Preparation and reactivity of iron complexes bearing anionic carbazole-based PNP-type pincer ligands toward catalytic nitrogen fixation

Article abstract of DOI:10.1039/c7dt04327a

Iron-chloride, -dinitrogen, and -methyl complexes bearing anionic carbazole-based PNP-type pincer ligands are designed, prepared and characterized by X-ray analysis. Some iron complexes are found to work as catalysts toward nitrogen fixation under mild re

Full text of DOI:10.1039/c7dt04327a

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Preparation and reactivity of iron complexes
bearing anionic carbazole-based PNP-type pincer
ligands toward catalytic nitrogen fixation†
Cite this: Dalton Trans., 2018, 47,
1117
Junichi Higuchi, Shogo Kuriyama, Aya Eizawa,
Kazunari Nakajima and Yoshiaki Nishibayashi
Kazuya Arashiba,
*
Received 16th November 2017,
Accepted 5th December 2017
Iron–chloride, –dinitrogen, and –methyl complexes bearing anionic carbazole-based PNP-type pincer
ligands are designed, prepared and characterized by X-ray analysis. Some iron complexes are found to
work as catalysts toward nitrogen fixation under mild reaction conditions.
DOI: 10.1039/c7dt04327a
rsc.li/dalton
Introduction
The development of catalytic transformations of nitrogen gas
in the presence of transition metal–dinitrogen complexes as
catalysts under mild reaction conditions is one of the most
1
important subjects in chemistry. Since the first discovery of
the direct and catalytic conversion of dinitrogen into ammonia
under ambient reaction conditions reported by Schrock and
2
co-workers in 2003, some successful examples have been
3
4
5
achieved by using molybdenum –, iron –, and cobalt –dinitro- Scheme 1 Iron-catalysed reduction of dinitrogen using 1.
gen complexes as catalysts. In 2010, we reported the catalytic
formation of ammonia from nitrogen gas under ambient reac-
tion conditions using a dinitrogen-bridged dimolybdenum–
β-protonated pyrrole moiety (2a) from a stoichiometric reac-
dinitrogen complex bearing pyridine-based PNP-type pincer
F
9
6
,7
tion of 1a with 1 equiv. of [H(OEt ) ][BAr ]. Unfortunately,
2 2 4
2
1
ligands as a catalyst. Quite recently, we have found a novel
nitrogen fixation system via a direct cleavage of the nitrogen–
nitrogen triple bond of molecular dinitrogen using molyb-
a has a lower catalytic activity toward nitrogen fixation than
a. This experimental result indicates that 2a is considered to
8
be one of the deactivated species in the catalytic reaction.
Quite recently, we have found that an iron–dinitrogen
complex bearing a dimethyl-substituted pyrrole-based PNP-
type pincer ligand (1b) worked as a more effective catalyst
than 1a toward the catalytic transformation, a mixture of
ammonia and hydrazine being produced (22.7 equiv. and 1.7
equiv. based on the catalyst, respectively; 26.1 equiv. of fixed
N atoms based on the catalyst) (Scheme 1). However, a sub-
stantial improvement has not yet been achieved in the modi-
fied reaction system.
denum–iodide complexes bearing a PNP-pincer ligand.
Based on our previous study, we have recently found that
an iron–dinitrogen complex bearing an anionic pyrrole-based
PNP-type pincer ligand ([Fe(N )(pyr-PNP)] 1a; pyr-PNP = 2,5-
2
bis(di-tert-butylphosphinomethyl)pyrrolide) worked as an
effective catalyst in the reaction of dinitrogen gas with KC as
a reductant and [H(OEt
at −78 °C for 1 h to give a mixture of ammonia and hydrazine
14.3 equiv. and 1.8 equiv. based on the catalyst, respectively;
7.9 equiv. of fixed atoms based on the catalyst)
Scheme 1). In this reaction system, we isolated and charac-
8
F
2
)
2
][BAr
4
] as a proton source in Et
2
O
1
0
(
1
(
N
9
Based on the previous findings, we have now newly
designed and prepared novel iron–dinitrogen complexes
bearing anionic carbazole-based PNP-type pincer ligands
terized the corresponding iron–dinitrogen complex bearing a
(
2
[Fe(N )(carb-PNP)] 3; carb-PNP = 1,8-bis(dialkylphosphino-
methyl)-3,6-di-tert-butyl-carbazolide) because the protonation
of the carbazole moiety may occur more hardly than that of
the pyrrole moiety such as 1 (Chart 1). Herein, we report the
preparation and reactivity of iron complexes bearing anionic
carbazole-based PNP-type pincer ligands.
