Technical Notes

Analysis of Hybrid Silica Materials with the Aid ofConventional NMR and GC/MS

Rossella Mello,†,‡ Andrea Olmos,‡ Teresa Varea,‡ and Marıa Elena Gonzalez-Nunez*,‡

Departamento de Quımica Organica, Facultad de Farmacia, Universidad de Valencia, Avda. Vicente Andres Estelless.n. 46100-Burjassot, Valencia, Spain, and Fundacion General de la Universidad de Valencia (FGUV), Valencia, Spain

Two simple and straightforward procedures for determin-ing the organic content of hybrid silica materials by meansof conventional NMR and GC/MS techniques are de-scribed. The methods involve dissolving the hybrid mate-rial either in a concentrated solution of sodium hydroxidein deuterated water containing a suitable reference or ina solution of hydrogen fluoride in water and extractingwith methylene chloride. These methods constitute usefulroutine techniques for obtaining immediate informationconcerning both the amount and chemical compositionof the organic compounds on the silica surface.

The development of hybrid materials with large specific surfaceareas and tunable morphology is currently a field of extensiveresearch, particularly with regard to potential applications in areassuch as adsorption, chromatography, heterogeneous catalysis,sensor technology, and gas storage.1,2

The organic functionalization of mesoporous silica is generallyachieved1-3 through postsynthesis grafting of either alkoxy- orchloroorganosilanes onto the surface of the presynthesized silicamaterial, copolymerization of an organosilane with a silica precur-sor, or the direct functionalization of the silica surface withorganolithium or organomagnesium reagents.4 Occasionally,however, depending on the complexity of the organic ligand tobe immobilized onto the silica surface or on the final applicationof the hybrid material itself, it is necessary to carry out some typeof chemical modification to the organic moieties anchored ontothe silica. This can include, for instance, acylation of pendantamino or hydroxyl groups as well as functional group intercon-version. As with any other process involving organic synthesis,

suitable techniques for quantifying and characterizing the sub-strate and the reaction products are necessary for obtaining validinformation concerning the efficiency of the reaction and thefactors influencing its course.

Several well-established analytical techniques5-7 have beenapplied to characterize hybrid silica materials, as, for instance,thermogravimetric analysis (TGA),6d,j,n,q,r differential scanningcalorimetry(DSC),6n,relementalanalysis,6c,h,j,pBrunauer-Emmett-Teller N2 adsorption isotherm measurements (BET),6c,d,h,m,o,p,x

transmission electron microscopy (TEM),6a,f,i,k,m scanning electronmicroscopy (SEM),6f,k,m atomic force microscopy,6c

electrophoresis,6w inductively coupled plasma atomic emissionspectroscopy (ICP-AES),6b,v extended X-ray absorption fine

* To whom correspondence should be addressed. E-mail: elena.gonzalez@uv.es.

† Fundacion General de la Universidad de Valencia (FGUV).‡ Departamento de Quımica Organica, Facultad de Farmacia, Universidad de

Valencia.(1) (a) Hoffmann, F.; Cornelius, M.; Morell, J.; Froba, M. Angew. Chem., Int.

Ed. 2006, 45, 3216. (b) Descalzo, A. B.; Martınez-Manez, R.; Sancenon,F.; Hoffmann, K.; Rurak, K. Angew. Chem., Int. Ed. 2006, 45, 5924.

(2) (a) Zhang, C; Li, H.; Jiang, D.; Yang, Q. Chem. Commun. 2007, 547. (b)McMorn, P.; Hutchings, G. J. Chem. Soc. Rev. 2004, 33, 108. (c) DeVos,D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Chem. Rev. 2002, 102, 3615.

(3) Sol-Gel Science. The Physics and Chemistry of Sol-Gel Processing; Brinker,C. J.; Sherer, G. W. Academic Press: London, 1990.

(4) (a) Yamamoto, K.; Tatsumi, T. Microporous Mesoporous Mater. 2001, 44-45, 459. (b) Yamamoto, K.; Tatsumi, T. Chem. Lett. 2000, 6, 624.

