Catalytic formation of ammonia from molecular dinitrogen by use of dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing pnp-pincer ligands: Remarkable effect of substituent at pnp-pincer ligand

Article abstract of DOI:10.1021/ja5044243

A series of dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing 4-substituted PNP-pincer ligands are synthesized by the reduction of the corresponding molybdenum trichloride complexes under 1 atm of molecular dinitrogen. In accordance with a theoretical study, the catalytic activity is enhanced by the introduction of an electron-donating group to the pyridine ring of PNP-pincer ligand, and the complex bearing 4-methoxy-substituted PNP-pincer ligands is found to work as the most effective catalyst, where 52 equiv of ammonia are produced based on the catalyst (26 equiv of ammonia based on each molybdenum atom of the catalyst), together with molecular dihydrogen as a side-product. Time profiles for the catalytic reactions indicate that the rates of the formation of ammonia and molecular dihydrogen depend on the nature of the substituent on the PNP-pincer ligand of the complexes. The formation of ammonia and molecular dihydrogen is complementary in the reaction system.

Full text of DOI:10.1021/ja5044243

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Article
Catalytic Formation of Ammonia from Molecular Dinitrogen by Use of
Dinitrogen-Bridged Dimolybdenum-Dinitrogen Complexes Bearing PNP-
Pincer Ligands: Remarkable Effect of Substituent at PNP-Pincer Ligand
Shogo Kuriyama, Kazuya Arashiba, Kazunari Nakajima, Hiromasa Tanaka,
Nobuaki Kamaru, Kazunari Yoshizawa, and Yoshiaki Nishibayashi
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja5044243 • Publication Date (Web): 04 Jun 2014
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Revised version of ja-2014-044243
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Catalytic Formation of Ammonia from Molecular Dinitrogen
by Use of Dinitrogen-Bridged Dimolybdenum-Dinitrogen
Complexes Bearing PNP-Pincer Ligands: Remarkable
Effect of Substituent at PNP-Pincer Ligand
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Shogo Kuriyama,† Kazuya Arashiba,† Kazunari Nakajima,† Hiromasa
Tanaka,‡ Nobuaki Kamaru,§ Kazunari Yoshizawa,*,‡§ and Yoshiaki
Nishibayashi*,†
†Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Yayoi,
Bunkyo-ku, Tokyo 113-8656, Japan
‡Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University,
Nishikyo-ku, Kyoto 615-8520, Japan
§Institute for Materials Chemistry and Engineering and International Research Center for
Molecular System, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
ABSTRACT:
A series of dinitrogen-bridged dimolybdenum-dinitrogen complexes
bearing 4-substituted PNP-pincer ligands are synthesized by the reduction of the
corresponding molybdenum trichloride complexes under 1 atm of molecular dinitrogen. In
accordance with a theoretical study, the catalytic activity is enhanced by the introduction of
an electron-donating group to the pyridine ring of PNP-pincer ligand and the complex bearing
4-methoxy-substituted PNP-pincer ligands was found to work as the most effective catalyst,
where 52 equiv of ammonia was produced based on the catalyst (26 equiv of ammonia based
on each molybdenum atom of the catalyst), together with molecular dihydrogen as a side-
product.
of ammonia and molecular dihydrogen depend on the nature of the substituent on the PNP-
pincer ligand of the complexes. The formation of ammonia and molecular dihydrogen is
complementary in the reaction system.
Time profiles for the catalytic reactions indicate that the rates of the formation
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1. Introduction
The nitrogen fixation under ambient reaction conditions is one of the most important
topics in chemistry. Since the finding of the first example of a transition metal-dinitrogen
1
complex, the preparation of various transition metal-dinitrogen complexes and their
stoichiometric transformation of the coordinated dinitrogen have so far been well investigated
toward the end of the achievement of nitrogen fixation system under mild reaction conditions.
In contrast to various studies on the stoichiometric reactivity of transition metal-dinitrogen
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complexes, the catalytic transformation of molecular dinitrogen using these complexes as
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catalysts under mild reaction conditions is limited to only a few examples.
The first successful example of the direct conversion of molecular dinitrogen into
ammonia catalyzed by a transition metal-dinitrogen complex was found by Schrock and his
co-worker in 2003, where a molybdenum-dinitrogen complex bearing triamidomonoamine as
a tetradentate ligand worked as a catalyst to produce less than 8 equiv of ammonia based on
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the catalyst at ambient conditions.
The isolation of some reactive intermediates and DFT
More
calculation clarified the detailed reaction pathway, what is called the Schrock cycle.
recently, we have found another successful example of the catalytic formation of ammonia
from molecular dinitrogen by using a dinitrogen-bridged dimolybdenum-dinitrogen complex
bearing tridentate PNP-type pincer ligands (1a) as a catalyst, where 23 equiv of ammonia
were produced based on the catalyst (12 equiv of ammonia based on each molybdenum atom
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of the catalyst) under ambient conditions.
The detailed reaction pathway remained
unclear at this stage and mononuclear molybdenum-dinitrogen complexes were considered to
work as catalysts. Quite recently, Peters and co-workers have reported the third example of
the conversion of molecular dinitrogen into ammonia catalyzed by iron-dinitrogen complexes
bearing triphosphineborane and triphosphinealkyl ligands, where 5-7 equiv of ammonia were
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produced based on the catalyst at an extremely low temperature such as -78 °C.
Although the whole catalytic cycle has not yet been clarified, this is the first successful
example of the iron-catalyzed direct transformation of molecular dinitrogen into ammonia
under mild reaction conditions.
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We have reported an experimental and theoretical study on a reaction pathway for the
formation of ammonia from molecular dinitrogen catalyzed by 1a, where the dinuclear
structure of the dinitrogen-bridged dimolybdenum-dinitrogen complex plays a decisive role in
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exhibiting the catalytic ability for the transformation of molecular dinitrogen into ammonia
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(Scheme 1).
