Fe-Mediated Nitrogen Fixation with a Metallocene Mediator:
Exploring p
K
a
Effects and Demonstrating Electrocatalysis
Matthew J. Chalkley
‡
,
Trevor J. Del Castillo
‡
,
Benjamin D. Matson
‡
, and
Jonas C. Peters
Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech),
Pasadena, California 91125, United States
Abstract
Substrate selectivity in reductive multi-electron/proton catalysis with small molecules such as N
2
,
CO
2
, and O
2
is a major challenge for catalyst design, especially where the competing hydrogen
evolution reaction (HER) is thermodynamically and kinetically competent. One common strategy
to achieve selectivity is to limit the direct reaction between acid and reductant with the intent of
slowing background HER. In this study, we investigate how the selectivity of a
tris(phosphine)borane iron(I) catalyst, P
3
B
Fe
+
, for catalyzing the nitrogen reduction reaction
(N
2
RR, N
2
-to-NH
3
conversion) versus HER changes as a function of acid p
K
a
. We find that there
is a strong correlation between p
K
a
and N
2
RR efficiency. Stoichiometric studies indicate that the
anilinium triflate acids employed are only compatible with the formation of early stage
intermediates of N
2
reduction (e.g., Fe(NNH) or Fe(NNH
2
)) in the presence of the metallocene
reductant Cp*
2
Co. This suggests that the interaction of acid and reductant is playing a critical role
in N–H bond forming reactions. DFT studies identify a protonated metallocene species as a strong
PCET donor and suggest that it should be capable of forming the early stage N–H bonds critical
for N
2
RR. Furthermore, DFT studies also suggest that the observed p
K
a
effect on N
2
RR efficiency
is attributable to the rate and thermodynamics, of Cp*
2
Co protonation by the different anilinium
acids. Experimental support for the hypothesis that Cp*
2
Co plays a critical role in P
3
B
Fe-
catalyzed N
2
RR comes from electrochemical studies. Inclusion of Cp*
2
Co
+
as a co-catalyst in
controlled potential electrolysis experiments leads to improved yields of NH
3
. The data presented
provide what is to our knowledge the first unambiguous demonstration of electrocatalytic nitrogen
fixation by a molecular catalyst (up to 6.7 equiv NH
3
per Fe at −2.1 V vs Fc
+/0
). While the
electrocatalysis is modest in terms of turnover, the comparatively favorable Faradaic efficiencies
for NH
3
(up to 31%) highlight the value of studying molecular N
2
RR catalysts to define design
criteria for selective N
2
RR electrocatalysis. Our collective results contribute to a growing body of
evidence that metallocenes may play multiple roles during reductive catalysis. While they can
*
Corresponding Author
. jpeters@caltech.edu.
ORCID
Jonas C. Peters: 0000-0002-6610-4414
‡
These authors contributed equally.
ASSOCIATED CONTENT
The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxx.
Computational models (MOL)
Experimental procedures, characterization data (PDF)
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
HHS Public Access
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J Am Chem Soc
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Published in final edited form as:
J Am Chem Soc
. 2018 May 16; 140(19): 6122–6129. doi:10.1021/jacs.8b02335.
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behave as single electron transfer (SET) reagents in the reductive protonation of small molecule
substrates, ring-functionalized metallocenes, previously considered as intermediates of
background HER, can also play a critical role in productive substrate bond-forming steps.
Graphical abstract
INTRODUCTION
There has been substantial recent progress in the development of soluble, well-defined
molecular catalysts for N
2
-to-NH
3
conversion, commonly referred to as the nitrogen
reduction reaction (N
2
RR).
1
Nevertheless, a significant and unmet challenge is to develop
molecular catalysts, and conditions, compatible with electrocatalytic N
2
RR. Progress in this
area could have both fundamental and practical benefits, including access to informative in
situ mechanistic studies via electrochemical techniques, and an electrochemical means to
translate solar or otherwise derived chemical currency (H
+
/e
−
) into NH
3
. The latter goal,
which has been the subject of numerous studies using heterogeneous catalysts, is key to the
long-term delivery of new ammonia synthesis technologies for fertilizer and/or fuel.
2
Many soluble coordination complexes are now known that electrocatalytically mediate the
hydrogen evolution reaction (HER),
3
the carbon dioxide reduction reaction (CO
2
RR),
4
and
the oxygen reduction reaction (O
2
RR).
5
The study of such systems has matured at a rapid
pace in recent years, coinciding with expanded research efforts towards solar-derived fuel
systems. In this context, it is noteworthy how little corresponding progress has been made
towards the discovery of soluble molecular catalysts that mediate electrocatalytic N
2
RR. To
our knowledge, only two prior systems address this topic directly.
6
,
7
,
8
More than three decades ago, Pickett and coworkers reported that a Chatt-type tungsten-
hydrazido(2−) complex could be electrochemically reduced to release ammonia (and trace
hydrazine), along with some amount of a reduced tungsten-dinitrogen product; the latter
species serves as the source of the tungsten-hydrazido(2−) complex (via its protonation by
acid).
6a
By cycling through such a process, an electrochemical, but not an electrocatalytic,
synthesis of ammonia was demonstrated. Indeed, efforts to demonstrate electrocatalysis with
this and related systems instead led to substoichiometric NH
3
yields.
6c
An obvious limitation to progress in electrochemical N
2
RR by molecular systems concerns
the small number of synthetic N
2
RR catalysts that have been available for study; it is only in
the past five years that sufficiently robust catalyst systems have been identified to motivate
such studies. In addition, the conditions that have to date been employed to mediate N
2
RR
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have typically included non-polar solvents, such as heptane, toluene, and diethyl ether
(Et
2
O), that are not particularly well-suited to electrochemical studies owing to the lack of
compatible electrolytes.
