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Perspective
Redox-Active Ligands in Electroassisted Catalytic H+ and CO2
Reductions: Benefits and Risks
Nicolas Queyriaux*
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ABSTRACT: In the past decade, the use of redox-active ligands
has emerged as a promising strategy to improve catalyst selectivity,
efficiency, and stability in electroassisted H+ and CO2 reductions.
Partial delocalization of the electrons within ligand-centered
orbitals has been proposed to serve as an electron reservoir, as a
catalytic trigger, or as a way to prevent deleterious low-valent metal
center formation. However, conclusive evidence of these effects is
still scarce, and open questions remain regarding the way redoxactive ligands may affect the catalytic mechanism. In this
Perspective, advances in redox-active ligands in electroassisted
catalytic H+ and CO2 reductions are discussed through recent
representative examples.
KEYWORDS: ligand design, redox-active ligand, electroassisted catalysis, hydrogen evolution, carbon dioxide reduction
1. INTRODUCTION
Owing to its ability to drastically decrease the addiction of our
modern societies to fossil resources, the electrochemical
conversion of abundant feedstocks, such as H2O or CO2, into
fuels or commodity chemicals has emerged, in the last decades,
as one of the most promising solutions offered by the academic
world to the urgent energy and environmental imperatives. As
simple as they may look on paper, these transformations are
complex. They involve multielectron, multistep processes and
generally require the assistance of catalytic systems to reduce
energy requirements and to increase selectivity. The ability of
transition-metal complexes to exhibit various oxidation states in
a relatively limited range of potential and their by-design
adjustable properties have contributed to their emergence as an
important class of electrocatalysts.
When reductive processes are considered, these complexes
thus accept electrons under an application of cathodic potential.
Nevertheless, where are the electrons hosted? Are the electrons
located on molecular orbitals that display a strong metal
character or on ligand-centered orbitals? Are they highly
delocalized, lying in extensively mixed molecular orbitals?
These questions are of great importance for those who wish to
better understand the mechanism underlying the catalytic
process. Answering these questions may also provide insights
regarding the nature of the deactivation pathways specific to the
catalyst under study. In recent years, a large and still-growing
body of work has been dedicated to electrocatalysts displaying
redox-active ligands, either for hydrogen evolution or CO2
reduction. Among the ligands that are able to store part of the
© 2021 American Chemical Society
supplementary electron density upon reduction of a coordination complex, the most commonly encountered in electroassisted reductive processes are polypyridines,1−4 iminopyridines,5−7 and other nitrogen-based heterocycles (quinoline,8,9
pyrazine10,11). Phthalocyanines,12−15 corroles,16,17 and porphyrins18−20 are also key ligands to build robust, efficient, and stable
electrocatalysts. Contradictory data are available in the literature
concerning the involvement of redox events located on ligandcentered orbitals over the electroassisted reduction of CO2 and
proton by their metal complexes (usually Fe and Co).21−26 In
the absence of a clear consensus, such compounds will thus not
be discussed further in this Perspective.
Different motivations have been invoked to explain the use of
redox-active ligands, from their role as electron reservoirs to
their potential ability to increase selectivity or stability.3,27−33 In
some rare cases, catalytic mechanisms relying only on ligandcentered processes have been observed, the role of the metallic
ion being dramatically decreased. However, the effect of the
redox moiety within the ligand framework is often difficult to
conclusively demonstrate. It is indeed challenging to draw
relevant comparisons between complexes, depending on
whether they display such motifs or not. Beyond the apparent
Received: January 17, 2021
Revised: March 5, 2021
Published: March 16, 2021
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should also be noted that catalytic systems with a redox-active
ligand commonly favor the formation of carbon monoxide, in
comparison to that of formate.1,5,43,44 The formation of formate
usually being triggered by the insertion of CO2 into a metal−
hydride bond, the elimination of hydride-type reaction
intermediates seems to be at work here as well. Rather than
only enhancing the selectivity of CO2 reduction over that of
protons, an approach that mitigates the metal center
nucleophilicity is thus a powerful tool to favor the formation
of a single C1 product.
2.1.1. Example 1: Bipyridyl-N-heterocyclic Carbene Donors
As Redox-Active Ligands. In a recent study, Jurss, Panetier, and
co-workers have investigated the electrocatalytic behavior of a
series of nickel complexes featuring bipyridyl-N-heterocyclic
carbene ligands (complexes I.1−3, Figure 1).28 These ligand
benefits, a number of studies have also highlighted the
occurrence of deactivation processes and decomposition
mechanisms specific to redox-active ligands. Dearomatization,
radical-driven reactivities (carboxylation, carbonation), and
electron trapping within ligand-centered orbitals or weakly
activated catalytic intermediates are among the most commonly
met deleterious side reactions.6,34−38
Understanding the effects of redox-active ligands on electrochemically driven catalytic processes can help in the rational
development of more efficient catalysts. To this end, this
Perspective examines molecular catalysts that include such
motifs in the context of electroassisted proton and CO2
reduction. Section 2 will be dedicated to the positive effects
that redox-active ligands may have on catalytic proton and CO2
reduction processes. Section 3 will explore their deleterious
consequences. Each section will be exemplified by recent ad hoc
developments from the literature. On the basis of these analyses,
some general guidelines will finally be proposed to inspire the
design of efficient catalysts bearing redox-active moieties for
electroassisted fuel generation.
