Generation of uniform and small particle size of palladium onto the SH-decorated SBA-15 pore-walls: SBA-15/(SH)_XPd–NP_Y as a recoverable nanocatalyst for Suzuki–Miyaura coupling reaction in air and water

Abstract

Uniform and small particle size of palladium was generated onto the SH-decorated SBA-15 pore-walls. The physicochemical properties of the SBA-15/(SH)_XPd–NP_Y were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray analysis (EDAX), N₂ adsorption/desorption isotherms (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermal gravimetric analysis (TGA). The uniform distribution of Pd-NPs into SBA-15 pores was obtained by reduction of SBA-15/(SH)₂PdCl₂ using NaBH₄. The resulting nanocatalyst was an effective catalyst in aerobic Suzuki–Miyaura cross coupling reaction in excellent yields with negligible leaching of Pd-NPs even in water. The SBA-15/(SH)_XPd-NP_Y, after easy separation from the reaction mixture, was reused 11 times with high product yields.

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1. Introduction

Transition metal (TMs) complexes, clusters and nanoparticles are extremely useful metals on account of their physical or chemical properties.¹ TM-catalysis makes possible a great many selective and atom-economical transformations. From a green chemistry perspective, immobilization of TMs is one of the most important applied catalysis processes.² However, existing technologies have their limitations. Palladium as an important transition metal catalyses a broad range of cross-coupling C–C forming reactions. The Suzuki–Miyaura coupling reaction produces various curative drugs and pharmacological products; this reaction is often used as a benchmark reaction to estimate the activity of different Pd-based catalysts.^1–4 Traditionally, the coupling reaction has been performed using homogeneous palladium complexes in the presence of various ligands. These homogeneous catalysts show difficulty in separation of the expensive heavy metal catalyst from the reaction mixture and suffer from problems like aggregation of the metallic centers and contamination of the final reaction products.⁵ In addition, the requirement for an inert atmosphere, the use of stoichiometric or relatively expensive reagents and generally unstable and toxic ligands, such as phosphine, are required to activate and stabilize Pd against agglomeration and formation of Pd-black.^2,5 Therefore, efforts have been focused on immobilization of homogeneous Pd complexes onto different solid supports to facilitate their separation from the products. Various kinds of porous supports, such as mesoporous silica, zeolites, metal–organic frameworks (MOFs), and carbon nanotubes (CNT), have been reported.⁶ Because of their properties, SBA-15 ordered mesoporous inorganic materials with uniform pore diameters, large surface areas, tunable structures and modifiable functional groups on the surface of the channels provide an appropriate scaffold for the immobilization of Pd.⁷

In recent years the importance of choosing the right ligand for the success of a catalytic system has been studied.⁸ Fryxell et al.^8b and Pinnavaia and Mercier^8c have demonstrated scavenging abilities of the SH-functionalized mesoporous materials for heavy metal elimination, and Crudden et al.⁹ have showed the efficiency of Pd encapsulated SH-functionalized mesoporous silicates in the Suzuki–Miyaura cross-coupling reaction, catalyzed with no leaching of Pd. Herein, we report our work on impregnating palladium nanoparticles (Pd-NPs) into SH-functionalized channels of a SBA-15 nanoreactor (Scheme 1).

Scheme 1 Studied pathways of Suzuki coupling.

2. Experimental

2.1. Materials and apparatus

Pluronic P123 (EO₂₀PO₇₀EO₂₀), tetraethyl orthosilicate (TEOS), potassium carbonate, and other compounds were obtained from Aldrich and were used without further purification. Mercaptopropyl trimethoxysilane (MPTMS) was obtained from Fluka, and was used as received. Solvents were of certified A.C.S. grade and used as received, though DMF was deoxygenated with bubbling Ar (g) prior to use. X-ray diffraction (XRD) data were collected with a Bruker-D8 Advance X-ray diffractometer using CuKα radiation with the capillary model, operated at 40 kV and 30 mA. The nitrogen sorption was measured using a Tristar 3000 Micromeritics apparatus. Melting points were measured using an Electrothermal 9100 apparatus. ¹H and ¹³C NMR spectra were measured (CDCl₃) with a Bruker DRX-300 AVANCE spectrometer at 300 and 75 MHz, respectively. The contents of palladium on the prepared solid catalysts and Pd leaching were measured by atomic absorption spectroscopy using residual contents in the solvents.

