Azaferrocene-Based PNP-Type Pincer Ligand: Synthesis of Molybdenum, Chromium, and Iron Complexes and Reactivity toward Nitrogen Fixation

Article abstract of DOI:10.1002/ejic.201601051

A series of azaferrocene-based PNP-type pincer ligands were designed and prepared as new PNP-type pincer ligands. Some transition-metal complexes, including molybdenum, chromium, and iron complexes, bearing these azaferrocene-based PNP-type pincer ligands were also prepared and characterized by X-ray analysis. The stoichiometric and catalytic reactivities of molybdenum–dinitrogen complexes toward nitrogen fixation were investigated in detail.

Full text of DOI:10.1002/ejic.201601051

A Journal of
Accepted Article
Title: Azaferrocene-Based PNP-type Pincer Ligand: Synthesis of Molybdenum,
Chromium, and Iron Complexes and Reactivity toward Nitrogen Fixation
Authors: Yoshiaki Nishibayashi; Shogo Kuriyama; Kazuya Arashiba; Kazunari Na-
kajima; Hiromasa Tanaka; Kazunari Yoshizawa
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201601051
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European Journal of Inorganic Chemistry
10.1002/ejic.201601051
COMMUNICATION
Azaferrocene-Based PNP-type Pincer Ligand: Synthesis of
Molybdenum, Chromium, and Iron Complexes and Reactivity
toward Nitrogen Fixation
Shogo Kuriyama,[a] Kazuya Arashiba,[a] Kazunari Nakajima,[a] Hiromasa Tanaka,[b] Kazunari
Yoshizawa,*[b] and Yoshiaki Nishibayashi*[a]
Abstract: A series of the azaferrocene-based PNP-type pincer
ligands are newly designed and prepared as new PNP-type pincer
ligands. Some transition metal- complexes such as molybdenum,
chromium, and iron complexes bearing the azaferrocene-based
PNP-type pincer ligands are also prepared and characterized by X-
ray analysis.
The stoichiometric and catalytic reactivity of
molybdenum-dinitrogen complexes toward nitrogen fixation is
investigated in details.
Pincer ligands, which coordinate to a metal center in a
meridional fashion, endow the corresponding transition metal
complexes with a high thermal stability. The precise design of
pincer ligands offers appropriate and unique reactivities to the
transition metal complexes. Hence, transition-metal complexes
bearing pincer ligands have been applied to various kinds of
catalytic reactions and small molecule activation.[1]
novel catalytic reactions and unique activity of small molecules
have been realized for the last two decades. As a result,
In fact,
continuous efforts have been paid on the development of novel
type of pincer ligands.
novel type of the metallocene-based PNP-type pincer ligand
(Figure 1 (c)).
Among a variety of pincer ligands, metallocene-based pincer
ligands, where a metallocene backbone is utilized for a central
coordination site, have been developed by several research
groups because the nature of the matallocene moiety can affect
the electronic property as well as the steric factor of the
coordination environment of another metal in the reaction center
(Figure 1).[2,3] Two major types of metallocene-based PCP-type
Figure 1. Metallocene-based pincer ligands. (a) Ferrocene- or ruthenocene-
based PCP-type pincer ligand. (b) Cationic CpM(η6-arene)-based PCP-type
pincer ligand. (c) Azaferrocene-based PNP-type pincer ligand (1). (d)
Phosphaferrocene-based PPP-type pincer ligand.
pincer ligands have been reported until now.
One contains
Azaferrocenes, which are isoelectronic analogues of
ferrocene, have a nitrogen atom instead of a carbon atom in a
ferrocene or ruthenocene skeleton (Figure 1 (a)).[2] The other
has a cationic (cyclopentadienyl)M(6-arere) fragment as a
backbone (Figure 1 (b)).[3] In sharp contrast to these anionic
PCP-type pincer ligands, no azaferrocene-based pincer ligand
has appeared as a neutral PNP-type pincer ligand. Here, we
have introduced an azaferrocene-based pincer ligand (1) as a
cyclopentadienyl ligand of ferrocene.[4]
Azaferrocenes are
typically redox-active compounds and have a unique electronic
property.[5]
Azaferrocenes are known to work as strong -
donating and poor -accepting ligands to transition-metal
complexes.[6]
More recently, optically active azaferrocene
derivatives have been used as organocatalysts and chiral
ligands in various asymmetric reactions.[7]
However, only
limited successful examples of multidentate ligands based on
the azaferrocene skeleton have been reported until now.[7b,7c,8]
In sharp contrast, multidentate ligands based on the
phosphaferrocenes, which are heavier analogues of the
azaferrocenes, have been widely studied.[7a,7b,9,10] Typically,
[a]
[b]
Dr. S. Kuriyama, Dr. K. Arashiba, Dr. K. Nakajima, Prof. Dr. Y.
