Fe3O4@SiO2 nanoparticle supported ionic liquid for green synthesis of antibacterially active 1-carbamoyl-1-phenylureas in water

In the present work, we have designed a novel, heterogeneous and recyclable magnetic Brønsted acidic ionic liquid based on 5-phenyl-1H-tetrazole. The {Fe3O4@SiO2@(CH2)35-phenyl-1H-tetrazole-SO3H/Cl} ([FSTet-SO3H]Cl) was prepared via the immobilization of 5-phenyl-1H-tetrazole-bonded sulfonic acid onto the surface of silica-coated magnetic nanoparticles using 3-chloropropyltriethoxysilane as a linker. The catalyst was characterized by XRD, TEM, FESEM, EDS, TG-DTA, and FT-IR. The ability and high activity of this catalyst were demonstrated in the synthesis of 1-carbamoyl-1-phenylureas with good to excellent yields via a new, simple and one-pot procedure in aqueous media under reflux conditions. This procedure has advantages such as high yields, short reaction times, a simple methodology and work-up process, green reaction conditions, high stability, catalytic activity, and easy preparation, separation and reusability of the catalyst. The synthesis of these compounds was confirmed by FT-IR, 1H NMR, 13C NMR and CHN. In addition, we investigated the biological properties of the 1-carbamoyl-1-phenylureas as newly synthesized compounds. The described catalyst could be easily separated from the reaction mixture by additional magnetic force and reused several times without a remarkable loss of its catalytic activity and any considerable changes in the product yield and the reaction time.


Introduction
Ionic liquids (IL) are a class of liquids that contain only ions. 1 In general, ionic liquids contain molten salts at a temperature above 800 C, however today, ionic liquids are known as salts that are liquid at temperatures below 100 C. 2 Over the past few years, ionic liquids have attracted the attention of many scientists due to their capabilities as catalysts, reaction media, reagents and solvents. 3 Ionic liquids have advantages such as a high thermal stability, being non-ammable, having a low vapor pressure, being recyclable and having excellent solvation properties. 4 However, ionic liquids have limited applications due to their toxic nature. 5 Recently, extensive studies have been conducted to eliminate the disadvantages associated with ionic liquids and replace them with safer and more productive ones. One of the best techniques is the combination of ionic liquids and magnetic nanoparticles (MNPs). MNPs act as a good support for the immobilization of the ionic liquids. 6 Magnetic ionic liquids have certain specications such as a large specic surface area, high stability, facile separation and recovery from the reaction mixture, good magnetic permeability and low toxicity and price. 7 Recently, solid acid catalysts have attracted a lot of attention in the eld of organic reactions. 8 In this regard, many Brønsted acids such as thiourea, TADDOL, amidinium and phosphate can be used as green and free-of-metal catalysts. 9 If an alkane sulfonic acid group is covalently tethered to the IL cation, the IL is converted into a strong Brønsted acid. 10 These SO 3 Hfunctionalized ionic liquids can act as good alternatives for homogenous and heterogeneous acidic catalysts due to their advantages such as being non-corrosive, nonvolatile and immiscible with many organic solvents. 11 From the past to present, the synthesis of organic compounds has attracted the attention of many chemists due to their special importance in biological and medical studies. Among the organic compounds, 1-carbamoyl-1-phenylureas are an important class of compounds. However, there is no report on the synthesis of 1-carbamoyl-1-phenylureas in literature so far.
Over the past few years, the hydration of cyanamides has attracted a lot of attention as one of the most important ways of synthesizing N-monosubstituted ureas. 12 However, the hydration of cyanamides suffers from several disadvantages including the use of corrosive bases or acids, low yields, the use of toxic organic solvents, long reaction times and tedious workup. 13 Therefore, the development of a new, easy and efficient method for the hydration of cyanamides is one of the most important challenges.
1-Carbamoyl-1-phenylureas are the parent compounds of a large and interesting class of organic substances. It is probable that the presence of three nitrogen atoms in their structure has led to greater biological properties, but still nobody has succeeded in synthesizing this important compound. In addition, they are important compounds which can be used as starting materials in coordination chemistry and organic synthesis in the future.
