Relative axial ligand orientation in bis(imidazole)iron(II) porphyrinates: Are picket fence derivatives different?

Article abstract of DOI:10.1021/ic702498c

The synthesis of three new bis(imidazole)-ligated iron(II) picket fence porphyrin derivatives, [Fe(TpivPP)(1-RIm)₂] 1-RIm = 1-methyl-, 1-ethyl-, or 1-vinylimidazole) are reported. X-ray structure determinations reveal that the steric requirements of the four α,α,α,α- o-pivalamidophenyl groups lead to very restricted rotation of the imidazole ligand on the picket side of the porphyrin plane; the crowding leads to an imidazole plane orientation eclipsing an iron-porphyrin nitrogen bond. An unusual feature for these diamagnetic iron(II) species is that all three derivatives have the two axial ligands with a relative perpendicular orientation; the dihedral angles between the two imidazole planes are 77.2°, 62.4°, and 78.5°. All three derivatives have nearly planar porphyrin cores. Moessbauer spectroscopic characterization shows that all three derivatives have quadrupole splitting constants around 1.00 mm/s at 100K.

Full text of DOI:10.1021/ic702498c

Inorg. Chem. 2008, 47, 3841-3850
Relative Axial Ligand Orientation in Bis(imidazole)iron(II) Porphyrinates:
Are “Picket Fence” Derivatives Different?
Jianfeng Li,† Smitha M. Nair,† Bruce C. Noll,† Charles E. Schulz,*,‡ and W. Robert Scheidt*,†
Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556, and Department of Physics, Knox College, Galesburg, Illinois 61401
Received December 26, 2007
The synthesis of three new bis(imidazole)-ligated iron(II) picket fence porphyrin derivatives, [Fe(TpivPP)(1-RIm)2]
1-RIm ) 1-methyl-, 1-ethyl-, or 1-vinylimidazole) are reported. X-ray structure determinations reveal that the steric
requirements of the four R,R,R,R-o-pivalamidophenyl groups lead to very restricted rotation of the imidazole ligand
on the picket side of the porphyrin plane; the crowding leads to an imidazole plane orientation eclipsing an
iron-porphyrin nitrogen bond. An unusual feature for these diamagnetic iron(II) species is that all three derivatives
have the two axial ligands with a relative perpendicular orientation; the dihedral angles between the two imidazole
planes are 77.2°, 62.4°, and 78.5°. All three derivatives have nearly planar porphyrin cores. Mössbauer spectroscopic
characterization shows that all three derivatives have quadrupole splitting constants around 1.00 mm/s at 100K.
Introduction
tions. A low-spin, high-spin equilibrium (S ) 1/2, S ) 5/2)
has been found for this polymorph.4 In the second poly-
morph, the projection of the two planar axial ligands nearly
eclipses an Fe-Np bond direction, and an intermediate-spin
state was found.5 The differences in physical properties were
clearly related to absolute orientation of the axial ligands,
and these results led to further study of the effects of axial
ligand orientation on the electronic structure of iron
porphyrinates.
Subsequent studies on a series of low-spin iron(III) hemes
led us to the conclusion that parallel orientations of the two
axial ligands, rather than perpendicular orientations, are
energetically favored.6 Low-spin species with axial ligands
Model compound studies of iron(II) and -(III) porphyri-
nates have been useful for understanding the properties of
the hemoproteins.1 A common goal in many such model
compound studies is to develop relationships between
structural and spectroscopic features in the expectation that
the connections developed can be directly applied to describ-
ing and understanding hemoproteins. One area where this
strategy has been quite successful has been the examination
of the relative and absolute orientation of planar axial ligands.
The effect of the axial ligand orientation in iron(III)
derivatives is especially clear, whereas the iron(II) porphy-
rinate systems are less well-understood.
The first example demonstrating the importance of axial
ligand orientation was that of [Fe(OEP)(3-ClPy)2]ClO4
(2) The following abbreviations are used in this paper: Porph, a generalized
porphyrin dianion; TMP, dianion of meso-tetramesitylporphyrin; TPP,
dianion of meso-tetraphenylporphyrin; TTP, dianion of meso-tetra-
tolylporphyrin, TpivPP, dianion of R,R,R,R-tetrakis(o-pivalamidophe-
nyl)porphyrin; TF5PPBr8, dianion of 2,3,7,8,12,13,17,18-octabromo-
5,10,15,20-tetrakis(pentafluorophenyl)porphyrin; HIm, imidazole;
1-MeIm, 1-methylimidazole; 2-MeHIm, 2-methylimidazole; 4-MeHIm,
4-methylimidazole; 1,2-Me2Im, 1,2-dimethylimidazole; 1-AcIm, 1-acetyl-
imidazole; 1-SiMe3Im, 1-(trimethylsilyl)imidazole; 1-VinylIm, 1-vi-
nylimidazole; 1-BzylIm, 1-benzylimidazole; RIm, generalized imida-
zole;Py, pyridine; 3-ClPy, 3-chloropyridine; 4-CNPy, 4-cyanopyridine;
3-CNPy, 3-cyanopyridine; 4-MePy, 4-methylpyridine; 4-NMe2Py,
4-(dimethylamino)pyridine; Pip, piperidine; Pyz, pyrazine; Im -,
imidazolate; 2-MeIm-, 2-methylimidazolate; Np, porphyrinato nitro-
gen; Nax, nitrogen of axial ligands; Nim, nitrogen of imidazole ligands;
EPR, electron paramagnetic resonance.
2
where two different crystalline polymorphs were isolated.3–5
In these complexes, the two pyridines maintained a relative
parallel orientation, but the absolute orientation of the axial
pyridines changes. The differences in the magnetic properties
between the two polymorphs can be related to the absolute
orientation of the pyridine ligands. In one polymorph, the
projection of the axial ligands nearly bisect an Np-Fe-Np
angle, that is, they are along the iron-meso-carbon direc-
* To whom correspondence should be addressed. E-mail: scheidt.1@
nd.edu.
(3) Scheidt, W. R.; Geiger, D. K. J. Chem. Soc., Chem. Commun. 1979,
1154.
†
University of Notre Dame.
Knox College.
‡
(4) Scheidt, W. R.; Geiger, D. K.; Haller, K. J. J. Am. Chem. Soc. 1982,
104, 495.
(1) Walker, F. A. Chem. ReV. 2004, 104, 589.
