PCET mediators for electrocatalytic
reduction of small molecules

M2, Takeshi Inoue, 2024/7/26

Contentsn Basics of
1. Electrochemistry
2. Proton-coupled electron transfer
n Main1. Cobaltocene-Brønsted base
• Development of PCET mediator• N2 fixation
2. Ferrocene-Brønsted base-Anthracenen Summary2

Contentsn Basics of
1. Electrochemistry
2. Proton-coupled electron transfer
n Main1. Cobaltocene-Brønsted base
• Development of PCET mediator• N2 fixation
2. Ferrocene-Brønsted base-Anthracenen Summary3

1. Basics of electrochemical reaction
Power source

Reference electrodePower“Controlled potential”
or “Controlled current”

For accurate measurementof the potential at working
electrode. (optional)

benefits gained from the hybrid process explain the higher number of studies selecting thisonfiguration (Table 3).

3.2.

Electrolyte

Cathode= Reduction

Reactor configuration: divided versus undivided cells

Improves conductivity
undivided cell (Figure 6b). Considering the photo-electrochemical process implementation, theby providing positive
main influence of efficiency comes from the electrochemical reactions. In divided cell, theand negative ionse–dvantage is that there is the possibility to avoid decomposition of oxidants at anode or cathode

Anode= Oxidation
A

Electroden electrolysis, there are two possibilities of operation, either in divided cell (Figure 6a) or incations
which increases
the faradic yield (Esquivelanions
et al. 2009; Ramirez et al. 2015). However, someB
Aedox reactions cannot be involved in the cell such as ferrous/ferric cycle that is useful for theelectrolyte(nBu) N ,+X , ClO ,Fe2+ regeneration4(Eq. 3) in an electro-Fenton process4 for example. In contrast, this catalytice–Bbehaviour of iron
species can be obtained in an undivided cell (Sirés et al. 2007). This latter
BF4 , PF6
Li , Na
onfiguration has been the most studied in photo-electrochemical reactors (Table 3), especiallyor its easier implementation at industrial scale if a stirred tank reactor is foreseen.

n Divided Cell

Choice of material can affectreactivity/selectivity.
(e.g., RVC, graphite, Pt, Zn…)

The reaction of interest occurs
on “Working electrode”.
The other electrode; “Counterelectrode”→ Product, substrate will be protected from the undesired
redox reaction at counter electrode
Frit or permeable membrane

Baran, P. S. et al. Acc. Chem. Res. 2020, 53, 72. 4

1. Redox potential
n Redox potential(E0) = The potential required to induce electron transfer
X (V)
1.

Ox

he Journal of Organic ChemistryOx; X = Redution potential of Ox= Oxidation potential of Ox

Red; Y = Oxidation potential of Red= Reduction potential of Red

Y (V)

2. Redpubs.acs.org/joc

JOCSynopsis

Single electron transfer
Ox

+

Red

Ox

+

Red

• X > Y ⇔ SET is thermodynamically favorable.
• X < Y ⇔ SET is thermodynamically unfavorable.
[Redox potential of representative functional groups]

Stahl, S. S. et al. J. Org. Chem.
2021, 86, 15875.

51. Cyclic voltammetry
pubs.acs.org/joc

l of Organic Chemistryn Determination of E0

JOCSynopn Analysis of ET in the reaction

| Epa

Epc |

• When the oxidized form is lostby a chemical reaction, the CVshows irreversible peak.
【Redox potential(E0)】※ Throughout this presentation, all redox
potentials are reported versus Fc+/0.

𝑬𝒑𝒂 + 𝑬𝒑𝒄𝒓𝟎=𝑬
e 3. (A) Understanding
potential
𝟏/𝟐 ≈ 𝑬 as a driving force. (B) A guide to developing an electro
𝟐
Baran, P. S. et al. Acc. Chem. Res. 2020, 53, 72.

6
Stahl, S. S. et al. J. Org. Chem. 2021, 86, 15875.
ox potentials measured for organic compounds at an electrode typically correspondto single electron-transfer steps rather than net 2

e rational
can proceed via either the stepwise or concerted transfer of an5
uld direct
2. andProton-coupledtransfer
electron
proton (Figure 4A).electron
For MS-PCET
reactions,ate redox
n H+ and e– transfer pathway,21,22,39,47
s of bothrs and thegure 3. In
Ambiguity in thedefinition remains…

• ET/PT, PT/ET; The stepwise transfer of H+ and e–.
• Concerted PCET; Synergistic transfer of H+ and e– in single kinetic step.
(This concerted pathway called PCET in this presentation).

n PCET is common mechanism not only insynthetic chemistry, but also in biologicalsystem. (e.g., hydrogenases)
Knowles, R. R. et al. Chem. Rev. 2022, 122, 2017.
Happe, T. et al. Proc. Natl. Acad. Sci. U. S. A. 2020, 117, 20520.

7

2. Kinetics of PCET (Concerted PCET)n pubs.acs.org/JACS
Potential energy surface Perspective

Figure 4. (A) Square scheme representation of PCET mechanisms
available for the cleavage of an E−H bond. The stepwise transfer ofelectrons and protons are shown on the edges of the square. The• The PCET pathway has lower activationenergy by the movement ofconcerted transfer of the electron and proton via concerted PCET is+using pKa andE1/2 values vs Fc /Fc charge)
in MeCN. *Denotes
protons(positivein a direction
counteracts
that ofelectrons
shown thatalong thediagonal. (B) Reaction
coordinateof stepwise vs
xcited state complex as the oxidant / reductant.
concerted PCET mechanisms. Stepwise PCET generates high-energy(negative charge).
charged intermediates which are avoided in a concerted PCET
pathway.
→ Advantages in efficiency and selectivity(directionality)of hydrogenich proton-coupled electron transfer can proceed. The edges of the squareottom, respectively. The pathway bisecting the square is concerted, where e−
Notethat the donor andacceptor units
forandET andPT can be(bottom)
the same orMS3.
Representativeoxidative(top)
reductive
PCET,
each
with
a
distincttransition
state.
reagents pairs and the corresponding eﬀective BDFEs in kcal

atomtransfer.
ain body ofthis Review,
we give E°, pKa, and BDFE
provided by the original authors and from the availableure, as they pertain to the speciﬁc synthetic method
discussion.

the diﬀerent stepwise pathways can lead to drastically diﬀerent

Forchemoselectivities.
detailed physical explanation,pleaseto the followings;
For instance,
anreferoxidative
PT/ET mecha-

nism can predominate
adequately
strongis chosen
Schiffer, A.ifH.anAcc.
Chem. Res.
2001,base34, 273.
8
Hammarström,
L. et al. J. Am.
Chem. Soc.
2021, 143, 560of thesuchthat a substantial
equilibrium
concentration

2. Bond dissociation free energy

Journal of the American Chemical Society

pubs.acs.org/JA

n Relationship between BDFE and pKa & redox potential.
M· + H ·BDFE

E0 (M–/M·)+
E0 (H+/H·)

M– + H +M H
pKa (M-H)

BDFE (kcal/mol) = 1.37·pKa(M-H) + 23.06·E0(M–/M·) + 23.06·E0(H+/H·)= CG (Constant)
X H +Y∆G0

X + Y H;CG; Solvent-dependent thermodynamic
constant to account for the energy
required to form H· from H+ and e–∆G0 = BDFE(XH) – BDFE(YH)

Harrelson, J. A. et al. J. Am. Chem. Soc. 1988, 110, 1229.
Nicholas, A. . P., Arnold, D. R. Can. J. Chem. 1982, 60, 2165.

9

Abbreviations
PCET;(Concerted) Proton-Coupled Electron Transfer

BDFE; Bond Dissociation Free Energy
BDE;

Bond Dissociation Enthalpy

HER;

Hydrogen Evolution Rection

N2RR;

N2(Dinitrogen) Reduction Reaction

CO2RR; CO2 Reduction Reaction
CSS;

Charge Separation StateCR;

Charge Recombination

SET;

Single Electron Transfer···
10Contentsn Basics of
1. Electrochemistry
2. Proton-coupled electron transfer
n Main1. Cobaltocene-Brønsted base
• Development of PCET mediator• N2 fixation
2. Ferrocene-Brønsted base-Anthracenen Summary11Hydrofunctionalization of unsaturated bondn Hydrogen atom transfer to unsaturated bondH+ and e–

X
R

X

R

R

R

X = CR2, NR, O

1. Conventional H· generation
SmI2
ROH

I2Sm
O H
R

PhSiH3,
NaBH4
Metal

H

• Synthetically versatile neutral radical
intermediate.
• BDFE = 20~40 kcal/mol→ Highly reactive H· is required.

2. Electrochemical reduction

H+ source

H

X

MIII

H

e–

R

H2 ↑

R

X

R

R

(Mn, Fe, Co)• BDFEO–H = 27~35 kcal/mol✕ Stoichiometric reductant✕ Competing HER✕ Electrode passivation

12

Development of PCET mediator
H+

Med H

X
R

R

PCET
H

e–
Med

Possible advantages

R

X
R• Facilitate the colocalization of H+ and e– at relatively anodic potential.
• Prevent electrode passivation by avoiding the formation of reactive
radical intermediates at the electrode surface.

Single electron
reduction
CoIII– 1.33 V

CoII

Acid
CoII
H
H

Sub

H

Sub

• Peters and co-workers
previously found that Cp2Co
can act as redox mediator.

Peters, J. C. et al. ACS Cent. Sci. 2017, 3, 217.
Peters, J. C. et al. J. Am. Chem. Soc. 2019, 141, 4721

nship between pK and formal potential for PCET donors with different BDFE values in13

Cp2Co mediatorn Hydrogen atom transfer to acetophenone
Cp2Co (2 mol%)
TsOH; H+ source (2 eq)[TBA][PF6] (4 eq)

O

2

Me

Me OH

DME (50 mM),
Controled potential (–1.30 V vs Fc+/0)

HO Me

TM; 5% yield
HER; 66% FE

n Detection of undesired process (HER)• Increasing the [Cp2Co]+ with maintaining [Acid](Inset; plot of I vs [Cp2Co]+)→ HER is first order in [Cp2Co]+

Cp2CoIII/II = –1.33 V(Med + Acid)

Peters, J. C. et al. Science, 2020, 369, 850.

14

Trials with Cp2Co mediator
1 e– reduction
CoII

CoIII

– 1.33 V
Sub

Undesiredprocess

H2 ↑

H+

H

[4]
Desiredprocess[1] Slow
H+
Sub

Further 1 e– reductionCo
H
H[2] Fast

[3]

CoII
H
H

[1] Slow due to the high reorganization energy; sp2 → sp3.
[2] Fast due to CpH ligand; anodic shift of E0 of [3] by π-accepting diene ligand.
[3] Inefficient hydrogen atom transfer[4] due to low concentration of H· donor[3].

Peters, J. C. et al. Science, 2020, 369, 850.

15

Development of novel PCET mediatorn Aniline appended cobaltocene

Co

II

H
NMe2

X
R

R

e–
PCET
H

H+

III

Co

NMe2

R

X
R

1. Rigid aniline-Brønsted base motif accelerates the rate of protonation
by minimized reorganization.
2. Insulate the CoIII/II redox couple; the immediate coordination sphereat cobalt remains unchanged.

Peters, J. C. et al. Science, 2020, 369, 850.

16

Development of novel PCET mediatorn BDFE was dramatically decreased by reduction.
(redox-induced bond weakening).
Co

E0 and pKa were determined by CV
and NMR(MeCN-d3), respectively.

III

NMe2

n Variable scan rate CVs(Med ＋Acid)

BDFE =
79 kcal/mol

E0 = 0.52 V

pKa = 8.6
III

Co

III

NMe2

H
NMe2

BDFE =
39 kcal/mol

E0 = –1.35 V

CoCo

pKa = 11.0

II

NMe2

• High reversibility
Fig. S30. Variable scan rate CVsa solution containing
1 mM [(Cp)Co→ ofSlower
reactivity
ferrocenein H
DME with 0.1 M [TBA][PF6] in the presence of 100 mM acidII
Co
HER. across the enti
highly reversible
process is observed
NMe2 electron transfertowardindicating slow reactivity of this species toward HER.
Peters, J. C. et al. Science, 2020, 369, 850.

