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aDepartment of Chemistry, Faculty of Scien

Iran. E-mail: mahmoudnasr81@gmail.com

32850953bYoung Researchers and Elite Club, Qom BrcDepartment of Chemistry, Buinzahra Bran

Qazvin, Iran

Cite this: RSC Adv., 2014, 4, 20351

Received 9th March 2014Accepted 8th April 2014

DOI: 10.1039/c4ra02052a

www.rsc.org/advances

This journal is © The Royal Society of C

Synthesis, characterization, antibacterial andcatalytic activity of a nanopolymer supportedcopper(II) complex as a highly active and recyclablecatalyst for the formamidation of arylboronic acidsunder aerobic conditions

Mahmoud Nasrollahzadeh,*a Ali Zahraei,b Ali Ehsania and Mehdi Khalajc

This paper reports on the synthesis and use of a nanopolymer supported copper(II) complex, as a separable

catalyst for the formamidation of arylboronic acids under aerobic conditions. The catalyst was

characterized using powder XRD, SEM, EDS, TGA-DTG and FT-IR spectroscopy. This method has the

advantages of high yields, elimination of homogeneous catalysts, simple methodology and easy work up.

Catalytic efficiency remains unaltered even after several repeated cycles. The synthesized catalyst is

found to be more highly toxic towards Gram-positive bacteria than Gram-negative bacteria.

Scheme 1 Copper-catalyzed formamidation of arylboronic acids.

Introduction

The ability to construct formamides efficiently is currently anactive area in organic synthesis due to the prevalence of thisstructural motif in a myriad of bioactive important productsand pharmaceutically interesting compounds.1–4 Among thevarious strategies developed to date, the copper-catalyzedChan–Lam coupling reaction has proven to be one of the mostconvenient synthetic routes for the synthesis of formamides.5

Chan–Lam coupling allows aryl carbon-heteroatom bondformation via an oxidative coupling of arylboronic acids, stan-nanes or siloxanes with N–H or O–H containing compounds inair. The reaction is induced by a stoichiometric amount ofcopper(II) or a catalytic amount of copper catalyst which isreoxidized by atmospheric oxygen.6–8

Recently, less expensive Cu catalysts has found muchinterest for the C–N bond formation.6–9 However, traditional Cu-catalyzed reactions also require the use of stoichiometricamounts of homogeneous copper catalysts, use of toxic ligands,harsh reaction conditions, strong bases and oen use of toxicpolar solvents. As a result, these drawbacks have limited theirlarge scale applications in industry. Therefore, it is desirable todevelop more efficient and convenient methods for the for-mamidation of arylboronic acids using heterogeneous catalystsunder aerobic conditions.

ce, University of Qom, Qom 37185-369,

; Fax: +98 25 32103595; Tel: +98 25

anch, Islamic Azad University, Qom, Iran

ch, Islamic Azad University, Buinzahra,

hemistry 2014

In continuation of our researches on the synthesis of tetra-zoles and application of heterogeneous catalysts,10–15 we report anew protocol for the preparation of the nanopolystyrene-anchored Cu(II) thiotetrazole complex [PS–ttet–Cu(II)] (2) and itsantimicrobial and catalytic applications as a novel and stableheterogeneous catalyst for the formamidation of arylboronicacids under aerobic conditions at 60 �C (Scheme 1). The PS–ttet–Cu(II) has become an affordable catalyst, as it can beprepared from inexpensive and readily available materials.Environmental acceptability, economic viability, and recycla-bility of the PS–ttet–Cu(II) is the advantages of this novelcatalyst.

Result and discussion

Catalyst was readily prepared in two steps. The synthesis of thepolymer-anchored Cu(II) complex catalyst is shown in Scheme 2.

Characterization of catalyst

The catalyst was characterized using the powder XRD, SEM,EDS, TGA-DTG and FT-IR spectroscopy.

Presence of the 1-phenyl-1H-tetrazole-5-thiol ligand onchloromethylated polystyrene and formation of PS–ttet–Cu(II) 2was conrmed by FT-IR spectra. The FT-IR spectrum of the 1-phenyl-1H-tetrazole-5-thiol and chloromethylated polystyrenewas compared with the polymer-supported thiotetrazole ligand

RSC Adv., 2014, 4, 20351–20357 | 20351

Scheme 2 Preparation of copper tetrazole-supported complex.