10.1021/ic702498c CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 9, 2008 3841
Published on Web 03/20/2008

Li et al.
having relative parallel orientations display normal rhombic
electron paramagnetic resonance (EPR) spectra with three
observed g-values consistent with an electronic configuration
of (dxy)2 (dxz,dyz)3.6–10 Bulky axial ligands such as 2-meth-
ylimidazole (or, as we later found, the combination of
tetrakis-(2,6-disubstituted phenyl)porphyrinates together with
pyridines11,12 or bulky imidazoles13), are required to force
the relative perpendicular orientation of planar axial ligands
in Fe(III) porphyrinates. These iron(III) species display a
different EPR spectrum9,11,13,14 with a single-feature signal
similar b hemes of cytochrome b6f of chloroplasts, one of
the c-type hemes of cytochrome c3,24 and the c-type heme
of cytochrome c′′ of Methylophilus methylotrophus.25
Although some effort was required to obtain iron(III)
species with perpendicular ligand orientations, we had
assumed that for the closed subshell configuration of low-
spin d6 Fe(II) porphyrinates two planar axial ligands would
prefer to align themselves in mutually perpendicular planes
to maximize the π-bonding interactions between the filled
dπ orbitals of Fe(II) and the π* orbitals of the ligands.
However, subsequent studies26 showed that obtaining iron(II)
derivatives with mutually perpendicular orientations was not
as readily achieved as in the iron(III) species. The use of
bulky axial ligands and a porphyrin with bulky peripheral
groups was required. The structure of [Fe(TMP)(2-MeHIm)2]
showed that the two axial ligands have a nearly perpendicular
ligand orientation and a very ruffled porphyrin core. Möss-
bauer characterization showed that the complex displayed a
large quadrupole splitting of ∼1.7 mm/s.27 These structural
and electronic features are markedly different from a series
of [Fe(TPP)(RIm)2] derivatives with parallel orientations and
quadrupole splitting of about 1.0 mm/s.28
Recently, we synthesized and crystallized the molecule
[Fe(TpivPP)(1-MeIm)2] as part of a larger study of vibra-
tional dynamics of iron porphyrinates. The X-ray structure
of this species revealed that the two imidazole ligands had
a relative perpendicular orientation. This unanticipated
feature led us to synthesize and characterize two additional
picket fence species with different imidazole ligands. These
two new species also showed the similar geometric feature
of relative perpendicular ligands. All three species were
characterized by Mössbauer spectroscopy. These measure-
ments have unequivocally resolved that the effects of
porphyrin core conformation rather than axial ligand orienta-
tion define the Mössbauer spectra in low-spin iron(II)
porphyrinates.
15
at g g 3.2, that has been called large gmax or highly
anisotropic low-spin (HALS).16 For example, for [Fe(TPP)(2-
MeHIm)2]+,6,17 the observed spectrum results from mutually
perpendicular axial ligands that lead to nearly degenerate
iron dπ (dxz,dyz) orbitals.
The application of the principles derived from the com-
bined X-ray and EPR spectroscopic studies of iron porphy-
rinate derivatives to the hemoproteins with two planar
imidazole (histidine) ligands showed that a number of
hemoproteins have relative parallel oriented ligands. Systems
for which this is shown include cytochromes b5,18 three of
the heme centers of cytochromes c3,19 the b hemes of sulfite
oxidase20 and flavocytochrome b2,21 and the heme a of
cytochrome oxidase.22 Other hemoproteins found to have
perpendicularly oriented axial ligands based on these devel-
oped spectroscopic probes include the b hemes of mitochon-
drial complex III, also known as cytochrome bc1,23 the
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Experimental Section
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General Information. All reactions were carried out using
standard Schlenk techniques under argon unless otherwise noted.
Tetrahydrofuran (THF), benzene, toluene, hexanes, and heptane
were distilled over sodium and benzophenone ketyl; all other
solvents were used as received (Fisher). [H2 (TpivPP)] and
[Fe(TpivPP)Cl] were prepared according to a local modification
of the reported synthesis.29
(12) Safo, M. K.; Gupta, G. P.; Watson, C. T.; Simonis, U.; Walker, F. A.;
Scheidt, W. R. J. Am. Chem. Soc. 1992, 114, 7066.
(13) Munro, O. Q.; Marques, H. M.; Debrunner, P. G.; Mohanrao, K.;
Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935.
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5644.
(15) Walker, F. A.; Reis, D.; Balke, V. L. J. Am. Chem. Soc. 1984, 106,
6888.
Mössbauer measurements were performed on a constant ac-
celeration spectrometer from 4.2 to 298 K with optional small field
(16) Magita, C. T.; Iwaizumi, M. J. Am. Chem. Soc. 1981, 103, 4378.
(17) Scheidt, W. R.; Kirner, J. L.; Hoard, J. L.; Reed, C. A. J. Am. Chem.
Soc. 1987, 109, 1963.
(18) Mathews, F. S.; Czerwinski, E. W.; Argos, P. In The Porphyrins;
Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. VII, pp
107–147.
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Chem. 1982, 257, 14341. (b) Higuchi, Y.; Kusunoki, M.; Matsuura,
Y.; Yasuoka, N.; Kakudo, M. J. Mol. Biol. 1984, 172, 109.
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D. L.; Xavier, A. V. Eur. J. Biochem. 1992, 208, 427. (c) Costa, H. S.;
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Scheidt, W. R. J. Am. Chem. Soc. 1997, 119, 9438.
(19) Czjzek, M.; Payan, F.; Haser, R. Biochimie 1994, 76, 546.
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J. H. Biochemistry 1988, 27, 2918.
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Xuong, N. H.; Lederer, F.; Mathews, F. S. Proc. Natl. Acad. Sci. U.S.A.
1987, 84, 2629. (b) Dubois, J.; Chapman, S. K.; Mathews, F. S.; Reid,
G. A.; Lederer, F. Biochemistry 1990, 29, 6393.
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3842 Inorganic Chemistry, Vol. 47, No. 9, 2008

Ligand Orientation in Bis(imidazole)iron(II) Porphyrinates
Table 1. Complete Crystallographic Details for [Fe(TpivPP)(1-MeIm)2]·1-MeIm, [Fe(TpivPP)(1-EtIm)2]·0.5C7H8, and [Fe(TpivPP)(1-VinylIm)2]·2C7H8
[Fe(TpivPP)(1-MeIm)2]·1-MeIm
[Fe(TpivPP)(1-EtIm)2]·0.5C7H8
[Fe(TpivPP)(1-VinylIm)2]·2C7H8
chemical formula
fw
a, Å
b, Å
c, Å
C76H82FeN14O4
1311.41
13.2358(3)
19.2487(5)
27.4149(6)
90
103.782(1)
90
6783.5(3)
P21/c
C
77.5H84FeN12O4
C88H92FeN12O4
1437.59
13.4924(4)
28.2916(9)
20.2542(6)
90
106.108(2)
90
7427.9(4)
P21/c
1303.44
13.1799(3)
13.3672(3)
22.2734(5)
75.524(1)
88.168(1)
65.322(1)
3440.79(13)
R, deg
ꢀ, deg
γ, deg
V, Å3
j
P1
2
space group
Z
4
4
temp, K
Dcalcd, g cm-3
µ, mm-1
final R indices
[I > 2σ (I)]
final R indices
(all data)
100
1.284
0.284
R1 ) 0.0585
wR2 ) 0.1513
R1 ) 0.0787
wR2 ) 0.1673
100
1.258
0.279
R1 ) 0.0444
wR2 ) 0.1114
R1 ) 0.0662
wR2 ) 0.1297
100
1.286
0.265
R1 ) 0.0555
wR2 ) 0.1372
R1 ) 0.0796
wR2 ) 0.1476
(Knox College). Samples for Mössbauer spectroscopy were prepared
by immobilization of the crystalline material in Apiezon M grease.
