Working Salts: Syntheses and Uses of Ionic Liquids Containing

Chapter 20

Downloaded by UCSF LIB CKM RSCS MGMT on September 13, 2014 | http://pubs.acs.org Publication Date: July 25, 2002 | doi: 10.1021/bk-2002-0818.ch020

Working Salts: Syntheses and Uses of Ionic Liquids Containing Functionalized Ions 1,2

JamesH.Davis, Jr. 1

2

Department of Chemistry, The University of South Alabama, Mobile, AL 36688 Center for Green Manufacturing, The University of Alabama, Tuscaloosa, AL 35487

The immobilization of specific chemical entities on supporting materials is a technology of phenomenal scope and importance (1-3). Long used industrially as a method for facilitating the recovery of precious metal catalysts, reagent immobilization plays central roles as well i n areas as diverse as water purification and solid-phase combinatorial synthesis. A basic rationale for using this technology is the ease of separation that is provided by the immobilization of a reagent or catalyst on a heterogeneous support. In petroleum cracking, this takes the form of a metal salt adsorbed onto a mineral support. Such catalyst-support systems are easily separated by physical means from gaseous or liquid products. In biochemistry, the immobilization of an amino acid on plastic beads allows additional amino acids to be added to the first i n a sequence specific manner. The desired polypeptide can subsequently be removed at the completion of the synthesis i n an environment free of byproducts or unreacted reagents. The increased ease of separation notwithstanding, the immobilization of

© 2002 American Chemical Society

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

247

248


catalysts and substrates on solid supports creates other problems (4). For example, i n the particular case of heterogeneous catalysts, low specificity, high reaction temperatures and temperamental kinetics characterize many systems. These problems stem chiefly from the encumbered access to the surface-bound catalyst that is presented to the substrate molecule. The simultaneous need for the diffusion of fresh substrate to and product away from the solid surface become mutually hindering processes. Moreover, the number of directions i n which the substrate may approach the catalytic site is sharply curtailed by the massive bulk of the support.

Solid Support

Porous Solid Support

Functionalized Ionic Liquid Figure 1. Reagent and catalyst heterogeneous type support systems. Attempts to circumvent these problems have largely focused on dispersing the supported reagents onto solids that have higher surface areas (e.g., porous solids or polymers), though other approaches such as using reagents "immobilized" i n aqueous and fluorous phase liquid supports are also being used (5). While these systems offer some clear advantages over classical solid supports (Figure 1), they still do not constitute ideal systems. A n arguably better system would be one i n which each molecular unit of the supporting



249 material bore functionality and was itself free to diffuse, while retaining a high degree of phase heterogeneity with other solvents, reagents, etc., allowing easy separation. Moreover, the utility of this liquid supporting medium would be enhanced by a diminished capacity for evaporative loss versus water or fluorocarbons. We are endeavoring to craft "task-specific" ionic liquids for use as just this type of system. Despite their formal similarity, a key characteristic distinguishes ionic liquids from other molten salts. Specifically, salts categorized as ionic liquids melt at temperatures many hundreds of degrees lower than do typical, wholly inorganic salts such as N a C l (mp = 800°C). Their liquidity at low temperature combined with vanishingly small vapor pressures and a remarkable capacity to solvate an array of organic and inorganic substrates have made ionic liquids materials of intense interest as potentially "green" solvents. As essentially non-evaporable entities, ionic liquids may be suitable for replacing V O C s (volatile organic compounds) i n a variety of applications. Given that the annual worldwide use of V O C s has a value of about six billion dollars, their replacement with these non-evaporating solvents could produce a considerable economic and environmental impact (6). Overall, research into ionic liquids has been largely limited to a small number of ion pairings (7). The cation of these salts is typically l-ethyl-3-methyl imidazolium, [emim] or l-butyl-3-methyl imidazolium, [4-mim] . A similarly small family of typical anions, mostly chloroaliuninates, fluorophosphate, fluoroborate or bis(trifluoromethyl)sulfonamide, is used as well. By judicious choice of the anion and cation, ionic liquids may be formulated that are immiscible with water, molecular organic solvents, or both (Figure 2). +

+

Figure 2. Typical ionic liquid ion pairs. In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


