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Intercalation synthesis of functional hybrid materials based on layered simplehydroxide hosts and ionic liquid guests – a pathway towards multifunctionalionogels without a silica matrix?

Emilie Delahaye,a Zailai Xie,b Andreas Schaefer,b Laurent Douce,c Guillaume Rogez,c Pierre Rabu,c

Christina Gunter,d Jochen S. Gutmanna,e and Andreas Taubert*b, f

Received 4th May 2011, Accepted 27th June 2011DOI: 10.1039/c1dt10841g

Functional hybrid materials on the basis of inorganic hosts and ionic liquids (ILs) as guests holdpromise for a virtually unlimited number of applications. In particular, the interaction and thecombination of properties of a defined inorganic matrix and a specific IL could lead to synergisticeffects in property selection and tuning. Such hybrid materials, generally termed ionogels, are thus anemerging topic in hybrid materials research. The current article addresses some of the recentdevelopments and focuses on the question why silica is currently the dominating matrix used for(inorganic) ionogel fabrication. In comparison to silica, matrix materials such as layered simplehydroxides, layered double hydroxides, clay-type substances, magnetic or catalytically active solids, andmany other compounds could be much more interesting because they themselves may carry usefulfunctionalities, which could also be exploited for multifunctional hybrid materials synthesis. Thecurrent article combines experimental results with some arguments as to how new, advanced functionalhybrid materials can be generated and which obstacles will need to be overcome to successfully achievethe synthesis of a desired target material.

Introduction

Ionic liquids (ILs) have attracted tremendous interest in the recentpast. The advantages of ILs have been put forward and discussedextensively,1–5 suffice to mention here that there is virtually noarea of chemical research, development, and engineering thathas not at least seen some attempts of making use of ILs. Afield, which has recently picked up momentum is the generalarea of (inorganic) materials chemistry in and with ILs. Initially,many publications have focused on the synthesis, stabilization, andapplication (in particular in catalysis) of inorganic nanoparticlesin and from ILs.6–13 Several studies have shown that it is possible tostabilize inorganic nanoparticles in ILs.6,14–25 Their properties suchas size and size distribution (and hence their optical and catalyticproperties) can be adjusted by a number of parameters such as thechemistry of the IL, the inorganic, the precursor for the inorganic,

aMax Planck Institute for Polymer Research, D-15528, Mainz, GermanybInstitute of Chemistry, University of Potsdam, Karl-Liebknecht-Str. 24/25,Building 26, D-14476, Golm, Potsdam, Germany. E-mail: ataubert@uni-potsdam.de; Tel: +49 (0) 331 977 5773cInstitut de Physique et Chimie des Materiaux de Strasbourg, UMR7504CNRS-Universite de Strasbourg, FrancedInstitute of Earth and Environmental Sciences, University of Potsdam, D-14476, Golm, GermanyePhysical Chemistry, University of Duisburg-Essen, D-45117, Essen,GermanyfMax Planck Institute of Colloids and Interfaces, D-14476, Potsdam,Germany

and the reaction process.23,26–33 In particular, the interaction ofthe IL with the growing inorganic surfaces appears to be a keypoint in the growth process; it is however so far only poorlyunderstood.23,32

Initially, relatively simple synthetic processes such as thechemical reduction of metal salts in ILs have been studied and awide variety of (mostly metal oxide and hydroxide) nanoparticleshas been reported.12–13 Later porous materials such as metalorganic frameworks (MOFs),8–9,34–35 mesoporous silica,36–40 andnanostructured rutile38,41 have also been synthesized from ILs.Recently, nanoparticle synthesis protocols with a more elegantexperimental design have been reported. For example, Janiakand coworkers and Santini and colleagues have published thesynthesis of inorganic nanoparticles, predominantly metals, usingzero-valent metal precursors such as metal carbonyls as startingmaterials.24,42–48 The nanoparticles form by thermal decompositionof the metal carbonyls to the respective metal particles. Theinteresting aspect of this approach is that the reactions neednot predominantly be diffusion limited, as is the case withconventional chemical reduction processes. This may not alwaysbe an important point as far as work in a research laboratory isconcerned. It is, however, important in the context of conductingreactions in large volumes, where the IL viscosity, which can beorders of magnitudes larger than that of conventional (molecular)solvents,1–2,4 makes the processing difficult. As a result, theapproach by Janiak, Santini, and others is one option towardsalleviating issues with the synthesis of nanoparticles in ILs.

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 9977–9988 | 9977

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Besides nanoparticle synthesis in ILs, the fabrication ofnanoparticle dispersions is an area that has, so far, largely beenignored. The authors are only aware of a few publications dealingwith the synthesis, structure, and properties of dispersions ofinorganic particles in ILs. Nakashima and Kawai showed thatCdTe nanocrystals with a positively charged surface are efficientlyextracted into the hydrophobic IL 1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide [Bmim][NTf2].49 Guerrero-Sanchez et al. and Altin et al. have studied the rheologicalbehavior of magnetic nanoparticle dispersions in ILs.50–51

Lunstroot et al. have reported the dispersion of luminescentnanoparticles in ILs52 and Khare et al. have synthesizeddispersions of magnetic Fe3C nanoparticles in ILs.53 As someof these dispersions exhibit rather interesting properties, furtherresearch and development into this area can be envisioned.54

Arguably, however, within the field of materials chemistry inand with ILs, ionogels have attracted the most rapidly growingattention. Ionogels are hybrid materials based on a (continuous)matrix and an IL.55 The most popular inorganic matrix at themoment is silica.56 Obviously this stems from the fact thatsilica chemistry is well established and countless strategies forthe synthesis, structuring, functionalization, and application ofsilica have been published.57 Vioux and colleagues have recentlyreviewed the state of the art in silica ionogel research,56,58 but a fewkey studies need to be discussed here as well.

