Metal-Ligand Cooperativity via Exchange Coupling Promotes Iron- Catalyzed Electrochemical CO2Reduction at Low Overpotentials

Catalyzed Electrochemical CO Reduction at Low Overpotentials

Article

Metal−Ligand Cooperativity via Exchange Coupling Promotes Iron-

Je ﬀ rey S. Derrick, Matthias Loipersberger, Ruchira Chatterjee, Diana A. Iovan, Peter T. Smith,

Khetpakorn Chakarawet, Junko Yano, Je ﬀ rey R. Long, Martin Head-Gordon, and Christopher J. Chang*

Cite This: https://dx.doi.org/10.1021/jacs.0c10664

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sı Supporting Information

ABSTRACT: Biological and heterogeneous catalysts for the

electrochemical CO reduction reaction (CO RR) often exhibit a

high degree of electronic delocalization that serves to minimize

overpotential and maximize selectivity over the hydrogen evolution

reaction (HER). Here, we report a molecular iron(II) system that

captures this design concept in a homogeneous setting through the

use of a redox non-innocent terpyridine-based pentapyridine ligand

(

tpyPY2Me). As a result of strong metal−ligand exchange coupling between the Fe(II) center and ligand, [Fe(tpyPY2Me)]

exhibits redox behavior at potentials 640 mV more positive than the isostructural [Zn(tpyPY2Me)] analog containing the redox-

inactive Zn(II) ion. This shift in redox potential is attributed to the requirement for both an open-shell metal ion and a redox non-

innocent ligand. The metal−ligand cooperativity in [Fe(tpyPY2Me)] drives the electrochemical reduction of CO to CO at low

−

overpotentials with high selectivity for CO RR (>90%) and turnover frequencies of 100 000 s with no degradation over 20 h. The

decrease in the thermodynamic barrier engendered by this coupling also enables homogeneous CO reduction catalysis in water

without compromising selectivity or rates. Synthesis of the two-electron reduction product, [Fe(tpyPY2Me)] , and characterization

by X-ray crystallography, Mossbauer spectroscopy, X-ray absorption spectroscopy (XAS), variable temperature NMR, and density

functional theory (DFT) calculations, support assignment of an open-shell singlet electronic structure that maintains a formal Fe(II)

oxidation state with a doubly reduced ligand system. This work provides a starting point for the design of systems that exploit

metal−ligand cooperativity for electrocatalysis where the electrochemical potential of redox non-innocent ligands can be tuned

through secondary metal-dependent interactions.

20−22

INTRODUCTION

states,

respectively. Such systems can thereby achieve

■

organization and separation of reducing equivalents in a way that

is challenging to design in molecular systems.

Previous work from our laboratories has explored the

The electrochemical carbon dioxide reduction reaction

(

CO RR) provides opportunities to synthesize value-added

products from this greenhouse gas in a sustainable manner.

Eﬃcient catalysts for this reaction are required to selectively

drive CO reduction over the thermodynamic and kinetically

competitive hydrogen evolution reaction (HER). Biological,

heterogeneous, and molecular catalysts have been studied for

CO reduction, and molecular systems in particular oﬀer

potential advantages such as their small size relative to enzymes

and the fact that they can be tuned with a level of atomic

precision that is inaccessible with heterogeneous materials.

However, despite advances in the development of molecular

catalysts with highly specialized ligand scaﬀolds, including

cyclams,

bonyl-bipyridines,

chemistry of the PY5Me ligand (2,6-bis(1,1-bis(2-pyridyl)-

3−25

25,26

ethyl)pyridine) and its molybdenum

and cobalt

derivatives as catalysts for the HER. We reasoned that by

27−29

incorporating a redox-active pendant such as terpyridine

within this ligand scaﬀold, it would be possible to increase

metal−ligand orbital mixing and delocalization of electron

density away from the metal center to favor CO reduction

(Figure 1). Precedent for the use of redox non-innocent ligands

in molecular electrochemical CO RR has shown the viability of

16,17,27−39

this strategy,

along with classic systems bearing strong

−10

11−15

metal−ligand cooperativity such as metal-dithiolenes for

porphyrins and phthalocyanines,

and tricar-

40−43

6,17

HER.

Indeed, the suppression of metal-centered reduc-

it remains a challenge to design

molecules that exhibit low overpotentials, high turnover

numbers, and compatibility with aqueous electrolytes to avoid

oﬀ-pathway HER processes. In this regard, one key advantage of

biological and heterogeneous catalysts is their ability to

minimize overpotential and maximize selectivity for CO₂

reduction via electronic delocalization, either through elec-

Received: October 7, 2020

8,19

tron-tunneling pathways

or a high local density of

XXXX American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

sca ﬀ old, inspired by the active site of mononuclear iron hydrogenase.

Article

Figure 1. Bioinspired design of a molecular iron polypyridyl complex for electrochemical carbon dioxide reduction catalysis. The molecular CO₂

reduction catalyst [Fe1]²⁺was designed to feature a mononuclear iron center supported by a redox-active pendant in a robust synthetic polypyridyl

Figure 2. Chemical structures of [Fe1]²⁺and the coordination complexes used as controls to disentangle the contributions of the ligand and the metal

center in catalysis. (a) Chemical structure of [Fe1]²⁺highlighting cooperativity between the open-shell iron center and redox non-innocent tpyPY2Me

ligand. (b) The diamagnetic analog containing the redox-silent Zn(II) center, [Zn1] , enables the role of the iron center to be probed with the same

redox non-innocent ligand scaﬀold. Likewise, (c) [Fe(PY5Me )(CH CN)] and (d) [Zn(PY5Me )(CH CN)] both feature the redox-innocent

PY5Me as a related pentapyridine ligand and thus aid in identifying the contributions of the redox-active terpyridine fragment within the polypyridyl

scaﬀold.

tions can limit the formation of oﬀ-pathway metal hydride

characterized by X-ray crystallography, Mossbauer spectrosco-

35,44

intermediates necessary for hydrogen evolution

and thereby

py, X-ray absorption spectroscopy (XAS), variable temperature

NMR, and density functional theory (DFT) calculations. The

collective data support assignment of an open-shell singlet

ground state that maintains a formal Fe(II) oxidation state,

comprised of an intermediate-spin iron(II) (S_F_e= 1)

antiferromagnetically coupled to a doubly reduced triplet

terpyridine ligand (Stpy = 1). This electronic structure also

enables the molecular catalyst to operate at low overpotentials in

both organic and neutral aqueous electrolytes, resulting in high

favor CO reduction catalysis. As part of a larger program in

,24,25,45−48

electrocatalysis via bioinorganic mimicry,

our

synthetic approach was inspired by mononuclear iron hydro-

genase enzymes, which catalyze the reduction of methenyl-

H MPT to methylene-H MPT in methanogenic archaea. In

these systems, heterolysis of H and hydride shuttling is enabled

by a single redox-innocent iron(II) site that synergistically

interacts with a redox non-innocent guanyl pyridinol cofactor in

the primary coordination sphere to provide reducing equivalents

selectivity for CO RR over HER.

