Structural and physical characterization of (nitrato)iron(III) porphyrinates [Fe(por)(NO3)] - Variable coordination of nitrate

Article abstract of DOI:10.1016/j.poly.2007.03.048

We report the X-ray crystal structures of two different iron(III) porphyrinates: [Fe(OEP)(NO₃)] and [Fe(TPP)(NO₃)]. The first complex has the nitrate ion coordinated by a single oxygen atom while the second derivative has the nitrate coordinated in a symmetric bidentate fashion. This latter structure is a redetermination that shows some differences from an earlier structure; the difference appears to be the result of an unrecognized nitrate ion disorder in the earlier structure determination. Changes in physical properties of three species [Fe(TPivP)(NO₃)], [Fe(OEP)(NO₃)], and [Fe-(TPP)-(NO₃)] as a function of coordination mode were examined by M?ssbauer and EPR spectroscopies; EPR spectra appear to be most sensitive to the change in coordination mode.

Full text of DOI:10.1016/j.poly.2007.03.048

Polyhedron 26 (2007) 4664–4672
www.elsevier.com/locate/poly
Structural and physical characterization of (nitrato)iron(III)
porphyrinates [Fe(por)(NO3)] – Variable coordination of nitrate
b
a,
*
Graeme R.A. Wyllie a,1, Orde Q. Munro a,2, Charles E. Schulz , W. Robert Scheidt
a
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, United States
b
Department of Physics, Knox College, Galesburg, IL 61401, United States
Received 17 January 2007; accepted 21 March 2007
Available online 10 April 2007
Abstract
We report the X-ray crystal structures of two different iron(III) porphyrinates: [Fe(OEP)(NO3)] and [Fe(TPP)(NO3)]. The first com-
plex has the nitrate ion coordinated by a single oxygen atom while the second derivative has the nitrate coordinated in a symmetric biden-
tate fashion. This latter structure is a redetermination that shows some differences from an earlier structure; the difference appears to be
the result of an unrecognized nitrate ion disorder in the earlier structure determination. Changes in physical properties of three species
[Fe(TPivP)(NO3)], [Fe(OEP)(NO3)], and [Fe-(TPP)-(NO3)] as a function of coordination mode were examined by Mo¨ssbauer and EPR
spectroscopies; EPR spectra appear to be most sensitive to the change in coordination mode.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Iron(III) porphyrinates; Nitrate binding mode; EPR spectroscopy; Mo¨ssbauer spectroscopy; Crystal structure; Electronic structure
1. Introduction
In addition to its physiological importance, the nitrate
ligand possesses a varied coordination chemistry. A num-
ber of coordination modes have been observed upon
interaction with a metal ion [7] with three of the more
common nonbridging modes illustrated in Scheme 1.
These coordination geometries range from the monoden-
tate mode (g1) (Scheme 1a) through the anisobidentate or
asymmetric bidentate (g2) mode (Scheme 1b) to the sym-
metric (g2) bidentate mode (Scheme 1c). An initial
attempt to classify these systems based upon geometry
was proposed by Addison et al. [8] and later refined by
Lever et al. [9] who showed such coordination modes
could be distinguished on the basis of spectroscopic
measurements.
Variable coordination modes of the nitrate ligand have
been observed in several series of complexes. Driessen
and co-workers [10] investigated a series of complexes
based upon the tripodal ligand amtd [11] and containing
a range of metal ions. Depending on the identity of the cen-
tral metal ion in [M(amtd)(NO3)2], variable coordination
modes of the nitrate are observed. A similar result is
Interactions between iron porphyrinates and the nitrate
ligand (NO3ꢀ) occur in a number of areas of bioinorganic
chemistry. Nitrate is a key component of the nitrogen cycle
in the fixation of atmospheric nitrogen to a more biologi-
cally accessible forms [1]. Effects of nitrate on mammalian
physiology are also closely studied, particularly in light of
its occurrence in drinking waters. Explications of the
debate as to the harmful or beneficial effects of nitrate
can be found in two recent publications for popular audi-
ences [2,3]. Nitrate can also be produced in vitro from
either oxidation of coordinated NO [4] or NO interaction
with coordinated dioxygen to yield nitrate that may ini-
tially be coordinated to heme [5,6].
*
Corresponding author. Fax: +1 574 631 4044.
E-mail address: Scheidt.1@nd.edu (W. Robert Scheidt).
Present address: Department of Chemistry, Concordia College,
1
Moorhead, MN 56562, United States.
2
Present address: School of Chemistry, University of KwaZulu-Natal,
Pietermaritzburg, South Africa.
