Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring p Ka Effects and Demonstrating Electrocatalysis

Article abstract of DOI:10.1021/jacs.8b02335

Substrate selectivity in reductive multielectron/proton catalysis with small molecules such as N₂, CO₂, and O₂ is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermodynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I) catalyst, P₃^BFe⁺, for catalyzing the nitrogen reduction reaction (N₂RR, N₂-to-NH₃ conversion) versus HER changes as a function of acid pK_a. We find that there is a strong correlation between pK_a and N₂RR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N₂ reduction (e.g., Fe(NNH) or Fe(NNH₂)) in the presence of the metallocene reductant Cp?₂Co. 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 N₂RR. Furthermore, DFT studies also suggest that the observed pK_a effect on N₂RR efficiency is attributable to the rate and thermodynamics of Cp?₂Co protonation by the different anilinium acids. Inclusion of Cp?₂Co⁺ as a cocatalyst in controlled potential electrolysis experiments leads to improved yields of NH₃. 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 of NH₃ per Fe at -2.1 V vs Fc^+/0).

Full text of DOI:10.1021/jacs.8b02335

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Fe-Mediated Nitrogen Fixation with a Metallocene Mediator:
Exploring pKa Effects and Demonstrating Electrocatalysis
Matthew J. Chalkley, Trevor J. Del Castillo, Benjamin D. Matson, and Jonas C. Peters
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02335 • Publication Date (Web): 18 Apr 2018
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Journal of the American Chemical Society
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Fe-Mediated Nitrogen Fixation with a Metallocene Mediator:
Exploring pKa Effects and Demonstrating Electrocatalysis
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Matthew J. Chalkley,‡ Trevor J. Del Castillo,‡ Benjamin D. Matson,‡ and Jonas C. Peters*
9
Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech), Pasadena, California
91125, United States
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ABSTRACT: Substrate selectivity in reductive multi-electron/proton catalysis with small molecules such as N2, CO2, and
O2 is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermo-
dynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I)
B
catalyst, P3 Fe+, for catalyzing the nitrogen reduction reaction (N2RR, N2-to-NH3 conversion) versus HER changes as a
function of acid pKa. We find that there is a strong correlation between pKa and N2RR efficiency. Stoichiometric studies
indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N2
reduction (e.g., Fe(NNH) or Fe(NNH2)) in the presence of the metallocene reductant Cp*2Co. This suggests that the inter-
action 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 N2RR. Furthermore, DFT studies also suggest that the observed pKa effect on N2RR efficiency is attributable to
the rate and thermodynamics, of Cp*2Co protonation by the different anilinium acids. Inclusion of Cp*2Co+ as a co-catalyst
in controlled potential electrolysis experiments leads to improved yields of NH3. 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
NH3 per Fe at −2.1 V vs Fc+/0).
of the tungsten-hydrazido(2−) complex (via its protonation by
acid).6a By cycling through such a process, an electrochemi-
cal, but not an electrocatalytic, synthesis of ammonia was
demonstrated. Indeed, efforts to demonstrate electrocatalysis
with this and related systems instead led to substoichiometric
NH3 yields.6c
An obvious limitation to progress in electrochemical
N2RR by molecular systems concerns the small number of
synthetic N2RR 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 ad-
dition, the conditions that have to date been employed to me-
diate N2RR have typically included non-polar solvents, such
as heptane, toluene, and diethyl ether (Et2O), that are not par-
ticularly well-suited to electrochemical studies owing to the
lack of compatible electrolytes.1
INTRODUCTION
There has been substantial recent progress in the devel-
opment of soluble, well-defined molecular catalysts for N2-to-
NH3 conversion, commonly referred to as the nitrogen reduc-
tion reaction (N2RR).1 Nevertheless, a significant and unmet
challenge is to develop molecular catalysts, and conditions,
compatible with electrocatalytic N2RR. Progress in this area
could have both fundamental and practical benefits, including
access to informative in situ mechanistic studies via electro-
chemical techniques, and an electrochemical means to trans-
late solar or otherwise derived chemical currency (H+/e−) into
NH3. 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
Nevertheless, several recent developments, including
ones from our lab, point to the likelihood that iron (and per-
haps other) molecular coordination complexes may be able to
mediate electrocatalytic N2RR in organic solvent. Specifi-
cally, our lab recently reported that a tris(phosphine)borane
Many soluble coordination complexes are now known
that electrocatalytically mediate the hydrogen evolution reac-
tion (HER),3 the carbon dioxide reduction reaction (CO2RR),4
and the oxygen reduction reaction (O2RR).5 The study of such
systems has matured at a rapid pace in recent years, coinciding
with expanded research efforts towards solar-derived fuel sys-
tems. In this context, it is noteworthy how little corresponding
progress has been made towards the discovery of soluble mo-
lecular catalysts that mediate electrocatalytic N2RR. To our
knowledge, only two prior systems address this topic di-
rectly.6,7,8
B
iron complex, P3 Fe+, that is competent for catalytic N2RR
with chemical reductants, can also mediate electrolytic N2-to-
NH3 conversion,6d with the available data (including that pre-
sented in this study) pointing to bona fide electrocatalysis in
Et2O.
B
Focusing on the P3 Fe+ catalytic N2RR system, a devel-
opment germane to the present study was its recently discov-
ered compatibility with reagents milder than those that had
been originally employed.1c Thus, decamethylcobaltocene
(Cp*2Co) and diphenylammonium acid are effective for N2RR
More than three decades ago, Pickett and coworkers re-
ported 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 tung-
sten-dinitrogen product; the latter species serves as the source
1
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catalysis; these reagents give rise to fast, and also quite selec-
Page 2 of 9
perchlorinated derivative ([per-ClPhNH3][OTf]) as the strong-
est. Importantly, good total electron yields (85.8 ± 3.3) were
obtained in all cases. As can be seen from the table, the NH3
efficiencies are found to be strongly correlated with pKa.10
In particular, a comparison of the efficiency for NH3 with
the pKa of the anilinium acid used gives rise to four distinct
activity regimes (Table 1, Figure 1). One regime that is com-
pletely inactive for N2RR, but active for HER, is defined by
the weakest acid, [4-OMePhNH3][OTf] (pKa = 8.8).11 A gradual
increase in observed NH3 yields, coupled with a decrease in
H2 yield, comprises a second regime, in which the acid is
strengthened from [PhNH3][OTf] (pKa = 7.8), to [2,6-
MePhNH3][OTf] (pKa = 6.8), to [2-ClPhNH3][OTf] (pKa = 5.6).