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
†
Electronic supplementary information (ESI) available. CCDC 1546201, 1546204,
546202 and 1546203. For ESI and crystallographic data in CIF or other elec-
1
tronic format see DOI: 10.1039/c7dt04327a
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Chart 1 Iron–dinitrogen complex bearing a novel PNP-type pincer
ligand.
Results and discussion
Gade and co-workers previously reported the preparation of
i
carbazole-based PNP-type pincer ligands bearing Ph
2
P, Pr
2
P,
1
1,12
and phospholane moieties.
According to their pro-
1,8-bis(bromomethyl)-3,6-di-tert-butyl-9H-carba-
1
1,12
cedure,
zole (4) was prepared as a precursor of the corresponding PNP-
type pincer ligands. The reaction of 4 with di-tert-butylphos-
phine and di-1-adamantylphosphine in acetone at reflux
temperature gave 1,8-bis(di-tert-butylphosphinomethyl)-3,6-di-
tert-butyl-9H-carbazole (5a) and 1,8-bis(di-1-adamantylphos-
phinomethyl)-3,6-di-tert-butyl-9H-carbazole (5b) in excellent
yields, respectively (Scheme 2). The treatment of [FeCl
2
(thf)1.5]
with 1.0 equiv. of lithium carb-PNP, which were prepared from
n
5
a with 1.1 equiv. of BuLi in THF at 0 °C for 1 h or generated
in situ from 5b with 1.1 equiv. of LiN(SiMe in THF at room
3 2
)
temperature for 1 h, in toluene at room temperature afforded
the corresponding iron–chloride complexes bearing the carb-
PNP ligand [FeCl(carb-PNP)] (6a and 6b) in 82% and 72%
yields, respectively (Scheme 2). The molecular structures of 6a
and 6b were confirmed by X-ray analysis.‡ The unit cell of 6a
contains two crystallographically independent molecules and
one of them contains whole molecule disorder. The ORTEP
drawings of 6a and 6b are shown in Fig. 1. The crystal struc-
tures of both 6a and 6b have distorted tetrahedral geometries
Scheme 2 Preparation of iron–dinitrogen and –methyl complexes.
around the iron atoms (the geometry index τ
4
= 0.79 for 6a and
0
.80 for 6b, respectively), where τ = 0.00 for a perfect square
4
1
3
planar and τ
6
4
4
= 1.00 for a tetrahedral geometry. Complexes
and
.7 ± 0.3μ at 298 K, respectively, which are consistent with the
a and 6b have solution magnetic moments of 4.9 ± 0.3μ
B
B
spin-only value for an S = 2 spin state (4.9μ
B
).
Reduction of 6a with 1.1 equiv. of KC
8
in THF at room
temperature for 17 h under an atmospheric pressure of nitro-
gen gas gave a dinitrogen-bridged diiron complex bearing
carb-PNP ligands [Fe(carb-PNP)]
(
2
(μ-N
2
) (7a) in 56% yield Fig. 1 ORTEP drawings of 6a (a) and 6b (b). Thermal ellipsoids are
Scheme 2). The molecular structure of 7a was confirmed by
shown at the 50% level. Hydrogen atoms are omitted for clarity.
X-ray analysis.‡ An ORTEP drawing of 7a is shown in Fig. 2(a).
