An NADH-Inspired Redox Mediator Strategy to Promote Second-Sphere Electron and Proton Transfer for Cooperative Electrochemical CO2 Reduction Catalyzed by Iron Porphyrin

Article abstract of DOI:10.1021/acs.inorgchem.0c01162

We present a bioinspired strategy for enhancing electrochemical carbon dioxide reduction catalysis by cooperative use of base-metal molecular catalysts with intermolecular second-sphere redox mediators that facilitate both electron and proton transfer. Fu

Full text of DOI:10.1021/acs.inorgchem.0c01162

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Article
An NADH-Inspired Redox Mediator Strategy to Promote Second-
Sphere Electron and Proton Transfer for Cooperative
Electrochemical CO2 Reduction Catalyzed by Iron Porphyrin
Peter T. Smith, Sophia Weng, and Christopher J. Chang*
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ABSTRACT: We present a bioinspired strategy for enhancing
electrochemical carbon dioxide reduction catalysis by cooperative
use of base-metal molecular catalysts with intermolecular second-
sphere redox mediators that facilitate both electron and proton
transfer. Functional synthetic mimics of the biological redox
cofactor NADH, which are electrochemically stable and are
capable of mediating both electron and proton transfer, can
enhance the activity of an iron porphyrin catalyst for electro-
chemical reduction of CO2 to CO, achieving a 13-fold rate
improvement without altering the intrinsic high selectivity of this
catalyst platform for CO2 versus proton reduction. Evaluation of a
systematic series of NADH analogues and redox-inactive control additives with varying proton and electron reservoir properties
reveals that both electron and proton transfer contribute to the observed catalytic enhancements. This work establishes that second-
sphere dual control of electron and proton inventories is a viable design strategy for developing more effective electrocatalysts for
CO2 reduction, providing a starting point for broader applications of this approach to other multielectron, multiproton
transformations.
outer-sphere electron reservoir to enhance activity.47 We have
INTRODUCTION
■
also shown that reticular materials can influence CO2
Catalyzing the CO2 reduction reaction by electrochemical
selectivity and activity by electronic delocalization in the
means offers an approach for converting this greenhouse gas
secondary coordination sphere.48,49 Redox-active ligands based
into value-added chemical products with sustainable energy
on polypyridines,50−55 imines,56 and porphyrins57,58 can also
input.1−15 Efforts in molecular electrocatalysts for CO2
be employed for CO2 reduction, where ligand-centered
reduction have predominantly focused on the two-electron/
reductions offer the advantage of removing electron density
two-proton reduction of CO2 into CO, which can then be fed
away from the metal center to diminish metal hydride
into numerous existing industrial processes for chemical
formation and off-pathway H2 evolution.59,60 Our laboratory
synthesis. Iron porphyrins, in particular, have been the subject
and Miller’s laboratory have identified molecular terpyridine-
of extensive research owing to their high selectivity for CO2
based iron complexes that are highly active CO2-to-CO
versus proton reduction, structural tunability, and compati-
electrocatalysts that operate at extremely low overpotentials
bility with various electrolyte media.16−24 Beyond this platform
due to electronic delocalization.54,55 Finally, bulk electron
and other privileged primary coordination sphere motifs,
transfer can be improved by electrode surface attachment.61−65
approaches to enhance CO2 reduction catalysis with secondary
Against this backdrop of literature showing benefits of either
coordination sphere pendants have largely focused on
second-sphere proton or electron control for electrochemical
improving the delivery of protons using intramolecular proton
CO2 reduction catalysis, we sought to develop a second-sphere
relays and/or stabilization of metal-bound CO2 intermediates
approach that enables dual management of both electron and
proton inventories. Our synthetic efforts are inspired by
via hydrogen bonding.17,18,25−39 Additionally, reduced CO2
intermediates can be captured by inter/intramolecular Lewis
acid cations40−42 or intramolecular charge-compensation
moieties, such as ammonium21,43 or imidazolium groups.44−46
Received: April 20, 2020
In addition to proton control, strategies for facilitating
electron delivery from the secondary coordination sphere for
CO2 reduction have also been explored. One elegant example
from Shafaat’s laboratory embeds nickel cyclam, a classic CO2
reduction catalyst, within a blue copper protein that acts as an
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Scheme 1. Bioinspired Design of Second-Sphere Additives That Enable Dual Electron and Proton Control for Facilitating
Electrochemical CO2 Reduction Catalyzed by Iron Tetraphenylporphyrin (Fe-TPP)
biological CO2 reduction enzymes. Included are carbon
monoxide dehydrogenases (CODH)66 and formate dehydro-
genases67 that couple primary active sites with properly
positioned second-sphere proton relays and electron reservoirs
to enable catalysis near thermodynamic potentials (Scheme 1).
Such enzymes can be utilized for electrochemical CO2
reduction,68,69 but challenges associated with direct electron
transfer from electrodes to buried active sites within these
macromolecules in the absence of their native redox partners in
the cell require the addition of small-molecule redox mediators
like ferrocenes, viologens, and quinones.70,71 To pursue such
biology-to-chemistry concept transfer, we turned to the use of
organic cofactors such as nicotinamide adenine dinucleotide
(NAD+/NADH) that can facilitate both electron and proton
transfer. However, NADH and various synthetic organic
analogues are often incompatible with electrochemically driven
catalytic cycles due to formation of radical intermediates
leading to decomposition.72 As such, we sought to overcome
this challenge by judicious choice of catalyst and electron−
proton mediator components with matched reduction
potentials to avoid these off-cycle decomposition pathways.
In this report, we present a cooperative electrochemical CO2
reduction strategy that combines a base-metal molecular
catalyst in the primary coordination sphere with intermolecular
NADH-like redox mediators that facilitate electron and proton
transfer from the secondary coordination sphere (Scheme 1).
We chose N-phenylnicotinamide (1) as a dual electron−
proton mediator that possesses a basic pyridine and promotes
two-electron/two-proton transfer pathways, coupled to the
classic iron tetraphenylporphyrin (Fe-TPP) electrocatalyst
system for CO2 reduction. As reduction of pyridine hetero-
cycles can give 1,2- or 1,4-dihydropyridine regioisomers,73 we
exploited this property to evaluate the N-phenylisonicotina-
mide (2) isomer as an additional electron−proton mediator.
When added in catalytic amounts, both 1 and 2 significantly
enhance CO formation during electrochemical CO2 reduction
catalyzed by Fe-TPP with up to a 13-fold improvement in
catalytic activity without sacrificing CO2 versus proton
substrate selectivity. Further studies with a systematic series
of control additives reveals that both proton and electron
reservoir capacities contribute to the observed second-sphere
driven improvements in electrochemical CO2 reduction
efficiency. This work provides a starting point for designing
catalysts for a broader range of multielectron, multiproton
transformations with second-sphere additives or pendants that
can manage both electron and proton inventories.
