Isolation of crystalline carbene-stabilized P₂-radical cations and P₂-dications

The discovery in 1900 by Gomberg that the trityl radical (Ph₃C •) exists at room temperature is often considered to be the beginning of radical chemistry. Since then, persistent and even room-temperature stable radicals based on second-row and heavier elements have been synthesized. However, few of them have been characterized crystallographically, because they are either too reactive or dimerize in the solid state. Here, we show that a P₂ fragment, capped with two bulky, strongly electron-releasing singlet carbenes (dicoordinate carbon compounds with only six valence electrons), can undergo one-electron oxidation, giving rise to room-temperature stable radical cations. Moreover, when N-heterocyclic carbenes are used, two-electron oxidation can also be performed, producing the corresponding stable dicationic diphosphene, which has to be regarded as a P₂ ²⁺ fragment coordinated by two carbenes. These results reveal a new application of stable singlet carbenes, the stabilization of paramagnetic species and electron-poor fragments.