Research Article pubs.acs.org/acscatalysis
Highly Efficient Epoxidation of Allylic Alcohols with Hydrogen Peroxide Catalyzed by Peroxoniobate-Based Ionic Liquids Chen Chen,†,§ Haiyang Yuan,†,§ Haifeng Wang,*,† Yefeng Yao,‡ Wenbao Ma,† Jizhong Chen,† and Zhenshan Hou*,† †
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ‡ Physics Department and Shanghai Key Laboratory of Magnetic Resonance, East China Normal University, Shanghai 200062, People’s Republic of China S Supporting Information *
ABSTRACT: This work reports new kinds of monomeric peroxoniobate anion functionalized ionic liquids (ILs) designated as [A+][NbO(OO)(OH)2] (A+ = tetrapropylammonium, tetrabutylammonium, or tetrahexylammonium cation), which have been prepared and characterized by elemental analysis, HRMS, NMR, IR, TGA, etc. With hydrogen peroxide as an oxidant, these ILs exhibited excellent catalytic activity and recyclability in the epoxidation of various allylic alcohols under solventfree and ice bath conditions. Interestingly, subsequent activity tests and catalyst characterization together with first-principles calculations indicated that the parent [NbO(O-O)(OH)2]− anion has been oxidized into the anion [Nb(O-O)2(OOH)2]− in the presence of H2O2, which constitutes the real catalytically active species during the reaction; this anion has higher activity in comparison to the analogous peroxotungstate anion. Moreover, the epoxidation process of the substrate (allylic alcohol) catalyzed by [Nb(O-O)2(OOH)2]− was explored at the atomic level by virtue of DFT (density functional theory) calculations, identifying that it is more favorable to occur through a hydrogen bond mechanism, in which the peroxo group of [Nb(OO)2(OOH)2]− serves as the adsorption site to anchor the substrate OH group by forming a hydrogen bond, while OOH as the active oxygen species attacks the CC bond in substrates to produce the corresponding epoxide. This is the first example of the highly efficient epoxidation of allylic alcohols using a peroxoniobate anion as a catalyst. KEYWORDS: peroxoniobate anion, ionic liquid, epoxidation, allylic alcohols, DFT
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INTRODUCTION
the ability to release oxygen in active form, either by chemical means or on irradiation, and act in oxidations of different inorganic and organic substrates such as sulfides, sulfur dioxide, alkenes, alcohols, aromatic and aliphatic hydrocarbons, and phosphines.3−8 Among these studies, however, the work on epoxidation of allylic alcohols over Nb-containing catalysts was still very scarce,5 possibly because Nb-containing catalysts seemed to afford a slightly lower oxidation activity in comparison with the tungsten peroxo complex analogue. Specifically, we reported recently an active Nb-based heterogeneous catalyst for selective epoxidation of allylic alcohols under very mild conditions.9 However, over this Nb2O7-derived heterogeneous catalyst, a very large fraction of Nb atoms were geometrically inaccessible to reactants. Thus, the development of a monomeric Nb catalyst would be highly desirable from the view of catalytic efficiency. Furthermore, the actual active site on the heterogeneous catalyst is still not
The epoxidation of allylic alcohols is of great importance in industry and synthetic chemistry for purposes such as the synthesis of natural products and chiral molecules.1 Much effort has been devoted to the development of this epoxidation process using environmentally benign oxidizing agents and ecofriendly solvents. Among them, transition-metal peroxo complexes with H2O2 have attracted much attention, especially tungsten peroxo complex based oxidation systems. Although some transition metals, especially tungsten peroxo complexes, were efficient for the H2O2-based epoxidation of allylic alcohols,2 these systems have some drawbacks: (1) an excess of H2O2 was used with respect to the substrates in some cases, (2) turnover frequency (TOFs) was also limited, (3) solvents or additives were normally required, (4) most of these catalysts are homogeneous complexes and the separation and recycling of the expensive catalysts still remained a huge challenge. Apart from the tungsten compound, in recent years catalysis by peroxo niobium compounds has become a field of growing interest as well. These niobium-containing compounds show © XXXX American Chemical Society
Received: March 17, 2016
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DOI: 10.1021/acscatal.6b00786 ACS Catal. 2016, 6, 3354−3364
Research Article
ACS Catalysis Scheme 1. Synthetic Route of Different IL Catalysts and Their Designation
Figure 1. Analysis of the outlet gases from the decomposition of ionic liquids by TG-MS: (a) TPAIL; (b) TBAIL; (c) THAIL.