2. BENEFICIAL EFFECTS OF REDOX-ACTIVE LIGANDS
ON THE ELECTROCATALYTIC BEHAVIOR
The assessment of an electrocatalyst’s performances relies on a
number of factors of merit. Some of the most commonly
considered are (i) the selectivity (i.e. the ability of the catalysts
to drive the formation of a unique product, usually described by
close-to-unity Faradaic yields), (ii) the overpotential requirement (i.e. the difference between the reaction thermodynamic
potential and the potential at which the catalytic event is
effectively detected, typically denoted η and measured at the
midwave potential39,40), (iii) the turnover frequency (i.e. the
rate at which the complex performed a given catalytic cycle), and
(iv) the stability (i.e. the durability of the catalysts under
working conditions, commonly described by the turnover
number or TON). Should the introduction of a redox-active
moiety in the ligand scaffold have any positive effect, a significant
improvement of at least one of these parameters should be
noticed without substantial worsening of the others. In the
following paragraphs, we have sought to identify a collection of
catalytic systems for which the use of redox-active ligands
resulted in clear benefits.
2.1. Increasing Selectivity. In the course of electroassisted
CO2 reduction, product selectivity may represent quite a
challenge. Contributing factors include the variety of chemicals
that can be generated in a fairly narrow range of potential (CO at
−0.53 V vs NHE, HCOOH at −0.61 V vs NHE, HCHO at
−0.48 V vs NHE, CH3OH at −0.38 V vs NHE, CH4 at −0.24 V
vs NHE41) and the competitive reduction of protons to H2 (at
−0.42 V vs NHE42). Mixtures of two-electron-reduction
products (CO/HCOOH/H2) are frequently encountered with
electroassisted molecular systems, calling for a rationalization of
the parameters enabling selective systems to be accessed. In the
absence of a redox-active motif, most of the electronic density
gained from the reduction processes is located at the metal
center. The associated Lewis basicity buildup has typically been
reported to favor metal hydride formation.28,29 Such reactivity is
a major step on the path to proton reduction. Electron capture in
ligand-based orbitals thus appears promising because it is
expected to substantially lower the nucleophilicity of the metal
center, thereby disfavoring the hydride pathway. This strategy
results in an increase in the selectivity of CO2 reduction in
comparison to that of protons. For an identical metal center, it
Figure 1. Structures of the nickel complexes I.1−3.
scaffolds, when they are macrocyclic, display variable lengths of
the alkyl bridging group. An increased rigidity of the redox-active
macrocycle leads to significant structural constraints: whereas
complex I.1 adopts a distorted-tetrahedral geometry, squareplanar environments are observed in the case of two other
complexes. Such a structural shift is expected to significantly
affect the electronic structure of the different complexes.
Interestingly, the authors were able to draw a clear trend
between the degree of rigidity of the ligands, the electronic
structures of the one- and two-electron-reduced complexes and
their ability to efficiently drive the electrocatalytic reduction of
CO2. After two reductions, the non-macrocyclic complex I.1 is
characterized by a nickel(I) center associated with a
monoreduced ligand. In contrast, the doubly reduced complex
I.3 exhibits a virtually unchanged nickel(II) center supported by
a biradical ligand. DFT calculations suggest an intermediate
electronic structure between these two limiting forms, for
complex I.2. These results should be compared with improved
Faradaic yields for the CO2-to-CO conversion observed
throughout the series: 5% (I.1), 56% (I.2), and 87% (I.3).
The authors propose that the increasing difficulty in generating
nickel hydrides from the reduced states along this series is the
main reason for the observed selectivity improvement. The
study of this ligand set was then extended to analogous
cobalt(II) complexes, with similar results.29 Within this new
series of compounds, the importance of a balanced distribution
of the electron density between the metal center and the redoxactive ligand was highlighted: the metal must acquire sufficient
nucleophilicity to allow further formation of a CO2 adduct, but
an excessive increase in the Lewis basicity promotes hydrogen
evolution.
2.1.2. Example 2: Functionalized Terpyridines As RedoxActive Ligands. Very recently, Chang and co-workers have
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Figure 2. Structures of the iron complexes I.4 and I.5 and their simplified molecular orbital diagrams.
2.2. Decreasing Overpotential Requirement. Reducing
protons to hydrogen, such as reducing CO2 to CO or formate,
requires two electrons that temporarily transit on the catalyst.
Various sequences of events may be considered, depending on
the catalytic mechanism (EECC, ECEC, and ECCE as examples,
where E corresponds to an electron transfer step and C to a
chemical reaction).46 In a number of situations,18−20,47−49
catalysts have to be able to successively accommodate two
supplementary electrons. To be energy-efficient, those electrons
should be transferred at balanced electrode potentials: cathodic
enough to overcome the kinetic barrier but as close as possible to
the thermodynamic equilibrium. However, it can sometimes
prove challenging to generate the required low-valent metal
species (cobalt(0) or iron(0), as examples) at potentials that are
not excessively negative. In this context, the introduction of new
redox events, centered on the ligand scaffold, may appear as an
appealing alternative to trigger catalytic processes at valuable
potentials.
