Article pubs.acs.org/Organometallics
Direct Synthesis of Imidazolinium Salt on Magnetic Nanoparticles and Its Palladium Complex Application in the Heck Reaction Agnieszka Z. Wilczewska* and Iwona Misztalewska Institute of Chemistry, University of Bialystok, Hurtowa 1, 15-399 Bialystok, Poland S Supporting Information *
ABSTRACT: Synthesis of magnetically separable imidazolinium salts (Nheterocyclic carbene precursor) from amine-terminated magnetic nanoparticles by a direct “grafting from” three-step approach was achieved. Purification after each step of the synthesis was easily performed using magnetic separation of products from the reaction mixtures. Next, the magnetic imidazolinium salts were used as ligands in transition-metal complexes. The catalytic properties of nanoparticles−Pd complexes in the Heck cross-coupling reaction were tested. In the majority of tested reactions, the catalyst allowed obtaining styrene derivatives in high yields. It was proven that the catalysts can be reused several times without loss of their activity.
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INTRODUCTION The N-heterocyclic carbenes (NHCs) are widely used in organocatalysis1 also as excellent ligands in transition-metal complexes.2 They successfully replaced phosphines in this area because of their lower toxicity and better electron-donating properties.3 Some of the NHCs are crystalline species, stable for several weeks at the inert atmosphere.4 The five-membered NHC are mainly formed by deprotonation of azolium salts, e.g., imidazolium or imidazolinium salts.5 The stabilization of the carbene center in the 2-position in the imidazolin-2-ylidenes (NHC formed from imidazolium salt) is caused by the electron-rich imidazole nucleus,5,6 whereas the stabilization of imidazolidin-2-ylidenes (NHC formed from imidazolinium salt) is based on the inductive and the mesomeric effects that cause N1-C2-N3 σ delocalization.5,7 Both of them can form transition-metal complexes and can be utilized as catalysts in a wide range of reactions,8 e.g., palladium complexes in the crosscoupling reactions.9 According to the green chemistry principles,10 the “green” catalyst is the one that can be simply removed from the reaction mixture and reused. In this field, magnetic nanoparticles have been the focus of great attention as a magnetically separable matrix for catalysts.11 Several approaches for preparation of palladium catalysts on a magnetically retrievable phase were used. Chemical adsorption of palladium salts on the MNP surface with subsequent reduction and formation of palladium nanoparticles is one of them.12 Another one is anchoring of palladium compounds on the magnetic nanoparticles’ surface by complexing agents grafted onto MNP, e.g., phosphines,13 amines,14 or imidazolium salts.15 The immobilization of the Pd−imidazolium complexes on the MNP surface was also applied.16 © XXXX American Chemical Society
The NHC precursorsimidazolium saltsimmobilization on the solid phase17 was achieved in three ways. The first one relied on the formation of a silane shell with a terminal imidazole moiety.18 In the second one, the creation of a silane shell (with chlorine-terminated alkyl groups) was followed by alkylation of the nitrogen atom of the imidazole ring.15b,19 Another one applies polymerization reaction of vinyl imidazole as a way to immobilize an NHC precursor on the magnetic nanoparticles’ surface.20 All of them have led to obtaining imidazolium salts on the magnetic surface. Such magnetically immobilized salts were utilized as catalysts (or ligands for catalysts) in a variety of reactions, e.g., C−C cross coupling reactions,15b,18,19a,20b oxidation of alkenes,19c enantioselective allylation,19b and synthesis of 1-amidoalkyl-2-naphthols,19d αaminophosphonates,19e and 4H-benzo[b]pyrans.20a To the best of our knowledge, the “grafting from” immobilization of imidazolinium salts on magnetic nanoparticles, its complexation with palladium ions, and application in the Heck reaction, considered in this paper, are presented for the first time.
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RESULTS AND DISCUSSION In this paper, we report the use of magnetic nanoparticles (MNPs) with an aminosilane shell (MNP@APTMS)21 as a starting material in the synthesis of saturated imidazolium salt (SIS), i.e., imidazolinium salt. We have chosen imidazolinium salt as a precursor for the NHC ligand because of the stronger electron-donating properties of imidazolidin-2-ylidenes in comparison to imidazolin-2-ylidenes.22 The magnetic phase was prepared according to the Massart’s method23 based on Received: May 13, 2014
A
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SIS, the peak at 1654 cm−1 indicates the existence of C−H bonds in the precarbenic center. Our further research was focused on the N-heterocyclic carbene palladium acetate complex (MNP@NHC-Pd) (Figure 3).
precipitation of magnetite nanoparticles from the mixture of iron(III) chloride and iron(II) chloride by ammonia (25% solution in water). The silane shell was obtained by condensation reaction of APTMS ((3-aminopropyl)trimethoxysilane) on the MNP surface. Next, a three-step synthetic path24 (Figure 1), including formation of a formamidinium unit, led to obtaining a magnetic N-heterocyclic carbene precursor (MNP@SIS).
