of 28
A Synthetic Single-Site Fe Nitrogenase: High Turnover, Freeze-
Quench
57
Fe Mössbauer Data, and a Hydride Resting State
Trevor J. Del Castillo
,
Niklas B. Thompson
, and
Jonas C. Peters
*
Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech),
Pasadena, California 91125, United States
Abstract
The mechanisms of the few known molecular nitrogen-fixing systems, including nitrogenase
enzymes, are of much interest but are not fully understood. We recently reported that Fe-N
2
complexes of tetradentate P
3
E
ligands (E = B, C) generate catalytic yields of NH
3
under an
atmosphere of N
2
with acid and reductant at low temperatures. Here we show that these Fe
catalysts are unexpectedly robust and retain activity after multiple reloadings. Nearly an order of
magnitude improvement in yield of NH
3
for each Fe catalyst has been realized (up to 64 equiv
NH
3
produced per Fe for P
3
B
and up to 47 equiv for P
3
C
) by increasing acid/reductant loading
with highly purified acid. Cyclic voltammetry shows the apparent onset of catalysis at the P
3
B
Fe-
N
2
/P
3
B
Fe-N
2
couple and controlled-potential electrolysis of P
3
B
Fe
+
at −45 °C demonstrates that
electrolytic N
2
reduction to NH
3
is feasible. Kinetic studies reveal first-order rate dependence on
Fe catalyst concentration (P
3
B
), consistent with a single-site catalyst model. An isostructural
system (P
3
Si
) is shown to be appreciably more selective for hydrogen evolution. In situ freeze-
quench Mössbauer spectroscopy during turnover reveals an iron-borohydrido-hydride complex as
a likely resting state of the P
3
B
Fe-catalyst system. We postulate that HER activity may prevent
iron hydride formation from poisoning the P
3
B
Fe-system. This idea may be important to consider
in the design of synthetic nitrogenases and may also have broader significance given that
intermediate metal-hydrides and hydrogen evolution may play a key role in biological nitrogen
fixation.
Graphical Abstract
Corresponding Author: jpeters@caltech.edu.
Author Contributions:
T.J.D.C. and N.B.T. contributed equally to this work.
The authors declare no competing financial interests.
Supporting Information
Experimental procedures and data for individual experiments. The Supporting Information is available free of charge on the ACS
Publications website at
http://pubs.acs.org
.
HHS Public Access
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J Am Chem Soc
. Author manuscript; available in PMC 2017 April 27.
Published in final edited form as:
J Am Chem Soc
. 2016 April 27; 138(16): 5341–5350. doi:10.1021/jacs.6b01706.
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1. INTRODUCTION
The fixation of molecular nitrogen (N
2
) into ammonia (NH
3
) is a transformation of
fundamental importance to both biology and industry,
1
a fact which has prompted
mechanistic study of the few known systems capable of catalyzing this reaction. The
industrial Haber-Bosch process has been the subject of exhaustive investigation, resulting in
a detailed mechanistic understanding in large part supported by surface spectroscopic studies
on model systems.
2
The nitrogenase family of enzymes provides an example of catalytic N
2
conversion under ambient conditions and has also been studied extensively. While many
questions remain unanswered regarding the mechanism of nitrogenase, a great deal of
kinetic and reactivity information has been collected.
3
Additionally, important insights have
been provided by protein crystallography, X-ray emission spectroscopy, and site-
mutagenesis studies, as well as in situ freeze-quench ENDOR and EPR spectroscopy.
4
,
5
Hypotheses underpinning the mechanisms of both of these systems are bolstered by
synthetic model chemistry and efforts to develop molecular N
2
conversion catalysts.
6
This
search has yielded systems capable of the catalytic reduction of N
2
to hydrazine (N
2
H
4
),
7
tris(trimethylsilyl)amine,
8
and a few examples of the direct catalytic fixation of N
2
to NH
3
(Chart 1).
8g
,
9
,
10
,
11
,
12
While a wealth of mechanistic information for the original Mo catalyst
system developed by Schrock has been derived from stoichiometric studies and theory,
13
,
14
in situ spectroscopic studies during catalysis were not reported. These synthetic catalysts
operate under heterogeneous conditions and are likely to generate mixtures of intermediate
species that are both diamagnetic and paramagnetic, making it challenging to reliably
determine speciation under turnover. This latter limitation is also true of biological
nitrogenases. While CW and pulsed-EPR techniques can and have been elegantly applied,
5
such studies are inherently limited in that species/intermediates not readily observable by
these techniques will go unnoticed.
Iron is the only transition metal that is essential in the cofactor for nitrogenase function, and
this fact has motivated a great deal of recent interest in Fe-N
2
model chemistry.
15
In recent
years we have focused on a family of Sacconi-type tetradentate ligands, P
3
E
, in which three
phosphine donors are bonded to a central atom through an
ortho
-phenylene linker (E = B, Si,
C). We have shown that P
3
E
M (M = Fe, Co) complexes promote the binding and activation
of N
2
, as well as the functionalization of bound N
2
with various electrophiles.
