Introduction The direct observation of long-lived carbocations has represented a fascinating topic for chemists. In this context, the preparation of fairly stable radical cations by one-electron oxidation of monocyclic aromatic molecules is still a difficult task [1]. On the other hand, the protonation of monocyclic arenes is a reaction generally requiring strong acidic systems and leading to arenium salts difficult to isolate at room-temperature in the solid state [2]. Results and Discussion In the course of our studies on the noticeable chemistry of the pentahalides of niobium and tantalum [3], we have found that room-temperature long-lived radical cation salts of monocyclic arenes can be prepared according to the reaction: 3NbF5 + Arene → [Arene][Nb2F11] + NbF4 [4]. The driving force of this unexpected redox process is supplied mainly by the interaction established between the anion [Nb2F11] and the -density of the cation. X-Ray structure of very rare radical cation salt isolated in the solid state at room temperature (Figure 1) and unprecedented EPR identification of a [C6H6]•+ salt as a non-transient species in solution at room temperature (Figure 2) are shown. The redox process indicated above has been further exploited for the convenient preparation of thermally stable [M2F11] (M = Nb, Ta) salts of protonated 1,3-dimethoxybenzene (Figure 3) [5]; the protonation reaction proceeds with monoelectron oxidation of the arene followed by hydrogen transfer from the solvent (CHCl3) to the resulting radical cation. Figure 3. X-Ray structure of [2,4-(OMe)2C6H5][M2F11] (M = Nb, Ta). Conclusions In spite of the fact that the chemistry of niobium and tantalum pentafluorides was scarcely explored in the past, these compounds have recently revealed unique characteristics which permit the straightforward synthesis of ionic arene derivatives in mild conditions and their stabilization by means of the highly innocent M2F11 anions (M = Nb, Ta).

MF5 (M=Nb, Ta) as Monoelectronic Oxidants and Precursors of Innocent anions: Generation of Long-lived Radical Cations of Monocyclic Arenes and Thermally Stable Arenium Salts

MARCHETTI, FABIO;PAMPALONI, GUIDO;
2011

Abstract

Introduction The direct observation of long-lived carbocations has represented a fascinating topic for chemists. In this context, the preparation of fairly stable radical cations by one-electron oxidation of monocyclic aromatic molecules is still a difficult task [1]. On the other hand, the protonation of monocyclic arenes is a reaction generally requiring strong acidic systems and leading to arenium salts difficult to isolate at room-temperature in the solid state [2]. Results and Discussion In the course of our studies on the noticeable chemistry of the pentahalides of niobium and tantalum [3], we have found that room-temperature long-lived radical cation salts of monocyclic arenes can be prepared according to the reaction: 3NbF5 + Arene → [Arene][Nb2F11] + NbF4 [4]. The driving force of this unexpected redox process is supplied mainly by the interaction established between the anion [Nb2F11] and the -density of the cation. X-Ray structure of very rare radical cation salt isolated in the solid state at room temperature (Figure 1) and unprecedented EPR identification of a [C6H6]•+ salt as a non-transient species in solution at room temperature (Figure 2) are shown. The redox process indicated above has been further exploited for the convenient preparation of thermally stable [M2F11] (M = Nb, Ta) salts of protonated 1,3-dimethoxybenzene (Figure 3) [5]; the protonation reaction proceeds with monoelectron oxidation of the arene followed by hydrogen transfer from the solvent (CHCl3) to the resulting radical cation. Figure 3. X-Ray structure of [2,4-(OMe)2C6H5][M2F11] (M = Nb, Ta). Conclusions In spite of the fact that the chemistry of niobium and tantalum pentafluorides was scarcely explored in the past, these compounds have recently revealed unique characteristics which permit the straightforward synthesis of ionic arene derivatives in mild conditions and their stabilization by means of the highly innocent M2F11 anions (M = Nb, Ta).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/205220
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