Nishibayashi
Department of Systems Innovation, School of Engineering,
The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
E-mail: ynishiba@sys.t.u-tokyo.ac.jp
Associate Prof. Dr. H. Tanaka, Prof. Dr. K. Yoshizawa
Institute for Materials Chemistry and Engineering,
Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
Elements Strategy Initiative for Catalysts and Batteries (ESICB),
Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japan
Mathey, Le Floch, and their coworkers reported
phosphaferrocene-based PPP-type pincer ligand and its
rhodium complex (Figure 1 (d)).[11]
However, no PNP-type
pincer ligand based on the azaferrocene skeleton has appeared
until now, to the best of our knowledge, although the
a
Homepage: http://park.itc.u-tokyo.ac.jp/nishiba/ &
http://trout.scc.kyushu-u.ac.jp/yoshizawaJ/
Supporting information for this article is given via a link at the end of
the document.
azafferocene-based PNP-type pincer ligand can be expected to
work as not only an isoelectronic analogue of a pyridine-based
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European Journal of Inorganic Chemistry
10.1002/ejic.201601051
COMMUNICATION
PNP-type pincer ligand, whose complexes exhibit unique and
excellent catalytic activity,[1a,1b,1c,1e,1f] but also a redox-active
ligand. In addition, steric bulkiness of the azaferrocene moiety
may provide a unique coordination environment of another metal
in the reaction center, which can not be attainable by using
pyridine-based PNP-type pincer ligand. In this paper, we have
described synthesis of azaferrocene-based PNP-type pincer
ligands and their coordination ability toward some transition
metals such as molybdenum, chromium, and iron atoms.
Next, we tried to prepare an azaferrocene bearing two phenyl
groups on the phosphorous atoms (1c) in the same method
(Scheme 1). However, the reaction of [Cp*FeCl(tmeda)] with
lithium 2,5-bis(diphenylphosphinomethyl)pyrrolide (2c-Li) in THF
at room temperature for 18 h did not afford the corresponding
azaferrocene 2c but a half sandwich complex [Cp*Fe(2c-
2P,N)] (3c) quantitatively (62% isolated yield). The complex
3c was characterized by 1H and 31P{1H} NMR spectroscopic
analyses and X-ray crystallography, an ORTEP drawing being
shown in Figure 2 (b).[12] The pyrrolide ligand 2c coordinated to
the iron atom with the 1-pyrrolide and the two phosphine
moieties.
Treatment
pentamethylcyclopentadienyl,
tetramethylethylenediamine)
of
[Cp*FeCl(tmeda)]
(Cp*
=
=
5-
tmeda
with
N,N,N',N'-
lithium
2,5-
bis(dicyclohexylphosphinomethyl)pyrrolide (2a-Li), which was
generated in situ from the corresponding pyrrole derivative with
LiN(SiMe3)2, in tetrahydrofuran (THF) at room temperature for 18
h afforded an azaferrocene [Cp*Fe(5-2a)] (1a) in 86% yield
(Scheme 1). According to the same method, an azaferrocene
bearing two tert-butyl groups on the phosphorous atoms (1b)
was prepared in 58% yield. The molecular structure of 1a and
When a solution of 3c in C6D6 was warmed to 50 ºC, 3c was
gradually consumed together with the formation of a new
1
species, which was identified as an azaferrocene 1c by H and
31P{1H} NMR spectroscopic analyses.
Figure 3 shows time
profiles of the transformation of 3c into 1c. After 90 h, the
complete consumption of 3c was observed and 1c was formed
as a major product (76% yield) with the formation of FeCp*2 as a
1b were unambiguously confirmed by X-ray analysis.[12]
An
byproduct (5% yield).
