star-shaped quaternary alkylammonium polyhedral oligomeric silsesquioxane ionic liquids

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DOI:10.1002/ejic.201402005

Star-Shaped Quaternary Alkylammonium PolyhedralOligomeric Silsesquioxane Ionic Liquids

Sundar Manickam,[a] Paola Cardiano,[a] Placido G. Mineo,[b] andSandra Lo Schiavo*[a]

Keywords: Ionic liquids / Polyammonium salts / Ion exchange / Cage compounds / Silicon / Silsesquioxanes

New star-shaped quaternary ammonium (QA) polyhedraloligomeric silsesquioxane (POSS) salts based on octakis-(tetraalkylammonium) POSS cations bearing alkyl chains ofdifferent length (methyl, n-butyl, and n-octyl) and differentlypolarizable anions [iodide, bis(trifluoromethylsulfonyl)imide,and ibuprofen] have been synthesized and characterized.Differential scanning calorimetry (DSC) and thermogravime-tric analysis (TGA) investigations show that the nature of thecation plays a key role in determining the thermal behaviorof QA POSS salts featuring the longest alkyl chains, whereas

Introduction

Polyhedral oligomeric silsesquioxanes (POSSs) representa class of intriguing materials, which, thanks to their nano-meter size, shape, and versatile reactivity, have been largelyexploited as building blocks at the molecular scale for nano-technology applications and the development of high-per-forming hybrid nanomaterials. POSS units are cube-shapedspecies with diameters between 1 and 3 nm, characterizedby an inorganic inner siloxane core (SiO1.5)x surrounded byorganic substituents, which may be inert or confer thePOSS nanoparticles with specific chemical reactivity and/orphysicochemical properties.[1–4] These features make POSSsversatile nanofillers, which, unlike conventional ones, offeradvantages in terms of their monodisperse size, uniformdistribution, low density, and synthetically controlled func-tionality. Many literature reports show that the incorpora-tion of POSS units into polymeric matrices provides dra-matic improvements in thermal stability, oxidation resis-tance, surface hardening, mechanical, electric, and opticalproperties as well as flammability reduction. This explainswhy POSS-based materials have been the subject of innov-ative research in the last decade, which has resulted in appli-cations in aerospace, protective coatings, microelectronics,

[a] Dipartimento di Scienze Chimiche, University of Messina,Viale F. Stagno D’Alcontres 31, S. Agata, 98166 Messina, ItalyE-mail: [email protected]

[b] Dipartimento di Scienze Chimiche, University of Catania,Viale A. Doria 6, 95125 Catania, ItalySupporting information for this article is available on theWWW under http://dx.doi.org/10.1002/ejic.201402005.

Eur. J. Inorg. Chem. 2014, 2704–2710 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2704

the influence exerted by anions becomes relevant for thosebearing short and medium alkyl chains. All the QA POSSsalts display amorphous behavior and, with the exception ofoctakis(trimethylpropylammonium)- and octakis(tri-n-butyl-propylammonium)octasilsesquioxane iodide, may be classi-fied as ionic liquids. Dynamic contact-angle studies revealtheir hydrophilic attitude owing to the preferential orienta-tion of the tetraalkylammonium groups at the film–waterinterface.

catalysis, organic light-emitting diodes (OLEDs), and zeo-lite-like materials. POSS nanotechnology has also been ex-ploited to provide access to a large variety of biocompatiblehybrid materials (polymeric or not) for biomedical applica-tions and used for the development of cardiovascular im-plants, tissue engineering products, coatings for quantumdot nanocrystals, and so on.[2] Further, the POSS hydro-phobic Si–O “spherical” core, when combined with suitablefunctionalized substituents, leads to the formation of am-phiphilic materials, which have been utilized for drug/genedelivery and the formation of hydrogels.[2,3]

The current interest in materials based on ionic liquid(ILs) has led to the employment of the POSS silica core asa scaffold for the construction of hybrid nanostructuredILs. In recent years, ILs, that is, salts that melt below100 °C, have emerged as a new class of functional materialswith unique and synthetically tailored properties, such aslow vapor pressure, wide liquid range, excellent chemicaland thermal stability, good electrochemical stability, highionic conductivity, dispersant capabilities, tunable dielectricconstants, and biocompatibility; as a result, they have foundapplications in many chemical and industrial fields.[5–8]

Their immobilization on suitable substrates such as poly-meric networks and nanostructured scaffolds (nano-tubes,[9,10] graphene,[11] etc.) resulted in significant improve-ments in their processing capabilities, stability, and struc-ture control, which allowed some drawbacks arising fromtheir intrinsic liquid nature to be overcome.[12–14]