16
,
17
,
18
Moreover, we discovered that P
3
B
Fe (P
3
B
= tris(
o
-diisopropylphosphinophenyl)borane) and
P
3
C
Fe (P
3
C
= tris(
o
-diisopropylphosphinophenyl)methyl) complexes mediate the catalytic
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reduction of N
2
to NH
3
at low temperature using a strong acid—[H(OEt
2
)
2
][BAr
F
4
]
(HBAr
F
4
)—and a strong reductant—potassium graphite (KC
8
) (Chart 1, D).
12
One unique
aspect of these Fe-based systems is their suitability for in situ spectroscopic study by freeze-
quench
57
Fe Mössbauer spectroscopy. In principle, this technique enables observation of the
total Fe speciation as frozen snapshots during turnover.
19
For single-site Fe nitrogenase
mimics of the type we have developed, analysis of such data is far simpler than in a
biological nitrogenase where many iron centers are present.
20
For the most active P
3
B
Fe catalyst system, many P
3
B
Fe-N
x
H
y
model complexes that may be
mechanistically relevant (e.g., Fe
+
, Fe-N
2
, Fe=NNH
2
+
, Fe-NH
3
+
) have now been
independently generated and characterized, including by
57
Fe Mössbauer spectroscopy, and
these data facilitate interpretation of the freeze-quench Mössbauer data reported here. In
combination with chemical quenching methods that we present to study the dynamics of
product formation, it becomes possible to attempt to correlate the species observed
spectroscopically with the N
2
fixing activity to gain a better understanding of the overall
catalytic system. Such a strategy complements the studies of model complexes and
stoichiometric reactions steps that we have previously undertaken and offers a fuller
mechanistic picture. While many questions remain, this approach to studying N
2
-to-NH
3
conversion mediated by synthetic iron catalysts is a mechanistically powerful one.
Here we undertake tandem spectroscopy/activity studies using the P
3
E
(E = B, C, Si) Fe
catalyst systems and report the following: (i) two of these Fe-based catalysts (E = B, C) are
unexpectedly robust under the reaction conditions, demonstrating comparatively high yields
of NH
3
that are nearly an order of magnitude larger than in initial reports at lower acid/
reductant loadings; (ii) based on electrochemical measurements the dominant catalysis by
the P
3
B
Fe system likely occurs at the formal P
3
B
Fe-N
2
/P
3
B
Fe-N
2
couple, corroborated by
demonstrating catalysis with Na/Hg and electrolytic N
2
-to-NH
3
conversion in a controlled-
potential bulk electrolysis; (iii) the P
3
B
Fe system shows first order rate dependence on iron
catalyst concentration and zero order dependence on acid concentration; (iv) kinetic
competition between rates of N
2
versus H
+
reduction are a key factor in determining whether
productive N
2
-to-NH
3
conversion is observed; and (v) a metal hydride-borohydride species
is a resting state of the P
3
B
Fe catalysis system.
2. RESULTS AND DISCUSSION
2.1. Increased turnover of Fe-catalyzed N
2
fixation and evidence for catalysis at the P
3
B
Fe-
N
2
/P
3
B
Fe-N
2
couple
Following our initial discovery that the addition of excess HBAr
F
4
and KC
8
to the anionic
dinitrogen complex [P
3
B
Fe(N
2
)][(12-crown-4)
2
Na] (
1
) at low temperature in Et
2
O under an
atmosphere of N
2
furnishes catalytic yields of NH
3
, we pursued the optimization of this
system for NH
3
yield (Eqn. 1).
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(1)
Under our initially reported conditions (in Et
2
O at −78 °C with 48 equiv HBAr
F
4
and 58
equiv KC
8
) the catalysis furnishes 7.0 ± 1.0 equiv of NH
3
per Fe atom, corresponding to
44% of added protons being delivered to N
2
to make NH
3
. Initial attempts at optimization
showed that neither the overall concentration of the reactants nor the mole ratio of the
catalyst substantially altered the yield of NH
3
with respect to proton equivalents.
We have since examined whether the post-reaction material retained any catalytic
competence when more substrate was delivered. We found that if, after stirring at −78 °C for
1 hour the reaction mixture was frozen (at −196 °C), delivered additional substrate, and then
thawed to −78 °C, significantly more NH
3
was formed. Iterating this reloading process
several times resulted in a steady increase in the total yield of NH
3
per Fe atom (Figure 1),
demonstrating that some active catalyst remains at −78 °C, even after numerous turnovers.
This result implies that the yield of NH
3
is limited by competitive consumption of substrate
in a hydrogen evolving reaction (HER).