The POM-ILs can offer a highly effective approach to the separation of products from ionic liquids due to immiscibility with nonpolar solvents, which is the motivation for inventing novel POM-ILs. However, to date, no investigation has been dedicated to peroxoniobate anion based ionic liquids and their catalytic applications. Keeping in mind the previous research of our group and others on the epoxidation of olefins with peroxotungstate anion based ionic liquid catalysts,15,16 herein we have developed a new kind of ionic liquid consisting of quaternary ammonium cations and a monomeric peroxoniobate anion. The new peroxoniobate anion based ionic liquids were utilized as catalysts for the epoxidation of allylic alcohols for the first time. These ionic liquids were easy to synthesize and demonstrated high activity under very mild conditions. Especially, the structural forms of the monomeric peroxoniobate anion in the presence of H2O2 were first recognized and the reaction transition state was proposed through both experimental methods and DFT calculations.
known and understanding the reaction mechanism remains a huge challenge. With respect to the epoxidation mechanism of allylic alcohols catalyzed by transition-metal peroxo complexes, previous works have speculated that it occurs in most cases through metal−alcoholate intermediate species to form a pentacoordinate configuration,10 while it was also argued that the transition state could be stabilized by hydrogen bonding between the catalytically active species and the substrates rather than forming metal−alcoholate species, which even results in higher activity for the epoxidation of homoallylic and allylic alcohols.11 As a result, the approach of substrate activation was possibly diverse, depending on the nature of the catalytically active site. On the other hand, ionic liquids that are liquid at room temperature and below are known as ambient-temperature or room-temperature ionic liquids, which are receiving keen interest currently due to their unique properties such as intramolecular forces, ionicity, polarity, and their inherent designer nature, and research involving ILs is expanding to many different areas of knowledge.12 The most common ionic liquids in use are those with alkylammonium, alkylphosphonium, N-alkylpyridinium, and N,N-dialkylimidazolium cations and simple inorganic anions such as Cl−, Br−, BF4−, PF6−, etc. Recently, a variety of functionalized ILs with functional groups tethered to their cations or anions have been developed,13 but most of them were designed for the functionalization of cations, while less attention has been paid to the functionality of anions. With respect to the functionalization of anions, polyoxometalate anion based ionic liquids (POM-ILs) have been synthesized and utilized in the past decade for the efficient acidic and oxidation catalysis.14 In this respect, our group had developed some reaction-induced phase separation and thermoregulated phase-separable peroxotungstate anion based ionic liquids (POM-ILs) as the catalysts for the epoxidation of various olefins, where the IL anions serve as catalytically active sites while the cations provide some specific functionality.15 Definitely, the emergence of functionalized ILs will endow the ILs with huge diversity in both quantity and properties, with more space for further development.
