chemical reviews volume 99 issue 8 1999 [doi 10.1021%2fcr980032t] welton, thomas -- room-temperature...

8/10/2019 Chemical Reviews Volume 99 Issue 8 1999 [Doi 10.1021%2Fcr980032t] Welton, Thomas -- Room-Temperature Ionic Liquids. Solvents for Synthesis and Cat

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Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis

Thomas Welton

Department of Chemistry, Imperial College of Science Technology and Medicine, South Kensington, London SW7 2AY, U.K.

Received November 23, 1998 (Revised Manuscript Received April 13, 199

Contents I. Introduction 2071II. Preparation 2072III. Handling 2073IV. Solvent Properties 2073V. Organic Reactions 2073

A. Diels Alder Reactions 2073B. Alkylation of Sodium -Naphthoxide 2073

VI. Transition-Metal-Mediated Catalysis 2074A. Hetrogenization of Homogeneous Catalysts 2074B. Hydrogenation Reactions 2074

C. Hydroformylation 2075D. Dimerization of Butadiene 2075E. Heck Reactions 2075

VII. Halogeno and Alkylhalogenoaluminate(III) IonicLiquids

2075

A. Preparation 2076B. Handling 2076C. Chloroaluminate(III) Species 2076

VIII. Reactions with Water 2077A. Oxide-Containing Species 2077B. Proton-Containing Species 2078

IX. Transition-Metal Halide Chemistry 2078X. Organic Reactions in Chloroaluminate(III) Ionic

Liquids2079

A. Electrophilic Substitutions 2079B. Reactions with Protons in Acidic

Chloroaluminate(III) Ionic Liquids2080

XI. Organometallic Reactions in Chloroaluminate(III)Ionic Liquids

2081

XII. Transition-Metal-Mediated Catalysis inChloroaluminate(III) Ionic Liquids

2081

A. Olefin Dimerization 2081B. Olefin Polymerization 2082C. Olefin Hydrogenation 2082

XIII. Conclusions 2082XIV. References 2082

I. Introduction Chemistr y is dominat ed by the st udy of species in

solut ion . Al though a ny l iqu id ma y be used as asolvent, relatively few are in general use. However,as the introduction of cleaner technologies has be-come a ma jor concern t hroughout both industry an dacademia , the search fo r a l te rna t ives to the mos tdamaging solvents has become a high priority. Sol-vents a re high on the list of dama ging chemicals for

tw o s imple r e a s on s : (i ) t h ey a r e u sed in h u geam ounts a nd (ii) they a re usually volat ile liquids tha tare difficult to contain.

Fused salts are liquids containing only ions, ionicliquids. It is possible, by careful choice of startingmat erials , to prepar e ionic liquids t hat ar e l iquid a ta n d b elow room t empera tu r e. I t i s t h es e room -t empera tur e ion ic l iqu ids 1 tha t a re the subject of thisreview. 2 Ionic liquids a re not new; some of them ha vebeen known for many years , for instance [EtNH 3]-[NO 3], which has a melting point of 12 C, was firstdescribed in 1914! 3 F or s ome t ime, i t h a s b eenproposed t hat these ionic l iquids provide a usefulextension to the range of solvents that are availablefor synthetic chemistry. However, i t is only in thepast few year s tha t significant litera ture ha s becomeava i lab le in th is a rea . Some more recen t rev iewsshowing an overview of the potential of ionic liquidsas solvents for synthesis and cat a lysis are ava ilable.4Reviews of the use of higher melting salt s in sy nthe-sis are a lso ava ilable .5

Some simple physical properties of the ionic liquidstha t ma ke them interesting as potentia l solvents forsyn thes is a re the fol lowing : (1) They a re goodsolvents for a w ide ra nge o f both inorgan ic and

Tom Welton was born in London in January 1964. He studied Chemiat the University of Sussex, where he graduated in 1985. He stayed the University of Sussex to study for his DPhil on Chemistry Spectroscopy in Ionic Liquids, with Dr. K. R. Seddon (now Prof. Seof The Queens University of Belfast). After three years as a post-doctfellow, he moved to Exeter University as the Demonstrator in InorgaChemistry. In 1995, he obtained a fellowship from the Lloyds of LonTercentenary Foundation which he used at Imperial College, where now works as a Lecturer in Inorganic Chemistry and is a member of Catalysis and Materials research group. Toms research interests continuto include the use of ionic liquids as solvents for synthesis and catalyHe also has interests in the use of metallodendrimers in catalysis.

2071Chem. Rev. 1999, 99, 2071 2083

10.1021/cr980032t CCC: $35.00 1999 American Chemical SocietyPublished on Web 07/07/1999


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organ ic mate r ia ls, and unusua l combina t ions ofreagents can be brought into the same phase. (2)They are often composed of poorly coordinating ions,so they have the po ten t ia l to be h igh ly po la r ye tnoncoordinating solvents. (3) They are immisciblewi th a number of organ ic solvents and prov ide anonaq ueous, polar a lterna tive for tw o-phase syst ems.Hydrophobic ionic l iquids can also be used as im-miscible pola r pha ses wit h w a ter. (4) Ionic liquids a re

nonvolatile, hence they may be used in high-vacuumsystems a nd elimina te ma ny conta inment problems.They do n ot evapora te!

The development of ionic liquids that are air andmoisture st a ble has provided renew ed vigor in ionicliquid chemistry , an d th e emerging use of these ionicliquids w ill be considered first. H ow ever, during theperiod covered by this review, the halogenoalumi-nat e(III) and the closely relat ed alkylhalogenoalu-minate(III) ionic liquids have been by far the mostwidely s tudied and they wil l be considered sepa-ra tely. Where it is of interest, some salts t ha t a re onlyliquid a bove room t empera ture will a lso be includedin t he discussion.

II. Preparation U ntil recently room-tempera tur e ionic liquids w ere

considered to be ra re, but it is now known t ha t ma nysalts form l iquids a t or c lose to room temperat ure.Invariably, these ionic liquids are either organic saltsor mixtures consisting of at least one organic com-ponent. The most common sa lts in use a re those wit halkylammonium, alkylphosphonium, N -alkylpyridin-ium, and N ,N -dialkylimidazolium cations (see Figure1). There is no reliable way to predict the precisemelting point of organic salts, and identification ofnew room-temperature ionic liquids is a somewhat

hit a nd miss a ffair. After a ll, there is nothing specia labout room temperat ure, i t just happens t o be thet e mp e ra tu r e a t w h ic h ro o ms a r e , a n d s a l t s w i thmelting points of 20 C a nd 30 C a re unlikely to havegreat differences in their structures and interionicinteractions.