Silica ionogels are interesting because they are easily syn-thesized, transparent, and exhibit properties such as highionic conductivity,55,59 luminescence (if doped with a lumines-cent species),52,60–61 or lithium ion transport (if containing aLi source).62–63 Ionogels are often thermally stable (until ca.250◦ C)55,59,64–66 or exhibit an interesting change of the IL propertiesupon confinement.65–68 The interaction strength between the ILand the silica influences the dynamics (ionic mobility) of thewhole system and can be affected by sample preparation (insitu23,32,55,59–60,67,69–70 or post-synthesis functionalization65–66) but alsoby the structure and chemical composition of the IL. Neouzeet al. found a liquid-like behavior of [Bmim][NTf2] confinedin silica.59,70 Liu et al. have shown that the surface of SiOx

nanoparticles leads to a melting point depression of imidazoliumILs.71 Sha et al. have used computer simulations to study theinteraction of ILs with inorganic surfaces; they suggest thatnegative surface charges lead to a bilayer formation in 1-ethyl-3-methylimidazolium hexafluorophosphate [Emim][PF6].72 Neouzeand colleagues have demonstrated that organically crosslinkedsilica nanoparticle networks exhibit a short range order withinthe organic bridges and that these hybrid materials are capableof anion exchange.73–75 Finally, Gobel et al. have shown thatthe pore wall of silica ionogels can readily be functionalizedwith organic pendant groups.65 These groups strongly affect thebehavior of the ILs in the ionogels. For example, additionalphase transitions appear in the DSC. These changes dependon both the pore wall chemistry and the IL composition. Theabove studies therefore suggest that the surface/IL interaction isa key parameter for property tuning of the IL (and therefore theionogel). As a result, pore wall modification of silica constitutesan attractive, yet largely unexplored, approach for property tuningof ionogels for a large variety of applications (of course, porewall modification as such has been explored in silica materialssynthesis).76–82

Despite the many advantages of silica, it is evident to theinorganic and hybrid materials chemist that there must be morethan one matrix material, silica, which is suitable for the generationof useful ionogels. There are, however, surprisingly few examples,where inorganic matrices other than silica have been used forionogel formation (there are of course polymeric or organicgelators,56,61,83–90 but this is not the topic of this article).

A few studies report on the behavior of ILs in carbon nanotubesand graphite. Chen et al. demonstrated that confinement of 1-butyl-3-methylimidazolium hexafluorophosphate [Bmim][PF6] ina carbon nanotube raises the IL melting point by 200 ◦C.91

Graphite also strongly affects the phase behavior of ILs.92–94 Inboth cases, p–p interactions between the matrix and the IL cationshave been held responsible for the changes in IL properties. Indeed,Dou et al. have recently shown that ILs form an ordered layer closeto a graphite surface, which strongly attaches to the graphite andleads to a pronounced diffusion of the IL components parallel tothe graphite surface.95 Exploiting this interaction, ILs have beenused for the exfoliation of graphite into graphene-like sheets.96–97

Besides silica and carbon, there are only very few examples ofinorganic ionogel matrix materials.40,49–51 Using a somewhat exoticmatrix, mesoporous silver, Neouze and coworkers have postulatedthat silver-confined ILs exhibit a two-phase regime, where the ILclose to the pore wall is solid, and the IL in the center of the poreis liquid-like.98 The porosity of the silver matrix is rather low andthe IL fraction correspondingly small. The investigation of suchan ionogel is therefore quite a challenge. Vioux and coworkershave reported on an SnO2-based ionogel, although the focus ofthis study was on the synthesis of porous SnO2 (rather than thestructure and properties of the initial ionogel) using different ILsas templates, which were subsequently removed.68

Finally, there are two reports on the synthesis, structure, andproperties of IL-intercalated organic/inorganic hybrid materials.Wu et al. have synthesized zinc hydroxyfluoride nanofibers fromZn5(OH)8(NO3)2 using a microwave approach in the IL 1,2,3-trimethylimidazolium tetrafluoroborate [Tmim][BF4].99 These au-thors claim that their reaction product forms and organizes due tohydrogen bonding and p–p stacking of the imidazolium layers andthe BF4

- anions. Unfortunately, no evidence of either interactionwas presented in the publication. It is thus not clear at the momentif the structure proposed by the authors is the one actually presentin the material. The hydrogen bonding in particular must be viewedwith care, because the authors used a tri-substituted imidazoliumcation where the most acidic proton in the imidazolium moiety(the 2-H) is replaced by a much less efficient hydrogen donor, amethyl group.

Oaki et al. report on a fundamentally different approachtowards an IL/inorganic hybrid material.100 Unlike the approachesdescribed above, it is not based on the intercalation of the completeIL. Rather, the authors intercalated the respective imidazole intothe interlayers of the host material, a-Co(OH)2, and subsequentlytransformed it to the desired imidazolium cation by reactionwith the corresponding alkyl bromide. The authors claim to havegenerated a novel hybrid material based on a layered inorganicmatrix and an intercalated IL. However, the X-ray data of thetwo materials before and after quaternization of the imidazoleare essentially identical. Moreover, there is no additional analysisexcept energy dispersive X-ray (EDX) spectra. The spectra doprovide evidence for the presence of Br (although not for the

9978 | Dalton Trans., 2011, 40, 9977–9988 This journal is © The Royal Society of Chemistry 2011

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presence of the bromide anion), but the authors do not discusselemental analysis or IR spectroscopy data; the entire structureassignment therefore remains somewhat inconclusive.

Overall, the few data available on non-siliceous and non-carbonaceous IL/inorganic hybrid materials68,98–100 indicate thatfurther investigation into this topic is a worthy endeavor. Inparticular, the combination of a functional matrix (instead ofsilica, which typically just sits there and holds the ionogel together)could add a tremendous potential to the respective ionogels.Why is it, then, that so little work has been done in that area?Presumably there is more than one answer to this question. Wewill in the remainder of the article discuss reasons and combineexperimental and theoretical arguments to evaluate promisingpathways towards multifunctional IL hybrid materials based on(crystalline) inorganic matrix materials.

Some of the authors of this article have a history of magnetichybrid materials research101–103 and it is thus straightforward to usemagnetic materials such as layered hydroxides or hydroxyacetatesas test cases towards new ionogels with non-siliceous and non-carbonaceous matrices. This is particularly true because theinsertion of a wide variety of organic molecules into the inter-layers of, for example, Zn5(OH)8(OAc)2, affects the structure andproperties of the resulting new hybrid materials.101–106 More workhas been done on zinc or nickel/zinc hydroxynitrates, carbonates,or chlorides.107–114 With ILs instead of “conventional” organicguest molecules, such experiments would enable an identification,quantification, and rationalization of the effects of chemical andphysical properties of a specific IL on the structure and propertiesof the resulting hybrid material and therefore the constructionof entirely new hybrid materials. We have therefore investigatedprotocols for incorporation of a series of ILs into several layeredmagnetic host materials. In a first attempt to answer the abovequestion – why there are so few reports on inorganic non-silica-based ionogels – we will in the following address some of theissues by combining experimental work with comparison of thelittle published data available and identify the major reasonsobstructing a more efficient synthesis and application of ionogelsbased on matrices other than silica and carbon.

It must at this point clearly be stated that there is a significantdifference between the “classical” ionogels in silica and the ma-terials discussed here. The inorganic matrix of the former (silica)is typically amorphous and mesoporous, while the host materialsdiscussed here are crystalline and highly ordered. Because of thesignificant differences in the order and the chemical compositionof the two types of matrix materials, it is at the moment (also due tothe lack of information on the latter) not clear how closely relatedthe resulting IL/inorganic hybrids and silica ionogels are. We willtherefore at the moment assume that both are ionogels althoughthere will certainly be differences that will need to be worked outin detail in the future.