Tanaka et al. have demonstrated that the star-shaped oc-tacarboxy–POSS-based IL featuring imidazolium groups ascountercations, namely, [POSS(COO–)8][B-mim+]8 (B-mim

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= methylbutylimidazolium), has a lower melting tempera-ture and enhanced thermal stability with respect to thoseof the corresponding discrete imidazolium IL.[15] These fin-dings have been explained in terms of both the large ex-clusion volume and star-shaped symmetrical distribution ofion pairs around the POSS core, which decrease the inter-molecular interactions among POSS units. Studies on POSScarboxy/imidazolium ILs bearing a variable number ofcarboxylate anions have also been performed and revealeda clear influence of “the number of ion pairs assembled onPOSS” on their thermal properties.[16] Octamethylimidazol-ium POSS species are also known, and their thermal fea-tures as a function of the anion were reported.[17] Further-more, monosubstituted imidazolium–POSS-based ILs havebeen successfully used as high performance electrolytes forfuel cells.[18]

Following our continuous interest in IL-based materi-als,[19–23] we focused our attention on tetraalkylammoniumPOSS ILs. Quaternary ammonium (QA) ILs, as shown byRogers and Seddon,[24] exhibit improved thermal, electro-chemical, miscibility, and solvating properties with respectto those of conventional imidazolium and pyridiniumILs.[25] QA-based ILs are also well known for their antimi-crobial activity; therefore, their introduction on a POSSplatform is of interest for pharmaceutical engineering appli-cations.[26] Further, QA multifunctional POSS ILs havegreat potential for the development of stationary phases ingas–liquid chromatography, as they combine the thermalstability induced by the POSS core with the wide liquidtemperature ranges and low vapor pressure of ILs.[27,28]

In this context, we recently reported the synthesis, char-acterization, and amphiphilic properties of the trimethyl-propylammonium hepta(isooctyl)octasilsesquioxane cation(POSS-NMe3

+) and a variety of anions.[22] In contrast withthose of conventional ILs, the physicochemical and solutionproperties of this class of ILs depend on the bulky natureof the cation, which features a hydrophobic organic coronaon the siloxane core and a hydrophilic pendant head group.In agreement, POSS-NMe3

+ ILs exhibit a high solubility inorganic media (even alkanes) and good extraction capa-bility towards a variety of polyanions from aqueous solu-tions. The biphasic liquid–liquid extraction process may beproperly exploited to provide access to new functionalnanohybrid ILs and, more generally, to POSS ionic nanos-tructured self-assembled materials. As a result, POSS-NMe3

+ ILs readily extract tetraanionic meso-tetrakis-(4-sulfonatophenyl)porphyrin (TPPS) from water to form the1:4 ionic supramolecular adduct POSS@TPPS, which rep-resents the first example of a porphyrin–POSS ionic liquid.POSS@TPPS, analogously to the POSS-IL precursor, is asan amorphous material that is very soluble in organic sol-vents, in which, according to optical and resonance lightscattering investigations, the TPPS anion is monomeric andhas a remarkably long emission lifetime decay (1.0 ns).Nanostructured TPPS J-oligomer aggregates, still emissivein solution, are formed by irradiation (under acidic condi-tions) of solutions of POSS@TPPS in chlorinated sol-vents.[23] These findings are clearly ascribable to the bulky


nature of POSS-NMe3+, which reduces the spontaneous ag-

gregation tendency of TPPS and makes POSS@TPPS a po-tential material for application in optical limiting devicesand optoelectronics.[29]

As a continuation of our ongoing project on nanostruc-tured tetraalkylammonium POSS ILs, here we report on thesynthesis and characterization of new star-shaped octakis-(tetraalkylammonium) POSS salts featuring different chainlength alkyl groups (methyl, n-butyl, n-octyl) and a varietyof anions. These have been characterized by MALDI-TOFmass spectrometry, thermogravimetric analysis (TGA),differential scanning calorimetry (DSC), and conventionalspectroscopic techniques (FTIR, 1H and 29Si NMR), aswell as elemental analysis. The results show that the cationplays a major role in determining the properties of the star-shaped octakis(tetraalkylammonium) POSS species featur-ing the longest alkyl groups (i.e., n-octyl), whereas the influ-ence of the anion is observable and determines the ionicliquid behavior in all other cases.

As many applications of ILs are dependent on theirhydrophobic or hydrophilic properties,[8] dynamic contact-angle investigations on the new materials have been per-formed to provide information on the behavior of the filmsurface towards water as a function of the IL cation–anioncouple.