On initiation by ligand-centered processes, it is advantageous
to tailor the ligand’s redox properties through proper ligand
design (introduction of electron-withdrawing groups and use of
extended π-conjugated ligands displaying low-lying π* orbitals
as examples) to lower the energy requirements. In such
situations, most authors generally find themselves dealing with
scaling relationships: decreasing the catalyst overpotential leads
to a slowdown of the catalytic process.50,51 The reverse also
holds true, resulting in middle-ground compromises.
2.2.1. Example 3: Combining Strong σ-Donor Ligands and
Redox-Active Ligands to Break Scaling Relationships. To
avoid compromising between high catalytic rates and low
overpotential, Miller and co-workers recently proposed a
strategy to decouple electronic tuning of the metal center
nucleophilicity and catalyst redox potentials.30,52 In this context,
the authors have examined the electroassisted reduction of CO2
by two geometric isomers of a ruthenium complex with the
general formula [Ru(tpy)(Mebim-py)(NCCH3)]2+ (tpy =
2,2′;6′,2″-terpyridine, Mebim-py = 1-methylbenzimidazol-2ylidene-3-(2′-pyridine)). These isomers display either the
carbene donor53,54 or the pyridine donor trans to the acetonitrile
ligand (complexes I.6 and I.7, Figure 3).
The initial measurements revealed similar electrocatalytic
properties for both complexes in terms of selectivity, overpotential, and catalytic activity. Using high-scan-rate voltammetry, the authors evidenced a two-electron ligand-based reduction
that is able to induce isomerization of complex I.7 to I.6 by rapid
dissociation of the acetonitrile ligand. An in-depth mechanistic
understanding was achieved by the preparation of catalytically
relevant intermediates that were thoroughly examined by the
means of electrochemical tools. When they were taken together,
investigated electrochemical CO2 reduction assisted by an
iron(II) complex, displaying a pentapyridine ligand based on a
redox-active terpyridine motif (complex I.4, Figure 2).27 In the
presence of an excess of phenol (3.5 M) under an atmosphere of
CO2, large current enhancements were observed on the
voltammetric scale in acetonitrile. Bulk electrolysis experiments
were performed that ensure the catalytic nature of the process:
Faradaic yields as high as 94% for CO2-to-CO conversion were
determined at a 190 mV overpotential. Under identical
conditions, the iron complex I-5 (Figure 2)structurally
related but lacking a redox-active motifproved unable to
drive CO2 reduction and mostly displayed proton reduction
activity. In order to gain further insight into the remarkable
properties of this catalytic system, the authors have undertaken
the preparation and characterization of the two-electronreduced species that is believed to be crucial to the catalytic
cycle. Generated by chemical reduction, this complex has been
exhaustively characterized, revealing an iron(II) center (featuring the intermediate-spin SFe = 1) supported by a doubly
reduced polypyridine ligand (biradical in nature and displaying a
triplet character, SL = 1). As they are antiferromagnetically
coupled, these two components result in an open-shell singlet
ground state. Such an electronic structure, which promotes
strong metal−ligand cooperativity, is proposed as a convenient
way to limit hydrogen evolution pathways (i.e. by disfavoring
metal hydride formation), resulting in high selectivity toward
CO2 reduction.
In a related study, the competition between electroassisted
reduction of protons and CO2-to-CO conversion in the
presence of cobalt(II) terpyridine complexes has been studied
by Fontecave and co-workers.45 In the presence of a proton
source under an inert atmosphere, the authors were able to
determine the rate constants associated with proton reduction
for a series of complexes displaying terpyridine functionalized
with either electron-withdrawing or electron-donating groups. A
clear trend emerged: the more electron withdrawing groups led
to decreased rate constants toward H2 evolution. Interestingly,
when the competitive CO2-to-CO pathway was enabled by
saturating the electrolytic solutions with carbon dioxide, a
related trend was noticed: the more electron-withdrawing
groups led to an increased Faradaic yield for CO production. In
light of previous discussions, it is tempting to assign such
behaviors to a decreased contribution of the metal-centered
orbitals in the reduced form of the complexes. This would thus
result in a lower nucleophilicity of the cobalt center and disfavor
the formation of hydride derivatives. By fine-tuning the degree of
redox activity of the terpyridine ligands, it thus appears possible
to slow down catalytic pathways resulting in hydrogen evolution
to enhance selectivity toward CO2-to-CO reduction.
