(Nitro)Iron(III) Porphyrins. EPR Detection of a Transient Low-Spin Iron(III) Complex and Structural Characterization of an O Atom Transfer Product

Article abstract of DOI:10.1021/ic970855l

The reaction of BF₃·OEt₂ with the bis(nitro) complex of iron(III) picket-fence porphyrin, [K(18C6)(OH₂)][Fe-(TpivPP)(NO₂)₂], leads to the formation of a transient porphyrin intermediate, assigned on the basis of its rhombic low-spin EPR spectrum as the five-coordinate N-bound mono(nitro) iron(III) derivative, [Fe(TpivPP)(NO₂)]. This species is reactive and readily undergoes oxygen atom transfer to form [Fe^III(TpivPP)(NO₃)] and [Fe^II(TpivPP)-(NO)]. The reactions have been followed by EPR and IR spectroscopy. [Fe(TpivPP)(NO₂)] has a rhombic EPR spectrum (g = 2.60, 2.35, and 1.75) in chlorobenzene and CH₂Cl₂ and is spectroscopically distinct from the bis(nitro) starting material (g = 2.70, 2.50, and 1.57). Oxidation of the nitrosyl species to [Fe(TpivPP)(NO₃)] proceeds via an intermediate assigned as [Fe(TpivPP)(NO₂)J on the basis of its EPR spectrum. The crystal structure of one of the reaction products, [Fe(TpivPP)(NO₃)], has been determined. The nitrate ion of [Fe(TpivPP)(NO₃)] is bound to the iron(III) ion in a symmetric bidentate fashion within the ligand-binding pocket of the porphyrin pickets. Individual Fe-O distances are 2.123(3) and 2.226(3) A?. The dihedral angle between the plane of the nitrate ion and the closest N_p-Fe-N_p plane is 10.0°. The Fe-N_p bonds (and trans N_p-Fe-N_p angles) perpendicular and parallel to the plane of the axial ligand average to 2.060(5) A? (154.84(9)°) and 2.083(3) A? (146.14(9)°), respectively. Crystal data for [Fe(TpivPP)(NO₃)]: a = 23.530(2) A?, b = 10.0822(5) A?, c = 48.748-(3) A?, β = 92.145(5)°. monoclinic, space group 121 a, V= 11556.4(14) A?³, Z= 8, FeN₉O₇C₆₄H₆₄, 8798 observed data, R₁ = 0.0606, wR₂ = 0.1313, all observations at 127(2) K.

Full text of DOI:10.1021/ic970855l

2308
Inorg. Chem. 1998, 37, 2308-2316
(Nitro)Iron(III) Porphyrins. EPR Detection of a Transient Low-Spin Iron(III) Complex and
Structural Characterization of an O Atom Transfer Product
Orde Q. Munro and W. Robert Scheidt*
The Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556
ReceiVed July 8, 1997
The reaction of BF3‚OEt2 with the bis(nitro) complex of iron(III) picket-fence porphyrin, [K(18C6)(OH2)][Fe-
(TpivPP)(NO2)2], leads to the formation of a transient porphyrin intermediate, assigned on the basis of its rhombic
low-spin EPR spectrum as the five-coordinate N-bound mono(nitro) iron(III) derivative, [Fe(TpivPP)(NO2)]. This
species is reactive and readily undergoes oxygen atom transfer to form [FeIII(TpivPP)(NO3)] and [FeII(TpivPP)-
(NO)]. The reactions have been followed by EPR and IR spectroscopy. [Fe(TpivPP)(NO2)] has a rhombic EPR
spectrum (g ) 2.60, 2.35, and 1.75) in chlorobenzene and CH2Cl2 and is spectroscopically distinct from the
bis(nitro) starting material (g ) 2.70, 2.50, and 1.57). Oxidation of the nitrosyl species to [Fe(TpivPP)(NO3)]
proceeds via an intermediate assigned as [Fe(TpivPP)(NO2)] on the basis of its EPR spectrum. The crystal structure
of one of the reaction products, [Fe(TpivPP)(NO3)], has been determined. The nitrate ion of [Fe(TpivPP)(NO3)]
is bound to the iron(III) ion in a “symmetric” bidentate fashion within the ligand-binding pocket of the porphyrin
pickets. Individual Fe-O distances are 2.123(3) and 2.226(3) Å. The dihedral angle between the plane of the
nitrate ion and the closest Np-Fe-Np plane is 10.0°. The Fe-Np bonds (and trans Np-Fe-Np angles)
perpendicular and parallel to the plane of the axial ligand average to 2.060(5) Å (154.84(9)°) and 2.083(3) Å
(146.14(9)°), respectively. Crystal data for [Fe(TpivPP)(NO3)]: a ) 23.530(2) Å, b ) 10.0822(5) Å, c ) 48.748-
(3) Å, â ) 92.145(5)°, monoclinic, space group I2/a, V ) 11556.4(14) Å3, Z ) 8, FeN9O7C64H64, 8798 observed
data, R1 ) 0.0606, wR2 ) 0.1313, all observations at 127(2) K.
Introduction
also reacts with the dioxygen complexes of ferrous Hb and Mb
to form the corresponding high-spin iron(III) hemoproteins and
Nitric oxide (NO), nitrite (NO2-), and nitrate (NO3-) are three
metabolically linked oxides of nitrogen found in the tissues and
organs of living organisms.1 Nitric oxide is produced in ViVo
in mammals by heme-containing nitric oxide synthases from
L-arginine and functions as a smooth muscle relaxant, modulat-
ing vascular tone in endothelial tissues and synaptic plasticity
in neural tissues.2 However, nitric oxide has a short half-life3
and is rapidly metabolized to nitrite and nitrate;4 the latter is
excreted in urine and may be used to gauge total NO synthase
activity.5 One mechanism that has been proposed for the
oxidation of NO to nitrate in ViVo involves hemoglobin.
Specifically, NO released from cells into the blood is bound by
ferrous hemoglobin to form HbNO6 which is then oxidized to
NO3- by molecular oxygen.1 Kinetic studies on the oxidation
of MbNO are in agreement with this mechanism.7 Free NO
- 8
NO3
. Nitrite ion is known to interact with several hemopro-
teins. The reaction of NO2- with oxyhemoglobin in ViVo results
in stoichiometric formation of nitrate and methemoglobin.5,9
Nitrite is also the substrate of assimilatory nitrite reductases
which contain an isobacteriochlorin prosthetic group (siroheme)
and an Fe4S4 cluster and catalyze the six-electron reduction of
nitrite ion to ammonia.10 The reduced hemoprotein binds nitrite
ion to form an iron(II)-nitrosyl complex which is also observed
as a steady state intermediate in the catalytic cycle of the
enzyme.11 The dissimilatory nitrite reductases contain two
c-type and two d1-type hemes (dioxoisobacteriochlorins)12 and
reduce nitrite ion to nitrous oxide, nitric oxide, or dinitrogen.13
Commercially, ascorbate reduction of metMb coupled with
(6) Abbreviations: 18C6, 1,4,7,10,13,16-hexaoxacyclooctadecane; DMF,
N, N′-dimethylformamide; K(222), potassium complex of 4,7,13,16,-
21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane; Mb, ferrous myo-
globin; metMb, ferric myoglobin; OEP, dianion of 2,3,7,8,12,13,17,18-
octaethylporphyrin; TPP, dianion of 5,10,15,20-tetraphenylporphyrin;
TpivPP, dianion of R,R,R,R -tetrakis(o-pivalamidophenyl)porphyrin.
(7) Arnold, E. V.; Bohle, D. S. Methods Enzymol. 1996, 269, 41.
(8) Eich, R. F.; Li, T.; Lemon, D. D.; Doherty, D. H.; Curry, S. R.; Aitken,
J. F.; Mathews, A. J.; Johnson, K. A.; Smith, R. D.; Phillips, G. N.,
Jr.; Olson, J. S. Biochemistry 1996, 35, 6976.
(9) Kosaka, H.; Imaizumi, K.; Imai, K.; Tyuma, I. Biochim. Biophys. Acta
1979, 581, 184.
(10) (a) Vega, J. M.; Kamin, H. J. Biol. Chem. 1977, 252, 896. (b) Aparicio,
P. J.; Knaff, D. B.; Malkin, R. Arch. Biochem. Biophys. 1975, 169,
102. (c) Coleman, K. J.; Cornish-Bowden, A.; Cole, J. A. Biochem.
