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.