of 14
Tandem electrocatalytic N
2
fixation via proton-coupled electron
transfer
Pablo Garrido-Barros
1
,
Joseph Derosa
1
,
Matthew J. Chalkley
1,2
,
Jonas C. Peters
1,
1.
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
CA, USA.
2.
Present address: Department of Pharmaceutical Chemistry, University of California, San
Francisco, CA, USA.
Abstract
New electrochemical ammonia (NH
3
) synthesis technologies are of interest as a complementary
route to the Haber–Bosch process for distributed fertilizer generation, and towards exploiting
ammonia as a zero-carbon fuel produced via renewably sourced electricity
1
. Apropos of these
goals is a surge of fundamental research targeting heterogeneous materials as electrocatalysts for
the nitrogen reduction reaction (N
2
RR)
2
. These systems generally suffer from poor stability and
NH
3
selectivity; the hydrogen evolution reaction (HER) outcompetes N
2
RR
3
. Molecular catalyst
systems can be exquisitely tuned and offer an alternative strategy
4
, but progress has been thwarted
by the same selectivity issue; HER dominates. Here we describe a tandem catalysis strategy that
offers a solution to this puzzle. A molecular complex that can mediate an N
2
reduction cycle is
partnered with a co-catalyst that interfaces the electrode and an acid to mediate proton-coupled
electron transfer steps, facilitating N–H bond formation at a favourable applied potential (−1.2
V versus Fc
+/0
) and overall thermodynamic efficiency. Certain intermediates of the N
2
RR cycle
would be otherwise unreactive via uncoupled electron transfer or proton transfer steps. Structurally
diverse complexes of several metals (W, Mo, Os, Fe) also mediate N
2
RR electrocatalysis at the
same potential in the presence of the mediator, pointing to the generality of this tandem approach.
Molecular catalysts offer a number of favourable characteristics compared with
heterogeneous electrode materials for electrocatalytic nitrogen reduction reaction (N
2
RR)
studies. In particular, they can be carefully tuned to satisfy the electronic requirements
of N
2
binding and activation. They can also afford access to insightful mechanistic
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is available at
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.
Correspondence and requests for materials
should be addressed to Jonas C. Peters. jpeters@caltech.edu.
Author contributions
P.G.-B., M.J.C. and J.C.P. conceptualized the work. P.G.-B. designed and executed the experiments. J.D.
assisted with the execution of the catalytic experiments. All authors analysed and interpreted the data and co-wrote the manuscript.
Online content
Any methods, additional references, Nature Research reporting summaries, source data, extended data, supplementary information,
acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code
availability are available at
https://doi.org/10.1038/s41586-022-05011-6
.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information
The online version contains supplementary material available at
https://doi.org/10.1038/
s41586-022-05011-6
.
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studies at the level of critical bond-breaking and bond-making steps (Fig. 1a). Remarkable
progress has been made over the past two decades in terms of chemically driven N
2
RR
catalysis and associated mechanistic understanding using molecular systems
4
7
. Despite
this, bona fide N
2
RR electrocatalysis in this domain remains virtually unknown
8
10
; only
one such molecular electrocatalyst (a tris(phosphine)borane iron ((TPB)Fe) system from our
laboratory; Fig. 1b) has been reliably demonstrated, but it requires low temperatures (−35
°C) to mitigate the background hydrogen evolution reaction (HER) and a highly reducing
potential (−2.1 V versus ferrocenium/ferrocene, Fc
+/0
; all potentials herein are reported
versus Fc
+/0
)
11
. This state of affairs sharply contrasts the substantial progress that has
been made applying molecular systems towards electrocatalytic HER, the carbon dioxide
reduction reaction and the oxygen reduction reaction, among other transformations
12
14
.
