Catalytic Reactivity of Molybdenum–Trihalide Complexes Bearing PCP-Type Pincer Ligands

Article abstract of DOI:10.1002/asia.201900496

Molybdenum–iodide complexes bearing a PCP[1] ligand have been found to work as excellent catalysts toward ammonia formation under ambient reaction conditions among dinitrogen-bridged dimolybdenum complexes and other molybdenum complexes bearing PNP and PCP[2] ligands.

Full text of DOI:10.1002/asia.201900496

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Catalytic Reactivity of Molybdenum–Trihalide Complexes Bearing
PCP-Type Pincer Ligands
Authors: Yoshiaki Nishibayashi, Aya Eizawa, Kazuya Arashiba, Akihito
Egi, Hiromasa Tanaka, Kazunari Nakajima, and Kazunari
Yoshizawa
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10.1002/asia.201900496
Chemistry - An Asian Journal
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Catalytic Reactivity of Molybdenum–Trihalide Complexes Bearing
PCP-Type Pincer Ligands
Aya Eizawa,[a] Kazuya Arashiba,[a] Akihito Egi,[b] Hiromasa Tanaka,[c] Kazunari Nakajima,[d] Kazunari
Yoshizawa,*[b] Yoshiaki Nishibayashi*[a]
Abstract: Molybdenum-iodide complexes bearing PCP[1] ligand
have been found to work as the best catalyst toward ammonia
formation under ambient reaction conditions among dinitrogen-
bridged dimolybdenum complex and other molybdenum complexes
bearing PNP and PCP[2] ligands.
methylene linkers (PCP[1] ligand, PCP[1] = 1,3-bis((di-tert-
butylphosphino)methyl)benzimidazol-2-ylidene) and the other
has ethylene linkers (PCP[2] ligand, PCP[2] = 1,3-bis(2-(di-tert-
butylphosphino)ethyl)imidazol-2-ylidene) between the NHC and
two phosphine units. Interestingly, the use of a molybdenum–
dinitrogen
complex
bearing
the
PCP[1]
ligands
([{Mo(N2)2(PCP[1])}2(µ-N2)]: 2a) as a catalyst afforded 100 equiv
of ammonia from dinitrogen based on the molybdenum atom of
the catalyst under ambient reaction conditions, while the use of a
molybdenum–dinitrogen complex bearing the PCP[2] ligands
([{Mo(N2)2(PCP[2])}2(µ-N2)]: 3a) afforded only up to 1.6 equiv of
ammonia (Scheme 1a).[34] The different reactivity of 2a and 3a
depends on the stability of the dinitrogen-bridged dimolybdenum
structure in solution, where the catalytic reaction proceeds via
Ammonia is an essential compound for human beings since
ammonia is feedstock for fertilizer and nitrogen-containing
compounds. Nowadays ammonia is industrially produced by the
Haber-Bosch process, which requires dihydrogen derived from
fossil fuels and harsh reaction conditions namely high
temperature and high pressure.[1] In sharp contrast, nitrogenase
enzymes convert dinitrogen into ammonia under ambient reaction
The active sites of nitrogenase include
Mimicking the
active site, nitrogen fixation reaction using a stoichiometric
amount of transition metal–dinitrogen complexes under mild
reaction conditions has been widely investigated.[8–11]
In 2003, Schrock and a co-worker reported the first catalytic
conversion of dinitrogen into ammonia with a mononuclear
molybdenum–dinitrogen complex under ambient reaction
conditions.[12–15] Since the first report using a molybdenum–
dinitrogen complex, other research groups have succeeded in
catalytic conversion of dinitrogen into ammonia or hydrazine
under mild reaction conditions such as -78 °C employing iron[16–
25]–, cobalt[26]–, ruthenium[27]–, osmium[27]–, vanadium[28]–, and
titanium[29]–dinitrogen complexes.
conditions.[2,3]
molybdenum, iron, and vanadium atoms.[4–7]
sequential protonation and reduction processes.
