Electrocatalytic Nitrogen Reduction on a Molybdenum Complex
Bearing a PNP Pincer Ligand
Ammar F. Ibrahim
†
,
Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech),
Pasadena, California 91125, United States
Pablo Garrido-Barros
†
,
Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech),
Pasadena, California 91125, United States
Jonas C. Peters
Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech),
Pasadena, California 91125, United States
Abstract
Electrocatalytic nitrogen reduction (N
2
R) mediated by well-defined molecular catalysts is poorly
developed by comparison with other reductive electrocatalytic transformations. Herein, we explore
the viability of electrocatalytic N
2
R mediated by a molecular Mo-PNP complex. A careful
choice of acid, electrode material, and electrolyte mitigates electrode-mediated HER under direct
electrolysis and affords up to 11.7 equiv of NH
3
(Faradaic efficiency < 43%) at −1.89 V versus
Fc
+
/Fc. The addition of a proton-coupled electron transfer (PCET) mediator has no effect.
The data presented are rationalized by an initial electron transfer (ET) that sets the applied
bias needed and further reveal an important impact of [Mo] concentration, thereby pointing to
potential bimolecular steps (e.g., N
2
splitting) as previously proposed during chemically driven
N
2
R catalysis. Finally, facile reductive protonation of [Mo(N)Br(
H
PNP)] with pyridinium acids is
demonstrated.
Graphical Abstract
Corresponding Author
:
Jonas C. Peters
– Division of Chemistry and Chemical Engineering, California Institute of Technology
(Caltech), Pasadena, California 91125, United States; jpeters@caltech.edu.
†
These authors contributed equally to this study.
Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.2c04769
.
CCDC 2181571 (
CIF
)
General materials and methods, ammonia quantification, controlled potential electrolysis, cyclic voltammetry experiments, synthesis
and characterization, UV−vis spectroscopy, and references (
PDF
)
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.2c04769
The authors declare no competing financial interest.
HHS Public Access
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ACS Catal
. 2023 January 6; 13(1): 72–78. doi:10.1021/acscatal.2c04769.
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Keywords
nitrogen reduction; electrocatalysis; molecular catalysts; media effects; reaction mechanism
Because of the role of ammonia as chemical fertilizer feedstock and as a possible energy
vector, the nitrogen reduction (N
2
R) reaction offers promise in sustainable and decentralized
energy storage and fertilizer synthesis where the energy to drive N
2
R can be renewably
sourced.
1
–
3
Toward this end, significant attention is turning toward achieving efficient N
2
R
via electrocatalysis.
4
–
7
From the perspective of fundamental studies, molecular N
2
R electrocatalysts can offer
distinct advantages in terms of synthetic tunability, selectivity, and suitability toward
detailed mechanistic investigations.
8
However, their systematic development has been
hampered by the strong reduction potential required to activate nitrogen.
9
–
13
Under such
conditions in acidic media, background hydrogen evolution reaction (HER) at the surface
of the electrode usually dominates.
14
For instance, the tris(phosphino)borane iron system,
P
3
B
Fe, has been demonstrated to be a modest N
2
R electrocatalyst when interfaced with
a glassy carbon electrode and an ammonium acid but requires low temperature (−35 °C)
to mitigate background HER.
15
Relatedly, important progress has recently been made in
electrochemical N
2
splitting by molecular complexes to generate metal nitrides (M–N),
16
–
19
but further development is warranted to incorporate a proton source toward successful
electrocatalytic N
2
-to-NH
3
conversion.
While chemical reductants can lead to similar selectivity issues, a careful choice of
reagent cocktails and conditions can mitigate competing HER, and impressive selectivities
have been demonstrated for N
2
R catalysis via such approaches.
8
,
20
–
23
Analogous efforts
to attenuate HER and, hence, enhance the relative rate of N
2
R versus HER under
electrochemical conditions are essential.
24
,
25
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Very recently, our lab reported that a tandem electrocatalytic strategy based on a proton-
coupled electron transfer (PCET) mediator (Figure 1A) affords a promising approach.