250 It is the goal of our research to design and synthesize ionic liquids that possess enhanced capacities for interacting with particular types of substrates while retaining control over the I L miscibility with co-solvents. B y incorporating a capacity for selective interactions with substrates, we can create task-specific ionic liquids - ionic liquids capable of accomplishing specific tasks ranging from metal extraction from aqueous solutions to the solvation and catalysis of organic reactions. Indeed, by grafting functionality into these salts - ion by ion - and retaining phase heterogeneity with other solvents, we are formally creating the type of improved heterogeneous support systems previously outlined. In this effort, we expect to expand the number of situations where ionic liquids can be utilized i n place of organic solvents or solid support systems i n synthesis, separations and other applications. Both the cation and anion of ionic liquids are potential targets for structural and compositional modification. To date, the major focus of our efforts has been on cation modification, though ample opportunities exist for anion modification as well. Conceptually, the ion structures can be regarded as possessing two elements. The first element is a core that bears the ionic charge and serves as the locus for the second element, the substituent groups. The core of the cations may be as simple as a single atom such as Ν , Ρ or S, as found i n ammonium, phosphonium or sulfonium ions, respectively. Alternatively, the core of the ion may be (and more commonly is) a heterocycle such as imidazole or pyridine. Substituents are typically straight-chain alkyl groups, but may i n theory incorporate any type of organic group.

Δ

Ν

ΙΘ

+ acid base

benzaldehyde CH

benzoin condensation catalysis organic phase

3

ionic liquid phase

Scheme 1. A prototype task-specific ionic liquid (TSJL) system.


251 We first demonstrated a capacity for task-specific ionic liquid design and synthesis with the preparation of 3-butyl-4-methyl- and 3-butyl-5-methyl thiazolium salts (8). These cations are structural homologs of the [4-mim] cation, i n which the core of the ion is changed from an imidazole ring to a thiazole ring. Alkylated thiazolium cations such as these are known to undergo facile deprotonation at the ring C(2) position, generating a reactive intermediate with two limiting canonical structures, that of a carbene or an ylide (Scheme 1). These reactive intermediates are known to catalyze a C - C bond forming reaction between aldehydes known as an "acyloin condensation." Previously characterized thiazolium salts, featuring halide anions, were relatively high melting solids. We found that by the simple replacement of iodide by tetrafluoroborate, we could isolate these salts as room-temperature liquids. More importantly, as liquids both demonstrated the capacity to dissolve a typical aldehyde, benzaldehyde, and when treated with a small amount of base, each acts as both the solvent and catalyst source for the condensation of the dissolved benzaldehyde to benzoin (Scheme 1). Frequently, the incorporation of functionality into an ion slated for use i n formulating an ionic liquid is a multi-step process. Consequently, a number of issues must be considered i n planning the synthesis of the ion. As illustrated i n Scheme 2, the first issue is the choice of cationic core. For example, salts of phosphonium cations appear to generally exhibit the greatest thermal stability, but also generally possess higher melting points than salts of other cations. Thus, i f the desired ionic liquid is to be used i n a process that is conducted at 0°C, choosing to build the cation core around a phosphonium ion might not be the most useful approach. If the ionic liquid is to be used i n a metal catalyzed reaction, the use of an imidazolium based ionic liquid might be critical, especially i n light of the possible involvement i n some such reactions of imidazolidene carbenes originating with the IL solvent.


+

cation core (imidazole, phosphine.etc.)

leaving group

functional group

metathesis salt & solvent f|

reaction solvent (if any)

anion utility?

secondary functionalization?

Scheme 2. General plan for functionalized cation synthesis.



