Nb(V) and Ta(V) Haloanions as Effective Counterions for the Room Temperature Isolation of Salts of Uncommon Organic Cations

In the framework of our recent studies on the direct interaction of MX5 (M = Nb, Ta; X = F, Cl, Br) with oxygen compounds, we have been aware that M(V) haloanions possess outstanding capability of stabilizing uncommon organic cations. Reactions involving ketones or polyethers may be accompanied by CH bond activation, thus the salts [(MePhCO)2(-H)][TaCl6], 1 (Fig. A) [1], and [diglyme(H)][NbCl6], 2 [2], have been isolated in the respective cases. Otherwise the reaction of NbCl5 with CMe2(OMe)2 proceeds with CO cleavage and leads to selective formation of the methylated mesityl oxide species [Me2C=CHC(=OMe)Me][NbCl5(OMe)], 3 (Fig. B) [3]. A series of [Nb2F11] radical cation salts of monocyclic arenes have been prepared according to unexpected redox reaction including metal reduction [4]. These salts are surprisingly long-lived at room-temperature or above, due to stabilizing cation-anion interactions; the [C6H6]+ radical has been identified for the first time by EPR as a non-transient species in solution at room temperature. The same redox pathway has been exploited for the straightforward synthesis of thermally stable [M2F11] salts of protonated 1,3-dimethoxybenzene (Fig. C) [5].