Flash Communication: A Ferrous Adduct of a Phosphanylidene-σ4-phosphorane

The sterically encumbered phosphanylidene-σ⁴-phosphorane Mes*PPMe₃ (Mes* = 2,4,6-^tBu₃C₆H₂) smoothly displaces Et₂O in [(^tBupyrr₂py)Fe(OEt₂)] (1-OEt₂) (^tBupyrr₂py^2– = 3,5-^tBu₂-bis(pyrrolyl)pyridine) to form a rare example of a η¹-phospha-Staudinger adduct of Fe, namely, [(^tBupyrr₂py)Fe(Mes*PPMe₃)] (1-Mes*PPMe₃) in 64% yield. Complex 1-Mes*PPMe₃ is a ferrous, S = 2 system and quite thermally stable, but in the presence of B(C₆F₅)₃ and photolysis, it forms the phosphaindan ferrous adduct [(^tBupyrr₂py)Fe(phosphaindan)] (1-phosphaindan) (phosphaindan = HPCH₂C(Me₂)-2,4-^tBu₂C₆H₂) along with Me₃PB(C₆F₅)₃. In the absence of Lewis acid, photolysis of 1-Mes*PPMe₃ results instead in the formation of free phosphaindan and the PMe₃ ferrous adduct [(^tBupyrr₂py)Fe(PMe₃)] (1-PMe₃) thus suggesting that dissociation of the phosphanylidene-σ⁴-phosphorane precedes the formation of a transient phosphinidene fragment Mes*P. All phosphorus adducts of 1 were crystallographically characterized and show quite similar Fe–P distances (2.4685(5) for 1-Mes*PPMe₃; 2.5062(7) for 1-phosphaindan; and 2.4323(8) Å for 1-PMe₃).