Identifying catalytically active mononuclear peroxoniobate anion of ionic liquids in the epoxidation of olefins

Article abstract of DOI:10.1021/acscatal.7b04443

The organic carboxylic acid coordinated monomeric peroxoniobate-based ionic liquids (ILs) [TBA][NbO(OH)₂(R)] (TBA = tetrabutylammonium; R = lactic acid (LA), glycolic acid (GLY), malic acid (MA)) were prepared and fully characterized by elemental analysis, NMR, IR, Raman, TGA, ⁹³Nb NMR, and HRMS. These IL catalysts exhibited not only high catalytic activity for the epoxidation of olefins under very mild reaction conditions, as the turnover frequency of [TBA][NbO(OH)₂(LA)] reached up to 110 h^-1, but also satisfactory recyclability in the epoxidation by using only 1 equiv of hydrogen peroxide as an oxidant. Meanwhile, this work revealed that the ILs underwent structural transformation from [NbO(OH)₂(R)]^- to [Nb(O-O)₂(R)]^- (R = LA, GLY, MA) in the presence of H₂O₂ by a subsequent activity evaluation, characterization, and first-principles calculations. Moreover, the organic carboxylic acid coordinated monomeric peroxoniobate-based ILs were investigated using density functional theory (DFT) calculations, which identified that [Nb(O-O)₂LA]^- was more advantageous than [Nb(O-O)₂(OOH)₂]^- for the epoxidation of olefins. Due to the coordination between the α-hydroxy acids and the monomeric peroxoniobate anions, the functionalized ILs can efficiently catalyze the epoxidation of a wide range of olefins and allylic alcohols under very mild conditions. Additionally, the effect of solvents on the reaction is illustrated. It was found that methanol can lower the epoxidation barriers by forming a hydrogen bond with a peroxo ligand attached to the niobium center.

Full text of DOI:10.1021/acscatal.7b04443

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Identifying Catalytically Active Mononuclear Peroxoniobate
Anion of Ionic Liquids in the Epoxidation of Olefins
Wenbao Ma, Haiyang Yuan, Haifeng Wang, Qingqing Zhou,
Kang Kong, Difan Li, Yefeng Yao, and Zhenshan Hou
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04443 • Publication Date (Web): 17 Apr 2018
Downloaded from http://pubs.acs.org on April 17, 2018
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Identifying Catalytically Active Mononuclear Peroxoniobate Anion of
Ionic Liquids in the Epoxidation of Olefins
†,§
†,§
, †
†
†
Wenbao Ma, Haiyang Yuan, Haifeng Wang,* Qingqing Zhou, Kang Kong,
†
‡
,†
Difan Li , Yefeng Yao, and Zhenshan Hou*
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†
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis,
East China University of Science and Technology, Shanghai 200237, People’s
Republic of China, E-mail: houzhenshan@ecust.edu.cn; hfwang@ecust.edu.cn.
‡
Physics Department and Shanghai Key Laboratory of Magnetic Resonance, East China
Normal University, Shanghai 200062, People’s Republic of China.
Abstract: The organic carboxylic acid-coordinated monomeric peroxoniobate-based
ionic liquids (ILs) designated [TBA][NbO(OH)
2
(R)] (TBA= tetrabutylammonium;
R=lactic acid (LA), glycolic acid (GLY) or malic acid (MA)) were prepared and fully
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3
characterized by elemental analysis, NMR, IR, Raman, TGA, Nb-NMR, and HRMS.
These IL catalysts exhibited not only high catalytic activity for the epoxidation of
olefins under very mild reaction conditions as the turnover frequency of
-1
[
TBA][NbO(OH) (LA)] reached up to 110 h but also satisfactory recyclability in the
2
epoxidation by using only one equivalent of hydrogen peroxide as an oxidant.
Meanwhile, this work revealed that the ILs underwent structural transformation from
-
-
[NbO(OH)
2
(R)] to [Nb(O-O)
2
(R)] (R=LA, GLY, MA) in the presence of H
2
2
O by the
subsequent activity evaluation, characterization and the first-principles calculations.
Moreover, the organic carboxylic acid-coordinated monomeric peroxoniobate-based
ILs were investigated using density functional theory (DFT) calculations, which
-
(OOH)
]- for the
identified that [Nb(O-O)
2
LA] was more advantageous than [Nb(O-O)
2
2
epoxidation of olefins. Due to the coordination between the α-hydroxy acids and the
monomeric peroxoniobate anions, the functionalized ILs can efficiently catalyze the
epoxidation of a wide range of olefins and allylic alcohols under very mild conditions.
Additionally, the effect of solvents on the reaction is illustrated. It was found that
methanol can lower the epoxidation barriers by forming a hydrogen bond with a peroxo
ligand attached to the niobium center.
Keywords: Peroxoniobate, Ionic liquid, Epoxidation, Olefins, DFT
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Introduction
Oxidation reactions are essential in the synthesis of oxygen-containing
1
compounds, and the epoxidation of olefins was one of the most important oxidation
reactions in the fine chemical industry. Epoxides constitute important intermediates for
the production of valuable chemicals, especially for the synthesis of polymers,
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surfactants, and a variety of natural products and chiral molecules. Expensive and
environmentally harmful oxidants, such as peracids and hazardous organic solvents, are
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usually employed to produce epoxides. In contrast, hydrogen peroxide is an
environmentally benign oxidant because water is the only product of H
2
2
O reduction,
which obviates the need to sell a co-product alcohol when organic hydroperoxide is
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selected as an oxidant. The epoxidation of olefins with H
2
O usually requires the
2
presence of a catalyst. Catalytic epoxidation reactions using non-noble metals and
environmentally benign oxidants (e.g., H
2
O ) have been attracting much attention
2
5
recently. As an important class of crystalline metallosilicates, microporous titanium
silicate zeolite (e.g., TS-1) has a uniform and ordered network of micropores and is
widely applied in industry for the epoxidation of C=C double bonds with hydrogen
peroxide, which provides an environmentally friendly approach for producing
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epoxides. However, TS-1 has pores of 5.5 Å in diameter, and thus bulky substrates
cannot easily access the active sites located inside the pores. Additionally, the leaching
of active species and the stability of these catalysts towards H
2
O under the reaction
2
conditions is still a considerable problem. Therefore, the epoxidation of bulky alkenes
using microporous titanosilicate catalysts remains a significant challenge.7
In addition to titanosilicate catalysts, various transition metal complexes,
especially the complexes of Mn, Fe and Mo-bearing ligands such as porphyrins, salen,
pyridyl-amine and N-heterocyclic carbene have also been reported to be employed as
catalysts in these processes.8-15 However, the common issues associated with
homogeneous complex catalysts, such as their hydrolytic stability under oxidative
conditions and the difficulty in recycling these catalysts, largely restricts their
application. Moreover, expensive ligands and complicated preparation processes also
lead to these catalysts not being cost-effective. Although tungsten-based catalysts
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display a high activity for the epoxidation of olefins, they usually exist in the form of
polynuclear species, e.g., polyoxometalates (POMs) and peroxotungstate catalysts,
which can degrade further into active species under reaction conditions. This leads to
structure instability and the difficult characterization of catalysts under reaction
conditions. Moreover, these catalysts typically have a limited turnover frequency.16-19
In Group V, the use of vanadium complexes as oxidative catalysts has been more
often reported in previous research.20-24 The two heaviest elements, niobium and
tantalum, have applications in advanced materials and catalysis. For example, the
niobium compounds are normally involved in acid-catalyzing reactions and show high
activity and stability.25-28 Additionally, niobium- and tantalum-based heterogeneous
catalysts have been explored in the epoxidation of olefins.29-34 However, only a poor or
moderate catalytic activity has been obtained. On the other hand, homogeneous
catalysts have been widely studied owing to their high catalytic activity and remarkable
structure designability, which allows for accurate reaction mechanism research at the
molecular level. To our knowledge, only a limited number of niobium-based
homogeneous catalysts have been used in the epoxidation of olefins and allylic
alcohols.35
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Peroxo complexes of niobium have attracted considerable attention in the last
decade. They exhibit the potential ability to release active oxygen and can participate
in oxidations of different kinds of substrates.36,37 These complexes correspond either to
+
+,
homoleptic tetraperoxoniobates of the general formula (M)
3
[Nb(O
2
)
4
] (M= K , NH
4
+
+
Na and gu ) (gu=carbamidine) or to heteroleptic peroxoniobate compounds,
designated (M) [Nb(O (L)]·zH O, where L is tartrate, citrate, malate, o-
x
2
)
y
2
phenanthroline, oxalic acid, dipyridine, EDTA, DTPA, etc.38,39 Nevertheless, the
catalytic performance of these complexes in the epoxidation reaction has rarely been
explored.
