Catalytic N
2
-to-NH
3
(or -N
2
H
4
) Conversion by Well-Defined
Molecular Coordination Complexes
Matthew J. Chalkley
†
,
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Marcus W. Drover
†
,
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Jonas C. Peters
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Abstract
Nitrogen fixation, the six-electron/six-proton reduction of N
2
, to give NH
3
, is one of the most
challenging and important chemical transformations. Notwithstanding the barriers associated with
this reaction, significant progress has been made in developing molecular complexes that reduce
N
2
into its bioavailable form, NH
3
. This progress is driven by the dual aims of better
understanding biological nitrogenases and improving upon industrial nitrogen fixation. In this
review, we highlight both mechanistic understanding of nitrogen fixation that has been developed,
as well as advances in yields, efficiencies, and rates that make molecular alternatives to nitrogen
fixation increasingly appealing. We begin with a historical discussion of N
2
functionalization
chemistry that traverses a timeline of events leading up to the discovery of the first
bona fide
molecular catalyst system and follow with a comprehensive overview of d-block compounds that
have been targeted as catalysts up to and including 2019. We end with a summary of lessons
learned from this significant research effort and last offer a discussion of key remaining challenges
in the field.
Graphical Abstract
Corresponding Author: Jonas C. Peters —
jpeters@caltech.edu.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.chemrev.9b00638
†
Author Contributions:
M.J.C. and M.W.D. contributed equally.
The authors declare no competing financial interest.
HHS Public Access
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Chem Rev
. Author manuscript; available in PMC 2021 June 24.
Published in final edited form as:
Chem Rev
. 2020 June 24; 120(12): 5582–5636. doi:10.1021/acs.chemrev.9b00638.
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1. INTRODUCTION TO THE FIELD OF N
2
REDUCTION CATALYSIS
Catalytic nitrogen fixation is an essential chemical transformation in both biology and
industry as it represents the primary means by which nitrogen (N
2
) from the air becomes
bioavailable. This review focuses on the development and study of synthetic molecular
catalysts that mediate the catalytic conversion of nitrogen to ammonia (N
2
-to-NH
3
, often
abbreviated as the nitrogen reduction reaction or N
2
RR) in the presence of acid and
reductant under moderate temperatures and pressures.
1.1 Motivation for New Ammonia Synthesis Catalyst Technologies
Conventional ammonia synthesis (i.e., Haber-Bosch) is among the most significant
technological advances of the 20th century and has been critical to sustained global
population growth.
1
However, with operating pressures of 150–250 bar and temperatures of
400–500 °C, it has high cost demands for infrastructure leading to centralization of the
manufacturing process and thus requires a global distribution system.
2
This large-scale
distribution and the necessary temperature and pressure to form NH
3
from N
2
and H
2
over a
solid-state Fe catalyst necessitates significant fossil fuel input with related high carbon
dioxide (CO
2
) emissions. While estimates vary, approximately 1–2% of annual global
energy consumption is accounted for by conventional ammonia synthesis, with some 4% of
global methane (CH
4
) and 60% of global hydrogen going into its production. The generation
of the needed hydrogen via steam-reforming (providing for ~72% global ammonia
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production) accounts for nearly 0.5 gigatons of CO
2
released annually.
3
In addition, other
environmental consequences from fertilizer use are severe, including surface and
groundwater pollution from runoff, eutrophication of freshwater systems, and massive
killing of aquatic organisms in coastal regions that comprise so-called dead zones due to
depleted oxygen.
4
,
5
These consequences could be mitigated if ammonia synthesis were
electrified,
6
and hence produced on scale, and on demand, in a distributed fashion. On
demand distributed production of fertilizer could offset use (and hence production) of
fertilizer that is sourced conventionally and could also be generated locally and at a rate that
increases net absorption by crops (versus runoff), offering a possible environmental benefit
to conventional practices in fertilizer acquisition and use.
7
Although ammonia is a commodity chemical produced primarily for fertilizer (on a massive
scale, ~150 million metric tons annually), it has also been identified as a promising
alternative fuel. It is highly storable, easily liquefied, and has an energy density approaching
half that of gasoline far exceeding that of compressed hydrogen. It can also be used as a fuel
within an internal combustion engine (ICE) or via a solid-oxide fuel cell (SOFC). Moreover,
globally substantial ammonia transport infrastructure and related safe-handling protocols
already exist.
