ARTICLE
Received 21 Jan 2016 | Accepted 8 Jun 2016 | Published 20 Jul 2016
Catalytic transformation of dinitrogen into
ammonia and hydrazine by iron-dinitrogen
complexes bearing pincer ligand
Shogo Kuriyama
1
, Kazuya Arashiba
1
, Kazunari Nakajima
1
, Yuki Matsuo
2
, Hiromasa Tanaka
2
, Kazuyuki Ishii
3
,
Kazunari Yoshizawa
2,4
& Yoshiaki Nishibayashi
1
Synthesis and reactivity of iron-dinitrogen complexes have been extensively studied, because
the iron atom plays an important role in the industrial and biological nitrogen fixation. As a
result, iron-catalyzed reduction of molecular dinitrogen into ammonia has recently been
achieved. Here we show that an iron-dinitrogen complex bearing an anionic PNP-pincer ligand
works as an effective catalyst towards the catalytic nitrogen fixation, where a mixture of
ammonia and hydrazine is produced. In the present reaction system, molecular dinitrogen is
catalytically and directly converted into hydrazine by using transition metal-dinitrogen
complexes as catalysts. Because hydrazine is considered as a key intermediate in the nitrogen
fixation in nitrogenase, the findings described in this paper provide an opportunity to
elucidate the reaction mechanism in nitrogenase.
DOI: 10.1038/ncomms12181
OPEN
1
Department of Systems Innovation, School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan.
2
Institute for Materials Chemistry
and Engineering and International Research Center for Molecular System, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan.
3
Institute of Industrial
Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan.
4
Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto
University, Nishikyo-ku, Kyoto 615-8520, Japan. Correspondence and requests for materials should be addressed to K.Y. (email: kazunari@ms.ifoc.kyushu-
u.ac.jp) or to Y.N. (email: ynishiba@sys.t.u-tokyo.ac.jp).
NATURE COMMUNICATIONS | 7:12181 | DOI: 10.1038/ncomms12181 | www.nature.com/naturecommunications 1
F
rom a viewpoint of the function of molybdenum and iron
atoms in nitrogenase, the development of the catalytic
nitrogen fixation by using molybdenum- and iron-dinitro-
gen complexes as catalysts is one of the most important subjects
in chemistry
1
. After the extensive study on the preparation and
stoichiometric reactivity of various transition metal-dinitrogen
complexes
2–9
, the molybdenum-catalyzed nitrogen fixation by
using molybdenum-dinitrogen complexes as catalysts under
ambient reaction conditions has been achieved by Schrock and
co-workers
10,11
and our research group
12–15
. More recently, we
have found the most efficient catalytic nitrogen fixation system by
using molybdenum-nitride complexes bearing a tridentate
triphosphine (PPP ¼ bis(di-tert-butylphosphinoethyl)phenylpho-
sphine) as a ligand, where up to 63 equiv of ammonia were
produced based on the catalyst
16
.
In addition to the molybdenum-catalyzed nitrogen fixation, the
iron-catalyzed reduction of molecular dinitrogen by using iron
complexes under mild reaction conditions has recently been
achieved because iron-catalyzed nitrogen fixation has also
attracted attention from a viewpoint of the industrial nitrogen
fixation (the Haber–Bosch process)
17
. In 2012, we found the first
successful example of the iron-catalyzed reduction of molecular
dinitrogen under ambient reaction conditions, where simple iron
complexes such as [Fe(CO)
5
] and ferrocene derivatives worked as
effective catalysts towards the formation of silylamine as an
ammonia equivalent (up to 34 equiv based on the catalyst)
18
.In
2013, Peters and co-workers
19
reported the iron-catalyzed direct
reduction of molecular dinitriogen into ammonia under mild
reaction conditions (1 atm at 78 C), where a sophisticated
iron-dinitrogen complex bearing a triphosphine-borane as a
ligand worked as a catalyst (up to 7 equiv of ammonia based on
the catalyst). More recently, Peters and co-workers
20,21
have
found the other iron-catalyzed nitrogen fixation system by using
iron-complexes bearing a triphosphinealkyl ligand and two cyclic
carbene ligands, where up to 4.6 equiv and 3.4 equiv of ammonia
were produced based on the catalyst, respectively. However, the
detailed reaction pathway has not yet been reported in all the
iron-catalyzed nitrogen fixation systems
22
.