J. 1978, 175, 483.
* To whom correspondence should be addressed. Electronic mail:
Scheidt.1@nd.edu.
(1) Inoue, M.; Minamiyama, Y.; Takemura, S. Methods Enzymol. 1996,
269, 474.
(2) (a) Hibbs, J. B., Jr.; Vavrin, Z.; Taintor, R. R. J. Immunol. 1987, 138,
550. (b) Hibbs, J. B., Jr.; Taintor, R. R.; Vavrin, Z. Science 1987,
235, 473. (c) Lyengar, R.; Stuehr, D. J.; Marletta, M. A. Proc. Natl.
Acad. Sci. U.S.A. 1987, 84, 6369.
(3) (a) Palmer, R. M. J.; Ashton, D. S.; Moncada, S. Nature (London)
1988, 333, 664. (b) Marletta, M. A.; Yoon, P. S.; Lyengar, R.; Leaf,
C. D.; Wishnok, J. S. Biochemistry 1988, 27, 8706.
(4) (a) Granger, D. L.; Hibbs, J. B., Jr.; Broadnax, L. M. J. Immunol.
1991, 146, 1294. (b) Hibbs, J. B., Jr.; Westenfelder, C.; Taintor, R.
R.; Vavrin, Z.; Kablitz, C.; Baranowski, R. L.; Ward, J. H.; Menlove,
R. L.; McMurry, M. P.; Kushner, J. P.; Samlowski, W. E. J. Clin.
InVest. 1992, 89, 867.
(11) Lancaster, J. R.; Vega, M. J.; Kamin, H.; Orme-Johnson, N. R.; Orme-
Johnson, W. H.; Krueger, R. J.; Siegel, L. M. J. Biol. Chem. 1979,
254, 1268.
(5) Granger, D. L.; Taintor, R. R.; Boockvar, K. S.; Hibbs, J. B., Jr.
Methods Enzymol. 1996, 268, 142.
(12) Chang, C. K. J. Biol. Chem. 1985, 260, 9520.
S0020-1669(97)00855-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/15/1998

(Nitro)Iron(III) Porphyrins
Inorganic Chemistry, Vol. 37, No. 9, 1998 2309
coordination of nitrite ion forms the basis of cured meat
preservation and coloration.14
mono(nitro) iron(III) complex, [Fe(TpivPP)(NO2)]. We have
used EPR spectroscopy to characterize the reaction system. Our
EPR data show the presence of an intermediate, possibly the
desired five-coordinate mono(nitro) iron(III) complex which is
a low-spin species and probably N-bound. Although the
porphyrin picket groups provide some protection for the FeIII-
NO2- moiety, this species is unstable and oxygen atom transfer
occurs, yielding [Fe(TpivPP)(NO3)] and [Fe(TpivPP)(NO)] as
final products. The molecular structure of [Fe(TpivPP)(NO3)]
has been determined from low-temperature X-ray diffraction
data.
Over the past decade, several studies on iron(III) porphyrins
in nonaqueous media in the presence of nitrite ion have
appeared. Fernandes et al.15 reported spectroscopic evidence
for stepwise formation of [Fe(TPP)(NO2)] and [Fe(TPP)(NO2)2]-
in DMF, with the five-coordinate low-spin N-bound nitro
complex apparently stable under these conditions. However,
we found that the addition of nitrite ion to a solution of [Fe-
(TPP)(OClO3)] in DMF produced the corresponding Fe(II)-
NO complex rather than [Fe(TPP)(NO2)] or [Fe(TPP)-
(NO2)2 ]-.16 Furthermore, formation of the iron(II) nitrosyl
species was accompanied by formation of the corresponding
nitrato complex, particularly at low nitrite/(porphinato)iron(III)
ratios. More recently, Castro et al.17 reported that [Fe(OEP)-
Cl] stoichiometrically inserts oxygen into substrates with allylic,
Experimental Section
General Information. All manipulations, except oxidation reac-
tions, were carried out under argon using a double-manifold vacuum
line, Schlenkware, and cannula techniques. Chloride ion was removed
from chlorobenzene by washing with sulfuric acid prior to drying by
distillation over P2O5. Methylene chloride and pyridine were distilled
over CaH2, and hexanes over sodium benzophenone. Boron trifluoride
etherate (Fisher) was distilled from a slight excess of diethyl ether over
CaH2 immediately before use.22 KNO2 (Fisher) was recrystallized twice
from distilled water, dried overnight at 75 °C, and stored under argon.
18-Crown-6 (Aldrich) was recrystallized from CH3CN and dried under
vacuum. The free base porphyrin, meso-R,R,R,R-tetrakis(o-pivala-
midophenyl)porphyrin (H2 TpivPP), and the corresponding chloro and
triflate iron(III) derivatives were synthesized using literature proce-
dures.20,23 [K(18C6)(OH2)][Fe(TpivPP)(NO2)2] was prepared from
[Fe(TpivPP)(OSO2CF3)(OH2)] using the method described by Nasri et
al.21
Electronic spectra were recorded with Perkin-Elmer Lambda 19 or
Lambda 6 spectrometers using chlorobenzene or methylene chloride
solutions in 1.0 and 0.1 cm pathlength cuvettes. IR spectra were
recorded with a Perkin-Elmer Model 883 spectrometer as Nujol mulls
on CsBr disks or as KBr pellets. EPR spectra were recorded at 77 K
with X-band Varian E-12 and Bruker ER 100E spectrometers. The
magnetic field was calibrated with an external DPPH standard (g )
2.0037). Solid samples were ground single crystals; frozen solution
spectra were obtained in chlorobenzene, methylene chloride, and DMF.
Freshly opened bottles of DMF were used, but no further attempts were
made to dry this solvent for EPR spectroscopy.
Attempted Synthesis of [Fe(TpivPP)(NO2)] from [Fe(TpivPP)-
(NO2)(Py)]. [Fe(TpivPP)(NO2)(Py)] was prepared from [K(18C6)-
(OH2)][Fe(TpivPP)(NO2)2] using an excess of pyridine in dry chlo-
robenzene as described previously.24 Attempts to remove iron-bound
pyridine from single crystals of [Fe(TpivPP)(NO2)(Py)] under vacuum
(<0.05 mmHg) overnight were unsuccessful; the EPR spectrum of
several crushed crystals remained unchanged ([Fe(TpivPP)(NO2)(Py)];
g1 ) 2.94, g2 ) 2.36, g3 ) 1.36).24 Finely ground (powder) samples
of [Fe(TpivPP)(NO2)(Py)] were equally inert to pyridine loss under
vacuum (<0.05 mmHg), both after 14 h at ambient temperature and
after 16 h at 73 °C. However, partial decomposition of [Fe(TpivPP)-
(NO2)(Py)] to form a high-spin product (g ) 5.66) and an {FeNO}7
species, [Fe(TpivPP)(NO)] (g3 ) 2.01, a3 ≈ 16 G), was observed after
16 h at 73 °C.25,26
benzylic, and aldehydic C-H bonds in the presence of a slight
-
excess (2-4 equiv) of NO2
. Other substrates similarly
oxidized included NO, O2, CO, Ph3P, and Me2S.18 Formation
of [FeII(OEP)(NO)], which is produced by oxygen atom transfer
from a species assumed to be [Fe(OEP)(NO2)], apparently drives
the reaction thermodynamically.17 Frangione et al.19 have
recently published the visible spectrum of a species assigned
as [Fe(TpivPP)(NO2)] in acetonitrile. However, their equilib-
-
rium model analysis of the NO2 binding equilibria suggested
that the “mono(nitro)” complex was in fact the six-coordinate
mixed-ligand species, [Fe(TpivPP)(NO2)(ClO4)]-.
Our earlier studies with [Fe(TPP)(OClO3)]16 and those of
Castro and co-workers with [Fe(OEP)Cl]17 suggest that, under
conditions which favor a nonzero concentration of the fiVe-
coordinate (nitro)iron(III) species (low nitrite ion concentration),
oxygen atom transfer from iron-bound nitrite to a variety of
substrates, including NO2 ,
- 16 occurs. This behavior appears to
be more limited in the presence of a large excess of nitrite ion,
particularly with sterically hindered porphyrins such as picket-
fence porphyrin20 (and [K(18C6)]+ as the counterion), since
we have been able to isolate and structurally characterize the
bis(nitro) iron(III) complex, [K(18C6)(H2O)][Fe(TpivPP)-
(NO2)2].21 However, the controversial question of whether a
bona fide mono(nitro) iron(III) porphyrin is stable enough to
be isolated and spectroscopically and/or structurally character-
ized clearly remains unanswered.