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.03.048

G.R.A. Wyllie et al. / Polyhedron 26 (2007) 4664–4672
4665
of the ferric porphyrinates. Based upon the reported vari-
ations in coordination geometry, the complexes
[Fe(por)(NO3)] provide a means of studying systematic
changes in the electronic environment of the iron as the
coordination geometry changes. However, seemingly
contradictory EPR results were obtained, and led us to
reexamine the structure of the asymmetric bidentate coor-
dinated species [Fe(TPP)(NO3)] [16]. The structure was
redetermined at 100 K with crystals having comparable
cell constants to those reported previously. Our new struc-
ture demonstrates that, rather than a single asymmetric
bidentate coordination mode, the nitrate ligand is in fact
disordered over two positions. This new structure is dis-
cussed particularly in relation to the previous room tem-
O
N
O
N
O
N
a
c
b
O
O
O
O
O
O
M
M
M
Scheme 1.
observed by Parkin and co-workers [12] in the study of
model complexes of the enzyme carbonic anhydrase. Once
again, the identity of the metal ion in a tripodal ligand
frameworks determines the coordination geometry of the
nitrate ligand.
perature example [16]. We also present
a second
Yet, in these examples, comparing the electronic proper-
ties of the coordinated ligand across such a series of com-
plexes is complicated by the change in metal identity. In
order to accurately investigate the effects on the electronic
structure of the complexes upon changes in coordination
geometry would require a series of related ligand frame-
works (excluding the nitrate), all containing the same cen-
tral metal, but which still allows variations in nitrate
geometry. To our knowledge, the ferric porphyrinate com-
plexes [Fe(por)(NO3)] provide the only example of such a
series. Hence we undertook a study of the electronic struc-
ture for such a series of complexes aiming to correlate
changes in both the Mo¨ssbauer and EPR spectra with
changes in ligand geometry.
A number of porphyrinate complex structures contain-
ing a coordinated nitrate ligand have been reported [13]
with a range of coordination modes reported therein.
Characterization of a five-coordinate monodentate ferrous
complex [Fe(TpivPP)(NO3)]ꢀ has been published [14]. The
structures of the ferric porphyrinate complexes
[Fe(por)(NO3)] based upon the porphyrinates OEP, [15]
TPP, [16] and TpivPP [17] demonstrate the geometries
illustrated in Scheme 1. Initial attempts at forming a
nitrite-coordinated heme complex had resulted in forma-
tion of free nitrate as determined by IR spectroscopy fol-
lowing an oxygen atom transfer reaction [18]. A related
reaction utilizing picket fence porphyrin (TpivPP) with a
protected ligand binding site resulted in nitrate formation
with nitrate coordinated to the heme [17]. The crystal struc-
ture of [Fe(TpivPP)(NO3)] [17] displays a symmetrically
coordinated bidentate nitrate ligand with the projected
plane of the nitrate eclipsing a trans pair of porphyrin
nitrogen atoms. Asymmetric bidentate coordination has
been reported for [Fe(TPP)(NO3)] [16] with the projected
plane of the nitrate bisecting adjacent pairs of porphyrin
nitrogen atoms. The triclinic [Fe(OEP)(NO3)] [15] exhibits
a monodentate nitrate ligand. We have subsequently deter-
mined the crystal structure of a monoclinic form of
[Fe(OEP)(NO3)] (reported herein) which also possess a
monodentate nitrate ligand.
crystalline form of [Fe(OEP)(NO3)], which like the previ-
ously reported structure, [15] contains a monodentate
nitrate ligand. In addition to the two new crystal struc-
tures, we provide electronic characterization of all three
[Fe(por)(NO3)] complexes both in solution and solid state.
These reveal marked differences in nitrate coordination
geometry between the solid and solution coordination
modes for both of the bidentate systems.
2. Experimental
2.1. General information
All reactions were carried out using standard Schlenk-
ware techniques unless otherwise noted. Tetrahydrofuran
and hexanes were distilled over sodium and benzophenone
prior to use. Methylene chloride was distilled over calcium
hydride. H2OEP was purchased from Midcentury
Chemicals. H2TPP was synthesized according to the
method of Adler et al. [19] Metallation reactions to yield
[Fe(OEP)(Cl)] and [Fe(TPP)(Cl)] were carried out accord-
ing to Adler et al. [20] H2TpivPP was prepared according
to a local modification of the reported synthesis [21]. [Fe-
(TpivPP)(Cl)] was obtained by stirring 150 mg of the free-
base overnight with 100 mg of FeCl2 in 25 mL of THF
and 0.2 mL of 2,6-lutidine.
Solution UV–Vis spectra were measured on a Perkin–
Elmer Lambda 19 UV–Vis–near-IR spectrometer. IR
spectra were collected on single crystals ground between
two sodium chloride plates with a small amount of Nujol.
Spectra were collected on a Nicolet Nexus 870 FT-IR
spectrometer. EPR measurements on ground single crys-
tals of [Fe(TpivPP)(NO3)] were made at 77 K on a Bruker
ER 100E spectrometer calibrated with a dpph standard.
All other EPR measurements were made on a Bruker
EMX-EPR spectrometer. Measurements were made on
either frozen methylene chloride solutions or finely
ground crystalline solids. Samples were prepared for
Mo¨ssbauer spectroscopy by grinding approximately
40 mg of crystals, immobilizing in Apiezon M grease to
form a mull which was subsequently sealed in an airtight
Mo¨ssbauer cup.
Initially we undertook to investigate the effects of
nitrate coordination mode upon the electronic structure

4666
G.R.A. Wyllie et al. / Polyhedron 26 (2007) 4664–4672
2.2. Synthesis of [Fe(OEP)(NO3)]
Determination of the cell constants of the crystals revealed
comparable values to those previously reported [16].