Yet stronger acids, [2,5-ClPhNH3][OTf] (pKa = 4.3), [2,6-
ClPhNH3][OTf] (pKa = 3.4), and [2,4,6-ClPhNH3][OTf] (pKa =
2.1), constitute another, most active N2RR regime, one in
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tive (> 70% vs HER), N2RR 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*(η4-C5Me5H)Co+, may be an important intermediate of
N2RR catalysis under such conditions. Indeed, we have sug-
gested that Cp*(η4-C5Me5H)Co+ may serve as a proton-cou-
pled-electron-transfer (PCET) donor (BDEC–H(calc) = 31 kcal
mol−1), thereby mediating net H-atom transfers to generate N–
H bonds during N2RR.9 The presence of a metallocene medi-
ator might, we therefore reasoned, enhance N2RR during elec-
trocatalysis.
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We present here a study of the effect of pKa on the selec-
B
tivity of P3 Fe+ for N2RR vs HER. By using substituted ani-
linium acids, we are able to vary the acid pKa over 9 orders of
magnitude and find that the selectivity is highly correlated
with the pKa. In our efforts to investigate the origin of the ob-
served pKa effect, we found, to our surprise, that in stoichio-
metric reactions, the catalytically competent anilinium triflate
acids are unable to faciltate productive N–H bond formation
with early-stage N2-fixation intermediates. We therefore hy-
pothesize that the formation of a protonated metallocene spe-
cies, Cp*(η4-C5Me5H)Co+, plays a critical role in N–H bond-
forming reactions, either via PCET, PT, or a combination of
both during N2RR catalysis. DFT studies support this hypoth-
esis and also establish that the observed pKa correlation with
N2RR selectivity can be explained by the varying ability of
the acids to protonate Cp*2Co. The suggested role of this pro-
tonated metallocene intermediate in N–H bond forming reac-
tions led us to test the effect of Cp*2Co+ as an additive in the
which the H2 yields are nearly invariant.12 The highest selec-
2,5-
tivity for N2RR (~ 78%) was observed using
[
ClPhNH3][OTf] as the acid. A final regime of very low N2RR
activity is encountered with [per-ClPhNH3][OTf] (pKa = 1.3) as
the acid. We suspect this last acid undergoes unproductive
B
electrolytic synthesis of NH3 mediated by P3 Fe+. We find that
the addition of co-catalytic Cp*2Co+ enhances the yield of
NH3 without decreasing the Faradaic efficiency (FE), and fur-
nishes what is to our knowledge the first unambiguous
demonstration of electrocatalytic N2RR mediated by a solu-
ble, molecular coordination complex.
RESULTS AND DISCUSSION
pKa studies.
B
In our recent study on the ability of P3 Fe+ to perform
N2RR with Cp*2Co as the chemical reductant,9 we found that
there was a marked difference in efficiency for NH3 genera-
tion with diphenylammonium triflate ([Ph2NH2][OTf]) versus
anilinium triflate ([PhNH3][OTf]). In that study, we posited
that this difference could arise from several possibilities, in-
cluding the differential solubility, sterics, or pKa’s of these ac-
ids.9
To investigate the last possibility, we have studied the ef-
ficiency of the catalysis by quantifying the NH3 and H2 pro-
duced when using substituted anilinium acids with different
pKa values (Table 1). The table is organized in increasing acid
strength, from [4-OMePhNH3][OTf] as the weakest acid to the
Figure 1. (top) Percentage of electrons being used to form NH3
or H2 at different pH values by the FeMo-nitrogenase in A. vine-
landii. 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
B
form NH3 or H2 at different pKa values by P3 Fe+.
Table 1. Literature and calculated pKa values and efficiencies observed in catalytic N2-to-NH3 conversion
calc
pKaexp
pKacalc
pKd
Equiv of
NH3/Fe
0.04 ± 0.01
7.3 ± 0.1
8.6 ± 0.7
―
% yield of
NH3/e-
0.2 ± 0.1
40.4 ± 0.5
47.5 ± 4.0
―
% yield of
H2/e-c
89.1 ± 0.2
48.6 ± 0.7
37.8 ± 0.2
―
Total
Yield/e-
89.3
87.5
85.6
(THF)10 (298 K)a (195 K)b
[
4-OMePhNH3][OTf]
[PhNH3][OTf]
2,6-MePhNH3][OTf]
[Cp*(exo-η4-C5Me5H)Co][OTf]
8.8
7.8
6.8
9.6
7.7
7.3
9.2
15.7
13.8
13.2
11.8
[
N/A
―
2
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Journal of the American Chemical Society
[
[
[
[
[
2-ClPhNH3][OTf]
2,5-ClPhNH3][OTf]
2,6-ClPhNH3][OTf]
2,4,6-ClPhNH3][OTf]
per-ClPhNH3][OTf]
5.6
4.3
3.4
2.1
1.3
5.6
4.0
3.4
2.7
0.8
6.0
5.0
3.4
1.8
0.4
10.7 ± 0.1
13.9 ± 0.7
13.8 ± 0 .9
12.8 ± 0.4
3.6 ± 0.1
53.9 ± 0.4
77.5 ± 3.8
76.7 ± 4.9
70.9 ± 2.2
19.9 ± 0.5
26.1 ± 1.9
10.5 ± 1.1
12.6 ± 2.5
12.0 ± 0.8
63.5 ± 1.1
80.0
87.7
89.3
83.1
83.5
1
2
3
4
5
6
7
8
[
4-OMePhNH3][OTf] = 4-methoxyanilinium triflate, [PhNH3][OTf] = anilinium triflate, [2,6-MePhNH3][OTf] = 2,6-dimethylanilinium triflate,
[
2-ClPhNH3][OTf] = 2-chloroanilinium triflate, [2,5-ClPhNH3][OTf] = 2,5-dichloroanilinium triflate, [2,6-ClPhNH3][OTf] = 2,6-dichloroanilin-
ium triflate, [2,4,6-ClPhNH3][OTf] = 2,4,6-trichloroanilinium triflate, [per-ClPhNH3][OTf] = 2,3,4,5,6-pentachloroanilinium triflate. aAcidities
calculated at 298 K in THF and referenced to the known literature value for [2,6-ClPhNH3][OTf]. All species calculated as the ion-paired
b
OTf− species in Et2O at 195 K and referenced to the known literature value for [2,6-ClPhNH3][OTf] in THF.