(5) Willard, H. H.; Merrit, L. L., Jr.; Dean, J. A.; Settle, F. A., Jr. InstrumentalMethods of Analysis, 7th ed.; Wadsworth Publishing Co.: Belmont, CA, 1988.

(6) See for instance: (a) Sharma, K. K.; Anan, A.; Buckley, R. P.; Ouellette,W.; Asefa, T. J. Am. Chem. Soc. 2008, 130, 218. (b) Yang, C.-M.; Lin, H.-A.; Zibrowius, B.; Spliethoff, B.; Schuth, F.; Liou, S.-C.; Chu, M.-W.; Chen,C.-H. Chem. Mater. 2007, 19, 3205. (c) Shin, J. H.; Metzger, S. K.;Schoenfisch, M. H. J. Am. Chem. Soc. 2007, 129, 4612. (d) Cheng, K.;Landry, C. C. J. Am. Chem. Soc. 2007, 129, 9674. (e) Blanc, F.; Thivolle-Cazat, J.; Basset, J.-M.; Coperet, C.; Hock, A. S.; Tonzetich, Z. J.; Schrock,R. R. J. Am. Chem. Soc. 2007, 129, 1044. (f) Ling, X. Y.; Reinhoudt, D. N.;Huskens, J. Langmuir 2006, 22, 8777. (g) Budny, A.; Novak, F.; Plumere,N.; Schetter, B.; Speiser, B.; Straub, D.; Mayer, H. A.; Reginek, M. Langmuir2006, 22, 10605. (h) Corma, A.; Gutierrez-Puebla, E.; Iglesias, M.; Monge,A.; Perez-Ferreras, S.; Sanchez, F. Adv. Synth. Catal. 2006, 348, 1899. (i)Yang, Q.; Yang, J.; Liu, J.; Li, Y.; Li, C. Chem. Mater. 2005, 17, 3019. (j)Acosta, E. J.; Carr, C. S.; Simanek, E. E.; Shantz, D. F. Adv. Mater. 2004,16, 985. (k) Radu, D. R.; Lai, C.-Y.; Wiench, J. W.; Pruski, M.; Lin, V. S.-Y.J. Am. Chem. Soc. 2004, 126, 1640. (l) Robbins, M. E.; Schoenfisch, M. H.J. Am. Chem. Soc. 2003, 125, 6068. (m) Liy, C.; Lambert, J. B.; Fu, L.J. Am. Chem. Soc. 2003, 125, 6452. (n) Dufaud, V.; Davis, M. E. J. Am.Chem. Soc. 2003, 125, 9403. (o) Armatas, G. S.; Salmas, C. E.; Louloudi,M.; Routsopoulos, G. P.; Pomonis, P. J. Langmuir 2003, 19, 3128. (p)Antochshuk, V.; Jaroniec, M. Chem. Commun. 2002, 258. (q) Descalzo,A. B.; Jimenez, D.; Marcos, M. D.; Martınez-Manez, R.; Soto, J.; El Haskouri,J.; Guillem, C.; Beltran, D.; Amoros, P.; Borrachero, M. V. Adv. Mater.2002, 14, 966. (r) Jarupatrakorn, J.; Tilley, T. D. J. Am. Chem. Soc. 2002,124, 8380. (s) Corma, A.; Garcıa, H.; Moussaif, A.; Sabater, M. J.; Zniber,R.; Redouane, A. Chem. Commun. 2002, 1058. (t) Xiang, S.; Zhang, Y.;Xin, Q.; Li, C. Angew. Chem., Int. Ed. 2002, 41, 821. (u) Liu, A. M.; Hidajat,K.; Kawi, S.; Zhao, D. Y. Chem. Commun. 2000, 1145. (v) Dai, S.; Burleigh,M. C.; Shin, Y.; Xue, Z. Angew. Chem., Int. Ed. Engl. 1999, 38, 1235. (w)Beck, C.; Hartl, W.; Hempelmann, X. X. Angew. Chem. Int. Ed. Engl. 1999,38, 1297. (x) Fowler, C. E.; Lebeau, B.; Mann, S. Chem. Commun. 1998,1825. (y) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemner,K. M. Science 1997, 276, 923.

(7) Laws, D. D.; Bitter, H.-M. L.; Jerschow, A. Angew. Chem., Int. Ed. 2002,41, 3096.