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and nanopolymer-supported [PS–ttet–Cu(II)] catalyst in order toconrm the coordination of the copper with the 1-phenyl-1H-tetrazole-5-thiol ligand (Fig. 1). As shown in Fig. 1B, the sharp

Fig. 1 FT-IR spectra of: (A) 1-phenyl-1H-tetrazole-5-thiol, (B) polystyren

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C–Cl peak (due to CH2Cl groups) at 698 cm�1 and the strongpeak at 1274 cm�1 corresponding to the H–C–Cl wagging modesin the starting polymer practically disappeared aer

e–thiotetrazole ligand and (C) supported Cu–thiotetrazole complex.

This journal is © The Royal Society of Chemistry 2014

Fig. 3 SEM image of PS–ttet–Cu(II) 2.

Fig. 4 EDS spectrum of PS–ttet–Cu(II) 2.

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introduction of the thiotetrazole ligand on the polymer. Thestretching vibrations of the N]N double bonds appear at 1449cm�1 and 1403 cm�1 for polystyrene–tetrazole ligand (Fig. 1B)and complex (Fig. 1C), respectively. Signals ranging under 3000to 2800 cm�1 identify the methylene stretching bonds. Thefunctionalized beads exhibited a band at 1652 cm�1 due tostretching vibrations of the C]N double bond, which wasshied to 1598 cm�1 in the anchored beads, indicating that thenitrogen atoms of the thiotetrazole is coordinated to copper.This suggests that the ligand acts as chelating agent coordi-nated through ‘N’ atom.

Phase investigation of the crystallized product was per-formed by powder XRD measurements and the powderdiffraction pattern of PS–ttet–Cu(II) 2 is presented in Fig. 2.Polystyrene exhibits a broad peak at about of 2q ¼ 19.0� and22.7�.16 The diffraction peaks at 38.2� and 46.5� correspond tocopper.

Scanning electron micrograph (SEM) image of PS–ttet–Cu(II)2 is shown in Fig. 3. As expected, the pure polystyrene bead hada smooth and at surface,17 while the anchored complexshowed roughening of the top layer. The presence of Cu hascaused changes, demonstrated by change in the polymerparticle size and roughness of the surface.

We used Energy Dispersive X-ray Spectroscopy (EDS) todetermine chemical composition of catalyst. The presence ofthe metals is conrmed by EDS. EDS results show that carbon,copper and oxygen concentrations are about 79.8%, 4.57% and11.89%, respectively (Fig. 4). The excess oxygen is due to phys-ical absorption of oxygen from environment during samplepreparation for SEM experiment.

Thermal stability of polymer-bound complex was investi-gated using TGA-DTG at a heating rate of 10 �Cmin�1 in air overa temperature range of 30–800 �C. The TGA curve of the poly-mer-supported copper complex is shown in Fig. 5. The complexis stable up to 200 �C, and above this temperature it decom-poses. The polymer-supported Cu(II) complex decomposed at300 �C.

Fig. 2 XRD pattern of PS–ttet–Cu(II) 2.

This journal is © The Royal Society of Chemistry 2014 RSC Adv., 2014, 4, 20351–20357 | 20353

Fig. 5 Thermal studies of PS–ttet–Cu(II) 2.

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Activity of PS–ttet–Cu(II) (2) catalyst for the formamidation ofarylboronic acids

The catalytic behavior of the PS–ttet–Cu(II) (2) was studied forthe formamidation of arylboronic acids.

Initially, we employed NH2CHO and phenylboronic acid asmodel substrates for the development of optimized conditions.Several solvents such as water, toluene, MeCN, MeOH andCH2Cl2 were examined. Control experiments show that there isno reaction in the absence of catalyst (Table 1, entry 12).However, addition of the catalyst to the mixture has rapidlyincreased the formamidation of phenylboronic acid in highyields. According to data given in Table 1, when the reaction wascarried out in the absence of solvent and only in the presenceNH2CHO (as both solvent & formylating agent), excellentconversion was achieved (Table 1, entries 6 and 7). The reactivityof the catalyst in the presence of different bases was alsoinvestigated (Table 1, entries 8–10). The results indicated thatbase had a demonstrative effect on the yield of product (Table 1,entry 11). Among the tested bases (K2CO3, Et3N, Na2CO3,NaOAc), K2CO3 was found to be superior for the highest yield ofN-phenyl formamide (Table 1, entry 7). The experimental resultsshow that the reaction times are reduced and the yields

