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Carboxyl-Functionalized Task-Specific

Ionic Liquids for Solubilizing Metal Oxides Peter Nockemann, Ben Thijs, Tatjana N. Parac-Vogt, Kristof Van Hecke, Luc Van Meervelt, Bernard Tinant, Ingo Hartenbach, Thomas Schleid, Vu Thi Ngan, Minh Tho Nguyen, and Koen Binnemans Inorg. Chem.

, 2008, 47 (21), 9987-9999? DOI: 10.1021/ic801213z ? Publication Date (Web): 08 October 2008

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Carboxyl-Functionalized Task-Speci?c Ionic Liquids for Solubilizing

Metal Oxides

Peter Nockemann,*,?Ben Thijs,?Tatjana N.Parac-Vogt,?Kristof Van Hecke,?Luc Van Meervelt,?

Bernard Tinant,?Ingo Hartenbach,§Thomas Schleid,§Vu Thi Ngan,?Minh Tho Nguyen,?

and Koen Binnemans?

Department of Chemistry,Katholieke Uni V ersiteit Leu V en,Celestijnenlaan200F,bus2404,

B-3001Leu V en,Belgium,Unite′CSTR,Uni V ersite′Catholique de Lou V ain,1Place Louis Pasteur,

1348Lou V ain-la-Neu V e,Belgium,and Institut fu¨r Anorganische Chemie,Uni V ersita¨t Stuttgart, Pfaffenwaldring55,D-70569Stuttgart,Germany

Received July1,2008

Imidazolium,pyridinium,pyrrolidinium,piperidinium,morpholinium,and quaternary ammonium bis(tri?uoromethyl-sulfonyl)imide salts were functionalized with a carboxyl group.These ionic liquids are useful for the selective dissolution of metal oxides and hydroxides.Although these hydrophobic ionic liquids are immiscible with water at room temperature,several of them form a single phase with water at elevated temperatures.Phase separation occurs upon cooling.This thermomorphic behavior has been investigated by1H NMR,and it was found that it can be attributed to the temperature-dependent hydration and hydrogen-bond formation of the ionic liquid components.

The crystal structures of four ionic liquids and?ve metal complexes have been determined.

Introduction

Ionic liquids are often considered as“supersolvents”for many classes of organic and inorganic compounds.1-5 Several remarkable reports on the superior solvent properties of ionic liquids have been published.For instance,cellulose dissolves in1,3-dialkylimidazolium chloride ionic liquids and can be regenerated from such an ionic liquid solution by the simple addition of water.6Also remarkable is the ability of chloroaluminate ionic liquids to dissolve kerogen,i.e.,the solid bituminous materials found in oil shales and which are very dif?cult to dissolve in conventional organic solvents.7 Although some ionic liquids can dissolve sul?dic ore metals,8 the inorganic chemist working with ionic liquids is facing the problem that the solubility of ionic inorganic compounds in ionic liquids is often very low.9,10This is a consequence of the fact that most of the ionic liquids described in the literature contain weakly coordinating anions like tetra?uo-roborate,11,12hexa?uorophosphate,13,14or bis(tri?uorometh-ylsulfonyl)imide.15These ionic liquids with?uorinated anions have lower melting points and viscosities than ionic liquids with coordinating anions like chloride or carboxylate ions,but their solvating abilities are very poor.16The presently used“second-generation”ionic liquids are much less moisture-sensitive than the so-called“?rst-generation”haloaluminate ionic liquids,4,17,18but they are inferior

*To whom correspondence should be addressed.E-mail:Peter. Nockemann@chem.kuleuven.be.

?Katholieke Universiteit Leuven.

?Universite′Catholique de Louvain.

§Universita¨t Stuttgart.

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solvents for inorganic salts.The low solubility of metal salts is a serious drawback for possible applications of ionic liquids that require high concentrations of dissolved metal salts,e.g.,the electrodeposition of metals19,20or solvents for the synthesis of nanoparticles.21,22Unfortunately,quantitative data on the solubility of metal salts in ionic liquids are still very scarce.The mechanism of the solubilization of metal ions in ionic liquids is an issue that needs to be further investigated because it has recently been reported that the coordination environments of the solvates in ionic liquids might be quite unique.23,24The solubility of metal salts in ionic liquids can be increased by mixing the ionic liquid with coordinating additives with a low vapor pressure.An example is the addition of poly(ethylene glycol)s to ionic liquids.25 Other examples are the“deep eutectic solvents”developed by Abbott and co-workers,like a mixture of choline chloride and urea26,27or a mixture of choline chloride and carboxylic acids.28Furthermore,metal-containing ionic liquids that incorporate the metal ion as a cation or anion exhibit a high miscibility with other ionic liquids.29-32Still another ap-proach to increase the solubility of metal salts in ionic liquids is to use ionic liquids with appending coordinating groups. These are the so-called task-speci?c ionic liquids.They were ?rst introduced by Davis and co-workers33,34and are actively being explored by other researchers.35-42As a rule,these task-speci?c ionic liquids are not used as single-component ionic liquids,but they are mixed with more conventional ionic liquids.The rationale for using mixtures rather than pure task-speci?c ionic liquids is that the task-speci?c ionic liquids often have a higher melting point and a higher viscosity than conventional ionic liquids.Moreover,the conventional ionic liquids are,in general,much cheaper than the task-speci?c ionic liquids.

Recently,we have reported on the task-speci?c ionic liquid betainium bis(tri?uoromethylsulfonyl)imide,[Hbet][Tf2N].43 This ionic liquid,bearing a carboxyl group,has a selective solubilizing ability for metal oxides.Soluble metal oxides are the trivalent rare earths uranium(VI)oxide,zinc(II)oxide, cadmium(II)oxide,mercury(II)oxide,nickel(II)oxide, copper(II)oxide,palladium(II)oxide,lead(II)oxide,and silver(I)oxide.Insoluble or very poorly soluble are oxides of iron,cobalt,aluminum,and silicon.Also,metal hydroxides can be solubilized in this ionic liquid.The metals can be stripped from[Hbet][Tf2N]by treatment of the ionic liquid with an acidic aqueous solution.After transfer of the metal ions to the aqueous phase,the ionic liquid can be recycled for reuse.Betainium bis(tri?uoromethylsulfonyl)imide forms one phase with water at high temperatures,whereas phase separation occurs below55.5°C(temperature-switch be-havior).The mixtures of the ionic liquid with water also show a pH-dependent phase behavior:two phases are formed at low pH,whereas one phase is present under neutral or alkaline conditions.