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of studies have been dedicated to metal dithiolene32,58−61 and
metal thiosemicarbazone33,62−64 complexes. The former
initiated early works on redox-active ligands, due to the inability
of conventional oxidation-state descriptors to provide a
satisfying depiction of their electronic structures.65
A recent review by Mitsopoulou and co-workers comprehensively surveys the different electro- and photocatalytic
behaviors of representative members of this wide family of
compounds.58 Although distinct mechanisms have been
proposed, depending on the nature of the dithiolene
derivatization, the most commonly encountered electrocatalytic
pathways rely on an ECCE sequence (where E corresponds to
an electron transfer step and C to a chemical reaction, here
protonation). The reduction of the redox-active ligand allows
two successive protonations of the ligand framework. A last
reduction yields a doubly protonated two-electron-reduced
species from which H2 typically evolves (complexes I.8M, Figure
4).32,59
2.4. Introducing New Catalytic Pathways. Redox-active
ligands disrupt the usual electron distribution within catalysts. In
reduced states, the metal center is no longer a potent nucleophile
capable of independently initiating reactions with substrates.
The increased electron density located on the ligand scaffold
may trigger mechanistic events whose occurrence is otherwise
not observed. Hence, Waymouth, Sarangi, and co-workers were
able to prevent the electroinduced dimerization of a manganese(I) tricarbonyl complex by the introduction of an azopyridine
derivative as a ligand.66 Direct formation of the two-electronreduced mononuclear complex was evidenced, bypassing the
redox-reluctant manganese dimer.67,68 A similar disfavoring of
dimer formation was observed by Kubiak and co-workers when
bulky tBu groups were used to induce metal-directed steric
hindrance.68 To another extent, Mulfort and co-workers have
considered intramolecular electron and proton transfers
between a cobalt center and its redox-active ligand, as part of
Figure 3. Structures of the ruthenium complexes I.6 and I.7.
the data pointed out a “reduction first” pathway, excluding the
intermediate formation of metal hydrides. According to the
authors, the high activity of the catalystobserved at mild
overpotentialresults from two main effects: (i) the use of
carbene as a strong σ-donor ligand allows a sufficiently high
nucleophilicity of the metal center to be reached, even when
most of the acquired electron density is located on the ligand
orbitals, and (ii) the presence of a redox-active motif triggers the
catalytic process at a lower overpotential. Further studies have
quantitatively investigated the effects of the redox-silent carbene
moiety on electroassisted CO2 reduction, highlighting its
important role in catalyst isomerization and ligand substitution
processes.55
2.3. Enhancing Local Concentration of Protons. The
introduction of basic sites capable of virtually increasing the local
proton concentration near the catalytic site is a robust strategy
for improving catalyst performances toward proton-assisted
multielectron processes, such as proton reduction56,57 and CO2
reduction.18 As such, redox-active ligands are interesting motifs.
Indeed, the additional electronic density acquired during their
reduction significantly boosts their basicity. Initially poorly basic
positions then become proton-responsive and may therefore
function as proton relays. Among the catalytic systems that
typically promote ligand protonation upon reduction, a number
Figure 4. Proposed ECCE mechanism for H2 evolution catalyzed by complexes I.8M (M = Co, Ni).
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Figure 5. Structures of the cobalt complexes I.9−11.
assessed, on the basis of bulk electrolysis measurements. Highly
reactive electrogenerated intermediates, such as Co(II) hydrides
and ligand-reduced species, negatively affects the stability of the
catalytic pathways they are involved in. This example is thus on
the borderline between the benefits and detriments of using
redox-active ligands. While the bipyridine motif enables the
activation of otherwise absent productive catalytic pathways, it
significantly limits the stability of the reaction intermediates
involved.
2.5. Improving Stability. When it comes to redox-active
ligands, stability gain is a general statement that can be found in a
number of reports. If prevention of the formation of low-valent
metal centers by distribution of the extra electron density all over
the ligand is commonly reported, substantial evidence that it
indeed positively affects the stability of the catalytic process is
still lacking.
In contrast, catalytic pathways involving the reduction of the
ligand scaffold sometimes revealed themselves as a source of
greater catalyst instability.3 Efforts should be made to extract
metrics that provide insight regarding the actual effects of the
ligand electronic involvement on the stability of catalytic
systems. In this regard, Savéant and co-workers have formulated
a useful descriptor that supplies valuable information about the
lifetime of an operating catalyst in the diffusion-reaction layer.76
the catalytic mechanism followed by H2-evolving catalysts I.9
and I.10 (Figure 5).31
Ultimately, the use of redox-active ligands may enable
catalytic pathways that completely shift the usual paradigm:
the main site for the catalytic event is no longer the metal center
but the ligand platform. Though examples of ligand-based
electroassisted catalysis are still relatively limited,69−74 they have
recently been exhaustively reviewed by Zhang and co-workers.75
2.4.1. Example 4: Redox-Active Ligand to Enable New
Catalytic Pathways for H2 Evolution. In a recent study, Artero,
Queyriaux, and co-workers have described the activity toward
electroassisted H2 evolution of a cobalt(II) complex combining
pendant bases and a redox-active moiety within a single
tetrapyridyl ligand (complex I.11, Figure 5).3 In DMF, this
complex features two successive monoelectronic reductions: a
reversible metal-centered processassociated with the Co(II)/
Co(I) couplefollowed by an irreversible ligand-centered (L/
L•−) process. Using proton sources of variable strength, the
authors evidenced the development of a range of catalytic
responses (Figure 6). Thus, strong acids enable protonation of
3. DELETERIOUS EFFECTS OF REDOX-ACTIVE
LIGANDS ON THE ELECTROCATALYTIC BEHAVIOR
Different unexpected side effects related to the involvement of
redox-active moieties throughout the electroassisted catalytic
cycle have been identified in the literature. In the following
paragraphs, we have grouped those different processes depending on their mode of operation.
