Direct Transformation of Molecular Dinitrogen into Ammonia Catalyzed by Cobalt Dinitrogen Complexes Bearing Anionic PNP Pincer Ligands

Article abstract of DOI:10.1002/anie.201606090

The direct formation of ammonia from molecular dinitrogen under mild reaction conditions was achieved by using new cobalt dinitrogen complexes bearing an anionic PNP-type pincer ligand. Up to 15.9 equivalents of ammonia were produced based on the amount of catalyst together with 1.0 equivalent of hydrazine (17.9 equiv of fixed nitrogen atoms).

Full text of DOI:10.1002/anie.201606090

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201606090
German Edition: DOI: 10.1002/ange.201606090
Nitrogen Fixation
Hot Paper
Direct Transformation of Molecular Dinitrogen into Ammonia
Catalyzed by Cobalt Dinitrogen Complexes Bearing Anionic PNP
Pincer Ligands
Shogo Kuriyama, Kazuya Arashiba, Hiromasa Tanaka, Yuki Matsuo, Kazunari Nakajima,
Kazunari Yoshizawa,* and Yoshiaki Nishibayashi*
Abstract: The direct formation of ammonia from molecular
dinitrogen under mild reaction conditions was achieved by
using new cobalt dinitrogen complexes bearing an anionic
PNP-type pincer ligand. Up to 15.9 equivalents of ammonia
were produced based on the amount of catalyst together with
1.0 equivalent of hydrazine (17.9 equiv of fixed nitrogen
atoms).
Lu and co-workers independently found that some cobalt
complexes are effective catalysts for the reduction of dini-
trogen gas into silylamine (N(SiMe3)3), which is regarded as
an ammonia equivalent, under ambient reaction condi-
tions.[10,11] In sharp contrast to the catalytic formation of
N(SiMe3)3, the cobalt-catalyzed direct formation of ammonia
from molecular dinitrogen has not been reported to date. In
2015, Peters and co-workers reported the “superstoichiomet-
ric” direct formation of ammonia by using a cobalt dinitrogen
complex bearing a tris(phosphine)borane ligand, but the
amount of formed ammonia (2.4 equiv) based on the amount
of catalyst used was not sufficient for the reaction to be
considered catalytic.[12]
I
n nature, molecular dinitrogen is converted into ammonia at
ambient temperature under atmospheric pressure by nitro-
genases. The active site of the FeMo cofactor in nitrogenases
has been found to contain molybdenum and iron atoms.[1] To
elucidate the reaction mechanism of nitrogen fixation cata-
lyzed by nitrogenases, the synthesis and reactivity of various
transition-metal dinitrogen complexes have been extensively
studied.[2,3] As a result, nitrogen fixation with molybdenum
dinitrogen complexes as the catalysts under ambient reaction
conditions was achieved by Schrock and Yandulov in 2003[4]
and by our groups in 2010.[5] Recently, Peters and co-workers
developed an iron-catalyzed reduction of molecular dinitro-
gen into ammonia using iron dinitrogen complexes as
catalysts under mild reaction conditions.[6] However, the
transformation of molecular dinitrogen into ammonia under
mild reaction conditions using other transition-metal dinitro-
gen complexes as the catalysts has not yet been achieved.[2,3]
The first cobalt dinitrogen complex, [Co(H)(N2)(PPh3)3],
was reported by Yamamoto and co-workers as the successful
example of the synthesis of a transition-metal dinitrogen
complex directly from dinitrogen gas.[7] Since the discovery of
this complex, detailed studies on the stoichiometric reactivity
of a variety of cobalt dinitrogen complexes have been carried
out by several research groups.[2c,d,8,9] In 2015, our group and
Based on our findings while developing a method for
catalytic nitrogen fixation under ambient reaction conditions,
we designed new cobalt dinitrogen complexes bearing an
anionic PNP-type pincer ligand, [Co(N2)(RPNP)] (1a: R =
tBu,
tBuPNP = 2,5-bis(di-tert-butylphosphinomethyl)pyrro-
lide; 1b: R = Cy, CyPNP = 2,5-bis(dicyclohexylphosphinome-
thyl)pyrrolide)[13] because the PNP ligand contains both hard
and soft donors to stabilize cobalt centers with various
oxidation states. We then investigated whether these com-
plexes show catalytic activity towards the direct formation of
ammonia from nitrogen gas under mild reaction conditions.