Based on our previous findings of the unique catalytic activity
of molybdenum complexes bearing mer-tridentate ligands such as
PNP
0
-pincer ligands (PNP
0
¼ 2,6-bis(di- tert-butylphosphino-
methyl)pyridine)
12–15
and PPP
16
ligand, we have designed iron-
dinitrogen complexes bearing PNP
0
-pincer and PPP ligands as
catalysts towards the iron-catalyzed nitrogen fixation. Although
we have not yet succeeded in preparing the corresponding iron-
dinitrogen complexes bearing PNP
0
-pincer and PPP ligands, we
have been successful to prepare a similar iron-dinitrogen complex
bearing an anionic PNP-pincer ligand (PNP ¼ 2,5-bis(di-tert-
butylphosphinomethyl)pyrrolide)
23–25
([Fe(N
2
)(PNP)]: 1). As a
result, an iron-dinitrogen complex as well as its precursors iron-
hydride and -methyl complexes have been found to work as
effective catalysts towards the catalytic nitrogen fixation under
mild reaction conditions. Interestingly, a mixture of ammonia
and hydrazine was obtained as nitrogenous products in the
present reaction system. Herein, we report the catalytic reduction
of molecular dinitrogen into ammonia and hydrazine by using
the iron complexes bearing an anionic PNP-pincer ligand as
catalysts.
Results
Preparation and characterization of iron complexes.The
reaction of [FeCl
2
(thf)
1.5
] with lithium 2,5-bis(di-tert-butylpho-
sphinomethyl)pyrrolide, generated from 2,5-bis(di-tert-butylpho-
sphinomethyl)pyrrole and
n
BuLi, in toluene at room temperature
for 14 h gave an iron-chloride complex bearing PNP ligand,
[FeCl(PNP)], (2) in 85% yield (Fig. 1). Reduction of 2 with 1.1
equiv of KC
8
as a reductant in tetrahydrofuran (THF) at room
temperature for 13 h under an atmospheric pressure of dinitrogen
afforded a paramagnetic iron(I)-dinitrogen complex 1 in 68% yield.
Molecular structures of 1 and 2 were confirmed by X-ray analysis.
ORTEP drawings of 1 and 2 are shown in Fig. 2a,b. Crystal
structures of both 1 and 2 have a distorted square-planar geometry
around the iron atom (the geometry index t
4
¼ 0.13 and t
4
¼ 0.11,
respectively), where t
4
¼ 0.00 for a perfect square-planar and
t
4
¼ 1.00 for a tetrahedral geometry
26
. A dinitrogen ligand
coordinates to the iron atom in a terminal fashion with the
Fe–N distance of 1.764(2)Å and the N–N distance of 1.134(2) Å.
To our knowledge, only a few examples of square-planar iron
complexes bearing a terminal dinitrogen ligand, except for iron-
dinitrogen complexes bearing a 2,6-bis(imine)pyridine ligand
27–29
,
have been reported until now.
The infrared (IR) spectrum of 1 in solid state (KBr) shows a
strong v
NN
band at 1,964 cm
1
assignable to the terminal
dinitrogen ligand. The complex 1 in a THF solution shows a v
NN
band at 1,966 cm
1
, which is similar to that of 1 in a solid state.