Anal. Chem. 2008, 80, 9355–9359

10.1021/ac801248m CCC: $40.75 2008 American Chemical Society 9355Analytical Chemistry, Vol. 80, No. 23, December 1, 2008Published on Web 10/28/2008

structure,6y powder X-ray diffraction (XRD),6a,b,i,k,m,n,u-x matrix-assisted laser desorption/ionization mass spectrometry,6j Fouriertransform infrared spectroscopy (FT-IR),6e,f,j,m,r-t,x diffuse reflec-tance or absorption UV-vis spectroscopy,6a,b,d,g,h,l,s,v,x dynamiclight scattering (DLS),6f,g,w and cross-polarization and magic spinangle nuclear magnetic resonance (CP/MAS NMR),6,7 amongothers.6 Although these techniques can provide valuable informa-tion about the shape, active surface, size, porosity, and aggregationof the particles, structure, and topological distribution of theorganic ligands bonded to the silica matrix and also about thephysicochemical properties of the hybrid material, none of themcan be considered simple or easily accessible routine methodsfor obtaining immediate preliminary information about the amountand structure of the organic ligands anchored onto the silicasurface.

In this paper, we describe two efficient and straightforwardroutine methods involving conventional NMR and GC/MS tech-niques for determining the organic ligands anchored onto meso-porous silica (eq 1). The first method entails dissolving the hybridsilica material in a concentrated solution of sodium hydroxide indeuterated water containing a known amount of a suitablestandard. Subsequent 1H and 13C NMR analysis of the samplesallows for characterization and quantification of the organiccontent of the silica materials. A second, complementary approachinvolves dissolving the hybrid silica material in aqueous hydro-fluoric acid or in hydrogen fluoride-pyridine complex and thenextracting the resulting mixture with an organic solvent andanalyzing the organic solution with the aid of conventional gaschromatography-mass spectrometry techniques. These methodscan be routinely applied to determine the structural changes tothe organic ligands resulting from the chemical reactions per-formed on hybrid materials, the molar ratio of the different ligandsanchored onto the silica surface, the availability of free silanolgroups on the silica surface, and the amount of surfactant presentin the mesoporous material.

EXPERIMENTAL SECTIONNMR Analysis of Hybrid Silica. A suspension of 50 mg of

hybrid silica in 1 mL of a 4.9 M NaOH and 0.15 M methanolsolution in deuterated water was sonicated at room temperatureuntil complete dissolution of the solid material (from 5 min toseveral hours, depending on the hydrophobicity of the hybridmaterial). The solution was transferred to an NMR tube, and thespectra were recorded under conventional conditions. Ethyleneglycol, ethanol, and dioxane are all suitable alternatives tomethanol as a standard.

GC/MS Analysis of Hybrid Silica. (A) To a suspension of0.1 g of hybrid silica in 2 mL of bidistilled water placed in a Teflonor polypropylene tube at 0 °C, 1 mL of a cold, 35% aqueous solutionof hydrogen fluoride was carefully added under stirring. The

mixture was stirred at 0 °C until complete dissolution of the solid(from 1 min to 1 h, depending on the hydrophobicity of the hybridmaterial), after which it was treated with 1 mL of methylenechloride under stirring for 5 min at 0 °C. The mixture was allowedto decant, and the organic layer was transferred to a vial containinga mixture of sodium bicarbonate or potassium hydrogen phos-phate and anhydrous magnesium sulfate with the aid of apolypropylene cannula. The organic solution was then filtered andanalyzed by means of GC/MS.

(B) To 0.1 g of hybrid silica placed in a Teflon or polypropylenetube, 0.5 mL of hydrogen fluoride-pyridine complex was addedat once under stirring at 0 °C. The mixture was stirred untilcomplete dissolution of the solid (30 s in all cases), after which itwas treated with 1 mL of methylene chloride under stirring for 5min at 0 °C. The mixture was allowed to decant, and the organiclayer was transferred to a vial containing a mixture of sodiumbicarbonate or potassium hydrogen phosphate and anhydrousmagnesium sulfate with the aid of a polypropylene cannula. Theorganic solution was filtered and analyzed by means of GC/MS.