17

Model reaction using acetophenone
Mediator (2 mol%)
TsOH; H+ source (2 eq)[TBA][PF6] (4 eq)

O
Me

2

Me OH

DME (50 mM),
Controled potential (–1.30 V vs Fc+/0)

TfO
III

Co

HO Me

NMe2

[CpCoCpN]+

Entry

Mediator

TM
(%)

TM
(FE %)

rSM
(%)

HER
(FE %)

TONs

1

Cp2Co

5

3

0

66

3

2

-

10

47

0

-

-

3

[CpCoCpN]+

83

39

11

45

~40

[Entry 3]• PCET mediator kinetically attenuates the thermally favorable HER reaction.
• HER reaction likely begins to dominate at low ketone concentrations.
𝐹𝐸 = 𝑁!"#$%&' (𝑚𝑜𝑙) ÷𝑄 𝐶𝐹𝑎𝑟𝑎𝑑𝑎𝑦 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (𝐶 ⁄𝑚𝑜𝑙)Peters, J. C. et al. Science, 2020, 369, 850.

18

Thermodynamical and kinetical characteristics

RES EARCH | R E P O R T

n DFT calculation• DFT calculation supports the thermodynamic
preference of PCET pathway.
(Circumvent high energy intermediates in PT/ET
and ET/PT.)

Downloaded from https://www.science.org at University of To

n Dependence of PCET kinetics

PCET med., Substrate= 1st order dependence
Proton source= 0th orderFig. 3. Mechanistic details of electrocatalytic reductive CPET. (A) DFT
calculated DG (in kilocalories per mole) for initial ET, PT, and CPET from[(Cp)Co(CpNH)]+ to acetophenone. (B) CVs showing the increase in current
with increasing concentration of [(Cp)Co(CpN)]+, with an inset demonstrating(Substrate + Med + Acid)

S7). (D) Demonstration of the redox couple of [(Cp)Co(CpNH)]2+/+ (red) and[(Cp)Co(CpN)]+/0 (gray), and a CV of [(Cp)Co(CpNH)]2+ in the presence of
acetophenone (blue). (E) Comparison of CVs taken at 10 mV s−1 with
[4-CNPhNH3]+ (red) and [4-CNPhND3]+ (purple). The inset shows the plot used

19

Thermodynamical and kinetical characteristics
n Kinetic isotope effectreductive CPET. (A) DFT
al ET, PT, and CPET fromwing the increase in current+, with an inset demonstrating
entration. (C) Plot of thethe square root of thend [4-CNPhNH3]+ (red)nce, respectively (eqs. S5 ton Hammett plotby the reduction of the resulting [(Cp)Co(CpN)]+
product (Fig. 3D).
To establish the concerted nature of the
proton-electron transfer in the rate-determining
step, the kinetic isotope effect (KIE) of thecatalysis was determined by comparing thecatalytic current at different scan rates inexperiments using either [4-CNPhNH3]+ or[4-CNPhND3]+ (Fig. 3E). The observed KIE

Downloaded from https://www.science.org at Univ

se
ong
ts,
atET
in
3,
e-

S7). (D) Demonstration of the redox couple of [(Cp)Co(CpNH)]2+/+ (red) and[(Cp)Co(CpN)]+/0 (gray), and a CV of [(Cp)Co(CpNH)]2+ in the presence of
acetophenone (blue). (E) Comparison of CVs taken at 10 mV s−1 with
[4-CNPhNH3]+ (red) and [4-CNPhND3]+ (purple). The inset shows the plot used
to determine the corresponding rates for evaluation of the KIE, where ip0 is
the current intensity of the one electron reduction for [(Cp)Co(CpNH)]2+/+ and v isthe scan rate. R2, coefficient of determination. (F) Hammett plot for the CPET
reaction with 4-substituted acetophenones against the sp parameter.

Downloaded from https://www.science.org at University of Tokyo on June 18, 2024

• Large KIE; 4.9 ± 0.7 was observed.
→ Rate-determining PCET process• Modest negative slope
More electron rich substrate ⇔ faster PCET
More electron deficient substrate ⇔ slower PCET

R is CF3, Cl, H, Me, or OMe] was also studied.
A Hammett plot against sp (28) reveals a linear
relationship with a modest negative slope (r =−0.50 ± 0.04; Fig. 3F). This observation suggests a slight buildup of positive charge on theacetophenone unit in the transition state (TS)and may be consistent with an asynchronous
CPET reaction in which the TS involves slightly
more proton-transfer than electron-transfer

This PCET process(TS) involves more protontransfer than electron transfer character.
20

Contentsn Basics of
1. Electrochemistry
2. Proton-coupled electron transfer
n Main1. Cobaltocene-Brønsted base
• Development of PCET mediator• N2 fixation
2. Ferrocene-Brønsted base-Anthracenen Summary21

Introduction of N2 fixation

Carl BoschThe Novel Prize in Chemistry 1931

n Haber-Bosch process
N2

+

3H2

cat. Fe3O2/K2O/Al2O3
200 ~ 500 atom, 500 ~ 600 °C

2NH3Fritz Haber
The Novel Prize in Chemistry 1918

• This reaction enables continuous population growth through the
production of fertilizers.
• Very high temperatures and pressures are required. (2 % of global
energy consumption)n Alternative methods have been reported using transition metal catalysts.
e.g.)
N2

+

6SmI2

+

(1 atom)

6ROH

Mo cat.
THF, r.t., 18 h

2NH3

+

6SmI2(OR)

+

H2

···

Peters, J. C. et al. Chem. Rev. 2020, 120, 5582.
Nishibayashi, Y. et al. Nature. 2019, 568, 536.
Hetterscheid, D. G. H. et al. Chem. Soc. Rev. 2014, 43, 5183

22

Electrocatalytic N2 fixation (N2RR)
Article

Power

a

b

Representative N2RR catalytic cycle
NH3

N2
[M]

H•

H•

VI

I

NH

NH2

N

[M]

[M]

e–

Cathode= Reduction

Anode= Oxidation
H

H•

Distal mechanism
for N2RR

V

e–

H•

NH2

NH

N2RR

N

[M]

IV
H•

H+ source

II

N

III
H•

[M]

[M]

NH3

e–
electrolytedVThis work
PCETN2-selectivity✕ Electrocatalytic reduction in acidic conditions suffers frommediatorover hydrogen evolution reaction(HER).

; HER is thermodynamically and often kinetically dominant. (BDFEH–H=104 kcal/mol)
Molecular

Electro-Peters,NJ.
C. et al. Nature,2022, 609, 71.
catalysis
2RR

23

icle

Electrochemical N2 reduction reactionb
Representative N2RR catalytic cycleb Demonstration of electrocatalytic N2RR
by an iron complex
N2
of electrocatalytic N RR
bNH3 Demonstrationn Precedent - 1

H•

H•

VI

NH

I

N2

M]

iPr2P
e– mechanism
Distalfor N2RR

V

2

B
IV

III

N

H•

PiPr2

BFe

NH2

P
Ph2

PiPr2

B
PhW
2
P

N2

N2

Ph2
P

Via
N2RR
cycle

W

Via
N2RR
cycle

2

E = –2.1 V

2

+

P
Ph2

2

2

+
CoII

(HOTs)(HOTs)

P
Ph2

W

OTs

P
Ph2

(TsO)W(NNH2)

2

CoII

E = –1.2 V

NMe2

N2RR
H

H+

Co(II,NH) + (HOTs)

2

[M]

PCET
+

Co(III,N)+

H•
NMe2

H

N2RR
NH3

N2PCET
RR

HigherthermodynamicH•efficiency

Co(III,N)+

N2[M]

Higherthermodynamic
efficiency

Higher
M = W, Mo, Os, Fe
thermodynamic
efficiency

M = W, Mo, Os,
Fe
CoIII

[M]

✕ Highly reducing
potetntial (– 2.6 V) inNHtheabsence of acid.
3

NMe
NH
2 3

N2 +

+
CoIII

= –2.6
E =E–1.2
V V

Co(II,NH) +N2

H•

E = –2.6 V

Step 2

H

e–
NMe

PCET

e–

H+

Ph2
P

E = –2.6 V 3

0.21 equiv. NH3 +Co(II,NH) +Electrocatalysis

NW

2

0.21 equiv. NH3

CoII

H+

OTs
W(N
2)2

Ph2
P

Ph2
P

+
Via
N2RR
cycle

Tandem PCET for N2RR electrocatalysis

Tandem PCET for N2RR electrocatalysis

Molecular
N2RR

P

22

V
–1.2 Vwork+
nV ThisThis
work; Tandem
PCET forE = N
2RR.

e–

WPh
N2 2

P
(TsO)W(NNH
)
Ph2 2

2

Tandem PCET for N2RR electrocatalysis
E = –2.1 V

V

N
W

Ph2
P
PhNH
2 2
P
PN
Ph2
P
Ph2

+

NH2

+

2

H+
(Ph2NH2+)

PCET
mediator

N2
NH
2

P
✕ Requires
N low
N
Hg-electrode/no
acid
Ph
OTs
N
Hg-electrode/no
acid
+
temp.
(–35
°C)
to
Step 2
HW(N )(TsO)W(NNH )Step 2(Ph NH )E = –2.1 V
suppress HER.
N
Hg-electrode/no acid0.21 equiv. NH
2–35 ºC
[M]
H•

P
Ph2

Ph2
P
Ph2
P
P
Ph2
P
Ph2

2 HOTs

PiPr2
Ph2
PPiPr2

P–35 ºC
W(NPh 2)2NH+–35 ºC+(Ph2NHNH
2 )3

[M]

N2
Fe

iPr
Ph22P
P

e–

2 HOTs2 HOTsStep 1

N2

II PiPrH•

NH

M]

[M]

Step 1

NH
Step
13

NH3

N PiPr2
Fe 2

iPr2P

e–

Demonstration of electrolytic NH3 generation
by a tungsten complex

NH3

N

NH2

n Precedent - 2

c2

by an iron complex[M]

Demonstration of electrolytic NH3 generation
Demonstration of electrocatalytic N2RRcby a tungsten complex
by
an
iron
complex
DemonstrationofelectrolyticNH
generationc3
by a tungsten complex

NMe2

N2

N2

M = W, Mo, Os, Fe

protonation step had to be performed separately from the reduction step
Approaches to N2RR electrocatalysis. a, A representative distal cycle
CoIII reported
protonation
step
had to beNperformed
separately
the reduction
step. a, A representative
distal cycleb, A previously
molecular
RR
W(N2)2 after
one cycle; 0.73 equiv. total NH3 per W(N2)2(0.21 equiv.fromNH3 peralyticN2-to-NH3 conversion.
2
NMePeters,J. C.
etbasedal. Nature,2022, 609, 71.
2 2)2 after
usly reported
N2RR
W(Nequiv.
total
per
W(N
(0.21 equiv.
NH3 perNocatalyst
basedmolecularon the tris(phosphine)borane
Fe system,
operatingat one cycle;
after0.73
threecycles).
d,NHTandemcatalysis
described
in this
work
on
3
2)2
2
1); Peters, J. C.
+
et
al.
J.
Am.
Chem.
Soc.
2018,
140, 6122.
boraneFesystem,operating
at
after
three
cycles).
d,Tandemcatalysis
described
in
this
work
based
on
+
on a glassy carbon electrode using a temperature
coupling the PCET mediator, Co(II,NH) , with molecular N2RR catalysts
Co(III,N)of −35 °C. c, Early work+2)
, withmolecular
N2RRelectrocatalysis
catalysts
mperatureof −35 °C.electrosynthesisc, Early work
coupling
thethePCET
mediator,
Co(II,NH)to; Pickett,atC.
J. & Talarmin,
J. Nature. 1985, 317, 652.
molecular
complexenablewell-definedcomparatively
mild potentials
ett demonstratingof
NH3 using24

Result of N2RR with PCET mediator
Co(III, N)+ (1 eq)
W(N2)2 (1 eq)
TsOH (100 eq)