There ar e t wo basic methods for the prepara tionof ionic liquids: meta thesis of a ha lide salt wit h, forinstance, a silver, group 1 metal or ammonium saltof the desired anion and acid - base neutral izat ionreactions.

Many alkylammonium halides are commercial lyava i lab le o r they can be p repared s imply by thereaction of the appropriate halogenoalkane and amine.

Prepara t ion of the pyr id inium and imidazol iumhal ides can be ach ieved s imi la r ly. 6,7 For volat i leha logenoa lkanes, the low boi ling poin ts lead topreparations requiring either a sealed tube, such asin the synt hesis of [emim]Cl (w here [emim] + i s the1-ethy l-3-meth ylimida zolium cat ion), 8 or a n elaboratereaction st i l l . Hence, the use of sal ts with longerchain subst ituents, such a s [bmim]Cl (where [bmim] +is the 1-butyl-3-methylimidazolium cation), that canbe prepared in conventional glassware by heatingunder r eflux ha s become popular. 9

In 1992 the first of the new ionic liquids, [emim]-[B F 4], was prepared via metathesis of [emim]I withAg[BF 4] in m etha nol. 10 This sal t a lso has a melt ing

point of 12 C and may be prepared considerablymore cheaply us ing [NH 4][B F 4] in acetone. 11 This easeof preparation, together with i ts re lat ive moistures tab i li ty and i t s immiscibi li ty w i th a number o forganic solvents is leading to i ts increasing use inbipha sic cat a lysis (see below). The prepara tion of[emim][P F 6] short ly followed; this t ime i t was pre-pared by r eaction of [emim]Cl w ith H P F 6.12 This sa ltha s a m elting point of 60 C , which ma kes it slightlyless a t tra ct ive than t he [BF 4]- sa lt, if room t emper-ature working is desired. Since then, thiocyanate ,nonafluorobuta nesulfonat e, bis((tr ifluoromethy l)sul-fonyl)imide, tr is((tr ifluorometh yl)sulfonyl)methide,trifluoroaceta te, a nd heptafluorobuta noat e salts haveall been prepared by metathesis reactions. 13 - 15 Thesemetat hesis reactions are good candidates for thosepreparing new ionic liquids for the first time. How-ever, they can leave the ionic liquids contaminatedwi th a smal l amount of ha l ide ions tha t may reactwith solute mat erials . 16

Tetraalkylammonium tetraalkylborides are usuallyprepared by the metathesis reaction of the tetraalkyl-ammonium bromide and the l ith ium te t raa lky l -boride. 2,17 Since both the a mmonium cation and thebor ide an ion a re asymmetr ica l ly subs t i tu ted , thes t a r t i n g ma te r i a l s a r e u s u a l ly ma d e r a th e r t h a npurchased. Reac t ion of a t r ia lky lborane wi th thea ppropriat e alkyllithium reagent in dr y hexane yieldsthe required lithium tetraalkylboride. 18

Other ionic l iquids are made by the quarterner-ization of the appropriate amine. This has been usedto prepar e a number of 1-a lkyl-3-meth ylimida zoliumtrifluoromethanesulfonate salts. 15 Methyl t r if la t e isreacted w ith a stoichiometric am ount of the 1-a lkyl-imidazole in 1,1,1-trichloromethane. Since the methyltriflat e is sensitive to moisture, the rea ction must becarried out under anhydrous conditions.

Monoalkylammonium nitrate sal ts are best pre-pared by the neutralization of aqueous solutions ofthe a mine with nitr ic acid.3b,13 The ionic liquids areisolated by removing excess water in vacuo. In asimilar reaction, t e traa lkylamm onium sulfonat eshave been prepared by mixing equimolar amounts

of the sufon ic ac id and the te t raa lky lammoniumhydroxide. 19 Again , excess wate r was removed inva cuo. To ensur e the purit y of th e ionic liquids, t heywere dissolved in either acetonitrile or tetrahydro-furan a nd treat ed wit h activat ed charcoal for at least24 h, a nd fina lly the organic solvent w a s removed invacuo.

The final method for the synthesis of ionic liquidsis direct combination of a halide sal t with a metalha lide. This is how the ha logenoa luminat e(III ) (seebelow) and the chlorocuprate(I) ionic l iquids ar eprepared. 2,20 The chlorocuprat e(I) ionic liquids a reparticularly sensitive to oxygen and have not foundwidespread use in synthesis. 21

Figure 1. (a) Alkylammonium, (b) alkylphosphonium, (c)N ,N -dialkylimidazolium, and (d) N -alkylpyridinium cat-ions.

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III. Handling I t i s w ide ly c la imed tha t many of the new ion ic

l iquids are both air and moisture s table; some areeven hydr ophobic. While it is t rue to sa y t ha t t he new liquids a re free from ma ny of the hy drolysis problemstha t ma ke the ha logenoa luminat es(III) so difficult tohandle, most a mmonium a nd imidazolium sa lts a rehygroscopic and if used in open vessels, hydrationwill almost certainly occur. The degree to which thisis a problem will depend on t he use to w hich the ionicliquid is being put and what solutes are being used.For instance, the small amounts of highly reactivespecies that are used as catalysts can be deactivatedby even the smal les t amounts o f wate r. 16 I wouldrecommend handling under an inert atmosphere ifthe ionic liquids a re t o be used for a ir- or moisture-sensitive solutes. Nonetheless, th ese new ionic liquidsare much ea sier t o handle tha n t he ha logenoalumi-nat e(III) systems a nd a re opening up new avenuesfor r esea rch, part icular ly in homogeneous cat alysis.

Among the room-tempera tur e ionic liquids t ha t a recur ren t ly rece iv ing a t ten t ion a re [E tNH 3][NO 3],

[emim][NO 3], a nd [emim ][ClO 4]. Organic nitrat es andperchlora tes are potential ly explosive, especial lywhen rigorously dried. Although no problems havebeen reported, car e should be used at a ll times w henhandling them.

IV. Solvent Properties Solvent pola rity is th e most commonly used solvent

classification. Even when considering molecular sol-vents i t is poorly understood and often confused.Terms such as polar, apolar, and nonpolar are usedindiscr imina te ly to apply to va lues of d ielect r icconst a nts , dipole moments, a nd pola riza bilities, even

though none of these are directly correlated in asimple way. The simplest quali ta t ive definit ion istha t a po la r so lven t i s one tha t w i l l d isso lve ands tab i lize d ipola r or charged solutes . I t i s w idelythought, though yet to be generally demonstrated,that under this definition, ionic liquids will be highlypolar solvents.