In this reaction system, synergy between two molybdenum moieties
connected with a bridging dinitrogen ligand is observed at the first protonation of the
coordinated dinitrogen ligand. A molybdenum core donates an electron to the active site of
the other core through the bridging dinitrogen ligand, and thereby a terminal dinitrogen at the
active site is reductively activated to accept a proton. This proposal indicates that a
mononuclear unit of the dinuclear molybdenum-dinitrogen complex bearing PNP-type pincer
ligands works as a mobile ligand to the other unit as an active site.
This result is in sharp
contrast to our previous proposal, where mononuclear molybdenum-dinitrogen complexes
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were proposed to work as key reactive intermediates.
The proposed catalytic mechanism prompted us to design and prepare more reactive
dinitrogen-bridged dimolybdenum-dinitrogen complexes as catalysts.
Based on the
theoretical finding that the first protonation of 1a is considered to be one of the rate-
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determining steps in the catalytic cycle, we have envisaged enhancing the electron-donating
ability of the molybdenum center in order to accelerate the first protonation process. The
introduction of an electron-donating group to the pyridine ring of 1a is expected to intensify
the back-donating ability of the molybdenum atom to the coordinated dinitrogen.
the nature of substituents on the PNP-pincer ligand has substantially affected the electronic
and electrochemical properties of 1. In accordance with our design, the introduction of an
In fact,
electron-donating substituent such as 4-methoxy moiety into the pyridine ring of the PNP-
pincer ligand has dramatically increased the catalytic activity toward the formation of
ammonia.
In this article, we describe the preparation and reactivity of a series of
dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing 4-substituted PNP-pincer
ligands. The catalytic behavior of the modified complexes is also investigated by means of
experimental and theoretical methods.
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2. Results and Discussion
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2.1. Synthesis and Properties of Dinitrogen-Bridged Dimolybdenum-Dinitrogen
Complexes
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A series of the dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing 4-
substituted PNP-pincer ligands were newly prepared according to the previous method.
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Treatment
of
[MoCl3(RPNP)]
(2;
R-PNP
=
4-substituted
2,6-bis(di-tert-
butylphosphinomethyl)pyridine), which were prepared by the reaction of [MoCl3(THF)3] with
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R-PNP in tetrahydrofuran (THF) at 50 °C for 18 h, with 6 equiv of Na-Hg in THF at room
temperature for 12 h under an atmospheric pressure of molecular dinitrogen gave the
corresponding substituted dinitrogen-bridged dimolybdenum-dinitrogen complexes
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[Mo(N2)2(R-PNP)]2(µ-N2) (1) in good yields (Scheme 2).
A variety of substituents such
as phenyl, trimethylsilyl, tert-butyl, methyl, and methoxy moieties could be introduced to the
4-position of the PNP-pincer ligand. These new dinitrogen-bridged dimolybdenum-
dinitrogen complexes were characterized spectroscopically.
Detailed molecular structures
of these complexes except for 1c were determined by X-ray crystallography.
As a typical
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example, an ORTEP drawing of 1f is shown in Figure 1.
Selected bond distances and
angles are shown in Table S8.
No distinct differences were observed in the bond distances
This result indicates that the introduction of a substituent
and angles of these complexes 1.
at the 4-position of the pyridine ring in 1 does not significantly change the coordination
environment around the molybdenum atom.
In the IR spectra, the complexes 1 exhibited strong absorptions derived from the
corresponding terminal dinitrogen ligands, where the nature of substituents has affected
absorptions (Table 1).
The presence of an electron-donating substituent at the PNP-pincer
For example, the dinitrogen stretching frequency of 1f is
ligands in 1 caused a red-shift.
smaller by 12 cm-1 than that of 1a, indicating that the introduction of an electron-donating
substituent to the pyridine ring may increase the back-donating ability of the molybdenum
atom in 1 to the coordinated dinitrogen ligand.
The nature of a substituent at the PNP-pincer ligands in 1 much affected the
electrochemical property of 1.
The cyclic voltammetry of 1 revealed two successive
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oxidation waves in the anodic scan and one reduction wave in the reverse cathodic scan
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(Figure S6 and Table S9).
As the electron donating ability of a substituent in 1 increased,
These redox properties of 1 are
represented by first oxidation potential E1 values in Table 1. For an example, the E1
these redox events were shifted more negatively.
pa
pa
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value of 1f is lower than that of 1a.
of the dinitrogen ligands of 1 and the oxidation potentials of 1 was observed as shown in
Figure 2. These results of the electronic and electrochemical properties of 1 indicate that
A good linear relationship between the IR absorbance
complexes bearing a more electron-donating substituent such as 1e and 1f have more
activated dinitrogen ligands, but the reduction steps of catalytic reactions by using 1e and 1f
do not proceed smoothly and effectively.
2.2. DFT Calculations on the First Protonation Process
DFT calculations performed in our previous study demonstrated that the first
protonation of a terminal dinitrogen ligand in 1 yielding a diazenide (–NNH) intermediate is
energetically the most unfavorable process in the formation of ammonia by the use of 1 as a
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catalyst.
When 2,6-lutidinium trifluoromethanesulfonate ([LutH]OTf; Lut = 2,6-lutidine;
OTf = OSO2CF3) is adopted as a proton donor, the first protonation process involves three
elementary reaction steps as shown in Figure 3: (step A) protonation of a terminal dinitrogen
ligand in 1, (step B) elimination of the dinitrogen ligand trans to the generated NNH group to
afford a five-coordinate diazenide intermediate, and (step C) coordination of an OTf anion to
the vacant coordination site.
The second and third steps should be considered as a part of
the first protonation process because the first step is an endothermic reaction and thus the
generated diazenide intermediate must be thermodynamically stabilized by ligand exchange
of the trans dinitrogen ligand by OTf–. In this article, we focus on the first protonation
process for the evaluation of the substituent effects in the catalytic transformation of
molecular dinitrogen.
Firstly, we have theoretically investigated energy profiles of the first protonation
process in the transformation of molecular dinitrogen catalyzed by dinitrogen-bridged
dimolybdenum-dinitrogen complexes bearing unsymmetrical PNP-pincer ligands.