The first
protonation step of the terminal dinitrogen ligand on the
dinitrogen-bridged dimolybdenum core of 2a took place smoothly,
but no protonation of the corresponding mononuclear dinitrogen
complexes generated from 3a in solution occurred at all under the
same reaction conditions.
a) Catalytic conversion of dinitrogen to ammonia with molybdenum–dinitrogen complex
bearing PCP ligands
cat.
6 e–
6 H+
N2
+
+
2 NH3
rt
1 atm
N
N
cat.
N
N
N
N
P
P
P
N
N
N
P
N
N
N
Mo
N
N
Mo
Mo N N Mo
N
N
N
N
P
N
P
N
N
P
N
N
N
N
P
N
PCP[1] ligand
PCP[2] ligand
In 2010, our group found that a dimolybdenum–dinitrogen
complex bearing PNP-type pincer ligands ([{Mo(N2)2(PNP)}2(µ-
N2)]: 1a, PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine)
worked as more effective catalyst under ambient reaction
conditions, where up to 12 equiv of ammonia were produced
based on the molybdenum atom of the catalyst.[30–33] Based on
the research background, we designed two kinds of PCP-type
pincer ligands, which includes one N-heterocyclic carbene (NHC)
unit between two phosphine units.[34] One of the PCP ligands has
2a, 100 equiv/Mo
3a, 1.6 equiv/Mo
P = PtBu2
b) Catalytic conversion of dinitrogen to ammonia via cleavage of nitrogen–nitrogen triple bond
cat. 1b
6 e–
6 H+
N2
+
+
2 NH3
up to 415 equiv/Mo
rt
1 atm
P
I
Mo
N
N
N
Mo
N
P
N
P
P
2 e–, N2
N
I
I
N
P
P
P
N
P
Mo
I
N
P
Mo
I
N
P
Mo
I
I
NH3
P
1b
N
Mo
NH3
3 H+, 3 e–
I
P
N2
P = PtBu2
[a]
[b]
Dr. A. Eizawa, Dr. K. Arashiba, Prof. Dr. Y. Nishibayashi
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
c) Stoichiometric reaction of molybdenum-triiodide complex to nitride complex
N
I
P
N2, 2 e–
rt
P
N
P
Mo
I
N
P
Mo
I
A. Egi, Prof. Dr. K. Yoshizawa
I
1b
P
Institute for Materials Chemistry and Engineering,
Kyushu University, Nishi-ku, Fukuoka, 819-0395 (Japan)
E-mail: kazunari@ms.ifoc.kyushu-u.ac.jp
Dr. H. Tanaka
School of Liberal Arts and Sciences, Daido University
Takiharu-cho, Minami-ku, Nagoya, 457-8530 (Japan)
Dr. K. Nakajima
I
Mo
N
N
N
N
P
P
Mo
P
[c]
[d]
P = PtBu2
I
Scheme 1. Previous work: catalytic nitrogen fixation with dimolybdenum-
Frontier Research Center for Energy and Resources,
School of Engineering, The University of Tokyo
Hongo, Bunkyo-ku, Tokyo, 113-8656 (Japan)
Supporting information for this article is given via a link at the end of
the document.
dinitrogen complexes bearing PNP-type pincer ligands.
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10.1002/asia.201900496
Chemistry - An Asian Journal
COMMUNICATION
More recently, our group has reported the first catalytic
reaction of dinitrogen into ammonia under ambient reaction
conditions which involves direct cleavage of nitrogen–nitrogen
triple bond of the bridging dinitrogen ligand as a key reaction step,
where molybdenum–triiodide complex bearing the PNP ligand
([Mol3(PNP)]: 1b) was used as a precatalyst (Scheme 1b). In this
reaction system, up to 415 equiv of ammonia were produced
based on the molybdenum atom of the catalyst and the catalytic
activity of 1b is 35 times higher than that of 1a.[35] Two-electron
reduction of 1b afforded a molybdenum(IV)–nitride complex via
direct cleavage of the bridging dinitrogen ligand on the two
molybdenum complexes (Scheme 1c). Results of stoichiometric
and catalytic reactions indicate that the molybdenum–nitride
complex is involved as a key reactive intermediate.