9
P3 FeN2− shows P3 FeNNH2+ formation, and the detection of
fixed-N products upon warming (0.20 ± 0.04 eq. NH3 per Fe).
These observations must next be reconciled with the
seemingly contradictory observation that comparatively effi-
cient N2RR catalysis is observed when [2,6-ClPhNH3][OTf],
and other anilinium triflate acids, are employed under cata-
lytic conditions. For example, [Ph2NH2][OTf] leads to better
B
B
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reduction via ET, thereby short-circuiting N2RR. The only
other N2RR system for which this type of acid-dependent cor-
relation has been systematically studied is the enzyme MoFe-
nitrogenase.13,14 As shown in Figure 1, the N2RR vs HER ac-
tivity of P3 Fe+ as a function of acid strength, is, in broad
B
terms, similar to the behavior of the enzyme13 across a wide
pH range.
efficiency for NH3 formation versus [Ph2NH2][BArF ] (72 ±
4
In a previous study of Cp*2Co-mediated N2RR by
3% and 42 ± 6%, respectively). A key difference between the
stoichiometric reactions described above, and the catalytic re-
action, is the presence of Cp*2Co in the latter.
P3 Fe+,9 we identified that P3 FeN2 forms under the catalytic
conditions. Earlier studies on the reactivity of P3 FeN2 with
an excess of soluble acids, including HOTf and
[H(OEt2)2][BArF ] (HBArF , BArF4 = tetrakis(3,5- bis(trifluo-
B
B
−
B
−
We have suggested that Cp*2Co can be protonated under
the catalytic reaction conditions, to form Cp*(η4-
C5Me5H)Co+,9 which may then play a role in N–H bond form-
ing steps.17 The results presented here (and below) suggest
that such a mechanism is not only plausible, but is likely nec-
essary, to explain the observed catalytic results with anilinium
triflate acids. Given the effect of pKa on the efficiency for
N2RR, we now hypothesize that this effect can arise from the
relative energetics and rates of Cp*2Co protonation by the dif-
ferent anilinium triflate acids.
4
4
romethyl)phenyl)borate)), at −78 °C in Et2O, established rapid
B
+ 15
formation of the doubly protonated species, P3 FeNNH2 .
Recent computational work from our group suggests that, un-
der catalytic conditions with a soluble acid, different efficien-
cies for N2RR (versus HER) by P3 Fe catalysts (E = B, C, Si)
are likely correlated to the rate of formation and consumption
E
E
of early N2RR intermediates (i.e., P3 FeNNH and
P3 FeNNH2+/0).16 Thus, we were interested in the reactivity of
E
B
−
these anilinium triflate acids with P3 FeN2 , reasoning they
may show differential efficiency in the formation of
P3 FeNNH2 .
To our surprise, a freeze-quench EPR spectrum of the re-
action of excess [2,6-ClPhNH3][OTf] (high N2RR efficiency re-
Computational Studies.
B
+
To investigate the kinetics and thermodynamics of
Cp*2Co protonation by anilinium triflate acids we turned to a
computational study. DFT-D318 calculations were undertaken
at the TPSS/def2-TZVP(Fe); def2-SVP19 level of theory, as
B
−
gime) at −78 °C in Et2O with P3 FeN2 does not show any
B
+
P3 FeNNH2 . Also, freeze-quench Mössbauer analysis shows
B
used previously for studies of this P3 Fe+ system.20 The free
the formation of the oxidized products P3 FeN2 and P3 Fe+,
B
B
energy of H+ exchange (ΔGa) was calculated for all of the ani-
linium acids used (representative example shown in eq 4), and
also for Cp*(exo-η4-C5Me5H)Co+, in Et2O at 298 K. These
free energies were then used to determine the acid pKa’s, with
inclusion of a term to reference them to the literature pKa value
for [2,6-ClPhNH3][OTf] at 298 K in THF (eq 5).
B
+
but nothing assignable to P3 FeNNH2 (see SI for relevant
spectra). Analysis of such a reaction for NH3 or N2H4 after
warming shows no fixed-N products. The observation of ex-
clusive oxidation, rather than productive N–H bond for-
mation, is analogous to what is observed upon addition of 1
equiv of a soluble acid (HBArF or HOTf) to P3 FeN2 . We
B
−
4
B
have previously suggested that if unstable P3 FeNNH is
formed (eq 1) without excess acid to trap it (to form more sta-
ble P3 FeNNH2 , eq 2), then it can decay bimolecularly with
the loss of 1/2 H2 to form P3 FeN2 (eq 3).
+
+
(4) PhNH2 + 2,6-ClPhNH3  PhNH3
+
2,6-ClPhNH2
B
+
+
+
(5) pKa(PhNH3 ) = −ΔGa/(2.303×RT) + pKa(2,6-ClPhNH3 )
B
B
−
B
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 po-
larity solvent), we additionally calculated the free energy for
net HOTf exchange reactions (ΔGd) at 195 K in Et2O (repre-
sentative example shown in eq 6). The free energies of these
reactions can then be used to determine a pKd, referenced to
the pKa value for [2,6-ClPhNH3][OTf] at 298 K in THF, for ease
of comparison (eq 7). Hereafter, we use these pKd values for
discussion, but note that use of the pKa values instead does not
substantively alter the conclusions drawn.