Solutions of organic compounds carrying trifluorosilyl groupsin methylene chloride are stable only for ∼30 min at 0 °C andshould be analyzed immediately after their preparation in orderto obtain valid and reproducible results. The impact of theseanalytes on GC column performance is similar to that of any otherfluorinated compound provided that samples have been carefullyneutralized prior to analysis.

RESULTS AND DISCUSSIONThe analysis of hybrid materials with the aid of NMR

techniques is illustrated in the following example. A sample of 58mg of [4-carboxybutyl]-functionalized silica (1a), previouslyprepared through acid hydrolysis of [5-ethoxy-4-(ethoxycarbonyl)-5-oxobutyl]-functionalized silica (1b), was completely dissolvedin 1 mL of a 4.9 M NaOH solution in D2O containing methanol asa standard (0.15 M) at room temperature under sonication. 1Hand 13C NMR spectra of the aqueous sample were recorded. The1H NMR spectrum (a, Figure 1) showed well-resolved multipletsat 0.43, 1.38, 1.58, and 2.21 ppm, which correspond to themethylene groups of the ionized 4-carboxybutylsilanetriol, as well

Figure 1. 1H NMR spectra of (a) [4-carboxybutyl]-functionalized silica(1a), (b) [5-ethoxy-5-oxopentyl]-functionalized silica (1c), and (c)dimethylsilylated [5-ethoxy-5-oxopentyl]-functionalized silica (1e).

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as a singlet at 3.43 ppm, which corresponds to methanol.Integration of these signals indicated a 0.209 M concentration forthe organic silanetriol in the sample, which corresponds to a ratioof 3.62 mmol of organic ligand per gram of hybrid material.

The presence of the ionizable silanetriol group bonded to theorganic ligand favors its solubility in basic aqueous solution.However, because the strong basic medium required to dissolvethe silica framework can lead to the chemical modification of theorganic ligands anchored onto the silica surface, the eventualoccurrence of base-promoted reactions should be taken intoaccount when interpreting the NMR spectra. For instance, theNMR analysis of 50 mg of (5-ethoxy-5-oxopentyl]-functionalizedsilica (1c), which was prepared by treating the hybrid silica 1awith a 1.1 M solution of sulfuric acid in anhydrous ethanol underreflux for 20 h followed by filtering, thorough washing with water,and drying under vacuum, showed the products derived from thehydrolysis of the ester groups as a result of the basic conditionsused in this sample preparation (Figure 1b). Integration of thesignals corresponding to the methylene groups adjacent to thecarboxy and hydroxy functionalities showed that the conversionof the carboxylic acids into esters reached 91% under the reactionconditions used.

1H NMR analysis was also used to determine the number oforganic ligands grafted onto silica through the reaction ofmesoporous MCM-41, which was prepared following reportedprocedures,8 with 3-aminopropyltriethoxysilane in toluene underreflux for 4 h. Thus, the 1H NMR spectrum of a solution of 50.4mg of [3-aminopropyl]-functionalized silica (1d) in 1 mL of 4.9 MNaOH solution in D2O (Figure 2a) indicated the presence of 3.27mmol of organic ligands per gram of material.

Acylation of alkylamino groups anchored to silica is a conve-nient method for grafting complex organic structures onto a silicasupport. When the grafted moiety is intended to serve as a ligandfor a transition metal, care should be taken to avoid the presenceof free amino groups, which can interfere in the preparation of awell-defined organometallic compound. In these instances, theavailability of a routine method for quantifying the conversion ofthe ligands is critical. In order to establish the usefulness of ouranalytical method in these cases, we reacted [3-aminopropyl]-

functionalized silica (1d) with 2 equiv of benzoyl chloride indichloromethane in the presence of 1.3 equiv of pyridine at roomtemperature for 3 h. The solid was then washed with dichlo-romethane and dried under vacuum. 1H NMR analysis of thismaterial (Figure 2b) indicated that 74% of the ligands on the silicasurface were free 3-aminopropyl ligands while 26% were 3-ben-zoylaminopropyl ligands. In all likelihood, the free silanol groupson the silica surface react with the acylating reagent, therebydiminishing the efficiency of the reaction.