Table 1 The formamidation of phenylboronic acid with NH2CHO under

Entry PS–ttet–Cu(II) (g) Base Solvent

1 0.05 K2CO3 H2O2 0.05 K2CO3 MeOH3 0.05 K2CO3 CH2Cl24 0.05 K2CO3 MeCN5 0.05 K2CO3 Toluene6 0.05 K2CO3 —7 0.05 K2CO3 —8 0.05 NaOAc —9 0.05 Na2CO3 —10 0.05 Pyridine —11 0.05 — —12 — K2CO3 —13 0.03 K2CO3 —14 0.08 K2CO3 —

a Reaction conditions: phenylboronic acid (1.0 mmol), NH2CHO (2.5 mmol

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increased under thermal conditions (Table 1, entry 7). Adecrease in the catalyst loading from 0.05 to 0.03 g afforded theproduct in lower yield (Table 1, entry 13). No signicantimprovement on the yield was observed using higher amountsof the catalyst (Table 1, entry 14) and 0.05 g of the catalyst wasfound to be optimum. The best result was obtained with 1.0mmol of phenylboronic acid, 2.5 mmol of NH2CHO, 0.05 g ofPS–ttet–Cu(II) (2) and 1.3 mmol of K2CO3 at 60 �C, which gavethe product in an excellent yield.

Next, the reactivity of several arylboronic acids was tested inthe formamidation reaction under at 60 �C and the results wereindicated in Table 2. To study the effects of the nature of thesubstituent groups on the benzene ring of phenylboronic acids,various formamidation were performed from different phenyl-boronic acids containing both electron-releasing and electron-withdrawing groups with NH2CHO in high yields under thermalconditions (Table 2). The formation of formamides wasconrmed by melting point and IR spectra, which showed twocharacteristic peaks, one between 3300 and 3400 cm�1

(secondary NH) and the other between 1640 and 1680 cm�1 (N-formyl, C]O).3,4,18

Antibacterial studies

In vitro antibacterial activity of the PS–ttet–Cu(II) (2) was evalu-ated against two bacterial species using the standard welldiffusion method. The results presented in Table 3 showed theantibacterial effects of PS–ttet–Cu(II) against E. coli (Escherichiacoli) and S. aureus (Staphylococcus aureus). The antibacterialactivity is estimated by the zone of inhibition. The diameter ofthe zone is measured to the nearest millimeter (mm). Thestandard error for each assay is presented in the parenthesis(Table 3). As shown in Table 3, PS–ttet–Cu(II) (2) has shown highantibacterial effect. The present study clearly indicates that thePS–ttet–Cu(II) (2) has good antibacterial action against Grampositive (Staphylococcus aureus) organism than Gram negative(Escherichia coli) organisms. It is possible that PS–ttet–Cu(II) notonly interact with the surface of membrane, but can alsopenetrate inside the bacteria.

different reaction conditionsa

Temperature (�C) Time (h) Yieldb (%)

r.t. 20 37r.t. 20 67r.t. 20 33r.t. 20 60r.t. 20 58r.t. 8 8960 5 9360 5 4260 5 8160 5 7960 5 3260 5 060 5 8060 5 92

), base (1.3 mmol), PS–ttet–Cu(II) (0.05 g), solvent (2 mL). b Isolated yield.

This journal is © The Royal Society of Chemistry 2014

Table 2 Formamidation of different arylboronic acids by the PS–ttet–Cu(II)

Entry R Time (h) Yielda%

1 C6H5 5 932 2,4-(Me)2-C6H3 5 903 2-Me-C6H4 5 914 4-Me-C6H4 5 915 4-OMeC6H4 5 936 2-Cl-C6H4 9 887 4-Br-C6H4 9 918 4-Cl-C6H4 9 929 4-COMe-C6H4 9 8110 3-CF3-C6H4 9 8011 4-NO2-C6H4 9 7912 1-Naphthyl 5 92

a Yields are aer work-up.

Table 3 Zone of inhibition (mm) of PS–ttet–Cu(II) against bacterialpathogens

Diameter of inhibition zone (mm)

Bacterial 100 ppm 200 ppm 300 ppm 400 ppm 500 ppmE. coli(Gram �ve)

12 (�3) 16 (�3) 21 (�3) 25 (�3) +35

S. aureus(Gram +ve)

17 (�3) 22 (�3) 23 (�3) 28 (�3) +35

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Catalyst recyclability

The reusability of the catalyst is one of the most importantbenets and makes them useful for commercial applications.From an economic point of view, the stability and sustainedactivity of the catalysts are of great importance. Thus, therecovery and reusability of the PS–ttet–Cu(II) (2) catalyst wasexamined by applying it to the C–N coupling of NH2CHO withphenylboronic acid under the present reaction conditions. Aerthe rst run completed, the catalyst was separated by ltration,

Fig. 6 Reusability of PS–ttet–Cu(II) for the formamidation of phenyl-boronic acid.

