electrospray ionization in the study of the polycondensation of ti(o-i-c3h7)4 and ti(o-n-c4h9)4

Electrospray ionization in the study of thepolycondensation of Ti(O-i-C3H7)4 andTi(O-n-C4H9)4

Simone Cristoni1, Lidia Armelao 2, Silvia Gross3, Eugenio Tondello3 and Pietro Traldi 2*1CNR, Area di Ricerca, Corso Stati Uniti 4, I-35100 Padova, Italy2CNR, Centro di Studio sulla Stabilita` e Reattivitadei Composti di Coordinazione, Via Marzolo 1, I-35100 Padova, Italy3Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, Via Loredan 4, I-35100 Padova, Italy

The polycondensation of Ti(O-i-C3H7)4 (1) and Ti(O-n-C4H9)4 (2), precursors widely employed in sol-gelprocesses, has been investigated by electrospray ionization mass spectrometry. By analysis of 10ÿ6 Mmethanol solutions of compounds 1 and 2, the same ionic species are detected, proving that the first step inthe polycondensation reaction is thei-propyl (or n-butyl) alcohol-methanol complete exchange. Thisreaction leads to the Ti(OCH3)4 (3) species, representing the synthon of the polycondensation. Variousoligomers of 3 have been detected and characterized by MS/MS experiments, and the related mechanismshave been discussed. A minor oligomeric series due to hydroxyl-containing polycondensation products hasalso been characterized. Copyright# 2000 John Wiley & Sons, Ltd.

Received 5 January 2000; Revised 22 February 2000; Accepted 23 February 2000

Metal alkoxides represent the most versatile molecularprecursors for the sol-gel synthesis of oxide-based materialswhich are currently of interest in optics, catalysis and for thedevelopment of gas-sensing devices.1,2 One unique featureof the sol-gel process is the ability to go all the way from themolecular precursor to the product, thus allowing bettercontrol of the whole process and the synthesis of ‘tailor-made’ materials.3 The chemistry of the sol-gel process ismainly based on hydrolysis and polycondensation of metalalkoxides to form extended networks with an oxideskeleton. Depending on the chemical conditions underwhich such compounds are polymerized, very differentstructures can be obtained which range from colloidalparticles to randomly branched polymers. This variabilitycomes from the many different ways in which monomerscan be linked and organized when they are dispersed in asolvent.4 While the mechanisms of hydrolysis and poly-condensation reactions have been extensively studied in thecase of silicon alkoxides, much less data are available fortransition-metal oxide precursors. In recent years titaniumoxo-clusters and oxopolymers have attracted considerableinterest, due to the various appealing properties of TiO2-based materials. A few papers in the literature concern thehydrolysis and condensation behaviour of simple ororganically modified titanium alkoxides.5 Further studiesare required in order to clearly understand the chemistryinvolved after the primary reaction steps, leading toproducts from small oxide clusters up to colloidal oxo-clusters.

In previous studies, mass spectrometry has been em-ployed to study the polycondensation reaction ofSi(OC2H5)4, either in the gas phase via chemical ionization(CI) experiments,6 or in the condensed phase by continuousflow fast atom bombardament (FAB)7 and electrospray

ionization (ESI).8 In the latter investigation,8 it was shownthat the polycondensation reaction is initiated by theprotonated molecule. Reaction intermediates, correspond-ing to pentacoordinated Si species containing H2O andCH3OH, already hypothesized as activated complexes, weredetected, together with products in which OHÿ OC2H5

exchanges had taken place. The reported data8 indicated thatESI-MS can be validly employed to obtain information onthe reactivity of polycondensation precursors and on thestructures of reaction intermediates. In the present paper,results obtained by the same method in the study of thepolycondensation reactions of Ti(O-i-C3H7)4 and Ti(O-n-C4H9)4 are reported and discussed.

EXPERIMENTAL

Compounds1 and 2 were purchased from Sigma Aldrich(Milan, Italy).