N2

DME (cat.; 0.05 mM), r.t.,
Controlled potential (V = – 1.35 V), 5.5 h

1 atomaN2+

j (mA cm–2)

6 e–

+

BDD
(100 equiv.)
electrode
TsOH(1 atm)b

6 H+

Co(III,N)+ (0.05 mM)
W(N2)2 (0.05 mM)
DME–1.35 V versus Fc+/0

n Current profile
0.2

c

+

Co(III,N) + W(N2)2W(N2)2

•

III

Co

NMe2

Co(III, N)+

The identity of the latter species was confirmed by

NH3; 11.3 ± 0.4generationequiv. via the protonation of the nitride pre
44.5± 1.9%FE (Supplementary Fig. 56)22–24. Although ot
with TsOH
2 NH
3presumably operating in this reaction (for example
HER;
11.3
± 0.539%
equiv. FE

transfer (PT) steps to other W−NxHy intermediates), t
well with the CV experiment noted above, sugges(TsO)W(NNH2)+ occurs, presumably followed by N
14N
NH3 release (equation (1) below depicts one plausible
2
15N
2
PCET step helps to explain why the system can be tur
Ar
whereas one-electron reduction of (TsO)W(NNH2)+ reBoth catalysts are required.
of −1.9 V.
44.5 ± 1.9 % FE

+

+

+

(TsO)W(NNH2) + Co(II, NH) → (TsO)W(N) + Co(• No current was observedin the absence of mediator.
→ Uncoupled ET/PT Related
is nottofacileat this potential.
this, CV of (TsO)W(NH)+ (Fig. 3d), a presu

0.1

0

2NH3

0

2

t (h)

4

6

7.4 7.3 7.2 7.1 7.0 6.9 6.8 6.7
1H NMR δ (ppm)Fig. 2 | Electrocatalytic N2RR via tandem catalysis. a, Electrocatalytic N2RR
on CPC at −1.35 V versus Fc+/0 in 0.1 M [Li][NTf 2] DME solution containing+

intermediate of the catalytic cycle generated by inof the nitride complex (N3)W(N), shows a similar
behaviour in the presence of Co(III,N)+. Additional(N3)W(N) with 4 equiv. Co(II,N) in the presence of exce
0.70 equiv. NH3 per W, along with (TsO)W(NNH2)+ a
detected by 31P NMR spectroscopy (Supplementar
Peters, J. C. et al. Nature, 2022, 609, 71.
25
PCET from Co(II,NH)+ to (TsO)W(NH)+ is probabl

ArticleaMechanistic insightsb

Representative N2RR catalytic cycle

Demonstration of electrocatalytic N2RR
by an iron complex

n Intermediate (TfO)W(NNH2)+
[M]

H•

VI

H•

I

NH2

Article[M]
Article

N

NH
[M]

IV

[M]

H•

d

This work

NH

N
PCET N[M]
H• mediator
[M]
IV

NH2
NH2
N
N
III
[M]
III
[M]
H•
H•
NH3
NH3

d
Molecular
ElectroN2RRThis work catalysis
This workn Detection by NMR

PCET
PCET
mediator
mediator

H•
II

H•

Ph2
P

N2

+

NH2
Ph2
P

Ph2 N
Ph2
P
P
Via
PiPr2
W
Co
N2RR
Demonstration of electrolytic
NH3 generation W
b Demonstration of electrocatalytic N2RR
c
NMe
2
B
P
Demonstrationof cycleelectrolytic
NH3 generation
by a tungsten
complexof electrocatalytic
N2RR
by an ironcomplexb Demonstration
cPPh
P
P
Ph2
2
by a tungsten complex Ph2
N2
by an iron complex
Ph2
OTs
+
Co(III, N)
Step
1
W(N2)2(TsO)W(NNH2) )+NH3(TsO)W(NNH
Step 1
2–35 ºCNH3
2 HOTsN2
Hg-electrode/no acid
+
2
HOTs
NH2
N2
+
NH
N2
Step 2H2H+N2Ph2
N2 Ph2N(Ph2NH2+)
PiPr2
Ph2
Ph2
P Ph2
P Ph2
E = –2.1 V
iPr2P Fee–P Ph2 E =N–2.6
P VPh2
PiPr
Via
H
0.21
equiv. III
NHP3
P W
iPr2P
Fe PiPr2 2
e–
P
P
RR
N
Via
W
2
Co
PiPr2
W P
cycle
N2RR2
B
P
NMe
W
P
P
Ph2P
Ph2P
Tandem PCET for NB2RR electrocatalysis
cycle
N
Ph2P
Ph2P
Ph2 2
Ph2
OTs
N2 E = –1.2
Ph2
Ph2
2+
V
OTs
Co(III,
NH)+
W(N2)2(TsO)W(NNH2) +(TsO)W(NH)V)
W(N–35 ºC
22(TsO)W(NNH2) )+(TsO)W(NNH2H–35 ºCN2
Hg-electrode/no acidNH
CoII
3
N2
Hg-electrode/no acid
NMe2
Step 2H+
N2RR+
Step 2(Ph2NHH2+)
E = –2.1 V
E = –2.6 V+(Ph2NH2+)
Co(II,NH)
0.21 equiv. NH3N2
E = –2.1 V
E = –2.6 V
Higher
0.21 equiv. NH3
thermodynamic–e PCET for N RR electrocatalysis
Tandem2
efficiency
PCET
H•
[M]
PCET for N2RR electrocatalysis+
HTandem
E = –1.2 V+
E = –1.2 V
V (HOTs)+ +V
M = W, Mo, Os, Fe
H
NH3
CoII
H
NH3
CoIIICoII
NMe2
N2RR
NMe
NMe
2 2
N
N RR
2
e–

II

2 HOTsN2[M]

Distal mechanism
Distalfor N2mechanism
RR
for N2RR

Step 1NH3

NH

H•
a Representative
DistalNmechanism
2RR catalytic cycle II
for NN2RR
H• aV Representative
2RR catalytic cycle
N2
NH3
N2
NH3
NH2
NH
[M]
H•
H•
NH
[M]
N
H•
[M]
H•
VI
I
NH
IV VI N
III I
N
NH2
[M]
NH2H•
H• [M] N
[M]
[M]
[M]
[M]
NH3

V

Demonstration of electrolytic NH3 generation
by a tungsten complex

N2

NH3

dH• H•V

c

iPr2P

Fe

PiPr2

e– distal cycle
Fig. 1 | Approaches to N2RR electrocatalysis. a, A representative
e–
+
N2RR
for catalytic N2-to-NH3 conversion. b, A previously reported molecular
H+
H
Molecular
Electroelectrocatalyst
based
on
the
tris(phosphine)boraneFesystem,operatingatElectro(HOTs)
NMolecularcatalysis
2RR−2.1 V on a glassy
carbon electrodeusing a temperature of −35(HOTs)°C. c, Early work
N2RR
catalysis
by Pickett demonstrating electrosynthesis of NH3 using the molecular complex
W(N2)2 at an applied potential of −2.6 V on a Hg-pool electrode using TsOH. The

by Pickett demonstrating electrosynthesis of NH3 using the molecular complex

2+
Co(II,NH)
Co(III,N)+
Co(II,NH) +N2

HigherHigherN2

thermodynamic
protonation step had to be performed separately from thereduction step
thermodynamic
efficiency
PCET
H•
[M]
)
after
one
cycle;
0.73
equiv.
total
NH(0.21PCET
equiv. NH3 per W(Nefficiency
3 per W(N2)2
H•2 2
[M]
after three cycles). d, Tandem catalysis described in this work based on+++ the PCET mediator, M, withcoupling
Co(II,NH)= W, Mo, Os,
Fe molecular N2RR catalysts
M = W, Mo, Os, Fe
toIIIenable well-defined electrocatalysis at comparatively mild potentials
Co III
Co V using TsOH).
(−1.2
NMe
2
NMe
2

Co(III,N)+ +Co(III,N)
4 A representative distal cycle
Fig. 1example,| Approaches
to N2RRM=NNH
electrocatalysis.
a,(for
M−N=NH,a mediator thatinter2, M=NH) , with
Fig. 1 | Approaches
to N2RR electrocatalysis.
a, A representative
distalcycle-to-NHconversion.
b,
Apreviously
reported
molecular
N2RR
for
catalytic
N
2
3
faces
the electrode
and
the
acid
with
the
N
reduction
cycle
via
PCET
2
conversion.
b,
Apreviously
reported
molecularN
for catalytic
N2-to-NH
3
2RR
electrocatalyst based on the tris(phosphine)borane Fe system, operating atsteps.
Importantly,
certainNtris(phosphine)borane
to moveelectrocatalyst
based
on theFechallenging
system, operatingat
2RR intermediates are−2.1 V on a glassy carbon electrode using a temperature of −35 °C. c, Early workthroughthecycle;
theyelectrodecan be difficult
to independentlyor−2.1 V ona glassy
carbonusing a temperatureof −35 °C.reducec, Early work

III

N2

N2

protonation
step had(DME)
to be performed
separately
fromthe reduction
step
dimethoxyethaneandlithiumbistriflimide([Li][NTfprotonation step had
to besolvent
performed
separatelyfrom the reduction
step 2])
per
W(N
)
after
one
cycle;
0.73
equiv.
total
NH
per
W(N
(0.21
equiv.
NH
3
2
2
3
2)2
electrolyte.
conditions
(BDD,
0.1
M
[Li][NTf],
W(N2)optimized
after
one
cycle;
0.73
equiv.
total
NH
per
W(N
(0.21 equiv.Under
NH3 perthese
2
3
2)2 2
after three cycles). d, Tandem catalysis described in thiswork based on+DME,
mM cycles).
TsOH, d,0.05
mM catalysis
W(N2)+ 2/Co(III,N))N
electrocatalysis
after5three
Tandemdescribed in
this
work
based on
2RRPeters,J. C.
et al.Co(II,NH)
Nature,
2022,
609,
71.
, withmolecular
N2RR
catalysts
coupling
the PCET
mediator,+
notably
improved,with 11.3 ±0.5 equiv.
NHmolecular(44.5 ± 1.9%
, withN22)RR
catalysts
coupling
the PCET mediator,
Co(II,NH)
3 per W(N2/Co
to enable well-defined electrocatalysis at comparatively mild potentials

26

b

31P NMR δ (ppm)
E versus Fc+/0 (V)Studies of insights(TsO)W(NH)
MechanisticcKinetic
analysis
Studiesof (TsO)W(NH)++dnd Intermediate (TsO)W(NH)edKinetic analysis

35

i (μA)

2

2.1

–2.5

i (μA)H•

25

i (μA)]–0.6PCET for N2RR–1.0
Tandemelectrocatalysis –1.4
Tandem PCET for N2RR electrocatalysis+/0

i (μA)
icat (μA)

H

15(N )W(N) + TsOH =Co(III,N) + (N )W(N) + TsOH(N )W(N) + 1 equiv. TsOH(N )W(N)In situ generationof (TsO)W(NH)–1.8 544(V)