Similar to candidate heterogeneous electrocatalysts, molecular systems typically mediate
HER in preference to N
2
RR, and/or operate at such reducing potentials that
background HER at a working electrode dominates. Pioneering research from 1985
by Pickett and co-workers underscored this point (Fig. 1c). In a study involving a
bis(diphenylphosphinoethane)tungsten system (abbreviated throughout as
W
), they showed
that the hydrazido complex (TsO)
W
(NNH
2
)
+
(TsO, tosylate), generated via protonation of
the bis-N
2
adduct
W
(N
2
)
2
by tosic acid (TsOH), releases NH
3
(0.21 equiv. NH
3
per
W
) after
application of a highly reducing potential (−2.6 V on a Hg-pool electrode), but only in the
absence of the acid
15
,
16
. Positively shifting the overpotential of the candidate electrocatalyst
would attenuate the HER, but the linear free energy relationship between overpotential and
catalytic rate requires an additional strategy for maintaining an appreciable N
2
RR rate at a
lower driving force
17
.
Recent work from our laboratory introduced a strategy for attenuating the rate of
(electro)catalytic HER. By using a proton-coupled electron transfer (PCET) mediator,
consisting of cobaltocenium modified by a tethered Brønsted base (abbreviated herein as
Co
(III,N)
+
; Fig. 1d)
18
, the catalysed HER is prevented. This mediator design spatially
and electronically separates the proton and electron relays, which is key to storing highly
reactive H atom equivalents in
Co
(II,NH)
+
at a potential that is sufficiently mild to
also mitigate background HER at the electrode. Initial model studies using this mediator
established that concerted proton–electron transfer (CPET) provides a means to reduce
unsaturated organic substrates electrochemically at comparatively mild potentials in the
presence of tosic acid
18
,
19
.
Although these results point to the possibility of applying such a mediator towards
electrocatalytic N
2
RR, the mediator itself does not react with N
2
, in contrast to some
unsaturated organic substrates. Hence, we have pursued a tandem catalysis strategy (Fig.
1d)
20
,
21
, pairing a candidate molecular catalyst that can bind N
2
(M−N
2
) and facilitate
its multistep reduction to NH
3
through various M−N
x
H
y
intermediates (for example,
M−N=NH, M=NNH
2
, M=NH)
4
, with a mediator that interfaces the electrode and the acid
with the N
2
reduction cycle via PCET steps. Importantly, certain N
2
RR intermediates are
challenging to move through the cycle; they can be difficult to independently reduce or
protonate (see below). In principle, a PCET step can circumvent this issue and favourably
shift the overpotential needed to drive the net electrochemical N
2
RR process. Here we
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show the feasibility of this tandem catalysis strategy at room temperature and atmospheric
pressure.
As a model system to test our tandem approach we adopted the classic tungsten system
studied by Pickett
15
. Using
W
(N
2
)
2
and the same solvent (tetrahydrofuran; THF), electrolyte
(0.2 M [TBA][BF
4
]; TBA, tetra-
N
-butylammonium) and acid (100 equiv. TsOH), in the
presence of the cobalt PCET mediator,
Co
(III,N)
+
, controlled potential coulometry (CPC)
produced 4.7 ± 0.3 equiv. NH
3
at 18 ± 2% Faradaic efficiency (FE) over a period of 11 h
using a glassy carbon (GC) electrode at −1.35 V (see Supplementary Section 4). Reloading
the system with an additional 100 equiv. TsOH furnished a total of 7.6 equiv. NH
3
. These
initial results show that inclusion of the
Co
(III,N)
+
mediator turns on electrocatalysis by
W
(N
2
)
2
, at a potential that is 1.25 V positive of Pickett’s original work (Fig. 1c)
15
. In
the absence of the mediator, electrocatalysis is not observed; N
2
RR is not kinetically
competitive under these conditions, presumably because an uncoupled ET-PT pathway is
not facile (see Supplementary Section 13).
