Enhanced Ammonia Oxidation Catalysis by a Low-Spin Iron
Complex Featuring
Cis
Coordination Sites
Michael D. Zott
,
Jonas C. Peters
*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Abstract
The goal of using ammonia as a solar fuel motivates the development of selective ammonia
oxidation (AO) catalysts for fuel cell applications. Herein we describe an Fe-mediated AO
electrocatalyst, [(bpyPy
2
Me)Fe(MeCN)
2
]
2+
, that exhibits the highest turnover number (TON)
reported to date for a molecular system. To improve on our recent report of a related iron AO
electrocatalyst, [(TPA)Fe(MeCN)
2
]
2+
(TON of 16), the present [(bpyPy
2
Me)Fe(MeCN)
2
]
2+
system (TON of 149) features a stronger-field, more rigid auxiliary ligand that maintains
cis
-labile
sites and a dominant low-spin population at the Fe(II) state. The latter is posited to mitigate
demetallation and hence catalyst degradation by the presence of a large excess of ammonia under
the catalytic conditions. Additionally, the [(bpyPy
2
Me)Fe(MeCN)
2
]
2+
system exhibits a
substantially faster AO rate (ca. 50x) at significantly lower (~250 mV) applied bias compared to
[(TPA)Fe(MeCN)
2
]
2+
. Electrochemical data are consistent with an initial E
1
net H-atom
abstraction step that furnishes the
cis
amide/ammine complex [(bpyPy
2
Me)Fe(NH
2
)(NH
3
)]
2+
,
followed by the onset of catalysis at E
2
. Theoretical calculations suggest the possibility of N–N
bond formation via multiple thermodynamically plausible pathways, including both reductive
elimination and ammonia nucleophilic attack. In sum, this study underscores that Fe, an earth-
abundant metal, is a promising metal for further development in metal-mediated AO catalysis by
molecular systems.
Graphical Abstract
*
Corresponding Author
: jpeters@caltech.edu.
The authors declare no competing financial interests.
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website.
Experimental procedures, compound characterization data, and computational details (PDF)
X-ray data (CIF)
HHS Public Access
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Published in final edited form as:
J Am Chem Soc
. 2021 May 26; 143(20): 7612–7616. doi:10.1021/jacs.1c02232.
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Ammonia is produced at industrial scale for use in fertilizer and chemical synthesis,
1
,
2
but
could become a promising carbon-free fuel if its selective and efficient catalytic oxidation to
nitrogen can be achieved. Catalysts sufficiently active and stable for fuel cell applications are
still needed.
2
,
3
,
4
Platinum-based materials, perhaps the current best current candidates,
5
,
6
,
7
suffer from low current densities due to side reactions that can result at moderate applied
bias.
Molecular systems offer several advantages with respect to fundamental studies that address
both activity and selectivity in AO.
8
The first molecular AO catalysts were reported in
2019.
9
,
10
,
11
,
12
,
13
Thus far, ruthenium catalysts have shown the highest turnover number
14
(TON; ~120 for [(TMP)Ru(NH
3
)
2
]
2+
using phenoxyl HAA reagents),
13
and the lowest
demonstrated onset potential for electrocatalysis (E
onset
= 0.04 V vs Fc/Fc
+
for [(bpydma)
(tpy)Ru(NH
3
)]
2+
; TON = 2).
9
We reported a distinct example of a first-row metal
electrocatalyst, [(TPA)Fe(NH
3
)
2
]OTf
2
, with a TON of 16 and a comparatively very fast rate
(10
7
M
−1
·s
−1
), but requiring a substantial E
onset
bias of 0.7 V (all potentials are reported vs
Fc/Fc
+
).
12
To improve on the AO activity of [(TPA)Fe(NH
3
)
2
]OTf
2
, we targeted an iron system that
would display enhanced catalyst stability while showing higher activity at a lower applied
bias. Catalyst degradation with [(TPA)Fe(NH
3
)
2
]OTf
2
appears to initiate from substitution
of the TPA ligand, an equilibrium process under the catalytic conditions that is likely favored
by the presence of a large excess of NH
3
. The extent of TPA displacement from
[(TPA)Fe(NH
3
)
2
]OTf
2
is likely increased by the complex’s dominant high-spin population
(
S
= 2) at RT, which results in more labile M–L bonds.
For the present system, given that the initial iron species in bulk solution during catalysis is
[(TPA)Fe(NH
3
)
2
]OTf
2
, we explored whether modifying the auxiliary ligand (L
aux
) in such a
fashion so as to support a low-spin (L
aux
)Fe(II)–NH
3
adduct might limit substitution by NH
3
and hence enhance overall stability, while maintaining high catalyst activity. We decided to
replace the weakfield tertiary amine donor of TPA, along with one of its pyridyl arms, with a
bipyridine ligand (Scheme 1); bipyridine has similar
σ
-donating properties to pyridine but
enhanced
π
-accepting properties.
