Isolation, characterization and reactivity of three-coordinate phosphorus dications isoelectronic to alanes and silylium cations

Three-coordinate main group Lewis acids are exceedingly important reagents in chemical synthesis. In contrast to the well-established chemistries of neutral group 13 and cationic group 14 species, isoelectronic group 15 element dications are unknown. In this work, we use stabilizing N-heterocyclic imine substituents to isolate and characterize phosphorandiylium dications ([R3P]2+) and show that the electrophilicity at the phosphorus atoms is controlled by the π-electron-donating ability of these subtituents. Structural, spectroscopic and theoretical results reveal that the phosphorus dications adopt a perfectly trigonal-planar geometry with the electron-deficient phosphorus centres being well separated from the borate anions. The reactivity of the dications reveal their exceptional Lewis acidity at phosphorus; the adjacent nitrogen atoms, however, are weakly basic, resulting in transformations such as chloride ion abstraction from Me3SiCl and the selective monodefluorination of trifluoromethyl groups. The isoelectronic series of alanes [R3Al] and silylium cations [R3Si]+ has now been extended with the synthesis and characterization of two phosphorandiylium dications ([R3P]2+), which are trigonal planar Lewis superacids. The electrophilicity at the phosphorus atom is governed by the π-electron-donating ability of the attached N-heterocyclic imine substituents.