Examination of the Magneto-Structural Effects of Hangman Groups on Ferric Porphyrins by EPR

Article abstract of DOI:10.1021/acs.inorgchem.9b02348

Ferric hangman porphyrins are bioinspired models for haem hydroperoxidase enzymes featuring an acid/base group in close vicinity to the metal center, which results in improved catalytic activity for reactions requiring O-O bond activation. These functional biomimics are examined herein with a combination of EPR techniques to determine the effects of the hanging group on the electronics of the ferric center. These results are compared to those for ferric octaethylporphyrin chloride [Fe(OEP)Cl], tetramesitylporphyrin chloride [Fe(TMP)Cl], and the pentafluorophenyl derivative [Fe(TPFPP)Cl], which were also examined herein to study the electronic effects of various substituents. Frequency-domain Fourier-transform THz-EPR combined with field domain EPR in a broad frequency range from 9.5 to 629 GHz allowed the determination of zero-field splitting parameters, revealing minor rhombicity E/D and D values in a narrow range of 6.24(8) to 6.85(5) cm^-1. Thus, the hangman porphyrins display D values in the expected range for ferric porphyrin chlorides, though D appears to be correlated with the Fe-Cl bond length. Extrapolating this trend to the ferric hangman porphyrin chlorides, for which no crystal structure has been reported, indicates a slightly elongated Fe-Cl bond length compared to the non-hangman equivalent.

Full text of DOI:10.1021/acs.inorgchem.9b02348

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Examination of the Magneto-Structural Effects of Hangman Groups
on Ferric Porphyrins by EPR
Joscha Nehrkorn,*,†,‡,§ Shannon A. Bonke,*,†,§ Azar Aliabadi,§ Matthias Schwalbe,∥
and Alexander Schnegg†,§
†EPR Research Group, Max-Planck-Institut fu
Germany
‡Institut fu
r Chemische Energiekonversion, Stiftstraße 34-36, 45470 Mulheim an der Ruhr,
̈ ̈
̈
r Anorganische und Angewandte Chemie, Universitat Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany
̈
§Institut Nanospektroskopie, Helmholtz-Zentrum Berlin fu
̈
r Materialien und Energie, Kekulestraße 5, 12489 Berlin, Germany
́
∥Institut fu
̈
r Chemie, Humboldt Universitat zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany
̈
S
* Supporting Information
ABSTRACT: Ferric hangman porphyrins are bioinspired models
for haem hydroperoxidase enzymes featuring an acid/base group in
close vicinity to the metal center, which results in improved
catalytic activity for reactions requiring O−O bond activation.
These functional biomimics are examined herein with a
combination of EPR techniques to determine the effects of the
hanging group on the electronics of the ferric center. These results
are compared to those for ferric octaethylporphyrin chloride
[Fe(OEP)Cl], tetramesitylporphyrin chloride [Fe(TMP)Cl], and
the pentafluorophenyl derivative [Fe(TPFPP)Cl], which were also
examined herein to study the electronic effects of various
substituents. Frequency-domain Fourier-transform THz-EPR com-
bined with field domain EPR in a broad frequency range from 9.5 to 629 GHz allowed the determination of zero-field splitting
parameters, revealing minor rhombicity E/D and D values in a narrow range of 6.24(8) to 6.85(5) cm−1. Thus, the hangman
porphyrins display D values in the expected range for ferric porphyrin chlorides, though D appears to be correlated with the Fe−
Cl bond length. Extrapolating this trend to the ferric hangman porphyrin chlorides, for which no crystal structure has been
reported, indicates a slightly elongated Fe−Cl bond length compared to the non-hangman equivalent.
reduction of CO2 to CO and also allow the reduction of
CO2 to CH4.2 Biomimetic models have evolved from planar
porphyrin macrocycles to bimetallic systems3,4 and cofacial
biporphyrins such as the Pacman class of structures.5 The latter
is aptly named as it features two porphyrin units that are
anchored by a single rigid pillar, which enables a small
substrate molecule to move between the two porphyrin
planes,5 i.e., be eaten by the Pacman. The design envisions
the substrate forming a dative bond to one or both metal
centers, with electrostatic interaction with the second
porphyrin making this substrate activation more favorable
and stabilizing the process. First generation Pacman structures
were diporphyrin anthracene (DPA) and diporphyrin
biphenylene (DPB), referring to the linking pillar.1,5,6 The
rigid backbone of these structures results in near-parallel
porphyrin units with limited flexibility. The linking pillars of
second generation Pacman biporphyrins,1 diporphyrin xan-
thene (DPX)7 and diporphyrin dibenzofuran (DPD),8 allowed
greater flexibility in the angle between the two porphyrin units,
INTRODUCTION
■
Enzymatic catalysis typically utilizes a metal center (Lewis
acid) to coordinate a substrate, with Brønsted acid−base
groups often present to assist substrate positioning and
orientation. Crucially, these groups abstract or supply protons
for proton coupled electron transfer steps (PCET). The
catalytic activity and selectivity of cytochrome c oxidase toward
the PCET steps of the oxygen reduction reaction has led to it
being coined “the mammalian fuel cell.”1 Key to its catalytic
activity are porphyrin structures of the haem structural family,
and the importance of the reaction has inspired synthetic
models. Biomimetic models aim to resemble the active site not
only by the similar redox potential of the metal center but also
by proximal sites for proton delivery/abstraction. These are
essential for small molecule activation and PCET reactions,
which are a foundation of efficient energy storage and
utilization.1
Functional enzyme-inspired porphyrins have been synthe-
sized with increasing sophistication and examined for catalytic
activity toward a range of reactions (many requiring PCET
steps). For example, planar ferric porphyrins have been
reported as being active and selective catalysts for the
Received: August 2, 2019
© XXXX American Chemical Society
A
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allowing the Pacman to open and have greater interaction with
substrate species.9 This correlated with improved (photo)-
catalytic activity toward certain oxidation reactions.9,10
The subsequently developed hangman structures aimed to
provide a Brønsted acid−base group proximal to the porphyrin
metal center, as is present in many enzymes. This was
synthetically achieved by retaining the rigid pillar of a Pacman
biporphyrin but replacing the second porphyrin with a
carboxylic acid group that could hang above the center of a
porphyrin (Scheme 1).11 The Brønsted acid−base group
better understand the species.17,18 These magnetic properties
are highly sensitive to the electronic structure and can
subsequently be related back to chemical properties via
magneto-structural correlations, which are rapidly improving
in accuracy due to developments in theory and computational
approaches.18−20 This information is especially valuable for
porphyrins that are hitherto unable to be crystallized such as
the mesityl substituted hangman porphyrins with chloride axial
ligands examined herein.
