Characterization of the mixed axial ligand complex (4-cyanopyridine)(imidazole)(tetramesitylporphinato)iron(III) perchlorate. Stabilization by synergic bonding

Article abstract of DOI:10.1142/S1088424616500206

The reaction of [Fe(TMP)(OClO3)], where TMP is the dianion of tetramesitylporphyrin, with a combination of a strong-acceptor ligand and a-donating imidazole can lead to the preparation of mixed-ligand complexes [Fe(Porph)(4-CNPy)(L)]+ where L is imidazole itself or 1-acetylimidazole and 4-cyanopyridine is the strong acceptor ligand. The stability of the new mixed-ligand pair is the presumed result of synergic bonding between the two axial ligands. The molecular structure and other characterization of the new mixed axial ligand complex, [Fe(TMP)(4-CNPy)(HIm)]ClO₄ is described. The axial ligands have a relative perpendicular arrangement with Fe-N(imidazole) = 1.945 ? and Fe-N(pyridine) = 2.021 ?. The average equatorial Fe-Np distance is 1.963 ?, which is consistent with the S₄-ruffled TMP core. Despite the relative perpendicular arrangement of axial ligands, the EPR spectrum of the complex is a rhombic signal and not a large g_max signal. The EPR g-values are g1 = 3.05, g2 = 2.07, and g3 = 1.22. A quadrupole doublet was seen in the M?ssbauer spectrum with an isomer shift of 0.197 mm/s and quadrupole splitting of 1.935 mm/s. Two crystalline forms of [Fe(TMP)(4-CNPy)(HIm)]ClO₄ have been characterized; the two forms differ only in the solvent content of the lattice. Crystal data for form A: a = 15.432 (12) ?, b = 20.696 (2) ?, c = 19.970 (5) ?, and β = 99.256 (14)deg, monoclinic, space group P2_1/n, V = 6295 (2) ?3, Z = 4, formula FeCl₃O₄N₈C69H69, 8397 observed data, R1 = 0.086, wR2 = 0.210, refinement on F2. Crystal data for form B: a =15.267 (3) ?, b =20.377 (6) ?, c = 19.670 (4) ?, and β = 98.14 (1)deg, monoclinic, space group P21/n, V = 6058 (4) ?3, Z = 4, formula C65.25H60.5Cl1.5FeN₈O₄, 5464 observed data, R1 = 0.096, wR2 = 0.112, refinement on F.

Full text of DOI:10.1142/S1088424616500206

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2016; 20: 1–11
DOI: 10.1142/S1088424616500206
Published at http://www.worldscinet.com/jpp/
Characterization of the mixed axial ligand complex
(4-cyanopyridine)(imidazole)(tetramesitylporphinato)iron(iii)
perchlorate. Stabilization by synergic bonding
Judith A. Serth-Guzzoa, Ilona Turowska-Tyrka, Martin K. Safoa, F. Ann Walkerb,
Peter G. Debrunnerc and W. Robert Scheidt*,a,◊
a Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana USA 46556
b Department of Chemistry and Biochemistry, University of Arizona, Tuscon, Arizona USA 85721-0041 and
c Department of Physics, University of Illinois, Urbana, Illinois USA 61801
Received 25 October 2015
Accepted 24 November 2015
ABSTRACT: The reaction of [Fe(TMP)(OClO3)], where TMP is the dianion of tetramesitylporphyrin,
with a combination of a strong p-acceptor ligand and a p-donating imidazole can lead to the preparation
of mixed-ligand complexes [Fe(Porph)(4-CNPy)(L)]+ where L is imidazole itself or 1-acetylimidazole
and 4-cyanopyridine is the strong p acceptor ligand. The stability of the new mixed-ligand pair is the
presumed result of synergic bonding between the two axial ligands. The molecular structure and other
characterization of the new mixed axial ligand complex, [Fe(TMP)(4-CNPy)(HIm)]ClO4 is described.
The axial ligands have a relative perpendicular arrangement with Fe–N(imidazole) = 1.945 Å and Fe–
N(pyridine) = 2.021 Å. The average equatorial Fe–Np distance is 1.963 Å, which is consistent with the
S4-ruffled TMP core. Despite the relative perpendicular arrangement of axial ligands, the EPR spectrum
of the complex is a rhombic signal and not a large gmax signal. The EPR g-values are g1 = 3.05, g2 = 2.07,
and g3 = 1.22. A quadrupole doublet was seen in the Mössbauer spectrum with an isomer shift of 0.197
mm/s and quadrupole splitting of 1.935 mm/s. Two crystalline forms of [Fe(TMP)(4-CNPy)(HIm)]ClO4
have been characterized; the two forms differ only in the solvent content of the lattice. Crystal data for
form A: a = 15.432 (12) Å, b = 20.696 (2) Å, c = 19.970 (5) Å, and b = 99.256 (14)°, monoclinic, space
group P21/n, V = 6295 (2) Å3, Z = 4, formula FeCl3O4N8C69H69, 8397 observed data, R1 = 0.086, wR2 =
0.210, refinement on F2. Crystal data for form B: a = 15.267 (3) Å, b = 20.377 (6) Å, c = 19.670 (4) Å,
and b = 98.14 (1)°, monoclinic, space group P21/n, V = 6058 (4)Å3, Z = 4, formula C65.25H60.5Cl1.5FeN8O4,
5464 observed data, R1 = 0.096, wR2 = 0.112, refinement on F.