Generalapproach
Kinetic
analysis+

NH2

Demonstration of electrolytic NH3 generation 2 equiv. Co(II,N)
Demonstrationof electrocatalytic N2RR
c
3
Demonstrationof electrolytic
NH3 generation
by a tungsten
complex
Demonstrationof electrocatalytic
N2RR
by an ironcomplexc2+ + (TsO)W(NNH )+N complex
10 equiv. TsOHby
Co(III,NH)
by an ironcomplex
Pha2 tungsten
Ph2
2
P
P
40
+
35
Step
Co(III,N)
351
NH3 (+ H2)
NH3
W 1
Step
2+ + TsOH
(N
)W(N)
THF
Co(III,NH)
NH
2 HOTs(NCo(III,N)+ TsOH3233)W(N) +39%
P+4 h
2 HOTs P3
NH2 + 0 ºC,
Co(III,N)+ + (N3)W(N) + TsOH
N2
Ph2 OTs Ph2 Co(III,N)++(N3)W(N)+ TsOH+
W(N2)NH
N2
H
N2
2
2
3
(N )W(N) + 1 equiv. TsOH
Ph2
N2 Ph2
N
25
(N
)W(N)
+Ph12 equiv. TsOH
PiPr23
Ph3
25 30
2
P Ph2
P Ph2
iPr
P
Fe
–
N
2
e
P Ph2
P Ph2
Via
(NPiPr
2
3 (N
P W
P
iPr2P
Fe PiPr32 )W(N)
e– 1
P
N2RR
Via
3P)W(N)
W
PiPr2
W
cycle
N2RR
B
P
P
+
W P
3
)
(TsO)W(NNH
P
Ph
Ph
cycle
B
2P N
2P
Ph22P
Ph2P
Ph2 2
Ph2
OTs
(TsO)W(NH)+
N2
Ph2
Ph2
20
15
OTs
15
W(N2)2+(TsO)W(NNH2)(TsO)W(NH)
W(N2)2–35 ºC(TsO)W(NNH2) )+(TsO)W(NNH02–35 ºCN2
Hg-electrode/no acidN2
Hg-electrode/no acid+
Step 2
H
eN2RR
eN2RR
Step 10
HER
H++
(Ph5
HER +
5 2
2NH2 )
E=–2.1
V
E = –2.6 V(Ph2NH2+)
0.21 equiv. NH3
E = –2.1 V
E = –2.6 V–1
0.21 equiv. NH3+b2) + TsOH

H•

Studies of (TsO)W(NH)+E versus Fc

(V)

+/0
E Eversus
versusFcFc+/0(V)(V)eN42
2RR40 38 HER
36 34 32 30 28 26

e ef
40 40
35

30

20

30
25

Background++
Co(III,N)(PPh
Co(III,N)
2Me)4Mo(N2)2(2)W(N2)W(N22)2

W(N2)2(P3Si)Os(N2) (5)(P3B)Fe(N2) (6)

15

icat

b

E versus Fc

icat (μA)
i (μA)(μA)

E versus Fc+/0 (V)Studies of (TsO)W(NNH2)+

20

10 5

10

P NMR δ (ppm)
0 –5• –1.0Consideringthat
thereduction
potential
–0.5
–0.7
–0.9
–1.1
–1.3
1 –1.5
2
3
–2.0
–2.5
0
1
2
Kinetic
analysis
GeneralapproachE versusFc (V)[Conc.](mM)
E versusof EW-catalysts
are < – 1.8 V, PCET
is(mM)Fc (V)versusFc(V)[Conc.]
0–5
N RR
f of 50 mMthe
N RRa, CV
Fig.e3 | Mechanistic insights into tandem PCET NFig.
RR.
TsOH–1.5
t
50
mM
TsOH
(purple).
of thecatalytic current
key
for
efficienttransferat
3 | Mechanistic
intowith
tandemPCET
N –2.5
RR.electrona, CVe,ofPlot
50 mM
TsOH
with
TsOH2(p–0.5–1.0 insights–2.00
1 50(imM) versus
40 0.5 mM Co(III,N) with 50 mM TsOH(dashed(black);grey);
0.5 mM(dashed grey);
concentrationof thedifferent
co-catalysts. f, CVs ofseveral represenwith
50 mM TsOH
(black);0.5 mM
0.5 mM
Co(III,N)
concentrationof th+/0 (V)–versus
1.2
V.
FcM(N
[Conc.](mM)
Background(dashed) catalysts (see Fig. 4a for their chemical structures)studiedund(TsO)W(NNH ) (solid red); 0.5 mM (TsO)W(NNH ) with 50 mM TsOH E
V

–5
–0.5

V

E versus Fc (V)–1.0 + +–1.5
–2.0
CoII
CoII

E = –1.2 V
E = –1.2 V
–2.5

H
+/0
H
NMe2
NMe2

31

NH3
NH3

2

2

Co(II,NH) + +Co(II,NH) +

e–

–5 0
–0.5 0

2

N2

+

+

+/0

+/0

2

HigherHigherthermodynamic

N2

cat

+

+
+
2
thermodynamicred); 0.5 mM (TsO)W(NNH(TsO)W(NNH
M(N2) catalysts (seeefficiency+
352) (solidPCET2) with 50 mM TsOH (dashedH•[M]
Co(III,N)
+
+
+ H•
+
efficiency
PCET
H
(PPh
Me)
Mo(N
)
(2)
[M]
+at
+2 2 electrocatalytic conditions in the presence of Co(III,N) andTsOH(purple).
b,CVred); 0.5HmM
Co(III,N) /(TsO)W(NNH2) with 50 mM
standard
OH24+red); 0.5 mM Co(III,N) /(TsO)W(NNH2) with 50 mM TsOH (purple). b, CV atstandard electroca
W(N2)2Fig.
3|
Mechanistic
insights
intotandem
PCET
N
RR.
a,
CV
of
50
mM
TsOH
with
50
m
−1
2+
+
2
)2 showing a multi-electron(HOTs)+
of30
0.5 mM Co(III,NH)/(TsO)W(NNH2) (purple
trace)
comparedtoW(N22+each caseat the sa
5 mVs(HOTs)catalytic
wavetofor N2RR in
mM Co(III,NH)/(TsO)W(NNH2)+ (purple trace)comparedmVOs,
s−1Feof 0.5
showinga multi-ele+
M = W,5Mo,+
Si(P3 )Os(N(5)
M =grey);
W,+Mo, Os,0.5
Fe
with
50 mM
mM
TsOH
(black);0.5thatmM(dashedmMc,Co(III,N)
concentr
2++
Co(III,N)
trace).
ChemicalNote
all CVs in a–f were
performed
at 10
0.5 mM Co(III,NH)2+ (redIIItrace) and 0.5 mM
appliedbias
as for W(N2) 0.52)2 .trace).
25(red trace)andCo(III,N)(blue
c, Chemical
0.5(bluemM Co(III,NH)
appliedbias as forW
Co III
B
+
+
+
(P
)Fe(N
)
(6)
Co
+
)
in
THF
with
2
equiv.
Co(II,N)
in
THF
in
the
]
THF
solution
using
a
BDD
reaction of 0.5 mM (TsO)W(NNH(unlessotherwise
stated)in
0.1
M[Li][NTf
3 mM 2)(solidred);
0.5(TsO)W(NNH
)
with
50
mM
TsOH
(dashed)ca
M(N2 NMe2 (TsO)W(NNH
2
2
2
2
)
in
THF
with
2
equiv.
Co(II,N)
in
THF
in
the
reactionof
0.5mM(TsO)W(NNH(unlessotherwise
s
N
2
2
NMe2
N
31
+ CV2 of+31
P presence
NMR
spectrum.
d,presence of excess acid andthethe)working
electrode,Pt diskas thecounterandstandard
Ag/AgOTf+ corresponding
Co(III,N)
P NMR
spectrum.
d, CV
of CVelectrodeofexcessacid
andthecorresponding
electro/(TsO)W(NNH
TsOH(purple).
b,at the working
red);
0.5
mM
Co(III,N)20+
2 with 50 mM
Co(III,N))W(N)(black);
0.5
mM
(N
)W(N)
and
1
equiv.
TsOH(solidred);
RR,electrocatalyticNRR.
0.5 mM (N3asthereferenceelectrode.
eN
15
3
2
2
−1 0.5 mM (N3)W(N)2++(black);
0.5
mM
(N
)W(N)
and
1
equiv.
TsOH(solidred);asthereferenceele
3
protonation step had5
tomV
be performed
separately
the reduction/(TsO)W(NNH
stepsis. a, A representative distal cycleof 0.5mM from
Co(III,NH)
sperformed
showing2) (purple trace) compared to+ the reduction step
protonation
step
had to be
separately
fromalysis. a, A representative
distal cycle
+
)W(N)
with
50
mM
TsOH
(dashed
red);
0.5
mM
Co(III,N)
/(N
)W(N)
0.5
mM
(N
3
3
viously reported molecular N2RR
cycle;
0.73
NH350
permM
W(N2TsOH)2(0.21 equiv. NH3 per W(N2)2 after one
with(dashed red); 0.5 mM
Co(III,N) /(N3)W(N)
0.5mM(Nequiv.
2+ total+3)W(N)
reviously reported molecular N RR
equiv.
total NH
per W(N)(0.21 equiv. NH per W(N ) after one cycle; 0.73
22

• Positive order for both co-catalysts.
→ Two rate-contributing PCET steps andand 0.5 mM Co(III,N)protonation(blue trace). c, Chemical
0.5 mM Co(III,NH) (redatrace)
rate-contributing
step.
after three cycles). d, Tandem catalysis described in this work
basedonafter three cycles). d, Tandem catalysis described in this work based oni (μA)

icat (μA)

e–

2

3

2 2

3

2 2

applied be)borane Fe system, operating athine)borane Fe system, 10
operating at, withmolecular
N2RR catalysts )+ in THF with 2 equiv. Co(II,N) in THF in theemperature of −35 °C. c, Early work
coupling the PCET mediator,
Co(II,NH)
5(TsO)W(NNH+of +0.5mM(unless o, withmolecular
N2RR catalysts2
a temperature of −35 °C. c, Early work
coupling the PCET reaction
mediator, Co(II,NH)
to enable well-defined electrocatalysis at comparatively mild potentials
of NH3 using the molecular complex
31
to enable well-definedelectrocatalysis
atcomparatively
mildpotentials
sis of NH3 using the molecular complex
P NMR
spectrum.
d, CVofpresence of (SiP
excessacid
and the corresponding
the work
Hg-pool electrodeusingtris(phosphine)silyl
TsOH. The(−1.2 V using TsOH).
related14
equiv.complex,
NH3 (51
andFE for NH
3)Fe(N2), is instead3), respectively.
n a Hg-poolA
electrodeusing TsOH. The(−1.2 V usingiron–N
TsOH). 2 complex,
A relatedtris(phosphine)silyl
iron–N(SiP55%14 equiv.The
NHdinu23)Fe(N2), is instead
3 (51

)W(N)
(black);0.5ofmM
and 1 equiv.
TsOH (solid
red);
0.5 mMdue(N3toasthesethe ref3)W(N) systemelectrocatalyticallyinactive, presumably
the generationan (Ncatalyst
(3)to
also
electrocatalysis
under
–5
0
electrocatalyticallyinactive,
presumably
duethedisplays
generationof an catalyst
system co(3+
7 2 0.5 mM (N
6Co(III,N) /(N )W(N)–0.550 –0.7
–0.9 (8.7
–1.1
–1.3
–1.5
)W(N)
with
mM
TsOH
(dashed
red);
0.5
mM
–2.5undesired 01
3
7 equiv.
(SiP)Fe(H)(N
)
state
.
NH
per
Mo)
.
By
contrast,
themononuclear
triiod334323
undesired(SiP.
(8.7 equiv. NH3 peNH) , witha mediatorthatinter(DME)
solventandlithiumbistriflimide([Li][NTf43)Fe(H)(N2) state2]) ])
M=NH), witha mediatorthatinter- dimethoxyethane
dimethoxyethane(DME)
solventandlithiumbistriflimide([Li][NTf2+/0Peters,J.has
C.beenet al.demonstrated
Nature, 2022,to609,
71. active
plex
(4),
which
be
highly[Conc.](mM)
E
versus
Fc
(V)
27 fo
the
N
reduction
cycle
via
PCET
electrolyte.
Under
these
optimized
conditions
(BDD,
0.1
M
[Li][NTf],
plex(4), whichh2− Under these− optimized conditions (BDD, 0.1 M [Li][NTfth the2N2 reduction cycle via PCET Melectrolyte.
− 2],
−
−
N
+
e
→
M
−
N
(−2
V)
+
(2)
2
M − N + e → M − N (−2 V)(2)mediates are challenging to move DME, 52mM TsOH, 0.05 mM
W(N ) /Co(III,N) ) N+ RR electrocatalysis

44

8

42

40

38

36

34

32

30

28

Si

26

Other metal catalysts

31P NMR δ (ppm)

General approachnf CV with representative catalysts
Background(PPh2Me)4Mo(N2)2(2)W(N2)2(P3Si)Os(N2) (5)(P3B)Fe(N2) (6)35i (μA)

25+ Co(III, N)+,
TsOH

I

4 (P = tBu2P)

5n Results of N2RRbc[Co] (0.05 mM)[Cat] (0.05 mM)
N2 + HOTs

NH3(1 atm) (100 equiv.)

