Yingxiao Zongab,
Junke Wang*b,
Yubi Heb,
Guoren Yueb,
Xicun Wang‡
*a and
Yi Panc
aGansu Key Laboratory of Polymer Materials, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
bKey Laboratory of Hexi Corridor Resources Utilization of Gansu, College of Chemistry and Chemical Engineering, Hexi University, Zhangye 734000, China. E-mail: wangxicun@nwnu.edu.cn; wangjk@hxu.edu.cn; Fax: +86-936-8287080
cCollege of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
First published on 5th September 2016
A water soluble and efficient ionic compound-supported palladium complex was prepared. The structure and composition of the complex was characterized by FT-IR, NMR, ICP-OES and mass spectrometry. The complex exhibits high catalytic activity for Suzuki–Miyaura reactions in aqueous media. Moreover, the catalyst shows outstanding stability and reusability, and it can be recovered simply and effectively and reused six times without much activity decrease.
From the standpoint of green chemistry, water is known to be a potential replacement for organic solvents.19–21 The major advantages associated with water are its low cost, non-flammability and low toxicity, and there is no need to desiccate substrates prior to the reaction.22 Combining the use of water as a solvent with hydrophilic metal catalysts results in easy separation and recycling of the catalyst from the product. In general, it has been observed that neutral catalysts are not retained in water and are lost during product isolation. Therefore, an effective approach is to use metal complexes with charged groups to avoid catalyst leaching into the organic layer.23
Presently, ionic liquid chemistry is getting into the epoch-making development, and has been attracting significant attention as alternative solvents and catalysts for chemical reactions because of low melting point, negligible vapour pressure, non-flammability, high conductivity, good solubility, and potential recyclability.24 The functionalized ionic liquids formed through the incorporation of additional functional groups, has recently gained remarkable interest.25 Some ligands supported to imidazolium-based ionic liquids have been reported, however, these needed to be dissolved in another ionic liquid for catalytic applications (Fig. 1).26,27 SiO2-supported imidazolium ionic liquid-immobilized PdEDTA has been reported, and is an efficient and reusable catalyst for the Suzuki reaction in water.28 These catalysts can strongly retained, when the extraction of product was carried out.
Metal complexes of Schiff bases are widely applied in the fields of synthesis and catalysis.29 Although ionic liquid supported Schiff base with hexafluorophosphate as ligand can promote the Pd-catalyzed Suzuki–Miyaura coupling reaction, this reaction proceed in organic solvent and under reflux.30 Based on the extensive research focused on the modification of Schiff base ligands and our previous work on the self-assembled catalyst,31,32 we became interested in the synthesis of water soluble ionic compound-supported metal complexes. Herein, we report an efficient, water soluble and reusable, ionic compound-supported palladium complex catalyst, and investigate the catalytic properties for Suzuki–Miyaura coupling reactions in aqueous media (Fig. 2).
As illustrated in Scheme 1, the synthesis of the imidazolium ionic compound-supported Schiff base is a straightforward process starting from 5-chloromethylsalicylaldehyde, which are prepared from the concentrated hydrochloric acid, poly formaldehyde and salicylic aldehyde. 5-Chloromethylsalicylaldehyde was treated with 1-methylimidazole resulted in the imidazolium ionic compound-supported salicylic aldehyde 1 in almost quantitative yield. This intermediate can be used to implement the condensation of 1 with 4-methylaniline in ethanol to give the imidazolium ionic compound-supported Schiff base 2 in 97% yield. In the 1H NMR spectrum of 2, a peak representing the azomethine proton (CHN) appeared at δ 9.23, and the hydroxyl proton resonated at δ 13.20 (Fig. 3). The FT-IR spectrum of 2 exhibited absorption bands at 1630 cm−1 (C
N stretch) and 3410 cm−1 (OH stretch) (Fig. 4). Peaks corresponding to other protons were also shown. A peak at m/z 306.16 in the HR-MS spectrum of 2 corresponding to the [M − Cl]+ ion further confirmed the structure of 2.
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Fig. 4 Comparison between the FT-IR of imidazolium ionic compound-supported Schiff base and the imidazolium compound-supported palladium Schiff base complex. |
Subsequently, we focus on the synthesis of the imidazolium ionic compound-supported palladium complex 3. Pd(OAc)2 was added to the solution of the imidazolium ionic compound-supported Schiff base 2 in methanol, and then a pale yellow powder was obtained as the catalyst 3. After washing with solvent (methanol) thoroughly and drying, the solid remains pale yellow. However, the imidazolium ionic compound-supported palladium Schiff base complex 3 between the ligand and palladium acetate was synthesized without using any deprotonating base. The compound has a high melting point of above 200 °C.