Canvassing factors to improve electrocatalytic N
2
RR by this tandem
W
(N
2
)
2
/
Co
(III,N)
+
co-catalyst system (see Supplementary Section 5) led us to adopt a boron-doped diamond
(BDD) working electrode, dimethoxyethane (DME) solvent and lithium bistriflimide ([Li]
[NTf
2
]) electrolyte. Under these optimized conditions (BDD, 0.1 M [Li][NTf
2
], DME, 5
mM TsOH, 0.05 mM
W
(N
2
)
2
/
Co
(III,N)
+
) N
2
RR electrocatalysis notably improved, with
11.3 ± 0.5 equiv. NH
3
per
W
(N
2
)
2
/
Co
(44.5 ± 1.9% FE) being generated at −1.35 V over 5.5
h (Fig. 2a,b). Quantification of the H
2
in the headspace after an equivalent CPC experiment
results in 39% FE for HER. A higher turnover number per
W
(N
2
)
2
/
Co
(III,N)
+
(up to 39.5
equiv.) was demonstrated by using a higher surface area GC foam electrode and lowering
the catalyst concentration (Supplementary Table 1). Control experiments to demonstrate that
both catalysts are required, to rule out the presence of catalytically active decomposition
products and to show that N
2
is the source of the NH
3
generated (Fig. 2c) are provided in the
Supplementary Information.
To assess the electrochemical behaviour of the
W
(N
2
)
2
/
Co
(III,N)
+
co-catalyst system,
a series of cyclic voltammograms (CVs) were collected. Following previous studies,
dissolution of
W
(N
2
)
2
in THF with added TsOH quantitatively produces the doubly
protonated hydrazido complex (TsO)
W
(NNH
2
)
+
(ref.
16
). CVs of (TsO)
W
(NNH
2
)
+
in a 0.1
M [Li] [NTf
2
] THF solution on a BDD working electrode show two irreversible one-electron
waves at low potential (less than −1.9 V; Fig. 3a). These waves are due to the generation of
W
(NNH
2
)
+
and
W
(NNH
2
), respectively
15
, in which the strongly reducing potential reflects
the challenge in reducing the 18-electron, closed-shell (TsO)
W
(NNH
2
)
+
complex. Although
addition of excess TsOH (100 equiv.) to the solution containing (TsO)
W
(NNH
2
)
+
leads to
an increase in current (irreversible) with an onset at −1.3 V, the same response is observed
without added
W
and is due to background HER at the electrode (Fig. 3a). The independent
CV of
Co
(III,N)
+
in THF shows a reversible Co
III/II
couple at −1.35 V (Fig. 3b), assigned to
Co
(III,N)
+
/
Co
(II,N)
18
. This couple shifts to −1.21 V when the mediator is protonated at the
tethered dimethylaniline group (that is,
Co
(III,NH)
2+
/
Co
(II,NH)
+
) (Fig. 3a). Gratifyingly,
CVs of (TsO)
W
(NNH
2
)
+
in the presence of
Co
(III,N)
+
and TsOH result in an irreversible
multi-electron wave at −1.2 V (Fig. 3a), consistent with electrocatalytic N
2
RR. At 100
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mV s
−1
an approximately threefold increase in the catalytic current is observed, compared
with the current of the one-electron reduction wave of
Co
(III,NH)
2+
, which is indicative
of relatively slow N
2
RR electrocatalysis. When the scan rate is reduced to 25 mV s
−1
,
the increase is 6.5-fold. Therefore, on this timescale, NH
3
is expected to be produced not
only at the surface of the electrode, as supported by rotating ring-disk electrode (RRDE)
experiments (see Supplementary Information), but also in the bulk solution, as evidenced
by chemical reactions between
Co
(II,NH)
+
and (TsO)
W
(NNH
2
)
+
and (TsO)
W
(NH)
+
(see
below). The reversible CV response of the
Co
(III,NH)
2+
/
Co
(II,NH)
+
couple at −1.21 V (in
the absence of TsOH) is noticeably altered as (TsO)
W
(NNH
2
)
+
is added; if scanning at
a slow rate (for example, 5–25 mV s
−1
), the presence of the
Co
(III,N)
+
/
Co
(II,N) couple
becomes clearly evident (Fig. 3b). The implication is that, as
Co
(II,NH)
+
is generated in
the presence of (TsO)
W
(NNH
2
)
+
, a PCET step occurs that generates
Co
(III,N)
+
, the CV
response of which becomes apparent at scan rates well matched to the kinetics of this
chemical step in the absence of acid. On the basis of kinetic analysis viacyclic voltammetry
(see Supplementary Section 13), a rate of approximately 0.5 s
−1
is estimated for this PCET
reaction, which is consistent with the rate-contributing nature of this step and relatively slow
catalysis overall.