Experimental

Chemicals

Cu(OAc)2·H2O, Zn(OAc)2·2H2O, and Co(OAc)2·4H2O were pur-chased from ABCR. Sodium dodecylsulfonate (NaDS0) waspurchased from VWR. The IL 1-ethyl-3-methylimidazoliumhexafluorophosphate [Emim][PF6] was purchased from IoLiTec.

All products were used as received. 3-(1-Methylimidazolium-3-yl)propane-1-sulfonate PmimSO3

115–116 and the ILs 1-methyl-3-(3-carboxypropyl-)-imidazolium tetrafluoroborate [PmimCO2H]-[BF4],117 1-methyl-3-(3-sulfopropyl-)-imidazolium paratoluenesul-fonate [PmimSO3H][PTS]115–116 and 1-methyl-3-(3-cyanopropyl-)-imidazolium chloride [PmimCN][Cl]118 were synthesized usingmodified published protocols. Scheme 1 shows the compoundsused in this work.

Scheme 1 Compounds used in this work: 1-ethyl-3-methylimidazoliumhexafluorophosphate [Emim][PF6], 1-methyl-3-(3-carboxypropyl-)-imidazolium tetrafluoroborate [PmimCO2H][BF4], 1-methyl-3-(3-sulfopropyl-)-imidazolium paratoluenesulfonate [PmimSO3H][PTS], and1-methyl-3-(3-cyanopropyl-)-imidazolium chloride [PmimCN][Cl], andthe betaine 3-(1-methylimidazolium-3-yl)propane-sulfonate PmimSO3.

Preparation of 3-(1-methylimidazolium-3-yl)propane-sulfonatePmimSO3.115–116

A solution of 1-methylimidazole (1.642 g, 20 mmol) in acetone(20 mL) was prepared in a three necked round bottom flask. Thena solution of 1,3-propanesultone (2.443 g, 20 mmol) in acetone(20 mL) was added slowly to this solution. The mixture wasstirred at room temperature under N2 for 4 days and a whiteprecipitate progressively formed. The mixture was filtered andthe white powder was washed with acetone, followed by dryingunder vacuum. Yield = 85%. Elemental analysis: C7H12N2SO3 (M =204.25 g/mol): found (calc., %): C 41.17 (41.16); H 5.91 (5.92); N13.68 (13.72); S 15.75 (15.70). 1H-NMR (D2O): 8.65 (s, 1), 7.45(d, 2), 7.35 (d, 1), 4.70 (d, 1), 3.80 (s, 3), 2.8 (d, 2), 2.25 (t, 2). IR(transmittance, KBr pellet, cm-1): 3453 w, 3133 s, 3075 s, 3035 s,2951 m, 2928 w, 2871 w, 2840 w, 1800 w, 1766 sh, 1688 w, 1572 m,1561 m, 1388 w, 1344 m, 1266 m, 1238 s, 1211 s, 1144 s, 1116 s,1077 m, 1033 s, 877 m, 805 s, 755 m, 733 s, 655 s, 627 s, 594 s, 527s, 444 m.

Preparation of 1-methyl-3-(3-sulfopropyl-)-imidazoliumparatoluenesulfonate [PmimSO3H][PTS].115–116

PmimSO3 (2.041 g, 10 mmol) was added to solid para-toluenesulfonic acid (1.901 g, 10 mmol) and the mixture was stirredat 110 ◦C overnight. A viscous transparent liquid was obtained.After a few weeks, this viscous liquid turned into a white solid.This phenomenon has already been described for ILs bearing -SO3

- groups.119 Yield = 80%. Elemental analysis: C14H20N2S2O6

(M = 376.45 g/mol): found (calc., %): C 44.41 (44.67); H 5.37

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 9977–9988 | 9979

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(5.35); N 7.13 (7.44); S 16.93 (17.04). 1H-NMR (D2O): 8.60 (s, 1),7.55 (d, 2), 7.35 (s, 1), 7.30 (s, 1), 7.20 (d, 2), 4.15 (d, 2), 3.70 (s,3), 2.70 (d, 2), 2.25 (s, 3), 2.15 (t, 2). 13C-NMR (D2O + 5 dropsof DMSO-d6): 144, 140, 136, 130, 126, 124, 122, 48, 37, 27, 21.IR (transmittance, KBr pellet, cm-1): 3383 m, 3161 s, 3111 s, 2966w, 2922 w, 1702 m, 1600 w, 1573 m, 1498 w, 1453 w, 1231 s, 1164s, 1120 s, 1035 s, 1006 s, 911 w, 817 m, 746 m, 680 s, 622 w, 567m, 522 w. IR (reflectance, cm-1): 3458 w, 3153 m, 3111 m, 2959 w,2923 w, 2861 w, 1694 m, 16040 w, 1569 m, 1494 w, 1453 m, 1231 s,1168 s, 1122 s, 1031 s, 1004 s, 904 w, 818 m, 742 m, 677 m, 620 w.

Synthesis of 1-methyl-3-(3-cyanopropyl-)-imidazolium chloride[PmimCN][Cl]*1.5 H2O.118

1-Methylimidazole (4.105 g, 50 mmol) and 4-chlorobutyronitrile(6.215 g, 60 mmol) were mixed in a round bottom flask and themixture was stirred at 80◦ C for 24 h. After cooling, the resultingoily and slightly yellow liquid was washed with diethyl ether (3 *15 mL) and dried under vacuum. Yield = 94%. Elemental analysis:C8H14.6N3O1.3Cl (M = 206.68 g/mol): Found (calc., %): C 46.03(45.96); H 7.01 (7.04); N 19.92 (20.10); Cl 16.99 (16.96). 1H-NMR(D2O): 8.60 (s, 1), 7.55 (d, 1), 7.35 (d, 1), 4.25 (t, 2), 3.75 (s, 3), 2.65(t, 2), 2.20 (q, 2). 13C-NMR (D2O): 123.98, 122.27, 120.19, 48.08,35.88, 25.16, 13.84. IR (reflectance, cm-1): 3375 m, 3074 m, 2246m, 1637 m, 1574 s, 1453 m, 1426 m, 1339 w, 1166 s, 1022 w, 847m, 759 m, 648 m, 621 s, 556 m.