(

Figure 1).

RESULTS AND DISCUSSION

Design, Synthesis, and Characterization of the Redox

Non-Innocent tpyPY2Me Ligand and Its Metal Coordi-

nation Complexes Showing Deviations from Idealized

Octahedral Geometry. The tpyPY2Me ligand and corre-

Here we report an iron polypyridine complex [Fe-

tpyPY2Me)(CH CN)] ([Fe1] ) (tpyPY2Me = 6-(1,1-

■

(

di(pyridin-2-yl)ethyl)-2,2′:6′,2″-terpyridine), which features a

redox-active terpyridine fragment and demonstrates this

principle of electronic delocalization. In particular, metal−

ligand cooperativity promotes strong exchange coupling

between the redox-active tpyPY2Me ligand and iron(II) center

where the [Fe(tpyPY2Me)]²complex is able to accept two

electrons at reduction potentials that are 640 mV more positive

than the isostructural [Zn(tpyPY2Me)] analog containing the

redox-inactive Zn(II) ion, showing that a redox non-innocent

ligand alone is not suﬃcient to promote multielectron redox

chemistry with low activation barriers. Upon two-electron

reduction of [Fe(tpyPY2Me)] , strong antiferromagnetic

coupling facilitates electrocatalytic reduction of CO to CO at

low overpotentials. Under these conditions, [Fe1] exhibits

high selectivity for CO production (greater than 90%) at high

turnover frequencies. Synthetic reduction of the [Fe-

sponding iron(II) complex [Fe(tpyPY2Me)] , abbreviated as

[Fe1] , were prepared following modiﬁcations of previously

reported procedures for the pentapyridine PY5Me ligand and

its metal complexes (see Supporting Information ). Brieﬂy, the

pentapyridine ligand tpyPY2Me bearing a redox non-innocent

terpyridine fragment was formed by a lithium-mediated

coupling between 6-bromo-2,2′:6′,2″-terpyridine and 2,2′-

(ethane-1,1-diyl)dipyridine. A series of control coordination

complexes were synthesized and evaluated to disentangle the

contributions of the redox-active ligand from the metal center

(Figure 2). In order to decipher the role of the iron center in

electrochemical behavior and subsequent catalysis, we prepared

the diamagnetic, isostructural zinc(II) analog [Zn-

(

tpyPY2Me)] complex enables access to the hypothesized

(tpyPY2Me)] ([Zn1] ) that is equipped with the same

catalytically active species, [Fe(tpyPY2Me)] , which has been

redox-active tpyPY2Me ligand but with a redox-silent zinc

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

tortion also signiﬁcantly compresses the axial bond angle in

Article

center. To aid in identifying the role of the redox-active

[Fe1]²⁺(173.64(6)°) whereas the corresponding angle in

terpyridine fragment of the polypyridyl scaﬀold, [Fe(PY5Me )-

[Fe(PY5Me )(CH CN)] is nearly linear (178.53(7)°)

2 3

2+ 26

(

CH CN)]

and [Zn(PY5Me )(CH CN)]

complexes

(Figure 3a). Distortion of the primary coordination sphere as

a result of the enhanced ligand rigidity of tpyPY2Me is also

observed for the control analog bearing the redox-silent Zn(II)

bearing the related pentapyridine PY5Me₂ligand without

polypyridine conjugation were synthesized. Indeed, irreversible

reduction of the PY5Me₂ligand occurs at very negative

center, [Zn1] . The solid-state structure of [Zn1] (Figure S1;

Tables S3 and S4 ) shows that tpyPY2Me enforces a severely

distorted square pyramidal coordination geometry with the

terpyridine nitrogen atoms occupying equatorial positions. This

potentials (between −2.0 and −2.5 V vs Fc/Fc ) and is coupled

to catalyst decomposition; thus it serves as a polypyridyl redox-

innocent ligand control (Figure 2).

structure departs quite signiﬁcantly from the O geometry

A comparison of the solid-state structures of [Fe1] and

2+(50)

observed for [Zn(PY5Me )(CH CN)] .

[

Fe(PY5Me )(CH CN)]

obtained via single-crystal X-ray

2 3

The structure of [Fe1]²remains more distorted than that of

diﬀraction (XRD) revealed that the enhanced rigidity of the

tpyPY2Me ligand enforces a more distorted pseudo-octahedral

[

Fe(PY5Me )(CH CN)] in solution, as veriﬁed by H NMR

2 3

spectra collected for both complexes in CD CN at 295 K. The

(

O ) geometry in [Fe1] relative to [Fe(PY5Me )-

H NMR spectrum of Fe(PY5Me )(CH CN)] features ﬁve

2 3

CH CN)] , which adopts an almost ideal O geometry

Figure 3a; Tables S1 and S2). In particular, occupation of the

sharp signals consistent with a C symmetric complex with low-

spin Fe(II) (S = 0), whereas the H NMR spectrum of [Fe1]

features slightly more broadened resonances with no well-

deﬁned symmetry (Figure S2). Alternatively, [Zn1] shows a

iron equatorial plane by the three terpyridine nitrogen atoms

and coordination of the pyridine arms to axial and equatorial

positions induces severe axial distortions in [Fe1] that are

H NMR spectrum with all expected ligand resonances

absent in [Fe(PY5Me )(CH CN)] (Table S2 ). This dis-

accounted for, consistent with its diamagnetic ground state

(

see Supporting Information). Using the Evans method, we

measured an eﬀective magnetic moment (μ ) of 1.27 μ for

eff

[

Fe1] at 293 K, which is indicative of a low-spin Fe(II) ground

state (S = 0) with a population of thermally accessible spin

excited states at room temperature (Figure S3). Because the

[

Fe1]²⁺bond lengths measured at 100 K are in agreement with

those of other low-spin Fe(II) polypyridyl complexes, the

collective data suggest that [Fe1]²⁺undergoes a spin-state

transition between 100 K and room temperature. To further

probe the spin-state transition of [Fe1] , variable-temperature

(

VT) H NMR spectra were collected in CD CN across a

temperature range of 233 to 353 K (Figure S4). At 293 K,

resonances at 9.47, 7.73, 7.12, and 6.74 ppm appear as very

broad singlets and upon heating the sample to 353 K, these

peaks become undetectable due to broadening into the baseline

(

Figure S4b ). Upon stepwise cooling, the resonances start to

become fully resolved. At 233 K, integration of the aryl region

accounts for the 18 proton signals expected for [Fe1] . This

temperature-dependent signal broadening is in line with our

interpretation of a population of thermally accessible spin

excited states. Due to the absence of systematic, temperature-

dependent paramagnetic shifts of the [Fe1]²⁺resonances, we

were unable to quantify the spin equilibrium from the H NMR

data alone. Nevertheless, variable temperature Evans method

data further revealed a gradual increase in μ upon heating, in

eff

agreement with the occurrence of a spin transition process in

solution (Figure S5 ). The limited temperature range available in

acetonitrile and limited solubility of [Fe1]²⁺in alternative

solvents to expand this temperature range prevents isolation of a

pure population of low-spin Fe(II) (μ of 1.20 μ at 233 K) and

eff

the transition is incomplete at the highest available temperature

μ of 1.33 μ at 353 K), precluding accurate determination of

(

eff

the spin-equilibrium transition (T₁_/₂).