[Fe(OEP)(Cl)] (109.5 mg, 0.176 mmol) was placed in a
Schlenk flask with finely ground AgNO3 (35.8 mg,
0.211 mmol). THF (25 mL) was added and the solution
stirred overnight. The solvent was removed in vacuo
and the reddish-brown solid redissolved in methylene
chloride, filtered to remove the inorganic salts and lay-
ered with hexanes. X-ray quality crystals were obtained
after 7 days.
2.4. Synthesis of [Fe(TpivPP)(NO3)]
[Fe(TpivPP)(Cl)] (150 mg, 0.14 mmol) was placed in a
Schlenk flask with 27 mg (0.16 mmol) of AgNO3 and
15 mL of THF. The mixture was stirred overnight and
the solvent evaporated prior to redissolving in CH2Cl2
and filtration. UV–Vis spectroscopy of both the THF
and CH2Cl2 solutions confirmed the formation of [Fe-
(TpivPP)(NO3)] yet attempts to obtain a crystalline product
were unsuccessful in spite of multiple attempts using varia-
tions in crystallization conditions.
2.3. Synthesis of [Fe(TPP)(NO3)]
Large single crystals of [Fe(TPP)(NO3)] were obtained
using a similar metathesis pathway. [Fe(TPP)(Cl)] (150
mg, 0.24 mmol) was stirred overnight in 15 mL of THF
with 48 mg (0.29 mmol) of AgNO3. Subsequent solvent
removal, dissolution in methylene chloride and filtration
was carried out prior to layering with hexanes. Large single
crystals of up to 2.5 mm length were obtained after 5 days.
2.5. X-ray structure determination
Structures of [Fe(OEP)(NO3)] and [Fe(TPP)(NO3)] were
solved using the direct methods program SHELXS [22]. All
heavy atoms were located using subsequent difference Fou-
rier syntheses. The structures were refined against F2 with
the program SHELXL [23], in which all data collected were
used including negative intensities. All nonhydrogen atoms
were refined anisotropically. All hydrogen atoms were ide-
alized using the standard SHELXL idealization method.
Complete crystallographic details, atomic coordinates,
anisotropic thermal parameters and fixed hydrogen coordi-
nates for both are included in the Supplementary material.
Table 1 gives brief crystallographic details.
Table 1
Crystallographic details for [Fe(OEP)(NO3)] and [Fe(TPP)(NO3)]
[Fe(OEP)(NO3)]
36H44FeN5O3
[Fe(TPP)(NO3)]
Formula
C
C44H28FeN5O3
730.56
FW (amu)
650.61
˚
a (A)
13.209(3)
13.774(3)
19.258(4)
105.50(3)
3376.2(12)
P21/c
10.0271(3)
16.1341(5)
16.1341(5)
90.330(2)
3399.24(17)
P21/n
˚
b (A)
A single crystal of [Fe(OEP)(NO3)] was mounted on a
glass fiber with the long axis approximately collinear with
the axis of the fiber. All measurements were performed
with graphite monochromated Mo Ka radiation
˚
c (A)
b (ꢁ)
3
˚
V (A )
Space group
Z
4
4
˚
(k = 0.71073 A) on an Enraf-Nonius FAST area detector
l (mmꢀ1
)
0.489
0.495
diffractometer at 127 K as previously described [24]. Inten-
sities of all reflections were reduced using Lorentz and
polarization correction factors; the data were also cor-
rected for absorption by an empirical (w scans) absorption
correction (l = 0.489 mmꢀ1). Two positions of the methyl
carbons of ethyl groups 6 and 7 were observed with this
ꢀ
ꢀ
Radiation
Mo Ka, k ¼ 0:71073
Mo Ka, k ¼ 0:71073
Temperature (K)
Final R indices
[I > 2r(I)]
Final R indices (all
data)
293
100(2)
R1 = 0.0502,
wR2 = 0.1140
R1 = 0.0802,
wR2 = 0.1311
R1 = 0.0447,
wR2 = 0.1167
R1 = 0.0704,
wR2 = 0.1405
Table 2
Details of FeNO3 geometry for all [Fe(por)(NO3)] complexes
Complex
Fe–O1a
Fe–O2a
da,b
N5–O1a
N5–O2a
N5–O3a
Ref.
[Fe(OEP)(NO3)]c
[Fe(OEP)(NO3)]d
[Fe(TPP)(NO3)]e
[Fe(TPP)(NO3)]f
[Fe(TPP)(NO3)]g
[Fe(TpivP)(NO3)]
1.996(2)
2.016(3)
2.019(4)
2.125(3)
2.117(4)
2.123(3)
3.042(2)
2.644(3)
2.323(8)
2.268(3)
2.121(4)
2.226(3)
1.076
0.628
0.304
0.143
0.004
0.103
1.301(3)
1.206(5)
1.203(6)
1.199(4)
1.334(4)
1.271(4)
1.212(3)
1.208(6)
1.176(6)
1.300(3)
1.271(4)
1.252(4)
1.199(3)
1.198(4)
1.188(5)
1.217(3)
1.217(3)
1.214(3)
[15]
tw
[16]
tw
tw
[17]
Coordination geometries are illustrated in Fig. 4.
a
˚
Value in A.
b
Difference between Fe–O1 and Fe–O2.