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RESULTS AND DISCUSSION Structure Identification of the Ionic Liquids. At first, A+OH− (A+ = quaternary ammonium cation) was prepared with A+Cl− as a starting material by anion exchange. Then the monomeric peroxoniobate anion based ionic liquids were successfully obtained by stirring A+OH and (NH4)3[Nb(O2)4] in EtOH at 40 °C. After evaporation at low temperature (40 °C) for a period, the resulting products were obtained as yellow to brownish red viscous liquids at room temperature (Figure S1 in the Supporting Information). The composition of the synthesized materials was examined by 1H NMR, elemental analysis, inductively coupled plasma atomic emission spectroscopy (ICP-AES), and potential difference titration of Ce3+/ Ce4+. The analyses revealed that the organic cations were integrated and the structure of peroxoniobate anion [Nb(O2)4]3− had changed in the course of IL preparation. It was found that [Nb(O2)4]3− in (NH4)3[Nb(O2)4] decomposed with the liberation of active oxygen to form the monoperoxoniobium complex [NbO(O-O)(OH)2]− (Scheme 1).17 As a result, there was only one peroxo group (O-O) in the anion of 3355
DOI: 10.1021/acscatal.6b00786 ACS Catal. 2016, 6, 3354−3364
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ACS Catalysis
O) band and the single ν(O-O) band near 850 cm−1 indicated the presence of only one kind of (O-O) group. The different shifts were probably due to the change of the coordination environment.23 Additionally, the bands around 890 cm−1 of the ILs were attributed to the vibration of NbO.17 Those bands of the anions were not overlapped with that of the hydroxide precursor (Figure S3a−c in the Supporting Information). Moreover, the bands at 2869 and 2950 cm−1 were attributed to ν(C−H) and those at 1300−1600 cm−1 were attributed to δ(C−N), which showed no differences from those of the precursor (Figure S3a−c), indicating the presence of the organic cation (Figure 2c−g). Structural Evolution of Peroxoniobate Anion in the Presence of H2O2. It was observed that there was an obvious exothermic phenomenon when the ILs were mixed with H2O2 at room temperature, which implied that the anion of ILs could be oxidized by H2O2 to create a new structure form. To identify the anion structure, the TBAIL was adopted as a starting material. First, the transformation of the peroxoniobium anion was examined by 93Nb NMR spectra. As shown in Figure 3,
IL, which was confirmed by titration of Ce3+/Ce4+. The thermogravimetric analysis (TGA) trace of the resulting ILs underwent a one-step degradation into Nb2O5, and the weight loss at the beginning could be attributed to the residual solvent. Figure 1 shows obvious weight losses of approximately 61.0%, 67.9% ,and 74.5%, respectively, between 100 and 900 °C, which could be related to the decomposition of the organic part and the peroxo group (O-O) to the final Nb2O5. A coupled TG-MS analysis revealed that the main evolved gases were carbon dioxide (CO2, m/z 44) and steam (H2O, m/z 18). The results of the TGA matched perfectly with those of the elemental analysis. It is worth noting that the HRMS results of peroxoniobate [NbO(O-O)(OH)2]− appeared at m/z 141, which was attributed to NbO3− (Figure S2 in the Supporting Information).18 The results of HRMS showed that peroxoniobate anions could be decomposed through loss of labile peroxo bonds during HRMS experiments.19 Moreover, the synthesis of imidazolium (1-butyl-3-methylimidazolium or 1dodecyl-3-methylimidazolium) cation based ILs failed, possibly because imidazolium cations might be transformed into carbene structures in the preparation of ILs under basic conditions.20 Then the different ILs were studied sequentially by using IR spectroscopy. As shown in Figure 2, the as-synthesized Nb2O5·
Figure 2. IR spectra of different niobium peroxo compounds: (a) Nb2O5·nH2O; (b) (NH4)3[Nb(O2)4]; (c) TBAIL; (d) the spent TBAIL after four recycles; (e) the spent TBAIL after eight recycles; (f) TPAIL; (g) THAIL.
Figure 3. 93Nb NMR spectra of different niobate anions in D2O: (a) K3[Nb(O2)4]; (b) K3[Nb(O2)4] + 40 equiv of H2O2 + several drops of HOAc; (c) TBAIL + 40 equiv of H2O2. All mixtures were stirred fully and then measured at room temperature snf pH 3−4. Insert: 93Nb NMR spectrum of TBAIL.