Synthesis of [Fe(TpivPP)(1-MeIm)2]·1-MeIm. [Fe(TpivPP)Cl]
(55 mg, 0.05 mmol) and fresh zinc amalgam (10% zinc, 1 mL)
were dried in vacuum for 30 min. 1-Methylimidazole (0.04 mL)
and toluene (∼15 mL) were then transferred into the Schlenk by
cannula. This mixture was stirred under an argon atmosphere at
30–40 °C for 2 h. After standing overnight, this red solution was
filtered. Hexanes were then allowed to diffuse slowly into the
filtrate. One week later, a needle crystalline product was collected.
Synthesis of [Fe(TpivPP)(1-EtIm)2]·0.5C7H8. Similar reaction
procedures as above were performed, heptane was used as the
nonsolvent to diffuse into the reaction solution. Several days later,
a block crystalline product was collected.
[Fe(TpivPP)(1-MeIm)2]·1-MeIm. A translucent red needle
crystal with the dimensions of 0.32 × 0.21 × 0.18 mm3 was used
for the structure determination. The asymmetric unit contains one
bis-ligated “picket fence” porphyrin complex and one 1-methylimi-
dazole molecule. One of the tert-butyl groups on the four pivalamide
“pickets” was disordered. Thus, the four butyl tert-carbon atoms
are restrained by “similar Uij ” (SIMU) to constrain the anisotropic
displacement parameters.
[Fe(TpivPP)(1-EtIm)2]·0.5C7H8. A dark purple block crystal
with the dimensions of 0.32 × 0.19 × 0.10 mm3 was used for the
structure determination. The asymmetric unit contains one bis-
ligated “picket fence” porphyrin complex and half of a toluene
solvent molecule. The toluene molecule was disordered around an
inversion center; thus, the occupancy of the methyl group is refined
as 0.5, and the three ring carbons were refined as 1.0.
[Fe(TpivPP)(1-VinylIm)2]·2C7H8. A translucent red plate crys-
tal with the dimensions of 0.28 × 0.16 × 0.12 mm3 was used for
the structure determination. The asymmetric unit contains one bis-
ligated “picket fence” porphyrin complex and two toluene solvent
molecules. Two of the carbon atoms (C6 and C7) on one of the
imidazole rings (C6-C7-N8-C8-N7) exhibited unusual thermal
motions suggesting some small disorder that could not be success-
fully modeled further.
Synthesis of [Fe(TpivPP)(1-VinylIm)2]·2C7H8. Similar reaction
procedures as above were performed; hexanes were used as the
nonsolvent to diffuse into the reaction solution. Several days later,
a block crystalline product was collected.
X-ray Structure Determinations. Single crystal experiments
were carried out on Bruker Apex II systems with graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å). The crystalline
samples were placed in inert oil, mounted on a glass pin, transferred
to the cold gas stream of the diffractometer, and crystal data
collected at 100 K. The structures were solved by direct methods
(SHELXS-97)30 and refined against F2 using SHELXL-97.31,32
Subsequent difference Fourier syntheses led to the location of all
remaining nonhydrogen atoms. For the structure refinement, all data
were used including negative intensities. All nonhydrogen atoms
were refined anisotropically if not remarked upon otherwise below.
Hydrogen atoms were idealized with the standard SHELX idealiza-
tion methods. The program SADABS33 was applied for the
absorption correction. Complete crystallographic details, atomic
coordinates, anisotropic thermal parameters, and fixed hydrogen
atom coordinates are given in the Supporting Information Tables
for all three structures; a brief sumary of crystallographic details is
given in Table 1.
Results
The synthesis, molecular structures, and Mössbauer spectra
of three new six-coordinate bis-imidazole “picket fence”
porphyrinates, [Fe(TpivPP)(1-MeIm)2] · 1-MeIm, [Fe-
(TpivPP)(1-EtIm)2] · 0.5C7H8,
and
[Fe(TpivPP)(1-
VinylIm)2] · 2C7H8 are reported. Crystalline [Fe(TpivPP)(1-
MeIm)2]·1-MeIm contains one bis-ligated iron porphyrinate
and one 1-methylimidazole molecule. Two labeled Oak
Ridge thermal ellipsoid plot (ORTEP) diagrams of [Fe(T-
pivPP)(1-MeIm)2] are given in Figures 1 and 2.
ORTEP diagrams of the molecular structures of the other
two bis-ligated “picket fence” iron porphyrinates, [Fe(T-
pivPP)(1-EtIm)2] and [Fe(TpivPP)(1-VinylIm)2], are shown
in the Supporting Information Figures S1-S4. A brief
summary of the crystallographic data is given in Table 1,
and the complete crystallographic details, atomic coordinates,
bond distances, bond angles of these three structure are given
in the Supporting Information Tables S1-S18.
(30) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(31) Sheldrick, G. M. Program for the Refinement of Crystal Structures;
Universität Göttingen: Germany, 1997.
(32) The conventional R-factors R1 are 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.
(33) Sheldrick, G. M. Program for Empirical Absorption Correction of
Area Detector Data; Universität Göttingen: Germany, 1996.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3843

Li et al.
study of vibrational dynamics of iron porphyrinates. The
structure determination of this complex showed unexpected
and noteworthy features.
Two ORTEP diagrams of [Fe(TpivPP)(1-MeIm)2] have
been presented-Figure 1 gives the view normal to the
porphyrin plane, whereas Figure 2 is the edge-on view. It
can be readily noted that the two imidazole ligands are both
perpendicular to the porphyrin plane, and surprisingly, the
two imidazole planes have a relative perpendicular orienta-
tion. The actual dihedral angle between the two imidazole
planes is 77.2°. The two axial imidazole ligand planes almost
align with the Fe-Np axes, with the 1-MeIm ligand planes
on the “picket fence” side making a dihedral angle of 8.5°;
and the imidazole on the other side making a 21.1° angle to
the closest Fe-Np vector. Additional quantitative information
is given in the top panel of Figure 3, which displays the
detailed displacements of each porphyrin core atom (in units
of 0.01 Å) from the 24-atom mean plane. The orientation of
the two 1-MeIm ligands including the values of the dihedral
angles are also shown; the circle represents the position of
the methyl group on each ligand.