Monoclinic form.
Triclinic form.
Room temperature structure.
100 K structure, major orientation.
100 K structure, minor orientation.
c
d
e
f
g

G.R.A. Wyllie et al. / Polyhedron 26 (2007) 4664–4672
4667
disorder arising from rotation about the Cm–C(6) and
Cm–C(7) bonds. This was resolved by allowing the two
possible methyl group positions to refine giving relative
occupancies of 65% and 35%. Relevant bond lengths and
angles for [Fe(OEP)(NO3)] are summarized in Table 2.
A single crystal of [Fe(TPP)(NO3)] was placed in inert
oil, mounted on a glass fiber attached to a brass mounting
pin and transferred to the cold stream of the diffractometer.
Crystal data were collected and integrated using a Bruker
Apex system with graphite-monochromated Mo Ka
˚
(k = 0.71073 A) radiation. Data collection was carried
out at 100 K. [Fe(TPP)(NO3)] was found to possess a dis-
ordered nitrate ligand with two orientations. The two
nitrate orientations were refined and determined to have
relative occupancies of 60% and 40%. Oxygen atoms of
the major orientation are labelled O1A and O2A whilst
the minor orientation is labelled O1B and O2B. The central
nitrogen atom and the uncoordinated oxygen of the nitrate
ligand did not display any disorder. Relevant bond lengths
and angles for [Fe(TPP)(NO3)] are summarized in Table 2.
3. Results
The crystal structures of two five-coordinate nitrate-
ligated iron(III) porphyrinate complexes have been
obtained. The first of these, monoclinic [Fe(OEP)(NO3)],
represents a second crystalline example of a monodentate
nitrate coordinated system in an iron(III) porphyrinate
[15]. The second structure, [Fe(TPP)(NO3)], is a redetermi-
nation of a previously reported room temperature structure
[16]. Here, we report a revised structure from data collected
at 100 K.
Fig. 1 shows the ORTEP diagram and a partial labelling
scheme for the monoclinic form of [Fe(OEP)(NO3)] clearly
illustrating the monodentate coordination geometry of the
nitrate ligand. Fig. 2a is a schematic representation of the
porphyrin core showing the perpendicular displacements
Fig. 2. Formal diagram of the porphyrinato core of (a) [Fe(OEP)(NO3)]
and (b) [Fe(TPP)(NO3)] displaying perpendicular displacement (in units of
0.01 A) of the core atoms from the 24-atom mean porphyrin plane.
Positive displacements are towards the nitrate coordinated face of the
porphyrin. The projections of the nitrate ligand upon the porphyrin plane
are also shown. For [Fe(TPP)(NO3)], the two orientations of the nitrate
ligand are shown. (O1A and O2A represent the oxygen atom positions of
the major orientation.)
˚
O(2)
O(3)
N(5)
C(62)
C(b6)
O(1)
C(72)
C(51)
C(52)
C(m2)
C(a4)
C(b5)
C(61)
C(a6)
C(m3)
C(a7)
C(82)
C(41)
C(71)
C(b7)
C(b4)
C(b3)
N(3)
C(a5)
N(2)
N(4)
Fe
C(42)
N(1)
C(m4)
C(a1)
C(a8)
C(b8)
C(a3)
C(m1)
C(81)
C(31)
C(12)
C(a2)
C(b2)
C(21)
C(b1)
C(32)
C(11)
C(22)
Fig. 1. ORTEP diagram of the monoclinic form of [Fe(OEP)(NO3)] with 50% probability ellipsoids. The atom labeling scheme is also shown.

4668
G.R.A. Wyllie et al. / Polyhedron 26 (2007) 4664–4672
O3
N5
O1A
O1B
Fe
Fig. 3. ORTEP diagram of [Fe(TPP)(NO3)] with 50% probability ellipsoids. Both orientations of the nitrate ligand is shown with the major orientation
labelled O1A, O1B.
illustrated with the coordinating oxygens of the major ori-
entation being labeled O1A, O2A. The minor orientation is
rotated approximately 70ꢁ from the major with both lying
close to a pair of opposite Fe–Np bonds. The dihedral
angles (/) which represent the relative orientation of the
nitrate ligand with respect to the closest Fe–Np vector are
13ꢁ and 8ꢁ for the major and minor orientations respec-
tively. In contrast, for monoclinic [Fe(OEP)(NO3)] struc-
ture, / is 41ꢁ with the nitrate ligand lying almost midway
between a pair of Fe–Np bonds. For both the OEP and
TPP systems, the nitrate plane lies perpendicular to the
plane of the porphyrin. Selected bond lengths for these
complexes and related systems are given in Table 2.
Table 3 reports UV–Vis and IR spectroscopic data for
the [Fe(por)(NO3)] complexes. EPR measurements for
both ground crystalline solids and solutions were made
on all three systems. These data are summarized in Table
4. Solution measurements on all three porphyrin systems
made at 4 K are clearly axial with the major feature
g^ = 5.7–6.0 and the minor feature, gi ꢁ 2. Powder mea-
Table 3
UV–Vis and IR spectroscopic data for
complexes
a
series of [Fe(por)(NO3)]
m(NO3)b (cmꢀ1
Complex
UV–Visa (nm)
)
[Fe(OEP)(NO3)]
[Fe(TPP)(NO3)]
[Fe(TpivP)(NO3)]
a
385, 502, 534, 583, 636
412, 513, 574, 658
415, 509, 574, 639
1515, 1385, 1276.
