of 27
Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic
Insights Regarding H
2
Elimination, HER, and NH
3
Generation
Dirk J. Schild
,
Jonas C. Peters
*
Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech),
Pasadena, California 91125, United States
Abstract
Despite their proposed accumulation at the Fe sites of the FeMo-cofactor of MoFe-nitrogenase, the
presence of hydride ligands in molecular model systems capable of the nitrogen reduction reaction
(N
2
RR) appears to diminish the catalytic N
2
-to-NH
3
conversion. We find that for an iron-based
system bearing the trisphosphine ligand P
2
P
Ph
, a dramatic difference in yields is observed for
N
2
RR catalyzed by precatalysts with zero, one, or two hydride ligands; however, irradiating the
three different catalysts with a mercury lamp results in similar yields. Although the efficacy for
N
2
RR versus the hydrogen evolution reaction (HER) is modest for this system by comparison to
certain iron (and other metal) catalysts, the system provides an opportunity to study the role of
hydrides in the selectivity for N
2
RR versus HER, which is a central issue in catalyst design.
Stochiometric reactions with hydride containing precatalysts reveal a hydrogen evolution cycle in
which no nitrogen fixation occurs. Irradiation of the dihydride precatalysts, observed during
turnover, results in H
2
elimination and formation of (P
2
P
Ph
)Fe(N
2
)
2
, which itself is unreactive with
acids at low temperature. N
2
functionalization does occur with acids and silyl electrophiles for the
reduced species [(P
2
P
Ph
)Fe(N
2
)]
and [(P
2
P
Ph
)Fe(N
2
)]
2−
, which have been characterized
independently. The requirement of accessing such low formal oxidation states explains the need
for strong reductants. The low selectivity of the system for functionalization at N
β
versus Fe
creates off-path hydride species that participate in unproductive HER, helping to explain the low
selectivity for N
2
RR over HER. The data presented here hence lends further insight into the
growing understanding of the selectivity, activity, and required driving force relevant to iron (and
other) N
2
RR catalysts.
SYNOPSIS TOC
*
Corresponding Author
jpeters@caltech.edu.
ASSOCIATED CONTENT
Supporting Information
The supporting information is available free of charge via the internet at
http://pubs.acs.org
.
Experimental procedures and compound characterization, X-ray data and Cartesian coordinates. CCDC 1886643–1886649
HHS Public Access
Author manuscript
ACS Catal
. Author manuscript; available in PMC 2020 May 03.
Published in final edited form as:
ACS Catal
. 2019 May 3; 9(5): 4286–4295. doi:10.1021/acscatal.9b00523.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
Keywords
ammonia synthesis; nitrogen fixation; N
2
RR; nitrogenase; hydrogen evolution; HER; iron
catalysis; photocatalysis
Introduction
Substantial progress has been made in the development and understanding of molecular
catalysts for N
2
-to-NH
3
conversion, commonly referred to as the nitrogen reduction reaction
(N
2
RR).
1
3
The number of well-defined complexes capable of N
2
RR is expanding rapidly,
and significant improvements in turnover and efficiency have been made.
4
,
5
With the
growing number of systems available, it becomes increasingly possible to uncover general
design principles that will aid in further progress for the field. The selectivity of N
2
RR
versus the competing hydrogen evolution reaction (HER) is a central selectivity issue in
need of model studies.
6
Competing HER not only limits the efficiency of molecular catalyst
systems but also limits nitrogenase enzymes.
7
,
8
Additionally, and relatedly, a deeper
understanding as to why seemingly related synthetic catalysts often require very different
reductant and acid combinations to be competent for N
2
RR is needed.
HER can occur by the background reaction between the reductant and acid; synthetic N
2
RR
catalysts depend on limiting the rate of background HER relative to the catalytic N
2
RR rate.
A catalyzed HER process, presumably accessible and competitive for many N
2
RR catalysts,
can also limit the efficacy of N
2
RR selectivity. Both scenarios can be at play.
4
,
9
For a complex that catalyzes both N
2
RR and HER, numerous pathways for the latter process
are possible. H
2
may evolve via protonation of a metal-bound hydride,
3
,
9
a commonly
proposed pathway for synthetic HER catalysts. Accordingly, the build-up of M–H species
has been observed both during and after catalytic N
2
RR experiments.
2
,
9
,
10
The accumulation
of M–H species is generally thought to attenuate N
2
RR activity, and hydride precatalysts can
give rise to diminished yields for N
2
RR.
2
,
3
,
9
When hydride precursors serve as active
precatalysts for N
2
RR, it is presumed they react with acid and reductant to release H
2
,
thereby generating a species that is on-path for N
2
RR.
3
,
9
As an example of this, for a
tris(phosphine)borane iron catalyst system studied extensively by our lab, [(P
3
B
)Fe(N
2
)]
(P
=
o
-(P
i
Pr
2
)
2
C
6
H
4
), a dihydride intermediate was observed as an off-path resting state of the
Schild and Peters
Page 2
ACS Catal
. Author manuscript; available in PMC 2020 May 03.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
system when KC
8
and HBAr
F
4
(Et
2
O) (BAr
F
4
= tetrakis-(3,5-
bis(trifluoromethyl)phenyl)borate) were employed (Figure 1).