RESULTS AND DISCUSSION
NADH Analogues Show Enhanced Current Activity in
■
the Presence of External Proton Sources. We initiated our
studies by evaluating the electrochemical behavior of NADH
analogues in the presence of external proton sources with the
goal of developing electron−proton redox mediators with
multielectron, multiproton capabilities and electrochemical
stability. As a starting point, the cyclic voltammogram (CV) of
1 under a N2 atmosphere in anhydrous DMF electrolyte shows
an irreversible reduction with a peak potential at −2.52 V vs
Fc/Fc+ (Figure 1). This result suggests an EC mechanism, in
Figure 1. Cyclic voltammograms of 5 mM NADH analogues 1 (blue)
and 2 (red) in 0.1 M TBAPF6 DMF electrolyte under a N2
atmosphere with titration of 0−100 mM phenol. Scan rate = 100
mV/s.
which the one-electron reduced species undergoes a rapid
chemical reaction. The CV of the isomeric analogue 2 shows a
similar EC wave but at a more positive peak potential of −2.21
V. Upon titration of phenol as a proton source into the
solution of 1 or 2 under a N2 atmosphere, we observe a
positive shift in peak potential and concomitant increase in
peak current in a dose-dependent manner until a saturation
B
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level is reached above 5 equiv of added phenol. The peak
potentials for 1 and 2 in the presence of excess phenol are
−2.40 and −2.14 V, respectively. Integration of the reduction
waves for 1 and 2 in the absence of phenol gives 29.8 and 29.6
μC of charge, respectively, whereas in the presence of 100 mM
phenol, electrochemical reductions of 1 and 2 consume 63.0
and 72.6 μC, respectively. These data suggest a transition from
a one-electron to a two-electron reduction process with added
acid, which is reminiscent of the ECEC mechanism that is
classically observed for quinone species in the presence of
proton sources.74 We speculate that hydrogen bonding
between the basic pyridine nitrogen and acidic phenol triggers
the positive shift in onset potential, and protonation after the
initial one-electron reduction step produces a species that is
more easily reduced than 1, so an overall two-electron/two-
proton reduction process is facilitated as more phenol is added.
This reaction leads to the proposed dihydropyridines 1′ and 2′
as shown in Scheme 2. A new oxidation wave is also observed
solutions of 1 and 2 at −2.4 V vs Fc/Fc+ with 500 mM phenol
under a CO2 atmosphere. The CPE data show that the overall
charge consumed is close to the two electrons per molecule
stoichiometry that would theoretically yield the reduced
dihydropyridines 1′ or 2′, respectively (Figure 2). The
Figure 2. Controlled potential electrolyses showing charge con-
sumption of 5 mM NADH analogues 1 (blue) and 2 (red) at −2.4 V
vs Fc/Fc+ in 0.1 M TBAPF6 DMF electrolyte containing 500 mM
phenol under a CO2 atmosphere. The gray dashed line denotes the
charge required for complete two-electron reduction of 1 and 2.
Scheme 2. Reductions of NADH Analogues 1 and 2 Produce
the Corresponding 1,4-Dihydropyridines, 1′ and 2′
catholyte consisted of 10 mL of 5 mM solutions of each
additive, which would require 9.6 C for a complete two-
electron reduction. Electrolysis of 1 led to rapid charge
consumption during the first 5 h but then required over 25 h of
electrolysis for complete reduction (Figure 2). This behavior is
likely due to low current density at this applied potential,
which is at the peak potential for the reduction of 1 (Figure 1).
Notably, more negative applied potentials were avoided to
minimize background hydrogen evolution at the glassy carbon
electrode. In contrast, 2 is fully reduced after 1.5 h of
electrolysis, quickly consuming a full 9.6 C as a result of the
more positively shifted reduction potential of 2 relative to 1,
where the electron-withdrawing group of the former is now
located on the 4-position. The applied potential of −2.4 V is
now 260 mV more negative than the peak potential for the
reduction of 2 so that the rate of reduction of 2 is primarily
limited by diffusion, whereas reduction of 1 is limited by
diffusion as well as electron transfer. We attribute the slight
excess of charge consumed to background hydrogen evolution,
which was detected by gas chromatography (GC). No reduced
carbon products were detected, confirming that the electro-
chemically generated dihydropyridines do not directly react
with CO2, as has been observed in other organohydride
systems.75 Indeed, the hydricity values of a range of substituted
dihydropyridines have been reported and are not strong
enough to reduce CO2 into formate.76
We also performed a direct chemical reduction of 1 using
LiAlH4 with an anaerobic aqueous workup to give 1′. ESI mass
spectrometry shows an m/z increase of 2, consistent with
dihydropyridine formation. Moreover, the UV/vis absorption
spectrum of starting additive 1 shows a single absorption at
250 nm, but spectra of the products of both chemical and
electrochemical reduction of 1 show the same prominent
absorption band at 355 nm (Figure 3A). Interestingly, this
absorption behavior is similar to that of NAD+/NADH,77
suggesting a similar change to the aromatic pyridine π system
in this synthetic analogue. Furthermore, the 1H NMR
spectrum of chemically synthesized 1′ shows nearly complete
consumption of starting material and the formation of new
peaks at 3.10, 4.56, 5.94, and 7.08 ppm consistent with allylic
and vinylic protons in the proposed dihydropyridine structure
at −0.61 V for 1 and at −0.24 V for 2 (Figures S1−S3),
indicative of oxidation of the incipient dihydropyridine
intermediate. After titration of 5 equiv of phenol, the CVs
show complete conversion to an ECEC mechanism, establish-
ing the potential for NADH analogues 1 and 2 in the presence
of complementary acid sources to act as two-electron/two-
proton mediators while also tuning redox potentials to more
positive potentials.
CVs of the structurally similar N-phenylpicolinamide (3)
exhibits a similar ECEC-type wave upon phenol titration with a
peak potential of −2.33 V (Figure S3). Interestingly,
unsubstituted nicotinamide exhibits different behavior with
CVs revealing a more negative reduction potential of −2.57 V
in the presence of 100 mM phenol and a peak area that does
not double as expected for a two-electron process, suggesting
an undesired reduction mechanism (Figure S7). The ECEC
behavior observed in CVs of additives 1−3 with an added
proton source remains unchanged up to 500 mM phenol in
both N2 and CO2 saturated electrolyte (Figures S8−S10).
Taken together, these observations establish that these
synthetic NADH analogues can serve as two-electron/two-
proton reservoirs in the presence of a complementary proton
source and that the reduced additives neither behave as CO2
reduction nor proton reduction electrocatalysts on the CV
time scale.
Controlled Potential Electrolysis Experiments on
NADH Analogues Provide Evidence for Dihydropyr-
idine Formation. To provide further evidence to support the
two-electron behavior in these synthetic NADH analogues, we
performed controlled potential electrolysis (CPE) on 5 mM
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Figure 3. (A) Overlaid UV/vis spectra in DMF solution of 1 (solid blue) and 1′ synthesized by either chemical reduction with LiAlH4 (dashed
blue) or by electrochemical reduction at −2.4 V in the presence of excess phenol (black). (B) Overlaid 1H NMR spectra in DMSO-d6 of 1 and 1′
synthesized by chemical reduction with LiAlH4 and 1 obtained by oxidation of 1′ upon exposure to air. Peaks labeled with asterisks are assigned to
the newly formed allylic and vinylic protons. Peaks between 3.3 and 3.5 ppm are assigned to residual water.
(Figure 3B). Full peak assignment is provided in Figure S14.