1
Nevertheless, several recent developments, including ones from our lab, point to the
likelihood that iron (and perhaps other) molecular coordination complexes may be able to
mediate electrocatalytic N
2
RR in organic solvent. Specifically, our lab recently reported that
a tris(phosphine)borane iron complex, P
3
B
Fe
+
, that is competent for catalytic N
2
RR with
chemical reductants, can also mediate electrolytic N
2
-to-NH
3
conversion,
6d
with the
available data (including that presented in this study) pointing to bona fide electrocatalysis in
Et
2
O.
Focusing on the P
3
B
Fe
+
catalytic N
2
RR system, a development germane to the present study
was its recently discovered compatibility with reagents milder than those that had been
originally employed.
1c
Thus, decamethylcobaltocene (Cp*
2
Co) and diphenylammonium
acid are effective for N
2
RR catalysis; these reagents give rise to fast, and also quite selective
(> 70% vs HER), N
2
RR catalysis at low temperature and pressure in ethereal solvent. In
addition, based on preliminary spectroscopic evidence and density functional theory (DFT)
predictions, it appears that a protonated metallocene species, Cp*(
η
4
-C
5
Me
5
H)Co
+
, may be
an important intermediate of N
2
RR catalysis under such conditions. Indeed, we have
suggested that Cp*(
η
4
-C
5
Me
5
H)Co
+
may serve as a proton-coupled-electron-transfer
(PCET) donor (BDE
C–H
(calc) = 31 kcal mol
−1
), thereby mediating net H-atom transfers to
generate N–H bonds during N
2
RR.
9
The presence of a metallocene mediator might, we
therefore reasoned, enhance N
2
RR during electrocatalysis.
We present here a study of the effect of p
K
a
on the selectivity of P
3
B
Fe
+
for N
2
RR vs HER.
By using substituted anilinium acids, we are able to vary the acid p
K
a
over 9 orders of
magnitude and find that the selectivity is highly correlated with the p
K
a
. In our efforts to
investigate the origin of the observed p
K
a
effect, we found, to our surprise, that in
stoichiometric reactions, the catalytically competent anilinium triflate acids are unable to
faciltate productive N–H bond formation with early-stage N
2
-fixation intermediates. We
therefore hypothesize that the formation of a protonated metallocene species, Cp*(
η
4
-
C
5
Me
5
H)Co
+
, plays a critical role in N–H bond-forming reactions, either via PCET, PT, or a
combination of both during N
2
RR catalysis. DFT studies support this hypothesis and also
establish that the the observed p
K
a
correlation with N
2
RR selectivity can be explained by the
varying ability of the acids to protonate Cp*
2
Co. The suggested role of this protonated
metallocene intermediate in N–H bond forming reactions led us to test the effect of Cp*
2
Co
+
as an additive in the electrolytic synthesis of NH
3
mediated by P
3
B
Fe
+
. We find that the
addition of co-catalytic Cp*
2
Co
+
enhances the yield of NH
3
without decreasing the Faradaic
efficiency (FE), and furnishes what is to our knowledge the first unambiguous demonstration
of electrocatalytic N
2
RR mediated by a soluble, molecular coordination complex.
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RESULTS AND DISCUSSION
p
K
a
studies
In our recent study on the ability of P
3
B
Fe
+
to perform N
2
RR with Cp*
2
Co as the chemical
reductant,
9
we found that there was a marked difference in efficiency for NH
3
generation
with diphenylammonium triflate ([Ph
2
NH
2
][OTf]) versus anilinium triflate ([PhNH
3
][OTf]).
In that study, we posited that this difference could arise from several possibilities, including
the differential solubility, sterics, or p
K
a
’s of these acids.
9
To investigate the last possibility, we have studied the efficiency of the catalysis by
quantifying the NH
3
and H
2
produced when using substituted anilinium acids with different
p
K
a
values (Table 1). The table is organized in increasing acid strength, from [
4-OMe
PhNH
3
]
[OTf] as the weakest acid to the perchlorinated derivative ([
per-Cl
PhNH
3
][OTf]) as the
strongest. Importantly, good total electron yields (85.8 ± 3.3) were obtained in all cases. As
can be seen from the table, the NH
3
efficiencies are found to be strongly correlated with
p
K
a
.
10
In particular, a comparison of the efficiency for NH
3
with the p
K
a
of the anilinium acid used
gives rise to four distinct activity regimes (Table 1, Figure 1). One regime that is completely
inactive for N
2
RR, but active for HER, is defined by the weakest acid, [
4-OMe
PhNH
3
][OTf]
(p
K
a
= 8.8).
11
A gradual increase in observed NH
3
yields, coupled with a decrease in H
2
yield, comprises a second regime, in which the acid is strengthened from [PhNH
3
][OTf]
(p
K
a
= 7.8), to [
2,6-Me
PhNH
3
][OTf] (p
K
a
= 6.8), to [
2-Cl
PhNH
3
][OTf] (p
K
a
= 5.6). Yet
stronger acids, [
2,5-Cl
PhNH
3
][OTf] (p
K
a
= 4.3), [
2,6-Cl
PhNH
3
][OTf] (p
K
a
= 3.4), and
[
2,4,6-Cl
PhNH
3
][OTf] (p
K
a
= 2.1), constitute another, most active N
2
RR regime, one in
which the H
2
yields are nearly invariant.
