Ammonia Formation Catalyzed by a Dinitrogen-Bridged Dirhenium Complex Bearing PNP-Pincer Ligands under Mild Reaction Conditions**

Article abstract of DOI:10.1002/anie.202102175

A series of rhenium complexes bearing a pyridine-based PNP-type pincer ligand are synthesized from rhenium phosphine complexes as precursors. A dinitrogen-bridged dirhenium complex bearing the PNP-type pincer ligands catalytically converts dinitrogen into ammonia during the reaction with KC₈ as a reductant and [HPCy₃]BAr^F₄ (Cy=cyclohexyl, Ar^F=3,5-(CF₃)₂C₆H₃) as a proton source at ?78 °C to afford 8.4 equiv of ammonia based on the rhenium atom of the catalyst. The rhenium-dinitrogen complex also catalyzes silylation of dinitrogen in the reaction with KC₈ as a reductant and Me₃SiCl as a silylating reagent under ambient reaction conditions to afford 11.7 equiv of tris(trimethylsilyl)amine based on the rhenium atom of the catalyst. These results demonstrate the first successful example of catalytic nitrogen fixation under mild reaction conditions using rhenium-dinitrogen complexes as catalysts.

Full text of DOI:10.1002/anie.202102175

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How to cite:
International Edition: doi.org/10.1002/anie.202102175
German Edition: doi.org/10.1002/ange.202102175
Homogeneous Catalysis
Ammonia Formation Catalyzed by a Dinitrogen-Bridged Dirhenium
Complex Bearing PNP-Pincer Ligands under Mild Reaction
Conditions**
Fanqiang Meng, Shogo Kuriyama, Hiromasa Tanaka, Akihito Egi, Kazunari Yoshizawa,* and
Yoshiaki Nishibayashi*
Abstract: A series of rhenium complexes bearing a pyridine-
based PNP-type pincer ligand are synthesized from rhenium
phosphine complexes as precursors. A dinitrogen-bridged
dirhenium complex bearing the PNP-type pincer ligands
catalytically converts dinitrogen into ammonia during the
During the last decade, substantial progress has been
made in the study on the preparation and reactivity of
rhenium-dinitrogen complexes.[18–21] The typical examples are
as follows. An anionic rhenium(I) complex reduced dinitro-
gen to form a rhenium(III)-silyldiazenide complex upon
silylation.[18] Rhenium(II) species cleaved the N-N triple bond
of dinitrogen to afford the corresponding rhenium(V)-nitride
complexes, which were converted into nitrogen-containing
organic compounds and ammonia.[19,20] Unfortunately, all of
these transformations are stoichiometric reactions that use
rhenium-dinitrogen complexes. Compared to stoichiometric
reactions, until now, there have been no successful examples
of catalytic reduction of dinitrogen using rhenium-dinitrogen
complexes as catalysts.
In 2011, we reported that a dinitrogen-bridged dimolyb-
denum complex bearing PNP-type pincer ligands [Mo(N2)2-
(PNP)]2(m-N2) (1; PNP = 2,6-bis(di-tert-butylphosphinome-
thyl)pyridine) worked as an effective catalyst for the forma-
tion of ammonia from dinitrogen under ambient reaction
conditions (Scheme 1).[5b,22] Considering that Mo0 and ReI
adopt a d6 configuration, we speculated that a dinitrogen-
bridged dirhenium(I) complex bearing the same PNP-type
pincer ligands [ReCl(N2)(PNP)]2(m-N2) (2) would have cata-
lytic activity toward the formation of ammonia from dini-
trogen (Scheme 1). Herein, we report the preparation of 2 and
its catalytic activity for the reduction of dinitrogen into
ammonia and tris(trimethylsilyl)amine [N(SiMe3)3] under
mild reaction conditions.
reaction with KC8 as a reductant and [HPCy3]BArF (Cy =
4
cyclohexyl, ArF = 3,5-(CF3)2C6H3) as a proton source at
À788C to afford 8.4 equiv of ammonia based on the rhenium
atom of the catalyst. The rhenium-dinitrogen complex also
catalyzes silylation of dinitrogen in the reaction with KC8 as
a reductant and Me3SiCl as a silylating reagent under ambient
reaction conditions to afford 11.7 equiv of tris(trimethylsilyl)-
amine based on the rhenium atom of the catalyst. These results
demonstrate the first successful example of catalytic nitrogen
fixation under mild reaction conditions using rhenium-dini-
trogen complexes as catalysts.
