Hydroalumination of alkynyl-aminophosphines as a promising tool for the synthesis of unusual phosphines: P–N bond activation, a transient phosphaallene, a zwitterionic AlP₂C₂ heterocycle and a masked Al/P-based frustrated Lewis pair

Treatment of the new alkynyl-chlorophosphine, Mes-P(Cl)–CC–CMe₃, with LiNR₂ afforded various unprecedented aminophosphines, Mes-P(NR₂)–CC–CMe₃, which showed a fascinating diversity in their reactivity towards H-Al^tBu₂. NMe₂ and NEt₂ derivatives yielded the hydroalumination products Mes-P(NR₂)–C(Al^tBu₂)C(H)–CMe₃ which have an Al–N and an activated P–N bond. Elimination of aluminium amide yielded the transient 3H-phosphaallene, Mes-PCC(H)-CMe₃, which finally afforded a five-membered AlP₂C₂ heterocycle with an Al–P bond and two exocyclic CC bonds. This heterocycle is directly formed with the sterically shielded ⁱPr₂N- and dimethylpiperidinophosphines. A unique R₂AlH adduct resulted from the NPh₂ and N(SiMe₃)₂ substituted phosphines. It may be viewed as an Al/P-based frustrated Lewis pair (FLP) which coordinates an R₂AlH moiety. The heterocyclic AlP₂C₂ compound is formed in the final step of this reaction. Hydroalumination with Et₂AlH yielded [Mes-P(H)–C(AlEt₂)C(H)–CMe₃]₂ which features an Al₂P₂C₂ heterocycle and two Al–P bonds. This dimer resembles the class of hidden or masked FLPs which show a reactivity similar to uncoordinated FLPs. Its unique structural motif is a P–H bond which may result in a new type of FLP chemistry.