of 11
Relating N–H Bond Strengths to the Overpotential for Catalytic
Nitrogen Fixation
Matthew J. Chalkley
[a]
,
Jonas C. Peters
[a]
[a]
Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E.
California Blvd., Pasadena, CA 91125, USA
Abstract
Nitrogen (N
2
) fixation to produce bio-available ammonia (NH
3
) is essential to all life but is a
challenging transformation to catalyse owing to the chemical inertness of N
2
. Transition metals
can, however, bind N
2
and activate it for functionalization. Significant opportunities remain in
developing robust and efficient transition metal catalysts for the N
2
reduction reaction (N
2
RR).
One opportunity to target in catalyst design concerns the stabilization of transition metal diazenido
species (M-NNH) that result from the first N
2
functionalization step. Well-characterized M-NNH
species remain very rare, likely a consequence of their low N–H bond dissociation free energies
(BDFEs). In this essay, we discuss the relationship between the BDFE
N–H
of a given M-NNH
species to the observed overpotential and selectivity for N
2
RR catalysis with that catalyst system.
We note that developing strategies to either increase the N–H BDFEs of M-NNH species, or to
avoid M-NNH intermediates altogether, are potential routes to improved N
2
RR efficiency.
Graphical Abstract
The reduction of N
2
to NH
3
(N
2
RR) is a globally significant reaction that is challenging due to the
inertness of N
2
. Transition metals can activate N
2
and mediate catalytic N
2
RR but challenges
remain with respect to catalytic efficiency in terms of overpotential and selectivity. We discuss the
role of the N–H bond dissociation free energy (BDFE) of metal diazenidos (M-NNH), the first
intermediates of N
2
RR, plays in determining N
2
RR efficiency.
jpeters@caltech.edu.
HHS Public Access
Author manuscript
Eur J Inorg Chem
. Author manuscript; available in PMC 2021 April 30.
Published in final edited form as:
Eur J Inorg Chem
. 2020 April 30; 2020(15-16): 1353–1357. doi:10.1002/ejic.202000232.
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Keywords
Nitrogen fixation; Catalysis; Inorganic chemistry; Proton transfer-electron transfer;
Thermochemistry
1. Introduction
Despite the abundance of nitrogen (N
2
) in the atmosphere, its bioavailability limits growth in
many environments.
[
1
]
Thus, the fixation of N
2
into its most common bioavailable form,
ammonia (NH
3
), is essential to all life and is among the most important (and fascinating)
chemical transformations on the planet. N
2
-to-NH
3
conversion (commonly called the
nitrogen reduction reaction and abbreviated as N
2
RR) is challenging owing to the chemical
inertness of N
2
with respect to proton transfer (PT), electron transfer (ET), and hydrogen
atom transfer (Figure 1).
Its high stability is due to a very strong N
N triple bond and the absence of a dipole (distinct
from isoelectronic CO).
[
2
]
N
2
can be rendered far more chemically reactive by binding to a
transition metal. The
σ
-lone pair donation from N
2
to the metal and corresponding
π
-
backdonation from the metal into the empty
π
* orbitals of N
2
can weaken the N
N bond
and induce a dipole, priming the N
2
ligand for functionalization.
[
3
]
Nonetheless, the first
functionalization step to form a metal diazenido (M-NNH) intermediate remains a
significant challenge in catalytic N
2
RR. In accord with their highly reactive nature, very few
M-NNH species have been reliably characterized.
[
4
7
]
In this short perspective, we draw
attention to the significance of the M-NNH intermediate in terms of overall catalytic N
2
RR
efficiency with respect to overpotential and likely also selectivity. We formulate a semi-
quantitative relationship between the stability of the M-NNH species, assessed via its N–H
Chalkley and Peters
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bond dissociation free energy (BDFE
N-H
), and the net overpotential required for catalytic
N
2
RR (Figure 1).
2. Bonding in M-N
2
H
y
Species
Chatt and coworkers were the first to synthesize well-defined M-N
x
H
y
coordination
complexes and to demonstrate stoichiometric ammonia generation via the protonation of N
2
complexes, with particular emphasis on bis(phosphine)-supported N
2
complexes of the
group 6 metals Mo and W (
e.g.
, (depe)
2
W(N
2
)
2
, depe = diethylphosphinoethane; Figure 2).
[
4
,
11
]
Later, other groups established that such systems could serve as precursors for
electrosynthetic (but not electrocatalytic) NH
3
formation.
