Sharing the π-bonding. An iron porphyrin derivative with trans, π- accepting axial ligands. Synthesis, EPR and Mossbauer spectra, and molecular structure of two forms of the complex nitronitrosyl(α,α,α,α-tetrakis(o- pivalamidophenyl)porphinato)ferrate(II)

Article abstract of DOI:10.1021/ja963871a

The reaction of the iron(II) picket fence porphyrin derivative [Fe(TpivPP)(NO₂)]^- (Nasri et al. J. Am. Chem. Soc. 1991, 113, 717) with NO has been examined. The result is a new hexacoordinated iron(II) porphyrin complex, [Fe(TpivPP)(NO₂)(NO)]^-. This compound has been characterized by UV-vis, IR, EPR, and Mossbauer spectroscopies. All data show that the species is low spin (S = 1/4 ). The EPR spectrum of [Fe(TpivPP)(NO₂)(NO)]^-(frozen chlorobenzene solution) is similar to that of five-coordinate nitrosyl derivatives of iron(II) porphyrinates with g(x) = 2.085, g(y) = 2.032, g(z) = 2.01, a(x) (¹4N) = 24 G, a(y) = 16.5 G, and a(z) = 16 G. Two crystalline forms of [Fe(TpivPP)(NO₂)(NO)]^- were obtained and their molecular structures determined. In both crystalline forms, the axial nitro ligand is inside the 'pocket' of the picket fence porphyrin derivative. In the first crystalline form isolated, there are two independent molecules, one of which has disorder in the nitro and nitrosyl groups while the other molecule has completely ordered axial ligands. In this first form, the plane of the nitro group is nearly perpendicular to the plane containing the nitrosyl group and the iron. In the second crystalline form, however, the two axial ligand planes are nearly parallel. Mossbauer investigations (solid state) show that the iron centers in the two different crystalline forms have different electronic properties. The Mossbauer parameters for the perpendicular form are δ = 0.22 mm/s and ΔE(q) = 1.78 mm/s (200 K) and for the parallel form δ = 0.35 mm/s and ΔE(q) = 1.2 mm/s (4.2 K). Mossbauer data are also presented for a related complex: five-coordinate [Fe(TPP)(NO)] (TPP = mesotetraphenylporphinato). IR spectra for the two forms also reveal distinctly different interactions of the trans (nitro) ligand.

Full text of DOI:10.1021/ja963871a

6274
J. Am. Chem. Soc. 1997, 119, 6274-6283
Sharing the π-Bonding. An Iron Porphyrin Derivative with
Trans, π-Accepting Axial Ligands. Synthesis, EPR and
Mo¨ssbauer Spectra, and Molecular Structure of Two Forms of
the Complex Nitronitrosyl(R,R,R,R-tetrakis(o-pivalamidophenyl)-
porphinato)ferrate(II)
Habib Nasri,†,‡ Mary K. Ellison,† Shuxian Chen,§ Boi Hanh Huynh,*,§ and
W. Robert Scheidt*,†
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556, and Department of Physics, Emory UniVersity,
Atlanta, Georgia 30322
ReceiVed NoVember 7, 1996. ReVised Manuscript ReceiVed March 15, 1997X
Abstract: The reaction of the iron(II) picket fence porphyrin derivative [Fe(TpivPP)(NO2)]- (Nasri et al. J. Am.
Chem. Soc. 1991, 113, 717) with NO has been examined. The result is a new hexacoordinated iron(II) porphyrin
complex, [Fe(TpivPP)(NO2)(NO)]-. This compound has been characterized by UV-vis, IR, EPR, and Mo¨ssbauer
spectroscopies. All data show that the species is low spin (S ) 1/2). The EPR spectrum of [Fe(TpivPP)(NO2)(NO)]-
(frozen chlorobenzene solution) is similar to that of five-coordinate nitrosyl derivatives of iron(II) porphyrinates
with gx ) 2.085, gy ) 2.032, gz ) 2.01, ax (14N) ) 24 G, ay ) 16.5 G, and az ) 16 G. Two crystalline forms of
[Fe(TpivPP)(NO2)(NO)]- were obtained and their molecular structures determined. In both crystalline forms, the
axial nitro ligand is inside the “pocket” of the picket fence porphyrin derivative. In the first crystalline form isolated,
there are two independent molecules, one of which has disorder in the nitro and nitrosyl groups while the other
molecule has completely ordered axial ligands. In this first form, the plane of the nitro group is nearly perpendicular
to the plane containing the nitrosyl group and the iron. In the second crystalline form, however, the two axial ligand
planes are nearly parallel. Mo¨ssbauer investigations (solid state) show that the iron centers in the two different
crystalline forms have different electronic properties. The Mo¨ssbauer parameters for the perpendicular form are δ
) 0.22 mm/s and ∆Eq ) 1.78 mm/s (200 K) and for the parallel form δ ) 0.35 mm/s and ∆Eq ) 1.2 mm/s (4.2 K).
Mo¨ssbauer data are also presented for a related complex: five-coordinate [Fe(TPP)(NO)] (TPP ) meso-
tetraphenylporphinato). IR spectra for the two forms also reveal distinctly different interactions of the trans (nitro)
ligand.
Introduction
Enzymology series.4 It is clear that understanding of the
biological functions has been greatly aided by earlier studies
of the interaction of nitric oxide with iron porphyrinates that
are cited below.
There has been a surge of renewed interest in the chemistry
of metal complexes with nitrogen oxide (NO) ligands, especially
those of iron porphyrin derivatives and nitric oxide. This is
the result of the important roles the ligands play in biological
systems which include, among others, neurotransmission,
vascular dilation, cytotoxicity, and activation of biochemical
processes.1 Recently a NO-bound ferric hemoprotein was found
in a blood-sucking insect that uses the protein to aid in its need
to obtain a blood meal.2 There is of course, a biological
production enzyme, NO-synthase, based on a heme protein.3
Indeed, the biological roles of NO are so extensive that they
have been the subject of two recent volumes in the Methods in
The focus in the first surge of interest in nitric oxide
complexes was in understanding the bonding between metal and
the diatomic ligand NO. The unusual nature of the geometry
of the M-N-O linkage, which was found to be sometimes
linear and sometimes bent,5 is one important feature of the
bonding in these species. The controlling factor in the bonding
geometry is the electronic configuration, which in terms of the
suggested Enemark and Feltham notation5 can be expressed as
{MNO}n where n is the number of d electrons on the metal
plus the unpaired electron from NO. For five- or six-coordinate
nitrosylmetalloporphyrin derivatives, when n e 6, as in the
manganese(II) derivatives [Mn(TTP)(NO)]6,7 and [Mn(TPP)(4-
MePip)(NO)]8 and the iron(III) derivatives [Fe(OEP)(NO)]+ and
[Fe(TPP)(H2O)(NO)]+,9 the M-N-O linkage is linear. In these
porphyrin derivatives, the five-coordinate derivatives have a high
affinity for a sixth axial ligand. For cases where n ) 8, as in
five-coordinate [Co(TPP)(NO)],10 the Co-N-O linkage is
strongly bent. Furthermore, there is little or no affinity for
another axial ligand and only five-coordinate species are known.
† University of Notre Dame.
‡ Present address: Faculte´ des Sciences de Monastir, 5000 Monastir,
Tunisia.
§ Emory University.
X Abstract published in AdVance ACS Abstracts, June 15, 1997.
(1) Snyder, S. H. Science 1992, 257, 494. Butler, A. R.; Williams, D. L.
H. Chem. Soc. ReV. 1993, 233. Yu, A. E.; Hu, S.; Spiro, T. G.; Burstyn, J.
N. J. Am. Chem. Soc. 1994, 116, 4117. Ignarro, L. J. Circ. Res. 1989, 65,
1. Moncada, S.; Palmer, R. M. J.; Higgs, E. A. Pharmacol. ReV. 1991, 43,
109.
(2) Ribeiro, J. M. C.; Hazzard, J. M. H.; Nussenzveig, R.; Champagne,
D.; Walker, F. A. Science 1993, 539-541.
