Atropisomeric Hydrogen Bonding Control for CO2 Binding and Enhancement of Electrocatalytic Reduction at Iron Porphyrins

Article abstract of DOI:10.1002/anie.202010859

The manipulation of the second coordination sphere for improving the electrocatalytic CO₂ reduction has led to breakthroughs with hydrogen bonding, local proton source, or electrostatic effects. We have developed two atropisomers of an iron por

Full text of DOI:10.1002/anie.202010859

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Accepted Article
Title: Atropisomeric Hydrogen Bonding Control for CO2 Binding and
Electrocatalytic Reduction Enhancement at Iron Porphyrins
Authors: Ally Aukauloo, Philipp Gotico, Loïc Roupnel, Régis Guillot,
Marie Sircoglou, Winfried Leibl, and Zakaria Halime
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Atropisomeric Hydrogen Bonding Control for CO2 Binding and
Electrocatalytic Reduction Enhancement at Iron Porphyrins†
Philipp Gotico,[a,b] Loïc Roupnel,[a] Regis Guillot,[a] Marie Sircoglou,[a] Winfried Leibl,[b] Zakaria Halime,*[a]
Ally Aukauloo*[a,b]
In memoriam: Pr. Jean-Michel Savéant
Abstract: The manipulation of the second coordination sphere for
improving the electrocatalytic CO2 reduction has led to amazing
breakthroughs with hydrogen bonding, local proton source, or
electrostatic effects. We have developed two atropisomers of an iron
porphyrin complex holding two urea functions acting as multiple
hydrogen bonding tweezers to lock the metal bound CO2 in a similar
fashion found in the carbon monoxide dehydrogenase (CODH)
enzyme. We found that the aa topological isomer with the two urea
groups on the same side of the porphyrin platform provides a stronger
binding affinity to tether the incoming CO2 substrate in comparison to
the ab disposition. However, the electrocatalytic activity of the ab
atropisomer outperforms its congener with one of the highest reported
turnover frequency at low overpotential. The strong H/D KIE observed
for the aa system indicates the existence of a tight water hydrogen
bonding network for proton delivery which is disrupted upon addition
of exogenous acid source. While the small H/D KIE for the ab isomer
and the enhanced electrocatalytic performance upon addition of
stronger acid pertain the free access of protons to the bound CO2 on
the opposite side of the urea arm.
Iron porphyrins are among the most efficient catalysts for
CO2 electrocatalytic reduction (CO2ECR) to CO.[16,19–28] The
simplified mechanism for this reaction can be described in four
main steps (Scheme I) albeit more detailed mechanism with
proton-coupled electron transfer processes have also been
proposed. Starting from the air-stable Porph-FeIII oxidation state,
the first step is the generation of the catalytically active species
Porph-Fe0 by a multiple electron transfer from the electrode to the
catalyst. The second step (II) is the substrate (CO2) binding to the
active Porph-Fe0 followed by the third step (III) where the Porph-
FeI-CO2¯ intermediate is protonated twice to yield Porph-FeII-CO
and a water molecule. In the last step (IV), two electrons are
needed to release the reaction product (CO) and regenerate the
active species. In this scenario, because they are mainly electron
transfer processes, the first (I) and last steps (IV) are considered
to be fast at the potential where the electrocatalytic reaction
proceeds and do not contribute to the overall rate of the
CO2ECR.[29,30] Therefore, CO2 binding and proton transfer in steps
II and III respectively are the two processes with a significant
contribution to the overall kinetics of the reaction.
Closing the carbon dioxide (CO2) cycle by transforming it
into fuel in order to mitigate its accumulation in the atmosphere is
one of the most important challenges of our fossil-fuel-dependent
society.[1–6] This solution was inspired by nature itself where green
plants efficiently capture sunlight to convert CO2 and water into
various hydrocarbons and O2 by photosynthesis.[7,8] After eons of
evolution, the different steps of capture, activation, and
transformation of the thermodynamically very stable CO2
molecule were optimized in sophisticated metalloenzymes.[9–12]
Similarly, an artificial transformation of CO2 requires a catalyst
that can capture and activate efficiently CO2 before reducing it in
proton-coupled multi-electron processes to skirt the formation of
thermodynamically unfavorable high-energy intermediates.[13]
Principally, the search for homogenous catalysts for CO2
conversion encompasses noble metals- (Ru, Rh, Pd, Re and Ir)
and first-row transition metals- (Mn, Fe, Co, Ni and Cu) based
complexes with mainly nitrogen-based ligands.[14–18]
Scheme 1. Proposed simplified mechanism for the reduction of CO2 to CO in
iron porphyrins. In this work, we shed light on the importance of controlling the
rate-determining step (RDS) in the overall kinetics of the catalytic reaction
through architectural second coordination sphere manipulations.