(1) P3 FeN2 + H+  P3 FeNNH
(2) P3 FeNNH + H+  P3 FeNNH2
B
B
+
B
B
(3) P3 FeNNH  P3 FeN2 + 1/2 H2
The low solubility of the anilinium triflate acids studied
herein, in excess (50 equiv) and under the catalytically rele-
vant conditions (1 mL Et2O, −78 °C), likely leads to a similar
B
scenario; consequently, P3 FeNNH that is generated is not ef-
ficiently captured by excess acid, leading instead to bimolec-
ular H2 loss. In accord with this idea, a freeze-quench EPR
(6) PhNH2 + [2,6-ClPhNH3][OTf] 
spectrum of the addition of 50 equiv of [2,6-ClPhNH3][BArF ],
4
[PhNH3][OTf] + 2,6-ClPhNH2
a far more ether soluble derivative of the same anilinium, to
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B
(7) pKd([PhNH3][OTf])
=
−ΔGd/(2.303×RT)
+
pKa(2,6-
given P3 FeNxHy species with an anilinium acid, better cap-
tures the collected data available.21
+
ClPhNH3 )
1
2
3
4
5
6
7
8
[Cp*(exo-η4-C5Me5H)Co][OTf], characterized by a
very weak C–H bond, should bea strong PCET donor, and we
presume it serves such a role under the catalytic conditions
Calculations of the pKd of all of the relevant species (Ta-
ble 1) shows that the pKd of [Cp*(exo-η4-C5Me5H)Co][OTf]
(pKdcalc = 11.8; Table 1) falls within the range of the anilinium
acids studied (0.4 ≤ pKdcalc ≤ 15.7), suggesting there should be
a significant acid dependence on the kinetics and thermody-
namics of Cp*2Co protonation. To better elucidate the differ-
ences in Cp*2Co protonation between the acids studied, we
investigated the kinetics of protonation for three acids, [2,6-
being discussed herein.22 Its reactions with P3 FeNxHy inter-
B
mediates may occur in a synchronous fashion, akin to HAT,
or in an asynchronous fashion more akin to a PT-ET reac-
tion.23 While many P3 FeNxHy intermediates may, at least in
B
part, be generated via PCET with [Cp*(exo-η4-
C5Me5H)Co][OTf],24 available experimental data point to a
critical role for such a reaction via trapping of the highly re-
9
ClPhNH3][OTf] (high selectivity; pKd
=
3.4), [
calc
2,6-
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MePhNH3][OTf] (modest selectivity; pKd = 13.2), and [4-
calc
B
active first fixed intermediate, P3 FeNNH (Figure 3), before
it can bimolecularly release H2 (eq 3). We hence investigated
this reaction in more detail.
OMePhNH3][OTf] (poor selectivity; pKdcalc = 15.8).
Transition states for Cp*2Co protonation were located for
all three acids. To confirm that these transition states accu-
rately reflect proton transfer, internal reaction coordinates
(IRC) were followed to determine the reactant (IRC-A) and
product (IRC-B) minima (Figure 2). 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-OMePhNH3][OTf], +4.5; [2,6-
MePhNH3][OTf], +3.8; [2,6-ClPhNH3][OTf], +1.3).
Figure 3. The calculated thermodynamics and kinetics of syn-
chronous PCET and asynchronous PCET (PT-ET), between
B
P3 FeNNH and [Cp*(exo-η4-C5Me5H)Co][OTf] to generate
B
P3 FeNNH2. Note: krel for ET is defined as 1 M−1 s−1.
Figure 2. The kinetics and thermodynamics of protonation of
Cp*2Co for three acids from different catalytic efficiency regimes
([4-OMePhNH3][OTf] = poor selectivity]; [2,6-MePhNH3][OTf] =
modest selectivity; [2,6-ClPhNH3][OTf] = high selectivity).
Both a synchronous PCET (ΔGPCET = −17.3 kcal mol−1;
eq 9) and an asynchronous PCET path (ΔGPT = −5.7 kcal
mol−1, ΔGET = −11.6 kcal mol−1; eq 10 and 11), are predicted
to be thermodynamically favorable.
This suggests that Cp*2Co protonation is kinetically accessi-
ble in all cases, in agreement with the experimental observa-
tion of background HER with each of these acids (see SI).
The small differences in rate, and the large variance in
the equilibrium constant Keq defined in eq 8, points to a sig-
nificant difference in the population of protonated metallo-
cene, [Cp*(exo-η4-C5Me5H)Co][OTf], for these anilinium ac-
ids during catalysis.
B
(9) P3 FeNNH + [Cp*(exo-η4-C5Me5H)Co][OTf] 
B
P3 FeNNH2 + [Cp*2Co][OTf]
B
(10) P3 FeNNH + [Cp*(exo-η4-C5Me5H)Co][OTf] 
B
[P3 FeNNH2][OTf] + Cp*2Co
B
(11) [P3 FeNNH2][OTf] + Cp*2Co 
B
P3 FeNNH2 + [Cp*2Co][OTf]
R
∗
4
+
[
PhNH −Cp (푒푥표−η −C Me H)Co ]
2 5 5
(8)
퐾eq
=
R
+
∗
[
PhNH –Cp Co]
3
2
To evaluate the kinetics of these reactions the Marcus
theory expressions25 and the Hammes-Schiffer method26 were
used to approximate relative rates of bimolecular ET and
We reason that the low solubility of the anilinium triflate
B
acids, and the low catalyst concentration (2.3 mM P3 Fe),
leads to a scenario in which the interaction between the acid
and the Cp*2Co, the latter being present in excess relative to
the iron catalyst (measured solubility of Cp*2Co at −78 °C in
Et2O is ~ 6 mM, see SI), significantly affects the overall ki-
netics of productive N–H bond formation. As such, the differ-
ence in [Cp*(exo-η4-C5Me5H)Co][OTf] concentration and
formation rate should relate to, and likely dominate, the origin
of the observed pKa effect. This explanation, rather than one
that involves differences in rates for the direct interaction of a
PCET. We find that there is a slight kinetic preference for the
PCET
fully synchronous PCET reaction (krel
~ 3 × 103 M−1s−1)
PT-ET
compared to the fully asynchronous PT-ET reaction (krel
≈ krelET ≡ 1 M−1 s−1; Figure 3).27
The above discussion leads to the conclusion that the ef-
ficiency for NH3 formation in this system is coupled to the
kinetics and/or thermodynamics of the reaction between the
anilinium triflate acid and the Cp*2Co reductant. This conclu-
sion is counterintuitive, as the protonation of Cp*2Co is also
the requisite first step for background HER.28 The fact that a
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key HER intermediate can be intercepted and used for produc-
tive N2RR steps is an important design principle for such ca-
talysis. Similar design strategies are currently being used to
repurpose molecular cobalt HER catalysts for the reduction of
unsaturated substrates.29
Cp*2Co+ (panel A, yellow trace), Cp*2Co+ with the addition of
B
ten equiv of [Ph2NH2][OTf] (panel A, green trace), P3 Fe+
1
2
3
4
5
6
7
8
B
(panel B, dark blue trace), P3 Fe+ with the addition of ten
B
equiv of [Ph2NH2][OTf] (panel B, light blue trace), and P3 Fe+
with the addition of one equiv of Cp*2Co+ and ten equiv of
[Ph2NH2][OTf] (both panels, red trace).