(3) Richards, M. K; Clague, M. J.; Marletta, M. A. Biochemistry 1996,
35, 7772 and references cited therein.
(4) Nitric Oxide Parts A and B in Methods in Enzymology; Packer, L.,
Ed.; Academic Press: San Diego, CA, 1996; Vols. 268 and 269.
(5) Enemark, J. H.; Feltham, R. D. Coord. Chem. ReV. 1974, 13, 339.
S0002-7863(96)03871-1 CCC: $14.00 © 1997 American Chemical Society

Sharing π-Bonding
J. Am. Chem. Soc., Vol. 119, No. 27, 1997 6275
Experimental Section
The intermediate system, where n ) 7, is exemplified by iron-
(II) derivatives. Five-coordinate [Fe(TPP)(NO)]11 and the six-
coordinate derivatives [Fe(TPP)(1-MeIm)(NO)]12 and [Fe(TPP)-
(4-MePip)(NO)]13 (two crystalline forms) have intermediate
values for the Fe-N-O angle. In these derivatives, it is clear
that the coordinated nitric oxide ligand exerts a strong trans
effect; the Fe-N bond distances trans to NO are g0.20 Å longer
than normal. Moreover, there are large variations in the trans
Fe-N distance as seen in the two forms of [Fe(TPP)(4-MePip)-
(NO)],13 where these distances are 2.328(10) and 2.463(7) Å.
General Information. All manipulations were carried out under
argon using a double-manifold vacuum line, Schlenkware, and cannula
techniques. Chlorobenzene was purified by washing with sulfuric acid
and then distilled over P2O5. Pentane was distilled over CaH2. Toluene,
benzene, and hexanes were distilled over sodium benzophenone. KNO2
was recrystallized twice from distilled water, dried overnight at 75 °C,
and stored under argon. Kryptofix-222 (Aldrich) was recrystallized
from benzene and stored under argon in the dark. NO gas was purified
by passing it through a KOH column or through 4A molecular sieves
immersed in a dry ice/ethanol slush bath to remove higher oxides of
nitrogen.19 The free base, meso-R,R,R,R-(o-pivalamidophenyl)porphy-
rin (H2TpivPP) and the corresponding iron(III) chloro and triflate
derivatives were synthesized by literature methods.20,21
UV-vis spectra were recorded on a Perkin-Elmer Lambda 6
spectrometer and IR spectra on a Perkin-Elmer model 883 as KBr pellets
and Nujol mulls. EPR spectra were obtained at 77 K on a Varian E-12
spectrometer operating at X-band. Both the strong- and weak-field
Mo¨ssbauer spectrometers were operated in a constant acceleration mode
with a transmission arrangement and have been described elsewhere.22
Zero velocity of the Mo¨ssbauer spectra are referred to the centroid of
the room temperature spectrum of a metallic iron foil. Samples of
[Fe(TpivPP)(NO2)(NO)]- for Mo¨ssbauer spectroscopy were prepared
by immobilization of the crystalline material (crystals not ground) in
paraffin wax (m.p. 78° C) in the dry box.
The M-N(NO) bond distances in the porphyrin derivatives
are all quite short and vary in the order Mn(II) = Fe(III) <
Fe(II) < Co(II). These distances, along with the observation
that all of the above complexes are low-spin species, are
manifestations of the strong M f NO π-bonding. The
π-accepting NO ligand forms either one π-bond (in the bent
systems) or two π-bonds (in the linear systems). The nitrite
complexes of iron(II)14 and iron(III)15-17 porphyrins are also
found to have electronic structures that are only consistent with
the N-bound nitrite ligands acting as strong π-accepting ligands.
The five-coordinate iron(II) complex [Fe(TpivPP)(NO2)]-14 is
especially noteworthy. This complex has a quite short Fe-
N(NO2) bond and an unusually large quadrupole splitting; both
result from the extremely strong M f N(NO2) π-bonding which
leads to strong differentiation between the two dπ orbitals of
the iron(II) porphyrin.
Preparation of [K(222)][Fe(TpivPP)(NO2)(NO)]. [Fe(TpivPP)(SO3-
CF3)(H2O)] (100 mg, 0.08 mmol) and about 1 mL of zinc amalgam
were stirred for 1 h under argon in 10 mL of C6H5Cl. This deep red
solution ([FeII(TpivPP)]) was then filtered into a second solution that
was made by stirring (overnight) 300 mg of Kryptofix-222 (0.8 mmol)
and 207 mg of KNO2 (2.4 mmol) in 10 mL of C6H5Cl. A stream of
NO gas was passed (for about 15 min) through this red-yellow solution
of the pentacoordinate iron(II) species [Fe(TpivPP)(NO2)]-.14
To further explore these issues of bonding and electronic
structure, we have prepared and characterized new (porphinato)-
iron(II) complexes that have nitric oxide and nitrite as the trans
axial ligands. The syntheses of these {FeNO}7 complexes
begins with iron(II) nitrite derivatives, and to avoid possible
problems with reactions of coordinated nitrite seen in iron(III)
systems,18 picket fence porphyrin was used as the porphyrin
ligand. The complexes have been characterized by a combina-
tion of spectroscopic methods and by X-ray diffraction studies.
Two different crystalline forms of [K(222)][Fe(TpivPP)(NO2)-
(NO)] have been prepared; the two structural forms further
demonstrate the importance of relative axial ligand orientation
effects in defining electronic structure in iron porphyrins.
The color changes to light red as a result of forming the hexacoor-
dinate product [Fe(TpivPP)(NO2)(NO)]-. Single crystals of this
complex were prepared by slow diffusion of dry pentane into the
chlorobenzene solution. A mass of single crystals of the desired
complex and excess KNO2 and Kryptofix-222 resulted. Crystals for
X-ray analysis were selected from the mass under the microscope.
Mo¨ssbauer samples were prepared by collecting the mass on a fritted-
glass filter, washing with pentane, and then washing with a small
quantity of degassed H2O, leaving bulk crystalline material. Alterna-
tively, the mass was washed directly in the reaction vessel and wash
solvents were removed by cannula filtration. To our knowledge, each
bulk sample was homogenous in iron(II) product. However, two
different crystalline species, each with the same ligand set, were
obtained. The initial crystalline form was obtained several times over
the course of several months, followed approximately 1 year later by
the production of a second crystalline form. This second crystalline
form has then been exclusively obtained in all subsequent experiments.
UV-vis in C6H5Cl λmax (log ꢀ) 544 (4.01), 426 (4.96). EPR (frozen
chlorobenzene solution): gx ≈ 2.1, gy ≈ 2.07, and gz ) 2.01. IR (KBr)
(form 1): ν (NO) 1616 (m) cm-1; ν (NO2-) 1380 (w) cm-1, 1310 (m)
cm-1. IR (Nujol) (form 2): ν (NO) 1668 (s) cm-1; ν (NO2-) 1346
(6) Abbreviations: Porph, a generalized porphyrin dianion; TpivPP, the
dianion of meso-R,R,R,R-tetrakis(o-pivalamidophenyl)porphyrin; TPP, di-
anion of meso-tetraphenylporphyrin; T p-OCH3PP, dianion of meso-tetrakis-
(p-methoxyphenyl)porphyrin; TTP, the dianion of meso-tetratolylporphyrin;
OEP, the dianion of octaethylporphyrin; TMP, the dianion of meso-
tetramesitylporphyrin; OBTPP, dianion of octabromotetraphenylporphyrin;
TDCPP, dianion of meso-tetrakis(2,6-dichlorophenyl)porphyrin; Kryptofix-
222 or 222, 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane;
4-MePip, 4-methylpiperidine; Py, pyridine; HIm, imidazole; PMS, pen-
tamethylene sulfide; 1-MeIm, 1-methylimidazole; EPR, electron paramag-
netic resonance; Np, porphyrinato nitrogen.
(7) Scheidt, W. R.; Hatano, K.; Rupprecht, G. A.; Piciulo, P. L. Inorg.
