Ionic Liquid Stabilized Niobium Oxoclusters Catalyzing Oxidation of Sulfides with Exceptional Activity

Article abstract of DOI:10.1002/chem.201806178

We present here a new class of niobium oxoclusters that are stabilized effectively by carboxylate ionic liquids. These functionalized ILs are designated as [TBA][LA], [TBA][PA], and [TBA][HPA] in this work, in which TBA represents tetrabutylammonium and LA, PA, and HPA refer to lactate, propionate, 3-hydroxypropionate anions, respectively. The as-synthesized Nb oxoclusters have been characterized by use of elemental analysis, NMR, IR, XRD, TGA, HRTEM. It was found that [TBA][LA]-stabilized Nb oxoclusters (Nb?OC@[TBA][LA]) are uniformly dispersed with an average particle size of 2–3 nm and afforded exceptionally high catalytic activity for the selective oxidation of various thioethers. The turnover number with Nb?OC@[TBA][LA] catalyst was over 56 000 at catalyst loading as low as 0.0033 mol % (1 ppm). Meantime, the catalyst also showed the high activity for the epoxidation of olefins and allylic alcohols by using only 0.065 mol % of catalyst (50 ppm). The characterization of ⁹³Nb NMR spectra revealed that the Nb oxoclusters underwent structural transformation in the presence of H₂O₂ but regenerated to their initial state at the end of the reaction. In particular, the highly dispersed Nb oxoclusters can absorb a large amount of polar organic solvents and thus were swollen greatly, which exhibited “pseudo” liquid phase behavior, and enabled the substrate molecules to be highly accessible to the catalytic center of Nb oxocluster units.

Full text of DOI:10.1002/chem.201806178

A Journal of
Accepted Article
Title: Ionic Liquid-Stabilized Niobium Oxoclusters Catalyzing Oxidation
of Sulfides with Exceptional Activity
Authors: Zhenshan Hou, Qingqing Zhou, Man Ye, Wenbao Ma, Difan
Li, Bingjie Ding, Manyu Chen, Xueqing Gong, and Yefeng
Yao
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Table of Contents
A new class of niobium (Nb) oxocluster has been developed through coordination
stabilization with carboxylate ionic liquids (ILs), which is uniformly dispersed with an
average particle size of 2−3 nm and afforded high catalytic activity for the selective
oxidation of thioethers and olefins, respectively with ultra-low catalyst loading. The
characterization revealed that the Nb oxoclusters underwent structural transformation
in the reaction but regenerated their initial state at the end of reaction. Especially, the
highly dispersed Nb oxoclusters can absorb a large amount of polar organic solvents
and were swollen greatly, which thus exhibited “pseudo” liquid phase behavior, and
enabled the substrate molecules to be highly accessible to the catalytic center of Nb
oxocluster units.
1
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Ionic Liquid-Stabilized Niobium Oxoclusters Catalyzing Oxidation of
Sulfides with Exceptional Activity
Qingqing Zhoua, Man Yeb, Wenbao Maa, Difan Lia, Bingjie Dinga, Manyu Chena, Yefeng Yaob,*,
Xueqing Gonga and Zhenshan Houa *
a Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China
University of Science and Technology, Shanghai 200237, China
b Physics Department and Shanghai Key Laboratory of Magnetic Resonance, East China Normal
University, Shanghai 200062, China.
* Corresponding author. E-mail address: houzhenshan@ecust.edu.cn (Z. Hou).
Abstract: We present here a new class of niobium oxoclusters, which are stabilized
effectively by carboxylate ionic liquids. These functionalized ILs are designated as
[TBA][LA], [TBA][PA] and [TBA][HPA] in this work, where TBA represents
tetrabutylammonium and LA, PA and HPA refer to lactate, propionate, 3-
hydroxypropionate anions, respectively. The as-synthesized Nb oxoclusters have been
characterized by use of elemental analysis, NMR, IR, XRD, TGA, HRTEM. It was
found that [TBA][LA]-stabilized Nb oxoclusters (Nb-OC@[TBA][LA]) are uniformly
dispersed with an average particle size of 2−3 nm and afforded exceptionally high
catalytic activity for the selective oxidation of various thioethers. The turnover number
with Nb-OC@[TBA][LA] catalyst was over 56000 at catalyst loading as low as 0.0033
mol% (1 ppm). Meantime, the catalyst also showed the high activity for the epoxidation
of olefins and allylic alcohols by using only 0.065 mol% of catalyst (50 ppm). The
characterization of 93Nb NMR spectra revealed that the Nb oxoclusters underwent
structural transformation in the presence of H2O2 but regenerated their initial state at
the end of reaction. Especially, the highly dispersed Nb oxoclusters can absorb a large
amount of polar organic solvents and thus were swollen greatly, which exhibited
“pseudo” liquid phase behavior, and enabled the substrate molecules to be highly
accessible to the catalytic center of Nb oxocluster units.
Keywords: Peroxoniobium species; Oxocluster; Oxidation; Sulfide; Hydrogen
peroxide
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Introduction
The peroxo compounds of transition metals have attracted great attention in the
last decade because their variable coordination chemistry has helped a lot to develop
new methods aiming at incorporating these elements in specific materials, or to improve
the existing ones. Especially, peroxo species of d0 transition metals (e.g., V(V), Ti(IV),
Mo(VI), W(VI), Nb(V) and Ta(V)) are potential oxygen donors to organic substrates in
the liquid phase oxidation, and thus have attracted considerable attention recently.[1]
These compounds can release oxygen in active form, either chemically or on irradiation,
and play a key role in oxidations of different inorganic and organic substrates like
sulfides, sulfur dioxides, olefins, alcohols, aromatic and aliphatic hydrocarbons and
phosphines.[2] These processes, either stoichiometric or catalytic, use hydrogen
peroxide or alkyl hydroperoxides as oxidant, are usually carried out under mild
conditions and are associated to good yield and selectivity. This “peroxo way” has been
shown to be a valuable method for the synthesis of mixed oxides containing niobium
and/or tantalum from well defined and pre-isolated complexes.[1e] Recently, our group
has developed several kinds of monomeric peroxoniobate anion functionalized ILs by
this manner, which is the first example of the highly efficient epoxidation of allylic
alcohols using a peroxoniobate anion as a catalyst.[3]
Organic-inorganic nanostructured materials are a wide, manifold and exciting
class of systems which derives from an intimate combination, often mediated by the
formation of a chemical bond, of inorganic and organic building blocks.[4] Among
inorganic nanosized building blocks, oxide-based clusters (e.g., polyhedral
silsesquioxanes (POSS),[5] metal oxoclusters,[6] polyoxometalates (POM),[7]) are
attracting more and more attention. The wide range of appealing properties of oxide-
based clusters results from the chemical variability of both constituent metals and
structure, as well as from the different functionalization strategies aimed at developing
hybrid derivatives. Particularly, metal oxoclusters and polyoxometalates containing
early transition metals in their highest oxidation state display attractively catalytic
properties.[8] Polyoxoniobates (PONbs) as a burgeoning subclass, have attracted
increasing interest because of their unique characteristics (high charge, oxygen-rich
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surface, rich redox properties, nanoscale size) and potential applications in nuclear-
waste treatment and virology.[9] A number of niobium-based polyoxometalate clusters
hybrid systems have been so far synthesized and characterized in the last decade, while
their catalytic performance is rarely reported.[10] Recently, a novel double-anion
complex has been reported, H13[(CH3)4N]12−[PNb12O40(VVO)2·(VIV O12)2]·22H2O,
4
based on bicapped polyoxoniobate and tetranuclear polyoxovanadate for the efficient
catalytic selective oxidation of the sulfur mustard simulant in the presence of hydrogen
peroxide.[11] However, two different polyoxometalate anions structure complicated the
catalytic oxidation process. Especially, to our knowledge, no sole polyoxoniobate has
been reported for selective oxidation of both sulfides and olefins.