An IL is an organic salt, containing an organic cation and a suitable anion, in the
40
liquid state. ILs are currently of great interest owing to their unique properties, such
as a large liquid state range, high ionic conductivity, strong polarity, the contact ion
pairs effect and their inherent nature, which has expanding ILs into many important
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areas.41 In particular, ILs can be used as novel green solvents or benign catalytic
materials. Therefore, the functionalization of the IL cations and anions will surely
provide an effective approach to impart particular capabilities to ILs and encourage
more sustainable and rational development in this field.42-45
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Recently, we have developed a series of highly catalytically active peroxoniobate
(
peroxotantalate)-based IL catalysts for the selective epoxidation of allylic alcohols.46,47
These IL catalysts showed a higher turnover frequency (TOF) compared to the
previously reported Nb/Ta-based heterogeneous catalysts and dinuclear tungstate-based
catalysts. However, these peroxoniobate (peroxotantalate)-based IL catalysts exhibited
only a very limited catalytic activity for the epoxidation of olefins, which is an even
more important reaction than the epoxidation of allylic alcohols in the chemical industry.
On the basis of our previous work, in this work we have attempted to design several
novel organic acid-coordinated peroxoniobate-based ILs. The new IL catalysts
demonstrated excellent activity and reusability in the epoxidation of various kinds of
olefins and allylic alcohols. To our knowledge, this is the first work that Nb-based
catalysts have afforded a high catalytic activity for the epoxidation of various olefins.
In particular, the structure of the monomeric peroxoniobate anion in the presence of
H
2
2
O was identified in detail, and the reaction transition state was suggested through
experimental measurements and DFT calculations. Furthermore, the role of methanol
in the epoxidation has also been illustrated.
Results and Discussion
Catalyst Preparation and Characterization
A series of mononuclear peroxoniobate-based ionic liquids were synthesized by
introducing L-(+)-lactic acid (LA), glycolic acid (GLY) and L-(-)-malic acid (MA) into
the parent peroxoniobate anion [NbO(η-O
acid-coordinated peroxoniobate-based
TBA][NbO(OH) (GLY)] and [TBA][NbO(OH)
2
)(OH)
2
]-, which afforded the carboxylic
ILs [TBA][NbO(OH) (LA)],
(MA)], respectively. These
2
[
2
2
compounds were dark red in color and showed the general characteristics of ILs, such
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as a viscous liquid at room temperature, the immiscibility with low polar solvents
(
alkane, toluene, diethyl ether, etc.) and non-volatility (Figure S1). The viscosity of the
three IL catalysts, [TBA][NbO(OH) (LA)], [TBA][NbO(OH) (GLY)] and
TBA][NbO(OH) (MA)], was determined (Table 1). In addition, the melting point of
the ILs was measured by DSC, which is shown in Table 1. The melting points of
2
2
[
2
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[
TBA][NbO(OH)
2
(GLY)] and [TBA][NbO(OH)
2
(MA) were found to be approximately
(LA)
-30.2 °C and -42.7 °C, respectively, while the melting point of [TBA][NbO(OH)
2
was below -75 °C.
In the first step of their synthesis, TBAOH was obtained from TBACl and KOH
by anionic exchange, where TBA represented a quaternary ammonium cation. Slowly
mixing TBAOH with (NH
4
)
3
[Nb(O
2
4
) ] in dry ethanol resulted in the ionic liquid
[
TBA][NbO(η-O
ionic liquids were obtained accordingly via the reaction between [TBA][NbO(η-
)(OH) ] and the different carboxylic acids. The structures of the synthesized catalysts
were examined by H NMR, C NMR, inductively coupled plasma-atomic emission
2
)(OH)
2
]. Next, the organic acid-coordinated peroxoniobate-based
O
2
2
1
13
3
+
4+
spectroscopy (ICP-AES), elemental analysis, potential difference titration of Ce /Ce ,
and HRMS. The number of peroxide bonds were 0.1, 0.12, and 0.09 in
[
TBA][NbO(OH)
respectively by potential difference titration of Ce /Ce , suggesting that the peroxo
band in [NbO(η-O )(OH) ] was substituted by carboxylato ligands in the course of
catalyst preparation (Scheme 1). As a result, the reaction between [TBA][NbO(η-
)(OH) ] and α-hydroxy acids in a 1:1 molar ratio led to new organic acid-coordinated
2
(LA)], [TBA][NbO(OH)
2
(GLY)] and [TBA][NbO(OH) (MA)],
2
3+
4+
2
2
O
2
2
ILs, where the α-hydroxy acids act as a bidentate ligand, coordinating with the Nb site
by two oxygen atoms from the carboxylato and vicinal hydroxyl groups. Complexation
with the metal atom therefore resulted in a five-membered chelate ring, which exhibited
a similar structure as that of the peroxo-carboxylato Nb(V) complexes.48,49
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Scheme 1. Synthetic route of different IL catalysts.
These IL catalysts were first investigated by using FT-IR spectroscopy. As shown
in Figure 1a (left), the band in [TBA][NbO(η-O
2
)(OH)
2
] at approximately 570 cmÞ1
Þ1
50
was related to ꢀas(Nb(O
2
)), and the band near 850 cm was characteristic for ꢀ(O-O).
Þ1
51
In addition, the band at approximately 886 cm was related to ꢀ(Nb=O), and the band
-
1
52
at 1640 cm was assigned to the adsorption of water molecules on the catalyst. For
the different organic acid-coordinated ILs (Figure 1b, 1c and 1d, left), the broadening
of the ꢀas(COO) band and its significant shift from a value of approximately 1700 cm-1
-1
in the free carboxylic acid to approximately 1665 cm in the coordination complex
39
indicated the coordination of the Nb site with a carboxylato group. In addition, the
Þ1
Þ1
bands at 570 cm and 850 cm that arose from (O-O) species were not observed in the
organic acid-coordinated ILs, revealing the absence of (O-O) species after coordination,
which was well in agreement with that of the potential difference titration analysis. The
Þ1
53
band at approximately 1100 cm of these ILs was related to the vibration of C-O; the
Þ1
Þ1
band at approximately 882 cm was attributed to v(Nb=O); and the band near 585 cm
-1
54
and 545 cm was characteristic for the ꢀ(Nb-O) in the ILs. The absence of the
-1
stretching of the Nb-O-Nb bridge at 835 cm indicated that the peroxoniobate anion
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existed as a mononuclear instead of a multinuclear form. Additionally, the band at
Þ1
approximately 1734 cm in [TBA][NbO(OH) (MA)] (Figure 1d, left) assigned to the
2
vibration of the free carboxylic group of malic acid still existed, indicating that only
one carboxylato group and the vicinal hydroxyl group in the malic acid molecule
participated in the formation of complexes.56
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86
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580
a
1640
50
570
a
b
b
c
c
d
d
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1100 882
585545
1
676
1
734 1665
1058 880 603
2
400 2100 1800 1500 1200 900-1 600 2400 2100 1800 1500 1200 90-10 600
Wavenumber (cm )
Raman Shift (cm )
Figure 1. FT-IR spectra (left) and Raman spectra (right) of (a) [TBA][NbO(η-
)(OH) ]; (b) [TBA][NbO(OH) (LA)]; (c) [TBA][NbO(OH) (GLY)]; and (d)
TBA][NbO(OH) (MA)].
