A Phosphorus Analogue of p-Quinodimethane with a Planar P₄ Ring: A Metal-Free Diphosphorus Source

Para-quinodimethane (pQDM) is a fundamental structural component in many π-conjugated organic molecules and materials. The incorporation of phosphorus atom into π-conjugated frameworks offers unique opportunities for controlling the properties of derived species. A phosphorus analogue of p-quinodimethane (pQDM), (IPrC)₂ P₄ [5, IPr=C{N(Ar)CH₂ }₂ ; Ar=2,6-iPr₂ C₆ H₃ ] featuring a planar P₄ ring, was readily accessible by KC₈ -reduction of (IPrC)(PCl₂ )₂ (2). Base-mediated C-H functionalization of IPrCH₂ (1) with PCl₃ afforded 2. The formation of 5 was expected to occur through a dimerization of the transient 3H-diphosphirene (IPrC)P₂ (4), which was theoretically suggested to have an intermediate diradical character. Compound 5 underwent photo-induced ring-contraction reaction to form the singlet diradicaloid (IPrCP)₂ VI and white phosphorus (P₄ ). The formation of and VI and P₄ suggested the formal diphosphorus (P₂ ) elimination from 5. Indeed, photolysis of a mixture of 1,3-cyclohexadiene (CHD) and 5 led to the formation of P₂ -entrapped product (CHD)₂ P₂ (6). The compound 5 represents the first organophosphorus species that functions as a P₂ source.