Frustrated Lewis Pair Oxidation Permits Synthesis of a Fluoroazaphosphatrane, [FP(MeNCH₂CH₂)₃N]

Proazaphosphatranes, also known as Verkade's superbases, are among the strongest nonionic bases available. Their extreme basicity derives in part from their ability to form a P-N transannulation upon interaction of the P atom with an electrophile. Although haloazaphosphatrane cations of the form [XP(RNCH₂CH₂)₃N]⁺ have previously been reported for X = Cl, Br, and I, no fluoroazaphosphatranes (X = F) have been prepared. Unlike treatment with Cl₂, Br₂, I₂, and surrogates thereof, reaction of proazaphosphatranes with XeF₂ results in decomposition. Analysis of the decomposition products suggested that fluoride ions may be the destructive agent. However, oxidation of a proazaphosphatrane/BPh₃ frustrated Lewis pair affords [FP(RNCH₂CH₂)₃N][FBPh₃]. Systematic trends in the experimental and computed NMR and structural data are considered. A computational analysis suggests that the transannular P-N distance varies as a result of the flexibility of the molecules and their capacity to deform in the solid state.