252 The second element of general importance i n the creation of the task-specific ionic liquid is the source of the functional group that is to be incorporated. In the thiazolium based ionic liquids described earlier, this consideration was coincident with the choice of the cationic core. In our hands, however, it is more common to graft functionality onto the cation core. Doing so thus requires the chemist to identify a substrate that contains two functional groups with different reactivity, one which allows the attachment of the substrate to the core, and the other of which either is the functional group of interest or is modifiable to the group of interest. We have generally used functionalized alkyl halides i n this capacity, although the triflate esters of functionalized alcohols work as well. The choice of reaction solvent is also of concern i n the synthesis of new ionic liquids. In our hands, toluene and acetonitrile have proven to be the most versatile, our choice i n any given synthesis being dictated by the relative solubility of the starting materials. This element of the synthesis of ionic liquids is decidedly the least "green." Significantly, recent developments i n the area of the solventless synthesis of ionic liquids promise to improve this situation (9). The choice of the anion that is to ultimately be an element of the ionic liquid is of key significance. Perhaps more so than any other single factor, it appears that the anion of the ionic liquid exercises a significant degree of control over the molecular solvents (water, ether, etc.) with which the ionic liquid will form two-phase systems. For example, nitrate salts are typically water miscible and those of hexafluorophosphate are not; those of tetrafluoroborate may or may not be, depending on the nature of the cation. Certain anions such as hexafluorophosphate are subject to hydrolysis at higher temperatures, while those such as bis(trifluoromethane)sulfonate are not, but are extremely expensive. Additionally, the cation of the salt used to perform any anion metathesis is important. While salts of potassium, sodium and silver are routinely used for this purpose, we have found the use of ammonium salts i n acetone to be the most convenient A l l of these factors must be weighed when making a decision about the appropriate choice of anion for an IL formulation. As mentioned, though our first endeavor i n the preparation of a task-specific ionic liquid featured the incorporation of function at the cation core, most of our subsequent efforts have focused on the incorporation of functionality into a branch appended to the core. While a number of the task-specific ionic liquids that we have prepared have been built-up from 1-methyl and 1-butyl imidazole, many of our more recently prepared functionalized cations have been constructed upon a relatively inexpensive, commercially available starting "scaffold," l-(3-aminopropyl) imidazole. The appended amino group i n this material is a versatile reactive site that lends itself to conversion into a variety of derivative functionalities (Scheme 3).



253

Scheme 3. Representative cations derivedfroml-(S-aminopropyl) imidazo Reaction of l-(3-aminopropyl) imidazole with isocyanates and isothiocyanates gives urea and thiourea derivatives, respectively (Scheme 4). These elaborated imidazoles are then ring quaternized by reaction with alkyl iodides to produce the corresponding N(3 )-alkylimidazolium salts. However, the alkylation reactions must be conducted within relatively narrow temperature and solvent parameters to accomplish the alkylation at the imidazole nitrogen and minimize the formation of the undesired alkyl isourea or alkyl isothiourea by alkylation at C=0 or O S . Similar care must be exercised in the synthesis of IL cations with appended acetamide and formamide groups. H I

Hι S

2. NH PF , acetone 4

6

2

W

©

PF

Θ 6

s

2

Scheme 4. Synthetic plan for Hg * and Cd * binding TSIL A variation on this overall synthetic approach allows the formation of related EL - ureas by first converting l-(3-aminopropyl) imidazole into an isocyanate, followed by its reaction with an amine and alkylating agent The latter approach has been used successfully to append both amino acids and nucleic acids onto the imidazolium cation skeleton.


254 The movement of hydrated metal ions from an aqueous phase into a conventional ionic liquid second phase is decidedly unfavorable. Indeed, partition coefficients for several different metal ions i n water - [4-mim]PF biphasic systems are 1 0 or less. We reasoned that task-specific ionic liquids with appropriately chosen Lewis base appendages might be used to increase the movement of metal ions into an ionic liquid phase. Accordingly, we have used several of the urea and thiourea ionic liquids as extractants of heavy metals, especially H g and C d , from an aqueous co-phase, with dramatic results (10). Depending upon the p H of the aqueous phase, partition of these metals into a mixed system of urea or thiourea TSIL and [4-mim]PF resulted i n partition coefficients of up to 10 for movement of the metal ions into the IL phase from water. The removal of actinides from aqueous waste is a major technical challenge to be addressed i n the remediation of wastewater at nuclear processing sites such as that at Hanford, Washington. A major consideration i n the application of ionic liquids to this task are the poor distribution coefficients of the aquated metal ions into an IL phase. Consequently, ligands are added to the system to complex the metal ion and render it more soluble i n the ionic liquid. The resulting complexes may still, however, retain a high degree of water solubility. We reasoned that the approach we utilized previously to facilitate the sequestration of H g and C d from water - the anchoring of the complex into the I L phase by appending the ligand to an "IL-like" ion - might similarly prove useful i n the removal of actinides from water. 6

1


2 +

2+

6

3

2 +

2 +

Ο 11

Ο N

W

N H 2

*af Ph

—

h

Ο

W

"

II

3

^ A'Ph N

H

P

h

NH PF6/acetone 4

τ©

Ο

Etl/CH CN H

p

h

Scheme 5. Synthetic plan for actiniae binding TSIL. Among the most avidly binding ligand types for actinide ions are phosphoramides and phosphine oxides (11). The former are readily synthesized via reaction of phosphorous (V) oxyhalides and primary or secondary amines. Using just such an approach, 1 -(3 -aminopropyl) imidazole was allowed to react with commercially available (CeHs^POCk i n