Efforts are often undertaken to suppress background re-
activity between acid and reductant in catalytic N2RR sys-
tems.1a-b We were hence particularly interested to explore
whether the inclusion of a metallocene co-catalyst, in this case
Cp*2Co, might improve the yield, and/or the Faradaic effi-
ciency (FE), for N2RR versus HER, in controlled potential
Table 2. Yields and Faradaic Efficiencies of NH3 from
B
CPE Experiments with P3 Fe+
9
B
electrolysis (CPE) experiments with P3 Fe+ under N2.
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Electrolysis studies.
To set the context for this section of the present study, we
had shown previously that ~ 2.2 equiv NH3 (per Fe) could be
generated via controlled potential electrolysis (CPE; −2.3 V
vs Fc+/0) at a reticulated vitreous carbon working electrode,
B
using P3 Fe+ as the (pre)catalyst in the presence of HBArF
4
(50 equiv) at −45 °C under an atmosphere of N2. This yield of
NH3 corresponded to a ~ 25% FE which, while modest in
terms of overall chemoselectivity, compares very favorably to
FE’s most typically reported for heterogeneous electrocata-
lysts for N2RR that operate below 100 °C (< 2%).2,30
Equiv
Equiv NH3
(per Fe)
Equiv NH3
(per Co)
―
NH3 FE
(%)
Entry
1
Cp*2Co+
0
0
1
5
5
10
5
5
2.6 ± 0.3d
2.6 ± 0.6
4.0 ± 0.6
4.0 ± 0.6d
5.5 ± 0.9e
4 ± 1
24 ± 5
18 ± 1
28 ± 5
25 ± 3
19 ± 1
24 ± 7
10 ± 1
6 ± 3
2a
3
―
To further explore the possibility of using P3 Fe+ as an
B
4.0 ± 0.6
0.8 ± 0.1
1.1 ± 0.2
0.4 ± 0.1
0.4 ± 0.1
0.2 ± 0.1
electrocatalyst for N2RR, various conditions were surveyed to
determine whether enhanced yields of NH3 could be obtained
from CPE experiments. For example, various applied poten-
tials were studied (ranging from −2.1 to −3.0 V vs Fc+/0), the
4
5a
6
7b
8c
1.9 ± 0.2
0.9 ± 0.4
B
concentrations of P3 Fe+ and HBArF4 were varied, the ratio of
All CPE experiments conducted at −2.1 V vs Fc+/0 with 0.1 M
acid to catalyst was varied, and the rate at which acid was de-
livered to the system was varied (e.g., initial full loadings,
batch-wise additions, reloadings, or continuous slow addi-
tions). None of these studies led to substantial improvement
in N2RR; in all cases, < 2.5 equiv of NH3 was obtained per
NaBArF in Et2O as solvent at −35 °C under an N2 atmosphere,
4
featuring a glassy carbon plate working electrode, Ag+/0 reference
couple isolated by a CoralPorTM frit referenced externally to Fc+/0
,
and a solid sodium auxiliary electrode. Working and auxiliary
chambers separated by a sintered glass frit. See SI for further ex-
perimental details, controls, and additional data. Averages repre-
B
P3 Fe+. Attempts to vary the ratio of the electrode surface area
to the working compartment solution volume, either by em-
ploying smaller cell geometries or by using different morphol-
ogies of glassy carbon as the working electrode (e.g., reticu-
lated porous materials of different pore density or plates of
different dimensions), also failed to provide substantial im-
a
sent two runs unless noted. After initial electrolysis with 50
equiv [Ph2NH2][OTf], an additional 50 equiv [Ph2NH2][OTf] in
0.1 M NaBArF4 Et2O solution was added to the working chamber,
via syringe through a rubber septum, followed by additional CPE
at −2.1 V vs Fc+/0. The listed Equiv NH3 (per Fe) for these runs is
provement in NH3 yield. The replacement of HBArF , the orig-
4
the total yield after both electrolysis experiments are completed.
inal acid used in our electrolysis studies,6d by 50 equiv of
[Ph2NH2][OTf] led to similar yields of NH3 (Table 2, entry 1).
We next investigated the effect of Cp*2Co+ as an additive
on the electrolysis/electrocatalysis. Traces of relevant cyclic
voltammograms (Figure 4A and 4B) collected with glassy car-
bon as the working electrode in Et2O under glovebox atmos-
phere N2 at −35 °C are provided. Background traces including
only [Ph2NH2][OTf] are present in both panels (gray traces).
c
[
b 2,6-ClPhNH3][OTf] employed as the acid. [PhNH3][OTf] em-
ployed as the acid. dAverage of three runs. eAverage of five runs.
The cyclic voltammogram of Cp*2Co+ is shown in panel
A (yellow trace), displaying the reversible Cp*2Co+/0 couple
at −2.0 V. The addition of [Ph2NH2][OTf] to Cp*2Co+ causes
an increase in current at this potential, consistent with HER
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Figure 4. A) Cyclic voltammograms of 10 equiv [Ph2NH2][OTf] (gray trace), 1 equiv [Cp*2Co][BArF ] (Cp*2Co+) (yellow trace), 1 equiv
4
Cp*2Co+ with 10 equiv [Ph2NH2][OTf] (green trace), and P3 Fe+ with 1 equiv of Cp*2Co+ and 10 equiv [Ph2NH2][OTf] (red trace). B) Cyclic
B
B
B
voltammograms of 10 equiv [Ph2NH2][OTf] (gray trace), P3 Fe+ (dark blue trace), P3 Fe+ with 10 equiv [Ph2NH2][OTf] (light blue trace),
and P3 Fe+ with 1 equiv of Cp*2Co+ and 10 equiv [Ph2NH2][OTf] (red trace). All spectra are collected in 0.1 M NaBArF4 solution in Et2O at
B
−35 °C using a glassy carbon working electrode, and externally referenced to the Fc+/0 couple. Scan rate is 100 mV/s.
catalyzed by Cp*2Co+ (panel A, green trace).