The synthetic versatility of tetraarylporphyrins has been
determinantal for implementing specific functionalities in the
second coordination sphere of the metal center as to replicate the
function of cofactors at the active sites of enzymatic systems, for
instance, amino acids that directly participate in the breaking and
making of bonds. Important breakthroughs in the CO2ECR rate
have been accomplished by adjoining proton relays or cationic
[a]
Dr. P. Gotico, Loïc Roupnel, Dr. R. Guillot, Dr. M. Sircoglou, Dr. Z.
Halime, Prof. A. Aukauloo
Université Paris-Saclay, CNRS, Institut de chimie moléculaire et des
matériaux d'Orsay, 91405, Orsay, France.
E-mail: zakaria.halime@universite-paris-saclay.fr,
ally.aukauloo@universite-paris-saclay.fr
groups that resulted in the gain of order of magnitudes in the
[31–34]
reactivity pattern of the iron porphyrin catalysts.
A boost of
the catalytic reaction rate was also observed when the porphyrin
contour was decorated with hydrogen bond donors to improve
CO2 capture by stabilizing the Porph-FeI-CO2¯ intermediate.[35,36]
The most flagrant example of such effect on CO2 binding rate
constant and alongside with a marked repercussion on the rate
[b]
†
Dr. P. Gotico, Dr. W. Leibl, Prof. A. Aukauloo
Service de Bioénergétique, Biologie Structurale et Mécanismes
(SB2SM), CEA/DRF/JOLIOT, Université Paris-Saclay, 91191, Gif-
sur-Yvette, France
In memoriam to Pr. Jean-Michel Savéant
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10.1002/anie.202010859
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S28 to S41) of the studied complexes are gathered in the
Supplementary Information (SI). Our synthetic design was guided
to keep the global chemical formulation of the ligand sets
unchanged while maintaining a similar inherent withdrawing
inductive effect from two urea groups on the porphyrin cycle. X-
ray diffraction analysis of catalyst ab-FeTPP-Ur were performed
on single crystals grown by slow evaporation of a solvent mixture
(MeOH/H2O, 4:1) (Figure 1 and see Table S4 for a more detailed
description of the structure). The X-ray structure of ab-FeTPP-Ur,
as well as that of its corresponding ligand (Figure S26), confirms
the ab disposition of the tethered Ur arms on the periphery of the
porphyrin skeleton. No crystal of sufficient quality were obtained
for the aa-FeTPP-Ur derivative but the layout of the urea groups
should resemble to the previous reported X-ray structure of the
abab-FeTPP-Ur.[37] Some interesting structural features are to be
noted from the crystallographic structure of ab-FeTPP-Ur. Unlike
most of the chloro Fe(III) porphyrinato where the strong binding
Cl¯ sits on the metal ion, herein the Cl¯ ligand is knocked out of
the metal coordination site through strong hydrogen bonding
interactions with the NH functions of the urea groups. The axial
positions are then filled by two weakly coordinating solvent
methanol molecules. The iron center is located within the
molecular plane of the porphyrin, a coordination scheme found for
low spin hexacoordinated iron (III) complexes with a set of Fe-N
bond lengths in the range of 2.04 Å. A crystallization water
molecule relays the hydrogen bonding interactions between one
bound MeOH and the NH groups of the urea.[38] The overlay of
the structures of the ab-FeTPP-Ur and the metal-free ligand
(Figure S27) depicts minimal overall structural changes for both
the urea arms and the porphyrin macrocycle pertaining that the
frame of the catalyst can adapt the binding of small molecules with
a minimal reorganization energy.