T
he reduction of dinitrogen into ammonia and its equiv-
alents under ambient reaction conditions has been a challeng-
ing task in chemistry. Recently, considerable attention has
been paid to the development of homogeneous molecular
catalysis that is capable of nitrogen fixation under mild
reaction conditions.[1,2] Currently, well-defined transition
metal complexes (e.g., Ti-, V-, Mo-, Fe-, Ru-, Os-, and Co-
complexes) worked as effective catalysts for the transforma-
tion of dinitrogen into ammonia and/or hydrazine under mild
reaction conditions.[3–8] Furthermore, the catalytic reduction
of dinitrogen into silylamines was achieved using Ti-, V-, Cr-,
Mo-, W-, Mn-, Fe-, Co-, Rh-, Ni-, and U-complexes as
catalysts.[9–17]
The reaction of [ReCl3(PPh3)2(MeCN)] with 1 equiv of
PNP-pincer ligand in THF at 808C for 4 h gave a rhenium-
[*] F. Meng, Dr. S. Kuriyama, Prof. Dr. Y. Nishibayashi
Department of Applied Chemistry, School of Engineering
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: ynishiba@g.ecc.u-tokyo.ac.jp
Prof. Dr. H. Tanaka
School of Liberal Arts and Sciences, Daido University
Minami-ku, Nagoya 457-8530 (Japan)
A. Egi, Prof. Dr. K. Yoshizawa
Institute for Materials Chemistry and Engineering
Kyushu University, Nishi-ku, Fukuoka 819-0395 (Japan)
E-mail: kazunari@ms.ifoc.kyushu-u.ac.jp
[**] A previous version of this manuscript has been deposited on
a preprint server (http://doi.org/10.26434/chemrxiv.12162270.v1).
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202102175.
Scheme 1. Catalytic formation of ammonia from dinitrogen using
dinitrogen-bridged d6 metal complexes bearing PNP-type pincer
ligands.
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fragments are bridged by one end-on dinitrogen ligand and
twisted around the Re-N-N-Re axis with each other. The
dihedral angle between the planes defined by P1-Re1-P1*-N1
and P1**-Re1*-P1***-N1* of 2 is 80.79(3)8, which is larger
than that of 1 [61.41(9)8].[5b]
1
A H NMR spectrum of 2 exhibits peaks consistent with
its dinuclear structure in paramagnetic region, suggesting
paramagnetic nature of 2. The measurement of magnetic
susceptibility of 2 at variable temperature (5 K to 300 K) may
suggest the TIP behavior as observed in 3 and other reported
rhenium complexes (Figure S2).[19c,d,20,24,25]
The IR spectrum of a solid sample in KBr of 2 (Figure 2)
exhibits two strong N2 stretching absorptions (1888 and
1949 cmÀ1) and a weak N2 stretching absorption (1993 cmÀ1).