Preparation of 1-methyl-3-(3-carboxypropyl-)-imidazoliumtetrafluoroborate [PmimCO2H][BF4]*0.5 H2O.120

Under an inert nitrogen atmosphere, equimolar quantities of 1-methylimidazole and 3-chloropropionic acid were dissolved inmethanol at room temperature. The mixtures were heated to 75 ◦Cwith stirring and allowed to react for 20 h. The resulting reaction

mixture was treated with NaBF4 (1.2 equiv) and stirred for 18 hat room temperature. After removing the precipitated NaCl thefiltrate was concentrated by rotary evaporation. The yellow andviscous product was washed with diethyl ether and dried undervacuum to afford the neat IL. Yield = 68%. Elemental analysis:C8H14N2O2.5BF4 (M = 265,1 g/mol): found (calc., %): C 36.03(36.26); H 5.56 (5.32); N 10.84 (10.57). 1H-NMR (D2O): 8.60 (s,1), 7.45 (s, 1), 7.40 (s, 1), 4.20 (m, 2), 3.80 (s, 3), 2.40 (m, 2), 2.10(m, 2). IR (reflectance, cm-1): 3330 w, 3152 w, 3111 w, 2966 w,1722 m, 1566 m, 1430 w, 1166 m, 1028 s, 861 m, 750 m, 625 m,513 m.

Intercalation

The layered simple hydroxides (LSH) Cu2(OH)3(OAc),121

Co2(OH)3(OAc),122 Zn5(OH)8(OAc)2,104,107–108,123 andCo2(OH)3(DS0)123–124 (where DS0 is dodecylsulfonate) wereprepared as previously described. In a typical procedure, theLSH was reacted with an excess of IL (around 4 equivalents ofIL per equivalent of LSH) suspended in aqueous NaOH (pHwas adjusted to 8–9.5 with 1 M NaOH). Upon completion ofthe reaction, the powder was filtered and washed with water andethanol. All reactions were done under argon with the reactiontime and temperature being variable parameters (see Table 1below). In the case of [Emim][PF6], a few drops of ethanol wereadded to the aqueous medium to ascertain complete dissolutionof the IL; no base was used in this case and the pH of the finalsolution was around 7.7.

Elemental analysis

C, H, N, S, and metals analysis were carried out at the AnalytischeLaboratorien GmbH Lindlar.

Table 1 Reaction parameters and products

Starting materials

Entry IL Hydroxide pH Temperature Time Final product

1 [Emim][PF6] Zn5(OH)8(OAc)2 ª 8 RT 4h Zn(OH)2 + LSH Zn2 Zn5(OH)8(OAc)2 ª 8 40◦ C 4h Zn(OH)2 + LSH Zn3 Zn5(OH)8(OAc)2 ª 8 90◦ C 4h ZnO4 Co2(OH)3(DS0) ª 8 RT 4h no reaction5 Co2(OH)3(DS0) ª 8 40◦ C 4h no reaction6 [PmimCO2H][BF4] Co2(OH)3(DS0) ª 7.7 RT 6h no reaction7 Co2(OH)3(DS0) ª 7.7 40◦ C 6h no reaction8 Co2(OH)3(DS0) ª 8.7 40◦ C 6h no reaction9 Co2(OH)3(DS0) ª 7.7 80◦ C 6h no reaction10 Cu2(OH)3(OAc) ª 8.7 90◦ C 2h CuO11 Zn5(OH)8(OAc)2 ª 7.7 90◦ C 2h ZnO12 [PmimSO3H][PTs] Co2(OH)3(DS0) ª 8.8 50◦ C 6h no reaction13 Co2(OH)3(DS0) ª 8.8 50◦ C 24h no reaction14 Zn5(OH)8(OAc)2 ª 9.3 50◦ C 4h Zn/PTS + LSH Zn15 Zn5(OH)8(OAc)2 ª 9.3 50◦ C 17h Zn/PTS + LSH Zn16 Zn5(OH)8(OAc)2 ª 9.3 50◦ C 48h Zn/PTS17 Co2(OH)3(OAc) ª 9.3 50◦ C 4h Co/PTS18 Cu2(OH)3(OAc) ª 9.3 50◦ C 4h Cu/PTS19 PmimSO3 Zn5(OH)8(OAc)2 ª 9.5 50◦ C 24h Zn(OH)2

19 Cu2(OH)3(OAc) ª 9.5 50◦ C 24h CuO21 [PmimCN][Cl] Cu2(OH)3(OAc) ª 6.6 50◦ C 24h Cu2(OH)3(Cl)22 Cu2(OH)3(OAc) ª 9.5 50◦ C 24h Cu2(OH)3(Cl)23 Zn5(OH)8(OAc)2 ª 6.6 50◦ C 4h Zn5(OH)8(Cl)2

24 Zn5(OH)8(OAc)2 ª 9.5 50◦ C 24h Zn5(OH)8(Cl)2

9980 | Dalton Trans., 2011, 40, 9977–9988 This journal is © The Royal Society of Chemistry 2011

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X-ray diffraction

Powder XRD patterns were collected on a Siemens/Bruker D5005diffractometer (Cu Ka1 = 1.540598 A) between 2 and 70 degrees2q.

Spectroscopy

Infrared spectra were recorded on KBr pellets on a NicoletFT-IR Nexus 470 or in ATR mode on a Nicolet FT-IR 730.UV/Vis/NIR spectra were recorded on a Perkin-Elmer Lambda950 spectrometer (reflection mode using a 150 mm integratingsphere with a mean resolution of 2 nm and a sampling rate of300 nm min-1). Liquid NMR was measured using a Bruker Avance250 spectrometer. The spectra were calibrated with respect to theH2O peak (4.70 ppm) from the D2O solvent.

Scanning Electron Microscopy

SEM images were obtained with a JEOL 6700F with a fieldemission gun, operating at 3 and 15 kV in the SEI mode. Priorto SEM, powder samples were deposited on a double-faced stickytape and coated with an evaporated carbon layer.

Thermogravimetric analysis

TGA was done on a Mettler Toledo TGA/SDTA 851 in air from20 to 900 ◦C with a heating rate of 10 ◦C min-1.

Differential scanning calorimetry

DSC experiments were done on a Mettler Toledo DSC 822.Samples of about 5 mg were placed in aluminum pans with piercedlids. DSC traces were recorded from -40 to 200◦ C with a heatingrate of 10 K min-1. To reduce water traces, the samples were heatedto 200◦ C before the first cooling cycle. Heating and cooling cycleswere repeated twice for reproducibility. Isothermal times were5 min.

Results

We have used a series of ILs and layered simple hydroxides (LSHs)to explore the parameter space for the synthesis of well-defined andsingle phase IL/LSH hybrid materials. For clarity, the results arediscussed in order of the ILs used in the study. With the exceptionof [Emim][PF6], materials were synthesized by dispersing the LSHin a basic aqueous solution containing the IL (see Experimentalfor details).