Zero-ﬁeld Fe Mossbauer spectra collected for [Fe1] at 5

Figure 3. Structural characterization of iron polypyridyl complexes. (a)

and 295 K further support such a spin state transition for the

Fe(II) center (Figure S6 and Tables S5 ). At 5 K, the spectrum

features a symmetric Lorentzian doublet with isomer shift (δ)

Solid state structures of [Fe(PY5Me )(CH CN)] , featuring the

redox-innocent PY5Me ligand, and [Fe1] , as determined via X-ray

diﬀraction analysis. Thermal ellipsoids are plotted at 80% probability

level; orange, blue, and gray ellipsoids represent Fe, N, and C atoms,

respectively. Non-coordinating solvent and anions are omitted for

clarity. (b) Simpliﬁed Walsh diagram illustrating the eﬀect of axial

distortion away from the near ideal O symmetry in [Fe(PY5Me )-

and quadrupole splitting (|ΔE |) values of 0.3562(4) and

.0203(8) mm/s, respectively, consistent with a low-spin

iron(II) (Figure S6a and Tables S5 ). The density functional

theory (DFT) computed Mossbauer parameters closely match

(

CH CN)] toward C₂_vsymmetry in [Fe1] .

the experimental values within the well-established uncertainty

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Figure 4. Electrochemical characterization. Cyclic voltammograms collected under Ar of (a) [Fe1] (red trace) and [Zn1]²⁺(black trace) complexes

and (b) [Fe(PY5Me )(CH CN)] (red trace) and [Zn(PY5Me )(CH CN)] (black trace) complexes supported by redox-innocent PY5Me . These

data show that the unique combination of Fe and redox non-innocent tpyPY2Me ligand lead to electrochemically reversible two-electron chemistry,

with the Zn-tpyPY2Me analog exhibiting two reversible one-electron reduction waves with the second reduction at potentials 640 mV more negative

than the iron analog. The Fe(II) and Zn(II) PY5Me analogs both exhibit irreversible electrochemical reductions at potentials more negative than −2.0

V vs Fc/Fc . Cyclic voltammograms of (c) [Fe1] and (d) [Fe(PY5Me )(CH CN)] collected under Ar (black), CO (red), and with added phenol

(

3.5 M, blue). Voltammograms were collected with a scan rate of 100 mV/s with an electrolyte of 0.10 M TBAPF dissolved in CH CN.

6 3

1,52

ranges (∼0.10 mm/s for δ and ∼0.3 mm/s for |Δ E |),

thus

the Redox Non-Innocent Terpyridine Fragment. The

corroborating the low-spin Fe(II) assignment (Figure S7 and

ssbauer spectrum measured at

95 K features an asymmetric quadrupole doublet that can be ﬁt

to two diﬀerent iron sites (Figure S6b and Table S5 ). The

parameters of the major site are nearly identical to the 5 K

spectrum when accounting for second-order Doppler shifts

presence of a redox-active terpyridine moiety in tpyPY2Me also

Table S6 ). In contrast, the Mo

gives rise to unique electrochemical behavior for [Fe1] and

[Zn1]²⁺relative to their PY5Me₂analogues. The room-

temperature cyclic voltammogram (CV) of [Fe1] collected

in CH CN under Ar exhibits three reversible features: a one-

electron oxidation at E₁_/₂= 0.66 V (versus Fc/Fc ) assigned to

(

SODS) (δ = 0.285(1) mm/s, |ΔE | = 1.056(2) mm/s) and are

the iron(III/II) couple and two, closely spaced, one-electron

redox processes that are centered at −1.43 V (Figure 4a).

Further chemical characterization of this redox behavior

indicates that these redox events are two sequential ligand-

therefore consistent with the same low-spin Fe(II) species

observed at 5 K. The isomer shift of the minor site is signiﬁcantly

more positive (0.62(1) mm/s) and is indicative of a higher spin

/1−

1−/2−

iron(II) species. DFT predicted Mo

ssbauer parameters for these

based reductions (tpyPY2Me

and tpyPY2Me

) (vide

−

higher-spin Fe(II) species correspond closely to the exper-

imental values for the minor site measured at 295 K, further

supporting this assignment (Figure S7 and Table S6).

infra). During constant potential coulometry of 7.6 × 10

mmol of [Fe1]²⁺conducted at −1.60 V vs Fc/Fc under Ar

atmosphere, 1.58 C of charge was passed equating to 2.1

The distinct iron(II) spin states in [Fe1] and [Fe(PY5Me )-

electrons per molecule of [Fe1] supporting this assignment as

(

CH CN)] can be understood from the simpliﬁed Walsh

two closely spaced one-electron redox waves centered at −1.43

diagram presented in Figure 3b. Axial distortions engendered by

the rigid tpyPY2Me ligand lead to a decrease in symmetry from

idealized O to C , and this distortion signiﬁcantly stabilizes the

V vs Fc/Fc (Figure S8). No additional features were observed

when scanning to more negative potentials.

In contrast, under the same conditions, the cyclic voltammo-

gram of [Zn1]²⁺exhibits two, well-separated, reversible, one-

electron ligand-centered reductions with E₁_/₂of −1.56 and

−2.07 V as well as an additional irreversible redox event

d orbital and destabilizes the d through z-component mixing

of the t and t orbitals. The stabilization of the d orbital and

disruption of t degeneracy can then facilitate the formation of

thermally accessible spin excited states near room temperature.

We hypothesize that these marked diﬀerences in the ground-

state electronic structures of [Fe1] and Fe(PY5Me )-

−/3−

(tentatively assigned as tpyPY2Me

) at −2.41 V (Figure 4a).

The ﬁrst ligand centered reduction of [Zn1] occurs at a

comparable potential to that of [Fe1] but the second ligand

(

CH CN)] contribute to their distinct electrochemical and

centered reduction is shifted 640 mV more negative and it is this

catalytic behavior (vide infra).

disparate electrochemical behavior between [Fe1] and

[Zn1]²⁺that indicates a degree of communication between

the reduced tpyPY2Me ligand and the open-shell iron center

Cyclic Voltammetry of tpyPY2Me Fe(II) and Zn(II)

Complexes and Comparison to PY5Me Analogs Lacking

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

mA/cm (Figure 5 a and Figures S11 and S12 ; Table S7 ). The

Article

that is not possible in the isostructural complex containing the

closed-shell, redox-silent zinc center. In contrast, ligand-based

experiments in both overpotential and versus Fc/Fc and use a

conservative estimate of E

(−1.27 V vs Fc/Fc )

CO /CO

reductions in [Fe(PY5Me )(CH CN)] and [Zn(PY5Me )-

obtained from measured p K values for acetonitrile−water

mixtures (see Supporting Information for additional details). It

should be noted that this value is 770 mV more positive than for

dry” acetonitrile and 70 mV more positive than the previously

estimated standard redox potential for “wet” acetonitrile derived

(

CH CN)] are electrochemically irreversible and occur at

potentials negative of −2.0 V (Figure 4b), highlighting the

crucial role of the terpyridyl fragment in achieving electro-

chemically reversible reduction events at relatively positive

potentials.