12
The highest selectivity for N
2
RR (~ 78%) was
observed using [
2,5-Cl
PhNH
3
][OTf] as the acid. A final regime of very low N
2
RR activity is
encountered with [
per-Cl
PhNH
3
][OTf] (p
K
a
= 1.3) as the acid. We suspect this last acid
undergoes unproductive reduction via ET, thereby short-circuiting N
2
RR. The only other
N
2
RR system for which this type of acid-dependent correlation has been systematically
studied is the enzyme MoFe-nitrogenase.
13
,
14
As shown in Figure 1, the N
2
RR vs HER
activity of P
3
B
Fe
+
as a function of acid strength, is, in broad terms, similar to the behavior of
the enzyme
13
across a wide pH range.
In a previous study of Cp*
2
Co-mediated N
2
RR by P
3
B
Fe
+
,
9
we identified that P
3
B
FeN
2
−
forms under the catalytic conditions. Earlier studies on the reactivity of P
3
B
FeN
2
−
with an
excess of soluble acids, including HOTf and [H(OEt
2
)
2
][BAr
F
4
] (HBAr
F
4
, BAr
F
4
=
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate)), at −78 °C in Et
2
O, established rapid
formation of the doubly protonated species, P
3
B
FeNNH
2
+
.
15
Recent computational work
from our group suggests that, under catalytic conditions with a soluble acid, different
efficiencies for N
2
RR (versus HER) by P
3
E
Fe catalysts (E = B, C, Si) are likely correlated to
the rate of formation and consumption of early N
2
RR intermediates (i.e., P
3
E
FeNNH and
P
3
E
FeNNH
2
+/0
).
16
Thus, we were interested in the reactivity of these anilinium triflate acids
with P
3
B
FeN
2
−
, reasoning they may show differential efficiency in the formation of
P
3
B
FeNNH
2
+
.
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To our surprise, a freeze-quench EPR spectrum of the reaction of excess [
2,6-Cl
PhNH
3
][OTf]
(high N
2
RR efficiency regime) at −78 °C in Et
2
O with P
3
B
FeN
2
−
does not show any
P
3
B
FeNNH
2
+
. Also, freeze-quench Mössbauer analysis shows the formation of the oxidized
products P
3
B
FeN
2
and P
3
B
Fe
+
, but nothing assignable to P
3
B
FeNNH
2
+
(see SI for relevant
spectra). Analysis of such a reaction for NH
3
or N
2
H
4
after warming shows no fixed-N
products. The observation of exclusive oxidation, rather than productive N–H bond
formation, is analogous to what is observed upon addition of 1 equiv of a soluble acid
(HBAr
F
4
or HOTf) to P
3
B
FeN
2
−
. We have previously suggested that if unstable P
3
B
FeNNH
is formed (eq 1) without excess acid to trap it (to form more stable P
3
B
FeNNH
2
+
, eq 2), then
it can decay bimolecularly with the loss of 1/2 H
2
to form P
3
B
FeN
2
(eq 3).
P
3
B
FeN
2
−
+ H
+
P
3
B
FeNNH
(1)
P
3
B
FeNNH + H
+
P
3
B
FeNNH
2
+
(2)
P
3
B
FeNNH
P
3
B
FeN
2
+ 1/2 H
2
(3)
The low solubility of the anilinium triflate acids studied herein, in excess (25 equiv) and
under the catalytically relevant conditions (Et
2
O, −78 °C), likely leads to a similar scenario;
consequently, P
3
B
FeNNH that is generated is not efficiently captured by excess acid, leading
instead to bimolecular H
2
loss. In accord with this idea, a freeze-quench EPR spectrum of
the addition of 25 equiv of [
2,6-Cl
PhNH
3
][BAr
F
4
], a far more ether soluble derivative of the
same anilinium, to P
3
B
FeN
2
−
shows P
3
B
FeNNH
2
+
formation, and the detection of fixed-N
products upon warming (0.20 ± 0.04 eq. NH
3
per Fe).
These observations must next be reconciled with the seemingly contradictory observation
that comparatively efficient N
2
RR catalysis is observed when [
2,6-Cl
PhNH
3
][OTf], and other
anilinium triflate acids, are employed under catalytic conditions. For example, [Ph
2
NH
2
]
[OTf] leads to better efficiency for NH
3
formation versus [Ph
2
NH
2
][BAr
F
4
] (72 ± 3% and 42
± 6%, respectively). A key difference between the stoichiometric reactions described above,
and the catalytic reaction, is the presence of Cp*
2
Co in the latter.
We have suggested that Cp*
2
Co can be protonated under the catalytic reaction conditions, to
form Cp*(
η
4
-C
5
Me
5
H)Co
+
,
9
which may then play a role in N–H bond forming steps.
17
The
results presented here (and below) suggest that such a mechanism is not only plausible, but
is likely necessary, to explain the observed catalytic results with anilinium triflate acids.
Given the effect of p
K
a
on the efficiency for N
2
RR, we now hypothesize that this effect can
arise from the relative energetics and rates of Cp*
2
Co protonation by the different anilinium
triflate acids.
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Computational Studies
To investigate the kinetics and thermodynamics of Cp*
2
Co protonation by anilinium triflate
acids we turned to a computational study. DFT-D
3
18
calculations were undertaken at the
TPSS/def2-TZVP(Fe); def2-SVP
19
level of theory, as used previously for studies of this
P
3
B
Fe
+
system.