O
2
2
2
2
[
2
Raman spectroscopy gave similar results. As shown in Figure 1a (right), the band
Þ1
in [TBA][NbO(η-O
2
)(OH)
2
] at 580 cm was related to ꢀ
s
(Nb(O
2
)), and the band near
Þ1
50
Þ1
8
65 cm was characteristic for ꢀ(O-O). The band at approximately 880 cm of the
IL was related to v(Nb=O). After the IL [TBA][NbO(η-O
2
)(OH)
2
] was coordinated by
Þ1
Þ1
carboxylic acid (Figure 1b, 1c and 1d, right), the bands at 580 cm and 865 cm ,
attributed to the vibration of peroxo group, disappeared, as seen from the FT-IR spectra.
Þ1
-
The band at approximately 1676 cm was related to the vibration of COO coordinating
Þ1
with the niobium atom, while the band at approximately 1058 cm was assigned to the
Þ1
-1
vibration of C-O. The bands at approximately 880 cm and 603 cm of the ILs were
related to ꢀ(Nb=O) and ꢀ(Nb-O), respectively.56
The TGA curves of the three ILs are sequentially shown in Figure S2. Obvious
weight losses of approximately 72.5%, 71.7%, and 75.1% were observed between
4
0 °C and 700 °C, which could be assigned to the decomposition of the cation part and
the carboxylato group in the anion moieties. Unlike [TBA][NbO(η-O )(OH) ], which
2
2
was mostly degraded at 200 °C,46 the organic acid-coordinated ILs decomposed
completely at a higher temperature (approximately 300 °C), indicating a higher thermal
stability. Table 1 summarizes the main physico-chemical data of the three ILs.
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1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 1. The physico-chemical data of the three IL catalysts.
T
m
T
d
Viscosity
IR
Raman
IL
Color
Pa.s[a]
cm-1
cm-1
°
C
°C
[
TBA][NbO(OH)
2
(LA)] Dark red < -75.0
334
1.25
1.37
1.46
543, 589 (ꢀas(Nb-O))
1660 (ꢀas(COO))
604 (ꢀas(Nb-O))
1667 (ꢀas(COO))
1059 (v(C-O))
880 (v(Nb=O))
604 (ꢀas(Nb-O))
1663 (ꢀas(COO))
1059 (v(C-O))
878 (v(Nb=O))
591 (ꢀas(Nb-O))
1680 (ꢀas(COO))
1057 (v(C-O))
879 (v(Nb=O))
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
123 (v(C-O))
83 (v(Nb=O))
543, 579 (ꢀas(Nb-O))
669 (ꢀas(COO))
091 (v(C-O))
8
[
TBA][NbO(OH)
2
(GLY)] Dark red
-30.2
330
1
1
883 (v(Nb=O))
[
TBA][NbO(OH)
2
(MA)] Dark red
-42.7
329
542, 583 (ꢀas(Nb-O))
1
667 (ꢀas(COO))
100 (v(C-O))
81 (v(Nb=O))
1
8
T
m
and T
d
refer to the melting point and decomposition temperature, respectively. [a] viscosity
data at 25 °C.
High-resolution electrospray ionization mass spectrometry (HR ESI-MS) analysis
in the negative mode was also applied to determine the structure of these catalysts. The
ESI-MS of [TBA][NbO(OH)
corresponding to [NbO(OH)
2
(LA)] a¡orded an ion centered at 230.9223 m/z,
-
2
(LA)] . Similarly, the ESI-MS of [TBA][NbO(OH)
2
(GLY)]
-
showed an ion centered at 216.9066 m/z, corresponding to [NbO(OH)
2
(GLY)] and an
ion centered at 274.9121 m/z for [TBA][NbO(OH)
2
(MA)] was attributed to
-
[
NbO(OH)
2
(MA)] (Figure S3-S5).
Finally, [TBA][NbO(OH)
chemical environment changes of the Nb sites by using Nb NMR spectra (Figure 2).
The [TBA][NbO(η-O )(OH) ] IL displayed a resonance signal at Þ1000 ppm (Figure
a). However, when coordinated by lactic acid, the signal was shifted to the low field
2
(LA)] was chosen as a model IL to examine the
9
3
2
2
2
and the chemical shift moved to approximately Þ890 ppm (Figure 2b). This was
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probably due to the decreasing electron density of the niobium atom when the
peroxoniobate anions were coordinated by α-hydroxy acids, which acted as the electron
5
7
withdrawing group. These results were highly consistent with those of the previous
characterization, including FT-IR, Raman, titration analysis and HRMS, which all
showed that the α-hydroxy acids functioned as a bidentate ligand. As a result, niobium
exhibited coordination by five oxygen atoms belonging to one bidentate group, two
hydroxyl groups and one doubly bound O atom in the three IL catalysts.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1000
a
0
-250 -500 -750 -1000 -1250 -1500 -1750 -2000
890
-
b
0
0
-250 -500 -750 -1000 -1250 -1500 -1750 -2000
1116
-
c
-250 -500 -750 -1000 -1250 -1500 -1750 -2000
Chemical Shift
9
3
Figure 2. Nb NMR spectra of different niobate anions in CD
)(OH) ]; (b) [TBA][NbO(OH) (LA)]; and (c) oxidizing species of
TBA][NbO(OH) (LA)].
3
OD: (a) [TBA][NbO(η-
O
2
2
2
[
2
The Structural Identification of Peroxoniobate Anion in the Presence of H
It was visually observed that the color of the ILs quickly changed from dark red to
colorless when the IL catalysts were treated with H , which revealed that the anion
of the ILs could be oxidized by H to create a new species. To identify the new anion
structure form, the IL [TBA][NbO(OH) (LA)] (0.2 mmol) was added to methanol (1
ml), and then hydrogen peroxide (2 mmol) was added to the solution. After stirring for
0 min at 273 K, diethyl ether was rapidly poured into the solution to precipitate the
2
O
2
2
O
2
2
O
2
2
1
oxidizing species, which was washed carefully with a small amount of diethyl ether,
sequentially. The resultant material was then characterized. In the IR spectra (Figure
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2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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6
-1
3
a), the vibration at approximately 882 cm , which was related to ꢀ(Nb=O),
Þ1
disappeared, and a single band near 865 cm emerged. This could be attributed to ꢀ(O-
5
0
O). Only one vibration of ꢀ(O-O) was observed, indicating that the oxidizing species
-1
only contained one type of (O-O) group. Additionally, a strong peak at 576 cm ,
referring to ꢀ[Nb(O )], appeared simultaneously. The Raman spectra gave similar
information (Figure 3b). The oxidizing species displayed one strong ꢀ(O-O) band at
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
Þ1
Þ1 50
approximately 870 cm and one broad v[Nb(O
2
)] band near 585 cm . In addition to
IR and Raman spectroscopy, Nb NMR of the oxidizing species was also carried out.
As shown in Figure 2c, when [TBA][NbO(OH) (LA)] was treated with H , the signal
was shifted to high field (-1116 ppm), which was obviously different from that of both
the IL [TBA][NbO(η-O )(OH) ] and the coordinated IL [TBA][NbO(OH) (LA)]. This
indicated that the chemical environment around niobium changed, and a new
9
3
2
2
O
2
2
2
2
peroxoniobate species was formed in the presence of H
2
2
O .
a
8
65
70
576
85
8
5
b
2240 1960 1680 1400 1120 840 560
-1
Wavenumber (cm )
Figure 3. (a) IR spectrum and (b) Raman spectrum of oxidizing species of
[TBA][NbO(OH) (LA)].
2
Next, the peroxo number of the oxidizing species was determined by potential
3
+
4+
difference titration of Ce /Ce . The number of O-O was approximately 1.8,
demonstrating that the oxidizing species could contain approximately two peroxo
groups (O-O) in derived oxidizing species. Furthermore, HR ESI-MS was employed to
determine the structure of the oxidizing intermediates. The ESI-MS of
[
TBA][NbO(OH) (LA)] containing hydrogen peroxide a¡orded an ion centered at
2
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44.9015 m/z corresponding to [Nb(O-O)
reaction intermediate (Figure S6).