As in planar porphyrins, coordination of iron into a
hangman structure often results in a 5-coordinate, square
pyramidal geometry, combining the tetradentate porphyrin
macrocycle and an additional ligand (e.g., Cl− or OH−). The
metal center is displaced from the plane of the porphyrin, thus
preventing other ligands from approaching the final (sixth)
coordination site through repulsion by the delocalized negative
charge of the conjugated porphyrin. If the hangman group is
sterically bulky, the axial ligand may be more stable on the
opposite side of the porphyrin planethough this would
diminish any catalytic benefits. Contrarily, hydrogen bonding
between the hangman motif and the axial ligand has been
demonstrated to result in them sharing the same porphyrin
face in the case of a hangman porphyrin crystal structure with a
hydroxide axial ligand and hanging carboxylic acid group.11
For paramagnetic systems with S ≥ 1, a zero-field splitting
(ZFS) can often be observed. ZFS arises from dipolar and
spin−orbit couplings splitting otherwise degenerate spin states,
and resulting in differences in spin state energy levels absent a
magnetic field. Ferric porphyrins have a 3d5 electron
arrangement and a distorted square pyramidal geometry,
which results in an S = 5/2 spin system and significant ZFS.
The ZFS provides an additional spectroscopic handle since it is
sensitive to the coordination environment. The magnetic
properties of ferric porphyrins can be described by the
following spin Hamiltonian (SH, eq 1)
Scheme 1. Structures of Ferric Porphyrins Spectroscopically
Examined Herein
a
i
j
j
2
z
1
3
2
y
z
z
z
2
x
2
y
̂
̂
̂
̂ ̂
̂
+ E(S − S ) + μ B ·g·S
B
H = D S −
S
j
0
(1)
k
{
The SH as shown contains two magnetic field-independent
terms. The first describes the axial ZFS (as depicted in Scheme
2), which is defined by D, and splits the 6-fold degenerate S =
5/2 multiplet into three doublets (viz., MS = 1/2, 3/2, and
5/2). An energy difference of 2D separates the ground state
from the first excited doublet ( 1/2 to 3/2), while 4D
separates the second from the third excited doublets ( 3/2 to
5/2). The second term shown in the SH describes the
rhombic ZFS, defined by the parameter, E, which changes the
energies (and wave functions) but does not split the doublets
due to Kramer’s theorem. By convention, the coordination
frame is chosen such that |E/D| ≤ 1/3. The third term
describes the Zeeman interaction, i.e., the interaction between
spins and the external magnetic field (B0), with μB being the
Bohr magneton. The tensor g is approximated as axial in the
ZFS frame, i.e., g = diag(g⊥, g⊥, g||), with ⊥ and || referring to
the main ZFS axis.
a
Hangman architecture based on a mesityl-substituted porphyrin with
chloride axial ligand, xanthene pillar, and hanging carboxylic acid
[Fe(HPX-COOH)Cl] or methyl carboxylate [Fe(HPX-COOMe)Cl]
(upper). Ferric tetra-arylporphyrins with phenyl, mesityl, or
pentafluorophenyl substituents, with axial chloride ligand omitted,
and ferric octaethylporphyrin (lower).