In this research, we have designed a heterogeneous and recyclable magnetic Brønsted acidic ionic liquid catalyst by using 5-phenyl-1H-tetrazole. Although many ionic liquid based imidazoles are known, only very few ionic liquid based tetrazoles have been described. 14 However, it is noteworthy that there is no report on the synthesis of magnetic ionic liquid based tetrazoles. Therefore, this report could create a new approach for the production of magnetic ionic liquid based tetrazoles. Next, we investigated the catalytic activity of [FSTet-SO 3 H]Cl in the synthesis of 1-carbamoyl-1-phenylureas via a one-pot procedure in aqueous media under reux conditions (Scheme 1). The results show that the catalyst has excellent catalytic activity in this reaction. The products were prepared in good to excellent yields and characterized by FT-IR, 1 H NMR, 13 C NMR, CHN and melting point determination.

Reagents and methods
All materials of commercial reagent grade were purchased from the Merck and Aldrich companies and used without further purication. FT-IR spectra were recorded on a Nicolet 370 FT/IR spectrometer (Thermo Nicolet, USA) using pressed KBr pellets. X-ray diffraction (XRD) measurements were carried out with a Philips powder diffractometer type PW 1373 goniometer. It was equipped with a graphite monochromator crystal. The X-ray wavelength was 1.5405 A and the diffraction patterns were recorded in the 2q range  with a scanning speed of 2 min À1 . TEM images were taken using a Philips EM208 transmission electron microscope with an accelerating voltage of 90 kV. Scanning electron microscopy (SEM) of the {Fe 3 O 4 @-SiO 2 @(CH 2 ) 3 5-phenyl-1H-tetrazole-SO 3 H/HCl} was performed on a Cam scan MV2300. The chemical compositions of the synthesized catalyst were determined by EDS (energy dispersive X-ray spectroscopy) performed in SEM. Thermal analysis (TG-DTG) was carried out using an STA 1500 Rheometric Scientic (England). The ow rate of air was 120 mL min À1 and the ramping rate of the sample was 2 C min À1 . VSM measurements were recorded using a SQUID magnetometer at 298 K (Quantum Design MPMS XL). Melting points were taken in open capillaries using BUCHI 510 melting point apparatus and are uncorrected.

Results and discussion
Our recent studies have shown that arylcyanamides and tetrazoles could be used as highly active compounds in organic synthesis. 8,15 However, a lack of convenient methods for the preparation of the arylcyanamides and ionic liquid based tetrazoles strongly restricts their potential application in organic synthesis. In this work, the arylcyanamides and 5-phenyl-1Htetrazole were prepared according to our recent work on aryl amines 15c and benzonitrile 15d (Scheme 2).
Due to our interest in green protocols, in the present work, for the rst time we have reported the synthesis of 1-carbamoyl-1-phenylureas using [FSTet-SO 3 H]Cl as a magnetic Brønsted acidic ionic liquid catalyst in water as a green solvent.
In this work, by combining the advantages of a Brønsted acidic ionic liquid as a H + source and silica-coated magnetic nanoparticles (Fe 3 O 4 @SiO 2 ) with high surface area, excellent thermal stability, low toxicity, and easy synthesis and separation from the reaction mixture by an external magnet, a [FSTet-SO 3 H]Cl nanocomposite has been produced using a simple method. To the best of our knowledge, this is the rst report wherein an ionic liquid based tetrazole has been immobilized on the Fe 3 O 4 @SiO 2 surface as a powerful catalytic support.
The thermal stability of [FSTet-SO 3 H]Cl was also analyzed by TGA (thermogravimetric analysis) and DTA (differential thermal analysis) experiments in air ow (120 mL min À1 ) at a heating rate of 2 C min À1 on an autonomic TG-DTA. As shown in Fig. 4, three weight loss stages were observed in air ow for [FSTet-SO 3 H]Cl. The rst weight-loss step mainly happened at 120-190 C and is associated with desorbed water or other organic solvents which were employed during the preparation steps of the catalyst. In the second stage at 218-300 C, a weight loss is observed which can be attributed to the thermal decomposition of 5-phenyl-1H-tetrazole functionalized with chlorosulfonic acid on the surface of  the silica coating. Finally, the weight loss observed at 650-750 C is due to the decomposition of the catalyst.