In argon-degassed DMF containing 0.1 M of tetra-N-
butylammonium hexafluorophosphate ([Bu4N]PF6), both catalysts
aa-FeTPP-Ur and ab-FeTPP-Ur show three reversible redox
waves corresponding to the formal FeIII/II, FeII/I, and FeI/0 couples
(Figure 2a, Table S1). As expected, due to the withdrawing
inductive effect of the two urea groups, the redox potentials of
these three waves are located in between those of the
corresponding waves of the nonfunctionalized iron porphyrin
(FeTPP) and the abab-FeTPP-Ur with four urea groups. The only
significant difference in the electrochemical response between
the two catalysts is a 53 mV cathodic shift of the third wave (FeI/0
couple) for the ab-FeTPP-Ur compared to the aa-FeTPP-Ur. This
could be attributed to the difference in the dipole moment of the
two isomers inducing a less optimal orientation of the former
porphyrin toward the electrode and thus less favorable electron
transfer process (Figure S42).
Figure 1. Molecular drawings of the previously reported abab-FeTPP-Ur (top
left) and newly synthesized aa-FeTPP-Ur (top right) and ab-FeTPP-Ur (bottom
right). Wireframe and capped sticks representation of ab-FeTPP-Ur X-ray
structure (bottom right). Only urea protons are explicitly shown; solvent
molecules are omitted for clarity.
dependence with the protonation process was observed with a
four-pillared porphyrin with urea groups acting as multipoint
hydrogen bonding clefts. For this model, we flanked the porphyrin
core with substituted urea functions in a face-to-face manner on
each side of the molecular plane (noted as abab-FeTPP-Ur in
Figure 1).[37] Inspired by subtle dissymmetric functional features
encompassing the catalytic enzymatic site of CODH, ie. substrate
binding and proton channel, our curiosity was driven to interrogate
how the topological arrangement of the urea groups, the hydrogen
bonding tweezers, may affect the CO2ECR. Our underlying target
is to have only one set of hydrogen bond donor to participate in
the stabilization process, leaving one side of the metal bound CO2
free upon activation. We have developed two porphyrin ligand
sets holding only two urea groups (Figure 1). For the aa
configuration, the two urea groups are disposed in a face-to-face
fashion on the same side of the porphyrinic plane while for the
ab configuration these groups are set in trans manner on both
sides of the tetrapyrrolic macrocycle (see Figure 1). We found that
this subtle modification of the second coordination sphere of the
iron porphyrin complexes induces a switch from a catalyst with
high affinity for CO2 binding for the aa model to an ab catalyst
with a lesser binding aptitude for CO2 but importantly with one of
the highest turnover frequency (TOF) reported in the literature for
CO2ECR to CO. In what follows we provide experimental and
theoretical supports to explain these compounded results in the
management of the binding of the CO2 and the proton delivery.
Catalysts aa-FeTPP-Ur and ab-FeTPP-Ur were prepared
using previously reported procedures for the abab-FeTPP-Ur
catalyst but starting from the two atropisomers (aa and ab) of the
In catalytic conditions, i.e. under a CO2 atmosphere and in
presence of 2.22 M of H2O as a proton source, the difference
between the two catalysts becomes more apparent with ab
-
FeTPP-Ur displaying a much more intense catalytic current for
the two-electron reduction of CO2 to CO than aa-FeTPP-Ur
(Figure 2b). The ab-FeTPP-Ur displays an even more intense
catalytic current than that of FeTPP which has more than 130 mV
more negative onset catalytic potential. This indicates that ab
-
FeTPP-Ur has a higher rate for the electrocatalytic reduction of
CO2. The Foot-of-the-Wave (FOW) analysis of the catalytic
currents confirmed this initial observation with five times higher
reaction rate constant for ab-FeTPP-Ur (kcat = 5.14 × 104 s-1) than
5,15-bis-(2-aminophenyl)-10,20-bis-(phenyl)-porphyrin.
