglas R. Powell,b Masood A. Khana and George B. Richter-Addo*a
a Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman, OK 73019,
USA. E-mail: grichteraddo@ou.edu
b X-ray Structural Laboratory, Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison,
WI 53706, USA
Received (in Cambridge, UK) 18th August 2000, Accepted 17th October 2000
First published as an Advance Article on the web 8th November 2000
The picket fence porphyrin complex [Fe(tpivpp)(NO)]
undergoes complete conversion to the isolable N-bound
nitrite derivative [Fe(tpivpp)(NO2)(py)] in air in the pres-
ence of pyridine: reconversion to the starting nitrosyl
complex is achieved using triphenylphosphine as reduc-
tant.
monitoring the reaction by UV–VIS spectroscopy). We have
also confirmed an earlier observation that air oxidation of the
non-picket fence [Fe(por)(NO)] compounds (por = tpp, ttp,
oep) results in the isolation of the [Fe(por)]2O oxo dimers,4
suggesting that the picket fence pocket is indeed crucial for the
stabilization of the singly-oxidized nitrite product.
Deoxygenation of the coordinated nitrite to nitrosyl in this
picket fence [Fe(tpivpp)(NO2)(py)] complex in CHCl3 (under a
nitrogen atmosphere) is achieved by the addition of triphenyl-
phosphine. The reaction is complete within 15 min, and the five-
coordinate nitrosyl [Fe(tpivpp)(NO)] complex is obtained in
80% isolated yield, with triphenylphosphine oxide as the
oxidized product. The overall reaction is shown in eqn. (1):
The study of the reactions of nitric oxide ligands in iron nitrosyl
porphyrins is receiving increased attention in bioinorganic
chemistry.1 This is due, in part, to a need to understand the fate
of the NO group after it binds to the metal center in heme
biomolecules and related model complexes, and to elucidate the
chemical processes involved in the conversion of the co-
ordinated NO group to nitrite. There are very interesting results
in the literature regarding the formation of coordinated NO2
groups in iron porphyrins.† For example, Scheidt and cowork-
ers have prepared and structurally characterized a series of
[Fe(por)(NO)(NO2)] complexes by the reaction of [Fe(por)]2O
or [Fe(tpivpp)(NO)] with an excess of NO,2 and they have
proposed the intermediacy of reactive nitrite compounds during
the formation of nitrate [Fe(por)(NO3)] products (por = tpivpp,
oep, tpp, ttp) from their nitrosyl precursors.3,4 Ford and
Lorkovic have demonstrated that NO2 (N2O3; derived from
reaction of NO gas with trace oxygen) reacts with
[Fe(tpp)(NO)] to give [Fe(tpp)(NO)(NO2)].5
Surprisingly, and although it is of fundamental interest in
heme–NO chemistry, no direct chemical evidence has been
reported on the straightforward conversion of the coordinated
NO group in an iron nitrosyl porphyrin to an isolable nitrite
complex.‡ We now produce the first unambiguous experimental
evidence that the [Fe(tpivpp)(NO)] compound converts to the
[Fe(tpivpp)(NO2)(py)] derivative in air and in the absence of
added NO.