Cyclic voltammetry of 1 in THF with [N
n
Bu
4
]PF
6
as a supporting
electrolyte revealed an irreversible reduction at 2.9 V versus
ferrocene
0/ þ
and an irreversible oxidation at 0.9 V
(Supplementary Fig. 1). The reduction and oxidation can be
assignable to Fe(0/I) and Fe(I/II) respectively. Electron para-
magnetic resonance (EPR) measurements were carried out at 10
N
Fe
P
t
Bu
2
P
t
Bu
2
CH
3
N
Fe
P
t
Bu
2
P
t
Bu
2
H
3
4
N
Fe
P
t
Bu
2
P
t
Bu
2
Cl N
Fe
P
t
Bu
2
P
t
Bu
2
N
12
KC
8
, N
2
(1 atm)
THF, rt, 13 h
Et
2
O, rt, 1 h
THF, rt, 1 h
MeMgCl
KBHEt
3
1) [H(OEt
2
)
2
]BAr
F
4
(− CH
4
)
2) KC
8
, N
2
(1 atm), Et
2
O, rt
1) [H(OEt
2
)
2
]BAr
F
4
(− H
2
)
2) KC
8
, N
2
(1 atm), Et
2
O, rt
N
N
Li
P
t
Bu
2
P
t
Bu
2
[FeCl
2
(thf)
1.5
]
Toluene
rt, 14 h
Figure 1 | Synthesis and reactivity of iron complexes. The reaction of iron(II) chloride with PNP-Li afforded 2. Reduction of 2 under N
2
atmosphere gave 1.
Reactions of 2 with KBHEt
3
and MeMgCl afforded 3 and 4, respectively. The complexes 3 and 4 were converted into 1 on protonation and reduction under
N
2
atmosphere.
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12181
2 NATURE COMMUNICATIONS | 7:12181 | DOI: 10.1038/ ncomms12181 | www.nature.com/naturecommunications
and 296 K in toluene to investigate the spin state of the complex
1. The complex 1 shows a typical single EPR signal at room
temperature, which is attributable to an S ¼ 1/2 system (Fig. 3).
The g value (g ¼ 2.25) of 1 is largely deviated from that (2.0023)
of a free electron. The width between extreme slope (140 G) of
this EPR signal is much broader than that of conventional organic
radicals, and therefore, any hyperfine structures could not be
seen. The deviation of the g value and the broad bandwidth are
characteristic features of EPR of metallocomplexes. In the EPR
spectrum of 1 at 10 K (Supplementary Fig. 2), anisotropic EPR
signals are observed at around g ¼ 2, but no EPR signal is seen at
around g ¼ 4, suggesting S ¼ 1/2. Reproducible EPR signals at
g ¼ 2.6 and 2.2 are attributable to the EPR of 1, which are similar
to the previous EPR spectra observed for the low-spin iron(I)
complexes with a square-planar geometry
30
. The complex 1 has a
solution magnetic moment of 3.0
±
0.2 m
B
at 298 K. The measured
magnetic moment is larger than spin-only value for an S ¼ 1/2
spin state (1.73 m
B
), but still within the range of the reported low-
spin square-planar iron(I) compounds
30,31
. We consider that the
large shift of the magnetic moment of 1 may be a result of the
spin-orbit coupling.
Density functional theory (DFT) calculations at the B3LYP-D3
level of theory
32
have been carried out to discuss the ground spin
state structure of 1. Optimized structures of 1 in the doublet and
quartet states are depicted in Fig. 4, together with their selected
geometric parameters. The result of the B3LYP-D3 calculations
indicates that the ground spin state of 1 is doublet and the quartet
state lies above 9.2 kcal mol
1
. The Fe–N
2
and N–N distances are
calculated to be 1.779 and 1.135 Å in the doublet state and 1.987
and 1.127 Å in the quartet state, respectively, the former of which
are close to those in the crystal structure of 1 shown in Fig. 2a.
The geometry index t
4
in the doublet state (0.15) well reproduces
a slightly distorted square-planar geometry around the iron atom
in the crystal structure, while the quartet state structure has a
larger value of t
4
(0.41). All the results strongly support the
experimental finding that the ground spin state of 1 is doublet.