20
The free energy of H
+
exchange (ΔG
a
) was calculated for all of the
anilinium acids used (representative example shown in eq 4), and also for Cp*(
exo
-
η
4
-
C
5
Me
5
H)Co
+
, in Et
2
O at 298 K. These free energies were then used to determine the acid
p
K
a
’s, with inclusion of a term to reference them to the literature p
K
a
value for
[
2,6-Cl
PhNH
3
][OTf] at 298 K in THF (eq 5).
PhNH
2
+
PhNH
3
+
2, 6 − Cl
PhNH
3
+
+
PhNH
2
2, 6 − Cl
(4)
p
K
a
(PhNH
3
+
) = − ΔG
a
/(2.303 × RT) + p
K
a
(
PhNH
3
+
2, 6 − Cl
)
(5)
Because we presume that variable triflate hydrogen bonding effects (0.5–10 kcal mol
−1
) are
likely to be important under the catalytic conditions (low temperature and low polarity
solvent), we additionally calculated the free energy for net HOTf exchange reactions (ΔG
d
)
at 195 K in Et
2
O (representative example shown in eq 6). The free energies of these
reactions can then be used to determine a p
K
d
, referenced to the p
K
a
value for [
2,6-Cl
PhNH
3
]
[OTf] at 298 K in THF, for ease of comparison (eq 7). Hereafter, we use these p
K
d
values for
discussion, but note that use of the p
K
a
values instead does not substantively alter the
conclusions drawn.
PhNH
2
+ [
PhNH
3
2, 6 − Cl
][OTf]
[PhNH
3
][OTf] +
PhNH
2
2, 6 − Cl
(6)
p
K
d
([PhNH
3
][OTf]) = − ΔG
d
/(2.303 × RT) + p
K
a
(
PhNH
3
+
2, 6 − Cl
)
(7)
Calculations of the p
K
d
of all of the relevant species (Table 1) shows that the p
K
d
of
[Cp*(
exo
-
η
4
-C
5
Me
5
H)Co][OTf] (p
K
d
calc
= 11.8; Table 1) falls within the range of the
anilinium acids studied (0.4 ≤ p
K
d
calc
≤ 15.7), suggesting there should be a significant acid
dependence on the kinetics and thermodynamics of Cp*
2
Co protonation. To better elucidate
the differences in Cp*
2
Co protonation between the acids studied, we investigated the
kinetics of protonation for three acids, [
2,6-Cl
PhNH
3
][OTf] (high selectivity; p
K
d
calc
= 3.4),
[
2,6-Me
PhNH
3
][OTf] (modest selectivity; p
K
d
calc
= 13.2), and [
4-OMe
PhNH
3
][OTf] (poor
selectivity; p
K
d
calc
= 15.8).
Transition states for Cp*
2
Co protonation were located for all three acids. To confirm that
these transition states accurately reflect proton transfer, internal reaction coordinates (IRC)
were followed to determine the reactant (IRC-A) and product (IRC-B) minima (Figure 2).
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These minima represent hydrogen bonded arrangements of the reactants and products.
Protonation is found to have only a moderate barrier (ΔG
‡
in kcal mol
−1
) in all three cases:
([
4-OMe
PhNH
3
][OTf], +4.5; [
2,6-Me
PhNH
3
][OTf], +3.8; [
2,6-Cl
PhNH
3
][OTf], +1.3). This
suggests that Cp*
2
Co protonation is kinetically accessible in all cases, in agreement with the
experimental observation of background HER with each of these acids (see SI).
The small differences in rate, and the large variance in the equilibrium constant
K
eq
defined
in eq 8, points to a significant difference in the population of protonated metallocene,
[Cp*(
exo
-
η
4
-C
5
Me
5
H)Co][OTf], for these anilinium acids during catalysis.
K
eq
=
[ PhNH
2
R
− Cp
∗
(
exo
− η
4
− C
5
Me
5
H)Co
+
]
[ PhNH
3
+
R
− Cp
2
∗
Co]
(8)
We reason that the low solubility of the anilinium triflate acids, and the low catalyst
concentration, leads to a scenario in which the interaction between the acid and the Cp*
2
Co,
the latter being present in large excess relative to the iron catalyst, significantly affects the
overall kinetics of productive N–H bond formation. As such, the difference in [Cp*(
exo
-
η
4
-
C
5
Me
5
H)Co][OTf] concentration and formation rate should relate to, and likely dominate,
the origin of the observed p
K
a
effect. This explanation, rather than one that involves
differences in rates for the direct interaction of a given P
3
B
FeN
x
H
y
species with an anilinium
acid, better captures the collected data available.
21
[Cp*(
exo
-
η
4
-C
5
Me
5
H)Co][OTf], characterized by a very weak C–H bond, should be a
strong PCET donor, and we presume it serves such a role under the catalytic conditions
being discussed herein.
22
Its reactions with P
3
B
FeN
x
H
y
intermediates may occur in a
synchronous fashion, akin to HAT, or in an asynchronous fashion more akin to a PT-ET
reaction.
23
While many P
3
B
FeN
x
H
y
intermediates may, at least in part, be generated via
PCET with [Cp*(
exo
-
η
4
-C
5
Me
5
H)Co][OTf],
24
available experimental data point to a critical
role for such a reaction via trapping of the highly reactive first fixed intermediate,
P
3
B
FeNNH (Figure 3), before it can bimolecularly release H
2
(eq 3). We hence investigated
this reaction in more detail.
Both a synchronous PCET (ΔG
PCET
= −17.3 kcal mol
−1
; eq 9) and an asynchronous PCET
path (ΔG
PT
= −5.7 kcal mol
−1
, ΔG
ET
= −11.6 kcal mol
−1
; eq 10 and 11), are predicted to be
thermodynamically favorable.