Figure 3. Synthesis of MNP@NHC-Pd (6).
The catalyst was obtained according to the known method of preparation of the NHC-Pd complex from imidazolium salts.18,19b The nanoparticles 6 were prepared in reaction with palladium acetate and sodium carbonate as a base. The MNP@ NHC-Pd complex was characterized via ATR-FT IR spectroscopy. In Figure 4, the MNP@NHC-Pd (6) and Pd(OAc)2
Figure 1. Synthesis of NHC ligands on magnetic nanoparticles: (1) MNP - Fe3O4 magnetic nanoparticle; (2) MNP@APTMS; (3) MNP@NOEt; (4) MNP@NCH-NH-C8H9; (5) MNP@SIS.
The formamidinium unit (4) was prepared by condensation reaction of an amine-terminated MNP with triethyl orthoformate catalyzed by formic acid, followed by reaction with 2,6dimethylaniline. In the alkylation reaction of nitrogen atoms with 1,2-dichloroethane, and with DIPEA as a base, the final product was obtained. Each step products were magnetically separated from the reaction mixtures (NdFeB magnet) and washed several times with appropriate solvents. The number of reactive −NH2 groups on the MNP@APTMS surface estimated by acid−base titration was 0.42 mmol per 1 g of nanoparticles.21 All products were analyzed via ATR-FT IR (attenuated total reflectance-Fourier transform infrared spectroscopy) (Figure 2). In the spectra, the characteristic peaks at
Figure 4. ATR-FT IR spectra of MNP@NHC-Pd and Pd(OAc)2.
infrared spectra are presented. The absence of C−H bond signal stretching at the MNP@NHC-Pd spectrum (in comparison to the MNP@SIS spectrum) indicates successful modification of the NHC precursor. Additionally, several signals characteristic of vibrations of acetic group bonds were detected (1645, 1458, 685 cm−1). TEM/EDX (transmission electron microscopy/energy-dispersive X-ray) investigations were carried out to characterize the MNP@SIS and MNP@NHC-Pd particles. Figure 5 shows the TEM images of the starting material (MNP@APTMS) and its palladium complex MNP@NHC-Pd. The images clearly show the silane shell that surrounds the particles. The
Figure 2. ATR-FTIR spectra of magnetic nanoparticles after each step of the synthesis.
around 560 and 1100 cm−1 can be assigned to Fe−O and Si−O bonds stretching. In the spectrum of formamidate (MNP@N CH-OEt), the signal of CN bond stretching (1593 cm−1) occurred. Furthermore, the presence of characteristic peaks of C−H aromatic bond stretching (1214, 1152 cm−1) also confirms formamidine formation. In the spectrum of MNP@
Figure 5. TEM images of starting material MNP@APTMS (on the left) and MNP@NHC-Pd (on the right). B
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formation reaction: the Heck cross-coupling reaction (Figure 7).
TEM/EDX quantification of the NHC-palladium modified MNP confirmed the presence of all expected elements (Fe, Pd, O, Si), and the determined Pd content was 6 wt %. The formation of agglomerated palladium black nanoparticles on iron oxide nanoparticles has not been observed. The DSC (differential scanning calorimetry) and TGA (thermogravimetric analysis) for MNP@APTMS, MNP@N CH-OEt, MNP@NCH-NH-C8H9, MNP@SIS, and MNP@ NHC-Pd nanoparticles were performed. The TGA, DTG, and DSC curves are presented in Figure 6. The weight of the
Figure 7. Heck cross-coupling reaction.