As expected, these complexes were indeed found to work as
catalysts for the direct reduction of dinitrogen gas into
ammonia under mild reaction conditions. Up to 15.9 equiv of
ammonia were produced based on the amount of catalyst
together with 1.0 equiv of hydrazine (17.9 equiv of fixed
N atoms). Herein, we detail the preparation and character-
ization of these novel cobalt dinitrogen complexes and their
catalytic behavior.
Treatment of CoCl2(thf)1.5 with lithium 2,5-bis(dialkyl-
R
À
phosphinomethyl)pyrrolide ( PNP Li) in toluene at room
[*] Dr. S. Kuriyama, Dr. K. Arashiba, Dr. K. Nakajima,
Prof. Dr. Y. Nishibayashi
temperature for 13 h gave the paramagnetic cobalt chloride
complexes CoCl(tBuPNP) (2a) and CoCl(CyPNP) (2b) in 88%
and 71% yield, respectively (Scheme 1). The molecular
Department of Systems Innovation
School of Engineering, The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: ynishiba@sys.t.u-tokyo.ac.jp
Prof. Dr. H. Tanaka, Y. Matsuo, Prof. Dr. K. Yoshizawa
Institute for Materials Chemistry and Engineering
Kyushu University, Nishi-ku, Fukuoka 819-0395 (Japan)
and
Elements Strategy Initiative for Catalysts and Batteries (ESICB)
Kyoto University, Nishikyo-ku, Kyoto 615-8520 (Japan)
E-mail: kazunari@ms.ifoc.kyushu-u.ac.jp
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201606090.
Scheme 1. Synthesis of cobalt dinitrogen complexes with anionic PNP
pincer ligands.
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Table 1: Cobalt-catalyzed reduction of dinitrogen into ammonia under
structures of 2a and 2b were confirmed by X-ray analysis (for
ORTEP drawings of 2a and 2b, see the Supporting Informa-
tion, Figures S3 and S4).[14] The crystal structures of both 2a
and 2b show a distorted square-planar geometry around the
cobalt atom with geometry indexes t4 of 0.10 and 0.12,
respectively (t4 = 0.00 for a perfectly square-planar and t4 =
1.00 for a tetrahedral geometry).[15] No distinct differences
were observed for the metrical parameters in 2a and 2b. At
296 K, complexes 2a and 2b have solution magnetic moments
of 2.3 Æ 0.2 and 2.1 Æ 0.1 mB, respectively. These measured
magnetic moments are larger than the spin-only value for an
S = 1/2 spin state (1.73 mB), but still within the range of values
reported for square-planar cobalt(II) complexes with a low-
spin state.[16]
Reactions of 2a and 2b with 1.1 equiv of KC8 as
a reductant in THF at room temperature for 13 h under an
atmospheric pressure of dinitrogen afforded the correspond-
ing diamagnetic cobalt(I) dinitrogen complexes 1a and 1b in
68% and 35% yield, respectively (Scheme 1). The molecular
structure of 1a was confirmed by X-ray analysis (Figure 1).
various reaction conditions.[a]
Entry Cat. Solvent Reductant NH3
NH2NH2 Fixed N atoms
[equiv][b] [equiv][b] [equiv][c]
1
1a Et2O
KC8
KC8
4.2Æ0.1
1.3Æ0.3
0
0
4.2
1.3
2[d] 1a Et2O
3
4
5
6
7
8
9
10
11
1a MTBE KC8
4.0Æ0.3 0.1Æ0.1 4.2
1a THF
1a Et2O
1a Et2O
1b Et2O
3a Et2O
4a Et2O
2a Et2O
2b Et2O
KC8
CoCp*2
K
KC8
KC8
KC8
KC8
KC8
1.7Æ0.1 0.2Æ0.1 2.1
0.6
0
0
0
0.6
0
3.1Æ0.1 0.1Æ0.1 3.3
2.2Æ0.1
2.0Æ0.2
1.2
0
0
0
0
2.2
2.0
1.2
0.8
0.8
[a] A mixture of catalyst (0.010 mmol, 1.0 equiv), reductant (0.40 mmol,
40 equiv), and [H(OEt2)2]BArF (0.38 mmol, 38 equiv) was stirred in the
4
indicated solvent at À788C for 1 h under 1 atm of N2 and then at room
temperature for 20 min. [b] Based on the amount of catalyst. [c] Number
of fixed nitrogen atoms (equiv)=[NH3 (equiv)]+2[NH2NH2 (equiv)].
