Effect of substituents on molybdenum triiodide complexes bearing PNP-type pincer ligands toward catalytic nitrogen fixation

Article abstract of DOI:10.1039/c8dt04975k

Molybdenum triiodide complexes bearing various substituted pyridine-based PNP-type pincer ligands are prepared and characterized by X-ray analysis. Their catalytic activity is investigated toward the reduction of nitrogen gas into ammonia under ambient reaction conditions.

Full text of DOI:10.1039/c8dt04975k

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DOI: 10.1039/C8DT04975K
Journal Name
COMMUNICATION
Effect of substituents on molybdenum triiodide complexes
bearing PNP-type pincer ligands toward catalytic nitrogen fixation
a
a
a
a
b
Takayuki Itabashi, Ikki Mori, Kazuya Arashiba, Aya Eizawa, Kazunari Nakajima, and Yoshiaki
Nishibayashi*
Received 00th January 20xx,
Accepted 00th January 20xx
a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Molybdenum triiodide complexes bearing various substituted heterocyclic carbene skeleton in place of the PNP-type pincer
pyridine-based PNP-type pincer ligands are prepared and ligands.9
,10
1
1
characterized by X-ray analysis.
investigated toward reduction of nitrogen gas into ammonia under
ambinet reaction conditions.
Their catalytic activity is
As an extensive study of our work, we more recently found
that a molybdenum triiodide complex bearing the same PNP-
type pincer ligand worked as a more effective catalyst toward
catalytic nitrogen fixation, where up to 415 equiv of ammonia
1
2
were produced based on the Mo atom of the catalyst, than
the dinitrogen-bridged dimolybdenum complex. Interestingly,
this novel reaction system is proposed to proceed via direct
cleavage of the nitrogen-nitrogen triple bond of the
Ammonia is a typical and well-known raw material as nitrogen
source of fertilizers, dyes, chemical fibres, and
1
pharmaceuticals. Recently, ammonia has attracted attention
as energy and hydrogen carriers because ammonia has unique
properties such as high energy density, low flammability, and
1
2
coordinated dinitrogen ligand on the dimolybdenum atoms.
This remarkable catalytic activity in the novel reaction system
prompted us to prepare molybdenum triiodide complexes
bearing various substituted PNP-type pincer ligands and
no carbon content.2
Therefore, development of a novel
ammonia production process under mild reaction conditions is
eagerly anticipated to replace the Haber-Bosch process as the investigate their catalytic activity toward ammonia production
industrial ammonia production requiring harsh reaction under ambient reaction conditions. As a result, we have found
conditions. Since the first successful example discovered by that the introduction of phenyl and ferrocenyl groups to the
3
pyridine ring of the PNP-type pincer ligand substantially
increased the catalytic activity. Herein, we have reported
preliminary results.
According to the previous procedure, we newly prepared a
variety of molybdenum triiodide complexes bearing various
Schrock and his coworker, catalytic ammonia production from
molecular dinitrogen under mild reaction conditions using
transition metal–dinitrogen complexes as catalysts has been
developed for the last decade.
In 2010, we found that dinitrogen-bridged dimolybdenum
complex bearing PNP-type pincer ligands based on the pyridine
skeleton worked as an effective catalyst toward ammonia
production under ambient reaction conditions. Recently, we
have improved the catalytic activity by the use of PPP-
triphosphines and PCP-type pincer ligands based on the N-
1
2
4
-7
substituted PNP-type pincer ligands. Treatment of [Mol
3 3
(thf) ]
(
thf tetrahydrofuran) with 2,6-bis(di-alkyl or di-
=
phenylphosphinomethyl)pyridines bearing two alkyl and phenyl
groups on the phosphorous atom and 2,6-bis(di-tert-
butylphosphinomethyl)pyridines bearing substituents at the 4-
8
R’
position of the pyridine moiety (R-PNP , 1) in THF at 50 °C for
1
8 h gave the corresponding molybdenum triiodide complexes
aDepartment of Systems Innovation, School of Engineering,
R'
I
R-PNP
PR'2
I
1
The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656,
Japan
MoI (thf)3
R
N Mo
3
THF
o
P
I
5
0 C, 18 h
R'2
b
Frontier Research Centre for Energy and Resources, School of
isolated yield
Engineering, The University of Tokyo, Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
Fc- Fe
a: R = H, R' = Bu 64%a 2e: R = Ph, R' = Bu
b: R = H, R' = Ad 18% 2f: R = Me, R' = Bu 55%
c: R = H, R' = Pr 55% 2g: R = OMe, R' = Bu 40%
d: R = H, R' = Ph 30% 2h: R = Fc, R' = Bu
t
t
25%
2
t
2
2
2
†Footnotes relating to the title and/or authors should appear
i
t
here.