The Heck reaction involved an aryl halogen (0.2 mmol), vinyl compound (0.1 mmol), and NaHCO3 (0.2 mmol) in DMF in the presence of MNP@NHC-Pd complex 6 (0.56 mol % Pd) at 140 °C (Table 1). It can be noted (Table 1) that the MNP@NHC-Pd catalyzed Heck cross-coupling reactions resulted in various yields (in most cases, very high). In comparison to the known activity of palladium catalysts immobilized on a magnetic surface,25 synthesized catalyst 6 in the majority of reactions presents comparable or even higher activity (4-(n-butyl acrylate)acetophenon - 82%). For example, for the reaction 1 and 2 (Table 1) with the palladium−phosphine complex immobilized on MNP as catalyst,13a the reported yield was 95% and 97%, respectively (catalyst 6 - 96% and 86%). The 4-nitro-nbutylcinnamate with the palladium−DABCO complex catalyst supported on γ-Fe2O3 magnetic nanoparticles14 was obtained with 84% yield (catalyst 6 - 72%). However, it is worth mentioning that the amount of catalyst used for the presented reactions was twice lower than that used in cited papers. The products of other reactions (entries 3−5, 7, 8, 10, Table 1) obtained by Heck cross-coupling reaction with magnetically recoverable catalysts are presented for the first time. For comparison of complex 6 and Pd(OAc)2 catalytical activities, reaction of iodobenzene and butyl acrylate was performed in equal conditions and the same amounts of palladium (0.58 mol % Pd, 0.012 mmol of Pd(OAc)2). The yield of the reaction carried out with palladium(II) acetate was significantly lower than that with MNP@NHC-Pd, 63% to 86%, respectively. Additionally, the catalyst 6 was easily separated from the reaction mixture by using an external magnetic field. The reactivity of the MNP@NHC-Pd catalyst was also compared to the reactivity of the MNP@APTMS-Pd complex prepared directly by the complexation of Pd(II) by amine groups present on the magnetic nanoparticles’ surface. This complex was prepared according to the method previously mentioned for catalyst 6 (Pd(OAc)2 with Na2CO3). It was revealed that the MNP@APTMS-Pd complex is also active in the Heck reaction of iodobenzene with butyl acrylate. However, the expected product was obtained with poor yield (less than 30%). Moreover, this catalyst cannot be reused without loss of activity, which was also investigated. The measured recovery of catalyst, after each cycle, was more than 80%. Then, the recovered catalyst was washed with diethyl ether, water, and ethanol to remove any adsorbed starting material, product or byproducts, e.g., iodine. After washing, MNP@NHC-Pd was successfully reused at least five times without any loss of activity (Table 2). Recyclability of the catalyst was investigated in reaction of iodobenzene with butyl acrylate. The TEM and FT IR analyses of recovered catalyst were done and are presented in the Supporting Information. No changes in catalyst structure were observed. Atomic absorption spectrometry investigations of palladium concen-
Figure 6. TGA, DTG, and DSC (at the bottom) curves of MNP@ APTMS, MNP@NCH-OEt, MNP@NCH-NH-C8H9, MNP@ SIS, and MNP@NHC-Pd.
organic shell (TGA curves) increases after each step of the synthesis from 10, 16, 19 to 21% for MNP@APTMS, MNP@ NCH-OEt, MNP@NCH-NH-C8H9, and MNP@SIS, respectively, which confirms the modifications of the magnetic nanoparticles’ surface. In the thermogram of aminosilane-coated nanoparticles 1, a broad degradation region of the methyl and aminopropyl groups between 200 and 730 °C is observed. In the case of modified nanoparticles 2−5, decomposition reach temperature of 880 °C. The TGA curves of MNP@NHC-Pd show a total weight loss of about 17%. The lower weight loss observed on the TGA curve of the complex compared to MNP@SIS (21%) is associated with the palladium residuals. The TGA/DTG curves present a two-stage weight loss in the temperature ranges at 100−460 °C and 460−880 °C. Differences in the shape of DSC heating curves of modified nanoparticles confirm changes in the chemical nature of the coating. The obtained MNP@NHC-Pd catalyst was air- and moisture-stable and was tested in a carbon−carbon bond C
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Table 1. Heck Couplings of Various Aryl Halides and Vinyl Compounds under MNP@NHC-Pd (Figure 7)a,b,c,d
a
The isolated yield. bReaction was carried out using 1.0 equiv of diiodobenzene, 1.7 equiv of butyl acrylate, and 5.0 equiv of NaHCO3. cReaction was carried out using 4.0 equiv of iodobenzene, 1.0 equiv of divinylbenzene, and 6.0 equiv of NaHCO3. dLiterature references related to palladium catalysts immobilized on magnetically separable phase.