[d] At room temperature for 1 h.
together with 2.1 equiv of molecular dihydrogen as a side
product (Table 1, entry 1; see also Table S9, run 1). In this
reaction, the formation of hydrazine was not detected. When
the reaction was carried out at room temperature for 1 h, only
a stoichiometric amount of ammonia was produced based on
the amount of catalyst (entry 2). We separately confirmed
that ammonia was not formed at all in the absence of 1a or
KC8 under similar reaction conditions. These results indicate
that the use of 1a as a catalyst and KC8 as a reductant at low
reaction temperatures such as À788C is necessary to promote
the catalytic formation of ammonia from molecular dinitro-
gen.[18] We also confirmed by using 15N2 gas instead of 14N2 gas
that molecular dinitrogen was certainly converted into
ammonia (see the Supporting Information for details).
Figure 1. ORTEP drawing of 1a. Hydrogen atoms are omitted for
clarity.
The crystal structure of 1a shows a distorted square-planar
geometry around the cobalt atom (t4 = 0.11) with a terminal
dinitrogen ligand. The structure of 1a is similar to those of
reported cobalt(I) dinitrogen complexes with a distorted
square-planar geometry.[17] The IR spectra of 1a and 1b in the
solid state (KBr) show strong nNN bands at 2016 cmÀ1 and
2020 cmÀ1, which were assigned to the terminal dinitrogen
ligand. In solution (THF), complexes 1a and 1b show nNN
bands at 2017 cmÀ1 and 2023 cmÀ1, which are similar to those
in the solid state. These nNN values are within the range of
those reported for cobalt(I) dinitrogen complexes.[17]
We then investigated the catalytic reduction of molecular
dinitrogen using 1a as a catalyst under the reaction conditions
developed by Peters and co-workers.[6,12] Representative
results are shown in Table 1. The reaction of an atmospheric
pressure of dinitrogen with KC8 (40 equiv with respect to 1a)
as a reductant and [H(OEt2)2]BArF4 (38 equiv with respect to
1a; ArF = 3,5-bis(trifluoromethyl)phenyl) as a proton source
in the presence of 1a as a catalyst in Et2O at À788C for 1 h
gave 4.2 equiv of ammonia based on the amount of catalyst
The amount of produced ammonia depends on the nature
of the solvent. When MeOtBu (MTBE) was used instead of
Et2O, a similar amount of ammonia (4.0 equiv) was produced
based on the catalyst together with a small amount of
hydrazine (0.1 equiv; entry 3). Compared to the reactions in
Et2O and MTBE, a lower amount of ammonia (1.7 equiv) was
produced when the reaction was carried out in THF (entry 4).
We surmise that the stronger coordination ability of THF may
reduce the catalytic activity. The use of a strong reductant
[19]
such as KC8 was necessary to render the catalytic reaction
effective. In fact, CoCp*2 (Cp* = pentamethylcyclopenta-
dienyl), which was used as an effective reductant for our
molybdenum-catalyzed nitrogen fixation,[5] and elemental
potassium were not suitable as reductants (entries 5 and 6).
Next, we investigated other cobalt complexes as catalysts
under the same reaction conditions. The cobalt dinitrogen
complex 1b was also an effective catalyst, and 3.1 equiv of
ammonia were produced based on the number of cobalt
atoms together with a small amount (0.1 equiv) of hydrazine
(entry 7). When cobalt hydride and cobalt methyl complexes
2
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(3a and 4a), which were prepared from reactions of 2a with
Interestingly, other cobalt complexes did not truly cata-
KBHEt3 in THF and MeMgCl in Et2O at room temperature
for 1 h (see the Supporting Information for details), were
employed, only a stoichiometric amount of ammonia was
produced based on the complexes (2.2 and 2.0 equiv, respec-
tively; entries 8 and 9). We separately confirmed that the
lyze ammonia formation. For example, when [Co(H)(N2)-
(PPh3)3][7] was used as the catalyst, only a stoichiometric
amount of ammonia was obtained (0.6 equiv of ammonia
based on the catalyst). Recently, Peters and co-workers
evaluated the catalytic activity of some cobalt dinitrogen
complexes towards ammonia formation using excess amounts
reaction of 3a with 1 equiv of [H(OEt2)2]BArF in Et2O at
4
room temperature and subsequent addition of 1 equiv of KC8
under N2 (1 atm) gave 1a together with molecular dihydrogen
(see the Supporting Information for details). The starting
cobalt chloride complexes 2a and 2b did not work as catalysts
of this process as 1.2 and 0.8 equiv of ammonia were formed
based on the complexes, respectively (entries 10 and 11).
