Diversion of Catalytic C-N Bond Formation to Catalytic Oxidation of NH3 through Modification of the Hydrogen Atom Abstractor

Article abstract of DOI:10.1021/jacs.9b13706

We report that (TMP)Ru(NH₃)₂ (TMP = tetramesitylporphryin) is a molecular catalyst for oxidation of ammonia to dinitrogen. An aryloxy radical, tri-tert-butylphenoxyl (ArO·), abstracts H atoms from a bound ammonia ligand of (TMP)Ru(NH₃)₂, leading to the discovery of a new catalytic C-N coupling to the para position of ArO· to form 4-amino-2,4,6-tri-tert-butylcyclohexa-2,5-dien-1-one. Modification of the aryloxy radical to 2,6-di-tert-butyl-4-tritylphenoxyl radical, which contains a trityl group at the para position, prevents C-N coupling and diverts the reaction to catalytic oxidation of NH₃ to give N₂. We achieved 125 ± 5 turnovers at 22 °C for oxidation of NH₃, the highest turnover number (TON) reported to date for a molecular catalyst.

Full text of DOI:10.1021/jacs.9b13706

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Communication
Diversion of Catalytic C-N Bond Formation to Catalytic Oxidation
of NH3 Through Modification of the Hydrogen Atom Abstractor
Peter Dunn, Samantha I Johnson, Werner Kaminsky, and R. Morris Bullock
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b13706 • Publication Date (Web): 01 Feb 2020
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Diversion of Catalytic C-N Bond Formation to Catalytic Oxidation of
NH3 Through Modification of the Hydrogen Atom Abstractor
Peter L. Dunn‡, Samantha I. Johnson‡, Werner Kaminsky§, and R. Morris Bullock‡*
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‡Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, Richland, WA 99352, USA
§Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, USA
Supporting Information Placeholder
catalytically oxidize NH3 to N2 (Figure 1, top);27-30 all
ABSTRACT: We report that (TMP)Ru(NH3)2 (TMP =
have turnovers less than 20.
tetramesitylporphryin) is a molecular catalyst for
oxidation of ammonia to dinitrogen. An aryloxy radical,
tri-tert-butylphenoxyl (ArO•), abstracts H atoms from a
bound ammonia ligand of (TMP)Ru(NH3)2, leading to the
discovery of a new catalytic C-N coupling to the para
position of ArO•, forming 4-amino-2,4,6-tri-tert-
butylcyclohexa-2,5-dien-1-one. Modification of the
aryloxy radical to contain a trityl group at the para
NH3
2+
+
2+
L
NH3
Fe
N
Ph
NH3
N
N
N
N
N
tBu
P
N
N
N
N
Ru
N
O
O
Ru
L
Ru
O
O
NH3
P
tBu
N
NMe2
N
Ph
N
L =
NMe2
L
L
NH3
Ru
Ru
- 6 H+, 6 e-
N
Ru
N
H3N
N
N
NH3
N
N
Ru
L
Ru
L
NH3
position,
2,6-di-tert-butyl-4-tritylphenoxyl
radical,
catalytic, TON = 125
stoichiometric oxidation of NH3
prevents C-N coupling and diverts the reaction to
catalytic oxidation of NH3 to give N2. We achieve
125(±5) turnovers at 22 °C for oxidation of NH3, the
highest reported to date for a molecular catalyst.
this work
Collman et al.
Figure 1:
oxidation by molecular catalysts.