Figure 3 also shows that the core conformation of
[Fe(TpivPP)(1-MeIm)2] has a modestly ruffled deformation,
with two of the meso-carbon atoms having displacements
of 0.14 and 0.15 Å from the mean plane. The iron center is
slightly out of the mean plane (0.04 Å) toward the picket
side of the porphyrin plane; the average Np-Fe-Np angle
is ideal at 90.00(18)°. The average Fe-Np distance of
1.9924(28) Å is a typical value for low-spin (porphinato)iro-
n(II) derivatives.34
Figure 1. ORTEP diagram of [Fe(TpivPP)(1-MeIm)2] displaying the atom
labeling scheme. Thermal ellipsoids of all atoms are contoured at the 50%
probability level. Hydrogen atoms have been omitted for clarity. The
porphyrin plane is in the plane of the paper.
Although most of these structural features are expected
for low-spin bis(imidazole) derivatives, one feature was quite
unexpected: the relative perpendicular orientation of the two
imidazole rings. To ascertain if this is a general feature of
picket fence porphyrin derivatives, we characterized two
additional examples with 1-ethyl- or 1-vinylimidazole as the
axial ligands. [Fe(TpivPP)(1-EtIm)2] · 0.5C7H8 and [Fe(T-
pivPP)(1-VinylIm)2]·2C7H8 were thus synthesized and struc-
turally characterized. ORTEP diagrams for both are available
in the Supporting Information.
As can be seen from both the ORTEP drawings and the
formal diagrams given in Figure 3, the pattern of the
orientation of the two imidazoles in these new species is
very similar to that seen in [Fe(TpivPP)(1-MeIm)2]. Both
ligand planes have a roughly eclipsed orientation with a
closest Fe-Np axis in all derivatives. Moreover, all deriva-
tives have the two axial imidazoles with relative perpen-
dicular orientations. Both derivatives also have modestly
saddled core conformations but with a different orientation
with respect to the axial ligands. We can conclude from this
that the axial ligand must have very little effect on the overall
core conformation, but the porphyrin substituents must have
significant effects on the axial ligand orientations.
Figure 2. A edge-on ORTEP diagram of [Fe(TpivPP)(1-MeIm)2] displaying
the atom labeling scheme. Thermal ellipsoids of all atoms are contoured at
the 50% probability level. Hydrogen atoms have been omitted for clarity.
The porphyrin plane is perpendicular to the plane of the paper.
Solid-state Mössbauer spectra were measured for all three
complexes and taken at several different temperatures from
20 to 298 K; they will be discussed subsequently.
Discussion
Structures. The bis(imidazole)-ligated iron(II) “picket
fence” porphyrinates were first presented by Collman and
co-workers29 in 1975 as part of their well-known work on
“synthetic models of oxygen-binding hemoproteins.” How-
ever, no member of the bis(imidazole)-ligated set of species
has ever been structurally studied. Recently, we synthesized
and crystallized [Fe(TpivPP)(1-MeIm)2] as part of a larger
There are several common structural features observed for
the three new structures. First, as already noted, the two axial
ligands are close to eclipsing the closest Fe-Np vector, and
(34) Scheidt, W. R.; Reed, C. A. Chem. ReV. 1981, 81, 543.
3844 Inorganic Chemistry, Vol. 47, No. 9, 2008

Ligand Orientation in Bis(imidazole)iron(II) Porphyrinates
compared to those of the unhindered side. The differences
are 0.004 Å in ([Fe(TpivPP)(1-MeIm)2]), 0.030 Å in ([Fe-
(TpivPP)(1-EtIm)2]), and 0.011 Å in ([Fe(TpivPP)(1-Vi-
nylIm)2]). This bond length asymmetry, especially prominent
in iron(III) species, had been explained by the nonbonded
interactions between coordinated imidazole and porphinato
atoms.35,36
The key structural parameters of all structurally character-
ized [Fe(II)(Porph)(L)2] structures with “L” being a nitrogen
donor axial ligand are shown in Table 2. All but one of these
derivatives have planar axial ligands. Listed in the table is
the crystallographically required symmetry at the iron center,
the absolute ligand orientation given by the dihedral angle
between the axial ligand plane and the closest Nax-Fe-Np
plane, conventionally denoted by ꢁ. Also given is the relative
ligand orientation, the dihedral angle between the two axial
ligand planes and denoted by θ.
Excluding the current derivatives, there are a total of 16
reported iron(II) structures in Table 2. Ten of these have
crystallographically required inversion symmetry at iron and
necessarily have relative parallel axial ligand pairs. The last six
species of Table 2 have no symmetry at iron (five) or C2
symmetry (one). Most have relatively large values of the
dihedral angle between the two axial ligands. Are there
explanations for the six derivatives that are different from those
for the ten derivatives? We believe that the differences appear
to reflect the contrast in core conformation. The ten derivatives
with relative parallel axial ligand orientations also have near
planar porphyrin cores consonant with the observed inversion
symmetry, whereas the six derivatives with nonzero dihedral
angles between the two axial ligands have strongly ruffled or
saddled porphyrin core conformations that do not allow the
possibility of an inversion center. A combination of porphyrin
peripheral groups, porphyrin core conformation, and/or axial
ligand substitution in these six derivatives are the reason for
the absence of the inversion center. We consider the stereo-
chemical features of these derivatives in turn.
In six-coordinate [Fe(TMP)(2-MeHIm)2], a near-perpen-
dicular relative ligand orientation results from coordination
of two bulky 2-methylimidazole axial ligands.27 The 2-meth-
yl groups of the axial ligands induce a strongly ruffled
porphyrin core in which the ligands must bind in mutually
perpendicular ligand binding pockets if the species is to
become low spin. The sterically crowded system leads to
slight increases in the axial Fe-NIm bond distances relative
to the other imidazole derivatives in Table 2. An electronic
reason for ruffling is precluded in these structures as the low-
spin d6 electron configuration minimizes possible porphyrin
f Fe π donation. This is in contrast to the Fe(III) case where
PorfFe(III) π donation from the porphyrin a2u (π) or-
bital requires ruffling26,37,38 when the ground-state is
(dxz,dyz)4(dxy)1.
