Electronic Structure of a Formal Iron(0) Porphyrin Complex Relevant to CO2 Reduction

Article abstract of DOI:10.1021/acs.inorgchem.7b00401

Iron porphyrins can act as potent electrocatalysts for CO₂ functionalization. The catalytically active species has been proposed to be a formal Fe(0) porphyrin complex, [Fe(TPP)]^2- (TPP = tetraphenylporphyrin), generated by two-electron reduction of [Fe^II(TPP)]. Our combined spectroscopic and computational investigations reveal that the reduction is ligand-centered and that [Fe(TPP)]^2- is best formulated as an intermediate-spin Fe(II) center that is antiferromagnetically coupled to a porphyrin diradical anion, yielding an overall singlet ground state. As such, [Fe(TPP)]^2- contains two readily accessible electrons, setting the stage for CO₂ reduction.

Full text of DOI:10.1021/acs.inorgchem.7b00401

Article
pubs.acs.org/IC
Electronic Structure of a Formal Iron(0) Porphyrin Complex Relevant
to CO2 Reduction
Christina Romelt,† Jinshuai Song,†,‡ Maxime Tarrago,† Julian A. Rees,†,§ Maurice van Gastel,†
̈
Thomas Weyhermuller,*,† Serena DeBeer,*,†,∥ Eckhard Bill,*,† Frank Neese,*,† and Shengfa Ye*,†
̈
†Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34−36, 45470 Mulheim an der Ruhr, Germany
̈
‡Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China
§Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
∥Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States
S
* Supporting Information
ABSTRACT: Iron porphyrins can act as potent electro-
catalysts for CO2 functionalization. The catalytically active
species has been proposed to be a formal Fe(0) porphyrin
complex, [Fe(TPP)]2− (TPP = tetraphenylporphyrin), gen-
erated by two-electron reduction of [FeII(TPP)]. Our
combined spectroscopic and computational investigations
reveal that the reduction is ligand-centered and that [Fe-
(TPP)]2− is best formulated as an intermediate-spin Fe(II)
center that is antiferromagnetically coupled to a porphyrin
diradical anion, yielding an overall singlet ground state. As
such, [Fe(TPP)]2− contains two readily accessible electrons,
setting the stage for CO2 reduction.
There is, however, controversy about the exact nature of the
electronic ground states of [Fe(TPP)]− and [Fe(TPP)]2−,
because the reductions can be either metal- or ligand-centered.
INTRODUCTION
■
Primarily because of the vast use of carbon-based fuels in
combustion processes, the amount of CO2 in the atmosphere
has increased by almost 40% over the past 100 years,1
contributing significantly to global warming. Therefore, to cope
with present environmental and energy issues, the reconversion
of CO2 into building blocks that can be employed for further
use like formaldehyde, formic acid, oxalic acid, methanol, and
CO has attracted much interest. Because of its thermodynamic
stability and kinetic inertness, facile CO2 reduction requires not
only high energy input but also suitable catalysts.2 One area of
growing interest is electrochemical CO2 functionalization, in
part because such processes could be powered by renewable
energy from wind and solar power plants.3 In addition to
transition metal complexes based on cyclam4 or bipyridine5
ligands, iron porphyrins are known to act as potent catalysts for
activating CO2.6 Iron(II) tetraphenylporphyrin ([Fe(TPP)]0),
featuring a triplet ground state, can undergo two reversible
single-electron reductions, yielding formal iron(I) ([Fe-
(TPP)]−) and iron(0) complexes ([Fe(TPP)]2−). The latter
has been proposed to be the catalytically active species for CO2
reduction.6,7 In the presence of external or internal proton
sources, [Fe(TPP)]2− and its derivatives have been reported to
exhibit one of the highest catalytic activities and turnover
frequencies observed so far.6a,b,d,e To pinpoint the pivotal
features responsible for the high efficiency of [Fe(TPP)]2−,
understanding its electronic structure is a prerequisite.