KEYWORDS:
from catalysis, electron transfer, to oxygen transfer and
INTRODUCTION
utilization [1].
The electronic structures of the iron porphyrinates
For the bis-ligated iron(III) porphyrinates with
are rich and diverse. Such diversity includes, but is not
two equivalent ligands, the effects of the axial ligand
limited to, a variety of oxidation states. Such richness is
orientation are substantial and well understood. The
also mirrored the diversity of biological functions ranging
absolute axial ligand orientation has been defined as the
dihedral angle between the ligand plane and the plane
defined by the donor atom, the iron and the porphyrinato
nitrogen atom closest to the ligand (sometimes the Fe–Np
◊SPP full member in good standing
bond direction is used instead). This angle is frequently
called φ. The relative ligand orientation is defined as the
*Correspondence to: W. Robert Scheidt, email; Scheidt.1@nd.edu,
fax: +1 574-631-5938
Copyright © 2016 World Scientific Publishing Company

2
J. A. SERTH-GUZZO ET AL.
dihedral angle between the two axial ligand planes. Most
such dihedral angles are close to either 0° or 90°; these
limiting forms are termed as the (relative) parallel or
perpendicular orientation, respectively.
Synthesis of [Fe(TMP)(4-CNPy)(HIm)] [ClO4]
At least two crystalline forms of [Fe(TMP)(4-CNPy)
(HIm)]ClO4 have been prepared by slight variation in
crystallization procedures. Crystal form B was synthesized
as follows: [Fe(TMP)OClO3] (30 mg, 0.032 mmol)
and 4-CNPy (20 mg, 0.192 mmol) were placed in an
argon-purged, 15 × 1.5 cm test tube. Dichloromethane
(2 mL) was added and the solution was stirred for 3 min.
Imidazole (2.4 mg, 0.035 mmol) was added and stirred for
an additional 3 min. The solution was then layered with
6 mL of hexane. Crystal form A was subsequently prepared
in the following manner: [Fe(TMP)OClO3] (120 mg,
0.127 mmol) and 4-CNPy (79.5 mg, 0.763 mmol) were
placed in a 25-mL Schlenk flask. Dichloromethane
(~8 mL) was added and the solution was stirred for 10 min.
The UV-vis (CH2CI2) spectrum (410, 534, 571.5 (sh) nm) is
that of [Fe(TMP)(4-CNPy)2][ClO4] [6]. A dichloromethane
solution of HIm (0.127 mmol) was added to the solution
with syringe and stirred for an additional 20 min. A new
UV-vis spectrum results (CH2CI2) λmax: 414 (Soret), 546,
580 (sh) nm. The reaction mixture was transferred to four
15 × 1.5 cm test tubes and layered with 15 mL of hexane.
X-ray quality crystals formed after 4 days.
Changes in the absolute orientation of the two ligands
can lead to the differing spin states of the iron with high-
spin, intermediate-spin and low-spin species resulting
that are clearly related to the absolute ligand orientations
[2, 3]. In addition for low-spin species, the relative
orientation of the two axial ligands has substantial effects
on the energies of the three lowest energy d orbitals [4–11].
We report in this paper strategies for the isolation
and characterization of mixed axial ligand complexes of
iron(III) porphyrinates, which to our knowledge have not
been previously reported. We have used appropriately
chosen imidazole and pyridine ligand pairs to accomplish
this. In addition to their novelty, such mixed-ligand
species are interesting because one can study both the
orientation effects (particular orientation of the axial
imidazole and pyridine with respect to the heme axes,
and the relative orientation of the two ligand planes
with respect to each other) and the crystal field effects
(splitting of the d orbitals) of the axial ligands.
The major complex reported herein is [(imidazole)
(4-cyanopyridine)(tetremesitylporphinato)iron(III)]
perchlorate, [Fe(TMP)(4-CNPy)(HIm)][ClO4] [12]. Two
crystalline forms of this complex have been isolated that
differ only in solvent content. Their preparation and
characterization are described here.
X-ray structure determinations
Two black, crystalline forms of [Fe(TMP)(4-CNPy)-
(HIm)]ClO4 were examined on an Enraf-Nonius FAST
area detector diffractometer at 127 K with graphite-
monochromated MoKα radiation. Unit cell determination
and data collection procedures with the area detector
have been described previously [15]. A summary of
cell constants and refinement results is given in Table 1;
complete details are given in Table S1. Crystal form B
was the first form investigated, but form A also confirms
the preparation of the same mixed-ligand species. Slight
variations in data collection instrument settings were
used owing to differing crystal quality. Both data sets
were corrected for Lorentz-polarization and absorption
effects [16]. The structures were solved by Patterson
methods with the SHELXS program [18]. During the
course of structure solution and refinement, the solvent
content of form B was found to be a single, partially
occupied, methylene chloride molecule, located near
an inversion center. This leads to the idealized formula
EXPERIMENTAL SECTION
General information
All reactions were performed under an argon atmosphere
with Schlenk-ware and cannula techniques. All solvents
weredistilledunderargonpriortouse.Dichloromethaneand
hexaneweredistilledfromCaH2andsodium/benzophenone,
respectively. 4-Cyanopyridine was recrystallized from
diethyl ether and imidazole was recrystallized from
dichloromethane. Tetramesitylporphyrin was prepared by
a modified version of the procedure published by Lindsey
et al. [13] and iron was inserted into H2TMP [12] by
standard techniques [14]. [Fe(TMP)OClO3] was prepared
as previously described [6]. Caution! These perchlorate
salts can detonate spontaneously and should be handled
only in small quantities; other safety precautions are also
warranted. UV-vis spectra were recorded on a Perkin-Elmer
Lambda 6 spectrophotometer. Mössbauer samples were
prepared from ground single-crystal samples as mulls in
Apiezon L grease, as previously described [7]. Mössbauer
measurements were made on a constant acceleration
spectrometer. The spectra were fitted with Lorentzian line
shapes. Isomer shifts are quoted relative to metallic iron at
room temperature.