Figure 3. Formal diagrams of the porphinato cores of [Fe(TpivPP)(1-
MeIm)2] (top), [Fe(TpivPP)(1-EtIm)2] (middle), and [Fe(TpivPP)(1-Vi-
nylIm)2] (bottom). Averaged values of the chemically unique bond distances
(in Å) and angles (in degrees) are shown. The numbers in parentheses are
the esd’s calculated on the assumption that the averaged values were all
drawn from the same population. The perpendicular displacements (in units
of 0.01 Å) of the porphyrin core atoms from the 24-atom mean plane are
also displayed. In all diagrams, positive values of the displacement are
towards the hindered porphyrin side while the dashed line indicates the
imidazole on the unhindered porphyrin side.
second, the two axial ligands have relative perpendicular
orientations. Third, the central iron atom has a small out-
of-plane displacement of 0.04 to 0.05 Å with all displace-
ments toward the hindered (picket) porphyrin side. Fourth,
the imidazole plane on the hindered side always has a smaller
dihedral angle with the closest Fe-Np axis. Thus, the three
structures have dihedral angles in the range of 6.6-11.2°
on the picket side, and 20.7–24.5° on the other side. The
Fe-Nax distances on the picket side are slightly elongated
Two additional examples have near-perpendicular ligand
39
orientation: [Fe((C3F7)4)P)(Py)2
and [Fe(TF5PPBr8)-
(Py)2].40 These two complexes have extremely ruffled
([Fe((C3F7)4)P)(Py)2]) or saddled ([Fe(TF5PPBr8)(Py)2]) core
(35) Scheidt, W. R.; Chipman, D. M. J. Am. Chem. Soc. 1986, 108, 1163.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3845

Li et al.
Table 2. Selected Structural Parameters for [Fe(II)(Porph)(L)2]a
c d
,
d
f g
,
f h
,
Fe S. S.b
(Fe-Np)av
Fe-Nax
core conf.d e
ꢁ1,2
θ
ref.
tw
,
complex
[Fe(TpivPP)(1-MeIm)2]
C1
1.992(3)
1.9958(19)
1.9921(18)
2.0244(18)
1.9940(19)
1.9979(19)
1.9866(18)
2.004(2)
2.017(4)
2.014(5)
2.0154(8)
1.996(2)
2.026(2)
2.010(2)
2.039(1)
2.037(1)
2.127(3)
2.030(3)
2.047(3)
2.032(3)
2.028(3)
2.007(6)
1.996(6)
2.007(7)
2.016(7)
2.000(3)
2.040(3)
2.010(3)
1.970(3)
near-Pla
(Cm,0.12)
near-Pla
(Cꢀ,0.05)
Near-Pla
(Cꢀ,0.17)
near-Pla
near-Pla
near-Pla
near-Pla
near-Pla
near-Pla
near-Pla
near-Pla
near-Pla
near-Pla
Ruf(0.51)
8.5
77.2
62.4
78.5
21.1
6.6
20.7
11.2
24.5
14
26
15
0.7
40
42
[Fe(TpivPP)(1-EtIm)2]
C1
C1
1.993(6)
1.988(5)
tw
tw
[Fe(TpivPP)(1-VinylIm)2]
[Fe(TPP)(1-VinylIm)2]
[Fe(TPP)(1-BzylIm)2]
[Fe(TPP)(1-MeIm)2]
[Fe(TPP)(4-MeHIm)2]
[Fe(TMP)(4-CNPy)2]
[Fe(TMP)(3-CNPy)2]
[Fe(TMP)(4-MePy)2 ]
[Fe(TPP)(Py)2] Py
[Fe(TPP)(Py)2]
[Fe(TPP)(Pip)2]
[Fe(TMP)(2-MeHIm)2]
(mol 1)
[Fe(TMP)(2-MeHIm)2]
(mol 2)
[Fe((C3F7)4)P)(Py)2]
Ci
Ci
Ci
Ci
Ci
Ci
Ci
CI
Ci
Ci
Ci
2.001(2)
1.993(9)
1.997(6)
0i
28
28
42
43
26
26
26
44
45
46
27
0i
0i
1.9952(8)
1.993(2)
1.996(2)
1.988(2)
1.993(6)
2.001(2)
2.004(6)
1.964(5)
0i
0i
0i
41
34.4
45
0i
0i
0i
0i
41.1
41.4
44.8
37.9
41.3
46.0
1.5
22.2
36.8
33.7
3.9
82.4
C1
C1
C1
C2
C1
1.961(7)
1.958(4)
1.963(4)
1.976(2)
1.987(8)
Ruf(0.50)
Ruf(0.62)
Sad(0.97)
Sad(0.67)
Sad(0.14)
84.4
87.5
68.3
19.2
40.9
27
39
40
47
48
[Fe(TF5PPBr8)(Py)2]
[Fe(TPPBr4)(Py)2]
[Fe(TPP)(Pyz)2]
37.0
a Estimated standard deviations are given in parentheses. b Site symmetry of Fe. c Averaged value. d Value in Å. e Number in parenthesis is the average
displacement of the methine carbons (Cm) or pyrrole ꢀ carbons (Cꢀ) from the 24-atom mean plane for ruffling (Ruf) or saddling (Sad) deformation, respectively;
near-Pla indicates an almost planar porphyrin plane in a centrosymmetric or picket fence structure. f Value in degrees. g Dihedral angle between the plane
of the closest Np-Fe-Nax and the ligand plane. h Dihedral angle between two axial ligands. i Exact value required by symmetry.
conformations. These core conformations are the result of
the peripheral substituents. The final two examples have
smaller dihedral angles between the two axial ligands;
however, only the last example could actually have inversion
symmetry for the molecule as a whole.
The picket fence derivatives are another set of porphyrin
systems that lead to relative perpendicular orientations of
the two axial ligands in iron(II) species; however, the basis
for this effect appears to be distinct from any of the systems
given above.
axis (ꢁ): 8.5° (1-MeIm), 6.6° (1-EtIm), and 11.2° (1-
VinylIm). This effect is clearly illustrated by the space-filling
models shown in Figure 4 for the three species. A close
examination of the t-butyl group orientations in the three
species shows that there are small differences that reflect
the differing sizes of the axial imidazoles. This is most clearly
seen in the differences between the 1-vinylimidazole species
and the other two; in the vinylimidazole system, the pickets
form two “walls” rather than the “V-shaped” cavity of the
other two derivatives.