15
,
16
,
17
We also sought to maintain the
cis
-labile sites
present in [(TPA)Fe(NH
3
)
2
]OTf
2
,
18
,
19
,
20
,
21
which may facilitate intramolecular N–N bond
formation. A rigid ligand containing each of these characteristics, bpyPy
2
Me (Scheme 1),
has been reported,
22
as has its iron(II) complex, [(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
. The latter
has been studied in the context of alkane oxidation.
21
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We first compared the electronic structure of both [(TPA)Fe(MeCN)
2
]OTf
2
and
[(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
in the presence of NH
3
in solution by the Evans method,
using trimethoxybenzene as an inert reference signal. At room temperature in the absence of
NH
3
, both systems display NMR spectra with resonances in the typical diamagnetic window,
and bulk magnetic moments of 0.7–0.8
μ
B
(see SI), indicating a dominant low-spin
population. In the presence of 75 equivalents NH
3
(~0.8 M at NMR concentrations),
however, the solution prepared with [(TPA)Fe(MeCN)
2
]OTf
2
gives rise to a spin-only
magnetic moment of 5.2
μ
B
, indicative of a fully populated
S
= 2 state. By contrast, under
identical conditions, a solution prepared with [(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
produces a
bulk magnetic moment of 1.2
μ
B
. Assuming a mixture of
S
= 0 and
S
= 2 species at spin-only
values, this moment corresponds to a 94:6 mixture in favor of the low-spin derivative in the
presence of NH
3
.
To assess the stability of [(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
to substitution by NH
3
in MeCN,
we monitored its speciation by UV-vis spectroscopy while titrating in NH
3
. A monotonic
decrease in the absorbance for [(bpyPy
2
Me)Fe(L)
2
]OTf
2
(L = MeCN, NH
3
), as well as a
loss of isosbestic behavior, becomes discernable in the presence of > 600 equivalents NH
3
(see SI). By contrast, [(TPA)Fe(MeCN)
2
]OTf
2
begins showing demetallation with > 200
equivalents NH
3
.
12
We next assessed catalytic AO by [(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
via cyclic voltammetry
(CV) and controlled potential coulometry (CPC) using boron-doped diamond (BDD)
working electrodes. CV of [(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
with added NH
3
as substrate
shows a precatalytic one-electron feature E
1
at 0.24 V and an irreversible multi-electron E
2
wave at 0.79 V (Fig. 1; see SI for DPV data), which replace the reversible one-electron wave
observed in the absence of NH
3
(E
1/2
= 0.82 V); this behavior mirrors that of
[(TPA)Fe(NH
3
)
2
]OTf
2
.
12
The catalytic onset potential of 0.45 V for
[(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
is ~250 mV cathodic of that for [(TPA)Fe(MeCN)
2
]OTf
2
,
and the catalytic current is ~fourfold higher. By contrast, applying less potential bias most
typically results in a concomitant decrease in catalytic current.
23
,
24
CPC confirms that [(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
is a highly active AO catalyst. With a
0.05 mM [(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
solution containing 400 equivalents NH
3
in MeCN
with NH
4
OTf supporting electrolyte (0.05 M), holding the bias at 0.85 V produces N
2
with a
high faradaic efficiency (FE) of 87%. After 24 h, a TON of 93 (average of 4 runs; STD = 8)
was measured. Furthermore, active catalyst remains after 24 h; a reload experiment was
performed in which the BDD electrode was cleaned and the NH
3
concentration was reset to
its original value; after an additional 24 h, another 56 equivalents N
2
were detected (average
of 2 runs), resulting in a net TON of 149. With respect to TON, this value is a marked
improvement on both the previously reported Ru AO electrocatalyst (TON of 2) and
[(TPA)Fe(MeCN)
2
]OTf
2
(TON of 16).
9
,
12
CPC with
15
NH
3
(
15
N = 99%) produces >90%
30
N
2
by GC-MS, indicating NH
3
as the source of nitrogen in the liberated N
2
. Post-catalysis,
a thoroughly rinsed electrode showed no catalytic activity, under the same conditions but
without added [(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
.
25
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To probe mechanistic issues for the [(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
system, we further
investigated the E
1
process. By CV, as the concentration of NH
3
is increased, the E
1
potential shifts cathodically. This is characteristic of an EC mechanism (single electron
transfer followed by a chemical step).
26
,
27
For an EC mechanism in the observed kinetic
regime (KE), the peak potential of such a process obeys Eq. 1 (Scheme 2). Two plausible
stoichiometries are provided, involving either one or two molecules of NH
3
in the forward
reaction (Scheme 2a and 2b, respectively). Plotting E
1
versus either [NH
3
] or [NH
4
+
]
(Scheme 2c and 2d, respectively), the respective slopes support stoichiometries of two NH
3
in the forward reaction and one NH
4
+
in the backward reaction, matching Scheme 2b.