Results and Discussion

All of the new compounds reported here are representedin Scheme 1. The parent iodide species, namely, POSS-C1-I, POSS-C4-I, and POSS-C8-I, were prepared in yields of45 to 74% by simple alkylation of OctaAmmonium POSS®

with iodomethane, 1-iodobutane, and 1-iodooctane, respec-tively, in the presence of potassium carbonate. Quaternaryammonium POSS iodides have been easily converted into

Scheme 1. Chemical structures of octakis(tetraalkylammonium)POSS salts.


other POSS-Cn derivatives (n = 1, 4, 8), namely, POSS-C1-Tf2N [Tf2N = bis(trifluoromethylsulfonyl)imide], POSS-C4-Tf2N, POSS-C4-IBU (IBU = ibuprofen), POSS-C8-Tf2N,and POSS-C8-IBU, by metathesis reactions with lithium bi-s(trifluoromethylsulfonyl)imide or ibuprofen sodium salt,and the yields ranged from 45 to 92 %. All attempts toachieve complete anion exchange between iodine and ibup-rofen for POSS-C1-I were unsuccessful. In particular,POSS-C4-Tf2N, POSS-C4-IBU, POSS-C8-Tf2N, and POSS-C8-IBU have been obtained by a bilayer chloroform–aqueous process, as described in the Experimental Section.POSS-C1-I, POSS-C4-I, POSS-C1-Tf2N, and POSS-C4-Tf2N are solids at room temperature, whereas POSS-C4-IBU, POSS-C8-I, POSS-C8-Tf2N, and POSS-C8-IBU areviscous liquids. The solubility of these QA salts is deter-mined by the combination of the ammonium alkyl chainslength and the nature of the anion, so that POSS-C1-I dis-solves readily in water and methanol, POSS-C8 salts aresoluble in chlorinated and low-polarity solvents [toluene,tetrahydrofuran (THF), acetone] and not in alcohols oracetonitrile, and all of the other compounds display goodsolubility in all of the aforementioned solvents with the ex-ception of water.

The new materials have been unambiguously charac-terized by 1H and 29Si NMR spectroscopy in solution,FTIR spectroscopy, elemental analysis, and MALDI-TOFmass spectrometry. In detail, as far as the parent POSS-Cn

iodides are concerned, the 1H NMR experiments clearlyshow that the resonance of the –CH2NH3

+ protons of thestarting OctaAmmonium POSS® at δ = 2.89 ppm are, asexpected, downfield shifted as a consequence of the alkyl-ation process. Even the signals of the other methylenegroups are diagnostic of the occurrence of the reaction, asthey are downfield shifted with respect to those of the start-ing iodoalkanes. Moreover, as a common feature, all of the1H NMR spectra of the POSS-Cn derivatives displayslightly shifted peaks with respect to those of the relatedparent iodides. In the 1H NMR spectra of POSS-Cn-IBU,additional alkyl and aryl signals with the expected integralratio are observed. The 29Si NMR spectra show the typicalsingle resonance in the correct range, that is, between δ =–67.67 and –67.93 ppm, which indicates that all of the sili-con atoms in the synthesized compounds feature the samechemical environment.

All of the new POSS-Cn compounds have also been char-acterized by MALDI-TOF mass spectrometry. The positiveMALDI-TOF mass spectra feature a single signal ascribedto the cationic fragments owing to the loss of the anionduring the MALDI desorption phenomenon. As an exam-ple, the spectrum of POSS-C8-Tf2N is displayed in Figure 1and features the [M]+ signal at m/z = 5532 owing to the lossof the Tf2N anion from the molecular species. TheMALDI-TOF mass spectra of POSS-C1-Tf2N and POSS-C4-Tf2N are reported in the Supporting Information (Fig-ures S1 and S2).

The thermal behavior of the new POSS-Cn species hasbeen ascertained by DSC and TGA. The results are shownin Table 1.


Figure 1. MALDI-TOF mass spectrum of POSS-C8-Tf2N.

Table 1. Thermal properties of octakis(tetraalkylammonium) POSSsalts.

Tg [°C] Tonset [°C] Tmax [°C][a] Residue [%][b]

POSS-C1-I – 253.8 334.2 25.7POSS-C1-Tf2N 24.6 409.2 455.1 19.8POSS-C4-I – 211.9 257.4 18.4POSS-C4-Tf2N 2.73 357.2 417.2 10.3POSS-C4-IBU 24.11 150.9 234.9 9.6POSS-C8-I – 207.7 235.7 7.6POSS-C8-Tf2N – 348.8 417.2 7.5POSS-C8-IBU – 175.1 230.6 8.8

[a] Temperature of maximum rate of degradation. [b] Residue at800 °C under N2 flow.