9
This dihydride species can be
converted to an on path intermediate by reductive protonation.
9
A conceptually similar
pathway has been described by Nishibayashi and coworkers for a (PNP)Fe system (Figure 1,
left).
3
Another possible competing HER pathway present within N
2
RR systems that has been
considered by our lab involves a bimolecular, N
x
H
y
ligand-mediated step wherein two
Fe(N
x
H
y
) intermediates that feature weak N–H bonds evolve H
2
(Eqs. 1 and 2).
6
2 Fe‐NNH
2 Fe‐N
2
+ H
2
(Eq. 1)
2 Fe‐NNH
2
2 Fe‐NNH+ H
2
(Eq. 2)
Protonation at the metal versus at coordinated N
2
to form a metal hydride should be
thermodynamically favored,
6
though the kinetic site of protonation can involve the
coordinated N
2
ligand. Even if protonation at N
2
is kinetically favored, this can be followed
by intra- or intermolecular H-atom/hydride/proton transfer to form a metal hydride.
11
Initial
protonation at a site on the auxiliary ligand can also be kinetically favored.
12
Protonation at
the terminal nitrogen (N
β
) is desired for selectivity towards nitrogen fixation. For the P
3
B
Fe-
system, iron is by far the thermodynamically favored site for protonation. However, the
steric profile of the complex and the acids used appear to render functionalization at N
β
kinetically favorable.
13
Recently, our group reported two related iron-based complexes bearing hydride ligands,
(P
2
P
Ph
)Fe(H)]
2
(μ–N
2
) (
1
) and (P
2
P
Ph
)Fe(N
2
)(H)
2
(
2
) (P
2
P
Ph
= bis(
o
-diisopropylphosphino-
phenyl)-phenylphosphine), that are modestly active systems for N
2
RR (Figure 1, right).
2
For
complex
2
, photo-induced H
2
elimination was proposed to yield a more activated Fe–N
2
species that could undergo subsequent reductive protonation steps to generate NH
3
. An H
2
-
elimination step can also be observed for (P
3
B)(μ-H)Fe(N
2
)(H), resulting in the formation of
(P
3
B)Fe(N
2
).
2
,
14
Although precatalysts
1
and
2
are significantly less efficient for N
2
RR than [(P
3
B
)Fe(N
2
)]
and certain other metal catalysts, they provide a fascinating model system for in-depth study
because they have been shown to display substantial enhancement for N
2
RR under
irradiation.
2
Furthermore, these catalysts bear hydride ligands but are nonetheless active for
N
2
RR, affording an opportunity to investigate the role of the hydride ligands in N
2
RR and
competing HER. Finally, a better understanding of the electronic and structural factors that
influence the required redox potential for N
2
RR in this phosphine-iron catalyst system
compared to other systems can aid in the development of selective catalysts that operate at a
comparatively low net driving force.
Schild and Peters
Page 3
ACS Catal
. Author manuscript; available in PMC 2020 May 03.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
Results and Discussion
In our prior communication we proposed that the product of H
2
elimination from well-
characterized dihydride
2
might be “(P
2
P
Ph
)Fe(N
2
)”.
2
Reasoning that this or a related
species might be on-path for N
2
RR, we targeted an independent synthesis. (P
2
P
Ph
)FeBr
2
(
3
)
provided a logical starting point. Treatment of
3
with 1.05 equiv sodium mercury amalgam
resulted in the formation of (P
2
P
Ph
)FeBr (
4
) in 65% yield (Scheme 1).
4
exhibits
C
s
symmetry in solution based on its
1
H NMR spectrum and a distorted tetrahedral geometry
(
τ
4
= 0.77)
15
in the solid state (See Figure S1 and S62).
Bromide
4
is a useful synthon for several complexes of present interest. For example,
treatment of
4
with NaHBEt
3
in toluene at −78 °C provides a more favorable route to the
diiron complex
1
(Scheme 1), whose preparation was previously described by NaHBEt
3
reduction of
3
. Furthermore,
4
can be reduced with sodium mercury amalgam in either
benzene or THF to provide a new, maroon red complex (P
2
P
Ph
) Fe(N
2
)
2
(
5
).
5
can be
alternatively prepared by reduction of
3
with excess sodium mercury amalgam in benzene
(Scheme 1).
18-electron
5
exhibits two intense bands in its IR spectrum (thin film;
ν
symm
= 2065 cm
−1
,
ν
asymm
= 2005 cm
−1
) and its solid-state crystal structure (See Figure S63), reveals a
distorted trigonal bipyramidal geometry at iron (
τ
5
= 0.54)
16
with Fe–P distances ~0.15 Å
shorter than in
3
, reflecting its singlet ground state.
2
The structure and stretching frequencies
of the five-coordinate N
2
complex
5
is closely related to recently reported (P
R
P
Cy
2
)Fe(N
2
)
2
(P
R
P
Cy
2
= RP(CH
2
CH
2
PCy
2
)
2
, R = Ph, tBu) complexes.
17
In the latter N
2
complexes, facile
N
2
dissociation hampers their isolation. Although
5
is also susceptible to N
2
dissociation
(
vide infra
) it can be readily isolated by evaporation of the solvent
in vacuo
followed by
extraction with pentane.
N
2
-Binding Equilibria of 5.