A variety of spectroscopic studies, including 57Fe Mossbauer,8
̈
electron paramagnetic resonance (EPR),8,9 resonance Raman,10
nuclear magnetic resonance (NMR),9a,11 X-ray diffraction,8 and
ultraviolet/visible (UV/vis)9b,10a,11a,12 experiments, have been
undertaken. Interestingly, different techniques seem to support
distinct electronic structure descriptions of the ground state,
and no consensus has yet been reached. Herein, we present a
detailed electronic structure investigation of [FeTPP]2− by
using a combined spectroscopic and computational approach.
EXPERIMENTAL DETAILS
■
Synthesis. All reactions were performed in an inert glovebox argon
atmosphere. Stabilizer-free tetrahydrofuran (THF) was purchased
from Acros Organics, stirred over sodium for 2 days, and stored over
molecular sieves (4 Å). Stabilizer-free 2-MeTHF was purchased from
Acros Organics. Before use, it was degassed by a freeze−pump−thaw
technique (three cycles), stirred over sodium for 2 days, and stored
over molecular sieves (4 Å). Heptane was purchased from Acros
Organics, degassed by being bubbled with argon for 1 h, and stored
over molecular sieves (4 Å). FeClTPP was purchased from Sigma-
Aldrich with 95% purity (porphyrin residue). Sodium anthracenide
was prepared by a previously described procedure.13 If reactions were
performed in THF, 0.2 M sodium anthracenide in THF was used as a
Received: February 17, 2017
© XXXX American Chemical Society
A
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Inorganic Chemistry
Article
reducing agent. If reactions were performed in MeTHF, 0.2 M sodium
anthracenide in MeTHF was used as a reducing agent. After reduction,
all samples were stored at −40 °C inside the glovebox. All iron
porphyrin species were prepared according to the published
procedure.8
previously published procedure.22 The list of complexes together with
their experimental pre-edge energies as well as the herein calculated
energies (shifted by a constant value of 22.37 eV for the used
functional/basis set) is given in Table S1. Geometry optimizations of
all iron complexes were performed on the basis of experimental X-ray
structures employing the B3LYP functional with def2-TZVP basis
sets,23 followed by time-dependent density functional theory (TD-
DFT) calculations using the CAM-B3LYP functional24 together with
the def2-TZVP auxiliary basis set.25 During integral generation and
transformation, the RI19a and chain-of-sphere (COSX)26 approxima-
tions were applied for Coulomb and exchange integrals, respectively.
̈
Mossbauer Spectroscopy. Mossbauer spectra were recorded on
̈
a conventional spectrometer with alternating constant acceleration of
the γ-source. The minimum experimental line width was 0.24 mm/s
(full width at half-height). The sample temperature was kept constant
in an Oxford Instruments Variox or in an Oxford Instruments
Mossbauer-Spectromag. The latter is a split-pair superconducting
̈
magnet system for applied fields up to 8 T where the temperature of
the sample can be varied in the range of 1.5−250 K. The field at the
sample is perpendicular to the γ-beam. The 57Co/Rh source (1.8 GBq)
was positioned at room temperature inside the gap of the magnet
system at a zero-field position, by using a re-entrant bore. Isomer shifts
RESULTS AND DISCUSSION
■
Preparation of all reduced species was performed by previously
published procedures (for further details, see the Supporting
Information).8 Specifically, sequential reduction of the Fe(III)
precursor, [Fe(TPP)Cl] (1), leads to isolation of ferrous
[Fe(TPP)(THF)2] (2a) (S = 2), [Fe(TPP)] (2b) (S = 1), and
the two- and three-electron-reduced sodium solvate complexes,
are quoted relative to iron metal at 300 K. Magnetic Mossbauer
̈
spectra of fully reduced compound 4b were simulated with the
program MX (by E.B.) by using the usual nuclear Hamiltonian with an
applied field only and zero internal field due to electronic spin S = 0.14
Zero-field spectra were measured at 80 K. For solid-state
measurements, the obtained solid was filled into a self-sealing Delrin
capsule in the glovebox. The capsule was additionally wrapped airtight
in aluminum foil and placed in liquid nitrogen directly after being
discharged from the glovebox. Handling and mounting of the samples
were performed under liquid nitrogen. For solution measurements, the
preparation of the reduced species was performed according to the
previously described procedures. The starting compound was enriched
with 20% [57Fe(TPP)Cl]. The reaction solution was filtered and
1
[Fe(TPP)][Na(THF)3] (3a) (S = /2) and [Fe(TPP)][Na-
(THF)3]2 (4a) (S = 0), respectively. The crystal structure8 of
4a shows the coordination of two [Na(THF)3]+ moieties to a
pyrrole nitrogen on each side of the porphyrin plane. To
circumvent the complexity arising from this additional
interaction, we prepared solution samples of [Fe(TPP)]−
(3b) and [Fe(TPP)]2− (4b) by using 18-crown-6 to trap Na+
and dissolving 3a and 4a into an essentially noncoordinating
solvent, 2-methyl-THF.