It was already reported that 1 to 5
ORTEP drawing of 1a is shown in Figure 2 (a). The 1a and 1b
have a sandwich structure with the 5-pyrrolide ligand. The Cp*
and pyrrolide ligands in 1a and 1b are almost parallel with the
interplanar angles of 1.3(2)º (1a) and 1.5(2)º (1b), respectively.
slippages of pyrrolide ligands caused the conversion of half
sandwich complexes into azeferrocenes.[13] Interestingly, we
found that addition of MgCl2 as a Lewis acid promoted the
transformation of 3c into 1c.
After treatment of 3c with an
excess amount of MgCl2 in THF at 50 ºC for 3 h, the complete
consumption of 3c was observed and 1c was obtained in 82%
PR2
PR2
PR2
NLi
yield after extractive workup (Scheme 1).
No formation of
N
FeCp*2 was observed in the presence of MgCl2, in contrast to
the reaction in the absence of MgCl2. We consider that the
coordination of MgCl2 to the phosphorous or the nitrogen atoms
in 3c promotes the isomerization into 1c. In fact, the formation
of a magnesium complex between 1c and MgCl2 was suggested
by 31P{1H} NMR spectroscopy after the reaction of 3c with MgCl2
in THF at 50 ºC.
+
[Cp*FeCl(tmeda)]
Fe
THF
rt, 18 h
PR2
(1 equiv)
R = Cy
R = tBu
2a-Li (R = Cy)
2b-Li (R = tBu)
2c-Li (R = Ph)
1a (R = Cy)
1b (R = tBu)
1c (R = Ph)
THF
rt, 18 h
R = Ph
MgCl2
(excess)
Ph2
P
N
Fe
THF
50 °C, 3 h
PPh2
3c
Scheme 1. Synthesis of azaferrocene-based PNP-type pincer ligands.
Figure 3. Time profiles of the transformation of 3c into 1c and FeCp*2.
This result suggests that the transformation of 1 and 3
depends on the steric property of the substituents on the
phosphorous atoms. The presence of large alkyl groups such
as cyclohexyl and tert-butyl groups on the phosphorous atom in
the pyrrolides inhibited the formation of the corresponding half
sandwich complexes.
derivative bearing phenyl groups kinetically formed the half
sandwich complex such as 3c. DFT calculations at the M06
On the other hand, the pyrrolide
Figure 2. ORTEP drawings of (a) 1a and (b) 3c. Thermal ellipsoids are shown
at the 50% probability level. Hydrogen atoms are omitted for clarity.
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European Journal of Inorganic Chemistry
10.1002/ejic.201601051
COMMUNICATION
level of theory[14] demonstrated that 1a containing bulkier
cyclohexyl groups is 14.5 kcal/mol more stable than 3a in THF at
Scheme 2. Synthesis of transition metal-complexes bearing azaferrocene-
based pincer ligands.
25 ºC.
Azaferrocene 1c containing phenyl groups is 4.1
kcal/mol more stable than the half sandwich complex 3c in THF
at 50 ºC, while 1c is slightly less stable (0.7 kcal/mol) than 3c in
vacuo without thermal corrections.
Optimized structures are
shown in Figure S12. The bulkiness of the Cp* ligand on Fe is
also an important factor for determining a thermodynamically
stable structure. When the Cp* ligand in 1a is replaced by a
smaller cyclopentadienyl (Cp) ligand, the corresponding
azaferrocene 1a′ is 3.8 kcal/mol less stable than the half-
sandwich complex 3a′ in THF at 25 ºC.[15]
Next, we investigated coordination behaviors of 1.
Treatment of [MoCl3(thf)3] with 1a in THF at 50 °C for 18 h gave
the corresponding molybdenum-pincer complex [MoCl3(1a)] (4a)
in 86% yield (Scheme 2). Similarly, the reaction of [MoCl3(thf)3]
with 1c, which was generated in situ from 3c, under the same
reaction conditions afforded [MoCl3(1c)] (4c) in 71% yield
(Scheme 3). On the other hand, no formation of [MoCl3(1b)]
(4b) was observed from the reaction of [MoCl3(thf)3] with 1b,
where only an unidentified mixture was obtained.