directly above the metal center is designed to orient the
substrate above a catalytic active site to facilitate PCET. This is
specifically interesting for small molecule activation, such as
H2O2 dismutation (catalase reaction) or O2 activation.1,12 For
iron hangman porphyrins as well as corroles, it has been
demonstrated that the carboxylic acid group had a positive
influence on the catalytic activity.13−16
Catalytic activity is defined by the structural and electronic
properties of a molecule or material. As such, determining
these parameters with accuracy is crucial to rational catalyst
development. For paramagnetic metalloporphyrins, such as
hemes, the magnetic properties offer a spectroscopic handle to
The determination of the axial ZFS parameter, D, of ferric
porphyrins has been the subject of many papers utilizing a
variety of techniques, including infrared spectroscopy (IR),
̈
Moßbauer spectroscopy, inelastic neutron scattering (INS),
and electron paramagnetic resonance (EPR). The rhombic
ZFS is typically small/negligible for these complexes, meaning
it is often not detected at all,21−23 leading most studies to focus
on the axial ZFS component. Key early work examining ZFS
B
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using INS for D = 6.33(8) cm−1.29 The groups of Neese and
Xue have systematically examined D for [Fe(TPP)X], where X
is F−, Cl−, Br−, and I− and D was respectively determined as
4.49(9), 6.33(8), 8.8(2) (E = 0.1(2)), and 13.4(6) (E = 0.3(2)
cm−1).18 For the closely related ferric octaethylporphyrin
Scheme 2. (A) Spin Energy Diagram of the S = 5/2
Multiplet Absent an External Magnetic Field and (B and
a
C) the Splitting of the Ground State Doublet with an
Applied Magnetic Field for g⊥ = g|| and D > 0, in the Low-
Field Limit (μBB0 ≪ D) and Intermediate Region (μBB0 ≲
b
chloride, D = 8(1) cm−1 was found by Moßbauer spectros-
D)
̈
copy,28 while D = 7 cm−1 was later concluded by EPR.30 The
group of Franke employed scanning tunnelling microscopy
(STM) to determine D as ca. 6 cm−1 when the same porphyrin
was grafted onto a superconducting Pb surface.31 Considering
other ferric porphyrin chlorides, the group of Ohta has used
EPR to determine D = 7.2 cm−1 and E = 0.0 cm−1 for hemin
(FeIII protoporphyrin IX chloride),21 which was subsequently
refined to D = 6.90(1) cm−1 and |E| = 0.055(5) cm−1.22
EPR is the ideal method to examine the magnetic properties
of transition metal ions,32,33 and is exceptionally powerful for
bioinorganic chemistry.34−37 This includes work on oxy-
genated hemoglobin and myoglobin, which allowed the
determination of the haem group orientation relative to the
protein prior to the availability of suitable diffraction
patterns.38,39 EPR probes unpaired electrons, and in principle
the ZFS can be directly measured by EPR. However,
conventional EPR is unable to precisely determine a ZFS
that is much larger than the incident microwave frequency/
energy of the spectrometer. This requirement makes variable
frequency EPR and frequency-domain EPR very promising
though instrumentally challenging. For a ferric cation with
positive D (and E = 0), transitions between the ground state
and the first excited doublet (transition I, MS = 1/2 to 3/2)
as well as between the first and second excited doublet
(transition II, MS = 3/2 to 5/2) can be observed at zero
field (Scheme 2A). Measurements at elevated temperatures are
necessary to observe the latter transition (II), and the
observation of both transitions I and II is required to
determine both D and E in zero field. A negative D results
in an inversion of the energy levels and consequently to a
reversed temperature dependence of transition I and II, i.e.,
transition II can be observed at lowest temperatures, while
elevated temperatures are required to observe transition I. The
sign of D can therefore be clearly determined from the
temperature dependence of transitions I and II (Scheme 2A).
Ferric porphyrins usually have positive D values, so subsequent
discussion will focus on this scenario. The application of a
magnetic field splits the doublets resulting in EPR allowed
transitions within the ground state. Depending on the
orientation of the molecule (and consequently its magnetic
axis), the magnetic field value at which a certain transition
energy is reached can differ quite strongly. If the applied field
energy is much lower than D (μBB0 ≪ D), then the splitting of
the doublet is approximately linear with the magnetic field
(Scheme 2B) and similar to an S = 1/2 system, thus allowing
effective g tensors to be introduced.40 The fingerprint-like
signals in X-band EPR spectra of ferric porphyrins at around
115 and 340 mT (III⊥ and III||) correspond to effective g
values of around 6 and 2. However, if the energy of the applied
field approaches D (μBB0 ≲ D, e.g., for larger magnetic fields),
the splitting deviates from linear field dependency (Scheme
2C),17,40 which allows the magnitude of D to be estimated.
The presence of a significant rhombic component to the ZFS,
E, induces splitting of transition III⊥. Hence, the observation of
two nearby resonances at low fields allows |E/D| to be
determined.
a
The six states are degenerate in the absence of ZFS (D and E = 0).
The multiplet is split into three doublets if the ZFS is solely axial (D
≠ 0 and E = 0) and MS is a good quantum number (with MS the
eigenvalue of the operator Sz). A positive D results in a ground state
̂
doublet formed by the states with MS
=
1/2. The MS
=
3/2
doublet is 2D higher in energy, while the MS = 5/2 doublet is a
further 4D higher. EPR transitions are only allowed between states
differing in MS by 1, with the allowed zero-field transitions denoted as
b
I and II (blue and red arrows, respectively). In both cases, the
splitting is different for magnetic fields applied along (solid black line)
or perpendicular (dashed red line) to the magnetic z-axis of the
molecule. Allowed EPR transitions, III|| and III⊥, are shown as green
arrows positioned at magnetic fields such that they have an identical
transition energy (frequency). The allowed EPR transitions differ in
resonance field (despite g⊥ = g|| for the S = 5/2 multiplet), due to the
presence of D. In the low-field limit, they are in first order
perturbation theory given by hνIII = 3g⊥μBB0 and hνIII = g||μBB0.
⊥
||
The former is often referred to as an effective g = 6 signal and
characteristic for ferric porphyrins.