In a
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previous experimental work, a remarkable effect of substituents at the phosphorus atom in
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the PNP-pincer ligand was observed: a dinitrogen-bridged dimolybdenum-dinitrogen complex
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bearing
AdPNPtBu
(AdPNPtBu
=
2-(di-1-adamantylphosphino)methyl-6-(di-tert-
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butylphosphino)methylpyridine) as PNP-pincer ligand (1h) worked as a slightly more
effective catalyst than 1a, while a dimolybdenum-dinitrogen complex bearing tBuPNPPh
(tBuPNPPh = 2-(di-tert-butylphosphino)methyl-6-(diphenylphosphino)methylpyridine) (1i) did
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not work at all (Scheme 3).
Figure 3 shows energy profiles of the first protonation process
Details on optimized structures of intermediates and
calculated for 1a, 1h, and 1i.
transition-state structures are described in the Supporting Information.
For step A, the
substitution of the tert-butyl groups by the adamantyl groups (1h) decreases the activation
energy (Ea) of the protonation step to 6.8 kcal/mol, while the substitution by the phenyl
groups (1i) does not change the value of Ea (8.5 kcal/mol).
exhibits a significant difference between 1a (1h) and 1i.
The energy profile for step B
The elimination of the dinitrogen
ligand trans to the NNH group proceeds in an exothermic way for 1a (ΔE = –2.6 kcal/mol)
and 1h (ΔE = –4.2 kcal/mol), while it is an endothermic reaction for 1i (ΔE = 3.3 kcal/mol).
Additionally, the activation energy for the backward reaction (III → II) is calculated to be
only 0.1 kcal/mol in the phenyl-substituted system.
The quite low activation energy
calculated for the coordination of dinitrogen to III-i indicates that the five-coordinate
diazenide complex III-i should immediately accept the released dinitrogen before reacting
with OTf– in the reaction solution. The regeneration of II-i will result in the loss of the
proton added at step A because the proton detachment from II-i is an exothermic reaction (ΔE
= –7.7 kcal/mol) with a very low activation energy of 0.8 kcal/mol.
On the other hand, III-
a and III-h are likely to have much opportunity to accept OTf– because of the endothermic
reaction for the coordination of dinitrogen to afford II as well as the highly exothermic
reaction for the coordination of OTf– to afford IV (ΔE = –15.6 kcal/mol for III-a and –11.2
kcal/mol for III-h).
Consequently, the lack of catalytic ability of 1i observed in the
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previous work can be rationalized by the thermodynamic instability of the diazenide
complexes II-i and III-i.
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Next, we have theoretically examined the impact of introduction of electron-donating
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groups to the 4-position of the pyridine ring in the PNP-pincer ligand.
energy profiles of the first protonation process calculated for 1e and 1f, together with that of
1a. Details on optimized structures of intermediates and transition-state structures are
described in the Supporting Information. The introduction of methyl and methoxy groups
to the pyridine ring does not significantly affect the trend of energy changes in the protonation
process. The activation energies for the protonation of 1 are decreased to 7.4 kcal/mol for
Figure 4 describes
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1e and 7.0 kcal/mol for 1f by the introduction of an electron-donating group and those for the
proton detachment from II are increased to 2.4 kcal/mol for II-e and 2.7 kcal/mol for II-f.
The methyl and methoxy groups introduced to the pyridine ring can accelerate the protonation
of 1, although this reaction step is still endothermic.
Table 1 summarizes the calculated
N≡N stretching frequency and the sum of atomic charges assigned to terminal dinitrogen
ligands in 1a, 1e, and 1f. The atomic charges were obtained with the natural population
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analysis (NPA).
The N≡N stretching frequency calculated for 1a (1960 cm–1) is slightly
red-shifted by introducing the methyl group (1e, 1958 cm–1) and the methoxy group (1f, 1954
cm–1), which reasonably agrees with the experimentally-observed trend in Table 1. The
sum of the NPA charges on dinitrogen is slightly increased by the introduction of an electron-
donating group (–0.125 (1a) → –0.127 (1e) → –0.133 (1f)).
stretching frequency and the increased negative NPA charge on dinitrogen indicate reductive
activation of the coordinated dinitrogen through an enhanced π-back donation. The
The red-shifted N≡N
elimination of the dinitrogen ligand (step B) proceeds in an exothermic way in all the three
system, followed by an exothermic reaction of the coordination of OTf– (step C).
Interestingly, the methyl and methoxy-substituted PNP-pincer ligands do not influence the
energetic of step B: the values of ΔE (Ea) in kcal/mol are calculated to be –2.6 (4.4) for 1a, –
2.6 (4.5) for 1e, and –2.8 (4.5) for 1f. This result implies that the substitution of the 4-
position of the pyridine ring in the PNP-pincer ligand mainly contributes to the protonation
step in the first protonation process.
When focusing on the first protonation process, we
can expect that the catalytic ability toward the formation of ammonia from molecular
dinitrogen will be improved by introducing an electron-donating group to the PNP-pincer
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ligand and methoxy-substituted dimolybdenum complex 1f would serve as one of the most
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effective catalysts for the transformation of molecular dinitrogen into ammonia.
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2.3. Catalytic Formation of Ammonia from Molecular Dinitrogen
2.3.1. Effect of Substituent in PNP-Pincer Ligand
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We carried out the catalytic reduction of molecular dinitrogen into ammonia using 1 as
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catalysts, according to the following modified procedure of the previous method.
To a
mixture of 1 and 2,6-lutidinium trifluoromethanesulfonate (288 equiv to 1) as a proton source
in toluene was added a solution of cobaltocene (216 equiv to 1; CoCp2; Cp = η5-C5H5) as a
reductant in toluene via a syringe pump at room temperature over a period of 1 h, followed by
stirring at room temperature for another 19 h under an atmospheric pressure of dinitrogen.
After the reaction, the formation of ammonia together with the formation of molecular
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dihydrogen was observed, the amount of which being determined by indophenol method
and GC, respectively.
Typical results are shown in Table 2. The yield of ammonia and
molecular dihydrogen was estimated based on cobaltocene.
observe the formation of other products such as hydrazine.