2.1.1. Synthesis of mesoporous silica materials. SBA-15 was synthesized, as previously reported.^7d,9 Typically, 4 g of P123 was dissolved in acidic water (pH 2) before the addition of the silica source (9.5 mL TEOS). Condensation of the silica network about the polymer template proceeded for 20 h at 35 °C before being treated hydrothermally at 100 °C for 40 h in a sealed vessel. Surfactant was removed from the as-made material by calcination (1 °C min⁻¹ ramp, 600 °C for 10 h). Propyl thiols were tethered to the silica pore walls by dispersing the as synthesized SBA-15 in dry toluene, containing an excess of MPTMS and refluxing the mixture for 20 h at 100 °C (see ESI†). Sulfur loadings, which were determined by elemental analysis, were 0.7 mmol S g⁻¹ material. Sufficient PdCl₂ or Pd(OAc)₂ to afford a 2 : 1 S : Pd catalyst was then added as a solution in water, then after 1 hour the Cat. A₁ or Cat. A₂ was separated from the solutions. For our main goal, Pd(II) ions present in mesoporous silica matrix of Cat. A₁, was treated with NaBH₄ to produce the highly dispersed palladium nanocatalyst (SBA-15/(SH)_XPd-NP_Y as Cat. A₃). The elemental analysis indicated that the ratio of S : Pd was 3.7 : 1 (for catalyst abbreviations see Fig. 4).

Fig. 1 FTIR spectra of SBA-15 (red), SBA-15/SH (green), SBA-15/SH·PdCl₂ (brown) and SBA-15/SH·Pd-NPs (blue).

Fig. 2 SEM (a) and TEM (c) images and XRD pattern (d) of SBA-15. TEM (e) images of SBA-15/(SH)₂PdCl₂. SEM (b), TEM (f) images and particle size histogram (h) of SBA-15/(SH)_XPd-NP_Y nanocatalyst. TEM (j) images and particle size histogram (i) of recycled SBA-15/(SH)_XPd-NP_Y nanocatalyst.

Fig. 3 600 dpi in TIF format)??>SBA-15/(SH)_XPd-NP_Y nanocatalyst: TGA/DTG analysis (a), N₂ physisorption isotherms (brown) and EDAX patterns (b) and (c), respectively.

Fig. 4 Schematic representation of catalyst preparation (Cat. A₁–Cat. A₃).

2.1.2. Synthesis of biaryls 3. In a test tube equipped with a magnetic stirrer bar, the aryl halides 1 (1 mmol) was mixed with boronic acid 2 (1.2 mmol), K₂CO₃ (2 mmol), and the Pd-catalyst (0.5 mol% Pd) in 3 mL of 1 : 1 mixture of DMF–H₂O, in air. The reaction mixture was than heated to 80 °C. After completion of the reaction, the reaction mixture was cooled to room temperature, and the catalyst was removed by filtration. The catalyst was then washed with Et₂O (3 × 8 mL). The organic layer was separated and dried over anhydrous MgSO₄. The solvent was evaporated under reduced pressure to give the corresponding biaryl compounds (ESI†). As an example, 4-methylbiphenyl: ¹H NMR: 7.28–7.63 (9H), 2.44 (3H). ¹³C NMR: 141.18, 138.38, 137.03, 129.50, 128.73, 127.23, 127.18, 127.00, 21.11.

2.2. Material characterization

In the FT-IR spectrum of the SBA-15 family materials, the band from 799 and ∼1100 cm⁻¹ belonged to the vibrations of the (Si–O–Si) bond, and the small band at about 960 cm⁻¹ was assigned to the functionalized (Si–OH) bond and the SiO–H groups are represented by the very broad IR absorption band in the 3000–3700 cm⁻¹ region (Fig. 2). The presence of several bands with low intensity in the 1400 and 2900 cm⁻¹ regions is allocated to SH and its complex groups. The decrease of the small band in the 850–900 and 1600 cm⁻¹ bands, in the spectrum of the SBA-15, indicates the functionalization of the silica surfaces (Fig. 1).