directly used for Mossbauer experiments. The solution capsule was
̈
placed in a Schlenk flask, discharged from the glovebox, and placed in
liquid nitrogen until the solution was frozen. Handling and mounting
of the samples were performed under liquid nitrogen.
Iron K-edge X-ray absorption spectroscopy (XAS) provides
an experimental means of assessing changes in the iron
oxidation state within a series of related complexes. Figure 1A
shows the normalized XAS spectra for solid samples of all
complexes under investigation. The XAS spectra are comprised
of three primary features: (1) weak 1s to 3d pre-edge feature in
the ∼7111−7113 eV region, (2) a rising edge shoulder at
∼7118 eV, and (3) the rising edge inflection point (at ∼7123
eV). Complex 1 clearly has the highest-energy pre-edge feature
(Figure 1A and Table 1) and the highest-energy rising edge
position (Figure 1A), consistent with ferric iron.27 In contrast,
complexes 2−4 have lower-energy pre-edges [with the lowest
1s to 3d feature at ∼71111 eV (Table 1)] and a lower-energy
rising edge position. These observations suggest that the iron
centers in complexes 2−4 feature the same valence state
(ferrous iron), thus pointing to ligand-centered reductions for
complexes 3 and 4. We note, however, that significant changes
are observed in the spectra of 2−4 in the ∼7118 eV region. In
the case of complex 2b, this feature can be attributed to a low-
lying 1s to 4pz transition due to the four-coordinate
environment at the Fe. Similar features are also observed for
complexes 3 and 4, reflecting the fact that [Na(THF)3] is only
weakly interacting. In contrast, for six-coordinate complex 2a,
this feature is absent as the 4pz orbital is shifted to higher
energies because of axial coordination. Using TD-DFT
calculations, we are able to generally reproduce the trends
observed in the energies and intensities of both the lowest-lying
1s to 3d pre-edge transitions (Figure 1B) and the ∼7118 eV 1s
to 4p feature (see Figure S4). As shown in Table 1, the
calculations indicate similar pre-edge transition energies for 2a,
2b, 3a, and 4a and a considerably higher pre-edge transition
energy for 1, consistent with the experimental observations. We
note that the additional feature at ∼7113.5 eV observed in the
XAS spectrum of 2a is not predicted computationally. This may
suggest that this feature originates from charge transfer
processes, which are not well modeled by TD-DFT approaches.
UV/Vis. All UV/vis experiments were performed in MeTHF.
Preparation as well as measuring of the samples was performed inside
the glovebox with an optical fiber, using a 1 mm cuvette. Handling of
the samples has to be performed without delay because of the high
reactivity of the samples in solution. UV/vis spectra were recorded on
a HP 8453 Diode Array system (190−1100 nm).
X-ray Absorption Spectroscopy (XAS). For solid-state XAS
experiments, the samples were diluted in boron nitride, sealed in an
aluminum spacer (1 mm layer thickness) between 38 μm Kapton tape
windows, and stored in liquid nitrogen. Handling and transportation of
the samples were performed under a nitrogen atmosphere. XAS
measurements were performed on ESRF beamline BM23, which is
equipped with a liquid N2-cooled Si(111) double-crystal mono-
chromator, which was used for energy selection. Energy calibration
was performed by setting the first inflection of an Fe foil to 7111.2 eV.