Ionic liquids (ILs) are known to have the advantages of small volatility, high
solubility, good thermal stability and diverse structure. Accordingly, ILs are very
attractive media in the synthesis of zero- and one-dimensional nanostructures. ILs can
act as solvents, cosolvents, promoters, reducing agents, stabilizing agents and
surfactants. Especially, the ILs have been regarded as a promising solvent candidate for
the stabilization of soluble metal nanoparticles and carbon nanospheres without the
need for additional organic ligands.[12] Nevertheless, the use of ILs to achieve the
nanosized transition-metal oxides/oxoclusters remains a huge challenge.
In our previous work, we have confirmed that the successful coordination between
the peroxoniobate and organic carboxylic acid, leading to forming mononuclear
peroxoniobate anion.[13] Herein we report the use of organic carboxylate IL as a
stabilizing agent to generate the nanosized Nb oxoclusters, where the formed
polyoxoniobate anions act both as oxygen transfer catalysts and structural building
blocks. In this work, we initially attempted to synthesize peroxo complexes of niobium
by using the carboxylate IL as a ligand. Unexpectedly, it was found that the precusor
peroxy-niobic acid was transformed into the nanosized oxoclusters during the course
of preparation. The as-synthesized nanosized Nb oxocluster featuring an enhanced
dispersion, high swelling ability and excellent stability endowed by the organic
carboxylate ILs, has been applied as an independent confined nanoreactor for the
selective oxidation of organic sulfides to the corresponding sulfoxides or sulfones and
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olefins into epoxides with hydrogen peroxide, respectively. Particularly, the Nb
oxocluster exhibited exceptionally high catalytic activity in sulfoxidation with turnover
numbers (TONs) more than 56000.
Results and Discussion
Preparation and Characterization of niobium oxoclusters
A series of biological carboxylate ILs including tetrabutylammonium lactate
([TBA][LA]), tetrabutylammonium propionate ([TBA][PA]) and tetrabutylammonium
3-hydroxypropionate ([TBA][HPA]) have been prepared, which presented as colorless
and transparent liquid. These compounds showed general characteristics of ILs such as
being a viscous liquid at room temperature, immiscibility with weakly polar solvents
(alkanes, toluene, diethyl ether, etc.) and nonvolatility. For their synthesis, TBAOH was
firstly obtained from TBACl and KOH by ionic exchange. Then, a simple neutralization
by adding equal equivalent different biological organic acids resulted in these ILs. Next,
the prepared peroxy-niobic acid solution was added to the as-synthesized ILs to obtain
these oxoclusters. The oxoclusters with different ILs were labeled as Nb-
OC@[TBA][LA], Nb-OC@[TBA][PA] and Nb-OC@[TBA][HPA], respectively. Then
their solubility and dispersion were summarized in Table S1 and the photographs of a
methanol dispersion were displayed in Figure S1. It can be seen that only Nb-
OC@[TBA][LA] was soluble in water and well dispersed in methanol. In contrast, the
other two oxoclusters were not soluble in any used solvents and displayed very poor
dispersion in methanol. Therefore, the present research focused mainly on the structure,
properties and catalytic oxidation performance of the lactate IL-stabilized Nb
oxocluster Nb-OC@[TBA][LA].
Notably, the properties of Nb-OC@[TBA][LA] are distinctly different from those
reported mononuclear peroxyniobate anion based ILs [TBA][Nb=O(O-O)(OH)2] and
[TBA][NbO(OH)2(LA)].[3,13] As shown in Figure S2, the latter two ILs were miscible
completely with methanol, and there was no obvious change after adding an equivalent
amount of H2O2. Nevertheless, the oxocluster Nb-OC@[TBA][LA] was uniformly
dispersed but not dissolved in either methanol or H2O2 solution in methanol, which
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revealed that the oxocluster Nb-OC@[TBA][LA] was greatly different from
mononuclear peroxyniobate species, and it did not degrade into mononuclear Nb
complexes even if addition of H2O2. This illustrated that the Nb oxocluster owned high
chemical stability through the stabilization of the carboxylate IL.
The obvious distinction of solubility and dispersion among three Nb oxoclusters
was most likely due to the coordination interaction of carboxylate anion with niobium
site. Based on our previous research, only one carboxylate group and the vicinal alkoxy
group in organic acid molecule can participate in forming stable Nb complexes. The -
hydroxyl carboxylate anion (e.g. lactate anion) acted as bidentate ligands, coordinating
with the niobium site results in a five-membered chelate ring.[13, 14] During the synthesis
of Nb oxocluster, the precursor peroxy-niobic acid was transformed into
polyoxoniobate anion due to condensation under slightly high temperature. In this case,
the IL actually played a crucial role in stabilizing the uniformly dispersed Nb
oxoclusters. The composition of the as-synthesized materials were analyzed by 13C
NMR, 93Nb NMR, elemental analysis, inductively coupled plasma atomic emission
spectroscopy (ICP-AES), and potential difference titration of Ce3+/Ce4+. The number of
peroxy bonds ranged from 0.13 to 0.15 per Nb atom in the three Nb oxoclusters by
potential difference titration of Ce3+/Ce4+, suggesting that there still remained a trace of
peroxy bond in the Nb oxoclusters even after treatment under high vacuum at the
elevated temperature. In the light of these results, it suggested rationally that the anions
(lactate) form a layer around the surface of oxocluster, leading to a center of negatively
charge, and thus the cations of IL (TBA) existed at the outer layer for charge balance
(Scheme 1). Recently, some other groups and ours have demonstrated that the ILs
possess pre-organized structures, which can create an external layer around the
nanostructure to avoid aggregation and/or agglomeration through both steric and
electronic protection.[12d-h]
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Scheme 1. The IL [TBA][LA]-stabilized Nb oxocluster.
Next, these carboxylate ILs and oxoclusters were characterized by using FT-IR
spectroscopy, respectively. On the one hand, the FT-IR spectra of three carboxylate ILs
were displayed in Figure 1 (left). The bands at 2869 and 2950 cm−1 were attributed to
ν(C−H) and those at 1300−1500 cm−1 were attributed to δ(C−N), indicating the
presence of the tetrabutyl ammonium cation. The bands at 650−1000 cm−1 and the band
around 1100 cm−1 of these ILs were attributed to the vibrations of σ(C−H) and C−O,
respectively.[15] The carboxylate vibrational peaks located at 1567 cm−1, 1573 cm−1,
1606 cm−1 belonged to propionate, 3-hydroxy propionate and lactate, respectively.[16]
Moreover, a peak at 3420 cm−1 was assigned to ν(O−H) and a typical band at 1650 cm−1
was related to the adsorbed water molecules in the ILs.[17] On the other hand, the FT-IR
spectra of the niobic acid and IL-stabilized oxoclusters were shown in Figure 1 (right).