We found that inclusion of 1.0 equiv of Cp*2Co+ en-
hanced the NH3 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 une-
quivocal, N2RR electrocatalysis. In single run experiments,
the highest NH3 yield in the absence of Cp*2Co+ was 2.8
equiv, compared with 4.4 equiv in the presence of 1 equiv of
Cp*2Co+. Conversely, the lowest single run NH3 yield in the
absence of Cp*2Co+ was 2.3 equiv, compared with 3.5 equiv
in the presence of 1 equiv of Cp*2Co+.
B
Panel B provides the cyclic voltammogram of P3 Fe+ in
the absence (dark blue trace, showing previously assigned and
B
0/−
quasi-reversible P3 FeN2 couple at ~ −2.1 V) and in the
presence (light blue trace) of [Ph2NH2][OTf].31 The latter is
indicative of modest HER and N2RR. Also evident upon the
addition of acid is the disappearance of a wave corresponding
B
to the P3 Fe+/0 couple at ~ −1.6 V. This wave, in the absence
of acid, is broad and shows a large peak-to-peak separation,
B
B
likely due to the presence of both P3 Fe+ and P3 FeN2+ in so-
lution at −35 °C. The addition of a large excess of
[Ph2NH2][OTf] presumably results in triflate binding (to gen-
Increasing the amount of added Cp*2Co+ did not affect
the NH3 yield (entry 4). However, the addition of a second
loading of [Ph2NH2][OTf] following the first electrolysis (en-
try 5), followed by additional electrolysis, led to an improved
yield of NH3, suggesting that some active catalyst is still pre-
sent after the first run.6d,9 Even higher Cp*2Co+ loading did
not lead to higher NH3 yields (entry 6).
B
erate P3 FeOTf, thereby attenuating the waves associated with
B
B
+
the reduction of P3 Fe+ and P3 FeN2 ). The red trace of Panel
A is reproduced in Panel B to illustrate the marked increase
incurrent observed when Cp*2Co is added.
CPE studies were undertaken to characterize the reduc-
tion 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 CoralPorTM
frit and referenced externally Fc+/0 redox couple, and a solid
sodium auxiliary electrode. The latter was used to avoid ex-
cessive, non-productive redox cycling between the working
and auxiliary chambers.32 Unless otherwise noted, CPE exper-
iments were performed at −2.1 V versus Fc+/0, with 0.1 M
NaBArF4 as the ether-soluble electrolyte, under a glovebox N2
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.
B
CPE of P3 Fe+ in the presence of Cp*2Co+ was also ex-
plored with other acids. Replacing [Ph2NH2][OTf] in these ex-
periments with [2,6-ClPhNH3][OTf] led to lower yields of NH3,
and with [PhNH3][OTf] even lower yields of NH3 were ob-
served (entries 7 and 8 respectively). The lower, but nonzero,
yield of NH3 provided by [PhNH3][OTf] in these CPE exper-
iments is consistent with chemical trials employing various
acids (vide supra) and can be rationalized by the relative pKa
of the acids (Table 1). The intermediate yield of NH3 provided
by [2,6-ClPhNH3][OTf] in these CPE experiments is less con-
sistent with simple pKa considerations, suggesting that addi-
tional factors are at play, perhaps including the relative stabil-
ity of the acid or conjugate base to electrolysis.
CPE experiments were conducted with the inclusion of 0,
To probe whether electrode-immobilized iron might con-
tribute to the N2RR electrocatalysis, X-ray photoelectron
spectroscopy (XPS) was used to study the electrode. After a
B
1, 5, and 10 equiv of Cp*2Co+ with respect to P3 Fe+, using
excess [Ph2NH2][OTf] as the acid. In the absence of added
Cp*2Co+, a significant amount of NH3 was generated (2.6 ±
0.3 equiv per Fe, entry 1), consistent with the previous finding
B
standard CPE experiment with P3 Fe+, 5 equiv of Cp*2Co+,
and 50 equiv [Ph2NH2][OTf], the electrode was removed,
B
that, in the presence of a strong acid, P3 Fe+ can electrolyti-
washed with fresh 0.1 M NaBArF Et2O solution, then fresh
4
cally mediate N2-to-NH3 conversion.6d Notably, when a CPE
experiment that did not include Cp*2Co+ was reloaded with
additional acid after electrolysis and electrolyzed again, the
total yield of NH3 (2.6 ± 0.6 equiv NH3 per Fe, entry 2) did
not improve.
Et2O, 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
B
a detectable but likely small degree of degradation of P3 Fe+
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over the course of a 15 hour CPE experiment. Worth noting is
that no Co was detected on the post-electrolysis electrode.
This may be consistent with the known stability of metallo-
cenes and the recently discovered stability of protonated-Cp*
ligands on Rh.33
bonds of early-stage N2RR intermediates such as
B
+
1
2
3
4
5
6
7
8
P3 FeNNH2 . 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,
B
P3 FeNNH. Under catalytic conditions, we believe that the
To test whether the small amount of deposited Fe mate-
rial might be catalytically active for N2RR, following a stand-
ard CPE experiment the electrode was removed from the cold
electrolysis solution, washed with fresh 0.1 M NaBArF4 Et2O
at −35 °C (the electrode itself was maintained at −35 °C at all
times), and then used for an additional CPE experiment, under
presence of the metallocene reductant (Cp*2Co) is essential,
as this species can be protonated in situ to form Cp*(η4-
C5Me5H)Co+, which in turn is effective in N–H bond for-
mation with early intermediates. This leads to the intriguing
conclusion that an intermediate of the background HER path-
way is redirected for productive N2RR chemistry during ca-
talysis.
9
B
identical conditions except that P3 Fe+ was excluded. This
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CPE experiment yielded no detectable NH3. The charge
DFT studies illustrate that the pKa effect on the N2RR ef-
ficiency may be explained by the variation in the kinetics and
thermodynamics of Cp*2Co protonation by the different acids.