Rc- Ru
t
62%
61%
t
Electronic Supplementary Information (ESI) available: [details of
any supplementary information available should be included
here]. See DOI: 10.1039/x0xx00000x
2i: R = Rc, R' = Bu
Scheme 1 Synthesis of molybdenum triiodide complexes bearing various
a
substituted PNP-type pincer ligands. Ref 12
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Pleas eD da lot o nn oT tr aa nd sj au c st ti o mn sa rgins
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COMMUNICATION
Journal Name
R’
[
MoI
3
(R-PNP )] (2; R = H, Ph, Me, MeO, ferrocenyl (Fc), and Table 2
View Article Online
Molybdenum complexes bearing functional groups at
DOI: 10.1039/C8DT04975K
ruthenocenyl (Rc); R’ = t-Bu, adamantyl (Ad), i-Pr, and Ph) in
moderate to good yields (Scheme 1). All molecular structures
except for 2b were confirmed by X-ray analysis (see
Supplementary Information).
The catalytic ammonia production was carried out according
to the following procedure of our previous method using 2a as
a catalyst.12 To a mixture of 2b as catalyst and 2,4,6-collidinium
trifluoromethanesulfonate (48 equiv to the Mo atom of the
catalyst; [ColH]OTf) as proton source in toluene was added a
4
-position of pyridine ring catalyzed reduction of dinitrogen to
ammoniaa
OTf
Catalyst
N2
+
6
Co
+
6
2 NH3 (+ H2)
toluene
rt, 20h
N
H
(1 atm)
(360 equiv/Mo) (480 equiv/Mo)
Entry Catalyst
NH3
equiv)b
NH3
(%)c
H2
(equiv)b
H2
(%)c
(
1
2
2a
2e
2f
66 ± 3
90 ± 1
59 ± 0
55 ± 2
75 ± 1
49 ± 0
20 ± 1
69 ± 1
62 ± 1
57
31 ± 1
27 ± 1
61 ± 5
103 ± 5
44 ± 3
30 ± 4
45
17 ± 1
15 ± 0
34 ± 3
57 ± 3
25 ± 2
17 ± 2
25
solution of decamethylcobaltcene (36 equiv to the Mo atom of
the catalyst; CoCp*
5
2
(Cp* =  -C
5 5
Me )) as reductant in toluene
3
4
2g
2h
24 ± 1
via a syringe pump at room temperature over a period of 1 h
under atmosphere pressure of nitrogen gas, followed by stirring
at room temperature for another 19 h. After the reaction, the
amounts of ammonia (8.4 equiv based on the Mo atom of the
catalyst) and molecular hydrogen (1.3 equiv based on the Mo
atom of the catalyst) were determined by indophenol method
and GC, respectively, where ammonia and molecular
dihydrogen were obtained in 69% and 7% yields, respectively,
5
83 ± 0
75 ± 1
69
6
2i
2ad
7
2ae
8
65
54
53
29
9
3e
3h
87
73
19
11
1
3
10
a
59
49
65
36
To a mixture of molybdenum complex (0.002 mmol) and [ColH]OTf (0.96
*
mmol) in toluene (1 mL) was added a solution of CoCp 2 (0.72 mmol) in
toluene ( 4 mL) at room temperature over a period of 1 h, followed by stirring
based on the CoCp*
2
(Table 1, Entry 2). When 2c and 2d were
at room temperature for another 19 h under N2. bEquiv based on the
molybudenum atom. cYield based on CoCp*2. dFeCp2 (1equiv. to 1a) was
added. eRuCp2 (1equiv. to 1a) was added.