Reactions of 2 with KBHEt
3
in THF and MeMgCl in Et
2
Oat
room temperature for 1 h afforded paramagnetic iron(II)-hydride
and -methyl complexes, [FeH(PNP)] (3) and [FeMe(PNP)] (4), in
62 and 81% yields, respectively (Fig. 1). Molecular structures
of 3 and 4 were confirmed by X-ray analysis. ORTEP drawings
of 3 and 4 are shown in Fig. 2c,d. Crystal structures of both 3 and
4 have a distorted square-planar geometry around the iron atom
(t
4
¼ 0.11 and t
4
¼ 0.12, respectively). The iron(II) complexes
2–4 have solution magnetic moments of 3.7
±
0.2, 3.6
±
0.2 and
3.8
±
0.2 m
B
at 296 K, respectively. The measured magnetic
moments are larger than spin-only value for an S ¼ 1 spin state
(2.83 m
B
), but still within the range of the reported intermediate-
spin square-planar iron(II) compounds
33–35
. When the magnetic
moments and the square-planar structures are taken into account,
P2
P1
P1
P2
N1
Fe1
H43
Fe1
Cl1
N1
N1
P1
P2
Fe1
C23
P2
P1
N1
Fe1
N2
N3
a
b
cd
Figure 2 | ORTEP drawings of the iron complexes. (a) chloride complex 2,
(b) dinitrogen complex 1,(c) hydride complex 3 and (d) methyl complex 4.
Thermal ellipsoids are shown at the 50% level. Hydrogen atoms except for
H43 in 3 are omitted for clarity.
2,000 2,500 3,000 3,500 4,000
Ma
g
netic field (G)
Figure 3 | X-band EPR spectra of 1. The spectrum collected at room
temperature in a toluene solution at a microwave frequency 9.44 GHz (red)
and the simulated EPR spectrum of 1 (black).
1.779
1.135
1.987
1.127
N(Py)–Fe–N = 176.3°
P–Fe–P = 162.9°
N(Py)–Fe–N = 147.7°
P–Fe–P = 154.4°
1 (doublet) 1 (quartet)
a
b
IV (open-shell sin
g
let)
N(Py)–Fe–N = 176.9°
P–Fe–P = 162.5°
1.767
1.165
Figure 4 | Optimized structures of 1 and IV. (a) The structures of 1 in the
doublet and quartet states and (b) the structure of IV in the open-shell
singlet state. Bond distances are presented in Å. Hydrogen atoms are
omitted for clarity.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12181 ARTICLE
NATURE COMMUNICATIONS | 7:12181 | DOI: 10.1038/ncomms12181 | www.nature.com/naturecommunications 3
the complexes 2–4 can be assigned as intermediate-spin S ¼ 1
states. We consider that the spin-orbit coupling may contribute
the large magnetic moments of 2–4.
Reactivity of iron complexes. At first, the catalytic reaction was
carried out by using 1 as a catalyst under our reaction conditions,
where CoCp
2
and 2,6-lutidinium trifluoromethanesulfonate
([LutH]OTf) were used as a reductant and a proton source at
room temperature
12–15
. However, no formation of ammonia was
observed at all. Then, we investigated the catalytic reaction under
the reaction conditions previously reported by Peters and co-
workers
19–22
. Typical results are shown in Table 1. The reaction
of an atmospheric pressure of dinitrogen with KC
8
(40 equiv to 1)
as a reductant and [H(OEt
2
)
2
]BAr
F
4
(38 equiv to 1;Ar
F
¼ 3,
5-bis(trifluoromethyl)phenyl) as a proton source in the presence
of 1 as a catalyst in Et
2
Oat 78 C for 1 h gave 4.4 equiv of
ammonia and 0.2 equiv of hydrazine based on the iron atom of
the catalyst, respectively (Table 1, run 1). When the reaction was
carried out at room temperature, the formation of ammonia and
hydrazine was not observed, but only molecular dihydrogen
(5.2 equiv) was produced. The use of larger amounts of both
reductant and proton source increased the amounts of both
ammonia and hydrazine, up to 10.9 equiv and 1.6 equiv,
respectively (Table 1, runs 2 and 3). The largest amounts of
ammonia and hydrazine (14.3 equiv of ammonia and 1.8 equiv of
hydrazine) were obtained by using 200 equiv of KC
8
and 184
equiv of [H(OEt
2
)
2
]BAr
F
4
under the same reaction conditions
(Table 1, run 4). Separately, we confirmed the direct conversion
of molecular dinitrogen into ammonia and hydrazine using
15
N
2
gas instead of
14
N
2
gas. After the catalytic reaction, we could
not identify any iron complexes and only the formation of free
PNP-H was observed by nuclear magnetic resonance (NMR).