DME
BDD(–)/Zn(+)–1.35 V versus Fc+/0

15

5

3

P

–5
–0.5

–0.7

–0.9

–1.1

–1.3

–1.5

E versus Fc+/0 (V)aMolecular
N2RR thecatalysts
purple).
e, Plot of the catalytic current(i ) versus
cat

he different co-catalysts.
f, CVs of several
P
N2 representative
N2
N2
Ph2 chemical
Ph2
PN2
e Fig. 4a for theirstructures)studied
under
the
PMePh2Ph2MePPP+Mo
Mo in the presence of Co(III,N) and TsOH,N Mo
N N
Mo N
talytic conditions
PMePh2Ph2MeP
P
P
RR
in
each
case
at
the
same
ectron catalytic
wave
for
N
N2
P
Ph2
Ph2
2
N2
N2
N
P
W(N2)2. Note that all CVs in a–f were performed at 100 mV s−1 2tated) in 0.1 M [Li][NTf 2] THF solution using a BDD disk as
1
2
3 (P = tBu2P)
ode, Pt disk as the counter electrode and Ag/AgOTf (5 mM)
N2
ctrode. eN2RR, electrocatalytic N2RR.N2

Catalyst

Equiv. NH3/Cat

FE (%)

W(N2)2

11.3 ± 0.5

44.5 ± 1.9

W(N2)2*

40

43

1

13

51

2

14

55

3

8.7

34

4

<0.1

<1.0

5

5.6

22

6

4.5

18

7

1.5

4.5

PiPr 2Fig.
4 | Electrocatalytic
N2RR using reported molecular catalysts.
iPr2P Fe
PiPr 2PiPra, Molecular
N22RR catalysts explored in combination with the PCET mediator
N Mo I• Broad applicability.
SiE
under electrocatalytic conditions. b, Results of the electrocatalyticPI 3), respectively. The dinuclear Mo
and 55% FE for NH
experiments for each molecular catalyst on CPC at −1.35 V versus Fc+/0 in+
3) also displays electrocatalysis
under these conditions
0.16M(E[Li][NTf2] DME solution containing 0.05 mM Co(III,N) , 0.05 mM of N2RR
4 (P = tBu2P)
5= B)
er Mo)6. By contrast, the mononuclear triiodide com7 (E = Si)
catalyst
and 5 mM TsOH, using a BDD plate working electrode. Average result31
as been demonstrated to be highly active for N2RR ,
I

P

iPr2P

Os

PiPr 2

for W
elect
and t
c, Est
repo
sourc
28

4 (P = tBu2P)

catalyst
and 5 mM TsOH, using
5
6 (E = B)a BDD plate working electrode. A

Short summaryb

7 (E = Si)c[Co] (0.05 mM)[Cat] (0.05 mM)

n PCET mediator for N2 fixation
N + HOTs

OverpotentialnMinimized
overpotential
NH3
acid cocktails
usedwith synthetic
N2RR catalyst syst
−1
DME SmI /H O (75 kcal mol ), Cp* Co/[Ph NH ]+ (77 kcal mol(1 atm) (100 equiv.)
2
2
2 +
2
2
CoCp2/[LutH]
BDD(–)/Zn(+)−1
F
7,27,30. A crud–1.35 V versus (196
Fc+/0 kcal mol ) (BAr 4, B(3,5-(CF3)2-C6H3)4)
PCET mediatorwith the biological nitrogenases(approximately 117 kcal
32
Equiv. NH3/Cat ing
FEfor(%) ATP) is also favourable . Likewise, heterogeneous
Catalyst
SmI2/H2O
+
+/0
on
Li
/Li
(E°(Li
)
<
−3.7
V),
which
commonly utilize etha
W(N2)2
11.3 ± 0.5
44.5 ± 1.9
+
−1
CoCp*
2/[Ph)2NH
2]
operateatanestimated
∆∆G
(NH
=
133
kcal
mol
(re
II/III W(N ) H
f
3
40
43
2 2*
Co
ingly,
one combination of reductant
and acid, Cp2Co a
NMe2
13
51
1
Nitrogenasefirst
14
55studied in the Schrock system and later applied2e–
Li0/EtOH
catalysis
with the Nishibayashi bis(phosphine)pyridine
8.7
34
3
2NH
3
catalyst
studiedherein (complex 3 in Fig. 4)5,6, is thermall
<0.1
<1.0
4
KC8/HBArF4
5
5.6
22
comparison(∆∆Gf(NH3) = 26 kcal mol−1). Thissuggests t
Molecular N2RR catalyst,
r.t., 4.5
1 atom6
18 and mediator designs may yet improve the efficien
acids
7
0
50
100
150
200
250
1.5
4.5
by tandem
electrocatalysis.
2

N2

+

3H

ΔΔGf (kcal mol–1)∆∆G (NH ) = 3[BDFE (H2)/2 − BDFEeff ]corr
Fig. 4 | Electrocatalytic N2RR using reported molecular catalysts.
deviation)
for W(Nf2)2 and3its associated error (standarda, Molecular N2RR catalysts explored in combination with the PCET mediator
electrocatalytic runs. *A GC foam was used as the working elunder electrocatalytic conditions. b, Results of the electrocatalytic
and the concentration was 0.01 mM for both co-catalysts an
2
Ininclosing, c,itEstimatedis widelyappreciated
that
PCET
steps
N2RR,
includingthe taexperiments for each molecular catalyst on CPC at −1.35 V versus Fc+/0
overpotential(∆∆Gf ) for+
mM of N2RR
0.1 M [Li][NTfreported here, a nitrogenase
enzyme,Li-mediated
N2RR
advantages
relative to
distinct
ET-PT
orusi
PT
2] DME solution containing 0.05 mM Co(III,N) , 0.05modynamic
2
catalyst and 5 mM TsOH, using a BDD plate working electrode. Average result
source and various reductant and acid partners used in chem

• First example of electrocatalytic N fixation by a tandem approach.
• High FE of N fixation in mild conditions; room temperature & atmosphericin enzyme catalysis, in which multi-electron redox reacpressure.
driven at biologically accessible potentials26, and als• Reducing competing hydrogen evolution reaction (but, still 39% FE of HER).
acid cocktails used with synthetic N RR catalyst systems (Fig. 4c): catalyst systems . The tandem catalytic approach
SmI /H O (75 kcal), Cp* Co/[PhNHmore] (77 kcalextensivemol ), KC /HBAr applications.
trochemical PCET described herein provides a viv• Higher reactivity
is molrequired
for
2

2

−1

4

2

2

2

2

+

−1

8

F
4

(196 kcal mol−1) (BArF4, B(3,5-(CF3)2-C6H3)4)7,27,30. A crude comparison latter, in which a PCET step that we suggest is largelwith the biological nitrogenases (approximately 117 kcal mol−1 account- on catalysis that is otherwise inaccessible at the aPeters,J. C. et al. Science,
2020, 369,
850. is illust
ing for ATP) is also favourable32. Likewise, heterogeneous systems based
A comparison
with nitrogenase
enzymes++/0
C. et al.
Nature,2022, 609,
71.
on Li /Li (E°(Li ) < −3.7 V), which commonly utilize ethanol as the acid, Peters,
been J.
positedthatthe active-site
cofactors
of ni
29
−1
33
Walker,A.,
Mougel,V.
Chimia.
2024,
78,
251.
operate at an estimated ∆∆Gf(NH3) = 133 kcal mol (ref. ). Interest- up proton and electron equivalents via H atoms

ween pKa and formal potential for PCET donors with different BDFE values inContentsn Basics of
1. Electrochemistry
2. Proton-coupled electron transfer
n Main1. Cobaltocene-Brønsted base
• Development of PCET mediator• N2 fixation
2. Ferrocene-Brønsted base-Anthracenen Summary

30

Phtotoelectrochemical PCET mediator

Article Recommendations

sı Supporting Information*pled electron transfer
hotocatalysis requires
n evolution reaction.
harge-separated stateor capable of being
ely anodic potential
er, the challenge isived excited state forpowerful ferroceneiator exhibiting an
addition to detailed
iometric and catalytic proton-coupled reductive transformations are presented, which• More powerful H-atom donating capacity by photoexcitation.
(BDFE ~ 17 kcal/mol)

E is anodic of the electrode-mediated HER window for acids• Even lowerappcathodic potential. (Higher substrate-selectivity over HER)
tuned to the mediator’s pKa (∼8.6 in MeCN), enabling
trates permeate
substrate reductions both in the absence20−22 and presence23,24
coupled electron• Sufficiently
excitedstate
for theareintermolecularof along-lived
tandem cocatalyst;
thesereductions
not feasible in thereactivity
ss high energy
utilizing Marcus
inverted
effect.
absence of
the PCET
mediator at the same Eapp.12/proton transfer
The linear relationship between the driving force (ΔBDFE)a considerable
and the formal potential required to regenerate the reactive
conditions areform of the PCET mediator can limit its scope. This can beaturing weak X−
illustrated using a Bordwell analysis,4,25 which shows that< 50 kcal
FE

31

Energy

Potential energy surfaces
r

•

Ea
R

P

EnergyReaction;• When the vibrational levels can beapproximated as a harmonic
oscillator, it is represented by aquadratic function.

The chemical reaction proceeds
through the intersection of theenergy surfaces of the reactants
and products

n Arrhenius equation

R

Ea

P
∆Gº

Reaction coordinate𝐸&
𝑘 = 𝐴 ) 𝑒𝑥𝑝 −𝑘' 𝑇32Mechanisms of Electron transfer
Single electron transfer
A + B

Energyn Inner-sphere machanism
A + B

n Outer-sphere mechanism

A + B

A·B

A + B

A + B

A ·B

∆E

• ∆E > 4 kJ/mol•

Adiabatic process; κ = 1(e.g., EDA complex …)• ∆E ≤ 4 kJ/mol• Non-adiabatic process; κ <1(e.g., Ir photocatalyst …)

Ø Marcus theory was developed to explain the outer-sphere process.
33

Reorganization energy in SET
n Franck–Condon principle;
There is no change in the arrangement of the nuclei of reactants and surrounding
solvent molecules at the moment of electron transfer. (ET rate >> motion of nuclei)
Self-exchange reaction;+[FeII(H2O)6]2+[FeIII(H2O)6]3+

Energy① Reorganization of structure[FeIII(H2O)6]3++[FeII(H2O)6]2+

Total reorganization energy of
SET(λ);
λ = λinner(structure) + λouter(solvent)Fe3+ + Fe2+

SET

Relaxation

Fe2+ + Fe3+

Fe3+ + Fe2+λλ
② Effect of solvent molecules
SET
Fe2+

+

Fe3+

Fe3+

δ+

δ-

+

Fe2+

; SolventR

0
P ∆G = λ/434

Very brief introduction of Marcus theoryn Normal region (∆G0 < λ )Energy

A + B

Ea

A + B

Ea
∆G
Rudolph A. Marcus∆Gº

The Novel Prize in Chemistry 1992

∆GºReaction coordinate

Decrease of ∆G0; More exergonic⇔ Decrease of ∆G‡; Faster SET
35

Very brief introduction of Marcus theory

Energyn Barrierless region (∆G0 = λ )
A + B

E =0
E∆G
a =a0

The most stable structure ofreactant coincides with theactivated state for SET
A + B

∆Gº
=λ
∆Gº
=λ

Reaction coordinate∆G0 = λ ⇔ ∆G‡ = 0; Electron transfer proceeds rapidly
without an energy barrier.
36

Very brief introduction of Marcus theory

Energyn Inverted region (∆G0 > λ )
A + B

Ea
∆G

Decrease of ∆G0; More exergonic⇔ Increase of ∆G‡; Slower SET(Counterintuitive)

A + B

∆Gº

Reaction coordinate37

Very brief introduction of Marcus theory
Log(kET)n ∆Gº dependence of SET rate
λ1

λ2
)
*
Δ𝐺
+
λ
Δ𝐺 ‡ =
4λk2

Δ𝐺 ) + λ *𝑘+, = 𝐴 ) exp −4λ𝑘' 𝑇

k1

– ∆G1

•
•

– ∆Gº

Small reorganization energy; λ1 ⇔ Inverted region at ∆G1
Large reorganization energy; λ2 ⇔ Normal region at ∆G1

Ø Minimizing the reorganization energy (rigid structure...) is importantfor slowing down the SET rate by utilizing the inverted region.
38

Incorporation of photosensitizern Catalyst design• Generation of highly reductive
H· donor by photoinduced
intramolecular SET.
• Regeneration of mediator atsubstantially anodic potential.