[
12
,
13
]
Most recently, catalytic
N
2
RR in the presence of SmI
2
and H
2
O or ethylene glycol has been established using these
types of phosphine-supported Mo complexes.
[
14
]
Detailed vibrational spectroscopy and analysis by Tuczek and coworkers mapped the
evolution of the W–N and N–N bonding as a function of the protonation state (e.g., N
2
vs
NNH vs NNH
2
vs NNH
3
) in a family of these foundational group 6 systems. Their studies
underscore the challenge of stabilizing a M-NNH intermediate.
[
15
17
]
To summarize their
findings, the N
N bond in free N
2
is characterized by a force constant (
f
) of 22.4 mdyn•Å
−1
.
The formal reduction to isodiazene (NNH
2
) generates a double bond (
f
= 11.7) and,
ultimately, reduction to hydrazine (N
2
H
4
) yields a single bond (
f
= 4.3, Figure 2). To
compare, in (depe)
2
W(N
2
)
2
, backdonation from the zerovalent W center reduces the N–N
bond order, evinced by the reduction of
f
N–N
to 16.4 mdyn•Å
−1
and an increase in W–N
bond character (
f
W–N
= 2.9 mdyn•Å
−1
). Monoprotonation to form (depe)
2
W(F)(NNH)
significantly attenuates the N–N
π
-bonding (
f
N–N
= 8.3 mdyn•Å
−1
), with W–N bonding
increasing only slightly (
f
W–N
= 4.5 mdyn•Å
−1
). Further protonation to yield [(depe)
2
W(F)
(NNH
2
)]
+
decreases the N–N bonding only negligibly (Δ
f
N–N
= −1 mdyn•Å
−1
), which is
compensated for by a further increase in the W–N bonding (Δ
f
W–N
= +1.5 mdyn•Å
−1
).
[
15
]
Lastly, in [(depe)
2
W(F)(NNH
3
)]
2+
, its formal W–N triple bond has a
f
W–N
= 7.3 mdyn•Å
−1
,
and its formal N–N single bond has a
f
N–N
= 6.0 mdyn•Å
−1
(Figure 2).
[
17
]
If we compare the first, second, and third PT steps, the first PT causes a much more
significant reduction in the N–N bonding then the next two. Indeed, after the first step the
N–N force constant is already significantly less than that of the N=N double bond in free
isodiazene (NNH
2
). However, the increase in the W–N force constant for each PT is
qualitatively similar. Thus, there is a relatively uncompensated loss of N–N
π
-bonding
following the first PT step to generate the M-NNH, and, hence, we can expect the resultant
N–H bond in the resulting diazenido species to be homolytically weak. Indeed, whereas N–
H bonds in amines typically have BDFEs between 90 and 100 kcal•mol
−1
,
[
8
]
density
functional theory (DFT) studies on the BDFE
N–H
of several diazenido species have
consistently predicted values less than 50 kcal•mol
−1
.
[
18
21
]
This is an important value to
keep in mind, because below 50 kcal•mol
−1
N–H (and other X-H) bonds are
thermodynamically prone to bimolecular reactions that release H
2
(BDFE(H
2
) = 102.3
kcal•mol
−1
in MeCN).
[
8
]
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Thus, although a transition metal can significantly stabilize an NNH species via coordination
compared to its free state (calculated gas-phase BDE
N–H
(NNH) = −4 kcal•mol
−1
),
[
8
]
M-
NNH species remain susceptible to the hydrogen evolution reaction (HER,
vide infra
). DFT
calculations
[
19
21
]
and fundamental chemical intuition suggest that reductive protonation
steps that occur after N–N bond cleavage are comparatively easier. Therefore, in N
2
RR
reactions proceeding from terminal N
2
complexes, the most challenging functionalization
step is likely to be formation of the M-NNH intermediate. What is more, we suggest that the
necessary overpotential for catalytic N
2
RR with a particular catalyst can be roughly
estimated from the BDFE
N–H
of the catalytically relevant diazenido intermediate. To explore
this idea further, we test here the validity of this approach for two cases of N
2
RR catalysts
(Mo and Fe) that proceed through an M-NNH intermediate.
3. M-NNH Intermediates and Catalytic N
2
RR
In 2003, Schrock and coworkers published the first example of a well-defined,
organometallic N
2
RR catalyst.