The apparent stability of at least some of the catalyst at low temperature suggested that it
may be possible to observe higher turnover numbers if the catalyst is delivered more
substrate at the beginning of the reaction. Indeed, as shown in Table 1, addition of increasing
equivalents of HBAr
F
4
and KC
8
to
1
at low temperature furnished steadily increasing yields
of NH
3
relative to catalyst, with a current maximal observed yield of 64 equivalents of NH
3
per Fe atom (average of 59 ± 6 over 9 iterations, Table 1, Entries 1–5) at 1500 equiv acid
loading. This yield is nearly an order of magnitude larger than that reported at the original
acid loading of 48 equiv. We note that the yields of NH
3
under these conditions are highly
sensitive to the purity of the acid source, unsurprising given the high acid substrate loading
relative to catalyst (~1500 equiv HBAr
F
4
). To obtain reproducible yields, we have developed
a tailored protocol for the synthesis of sufficiently pure NaBAr
F
4
/HBAr
F
4
, which is detailed
in the Supporting Information. It is also important to ensure good mixing and a high gas-
liquid interfacial surface area to enable proper mass transfer in the heterogeneous reaction
mixture.
Having discovered that P
3
B
Fe-N
2
1
is a significantly more robust catalyst than originally
appreciated, we investigated the activity of the alkyl N
2
anion [P
3
C
Fe-N
2
][(Et
2
O)
0.5
K] (
2
)
toward N
2
fixation at higher substrate loading. Significantly higher yields of NH
3
per Fe are
also attainable using
2
as a catalyst, albeit with roughly 2/3 the activity of
1
(Table 1, Entries
6–10). As a point of comparison, we also submitted the silyl congener [P
3
Si
Fe-N
2
][(12-
crown-4)
2
Na] (
3
) (P
3
Si
= tris(
o
-diisopropylphosphinophenyl)silyl) to these conditions and
observed dramatically lowered yields of NH
3
, consistent with earlier reports (Table 1,
Entries 11,12). Although the P
3
Si
Fe-N
2
system
3
displays worse selectivity for NH
3
formation vs. HER than
1
(
vide infra
),
3
still demonstrates catalytic yields of NH
3
under
sufficiently high substrate loading (up to 4 equiv NH
3
per Fe, Table 1 Entry 12).
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Table 1 also contains data for catalytic trials with the borohydrido-hydrido complex (P
3
B
)(μ-
H)Fe(H)(N
2
) (
4-N
2
) as a catalyst in mixed Et
2
O/toluene solvent. In the presence of admixed
toluene
4-N
2
is observed to be partially soluble and demonstrates competence as a catalyst
(Table 1, Entry 14); in the absence of toluene
4-N
2
shows poor solubility and lower than
catalytic yields of NH
3
were observed under the originally reported catalytic conditions
(0.50 ± 0.1 equiv NH
3
per Fe).
12a
The significance of these observations is discussed below
(section 2.4).
The efficiency of NH
3
production with respect to acid substrate decreases under increasingly
high turnover conditions for these iron systems. Our understanding of the HER kinetics
(
vide infra
) rationalizes this phenomenon in that under comparatively low catalyst loading
(which engenders higher turnover) the background HER should be increasingly competitive,
thereby reducing N
2
-fixing efficiency. The product of the reaction (NH
3
) may also act as an
inhibitor of catalysis. To test this latter possibility, catalytic runs with 150 equiv HBAr
F
4
and
185 equiv KC
8
in the presence of
1
were conducted with the inclusion of 25 equiv of NH
3
(Table 1, Entry 15). The fixed N
2
yield of this reaction is substantially lower than the
comparable experiment without added NH
3
(Table 1, Entry 3). One contributing cause for
NH
3
inhibition is that it sequesters HBAr
F
4
as [NH
4
][BAr
F
4
]; however, the yield of NH
3
observed in Entry 15 is suppressed compared to an experiment with only 100 equiv HBAr
F
4
.
This observation indicates that NH
3
inhibits the catalytic reaction, and that the degree of
inhibition is more substantial than stoichiometric leveling of the acid strength.
We also sought to establish the minimum reducing potential required to drive catalysis with
P
3
B
Fe-N
2
1
. We have shown in previous work that
1
reacts favorably with HBAr
F
4
in Et
2
O
at −78 °C along a productive N
2
fixation pathway.
18
In brief,
1
can be doubly protonated in
Et
2
O at −78 °C to generate P
3
B
Fe=NNH
2
+
(Eqn. 2). If only stoichiometric acid is present,
1
is instead unproductively oxidized to P
3
B
Fe-N
2
(Eqn. 3). We have only observed net
oxidation in the reaction of the neutral P
3
B
Fe-N
2
state with HBAr
F
4
in Et
2
O to produce
P
3
B
Fe
+
(Eqn. 4).
(2)
(3)
(4)
These observations suggest that N
2
fixing catalysis likely occurs at the P
3
B
Fe-N
2
/P
3
B
Fe-N
2
redox couple (−2.2 V vs Fc/Fc
+
), but not at the P
3
B
Fe
+
/P
3
B
Fe-N
2
couple (−1.5 V vs Fc/Fc
+
).
We have explored this hypothesis via cyclic voltammetry (CV) experiments. Figure 2 shows
electrochemical data for P
3
B
Fe
+
dissolved in Et
2
O at −45 °C under 1 atm N
2
in the presence
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of 0.1 M NaBAr
F
4
as a soluble electrolyte to create a modestly conductive ethereal
solution.