In the present work, we have specifically attempted to
synthesize the sterically protected five-coordinate N-bound
(13) (a) Firestone, M. K.; Firestone, R. B.; Tiedje, J. M. Biochem. Biophys.
Res. Commun. 1979, 91, 10. (b) Johnson, M. K.; Thomson, A. J.;
Walsh, T. A.; Barber, D.; Greenwood, C. Biochem. J. 1980, 189,
285.
(14) (a) Tsukahara, K.; Yamamoto, Y. J. Biochem. (Tokyo) 1983, 93, 15.
(b) Fox, J. B.; Thomson, J. S. Biochemistry 1963, 2, 465. (c) Giddings,
G. G. J. Food Sci. 1977, 42, 288. (d) Cassens, R. G.; Greaser, M. L.;
Ito, T.; Lee, M. Food Technol. 1979, 33, 46.
Reaction of [K(18C6)(OH2)][Fe(TpivPP)(NO2)2] with BF3‚OEt2.
To 28.1 mg (19 µmol) of dry, powdered [K(18C6)(OH2)][Fe(TpivPP)-
(15) Fernandes, J. B.; Feng, D. W.; Chang, A.; Keyser, A.; Ryan, M. D.
Inorg. Chem. 1986, 25, 2606.
(16) Finnegan, M. G.; Lappin, A. G.; Scheidt, W. R. Inorg. Chem. 1990,
29, 181.
(17) (a) O’Shea, S. K.; Wang, W.; Wade, R. S.; Castro, C. E. J. Org. Chem.
1996, 61, 6388. (b) Castro, C. E.; O’Shea, S. K. J. Org. Chem. 1995,
60, 1922.
(18) In the case of the putative five-coordinate (nitro)iron(III) complex [Fe-
(OEP)(NO2)], dioxygen appears to be reversibly oxidized by direct O
atom transfer from N-bound nitrite: (OEP)FeIIINO2 + O2 h (OEP)FeII-
NO + O3.17a
(19) Frangione, M.; Port, J.; Baldiwala, M.; Judd, A.; Galley, J.; DeVega,
M.; Linna, K.; Caron, L.; Anderson, E.; Goodwin, J. A. Inorg. Chem.
1997, 36, 1904.
(22) Fieser, L. F.; Fieser, M. Reagents for Organic Synthesis; John Wiley
& Sons: New York, 1967; Vol. 1, p 70.
(23) Gismelseed, A.; Bominaar, E. L.; Bill, E.; Trautwein, A. X.; Nasri,
H.; Doppelt, P.; Mandon, D.; Fischer, J.; Weiss, R. Inorg. Chem. 1990,
29, 2741.
(24) Nasri, H.; Wang, Y.; Huynh, B. H.; Walker, F. A.; Scheidt, W. R.
Inorg. Chem. 1991, 30, 1483.
(25) The {MNO}n notation is that of Enemark and Feltham: Enemark, J.
H.; Feltham, R. D. Coord. Chem. ReV. 1974, 13, 339. n corresponds
to the number of d electrons plus the unpaired electron of the NO
ligand.
(20) Collman, J. P.; Gagne, R. R.; Halbert, T. R.; Lang, G.; Robinson, W.
T. J. Am. Chem. Soc. 1975, 97, 1427.
(21) Nasri, H.; Goodwin, A.; Scheidt, W. R. Inorg. Chem. 1990, 29, 185.
(26) Given the nature of the product mixture observed by EPR spectroscopy,
no attempt was made to obtain a DSC thermogram of pyridine
thermolysis from [Fe(TpivPP)(NO2)(Py)].

2310 Inorganic Chemistry, Vol. 37, No. 9, 1998
Munro and Scheidt
Table 1. Crystallographic Data for [Fe(TpivPP)(NO3)]a
(NO2)2] in a Schlenk tube under argon was added ∼4 mL of
chlorobenzene and 3.0 µL (24 µmol) of freshly distilled BF3‚OEt2. The
deep red solution turned red-brown on swirling at ambient temperature.
The EPR spectrum of a reaction mixture aliquot drawn after 25 min
indicated the coexistence of three species: a high-spin species with a
signal at g ) 5.57 (with a slight shoulder to lower field, g ≈ 7.6), a
low-spin species with g1 ) 2.60, g2 ) 2.37, and g3 ≈ 1.7, and a nitrosyl
species with an anisotropic signal centered around g ≈ 2.06 (g3 ) 2.01,
a3 ≈ 17 G). After 48 h, the chlorobenzene solution was layered with
hexanes and crystallization was allowed to proceed in the dark; X-ray-
quality crystals formed after 3 days. A low-temperature X-ray
diffraction study revealed that one crystalline reaction product was [Fe-
(TpivPP)(NO3)]. UV-vis (C6H5Cl): λmax, nm (ꢀ, M-1 cm-1) 349
(2.60 × 104), 415 (7.55 × 104), 509 (9.0 × 103), 574 (3.4 × 103), 639
(2.0 × 103), 671 (1.7 × 103). IR (C6H5Cl): ν (NO3) 1520 cm-1 (s),
1250 cm-1 (br), 1047 cm-1 (m). IR (KBr pellet): ν(NO3) 1515 cm-1
(s), 1255 cm-1 (br), 1072 cm-1 (m).
formula
FW, amu
C64H64FeN9O7
1127.12
a, Å
b, Å
23.530(2)
10.0822(5)
c, Å
48.748(3)
92.145(5)
11556.4(14)
â, deg
V, Å3
space group
Z
I2/a
8
1.296
0.323
Dc, g/cm3
µ, mm-1
radiation
temperature, K
final R indices [I > 2σ (I)]b
final R indices (all data)
Mo KR, ) 0.710 73 Å
127(2)
R1 ) 0.0606, wR2 ) 0.1313
R1 ) 0.0994, wR2 ) 0.1546
a The estimated standard deviations of the least-significant digits are
given in parentheses. b R1 ) Σ||Fo| - |Fc||/Σ|Fo| and wR2 ) {Σ[w(Fo
2
EPR Study of the Reaction of [K(18C6)(OH2)][Fe(TpivPP)-
(NO2)2] with BF3‚OEt2. All Schlenk and EPR tubes were cooled to
2 ( 2 °C in ice-water baths, and solution transfers were carried out
under argon using cannula techniques. In a typical reaction, 3.7 µL
(30 µmol) of freshly distilled BF3‚OEt2 was injected into 7.6 mL of
dry chlorobenzene and the system allowed to attain thermal equilibrium.
The reaction was started by transferring the chlorobenzene solution of
BF3‚OEt2 to a second Schlenk tube containing 34.1 mg (23 µmol) of
powdered [K(18C6)(OH2)][Fe(TpivPP)(NO2)2]. Samples for EPR
spectroscopy (∼60-70 µL) were withdrawn from the reaction mixture
at set time intervals (up to 25.7 h) and, upon complete transfer,
immediately frozen in liquid nitrogen. An EPR spectrum of the reaction
mixture at t∞ was obtained after maintaining the solution at 4 °C for
10 days under argon. The reaction was also studied in CH2Cl2 and
DMF.
- Fc2)2]/Σ[wFo ]}1/2. R factors R1 are based on F, with F set to zero
4
for negative F2. The criterion of F2 > 2σ(F2) was used only for
calculating R1. R factors based on F2 (wR2) are statistically about twice
as large as those based on F.
in the least-squares process. Standard SHELXL-93 idealization
parameters were used. At the final stages of refinement, a difference
Fourier synthesis located several peaks with heights >1 e/Å3 near C(7),
C(8), C(11), and O(1), apparently due to a rotational disorder of the
o-pivalamido group about the C(6)-N(5) bond.31 The final refinement
converged to the R values given in Table 1. The maximum and
minimum electron densities on the final difference Fourier map were
0.61 and -0.39 e/Å3, respectively. Complete crystallographic details,
fractional atomic coordinates, anisotropic thermal parameters, bond
lengths, and bond angles for [Fe(TpivPP)(NO3)] are given in the
Supporting Information (Tables S1-S6).
