Towards Catalytic Ammonia Oxidation to Dinitrogen: A Synthetic Cycle by Using a Simple Manganese Complex

Article abstract of DOI:10.1002/chem.201703153

Oxidation of the nucleophilic nitride, (salen)Mn≡N (1) with stoichiometric [Ar₃N][X] initiated a nitride coupling reaction to N₂, a major step toward catalytic ammonia oxidation (salen=N,N'-bis(salicylidene)-ethylenediamine dianion; Ar=p-bromophenyl; X=[SbCl₆]^? or [B(C₆F₅)₄]^?). N₂ production was confirmed by mass spectral analysis of the isotopomer, 1-¹⁵N, and the gas quantified. The metal products of oxidation were the reduced Mn^III dimers, [(salen)MnCl]₂ (2) or [(salen)Mn(OEt₂)]₂[B(C₆F₅)₄]₂ (3) for X=[SbCl₆]^? or [B(C₆F₅)₄]^?, respectively. The mechanism of nitride coupling was probed to distinguish a nitridyl from a nucleophilic/electrophilic coupling sequence. During these studies, a rare mixed-valent Mn^V/Mn^III bridging nitride, [(salen)Mn^V(μ-N)Mn^III(salen)][B(C₆F₅)₄] (4), was isolated, and its oxidation-state assignment was confirmed by X-ray diffraction (XRD) studies, perpendicular and parallel-mode EPR and UV/Vis/NIR spectroscopies, as well as superconducting quantum interference device (SQUID) magnetometry. We found that 4 could subsequently be oxidized to 3. Furthermore, in view of generating a catalytic system, 2 can be re-oxidized to 1 in the presence of NH₃ and NaOCl closing a pseudo-catalytic “synthetic” cycle. Together, the reduction of 1→2 followed by oxidation of 2→1 yield a genuine synthetic cycle for NH₃ oxidation, paving the way to the development of a fully catalytic system by using abundant metal catalysis.

Full text of DOI:10.1002/chem.201703153

A Journal of
Accepted Article
Title: Towards Catalytic Ammonia Oxidation to Dinitrogen: A Synthetic
Cycle Using a Simple Manganese Complex
Authors: Megan Keener, Madeline Peterson, Raúl Hernández
Sánchez, Victoria F. Oswald, Guang Wu, and Gabriel Menard
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To be cited as: Chem. Eur. J. 10.1002/chem.201703153
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Towards Catalytic Ammonia Oxidation to Dinitrogen: A Synthetic
Cycle Using a Simple Manganese Complex
Megan Keener, Madeline Peterson, Raúl Hernández Sánchez, Victoria F. Oswald, Guang Wu, and
Gabriel Ménard*
Abstract: Oxidation of the nucleophilic nitride, (salen)Mn≡N (1) with
stoichiometric [Ar3N][X] initiates a nitride coupling reaction to N2, a
major step toward catalytic ammonia oxidation (salen = N,N′-
bis(salicylidene)-ethylenediamine dianion; Ar = p-bromophenyl; X =
[SbCl6]- or [B(C6F5)4]-). N2 production was confirmed by mass
spectral analysis of the isotopomer, 1-15N, and the gas quantified.
The metal products of oxidation were the reduced MnIII dimers,
[(salen)MnCl]2 (2) or [(salen)Mn(OEt2)]2[B(C6F5)4]2 (3) for X = [SbCl6]-
or [B(C6F5)4]-, respectively. The mechanism of nitride coupling was
probed to distinguish a nitridyl from a nucleophilic/electrophilic
coupling sequence. During these studies, a rare mixed-valent
MnV/MnIII bridging nitride, [(salen)MnV(-N)MnIII(salen)][B(C6F5)4] (4),
was isolated and its oxidation state assignment confirmed by X-Ray
diffraction (XRD) studies, perpendicular and parallel-mode EPR and
UV-Vis-NIR spectroscopies, as well as SQUID magnetometry. We
found that 4 could subsequently be oxidized to 3. Furthermore, in
view of generating a catalytic system, 2 can be re-oxidized to 1 in
120 Mt/year), NH3 is ideal for storing H2.[2] Furthermore, the
energy loss of the HB process (1.5 GJ/t) is also easily countered
by that stored in NH3 (28.4 GJ/t).[2]
Splitting NH3 to N2/H2 under mild conditions is of particular
2b]
interest in catalysis science today (eq. 1).[2a,
While
electrochemical and heterogeneous systems have shown
promise,[3] homogeneous systems may provide a molecular-level
probe to understanding the mechanism of the 6 e- oxidation of
NH3 (eq. 2), key to overall splitting. While this strategy has been
extensively used for N2 reduction chemistry,[4] NH3 oxidation
using well-defined metal complexes remains far less explored.