7
Through the use of a cobaltocene-derived PCET mediator [
Co
(III,N)
+
; Figure 1],
26
H
+
and e
−
equivalents (net H atoms) are transferred to certain M-N
x
H
y
intermediates at milder
potentials than would otherwise be required via stepwise electron transfer–proton transfer
(ET−PT) steps. This strategy enables N
2
R electrocatalysis at room temperature (rt) pinned
to the potential of the PCET mediator (onset at −1.2 V vs Fc
+
/Fc; all potentials herein are
referenced Fc
+/0
) using a variety of molecular complexes (e.g., Fe, Os, Mo, W) with tosic
acid as the H
+
source; background HER is attenuated at this potential.
Certain ET steps during N
2
R may not be immediately followed by protonation; the utility of
a PCET mediator would be limited in cases where discrete ET steps determine the applied
potential needed to drive electrocatalysis. An illustrative example is the family of fascinating
N
2
R precatalysts described by Nishibayashi and co-workers on the basis of the [Mo
III
X
3
(
H
PNP)] platform (
H
PNP = 2,6-bis(di-
tert-
butylphosphinomethyl)pyridine; X = Cl, Br, I;
Figure 1B).
27
,
28
These complexes have been proposed to undergo bimolecular dinitrogen
splitting upon accessing a [Mo
I
X (
H
PNP)] state to furnish a terminal nitride [Mo
IV
(N)X-
(
H
PNP)] (Figure 1C). Further reductive protonation of the nitride intermediate releases NH
3
.
In this case, N
2
binding and activation is gated by a limiting ET step associated with the
Mo
III/II
redox couple (
E
° < −1.70 V).
Relatedly, an elegant mechanistic study by Miller and coworkers demonstrated
electrochemical N
2
splitting using this same platform, where [Mo
IV
(N)Br(
H
PNP)] is
generated from a net two-electron reduction of [Mo
III
Br
3
(
H
PNP)] at −1.89 V (Figure
1B).
16
However, the use of [Mo
III
X
3
(
H
PNP)] as an electrocatalyst has remained elusive
despite its remarkable activity for chemical N
2
R. Here, we explore factors that enable N
2
R
electrocatalysis using [Mo
III
Br
3
(
H
PNP)] and find that the electrode, the acid, the electrolyte,
and the solvent play essential roles in enhancing N
2
-to-NH
3
conversion.
We first explored the impact of incorporating the
Co
(III,N)
+
PCET-mediator (Figure 1A)
to facilitate N−H bond-forming steps toward electrocatalytic N
2
R with [Mo
III
X
3
(
H
PNP)]
catalysts. Distinct from other catalysts canvassed,
7
at −1.35 V, only trace NH
3
could be
detected. This suggests a required applied potential of <−1.70 V for [Mo
III
X
3
(
H
PNP)] in
order to access an on-path N
2
R intermediate via an ET step. We wondered whether a
downstream PCET step (e.g., reductive protonation of a Mo−N intermediate) might still
play an important role in enabling N
2
R; hence, we evaluated the viability of this tandem
PCET-mediated approach under more reducing conditions (Figure 2A).
Controlled potential electrolysis (CPE) at −1.89 V yielded substoichiometric amounts
of ammonia (0.7 equiv NH
3
) using similar conditions as previously reported: 0.1 mM
Co
(III,N)
+
, 0.1 mM [Mo
III
Br
3
(
H
PNP)], 100 equiv of tosic acid (TsOH·H
2
O), and 0.1
M [Li][NTf
2
] electrolyte in THF solution with a boron-doped diamond (BDD) working
electrode (see Supporting Information (SI) for electrochemical details and electrode sizes).
An analogous CPE experiment in the absence of
Co
(III,N)
+
produced similar results (0.8
equiv NH
3
), which suggests that PCET mediation does not enhance the N
2
R efficiency of
this catalyst system.
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We next explored other factors that might enable electro-catalysis with [Mo
III
Br
3
(
H
PNP)].
