Deprotonation and oligomerization in photo-, radiolytically, and electrochemically induced redox reactions in hydrophobic alkylalkylimidazolium ionic liquids.
Radical chemistry initiated by one-electron reduction of 1-methyl-3-alkylimidazolium cations in the corresponding ionic liquids (ILs) is examined. The reaction scheme is examined in light of the recent experimental data on photo-, radiation-, and electrochemically induced degradation of the practically important hydrophobic alkylimidazolium ILs. It is suggested that the primary species leading to the formation of the oligomers and acidification of the IL is a sigmasigma* dimer radical cation that loses a proton, yielding a neutral radical whose subsequent reactions produce C(2)-C(2) linked oligomers, both neutral and charged. The neutral oligomers (up to the tetramer) account for the features observed in the NMR spectra of cathodic liquid generated in electrolytic breakdown of the IL solvent. In photolysis and radiolysis, these neutral species and/or their radical precursors are oxidized by radical (ions) derived from the counteranions, and only charged dimers are observed. The dication dimers account for the features observed in the mass spectra of irradiated ILs. The products of these ion radical and radical reactions closely resemble those generated via carbene chemistry, without the formation of the carbene via the deprotonation of the parent cation. As the loss of 2-protons increases the proticity of the irradiated IL, it interferes with the extraction of metal ions by ionophore solutes, while the formation of the oligomers modifies solvent properties. Thus, the peculiarities of radical chemistry in the alkylimidazolium ILs have significant import for their practical applications.
Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, USA. shkrob@anl.gov
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