used as catalysts in place of 2b, 5.6 equiv. and 1.9 equiv. of
ammonia were produced based on the Mo atom of the
catalysts, together with 4.0 equiv. and 7.0 equiv. of molecular
hydrogen, respectively (Table 1, Entries 3 and 4). These results
indicate that adamantyl, iso-propyl, and phenyl groups on the
phosphorous atom of the PNP-type pincer ligands were less
effective than tert-butyl group. Thus, molybdenum complex
bearing tert-butyl groups on the phosphorous atom was found
to work as the most effective catalyst.
Next, we investigated the nature of substituents at the 4-
postion of the pyridine ring in the PNP-type pincer ligand
toward the catalytic nitrogen fixation. At first, we carried out
reactions using 36 equiv of CoCp*
ColH]OTf as proton source in the presence of molybdenum
triiodide complexes bearing substituents at the 4-postion of the
pyridine ring in the PNP-type pincer ligand. However,
NN / cm-1
1944
R
N
N
N
H
P
Ph
Me
1950
P
N
1939
R
N
Mo N
N Mo N
R
OMe 1932
P
N
N
N
Fc
1944
P
t
P = P Bu
N
2
Rc
1944
Fig. 1 IR absorbance of terminal dinitrogen ligand in dinitrogen-bridged
dimolybdenum complexes.
amount of the produced ammonia under the same reaction
conditions (Table 1, Entries 5–7). Based on these results, we
used larger amounts of CoCp*
2
(360 equiv) as reductant and
2
as reductant and 48 equiv of
[
ColH]OTf (480 equiv) as proton source in the presence of the
[
molybdenum triiodide complexes. Typical results are shown in
Table 2. When 2e was used as a catalyst, 90 equiv. of ammonia
and 27 equiv. of molecular hydrogen, respectively, were
produced based on the Mo atom of the catalyst (Table 2, Entry
unfortunately, we did not find a significant difference in the
Table 1 Molybdenum triiodide complexes bearing various
functional groups catalyzed reduction of dinitrogen to ammoniaa
2
). For comparison, the amount of the produced ammonia
using 2e as a catalyst was substantially higher than that using
2a as a catalyst (Table 2, Entry 1). On the other hand, when 2f
and 2g were used as catalysts, only smaller amount of ammonia
was produced than that using 2a as a catalyst together with
larger amount of molecular hydrogen (Table 2, Entries 3 and 4).
OTf
Catalyst
N2
+
6
Co
6
2 NH3 (+ H2)
+
toluene
rt, 20h
N
H
(1 atm)
(36 equiv/Mo)
(48 equiv/Mo)
8
b
Entry Catalyst
NH3
equiv)b
NH3
(%)c
H2
(equiv)b
H2
(%)c
In our previous study, we confirmed that Ph group and
MeO group at the 4-postion of the pyridine ring in the PNP-type
pincer ligand worked as electron-withdrawing and electron-
donating groups, respectively, based on the comparison of the
dinitrogen stretching frequency of the terminal dinitrogen
ligand of the corresponding dinitrogen-bridged dimolybdenum
complexes bearing the PNP-type pincer ligands (Figure 1). Both
our previous result and the experimental results shown in Table
2 indicate that the presence of an electron-withdrawing group
such as Ph group and an electron-donating group such as MeO
group to the pyridine ring in the PNP-type pincer ligand
increased and decreased the catalytic activity, respectively.