In this paper,several novel structural derivatives of betainium bis(tri?uoromethylsulfonyl)imide(Figure1)are described.All of these variants bear a carboxyl group attached to a positively charged nitrogen atom.Cationic cores that were considered are the imidazolium,pyridinium, pyrrolidinium,piperidinium,and morpholinium heterocycles. In addition,analogues of betainium bis(tri?uoromethylsul-fonyl)imide by replacement of one of the methyl groups of the betainium cation by a longer alkyl chain were synthe-sized.The rationale for synthesizing these derivatives was to explore the in?uence of the cation on the properties of these task-speci?c ionic liquids.A change of the cation not only in?uences the physical properties like the melting point, viscosity,and hydrophobicity but also the acidity of the carboxyl group and hence the selectivity of the metal oxide dissolution process.

Experimental Section

General Techniques.Elemental analyses(carbon,hydrogen,and nitrogen)were performed using a CE Instruments EA-1110 elemental analyzer.Fourier transform infrared(FTIR)spectra were recorded on a Bruker IFS-66spectrometer.The samples were measured using the KBr pellet method or as a thin?lm between

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4265–4279.

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Rogers,R.D.New J.Chem.2008,32,872–877.

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Aspects of Ionic Liquids;Ohno,H.Ed.;Wiley:New York,2005;

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https://www.wendangku.net/doc/2789654.html,mun.2003,70–71.

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J.Am.Chem.Soc.2004,126,9142–9147.

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Meervelt,L.;Binnemans,K.J.Am.Chem.Soc.2006,128,13658–13659.

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Nockemann et al.

9988Inorganic Chemistry,Vol.47,No.21,2008

KBr windows.1H NMR spectra were recorded on a Bruker Avance 300spectrometer (operating at 300MHz for 1H NMR).The water content of the ionic liquids was determined by a coulometric Karl Fischer titrator (Mettler Toledo coulometric Karl Fischer titrator model DL39).The viscosity of the ionic liquids was measured at room temperature by the falling-ball method (Gilmont Instruments).Differential scanning calorimetry (DSC)measurements were carried out on a Mettler-Toledo DSC822e module (scan rate of 10°C/min under a helium ?ow).High-temperature dissolution experiments of the metal oxides were performed in a poly(tetra?uoroethylene)(PTFE)-lined acid digestion bomb (No.4744,45mL,Parr Instru-ment Co.).Lithium bis(tri?uoromethylsulfonyl)imide was purchased from IoLiTec.All other chemicals were obtained from Acros Organics or from Aldrich-Sigma.

Crystallography.X-ray intensity data for [HbetPy][Tf 2N],[HbetmPyr][Tf 2N],[HbetmIm][Tf 2N],[EtbetmMor][Tf 2N],and [Cu 2(betmMor)4][Tf 2N]4were collected on a SMART 6000dif-fractometer equipped with a CCD detector using Cu K R radiation (λ)1.54178?).The images were interpreted and integrated with

the program SAINT from Bruker.44The intensity data sets for the single crystals of [Cu 2(mbetIm)4(H 2O)2][Tf 2N]4(H 2O)and [Eu 2(betmMor)6(H 2O)4][Tf 2N]6were collected on a Nonius Kappa CCD diffractometer using graphite-monochromatized Mo K R radiation (λ)0.71069?).[Cu 2(betmPyr)4(H 2O)2][Tf 2N]4and [Cd(betPy)2(H 2O)2][Tf 2N]2were measured on a Rigaku diffracto-meter equipped with a RU200rotating anode and a MAR345image plate using graphite-monochromatized Mo K R radiation (λ)0.71073?).All nine structures were solved by direct methods and re?ned by full-matrix least squares on F 2using the SHELXTL program package.45Non-hydrogen atoms were anisotropically re?ned and the hydrogen atoms in the riding mode with isotropic temperature factors ?xed at 1.2U (eq)of the parent atoms (1.5times for methyl groups).CCDC 698586-698594contain the supple-mentary crystallographic data for this paper and can be obtained free of charge via https://www.wendangku.net/doc/2789654.html,/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre,12Union Road,Cambridge CB21EZ,U.K.;fax +44-1223-336033;or deposit@https://www.wendangku.net/doc/2789654.html,).The crystallographic data of the complexes are summarized in Tables 1and 2.

Computational Methods.Density functional theory (DFT)calculations were performed by making use of the Gaussian03package.46The DFT B3LYP was used for all calculations.The large basis set 6-311++G(d,p),a triplet-split valence basis set with additional diffuse sp functions,and one d function on heavy atoms and diffuse s functions and one p function on hydrogen atoms were used for single-point calculations.The optimizations were carried out using the DGauss valence double- basis set,called DGDZVP2.Synthesis.The synthesis of the ionic liquids and the metal complexes is described in the Supporting Information.

Results

Synthesis and Characterization.Different structural variants of the previously described task-speci?c ionic liquid [Hbet][Tf 2N]were synthesized.All of these variants are bearing a carboxyl group attached to the positively charged nitrogen atom of the cation.A ?rst modi?cation was the replacement of a methyl group of the betainium cation by a longer alkyl chain (a butyl or hexyl chain).By the addition of a longer alkyl chain to the nitrogen atom,it can be expected that the polarity of the ionic liquid decreases and thereby the solubility properties change (higher solubility in nonpolar solvents).On the other hand,alkyl chains are electron-donating groups,and their presence decreases the acidity of the carboxyl group.The betainium compounds with a butyl or hexyl chain were prepared by the reaction of the corresponding bromoalkane with the ethyl ester of glycine betaine under solventless conditions.The hydrobromide salt was recrystallized from methanol and dissolved in water.An aqueous solution of lithium bis(tri?uoromethylsulfonyl)imide was added,and the hydrophobic ionic liquid phase separated from the water phase after the metathesis reaction.The synthesis of the ionic liquids with heterocyclic cations was partially based on literature procedures.For the N -carboxymethyl-(44)SAINT Manual Version 5/6.0;Bruker Analytical X-ray Systems Inc.:

Madison,WI,1997.

(45)SHELXTL-PC Manual Version 5.1;Bruker Analytical X-ray Systems

Inc.:Madison,WI,1997.

(46)Frisch,M.J.;et al.Gaussian 03,revision C.02;Gaussian,Inc.:

Wallingford,CT,

2004.

