Specific Effects of Room Temperature Ionic Liquids on Cleavage Reactivity:  Example of the Carbon−Halogen Bond Breaking in Aromatic Radical Anions

Specific solvation effects of ionic liquids have been evidenced on the chemical reactivity of radical anions with three different ionic liquids (1-butyl-3-methylimidazolium, trimethylbutylammonium, and triethylbutylammonium cations associated with the same anion (bis(trifluoromethylsulfonyl)imide). Large modifications depending on the localization of the negative charge in the radical anions and, to a less extent, on the nature of the ionic liquids cations are reported. When the charge is spread out over the entire molecule as in the 9-chloroanthracene radical anion, an acceleration of the carbon−halogen bond cleavage when passing from acetonitrile to the ionic liquid is observed. On the contrary, in the case of 4-chlorobenzophenone radical anion where the negative charge is more localized on the oxygen atom of the carbonyl group, a large decrease of the C−Cl cleavage rate occurs in relation with a positive shift of the reduction standard potentials. These effects can be explained by specific ion-pair associations between the radical anion and the cation of the ionic liquid that stabilizes the unpaired electron in the π* orbital of the aromatic system and thus decreases its presence in the σ* bond breaking. The experimental results can be rationalized using Marcus-type formalism (Savéant's model describing the dynamics of electron transfers and bond cleavage) and agree well with the calculated ion-pair stabilization energies estimated with density functional theory (B3LYP). Besides the decrease of the cleavage rate, the ion pairing favors the dimerization between two radical anions that prevails over the cleavage reaction, leading to a different mechanism.