Enhanced Ammonia Oxidation Catalysis by a Low-Spin Iron Complex Featuring Cis Coordination Sites

Article abstract of DOI:10.1021/jacs.1c02232

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 Fe-mediated AO electrocatalysis with [(bpyPy2Me)Fe(MeCN)2]2+, exhibiting 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 [(bpyPy2Me)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 demetalation and hence catalyst degradation by the presence of a large excess of ammonia under the catalytic conditions. Additionally, the [(bpyPy2Me)Fe(MeCN)2]2+ system exhibits a substantially faster AO rate (ca. 50×) at significantly lower (～250 mV) applied bias compared to [(TPA)Fe(MeCN)2]2+. Electrochemical data are consistent with an initial E1 net H-atom abstraction step that furnishes the cis amide/ammine complex [(bpyPy2Me)Fe(NH2)(NH3)]2+, followed by the onset of catalysis at E2. 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.

Full text of DOI:10.1021/jacs.1c02232

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Enhanced Ammonia Oxidation Catalysis by a Low-Spin Iron
Complex Featuring Cis Coordination Sites
Michael D. Zott and Jonas C. Peters*
Cite This: J. Am. Chem. Soc. 2021, 143, 7612−7616
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sı Supporting Information
ABSTRACT: The goal of using ammonia as a solar fuel motivates the development of selective ammonia oxidation (AO) catalysts
2
+
for fuel cell applications. Herein, we describe Fe-mediated AO electrocatalysis with [(bpyPy Me)Fe(MeCN) ] , exhibiting the
2
2
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 Me)Fe(MeCN) ] system (TON of 149) features a
+
2+
2
2
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 demetalation and hence catalyst degradation by the presence of a large excess of ammonia under the
2
+
catalytic conditions. Additionally, the [(bpyPy Me)Fe(MeCN) ] system exhibits a substantially faster AO rate (ca. 50×) at
2
2
2
+
significantly lower (∼250 mV) applied bias compared to [(TPA)Fe(MeCN) ] . Electrochemical data are consistent with an initial
E net H-atom abstraction step that furnishes the cis amide/ammine complex [(bpyPy Me)Fe(NH )(NH )] , followed by the onset
2
2
+
1
2
2
3
of catalysis at E . Theoretical calculations suggest the possibility of N−N bond formation via multiple thermodynamically plausible
2
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.
mmonia is produced at industrial scale for use in fertilizer
and chemical synthesis, but could become a promising
For the present system, given that the initial iron species in
bulk solution during catalysis is [(TPA)Fe(NH ) ]OTf , we
1
,2
A
3
2
2
carbon-free fuel if its selective and efficient catalytic oxidation
to nitrogen can be achieved. Catalysts sufficiently active and
explored whether modifying the auxiliary ligand (L ) in such
aux
a fashion so as to support a low-spin (L )Fe(II)−NH adduct
aux
3
2
−4
stable for fuel cell applications are still needed.
Platinum-
might limit L substitution by NH and hence enhance
aux 3
5
−7
based materials, perhaps the current best candidates,
suffer
overall stability, while maintaining high catalyst activity. We
decided to replace the weak-field tertiary amine donor of TPA,
along with one of its pyridyl arms, with a bipyridine ligand
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
(
Scheme 1); bipyridine has similar σ-donating properties to
15−17
pyridine but enhanced π-accepting properties.
We also
8
ammonia oxidation (AO). The first molecular AO catalysts
sought to maintain the cis-labile sites present in [(TPA)Fe-
9
−13
1
8−21
were reported in 2019.
Thus, far, ruthenium catalysts have
(
NH ) ]OTf ,
which may facilitate intramolecular N−N
3
2
2
14
shown the highest turnover number (TON; ∼120 for
bond formation. A rigid ligand containing each of these
characteristics, bpyPy Me (Scheme 1), has been reported, as
2+
22
[
(TMP)Ru(NH ) ] using phenoxyl hydrogen atom abstrac-
3 2
13
2
tion (HAA) reagents) and the lowest demonstrated onset
potential for electrocatalysis (E = 0.04 V (all potentials are
has its iron(II) complex, [(bpyPy Me)Fe(MeCN) ]OTf . The
2
2
2
21
onset
latter has been studied in the context of alkane oxidation.