255 dichloromethane. After isolation, the resulting phosphoramide was then quaternized at the imidazole N(3) position by reaction with ethyl iodide (Scheme 5). The product salt, a viscous oil, mixes readily with more conventional ionic liquids such as [6-mim]PF , yielding a more tractable material. Agitation of this liquid with a brilliant yellow aqueous solution of uranyl acetate results i n the immediate and dramatic movement of the yellow color into the ionic liquid. Similar contact between aqueous uranyl acetate and pure [6-mim]PF results i n no apparent movement of color into the I L phase. Detailed measurements of the capacity of ionic liquid mixtures incorporating the phosphoramide-appended IL to remove actinides from aqueous solution are in progress with A m , U 0 and P u . Preliminary results suggest partition coefficients for these metals with the phosphoramide TSIL are i n the same range as those found for H g and C d with the urea and thiourea IL. We have taken a different synthetic approach for the preparation of a second class of ionic liquids / EL additives designed for the sequestration of actinides. These simple, hybrid phosphonium ion - phosphine oxide species are easily prepared beginning with a commercially available starting material, 1,4bis(diphenylphosphino) butane. The bisphosphine is reacted i n a 1:1 stoichiometry with an alkylating agent such as methyl iodide or methyl triflate, producing the expected statistical distribution of mono- and diphosphonium ions, as well as unreacted starting material. Solubility differences between these products facilitate their relatively straightforward separation. After anion exchange (in the case of iodide salts), the monophosphonium s$lt is dissolved i n dichloromethane and stirred together with 30% aqueous hydrogen peroxide. Décantation of the organic layer followed by solvent evaporation leaves a white residue of phosphonium - phosphine oxide salt. Though solids at room temperature, these salts are soluble i n conventional ionic liquids. And, like solutions of phosphoramide IL and [6-mim]PF mentioned earlier (vide supra), solutions of the hybrid phosphonium-phosphine oxide ILs i n the latter also result i n the apparent partition of uranyl cation into the ionic liquid phase. 6


6

3 +

2 +

4+

2

2 +

2 +

6

Task-specific ionic liquids designed for the binding of metal ions need not be only monodentate i n nature. Taking a hint from classical coordination chemistry, we have recently prepared our first bidentate TSIL. This species is readily prepared i n a two step process. First, 1 -(3 -aminopropyl) imidazole is condensed under Dean-Stark conditions with 2-salicylaldehyde, giving the corresponding Schiff base. This species is readily alkylated i n acetonitrile to form the imidazolium salt. Mixed as the PF " salt i n a 1:1 (v/v) fashion with [6-mim]PF , this new TSIL quickly decolorizes green, aqueous solutions containing N i with which it is placed into contact, the color moving completely into the IL phase (Scheme 6). We are currently working to isolate and characterize the putative N i - TSIL complex. 6

6

2 +


256


H

2+

Scheme 6. Synthetic scheme and proposed mode of binding of Ni bidentate Schiffbase TSIL The utilization of functionality in ionic liquid formulation need not be limited to effecting the solubilization of metals in ionic liquid phases. For example, we recently developed a family of ionic liquids that contain imidazolium cations with long, appended fluorous tails. While the solubility of these species in conventional ionic liquids is rather limited (saturation concentrations of about 5 mM), the TSIL apparently formfluorousmicelles in the latter. Thus, when a conventional ionic liquid doped with the fluorous TSIL is mixed with perfluorocarbons, extremely stable emulsions can be formed (Figure 3). These may be of use in developingfluorous- ionic liquid two-phase reaction systems U2).

While the majority of our focus has concerned the modification of ionic liquid cations, we have also begun to probe the use of a rather conceptually unorthodox anion class, the sheet layers of aluminosilicate clays. Ionexchangeable clays are a class of naturally occurring materials that consist of sheet-like layers of aluminum and silicon atoms bridged and terminated by oxygen atoms and hydroxy! groups. The isomorphous replacement of sheet


by a

257 3+


metals by ions such as F e gives rise to permanent charges on the sheet structures. In the natural systems, these negative charge on these structures are offset by alkali metal or alkaline earth cations that are interspersed between the sheet structures i n areas referred to as galleries (13).