for ferric coordination complexes employed IR,24,25 including a
focus on biochemically relevant ferric porphyrins.26,27 This
included studies of met-myoglobin wherein D = 9.5(2) cm−1
was found, which decreases to D = 5.94(8) cm−1 by
exchanging the aquo for a fluoro ligand.26 The corresponding
met-hemoglobin with a F− ligand showed D = 6.3(1) cm−1.26
These values have been more recently confirmed by EPR as D
= 9.2(4) cm−1 for met-myoglobin, while the fluoro ligand
decreases this to D = 5.0(1) cm−1.17 The corresponding values
for met-hemoglobin were D = 10.4(2) cm−1 (H2O) and D =
5.1(1) cm−1 (F−).17
A well-studied example of ZFS determination for ferric
porphyrins is ferric tetraphenylporphyrin chloride, [Fe(TPP)-
̈
Cl] (Scheme 1), for which Moßbauer spectroscopy indicated
D = 7(1) cm−1,28 while a more recent determination was made
C
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Ferric porphyrins are examined herein with a combination of
EPR approaches to provide a full data set for analysis. For this
approach, measurements were undertaken at X-band (ca 9.5
GHz), J-band (263 GHz), multi-high-frequency EPR (MHF-
EPR) and with frequency-domain Fourier-transform THz-EPR
(FD-FT THz-EPR). MHF-EPR in transmission mode with
frequency multipliers allows for a series of measurements at
high frequencies (79−629 GHz used herein). FD-FT THz-
EPR allows EPR analysis in the frequency domain under a
varied magnetic field, with frequencies from 150 GHz to 1.2
THz (5 to 40 cm−1).17,33,41,42 An excitation energy range from
the microwave through to far-infrared means both spin
transitions and soft vibrational modes may be excited, though
these absorptions are separated by measuring their magnetic
field dependence.33 Thus, the frequency dependent absorption
is first measured absent an external magnetic field and then
repeated at applied fields, confirming the magnetic transitions.
Comparing experimental and simulated spectra across multiple
frequencies and fields allows the accuracy of the simulation
parameters to be thoroughly scrutinized.
The analysis is focused on ferric hangman porphyrins, along
with complementary planar porphyrins, to examine fine-tuning
of the porphyrin electronics and effects of the hanging group.
This is key to establishing if improved catalytic activity
observed by hangman porphyrins is due to the Brønsted acid/
base group, or if changes in porphyrin electronic structure also
contribute. The studied ferric hangman porphyrins have
architectures based on tetramesitylporphyrin with a xanthene
pillar functionalized with either a carboxylic acid or a methyl
carboxylate ([Fe(HPX-COOH)Cl] and [Fe(HPX-COOMe)-
Cl] respectively, where HPX is hangman porphyrin xanthene
and Me = methyl; Scheme 1). Changes to the porphyrin
substituents in either the meso- or β-position alter the
electronics, so for comparison and completeness, D was also
determined for a systematic series of ferric porphyrins with
axial chlorides (Scheme 1), where the porphyrin is
tetraphenylporphyrin (TPP), octaethylporphyrin (OEP),
tetramesitylporphyrin (TMP), or tetra(pentafluorophenyl)-
porphyrin (TPFPP).
RESULTS
■
Focusing initially on the [Fe(OEP)Cl] FD-FT THz-EPR
spectra, the magnetic field dependence of signals was examined
from 0 to 7 T at a temperature of 2.5 K, along with the
temperature dependence at four cryogenic temperatures with
applied fields of 0 and 7 T. The absorbance spectra at zero-
field are obtained by dividing a reference measurement at a
higher temperature (30 K) by the measurement at the
temperature of interest. The different spin populations of
energy levels at the two temperatures lead to the observation of
absorbance (as discussed in a recent review41 and described
briefly in the Experimental Section herein). The 7.5−35 cm−1
energy range was chosen due to the vanishing intensity of the
raw spectra at lower energies (see Figure S1). The absorbance
spectra at zero-field display a maximum at 14.1 cm−1 that
decreases with temperature, as well as a minimum at 27.3 cm−1
that increases with temperature (Figure 1, lower). These can
be directly identified as transitions I and II, respectively
(Scheme 2A), from which D and E may be determined.
However, this is inaccurate as the line shape of I is distorted,
probably due to overlap with a nonmagnetic absorption that is
partially resolved at 16 K. I is observed as a maximum that
exhibits decreasing absorbance with increasing temperature, as
Figure 1. FD-FT THz-EPR absorbance spectra of [Fe(OEP)Cl] at
magnetic fields of 7 T (top) and 0 T (bottom) with the indicated
temperatures. The main panel shows spectra offset to the
corresponding magnetic field and measured at a temperature of 2.5
K. These spectra were divided by a reference spectrum measured at 0
T and 30 K (fields below 0.5 T) or 7 T and 30 K (spectra measured at
>0.5 T), as specified in Table S1. Experimental spectra (black) are
overlaid on simulations (red; simulation parameters specified in Table
1). Simulated transition energies for magnetic fields parallel to the x
(dashed), y (solid), and z (dotted) axes are shown as gray background
lines for transitions that contribute significantly to the simulated
spectra.
the absorption is highest at low temperatures. This is the
opposite of II, which displays the highest absorption at
elevated temperatures. At low temperatures, although there is
minor absorption, this is observed as minimum in the
absorbance spectrum, due to the much stronger absorption
D
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in the reference spectrum (30 K). This temperature depend-
ence of I and II is synonymous with a positive D value.
Under application of a magnetic field, the line arising from
transition I broadens in an unsymmetrical manner (main panel
of Figure 1). The influence of the applied field upon the line
shape clearly distinguishes it from a nonmagnetic absorption
(like vibrations or phonons).
The line arising from transition II is seemingly unaffected by
magnetic fields less than 0.5 T and absent from spectra
measured with higher fields. This is because spectra measured
with magnetic fields less than 0.5 T use the zero-field 30 K
reference spectrum, while the measurements at 0.5−7.0 T use
a reference measured at 7 T and 30 K. Because the minimum
assigned to II is visible due to the reference spectrum (0 T, 30
K), it is seemingly unaffected by the applied field. The signal is
identified as an EPR transition by comparison with the high-
field reference (7 T, 30 K), in which the signal is shifted to an
energy exceeding the analysis range. Furthermore, an addi-
tional EPR line is observable at the low end of the energy range
when the field is 4−7 T. This signal can be directly assigned to
transition III⊥, concordant with simulations, and its nonlinear
field dependence indicates μBB0 ≲ D within the measured field
range (see Scheme 2C).