(c)
(d)
The molecular structures of 4a and 4c were determined by X-
ray crystallography (Figure 4).[12] The crystal structures of 4a
and 4c display a distorted octahedral geometry around the
Figure 4. ORTEP drawings of (a) 4a, (b) 4c, (c) 6a, and (d) 7a. Thermal
ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted
for clarity.
molybdenum center.
The ligands 1 coordinated to the
Bond lengths
molybdenum atoms in a meridional fashion.
MoP (2.543(3)-2.555(3) Å for 4a; 2.539(1)-2.543(1) Å for 4c)
and MoN (2.140(8)-2.162(9) Å for 4a; 2.150(3) Å for 4c) in 4
are shorter than those of [MoCl3(PNP)] (5, PNP = 2,6-bis(di-tert-
butylphosphinomethyl)pyridine; 2.589(2)-2.618(1) Å for MoP;
2.199(4) Å for MoN).[16] In addition, 4 have smaller PMoP
bite angles (155.01(9)-155.72(9)º for 4a; 155.80(3)º for 4c) than
5 (155.97(8)-156.79(8)º). We consider that the difference in
ring size of the skeletons causes these structural features. The
pyrrolide and Cp* ligand planes in 4a and 4c are inclined
towards each other by 9.5(5)-11.2(5)º and 8.9(2)º, respectively.
This inclination seems to be due to the steric repulsion between
the molybdenum fragment and the Cp* rings in 4. The slightly
larger deviation from the parallel structure in 4a than that in 4c
reflects the difference in size of the PR2 groups. This steric
hindrance also causes shift of the molybdenum atom relative to
the pyrrolide ring (0.87(2)-0.97(2) Å in 4a and 0.81(1) Å in 4c).
Next, we prepared other transition-metal complexes such as
chromium and iron complexes bearing the azaferrocene-based
PNP-pincer ligand. The reaction of [CrCl3(thf)3] with 1a in THF
at room temperature for 18 h gave a chromium complex
[CrCl3(1a)] (6a) in 81% yield (Scheme 2). We confirmed that 6a
has an octahedral structure around the chromium atom similar to
that of the molybdenum complex 5 by X-ray analysis.[12]
Reactions of [FeI2] and [FeBr2(thf)2] with 1a in THF at reflux
temperature for 15 h gave the corresponding iron complexes
[FeX2(1a)] (7a: X = I; 8a: X = Br) in 90% and 64% yields,
respectively (Scheme 2).
X-ray analysis of 7a reveals that
these iron complexes have a distorted trigonal bipyramidal
structure around the iron atom.[12]
We carried out electrochemical measurement of the
azaferrocene pincer ligand 1a and its molybdenum complex 4a,
in order to obtain an electronic property of the azaferrocene-
based PNP-pincer ligand 1. Cyclic voltammogram of 1a in THF
using [NBu4]BArF (ArF = 3,5-bis(trifluoromethyl)phenyl) as an
Cl
[MoCl3(thf)3]
PR2
4
4
or
N
M
Cl
supporting electrolyte showed an irreversibly oxidation wave at
Epa = 0.15 V vs FeCp2, which was assigned to Fe(II/III) (Figure
S1).[17] On the other hand, the molybdenum complex 4a showed
a reversible oxidation wave at E1/2 = 0.07 V and an irreversible
oxidation wave at Epa = +0.68 V. Based on the previous result
that similar molybdenum complex [MoCl3(PNP)] 5 exhibited a
reversible oxidation at E1/2 = +0.14 V,[18] we assigned the former
oxidation wave to Mo(III/IV). Thus, the potential of Mo(III/IV)
couple of 4a was more negative than that of 5. This result
suggests that the azaferrocene-based PNP-pincer ligand 1a has
a stronger electron-donating ability than the corresponding
pyridine-based pincer ligand.
