Phosphaarsenes - Combining Phospha- and Arsa-Wittig-Reagents

Dipnictenes of the type RPn=PnR (Pn=P, As, Sb, Bi) can be viewed as dimers of the corresponding pnictinidenes R-Pn. Phosphanylidene- and arsanylidenephosphoranes (R-Pn(PMe₃); Pn=P, As) have been shown to be versatile synthetic surrogates for the delivery of pnictinidene fragments. We now report that thermal treatment of 1 : 1 mixtures of R-P(PMe₃) and R'-As(PMe₃) gives access to arsaphosphenes of the type RP=AsR'. Three examples are presented and the properties and reactivity of Mes*P=As^DipTer (1) (Mes*=2,4,6-tBu₃-C₆H₂; ^DipTer=2,6-(2,6-iPr₂C₆H₃)₂-C₆H₃) were studied in detail. Solid state ³¹P NMR spectroscopy revealed a large ³¹P NMR chemical shift anisotropy with a span of ca. 920 ppm for 1 while computational methods were employed to investigate this pronounced magnetic deshielding of the P atom in 1. In the presence of the carbene IMe₄ (IMe₄=:C(MeNCMe)₂) 1 is shown to be split into the corresponding NHC adducts Mes*P(IMe₄) and ^DipTerAs(IMe₄), which is additionally shown for diarsenes.