.
[Fe(TMP)(4-CNPy)(HIm)]ClO4 1/4CH2Cl2 for form B.
Similarly, the formula for form A was established
during structure solution as [Fe(TMP)(4-CNPy)(HIm)]
.
.
ClO4 CH2Cl2 1/2C6H14.
Least-squares refinement of the structural model for
form B was carried out with a traditional refinement on
F using the “observed” data, while that for form A was
on F2 using all the unique, measured data including the
reflections with negative intensities. In both structures,
all nonhydrogen atoms were refined with atomic
displacement factors. Hydrogen atoms were included as
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CHARACTERIZATION OF THE MIXED AXIAL LIGAND COMPLEX
3
Table 1. Brief crystallographic data and data collection parameters for the two forms of [Fe(TMP)
(4-CNPy)(HIm)]ClO4
[Fe(TMP)(4-CNPy)(HIm)]
ClO4 CH2Cl2 0.5C6H14 Form A
[Fe(TMP)(4-CNPy)(HIm)]
ClO4 0.25 CH2Cl2 Form B
Molecule
.
formula
FW, amu
a, Å
C69H69Cl3FeN8O4
1236.57
15.4318 (12)
20.696 (2)
19.970 (5)
99.256 (14)
6295 (2)
P21/n
C65.25H60.5Cl1.5FeN8O4
1129.78
15.267 (3)
20.377 (6)
19.670 (4)
98.14 (1)
6058 (4)
P21/n
b, Å
c, Å
b, deg
V, Å3
space group
Z
4
4
Dc, g/cm3
m, mm-1
radiation, MoKα, λ
temperature, K
unique data
1.30
1.24
0.420
0.365
0.71073 Å
127 (2)
0.71073 Å
127 (2)
16241
13051
unique observed data [I > 2s(I)] 8397
5464
refinement method
Full-matrix least-squares on F2 Full-matrix least-squares on F
final R indices [I > 2s(I)]
final R indices (all data)
R1 = 0.086, wR2 = 0.210
R1 = 0.168, wR2 = 0.262
R1 = 0.096, wR2 = 0.112
fixed, idealized contributors. The structures were then
refined to convergence with the discrepancy indices
listed in Table 1. Since the crystal and data quality of
form A proved to be superior to that of form B and the
structures are essentially identical, we report more details
for form A herein. Complete sets of atomic coordinates,
atomic displacement factors, bond distances, and bond
angle tabulations for both A and B forms are given in the
Supplementary Information.
perpendicular orientation. The actual dihedral angle
between the two axial ligands is 85.4°. (In form B, this
angle is 84.9°). The projection of the imidazole ligand
plane onto the porphyrin plane makes an angle of 42°
with the closest Fe–Np vector; the corresponding value for
the pyridine ligand is 38.2°. These angles are frequently
denoted by the symbol φ.
Consistent with observations of [FeIII(TMP)L2]+
derivatives having axial ligands with relative perpendi-
cular orientations, the porphinato core exhibits an
S4-ruffling, which is illustrated in the formal diagram
of Fig. 2. This figure displays the perpendicular
displacements of each atom (in units of 0.01 Å) from
the mean plane of the core. Also consistent with the
ruffled core are the relatively short equatorial Fe–Np
bond distances which average to 1.963 (10) Å. The axial
(FeIII–N) distances are 2.021 (4) Å to the pyridine ligand
and 1.945 (4) Å to the imidazole ligand. (For form B,
the axial (FeIII–N) distances arc 2.026 (9) Å and 1.933
(8) Å, respectively). The axial N–Fe–N angle is 177.6
(2)°; the Nax–Fe–Np angles range from 88.4 (2) to 92.4
(2)°. Individual values of the bond distances and bond
angles in form A are given in the Supporting Information
and in the CIF file. Averaged values of the chemically
equivalent bond distances and angles in the core are
entered on Fig. 2. The dihedral angles between the
peripheral mesityl groups and the porphinato core are
RESULTS
We have prepared the mixed axial ligand complex,
[Fe(TMP)(4-CNPy)(HIm)]ClO4, as a crystalline solid
and have characterized it by single-crystal X-ray structure
determinations and by Mössbauer and EPR spectroscopies.
[Fe(TMP)(4-CNPy)(HIm)]ClO4 has been obtained in two
crystalline forms which we call form A and form B; both
have been characterized by single-crystal X-ray structure
determinations. As we will later discuss, the two forms
differ only in solvent content of the lattice. Hence we report
details for form A only and summary information for form
B. The molecular structure of form A is shown in Fig. 1
which also illustrates the atom labeling scheme used for all
tables. The ORTEP diagram shows the interesting features
of the molecule, viz., that there are two nonequivalent
ligands and that the two planar axial ligands have a relative
Copyright © 2016 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2016; 20: 3–11

4
J. A. SERTH-GUZZO ET AL.
Fig. 1. ORTEP diagram of [Fe(TMP)(4-CNPy)(HIm)]ClO4 (Crystal form A). Thermal ellipsoids are drawn at the 50% probability
level. Porphyrin hydrogen atoms have been omitted for clarity
reasonably close to perpendicular (77.0, 88.5, 89.5, and
84.1°). Equivalent drawings for form B are given in
Figs S1 and S2 of Supplementary Information.