Taking the iron species to be [(bpyPy
2
Me)Fe(MeCN)(NH
3
)]OTf
2
, we thus propose that the
product of this EC reaction is [(bpyPy
2
Me)Fe(NH
2
)(NH
3
)]OTf
2
, formed via substitution and
net hydrogen atom abstraction. This behavior parallels [(TPA)Fe(NH
3
)
2
]OTf
2
, which
follows Scheme 2a at a nearly identical potential.
12
The iron speciation deduced from the above electrochemical data, favoring
[(bpyPy
2
Me)Fe(MeCN)(NH
3
)]OTf
2
prior to E
1
, is notionally consistent with a solid-state
XRD study of a crystal grown from an ammoniacal MeCN solution (Fig. 2). The short Fe–
N
bpy
bond length
trans
to MeCN of 1.89
Å
also underscores tight binding of the bpyPy
2
Me
ligand.
To understand the character of the turnover-limiting E
2
step, we studied the rate dependence
on [Fe] and [NH
3
] concentrations. [(bpyPy
2
Me)Fe(MeCN)(NH
3
)]OTf
2
demonstrates first-
order behavior for both [Fe] and [NH
3
] (SI). The concentration ranges studied ([Fe] = 0.05–
2 mM, [NH
3
] = 0–0.5 M) span the conditions employed for both CV and CPC experiments.
Using the foot-of-the-wave analysis with a standard EC
cat
scheme to simplify the multi-
electron, multi-proton wave,
28
,
29
the first-order dependence on iron was recapitulated;
however, while a clear dependence on [NH
3
] is evident from the FOWA, ascertaining the
quantitative dependence on [NH
3
] is hindered by uncertainty in E°
cat
at high NH
3
concentrations. Still, we are able to compare the intrinsic AO reaction rates for
[(TPA)Fe(NH
3
)
2
]
2+
and [(bpyPy
2
Me)Fe(MeCN)(NH
3
)]
2+
. We previously reported a second-
order rate constant (k
′
obs
) of 3.7 × 10
7
M
−1
· s
−1
for [(TPA)Fe(NH
3
)
2
]
2+
;
12
for the present
catalyst [(bpyPy
2
Me)Fe(MeCN)(NH
3
)]
2+
, the average k
′
obs
is 1.8 × 10
9
M
−1
· s
−1
. Thus,
[(bpyPy
2
Me)Fe(MeCN)(NH
3
)]
2+
is ca. 1.5 orders of magnitude faster than
[(TPA)Fe(NH
3
)
2
]
2+
.
The aforementioned electrochemical data are limited in mechanistic utility with respect to
the various steps that follow E
2
, governing the pathway for N–N bond formation. Literature
precedent for N–N formation in systems applied to AO, whether mono- or bimolecular in
nature with respect to the metal complex, suggests two broad scenarios for consideration: (1)
interaction of two nitrogen ligands (I2N), as via nitride,
8
,
11
,
30
,
31
,
32
imide, or amide
33
,
34
coupling, or (2) ammonia nucleophilic attack (ANA) on an electrophilic nitrido or imido
ligand.
9
,
10
To begin to explore these issues for the present iron system, we have undertaken a
theoretical study (Scheme 3A,B), using density functional theory due to the size of the
present system, and the TPSS functional owing to its minimal bias for Fe
2+
versus Fe
3+
states.
35
,
36
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As an initial point of calibration, our chosen method reliably predicts the low-spin ground
state of [(bpyPy
2
Me)Fe(MeCN)(NH
3
)]
2+
and also its E
1
potential (0.24 V calcd; see SI),
which is analogous to that experimentally observed at 0.2 M NH
3
. The latter result is
encouraging as it involves both a change in oxidation state and a chemical step (to produce
[(bpyPy
2
Me)Fe(NH
2
)(NH
3
)]
2+
, in accordance with our electrochemical data).
From the E
1
product, [(bpyPy
2
Me)Fe(NH
2
)(NH
3
)]
2+
, one can consider a subsequent 1-
electron oxidation step that determines the E
2
potential (0.79 V by DPV). Calculations
suggest oxidation to [(bpyPy
2
Me)Fe(NH
2
)(NH
3
)]
3+
requires a potential of 1.10 V, well
above 0.79 V. However, a proton-coupled oxidation step to instead generate a
cis
-bis-amido
complex, [(bpyPy
2
Me)Fe(NH
2
)(NH
2
)]
2+
, occurs at 0.81 V (Scheme 3A, (a)). Alternatively,
a proton-coupled oxidation to generate the imido complex [(bpyPy
2
Me)Fe(NH)(NH
3
)
2
]
2+
occurs at 0.91 V (Scheme 3A, (b)), from which a subsequent proton-coupled oxidation to
produce the nitride species [(bpyPy
2
Me)Fe(N)(NH
3
)]
2+
can occur at much lower potential
(0.24 V, Scheme 3A, (c)). On thermodynamic grounds, both scenarios remain plausible in
working towards a mechanistic model.