[
22
]
Their triamidoamine Mo system, (HIPTN
3
N)Mo(N
2
)
([HIPTN
3
N]
3−
= [HIPTNCH
2
CH
2
)
3
N]
3−
, HIPT = 3,5-(2,4,6-
i
Pr
3
C
6
H
2
)
2
C
6
H
3
)), is capable
of forming NH
3
catalytically (63%) concomitant with H
2
formation (33%) in the presence of
excess Cp*
2
Cr (−1.45 V vs Fc
+/0
in MeCN) and [Lut-H]
+
([Lut-H]
+
= 2,6-
dimethylpyridinium, p
K
a
= 14.13 in MeCN).
[
22
,
23
]
Later, Schrock and coworkers established
that lower but still catalytic yields of NH
3
(3.6 equiv NH
3
per Mo) could be obtained using
the same acid and the weaker reductant Cp
2
Co (−1.33 V vs Fc
+/0
in MeCN, Figure 3).
[
24
]
As
articulated by Mayer and coworkers,
[
8
,
25
]
the equation (Eq 1), in which C
G
is a solvent
dependent constant (C
G
= 54.9 kcal•mol
−1
in MeCN), that is typically used to determine
BDFEs can also be used to predict an effective BDFE (BDFE
eff
) for any acid/reductant pair.
[
8
]
Such estimates are inherently limited by the lack of available data with respect to
E
°, p
K
a
,
and C
G
values in the types of non-polar solvents typically used in N
2
RR catalysis. However,
since net H• transfer reactions do not change the overall charge of an intermediate, solvent-
dependence is anticipated to be moderate and likely does not significantly affect the
estimates presented here.
BDFE
eff
= 1.37 × p
K
a
acid
+ 23.06 ×
E
°
reductant
+ C
G
(1)
By comparing the BDFE
eff
value of the Cp
2
Co/[LutH]
+
pair (43.6 kcal•mol
−1
) to the
strength of an H• derived from H
2
,
[
8
]
one can derive the excess energy used relative to the
Gibbs free energy of formation for NH
3
(ΔΔ
G
f
(NH
3
), Eq 2) in this reaction. Since the
reduction of N
2
with H
2
to form NH
3
is nearly thermoneutral (
i.e.
, Δ
G
f
(NH
3
) ~ 0 kcal•mol
−1
, Figure 1), this analysis provides a good estimate of the net overpotential needed to drive
N
2
RR.
[
18
,
19
,
26
,
27
]
For the present case, the deduced ΔΔ
G
f
(NH
3
) of 23 kcal•mol
−1
provides
an experimental lower limit for N
2
RR by the Schrock Mo catalyst; in accord with this
notion, their efforts to perform catalysis using weaker acids, such as [Et
3
NH]
+
(p
K
a
= 18.82
in MeCN),
[
23
]
proved unsuccessful.
[
22
]
ΔΔG
f
NH
3
= 3 ×
BDFE
H
2
/2 – BDFE
eff
(2)
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An estimate of the BDFE
N–H
of the parent diazenido species in this same system,
(HIPTN
3
N)Mo(NNH), can be ascertained from experimental data previously reported by
Schrock and coworkers (Figure 3).
[
5
,
18
,
23
]
In particular, [(HIPTN
3
N)Mo(N
2
)]
/
(HIPTN
3
)Mo(NNH) were shown to be in a PT equilibrium with [DBU-H]
+
/DBU (DBU =
1,8-diazaobicyclo[5.4.]undec-7-ene) in THF (p
K
a
([DBU-H]
+
) = 18.5 in THF).
[
28
,
29
]
The
[(HIPTN
3
N)Mo(N
2
)]
0/−
redox couple is −1.81 V vs Fc
+/0
in THF.
[
23
]
From these
observations, the BDFE
N–H
(Mo-NNH) can be approximated as 45 kcal•mol
−1
(C
G
in THF ~
61 kcal•mol
−1
).
[
30
]
This value accords both with computational studies
[
18
,
19
]
and its solution
instability; (HIPTN
3
N)Mo(NNH) slowly decays via formal
β
-hydride elimination to release
N
2
and form (HIPTN
3
N)Mo(H) (Figure 3).
27
One can then predict a ΔΔ
G
f
(NH
3
) for N
2
RR
catalysis by (HIPTN
3
N)Mo, using as a basis the BDFE
N–H
((HIPTN
3
N)Mo(NNH)) and
comparing this to BDFE(H
2
) (Eq 3). This analysis leads to the prediction of a minimum
ΔΔ
G
f
(NH
3
) for (HIPTN
3
N)Mo to be 20 kcal•mol
−1
, which compares quite well with the 23
kcal•mol
−1
value deduced above using the BDFE
eff
for Cp
2
Co/[LutH]
+
.