A beam size of 500 μm × 3 mm was used. A continuous flow dual-
chamber liquid He cryostat at 10 K was used. All samples were
continuously monitored for signs of radiation damage throughout the
course of data collection. Data were measured over an energy range of
7056−7970 eV. Data calibration and averaging were performed using
Athena from the IFEFFIT package. Peak positions of the pre-edge and
rising edge were determined by calculating the center of mass of the
second-derivative minima of five-point smoothed spectra.
Theoretical Calculations. All calculations were performed with
the ORCA program package.15 Geometry optimizations were
performed on experimental X-ray structures using the BP86
functional16 with TZVP17 and ZORA-TZVP basis sets18 on all
atoms. To accelerate calculations, the resolution of identity (RI)
approximation is used in combination with the TZVP auxiliary basis
set.19 Calculations of Mossbauer parameters were then performed
̈
using the B3LYP functional20 together with the TZVP auxiliary basis
set. The isomer shift was predicted by employing the calibration
parameters previously published for a given combination of functionals
and basis sets.21 An example of the input file for geometry
optimization and parameter calculations is given in the Supporting
Information. For XAS, calibration calculations were conducted on a
series of literature known and measured iron complexes according to a
B
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Inorganic Chemistry
Article
Table 2. Experimental (exp.) and Calculated (calcd) Isomer
Shifts, δ (millimeters per second), and Quadrupole
Splittings, ΔEQ (mm/s), of Iron Tetraphenylporphyrin
Species under Investigation
complex
δ exp.
0.95
δ calcd
|ΔEQ| exp.
2.64
|ΔEQ| calcd
2a32
2b28a,b
38
0.90
0.55
0.48
2.34
1.11
0.32
0.75
a
b
a
b
0.57 (0.50 )
2.21 (1.51 )
c
d
c
d
0.65 (0.52 )
2.23 (2.21 )
48
0.49 (0.45 )
0.59
1.35 (1.50 )
e
f
e
f
a
b
Values for 2b with a saddled core. Values for 2b with a ruffled core.
c
d
e
f
Values for 3a. Values for 3b. Values for 4a. Values for 4b.
more positive isomer shift of 0.95 mm/s that is typical for six-
coordinate high-spin ferrous porphyrin complexes.29 This
experimental finding provides strong support for the notion
that the iron centers in 2b, 3b, and 4b feature not only the
identical oxidation state, consistent with the XAS results, but
also the same local spin state (SFe = 1), as previously assigned
for four-coordinate Fe(II) porphyrins.8,28,29 Should the
reduction be iron-centered, the changes in the isomer shift
should be more pronounced than what is found for 3b and 4b,
because the isomer shift is a sensitive probe of the local
electronic structure change at the iron center. The somewhat
different isomer shifts between the measurements performed
on the solid samples and the frozen solutions found for
complexes 3 and 4 may be attributed to the weak coordination
of [Na(THF)3]+ to the porphyrin ring in the solid state.8 For
all investigated systems, the DFT-computed isomer shifts
match the experimental values (in solution) reasonably well
within the well-established uncertainty range (∼0.1 mm/s).21
According to our earlier experiences,30 the large deviations (>1
mm/s) between computed quadrupole splittings and exper-
imental values are not unusual, especially for compounds
involving a flexible first coordination sphere, such as porphyrin
complexes, for which porphyrin may adopt several nearly
isoenergetic core conformations.31 Such different core geo-
metries may manifest themselves in the quadrupole splittings,
as found for the two conformers of complex 2b (Table 2),
whereas the influence on the isomer shift is insignificant. This
may explain the particularly large error in the calculated
quadrupole splitting of complex 3.
Figure 1. (A) Experimental XAS spectra of iron tetraphenylporphyrins
in all known oxidation states with an enlarged pre-edge region shown.