The ESI mass spectra of Ti(O-i-C3H7)4 (1) and Ti(O-n-C4H9)4 (2) were obtained using a LCQ instrument(Finnigan, Palo Alto, CA, USA). The entrance capillarytemperature was 200°C and the capillary voltage was 5 kV.10ÿ6 M methanol solutions of samples1 and 2 wereintroduced by direct infusion at a flow rate of 8mL/min.MS/MS and MS3 experiments were performed by selectionof the ions of interest and by their resonance excitation bysupplementary rf voltage in the range 30–40% of itsmaximum value (5 V peak to peak) for 200 ms. The Hepressure inside the trap was kept constant; the pressuredirectly read by ion gauge, in the absence of the N2 stream,was 2.8� 10ÿ5 Torr

RESULTS AND DISCUSSION

The high reactivity of titanium alkoxides towards nucleo-philic reagents (in particular water) is well known. Merelyupon adding water, the occurrence of a rapid polymerizationis immediately observed. Thus, in order to study their

*Correspondence to: Pietro Traldi, CNR, Centro di Studio SullaStabilitae Reattivitadei Composti di Coordinazione, Via` Marzolo 1, I-35100 Padova, Italy

Copyright# 2000 John Wiley & Sons, Ltd.

RAPID COMMUNICATIONS IN MASS SPECTROMETRYRapid Commun. Mass Spectrom.14, 662–668 (2000)

Figure 1.ESImassspectrumobtainedby injectionof a10ÿ6 M solutionof 1 in methanol(for thestructureions leadingto peaksk, l andy seethe text).

Figure 2. Isotopicdistributionof theclusterexperimentallydetected,centeredat m/z645(a) comparedwiththe theoreticalisotopicdistributionof Ti5 (b) andC11H33O15Ti5 (c).

Copyright# 2000JohnWiley & Sons,Ltd. Rapid Commun.MassSpectrom.14, 662–668(2000)

ESI-MS STUDY OF POLYCONDENSATION OF TI(OR) 663

reactivity by ESI, theinjection of highly diluted solutionsisrequired. For this reasondifferent sampleconcentrationswerefirst evaluated,andthe bestresultswereachievedbyanalysesof 10ÿ6 M solutions of 1 and2 in methanol. Forsuch diluted conditions the formation of gel, reasonably

inducedby water traces, is strongly inhibited. However, itmustbestressedthatthis representstheconcentration of thebulk solutioninjecteddirectly into theESIsource.After thespray formation, the sample concentration increasessig-nificantly as the result of evaporation due to the heating

Scheme1.

Figure 3. ESI massspectrumobtainedby injection of a 10ÿ6 M solutionof 2 in methanol(Fig. 1).

Rapid Commun.MassSpectrom.14, 662–668(2000) Copyright# 2000JohnWiley & Sons,Ltd.

664 ESI-MS STUDY OF POLYCONDENSATION OF TI(OR)

experiencedin the entrance capillary (200°C) and to thereducedpressuresexperiencedby thesprayeddroplet.Thus,during the droplet life, it is reasonableto assumethat thecritical concentrationusuallyadoptedfor theoccurrenceofpolycondensation reactions(10ÿ1 M) canbe reached.

The spectrum obtainedby injection of a 10ÿ6 M solutionof 1 in methanolis shownin Fig. 1. Numerous peaksaredetectableup to m/z1400. Eachof these peaksshows theisotopic clusterscharacteristicof Ti-containing ions. Fromthe different abundance ratios, the number of Ti atoms

Figure 4. ESI massspectrumobtainedby injection of a 10ÿ5 M solutionof 1 in acetonitrile.

Figure 5. MS/MS (a) andMS3 (b) spectraof ESI generatedionsat m/z645.



present in each cluster can be easily determined. As anexample, in Fig. 2, an enlarged view of the ionic clustercentered at m/z 645 is shown (a), together with thetheoretical distributions calculated for Ti5 (b) andC11H33O15Ti5 (c). On thebasisof thenumbersof Ti atoms,it follows thatnoneof thedetectedpeakscanbeassignedtospeciesstill containing theoriginal i-propoxy groups. Theycanberationalized only by takinginto account theexchangeof the i-propoxy by methoxy groups,through a reactionoccurring with the methanol solvent. Under the presentconditions this reaction reasonably takes place via theformation of a pentacoordinated species,as shown inScheme 1. As alreadydescribedin the literature,3 in thecaseof acidic solution the reaction rate is catalyzed; thisbehaviourhasbeenrationalized by protonation of analkoxyligand which makesit a good leaving groupandincreasesthe positive chargeon the metalatom.

