ELECTROCHEMICAL PREPARATIONOF NEW ORGANOSILICONE COMPOUNDS

FOR FUNCTIONALIZING OF MESOPOROUS SILICA

HABIBEH HADAD DABAGHIDepartment of Science, Karaj Branch

Islamic Azad University, P.O. Box 31485-313Karaj, Iran

habibeh.hadad@kiau.ac.ir

MAHMOOD KAZEMZAD, YADOLAH GANJKHANLOUand AMIR ALI YUZBASHI

Materials and Energy Research CenterP.O. Box 14155-4777, Tehran-Iran

Received 31 December 2012; Accepted 10 March 2013; Published 30 April 2013

Electrochemistry has been introduced as a powerful tool in order to prepare new organometallic reagents for functionalizing ofmesoporous silica. Preparation of the reagents was based on electrochemical oxidation of dihydroxybenzene derivatives in thepresence of 3-(trimethoxysilyl)-1-propanethiol as a nucleophile. The mechanisms of electrochemical reactions were studied byvoltammetric studies. Mesoporous silica SBA-15 was also synthesized in this work through sol-gel hydrothermal method usingGenapol PF-10 as structure directing compound. The prepared mesoporous silica was characterized by FT-IR analysis andBarrett�Joiner�Halenda (BJH) pore size and Brunauer�Emmett�Teller (BET) surface area measurement methods using N2

adsorption�desorption isotherm. Finally, the organometallic reagent was covalently grafted on the surface of mesoporous silica.Functionalizing of this material with the new reagent was confirmed by Fourier transform infrared (FT-IR) spectroscopy. Thefunctionalized mesoporous silica by new reagent can be utilized in biological applications.

Keywords: Mesoporous silica; cyclic voltammetry; electrochemical synthesis; functionalized materials; hydrothermal synthesis.

Modification of solid surface by organic functionalizingreagents is important in catalytic,1�3 adsorbent2�4 or inbiological applications.5,6 There are many attempts to find newmethods or new functionalizing reagents in order to enhancethe surface activities for a variety of applications.7 However,serious limitations may be encountered for functionalizing ofmetal oxides because of dissimilarities between inorganicsupport and organic reagents. Varieties of organometalliccompounds are generally utilized for surface modification,among them organosilane compounds are preferred. This isdue to the fact that the position of silicone just below thecarbon in periodic table satisfies suitable matching of organichydrocarbons with inorganic support. Nevertheless, because ofcomplication of conventional synthetic methodologies appliedfor preparation of organometallic compounds, there are manyattempts to develop easy and controlled synthetic methodsfor preparation of new organosilicone compounds.8 On theother hand, electrochemistry is a suitable technique for bothsynthesis and analysis of organic and inorganic chemicals.

Electrochemical synthesis is a powerful technique forpreparation of various compounds due to its capability incontrolling both chemical and electrical parameters affectingthe whole reaction.9 In fact, it is usually the preferred methodwhenever there is a risk of the side reactions which produceundesired products. It is also the favorite technique to createunstable intermediates reacting with the target molecules.10

Moreover, electrochemical methods can significantly con-tribute to the protection of the environment through the min-imization of waste and toxic materials production. Therefore,electrochemistry is known as environmentally friendly methodfor synthetic attempts.11

Silicone oxides with the micro-, meso-, or macro porosityare very important group of metal oxide compounds withrespect to their applications as adsorbents,12 sensors,13 ionexchangers14, and heterogeneous catalysts.15�17 Their effi-ciency in all of these applications can be enhanced by modi-fication of their surface as well as controlling the pore size andshape.

Functional Materials LettersVol. 6, No. 3 (2013) 1350031 (4 pages)© World Scientific Publishing CompanyDOI: 10.1142/S1793604713500318

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Biological application of functionalized materials needs thesurfaces to be modified with biologically reactive moleculeslike dihydroxybenzenes. These compounds have biologicalcharacteristics such as anti- and pro-oxidant behavior.18

The nuclephilic addition of the sulfur compounds toquinone has been comprehensibly reviewed.19,20 Quinones,containing a polarized double bond, are considered as Michaelacceptors, whereas the thiol nucleophiles are Michael donors.Traditionally the 1,4-addition of mercaptans are catalyzed bystrong bases such as alkali metal alkoxides, hydroxides,21 andamines.22 Scheme 1 represents the Michael addition of asulfur anion to oxidized form of hydoquinone which can besimply produced on the surface of an electrode. There arelimited numbers of reports on C�C bond formation throughelectrochemical addition of mercaptanes.23 It may be relatedto complexity arising from the risk of free radical couplingof sulfur compounds whenever they are electrochemicallyoxidized at the surface of electrodes.