21
The blue trace shows the expected irreversible P
3
B
Fe
+
/P
3
B
Fe-N
2
couple centered
around ~ −1.7 V and the P
3
B
Fe-N
2
/P
3
B
Fe-N
2
couple at −2.2 V, as previously reported.
17
a
The red trace shows the electrochemical behavior of P
3
B
Fe
+
in the presence of 5 equiv
HBAr
F
4
. The data reveal a sharp plateaued increase in current coincident with the P
3
B
Fe-
N
2
/P
3
B
Fe-N
2
redox couple, and very little increase in current at the P
3
B
Fe
+
/P
3
B
Fe-N
2
couple. The onset of an apparent catalytic response at the P
3
B
Fe-N
2
/P
3
B
Fe-N
2
couple
intimates that electrocatalysis may be feasible, and that chemical reductants with weaker
reduction potentials than KC
8
may also be competent for N
2
-to-NH
3
conversion catalyzed
by P
3
B
Fe-N
2
. Also, the potential of the apparent catalytic response does not shift from the
P
3
B
Fe-N
2
/P
3
B
Fe-N
2
couple in the absence of acid, indicating that this reduction precedes
the first protonation event.
To determine whether electrolytic N
2
-to-NH
3
conversion contributes to the catalytic feature
observed in the CV data, a controlled-potential bulk electrolysis of P
3
B
Fe
+
and 10 equiv
HBAr
F
4
in Et
2
O at −45 °C under 1 atm N
2
in the presence of 0.1 M NaBAr
F
4
electrolyte
with a reticulated vitreous carbon working electrode was performed. The electrolysis was
held at −2.6 V (vs Fc/Fc
+
) for 4.6 hours, after which time 5.85 C of charge had been passed.
Product analysis revealed the formation of NH
3
(18% faradaic efficiency) as well as H
2
(58% faradaic efficiency). The amount of NH
3
generated in this experiment corresponds to
0.5 equiv with respect to Fe and 14% yield with respect to acid. When the experiment was
performed at higher acid loading (50 equiv), the NH
3
yield increased substantially (1.4 equiv
per Fe; 8% faradaic efficiency; electrolysis held at −2.3 V in this instance with 13.04 C
charge passed over 9 hrs). While these yields of NH
3
with respect to Fe do not demonstrate
formal turnover, they do suggest that electrocatalytic N
2
-to-NH
3
conversion by this iron
system may be feasible. That the NH
3
yield increases with increased acid correlates well
with our results in the chemical system. Studies to more thoroughly explore the
electrocatalytic N
2
-to-NH
3
conversion behavior of P
3
B
Fe-species are underway.
The electrochemical data presented in Figure 2 also suggest that chemical reductants with
weaker reduction potentials than KC
8
may be competent for N
2
-to-NH
3
conversion catalysis
by
1
. Consistent with this notion we find that catalytic yields of NH
3
(5 equiv per Fe) are
obtainable using
1
in the presence of 150 equiv HBAr
F
4
and 1900 equiv 10 wt% Na/Hg
amalgam under ~1 atm N
2
at −78 °C in Et
2
O (Table 1, Entry 16; a larger excess of 10 wt%
Na/Hg amalgam was employed to compensate for the lower surface area of the reagent).
This result demonstrates that the catalysis is not unique to the presence of either potassium
or graphite. KC
8
is a stronger reductant than is needed for N
2
-to-NH
3
conversion, but shows
more favorable selectivity for N
2
reduction relative to H
2
generation than other reductants
we have thus far canvassed.
2.2 Kinetics of ammonia and hydrogen formation
To better understand the competing NH
3
- and H
2
-forming reactions that occur during
catalysis, we measured the time profiles of product formation using the most active catalyst,
P
3
B
Fe-N
2
1
. Our method for quenching catalytic NH
3
production uses rapid freeze-
quenching of reactions to −196 °C, followed by addition of
tert
-butyllithium (
t
BuLi), and
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subsequent annealing to −78 °C. Employing this method allows for the measurement of NH
3
production as a function of time. The time courses of NH
3
formation obtained for the
previously reported substrate loading (blue trace)
12a
as well as a higher substrate loading
(red trace) are shown in Figure 3.
Under both substrate loadings shown in Figure 3, the reaction proceeds to completion at
−78 °C. Furthermore, under the higher-turnover conditions (with 150 equiv HBAr
F
4
and 185
equiv KC
8
, Figure 3, red triangles) the reaction proceeds to completion over ~45 min, a
time-scale that enables us to measure the dependence of d[NH
3
]/d
t
on the concentrations of
the soluble reagents—
1
and HBAr
F
4
—via the method of initial rates. As shown in Figure 4
(left), an initial rates analysis demonstrates that the reaction is first order in [Fe], which is
consistent with the involvement of a monomeric P
3
B
Fe species in the turnover-limiting step
for NH
3
formation. Comparing conditions ranging from 15 mM to 250 mM [HBAr
F
4
]
revealed no significant correlation between initial [HBAr
F
4
] and initial NH
3
production rate;
for instance, there is no measurable difference in the amount of NH
3
produced after five
minutes. This observation suggests zero-order rate dependence on acid concentration, which
is borne out by the initial rates analysis (Figure 4, right).