As an extensive study of our novel reaction system, we have
now envisaged that molybdenum–triiodide complex bearing
PCP[1] ligand ([MoI3(PCP[1])]: 2b) may work as a more effective
catalyst under ambient reaction conditions than 2a and 1b
(Scheme 2). As a result, we have found that the catalytic activity
of 2b is higher than those of 1b, 2a, and molybdenum–triiodide
complex bearing PCP[2] ligand ([Mol3(PCP[2])]: 3b) (Scheme 2).
As a result, we have found that [MoI3(PCP[1])] 2b is the best
catalyst for the molybdenum-catalyzed ammonia formation under
([MoBr3(PCP[1])] (2c) and [MoCl3(PCP[1])] (2d)) as catalysts
under the same reaction conditions afforded 49 and 34 equiv of
ammonia based on the molybdenum atom of the catalysts,
respectively (Table 1, entries 4 and 5). These results indicate
that 2d worked as a less effective catalyst than 2b and 2c.
However, addition of 3 equiv of NaI to 2d dramatically improved
catalytic activity and selectivity for ammonia formation under the
same reaction conditions, where 50 equiv of ammonia and 2.9
equiv of dihydrogen were produced based on the molybdenum
atom of 2d (Table 1, entry 6). When we added 3 equiv of NaI to
2b and 2c under the same reaction conditions, a slightly higher
catalytic activity and selectivity for ammonia formation were
observed in both cases (Table 1, entries 7 and 8). At present,
we consider that the ligand exchange of chloride and bromide
ligands on [MoX3(PCP[1])] with iodide ligand derived from NaI
may readily occur to afford the same molybdenum–iodide species
with 2b under the catalytic reaction conditions.[36] This result
indicates that we can use a mixture of 2d and 3 equiv of NaI to
check the reactivity of 2b because 2d is more readily available
than 2b.
Table 1. Catalytic nitrogen fixation reaction with molybdenum–PCP[1]
complexes
OTf
cat.
ambient reaction conditions.
preliminary results.
Herein, we have reported
N2
+
+
6
6
Co
2 NH3
toluene
rt, 20 h
N
H
1 atm
180 equiv/Mo
240 equiv/Mo
cat.
This work
I
Entrya
Cat.
NaI
(equiv/Mo)
NH3
(equiv/Mo)
NH3
H2
(equiv/Mo)
H2
(%)b
I
P
P
cat.
N
N
(%)b
82
48
87
82
57
83
83
88
85
72
50
85
6 e– 6 H+
+
N2
+
2 NH3
Mo
I
Mo
I
N
N
rt
P
1 atm
P
I
I
P = PtBu2
1
2b
2a
2a
2c
2d
2d
2b
2c
1b
2d
1b
4’
0
0
3
0
0
3
3
3
0
3
0
0
49
29
52
49
34
50
50
53
51
167
121
51
3.6
4
2b
3b
Scheme 2. Conversion of dinitrogen to ammonia under mild conditions.
2
22
24
4
3
3.7
First, we compared the reactivity of 2b with that of 2a under
the same reaction conditions. In the presence of a catalytic
amount of 2b or 2a, reactions of atmospheric pressure of
dinitrogen with 180 equiv of decamethylcobaltocene (CoCp*2; Cp*
= η5-C5Me5) as a reducing reagent and 240 equiv of 2,4,6-
collidinium trifluoromethanesulfonate ([ColH]OTf; Col = 2,4,6-
collidine, OTf = trifluoromethanesulfonate) as a proton source in
toluene at room temperature for 20 h afforded 49 or 29 equiv of
ammonia based on the molybdenum atom of the catalyst together
with 3.6 or 22 equiv of dihydrogen, respectively (Table 1, entries
1 and 2). This result indicates that 2b has a higher catalytic
activity and selectivity for ammonia formation than 2a under the
same reaction conditions. Surprisingly, addition of 3 equiv of
sodium iodide (NaI) to the molybdenum atom of 2a improved the
catalytic activity and selectivity for ammonia formation. In this
case, 52 equiv of ammonia and only 3.7 equiv of dihydrogen were
produced based on the molybdenum atom of 2a (Table 1, entry
3). Based on the dramatic effect of NaI, we consider that the
same molybdenum–iodide species with 2b may be generated
from 2a and NaI. In fact, we observed a similar phenomenon
with the reaction using a mixture of 1a and 3 equiv of NaI as
catalyst in our previous reaction system, where the corresponding
molybdenum–iodide species from 1b may be generated from 1a
and NaI.[35]
4
3.6
4
5
9.4
10
3
6
2.9
7
1.9
2
8
2.1
2
9c
10d
11d
12
0.5
0.6
22
21
3
81
77
3.1
a
To a suspension of catalyst (0.002 mmol) and [ColH]OTf (0.48 mmol) in
toluene (1 mL) was added a solution of CoCp*2 (0.36 mmol) in toluene (4 mL)
at a rate of 4 mL per hour via a syringe pump and the mixture was stirred for
another 19 h at room temperature under atmospheric pressure of dinitrogen.