Chem. 1979, 18, 292.
(8) Piciulo, P. L.; Rupprecht, G.; Scheidt, W. R. J. Am. Chem. Soc. 1974,
96, 5293.
(9) Scheidt, W. R.; Lee, Y. J.; Hatano, K. J. Am. Chem. Soc. 1984, 106,
3191.
(10) Scheidt, W. R.; Hoard, J. L. J. Am. Chem. Soc. 1973, 95, 8281.
(11) Scheidt, W. R.; Frisse, M. E. J. Am. Chem. Soc. 1975, 97, 17.
(12) Piciulo, P. L.; Scheidt, W. R. J. Am. Chem. Soc. 1976, 98, 1913.
(13) Scheidt, W. R.; Brinegar, A. C.; Ferro, E. B.; Kirner, J. F. J. Am.
Chem. Soc. 1977, 99, 7315.
(m) cm-1, 1305 (m) cm-1
.
X-ray Diffraction Studies of [Fe(TpivPP)(NO2)(NO)]-.23 Both
crystalline forms of this complex have been used in X-ray diffraction
studies. A summary of crystal data, intensity collection data, and least-
squares refinement parameters of these new hexacoordinate iron(II)
porphyrin species are given in Tables 1 and S1 (Supporting Informa-
tion).
Investigations on the first crystalline form were carried out at 124
K using graphite-monochromated Mo KR radiation on an Enraf-Nonius
CAD4 diffractometer. Data were processed with the Blessing suite24
(14) Nasri, H.; Wang, Y.; Huynh, B. H.; Scheidt, W. R. J. Am. Chem.
Soc. 1991, 113, 717.
(15) Nasri, H.; Goodwin, J. A.; Scheidt, W. R. Inorg. Chem. 1990, 29,
185.
(16) Nasri, H.; Wang, Y.; Huynh, B. H.; Walker, F. A.; Scheidt, W. R.
Inorg. Chem. 1991, 30, 1483.
(17) Nasri, H.; Haller, K. J.; Wang, Y.; Huynh, B. H.; Scheidt, W. R.
Inorg. Chem. 1992, 31, 3459.
(19) Dodd, R. E.; Robinson, P. L. Experimental Inorganic Chemistry;
Elsevier: New York, 1957; pp 233-234.
(20) Collman, J. P.; Gagne, R. R.; Halbert, T. R.; Lang, G; Robinson,
W. T. J. Am. Chem. Soc. 1975, 97, 1427.
(21) Gismelseed, A.; Bominaar, E. L.; Bill, E.; Trautwein, A. X.; Nasri,
H.; Doppelt, P.; Mandon, D; Fischer, J.; Weiss, R. Inorg. Chem. 1992, 31,
1845.
(18) Finnegan, M. G.; Lappin, A. G.; Scheidt, W. R. Inorg. Chem. 1990,
29, 181.
(22) Kretchmar, S. A.; Teixeira, M.; Huynh, B. H.; Raymond, K. N.
Biol. Met. 1988, 1, 26.

6276 J. Am. Chem. Soc., Vol. 119, No. 27, 1997
Table 1. Crystallographic Details
Nasri et al.
form 1
form 2
formula
[K(O6N2C18H36)][Fe(O4N8C64H64)-
(NO)(NO2)]‚(C5H12)0.36‚(C6H5Cl)0.64
1654.7
[K(O6N2C18H36)][Fe(O4N8C64H64)-
(NO)(NO2)]‚(C5H5Cl)0.5‚(H2O)0.5
1622.0
FW
a, Å
b, Å
c, Å
33.34(3)
18.80(5)
27.10(4)
95.30(9)
16916(56)
8
13.070(12)
21.502(19)
31.848(29)
98.21(5)
8858(25)
4
â, deg
V, Å3
Z
space group
P21/c
1.301
P21/n
1.22
Dc, g/cm3
F(000)
7024
0.317
3436
0.308
µ, mm-1
crystal dimens, mm
diffractometer
λ, Å
0.80 × 0.30 × 0.23
0.59 × 0.43 × 0.39
CAD4
FAST
0.71073
124
0.71073
127
T, K
total data colld
no. of unique data
no. of unique obsd data
[I > 2σ(I)]
final R indices [I > 2σ(I)]
final R indices (for all data)
27181
26673 (Rint ) 0.0386)
18562
26458 (Rint ) 0.029)
9398
R1 ) 0.0947; wR2a ) 0.1904
R1 ) 0.1498; wR2a ) 0.3179
R1 ) 0.094; wR2b ) 0.107
a Based on F2. b Based on F.
of data reduction programs. Very narrow diffraction profiles were used
to avoid reflection overlap. The structure was solved in the monoclinic
space group P21/c with the program MULTAN. There are two
[Fe(TpivPP)(NO2)(NO)]- anions and two potassium Kryptofix-222
counterions in the asymmetric unit of structure. The final structural
refinement was made with F2 data with the program SHELXL-93,25 in
which all data collected were used, including reflections with negative
intensities. One porphyrin, one cryptand, and one chlorobenzene
molecule are completely ordered. There is disorder in the axial ligands
of the second porphyrin molecule. The oxygen atom of the nitrosyl
ligand is disordered on two positions with refined occupancies of 0.77-
(4) and 0.23(4). The NO2 group displays linkage isomerism as it is
coordinated through an oxygen atom (60%) or a nitrogen atom (40%).
The coordinated atom (O, N) occupies the same site in both cases, and
the scattering factor of nitrogen was used for this fully occupied atom.
A tert-butyl group of a picket occupies two alternate positions with
converged occupancies of 0.66(2) and 0.34(2). One strand of the second
cryptand molecule is disordered on two positions with refined oc-
cupancy coefficients converged to 0.727(10) and 0.273(10). A pentane
and chlorobenzene molecule alternately occupy the same site. The
occupancy of the pentane molecule converged to 0.728(7). The
chlorobenzene molecule was refined as a rigid body with an occupancy
of 0.272(7) at convergence. All non-hydrogen atoms were refined
anisotropically except the minor component of the disordered tert-butyl
group, the minor component as well as several atoms of the major
component of the disordered cryptand strand, and the partially occupied
chlorobenzene molecule. All hydrogen atoms were idealized with the
0.095 and wR2 ) 0.190.26 A final difference Fourier map was judged
to be significantly free of features with the largest positive peak having
a height of 0.90 e/Å3. Complete listings of atomic coordinates, thermal
parameters, and final hydrogen atom coordinates are given in the
Supporting Information (Tables S2, S3, and S4).
A second form was subsequently obtained with the same crystal-
lization methods (and the same experimenter). A dark purple crystal
of this form with dimensions 0.39 × 0.43 × 0.59 mm was mounted on
the end of a glass 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. Our detailed
methods and procedures for small molecule X-ray data collection with
the FAST system have been described.27 Data were corrected for
Lorentz and polarization factors but not for absorption. A total of
26 458 observed reflections (Fo g 2.0σ(Fo)) were measured and
averaged to 9398 unique reflections.
The structure of this crystalline form was solved in the centrosym-
metric space group P21/n with the direct methods program MULTAN.