Exposure of 1′ to air leads to rapid reformation of 1 as
observed by 1H NMR, indicating the reversible redox nature of
this additive.
which is reflected by a larger difference in catalytic current with
added 1 at more negative potentials in Figure 4. In contrast,
additive 2 has a peak reduction potential of −2.14 V under
these conditions, which more closely matches where Fe-TPP
catalyzes CO2 reduction. This redox matching results in a more
significant increase in peak catalytic current in the presence of
1 equiv of NADH analogue 2 compared to 1, with further
amplification in the presence of 5 equiv of additive. Taken
together, these results suggest that additives 1 and 2 can
participate as cocatalysts with Fe-TPP for cooperative
electrochemical CO2 reduction.
Controlled Potential Electrolysis Experiments Con-
firm That NADH Analogues Enhance Electrochemical
CO2 Reduction Catalyzed by Iron Porphyrin. We then
performed CPE experiments to quantify product formation for
electrochemical CO2 reduction reactions catalyzed by Fe-TPP
in the absence or presence of the NADH analogues 1 and 2 in
addition to a broader set of NADH mimics (Scheme 3). CPE
Synthetic NADH Analogues Promote Higher Cata-
lytic Current for Iron Porphyrin in the Presence of CO2.
We next tested the ability of 1 to act as an electron−proton
transfer mediator for the CO2 reduction reaction catalyzed by
Fe-TPP. The peak reduction potentials for 1 and 2 in the
presence of excess phenol are −2.40 and −2.14 V, respectively,
which lie in the same range as the formal FeI/Fe0 reduction at
−2.15 V with Fe-TPP where CO2 reduction occurs.16
Titration of phenol into a DMF electrolyte solution of Fe-
TPP saturated with CO2 shows the expected significant
increase in catalytic current with peak shaped waves indicative
of fast catalysis (Figure S15). We then repeated this CV
experiment with the addition of either 1 or 5 equiv of additives
1 and 2 (Figures S16−S19). At low concentrations of phenol,
the reductions of 1 and 2 are clearly observable in the range of
the catalytic CO2 reduction wave. Figure 4 shows the overlaid
CV traces at the highest phenol concentration of 500 mM with
the catalytic currents normalized to the one-electron peak
Scheme 3. Chemical Structures of Synthetic NADH-Type
Additives 1−7
height for the formal FeII/FeI reduction of Fe-TPP, ip . With 1
0
equiv of 1, a modest increase in peak catalytic current is
observed at −2.25 V relative to Fe-TPP alone. However, 1 has
a peak reduction potential of −2.40 V under these conditions,
experiments were conducted with 0.5 mM Fe-TPP, 0.5 mM
additive, and 500 mM phenol under CO2 for 16 h at −2.4 V vs
Fc/Fc+. Charge accumulation over time is plotted in Figure 5,
and a summary of the data is listed in Table 1. With no
additive, 4.9 mL of CO was generated. In comparison, addition
of 1 equiv of 1 yielded over 3 times as much CO under the
same conditions, establishing the ability for 1 to increase the
catalytic activity of Fe-TPP for CO2 reduction. Moreover,
addition of 1 equiv of 2 yields 3.7 times as much CO as Fe-
TPP alone, highlighting the redox tunability of these
nicotinamide additives. Additive 3 yields a similar increase in
CO production. Current density over time is plotted in Figure
S22, showing that the current for each experiment drops
Figure 4. CVs of Fe-TPP (black) in the presence of 1 or 5 equiv of
additives 1 (blue) or 2 (red) in 0.1 M TBAPF6 DMF electrolyte
containing 500 mM phenol under a CO2 atmosphere. Current is
0
normalized to the formal FeII/FeI reduction of Fe-TPP, ip .
D
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The observed rate constants (kobs) can be estimated from
the CO-specific current densities (jCO), since the charge
consumed is directly associated with catalyst turnover.81 The
common foot-of-the-wave analysis determining rate constants
from CV traces is not applicable in this system, because the
reduction of 1 occurs more negative than the Fe-TPP foot-of-
the-wave, whereas the reduction of 2 occurs more positive than
the Fe-TPP foot-of-the-wave, which obscures this type of
analysis. As such, we used the average jCO from direct product
detection and quantification with the assumption that the
redox mediator did not change the rate-determining step for
Fe-TPP catalysis as product distributions were identical with
and without the additive. Using this method, the maximum
turnover frequency (TOFmax = kobs) for 0.5 mM Fe-TPP under
CO2 in the presence of 500 mM phenol was estimated to be
5.24 × 104 s−1, which is in agreement with literature reports
under conditions of large proton excess.21,36,45,81 In the
presence of 1 equiv of 1 or 2, patent increases in the Fe-
TPP TOFmax are observed with estimated values of 5.00 × 105
and 7.01 × 105 s−1, respectively. These values correspond to
9.5- and 13.4-fold increases in rate, respectively. Catalytic Tafel
plots could then be constructed to visualize the relationship
between TOF and overpotential.81 Figure 6 depicts the Tafel
Figure 5. CPEs of 0.5 mM Fe-TPP in the presence of 1 equiv of
additives 1−7 in 0.1 M TBAPF6 DMF electrolyte containing 500 mM
phenol under CO2 performed at −2.4 V vs Fc/Fc+.
Table 1. CPE Data Summarizing Effects of NADH-Type
Additives on Electrochemical CO2 Reduction Catalyzed by
Iron Porphyrin
charge
(C)
vol. CO
b
FECO
b
avg. jCO
TOFmax
d
c
catalysts
(mL)
(%)
(mA cm−2
)
(s−1
)
Fe-TPP
+1
+2
+3
+4
+5
+6
+7
41.8
119.2
141.0
132.7
50.4
46.0
94.5
112.5
4.9
15.0
17.9
16.8
6.0
5.4
11.9
14.2
93
86
108
87
94
92
0.67
2.07
2.45
2.31
0.82
0.74
1.63
1.95
5.24 × 104
5.00 × 105
7.01 × 105
6.23 × 105
7.85 × 104
6.39 × 104
3.10 × 105
4.44 × 105
104
100
a
CPEs performed with 0.5 mM Fe-TPP, with or without a 0.5 mM
concentration of the synthetic NADH analogue as an additive, at a
potential of −2.4 V vs Fc/Fc+ in 0.1 M TBAPF6 DMF electrolyte
containing 500 mM phenol under a CO2 atmosphere for 16 h.
b
Determined using gas chromatography equipped with a flame
ionization detector and a known calibration curve. Faradaic efficiency
c
(FE) values are estimated to have an error of 10%. Determined by
averaging the observed current density during each 16 h electrolysis
d
experiment and multiplying by the FECO. Estimated from the average
jCO during each electrolysis (see Supporting Information).