We start with an examination from the hindered porphyrin
side of the molecules. The “picket fence” environment has
strong steric effects on the axial ligand orientation. The
crowding between the four bulky t-butyl groups of the pickets
and the axial imidazole will allow only a limited rotational
motion of the axial ligand around the Fe-NIm bond. This
crowding forces the axial ligands to align in specific
orientations, always near the axes defined by the porphyrin
nitrogen atoms, and prevents the imidazoles from freely
rotating. The three different imidazoles on the picket side
all show very small dihedral angles with the closest Fe-Np
We then need to consider the orientation of the second
imidazole on the opposite, unhindered porphyrin side. As
shown in the three 24-atom mean plane diagrams (Figure
3), the second imidazoles all have small ꢁ angles ranging
from 20.7 to 24.5° with the closest Fe-Np axis. Space-filling
models showing the opposite side of the porphyrin plane in
the three derivatives are given in Figure 5 where the small
ꢁ values are quite evident.
Selected structural parameters and spin states for five- and
six-coordinate [FeII(TpivPP)(L1)(L2)] imidazole derivatives
are listed in Table 3. L1 and L2 denote the ligands on the
hindered and unhindered sides of the porphyrin plane,
respectively. We first consider the situation for six-coordinate
complexes, all of which are low spin. It is noted that the
two structures with anionic ligands, [Fe(TpivPP)(NO2)(HIm)]
and [Fe(TpivPP)(Im-)(HIm)]-, have the smallest ꢁ2 angles.
Even if these two are not considered, it is perhaps surprising
to find that the other complexes have ꢁ2 angles in the very
(36) Collins, D. M.; Countryman, R.; Hoard, J. L. J. Am. Chem. Soc. 1972,
94, 2066.
(37) Safo, M. K.; Walker, F. A.; Raitsimring, A. M; Walters, W. P.; Dolata,
D. P.; Debrunner, P. G.; Scheidt, W. R. J. Am. Chem. Soc. 1994, 116,
7760.
(38) Walker, F. A.; Nasri, H.; Turowska-Tyrk, H.; Mohanrao, K.; Watson,
C. T.; Shokhirev, N. V.; Debrunner, P. G.; Scheidt, W. R. J. Am.
Chem. Soc. 1996, 118, 12109.
(39) Moore, K. T.; Fletcher, J. T.; Therien, M. J. J. Am. Chem. Soc. 1999,
121, 5196.
(40) Grinstaff, M. W.; Hill, M. G.; Birnbaum, E. R.; Schaefer, W. P.;
Labinger, J. A.; Gray, H. B. Inorg. Chem. 1995, 34, 4896.
(41) CrystalMaker Version 7.2.3; CrystalMaker Software: Bicester, Ox-
fordshire OX26 3TA, England, 2007.
3846 Inorganic Chemistry, Vol. 47, No. 9, 2008

Ligand Orientation in Bis(imidazole)iron(II) Porphyrinates
Figure 5. Spacing-filling diagrams (view perpendicular to the porphyrin
core) of [Fe(TpivPP)(1-MeIm)2] (top), [Fe(TpivPP)(1-EtIm)2] (middle), and
[Fe(TpivPP)(1-VinylIm)2] (bottom), showing the unhindered porphyrin side
(180 °; rotated from Figure 4) (Carbon, dark grey; imidazole carbon, brown;
Hydrogen, light grey; Nitrogen, blue; Oxygen, red; drawn with Crystal-
maker41).
Figure 4. Spacing-filling diagrams (view perpendicular to the porphyrin
plane) of [Fe(TpivPP)(1-MeIm)2] (top), [Fe(TpivPP)(1-EtIm)2] (middle),
and [Fe(TpivPP)(1-VinylIm)2] (bottom), showing the pocket porphyrin side
(Carbon, dark grey; imidazole carbon, brown; Hydrogen, light grey;
Nitrogen, blue; Oxygen, red; drawn with Crystalmaker41).
porphyrin plane side. However, these absolute orientations
could equally lead to either relative parallel or relative
narrow range of 20–24.5°. This result indicates that the
imidazole on the unhindered porphyrin side is very likely to
have a dihedral angle e ∼22° with the closest Fe-Np axis,
irrespective of the ligands on the opposite side. This is seen
even for five-coordinate high-spin [Fe(TpivPP)(2-MeHIm)],
where the bulky imidazole has a ꢁ2 angle of 22.8°.
(42) Steffen, W. L.; Chun, H. K.; Hoard, J. L.; Reed, C. A. Abstracts of
Papers, 175th National Meeting of the American Chemical Society,
Anaheim, CA, March 13, 1978; American Chemical Society: Wash-
ington, DC, 1978; INOR 15.
(43) Silvernail, N. J.; Noll, B. C.; Scheidt, W. R. Acta Crystallogr., Sect.
E 2005, E61, m1201.
These observations allow us to rationalize the absolute
orientation of the imidazole ligands on the unhindered
Inorganic Chemistry, Vol. 47, No. 9, 2008 3847

Li et al.
Table 3. Selected Structural Parameters for [Fe(TpivPP)(L1)(L2)] Imidazole Derivativesa b
,
c d
,
e
d
d
f g
,
f g
,
f h
,
complex
spin state
∆
(Fe-Np)av
Fe-L1
Fe-L2
ꢁ1
ꢁ2
θ
ref.
Five-Coordinate
[Fe(TpivPP)(2-MeHIm)]
[Fe(TpivPP)(2-MeIm-)]-
HS
HS
-0.43
0.65
2.072(2)
2.106(20)
2.095(6)
22.8
49
50
2.002(15)
14.7
Six-Coordinate
[Fe(TpivPP)(1-MeIm)2]
[Fe(TpivPP)(1-EtIm)2]
LS
LS
LS
LS
LS
LS
LS
0.04
0.05
0.05
-0.11
0.02
0.01
-0.01
1.9924(28)
1.9958(19)
2.0244(18)
1.9979(19)
1.898(7)
1.75(2)
1.930(5)
1.9921(18)
1.9940(19)
1.9866(18)
2.107(4)
2.07(2)
1.953(5)
8.5
6.6
11.2
42.9,43.3
41.7,42.4
25.1
21.1
20.7
24.5
22.2
20
77.2
64.3
78.6
24.4,69.4
27.4,62.9
36.8
tw
tw
tw
49
51,52
50
53
1.993(6)
1.988(5)
1.996(2)
1.98(1)
1.995(9)
1.970(4)
[Fe(TpivPP)(1-VinylIm)2]
[Fe(TpivPP)(O2)(2-MeIm)]
[Fe(TpivPP)(O2)(1-MeIm)]
[Fe(TpivPP)(Im-)(HIm)]-
[Fe(III)(TpivPP)(NO2)(HIm)]
11.7
16.4
1.949(10)
2.037(10)
37
69
a Estimated standard deviations are given in parentheses. b L1 denotes the ligand in the pocket of the porphyrin, L2 denotes the ligand on the unhindered
porphyrin side. c Displacement of iron atom from the 24-atom mean plane, a positive value is toward to the hindered porphyrin side. d Value in Å. e Average
value in Å. f Value in degree. g Dihedral angle between the planes defined by the closest Np-Fe-N and the ligand plane. h Dihedral angle between two
ax
axial ligands.
perpendicular orientations of the axial imidazole pairs. What
bonding feature gives the observed relative perpendicular
orientation to three distinct picket fence derivatives?