The physicochemical properties of ammonium salts (QA)are dependent on the proper cation–anion combination.Therefore, the presence of a bulky cation coupled with ahighly polarizable anion contributes significantly to the de-termination of the phase–phase transitions and thermal sta-bility.[30,31] The incorporation of a POSS core into QA saltscould result in both lower melting points and enhancedthermal stability as a result of the intrinsic low density ofPOSS.[15,16]

The DSC measurements have been performed under anitrogen atmosphere in the range –90 to 120 °C. As an ex-ample, the DSC trace for POSS-C1-Tf2N is displayed inFigure 2. A common feature arising from the DSC studiesis that most of the QA POSS salts under investigation dis-plays amorphous behavior and do not reorganize into crys-talline materials upon cooling. This is not unexpectedbecause, as already suggested for [POSS(COO–)8][B-mim+]8,[16] whenever a high number of ion pairs is distrib-uted around the POSS core, a star-shaped symmetricalstructure with a larger exclusion volume may be obtained.As a result, when QA POSS cations are combined with asuitable polarizable anion, no crystallization occurs. Glasstransitions are evident only for POSS-C1-Tf2N, POSS-C4-Tf2N, and POSS-C4-IBU at Tg = 24.6, 2.73, and 24.11 °C,respectively.


Figure 2. DSC trace of POSS-C1-Tf2N.

From Table 1, it emerges that for the POSS-C8 series,characterized by the presence of the longest alkyl chains,the cation plays a major role. This is corroborated by thedifferent behavior observed in the POSS-Cn-Tf2N series, inwhich POSS-C8-Tf2N is the only one that does not exhibitany Tg, and in POSS-Cn-I series, in which POSS-C8-I is theonly one that appears as a viscous liquid at room temp. Themobility imparted by the long alkyl chains may be invokedto explain such experimental evidence. In contrast, for thePOSS-C1 and POSS-C4 series, which feature short and me-dium length alkyl chains, the influence of the anion is rel-evant. Consistently, the POSS-C1 and POSS-C4 iodide de-rivatives, which are solids at room temp. do not exhibit anyphase–phase transition in the investigated range, althoughwe cannot exclude the possibility that a thermal phenome-non occurs above 120 °C. Likewise, the data collected forPOSS-C1-Tf2N and POSS-C4-Tf2N/IBU suggest that thephase transitions observed (i.e., glass transitions) are theresult of the combination between the lower flexibility ofthe alkyl groups and the polarization capability of theanion. On this basis, we can conclude that all of the POSSQA species reported here may be classified as ionic liquids,with the exception of POSS-C1 and POSS-C4 iodides.

The TGA investigations (Figures 3 and 4, Table 1), per-formed under nitrogen, clearly indicate that the presence ofthe POSS core confers the expected thermal stability withrespect to discrete QA salts; this is mainly dependent on thenature of the anion and follows the trend POSS-Cn-Tf2N �POSS-Cn-I � POSS-Cn-IBU. The contribution of the cat-ionic counterpart, as a function of the alkyl chain length,is also well detectable as the shortest ones (i.e., POSS-C1)confer the highest thermal stability. The presence of longeralkyl groups (i.e., n-butyl and n-octyl) on QA results in adecrease of the intermolecular interactions and, hence, in alower resistance to pyrolysis.

The temperatures of the maximum rates of volatilization(Tmax) show a trend quite different from that displayedby Tonset: POSS-C1-Tf2N � POSS-C4-Tf2N � POSS-C1-I� POSS-C4-I � POSS-C8-I ≈ POSS-C4-IBU � POSS-C8-IBU. The rates of volatilization, involved in the main ther-


Figure 3. TGA traces of POSS-Cn salts under nitrogen.

Figure 4. Derivative TGA curves of POSS-Cn salts under nitrogen.

mal event, fall in the range 9–22 %min–1 for all of thePOSS-based derivatives. As expected, the samples featuringthe IBU anion are more susceptible to thermal degradationas the decarboxylation process occurs even at relatively lowtemperatures. As a consequence, the first degradationevents for POSS-C4-IBU and POSS-C8-IBU are observed atTmax = 234.9 and 230.6 °C, respectively, and are followedby other thermal events, as can be inferred from Figures 3and 4. A multistep thermodegradation behavior is alsoshown by the samples with the longest alkyl groups on thenitrogen atoms, that is, the POSS-C8 compounds. On thecontrary, the decomposition process occurs in a single stepfor POSS-C1-I, POSS-C1-Tf2N, POSS-C4-I, and POSS-C4-Tf2N. For all POSS-Cn samples, a silica residue (from 7 to26%) is observed at 800 °C.