1. Intercalation of 1-ethyl-3-methylimidazoliumhexafluorophosphate [Emim][PF6]

1.1 Intercalation into Zn5(OH)8(OAc)2. Fig. 1 shows X-raydiffraction (XRD) patterns of products obtained after 4 h ofreaction between Zn5(OH)8(OAc)2 and [Emim][PF6] at differenttemperatures. Samples obtained at room temperature (RT) andat 40 ◦C exhibit a progressive disappearance of the lamellarstarting compound Zn5(OH)8(OAc)2. Accordingly, a new phase isobserved, Zn(OH)2 (wuelfingite, JCPDS 38-385). Upon treatmentat 90 ◦C, the starting lamellar phase Zn5(OH)8(OAc)2 completelytransforms into ZnO (wurtzite, JCPDS 36-1451).

Fig. 1 Powder XRD patterns of the starting compound Zn5(OH)8(OAc)2

(black) and of the compounds obtained after 4 h of reaction betweenZn5(OH)8(OAc)2 and [Emim][PF6] at RT (red), at 40◦ C (blue), and at90◦ C (green).

Fig. 2 shows the corresponding scanning electron microscopy(SEM) data. Samples treated at 40 ◦C contain rather large crystalswith well-developed faces and edges. This is consistent with(1) the observation of sharp reflections in the XRD patternsand (2) with earlier reports on the morphology of wuelfingiteZn(OH)2.123 Samples treated at 90 ◦C contain particles with apoorly defined hexagonal prismatic morphology. Most particlesexhibit the central grain boundary known from other reportson ZnO precipitated from aqueous solution.125–128 SEM furthersuggests that the particles are not single crystals, but mesocrystals,similar to other examples known from the literature.129–133

Fig. 2 SEM images of the compounds obtained after for 4 h of reactionbetween Zn5(OH)8(OAc)2 and [Emim][PF6] at 40 ◦C (left) and at 90 ◦C(right). Scale bars are 1 mm.

Fig. 3 shows the corresponding IR spectra. IR spectroscopyclearly shows the disappearance of the vibration bands at1540 cm-1 and 1388 cm-1 corresponding to the antisymmetricand symmetric stretch vibration of the carboxylate groups ofthe starting LSH Zn5(OH)8(OAc)2. Moreover, IR spectra of theproducts obtained at RT and 40 ◦C show new bands at 1090,1032, 718, and 484 cm-1. None of these bands is present in theoriginal LSH. They can be attributed to asymmetric stretchingvibrations of Zn–O–Zn and bridge-oxygen in the orthorhombicstructure (1090 and 1032 cm-1), to nO–H groups (718 cm-1) and tothe lattice vibrations of Zn–O metal–oxygen bonds (484 cm-1).134

The compound obtained at 90 ◦C exhibits a new band at 545 cm-1

which can be assigned to Zn–O vibrations.134 However, noneof the compounds exhibits bands of the IL, in particular thehexafluorophosphate anion around 830 cm-1, which is clearlyvisible in the original IL. In summary, therefore, XRD, SEM,

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Fig. 3 Infrared spectra of the starting compound Zn5(OH)8(OAc)2

(black) and of the compounds obtained after 4 h of reaction betweenZn5(OH)8(OAc)2 and [Emim][PF6] at RT (red), at 40◦ C (blue), at 90◦ C(green) and neat IL [Emim][PF6] (grey).

and IR spectroscopy show that [Emim][PF6] does not intercalateinto the LSH Zn5(OH)8(OAc)2. Simple adaptation of intercalationprotocols from layered hybrid materials is therefore not possiblefor IL/LSH hybrid materials.

1.2 Intercalation into Co2(OH)3(DS0) [that is, Co2(OH)3(OAc)pre-intercalated with dodecylsulfonate DS0]

Fig. 4 shows the IR spectra and XRD patterns of the start-ing material and the products obtained after 4 h of reactionbetween [Emim][PF6] and pre-intercalated Co2(OH)3(DS0). Thedifference to the above example is that here, the inorganic matrix,Co2(OH)3(OAc), has been treated with sodium dodecylsulfonateprior to IL intercalation. The goal of this approach was to generatea more hydrophobic interlayer environment, which, possibly,would be more suitable for IL intercalation. Unlike the examplediscussed above, there is no phase transition of the inorganic.The only modification observed in XRD is a decreasing reflectionintensity, in particular of the (001) reflection. The exact reasonfor this reduction is hard to identify: either, a partial removal ofthe dodecylsulfonate from the LSH hosts leads to a lower electrondensity near the basal planes of the inorganic layer, which in turnleads to a lower signal intensity. Alternatively, the reduced intensityis due to a reduced order in the layers, although, possibly, in this

Fig. 4 XRD patterns (left) and IR spectra (right) of the startingcompound Co2(OH)3(DS0) (black) and the compounds obtained after 4 hof reaction between Co2(OH)3(DS0) and [Emim][PF6] at RT (red) and at40◦ C (blue). The infrared spectrum of the neat IL [Emim][PF6] is shownin the right part in grey. A modification of the pH from 7.7 to 8.7 does notchange the final product (data not shown).

case a line broadening should also be observed. There is, however,no indication of IL incorporation into the inorganic.

IR spectroscopy supports XRD in that also here essentially thesignals of the starting compound Co2(OH)3(DS0) are observed.Moreover, no band originating from the IL (in particular thePF6 band around 830 cm-1) is present in the IR spectra of thecompounds obtained after reaction with [Emim][PF6].

2. Intercalation of 1-carboxypropyl-3-methylimidazoliumtetrafluoroborate [PmimCO2H][BF4]

It has been demonstrated above that the LSH Zn5(OH)8(OAc)2

is not stable under the experimental conditions studied sofar (Fig. 1). Co2(OH)3(DS0) is stable (Fig. 4), but does notshow intercalation with [Emim][PF6]. One of the reasons forthe failed intercalation could be an unfavorable interaction ofthe IL cation with the LSH hosts. We have thus exchanged[Emim][PF6] for 1-carboxypropyl-3-methylimidazolium tetrafluo-roborate [PmimCO2H][BF4] which bears an additional carboxylicacid functionality in the side chain. The carboxylate groupstrongly interacts with LSHs,101,103–105,107–108,110,122135–136 and a betterincorporation into the LSH can therefore be envisioned.