The crystal structure of 7a has a distorted tetrahedral geometry
gen-bridged diiron complexes with a tetrahedral geometry
around the iron atoms have already been reported by other
research groups. Iron complex 7a has a solution magnetic
moment of 7.5 ± 0.7 at 298 K. When the magnetic moment
and the tetrahedral structure are taken into consideration,
around the iron atom (τ = 0.77 and 0.71 in two iron atoms,
4
respectively) with a bridged dinitrogen ligand in an end-on
fashion. The N–N bond length of 1.106(4) Å in 7a indicates the
weak activation of the coordinated dinitrogen. Similar dinitro-
1
4
B
complex 7a can be assigned an S = 3 (6.9μ ) state. No absor-
bance assignable to the terminal dinitrogen ligand in the IR
‡
Crystal structures of 6a, 6b, 7a, and 8a were determined by X-ray analysis. See
the ESI† for details.
spectra of a THF solution of 7a or in a solid state (KBr) indi-
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As shown in the previous sections, molecular structures are
quite different between iron–dinitrogen complexes bearing
pyr- and carb-PNP-type pincer ligands. While 1a with a low
9
spin square planar state adopted a mononuclear structure, 7a
with a high spin tetrahedral state has been found to have a
dinitrogen-bridged dinuclear structure in solid and solution
states. These results indicate that the geometry around the
iron atom determines the formation of a mononuclear or a
dinuclear structure in the present system. At present, we con-
sider that the tetrahedral configuration of 7a decreases the
steric hindrance around the coordinated dinitrogen to form a
Fig. 2 ORTEP drawings of 7a (a) and 8a (b). Thermal ellipsoids are dinitrogen-bridged dinuclear structure.
shown at the 50% level. Hydrogen atoms are omitted for clarity.
Next, we investigated the catalytic reaction in the same
9
experimental procedure originally reported by Peters and co-
4
workers. The reaction of atmospheric dinitrogen gas with 40
F
4
cates that the dinuclear structure remains in a THF solution of equiv. of KC as a reductant and 38 equiv. of [H(OEt ) ][BAr ]
8
2 2
7
a. Unfortunately, we cannot isolate the corresponding dinitro- as a proton source in the presence of a catalytic amount of 7a
gen complex after the reduction of 6b with KC8 under an in Et O at −78 °C for 1 h gave only 0.5 equiv. of ammonia
2
atmospheric pressure of nitrogen gas, where only an un- based on the iron atom of the catalyst together with 1.6 equiv.
identified mixture was obtained.
Reactions of 6a and 6b with 1.1 equiv. of MeMgCl in Et O was observed when THF was used as a solvent in place of Et O
of hydrogen gas (Table 1, entry 1). No formation of ammonia
2
2
and THF at room temperature for 1–3 h afforded the corres- under the same reaction conditions, where only hydrogen (2.8
ponding methyl complexes [FeMe(carb-PNP)] (8a and 8b) in equiv. based on the iron atom of the catalyst) gas was observed
8
5% and 64% yields, respectively (Scheme 2). The molecular (Table 1, entry 2). The use of methyl complex 8a as a catalyst
structure of 8a was confirmed by X-ray analysis.‡ An ORTEP gave 1.9 equiv. of ammonia and 3.2 equiv. of hydrogen gas
drawing of 8a is shown in Fig. 2(b). The crystal structure of 8a based on the iron atom, respectively (Table 1, entry 3).
has a distorted tetrahedral geometry around the iron atom Interestingly, methyl complex bearing adamantyl groups at the
with τ of 0.81. Iron complexes 8a and 8b have the same solu- phosphorus atom 8b has a catalytic activity, where 3.2 equiv.
4
tion magnetic moment of 4.7 ± 0.3μ
plexes, the spin state can be assigned S = 2 (4.9μB).
In sharp contrast to an intermediate spin square planar larger amounts of KC8 and [H(OEt ) ][BAr ] were used,
B
at 296 K. In both com- of ammonia and 2.1 equiv. of hydrogen gas were produced
based on the iron atom, respectively (Table 1, entry 4). When
F
2 2
4
9
state of iron(II) complexes bearing a pyr-PNP pincer ligand,
respectively, as a reductant and as a proton source, a mixture
the iron(II) complexes bearing a carb-PNP pincer ligand of ammonia and hydrazine (3.2 equiv. and 0.8 equiv. based on
adopted a high spin tetrahedral geometry around the iron the iron atom, respectively) was obtained together with 3.4
atom. We consider that the size of the chelate ring rather than equiv. of hydrogen gas (Table 1, entry 5). When 8 were used as
the nature of the N-donor of the PNP-type pincer ligands deter- pre-catalysts, the corresponding anionic mononuclear iron(0)–
mines the configuration of the iron complexes. As a result, the dinitrogen complexes such as 9 may be formed under catalytic
rigid nature of the pyr-PNP pincer ligand may favour the reaction conditions (Scheme 3).