The longest wavelength absorption band of Rei-chard ts dye (2,4,6-tr iphenylpyridinium N -4-(2,6-diphenylphenoxide) beta ine shows one of th e lar gestsolva tochromic shifts known (375 nm betw een diphen-yl e ther and water) .22 It can register effects arisingfrom the solvent dipolarity, hydrogen bonding, andLewis a cidity a nd is considered to be a good general

polarity scale. The E TN

values of a small number ofalkylammonium nitra te , 23 thiocyanate ,23 a n d s u l -fonate19 salts have been recorded. Values of ca. 0.95 -1.01 for monoalkylammonium nitrat es and thiocy-anates are close to that of water (1.00, by definition),whereas quaternary ammonium sulfonates give lowervalues of ca. 0.45- 0.65 which are more typical ofpolar organic solvents such as DMSO. An at temptwa s a lso ma de to separa te dipole- dipole polariza bil-ity effects from hydrogen-bonding effects by using the * scale of dipolar ity/polariza bility, the R scale ofhydrogen-bond-donor acidity, and the scale ofhydrogen-bond basicity. 19 Although some differencewas seen between the * values for monalkylamm o-

nium salts and the quat ernary a mmonium salts , thedifference in the hydrogen-bond acidities and basici-t ies was far more marked.

The solvent properties of these ionic liquids havealso been investigated using chromatographic tech-niques. 19,23,24 I t wa s genera l ly found tha t the ion icliquids could be considered to be polar phases wit hthe solvent properties being largely determined bythe a bility of the sa lt t o a ct a s a h ydrogen-bond donor

a nd/or a cceptor a nd t he degree of loca lizat ion of th echarge on the anions. However, the ammonium andphosphonium salts that were used would not of hadlarge differences in the delocalization of charge onthe cation, and this may be an important effect forother salts, such as the pyridinium-and imidazolium-based ionic liquids. Furthermore, it was found thatincreasing the chain length of alkyl substituents onboth cations a nd a nions leads to great er lipophilicityof the ionic liquids.23,25 Also, the influence of hy drogenbonding can be diminished by fluorina ting the ionicliquids.26

V. Organic Reactions

A. Diels Alder ReactionsThe possibility of using ionic liquids a s a substit ute

for water, which has become a popular solvent tocarry out Diels- Alder cycloaddition reactions, hasbeen explored. The first study was of the reaction ofcyclopentadiene with methyl acrylate and methylvinyl ketone in [Et NH 3][NO 3].27 These rea ctions leadto a m ixture of exo a nd endo products, a nd t he solventinfluences on the endo /exo selectivity of the reactiona re w ell understood. The effect can be at tributed t osolvophobic interactions tha t genera te an interna lpressure and promote the a ssociat ion of the reagent sin a solvent cavity during the activation process.The rea ctions showed a str ong preference for t he endo product and an acceleration of the reaction in com-parison to nonpolar organic solvents. Although theincreased rate and selectivities were not as great asthose seen in w at er, the ionic liquid has the a dvan-ta ge tha t moisture-sensit ive reagents may be used,Figure 2.

The same react ion has been invest iga ted in anumb er of different ionic liquids ([emim][B F 4], [emim]-[ClO4], [emim][CF 3S O3], [emim][NO 3], and [emim]-[P F 6]), a ll of w hich showed t he sa me genera l trend. 28The effect was slightly weaker in these ionic liquids,and th is may be due to s t ronger (N - H) hydrogenbonding leading to stronger solvophobic effects in[EtNH 3][NO 3].

B. Alkylation of Sodium -NaphthoxideThe C vs O alkylation of sodium 2-naphthoxide in

simple molten phosphonium and ammonium halides

Figure 2. Diels- Alder cycloaddition of cyclopentad ieneand methyl acrylate.

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mixture of C 1- C 5 pa ra ff ins and C2- C4 olefins andsma ll amounts of part ially hydrogenat ed products. 36The catalyst quickly deactivates, but its life can beextended if CoCl2 or C uCl2 is a dded to the reactionmixture. Almost certa inly [P dCl 4]2- is formed oninitial dissolution of the PdCl2 in the [bu 4N]Cl.

The hy drogena tion of carbon m onoxide a t 220 Cand 430 a tm pressure has been inves t iga ted in avariety of qua ternary group 15 halides.37 The addi-

tion of either ruthenium(IV) oxide or ruthenium(III)acetylacetonate led to solutions that would catalyzethe reaction to y ield m ixtures of ethylene glycol, itsmonoa lky l e thers (up to p ropyl ), methanol, andethanol. The best results were achieved with qua-terna ry phosphonium sa lts. E vidence suggested tha tthe ac t ive ca t a ly t ic species in solut ion were theanionic cluster [HRu 3(CO)11 ]- and, after multicycling,[H 3Ru 4(CO)12 ]- , [H Ru 3(CO)13 ]- , and possibly [HRu 3-(CO)11 ]- .

C. HydroformylationRh(acac)(CO)2w i t h P P h3 in a r a nge of ionic liquids

([P F 6]- , [SbF6]- , [AsF 6]- , [B F4]- ) froms a solution tha twi l l ca ta lyze the hydroformyla t ion of olef ins .16,38Although the solution showed high cata lytic activitya nd could be reused, some of the cat a lyt w a s lost w itheach run. To avoid this, sulfonated triphenylphos-phine derivat ives w ere used as t he phosphine; how-ever, this lead to reductions in reaction rates. Thishighlights the possibili ty of using t hese l igands,which are widely used to solublize transition-metalcomplexes in wa ter, for the same purpose in ionicliquids. This w ill great ly extend the r an ge of ma teri-als that can be investigated.

Solutions of rut henium clusters in q uart ernaryammonium a nd phosphonium sa lts have also beenused as hydroformyla t ion ca ta lysts .39,40 Improvedyields were reported when bidentat e N-donor orP -donor liga nds w ere added t o the reaction mixturesan d t he principal cat a lytic species were [HR u 3(CO)9-(L - L)]- (w here (L - L) ) bidentate ligand).