8
For these reasons, ammonia synthesis is an extremely attractive target for
electrification, especially via renewable energy technologies, requiring major advances in
catalyst development. N
2
RR electrification would enable surplus energy in the grid, at times
of excess supply, to be converted to fertilizer and/or to a storable and transportable fuel,
particularly desirable in areas where wind and solar resources are vast. The eventual
realization of an “ammonia fuel economy” that can contribute to diverse future energy
strategies, along with technologies for on-site and on-demand ammonia fertilizer generation,
will require breakthrough research discoveries in catalysis.
9
1.2. Inspiration for Organometallic and Inorganic Chemistry
In contrast to the forcing conditions required in the Haber–Bosch process, certain
microorganisms can fix N
2
under ambient conditions, using extensive hydrolysis of
adenosine triphosphate (ATP) to power the delivery of H
+
and e
−
equivalents to N
2
. These
enzymes may hold important clues as to how H
+
/e
−
currency, potentially derived from
photosynthetic water splitting, could be efficiently delivered to N
2
via an appropriate
catalyst. Housed within any given nitrogen-fixing organism are conserved sets of proteins—
the nitrogenase enzymes—that bind and convert N
2
-to-NH
3
. Nitrogenases appear to require
iron as an essential transition metal and typically contain molybdenum (FeMo-nitrogenase,
most common form), with either vanadium (VFe-nitrogenase) or Fe (FeFe-nitrogenase)
being assembled (and functionally active) in the absence of Mo.
10
,
11
FeMo-nitrogenase was
the first to be discovered and has been by far the most widely studied.
12
,
13
In addition to
various exogenous cofactors required for its function, this enzyme consists of an Fe-protein
that delivers reducing equivalents and a MoFe-protein.
10
The latter contains two structurally
unique clusters. The first is the P-cluster (Fe
8
S
7
), which serves as an electron relay to the Fe-
protein. The second is the M-cluster (MoFe
7
S
9
C-homocitrate), an inorganic FeMo cofactor
(FeMoco, Figure 1) that mediates the catalytic bond-breaking and making steps.
14
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Inorganic chemists have long puzzled (and engaged in spirited debate!) over how this
cofactor operates
15
–
19
and, in collaboration with biochemists, microbiologists, structural
biologists, and spectroscopists, have pursued molecular model systems as a means of
constraining hypotheses regarding viable inorganic mechanisms for catalytic N
2
RR. This
has proven to be a remarkably rich research area. The list of talented chemists who have
significantly contributed to what is now an enormous body of knowledge reads like a
“Who’s Who of Inorganic Chemistry.” Fortunately, much of this work has been reviewed
previously.
15
,
20
–
29
The goal of this review is to be comprehensive only with respect to the comparatively recent
body of literature pertaining to catalytic N
2
RR that is mediated by nominally well-defined
synthetic complexes, in the presence of H
+
/e
−
sources. We acknowledge at the outset that
much of the fascinating literature preceding such catalyst discoveries will not be detailed,
except in cases where introducing background is needed to set the stage for the catalyst
discoveries that will be covered. One such case is the early and pioneering work of Joseph
Chatt and his co-workers.
15
,
30
Given how central the research and ideas he and his team
espoused were to the development of the broader field of N
2
RR catalysis, we felt it
appropriate to briefly summarize some of this historical context here. The interested reader
should consult a number of excellent reviews for a deeper dive into this and other early
literature.
15
,
31
–
34
2. DISCUSSION OF THE CHATT CYCLE
Coordination chemists began to think seriously about alternative catalyst technologies to the
Haber–Bosch ammonia synthesis in the early 1960s (Figure 2). Several factors had set the
stage; key among these was that, in 1963, the British Agricultural Council, led by Secretary
Sir Gordon Cox, himself a coordination chemist by training, appointed the British inorganic
chemist Joseph Chatt to oversee a multidisciplinary research unit dedicated toward
understanding the mechanism of biological nitrogen fixation.
35
It was by this time presumed
that Fe and Mo were present within the active site of the single nitrogenase that was then
known.
36
Refreshingly, the Secretary of Agriculture must have intuited that, at its heart, the
mechanism of nitrogen fixation was an inorganic chemistry problem. This took imagination
and foresight, as it was not until two years later (1965) that Allen and Senoff reported their
landmark (and fortuitous) discovery that N
2
could coordinate as a ligand to a transition
metal, via the isolation and characterization of [(NH
3
)
5
Ru
(N
2
)]
2+
(Figure 2).