A solution equilibrium exists between
5
and a dinuclear, mono-N
2
-bridged complex
6
(Scheme 1). This is clearly gleaned from
1
H and
31
P NMR spectroscopies. For example, the
1
H NMR signal intensities for
5
decay upon degassing the solution in a J-Young NMR tube
and the signals corresponding to
6
grow in (See Figure S23). Addition of N
2
regenerates
5
.
The absence of an N
2
stretch in the IR spectrum of
6
, and the release of 1.5 equiv of N
2
per
Fe on conversion of
5
to
6
, as measured by a Toepler pump experiment, are consistent with
our formulation of
6
(Scheme 1; Eq. 3):
2
P
2
P
Ph
Fe
N
2
2
P
2
P
Ph
Fe
2
μ–N
2
+ 3/2 N
2
(Eq. 3)
Monitoring the
1
H and
31
P chemical shifts of
6
over a 130 °C range under vacuum reveals
deviation from Curie-behavior (Figure 2 and S31–S33). The singlet ground state of
6
likely
arises from antiferromagnetic coupling of two S = 1 iron nuclei. The dramatic shifts in the
NMR spectra are therefore attributed to partial population of triplet and quintet states
separated by 2
J
and 6
J
from the ground state, respectively (as obtained for the Heisenberg-
Schild and Peters
Page 4
ACS Catal
. Author manuscript; available in PMC 2020 May 03.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
Dirac-VanVleck Hamiltonian in the notation
H
HDVV
= −2J
S
1
S
2
). Fitting of the appropriate
Boltzmann function to the experimental data yields
J =
−940 ± 9.4 cm
−1
(Equation S1).
18
Antiferromagnetic coupling for an N
2
–bridged diiron species has been observed previously.
19
Increased Turnover with Non-Hydride Precatalysts and Identification of Off-Path Species.
Previous N
2
RR studies using the (P
2
P
Ph
)Fe-system were performed with the hydride
complexes
1
and
2
as (pre)catalysts. We wondered whether increased turnover numbers
might be realized with (P
2
P
Ph
)Fe(N
2
)
2
,
5
, devoid of hydride ligands. Indeed, catalysis under
the same conditions with
5
resulted in significantly higher NH
3
yields than those afforded by
1
and
2
. For example, at a loading of 150 equivalents acid and 180 equivalents reductant at
−78 °C in Et
2
O, in the absence of light, complexes
1
and
2
catalyzed only 3.6 ± 0.6 and 2.6
± 0.01 equiv NH
3
per iron, respectively, whereas
5
catalyzed the generation of 6 ± 0.5 equiv.
Interestingly, a comparable NH
3
yield (5.1 ± 0.02 equiv per iron center) could be realized
with
5
using only 1/3 as much reductant and acid (50 equiv HBAr
F
4
(Et
2
O) and 60 equiv
KC
8
), which was not the case for either
1
or
2
. A possible explanation for this difference is
that HER catalysis from the hydrides, which are present in the highest concentration at the
onset of runs with
1
and
2
, outcompetes N
2
RR.
Similar NH
3
yields were obtained for the three different precatalysts
1
,
2
, and
5
in catalytic
experiments irradiated with a mercury lamp. We presume that dihydride
2
releases H
2
upon
irradiation with light to yield
5
, and that this transformation occurs rapidly under turn-over
conditions as all (pre)catalysts give similar yields. The consumption of hydride species via
photolysis reduces HER catalyzed by the hydrides, thus increasing overall efficiency for
NH
3
.
To determine whether catalyzed HER contributes to the low yields of NH
3
obtained with
dihydride
2
, hydrogen evolution was measured under catalytically relevant conditions. As
shown in Figure 3, the initial rate of H
2
evolution at −78 °C, is significantly enhanced by the
presence of either
2
or
5
. These data suggests both
2
and
5
are comparatively competent
catalysts for HER, whereas complex
5
is a more effective (pre)catalyst for N
2
RR. Indeed,
the fact that most of the acid is consumed within 30 minutes at −78 °C speaks to how rapidly
5
must catalyze NH
3
production for N
2
RR to be kinetically competitive.
Additional evidence for the active role of
2
in HER is obtained from Mossbauer studies of
freeze-quenched samples. Freeze-quenching of a catalytic run using
57
Fe-labeled
2
, 50 equiv
acid, and 60 equiv reductant, shows its disappearance within 5 minutes (Figure 4, middle
trace). A new broad feature, likely due to the overlap of several species, is observed. Freeze-
quenching the reaction after 30 minutes provides a similarly broad signal (Figure 4, bottom
trace), but one that also contains
2
, with its characteristic small quadrupole splitting
(constituting ~ 40% of the total). Experiments using
5
as the precatalyst provide an
analogously broad signal after 5 and 30 minutes (See Figure S59–S60). Notably, dihydride
2
is always observed at the end of a catalytic experiment, once the sample has been warmed to
room temperature. It is the major species present (typically ~ 90% by Mössbauer
spectroscopy). Furthermore, IR and NMR spectra recorded after runs using
5
as the
Schild and Peters
Page 5
ACS Catal
. Author manuscript; available in PMC 2020 May 03.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
precatalyst show
2
as the only identifiable species upon warming. These data collectively
suggest that the catalytic system converts to a Fe–H species (
2
), which is on path for HER
(
vide infra
), as the major product.