A second example of a mixed-ligand system,
[Fe(TMP)(4-CNPy)(1-AcIm)]ClO4, has also been
obtained as a solid-state species but is less definitively
characterized than the unsubstituted imidazole derivative.
A combination of relatively poor crystal specimens and
solvent disorder severely limit the quality of the structure
determination [19]. However, the structural results are
reasonably interpreted in terms of a mixed-ligand species
with all structural features similar to those observed for
the two forms of [Fe(TMP)(4-CNPy)(HIm)]ClO4.
Mössbauer spectroscopic measurements have been
made on form A of [Fe(TMP)(4-CNPy)-(1-AcIm)]
ClO4 and on [Fe(TMP)(4-CNPy)(1-AcIm)]ClO4. The
Mössbauer spectrum of polycrystalline form A taken
at 170 K is shown in Fig. 3. Form A has a quadrupole
doublet with a splitting (∆Eq) of 1.935 (8) mm/s and
an isomer shift (δ) of 0.197 (7) mm/s. The Mössbauer
parameters for [Fe(TMP)(4-CNPy)(1-AcIm)]ClO4
are 1.82 (4) mm/s and an isomer shift of 0.18 (3). See
Fig. S3 of the Supporting Information.
Fig. 2. Formal diagram of the porphinato core in [Fe(TMP)
(4-CNPy)(HIm)]ClO4 for form A displaying displacement
of each unique atom from the 24-atom mean plane. All
displacements are given in units of 0.01 Å. Negative values
of atom displacements are towards the imidazole ligand. Also
entered on the diagram are the averaged values of all bond
distances and angles of the core. The orientations of the two
planar ligands with respect to core atoms are also displayed
DISCUSSION
Although there are a number of mixed-ligand species
of the general form [FeIII(Porph)(L)(X)], where X is an
anionic ligand and L a neutral nitrogen donor, there are
to our knowledge no reports of the preparation of species
of the general form [FeIII(Porph)(L)(L/)]+ where L and
L′ are different neutral donors. The reaction of iron(III)
porphyrinates with neutral axial nitrogen donors, L, has
been long known to lead to six-coordinate complexes,
[FeIII(Porph)L2]ClO4:
K1
[FeIII (Porph)(OClO )]+ L
[FeIII (Porph)L(OClO3 )]
ꢀꢀꢀꢁ
ꢂ ꢀ ꢀꢀ
[FeIII (Porph)L2 ] ClO4
3
K2
ꢀꢀꢀꢁ
+ L
ꢂ ꢀ ꢀꢀ
(1)
Copyright © 2016 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2016; 20: 4–11

CHARACTERIZATION OF THE MIXED AXIAL LIGAND COMPLEX
5
Fig. 3. Mössbauer spectrum of [Fe(TMP)(4-CNPy)(HIm)]ClO4 (Crystal form A) at 170 K and in a field of 2.2 kG perpendicular to
the g beam. The solid line is a Lorentzian fit with parameters given in the text with a quadrupole splitting ∆Eq = 1.935 mm/s and an
isomer shift δ of 0.197 mm/s
For any given ligand L, the first binding constant, K1,
is generally much smaller than the second (K2) such that
typically only logb2 (K1K2) values can be measured [20,
21]. For neutral nitrogen donors the magnitude of the
binding constant b2 is known to vary over a number of
orders of magnitude and is also generally related to ligand
pKa [20, 21]. In general, the differing values of binding
constants suggest that for any arbitrarily chosen pair of
potential ligands, one member of the ligand pair will
have a much higher affinity for the iron(III) center that
the other and the reaction will lead to the preparation of
one of the possible bis-symmetrically ligated complexes.
In particular, the binding constants for 4-cyanopyridine
with iron(III) porphyrinates are known to be quite small,
on the order of 108 smaller than imidazoles in DMF [21]
and a 4-cyanopyridine ligand might thus be expected
to be readily supplanted by almost any other neutral
nitrogen donor.
However, our prior synthetic and structural work and
that of others shows that there are large differences in
the bonding characteristics of neutral nitrogen ligands
ranging from strong p-donating to s-donating to strong
p-accepting character. Moreover, these differences have
real effects on the electronic structure of the iron(III)
center as revealed by EPR and Mössbauer spectroscopy
in this work. Our strategy in the investigation of possible
mixed-ligand species was to make use of the (expected)
synergicbondingcharacteristicsofdifferingaxialligands.
In this study we used a p-accepting (4-cyanopyridine) and
a s- and p-donating (imidazole) combination. We first
allowed up to two 4-CNPy ligands to bind to the iron(III)
porphyrinate by using a modestly high concentration of
that ligand in concentrated solution, followed by adding
only one equivalent of the imidazole, which we hoped
would thus only replace one of the 4-CNPy ligands.
Clearly this strategy worked to produce the mixed-ligand
complex.
The crystal structure determinations of the two forms
of [Fe(TMP)(4-CNPy)(HIm)]ClO4 and the preliminary
results for [Fe(TMP)(4-CNPy)(1-AcIm)]ClO4 clearly
demonstrate that ferric porphyrinates with mixed, neu-
tral nitrogen ligands can be prepared as solid-state spe-
cies. Indeed, the relative perpendicular orientation of the
mixed axial ligands is that expected for such a synergic
p-bonding ligand pair where the p-accepting ligand
interacts with the filled dxz orbital and the p-donating
ligand with the orthogonal, half-filled dyz orbital.