The structure of the synthesized palladium complex 3 was characterized using different techniques such as IR, NMR and HR-MS. The 1H NMR spectral difference between the ligands and their metal complexes is one of the important methods for characterization of complexes in the literature.33,34 By the comparison of the 1H NMR of imidazolium ionic compound-supported Schiff base 2and the resulting Pd complex 3, the shift of the peak for the azomethine proton to δ 8.45 and the absence of a peak (δ 13.20) for the hydroxyl proton suggested that complete complexation with the metal had occurred through the nitrogen of the CN group and oxygen of the OH group (Fig. 3). This result is in accordance with our previous work.31,32,35 In the FT-IR spectrum of 3, the C
N peak was shifted to 1624 cm−1, and the peak in the region of 3410 cm−1 (OH stretch) in the spectrum of 2 disappeared. The HR-MS spectrum of 3 showed a peak at m/z 753.1833 that corresponds to the [M + 2H − Cl]+ ion. In order to further confirm the structure of the catalyst, ICP-OES was used to analyzed for Pd content (found: 13.46% calcd: 13.50). All of these facts indicated that the ligand had coordinated with the metal ion through the nitrogen of the C
N group (Fig. 4).36
Next, we concentrate on investigating the catalytic applications of 3 for SM reactions. In the beginning, we chose a model reaction between p-bromoanisole and phenylboronic acid. Since water solvent endows the reaction with green and safe properties,37,38 we focused on the use of water to replace organic solvents, and the results are summarized in Table 1. It was found that carrying out the reaction at r.t. with 0.05 mol% of 3 in the presence of K2CO3 gave the coupled product in 75% yield (Table 1, entry 1). The use of an organic base, such as triethylamine and pyridine resulted in a poor yield of the coupled product (Table 1, entry 5, 6). After both of the amounts of the catalyst and the reaction time were optimized, the best condition is the combination of catalyst of 0.05 mol% and reaction time of 4 h. It is noteworthy that the yield of coupled product was found to be approximately 92% when EtOH was used as the reaction medium and K2CO3 was used as base (Table 1, entry 13). However, under the same condition, the yield of the coupled product is 78% in pure DMF within 12 h (Table 1, entry 14). When the mixture of water and DMF was used as solvent, the yield can significantly be improved. This fact shows that water play an important role in this reaction (Table 1, entry 15).
Entry | Solvent | Catalyst (Pd mol%) | Base | Time (h) | Yieldb (%) |
---|---|---|---|---|---|
a Reaction conditions: the catalyst 3, 1 mmol of p-bromoanisole, 1.2 mmol of phenylboronic acid, 2 mmol of base, 5 mL of solvent, r.t. in air.b Isolated yield. | |||||
1 | H2O | 0.05 | K2CO3 | 2 | 75 |
2 | H2O | 0.05 | KOAc | 2 | 70 |
3 | H2O | 0.05 | Na2CO3 | 2 | 54 |
4 | H2O | 0.05 | NaOAc | 2 | 40 |
5 | H2O | 0.05 | TEA | 2 | 34 |
6 | H2O | 0.05 | Pyridine | 2 | 20 |
7 | H2O | 0 | K2CO3 | 2 | 0 |
8 | H2O | 0.01 | K2CO3 | 2 | 61 |
9 | H2O | 0.08 | K2CO3 | 2 | 85 |
10 | H2O | 0.10 | K2CO3 | 2 | 92 |
11 | H2O | 0.10 | K2CO3 | 4 | 98 |
12 | H2O | 0.10 | K2CO3 | 5 | 98 |
13 | EtOH | 0.10 | K2CO3 | 8 | 92 |
14 | DMF | 0.10 | K2CO3 | 12 | 78 |
15 | DMF/H2O (v/v, 1![]() ![]() |
0.10 | K2CO3 | 8 | 94 |
Based on the above investigations, the coupling of various aryl halides and arylboronic acids were investigated under the optimized conditions to investigate the general applicability of this protocol. As summarized in Table 2, it is found that the electronic effects of either coupling partner produced no significant impact on the reaction yields (Table 2, entries 1–10). These results suggested that the ionic compound-supported palladium complex 3 display the outstanding catalytic activities for the coupling reactions. However, ortho-substituted aryl bromides and arylboronic acids were coupled in very lower yields (Table 2, entry 11). This fact indicates that the steric hindrance of coupling partners has a definite influence on this coupling reaction. Even if the reaction time was extended, the yield was not improved. Highly sterically hindered di-ortho substituted biphenyls cannot be synthesized under the same conditions (Table 2, entry 12). In order to show the effect of the ligand in this reaction, we used Pd(OAc)2 as catalyst instead of the Pd complex 3, but the target compound can be obtained in lower yield. The results showed that in the absence of the ligand, the reaction was sluggish and only 22% yield was obtained (Table 2, entry 13). From these studies we can predict that this catalyst can be applied to a wide range of aryl iodides and bromides.