Figure 1.Structures of the ionic liquids.I :betainium bis(tri?uorometh-ylsulfonyl)imide,[HBet][Tf 2N](R )CH 3).II :N -butyl-N -dimethylbetainium bis(tri?uoromethylsulfonyl)imide,[C 4HBet][Tf 2N](R )C 4H 9).III :N -hexyl-N -dimethylbetainium bis(tri?uoromethylsulfonyl)imide,[C 6HBet][Tf 2N](R )C 6H 13).IV :N -carboxymethyl-N -methylpyrrolidinium bis(tri?uorom-ethylsulfonyl)imide,[HbetmPyr][Tf 2N].V :N -carboxymethyl-N -methylpi-peridinium bis(tri?uoromethylsulfonyl)imide,[HbetmPip][Tf 2N].VI :N -carboxymethyl-N -methylmorpholinium bis(tri?uoromethylsulfonyl)imide,[HbetmMor][Tf 2N].VII :N -carboxymethyl-N -methylmorpholinium ethyl ester bis(tri?uoromethylsulfonyl)imide,[EtHbetmMor][Tf 2N].VIII :N -carboxym-ethylpyridinium bis(tri?uoromethylsulfonyl)imide,[HbetPy][Tf 2N].IX :1-carboxymethyl-3-methylimidazolium bis(tri?uoromethylsulfonyl)imide,[HbetmIm][Tf 2N].

Carboxyl-Functionalized Task-Speci?c Ionic Liquids

Inorganic Chemistry,Vol.47,No.21,2008

9989

functionalized morpholinium,pyrrolidinium,piperidinium, and imidazolium compounds,a method earlier reported by Dega-Szafran and Przybylak was used.47The cations were prepared by quaternizing the heterocycle with the ester of chloroacetic acid.After quaternization,the ester was converted into the corresponding acid,and the chloride anion was exchanged for a bis(tri?uoromethyl-sulfonyl)imide anion by a metathesis reaction.These hydrophobic bis(tri?uoromethylsulfonyl)imide ionic liq-uids separated from the water phase.For the synthesis of

Table1.Summary of the Crystallographic Data of the Ionic Liquids

[HbetPy][Tf2N][HbetmPyr][Tf2N][HbetmIm][Tf2N][EtbetmMor][Tf2N] formula C9H8F6N2O6S2C9H14F6N2O6S2C8H9F6N3O6S2C11H18F6N2O7S2 MW418.31424.36421.32468.41

dimens(mm3)0.3×0.3×0.150.3×0.2×0.10.5×0.4×0.10.4×0.2×0.2 cryst syst triclinic monoclinic triclinic monoclinic space group P1(No.1)P21/c(No.14:b1)P j1(No.2)P21/c(No.14:b1) a(?)8.0812(3)15.6959(16)9.3990(7)13.0236(8) b(?)8.1092(3)7.0808(8)10.4524(7)8.5641(7) c(?)13.7815(5)15.3090(13)17.0877(13)17.3436(12) R(deg)77.576(2)90.00096.992(4)90.000

(deg)76.722(2)108.721(4)92.566(5)110.307(3)γ(deg)62.6590(10)90.000108.232(4)90.000

V(?3)774.49(5)1611.4(3)1576.5(2)1814.2(2) Z2444

D calcd(g/cm3) 1.794 1.749 1.775 1.715

temp(K)100100100100

limiting indices-9e h e9-18e h e19-11e h e11-13e h e15

-9e k e9-8e k e8-12e k e12-9e k e10

-16e l e16-18e l e18-19e l e20-21e l e20μCu K R(mm-1) 4.109 3.951 4.059 3.617

abs corrn multiscan multiscan multiscan multiscan F(000)420864848960

no.of measd re?ns13228152251838016567

no.of unique re?ns4947310659473414

no.of obsd re?ns[I0<2σ(I0)]4869268950302720

no.of param re?ned454236459255

GOF on F2 1.029 1.079 1.029 1.043

R10.04470.03740.05000.0420

wR20.10930.09620.13770.0981

R1(all data)0.04530.04350.06150.0569

wR2(all data)0.10990.09990.16430.1067

Table2.Summary of the Crystallographic Data of the Metal Complexes

[Cu2(betmMor)4]-

[Tf2N]4[Cu2(betmPyr)4(H2O)2]-

[Tf2N]4

[Cu2(betmIm)4(H2O)2]-

[Tf2N]4

[Cd(betPy)2(H2O)2]-

[Tf2N]2

[Eu2(betmMor)6(H2O)4]-

[Tf2N]6

formula C36H52Cu2F24N8O28S8C36H56Cu2F24N8O26S8C32H40Cu2F24N12O28S8C18H14Cd F12N4O14S4C54H78Eu2F36N12O48S12 MW1884.521856.551880.34979.023036.04

dimens(mm3)0.4×0.3×0.20.4×0.2×0.180.4×0.2×0.10.18×0.15×0.10.6×0.6×0.1

cryst syst monoclinic triclinic triclinic monoclinic monoclinic

space group P21/n(No.14:b2)P j1(No.2)P j1(No.2)P21/n(No.14:b2)P21/n(No.14:b2)

a(?)14.8237(5)10.363(2)8.2389(5)7.8760(16)16.050(3)

b(?)15.5544(5)11.112(2)14.2055(9)26.280(5)14.729(3)

c(?)14.8893(5)16.118(3)14.649(1)16.588(3)22.604(5)

R(deg)90.0075.95(3)81.659(4)90.0090.00

(deg)92.131(2)78.11(3)74.037(5)103.20(3)94.873(3)

γ(deg)90.0089.73(3)75.644(4)90.0090.00

V(?3)3430.7(2)1759.9(7)1591.6(2)3342.7(12)5324.3(19)

Z21142

D calcd(g/cm3) 1.824 1.752 1.962 1.945 1.894

temp(K)100120100120100

limiting indices-18e h e18-12e h e12-9e h e10-8e h e8-19e h e19 0e k e18-13e k e13-17e k e17-30e k e30-17e k e17

0e l e18-20e l e200e l e18-19e l e19-27e l e27

μCu K R(mm-1) 4.4200.983 1.093 1.038 1.556

abs corrn multiscan none refdelf none multiscan

F(000)190093894219283024

no.of measd re?ns315033740062342517270910

no.of unique re?ns648069235071511113135

no.of obsd re?ns

[I0<2σ(I0)]

558463436234442410937

no.of param re?ned480478492580865

GOF on F2 1.033 1.058 1.140 1.100 1.065

R10.04880.05720.06620.04560.0600

wR20.12600.15920.17170.11830.1707

R1(all data)0.05720.06050.08450.05460.0739

wR2(all data)0.13320.16290.18400.12560.1839

Nockemann et al. 9990Inorganic Chemistry,Vol.47,No.21,2008

the N-carboxymethylpyridinium bis(tri?uoromethylsulfo-nyl)imide,a metathesis reaction was used to exchange the chloride anions for a bis(tri?uoromethylsulfonyl)imide anion,starting from the commercially available N-car-boxymethylpyridinium hydrochloride.