The E-map and subsequent difference Fourier syntheses led to the
location of all heavy atoms. A chlorobenzene and a water solvent
molecule were found; the two molecules share the same region of the
unit cell and must be alternately populated (occupancies were fixed at
0.5 for both molecules). Thus the complete formula for the structure
is [K(222)][Fe(TpivPP)(NO2)(NO)]‚1/2C6H5Cl‚1/2H2O. A minor dis-
order was found for the nitro and nitrosyl axial ligands. The
occupancies of the second positions of the two oxygen atoms of the
nitro group and the oxygen atom of the nitrosyl were fixed at 0.1. The
orientation of each minor moiety is approximately orthogonal to its
corresponding major one. After full-matrix least-squares refinement
was carried to convergence, a difference Fourier suggested possible
locations for most hydrogen atoms of the Kryptofix-222 and the
porphyrin. Hydrogen atoms were included in subsequent cycles of the
least-squares refinement as fixed, idealized contributions (C-H ) 0.95
Å, N-H ) 0.90 Å, B(H) ) B(C,N) × 1.3). Final cycles of full-matrix
least-squares refinement used anisotropic temperature factors for all
heavy atoms except (i) the oxygen atoms of the second positions of
the nitro and nitrosyl groups: O(5b), O(6b), and O(7b), and (ii) C(63),
standard SHELXL-93 idealization methods. At convergence, R1
)
(23) Programs used in this study include (a) MADNES routines for FAST
data collection (J. W. Pflugrath, Cold Spring Harbor, and A. Messerschmidt,
Max-Plank-Institute fu¨r Biochemie, Martinsried, unpublished) and data
reduction the program ABSURD (I. Tickle, Birbeck College, and P. Evans,
MRC, unpublished), (b) direct method program MULTAN (Main, P.; Hull,
S. E.; Lessinger, L.; Germain, G.; Declerq, J. P.; Woolfson, M. M.
MULTAN, a system of computer programs for the automatic solution of
the crystal structures from X-ray diffraction data. Universities of York, U.K.,
and Louvain, Belgium), (c) Zalkin’s FORDAP for difference Fourier
syntheses, (d) local modified least-square refinement (Lapp, R. L.; Jacobson,
R. A. ALLS, a generalized crystallographic least-squares program. National
Technical Information Services IS-4708 UC-4, Springfield, VA), (e) Busing
and Levy’s ORFFE and ORFLS and Johnson’s ORTEP2 and ORTEP3, (f)
atomic form factors from: Cromer, D. T.; Mann, J. B. Acta Crystallogr.,
Sect. A 1968, 24, 321, real and imaginary corrections for anomalous
dispersion in the form factor of iron and the potassium and the chlorine
atoms from Cromer, D. T.; Liberman, D. J. J. Chem. Phys. 1970, 53, 1891,
and scattering factors for hydrogen from Stewart, R. F.; Davidson, E. R.;
Simpson, W. T. J. Chem. Phys. 1959, 42, 3175.
2 2
4
(26) R1 ) ∑||Fo| - |Fc||/|Fo| and wR2 ) {∑[w(Fo2 - Fc ) ]/∑ [wFo ]}1/2
(for refinement with F2 data). The conventional R factor R1 is based on F,
with F set to zero 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, and R factors based on ALL data will
be even larger. wR2 ) [∑w(|Fo| - |Fc|)2/∑w(Fo)2]1/2 (for refinement with
data on F).
(27) Scheidt, W. R.; Turowska-Tyrk, I. Inorg. Chem. 1994, 33, 1314.
(28) Mu, X. H.; Kadish, K. M. Inorg. Chem. 1988, 27, 4720.
(29) Nasri, H.; Scheidt, W. R. Manuscript in preparation.
(24) Blessing, R. H. Crystallogr. ReV. 1987, 1, 3.
(25) Sheldrick, G. M. J. Appl. Crystallogr. Manuscript in preparation.

Sharing π-Bonding
J. Am. Chem. Soc., Vol. 119, No. 27, 1997 6277
Table 2. Electronic Spectral Data for Selected Iron(II) and Iron(III) Tetraarylporphyrinsa
λ
max (nm)
Soret region (log ꢀ)
R, â region (log ꢀ)
complex
ref
A. Five-Coordinate Iron(II)
475 (sh)
477 (sh) (4.35)
477 (sh)
433 (sh) (4.94)
[Fe(TPP)(NO)]b
406 (4.97)
538 (4.00)
539 (4.12)
539 (3.04)
543 (sh) (3.89) 567 (4.02)
604 (3.45) 11
607 (3.62) 17
611 (3.84) 28
608 (3.59) 14
[Fe(TpivPP)(NO)]c
[Fe(TMP)(NO)]d
407 (5.15)
408 (4.96)
444 (5.12)
[Fe(TpivPP)(NO2)]- c
B. Six-Coordinate Iron(II)
478 (sh) (4.14) 544 (4.01)
[Fe(TpivPP)(NO2)(NO)]- c
[Fe(TpivPP)(NO2)(Py)]- c
[Fe(TpivPP)(NO2)(PMS)]- c
[Fe(TPP)(NO)(1-MeIm)]b
410 (sh) (4.91) 426 (4.96)
413 (sh) (4.83) 430 (5.23)
418 (sh) (4.88) 432 (5.07)
415 (5.26)
this work
533 (4.21)
537 (4.18)
545 (3.99)
560 (sh) (3.66) 653 (2.83) 29
558 (sh) (3.81) 653 (3.12) 29
580 (sh) (3.78) 643 (3.05) 12
460 (4.26)
C. Five-, Six-Coordinate Iron(III)
[Fe(TpivPP)(NO2)2]- c
[Fe(TpivPP)(NO2)(Py)]c
[Fe(TPP)(NO)]+ d,e
364 (sh)
379 (sh)
426 (5.11)
464 (sh)
459 (sh)
553 (4.15)
550 (4.11)
546 (4.01)
15
16
28
36a
f
422 (5.26)
411 (4.85)
433 (5.34)
437
680 (sh)
[Fe(TPP)(NO2)(NO)]b
[Fe(T p-OCH3PP)(NO2)(NO)]c
[Fe(TpivPP)(NO2)(NO)]c
510 (sh) (3.70) 545 (4.20)
577 (sh) (3.70)
586
581
549
542
432
g
a All spectra were taken at 25 °C. b Chloroform solution. c Chlorobenzene solution. d Dichloromethane solution. e Thin-layer spectroelectrochemical
oxidation of the corresponding Fe(II) species. f Ellison, unpublished. g Nasri, unpublished.
C(64), and C(68) of the chlorobenzene solvent. At convergence, R1 )
0.094 and wR2 ) 0.107 and the final data/variable ratio was 8.8.26
A
final difference Fourier map was judged to be significantly free of
features with the largest positive peak having a height of 0.89 e/Å3.
Final atomic coordinates, anisotropic and isotropic thermal parameters
for all heavy atoms, and fixed hydrogen atom coordinates are available
in the Supporting Information (Tables S5, S6, and S7).
Results and Discussion
The
synthesis
of
the
novel
complex
anion
[Fe(TpivPP)(NO2)(NO)]- with two (trans) axial nitrogen oxide
ligands is straightforward but requires substantial experimental
care owing to the oxygen sensitivity of the product and
intermediates in solution. The preparative reaction makes use
of five-coordinate [K(222)][Fe(TpivPP)(NO2)],14 prepared in situ
by reaction of [FeII(TpivPP)] with an excess of Kryptofix-222-
solubilized KNO2, followed by reaction with gaseous NO. The
complex has been characterized by IR, UV-vis, EPR, and
Mo¨ssbauer spectroscopy as well as by single-crystal structure
determinations. The product, [Fe(TpivPP)(NO2)(NO)]-, appears
quite stable once it has been isolated (as single crystals) although
crystals of [K(222)][Fe(TpivPP)(NO2)(NO)]‚1/2C6H5Cl‚1/2H2O
do seem to effloresce solvent but not NO on long standing (>3
mo). Grinding the crystalline product, however, does appear
to cause a loss of NO. In solution, in the presence of excess
nitrite ion, the complex appears to be easily oxidized to initially
yield the bis(nitro)iron(III) complex [Fe(TpivPP)(NO2)2]-. The
instability of solutions of the nitrite-iron(III) porphyrin system
has been previously noted,18 and it is to be expected that other
products will also ensue. In the absence of oxygen, the
[Fe(TpivPP)(NO2)(NO)]- anion is a stable species that persists
in solution as a six-coordinate complex. As shown in Table 2
and Figure 1, the spectral properties of this six-coordinate,
mixed-ligand complex are distinctively different from all other
known iron(II) or -(III) tetraarylporphyrinate derivatives con-
taining nitrite or nitrosyl ligands. It can be seen from the data
in Table 2 that a Soret band near 430 nm is characteristic of
six-coordinate species containing a good π-accepting ligand.