TsOH is well matched to the protonation of
Co
(III,N)
+
but also leads to significant
background HER (
E
onset
= −1.5 V). This HER likely outcompetes the alternative reduction
of [Mo
III
Br
3
(
H
PNP)] and corresponding N
2
-derived intermediates. Thus, conditions were
explored to attenuate electrode-mediated HER. From the pyridinium acids typically used in
chemical N
2
R with Mo-based catalysts,
29
we found that collidinium triflate ([ColH]-[OTf])
provides a wide redox window for operation.
Cyclic voltammetry (CV) on a BDD working electrode at 100 mV/s in the presence of
50 mM [ColH][OTf] in 100 mM [Li][NTf
2
] electrolyte THF solution revealed an increase
in current associated with electrode-mediated HER starting at −1.80 V. CV of 0.5 mM
[MoBr
3
(
H
PNP)] (Figure 2C) in the same electrolyte solution but in the absence of acid
featured a reduction wave at −1.80 V associated with the Mo
III/II
redox couple, which is
consistent with previous observations. The addition of [ColH][OTf] (50 mM) resulted in
a current increase at the Mo
III/II
redox wave that is suggestive of N
2
R electrocatalysis at
potentials sufficiently anodic of the background HER. Accordingly, a controlled potential
electrolysis (CPE) using 0.10 mM [Mo] and 100 equiv of [ColH][OTf] at −1.89 V furnished
9.4 ± 0.3 equiv of ammonia per Mo atom (FE = 34 ± 1%) over 10 h (Figure 2B). CPE
experiments in the absence of [MoBr
3
(
H
PNP)] failed to produce significant amounts of NH
3
.
Control experiments corroborated the electrocatalytic nature of this system for N
2
-to-NH
3
conversion (see SI).
Comparative analysis of other pyridinium acids revealed a strong impact on the basis of
the acid choice during electrocatalytic N
2
R, as inferred from CV data (SI) and measured
NH
3
yields from CPE experiments (Table 1). For example, the use of more acidic lutidinium
triflate ([LutH]-[OTf]) enhanced background HER and led to a corresponding drop in the FE
to ~ 16% (4.5 equiv NH
3
), despite the fact that [LutH][OTf] can perform well as the acid
under chemically (as opposed to electrochemically) driven catalysis with Mo complexes.
30
Picolinium triflate ([PicH][OTf]) afforded the highest background HER current and very
low NH
3
conversion (5% FE, 1.3 equiv NH
3
). Therefore, the use of weaker acids that are
still compatible with N
2
R proves to be essential to optimize the FE of electrocatalytic NH
3
formation.
The electrode material also influences the kinetics of competitive HER. The use of a
glassy carbon (GC) electrode instead of BDD in the presence of [ColH][OTf] led to a
2-fold increase in current at the catalytically relevant potential (−1.89 V). This trend was
reproduced when [LutH][OTf] and [PicH][OTf] were employed, with an HER current
response approximately double that obtained with BDD electrodes. Consistently, CPE
experiments using [MoBr
3
(
H
PNP)] and a GC plate electrode (Table 1) resulted in a
decreased NH
3
yield (5.2 equiv; 19%) relative to BDD.
We also explored the viability of electrocatalysis using the analogous [Mo
III
Br
3
(
Me
PNP)]
complex. The substituted MePNP ligand has been shown to improve turnover frequency
and turnover number during chemically driven N
2
R using related Mo
0
complexes.
31
The introduction of an electron-donating Me group shifts the reduction potential of
[Mo
III
Br
3
(
Me
PNP)] cathodically by 60 mV with respect to [Mo
III
Br
3
(
H
PNP)], as evidenced
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by cyclic voltammetry (see SI). In this case, the higher HER background expected to be
associated with operating at a lower potential (−1.95 V) is potentially offset by the higher
TOF of this catalyst, thereby generating 8.1 equiv of NH
3
(30% FE) after only 4 h of CPE.
Additionally, a subsequent CPE experiment performed with a reloading of 100 equiv of
[ColH][OTf] resulted in a total of 13.0 equiv of NH
3
per Mo atom (FE = 24% overall; Table
S1).
Bimolecular steps have shown to play an important role in the mechanism of both chemical
N
2
R catalysis and electro-chemical N
2
splitting by the [Mo
III
Br
3
(
H
PNP)] catalyst.
