Dimer Radical Anions of Polyfluoroarenes. Two More to a Small Family.

While there is a body of experimental data concerning dimers formed by an aromatic molecule and its radical cation, information on the corresponding dimer radical anions (DRAs) is scarce. In this work, evidence for the formation of the DRAs of decafluorobiphenyl and 4-aminononafluorobiphenyl has been obtained by the optically detected electron paramagnetic resonance and the time-resolved magnetic field effect techniques. Theoretical investigation (DFT B3LYP-D3/6-31+G*) of these DRAs and the DRAs of octafluoronaphtalene and 1,2,4,5-tetrafluorobenzene previously detected by Werst has been undertaken to gain greater insight into the structure of the polyfluoroarene DRAs. Without substituents different from a fluorine atom, an extra electron is evenly delocalized over two fragments; the bonding interaction is π stacking. On the potential energy surfaces (PES), there are two minima of nearly equal energy corresponding to the structures of perfect and parallel displaced sandwiches. Such a PES structure is due to a conical intersection between two electronic states of different symmetry. The DRA of 4-aminononafluorobiphenyl is an ion-molecular associate stabilized by electrostatic interactions involving NH2 groups. The complex cyclic structure of the PES of this DRA suits the successive electron transfers between the dimer fragments. The calculated hyperfine coupling constants averaged over the PES minima agree well with the experimental ones.

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.

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.