[C6Hbet][Tf2N]was obtained as a highly viscous liquid at room temperature,but it cannot be excluded that it is a supercooled liquid that would crystallize upon standing for a prolonged time.All of the other compounds were also obtained?rst as viscous liquids,but all crystallized.However, their melting points are rather low,and the crystalline compounds were found to be hygroscopic.When the ionic liquids are arranged according to increasing melting point, one?nds N-carboxymethyl-N-pyridinium bis(tri?uoromethyl-sulfonyl)imide(mp)32°C),N-carboxymethyl-N-methyl-piperidinium bis(tri?uoromethylsulfonyl)imide(mp)32

°C),N-carboxymethyl-N-methylpyrrolidinium bis(tri?uo-romethylsulfonyl)imide(mp)39°C),1-carboxymethyl-3-methylimidazolium bis(tri?uoromethylsulfonyl)imide(40

°C),N-carboxymethyl-N-methylmorpholinium bis(tri?uo-romethylsulfonyl)imide(mp)55°C),and betainium bis-(tri?uoromethylsulfonyl)imide(mp)55°C).These melting point values correspond to the onset temperature as measured by DSC.Replacement of the carboxyl group by an ester group reduces the melting point:the melting point of the ester[EtbetmMor][Tf2N]is46°C,whereas that of the corresponding carboxylic acid[HbetmMor][Tf2N]is55°C. The molten task-speci?c ionic liquids with the COOH functional group are highly viscous liquids.To make these ionic liquids easier to handle,they can be mixed(diluted) with other ionic liquids.For instance,the ionic liquid 1-carboxymethyl-3-methylimidazolium bis(tri?uoromethyl-sulfonyl)imide was found to be miscible in all mass propor-tions with the ionic liquid1-butyl-3-methylimidazolium bis(tri?uoromethylsulfonyl)imide,and these mixtures have

a lower viscosity than the task-speci?c ionic liquid itself. Solid-State Structures of Ionic Liquids.Single crystals suitable for X-ray diffraction study were obtained for [HbetPy][Tf2N],[HbetmPyr][Tf2N],[HbetmIm][Tf2N],and [EtbetmMor][Tf2N].The ionic liquids crystallized spontane-ously after the liquids were cooled to room temperature and left to stand for about3days.

The crystal structure of[HbetPy][Tf2N]consists of pro-tonated N-carboxymethylpyridinium cations and bis(tri?uo-romethylsulfonyl)imide anions(Figure2).In contrast to the crystal structure of[Hbet][Tf2N],43no direct O-(H)···O hydrogen bonds between the anions and the carboxylic function could be observed.Instead,two N-carboxymeth-ylpyridinium cations are forming dimers that are connected by strong hydrogen bonds[O-(H)···O distance of1.79-1.80?or O···O distance of 2.63-2.64?].The molecular structure of such a cation-cation pair of[HbetPy][Tf2N]is shown in Figure2a.The O-H···O angles of the hydrogen bonds are175.1and179.7°.The O-C-O angles of the carboxyl groups in[HbetPy][Tf2N]are127.0and126.7°. Besides the strong hydrogen bonding between the cations,several short C-H···O and C-H···F contacts between [Tf2N]-anions and the cations can be observed,ranging from 2.52to2.96?and from2.47to2.91?,respectively. The crystal structure of N-carboxymethyl-N-methylpyr-rolidinium bis(tri?uoromethyl-sulfonyl)imide,[HbetmPyr]-[Tf2N],consists of protonated N-carboxymethyl-N-methyl-pyrrolidinium cations and[Tf2N]-anions(Figure3).Similarly to the crystal structure of[HbetPy][Tf2N],the cations form pairsthatareconnectedbystronghydrogenbonds[O-(H)···O distance of1.86?or O···O distance of2.67?).The O-H···O angle of the hydrogen bond is almost linear with 178.4°.The O-C-O angle of the carboxyl group in [HbetmPyr][Tf2N]is125.6°.Several short C-H···O and C-H···F contacts between[Tf2N]-anions and the cations can be observed,ranging from2.48to2.92?and from2.64 to2.90?,respectively.The carboxymethyl group is in the equatorial position with respect to the pyrrolidinium ring, whereas the methyl group is in the axial position.

A similar arrangement of anions and cations was also found in the crystal structure of N-carboxymethyl-N-meth-ylimidazolium bis(tri?uoromethylsulfonyl)imide,[HbetmIm]-[Tf2N](Figure4).The cations form pairs that are connected by strong hydrogen bonds with an O-(H)···O distance of

(47)Dega-Szafran,Z.;Przybylak,R.

J.Mol.Struct.1997,437,107–121.

Figure 2.(a)Molecular structure of N-carboxymethyl-N-pyridinium

bis(tri?uoromethylsulfonyl)imide,[HbetPy][Tf2N].(b)Hydrogen-bonding

interactions of the cation with the surrounding bis(tri?uoromethylsulfo-

nyl)imide anions.

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Inorganic Chemistry,Vol.47,No.21,20089991

1.75-1.89?and an O ···O-distance of

2.62-2.65?,respectively.The O -H ···O angles of the hydrogen bonds are 165.4and 171.0°.The O -C -O angle of the carboxyl groups in [HbetmIm][Tf 2N]are both 126.3°.Several short

C -H ···O and C -H ···F contacts between [Tf 2N]-anions and the acidic hydrogen atoms of the cation can be observed,ranging from 2.35to 2.65?and from 2.37to 2.80?,respectively.These “second-order”interactions are shorter compared to the interactions in the crystal structures of [HbetPy][Tf 2N]and [HbetmPyr][Tf 2N].The short contacts in the surroundings of the N -carboxymethyl-N -methylimi-dazolium cation are shown in Figure 4b.