Figure 6. Catalytic Tafel plots for Fe-TPP in the absence or presence
of 1 equiv of NADH mimics 1−7 derived from the kobs values
estimated from controlled potential electrolysis experiments.
significantly over the long-term CPE experiments, which may
be due to the relatively negative applied potential of −2.4 V
that accelerates catalyst decomposition. All experiments
revealed CO to be the major product accounting for
approximately 90−100% Faradaic efficiency (FE), consistent
with the high selectivity for iron porphyrin catalysts, and
showed that these cooperative electron−proton redox
mediators improved catalytic activity without sacrifices in
catalytic selectivity.16,18,23,24 Indeed, only trace amounts of H2
were detected by GC (Table S1), and no liquid products were
detected by 1H NMR spectroscopy. Noting that some
dihydropyridines may be capable of reducing CO2 into
formate via hydride transfer,78 the formation of CO as the
sole reduced carbon product in this system suggests a different
mechanism for enhanced electron and proton transfer with
catalysis occurring at Fe-TPP. Along these lines, an elegant
related study by Anson and Stahl showed an increase in
current density for reduction of O2 into H2O2 and H2O using
quinones as electron−proton mediators.79 Similarly, aminoxyl
radicals can aid in electron−proton transfer for catalytic
electrochemical oxidations.80
plots for Fe-TPP in the absence and presence of synthetic
NADH analogues 1 and 2, further demonstrating the
substantial rate enhancement for CO production by Fe-TPP
in the presence of these two additives.
We next expanded these studies to the broader set of NADH
mimics to decipher contributions of electron and proton relays
in a systematic fashion (Scheme 3). First, pyridine additive 4
was tested to probe the role of the basic nitrogen atom in the
observed catalytic enhancements. It has previously been
hypothesized that pyridine additives can coordinate to cobalt
phthalocyanine catalysts to increase their nucleophilicity and
CO2 reduction activity.82 However, CPE of Fe-TPP with 1
equiv of 4 does not show a significant increase in charge
consumption and CO production (Figure 5 and Table 1)
relative to Fe-TPP alone. Second, the methylated pyridinium
additive 5, a closer structural mimetic of NAD+, was used to
determine the importance of the ECEC mechanism, because
E
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this charged pyridinium undergoes two stepwise reductions,
the first of which is proton-independent (Figure S5). CPE of
Fe-TPP with 1 equiv of 5 shows a similarly high rate of charge
consumption and current density as observed with additives
1−3 (Figures 5 and S22). However, after this initial increase,
the catalytic current decreases dramatically, and the cumulative
CO production is comparable to that of Fe-TPP alone, which
we attribute to the formation of radical anions that dimerize or
decompose catalyst. Figure 6 further shows that the log-
(TOF)−overpotential relationship for Fe-TPP is not signifi-
cantly changed upon addition of 4 or 5.
Additional synthetic NADH analogues were then inves-
tigated as additives to probe contributions of second-sphere
electron and proton relays to enhanced CO2 catalysis. CPE
experiments were performed using benzanilide 6 as a redox-
inactive additive that retains the amide substituent. Previous
work from our laboratory demonstrated that intramolecular
amide pendants can significantly enhance the reduction of CO2
to CO with proper second-sphere positioning.29 In agreement
with that study, the CPE experiments performed with Fe-TPP
and 1 equiv of 6 lead to improved rates of CO production (5.9-
fold increase) but not to the levels achieved with additives 1−3
seen in Figure 6 that possess both electron and proton transfer
capabilities (9.5- to 13.4-fold rate increases). Indeed, CVs of 6
in the presence of phenol do not show the ECEC behavior
observed with additives 1−3. Instead, a catalytic wave is
observed with a peak area that more than triples with
increasing phenol concentration, indicating that additive 6
behaves solely as a proton source through the amide NH
moiety (Figure S6). Finally, we tested pyridine amide additive
7, where the amide nitrogen atom is methylated to remove
benefits of hydrogen bonding and/or proton relays but retains
the redox-active reservoir. As observed with proton-only 6,
electron-only additive 7 increases charge consumption and
rates of CO production compared to Fe-TPP alone (8.4-fold)
but not to the same extent as NADH analogues 1−3 that have
the capacity for both electron and proton management
(Figures 6 and Figure S24). Interestingly, the enhanced
catalysis with additive 1 is not simply a summation of the
benefits provided by hydrogen bonding or proton donation
with 6 and by electron transfer with 7, suggesting a more
complex interplay. Nevertheless, these data suggest the
participation of both electron and proton relay components
in enhancing electrochemical CO2 reduction catalysis, as
NADH analogues that possess dual electron−proton redox
reservoir capabilities are superior to additives that only have
either electron or proton transfer capacity.
Finally, as NADH analogue 2 showed the highest activity
enhancements for electrochemical CO2 reduction, we
performed further dose-dependent CPE experiments with
this additive. Figure 7 compares relative current density and
charge accumulation values during 1 h of electrolysis at −2.4 V
with Fe-TPP under a CO2 atmosphere and 0, 1, 5, 10, or 15
equiv of additive 2. Addition of 1 equiv of 2 leads to a
significant enhancement in average current density to 4.19
mA/cm2, compared to 1.64 mA/cm2 with Fe-TPP alone.
However, increasing doses of 2 to 5, 10, or 15 equiv provides
only a modest further current density enhancement. We
speculate that this saturation behavior could be due to inherent
rate limitations of the parent Fe-TPP catalysis and/or
insufficient electrolysis to reduce enough 2 to couple to Fe-
TPP to be effective in the catalytic cycle, as the two
components must interact intermolecularly. Further experi-
Figure 7. CPEs of 0.5 mM Fe-TPP in the presence of 0−15 mM
additive 2 in 0.1 M TBAPF6 DMF electrolyte containing 500 mM
phenol under a CO2 atmosphere plotting (A) current density and (B)
charge accumulation over a period of 1 h at −2.4 V vs Fc/Fc+.
ments beyond the scope of this work are required to elucidate
such complex mechanistic details.
CONCLUDING REMARKS
■
In summary, we have presented a strategy for enhancing
electrochemical CO2 reduction catalysis through the use of
second-sphere additives that have dual electron−proton
reservoir capabilities. We establish this concept using synthetic
NADH analogues as electron−proton mediators with the
molecular CO2 reduction catalyst Fe-TPP. Indeed, although
extensive efforts have provided valuable design strategies for
facilitating second-sphere proton transfer for electrochemical
CO2 reduction, our findings show that such electrocatalysts
can be further augmented by adding electron transfer relays as
well. Specifically, the NADH-inspired nicotinamide-based
additives 1−3 undergo an ECEC reduction pathway to
subsequently serve as intermolecular electron−proton sources
for CO2 reduction catalyzed by Fe-TPP. Dihydropyridine
intermediates, which can be generated by chemical or
electrochemical means, can be reoxidized in air thermally or
through anodic electrochemistry back to the starting
nicotinamide to complete the electrochemical cycle.
A key design feature that emerges from this study is to
precisely match the redox potentials of the primary-sphere
molecular catalyst and second-sphere electron−proton medi-
ator to promote productive multielectron, multiproton
chemistry. For the two-electron/two-proton conversion of
CO2 to CO, catalytic rates for reduction can be augmented by
up to 13.4-fold in the presence of the electron−proton
mediator without sacrificing the high selectivity of Fe-TPP for
CO2 versus proton reduction. The cooperative second-sphere
approach complements work on CO2 hydrogenation catalysts,
where metal hydrides simultaneously deliver electrons and
protons.59,83−85 Indeed, few organic hydrides are strong
enough donors to reduce CO2 directly,73,76 and those that
can be regenerated encounter slow rates of hydride transfer
that limit integration into photo- or electrocatalytic cycles.75,86
This cooperative, bioinspired approach to electrocatalysis,
which leverages primary metal centers with secondary dual
electron−proton reservoirs, should be applicable to a broader
array of chemical transformations where bond activation and
catalytic turnover rely on controlling both electron and proton
inventories.
