Xenon difluoride cations from

Radical cations derived from Mrfluoro aromatic molecules were first described by Bazhin et al. > The species were made in super acid or oleum solutions and were unambiguously characterized by ESR spectroscopy. Such species have also been proposed as interm tates in the oxidative fluorination of aromatic molecules by high-valent metal fluorides such as cobalt trtfluoride, by bromine trifluoride, and by xenon difluoride. They have also been invoked in the electrochemical fluorination of aromatic molecules in the presence of F. Salts of such radical cation species were unknown prior to the synthesis of C6F6 AsF6 in these laboratories. ... 

The effectiveness of xenon difluoride as an oxidative fluorinator in the presence of fluoride ion acceptors is probably due to the formation of [XeF]+ or [XejFj]. The electron affinity of the cation, E[XeF+fg)] is estimated from the following enthalpy cycle ... 

The formulation of complexes as salt-like species containing well-defined cations or anions should be approached with a certain amount of caution. This is particularly well illustrated with xenon difluoride adducts which span the gamut of complexes from salt-like species such as [XeF]+[Sb2Fn] to covalent adducts like XeF2 XeOF4, In the latter the components preserve their molecular identities and dimensions and the adduct is clearly a covalent adduct, but even in the former the relative short Xe F distance between the [XeF]+ cation and the [Sb2Fu] anion (2.34 A) implies considerable covalent character. 

If the [Xe2F3]+ cation is. considered in terms of an assemblage of two [XeF]+ and one F ions, each [XeF]+ species receives electron density from the associated F and the assembly becomes a multicentre bonded system with a lower overall electron affinity than that of [XeF]+ 78). The measured fluoride ion donor ability of xenon difluoride, (AH0 = 9.45 eV)14), is greater than that of XeF4 but less than that of XeF6 (see Section 2a, p. 65). The increase in Xe—F bond energy in cation formation [195.9 kJ mol-1 in [XeF]+ c.f. 135.4 kJmol in XeF2, 4)] contributes to... 

The mechanism for the fluorination of benzene and its derivatives with xenon difluoride in the presence of hydrogen fluoride has been discussed independently in more detail, but similar conclusions about the participation of radical-cations in the early stages of the reaction have been reached. However, the effects of added chlorine and of hydrogen chloride on fluori-nations by this method indicate that free radical-cations are involved only in the formation of biaiyls, the halogenobenzenes themselves being formed directly from a complex of the substrate and fluorinating agent. 

Radical-cations may also be involved in the formation of phenyl esters from aromatic carboxylic acids and benzene in the presence of xenon difluoride, and can apparently be detected by e.s.r. spectroscopy during the reaction of iViV-dimethylaniline with perfluoropiperidine. The last reagent gives a low yield of o- and p-fluorophenol in the ratio 3.5 1 in its reaction with sodium phenoxide, and the predominance of ortho substitution is ascribed to a cage mechanism of the type shown in (3). However, towards 2,4,6-tri-t-butylphenoxyl radicals the piperidine acts as a source of fluorine atoms to give the dienone (4). ... 

One electron oxidation of silyl enol ether 44 leads to the formation of a radical ion pair of 45 and the xenon difluoride radical anion. Subsequent transfer of a fluoride radical yields cation 46, which reacts by loss of the trimethylsilyl cation to yield the fluoroketone 47. The formation of ketone 49 is explained by a [1,5] -hydrogen migration from the trimethylsilyl group to the radical cationic moiety of 45, leading to the formation of 48. 

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