“

13,14

by assigning unusual gas standard states.

Tafel plots that

Electrochemical Reduction of CO Catalyzed by

compare catalyst activity are plotted to show both the calculated

overpotential as well as the directly measured potential versus

the reference electrode used in the experiments. We reason that

reporting both values should lead to more clarity in

[

Fe(tpyPY2Me)] in Organic Solution. Under an atmos-

phere of CO , CV data collected for [Fe1] exhibit only a subtle

increase in current relative to scans under Ar; however, when

excess phenol (3.5 M) is added as a proton source, two large₊

catalytic waves appear at ca. −1.50 and −2.0 V versus Fc/Fc

benchmarking catalysts.

(

Figure 4c). The PY5Me analog [Fe(PY5Me )(CH CN)]

[Fe1] is highly selective for CO

reduction over hydrogen

also displays catalytic currents upon addition of CO and

evolution across the entire potential window examined (Figure

5a and Figures S11 and S12 ; Table S7 ), with averaged speciﬁc

current densities for CO production (j _C _O ) reaching as high as 3.6

phenol, but the onset current for catalysis is >500 mV more

negative than that observed for [Fe1] (Figure 4d). Phenol

titrations conducted with solutions of [Fe1]²under a CO₂

atmosphere revealed a dose-dependence on phenol concen-

tration (Figure S9a ). The electrochemical response is mitigated

when control phenol titrations are collected under an Ar

atmosphere, suggesting that [Fe1]²selectively catalyzes

electrochemical CO reduction (CO RR) over the hydrogen

evolution reaction (HER), even at high concentrations of

organic acid (Figure S9b). As anticipated, the isostructural

Zn(II) analog [Zn1]²does not show similar increases in

catalytic current at such positive potentials (Figure S10 ),

establishing that the redox non-innocent tpyPY2Me ligand alone

with a Lewis acid center is not suﬃcient for promoting CO RR

activity.

Controlled Potential Electrolysis Studies for Direct

Product Measurements of Electrochemical CO RR Cata-

lyzed by [Fe(tpyPY2Me)] in Organic Solution. Prepara-

tive-scale controlled potential electrolysis (CPE) experiments

were conducted in CO -saturated acetonitrile with 3.5 M phenol

across a range of applied potentials (−1.35 to −1.98 V vs Fc/

Fc ), and the products were quantiﬁed by gas chromatography.

We ﬁnd that CO is the major product of CO RR. Importantly,

we note that subsequent turnover number and turnover rate

values reported in this work are derived from these direct

measurements of product, as opposed to indirect methods

derived from transient CV currents with electrochemical

assumptions.

Because the utility of a catalyst is benchmarked by its rate and

overpotential (η), the applied potentials were also converted

into overpotentials following the methods reported by Mayer

and Matsubara and co-workers.

4,55

Accurate conversion

necessitates that the thermodynamic potential for CO₂

reduction to CO is known under the experimental conditions.

Whereas the aqueous standard potential is known (−0.106 V vs

SHE; −0.730 V vs Fc/Fc ), it is diﬃcult to experimentally

determine in organic solvents where the identity and amount of

the proton source complicate the measurements. This caveat

often leads to the use of estimated values that do not match the

experimental conditions or rely on thermodynamic assumptions

that do not rigorously apply, such as taking the CO and CO

13,14

Figure 5. Electrochemical CO

reduction performance of [Fe1] . (a)

standard states to be 1 M

or assuming anhydrous conditions

Selectivity of [Fe1]²for CO reduction to CO at varying over-

^p^o^t^eⁿ^tⁱ^a^l^s^aⁿ^d^s^p^e^cⁱ^ﬁ^c^c^u^r^r^eⁿ^t^d^eⁿ^sⁱ^tⁱ^e^s^f^o^r^C^O⁽^jCO) production

averages of three experiments). (b) Catalytic Tafel plot for [Fe1]

obtained from CPE experiments in acetonitrile with added phenol (3.5

M). CPE experiments were collected in an electrolyte of 0.10 M

TBAPF dissolved in CH CN.

without reporting measured water concentrations or using

anhydrous organic acids, leading to potential inconsistencies

in the literature with regard to reported overpotentials. With the

goal of increasing standardization and transparency in molecular

electrocatalysis, we report the applied potentials from our CPE

(

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Figure 6. Synthetic preparation and characterization of the two-electron reduced species [Fe1] . The collective structural and spectroscopic data

support assignment to an open-shell singlet species that maintains a formal Fe(II) oxidation state. (a) Two-electron reduction of [Fe1] generates the

[

Fe1] complex that is a putatively active species for CO reduction. (b) H NMR spectrum of [Fe1] (black) and H NMR spectrum of the product

resulting from chemical reduction of [Fe1] with CoCp* (purple); both spectra were in CD CN at room temperature. (c) Zero-ﬁeld Fe Mossbauer

analysis of [Fe1] (black) and [Fe1] (purple) measured at 5 K. (d) Solid-state structure of [Fe1] from single-crystal X-ray diﬀraction analysis.

Thermal ellipsoids are plotted at 80% probability level; orange, blue, and gray ellipsoids represent Fe, N, and C atoms, respectively. Residual solvent

and co-crystallized CoCp* are omitted for clarity.

Faradaic eﬃciency for CO production (FE_C_O) reaches 94% at an

overpotential of 190 mV (−1.46 V vs Fc/Fc ) and increases to

inversion of product selectivity, with H as the predominant

product and negligible generation of CO (Figure S19 ).

00% with the application of overpotentials between 320 and

10 mV (−1.59 and −1.98 V vs Fc/Fc ) (Figure 5a and Figure

Kinetic Analysis of Electrochemical CO RR Catalyzed

by [Fe(tpyPY2Me)] in Organic Solution. Encouraged by

the high selectivity of [Fe1]²⁺for electrochemical CO

RR over

S11 ; Table S7 ). This high activity and selectivity for CO

production at applied potentials as low as −1.46 V vs Fc/Fc is a

substantial decrease in the energy requirements compared to

competing HER processes, we sought to evaluate its kinetic

performance. Cyclic voltammograms acquired for [Fe1]²

other molecular catalysts and sits among the most eﬃcient

under CO -saturated electrolyte in the presence of 3.5 M

4,27−29,59−61

reported catalysts.