The UV-vis spectral data thus show that it is highly unlikely
that either of the axial ligands of [Fe(TpivPP)(NO2)(NO)]-
dissociates in chlorobenzene solution. The strong similarities
of the solid-state and frozen solution EPR spectra are also highly
suggestive concerning this point.
Figure 1.
UV-vis spectra of [Fe(TpivPP)(NO2)(NO)]-,
[Fe(TpivPP)(NO2)]-, and [Fe(TpivPP)(NO)] (chlorobenzene solution,
approximately equivalent concentrations).
forms differ in the relative orientation of axial ligands and
solvent content; the effect of ligand orientation appears to have
significant effects on the electronic structure. As further detailed
below, solid-state Mo¨ssbauer spectra of the two crystalline forms
clearly show that the two are different not only at the single-
crystal level but at the level of the bulk samples. Crystals of
form 1 contain two independent complex anions in the asym-
metric unit of structure. Their structures are illustrated in
Figures 2 (anion 1) and 3 (anion 2). The ORTEP diagram
shown for anion 2 also displays the disorder that results from
the linkage isomerism of binding the nitrite anion. The common
labeling scheme used for the two anions is given in Figure 4,
which is an alternate view of anion 1. The structure of the
complex anion found in crystal form 2 is given in Figure 5.
The ligands to iron are the four porphinato nitrogen atoms, the
nitrogen atom of nitric oxide, and a donor atom from nitrite
ion. In two complex anions, the nitrite ion donor atom is solely
nitrogen, while in the third case, the axial NO2- ligand exhibits
disorder with the donor atom being either a nitrogen or an
oxygen atom. In this complex, there is also a correlated disorder
in the tert-butyl group of one of the pickets. It is seen that the
nitrite anion is in the ligand binding pocket of picket fence
porphyrin in all three complex anions.
Two crystalline forms of [Fe(TpivPP)(NO2)(NO)]- have been
obtained and studied by X-ray diffraction. The two crystalline

6278 J. Am. Chem. Soc., Vol. 119, No. 27, 1997
Nasri et al.
coordination group of the three independent complex anions.
Differences in bond distances between the three anions in
[Fe(TpivPP)(NO2)(NO)]- are small, and only those of the Fe-
N(NO) distances are possibly significant. In all three complex
anions, the nitric oxide bonds in a bent fashion; the Fe-N-O
angles are all near 137°. Figure 6 gives formal diagrams for
the three porphinato cores in the two crystalline forms. The
diagrams display the displacement of each atom (in units of
0.01 Å) in the core from the mean plane of the 24-atom core.
The porphinato ring in all anions shows S4 ruffling. In all three
complex anions, the iron atom is slightly displaced (0.08-0.10
Å) out of the plane toward the nitric oxide ligand. The diagrams
also display averaged30 values of each chemically distinct bond
distance and angle type in the core. Finally, Figure 6 gives the
-
orientation of the NO2 and FeNO planes with respect to the
-
porphinato core. The projection of the NO2 plane onto the
porphinato core is seen to lead to near bisecting of an Np-Fe-
Figure 2. ORTEP diagram illustrating the structure of the
[Fe(TpivPP)(NO2)(NO)]- anion (30% probability ellipsoids, form 1,
anion 1). The two ordered axial ligands have been labeled. The angle
between the NO2 and the Fe-N-O planes is 85.4°.
Np angle in both crystalline forms. However, in each of the
-
form 1 complexes, the angle between the NO2 and FeNO
planes is seen to be near 90°, while in form 2 the analogous
angle is 21°.
Metal complexes with nitrogen oxide ligands that have
nitrogen atoms in two different oxidation states are relatively
rare, and only a few have been structurally characterized. All
such complexes have nitrosyl/nitrite ligand mixtures.31-37
Although [Fe(TpivPP)(NO2)(NO)]- should thus be described
as an unusual species, it should be noted that the analogous
iron(III) species [Fe(TPP)(NO2)(NO)] has been known for some
time,36 although it has not been structurally characterized. All
of the [Fe(TpivPP)(NO2)(NO)]- complex anions are found to
have a coordinated bent nitrosyl; two are found to have trans
N-bound nitrite exclusively, while the third has an unusual
linkage isomerism disorder with both O- and N-bound nitrite.
All of the previously structurally characterized mixed ligand
species have N-bound nitrosyl ligands; however, many have
an O-bound (nitrito) rather than an N-bound (nitro) NO2- ligand.
The metal-nitrosyl (M-N-O) angle is generally close to linear.
The majority of the known mixed complexes would be
considered low-valent species. A major point of interest in such
mixed species is their potential in metal nitrosyl oxidations of
organic substrates.38 Interestingly, many, but not all, of the
mixed ligand species can be prepared by oxygen atom transfer
reactions of coordinated nitrite and nitric oxide.31,32 The only
iron complex32 is an iron(III) complex with cis nitrosyl and
Figure 3. ORTEP diagram illustrating the structure of the
[Fe(TpivPP)(NO2)(NO)]- anion (30% probability ellipsoids, form 1,
anion 2). The disorder in the two axial ligands is shown.
(30) The numbers in parentheses following each averaged value is the
estimated standard deviation calculated on the assumption that all averaged
values are drawn from the same population.
(31) Kriege-Simondsen, J.; Elbaze, G.; Dartiguenave, M.; Feltham, R.
D.; Dartiguenave, Y. Inorg. Chem. 1982, 21, 230. Kriege-Simondsen, J.;
Feltham, R. D. Inorg. Chim. Acta 1983, 71, 185.
(32) Ileperuma, O. A.; Feltham, R. D. Inorg. Chem. 1977, 16, 1876.
(33) Wester, D.; Edwards, R. C.; Busch, D. Inorg. Chem. 1977, 16, 1055.
(34) (a) Abraham, F.; Nowogrocki, G.; Sueur, S.; Bremard, C. Acta
Crystallogr., Sect. C 1983, C39, 838. (b) Carrondo, T.; Rudolph, P. R.;
Skapski, A. C.; Thornback, J. R.; Wilkinson, G. Inorg. Chim. Acta 1977,
24, L95.
(35) Lukehart, C. M.; Troup, J. M. Inorg. Chim. Acta 1977, 22, 81.
Wilson, R. D.; Bau, R. J. Organomet. Chem. 1980, 191, 123. Calderon, J.
L.; Cotton, F. A.; DeBoer, B. G.; Maretinez, N. J. Chem. Soc., Chem.
Commun. 1971, 1476. Bell, L. K.: Mason, J.; Mingos, D. M. P.; Tew, D.
G. Inorg. Chem. 1983, 22, 3497.
(36) (a) Yoshimura, T. Inorg. Chim. Acta 1984, 83, 17. (b) Settin, M.
F.; Fanning, J. C. Inorg. Chem. 1988, 27, 1431.
(37) (a) Fajer, J. Personal communication. (b) Kadish, K. M.; Adamian,
V. A.; Caemelbecke, E. V.; Tan, Z.; Tagliatesta, P.; Bianco, P.; Boshi, T.;
Yi, G.-B.; Khan, M.; Richter-Addo; G. B. Inorg. Chem. 1996, 35, 1343-
1348. (c) Ford, P. C.; Miranda, K.; Lee, B.; Bourasssa, J.; Kudo, S. Abstracts
of Papers, 211th National Meeting of the American Chemical Society, New
Orleans, LA; American Chemical Society: Washington, DC, 1996; Abstract
INOR 626. (d) Bohle, D. S.; Gopodson, P. A.; Smith, B. D. Polyhedron
1996, 15, 3147.
Figure 4. ORTEP diagram illustrating the labeling scheme used for
the [Fe(TpivPP)(NO2)(NO)]- anions in form 1. Anion 1 (50% prob-
ability ellipsoids) is depicted; identical labels are used in both anions.
Complete listings of bond distances and bond angles in the
complex anions, cations, and solvent molecules are given in
the Supporting Information. Table 3 summarizes bond dis-
tances, angles, and the FeNO-NO2 dihedral angle in the
(38) Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford Univer-
sity Press: New York, 1992.

