New strategy for chemically attachment of Schiff base complexes on Multiwalled Carbon Nanotubes surfaces

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Introduction
Until the 1980s, the carbon universe was built on the well-known modifications, graphite and diamond.This perspective totally changed with the discovery of the carbon allotropes such as fullerenes, carbon nanotubes 1 (CNTs), carbon nanohorns, and carbon onions.Because of so many outstanding performances, CNTs exhibit great promise for potential applications in many technological fields such as hydrogen storage 2-4 , catalyst supports 5,6 , chemical sensors 7,8 and nanoelectronic devices 9 .One of the most powerful approaches to improve CNT handling is the covalent functionalization of their side walls and tips [10][11][12][13] .A wide variety of reactions has been described for functionalization of CNTs.Because amine groups are versatile and undergo a variety of reactions, particular interest in functionalization processes is the attachment of amino groups (-NHR or NH 2 groups) on CNT surfaces.In fact, the aminated MWCNTs have some advantageous for several applications, such as the attachment of nanoparticles to single and multiwalled carbon nanotubes [14][15][16] , the attachment of DNA to SWCNTs 17,18 and carbon nanofibers 19,20 and also immobilization of Schiff base complexes 21 on CNT surfaces.The Schiff bases and their metallic complexes have catalytic properties and can be used as homogen and heterogeneous catalysts in wide variety of chemical reactions [22][23][24][25][26] .The homogeneous catalysts, have some drawbacks such as difficulties in the catalyst recovery and product separation in contrast to heterogeneous catalysts, that have many advantages such as easy separation and facile recovery of the solid catalyst from the reaction mixture.In recent years, the interest of many researchers has been focused on heterogenizing of homogeneous catalysts, and there are a number of successful demonstrations of immobilized homogeneous complexes on supports.Heterogenization of homogeneous catalysts with similar activities affords an appealing methodology to expand the industrial application of homogeneous catalysts; for this purpose, immobilization of metal Schiff base complexes on organic or inorganic supports had been widely reported [27][28][29] .Immobilization via covalent bond on the support is more advantageous and improving the long-term stability of the solid catalyst.
Here we report a simple convenient method for chemically attachment of salen Schiff bases and Schiff base complexes to MWCNTs surfaces.In the first step of this method, we try to aminated of MWCNTs through a new one pot reaction and in second step, attachment of synthetic chlorinated Schiff bases (on CNT surfaces) performed under a nucleophilic substitution reaction.Complexation of attached salen Schiff base on MWCNTs (Salen complex@MWCNTs) has been occurred through the reaction of transition metal salts and salen@MWCNTs.

Results and discussion
The present method is containing to several steps: 1. amination of MWCNTs, 2. synthesis of chlorinated Schiff bases, 3. chemically attachment of Schiff bases to functionalized MWCNTs and 4. complexation of attached Schiff bases on MWCNTs.The products of every step have been characterized with FTIR, UV-Vis absorption, XRD, SEM and EDX methods.In continue, we discuss about the obtained results.

Conclusion
Here we present a new method for chemically attachment of salen complexes to MWCNTS surfaces through a clean, convenient and fast method.Functionalization of MWCNTs with amino groups in first step, provide a useful material for attachment of chlromethylated salen Schiff bases via a nucleophilic substitution reaction.Analysis results show that CNTs were functionalized in a high concentration.After the immobilizing the Schiff bases to CNT surfaces, reaction of them with Cu and Ni salts, leads to Salen complex@MWCNTs.
Our method in functionalization of CNT surfaces has some advantages in compare with other reported results 6, 21 , because of; here we don't use acids and hazardous reagents in procedure of the attachment of Schiff bases on CNT surfaces.Also aminated CNTs were prepared in fast and clean conditions.In fact, this new procedure provides functionalized CNTs (salen@MWCNTs and Salen complex@MWCNTs) with high concentration of functional groups.The final reaction products can be used as heterogeneous catalysts in chemical reactions.The process has proved to be very effective, safe and easy to operate and also the scale up of this method is easy.