Investigation of the reactivity of Cp*(exo-η4-C5Me5H)Co+
B
passed, and H2 yield, were very similar to a “no P3 Fe+” con-
trol experiment conducted with a freshly cleaned electrode
(See SI for further details). Accordingly, a CPE experiment in
B
B
the absence of P3 Fe+ demonstrated that Cp*2Co+ serves as an
with the P3 FeNNH intermediate revealed that PCET reactiv-
effective electrocatalyst for HER with [Ph2NH2][OTf] as the
acid source, but does not catalyze the N2RR reaction (0% FE
for NH3, 75% FE for H2; see SI). This background HER, and
the observed catalytic response to the addition of
[Ph2NH2][OTf] at the Cp*2Co+/0 couple, provides circumstan-
tial evidence for the formation of a protonated decamethylco-
baltocene intermediate, Cp*(η4-C5Me5H)Co+, on a timescale
ity, either synchronous or asynchronous, is favorable and may
B
proceed with only a small barrier, suggesting that P3 FeNNH
can be rapidly trapped by Cp*(exo-η4-C5Me5H)Co+. We sus-
pect Cp*(η4-C5Me5H)Co+ is likely involved in a variety of N–
H bond forming reactions during the overall catalysis, includ-
ing reactions with late-stage nitrogen fixation intermediates.
Despite the fact that Cp*2Co+ itself catalyzes HER under
the conditions employed for electrocatalytic N2RR, we found
B
similar to that of the N2RR mediated by P3 Fe+.
B
To probe whether the sodium auxiliary electrode used in
the CPE experiments might play a non-innocent role as a
that its inclusion in CPE experiments containing P3 Fe+ and
acid under an N2 atmosphere led to modest improvements in
the overall catalytic yield of NH3. This system represents what
is to our knowledge the first unambiguous example of electro-
catalytic N2RR mediated by a soluble, molecular coordination
complex.
B
chemical reductant, a standard CPE experiment with P3 Fe+,
5 equiv Cp*2Co+,and 50 equiv [Ph2NH2][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 NH3 (rela-
tive to Fe), suggesting that background N2RR due to the so-
dium auxiliary electrode is very minor.
ASSOCIATED CONTENT
To ensure the NH3 produced was derived from the N2 at-
mosphere during these electrolysis experiments, as opposed
to degradation of the anilinium acid used, a standard CPE ex-
The Supporting Information is available free of charge via the In-
ternet at http://pubs.acs.org.
B
periment using P3 Fe+, 5 equiv Cp*2Co+, and 50 equiv of
Computational models (MOL)
[Ph215NH2][OTf] was performed. Only 14NH3 product was de-
tected.
Experimental procedures, characterization data (PDF)
We also sought to compare the chemical N2RR catalysis
AUTHOR INFORMATION
B
efficiency of the P3 Fe+ catalyst under conditions similar to
those used for electrocatalysis. Hence, chemical catalysis with
Corresponding Author
B
P3 Fe+, employing Cp*2Co as a reductant and [Ph2NH2][OTf]
* jpeters@caltech.edu
as the acid at −35 °C instead of the more typical temperature
of −78 °C, in a 0.1 M NaBArF4 Et2O solution, afforded lower
yields of NH3 (1.8 ± 0.7 equiv of NH3 per Fe) than the yields
observed via electrolysis with Cp*2Co+ as an additive. The
lower yields of NH3 in these chemical trials, compared with
our previously reported conditions (12.8 ± 0.5 equiv of NH3
per Fe at −78 °C),9 may be attributable to increased competi-
tive HER resulting from a more solubilizing medium (0.1 M
ORCID
Jonas C. Peters: 0000-0002-6610-4414
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the man-
uscript.
NaBArF Et2O vs pure Et2O) and a higher temperature (−35
4
‡These authors contributed equally.
°C vs −78 °C).9 These results suggest that an electrochemical
approach to NH3 formation can improve performance, based
on selectivity for N2RR, of a molecular catalyst under compa-
rable conditions.
ACKNOWLEDGMENTS
This work was supported by the NIH (GM-075757) 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 Interac-
tions (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.
CONCLUSION
Herein we described the first systematic pKa studies on
a synthetic nitrogen fixation catalyst and find a strong corre-
lation between pKa and N2RR 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
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REFERENCES33
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20
For the P3BFeNxHy 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.
10
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21
In all cases where the basicity of P3BFeNxHy 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 P3BFeNxHy intermediates
on the N2RR pathway have N–H bonds stronger than the C–H bond in
Cp*(exo-η4-C5Me5H)Co+, suggesting that, at least thermodynamically,
the formation of these N–H bonds by PCET is favorable. See reference
16.
2 (a) Shipman, M. A.; Symes, M. D. Catal. Today 2017, 286, 57. (b) Kyr-
iakou, V.; Garagounis, I.; Vasileiou, E.; Vourros, A.; Stoukides, M. Catal.
Today 2017, 286, 2.
3 (a) McKone, J. R.; Marinescu, S. C.; Brunschwig, B. S.; Winkler, J. R.;
Gray, H. B. Chem. Sci. 2014, 5, 865. (b) Bullock, R. M.; Appel, A. M.;
Helm, M. L. Chem. Commun. 2014, 50, 3125.
23 The reactivity of ring-functionalized Cp* rings has been discussed pre-
viously in the context of electrocatalytic HER by 4d and 5d metals, but
via a mechanism involving hydride transfer, rather than via PCET. See:
(a) Pitman, C. L.; Finster, O. N. L.; Miller, A. J. M. Chem. Commun.
2016, 52, 9105. (b) Quintana, L. M. A.; Johnson, S. I.; Corona, S. L.;
Villatoro, W.; Goddard, W. A.; Takase, M. K.; VanderVelde, D. G.; Win-
kler, J. R.; Gray, H. B.; Blakemore, J. D. Proc. Natl. Acad. Sci. 2016, 113,
6409. (c) Peng, Y.; Ramos-Garcés, M. V.; Lionetti, D.; Blakemore, J. D.
Inorg. Chem. 2017, 56, 10824.
4
(a) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Chem.