2
This finding is similar to that of [(P
3
B
)Fe(N
2
)]
-
, which
also ends tied-up in an off-path hydride-borohydride state (Figure 1, left).
9
Oxidative addition and reductive elimination of H
2
.
To investigate potential pathways by which hydride species form during catalysis,
stoichiometric reactions were performed with dihydride
2
, the bis–N
2
complex
5
, and
dinuclear
6
. Addition of H
2
to
5
(or
6
), followed by N
2
, resulted in the quantitative
formation of
2
(Scheme 2). However, addition of H
2
to
5
at −78 °C in a J-Young tube for one
hour resulted in the appearance of a trace amount of
2
. Full conversion was only observed
upon warming to room temperature (see Figure S25). The latter result strongly suggests that
the formation of
2
under the catalytic conditions at −78 °C does not occur by a reaction
between
5
and H
2
.
Irradiating solutions of
2
with a 100 W mercury lamp at −78 °C or room temperature results
in darkening of the solution and the formation of
5
(Scheme 2). Complete disappearance of
2
is not observed, suggesting the reaction is reversible (Figure S27). A possible 16-electron
intermediate, such as “(P
2
P
Ph
)Fe(N
2
)”,
2
could not be identified by NMR, IR, or Mössbauer
spectroscopy.
H
2
elimination from
2
to
5
leads to a significant decrease in
ν
(NN) stretching frequencies
due to increased backbonding upon H
2
elimination. A similar effect was observed previously
for (P
3
B
)(μ-H)Fe(N
2
)(H).
2
In this context, these systems crudely model a proposed N
2
binding/activation via H
2
elimination at the E
4
state of the iron-molybdenum cofactor
(Figure 5).
20
Clearly, increased N
2
activation upon H
2
elimination observed for this
P
2
P
Ph
Fe-system would be even more pronounced for the unobserved, but perhaps
catalytically relevant mono-N
2
adduct “P
2
P
Ph
Fe(N
2
)” (
vide infra
).
Stoichiometric reactivity and hydrogen evolution.
To further probe HER catalysis by the present system, HBAr
F
4
was added to (P
2
P
Ph
)Fe(N
2
)
2
5
at −78 °C, causing a color change from maroon to dark yellow upon warming. The product
of protonation at iron was identified as [(P
2
P
Ph
)Fe(N
2
)
2
(H)][BAr
F
4
] (
7
) (Scheme 3),
featuring a diagnostic
1
H NMR hydride resonance (−17 ppm) and bands at 2069, 2194 and
2264 cm
−1
, corresponding to the
ν
(Fe–H) and
ν
(NN) IR stretches. Its solid-state structure
was also determined (see Figure S64). This complex can also be obtained by oxidation of
{(P
2
P
Ph
)Fe(H)}
2
(μ–N
2
)
1
with either FcBAr
F
4
(Fc = bis(
η
5
-cyclopentadienyl)iron) or
HBAr
F
4
(Scheme 3). Of primary interest, protonation of
2
with HBAr
F
4
likewise generates
7
with concomitant H
2
release, possibly via an “[Fe(H
2
)(H)]
+
” adduct.
21
Monohydride
7
can be cleanly reduced to dinuclear
1
using either Cp*
2
Co or stoichiometric
KC
8
. Reduction of
7
with an excess of KC
8
by contrast generates a different diamagnetic
species which, following addition of 18-crown-6, could be isolated in pure form as
[P
2
P
Ph
Fe(N
2
)(H)][K(18-crown-6)] (
8
) (Scheme 3). Complex
8
features a diagnostic hydride
resonance in its
1
H NMR spectrum (
δ
= −9.69 ppm, dt), and its solid-state structure displays
Schild and Peters
Page 6
ACS Catal
. Author manuscript; available in PMC 2020 May 03.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
a short Fe–N (1.774(1) Å) and an elongated N–N (1.139(2) Å) bond. A high degree of
activation of N
2
is reflected by its
ν
(NN) (1924 cm
−1
).
Stoichiometric mixing of cation
7
and anion
8
resulted in comproportionation to
1
(> 90%
yield). Proton transfer (PT) from
7
to
8
might have alternatively resulted in the formation of
2
and
5
(Scheme 4), but this was not observed. This may be rationalized by low acidity of
the hydride ligand in
7,
which is not deprotonated by NaO
t
Bu. Relatedly,
8
is weakly basic
and is not protonated by MeOH at −78 °C. The absence of proton transfer between
7
and
8
makes this an unlikely step for (re)generating
2
and
5
under turnover conditions. Dihydride
2
can, however, be obtained readily by protonation of
8
with HBAr
F
4
at low temperature
(Scheme 4).
Apparent differences in reactivity of the hydrides with respect to N
2
RR can be rationalized
by the availability, or lack of, kinetically competent pathways for the hydrides to be
converted to on-path Fe–N
2
species at −78 °C. For the P
2
P
Ph
system, no pathway has been
identified via which hydrides convert back to on-path Fe–N
2
species. Instead, in
stoichiometric reactions, the different hydrides interconvert in an HER cycle (Figure 6).
However, the observation of NH
3
production during catalytic experiments with
1
or
2
as
precatalysts indicates that there must be some pathway to a species active for N
2
RR, even if
comparatively inefficient.