Molecular mediators to abstract or donate H
Stoichiometric and catalytic ammonia
atoms have been increasingly beneficial in the catalytic
reduction of O2,31-32 using a variety of mediators that span
a range of bond strengths.12 We have recently shown that
tri-tert-butylphenoxyl (ArO•)12, 33 accomplishes H atom
abstraction (HAA) from ammonia ligands: triple HAA
from Mn diphosphine complexes resulting in
cyclophosphazeniums,34 triple HAA from a Mo(NH3)
complex, generating a metal imido formed by C-N
coupling,35 sub-stoichiometric ammonia oxidation from a
Mo polypyridyl complex,36 and ammonia oxidation
catalyzed by a Cp*Ru complex.27
Carbon-free fuels are attractive to meet
worldwide demands for sustainable energy.1 Ammonia is
produced on a huge scale by the Haber-Bosch process;2 it
has a high energy density, and is readily transported as a
liquid using a well-established infrastructure.3 Direct
ammonia fuel cells, where electricity is generated through
oxidation of ammonia to N2, offers appealing attributes
compared to carbon-based fuels.4-7 Ammonia has also
been used as an H2 storage medium.8-11 However, the
study of molecular ammonia oxidation catalysts has
lagged behind studies of its microscopic reverse, nitrogen
reduction.
Collman and co-workers reported elegant studies
on stoichiometric ammonia oxidation to dinitrogen
(Figure 1), using cofacial diruthenium porphryins.37-38
Starting from a [Ru(NH3)]2 complex, bridging hydrazine,
diazene, and dinitrogen complexes were synthesized
through double oxidation and deprotonation reactions.
Inspired by their results, we investigated the reactivity of
a Ru complex with a more readily accessible ligand.39
The first N-H bond dissociation free energy
(BDFE) of ammonia is 99.4 kcal/mol,12 but the energy is
decreased by coordination to a metal. Several approaches
in transition metal systems have overcome the high bond
strength to cleave N-H bonds, including N-H oxidative
addition,13 metal-ligand cooperativity,14-15 1,2 addition
across a metal-metal bond,16 and H atom abstraction.17-20
Formation of an N-N bond is a requisite step in oxidation
of NH3 to N2, so understanding the factors that control
that reaction are crucial in the design of metal catalysts
for oxidation of NH3.21-26 To the best of our knowledge,
only four molecular systems have been reported to
Treatment of (TMP)Ru(CO) (TMP
=
tetramesitylporphyrin) with NH3 (1 atm) results in
formation of (TMP)Ru(CO)(NH3). Photolysis of a
solution of (TMP)Ru(CO)(NH3) with excess NH3 leads to
(TMP)Ru(NH3)2 in 96% yield (Scheme 1). The 1H NMR
spectrum of (TMP)Ru(NH3)2 exhibits an upfield shifted
ammonia resonance at -6.88 ppm due to porphyrin ring
currents.40 Single crystals of (TMP)Ru(NH3)2 were grown
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from slow evaporation of a concentrated THF solution,
and the structure was determined by X-ray diffraction
(Figure 2).
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Scheme 1. Synthesis of (TMP)Ru(NH3)2
Scheme 3. Oxidation of ammonia to N2 catalyzed by
(TMP)Ru(NH3)2.
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Recognizing the possibility of thwarting the C-N
coupling by increasing the steric bulk at the para position
of ArO•, we prepared an aryloxy radical with a trityl
substituent in the para position, the previously reported
2,6-di-tert-butyl-4-tritylphenoxyl radical (Ph3C-ArO•)
(Figure 3).48 Ph3C-ArO• was isolated as dark green
crystals; it shows good stability over months when stored
as a solid at -35ºC.49
Figure 2: 50% thermal ellipsoid drawing of
(TMP)Ru(NH3)2. Solvent and H atoms, except N-H
bonds, are not shown. Bond distances (Å): Ru-N1:
2.108(3), Ru-N2: 2.037(2), and Ru-N3: 2.031(2).
No
reaction
was
observed
when
(TMP)Ru(CO)(NH3) was treated with ArO• and excess
ammonia. In contrast, in the presence of NH3 and ArO•
at room temperature, (TMP)Ru(NH3)2 quenched the blue
1
color of ArO•. H NMR spectroscopic analysis showed
formation of ArOH and 4-amino-2,4,6-tri-tert-
butylcyclohexa-2,5-dien-1-one (RNH2) that results from
41-42
C-N bond formation with NH3
(Scheme 2). Small
1
amounts (<4%) of isobutylene are also observed by H
NMR spectroscopy; it was previously reported from
decomposition of ArO•.43 The catalytic reaction was
optimized with as low as 0.05 mole % loading of Ru,
resulting in 610±20 turnovers RNH2, which can be
isolated as a white solid (65% yield). Control reactions
with ArO• and 15NH3 in C6D6 (no Ru) show no coupling
product or 15N2 as measured by 15N NMR spectroscopy.