We consider that these three absorptions of 2 are derived
from conformers of 2 in the crystals. The crystallographic data
Scheme 2. Synthesis of rhenium complexes bearing the PNP-pincer
ligand.
trichloride complex bearing the PNP-type ligand [ReCl3-
(PNP)] (3) in 98% yield (Scheme 2). We confirmed the
molecular structure of 3 by X-ray analysis, and an ORTEP
drawing of 3 is shown in Figure 1a.[23] Complex 3 shows
Figure 2. IR spectra of 2 in KBr (black) and THF (gray). Absorptions
observed around 1930 and 2030 cmÀ1 were machinery noise.
of 2 indicates that the two conformers coexist in the crystals of
2 due to the disorder of the terminal dinitrogen (N3-N4) and
the chloride ligands (Cl1), which is caused by the rotation of
[ReCl(N2)(PNP)] units about the bridging dinitrogen ligand
(N2-N2*). Because the dihedral angles between the planes
defined by N2-Re1-N3 and N2*-Re1*-N3* of the two isomers
are different, each isomer is expected to have two N2
stretching absorptions (symmetric and asymmetric stretches),
like a similar dirhenium-dinitrogen complex [Re(N2)(NNN)]2
Figure 1. ORTEP drawings of 3 (a) and 2 (b). Ellipsoids are set at 50%
probability. Hydrogen atoms are omitted for clarity.
a strongly shifted signal at À1587 ppm in a 31P{1H} NMR
spectrum. The solution magnetic moment of 3 is estimated to
be 1.5 mB by Evansꢀ method, which is much smaller than
a spin-only value for the triplet state. These observations
indicate that an energetically well-separated ground state
causes temperature-independent paramagnetism (TIP). The
magnetic susceptibility measurement of solid-state in the
temperature range from 300 K to 5 K also suggests the TIP
behavior (Supporting Information, Figure S1). Large contri-
butions from TIP are often observed in various Re complexes
including a ReIII state.[19c,d,20,24]
The reaction of 3 with 2 equiv of KHBEt3 in THF at room
temperature for 8 h under an atmospheric pressure of
dinitrogen afforded the dinitrogen-bridged dirhenium(I)
complex 2 in 26% yield (Scheme 2). The molecular structure
of 2, which was confirmed by X-ray analysis, reveals that each
rhenium center has an octahedral geometry with end-on
dinitrogen and chloride ligands in axial positions.[23] An
ORTEP drawing of 2 is shown in Figure 1b. The two rhenium
(NNN = CH3N(CH2CH2NC6F5)2À)
exhibited
(nNN =
2031 cmÀ1 and 2015 cmÀ1).[26] The partial overlap of absorp-
tions is considered to result in the observed three N2
stretching bands. In fact, Harman and co-workers reported
that a dinitrogen-bridged dirhenium complex [TpRe(CO)2]-
(m-N2) (Tp = hydridotris(pyrazolyl)borate) showed three CO
stretching absorptions derived from the rotational conform-
ers.[27] DFT calculations also suggest that 2 can access two
energetically-degenerated rotational conformers and that the
conformers show different patterns of N2 absorptions in IR
(Figure S20). The IR spectrum in a solution state (THF) of 2
shows only two absorptions of dinitrogen ligands at 1892 cmÀ1
and 1956 cmÀ1 (Figure 2). The Raman spectrum of a THF
solution of 2 exhibits a strong band at 1890 cmÀ1 (Figure S6).
The spectroscopic data in solution suggest that only one of the
two conformers exist in solution. When the solution of 2 in
THF was exposed to 15N2 (1 atm) at 508C for 2 h, no ligand
2
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exchange of the original 14N2 in 2
for 15N2 was observed. This reac-
tivity of 2 is in sharp contrast to
that of 1, which underwent ligand
exchange of coordinated 14N2 for
15N2.[5b] Instead, the reduction of 3
with KHBEt3 under 15N2 atmos-
phere produced 15N2-labeled com-
plex 2-15N2, which shows shifted
dinitrogen stretching absorptions
in IR spectrum as expected (Fig-
ure S8).
To obtain more information on
the properties of 2, we carried out
density functional theory (DFT)
calculations at the B3LYP-D3
level of theory. For geometry opti-
Table 1: Catalytic formation of ammonia from dinitrogen by 2.