The crystal structure of N -carboxymethyl-N -methylmor-pholinium ethyl ester bis(tri?uoromethylsulfonyl)imide,[EtbetmMor][Tf 2N],could also be determined (Figure 5).The structure of this compound allows one to obtain more information on the interactions of the ions in the solid state in the absence of a strong hydrogen-bond donor like the carboxyl function.As expected,no strong hydrogen bonding could be observed.Despite the absence of “classical”hydrogen bonding,short C -H ···O and C -H ···F contacts are present.Oxygen atoms of the bis(tri?uoromethylsulfo-nyl)imide anion are directed toward the hydrogen atoms in the 2and 3positions of the morpholinium ring within distances ranging from O -H )2.47to 2.70?.The carbonyl oxygen atom of the ester group is also directed toward neighboring hydrogen atoms of the morpholinium ring in the 2position within a distance of 2.61-2.65?.The oxygen of the morpholinium ring interacts weakly with the hydrogen on the 2and 3positions of a neighboring ring (2.44-

2.65

Figure 3.Molecular structure of N -carboxymethyl-N -methylpyrrolidinium bis(tri?uoromethylsulfonyl)imide,[HbetmPyr][Tf 2

N].

Figure 4.(a)Molecular structure of 1-carboxymethyl-3-imidazolium bis(tri?uoromethylsulfonyl)imide,[HbetmIm][Tf 2N].(b)Hydrogen-bonding interactions of the cation with the surrounding bis(tri?uoromethylsulfo-nyl)imide

anions.

Figure 5.(a)Molecular structure of N -carboxymethyl-N -methylmorpho-linium ethyl ester bis(tri?uoromethylsulfonyl)imide,[EtHbetmMor][Tf 2N].(b)Packing of the molecules in the crystal structure of N -carboxymethyl-N -methylmorpholinium ethyl ester bis(tri?uoromethylsulfonyl)imide,[EtHbetmMor][Tf 2N].

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?,respectively).The short contacts around a N -carboxym-ethyl-N -methylmorpholinium ethyl ester cation are also visible in Figure 5a.The morpholine ring has a chair conformation with the carboxymethyl group in the equatorial postion and the methyl group in the axial position.In the packing of the crystal structure,the cations are arranged into layers parallel to the (100)plane and alternate with layers of the bis(tri?uoromethylsulfonyl)imide anions (Figure 5b).Thermomorphic Behavior.The ionic liquid [Hbet][Tf 2N]displays a thermomorphic “phase-switching”behavior at 55°C.43Above this critical temperature,a two-phase water/ionic liquid mixture turns into a one-phase system (Figure 6).Upon cooling below 55°C,the one-phase mixture separates again into two immiscible phases.For the tem-perature-dependent miscibility studies,a 1:1ratio (mass/mass)of the ionic liquid and water was considered.

In [C 4Hbet][Tf 2N]and [C 6Hbet][Tf 2N],one methyl group on the nitrogen atom of the betainium cation is replaced by butyl and hexyl chains,respectively.This alkyl chain makes the resulting ionic liquids more hydrophobic than [Hbet][Tf 2N],and they in?uence the thermomorphic behavior of these ionic liquids:by heating of a two-phase system of [C 4Hbet][Tf 2N]/H 2O or [C 6Hbet][Tf 2N]/H 2O,the two phases remain immiscible,even at elevated temperatures.The nonpolar alkyl chains hinder the hydration of the ions and prevent the formation of a one-phase system with water upon heating.

[HbetmPyr][Tf 2N]does not form one phase with water,even at elevated temperatures.The same behavior was observed for [Hbetmpip][Tf 2N].[HbetmMor][Tf 2N]shows a tendency to dissolve in a water phase at elevated temper-atures.The critical temperature is 52°C.The phase transition of the temperature-dependent miscibility was not de?ned as sharp as it was in the case for [Hbet][Tf 2N]:instead of a distinct transition to a homogeneous phase,a clouding was observed at the transition temperature,which slowly clari?ed after about 5min.Replacement of the carboxyl function of [HbetmMor][Tf 2N]by an ester group prevents the thermo-morphic behavior:a two-phase region was observed at all temperatures.[Hbetmim][Tf 2N]switches from a two-phase ionic liquid/water system to an one-phase system at 64°C.[HbetPy][Tf 2N]exhibits thermomorphic behavior at tem-peratures of around 55°C.

Nuclear magnetic resonance (NMR)studies have been performed in order to gain additional insight into the nature

of the temperature-dependent miscibility of ionic liquids with water.1H NMR spectra of the pure ionic liquid [HBet][Tf 2N]recorded at 25and 70°C showed resonances at 3.95and 4.84ppm,which could be unambiguously assigned to three equivalent CH 3groups and a NCH 2group,respectively.In addition,at both temperatures a sharp resonance at 10.58ppm was observed,indicating involvement of a carboxylic proton in a strong hydrogen bonding (Figure 7).This is in good agreement with the solid-state structure of [HBet][Tf 2N]in which hydrogen bonds between carboxylic protons of the betainium cations and the nitrogen atom of the bis(tri?uo-romethylsulfonyl)imide anion have been observed.431H NMR spectra of [HBet][Tf 2N]recorded in the presence of an equal volume of water at room temperature were identical with the spectrum of this compound recorded in the absence of water.However,when the sample of [HBet][Tf 2N]containing water was heated to 70°C,at which point a one-phase system was formed,the resonance at 10.58ppm disappeared,indicating the absence of hydrogen bonding between the anion and the cation of [HBet][Tf 2N]upon miscibility of water.In Figure 8,the phase diagram of the binary mixture of [HbetmMor][Tf 2N]and water is shown;the line indicates the phase transition temperature at different compositions.

An analogous behavior was observed for [HbetmMor]-[Tf 2N],in which 1H NMR spectra indicated the loss of hydrogen bonding upon formation of a one-phase system with water.Interestingly,the hydrogen bonding detected in the 1H NMR spectra of [HbetmPip][Tf 2N],which does not exhibit temperature-dependent miscibility with water,re-mained unaffected upon the addition of water and heating of the sample up to 100°C.These results indicate that the formation of one phase with water above the critical temperature results in the breakage of hydrogen bonding that exists in a pure ionic liquid.It is likely that,upon formation of a one-phase system,hydrogen bonds between the ionic liquid components and water are formed.However,these are dif?cult to assess,most likely because of the fast exchange of carboxylate protons on the NMR time scale.So,it can be concluded that the thermomorphic behavior can be explained by the total solvation of the anion and cation of the ionic liquid by water molecules.The hydrogen bonds between the betainium cation and the bis(tri?uoromethyl-sulfonyl)imide anion or between two betainium cations,respectively,break,and each ion will be surrounded by a hydration shell of water molecules.