In all cases, we did not
In either reaction using 1b or 1c as a catalyst, a similar amount of ammonia was
produced (Table 2, runs 2 and 3). Use of 1d and 1e as catalysts increased the amount of
ammonia, where 28 equiv and 31 equiv of ammonia were obtained based on the catalyst
(Table 2, runs 4 and 5). Further, 34 equiv of ammonia were formed based on the catalyst
when 1f was used as a catalyst (Table 2, run 6). Thus, the largest amount of ammonia was
formed when the most electron-donating moiety (MeO) was introduced to the PNP-pincer
ligands in 1, showing that the electronic and electrochemical properties of the substituent at
the PNP-pincer ligand directly affect the catalytic activity of 1.
Separately, we confirmed
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the direct conversion of molecular dinitrogen into ammonia by using N2 gas in place of
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normal N2 gas.
2.3.2. Time Profiles of Catalytic Formation of Ammonia
To obtain further information on the catalytic behavior, we monitored three catalytic
reactions using 1a, 1e, and 1f as catalysts, respectively.
Results are shown in Figure 5.
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The catalytic conversion of molecular dinitrogen into ammonia occurred rapidly in the
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presence of 1a or 1e as a catalyst, where the formation of ammonia was almost completed
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within 2 h.
Turnover frequencies (TOFs) by 1a and 1e, which were determined as mols of
-1
-1
ammonia produced in initial 1 h per catalyst, are 17 h and 28 h , respectively.
In
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contrast, the formation of ammonia proceeded more slowly using 1f as a catalyst, where a
longer reaction time such as 10 h was necessary to be completed in ammonia formation.
-1
TOF by 1f as a catalyst is 7 h , which was lower than those of 1a and 1e.
Results of the
time profiles indicate that the catalytic behavior of 1f is quite different from those of 1a and
1e.
Based on the result of the time profile of 1f (Figure 5), we consider that the reaction
mixture was overstocked with cobaltocene at the early stage of the catalytic reaction. The
excess amount of cobaltocene may react with a proton source to form the corresponding
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protonated cobaltocene or molecular dihydrogen via the protonated cobaltocene.
To
prevent the presence of an excess amount of cobaltocene at the early stage, we carried out the
reaction by slower addition of a toluene solution of cobaltocene via a syringe pump over a
longer period such as 5 h, followed by stirring at room temperature for another 15 h.
this case, a slightly larger amount of ammonia (36 equiv) was produced based on the catalyst
(Figure 6). To confirm our proposal, we monitored the catalytic reaction over a longer
period (Figure 6). As shown in Figure 6, slower addition of cobaltocene to the reaction
mixture lengthened the lifetime of the catalyst 1f, where ammonia was produced even after 10
h. Finally, 52 equiv of ammonia based on 1f (43% yield based on the cobaltocene) were
In
produced in a similar reaction using larger amounts of CoCp (360 equiv to 1f) and
2
[LutH]OTf (480 equiv to 1f). This is so-far the most effective catalytic formation of
ammonia from molecular dinitrogen using a transition metal-dinitrogen complex as a catalyst.
2.3.3. Effect of Reducing Reagents and Proton Sources
The reduction steps of catalytic reactions using 1e and 1f as catalysts did not proceed
smoothly, as shown in the previous paragraphs.
This result encouraged us to investigate
the catalytic reaction using metallocene of a higher reducing ability such as CrCp* (Cp* =
2
5
η -C Me ) in place of CoCp .
When CrCp* was used as a reducing reagent in the
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catalytic reaction in the presence of 1e as a catalyst, almost the same amount of ammonia was
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produced based on the catalysts (Table 3, run 2). In contrast to the reaction catalyzed by 1e,
only a slightly lower amount of ammonia was produced based on the catalyst when 1f was
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used as a catalyst (Table 3, run 4).
mixture increased the amount of ammonia, where up to 33 equiv of ammonia were produced
based on the catalyst (Table 3, run 5). These results indicate that the use of the higher
However, slower addition of CrCp* to the reaction
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reducing reagent such as CrCp* did not affect much the catalytic activity of 1e and 1f.
2
We have then investigated the effect of the acidity of a proton source on catalytic
reactions by using a higher acidic proton source such as [Cl-LutH]OTf (Cl-Lut = 4-chloro-
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2,6-dimethylpyridine) in place of [LutH]OTf.
When [Cl-LutH]OTf was used as a proton
source in the presence of 1a as a catalyst, only a stoichiometric amount of ammonia was
produced based on the catalysts (Table 4, run 2). We consider that the protonation of
cobaltocene with [Cl-LutH]OTf may proceed more smoothly because only a small amount of
molecular dihydrogen was observed from the reaction mixture (see reference 18 for the
acidity of the protonated cobaltocene).
On the other hand, only a lower amount of
ammonia (10 equiv) was produced based on the catalysts together with a larger amount of
molecular dihydrogen (53 equiv) when a lower acidic proton source such as [Me-LutH]OTf
19
(Me-Lut = 2,4,6-trimethylpyridine) was used in place of [LutH]OTf in the presence of 1a as
a catalyst (Table 4, run 3).
These results indicate that [LutH]OTf works as the most
suitable proton source in our reaction system using 1 as catalysts.
2.4. Mechanistic Investigation on Catalytic Formation of Ammonia
2.4.1. Effect of Concentrations of Catalyst and Cobaltocene
In the previous section, we monitored the time profiles of catalytic reactions by using
three different kinds of dimolybdenum-dinitrogen complexes (1a, 1e, and 1f) as catalysts.
As a result, the catalytic behavior of 1f was revealed to be quite different from those of 1a and
1e.
These experimental results promoted us to investigate the catalytic behavior of both 1a
and 1f from a viewpoint of kinetic study.
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At first, we estimated the kinetic data of 1a and 1f for different initial concentrations of
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the catalyst at the early stage of catalytic reactions.
The plot of the v(NH ) against the
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concentration of the molybdenum ([Mo]) is shown in Figure 7 (a). This plot shows that the
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v(NH ) apparently depended on [Mo] when 1a was used as a catalyst, while v(NH ) was
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independent on [Mo] when 1f was used as a catalyst.