Figure 3. Structural drawings (50% thermal ellipsoids) of
Ph3C-ArO• (top left) and Ph3C-ArOH (top right).
Solvent of crystallization is not shown. Bond distance (Å)
comparisons (bottom) are consistent with a radical
centered on the para carbon: a dienone-like structure.33
Scheme 2. C-N coupling catalyzed by (TMP)Ru(NH3)2.
RNH2 is proposed to result from C-N coupling
between the para position of the aryl radical and a Ru-
NHx (x = 0, 1, 2) species that has significant radical
character on the nitrogen atom.44 Mayer and co-workers
noted that the predominant resonance structure of ArO•
has significant radical character on the para carbon.33
Similar C-N coupling was observed in a CpMo system,
where H atom abstractions from an ammonia ligand led
to the formation of a Mo-NHx (x = 0, 1, 2) species that
couples with the aryloxy radical.35 Oxidation at the 2- or
4- positions has been observed,45-46 and coupling between
anilido radicals and ArO• resulting in C-N bond
formation at the para position has also been reported.43
Attempts to use TEMPO or ABNO,47 which form weaker
O-H bonds,12 did not lead to product formation in our
system.
When (TMP)Ru(NH3)2 was treated with Ph3C-
ArO• and excess 15NH3, the intense green color of the
radical dissipated over the course of a day. A 15N NMR
spectrum in C6D6 revealed a sharp singlet at -71.7 ppm
(vs. CH315NO2 = 0 ppm), consistent with the formation of
1
free 15N2 (Scheme 3). The H NMR spectrum showed
conversion of Ph3C-ArO• to Ph3C-ArOH. Importantly,
no C-N coupling product is observed, although, minor
amounts of isobutylene are again observed. In a control
experiment, we observed no resonance for 15N2 after
treating Ph3C-ArO• with 15NH3 in C6D6 for one month at
22 °C, as judged by an overnight 15N NMR spectrum.
Catalytic turnover was determined by
quantifying the liberated N2 gas in the headspace by GC
(Figure S19, SI).27,36 Treatment of a 1.0 mM solution of
(TMP)Ru(NH3)2 with Ph3C-ArO• (450 equiv.), then
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saturated with NH3, produces ~3.3  10-5 moles of N2,
giving 40(±0.5) turnovers of N2 over the course of one
day. Decreasing the catalyst concentrations to 0.5 mM or
0.25 mM gives increased turnovers, reaching 125(±5) per
Ru (Table S1, SI). Recharging a spent 0.5 mM reaction
with 540 more equivalents of Ph3C-ArO• and NH3
produces ~20 more turnovers of N2, showing that the
ruthenium porphyrin catalyst is still active. The current
system is limited by the amount and stability of Ph3C-
ArO• in solution, as Ph3C-ArO• has a half-life of ~ 1 day
in C6D6, as shown by UV-Vis spectroscopy. Analysis of
uphill, previous studies have also found this trend; when
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coupled to a more favorable step (C-N, N-N coupling or
CO loss), thermodynamically unfavorable HAAs can
occur.34-35
Computations show that C-N coupling is
favorable from both the amido and imido complexes, by
0.9 and 18.5 kcal/mol (Fig. 4). These favorable steps
appear to drive the reaction, leading to formation of
RNH2. Release of the RNH2 product and binding a fresh
ammonia molecule is exergonic by 10.9 kcal/mol. While
C-N coupling is more favorable from a RuIV=NH
intermediate, generation of this species necessitates a
second, more uphill HAA. Additionally, in order for
product formation to occur via RuIV=NH, an additional H
atom would have to be donated (not shown in Figure 4)
from either previously formed ArOH, or possibly another
Ru(NH)x (x = 2 or 3) species. Therefore, we favor the
simplest and lowest energy route for C-N bond formation:
C-N coupling from RuIII-NH2.