When 1a was employed as the catalyst, the use of larger
of KC8 and [H(OEt2)2]BArF in Et2O at À788C.[12] In all
4
reactions, almost a stoichiometric amount of ammonia was
produced based on the catalyst (up to 2.4 equiv of ammo-
nia).[12,21] Although a cobalt dinitrogen complex bearing an
anionic PBP-type pincer ligand (5) has a similar molecular
structure to 1a and a similar nNN value of 2013 cmÀ1,[17c] this
complex did not work well as a catalyst, and only 0.4 equiv of
ammonia were produced based on the amount of catalyst.
This result indicates that the presence of PNP-type pincer
ligands with the pyrrole skeleton is important to promote the
cobalt-catalyzed nitrogen fixation.
amounts of both KC8 and [H(OEt2)2]BArF dramatically
4
increased the amount of nitrogenous products. The use of
200 equiv of KC8 and 184 equiv of [H(OEt2)2]BArF4 gave the
largest amounts of ammonia and hydrazine (15.9 and
1.0 equiv based on the catalyst, respectively; Scheme 2).
These results indicate that the cobalt dinitrogen complexes
bearing an anionic PNP-type pincer ligand are effective
catalysts for the conversion of molecular dinitrogen into
ammonia and hydrazine, and up to 17.9 equiv of N atoms
were fixed based on the amount of catalyst.
We performed DFT calculations to compare the geo-
metric and electronic structures of the cobalt dinitrogen
complexes 1a and 5. Figure 2a shows their optimized
Scheme 2. Catalytic reduction of dinitrogen with 1a as the catalyst.
We carried out several experiments to gain insight into the
reaction pathway. The reaction of 1a with excess amounts of
KC8 (10 equiv relative to 1a) and [H(OEt2)2]BArF4 (9.5 equiv
relative to 1a) in Et2O at À788C for 1 h under 1 atm of N2
gave no cobalt-containing products except for 1a. We could
not detect the free PNP-H ligand. We believe that the
formation of a small but certain amount of hydrazine in the
present catalytic reaction provides an important key to
elucidate the reaction pathway. Based on the experimental
result that hydrazine is formed as a nitrogenous product, we
suggest that a cobalt hydrazine complex may be formed as
a key reactive intermediate in the catalytic reaction.[8b,20] We
thus carried out a control experiment using hydrazine as the
substrate. The reaction of hydrazine (4 equiv relative to 4a)
with excess amounts of KC8 (40 equiv relative to 4a) and
[H(OEt2)2]BArF (38 equiv relative to 4a) in the presence of
4
4a as the catalyst in Et2O at À788C for 1 h under 1 atm of Ar
gave 0.8 equiv of ammonia based on 4a together with the
recovery of hydrazine in 60% (2.4 equiv to 4a). The
formation of molecular dinitrogen was not detected in this
reaction system. This result indicates that hydrazine is
partially converted into ammonia when hydrazine is formed
in the catalytic reaction (see the Supporting Information).
Further studies are necessary to elucidate the detailed
reaction pathway.
Figure 2. a) Optimized structures of 1a and 5 in the closed-shell
singlet state. Bond lengths are given in ꢀ. The Mayer bond orders are
given in parentheses. Hydrogen atoms are omitted for clarity. b) The
molecular orbitals of 1a and 5 corresponding to the s bond between
Co and the pincer ligands (HOMO-7 for 1a and HOMO-6 for 5) and
the p bond (HOMO-4).
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structures in the ground spin state (closed-shell singlet),
together with the Mayer bond orders[22] of selected bonds. The
optimized structures well reproduce the crystal structures. A
À
striking finding is that the Co N2 distance of 1a (1.743 ꢀ) is
À
much shorter than that of 5 (1.780 ꢀ). The long Co N2 bond
in 5 implies a significant trans influence of the PBP ligand,
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ꢀ
by the pincer ligands: The N N bond distances (Mayer bond
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Figure 2b shows the molecular orbitals corresponding to the
s and p bonds between the cobalt center and the pincer
ligands of 1a and 5. Comparing HOMO-7 of 1a and HOMO-6
of complex 5 suggests that the PBP ligand serves as a strong
s donor. As shown in Figure 2a, the Mayer bond order (b.o.)