[a]
Entry
Reductant
(X)
Proton source
(Y)
Solvent
T
NH3
1[b]
CoCp2 (36)
CoCp*2 (36)
SmI2 (36)
KC8 (400)
CoCp*2 (400)
KC8 (400)
KC8 (800)
KC8 (800)
KC8 (800)
KC8 (800)
KC8 (800)
[LutH]OTf (48)
[ColH]OTf (48)
H2O (36)
toluene
toluene
THF
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
rt
rt
rt
0
0
0.4
2[b]
3[c]
4[d]
[HPCy3]BArF (400)
À788C
À788C
À788C
À788C
À788C
À788C
À788C
À788C
3.9Æ0.2[e]
0.1
4
5[d]
[HPCy3]BArF (400)
4
6[d]
[H(OEt2)2]BArF (400)
3.7Æ0.4[e]
8.4Æ0.9[f]
4.9Æ0.1[e]
7.5Æ0.6[g]
6.0Æ1.2[g]
8.2Æ0.4[e]
4
7[d]
[HPCy3]BArF (800)
4
8[d]
[H(OEt2)2]BArF (800)
4
9[d]
[HPtBu3]BArF (800)
4
10[d]
11[d,h]
[HPnBu3]BArF (800)
Et2O
Et2O
4
[HPCy3]BArF (800)
4
[a] Equiv based on the Re atom of 2. [b] For 20 h. [c] For 18 h. [d] At À788C for 2 h then at room
temperature for 15 h. [e] An average of three runs. [f] An average of four runs. [g] An average of two runs.
[h] Molybdenum complex 1 was used as a catalyst instead of 2.
mization
calculations,
we
employed the SDD basis set for
rhenium and chlorine atoms and
the 6-31G(d) basis set for other
atoms (see Supporting Information for details). Calculated
nNN values are 1963 cmÀ1 and 1970 cmÀ1 for the terminal
dinitrogen ligands, and 2023 cmÀ1 for the bridging dinitrogen
ligands. The asymmetric and symmetric vibrational modes of
the terminal dinitrogen ligands are IR active, while the
symmetric vibrational modes of both the terminal and
bridged dinitrogen ligands are Raman active. Owing to the
low p-accepting ability of Cl ligands trans to the axial N2
ligand,[28] the NPA charge of terminal dinitrogen ligands of 2
(À0.18) is larger than that of 1 (À0.09). Because the NPA
charge of terminal dinitrogen ligand is more negative than
that of the bridging one (À0.02), terminal dinitrogen ligands
are more activated. The bond dissociation free energy for
and 48 equiv of [LutH]OTf (Lut = 2,6-lutidine, OTf =
OSO2CF3) as a proton source in the presence of a catalytic
amount of 2 in toluene at room temperature for 20 h afforded
neither ammonia nor hydrazine (Table 1, entry 1). The use of
the combination of CoCp*2 (Cp* = h5-C5Me5) and [ColH]OTf
(Col = 2,4,6-collidine) did not produce ammonia (Table 1,
entry 2). The use of SmI2-H2O produced only a small amount
of ammonia (Table 1, entry 3). Then, we investigated the
catalytic reaction at low temperature.[3,6a,h] The reaction of N2
(1 atm) with 400 equiv of KC8 and 400 equiv of [HPCy3]BArF
4
(Cy = cyclohexyl, ArF = 3,5-(CF3)2C6H3) in the presence of 2
in Et2O at À788C afforded 3.9 equiv of ammonia based on the
rhenium atom of 2 (Table 1, entry 4). The formation of
hydrazine was not observed in this reaction, while 50 equiv of
dihydrogen were produced as a byproduct. Separately, we
À
a Re N bond between the Re atom and bridging dinitrogen
ligand is sufficiently high (24.7 kcalmolÀ1) to maintain the
dinuclear structure in solution.
confirmed that [NH2NH3]BArF was partially converted into
4
We examined the stoichiometric reactivity of 2 toward the
formation of ammonia. The protonation of 2 with an excess
amount of sulfuric acid in THF at room temperature for 24 h
produced 0.2 equiv of ammonia and no hydrazine based on
the Re atom [Eq. (1)]. The obtained amounts of ammonia
and hydrazine are smaller than those from protonation of
dinitrogen-bridged dimolybdenum complex 1.[29] This result
suggests the lower reactivity of dinitrogen ligands in 2 toward
protonation than that in 1.
ammonia in the presence of KC8 and [HPCy3]BArF without
4
catalyst in Et2O at À788C. This result indicates that hydrazine
can be converted to ammonia under the catalytic reaction
conditions even if hydrazine is produced during the catalytic
reaction.