This type of NMR analysis is also useful for determining thepresence of surfactant in mesoporous silica.8,9 A sample of MCM-41 prepared in accordance with reported procedures8 was washedsuccessively with water and ethanol in a Soxhlet extractor forseveral days. The solid was then dried under vacuum until aconstant weight was reached. 1H NMR analysis of this materialwas performed as described above. Integration of the signalscorresponding to methanol and the methylene group adjacent tothe ammonium moiety showed the presence of 0.58 mmol ofcetyltrimethylammonium salt/g of material. After treating10 thesolid twice with a mixture of methanol and hydrochloric acid underreflux for 3 h, followed by washing with distilled water and dryingunder vacuum, the 1H NMR spectrum of the material displayedthe total absence of surfactant.

NMR analysis can also be used to determine the availabilityof free silanol groups on the silica surface for reactions with chloro-or alkoxysilanes. Thus, a sample of mesoporous MCM-41 wasprepared following a standard procedure,8 and a portion of itwas calcined at 500 °C for 5 h. A portion of the calcined materialwas then rehydrated through treatment with 20% aqueous sulfuricacid under reflux for 60 h, filtered, washed with bidistilled water,and dried under vacuum at room temperature. Samples of thesethree materials were then treated with an excess of dichlorodi-methylsilane under reflux for 4 h, washed with dichloromethane,and dried under vacuum. 1H NMR analyses of the resultingmaterials were performed as described above. Integration of thesignals corresponding to methanol and to the dimethylsilyl ligandsin the 1H NMR spectra indicated the presence of 11 mmol of freesilanol groups/g of the original MCM-41, 4.7 mmol of free silanolgroups/g of the calcined MCM-41, and 8.2 mmol of free silanolgroups/g of the rehydrated MCM-41. Interestingly, we found thattreatment of a sample of silica MCM-418 containing a significantamount of surfactant with an excess of dichlorodimethylsilaneunder reflux, followed by washing with dichloromethane anddrying under vacuum, led to a silylated material that wascompletely free of surfactant.

The strongly hydrophobic nature of dimethylsilylated hybridsilicas makes their dissolution in the strongly basic aqueoussolution required for preparing NMR samples extremely difficult.For example, a sample of 51.4 mg of dimethylsilylated [5-ethoxy-5-oxopentyl]-functionalized silica (1e) required 10 h under soni-cation to dissolve in 1 mL of a 4.9 M aqueous solution of NaOH.The 1H NMR spectrum did not exhibit any significant degradationof the organic ligands (Figure 1c) and indicated the presence of2.28 mmol of free silanol groups on the surface of the function-alized silica per gram of the solid. However, for hybrid silicas

(8) (a) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, E. M.; Cresge, C. T.;Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen,S. B.; Higgins, J. B.; Schelenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834.(b) Zhao, X. S.; Lu, G. Q.; Millar, G. J. Ind. Eng. Chem. Res. 1996, 35,2075.

(9) Wan, Y.; Zhao, D. Chem. Rev. 2007, 107, 2821.(10) Kumar, R.; Chen, H.-T.; Escoto, J. L. V.; Lin, V. S.-Y.; Pruski, M. Chem.

Mater. 2006, 18, 4319.

Figure 2. 1H NMR spectra of (a) [3-aminopropyl]-functionalized silica(1d) and (b) hybrid material formed by reaction of 1d with benzoylchloride in pyridine.

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carrying sensible organic ligands, an alternative, gentler methodof analysis would be desirable.