(B) Calculated pre-edge features of 1−4.
Table 1. Experimental (exp.) and Calculated (calcd) Pre-
Edge Energies (eV)
a
complex
S
exp.
calcd
bonding
5
1
/
7112.9
7110.5
7110.5
7110.5
7110.1
7113.2
7111.1
7111.3
7111.6
7111.6
HS-Fe(III)(TPP)2−
HS-Fe(II)(TPP)2−
IS-Fe(II)(TPP)2−
IS-Fe(II)(TPP)3−•
IS-Fe(II)(TPP)4−••
2
2a
2b
3a
4a
2
1
1
The magnetic Mossbauer spectrum of complex 4b recorded
̈
/
2
at 4.2 K with an applied magnetic field of 4 T (Figure 2)
0
confirms the diamagnetic ground state of this compound.8 In
a
Abbreviations: HS, high spin; IS, intermediate spin.
light of the consistent Mossbauer data of complex 2b, we
̈
suggest that the S = 0 ground state of complex 4b arises from a
strong antiferromagnetic intramolecular exchange coupling of
an intermediate-spin iron(II) (SFe = 1) with a porphyrin triplet
anion diradical (STPP = 1).
In any case, the similarity in the lowest-energy 1s to 3d
transitions supports a similar electronic structure at the iron in
complexes 2−4, providing experimental evidence of ligand-
centered reductions across this series.
Our spectroscopic results clearly evidence that complexes 3
and 4 contain intermediate-spin ferrous ions, the same as that
found for 2b.28,33 In accordance with this conclusion, a
computational attempt to locate a low-spin d7 Fe(I) state for 3b
from different initial guesses failed, indicating that the Fe(I)
state is much higher in energy. A low-spin d8 Fe(0) state for 4b
can be obtained from the restricted Kohn−Sham calculations;
however, this state is 58 kcal/mol above a broken-symmetry
state discussed below. More importantly, our computed
electronic structures also predict accurate spectroscopic
In addition to our XAS studies, Mossbauer studies were
̈
performed on the complete series of complexes and further
correlated to computations. The experimental Mossbauer
̈
parameters for the solid samples of 2a, 2b, 3a, and 4a have
been previously reported;8,28 however, to the best of our
knowledge, a detailed correlation of the experimental
Mossbauer parameters to the electronic structures has not
̈
been made. Our Mossbauer measurements (Table 2)
̈
reproduced all previously published data. The measured isomer
shifts for complexes 2b−4 all cluster around a remarkably low
value of ∼0.5 mm/s, having only small differences of ∼0.1 mm/
s. In contrast, six-coordinate complex 2a has a significantly
parameters when compared to the experimental Mossbauer
̈
isomer shifts and XAS data, whereas the computed isomer shift
for the hypothetical d8 Fe(0) species (0.38 mm/s) shows a
C
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
large S values (Figure 3). It has been reported that complex 4
possesses a diamagnetic ground state even at room temper-
ature.8 Remarkably, this exchange interaction fixes the iron
2
22
1
1
electron configuration to be dxy dz dxz dyz , consistent with the
negligibly small asymmetry parameter (η) determined by
applied field Mossbauer spectroscopy (Figure 2). In contrast,
̈
there remains intense debate about the exact electron
configuration for the ground state of complex 2b, for which
two different states have been proposed, either
28,33
2
22
1
1
2
21
A2g(dxy dz dxz dyz ) or Eg(dxy dz (dxzdyz)3).