bYields based on reductant. cReference 35 dCoCp*2 (720 equiv) and ColHOTf
(960 equiv) were used.
The catalytic activity of [MoX3(PCP[1])] is as follows:
[MoI3(PCP[1])] 2b > [MoBr3(PCP[1])] 2c >> [MoCl3(PCP[1])] 2d.
The tendency is consistent with the previous reaction using
[MoX3(PNP)] as catalysts. The catalytic activity of [MoX3(PNP)]
is as follows: [MoI3(PNP)] 1b > [MoBr3(PNP)] 1c > [MoCl3(PNP)]
1d[35] because iodide worked as more electron-withdrawing ligand
Next, we investigated the nature of halide ligand of the
molybdenum complex [MoX3(PCP[1])]. The use of molybdenum–
tribromide and –trichloride complexes bearing PCP[1] ligand
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than bromide and chloride as ligand in transition metal
with 2b under the same reaction conditions (Table 1, entry 12).
Based on the result of the stoichiometric and catalytic reactions,
we believe that ammonia formation using 2b as a catalyst
proceeded via molybdenum–nitride complex such as 4 which was
generated from direct cleavage of the bridging dinitrogen ligand
on the two molybdenum complexes.
complexes.[37]
Thus, the presence of electron-withdrawing
groups increased the catalytic activity. We have already found
that the presence of electron-withdrawing groups increased the
catalytic activity in our previous papers.[35,38]
To get more information on the different reactivity of 2b and
2a, we monitored the time profile for the catalytic conversion of
dinitrogen into ammonia using a mixture of 2d and 3 equiv of NaI
and 2a as catalysts, respectively. The reaction with 108 equiv of
CoCp*2 as a reducing reagent and 144 equiv of [ColH]OTf as a
proton source was carried out in toluene at room temperature.
OTf
cat.
N2
+
+
2 NH3
Co
N
toluene
rt
H
108 equiv/Mo 144 equiv/Mo
a)
b)
Typical results are shown in Figure 1a.[39]
ammonia formation completed within 1 h.
In both cases,
Substantially,
30
30
ammonia formation using a mixture of 2d and 3 equiv of NaI
proceeded more rapidly than that using 2a. These experimental
results support that [MoI3(PCP[1])] 2b has a better ability than
[{Mo(N2)2(PCP[1])}2(µ-N2)] 2a as a catalyst.
20
20
c
10
0
10
0
Next, we compared the reactivity of 2b with that of
[Mol3(PNP)] 1b. As described in the Introduction of the present
paper, we already reported the catalytic activity of 1b, where 51
equiv of ammonia and 0.5 equiv of dihydrogen were produced
based on the molybdenum atom of 1b (Table 1, entry 9).[35] For
comparison, the time profile for ammonia formation using a
mixture of 2d and 3 equiv of NaI as catalyst, in place of 2b, was
investigated under the same reaction conditions. Typical results
are shown in Figure 1b together with the time profile for ammonia
formation using 1b as catalyst. Although the rate of ammonia
formation for 2b is almost the same with that of 1b, a slightly
higher amount of ammonia was produced based on the
molybdenum atom of the catalyst. In addition, when we used
larger amounts of CoCp*2 (720 equiv) as a reducing reagent and
[ColH]OTf (960 equiv) as a proton source, a substantially different
0
2
4
20
0
2
4
20
Time / h
Time / h
a
To a suspension of catalyst (0.0033 mmol) and [ColH]OTf (0.48 mmol)
in toluene (1 mL) was added a solution of CoCp*2 (0.36 mmol) in toluene (4 mL)
at a rate of 4 mL per hour via a syringe pump and the mixture was stirred
for another 19 h at room temperature under atmospheric pressure of dinitrogen.