To independently probe the PCET step, we generated (TsO)
W
(NNH
2
)
+
in THF with
excess TsOH present, and added 2 equiv.
Co
(II,N) to the solution, conditions under
which
Co
(II,NH)
+
is instantly generated. Such a reaction liberates 0.39 equiv. NH
3
per (TsO)
W
(NNH
2
)
+
(Fig. 3c) over 4 h (note: this experiment was performed at 0
°C to attenuate the competing HER). Analysis of the reaction mixture by
31
P NMR
spectroscopy showed some remaining (TsO)
W
(NNH
2
)
+
starting material and also a new
peak corresponding to the imido complex (TsO)
W
(NH)
+
. The identity of the latter
species was confirmed by its independent generation via the protonation of the nitride
precursor (N
3
)
W
(N) with TsOH (Supplementary Fig. 56)
22
24
. Although other processes
are presumably operating in this reaction (for example, HER and proton transfer (PT) steps
to other W−N
x
H
y
intermediates), the rate correlates well with the CV experiment noted
above, suggesting that PCET to (TsO)
W
(NNH
2
)
+
occurs, presumably followed by N−N
cleavage and NH
3
release (equation (1) below depicts one plausible scenario). Such a PCET
step helps to explain why the system can be turned over at −1.2 V, whereas one-electron
reduction of (TsO)
W
(NNH
2
)
+
requires a potential of −1.9 V.
TsO
W
(NNH
2
)
+
+
Co
II, NH
+
TsO
W
N
+
+
Co
III, N
+
+ NH
3
(1)
Related to this, CV of (TsO)
W
(NH)
+
(Fig. 3d), a presumed downstream intermediate of the
catalytic cycle generated by in situ protonation of the nitride complex (N
3
)
W
(N), shows a
similar electrocatalytic behaviour in the presence of
Co
(III,N)
+
. Additionally, the reaction of
(N
3
)
W
(N) with 4 equiv.
Co
(II,N) in the presence of excess TsOH afforded 0.70 equiv. NH
3
per
W
, along with (TsO)
W
(NNH
2
)
+
and (TsO)
W
(NH)
+
detected by
31
P NMR spectroscopy
(Supplementary Fig. 59). Again, PCET from
Co
(II,NH)
+
to (TsO)
W
(NH)
+
is probably key,
given the a one-electron reduction potential for a (TsO)
W
(NH)
+
species of approximately
−1.8 V (Fig. 3d).
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By independently varying the concentration of the
W
(N
2
)
2
and
Co
(III,N)
+
we could
determine a positive order for both co-catalysts in the electrocatalytic response (Fig. 3e).
Interestingly, a positive order in acid was also evident (Supplementary Fig. 73), as was a
primary kinetic isotope effect (2.5) when comparing TsOH versus TsOD (Supplementary
Fig. 79). These electrochemical and chemical data are consistent with one or perhaps two
rate-contributing PCET steps, involving (TsO)
W
(NNH
2
)
+
and possibly also (TsO)
W
(NH)
+
, and a rate-contributing protonation step (such as initial protonation of
W
(N
2
)
2
; see
Supplementary Section 9). Thermodynamic arguments using associated rate estimates (see
Supplementary Section 13)
25
,
26
point to a probable concerted PCET step as being turnover
limiting in this catalysis, akin to simpler reactions using this mediator with unsaturated
organic substrates that we have previously attributed to CPET
18
,
19
.
To explore our strategy more broadly, we turned our attention to a series of complexes
known to mediate catalytic N
2
RR in the presence of various reductant/acid reagents (Figs.
3f and 4a). We opted to test them under the standard conditions (Fig. 2a), reasoning that
some degree of electrocatalysis might turn on at −1.2 V if PCET steps from
Co
(II,NH)
+
can
similarly circumvent the need for challenging ET steps requiring more negative potentials.