3.1. Ligand-Centered Chemical Reactivity. Uptake of an
electron within molecular orbitals displaying a strong ligand
character may open up undesired reactivities of the reduced
catalyst. The electrogenerated species can generally be described
as radical anions and are indeed potent nucleophiles. Rather
than simple sequestration of this additional electron density, a
number of ligands have been shown to react with electrophilic
substrates such as protons and carbon dioxide. As a main effect,
the electron within the ligand platform will be trapped through
the formation ofusually irreversiblechemical bonds: C−C
(carboxylation), C−O (carbonation), or C−H (dearomatization) bonds. Now engaged in a covalent bond, this electron can
no longer participate in the catalytic process. Further reductions
are thus needed for catalysis to occur. Consequently, the
overpotential requirement associated with the catalytic process
may be increased. In many cases, these types of reactions remain
“invisible” on a macroscopic scale, as long as the catalytic system
displays satisfactory performance.
Figure 6. Proposed mechanistic pathways followed by complex I.11
during electroassisted H2 evolution in the presence of acids of various
strengths in DMF. The bipyridine redox-active motif is notably
involved in the presence of weak acids (Et3NH+ − green path−and
CH3COOH − blue path). Reprinted with permission from ref 3.
Copyright 2020 American Chemical Society.
the electrogenerated cobalt(I) center, resulting in the formation
of catalytically competent Co(III) hydrides. Kinetically
reluctant, this compound may be further reduced to promote
faster H2 evolution. Weaker acids are unable to trigger metal
protonation upon one-electron reduction. The redox activity of
the ligand scaffold proves crucial in initiating a catalytic
sequence of events (EECC when acetic acid is used or ECEC
in the case of HNEt3+). Interestingly, the relative stabilities of
these different catalytic pathways have been quantitatively
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Figure 7. Structures of some complexes displaying redox-active ligands that have been shown to undergo processes detrimental to an electroassisted
catalytic reaction.
Figure 8. Example of catalyst decomposition proposed by McCrory and co-workers. Adapted with permission from ref 6. Copyright 2018 Royal
Society of Chemistry.
Similarly, carboxylation of imino carbon centers was proposed
as a deactivation pathway in a number of reports using redoxactive ligands (complex II.5, Figure 8).6,79 Electrochemical
carboxylation reactions have been exploited as early as 1981 to
achieve electrosynthesis of N-carboxylated heteroaromatic
compounds.80 Since then, many examples have been reported,
some of them involving structures usually employed as ligands in
electroassisted catalysis.81
3.1.2. Example 6: Electrochemical Deactivation Processes
in Proton-Responsive Functionalized Redox Ligands. The
incorporation of acidic or basic moieties near the metal center of
CO2 reduction or H2 evolution catalysts is a well-established
strategy. It has opened the way to the development of highperformance catalysts by increasing the local proton concentration,18 providing proton relays3,57,82 or stabilizing catalytic
adducts.83−85 In most cases, these functional groups assisting the
catalytic process were attached on electrochemically innocent
ligands. However, Fujita and co-workers have shown that the
combination of a redox-active moiety and proton-responsive
groups within the same ligand resulted in new deactivation
pathways through the formation of catalytically unproductive
species.35 In an attempt to foster catalytic processes leading to
CO2-to-CO conversion in the well-described [Ru(tpy)(bpy)(S)]2+ system (tpy = 2,2′:6′,2″-terpyridine, bpy = 2,2′bipyridine, and S = a neutral solvent molecule), the authors
introduced hydroxyl groups in the 6,6′-positions of the
bipyridine ligand (complex II.2, Figure 7).
This design was intended to stabilize a metal−CO2 adduct via
hydrogen bonding or to facilitate the protonation of this
metallocarboxylate intermediatea key step toward the release
of CO. Controlled-potential electrolysis (CPE) experiments
were performed on a mercury-pool electrode to assess the ability
of this complex to assist the reduction of CO2 in acetonitrile.
Faradaic yields as low as 4.1% for CO and 5.9% for formate were
observed, suggesting the existence of processes undermining the
catalysis. A combination of experimental techniques and DFT
calculations was used to shed light on the deactivation pathway
However, these interfering reactivities sometimes have a
deeper effect. In some situations, they have been found to
impedeor even preventfurther catalytic processes by
modification of the structural integrity of the ligand. Even if
they do not result in an immediate demetalation, ligand
modifications can still have damaging consequences on the
catalysis. Changes in the accessibility of the metal center, the
overall charge of the complex, or the electronic balance within
the catalystthis list is not exhaustiveare thus prone to occur
and to significantly hamper the desired reactivity, resulting in
low Faradaic yield. These extreme situations sometimes allow
pointing out the adverse effects of certain redox-active ligands.