F
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electrosynthesis to displace petrochemical processes? Science 2019,
364 (6438), No. eaav3506.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01162.
■
sı
(8) Nitopi, S.; Bertheussen, E.; Scott, S. B.; Liu, X.; Engstfeld, A. K.;
Horch, S.; Seger, B.; Stephens, I. E. L.; Chan, K.; Hahn, C.; Norskov,
J. K.; Jaramillo, T. F.; Chorkendorff, I. Progress and Perspectives of
Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte.
Chem. Rev. 2019, 119 (12), 7610−7672.
(9) Zhao, S.; Jin, R. X.; Jin, R. C. Opportunities and Challenges in
CO2 Reduction by Gold- and Silver-Based Electrocatalysts: From
Bulk Metals to Nanoparticles and Atomically Precise Nanoclusters.
ACS Energy Lett. 2018, 3 (2), 452−462.
(10) Ross, M. B.; De Luna, P.; Li, Y.; Dinh, C.-T.; Kim, D.; Yang, P.;
Sargent, E. H. Designing materials for electrochemical carbon dioxide
recycling. Nat. Catal. 2019, 2 (8), 648−658.
(11) Weekes, D. M.; Salvatore, D. A.; Reyes, A.; Huang, A.;
Berlinguette, C. P. Electrolytic CO2 Reduction in a Flow Cell. Acc.
Chem. Res. 2018, 51 (4), 910−918.
Experimental methods, additional electrochemistry data,
and synthetic procedures (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Christopher J. Chang − Department of Chemistry and
Department of Molecular and Cell Biology, University of
California, Berkeley, California 94720, United States; Chemical
Sciences Division, Lawrence Berkeley National Laboratory,
Berkeley, California 94720, United States; orcid.org/0000-
0001-5732-9497; Email: chrischang@berkeley.edu
(12) Armstrong, F. A.; Hirst, J. Reversibility and efficiency in
electrocatalytic energy conversion and lessons from enzymes. Proc.
Natl. Acad. Sci. U. S. A. 2011, 108 (34), 14049−14054.
(13) Kornienko, N.; Ly, K. H.; Robinson, W. E.; Heidary, N.; Zhang,
J. Z.; Reisner, E. Advancing Techniques for Investigating the Enzyme-
Electrode Interface. Acc. Chem. Res. 2019, 52 (5), 1439−1448.
(14) Chen, H.; Dong, F.; Minteer, S. D. The progress and outlook of
bioelectrocatalysis for the production of chemicals, fuels and
materials. Nat. Catal. 2020, 3 (3), 225−244.
(15) Smith, P. T.; Nichols, E. M.; Cao, Z.; Chang, C. J. Hybrid
Catalysts for Artificial Photosynthesis: Merging Approaches from
Molecular, Materials, and Biological Catalysis. Acc. Chem. Res. 2020,
53 (3), 575−587.
Authors
Peter T. Smith − Department of Chemistry, University of
California, Berkeley, California 94720, United States; Chemical
Sciences Division, Lawrence Berkeley National Laboratory,
Berkeley, California 94720, United States
Sophia Weng − Department of Chemistry, University of
California, Berkeley, California 94720, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01162
Notes
(16) Bhugun, I.; Lexa, D.; Saveant, J.-M. Ultraefficient selective
homogeneous catalysis of the electrochemical reduction of carbon
dioxide by an iron(0) porphyrin associated with a weak Broensted
acid cocatalyst. J. Am. Chem. Soc. 1994, 116 (11), 5015−5016.
(17) Costentin, C.; Drouet, S.; Robert, M.; Saveant, J. M. A local
proton source enhances CO2 electroreduction to CO by a molecular
Fe catalyst. Science 2012, 338 (6103), 90−94.
(18) Costentin, C.; Passard, G.; Robert, M.; Saveant, J. M.
Ultraefficient homogeneous catalyst for the CO2-to-CO electro-
chemical conversion. Proc. Natl. Acad. Sci. U. S. A. 2014, 111 (42),
14990−14994.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Director, Office of Science,
Office of Basic Energy Sciences, and the Division of Chemical
Sciences, Geosciences, and Bioscience of the U.S. Department
of Energy at Lawrence Berkeley National Laboratory (DE-
AC02-05CH11231). C.J.C. is a CIFAR Senior Fellow. P.T.S.
thanks the NSF for a graduate research fellowship, and S.W.
thanks the UC Berkeley College of Chemistry for a summer
undergraduate research award.
(19) Hod, I.; Sampson, M. D.; Deria, P.; Kubiak, C. P.; Farha, O. K.;
Hupp, J. T. Fe-Porphyrin-Based Metal-Organic Framework Films as
High-Surface Concentration, Heterogeneous Catalysts for Electro-
chemical Reduction of CO2. ACS Catal. 2015, 5 (11), 6302−6309.
(20) Mohamed, E. A.; Zahran, Z. N.; Naruta, Y. Efficient
electrocatalytic CO2 reduction with a molecular cofacial iron
porphyrin dimer. Chem. Commun. 2015, 51 (95), 16900−16903.
(21) Azcarate, I.; Costentin, C.; Robert, M.; Saveant, J. M. Through-
Space Charge Interaction Substituent Effects in Molecular Catalysis
Leading to the Design of the Most Efficient Catalyst of CO2-to-CO
Electrochemical Conversion. J. Am. Chem. Soc. 2016, 138 (51),
16639−16644.
REFERENCES
■
(1) Kumar, B.; Llorente, M.; Froehlich, J.; Dang, T.; Sathrum, A.;
Kubiak, C. P. Photochemical and Photoelectrochemical Reduction of
CO2. Annu. Rev. Phys. Chem. 2012, 63 (1), 541−569.
(2) Appel, A. M.; Bercaw, J. E.; Bocarsly, A. B.; Dobbek, H.; DuBois,
D. L.; Dupuis, M.; Ferry, J. G.; Fujita, E.; Hille, R.; Kenis, P. J.;
Kerfeld, C. A.; Morris, R. H.; Peden, C. H.; Portis, A. R.; Ragsdale, S.
W.; Rauchfuss, T. B.; Reek, J. N.; Seefeldt, L. C.; Thauer, R. K.;
Waldrop, G. L. Frontiers, opportunities, and challenges in biochemical
and chemical catalysis of CO2 fixation. Chem. Rev. 2013, 113 (8),
6621−6658.
(22) Rao, H.; Schmidt, L. C.; Bonin, J.; Robert, M. Visible-light-
driven methane formation from CO2 with a molecular iron catalyst.
Nature 2017, 548 (7665), 74.
(3) Costentin, C.; Robert, M.; Saveant, J.-M. Catalysis of the
electrochemical reduction of carbon dioxide. Chem. Soc. Rev. 2013, 42
(6), 2423−2436.