It is well-known that silica readily dissolves in hydrofluoric acid.This reactivity allowed us to devise a second method for analyzingthe organic ligands anchored on the silica surface to complementthe NMR analysis described above. The protocol is illustratedin the following example: a sample of 100 mg of 1e was treatedwith 3 mL of a 10% aqueous solution of hydrogen fluoride at 0 °Cunder stirring. The solid was completely dissolved after 20 min;the mixture was then extracted with 1 mL of methylene chloride.The organic layer was treated with sodium bicarbonate andanhydrous magnesium sulfate and then analyzed with the aid ofconventional gas chromatography-mass spectrometry techniques.The mass spectra of the peak at 9.91 min showed characteristicions that could be assigned to (5-ethoxy-5-oxopentyl)trifluorosi-lane: m/z 214 [M+•], 129 [M+• - SiF3], and 187 [M+• - C2H5O].The GC/MS data indicated the presence of traces of difluorodi-methylsilane in the organic solution. Because trifluorosilyl-substituted organic compounds formed in the reaction of hybridsilicas with hydrogen fluoride tend to decompose in methylenechloride solution either by condensing to form silica in thepresence of traces of water or by grafting onto the glass surfaceof the container, it is essential that both the sample preparationand the analysis be carried out rapidly in order to obtain validand reproducible results. In addition, we found that extremelyhydrophobic hybrid silica materials, which were reluctant to reacteven with aqueous hydrogen fluoride, dissolved readily in hydro-gen fluoride-pyridine complex at 0 °C. The solution was extractedwith methylene chloride and then, after treating the organicsolution with sodium bicarbonate and anhydrous magnesiumsulfate, analyzed with the aid of GC/MS.

An obvious limitation of this method, and one that must betaken into account in the interpretation of the results, is that thereaction conditions required for liberating the organic ligands fromthe silica matrix can modify the structure of the ligands them-selves. Moreover, the involvement of an extractive process in thisprotocol introduces some uncertainty in the quantitation of thereaction components. In our tests, for example, the GC/MSanalysis of [4-carboxybutyl]-functionalized silica (1a) failed todetect any (4-carboxybutyl)trifluorosilane in the organic solution.

The protocols for NMR and GC/MS analysis of hybrid silicamaterials described herein provide valuable information forefficiently monitoring the chemical transformations undergone bythe organic ligands on the silica surface. In order to furtherillustrate their application, we optimized a series of simplechemical reactions on hybrid materials (Scheme 1), which maybroaden the array of chemical strategies for anchoring organicligands onto silica surfaces. Detailed experimental procedures canbe found in the Supporting Information.

[4-Carboxybutyl]-functionalized MCM-41 silica (1a) was treatedwith an excess of dimethyldichlorosilane under reflux to obtainthe corresponding dimethylsilylated material 1f. Capping thesilanol groups on the silica surface with dimethylsilyl groupsallowed us to perform the reduction of the carboxylic functional-ities of 1f with borane-dimethyl sulfide complex in THF underreflux to give dimethylsilylated [5-hydroxypentyl]-functionalizedMCM-41 silica (1g). NMR analysis of the hybrid materialsobtained under various reaction conditions led to the conclusion

that the reaction requires 2 equiv of the borane complex to achievecomplete conversion of the organic ligands. GC/MS analysis of1g indicated that the only product in the organic solution was1,1-difluoro-2-oxa-1-silacycloheptane, derived from the intramo-lecular cyclization of the intermediate (5-hydroxypentyl)trifluo-rosilane formed in the reaction of 1g with aqueous hydrogenfluoride.

NMR analysis of the hybrid material 1g showed no significantloss of dimethylsilyl groups under the reaction conditions used.In contrast, the reaction of dimethylsilylated [5-ethoxy-5-oxopen-tyl]-functionalized silica 1e with 1.1 equiv of LiAlH4 in THFfollowed by hydrolysis with a 20% aqueous solution of potassiumtartrate, which quantitatively converted the ester groups into thecorresponding alcohols, led to a 62% loss of the dimethylsilylprotecting groups on the silica surface.

[5-Hydroxypentyl]-functionalized silica (1g) was quantitativelyconverted into [5-acetyloxypentyl]-functionalized silica (1h), [5-bro-mopentyl]-functionalized silica (1i), or [5-iodopentyl]-functional-ized silica (1j) upon reaction with 2.6 equiv of acetyl chloride inpyridine, PBr3, or aqueous hydrogen iodide under reflux, respec-tively (Scheme 1). Functionalized silica 1j can be also obtainedquantitatively by reacting 1i with sodium iodide in acetone.Halogen-substituted ligands in hybrid silicas 1i and 1j could onlybe analyzed with the aid of GC/MS since treatment of thesematerials with the strong basic aqueous solution needed to preparethe NMR samples gave rise to the corresponding hydroxy-substituted ligands. These results show that 5-substituted-pentylligands on dimethylsilylated silica are able to undergo SN2reactions, thus opening up new opportunities for grafting complexligands onto silica through the use of suitable nucleophiles.