Oxidation Studies. [Fe(TpivPP)(NO)] produced in the reaction of
[K(18C6)(OH2)][Fe(TpivPP)(NO2)2] with BF3‚OEt2 could be oxidized
in air or with pure oxygen. [K(18C6)(OH2)][Fe(TpivPP)(NO2)2] (32.5
mg, 22.0 µmol) was dissolved in ∼7 mL of chlorobenzene under argon,
and 3.2 µL (25.9 µmol) of BF3‚OEt2 was added. The solution was
stirred at 40 °C under argon overnight. An aliquot of the reaction
mixture was withdrawn for EPR spectroscopy (t0 sample), prior to
passing pure oxygen through the solution at 40 °C (stainless steel
needle) and periodically withdrawing and freezing additional samples.
Results and Discussion
Nitrite ion is known to bind to [Fe(TpivPP)(OSO2CF3)(OH2)]
in a stepwise manner.21 However, since the binding constants
K1 and K2 are comparable in magnitude, the bis(nitro) complex
is formed even at low concentrations of added nitrite ion.21
Because of the stability of low-spin [Fe(TpivPP)(NO2)2]-,
attempts to synthesize the five-coordinate mono(nitro) iron-
X-ray Diffraction Study of [Fe(TpivPP)(NO3)]. Single crystals
of [Fe(TpivPP)(NO3)] grown by slow diffusion of hexane into a
chlorobenzene solution of the metalloporphyrin were flat needles. A
dark purple-brown crystal with the dimensions 0.60 × 0.13 × 0.07
mm was mounted on the end of a glass fiber with its long axis
approximately collinear with the axis of the fiber. All measurements
were performed with graphite-monochromated Mo KR radiation (λh )
0.710 73 Å) on an Enraf-Nonius FAST area detector diffractometer at
127 K as described previously.27 Data were corrected for Lorentz and
polarization factors but not for absorption (µ ) 0.32 mm-1). A total
of 36 609 observed reflections (Fo g 2.0σ(Fo)) were measured and
averaged to 12 737 unique reflections.
-
(III) complex by direct addition of NO2 to a solution of
[Fe(TpivPP)(OSO2CF3)(OH2)] are unlikely to succeed. We
have therefore attempted to isolate [Fe(TpivPP)(NO2)] by (i)
stoichiometric removal of pyridine from single crystals and
powdered samples of the mixed-ligand complex [Fe(TpivPP)-
-
(NO2)(Py)] and by (ii) stoichiometric removal of NO2 from
[K(18C6)(OH2)][Fe(TpivPP)(NO2)2] in solution. The reactions
were characterized by EPR spectroscopy (IR, and electronic
spectroscopy when warranted) and a structure determination of
one product.
The structure was solved in the monoclinic space group I2/a with
the direct methods program XS,28 as implemented in SHELXTL.29 The
E-map and subsequent difference Fourier syntheses led to the location
of all non-hydrogen atoms, including those of the axial ligand. The
structure was refined against F2 using the program SHELXL-93.30 All
data collected were used including negative intensities. The metal and
porphyrin non-hydrogen atoms were refined anisotropically. A dif-
ference Fourier synthesis located the eight hydrogen atoms attached to
the porphyrin â carbons and most of the hydrogens of the o-
pivalamidophenyl groups; all were included as idealized contributors
Heating samples of [Fe(TpivPP)(NO2)(Py)] to 73 °C in Vacuo
resulted in partial loss of coordinated pyridine. However,
pyridine dissociation was always accompanied by O atom
transfer from iron-bound nitrite. Concomitant formation of a
nitrosyl species with an anisotropic EPR signal centered around
g ) 2.07 (g3 ) 2.01, a3 ≈ 16 G) and a high-spin species (g )
(31) The disorder was resolved by allowing atoms C(10) and C(10b) to
share the same site with the same anisotropic displacement parameters;
idealized hydrogens were not included for either atom. Atoms C(8)
and C(8b) were also constrained to have the same anisotropic
displacement parameters. The atom pairs O(1)/O(1b), C(9)/C(9b), and
C(11)/C(11b) were restrained to have the same Uij components to
within an effective standard deviation of 0.1. Idealized hydrogens were
not added to C(11b). Atoms C(7) and C(7b) were similarly treated,
but to within an effective standard deviation of 0.05. The site
occupancy factor for the two components of the disordered o-
pivalamido group was allowed to refine, converging to 0.839(5).
(27) Scheidt, W. R.; Turowska-Tyrk, I. Inorg. Chem. 1994, 33, 1314.
(28) (a) The direct methods component of the program XS implements
Sheldrick’s SHELX-90 “phase annealing” algorithm. Sheldrick, G.
M. Acta Crystallogr., Sect. A 1990, A46, 467.
(29) SHELXTL, Version 5; Siemens Industrial Automation, Inc., Analytical
Instrumentation: Madison, WI, 1994; Part Number 269-015900.
(30) Sheldrick, G. M. J. Appl. Crystallogr., in preparation.

(Nitro)Iron(III) Porphyrins
Inorganic Chemistry, Vol. 37, No. 9, 1998 2311
Figure 2. X-band EPR spectrum of the nitrosyl product produced in
the reaction between [K(18C6)(OH2)][Fe(TpivPP)(NO2)2] and BF3‚OEt2
after 2.7 h in chlorobenzene at ∼2 °C.
Figure 1. Selected X-band EPR spectra of samples (∼60 µL) of a
reaction mixture comprising a chlorobenzene solution of [K(18C6)-
(OH2)][Fe(TpivPP)(NO2)2] (initial concentration 3.0 mM) and BF3‚OEt2
(3.9 mM) at ∼2 °C. The times indicated are the freezing point times
after the start of the reaction.
(g1), 2.50 (g2), and 1.57 (g3).21 However, a new low-spin species
with g2 ) 2.36 and g3 ) 1.75 is also evident. A high-spin
species with g ) 5.50 indicates formation of a second product,
while the feature at g ∼ 2 suggests formation of a third product,
a nitrosyl species.
5.7) was observed in all cases. Irreversible formation of the
iron(II) nitrosyl species [Fe(TpivPP)(NO)]32 therefore appears
to be favored over simple dissociation of pyridine under these
conditions. The identity of the high-spin species produced in
the reaction is unclear; possible assignments include O-bound
nitrite, or, more likely,16 a nitrato complex produced by O atom
transfer to iron-bound nitrite. Importantly, no spectroscopic
evidence consistent with the formation of [Fe(TpivPP)(NO2)]
was obtained.
We anticipated that addition of 1 equiv of BF3‚OEt2 to a
chlorobenzene solution of [K(18C6)(OH2)][Fe(TpivPP)(NO2)2]
would lead to clean formation of a stable adduct between BF3
and NO2- and the desired five-coordinate complex [Fe(TpivPP)-
(NO2)]. The reaction of [K(18C6)(OH2)][Fe(TpivPP)(NO2)2]
with BF3‚OEt2 was initially studied at room temperature; the
EPR spectra of reaction mixture aliquots indicated the coexist-
ence of several products, including a high-spin species (g )
5.6), a nitrosyl species (g ≈ 2.01), and a new rhombic low-
spin iron(III) species (g1 ) 2.60, g2 ) 2.37). To differentiate
spectroscopically between this new low-spin species and the
low-spin starting material ([Fe(TpivPP)(NO2)2]- with g1 ) 2.7,
g2 ) 2.5, and g3 ) 1.6), subsequent reactions of [K(18C6)-
(OH2)][Fe(TpivPP)(NO2)2] with BF3‚OEt2 were studied at ∼2
°C to slow the rate of transformation of starting material to
products.
After 21 min, the concentrations of the three product species
have increased at the expense of the starting material; the
new low-spin species clearly has a rhombic EPR spectrum
with g1 ) 2.60, g2 ) 2.35, and g3 ) 1.75. The signal at
g ∼ 2 is sufficiently resolved to reveal a three-line hyperfine
splitting pattern, while the signal from the high-spin species
(g ) 5.61) is broad and exhibits a low-field shoulder (g ∼
6.8).