Sharing π-Bonding
J. Am. Chem. Soc., Vol. 119, No. 27, 1997 6279
Figure 5. ORTEP diagram illustrating the molecular structure of [Fe(TpivPP)(NO2)(NO)]- (30% probability ellipsoids, form 2). The labels of the
crystallographically unique atoms are shown along with the values of the bond distances in the coordination group. The near parallel orientation of
the nitrite ion plane and the iron-nitrosyl plane is evident (the dihedral angle is 20.9°).
Table 3. Selected Bond Distances (Å) and Angles (deg) for
projection of the nitrite ion plane onto the porphinato core
effectively bisects a porphyrin Np-Fe-Np bond angle. Analysis
of Mo¨ssbauer spectra of these nitrite complexes, obtained in
an applied magnetic field, requires an unusual rotation of the g˜
and A˜ tensor axes in order to fit the spectra. This strongly
suggests that the two dπ orbitals are perpendicular to the heme
plane and between the Fe-Np directions.16,17,39 In other words,
the planes of the two dπ orbitals do not correspond to the
directions taken from the heme geometry. However, this rather
unusual orientation is exactly that required for iron π-donation
to nitrite. Moreover, in five-coordinate [K(222)][Fe(TpivPP)-
(NO2)], the single axial nitrite ligand provides a sufficient axial
ligand field to yield a low-spin iron(II) complex, consistent with
a strong π-acid ligand.
Although the evidence is perhaps not as direct, M f L
π-bonding is also significant for the nitrosyl ligand. Further-
more, in the six-coordinate iron(II) systems, the nitrosyl ligand
has a strong trans effect. Thus, while it is certainly to be
expected that the present nitronitrosyl complexes will be low
spin, it is not clear which of the axial ligands, nitric oxide or
nitrite (or both), will predominate in defining the axial ligand
bonding. Two limiting possibilities, consistent with the syn-
thetic chemistry, can be enumerated. If NO dominates the axial
π-bonding to the near exclusion of nitrite, it would be expected
that the trans Fe-N(NO2) distance would be quite long as the
NO ligand in this iron(II) derivative showed its usual structural
trans effect. If both axial ligands were to exhibit strong
π-accepting behavior, it is to be expected that two axial ligands
should be found in planes orthogonal to each other, so as to
maximize the M f L π-bonding.
[Fe(TpivPP)(NO2)(NO)]- a,b
form 1, anion 1 form 1, anion 2
Lengthsc
form 2
Fe-N(1)
2.003(6)
1.993(6)
1.996(6)
1.962(6)
2.086(8)
1.792(8)
1.246(7)
1.245(8)
1.176(8)
1.992(6)
1.991(6)
2.008(6)
1.994(6)
2.080(8)
1.774(8)
1.17(2)
1.984(6)
1.983(6)
1.982(6)
1.996(6)
2.060(7)
1.840(6)
1.202(8)
1.259(9)
1.134(8)
Fe-N(2)
Fe-N(3)
Fe-N(4)
Fe-N(9)
Fe-N(10)
N(9)-O(5)
N(9)-O(6)
N(10)-O(7)
1.21(2)
1.156(10)
Anglesd
120.3(5)
121.8(5)
137.4(6)
117.8(6)
Fe-N(9)-O(5)
Fe-N(9)-O(6)
Fe-N(10)-O(7)
O(5)-N(9)-O(6)
122.9(11)
122.0(6)
123.6(9)
139.4(7)
113.0(13)
120.7(5)
137.4(6)
117.2(7)
Dihedral Anglesd
FeNO-NO2
NpFeN-NO2
NpFeN-FeNO
85.4
44.1
41.3
76.2
41.4
35.2
20.9
44.0
25.2
a Estimated standard deviations of least significant digits are given
in parentheses. b In cases where disorder is present, bond lengths and
angles presented represent the major nitro component. c Value in
angstroms. d Value in degrees.
nitrite ligands and two dithiocarbamate ligands. The complexes
closest in structure to the present case are an octahedral Cr(I)
macrocyclic complex and a ruthenium(III) porphyrin. The Cr-
(I) complex has a linear nitrosyl and a trans nitro ligand; the
two axial Cr-N distances are quite different with Cr-N(NO)
) 1.679(5) Å and Cr-N(NO2) ) 2.204(5) Å.33 The ruthenium
system, which has been independently synthesized by at least
In the actual case, the structures in the two crystalline forms
of [Fe(TpivPP)(NO2)(NO)]- show different bonding geometries
from the limiting possibilities given above. The geometries
appear to reflect differences in bonding of the axial ligands;
furthermore, the differences appear to be manifested in physical
four different groups, has a linear nitrosyl and a trans O-bound
- 37
NO2
.
Prior investigations of nitrite-complexed iron por-
phyrinates14-17,29 have made clear that the nitrite ion plays a
dominant role in the bonding. In all of these derivatives the
nitrite ion has nearly the same absolute orientation: the
(39) We assume that the z direction is the heme normal. This assumption
has been confirmed by single-crystal EPR measurements (S. Lloyd, Emory
University, unpublished observations).

6280 J. Am. Chem. Soc., Vol. 119, No. 27, 1997
Nasri et al.
properties and in the electronic structure at iron. The NO group
geometries are well determined in both crystalline forms,
although two of the nitrosyl ligand descriptions are marred by
minor crystallographic disorder. The observed Fe-N-O angles
are similar and bent (137°) as expected for these iron(II)
{FeNO}7 species. The two form 1 anions have the two axial
ligands in near orthogonal planes with both ligand planes close
to bisecting an Np-Fe-Np angle. The two axial ligands in
crystals of form 2 are close to being in the same plane, but
again, the two ligand planes nearly bisect an Np-Fe-Np angle;
these dihedral angles are summarized in Table 3. The absolute
orientations observed for the axial ligands suggest that they exert
a prominent role in defining the orientation of the dπ orbitals
as was previously noted for nitroiron(III) species.16,17
The distances and angles of Table 3 may be compared with
the observed values for nitro- and nitrosylmetalloporphyrin
derivatives given in Table 4. Several generalizations are
apparent. The Fe-Np bond distances are comparable to those
of the other low-spin derivatives reported in Table 4. The
observed Fe-N-O angles are smaller than those previously
observed in the several iron(II) species. The data given in Table
4 suggest that Fe-N(NO) bond lengths might increase slightly
upon change from five- to six-coordination, but those of the
nitronitrosyl derivatives are clearly longer (by 0.08-0.12 Å)
than those found in other nitrosyliron(II) derivatives. Com-
parison of the Fe-N(NO2) distances in [Fe(TpivPP)(NO2)(NO)]-
with the values for the nitroiron(II) or -iron(III) derivatives
presented in Table 4 shows that these distances are significantly
longer (by ∼0.13 Å). Thus both types of axial distances are
found to have similar, increased lengths compared to appropriate
reference complexes. Nitric oxide appears to exert at most a
modest trans effect in the nitronitrosyl complexes unlike the
other six-coordinate nitrosyliron(II) complexes.
Thus, the structural data for the two forms of
[Fe(TpivPP)(NO2)(NO)]- are consistent with relatively strong
NO bonding, though probably weaker than in the previously
characterized nitrosyliron(II) derivatives. The axial nitrite ion
bonding is also probably slightly weaker than in other studied
nitrite complexes. It is reasonable to ascribe this decreased
bonding to competition between the two strong axial π-accepting
ligands. The most significant difference in the three species is
that of the relative axial ligand orientations; bond distances are
quite similar, suggesting that the various species have equivalent
electronic structures. However, infrared and Mo¨ssbauer spectra
provide evidence for real differences in electronic structure (and
hence bonding) in the two crystalline forms. The differences
in electronic structure, however, must be small in overall energy
terms.