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the reaction solution by H NMR spectroscopy after
catalysis shows only two main organic products: Ph3C-
ArOH (~90%) and isobutylene (~2%) (Table S2, SI).
We propose that the dramatically divergent
catalysis mediated by Ph3C-ArO• relative to ArO• is
caused by steric differences. While the mesityl groups on
the porphyrin ring were initially chosen to prevent
dimerization, the added steric bulk appears to have a
cooperative effect with Ph3C-ArO• to prevent C-N
coupling. From the crystal structure of (TMP)Ru(NH3)2,
the porphyrin pocket, from ortho-methyl to ortho-methyl,
is approximately ~8.5 Å across and ~3.5 Å deep.
Comparing the crystal structures of ArO• and Ph3C-
ArO• lends insight. The height (oriented as in Figure 3)
of ArO• is ~7.4 Å. Replacement of the tert-butyl by a
trityl group increases the length to ~9.1 Å and the depth
from ~4.1 to ~8.4 Å in Ph3C-ArO•, making it too big to
easily fit in the porphyrin pocket (see Figure S22, SI, for
additional explanation and drawings). We suggest that
this increase in size impedes the relative rate of C-N
coupling compared to that of N2 formation, such that
oxidation to give N2 predominates.
Figure 5. Possible mechanism for ammonia oxidation
catalyzed by (TMP)Ru(NH3)2. L = NH3 or vacant site.
Two types of mechanisms have been invoked for
N-N bond formation in ammonia oxidation by molecular
complexes: bimetallic N-N coupling, or nucleophilic
attack of ammonia on an electrophilic M(NH)x (x = 0, 1,
or 2) intermediate.21-24, 27-30, 50-54 Multiple mechanistic
possibilities should be considered, since bimetallic N-N
coupling could occur at any of the possible intermediates:
amide, imide, or nitride ligand on the metal. Using
cofacial Ru porphryins, Collman and co-workers reported
stoichiometric N-N coupling through
a
cofacial
[RuIII(NH2)]2 intermediate.37-38 Their proposal, in
conjunction with our observed C-N coupling from a
RuIII(NH2) intermediate, leads us to favor N-N bond
formation by a bimetallic coupling route of two Ru
amides, forming a bridging hydrazine ligand (Figure 5).
We are not aware, however, of additional precedents for
coupling of two metal amides. Subsequent HAAs could
then proceed from the bridged, bimetallic complex, akin
to that proposed by Collman, or dissociation of a Ru
center could lead to a terminal Ru hydrazine species.
Figure 4. Computed free energies in benzene solvent.
DFT calculations provide further understanding
of the observed reactivity (Figure 4). The first two N-H
BDFEs were computed to be 81.7 and 92.9 kcal/mol,
which are thermodynamically uphill by 5.0 and 16.2
kcal/mol, respectively, for hydrogen atom abstraction by
ArO• (ArO-H BDFE = 77 kcal/mol).12 The third BDFE
is lower (74.8 kcal/mol). While the second BDFE is
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Schüth, F.; Palkovits, R.; Schlögl, R.; Su, D. S., Ammonia
HAA from both possible hydrazine intermediates should
be facile compared to their NH3 analogues. HAAs from
the terminal hydrazine species would access higher
oxidation states of Ru, which are well-documented for
ruthenium porphryins bearing nitrogen-based axial
ligands.40, 55-58
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In conclusion, we report a new catalyst,
(TMP)Ru(NH3)2, that catalyzes the oxidation of NH3 to
N2 under mild conditions using Ph3C-ArO•. The
scalability (>10 g) and ease of preparation of Ph3C-ArO•
make it an attractive alternative to ArO•. This catalytic
system achieves 125(±5) turnovers for oxidation of NH3,
a significant improvement compared to previously
reported molecular catalysts. Ongoing work is focusing
on investigating the mechanism of N-N coupling,
translating this system to work with earth-abundant
metals, and converting this system to work under
mediated electrocatalytic conditions wherein the
phenoxyl radical can be regenerated.