The
synthetic procedures (Figure S1) and characterization (Figures
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Figure 2. (a) Cyclic voltammograms of modified iron porphyrin complexes under investigation: abab-FeTPP-Ur (magenta), aa-FeTPP-Ur (blue), ab-FeTPP-Ur
(red), and the nonfunctionalized FeTPP (black), at a concentration of 1 mM in dimethylformamide (DMF) containing 0.1 M [Bu4N]PF6 at 25 °C under argon and
(b) under CO2 with 2.22 M water as proton source (inset shows the onset of the catalytic waves). (c) Comparison of the catalytic rate constant calculated from
FOW analysis as a function of catalytic overpotentials (E0
= -0.74 V vs NHE).[39] (d) Comparison of the estimated CO2 binding rate constant and DFT-
CO₂/CO
calculated CO2 binding enthalpy for the three atropisomers of FeTPP-Ur (*Experimental estimation for ab-FeTPP-Ur may not be accurate, see SI). (e) Kinetic
Isotope Effect (KIE) when using H2O/D2O as a proton source for the catalytic CO2 reduction. (f) Relationship between the catalytic rate constant, kcat, as a function
of the pKa of the proton source (0.28 M). All color legends for (b) to (f) correspond to the same indications as in (a). Further details are reported in the SI.
that of aa-FeTPP-Ur (kcat = 1.21 × 104 s-1) and thirty times higher
than that of FeTPP (kcat = 5.80 × 102 s-1) (Figure 2c). Electrolysis
experiments show that both catalysts display a stable average
catalytic current density of 1.25 mA cm-2 and 2.60 mA cm-2 with
an exclusive production of CO at Faradaic efficiencies of 91% and
95% for aa-FeTPP-Ur and ab -FeTPP-Ur, respectively (Figures
S14 – S15).
binding enthalpy and entropic effect were found when two urea
clefts grip the metal bound CO2 as compared to a single binding
unit (Figure 2d and Table S9).
These counter intuitive results on the binding aptitudes
towards CO2 compared to the corresponding catalytic rate
constants of these two atropisomers led to further investigation to
provide a rationale for these findings. We have measured an
average 7.60 normal kinetic isotope effect (KIE) for the
electrocatalytic reduction of CO2 by aa-FeTPP-Ur with H2O/D2O
as a proton source indicating that proton transfer is involved in the
rate determining step (Figures 2e and S24 and Table S3). This
value is particularly elevated in comparison to values ca. 1.5 – 2.5
commonly found for the CO2 reduction with different proton
sources (H2O, trifluoroethanol, phenol, and acetic acid) and is
similar to the one observed for abab-FeTPP-Ur.[29,35] Such an
important KIE translates the presence of a tight hydrogen bonding
network implicated in the proton transfer process. Hint for this
argument comes from the X-ray studies where for both systems
we found that a water molecule of crystallization was lodged
within the urea binding cleft. Hence, this result backs the
important dependence of the reaction rate on the protonation
An estimation of the binding rate constant for CO2 at the
reactive Fe(0) species was obtained by analysis of the CVs under
argon and in dry DMF (see SI for details). For aa-FeTPP-Ur
where the molecular clamp drawn by the two urea groups on the
same side of the porphyrin platform is conserved, the observed
CO2 binding rate constant is as high as kCO₂ = 90.6 M-1s-1 (Figures
2d and S20) in comparison to kCO₂ of 58.0 M-1s-1 for abab-FeTPP-
Ur (Figure S22) and kCO₂ = 0.1 M-1s-1 for FeTPP (Figure S23). We
assigned this to the more pronounced nucleophilic character of
the Porph-Fe0 species (generated at a more negative potential)
for the aa-FeTPP-Ur than the abab-FeTPP-Ur. However, having
only one available urea arm on each side of the porphyrin plane
in the case of the ab-FeTPP-Ur atropisomer causes a remarkable
drop in the ability of the Porph-Fe0 species for CO2 binding with
an estimated rate constant of only kCO₂ = 3.6 M-1s-1 (Figures 2d
and S21). Of note, this value is still one order of magnitude higher
than the nonfunctionalized FeTPP, indicating the need for the
urea arms for improved CO2 binding. Our DFT calculations
outcome also support these experimental facts. Indeed, a higher
process (Step III in Scheme 1). Unlike its two analogues, ab
-
FeTPP-Ur displays a smaller KIE = 1.04 (Figures 2e and S25 and
Table S3). This result combined with the fact that ab-FeTPP-Ur
has a significantly higher overall reaction rate indicates that the
proton transfer became in this case a much faster process. Such
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an observation reflects the structure/reactivity relationship of our
designed models whereby for the ab system in contrast with the
aa, the lack of a urea arm opens the way for the rapid convoy of
protons to the “loose” oxygen atom of the metal bound CO2 under
electrochemical reduction process (Figure 3).