MHF-EPR spectra of [Fe(OEP)Cl] all show a strong,
slightly split line that displays an increasing resonance field
consistent with the increasing frequency (Figures 2 and S2−
S4). Its resonance field shifts from 1.0 T at 79 GHz to 11.8 T
at 629 GHz concordant with simulations, and it is
consequently assigned to transition III⊥. Its nonlinear field
dependence indicates μBB0 ≲ D (see Scheme 2C). An
additional set of lines is observed at low fields for frequencies
around 420 GHz (14 cm−1), which can be assigned to
transition I. These results are coherent with those from FD-FT
THz-EPR with the same simulation parameters used for
modeling.
Magnetic parameters were obtained by systematically
optimizing simulations in both frequency and field domains
against the experimental spectra (Table 1), yielding values of
g⊥ and g|| = 1.99(3) and D = 6.85(5) cm−1 for [Fe(OEP)Cl].
The uncertainties were also estimated heuristically by
individually varying each parameter in simulations and
comparing the agreement with experimental spectra. Simu-
lations of FD-FT THz-EPR absorbance visually disagree in the
position of the zero-field maximum, which is likely due to the
aforementioned overlap with a nonmagnetic resonance. The
final simulation parameters result in overall excellent agree-
ment with the experimental MHF-EPR spectra (Figure 2), and
the mismatch for a few relative intensities is probably due to
the presence of microcrystallites in the sample (vs a perfect
powder).
Figure 2. MHF-EPR spectra of [Fe(OEP)Cl]. Spectra are rescaled for
visibility and offset according to the MW energy at which they were
measured with the frequency indicated. Experimental spectra (black)
are overlaid on simulations (red; simulation parameters specified in
Table 1). Simulated transition energies for magnetic fields parallel to
the x (dashed), y (solid), and z (dotted) axes are shown as gray
background lines for transitions that contribute significantly to the
simulated spectra. All spectra were measured at 5 K.
Table 1. Magnetic Parameters Determined Herein Using eq
1
a
ferric porphyrin
D/cm−1
|E/D|
g⊥
g||
b
[Fe(TPP)Cl]
6.465(3)
6.85(5)
6.30(4)
6.24(8)
6.6(2)
0.003(2)
0.003(1)
0.000(5)
0.00(2)
0.00(2)
0.00(1)
2.00(5)
1.99(3)
1.992(5)
1.98(5)
2.019(5)
2.024(5)
1.95(5)
[Fe(OEP)Cl]
[Fe(TMP)Cl]
[Fe(TPFPP)Cl]
[Fe(HPX-COOH)Cl]
[Fe(HPX-COOMe)Cl] 6.55(3)
1.984(5)
2.005(5)
2.005(5)
a
b
Chemical structures of each complex depicted in Scheme 1. Values
from a previous study using the same techniques.23
The FD-FT THz-EPR spectra of [Fe(TMP)Cl] resemble
those of [Fe(OEP)Cl] with slight shifts of the peak energies
(cf. Figure 3 and Figure 1; note that, due to technical reasons,
the spectra are shown as magnetic division spectra (MDS) and
absorbance, respectively; raw spectra are shown in Figure S5).
[Fe(TMP)Cl] was investigated with field-domain EPR at
frequencies in X- and J-bands. The X-band EPR spectrum was
observed as typical for an S = 5/2 system with large axial ZFS
(Figure 4). A strong EPR line at 115 mT dominates the
spectrum, while a weaker line is observed at 340 mT (the
additional signal at 160 mT was assigned to an impurity).
Simulations indicate these two lines are due to transitions III⊥
and III∥, respectively (Scheme 2C). A line at 3.6 T was
observed in the J-band spectrum and assigned to III⊥, while the
line corresponding to III∥ is expected to be much weaker than
the III⊥ line (based on the X-band EPR spectrum) and was not
observed. Neither at X- nor J-band was a splitting of III⊥
observed, which provides an upper limit for the |E/D| ratio of
0.005. For an axial S = 5/2 system where D ≫ 0.33 cm−1 (10
GHz), g⊥ and g|| can be directly obtained from the X-band
spectrum and are assigned as 1.992(5) and 1.984(5),
respectively (Table 1). The D value can be read off the FD-
FT THz-EPR absorbance spectrum at zero field as 6.30(4)
cm−1. Simulations with these parameters accurately reproduce
E
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Figure 4. Field-domain EPR spectra of [Fe(TMP)Cl]. The X-band
EPR spectrum (black line) and the relevant region of the J-band EPR
spectrum (blue line on inset axis) are shown overlaid upon
simulations (red lines). T = 5 K for both experiments.
The hangman porphyrin [Fe(HPX-COOMe)Cl] shows
comparable spectral features to the aforementioned [Fe-
(TMP)Cl], though the resonance positions are slightly shifted
in the FD-FT THz-EPR and X-band spectra (Figures 5, S8,
and S9, respectively). The obtained magnetic parameters are of
course also similar (Table 1), with simulations using D =
6.55(3) cm−1 and g⊥ and g|| as 2.024(5) and 2.005(5), yielding
spectra concordant with the experimental results. The second
hangman porphyrin, [Fe(HPX-COOH)Cl], revealed rather
weak EPR lines by FD-FT THz-EPR and at X-band (Figures
S10, S11, and S12). The resonance positions very closely
resemble those observed for [Fe(HPX-COOMe)Cl], and
consequently the magnetic parameters show only small
differences (Table 1).