To probe this, we examined the group VIII complexes
Fe
(N
2
) and
Os
(N
2
), where
Fe
and
Os
feature tris(phosphine)borane and silane ligands, respectively (Fig. 4a). Each mediates
chemical N
2
RR at −78 °C, but requires a comparatively strong reductant (Cp*
2
Co at −2 V)
owing to an M−N
2
0/−
couple that is key to moving through their respective N
2
RR cycles
via uncoupled electron transfer-proton transfer steps (equations (2) and (3))
27
,
28
. Strikingly,
both
Fe
and
Os
display an electrocatalytic wave at −1.2 V (Fig. 3f), akin to
W
(N
2
)
2
, and
CPC at −1.35 V produced 5.6 and 4.5 equiv. NH
3
, respectively (Fig. 4b). Despite their
relatively lower selectivity for NH
3
generation compared to
W
(N
2
)
2
under these conditions,
the electrocatalysis observed represents a remarkable shift in thermodynamic efficiency for
the overall N
2
RR cycle relative to previously reported conditions (see below). To explain
this, we posit that the neutral M−N
2
adduct species are converted directly to M−N=NH
intermediates via PCET from
Co
(II,NH)
+
(equation (4)), circumventing the M−N
2
0/−
couple
in the cycle. For
Fe
specifically, generation of the on-path
Fe
(N
2
) species requires reduction
of the
Fe
+
pre-catalyst used here, which occurs at about −1.4 V (see Supplementary
Section 14). Thus, applying slightly more bias in the CPC (−1.45 V instead of −1.35 V)
results in improved NH
3
yield (9.3 equiv. NH
3
per
Fe
). A related tris(phosphine)silyl iron–
N
2
complex, (SiP
3
)Fe(N
2
), is instead electrocatalytically inactive, presumably due to the
generation of an undesired (SiP
3
)Fe(H)(N
2
) state
7
M
− N
2
+ e
M
− N
2
( − 2V)
(2)
M
− N
2
+ H
+
M
− N
2
H
(3)
M
− N
2
+
Co
(II, NH)
+
M
− N
2
H +
Co
(III, N)
+
(4)
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We also explored a series of Mo complexes, including two tetrakis (phosphine) systems
that are structurally related to
W
(N
2
)
2
(compounds
1
and
2
in Fig. 4a), and highly
active pincer-type bis(phosphine)pyridine complexes (
3
and
4
in Fig. 4a) pioneered more
recently
6
,
29
. Among the reductants that have proved to be effective for these systems,
SmI
2
/H
2
O has led to the most impressive results in chemically driven catalysis
30
. We find
that the
Mo
(N
2
)
2
complexes
1
and
2
are both effective co-electrocatalysts with favourable
selectivities, furnishing 13 and 14 equiv. NH
3
(51 and 55% FE for NH
3
), respectively. The
dinuclear Mo catalyst system (
3
) also displays electrocatalysis under these conditions (8.7
equiv. NH
3
per Mo)
6
. By contrast, the mononuclear triiodide complex (
4
), which has been
demonstrated to be highly active for N
2
RR
31
, is electrocatalytically inactive under these
conditions (<0.1 equiv. NH
3
detected). The latter observation is readily explained; the strong
reduction potential (−1.8 V) required to access an on-path N
2
RR intermediate by iodide loss
is not accessible at −1.35 V.
The free energy for the electrocatalytic N
2
RR processes described here compares quite
favourably to estimates for other systems that mediate catalytic and electrocatalytic N
2
RR.
This can be readily quantified by ΔΔ
G
f
(NH
3
), a term that compares the energetic input for
N
2
RR relative to a reaction that derives the needed protons and electrons from H
2
(equation
(5))
26
. Using the bond dissociation free energy (BDFE) for H
2
(102.5 kcal mol
−1
)
32
and
that of the PCET mediator
Co
(II,NH)
+
(38.9 kcal mol
−1
)
18
, the ΔΔ
G
f
(NH
3
) is 36.5 kcal
mol
−1
for the electrocatalysis observed at −1.2 V by our CV studies. This net driving
force is at least 50 kcal mol
−1
lower than has been reported for most other reductant/acid
cocktails used with synthetic N
2
RR catalyst systems (Fig. 4c): SmI
2
/H
2
O (75 kcal mol
−1
),
Cp*
2
Co/[Ph
2
NH
2
]
+
(77 kcal mol
−1
), KC
8
/HBAr
F
4
(196 kcal mol
−1
) (BAr
F
4
, B(3,5-(CF
3
)
2
-
C
6
H
3
)
7
,
27
,
30
. A crude comparison with the biological nitrogenases (approximately 117 kcal
mol
−1
accounting for ATP) is also favourable
32
. Likewise, heterogeneous systems based
on Li
+
/Li (
E
°(Li
+/0
) < −3.7 V), which commonly utilize ethanol as the acid, operate at an
estimated ΔΔ
G
f
(NH
3
) = 133 kcal mol
−1
(ref.