(1)
The conversion of coordinated nitrite to nitrosyl has precedent
in iron porphyrin chemistry. For example, (i) the anionic bis-
nitro [Fe(tpivpp)(NO2)2]2 complex undergoes rearrangement
in the presence of boron trifluoride to [Fe(tpivpp)(ONO2)] and
[Fe(tpivpp)(NO)] in some organic solvents,3 and (ii) oxygen-
atom transfer to organic compounds by the [Fe(oep)Cl]/nitrite
mixture is believed to occur via the intermediacy of
‘[Fe(oep)(NO2)]’, although this intermediate was not unambi-
guously identified.14
The observation of this base-assisted stabilization of an iron-
bound nitrite product derived from its nitrosyl precursor has
implications for the reactivity of the bound NO group in nitrosyl
heme proteins containing trans axial N-bases, and suggests that
The X-ray structure of [Fe(tpivpp)(NO)] is shown in Fig. 1.§¶
Reaction of this compound in CHCl3 with air in the presence of
pyridine results in its complete conversion to [Fe(tpivpp)-
(NO2)(py)]∑** within 1 h as judged by IR spectroscopy of KBr
pellets of dried aliquots.†† When the reaction of [Fe(tpivpp)-
(NO)] (20–50 mg, 0.018–0.040 mmol) with air and pyridine is
performed in a CHCl3–heptane mixture, and the solvent
allowed to evaporate over a 3 d period, needle-shaped crystals
of the [Fe(tpivpp)(NO2)(py)]·py product were obtained in
70–80% isolated yields. The observed Fe–NO2 distances of
1.92–1.97 Å in the [Fe(tpivpp)(NO2)(py)] products are longer
than the axial Fe–NO distance in the precursor [Fe(tpivpp)-
(NO)] complex (1.65–1.74 Å), and the nitrite O-atoms point
towards the HN groups of the picket fence (O…N distances of
3.7–3.8 Å) suggesting possible stabilization of the bound nitrite
through weak H-bonding interactions.
The added base is essential for the generation of the stable
ferric nitrite derivative, since the reaction of [Fe(tpivpp)(NO)]
with air in CHCl3 solution (without added pyridine) results in its
quantitative conversion to the known ferric chloro [Fe(tpivpp)-
(Cl)] complex (lmax = 417 nm).12,13 We have found that this
latter reaction is also enhanced by UV–VIS light (e.g. when
Fig. 1 Molecular structure of [Fe(tpivpp)(NO)]. The nitrosyl group is
disordered over two positions, and only one of these positions is shown.
Hydrogen atoms have been omitted for clarity. Selected bond distances (Å)
and angles (°): Fe(1)–N(por) 1.991(5)–2.007(5), Fe(1)–N(5) 1.65(5), Fe(1)–
N(5A) 1.74(6), N(5)–O(1) 1.17(5), N(5A)–O(1A) 1.20(7); Fe(1)–N(5)–O(1)
149(4), Fe(1)–N(5A)–O(1A) 137(4).
DOI: 10.1039/b006775j
Chem. Commun., 2000, 2301–2302
This journal is © The Royal Society of Chemistry 2000
2301

coordinate [Fe(tpivpp)(NO2)]2 compound is known.10 Anionic iron
porphyrin nitrites also have rich electrochemistry.11
iron-bound nitrite could be one of several products or
intermediates in some heme–NO ligand oxidations.
We are grateful to the National Science Foundation (CHE-
9625065) and the National Institutes of Health (GM-53586),
and the Oklahoma Center for the Advancement of Science and
Technology (HN97-088) for support for this research. We thank
Professor W. Robert Scheidt (U.S.A.) for helpful discussions.
†† KBr pellets were used to observe the nNO (1675 cm21) and nNO (1307
cm21) bands. These bands were confirmed by the use of the 15N-2labeled
[Fe(tpivpp)(15NO)] (lmax = 409 nm) precursor: n NO of 1641 cm21 for the
15
nitrosyl complex, and n
1289 cm21 for the nitrite derivative. We have
15NO2
not been able to determine the position of the second nNO band owing to
2
extensive overlap with the porphyrin bands. Also, the UV–VIS spectrum of
[Fe(tpivpp)(NO2)(py)] (lmax = 419 nm) is very similar to that of the six-
coordinate nitrosyl [Fe(tpivpp)(NO)(py)] complex (lmax
= 419 nm),
making the monitoring of the reaction by UV-VIS spectroscopy very
difficult.
Notes and references
† Abbreviations: por = porphyrinato dianion, tpivpp = ‘picket fence’
1 L. Cheng and G. B. Richter-Addo, in The Porphyrin Handbook, ed.
K. M . Kadish, K. M. Smith and R. Guilard, Academic Press, New York,
2000, ch. 33.
2 M. K. Ellison, C. E. Schulz and W. R. Scheidt, Inorg. Chem., 1999, 38,
100.