37
,
38
Hence,
from the outset, the systematic study of inorganic and organometallic complexes for N
2
RR
was established as an area of bioinorganic model chemistry.
Fundamental studies toward understanding the binding, activation, and conversion of N
2
to
protonated intermediates and/or products at well-defined transition metal centers were
deemed essential to helping formulate and constrain hypotheses concerning the biological
process. The Unit of Nitrogen Fixation, first located at the University of London but soon
thereafter relocated to Sussex, was highly innovative in its approach and comprised not just
chemists but also microbiologists, biochemists, and geneticists, working collectively.
39
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Prior to his appointment leading the Unit of Nitrogen Fixation, Chatt (with Duncanson),
contemporaneously with Dewar,
40
played a key role in defining and generalizing the
bonding of olefins to transition metals, describing the interaction by both sigma donation
(from the olefin
π
-electrons to the metal) and
π
-bonding (from the metal to the olefin
π
-
antibonding orbital).
41
This type of bonding is today often referred to as the Dewar–Chatt–
Duncanson model.
42
This proposal paralleled what had already been developed for metal
carbonyls
43
and anticipated the type of bonding interactions in metal dinitrogen complexes.
Following Allen and Senoff’s isolation of a terminally bound
Ru
(N
2
) complex,
38
it was not
much of a stretch to postulate that synthetic metal complexes might be able to bind and
catalyze N
2
RR under suitable conditions. A surge of relevant research activity thus followed,
not just in the UK but around the world, and within just a decade profound progress was
made.
15
,
33
,
34
For instance, Shilov and his collaborators in the former Soviet Union reported
the exciting discovery that certain transition metal mixtures containing, for example,
molybdenum precursors mixed with Mg(OH)
2
, could in fact catalyze N
2
reduction to N
2
H
4
and NH
3
in alcohol/water mixtures in the presence of reducing agents such as sodium
amalgam.
20
,
44
–
47
But it was the work of Chatt and his team at Sussex, via their careful,
rigorous preparation and study of
M
(N
2
) and
M
(N
x
H
y
) complexes, that most directly laid
the groundwork for the well-defined synthetic N
2
RR catalysts that have emerged thus far.
15
,
32
2.1. The Chatt Cycle As It Is Commonly Known
While M(N
2
) complexes for a range of TMs (e.g., Ru, Re, Os, Co) were known by the end
of the 1960s, much of the early biomimetic work focused on
Mo
(N
2
) (and related
W
(N
2
))
complexes.
32
–
34
,
57
Since the work of Bortels in 1930, it was long-held dogma that
molybdenum was essential to nitrogen fixation
48
and this fact, combined with the practical
reality that so many
M
(N
x
H
y
) complexes featuring Mo (and W) proved accessible, helped
focus model chemistry research in this area. Relatedly, biochemical experiments had
suggested that nitrogenase activity was highly sensitive to alteration at or near the
molybdenum site, such as at the homocitrate ligand.
10
Chatt’s team established N
2
binding
and activation at Mo (and W), and showed that the coordinated N
2
ligand could be
protonated to release NH
3
(and hydrazine) in variable yield (as high as 90% for
W
(PMe
2
Ph)
4
(N
2
)
2
with H
2
SO
4
, assuming one N
2
equiv is released as gas)
58
depending on a
range of factors (Figure 3). What is more, the group was able to identify a number of
M
(N
x
H
y
) complexes, including
M
(N),
59
M
(NNH
2
),
60
–
64
M
(NHNH
2
),
65
M
(NNH),
60
,
66
,
67
M
(NNH
3
)
68
,
69
species (
M
= Mo or W), that likewise underwent protonation to release NH
3
in similar yields.
These findings led Chatt to propose a simple scenario in which triple functionalization at N
β
leads to the release of NH
3
and a nitride intermediate. This scenario is now known as the
Chatt cycle, and a simplified version is depicted below (Figure 4).
30
Worth noting is that
while examples of the types of
M
(N
x
H
y
) species invoked in the Chatt cycle could be
generated, most typically these species were not characterized in the same formal state of
oxidation as invoked in the catalytic scheme.
26
It would not be until Schrock’s discoveries
some 30 years later
50
,
70
that well-characterized examples of the proposed catalytic
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