+
2+
reported vs Fc/Fc ) for [(bpydma)(tpy)Ru(NH )] ; TON =
3
We first compared the electronic structure of both
9
2
). We reported a distinct example of a first-row metal
electrocatalyst, [(TPA)Fe(NH ) ]OTf , with a TON of 16 and
[
(TPA)Fe(MeCN) ]OTf and [(bpyPy Me)Fe(MeCN) ]-
2
2
2
2
3
2
2
OTf in the presence of NH in solution by the Evans
2
3
7
−1 −1
a comparatively very fast rate (10 M ·s ), but requiring a
method, using trimethoxybenzene as an inert reference signal.
1
2
substantial Eonset bias of 0.7 V.
To improve on the AO performance of [(TPA)Fe(NH ) ]-
At room temperature in the absence of NH , both systems
3
3
2
display NMR spectra with resonances in the typical
OTf , we targeted an iron system that would display enhanced
2
diamagnetic window, and bulk magnetic moments of 0.7−
catalyst stability while showing higher activity at a lower
applied bias. Catalyst degradation with [(TPA)Fe(NH ) ]-
0
.8μ (see Supporting Information (SI)), indicating a
B
3
2
OTf appears to initiate from substitution of the TPA ligand,
an equilibrium process under the catalytic conditions that is
2
Received: February 28, 2021
Published: May 17, 2021
likely favored by the presence of a large excess of NH . The
3
extent of TPA displacement from [(TPA)Fe(NH ) ]OTf is
3
2
2
likely increased by the complex’s dominant high-spin
population (S = 2) at rt, which results in more labile M−L
bonds.
©
2021 American Chemical Society
https://doi.org/10.1021/jacs.1c02232
J. Am. Chem. Soc. 2021, 143, 7612−7616
7
612

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Scheme 1. Targeting Enhanced Fe-Mediated AO via an
Alternative Auxiliary Ligand Strategy
Scheme 2. Evidence Supporting an EC Mechanism at the E1
Potential
a
a
Possible stoichiometries for the E process are shown in (a) and (b).
1
Plots of E potential versus the natural logarithm of (c) NH or (d)
1
3
Figure 1. CV of MeCN solutions containing 0.2 M NH (400 equiv),
+
3
NH4 concentration for [(bpyPy Me)Fe(MeCN) ]OTf .
2 2 2
0
.05 M NH OTf, and 0.5 mM [(TPA)Fe(MeCN) ]OTf or
4
2
2
[
5
(bpyPy Me)Fe(MeCN) ]OTf with BDD working, Pt counter, and
mM Ag/AgOTf reference electrodes.
2 2 2
dominant low-spin population. In the presence of 75 equiv
NH (∼0.8 M at NMR concentrations), however, the solution
3
prepared with [(TPA)Fe(MeCN) ]OTf gives rise to a spin-
2
2
only magnetic moment of 5.2μ , indicative of a fully populated
B
S = 2 state. By contrast, under identical conditions, a solution
prepared with [(bpyPy Me)Fe(MeCN) ]OTf produces a
2
2
2
bulk magnetic moment of 1.2μ . Assuming a mixture of S =
B
0
and S = 2 species with spin-only values, this moment
corresponds to a 94:6 mixture in favor of the low-spin
derivative in the presence of NH3.
To assess the stability of [(bpyPy Me)Fe(MeCN) ]OTf to
Figure 2. Solid-state crystal structure of [(bpyPy Me)Fe(MeCN)-
2
2
2
2
substitution by NH in MeCN, we monitored its speciation by
(NH )]OTf at 100 K, with select bond lengths labeled in angstroms.
3 2
3
Thermal ellipsoids are shown at 50% probability. Triflate counterions
and Laux hydrogen atoms are omitted for clarity.
UV−vis spectroscopy while titrating in NH . A monotonic
3
decrease in the absorbance for [(bpyPy Me)Fe(L) ]OTf (L =
2
2
2
MeCN, NH ), as well as a loss of isosbestic behavior, becomes
3
discernible in the presence of >600 equiv NH (see SI). By
catalytic onset potential of 0.45 V for [(bpyPy Me)Fe-
3
2
contrast, [(TPA)Fe(MeCN) ]OTf begins showing demetala-
tion with >200 equiv of NH3.