These data provide an estimate of initial turnover frequency (TOF, determined as moles of
NH
3
produced per minute per Fe atom) of this catalyst system of 1.2 ± 0.1 min
−1
. While the
TOF of this catalyst is not directly comparable to other N
2
-to-NH
3
conversion catalysts due
to differences in conditions and substrate, it is notable that
1
under the conditions used here
furnishes a substantially higher TOF than the other synthetic systems in Chart 1 for which
data is available (Table 2), despite operating over 100 °C lower in temperature (albeit with
the benefit of a stronger reductant). MoFe nitrogenase purified from
Klebsiella pneumoniae
exhibits a TOF of approximately 80 min
−1
,
22
nearly two orders of magnitude larger than the
present synthetic Fe system, while operating at room temperature.
To determine potential HER activity of P
3
B
Fe-N
2
1
, we measured the time course of H
2(g)
formation from HBAr
F
4
and KC
8
in the absence and presence of
1
, under catalytically
relevant conditions. As shown in Figure 5, the initial rate of H
2(g)
evolution at −78 °C is
enhanced by the presence of
1
. The Fe-catalyzed HER is > 85% complete within the first
hour with a final yield of ~40% (blue trace).
23
Quantifying the NH
3
produced in this
reaction (34% yield based on HBAr
F
4
) accounts for 74% of the acid added. We also confirm
that there is significant background HER from HBAr
F
4
and KC
8
(black trace), as expected.
We conclude that both catalyzed and background HER are competing with NH
3
formation in
the catalyst system.
As a point of comparison, we also measured the rate of H
2
evolution in the presence of
P
3
Si
Fe-N
2
(
3
). As shown in Figure 5, 3 also catalyzes HER, with an initial rate that is
comparable to
1
. However, in this case, H
2
evolution approaches completion over two hours,
resulting in a final measured yield of 88%. This is consistent with the low N
2
-fixing activity
of
3
; in the absence of a competitive NH
3
-producing reaction,
3
catalyzes the reduction of
protons to H
2
. Understanding the fundamental differences that give rise to the divergent
selectivity of these Fe catalysts is an important goal in the context of designing selective N
2
reduction catalysts.
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2.3 Spectroscopic characterization of Fe speciation under turnover
Considering the relatively slow rate of NH
3
formation ascertained from low-temperature
quenching experiments, we sought to determine the Fe speciation under turnover using the
P
3
B
Fe-N
2
catalyst
1
. By rapidly freeze-quenching reaction mixtures using
57
Fe-enriched
1
as a catalyst, time-resolved Mössbauer spectra can be obtained reflective of catalysis.
24
The Mössbauer parameters of some independently synthesized P
3
B
Fe species that may be
relevant to the present catalysis have been measured and are collected in Table 3. Mössbauer
isomer shifts (
δ
) can often be used to assign the relative oxidation state of structurally
related compounds,
16b
,
25
yet in this series of P
3
B
Fe compounds there is a poor correlation
between
δ
and formal oxidation state assignments (e.g. Fe-N
2
and Fe=NNH
2
+
species have
nearly identical isomer shifts). This fact reflects the high degree of covalency present in
these P
3
B
Fe-N
x
H
y
complexes, skewing classical interpretations of the Mössbauer data; the
degree of true oxidation/reduction at the iron centers in P
3
B
Fe-N
x
H
y
species is buffered by
strong covalency with the surrounding ligand field.
26
,
27
We do, however, find a useful linear
correlation (
r
2
= 0.90) between the measured ground spin states
(S
) of P
3
B
Fe-N
x
H
y
compounds and
δ
(Figure 6),
28
providing an empirical relationship that guides analysis of
Mössbauer spectra obtained from catalytic reactions. Ground spin states can be reliably
correlated with the type of N
x
H
y
ligand, and possibly the presence of hydride ligands,
coordinated to a P
3
B
Fe center. This knowledge, combined with freeze-quench Mössbauer
data, enables us to predict with some confidence the type(s) of Fe species that are present in
a spectrum obtained after freeze-quenching during turnover.
Figure 7 shows time-resolved Mössbauer spectra of freeze-quenched catalytic reaction
mixtures of P
3
B
Fe-N
2
anion
1
with 48 equiv of HBAr
F
4
and 58 equiv of KC
8
. Figure 7A
shows the spectrum of catalyst
1
as a 0.64 mM solution in THF, which features a sharp,
asymmetric quadrupole doublet at 80 K in the presence of a 50 mT external magnetic field.
Figure 7B shows the spectrum of a catalytic reaction mixture freeze-quenched after 5
minutes of stirring, revealing the major Fe species (blue, representing ca. 60% of all Fe)
present during active turnover to have parameters
δ
= 0.16 ± 0.2 mm s
−1
and ΔE
Q
= 1.63
± 0.03 mm s
−1
, which, within the error of the simulation, is consistent with the diamagnetic
borohydrido-hydrido species (P
3
B
)(μ-H)Fe(H)(L) (
4-L
), where L = N
2
or H
2
.
