Ionic Liquids as Green Solvents - ACS Publications - American

Chapter 9

Commercially Available Salts as Building Blocks for New Ionic Liquids

Downloaded by COLUMBIA UNIV on July 26, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch009

James H . Davis, Jr.

1,2,*

and Phillip A . F o x

1

1

Department of Chemistry, University of South Alabama, Mobile, AL 36688 The Center for Green Manufacturing, The University of Alabama, Tuscaloosa,AL35487 2

Imidazolium salts are by far the most widely used ionic liquids, and several of them are now commercially available. Nevertheless, there are a large number of other commercially available salts that have low melting points and that are of potential use as ionic liquids. In this article, we offer some suggestions as to possible avenues for their exploitation, and provide a tabulation of over one hundred of these salts.

Ask a dozen researchers working in the area to provide a definition for the term "ionic liquid" and you may get a dozen different answers. While virtually all will concur that the liquid must be composed wholly of ions, the temperature at which the material must become a liquid is less universally agreed upon. There is widespread agreement that any salt that melts at "room-temperature" or below meets the definition. There remains, however, some debate as to what temperature should be considered as the upper melting point limit for a salt to be described as an ionic liquid. Upper melting point limits of 100°C or 150°C are commonly favored. Advocates of the former point to the boiling point of water

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© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


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as being a universal thermal benchmark, while those favoring the latter do so because completely inorganic salts with melting points below that value are virtually unknown. Discussions of the issue can quickly take on a "how many angels can dance on the head of a pin" flavor. So why all the fuss over a seemingly esoteric class of salts? It's because these unorthodox liquids have captured the imaginations of a large and still growing number of researchers, a fact manifested by the rapidly increasing number of papers published on the topic. (1) The interest in these salts appears to derive mostly from three factors. First, because ionic liquids usually have negligible vapor pressure, and since evaporative loss is a major mode of discharge of molecular solvents into the environment, the use of select ionic liquids as solvents may offer a degree of environmental benignity to industrial processes in which they are utilized. Secondly, a growing number of studies report that when used in place of molecular solvents in certain processes, ionic liquids provide improvements in product yields, selectivities and ease of product/solvent separation. Finally, these materials are wholly unconventional. By virtue of our training, most of us were simply not exposed to the notion that salts could be liquids under anything but rarified conditions. Hence, the prospect of working with materials that are so radically different from those to which we are accustomed is intriguing. Probably the most widely used ionic liquids are l-butyl-3-methyl imidazolium hexafluorophosphate, [bmim]PF , and a handful of close analogs with different n-alkyl chain lengths. To prove the point, consider the following figures. Of papers accepted or published by Elsevier imprint journals between January and May of 2002 dealing topically with ionic liquids, fourteen of twenty-one studies used [bmim]PF . A survey of papers published in or accepted for publication by ACS journals for the same period showed that of nineteen studies topically concerning ionic liquids, six used [bmim]PF . Figures from RSC journals are comparable. Much of the focus on this salt is doubtless driven by its demonstrated utility in earlier studies - a case of success begetting success. Too, this ionic liquid is one of the most frequently mentioned in trade magazine (e.g., Chemical & Engineering News, etc.) articles on ionic liquids; Anecdotally, such articles are the first exposures to the subject that many of us have had. Finally, and perhaps most significantly, using this and related salts for studies of known reactions is easy. Members of the imidazolium family of ionic liquid salts are readily prepared, and several of them (including fbmimJPFô) are now commercially available. 6

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There is great value in this ongoing, wide-ranging survey of reactions in [bmim]PF and its congeners. These studies provide important insights into the comparative behavior of a more or less standard ionic liquid versus conventional molecular solvents. Such a survey will doubtless lead to discoveries that completely change the standard way in which we do one reaction or another. 6

In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


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Still, two irksome questions persist: is this small family of ionic liquids the only one of possible utility as solvents and are these the only ionic liquids that are readily available for experimentation with as solvents? We maintain that the answer to both questions is simple: No. Due to the roundabout way in which we came to be involved in ionic liquid research, our focus from the outset has been on developing new ionic liquids that are not simple Ν,Ν'-dialkyl imidazolium salts. While many of the ionic liquids that we have developed have imidazolium rings as their locus of positive charge, our salts invariably feature somewhere within their structures a functional group such as an amine or sulfonic acid group. In other words, our interest is in the development of functionalized ionic liquids, capable of acting as not only solvents but also as reagents or catalysts for particular processes. We refer to these species as "task-specific" ionic liquids and we have had some gratifying successes in their development. Among other things, we have developed amine appended ionic liquids that can scrub C 0 from gas streams, sulfonic acid appended ionic liquids that can catalyze an array of organic reactions, thiazolium ion based ionic liquids that solvate and catalyze the benzoin condensation, thiourea appended ionic liquids that can pull dissolved metal ions from aqueous co-phases, and fluorous ILs that function as surfactants. (2, 3, 4, 5, 6) Still, all of these projects involve the ground-up design and synthesis of the ionic liquids for the tasks at hand. About a year ago, we noted that a compound of interest to us as a starting material for introducing a functional group into a new ionic liquid chloroacetamidine hydrochloride - was not only commercially available but might possibly be categorizable as an ionic liquid itself, having a reported mp = 95°-98°C. Intrigued by this discovery, we set out to survey retail chemical catalogs item by item for other salts with low melting points. Most of these compounds are species pairing organic cations and small, hard inorganic anions, commonly halides. Given that it is commonly observed in imidazolium salts that the exchange of halide for anions like PF " or BF " gives rise to salts with lower melting points, these commercial species may constitute valuable platforms for the creation of new ionic liquids. This general principal of ionic liquid synthesis through the pairing of known ions has been previously articulated by Seddon, and has already been exploited by his group. (7) Nevertheless, fewer groups than might be expected have yet to explore the possibilities, and it is the purpose of this paper to renew the call to exploration, and to point out some pathways leading into the shadowy wood. In our survey, we looked at every listing in the most recently published catalogs from Aldrich, Lancaster, and Fluka, three large fine chemicals retail outlets. We also requested and received lists of low melting quaternary ammonium and phosphonium salts from two specialty manufacturers, Sachem and Cytec, which they generously supplied. The latter are highly useful ionic 2