A comparison with analogous imidazolium POSS cannotbe performed as the only data available is for octakis-(methylpropylimidazolium chloride)- and octakis(methyl-propylimidazolium dodecyl sulfate)octasilsesquioxane,[17]

for which the role played by the anion in determining theIL behavior was evidenced. Although, to the best of ourknowledge, no thermal studies on POSS bearing a variable


number of tetraalkylammonium cations have been reported,from the comparison with the data obtained for trimethyl-propylammoniumhepta(isooctyl) POSS iodide and Tf2N,[22]

it can be clearly deduced that, as expected, the presence of ahigh number of ion pairs on a POSS platform significantlyenhances the thermal stability.[3,16,32] For POSS-C1-I andPOSS-C1-Tf2N, the Tonset values are observed at 253.8 and409.2 °C versus 120.1 and 277.0 °C for mono(trimethyl-ammonium) POSS species, respectively.

Contact-Angle Investigations

The potential of an IL for practical applications is veryoften related to its hydrophobic or hydrophilic properties.For example, for their employment as electrolytes,[8] hydro-phobic ILs are usually preferred, whereas amphiphilic ILsare of great interest for drug delivery applications.[33,34]

To gain insight into the water affinity of this new classof quaternary ammonium POSS core amphiphilic materials,contact-angle investigations were performed by theWilhelmy method. The advancing (θadv) and receding (θrec)contact angles provide information about the most hydro-phobic and hydrophilic portions, respectively, of a surface.Hydrophilic surfaces display relatively small contact angles(θ�90°), whereas hydrophobic ones have large water con-tact angles (θ � 90°).

From Table 2, it can be concluded that this family of oc-takis(tetraalkylammonium) POSS species has an overall hy-drophilic attitude. Such a tendency is particularly pro-nounced for POSS-C1-I, which readily dissolves in water.

Table 2. Dynamic contact angles measured on coated glass slidesby using a Wilhelmy balance.

θadv [°] θrec [°]

POSS-C1-Tf2N 73.5�1.4 2.6�0.3POSS-C4-I 52.0�0.9 18.2� 0.7POSS-C4-Tf2N 101.2�2.5 36.8�1.6POSS-C4-IBU 78.8�1.8 31.3�0.4POSS-C8-I 88.1�2.7 39.6�2.0POSS-C8-Tf2N 92.9 �3.4 46.9�1.9POSS-C8-IBU 89.6�2.1 55.6�0.5

In detail, for the POSS-C8 series, the θadv values wereclose to 90°, and θrec had an average value of 40°. Thesefindings suggest that, despite the presence of the long n-octyl chains on the nitrogen atoms, for which the mosthydrophobic behavior should be expected, the observed ad-vancing contact angles show borderline values betweenhydrophobic and hydrophilic domains, whereas the recedingones are consistent with a hydrophilic material. The affinityof ILs towards water is mainly dependent on the nature ofthe anion. In our case, however, such data are indicative ofa competition between the orientation of the anion and cat-ion domains, and the latter clearly predominates in de-termining an overall hydrophilic attitude. For the POSS-C4

series, the iodide and ibuprofen derivatives display hydro-philic behavior and have low contact angles both in advanc-ing and in receding. Although POSS-C4-Tf2N displays the


highest θadv (101°), as expected, it still exhibits a low valuefor the receding contact angle (37°). Despite the presenceof the hydrophobic Tf2N anion, the hydrophilic characterof POSS-C1-Tf2N has been observed.

The whole of the collected data show a general hydrophi-licity, which is not surprising as the presence of water canprovide a driving force for the reorganization of the QAPOSS groups to enhance the interactions between tetraalk-ylammonium groups and water molecules.[35]

ConclusionsIn the context of our ongoing project on nanostructured

tetraalkylammonium POSS ILs, we have reported here onthe synthesis and characterization of a series of star-shapedoctakis(tetraalkylammonium) POSS salts.

The new QA POSS materials feature different alkyl-ammonium chain lengths (methyl, n-butyl, and n-octyl) anda variety of anions [iodide, bis(trifluoromethylsulfonyl)-imide, and ibuprofen], which were chosen on the basis oftheir different polarization capability. All of the salts displayamorphous behavior and, with the exception of POSS-C1

and POSS-C4 iodides, may be classified as ionic liquids.The presence of QA moieties on a POSS platform pro-

duced nanohybrid materials with enhanced thermal sta-bility and lower melting points with respect to those of dis-crete QA units, as a consequence of the low density inducedby the POSS core. The present study also demonstrates theinfluence of the ammonium alkyl chain length on the ther-mal and, hence, IL properties and how these are modulatedby the nature of the anion.

Dynamic contact-angle investigations (Wilhelmymethod) reveal an overall hydrophilic tendency for all ofthe QA POSS salts, and POSS-C1 iodide, characterized bythe shortest alkyl ammonium chain and the less polarizableanion, dissolves readily in water. This behavior may be at-tributed to the ability of water to induce self-organizationof the polar QA groups on a film surface. Other propertiesof these and other QA POSS ILs such as their conductivityand viscosity are under investigation. The large body of cur-rent research on POSS-based ILs prompted us to publishthis report.