TOF (h–1) is defined as the amount of ammonia (equiv/Mo) produced in the initial 1 h.
[LutH]OTf and CrCp*2 were used instead of [ColH]OTf and CoCp*2 (Lut = 2,6-lutidine).
b
c
Figure 1. Time profile of conversion of dinitrogen to ammonia catalyzed by
molybdenum complexes bearing PCP[1] or PNP ligand.
(a) Stoichiometric reduction of molybdenum-triiodide complex under N2.
N2 (1 atm)
2 CoCp*2 (2.2 equiv)
I
N
P
P
Mo
N
N
Mo
I
N
toluene, rt, 1 h
N
P
I
P
I
2b
4
(b) Conversion of nitride ligand of complex 4' to ammonia.
reactivity was observed.
In this case, a larger amount of
Ar
I
N
OTf
ammonia (167 equiv) was formed together with 81 equiv of
dihydrogen in the reaction using a mixture of 2d and 3 equiv of
NaI as catalyst (Table 1, entry 10). Under the same reaction
conditions using 1b as a catalyst, 121 equiv of ammonia and 77
equiv of dihydrogen were produced based on the molybdenum
atom of the catalyst (Table 1, entry 11). These experimental
results indicate that 2b worked as a more effective catalyst than
1b under the same reaction conditions.
At present, we believe that the reaction pathway for 2b is
almost the same with that for 1b in our previous work.[35] Thus,
ammonia formation using 2b as a catalyst proceeds via direct
cleavage of nitrogen–nitrogen triple bond of the bridging
dinitrogen ligand as a key reaction step. To support our proposal,
we investigated the following stoichiometric and catalytic
reactions of 2b. Reduction of 2b with 2.2 equiv of CoCp*2 in
toluene at room temperature for 1 h under 1 atm of N2 afforded
the corresponding molybdenum–nitride complex (4) (Scheme 3a).
Unfortunately, we did not isolate the molybdenum–nitride
complex 4 as a pure form, however, we confirmed the formation
of the molybdenum–nitride complex 4 by ESI-TOF-MS (See the
(1 atm)
P
NH3
+
+
Co
N
Mo
toluene
rt, 1 h
N
H
N
I
P
87%
4 equiv/Mo
4 equiv/Mo
4'
Scheme 3. Stoichiometric reactivity of molybdenum complexes bearing PCP[1]
ligands.
Next, we checked the catalytic activity of [Mol3(PCP[2])] 3b.
We envisaged that 3b may work as an effective catalyst because
the reaction pathway using 3b may proceed via direct cleavage of
nitrogen–nitrogen triple bond of the bridging dinitrogen ligand,
which is completely different from the reaction pathway using 3a
as a catalyst.
We used [MoCl3(PCP[2])] (3d) in place of
[Mol3(PCP[2])] 3b because we could not prepare 3b as a pure
form. The use of 3d as a catalyst under the same reaction
conditions afforded 39 equiv of ammonia based on the
molybdenum atom of the catalyst together with 12 equiv of
dihydrogen (Table 2, entry 1). When a mixture of 3d and 3 equiv
of NaI was used as a catalyst, higher catalytic activity and
selectivity for ammonia formation were achieved, where 44 equiv
of ammonia were produced based on the molybdenum atom of
the catalyst (Table 2, entry 2).
Interestingly, addition of 3 equiv of NaI to the molybdenum
atom of 3a substantially increased the amount of ammonia (Table
2, entry 3). This result is in sharp contrast to our previous work
that 3a did not work as a catalyst at all, as described in the
Introduction (vide supra). Finally, we compared the reactivity of
Supporting Information for details).