33
). Interestingly, one combination of reductant
and acid, Cp
2
Co and lutidinium, first studied in the Schrock system and later applied
towards N
2
RR catalysis with the Nishibayashi bis(phosphine)pyridine molybdenum catalyst
studied herein (complex
3
in Fig. 4)
5
,
6
, is thermally favourable by comparison (ΔΔ
G
f
(NH
3
)
= 26 kcal mol
−1
). This suggests that alternative acids and mediator designs may yet improve
the efficiency achievable by tandem electrocatalysis.
ΔΔG
f
NH
3
= 3
BDFE
H
2
/2 − BDFE
eff
(5)
In closing, it is widely appreciated that PCET steps can offer thermodynamic advantages
relative to distinct ET-PT or PT-ET pathways in enzyme catalysis, in which multi-electron
redox reactions must be driven at biologically accessible potentials
26
, and also in synthetic
catalyst systems
4
. The tandem catalytic approach to N
2
RR via electrochemical PCET
described herein provides a vivid example of the latter, in which a PCET step that we
suggest is largely concerted turns on catalysis that is otherwise inaccessible at the applied
potential. A comparison with nitrogenase enzymes is illustrative here. It has been posited
that the active-site cofactors of nitrogenases store up proton and electron equivalents via
H atoms bound at or near to the active-site cluster, to be able to mediate N
2
reduction
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at a single redox potential (set by the potential of the Fe protein)
34
. Our two-component
tandem catalyst system functions in a conceptually similar manner, in which the mediator
independently stores an H atom equivalent at its own redox potential for delivery to a
synthetic M−N
2
active site.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgements
We thank the Dow Next Generation Educator Funds and Instrumentation Grants for their support of the NMR
facility at Caltech. We also thank the Resnick Water and Environment Laboratory (WEL) and the Molecular
Materials Resource Center at Caltech for the use of their instrumentation. We thank the following funding agencies:
Department of Energy, Office of Basic Energy Sciences (DOE-0235032), Catalysis Science Program (for the
development and applications of CPET mediators); National Institutes of Health (R01 GM-075757) (for studies of
Fe-mediated N
2
RR). P.G.-B. thanks the Ramón Areces Foundation for a postdoctoral fellowship. J.D. thanks the
Arnold and Mabel Beckman Foundation for a postdoctoral fellowship. M.J.C. thanks the Resnick Sustainability
Institute for a graduate fellowship.
Data availability
Details on the procedures and the corresponding datasets generated during and/or analysed
during the current study are available within the paper and its Supplementary Information
files, and from the corresponding author on reasonable request.
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Fig. 1 |. Approaches to N
2
RR electrocatalysis.
a
, A representative distal cycle for catalytic N
2
-to-NH
3
conversion.
b
, A previously reported
molecular N
2
RR electrocatalyst based on the tris(phosphine)borane Fe system, operating at
−2.1 V on a glassy carbon electrode using a temperature of −35 °C.
c
, Early work by Pickett
demonstrating electrosynthesis of NH
3
using the molecular complex
W
(N
2
)
2
at an applied
potential of −2.6 V on a Hg-pool electrode using TsOH. The protonation step had to be
performed separately from the reduction step (0.21 equiv. NH
3
per
W
(N
2
)
2
after one cycle;
0.73 equiv. total NH
3
per
W
(N
2
)
2
after three cycles).
d
, Tandem catalysis described in this
work based on coupling the PCET mediator,
Co
(II,NH)
+
, with molecular N
2
RR catalysts to
enable well-defined electrocatalysis at comparatively mild potentials (−1.2 V using TsOH).