The solid-state IR spectrum of form 1 shows bands from the
NO ligand (1616 cm-1) and the nitrite ion (1380 and 1310
cm-1). Form 2 also shows bands from the same moieties but
at different frequencies: the band assigned to the NO ligand is
at 1668 cm-1, while the bands arising from the nitrite ion are
at 1346 and 1305 cm-1. NO stretching frequencies seen in
nitrosyliron(II) porphyrin derivatives are a sensitive reporter of
the bonding at iron. The NO stretching frequency found for
five-coordinate [Fe(TpivPP)(NO)]17 is 1665 cm-1, and that for
five-coordinate [Fe(TPP)(NO)] is 1670 cm-1 11,41
Bohle and
.
Hung40 have reported that the stretching frequencies for a
number of different five-coordinate nitrosyliron(II) derivatives
Figure 6. Formal diagram of the porphinato core of the
[Fe(TpivPP)(NO2)(NO)]- illustrating the displacement of each unique
atom from the mean plane of the 24-atom porphinato core in units of
0.01 Å. Negative values of displacement are toward the nitro ligand.
Also displayed on the diagram are the averaged values of each type of
bond distance and angle in the porphinato core. The relative orientation
of the two axial ligands with respect to the porphyrin core (and to each
other) are also schematically displayed. Top: form 1, anion 1.
Middle: form 1, anion 2. Bottom: form 2.
range from 1675 to 1705 cm-1
. Differences appear to be
qualitatively related to ease of oxidation of the metal. Most
importantly, there is a significant effect on the NO stretching
frequency of adding a trans sixth ligand. The addition of an
(40) Bohle, D. S.; Hung, C.-H. J. Am. Chem. Soc. 1995, 117, 9584.
(41) Wayland, B. B.; Olson. L. W. J. Am. Chem. Soc. 1974, 96, 6037.

Sharing π-Bonding
J. Am. Chem. Soc., Vol. 119, No. 27, 1997 6281
Table 4. Summary of Coordination Group Geometry for Nitrosyl and Nitro Metalloporphyrin Derivatives
Metal Nitrosyl Derivatives
complexa
M-Np
M-NNO
MNO
M-L
ref
[CoII(TPP)(NO)]
1.978(4)
2.001(3)
1.981(26)
1.986(23)
2.004
2.008(12)
2.004(9)
1.998(10)
2.004(5)
2.027(3)
1.994(1)
1.999(6)
1.833(53)
1.717(7)
1.716(15)
1.75(6)
1.703(8)
1.743(4)
1.721(10)
1.740(7)
1.641(2)
1.644(5)
1.644(3)
1.652(5)
∼130
10
11
17
40
40
12
13
13
7
[FeII(TPP)(NO)]
149.2(6)
143.8(17)
146(2)
[FeII(TpivPP)(NO)]
[FeII(OBTPP)(NO)]
[FeII(TDCPP)(NO)]
138.8(9)
142.1(6)
138.5(11)
143.7(6)
177.8(3)
176.2(5)
176.9(3)
174.4(10)
[FeII(TPP)(NO)(1-MeIm)]
[FeII(TPP)(NO)(4-MePip)]
(second form)
2.180(4)
2.328(10)
2.463(7)
[MnII(TTP)(NO)]
[MnII(TPP)(NO)(4-MePip)]
[FeIII(OEP)(NO)]+
2.206(5)
7
9
9
[FeIII(TPP)(NO)(H2O)]+
2.001(5)
2.075(5)
[FeII(TpivPP)(NO2)(NO)]- (average of three values)
1.990(12)
1.802(34)
138.1(12)
this work
Metal Nitro Derivatives
complexa
Fe-Np
Fe-NNO
M-L
ref
2
[FeIII(TpivPP)(NO2)2]-
1.992(1)
1.985(3)
1.974(2)
1.980(7)
1.970(4)
1.990(15)
1.990(6)
1.970(5)
1.960(5)
1.949(10)
1.990(7)
1.849(6)
1.951(5)
1.937(3)
2.001(6)
2.093(5)
2.037(10)
2.277(2)
15
16
16
17
14
29
29
[FeIII(TpivPP)(NO2)(Py)]
[FeIII(TpivPP)(NO2)(HIm)]
[FeIII(TpivPP)(NO2)(SC6HF4)]-
[FeII(TpivPP)(NO2)]-
[FeII(TpivPP)(NO2)(Py)]-
[FeII(TpivPP)(NO2)(PMS)]-
2.032(5)
2.380(2)
a Formal oxidations states based on neutral NO.
imidazole ligand leads to a decrease in the NO stretch to 1625
cm-1 12,42
Moreover, the NO stretch appears to be quite
.
sensitive to the Fe-L bonding trans to NO in [Fe(Porph)(NO)-
(L)]; an increased NO stretching frequency is seen as the trans
interaction decreases. Scheidt and co-workers13 found that two
solid state forms of [Fe(TPP)(NO)(4-MePip)] had NO stretching
frequencies of 1640 and 1653 cm-1; these are correlated with
the trans Fe-N(4-MePip) bond distances of 2.328(10) and
2.463(7) Å, respectively. Thus the fact that the two crystalline
forms of [Fe(TpivPP)(NO2)(NO)]- display differing NO stretch-
ing frequencies appears significant. The most straightforward,
but perhaps not complete, interpretation of the solid-state IR
differences is that the NO stretch variation reflects differences
in the bonding to iron trans to NO. Thus the orthogonal
orientation of the two ligand planes in the form 1 allows stronger
π-bonding to both ligands than that in form 2 where the ligand
planes are effectively parallel and nitric oxide bonding is more
dominant.
Mo¨ssbauer spectra of a sample of form 2, recorded at
temperatures ranging from 4.2 to 300 K in the absence or
presence of a weak applied field (50 mT), show broad and
featureless spectra, indicating that the electronic relaxation time
of the complex is comparable to the Mo¨ssbauer sampling time
(10-7 s). To obtain the characteristic hyperfine parameters, a
spectrum was recorded at 4.2 K in the presence of a strong (8
T) magnetic field applied parallel to the γ-beam (Figure 7A).
The strong field removes the degeneracy of the S ) 1/2 doublet
such that the separation of the two energy levels is large in
comparison to the sample temperature and the effects of
electronic relaxation are minimized. The spectrum can then
be analyzed by a spin Hamiltonian formalism commonly applied
for the study of iron-containing proteins.43 The solid line in
Figure 7A is a theoretical simulation assuming fast electronic
relaxation and using the following parameters: quadrupole
splitting, ∆Eq ) 1.2 ( 0.2 mm/s; isomer shift, δ ) 0.35 (
0.05 mm/s; asymmetry parameter, η ) 0.5 ( 0.1; magnetic
hyperfine tensor, Axx/gnân ) -23 T, Ayy/gnân ) -(8.5 ( 1.5)
T, Azz/gnân ) +(14.5 ( 0.5) T. The theoretical simulation was
Figure 7. (A) Mo¨ssbauer spectrum of [K(222)][Fe(TpivPP)(NO2)(NO)]
(form 2) at 4.2 K in a parallel applied field of 8 T. The solid line
shows the fit based on the parameters given in the text. (B) Mo¨ssbauer
spectrum of [Fe(TPP)(NO)] at 4.2 K in a parallel applied field of 8 T.
The solid line shows the fit based on the parameters given in the text.
found to be insensitive to the parameter Axx/gnân within the range
-26 to -19 T. Except for the rhombicity detected in the
xy plane, the hyperfine parameters determined for the
[Fe(TpivPP)(NO2)(NO)]- complex compare reasonably well in
both signs and magnitudes with those observed for NO-bound
heme proteins.44-46 For example, the corresponding parameters
(42) Maxwell, J. C.; Caughey, W. S. Biochemistry 1976, 15, 388.
(43) Huynh, B. H.; Kent, T. A. AdV. Inorg. Biochem. 1984, 6, 163.
(44) Liu, M.-C.; Huynh, B. H.; Payne, W. J.; Peck, H. D., Jr.;
DerVartanian, D. V.; LeGall, J. Eur. J. Biochem. 1987, 169, 253.