DISCUSSION
■
Ferric porphyrin chlorides typically exhibit positive axial ZFS
parameters of D = 6−7 cm−1, with a rhombic ZFS component
of E < 1 cm−1.17,18,21−23,28,29 All the complexes analyzed herein
fall into this expected range, including the hangman structures
(see Table 1). The subtle differences in ZFS due to changes in
the porphyrin substituents were able to be determined due to
the sensitivity of the employed approach. These types of
structural modifications have been described as providing the
ability to fine-tune the catalytic activity of metalloporphyrins,
while the presence of a hangman group has a more significant
influence on catalytic activity.
Figure 3. Main panel: FD-FT THz-EPR magnetic division spectra of
[Fe(TMP)Cl]. Spectra are offset to the corresponding magnetic field
and measured at a temperature of 2 K. Experimental spectra (black)
are overlaid on simulations (red; simulation parameters specified in
Table 1). Simulated transition energies for magnetic fields parallel
(dotted) and perpendicular (solid) to the z axis are shown as gray
background lines for transitions that contribute significantly to the
simulated spectra. The absorbance at the indicated temperatures
absent an applied magnetic field is shown in the bottom panel.
[Fe(OEP)Cl] was herein determined to have a D value of
6.85(5) cm−1, which is comparable to the literature value for
hemin (FeIII protoporphyrin IX chloride) of 6.90(1) cm−1.22
These values are significantly higher than D for [Fe(TPP)Cl],
which has been examined in great detail using complementary
techniques (vide supra), with 6.465(3) cm−1 having been
determined by FD-FT THz-EPR.23 These values are also
consistent with previous findings that the ZFS was higher for
[Fe(OEP)Cl] than [Fe(TPP)Cl].28
the field-domain EPR spectra (Figure 4), though it is not
understood why they diverge from the FD-FT THz-EPR MDS
spectra (Figure 3). [In the simulation of the X-band EPR
spectrum, a 4.9(5)% impurity was included with S = 3/2, large
D (6.3−10 cm−1), g⊥ = 2.16(1), and g|| = 2.01(1).]
[Fe(TPFPP)Cl] was probed with FD-FT THz-EPR and
exhibited spectral features comparable to those of [Fe(OEP)-
Cl] and [Fe(TMP)Cl] (see SI, Figures S6 and S7).
Simulations of the FD-FT THz-EPR spectra allowed the
spectra to be well reproduced using g⊥ and g|| equal to 1.98(5)
and a D of 6.24(8) cm−1 (Table 1).
Exchanging the phenyl substituents of [Fe(TPP)Cl] for
mesityl substituents gives the structurally similar but sterically
more congested [Fe(TMP)Cl], for which D = 6.30(4) cm−1
F
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the similarity of D and gives an indication of the magnitude of
catalytic fine-tuning.
The aforementioned study of [Fe(TPP)X] by Neese and
colleagues showed that larger halogens (X = F−, Cl−, Br−, I−)
and consequently softer ligands systematically increase the
axial ZFS (D = 4.49(9), 6.33(8), 8.8(2), 13.4(6) cm−1,
respectively).18 It is elegantly described therein that D is
correlated with the increasingly covalent nature of the Fe−X
bond, and with larger spin orbit coupling. Indeed, the length of
the Fe−X bond increases along the series from 1.79,46 to
2.21,43 to 2.35,47 to 2.55 Å,48 while the Fe displacement from
the N4 plane is essentially unchanged at 0.48 0.02 Å (Table
2).43,46−48 Work by Franke and colleagues on [Fe(OEP)Cl]
using an STM tip to attract the chloride also indicated that
increasing the Fe−X bond length led to an increase in D.31,49
Table 2. Selected Crystal Structure Parameters for Ferric
Porphyrins Taken from the Literature
Fe−halide bond
Fe displacement from
plane/Å
ferric porphyrin
distance/Å
[Fe(TPP)F]46
1.79
2.19
2.21
2.22
2.23
2.35
2.55
0.47
0.45
0.49
0.48
0.49
0.49
0.46
[Fe(TPFPP)Cl]44,45
[Fe(TPP)Cl]43
a
50
[Fe(PPIX)Cl]
[Fe(OEP)Cl]51,52
[Fe(TPP)Br]47
[Fe(TPP)I]48
a
PPIX = protoporphyrin IX.
Correlations between bond lengths and D values were also
explored herein and extrapolated to include porphyrin
structures where crystal structures are hitherto not available.
[Fe(OEP)Cl] crystallizes with a 2.23 Å Fe−Cl bond,51,52 while
for hemin the length is 2.22 Å.50 Both have D values that are
relatively high for ferric porphyrin chlorides, and slightly longer
axial bonds than [Fe(TPP)Cl]. The Fe−Cl bond length in
[Fe(TPFPP)Cl] is 2.19 Å,44 with the lower D value
accompanying a shorter bond length. Regrettably, a crystal
structure could not be found for [Fe(TMP)Cl], though the D
value supports a logical and intuitive Fe−Cl distance between
those of [Fe(TPFPP)Cl] and [Fe(TPP)Cl] (i.e., 2.20 Å).
The carboxylic acid and methyl carboxylate functionalized
hangman porphyrins examined herein were determined to have
respective D values of 6.6(2) and 6.55(3) cm−1 ([Fe(HPX-
COOH)Cl] and [Fe(HPX-COOMe)Cl] in Table 1). The
hangman porphyrins are structurally equivalent to the planar
[Fe(TMP)Cl], aside from the replacement of a mesityl
substituent with the hangman pillar (Scheme 1). Conse-
quently, as for the planar porphyrins, strong electron donation
or withdrawal via the aromatic π orbital system is prohibited by
the sterically enforced angle of the bulky porphyrin
substituents. However, D is significantly higher for the
hangman functionalized porphyrins than [Fe(TMP)Cl] (6.6
vs 6.3 cm−1), indicating they have Fe−Cl bond lengths
between those of [Fe(TMP)Cl] and [Fe(OEP)Cl] (i.e., 2.20−
2.23 Å).