UV-vis spectra and solution EPR spectra, taken under
conditions as similar to our preparative reaction conditions
as possible, provides evidence that a mixed-ligand species
exists to a significant extent in solution. The spectra of
solutions of [Fe(TMP)(OClO3)] containing, respectively,
10 equivalents of 4-CNPy, 10 equivalents of 4-CNPy and
1 equivalent of imidazole, or 10 equivalents of imidazole
per equivalent of iron(III) are shown in Fig. 4. The figure
clearly shows that the spectrum of the solution containing
both 4-CNPy and imidazole is not simply the sum of the
two end species, suggesting the presence of a significant
concentration of the mixed–ligand species in solution.
The fact that any mixed–ligand complex forms at all in
solution is remarkable given that the overall equilibrium
constant (b2) for 4-CNPy or imidazole with [Fe(TMP)
(OClO3)] differs by up to eight orders of magnitude.
The structural features of [Fe(TMP)(4-CNPy)(HIm)]
ClO4 are those generally expected for a low-spin iron(III)
porphyrinate. The average Fe–Np distance of 1.963 (10) Å
(form A) or 1.964 (12) Å (form B) are within the range of
values observed for derivatives with significantly S4-ruffled
cores and the magnitude of the ruffling is within the range
observed previously for FeIIITMP derivatives [22].
We tabulate, for comparison, selected structural
parameters for the known low-spin derivatives with at
least one imidazole or pyridine as axial ligand in Table 2.
The absolute orientation of the pyridine ligand (φ = 38.2°)
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6
J. A. SERTH-GUZZO ET AL.
Fig. 4. Electronic spectra of a 9.6 × 10-6 M (300–500 nm) or 1.28 × 10-3 M (480–800 nm) CHCl3 solutions of [Fe(TMP)(OClO3)]
…
containing, respectively, 10 equivalents of 4-CNPy( ), 10 equivalents of imidazole (—), or 10 equivalents of 4-CNPy and 1 equiva-
lent of imidazole (---), per equivalent of iron(III)
Table 2. Summary of structural parameters and EPR for low-spin six-coordinate iron(iii) imidazole and pyridine derivatives
Relative Porphyrin core
a,b
ax
c
Complex
Fe–Npa
Fe–N
φ
EPR type
NRe
Ref.
23
orientc,d
conformation
[Fe(TMP)(1,2-Me2HIm)2]ClO4
1.937 (12)
2.004 (5)
2.004 (5)
44
44
89
S4-ruffled
[Fe(TPP)(2-MeHIm)2]ClO4
[Fe(T2,6Cl2PP)(1-VinIm)2]ClO4f
[Fe(TPP)(1-MeIm)2]ClO4
[Fe(TMP)(1-MeIm)2]ClO4h
[Fe(TPP)(HIm)2]Cl
1.970 (4)
1.972 (6)
1.982 (11)
2.010 (4)
2.015 (4)
33
32
89
S4-ruffled
S4-ruffled
S4-ruffled
planar
gmax
24
4
1.968 (4)
1.976 (4)
5
6
76
rhombic
gmax
14, 20
1.970 (3)
1.978 (3)
32
22
10
rhombic
rhombic
NRe
25
6
1.987 (1)
1.988 (20)
1.965 (3)
1.975 (3)
42
23
0g
0g
1.989 (8)
1.957 (5)
1.991 (5)
39
18
57
13
0g
S4-ruffled
S4-ruffled
planar
26
27
[Fe(Proto IX)(1-MeIm)2]
1.990 (16)
1.966 (5)
1.988 (5)
16
3
NRe
[Fe(TPP)(tMU)2]SbF6
[Fe(TPP)(cMU)2]SbF6h
1.992 (5)
1.983 (4)
22
rhombic
rhombic
8
8
1.996 (10)
1.967 (7)
1.979 (7)
29
15
0g
0g
planar
planar
[Fe(TPP)(HIm)2]Cl·H2Oh
1.993 (7)
1.964 (3)
1.977 (3)
41
6
0g
0g
planar
rhombic
28
[Fe(TPP)(CuIm)2]+
2.00 (3)
1.98 (1)
7
9
S4-ruffled
S4-ruffled
NRe
29
9
[Fe(TPP)(4-CNPy)2]ClO4
1.952 (7)
2.008 (4)
1.997 (4)
35
36
89
axial
[Fe(TMP)(4-CNPy)2]ClO4
1.961 (6)
2.001 (5)
2.021 (6)
44
43
90
S4-ruffled
axial
10
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CHARACTERIZATION OF THE MIXED AXIAL LIGAND COMPLEX
7
Table 2. (Continued)
[Fe(TMP)(3-EtPy)2]ClO4
[Fe(TMP)(3-ClPy)2]ClO4
[Fe(TMP)(4-NMe2Py)2]ClO4
1.964 (4)
1.968 (7)
1.964 (10)
1.989 (4)
2.002 (4)
44
44
90
77
79
S4-ruffled
S4-ruffled
S4-ruffled
gmax
gmax
gmax
10
10
6
2.006 (7)
2.018 (7)
42
29
1.978 (4)
1.989 (4)
42
37
[Fe(OEP)(4-NMe2Py)2]ClO4
[Fe(TPP)(Py)2]ClO4
2.002 (4)
1.982 (6)
1.995 (3)
36
0g
planar
rhombic
6
2.001 (5)
2.005 (5)