Soc. Rev. 2009, 38, 89. (b) Appel, A. M.; Bercaw, J. E.; Bocarsly, A. B.;
Dobbek, H.; DuBois, D. L.; Dupuis, M.; Ferry, J. G.; Fujita, E.; Hille, R.;
Kenis, P. J. A.; Kerfeld, C. A.; Morris, R. H.; Peden, C. H. F.; Portis, A.
R.; Ragsdale, S. W.; Rauchfuss, T. B.; Reek, J. N. H.; Seefeldt, L. C.;
Thauer, R. K.; Waldrop, G. L. Chem. Rev. 2013, 113, 6621. (c) Francke,
R.; Schille, B.; Roemelt, M. Chem. Rev. 2018, DOI:
10.1021/acs.chemrev.7b00459.
24
Productive N–H bond formation via PCET with models of late-stage
5
Zhang, W.; Lai, W.; Cao, R. Chem. Rev. 2017, 117, 3717.
N2 fixation intermediates (i.e., M≡N or M–NH2) has been observed pre-
viously: (a) Scepaniak, J. J.; Young, J. A.; Bontchev, R. P.; Smith, J. M.
Angew. Chem. Int. Ed. 2009, 48, 3158. (b) Pappas, I.; Chirik, P. J. J. Am.
Chem. Soc. 2015, 137, 3498. (c) MacLeod, K. C.; McWilliams, S. F.;
Mercado, B. Q.; Holland, P. L. Chem. Sci. 2016, 7, 5736. (d) Lindley, B.
M.; Bruch, Q. J.; White, P. S.; Hasanayn, F.; Miller, A. J. M. J. Am. Chem.
Soc. 2017, 139, 5305.
6 (a) Pickett, C. J.; Talarmin, J. Nature 1985, 317, 652. (b) Al-Salih, T. I.;
Pickett, C. J. J. Chem. Soc. Dalton Trans. 1985, 1255. (c) Pickett, C. J.;
Ryder, K. S.; Talarmin, J. J. Chem. Soc. Dalton Trans. 1986, 1453. (d)
Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. J. Am. Chem. Soc. 2016,
138, 5341.
7
In this context, a recent report in which the bioelectrosynthesis of am-
25 Marcus, R. A. J. Chem. Phys. 1956, 24, 966.
monia by nitrogenase is coupled to H2 oxidation is also noteworthy: Mil-
ton, R. D.; Cai, R.; Abdellaoui, S.; Leech, D.; De Lacey, A. L.; Pita, M.;
Minteer, S. D. Angew. Chem. Int. Ed. 2017, 56, 2680.
26
Iordanova, N.; Decornez, H.; Hammes-Schiffer, S. J. Am. Chem. Soc.
2001, 123, 3723.
27
8
We have assumed a PT-ET mechanism in which ET is rate limiting
Very recently there was a report of electrolytic NH3 synthesis by
based on significantly lower reorganization energies and barriers for PT
compared to ET. See SI for full description.
Cp2TiCl2. Although rates and Faradaic efficiencies are discussed, no
yields of NH3 are reported: Jeong, E.-Y.; Yoo, C.-Y.; Jung, C. H.; Park,
J. H.; Park, Y. C.; Kim, J.-N.; Oh, S.-G.; Woo, Y.; Yoon, H. C. ACS Sus-
tainable Chem. Eng. 2017, 5, 9662.
28
Koelle, U.; Infelta, P. P.; Graetzel, M. Inorg. Chem. 1988, 27, 879.
Call, A.; Casadevall, C.; Acuna-Pares, F.; Casitas, A.; Lloret-Fillol, J.
29
9
Chem. Sci. 2017, 8, 4739.
Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters,
30
Very recently there has been a study of electrocatalytic N2RR under
J. C. ACS Cent. Sci. 2017, 3, 217.
10 In some cases, the pKa of a particular anilinium acid was already known
in THF in which case this value was used. In cases where the pKa has not
been reported in THF a literature procedure was used to appropriately
convert the pKa from the solvent in which it was measured into a value
for THF. See SI for details.
ambient conditions in ionic liquids with Fe nanoparticles that reports
FE’s for NH3 as high as 60%: Zhou, F.; Azofra, L. M.; Ali, M.; Kar, M.;
Simonov, A. N.; McDonnell-Worth, C.; Sun, C.; Zhang, X.; MacFar-
lane, D. R. Energy Environ. Sci. 2017, 10, 2516.
31 We believe that the quasi-reversible nature of the electrochemical cou-
ple results from a high reorganization energy; the P3BFeN2 couple is
0/−
11
Consistent with this observation is that efforts to use other weak, non-
fully reversible using chemical reagents: Moret, M.-E.; Peters, J. C. An-
gew. Chem. Int. Ed. 2011, 50, 2063.
anilinium acids such as benzylammonium triflate (pKa in THF of 13.2)
and collidinium triflate (pKa in THF of 11.2) also led to no observed NH3
formation.
32
Due to the extensive diffusion between the working and auxiliary
12
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 oxida-
tion (Na+) is stable to the CPE conditions.
These results are also consistent with our previous observation of
[Ph2NH2][OTf] (pKa in THF of 3.2) yielding 72 ± 3 % NH3. See reference
9.
13
Pham, D. N.; Burgess, B. K. Biochemistry 1993, 32, 13275.
In some other reports on N2RR by molecular catalysts, efficiencies for
14
NH3 have been reported for several acids, but typically these acids span
only a small pKa range, electron yields are inconsistent, and variations are
not explained. For a representative example, see Reference 1b.
15 Anderson, J. S.; Cutsail III, G. E.; Rittle, J.; Connor, B. A.; Gunderson,
W. A.; Zhang, L.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2015,
137, 7803.
33 Henke, W. C.; Lionetti, D.; Moore, W. N. G.; Hopkins J. A.; Day, V.
W.; Blakemore, J.D. ChemSusChem. 2017, 10, 4589.
16
Matson, B. D.; Peters, J. C. ACS Catal. 2018, 8, 1448.
Recent results on a Cr–N2 species also support the role of PCET in the
17
formation of early-stage N2 fixation intermediates in the presence of col-
lidinium triflate and a cobaltocene: Kendall, A. J.; Johnson, S. I.; Bullock,
R. M.; Mock, M. T. J. Am. Chem. Soc. 2018, 140, 2528.
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