ΔΔG
f
NH
3
predicted
= 3 ×
BDFE
H
2
/2 − BDFE
N–H
M‐NNH
(3)
In 2013, our lab at Caltech reported the first example of a molecular iron catalyst for N
2
RR,
(
iPr
P
3
B
)Fe (
iPr
P
3
B
= tris(
o
-diisopropylphosphinophenyl)-borane). This was the first non-Mo
system to be identified for catalytic N
2
RR and was initially shown to be active using
HBAr
F
4
and KC
8
at low temperature in diethyl ether.
[
31
,
32
]
It was later shown that this same
catalyst was competent for N
2
RR with Cp*
2
Co (
E
° = −1.91 V vs Fc
+/0
in MeCN) and
substituted anilinium acids (Figure 4).
[
18
]
A comparison of the N
2
RR efficacy versus the
strength of the acid revealed that the weakest anilinium acid for which catalysis could be
observed (7.3 equiv NH
3
per Fe at the loading tested) was [PhNH
3
]
+
(p
K
a
= 10.62 in
MeCN).
[
33
]
Efforts to achieve catalysis with milder reductants, such as Cp*
2
Cr or Cp
2
Co,
were unsuccessful.
[
18
]
Using the above
E
° and p
K
a
values for Cp*
2
Co and [PhNH
3
]
+
,
respectively, and Eqs 1 and 2, allows one to deduce a BDFE
eff
= 25.4 kcal•mol
−1
, and hence
an experimental ΔΔ
G
f
(NH
3
) of 75 kcal•mol
−1
for this acid/reductant pair.
Efforts to synthesize (
iPr
P
3
B
)Fe(NNH) from [(
iPr
P
3
B
)Fe(N
2
)]
via monoprotonation lead
instead to the instantaneous formation of H
2
and oxidation at Fe, even at very low
temperature,
[
34
]
highlighting that the Fe-NNH species is a potential source of H
2
formation
during catalytic N
2
RR. However, the intermediacy of (
iPr
P
3
B
)Fe(NNH) in N
2
RR is
supported by the double protonation of [(
iPr
P
3
B
)Fe(N
2
)]
in the presence of excess acid to
afford the isodiazene intermediate, [(
iPr
P
3
B
)Fe(NNH
2
)]
+
.
[
33
,
34
]
Additionally, direct
spectroscopic characterization of a related diazenido species, (
Ar
P
3
B
)Fe(NNH) (
Ar
P
3
B
=
tris(
o
-di-(3,5-diisopropyl-4-methoxy)phenylphosphinophenyl)-borane), was accomplished at
cryogenic temperatures (−136 °C) with a ligand featuring bulkier phosphine substituents.
[
7
]
While the solution instability of (
R
P
3
B
)Fe(NNH) species has to date precluded the
experimental determination of a BDFE
N–H
, DFT calculations predict a BDFE
N–H
of ~31
kcal•mol
−1
. This theoretical approach has been calibrated with experimentally derived
BDFE values derived from similar iron complexes.
[
18
,
20
]
Using this BDFE
N–H
in Eq 3 leads
to a predicted minimum ΔΔ
G
f
(NH
3
) of 60 kcal•mol
−1
for the (
iPr
P
3
B
)Fe-catalyst system.
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4. Conclusion
Note that in both of the case studies presented here, the experimentally observed minimum
ΔΔ
G
f
(N
2
RR) is higher than that estimated from the BDFE
N–H
of the M-NNH species. This
“extra” driving force might be necessary because steps that are not related to N
2
functionalization, such as NH
3
/N
2
exchange, are instead thermodynamically limiting.
Alternatively, as most N
2
RR catalysts are also HER catalysts,
[
23
,
32
,
33
,
36
]
this excess driving
force may be necessary to form (and consume) reactive intermediates at a sufficient rate to
prevent their decay to H
2
.
[
20
,
21
]
For our iron catalyst system, it may remain to be discovered
that a modestly weaker acid (or reductant) is compatible with N
2
RR, such that an
overpotential closer to the predicted ΔΔ
G
f
(NH
3
) of 60 kcal•mol
−1
can be achieved. Our view
is that strategies to improve not only the thermodynamic stability but also the kinetic
stability of the key M-NNH intermediate will offer a means of achieving N
2
RR catalysis at
lower overpotentials and with higher selectivities. These efforts should be complemented by
further development of N
2
RR catalysts that can circumvent such an M-NNH intermediate,
such as those recently proposed for Mo by Nishibayashi and coworkers
[
27
,
37
]
and for Ti by
Liddle and coworkers.