Metal complexes of Mn,[5] Co,[6] Mo,[7] Ru,[8] and Os[9] among
others,[10] have been used to oxidize NH3 chemically or
electrochemically to metal-nitride, dimetallic -N2, or N2 ( A→B,
A→C or A→D, respectively, Scheme 1, top). For the latter N–N
bond formation pathways, intermediate metal-nitride coupling
(B→C) – either through proposed bimolecular nitridyl[11] (M≡N•)
the presence of NH3 and NaOCl closing
a pseudo-catalytic
“synthetic” cycle. Together, the reduction of 1→2 followed by
oxidation of 2→1 yield a genuine synthetic cycle for NH3 oxidation,
paving the way to the development of a fully catalytic system using
abundant metal catalysis.
or nucleophilic (M≡N)
+
electrophilic (M≡N) coupling[12]
(Scheme 1, bottom) – is proposed as the main pathway to -N2
or N2 formation. Other nitrides, generated by N-atom transfer
-
chemistry (ex. N3 photolysis), have also shown similar coupling
Renewable energy production is rapidly growing; however,
with it will come an increased demand for new energy storage
technologies. Up to 100 times more energy can be stored in
chemical bonds than today’s batteries making them an ideal
platform for storage and delivery (50-140 MJ/kg vs. ~ 0.5-1
MJ/kg).[1] Hydrogen has a high gravimetric energy density (143
MJ/kg); however, handling and storing it remains a high barrier
to implementing it as our primary energy currency. This problem
can be overcome by the Haber-Bosch (HB) process to produce
NH3, an efficient H2 storage reaction. With its high capacity
(17.6 % wt. H2), low cost (~ $1.5/kg H2), ease of handling (-33 °C
b.p.), and established production/distribution infrastructures (>
behavior.[11a, 11b, 12-13] While significant progress has been made
towards NH3 oxidation to various products (B, C, D), closing the
cycle in a fully catalytic or pseudo-catalytic “synthetic” cycle,[14] is
rare.[5c] Herein, we report a complete synthetic cycle for NH3
oxidation at an abundant first-row metal complex of Mn.
Preliminary mechanistic investigations into the coupling pathway
(Scheme 1, bottom) are also reported.
[a]
M. Keener, M. Peterson, Dr. G. Wu, Prof. G. Ménard
Department of Chemistry and Biochemistry
University of California, Santa Barbara
Santa Barbara, California, 93106, USA
E-mail: menard@chem.ucsb.edu
Dr. R. Hernández Sánchez
Department of Chemistry
Columbia University
New York, New York, 10027, USA
V. F. Oswald
Scheme 1. Top: reported conversions of NH3 to N2. Bottom: proposed
mechanisms for nitride coupling to N2.
[b]
[c]
The known nucleophilic MnV-nitride, (salen)Mn≡N (1),
generated through NH3 oxidation with NaOCl,[5b] represents the
first step of a potential synthetic cycle (A→B, Scheme 1, top and
Fig. 1). We were interested in determining if chemical oxidation
of 1 to 1+ would induce nitride radical character (nitridyl) or
render it electrophilic and susceptible to nucleophilic attack by
Department of Chemistry
University of California, Irvine
1102 Natural Science II, Irvine, California, 92697, USA
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201703153
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an equivalent of 1 (Scheme 1, bottom).[11a, 12] Electrochemical
analysis of 1 using cyclic voltammetry in DCM revealed a quasi-
reversible oxidation event at 0.53 V relative to the Fc/Fc+ (Fc =
ferrocene) couple (Fig. 1 (right)), accessible using the known
14N2) (Fig. S2a). Similar to other reports,[5c, 11a] we were unable to
completely eliminate 14N2 from the background. Treatment of a
1:1 mixture of 1:1-15N to an equivalent of oxidant also resulted in
the appearance of the mixed isotopomer 14N≡15N, in addition to
15N2, in an approximate 2:1 ratio (19 % and 9.1 %, respectively),
confirming the expected bimolecular coupling sequence (Fig.
S2b). Similar results were obtained for the 1→2 conversion (Fig.
