Notably, crystallographic disorder of the cation is observed in all but one of these salts. Nine of the thirteen salts have been the subject of X-ray crystal structural determination. C-Alkylation of the anion can also contribute to low melting points by the introduction of a further packing inefficiency. From trends in melting points, which range from 156 ☌ down to 45 ☌, it is clear that the alkylation pattern on the imidazolium cation is the main determinant of melting point and that packing inefficiency of the cation is the intrinsic cause of low melting points. The cations and anions have been systematically varied with combinations of 1-ethyl-3-methyl- (EMIM +), 1-octyl-3-methyl- (OMIM +), 1-ethyl-2,3-dimethyl- (EDMIM +), and 1-butyl-2,3-dimethyl- (BDMIM +) imidazolium cations and CB 11H 12 -, CB 11H 6Cl 6 -, and CB 11H 6Br 6 - carborane anions to elucidate the factors which affect their melting points. A new class of low-melting N, N‘-dialkylimidazolium salts has been prepared with carborane counterions, some of the most inert and least nucleophilic anions presently known.
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