Figure 5. Main panel: FD-FT THz-EPR magnetic division spectra of
[Fe(HPX-COOMe)Cl]. Spectra are offset corresponding magnetic
field and measured at a temperature of 2 K. Experimental spectra
(black) are overlaid on simulations (red; simulation parameters
specified in Table 1). Simulated transition energies for magnetic fields
parallel (dotted) and perpendicular (solid) to the z axis are shown as
gray background lines for transitions that contribute significantly to
the simulated spectra. The absorbance at the indicated temperatures
absent an applied magnetic field is shown in the bottom panel.
was determined herein. To systematically probe the effect of a
stronger electron withdrawing substituent on the porphyrin
electronics, the phenyl moiety was also exchanged for the
penta-fluorinated variant to yield [Fe(TPFPP)Cl]. This had
the effect of lowering the measured D value to 6.24(8) cm−1.
These three values do not indicate a correlation of D with the
electron donating/withdrawing character in the porphyrin
substituents (TMP, TPP, TPFPP). This is probably because
the steric bulk of the aromatic substituents ensures they are
near perpendicular to the porphyrin plane (crystal structures
exist for the latter two),43−45 thus hindering their conjugation
with the π orbitals of the porphyrin. The more subtle
electronegativity effect through the σ bonds likely explains
Unfortunately, no crystal structures are available for ferric
hangman porphyrins with chloride axial ligands, though one
exists for the equivalent structure with an axial hydroxyl
ligand.11 Therein, the hangman group and axial ligand are
located on the same porphyrin plane, with water present
between the hangman group and axial ligand. A similar
G
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the reference spectrum) and magnetic field division spectra (MDS, M
= [I(B0)]/[I(B0 + 0.5T)], with I(x) the spectrum measured at the
magnetic field x and both spectra measured at the same temperature).
The expected shapes of such obtained spectra are described in ref 41.
hydrogen bonding arrangement could be considered here,
whereby hydrogen bonding withdraws electron density to
make Cl− a softer ligand and thus increases D. Hydrogen
bonding has been shown to increase Fe−Cl bond lengths from
2.23 to 2.36 Å (comparable to an Fe−Br bond) in octaethyl-
tetraarylporphyrin.53 However, an Fe−Cl bond length outside
the typical ferric porphyrin chloride range is incoherent with
the D values measured herein, thus implying that any hydrogen
bonding effects are weaker in the hangman porphyrins than in
the aforementioned study. It may also be expected that the
strength of such an interaction would vary between the
carboxylic acid and methyl carboxylate moieties, though it is
interesting that no significant difference in D was detected.
It is also possible that the changes in D are due to the
different electronics of the xanthene pillar compared to the
mesityl group, in the same manner that changing the phenyl
substituents to mesityl or pentafluorophenyl groups induced a
change in D of the same order (vide supra). In this scenario, D
would be largely insensitive to changing the hangman moiety
from a carboxylic acid to a methyl carboxylate, which is
consistent with the determined values.
I
ref for absorbance spectra was measured at a temperature of 30 K and
in the absence of a magnetic field, except for absorbance spectra of
[Fe(OEP)Cl] with magnetic fields of more than 0.5 T. For those, Iref
was measured at 30 K and 7 T.
MHF-EPR. Multi-high-frequency EPR (MHF-EPR) was measured
at the EMR facility of the National High Magnetic Field Laboratory,
Tallahassee, FL. A transmission type spectrometer with a phase-
locked Virginia Diodes source combined with a series of frequency
multipliers, covering the range from 50 to 635 GHz, was used.54 The
sample is placed inside a 17 T superconducting magnet (Oxford)
equipped with a continuous-flow cryostat. The signal was field-
modulated at 50 kHz and 10 G amplitude. For detection, an InSb hot-
electron bolometer (QMC) was used.
J-band EPR was performed on a Bruker Elexsys E780 EPR
spectrometer using a nonresonant sample insert and a continuous-
flow cryostat. Experiments were performed at a temperature of 5 K.
MW frequency was 262.354 GHz, and the nominal output power was
15 mW. Field modulation at 100 kHz with 40 G amplitude was used.
X-band EPR was performed on a Bruker ESP300, equipped with a
continuous flow cryostat and a TE011 super high Q resonator. Spectra
were measured at a temperature of 5 K and with field modulation at
100 kHz with 10 G amplitude. MW frequencies and powers were
9.393, 9.392, and 9.392 GHz and 0.8, 0.8, and 0.2 mW for
[Fe(TMP)Cl], [Fe(HPX-COOMe)Cl], and [Fe(HPX-COOH)Cl],
respectively.
CONCLUSION
■
A series of ferric porphyrin chlorides was studied with FD-FT
THz-EPR and field domain EPR at frequencies between 9.3
and 629 GHz. This allowed subtle differences in the magnetic
properties to be distinguished and values determined with high
accuracy. The experimentally obtained D values span a range
from 6.24 to 6.85 cm−1, with vanishing rhombicity (maximum
|E/D| = 0.003). Changes to the porphyrin substituents,
including the presence of a hangman motif, did not shift the
ZFS parameters beyond this range. The accuracy of the
employed techniques in determining D values allowed
correlations with Fe−halide bond lengths to be examined for
different porphyrin chlorides rather than different halides. This
correlation prompted estimates of the Fe−Cl bond length
based on the D values of the hangman porphyrins for which no
crystal structures are known. The high precision of the
combination of FD-FT THz-EPR and field domain EPR over a
wide range of frequencies has thus been exemplified for ferric
porphyrins in particular.
