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J. Phys. Chem. B 2010, 114, 368–375
Deprotonation and Oligomerization in Photo-, Radiolytically, and Electrochemically Induced Redox Reactions in Hydrophobic Alkylalkylimidazolium Ionic Liquids Ilya A. Shkrob* Chemical Sciences and Engineering DiVision, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439 ReceiVed: September 2, 2009; ReVised Manuscript ReceiVed: September 29, 2009
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 σσ* 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. 1. Introduction Room-temperature ionic liquids (ILs) find a growing number of applications,1 including their use as extraction solvents for reprocessing of spent nuclear fuel2,3 and electrolytes in photoelectrochemical4 and electrochemical5-7 cells. Both of these applications depend on the stability of the ILs under the conditions favoring redox reactions involving the constituent ions. In particular, radiolytically, photolytically, or electrochemically induced one-electron reduction of the constituent cations causes the formation of high molecular weight products increasing solvent viscosity8 and the concomitant acidification of the IL.3 Mitigation of these detrimental changes requires mechanistic insight into such reactions. The intent of this paper is to provide such an insight and compare the mechanistic predictions with the experimental results available in the literature. The aim is to provide a unified framework that can guide the experiment. The most important implication of this examination is that the ILs consisting of alkylimidazolium cations lacking the protective 2-alkyl group are inherently unstable toward the deprotonation that occurs in the reaction intermediates and the resulting dimer/oligomer formation. That such deprotonation (leading to the formation of reactive carbenes) can be induced by the addition of strong bases to the alkylimidazolium ILs is well-known.9 We content that this deprotonation occurs even under weakly acidic conditions due to the occurrence of radical and ion radical reactions that lead to the formation of products that are closely resembling or identical to those occurring via the more familiar Nheterocyclic carbene chemistry.9 * Author to whom correspondence should be addressed. E-mail:
[email protected].
The ILs are salts composed of oddly shaped organic and inorganic ions. 1-Methyl-3-alkylalkylimidazolium (Rmim+) is a popular choice for the IL cations. Changing the length of the 3-alkyl arm allows tuning of the hydrophobicity of the IL;1-5 this tuneability and the ease of the IL synthesis account for the popularity of this class of the ILs. Rmim cations form moderately viscous liquids (
