Novel synthesis of ClF6+ and BrF6+ salts

For a compound in a given oxidation state, its oxidizing strength increases from its anion to the neutral parent molecule to its cation. Similarly, an anion is more easily oxidized than its neutral parent molecule, which in turn is more easily oxidized than its cation. This concept was systematically exploited in our search for new superoxidizers. Transition metal fluoride anions were prepared in their highest known oxidation states by high temperature/high pressure fluorinations with elemental fluorine and subsequently converted to their more strongly oxidizing cations by a displacement reaction with a strong Lewis acid. The application of this principle resulted in new syntheses for ClF6+AsF6- and BrF6+AsF6- using the highly reactive and thermally unstable NiF3+ cation that was prepared from the reaction of the NiF62- anion with AsF5 in anhydrous HF. Attempts to prepare the known KrF+ and ClO2F2+ cations and the yet unknown XeF7+ cation by the same method were unsuccessful. The results from this and previous studies show that NiF3+ is a stronger oxidative fluorinator than PtF6, but whether its oxidizing strength exceeds that of KrF+ remains unclear. Its failure to oxidize Kr to KrF+ might have been due to unfavorable reaction conditions. Its failure to oxidize ClO2F to ClO2F2+, in spite of its favorable oxidizer strength, is attributed to the high Lewis basicity of ClO2F which results in a rapid displacement reaction of NiF3+ by ClO2F, thus generating the weaker oxidizer NiF4 and the more difficult to oxidize substrate ClO2+. Therefore, the general applicability of this approach appears to be limited to substrates that exhibit a weaker Lewis basicity than the neutral transition metal parent molecule. Compared to KrF+- or PtF6-based oxidations, the NiF3+ system offers the advantages of commercially available starting materials and higher yields, but product purification can be more difficult and tedious than for KrF+.