The use of CoCp*2 (400 equiv/Re) as a reductant instead
of KC8 afforded only a trace amount of ammonia (Table 1,
entry 5). The use of [H(OEt2)2]BArF (400 equiv/Re) pro-
4
duced an amount of ammonia that was comparable to that of
[HPCy3]BArF (Table 1, entry 6). When larger amounts
4
(800 equiv/Re) of the reductants and proton sources were
used, [HPCy3]BArF worked better than [H(OEt2)2]BArF as
4
4
a proton source, giving 8.4 equiv of ammonia based on the Re
atom of 2 (Table 1, entries 7 and 8). Similar amounts of
ammonia were obtained by using [HPtBu3]BArF and
4
[HPnBu3]BArF as weaker and stronger proton sources,
4
Next, the catalytic transformation of dinitrogen into
ammonia using 2 as a catalyst was investigated under our
previous reaction conditions, where dinitrogen-bridged dimo-
lybdenum complex 1 worked as an efficient catalyst toward
the formation of ammonia.[5b,e,f,22] However, the reaction of N2
(1 atm) with 36 equiv of CoCp2 (Cp = h5-C5H5) as a reductant
respectively (Table 1, entries 9 and 10).[3,30] To our knowledge,
rhenium-dinitrogen complex 2 represents the first successful
example of the rhenium-catalyzed reduction of dinitrogen
into ammonia under mild reaction conditions. Additionally,
we confirmed that the formation of ammonia was derived
from dinitrogen using 15N2 gas instead of 14N2 gas and 15N2-
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labeld complex 2-15N2 as a catalyst (see the Supporting
Information for detail).
(OTf) unit through the bridging dinitrogen ligand. Thus, the
ꢀ
structure of the dirhenium core can be described as [Re(
Next, we examined other rhenium complexes as catalysts
for the reduction of dinitrogen under optimized reaction
conditions (Table 2). The use of [ReCl3(PNP)] 3 afforded
more than a stoichiometric amount of ammonia (Table 2,
entry 2), while only 1.2 equiv of ammonia were produced of
the reaction using [ReCl3(PPh3)2(MeCN)] as a catalyst
(Table 2, entry 3). Simple rhenium complexes, such as [ReCl-
(CO)5] and [ReI3], did not exhibit catalytic activity under the
same reaction conditions (Table 2, entries 4 and 5).
N)(PNP)](m-N2)[Re(N2)(OTf)(PNP)]. Interestingly, in the
Re-nitride unit, the nitride ligand occupies the coordination
site opposite to the N atom of PNP. As a result, the dinitrogen
ꢀ
ligand of cis-[Re( N)(N2)(PNP)] bridges to trans-[Re(N2)-
(OTf)(PNP)] in an end-on fashion. We previously reported
dinitrogen-bridged dinuclear molybdenum dinitrogen com-
plexes which have similar cis,trans-structures.[22a,31] The bond
dissociation free energy for Re-N(bridging) bonds is esti-
mated to be at least 27 kcalmolÀ1 at 195 K in Et2O, which is
high enough to maintain the dinuclear structure. These results
suggest that complex 2 is converted into a dinitrogen-bridged
dinuclear rhenium-nitride complex upon successive protona-
tion and reduction during the catalytic reaction.