[CrCl3(thf)3]
THF
P
Fe
Cl
R2
PR2
PR2
N
4a (M = Mo; R = Cy)
4c (M = Mo; R = Ph)
6a (M = Cr; R = Cy)
Fe
[FeI2]
or
[FeBr2(thf)2]
X
1
PCy2
(1 equiv)
N Fe
P
X
THF
Fe
Cy2
7a (X = I)
8a (X = Br)
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European Journal of Inorganic Chemistry
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COMMUNICATION
In order to evaluate the catalytic activity of transition metal
complexes bearing the azaferrocene-based PNP-pincer ligand,
we have chosen molybdenum-catalyzed reduction of molecular
dinitrogen into ammonia under ambient reaction conditions
because quite recently we have found that a dinitrogen-bridged
dimolybdenum-dinitrogen complex bearing PNP-pincer ligands
[Mo(N2)2(PNP)]2(-N2) (9) worked as an efficient catalyst for the
formation of ammonia, where up to 12 equiv based on the Mo
atom of the catalyst (23 equiv of ammonia based on the catalyst)
were produced.[16] In addition, more recently, we have found
that introduction of electron-donating groups such as a methoxy
group or redox active moieties such as ferrocene at the 4-
position of the pyridine ring on the PNP-pincer ligand
dramatically increased the catalytic activity.[18,19] As a result, we
have envisaged that the azaferrocene-based PNP-pincer ligands
1 may work as an electron-donating and redox active ligand in
molybdenum-dinitrogen complexes.
N
N
(b)
(a)
N2
(1 atm)
Na-Hg
(4 equiv)
PCy2
N
N
Mo
N
4
P
Cy2
Fe
N
N
THF
rt, 14 h
10a
N2
(1 atm)
Na-Hg
(4 equiv)
N
N
N
N
Ph2P
N
PPh2
+
Mo N N
Fe
N
PPh2
N
N
Mo
N
N Mo
N
THF
P
rt, 14 h
P
Ph2
Fe
N
N
N
P
Ph2
Fe
N
N
Ph2
N
10c'
10c
At first, we prepared molybdenum-dinitrogen complexes
bearing the azaferrocene-based PNP-pincer ligands. Reduction
of 4a with 4 equiv of Na-Hg in THF at room temperature for 14 h
under an atmospheric pressure of dinitrogen gave a diamagnetic
tris(dinitrogen) molybdenum complex mer-[Mo(N2)3(1a)] (10a) in
39% yield (Scheme 3 (a)). We could not detect other products
in the crude mixture by means of 31P{1H} NMR and IR
Scheme 3. (a) Synthesis of molybdenum-dinitrogen complexes. (b) An
ORTEP drawing of 10a. Thermal ellipsoids are shown at the 50% probability
level. Hydrogen atoms are omitted for clarity.
Reduction of 4c with 4 equiv of Na-Hg in THF at room
temperature for 14 h under an atmospheric pressure of
dinitrogen gave novel complexes showing a broad singlet at
62.6 ppm and a sharp singlet at 60.6 ppm in 31P{1H} NMR
spectroscopic analyses.
clarified by X-ray crystallography, an ORTEP drawing being
shown in Scheme 3 (b).[12]
The molecular structure of 10a
contains a molybdenum moiety bearing three terminal dinitrogen
ligands in a meridional form. Structural parameters of the
A detailed structure of 10a was
spectrum (Scheme 3 (a)).
Integrations of these signals
indicated that the ratio of two compounds was ca. 1 to 1. IR
spectrum of a crude mixture in a solution state (C6D6) exhibited
four NN bands at 2065, 1992, 1974, and 1959 cm-1, which are
assignable to the terminal dinitrogen ligands (Figure S3 (a)).
Based on these spectroscopic data, we consider the formation
of a mixture of a mononuclear molybdenum-dinitrogen complex
mer-[Mo(N2)3(1c)] (10c) and a dinuclear molybdenum-dinitrogen
dinitrogen ligands of 10a are similar to those of other mer-
Mo(N2)3 complexes.[20,21] The pyrrolide and Cp* ligand planes in
10a are inclined towards each other by 9.4(2)º.
molybdenum atom in 10a is located above the pyrrolide ring by
0.67(1) Å. These distortions of the structure suggest the
The
complex [Mo(N2)3(1c)]2(-N2) (10c') in
a
solution state.
presence of the contact between the PCy2 groups and the Cp*
ligand. IR spectrum of 10a in a solid state (KBr) showed strong
NN bands at 2039 and 1952 cm-1 assignable to the terminal
Recrystallization of the crude mixture afforded only a mixture of
10c and 10c' in 45% yield, where 10c' was formed as a major
product.
31P{1H} NMR spectrum of the crystallized product
dinitrogen ligands (Figure S2 (a)).