The same behavior is shown by the tetrabutoxyderivative, compound2. As can be seenby the spectrumreportedin Fig. 3, thesameionic speciesalreadydetectedinthe ESI spectrum of 1 (Fig. 1) are the only onespresent,even if in different abundance ratios. Mass values andisotopic cluster analysis again showthat noneof themcanbe justified by butoxy-containing species, thus indicatingthe occurrenceof an exchangereactionanalogousto thatdescribed for 1 in Scheme 1. Hencefor both 1 and 2 thepolycondensation precursor must be consideredto be theTi(OCH3)4 molecule(compound 3).

In order to verify this aspect,and to confirm that thepolycondensation observedin the ESI spectraof 1 and2 isnotactivatedby theESIconditionsbut is dueto theintrinsicreactivity of 1 and 2 with methanol,further experimentswere carried out by analyzing, using the same ESIconditions, 10ÿ6 M acetonitrile solutions of 1 and 2. Infact it is known that acetonitrile does not react in thecondensed phase with titanium alkoxides. Even whenvarying the sampleconcentration in the range10ÿ5–10ÿ7

M, significant spectrawerenot obtained. For example,Fig.4 showsthe ESI spectrum obtained by direct infusion of a10ÿ5 M acetonitrile solution of compound1. Low intensitypeaks(DAC valuesin therange103–104, comparewith thespectrashown in Figs1 and3 with DAC valuesof 105–106)

are detectable in the presenceof high levels of chemicalnoisebut isotopic cluster analysisdoesnot allow unambig-uousinterpretationin termsof Ti-containing ions.

In addition, useof acetonitrile/watersolventmixturesindifferentratios (up to 40:60v/v), in orderto achievea moreacidicmedium,resultedin noobservation of polycondensa-tion.

Theanalysis of thespectraof Figs1 and3 demonstratesthepresenceof threeseriesof oligomers,differingby amassincrementof 126u, correspondingto structurea.

Thus,a first seriesof ionic speciesis responsiblefor thepeaksat m/z267,393,519,645,771,897,1023,1149and1275,consistentwith structurek, with n rangingfrom 1 to 9.

MS/MS experimentsperformedon more abundantk ionsconfirm the proposedstructure.As an examplethe MS/MSspectrumof the ion at m/z645(n = 4) is shownin Fig. 5(a).Itshows the occurrenceof only one collisionally induceddecomposition(CID) pathway leading to ions at m/z 473,which can be explainedby the loss of a Ti(OCH3)4 neutral

Scheme2.



molecule.A possiblemechanismof this fragmentationprocessis shown in Scheme3.Thestructureof the[Mkÿ Ti(OCH3)4]

�

ion thusformed, which doesnot beara terminalTi(OCH3)3

group,is confirmedby the MS3 experimentsperformed ontheM�k ions. As shownin Fig. 5(b), the[Mk – Ti(OCH3)4]

�

ions are completely inhibited from undergoing theTi(OCH3)4 loss,andthe mostfavoreddecompositionrouteis now methanol loss. Thesefragmentation processes areobservedfor all theionsbelonging to thek series;however,the ions at m/z519 arealsoobservedto undergoa lossofTiO(OCH3)2 concurrent with that of Ti(OCH3)4.

In principle the k ions could originatevia two differentprocesses:(i) ESI-inducedfragmentation processes of the

polycondensation products; and (ii) Polycondensation ofESI-producedprecursor ions.

Assumingthatprocess(i) is theoperativeone,it must beconsidered that k ions could originate through the twoalternative processes shownin Scheme4. The mechanismlabeled j consistsof a first protonation reactionreasonablydue to tracesof water presentin the solvent followed byCH3OH loss; it can be considered valid only assuming ahigh instability for theprotonatedmolecules.In fact [MH]�

ionswereneverdetected,evenby varyingtheexperimentalconditions over quite wide ranges (different sampleconcentrations, different sheathgasflows, different sampleinjection flows, differententrance capillary temperatures).