Table 2: Catalytic formation of ammonia from dinitrogen by rhenium
complexes.[a]
Because the rhenium nitride complex was detected by
mass spectroscopy, we synthesized a mononuclear rhenium-
ꢀ
nitride complex [Re( N)Cl2(PNP)] (4) in 80% yield from the
ꢀ
reaction of [Re( N)Cl2(PPh3)2] with the PNP-type pincer
ligand (Scheme 3).[23] The catalytic reaction using 4 as
[b]
Entry
Cat.
NH3
1
2
3
4
5
6
[ReCl(N2)(PNP)]2(m-N2) (2)
[ReCl3(PNP)] (3)
[ReCl3(PPh3)2(MeCN)]
[ReCl(CO)5]
8.4Æ0.9[c]
3.9Æ0.6[d]
1.2
0.4
0
[ReI3]
[Re( N)Cl2(PNP)] (4)
3.6Æ0.7[d]
ꢀ
[a] A mixture of catalyst (1.0 mmol/Re), KC8 (0.80 mmol, 800 equiv/Re),
and [HPCy3]BArF4 (0.80 mmol, 800 equiv/Re) in Et2O (5 mL) was stirred
at À788C for 2 h then at room temperature for 15 h under 1 atm of N2.
[b] Equiv based on the rhenium atom of the catalyst. [c] An average of
four runs. [d] An average of three runs.
Scheme 3. Synthesis of rhenium-nitride complex 4.
a catalyst afforded 3.6 equiv of ammonia based on the Re
atom of 4 (Table 2, entry 6). The reaction of 4 with KC8
To obtain information on the reaction mechanism, we
investigated the reactivity of 2. No reaction was observed
upon the treatment of 2 with 1 equiv (based on the Re atom)
of [HPCy3]BArF4 in Et2O at À788C for 2 h. According to DFT
calculations, the protonation of the terminal dinitrogen ligand
of 2 by [HPCy3]+ is highly endergonic by 17.2 kcalmolÀ1 at
195 K in Et2O (see Figure S22). Owing to the low reactivity of
2 toward protonation and the weak acidity of [HPCy3]+,[3,30]
(200 equiv/Re) and [HPCy3]BArF (200 equiv/Re) in Et2O at
4
À788C for 2 h and rt for 15 h under Ar atmosphere (1 atm)
gave 0.8 equiv of ammonia based on the Re atom (Scheme 3).
This result indicates that the nitride ligand of 4 was converted
into ammonia under the catalytic conditions.
We consider that the rhenium-catalyzed formation of
ammonia from dinitrogen proceeds via a similar distal
pathway as our previously proposed one for 1.[28] Based on
the stoichiometric reaction of 2 with the reductant and the
acid, we propose that complex 2 maintains its dinitrogen-
bridged dinuclear structure during the catalytic reaction. The
stepwise reduction and protonation will lead to the formation
of a dinuclear nitride complex as an intermediate. The nitride
complex will produce ammonia by multiple reduction and
protonation together with a dinuclear dinitrogen complex. We
do not exclude the possibility of the reaction pathway where
monomeric species works as reactive intermediates. Because
some research groups have reported that the direct cleavage
of the nitrogen–nitrogen triple bond occurs at a dirhenium
structure,[5e,19,20] the formation of ammonia via an N2 splitting
pathway should be considered.
À
we consider that the N H bond formation by the reaction of 2
with [HPCy3]BArF may not proceed smoothly. The cyclic
4
voltammogram of 2 in THF shows an irreversible reduction
0/+
wave at À2.10 V vs. FeCp2 (Figure S11). The reaction of 2
with 1 equiv (based on the Re atom) of KC8 in Et2O at À788C
for 1 h completely consumed 2 to form new species, which
showed nNN bands at 2038 cmÀ1 and 1951 cmÀ1, although we
have not yet identified this species because of its instability.
However, these results suggest that 2 may be reduced during
the catalytic cycle.