It was seen that the as-obtained niobic acid displayed a broad band centered at 610 cm−1,
and a shoulder at 896 cm−1, which can be attributed to amorphous structure.[18] For the
IL-stabilized oxoclusters, the vibrational peaks of the carboxylic groups were red-
shifted to 1562 cm−1, 1571 cm−1 and 1601 cm−1,[19] respectively, which was different
from those of the carboxylate group in the sole ILs due to the coordination with niobium
site. In addition, the vibrational peak belonging to ν(Nb=O) at approximately 885 cm−1
might be overlapped by σ(C−H) in the ILs. The bands near 545 cm−1 were characteristic
for νas(Nb−O) of the catalysts.[13] The vibrational peak at 841 cm−1 might be assigned
to be stretching of the Nb−O−Nb bridge,[20] indicating that the polyoxoniobate anion
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existed as a multinuclear instead of a mononuclear form. Furthermore, the weak bands
at 570 cm−1 that arose from (O−O) species were still observed in the oxoclusters,
revealing the decomposition of labile peroxy bonds (O−O) ligands during the course of
the oxocluster preparation,[21] which was well in agreement with that of the potential
difference titration analysis.
Figure 1. FT-IR spectra of carboxylate ILs (left) and IL-stabilized Nb oxoclusters and
niobic acid (right).
Then, Nb-OC@[TBA][LA] was subjected to NMR characterization, which can
offer the information on the chemical environment changes of the carbon nuclear and
niobium site by using 13C NMR and 93Nb NMR spectra, respectively. On the one hand,
as shown in Figure 2a, it can be seen that the signals at 181.87 ppm and 68.20 ppm can
be attributed to carbon atom of the carboxyl group and -carbon atom in the lactate
IL,[22] respectively. After coordination with niobium site, these signals shifted slightly
to downfield and appeared at 182.39 ppm and 68.46 ppm, respectively. This confirmed
that the alcoholic group attached to -carbon atom could be deprotonated, leading to
formation of five-membered rings composed of metal atom and lactic acid
molecules[13,14c]. Meanwhile, the chemical shifts of other carbon atoms had not changed,
which indicated -hydroxy carboxylate indeed has a stable coordination effect on
niobium site. On the other hand, 93Nb NMR spectra also gave clear evidences. The
peroxy-niobic acid solution in D2O displayed only one resonance signal at −1312 ppm
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(Figure 2b, upper), attributing to peroxoniobate anion [Nb(O-O)2(OOH)2]− that was
survived under acidic condition according to our previous work.[3] However, upon
reaction with [TBA][LA], the signal moved to a lower field and evolved into three
different intensity signals appeared at −849 ppm, −1108 ppm and −1036 ppm (Figure
2b, bottom), indicating that there were three types of niobium sites in the Nb-
OC@[TBA][LA]. This may be due to the decrease of the electron density of niobium
nuclear when the peroxoniobate anion was coordinated by lactate as electron
withdrawing ligand. According to our previous research, the three signals could be very
similar to these of the [NbO(OH)2(LA)]−, [Nb(O-O)2(LA)]− and [Nb=O(O-O)(OH)2]−
anions, respectively,[3,13] which might correspond to outward reduced niobium site with
lactate coordination, the corresponding outward oxidized niobium site and niobium site
without lactate coordination, respectively. These results were highly consistent with the
characterizations of FT-IR and 13C NMR, which indicated that the -hydroxy
carboxylate served as a bidentate ligand and thereby stabilized the niobium site to form
highly dispersed Nb oxoclusters.
Figure 2. (a) 13C NMR spectra of before (upper) and after IL [TBA][LA] coordinated
with niobium site (bottom), respectively; (b) 93Nb NMR spectra of peroxy-niobic acid
(upper) and Nb-OC@[TBA][LA] (bottom) in D2O. The preparation of peroxy-niobic
acid was given in the experimental section.
In the next step, niobium oxides and IL-stabilized Nb oxoclusters were
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characterized by using XRD. In contrast to the commercial crystalline Nb2O5 (Figure
S3a),[23] the as-synthesized niobic acid was amorphous (Figure S3b). In addition,
although IL-stabilized Nb oxoclusters (Nb-OC@[TBA][PA], Nb-OC@[TBA][HPA],
Nb-OC@[TBA][LA]) also showed an amorphous structure (Figure S3c-e), the
diffraction peak of the Nb oxoclusters moved obviously to a smaller angle, indicating
that the introduction of IL exerted an impact on structure of Nb oxoclusters, as
compared with that of as-synthesized niobic acid.
Sequentially, the TGA curves of the three Nb oxoclusters were shown in Figure S4.
Obvious weight losses of approximately 85.2%, 88.4% and 88.3% were observed
between 50 and 800 °C for the catalysts, which were in well agreement with the results
of ICP-AES analysis. The weight loss could be assigned to the decomposition of the
tetrabutyl ammonium cation part and the moiety of the organic acid anion. The
complete decomposition temperatures of the three catalysts are 350, 380, 330 °C
respectively, which are slightly higher than those of the sole ILs,[24] implying that a
coordination interaction between the Nb oxocluster and ILs could improve the thermal
stability of ILs.
The particle distribution of the oxoclusters was evaluated by HRTEM. As shown
in Figure 3a and b, the as-prepared niobic acid showed an aggregate of irregular
particles. Meanwhile, the ILs like [TBA][PA] and [TBA][HPA]-stabilized Nb
oxoclusters have been prepared for the sake of comparison. HRTEM images exhibited
that the Nb-OC@[TBA][PA] oxocluster aggregated slightly with an average diameter
of 3.7 nm (Figure 3c and d), while Nb-OC@[TBA][HPA] oxocluster even agglomerated
more obviously with the average particle size of 3.4 nm (Figure 3e and f). This reflected
that the propionate or 3-hydroxypropionate IL could only generate labile coordination
with niobium site, leading to weak stabilization of oxoclusters, which was in agreement
with our previous research.[13] Interestingly, the IL [TBA][LA]-stabilized oxocluster
Nb-OC@[TBA][LA] existed as highly uniform nanoparticles with the average particle
size of 2.6 nm (Figure 3g and h), which is distinctly different from that of niobic acid.
This demonstrated clearly that -hydroxy carboxylate anion (lactate anion) can form a
stable coordination bond with niobium site, which enables the polyoxoniobates derived
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from the condensation of peroxy-niobic acid to be highly stable and uniformly
dispersed.
Figure 3. HRTEM images of niobic acid (a,b); Nb-OC@[TBA][PA] (c,d); Nb-
OC@[TBA][HPA] (e,f) and Nb-OC@[TBA][LA] (g,h) with different magnification.
Catalytic Activity
In the light of the characterization above, the catalysts have been screened for the
oxidation of thioanisole with H2O2 as an oxidant in methanol. As shown in Figure 4, it
indicated that niobic acid gave very poor activity, while Nb-OC@[TBA][HPA] and Nb-
OC@[TBA][PA] showed a much higher activity than that of niobic acid. Meanwhile,
both of mononuclear [TBA][Nb=O(O-O)(OH)2] and [TBA][NbO(OH)2(LA)] ILs
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showed a similar catalytic activity to these two Nb oxocluster. Notably, Nb-
OC@[TBA][LA] catalyst afforded the exceptional activity, and gave almost full
conversion within 30 min while the other Nb oxocluster or IL catalysts only afford
around 40%−66% conversion under the same conditions. The excellent dispersion of
Nb-OC@[TBA][LA] catalyst in methanol might be one of the reasons that account for
exceptionally high oxidation activity. The other IL-stabilized oxoclusters, as well as
niobic acid exhibited a poor dispersion or have sunk to the bottom of flask during the
course of the oxidation of thioanisole in methanol media.
