Reaction Kinetics in Ionic Liquids as Studied by Pulse Radiolysis:  Redox Reactions in the Solvents Methyltributylammonium Bis(trifluoromethylsulfonyl)imide and <i>N</i>-Butylpyridinium Tetrafluoroborate

Abstract

Rate constants for several reduction and oxidation reactions were determined by pulse radiolysis in three ionic liquids and compared with rate constants in other solvents. Radiolysis of the ionic liquids methyltributylammonium bis(trifluoromethylsulfonyl)imide (R 4 NNTf 2 ), N -butylpyridinium tetrafluoroborate (BuPyBF 4 ), and N -butyl-4-methylpyridinium hexafluorophosphate (BuPicPF 6 ) leads to formation of solvated electrons and organic radicals. In R 4 NNTf 2 the solvated electrons do not react rapidly with the solvent and were reacted with several solutes, including CCl 4, benzophenone, and quinones. In the pyridinium ionic liquids the solvated electrons react with the pyridinium moiety to produce a pyridinyl radical, which, in turn, can transfer an electron to various acceptors. The rate constant for reduction of duroquinone by the benzophenone ketyl radical in R 4 NNTf 2 ( k = 2 × 10 7 L mol -1 s -1 ) is much lower than that measured in water ( k = 2 × 10 9 L mol -1 s -1 ) due to the high viscosity of the ionic liquid. Rate constants for electron transfer from the solvent-derived butylpyridinyl radicals in BuPyBF 4 and BuPicPF 6 to several compounds ( k of the order of 10 8 L mol -1 s -1 ) also are lower than those measured in water and in 2-PrOH but are significantly higher than the diffusion-controlled rate constants estimated from the viscosity, suggesting an electron hopping mechanism through solvent cations. Electron transfer between methyl viologen and quinones takes place 3 or 4 orders of magnitude more slowly in BuPyBF 4 than in water or 2-PrOH and the direction of the electron transfer is solvent dependent. The driving force reverses direction on going from water to 2-PrOH and is intermediate in the ionic liquid. Radiolysis of ionic liquid solutions containing CCl 4 and O 2 leads to formation of CCl 3 O 2 • radicals, which oxidize chlorpromazine (ClPz) with rate constants near 1 × 10 7 L mol -1 s -1, i.e., much lower than in aqueous solutions and close to rate constants in alcohols. On the other hand, the experimental rate constants in the ionic liquids and in water are close to the respective diffusion-controlled limits while the values in alcohols are much slower than diffusion-controlled.