MF5 (M=Nb, Ta) as Precursors of Highly Innocent Anions: Synthesis of Long-Lived Monocyclic Arene Radical Salts and Thermally Stable Protonated Arene Salts

In the course of our recent studies on the chemistry of the pentahalides of niobium and tantalum [1], we have found that Nb(V) and Ta(V) haloanions are capable of providing outstanding inertness to uncommon organic cations, e.g. protonated/alkylated ketons [2]. Room-temperature inert radical cation salts of monocyclic arenes can be prepared according to the reaction: 3NbF5 + Arene → [Arene][Nb2F11] + NbF4 [3]. X-ray, EPR, UV-Vis and DFT analyses on [Arene][Nb2F11] have evidenced cation-anion interactions in solution, giving rise to the surprisingly long life of the radical species. Low steric hindrance at the cation favours interaction with the anion, enhancing the kinetic inertness of the salt; on account of this, the [C6H6]+ radical could be identified for the first time by EPR as a non-transient species in solution at room temperature. Furthermore, thermally stable [M2F11] (M = Nb, Ta) salts of protonated 1,3-dimethoxybenzene have been synthesized by straightforward reaction which does not require the use of a strong protonating agent [4]: initial formation of [1,3-(OMe)2C6H4][M2F11] (and MF4) is followed by hydrogen transfer from the reaction medium to the arene radical. The process may be viewed also as a convenient entry into the chemistry of MF4 (M = Nb, Ta); the first evidence for a TaF4 derivative has been supplied by EPR