After 2.7 h, the signal from [Fe(TpivPP)(NO2)2]- is weaker
than that from the new rhombic low-spin species. Both the
high-spin product (g ) 5.68) and the species characterized by
the g ∼ 2 signal appear to have increased in concentration; an
expansion of the high-field signal (Figure 2) clearly indicates
that the third product has a near-rhombic g tensor with g1 )
2.11 (a1 ≈ 14 G), g2 ≈ 2.07 (a2 ≈ 21 G), and g3 ) 2.01 (a3 ≈
17 G). The three-line splitting pattern of the g3 signal is strong
evidence for a hyperfine interaction involving a single 14N
nucleus.33 Since the g values and hyperfine coupling constants
match those previously reported for [Fe(TpivPP)(NO)] in
chlorobenzene at 77 K32 and are similar to those reported by
Wayland and Olson for [Fe(TPP)(NO)],34 [Fe(TpivPP)(NO)] is
clearly formed in the reaction. This assignment is further
supported by the IR spectrum of the reaction mixture taken
under argon after 26 h which shows an NtO stretching band
at 1655 cm-1, consistent with the value of ν(NO) observed for
Reaction of [K(18C6)(OH2)][Fe(TpivPP)(NO2)2] with
BF3‚OEt2. Selected X-band EPR spectra showing the changes
in iron porphyrin species during the reaction of [K(18C6)(OH2)]-
[Fe(TpivPP)(NO2)2] with BF3‚OEt2 (1.3 molar equiv) in
chlorobenzene are shown in Figure 1. The dominant species
at ∼3 min is the rhombic low-spin starting material,
[Fe(TpivPP)(NO2)2]-, as evidenced by the three g values 2.70
solid samples (KBr mulls) of [Fe(TpivPP)(NO)] (1665 cm-1 32
and [Fe(TPP)(NO)] (1670 cm-1 35
and the values of ν(NO)
)
)
(33) Palmer, G. In Iron Porphyrins; Physical Bioinorganic Chemistry Series;
Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA,
1983; Part 2, pp 43-88.
(34) Wayland, B. B.; Olson, L. W. J. Am. Chem. Soc. 1974, 96, 6037.
(35) Scheidt, W. R.; Frisse, M. E. J. Am. Chem. Soc. 1975, 97, 17.
(32) Nasri, H.; Haller, K. J.; Wang, Y.; Huynh, B. H.; Scheidt, W. R. Inorg.
Chem. 1992, 31, 3459.

2312 Inorganic Chemistry, Vol. 37, No. 9, 1998
Munro and Scheidt
recently reported by Bohle and Hung36 for several five-
coordinate (nitrosyl)(porphinato)iron(II) complexes (1675-1705
cm-1).
(g ≈ 2.07). The EPR spectrum of the reaction mixture after
73 h indicates that a small amount of the low-spin intermediate
remains. (The spectrum is similar to that obtained after 25.7 h
in chlorobenzene.) Interestingly, the low field signal (g ≈ 5.6)
from [Fe(TpivPP)(NO3)] is sharper in methylene chloride than
in chlorobenzene, mainly as a result of the absence of the
shoulder at g ≈ 6.8.41
At long times (t ) 25.7 h), the EPR signal from
[Fe(TpivPP)(NO2)2]- is not observable, leaving only the new
rhombic low-spin species (as a minor component), the high-
spin product (g ) 5.68), and the species displaying the 14N
hyperfine interaction at g ∼ 2 ([Fe(TpivPP)(NO)]). This product
distribution, with the new rhombic low-spin species remaining
as a minor component, is observed at t∞ under argon. Three
possible assignments for the high-spin iron(III) product include
(i) a five-coordinate N-bound nitro complex, (ii) a five-
coordinate O-bound nitrito complex, and (iii) a mono- or
bidentate nitrato complex. Since the IR spectrum of the reaction
mixture shows three discrete bands at 1520 cm-1 (s), 1250
cm-1 (m), and 1047 cm-1 (m), and the IR spectrum of
The EPR spectra of Figure 1 clearly indicate that [Fe-
(TpivPP)(NO)] and [Fe(TpivPP)(NO3)] are generated early on
in the reaction. A possible pathway by which [Fe(TpivPP)-
(NO2)2]- reacts with BF3 to form [Fe(TpivPP)(NO)] and [Fe-
(TpivPP)(NO3)] is shown in eqs 1 and 2.
[FeIII(TpivPP)(NO2)2]- + BF3‚OEt2 f
[FeIII(TpivPP)(NO2)] + [BF3NO2]- (1)
[Fe(TpivPP)(NO2)2]- (with both nitro ligands N-bound) exhibits
2[FeIII(TpivPP)(NO2)] f
two NO2 stretching modes at 1351 and 1315 cm-1 21
,
a five-
-
[FeIII(TpivPP)(NO3)] + [FeII(TpivPP)(NO)] (2)
coordinate N-bound nitro complex can be ruled out. Further-
more, since [FeII(TpivPP)(NO2)]- is known to be low-spin,37
-
NO2 is likely to form a low-rather than a high-spin complex
In the first step, BF3‚OEt2 acts as a competing Lewis acid
for one of the nitrite ligands of [Fe(TpivPP)(NO2)2]-, leading
to the formation of a new low-spin mono(nitro)iron(III) complex
and a BF3-nitrite adduct. Although we have no direct
in the iron(III) derivative. Nitrito complexes of transition metal
ions normally show two well-separated stretching modes, ν-
(NdO) and ν(N-O), at 1485-1400 and 1110-1050 cm-1
,
respectively.38 Since the three bands observed for the high-
spin reaction product are inconsistent with these modes, linkage
isomerism of NO2- does not account for the IR data. The three
IR bands are in fact consistent with the three stretching modes
expected for C2V nitrate bound in a bidentate manner to the
metal.38,39 Moreover, the two highest frequency bands are close
in energy to those reported by Goff and co-workers for [Fe-
(TPP)(NO3)] (1544 and 1275 cm-1).40 This suggests that the
high-spin product of the reaction is [Fe(TpivPP)(NO3)].
The reaction of [K(18C6)(OH2)][Fe(TpivPP)(NO2)2] and
BF3‚OEt2 follows a similar course in methylene chloride (Figure
S1). However, the formation of the transient rhombic low-spin
intermediate proceeds more rapidly so that it is the dominant
species in solution after 3 min. The signal due to this species
then decays slowly, with a concomitant increase in the signal
from the high-spin product (g ) 5.62) and the nitrosyl species
-
experimental evidence that the NO2 ion is N-bound in this
iron porphyrin intermediate, three factors are consistent with
this notion: (i) NO2- is N-bound in the analogous low-spin iron-
(II) complex, [K(222)][Fe(TpivPP)(NO2)],37 (ii) five-coordinate
iron(III) porphyrins with a single O donor axial ligand are
usually high-spin42-52 or admixed-spin (S ) 3/2, 5/2)53,54 species,
rather than low-spin species, and (iii) a crystal field analysis of
the EPR spectrum55 yields relative crystal field splittings of ∆/λ
) 3.13 (tetragonality) and V/∆ ) 0.997 (rhombicity). The
decrease in rhombicity from V/∆ ) 1.3432 in [Fe(TpivPP)(NO2)2]-
is consistent with an N-bound mono(nitro) complex.
However, [FeIII(TpivPP)(NO2)] is a reactive species. In the
solvents used (chlorobenzene, CH2Cl2, and DMF), [FeIII-
(TpivPP)(NO2)] converts into [Fe(TpivPP)(NO)] and [Fe-
(TpivPP)(NO3)] (eq 2), consistent with the EPR and IR spectral
data. Furthermore, [Fe(TpivPP)(NO2)2]- in chlorobenzene
solution (Vide infra) reacts to give the same products under an
argon atmosphere. The ratio of [Fe(TpivPP)(NO3)] to [Fe-
(TpivPP)(NO)] produced in the reaction of [Fe(TpivPP)(NO2)2]-
with BF3‚OEt2 appears to be solvent dependent. Apparently
(36) Bohle, S. D.; Hung, C.-H. J. Am. Chem. Soc. 1995, 117, 9584.
(37) Nasri, H.; Wang, Y.; Huynh, B. H.; Scheidt, W. R. J. Am. Chem.
Soc. 1991, 113, 717.
(38) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 3rd ed.; Academic: New York, 1977; pp 244-
247.
(42) Lecomte, C.; Chadwick, D. L.; Coppens, P.; Stevens, E. D. Inorg.
Chem. 1983, 22, 2982.
(43) Goff, H. M.; Shimomura, E. T.; Lee, Y. J.; Scheidt, W. R. Inorg.
Chem. 1984, 23, 315.
(44) Cocolios, P.; Lagrange, G.; Guilard, R.; Oumous, H.; Lecomte, C. J.