27
,
28
Specifically, reduction to [Mo
I
Br(
H
PNP)] leads to the formation of an N
2
-bridged Mo
complex, {[(
H
PNP)- Mo
I
(Br)]
2
(
μ
-N
2
)}, which undergoes exergonic N
≡
N bond cleavage
to form [Mo(N)Br(
H
PNP)]. We, thus, wondered whether concentration might play a
role in enhancing the efficiency of electrocatalytic N
2
R. CPE experiments at higher
Mo concentrations (0.2 mM; Table 1, entry 7) maintaining the same acid ratio (100
equiv) led to a significant increase in the NH
3
production (11.7 equiv) and FE (43%).
Conversely, a decrease of the concentration to 0.05 mM had the opposite effect, with the
FE dropping to 17% (4.7 equiv NH
3
). These results are consistent with (though do not
require) a bimolecular N
2
splitting step being kinetically relevant under our electrocatalytic
conditions. Importantly, further increase in the [Mo] concentration led to extensive THF
polymerization attributable to higher anodic current passed during the CPE, which imposes
practical limitations. A switch to solvents that provide convenient oxidation processes
without suffering from polymerization resulted in significantly attenuated electro-catalytic
performance (Table 1, entries 12−14), including 2-methyl tetrahydrofuran (2-MeTHF), 1,2-
dimethoxyethane (DME), or methanol (MeOH).
Miller and co-workers have previously shown that electro-chemical reduction
of [Mo
III
Br
3
(
H
PNP)] at −1.89 V can also lead to formation of low valent
{[(
H
PNP)Mo(N
2
)
2
]
2
(
μ-
N
2
)}.
16
Under acidic conditions, {[(
H
PNP)Mo(N
2
)
2
]
2
(
μ-
N
2
)} is
proposed to undergo N
2
fixation via a Chatt-type mechanism.
30
,
32
When we studied
{[(
H
PNP)Mo(N
2
)
2
]
2
(
μ-
N
2
)} during CPE experiments, we obtained only 1.7 equiv of NH
3
(6% FE; Table 1, entry 8); the addition of 3 equiv of LiBr under similar conditions doubled
the yield (to 3 equiv NH
3
). Previous observations have suggested that halide salts can
generate [Mo(N)X(
H
PNP)] from {[(
H
PNP)Mo(N
2
)
2
]
2
(
μ-
N
2
)} under catalytic conditions,
potentially involving a partial change to an N
2
splitting mechanism and associated
improvement of NH
3
yields.
27
Within a N
2
splitting scenario, the nitride complex [Mo(N)Br(
H
PNP)] should be a
relevant intermediate, which prompted us to independently evaluate its competence as
an electrocatalyst. CPE experiments with 0.1 mM [Mo(N)Br-(
H
PNP)] under our standard
conditions led to the catalytic formation of NH
3
(4.6 equiv) with a FE of 17% (Table
1, entry 9). While this yield is lower than those obtained with [Mo
III
Br
3
(
H
PNP)] as the
added catalyst, the addition of 2 equiv of [Li][Br] improved the FE of [Mo(N)Br(
H
PNP)] to
22% (6.0 equiv NH
3
). This observation might be attributable to an adverse anion exchange
equilibrium with OTf
−
, which would be partially prevented at higher [Br
−
], or instead to a
required generation of a higher valent [MoBr
n
(
H
PNP)] (
n
= 2 or 3) species.