[
38
]
Given the global importance of nitrogen fixation, and the rich
catalytic landscapes that are at play in the growing number of synthetic catalyst systems,
there are exciting opportunities to improve mechanistic understanding and develop more
robust and efficient catalyst systems.
[
39
]
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgements
This research was supported by the National Institutes of Health (GM-070757). M.J.C. thanks the Resnick
Sustainability Institute at Caltech for a graduate research fellowship.
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[36]. Kuriyama S, Arashiba K, Nakajima K, Tanaka H, Kamaru N, Yoshizawa K, Nishibayashi Y, J.
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[37]. Arashiba K, Eizawa A, Tanaka H, Nakajima K, Yoshizawa K, Nishibayashi Y, Bull. Chem. Soc.
Jpn 2017, 90, 1111–1118.
[38]. Doyle LR, Wooles AJ, Jenkins LC, Tuna F, McInnes EJL, Liddle ST, Angew. Chem. Int. Ed
2018, 57, 6314–6318.
[39]. Chen JG, Crooks RM, Seefeldt LC, Bren KL, Bullock RM, Darensbourg MY, Holland PL,
Hoffman B, Janik MJ, Jones AK, et al., Science 2018, 360, 873–879.
Chalkley and Peters
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Figure 1:
(A) Properties of free N
2
highlighting its recalcitrance to initial functionalization (e
, H
+
,
and H• affinity), which are similar to those of the noble gases, and its strong N
N triple
bond (BDE = bond dissociation enthalpy).
[
2
,
8
,
9
]
(B) N
2
terminally binds transition metals via
σ
-donation of its lone and acceptance of
π
-backdonation from the metal center. Binding
induces a dipole on N
2
and weakens the N
N bond, priming it for conversion to NNH.
Subsequent reductive protonation steps lead to the formation of NH
3
. (C) Comparison of the
standard (1 M or 1 atm, 298 K, 0 V vs NHE) free energy for the formation of NH
3
from both
H
2
G
°
f
) and H
+
/e
G
°(N
2
RR)).
[
10
]
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Figure 2:
(left) Force constants for the W–N and N–N normal modes in a PT series derived from
(depe)
2
W(N
2
)
2
([W]
(depe)
2
W). (right) Force constants for the N–N bond in free N
2
,
NNH
2
, and N
2
H
4
provided as standards for N–N triple, double, and single bonds
respectively.
[
15
,
17
]
Chalkley and Peters
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Figure 3:
(A) Lowest established overpotential catalytic conditions, using [LutH]
+
and Cp
2
Co as the
acid/reductant pair, for N
2
RR by a (HIPTN
3
N)Mo species ([Mo]).
[
24
]
(B) Hess’s law
scheme showing the thermodynamic equivalence between p
K
a
and
E
° for the separated acid/
reductant pair and the BDFE
eff
.
[
8
,
25
]
(C) Protonation equilibrium between
[(HIPTN
3
N)Mo(N
2
)]
/(HIPTN
3
N)Mo(NNH) and [DBU-H]
+
/DBU in THF, which allows the
p
K
a
to be estimated.
[
5
]
(D) The redox couple for [(HIPTN
3
N)Mo(N
2
)]
0/−
in THF.
[
23
]
(E)
Decay pathway
[
5
]
and estimated BDFE of (HIPTN
3
N)Mo(NNH).
Chalkley and Peters
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Figure 4:
(A) Lowest established overpotential conditions, using [PhNH
3
]
+
and Cp*
2
Co as the acid/
reducant pair, for N
2
RR catalysis observed with (
iPr
P
3
B
)Fe.
[
18
,
33
]
(B) Double protonation of
[(
iPr
P
3
B
)Fe(N
2
)]
leads to formation of the cationic isodiazene complex.
[
34
,
35
]
Efforts to
form the Fe-NNH diazenido via single protonation lead to formal H• loss (observed as 0.5
equiv H
2
), consistent with its low BDFE
N–H
.
[
18
,
20
,
34
]
Reduction of the resultant
(
iPr
P
3
B
)Fe(N
2
) closes a cycle for HER. (C) Bulkier (
Ar
P
3
B
)Fe platform allows for kinetic
stabilization (at −136 °C) of an Fe-NNH diazenido species, as elucidated by pulse electron
paramagnetic resonance (EPR) spectroscopy.
[
7
]
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