Journal of the American Chemical Society

pubs.acs.org

n Reduction potential
3＊

1＊

tripletsinglet+ 0.7 V– 0.6 Vvs Fcº/+
– 3.0

– 2.0

– 1.0

0.0

+ 0.0 V– 2.44 V

+ 1.0

–2.44

III

Fe
nd formal potential for PCET donors
with different BDFE values in+l of the Hground/H2 couple
is indicated by a dashed black line. CG value
state) mediator in comparison with this report of a photoelectrocatalyticPeters, J. C. et al. J. Am. Chem. Soc. 2024, 146, 12750.
arge-separated state (CSS). (CS = charge separation; CR = charge

39Working model

2. RESULTS AND DISCUSSION
To guide the following discussion, Figure 2A provides ourworking model for photoelectrochemical catalysis using {Fc−n Catalytic cycle[Hypothesis]n Jablonski diagram

Figure 2. (A) Thermodynamic parameters for the {Fc−N−an}system toward ground-state and excited-state PCET, including
calculated parameters and measured values. (B) Jablonski representation of the electronic states of {Fc−N−an}, indicating a stabilizationof the CSS in the presence of a salt. (kBPCET = rate of back PCET; kIC= rate of internal conversion; kISC = rate of intersystem crossing).

Figure 3. (A) Steady-state emission spectraof {Fc−NH −an} vs1• Appended mild reductant(ferrocene) enables+efficient quenching of anthracene*.
{NH −an}in thepresencephysicalof 15 mM
[PicH][OTf]following
NH+−an}and thepertinent
parameters.
Figure2B
excitation at 355 nm. (B) Spectroelectrochemistry data for the• CSS might be extended by Marcus inverted
effect.
providesa corresponding
representation
theM•−
reductionof {Fc−N−an} Jablonski
to {Fc−N−an} (0.4 mM) ofin 0.7+++
electronic
statesof {Fc−NH+−an}. Anodic cyclic voltammetry
TBAPF
6 (THF). Inlay includes the TA spectrum of {Fc −NH −III/II•−with{Fc−N−an}revealed
a reversible
Fe 146,redox
couple[E°an }J.from690
toJ.790
For thesteady-state
emission
data, [{Fc−Peters,C. etal.
Am.nm.
Chem.
Soc.
2024,
12750.
40+/0= 15 mM inN−an}] = [{NH+−an}] = 0.15 mM; [PicH][OTf]al of the American Chemical Society

pubs.acs.org/JACS

ArticleSpectroscopic analysis
SULTS AND DISCUSSIONde thepubs.acs.org/JACS
following discussion, Figure 2A providesArticle
ourspectracatalysis using {Fc−g modelnforEmission
photoelectrochemical
pubs.acs.org/JACS

Article•1anthracene* (400~475nm fluorescence)is heavily quenched by intramolecular
SET from ferrocene.
• Fast SET (kcs > 5 × 1010) at low driving
force (0.6 eV) implies a modest
reorganization barrier.

n Transient absorption spectrum

Figure 3. (A) Steady-state emission spectra of {Fc−NH+−an} vsNH+−an} in the presence of 15 mM [PicH][OTf] following
xcitation at 355 nm. (B) Spectroelectrochemistry data for the• Absorption of produced species is
consistent with the spectrum ofspectroelectrochemically prepared
anthracene radical anion.

Figure 3. (A) Steady-state emission spectra of {Fc−NH+−an} vs{NH+−an} in the presence of 15 mM [PicH][OTf] following
excitation
atC.355(B)Chem.
Spectroelectrochemistrydata for 41
thePeters, J.
et al.nm.
J. Am.
Soc. 2024,
146, 12750.
•−
reduction of {Fc−N−an} to {Fc−N−an } (0.4 mM) in 0.7 M2B) illustrates the complex array ofhence assignable to {Fc −NH −an } (CSS). TA kinetics
pathways available in {Fc−NH+−an
Spectroscopic
analysis An alternative assignment for themonitored at 700 nm further
reveal a relatively long-lived
species (τ ∼ 0.9 μs; Figure 4A, black trace). The signal has
a species resulting from protonationn Kinetic isotope effectn TA kinetics monitored at 700 nm
anion by acid ([PicH][OTf], 15 m
radical instead. Experimental prece
nation at C9 by [PicH][OTf] woulfragment with absorbance below 500
observed 700 nm absorbance. M
scrambling experiment, where a Dan} and [PicD][OTf] was irradiate
NMR spectra show no indication ointo the anthracene moiety (see Su
We next tested whether photoch
NH+−an•−} could undergo interm
using acetophenone (estimated BD
based on DFT calculations; see Sup
Figure 4. (A) Time-resolved TA decays
an initialoftest
substrate.
{Fc−N−anpresence
excessacidandquencher• This mediator has long-lived CSS;τ = 0.9 ~1.3 μs
of[PicH][OTf](10mM,
pKa(Mewith a 355 nm laser pulse and monitor([Ir(ppy)2dtbbpy]PF6; τ = 0.54 μs)*
reacta time-delay
with acetophenonemM
afterof 10 ns. (B)(10
Stern−V++
excitation,owing
to −NH(D)the large unfavo
rate
of decayof {Fc−an•−• Back electron transfer has high driving force(2.5 eV) andclosed shellconcentrations
ofexperimentsub,product.
relativewith
to thera
An analogous
390
sub,and anextracted
kine→ BET is likely to be in Marcus inverted region.
afforded29%of the deuterium
pinacol-coupl
Demonstrationof the tozero-orderdepe
net+ H atom transfer
acetophenon
the TAradical
data, [{Fc−N−an}[Hof ].theForα-ketyl
intermediat[PicH][OTf]; H+ source[OTf]] = 15 mM; [sub] = 18 mM in D
chemical quantum yield of 6% w
study, [[PicH][OTf]] + [[PicMe][OTH
conditions(see 2016,
SupportingN*Ochola, J. R., Wolf, M. O. Org.
Biomol. Chem.
14, 9088.Informa+
Peters, J. C. et al. J. Am.
Chem.
Soc. 2024,−an}146, 12750.
using{Fc−NMein the 42
pres
OTf
Me
{Fc−N−an} and a weaker acid inc
+

+

•−

Control experiment{Fc-N-an}; Mediator (1 eq)[PicH][OTf]; H+ source (10 eq)

O

2

Me

DME (sub; 10 mM), 390 nm LED

Me OH

Fe

II

N
Me

N
Me

{Fc-N-an}{N-an}HO Me(10 eq)Entry

Deviation

TM
(%)

1

No light

02{Fc-NMe+-an} instead of {Fc-N-an} & [PicH][OTf]<1

3

p-CF3-benzoic acid(1) instead of [PicH][OTf]<1

4

Cp2Fe + {N-an}(2) instead of {Fc-N-an}0(1); Weak acid incapable of protonating the amine group.
(2); Inefficient intermolecular reduction due to the short lifetime of 1anthracene*.

Peters, J. C. et al. J. Am. Chem. Soc. 2024, 146, 12750.

43

Stoichiometric photochemical reaction{Fc-N-an}; Mediator (1 eq)[PicH][OTf]; H+ source (10 eq)
Sub(10 eq)DME (sub; 10 mM), 390 nm LED, 1 h

Fe

Sub H

II

N
Me

{Fc-N-an}

Entry

Substrate
HO

1

BDFEX–H(kcal/mol)

Product

O

Me OH

Me

Me

H

NPh

29

Me OH

36 kcal/mol

PhHN

NPh

2

65
NHPh

50 kcal/mol
O

3

O
OPh

PhO

[(TfO)W(NNH2)][OTf]

H

O
OPh

PhO

O

4

TM
(%)

H

45 kcal/mol

O

-

OPh

PhO
H

42

O

[(TfO)W(NH)][OTf]~70• No HER was observed.
Peters, J. C. et al. J. Am. Chem. Soc. 2024, 146, 12750.

44

Photoelectrocatalytic reactionSub(1 eq){Fc-N-an}; Mediator (2 mol%)[PicH][OTf]; H+ source (2 eq)[TBA][PF6] (3 eq)
DME (50 mM), 390 nm LED,
Controlled potential (– 0.1 V vs Fc+/0), 24 h

Sub H

Fe

II

N
Me

{Fc-N-an}

Entry

Substrate

Product

O

1

TM
(%)

rSM
(%)

TONs

36 ± 451

18 ± 2

30

55

15

9

75

9

Me OH
Me
Me OH

NPh

PhHN

2
NHPh
O

3

H

O
OPh

PhO
O

OPh

PhO
H

O

• No HER was observed.
(Retention time; H2 = 4.3 min, N2 = 4.5 min)Peters, J. C. et al. J. Am. Chem. Soc. 2024, 146, 12750.

45

Contentsn Basics of
1. Electrochemistry
2. Proton-coupled electron transfer
n Main1. Cobaltocene-Brønsted base
• Development of PCET mediator• N2 fixation
2. Ferrocene-Brønsted base-Anthracenen Summary46

Summaryn PCET mediator for electrocatalytic activation of small molecules

e–

H

N2

X
R

PCET
mediator

R

CO2(+ Metal catalyst)

R

NH3

X
R

HCO2H

• Novel molecular design & comprehensive mechanistic analysis.
• High reactivity in mild conditions.
• Minimized hydrogen evolution reaction.
(?) Applicability to more synthetically valuable transformation;
Generality of this strategy.
(e.g., Hydrogen atom transfer to unactivated alkene · · · )47

Thank you for your kind attention.

48

Appendix【参考資料】•Walker, A., Mougel, V. Chimia. 2024, 78, 251.
•
分子光化学の原理（著者；Nicholas, J. T. 他 訳者；井上 晴夫 他）•
大学院講義有機化学 第２版 Ⅰ（著者；野依 良治 他）•
量子化学 基礎からのアプローチ（著者；真船 文隆）•
金属錯体の電子移動と電気化学 錯体化学選書９（著者；西原 寛）•
http://mol-chem.com/photoinduced_electron_transfer/index.html

49

Another example of PCET mediator~ Fe-S cluster for Electrocatalytic CO2RR ~CO2Metal cat.

CO

Acidic conditions

+

HCO2H

+

…

• HCO2H is the highest value-added CO2 reduction product.
(Novel H2 career, hydride source …)✕ Generally, CO is preferentially produced✕ Competing HER

ticlen Challenges in Selectivityae–[M(n–2)+]ET

H+Classical PCET M-H formationPT

two e– reduction of the metal
centrestrong H+ donor

ET–PT or PT–ET
[Mn+]

e–

[M(n–1)+]H+[M(n–1)+]-He–[Mn+-H]PT
ET
Figure 5. Scope of organic substrates
amenable to photochemical
reduction via pPCET using {Fc−NH+−an}. Photoelectrocatalytic reduction of+organic substrates by {Fc−NH −an} in the presence of [PicH][OTf] under irradiation at 390 nm and an Eapp of −0.1 V vs Fc+/0 using a carbon
CPET-mediated M-H formation
CPET
mediated
cloth cathode and either a Zn or GC
anode.
All reported yields are NMR yields– measured against an internal standard (see Supporting
one e reduction of the metal
Information). Only trace H2 (<2% Faradaic
H efficiency) was detected when sub = acetophenone. Calculated BDFE values of key organic
centre41
intermediates are reported as well. med-H
The inlayedplot is measured current weak
response
in the presence and absence of irradiation for the
H+ donormed+
photoelectrocatalytic reduction of acetophenone by {Fc−NH −an}. (SM = starting material).