3.1.1. Example 5: Electrochemically Induced Carboxylation of Polypyridyl-Based Transition-Metal Catalysts. Among
the wide diversity of molecular complexes developed to achieve
H2 evolution and CO2 reduction, polypyridyl-based transitionmetal complexes have played a major role.77,78 Of particular
interest to the scope of this Perspective is the study of the
electrocatalytic behavior of [M(tpy)2]2+ (M = Co, Ni, Zn)
complexes toward CO2 reduction in organic medium. In 2014,
Elgrishi et al. demonstrated that nickel(II) and cobalt(II) bisterpyridine complexes are capable of electroassisted CO2-to-CO
conversion, the catalytic cycle being initiated by ligand-based
reductions and loss of a terpyridine ligand.34 Although selective,
the cobalt(II) catalyst (complex II.1, Figure 7) suffers from
modest Faradaic yields (about 20%, under optimized conditions), which limits its scope. In order to better understand the
origin of this electron wastage, the authors sought to highlight
the existence of deleterious side processes or catalytic dead ends.
To this end, the behavior of the Zn(II) complex, a diamagnetic
analogue of the catalysts, was investigated under electrocatalytic
and photocatalytic conditions. Chemical trapping by iodomethane and 1H NMR experiments were performed and
strongly suggested deactivation reactions involving the terpyridine ligand, namely loss of aromaticity of the pyridine rings and
carboxylation.
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and the species involved. Collectively, these data suggest a first
step of reductive deprotonation to produce a doubly
deprotonated complex. Carbon dioxide was shown to efficiently
react with the now-deprotonated ligand to form carbonates
(Figure 9). Such an electroassisted reaction of carbon dioxide
Perspective
proton uptake over the catalytic cyclesusing electroresponsive
proton relaystheir role can also be potentially detrimental. In
a recent study, Hess and co-workers investigated the electrocatalytic behavior of [Co(Mabiq)(THF)]+, a cobalt(II)
complex displaying a macrocyclic biquinazoline ligand (complex
II.3, Figure 7).36 This complex exhibits the particular feature of
successively undergoing two reversible monoelectronic reductions centered on its largely conjugated ligand, leaving the metal
center virtually unchanged. Upon addition of p-cyanoanilinium,
this complex is capable of electroassisting the reduction of
protons in acetonitrile. However, unlike many other molecular
electrocatalysts such as cobaloximes,88−90 the authors noticed
that the development of the electrocatalytic wave proceeds at a
potential lower than that associated with the formation of the
formal CoI species. This behavior prompted them to use an
electrochemical technique that is only rarely employed in
electrocatalysis to explore the early steps of the catalytic
pathway. By combining the results of rotating ring-disk electrode
(RRDE) experiments and online electrochemical mass spectrometry (OLEMS) measurements, the authors were able to
evidence a nonproductive precatalytic event. In the presence of
acid, the complex thus undergoes a bielectronic reduction that is
coupled with at least one protonation. This results in the initial
storage of a hydride equivalent (2 e− and 1 H+) within the ligand
platform. Although the authors could not establish the exact
nature of the resulting species, they clearly showed its inability to
drive catalysis. A new, more energetic electron is finally required
to complete the catalytic cycle, resulting in an increased
overpotential. Similar results were obtained by Savéant and
co-workers when they investigated the ability of a Cu(II)
tetraphenylporphyrin (TPP) complex to drive H2 evolution in
DMF.47 When triethylammonium was used as a proton source,
the once monoelectronic wave becomes more complex,
displaying a 3 e− 3 H+ coupled process that has been assigned
to hydrogenation of the ring. In this case, however, no catalysis
could be achieved.
3.2. Metal-Centered Chemical Trapping. Catalytic cycles
are expected to be derived from a reactive intermediate formed
upon an inner-sphere reaction between the electrogenerated
active form of the catalyst and its substrate (here, CO2 or H+).
The introduction of redox-active ligands is likely to affect this
classical scheme by disrupting the electronic balance within the
catalytic system. The reduction of the ligand framework
produces a priori two main effects. First, the nucleophilicity of
the metal center is only slightly increased in comparison to what
it is in the absence of a redox-active ligand, i.e. metal-centered
reduction. Although some recent works have shed light on
effective ligand-centered electrocatalytic processes (vide supra),
it is important to remember that the vast majority of the catalytic
pathways resolved so far strongly rely on the metal center, the
electrochemically controlled nucleophilicity of which being the
main trigger enabling the formation of a primary catalyst−
substrate adduct, as a gateway to the catalytic cycle. A decreased
Lewis basicity of the metal center thus potentially hampers the
reactivity by limiting its interaction with electrophilic substrates.
Even if this reactivity is not completely inhibited, the resulting
adducts may display weakly activated substrates playing a kinetic
trap role. Second, the electron density acquired by the ligand
framework is sometimes the origin of interfering side reactions
capable of obstructing access to one of the catalyst coordination
sites. Such phenomena are likely deleterious to catalytic
turnover.