38
34
86
S4-ruffled
gmax
30
tri-[Fe(OEP)(3-ClPy)2]ClO4i
1.995 (6)
2.031 (2)
41
0g
planar
NRe
2
[Fe(TMP)(4-CNPy)(HIm)]ClO4j
1.963 (10)
1.954 (4) (Im)
2.021 (4) (Py)
42
38
85
S4-ruffled
rhombic
this
work
[Fe(TMP)(4-CNPy)(HIm)]ClO4k
1.964 (8)
1.933 (8) (Im)
2.026 (9) (Py)
44
41
85
S4-ruffled
NRe
this
work
[Fe(OEP)(1-MeIm)2]+
2.004 (2)
1.983 (4)
1.975 (2)
20
0g
planar
NR
31
11
paral-[Fe(TMP)(5-MeHIm)2]ClO4
1.978 (6)
1.961 (5)
20
10
30
S4-ruffled
rhombic
paral-[Fe(TMP)(5-MeHIm)2]ClO4
perp-[Fe(TMP)(5-MeHIm)2]ClO4
1.981 (5)
1.981 (7)
1.980 (5)
1.985 (5)
12
14
26
76
planar
rhombic
11
11
1.957 (6)
1.973 (6)
30
40
S4-ruffled
gmax
[Fe(TPP)(5-MeHIm)2]Cl
2.008 (2)
2.002 (2)
1.990 (2)
1.975 (2)
1.987 (3)
3.1
4.6
0
0
planar
planar
rhombic
rhombic
rhombic
32
32
33
[Fe(TPP)(5-MeHIm)2]Cl
paral-[Fe(OMTPP)(1-MeIm)2]Cl
1.975 (2)
2.016 (2)
13
6
19
S4-saddled
perp-[Fe(OMTPP)(1-MeIm)2]Cl
[Fe(OETPP)(1-MeIm)2]Cl
[Fe(OETPP)(2-MeHIm)2]+
[Fe(OETPP)(4-Me2Py)2]Cl
[Fe(OEP)(2-MeHIm)2]Cl
[Fe(TiPrP)(BzHIm)2]+
1.969 (7)
1.970 (7)
1.974 (9)
1.951 (5)
1.974 (4)
1.915 (5)
1.938 (3)
1.970 (4)
1.982 (10)
1.982 (10)
29
61
90
73
S4-saddled
S4-saddled
S4-saddled
S4-saddled
S4-ruffled
S4-ruffled
S4-ruffled
S4-ruffled
gmax
gmax
33
33
34
34
35
36
36
37
1.976 (3)
1.978 (3)
10
7
2.09 (2)
2.09 (2)
14
14
90
gmax
1.984 (5)
2.015 (6)
9
29
70
gmax
1.998 (2)
2.012 (2)
40.8
43.4
87.6
90
NR
2.070 (5)
45
44
axial
axial
rhombic
[Fe(TiPrP)(HIm)2]+
1.993 (3)
45
45
~90
69
[Fe(TpivPP)(NO2)(HIm)]
2.037 (10) (Im)
1.949 (10)
(NO2)
16
37
[Fe(TPP)(CN)(Py)]
[Fe(TPP)(NCS)(Py)]
1.970 (14)
1.988 (9)
2.075 (3) (Py)
40
39
—
—
S4-ruffled
S4-ruffled
NR
NR
38
39
2.082 (3) (Py)
1.942 (4)
(NCS)
[Fe(TPP)(N3)(Py)]
1.989 (6)
2.089 (6) (Py)
1.925 (7) (N3)
40
10
S4-ruffled
NR
40
aValues in Å. bAll independent axial distances are listed. cValues in degrees. dDihedral angle between pair of axial ligands. eNot reported. fDisordered
g
h
i
structure with relative parallel and perpendicular imidazoles. Value of 0º required by symmetry. Two independent molecules. Triclinic form at
100K. jForm A. kForm B.
Copyright © 2016 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2016; 20: 7–11

8
J. A. SERTH-GUZZO ET AL.
is within the range (24°–45°) apparently required to form
low-spin pyridine complexes. The absolute orientation
of imidazole ligands in observed in low-spin complexes
encompasses the entire possible range of 0°–45° and thus
no particular absolute orientation is needed to form a low-
spin imidazole complex (even with an S4-ruffled core).
Thus there appears to be no steric constraints on either
the absolute or relative imidazole orientation in order to
form the low-spin complex [Fe(TMP)(4-CNPy)(HIm)]
ClO4. We therefore believe that the relative perpendicular
orientation of the imidazole and pyridine ligands in low-
spin [Fe(TMP)(4-CNPy)(HIm)]ClO4 are the result of
bonding considerations and are not required by steric
considerations. The relative perpendicular orientation
is in agreement with the bonding requirements of the
p-acceptor ligand which requires a filled orbital (dxz) and
the p-donor ligand which interacts with the orthogonal
half-filled orbital (dyz).
Axial bond distance comparisons are also consistent
with the idea that the relative ligand orientation is
controlled by bonding. The axial Fe–N(imidazole)
distance of 1.945 (4) Å is shorter than that observed in
any bis-imidazole iron(III) derivative (Table 2), while
the axial Fe–N(pyridine) distance of 2.021 (4) Å is as
long as or longer than the Fe–N(pyridine) distances
observed in the bis-pyridine derivatives. The short
Fe–N(imidazole) distance is most consistent with very
strong p-bonding between the iron(III) and imidazole.