nH2O displayed a broad band centered at 655 cm−1 and a shoulder at 895 cm−1 (Figure 2a), which can be attributed to amorphous niobia.21 In addition, a peak at 3420 cm−1 was assigned to the OH stretching of Nb−OH and a typical band at 1636 cm−1 was related to the adsorbed water molecules on the Nb2O5 surface.22 However, no characteristic peaks of the peroxy group were observed. In contrast, the IR bands of the peroxoniobate anion could be assigned to the typical vibrational modes of the antisymmetric Nb−O stretching and the coordinated side-on-bonded peroxo ligand (O-O) stretching. Figure 2b shows the IR spectrum of the tetraperoxometalate compound (NH4)3[Nb(O2)4]. Two types of vibrational modes were expected. One broad ν(O-O) band around 818 cm−1 and another niobium−peroxo stretching vibration νas[Nb(O2)] near 550 cm−1 were observed. However, there were some shifts of the peak between the original (NH4)3[Nb(O2)4] and the derived ILs (Figure 2b−g). Unlike (NH4)3[Nb(O2)4], the band in ILs at approximately 570 cm−1 was attributed to the νas(Nb−
TBAIL showed a resonance signal at −1000 ppm (inset). However, after H2O2 was added to a TBAIL solution (H2O2/ TBAIL = 40, molar ratio) in D2O at room temperature, the signal was shifted to high field and chemical shifts moved to about −1362 ppm. The shift suggested that the [NbO(OO)(OH)2]− was transformed into a new structure. It was demonstrated that the well-known peroxoniobate anion [Nb(O-O)4]3− showed a chemical shift at −1522 ppm (Figure 3a). However, when acetic acid (HOAc) was added to an aqueous solution of K3[Nb(O2)4] and the pH was adjusted to 3−4 for the actual reaction conditions adopted in the epoxidation of allylic alcohols with H2O2, a white precipitate immediately formed, arising from the formation of hydrated niobium peroxide. Nevertheless, the precipitate disappeared gradually after 40 equiv of H2O2 was added, indicating that the insoluble hydrated niobium peroxide formed new soluble active species under a large excess of H2O2 at pH 3−4. 3356
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Figure 4. a) IR (top) and Raman (bottom) spectra of (a) K3[Nb(O2)4] and (b) the new Nb species.
principles calculations, and the free energy changes (ΔG) for the conversion from [NbO(O-O)(OH)2]− to [Nb(OO)m(OOH)4−m](m−1)− (m = 2−4) were calculated at 298 K (see details in the Supporting Information). Evidently, ΔG < 0 (>0) indicates that the conversion from [NbO(O-O)(OH)2]− to [Nb(O-O)m(OOH)4−m](m−1)− (m = 2−4) is thermodynamically favored (limited). As shown in Table 1,
Next, to identify the structure of this soluble peroxoniobate anion, acetone was used as anti-solvent to precipitate the watersoluble peroxoniobate. The resulting solid was dissolved in D2O and then characterized by 93Nb NMR spectra. As shown in Figure 3b, the chemical shifts showed an obvious difference with that of the parent K3[Nb(O2)4] solution, implying that the [Nb(O-O)4]3− has already transformed into new peroxoniobate species upon the addition of acetic acid. Interestingly, the 93Nb NMR spectrum of the resulting new Nb species was actually the same with that of IL pretreated with H2O2 (Figure 3b,c). These results demonstrate that whatever was used as starting material, the ILs or the salt K3[Nb(O2)4], were all transformed into the same new peroxoniobate species in the presence of a large excess of H2O2 at pH 3−4. The resulting solid was further subjected to various characterizations by use of titration of peroxide bonds and IR and Raman spectroscopy to clarify the structure of the new peroxoniobate anion accordingly. The titration with Ce3+/Ce4+ showed that there were still four peroxide bonds in the collected solid. However, as shown in Figure 4, the IR and Raman spectra were quite different from those of [Nb(OO)4]3−, which only exihited one ν(O-O) band at 817 cm−1 (Figure 4a, top). However, IR spectra of the new Nb species showed two vibration modes of the O-O stretching around 820 and 853 cm−1. Additionally, the peaks around 550 cm−1 (Figure 4a,b, top) of the two structures were quite similar, which were attributed to ν[M(O2)] vibrations. Raman spectroscopy gave the same information (Figure 4a,b, bottom), which meant that the new Nb species had an asymmetric structure but was quite similar to [Nb(O-O)4]3−.23 From the above characterization results, it could be speculated that the new niobate anion possesses a formula of [Nb(O-O)m(OOH)4−m](m−1)− (m = 2, 3) resulting from the reaction
Table 1. Gibbs Free Energy Changes (ΔG) of the Conversion from [NbO(O-O)(OH)2]− (A) to [Nb(OO)m(OOH)4−m](m−1)− (B; m = 2−4) in the Presence of H2O2 and the Estimated Concentrations of the Formed Anion (B) at Equilibriuma reaction A + 3H2O2 → B1 + 2H + 3H2O A + 3H2O2 → B2 + H+ + 3H2O A + 3H2O2 → B3 + 3H2O +
anion
C/mol L−1
1.57 0.16 −0.30
B1 B2 B3
1.89 × 10−10 1.30 × 10−4 9.99 × 10−1
a B1, B2, and B3 represent [Nb(O-O)4]3−, [Nb(O-O)3(OOH)]2−, and [Nb(O-O)2(OOH)2]−, respectively. The initial concentration of [NbO(O-O)(OH)2]− is taken as 1 mol L−1.