D. Dimerization of ButadieneP a lladium dichloride, a ceta te, and a cetyla cetonat e

in the [bmim][BF 4] or [bmim ][PF 6] ionic liquids ha vebeen used for t he hydrodimerization of buta diene toform octa -2,7-dien-1-ol a s the ma jor product a nd1,3,6-octa tr iene as t he minor product. 41 The reactionmixture was homogeneous under the reaction condi-t ions (70 C ), bu t the p roducts cou ld eas ily be

separa ted by cool ing the mixture t o be low 5 C .Again, the r ecovered ionic liquid an d ca ta lyst couldbe reused. In this reaction system hydrodimerizationwas achieved in the absence of CO2, which is neces-sa ry for reaction t o occur in conventional m olecularsolvents, a lthough both the degree of conversion ofthe 1,3-butadiene and the turnover frequencies weregreatly improved by i ts addit ion. In this case, theactive cata lyst is the neutra l t r a n s -bis(methylimida-zole)palladium(II) dichloride, which is formed fromthe ionic precursor [bmim][P dCl 4] w h e n w a t e r i sad ded to its solution in t he ionic liquid. 41

The cyclodimerization of but a diene t o 4-vinylcy-clohexene using Fe(NO) 2 in [bmim][AF 6] (A ) P or

Sb) ionic liquids ha s been reported t o show a rema rk-able solvent effect.4,42 Fe(NO) 2 wa s produced by thechemical or electrochemical reduction of Fe 2(NO) 4C l2to yield the cat alyst for this D iels- Alder dimeriza-tion.

E. Heck ReactionsPalladium(II) chloride and acetate have also been

used a s p reca t a lys ts in a number of t e t raa lky lam-monium a nd phosphonium bromide sa l ts for pal-ladium(0)-catalyzed C - C coupling, Heck, reactions. 43Since the reactions were conducted at 100 C , it didn ot ma t t e r i f t h e s olven t s w e re l iq u id s a t r oomtemperature. In the Heck reaction, C - C coupling ofa romat ic and vinylic systems occurs. The palla dium-cat alyzed reaction of bromobenzene with n -butylacrylate in phosphonium salts in the presence oftriethylamine at 100 C formed t r ans -cinamic n -butyle ther in good y ields; 5% of the ci s -isomer wa ssynthesized w hen sodium acetate w as added to thereaction mixture, Figure 4. 43

When th e complexes d ichlorobis(tr iphenylphos-phine)palladium(II) and palladium(II) acetate wereused as the cat a lyst precursors, sta ble solutions w ereformed w hich could be reused after the reactionwi thou t loss o f ca t a ly t ic ac t iv ity for a t leas t two

further runs. The products were removed from thesolution by distilla tion. High levels of conversion w ereobserved in these ionic liquids (over 99% in somecases). When palla dium(II) chloride w a s used, pre-cipita t ion of a palladium cluster which was deacti-va ted towa rd the ca ta lysis, occur red a f te r somehours.

VII. Halogeno and Alkylhalogenoaluminate(III) Ionic Liquids

Although this is changing, the halogenoaluminate-(I I I ) ion ic l iqu ids remain by fa r the mos t w idelystudied of a ll of the room-temperat ure ionic liquids

and justify separate treatment here. The first halo-genoa lumina tes(III ) to be liquid at room tempera tur ewere mixtures of 1-alkylpyridinium bromides withaluminum(III) chloride where X (AlCl3) ) 0.66. 7,44How ever, in other compositions, these mixtur es weresolid at room tempera ture. B y using 1-butylpyri-dinium chloride ([ n -bpy]Cl), a system w hich wa sliquid over a range X (AlCl3) ) 0.43 - 0.66 was ob-tained.45 Furt her improvements in liquid ra nges werea chieved by using a mixt ure of 1-ethy l-3-meth ylimi-da zolium chloride with a luminum(III ) chloride ([emim]-Cl- AlCl3).46,47 Since the first use of imidazolium sa lts,they have become the ionic l iquids of choice. I na ddition to t he chloroalumina te(III ) systems, [emim]-

Figure 4. Heck coupling reactions in tetraalkylammoniumand phosphonium bromide salts.



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The equilibrium const a nt for eq 2 ha s been measuredsevera l t imes for both the [emim]Cl - AlCl3 a nd[n -bpy]Cl - AlCl3 systems a nd l ies in t he range from10 - 16 to 10 - 17 a t 40 C .62 - 64 Heerman and DOlies-lager 65 measur ed potentiometric titra tion curves for[n -bpy]Cl - AlCl3 ion ic l iqu ids con ta in ing a la rgeexcess of a luminum(III ) chloride a nd calcula ted t heequilibrium const a nt for eq 3 to be 2.09 ( 0.06 10 - 3a t 40 C .

In t h e i on ic l iq u id , a n a n a log y i s of t en ma d ebetween equilibrium 2 and the autosolvolysis reactionof wa ter. Since Cl- is a Lewis ba se and [Al2C l7]- a nd[Al3C l10 ]- are both Lewis acids, the Lewis acidity/basici ty of the ionic l iquid may be manipulated bya ltering its composition. This leads to a nomenclatur eof t he liquids in wh ich compositions w ith a n excess

of Cl-

(i.e., X (AlCl3) < 0.5) a re called bas ic, th ose w ithan excess of [Al2C l7]- (i.e., X (AlCl3) > 0.5) ar e calledacidic, and those at the compound formation point(X (AlCl3) ) 0.5) a re called neutra l.

27 Al NMR spectra of the closely related ethylchlo-roalumina te(III ) system [emim]Cl - EtAlCl2 have alsobeen measured a nd used to propose th e presence of[EtAlCl3]- ions and molecular Et 2Al2C l4 in neu t ra lcompositions of the ionic liquid with an additionalsmall amount of [Et 2Al2C l5]- in a cidic (X (EtAlCl 2) )0.6) compositions. 66 However, both 1H N M R a n dRaman spectroscopy of [bmim]Cl - EtAlCl2 ionic liq-uids have shown that the distr ibution of the ethyl-chloroalumina te(III ) species follows the sa me pat terna s tha t found in chloroa luminat e(III ) ionic liquids. 67,68Hence, in basic ionic liquids C l- and [EtAlCl3]- ionsa re found, in moderat ely a cidic ionic liquids [Et AlCl3]-

and [Et 2Al2C l5]- ions a re present, a nd in highly a cidiccompositions [Et 3Al3C l7]- and f ina l ly E t2Al2C l4 be-come importa nt components . S imilar results ar efound for [bmim]Cl - E t 2AlCl ionic liquids.68 Thissimple model of t he ethylchloroalumina te(III) ionicliquids does not, however, give the full picture. Closerinspection of the Ra ma n spectra of acidic [bmim]Cl -EtAlCl2 ionic liquids reveals that the ions [AlCl4]-

and [EtAl 2C l6]- and the molecular species Et 2AlCland E t 3Al2Cl3 a re present. 68 Hence, exchan ge of ethy l

and chloride ligands must be taking place,68

e.g.,

This behavior is well-known for the molecularspec ies , and i t i s qu i te l ike ly tha t the l iqu ids a recomplex mix tures con ta in in g ion s of t h e t y pe[Et x AlCl4 - x ]- , [Et x Al2C l7- x ]- , and [E tx Al3C l10 - x ]- . Intw o-phase systems, the molecular ethylchloroalumi-nate(III) species EtAlCl 2, E t2AlCl, and Et3Al2C l3 a reextra cted into the orga nic pha se, leading t o a reduc-tion in the acidity of the ionic liquid a nd a n increasedconcentr at ion of t he chloroa luminat e(III ) species. 68

VIII. Reactions with Water Aluminum(III) halides are extremely sensitive to

even the smallest amounts of water. Hence, whenwa ter is a dded to a chloroalumina te(III ) ionic liquidof any composition, an exothermic reaction occurswith the evolution of HCl. The reaction producesoxide- and proton-containing species, both of whichcan interact with other solutes.

