Synthesis and Reactivity of Triphosphaallyl Cation Stabilized by N-Heterocyclic Carbenes

Reduction of the triphosphaallyl species 6[GaCl₄] with Ga^I[Ga₂Cl₇] affords the imidazoliumyl-substituted (L) bicyclo[2.1.1]-triphosphane 10[Ga₂Cl₇], featuring an unprecedented Ga₂Cl₅-bridged P₃ scaffold. Reactions of 10[Ga₂Cl₇] with nucleophiles (Cl^- or NHC) result in rare, selective P-C bond cleavages, affording Ga₂Cl_y-substituted triphosphiranes (LP₃Ga₂Cl_y, y =  5, 6) via an intramolecular ring closure mechanism. Protonation of 6[GaCl₄] gives rise to a similar ring closure, but without P-C bond cleavage, to afford the L₂P₃H⁺ salt 8[OTfGaCl₃]₂. Additionally, the palladium complex 26[GaCl₄], formed through the reaction of 10[Ga₂Cl₇] with [Pd(PPh₃)₄], presents a novel bicyclic P₃Pd moiety (LP₃Pd(PPh₃)₂[GaCl₄]). Comprehensive DFT calculations have been conducted to elucidate the bonding situation in 26[GaCl₄], uncovering significant metal-to-ligand π-back-donation and a distinctive 3-center-4-electron hyperbonding phenomenon in the P₃Pd framework. These findings offer valuable insights into chemistry of cyclic polyphosphorus compounds and, in particular, the reactivity, structural flexibility, and the coordination properties of cationic triphosphorus species.