(
1d
2
10.9 ± 0.2
8.4
91 ± 2
69
0.2 ± 0.1
1.3
1 ± 1
7
2a
2b
2c
2d
2e
2f
3
5.6
46
4.0
22
39
1
4
1.9
16
7.0
5
11.5
10.3
7.6
94
0.2
6
88
0.3
2
7
2g
63
1.6
9
a
To a mixture of molybdenum complex (0.010 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 room temperature over a period of 1 h, followed by stirring
at room temperature for another 19 h under N2. bEquiv based on the
molybudenum atom. cYield based on CoCp*2. dRef 12
2
| J. Name., 2012, 00, 1-3
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COMMUNICATION
Thus, the tendency of substituents to the pyridine ring of the
PNP-type pincer ligands in 2 shown in Table 2 is in sharp
contrast to that in dinitrogen-bridged dimolybdenum
comprexes.8b
Next, we carried out reactions in the presence of
molybdenum triiodide complexes bearing Fc and Ru groups at
the 4-postion of the pyridine ring in the PNP-type pincer ligand
Catalys tV iew Article Online
OTf
D
(
O
0.
I
0
:
0
1
2
0.1
m
0
m
39
o
/
l )C 8DT04975K
N2
+
6
Co
+
6
2 NH
3
toluene, rt
N
H
(
1 atm)
(180 equiv/Mo) (240 equiv/Mo)
18
16
14
12
10
8
6
4
2
0
(2h and 2i). The amounts of ammonia (83 and 75 equiv,
respectively) using 2h and 2i as catalysts were higher than that
using 2a as a catalyst (Table 2, Entries 5 and 6). To obtain more
information on the role of Fc and Ru groups to the catalysts, we
confirmed no change in the amount of ammonia production
when ferrocene and ruthenocene were added to the reaction
using 2a as a catalyst under the same reaction conditions (Table
2a
2e
2h
2
, Entries 7 and 8). These results indicate that the introduction
0
1
2
3
4
5
of Fc and Rc groups to the pyridine ring of the PNP-type pincer
ligand increased the catalytic activity.
Reaction time (min)
Fig. 2 Time profiles of ammonia production with 2a (blue), 2e (green) and
2h (red) as catalysts. CoCp* (0.36 mmol; 180 equiv / Mo) and [ColH]OTf
8
c
In our previous study, we confirmed that Fc and Rc groups
at the 4-postion of the pyridine ring in the PNP-type pincer
ligand did not work as electron-withdrawing groups, based on
the comparison of the dinitrogen stretching frequency of the
terminal dinitrogen ligand of the corresponding dinitrogen-
bridged dimolybdenum complexes bearing the PNP-type pincer
ligands (Figure 1). To obtain more information on the property
of Fc and Rc groups, cyclic voltammograms (CVs) of 2a, 2h, and
2
(
0.48 mmol; 240 equiv / Mo) was added a solution of catalyst (0.002 mmol)
in toluene (5 mL) at room temperature.
F
F
2
4 4 3 2 6 3
i were measured in THF with [NBu ]BAr (Ar = 3,5-(CF ) C H )
as supporting electrolyte. Typical results are shown in Table 3.
The CV of 2a showed an irreversible electron oxidation wave at
0
/+
Epa = +0.11 V vs FeCp
2
assignable to Mo (III/IV) (Table 3, Entry
1
). The CV of 2h showed an irreversible electron oxidation
wave at Epa = +0.13 V derived from Mo (III/IV) and a reversible
one-electron oxidation wave derived from Fe (II/III) couple at
0
/+
E
1/2 = + 0.40 V vs. FeCp
2
respectively (Table 3, Entry 2). Based
(1h) reported in our previous study
t-Bu
on the CV of Fc-PNP
(
Table 3, Entry 3),8 complexation with molybdenum-triiodide
c
led oxidation potential of Fc moiety at the pincer ligand to
positive sift by + 0.29 V in 2h. Based on the CVs of 2a, 2h and
1
h, we consider that an electronic interaction between the Mo
centre and Fe centre in 2h may play an important role to
increase the catalytic activity. As discussed in our previous
work,8 the direct interaction between the Mo centre and Fe
centre may accelerate the reduction step in the catalytic
ammonia production via an intramolecular electron transfer
from the Fe centre of the Fc moiety to the active site of the Mo
centre in the complex.