Scheme 1. Reactions on Functionalized MCM-41Silica Materials

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In addition, reaction of dimethylsilylated [4-carboxybutyl]-functionalized MCM-41 silica (1f) with an excess of thionylchloride followed by treatment with a 3-fold excess of either benzylalcohol or benzylamine in pyridine led to low conversions of thecarboxylic acid functionalities into ester (45%) and amide (65%)(Scheme 1). In this case, NMR and GC/MS analysis of [5-chloro-5-oxopentyl]-functionalized silica (1k) did not lead to any clearconclusions concerning the substrate conversion as the acylchloride moiety underwent hydrolysis in both sample preparationprocedures.

CONCLUSIONSThe methods involving the dissolution of the hybrid silica

material with aqueous basic solution or hydrofluoric acid whichhave been described herein provide immediate access to quantita-tive structural information concerning the organic ligands bondedto the silica surface.11 These methods complement currentlyavailable analytical techniques6 such as, for instance, TEM,6a,f,i,k,m

SEM,6f,k,m BET,6c,d,h,m,o,p,x DLS,6f,g,w TGA,6d,j,n,q,r DSC,6n,r

XRD,6a,b,i,k,m,n,u,v,x ICP-AES,6b,v or elemental analysis,6c,h,j,p whichprovide valuable information about the shape, size, active surface,porosity and aggregation of the particles, the phase transitionsor thermal degradation involving organic components of thematerial, and also about the elements present at the surface,6 butnot on the structure and composition of the organic ligands onthe hybrid silica materials. FT-IR6e,f,j,m,r-t,x is a valuable techniqueto identify the functional groups present in the organic ligands,but the data are not quantitative and become eventually obscuredby the signals from the silica matrix. Solid-state CP/MAS NMR6,7

is a powerful technique that provides direct structural information,but it can hardly be considered a routine technique for theimmediate analysis of the hybrid materials. On the other hand,the simplicity of the procedures reported herein and the availability

of conventional NMR and GC/MS equipments allow for the rapidscreening of series of samples, thus facilitating the optimizationof chemical reactions performed on hybrid materials either forpreparing or for modifying the organic ligands bonded to the silicasurface.

In summary, we have described two simple and straightforwardprocedures for determining the organic content of hybrid silicamaterials by means of conventional NMR and GC/MS techniques.The methods involve dissolving the hybrid material either in aconcentrated solution of sodium hydroxide in deuterated watercontaining a suitable reference or in a solution of hydrogenfluoride in water and extracting with methylene chloride. Whilethese methods do not provide information concerning the porestructure or the local environment of the ligands in the hybridmaterial, they constitute useful routine techniques for obtainingimmediate information concerning both the amount and chemicalcomposition of the organic compounds on the silica surface withinthe accuracy range of conventional NMR and GC/MS techniques.This information is useful for applying these types of materials inprocesses involving chemical reactions on organic ligands bondedto a silica surface.

ACKNOWLEDGMENTFinancial support by the Spanish Direccion General de Inves-

tigacion, CTQ2006-28333-E, and Consolider Ingenio 2010 (CSD2007-00006), as well as by the Generalitat Valenciana GV06217, isgratefully acknowledged. A.O. thanks the Spanish Ministerio deEducacion y Ciencia for a fellowship. We also thank the SCSIE(Universidad de Valencia) for being allowed access to theirinstruments and facilities.

SUPPORTING INFORMATION AVAILABLEDetailed experimental procedures, MS spectra, 1H and 13C

NMR spectra for materials 1a-m. This material is available freeof charge via the Internet at http://pubs.acs.org.

Received for review June 18, 2008. Accepted September26, 2008.

AC801248M

(11) Analogous approaches have been applied by other authors for characterizinghybrid materials. See for instance ref 5j and:(a) Houghten, R. A.; Yu, Y.J. Am. Chem. Soc. 2005, 127, 8582. (b) Templeton, A. C.; Hostetler, M. J.;Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes,M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845.

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