(23) Smith, P. T.; Benke, B. P.; Cao, Z.; Kim, Y.; Nichols, E. M.;
Kim, K.; Chang, C. J. Iron Porphyrins Embedded into a Supra-
molecular Porous Organic Cage for Electrochemical CO2 Reduction
in Water. Angew. Chem., Int. Ed. 2018, 57 (31), 9684−9688.
(24) Mondal, B.; Sen, P.; Rana, A.; Saha, D.; Das, P.; Dey, A.
Reduction of CO2 to CO by an Iron Porphyrin Catalyst in the
Presence of Oxygen. ACS Catal. 2019, 9 (5), 3895−3899.
(25) Loewen, N. D.; Thompson, E. J.; Kagan, M.; Banales, C. L.;
Myers, T. W.; Fettinger, J. C.; Berben, L. A. A pendant proton shuttle
on [Fe4N(CO)12]− alters product selectivity in formate vs. H2
production via the hydride [H−Fe4N(CO)12]−. Chem. Sci. 2016, 7
(4), 2728−2735.
(4) Francke, R.; Schille, B.; Roemelt, M. Homogeneously Catalyzed
Electroreduction of Carbon Dioxide-Methods, Mechanisms, and
Catalysts. Chem. Rev. 2018, 118 (9), 4631−4701.
(5) Corbin, N.; Zeng, J.; Williams, K.; Manthiram, K. Heterogeneous
molecular catalysts for electrocatalytic CO2 reduction. Nano Res.
2019, 12 (9), 2093−2125.
(6) Zhang, S.; Fan, Q.; Xia, R.; Meyer, T. J. CO2 Reduction: From
Homogeneous to Heterogeneous Electrocatalysis. Acc. Chem. Res.
2020, 53 (1), 255−264.
(7) De Luna, P.; Hahn, C.; Higgins, D.; Jaffer, S. A.; Jaramillo, T. F.;
Sargent, E. H. What would it take for renewably powered
G
https://dx.doi.org/10.1021/acs.inorgchem.0c01162
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
́
(26) Roy, S.; Sharma, B.; Pecaut, J.; Simon, P.; Fontecave, M.; Tran,
P. D.; Derat, E.; Artero, V. Molecular Cobalt Complexes with Pendant
Amines for Selective Electrocatalytic Reduction of Carbon Dioxide to
Formic Acid. J. Am. Chem. Soc. 2017, 139 (10), 3685−3696.
(27) Ngo, K. T.; McKinnon, M.; Mahanti, B.; Narayanan, R.; Grills,
D. C.; Ertem, M. Z.; Rochford, J. Turning on the Protonation-First
Pathway for Electrocatalytic CO2 Reduction by Manganese Bipyridyl
Tricarbonyl Complexes. J. Am. Chem. Soc. 2017, 139 (7), 2604−2618.
(28) Chabolla, S. A.; Machan, C. W.; Yin, J.; Dellamary, E. A.; Sahu,
S.; Gianneschi, N. C.; Gilson, M. K.; Tezcan, F. A.; Kubiak, C. P. Bio-
inspired CO2 reduction by a rhenium tricarbonyl bipyridine-based
catalyst appended to amino acids and peptidic platforms: incorporat-
ing proton relays and hydrogen-bonding functional groups. Faraday
Discuss. 2017, 198 (0), 279−300.
(29) Nichols, E. M.; Derrick, J. S.; Nistanaki, S. K.; Smith, P. T.;
Chang, C. J. Positional effects of second-sphere amide pendants on
electrochemical CO2 reduction catalyzed by iron porphyrins. Chem.
Sci. 2018, 9 (11), 2952−2960.
(30) Margarit, C. G.; Schnedermann, C.; Asimow, N. G.; Nocera, D.
G. Carbon Dioxide Reduction by Iron Hangman Porphyrins.
Organometallics 2019, 38 (6), 1219−1223.
(43) Wang, M.; Torbensen, K.; Salvatore, D.; Ren, S.; Joulie, D.;
Dumoulin, F.; Mendoza, D.; Lassalle-Kaiser, B.; Isci, U.; Berlinguette,
̧
C. P.; Robert, M. CO2 electrochemical catalytic reduction with a
highly active cobalt phthalocyanine. Nat. Commun. 2019, 10 (1),
3602.
(44) Sung, S.; Kumar, D.; Gil-Sepulcre, M.; Nippe, M. Electro-
catalytic CO2 Reduction by Imidazolium-Functionalized Molecular
Catalysts. J. Am. Chem. Soc. 2017, 139 (40), 13993−13996.
(45) Khadhraoui, A.; Gotico, P.; Boitrel, B.; Leibl, W.; Halime, Z.;
Aukauloo, A. Local ionic liquid environment at a modified iron
porphyrin catalyst enhances the electrocatalytic performance of CO2
to CO reduction in water. Chem. Commun. 2018, 54 (82), 11630−
11633.
(46) Sung, S.; Li, X.; Wolf, L. M.; Meeder, J. R.; Bhuvanesh, N. S.;
Grice, K. A.; Panetier, J. A.; Nippe, M. Synergistic Effects of
Imidazolium-Functionalization on fac-Mn(CO)3 Bipyridine Catalyst
Platforms for Electrocatalytic Carbon Dioxide Reduction. J. Am.
Chem. Soc. 2019, 141 (16), 6569−6582.
(47) Schneider, C. R.; Shafaat, H. S. An internal electron reservoir
enhances catalytic CO2 reduction by a semisynthetic enzyme. Chem.
Commun. 2016, 52 (64), 9889−9892.
(31) Chapovetsky, A.; Welborn, M.; Luna, J. M.; Haiges, R.; Miller,
T. F., 3rd; Marinescu, S. C. Pendant Hydrogen-Bond Donors in
Cobalt Catalysts Independently Enhance CO2 Reduction. ACS Cent.
ACS Cent. Sci. 2018, 4 (3), 397−404.
(32) Haviv, E.; Azaiza-Dabbah, D.; Carmieli, R.; Avram, L.; Martin,
J. M. L.; Neumann, R. A Thiourea Tether in the Second Coordination
Sphere as a Binding Site for CO2 and a Proton Donor Promotes the
Electrochemical Reduction of CO2 to CO Catalyzed by a Rhenium
Bipyridine-Type Complex. J. Am. Chem. Soc. 2018, 140 (39), 12451−
12456.
(48) Lin, S.; Diercks, C. S.; Zhang, Y. B.; Kornienko, N.; Nichols, E.
M.; Zhao, Y.; Paris, A. R.; Kim, D.; Yang, P.; Yaghi, O. M.; Chang, C.
J. Covalent organic frameworks comprising cobalt porphyrins for
catalytic CO2 reduction in water. Science 2015, 349 (6253), 1208−
1213.
(49) Diercks, C. S.; Lin, S.; Kornienko, N.; Kapustin, E. A.; Nichols,
E. M.; Zhu, C.; Zhao, Y.; Chang, C. J.; Yaghi, O. M. Reticular
Electronic Tuning of Porphyrin Active Sites in Covalent Organic
Frameworks for Electrocatalytic Carbon Dioxide Reduction. J. Am.
Chem. Soc. 2018, 140 (3), 1116−1122.