To gain more insight into the oxidizing species, five possible anions arising from
2
(LA)] , which can be recognized as the
-
-
-
[NbO(OH)
2
(LA)] in the presence of H
2
O
2
, i.e., [Nb(O-O)(OH)
2
LA] , [NbO(O-O)LA] ,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-
3-
-
[Nb(O-O) (LA)] , [Nb(O-O)
2
3
(LA)] and Nb[(O-O)(OOH)
2
(LA)] , were explored by
first principles calculations. Upon optimizing the structures of these five peroxoniobate
anions, the relative stability was compared through computing the Gibbs free energy
-
changes (ûG) for their formation reactions from [NbO(OH)
2
(LA)] at 298 K (Table 2).
Evidently, if ûG < 0 (> 0), the corresponding conversion is thermodynamically favored
(
hindered); the more negative ûG is, the more stable the generated Nb anion is. As
-
indicated in Table 2, the formation of [Nb(O-O)
2
LA] containing two peroxos is the
3
-
most thermodynamically favored (ûG = -0.51 eV), while [Nb(O-O) LA] is the least
3
stable with the most positive formation energy (ûG =1.72 eV). Therefore, [Nb(O-
-
O)
2
LA] can be theoretically expected to be the dominant form, which is in accordance
with the above experimental identification.
-
Table 2. Gibbs free energy changes (ûG) of the conversion from [NbO(OH)
2
LA] to
-
-
-
-
[
[
(
Nb(O-O)(OH)
2
LA] , [NbO(O-O)LA] , [Nb(O-O)
2
LA] , Nb[(O-O)(OOH)
2
LA] and
3-
Nb(O-O)
3
LA] , in the presence of H
2
O . Note that the solvation energy of the proton
2
+
H ) is set at the experimental value of approximately -11.53 eV.
Reaction
ûG /eV
-0.15
-0.24
-0.51
-0.18
1.72
-
-
[NbO(OH)
2
2
2
2
2
LA] + H
2
O
2
: [Nb(O-O)(OH)
2
LA] + H O
2
-
-
[
[
[
[
NbO(OH)
NbO(OH)
NbO(OH)
NbO(OH)
LA] + H
2
O
2
: [NbO(O-O)LA] + 2H O
2
-
-
LA] + 2H
2
2
2
O
O
O
2
2
2
: [Nb(O-O)
2
LA] + 3H
2
O
-
-
O + 2H+
LA] + 3H
: Nb[(O-O)(OOH)
2
LA] + 3H
2
-
3-
LA] + 3H
: [Nb(O-O)
3
LA] + 3H
2
O
In summary, according to the above experimental and theoretical results, the newly
-
evolved Nb anion from [NbO(OH)
2
R] oxidized by H
2
2
O in our system is proposed to
-
be [Nb(O-O)
2
LA] (R=LA, GLY, MA) (see Scheme 2). The optimized configuration of
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2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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6
-
[
Nb(O-O) LA] is illustrated in Figure 4.
2
R'
O
OH
R'
O
O
O
O
O
O
O
O
[
N4,4,4,4] O Nb
2
+ 2H O2
[N4,4,4,4
]
Nb
O
+ 3H O
2
OH
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
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1
2
3
4
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
R' = -CH ; [TBA][Nb(OO) (LA)]
3
2
-
-
H; [TBA][Nb(OO) (GLY)]
2
CH COOH; [TBA][Nb(OO) (MA)]
2
2
Scheme 2. The structure evolution of the IL catalysts [TBA][NbO(OH)
2
(LA)] in the
presence of hydrogen peroxide.
-
Figure 4. Ball-and-stick model of the optimized structure of [Nb(O-O)
2
LA] . White,
gray, red, blue balls indicate the H, C, O and Nb atoms, respectively.
Catalytic Performance
Catalytic Epoxidation of Olefins
The model substrate cyclooctene was epoxidized with H
2
2
O in the presence of
organic acid-coordinated peroxoniobate-based ionic liquids in methanol at 30 °C. The
results are shown in Table 3. The catalytic reaction could barely occur without catalysts
(
Table 3, entry 1), indicating that H
2
2
O could not oxidize the substrate without the
catalysts. TBACl was also inactive for the epoxidation of olefins (Table 3, entry 2). It
was clear that the organic acid itself could not catalyze the epoxidation reaction at all
(
Table 3, entries 3-5). In addition, (NH
epoxidation of cyclooctene (Table 3, entry 6). The (NH
nearly undissolved in the methanol, and the catalytic system displayed a liquid-solid
biphase under the reaction conditions. In contrast, the IL catalyst [TBA][NbO(
)(OH) ] was miscible with the methanol media and retained a homogeneous phase,
and it exhibited a much higher catalytic activity than that of (NH [Nb(O ] (Table 3,
4
)
3
[Nb(O
2
)
4
] afforded a very poor activity for the
4
)
3
[Nb(O
2
4
) ] compound was
η
-
O
2
2
4
)
3
2 4
)
1
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entries 6 vs 7). The results suggest that the catalytic activity could arise from the moiety
of peroxoniobate anions, while the substitution of the inorganic cation (NH +
) by a
4
quaternary ammonium cation was much more favorable to the epoxidation reaction due
to the better solubility of the catalyst in the organic solvent.58,59 Additionally, the
different anion structure could have contributed to the improved activity. Interestingly,
1
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1
1
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1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
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4
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5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
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9
0
1
2
3
4
5
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7
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
the epoxidation reaction proceeded very efficiently when [TBA][NbO(OH)
adopted as the catalyst (Table 3, entry 8). A similar reactivity could be obtained using
TBA][NbO(OH) (GLY)] and [TBA][NbO(OH) (MA)] (Table 3, entries 9 and 10). If
lactic acid was added directly into the parent IL, a high conversion of approximately
5.3% was still achieved (Table 3, entry 11). However, it was observed that the
2
(LA)] was
[
2
2
8
introduction of acetic acid, succinic acid or 3-hydroxypropionic acid afforded the same
catalytic activity as that of the parent IL (Table 3, entry 7 vs entries 12-14), implying
that the addition of these acids cannot exert a positive effect on the epoxidation of
olefins. These results clearly demonstrated that the α-hydroxy acids (LA, GLY and MA)
as ligands played a crucial role in the enhancement of catalytic activity. In particular,
these IL catalysts demonstrated their advantages in catalytic epoxidation compared with
that of the reported tungsten-containing catalysts such as [TBA]
2
W O11 or
2
[
TBA] 40 under similar reaction conditions (Table 3, entries 15 and 16).
3
PW12O
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
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5
6
Table 3. Epoxidation of Cyclooctene with Different Catalysts[a]
Con.
Sel. [%]
epoxide others[c]
b]
TOF/h-1
Entry
Catalysts
/[%][
1
2
3
4
5
6
7
8
9
None
0.3
0.7
≥99
≥99
≥99
≥99
≥99
≥99
≥99
≥99
≥99
≥99
≥99
≥99
≥99
≥99
≥99
≥99
< 1
< 1
< 1
< 1
< 1
< 1
É
É
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
TBACl
Lactic acid
Glycolic acid
Malic acid
0.4
É
0.2
É
0.5
É
(NH
[TBA][NbO(η-O
[TBA][NbO(OH)
[TBA][NbO(OH)
[TBA][NbO(OH)
4
)
3
[Nb(O
2
)
4
]
1.5
É
2
)(OH)
(LA)]
(GLY)]
(MA)]
2
]
52.1
98.1
97.3
95.6
88.3
57.7
58.1
56.0
62.4
3.5
< 1
22.6
110
105
107
90.8
30.2
35.3
36.7
25.5
É
2
< 1
< 1
< 1
<1
2
10
2
1
1[d] [TBA][NbO(η-O
2[e] [TBA][NbO(η-O
3[f] [TBA][NbO(η-O
4[g] [TBA][NbO(η-O
15 [TBA]
6[h]
[TBA]
2
2
2
2
)(OH)
)(OH)
)(OH)
)(OH)
2
]
2
]
2
]
2
]
1
< 1
< 1
< 1
< 1
< 1
1
1
2
W
2
O
11
40
1
3
PW12
O
[
a] Reaction conditions: substrate (3 mmol), H
solution), catalyst (0.05 mmol), T=30 °C, t=4 h, 2 ml of CH
conversion with n-butyl alcohol as an internal standard. Conversion=([R
R])/[R ], where [R ] refers to the initial concentration of cyclooctene and [R]
refers to the concentration during the reaction; [c] indicates cyclooctanediol;
2
O
2
(3 mmol, 30% aqueous
3
OH; [b] GC
0
]-
[
0
0
[
[
(
d] Lactic acid was an additive; [e] Acetic acid (0.05 mmol) was an additive;
f] Oxalic acid (0.025 mmol) was an additive; [g] 3-hydroxypropionic acid
0.05 mmol) was an additive; [h] CH CN was used as solvent.