Fig. 2 shows images of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of the as synthesized SBA-15 family sample, chosen as representative. The highly ordered mesoporous structure of SBA-15 observed by XRD (Fig. 2d) was confirmed by TEM images (Fig. 2c). Diffraction peaks the below 2° corresponding to the (1 0 0), (1 1 0), and (2 0 0) are readily recognized from the XRD pattern of SBA-15 (Fig. 2d). The observed diffraction peaks agree with the 2D-hexagonal SBA-15 family.^8,9

In fact, Fig. 2c shows the long-chain structure characteristic of the SBA-15 materials as it can be observed by the presence of straight mesochannels arraying along the long axis; moreover, the hexagonal arrangement of the unidirectional mesopores is very clear in Fig. 2c, where a frontal view of them is presented. It is important to note, that even after the loading of Pd ions and even reduction of the Pd to nanoparticles, the highly ordered structure of the SBA-15 support was retained (Fig. 2e–j). The average mesopore size estimated by TEM is around 7 nm. Transmission electron microscopy (TEM) images of Cat. A₃ showed that Pd nanoparticles were well dispersed inside the nanochannels of functionalized SBA-15 (Fig. 2f). The particle size histogram of SBA-15/(SH)_XPd-NP_Y shows that particle size ranged from 2 nm to 6 nm (Fig. 2h). For this sample, after the 11^th recycle, segregated Pd-nanoparticles are also observed on the external surface of the silicate and out of pores as it is evidenced by the darker areas in image j in Fig. 2 (Fig. 2j). The particle size histogram of recycled SBA-15/(SH)_XPd-NP_Y and particle size range are indicated in Fig. 2i.

To examine the thermal stability of the SBA-15/(SH)_XPd-NP_Y nanocatalyst, thermal gravimetric (TG) were carried out between 30 °C and 800 °C in a static atmosphere of nitrogen (Fig. 3a). TGA/DTG analysis of the SBA-15/(SH)_XPd-NP_Y nanocatalyst shows a weight loss due to the desorption of water below 100 °C followed by a second weight loss centered at 140 °C, which is due to the loss of inner water as well as the loss of CH₃OH upon further condensation of the unreacted methoxy groups. This is finally followed by a set of weight losses centered at 380 °C corresponding to the elimination of the surface bound organic propyl thiol groups (PrSH), which indicates that this nanocatalyst (Cat. A₃) is thermally stable up to 380 °C. For the as synthesized SBA-15 and SBA-15/(SH)_XPd-NP_Y the N₂ physisorption (Fig. 3b) showed type IV in nature with an H1 hysteresis loop at p/p_o = 0.6–0.9, which is typical for mesoporous solids, according to the IUPAC classification.¹⁰ The adsorption branch of each isotherm showed a sharp inflection at a relative pressure value of about 0.7, which was particular to capillary condensation within uniform pores.¹⁰ The position of the inflection point was clearly related to the diameter in the mesopore scope. The BET surface area for SBA-15/(SH)_XPd-NP_Y was 341 m² g⁻¹. EDAX was performed to further confirm the composition of the as-prepared products (Fig. 3c). Fig. 3c shows a sample of the SBA-15/(SH)_XPd-NP_Y nanocatalyst that is composed of Si, O, and Pd.

3. Results and discussion

Based on our preliminary work on the synthesis of mesoporous silica materials as catalysts,^7,11 mercapto groups (–SH) were first grafted onto the mesoporous silica support. The SBA-15/SH was then loaded with Pd nanoparticles with –SH serving as anchor points. Finally, the SBA-15/(SH)_XPd-NP_Y nanocatalysts were assessed for the Suzuki–Miyaura C–C cross coupling reaction. In fact, here, we present the results of our study on the efficiency of mercapto-stabilized small sized palladium nanoparticles embedded onto the pore wall of functionalized SBA-15 mesoporous nanoreactors as a highly efficient and green method for the synthesis of biaryls.