Reduction of (P
2
P
Ph
)Fe(N
2
)
2
.
The fact that there is no reactivity between
5
and HBAr
F
4
at −78 °C indicates that the N
2
ligands are not sufficiently activated to be protonated, in accord with comparatively high N
2
stretching frequencies for
5
(2065 and 2009 cm
−1
). To explore whether further reduction
might generate a more reactive and hence on-path species,
5
was stirred with 1 equiv
potassium naphthalide followed by the addition of 18-crown-6. This produced the anionic,
4-coordinate
S
= ½ (
μ
eff
= 1.80) complex [(P
2
P
Ph
)Fe(N
2
)][K(18-crown-6)] (
9
) (Scheme 5).
9
features a single and highly activated N
2
ligand (1872 cm
−1
). Its solid-state crystal structure
shows a disordered tetrahedral iron center (
τ
4
= 0.75), and CV measurements show a quasi-
reversible Fe
0/−
redox event centered at −2.5 V vs Fc/Fc
+
(Figure 7). At more negative
potential, an irreversible, presumably Fe
−2/−1
redox event is observed.
Reduction of
5
with an excess of KC
8
produced a diamagnetic species (
31
P NMR:
δ
=
113.13, doublet; 95.15 ppm, triplet) identified as [(P
2
P
Ph
)Fe(N
2
)][K
2
(THF)
3
] (
10
) (Scheme
5). Complex
10
is an unusual iron species in that it is isoelectronic with [Fe(CO)
4
]
2-
(
vide
infra
).
22
,
23
A
15
N-labeled analogue was synthesized by reduction of (P
2
P
Ph
)FeBr
2
under
15
N
2;
its
15
N NMR spectrum (2.36 and −26.23 ppm) rules out the possibility of a dinuclear
structure [{(P
2
P
Ph
)Fe}
2
(μ-N
2
)]K
2
.
24
Consistent with our assignment of dianion
10
, its IR
spectrum displays a
ν
(NN) at 1677 cm
−1
(1591 cm
−1
for
10
15
N
2
) that is broadened due to
ion-pairing, consistent with a very strongly activated N
2
ligand. Addition of 18-crown-6
resulted in intractable decomposition, suggesting tight ion-pairing is important to its
stability.
25
The structure of
10
in the solid-state (Figure 7) presents two distorted tetrahedral iron
centers (
τ
4
= 0.71) that are related by an inversion center within a dimeric unit. Tight ion
Schild and Peters
Page 7
ACS Catal
. Author manuscript; available in PMC 2020 May 03.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
pairing is evident from the close proximity of each iron center to the potassium cations (Fe–
K = 3.442 and 3.567 Å); each N
2
ligand interacts with three potassium ions. The Fe–N bond
is remarkably short (1.728(2) Å), ~ 0.1 Å shorter than the Fe–N bonds in
5
, reflective of
very strong backbonding. Relatedly, significant N–N elongation is also observed (1.189(3)
Å). The Fe–P bond distances are also highly contracted at 2.1494(6) Å, in line with the very
strong covalency expected of a d
10
tetrahedral iron center. Prior to this study, tetrahedral Fe
−II
species have been limited to complexes with very strong pi-acceptor ligands, such as CO,
22
,
23
PF
3
,
26
(C
2
H
4
),
27
,
28
(COD) (COD = cyclooctadiene),
27
and CNAr.
25
,
29
Additional
species bearing phosphorine
30
and nitrosyl
31
ligands have also been reported, however the
assignment of their oxidation state is ambiguous.
The Mössbauer spectrum of a perfectly tetrahedral d
10
iron complex, such as Na
2
[Fe(CO)
4
],
should show a singlet instead of a quadrupole doublet due to the spherical electric field
gradient at the iron nucleus;
22
,
32
any quadrupole splitting in Na
2
[Fe(CO)
4
] is barely
discernable.
32
Similarly, the Mössbauer spectrum of
10
shows an apparent singlet (Figure 7)
which can be fit by a small quadrupole splitting (
δ
= 0.27 mm/s, ΔE
Q
= 0.26 mm/s). The
very small quadrupole splitting in
10
, which is required to have at least a modest electric
field gradient owing to the presence of three unique types of donor ligands, indicates its
classification as a d
10
tetrahedral structure is appropriate, at least to the extent this
description is apt for Na
2
[Fe(CO)
4
] given the significant covalency in both species.
Functionalization of formal Fe
−I
and Fe
−II
species.
Current examples of Fe-mediated N
2
RR are thought to proceed through Fe–N
2
intermediates
with
ν
(NN) stretching frequencies below 1970 cm
−1
.
1d
,
3
,
33
The N
2
ligand of
[(P
2
P
Ph
)Fe(N
2
)][K(18-crown-6)] (
9
) has a stretching frequency of 1872 cm
−1
and in this
context should be activated enough to be functionalized. Attempts to protonate
9
with
stoichiometric HBAr
F
4
unfortunately resulted in complex product mixtures. Silylium ions
(R
3
Si
+
) have been used as surrogate electrophiles for protons to model unstable protonated
Fe-N
x
H
y
species.