When 2 was treated with 10 equiv (based on the Re atom)
of KC8 and 10 equiv of HOTf (based on the Re atom) in Et2O
at À788C for 2 h, a dinuclear-nitride complex bearing the
dinitrogen-bridged dirhenium core was observed by mass
spectrometry (Figure S15). We have carried out DFT calcu-
lations to discuss possible structures of the dirhenium core
(Figure S24). In the most probable candidate of the dirhe-
nium core, a ReIII-nitride unit is connected with a ReI(N2)-
As described herein, rhenium-dinitrogen complex 2
catalyzes the formation of ammonia from dinitrogen only at
low temperature. This result is in sharp contrast to the
catalytic reactivity of molybdenum-dinitrogen complex 1,
where the formation of ammonia from dinitrogen proceeded
4
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Table 3: Catalytic silylation of dinitrogen by rhenium complexes.[a]
even at room temperature. To compare the catalytic activity
of 2 at low temperature, we carried out the catalytic reaction
at À788C using 1 as a catalyst to afford 8.2 equiv of ammonia
based on the Mo atom of 1 (Table 1, entry 11). This amount of
ammonia produced by 1 is comparable to that produced by 2.
We would like to discuss the difference in the catalytic
activity of dimolybdenum complex 1 and dirhenium complex
2 on the basis of the reactivity toward the protonation of
coordinated dinitrogen. As described in the former section,
the protonation of 2 by [HPCy3]+ is highly endergonic.
Figure 3 shows the MO energies of HOMOÀ1, HOMO, and
[b]
Entry
Cat.
NH3
1
2
3
4
5
6
7
[ReCl(N2)(PNP)]2(m-N2) (2)
[ReCl3(PNP)] (3)
11.7Æ1.0[c] (10.3[d])
5.7Æ0.3[e]
ꢀ
[Re( N)Cl2(PNP)] (4)
5.1Æ1.0[e]
[ReCl3(PPh3)2(MeCN)]
[ReOCl3(PPh3)2]
2.7Æ0.5[e]
3.7
4.7
2.4
ꢀ
[Re( N)Cl2(PPh3)2]
[NaReO4]
8
[MeReO3]
5.4
9
[Re2(CO)10]
[ReI3]
Re powder
5.0
1.9
0
10
11
12
[Mo(N2)2(PNP)]2(m-N2) (1)
8.4Æ1.0[e]
[a] A mixture of catalyst (4.0 mmol/M), KC8 (2.4 mmol, 600 equiv/M),
and Me3SiCl (2.4 mmol, 600 equiv/M) in THF (10 mL) was stirred at
room temperature for 20 h under 1 atm of N2. [b] Equiv based on the
metal atom of the catalyst after acid hydrolysis of the reaction mixture.
[c] An average of three runs. [d] GC yield of N(SiMe3)3. [e] An average of
two runs.
did not inhibit the catalytic activity. This result suggests that
homogeneous rhenium species promotes the silylation of
dinitrogen under ambient reaction conditions.
Next, we examined other rhenium complexes as catalysts
for the silylation of dinitrogen. Yield of N(SiMe3)3 was
determined as ammonia after acid hydrolysis. ReIII complex 3
and ReV-nitride complex 4 worked as a less efficient catalyst
than 2, producing 5.7 equiv and 5.1 equiv of ammonia,
respectively (Table 3, entries 2–3). Rhenium complexes sup-
ported by PPh3 such as [ReCl3(PPh3)2(MeCN)], [ReOCl3-
Figure 3. MO energy diagrams of the frontier orbitals of 1 and 2.
LUMO of 1 and 2, together with spatial distributions of these
MOs of 2 (see Supporting Information for the corresponding
MOs of 1). The HOMO and HOMOÀ1 consisting of d
orbitals of the metal atoms and p* orbitals of dinitrogen
ligands play a role in the protonation step because a proton
donor attacks the distal nitrogen atom of coordinated
dinitrogen. Of note, the HOMO energy of 2 (À3.99 eV) is
much lower than that of 1 (À3.13 eV). The low energy level of
HOMO of 2 results in the large HOMO–LUMO energy gap
of 2 (2.69 eV) compared to that of 1 (2.13 eV). Currently, we
believe that the stabilization of HOMO of 2 can be associated
with the low reactivity toward the protonation of dinitrogen
coordinated to rhenium centers.