A. Oxide-Containing Species17O NMR spect ra of the [emim]Cl - AlCl3 ionic

liquids with added 17OH 2 show a single signal in ba siccompositions, the chemical shift of which is depend-en t on the amount o f 17OH 2 added and the exac tcomposition of the ionic liquid. 69 If proton impurityis removed by the addit ion of e thylaluminum(III)dichloride, 70 then a s ingle resonance a ppears , t hechemical shift of which is both composit ion andconcentr a tion independent. 20 These data suggest thatin basic ionic liquids, there is a single chlorooxoalu-mina te(III) species tha t, in t he presence of proton, is

in equilibrium w ith a chlorohydroxoa luminat e(III)species with the former being dominant a t lowerconcentrations of oxide. 69

Negative-ion FAB mass spectra of basic [emim]Cl -AlCl3 a n d [n -bpy]Cl - AlCl3 ionic l iquids show thepresence of [Al 2C l5O] - ions,71 and this is c learly thechlorooxoalumina te(III) species seen in th e 17O NMRspectra. 19 However, no hydroxo-conta ining specieswa s observed; in par ticular, t here is no evidence forthe [AlCl 3(OH)]- ion. The absence of [AlCl 3(OH)] - inthe FAB experiments indicates that the initial reac-tion of the basic ionic liquid with w a ter is tha t shownin eq 5. At h igher concentr a tions of proton, a s econdrea ction becomes import a nt , lea ding to the forma tion

of th e hy droxo species, possibly [Al 2C l6(OH)]-

.

In ac id ic ion ic l iqu ids , the 17O NMR spect ra 69clearly show thr ee oxide-conta ining species w hoserela t ive concent ra t ions a re sens it ive t o both theprecise composition of th e ionic liquid a nd t he a mountof 17OH 2 added . Aga in , us ing E tAlCl2 to removeproton, one of t hese ca n be shown to be a chlorohy-droxoaluminate(III) species ([Al 2Cl6(OH)] - ). The othertwo signals were at tr ibuted to [Al2C l5O] - and [Al 3-

C l6O2]-

ions.Nega tive-ion FAB ma ss spectr a of oxide-free [emim]-C l- AlCl3 ionic liquids have been recorded and thehydrolysis of the l iquids in the spectrometer ob-served. 72 After 30 min in t he spectrometer, six oxide-containing ions were visible in the spectrum. Theforma tion of [Al3C l8O] - ions (the st ructure of wh ichha s been determined crysta llogra phica lly 73 ) directlyfrom [Al 3Cl10]- wa s clear ly demonstra ted, as w as tha tof [Al2C l5O] - from [Al 2C l7]- . These initia l r eactionswere shown to be fol lowed by others, lead ing to[Al 2C l6(OH)] - a s t he domina nt chlorohydroxoa lumi-na te(III) ion a nd t o [Al 4C l9O 2]- , [Al3C l7O(OH)] - , and[Al3C l6O2]- ions a s the a mount of wa ter is increased.

[Al2Cl6] + Cl-

f [Al2Cl7]- (1)

2[AlCl 4]-

h [Al2Cl7]- + Cl- (2)

2[Al 2Cl7]-

h [Al3Cl10 ]- + [AlCl4]

- (3)

2[EtAl 2Cl6]-

h [Et 2Al2Cl5]- + [Al2Cl7]

- (4)

2[AlCl 4]- + H 2O f [Al2Cl5O]

- + H Cl2- + H Cl

(5)



8/14


9/14

aluminum(III) chloride containing ionic liquids wereused, a luminum co-deposited wit h the a lloy.

Despite the w idesprea d interest in t he ionic liquidsas solvents for halogenometalate species, very few chemical transformations of these complexes havebeen s tud ied in the ion ic l iqu ids . The on ly rea lexception to t his ha s been t he investigat ion of theoxo-exchange chemistry. 75,93,99 The use of phosgeneas a deoxochlorination agent combined with electro-chemistr y ha s been used to genera te both Nb(IV) a ndNb(V) complexes, Scheme 1. 92

More recently tr iphosgene, w hich is a solid andmuch easier to ha ndle, ha s been used t o deoxochlo-rina te both [VOCl 4]2 - and [VO 2C l2]- .75 In this s tudythe deoxochlorina tion w a s a ccompa nied by sponta ne-ous reduction of the products to [VCl 6]3- and [VOCl4]2- ,respectively. I t was also found that mild oxidantscould be used to generate [VOCl 4]2 - from [VCl6]3 - ,but the st ronger oxidizing a gent iodosobenzene w asrequired to produce [VO 2C l2]- .

X. Organic Reactions in Chloroaluminate(III) Ionic Liquids

A. Electrophilic SubstitutionsIt is unsurprising that electrophilic aromatic sub-

s t i tu t ions were t he f ir s t organ ic react ions to beinvestigated in the room-temperature chloroalumi-nate(III) ionic liquids. 95 The high concentration ofchloroalumina te(III) species coupled wit h the goodsolubility of simple arenes in the acidic ionic liquidsmakes t hem ideal solvents for t hese reactions, a ndit is possible to combine their function a s a solventand a cat alyst . Of the a renes tested by reaction w ith1-chloropropane, only n itr obenzene fa iled to rea ct. Aswith conventional systems, polyalkylation was com-mon, e.g., reaction betw een an excess of chloroetha ne

and benzene led to the formation of a mixture ofmon o (12%), d i (11%), t ri (33%), t et ra (24%), pen t a(17%), a nd hexa (2%) substit ut ed products . P olya lky-lation can be minimized by use of a large excess ofthe a rene bu t not tota l ly el imina t ed . Ba s ic ion icliquids do not provide adequa te cat a lytic activity foralkylation and acylation reactions to occur.

In order for the Friedel - Cra fts rea ctions t o occur,it is necessary to be able to form an electrophile inthe ionic liquid. Luer and B ar ta k96 demonstra ted thateven in a moderately a cidic (X (AlCl3) ) 0.52) ionicliquid, dissolution of chlorotriphenylmetha ne leadsto t he formation of t he t r iphenylmethyl carboniumion.