1c
,
34
38
Reacting
10
, generated
in situ
, with one equivalent of Me
3
SiCl at
−78 °C, results in an immediate color change from dark purple to dark orange. After work-
up, diamagnetic [(P
2
P
Ph
)Fe(NNSiMe
3
)]K ([
11
-NNSiMe
3
]
) was isolated as a dark brown
solid in 50% yield (Scheme 6). As for
10
, a tight ion-pair seems to be important for its
stability; addition of 18-crown-6 results in its decomposition. The solid-state structure of
[
11
-NNSiMe
3
]
reveals a four-coordinate iron center with a distorted tetrahedral geometry
(
τ
4
= 0.76). The Fe–N bond length of [
11
-NNSiMe
3
]
is even shorter than that in
10
(1.664(7) Å vs. 1.728(2) Å respectively), and the N–N bond length is much longer (1.270(9)
Å vs. 1.189(3) Å) in
10
.
Attempts to oxidize [
11
-NNSiMe
3
]
at −78 °C with cobaltocenium to generate the neutral
diazenido species (P
2
P
Ph
)Fe-NNSiMe
3
resulted in a mixture of species, presumably
complicated by the loss of Me
3
Si·. Based on low temperature EPR data (Figure 8), we
assign the major product of oxidation to be the iron-silyl complex (P
2
P
Ph
)Fe(SiMe
3
)(N
2
) (its
EPR signature is highly similar to that of (P
2
P
Ph
)Fe(N
2
)(H);
2
see Figure 8 ). There is also a
minor component in the EPR trace that can be tentatively assigned as the expected diazenido
(P
2
P
Ph
)Fe(NNSiMe
3
). Use of
i
Pr
3
SiOTf instead leads to the analogous [
11
-NNSi
i
Pr
3
]
Schild and Peters
Page 8
ACS Catal
. Author manuscript; available in PMC 2020 May 03.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
complex, but in this case its oxidation affords a clean EPR spectrum consistent with the
diazenido species
12
-NNSi
i
Pr
3
(Figure 8). Addition of
i
Pr
3
SiOTf to [(P
2
P
Ph
)Fe(N
2
)]
9
generates the same species as is evident by IR and EPR spectroscopy (Figure 8). The IR
spectrum of
12
-NNSi
i
Pr
3
displays an intense band corresponding to
ν
NN at 1660 cm
−1
,
characteristic of iron diazenido species.
34
,
37
,
39
In contrast with the -SiMe
3
derivative, the -
Si
i
Pr
3
species is stable for days. We suspect that for the less bulky -SiMe
3
derivative,
kinetically competitive N-to-Fe silyl migration is operative.
We intuit that
9
, or its further reduced state [(P
2
P
Ph
)Fe(N
2
)]
2-
10
, must be reached before
nitrogen functionalization occurs via protonation or silylation. The iron centers in
9
and
10
are exposed, and are therefore susceptible to direct protonation at iron, or to facile migration
from N-to-Fe. An N-protonated form of
9
(or
10
) can presumably react further under the
catalytic conditions to produce NH
3
, when both excess acid and reductant are present. Such
reactivity must be kinetically competitive with a step that produces an off-path hydride.
The need to access an anionic state of the system (either
9
or
10
) before functionalization at
N
2
can occur sets the requirement of a potent reductant for N
2
RR in the P
2
P
Ph
Fe-system.
The Fe
−I/0
couple of
9
is −2.47 V vs. Fc/Fc
+
, which is ~ 0.30 V more negative than the
corresponding Fe
−I/0
couple for [(P
3
B
)Fe(N
2
)]
0/−
; N
2
RR can be driven rather efficiently with
the latter system using Cp*
2
Co paired with anilinium acids, which are ineffective with this
precatalyst.
4
,
5
Conclusions
The present study highlights the detrimental effect of hydride ligands on an iron–catalyzed
N
2
RR model system whose efficiency is enhanced by irradiation. Stoichiometric reactivity
as well as freeze-quench Mossbauer studies reveal that off-path (P
2
P
Ph
)Fe(N
2
)
x
(H)
y
species
are formed but are not inert resting states. On the contrary, they rapidly produce hydrogen in
an HER cycle operating parallel to the desired N
2
RR cycle. In the absence of light, these
pathways compete with one another but operate along different cycles. Irradiation of
(P
2
P
Ph
)Fe(N
2
)(H)
2
2
results in photoinduced H
2
elimination and the formation of
(P
2
P
Ph
)Fe(N
2
)
2
(
5
), which is significantly more competent for N
2
RR. Thus, photolysis shifts
the speciation from favoring an unproductive HER cycle to one where N
2
RR becomes
kinetically competitive.
A deeper understanding of the required driving force for N
2
functionalization is obtained by
stoichiometric reactions with Fe–N
2
species. No protonation reactivity is observed with a
strong acid for (P
2
P
Ph
)Fe(N
2
)
2
at −78° C; further reduction is required before
functionalization can take place. Protonation experiments with the Fe
−I
and Fe
−II
species
9
and
10
provide complex mixtures, but silylation experiments are informative.