The Fe–N(imidazole) distance is also much shorter than
the 2.068 (4) Å distance found for the five-coordinate
high-spin complex [Fe(OEP)(2-MeHIm)]ClO4 where
strong axial bonding might be expected [42].
interesting and possibly distinctive ground state. A final
system with two strong p-accepting ligands ([Fe(TPP)
(4-CNPy)2]ClO4) has a quadrupole splitting value of
0.65 mm/s and an axial EPR spectrum [9].
EPR spectra of low-spin bis-ligated iron(III)
porphyrinates have been shown to be particularly
informative about the electronic structure. The relative
energies of the three d-orbitals lowest in energy can be
determined from the EPR g-values utilizing the Taylor
formulation [48, 49]. Moreover, the EPR spectral type
provides additional information. The observed type of
EPR spectrum for all complexes listed in Table 2 are also
given in the table when the spectrum has been measured.
When the two planar axial ligands have a relative parallel
orientation, rhombic spectra, with three distinct g-values,
are observed. This is the result of a modest energy
difference between the dxy and dyz orbitals since both
axial ligands interact with only one of the two. However,
when the two axial ligands planes have a dihedral angle
close to 90°, i.e. a relative perpendicular orientation, a
spectral type called gmax is observed. In this case, the
two d orbitals interact more or less equivalently with the
perpendicularly aligned ligands, leading to a very small
energy gap between the dxy and dxz orbitals. A third type,
with two strong p-acceptor ligands such as isocyanides,
leads to an axial spectrum being observed. This is the
result of the dxy orbital becoming the highest energy
orbital of the three t2g orbitals as the energies of two dp
orbitals are lowered because of the interaction with the
p-acceptor ligands. We know of no case where a relative
orientation of the two planar ligands leads to a rhombic
EPR spectrum.
Although we have found and characterized two
different crystalline forms of [Fe(TMP)(4-CNPy)(HIm)]
ClO4, it is to be noted that the crystal structure, as well as
the molecular structure, of both forms are quite similar.
The cell packing diagrams for form A and form B (see
Fig. 5) show that the packing of [Fe(TMP)(4-CNPy)
(HIm)]+ cations is essentially identical even though
the solvent content in the two lattices clearly requires
different cell volumes. Thus the two forms of [Fe(TMP)
(4-CNPy)(HIm)]ClO4 represent an additional set of
cases of lattice packing dominated by the large metallo-
porphyrin species [43–46].
The Mössbauer spectrum of polycrystalline form A
taken at 170 K is shown in Fig. 3. The spectrum illustrates
a doublet with a quadrupole splitting of ∆Eq = 1.935 (9)
mm/s and an isomer shift δ of 0.189 (7) mm/s. The isomer
shift is similar to values reported for other low-spin ferric
hemes [5, 6, 10, 47]. Quadrupole splitting values less than
~1.75mm/saretypicalforlow-spiniron(III)porphyrinates
with axial ligands in the perpendicular orientation and a
“pure” (dxy)2(dxz, dyz)3 ground state [5, 6, 10], while values
greater than ~2.00 mm/s are typical for species with
relative parallel orientation [5, 6]. With a quadrupole
splitting value for form A in the middle (between) of
the two limiting set of values, this is consistent with an
What type of EPR spectrum will [Fe(TMP)(4-CNPy)
(HIm)]ClO4 display? From the data of Table 2, two
distinct possibilities can be envisioned. The perpendicular
orientation of the two ligands in [Fe(TMP)(4-CNPy)
(HIm)]ClO4 suggests that a strong gmax spectrum would
be expected. However, the expected synergic bonding
of the two ligands, one p accepting and one p donating,
leads to a different prediction. The axial ligands and the
metal dxy and dxz orbitals are 90° apart. Thus, one ligand
(the p acceptor) will interact with the filled dxz orbital and
the other ligand (the p donor) will interact with the singly
occupied dyz orbital to give the two orbitals differing
energies and an expected rhombic EPR spectrum.
Single-crystal EPR spectral measurements for [Fe(TMP)
(4-CNPy)(HIm)]ClO4 reveal a rhombic spectrum with g
values of 3.05, 2.07 and 1.22. This clearly shows that the
dxz and dyz orbitals are separated in energy.
With the available g-values for [Fe(TMP)(4-CNPy)
(HIm)]ClO4 the Taylor formalism [48] can now be used
to evaluate the relative energies of the three lowest d
orbitals. These values, expressed in terms of the energy
of spin orbit coupling constant λ are shown on the right
hand side of Fig. 6. For comparison, the energies of a
bis-ligated 1-methylimidazole complex is shown at the
left [6]. Although the energy difference between the dxz
Copyright © 2016 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2016; 20: 8–11

CHARACTERIZATION OF THE MIXED AXIAL LIGAND COMPLEX
9
Fig. 5. Steroscopic packing diagrams of the forms of [Fe(TMP)(4-CNPy)(HIm)]ClO4. Top shows form A and the bottom shows
form B
Fig. 6. Diagram showing the relative energies of the three lowest d-orbitals for [Fe(TMP)(4-CNPy)(HIm)]ClO4 (right) and a com-
parison complex (left)
Copyright © 2016 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2016; 20: 9–11

10
J. A. SERTH-GUZZO ET AL.
and dyz orbitals is slightly smaller in the mixed-ligand
complex, the two orbitals are clearly still well-separated.