According to the plot of the
log(v(NH )) against log [Mo] shown in Figure 7 (b), the reaction orders with respect to the
3
molybdenum were estimated to be ca. 1 for 1a and 0 for 1f, respectively.
Thus, the
reaction order to the molybdenum depends on the nature of catalysts. At present, we
consider that both reactions using 1a and 1f as catalysts proceed via a similar reaction
pathway, but the rate-determining steps are quite different in two reaction pathways.
Next, we estimated the kinetic data of 1a and 1f for different initial concentrations of
cobaltocene at the early stage of catalytic reactions.
The plot of v(NH ) against the
3
concentration of cobaltocene ([CoCp ]) is shown in Figure 8 (a).
According to the plot of
log(v(NH )) against log [CoCp ] shown in Figure 8 (b), the reaction order with respect to the
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2
cobaltocene was ca. 1 for both cases.
observed in both reaction systems.
Thus, almost the first order to the [CoCp ] was
2
The rate constant for use of 1f (kobs(1f)) was
substantially smaller than that of 1a (kobs(1a)), where the relative ratio of kobs(1f)/kobs(1a) was
estimated to be 0.5. This result shows that the reduction step with CoCp should be
involved as one of the rate-determining steps in both reaction systems.
2
However, the
reduction step for use of 1f is considered to proceed more slowly than of 1a, due to the higher
oxidation potential of 1f than that of 1a.
2.4.2. Influence of Molecular Dihydrogen
To get more insight of the unique catalytic behavior of 1, we have investigated the
influence of molecular dihydrogen to the catalytic formation of ammonia.
As shown in the
previous sections, molecular dihydrogen was formed as a side-product in all reaction systems.
For example, the formation of 13 equiv of molecular dihydrogen (37% yield based on
cobaltocene) was observed in the catalytic reaction using 1a as a catalyst, where 12 equiv of
ammonia (49% yield based on cobaltocene) were produced based on the catalyst (Table 5).
In the absence of 1a, the formation of only 3 equiv of molecular dihydrogen (9% yield) was
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observed without the formation of ammonia under similar reaction conditions.
In sharp
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contrast, the formation of 24 equiv of molecular dihydrogen (67%) was observed under 1 atm
of Ar in place of N . These results indicate that 1 worked as an effective catalyst not only
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toward the formation of ammonia but also toward the formation of molecular dihydrogen.
At present, we consider that molecular dihydrogen should be formed via molybdenum-
dihydride or -dihydrogen complexes, which may be generated by the protonation of the
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molybdenum atom in 1 and the sequential reduction.
In fact, we have already reported the
transformation of dinitrogen-bridged dimolybdenum-tetrachloride complex into 1 and
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molecular dihydrogen by treatment with super-hydride under 1 atm of molecular dinitrogen.
To obtain more information on the catalytic ability of 1 toward the formation of
molecular dihydrogen, we monitored the formation of molecular dihydrogen in three catalytic
reactions using 1a, 1e, and 1f as catalysts.
Typical results are shown in Figure 9.
In all
cases, the formation of molecular dihydrogen proceeded simultaneously with the formation of
ammonia, but the rate of the formation of molecular dihydrogen depends much on the nature
of substituents at PNP-pincer ligands in 1.
evolution of molecular dihydrogen proceeded rapidly, where TOFs by 1a and 1e were 23 h-1
and 19 h-1, respectively.
In the presence of 1f as a catalyst, the evolution of molecular
When 1a and 1e were used as catalysts, the
dihydrogen proceeded slowly, where TOF by 1f is 5 h-1. These results indicate that the
introduction of an electron-donating group decreased the rate of the formation of molecular
dihydrogen, where the use of 1f as a catalyst suppressed the formation of molecular
dihydrogen.
catalysts are summarized in Table 6.
Next, we discuss the influence of the formation of molecular dihydrogen on the
ammonia formation in the catalytic reactions using 1a, 1e, and 1f. Yields of ammonia and
molecular dihydrogen using 1a, 1e, and 1f as catalysts are summarized in Figure 10. Use
TOFs (NH ) and TOFs (H ) in catalytic reactions using 1a, 1e, and 1f as
3 2
of 1f as a catalyst gave not only the higher yield of ammonia but also the lower yield of
molecular dihydrogen than those of 1a and 1e. These results indicate that the formation of
ammonia and molecular dihydrogen is complementary in the present catalytic reaction.
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The formation of molecular dihydrogen should be suppressed to achieve the effective
production of ammonia.
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Finally, we investigated the influence of molecular dihydrogen on 1 during the catalytic
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reaction.
(96/4; 14 equiv of molecular dihydrogen based on 1 were included in a reaction flask) gave
24 equiv and 23 equiv of ammonia based on the catalysts, respectively (Table 7). In both
cases, substantially lower yields of ammonia were obtained in comparison with those under 1
atm of N . In contrast, the reaction using 1e as a catalyst under 1 atm of N (removing the
Catalytic reactions using 1e and 1f as catalysts under 1 atm of a mixture of N /H
2 2
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2
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formed gases) gave a slightly higher amount of ammonia (34 equiv) based on the catalyst.
Based on these results, we consider that molecular dihydrogen works as an inhibitor to
produce ammonia in this catalytic reaction.
In fact, we observed the decomposition of 1
into unidentified complexes and free PNP-pincer ligands in the reaction mixture by NMR
spectroscopy when a benzene solution of 1 was exposed to molecular dihydrogen (1 atm).
We consider that the ligand exchange between the coordinated dinitrogen and molecular
dihydrogen may take place in the presence of a larger amount of molecular dihydrogen to 1.
As the presence of molecular dihydrogen may inhibit the formation of ammonia from the
catalytic reaction, the suppression of the formation of molecular dihydrogen is one of the
most important factors to produce ammonia efficiently in the present reaction system.