Further stoichiometric
reaction of cationic molybdenum–nitride complex (4’), which was
prepared from the reaction of 2b with 1 equiv of Me3SiN3 in
tetrahydrofuran (THF) at 50 °C for 21 h, with 4 equiv of CoCp*2
and 4 equiv of [ColH]OTf in toluene at room temperature for 1 h
under 1 atm of Ar afforded ammonia in 87% yield (Scheme 3b).
Separately, we confirmed that 4’ has a similar catalytic activity
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Chemistry - An Asian Journal
COMMUNICATION
[Mol3(PCP[1])] 2b with that of [Mol3(PCP[2])] 3b. When we used
larger amounts of CoCp*2 (720 equiv) as a reducing reagent and
[ColH]OTf (960 equiv) as a proton source in the presence of a
mixture of 3d and 3 equiv of NaI, 107 equiv of ammonia were
produced based on the molybdenum atom of the catalyst together
with 78 equiv of dihydrogen (Table 2, entry 4). For comparison,
the time profile for ammonia formation using a mixture of 3d and
3 equiv of NaI as catalyst was investigated under the same
reaction conditions. The reaction with 108 equiv of CoCp*2 as a
reducing reagent and 144 equiv of [ColH]OTf as a proton source
was carried out in toluene at room temperature. Typical results
are shown in Figure 2 together with the time profile for ammonia
formation using a mixture of 2d and 3 equiv of NaI as catalyst.
In both cases, the ammonia formation was completed within 1 h.
Substantially, ammonia formation using a mixture of 2d and 3
equiv of NaI proceeded more rapidly than that using a mixture of
3d and 3 equiv of NaI. These experimental results indicate that
[MoI3(PCP[1])] 2b has a better ability than [MoI3(PCP[2])] 3b.
OTf
cat.
N2
+
+
2 NH3
Co
N
toluene
rt
H
108 equiv/Mo 144 equiv/Mo
30
20
10
0
0
2
4
Time / h
20
a
b
To a suspension of catalyst (0.0033 mmol) and [ColH]OTf (0.48 mmol)
in toluene (1 mL) was added a solution of CoCp*2 (0.36 mmol) in toluene (4 mL)
at a rate of 4 mL per hour via a syringe pump and the mixture was stirred
for another 19 h at room temperature under atmospheric pressure of dinitrogen.
TOF (h–1) is defined as the amount of ammonia (equiv/Mo) produced in the initial 1 h.
Figure 2. Time profiles for catalytic ammonia formation reaction with 3d and 2d
In summary, we have compared the catalytic activity of
[MoI3(PCP[1])] 2b with that of [{Mo(N2)2(PCP[1])}2(µ-N2)] 2a,
[Mol3(PNP)] 1b, and [Mol3(PCP[2])] 3b in detail. For comparison,
the corresponding catalytic activity including the amount of
ammonia and the rate of ammonia formation is summarized in
Figure 3. As a result, [MoI3(PCP[1])] 2b has been found to work
as the best catalyst among the four molybdenum complexes from
viewpoints of ammonia production and the rate of ammonia
Table 2. Catalytic nitrogen fixation reaction with molybdenum–PCP[2]
complexes
OTf
cat.
N2
+
+
6
Co
6
2 NH3
toluene
rt, 20 h
N
H
1 atm
180 equiv/Mo
240 equiv/Mo
formation under the same reaction conditions.[40]
However,
Entry
a
Catalys
t
NaI
(equiv/Mo
)
NH3
(equiv/Mo
)
NH3
(%)
b
H2
H2
(%)
b
(equiv/Mo
)
unfortunately, the amount of ammonia did not exceed 415
equiv[35] based on the Mo atom of the catalyst when 2b was used
as a catalyst. We believe that the achievement described in this
paper provides valuable information to develop the ultimate
reaction system for ammonia production under mild reaction
conditions.