P
3
B
FeNNH + [Cp ∗ (
exo
− η
4
− C
5
Me
5
H)Co][OTf]
P
3
B
FeNNH
2
+ [ Cp∗
2
Co][OTf]
(9)
P
3
B
FeNNH + [Cp ∗ (
exo
− η
4
− C
5
Me
5
H)Co][OTf]
[P
3
B
FeNNH
2
][OTf] + Cp∗
2
Co
(10)
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[P
3
B
FeNNH
2
][OTf] + Cp∗
2
Co
P
3
B
FeNNH
2
+ [ Cp∗
2
Co][OTf]
(11)
To evaluate the kinetics of these reactions the Marcus theory expressions
25
and the Hammes-
Schiffer method
26
were used to approximate relative rates of bimolecular ET and PCET. We
find that there is a slight kinetic preference for the fully synchronous PCET reaction
(
k
rel
PCET
~ 3 × 10
3
M
−1
s
−1
) compared to the fully asynchronous PT-ET reaction (
k
rel
PT-ET
≈
k
rel
ET
≡
1 M
−1
s
−1
; Figure 3).
27
The above discussion leads to the conclusion that the efficiency for NH
3
formation in this
system is coupled to the kinetics and/or thermodynamics of the reaction between the
anilinium triflate acid and the Cp*
2
Co reductant. This conclusion is counterintuitive, as the
protonation of Cp*
2
Co is also the requisite first step for background HER.
28
The fact that a
key HER intermediate can be intercepted and used for productive N
2
RR steps is an
important design principle for such catalysis. Similar design strategies are currently being
used to repurpose molecular cobalt HER catalysts for the reduction of unsaturated
substrates.
29
Efforts are often undertaken to suppress background reactivity between acid and reductant in
catalytic N
2
RR systems.
1a-b
We were hence particularly interested to explore whether the
inclusion of a metallocene co-catalyst, in this case Cp*
2
Co, might improve the yield, and/or
the Faradaic efficiency (FE), for N
2
RR versus HER, in controlled potential electrolysis
(CPE) experiments with P
3
B
Fe
+
under N
2
.
Electrolysis studies
To set the context for this section of the present study, we had shown previously that ~ 2.2
equiv NH
3
(per Fe) could be generated via controlled potential electrolysis (CPE; −2.3 V vs
Fc
+/0
) at a reticulated vitreous carbon working electrode, using P
3
B
Fe
+
as the (pre)catalyst in
the presence of HBAr
F
4
(50 equiv) at −45 °C under an atmosphere of N
2
. This yield of NH
3
corresponded to a ~ 25% FE which, while modest in terms of overall chemoselectivity,
compares very favorably to FE’s most typically reported for heterogeneous electrocatalysts
for N
2
RR that operate below 100 °C (< 2%).
2
,
30
To further explore the possibility of using P
3
B
Fe
+
as an electrocatalyst for N
2
RR, various
conditions were surveyed to determine whether enhanced yields of NH
3
could be obtained
from CPE experiments. For example, various applied potentials were studied (ranging from
−2.1 to −3.0 V vs Fc
+/0
), the concentrations of P
3
B
Fe
+
and HBAr
F
4
were varied, the ratio of
acid to catalyst was varied, and the rate at which acid was delivered to the system was varied
(e.g., initial full loadings, batch-wise additions, reloadings, or continuous slow additions).
None of these studies led to substantial improvement in N
2
RR; in all cases, < 2.5 equiv of
NH
3
was obtained per P
3
B
Fe
+
. Attempts to vary the ratio of the electrode surface area to the
working compartment solution volume, either by employing smaller cell geometries or by
using different morphologies of glassy carbon as the working electrode (e.g., reticulated
porous materials of different pore density or plates of different dimensions), also failed to
provide substantial improvement in NH
3
yield. The replacement of HBAr
F
4
, the original
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acid used in our electrolysis studies,
6d
by 50 equiv of [Ph
2
NH
2
][OTf] led to similar yields of
NH
3
(Table 2, entry 1).
We next investigated the effect of Cp*
2
Co
+
as an additive on the electrolysis/
electrocatalysis. Traces of relevant cyclic voltammograms (Figure 4A and 4B) collected with
glassy carbon as the working electrode in Et
2
O under glovebox atmosphere N
2
at −35 °C are
provided. Background traces including only [Ph
2
NH
2
][OTf] are present in both panels (gray
traces). Cp*
2
Co
+
(panel A, yellow trace), Cp*
2
Co
+
with the addition of ten equiv of
[Ph
2
NH
2
][OTf] (panel A, green trace), P
3
B
Fe
+
(panel B, dark blue trace), P
3
B
Fe
+
with the
addition of ten equiv of [Ph
2
NH
2
][OTf] (panel B, light blue trace), and P
3
B
Fe
+
with the
addition of one equiv of Cp*
2
Co
+
and ten equiv of [Ph
2
NH
2
][OTf] (both panels, red trace).
The cyclic voltammogram of Cp*
2
Co
+
is shown in panel
A
(yellow trace), displaying the
reversible Cp*
2
Co
+/0
couple at −2.0 V. The addition of [Ph
2
NH
2
][OTf] to Cp*
2
Co
+
causes
an increase in current at this potential, consistent with HER catalyzed by Cp*
2
Co
+
(panel
A
,
green trace).