The catalytic reduction of dinitrogen into N(SiMe3)3 was
also investigated under ambient reaction conditions. We
carried out the reaction of dinitrogen with 600 equiv of KC8 as
a reductant and 600 equiv of Me3SiCl as a silylating reagent in
the presence of 2 in THF at atmospheric pressure at room
temperature for 20 h. After hydrolysis of the reaction
mixture, 11.7 equiv of ammonia based on the Re atom of 2
were obtained (Table 3, entry 1). Separately, the formation of
10.3 equiv of N(SiMe3)3 based on the Re atom of 2 was
confirmed by GC. This result represents the first successful
example of the rhenium-catalyzed formation of N(SiMe3)3
from dinitrogen under mild reaction conditions. According to
the time profiles of the formation of N(SiMe3)3 (Figure S16),
approximately 5 equiv of N(SiMe3)3 were produced during
the first 30 min, and the formation of N(SiMe3)3 almost
finished within 8 h. The addition of Hg to the reaction mixture
ꢀ
(PPh3)2], and [Re( N)Cl2(PPh3)2] afforded 2.7–4.7 equiv of
ammonia (Table 3, entries 4–6). Commercially available rhe-
nium complexes such as Na[ReO4], [MeReO4], [Re2(CO)10],
and [ReI3] exhibited moderate activities for nitrogen silyla-
tion (Table 3, entries 7–10), while use of Re powder as
a catalyst afforded no ammonia (Table 3, entry 11). These
results indicate that the PNP pincer ligand plays an important
role in the catalytic activity for the reduction of dinitrogen
into silylamine. Molybdenum complex 1 also worked as an
efficient catalyst for the formation of silylamine (Table 3,
entry 12).
In summary, we have achieved the first successful example
of the rhenium-catalyzed formation of ammonia and N-
(SiMe3)3 from dinitrogen under mild reaction conditions. The
current results show the potential that Group 7 metals work
as efficient catalysts for nitrogen fixation. Further studies for
the elucidation of the reaction mechanism and the develop-
ment of a more efficient catalytic nitrogen fixation system by
rhenium-dinitrogen complexes are ongoing in our laboratory.
Acknowledgements
The present project is supported by CREST, JST (Grant
JPMJCR1541). We acknowledge Grants-in-Aid for Scientific
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Research (Grants JP18K19093, JP18K05148, JP19K23645,
JP20H05671, JP20K21203, and JP20K15295) from JSPS and
MEXT. The SQUID measurement was performed using
facilities of the Cryogenic Research Center, the University of
Tokyo.
Conflict of interest
The authors declare no conflict of interest.
Keywords: ammonia · homogeneous catalysis ·
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Manuscript received: February 11, 2021
Revised manuscript received: April 8, 2021
Accepted manuscript online: April 9, 2021
Version of record online: && &&, &&&&
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Communications
Chemie
Communications
Homogeneous Catalysis
F. Meng, S. Kuriyama, H. Tanaka, A. Egi,
K. Yoshizawa,*
Y. Nishibayashi*
&&&& — &&&&
Ammonia Formation Catalyzed by
a Dinitrogen-Bridged Dirhenium Complex
Bearing PNP-Pincer Ligands under Mild
Reaction Conditions
The first successful example of catalytic
nitrogen fixation using a rhenium-dini-
trogen complex as a catalyst under mild
reaction conditions is described in this
paper. The dinitrogen-bridged dirhenium
complex worked as an effective catalyst
for the formation of ammonia and silyl-
amine, where up to 8.4 equiv of ammonia
and 11.7 equiv of silylamine were pro-
duced based on the Re atom of the cat-
alyst.
8
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