1
3d
3d
3a
3a
0
3
3
3
39
65
72
81
44
12
13
5
2
44
4.6
4.0
78
3
49
4
4c
107
21
a
To a suspension of catalyst (0.002 mmol) and [ColH]OTf (0.48 mmol) in
OTf
cat.
toluene (1 mL) was added a solution of CoCp*2 (0.36 mmol) in toluene (4 mL)
at a rate of 4 mL per hour via a syringe pump and the mixture was stirred for
N2
+
+
2 NH3
Co
N
toluene
rt
H
another 19
h at room temperature under atmospheric pressure of
(1 atm)
dinitrogen..bYields based on reductant. c CoCp*2 (720 equiv) and ColHOTf (960
equiv) were used.
cat.
2b
2aa
1b
3b
At present, we expect that nitrogen fixation in the presence
of 2b involves formation of a dinitrogen-bridged Mo complex such
as MoI(PCP[X])–N≡N–MoI(PCP[X]) and cleavage of its N≡N bond,
in a similar manner as proposed for 1b in Scheme 1c. Our
preliminary DFT calculations estimate that the bond dissociation
free energies (BDFEs) of a bridging Mo–N bond in MoI(PCP[X])–
N≡N–MoI(PCP[X]) are 23.2 kcal/mol for PCP[1] and 16.6 kcal/mol
for PCP[2] at 298.15 K. These BDFEs are larger than that of
MoI(PNP)–N≡N–MoI(PNP) (14.7 kcal/mol),[35] and thus the
formation of dinitrogen-bridged Mo complexes is energetically
favorable in the Mo-PCP[X] systems, especially in the Mo-PCP[1]
system.
TON
167c 100
121
107d
(equiv/Mo)b
TOF
(h–1 e
31
21
28
21f
)
a
b
CrCp*2 and [LutH]OTf were used in place of CoCp*2 and [ColH]OTf.
TON is defined as equivs of ammonia produced in the reaction of catalyst,
reductant (180 equiv) and proton source (240 equiv) for 20 h.
A mixture of 2d and 3 equiv of NaI was used as a catalyst in place of 2b.
A mixture of 3a and 3 equiv of NaI was used as a catalyst in place of 3b.
TOF is defined as equivs of ammonia produced in the reaction of catalyst,
reductant (108 equiv) and proton source (144 equiv) in the initial 1 h.
A mixture of 3d and 3 equiv of NaI was used as a catalyst in place of 3b.
c
d
e
f
Figure 3. Catalytic activity and turnover number of molybdenum–PCP
complexes.
Quite recently, we have reported that the introduction of
electron-donating groups at the 4-position of the pyridine ring in
the PNP pincer ligand substantially decreased the catalytic
activity.[38] The tendency is consistent with that 3b worked as a
less effective catalyst than 2b because PCP[2] ligand worked as
more electron-donating ligand than PCP[1] ligand.[34]
Acknowledgements
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The present project is supported by CREST, JST (JPMJCR1541).
We acknowledge Grants-in-Aids for Scientific Research (Nos.
JP17H01201, JP15H05798, JP18K19093 and JP18K05148) from
JSPS and MEXT. A.E. is a recipient of the JSPS Predoctoral
Fellowships for Young Scientists. We also thank the Research
Hub for Advanced Nano Characterization at The University of
Tokyo for X-ray analysis.
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In this paper, we have defined the best catalyst from viewpoints of
ammonia production and the rate of ammonia formation under the same
reaction conditions although there are many factors to determine the best
catalyst.
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
cat.
rt
6 H+
+
6 e‒
N2
+
2 NH3
Molybdenum-iodide complexes
bearing PCP[1] ligand have been
found to work as the best catalyst
toward ammonia formation under
ambient reaction conditions among
dinitrogen-bridged dimolybdenum
complex and other molybdenum
complexes bearing PNP and PCP[2]
ligands.
Aya Eizawa, Kazuya Arashiba, A. Egi,
Hiromasa Tanaka, Kazunari Nakajima,
Kazunari Yoshizawa,* Yoshiaki
Nishibayashi*
1 atm
cat.
X
X
P
P
N
N
Mo
X
Mo
X
Page No. – Page No.
N
N
P
Catalytic Reactivity of Molybdenum–
Trihalide Complexes Bearing PCP-
Type Pincer Ligands
P
X
PCP[1]
X
PCP[2]
P = PtBu2
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