Signiﬁcantly, no CO and only

phenol (Figure S20 ) exhibit scan rate dependence and

noncanonical peak-shaped waves, indicative of competitive,

nonideal processes (e.g., substrate consumption, product

inhibition, etc.). As a result, k_o_b_s(TOF_m_a_x) obtained from

small amounts of H (FE_H= 44%) were detected in analogous

CPE experiments with [Fe(PY5Me )(CH CN)] at much

larger overpotentials (Figure S13). Moreover, variable potential

catalytic plateau current analysis is not appropriate here and will

CPE experiments with [Zn1] conducted between −1.66 V and

lead to underestimations of the rates of CO reduction.

−

2.41 V vs Fc/Fc produced only trace amounts of CO (FE

Therefore, we instead extracted kinetic parameters directly from

our variable potential CPE experiments (Figure S11), as

−6%) and a small amount of H (FE_H= 1−20%), further

4,65

illustrating the importance of metal −ligand cooperativity for

described by Savea

nt and co-workers,

where the products

eﬃcient CO RR catalysis (Figure S14 ).

(CO and H ) were detected and quantiﬁed by gas

Control experiments strongly support the homogeneous

chromatography (Figures S21 and S22). The observed rate

molecular nature of [Fe1] . First, CO reduction was not

constants (k ) at each applied potential were extracted from

obs

observed in the absence of [Fe1]²(Figure S15 ). Furthermore,

cyclic voltammetry and UV−vis data collected before and after 1

h CPE experiments are indistinguishable, attesting to the bulk

the averaged speciﬁc current density for CO production (j_C_O

)

taken across the entire 1 h CPE experiments (Figure S11 ) that

were conducted in triplicate (see Supporting Information for

additional details).

stability of [Fe1] under electrolysis conditions (Figure S16).

Finally, the possible formation of an electrode-adsorbed active

catalyst was excluded through a series of control experiments.

A catalytic Tafel plot was constructed from our CPE

experiments with direct product quantiﬁcation (Figure 5b)

First, peak currents for [Fe1]²and [Zn1] scale linearly with

the square root of the scan rate, indicative of freely diﬀusing

species in solution (Figures S17 and S18 ). Second, CPE rinse

and is summarized in Table S7 . Tafel analysis identiﬁes [Fe1]

as one of the most active molecular electrocatalysts reported to

date with it achieving a turnover frequency (TOF) of 75,000 s⁻

at an applied overpotential as low as 190 mV (−1.46 V vs Fc/

tests which probe for electrode deposition, give complete

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Supporting Information ).

Article

−1

Fc ) and reaching TOFs > 500 000 s between overpotentials

of 560−710 mV (−1.81 and −1.98 V vs Fc/Fc ) (Figure S23).

Notably, this rapid, homogeneous CO reduction catalysis is

achieved without any modiﬁcations to the secondary coordina-

tion sphere to introduce hydrogen bond/proton-relay function-

13,14,47,66−69

alities to break electronic scaling relationships.

In parallel, we also attempted to apply foot-of-the-wave

4,65

analysis (FOWA) as pioneered by Savea

to estimate the

kinetics based solely on CVs collected in the presence and

absence of CO . FOWA estimates the idealized catalytic

reactivity by considering the initial portion of the catalytic

wave where the eﬀects of side phenomena are expected to be

minimal. Unfortunately, FOWA does not ﬁnd clear application

here because the linear region of the FOW is very small and is ill

deﬁned due to the appearance of a pre-wave feature at −1.36 V

Figure 7. Experimental Fe K-edge XAS spectra of [Fe1]²⁺(black) and

two-electron reduced [Fe1]⁰(red), supporting a formal Fe(II)

oxidation state assignment for both species.

vs Fc/Fc and a multi-segmented catalytic wave with apparent

maxima at −1.56 and −1.96 V vs Fc/Fc (Figures 4c and S24 ).

The existence of this multi-segmented catalytic wave may

suggest that the catalytic resting state is changing as more

negative potentials are applied. Therefore, the extraction of

kinetic data based on our CPE experiments results in more

accurate analysis as they are determined from preparative scale

experiments that directly detect and measure CO rather than

relying on estimations of observed rate constants determined

from CV data.

closed-shell singlet ligand (S_t_p_y_P_Y₂_M_e= 0) generated by both

electrons occupying the same π* orbital (i.e., [Fe-

2− 0

(

tpyPY2Me) ] ). The other possibility is an intermediate-

spin iron(II) center (S = 1) antiferromagnetically coupled to a

reduced ligand with an open-shell triplet conﬁguration (S_t_p_y_P_Y₂_M_e

1) through half occupation of both low-lying π* terpyridine

orbitals, leading to an overall metal−ligand spin-coupled

•• 2− 0

Finally, to assess the long-term catalytic stability and

electronic structure (i.e., [Fe(tpyPY2Me ) ] ). The two

electronic conﬁgurations are more clearly distinguished here

by specifying the spin state of the ligand as either a singlet

recyclability of [Fe1] , four consecutive 5 h CPE experiments

were conducted at −1.98 V (versus Fc/Fc ), the highest

2−

•• 2−

potential investigated, without addition of fresh [Fe1]

(

tpyPY2Me) or a triplet (tpyPY2Me ) in the chemical

between each experiment (Figure S25). [Fe1]²is remarkably

stable with 221 C of charge passed over the 20 h window without

^aⁿ^y^l^o^s^sⁱⁿ^s^e^l^e^c^tⁱ^vⁱ^t^y^f^o^r^C^O⁽^a^v^e^r^a^g^e^F^ECO = 87%).

formula. Because of the large anodic shift (640 mV) in the

ligand-based reductions of [Fe1]²⁺relative to [Zn1] (Figure

4a), the electrochemistry indicates a strong inﬂuence of the

metal center and supports assignment to a [Fe-

Chemical Reduction and Characterization of [Fe-

•• 2− 0

(

tpyPY2Me)] ([Fe1] ). We reasoned that the observed

(tpyPY2Me ) ] electronic conﬁguration. In further support

electrocatalytic eﬃciency of [Fe1] for CO RR stems from a

of this model, Mo

ssbauer data for [Fe1] revealed an isomer

synergistic eﬀect between the iron center and the redox-active

tpyPY2Me ligand, as both components are necessary to achieve

shift of δ = 0.2608(2) mm/s and a quadrupole splitting of |ΔE |

= 0.7637(4) mm/s, consistent with the formation of

intermediate-spin iron(II) (Figures 6c and S27 ). The

antiferromagnetic coupling between the reduced biradical

ligand, (tpyPY2Me ) , and the intermediate-spin iron(II) is

selective CO RR reactivity. To establish a molecular-level

framework for understanding this metal−ligand cooperativity,

•• 2−

we sought to synthesize, isolate, and characterize the two-

electron reduced complex, [Fe(tpyPY2Me)] ([Fe1] ), which

expected to be quite strong given that a diamagnetic H NMR

we hypothesize has putative catalytic relevance. Chemical

spectrum is observed at room temperature. Indeed, DFT

−1

reduction of [Fe1] with 2.2 equiv of decamethylcobaltocene

calculations predict a large coupling constant of −1023 cm ,

−1

(

Figure 6a) produced [Fe1] as a dark purple solid.