S3). A related paper by Lau and co-workers described a similar
reaction involving N2 production (and in some cases NH3) from
commercially available oxidant, [Ar3N][SbCl6] (Ar
=
p-
bromophenyl; Eo = 0.70 V vs. Fc/Fc+).[15] Oxidation of 1 with
[Ar3N][SbCl6] in fluorobenzene (FB) resulted in loss of the
diamagnetic signals of 1 and concomitant appearance of the
aromatic NAr3 signals in the 1H NMR spectrum. A major
paramagnetic product was subsequently isolated and
crystallized by vapor diffusion of MeCN/Et2O to yield single
crystals suitable for XRD studies. The solid state structure
revealed loss of the nitride with concurrent chloride capture,
presumably from [SbCl6]-, to yield the known[16] MnIII dimer
[(salen)MnCl]2 (2) in 82% yield. Similarly, treatment of 1 to 1 eq.
of the non-coordinating oxidant, [Ar3N][B(C6F5)4],[17] in FB again
resulted in replacement of the diamagnetic peaks of 1 for a set
MnV-imido
(Mn=NR)
intermediates
generated
through
electrophilic conversion of related MnV-nitrides (similar to 1).[22]
In addition, during the preparation of this manuscript, Clarke and
Storr reported the analogous nitride coupling with related bulkier
salen-type Mn-nitride species, observing in most cases nitride
coupling to N2.[5c] In both ours and Storr’s cases, oxidation of the
Mn-nitride precursors is key to initiating nitride coupling to N2, as
opposed to imide coupling proposed by Lau.[22]
of
3
paramagnetically shifted resonances at -8.7, -28.2,
Next, we were interested in probing the coupling pathway
at play (nitridyl vs. nucleophilic/electrophilic – Scheme 1, bottom).
With the pseudo-square pyramidal geometry and  orbital
overlap between Mn and the nitride in 1, oxidation of 1 should
and -32.2 ppm in the 1H NMR spectrum. Separation of NAr3 from
the reaction mixture resulted in the isolation of the paramagnetic
product in 80% yield which was identified by single crystal XRD
studies[18]
[(salen)Mn(OEt2)]2[B(C6F5)4]2 (3; Fig. S1), similar to previously
reported Evans’ method
(salen)MnIII complexes.[19]
measurements of the spin state of 3 in MeCN-d3 revealed an S =
2 spin state assuming a monomeric species in solution.[20]
as
the
Et2O-ligated
MnIII
dimer,
remove an electron from the non-bonding dxy orbital (Scheme 2)
23]
as seen in similar complexes.[5c,
Given the locus of orbital
oxidation (dxy) bears no  or * contributions, we wanted to
probe if the nitride in 1+ would have electrophilic or radical
character.
It
is
worth
noting
that
should
the
nucleophilic/electrophilic mechanism be at play, only half an
equivalent of oxidant – oxidizing 0.5 eq. of 1 to 1+ for subsequent
reaction with 0.5 eq. of nucleophilic 1 – would be required for full
conversion of all nitrides to N2 (Scheme 1, bottom). To test this,
we treated 1 to 0.5 eq. of [Ar3N][B(C6F5)4] in FB. Separation of
NAr3 from the reaction mixture resulted in the isolation of a red,
MeCN-soluble, 1H NMR-silent fraction. The complex was
crystallized by vapor diffusion of a FB:DCM mixture in 40 % yield
and identified by XRD studies as the bridged species,
[(salen)MnV(-N)MnIII(salen)][B(C6F5)4] (4) (Fig. 2). The mixed-
valency was assigned based on crystallographic, magnetometric,
and spectroscopic analyses.
Figure 1. Left: oxidation of 1 with [Ar3N][X] to yield 2 (X = Cl) or 3 (X =
B(C6F5)4) (monomeric forms are shown for simplicity). Right: cyclic
voltammogram of 1 (1.0 mM) with 0.1 M [Bu4N][PF6] electrolyte in DCM
(glassy C working electrode, scan rate 100 mV/s, referenced to Fc/Fc+).
The oxidation-initiated reduction of 1→2 or 1→3 resulted in
loss of nitride and subsequent 3 e- metal reduction via an
intermediate 1+ to MnIII (2 or 3). As observed in other systems,
such metal-nitride reduction is typically accompanied by N2
11b, 13d]
generation.[11a,
In order to unambiguously confirm this
here, we isotopically enriched the nitride in 1 to 1-15N using
15NH3 in the initial synthesis.[5b,
21]
The Mn≡N stretching
frequency shifts from 1046 cm-1 to 1018 cm-1 in the IR spectrum
for the 14N and 15N isotopomers, respectively. The headspace
gas of an oxidation reaction (1→3) performed under Ar was
sampled and analyzed by GC-MS using a gastight syringe. The
presence of 27.5 % 15N2 relative to background 14N2, marked a
significant deviation from natural abundance (0.13 % 15N2 vs.
