Use of KrF and PtF6 as Oxidative Fluorinating Reagents

The oxidative fluorination of NF3 with a KrF+ salt in place of elemental fluorine was first reported by Artyukhov and Khoroshev in 1977 (6). Stoichiometric amounts of NF3, KrF2, and Lewis acid (SbFs, NbF5, PF5, TiF4, or BF3) were allowed to react at room temperature. Alternatively, the preformed solid salt KrF+SbF6 was reacted with NF3 at a pressure of 1 atm. In each instance the corresponding tetrafluoro-ammonium salt resulted, though yields were not reported. The products were identified by their vibrational spectra and also by chemical analysis. Krypton was liberated in the reaction, e.g., 

The KrF+ cation is the most powerful oxidizer known, and attempts were made to fluorinate CF3NF2 by means of KrF+ to produce a salt of 

The same authors showed that platinum hexafluoride, which is a somewhat weaker oxidative fluorinating agent than KrF+, can also oxidize NF3, though the yield and purity of the NF4+ fluoroplatinate formed as a dark red solid were low. The pure salt was prepared for purposes of comparison by the thermal reaction at 125°C between NF3, F2, and PtF6. The reaction between NF3 and the hexafluoride was carried out either in HF solution at 25°C or under ultraviolet irradiation in the gas phase, also at ambient temperature. In each case the vibrational spectrum of the product showed the presence of a tetraflu-oroammonium salt, but the product was a mixture of fluoroplatinate and polyfluoroplatinate which could not be purified by extraction with liquid HF. 

Christe considers the three mechanisms set out below to be the most probable for the formation of NF4+ salts. 

The ionic oxidant (KrF+) gives an ionic mechanism, whereas the Lewis acid in association with F2 or PtF6, which are radical oxidants, results in a radical mechanism. In all the systems the one-electron (Lewis acid-F2 or PtF6) or two-electron (KrF+) oxidizer reacts with the substrate (NF3). This leads to an electron transfer to the oxidant. Either simultaneously (for KrF+) or subsequently (for Lewis acid-F2 or PtF6), the intermediate radical cation (NF3+) is fluorinated to give NF4+. 

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