giving rise to a singlet-quintet gap of 4400 cm (see the

Interestingly, room temperature H NMR analysis of [Fe1]

Further insights into the electronic structure of [Fe1] can be

in CD CN revealed that the compound is diamagnetic with the

aryl and aliphatic signals shifted upﬁeld (Figure 6b). In support

gleaned from the solid-state structure of [Fe1] . Single crystals

of this result, a nearly identical H NMR spectrum was obtained

of [Fe1] were obtained by slow evaporation of a saturated

for the product [Zn1] generated from the chemical reduction

acetonitrile solution, and X-ray diﬀraction analysis revealed a

distorted square pyramidal geometry (Figure 6d; Tables S8 and

S9 ) resulting from the loss of the axial acetonitrile ligand in

of [Zn1] with potassium graphite (see Figure S26 and details

in the Supporting Information). These data are consistent with

our assignment of the reduction features for both [Fe1] and

[Fe1] . This structure is in good agreement with the reduction

[

Zn] as ligand-centered events. Frozen solution Fe K-edge X-

pathway predicted by DFT (vide infra). The bond metrics of the

terpyridine moiety unambiguously supports the two-electron

reduction of the tpyPY2Me ligand in the solid state and thus the

iron(II) oxidation state assignment. In particular, the con-

traction of the intra-pyridine bond lengths (C −C′ ), from an

ray absorption near-edge spectroscopy (XANES) was used to

characterize metal oxidation states (Figure 7). Absorption edge

energies were determined from the second derivative zero

crossings, giving the following values (eV): 7126.9 ([Fe1] ) and

127.4 ([Fe1] ). These values are similar to compounds in the

average of 1.474(3) Å in [Fe1] to 1.434(4) Å in [Fe1] is

consistent with the metrics with which the oxidation levels of

terpyridine can be established as determined through structural

studies on a series of iron(II) and chromium(III) bis-terpyridine

literature leading to the assignment of the oxidation states of

both species as Fe(II).

Given the unique diamagnetic ground state of [Fe1] , two

2,73

major electronic structures are possible. The ﬁrst possibility is a

low-spin iron(II) center (S_F_e= 0) coordinated to a reduced

complexes reported by Wieghardt and co-workers.

Furthermore, average C−N bond lengths of the terpyridine

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Figure 8. DFT analysis and calculated molecular orbitals for [Fe1] and [Fe1] species. (a) DFT predicted pathway for reduction of [Fe1] to the

•• 2−

open-shell singlet [Fe1] , [Fe(tpyPY2Me ) ] . The computed reduction potentials are given for reduction of [Fe1] to [Fe1] and [Fe1] to

Fe1] . See Table S11 for the predicted reduction pathways for [Zn(1)]

density plot and (c) simpliﬁed molecular orbital diagram of the one electron reduced intermediate [Fe1] . (d) Spin density plot and (e) simpliﬁed

molecular orbital diagram of [Fe1] ([Fe(tpyPY2Me ) ] ). Localized metal orbitals are depicted in the Supporting Information.

2+/0

2− 0

[

and for the closed-shell singlet [Fe1] , [Fe(tpyPY2Me) ] . (b) Spin

•• 2− 0

moiety in [Fe1] (1.384(3) Å) depart quite signiﬁcantly from

([Fe(tpyPY2Me ) ] ) composed of an intermediate-spin

what would be expected for typical, neutral aromatic pyridine

iron(II) center (S_F_e= 1) that is antiferromagnetically coupled

to a doubly reduced tpyPY2Me (S_t_p_y= 1) ligand. This strong

exchange coupling shifts the tpyPY2Me-based reductions of

[Fe1]²⁺to more positive potentials relative to the analog with

the redox-silent Zn(II) center by 640 mV, which we hypothesize

rings (1.35 ± 0.1 Å, respectively) and what was observed in the

solid state structure of [Fe1]²(1.353(2) Å).

Distinguishing between the two possible electronic conﬁg-

urations of [Fe1] based on the single crystal X-ray diﬀraction

data is more challenging. The DFT predicted structure for the

ultimately promotes highly selective CO reduction by [Fe1] .

•

2−

2+/0

open-shell singlet ([Fe(tpyPY2Me ) ] ) more closely

To further elucidate the electronic structures of [Fe1] , we

matches the experimental bond metrics of [Fe1] than does

subsequently applied extensive DFT and multireference

the predicted structure of the closed-shell singlet ([Fe-

methods.

− 0

(

tpyPY2Me) ] ); however the solid state structures are

Electronic Structure Calculations Support Strong

Metal−Ligand Exchange Coupling Between Fe and

tpyPY2Me Units. We carried out DFT calculations using the

ωB97X-D functional to generate a more accurate molecular

susceptible to additional packing and dispersion forces that are

not accounted for by DFT calculations (Table S10). In theory,

the unique bonding interactions of the two terpyridine

2−74

LUMOs

should enable us to directly distinguish between

orbital picture of our [Fe1] system. Optimized structures and

the two possible electronic conﬁgurations of [Fe1] , but the

predicted redox potentials for [Fe1] and [Zn1] are in good

agreement with experimental data (Tables S11 and 12).

Interestingly, although the DFT data indicate the metal center

change in bond metrics between the dianionic, singlet ligand

−

(

tpyPY2Me ) and the triplet, π radical dianion (tpy-

••2−

PY2Me ) is expected to be quite subtle given the metrics

in [Fe1] is best described as low-spin iron(II), the calculated

reported by Wieghardt and co-workers on a series of reduced

singlet-quintet gap is small, explaining the observed thermal

population of a higher spin-state at room temperature (Tables

S13 and 14). The ﬁrst reduction of [Fe1] to [Fe1] is

homoleptic bis(terpyridine) complexes ([M(tpy) ] ; M = Cr,

2,73

Fe).

Attempts to crystallize [Zn1] have been unsuccessful

to date, thus preventing direct crystallographic comparison to

predicted to generate a ground state doublet composed of a low-

spin iron(II) and a tpyPY2Me-based radical (tpyPY2Me )

• 1−

[

Fe1] .

Although absolute determination of the electronic structure

with a predicted redox potential of −1.46 V (Figure 8a−c). The

should not be based solely on X-ray diﬀraction, when it is taken

together with the electrochemical, NMR, and Mossbauer results,

spin density plot of this [Fe1] species indicates a ligand-based

reduction with almost no excess spin on the iron center (Figure

8b). The second reduction populates a second low lying π*

orbital of tpyPY2Me and induces loss of the axial acetonitrile

the collective data support the iron(II) oxidation state

assignment of [Fe1] as an open-shell singlet ground state

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

CO reduction. However, homogeneous molecular CO

Article

−

molecule, providing access to an intermediate-spin iron(II) state

catalyst, [Re(bpy)(CO) ] (and its Mn derivative), where the

(

Figure 8a,d,e). The S_F_e= 1 iron center is strongly

quantum chemical calculations and spectroscopic data revealed

that one electron is placed into the π* orbital of the non-

innocent bpy ligand and the second electron is placed in the

antiferromagnetically coupled to the two electrons in the

tpyPY2Me π* orbitals (i.e., [Fe(tpyPY2Me ) ] ; Figure 8e).