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liquids that have been around for ages but that have been largely overlooked in the current frenzy of ionic liquid research. From these sources we compiled a list of commercially available salts (and some salt hydrates) that have a melting point of 150°C or less, where the listed melting point is not noted as being a decomposition temperature. Because our objective is to encourage the development of new ionic liquids, we have omitted most commercially available imidazolium based species from the list. The results of our survey - a list of over 100 compounds - are compiled in Table I. Certain general comments regarding the salts on our list bear making. First, their commercial availability is due to their existing use in some other type of application. For example, l,8-diazabicyelo[5.4.0]undec-7-ene hydrotribromide (DBU-HBr , mp 119 - 122°C) is a superb reagent for high-yield aromatic brominations. (8) Naturally, the pre-existing applications point to potential uses as IL for the compounds themselves or salts related to or derived from them. Second, salts on the list or their derivatives need not necessarily be used in a single cation type - single anion type form. Ionic liquids composed of a single cation but incorporating two different anions have recently been reported. (9) In addition, combinations of certain higher melting salts can give rise to interesting ionic liquid eutectics. Davies has recently demonstrated that combinations of acetylcholine chloride and select inorganic salts form eutectics with melting points as low as room temperature. (10) These eutectics constitute interesting new ionic liquids with built-in, water-stable Lewis acidic character. Many of the available salts on the list are biomolecular in nature. Examples include L-alanine ethyl ester hydrochloride (mp 78°C) and L-serine methyl ester hydrochloride (106°C). Like many other lower-melting salts, these are probably not ionic liquids in a strict sense. In water, they manifest equilibrium concentrations of neutral species, suggesting that their melts may also not be composed only of ions. Still, as melts in contact with a secondary, low-polarity organic phase, it is doubtful that any neutral melt component would partition into the latter, allowing the melt to function in a fashion similar to a true ionic liquid. Regardless, many of the ions in these low-melting, commercially available salts are potentially versatile skeletons that suggest themselves as starting points for modification into IL with built-in functional groups. (11) The intrinsic utility of any of the salts in Table 1 as ionic liquids or ionic liquid precursors is dictated by nature. However, the unmasking of that intrinsic utility is up to the research community. For those willing to take the risks that accompany the process of exploration, the potential rewards are high. Now, without further commentary, we urge the reader to peruse the list and to imagine the possibilities! 3


104 Table I. Commercially Available Low Melting Salts


Compound Name acetylcholine chloride acetyl choline bromide acetyl-P-methylcholine bromide [2-(acryloyloxy)ethyl](4-benzoylbenzyl) dimethylammonium DL-alanine ethyl ester hydrochloride L-alanine ethyl ester hydrochloride β-alanine ethyl ester hydrochloride D-alanine methyl ester hydrochloride L-alanine methyl ester hydrochloride trcaprylylmethylammonium chloride [Aliquat 336] O-allyl-N-benzylcinehodinium bromide aluminum potassium sulfate dodecahydrate aminoacetonitrile bisulfate aminoethanethiol hydrochloride [cysteamine hydrochloride] 2-aminoethyl methacrylate hydrochloride [90%] aminoguanidine nitrate 1-aminopyrrolidine hydrochloride 5-aminovaleric acid hydrochloride ammonium formate ammonium hydrogensulfate ammonium sulfamate ammonium trifluoroacetate DL-arginine hydrochloride monohydrate L-aspartic acid dimethyl ester hydrochloride benzamidine hydrochloride hydrate Ν-α-benzoyl-L-arginine ethyl ester hydrochloride N-benzoyl-L-threonine methyl ester benzylcetyldimethylammonium chloride monohydrate benzyldimethylstearylammonium chloride monohydrate benzyldimethyltetradecylammonium chloride dihydrate 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride N-benzylhydroxylamine hydrochloride benzyltributylammonium iodide benzyltrimethylammonium dichloroiodate benzyltrimethylammonium tribromide N,N,N ,N -bis(pentamethylene)chloroformamidiniumPF6 2-bromopyridine N-oxide hydrobromide ,