Aknowledgment
We are grateful to the University of Kashan Research Council for the partial support of this work and thankful to the Department of Organic Chemistry, Payame Noor University of Tehran

Materials
Multiwalled carbon nanotubes were obtained from Shenzhen Nanotechnology Co., Ltd.(China) The Purity of the CNTs was about 90-95%, with their diameters and lengths ranging between 20-40 nm and 5-15 μm, respectively.The other reagents and solvents were purchased from Merck Company.

Apparatus
FT-IR spectra were recorded on a Bruker ISS-88 spectrophotometer in KBr pellets. 1 H NMR (400 MHz) spectra were measured in CDCl 3 solvent and referenced to the solvent signals by Bruker Avance DPX instrument.The spectra in the UV-Vis range were taken using Cary 100 varian el 12092335 UV-Vis scanning spectrometer.ASTRA 3D Bath sonicator was applied for debundeling of MWCNTs also XRD instrument model Philips Analytical PC-APD was used for characterization of the reaction products.Cambridge 360 scanning electron microscope (SEM) with energy dispersive Xray spectroscopy (EDX) analysis was used for surface image measurements and chemical characterization of Cu and Ni salencomplex@MWCNTs.

General procedure for amination of MWCNTs
180 mg of AlCl 3 mixed with 10 mg MWCNTs and grind for 5 minutes and then added to 50 ml CH 2 Cl 2 .The mixture was sonicated for 10 minutes in bath sonicator.After that, the homogen mixture was irradiated in a microwave oven for 10 minutes with power of 900 watt.After this time, 200 mg of ammonium benzoate (as ammonia source) was added to the mixture and the reaction continues for 10 minutes under microwave irradiation with power of 900 watt.After this time, the mixture cool to room temperature and filtered.Then 50 ml warm ethanol were added and sonicated for 10 min to remove remained unreacted compounds, and then the mixture was filtered and the resulting products were dried at 80C for 10 hours (Scheme 1).

Chloromethylation of Salicylaldehyde
17.5 g (160 mmol) salicylaldehyde, 24 ml formaldehyde, 1.2 g ZnCl 2 and 100 ml of concentrate HCl were poured in a 250 ml flask and stirred for 24 hours at room temperature (under N 2 atmosphere).After this time, a white solid was formed .To isolate the product, [5-chloro-methyl-2hydroxy-Benzaldehyde (1)], the solids dissolved in diethyl ether, and the organic phase was washed by saturated sodium bicarbonate.For removal of water from organic phase, a small amount of magnesium sulfate was added and crystallization was performed in petroleum ether 30 (Scheme 2).

General procedure for synthesis of Schiff Bases
3.41 g (20 mmol) of 5-chloro-methyl-2-hydroxy-Benzaldehyde (1) was dissolved in a minimum of dichloromethane.Then, according to the stoichiometry of 1 to 2 of diamine to 1, the amounts of diamines was mesured and dissolved in minimum volume of dichloromethane and were added to 1.The mixture was stirred in room temperature for 1 to 3 hours (the completion of reaction monitored by TLC).After this time, the solution was filtered and the obtained precipitate was washed with dichloromethane and dried in 70 o C for 6 hours (Scheme 2).4.6.General procedure for synthesis of salen @MWCNTs 50 mg of aminated MWCNTs were added to 8 ml of methanol containing 100 mg of Schiff base and refluxed for 24 hours at 70°C.After this time, the reaction mixture was filtered and washed with methanol and finally was dried at 70°C in oven (Scheme 3).

Settlin
To estim Aminated M aminated M between am when hydro stability of d

UV-Vis absorption
From comparison of adsorption spectra of salen Schiff bases (Fig. 6) and salen@MWCNTs (Fig. 8), it clearly can be recognized, that the Schiff bases were chemically attached to the surfaces of functionalized CNTs.