Theoretical Results.The interaction energies between cations and anions have been calculated by DFT methods based on the crystal structures obtained for [HbetPyr][Tf 2N],[HbetPy][Tf 2N],and [HbetmIm][Tf 2N].In order to estimate the contributions of the different interactions in the crystal structures,pairs of two cations and their neighboring anions were virtually separated into neutral halves.The highest calculated total interaction energies [obtained by single-point B3LYP/6-311++G(d,p)calculations]were found for [HbetPy][Tf 2N](168.0kcal/mol)and [HbetmIm][Tf 2N](160.7kcal/mol)(see Table 3).The calculated total interac-tion energy for [HbetPyr][Tf 2N]is lower (140.6

kcal/mol),

Figure 6.Illustration of the temperature-dependent phase behavior of a binary ionic liquid/water mixture.The red color (methyl red,dissolved in the aqueous layer)was added to accentuate the phase boundaries.

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which can be explained by the weaker average cation -anion interaction energy (57.8kcal/mol compared to 63.1kcal/mol for [HbetPy][Tf 2N]and 72.4kcal/mol for [HbetmIm]-[Tf 2N]).Contrary to the nonaromatic pyrrolidinium-based cation [HbetPyr]+,the aromatic pyridinium-and imidazo-lium-based cations [HbetPy]+and [HbetmIm]+contain acidic ring hydrogen atoms that are able to form hydrogen bonds

to the anions.This is also demonstrated by optimizations of the structures of each one cation and one bis(tri?uorometh-ylsulfonyl)imide anion in the gas phase.The calculations were performed at the B3LYP/DGDZVP2level for [HbetPyr][Tf 2N]and [HbetmIm][Tf 2N](see Figure 9)and show that for the [HbetmIm]+cation the hydrogen bonding of the anion with the acidic ring hydrogen atom is preferred over the interaction with the carboxyl group.In the optimized structure for [HbetPyr][Tf 2N],an interaction with the car-boxyl function is found.

Solubilization of Metal Oxides.An interesting property of all of these ionic liquids functionalized with a carboxyl group is their solubilizing power toward metal oxides and metal hydroxides.A range of metal oxides and hydroxides dissolves in these ionic liquids.In a typical experiment,the ionic liquid (1g)was mixed with a stoichiometric amount of the metal oxide or hydroxide (or a slight excess of the metal oxide or hydroxide)and 5mL of water.A stoichio-metric amount of metal oxide M x O y is present if for every mole of M x O y the amount of ionic liquid equals x times the oxidation state of the metal.For instance,6mol of ionic liquid has to be added to 1mol of M 2O 3.If the amount of M 2O 3exceeds the stoichiometric amount,the excess of

metal

Figure 7.1H NMR spectrum of neat [Hbet][Tf 2N](lower spectrum)at 70°C and the 1H NMR spectrum of [Hbet][Tf 2N]dissolved in H 2O at 70°C (upper spectrum).The proton resonance at 10.58ppm (marked with an asterisk)in the neat sample disappears in the single-phase solution at 70°

C.

Figure 8.Phase diagram of the binary mixture of N -carboxymethyl-N -methylmorpholinium bis(tri?uoromethylsulfonyl)imide,[HbetmMor][Tf 2N],and water.

Table 3.Interaction energies (kcal/mol)for [HbetPyr][Tf 2N],[HbetPy][Tf 2N],and [HbetmIm][Tf 2N]a

ionic liquid

average cation -anion interaction energy

interaction energy between two neutral halves

total interaction energy

[HbetPyr][Tf 2N]

-57.8-25.0-140.6[HbetPy][Tf 2N]-63.1-41.8-168.0[HbetmIm][Tf 2N]-72.4-15.9-160.7a

These values are obtained by

single-point B3LYP/6-311++G(d,p)calculations from the crystal structures.

Figure 9.Optimized structures in the gas phase for [HbetPyr][Tf 2N](left)and [HbetmIm][Tf 2N](right).The calculations were performed at the B3LYP/DGDZVP2level.

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9994Inorganic

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oxide will remain undissolved.The mixture was stirred under re?ux for 12h.After ?ltration,water was evaporated under vacuum.The following oxides were found to be soluble in the ionic liquids:Sc 2O 3,Y 2O 3,La 2O 3,Pr 6O 11,Nd 2O 3,Sm 2O 3,Eu 2O 3,Gd 2O 3,Tb 4O 7,Dy 2O 3,Ho 2O 3,Er 2O 3,Tm 2O 3,Yb 2O 3,Lu 2O 3,UO 3,PbO,ZnO,CdO,HgO,CuO,Ag 2O,NiO,and PdO.The following hydroxides were found to be soluble in the ionic liquids:Pb(OH)2,Zn(OH)2,Cd(OH)2,Cu(OH)2,Ni-(OH)2,Fe(OH)2,Fe(OH)3,Co(OH)2,Cr(OH)3,Mn(OH)2,LiOH,NaOH,KOH,RbOH,CsOH,Mg(OH)2,Ca(OH)2,Sr(OH)2,and Ba(OH)2.Some of the corresponding metal complexes were characterized by means of element analysis and single-crystal X-ray diffraction (see the next section).As discussed in our previous paper,the presence of water facilitates the dissolution process.43This is probably due to the fact that the water molecule aids in the deprotonation of the carboxyl group.The solubility of the metal oxides or hydroxides in the task-speci?c ionic liquids is high because the ionic liquid allows dissolution of stoichiometric amounts of the oxide or hydroxide.Co 3O 4,CoO,Co 2O 3,Cr 2O 3,FeO,and Fe 2O 3were found to be insoluble in the ionic liquids under the experimental conditions used to dissolve the other oxides.However,these oxides could be solubilized in the ionic liquids (including in [Hbet][Tf 2N])by using a digestion bomb and higher temperatures.In a typical experiment,the ionic liquid (1g)was mixed with a stoichiometric amount of the metal oxide and 5mL of water.The mixture was treated by heating of the oxide/ionic liquid/water mixture in an oven at 140°C for 24h in a PTFE-lined acid digestion bomb.After ?ltration,water was evaporated under vacuum.Crystal Structures of Metal Complexes.The crystal structures of ?ve metal complexes have been determined by single-crystal X-ray diffraction:the copper(II)complexes of [HbetmMor][Tf 2N],[HbetmPyr][Tf 2N],and [HbetmIm]-[Tf 2N],the europium(III)complex of [HbetmMor][Tf 2N],and the cadmium(II)complex of [HbetPy][Tf 2N].Single crystals suitable for X-ray diffraction analysis were obtained by slow evaporation of aqueous solutions of these metal complexes at room temperature.