Figure 4. Conversion/time profiles of thioanisole oxidation reactions over different
catalysts. (■) Nb-OC@[TBA][LA]; ( ◆ ) Nb-OC@[TBA][HPA]; (◄) Nb-
OC@[TBA][PA]; (▼) [TBA][Nb=O(O-O)(OH)2]; (▲) [TBA][NbO(OH)2(LA)]; (●)
niobic acid. Reaction conditions: thioanisole (2.0 mmol), H2O2 (2.2 mmol, 30%
aqueous solution), catalyst (0.027 mol%), CH3OH (1.0 mL), 0 °C. The selectivity
toward phenyl methyl sulfoxide was always more than 95%.
With regard to the excellent catalytic performance of oxocluster Nb-
OC@[TBA][LA], which was optimized for the oxidation of thioanisole with different
molar ratio of IL [TBA][LA] to Nb atoms. As showed in Figure S5, it can be seen that
the molar ratio of IL to Nb atoms indeed exerted a significant effect on the catalytic
activity. Notably, the oxidation reaction can even complete within 30 min in the
presence of Nb catalyst (ca. 30 ppm) under very mild conditions (0 °C) in methanol
when the molar ratio of IL to Nb was 2. The molar ratio deviated from 2 seemed to be
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detrimental to the thioanisole oxidation (Figure S5). This can be explained that a small
amount of IL was not able to stabilize the oxocluster very well, while the coverage of
excessive ILs around the metal oxocluster probably prevented the substrates from
accessibility to active niobium site, resulting in poor activity.
The reaction solvents have an important effect on the oxidation of thioethers. As
shown in Table S2, a poor conversion was obtained in dichloromethane, toluene and
H2O (Table S2, entries 1−3), which could result from biphasic state under reaction
condition. Furthermore, the moderate activity was achieved (55% conversion) if
CH3CN was used as solvent (Table S2, entry 4), which was probably due to its polarity.
Especially, the conversion of thioanisole increased up to 67.5% and 98.7% in ethanol
and methanol, respectively (Table S2, entries 5 and 6). The results showed that this
sulfoxidation reaction was favored in strongly polar protic solvents (e.g., in methanol),
which was consistent with previous report that protic solvents probably formed a
hydrogen bond with the peroxo ligand attached to the metal center, which can lower the
25]
reaction barriers.[13,
Overall, methanol was an appropriate solvent for the
sulfoxidation reaction.
Based on the results above, it can be found that the catalyst Nb-OC@[TBA][LA]
exhibited extremely high catalytic performance on oxidation of sulfides. In contrast, the
oxidation reaction of thioanisole can hardly happen within 1 min and even only offered
only 15% conversion after 120 min in the absence of catalyst (Table 1, entry 1). This
revealed the reaction occurred difficultly without catalyst. When the molar loading of
the catalyst was 1.0 mol%, the oxidation reaction was almost completed rapidly within
15 seconds over the Nb-OC@[TBA][LA] catalyst (Table 1, entry 2). Obviously, while
the Nb-OC@[TBA][LA] catalyst was loaded as low as 0.1 mol%, the reaction can still
complete within 2 min and the catalyst actually showed the highest activity among the
IL-stabilized oxoclusters catalysts (Table 1, entries 3−5). Furthermore, it demonstrated
that the present Nb oxocluster catalysts displayed outstanding catalytic activity even at
a loading as low as 0.027 mol% (Table 1, entries 6−8). Especially, the Nb-
OC@[TBA][LA] catalyst afforded a satisfactory conversion with 3656 TON (Table 1,
entry 8). The result is also higher than that of our previous reported mononuclear
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peroxyniobate anion based ILs [TBA][Nb=O(O-O)(OH)2] and [TBA][NbO(OH)2(LA)]
(Table 1, entries 9 and 10). Afurther reduced catalyst loading of 0.016 mol% and 0.0065
mol% was sufficient to achieve a satisfactory conversion of 86.8%, 95.9% and TONs
of 5425, 14754, respectively (Table 1, entries 11 and 12). It is also worth noting that the
reaction is readily scalable. Unexpectedly, upon increasing the substrate scale from
milligram to gram scale, the catalytic performance of Nb-OC@[TBA][LA] even
increased obviously. For example, 4.5 mg of Nb-OC@[TBA][LA] was dissolved in 10
mL of methanol, and then 1 mL of the solution (0.0033 mol%, 1.0 ppm in whole
reaction system) was taken out and stirred with 2.484 g of thioanisole for 150 min at
0C. It was found that thioanisole was converted into its sulfoxide with a high
conversion of 93.2% and 96.2% of selectivity within 150 min with TONs up to 56485
(Table 1, entry 13). The present TONs is higher than those reported niobium-catalysts
for thioethers oxidation.[26] Notably, after the reaction, the Nb-OC@[TBA][LA]
catalyst can be removed by silica-gel column chromatography with n-hexane and ethyl
acetate(2:1, V/V) as an effluent. ICP-AES analysis proved that almost no residual
catalyst was found in the product (<5 ppb).
Table 1. The catalytic activity at ultra low-loading of Nb-based catalysts.[a]
Con.
[%]
Sel.
Entry
1[e]
Cat.
mol%[b] TON[c] t[min]
[%][d]
none
none
1.0
-
1(120) 0(15.0) 0(92.5)
2
3
Nb-OC@[TBA][LA]
Nb-OC@[TBA][PA]
Nb-OC@[TBA][HPA]
Nb-OC@[TBA][LA]
Nb-OC@[TBA][PA]
Nb-OC@[TBA][HPA]
Nb-OC@[TBA][LA]
[TBA][Nb=O(O-O)(OH)2]
[TBA][NbO(OH)2(LA)]
100
0.2
2
99.8
69.7
78.5
99.8
62.9
71.9
98.7
64.6
65.5
95.6
90.3
97.1
95.4
95.4
95.8
96.3
98.2
97.0
0.1
444
4
0.1
623
2
5
0.1
865
2
6
0.027
0.027
0.027
0.027
0.027
2330
2663
3656
2426
2404
40
40
40
40
40
7
8
9
10
14
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11
12
13
Nb-OC@[TBA][LA]
0.016
0.0065
0.0033
5425
14754
56485
60
86.8
95.9
93.2
95.9
96.5
96.2
Nb-OC@[TBA][LA]
Nb-OC@[TBA][LA]
120
150
[a] Reaction conditions: thioanisole (2.0 mmol), H2O2 (2.2 mmol, 30% aqueous
solution), CH3OH (1.0 mL), 0 °C. [b] Catalyst loadings were given in mol.%. [c]
Calculated turnover numbers. [d] The selectivity towards sulfoxide. [e] The values in
the parenthesis refer to the conversion and selectivity of thioanisole after reaction 120
min.
Kinetic Studies
The reaction kinetics catalyzed by Nb-OC@[TBA][LA] was then investigated in
detail for the oxidation of thioanisole. The relationship between the reaction rate (R0)
and the concentrations of the substrate and H2O2 were shown in Figure S5. All reaction
rates (R0) for the kinetic studies were determined at conversion lower than 15%. The
concentration of niobium catalyst was kept constant (ca. 30 ppm) in the kinetics study.
The study showed reasonable first-order plot for the concentration of substrate (1.5−3.0
M) (Figure S6a and b) and also first-order dependence on the concentration of H2O2
(0.5−2.0 M) (Figure S6c and d). Therefore, the reaction rate can be calculated by using
eq (1) at a constant concentration of niobium catalyst, where R0 and k represent the
reaction rate and rate constant, respectively.