CONCLUSION
■
In summary, our combined experimental and theoretical
investigations of the redox series of [Fe(TPP)]0/1−/2− show
that the reductions are not metal- but ligand-centered. Analysis
of the experimentally validated electronic structure of [Fe-
(TPP)]2−, the putative species for CO2 functionalization,
reveals that the two additional reducing electrons reside in π*
orbitals of the porphyrin ligand. These electrons are likely to be
responsible for CO2 reduction in the catalytic process. As these
electrons are delocalized onto the conjugated macrocycle, one
can anticipate that the corresponding electron transfer involves
marginal geometric distortions of the porphyrin ligand and
hence a low reorganizational energy. This contrasts with metal-
centered redox processes, which typically entail significant
geometric adjustments in the first coordination sphere,
especially for 3d transition metals. In other words, noninnocent
ligands such as porphyrin may function as electron buffers to
facilitate electron transfers in chemical transformations.35 An
analogous situation is found for N−H and O−H bond
activation mediated by a related corrole system.36 A detailed
mechanistic study of CO2 activation by [Fe(TPP)]2− is in
progress.
Figure 2. Applied magnetic field Mossbauer spectrum of [FeTPP]2− in
̈
MeTHF (4b) recorded at 4.2 K and 4 T. The green line shows a
simulation of the target compound with S = 0 and δ = 0.45 mm/s,
ΔEQ = +1.50 mm/s, and asymmetry parameter η = 0.1 (see Table 2).
The dotted line represents an unknown ferrous impurity (accounting
for 10% of the total iron) with δ = 1.02 mm/s, ΔEQ = +3.61 mm/s,
and η = 0.3, which also could be simulated with an S = 0 ground state
(we presume that the apparent diamagnetism may arise from an
exceptionally large zero-field splitting rendering an isolated ms = 0
ground sublevel, or from exchange coupling within a dimer). The red
line is the superposition of both subspectra.
substantial deviation from the experiment. Similarly, the
calculated pre-edge energy for the d8 Fe(0) state (Epre edge
=
7112.1 eV) is distinctly different from that experimentally
observed for complex 2b. As depicted in Figure 3, the iron
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00401.
Experimental procedures, details of sample preparation,
spectroscopic measurements, theoretical calculations, and
Mossbauer and XAS spectra (PDF)
̈
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: thomas.weyhermueller@cec.mpg.de.
*E-mail: serena.debeer@cec.mpg.de.
*E-mail: eckhard.bill@cec.mpg.de.
*E-mail: frank.neese@cec.mpg.de.
*E-mail: shengfa.ye@cec.mpg.de.
ORCID
Thomas Weyhermuller: 0000-0002-0399-7999
̈
Shengfa Ye: 0000-0001-9747-1412
Notes
The authors declare no competing financial interest.
Figure 3. Schematic molecular orbital diagram of 4.
2
22
1
1
center in complex 4 features a dxy dz dxz dyz electron
configuration that is antiferromagnetically coupled to a triplet
porphyrin diradical tetra-anion. Because the singly occupied Fe
dxz and dyz orbitals and the π* orbitals of the porphyrin
radical34 transform as the same irreducible representation (eg)
in the effective D4h point group, antiferromagnetic coupling is
achieved by symmetry-allowed overlap as quantified by the
ACKNOWLEDGMENTS
■
This project has received funding from the European Union’s
Horizon 2020 research and innovation programme under Grant
Agreement No. 675020 (675020-MSCA-ITN-2015-ETN to
M.T., F.N., and S.Y.). J.A.R. is funded by a graduate study
D
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(7) Mondal, B.; Rana, A.; Sen, P.; Dey, A. Intermediates Involved in
the 2e−/2H+ Reduction of CO2 to CO by Iron(0) Porphyrin. J. Am.
Chem. Soc. 2015, 137, 11214−11217.
(8) Mashiko, T.; Reed, C. A.; Haller, K. J.; Scheidt, W. R. Nature of
iron(I) and iron(0) tetraphenylporphyrin complexes. Synthesis and
molecular structure of (dibenzo-18-crown-6)bis(tetrahydrofuran)
sodium (meso-tetraphenylporphinato)ferrate and bis[tris-
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scholarship from the German Academic Exchange Service
(DAAD). We also gratefully acknowledge the financial support
from the Max-Planck Society. The XAS experiments were
performed on beamline BM23 at the European Synchrotron
Radiation Facility (ESRF) in Grenoble, France. We are grateful
to Dr. Sakura Pascarelli at the ESRF for providing technical
assistance.
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