(MeCN) ]OTf is ∼250 mV cathodic of that for [(TPA)Fe-
2
2
2
2
12
(MeCN) ]OTf , and the catalytic current is ∼4-fold higher. By
2
2
We next assessed catalytic AO by [(bpyPy Me)Fe-
contrast, applying less potential bias most typically results in a
2
23,24
(
MeCN) ]OTf via cyclic voltammetry (CV) and controlled
concomitant decrease in catalytic current.
CPC confirms that [(bpyPy Me)Fe(MeCN) ]OTf is a
2
2
potential coulometry (CPC) using boron-doped diamond
BDD) working electrodes. CV of [(bpyPy Me)Fe(MeCN) ]-
2
2
2
(
highly active AO catalyst. With a 0.05 mM [(bpyPy Me)Fe-
2
2
2
OTf with added NH as substrate shows a precatalytic one-
(MeCN) ]OTf solution containing 400 equiv of NH in
2
3
2
2
3
electron feature E at 0.24 V and an irreversible multielectron
MeCN with NH OTf supporting electrolyte (0.05 M), holding
1
4
E2 wave at 0.79 V (Figure 1; see SI for differential pulse
voltammetry (DPV) data), which replace the reversible one-
electron wave observed in the absence of NH (E = 0.82 V);
the bias at 0.85 V produces N with a high faradaic efficiency
2
(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
3
1/2
12
this behavior mirrors that of [(TPA)Fe(NH ) ]OTf . The
3
2
2
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Scheme 3. (A) Possible E Steps and Calculated E (V)
values; (B) Possible N−N Coupling Reactions
electrocatalyst (TON of 2) and [(TPA)Fe(MeCN) ]OTf
2 2
9,12
2
a
15
15
(TON of 16).
CPC with NH ( N = 99%) produces
3
3
0
>
90% N by GC-MS, indicating NH as the source of
2
3
nitrogen in the liberated N . Post-catalysis, a thoroughly rinsed
2
electrode showed no catalytic activity, under the same
conditions but without added [(bpyPy Me)Fe(MeCN) ]-
2
2
25
OTf2.
To probe mechanistic issues for the [(bpyPy Me)Fe-
2
(
MeCN) ]OTf system, we further investigated the E process.
2
2
1
By CV, as the concentration of NH is increased, the E
3
1
potential shifts cathodically. This is characteristic of an EC
mechanism (single-electron transfer followed by a chemical
26,27
step).
regime (KE), the peak potential of such a process obeys eq 1
Scheme 2). Two plausible stoichiometries are provided,
For an EC mechanism in the observed kinetic
(
involving either one or two molecules of NH in the forward
3
reaction (Scheme 2a and 2b, respectively). Plotting E versus
1
+
4
either [NH ] or [NH ] (Scheme 2c and 2d, respectively)
3
revealed the respective slopes support stoichiometries of two
+
4
NH in the forward reaction and one NH in the backward
3
reaction, matching Scheme 2b. Taking the iron species to be
(bpyPy Me)Fe(MeCN)(NH )]OTf , we thus propose that
[
2
3
2
the product of this EC reaction is [(bpyPy Me)Fe(NH )-
2
2
(
NH )]OTf , formed via substitution and net hydrogen atom
3 2
abstraction. This behavior parallels [(TPA)Fe(NH ) ]OTf ,
which follows Scheme 2a at a nearly identical potential.
3
2
2
12
The iron speciation deduced from the above electrochemical
data, favoring [(bpyPy Me)Fe(MeCN)(NH )]OTf prior to
2
3
2
E1, is notionally consistent with a solid-state XRD study of a
crystal grown from an ammoniacal MeCN solution (Figure 2).
The short Fe−N bond length trans to MeCN of 1.89 Å also
bpy
underscores tight binding of the bpyPy Me ligand.
2
To understand the character of the turnover-limiting E step,
2
we studied the rate dependence on [Fe] and [NH3]
concentrations. [(bpyPy Me)Fe(MeCN)(NH )]OTf demon-
2
3
2
strates first-order behavior for both [Fe] and [NH ] (SI). The
3
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
(
FOWA) with a standard EC scheme to simplify the
cat
28,29
multielectron, multiproton wave,
the first-order depend-
ence on iron was recapitulated; however, while a clear
dependence on [NH ] is evident from the FOWA, ascertaining
3
the quantitative dependence on [NH ] is hindered by
3
uncertainty in E°cat. at high NH concentrations. Still, we are
3
able to compare the intrinsic AO reaction rates for [(TPA)-
2
+
2+
Fe(NH ) ] and [(bpyPy Me)Fe(MeCN)(NH )] . We
3
2
2
3
previously reported a second-order rate constant (k′ ) of
obs
7
−1 −1
2+ 12
3
.7 × 10 M ·s for [(TPA)Fe(NH ) ] ; for the present
3
2
2
+
catalyst [(bpyPy Me)Fe(MeCN)(NH )] , the average k′ is
2
3
obs
2+
9
−1 −1
1
.8 × 10 M ·s . Thus, [(bpyPy Me)Fe(MeCN)(NH )] is
2
3
2
+
ca. 1.5 orders of magnitude faster than [(TPA)Fe(NH ) ] .