Localized orbital bonding analysis conﬁrmed that iron remains

•

• 2− 0

metal d_z

carbonyl ligands. Both electrons form a metal−ligand bond

delocalized metal−ligand orbital) with a singlet ground state

which is 15 kcal/mol lower in energy than the uncoupled triplet

orbital but is strongly delocalized into the three

in the 2 oxidation state during the second reduction process,

which has a predicted reduction potential of −1.51 V. This

predicted redox potential is in good agreement with that

observed experimentally.

In contrast, the other feasible closed-shell singlet conﬁg-

uration: a low-spin iron(II) center (S_F_e= 0) coordinated to a

(

state. Thus, the “initial” d-orbitals are not involved in the

stabilization of the excess electrons as they are in [Fe1] and

[Fe(TPP)].

reduced closed-shell doubly reduced singlet ligand (S

typPY2Me

Electrochemical CO RR Catalyzed by [Fe-

2− 0

) (i.e., [Fe(tpyPY2Me) ] ), does not match the experimental

(

tpyPY2Me)] in Water. Given the low overpotentials

data. The doubly reduced tpy ligand is strongly π basic and thus

required for [Fe1] to catalyze homogeneous electrochemical

delocalizes some electron density into the empty metal orbitals

CO reduction in organic solvent, we sought to evaluate its

(

distorted e -type). The computed reduction potential of −2.03

activity in water (Figure 9). For this purpose, we prepared the

+ 2− 0

V vs Fc/Fc using the [Fe(tpyPY2Me) ] electronic conﬁg-

uration does not match the experimental value of −1.43 V vs Fc/

Fc . This diﬀerence can be rationalized by the large free energy

more water-soluble nitrate analog of [Fe1] (see the

Supporting Information ), and X-ray diﬀraction analysis revealed

that anion exchange does not alter the catalyst structure (Figure

S29 ; Tables S15 and S16 ). As a cheap, abundant, and benign

solvent and proton source, water is an attractive medium for

gap (12 kcal/mol) between the broken symmetry solution

•

• 2− 0

(

[Fe(tpyPY2Me ) ] ) and the closed-shell singlet ([Fe-

tpyPY2Me) ] ) consisting of low-spin iron(II) (S = 0)

2− 0

and doubly reduced ligand (S_t_p_y_P_Y₂_M_e= 0) (Tables S13 and 14).

It should also be noted that a large variety of diﬀerent hybrid

density functionals all predict the broken symmetry con ﬁ g-

uration as the ground state on the singlet surface (see Table

S14 ).

In addition, multi-reference calculations using complete

active space self-consistent ﬁeld (CASSCF) conﬁ rmed the

antiferromagnetically coupled ground state (see the Supporting

Information ). This calculation reveals a strong entanglement of

the two ligand-based orbitals and the two metal d-orbitals. In

particular, the axial distortion away from octahedral symmetry

allows the d and d metal orbitals (or superposition of the two

orbitals) to interact with the two tpyPY2Me π* orbitals in a weak

π-type interaction (Figure S28 ). Hence, the novel ligand

framework with two low-lying π* orbitals, a distorted

coordination geometry, and a moderate ligand ﬁeld in

combination with the iron metal center stabilizes two excess

electrons eﬀectively. The spin transition alters both the

energetics of the d-orbitals and local spin on the central metal

to facilitate an eﬀective antiferromagnetic coupling. In contrast,

in the alternative closed-shell singlet electronic conﬁguration

− 0

(

[Fe(tpyPY2Me) ] ; S = 0 and S = 0), the stabilization of

Fe tpy

the doubly reduced tpyPY2Me π* (S_t_p_y= 0) by the high lying e_g*

of the low-spin iron(II) (S_F_e= 0) is marginal. This feature is

illustrated by the almost identical second ligand reduction

potentials of [Zn1]²and [Fe1] with a closed-shell singlet

electronic structure (−2.02 V for [Zn1] and −2.03 V vs Fc/

Fc for [Fe1] with S = 0 and S = 0).

tpy

We note that a similar electronic structure involving this

intramolecular antiferromagnetic coupling was proposed by

Neese and co-workers for the electronic structure of another

CO reduction catalyst, the doubly reduced iron tetraphenylpor-

2−

phryin complex, [Fe(TPP)] . Through quantum mechanical

calculations and spectroscopic data (Mossbauer and XAS) they

assign both reductions of [FeTPP]²to the porphyrin ligand

−

framework and show that it is coupled to an intermediate-spin

Figure 9. Aqueous CO

electrocatalyst. (a) Faradaic eﬃciencies for CO (blue) and H (red)

formation at varying applied potentials for 60 min (averages of three

reduction performance for the Fe-tpyPY2Me

iron center (S = 1) giving an overall singlet state. It should be

noted that the iron center in the un-reduced iron(II) complex,

[

Fe(TPP)] , is intermediate-spin and therefore the metal center

experiments). The average FE_C_O₂is indicated by the black dashed line.

does not undergo a spin transition upon reduction. This

(b) Catalytic Tafel plot for [Fe1] obtained from CPE experiments in

behavior can be contrasted to the group VII CO reduction

0.10 M NaHCO₃.

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

reduction catalysts typically require mercury working electrodes

to operate eﬀectively in aqueous electrolytes because their onset

potentials are too negative to be compatible with more desirable

carbon-based electrodes that preferentially catalyze competing

synergistic coupling between both metal and ligand compo-

nents.

The combination of distorted coordination geometry, low-

lying π* orbitals, and moderate ligand ﬁeld engendered by the

rigid, redox non-innocent tpyPY2Me ligand promotes strong

0,76−79

water reduction.

Owing to the 640 mV positive shift in 2-

electron reduction potential upon moving from Zn(II) to

metal−ligand cooperativity with the iron(II) center. [Fe1] is

Fe(II), we envisioned that CO RR could be favored over HER,

shown to catalyze CO reduction at extremely mild redox

even with water as both a solvent and proton source.

potentials (η = 190 mV; − 1.46 V vs Fc/Fc ) relative to the

control polypyridyl complexes presented in this study. The

Against this backdrop, we were pleased to observe that cyclic

voltammograms collected for [Fe1]²dissolved in 0.10 M

positive redox potentials at which CO RR catalysis occurs is

NaHCO with a carbon paste working electrode show the

interpreted through extensive experimental spectroscopic

formation of a catalytic wave when saturated with CO (Figure

(

NMR, Mossbauer, and XAS), XRD, and electrochemical

S30 ). Notably, variable potential CPE experiments (Figures

studies, along with electronic structure calculations, which

collectively point to a model in which the two-electron reduced

S31 −S35 ) revealed that [Fe1] maintains its activity and

selectivity for CO reduction to CO in water (average FE

iron complex, [Fe1] , possesses an open-shell singlet ground

2% and FE_H₂< 1%) with maximum turnover frequencies,

•

• 2− 0

state electronic structure ([Fe(tpyPY2Me ) ] ) resulting

−

determined from direct product detection, reaching >50 000 s

from a strong intramolecular antiferromagnetic coupling

at overpotentials of 500 mV (Figure 9b). Importantly, control

rinse tests passed negligible current during 1 h CPE experiments,

between a triplet reduced ligand (tpyPY2Me tpyPY2Me^•

•2−

;

S_t_p_y_P_Y₂_M_e= 1) and an intermediate-spin Fe(II) center (S = 1).

and the selectivity was inverted such that H is the major product

We propose that the multielectron chemistry of [Fe1] at

https://pubs.acs.org/doi/10.1021/jacs.0c10664.