c
On the other hand, the CV of 2i showed only one irreversible
electron oxidation wave at Epa = +0.10 V derived from Mo (III/IV)
without the oxidation wave derived from Ru (II/III) of Rc group
t-Bu
(
Table 3, Entry 4). Based on the CV of Rc-PNP (1i) reported
8
c
in our previous study, where one irreversible oxidation wave
was observed at Epa = + 0.78 V derived from Ru (II/III) (Table 3,
Entry 5), we did not obtain any direct evidence to support an
electronic interaction between the Mo centre and Ru centre in
2
i. Thus, unfortunately, we have not yet clarified the exact
reason why the introduction of Rc group to the pyridine ring of
the PNP-type pincer ligand increased the catalytic activity.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Table 3 Electrochemical data of molybdenum triiodide
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Dalton Transactions
Page 4 of 5
complexesa
Entries
Compd.
Mo (III/IV)/Vb
Fe (II/III)c
COMM
1
UNICATIO
2
N
a
+ 0.11
+ 0.13
Journal Name

+ 0.40
2
2h
1h
(
a) 3d
I
t
Bu2

+ 0.10
t
ammonia production described in the present manuscript
View Article Online
N Bu2
P
N2
P
e
DOI: 10.1039/C8DT04975K
4
5d
2i
Mo I +
+ 0.10
proceeded via the same reaction pathway, where direct
(
1 atm)
R
N
Co
R
N
+ 0.78f
Mo
1i

, 1a th
cleavage of the nitrogen-nitrogen triple bond of the bridging
dinitrogen ligand on the dimolybdenum atoms played a key role
to afford the corresponding nitride complexes as reactive
toluene
F
I
4
a
P
0.1 V s-1 with [NBu ]BAr as
F
Measured by cyclic voltammetry in T Hr t
P
t
I
t
4
Bu2
e: R = Ph
h: R = Fc
Bu
0/+
b
2
supporting electrolyte. Potentials are indicated vs. FeCp
.
E
value of
pa
2
2
2
2.2 equiv
/2
3e: R = Ph : 62%
molybdenum oxidation. E value of ferrocene center. dRef 8c. Oxidation
c
e
1
12
3h: R = Fc : 70%
f
intermediates.
wave of ruthenoce
t
ne was not observed in the electrochemical window. E
pa
Bu2
OTf
(b)
N
In summary, we have newly prepared and characterized a
series of molybdenum triiodide complexes bearing various
value of ruthenoce
P
ne oxidation.
Ar
(1 atm)
R
N
Mo
+
NH3
+
Co
I
toluene
rt, 1 h
substituted pyridine-based PNP-type pincer ligands.
The
P
3e: 59%
3h: 85%
t
N
H
Bu2
introduction of Ph and Fc groups at the 4-position of the
pyridine ring of the PNP-ligand as electron-withdrawing and
redox active groups substantially increased the catalytic
activity. The former tendency is in sharp contrast to our
previous reaction system that the electron-withdrawing groups
to the PNP-ligand of the dinitrogen-bridged dimolybdenum
3
3
e: R = Ph
h: R = Fc
3 equiv
4 equiv
*
Scheme 2 (a) Stoichiometric reduction of 2e and 2h with CoCp 2. Yield
was determined by NMR. (b) Stoichiometric reaction of 3e and 3h with
*
CoCp 2 and [ColH]OTf under Ar. Yield was determined by indophenol
method.
To verify the difference of the catalytic behavior of 2a, 2e, complexes decreased the catalytic activity.8b However, the
and 2h, we monitored time dependence of ammonia formation former tendency is consistent with that of our previous reaction
using 360 equiv of CoCp*
2
and 480 equiv of [ColH]OTf. system that iodide ligand as an electron-withdrawing group in
1
4
However, we did not observe any difference of the catalytic the molybdenum complexes worked more effective catalysts
activity because the initial rate within 40 min from the start of than other halide ligands such as bromide and chloride
the catalytic reaction was almost the same in all cases. Typical ligands.1 We believe that the achievement described in the
results are shown in the Supplementary Information (see present manuscript provides valuable information to improve
2
Supplementary Information in details).
experimental results, we modified the experimental procedure ambient reaction conditions.
of the slow addition of a solution of CoCp* over a period of 1 h
The present project is supported by CREST, JST (JPMJCR1541).
to the reaction mixture to the addition of CoCp*
to the reaction We thank Grants-in-Aid for Scientific Research (Nos.