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3. Conclusions
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We have newly prepared and spectroscopically characterized a series of dinitrogen-
bridged dimolybdenum-dinitrogen complexes bearing a variety of 4-substituted PNP-pincer
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ligands.
affects not only the electronic and electrochemical properties but also the catalytic properties
toward the formation of ammonia and molecular dihydrogen. The complex bearing 4-
The nature of substituents at PNP-pincer ligands in the complexes substantially
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methoxy-substituted PNP-pincer ligands 1f has been found to work as the most effective
catalyst toward the catalytic formation of ammonia from molecular dinitrogen, where 52
equiv of ammonia are produced based on the catalyst (26 equiv of ammonia based on each
molybdenum atom of the catalyst).
Mechanistic study indicates that the introduction of an
electron-donating group to the PNP-pincer ligand accelerates the protonation steps in the first
protonation process involved in the catalytic reaction. The formation of ammonia and
molecular dihydrogen is complementary in the present reaction system.
The suppression of
the formation of molecular dihydrogen leads to achieve the effective production of ammonia.
We believe that the result described in this paper provides valuable information to design a
more effective catalyst toward the catalytic formation of ammonia under mild reaction
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conditions.
system.
Further work is currently in progress to develop a more effective reaction
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EXPERIMENTAL SECTION
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General. 1H NMR (270 MHz), P{1H} NMR (109 MHz), were recorded on a JEOL
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Excalibur 270 spectrometer in suitable solvents, and spectra were referenced to residual
solvent (1H) or external standard (31P{1H}: H3PO4).
IR spectra were recorded on a JASCO
Raman spectra were recorded on a
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FT/IR 4100 Fourier Transform infrared spectrometer.
JASCO NRS-2000.
Evolved dihydrogen was quantified by gas chromatography using a Shimadzu GC-8A with a
TCD detector and a SHINCARBON ST (6 m × 3 mm). Elemental analyses were
performed at Microanalytical Center of The University of Tokyo. Mass spectra were
measured on a JEOL JMS-700 mass spectrometer. All manipulations were carried out
under an atmosphere of nitrogen by using standard Schlenk techniques or glovebox
techniques unless otherwise stated. Toluene was distilled from dark-blue
Na/benzophenone ketyl solution and degassed, and stored over molecular sieves 4A in a
Absorption spectra were recorded on a Shimadzu MultiSpec-1500.
nitrogen-filled glove box.
before use.
Other solvents were dried by general methods, and degassed
CoCp2 (Aldrich) was sublimed before use.
[Mo(N2)2(PNP)]2(µ-N2) (1a,
6a
6a
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PNP = 2,6-bis(di-t-butylphosphino)methylpyridine), [LutH]OTf, and CrCp*2, were
prepared according to the literature methods.
[Cl-LutH]OTf and [Me-LutH]OTf were
All other reagents were commercially
6a
prepared in a method similar to [LutH]OTf.
available.
Preparation of [Mo(N2)2(4-R-PNP)]2(µ-N2) (1).
preparation of 1b·0.5C6H6·0.5C6H14 is described below.
A typical procedure for the
To Na-Hg (0.5 wt%, 43.9 g, 10.2
mmol) were added THF (30 mL) and [MoCl3(4-Ph-PNP)]·0.5CH2Cl2 (1.00 g. 1.40 mmol),
and the mixture was stirred at room temperature for 12 h under N2 (1 atm).
supernatant solution was decanted off and the residue was washed with THF (10 mL).
The
The
combined extracts were dried in vacuo.
mL), the solution was filtered through Celite and the filter cake was washed with benzene
three times. After the combined filtrate was concentrated to ca. 10 mL, slow addition of
After the residue was dissolved in benzene (15
hexane (40 mL) afforded 1b·0.5C6H6·0.5C6H14 as brown crystals, which was collected by
filtration, washed with hexane (5 mL) and dried in vacuo to afford 1b·0.5C6H6·0.5C6H14 as a
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brown crystalline solid (490 mg, 0.360 mmol, 52% yield).
7.3 Hz, Ph-H2,6, 4H), 7.29-7.16 (m, Ph-H3,4,5, 6H), 7.05 (s, Py-H, 4H), 3.24 (br s, CH2PtBu2,
8H), 1.44 (pseudo t, J = 5.7 Hz, CH2PtBu2, 72H). 31P{1H} NMR (C6D6) δ 91.1 (s).
IR
(KBr, cm-1) 1943 (s, νNN). IR (THF, cm-1) 1950 (s, νNN). Raman (THF, cm-1) 1880
1H NMR (C6D6) δ 7.53 (d, J =
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(νNN).
Anal. Calcd for C64H104Mo2N12P4 (1b·0.5C6H6·0.5C6H14): C, 56.63; H, 7.72; N,
12.38.
Found: C, 56.92; H, 7.47; N, 11.77. The slightly low content of nitrogen is
considered to be due to the labile property of the coordinated molecular dinitrogen in 1.
[Mo(N2)2(4-Me3Si-PNP)]2(µ-N2)·0.5C5H12 (1c·0.5C5H12).
Recrystallization
from pentane at −35 ºC afforded 1c·0.5C5H12, where a 0.5 equiv of C5H12 was determined by
1H NMR.
44% Yield.
A green crystalline solid.
1H NMR (C6D6) δ 7.01 (s, Py-H,
4H), 3.24 (br s, CH2PtBu2, 8H), 1.41 (pseudo t, J = 5.7 Hz, CH2PtBu2, 72H), 0.22 (s, SiMe3,
18H). 31P{1H} NMR (C6D6) 91.5 (s). IR (KBr, cm-1) 1947 (s, νNN). IR (THF, cm-1)
1947 (s, νNN). Raman (THF, cm-1) 1886 (νNN).
(1c·0.5C5H12): C, 50.22; H, 8.35; N, 12.89. Found: C, 49.98; H, 8.09; N, 12.34.
[Mo(N2)2(4-tBu-PNP)]2(µ-N2)·0.5C6H14 (1d·0.5C6H14).
Recrystallization from
hexane at −35 ºC. 1d·0.5C6H14: 48% Yield. Purple crystals.