3
Investigation of the solvent effect on the epoxidation of cyclooctene using
[TBA][NbO(OH) (LA)] as a catalyst showed that a poor conversion was obtained in
2
1
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the solvent-free condition and in ethyl acetate and dichloromethane (Table S1, entries
1
-3), which was probably due to the biphasic state during the reaction. Additionally, the
conversion of cyclooctene reached 32.0%, 82.8% and 98.1% in acetonitrile, ethanol and
methanol, respectively (Table S1, entries 4-6). The results showed that this epoxidation
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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3
4
4
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4
4
4
4
4
4
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9
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9
0
6
0
reaction was favored in strong polar protic solvents (Table S1, entries 4-6). To further
understand the role of the protic solvent in the reaction, the reaction rate constant in
methanol solvent containing different concentrations of H
results are shown in Figure 5. It was observed that when the concentration of H
raised from 4.9 M (no extra water was added) to 7.0 M, the reaction rate constant
increased slightly. However, when the concentration of H O was greater than 7.0 M,
2
O was determined, and the
2
O was
2
the reaction rate constant decreased. This was because the additional water led to a
biphasic system, and thus the substrate was not easily accessible to the active center.
Accordingly, the substrate was hardly converted using solely H O as a solvent (Table
2
S1, entry 7). It was clear that using an appropriate protic solvent can indeed promote
epoxidation in this system. In addition, the effect of different oxidants, including
hydrogen peroxide, tert-butyl hydroperoxide and cumyl hydroperoxide, were also
investigated and the results are summarized in Table S2. The results demonstrated that
the epoxidation of the olefin could proceed very effectively with hydrogen peroxide as
the oxidant using [TBA][NbO(OH) (LA)], but the reaction only slightly occurred when
2
using either tert-butyl hydroperoxide or cumyl hydroperoxide as the oxidant under
similar reaction conditions (Table S2, entries 2-5). Notably, the epoxidation of
cyclooctene did not proceed even if [TBA][NbO(η-O
2
)(OH) ] was used as a catalyst
2
with tert-butyl hydroperoxide as an oxidant (Table S2, entry 6). Thus, the inhibition of
acid co-catalyst binding might be impossible. In general, a high reaction temperature is
usually needed when tert-butyl hydroperoxide or cumyl hydroperoxide are used as
oxidant.61 However, the present peroxoniobate species decomposed at high
temperatures. Therefore, in the present catalytic system, the catalytically relevant active
intermediate may be hard to generate under the very mild reaction conditions if tert-
butyl hydroperoxide or cumyl hydroperoxide is used as the oxidant.
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100
2.8
2.4
a
b
80
60
40
2.0
1.6
1.2
0.8
4.9 M
0
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5.9 M
7.0 M
7.5 M
8.0 M
20
0
0.4
0
50 100 150 200 250
5
6
7
8
t (min)
[H O] (M)
2
Figure 5. (a) The conversion of cyclooctene over time in CH
concentrations of H
mmol, 30% aqueous solution), IL (0.05 mmol), T=30 °C. (b) Effect of concentration of
3
OH containing different
2
O (4.9 M-8.0 M). Conditions: cyclooctene (3.0 mmol), H
2
O (3.0
2
H
2
O on the pseudo-first-order rate constant k.
There have been some reports regarding a radical mechanism in the oxidation
process catalyzed by niobium-based catalysts.62-63 Thus, a free radical inhibiter,
butylated hydroxytoluene (BHT), was added to the reaction mixture in the course of the
epoxidation reaction. As shown in Figure S7, the addition of BHT did not have any
effect on the reaction rate. This result demonstrated that a peroxo radical intermediate
could not be involved in the present epoxidation reaction.64
Kinetic Studies
When hydrogen peroxide was added to [TBA][NbO(η-O
solution, more bubbles were visually observed due to the H decomposition than in
the [TBA][NbO(OH) (LA)] solution. This implied that the decomposition rate of H
might be different in the two IL solutions. Subsequently, the effect of IL catalysts on
the decomposition rate of H was determined by quantifying the residual amount of
2
)(OH) ] methanol
2
2
O
2
2
O
2 2
2
O
2
hydrogen peroxide over time. As shown in Figure 6, it was found that the hydrogen
peroxide in methanol was not appreciably decomposed without catalyst. The hydrogen
peroxide was obviously more stable in three ILs ([TBA][NbO(OH)
2
(LA)],
[
TBA][NbO(OH) (GLY)] and [TBA][NbO(OH) (MA)]) than in [TBA][NbO(η-
2
2
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O
2
)(OH)
2
] (Figure 6). This revealed that the utilization efficiency of hydrogen peroxide
could be much higher when using organic acid-coordinated ILs as a catalyst in methanol.
Additionally, there was a lower decomposition rate of H in methanol, compared with
(LA)] (Figure S8).
2
O
2
that in ethanol or acetonitrile when using [TBA][NbO(OH)
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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0
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0
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0
1
2
3
4
5
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7
8
9
0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
100
200
300
400
500
t (min)
Figure 6. The changes of H
2
O
2
concentration [H
2
2
O ] in methanol over time in the
presence of the different ILs. (v) without catalyst; (x) [TBA][NbO(η-O
2
)(OH)
2
]; (”)
[
[
(
TBA][NbO(OH)
2
2
(LA)];
(z)
[TBA][NbO(OH)
2
(GLY)];
(
¹
)
TBA][NbO(OH)
(MA)]. Conditions: H
2
O (3.0 mmol, 30% aqueous solution), IL
2
0.05 mmol), CH OH (2 ml), T=30 °C. The residual amount of hydrogen peroxide was
3
3+
4+
measured by potential difference titration of Ce /Ce .
To further illustrate the decomposition rate of H
2
2
O in the two Nb-containing anion
systems ([TBA][NbO(OH)
compared the conversion process of H
NbO(η-O )(OH)
pathways for H
2
(R)] and [TBA][NbO(η-O
2
)(OH)
2
]), we calculated and
-
2
O
2
catalyzed by either [NbO(OH)
2
R] or
[
2
2
]- via the first principles simulations. The considered key reaction
decomposition (2H :2H O + O ) are schematized in Figure 7.