The ratio of ammonia to hydrazine depends on the nature of
solvents. When THF was used in place of Et
2
O, hydrazine was
produced as a major product together with ammonia as a minor
product based on the fixed N atom, where up to 2.4 equiv of
hydrazine and 2.9 equiv of ammonia were produced based on the
catalyst (Table 1, runs 5 and 6). Iron- and other transition
metal-dinitrogen complexes have been reported to produce a
stoichiometric amount of hydrazine on treatment of acids and
reductants
36–39
. This result shows that the dinitrogen complex 1
works as a catalyst for the formation of hydrazine directly from
dinitrogen. The use of much larger amounts of both reductant and
proton source did not increase the amounts of both hydrazine and
ammonia (Table 1, run 7). The formation of ammonia and
hydrazine was not observed from the reaction in Et
2
Oat 78 Cin
the absence of iron-complexes as catalysts (Table 1, runs 8 and 9).
Interestingly, iron-hydride and -methyl complexes 3 and 4
also worked as effective catalysts under the same reaction
conditions, where 3.0 equiv and 3.7 equiv of ammonia were
produced based on the iron atom of the catalyst, respectively
(Table 1, runs 10 and 11)
19,20,40
. We consider that the unique
reactivity of iron-hydride complex 3 provides useful information
to consider the reaction mechanism of nitrogenase because
iron-hydride complexes are reported to play an important
role as a key reactive intermediate in the catalytic cycle of
nitrogenase
1,41
. Separately, we confirmed the protonation of
3 and 4 with 1 equiv of [H(OEt
2
)
2
]BAr
F
4
in Et
2
O at room
temperature and then the addition of 1 equiv of KC
8
under
N
2
(1 atm) gave 1 together with molecular dihydrogen and
methane, respectively. These results indicate that 3 and 4 are easily
converted into 1 under the catalytic reaction conditions. Schrock
and a coworker previously reported a similar phenomenon that
protonation and reduction of a molybdenum-hydride complex
bearing a triamideamine ligand under N
2
(1 atm) gave the
corresponding molybdenum-dinitrogen complex, which was
worked as a catalyst for the formation of ammonia from
molecular dinitrogen
42
. After the submission of the manuscript,
Peters and co-workers
43
have reported that an iron-hydride
complex bearing a triphosphine-borane ligand was identified to
be catalytically competent when it was solubilized, and also was
identified to be a catalyst resting state, although Peters and
co-workers reported that iron-hydride complexes did not work as
catalysts in the previous papers
19,20,40
. On the other hand, only a
stoichiometric amount of ammonia was formed when 2 was used
as a catalyst (Table 1, run 12).
Table 1 | Iron-catalyzed reduction of dinitrogen to ammonia and hydrazine*.
N
2
1 atmðÞ
þ
KC
8
40 equivðÞ
þ
H OEt
2
ðÞ
2
BAr
F
4
38 equivðÞ
!