1H NMR (C6D6): δ 6.86
(s, Py-H, 4H), 3.26 (br s, CH2PtBu2, 8H), 1.44 (pseudo t, J = 5.7 Hz, CH2PtBu2, 72H), 1.13 (s,
Py-tBu, 18H). 31P{1H} NMR (C6D6) δ 92.3 (s). IR (KBr, cm-1) 1942 (s, νNN).
IR
Anal. Calcd for C54.5H108Mo2N12P4Si2
(THF, cm-1) 1939 (s, νNN).
Raman (THF, cm-1) 1899 (νNN).
Anal. Calcd for
C57H109Mo2N12P4 (1d·0.5C6H14): C, 53.55; H, 8.59; N, 13.15.
Found: C, 53.98; H, 8.48; N,
13.20.
[Mo(N2)2(4-Me-PNP)]2(µ-N2)·C6H14 (1e· CH14).
Recrystallization from
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benzene−hexane afforded X-ray suitable crystals of 1e·2C6H14, which was collected by
filtration, washed with hexane and dried in vacuo to afford 1e·C6H14 as a green crystalline
solid.
(s, Py-Me, 6H), 1.44 (pseudo t, J = 5.7 Hz, CH2PtBu2, 72H).
79% Yield. 1H NMR (C6D6) δ 6.44 (s, Py-H, 4H), 3.20 (br s, CH2PtBu2, 8H), 1.82
31P{1H} NMR (C6D6) δ 92.3
(s).
IR (KBr, cm-1) 1923 (s, νNN). IR (THF, cm-1) 1939 (s, νNN).
Raman (THF, cm-1)
1896 (νNN).
Anal. Calcd for C54H104Mo2N12P4 (1e·C6H14): C, 52.42; H, 8.47; N, 13.58.
Found: C, 52.90; H, 8.07; N, 12.83.
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[Mo(N2)2(4-MeO-PNP)]2(µ-N2)·0.5C6H14 (1f·0.5C6H14).
Recrystallization from
1
benzene−hexane afforded 1f·0.5C6H14, where a 0.5 equiv of C6H14 was determined by H
NMR.
52% Yield.
A brownish purple crystalline solid.
1H NMR (C6D6): δ 6.45 (s,
Py-H, 4H), 3.21 (br s, CH2PtBu2, 8H), 3.18 (s, OMe, 6H), 1.46 (pseudo t, J = 5.7 Hz,
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CH2PtBu2, 72H). 31P{1H} NMR (C6D6) δ 92.8 (s).
(THF, cm-1) 1932 (s, νNN). Raman (THF, cm-1) -.
IR (KBr, cm-1) 1924 (s, νNN). IR
Analytical pure and X-ray suitable
sample 1f·C6H14 was obtained by recrystallization from toluene−hexane. Anal. Calcd for
C54H104Mo2N12O2P4 (1f·C6H14): C, 51.10; H, 8.26; N, 13.24. Found: C, 51.11; H, 7.58; N,
12.53.
Catalytic Reduction of Dinitrogen to Ammonia under Ambient Conditions.
A typical experimental procedure for the catalytic reduction of dinitrogen into ammonia using
the dinitrogen complex 1f•0.5C6H14 is described below. In a nitrogen-filled glove box,
toluene (1.0 mL) was added to a mixture of 1f•0.5C6H14 (12.3 mg, 0.010 mmol) and
[LutH]OTf (740.9 mg, 2.88 mmol) in a 50 mL Schlenk flask.
(408.5 mg, 2.16 mmol) in toluene (4.0 mL) was slowly added to the stirred mixture in the
Schlenk flask with a syringe pump at a rate of 4.0 mL per hour. After the addition of
CoCp2, the mixture was further stirred at room temperature for 19 h. The amount of
dihydrogen of the catalytic reaction was determined by GC analysis. The reaction mixture
was evaporated under reduced pressure, and the distillate was trapped in dilute H2SO4
solution (0.5 M, 10 mL). Potassium hydroxide aqueous solution (30 wt%; 5 mL) was
added to the residue, and the mixture was distilled into another dilute H2SO4 solution (0.5 M,
Then a solution of CoCp2
10 mL).
Ammonia (NH3) present in each of the H2SO4 solutions was determined by the
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indophenol method.
The amount of ammonia was 0.043 mmol of NH3 collected before
base distillation of the reaction mixture and 0.296 mmol of NH3 collected after base
distillation to fully liberate NH3, respectively. The total amount of ammonia was 0.340 mmol
(34.0 equiv per 1f).
Computational Methods.
DFT calculations were performed to search all
intermediates and transition structures on potential energy surfaces using the Gaussian 09
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program.
Similar to the previous study, we adopted the B3LYP* functional, which is a
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reparametrised version of the B3LYP hybrid functional developed by Reiher and co-
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workers.
For all intermediates calculated in the present study, the minimum-energy
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structures have the lowest spin multiplicity (singlet or doublet). The B3LYP and B3LYP*
energy expressions are given as equation (1):
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B3LYP
HF
LSDA
B88
LYP
VWN
EXC
= a0EX
+ (1 – a0)EX
+ aXEX
+ acEC
+ (1 – ac)EC
(1)
HF
where a0 = 0.20 (B3LYP) or 0.15 (B3LYP*), ax = 0.72, ac = 0.81; and in which EX is the
LSDA
Hartree-Fock exchange energy; EX
is the local exchange energy from the local spin
B88
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density approximation (LSDA); EX
is Becke's gradient correction to the exchange
LYP
VMN
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functional; EC is the correlation functional developed by Lee, Yang, and Parr ; and EC
is the correlation energy calculated using the local correlation functional of Vosko, Wilk, and
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Nusair (VWN).
For optimization, the LANL2DZ and 6-31G(d) basis sets were chosen
All stationary-point structures were
found to have the appropriate number of imaginary frequencies. To determine the energy
for the Mo atom and the other atoms, respectively.
profile of the first protonation process, we performed single-point energy calculations at the
optimized geometries using the Stuttgart-Dresden pseudopotentials (SDD) and 6-311+G(d,p)
basis sets.
Zero-point energy corrections were applied for energy changes (ΔE) and
Solvation effects (toluene) were
activation energies (Ea) calculated for each reaction step.
taken into account by using the polarizable continuum model (PCM) in the single-point
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energy calculations.