2
O
2
2
O
2
2
2
First, two possible pathways were compared for the initial reaction of H
2
2
O on [Nb(O-
-
O)
2
LA] , corresponding to the O-O bond cleavage into two *OH groups or the peroxo-
assisted dehydrogenation, as shown in Figure 7. The latter pathway was kinetically
more favorable with a lower energy barrier of 0.79 eV compared with the former
pathway (1.79 eV). The transition state (TS1) for the dehydrogenation reaction is shown
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in Figure S9a. It can be seen that the O-H bond in H
2
O is elongated to 1.154 Å, and
2
the H-receiving peroxo (O-O) nearly breaks its one Nb-O bond at a long distance of
2.285 Å, implying the formation of two *OOH species. Second, as shown in Figure 7,
the second H
i) breaking the peroxo (O-O) bond in H
and breaking its O-O bond simultaneously (corresponding to H
O + O ), and (iii) dehydrogenation assisted by *OOH, forming an *O ligand and
OOH (H + *OOH : *OOH + H O + *O). The barriers of pathways (i) and (ii)
2
O
2
molecule could further participate in the reaction via three pathways:
, (ii) coupling with *OOH to form the H
+ *OOH : *OH +
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0
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3
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0
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0
(
2
O
2
2
O
2
O
2
H
2
2
*
2
O
2
2
were calculated to be as high as 1.82 eV and 2.04 eV, respectively, implying their
infeasibility. In contrast, pathway (iii) is advantageous with a reasonable barrier of 1.28
eV (ûG = -0.89 eV). The located transition state (TS2) for pathway (iii) is shown in
Figure S9b. Subsequently, the *OOH species can easily transfer its H to the nearby *O
by overcoming a low barrier of 0.28 eV (see TS3 in Figure S9c for the transition state),
leading to an *OH and O
2
molecule detaching from the Nb site (*OOH + *O: *OH +
O
2
). Third, the formed *OH captures H from the *OOH species to form a H O molecule
2
and recovers the peroxo group, finishing the catalytic cycle. This process is
thermodynamically favored with ûG = -0.50 eV and corresponds to a low barrier of
0
.38 eV (see TS4 for the transition sate in Figure S9d). According to the above analysis,
the optimal decomposition pathway of H
2
O
2
(2H
2
O
2
:2H
2
O+ O ) catalyzed by [Nb(O-
2
-
O)
2
LA] is shown by a light blue line in Figure 7, and the energy profile is illustrated in
Figure 7 (see blue). Similarly, the decomposition of H
2
O
2
in the [NbO(η-O
2
)(OH)
]-
2
-
system was also calculated and compared with [Nb(O-O)
)(OH) to [Nb(O-O) (OOH)
]- is easily oxidized by H
work,46 and thus we mainly investigated the decomposition of H
O) (OOH)
]-. The resulting energy profile is shown by the red line in Figure 7.
Comparing these two energy profiles in Figure 7, one can clearly see that the potential
2
LA] . Notably, [NbO(η-
O
2
2
2
O
2
2
2
]-, according to our previous
2
O on [Nb(O-
2
2
2
-
-
energy surface of [Nb(O-O)
directly demonstrating that the decomposition of H
difficult relative to that in [Nb(O-O) (OOH)
with the above experimental result that H
2
LA] is higher overall than that of [Nb(O-O)
2
(OOH) ] ,
2
-
2
O
2
in [Nb(O-O)
2
LA] is more
]-. This theoretical result is in accordance
2
2
2
O
2
has a lower decomposition rate in the
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-
]- system.
[
Nb(O-O)
2
LA] system than in the [Nb(O-O)
2
(OOH)
2
In addition, considering that the solvent molecule, especially the protonic solvent,
6
5
could play a role in affecting the H
O
2 2
decomposition, here we also investigated the
-
solvent effect of CH
was found that the presence of CH
dissociation (role I: facilitation via CH
inhibition due to the competitive adsorption of CH
3
OH on the critical H
2
O
2
dehydrogenation in [Nb(O-O)
2
LA] . It
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1
1
1
1
1
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2
2
2
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2
2
2
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0
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0
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2
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0
1
2
3
4
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8
9
0
1
2
3
4
5
6
7
8
9
0
3
OH solvent showed two types of roles in modulating
H
2
O
2
3
OH-assisted proton transfer; role II:
OH at the Nb site), which overall
3
resulted in a slightly increased barrier (see details in the Supplementary Note 1 and
Figure S18). Upon further comparison of the interaction energies of different solvent
-
molecules (CH
Table S3), we found that they follow the order of ûGCH
CN(Nb), implying that CH OH has the strongest interaction with the Nb
3
OH, CH
3
CH
2
OH and CH
3
CN) with the Nb site in [Nb(O-O)
2
LA] (see
3
OH(Nb)
<
ûGCH
3
CH
2
OH(Nb)
<
ûG
H
2
O
2
(Nb)
<
ûGCH
3
3
site and can suppress H
2
O dehydrogenation to the largest extent among these solvents,
2
which is in agreement with the experimental result shown in Figure S8.
Figure 7. Diagram and energy profile of the decomposition of H
2
O catalyzed by two
2
different Nb anions. In the energy profile, the blue and red line represent [Nb(O-
-
(OOH)
]-, respectively.
O)
2
LA] and [Nb(O-O)
2
2
Considering on the slow decomposition rate of H
2
2
O in methanol, the initial
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epoxidation rate of cyclooctene using the four types of ILs was examined. The results
are shown in Figure 8. In spite of the same concentration of hydrogen peroxide, a higher
initial reaction rate was achieved with organic acid-coordinated IL as the catalyst, in
comparison with that of the parent IL [TBA][NbO(η-O
2
)(OH) ]. Thus, the improved
2
0
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8
9
0
catalytic activity resulted from not only the stability of hydrogen peroxide but also the
nature of IL catalysts.
100
80
6
0
0
4
20
0
0
50 100 150 200 250 300 350 400
t (min)
Figure 8. Conversion/time profile of the epoxidation reaction of cyclooctene catalyzed
by (v) [TBA][NbO(OH) (LA)], (z) [TBA][NbO(OH) (GLY)], (x)
TBA][NbO(OH) (MA)] and (”) [TBA][NbO(η-O )(OH) ]. Reaction conditions:
cyclooctene (3.0 mmol), H (3.0 mmol), catalyst (0.05 mmol), CH OH (2 ml), T=
0 °C.
2
2
[
2
2
2
2
O
2
3
3
With regards to the results of other groups and our previous research, the
epoxidation of olefins showed reasonable first-order plots for the concentration of
substrate and zero-order dependence on the concentration of hydrogen peroxide at low
concentrations.46,47,66 To eliminate the diffusion limitations, the effect of stirring speeds
and the amount of catalysts on the reaction rate was investigated, and the results are
shown in Figure S10. Next, the reaction kinetics of each catalyst were investigated and
the activation energy was attained. The results are shown in Figure S11, where the
reaction rate constants were determined from the experimental data by assuming
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pseudo-first-order reaction kinetics. The activation energy of [TBA][NbO(OH) (LA)]
2
-1
under the present conditions (Ea=32.5 KJ⋅mol ) was much lower than that of
-1
[TBA][NbO(η-O
2
)(OH) ] (Ea=51.1 KJ⋅mol ). The activation energy of the present IL
2
catalyst was also lower than the previously reported metal-based catalysts, where
1
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1
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1
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1
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2
2
2
2
2
2
2
2
2
2
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0
1
2
3
4
5
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7
8
9
0
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2
3
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5
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7
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9
0
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2
3
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5
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0
1
2
3
4
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3
4
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9
0
activation energies (Ea) were in the range of 50-100 KJ⋅mol-1.67
Substrate Scope and Recyclability of the Catalyst
In the next step, a series of olefins were tested to synthesize the corresponding
epoxides to examine the substrate scope of the present catalyst, and the results are
shown in Table 4. A conversion of between 60% and 98.5% was obtained in the
epoxidation of cyclic olefins or aromatic olefins (Table 4, entries 1-6). Specifically, the
epoxidation of cyclic olefins afforded very high conversion and selectivity (Table 4,
entries 1-3), although small amounts of by-products were detected by GC-MS
(cyclohexane-1,2-diol and 2-methoxycyclohexanol) in the cyclohexene epoxidation.
The absence of cyclohexenol or cyclohexenone in the products demonstrated that the
epoxidation reaction did not proceed by a single electron transfer mechanism (Table 4,
entry 2). Furthermore, the catalyst was also proven to be effective for the epoxidation
of aromatic olefins, where corresponding aromatic aldehydes (benzaldehyde, p-
tolualdehyde, and p-chlorobenzaldehyde) were detected as the main by-products (Table
4
, entries 4-6). The superior reactivity of cyclic olefins prompted us to use our
optimized method for the epoxidation of more challenging linear terminal alkenes. The
reaction of 1-hexene resulted in approximately 60.3% conversion and the selective
formation of 1,2-epoxyhexane (Table 4, entry 7), while the epoxidation of 1-octene also
produced the corresponding epoxide very selectively, although the conversion was
slight lower (Table 4, entry 8). Considering the limited miscibility of 1-octene in the
methanol/H
(Table 4, entry 9). Moreover, a similar reaction activity could be obtained using
TBA][NbO(OH) (GLY)] or [TBA][NbO(OH) (MA)] as a catalyst (Table 4, entries 8,
0 and 11). With respect to linear internal alkenes, trans-2-octene a¡orded
approximately 66.1% conversion (Table 4, entry 12). Cis-2-octene provided
2
O
2
system, an improved catalytic activity could be obtained in ethanol
[
2
2
1
2
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approximately 63.6% conversion (Table 4, entry 13). The configuration around the C=C
moiety was retained in the corresponding epoxide, which also suggested that the free-
radical intermediates are not involved in this system.