Figure 9. Catalytic and deactivation pathways proposed to occur over
the course of the electroassisted reduction of CO2 by complex II.2.
with alcohols has been previously reported as an electrosynthetic
tool toward the preparation of organic carbonates.86 Interestingly, the modification of the ligand framework does not hamper
the ability of the metal center to assist the CO2-to-CO
conversion but seems to inhibit further CO release. The
resulting bis-carbonated carbonyl species have indeed been
identified from the precipitate that formed upon CPE
experiments.
With a parallel interest, Chardon-Noblat and co-workers have
studied the electro- and photocatalytic performances of a
manganese(I) tricarbonyl complex displaying [1,10]-phenanthroline-5,6-dione as a ligand.87 Upon two successive ligandbased reductions under a CO2 atmosphere, a bis-carbonated
adduct was identified. Here, the catalytic process remains
effective and bulk electrolysis experiments performed in
acetonitrile containing 5% water resulted in efficient CO2-toCO conversion (100% Faradaic yield).
3.1.3. Example 7: Hydride Sink Effect in a Cobalt(II)
Complex Bearing a Macrocyclic Biquinazoline Ligand. The
harmful changes that occur at the redox-active ligands are not
restricted to unwanted reactions with CO2 in the context of its
electroassisted reduction. Protonation processes have also been
shown to occur when hydrogen production was pursued.
Although these reactions are sometimes desired to facilitate the
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3.2.1. Example 8: Limited Reactivity of the Catalyst Due to
a Decreased Nucleophilicity of the Metal Center. Limiting the
increase in nucleophilicity of the metal centerthrough
sequestration of a significant part of the electron density in
ligand-centered orbitalshas recently been shown to affect
catalytic properties. Queyriaux, Hammarström, and co-workers
thus reported the electrocatalytic investigation of a polypyridylbased cobalt(II) complex toward CO2 reduction.38 Displaying a
redox-active moiety and basic sites in close proximity to the
metal center, this complex was previously shown to be an
efficient catalyst for the reduction of protons in aqueous and
organic media (complex I.11, Figure 5).91 On the basis of a
comparison with a Zn(II) analogue and DFT calculations, the
location of the two first reductions was unambiguously
established: the first being metal-centered and the second
mainly involving the ligand scaffold. Although voltammograms
displayed significant current enhancement in CO2-saturated
DMF solutions, CPE experiments revealed a rather inefficient
catalytic process suffering from both low selectivity and low
Faradaic yields (FYCO = 7.4% and FYHCOOH = 7.9%). In order to
gain some insight into the nature of the processes undermining
CO2 reduction catalysis, chemical reductions were coupled to
UV−vis and IR spectroscopy and DFT calculations. These
different techniques revealed the formation of an adduct
between the cobalt catalyst and a CO2 molecule, via the twoelectron-reduced complex. In addition, the formation of a
significant amount of CO trapped complexes was excluded. The
electronic nature of the CO2 adduct was investigated by DFT
calculations, which helped to establish that coordination of CO2
to the metal center does not result in a significant charge
redistribution within the newly bound CO2 ligand-centered
orbitals. In many systems, such an electronic reorganization has
been proposed to be a crucial step of the CO2 reduction, usually
resulting in a metallocarboxylate complex. The resulting metalboundbut poorly activatedCO2 molecule was expected to
hardly react with oxide acceptors (such as carbon dioxide or
protons) as required to close the catalytic cycle. The authors
suggested that this weak activation was caused by an effective
electron trapping within the ligand.
In a related study, Musgrave, Luca, and co-workers have
explored the CO2 reduction electroassisted by bis-NHC pincer
complexes of the type M(CO)3CNCBn (M = Re, Mn).37 While
the Mn derivative displays promising ability to reduce CO2 on
the voltammetric scale, the Re complex proves to be mostly
inactive. Electronic structures of the one-electron-reduced
catalysts were computed and provided insightful information
on the role played by the ligand scaffold. A spin population
density analysis revealed significantly different partitioning of
the electron density between ligand-centered and metal-based
orbitals, depending on the metal center. Whereas 66% of the
spin density resides on the metal in the manganese complex,
only 38% of it is located on the rhenium. This dramatic
modification of the metalloradical character of the catalytic
intermediates was proposed to be the key feature dictating the
ability of the pincer complexes to driveor notthe catalytic
process.
3.2.2. Example 9: Prevention of the Metal Center
Accessibility by Metallacycle Formation. Reactions of the
reduced ligand with reactants or reduction products are
potentially generating new species that display trapped
coordination sites. In 2015, Kubiak, Peters, and co-workers
investigated the electrocatalytic behavior of mononuclear
molybdenum complexes of the type [Mo(CO)4(PMI)] (PMI
Perspective
= pyridine monoimine), which exhibit two successive redox
processes centered on the pyridine monoketimine ligand
(complex II.4, Figure 7).92 Although they displayed encouraging current enhancement when the CVs were recorded under
CO2 saturation, the complexes were revealed to be rather
ineffective catalysts under continuous operation. Faradaic yields
toward CO formation were limited to 10% as a maximum. In
order to better understand the processes underpinning the
deactivation, chemical reductions, IR spectroelectrochemical
methods, and NMR investigations were used. These techniques
pointed out the formation of an unusual CO2 adduct in which
the CO2 carbon atom binds to the imine carbon atom of the
reduced complex (Figure 10). In addition to provide strong
Figure 10. (a) Metal−ligand cooperative capture of CO2 by complex
II.4 over the course of the electroassisted reduction of CO2 and (b)
metallacycle ring closing upon reduction of complex II.5.