1531, 1261, 1070
1515, 1276, 1071
Measured on CH2Cl2 solution.
Measured as KBr disk.
b
from the 24-atom mean plane. The iron is significantly
shifted out of the plane in the direction of the nitrate ligand
while the porphyrin core shows a modest combined
doming/tenting distortion. An ORTEP diagram for
[Fe(TPP)(NO3)] illustrating both orientations of the nitrate
ligand is shown as Fig. 3. The bidentate coordination mode
of the nitrate is clearly evident. Fig. 2b shows the porphyrin
core of [Fe(TPP)(NO3)] which also shows a large displace-
ment of the iron in the direction of the nitrate ligand. Pro-
jections of both orientations of the nitrate ligand are
Table 4
Summary of EPR Data for [Fe(por)(NO3)] complexes at 4 and 77 Ka
Complex
Sample
Temperature (K)
Spectrum appearance
g-values
[Fe(OEP)(NO3)]
[Fe(OEP)(NO3)]
[Fe(OEP)(NO3)]
[Fe(OEP)(NO3)]
[Fe(TPP)(NO3)]
[Fe(TPP)(NO3)]
[Fe(TPP)(NO3)]
[Fe(TPP)(NO3)]
[Fe(TpivPP)(NO3)]
[Fe(TpivPP)(NO3)]
[Fe(TpivPP)(NO3)]
[Fe(TpivPP)(NO3)]
[Fe(TpivPP)(NO3)]
a
cryst. powder
cryst. powder
solution
77
4
77
4
77
4
77
4
77
4
77
4
axial
axial
axial
axial
rhombic
rhombic
axial
axial
axial
axial
axial
axial
rhombic
g^ = 5.85
g^ = 5.81, g ¼ 2:018
k
k
k
g^ = 5.99, g ¼ 2:00
solution
g^ = 5.86, g ¼ 1:998
cryst. powder
cryst. powder
solution
g = 7.71, 4.04, 1.78
g = 8.5, 4.0, 1.8
g^ = 5.65, g 2:001
g^ = 5.82, g 1:99
k
k
k
k
k
k
solution
powderb
g^ = 5.45, g ¼ 1:98
powderb
g^ = 5.54, g ¼ 1:99
solution
solution
cryst. powder
g^ = 5.65, g ¼ 2:00
g^ = 5.71, g ¼ 2:00
77
g^ = 6.44, 3.97, 1.9
All powder samples were prepared from ground single crystals unless otherwise noted. All solution measurements were carried out in frozen CH2Cl2.
Powder sample of unknown phase.
b

G.R.A. Wyllie et al. / Polyhedron 26 (2007) 4664–4672
4669
Table 5
these systems and all other [Fe(por)(NO3)] complexes are
shown in Fig. 4. At the onset of the project, it was thought,
based on the reported structures [15–17] that the series of
complexes [Fe(por)(NO3)] exhibited distinct examples of
three coordination geometries (Scheme 1). However after
evaluation of the EPR data, we decided to redetermine
the structure of the asymmetric bidentate coordinated sys-
tem [Fe(TPP)(NO3)] [16]. The reported structure [16]
showed a nitrate ligand lying midway between a pair of
Fe–Np bonds with two very different Fe–O bond lengths
Mo¨ssbauer data for [Fe(por)(NO3)] complexes
Complex
Temperature (K)
DEq (mm/s)
d (mm/s)
[Fe(OEP)(NO3)]
4.2
15
50
100
150
200
RT
1.44
1.39
1.39
1.38
1.36
1.38
1.37
0.53
0.41
0.42
0.41
0.42
0.38
0.36
[Fe(TPP)(NO3)]
4.2
200
RT
0.97
0.98
0.97
0.61
0.47
0.38
˚
of 2.019(4) and 2.323(8) A and d, the difference between
˚
the Fe–O bond lengths, of 0.304 A. The porphyrin core dis-
plays a characteristic saddling distortion and the iron is sig-
nificantly displaced from the mean porphyrin plane in the
direction of the nitrate ligand. A similar saddling distortion
and iron out-of-plane displacement is observed in the por-
phyrin core of our redetermined structure at 100 K Fig. 2b.
In fact, if we overlay the porphyrin cores of the RT struc-
ture and our newly redetermined structure so as to align the
saddling patterns, we observe that the originally reported
nitrate ligand orientation lies midway between the two ori-
entations found in the 100 K structure. This is consistent
with the very large thermal ellipsoids observed for the
two coordinated oxygen atoms of the nitrate. Based upon
[Fe(TpivPP)(NO3)]
4.2
200
RT
1.0
1.10
1.07
0.56
0.49
0.37
surements on the monodentate [Fe(OEP)(NO3)] also reveal
an axial spectrum with g = 5.8, 2. By contrast, rhombic
spectra are observed for the crystalline bidentate TPP
and TpivPP complexes with g values as reported in Table
4. Mo¨ssbauer spectra were collected at 4.2 K in zero field
for ground crystalline samples of all three porphyrinates.