Panel B provides the cyclic voltammogram of P
3
B
Fe
+
in the absence (dark blue trace,
showing previously assigned and (pseudo)reversible P
3
B
FeN
2
0/−
couple at ~ −2.1 V) and in
the presence (light blue trace) of [Ph
2
NH
2
][OTf].
31
The latter is indicative of modest HER
and N
2
RR. Also evident upon the addition of acid is the disappearance of a wave
corresponding to the P
3
B
Fe
+/0
couple at ~ −1.6 V. This wave, in the absence of acid, is broad
and shows a large peak-to-peak separation, likely due to the presence of both P
3
B
Fe
+
and
P
3
B
FeN
2
+
in solution at −35 °C. The addition of a large excess of [Ph
2
NH
2
][OTf]
presumably results in triflate binding (to generate P
3
B
FeOTf, thereby attenuating the waves
associated with the reduction of P
3
B
Fe
+
and P
3
B
FeN
2
+
). The red trace of Panel A
reproduced in Panel B to illustrate the marked increase incurrent observed when Cp*
2
Co is
added.
CPE studies were undertaken to characterize the reduction products associated with the red
trace at ~ −2.1 V vs Fc
+/0
. These studies employed a glassy carbon plate electrode, a Ag
+/0
reference electrode that was isolated by a CoralPor™ frit and referenced externally Fc
+/0
redox couple, and a solid sodium auxiliary electrode. The latter was used to avoid excessive,
non-productive redox cycling between the working and auxiliary chambers.
32
Unless
otherwise noted, CPE experiments were performed at −2.1 V versus Fc
+/0
, with 0.1 M
NaBAr
F
4
as the ether-soluble electrolyte, under a glovebox N
2
atmosphere at −35 °C. The
electrolysis was continued until the current had dropped to 1% of the initial current
measured, or until 21.5 hours had passed. The Supporting Information provides additional
details.
CPE experiments were conducted with the inclusion of 0, 1, 5, and 10 equiv of Cp*
2
Co
+
with respect to P
3
B
Fe
+
, using excess [Ph
2
NH
2
][OTf] as the acid. In the absence of added
Cp*
2
Co
+
, a significant amount of NH
3
was generated (2.6 ± 0.3 equiv per Fe, entry 1),
consistent with the previous finding that, in the presence of a strong acid, P
3
B
Fe
+
can
electrolytically mediate N
2
-to-NH
3
conversion.
6d
Notably, when a CPE experiment that did
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not include Cp*
2
Co
+
was reloaded with additional acid after electrolysis and electrolyzed
again, the total yield of NH
3
(2.6 ± 0.6 equiv NH
3
per Fe, entry 2) did not improve.
We found that inclusion of 1.0 equiv of Cp*
2
Co
+
enhanced the NH
3
yield, by a factor of ~
1.5 (Table 2, entry 3) without decreasing the FE. The data provide a total yield, with respect
to both Fe and Co, that confirm modest, but still unequivocal, N
2
RR electrocatalysis. In
single run experiments, the highest NH
3
yield in the absence of Cp*
2
Co
+
was 2.8 equiv,
compared with 4.4 equiv in the presence of 1 equiv of Cp*
2
Co
+
. Conversely, the lowest
single run NH
3
yield in the absence of Cp*
2
Co
+
was 2.3 equiv, compared with 3.5 equiv in
the presence of 1 equiv of Cp*
2
Co
+
.
Increasing the amount of added Cp*
2
Co
+
did not affect the NH
3
yield (entry 4). However,
the addition of a second loading of [Ph
2
NH
2
][OTf] following the first electrolysis (entry 5),
followed by additional electrolysis, led to an improved yield of NH
3
, suggesting that some
active catalyst is still present after the first run.
6d
,
9
Even higher Cp*
2
Co
+
loading did not
lead to higher NH
3
yields (entry 6).
CPE of P
3
B
Fe
+
in the presence of Cp*
2
Co
+
was also explored with other acids. Replacing
[Ph
2
NH
2
][OTf] in these experiments with [
2,6-Cl
PhNH
3
][OTf] led to lower yields of NH
3
,
and with [PhNH
3
][OTf] even lower yields of NH
3
were observed (entries 7 and 8
respectively). The lower, but nonzero, yield of NH
3
provided by [PhNH
3
][OTf] in these CPE
experiments is consistent with chemical trials employing various acids (vide supra) and can
be rationalized by the relative p
K
a
of the acids (Table 1). The intermediate yield of NH
3
provided by [
2,6-Cl
PhNH
3
][OTf] in these CPE experiments is less consistent with simple p
K
a
considerations, suggesting that additional factors are at play, perhaps including the relative
stability of the acid or conjugate base to electrolysis.
To probe whether electrode-immobilized iron might contribute to the N
2
RR electrocatalysis,
X-ray photoelectron spectroscopy (XPS) was used to study the electrode. After a standard
CPE experiment with P
3
B
Fe
+
, 5 equiv of Cp*
2
Co
+
, and 50 equiv [Ph
2
NH
2
][OTf], the
electrode was removed, washed with fresh 0.1 M NaBAr
F
4
Et
2
O solution, then fresh Et
2
O,
and probed by XPS. A
very low
coverage of Fe (< 0.3 atom % Fe) was detected in the post-
electrolysis material; no Fe was detected on a segment of the electrode which was not
exposed to the electrolytic solution. This observation implies a detectable but likely small
degree of degradation of P
3
B
Fe
+
over the course of a 15 hour CPE experiment. Worth noting
is that no Co was detected on the post-electrolysis electrode.