Similar NN bands were
showed a strong broad signal at 62.6 ppm derived from 10c',
together with a weak singlet at 60.6 ppm derived from 10c. In
IR spectrum of the crystallized product, a major NN bands was
observed at 1974 cm-1 (Figure S3 (b)), which was attributable to
the terminal dinitrogen ligands in 10c'. These spectroscopic
features are consistent with the formation of 10c' as a major
observed at 2041 and 1953 cm-1 in a C6D6 solution of 10a
(Figure S2 (b)). 31P{1H} NMR spectrum of 10a in C6D6 exhibited
a singlet at 63.8 ppm due to the PCy2 groups.
These
spectroscopic data indicate that the complex 10a maintains the
mononuclear structure in a solution state.
product after recrystallization.
Unfortunately, we can not
separate these complexes 10c and 10c', most likely due to an
equilibrium between 10c and 10c'. We also tried to prepare the
corresponding chromium- and iron-dinitrogen complexes from 6-
8a, according to similar methods. However, no formation of the
dinitrogen complexes was observed although we observed the
free pincer ligand in the reaction mixture after the reaction.
We investigated the reactivity of these molybdenum-
dinitrogen complexes toward protonolysis. Treatment of 10a
with an excess amount of sulfuric acid in THF at room
temperature for 24 h afforded 0.48 equiv of ammonia based on
the molybdenum atom in 10a together with 0.02 equiv of
hydrazine. Typical results are shown in Table 1. Protonolysis
of 10c' under the same reaction conditions afforded only a trace
amount of ammonia (0.02 equiv/Mo).
The ammonia and
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European Journal of Inorganic Chemistry
10.1002/ejic.201601051
COMMUNICATION
hydrazine produced by 10a are almost the same as those
produced by [Mo(N2)2(PNP)]2(-N2) 9 (0.61 equiv of ammonia
and 0.06 equiv of hydrazine).[22]
10c' as a catalyst under the same reaction conditions gave 33
equiv of N(SiMe3)3 based on the molybdenum atom in 10c'.
These results indicate that molybdenum-dinitrogen complexes
bearing the azaferrocene-based PNP-pincer ligands worked as
effective catalysts for the catalytic silylation of molecular
Next, we carried out the catalytic reduction of molecular
dinitrogen into ammonia using these molybdenum-dinitrogen
complexes 10 as catalysts, according to our previous
method.[16,18,19] In the presence of a catalytic amount of 10a, the
reaction of an atmospheric pressure of dinitrogen with CoCp2
(36 equiv to the molybdenum atom in 10a;) as a reductant and
[LutH]OTf (Lut = 2,6-lutidine, OTf = trifluoromethanesulfonate,
48 equiv to the molybdenum atom in 10a) as a proton source in
toluene at room temperature for 20 h afforded 0.2 equiv of
ammonia based on the molybdenum atom in 10a. The reaction
by using 10c' as a catalyst under the same reaction conditions
gave only a trace amount of ammonia.
The result of catalytic reactions using 10 as catalysts is in
sharp contrast to that using [Mo(N2)2(PNP)]2(-N2) 9 as a
catalyst. In the reaction using 9 as a catalyst, 6.0 equiv of
ammonia were produced based on the molybdenum atom in
9.[16] The result of the DFT calculations on the catalytic reaction
by using 9 as a catalyst indicates that the dinuclear structure of
the dinitrogen-bridged dimolybdenum complex 9 plays an
essential role in the catalytic cycle,[23] where a mononuclear
dinitrogen complex such as mer-[Mo(N2)3(PNP)] can not be
protonated with [LutH]+.[23] We consider that the reason why
10a did not work as an effective catalyst is due to the
mononuclear structure of 10a in a solution state. The steric
hindrance around the molybdenum atom in 10a may inhibit the
formation of a dinuclear structure.
dinitrogen under ambient reaction conditions.
Further
investigation is necessary to elucidate the exact role of the
azaferrocene-based PNP-pincer ligands in the catalytic
reactions.
In summary, we have newly prepared and characterized a
series of the azaferrocene-based PNP-type pincer ligands and
the corresponding molybdenum, chromium, and iron complexes.