Figure 3. Micelle formation by fluorous ionic liquids. The replacement of gallery cations by alkyl ammonium and alkyl pyridinium salts is well precedented. The designed ion exchange of these cations by select organic cations has given rise to tailored clay materials that can be swelled by organic solvents. With properly chosen solvents, the clay sheets can be sufficiently separated to be considered to be exfoliated. That is, at interlamellar separations of 100 Â or so, the aluminosilicate sheets can be regarded as acting autonomously. This technology has allowed the incorporation of dispersed nanoscale mineral platelets into organic polymers, leading to significant improvements i n polymer properties (14). In addition to the one-to-one replacement of gallery cations by cationic organic species, the uptake into clay galleries of alkyl ammonium salts i n excess of the ion exchange capacity has been documented (13). This phenomenon is of considerable potential relevance to ionic liquids chemistry. Indeed, to the degree that such chemistry occurs i n an ionic liquid, the ionic liquid could be regarded as being composed in part of the mineral. In more practical terms, the environmental fate of ionic liquids is a topic of some considerable importance, but is an issue that has yet to be the subject of much study. Given that clays are a major soil component in many areas, a thorough study of clay interactions with ionic liquids is merited. Too, ionic liquid clay interactions are of potential significance since clays are both "green" and catalytic i n many types of organic reactions (15). We have recently determined that clay minerals such as bentonite and montmorillonite undergo complete cation exchange with the common ionic liquid cations [6-mim] and [4-mim] . Moreover, the ion-exchanged materials readily take up excesses of these cations when paired with P F \ In preliminary +

+

6


258


powder X-ray diffraction studies, we have observed ionic liquid induced interlayer separations as great as 60 Â. Given this capacity for the uptake of ionic liquids by clay minerals, these materials can be suggested as potential sequestering agents i n the event of ionic liquid spills. Moreover, these results suggest that the widespread dispersion of an ionic liquid into the soil at the site of a spill may be retarded i n soils with higher ion-exchangeable clay content. Further studies on this point, as well initial studies on the catalysis by clays of organic reactions i n ionic liquid solvents are i n progress.

References 1. Labadie, J. Current Opinion in Chemical Biology 1998, 2, 346-352. 2. Drewry,D.H.;CoeD.M.;Poon,S.;Med Res. Rev. 1999, 19, 97-148. 3. Y. de Miguel. J. Chem. Soc. Perkin Trans. 1, 2000, 4213. 4. Moore, J. W.; Pearson, R.G.; FrostA.A.Kinetics and Mechanism: A Study of Homogeneous Chemical Reactions, 3rd ed.; Wiley: New York, 1981. 5. Tundo, P.; Anastas, P.; Black, D. StC.; Breen, J.; Collins, T.; Memoli, S.; Miyamoto, J.; Polyakoff, M.; Tumas, W. Pure Appl. Chem. 2000, 72, 1207-1228. 6. Holbrey, J.D.; Seddon, K . R . Clean Products and Processes, 1999, 1, 223236. 7. Davis, J.H., Jr.; Wierzbicki, A . Proceedings of the Symposium on Advances in Solvent Selection and Substitution for Extraction; AIChE: New York, 2000; Paper 14F. 8. Forrester, K . J . ; Davis, J.H., Jr. Tetrahedron Letters 1999, 40, 1621-1622. 9. Rajender S. Varma, Vasudevan V . Namboodiri, Chem. Commun., 2001, 643-644. 10. Visser, A . E . ; Swatloski, R.P.; Reichert, W . M . ; Mayton, R.; Sheff, S.; Wierzbicki, Α.; Davis, J.H., Jr.; Rogers, R.D. Chem. Commun., 2001, 135136. 11. Ultrafast Chemical Separations; Committee on Nuclear and Radiochemistry, Board on Chemical Science and Technology, National Research Council: Washington, D.C., 1993; pp 114-272. 12. Merrigan, T . L . ; Bates, E . D . ; Dorman, S.C.; Davis, J.H., Jr. Chem. Commun. 2000, 2051-2052. 13. Chemistry of Clays and Clay Minerals; Newman, A . C . D . , E d . ; Minerological Society Monograph No. 6; Longman Scientific and Technical: Harlow, Essex, 1987. 14. Pinnavaia, T.J.; Lan, T.; Wang, Ζ.; Shi, Η.; Kaviratna, P . D . In Nanotechnology; Chow, G . - M . ; Gonsalves, K . E . , Eds.; A C S Symp. Series No. 622; A C S : Washington, D.C., 1995; pp 250 -261. 15. Balogh, M.; Laszlo, P. Organic Chemistry Using Clays; Springer: New York, 1993.


Working Salts: Syntheses and Uses of Ionic Liquids Containing

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