Previous publication of our group demonstrate electro-chemical oxidation of hydrocaffeic acid in one compartmentcell has capability to produce quinone as a reactive inter-mediate for Michael addition of neucleophiles.24,25 In currentreport, the same idea is developed for preparation of orga-nosilicone compounds utilizing dihydroxybenzene derivativesin the presence of 3-(trimethoxysilyl)-1-propanethiol (TMPT)as a sulfur containing neucleophile. The main concept ofthis work is to introduce electrochemistry as an easy tech-nique for preparation of new reagents from the commerciallyavailable compounds and also covalent grafting of thesereagents onto the mesoporous silica surface as a typicalnanostructured material. Finally, modification of surface bycatechol derivative as a biologically important material hasbeen carried out. It should be pointed out those catecholderivatives as well as other polyhydroxyaromatic compoundssuch as Caffeic and Gallic acid derivatives are considered fortheir potential HIV integrase inhibitory activity. Caffeic acidis found in all plants because it is a key intermediate in thebiosynthesis of lignin, one of the principal sources of bio-mass. Gallic acid also known as 3,4,5-trihydroxybenzoic acid,is found in gallnuts, sumac, witch hazel, tea leaves, oak bark,and other plants.26

The structure directing reagent Genapol PF-10 was pro-duct of Clariant Corporation. Tetraethylorthosilicate (TEOS),

Catechol, 3-(Trimethoxysilyl)-1-propanethiol (TMPT) and allother chemicals were purchased from Merck and used with-out further purification. The cyclic voltammetry (CV) anal-ysis was performed using a three electrode 747 MetrohmTrace analyzer with glassy carbon disc as working electrodeand Pt rode as auxiliary and Ag/AgCl as reference electrodes.The glassy carbon was polished on 0.05 micron alumina pasteand washed with copious amount of deionized water beforeeach experiment. Bulk electrolysis was performed using aPotentiostat-Galvanostat PAAR173 with graphite rods asworking and counter electrodes.

Mesoporous silica prepared according to a previouslyreported procedure.27,28 The procedure started briefly bydissolution of a desired amount of Genapol PF-10 in 0.4MHCl at 35○C for 6 h under vigorous stirring. Then, TEOSwas added to this solution and mixed at the same temperaturefor 24 h. The temperature of hybrid material was raised upto 100○C for 24 h following by filtering, washing, dryingand burning off the template at 250○C for 2 h and then at550○C for 2 h (Fig. 1). Barrett�Joiner�Halenda (BJH) poresize and Brunauer�Emmett�Teller (BET) surface areameasurement methods using N2 adsorption�desorption iso-therm have been performed by the Micromeritics Gimini III2375 instrument. Fourier transform infrared spectroscopy(FT-IR) spectra were obtained on Bruker v33 with KBr discs.

Glassy carbon electrodes polished on 0.05 micron alu-mina paste before each experiment. A 0.25mM solution of1,2 �dihydroxy benzene was prepared by dissolution of itsdesired amount in 0.2M phosphate buffer solution (PBS)with pH of 7.2. Then 15 μL of TMPT was added into theelectrochemical vessel containing 15mL of the above solu-tion which was stirred for 5min under nitrogen bubblingbefore each experiment. CVs recorded in different pHs andscan rates (10�500mv/s) (not shown here); the best resultsobtained at 50mv/s in PBS buffer solution with pH ¼ 7:2.Bulk electrolysis of 100mL solution containing 0.25mMcatechol and equivalent molar amount of TMPT was per-formed by PAAR 173 potentiostat-galvanostat system.