Scheme 2. Qualitative partial MO diagram for 1.
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interaction between redox-sites; Class II corresponds to a
partially delocalized mixed-valent state, and; Class III describes
a fully delocalized redox state.[26] The lack of an IVCT band in
the NIR spectrum of 4 between 800−3300 nm (Fig. 3b) is
consistent with a Class 1 assignment, with a localized (S = 2)
MnIII center and a diamagnetic (S = 0) MnV center (similar to the
diamagnetism observed in 1[5b]). Collectively, these data support
our mixed-valent oxidation state assignment with a localized
high-spin MnIII bridged to a diamagnetic MnV nitride center.
Figure 2. Solid-state structure of 4 with [B(C6F5)4]- and H atoms omitted for
clarity (Mn, teal; N, blue; O, red; C, black). Inset: the truncated 1-D chain of 4.
Pertinent bond lengths (Å) and angles (°): Mn1–N5 (1.550(7)), Mn2–N5
(2.419(3)), Mn2-O2 (2.276(3)), Mn1-N5-Mn2 (149.4(2)), N5-Mn2-O2
(175.20(11)).
The solid-state structure of 4 reveals a Mn1-N5 bond
length of 1.550(3) Å with a Mn center puckered out of the plane
of the salen ligand by 0.440(3) Å consistent with a slightly
elongated Mn≡N triple bond (Mn≡N = 1.512 Å and 0.451(1) Å
out of plane Mn in 1).[5b] In contrast, the Mn2 center sits mostly in
the plane of the salen ligand (out of plane by 0.071(2) Å) flanked
at the apical site by N5 to form an elongated Mn2–N5 bond of
2.419(3) Å. In addition to these bonds, the cations of 4 form an
infinite 1-D chain,[24] wherein Mn2 also connects at the other
axial site to O2 (2.276(3) Å) from the neighboring MnV-nitride
bound salen (Fig. 2 box). The long Mn2–O2 and Mn2–N5
linkages are consistent with a Jahn-Teller distorted MnIII center
in a pseudo-octahedral field.[24]
Figure 3. a) Variable temperature magnetic susceptibility measurements of 4
collected at 0.5 T (blue circles). Inset: Variable temperature, variable field
magnetization data collected from 2-10 K with fields from 1-5 T. b) UV-Vis-NIR
of 4 in MeCN (green). Inset: X-Band EPR spectra of 4 taken at 6 K in both
perpendicular (orange) and parallel-modes (purple).
The conversion of 1 to 4 with 0.5 eq. of oxidant suggests
that a nucleophilic/electrophilic coupling pathway is not at play
here (Scheme 1, bottom). Such a coupling would not be
expected to produce 4 and instead would involve 1+ reacting
with 1, resulting in complete nitride conversion to N2, as well as
generation of (salen)MnII and 3 (Scheme 3, right). Low valent
MnII production was observed in Lau’s report of MnV-imido
coupling to generate N2 (vide supra).[22] To further probe this, a
reaction mixture of 1 and 0.5 eq. of [Ar3N][B(C6F5)4] in MeCN
was analyzed by perpendicular-mode EPR spectroscopy and
compared to an authentic sample of the known (salen)MnII
starting material. Whereas the latter displayed a broad isotropic
feature at g = 2.1 consistent with literature reports,[27] the former
was featureless precluding the formation of any MnII by-products
(Fig. S4). These experimental results support Storr’s DFT
calculations suggesting that significant nitridyl character in 1+
(and similar species) may be expected and are likely operational
in this coupling.[5c] With this, we believe that production of 4 is
the result of a bimolecular coupling of two nitridyls (1+) to
generate 3 (Scheme 3, left) which rapidly react with the
remaining 1 in solution to generate 4. Lastly, we investigated
whether 4 could be further oxidized to generate 2 equivalents of
3. The CV of 4 displays a cathodically shifted oxidation event
(relative to 1 – Fig. 1) at +0.43 V (Fig. S5), which is again
accessible using the [Ar3N][B(C6F5)4] oxidant. Addition of 1.0 eq.
of oxidant to a solution of 4 in MeCN-d3 results in the immediate
formation of the paramagnetically shifted resonances of 3 in the
1H NMR spectrum, confirming that 4 is only an accessible
intermediate in the conversion of 1→3.