Based on these the catalytic activity toward ammonia production under
2
2
mixture in one portion. The ammonia production finished JP17H01201, JP15H05798, and JP18K19093) from JSPS and
within 5 min in all cases. Typical results are shown in Figure 2. MEXT. A.E. is a recipient of the JSPS Predoctoral Fellowships
Turnover frequencies (TOFs) for ammonia production using 2a, for Young Scientists.
2
e, and 2h as catalysts, which are determined as mols of
ammonia produced in initial 1 min per catalyst, were 5.6, 14 and
-1
Notes and references
7
.7 min , respectively.
These results indicate that the
introduction of Ph and Fc groups to the pyridine ring of the PNP-
type pincer ligand increased the reaction rate of ammonia
production. Thus, the rate-determining step of the catalytic
reaction described in this paper is not involved in protonation
step but in reduction step. At present, we consider that one
of the reduction steps of cationic imido, amido, and ammonia
complexes is the rate-determining step.
1
2
Ammonia synthesis catalysts: Innovation and Practice, Ed: H.
Liu, World Scientific, Beijing, 2013.
(a) J. Guo and P. Chen, Chem, 2017, 3, 709; (b) T. Zhang, H.
Miyaoka, T. Ichikawa and Y. Kojima, ACS Appl. Energy Mater.,
2
018, 1, 232.
1
2
3
(a) D. V. Yandulov and R. R. Schrock, Science, 2003, 301, 76;
(b) V. Ritleng, D. V. Yandulov, W. W. Weare, R. R. Schrock, A.
S. Hock and W. M. Davis, J. Am. Chem. Soc., 2004, 126, 6150;
(c) D. V. Yandulov and R. R. Schrock, Inorg. Chem., 2005, 44,
Finally, we carried out some stoichiometric reactions of 2e
and 2h for comparison of the reactivity of 2e and 2h with that
1103; (d) L. A. Wickramasinghe, T. Ogawa, R. R. Schrock and
P. Müller, J. Am. Chem. Soc., 2017, 139, 9132.
4
For selected recent reviews, see: (a) K. C. MacLeod and P. L.
Holland, Nat. Chem., 2013, 5, 559; (b) M. D. Fryzuk, Chem.
Commun., 2013, 49, 4866; (c) H. Broda, S. Hinrichsen and F.
Tuczek, Coord. Chem. Rev., 2013, 257, 587; (d) Y. Tanabe and
Y. Nishibayashi, Coord. Chem. Rev., 2013, 257, 2551; (e) H.-P.
Jia and E. A. Quadrelli, Chem. Soc. Rev., 2014, 43, 547; (f) C. J.
M. van der Ham, M. T. M. Koper and D. G. H. Hetterscheid,
Chem. Soc. Rev., 2014, 43, 5183; (g) C. Köthe and C. Limberg,
Z. Anorg. Allg. Chem., 2015, 641, 18; (h) N. Khoenkhoen, B. de
Bruin, J. N. H. Reek and W. I. Dzik, Eur. J. Inorg. Chem., 2015,
of 2a. Reduction of 2e and 2h with 2.2 equiv. CoCp*
in toluene
2
at room temperature under an atmospheric dinitrogen gas gave
t-Bu
the corresponding nitride complexes [MoI(N)(R-PNP )] (3e: R
=
Ph; 3h: R = Fc) in 62% and 70% NMR yields, respectively
Scheme 2a). Both the nitride complexes were isolated, and
the detailed molecular structure was confirmed by X-ray
analysis (see Supplementary Information). Further
stoichiometric reactions of the nitride complexes such as 3e and
h with 3 equiv. of CoCp*
and 4 equiv. of [ColH]OTf under
atmospheric pressure of argon gas gave ammonia in 59% and
6% yields based on the Mo atom of the complexes,
(
5
0
67; (i) R. J. Burford and M. D. Fryzuk, Nat. Rev. Chem., 2017,
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Dalton Transactions
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Journal Name
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