EPR spectra simulations employed the Matlab toolbox, EasySpin,55
with FD-FT THz-EPR utilizing a recently developed frequency-
domain extension included therein.23,56
Preparation of Samples for Analysis. FD-FT THz-EPR
experiments were performed using pellets of 10 mm diameter.
[Fe(OEP)Cl] was mixed with polyethylene (PE), ground, and
pressed into a pellet. The other porphyrins were dissolved in acetone;
PE was added to the solution and the solution dried out and pressed
into a pellet. For MHF-EPR, ground powder of [Fe(OEP)Cl] was
filled into a plastic holder, closed with a Teflon cap. For J-band EPR
experiments, [Fe(TMP)Cl] in acetone solution was filled in the
Teflon sample holder. After the acetone was dried off, the lid was
closed. X-band EPR experiments were performed on acetone
solutions filled in X-band quartz tubes.
Samples. [Fe(TPP)Cl] was previously measured by Nehrkorn et
al.,23 using the same instrumentation and approach. The sample,
chloro(5,10,15,20-tetraphenylporphyrin)iron(III), was purchased
from a commercial supplier (Sigma-Aldrich) and used as received
(CAS 16456-81-8). [Fe(OEP)Cl], 2,3,7,8,12,13,17,18-octaethyl-
21H,23H-porphine iron(III) chloride, was purchased from a
commercial supplier (Sigma-Aldrich) and used as received (CAS
28755-93-3).
Synthetic Procedure. The other ferric porphyrins were
synthesized according to published procedures that will only be
briefly outlined herein. For the hangman porphyrins, the free base
porphyrins HPX-COOH and HPX-COOMe were synthesized
according to a previously defined microwave synthesis protocol.57
Subsequent reaction with iron(II) chloride in dimethylformamide and
aerobic acid workup yields the corresponding chloroiron(III)
porphyrin complexes.12 [Fe(TMP)Cl] and [Fe(TPFPP)Cl] were
synthesized following the same metalation procedure from the known
ligands, TMP and TPFPP.58
EXPERIMENTAL SECTION
■
FD-FT THz-EPR. Frequency domain Fourier transform EPR (FD-
FT THz-EPR) spectroscopy experiments were conducted on the THz
beamline of the BESSY II storage ring, at the dedicated end station
that has been described elsewhere in detail,33,42 and recently
reviewed.41 Briefly, radiation is passed through an FT-IR spectrometer
(Bruker IFS 125), to be then transmitted through the sample placed
in the variable-temperature insert inside an optical magnet (Oxford
Optistat) and finally detected by a Si bolometer (Infrared
Laboratories, Far-Infrared bolometer) cooled down to 1.3 K.
Radiation used was either from a Hg-arc lamp ([Fe(OEP)Cl]) or
coherent synchrotron radiation extracted from BESSY II operated in
low-α storage-ring mode (all other porphyrins). The FT-IR
spectrometer was operated with resolution of 0.5 cm−1 and Mylar
beam splitter of 50 ([Fe(OEP)Cl], [Fe(TPFPP)Cl]) or 23 μm
([Fe(TMP)Cl], [Fe(HPX-COOMe)Cl], [Fe(HPX-COOH)Cl])
thickness. The detected signal is the power of the transmitted
radiation as a function of frequency. To account for varying spectral
density etc., a reference is required. In traditional FT-IR spectroscopy,
often a blank spectrum is used. The strong nonmagnetic absorption
forbids this for FD-FT THz-EPR. Therefore, spectra of the sample
under varied experimental conditions are used. Here, we used
absorbance (A = log10(Iref/I0) , with I0 the spectrum of interest and Iref
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02348.
■
S
Additional spectra for [Fe(OEP)Cl], [Fe(TMP)Cl], and
[Fe(HPX-COOMe)Cl], spectra for [Fe(TPFPP)Cl]
H
DOI: 10.1021/acs.inorgchem.9b02348
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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and [Fe(HPX-COOH)Cl] as Figures S1−S12 and Table
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for the Assembly of a Model Heme Water Channel. J. Am. Chem. Soc.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: Joscha.Nehrkorn@cec.mpg.de.
*E-mail: Shannon.Bonke@cec.mpg.de.
ORCID
Joscha Nehrkorn: 0000-0003-1156-5675
Shannon A. Bonke: 0000-0002-3285-4356
Matthias Schwalbe: 0000-0003-4209-1601
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ACKNOWLEDGMENTS
■
The authors are grateful to Dr. Samuel M. Greer (National
High Magnetic Field Laboratory, FL, USA), Dr. Karsten
Holldack for experimental assistance, Dr. Steffen Meyer for
productive discussions on iron chemistry (the latter are
affiliated with Helmholtz-Zentrum Berlin, Germany), and
Prof. Dr. Katharina J. Franke (Freie Universitat Berlin,
Germany) for inspiring the work on [Fe(OEP)Cl]. J.N. thanks
̈
̈
Prof. Dr. Carmen Herrmann (Universitat Hamburg, Germany)
for support and discussions. The National High Magnetic Field
Laboratory is supported by the National Science Foundation
through NSF/DMR-1157490 and the State of Florida.
Funding from the Deutsche Forschungsgemeinschaft through
SPP 1601 and a research fellowship to J.N. (grant no. NE
2064/1-1) is gratefully acknowledged. S.A.B. gratefully
acknowledges a fellowship from the Alexander von Humboldt
Foundation.
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DOI: 10.1021/acs.inorgchem.9b02348
Inorg. Chem. XXXX, XXX, XXX−XXX