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ASSOCIATED CONTENT
11.
Adli, N. M.; Zhang, H; Mukherjee, S.; Wu, G., Review—
The Supporting Information is available free of charge on
the ACS Publications website. Experimental procedures,
spectral data, computational procedures, molecular
coordinates, X-ray procedures (pdf).
Ammonia Oxidation Electrocatalysis for Hydrogen Generation and
Fuel Cells. J. Electrochem. Soc. 2018, 165, J3130-J3147.
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Warren, J. J.; Tronic, T. A.; Mayer, J. M.,
Thermochemistry of Proton-Coupled Electron Transfer Reagents and
its Implications. Chem. Rev. 2010, 110, 6961-7001.
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Zhao, J.; Goldman, A. S.; Hartwig, J. F., Oxidative
AUTHOR INFORMATION
Addition of Ammonia to Form a Stable Monomeric Amido Hydride
Complex. Science 2005, 307, 1080-1082.
Corresponding Author
R. Morris Bullock (morris.bullock@pnnl.gov)
14.
Margulieux, G. W.; Turner, Z. R.; Chirik, P. J., Synthesis
and Ligand Modification Chemistry of a Molybdenum Dinitrogen
Complex: Redox and Chemical Activity of a Bis(imino)pyridine
Ligand. Angew. Chem. Int. Ed. 2014, 53, 14211-14215.
Notes
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Gutsulyak, D. V.; Piers, W. E.; Borau-Garcia, J.; Parvez,
The authors declare no competing financial interest.
M., Activation of Water, Ammonia, and Other Small Molecules by
PCcarbeneP Nickel Pincer Complexes. J. Am. Chem. Soc. 2013, 135,
11776-11779.
ACKNOWLEDGMENT
This work was supported as part of the Center for Molecular
Electrocatalysis, an Energy Frontier Research Center funded
by the U.S. Department of Energy, Office of Science, Office
of Basic Energy Sciences. Pacific Northwest National
Laboratory (PNNL) is operated by Battelle for the U.S.
Department of Energy. Calculations were performed using
the Cascade supercomputer at EMSL, a national scientific
user facility sponsored by the DOE’s Office of Biological
and Environmental Research and located at PNNL. We
thank Dr. Eric Wiedner, Dr. Aaron Appel, Dr. Simone
Raugei, Dr. Spencer Heins, Dr. Ba Tran, Dr. Brian Cook, and
Dr. Thilina Gunasekara (PNNL), and Prof. Jim Mayer (Yale)
for insightful discussions and helpful technical expertise.
16.
Fafard, C. M.; Adhikari, D.; Foxman, B. M.; Mindiola, D.
J.; Ozerov, O. V., Addition of Ammonia, Water, and Dihydrogen
Across a Single Pd−Pd Bond. J. Am. Chem. Soc. 2007, 129, 10318-
10319.
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Bezdek, M. J.; Chirik, P. J., Interconversion of
Molybdenum Imido and Amido Complexes by Proton-Coupled
Electron Transfer. Angew. Chem. 2018, 130, 2246-2250.
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Bezdek, M. J.; Guo, S.; Chirik, P. J., Coordination-induced
weakening of ammonia, water, and hydrazine X–H bonds in a
molybdenum complex. Science 2016, 354, 730-733.
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Scheibel, M. G.; Abbenseth, J.; Kinauer, M.; Heinemann,
F. W.; Würtele, C.; de Bruin, B.; Schneider, S., Homolytic N–H
Activation of Ammonia: Hydrogen Transfer of Parent Iridium
Ammine, Amide, Imide, and Nitride Species. Inorg. Chem. 2015, 54,
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Margulieux, G. W.; Bezdek, M. J.; Turner, Z. R.; Chirik, P.
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