29
This
observation correlates well with the previously reported result that
4-N
2
is produced from
the reaction of
1
with smaller excesses of HBAr
F
4
and KC
8
.
12a
Further corroborating this
assignment, data collected at liquid He temperature with a small applied magnetic field
suggest that this species is a non-Kramer’s spin system,
30
and should be
S
= 0 given the
observed correlation between
δ
and
S
(
vide supra
). Also present in Figure 7B is a minor
component (~8%, shown in white) with parameters
δ
= 0.02 ± 0.2 mm s
−1
and ΔE
Q
= 0.97
± 0.2 mm s
−1
, and a broad residual absorbance centered at
δ ≈
0.9 mm s
−1
encompassing a
width of ~2 mm s
−1
(representing ca. 20–30% of all Fe in the sample, shown in grey). Due
to the broadness of the latter resonance (
Γ ≈
1 mm s
−1
), this feature could not be accurately
modeled. Nevertheless, the signal is consistent with several known
S
= 3/2 P
3
B
Fe species.
For example, the vacant cation, P
3
B
Fe
+
, and the cationic species P
3
B
Fe-N
2
H
4
+
and P
3
B
Fe-
NH
3
+
, are
S
= 3/2 species and give rise to quadrupole doublets that lie within the envelope
of this broad signal (Table 3, Entries 1–3).
31
The utility of freeze-quench
57
Fe Mössbauer
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spectroscopy is evident; in a single spectral snapshot the presence of P
3
B
Fe-components
with varied spin states including
S
= 0, 1/2, 1, and 3/2 are observed.
Figure 7C shows that the primary Fe species present after 25 minutes of reaction time is the
starting catalyst
1
(shown in red, representing ca. 70% of all Fe in the sample). Also present
is ca. 20% of the species we assign as
4-L
(
δ
= 0.22 ± 0.2 mm s
−1
and ΔE
Q
= 1.62 ± 0.03
mm s
−1
,
shown in blue), < 5% of the neutral dinitrogen complex, P
3
B
Fe-N
2
(green), and
~7% of an as-yet unknown species with parameters
δ
= 0.00 ± 0.02 mm s
−1
and ΔE
Q
= 2.97
± 0.06 mm s
−1
(white). Thus, as acid substrate is consumed in the reaction to produce NH
3
and H
2
, the mixture of Fe species shown in Figure 7B at an early time point evolves back to
the starting material
1
. A slight residual excess of KC
8
is needed to ensure recovery of the
active catalyst. These data help rationalize the results of the substrate reloading experiments
(
vide supra
).
The increasingly low Fe concentrations used to achieve the highest yields of NH
3
reported
here make the collection of well-resolved Mössbauer spectra under such conditions
challenging. Nonetheless, we repeated freeze-quench experiments for one set of higher-
turnover conditions (Figure 8). Although in this case the Fe speciation at intermediate times
appears more complex, these data exhibit the same gross behavior shown in Figure 7; under
active turnover the major Fe species present is consistent with hydride
4-L
(≥ 50%, average
parameters
δ
= 0.20 ± 0.2 mm s
−1
and ΔE
Q
= 1.49 ± 0.09 mm s
−1
, Figure 8A,B
32
), and as
the extent of reaction increases significant amounts of P
3
B
Fe-N
2
1
reform (Figure 8C
33
).
2.4 Precatalyst activity of (P
3
B
)(μ-H)Fe(H)(N
2
) (4-N
2
) and identification of a catalyst resting
state
The observations presented in section 2.3 suggest that hydride
4-L
builds up as the major
Fe-containing species during active turnover and appears to be converted back to the active
catalyst
1
when catalysis is complete. We previously observed that this species can form
under conditions that model the catalytic conditions (10 equiv acid/12 equiv reductant) and
our initial thinking that
4-N
2
may be a catalyst deactivation product was guided by the poor
activity of isolated
4-N
2
as a precatalyst under the standard conditions (generating only 0.5
± 0.1 equiv NH
3
per Fe at 50 equiv acid/60 equiv reductant).
12a
However, in that initial
report we also noted that isolated
4-N
2
is not solubilized under the catalytic conditions.
Therefore, in light of the current in situ spectroscopy, and the observation that
4-N
2
liberated
some NH
3
under the original conditions, we wondered whether its insolubility may be what
is responsible for its comparative low activity as an isolated precursor. If
4-N
2
is brought
into solution, or formed in solution during turnover, it may exhibit activity. To test this
hypothesis we explored the activity of
4-N
2
under modified catalytic conditions where a
toluene/Et
2
O mixture (which improves the solubility of
4-N
2
) was employed as the solvent.
In this case we find that
4-N
2
serves as a viable precatalyst (Table 1, Entries 13, 14). We
suppose then that under the standard conditions (in pure Et
2
O), if
4-L
is generated in
solution during catalysis, it should be able to react productively so long as it does not
irreversibly precipitate, which may be slow at −78 °C. Accordingly, we have observed that
the Mössbauer spectrum of a sample taken from a standard catalytic mixture as described in
section 2.3 can be filtered at low temperature and still displays substantial
4-L
.