The significant decrease of product yield in the sixth reuse of the catalyst is probably caused by intoxication of the catalyst by byproducts (e.g., iodine) generated during previous reactions.
tration in the reaction mixture after separation of catalyst (external magnetic field) were performed. After the first usage of catalyst, in the residues, only 3% of palladium was found (according to the amount of palladium present in the catalyst). The amount of free palladium leaching in reaction mixtures decreases after each usage of the catalyst to finally 0.15% in fourth and successive reuses.
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CONCLUSIONS In summary, this work presents the first example of the application of aminosilane-coated magnetic nanoparticles to the synthesis of NHC precursors directly from their surface. The D
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Synthesis of Nanoparticles 2. With the purpose of obtaining magnetic nanoparticles with an ultrathin aminosilane shell, 1 g of coated (with oleic acid) magnetic nanoparticles 1 was ultrasonically dispersed in 800 mL of EtOH and then diluted with 1200 mL of ethanol. The mixture was placed in a 2 L reactor and stirred for 15 min under an argon atmosphere, and then 4 mL of concentrated ammonia was added. After 5 min of stirring, 0.5 g of APTMS was added dropwise into the particle suspension, and then the suspension was vigorously stirred for 4 h at room temperature. After magnetic separation, 860 mg of product was obtained. ATR-IR, cm−1: 3389, 2928, 1557, 1452, 1006, 568. Synthesis of Nanoparticles 3. Silane-coated MNPs 2 (400 mg) were ultrasonically dispersed in triethylorthoformate26 (200 mL), and then several drops of formic acid were added and the mixture was refluxed over 20 h. The 428 mg of product was obtained. ATR-IR, cm−1: 3364, 2925, 1593, 1382, 1204, 1048, 564. Synthesis of Nanoparticles 4. MNPs 3 (428 mg) were ultrasonically dispersed in 2,6-dimethylaniline26 (20 mL) and then stirred at 100 °C over 20 h. After magnetic separation, 405 mg of product was obtained. ATR-IR, cm−1: 3372, 2926, 1594, 1383, 1214, 1152, 565. Synthesis of Nanoparticles 5. MNPs 4 (400 mg) were ultrasonically dispersed in 1,2-dichloroethane (100 mL), and then N-ethyl-N,N-diisopropylamine27 (3.1 mmol, 400 mg) was added. The mixture was refluxed over 20 h. After magnetic separation, 393 mg of product was obtained. ATR-IR, cm−1: 3382, 2924, 1653, 1209, 1152, 551. Preparation of Catalyst 6. MNPs 5 (100 mg) were ultrasonically dispersed in DMF (4 mL); next, Pd(OAc)2 (0.18 mmol, 40 mg) and 4 mL of 1% aqueous solution of Na2CO3 were added. The mixture was stirred over 18 h at 50 °C. After magnetic isolation, 108 mg of product was obtained. ATR-IR, cm−1: 3311, 2971, 2923, 1645, 1458, 1331, 1077, 685, 629, 551. General Procedure for the Heck Reaction. The 10 mg (0.56 mol % Pd) of catalyst 6 was ultrasonically dispersed in freshly distilled DMF (1 mL). Then, NaHCO3 (0.2 mmol), aryl halogen (0.2 mmol), and vinyl compound (0.2 mmol) was added. The mixture was stirred under reflux over varied times (3 h for iodoarenes, 22 h for bromoarenes, and 48 h for chlorobenzene). The mixture was cooled, and the catalyst was separated from the reaction mixture using an external magnet. The mixture was extracted with diethyl ether (3 times), and organic layers were collected and dried under anhydrous Na2SO4. Subsequently, the solvent was evaporated and the residue was purified on silica gel (eluent - hexane) or by crystallization (from hexane or hexane:dichloromethane). The products were obtained with various yields (2−96%).
Table 2. Reusability of Catalyst 6 in the Heck Reaction of Iodobenzene and Butyl Acrylate no. of cycle
yield [%]
1 2 3 4 5 6
86 90 87 86 85 15
separation and purification of products were easily performed by an external magnetic field. Finally, the N-heterocyclic carbene palladium acetate complex was formed and tested in the Heck cross-coupling reactions of various aryl halides, and vinyl compounds. The products were obtained with high yields (the isolated yield). Such a catalyst can be easily removed from the reaction mixture and reused in the next reaction (at least five times without loss of activity). Furthermore, this synthetic method can be applied to the preparation of other Nheterocyclic carbene precursors (e.g., chiral compounds) anchored to magnetic nanoparticles. Nanoparticles can be used for the preparation of other NHC−transition-metal complexes, widely applied as catalysts in organometallic chemistry.