The axial bonding to iron was thus controlled using
4-CNPy as a strong p acceptor (poor base) with low
affinity for the ferric porphyrinate, while HIm was used
as a strong p donor (good base) with high affinity for the
ferric porphyrinate. Thus the ligand pair chosen to obtain
a synergic effect or a “push–pull” system was found to be
a viable choice for a mixed axial ligand system.
3. Scheidt WR, Geiger DK, Hayes RG and Lang G.
J. Am. Chem. Soc. 1983; 105: 2625–2632.
4. Hatano K, Safo MK, Walker FA and Scheidt WR.
Inorg. Chem. 1991; 30: 1643–1650.
5. Walker FA, Huynh BH, Scheidt WR and Osvath
SR. J. Am. Chem. Soc. 1986; 108: 5288–5297.
6. Safo MK, Gupta GP, Walker FA and Scheidt WR.
J. Am. Chem. Soc. 1991; 113: 5497–5510.
7. Scheidt WR, Osvath SR, Lee YJ, Reed CA, Schae-
vitz B and Gupta GP. Inorg. Chem. 1989; 28:
1591–1595.
8. Quinn R, Valentine JS, Byrn MP and Strouse CE.
J. Am. Chem. Soc. 1987; 109: 3301–3308.
9. Safo MK, Walker FA, Raitsimring AM, Walters
WP, Dolata DP, Debrunner PG and Scheidt WR. J.
Am. Chem. Soc. 1994; 116: 7760–7770.
10. Safo MK, Gupta GP, Walker FA, Watson CT,
Simonis U and Scheidt WR. J. Am. Chem. Soc.
1992; 114: 7066–7075.
11. Munro OQ, Serth-Guzzo JA, Turowska-Tyrk I,
Mohanrao K, Shokhireva TKh, Walker FA, Debrun-
ner PG and Scheidt WR. J. Am. Chem. Soc. 1999;
121: 11144–11155.
Summary
The reaction of [Fe(TMP)(OClO3)] with 6 equivalents
of 4-CNPy followed by the addition of 1 equivalent of
imidazole yields the mixed-ligand complex [Fe(TMP)
(4-CNPy)(HIm)]ClO4. The two distinct ligands are
found to have a relative perpendicular orientation that
is consistent with a push–pull synergic effect of the
strongly p-accepting ligand 4-CNPy and the p and s
donating imidazole ligand. This is also consistent with
the Mössbauer and EPR spectra that show a classical
(dxy)2(dxz, dyz)3 ground state.
12. Abbreviations used: Porphyrin Ligands: TMP, 5,10,
15,20-tetramesitylporphyrin dianion; TPP, tetra-
phenylporphyrin dianion; OEP, octaethylporphyrin
dianion; OETPP, octaethyltetraphenylporphyrin
dianion; TCl2PP, meso-tetra(2,6-dichlorophenyl)
porphyrin dianion; Proto, protoporphyrin IX dian-
ion; OMTPP, octamethyltetraphenylporphyrin dian-
ion; TpivPP, picket fence porphyrin dianiom; TiPrP
tetra(i-propyl)porphyrin dianion. Other Ligands:
HIm, imidazole; 4-CNPy, 4-cyanopyridine;
1-AcIm, 1-acetylimidazole; 1-MeIm, 1-methyl-
imidazole; 2-MeHIm, 2-methylimidazole; 3-ClPy,
3-chloropyridine; 4-NMe2Py, 4-dimethylamino-
pyridine; Py, pyridine; 5-MeHIm, 5-methylimid-
azole; BzHIm, 1-benzimidazole; Nax, axial nitrogen
donor; Np, porphinato nitrogen.
13. Wagner RW, Lawrence DS and Lindsey JS.
Tetrahedron Lett. 1987; 28: 3069. Lindsey JS and
Wagner RW. J. Org. Chem. 1988; 54: 828–836.
14. Adler AD, Longo FR, Kampas F and Kim J.
J. Inorg. Nucl. Chem. 1970; 32: 2443–2448.
15. Scheidt WR and Turowska-Tyrk I. Inorg. Chem.
1994; 33: 1314–1318.
16. The process is based on an adaptation of the DIFABS
[17] logic to area detector geometry by Karaulov:
Karaulov AI. School of Chemistry and Applied
Chemistry, University of Wales, College of Cardiff,
Cardiff CF1 3TB, UK, personal communication.
17. Walker NP and Stuart D. Acta Crystallogr. 1983;
A39: 158–166.
18. Sheldrick GM. Acta Crystallogr. 2008; A64:
112–122.
19. Hughes C.; Unpublished data.
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 diffractometer was
provided through NIH Grant RR-06709.
Supporting Information Available
ORTEP and formal diagrams for form B (Figures S1
and S2) and the Mössbauer spectrum for [Fe(TMP)(4-
CNPy)(1-AcIm)]ClO4 (Fig. S3). Crystallographic details
for both forms A and B (Tables S1–S11) are given in
the supplementary material. This material is available
free of charge via the Internet at http://www.worldscinet.
com/jpp/jpp.shtml. Crystallographic data have been de-
posited at the Cambridge Crystallographic Data Centre
(CCDC) under number CCDC 1432125. Copies can be
obtained on request, free of charge, via www.ccdc.cam.
ac.uk/data_request/cif or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK (fax: +44 1223-336-033 or email: deposit@
ccdc.cam.ac.uk).
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