FT-IR analysis was carried out to conrm the structure and the formation of [FSTet-SO 3 H]Cl. Fig. 5 shows the FT-IR spectra of 5-phenyl-1H-tetrazole (A), Fe 3 O 4 @SiO 2 (B), Fe 3 O 4 @SiO 2 @ (CH 2 ) 3 Cl (C), Fe 3 O 4 @SiO 2 @(CH 2 ) 3 5-phenyl-1H-tetrazole (D) and [FSTet-SO 3 H]Cl (E). The structure of 5-phenyl-1H-tetrazole was in agreement with the FT-IR spectra data. The disappearance of one strong and sharp absorption band (CN stretching band) in benzonitrile, and the appearance of an NH stretching band in the FT-IR spectroscopy, was evidence for the formation of 5phenyl-1H-tetrazole (Fig. 5A). In Fig. 5B-E, the peak at 3600-3100 cm À1 is due to the O-H stretching mode. The appearance of two peaks at 570 cm À1 and 617 cm À1 (Fig. 5B) (Fig. 5B). The peak at 1619 cm À1 may be attributed to C]C and C]N vibrations of the tetrazolium moiety of the ionic liquid (Fig. 5E).
To determine the crystallographic structure of Fe 3 O 4 @SiO 2 , Fe 3 O 4 @SiO 2 @(CH 2 ) 3 Cl, Fe 3 O 4 @SiO 2 @(CH 2 ) 3 5-phenyl-1H-tetrazole and [FSTet-SO 3 H]Cl, XRD analysis was carried out (Fig. 6). As shown in Fig. 6   In order to study the magnetic properties of [FSTet-SO 3 H]Cl, a vibrating sample magnetometer (VSM) was used to characterize the as-prepared catalyst with a magnetometer at 298 K and with eld sweeping from À10 000 to +10 000 Oe. Fig. 7 shows a typical room temperature magnetization curve of [FSTet-SO 3 H]Cl. As demonstrated in Fig. 7, the magnetization curves of the as-prepared ionic liquid display no hysteresis loop which demonstrates its superparamagnetic characteristics. Therefore, at the end of the reaction, [FSTet-SO 3 H]Cl could simply be collected from the reaction mixture using an external magnet.
Following the synthesis of 1-arylureas 8 and in a further study on the preparation of nitrogen-containing compounds, we focussed on the synthesis of 1-carbamoyl-1-phenylureas as nitrogen-rich compounds from arylcyanamides and sodium cyanate (NaOCN) as starting materials. In 2008, our research group reported the synthesis of primary carbamates from the reaction between phenol or alcohols with NaOCN in the presence of HClO 4 -SiO 2 at room temperature or 55-65 C for an appropriate time in high yields and under solvent-free conditions (Scheme 4). 16 Herein, in the course of our research on the synthesis of the nitrogen-containing compounds, 15 we now wish to report the preparation of novel 1-carbamoyl-1-phenylureas by the reaction of NaOCN with arylcyanamides in the presence of a [FSTet-SO 3 H]Cl catalyst in water under reux conditions. Apparently, to the best of our knowledge, so far no methodology has been reported where NaOCN is used as an effective salt in the synthesis of the 1-carbamoyl-1-phenylureas.
We applied our catalyst to the synthesis of 1-carbamoyl-1phenylureas under reux conditions and 3-bromophenylcyanamide was chosen as a model substrate. Various reaction conditions including the amount of the [FSTet-SO 3 H]Cl catalyst and temperature were varied to examine the inuence on the compositions in the reaction mixture (Table 1). We observed that 1-(3-bromophenyl)-1-carbamoylurea (1)   With the optimized conditions in hand, we next tested the substrate scope of this transformation. Excellent yields could be achieved regardless of the substituents associated with the arylcyanamides ( Table 2). Different substituents such as Br, Cl, OMe and Me groups were compatible, and achieved a yield up to 90% (1-7). Both electron-withdrawing and electron-donating groups in the cyanamides were compatible. As shown in Table  2, the arylcyanamides with electron-donating groups were completed under reux conditions aer 3 h and the corresponding products were obtained in shorter reaction times because of their greater ability to attack the NaOCN. However, the species bearing electron withdrawing groups required This journal is © The Royal Society of Chemistry 2018 higher reaction times. As shown in Table 2, 1,4-phenylenecyanamide interestingly afforded the double-addition product (7), due to presence of two CN groups. The products were characterized by IR, 1 H NMR and 13 C NMR spectroscopy, elemental analysis and melting point determination.