To test whether the small amount of deposited Fe material might be catalytically active for
N
2
RR, following a standard CPE experiment the electrode was removed from the cold
electrolysis solution, washed with fresh 0.1 M NaBAr
F
4
Et
2
O at −35 °C (the electrode itself
was maintained at −35 °C at all times), and then used for an additional CPE experiment,
under identical conditions except that P
3
B
Fe
+
was excluded. This CPE experiment yielded
no detectable NH
3
. The charge passed, and H
2
yield, were very similar to a “no P
3
B
Fe
+
”
control experiment conducted with a freshly cleaned electrode (See SI for further details).
Accordingly, a CPE experiment in the absence of P
3
B
Fe
+
demonstrated that Cp*
2
Co
+
serves
as an effective electrocatalyst for HER with [Ph
2
NH
2
][OTf] as the acid source,
but does not
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catalyze the N
2
RR reaction (0% FE for NH
3
, 75% FE for H
2
; see SI). This background
HER, and the observed catalytic response to the addition of [Ph
2
NH
2
][OTf] at the Cp*
2
Co
+/0
couple, provides circumstantial evidence for the formation of a protonated
decamethylcobaltocene intermediate, Cp*(
η
4
-C
5
Me
5
H)Co
+
, on a timescale similar to that of
the N
2
RR mediated by P
3
B
Fe
+
.
To probe whether the sodium auxiliary electrode used in the CPE experiments might play a
non-innocent role as a chemical reductant, a standard CPE experiment with P
3
B
Fe
+
, 5 equiv
Cp*
2
Co
+
, and 50 equiv [Ph
2
NH
2
][OTf] was assembled, but was left to stir at −35 °C for 43
hours without an applied potential bias. This experiment yielded 0.3 equiv NH
3
(relative to
Fe), suggesting that background N
2
RR due to the sodium auxiliary electrode is very minor.
To ensure the NH
3
produced was derived from the N
2
atmosphere during these electrolysis
experiments, as opposed to degradation of the anilinium acid used, a standard CPE
experiment using P
3
B
Fe
+
, 5 equiv Cp*
2
Co
+
, and 50 equiv of [Ph
2
15
NH
2
][OTf] was
performed. Only
14
NH
3
product was detected.
We also sought to compare the chemical N
2
RR catalysis efficiency of the P
3
B
Fe
+
catalyst
under conditions similar to those used for electrocatalysis. Hence, chemical catalysis with
P
3
B
Fe
+
, employing Cp*
2
Co as a reductant and [Ph
2
NH
2
][OTf] as the acid at −35 °C instead
of the more typical temperature of −78 °C, in a 0.1 M NaBAr
F
4
Et
2
O solution, afforded
lower yields of NH
3
(1.8 ± 0.7 equiv of NH
3
per Fe) than the yields observed via electrolysis
with Cp*
2
Co
+
as an additive. The lower yields of NH
3
in these chemical trials, compared
with our previously reported conditions (12.8 ± 0.5 equiv of NH
3
per Fe at −78 °C),
9
may be
attributable to increased competitive HER resulting from a more solubilizing medium (0.1 M
NaBAr
F
4
Et
2
O vs pure Et
2
O) and a higher temperature (−35 °C vs −78 °C).
9
These results
suggest that an electrochemical approach to NH
3
formation can improve performance, based
on selectivity for N
2
RR, of a molecular catalyst under comparable conditions.
CONCLUSION
Herein we described the first systematic p
K
a
studies on a synthetic nitrogen fixation catalyst
and find a strong correlation between p
K
a
and N
2
RR vs HER efficiency. Chemical studies
reveal that, on their own, the anilinium triflate acids employed in the catalysis are unable to
generate the N–H bonds of early-stage N
2
RR intermediates such as P
3
B
FeNNH
2
+
. We
propose that the insolubility of these triflate acids prevents the sufficiently rapid proton
transfer necessary to capture the critical but unstable first fixed intermediate, P
3
B
FeNNH.
Under catalytic conditions, we believe that the presence of the metallocene reductant
(Cp*
2
Co) is essential, as this species can be protonated in situ to form Cp*(
η
4
-C
5
Me
5
H)Co
+
, which in turn is effective in N–H bond formation with early intermediates. This leads to
the intriguing conclusion that an intermediate of the background HER pathway is redirected
for productive N
2
RR chemistry during catalysis.
DFT studies illustrate that the p
K
a
effect on the N
2
RR efficiency may be explained by the
variation in the kinetics and thermodynamics of Cp*
2
Co protonation by the different acids.
Investigation of the reactivity of Cp*(
exo
-
η
4
-C
5
Me
5
H)Co
+
with the P
3
B
FeNNH
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intermediate revealed that PCET reactivity, either synchronous or asynchronous, is favorable
and may proceed with only a small barrier, suggesting that P
3
B
FeNNH can be rapidly
trapped by Cp*(
exo
-
η
4
-C
5
Me
5
H)Co
+
. We suspect Cp*(
η
4
-C
5
Me
5
H)Co
+
may be involved in
a variety of N–H bond forming reactions during the overall catalysis.
Despite the fact that Cp*
2
Co
+
itself catalyzes HER under the conditions employed for
electrocatalytic N
2
RR, we found that its inclusion in CPE experiments containing P
3
B
Fe
+
and acid under an N
2
atmosphere led to modest improvements in the overall catalytic yield
of NH
3
. This system represents what is to our knowledge the first unambiguous example of
electrocatalytic N
2
RR mediated by a soluble, molecular coordination complex.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments
This work was supported by the the NIH (GM-070757) and the Resnick Sustainability Institute at Caltech. MJC,
TJDC, and BDM are grateful for NSF Graduate Research Fellowships and MJC acknowledges a Caltech
Environment Microbial Interactions (CEMI) Fellowship. This work made use of the Extreme Science and
Engineering Discovery Environment (XSEDE), which is supported by the NSF Grant ACI-1053575. We also thank
Pakpoom Buabthong for technical assistance with XPS measurements.