The need to access an anionic or a dianionic state of the system before productive
functionalization at N
2
occurs sets the low reduction potential required for N
2
RR by this
P
2
P
Ph
Fe-system and explains why comparatively milder reductants such as Cp*
2
Co, which
are effective for a related (P
3
B
)Fe-catalyst system, are ineffective in the present case. Future
Schild and Peters
Page 9
ACS Catal
. Author manuscript; available in PMC 2020 May 03.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
catalyst designs for iron systems should focus on anodically shifting the needed redox
couple to generate an Fe–N
2
species while maintaining a strongly activated N
2
ligand.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
ACKNOWLEDGMENT
Dr. Michael Takase, Larry Henling and Dr. Marcus Drover are acknowledged for their assistance with
crystallographic studies. Dr. Niklas Thompson is thanked for assisting with fitting the variable temperature NMR
data.
Funding Sources
We are grateful to the NIH (GM-075757) for support of this research, and also to the National Science Foundation
for support of the Caltech EPR Facility via the NSF-MRI grant NSF-153194, and to the Dow Next Generation
Educator Fund. D.J.S acknowledges the support of the Resnick Sustainability Institute at Caltech for a Graduate
Fellowship.
REFERENCES
1 (a). Yandulov DV; Schrock RR Catalytic reduction of dinitrogen to ammonia at a single molybdenum
center. Science 2003, 301, 76–79. [PubMed: 12843387] (b)Arashiba K; Miyake Y; Nishibayashi
Y A molybdenum complex bearing PNP-type pincer ligands leads to the catalytic reduction of
dinitrogen into ammonia. Nat. Chem 2011, 3, 120–125. [PubMed: 21258384] (c)Anderson JS;
Rittle J; Peters JC Catalytic conversion of nitrogen to ammonia by an iron model complex.
Nature 2013, 501, 84–87. [PubMed: 24005414] (d)Hill PJ; Doyle LR; Crawford AD; Myers WK;
Ashley AE Selective catalytic reduction of N
2
to N
2
H
4
by a simple Fe complex. J. Am. Chem.
Soc 2016, 138, 13521–13524. [PubMed: 27700079] (e)Sekiguchi Y; Arashiba K; Tanaka H;
Eizawa A; Nakajima K; Yoshizawa K; Nishibayashi Y Catalytic reduction of molecular
dinitrogen to ammonia and hydrazine using vanadium complexes. Angew. Chem. Int. Ed 2018,
57, 9064–9068.(f)Doyle LR; Wooles AJ; Jenkins LC; Tuna F; McInnes EJL; Liddle ST Catalytic
dinitrogen reduction to ammonia at a triamidoamine-titanium complex. Angew. Chem. Int. Ed
2018, 57, 6314–6318.
2. Buscagan TM; Oyala PH; Peters JC N
2
-to-NH
3
Conversion by a triphos-iron catalyst and enhanced
turnover under photolysis. Angew. Chem. Int. Ed 2017, 56, 6921–6926.
3. Sekiguchi Y; Kuriyama S; Eizawa A; Arashiba K; Nakajima K; Nishibayashi Y Synthesis and
reactivity of iron–dinitrogen complexes bearing anionic methyl- and phenyl-substituted pyrrole-
based PNP-type pincer ligands toward catalytic nitrogen fixation. Chem. Commun 2017, 53, 12040–
12043.
4. Chalkley MJ; Del Castillo TJ; Matson BD; Roddy JP; Peters JC Catalytic N
2
-to-NH
3
Ccnversion by
Fe at lower driving force: A proposed role for metallocene-mediated PCET. ACS Cent. Sci 2017, 3,
217–223. [PubMed: 28386599]
5. Chalkley MJ; Castillo T. J. Del; Matson BD; Peters JC Fe-mediated nitrogen fixation with a
metallocene mediator: Exploring pK
a
effects and demonstrating electrocatalysis. J. Am. Chem. Soc
2018, 140, 6122–6129. [PubMed: 29669205]
6. Matson BD; Peters JC Fe-mediated HER vs N2RR: Exploring factors that contribute to selectivity in
P3EFe(N2) (E = B, Si, C) catalyst model systems. ACS Catal 2018, 8, 1448–1455. [PubMed:
30555733]
7. Simpson FB; Burris RH A nitrogen pressure of 50 atmospheres does not prevent evolution of
hydrogen by nitrogenase. Science 1984, 224, 1095–1097. [PubMed: 6585956]
8. Schubert KR; Evans HJ Hydrogen evolution: A major factor affecting the efficiency of nitrogen
fixation in nodulated symbionts. Proc. Natl. Acad. Sci 1976, 73, 1207–1211. [PubMed: 16592307]
Schild and Peters
Page 10
ACS Catal
. Author manuscript; available in PMC 2020 May 03.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
9. Del Castillo TJ; Thompson NB; Peters JC A synthetic single-site Fe nitrogenase: High turnover,
freeze-quench 57Fe Mössbauer data, and a hydride resting state. J. Am. Chem. Soc 2016, 138,
5341–5350. [PubMed: 27026402]
10 (a). Fajardo J; Peters JC Catalytic nitrogen-to-ammonia conversion by osmium and ruthenium
complexes. J. Am. Chem. Soc 2017, 139, 16105–16108. [PubMed: 29073760] (b)Kuriyama S;
Arashiba K; Nakajima K; Matsuo Y; Tanaka H; Ishii K; Yoshizawa K; Nishibayashi Y Catalytic
transformation of dinitrogen into ammonia and hydrazine by iron-dinitrogen complexes bearing
pincer ligand. Nat. Commun 2016, 7, 12181. [PubMed: 27435503] (c)Kuriyama S; Arashiba K;
Tanaka H; Matsuo Y; Nakajima K; Yoshizawa K; Nishibayashi Y Direct transformation of
molecular dinitrogen into ammonia catalyzed by cobalt dinitrogen complexes bearing anionic
PNP pincer ligands. Angew. Chem. Int. Ed 2016, 55, 14291–14295.(d)Creutz SE; Peters JC
Catalytic reduction of N
2
to NH
3
by an Fe–N
2
complex featuring a C-atom anchor. J. Am. Chem.