Fig. 5 shows the results of the intercalation experiments with[PmimCO2H][BF4] and Co2(OH)3(DS0). Somewhat surprisingly,the XRD patterns are strikingly similar to those of the productsof the reaction between [Emim][PF6] and Co2(OH)3(DS0), Fig. 4.Again, XRD suggests a degradation of Co2(OH)3(DS0) by DS0

release, but no IL incorporation. Comparison experiments at80 ◦C only yield a small amount of product, which could notbe analyzed further. However, the brown color of the resultingsolid suggests the formation of oxides. IR spectroscopy confirmsthe XRD data. The IR spectra of the two compounds obtainedafter the reaction are essentially identical to the spectra of thestarting compound. No vibration band originating from BF4

- or-CO2

- groups is present in the IR spectra of the products.So far, we did not observe any IL incorporation into the LSH

host materials. One possible explanation can be found in therather poor stability of the PF6

- and the BF4- anions against

hydrolysis. To determine the stability of the IL [PmimCO2H][BF4]we have performed an acid–base titration, Fig. 6. Clearly, the curvedisplays two steps. The first step corresponds to the deprotonationof the –CO2H group and the second step corresponds to thehydrolysis of the BF4 groups. Titration therefore clearly showsthat [PmimCO2H][BF4] is not stable in a neutral or basic aqueousmedium (which is used for our synthesis). This confirms previouswork on the stability of the PF6

- and BF4- anions.137

3. Intercalation of 1-propylsulfonate-3-methylimidazoliumpara-toluene sulfonate [PmimSO3H][PTS]

Intercalation into Cu2(OH)3(OAc), Zn5(OH)8(OAc)2, andCo2(OH)3(OAc). Besides carboxylates, sulfonates efficiently at-tach to the inorganic layers in LSHs. We have therefore, in a nextattempt towards stable IL/LSH hybrid materials, replaced thecarboxylate group for a sulfonate group and the labile anions PF6

-

and BF4- for the stable anion para-toluene sulfonate (tosylate).

Fig. 7 shows the IR spectra and XRD patterns of the prod-ucts obtained after 4 h of reaction at 50 ◦C between the IL[PmimSO3H][PTS] and different LSHs. All XRD patterns show

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Fig. 5 XRD patterns (left) and IR spectra (right) of the starting compound Co2(OH)3(DS0) (black) and the products obtained after 4 h of reactionbetween Co2(OH)3(DS0) and [PmimCO2H][BF4] at RT (red) and at 40 ◦C (blue). The IR spectrum of the neat IL [PmimCO2H][BF4] is shown in the rightpart in grey.

Fig. 6 Titration curve of the IL [PmimCO2H][BF4].

a new lamellar phase with an interlamellar distance of 13.5, 16.9,and 17.6 A for the LSH based on Cu, Zn and Co, respectively. Inthe case of the Zn–LSH the reaction is not complete and it wasnecessary to increase the reaction time to 48 h to obtain a singlephase with an interlamellar distance of 16.9 A (data not shown).IR spectra confirm XRD suggesting that here, for the first time,new hybrid materials have formed. The IR spectra can, however,not be assigned to the target hybrid materials. Rather, the signalssuggest that not the IL is incorporated but something else, leadingto a well-ordered material.

Fig. 8 shows IR and NMR spectra of a sample obtained from thereaction of Zn5(OH)8(OAc)2 and [PmimSO3H][PTS]. This samplewas dissolved in aqueous HCl to isolate the organic component in

the hybrid. 1H-NMR spectra can only be assigned to the para-toluenesulfonate (PTS-) counter ion of the original IL. NMRtherefore shows that not the IL, but only the PTS- anion is insertedin the LSH. Moreover, although NMR cannot readily be usedfor the Cu and Co LSHs since these metals are paramagnetic,the similarity of all IR spectra of the reaction products with thedifferent LSHs and [PmimSO3H][PTS] suggests that in all cases(Co, Cu, Zn) only the PTS- anion is inserted into the LSH. Thisis further confirmed by elemental analysis (data not shown).

Intercalation into the pre-intercalated LSH Co2(OH)3(DS0) with[PmimSO3H][PTS]. As demonstrated above for [Emim][PF6],(see Fig. 4), the pre-intercalation of an LSH with dodecylsulfonatedoes not improve the intercalation with the IL [PmimSO3H][PTS].Again, the XRD patterns and the IR spectra of the compoundsobtained after 6 or 24 h of reaction at 50 ◦C resemble the respectivedata of the starting compound (except for the intensity of the XRDpeaks, data not shown).

4. Intercalation of the betaine1-propylsulfonate-3-methylimidazolium [PmimSO3]

It has just been mentioned that IL intercalation intoCo2(OH)3(DS0) has so far not been successful. In contrast(although not the entire IL but only the anion PTS- has been incor-porated), intercalation into Cu2(OH)3(OAc) and Zn5(OH)8(OAc)2

is at least a partial success. We have therefore rationalized thatremoval of the PTS- anion could result in the incorporation of the

Fig. 7 XRD patterns (left) and IR spectra (right) of the starting compounds Cu2(OH)3(OAc) (black), Zn5(OH)8(OAc)2 (brown), Co2(OH)3(OAc) (grey),and of the compounds obtained after 4 h of reaction at 50 ◦C between [PmimSO3H][PTS] and Cu2(OH)3(OAc) (red), and Zn5(OH)8(OAc)2 (blue), andCo2(OH)3(OAc) (green). The IR spectrum of the neat IL [PmimSO3H][PTS] is shown on the right (grey).

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Fig. 8 NMR (left) and IR (right) spectra of the compound obtained from the reaction of Zn5(OH)8(OAc)2 and [PmimSO3H][PTS] (48 h at 50 ◦C). TheNMR spectrum was obtained after dissolution of the sample in aqueous HCl. The IR spectrum of the hybrid material is shown in blue.

[PmimSO3] betaine unit because the competing insertion of thePTS- anion is eliminated.

Fig. 9 shows the IR and XRD data of a sample obtained byreaction of Zn5(OH)8(OAc)2 with [PmimSO3] after 24 h at 50 ◦C.Unfortunately, as already observed for the case of [Emim][PF6],the XRD pattern of the product can be assigned to Zn(OH)2

(wuelfingite, JCPDS 38-385) and not to an IL/LSH hybridmaterial.

XRD is confirmed by IR spectroscopy. The bands corre-sponding to the antisymmetric and symmetric vibration of thecarboxylate -CO2

- (1540 and 1388 cm-1, respectively) in thestarting material disappear. New bands at 1090 and 1032 (asym-metric stretching vibration of Zn–O–Zn and bridge-oxygen in theorthorhombic structure, respectively), 718 (dO–H), and 508 and484 cm-1 (lattice vibrations of Zn–O) appear.134 But once again,there is no signal of an IL component, in this case PmimSO3, visiblein the reaction product. Both XRD and IR therefore suggest noIL incorporation. In the case of Cu2(OH)3(OAc) a brown powderindicative of CuO was obtained. This is confirmed by infraredspectroscopy (data not shown) but the very low yield preventeddiffraction analysis.