R0 = −[d(thioether)/dt] = k[thioether]1[H2O2]1
(1)
The results meant that the rate-limiting transition state could not only be involved
in the conversion of thioether, but also in the activation progress of H2O2, which is
consistent with that of sulfoxidation process reported previously.[26c, 27] Additionally,
the dependence of the reaction rate of thioanisole on the temperature (Arrhenius plots,
278−298 K) was shown in Figure 5. The linearity of the Arrhenius plots was examined
to determine the activation energy, which was Ea = 36.1 kJ mol−1. The activation energy
under the present conditions was lower than or comparable with that of previous work
on Nb-catalyzed thioether oxidation.[26c, 27b]
15
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Chemistry - A European Journal
Figure 5. a) Conversion/time profiles of thioether oxidation reactions catalyzed by Nb-
OC@[TBA][LA] at different temperatures. b) Arrhenius plots for the thioether
oxidation reaction of thioanisole and H2O2. The observed rate constants (k) were
calculated with the initial rates from Figure 5a at different temperatures. Reaction
conditions: thioanisole (2.0 mmol), H2O2 (2.2 mmol), dodecane (0.2 mmol), catalyst
(0.027 mol%), CH3OH (1.0 mL).
Substrate Scope and Recyclability of the Catalyst
In the next step, a series of thioethers were tested to selectively synthesize the
corresponding sulfoxides to examine the substrate scope of the present catalyst under a
low loading condition (0.027 mol%, ca. 30 ppm) under ice bath condition. As
demonstrated by the results in Table 2, the catalyst Nb-OC@[TBA][LA] displayed a
wide substrate scope and provided for a high selectivity of sulfoxides or sulfones,
respectively. A series of dialkyl sulfides, phenyl alkyl sulfides, phenyl benzyl sulfides
and dibenzyl sulfides could be converted into sulfoxides, regardless of the electronic
properties of the substituents. However, the results indicated that methyl ethyl sulfide
are easier to be attacked by electrophilic attack due to its higher electron density on
sulfur atom and small steric hindrance, and the reaction could be finished within 10
minutes (Table 2, entry 1). Also, substrates with electron-donating groups on benzene
rings are easier to react (Table 2, entries 2-6). This observation is consistent with a
mechanism that electrophilic active oxygen species are attacked directly by sulfide .[28]
Most reactions above remained homogeneous phase. However, the oxidation of
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diphenyl sulfides took a relatively longer time, which could result from biphasic state
at the beginning of reaction (Table 2, entry 7). Overall, all substrates could be converted
into corresponding sulfoxides or sulfones by adjusting the equivalent of H2O2, reaction
time and reaction temperature to obtain excellent selectivity (Table 2, entries 1-7).
Obviously, sulfoxides could be produced almost stoichiometrically, and moreover the
corresponding sulfones were achieved in an excellent selectivity under higher
temperature (50 C or 60 C) and more amounts of H2O2 conditions, which makes the
process a good substitute for sulfone production.[29]
Subsequently, the recyclability of the catalyst Nb-OC@[TBA][LA] was also
examined under a higher loading condition (0.1 mol%, ca. 120 ppm). As shown in
Figure S7, it indicated that the catalyst could be reusable for at least five times with
high selectivity to sulfoxide, although the activity had a slight decrease possibly due to
the slight aggregation of Nb-OC@[TBA][LA] in the consecutive recycles, as confirmed
by HRTEM indicated in Figure S8. However, the conversion of thioanisole can still
increase to a high level in the sixth recycle if the reaction time was prolonged only from
5 min to 20 min. Moreover, the ICP-AES analysis revealed that about 12 ppm of
niobium was leached after six cycles.
Table 2. Selective thioether oxidation of different substrates with Nb-OC@[TBA][LA]
catalyst.[a]
H2O2
Substrate [equiv.]
Con.
Sulfoxide
/Sulfone
Entry
1
t [min]
10
[%][b]
TON
3704
T[C]
1.1
2.5
1.1
2.5
1.1
2.5
1.1
2.2
0
100
100
100
100
100
100
91
87/13
0/100
96/4
50
0
60
40
3704
3704
3704
3704
3704
3370
3704
2
3
4
50
0
240
40
0/100
94/6
50
0
240
80
0/100
99/1
50
240
100
0/100
17
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5
6
7
1.1
2.5
1.1
2.5
1.1
2.5
0
120
400
120
240
210
420
87
100
91
97/3
0/100
98/2
3222
3704
3370
3704
3333
3704
50
0
50
25
60
100
90
0/100
95/5
100
0/100
[a] Reaction conditions: substrate (2.0 mmol), H2O2 (30% aqueous solution), dodecane
(0.2 mmol), Nb-OC@[TBA][LA] (0.027 mol%, ca. 30 ppm), CH3OH (1.0 mL). [b] GC
conversion with dodecane as an internal standard.
Interestingly, the catalyst Nb-OC@[TBA][LA] gave a moderate to good
conversion for epoxidation of olefins at ultra-low loading (0.065 mol%, ca. 50 ppm) in
CH3OH. As shown in Table 3, the epoxidation of chain allylic alcohols proceeded
facilely under this condition, which gave excellent chemoselectivity to the
corresponding epoxy alcohols without formation of the aldehydes and hydrolysis
products (Table 3, entries 1−4). Moreover, the regioselective epoxidation of geraniol
was preferred to the proximal 2,3-allyl double bond rather than the distal
unfunctionalized 6,7-double bond, probably due to the hydrogen bond interaction of
hydroxyl groups with the peroxo group of peroxoniobate species (Table 3, entry 4).[30]
In addition, cyclooctene, cyclohexene and styrene were also epoxidized with ca. 70%
conversion (Table 3, entries 5−7). Only tiny amounts of cyclohexenol or cyclohexenone
were detected in the epoxidation of cyclohexene (Table 3, entry 6), which meant that
the reaction did not proceed by a single electron transfer mechanism.[31] Some of by-
products were detected in styrene epoxidation mainly as benzaldehyde (Table 3, entry
7). Furthermore, the epoxidation of 1-octene, and internal olefins also produced the
corresponding epoxide very selectively and the configurations around the C=C moiety
were retained in the corresponding epoxides, although the conversion was slightly
lower likely owning to steric hindrance (Table 3, entries 8 and 9).[32]
Table 3. Epoxidation of different substrates with Nb-OC@[TBA][LA] catalyst.[a]
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Entry
Substrate
t [h]
12
Con. [%][b]
80.2
Sel. [%][c] TON
T[C]
25
1
2
3
99.9
99.9
99.9
1234
1209
1102
40
15
78.6
40
12
71.6
4
5
6
40
60
40
18
23
12
92.4
66.9
66.3
98.6
99.9
89.8
1422
1029
1020
7
8
9
60
60
60
24
24
24
69.9
18.8
14.1
61.4[d]
99.9
1075
289
99.0
217
[a] Reaction conditions: substrate (1.0 mmol), H2O2 (1.1 mmol, 30% aqueous solution),
catalyst (0.065 mol%), CH3OH (1.0 mL). [b] GC conversion with n-butyl alcohol as an
internal standard. [c] The selectivity towards epoxides; [d] The main by-product is
benzaldehyde.