The crystal structure of the [Cu 2(betmMor)4][Tf 2N]4complex consists of four μ2-bridging N -carboxymethyl-N -methylmorpholinium cations and four bis(tri?uoromethyl-sulfonyl)imide anions (Figure 10).The two copper(II)ions exhibit a distorted square-pyramidal coordination sphere.No coordinated water molecules could be observed.The copper -copper distance is quite short (2.63?),and the two metal centers are bridged by four μ2N -carboxymethyl-N -methylmorpholinium cations [d (Cu -O))1.96-1.97?].Two coordinating bis(tri?uoromethylsulfonyl)imide anions complete the coordination sphere [d (Cu1-O5))2.18?].The crystal structure obtained after dissolution of Eu 2O 3in [HbetmMor][Tf 2N]results in the dimeric complex [Eu 2-(betmMor)6(H 2O)4][Tf 2N]6that consists of [Eu 2(betmMor)8-(H 2O)4]6+cations and the bis(tri?uorosulfonyl)imide anions [Tf 2N]-(Figure 11).The two europium(III)ions in this centrosymmetric complex are linked by two μ2-bridging and two chelating-bridging carboxylate groups of the zwitterionic N -carboxymethyl-N -methylmorpholinium ligands.Each eu-

ropium(III)ion is further surrounded by one additional chelating ligand and two coordinating water molecules.The noncoordinating [Tf 2N]-anions form hydrogen bonds to these water molecules with O -O distances ranging from 2.71to 2.80?.

The crystal structure of the dimeric [Cu 2(betmPyr)4-(H 2O)2][Tf 2N]4complex consists of four μ2-bridging N -carboxymethyl-N -methylpyrrolidinium cations,two coordi-natingwatermolecules,andfourbis(tri?uoromethylsulfonyl)imide anions (Figure 12).Both copper(II)ions exhibit a distorted square-pyramidal coordination sphere.The copper(II)-copper(II)distance is 2.69?.In analogy to the [Cu 2-(betmPyr)4(H 2O)2][Tf 2N]4complex,there are slightly dif-ferent bond lengths found for the copper(II)-oxygen bond of the carboxylate groups [d (Cu -O))1.96-1.98?].Each copper(II)ion is coordinated to a water molecule [d (Cu -O))2.13?].Four bis(tri?uoromethylsulfonyl)imide anions are connected via the oxygen atoms by weak hydrogen bonds to the two coordinated water molecules [d (O -H ···O))2.85-2.98?].

The structure of [Cu 2(betmIm)4(H 2O)2][Tf 2N]4(H 2O)ex-hibits the same dimeric core of the complex consisting of four μ2-bridging N -carboxymethyl-N -methylimidazolium cat-ions and two coordinating water molecules.In contrast to the structure with the N -carboxymethyl-N -methylpyrroli-dinium cations,the four bis(tri?uoromethylsulfonyl)imide anions prefer hydrogen bonding with the acidic protons of imidazolium cations rather than with the coordinated water molecules (Figures 13and 14).These hydrogen bonds range from 2.36to 2.70?.Both copper(II)ions exhibit a distorted square-pyramidal coordination sphere.The copper(II)-copper(II)distance is 2.65?.Each copper(II)ion is coordinated to a water molecule [d (Cu -O))2.13?],which form hydrogen bonds to the oxygen atoms of a neighboring N -carboxymethyl-N -methylimidazolium ligand.This

hydro-Figure 10.Molecular structure of [Cu 2(betmMor)4][Tf 2N]4.The two noncoordinating bis(tri?uoromethylsulfony)imide anions are omitted for clarity.

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gen bonding arranges the dimers to strands along the [100]direction.

The crystal structure of the cadmium(II)N -carboxymethyl-pyridinium bis(tri?uoromethylsulfonyl)imide complex,[Cd(betPy)2(H 2O)2][Tf 2N]2,consists of in?nite rows of [Cd(betPy)]+units along the a axis surrounded by two noncoordinating bis(tri?uoromethylsulfonyl)imide anion as counterions.The average cadmium -cadmium distance is 4.00?.In Figure 13,the linkage of the μ2-bridging carboxylate groups of the N -carboxymethylpyridinium ligands coordinating to two cadmium(II)ions is shown,with typical Cd -O distances of 2.44-2.48?.Two water molecules are coordinated to each cadmium(II)ion,with an average bond length (Cd -O)of 2.33?.Discussion

Betainium salts of the type that we describe in this paper have been investigated in the past by several research

groups

Figure 11.(a)Molecular structure of [Eu 2(betmMor)6(H 2O)2][Tf 2N]6.The two noncoordinating bis(tri?uoromethylsulfony)imide anions are omitted for clarity.(b)Schematic presentation of the structure of [Eu 2(betmMor)6(H 2O)2][Tf 2N]6

.

Figure 12.Molecular structure of [Cu 2(betmPyr)4(H 2O)2][Tf 2N]4.Two bis(tri?uoromethylsulfony)imide anions are omitted for clarity.

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9996Inorganic Chemistry,Vol.47,No.21,

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and mainly by Dega-Szafran,Szafran,and coworkers.48-52These authors have studied the zwitterionic form of the complexes and salts of mineral acids.None of these salts are ionic liquids.It is only by the use of the bis(tri?uoro-methylsulfonyl)imide anion that salts with a suf?ciently low melting point can be obtained.These low melting points can be attributed to the conformational disorder of bis(tri?uo-romethylsulfonyl)imide anions and to the limited involvement of these anions in hydrogen-bond formation.The tempera-ture-dependent miscibility of the bis(tri?uoromethylsulfo-nyl)imide ionic liquids is a fascinating phenomenon.43Ko ¨ddermann and co-workers investigated by DFT calcula-tions and IR spectroscopy mixtures of different ionic liquids with water and showed that hydrogen bonding can play an important role for the phase behavior of these ionic liquids.53Different types of conformers were found for [C 2mim][Tf 2N].There exists a double-donor conformer in which the water molecules form two hydrogen bonds to one [Tf 2N]-group.In another conformer is a single-donor conformer in which the water molecules form a strong hydrogen bond to the anion.Therefore,the most probable explanation of the thermomorphic behavior observed for the [Hbet][Tf 2N]/water system (and for mixtures of similar ionic liquids with water)is the loss of the cation -anion hydrogen bonding upon heating.At high temperatures,the cation and anion are fully solvated by water molecules.We found evidence for this process by 1H NMR spectroscopy.An analogous behavior for [C 4mim][BF 4]has already been described by Dullius et

al.(1998).54Here,the temperature-dependent miscibility could only be observed upon cooling of the system to 5°C.Therefore,it is feasible to assume that the hydrogen bonds between anion and cation are relatively weak in [C 4mim][BF 4]and do only exist at low temperature.At higher temperatures,the solvation of the ions is favored.Rebelo et al.performed a detailed thermodynamic study of the [C 4mim][BF 4]/water system.These authors performed a case study to model other ionic liquid/water systems.55The most detailed studies on the temperature-dependent miscibil-ity of ionic liquids with organic solvents have focused on alcohols as solvents.56,57The temperature-dependent phase behavior of mixtures of these ionic liquids could be applied for chemical separations.There is currently a strong interest in the thermomorphic behavior of ionic liquids and organic solvents (or water)because of its importance for product separation.58It should also be mentioned that binary mixtures of different ionic liquids have been reported to be mutually immiscible.59