,

melting point (Celsius) 147-149 144-146 147-149 118-123 87-88 78-80 70-72 108-110 109-111 below RT 140-144 92 123-125 66-68 102-110 145-147 117-119 95-97 119-121 121-145 131-135 123-125 128-130 115-117 86-88 127-131 97-99 62-64 67-69 63-65 144-146 108-110 143-145 126-128 99-101 120-122 145-147


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Table I. Continued

Compound Name 2-bromopyridine N-oxide hydrochloride calcium hypochlorite 2-chloroethylamine hydrochloride 2-(2-chloroethyl)- l-methylpyrrolidine hydrochloride cresyl violet acetate 1 -cyclohexy l-3-(2-morpholinoethy l)carbodiimide pTS A cyclopropyldiphenylsulfonium tetrafluoroborate decyltriphenylphosphonium bromide 1,8-diazabicyclo[5.4.0]undec-7-ene hydrotribromide 3,5-dichloro-l-fluoropyridinium triflate (85%) 2-(2,4-dichIorophenoxy)aniline hydrochloride didecyldimethylammonium bromide 1,3-didecy 1-2-methy limidazolium chloride [ 1,4-dihydro-1 -(trifluoroacety l)Py ]dimethy lammonium TFA dimethylamine hydrobromide (dimethylaminomethylene)dimethylammonium chloride l-[3-(dimethylamino)propyl]-3-ethylcarbodiimide 4-(dimethylamino)pyridinium tribromide 1,3-dimethyl-2-fluoropyridinium 4-toluenesulfonate 3,4-dimethyl-5-(2-hydroxyethyl)thiazolium iodide Ν,Ο-dimethylhydroxylamine hydrochloride Ν,Ν-dimethylmethylene ammonium chloride (90%) dimethylphenylsulfonium fluoroborate dodecylisothiouronium chloride 1 -dodecyIpyridinium chloride hydrate dodecyltriphenylphosphonium bromide (dodecyldimethyl-2-phenoxyethyl)ammonium bromide l-heptyl-4-(4-pyridyl)pyridinium bromide hexyltriphenylphosphonium iodide L-cysteine ethyl ester hydrochloride [N-methylbis(2-chloroethyl)amine hydrochloride 4-methoxybenzenediazonium tetrafluoroborate 1 -methoxy-4-phenylpyridinium tetrafluoroborate N-methylhydroxylamine hydrochloride 2-methylthio-2-imidazoline hydroiodide methyltriphenoxyphosphonium iodide

melting point (Celsius) 131-134 100 143-146 101-104 140-143 113-115 136-138 90 119-122 107-112 96-98 149-151 82 102-103 126-128 130-139 111-113 131-133 111-114 85-97 112-115 146-148 78-80 127-130 66-70 85-88 117-119 125-128 131-133 123-125 108-110 142-144 90-95 86-88 144-146 142-146

Continued on next page. In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Table I. Continued.

Compound Name R-(-)-3-hydroxy-a-(methylaminomethyI)benzyl alcohol HC1 phenyltrimethylammonium tribromide 2-(chloromethyl)pyridine hydrochloride 3-(chloromethyl)pyridine hydrochloride 2-propylisoquinolinium bromide (R)-(-)-3-pyrrolidinol hydrochloride sarcosine tert-butyl ester hydrochloride sarcosine ethyl ester hydrochloride L-serine ethyl ester hydrochloride D-serine methyl ester hydrochloride DL-serine methyl ester hydrochloride L-serine methyl ester hydrochloride serinol hydrochloride (-)-sparteine sulfate pentahydrate tetrabutylammonium borohydride tetrabutylammonium bromide tetrabutylammonium chloride hydrate tetrabutylammonium iodide tetrabutylammonium tribromide tetraheptylammonium bromide tetraheptylammonium chloride tetrahexadecylammonium bromide tetrahexylammonium bromide tetrahexylammonium chloride tetrahexylammonium hydrogensulfate tetrakis(acetonitrile)silver(I) tetrafluoroborate tetraoctadecylammonium bromide tetraoctylammonium bromide tetraoctylphosphonium bromide tetrapentylammonium bromide trihexyl(tetradecyl)phosphonium chloride trihexyl(tetradecyl)phosphonium tetrafluoroborate trihexyl(tetradecyl)phosphonium hexafluorophosphate trihexyl(tetradecyl)phosphonium decanoate trihexyl(tetradecyl)phosphonium dicyanamide trihexyl(tetradecyl)phosphonium bromide tri-iso-butyl(methyl)phosphonium tosylate

melting point (Celsius) 143-145 114-116 125-129 137-143 143-146 104-104 137-141 127-128 130-132 163-166 134-136 163 (dec) 106-108 133-140 124-128 102-106 41-44 144-146 74-76 89-91 38-40 99-101 99-100 111-113 100-102 72-75 103-105 95-98 38-43 100-101 -68 26-28 29-31 -8 -18

Ionic Liquids as Green Solvents - ACS Publications - American

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