This journal is © The Royal Society of Chemistry 2014

washed with ethyl acetate and dried in a hot air oven at 100 �Cfor 2 h and employed for the next run of the reaction. Thecatalytic activity did not decrease considerably aer ve cata-lytic cycles (Fig. 6). This reusability demonstrates the highstability and turnover of catalyst under operating condition.

Conclusions

In conclusion, we have developed an efficient and simpleprocedure for preparation of nanopolystyrene-anchored Cu(II)thiotetrazole complex [PS–ttet–Cu(II)] (2) as an efficient, easilyrecoverable and reusable catalyst. The catalyst was character-ized by SEM, XRD, EDS, TGA-DTG and FT-IR analysis. Thiscatalyst demonstrated a high catalytic activity in promotingformamidation of phenylboronic acids to provide the form-amides in high yields. This method has the advantages of highyields, elimination of homogeneous catalysts, simple meth-odology and easy work up. Another important factor is thestability and recyclability of the catalyst under the reactionconditions used. The results show that the immobilized cata-lyst is slightly more active than its homogeneous analogue.This heterogeneous catalyst shows no signicant loss ofactivity in the recycling experiments. The active sites do notleach out from the support and thus can be reused withoutappreciable loss of activity, indicating that the anchoringprocedure was effective. The antibacterial activity of PS–ttet–Cu(II) was evaluated against E. coli and S. aureus, showingeffective bactericidal activity.

Experimental section

All reagents were purchased from the Merck and Aldrichchemical companies and used without further purication.Chloromethylated polystyrene (4–5% Cl and 2% cross-linkedwith divinylbenzene) was purchased from Merck company.Products were characterized by different spectroscopicmethods (FT-IR and 1H NMR spectra), elemental analysis(CHN) and melting points. 1H NMR spectra were recorded ona Bruker Avance DRX 90 and 300 MHz instrument. Thechemical shis (d) are reported in ppm relative to the TMS asinternal standard. J values are given in Hz. IR (KBr) spectrawere recorded on a Perkin-Elmer 781 spectrophotometer.Melting points were taken in open capillary tubes with aBUCHI 510 melting point apparatus and were uncorrected.The elemental analysis was performed using Heraeus CHN-O-Rapid analyzer. TLC was performed on silica gel polygramSIL G/UV 254 plates. X-ray diffraction measurements wereperformed with a Philips powder diffractometer type PW1373 goniometer. It was equipped with a graphite mono-chromator crystal. The X-ray wavelength was 1.5405 A and thediffraction patterns were recorded in the 2q range (10–60)with scanning speed of 2� per min. Morphology and particledispersion was investigated by scanning electron microscopy(SEM) (Cam scan MV2300). The chemical composition ofthe prepared nanostructures was measured by EDS per-formed in SEM.

RSC Adv., 2014, 4, 20351–20357 | 20355

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Preparation of nanopolystyrene-anchored Cu(II) thiotetrazolecomplex 2

The reaction was carried out in a round-bottomed ask of 250mL capacity. Chloromethylated polystyrene (2.0 g, 1.25 mmolg�1 of Cl) was stirred in 50 mL of DMF. Then 1-phenyl-1H-tet-razole-5-thiol (5.0 mmol) and K2CO3 (5.0 mmol) was added tothe above solution of the polymer, and the mixture was heatedfor 24 h at 100 �C. The polymer-anchored ligand (1) was lteredout, washed thoroughly with DMF and dried under vacuum for12 h. Polymer-anchored thiotetrazole ligand (1.0 g) was added toethanol (50 mL). CuCl2$2H2O (0.5 g) was added to the abovesuspension with constant stirring and then reuxed for 24 h.Aer cooling the reaction mixture to room temperature, theseparated bright green solid was ltered out, washed thor-oughly with ethanol and dried under vacuum to give PS–ttet–Cu(II) 2.

General procedure for the formamidation of arylboronic acids

A mixture of the appropriate arylboronic acid (1.0 mmol),NH2CHO (2.5 mmol), base (1.3 mmol) and PS–ttet–Cu(II) (0.05 g)was stirred at 60 �C for the appropriate time. Aer completion ofreaction (as monitored by TLC), the reactionmixture was cooledto room temperature, ethyl acetate and water was added andorganic layer was separated. Then, aqueous layer was againextracted with ethyl acetate three times. The combined organiclayers were washed with water, dried over MgSO4, ltered andevaporated under reduced pressure. The residue was puried bycolumn chromatography to give the desired pure products. Thephysical and spectral (IR, 1H NMR and 13C NMR) data of theknown products were found to be identical with those reportedin the literature.3,4,18

N-Phenyl formamide (Table 2, entry 1): 1H NMR (90 MHz,CDCl3): dH 7.61–7.12 (m, 5H), 8.29 (s, 1H, cis), 8.66 (d, 1H, trans),8.99 (brs, 1H, cis), 9.38 (brs, 1H, trans).