5. Intercalation of 1-cyanopropyl-3-methylimidazolium chloride[PmimCN][Cl]

Fig. 10 shows the results of the reaction of Zn5(OH)8(OAc)2 andCu2(OH)3(OAc) with [PmimCN][Cl] after 24 h at 50 ◦C. In both

cases, a new lamellar product with an interlamellar distance of 5.71(Cu) and 7.61 A (Zn) is obtained, suggesting that, possibly, hereindeed the IL is incorporated into the LSH matrix. Unfortunately,however, these interlamellar distances correspond to the well-known lamellar compounds Cu2(OH)3(Cl) and Zn5(OH)8(Cl)2.XRD therefore demonstrates that not the IL but, once again, onlythe anion (in this case chloride) is incorporated into the LSH host.IR spectroscopy confirms this finding, as mainly M–O vibrationbands, but no IL signals are observed. Further modifications ofthe conditions, such as altering the pH from 6.6 to 9.5 during thereaction do not further change this finding.

Discussion

For clarity, Table 1 summarizes the results of the experimentspresented above. The main outcomes are (1) that the synthesis ofIL/inorganic hybrid materials is much more difficult with LSHsthan with silica or carbon host materials, (2) that almost everycombination of IL and LSH host yields a different product, and(3) that the anion of the IL appears to intercalate much morereadily into the LSH than the entire IL, even in cases where theimidazolium moiety bears a coordinating function, i.e. carboxylateor sulfonate. There are however also finer details which will bediscussed below.

Overall, the current experiments and the two studies on relatedapproaches99–100 suggest that the synthesis of IL/inorganic hybrid

Fig. 9 Powder XRD patterns (left) and IR spectra (right) of the starting compound Zn5(OH)8(OAc)2 (black) and of the compound obtained after 24 hof reaction between Zn5(OH)8(OAc)2 and PmimSO3 at 50 ◦C (red). The IR spectrum of PmimSO3 is shown in the right part in grey.

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Fig. 10 Powder XRD patterns (left) and IR spectra (right) of the starting compounds Cu2(OH)3(OAc) (black), Zn5(OH)8(OAc)2 (brown), and of thecompounds obtained after 24 h of reaction at 50 ◦C between [PmimCN][Cl] and Cu2(OH)3(OAc) (red), and Zn5(OH)8(OAc)2 (blue). The IR spectrum ofthe IL [PmimCN][Cl] is shown on the right (grey).

materials is much more difficult once the inorganic host is acrystalline, well-ordered substance. In contrast to amorphoussilica55,59–60,65–67,69–70 or different carbon72,91–95 hosts, where a fairlylarge number of ILs has readily been incorporated, the insertioninto LSHs and related compounds appears to be much moredependent on the chemical nature of the host (LSH) and guest(IL) and the specifics of their interaction. Indeed, ILs show alarge number of different interactions138 and it is thus necessaryto evaluate these different interactions for their importance in thecurrent problem.

Among others, ILs exhibit ionic, hydrogen bonding, hydropho-bic, and p–p interactions both between IL components and thesurroundings such as the inorganic host material. In the caseof silica surfaces SiOx, some authors report a melting pointdepression,71 which is different than what has been reported forILs immobilized in carbon nanotubes.91 Sha et al. suggest thatnegative surface charges lead to a bilayer formation in [Emim][PF6]although they did not specify the chemical nature of the surface.72

One may thus argue (although this question has not been answeredconclusively as of now) that in some cases also positive surfacecharges may lead to specific interactions between the IL anionand matrix. Indeed, Gobel et al. have shown that the modificationof pore walls in mesoporous silica with neutral and positivelycharged pendant groups strongly affects the phase behavior of theconfined IL.65 This is one of the first experimental datasets of asurface charge effect on the behavior of a confined IL. However, thependant groups used in this study are aromatic and p–p stackingcould therefore also be of importance. Indeed, p–p stackingbetween the imidazolium ions and the p-system of graphite orcarbon nanotubes has been suggested as a key interaction affectingthe behavior of ILs.71,83–87 Further experiments are therefore neededto separate the contributions of the charge from the p–p stackingcontributions to the IL behavior in confinements.

The cases of silver98 and SnO268 hosts remain currently open,

because of the lack of data. Obviously, for example hydrogenbonding is a possible key interaction in the case of SnO2, butthis remains to be proven in the future. Overall these datanevertheless suggest that in carbon host materials p–p interactionsare important, while in all other inorganic matrices presumablyionic interactions and hydrogen bonding dominate.

Let us now turn to the problem at hand, the incorporation ofILs in a chemically and crystallographically well-defined inorganicmatrix such as LSHs. Essentially, our results suggest that in the

worst case, the matrix is not stable and transformation into anundesired inorganic such as wuelfingite or zincite occurs (Fig. 1–3). The other key outcome is that there is no incorporationof a molecular entity containing a cation. Neither simple ILcations such as [Emim] (Fig. 1) nor more complex moleculeslike [PmimCO2H] (Fig. 5) or the less highly charged [PmimCN](Fig. 10) are found anywhere in the reaction products. Pre-intercalation with dodecylsulfonate (Fig. 4) does not alter thisbehavior.

In contrast, anions like tosylate (Fig. 7 and 8) or chloride(Fig. 10) readily exchange with the acetate of the LSH. Thissuggests that there is a general problem with the concomitantincorporation of the cationic counterpart of the IL. Indeed, LSHsare anion exchange materials. As a result, an incorporation of ILswill strongly depend on the balance between electrostatic repulsionby the layers where the anion exchange occurs and the bondingbetween the two components of the ILs (the inorganic layers inLSH are neutral since the anionic interlayer species coordinate themetal ions; a transition state with locally positively charged layerscan therefore be envisaged during the anion exchange reaction.This would, however, not aid a cation exchange).

Indeed, recent work by Oaki et al. suggests that anotherapproach could be more successful.100 Instead of intercalating theIL, these authors have incorporated a carboxylated imidazole intolayered a-Co(OH)2. Only after incorporation, the imidazole wasreacted with an alkyl bromide. The rationale behind this approachis that cations are hard to incorporate into many inorganicmatrices (which is also confirmed by the experiments reportedhere). As pointed out in the introduction, however, there are issueswith the reported characterization of the resulting material. XRDpatterns of the products before and after imidazole quaternizationare virtually identical and except EDX data, no further analysis isreported. The structure assignment therefore remains somewhatambiguous and it is thus at the moment not possible to concludeon the potential of the intercalation-quaternization approachintroduced by Oaki et al.