Identification of active sites in IL-stabilized Nb oxocluster catalysts
93Nb NMR spectra has been proved a powerful tool to gain more insight of the
possible evolution of active niobium species during the course of the reaction.[33] As
shown in Figure 6, after H2O2 was added to the catalyst (H2O2/Nb-OC@[TBA][LA] =
10:1, molar ratio), the original signals at −849 and −1036 ppm disappeared (Figure 2b,
bottom), and the signal at −1108 ppm was enhanced significantly, accompanying with
a new wide peak appeared at −1331 ppm, which can be attributed to forming
peroxoniobate species similar to [Nb(O-O)2(OOH)2]−.[3] The shifts suggested that the
original niobium sites were transformed into new active peroxy niobium species, and
the resonance signal at −1108 ppm of the catalyst demonstrated that a part of niobium
sites without lactate anion coordination still existed in the Nb oxocluster. The signal
became much stronger after H2O2 was added (Figure 2b vs Figure 6a). Furthermore,
19
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when a small amount of thioanisole was added with molar ratio of
H2O2/thioanisole/catalyst = 10:1:1, the chemical shift at −1331 ppm almost disappeared,
indicating that the active oxygen on the peroxy niobium species was transferred into
sulfur and the reduced niobium sites formed and gave a wide weak peak around −840
ppm (Figure 6b). If more thioanisole was introduced (H2O2/thioanisole/catalyst =
10:11:1, molar ratio), the chemical shift at −1108 ppm also disappeared completely and
the two chemical shifts of the initial oxocluster catalyst were indeed observed (Figure
2b vs Figure 6c). These results demonstrated clearly that once the peroxy niobium sites
on the oxocluster completed the oxidation of thioethers, the catalyst can regenerate to
its original state.
Figure 6. The evolution of niobum sites on the Nb-OC@[TBA][LA] catalyst as
reflected by93Nb NMR spectra after reacting with H2O2 in D2O. a) H2O2 was added into
Nb-OC@[TBA][LA] solution in D2O (H2O2/catalyst = 10, molar ratio); b) thioanisole
was added to the mixture of Nb-OC@[TBA][LA] and H2O2 (H2O2/thioanisole/catalyst
= 10:1:1, molar ratio); c) more thioanisole was added to the reaction mixture
(H2O2/thioanisole/catalyst = 10:11:1, molar ratio).
To shed light on the origin of the exceptional activity of Nb-OC@[TBA][LA],
firstly, a free radical inhibitor, butylated hydroxytoluene (BHT), was added to the
reaction mixture in the course of the thioanisole oxidation reaction. As shown in Figure
S9, the addition of BHT actually had no effect on the reaction rate. The result indicated
20
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clearly that the current oxidation reaction did not proceed by a single electron transfer
mechanism but a direct oxidation route.
Furthermore, as we have observed visually during the reaction, Nb-
OC@[TBA][LA] was well dispersed in methanol, which inspired us to identify if there
exists a strong interaction between the present Nb oxocluster and the polar organic
solvents. All of Nb oxoclusters and niobic acid were sunk in the bottom of test tube due
to its poor immiscibility with n-hexane (Figure S10a). However, it can be seen that after
addition of a suitable of methanol, the volume of Nb oxocluster was expanded
significantly owning to selective adsorption of methanol (Figure S10, the top three
rows). This was evidenced that the volume-expansion coefficient (V) of Nb
oxoclusters was highly relevant to the dosage of methanol (Figure 7a, Figure S11a and
c). The expansion coefficient increased linearly with the addition of methanol. Notably,
Nb-OC@[TBA][LA] can adsorb 1.1 mL methanol in 7 mL n-hexane without phase
separation even after 4 hours (Figure S10, first row). The other two Nb oxoclusters
showed the same tendency for adsorbing methanol in n-hexane as that of Nb-
OC@[TBA][LA]. However, these two Nb oxocluster precipitated from lower phase
when the dosage of methanol was more than 0.7 ml (Figure S10, the second and third
row). Besides, as shown in Figure 7b, the methanol mass concentration (wt.%) in upper
n-hexane phase decreased sharply with time due to adsorption of the Nb oxocluster and
reached a distribution equilibrium within 5 min. The other two Nb oxoclusters showed
a similar tendency to that of Nb-OC@[TBA][LA] in spite of low adsorption capacity
of methanol (Figure S11b and d). As a comparison, the methanol mass concentration
retained almost unchanged because as-obtained niobic aciddid not adsorb any methanol
(Figure 7b and the bottom row of Figure S10). As a result, we consider that the structure
of the present Nb oxoclusters manifests themselves to exhibit the similar “pseudo-liquid
phase” behavior to that of polyoxoanions reported previously.[34]
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Figure 7. (a) The changes of volume-expansion coefficient (V) of Nb-based catalysts
with the dosage of methanol; (b) The methanol mass concentration (wt.%) in n-hexane
as a function of time. (■) Nb-OC@[TBA][LA]; (●) niobic acid. Conditions for (b): 7
mL n-hexane and 0.5 mL methanol, Nb oxoclusters and niobic acid (ca. 0.08 mmol)
were used.
The Nb oxocluster swollen by the polar solvents has also been confirmed by use of
DLS. The typical HRTEM images of the Nb oxocluster Nb-OC@[TBA][LA] have
shown a mean size about 2.6 nm (Figure 3g and h). In methanol solution, the Nb
oxocluster showed a distribution of hydrodynamic diameters centered at 59 nm as
measured by DLS. n-Butanol gave similar average hydrodynamic diameter although
the diameter distribution became slightly wider. Similar swelling behavior was also
observed in other polar solvents (CH2Cl2 and CH3CN) (Figure S12). This revealed that
although the coordination bond of Nb to lactate anions led to the excellent stability and
high dispersion, the structure of the nanosized Nb oxocluster was quite flexible and was
swollen easily by polar organic solvents. In this case, the solvent-swollen Nb oxocluster
actually serves as a nanoreactor, where the catalysis takes place in either the bulk or
surface of oxocluster with substrates and products transporting into and out of the
nanoreactor, as represented in Scheme 2. The mass transport of the substrates to the
catalytic site can be promoted due to the solvent swelling process.
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Scheme 2. The proposed reaction route for selective oxidation of sulfides and olefins
with Nb oxocluster catalysts.
Conclusion
In this work, we have demonstrated that the Nb oxocluster can be achieved due to
condensation of peroxy-niobic acid precursor in a convenient manner. The -hydroxyl
carboxylate anion in the IL played a critical role in stabilizing the nanosized Nb
oxoclusters through coordination with niobium sites. Moreover, the IL cation
(quaternary ammonium) was also indispensable in balancing/stabilizing the negatively
charged polyoxoniobate anion, providing an excellent dispersion in the catalysis media.
The as-synthesized Nb oxocluster was a highly efficient catalyst for selective oxidation
of thioethers and olefins. Especially, the Nb-OC@[TBA][LA] with average diameter
of 2.6 nm demonstrated exceptional activity for the selective oxidation of thioethers at
0 °C in methanol with 30% H2O2 with TONs more than 56000 at catalyst loading as
low as 0.0033 mol%, which actually outperforms all reported niobium-catalysts for the
oxidation of thioethers. Moreover, the present Nb oxocluster can also be extended for
the epoxidation of olefins by electrophilic oxygen transfer. Remarkably, a variety of
olefins can also be epoxidized at a catalyst loading as low as 0.065 mol% due to its
superior catalytic performance, with TONs reaching over 1000, even exceeding those
of homogeneous transition metal catalysts. This is mainly due to the highly dispersed
active peroxy niobium species coordinated by -hydroxyl carboxylate anion. In
addition, a strong protic solvent (methanol) can accelerate the sulfoxidation and
epoxidation reaction. It was found that the Nb oxocluster showed a“pseudo-liquid
phase” behavior and its structure was highly flexible, leading to easily swelling with
polar solvents. The swollen Nb oxocluster serves as a nanoreactor and promotes the
23
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accessibility of organic substrates to active sites in methanol environment. Meanwhile,
product molecules can leave the oxocluster units by fast diffusion. This work is the first
example of nanosized Nb oxocluster owning high catalytic activity for the oxidation of
thioethers and olefins. Undoubtedly, the design and construction of the novel
oxoclusters can open new avenues for achieving high redox-active and stable catalysts.