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EXPERIMENTAL SECTION
General Remarks. All reagents were purchased from Aldrich Chemical Co. and used as received. N,N-Dimethylformamide and dichloroethane were purchased from POCH S.A. and were distilled before use. Concentrated ammonia and concentrated formic acid were purchased from POCH S.A. The formation of magnetic nanoparticles, particle size, and morphology were confirmed by transmission electron microscopy (TEM). Energy-dispersive X-ray spectroscopy (EDX) analyses were collected from the samples imaged by TEM. Samples for TEM were prepared on holey carbon cooper grids. Surface modifications were confirmed by ATR-FT IR. Thermogravimetric analysis (TGA) was performed in the temperature range of 50−900 °C (10 °C/min). Differential scanning calorimetry (DSC) was performed in the temperature range of 50−480 °C (10 °C/min). Nitrogen was used as a purge gas (10 mL·min−1). The 1H NMR spectra were recorded on a 400 MHz spectrometer. An atomic absorption spectrometer equipped with an end-heated electrothermal atomizer (ETAAS) was used for the Pd determination. The Pd hollow cathode lamp was operated at a current of 8 mA. The integrated absorbance signal of Pd was measured at 247.6 nm with a spectral bandpass of 0.5 nm. The following optimized atomizer heating program was used for the Pd determination: drying at 100 °C for 30 s, ashing at 350 °C for 10 s and at 1100 °C for 10 s, and atomization at 2200 °C for 3 s. General Procedure for Preparing MNPs 1−6. Magnetic nanoparticles were separated from the reaction mixture by putting them into an external magnetic field and then were washed with water (products 1 and 6) and several times (minimum three) with ethanol and then dried under reduced pressure in 60 °C over 1 h. Synthesis of Magnetic Nanoparticles Coated with Oleic Acid 1. Magnetic nanoparticles were prepared according to the Massart’s method.23 The solutions of 2.15 g (10.8 mmol) of FeCl2·4H2O and 5.8 g (21.4 mmol) of FeCl3·6H2Oboth in 200 mL of deionic and deoxidaized waterwere mixed and heated to 80 °C on the ultrasonic bath. Afterward, concentrated ammonia was added until a pH ∼ 11 was obtained. After precipitation of MNP by ammonia, 4 g of oleic acid was added and the mixture was ultrasonically treated for 30 min. The 2.7 g of product was obtained after isolation. ATR-IR, cm−1: 3318, 2918, 1516, 1405, 560.
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ASSOCIATED CONTENT
S Supporting Information *
Characterization data for the Heck reaction products and NMR spectra, additional TEM images, and TEM/EDX results. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +48 85 745 75 88 (A.Z.W.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to Ms. E. Zambrzycka for ETAAS. This work was supported by funds from the Polish National Centre of Sciences, project no. NCN-2011/03/B/ST5/02691. E
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(20) (a) Pourjavadi, A.; Hosseini, S. H.; Amin, S. S. Chem.Eng. J. 2014, 247, 85−92. (b) Mohsen, E.; Jaber, J.; Mehdi, M. A.; Fatemeh, N. D. J. Iran. Chem. Soc. 2014, 11, 499−510. (21) Wilczewska, A. Z.; Markiewicz, K. H. Macromol. Chem. Phys. 2014, 215, 190−197. (22) Glorius, F., Ed. N-Heterocyclic Carbenes in Transition Metal Catalysis; Springer: Berlin, 2007; Vol. 21, pp 1−20. (23) Massart, R. IEEE Trans. Magn. 1981, 17, 1247−1248. (24) Misztalewska, I.; Wilczewska, A. Z. Patent aplication no. P.405339, Sept. 13, 2013. (25) Molnár, A. Chem. Rev. 2011, 111, 2251−2320. (26) Binobaid, A.; Iglesias, M.; Beetstra, D. J.; Kariuki, B.; Dervisi, A.; Fallis, I. A.; Cavell, K. J. Dalton Trans. 2009, 7099−7112. (27) Kuhn, K. M.; Grubbs, R. H. Org. Lett. 2008, 10, 2075−2077.
Analyses were performed in the Centre of Synthesis and Analysis BioNanoTechno of University of Bialystok.The equipment in the Centre of Synthesis and Analysis BioNanoTechno of University of Bialystok was funded by EU, as a part of the Operational Program Development of Eastern Poland 2007-2013, project: POPW.01.03.00-20-034/ 09-00.
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