The preparation procedure of the nanocatalyst (SBA-15/(SH)_XPd-NP_Y) is outlined in Fig. 4. The synthesis of the catalyst has been achieved in four main steps: in the first step we prepared the SBA-15 using a known procedure described by Zhao et al.,^6c then in the second step, thiol functionalization of the SBA-15 occurred by condensation of mercaptopropyl trimethoxysilane with surface silanols. In next two steps, adsorption of the palladium sources [Pd(OAc)₂ and PdCl₂] with the thiol groups and subsequently reduction of immobilized Pd^II with the MeOH/NaBH₄ system occurred, which lead to the production of highly dispersed Pd nanoparticles. The obtained SBA-15/(SH)_XPd-NP_Y nanocatalysts were investigated under both the Pd^II-anchored pre-catalyst (Cat. A₁ and Cat. A₂) and Pd-NPs pre-catalyst (Cat. A₃ by reduction of Cat. A₁) under Suzuki coupling reaction conditions (Fig. 4).

Different types of Pd@SBA-15/SH were investigated under both the Pd-anchored pre-catalyst (method I) and directly as ligands (method II) in Suzuki coupling reaction conditions (Table 1). For catalytically activity and efficiency purposes, 4-bromoacetophenone and phenylboronic acid were chosen as substrates in the coupling model reaction of the Suzuki reaction. In the model reaction, to obtain the cross coupled product 3a, we tested the reaction using catalyst A₁–A₃.

Table 1 Different catalysts in the model reaction of Suzuki coupling^a

Catalyst	Method (I or II)	[Pd]	Mol (%)Pd	S : Pd ratio	(%) Pd-leaching	(%) Yield (3a)
a Reaction scale: 1 mmol of substrates. K2CO3 (2 eq.), 80 °C, 60 min, DMF–H2O (1 : 1, 4 mL).b Pd-black. Reaction progress for the catalysts (a) and different amount of the Pd of SBA-15/(SH)XPd-NPY in reaction condition (b).
Cat. A1	Method I	SBA-15/SH·PdCl2	0.5	2 : 1	—	98
	Method II	PdCl2 + SBA-15/SH	0.5	2 : 1	0.61 + Pd-BL^b	99
Cat. A2	Method I	SBA-15/SH·Pd(OAc)2	0.5	2 : 1	0.09	94
	Method II	Pd(OAc)2 + SBA-15/SH	0.5	2 : 1	0.61 + Pd-BL^b	99
Cat. A3	—	SBA-15/(SH)XPd-NPY	0.05	3.7 : 1	0.05	96

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As shown in Table 1, the use of Pd@SBA-15/SH catalysts with different source of Pd (Table 1, catalysts A₁, A₂, A₃) for method II of Cat. A₁ and Cat. A₂ led to the desired product, but in this reaction both the homocoupling product 4 and Pd-black were also detected. Performing the reaction with method I resulted in the production of 3a in high yields without side products and Pd-black, but the amount of catalysts was 1 mol%. Performing the reaction with Pd-NPs embedded SBA-15/SH just with 0.05 mol% of Pd resulted in the production of 3a in high yields without any side products. These results shows the high efficiency of the SBA-15/(SH)_XPd-NP_Y catalyst (Cat. A₃) in both stability and activity (96% yield of product 3a with negligible leached Pd).

For optimization of the reaction conditions, the effect of various parameters, such as different solvents, bases, time and Pd mol% in the presence of Cat. A₃, was investigated (model reaction). As shown in Table 2, the best results were obtained using DMF–H₂O (1 : 1, 2 mL) as the solvent, and K₂CO₃ as base with 0.05 mol% Pd during 0.5 hour (Table 2, entry 7). For comparison, the model reaction was tested under the same conditions but with Cs₂CO₃ as base, the reaction was completed in 25 min with 97% yield of 3a, without any side products (Table 2, entry 13), but because of availability and price we selected potassium carbonate for future studies.

Table 2 Optimization of model reaction over the Cat. A₃ pre-catalyst^a

Entry	Base	Solvent	Mol (%) Pd	Temp (°C)	Time (min)	Yield (%)^b
a Reaction conditions: 1 mmol 4-bromoacetophenone, 1.2 mmol phenylboronic acid, 2 mmol base, 3 mL solvent.b Isolated yield.c Determined by GC.d Pd black formed.e DMF : H2O (1.5 : 1.5).
1	K2CO3	DMF–H2O^e	0.06	r.t.	180	Trace
2	K2CO3	DMF–H2O^e	0.06	r.t.	24	40
3	K2CO3	DMF–H2O^e	0.06	60	60	80
4	K2CO3	DMF–H2O^e	0.06	80	60	96
5	K2CO3	DMF–H2O^e	0.05	80	60	97
6	K2CO3	DMF–H2O^e	0.03	80	60	91
7	K2CO3	DMF–H2O^e	0.05	80	30	98 (100)^c
8	K2CO3	Toluene	0.05	80	20	79
9	K2CO3	DMF	0.05	80	20	89
10	K2CO3	H2O	0.05	80	20	84
11	NaOH^d	DMF–H2O^e	0.05	80	60	86
12	Cs2CO3	DMF–H2O^e	0.05	80	60	96
13	Cs2CO3	DMF–H2O^e	0.05	80	25	97 (100)^c
14	K3PO4	DMF–H2O^e	0.05	80	20	50
15	K3PO4	DMF–H2O^e	0.05	80	90	60
16	BaCO3	DMF–H2O^e	0.05	80	60	10
17	BaCO3	DMF–H2O^e	0.05	80	90	20

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Based on the discussed results, the catalytic activity of SBA-15/(SH)_XPd-NP_Y in the synthesis of various biaryls by the Suzuki cross-coupling reaction was also studied. Coupling of aryl halide 1 and aryl boronic acid 2 treated under the optimized conditions led to compound 3 (Table 3).

Table 3 Pd-NPs@SBA-15/SH catalyst Suzuki reactions under optimized conditions^a

Entry	Ar(R)–	X	Ar(Y)–	Time (min)	3	Yield (%)^b
a Reaction conditions: 1 mmol aryl halide, 1.2 mmol aryl boronic acid, 3 mmol K2CO3, DMF–H2O 3 mL, 80 °C.b Isolated yield and in parentheses are GC yield.c Containing 0.5 mol% of Pd with 0.5 mmol TBAB.
1	4-MeCOC6H4–	Br	C6H5–	20	3a	95
2	C6H5–	I	C6H5–	20	3b	97 (100)
3	C6H5–	I	4-MeC6H4–	20	3c	97 (100)
4	C6H5–	Br	C6H5–	35	3d	97
5	C6H5–	Br	4-MeC6H4–	35	3e	95 (98)
6	2-MeC6H4–	Br	C6H5–	35	3f	94
7	4-MeC6H4–	Br	C6H5–	35	3g	95
8	2-OHCC6H4–	Br	C6H5–	35	3h	96
9	3-OHCC6H4–	Br	C6H5–	35	3i	95
10	4-MeCOC6H4–	Br	C6H5–	20	3j	96
11	C6H5–	Cl	C6H5–	24 h	3k	42^c
12	4-MeCOC6H4–	Cl	C6H5–	24 h	3l	46^c
13	4-MeCOC6H4–	F	C6H5–	24 h	3m	Trace

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Coupling of aryl iodides and boronic acid derivatives gave excellent yields in 20 min. There was no significant difference in the yields of the products with Cat. A₃ and Cat. A₁ (Table 3, entries 2 and 3). Aryl bromides as well as aryl iodides show high reactivity with boronic acid derivatives with high product yields, within a short time (Table 3, entries 4–10).

When the reactions use solid catalysts, the reusability and recovery of the catalysts are important factors. In the model reaction, it was found that supported Pd(0) catalysts has been recovered and reused without considerable loss of theirs reactivity (Fig. 5).

Fig. 5 Recyclability study of SBA-15/(SH)_XPd-NP_Y for model reaction.

4. Conclusions

In summary, highly dispersed Pd-loaded mesostructure nanocatalysts using (3-mercaptopropyl)trimethoxysilane functionalized SBA-15 as support were synthesized. The anchoring thiol groups were effective for the attachment of the Pd nanoparticles onto the surface of the silica support. In fact, uniform and small size particles of palladium were generated onto the SH-decorated SBA-15 pore-walls. This SBA-15/(SH)_XPd-NP_Y efficiently catalyzes Suzuki–Miyaura cross-coupling of aryl halides and boronic acids in excellent yields with negligible leaching of Pd-NPs even in water. The SBA-15/(SH)_XPd-NP_Y, after easy separation from the reaction mixture, was reused 11 times with high product yields.

References

1. A. Suzuki, in Modern Arene Chemistry, ed. D. Astruc, Wiley-VCH, Weinheim, 2002, Ch. 3, pp. 53–106.
2. L. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133.
3. (a) A. Suzuki, J. Organomet. Chem., 1999, 576, 147; (b) N. Miyaura and A. Suzuki, Chem. Commun., 1979, 866–867; (c) N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457.
4. J.-P. Corbet and G. Mignani, Chem. Rev., 2006, 106, 2651.
5. (a) C. Baleizao, A. Corma, H. Garcia and A. Leyva, J. Org. Chem., 2004, 69, 439; (b) A. Corma, D. Das, H. García and A. Leyva, J. Catal., 2005, 229, 322.
6. (a) B. Karimi and P. Fadavi Akhavan, Chem. Commun., 2011, 47, 7686; (b) S. Rostamnia and H. Xin, Appl. Organomet. Chem., 2013, 27, 348; (c) R. Zhang, W. Ding, B. Tu and D. Zhao, Chem. Mater., 2007, 19, 4379; (d) S. Mandal, D. Roy, R. V. Chaudhari and M. Sastry, Chem. Mater., 2004, 16, 3714; (e) C. D. Wu, A. Hu, L. Zhang and W. B. Lin, J. Am. Chem. Soc., 2005, 127, 8940; (f) X. Pan, Z. Fan, W. Chen, Y. Ding, H. Luo and X. Bao, Nat. Mater., 2007, 6, 507; (g) E. Castillejos, P.-J. Debouttie` re, L. Roiban, A. Solhy, V. Martinez, Y. Kihn, O. Ersen, K. Philippot, B. Chaudret and P. Serp, Angew. Chem., Int. Ed., 2009, 48, 2529.
7. (a) A. Corma, Chem. Rev., 1997, 97, 2373; (b) A. Corma and H. Garcia, Adv. Synth. Catal., 2006, 348, 1391; (c) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 1992, 359, 710; (d) D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548; (e) S. Rostamnia, H. Xin, X. Liu and K. Lamei, J. Mol. Catal. A: Chem., 2013, 374–375, 85; (f) S. Rostamnia and F. Pourhassan, Chin. Chem. Lett., 2013, 24, 401.
8. (a) T. E. Barder, S. D. Walker, J. R. Martinelli and S. L. Buchwald, J. Am. Chem. Soc., 2005, 127, 4685; (b) X. Feng, G. E. Fryxell, L. Q. Wang, A. Y. Kim, J. Liu and K. M. Kemner, Science, 1997, 276, 923; (c) L. Mercier and T. J. Pinnavaia, Adv. Mater., 1997, 9, 500.
9. (a) C. M. Crudden, M. Sateesh and R. Lewis, J. Am. Chem. Soc., 2005, 127, 10045; (b) B. W. Glasspoole, J. D. Webb and C. M. Crudden, J. Catal., 2009, 265, 148; (c) C. M. Crudden, K. McEleney, S. L. MacQuarrie, A. Blanc, M. Sateesh and J. D. Webb, Pure Appl. Chem., 2007, 79, 247.
10. (a) K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscow, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 1985, 57, 603; (b) S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 2nd edn, 1982.
11. (a) S. Rostamnia and K. Lamei, Synthesis, 2011, 3080; (b) S. Rostamnia, K. Lamei, M. Mohammadquli, M. Sheykhan and A. Heydari, Tetrahedron Lett., 2012, 53, 5257; (c) S. Rostamnia and A. Zabardasti, J. Fluorine Chem., 2012, 144, 69; (d) S. Rostamnia, A. Hassankhani, H. G. Hossieni, B. Gholipour and H. Xin, J. Mol. Catal. A: Chem., 2014, 395, 463; (e) S. Rostamnia, X. Liu and D. Zheng, J. Colloid Interface Sci., 2014, 432, 86; (f) S. Rostamnia and E. Doustkhah, RSC Adv., 2014, 4, 28238; (g) S. Rostamnia, Z. Karimi and M. Ghavidel, J. Sulfur Chem., 2012, 33, 313.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09157d

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