6282 J. Am. Chem. Soc., Vol. 119, No. 27, 1997
Nasri et al.
observed for the NO-bound d1 heme in the diheme cytochrome
cd1 nitrite reductase from Thiobacillus denitrificans are ∆Eq )
0.8 mm/s, δ ) 0.34 mm/s, Axx/gnân ) Ayy/gnân ) -15 T, and
Azz/gnân ) +12 T.44 These values were suggested to indicate
a substantial σ-electron donation from the NO ligand to the Fe
2
dz orbital and the π-electron back-donation from the Fe (dxy
and dxz) to the NO. Most interestingly, a Mo¨ssbauer spectrum
similar to that of the NO-bound d1 heme is also observed for
the NO-bound siroheme in the hemoprotein subunit of the E.
coli sulfite reductase, where the siroheme is covalently linked
to a [4Fe-4S] cluster.45 These observations of similar Mo¨ss-
bauer spectra for NO-bound hemes of different macrocyclic
structures suggest that the electronic structure of an NO-bound
heme complex is predominantly determined by the Fe-NO
moiety.
We have also studied the Mo¨ssbauer spectrum of a crystalline
sample of the related five-coordinate nitrosyl complex [Fe(TPP)-
(NO)],11,41 for which the electron relaxation was found to be
relatively fast at 4.2 K. In the absence of an applied field, a
broad quadrupole doublet is detected (Figure 8A). A least-
squares fit of the data yields parameters (∆Eq ) 1.24 ( 0.03
mm/s and δ ) 0.35 ( 0.02 mm/s) that are indistinguishable to
those obtained from form 2 crystals of the nitronitrosyl complex,
suggesting similar electronic structure for the two compounds.
This similarity in electronic structure is further corroborated by
the Mo¨ssbauer spectra recorded in strong applied fields. A
spectrum of [Fe(TPP)(NO)] recorded at 4.2 K in an 8 T magnetic
field applied parallel to the γ-beam is shown in Figure 7B for
comparison with the spectrum of the six-coordinate nitronitrosyl
complex recorded under the same conditions (Figure 7A). The
spectra are almost identical showing only subtle differences.
Analysis of the spectrum shown in Figure 7B yielded magnetic
hyperfine parameters that are very similar to those of the six-
coordinate complex. The solid line in Figure 7B is a theoretical
simulation using the following parameters: ∆Eq ) 1.24 mm/s,
δ ) 0.35 mm/s, η ) 0.32 ( 0.05, Axx/gnân ) -(25 ( 4) T,
Ayy/gnân ) -(10 ( 2) T, and Azz/gnân ) 13.2 ( 0.5 T. This
strong resemblance in electronic structures observed for five-
coordinate [Fe(TPP)(NO)] and six-coordinate [Fe(TpivPP)-
(NO2)(NO)]- (form 2) complexes strongly suggests that nitric
oxide is the predominant axial ligand in bonding in form 2.
Figure 8. (A) Mo¨ssbauer spectrum of [Fe(TPP)(NO)] at 4.2 K in zero
field. (B) Mo¨ssbauer spectrum of [K(222)][Fe(TpivPP)(NO2)(NO)]
(form 1) at 200 K in zero field.
In a sample of form 1, the electronic relaxation in the
Mo¨ssbauer was found to be fast at high temperatures. Figure
8B shows the Mo¨ssbauer spectrum recorded for a bulk sample
at 200 K. One major and one minor quadrupole doublet are
observed, suggesting multiple iron environments. The major
species (∼75%, shown as a solid line in Figure 8B) has a broad
quadrupole doublet with apparent parameters ∆Eq ) 1.78 mm/s
and δ ) 0.22 mm/s; the value of δ at 4.2 K (corrected for the
second-order Doppler shift) is expected to increase to about 0.28
mm/s. The 75% abundance in the Mo¨ssbauer agrees well with
the independent estimate of 70% for the nitro linkage isomer
species, supporting the idea that the Mo¨ssbauer sample and the
single-crystal specimen represent the same sample. The samples
of forms 1 and 2 are seen to have similar isomer shifts but
significantly different quadrupole splittings and support the idea
that there are differences in the electronic structure of the two
crystalline forms. Unfortunately, the limited amount of the form
1 species available, along with the difficulties engendered by
the multiple iron environments, did not allow a complete study
in applied magnetic field. However, the value of the quadrupole
splitting in form 1 halfway between that of five-coordinate
Figure 9. Solid-state and frozen chlorobenzene solution (77 K) EPR
spectra of [K(222)][Fe(TpivPP)(NO2)(NO)]. The simulated spectrum,
using the parameters given in the text, is also displayed.
[Fe(TPP)(NO)]
(1.24
mm/s)
and
five-coordinate
[Fe(TpivPP)(NO2)]- (2.28 mm/s)14 is strongly suggestive of a
nitrite ion effect on the symmetry of the charge distribution at
iron in the form 1 species.
The low-spin iron(II) complex is paramagnetic owing to the
unpaired electron derived from the NO ligand. The EPR
spectrum of the complex (Figure 9) shows similar features in
solution and in both solid state forms; in solution, hyperfine
coupling from only one nitrogen atom (probably the nitrosyl
nitrogen) is apparent. Figure 9 also shows a simulated spectrum.
The following parameters were used in the spectral simulation:
gx ) 2.085, gy ) 2.032, gz ) 2.01, ax (14N) ) 24 G, ay ) 16.5
G, and az ) 16 G. It is to be noted that the low-field portions
of nitrosyliron porphyrin EPR spectra are particularly difficult
to simulate due to possible interactions from porphinato nitrogen
ligands.47
(45) Christner, J. A.; Mu¨nck, E.; Janick, P. A.; Siegel, L. M. J. Biol.
Chem. 1983, 258, 11147.
(46) Costa, C.; Moura, J. J. G.; Moura, I.; Liu, M. Y.; Peck, H. D., Jr.;
LeGall, J.; Wang, Y.; Huynh, B. H. J. Biol. Chem. 1990, 265, 14382.
(47) Henry, Y.; Ducrocq, C.; Drapier, J.-C.; Servent, D.; Pellat, C.;
Guissani, A. Eur. Biophys. J. 1991, 20, 1.

Sharing π-Bonding
J. Am. Chem. Soc., Vol. 119, No. 27, 1997 6283
The structures of the three complex cations, [K(222)]+, are
normal. Each potassium ion is coordinated by the usual six
oxygen atoms and two nitrogen atoms of the cryptand ligand.
One of the complex cations of form 1 has one disordered
ethylene oxide chain, but the observed K-O and K-N distances
are generally quite typical to that observed previously. The
average value of the K-N distances is 3.01 Å, and the average
K-O distance is 2.81 Å. (The range of K-O distances is 2.768-
(8)-2.860(8) Å.) ORTEP diagrams of the three cations are given
in the Supporting Information.
Summary. The synthesis and characterization of two
crystalline forms of the six-coordinate complex [Fe-
(TpivPP)(NO2)(NO)]- are reported. Structurally, the two forms
differ primarily in the relative orientation of the two axial
ligands. In form 1, the two axial ligands are in perpendicular
planes, which allows both ligands to more strongly bond to iron
than in form 2. In this form, the two ligands are in essentially
parallel planes and the nitric oxide ligand appears to be more
dominant ligand to iron. These orientation differences are also
manifested in differences in the Mo¨ssbauer and IR spectra.
Acknowledgment. We thank the National Institutes of
Health for support of this research under Grant GM-38401 to
W.R.S. and Grant GM-47295 to B.H.H. Funds for the purchase
of the FAST area detector diffractometer were provided through
NIH Grant RR-06709 to the University of Notre Dame.
Supporting Information Available: Tables S1-S11 giving
complete crystallographic details, atomic coordinates, anisotropic
temperature factors, fixed hydrogen atom positions, and com-
plete listings of bond distances and angles for both crystal-
lographic forms and Figures S1-S3 giving ORTEP diagrams
of the K(222) cations (75 pages). See any current masthead
page for ordering and Internet access instructions.
JA963871A