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Given the supported intermediacy of a nitride [Mo(N)Br-(
H
PNP)] complex, we further
evaluated its electrochemical behavior (Figure 3A−C). CV of [Mo(N)Br(
H
PNP)] in 100 mM
[Li][NTf
2
] THF solution showed a reversible Mo
V/IV
oxidation wave at −1.2 V followed
by a Mo
IV/III
reduction peak at approximately −2.5 V (Figure 3A). The highly cathodic
potential contrasts with the milder potential required for electrocatalysis (approximately
−1.9 V). The addition of [ColH][OTf] to this solution resulted in the attenuation of
the Mo
V/IV
redox couple and the appearance of an electrocatalytic wave similar to that
found with [Mo
III
Br
3
(
H
PNP)]. This result suggests a protonation equilibrium process via
formation of the imido complex [Mo(NH)Br(
H
PNP)]
+
, reduction of which takes place at
around −1.8 V, as evidenced by an increase in the current, thereby enabling the system
to enter the N
2
R catalytic cycle. A more anodic scan reveals the appearance of a small
redox feature at 0 V that we associate with the oxidation of [Mo(NH)Br(
H
PNP)]
+
to
[Mo(NH)Br(
H
PNP)]
2+
(Figure 3B). This behavior follows a square scheme where the
oxidation of [Mo(NH)Br(
H
PNP)]
+
promotes deprotonation to form [Mo(N)Br(
H
PNP)]
+
(Figure 3D). Conversely, the reduction of this cationic nitride to [Mo(N)Br(
H
PNP)] is
followed by protonation to produce [Mo(NH)Br(
H
PNP)]
+
.
This mechanism correlates well with experiments employing the stronger acid [LutH][OTf].
Under such conditions, the protonation equilibrium is further shifted toward the formation
of [Mo(NH)Br(
H
PNP)]
+
, thereby resulting in a larger oxidation peak at 0 V (Figure 3B).
In the subsequent cathodic scan, the corresponding reduction peak showed significantly
lower current, and the [Mo(N)Br(
H
PNP)]
+/0
reduction became clearer. Variable scan rate
CV experiments allow for the calculation of the kinetic constant for the protonation of
[Mo(N)Br(
H
PNP)]:
k
PT
= ~3.3 M
−1
s
−1
with [ColH][OTf] (see Figure 3C and the SI).
The rapid kinetics for the protonation of [Mo(N)Br(
H
PNP)] and the facile reduction of
[Mo(NH)Br(
H
PNP)]
+
at the applied potential (−1.89 V) suggest a minor influence of these
steps in the overall kinetics and efficiency of N
2
R.
We have previously shown that the use of electrolytes containing Li
+
as a Lewis
acid enhances the efficiency of NH
3
production, presumably by activation of key N
2
R
intermediates.
7
,
33
We, thus, questioned whether [Li][NTf
2
] plays a similar role in the present
catalytic system. A substitution of Li
+
with the commonly used cation tetrabutylammonium
TBA
+
in our electrocatalytic set up led to greater than stoichiometric amounts of NH
3
(2.8 equiv; Table 2, entry 1). The use of [K][NTf
2
] as the electrolyte yielded catalytic,
albeit lower, amounts of NH
3
(5.0 equiv; Table 2, entry 3) compared with [Li][NTf
2
];
the difference between K
+
and Li
+
could correlate with their different Lewis acidities. [Na]
[NTf
2
] performed more poorly and afforded only 1.9 equiv of NH
3
at 7% FE (Table 2, entry
2). Na
+
might serve as a more efficient bromide abstracting agent to precipitate [Na][Br],
thereby affecting the availability of Br
−
during the catalysis; the latter can impact N
2
R
efficiency (
vide supra
).
34
,
35
We next studied how the presence of Li
+
might impact the efficiency of the electrochemical
N
2
splitting reaction as a potential key step during electrocatalysis. Subjection of 0.1 mM
[Mo
III
Br
3
(
H
PNP)] to a CPE at −1.89 V using a BDD plate electrode in a 100 mM [Li]
[NTf
2
] THF solution afforded [Mo(N)Br(
H
PNP)] in ~30% yield, as calculated via CV
analysis after CPE (Figure 4 and SI); no {[(
H
PNP)Mo-(N
2
)
2
]
2
(
μ
-N
2
)} was observed. When
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[TBA][NTf
2
] was used, instead, at similar concentrations, neither of these reduced Mo
species were detected. These results suggest the possibility that a Lewis acid such as Li
+
may aid in lowering the barrier for the formation of key N
2
-bridged species and/or stabilize
the [Mo(N)Br(
H
PNP)] product of the N
2
splitting reaction. Changes in the UV−vis spectrum
of [Mo(N)Br(
H
PNP)] upon addition of [Li][NTf
2
] support this idea (see SI). Previous work
has shown that [Mo(N)Br(
H
PNP)] can also be produced with TBA electrolytes ([TBA][PF
6
])
but at significantly higher [Mo
III
Br
3
(
H
PNP)] loadings (4 mM),
16
presumably because of the
influence of concentration on the bimolecular reaction. Although these conditions are not
suitable for our electro-catalytic system, the results discussed herein show that the presence
of Li
+
facilitates this cleavage reaction at lower [Mo] concentrations.