−1−1e – + H+is kPCET = 3.2 ± 0.5 × This
10 work
M s (Figure 4B); theH
DkPCET/ kPCET KIE = 2.8 ± 0.4.
Because an H-bonding interaction has been proposedD7high selectivity

high withefficiency
N-phenylbenzylimine and diphenylfumarate
NH+−an}(Figure 5). Reaction with N-phenylbenzylimine, featuring50
afor its correspondCN π-bond and an associated BDFE

e – + H+

high selectivity
high efficiency

This work
PCET mediator
for efficient M-H formation

b

CO
OC

Electrocatalytic

Mn0

OC
N

Mn0

N

Br
N

MnI

N

CO

e–

CO

N

CO

N
Br–

[MnI-cat]

SPh
PhS

Fe

S
Fe
Fe

PhS

2–

S

CO

CO

Fe

N

CO2 + 2H+ + 2e–

CO

CO

H2O

CO

CO
[Mn0-dimer]

Photocatalytic[Mn0-cat]Electrocatalytic
H= Fe-SSS

Mn0e– (PS ) + H+

CON

Fe-S– + H+
SPh

H
N
N

Fe-S

MnI
CO

CO
CO

CO2 + H+

HCOOH

[MnI-H]e– + H+This work

ed metal hydride formation. a, Generation of M-Hoxidation states. b, Reaction pathways and associated prod
underelectro- and photo
proton-coupledelectrontransfer (PCET:ET–PTwasor PT–ET,• Biomimeticiron-sulfur
cluster
utilized Mn
asI-cat-catalyseda hydrogenCO
atom
transfer
mediator.
2 reduction
conditions.
blue box) processes. The superscripts indicate metal• Multiple site nature of PCET on Fe-S cluster, in which protonation occurs at a sulfur
base, whereas reduction occurs at a Fe centre, constitute a kinetic advantage by
the lower reorganization energy.

ngly reduced metal site (Fig. 1a, blue box). The thernetic parameters required for efficient M-H genera-

as CPET mediators. Synthetic Fe–S clusters hav
Mougel, V. et al. Nature. 2022, 607, 499.
51
exploited to mediate CPET steps in a catalytic coCatalytic activity and spectroscopic characterizationn CV chartabc

Mn0-dimer
Mn0-L
0 l-H
Mn
mer-MnI- MnI-cat
cat
HCOOH

b

20

10
2,100

Product
yield

–0.4

Mn0-dimer
Mn0-L
Mnl-H
mer-MnIcat
Faradaic
HCOOH
efficiency

HCOOH
CO
HCOOH
H2
CO
MnI-cat/Fe-S
2,000
1,900
1,800
H2
Q (cm–1)e100

MnI-cat/Fe-S

80
Faradaic efficiency (%)

–1.6
–1.2
–0.8
E (V) (versus Fc/Fc+)

Fe-S

1,700

20
0

MnI-cat/Fe-S (TFE/CO2)

2,000

1,900
1,800
Q (cm–1)

Best
performances

f 3

f 3

–3.12
ppm

[Mnl-H]

6.0

2

1

0

Mn-cat/Fe-S
1,900
1,800
(EQred(cm= –1.85
V)
–1)

2,000

[Mnl-H]c Mn -cat/Fe-S (TFE/Ar)
HCOOH
8.43 ppm

4 min

n Minimized overpotential

MnI-cat/Fe-S

40
1,600
12.0) 11.0 10.0 9.0 8.0 7.0(TFE/CO
MnI-cat/Fe-S
2
G (ppm)
20

–3.12
ppm

4 min

0

40 I
Mn -cat/Fe-S

Fe-S

I

Mn -cat/Fe-S (TFE/Ar)
60
HCOOH
8.43
ppm I
MnI-catMn -cat (TFE/Ar)
I

20

e

n Complete shift of product selectivity
–2.0

60

Faradaic efficiency (%)

80

Faradaic efficiency (%)

Product yield (μmol)

2

10

0Best
min

log((TOF/s–1)(FYHCOOH)2)

MnI-cat/Fe-S (1:1)
MnI-cat/Fe-S (1:2)

dProduct yield (μmol)

cat

80
Faradaic process at –1.85V resulting• Product
New redox
yieldefficiencycfrom
the synergistic
combination of Fe-S
60
HCOOH
0 min
100
CO
HCOOH
HMnI-catandCO
40log((TOF/s–1)(FYHCOOH)2)–2d

Ered = –1.65 V–2

20

Product
Faradaic
H2
20
yield
efficiency
MnI-catEred = –1.85 V
Fe-S
HCOOH
MnI-cat/Fe-S (1:1)
CO–3
MnI-cat/Fe-S (1:2)
HCOOH
H2
CO 0
Ered = –1.65
V
10
MnI-cat
–2.0
–1.6
–1.2
–0.8
–0.4
H2
+)
E Mn
(V) I(versus
Fc/Fc
-cat
Ered = –1.85 V
Fe-S

–1

–3

b

–1

Product yield (μmol)

0

I (mA cm–2)

0

a

0 min

100

2

[Fe 4N(CO) 12]–1

performances

1,700

1,700

[CoCp(P 2N2)]2+[Ni(qpdt2)] –[Ir(POCOP)] 2–[Co(imino-bpy)]+[(bpy)Co(PyS)]+ [CoCp(P 2N2)]2+
Mn-cat/Fe-S[FeP 4N2]2+(Ered = –1.85 V)
0 [Pt(dmpe)] 2+[Ni(qpdt2)] –4 min[FeN 5Cl2]+[Fe 4N(CO) 12]– Mn-cat/Fe-S [Ir(POCOP)] 2–+(Ered[Co(imino-bpy)]= –1.65 V)–1[(bpy)Co(PyS)]+–3.0[Pt(dmpe)] 2+02004N2]2+
400
600
[FeP
Overpotential K (mV)

800

Mn-cat/Fe-S [FeN 5Cl2]
1(E= –1.65 V)(orange) under
red conditions described in c. e, Ex situ H NMR spectra–1 the+cat/Fe-S(1:1)
g. 2 | Catalytic
activity and spectroscopic characterization of relevantI
cat/Fe-S (1:2)-cat
MnI-catI (TFE/Ar)a, Overlaid CV
termediates of0 CPET-mediated COMn
recorded after CPE of Mn I -cat (orange) and a 1:1 Mn I -cat/Fe-S mixture
2 RR activity of Mn -cat.
0
I-cat
I
01,700400
600
800
1,800absence
1,700
Mnin
Fe-Sand1,600
MnI-cat/Fe-S
2,000
12.0 11.0
8.0
7.0 6.01,900−1.65–3.0 1,800.8 2,100-cat (1 1,900mM)
thein thepresenceof10.0Fe-S9.0
along
with
aces
of–0.4
Mn2,000
V and additionof200
TFE (50 equiv.)under
Ar (blue)
and CO2 (green)
Overpotential K (mV)
Q (cm–1)
G (ppm)Q (cm–1)e CV of Fe-S alone (0.2 M TFE, 0.1 M TBAPF6 in CH3CN). b, Correspondingf, Comparison of the performance among reported catalytic CO2RR sy
10oducts,associatedFY obtained
during 90 minofCPE
at −1.85
using
a 1:2 underselective
forMougel,
HCOOH
production(Supplementary
SectionicMn
activity
andcharacterization
relevant
H NMR
spectra(orange)the conditions
described
in Nature.
c. e,
Ex situ
V. et al.
2022,
607,
499. 2.8 and-dimerand
52
espectroscopic
f 3 VBest
IMn0-L
I
I
I
performances-cat/Fe-S
mixture (0.2
M TFE,
0.1 MofTBAPF
in CO-saturated
CN).
Thick after
Supplementary
Table3). Thebars
indicate
the rangeof overpotential
activity
Mn -cat.
a,–3.12Overlaid
CVCH(orange)anda 1:1
Mn -cat/Fe-S
mixtureat
snof
CPET-mediated
CO RR
recorded
CPE of Mn -catMetal hydride [M-H]
formation

CPET cycle

regeneration

to CO2

1

Thermodynamic and kinetic considerations
e– + H+–+med-H] + e + H +[Mn+[M(n–1)+]

med

med-H

med-H + [M(n–1)+]

Free energyaCO2 + H+e–[M(n–1)+]

[Mn+]

O
H

med + [MOHb‡

CPET mediator
regeneration

Metal hydride [M-H]
formation

CPET cycle

O
Hydride 1transfer
to CO2O2ΔGCPET =(kCPET = A × exp‡–ΔGCPETkBT°(ΔGCPET+ O)24O) = A × exp
(
‡

(1)

°
–(ΔGCPET
+ O)2

ΔG2

4OkBT
‡

med + e– + H+ +n+-H][M(n–1)+[M]

e – + H+Free energy

med

b
O1

b

CO2 + H+]
med-H + [M(n–1)+and
M-H – BDFEmed-H
Fig. 3 | Thermodynamic
kinetic considerationsΔG
for°catalytic
2 = BDFE
CPET-mediated metal hydride formation. a, Schematic representation of themedgeneration+ [M(n–2)+-H] followed
reaction steps in a CPET-mediated M-H
by hydride
transfer
Reaction
coordinate
to CO2 and the associated free-energy parabola representation. b, Equations (1)and (2) describe the free-energy
barrier, ∆G‡CPET
c , and the reaction rate constant,
2
°
(ΔG
+
O)
‡
Boundary conditions for CPETCPET(1) CPET process; 𝜆 is the reorganization
ΔG,CPETfor a= weakly
coupled non-adiabatic
kCPET
mediated M-H formation:
4O
O2‡
energy,factor determined from overlap°–ΔGA is the Arrhenius–(ΔGpre-exponential
+ O)2

(

kCPET = A × exp

med + e– + H+ +[M(n–1)+]

BDFE = 1.346 × pKa + 23.06 × Eº +ΔGCG2
ΔG1° = BDFEmed-H

CPET

kBT

) = A × exp(

CPET

‡ 4OkBT

)

(2)

)

(2)
(3)

integrals
and prod
The Bord
acidity cpotentiac, Boundgenerati

BDFEM-H > BDFEmed-H

ΔG2
pKaH+-donor < pKamed-H(3)
BDFE = 1.346 × pKa + 23.06 × Eº + CG‡
ΔG2
ΔG1° =have
BDFEmed-H
Two CPET steps
to be considered:(1) the (re)generation of thereorganFig. 3 | Thermodynamic
kinetic
considerations
forspecies
catalyticof the electronic
andoverall
proton vibrational
statesb
med-H
with an electronintegrals
and a proton;and (2) thetrans- sic
geom]
med-H +and[M(n–1)+ΔG2° = BDFEM-H – BDFEmed-H
the spB
CPET-mediated metal hydride formation. a, Schematicof theand product,is the absolutetemperatureand kIn
B isthefer of anrepresentation
apparent hydrogenatom
to theTsingly
reducedmetal site(n−1)+
Thebe
Bordwellequationenables estimating
BDFEanvareaction steps in a CPET-mediated M-H generation
followedby hydride
transfer(n–2)+[M3a). Bothsteps caninterpretedin (3)a Marcus-typefor- thevary,
med + [M
-H] ] (Fig.
Reaction
coordinate
0b,Equations(1)asacidity
constantof
theprotonatedmediator
med-H,
E
to CO2 and the associated free-energy parabola representation.
malism (Fig. 3b), in which a symmetric dependence
between the rate will be tand (2) describe the free-energy barrier, ∆G‡CPET, and the reaction rate constant,potential of the med0/− couple, CG being °a solvent-depend
ofcthe reaction (kCPET) and the thermodynamic driving force (∆G CPET) to selec°(ΔGCPET+non-adiabaticO)2‡
Boundary
conditions for CPET-c, Boundary conditions to be met to ensure catalytic CPEa weaklycoupledCPET
process;
𝜆 isthe reorganization
kCPET, for ΔG(1) is expected,
resultingin a lower activation barrier than required for on CPE
CPET =
mediated
M-H formation:
4O
energy, A is the‡Arrhenius pre-exponential factor determined from overlap
generation.
2°
the stepwise ET–PT and PT–ET processes. This dependence may be
Secon–ΔGCPET–(ΔGCPET + O)
M-H
med-H
(2)
kCPET = A × exp()
= A × exp()
BDFE
> BDFE
diminished by the fact that botha protonan electron
syn- from m
kB T
4OkBT
Mougel,V. et andal. Nature.
2022,must
607, 499.
53
H+-donor
med-H
chronously
in a potentially higher pre-exponential BDFE ofpKatunnel,< resulting
pKa