evidence of the C−C bond formation between the reduced
ligand and a CO2 molecule, the crystal structure of this
intermediate revealed that the metal center is bound to an
oxygen atom of the newly formed carboxylate. By restriction of
access to one of the coordination sites of the molybdenum
center, the formation of this adduct is likely to be detrimental to
the electrocatalytic activity. In a related study, Vogt and coworkers have evidenced a similar reversible process on a
rhenium(I) tricarbonyl complex.93
CO-trapped intermediates have recently been shown to
efficiently act as catalytic dead ends by preventing coordination
site liberation.94−97 The potentially negative effect of these types
of intermediates could, however, be amplified by the presence of
functionalized redox-active ligands. As such, works by Tanaka
and co-workers may provide interesting insights.98 Indeed, in
2005, the authors reported the unusual electrochemical behavior
of a ruthenium carbonyl complex bearing a redox-active ligand
(complex II.5, Figure 10). Once it is reduced, this bipyridine
ligandfeaturing a quinone-like motifis capable of attacking
the carbon atom of the carbonyl ligand thanks to the increased
nucleophilicity of its pyridonato group. The resulting stable fivemembered metallacycle prevents reductive cleavage of the CO
ligand and thus inhibits any future reactivity at the metal center.
Although the behavior of this complex with respect to
electroassisted CO2 reduction has not been investigated so far,
this type of reactivity appears to be relatively common.99−101 As
far as redox-active ligands are considered, such metallacycle
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formation could play a major role in a number of catalytic
systems.
■
4. CONCLUSIONS AND OUTLOOK
Redox-active ligands are growing in importance among the
various strategies elaborated to design selective, efficient, and
stable catalysts for electroassisted catalytic reductions of CO2
and H+. For most systems, however, a quantitative analysis of
their exact roles on the catalyst behavior is still lacking. As a
consequence, no obvious relationship between catalyst metrics
improvement and the redox-active nature of a ligand can be
drawn. This observationpreviously made by Savéant102is
probably due to the variety of ways those motifs act.
Nevertheless, some specific observations can be made to
facilitate further development of complexes using redox-active
moiety for electroassisted catalysis:
(1) Depending on whether you want to reduce CO2 or
protons, the expected effects such motifs may have are
quite different. The nucleophilicity of the metal center
all along the catalytic cycleshould be considered as a
key element. Where typically high Lewis basicity of the
metal center is sought in the case of H2-evolving catalysts
to promote hydrides generation, this feature has to be
significantly more balanced when it comes to CO2
reduction to ensure selectivity. In this specific case,
redox-active ligands thus appear to be useful tools to tune
the electronics of the metal center by allocating part of the
electroacquired electron density to the ligand scaffold. In
the case of proton reduction, the main concern is typically
to lower the overpotential requirement of the catalytic
system, which often comes at the expense of the catalytic
rate.
(2) Although this is not a rule that can be generalized to all
systems, it seems that redox-active ligands capable of
delocalizing the excess electron density over largely
conjugated systems are less prone to side reactions.
Localization of the radical within a restricted part of the
ligand scaffold commonly leads to irreversible and
unproductive bond formation with the electrophilic
substrates.
(3) Whenever possible, a comparison with complexes bearing
redox-innocent ligands should be systematically carried
out. However, caution should be taken to ensure the
relevance of such benchmarking. If certain molecular
parameters (such as the rigidity of the ligand or the
geometry adopted by the complex, as examples) differ too
significantly between the two species, it may be tricky to
provide meaningful interpretations on the effects of
introducing redox-active ligands on the catalytic process.
Quantitative assessments of the effects of introducing
redox-active ligands on specific electrochemical events
(ligand exchange rate constants, stability descriptors, ...)
have recently been highlighted. Efforts on this front must
be pursued in order to comprehensively shed light on the
key parameters at stake when redox-active ligands are
involved.
(4) The identification of postelectrolysis solution contents is
crucial to evidence catalyst evolution under the working
conditions. When this is coupled to in situ probing
techniques and DFT calculations, it may help to more
clearly decipher catalytic dead ends and real active forms
of the catalysts. This knowledge is essential for a better
Perspective
understanding of the redox-active ligand role and may
inspire new efficient, selective, and stable systems.
AUTHOR INFORMATION
Corresponding Author
Nicolas Queyriaux − CNRS, LCC (Laboratoire de Chimie de
Coordination), 31077 Toulouse, France; orcid.org/00000002-8525-280X; Email: nicolas.queyriaux@lcc-toulouse.fr
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c00237
Notes
The author declares no competing financial interest.
■
ACKNOWLEDGMENTS
N.Q. expresses his gratitude to Dr. V. Artero, Dr. M. ChavarotKerlidou, Dr. N. Kaeffer, and Dr. C. Esmieu for fruitful
discussions regarding this Perspective.
■
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