To conclude, herein we have shown that Nishibayashi et al.’s [Mo
III
Br
3
(
H
PNP)] N
2
R
catalyst system can be adapted to electrocatalysis on careful consideration of the reaction
medium. Electrocatalytic N
2
R by this system is gated by an initial ET step at −1.80 V
that sets the needed applied bias for the observed catalysis. This is distinct from PCET-
mediated N
2
R electrocatalysis, where an anodically shifted potential can be used because
of a rate-limiting PCET step that instead sets the required bias of the system. We have
further shown how the nature of the working electrode, the acid, the electrolyte, and the
catalyst concentration contribute to the observed N
2
R electrocatalysis in the present system
by enhancing the relative kinetics for N
2
R versus background HER. In particular, the
use of a BDD electrode, together with a relatively weak pyridinium acid ([ColH][OTf]),
allows minimization of the influence of competitive HER. The use of an electrolyte with
a hard Lewis acid, such as Li
+
, also increases NH
3
yields, potentially because of the
stabilization and/or activation of key N
2
R intermediates. These conditions contrast those
often employed in small molecule electrocatalysis (e.g., employing GC electrodes and
[TBA][X] electrolytes). We anticipate the findings disclosed here will aid in the future
development of more efficient N
2
R electrocatalysts using coordination complexes.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
ACKNOWLEDGMENTS
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 and the Molecular Materials
Resource Center at Caltech for the use of their instrumentation. The authors thank Dr. Michael Takase for assistance
with X-ray crystallography. 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 PCET
mediators) and National Institutes of Health (R01 GM-075757) (for fundamental studies of N
2
R catalysis). P.G.B.
thanks the Ramón Areces Foundation for a postdoctoral fellowship.
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Figure 1.
(A) Tandem
e
PCET strategy for N
2
R. (B) Chemically driven N
2
R catalysis and
electrochemical N
2
splitting by [Mo
III
X
3
(
H
PNP)]. (C) Previously proposed mechanism for
N
2
splitting by [Mo
III
X
3
(
H
PNP)].
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Figure 2.
(A) Tandem attempts at electrocatalytic N
2
R by [Mo
III
X
3
(
H
PNP)] using a cobaltocene-
derived PCET mediator. (B) Optimized conditions for electrocatalytic NH
3
generation using
[Mo
III
Br
3
(
H
PNP)]. (C) Cyclic voltammograms of 0.5 mM [Mo] (red trace), 50 mM [ColH]
[OTf] (blue trace), and 0.5 mM [Mo] with 50 mM [ColH][OTf] (purple trace) in a THF
solution containing 100 mM [Li][NTf
2
]. A BDD working electrode was employed at a 100
mV/s scan rate.
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Figure 3.
(A) CV of [Mo(N)Br(
H
PNP)] in the presence and absence of [ColH][OTf], and comparison
with just [ColH][OTf] or [MoBr
3
(
H
PNP)] in the presence of [ColH][OTf]. (B) CV of
[Mo(N)Br(
H
PNP)] in the presence of either [ColH][OTf] or [LutH][OTf]. (C) Variable
scan rate CV analysis of [Mo(N)Br(
H
PNP)] in the presence of [ColH][OTf]. (D) Square
scheme mechanism showing the protonation and reduction processes associated to
[Mo(N)Br(
H
PNP)]. Note: CVs were performed at 100 mV/s (unless otherwise noted) in
a 0.1 [Li][NTf
2
] THF solution using a BDD disk working electrode.
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