Nanostructured POSS ammonium ILs have a largenumber of potential applications, as they combine the prop-erties of QA salts with those of POSS ILs. The possibilityto modulate their properties by varying the alkyl chainlength and/or the anion makes these species very appealing.Apart from the applications related to their microbiologicalactivity and in separation science, they may be properly ex-ploited as aggregation-reducing agents,[23] electrolytes forelectrochemical devices (solar and fuel cells),[18,36] IL nano-fillers,[37,38] platforms for ionic self assembly,[23] and for thedevelopment of nanoscale ionic materials.[39]

Experimental SectionMaterials: Iodomethane, 1-iodobutane, 1-iodooctane, potassiumcarbonate, lithium bis(trifluoromethylsulfonyl)imide, and ibuprofen


sodium salt were purchased from Sigma–Aldrich; OctaAmmoniumPOSS® was purchased from Hybrid Plastics; all of the compoundswere used as received.

Measurements: 1H and 29Si NMR solution spectra were recordedwith a Bruker AMX R-300 spectrometer operating at 300.13 and59.62 MHz, respectively. Elemental analyses were performed by theRedox Microanalytical Laboratory, Cologno Monzese, Milan(Italy). Thermogravimetric analyses were performed with a Perkin–Elmer TGA7 instrument in the temperature range 50–800 °C undernitrogen (50 mLmin–1) at a heating rate of 10 °Cmin–1. Differentialscanning calorimetry experiments were performed with a TA Q20instrument equipped with a refrigerant cooling system (RCS) in thetemperature range –90 to 120 °C at a heating rate of 10 °Cmin–1

under an anhydrous N2 atmosphere (60 mLmin–1). FTIR analyseswere performed with a Perkin–Elmer Spectrum One spectrometer;the samples were ground with a pestle and mortar, pressed intoKBr pellets (7 Tons), and then investigated. The positive MALDI-TOF mass spectra were acquired with a Voyager DE spectrometer(PerSeptive Biosystem) by using a delay extraction procedure(25 kV applied after 2600 ns with a potential gradient of454 V mm–1 and a wire voltage of 25 V) with ion detection in linearmode.[40,41] The instrument was equipped with a nitrogen laser(emission at 337 nm for 3 ns) and a flash analogue-to-digital (AD)converter (time base 2 ns). To avoid fragmentation of the macro-molecules, the laser irradiance was set slightly above the threshold(ca. 106 Wcm–2). Each spectrum was an average of 32 laser shots.The MALDI-TOF experiments were performed by loading thesample (0.1 mmol) and matrix {trans-3-indoleacrylic acid (IAA)or trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]-malonitrile (DCTB); 40 mmol} on the sample plate with dimethylsulfoxide (DMSO) or N,N-dimethylformamide (DMF) as solvent.5,10-Di(p-dodecanoxyphenyl)-15,20-di(p-hydroxyphenyl)porphyrin(C68H78N4O4, 1014 Da), tetrakis(p-dodecanoxyphenyl)porphyrin(C92H126N4O4, 1350 Da),[42] and a polyethylene glycol (PEG) sam-ple of known structure were used as external standards for m/zcalibration. Dynamic contact-angle investigations were performedon coated glass microscopy slides (26�76�1 mm by Prestige) witha KSV Sigma 700 tensiometer at the speed of 2 mmmin–1 in ultra-pure water by using the Wilhelmy method. The glass slides werecarefully cleaned with a sulfuric acid/potassium dichromate clean-ing solution before coating, washed with ultrapure water, and thenchecked by measuring the surface tension of water. The slides weredried and then dip coated at a constant rate (20 mmmin–1) withoutany delay between immersion and withdrawal; 3.75 wt.-% dichloro-ethane solutions were employed for all of the POSS-Cn salts (n =1, 4, 8) except for POSS-C1-I, which is soluble in water. The slideswere then aged for 10 d at 50 °C and contact-angle investigationswere performed. The reported contact angles correspond to theaverage of measurements on five glass slides treated in the sameway. Furthermore, to check the wetting effect of water on the sur-faces of the samples, five continuous immersion–withdrawal cycleswere performed on each coated slide, but no significant variationin the contact angles was detected.