It was encouraging that propylene could also be oxidized and afforded the
corresponding epoxide very selectively under mild conditions. The yield of propylene
epoxide was approximately 28.2%, even under only 2.5 bar of propylene pressure
0
1
2
3
4
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6
7
8
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0
1
2
3
4
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0
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5
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2
3
4
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
(Table 4, entry 14). The results above indicate that the cyclic olefins and aromatic
olefins give higher reactivity when compared with that of the linear alkenes (Table 4,
entries 1-6 vs 7-14). In addition, the allylic alcohols were also tested accordingly. The
epoxidation of allylic alcohols was favorable under solvent-free conditions (Table 4,
entry 15 vs 16). Specifically, the epoxidation of chain allylic alcohols afforded the
epoxy alcohols without the formation of aldehydes and hydrolysis products (Table 4,
entries 15-19). The IL catalyst also displayed an excellent catalytic activity for the
epoxidation of aromatic allylic alcohols, and the main by-products were the
corresponding aldehydes, including benzaldehyde and cinnamyl aldehyde (Table 4,
entry 20). It was noticeable that the present IL catalyst, [TBA][NbO(OH)
2
(LA)],
showed a higher activity than that of [TBA][NbO(η-O )(OH)
2
2
]46 towards allylic
alcohols under the solvent-free conditions (Table 4, entries 16-20). Interestingly, the
epoxidation of geraniol was regioselective, as only 2,3-epoxygeraniol was obtained
(Table 4, entry 18). This could be due to the coordination of hydroxyl groups with the
Nb anion.46 The configuration around the C=C moiety was also retained in the
epoxidation of allylic alcohols (Table 4, entries 17, 18 and 20). However, the catalyst
showed a low reactivity for allyl methyl ether, likely due to the presence of an electron-
withdrawing group (Table 4, entry 21). Subsequently, the recyclability of the catalyst
has also been examined under optimal conditions. As shown in Figure S12, the IL
catalyst can maintain high catalytic activity for at least eight cycles, demonstrating that
the catalyst was highly stable under the reaction conditions, as confirmed by FT-IR of
the spent catalyst (Figure S13) The ICP-AES analysis indicated that trace Nb leaching
(1.1 ppm after recycle) occurred.
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(LA)] Catalysts[a]
Table 4. Epoxidation of Different Substrates with [TBA][NbO(OH)
2
Entry
substrate
T/°C
t/h
Con./%[b]
Sel./%
epoxide others
1
30
4
98.5
≥99
<1
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3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
3
4
30
30
30
4
4
4
95.1
99.3
74.9
97.3
≥99
91.3
2.7
<1
8.7
5
6
30
30
4
4
77.6
67.7
85.1
86.4
14.9
13.6
7
8
40
40
40
40
40
40
10
10
10
10
10
10
60.3
40.2
59.3
39.7
36.4
66.1
≥99
≥99
≥99
≥99
≥99
≥99
<1
<1
<1
<1
<1
<1
9[c]
1
0[d]
11[e]
12
13
40
40
0
10
10
63.6
28.2[f]
98.7
≥99
≥99
≥99
<1
<1
<1
1
4[f]
5
1
0.5
1
1
1
6[g]
7[g]
8[g]
0
0
0
0
0.15
1.5
1.5
6
99.4
90.1
95.2
93.7
≥99
≥99
≥99
≥99
<1
<1
<1
<1
OH
19[g]
OH
2
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0[g]
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94.5
15.7
39.4
60.6
<1
Ph
OH
21
40
10
≥99
O
[
a] Reaction conditions: substrate (3 mmol), H
catalyst (0.05 mmol), T=30 °C, 2 ml of CH OH; [b] GC conversion with n-butyl alcohol
as an internal standard; [c] The solvent is ethanol; [d] The catalyst was
2
O (3 mmol, 30% aqueous solution),
2
0
1
2
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0
1
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1
2
3
4
5
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8
9
0
3
[
TBA][NbO(OH)
2
(GLY)]; [e] The catalyst was [TBA][NbO(OH)
2
(MA)]; [f] refers to
yield, propylene: 2.5 bar; [g] Solvent-free conditions.
Mechanism of Olefin Epoxidation
-
To shed light on the origin of the improved activity of [Nb(O-O)
cyclooctene epoxidation relative to [Nb(O-O) (OOH)
2
LA] toward the
]-, the kinetics of the cyclooctene
2
2
epoxidation reaction were calculated and compared using first-principles simulations.
-
This revealed that [Nb(O-O)
2
LA] can attack the C=C bond using one O (denoted as
O
1
) from the peroxo (O-O) group (denoted as O
1
-O ) to form the cyclooctene epoxide,
2
which corresponds to a barrier of 0.85 eV (see blue in Figure 9), and the overall reaction
is thermodynamically favored with a very negative ûG. From the optimized transition
state (see TS1 in Figure 9), one can see that the distances of the two C-O
.381 Å and 2.264 Å, while the peroxo (O -O ) bond is elongated to 1.764 Å from the
original 1.475 Å, and the Nb-O bond is shortened to 1.884 Å.
In contrast, it can be seen that the epoxidation of cyclooctene in [Nb(O-
(OOH)
]- requires overcoming a higher barrier of 0.97 eV (see TS2, red line in
1
bonds are
2
1
2
2
O)
2
2
Figure 9) or 1.12 eV (see TS3, olive line in Figure 9), corresponding to using the -OOH
group or the peroxo (O-O) as the reactive centers, respectively. Hence, the -OOH
species is more reactive than the peroxo (O-O) in [Nb(O-O)
2
(OOH)
]- in the
2
epoxidation process (TS2 < TS3, see Figure 9), which is similar to our previous report
46
of the allylic alcohol epoxidation system . The optimized structures of the transition
states TS2 and TS3 are shown in Figure S14a and S14b. However, comparing TS1 on
-
(OOH)
]- in Figure 9, one can clearly see that
[Nb(O-O)
[Nb(O-O)
2
2
LA] with TS2 on [Nb(O-O)
2
2
-
LA] has a lower barrier to promote cyclooctene epoxidation than [Nb(O-
2
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O)
O)
2
2
(OOH)
2
]- by 0.11 eV. This therefore demonstrates that the peroxo (O-O) in [Nb(O-
-
LA] is more advantageous for the epoxidation of cyclooctene, which agrees with
the experimental result.
We also examined the solvent effect on the epoxidation reaction catalyzed by
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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0
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2
3
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0
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5
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8
9
0
1
2
3
4
5
6
7
8
9
0
-
[
Nb(O-O)
of a CH
eV in a vacuum) owing to the hydrogen bond effect between the CH
reactive peroxo (O-O) group (see TS4 in Figure 9). By comparison, the other protic
solvent CH CH OH shows a weaker hydrogen bond stability effect (0.11 eV versus -
.02 eV in CH OH, see Table S4), and thus is expected to exhibit a lower facilitation of
epoxidation; such a theoretical trend is in good agreement with our experimental results
Table S1). Details on the computational results are given in the Supplementary Note 2.
2
LA] . Taking the epoxidation of cyclooctene as an example, in the presence
3
OH molecule the epoxidation barrier can be decreased to 0.80 eV (versus 0.85
3
OH and the
3
2
0
3
(
Figure 9. The energy profile of the epoxidation of cyclooctene assisted by [Nb(O-
-
O)
2
LA] with and without the involvement of the CH
3
OH solvent molecule (the black
and blue lines, respectively) and [Nb(O-O)
2
(OOH)
2
]- (the olive and red lines,
respectively). TS1 and TS4 correspond to the transition state configurations of
-
cyclooctene epoxidation in [Nb(O-O)
2
LA] without and with the CH
3
OH molecule
involved as a H-transfer bridge, respectively.
