still >75% unionised XeF2 after one week. The main products [CH2FCl (. -172.9); CHFCl2(. -84.0); CF2Cl2 (. +62.1); HF (. -193.3)] are those reported by Holloway and co-workers under similar conditions,17 although we consistently observe greater amounts of CF2Cl2. At all times the amount of CHFCl2 (H-F exchange) is small and exceeded by the amount of CF2Cl2, indicating the ease of H-F exchange in CHFCl2. Under these conditions XeF2 is more stable in CH2Cl2 than in CHCl3 probably because the H-CCl3 bond is more reactive (cf H-CFCl2 above) towards XeF2 than the H-CHCl2 bond. For this reason CH2Cl2 is a more suitable solvent for XeF2 reactions in plastic vessels. In PTFE- FEP/CFCl3 there is no detectable decomposition of XeF2 after one week and no evidence of CF2Cl2 formation. For this reason CFCl3 is an excellent solvent for XeF2 reactions, especially if solvent derived by-products need to be avoided. However, for environmental reasons this solvent is increasingly difficult to obtain.
(ii) Pyrex® and quartz. In Pyrex®/CH2Cl2 the 19F spectrum of XeF2 is unchanged after ten minutes but most has decomposed after one hour and decomposition is complete after two hours. The products are CHFCl2 (. -81), CH2FCl (. -170) and fluoride [. -149 (broad), -....(sharp).and -162 (sharp)]. When the tube was emptied and refilled with pure solvent the broad signal at . -149 remained suggesting that it is due to fluoride bound to the Pyrex® surface. Sometimes this signal resolves into two signals (. -148.5 and -149.1) suggesting two discrete binding sites. The signals at . -157 and . -162 are attributable to F- and HF2- in solution. The lifetime of XeF2 was considerably extended in quartz/CH2Cl2 with no decomposition after two hours, after which decomposition to fluoride [. -128 (bound) and -162 (unbound)], together with smaller amounts of CHFCl2 and CH2FCl, begins and is complete after about four hours. Significantly the bound fluoride in quartz is observed at a different position to that in Pyrex® (. -128 vs . -149) and is a sharp signal suggesting only one binding site. In both Pyrex® and quartz a pre-wash of the tube with 2N NaOH extends the lifetime of XeF2 in CH2Cl2 by approximately one hour. These results are very similar to those obtained for CHCl3 and CDCl3 under the same conditions,7 except that CHCl3 appears to be more reactive and therefore less suitable as a solvent. Similar stabilities in Pyrex® and quartz were obtained using CFCl3 as solvent except that only fluoride (bound and unbound) was detected as decomposition product and, as for PTFE-FEP reactions, this solvent is superior to CH2Cl2 and CHCl3.
Clearly the stability profiles of XeF2 in Pyrex®/CH2Cl2, CHCl3 and CFCl3 are quite different to those in PTFE-FEP. We observed no 19F NMR evidence of surface bound XeF2 and most of the XeF2 must be in solution. These results are consistent with XeF2 bonding to Lewis acid sites on the glass surface and the bound reagent (FXe.+---F.Pyrex..-) either rapidly reacting as an electrophile (.XeF+) with solvent (or substrate) or, alternatively, being reduced to fluoride (bound or unbound)(XeF2 + 2e- . Xe + 2F-). The latter reaction limits the lifetime of the XeF2 and for reactions in glass necessitates the use of more than one equivalent. These surface interactions do not occur in PTFE-FEP.
(b) CH3CN and CD3CN
(i) PTFE-FEP. Under PTFE-FEP/CD3CN conditions no decomposition of XeF2 had occurred after one week.
