Ionic liquids structurally characterised

Hunt PA, Kirchner B, Welton T (2006) Characterising the electronic structure of ionic liquids an examination of the l-butyl-3-methylimidazolium chloride ion pair. Chem Eur J 12 ... 

Thus, analytically identified or theoretically predicted transformation products of ionic liquids should be synthesised and (eco)toxicologically characterised, and included in the evaluation of the hazard and risk potential of their parent structures. 

Our survey simply reinforces and illustrates Seddon s conclusion, from his widely quoted [606] estimate of 10 possible ionic liquids, [cat][A], that a relatively small part of ionic liquid composition space has so far been investigated. Tables 11.4 and 11. S list, respectively, the 248 anions and 473 cations found in the salts listed in Tables 11.1.11.2 and 11.3. In the more restricted list of room-tenperature ionic liquids, that is, those materials with melting temperatures of 30°C or less, 47 cations and 43 anions are found. 70 (3.5%) of the possible 2021 [A] [cat] combinations have been crystallographically characterised. Salts with mpt. < 100°C contain 397 separate cations and 205 separate anions. The 623 salts for which crystal structures are reported thus represent less than 0.8% of the possible 81,385 combinations. A further 76 cations and 43 anions are found only in salts with melting temperatures in the range 100-ca. 110°C (and for which crystal structures are available), representing a surprisingly... 

Of associated relevance are investigations that show that the imidazolium cation of the ionic liquid may be reactive towards dissolved transition metal catalysts, with the isolation and characterisation of a carbene-platinum complex [642] (CSD refcode TETVOC), mpt. 193°C, from one such system This provides a link to the extensive literature [643-645] on imidazolylidene carbenes and related metal imidazolylidene complexes. More recendy, the crystal structures of [Ag(l-ethyl-3-methylimidazol-2-ylidene)2][B(CN)4] (mpt. 100-104°C) [464] and the lower-melting [Ag(l-ethyl-3-methylimidazol-2-ylidene)2][N(S02CF3)2]... 

Structurally characterised ionic liquids with chiral anions are less common. Examples include [HC2im][d-tartrate] (mpt. 41-43°C L7 l 462 kJ mor ) [130], [Hmim][d-tartrate] (mpt. 98-101°C UL 470 kJmol ) [130, 452] and [1-methyl-3- (2S)-2-methylbutyl imidazolium] [bis (2S)-2-o -3-methylbutanoato-0,0 borate] IFig. 11.141 (mpt. 89°C) [373]. The absolute configurations of the anion O-ethyl ethylphosphonothioate was determined from the... 

There are, currendy, no room-temperature ionic liquids of the type [cat][A]2, though this probably is a consequence of the absence of appropriate structural characterisation. The... 

Aspects of the structure that focus on the cation are considered in Sections 11.17 (pyridinium) and 11.18 (imidazolium). The focus of this section is on the anion. Many of the studies on [N(S02CF3)2] ionic liquids underline the point that the anion is characterised by extensive charge delocalisation that reduces specific interionic interactions, along with conformational flexibility and lability, usually characterised by a C—S... S—C torsion angle between planes... 

Quaternary ammonium salts containing 3-, 4-, 5-, 6- and 7-membered saturated rings have been structurally characterised, some of which are ionic liquids. 

It is probable that a very early preparation of an ionic liquid (if not its characterisation) occurred on the isolation of the so-called red oiT in Friedel-Crafts reactions [736]. While the isolation and structural characterisation of carbocations and other reactive cations from strongly acidic media are an extensive and important area of study, only those materials that are stable at ambient temperatures and which have a reported mpt. < 100°C are listed in Table 11.24. These include [CgMey]-" [375, 414, 415], [C6Me6CH2Cl]+ [393, 394] and [CgMegPh]-"... 

The boronium ionic liquid, [BH2(L )(L")][NTf2] (L = 3-methylimidazole, L" = trimethylamine), melting at 60°C, has been structurally characterised [193], revealing only weak cation-anion interactions and the anion in the low-energy trans conformation. The structure of the [BH2(CN)2] salt of [bis(l-methyl-lJT-imidazole-3-yl)dihydroboronium] ... 

Ionic liquids have been characterised crystallographically that contain anions and cations whose central atom can be drawn from each group of the periodic table other than from Groups 2 and 18. The structures of an increasing number of ionic liquid coordination conplexes are now appearing, though the palette of ligands used is currently limited. 

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