In the calculation of the N≡N stretching frequency of 1a, 1e, and 1f,
solvation effects (THF) were considered by using PCM.
The calculated vibrational
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frequencies were corrected with a scaling factor of 0.960.
Protonation of a terminal dinitrogen ligand in 1 by LutH+ (step A) and elimination of
the dinitrogen ligand trans to the NNH group (step B) were assessed from a kinetic aspect by
exploring reaction pathways.
Energy profiles of the coordination of OTf– (step C) were
calculated with a thermochemical equation, III + OTf– → IV.
ASSOCIATED CONTENT
Supporting Information.
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Full preparative, analytical, crystallographic, and computational details in pdf format. This
material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
kazunari@ms.ifoc.kyushu-u.ac.jp
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ynishiba@sogo.t.u-tokyo.ac.jp
ACKNOWLEDGMENT
This paper is dedicated to the memory of Dr. Ryuji Shimazaki of Fuel Cell System
Development Center, Toyota Motor Corporation, Japan.
Y.N. thanks the Funding Program
for Next Generation World-Leading Researchers (GR025) and Grants-in-Aid for Scientific
Research (Nos. 26288044, 26620075, and 26105708) from the Japan Society for the
Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and
Technology of Japan (MEXT).
K.Y. thanks Grants-in-Aid for Scientific Research (Nos.
22245028 and 24109014) from the Japan Society for the Promotion of Science (JSPS) and the
Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) and the
MEXT Projects of "Integrated Research on Chemical Synthesis" and "Elements Strategy
Initiative to Form Core Research Center".
S.K. 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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REFERENCES
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method.
However, 1g could not be isolated and the reaction afforded only a mixture of
unidentified complexes containing 1g.
(12) Other ORTEP drawings are shown in the Supporting Information.
(13) Detailed electrochemical data are shown in the Supporting Information.
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Scheme 1.
Proposed Catalytic Cycle of Formation of Ammonia from Dinitrogen.
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8
cat.
6 e-
6 H+
+
N2
2 NH3
+
rt
1 atm
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N
N
N
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P
P
N
N Mo N N Mo N
NH3
2 H+, e!
P
N
N
N
P
N
N2
1a, P = PtBu2
NH2
N
N
NH3
P
N
P
P
N
P
N
N Mo N N Mo N
N Mo N N Mo N
P
N
N
N
P
P
N
OTf
P
N
N
H+, e!
N
P
3 H+, 4 e!
[Mo(N2)3(PNP)]
NH3
[Mo(N2)3(PNP)]
N Mo
OTf
P
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Scheme 2.
Synthesis of Dinitrogen-Bridged Dimolybdenum-Dinitrogen Complexes
4
5
Bearing 4-R-PNP-Pincer Ligands (1).
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9
N
N
N
Cl
Na-Hg
(6 equiv)
P
P
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P
N
R
N Mo Cl
R
N Mo N N Mo N
R
THF
rt, 12 h
P
P
Cl
N
N
N
P
P = PtBu2
N2 (1 atm)
N
2b (R = Ph)
1b (R = Ph): 52%
1c (R = Me3Si): 44%
1d (R = tBu): 48%
1e (R = Me): 79%
1f (R = MeO): 52%
2c (R = Me3Si)
2d (R = tBu)
2e (R = Me)
2f (R = MeO)
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Scheme 3.
Catalytic Formation of Ammonia Using Dimolybdenum-Dinitrogen Complexes
4
5
Bearing Unsymmetric PNP-Pincer Ligand as Catalysts.
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7
8
9
OTf
cat.
N2
(1 atm)
+
6
+
2 NH3
Co
6
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N
toluene
rt, 20 h
H
(72 equiv)
(96 equiv)
N
cat.
NH3
NH3
(equiv) (%)
N
R'
cat.
N
R'2P
tBu
Ad
Ph
12
14
2
49
58
8
PR'2
N
1a
1h
1i
N Mo N N Mo N
P
N
N
N
P
tBu2
tBu2
N
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Figure 1.
ORTEP drawing of 1f.
Hydrogen atoms and solvent molecules are omitted
for clarity.
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Figure 2.
Relationship between dinitrogen stretching frequencies and oxidation potentials
of 1 (R2 = 0.98).
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OTf!
N2 elimination
(step B)
protonation
(step A)
coordination
(step C)
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9
11.0
10.5
10.9
10.1
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8.5
8.4
9.1
TSII/III
7.7
6.5
7.6
6.8
6.1
4.9
TS1/II
4.7
II!Lut
II
3.5
(+ Lut)
0.0
0.0
0.0
2.7
0.7
0.3
III
(+ N2)
1!LutH+
III!N2
step A
+
N
H
+
N
H
H
+
+
N
N
N
N
N
N
N
N
N
R'2P
R'2P
R'2P
PR'2
N
PR'2
N
PR'2
N
N Mo N N Mo N
N Mo N N Mo N
N Mo N N Mo N
P
N
P
N
N
N
P
P
N
N
P
tBu2
N
tBu2
N
N
P
!
tBu2
tBu2
tBu2
tBu2
N
N
N
N
-10.9
-12.9
1!LutH+
II!Lut
II
step B
H
H
+
+
N
N
N
N
N
N
N
R'2P
R'2P
PR'2
PR'2
N
-14.9
IV
II
N Mo N N Mo N
N Mo N N Mo N
N2
!
P
N
P
P
tBu2
N
P
tBu2
tBu2
tBu2
N
N
N
N
III
III!N2
N
N
N
R'2P
step C
III
PR'2
N
H
N Mo N N Mo N
N
N
N
P
N
N
N
P
tBu2
OTf!
R'2P
tBu2
PR'2
N Mo N N Mo N
N
N
1a (R' = tBu):
1h (R' = Ad):
1i (R' = Ph):
P
N
OTf
P
tBu2
tBu2
N
IV
Figure 3.
Energy profiles of the first protonation process of 1a, 1h, and 1i.
Energies
+
relative to 1-LutH are presented in kcal/mol.
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