2
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-
To realize more clearly the different activities of [Nb(O-O) LA] and [Nb(O-
2
O)
properties of two key active centers, i.e., the peroxo (O-O) in [Nb(O-O)
OOH species in [Nb(O-O) (OOH)
]-, were investigated. First, from the optimized
2
(OOH)
2
]- for the epoxidation of cyclooctene, the basic geometric and electronic
-
2
LA] and the
2
2
0
1
2
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0
structure of these two anions, one can find that the bond length of the peroxo (O-O) in
-
(OOH)
]- (1.478 Å vs 1.466
[
Nb(O-O)
2
LA] is longer than the O-OH bond in [Nb(O-O)
2
2
-
Å), and the peroxo (O-O) bond in [Nb(O-O)
2
LA] is expected to be relatively weaker,
implying its higher reactivity to attack the C=C bond of cyclooctene and accomplish
the epoxidation process. Second, the Mulliken charge analysis revealed that the Nb
-
center in [Nb(O-O)
2
LA] carries a more positive charge (+2.79 |e|) relative to that in
[
Nb(O-O)
2
(OOH)
]- (+2.47 |e|), while the O atom in the peroxo (O-O) in [Nb(O-
2
-
O)
O)
2
2
LA] shows a less negative charge compared with that in the OOH species of [Nb(O-
]-, being -0.58 |e| versus -0.67 |e|. This implies that in [Nb(O-O)
-
(OOH)
2
2
LA] the
organic ligand (LA) serves as an electron-withdrawing group to decrease the election
density of the Nb anion. Owing to the electrophilic addition mechanism of the
epoxidation process, the lower electron density of the reactive center (O atom) is more
6
8
likely to couple with the C=C bond. Moreover, the LUMO energy levels of these two
Nb anions and the HOMO energy level of cyclooctene are calculated (see Figure 10).
As shown in Figure 10, HOMO electrons in cyclooctene mainly distribute around the
-
C=C bond at -0.24 eV, and [Nb(O-O)
2
LA] gives a relatively lower LUMO compared
with [Nb(O-O)
2
(OOH)
2
]-, which leads to a smaller gap for the electron transfer from
-
the HOMO of cyclooctene to the LUMO of [Nb(O-O)
2
LA] . According to the frontier
-
molecular orbital theory, it can be rationalized that [Nb(O-O)
attack the C=C bond to complete the epoxidation process.
2
LA] is more active to
2
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0
LA]-
Figure 10. The calculated HOMO of cyclooctene and the LUMO of [Nb(O-O)
2
and [Nb(O-O)
2
(OOH)
2
]-, as well as their relative energy levels.
-
-
Notably, the anion [NbO(OH)
2
R] can be oxidized easily into [Nb(O-O)
2
LA] in the
present of excess of H
O
2 2
, even under very mild conditions (Scheme 2), while the
-
formation of Nb[(O-O)(OOH)
2
LA] was highly unfavorable (Table 2). As shown in
Figure 7, H
O
2 2
was decomposed with a high barrier of 1.58 eV over the [Nb(O-O) LA]-
2
-
based IL catalyst. However, the peroxo group (O-O) in [Nb(O-O)
2
LA] attacking the
C=C bond has a barrier of only 0.85 eV (Figure 9). As a result, the formation of TS1
would be more thermodynamically favorable in the presence of olefins (Figure 9)
compared with the decomposition pathway of H
2
2
O (Figure 7). This could be the reason
why excellent catalytic performance can be achieved in this catalytic system.
On the basis of the above epoxidation reaction results, the catalyst characterization
and the DFT calculations, the proposed mechanism for the epoxidation of olefins
catalyzed by organic acid-coordinated peroxoniobate-based ionic liquids is shown in
-
Scheme 3. In detail, the original anion [NbO(OH)
2
R] was first oxidized to [Nb(O-
-
O)
2
R] by hydrogen peroxide. The olefin then associated with the peroxo group (O-O)
to afford the epoxide through formation of a three-membered ring transition state. In
this process, the hydrogen bond between CH OH and the peroxo activated the peroxo
bond more, resulting in a lower barrier for cyclooctene epoxidation. With the desorption
3
2
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-
of the epoxide, [NbO(O-O)R] was produced and was then regenerated into the
oxidizing species with hydrogen peroxide, which completed the catalytic cycle. The
quaternary ammonium cation of the ILs plays an indispensable role in
balancing/stabilizing the negatively charged peroxoniobate anion, allowing the catalyst
to dissolve in organic solvents more easily. The existence of cation part also made the
charge of the peroxo ligand less negative, and this was favorable to the epoxidation
reaction to some extent. The details of the computational results are presented in
Supplementary Note 3 and Figure S19. As a kind of ionic liquid, the presence of the
“nano-inhomogeneity” induced by the ionic aggregates in the ionic liquid could allow
the substrate molecules to reside in less polar regions and enable the ionic species to
0
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undergo faster diffusion, which enhances the reaction rate.
Scheme 3. Proposed mechanism for the epoxidation of cyclooctene with H
2
O
2
catalyzed by carboxylic acid-coordinated mononuclear peroxoniobate-based ionic
liquids.
Conclusion
In summary, we have demonstrated that new carboxylic acid-coordinated mononuclear
peroxoniobate-based ionic liquids can be utilized as highly efficient and recyclable
catalysts for the epoxidation of both various olefins and allylic alcohols with hydrogen
peroxide as an oxidant under mild conditions. The structure of the functionalized ILs
was fully characterized, and the structural evolution was investigated in detail
2
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throughout the reaction. It was found that the parent IL [TBA][NbO(η-O
2
)(OH) ] can
2
-
coordinate with α-hydroxy acid to afford new ILs, such as [NbO(OH)
2
(R)] (R=LA,
GLY, MA) with five-membered chelate rings. The new ILs underwent further structural
-
-
transformation from [NbO(OH)
2
(R)] to [Nb(O-O)
2
(R)] in the presence of H
2
2
O , which
1
1
1
1
1
1
1
1
1
1
2
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2
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was proven by catalyst characterization and first-principles calculations. [Nb(O-
Þ
O)
2
(R)] serves as a highly active species to oxidize the C=C double bond, being more
]-, which was
advantageous for the epoxidation of olefins than [Nb(O-O)
2
(OOH)
2
identified at the atomic level by DFT calculations. The strong protic solvent can
accelerate the epoxidation reaction via the hydrogen bonding between the solvent and
peroxo ligand. It was demonstrated that the catalytically active anion [Nb(O-O)
can suppress the decomposition of H , compared with [NbO(η-O
2
(R)]-
-
2
O
2
2
)(OH)
2
] .
According to the reaction mechanism, there is no direct coordination interaction
between the olefins/allylic alcohol and the Nb site, and the epoxidation reaction occurs
by O-O active species oxidizing the C=C bond, where the peroxide species around the
Nb site can be converted reversibly. This shows that the structure of the catalytically
active anions is stable in principle and can be easily regenerated. This work is the first
example of peroxoniobate anions owning excellent catalytic activity for the epoxidation
of olefins. Undoubtedly, the design and construction of mononuclear peroxometalate-
based ILs can open new avenues for achieving high redox-active and stable catalysts.
Experimental Section
Materials
Tetrabutylammonium chloride (TBACl), potassium hydroxide (KOH) and
niobium pentaoxide (Nb
2
5
O ) were obtained from Sinopharm Chemical Reagent Co.,
Ltd. (Shanghai, People’s Republic of China). L(-)-malic acid and glycolic acid were
obtained from Shanghai Macklin Biochemical Co., Ltd. L(+)-Lactic acid was obtained
from Meryer Chemical Technology Co., Ltd. (Shanghai, People’s Republic of China).
Catalyst Characterization
FT-IR spectra were obtained using a Nicolet Magna 550 FT-IR spectrometer.
2
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