These are fit to a S = 5/2 model and data for this fit are
given in Table 5. Spectra are illustrated in Fig. 6.
O3
4. Discussion
a
O3
1.2174(25)
b
1.2174(25)
N
N
1.2706(43)
1.1991(36)
1.3337(42)
1.2997(33)
4.1. Synthetic aspects
O1A
O2A
2.2681(34)
O1B
O2B
2.1215(42)
2.1168(38)
2.1248(30)
We report a straightforward synthetic route for obtain-
ing ferric iron porphyrinate complexes containing a coordi-
nated nitrate ligand. A simple metathesis reaction yields the
desired [Fe(por)(NO3)] complex with the undesirable side-
products being removed by a simple filtration. The ease
of this synthetic pathway is emphasized when compared
with the previous reported methods which either proceeded
via an oxygen atom transfer [17], NO attack of a bridged l-
oxo-dimer [15] or in the case of the TPP derivative, reaction
of the l-oxo-dimer with aqueous nitric acid followed by a
three-week crystallization [16]. In the latter case, we are
able to successfully obtain large crystals in under one week.
The only problem encountered with the new reaction sys-
tem lay in the inability to obtain crystals of the picket-fence
porphyrin complex [Fe(TpivPP)(NO3)]. We were able to
show spectroscopically that the species could be obtained
in solution yet were unable to prepare a solid crystalline
sample. UV–Vis and IR spectroscopic data for all three
systems are given in Table 3. In all cases, IR bands associ-
ated with the free nitrate ligand were not found, confirming
the coordinated nature of the nitrate in all three cases.
N2
N2
N1
2.0805(18) Fe
N1
Fe
2.0805(18)
N4
N4
N3
N3
0.62
O3
0.62
O3
c
d
1.214(3)
1.188(5)
N
N
1.271(4)
1.252(4)
1.203(6)
1.176(6)
O1
2.123(3)
O2
O1
O2
2.019(4)
2.226(3)
2.323(8)
N3
N2
N2
N1 2.057(3)
Fe
Fe
2.063(2)
N1
N4
N4
N3
0.61
O3
0.60
O3
e
f
1.199(3)
1.198(4)
1.212(3)
1.301(3)
N
1.206(5)
N
1.208(6)
O2
O2
O1
1.966(2)
O1
3.042
2.644
2.016(3)
N2
N1
N2
N1
Fe
N4 2.056(2)
Fe
2.049(2)
N4
N3
N3
0.45
0.50
Fig. 4. Schematic representations of the N4FeNO3 geometry for all
[Fe(por)(NO3)] complexes. Structures represented are (a) [Fe(TPP)(NO3)]
100 K, major orientation; (b) [Fe(TPP)(NO3)] 100 K, minor orientation;
(c) [Fe(TPP)(NO3)]. Room temperature structure (Ref. [16]); (d) [Fe-
(TpivPP)(NO3)] (Ref. [17]); (e) [Fe(OEP)(NO3)] triclinic form (Ref. [15]);
(f) [Fe(OEP)(NO3)] monoclinic form.
4.2. Molecular structures
We report here a new crystalline form and a redeter-
mined structure for two high-spin ferric (nitrato)-porphyri-
nates. Schematic representations of the porphyrin core for

4670
G.R.A. Wyllie et al. / Polyhedron 26 (2007) 4664–4672
this observation, we conclude the previously reported
nitrate ligand orientation is in fact an averaging of the
two orientations we observe in our 100 K structure. These
two orientations present a more eclipsed nitrate position
both lying approximately 10ꢁ from a Fe–Np vector. A sim-
ilar orientation is observed in the other bidentate system
[Fe(TpivPP)(NO3)] [17] where the bidentate nitrate lies
10ꢁ from the closest Fe–Np vector. However, neither of
the two new nitrate orientations in the 100 K structure of
[Fe(TPP)(NO3)] display anywhere near the same extent of
asymmetric coordination as reported in the RT structure.
The major orientation shows the greatest disparity in the
nitrogens and the uncoordinated oxygen of the nitrate
ligand.
4.3. EPR spectroscopy
EPR spectra on both the powder and solutions of all
three nitrate complexes were collected at 77 and 4 K. These
data are summarized in Table 4 and displayed in Fig. 5.
For the OEP complexes, the EPR spectra for both solid
and solution at both temperatures are axial. At 77 K how-
ever, the minor gi feature was too weak to be detected.
When the temperature is reduced to 4 K, both g^ and gi
features are evident. The observation of axial spectra for
both the solid and the solution is consistent with a monod-
entate coordination mode in both states. Similar axial spec-
tra are observed in other five-coordinate high-spin systems
such as [Fe(por)(Cl)] [26]. We have already discussed the
differences in coordination mode of the nitrate ligand in
the TPP and picket-fence derivatives and we would expect
the EPR spectra for these to differ from the simple axial
spectra observed for the OEP derivative. Had the previ-
ously reported coordination mode of the nitrate in
[Fe(TPP)(NO3)], the highly asymmetric bidentate geome-
try, been representative of the nitrate geometry at lower
temperatures, we would perhaps have expected to see
EPR spectra for this system that differ from both that of
the OEP and TpivPP systems. Instead, following our new
crystallographic determination, we now expect the EPR
spectra of the TPP and TpivPP derivatives to be somewhat
similar. Indeed, this is true with the EPR of the ground
crystalline material displaying rhombic spectra for both
of the bidentate coordinated systems. [Fe(TPP)(NO3)] at
77 K exhibits a complex spectrum from which we identify
a major rhombic component has g = 7.71, 4.04 and 1.78
˚
two Fe–O bond length (d = 0.1433 A) whereas the bond
lengths in the minor orientation are practically equivalent
˚
(d = 0.0047 A). Based upon these results, we reclassify the
[Fe(TPP)(NO3)] system as containing a symmetrically
coordinated bidentate nitrate ligand.