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These results suggest the feasibility of the stoichiometric transformation of hydride
4-L
into
N
2
anion
1
under catalytically relevant conditions. In a previous report, we showed that
4-N
2
is stable for short periods to either HBAr
F
4
or KC
8
in Et
2
O at room temperature, again
noting its insolubility under these conditions.
12a
Given the results above, we have
reinvestigated this reactivity in Et
2
O/toluene mixtures. Thus the reaction of
4-N
2
with 1
equiv of HBAr
F
4
in 6:1
d
8
-toluene:Et
2
O results in consumption of the starting material along
with the appearance of several new, paramagnetically-shifted
1
H NMR resonances. We
hypothesize that protonolysis of either the terminal or bridging hydride moieties in
4-N
2
produces a cationic “P
3
B
Fe-H
+
” species, which may then be reduced to liberate 0.5 equiv of
H
2
and re-enter the catalytic manifold of {P
3
B
Fe-N
2
}
n
species under an N
2
atmosphere.
Indeed, the sequential addition of 1.5 equiv of HBAr
F
4
followed by 6 equiv of KC
8
to
4-N
2
at −78 °C in 3:1 Et
2
O:toluene produces substantial amounts of
1
(32% yield, unoptimized;
Scheme 1). This stoichiometric reactivity provides support for the idea that as
4-L
is formed
under the standard reaction conditions it can react with acid and reductant to produce the
starting catalyst
1
, consistent with the observations provided in section 2.3.
Given that (i)
4-L
appears to be the predominant Fe-containing species observed by freeze-
quench Mössbauer spectroscopy under turnover conditions at early time points, (ii) that this
species serves as a competent precatalyst when solubilized, and (iii)
4-N
2
can be
synthetically converted to
1
by HBAr
F
4
and KC
8
, we conclude that
4-L
is a major resting
state of the catalysis. This conclusion does not require
4-L
to be an “on path” intermediate;
we instead think
4-L
is more likely a resting state that ties up the catalyst, but one that
reversibly leaks into the on-path catalytic cycle in which
1
is ultimately protonated.
The observation of a hydride resting state for this synthetic Fe catalyst may have additional
relevance in the context of biological nitrogen fixation, where the intermediacy of metal
hydride species has been proposed on the basis of spectroscopic data obtained during
turnover.
5
It has further been proposed that the reductive elimination of hydrides as H
2
may
be a requisite component of N
2
binding to the nitrogenase active-site cofactor,
22
,
34
,
35
giving
rise to obligate H
2
evolution in the limiting stoichiometry of N
2
conversion to NH
3
.
36
The
results described here directly implicate the relevance of a synthetic iron hydride species to a
system capable of catalytic N
2
-to-NH
3
conversion. This in turn motivates complimentary
model reactivity studies on iron hydride species such as
4-L
, targets whose relevance might
otherwise be overlooked.
2.5 Summary of mechanistically relevant observations
To help collect the information presented here and in related studies of the P
3
B
Fe-system,
Scheme 3 provides a mechanistic outline for the key iron species and plausible
transformations we think are most relevant to the catalytic N
2
-to-NH
3
conversion cycle
catalyzed by P
3
B
FeN
2
1
. The complexes shown in blue, along with their respective spin
states
S
, have been thoroughly characterized. Also, the net conversions between complexes
that are indicated by solid blue arrows have been experimentally demonstrated. Those
complexes depicted in black have not (as yet) been experimentally detected.
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Several results are worth underscoring. (i) We have characterized the
S
= 3/2, substrate-free
state P
3
B
Fe
+
, and shown that it binds N
2
upon electron loading, generating
S
= 1 P
3
B
Fe-N
2
or
S
= ½ P
3
B
Fe-N
2
depending on reducing equivalents provided.
12a
,
17
P
3
B
Fe
+
is competent
for catalytic N
2
-to-NH
3
conversion,
12a
and facilitates this conversion electrolytically as
established herein. (ii) The most reduced state, P
3
B
Fe-N
2
1
, can be doubly protonated at
low temperature to generate P
3
B
Fe=N-NH
2
+
, a distal pathway intermediate.
18
This
S
= 1/2
species features a short Fe-N multiple bond (~1.65 Å). Its diamagnetic relative, P
3
sSi
Fe=N-
NH
2
+
, has very recently been structurally characterized.
37
(iii) The P
3
B
Fe=N-NH
2
+
intermediate anneals (in the absence of reductant) to generate significant amounts of P
3
B
Fe-
NH
3
+;
12a
,
18
(iv) P
3
B
Fe-NH
3
+
can also be generated by protonation of P
3
B
Fe-NH
2
, and
reductive displacement of NH
3
from P
3
B
Fe-NH
3
+
under N
2
regenerates P
3
B
Fe-N
2
−.