catalyst
solvent
78 C; 1 h
NH
3
þ NH
2
NH
2
Run Catalyst Solvent NH
3
(equiv)
w
NH
2
NH
2
(equiv)
w
fixed N atom (equiv)
z
1
y
1 Et
2
O 4.4
±
0.2 0.2
±
0.2 4.8
2
||
1 Et
2
O 6.7 0.8 8.3
3
z
1 Et
2
O 10.9
±
0.4 1.6
±
0.2 14.1
4
#
1 Et
2
O 14.3
±
0.4 1.8
±
0.2 17.9
5 1 THF 1.9
±
0.4 1.4
±
0.7 4.7
6
||
1 THF 2.9
±
0.2 2.4
±
0.1 7.7
7
z
1 THF 1.6 0.8 3.2
8—
**
Et
2
O00 0
9 PNP-H
ww
Et
2
O o0.1 0 0
10 3 Et
2
O 3.0
±
0.9 0.1
±
0.1 3.2
11 4 Et
2
O 3.7
±
0.5 o0.1 3.7
12 2 Et
2
O 1.1
±
0.6 0 1.1
13 5 Et
2
O 2.6
±
0.2 o0.1 2.7
*A mixture of a catalyst (0.010 mmol), KC
8
(0.40 mmol, 40 equiv based on the catalyst), and [H(OEt
2
)
2
]BAr
F
4
(0.38 mmol, 38 equiv based on the catalyst) was stirred in solvent at 78 C for 1 h under
1 atm of N
2
and then at room temperature for 20 min.
wEquiv based on the iron atom of a catalyst. Average of multiple runs (42 times) are shown unless otherwise stated.
zFixed N atom (equiv) ¼ [NH
3
(equiv)] þ 2[NH
2
NH
2
(equiv)]. Equiv based on the iron atom of a catalyst.
yAverage of 5 runs are shown.
||80 equiv of KC
8
and 76 equiv of [H(OEt
2
)
2
]BAr
F
4
were used.
z160 equiv of KC
8
and 152 equiv of [H(OEt
2
)
2
]BAr
F
4
were used.
#200 equiv of KC
8
and 184 equiv of [H(OEt
2
)
2
]BAr
F
4
were used.
**In the absence of a catalyst.
wwPNP-H ligand (0.010 mmol) was used as a catalyst.
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12181
4 NATURE COMMUNICATIONS | 7:12181 | DOI: 10.1038/ ncomms12181 | www.nature.com/naturecommunications
The formation of both ammonia and hydrazine is in sharp
contrast to that in the Peters’ reaction systems
19–22
, where no
formation of hydrazine was observed at all when iron-dinitrogen
complexes bearing triphosphine-borane and -alkyl ligands, and
two cyclic carbene ligands were used as catalysts. Separately, we
confirmed that the partial reduction
7,44–47
of hydrazine into
ammonia in the presence of a catalytic amount of 4 proceeded in
Et
2
Oat 78 C for 1 h (Supplementary Table 6). However, the
use of THF in place of Et
2
O relatively inhibited the partial
reduction of hydrazine into ammonia under the same reaction
conditions. These results indicate that some iron-hydrazine
complexes may be involved as key reactive intermediates in the
transformation of hydrazine into ammonia. We consider that the
result described in the present manuscript provides useful
information on the elucidation of the reaction mechanism in
nitrogenase because hydrazine may be formed as a key reactive
intermediate in the biological nitrogen fixation
1
.
Discussion on the catalytic reaction pathway. Based on the
results of experimental and DFT calculations on the molybde-
num-catalyzed nitrogen fixation under mild reaction conditions,
we proposed that the transformation of molecular dinitrogen into
ammonia under mild reaction conditions proceeds via hydrazide
and nitride complexes (B and C, respectively) as key reactive
intermediates as shown in Fig. 5a as a distal pathway
12–16
.
However, the formation of hydrazine is not possible from a distal
pathway. To explain the direct formation of hydrazine from
molecular dinitrogen in the present iron system, we now propose
an alternating pathway where the catalytic reaction proceeds via
hydrazine complex (E) as a key reactive intermediate as shown in
Fig. 5b
19,22
. In fact, the alternating pathway has been proposed
for the stoichiometric formation of ammonia and hydrazine from
iron-dinitrogen complexes based on the reactivity of isolated and
generated intermediates
36,37
. In this alternating reaction pathway,
dinitrogen complex (A) is converted into hydrazine complex (E)
via sequential protonation and reduction. Hydrazine is formed by
the ligand exchange of the coordinated hydrazine for molecular
dinitrogen to regenerate the starting complex A. On the other
hand, further protonation and reduction of hydrazine complex E
give ammonia together with amide complex (F). Then, amide
complex F is transformed into ammonia complex (D). Finally,
the ligand exchange of the coordinated ammonia for molecular
dinitrogen regenerates the starting complex A. As presented in
the previous section, the ratio of ammonia to hydrazine depends
on the nature of solvents. The ligand exchange of the coordinated
hydrazine for molecular dinitrogen might proceed more smoothly
in THF as solvent to give hydrazine as a major product. At the
present stage, however, we can not exclude the possibility of
ammonia formation via the distal reaction pathway. In addition,
reduction of dinitrogen via a hybrid of the distal and the
alternating pathways, where the hydrazide complex B was
converted into hydrazine complex E on protonation and
reduction, is also possible
48
.