only the formation of [Nb(O-O)2(OOH)2]− is thermodynamically stable with a ΔG value less than 0, while the [Nb(OO)4]3− species is the least stable, being strongly endothermic by 1.57 eV. In other words, the stabilities of these three Nb anions increase in the order [Nb(O-O)4]3− < [Nb(O-O)3(OOH)]2− < [Nb(O-O)2(OOH)2]−. Moreover, we quantitatively calculated the relative concentrations of these three anions at the equilibrium state to give a clear comparison (see details in the Supporting Information). As shown in Table 1, one can see that [Nb(O-O)2(OOH)2]− (B3) has the largest relative concentration (99.9%), which is much larger than the concentrations of the other species. Specifically, the concentration of [Nb(O-O)4]3− (B1) is almost 0, which proves the instability and absence of [Nb(O-O)4]3− under realistic experimental conditions, agreeing with the above experimental results. In other words, the ionic liquid anion [NbO(OO)(OH)2]− tends to convert into [Nb(O-O)2(OOH)2]− thermodynamically in the presence of H2O2. For further identification of [Nb(O-O)2(OOH)2]−, the IR vibration spectra of the above three anions were simulated and compared. We find that the simulated IR spectra of [Nb(O-
[NbO(O‐O)(OH)2 ]− + 3H 2O2 → [Nb(O‐O)m (OOH)4 − m ](m − 1) − + (m − 2)H+ + 3H 2O
ΔG/eV
(1)
To obtain a quantitative estimation and identification of this Nb anion, the structures of these two peroxoniobate anions together with [Nb(O-O)4]3− were optimized by virtue of first3357
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ACS Catalysis O)2(OOH)2]− are basically in accord with the experimental IR spectra (Figure S4 in the Supporting Information), despite some known numerical errors with DFT accuracy. Specifically, as shown in Figure S4, the two typical characteristic peaks (862 and 875 cm−1) attributed to the stretching of peroxo bonds (OO) and O−OH are similar to those of experimental IR spectra (820 and 853 cm−1, Figure 4b). In contrast, [Nb(O-O)4]3− and [Nb(O-O)3(OOH)]2− exhibit only one characteristic peak at 820 and 852 cm−1, respectively, resulting from the vibration of the peroxo group (Figure S4). Therefore, according to the above thermodynamic and IR analysis, it could be safely believed that [Nb(O-O)2(OOH)2]− (Figure 5) constitutes the characterized Nb species resulting from the conversion of [NbO(O-O)(OH)2]−.
Table 2. Solvent-Free Epoxidation of 3-Methyl-2-buten-1-ol with Different Catalystsa sel/% entry
catalyst
conversn/%b
epoxide
diol
1 2 3 4 5 6 7 8
none (NH4)3[Nb(O2)4] TPAIL TBAIL THAIL (TBA)2W2O11 K2W2O11 TBAILc
0.7 47.1 91.6 96.4 97.1 59.4 66.8 0.3
≥99 ≥99 ≥99 ≥99 ≥99 ≥99 ≥99 ≥99
