Dimethylimidazolium tetrafluoroborate

Another means of in situ metal-carbene complex formation in an ionic liquid is the direct oxidative addition of the imidazolium cation to a metal center in a low oxidation state (see Scheme 5.2-2, route b)). Cavell and co-workers have observed oxidative addition on heating 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPli3)4 in refluxing THF [32]. The Pt-carbene complex formed can decompose by reductive elimination. Winterton et al. have also described the formation of a Pt-car-bene complex by oxidative addition of the [EMIM] cation to PtCl2 in a basic [EMIM]C1/A1C13 system (free CP ions present) under ethylene pressure [33]. The formation of a Pt-carbene complex by oxidative addition of the imidazolium cation is displayed in Scheme 5.2-4. 

Symmetry is another factor to affect Tm. The salts with symmetric ions generally show higher Tm than those with asymmetric ones. For example, 1,3-dimethylimidazolium tetrafluoroborate showed higher Tm than 1-methylimi-dazolium or l-ethyl-3-methylimidazolium salts, as shown in Figure 3.1. In the case of tetraalkylammonium salts, their Tm also increased with increasing symmetry of the cation structure [18]. This tendency is understood to relate to the structural effect on crystallinity [19], i.e., highly symmetric ions are more efficiently packed into the crystalline structure than unsymmetric ones. Other kinds of chain structures such as polyether [20], perfluorocarbon [21], etc. [22] are obviously also effective in influencing thermal properties. 

Industrially performed catalytic oxidation reactions often suffer from drawbacks such as poor conversion and selectivity due to overoxidation, corrosive reaction media, lack of solvent and catalyst recycling, and negative environmental impact due to evaporation of the solvents. In order to provide a methodology that addresses these problems, ionic liquids have been investigated as reaction media. For example, the aerobic oxidation of benzyl alcohol and alkylbenzene to benzaldehyde and benzoic acids was performed in l-butyl-2,3-dimethylimidazolium tetrafluoroborate ([C4dmim][BF ]) using palladium and cobalt complexes respectively [34, 35]. 

De Long HC, Fox DM, Gilman JW, Trulove PC. 2005. TGA decomposition kinetics of 1-butyl-2,3-dimethylimidazolium tetrafluoroborate and the thermal effects of contaminants. J Chem Thermodynamics 57 900-905. 

The nature of the alkyl substituents was found to rule the rate of cleavage for the N-C bonds [63, 64]. Methyl substitution in the 2 position (i.e., between the two N atoms) enhances the thermal stability. This may be due to the strong acidic character of the C-2 proton. It was observed that the thermal stability of imidazolium was also affected by the type of isomeric structure of the alkyl side group. This was evidenced by the observation that both l-butyl-2,3-dimethylimidazolium tetrafluoroborate and l-butyl-2,3-dimethylimidazolium hexafluorophosphate salts had higher onset decomposition temperatures than l,2-dimethyl-3-isobutylimidazolium tetrafluoroborate and l,2-dimethyl-3-isobutylimidazolium hexafluorophosphate salts. This reaction presumably proceeds via SnI, as shown in Scheme 2.8. 

This interaction between Fc and IL ions has been theoretically studied by Yang et al. [50]. According to their MEP predictions, the possible interactions of ferrocene with 1,3-dimethylimidazolium tetrafluoroborate ([Cimim][BF4], used as an IL model system) are shown in Figs. 3.5 and 3.6. In Fig. 3.5, it is possible to observe that the IL cation binds to Fc both on the top and from the side. Meanwhile, the IL anion is only able to interact with Fc from the side (Fig. 3.6). 

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