We also observe some disparity in the Fe–O distances
in the two structures of [Fe(OEP)(NO3)]. This is most
evident in the nonbonded distance between the iron cen-
ter and the noncoordinating oxygen, O2, in Fig. 4e and f.
Although the Fe–O bond length for both of the monod-
entate nitrates are very similar, the nitrate position with
respect to the porphyrin core is significantly different.
In the previously reported triclinic form [15], the nitrate
adopts an orientation similar to the bidentate systems
with the Fe–O2 vector perpendicular to the porphyrin
plane and passing through the core center where the iron
sits. In the monoclinic form, the Fe–O bond lies more
perpendicular to porphyrin plane with the N–O(2) bond
shifted significantly from the centered normal to the por-
phyrin plane. This is reflected in a marked increase in the
distance between the iron center and the noncoordinated
˚
˚
O(3) from 2.644 A in the triclinic system to 3.042 A for
the monoclinic system. In both cases, the nitrate ligand
still lies close to midway between a pair of Fe–Np vectors
and equivalent iron out-of-plane displacements are
observed.
Fig. 4 illustrates the core geometry for all ferric
[Fe(por)(NO3)] systems reported to date. There are a num-
ber of features common to all the systems. Each system dis-
plays a significant out-of-plane displacement of the iron in
the direction of the nitrate ligand. large out-of-plane dis-
placements are common in five-coordinate ferric porphyri-
nates [25]. No significant differences are observed in the
Fe–Np distances for each system although the distances
for the two monodentate OEP derivatives are slightly
shorter than the meso-substituted porphyrinates.
The variation in the projection of the nitrate plane onto
the plane of the porphyrin has already been discussed
briefly. Comparing the data from all systems reveals when
the nitrate coordinates in a bidentate fashion, the projec-
tion of the ligand plane is close to a Fe–Np vector with a
dihedral angle (/) of ꢁ10ꢁ. When the nitrate coordinates
in a monodentate fashion, the orientation changes such
that it lies midway between the two Fe–Np vectors, possi-
bly to minimize steric interactions between the pyrrolic
Fig. 5. EPR spectra at 77 K of ground single crystals of (a)
[Fe(TpivPP)(NO3)], (b) [Fe(OEP)(NO3)], and (c) [Fe(TPP)(NO3)].

G.R.A. Wyllie et al. / Polyhedron 26 (2007) 4664–4672
4671
Fig. 6. Mo¨ssbauer spectra of (a) [Fe(OEP)(NO3)], (b) [Fe(TPP)(NO3)],
and (c) [Fe(TpivPP)(NO3)]. Spectra taken at room temperature.
Fig. 7. Mo¨ssbauer spectra and calculated fits of the spectra of (a)
[Fe(OEP)(NO3)], (b) [Fe(TPP)(NO3)], and (c) [Fe(TpivPP)(NO3)]. Spectra
taken at 4.2 K and an applied 4.5 T field parallel to the gamma ray beam.
with the largest g-tensor shifting to 8.5 at 4 K. Correspond-
ingly, the largest g-tensor in the EPR spectrum of the
ground crystalline [Fe(TpivPP)(NO3)] is smaller than that
of the TPP derivative (g = 6.44, 3.97, 1.9). EPR data from
a powder sample of [Fe(TpivPP)(NO3)] of unknown phase
which was prepared using a crude precipitation technique
exhibits a characteristic axial spectrum with g = 5.5, 2 pat-
tern. Based upon our observations from the OEP system,
this likely contains a monodentate nitrate ligand. Similarly,
axial EPR spectra are observed for solutions of the TPP
and picket fence derivatives. This suggests that the coordi-
nation mode of the nitrate ligand in both these systems
changes to monodentate upon solvation.
Fig. 7). All three systems displayed asymmetric quadru-
pole doublets at room temperature. Linewidths broadened
such that quadrupole splitting fits were inaccessible in the
temperature range from 150 K to below 18 K for the TPP
and TpivPP derivatives; the OEP derivative did not show
the same broadening behavior. The values for the quadru-
pole splitting are somewhat larger than other high-spin
ferric systems such as [Fe(por)(Cl)] (DEq ꢁ 0.9 mm/s).
Since the quadrupole splitting is dependent upon the sym-
metry of the metal center and we have already observed
significant differences in the nature of the g tensors in
the ground crystalline samples, we expect here to observe
substantial differences in the values of DEq for all three
systems. Although differences are observed, the fact that
the OEP derivative has the largest value of DEq is unex-
pected. Moreover, there remains some uncertainty about
the exact phase for the picket fence porphyrin
measurement.