Experimental Section
Dimethylformamide (DMF, Aldrich 99.9%), tetrabutyl-ammonium
hexafluorophosphate ([Bu4N]PF6, Aldrich 99%) were used as
received. All other chemical reagents used in the synthetic route
were obtained from commercial sources as guaranteed-grade
reagents and used without further purification. Cyclic voltammetry
measurements were performed in an electrochemical cell
composed of a glassy carbon (3 mm diameter) working electrode,
an aqueous standard calomel electrode (SCE) as the reference
electrode, and a platinum wire counter electrode using a
dimethylformamide (DMF) solvent containing 1 mM catalyst and
0.1 M [Bu4N]PF6. Bulk electrolysis was performed in a gas-tight
two-compartment cell with a glassy carbon working electrode
(effective surface area of 1.41 cm2), reference electrode
(Ag/AgNO3), and titanium grid as counter electrode. Products
analysis was analyzed using gas chromatography (GC - TraceGC
Ultra, ThermoScientific) equipped with a molecular sieve porous
layer open tubular (PLOT) column, helium carrier gas, and a
thermal conductivity detector (TCD).
Figure 3. Notional structures showing distinct proton delivery pathways
depending on the atropoisomer: a) through a water-cluster disrupting the urea-
CO2 binding, b) via free H2O approach (C-bonded hydrogen omitted for clarity)
Acknowledgements
We further investigated the effect of the nature of the acid
source on the electrocatalytic behavior of both atropisomeric
catalysts. We tested more acidic proton sources than water (pKa
= 31.5), such as trifluoroethanol (TFE, pKa = 24.0) or phenol
(PhOH, pKa = 18.8) to provide common grounds for comparison
with the literature. In the case of aa-FeTPP-Ur, our results single
out from the common observable trend for such a variation. In
effect, we noticed a lowering of the reaction rate upon addition
stronger acid (Figure 2f, S8 and S9 and Table S2). Such a
particularity pushes forward the scheme of formation of a
topological tight hydrogen bonding network containing the water
This work has been supported by the French National Research
Agency (ANR-19-CE05-0020-02, LOCO). We thank CNRS, CEA
Saclay, ICMMO and University Paris-Saclay for the financial
support. We also thank the synthesis support facility (Pôle
Synthèse) at ICMMO for their help with the synthesis of the
starting porphyrin platform. Computational work was performed
using HPC resources from GENCI-CINES (Grant A0070810977).
Keywords: carbon dioxide reduction • iron porphyrin • hydrogen
¯
molecules locked by the urea functions and the Porph-FeI-CO2
bonding • urea • second coordination
adduct while the other urea group maintains it in position during
the protonation process. Henceforth, the addition of stronger or
more sterically hindered acid may hinder the formation or disrupt
this network. For the ab-FeTPP-Ur the protonation process
seems to proceed with a linear dependence of catalytic reaction
rate constant with the pKa of the acid (Figure 2f, S6 and S7 and
Table S2), a classic trend observed for most reported molecular-
based electrocatalytic studies.[29] Accordingly, protons are
convoyed more efficiently with stronger acids to the mono-oxygen
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Entry for the Table of Contents
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Philipp Gotico, Loïc Roupnel, Regis
Guillot, Marie Sircoglou, Winfried Leibl,
Zakaria Halime,* Ally Aukauloo*
Page No. – Page No.
Atropisomeric Hydrogen Bonding
Control for CO2 Binding and
Electrocatalytic
Reduction Enhancement at Iron
Porphyrins
Hold me but not too tight! Adapting the 3D topological layout of an iron porphyrin
catalyst bearing urea groups as multipoint hydrogen bonding donors in the second
sphere can induce a switch from higher affinity for CO2 binding to faster reaction rate
for the CO2 electrocatalytic reduction to CO.
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