Chem. Soc., Dalton Trans. 1984, 567.
(45) Scheidt, W. R.; Lee, Y. J.; Finnegan, M. G. Inorg. Chem. 1988, 27,
4725.
(46) Hatano, K.; Uno, T. Bull. Chem. Soc. Jpn. 1990, 63, 1825.
(47) Helms, A. M.; Jones, W. D.; McLendon, G. L. J. Coord. Chem. 1991,
23, 351.
(48) Gonzales, J. A.; Wilson, L. J. Inorg. Chem. 1994, 33, 1543.
(49) Cheng, B.; Hobbs, J. D.; Debrunner, P. G.; Erlebacher, J.; Shelnutt,
J. A.; Scheidt, W. R. Inorg. Chem. 1995, 34, 102.
(50) Bohle, D. S.; Conklin, B. J.; Hung, C.-H. Inorg. Chem. 1995, 34, 2569.
(51) Hoffman, A. B.; Collins, D. M.; Day, V. W.; Fleischer, E. B.;
Srivastava, T. S.; Hoard, J. L. J. Am. Chem. Soc. 1972, 94, 3620.
(52) Lay, K. L.; Buchler, J. W.; Kenny, J. E, Scheidt, W. R. Inorg. Chim.
Acta 1986, 123, 91.
(53) Reed, C. A.; Mashiko, T.; Bentley, S. P.; Kastner, M. E.; Scheidt, W.
R.; Spartalian, K.; Lang, G. J. Am. Chem. Soc. 1979, 101, 2948.
(54) Masuda, H.; Taga, T.; Osaki, K.; Sugimoto, H.; Yoshida, Z.-I.; Ogoshi,
H. Inorg. Chem. 1980, 19, 950.
(39) The large separation of the two highest frequency bands (270 cm-1
)
-
is consistent with a symmetric bidentate NO3 complex;38 smaller
separations are observed for unidentate nitrato complexes such as the
monoclinic form of [Fe(OEP)(ONO2)] which shows three NO3- modes
at 1515, 1385, and 1276 cm-1 (Munro, O. Q.; Scheidt, W. R.
Unpublished results).
(40) Phillippi, M. A.; Baenziger, N.; Goff, H. M. Inorg. Chem. 1981, 20,
3904.
(41) The changes in the EPR spectrum of the reaction mixture initially
comprising [K(18C6)(OH2)][Fe(TpivPP)(NO2)2] and BF3‚OEt2 in
DMF at 2 °C were less well resolved. However, clear evidence for
the formation of the high-spin species (g ) 5.68) and a nitrosyl
species (g ∼ 2) within the first 3 min of the reaction was obtained.
The signal from the rhombic low-spin starting material,
[Fe(TpivPP)(NO2)2]-, had g1 ) 2.74, g2 ) 2.47, and g3 ) 1.67 in
DMF. The broad, asymmetric shape of the g1 signal obtained from
[Fe(TpivPP)(NO2)2]- suggested that signals from two low-spin iron-
(III) species were overlapped in this region of the spectrum. After 3
h, the high-spin product (g ) 5.68) had a sharp derivative signal
which appeared larger than the g1 feature from the low-spin starting
material and the (nitrosyl)iron(II) signal. In contrast to the reaction in
chlorobenzene or methylene chloride, the signal from the nitrosyl
product never became the most intense spectral feature, even after 5
h.
(55) Taylor, C. P. S. Biochim. Biophys. Acta 1977, 491, 137. We have
assumed that “z” is along the heme normal.

(Nitro)Iron(III) Porphyrins
Inorganic Chemistry, Vol. 37, No. 9, 1998 2313
differing nitrate/nitrosyl ratios were observed in chlorobenzene
(Figure 1 and CH2Cl2 (Figure S1). Since colorless crystals of
[K(18C6)][BF3‚NO3] were isolated56 along with crystals of [Fe-
(TpivPP)(NO3)], a third reaction (eq 3) apparently competes
with eq 2 for [FeIII(TpivPP)(NO2)] to form [FeII(TpivPP)(NO)].
[FeIII(TpivPP)(NO2)] + [BF3‚NO2]- f
[FeII(TpivPP)(NO)] + [BF3‚NO3]- (3)
-
Oxygen atom transfer from iron-bound nitrite to free NO2 is
well-known16 and appears to be favored to some extent here,
particularly in chlorobenzene.
Importantly, eqs 2 and 3, i.e., oxygen atom transfer from [FeIII-
(TpivPP)(NO2)], are consistent with (i) the tendency of related
RuIII-NO2 complexes to undergo O atom transfer to produce
RuIII-NO3 and RuII-NO,57 (ii) our previous spectroscopic
observations that [FeIII(TPP)(NO2)] readily forms [FeII(TPP)-
(NO)] in the presence of an O atom acceptor such as free nitrite
ion,16 and (iii) the recent findings of Castro and co-workers17
that [Fe(OEP)Cl] stoichiometrically oxidizes both inorganic and
organic substrates in the presence of a slight excess of nitrite
ion.58 The general significance of eqs 2 and 3 is that studies
aimed at using or characterizing five-coordinate nitro complexes
of iron(III) porphyrins are likely to be hampered by the inherent
instability of this species.
Oxidation Studies. Nitric oxide complexes of iron(II)
porphyrins are relatively unstable under aerobic conditions,
being oxidized by dioxygen to the corresponding iron(III)
nitrates.16 In the case of hemoproteins such as MbNO, dioxygen
reacts with coordinated NO to give an N-bound peroxynitrite
intermediate which then decays to metMbNO3.7 [Fe(TpivPP)-
(NO)] produced in the reaction of [K(18C6)(OH2)][Fe(TpivPP)-
(NO2)2] with BF3‚OEt2 under argon could be removed by
oxidation over a period of ∼60 h at 40 °C with gaseous oxygen.
X-band EPR spectra of aliquots from the reaction in chloroben-
zene are shown in Figure 3. Three significant time-dependent
changes are evident: (i) the signal due to [Fe(TpivPP)(NO)]
centered at g ∼ 2.07 decays with time, (ii) the signal
characteristic of the low-spin iron(III) intermediate increases
in intensity before diminishing to zero as the reaction approaches
Figure 3. Selected X-band EPR spectra showing oxidation (40 °C,
chlorobenzene) of the compound characterized by a 14N hyperfine-
split signal centered at g ∼ 2.07. The starting solution contains the
two products of the anaerobic reaction of [K(18C6)(OH2)][Fe(TpivPP)-
(NO2)2] and BF3‚OEt2 at t∞ as well as some residual [Fe(TpivPP)(NO2)].
match those of [Fe(TpivPP)(NO2)] (Figure 1), a simple scheme
which qualitatively accounts for the principal species observed
during the oxidation of [Fe(TpivPP)(NO)] is given in eqs 4 and
5.
2[FeII(TpivPP)(NO)] + O2 f 2[FeIII(TpivPP)(NO2)] (4)
5
completion, and (iii) the signal at low field from S ) /2 [Fe-
(TpivPP)(NO3)] (g ) 5.84) increases in intensity until it is the
principal component after 36 h. The growth and subsequent
decay of the rhombic low-spin iron(III) EPR signal suggests
that a porphyrin intermediate is produced upon oxidation of [Fe-
(TpivPP)(NO)] and ultimately consumed in the formation of
[Fe(TpivPP)(NO3)]. Since the g values of this intermediate
2[FeIII(TpivPP)(NO2)] f
[FeIII(TpivPP)(NO3)] + [FeII(TpivPP)(NO)] (5)
The FeII-NO complex produced in the second step (eq 5) is
then reoxidized until conversion to [Fe(TpivPP)(NO3)] is
complete. The EPR spectra shown in Figure 3 indicate that
the final product of the reaction is [Fe(TpivPP)(NO3)], which
is readily crystallized from chlorobenzene. Oxidation of [Fe-
(TpivPP)(NO)] is considerably slower than oxidation of [Fe-
(TPP)(NO)],16 consistent with coordination and steric protection
of NO within the pocket of the porphyrin.32 Alternatively, the
NO in-pocket/out-of-pocket equilibration rate could be slow.