3
2
The aforementioned electrochemical data are limited in
mechanistic utility with respect to the various steps that follow
E , governing the pathway for N−N bond formation.
2
a
ΔG (kcal/mol). DFT-predicted ground spin-state multiplicities are
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),
shown.
electrode was cleaned and the NH concentration was reset to
its original value; after an additional 24 h, another 56 equiv of
3
8
,11,30−32
33,34
as via nitride,
imide, or amide
coupling, or (2)
N 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
ammonia nucleophilic attack (ANA) on an electrophilic
2
9
,10
nitrido or imido ligand.
To begin to explore these issues
for the present iron system, we have undertaken a theoretical
7
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study (Scheme 3A,B), using density functional theory due to
the size of the present system, and the TPSS functional owing
Accession Codes
CCDC 2065372−2065373 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
to its minimal bias for Fe2 versus Fe states.
+
3+
35,36
As an initial point of calibration, our chosen method reliably
predicts the low-spin ground state of [(bpyPy Me)Fe(MeCN)-
2
2
+
(
NH )] and also its E potential (0.24 V calcd; see SI), which
3 1
is analogous to that experimentally observed at 0.2 M NH3.
The latter result is encouraging, as it involves both a change in
oxidation state and a chemical step (to produce [(bpyPy Me)-
AUTHOR INFORMATION
2
■
2
+
Fe(NH )(NH )] , in accordance with our electrochemical
2
3
Corresponding Author
data).
From the E product, [(bpyPy Me)Fe(NH )(NH )] , one
Jonas C. Peters − Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena,
California 91125, United States; orcid.org/0000-0002-
6610-4414; Email: jpeters@caltech.edu
2
+
1
2
2
3
can consider a subsequent one-electron oxidation step that
determines the E potential (0.79 V by DPV). Calculations
2
3
+
suggest oxidation to [(bpyPy Me)Fe(NH )(NH )] requires
2
2
3
a potential of 1.10 V, well above 0.79 V. However, a proton-
Author
coupled oxidation step to instead generate a cis-bis-amido
Michael D. Zott − Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena,
California 91125, United States; orcid.org/0000-0003-
0535-0512
2+
complex, [(bpyPy Me)Fe(NH )(NH )] , occurs at 0.81 V
2
2
2
(
Scheme 3A, (a)). Alternatively, a proton-coupled oxidation to
2
+
generate the imido complex [(bpyPy Me)Fe(NH)(NH ) ]
2
3 2
occurs at 0.91 V (Scheme 3A, (b)), from which a subsequent
proton-coupled oxidation to produce the nitride species
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c02232
2
+
[
(bpyPy Me)Fe(N)(NH )] can occur at much lower
2 3
potential (0.24 V, Scheme 3A, (c)). On thermodynamic
grounds, both scenarios remain plausible in working toward a
mechanistic model.
Notes
The authors declare no competing financial interest.
We have also probed subsequent N−N bond formation
ACKNOWLEDGMENTS
■
steps. For example, we investigated both reductive elimination
(
The authors thank the National Institutes of Health (NIH
GM070757). M.D.Z. 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.
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 Me)Fe(MeCN) ]OTf . From [(bpyPy Me)Fe-
2
2
2
2
2
+
2
(
NH )(NH )] , N−N reductive elimination to form the η -
hydrazine adduct [(bpyPy Me)Fe(η -N H )] (Scheme 3B,
2
2
2 2+
2
2
4
(
d)) is exergonic by 6.3 kcal/mol. Alternatively, ANA at either
the imido or nitrido (Scheme 3B, (e) and (f)) is exergonic by
6.0 or 28.7 kcal/mol, respectively, affording another plausible
1
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