4.4. Mo¨ssbauer spectroscopy
Table 5 summarizes the Mo¨ssbauer measurements car-
ried out on each of the nitrate complexes collected over a
range of temperatures. All three systems were studied at
4 K in an applied magnetic field and the spectra could
be satisfactorily fit to a S = 5/2 model (Table 6 and
Table 6
Data from S = 5/2 fit to 4.2 K Mo¨ssbauer spectra of [Fe(por)(NO3)] derivatives
Complex
DEq (mm/s)
d (mm/s)
g
C (mm/s)
D (cmꢀ1
)
E (cmꢀ1
)
Aii/gNbN (T)
[Fe(OEP)(NO3)]
[Fe(TPP)(NO3)]
[Fe(TpivPP)(NO3)]
1.44
0.97
1.0
0.53
0.61
0.56
0.06
0.0
0.4
0.52
0.42
0.57
5.7
6.9
10
0.37
0.6
0.2
ꢀ14.9, ꢀ17.3, ꢀ2.7
ꢀ21.8, ꢀ20.0, ꢀ10.4
ꢀ23.1, ꢀ20.9, ꢀ2.0

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G.R.A. Wyllie et al. / Polyhedron 26 (2007) 4664–4672
[6] R.F. Eich, T. Li, D.D. Lemon, D.H. Doherty, S.R. Curry, J.F.
5. Summary
Aitken, A.J. Mathews, K.A. Johnson, R.D. Smith, G.N. Phillips Jr.,
J.S. Olson, Biochemistry 35 (1996) 6976.
Nitrate can bind to the iron center in the iron(III) por-
phyrinates in either a symmetric bidentate or monodentate
mode. An earlier report of an asymmetric binding mode is
probably not correct. EPR spectra display sensitivity to the
coordination mode. An axial high-spin signal is seen for
monodentate coordination, whereas a rhombic high-spin
spectrum is observed for the bidentate species. Mo¨ssbauer
spectra do not appear to be as sensitive to the coordination
mode.
[7] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced
Inorganic Chemistry, sixth ed., Wiley-Interscience, New York, NY,
1999, p. 489.
[8] C.C. Addison, N. Logan, S.C. Wallwork, C.D. Garner, Quart. Rev.
25 (1971) 289.
[9] (a) B.M. Gatehouse, S.E. Livingstone, R.S. Nyholm, J. Chem. Soc.
(1957) 4222;
(b) A.B.P. Lever, E. Mantovani, B.S. Ramaswamy, Can. J. Chem. 49
(1971) 1957.
[10] (a) G.J. Van Driel, W.L. Driessen, J. Reedjik, Inorg. Chem. 24 (1985)
2919;
(b) G.J. Kleywegt, W.G.R. Wiesmeijer, G.J. Van Driel, W.L.
Driessen, J. Reedjik, J. Chem. Soc., Dalton Trans. (1985) 2717.
[11] Abbreviations used in this paper Por—a generalized porphyrin
dianion, OEP dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin,
TPP dianion of 5,10,15,20-tetraphenylporphyrin, TpivPP dianion of
Acknowledgements
We thank the National Institutes of Health for support
of this research under Grant GM-38401. Funds for the
purchase of the FAST area detector were provided through
NIH Grant RR-06709. We thank the NSF for EPR sup-
port through instrumentation Grant NSF-98-70990.
meso-tetrakis(a-a-a-a-pivalamidophenyl)porphyrin,
Np—pophyri-
nato nitrogen atom, amtd tris-((3,5-dimethylpyrazoyl-1-yl)methyl)-
amine, dpph diphenylpicrylhydrazl.
[12] (a) R. Han, G. Parkin, J. Am. Chem. Soc. 113 (1991) 9707;
(b) C. Kimblin, V.J. Murphy, T. Hascall, B.M. Bridgewater, J.B.
Bonanno, G. Parkin, Inorg. Chem. 39 (2000) 967.
Appendix A. Supplementary material
[13] G.R.A. Wyllie, W.R. Scheidt, Chem. Rev. 102 (2002) 1067.
[14] H. Nasri, M.K. Ellison, B. Shaevitz, G.P. Gupta, G. Lang, W.R.
Scheidt, Inorg. Chem. 45 (2006) 5284.
[15] M.K. Ellison, M. Shang, J. Kim, W.R. Scheidt, Acta Crystallogr.,
Sect. C 52 (1996) 3040.
[16] M.A. Phillippi, N. Baenziger, H.M. Goff, Inorg. Chem. 20 (1981) 3904.
[17] O.Q. Munro, W.R. Scheidt, Inorg. Chem. 37 (1998) 2308.
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[19] A.D. Adler, F.R. Longo, J.D. Finarelli, J. Goldmacher, J. Assour, L.
Korsakoff, J. Org. Chem. 32 (1967) 476.
CCDC 633779 and 633780 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at
doi:10.1016/j.poly.2007.03.048.
[20] A.D. Adler, F.R. Longo, F. Kampus, J. Kim, J. Inorg. Nucl. Chem.
32 (1970) 2443.
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Am. Chem. Soc. 97 (1975) 1427.
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