Rea ctions w ith 1-chloropropa ne a nd 1-chlorobuta nelead to the format ion of p roducts resu lt ing f romsecondary carbonium ions, which implies that alky-lation occurs via the dissociated carbonium ions. 95Mixing benzene and hexamethylbenzene in an acidicionic liquid did not lead to t he format ion of t oluene,xylenes, mesitylenes, etc. , showing that there is nodissocia tion of th e methyl subst itutes w hen th e ionicliquid does not conta in protic impurities.

Friedel - Craf ts acy la t ion react ions of a romat iccompounds ha ve a lso been car ried out in t he [emim]-C l- AlCl3 ionic liquids.95 For the reaction of acetylchloride with benzene, i t was determined that therate at which acetophenone was produced was de-pendent on the Lewis acidity of the ionic liquid, whichis in turn dependent on the ionic liquid composition.The rea ction betw een a cety l chloride a nd t he a cidicionic liquid was followed by 1H NMR. The resultssuggested a s toichiometric reaction betw een CH 3-COCl and [Al2C l7]- .95

Indeed, it is possible to isolate solid [CH 3CO][AlCl 4]from t he ionic liquids.

The alkylation of coal has been investigated in apyridinium chloride based ionic liquid (X(AlCl 3) )0.65). 97 The coal chosen for study was a high-sulfur,high-volatile C bituminous coal, and the alkylatingagent employed was 2-propanol. Autoalkylation of2-propanol yielded a series of high molecula r w eightpolymers, most of w hich w ere nonvolat ile. No a lky-lated pyridines, from reaction with the ionic liquid

itself, were formed. Reaction between the deminer-al ized coal and 2-propanol was at tempted, and theFriedel - Crafts a lkylat ion was accompanied by de-polymerizat ion of t he coa l.

Isobutan e alkylat ion ha s also been investiga ted inionic l iquids. Again, i t wa s the a djustable na ture ofthe Lewis ac id i ty o f the sys tems tha t made themattractive.98 The process involved a r eaction mixt ureconsisting of isobutane, butane, and but-2-ene (mix-tu re o f c is and t ra ns) in a [bmim]Cl - AlCl3 ionicliquid. The products of th e rea ction included included2,2,4-trimet hylpenta ne, 2,5-dimethylhexane, lightends (C 5- C7 i sopara ff ins ), and heavy ends (C9+isopara ffins). The ra te a nd selectivity of the a lkyla-

t ion reac t ion wa s found to be dependen t on theprecise composition of the ionic liquid, r eact ion t em-perature, feed rate into the reactor, and residencetime of the m ixture in t he ionic liquid.

In a n investiga tion of the electrochemica l oxida tionof hexamethy lbenzene in mixtures of an ac id ic(X (AlCl3) ) 0.67) ethylpyridinium bromide - alumi-num(III) chloride ionic liquid with benzene, 99 i t w a sfound tha t a p roduct mixture con ta in ing pen ta -,te tra -, a nd tr imethylbenzene a nd diphenylmethanewa s formed. The aut hors postulat ed tha t t he mixturewa s formed by a series of Friedel- Crafts reactionsinitiat ed by a tw o-electron oxidat ion of hexamethy l-benzene.

Scheme 1. Oxoexchange Chemistry of NiobiumHalides in a Basic [emim]Cl - AlCl 3 Ionic Liquid

[VO 2Cl2]- y\

z triphosgene

PhI O [VOCl4]

2 - y\

z triphosgene

O2 or LiCO3[VCl 6]

3 -

(6)

P h3CCl + [Al2Cl7]-

f [P h 3C]+ + 2[AlCl 4]

- ;

k eq m ) 20.4 ( 0.3 mol dm- 3 (7)

CH 3COCl + [Al2Cl7]-

f [CH 3CO]+ + 2[AlCl 4]

-

(8)



10/14

Ca rboca tions are not t he only electrophile that canbe genera ted in t he chloroalumina te(III ) ionic liquids,and both chlorination and nitrat ion reactions havebeen observed in [emim]Cl - AlCl3 ionic liquids.100Chlorination occurs in both acidic and basic ionicl iquids but not in pure molten [emim]Cl, whichindica tes th a t chloroalumina te(III) ions a re requiredfor reactions to occur. However, a lmost certa inlyreactions proceed via different mechanisms in acidica nd ba sic ionic liquids. In both rea ctions, the prima ryproduct was chlorobenzene. In acidic compositions,large a mounts of a t least tw o dichlorobenzene iso-mers were synthesized, as well as some trichloroben-zene products. In basic compositions, there was noforma tion of polychlorobenzenes but s ignif icant

amounts of te t ra ch lorocyc lohexene i somers andhexachlorocyclohexane isomers were observed.In acidic composit ions, i t is obvious that a very

reactive electrophile is observed, suggested by thesubstantial polychlorination and the authors proposethe formation of Cl+ .100

Simple electrophilic chlorina tion via Cl + is lesslikely in basic ionic liquids, since reaction with Cl-would almost defini tely destroy the electrophile.Hence, C l3 - , formed by the r eaction of Cl2 w i th C l- ,

wa s proposed a s the chlorination a gent, Scheme 2.Nitrat ion of aromat ic compounds in [emim]Cl -AlCl3 ionic liquids used K NO 3 as t he source of NO2+ ,wh ich a cts a s th e electrophile and resulted in a 55%yield of nitrobenzene. 100 NH 4NO 3 and NO2B F4 werealso tr ied as ni trat ing agents but gave much loweryields.

B. Reactions with Protons in AcidicChloroaluminate(III) Ionic Liquids

It has been demonstrated that protons present in[emim]Cl - AlCl3 ionic liquids are superacidic, withHa mmet t ac id i t ies up to - 18. 76 Hence , they a reexpected to be highly r eactive.

Initial investigations 76 showed t he a bility of [emim]-C l- AlCl3- HCl systems to protonate aromatic sub-stra tes a nd generat e ca tionic species. Anth ra cene hasbeen shown t o excha nge deuterium with DC l in acidic[emim]Cl - AlCl3 ionic liquids.101 It is, therefore, un-surpris ing tha t i t ha s been shown tha t the [emim] +cations undergo H - D excha nge in a cidic [emim]Cl -AlCl3 solutions of DC l.102 Since the exchange occurs

by an e lect rophi lic subs t i tu t ion mechan ism, theexchange occurs a t the 4 a nd 5 pos it ions of theimidazolium ring ra ther t han the 2 posit ion, w hichhas a great er posit ive charge.