Octakis(trimethylpropylammonium iodide)octasilsesquioxane(POSS-C1-I): Potassium carbonate (2.8 g, 20 mmol) was added toOctaAmmonium POSS® (1 g, 0.85 mmol) suspended in acetonitrile(50 mL) under an inert atmosphere. The dispersion was coveredwith aluminium foil and stirred at room temperature for 30 min.Iodomethane (1.1 mL, 27 mmol) was then added dropwise, and thetemperature was slowly increased to 85 °C; the mixture was heatedunder reflux for 48 h. After extraction from methanol, the productwas washed with acetone (three times) and dried with a rotaryevaporator to afford a white solid. Owing to the presence of potas-


sium salt impurities, the product was further treated with 18-crown-6 ether in methanol and precipitated in acetone to give POSS-C1-Iin a 65% yield. C48H120I8N8O12Si8 (2239.9): calcd. C 25.72, H 5.40,N 5.00; found C 25.63, H 4.97, N 4.96. IR (KBr pellets): ν̃ = 1203–1017 (SiOSi), 795 (SiC) cm–1. 1H NMR (D2O): δ = 0.42 (t, 2 H),1.75 (m, 2 H), 2.99 (s, 9 H), 3.19 (m, 2 H) ppm. 29Si{1H} NMR(D2O): δ = –67.67 ppm.

Octakis[trimethylpropylammonium bis(trifluoromethylsulfonyl)-imide]octasilsesquioxane (POSS-C1-Tf2N): Bis(trifluoromethyl-sulfonyl)imide lithium salt (0.29 g, 1 mmol) was added to a suspen-sion of POSS-C1-I (0.25 g, 0.11 mmol) in methanol (5 mL). Thereaction mixture was stirred for 12 h at 65 °C, the solvent was re-moved, and the crude product was washed with water and driedto afford POSS-C1-Tf2N as a white solid, yield 92%.C64H120F48N16O44S16Si8 (3464.1): calcd. C 22.17, H 3.49, N 6.46;found C 22.22, H 3.30, N 6.48. IR (KBr pellets): ν̃ = 1220–1050(SiOSi), 789 (SiC), 1350, 1190, 1132, 1054 (anion) cm–1. 1H NMR(CD3OD): δ = 0.78 (t, 2 H), 1.88 (m, 2 H), 3.00 (s, 9 H), 3.32 (m,2 H) ppm. 29Si{1H} NMR (CD3OD): δ = –67.78 ppm.

Octakis(tri-n-butylpropylammonium iodide)octasilsesquioxane(POSS-C4-I): Potassium carbonate (5.65 g, 41 mmol) was added toa suspension of OctaAmmonium POSS® (1 g, 0.85 mmol) in aceto-nitrile (50 mL) under a nitrogen atmosphere and covered with alu-minium foil. After 1 h, 1-iodobutane (3.1 mL, 27 mmol) was addeddropwise to the dispersion; the reaction mixture was stirred at roomtemperature for 5 h. The temperature was then slowly increased to85 °C, and the mixture was heated under reflux for 48 h. After re-moval of the acetonitrile, the product was extracted in CHCl3 andthen obtained as a white solid after removal of the solvent with arotary evaporator, yield 74%. C120H264I8N8O12Si8 (3249.2): calcd.C 44.33, H 8.18, N 3.45; found C 44.03, H 8.21, N 3.47. IR (KBrpellets): ν̃ = 1206–1043 (SiOSi), 795 (SiC) cm–1. 1H NMR (CDCl3):δ = 0.77 (t, 2 H), 0.98 (t, 9 H), 1.42 (m, 6 H), 1.67 (m, 6 H), 1.85 (m,2 H), 3.37 (m, 6 H), 3.51 (m, 2 H) ppm. 29Si{1H} NMR (CDCl3): δ= –67.68 ppm.

Octakis[tri-n-butylpropylammonium bis(trifluoromethylsulfonyl)-imide]octasilsesquioxane (POSS-C4-Tf2N): A solution of POSS-C4-I (0.2 g, 0.06 mmol) in CHCl3 (4 mL) was added dropwise to asolution containing bis(trifluoromethylsulfonyl)imide lithium salt(0.16 g, 0.61 mmol) in water (4 mL). The biphasic reaction mixturewas stirred at room temperature for 1 d. The organic phase wasseparated, washed several times with deionized water, and thendried to afford the product as a white solid (yield 60 %).C136H264F48N16O44S16Si8 (4473.2): calcd. C 36.48, H 5.94, N 5.01;found C 36.41, H 5.84, N 5.02. IR (KBr pellets): ν̃ = 1220–1037(SiOSi), 786 (SiC), 1349, 1188, 1131, 1053 (anion) cm–1. 1H NMR(CDCl3): δ = 0.73 (t, 2 H), 0.97 (t, 9 H), 1.37 (m, 6 H), 1.59 (m, 6H), 1.73 (m, 2 H), 3.13 (b, 8 H) ppm. 29Si{1H} NMR (CDCl3): δ= –67.72 ppm.

