Structure and Stability of Pentafluoroaniline and 4-Aminononafluorobiphenyl Radical Anions: Optically Detected Electron Paramagnetic Resonance, Time-Resolved Fluorescence, Time-Resolved Magnetic Field Effect, and Quantum Chemical Study.

Abstract

Radical anions (RAs) are the key intermediates of the selective hydrodefluorination of polyfluoroarenes. We used the techniques of optically detected electron paramagnetic resonance (OD EPR), time-resolved fluorescence, time-resolved magnetic field effect (TR MFE), and the density functional theory to study the possibility of RAs formation from 4-aminononafluorobiphenyl (1) and pentafluoroaniline (2) and estimate their lifetimes and decay channels. To our knowledge, both RAs have not been detected earlier. We have registered the OD EPR spectrum for relatively stable in nonpolar solutions 1(-•) but failed to register the spectra for 2(-•). However, we have managed to fix the 2(-•) by the TR MFE method and obtained its hyperfine coupling constants. The lifetime of 2(-•) was found to be only a few nanoseconds. The activation energy of its decay was estimated to be 3.6 ± 0.3 kcal/mol. According to the calculation results, the short lifetime of 2(-•) is due to the RA fast fragmentation with the F(-) elimination from ortho-position to the amine group. The calculated energy barrier, 3.2 kcal/mol, is close to the experimental value. The fragmentation of 2(-•) in a nonpolar solvent is possible due to the stabilization of the incipient F(-) anion by the binding with the amine group proton.