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Fig. 2 |. Electrocatalytic N
2
RR via tandem catalysis.
a
, Electrocatalytic N
2
RR on CPC at −1.35 V versus Fc
+/0
in 0.1 M [Li][NTf
2
] DME
solution containing 0.05 mM
Co
(III,N)
+
, 0.05 mM
W
(N
2
)
2
and 5 mM TsOH, using a
BDD plate working electrode.
b
, Current (
j
) profile for the CPC experiment described
(black trace) and a similar CPC experiment in the absence of the
Co
(III,N)
+
mediator.
c
,
Quantification of NH
3
following CPC via
1
H NMR spectroscopy using either
14
N
2
,
15
N
2
or
an argon atmosphere. Average result for
W
(N
2
)
2
and its associated error (standard deviation)
corresponds to three electrocatalytic runs.
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Fig. 3 |. Mechanistic insights into tandem PCET N
2
RR.
a
, CV of 50 mM TsOH (dashed grey); 0.5 mM
Co
(III,N)
+
with 50 mM TsOH (black);
0.5 mM (TsO)
W
(NNH
2
)
+
(solid red); 0.5 mM (TsO)
W
(NNH
2
)
+
with 50 mM TsOH
(dashed red); 0.5 mM
Co
(III,N)
+
/(TsO)
W
(NNH
2
)
+
with 50 mM TsOH (purple).
b
, CV
at 5 mV s
−1
of 0.5 mM
Co
(III,NH)
2+
/(TsO)
W
(NNH
2
)
+
(purple trace) compared to 0.5 mM
Co
(III,NH)
2+
(red trace) and 0.5 mM
Co
(III,N)
+
(blue trace).
c
, Chemical reaction of 0.5
mM (TsO)
W
(NNH
2
)
+
in THF with 2 equiv.
Co
(II,N) in THF in the presence of excess
acid and the corresponding
31
P NMR spectrum.
d
, CV of 0.5 mM (N
3
)
W
(N) (black); 0.5
mM (N
3
)
W
(N) and 1 equiv. TsOH (solid red); 0.5 mM (N
3
)
W
(N) with 50 mM TsOH
(dashed red); 0.5 mM
Co
(III,N)
+
/(N
3
)
W
(N) with 50 mM TsOH (purple).
e
, Plot of the
catalytic current (
i
cat
) versus the concentration of the different co-catalysts.
f
, CVs of several
representative
M
(N
2
) catalysts (see Fig. 4a for their chemical structures) studied under the
standard electrocatalytic conditions in the presence of
Co
(III,N)
+
and TsOH, showing a
multi-electron catalytic wave for N
2
RR in each case at the same applied bias as for
W
(N
2
)
2
.
Note that all CVs in
a
f
were performed at 100 mV s
−1
(unless otherwise stated) in 0.1 M
[Li][NTf
2
] THF solution using a BDD disk as the working electrode, Pt disk as the counter
electrode and Ag/AgOTf (5 mM) as the reference electrode. eN
2
RR, electrocatalytic N
2
RR.
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Fig. 4 |. Electrocatalytic N
2
RR using reported molecular catalysts.
a
, Molecular N
2
RR catalysts explored in combination with the PCET mediator under
electrocatalytic conditions.
b
, Results of the electrocatalytic experiments for each molecular
catalyst on CPC at −1.35 V versus Fc
+/0
in 0.1 M [Li][NTf
2
] DME solution containing
0.05 mM
Co
(III,N)
+
, 0.05 mM of N
2
RR catalyst and 5 mM TsOH, using a BDD plate
working electrode. Average result for
W
(N
2
)
2
and its associated error (standard deviation)
corresponds to three electrocatalytic runs. *A GC foam was used as the working electrode
instead and the concentration was 0.01 mM for both co-catalysts and 1 mM TsOH.
c
,
Estimated overpotential (ΔΔ
G
f
) for N
2
RR, including the tandem PCET strategy reported
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here, a nitrogenase enzyme, Li-mediated N
2
RR using EtOH as the H
+
source and various
reductant and acid partners used in chemically driven N
2
RR.
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