The stoichiometric and catalytic reactivity of molybdenum-
dinitrogen complexes toward nitrogen fixation has been
investigated in details. We believe that the results described in
this article provide fundamental insight to design a novel type of
transition-metal complexes bearing metallocene-based pincer
ligands.[26]
Acknowledgements
The present project is supported by CREST, JST and Mitsubishi
Foundation.
We thank JSPS KAKENHI (Nos. JP26288044,
JP26105708, JP15K13687, JP15H05798 to Y.N., No.
JP24109014 to K.Y., and No. JP26888008 to H.T.). S.K. is a
recipient of the JSPS Predoctoral Fellowships for Young
Scientists.
Table 1: Protonolysis of molybdenum-dinitrogen complexes (10).
Keywords: Ligand design • Tridentate ligands • Metallocenes •
H2SO4 (ecxess)
Mo-N2 complex
Molybdenum • Nitrogen fixation • Pincer ligand
+
NH2NH2
NH3
(0.040 mmol)
THF, rt, 24 h
NH3
(mmol) (equiv/Mo)[a]
[1]
a) Topics in Organometallic Chemistry Vol. 40: Organometallic Pincer
Chemistry (Eds.: G. van Koten, D. Milstein), Springer, Berlin, 2013; b)
Pincer and Pincer-Type Complexes: Applications in Organic Synthesis
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NH2NH2
NH2NH2
(mmol) (equiv/Mo)[a]
NH3
Entry Complex
0.02
0
0.06
1
2
3b
10a
10c'
9
0.48
0.02
0.61
0.001
0
0.005
0.019
0.001
0.049
[a] Equiv based on Mo atom of the complex.
[b] Reference 22.
On the other hand, no formation of ammonia was observed in
both stoichiometric and catalytic reactions using 10c'. Although
we have not yet obtained the exact reason, we consider that the
coordination ability of terminal dinitrogen ligands to the
molybdenum atoms is too low to form N–H bonds by the
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protonation of 10c'.
In fact, we previously reported that a
dinitrogen-bridged dimolybdenum-dinitrogen complex bearing
unsymmetric PNP-pincer ligands with phenyl groups on the
phosphorous atoms did not work as a catalyst for the ammonia
formation.[24]
Finally, we carried out the catalytic formation of silylamine
from molecular dinitrogen using 10 as catalysts, according to our
previous method.[25] In the presence of a catalytic amount of
10a, the reaction of an atmospheric pressure of dinitrogen with
Na (600 equiv to the molybdenum atom in 10a) as a reductant
and Me3SiCl (600 equiv to the molybdenum atom in 10a) in THF
at room temperature for 20 h afforded 41 equiv of N(SiMe3)3
based on the molybdenum atom in 10a. The reaction by using
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European Journal of Inorganic Chemistry
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COMMUNICATION
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European Journal of Inorganic Chemistry
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Shogo Kuriyama, Kazuya Arashiba,
Kazunari Nakajima, Hiromasa Tanaka,
Kazunari Yoshizawa,* and Yoshiaki
Nishibayashi*
Page No. – Page No.
Azaferrocene-Based PNP-type Pincer
Ligand: Synthesis of Molybdenum,
Chromium, and Iron Complexes and
Reactivity toward Nitrogen Fixation
Some transition metal- complexes such as molybdenum, chromium, and iron
complexes bearing the azaferrocene-based PNP-type pincer ligands have been
prepared and characterized by X-ray analysis. The stoichiometric and catalytic
reactivity of molybdenum-dinitrogen complexes toward nitrogen fixation has been
investigated in details.
N
N
PR2
PR2
PR2
N
N
Mo
N N
Fe
P
X Fe
+
Fe
R
N
2 N
N
R2P
PR2
Azaferrocene-Based
PNP-type Pincer Ligand
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European Journal of Inorganic Chemistry
10.1002/ejic.201601051
COMMUNICATION
Table 1: Protonolysis of molybdenum-dinitrogen complexes (10).
Entry Complex
NH3 (mmol) NH3 (equiv/Mo)[a] NH2NH2 (mmol) NH2NH2 (equiv/Mo)[a]
1
10a
10c'
9
0.019
0.001
0.049
0.48
0.02
0.61
0.001
0
0.005
0.02
0
0.06
2
3b
[a] Equiv based on Mo atom of the complex.
[b] Reference 22.
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