We have also probed subsequent N–N bond formation steps. For example, we investigated
both reductive elimination (I2N) from the
cis
-bis-amido and ANA from the imido/nitrido
species; the first scenario highlights a
cis
-labile catalyst design, as in
[(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
. From [(bpyPy
2
Me)Fe(NH
2
)(NH
2
)]
2+
, N–N reductive
elimination to form the
η
2
-hydrazine adduct [(bpyPy
2
Me)Fe(
η
2
-N
2
H
4
)]
2+
(Scheme 3B, (d))
is exergonic by 6.3 kcal/mol. Alternatively, ANA at either the imido or nitrido (Scheme 3B,
(e) and (f)) is exergonic by 16.0 or 28.7 kcal/mol, respectively, affording another plausible
path towards N–N bond formation. Other pathways, such as those including bimolecular N–
N coupling (e.g., from NH
2
, NH, or N intermediates), may also be plausible (see SI for
additional details).
In conclusion, [(bpyPy
2
Me)Fe(MeCN)(NH
3
)]OTf
2
is an effective AO catalyst, yielding a net
TON of 149 after 48 h, which is the highest TON value reported to date for a molecular
catalyst. Compared to its related iron congener, [(TPA)Fe(NH
3
)
2
]OTf
2
,
[(bpyPy
2
Me)Fe(MeCN)(NH
3
)]OTf
2
is substantially more stable and operates at a higher rate
at significantly lower overpotential. While a number of mechanistic insights have been
discussed, including a net H-atom abstraction at E
1
to furnish [(bpyPy
2
Me)Fe(NH
2
)
(NH
3
)]
2+
prior to the onset of catalysis at E
2
, future efforts are needed to probe mechanistic
aspects of the N–N bond-forming step(s), guided by the thermodynamic considerations from
the experiments and theory discussed herein.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
ACKNOWLEDGMENT
The authors thank the National Institutes of Health (NIH GM070757). MDZ acknowledges the Resnick
Sustainability Institute at Caltech and NSF for support via Graduate Fellowships. The Beckman Institute at Caltech
supports the X-ray crystallography facility. We also acknowledge Dr. Matthew J. Chalkley and Dr. Pablo Garrido-
Barros for insightful discussions.
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33. Collman JP; Hutchison JE; Ennis MS; Lopez MA; Guilard R Reduced Nitrogen Hydride
Complexes of a Cofacial Metallodiporphyrin and Their Oxidative Interconversion. An Analysis of
Ammonia Oxidation and Prospects for a Dinitrogen Electroreduction Catalyst Based on Cofacial
Metallodiporphyrins. J. Am. Chem. Soc 1992, 114, 8074–8080.
34. Gu NX; Oyala PH; Peters JC Hydrazine Formation via Coupling of a Nickel(III)–NH
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Angew. Chem. Int. Ed 2021, 60, 4009–4013.
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36. See SI for full details.Calculations considered all possible spin multiplicities and plausible isomers.
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Figure 1.
CV of MeCN solutions containing 0.2 M NH
3
(400 equivalents), 0.05 M NH
4
OTf, and 0.5
mM [(TPA)Fe(MeCN)
2
]OTf
2
or [(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
with BDD working, Pt
counter, and 5 mM Ag/AgOTf reference electrodes.
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Figure 2.
Solid-state crystal structure of [(bpyPy
2
Me)Fe(MeCN)(NH
3
)]OTf
2
at 100 K, with select
bond lengths labeled in angstroms. Thermal ellipsoids are shown at 50% probability. Triflate
counterions and L
aux
hydrogen atoms are omitted for clarity.
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Scheme 1.
Targeting enhanced Fe-mediated AO via an alternative auxiliary ligand strategy.
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Scheme 2.
Evidence supporting an EC mechanism at the E
1
potential. Possible stoichiometries of the
E
1
potential are shown in (a) and (b). Plots of E
1
potential versus the natural logarithm of (c)
NH
3
or (d) NH
4
+
concentration for [(bpyPy
2
Me)Fe(MeCN)
2
]OTf
2
.
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Scheme 3.
(A) Possible E
2
steps and calculated E (V) values. (B) Possible N–N coupling reactions; ΔG
(kcal/mol). DFT-predicted ground spin-state multiplicities are shown.
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