Stabilization of the Triphosphallyl Ligand η³-P₃{P(O)H} at Iridium via Alkaline Activation of P₄

The selective functionalization of the polyphosphorus moiety Ph₂PCH₂PPh₂PPPP present as a tetrahapto-ligand in complex [Ir(dppm)(Ph₂PCH₂PPh₂PPPP)]⁺ (1, dppm=Ph₂PCH₂PPh₂) was obtained by reaction of 1 with water under basic conditions at room temperature. The formation of the new triphosphaallyl moiety η³-P₃{P(O)H} was determined in solution by NMR spectroscopy, and confirmed in the solid state by a single-crystal X-ray structure of the stable product [Ir(κ²-dppm)(κ¹-dppm)(η³-P₃{P(O)H})] (2). In solution, 2 has a fluxional behavior attributable to the four P atoms belonging to the tetraphosphorus moiety in 1 and exhibits a chemical exchange process involving the two PPh2 moieties of the same bidentate ligand, as determined by 1D and 2D NMR spectroscopy experiments carried out at variable temperature. The mechanism of the reaction was investigated at the DFT level, which suggested a selective attack of an in-situ generated OH− anion on one of the non-coordinated phosphorus atoms of the P4 moiety. The reaction then evolves through an acid-assisted tautomerization, which leads to the final compound 2. Bonding analysis pointed out that the new unsubstituted P3-unit in the η₃-P₃{P(O)H} moiety behaves as a triphosphallyl ligand.