N-(2,4-Dimethylphenyl) formamide (Table 2, entry 2): 1HNMR (300 MHz, DMSO-d6): dH 2.28 (s, 3H), 2.29 (s, 3H), 7.66 (d,1H, J ¼ 8.7), 8.43 (d, 1H, J ¼ 15.5).

N-(2-Methylphenyl) formamide (Table 2, entry 3): 1H NMR(90 MHz, acetone-d6): dH 8.41 (s, 1H), 7.95 (d, 1H), 6.98–7.29 (m,4H), 2.29 (s, 3H).

N-(4-Methylphenyl) formamide (Table 2, entry 4): 1H NMR(300 MHz, CDCl3): dH 2.29 (S, 3H), 6.98–8.32 (m, 4H), 8.62 (s,1H), 8.89 (s, 1H).

N-(2-Chlorophenyl) formamide (Table 2, entry 6): 1H NMR(90 MHz, acetone-d6): dH 6.95–8.36 (m, 5H), 8.93 (s, 1H).

N-(4-Bromophenyl) formamide (Table 2, entry 7): 1H NMR(90 MHz, CDCl3): dH 6.31 (s, 1H), 6.52–7.46 (m, 4H), 8.61 (s, 1H).

N-(4-Chlorophenyl) formamide (Table 2, entry 8): 1H NMR(300 MHz, CDCl3): dH 7.02–7.67 (m, 4H), 8.34 (s, 1H), 8.65 (s,1H).

N-(4-Acetylphenyl) formamide (Table 2, entry 9): 1H NMR (90MHz, acetone-d6): dH 2.42 (s, 3H), 7.65 (d, J ¼ 8.8 Hz, 2H), 7.86(d, J ¼ 9.0 Hz, 2H), 8.32 (s, 1H), 9.42 (s, 1H).

N-(3-Fluorophenyl) formamide (Table 2, entry 10): 1H NMR(90 MHz, CDCl3): dH 7.42–8.33 (m, 4H), 8.89 (d, 1H, J ¼ 10.6),10.49 (s, 1H).

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N-(4-Nitrophenyl) formamide (Table 2, entry 11): 1H NMR (90MHz, acetone-d6): dH 7.91 (d, J ¼ 8.7 Hz, 2H), 8.25 (d, J ¼ 9.0 Hz,2H), 8.49 (s, 1H), 9.77 (s, 1H).

N-(1-Naphthyl) formamide (Table 2, entry 12): 1H NMR (300MHz, CDCl3): dH 7.27–8.06 (m, 7H), 8.65 (d, 1H), 8.83 (s, 1H).

Procedure for antibacterial studies

Antibacterial activity of the catalyst was tested by using thestandard well diffusion method. The bactericidal activities ofPS–ttet–Cu(II) was investigated against E. coli (ATCC 25922) andS. aureus (ATCC 25923) bacterial as Gram negative and Grampositive models, respectively. Before each experiment, all thesamples and glassware were sterilized by autoclaving at 120 �Cfor 10 min. Mueller-Hinton agar (MHA) (Merck) was used asbase medium and sterile saline was used for the preparation ofinoculum. Various concentrations (100, 200, 300, 400 & 500ppm) of each compound were made. For each concentrationseparate Petri plate with preinoculated bacteria was used. Fourto ve isolated colonies of tested organisms were picked bysterile inoculating loop and inoculated in tubes of sterile saline(5 mL in each). The inoculated tubes were incubated at 35–37 �Cfor 10 minutes and matched with 0.5 McFarland nephelometerturbidity standards. The surface of MHA was completelycultured using a cotton swab which steeped in preparedsuspension of bacterium. On each plate ve cups were madeand 25 mL of each test compound were poured on each cupcarefully. Aer incubation at 35–37 �C for 24 hours, the differentlevels of zone of inhibition of bacteria were measured in milli-meter (mm), and were recorded as mean � SD of the triplicateexperiment.

Acknowledgements

We gratefully acknowledge the Iranian Nano Council andUniversity of Qom for the support of this work.

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