Reaction Kinetics in Ionic Liquids as Studied by Pulse Radiolysis:  Redox Reactions in the Solvents Methyltributylammonium Bis(trifluoromethylsulfonyl)imide and N-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 (R4NNTf2), N-butylpyridinium tetrafluoroborate (BuPyBF4), and N-butyl-4-methylpyridinium hexafluorophosphate (BuPicPF6) leads to formation of solvated electrons and organic radicals. In R4NNTf2 the solvated electrons do not react rapidly with the solvent and were reacted with several solutes, including CCl4, 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 R4NNTf2 (k = 2 × 107 L mol-1 s-1) is much lower than that measured in water (k = 2 × 109 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 BuPyBF4 and BuPicPF6 to several compounds (k of the order of 108 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 BuPyBF4 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 CCl4 and O2 leads to formation of CCl3O2 • radicals, which oxidize chlorpromazine (ClPz) with rate constants near 1 × 107 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.

Systematic Dielectric and NMR Study of the Ionic Liquid 1‐Alkyl‐3‐Methyl Imidazolium

“…Remaining unreacted alkyl bromide was evaporated from the resultant solutions. When preparing C n MIMBF 4 , [45,46] C n MIMBr was mixed with > 10 % excess NH 4 BF 4 in acetonitrile at r.t. and stirred for at least 4 days. The white precipitates of unreacted NH 4 BF 4 and NH 4 Br (a byproduct) were filtered and acetonitrile was completely removed from the resultant liquids.…”

Molar absorption coefficient and radiolytic yield of solvated electrons in diethylmethyl(2-methoxy)ammonium bis (trifluoromethanesulfonyl)imide ionic liquid

“…Several pulse radiolysis studies on ionic liquids (Behar et al, 2001;Behar et al, 2002;Grodkowski and Neta, 2002a-c;Wishart and Neta, 2003;Grodkowski et al, 2003;Wishart et al, 2005;Kato, et al, 2007;Takahashi et al, 2007;Kimura et al, 2007) have been reported. The researches indicate that, for the imidazolium ionic liquids, dry electrons produced by ionization before full solvation are rapidly scavenged by the solvent molecules.…”

Ionic Manganese Porphyrins with S-containing Counter Anions: Mimicking Cytochrome P450 Activity for Alkene Epoxidation

“…As to the chalcones (Entries 1-3, 6-8) with the internal C=C bonds, the epoxides were the only products, implicating the facial access of the substrate to oxomanganese(V) porphyrin domain [31]. And due to the bulky hindrance, the conversions of the chalcones were relatively lower due to the unfavorable accessibility of C=C site to the manganese porphrin center.…”