Scheme4.

Scheme3.



Alternatively to processj , the direct electrochemicaloxidation of the titanium compound,leadingto the radicalcation could be proposed. From M� the loss of CH3Oradicalwould lead to ionsk (processw of Scheme 4).

Mechanismsanalogousto j andw could apply directly toTi(OCH3)4, thus leading to �Ti(OCH3)3 specieswhichrepresent the initiator of the polycondensation reaction[process(ii)], following themechanismreportedin Scheme2. It must be emphasizedthat Ti(OCH3)3 ions wereneverdetected;however, the presenceof dimeric moieties at m/z267 could be good evidencefor the possibleoccurrence ofthis mechanism. Furthermore, it mustbetakeninto accountthat the polycondensationdid not take place in the dilutesolutions of 1 and 2 without ESI activation. However, inview of themarked increase in sample concentration in thesprayed droplets due to solvent evaporation, one cannotexclude the occurrence of polycondensation reactionsfollowed by electrosprayionization (mechanismsj and wof Scheme4).

Another seriesof ionic speciesevidentin Fig 1 and3 isthatdueto theionsatm/z473,599,725and851.Analysisofisotopic clusters and MS/MS spectra suggest structures 1(with 2< = n < = 5) asthe most reasonablefor these ions.Thesestructuresareexactly thesameastheCID productsofk ionsdescribedabove,andreasonably originatefrom themvia the mechanismshown in Scheme 3.

Finally a further seriesof peaks(y) is dueto ions at m/z691,817,943, 1069and1195,andis particularly abundant

for 2 (seeFigs 1 and3). Again the spacingscorrespondtounits a, and can be rationalized only by invoking, incompetition with the (OR)-(OCH3) exchanges, extensive(OR)-(OH) exchanges,reasonably due to the presenceofresidualwater. Theextent of thelatter reactionswill dependon theR substituent (1:R = iC3H7;2:R = C4H9), andappearsto bemorefavoredin thecaseof 2. Thus,for example, thespeciesat m/z 691, representing the base peak of thespectrum of Fig. 3, could be assigned as the[Ti 6O5(OCH3)6(OH)8H]� cation; isotopic cluster analysissupportsthis elemental formula.

In conclusion the data discussedaboveshow that ESImassspectrometry representsa valid tool to investigatethepolycondensationprocessesof metalalkoxides.In thecasesof 1 and2, it wasproved thatsuchprocessesareprecededbythe formation, throughreaction with methanol solvent,ofTi(OCH3)4 (3), representing the starting point of theoligomerization reaction. Using a different solvent (aceto-nitrile) no polycondensation was observed.Furthermore,both1 and2 leadto the sameoligomerizationproducts.

Threedifferent seriesof ionic productsare observed inthe massspectra:speciessimply originating from poly-condensation of 3, speciesbearinga terminal TiO(OCH3)group and, to a minor extent, speciesin which alkoxy-hydroxyexchangehastakenplace.

REFERENCES

1. Transition Metal Oxides:SurfaceChemistryand Catalysis, KungHH (ed.).Elsevier:Amsterdam,1989.

2. TheSurfaceScienceof Metal Oxides, HenrichVE, Cox PA (eds).CambridgeUniversity Press:Cambridge,1994.

3. Sol-GelScience:ThePhysicsandChemistryof Sol-GelProcessing,Brinker CJ,SchererGW (eds).AcademicPress:New York, 1990.

4. Kallala M, SanchezC, CabaneB. Phys.Rev.E. 1993;48: 3692.5. SanchezC, Livage J, HenryM, BabonneauF. J. Non-Cryst.Solids

1988;100: 650.6. CampostriniR, CarturanG, Pelli B, Traldi P. J. Non-Cryst.Solids

1989;108: 143.7. CampostriniR, CarturanG, SoraruG, Traldi P.J. Non-Cryst.Solids

1989;108: 315.8. Cristoni S, Armelao L, Tondello E, Traldi P. J. MassSpectrom.

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electrospray ionization in the study of the polycondensation of ti(o-i-c3h7)4 and ti(o-n-c4h9)4

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