To obtain information on reactive species in the present
catalytic reaction by using 1, we carried out the protonation of 1
under the following reaction conditions. The protonation of iron-
dinitrogen complex 1 with 1 equiv of [H(OEt
2
)
2
]BAr
F
4
in Et
2
Oat
room temperature for 10 min gave the corresponding protonated
complex (5) in 90% yield (Fig. 6). A v
NN
peak at 2,034 cm
1
assignable to the terminal dinitrogen ligand appeared at the IR
spectrum of the protonated complex in solid state (KBr) although
no v
NH
peak was observed. Based on the experimental result, we
characterized 5 as iron-dinitrogen complex including the
protonated pyrrole ring of PNP ligand. The result reveals that
the first protonation at 1 may occur not at the coordinated
dinitrogen ligand but at the pyrrole moiety in 1 (ref. 49). When
we attempted to reduce 5 with 1 equiv of KC
8
as a reductant in
Et
2
O at room temperature for 10 min, a mixture of 1 and free
PNP-H was observed in 38 and 27% yields by
1
H NMR,
respectively, together with the formation of 3 in 3% yield.
Separately, we confirmed that the protonated iron-dinitrogen
complex 5 has also a slightly lower catalytic activity towards the
nitrogen fixation than 1 (Table 1, run 13). These experimental
results indicate that the protonated iron-dinitrogen complex 5 is
considered to be one of deactivated species in the present catalytic
reaction.
DFT calculations on the reactivity of iron-dinitrogen complexes.
To get further information on the reaction pathway, we have
carried out DFT calculations on the first protonation of 1 with
H
þ
(OEt
2
)
2
, according to the experimental result shown in Fig. 6.
M NH
2
NH
2
Hydrazine complex (E)
Amide complex (F)
M
NH
2
N
2
NH
2
NH
2
M
N N
M
N
Dinitrogen complex (A)
Hydrazide complex (B)
M
NH
3
Nitride complex (C)
M
N
N
2
NH
3
NH
3
Ammonia complex (D)
2 H
+
, 2 e
–
H
+
, e
–
3 H
+
, 3 e
–
NH
2
Distal pathway
Alternating pathway
M
N N
Dinitrogen complex (A)
M
NH
3
N
2
NH
3
NH
3
Ammonia complex (D)
4 H
+
, 4 e
–
H
+
, e
–
H
+
, e
–
a
b
Figure 5 | Reaction pathway for formation of ammonia and hydrazine from molecular dinitrogen by using transition metal-dinitrogen complexes as
catalysts. (a) Distal pathway and (b) alternating pathway.
N
Fe
P
t
Bu
2
P
t
Bu
2
N
N
Fe
P
t
Bu
2
P
t
Bu
2
N
H
H
BAr
F
4
Et
2
O, rt, 10 min
5, ν
NN
= 2,034 cm
–1
[H(OEt
2
)
2
]BAr
F
4
N
N
1, ν
NN
= 1,964 cm
–1
Figure 6 | Reactivity of iron-dinitrogen complex 1. Protonation of 1 with
[H(OEt
2
)
2
]BAr
F
4
occurred at the pyrrole ring to give 5.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12181 ARTICLE
NATURE COMMUNICATIONS | 7:12181 | DOI: 10.1038/ncomms12181 | www.nature.com/naturecommunications 5