(33) Sinha, S.; Warren, J. J. Unexpected Solvent Effect in
Electrocatalytic CO2 to CO Conversion Revealed Using Asymmetric
Metalloporphyrins. Inorg. Chem. 2018, 57 (20), 12650−12656.
(34) Sen, P.; Mondal, B.; Saha, D.; Rana, A.; Dey, A. Role of 2nd
sphere H-bonding residues in tuning the kinetics of CO2 reduction to
CO by iron porphyrin complexes. Dalton Trans. 2019, 48 (18),
5965−5977.
(35) Nichols, E. M.; Chang, C. J. Urea-Based Multipoint Hydrogen-
Bond Donor Additive Promotes Electrochemical CO2 Reduction
Catalyzed by Nickel Cyclam. Organometallics 2019, 38 (6), 1213−
1218.
(36) Gotico, P.; Boitrel, B.; Guillot, R.; Sircoglou, M.; Quaranta, A.;
Halime, Z.; Leibl, W.; Aukauloo, A. Second-Sphere Biomimetic
Multipoint Hydrogen-Bonding Patterns to Boost CO2 Reduction of
Iron Porphyrins. Angew. Chem., Int. Ed. 2019, 58 (14), 4504−4509.
(37) Hellman, A. N.; Haiges, R.; Marinescu, S. C. Rhenium
bipyridine catalysts with hydrogen bonding pendant amines for CO2
reduction. Dalton Trans. 2019, 48 (38), 14251−14255.
(50) Chen, Z.; Chen, C.; Weinberg, D. R.; Kang, P.; Concepcion, J.
J.; Harrison, D. P.; Brookhart, M. S.; Meyer, T. J. Electrocatalytic
reduction of CO2 to CO by polypyridyl ruthenium complexes. Chem.
Commun. 2011, 47 (47), 12607−12609.
(51) Benson, E. E.; Sampson, M. D.; Grice, K. A.; Smieja, J. M.;
Froehlich, J. D.; Friebel, D.; Keith, J. A.; Carter, E. A.; Nilsson, A.;
Kubiak, C. P. The Electronic States of Rhenium Bipyridyl Electro-
catalysts for CO2 Reduction as Revealed by X-ray Absorption
Spectroscopy and Computational Quantum Chemistry. Angew. Chem.,
Int. Ed. 2013, 52 (18), 4841−4844.
(52) Elgrishi, N.; Chambers, M. B.; Artero, V.; Fontecave, M.
Terpyridine complexes of first row transition metals and electro-
chemical reduction of CO2 to CO. Phys. Chem. Chem. Phys. 2014, 16
(27), 13635−13644.
(53) Gonell, S.; Massey, M. D.; Moseley, I. P.; Schauer, C. K.;
Muckerman, J. T.; Miller, A. J. M. The Trans Effect in Electrocatalytic
CO2 Reduction: Mechanistic Studies of Asymmetric Ruthenium
Pyridyl-Carbene Catalysts. J. Am. Chem. Soc. 2019, 141 (16), 6658−
6671.
(54) Derrick, J.; Loipersberger, M.; Iovan, D.; Smith, P. T.;
Chakarawet, K.; Long, J. R.; Head-Gordon, M.; Chang, C. Metal−
Ligand Exchange Coupling Promotes Iron-Catalyzed Electrochemical
CO2 Reduction at Low Overpotentials. ChemRxiv 2020.
DOI: 10.26434/chemrxiv.11923176.v1.
(55) Gonell, S.; Lloret, J.; Miller, A. An Iron Pyridyl-Carbene
Catalyst for Low Overpotential CO2 reduction to CO: Mechanistic
Comparisons with the Ruthenium Analogue and Photochemical
Promotion. ChemRxiv 2020. DOI: 10.26434/chemrxiv.11911383.v1.
(56) Lacy, D. C.; McCrory, C. C. L.; Peters, J. C. Studies of Cobalt-
Mediated Electrocatalytic CO2 Reduction Using a Redox-Active
Ligand. Inorg. Chem. 2014, 53 (10), 4980−4988.
(57) Romelt, C.; Song, J.; Tarrago, M.; Rees, J. A.; van Gastel, M.;
Weyhermuller, T.; DeBeer, S.; Bill, E.; Neese, F.; Ye, S. Electronic
Structure of a Formal Iron(0) Porphyrin Complex Relevant to CO2
Reduction. Inorg. Chem. 2017, 56 (8), 4745−4751.
(58) Wu, Y.; Jiang, J.; Weng, Z.; Wang, M.; Broere, D. L. J.; Zhong,
Y.; Brudvig, G. W.; Feng, Z.; Wang, H. Electroreduction of CO2
(38) Zee, D. Z.; Nippe, M.; King, A. E.; Chang, C. J.; Long, J. R.
Tuning Second Coordination Sphere Interactions in Polypyridyl-Iron
Complexes to Achieve Selective Electrocatalytic Reduction of Carbon
Dioxide to Carbon Monoxide. Inorg. Chem. 2020, 59 (7), 5206−5217.
(39) Guo, K.; Li, X.; Lei, H.; Zhang, W.; Cao, R. Unexpected Effect
of Intramolecular Phenolic Group on Electrocatalytic CO2 Reduction.
ChemCatChem 2020, 12 (6), 1591−1595.
́
(40) Bhugun, I.; Lexa, D.; Saveant, J.-M. Catalysis of the
Electrochemical Reduction of Carbon Dioxide by Iron(0) Porphyrins.
Synergistic Effect of Lewis Acid Cations. J. Phys. Chem. 1996, 100
(51), 19981−19985.
(41) Sampson, M. D.; Kubiak, C. P. Manganese Electrocatalysts with
Bulky Bipyridine Ligands: Utilizing Lewis Acids To Promote Carbon
Dioxide Reduction at Low Overpotentials. J. Am. Chem. Soc. 2016,
138 (4), 1386−1393.
(42) Hong, D.; Kawanishi, T.; Tsukakoshi, Y.; Kotani, H.; Ishizuka,
T.; Kojima, T. Efficient Photocatalytic CO2 Reduction by a Ni(II)
Complex Having Pyridine Pendants through Capturing a Mg2+ Ion as
a Lewis-Acid Cocatalyst. J. Am. Chem. Soc. 2019, 141, 20309.
H
https://dx.doi.org/10.1021/acs.inorgchem.0c01162
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Catalyzed by a Heterogenized Zn-Porphyrin Complex with a Redox-
Innocent Metal Center. ACS Cent. Sci. 2017, 3 (8), 847−852.
(59) Ceballos, B. M.; Yang, J. Y. Directing the reactivity of metal
hydrides for selective CO2 reduction. Proc. Natl. Acad. Sci. U. S. A.
2018, 115 (50), 12686−12691.
(60) Su, X.; McCardle, K. M.; Panetier, J. A.; Jurss, J. W.
Electrocatalytic CO2 reduction with nickel complexes supported by
tunable bipyridyl-N-heterocyclic carbene donors: understanding
redox-active macrocycles. Chem. Commun. 2018, 54 (27), 3351−
3354.
(61) Maurin, A.; Robert, M. Noncovalent Immobilization of a
Molecular Iron-Based Electrocatalyst on Carbon Electrodes for
Selective, Efficient CO2-to-CO Conversion in Water. J. Am. Chem.
