xenon difluoride forms a dative bond (FXeF + A . FXe.+---F.A..-). The polarised xenon difluoride may then react via an electrophilic mechanism in which the electrophile can be formally regarded as an [FXe+] equivalent. This proposal is consistent with borosilicate glass, such as Pyrex®, containing 13% B2O3 and 2% Al2O3.
Even in a Pyrex® flask the reactions shown in Equation 1, and other reactions, are completely inhibited if the solvent is acetonitrile. Since nitriles are weak Lewis bases, we believe that this solvent occupies all the acidic sites on the glass surface and prevents catalysis. Under these conditions, or in quartz or FEP flasks, xenon difluoride remains covalent/unpolarised and appears to react via a single electron transfer (SET) mechanism (M + XeF2 . M•+ + XeF2•- . XeF• + F-)5,10 to give products via radical intermediates. These mechanistic conclusions are supported by our studies of reactions of other substrates in which the product composition is determined by the combination of solvent and vessel. These include studies of TMS benzoates,4 enol ethers6 and carboxylic acids.8
With the intention of supporting our preparative and mechanistic studies by identifying the species present in solution under organic reaction conditions, we have determined the 19F NMR and UV spectra of xenon difluoride in various solvent/cell systems. In this paper we report the results and demonstrate their relevance to (i) interpreting reaction mechanisms of xenon difluoride with organic substrates and (ii) choice of suitable reaction conditions. An examination of the literature reveals that this type of spectroscopic analysis in organic solvents is limited. The choice of NMR solvents for inorganic studies of xenon species has been briefly discussed.11 We have previously reported an NMR study of the decomposition of XeF2 in chloroform under various conditions,7 and results for this solvent are not included here except for direct comparison with those for dichloromethane.
Results and Discussion
19F NMR Spectroscopy
Several groups have previously studied aspects of the 19F NMR spectrum of XeF2 using the following systems: aqueous HF/Teflon12 or Kel-F,13 MeCN/glass,14-16 SO2ClF/glass,11 BrF5/glass,11 HF/FEP,11 hydrohalogenocarbons/FEP 17 and solid XeF2/quartz.18 However, the influence of vessel surface on stability and decomposition over time has not previously been studied and discussed. We have studied XeF2 stability in four discrete solvent groups: (a) CH2Cl2, CHCl3 and CFCl3; (b) CH3CN and CD3CN; (c) H2O, D2O and D2O/CD3CN; (d) C6F6. For each group we have studied stability in (i) Pyrex® tubes containing a PTFE-FEP liner and (ii) Pyrex® and quartz tubes (including tubes pre-washed with alkali (aq. NaOH). The 19F NMR spectrum of unionised XeF2 is characterised by both a singlet and a doublet. The latter arises from coupling of fluorine with the isotope 129Xe (JF-Xe ca. 5600 Hz), which occurs in 26.4% natural abundance, and the 19F signal therefore appears as a pseudo-triplet (. = ca - 175).
(a) CH2Cl2, CHCl3 and CFCl3
(i) PTFE-FEP. Under PTFE-FEP/CH2Cl2 conditions almost no decomposition of XeF2 has occurred after twenty-four hours and decomposition only begins to be detectable after two days. The sample is