Finally, Wu et al. reported on zinc hydroxyfluoride nanofiberssynthesized with ILs.99 The authors draw a rather specific imageof how and where the IL anion (BF4

-) and the IL cation(1,2,3-trimethylimidazolium) are located in their material. It musthowever be concluded that the analysis (as it is reported in thepublication) does not entirely support this claim. For example,there is no discussion of the d-spacings vs. the sizes of the IL

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anions and cations or with respect to a non-substituted (that is,without organic component) zinc hydroxyfluoride control sample.In spite of these shortcomings the work by Wu et al. is interestingbecause it is the first report of a non-siliceous hybrid materialwhere a 1,2,3-trisubstituted imidazolium IL has been used. As the2-H site in imidazolium cations is quite acidic, this work may serveas a prototype for materials where this 2-H acidity is a problem.This possibly also includes some of the materials that have beenstudied by us in the current manuscript.

Having said all the above, it is important to note that there isa type of crystalline inorganic matrix, which has been used quitesuccessfully for the fabrication of inorganic/IL hybrid materials:clays. Initially, a few studies have focused on the fabrication ofthese materials, mostly based on kaolinite or montmorillonite.While an early study focused on the sorption properties of severalinorganic surfaces,139 subsequent publications focused on the op-timization of the intercalation procedure and a detailed structuralcharacterization.140–142 Among others these studies clearly showthat successful IL intercalation often, but not exclusively, needs asuitable pre-intercalation, for example with DMSO140 or urea.141

This suggests that intercalation may also be possible with thematerials used in the current study or those used by Oaki et al.100

and Wu et al.,99 but that a suitable pre-intercalation process is yetto be developed. Furthermore, application oriented-approacheshave then explored the potential of clay/IL hybrid materials forcatalysis,143–146 sensing,147–149 electrochemistry,150 and mechanicalreinforcement.151 It must be noted at this point that not in allcases the interaction of the IL with the clay has been studied indetail; it is therefore not possible to draw a final conclusion as towhy, for example, in some cases pre-intercalation is necessary forsuccessful fabrication of a homogeneous and well-defined hybridmaterial and not in others.

Nevertheless, based on the currently available data it is possibleto formulate some first, basic rules or hypotheses for the synthesisof IL/inorganic hybrid materials which will need to be furtherevaluated in the future. (1) The ILs (including the cations, seethe discussion on the 2-H acidity problem in imidazolium above)must be stable in the medium chosen for the reaction. This seemstrivial, but as shown in this study, even a slow degradationof IL components (for example [BF4] or [PF6])99 releases ionsor molecules that can dramatically change the outcome of thereaction, Fig. 1–3. (2) Pre-intercalation does, at least until now,not seem a useful strategy for successful intercalation in the caseof metal hydroxides, although it has been successfully used inclays.140–141 Even relatively hydrophobic ILs such as [Emim][PF6]are not incorporated into the hydrophobic pre-intercalated in-terlayer spaces of the metal hydroxides, Fig. 4. This suggeststhat, possibly, hydrophobic interactions are less important for ILincorporation than some others. (3) Electrostatic interactions area key parameter to investigate further, even though they may berather short ranged, because of the high ion concentration.100

This is clearly supported by some work on clays, which havecharged inorganic layers.142,144–146,148,151 The current study, wherethe inorganic layers are electrically neutral, shows a clear selectionbetween anion and cation incorporation, e.g. Fig. 7 and 8. Onlyanions such as the [PTS] ion are incorporated into the interlayer,but neither simple cations like [Emim] nor betaine-like moleculessuch as [PmimSO3] or functionalized cations such as [PmimCO2H]are found in the products. (4) Possibly, competing intercalation of

two chemically similar IL components (for example [PmimSO3H]and [PTS]-, Scheme 1) is another factor to consider. If the[PTS]- anion is incorporated much faster, the [PmimSO3H] cationmay not stand a chance for incorporation because the finalinteraction with the inorganic matrix is roughly the same (both viaa sulfonate group) and there is thus no net energy gain for a [PTS]–[PmimSO3H] exchange. This may be different if the IL anion ismuch bulkier than the betaine. (5) Finally, if the IL contains ions,in particular anions, that efficiently coordinate to the inorganiclayer they must be selected such that they do not alter the structureinto an undesired compound. For example, ILs with a chloride orbromide anion may generally not be suitable, because they willlead to, for example, Cu2(OH)3(Cl) and Zn5(OH)8(Cl)2, which arenot the desired products, but are hard to transform any further.

The current study therefore shows that, although there aretwo reports99–100 on the successful formation of IL/inorganichybrid materials, there is so far no generic approach towards thisinteresting class of hybrid materials (with the exception of someclays). There is thus a need to further investigate the interaction ofILs with different inorganic hosts and to continue the developmentof synthesis protocols. One possibility may indeed be the approachput forward by Oaki et al.100 of first intercalating a precursor intothe matrix followed by formation of the cation. Other alternativesinclude (1) the use of other matrix materials, for example layereddouble hydroxides, oxalates, perovskites, or mesoporous metaloxides,152–153 (2) other synthetic processes such as ionothermalsynthesis8–9 or (3) exfoliation and casting of the hybrid materialsfrom an inorganic/IL mixed solution. So far there are no reportson any of these approaches, although two versatile exfoliationprotocols for inorganic solids have recently been reported.154–155

Overall, the combination of many interesting matrix materialswith the well known properties of ILs could lead to multifunctionalhybrid materials with combined, possibly synergistic propertiessuch as a (switchable) coupling between the magnetic layers ofa matrix material mediated by the IL in the interlayer spacings.These properties will obviously depend on the exact compositionof both the host (the inorganic) and the guest (the IL) and willneed to be evaluated in much greater detail in the future.

Conclusion

IL/inorganic hybrid materials, predominantly in the form of silicaionogels, have attracted increasing attention from the materialscommunity. Related, but chemically and structurally very differentmaterials based on layered, crystalline inorganic matrix materialssuch as layered simple hydroxides (LSHs) hold promise for theconstruction of highly advanced IL/inorganic hybrid materials.As discussed in this article, however, the synthesis of thesematerials is significantly more complicated than the synthesisof silica ionogels. Up to now there are interesting concepts –but no generic approaches towards such materials – availablein the literature. The fabrication, and consequently also thecharacterization and application therefore remain a challenge forthe materials chemistry community.

Acknowledgements

E.D. acknowledges an MPG-CNRS postdoctoral fellowship. Wethank Dr E.I. Unuabonah for useful discussion. Financial support

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by the University of Potsdam, the “Fonds der ChemischenIndustrie”, the MPI of Colloids and Interfaces (Colloid ChemistryDepartment), and the MPI for Polymer Research is gratefullyacknowledged.

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9988 | Dalton Trans., 2011, 40, 9977–9988 This journal is © The Royal Society of Chemistry 2011

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