Soc. 2016, 138 (8), 2492−2495.
(62) Oh, S.; Gallagher, J. R.; Miller, J. T.; Surendranath, Y. Graphite-
Conjugated Rhenium Catalysts for Carbon Dioxide Reduction. J. Am.
Chem. Soc. 2016, 138 (6), 1820−1823.
(63) Hu, X. M.; Ronne, M. H.; Pedersen, S. U.; Skrydstrup, T.;
Daasbjerg, K. Enhanced Catalytic Activity of Cobalt Porphyrin in
CO2 Electroreduction upon Immobilization on Carbon Materials.
Angew. Chem., Int. Ed. 2017, 56 (23), 6468−6472.
(64) Wang, M.; Chen, L.; Lau, T. C.; Robert, M. A Hybrid Co
Quaterpyridine Complex/Carbon Nanotube Catalytic Material for
CO2 Reduction in Water. Angew. Chem., Int. Ed. 2018, 57 (26),
7769−7773.
(65) Zhanaidarova, A.; Jones, S. C.; Despagnet-Ayoub, E.; Pimentel,
B. R.; Kubiak, C. P. Re(tBu-bpy)(CO)3Cl Supported on Multi-Walled
Carbon Nanotubes Selectively Reduces CO2 in Water. J. Am. Chem.
Soc. 2019, 141 (43), 17270−17277.
(66) Jeoung, J. H.; Dobbek, H. Carbon dioxide activation at the
Ni,Fe-cluster of anaerobic carbon monoxide dehydrogenase. Science
2007, 318 (5855), 1461−1464.
(78) Lim, C. H.; Holder, A. M.; Hynes, J. T.; Musgrave, C. B.
Reduction of CO2 to methanol catalyzed by a biomimetic organo-
hydride produced from pyridine. J. Am. Chem. Soc. 2014, 136 (45),
16081−16095.
(79) Anson, C. W.; Stahl, S. S. Cooperative Electrocatalytic O2
Reduction Involving Co(salophen) with p-Hydroquinone as an
Electron-Proton Transfer Mediator. J. Am. Chem. Soc. 2017, 139
(51), 18472−18475.
(80) Wang, F.; Stahl, S. S. Electrochemical Oxidation of Organic
Molecules at Lower Overpotential: Accessing Broader Functional
Group Compatibility with Electron−Proton Transfer Mediators. Acc.
Chem. Res. 2020, 53 (3), 561−574.
(81) Costentin, C.; Drouet, S.; Robert, M.; Saveant, J. M. Turnover
numbers, turnover frequencies, and overpotential in molecular
catalysis of electrochemical reactions. Cyclic voltammetry and
preparative-scale electrolysis. J. Am. J. Am. Chem. Soc. 2012, 134
(27), 11235−11242.
(82) Liu, Y.; McCrory, C. C. L. Modulating the mechanism of
electrocatalytic CO2 reduction by cobalt phthalocyanine through
polymer coordination and encapsulation. Nat. Commun. 2019, 10 (1),
1683.
(83) Kang, P.; Cheng, C.; Chen, Z.; Schauer, C. K.; Meyer, T. J.;
Brookhart, M. Selective Electrocatalytic Reduction of CO2 to Formate
by Water-Stable Iridium Dihydride Pincer Complexes. J. Am. Chem.
Soc. 2012, 134 (12), 5500−5503.
(84) Taheri, A.; Thompson, E. J.; Fettinger, J. C.; Berben, L. A. An
Iron Electrocatalyst for Selective Reduction of CO2 to Formate in
Water: Including Thermochemical Insights. ACS Catal. 2015, 5 (12),
7140−7151.
(85) Waldie, K. M.; Ostericher, A. L.; Reineke, M. H.; Sasayama, A.
F.; Kubiak, C. P. Hydricity of Transition-Metal Hydrides:
Thermodynamic Considerations for CO2 Reduction. ACS Catal.
2018, 8 (2), 1313−1324.
(86) Ohtsu, H.; Tanaka, K. An organic hydride transfer reaction of a
ruthenium NAD model complex leading to carbon dioxide reduction.
Angew. Chem., Int. Ed. 2012, 51 (39), 9792−9795.
(67) Boyington, J. C.; Gladyshev, V. N.; Khangulov, S. V.; Stadtman,
T. C.; Sun, P. D. Crystal Structure of Formate Dehydrogenase H:
Catalysis Involving Mo, Molybdopterin, Selenocysteine, and an Fe4S4
Cluster. Science 1997, 275 (5304), 1305−1308.
(68) Parkin, A.; Seravalli, J.; Vincent, K. A.; Ragsdale, S. W.;
Armstrong, F. A. Rapid and efficient electrocatalytic CO2/CO
interconversions by Carboxydothermus hydrogenoformans CO
dehydrogenase I on an electrode. J. Am. Chem. Soc. 2007, 129 (34),
10328−10329.
(69) Reda, T.; Plugge, C. M.; Abram, N. J.; Hirst, J. Reversible
interconversion of carbon dioxide and formate by an electroactive
enzyme. Proc. Natl. Acad. Sci. U. S. A. 2008, 105 (31), 10654−10658.
́
(70) Fourmond, V.; Leger, C. Protein Electrochemistry: Questions
and Answers. Adv. Biochem. Eng./Biotechnol. 2016, 158, 1−41.
(71) Yates, N. D. J.; Fascione, M. A.; Parkin, A. Methodologies for
“Wiring” Redox Proteins/Enzymes to Electrode Surfaces. Chem. - Eur.
J. 2018, 24 (47), 12164−12182.
(72) Ilic, S.; Alherz, A.; Musgrave, C. B.; Glusac, K. D. Importance of
proton-coupled electron transfer in cathodic regeneration of organic
hydrides. Chem. Commun. 2019, 55 (39), 5583−5586.
(73) McSkimming, A.; Colbran, S. B. The coordination chemistry of
organo-hydride donors: new prospects for efficient multi-electron
reduction. Chem. Soc. Rev. 2013, 42 (12), 5439−5488.
(74) Costentin, C. Electrochemical approach to the mechanistic
study of proton-coupled electron transfer. Chem. Rev. 2008, 108 (7),
2145−2179.
(75) Lim, C. H.; Ilic, S.; Alherz, A.; Worrell, B. T.; Bacon, S. S.;
Hynes, J. T.; Glusac, K. D.; Musgrave, C. B. Benzimidazoles as Metal-
Free and Recyclable Hydrides for CO2 Reduction to Formate. J. Am.
Chem. Soc. 2019, 141 (1), 272−280.
(76) Ilic, S.; Alherz, A.; Musgrave, C. B.; Glusac, K. D.
Thermodynamic and kinetic hydricities of metal-free hydrides.
Chem. Soc. Rev. 2018, 47 (8), 2809−2836.
(77) Margolis, S. A.; Howell, B. F.; Schaffer, R. Purification and
analysis of the purity of NADH. Clin. Chem. 1976, 22 (8), 1322−
1329.
I
https://dx.doi.org/10.1021/acs.inorgchem.0c01162
Inorg. Chem. XXXX, XXX, XXX−XXX