Evaluation of Donor Property of 1-Butyl-1-Methylpyrrolidinium Bis(Trifluoromethylsulfonyl)Amide Ionic Liquid Based on Electrode Potentials of Metal Redox Couples

The formal potentials for Ag(I)/Ag, Pb(II)/Pb, Sn(II)/Sn, Fe(III)/Fe(II) and [Fe(bpy)(3)](3+)/[Fe (bpy)(3)](2+) (bpy = 2,2'-bipyridine) were investigated in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) ionic liquid at 25 degrees C. The formal potentials and thermodynamic properties of metal redox couples were evaluated on the basis of the potential of ferrocene/ferrocenium couple in various electrolytes including organic solvents and ionic liquids. The formal potentials for Ag(I)/Ag, Pb(II)/Pb and Sn(II)/Sn are more positive in BMPTFSA than in conventional organic solvents, resulting in the positive Gibbs energy of transfer for metal ion from the organic solvents into BMPTFSA. The donor number of BMPTFSA was estimated from the correlation with the formal potentials of metal redox couples. The estimated donor number of BMPTFSA is consistent with the literature values based on other methods, such as the lanthanide redox potentials and the chemical shift of Na-23 NMR. The donor ability of TFSA(-)-type ionic liquids was located between nitromethane and acetonitrile, suggesting that the interaction between metal ion and TFSA(-) is weak as compared with other media.