square planar geometry around the iron atom. On the other
As shown in the previous section, the newly prepared dinuc-
hand, the carb-PNP pincer ligand, which has two more methyl- lear dinitrogen-bridged iron complex 7a has no catalytic
ene groups in the chelate ring, is considered to adopt a tetra- activity under the same reaction conditions. On the other
hedral structure around the iron atom to decrease the strain of hand, mononuclear methyl complexes 8 have a higher activity
the chelate ring.
toward nitrogen fixation than 7a, where 8b with bulkier substi-
a
Table 1 Iron-catalyzed reduction of dinitrogen to ammonia and hydrazine
Catalyst
ꢁ!
F
4
N2 þKC8 þ ½HðOEt2Þ ꢀ½BAr ꢀ
NH3 þ N2H4
2
1
atm
Et2O; ꢁ78 °C
1
h
b
F
b
b
b
b
Entry
1
Catalyst
KC
8
(equiv.)
[H(OEt
2 2
) ][BAr
4
] (equiv.)
NH
3
(equiv.)
N
2
H
4
(equiv.)
H
2
(equiv.)
c
c
c
7a
7a
8a
8b
8b
40
40
40
40
80
38
38
38
38
76
0.5 ± 0.3
<0.1
1.9 ± 0.1c
3.2 ± 0.1c
3.2 ± 0.7
<0.1
0
1.6 ± 0.7
2.8
3.2 ± 0.2c
2.1 ± 0.3c
3.4 ± 0.1
d
2
c
c
c
3
4
5
0
c
0
c
0.8 ± 0.1
a
F
To a mixture of catalysts, KC
8
and [H(OEt
2 2
) ][BAr
4 2
] was added Et O at −78 °C, and then the resultant mixture was stirred at −78 °C for 1 h.
b
c
d
Equiv. based on the Fe atom. An average of multiple runs (>2 times) is shown. THF was used in place of Et
2
O.
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JSPS and MEXT. A. E. is a recipient of the JSPS Predoctoral
Fellowships for Young Scientists. We also thank Dr Yoshiaki
Tanabe of the University of Tokyo for his assistance in X-ray
analysis.
Scheme 3 Formation of anionic mononuclear iron–dinitrogen
complex 9 from 8.
Notes and references
1
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tuents such as adamantyl groups at the phosphorus atom
worked as a more effective catalyst than 8a. These results indi-
cate that the formation of anionic mononuclear iron(0)–di-
nitrogen complexes as key reactive intermediates of catalytic
nitrogen fixation, which was proposed in the previous report
9
by our group, is more favoured from 8 than 7a.
For a comparison of the catalytic activity of iron complexes
bearing pyr- and carb-PNP-type pincer ligands, the molecular
structure of the iron complexes is considered to be one of the
most important factors toward catalytic nitrogen fixation. In
fact, iron complexes bearing a pyr-PNP-type pincer ligand
provide a square planar geometry around the iron atom. On
the other hand, iron complexes bearing a carb-PNP-type pincer
ligand provide a tetrahedral geometry around the iron atom.
Although no direct relationship between the molecular struc-
ture and reactivity of iron complexes has yet been clarified in
the present reaction system, we believe that the molecular
structure of iron complexes has a great influence on the cata-
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anionic carbazole-based PNP-type pincer ligands. Iron–chlor-
ide, –methyl, and –dinitrogen complexes bearing these PNP-
type pincer ligands have also been prepared and characterized
by X-ray analysis. As a result, the iron–methyl complex bearing
more bulkier substituents such as adamantyl groups at the
phosphorus atom has been found to work as a catalyst toward
nitrogen fixation under mild reaction conditions, where up to
5
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