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7. In this context, a recent report in which the bioelectrosynthesis of ammonia by nitrogenase is
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2
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2
TiCl
2
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10. In some cases, the p
K
a
of a particular anilinium acid was already known in THF in which case this
value was used. In cases where the p
K
a
has not been reported in THF a literature procedure was
used to appropriately convert the p
K
a
from the solvent in which it was measured into a value for
THF. See SI for details.
11. Consistent with this observation is that efforts to use other weak, non-anilinium acids such as
benzylammonium triflate. (p
K
a
in THF of 13.2) and collidinium triflate. (p
K
a
in THF of 11.2) also
led to no observed NH
3
formation.
12. These results are also consistent with our previous observation of [Ph
2
NH
2
][OTf]. (p
K
a
in THF of
3.2) yielding 72 ± 3 % NH
3
. See reference
9
.
13. Pham DN, Burgess BK. Biochemistry. 1993; 32:13275.
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2
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3
have been reported for
several acids, but typically these acids span only a small p
K
a
range, electron yields are
inconsistent, and variations are not explained. For a representative example, see Reference
1b
.
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20. For the P
3
B
FeN
x
H
y
and related systems, this combination of functional and basis sets is able to
reproduce not only crystallographic details, but also experimentally measured singlet-triplet gaps,
reduction potentials, and N–H BDFE’s, as described in reference
16
.
21. In all cases where the basicity of P
3
B
FeN
x
HS intermediates has been evaluated, they are predicted
to be readily protonated by the anilinium triflate acids employed. (see SI for details).
22. DFT calculations suggest that almost all of the P
3
B
FeN
x
H
y
intermediates on the N2RR pathway
have N–H bonds stronger than the C–H bond in Cp*(
exo
-
η
4
-C
5
Me
5
H)Co
+
, suggesting that, at
least thermodynamically, the formation of these N–H bonds by PCET is favorable. See reference
16
.
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2
fixation intermediates.
(i.e., M
≡
N or M–NH
2
) has been observed previously: Scepaniak JJ, Young JA, Bontchev RP,
Smith JM. Angew. Chem. Int. Ed. 2009; 48:3158.Pappas I, Chirik PJ. J. Am. Chem. Soc. 2015;
137:3498. [PubMed: 25719966] MacLeod KC, McWilliams SF, Mercado BQ, Holland PL. Chem.
Sci. 2016; 7:5736. [PubMed: 28066537] Lindley BM, Bruch QJ, White PS, Hasanayn F, Miller
AJM. J. Am. Chem. Soc. 2017; 139:5305. [PubMed: 28383261]
25. Marcus RA. J. Chem. Phys. 1956; 24:966.
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26. Iordanova N, Decornez H, Hammes-Schiffer S. J. Am. Chem. Soc. 2001; 123:3723. [PubMed:
11457104]
27. We have assumed a PT-ET mechanism in which ET is rate limiting based on significantly lower
reorganization energies and barriers for PT compared to ET. See SI for full description.
28. Koelle U, Infelta PP, Graetzel M. Inorg. Chem. 1988; 27:879.
29. Call A, Casadevall C, Acuna-Pares F, Casitas A, Lloret-Fillol J. Chem. Sci. 2017; 8:4739.
30. Very recently there has been a study of electrocatalytic N
2
RR under ambient conditions in ionic
liquids with Fe nanoparticles that reports FE’s for NH
3
as high as 60%: Li S-J, Bao D, Shi M-M,
Wulan B-R, Yan J-M, Jiang Q. Adv. Mater. 2017; 29:1700001.
31. Moret M-E, Peters JC. Angew. Chem. Int. Ed. 2011; 50:2063.
32. Due to the extensive diffusion between the working and auxiliary chambers, production of an
oxidation product which can diffuse to the working electrode and be re-reduced at −2.1 V vs Fc
+/0
leads to excessive, nonproductive redox cycling between chambers over the course of the lengthy
CPE experiments. Sodium metal as an electrode material provides a suitable solution to this
technical challenge, as the product of its oxidation. (Na
+
) is stable to the CPE conditions.
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Figure 1.
(top) Percentage of electrons being used to form NH
3
or H
2
at different pH values by the
FeMo-nitrogenase in
A. vinelandii
. Reprinted with permission from Pham, D. N.; Burgess,
B. K.
Biochemistry
1993
,
32
, 13725. Copyright 1993 American Chemical Society. (bottom)
Percentage of electrons being used to form NH
3
or H
2
at different p
K
a
values by P
3
B
Fe
+
.
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Figure 2.
The kinetics and thermodynamics of protonation of Cp*
2
Co for three acids from different
catalytic efficiency regimes ([
4-OMe
PhNH
3
][OTf] = poor selectivity]; [
2,6-Me
PhNH
3
][OTf] =
modest selectivity; [
2,6-Cl
PhNH
3
][OTf] = high selectivity).
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Figure 3.
The calculated thermodynamics and kinetics of synchronous PCET and asynchronous PCET
(PT-ET), between P
3
B
FeNNH and [Cp*(
exo
-
η
4
-C
5
Me
5
H)Co][OTf] to generate
P
3
B
FeNNH
2
. Note:
k
rel
for ET is defined as 1 M
−1
s
−1
.
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