Entry | ArX | R | Time (h) | Yieldb (%) |
---|---|---|---|---|
a Reaction conditions: the catalyst 3 containing 0.10 mol% Pd, 1 mmol of aryl halide, 1.2 mmol arylboronic acid, 2 mmol of K2CO3, 5 mL of H2O, r.t. in air.b Isolated yield.c 1 mol% Pd was used.d Pd(OAc)2 used as catalyst. | ||||
1 | ![]() |
4-Me | 4 | 99 |
2 | ![]() |
4-Cl | 4 | 99 |
3 | ![]() |
H | 4 | 89 |
4 | ![]() |
4-Me | 4 | 97 |
5 | ![]() |
4-Cl | 4 | 96 |
6 | ![]() |
2-Cl | 4 | 96 |
7 | ![]() |
4-Cl | 4 | 88 |
8 | ![]() |
H | 4 | 98 |
9 | ![]() |
2-Cl | 4 | 97 |
10 | ![]() |
4-Me | 4 | 96 |
11 | ![]() |
H | 4 | Trace |
12 | ![]() |
H | 4 | 0 |
13 | ![]() |
H | 4 | 22d |
14 | ![]() |
H | 4 | 43(72c) |
15 | ![]() |
4-Me | 4 | 34(68c) |
16 | ![]() |
H | 2 | 98 |
17 | ![]() |
4-Cl | 2 | 96 |
Aryl chlorides are usually cheaper but less active than aryl bromides. Unfortunately, for coupling reactions of aryl chlorides, which are much more difficult to activate than aryl iodides and bromides, only low yields were obtained under the same conditions (Table 2, entries 14 and 15). When the reaction temperature was raised to 95 °C and the catalyst loading was increased to 1 mol%, the corresponding coupling products were obtained in moderate yields. However, the coupling of aryl iodides with phenylboronic acid can be completed within a shorter time (Table 2, entries 16 and 17).
In another investigation, the reusability of the catalyst was also evaluated in the coupling of methyl 4-bromobenzoate with phenylboronic acid. In detail, after the first run of the reaction was completed, ethyl acetate was directly added to extract the target compound, and the aqueous solution containing the catalyst was reused in the next cycle of the coupling reactions by mixing them with a new substrate and base. It is noteworthy that the catalytic activity was maintained at least until the six uses without much loss in activity (Fig. 5). To avoid deactivation of catalyst owing to the accumulation of potassium bromide, which makes the solution saturated, glycerol was used to wash potassium bromide after the second run was completed and the aqueous solution was concentrated under vacuum. The gradual decrease in the yield of the product during recycling could be mainly attributed to the leaching of the catalyst during extraction of the catalyst. The leaching of the catalyst could be detected by atomic absorption spectrometry (AAS) (Table 3). Pd complex species in the extraction was still the catalyst 3 confirmed by HR-MS. In order to confirm the stability of the Pd complex, the 1H NMR of the spent catalyst after six run were check. It was found that the used Pd complex catalyst can retain its structure, indicating a good stability and recyclable applicability for the Suzuki cross-coupling reaction in aqueous media (see ESI†).
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Fig. 5 Reusability of the imidazolium ionic compound-supported palladium complex in the Suzuki coupling of methyl 4-bromobenzoate with phenylboronic acid over 6 runs. |
Run | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
The leaching amount (μg) | 9 | 10 | 8 | 11 | 7 | 8 |
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19850c |
‡ Xicun Wang is first communication author. |
This journal is © The Royal Society of Chemistry 2016 |