Oxidation of metallonitrosyls to nitro complexes is not
unprecedented. For example, Clarkson and Basolo59 have
shown that the mechanism of oxidation of several CoII-NO
salen analogues is first order in both metal nitrosyl and oxygen
and leads to the formation of an N-bound peroxynitrite
intermediate. This species rapidly attacks a second metal
(56) The IR spectrum (KBr mull) of colorless crystals of [K(18C6)]-
[BF3‚NO3] showed diagnostic absorption bands at 2922, 2856, and
1493 cm-1 (m, -CH2 -), 1733 cm-1 (br, B-O), 1385 cm-1 (s, NO3-),
1105 cm-1 (s, CH2-O-CH2), and 962 cm-1 (m, B-F). The K(18C6)
-
and NO3 modes assigned here are in good agreement with those
reported for the structurally characterized (2S,6S)-2,6-dimethyl-1,4,7,-
10,13,16-hexaoxacyclooctadecane complex of KNO3 (Dyer, R. B.;
Ghirardelli, R. G.; Palmer, R. A.; Holt, E. M. Inorg. Chem. 1986, 25,
3184).
(57) (a) Keene, F. R.; Salmon, D. J.; Meyer, T. J. J. Am. Chem. Soc. 1977,
99, 2384. (b) Keene, F. R.; Salmon, D. J.; Walsh, J. L.; Abruner, H.
D.; Meyer, T. J. J. Am. Chem. Soc. 1977, 99, 4821. (c) Keene, F. R.;
Salmon, D. J.; Walsh, J. L.; Abruner, H. D.; Meyer, T. J. Inorg. Chem.
1980, 19, 1896.
(58) Depending on the values of K1 and K2 for nitrite coordination by [Fe-
(OEP)Cl] in N-methylpyrrolidone-1% acetic acid solution, a slight
excess (2-4 molar equiv)17 of [K(18C6)][NO2] presumably results
in a significant fraction of the reactive five-coordinate mono(nitro)
species, [Fe(OEP)(NO2)], in addition to the six-coordinate bis(nitro)
complex.
(59) (a) Clarkson, S. G.; Basolo, F. Inorg. Chem. 1973, 12, 1528. (b)
Clarkson, S. G.; Basolo, F. J. Chem. Soc., Chem. Commun. 1972, 670.

2314 Inorganic Chemistry, Vol. 37, No. 9, 1998
Munro and Scheidt
nitrosyl to form an unstable peroxo-bridged intermediate (N-
O-O-N) which decomposes to the nitro complex.
EPR Study of the Decomposition of [K(18C6)(OH2)][Fe-
(TpivPP)(NO2)2]. An initially 2.52 mM chlorobenzene solution
of [K(18C6)(OH2)][Fe(TpivPP)(NO2)2] was investigated by
periodically withdrawing samples, freezing, and running the EPR
spectrum.60 The EPR spectrum of the solution 3.5 min after
dissolution indicated formation of three new species; a high-
spin species with g ) 5.64, a rhombic low-spin species (g1 )
2.60, g2 ) 2.36, and g3 ) 1.77) spectroscopically distinct from
the starting material (g1 ) 2.70, g2 ≈ 2.5, and g3 ) 1.56), and
a species with a three-line 14N hyperfine-split signal at g ∼ 2.01.
The intensities of the product signals increased with time
(as in the reaction with BF3‚OEt2); after 7.5 min, the most
intense signal centered at g ∼ 2.08 had a similar 14N hyperfine
splitting pattern to that observed in the reaction of [K(18C6)-
(OH2)][Fe(TpivPP)(NO2)2] with BF3‚OEt2 (Figure 2). The
g values (and hyperfine coupling constants) of this product
were g1 ≈ 2.11 (a1 ≈ 14 G), g2 ≈ 2.08 (a2 ≈ 24 G), and g3
) 2.01 (a3 ≈ 17 G). The spectrum of the solution after 28 h
at room temperature was similar to that shown in Figure 1 for
the product mixture generated with BF3‚OEt2 after 25.7 h at
2 °C. These observations suggest that partial dissociation of
nitrite ion from the bis(nitro) complex affords [Fe(TpivPP)-
(NO2)], which undergoes oxygen atom transfer to yield
[Fe(TpivPP)(NO3)] and [Fe(TpivPP)(NO)] as before. In
contrast, the synthesis of [K(18C6)(H2O)][Fe(TpivPP)(NO2)2]
uses a large excess of NO2-, which clearly minimizes the
concentration of the reactive five-coordinate iron(III) species
and allows isolation of the stable bis(nitro) complex.21 It is
noteworthy that [FeII(TpivPP)(NO2)], unlike the mono(nitro)
iron(III) derivative, is stable under anaerobic conditions in
chlorobenzene.37
Figure 4. ORTEP diagram showing a labeled edge-on view of the
structure of [Fe(TpivPP)(NO3)] (50% probability surfaces). Hydrogen
atoms have been omitted for clarity. The near-perpendicular orientation
of the bidentate nitrato ligand relative to the N(2)-Fe-N(4) plane is
clearly evident (the dihedral angle is 80.3°).
Molecular Structure of [Fe(TpivPP)(NO3)]. The identity
of the high-spin iron(III) product generated in the reaction of
[K(18C6)(OH2)][Fe(TpivPP)(NO2)2] with BF3‚OEt2 and in the
reaction of [Fe(TpivPP)(NO)] with dioxygen was established
unambiguously from low-temperature X-ray diffraction mea-
surements. Figure 4 shows an ORTEP drawing of [Fe(TpivPP)-
(NO3)]. The nitrate ion occupies the coordination position
within the pocket of the porphyrin and is bound to the iron in
a near-symmetric bidentate fashion with individual Fe-O
distances of 2.226(3) and 2.123(3) Å. Averaged values of the
bond distances and bond angles within the porphyrin core are
shown in Figure 5. Individual values of bond distances and
angles for [Fe(TpivPP)(NO3)] are given in the Supporting
Information, and selected bond lengths and angles within the
coordination group are shown in Table 2.
As shown in Figure 6, the dihedral angles between the plane
of the axial nitrato ligand and the N(1)-Fe-N(3) and N(2)-
Fe-N(4) planes are 10.0 and 80.3°, respectively; the coordinat-
ing nitrate oxygens are thus positioned approximately above
the N(1)-Fe and N(3)-Fe bonds. The mean plane of the nitrate
ion is essentially orthogonal (89.1°) to the 24-atom porphyrin
mean plane. The trans Np-Fe-Np angles are significantly
different, with N(1)-Fe-N(3) ) 146.14(9)° and N(2)-Fe-
N(4) ) 154.84(9)° (Table 2). Commensurate differences in the
Fe-Np bond distances are observed; those involving the more
acute angle (N(1)-Fe-N(3)) average 2.083(3) Å and are longer
Figure 5. Formal diagram of the porphinato core of [Fe(TpivPP)-
(NO3)] illustrating the displacement of each unique atom (in units of
0.01 Å) from the mean plane of the 24-atom core. Positive values of
displacement are toward the axial nitrato ligand. Also shown on the
diagram are the averaged values of each chemically unique class of
bond distance and angle in the porphinato core. The orientation of
the axial nitrato ligand and individual Fe-Np bond distances are
indicated.
(by ∼4 standard deviations) than those involving the more
obtuse angle which average 2.060(5) Å. This distortion of the
coordination geometry apparently reflects both the bidentate
mode of nitrate coordination and the orientation of the axial
ligand relative to the Fe-Np bonds.