Ca rboca tion forma tion by protona tion of ar enes ha sbeen observed, using both electronic absorption and1H NMR spectroscopies, for a number of substratesin trimethylsulfonium bromide - a luminum(III) chlo-ride and t rimethylsulfonium bromide - aluminum(III)bromide ( X (AlCl3) ) 0.67) ionic liquids with addedH B r.103 As one w ould expect, th e a ll-bromide syst emproved to be the more acidic solvent. Also observedwere the formation of the 1,1-diphenylethyl cationby protonation of 1,1-diphenylethene and the diphen-ylmethyl ca tion from chlorodiphenylmetha ne in t hea ll-bromide ionic liquid. All of the solutions of car-bocations were shown to be stable in the absence ofmoisture for a t least several da ys.

P rotonation of benzene is a lso probably the f irs tstep in the chemical synthesis of polyphenylene. 104The rea ction requires the addit ion of C uCl 2 whichacts as the oxidizing agent in this dehydropolycon-densat ion of benzene. Alterna tively, polyphenylenecan be prepar ed by the electropolymerization ofbenzene in neutral and acidic ionic l iquids. 105 - 107Varying the precise composition of the ionic liquidhad a minimal effect on the polymerization potential,suggesting that there is l i t t le interaction betweenbenzene and chloroalum ina te(III) species in the ionicliquid. This reaction has also been carried out in anovel [n -bpy]Cl - AlCl2(OC 2H 3) ionic liqu id.108

P rotons in a cidic chloroalum ina te(III) ionic liquidsha ve been known to cat a lyze the format ion of oligo-mers , w i th molecu la r weigh ts chara cte r is t ic o f acationic reaction, from olefins. Indeed, attempts to

avoid this reaction lead to the f irs t preparation ofa lkylchloroalumina te(III ) ionic liquids. 109 The elec-trochemical oligomerization of ethene and propenein t he t hree room-temperat ure ionic liquids [ n -bpy]-C l- AlCl3, [pyH]Cl- AlCl3, a nd [emim]Cl - AlCl3 havealso been investigated. 110 The main products ofoligomerization were C 3- C6 hydr ocarbons. The re-sults indicated th a t t he [emim]Cl - AlCl3 ionic liquidleads to much greater selectivity of unsaturated C - Chydrocarbons than the more acidic [ n -bpy]Cl - AlCl3ionic liquids, in the case of both ethene and propeneconver sion. The [emim]Cl - AlCl3 an d [pyH]Cl - AlCl3ionic liquids ga ve similar selectivities during propeneoligomerizat ion. However, the [pyH]Cl - AlCl3 ionicl iquid shows by far the best cat alytic s ta bili ty a nd istherefore the best system for these conversions. 110The average conversion of propene is greater thantha t of ethene in this ionic liquid, but t his is expectedas propene is generally more reactive than ethene.Similar e lectrochemical polymerization has beenutilized to synt hesize polya niline using [emim]Cl -AlCl3 ionic liquids.111

This chemist ry has led to a number o f pa ten tapplications for the polymerization of raffinate gas(mixtures of butenes) 112 - 114 and the preparation ofbranched ol igomer ic fa t ty ac ids f rom l inear fa t tyacids.115

Scheme 2. Proposed Mechanism for Chlorinationof Benzene in a Basic [emim]Cl - AlCl 3 Ionic L iquid

Cl2 + [Al2Cl7]-

f Cl+ + 2[AlCl 4]- (9)

NO 3- + [Al2Cl7]

-f

NO 2+ + [AlOCl 3]

2 - + [AlCl4]- (10)



11/14

XI. Organometallic Reactions in Chloroaluminate- (III) Ionic Liquids

Another interesting Friedel - Crafts reaction per-formed in room-temperat ure ionic l iquids is theacylation of ferrocene.49 The a cylation of ferrocenewit h a cetic anh ydride in a [emim]I - AlCl3 ionic liquidand in the l iquid clathra te , prepared from t he addi-tion of toluene to the ionic liquid, w a s investiga ted.Interest ingly, the authors used commercial a lumi-num(III) chloride, without further purif icat ion, toprepare the ir ion ic l iqu ids . I t wa s cla imed tha tmonoacetylferrocene wa s the only product of theacylat ion reaction using both reaction media, withthe highest yields being acquired using the l iquidclathrate .

Arene exchange reactions of ferrocene are well-known to be catalyzed by aluminum(III) chloride.Hence, t he a cidic (X (AlCl3) ) 0.65) [bmim]Cl - AlCl3ionic l iquid ha s been used t o prepare a number ofarene(cyclopentadienyl)iron(II) complexes, [Fe(C 5H 5)-(arene)] + , fr om ferrocene. 116 The ionic liquid a cts a sboth a solvent and Lewis acid source. However, since

the ionic liquids used were completely aprotic, theproducts were only formed on addition of a protonsource, [bmim][HCl 2]. This w ould probably h a ve beenunnecessary if commercial aluminum(III) chloridehad been used without sublimation, Figure 6.

The reductive carbonylat ion of tita nocene dichlo-ride (Cp 2TiCl 2) in a n a cidic (X (AlCl3) ) 0.60) [emim]-C l- AlCl3 ionic liquid has also lead to the formationof a meta l- carbon bond. 49 A reducing mixture w asprepa red by a dding sodium meta l to the ionic liquid.This lead to th e precipita tion of aluminum from t heionic l iquid in a form that was more reactive thancommercial granular a luminum. Init ia l dissolutionof titanacene dichloride in the [emim]Cl - AlCl3 ionicliquid has been shown 117 to lead to the formation ofC p2Ti(AlCl 4)Cl. In the r educing mixture used in t hisprocess, this rapidly reduces to the Ti(III) species([Cp 2Ti(AlCl 4)2]- ). [AlCl4]- i s a lab ile ligand and i sea s i ly s ub st i t ut ed b y C O b y p a ss a g e o f C O g a sthr ough the solution formed.

Fur ther reduct ion of th is complex lead to theforma tion of Cp2Ti(CO) 2, which precipita t ed fromsolution.

XII. Transition-Metal-Mediated Catalysis in Chloroaluminate(III) Ionic Liquids

In homogeneous catalysis , the solvent can oftencontr ol the course of a reaction by a ffecting rea ctionrat es and improving chemo-, regio-, s tereo-, and

enantioselectivities of reaction products. In transi-t ion-metal-based cat alysis , i t is importa nt for thesolvent to solubilize and st a bilize the a ctive ca ta lyticspecies bu t remain unreac t ive toward the ac t ivecatalytic s i te , i . e . , to behave as a noncoordinatingsolvent . H ence, a cidic compositions of the chloroalu-minate(III) ionic liquids, with their poorly coordinat-ing a nions, offer a potent ia lly exciting n ovel mediumfor catalysis.