To gain further insight into the electronic structure of 4, we
collected variable temperature magnetic susceptibility (

MT) and
low temperature magnetization data (Fig. 3a). The MT data

collected at 0.5 T remains almost constant going from 3.16 to
3.31 cm3K/mol when scanned from 30 to 300 K, respectively.
This is in agreement with the spin-only value of an ideal S = 2
(3.0 cm3K/mol) system. Similarly, magnetization data at low
temperature indicates the effect of zero-field splitting since data
at 2 K and 5 T saturates at 3.12 B, which is lower than the ideal
value of M = gS of 4 B for an S = 2 (Fig. 3a, inset). Both MT

and reduced magnetization data were fit well to an S = 2 (red
and black continuous lines in Fig. 3a), as described in the SI.
The 

MT fit parameters that best describe the data are: g = 2.04
and TIP
=
5.1
x
10–4 cm3/mol; while for the reduced
magnetization data we obtained: g = 2.42 and D = –8.5 cm–1,
where D corresponds to the axial zero-field splitting parameter.
The S = 2 assignment for 4 is further supported by X-band
EPR measurements. We analyzed 4 by perpendicular- and
parallel-mode EPR spectroscopy in MeCN at 6 K (Fig. 3b, inset).
The perpendicular-mode spectrum was featureless as expected
for an integer spin system, whereas the parallel-mode spectrum
displayed a characteristic signal at g = 7.84, indicative of a high-
spin (S = 2) MnIII center.[25] Lastly, the extent of redox
localization was probed by UV-Vis-NIR (NIR
= near-IR)
spectroscopy (Fig. 3b). Intervalence charge transfer (IVCT)
bands in the NIR appear due to inner-sphere electron charge
transfer (thermally or photoinduced) between mixed-valent
species. Three categories are used to describe the degree of
communication: Class I equates to localized redox states with no
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Scheme 3. Proposed coupling pathways (salen and anions omitted for
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As part of our endeavor to generate a catalytic system for
NH3 oxidation to N2, we turned to our initial results for the
conversion of 1→2 (Fig. 1, left). While the oxidants, [Ar3N][X] (X
= SbCl6, B(C6F5)4), are incompatible with NH3, 2 may be re-
oxidized to 1 using NH3 and NaOCl following a literature
procedure,[28] effectively closing the synthetic cycle, D→B
(Scheme 1, top; Scheme 4). We performed several large scale
oxidations of 1 using [Ar3N][SbCl6], quantified the gas evolved,
and found the reactions to produce between 70-84% N2 within
10 minutes of mixing. Furthermore, removal of the solvent,
followed by treatment with NH3/NaOCl in a DCM:MeOH solvent
mixture, followed by purification,[28] resulted in the isolation of
pure 1 in varying yields from 50-63 %. This represents a closed
synthetic cycle for NH3 oxidation (Scheme 4) and is to our
knowledge the first example using an abundant metal.
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Scheme 4.
In summary, we have described the oxidation-initiated 6 e-
reductive nitride coupling reaction to N2 at Mn, including the
experimental elucidation of the likely nitridyl coupling pathway, in
support of previous theoretical work.[5c] Isolation of a rare mixed
valent MnIII/MnV intermediate, including full characterization of
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species can be further oxidized with NH3 and oxidant, closing
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We thank the University of California, Santa Barbara for financial
support. Dr. Jiaxiang Chu, Clayton Silva, and Ali Chamas are
thanked for experimental support. Profs. T. W. Hayton, M. M.
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Keywords: ammonia oxidation • hydrogen storage • nitride
coupling • mixed-valent manganese • synthetic cycle
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10.1002/chem.201703153
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Megan Keener, Madeline Peterson, Raúl
Hernández Sánchez, Victoria F. Oswald,
Guang Wu, Gabriel Ménard*
Page No. – Page No.
Towards
Catalytic
Ammonia
Oxidation to Dinitrogen: A Synthetic
Text for Table of Contents
Cycle Using
Complex
a Simple Manganese
Store that H2! Oxidation of a simple (salen)Mn≡N results in the 6 e- nitride coupling reaction, central to NH3 oxidation to N2. A
complete synthetic cycle is established with implications for catalytic NH3 oxidation for H2 storage applications. An unusual mixed-
valent MnV-MnIII species is also isolated and characterized.
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