Figures S31 −S36). UV−visible spectra collected before and

relatively positive potentials is facilitated through this electronic

structure, which in turn promotes its exceptional selectivity

after electrolysis are nearly indistinguishable; however, cyclic

voltammograms change quite drastically following the 1 h CPE

(

^F^ECO > 90%) through suppression of HER pathways.

(Figure S37) indicative of some degree of catalyst deposition

and decomposition on the glassy carbon electrode. In line with

this observation, electrochemical stripping behavior is also

observed, which may contribute to slower catalysis in water

relative to organic solvent (Figure S38). Nevertheless, these data

Furthermore, variable potential CPE experiments with direct

product detection show that [Fe1] operates at fast rates in

−

both organic (TOF > 100 000 s ) and aqueous electrolytes

−

(TOF > 50 000 s ) with robust long-term stability and

show that [Fe1] is an eﬀective molecular catalyst for

recyclability.

homogeneous CO reduction in water. Crucially, given its

The ability to create molecular compounds with strong

exchange coupling between synergistic, redox non-innocent

ligands and metal centers has broad implications for catalyst

design, including enabling control over the density and

arrangement of functional active sites. Indeed, redox balancing

through electronic delocalization is pivotal for biological and

materials catalysts to achieve complex chemical transformations

at or near thermodynamic potentials. However, discrete

molecular compounds oﬀer a level of tunability via coordination

chemistry that remains unparalleled in protein design and

nanostructure engineering. Thus, inorganic molecules are

uniquely positioned to open access to a range of new properties

and oﬀer a desirable alternative strategy for lowering energy

barriers for redox catalysis. The observed 640 mV electro-

molecular nature, the activity, stability, and overpotential of this

catalyst can in principle be tuned through ligand modiﬁcations

accessible by synthetic chemistry. Further, the study of this

catalyst and related systems has the potential to shed important

insights on the behavior of materials catalysts bearing atomically

dispersed iron sites.

CONCLUDING REMARKS

■

In summary, inspired by the beneﬁcial feature of electronic

delocalization exhibited by biological and heterogeneous

systems for electrochemical CO RR catalysis to minimize

overpotentials and favor CO RR over competing HER path-

ways, even in aqueous media, we report a molecular system that

captures this design concept. A new ligand framework that

incorporates a redox-active terpyridine moiety into a modular

polypyridyl scaﬀold can achieve highly selective and eﬃcient

chemical potential shift and associated turn-on of CO RR

catalysis upon substituting Zn(II) for Fe(II) for the same redox

non-innocent ligand emphasizes metal−ligand cooperativity for

redox balancing that goes beyond redox activity dominated at

either ligand or metal sites alone.

iron-catalyzed electrochemical CO reduction to CO in both

organic and aqueous solutions with high speciﬁcity over

competing hydrogen evolution pathways. Importantly, the

combination of both an open-shell metal ion using an earth-

abundant, ﬁrst-row transition metal in conjunction with a redox

non-innocent ligand enables metal-cooperativity through strong

exchange coupling that shifts electrochemical reduction

ASSOCIATED CONTENT

■

sı Supporting Information

The Supporting Information is available free of charge at

potentials to promote CO RR catalysis. Speciﬁcally, the Fe(II)

tpyPYMe2 analog exhibits a 640 mV positive shift in the 2-

electron reduction potential relative to the isostructural analog

bearing a redox-silent Zn(II) center, poising it for multielectron

electrocatalysis with minimized overpotentials. Control experi-

ments with Fe(II) and Zn(II) complexes bearing a related

pentapyridine ligand lacking the redox-active terpyridine

fragment and the same redox non-innocent ligand but with a

X-ray crystallographic data for [Fe1] , [Zn1] , [Fe1] ,

and [Fe(tpyPY2Me)(NO )][NO ] (CIF)

Experimental and computational details, compound

characterization, supplemental electrochemical and spec-

troscopic data, and DFT geometry optimized atomic xyz

coordinates (PDF)

redox-silent Zn(II) center show that CO RR relies on

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Laboratory, Berkeley, California 94720, United States;

Article

AUTHOR INFORMATION

Collection and interpretation of Mo

ssbauer spectra were

■

supported by National Science Foundation CHE-1800252 to

J.R.L. C.J.C. is a CIFAR Fellow. P.T.S. acknowledges the NSF

for a graduate research fellowship. We thank Dr. Hasan Celik

and UC Berkeley’s NMR facility in the College of Chemistry

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

orcid.org/0000-0001-5732-9497; Email: chrischang@

berkeley.edu

(

CoC-NMR) and Dr. Ping Yu and UC Davis’s NMR facility for

spectroscopic assistance. Instruments in the CoC-NMR are

supported in part by NIH S10OD024998. We thank Dr. N.

Settineri for assistance with X-ray crystallography. We thank R.

Murphy, C. Gould, C. Stein, and Dr. A. Wuttig for fruitful

discussions and Dr. K. R. Meihaus for editorial assistance. We

dedicate this manuscript to Prof. Dick Andersen.

Authors

Jeﬀrey S. Derrick − Department of Chemistry, University of

California, Berkeley, California 94720, United States;

Chemical Sciences Division, Lawrence Berkeley National

Laboratory, Berkeley, California 94720, United States

Matthias Loipersberger − Department of Chemistry, University

of California, Berkeley, California 94720, United States;

orcid.org/0000-0002-3648-0101

Ruchira Chatterjee − Molecular Biophysics and Integrated

Bioimaging Division, Lawrence Berkeley National Laboratory,

Berkeley, California 94720, United States

Diana A. Iovan − Department of Chemistry, University of

California, Berkeley, California 94720, United States;

orcid.org/0000-0001-9889-7183

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

Khetpakorn Chakarawet − Department of Chemistry,

University of California, Berkeley, California 94720, United

States; orcid.org/0000-0001-5905-3578

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Jeﬀrey R. Long − Department of Chemistry and Department of

Chemical and Biomolecular Engineering, University of

California, Berkeley, California 94720, United States;

Materials Sciences Division, Lawrence Berkeley National

Laboratory, Berkeley, California 94720, United States;

orcid.org/0000-0002-5324-1321

Martin Head-Gordon − Department of Chemistry, University

of California, Berkeley, California 94720, United States;

Chemical Sciences Division, Lawrence Berkeley National

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