Soc 2014, 136, 1105–1115. [PubMed: 24350667]
11 (a). Yandulov DV; Schrock RR; Rheingold AL; Ceccarelli C; Davis WM Synthesis and reactions of
molybdenum triamidoamine complexes containing hexaisopropylterphenyl substituents. Inorg.
Chem 2003, 42, 796–813. [PubMed: 12562193] (b)Yandulov DV; Schrock RR Studies relevant to
catalytic reduction of dinitrogen to ammonia by molybdenum triamidoamine complexes. Inorg.
Chem 2005, 44, 1103–1117. [PubMed: 15859292]
12. Kinney RA; McNaughton RL; Chin JM; Schrock RR; Hoffman BM Protonation of the dinitrogen-
reduction catalyst [HIPTN3N]MoIII investigated by ENDOR spectroscopy. Inorg. Chem 2011, 50,
418–420. [PubMed: 21155580]
13. Anderson JS; Cutsail GE; Rittle J; Connor BA; Gunderson W; Zhang L; Hoffman BM; Peters JC
Characterization of an Fe
N–NH
2
intermediate relevant to catalytic N
2
reduction to NH
3
. J. Am.
Chem. Soc 2015, 137, 7803–7809. [PubMed: 26000443]
14. (a)Other molecular Fe(H)x complexes undergo photoinduced reductive H2 elimination with
associated N2 binding. See: Sacco A; Aresta M Nitrogen fixation: Hydrido- and hydrido-nitrogen-
complexes of Iron(II). Chem. Commun 1968, 1223–1224.(b)Whittlesey MK; Mawby RJ; Osman
R; Perutz RN; Field LD; Wilkinson MP; George MW Transient and matrix photochemistry of
Fe(dmpe)2H2 (dmpe = Me2PCH2CH2Me2): dynamics of C-H and H-H activation J. Am. Chem.
Soc 1993, 115, 8627–8637.(c)Perutz RN; Procacci B Photochemistry of Transition Metal
Hydrides, Chem. Rev 2016, 116, 8506–8544. [PubMed: 27380829]
15. Okuniewski A; Rosiak D; Chojnacki J; Becker B Coordination polymers and molecular structures
among complexes of mercury(II) halides with selected 1-benzoylthioureas. Polyhedron 2015, 90,
47–57.
16. Addison AW; Rao TN; Reedijk J; van Rijn J; Verschoor GC Synthesis, structure, and spectroscopic
properties of copper(II) compounds containing nitrogen-sulphur donor ligands; the crystal and
molecular structure of Aqua[l,7-bis(N-methylbenzimidazol-2’-yl)-2,6-dithiaheptane]copper(II)
Perchlorate. J. Chem. Soc., Dalt. Trans 1984, 0, 1349–1356.
17. Cavaillé A; Joyeux B; Saffon-Merceron N; Nebra N; Fustier-Boutignona M; Mézailles N Triphos–
Fe dinitrogen and dinitrogen–hydride complexes: relevance to catalytic N
2
reductions, Chem.
Commun 2018, 54, 11953–11956.
18. a)Examples of deriving the coupling between metal centers by NMR can be found in Pfirrmann S;
Limberg C; Herwig C; Knispel C; Braun B; Bill E; Stösser R A reduced
β
-diketiminato-ligated
Ni3H4 unit catalyzing H/D Exchange. J. Am. Chem. Soc 2010, 132, 13684–13691. [PubMed:
20828129] b) Tepper AWJW; Bubacco L; Canters GW Paramagnetic properties of the halide-
bound derivatives of oxidised tyrosinase investigated by 1H NMR spectroscopy. Chem. Eur. J
2006, 12, 7668–7675. [PubMed: 16927257]
19. Stoian SA; Vela J; Smith JM; Sadique AR; Holland PL; Münck E; Bominaar EL Mössbauer and
computational study of an N
2
-bridged diiron diketiminate complex: parallel alignment of the iron
spins by direct antiferromagnetic exchange with activated dinitrogen. J. Am. Chem. Soc 2006, 128
(31), 10181–10192. [PubMed: 16881648]
20 (a). Hoffman BM; Lukoyanov D; Yang ZY; Dean DR; Seefeldt L C. Mechanism of nitrogen
fixation by nitrogenase: the next stage. Chem. Rev 2014, 114, 4041–4062. [PubMed: 24467365]
(b)Lukoyanov D; Khadka N; Yang ZY; Dean DR; Seefeldt LC; Hoffman BM Reversible
photoinduced reductive elimination of H2 from the nitrogenase dihydride state, the E4(4H) janus
Schild and Peters
Page 11
ACS Catal
. Author manuscript; available in PMC 2020 May 03.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript