A new and green synthesis of formamidines by γ-Fe2O3@SiO2–HBF4 nanoparticles as a robust and magnetically recoverable catalyst

Journal of Molecular Structure 1027 (2012) 156–161

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A new and green synthesis of formamidines by c-Fe2O3@SiO2–HBF4 nanoparticles as a robust and magnetically recoverable catalyst Mehdi Sheykhan, Mohsen Mohammadquli, Akbar Heydari ⇑ Chemistry Department, Tarbiat Modares University, P.O. Box 14155-4838, Tehran, Iran

h i g h l i g h t s " Magnetically recoverable catalyst afforded in enhancement simplicity. " Product’s purity enhanced by efficient magnetic recovery. " Green Brønsted acid catalyst. " Mild, cost-effective and applicable efficient reusable heterogeneous catalyst. " Environmentally benign method for synthesis of formamidines.

a r t i c l e

i n f o

Article history: Received 19 February 2012 Received in revised form 25 April 2012 Accepted 1 June 2012 Available online 12 June 2012 Keywords: Spectroscopic characterization Formamidines Immobilized catalysts Magnetic recovery

a b s t r a c t A series of Brønsted acids (HA) were immobilized on superparamagnetic c-Fe2O3@SiO2. The synthesized [c-Fe2O3@SiO2–HA] nanocrystallites were fully characterized by spectroscopic techniques (FT-IR, XRD, SEM, EDX) and used as solid acid catalysts in the synthesis of biologically important formamidines. The results were excellent in yield and time of reaction. Activity of various Brønsted acids on superparamagnetic support as catalyst were also evaluated and among them immobilized HBF4 showed the best catalytic properties. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Formamidines have been used extensively as pharmacological agents [1–3] and are interesting compounds in the biosyntheses of purines, imidazoles and also in the catabolism of histidine [4]. Formamidines and their anions are common ligands in coordination chemistry since they bind to transition metals via monodentate, chelating or bridging modes [5–11]. Also, benzimidazole heterocycles are high important compounds for a wide range of applications including biology and biomedicine. They have biological activities such as antiviral, antihypertension, anticancer properties [12,13] and also their application against several viruses such as HIV [14,15], Herpes (HSV-1) [16], human cytomegalovirus (HCMV) [17] and influenza [18] has demonstrated. They have also been used as ligands for asymmetric transformations [19]. Traditionally, the synthesis of formamidines has been achieved through reduction of carbodiimides with sodium borohydride in ⇑ Corresponding author. Tel.: +98 21 82883444; fax: +98 21 82883455. E-mail address: [email protected] (A. Heydari). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.06.009

isopropanol [20], the exchange of N,N-dimethylformamidines or acetamidines by a variety of amines [21], the exchange of N,Ndimethylformamidines in non-protic solvents [22], the reaction of triethylorthoformate or orthoacetate with amines in acetic acid under reflux at 140–150 °C for 1.5–94 h [23], and by use of aldehydes [24–27], acid chlorides [28], o-dinitrobenzene [29], Gold’s reagent [27] and 2-nitroanilines [28] as the starting materials. Unfortunately, these protocols have suffered from several problems such as the use of toxic organic solvents (either in conducting of reaction or working-up of product), high temperatures, long times of reaction, strong acidic conditions, low yields of the products, tedious work-up and using excess amounts of reagents. The use of Lewis and Brønsted acid catalysts has high priority from laboratories to chemical manufacturing plants. Proton, H+, arguably is the most versatile catalyst for extraordinary range of organic reactions and biological and synthetic transformations [29]. Solid acid catalysts are important functional materials in the petroleum refining industry, catalytic cracking processes and production of chemicals [30–32]. In contrast, a significant number of reactions such as Friedel–Crafts, several esterifications,

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hydration, and hydrolysis, are still carried out using liquid acid catalysts. Using liquid acid catalysts have different limitations such as difficulties of catalyst separation, waste production, corrosion of reactors and their inability to be recovered and reused [33]. Thus, one of the great challenges in catalysis is to immobilize the Brønsted acids on an insoluble, separable and high surface area solid. In recent years, solid supported catalysts have attracted much attention due to their low toxicity, high catalytic activity, moisture and air tolerance, ease of separation, recyclability, low corrosion and also for environmental (eco-friendly) concepts [34,35]. Due to their high surface area, nanoparticles have received increasing attention as an alternative catalytic support [36,37]. Although, tedious recycling of nanocatalysts via filtration and inevitable loss of solid nanocatalysts during the separation process have strongly limited their applications. Furthermore, nanometer-sized catalysts are often easily dispersible in solution by forming stable suspensions and therefore, expensive ultra-centrifugation is often the only way to separate them from the products [38–40]. Fabrication of core–shell magnetic nanocatalysts having a magnetic core and an inorganic shell is one of the solutions to overcome this difficulty. Such core–shell materials combine the unique magnetic properties of the core with the functionalization possibilities of inorganic surface [41]. It was reported that formation of a passive coating of magnetic nanoparticles could prevent their aggregation in solutions and thus improve their chemical stability [42]. Owing to its good stability, high surface area and easy synthesis, silicaencapsulated magnetic c-Fe2O3 nanocrystallities have recently been used as heterogeneous catalytic supports [43]. Following high

importance of formamidines and strong demand for a highly efficient and environmentally benign method in their synthesis, the present work illustrates the immobilization of different Brønsted acids on c-Fe2O3@SiO2 for use as solid acid catalyst in the synthesis of biologically important formamidines. 2. Experimental 2.1. General procedure for formamidine synthesis For comparison of the catalytic activity of different c-Fe2O3@SiO2–HA catalysts, to a glass flask containing 1.2 mmol triethyl

Scheme 1. The synthesis of formamidines in the presence of c-Fe2O3@ SiO2–HA.

Fig. 2. FT-IR of selected catalyst before and after using.

Fig. 1. FT-IR spectrums of catalysts.

157

Fig. 3. Powder XRD of catalysts.

158

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Fig. 4. SEM and particle size histogram of (a) c-Fe2O3@SiO2–HBF4; (b) c-Fe2O3@SiO2–HClO4; (c) c-Fe2O3@SiO2–H3PW12O40.

orthoformate was added a catalytic amount of c-Fe2O3@SiO2–HA (2.5 mol%) and stirred solvent free at r.t. After 15 min 2 mmol aniline was added to this mixture and the reaction carried out until appropriate time. The reaction was monitored by thin layer chromatography (TLC) with n-hexane and ethyl acetate (4:1 or 7:3 for diamines). After completion of the reaction, 5 mL CH2Cl2

(EtOAC for entry 8&9) was added to the mixture and then the catalyst was separated by an external magnet to be utilized in next runs and the resulting solution was concentrated on a rotary evaporator to obtain purified corresponding solid products. All isolated products gave predictable spectral data (1H NMR and 13C NMR) and compared with those reported in literature (Scheme 1).

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2.2. General procedure for recycling of c-Fe2O3@SiO2–HBF4

3. Results and discussion

After the reaction completed, the catalyst was easily separated from the products (by a magnet), washed twice with methanol and dried in oven to be used in next run. In every run, the yield of product was evaluated in constant time.

Silica-coated ultrafine and uniform core–shell nanoparticles were synthesized. The magnetite (Fe3O4) nanoparticle was prepared via facile chemical co-precipitation method from ferric and ferrous ions in alkaline conditions by reported procedure. Coating of a layer of silica on the surface of the Fe3O4 nanoparticles was achieved through hydrolysis of TEOS. Having decanted by a permanent magnet, the silica-coated nanoparticles were washed three times with EtOH, diethyl ether and calcinated at 300 °C in vacuum for 3 h to obtain c-Fe2O3@SiO2 and remove organic residues. Energy-dispersive X-ray spectroscopy (EDX) analysis was used to determine the ratio of SiO2 and c-Fe2O3 in c-Fe2O3@SiO2 and showed 43% and 57%, respectively. The loading of Brønsted acids measured by back titration and pH analyses [c-Fe2O3@SiO2–HA] nanocrystallites were characterized by FT-IR (Figs. 1 and 2), Xray diffraction (XRD) spectroscopy (Fig. 3) and scanning electron microscopy (SEM) (Fig. 4). In all of [c-Fe2O3@SiO2–HA] Brønsted acids, the band at 400–650 cm1 is assigned to the stretching vibrations of (Fe–O) bond in c-Fe2O3, and the band at about 1100 cm1 is ascribed to the stretch of (Si–O) bond. The bound Si–OH groups are characterized by the very broad IR absorption band in the 2800–3700 cm1 region also the stretching band at 1635 cm1 indicates the presence of residual physisorbed water molecules. FT-IR spectra of the selected catalyst before and after using are presented in Fig. 3. Diffraction peaks at around 35.5°, 43.1°, 54.0°, and 62.8° related to the (3 1 1), (4 0 0), (4 4 0), and (5 1 1) are readily recognized from the XRD patterns (Fig. 3). The observed diffraction peaks agree well with the tetragonal structure of maghemite in every four catalysts (1999 JCPDS file No. 13-0458). New peaks rised in the XRD of c-Fe2O3@SiO2–H3PW12O40 are due to presence of crystalline network structure of Keggin-type heteropoly acid ‘Hydrogen Tungsten Oxide Phosphate’. Diffraction peas at 11.9°, 14.6°, 16.9°, 20.8°, 24.0°, 25.5°, 26.9°, 29.5°, 32.0°, 34.2°, 38.4°, 44.1°, 54.0°, 62.8° and 64.1° related to the (1 1 0), (1 1 1), (2 0 0), (2 1 1), (2 2 0), (2 2 1), (3 1 0), (2 2 2), (3 2 1), (4 0 0), (4 2 0), (5 1 0), (5 3 2), (5 5 0) and (6 4 0), are readily recognized from the XRD patterns. The diffraction peaks agree well with the cubic structure of H3PW12O40 (1999 JCPDS file No. 75-2125).

2.3. Product spectroscopic data 1. N,N0 -Diphenyl-formamidine: FTIR: 512 cm1, 745 cm1, 1 1 1 1 827 cm , 990 cm , 1195 cm , 1512 cm , 1609 cm1, 1675 cm1, 3441 cm1. 1H NMR (CDCl3, 500 MHz): d 6.90– 7.35 (m, 10H), 8.21 (s, 1H), 9.22 (brs, NH); 13C NMR (CDCl3, 125 MHz): d = 119.1, 129.3, 132.6, 143.0, 149.5. MS m/z: 196 (M+). 2. N,N0 -Di-(p-tolyl)-formamidine: FTIR: 515 cm1, 754 cm1, 820 cm1, 988 cm1, 1190 cm1, 1370 cm1, 1462 cm1, 1508 cm1, 1611 cm1, 1671 cm1, 3426 cm1. 1H NMR (CDCl3, 500 MHz): d 2.35 (s, 6H), 6.95 (d, J = 8.1 Hz, 4H), 7.12 (d, J = 8.1 Hz, 4H), 8.19 (s, 1H), 9.32 (br s, NH); 13C NMR (CDCl3, 125 MHz): d 20.7, 119.0, 129.8, 132.6, 143.0, 149.5. MS m/z: 224 (M+). 3. N,N0 -Bis-(4-flouro-phenyl)-formamidine: (142–144 °C in literature), FTIR: 509 cm1, 588 cm1, 691 cm1, 755 cm1, 831 cm1, 995 cm1, 1092 cm1, 1202 cm1, 1310 cm1, 1501 cm1, 1600 cm1, 1667 cm1, 2923 cm1, 3431 cm1. 1H NMR (CDCl3, 500 MHz): d = 7.00 (s, 8H), 8.02 (s, 1H) [44]. MS m/z: 232 (M+). 4. N,N0 -Bis-(4-bromo-phenyl)-formamidine: FTIR: 499 cm1, 1 1 1 1 555 cm , 620 cm , 715 cm , 835 cm , 998 cm1, 1066 cm1, 1189 cm1, 1510 cm1, 1604 cm1, 1670 cm1, 2928 cm1, 3420 cm1. 1H NMR (CDCl3, 500 MHz): d 8.04 (s, 1H), 7.48(d, J = 10.0 HZ, 4H) 7.40 (d, 2H, aromatic), 6.93 (d, 2H, aromatic), 6.91(d, 2H, aromatic) [45]. MS m/z: 354 (M+). 5. N-Phenyl-N0 -(p-tolyl)-formamidine: FTIR: 511 cm1, 750 cm1, 813 cm1, 1001 cm1, 1192 cm1, 1374 cm1, 1465 cm1, 1503 cm1, 1606 cm1, 1678 cm1, 3430 cm1. 1H NMR (CDCl3, 500 MHz): d 2.36 (s, 3H), 6.92–7.35 (m, 9H), 8.24 (s, 1H), 9.12 (br s, NH); 13C NMR (CDCl3, 125 MHz): d 20.8, 119.1, 123.3, 129.2, 129.4, 129.7, 132.8, 142.7, 145.3, 149.8. MS m/z: 210 (M+). 6. 1H-Benzimidazole: FTIR: 623 cm1, 749 cm1, 878 cm1, 953 cm1, 1001 cm1, 1128 cm1, 1198 cm1, 1244 cm1, 1302 cm1, 1407 cm1, 1459 cm1, 1589 cm1, 1796 cm1, 2791 cm1, 2860 cm1, 2940 cm1, 3439 cm1. 1HNMR (CDCl3, 500 MHz): d 7.10–7.32 (m, 2H), 7.44–7.66 (m, 2H), 8.11 (s, 1H), 10.75 (br s, NH); 13C NMR (CDCl3, 125 MHz): d 115.3, 121.3, 137.1, 140.5. MS m/z: 118 (M+). 7. 1H-perimidine: FTIR: 642 cm1, 750 cm1, 815 cm1, 865 cm1, 1027 cm1, 1202 cm1, 1290 cm1, 1333 cm1, 1372 cm1, 1440 cm1, 1478 cm1, 1588 cm1, 1632 cm1, 2701 cm1, 2860 cm1, 3044 cm1, 3110 cm1, 3188 cm1, 3424 cm1. 1H NMR (CDCl3, 500 MHz): d 5.97 (br s, NH), 6.40 (d, J = 5.0 Hz, 2H), 7.10 (m, 4H), 7.31 (s, 1H); 13C NMR (CDCl3, 125 MHz): d 108.5, 120.3, 123.2, 128.1, 135.5, 139.2, 145.2. MS m/z: 168 (M+). 8. 1H-imidazo [4, 5-b] pyridine: FTIR: 620 cm1, 723 cm1, 850 cm1, 934 cm1, 1011 cm1, 1120 cm1, 1186 cm1, 1240 cm1, 1312 cm1, 1401 cm1, 1452 cm1, 1600 cm1, 1790 cm1, 2775 cm1, 2856 cm1, 3422 cm1. 1H NMR (CDCl3, 500 MHz): d 7.3 (s, 1H), 8.1 (d, J = 10.0 Hz, 1H), 8.3 (s, 1H), 8.4(s, 1H); 13C NMR (CDCl3, 125 MHz): d 118.4, 127.3, 133.4, 142.3, 143.9, 148.2. MS m/z: 119 (M+).

Fig. 5. VSM diagram of c-Fe2O3@SiO2–HBF4 nanoparticles.

Table 1 Comparisons of catalysts activities. Entry

Catalyst

Time (h)

Yield (%)

1 2 3 4 5

c-Fe2O3@SiO2–HBF4 c-Fe2O3@SiO2–HClO4 c-Fe2O3@SiO2–H3PO4 c-Fe2O3@SiO2–H3PW12O40

4 4 9 9 9

95 51 22 5 86

Fe3O4

160

M. Sheykhan et al. / Journal of Molecular Structure 1027 (2012) 156–161

Table 2 The synthesis of formamidines in the presence of c-Fe2O3@SiO2–HBF4 as a magnetically recoverable catalyst. Entry

Amines

Product

Time (h)

NH2

1

Temperature (°C) a

4

95 (92,92,90,91) TON = 184

r.t.

4

95

r.t.

8

95

70

4

70

70

4

95

r.t.

3

100

r.t.

NH

3

100

r.t.

H N

1.5

80

70

N

N H

NH2

2

Yield

N

N H

CH3 NH2

3

F

F N

N H

F

Br

Br

4

Br N H

N

NH2 NH2

5

N H

N

CH3 NH2

NH2

6

H N

NH2

N

NH2 NH2

7

NH2

8

N a

N

NH2 N

N

Yields after recycle of catalyst for run 2–5.

In the case of [c-Fe2O3@SiO2–H3PW12O40] the solid supported acid (Keggin-type structure of heteropoly acid) is clearly revealed in the XRD pattern. The SEM graphs of c-Fe2O3@SiO2–HA Brønsted acids are presented in Fig. 4. Also, particle size distribution histogram of every

c-Fe2O3@SiO2–HA Brønsted acid obtained from its 50 observed particles in SEM is shown in each case. Superparamagnetic properties of [c-Fe2O3@SiO2–HBF4] were characterized by a home-made vibrating sample magnetometer (VSM) (Fig. 5). The magnetic saturation (Ms) is about 3.6 emu g1

M. Sheykhan et al. / Journal of Molecular Structure 1027 (2012) 156–161

and hysteresis loop was not observed, confirmed that catalyst is a superparamagnetic material. Four aforementioned catalysts and also 40 mg naked Fe3O4 (for comparison) as catalyst were examined in a model reaction. The activity of the catalysts was also investigated from time and yield point of view. The model reaction was carried out by the following protocol: 1.2 mmol triethyl orthoformate was activated by 0.05 mmol catalysts and then 2 mmol aniline was added at room temperature. After investigation, we found that c-Fe2O3@SiO2– HBF4 is more efficient catalyst for the synthesis of formamidine derivatives (Table 1). The catalyst was extended to diamines and mixtures of amines resulted in the formation of desired products (Table 2). In addition, the recyclability of catalyst was considered in 5 consecutive runs without significant loss of activity.

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4. Conclusion [15]

In summary the synthesis, spectroscopic characterization and catalytic application of four Brønsted acids supported on c-Fe2O3@SiO2 was described. Synthesized materials act as robust and green heterogeneous catalysts for the synthesis of some formamidine derivatives. Characterizations of [c-Fe2O3@SiO2–HA] Brønsted acids were done using several spectroscopic techniques such as FT-IR, SEM–EDX, XRD and VSM analyses. In all the cases, no impurities were detected. The magnetic supported acids show high activities in synthesis of formamidines. Evaluation of various supported acids represent that c-Fe2O3@SiO2–HBF4 has the best catalytic activity, having high superparamagnetic property as well as good turnover number. It seems that the activities are inversely related to the catalyst acidity (HBF4 > HClO4 > H3PO4 > H3PW12O40). HBF4 supported catalyst having weakest acidity showed the best activity for this reaction. We think that the stronger acids can suppress the amine (nucleophile) more easily and therefore the mild acid catalyst HBF4 shows better catalytic activity. Immobilization of Brønsted acids on magnetic silica surface eliminated the requirement for tedious catalyst separation procedures such as filtration and centrifugation which caused time, money and catalyst loss. Beside aforementioned factors, clean separation of catalyst is important to ‘‘green chemistry’’. Recyclable and easily separable c-Fe2O3@SiO2–HBF4 catalyst is appropriate for most formamidine derivatives due to easy work-up and excellent purity of products. In addition, the method is extendable for diamines such as orthophenylenediamine and 1,8-naphtalenediamine and mixtures of amines. The procedure is economically and environmentally a favorable method for the synthesis of formamidine derivatives.

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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2012. 06.009.

161

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