12a
Also worth emphasizing is that diamagnetic P
3
Si
Fe=N-NH
2
+
can be reduced at low
temperature to
S
= 1/2 P
3
Si
Fe=N-NH
2
, and this species in the presence of additional acid
and reductant evolves to a mixture of P
3
Si
Fe-N
2
H
4
+
and P
3
Si
Fe-NH
3
+.
37
P
3
Si
Fe-N
2
H
4
+
, and
also P
3
B
Fe-N
2
H
4
+
, readily disproportionate the bound N
2
H
4
to generate the corresponding
NH
3
adducts P
3
Si
Fe-NH
3
+
and P
3
B
Fe-NH
3
+
,
12a
,
16b
each of which evolves NH
3
upon
reduction to regenerate (under N
2
) P
3
B
Fe-N
2
1
and P
3
Si
Fe-N
2
, respectively. The reaction
pathway observed for P
3
Si
Fe=N-NH
2
+
, more readily studied than for P
3
B
Fe=N-NH
2
+
because P
3
Si
Fe=N-NH
2
+
can be isolated in pure form, highlights the possibility of a hybrid
crossover mechanistic pathway wherein a distal intermediate (Fe=N-NH
2
) traverses to an
alternating intermediate (Fe-N
2
H
4
) that may then be converted to NH
3
, possibly via
disproportionation.
18
,
37
By demonstrating first-order rate dependence on the concentration
of P
3
B
Fe-N
2
, [
1
], the present study remains consistent with our hypothesis that a single-site
mechanism is likely operative during N
2
-to-NH
3
conversion catalysis. The direct observation
of both
1
and its neutral form P
3
B
Fe-N
2
in catalytic mixtures by freeze-quench Mössbauer
spectroscopy lends further credence to this idea.
A plausible pathway for the formation of the putative resting state species
4-L
would be
hydrogenation of P
3
B
Fe-N
2
by evolved H
2
side-product during catalysis. This process has
been demonstrated independently at room temperature in benzene.
29
Follow-up control
experiments in
d
8
-toluene, however, suggest that this reaction is not kinetically competent at
−78 °C. One alternative pathway for the formation of
4-L
during catalysisis via bimolecular
H-atom transfer from the unobserved intermediate P
3
B
Fe-N
2
H (e.g., 2 P
3
B
Fe-N
2
H
4-L
+
P
3
B
Fe-N
2
), a process that could compete with productive protonation to generate P
3
B
Fe=N-
NH
2
+
. Efforts are ongoing to find conditions under which P
3
B
Fe-N
2
H can be generated,
characterized, and studied.
3. CONCLUSION
In the present study we have shown that N
2
-fixing catalyst systems with P
3
E
Fe (E = B, C,
Si) species give rise to high yields of NH
3
if supplied with sufficient acid and reductant.
These yields (for E = B and C) compare very favorably to the most active known Mo
catalysts and are almost an order of magnitude greater than the yields presented in our
previous reports. While we do not rule out some degree of catalyst degradation at −78 °C,
these iron catalysts are unexpectedly robust and it is possible that the lower efficiency of
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catalysis at higher turnover is in part due to build-up of NH
3
product, which is an inhibitor.
We have also provided new mechanistic insights for reactions with catalyst P
3
B
Fe-N
2
1
,
such as the observation that catalysis proceeds at −78 °C, the demonstration of first-order
rate dependence on catalyst concentration, the demonstration of zeroth-order rate
dependence on HBAr
F
4
concentration, and the observation that
1
catalyzes HER as well as
NH
3
formation. Preliminary electrochemistry data suggests that catalysis by the P
3
B
Fe
system can be driven at the formal P
3
B
Fe-N
2
/P
3
B
Fe-N
2
couple around −2.2 V vs Fc/Fc
+
,
consistent with Na/Hg also serving as a viable reductant for catalytic turnover. Cyclic
voltammetry and controlled potential electrolysis of P
3
B
Fe
+
at −45 °C demonstrate that
electrolytic N
2
reduction is possible.
The present study has also demonstrated the utility of coupling in situ freeze-quench
57
Fe
Mössbauer spectroscopy with kinetic analysis of product formation as a powerful tool for
the mechanistic study of Fe-catalyzed N
2
fixation. To date, no synthetic molecular N
2
-to-
NH
3
conversion catalyst system had been studied spectroscopically under active turnover
conditions. Our freeze-quench Mössbauer results suggest that
4-L
is a resting state of the
overall catalysis; this hydride species, which we previously posited to be primarily a catalyst
sink, can instead reenter the catalytic pathway via its conversion to catalytically active
P
3
B
Fe-N
2
1
. This observation underscores the importance of understanding metal hydride
reactivity in the context of Fe-mediated nitrogen fixation. It may be that HER activity
provides a viable strategy for recovering catalytically active states from the unavoidable
generation of iron hydride intermediates.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments
This work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. T.J.D.C.
acknowledges the support of the NSF for a Graduate Fellowship (GRFP) and N.B.T. acknowledges the support of
the Resnick Sustainability Institute at Caltech for a Graduate Fellowship.
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