Prior density functional theory (DFT) calculations have
suggested that [Fe(II)(Porph)(L)2] complexes, especially
when the axial ligands are imidazoles, do not have a strong
orientational preference.54,55 The two idealized ligand ori-
entations, relative parallel versus relative perpendicular, are
calculated to be isoenergetic conformers to within 1 kcal/
mol. These calculations start by assuming that the projection
of the first imidazole ligand will bisect an equatorial
Np-Fe-Np angle yielding a ꢁ angle of ∼45°, much larger
than those observed here. In an earlier study, Scheidt and
Chipman35 had suggested that a metal pπ-imidazole pπ
interaction would favor eclipsed orientations for both five-
and six-coordinate imidazole-ligated species.
The values of the axial Fe-NIm bonds given in Table 2
suggest that, in the picket fence derivatives with relative
perpendicular orientations, the axial bonding is slightly
stronger than the four imidazole-ligated complexes with
relative parallel orientations. The axial distances found
for the relative parallel orientation species are systemati-
cally larger.35,36 However, the differences are marginal
and are certainly not significant at the three sigma level.
Nonetheless, the systematic orientational behavior of the
picket fence derivatives suggests an underlying effect. The
most reasonable explanation for the apparently systematic
effect in the picket fence porphyrin derivatives is that a
very modest π-bonding between the imidazole and iron
will be maximized only if the two axial ligands have a
relative perpendicular orientation. Like the differences in
the axial bond lengths, the effect must be a very modest
one. The evidence presented here suggests that the effect
starts with the enforced orientation of the axial ligand
nearly eclipsing a coordinating Fe-Np bond. The steric
effects of the pickets prevents the rotation of the imida-
zole. Such rotations are known to be quite rapid in a
number of model heme complexes in homogeneous
solution.56 This suggests that any further theoretical studies
of axial ligand orientation effects should include consid-
eration of the absolute as well as the relative ligand
orientation.
Mössbauer Spectra. Mössbauer spectra were measured
for the three new complexes in the solid state from room
temperature to 20 K; the 200 K spectra are illustrated in
Figure 6. The spectra are very similar and consist of a single
quadrupole doublet at all temperatures. The quadrupole
splitting decreases slightly with decreasing temperature with
a maximum value of 1.07 mm/s at room temperature and a
minimum of 0.99 mm/s at 20 K. The isomer shift increases
slightly from 0.35 to 0.45 mm/s. Both the quadrupole
splitting and the isomer shift values are those expected for
low-spin iron(II) complexes.
Table 4 shows the Mössbauer data observed for the current
complexes and the values observed for all known, related
six-coordinate low-spin species. An examination of all
quadrupole splitting values shows that almost all of these
species have quadrupole splitting values in the range of
1.0–1.2 mm/s, with the pyridine derivatives having slightly
higher values than those of the imidazoles. Only one set of
imidazole derivatives, those with a sterically bulky 2-sub-
stituent on the imidazole ligand, are seen to have quadrupole
splitting constants greater than ∼1.60 mm/s. Only one such
(44) Li, N.; Petricek, V.; Coppens, P.; Landrum, J. Acta Crystallogr., Sect.
C 1985, C41, 902.
(45) Li, N.; Coppens, P.; Landrum, J. Inorg. Chem. 1988, 27, 482.
(46) Radonovich, L. J.; Bloom, A.; Hoard, J. L. J. Am. Chem. Soc. 1972,
94, 2073.
(47) Scheidt, W. R.; Noll, B. C. Acta Crystallogr., Sect. E 2006, E62,
m1892.
(48) Hiller, W.; Hanack, M.; Mezger, M. G. Acta Crystallogr., Sect. C
1987, C43, 1264.
(49) Jameson, G. B.; Molinaro, F. S.; Ibers, J. A.; Collman, J. P.; Brauman,
J. I.; Rose, E.; Suslick, K. S. J. Am. Chem. Soc. 1980, 102, 3224.
(50) Mandon, D.; Ott-Woelfel, F.; Fischer, J.; Weiss, R.; Bill, E.; Trautwein,
A. X. Inorg. Chem. 1990, 29, 2442.
(51) Collman, J. P.; Gagne, R. R.; Reed, C. A.; Robinson, W. T.; Rodley,
G. A. Proc. Natl. Acad. Sci. U.S.A. 1974, 71, 1326.
(52) Jameson, G. B.; Rodley, G. A.; Robinson, W. T.; Gagne, R. R.; Reed,
C. A.; Collman, J. P. Inorg. Chem. 1978, 17, 850.
(53) Nasri, H.; Wang, Y.; Huynh, B. H.; Walker, F. A.; Scheidt, W. R.
Inorg. Chem. 1991, 30, 1483.
(56) (a) Nakamura, M.; Groves, J. T. Tetrahedron 1984, 44, 3225. (b)
Walker, F. A.; Simonis, U. J. Am. Chem. Soc. 1991, 113, 8652. (c)
Shokhirev, N. V.; Shokhireva, T. Kh.; Polam, J. R.; Watson, C. T.;
Raffii, K.; Simonis, U.; Walker, F. A. J. Phys. Chem. A 1997, 101,
2778. (d) Momot, K. I.; Walker, F. A. J. Phys. Chem. A 1997, 101,
2787. (e) Nakamura, M.; Tajima, K.; Tada, K.; Ishizu, K.; Nakamura,
N. Inorg. Chim. Acta 1994, 224, 113. (f) Polam, J. R.; Shokhireva, T.
Kh.; Raffii, K.; Simonis, U.; Walker, F. A. Inorg. Chim. Acta 1997,
263, 109.
(54) Medakovic, V.; Zaric, S. D. Inorg. Chim. Acta 2003, 349, 1.
(55) Ghosh, A.; Gonzales, E.; Vangberg, T. J. Phys. Chem. B 1999, 103,
1363.