Experimental Section
Materials
Potassium hydroxide (KOH), Tetrabutylammonium chloride (TBACl) and
commercial niobium pentoxide (Nb2O5) and various substrates were obtained from
Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Lactic acid (85.0%) and
Propionic acid (AR) was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd.
3-hydroxypropionic acid (97%) was obtained from Adamas Reagent Co., Ltd.
(Shanghai, China).
Characterization
The FT-IR spectra were recorded at room temperature on a Nicolet Magna 550 FT-
IR spectrometer. The X-ray diffraction (XRD) analysis was performed with D/MAX
2550 VB/PC using CuKα radiation (λ = 1.5406 Å) operated at 40 kV and 200 mA (scan
rate: 6 ° min−1; scan area: 10−80°). The thermal stability of catalysts was determined
by TGA method. A Perkin Elmer Pyris Diamond was used for the TGA measurements.
All samples were vacuumed at 60 °C for 1 h to remove the solvent molecules before
TGA. The samples were heated from 50 °C to 800 °C (heating rate: 10 °C min−1) under
the flow of anhydrous air (flow rate: 20 mL min−1). The elemental analysis of C, H, N
was performed using an Elementar vario EI III C H N O S elemental analyzer and the
ICP-AES analysis of niobium on a Varian 710 instrument, respectively. High-resolution
transmission electron microscopy (HRTEM) was performed in a JEM 2100 instrument
(JEOL Corporation, Japan) operating at 200 kV with a nominal resolution of 0.25 nm.
The samples for TEM were prepared by dropping the aqueous solutions containing the
NPs onto the carbon-coated Cu grids. All NMR spectras were recorded on a Bruker
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1
13
AVANCE 400 MHz instrument (400 MHz H NMR, 100 MHz C NMR) using D2O
as solvents and TMS as the reference, respectively. 93Nb NMR spectras were obtained
at room temperature on a Varian 700 MHz spectrometer operating at 171.05 MHz.
−
+
Chemical shifts in ppm are referred to an external CD3CN solution of NbCl6 N(Et)4
(93Nb), synthesized from NbCl5.[35] The reaction products were analyzed by using Gas
chromatography (GC) [a Shimadzu GC-2014 gas chromatograph equipped with the
KB-50 capillary column (30 m × 0.32 mm × 0.50 mm)] and GC−MS [Agilent
6890/5973 GC–MS equipped with the HP-5MS column (30 m × 0.25 mm × 0.25 mm)].
The size distribution of the aggregates was determined by dynamic light scattering
(DLS) measurements by using a Malvern NanoZetaSizer.
Preparation of carboxylate ILs
[TBA][LA] was prepared by a two-step process. KOH (0.392 g, 7 mmol) was
added to a solution of tetrabutylammonium chloride (TBACl; 1.39 g, 5 mmol) in dry
ethanol (10 mL) under N2, and the mixture was stirred vigorously at room temperature
for 12 h. The solution was allowed to stand in an ice bath for 12 h. The precipitated KCl
and redundant KOH were filtered off. This step could be repeated until no chloride ions
in the filtrate were detectable by a silver nitrate test. In the second step, the
neutralization of lactic acid with TBAOH (1:1 molar ratio) in ethanol gave the
corresponding ILs.[24] After the solution was stirred for a few hours, followed by drying
under vacuum at 60 °C for 1 h, the IL was obtained as a colorless viscous liquid.
[TBA][LA], 1H NMR (400 MHz, D2O, Me4Si) δ 4.01 (m, 1H), 3.08 (m, 8H), 1.54 (m,
8H), 1.28 (m, 3H), 1.25 (m, 8H), 0.84 (dt, J = 8.0 Hz, 12H). 13C NMR (100 MHZ, D2O,
Me4Si): δ 181.87, 68.20, 58.04, 23.07, 19.97, 19.10, 12.80. Anal. Calcd for C19H41NO3
([TBA][LA]) (333.54): C, 68.8; H, 12.5; N, 4.2. Found: C, 67.4; H, 13.2; N, 4.0.
[TBA][PA] was prepared by a similar method, except that lactic acid was replaced
1
by propionic acid. The product was a colorless viscous liquid. ([TBA][PA]): H NMR
(400 MHz, D2O, Me4Si) δ 3.08 (m, 8H), 2.08 (m, 2H), 1.54 (m, 8H), 1.25 (m, 8H), 0.94
(m, 3H), 0.83 (dt, J = 8.0 Hz, 12H). 13C NMR (100 MHZ, D2O, Me4Si): δ 184.18, 58.08,
30.38, 23.10, 19.12, 12.83, 10.03.Anal. Calcd for C19H41NO2 (TBA-PA) (315.54): C,
72.3; H, 13.1; N, 4.4. Found: C, 70.8; H, 14.0; N, 4.2.
25
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[TBA][HPA] was prepared by a similar method, except that lactic acid was
replaced by 3-hydroxy propionic acid. The product was a colorless viscous liquid.
(TBA-HPA): 1H NMR (400 MHz, D2O, Me4Si) δ 3.68 (m, 2H), 3.09 (m, 8H), 2.33 (m,
2H), 1.54 (m, 8H), 1.25 (m, 8H), 0.84 (dt, J = 8.0 Hz, 12H). 13C NMR (100 MHZ, D2O,
Me4Si): δ 180.25, 67.64, 58.07, 40.02, 23.10, 19.13, 12.86. Anal. Calcd for C19H41NO3
([TBA][HPA]) (331.54): C, 68.8; H, 12.5; N, 4.2. Found: C, 67.8; H, 13.1; N, 4.0.
Preparation of Niobic Acid
Niobic acid (Nb2O5·nH2O) was prepared by using a previously reported method
but with some modifications.[36] In brief, solid KOH (3.92 g, 70 mmol) and commercial
niobium pentoxide Nb2O5 (1.86 g, 7 mmol) were fused in a nickel crucible for 6 h at
550 °C. When the mixture was cooled, abundant water was poured into the system. The
unreacted Nb2O5 was filtered and the pH value of the clear filtrate was adjusted to 5 by
slowly adding acetic acid. After that, the white precipitate was obtained, which was
washed fully with distilled water until neutral pH was attained, followed by drying at
50 °C for 0.5 h. The resultant niobic acid containing ca. 84% of water was used for
preparing Nb oxocluster. The as-obtained niobic acid was further dried at 80 °C for 12
h prior to the characterizations by FT-IR, XRD, HRTEM etc.
