Room-temperature long-lived [Nb2F11] salts of radical cations of simple arenes: EPR, UV-Vis and DFT results

The computed structures of the long-lived radical cation salts [Arene][Nb₂F₁₁] [Arene = 1,4-F₂-2,5-(OMe)₂C₆H₂, 2; 1,4-(OMe)₂C₆H₄, 3; 2,5-(OEt)₂(Me)C₆H₃, 4; C₆H₆, 5] and that of the transient [1,3-(OMe)₂C₆H₄][Nb₂F₁₁], 6, obtained for the gas-phase by DFT at the B3LYP/6-31G∗∗ level, are presented. The degree of inertness observed in chloroform solution seems to increase on decreasing the steric demand of the ring substituents, and may be correlated to the calculated distance between the cation-centroid and the niobium atoms. The room-temperature EPR spectra of 2-4, in CHCl₃, are described in detail; the spectrum of 3 is compared to those of analogous 1,4-dimethoxybenzene radical species reported previously. The EPR spectra display a hyperfine structure due to coupling of the unpaired electron with nuclei belonging to both the cation (H and F in the case of 2) and the anion (F and eventually Nb). The UV-Vis spectra of 2-4 exhibit one strong absorption attributed to the cation and one anion-to-cation charge-transfer band (e.g. for 3 at 398 and 589 nm, respectively). Thermodynamic calculations indicate that the low yield formation of the benzene radical salt 5 occurs with Gibbs free energy variation significantly higher than those involved in the synthesis of 2-4 and 6.