À
of the Co Npy bond (0.69) is smaller than that of the
À
corresponding Co B bond (0.84). On the other hand, the
strong trans influence of the PBP ligand in 5 weakens the
À
binding of dinitrogen in the trans position. The Co N2 bond in
1a (b.o. = 0.71) is stronger than that in 5 (b.o. = 0.64). For
dissociation of dinitrogen from the cobalt center in the singlet
state, the free energy changes at 195 K (DG195) in Et2O were
calculated to be 19.1 kcalmolÀ1 for 1a and 16.5 kcalmolÀ1 for
5. The moderate s-donating ability of the PNP ligand[24] might
be essential in the effective catalytic transformation of
dinitrogen into ammonia by 1a, although the exact role of
the PNP ligand has not yet been clarified.[25]
In summary, we have designed new cobalt dinitrogen
complexes with anionic PNP-type pincer ligands that are
effective catalysts for the reduction of molecular dinitrogen
into ammonia under mild reaction conditions. Whereas
molybdenum- and iron-catalyzed variants have been reported
previously,[26] this is the first clear-cut example of cobalt-
catalyzed direct formation of ammonia from dinitrogen under
mild reaction conditions. We believe that the present findings
provide valuable insight for the development of nitrogen
fixation systems using transition-metal dinitrogen complexes
as catalysts.
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Acknowledgements
The present project was supported by CREST, JST, and the
Mitsubishi Foundation. We acknowledge Grants-in-Aid for
Scientific Research (JP26288044, JP26105708, JP15K13687,
and JP15H05798 to Y.N., JP24109014 to K.Y., and
JP26888008 to H.T.) from the JSPS and MEXT. S.K. is the
recipient of a JSPS Predoctoral Fellowship for Young
Scientists.
Keywords: ammonia · catalysis · cobalt · dinitrogen ·
pincer ligands
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[18] To clarify whether or not cobalt nanoparticles generated in situ
act as heterogeneous catalysts in the present reaction, we carried
out the catalytic reaction in the presence of cobalt powder. The
reaction only gave a trace amount of ammonia based on the
amount of cobalt.
[25] The result described in the Peters paper[12] indicates that the
corresponding anionic cobalt dinitrogen complexes may play
a major role in the transformation of molecular dinitrogen into
ammonia. In fact, our preliminary calculations have shown that
protonation of the dinitrogen ligand in neutral 1a with
[H(OEt2)2]+ is endergonic. We also computationally observed
that reduction of the neutral cobalt dinitrogen complex 1a
significantly increases the negative NPA charge on dinitrogen.
Cyclovoltammetric analysis of 1a also suggests the formation of
an anionic cobalt dinitrogen complex under the catalytic
reaction conditions (see the Supporting Information for details).
[26] a) During the preparation of the present manuscript, Peters and
co-workers reported the formation of 59 equiv of ammonia
based on the iron atom of their catalyst upon modifying the
reaction conditions; see: T. J. Del Castillo, N. B. Thompson, J. C.
Peters, J. Am. Chem. Soc. 2016, 138, 5341; b) S. Kuriyama, K.
Arashiba, K. Nakajima, Y. Matsuo, H. Tanaka, K. Ishii, K.
Yoshizawa, Y. Nishibayashi, Nat. Commun. 2016, 7, 12181.
Received: June 23, 2016
[19] I. S. Weitz, M. Rabinovitz, J. Chem. Soc. Perkin Trans. 1 1993,
117.
Revised: July 19, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Nitrogen Fixation
S. Kuriyama, K. Arashiba, H. Tanaka,
Y. Matsuo, K. Nakajima, K. Yoshizawa,*
Y. Nishibayashi*
&&&& — &&&&
Direct Transformation of Molecular
Dinitrogen into Ammonia Catalyzed by
Cobalt Dinitrogen Complexes Bearing
Anionic PNP Pincer Ligands
Cobalt dinitrogen complexes with an
anionic PNP-type pincer ligand enable the
direct formation of ammonia from
molecular dinitrogen under mild reaction
conditions. Up to 15.9 equivalents of
ammonia were produced based on the
amount of catalyst together with one
equivalent of hydrazine (17.9 equivalents
of fixed N atoms).
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!