Preparation of IL-stabilized Nb oxoclusters
Firstly, the peroxy-niobic acid was prepared according to a previously reported
method.[37] The as-synthesized wet niobic acid (0.831g, 0.5 mmol) was mixed with a
30 wt.% solution of H2O2 (4 mL) under the condition of ice-bath to obtain a pale yellow
clear solution. Next, the peroxy-niobic acid solution was added to the already
synthesized IL [TBA][LA] (0.663g, 2 mmol) and stirred for several hours under the
condition of ice-bath. Then, the solution was warmed to 40 °C and stirred overnight to
evaporate water. Finally, the resulting material was dried further at 65°C for 1 h to
obtain a white viscous liquid as Nb-OC@[TBA][LA]. 1H NMR (400 MHz, D2O, Me4Si)
δ 3.95 (m, 1H), 3.06 (m, 8H), 1.51 (m, 8H), 1.23 (m, 3H), 1.19 (m, 8H), 0.81 (dt, J =
8.0 Hz, 12H). 13C NMR (100 MHZ, D2O, Me4Si): δ 182.39, 68.46, 58.09, 23.11, 20.05,
26
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19.13, 12.82. Found: C, 54.0; H, 12.4; N, 3.9; Nb, 13.7. Number of peroxy bonds: 0.13
per Nb atom.
Nb-OC@[TBA][PA] was prepared by a similar method, except that IL [TBA][PA]
was adopted instead. The product was a yellow viscous liquid. No NMR spectra was
measured due to very poor solubility in water and organic solvents. Found: C, 54.1; H,
12.5; N, 3.9; Nb, 14.0. Number of peroxy bonds: 0.15 per Nb atom.
Nb-OC@[TBA][HPA] was prepared by a similar method, except that the IL
[TBA][HPA] was adopted instead. The product was a yellow viscous liquid. No NMR
spectra was measured due to very poor solubility in water and organic solvents. Found:
C, 54.4; H, 12.3; N, 4.0; Nb, 13.4. Number of peroxy bonds: 0.14 per Nb atom.
Preparation of mononuclear peroxoniobate-based ILs
The IL [TBA][Nb=O(O-O)(OH)2] was prepared according to a previously
reported method for the sake of comparsion.[3] Briefly, ionic exchange between KOH
(0.156 g, 2.8 mmol) and tetrabutylammonium chloride (TBACl; 0.556 g, 2 mmol) in
dry ethanol (10 mL) under N2 afforded TBAOH. The as-obtained solution was cooled
to 0 °C and stored for 12 h. The white precipitate was filtered off, and then
(NH4)3[Nb(O2)4] (1.868 g, 6.8 mmol) was added to the solution and stirred at 40 °C for
2 days until the pH value was 7. The solution was filtered to remove insoluble materials
and was then evaporated at 40 °C for 3 h. The crude product was purified with CH2Cl2
to obtain a brownish red liquid as [TBA][Nb=O(O-O)(OH)2].
The lactate coordinated peroxoniobate-based IL was prepared according to a
previously reported method.[13] The peroxoniobate-based IL [TBA][Nb=O(O-O)(OH)2]
(2.0 mmol) was dissolved in methanol (10 mL). Next, lactic acid (0.192 g, 2.0 mmol)
was added to the solution. The mixture was stirred vigorously at 30 °C for 6 h, and the
solvent was then evaporated. The crude product was washed with diethyl ether. The
resulting product was placed under vacuum at 25 °C for 1 h to give a dark red viscous
liquid as [TBA][NbO(OH)2(LA)].
Typical procedure for catalytic oxidation
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10.1002/chem.201806178
Chemistry - A European Journal
The method for the catalytic oxidation of sulfides was given as follows. Firstly,
prepare a stock solution of catalyst Nb-OC@[TBA][LA] in CH3OH. The catalyst Nb-
OC@[TBA][LA] (9.2 mg) was dissolved in CH3OH (25.0 mL) and then stored for uses.
Next, a mixture of sulfides (2.0 mmol), 30% aqueous H2O2 (2.2 mmol) and the as-
prepared Nb-OC@[TBA][LA] solution (0.54 mM) in methanol solution (1.0 mL, ca.
30 ppm in the whole system) were placed in a 5 mL sealed vial equipped with a
magnetic stirrer and then stirred under the condition of ice-bath for the required time.
The reaction progress was monitored by GC, and the yield of products was determined
by GC analysis using dodecane as the internal standard. When the reaction was finished,
the catalyst was removed by a short silica gel column and then analyzed by using GC.
For catalyst recycling, after the reaction the solvent was removed by rotary evaporation
at 40 °C and then the reaction mixture was extracted with ethyl ether. The recovered
catalyst was dried, followed by adding fresh substrate and methanol for the next run.
In the catalytic epoxidation of olefins, a stock solution of catalyst Nb-
OC@[TBA][LA] in CH3OH was firstly prepared. In brief, the Nb-OC@[TBA][LA]
(11.0 mg) was dissolved in CH3OH (25.0 mL) and then stored for uses. Next, a mixture
of olefins (1.0 mmol), 30% aqueous H2O2 (1.1 mmol) and the as-prepared Nb-
OC@[TBA][LA] solution (0.65 mM) in methanol (1.0 mL ca. 50 ppm in the whole
system) were placed in a 5 mL sealed vial equipped with a magnetic stirrer and then
stirred at a certain temperature for the required time. The reaction progress was
monitored by GC, and the yield of products was determined by GC analysis using n-
butyl alcohol as the internal standard. After completion of the reaction, using the
method similar to above for analysis.
Reaction Kinetics
The procedure for investigating the kinetic parameters for the oxidation of
thioanisole was shown as follows. The concentration of catalyst was constant in all
experiments (ca. 30 ppm). In order to examine the effect of substrate concentration on
the rate of sulfoxidation, the concentration of sulfides was varied from 1.5 to 3.0 M,
while the concentration of H2O2 was 1.0 M. Similarly, the effect of H2O2 concentration
28
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10.1002/chem.201806178
Chemistry - A European Journal
on the rate of sulfoxidation was investigated through adjusting the concentration of
H2O2 from 0.5 to 2.0 M, while the concentration of sulfides was 2.0 M. The other
reaction conditions have been given in the figure captions (Figure 5). The reaction was
monitored using GC. Reaction rates (R0) for the kinetic studies were determined from
the slopes of reaction profiles ([substrate]0−[substrate]t vs time) at low conversions
(<15%, initial rate method). The activation energy for the process was determined using
the Arrhenius equation based on the calculated rate constants at different temperatures.
The measurement of volume-expansion coefficient
A certain amount of Nb oxoclusters and niobic acid (ca. 0.08 mmol) were added
into a sealed graduated test tube containing 7 mL of n-hexane, following by adding a
suitable amount of methanol. Then the volume of the swollen Nb oxocluster were
measured accordingly. The volume-expansion coefficient (V) was calculated on the
basis of the volumes of bottom phase (Nb oxocluster) before and after the adsorption
of methanol according to eq (2):
V=(V−V0)/V0
(2)
where V and V0 are the volumes of the methanol-adsorbed and methanol-free Nb
oxocluster at different dosage of methanol.
At the same time, the measurement of phase equilibrium demonstrated that 7 mL
n-hexane and 0.5 mL methanol were completely miscible. After mixing them, a certain
amount of Nb oxoclusters or niobic acid (ca. 0.08 mmol) was then added and stirred
vigorously. The change of methanol mass concentration (wt.%) in n-hexane was
monitored by GC consecutively.
ACKNOWLEDGMENTS
The authors are grateful for financial support from the National Natural Science
Foundation of China (21373082, 21773061), and the Innovation Program of the
Shanghai Municipal Education Commission (15ZZ031).
References
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