3 O. Q. Munro and W. R. Scheidt, Inorg. Chem., 1998, 37, 2308.
4 M. G. Finnegan, A. G. Lappin and W. R. Scheidt, Inorg. Chem., 1990,
29, 181.
5 I. M. Lorkovic and P. C. Ford, Inorg. Chem., 2000, 39, 632.
6 T. S. Kurtikyan, Russ. J. Inorg. Chem., 1999, 25, 28; P. G. Jene and J. A.
Ibers, Inorg. Chem., 2000, 39, 3823.
tetra(a,a,a,a-o-pivalamidophenyl)porphyrinato dianion, oep
= octa-
ethylporphyrinato dianion, tpp = tetraphenylporphyrinato dianion, ttp =
tetratolylporphyrinato dianion.
‡ Related conversions of cobalt6 and rhodium7 nitrosyl porphyrins to nitrite
derivatives have been reported.
§ Crystal data for [Fe(tpivpp)(NO)] were collected on a Bruker (Siemens)
SMART/CCD diffractometer with Mo-Ka radiation (l = 0.71073 Å). The
structure was solved by direct methods using the SHELXTL system and
refined by full-matrix least squares on F2. Crystal data: C64H64FeN9O5, M
=
1095.09, monoclinic, space group P21/n, a = 17.7905(14), b =
17.6238(13), c = 20.2731(16) Å, b = 99.390(2)°, V = 5913.3(8) Å3, Z =
4, Dc = 1.230 g cm23, T = 158(2) K, absorption coefficient = 0.311
mm21, 7705 independent reflections (Rint = 0.0758). Final R (F obs. data)
= 0.0709 for 4338 ‘observed’ reflections with [I > 4s(I)].
CCDC 182/1820. See http://www.rsc.org/suppdata/cc/b0/b006775j/ for
crystallographic files in .cif format.
¶ The X-ray structure of the related [Fe(tpivpp)(NO)][K(NO2)(18-crown-
6)] has been reported.8 The [Fe(por)(NO)] compounds are commonly
referred to as ferrous nitrosyls.1
7 B. B. Wayland and A. R. Newman, Inorg. Chem., 1981, 20, 3093.
8 H. Nasri, K. J. Haller, Y. Wang, B. H. Huynh and W. R. Scheidt, Inorg.
Chem., 1992, 31, 3459.
9 H. Nasri, Y. Wang, B. H. Huynh, F. A. Walker and W. R. Scheidt, Inorg.
Chem., 1991, 30, 1483.
10 H. Nasri, Y. Wang, B. H. Huynh and W. R. Scheidt, J. Am. Chem. Soc.,
1991, 113, 717.
11 J. B. Fernandes, D. Feng, A. Chang, A. Keyser and M. D. Ryan, Inorg.
Chem., 1986, 25, 2606.
∑ This compound was initially prepared from the reaction of the anionic bis-
12 A. Gismelseed, E. L. Bominaar, E. Bill, A. X. Trautwein, H. Winkler, H.
Nasri, P. Doppelt, D. Mandon, J. Fischer and R. Weiss, Inorg. Chem.,
1990, 29, 2741.
13 J. P. Collman, R. R. Gagne, C. A. Reed, T. R. Halbert, G. Lang and
W. T. Robinson, J. Am. Chem. Soc., 1975, 97, 1427.
14 S. K. O’Shea, W. Wang, R. S. Wade and C. E. Castro, J. Org. Chem.,
1996, 61, 6388.
nitro [Fe(tpivpp)(NO2)2]– complex with pyridine.9
** We have obtained three solid-state X-ray structures of this [Fe(tpivpp)-
(NO2)(py)]·solvate product from these reactions that differ in the nature of
the solvate (pyridine, chloroform or dichloromethane). These structures are
available in the above .cif file, and are similar to the previously reported
structure of [Fe(tpivpp)(NO2)(py)]·PhCl.9 The structure of the anionic five-
2302
Chem. Commun., 2000, 2301–2302