Octakis(tri-n-butylpropylammonium ibuprofen)octasilsesquioxane(POSS-C4-IBU): POSS-C4-IBU was obtained as a colorless vis-cous liquid by using a similar procedure to that for POSS-C4-Tf2N.Ibuprofen sodium salt dissolved in water was employed as the reac-tant for the exchange reaction (yield 65 %). C224H400N8O28Si8

(3874.8): calcd. C 69.37, H 10.40, N 2.89; found C 69.31, H 10.61,N 2.82. IR (KBr pellets): ν̃ = 1200–1028 (SiOSi), 795 (SiC), 1728(anion) cm–1. 1H NMR (CDCl3): δ = 0.65 (t, 2 H), 0.86 (d, 6 H),0.94 (t, 9 H), 1.23 (d, 3 H), 1.35 (m, 6 H), 1.53 (m, 6 H), 1.77 (m,1 H, 2 H, overlapped), 2.37 (d, 2 H), 3.10 (b, 8 H), 3.46 (m, 1 H),6.96 (d, 2 H), 7.24 (d, 2 H) ppm. 29Si{1H} NMR (CDCl3): δ =–67.91 ppm.


Octakis(tri-n-octylpropylammonium iodide)octasilsesquioxane(POSS-C8-I): Compound POSS-C8-I was obtained as a brown vis-cous liquid by employing a similar work-up as that for POSS-C4-Iwith 1-iodooctane as the alkylating agent (yield 45 %). C216H450I8-N8O12Si8 (4588.5): calcd. C 56.50, H 9.88, N 2.44; found C 56.49,H 9.89, N 2.41. IR (KBr pellets): ν̃ = 1201–1010 (SiOSi), 788 (SiC).1H NMR (CDCl3): δ = 0.78 (t, 2 H), 0.86 (t, 9 H), 1.26 (m, 24 H),1.38 (m, 6 H), 1.73 (m, 6 H), 1.92 (m, 2 H), 3.42 (m, 6 H), 3.60(m, 2 H) ppm. 29Si{1H} NMR (CDCl3): δ = –67.75 ppm.

Octakis[tri-n-octylpropylammonium bis(trifluoromethylsulfonyl)-imide]octasilsesquioxane (POSS-C8-Tf2N): Compound POSS-C8-Tf2N was obtained as a colorless viscous liquid by using a pro-cedure similar to that for POSS-C4-Tf2N. Bis(trifluoromethylsulf-onyl)imide lithium salt was dissolved in methanol, and the reactionmixture was stirred for 12 h at 60 °C. The solvent was removed,and the residue obtained was dissolved in CHCl3, washed severaltimes with water, and finally dried in a rotary evaporator (yield90%). C232H450F48N16O44S16Si8 (5812.6): calcd. C 47.90, H 7.80,N 3.85; found C 47.81, H 7.84, N 3.92. IR (KBr pellets): ν̃ = 1220–1044 (SiOSi), 787 (SiC), 1346, 1187, 1131, 1056 (anion) cm–1. 1HNMR (CDCl3): δ = 0.72 (t, 2 H), 0.88 (t, 9 H), 1.26 (m, 24 H),1.30 (m, 6 H), 1.60 (m, 6 H), 1.71 (m, 2 H), 3.12 (b, 8 H) ppm.29Si{1H} NMR (CDCl3): δ = –67.85 ppm.

Octakis(tri-n-octylpropylammonium ibuprofen)octasilsesquioxane(POSS-C8-IBU): Compound POSS-C8-IBU was obtained as a col-orless viscous liquid by using a work-up method similar to that forPOSS-C4-Tf2N. Ibuprofen sodium salt was employed as the reac-tant for the exchange reaction, and the reaction mixture was stirredfor 2 d at room temperature (yield 45%). C320H586N8O28Si8(5214.3): calcd. C 73.65, H 11.32, N 2.15; found C 73.71, H 11.34,N 2.12. IR (KBr pellets): ν̃ = 1200–1019 (SiOSi), 798 (SiC), 1724(anion) cm–1. 1H NMR (CDCl3): δ = 0.64 (t, 2 H), 0.88 (b, 6 H, 9H, overlapped), 1.27 (b, 30 H), 1.37 (d, 3 H), 1.47 (m, 6 H), 1.78(m, 1 H, 2 H, overlapped), 2.39 (d, 2 H), 3.12 (b, 8 H), 3.53 (m, 1H), 6.99 (d, 2 H), 7.21 (d, 2 H) ppm. 29Si{1H} NMR (CDCl3): δ =–67.93 ppm.

Supporting Information (see footnote on the first page of this arti-cle): MALDI-TOF mass spectra.

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29, 5520–5528.Received: February 4, 2014

Published Online: April 28, 2014

star-shaped quaternary alkylammonium polyhedral oligomeric silsesquioxane ionic liquids

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