The structures of the 1-carbamoyl-1-phenylureas were in agreement with their FT-IR and NMR spectra. In the FT-IR spectra of the 1-carbamoyl-1-phenylureas, three new peaks appeared corresponding to C]O and NH 2 groups absorption vibrations and the CN peak had disappeared (Fig. 8). The 1 H NMR spectra showed one characteristic peak belonging to the NH 2 group (Fig. 9). The appearance of a carbon signal corresponding to a carbonyl group in the 13 C NMR spectra is evidence of the formation of 1-carbamoyl-1-phenylureas (Fig. 10).
Escherichia coli is a common inhabitant of the intestinal tract of humans and warm-blooded animals. Most strains of E. coli are harmless and are a part of the normal intestinal microora. These strains serve a useful function in the body by suppressing the growth of harmful bacteria and by synthesizing appreciable amounts of vitamins. However, several pathogenic E. coli strains have emerged which cause disease in humans. Pathogenic E. coli can be divided into intestinal pathogens causing diarrhoea, and extra intestinal E. coli causing a variety of infections in both humans and animals. 18 Through this report we investigated the antibacterial activity of synthesized 2-(4-(1-carbamoylureido)phenyl)malonamide (7) against pathogenic E. coli as following.
The antibacterial activity of the sample was studied against Escherichia coli bacteria by disk diffusion method using a Muller Hinton agar culture. The concentrations used for investigations of the sample were 1% (10 mg mL À1 ), 5% (50 mg mL À1 ), 10% (100 mg mL À1 ), 15% (150 mg mL À1 ) and 20% (200 mg mL À1 ) respectively. The results were compared with chloramphenicol as a positive control. For reporting the results of the antibiogram test the minimum protection zone per millimeter (mm) was reported for each test. Furthermore, the frequency of the test was in triplicate for each concentration of the sample. According to Table 3 and Fig. 12, the sample demonstrated no antibacterial activity against E. coli in concentrations lower than 15% but showed an antibacterial activity with good potential for concentrations equal and greater than 15%.
As shown from Fig. 12, for both concentrations of 15% and 20% the sample showed a suitable antibacterial activity but in concentrations lower than 15% no antibacterial results were detected. Therefore, the study conrmed that the concentration of the compound is an important factor concerning its biological activity against E. coli, and in concentrations greater than 15% the sample demonstrated a good antibacterial activity against the mentioned bacteria compared to the positive control.
One of the most important points in the area of nanocatalysis is the stability and recyclability of heterogeneous catalysts. In order to show the effectiveness of [FSTet-SO 3 H]Cl, catalyst recycling experiments were carried out using 1-(4methoxyphenyl)urea as the model substrate under optimized conditions. Aer each cycle, [FSTet-SO 3 H]Cl was separated with an external magnet, washed with ethanol, dried and then reused at least ve times without signicant loss of catalytic activity (Fig. 13). Easy separation and reusability of the catalysts is one of the most important benets. As shown in FE-SEM and TEM images of the recycled catalyst ( Fig. 14 and 15), no obvious change in the morphology of [FSTet-SO 3 H]Cl was observed.

Conclusion
In conclusion, we have developed a novel and highly efficient protocol for the preparation of a heterogeneous and recyclable magnetic Brønsted acidic ionic liquid catalyst using 5-phenyl-1H-tetrazole. To the best of our knowledge, this is the rst report on the synthesis of a tetrazole-based ionic liquid stabilized on the surface of silica-coated magnetic nanoparticles using 3-chloropropyltriethoxysilane as a linker. In this work, we have developed a new strategy for the synthesis of 1-carbamoyl-1-phenylureas from arylcyanamides via a one-pot procedure in aqueous media under reux conditions. A wide range of substituted arylcyanamides as substrates were employed and afforded the desired products in excellent yields. The high yield of products, the efficiency, generality, short reaction time, clean reaction prole, the use of water as a green solvent, the use of a relatively inexpensive catalyst, simplicity and easy work-up procedure, recyclability and reusability of the catalyst, and the straightforward isolation of the products are the advantages of this protocol. The wide substrate scope, green reaction conditions, high yield of products and recovery and recyclability of the catalyst offer the potential for scale-up in pharmaceutical applications. Further studies and the development of other methodologies for the arylcyanamides' and 1-carbamoyl-1phenylureas' reactivities are in progress.

Conflicts of interest
There are no conicts to declare.