3848 Inorganic Chemistry, Vol. 47, No. 9, 2008

Ligand Orientation in Bis(imidazole)iron(II) Porphyrinates
Table 4. Selected Mössbauer Parameters for [Fe(II)(Porph)(L)2] Derivatives
a
a
complex
∆EQ
δFe
sample phase
T, K
θb
77.2
conf.c
ref.
tw
[Fe(TpivPP)(1-MeIm)2]
0.99
1.00
1.02
1.05
1.03
1.04
1.05
1.07
1.02
1.02
1.03
1.07
1.70
1.07
0.97
1.02
1.02
1.04
1.15
1.13
1.11
1.13
1.23
1.12
1.27
1.64
1.73
1.67
1.02
1.10
0.96
0.44
0.43
0.42
0.35
0.45
0.44
0.42
0.37
0.43
0.42
0.41
0.35
0.42
0.47
0.45
0.45
0.45
0.46
0.40
0.46
0.43
0.41
0.43
0.42
0.36
0.39
0.39
0.34
0.45
0.32
0.46
cryst solid
20
100
200
298
20
100
200
298
20
100
200
298
100
77
near-Pla
[Fe(TpivPP)(1-EtIm)2]
cryst solid
cryst solid
64.3
78.6
near-Pla
near-Pla
tw
tw
[Fe(TpivPP)(1-VinylIm)2]
[Fe(TMP)(2-MeHIm)2]
[Fe(TPP)(1-MeIm)2]
[Fe(TPP)(1-AcIm)2]
[Fe(TPP)(1-VinylIm)2]
[Fe(TPP)(1-BzylIm)2]
[Fe(TPP)(1-SiMe3Im)2]
[Fe(TPP)(Py)2]
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
frozen soln
frozen soln
frozen soln
frozen soln
frozen soln
frozen soln
82.4/84.4
Ruf
near-Pla
d
near-Pla
near-Pla
d
near-Pla
d
d
near-Pla
near-Pla
near-Pla
near-Pla
e
e
e
d
d
d
27
28
28
28
28
28
59
60
26
26
26
26
26
58
58
58
58
58
58
0
d
0
0
d
0
d
d
0
0
0
0
e
77
77
77
77
77
[Fe(OEP)(Py)2]
4.2
120
120
120
120
120
77
77
77
77
77
[Fe(TMP)(1-MeIm)2]
[Fe(TMP)(4-CNPy)2]
[Fe(TMP)(3-ClPy)2]
[Fe(TMP)(4-MePy)2]
[Fe(TMP)(4-NMe Py)2]
[Fe(TMP)(2-MeHIm)2]
[Fe(TMP)(1,2-Me2 Im)2]
[Fe(OEP)(2-MeHIm)2]
[Fe(OEP)(4-NMe2Py)2]
[Fe(OEP)(4-CNPy)2]
[Fe(OEP)(1-MeIm)2]
e
e
d
d
d
77
a mm/s. b Dihedral angle between two axial ligands. c Predominant core conformation contribution; Pla, planar; Ruf, ruffled. d Not determined, presumed
parallel and planar. e Not determined, presumed perpendicular and ruffled.
dicular ligand orientations, whereas lower values (e1.25 mm/
s) result from relative parallel orientations. On the other hand,
Grodzicki et al.57 had suggested that the large quadrupole
splitting constant was the result of porphyrin core ruffling.
Their conclusion was based on the presumed ruffled core
conformation for a series of complexes studied in frozen
solution;58 no crystal structure data were then available.
The new picket fence derivatives with their relative
perpendicular axial ligand orientations and nearly planar
cores allow us to examine the issue further. Table 4 displays,
in addition to the Mössbauer quadrupole splitting and the
isomer shift values, the relative axial ligand orientation (θ)
and the core conformations for all bis-ligated derivatives.
The entries in the table clearly show that the common
geometric factor for a large quadrupole splitting constant is
a ruffled core conformation and not ligand orientation.
Moreover, since the quadrupole splitting values are most
sensitive to d-electron configuration, it is clear that the
aggregate d-electron configuration in the picket fence deriva-
Figure 6. Mössbauer spectra in 500 mT field of (a) [Fe(TpivPP)(1-MeIm)2],
(b) [Fe(TpivPP)(1-EtIm)2], and (c) [Fe(TpivPP)(1-VinylIm)2] at 200 K.
tives and the imidazole-ligated derivatives with parallel axial
ligand orientations are very similar, and consistent with the
conclusion that a major influence in defining the eclipsed
axial imidazole orientations is an iron pπ-imidazole pπ
interaction.35
species showing a large quadrupole doublet has been
structurally characterized, the [Fe(TMP)(2-MeHIm)2] deriva-
tive.27 The two independent molecules in the structure both
had the two axial ligands in a relative perpendicular
arrangement. We suggested that the quadrupole splitting
value for low-spin bis(planar axial ligand) iron(II) systems
was sensitive to the relative orientation of the two axial
ligands.27 Thus, larger values of the quadrupole splitting
(g1.5 mm/s) result from the presence of relative perpen-
(57) Grodzicki, M.; Flint, H.; Winkler, H.; Walker, F. A.; Trautwein, A. X.
J. Phys. Chem. A 1997, 101, 4202.
(58) Polam, J. R.; Wright, J. L.; Christensen, K. A.; Walker, F. A.; Flint,
H.; Winkler, H.; Grodzicki, M.; Trautwein, A. X. J. Am. Chem. Soc.
1996, 118, 5272.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3849

Li et al.
Summary
obtained for all three derivatives. A comparison of the values
obtained demonstrated that the quadrupole splitting values
are sensitive to porphyrin core conformation and not to the
relative axial ligand orientation.
Three bis(imidazole-ligated)iron(II) “picket fence” por-
phyrinates have been prepared and characterized by X-ray
structure determinations and multitemperature Mössbauer
spectroscopy. The steric demands on the picket side of the
porphyrin require that the projection of the imidazole plane
on that side to nearly eclipse an Fe-Np bond direction. All
three derivatives have the two axial ligand planes with a
relative perpendicular orientation. This is the first time that
such a ligand orientation with a nearly planar porphyrin core
and unhindered axial ligands has been observed and may
result from the restriction of the rotation of the axial ligand
on one side of the porphyrin plane. Mössbauer spectra were
Acknowledgment. We thank the National Institutes of
Health for support of this research under Grant GM-38401
(W.R.S). We thank the NSF for X-ray instrumentation
support through Grant CHE-0443233.
Supporting Information Available: Crystallographic data in
CIF format, figures with ORTEP diagrams, and tables with complete
crystallographic details, atomic coordinates, bond lengths, bond
angles, and anisotropic/isotropic displacement parameters (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
(59) Kobayashi, H.; Maeda, Y.; Yanagawa, Y. Bull. Chem. Soc. Jpn. 1970,
43, 2342.
(60) Dolphin, D.; Sams, J. R.; Tsin, T. B.; Wong, K. L. J. Am. Chem. Soc.
1976, 98, 6970.
IC702498C
3850 Inorganic Chemistry, Vol. 47, No. 9, 2008