All of electrochemically produced compound dried atroom temperature. Then it was added to the solution con-taining 100mg of the prepared SBA-15 dispersed in 50mLof absolute ethanol. The mixture was stirred for 24 h underreflux condition. Finally, the precipitate was filtered andwashed with copious amounts of water, ethanol and acetone,respectively. The powder was dried in an oven at 50○C for5 h and investigated by FT-IR spectroscopy (Fig. 1).

CVs of TMPT at various pHs (not demonstrated here)showed no electrochemical activity in the potential windowselected for electro-oxidation of catechol. However, there is arisk of blocking the surface due to the hydrolysis of alkox-ysilanes in extreme pH values particularly in long time bulkelectrolysis reaction. Moreover, many biologic reactions are

Scheme 1. Addition of sukfur containing neucleophile to quinone.

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sensitive to harsh pH conditions. So, electrochemical reac-tions had been performed under neutral pHs.

Figure 2 shows that the reduction peak of catechol iscompletely disappeared in the presence of TMPT with scanrate of 50mv/s at pH ¼ 7:2 indicating covalent addition ofthe nucleophile to this substrate.

Figure 3 shows the multiple voltammograms recordedin the same condition. It can be observed that there are noadditional peaks after nucleophilic attack. Otherwise, electro-active chemicals with oxidation potentials lower than pre-cursors may be produced which cause formation of undesiredproducts. The obtained data supports applicability of themethod for preparation of functionalizing reagent through

bulk electrolysis due to electrochemical stability of theproduct.

This result is also pointing to elechrochemical-chemical(EC) mechanism of the reaction which is shown in Scheme 2.The nucleophilic addition to catechol is mainly occurred inortho and para positions. However, ortho position of both OHgroups in catechol is deactivated by another OH. Basedon theabove discussion, the main nucleophilic addition should betake place in para position.

Adsorption�desorption isotherm of N2 on surface ofprepared mesoporous silica has also depicted in Fig 4. Theadsorption�desorption isotherm satisfying to type IV iso-therm which indicates meso-structure of prepared silica. Thesurface area calculated from this isotherm using BET modelsindicates that the sample has estimated surface area of640m2/g. In addition, the average pore size has also beencalculated by BJH method for this sample which is about6 nm. This results clearly point toward highly porous structureof synthesized mesoporous silica and therefore, this samplehas been selected for functionalizing by the new reagentprepared in the current work.

In order to check functionalizing of mesoporous silica theFT-IR spectroscopy has been employed. Figure 5 shows FT-IR

Fig. 1. Flowchart of experiments.

Fig. 3. Catechol oxidation in the presence of triethoxysilylproanethiol(additional cycles) (50mv/s).

Fig. 2. Catechol oxidation in the absence and presence of triethoxysilyl-proanethiol (50mv/s). Scheme 2. Suggested EC mechanism.

Electrochemical Preparation of New Organosilicone Compounds

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spectrum of mesoporous silica before and after functiona-lizing by electrochemically produced catechol product. Thepeaks related to stretching vibration modes of CH2, C¼S,C¼C aromatic bonds and also bending modes of C�H bondappeared after functionalizing which clearly confirms thecovalent binding of new reagent to mosoporous silica sur-face. Scheme 3 shows the mechanism of functionalizing ofmesoporous silica by the new reagent.

The results of this work showed that TMPT can be utilizedas a new nucleophile to attack o-quinonone which is unstableintermediate product of electrochemical oxidation of cate-chols to produce new stable reagents. The electrochemicalreaction was simply performed in a beaker as an undividedcell and prepared reagents can be applied in modification ofthe surface for different applications.

By our knowledge, this is the first report on electro-chemical preparation of new organosilicon compounds formodification of various surfaces. Considering the high inter-action of catechol molecules and its derivatives with biolog-ical molecules, we believe that the nanostructured compoundfunctionalized in this work may have potential applications inbiological issues.

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Fig. 5. FT-IR spectrums of Catechol þ triethoxysilylproanethiol, Catechol,and propanethiol modified SBA-15.

Scheme 3. Mechanism of functionalizing of mesoporous silica by newreagent.

Fig. 4. Absorption�desorption isotherm of the prepared mesoporous silicasample (SBA-15).

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