The solubility for metal oxides is comparable for all ionic liquids that we investigated;the oxides of the trivalent lanthanide ions,uranium(VI)lead(II),zinc(II),cadmium(II),mercury(II),copper(II),silver(II),nickel(II),palladium(II),and manganese(II),are good to reasonably soluble in these ionic liquids.This similar solubilizing ability is also re?ected by the proton acceptor properties of betaine (p K a )1.73),pyridine betaine (p K a ) 1.99),and N -methylmorpholine betaine (p K a )2.04),which are in a comparable range.60,62To aid in the solubilization process,the addition of water is necessary.Although the oxides Co 3O 4,CoO,Co 2O 3,Cr 2O 3,FeO,and Fe 2O 3are not soluble in the ionic liquids under normal conditions,solubilizing could be achieved in a Te?on-lined digestion bomb at 160°C.This means that virtually all of the metal oxides can be solubilizing in these types of ionic liquids thanks to the presence of the carboxyl function.

Some other carboxyl-functionalized ionic liquids have been described earlier in the literature.Li and co-workers pub-lished on a N -carboxyalkylpyridinium-functionalized ionic liquid with [BF 4]-,[PF 6]-,and [CF 3SO 3]-anions.61The authors determined the physical and chemical properties,but they did not comment on the complex-forming abilities of these types of ionic liquids.Szafran and co-workers func-(48)Dega-Szafran,Z.;Dulewicz,E.;Szafran,M.

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J.;Fischer,J.;De Cian,https://www.wendangku.net/doc/2789654.html,anometallics 1998,17,815–819.(55)Rebelo,L.P.N.;Najdanovic-Visak,V.;Visak,Z.P.;Nunes da Ponte,

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Figure 13.Molecular structure of [Cu 2(betmIm)4(H 2O)2][Tf 2N]4(H 2O).

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tionalized the piperidinium cation with a carboxylic group,but these compounds are not ionic liquids.62-65The authors found for chloride salts that were investigated that the acidic hydrogen atom links two molecules of the carboxylated N -methylpiperidinium into a centrosymmetric cation through a very short O -H ···O hydrogen bond.The position of the hydrogen-bonded proton is controlled by the halide ions.Imidazolium ionic liquids bearing a carboxylic group in combination with [BF 4]-,[PF 6]-,and [CF 3SO 3]-anions have been synthesized for use as an extractant,a catalyst,and a solvent for chemical reactions.66,67Bartsch and Dzyuba described the N -carboxymethyl-N -methylimidazolium bis-(tri?uoromethylsulfonyl)imide ionic liquid as a solvent for

Diels -Alder reactions.68These authors reported that this ionic liquid is a highly viscous liquid at room temperature.However,we found that the compound after removal from a refrigerator (-25°C),where it was kept for several weeks,crystallized when it was allowed to warm up to room temperature.A melting point of 40°C was found,although the melting point was dif?cult to determine because of the hygroscopicity of the ionic liquid.Differences in the melting points can be due to the fact that ionic liquids are often obtained as supercooled liquids that crystallize only in the presence of seed crystals.On the other hand,water or halide impurities might also cause different melting points because the presence of these impurities is known to strongly affect the physical properties of ionic liquids.69

The crystal structural data of the ionic liquids and their metal complexes complement the data available in the literature on similar systems (which are not ionic liquids).Dega-Szafran et al.published the crystal structure of bis(py-ridiniumbetainium)perchlorate.70Only one hydrogen bond between to the N -carboxymethylpyridinium cation and the zwitterionic derivative could be observed,with a bond length of 2.45?.The perchlorate anions were not involved in the hydrogen bonding.The same authors found a hydrogen bond [d (O ···Cl -))2.92?]with the chloride anion for the N -carboxymethylpyridinium hydrochloride.39Dega-Szafran et al.also reported on the crystal structure of N -morpholine betaine complexes with phosphate and picric acid.71,72

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Figure 14.(a)Part of the polymeric crystal structure of [Cd(betPy)2(H 2O)2][Tf 2N]2.(b)Polymeric chain in the structure viewed along the [100]direction.

Nockemann et al.

9998Inorganic Chemistry,Vol.47,No.21,2008

Conclusions

This work illustrates that the principle of using a task-speci?c ionic liquid with a carboxyl functional group for solubilizing metal oxides is not only applicable to the betainium bis(tri?uoromethylsulfonyl)imide ionic liquid that we described previously43but to similar ionic liquids as well. However,the temperature-dependent miscibility with water, including the formation of one phase above the critical temperature,is not always as pronounced as it is in the archetype[Hbet][Tf2N]ionic liquid.The best performance of temperature-dependent miscibility was observed for the N-carboxymethyl-N-methylimidazolium bis(tri?uoromethyl-sulfonyl)imide and for the N-carboxymethylpyridinium bis-(tri?uoromethylsulfonyl)imide ionic liquids.The presence of the carboxylic group gives to this class of ionic liquids a good solubilizing ability for metal oxides and metal hydrox-ides.Further research will be directed toward the develop-ment of new types of task-speci?c ionic liquids with coordinating groups that show a high speci?city for a given metal ion.

Acknowledgment.The authors acknowledge FWO-Flanders for?nancial support(Project G.0508.07).Financial support by Katholieke Universiteit Leuven is acknowledged as well(Projects GOA03/03and IDO/05/005).IR spectra and CHN analyses were recorded by Dirk Henot.The authors wish to thank IoLiTec(Denzlingen,Germany)for support of this research.

Supporting Information Available:Synthetic procedure and analytical data for the ionic liquids and metal complexes,solubility of the ionic liquids in organic solvents,and a CIF?le of the crystal structures.This material is available free of charge via the Internet at https://www.wendangku.net/doc/2789654.html,.

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