A. Olefin DimerizationThe chloroaluminate(III) ionic liquids were first

used as solvents for the dimerization of propene tohexenes using nickel(II) complexes as the ca ta lysts. 109In these reactions, the product hexenes separatedf rom the ion ic l iqu id phase and cou ld eas ily beremoved by decan ta t ion . I t wa s found t ha t on lycat alysts w ith a nickel- carbon bond w ere active andtha t the products were conta minated w ith t he prod-ucts of a cat ionic side reaction. B y using ethyla lumi-num(III ) dichloride-ba sed ionic liquids , both of th eseproblems were avoided. This allowed a greater num-ber of poten t ia l ca ta lysts to be employed for thereaction, par ticularly nickel phosphine diha lides.

Cationic -nickel(II) complexes containing phos-phine l igands a re useful cata lysts for the dimeriza-tion of propene in solution in chlorinat ed or a roma tichydrocarbons. 118 The nature of the phosphine liganddetermines the regioselectivity of the dimerizationreaction, s ter ical ly demanding l igands lead to theforma tion of 2,3-dimeth ylbut enes (ta il-to-ta il dimers),which can be u t i l ized as s ta r t ing a lkenes fo r theproduction of fine chemicals, wherea s cat a lysts w ithundemanding l igands yield oligomers with uncon-trolled regioselectivity.

A number of air-stable NiCl 22L complexes (L )

P(Bu)3, P(i-Pr)3, P(cyclohexyl)3, and pyridine) havebeen successfully used as catalysts for the regiose-lective dimerizat ion of propene in chloroalumina te-(III) and ethylchloroaluminate(III) ionic liquids. 119 Acosolvent of hepta ne is used to improve productsepa ra tion. The upper la yer conta ining the productscan be removed easi ly, and the cata lyst remains inthe ionic phase a nd ca n be reused. However, deacti-vat ion of t he syst em is observed if pure ethylchloro-aluminate(III) ionic liquids are used as the solventdue to t he extra ction of th e dimeric [Et 4Al2C l2] intothe organic layer. This is avoided by the use of the[bmim]Cl - AlCl3 ionic liquid wit h a sma ll amount ofdichloroethy laluminum(III) a dded.

This work has been extended to the oligomerizationof butenes 120 and to the selective dimerizat ion ofethene. 121 In t his la t ter example, quanti ta t ive ana ly-sis of the nickel content of the tw o phases indicat edthat over 98%of Ni remained in the ionic layer.

Optimum dimerization activi ty and selectivi ty isachieved when toluene is used as the cosolvent forthe reactions. 119,121 When other organic solvents suchas heptane are used or a cosolvent was omitted, amixture of high molecular weight ethene oligomerswa s synthesized. I t is feasible that this occurs a s aresult of coordination of AlCl3 to the aroma tic ring.122This w ould r educe th e a cidity of the ionic liquid, soattenuating the activi ty of the catalyst .

Figure 6. Arene exchange reactions of ferrocene.

[Cp 2Ti(AlCl 4)2]- + 2 CO f

[Cp 2Ti(CO) 2]+ + 2[AlCl 4]

- (11)



12/14

B. Olefin PolymerizationIn t wo studies of tita nium chemistry in chloroalu-

mina te(III ) ionic liquids t o wh ich a lkylchloroalumi-na te (I I I ) d ry ing a gen ts had been added, e thenepolymerization has been observed. When AlEtCl 2 a ndTiCl 4 a re added to an ac id ic (X (AlCl3) ) 0.52) ionicl iqu id, a deep red solu t ion i s formed which wi l lpolymerize ethene to give poly(ethene) with a meltingpoint in the region 120 - 130 C.123 B etter yields wereob ta ined wi th solut ions of Cp2TiCl 2 i n t h e s a m ecomposition ionic liquid with Al 2Me 3C l3 act ing as thealkylat ing agent.124 Interest ingly, neither Cp 2ZrCl2or Cp2HfCl2 were ca ta ly t ica l ly ac t ive in the ion icl iquids. Unfortunately, this work does not seem tohave been followed up.

C. Olefin HydrogenationAnother example of the use of this system is the

catalytic hydrogenation of cyclohexene by rhodiumcomplexes. 33 The use of an acidic chloroaluminate-(III) ionic l iquid (X (AlCl3) ) 0.54) leads to thepolymeriza tion of cyclohexene (see a bove). H owever,

upon d issolving Wilkinsons ca ta lyst (RhC l(P P h 3)3) ina ba sic (X (AlCl3) ) 0.45) ionic liq uid, th e cyclohexeneundergoes hydrogenat ion, 33 and the product collectsin a separate phase, which can easi ly be decanted.Wilkinsons cat alyst forms a sta ble solution, a nd t hea ctive species (more th a n 98%) is r eta ined in t he ionicphase.

XIII. Conclusions The chemistry of room-temperat ure ionic liquids

is at a n incredibly exciting st a ge in its development.No longer m ere curiosities, ionic liquids a re beginningto be used a s solvents for a wide ra nge of synthetic

procedures. The a dvent of systems tha t ar e easy tohandle will allow those without specialist knowledgeof th e field to use them for t he first t ime. The sma llnumber of reactions that have been investigated sofar show t he potent ial of the ionic liquids but a re justa beginning. The solvent environment that is pro-vided by the ionic liquids is quite unlike any otheravailable at or close to room temperature. Already,startling differences have been seen between reac-tions in ionic liquids a nd m olecular solvents. As thenumber of investigations increases, we will be ableto te l l i f there is any general ionic l iquid effect .P otential ly any reaction may produce interest ingresults in ionic liquids, a nd t he discovery of the n ew

chemistry wait ing to be found wil l be a mammothta sk. It ha s been 15 years since Chuck Hussey wrotethe f irs t major review of room-temperat ure ionicliquids,2 I hope tha t th is rev iew has in some wayupdated tha t w ork a nd look forw ar d wit h excitementto the next .

XIV. References (1) Note on nomenclatur e: room-tempera tur e ionic liquid, nona que-

ous ionic liquid, molten salt, liquid organic salt, and fused salthave a l l been used to descr ibe sa l ts in t he l iquid phase . Withthe increase in e lectronic databases , the use of keywords assearch tools is becoming ever more important. While authorsare f ree to choose any na me that they wish for their systems, Iwould suggest tha t th ey at least include the term ionic liquid in

keyword lists. In this paper, I allow the term ionic liquid to implythat the sa l t is low melt ing.

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chemical reviews volume 99 issue 8 1999 [doi 10.1021%2fcr980032t] welton, thomas -- room-temperature...

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