The iron(III) ion is displaced 0.61 Å from the 24-atom mean
plane of the porphyrin core toward the axial nitrato ligand
(Figure 5). The porphinato core displays a combination of
doming and S4 ruffling as evidenced by (i) the tilt of the four
pyrrole rings relative to the porphyrin mean plane (5.6, 6.6, 5.5,
and 3.1° for the rings containing N(1) to N(4), respectively)
and (ii) the displacement of the meso-carbons alternately above
and below the porphyrin mean plane (Figure 5). The dihedral
angles between the meso-phenyl groups and the porphyrin mean
plane are 70.7, 82.7, 72.0, and 81.6° for the phenyl rings attached
to meso-carbons 1 to 4, respectively. Figure 6 shows that the
amide groups of the pivalamido substituents are not coplanar
(60) The solution of [K(18C6)(OH2)][Fe(TpivPP)(NO2)2] used for this study
was prepared by rapidly dissolving ground crystals of [K(18C6)(OH2)]-
[Fe(TpivPP)(NO2)2] in chlorobenzene under argon; the solution
therefore does not contain an excess of NO2-, unlike solutions used
for growing crystals of [K(18C6)(OH2)][Fe(TpivPP)(NO2)2] from
chlorobenzene and hexane.21

(Nitro)Iron(III) Porphyrins
Inorganic Chemistry, Vol. 37, No. 9, 1998 2315
found for the bidentate peroxo ligands of [Ti(OEP)(O2)],61
[K(K222)][Mn(TPP)(O2)],62 and [Mo(TTP)(O2)2].63 The nitrate
orientation in [Fe(TpivPP)(NO3)] apparently leads to the statisti-
cally significant difference between the N(1)-Fe-N(3) and
N(2)-Fe-N(4) angles. This distortion of the coordination
sphere geometry is also observed in the two nominally five-
coordinate peroxo complexes, [Ti(OEP)(O2)]61 and [K(K222)]-
[Mn(TPP)(O2)].62 Interestingly, the short Fe-Np distances
observed for the bonds perpendicular to the nitrate ion in [Fe-
(TpivPP)(NO3)] parallel the coordination geometry in [Ti(OEP)-
(O2)]61 but not that in [Mn(TPP)(O2)]-,62 which shows the
reverse pattern (long Mn-Np bonds perpendicular to the Mn-
peroxo plane).
Table 2. Selected Bond Lengths and Angles for
[Fe(TpivPP)(NO3)]a
(A) Bond Lengths
Fe-N(1)
Fe-N(3)
Fe-O(5)
N(9)-O(5)
N(9)-O(7)
2.085(2)
2.081(2)
2.123(3)
1.271(4)
1.214(3)
Fe-N(2)
Fe-N(4)
Fe-O(6)
N(9)-O(6)
2.063(2)
2.056(2)
2.226(3)
1.252(4)
(B) Bond Angles
N(1)-Fe-N(2)
N(1)-Fe-N(4)
N(2)-Fe-N(4)
N(4)-Fe-O(5)
N(3)-Fe-O(5)
N(4)-Fe-O(6)
N(3)-Fe-O(6)
O(5)-Fe-O(6)
N(9)-O(5)-Fe
86.54(9)
86.28(9)
154.85(9)
97.70(10) N(2)-Fe-O(5)
80.20(10) N(1)-Fe-O(5)
106.66(10) N(2)-Fe-O(6)
136.93(9)
57.75(10) N(9)-O(5)-Fe
96.9(2)
N(1)-Fe-N(3)
N(2)-Fe-N(3)
N(3)-Fe-N(4)
146.14(9)
86.34(9)
86.29(9)
104.68(10)
133.56(10)
95.06(10)
76.70(9)
92.5(2)
Currently, there are four structurally characterized (nitrato)-
(porphinato)iron(III) complexes: [Fe(TpivPP)(NO3)], [Fe(TPP)-
(NO3)],40 triclinic [Fe(OEP)(NO3)],64 and a monoclinic form of
[Fe(OEP)(NO3)].65 All have unique nitrate coordination ge-
ometries. These range from “symmetric” monodentate in the
monoclinic form of [Fe(OEP)(NO3)]65 to “symmetric” bidentate
in [Fe(TpivPP)(NO3)]. This changing mode of nitrate coordina-
tion is accompanied by an increase in the out-of-plane displace-
ment of the iron(III) ion (0.45 f 0.61 Å) and a concomitant
increase in the mean Fe-Np bond distance from 2.047(6) Å in
monoclinic [Fe(OEP)(NO3)] to 2.071(14) Å in [Fe(TpivPP)-
(NO3)]. The Fe-Np bonds become more asymmetric as the
mode of nitrate coordination switches from mono- to bidentate.
Each species has a unique EPR spectrum which is apparently
related to the symmetry of the iron(III) center; the complete
electronic structures of these species are currently under
investigation and will be reported elsewhere.
N(1)-Fe-O(6)
O(7)-N(9)-O(5) 112.8(3)
O(7)-N(9)-O(6) 124.0(3)
O(7)-N(9)-O(5) 123.1(3)
a The estimated standard deviations of the least-significant digits are
given in parentheses.
Conclusions
The reaction of [K(18C6)(OH2)][Fe(TpivPP)(NO2)2] with
BF3‚OEt2 under argon leads to the formation of a new low-
spin iron(III) porphyrin species assigned on the basis of its
EPR spectrum in several solvents as the N-bound mono-
(nitro) complex, [Fe(TpivPP)(NO2)]. This five-coordinate
iron(III) nitro species is, however, reactive and readily trans-
fers an oxygen atom to form [FeII(TpivPP)(NO)] and [FeIII-
(TpivPP)(NO3)]. The low-temperature X-ray structure of [Fe-
(TpivPP)(NO3)] shows that the nitrate ion is bound within the
pocket of the porphyrin in a near-symmetric bidentate fashion
in which the coordinating oxygens are positioned approximately
over a trans pair of Fe-Np bonds. This mode of nitrate
coordination considerably lowers the symmetry of the iron(III)
center.
Figure 6. ORTEP diagram showing the structure of [Fe(TpivPP)(NO3)]
(50% probability surfaces) looking down the heme normal from the
same side as the axial nitrato ligand. Hydrogen atoms have been omitted
for clarity. The dihedral angle between the axial ligand and N(2)-
Fe-N(4) planes is 10.0°.
with the phenyl rings to which they are bonded; the individual
dihedral angles are 35.5, 80.1, 58.6, and 16.1° for the amide
linkages of phenyl rings 1 to 4, respectively. The o-pivalamido
group of the phenyl ring attached to C(m1) is disordered,
displaying two orientations relative to the plane of the phenyl
group, apparently as a result of rotation about the C(6)-N(5)
bond. The dihedral angle between the amide group of the major
component (84%) and the minor component (16%) is 61.4°.
An ORTEP diagram showing the minor component of the
disordered group is given in the Supporting Information (Figure
S2).
Acknowledgment. We thank the National Institutes of
Health for support of this research under Grant GM-38401
(W.R.S). Funds for the purchase of the FAST area detector
diffractometer were provided through NIH Grant RR-06709.
We thank Dr. G. J. Ferraudi of the Radiation Laboratory
(University of Notre Dame) for the use of the Bruker EPR
spectrometer.
(61) Guilard, R.; Latour, J.-C.; Lecomte, C.; Marchon, J.-C.; Protas, J.;
Ripoll, D. Inorg. Chem. 1978, 17, 1228.
(62) VanAtta, R. B.; Strouse, C. E.; Hanson, L. K.; Valentine, J. S. J. Am.
Chem. Soc. 1987, 109, 1425.
(63) Chevrier, B.; Diebold, T.; Weiss, R. Inorg. Chim. Acta 1976, 19, L57.
(64) Ellison, M. K.; Shang, M.; Kim, J.; Scheidt, W. R. Acta Crystallogr.,
Sect. C 1996, C52, 3040.
(65) The monoclinic form of [Fe(OEP)(NO3)] has an Fe-O distance of
1.966(2) Å and a mean Fe-Np distance of 2.047(6) Å; the iron(III)
ion is displaced 0.45 Å from the 24-atom porphyrin mean plane.
(Munro, O. Q.; Scheidt, W. R. Unpublished results.).
The structure of [Fe(TpivPP)(NO3)] is unusual in several
respects. Although bidentate coordination of nitrate is known
for [Fe(TPP)(NO3)],40 the plane of the nitrate ion in the TPP
derivative bisects an opposite pair of cis Np-Fe-Np angles.
The acute nitrate orientation (10°) observed in [Fe(TpivPP)-
(NO3)] is clearly more akin to the eclipsing orientations (0°)

2316 Inorganic Chemistry, Vol. 37, No. 9, 1998
Munro and Scheidt
Supporting Information Available: Complete crystallographic
details, atomic coordinates, anisotropic thermal parameters, fixed
hydrogen atom coordinates, bond lengths, and bond angles for [Fe-
(TpivPP)(NO3)] (Tables S1-S6); X-band EPR spectra following the
reaction of [K(18C6)(OH2)][Fe(TpivPP)(NO2)2] and BF3‚OEt2 in CH2-
Cl2 (Figure S1); and an ORTEP diagram of [Fe(TpivPP)(NO3)] showing
the rotational disorder of one o-pivalamido group (Figure S2) (18
pages). An X-ray crystallographic file, in CIF format, is available on
the Internet only. Ordering and access information is given on any
current masthead page.
IC970855L