Nucleophilicity in ionic liquids. 3. Anion effects on halide nucleophilicity in a series of 1-butyl-3-methylimidazolium ionic liquids.

We have continued the study of halide nucleophilicity in ionic liquids, concentrating on the effect of changing the anion (BF(4), PF(6), SbF(6), OTf, and N(Tf)(2)) when the cation is bmim (where bmim = 1-butyl-3-methylimidazolium). It was found that the nucleophilicities of all the halides were lower in all of the ionic liquids than in dichloromethane. Changing the anion affected the order of halide nucleophilicity, e.g., in [bmim][BF(4)] the order of nucleophilicity was Cl(-)>Br(-)>I(-) while in [bmim][N(Tf)(2)] the order was Cl(-)<Br(-)<I(-). It was also found that the nucleophilicity of each halide was different in each ionic liquid, with chloride being almost four times as nucleophilic in [bmim][BF(4)] as in [bmim][SbF(6)]. Similarly bromide was more than four times as nucleophilic in [bmim][BF(4)] as in [bmim][PF(6)]. The activation parameters DeltaG++, DeltaH++, and DeltaS++ have been measured for the reaction of chloride in each of the ionic liquids, plus the reaction of bromide in [bmim][BF(4)] and [bmim][PF(6)]. These data were also compared to each other as well as to a similar reaction in dichloromethane (where these parameters have been estimated for both the free ion and the ion-pair). These studies show that the reaction in the ionic liquids has a high activation free energy barrier, due to the solvent-solute interactions within the ionic liquids. These interactions are described and discussed.