Far

Prod

15

68.0

CO2RR activity with various PCET mediators

10

No
HCOOH

20

5
0

PhOH

40

n Catalytic cycle
Ru-NH Ru-NH* DTH2Q

Fe-S

H2Q

0

PhOH

Ru-NH Ru-NH* DTH2Q

Fe-S

H2Q

N
N

d

MnI
CO

CO
CO

[MnI-H]

65.8

Mediator
regeneration cycle

Formic acidproduction cycle[Fe-S]

3]

CO2 + H+HCOOH

TOF = 0.4 s–1

e– + H+

Ered = –1.35 V

Ered ≤ –1.85 V

HCOOH> BDFE Mn-H >65.8 kcal mol–1

Ered ≥ –1.65 V[Fe-S]H
pKa = 29.9

HO

62.9

[(bpy) MnI(CO) 3]+

DTH2Q te–

CO2 + H+cle
O RR activityArticleof Mn -cat in the presence of various CPET mediators.
I

RuII

TOF = 20 s–1

HCOOH

OH

Bu
Ø HCO
62.1 2H was observed ina faradaic efficiency
of
O
O
N
61.0
92%!
O
N
HN

[(bpy)Mn0H(CO) 3]–[(bpy) Mn0(CO) 3]

H

tBu

63.5

67.3 kcal mol–1

BDFE [Fe-S]H =63.5 kcal mol–1

SPh
Fe
2–
PhS Fe
S
S
Fe
PhS
Fe
S
Ph
[Fe-S]H S
Se–[(bpy) MnIH(CO)

OH

H2Q

67.3

H

PhOH

HO

O

Ru-NH

indicate the systematic error of duplicate independent CPE measurements.
c, Scaling relation of BDFEmed-H with formic acid production, which led to the2ca of various CPET mediators.
b entries fromcabin the presence
The first three
shownat theH top. The BDFE value
determinationof BDFEMn-H via the reaction
med
med-H
med
med-H
CO
CO
N
N
HCOOH
HCOOH
CO
N
I
HCOOH of a protonorrespond to35experiments carried out in the presence
mentioned hereHCOOH
are summarized
Section
6. d, ProposedN
Mn0 in Supplementary
Mn0 Mn
CO
CO
N
N
CO
35
CO
CO
N
N
CO CO
100
CO
100
H2 where 1.5 M H2O wasandsubsequent tran
0.2 M TFE, except for Ru-NH*
mechanistic
cycle
Mn I-H formation
CO CO
H2 for Fe-S-mediatedH2 used), whereas theH2
30
I-H][Mn0-cat][Mn0[Mn-cat][M
tries relate to catalytic tests carried30out in the absence of a protonof the hydride to CO2.
80
80
25
med-H
25 mediator (0.1–0.5 M), error barsnd at high concentration of the CPET
BDFE (kcal mol–1)
med-H
BDFE (kcal mol–1)
15
10

20
15

n Faradaic yield

Faradaic yield (%)

20

Faradaic yield (%)

Product yield (μmol)

Product yield (μmol)

n Product yieldduct yields (a) and associated Faradaic yields (b) for CO RR using
2

60
40

10

60

OH

87.8
PhOH

40

68.0

OH

87.8
PhOH

HO

OH

68.0

HO
OH
No
No
I
was proposed to be HCOOH
driven by67.3
the formation
of
a
Mn
-OCHO
specie
H
Q
2
H2Q
HCOOH
67.3

20

20

of 30.3 ± 0.3. This value lies in
5 the expected range for this0s assuming thata one-electron0 transfer increases
To demonstratethat suchHa transfer could occur in the specific cas
0 the pKa by0
H
Ru-NH Ru-NH* DTH Q
Fe-SRu-NH
H QRu-NH*
PhOHDTH Q
Ru-NH HRu-NH*
DTH Q
Fe-S
H QRu-NH*
PhOH
Fe-S
Q
PhOH
DTH Q
Fe-S
H CO
Q
PhOH
I Ru-NHI
N
CO CPE under CO ,N
I-H]
its compared to the oxidized species, as observed for syn- Mn -H, we reacted a solutionMn-H preparedby
Mn of [Mn2
Mn
[MnI-H]
N
CO
N
CO
25
Mn-H
I
CO
Fe2S2 clusters . The lower limit of BDFE
was estimated as observed the consumption of the Mn -H species
and the appeara
COd65.8499.
d 1H NMR data (Supplementary of [MnI(bpy)(CO)
Mougel,V. et 65.8
al.I Nature. 2022,SPh
607,
5419SP
al mol−1 on the basis ofMediator
CV andFigs.
3(OCHO)] (Mn -OCHO). (Supplementary
Formic acid
5

2

2

2

Mediator222

Formic acid

2

I

2

I

S

Fe

Synthesis of PCET mediator

NMe2
Li
CoIII

CoIII
H

40% yield
Ar

AgOTf

II

reflux
59% yield

Co

NMe2

74% yield

III

Co

H+
Low temp.

Unstable,
Detected by EPR, UV-vis
Co

II

H
NMe2

NMe2

HOTf
50% yielde–
Low temp.

III

Co

H
NMe2

The reduction of catalyst can also be performed electrochemically.

Peters, J. C. et al. Science, 2020, 369, 850.

55

Reaction set-up
n Peters, J. C. et al. Nature, 2022, 609, 71.

56

Reaction set-up
n Peters, J. C. et al. J. Am. Chem. Soc. 2024, 146, 12750.
S6. Stoichiometric photochemical reactions

1. Stoichiometric photochemical reaction

General set up for a typical photochemical experiment: in a N2 filled glove box, the corresponding
mass of the {Fc‒N‒an} mediator, the [PicH][OTf] acid and the substrate was weighed and placedin a vial with a magnetic stir bar. Subsequently, 1 ml of dried and degassed THF was added andthe vial was closed with a septum to ensure inert atmosphere. The vial was brought out of the glove
box and irradiated with a 390 nm LED lamp during 1 h. Stirring and refrigeration with a fan wasmaintained throughout the experiment.
General work up methodology: after the reaction, the reaction vessel was open to air and 1 ml of
HCl in diethyl ether was delivered. After stirring for 5 min, the mixture was evaporated to dryness.
The remaining solids were extracted with 5 ml of diethyl ether, which was subsequently filtered,washed with concentrated solution of saturated NH4Cl and water, dried over sodium sulfate andevaporated to dryness. Subsequently, 1 ml of CDCl3 and 5 µL of CH2Br2 as internal standard wereadded and 1H NMR was performed to analyze for product formation.

57

NaHCO3 (aq, N-phenylbenzaldimine), or 0.1 M Na2CO3 (aq, diphand extracted with ether (5 x 20 mL). The organic layers were coconcentrated in vacuo. A crude 1H-NMR was taken with CH2Br2
where ferrocene oxidation was used as the anodic reaction, an amthat of the acid in the working compartment was dissolved in t
n Peters, J. C. et al. J. Am. Chem. Soc. 2024, 146,completion
12750.of the CPC, the counter compartment is a deep blue c
Fc+.
Because of the photo-induced reduction of Ag+ to form Ag
S12. Photoelectrocatalytic
reactions
2. Photoelectrocatalytic reactiona Pt-wire pseudo-reference that was calibrated from the Fe(III/II)
before electrolysis.

Reaction set-up

General Controlled Potential Coulometry (CPC) Procedure: Carbon cloth electrodes were cut
to 5 cm2 and used as obtained from the supplier. Controlled potential coulometry (CPC)
experiments were carried out in a gas-tight two compartment cell using a carbon cloth (dimensions
1 x 2 cm) working electrode, a Pt wire pseudo-reference electrode, and a carbon cloth (5 cm2) or
Zn foil (10 cm2) counter electrode. The solution was stirred throughout the CPC. In a typicalexperiment with an organic substrate, a 6 mL solution of 0.15 M [TBA][PF6] with two equivalentsof acid (based on desired number of H-atom equivalents delivered to substrate) in DME was added
to the solution which was added to the electrochemical cell distributing evenly between bothcompartments. Then, an appropriate amount of substrate (50 mM in 3 mL) and {Fc‒N‒an} (1 mM
in 3 mL) was added to the working compartment. After the appropriate reaction time, the solution
was quenched with 2 M HCl in ether (1 mL) except for the acid sensitive N-phenylbenzaldimine.
After 5 min of stirring, the solvent was evaporated under reduced pressure until dryness. The
resulting solids were dissolved in diethyl ether (20 mL). For the diphenylfumarate reaction, ethyl
Fig. S96. Photoelectrocatalysis setup as described, with a two coacetate (10 mL) was first added to the carbon cloth working electrode, thensonicated to extract
separated by a fine pore frit, 390 nm Kessil H150 lamp, and fan c
the remaining starting material (sparingly soluble in diethyl ether) before 100 mL of diethyl ether
was added. The solution was subsequently washed with either 1 M HCl (aq, acetophenone), sat.
NaHCO3 (aq, N-phenylbenzaldimine), or 0.1 M Na2CO3 (aq, diphenylfumarate) solution (20 mL)
and extracted with ether (5 x 20 mL). The organic layers were combined, dried over Na2SO4, andconcentrated in vacuo. A crude 1H-NMR was taken with CH2Br2 as internal standard. In reactions
where ferrocene oxidation was used as the anodic reaction, an amount of ferrocene equivalent tothat of the acid in the working compartment was dissolved in the counter compartment. Uponcompletion of the CPC, the counter compartment is a deep blue color, indicating the formation ofFc+.
Because of the photo-induced reduction of Ag+ to form Ag0 nanoparticles, we opted to use
a Pt-wire pseudo-reference that was calibrated from the Fe(III/II) couple of {Fc‒NH+‒an} in a CV
58

Other applications of PCET mediator
1.

Stahl S. S. et al. Cooperative electrocatalytic alcohol oxidation with electron-proton-transfer mediators.
Nature. 2016, 535, 406.

2.

Peters, J. C. et al. Electrocatalytic Reduction of C−C π-Bonds via a Cobaltocene- Derived Concerted
Proton−Electron Transfer Mediator: Fumarate Hydrogenation as a Model Study. J. Am. Chem. Soc.
2021, 143, 9303

3.

Waymouth, R. M. et al. Electron-Rich Phenoxyl Mediators Improve Thermodynamic Performance ofElectrocatalytic Alcohol Oxidation with an Iridium Pincer Complex. J. Am. Chem. Soc. 2020, 142,
19368.

4.

Peters, J. C. et al. Electrocatalytic Ketyl-Olefin Cyclization at a Favorable Applied Bias Enabled by aConcerted Proton−Electron Transfer Mediator. Inorg. Chem. 2022, 61, 6672

5.

Peters, J. C. et al. Use of a PCET Mediator Enables a Ni-HER Electrocatalyst to Act as a Hydride
Delivery Agent. J. Am. Chem. Soc. 2022, 144, 20118

6.

Meyer, G. J. et al. Direct Evidence for a Sequential Electron Transfer−Proton Transfer Mechanism in
the PCET Reduction of a Metal Hydroxide Catalyst. J. Am. Chem. Soc. 2024, 146, 1742.

7.

Peters, J. C. et al. Catalytic Reduction of Cyanide to Ammonia and Methane at a Mononuclear Fe Site.
J. Am. Chem. Soc. 2024, 146, 5343

8.

Waymouth, R. M. et al. Cobaltocene-Mediated Catalytic Hydride Transfer: Strategies forElectrocatalytic Hydrogenation. J. Am. Chem. Soc. 2024, 146, 17075.

···
59