White phosphorus (P4) reacts with two equivalents of sodium in refluxing diglyme to give the sodium salt of the aromatic pentaphospholide anion [cyclo-P5], which is considered as the inorganic analog of the classical cyclopentadienyl anion [Cp]. Indeed, [cyclo-P5] forms sandwich complexes with the η5-coordination mode similar to isolobal [Cp]. However, due to the presence of the energetically and sterically accessible lone pairs on the phosphorus atoms, an additional η1-coordination is also feasible. Apart from the coordination to different transition metal centers little is known about the chemical behavior of [cyclo-P5].
In this work, the reactivity of the pentaphospholide anion towards alkynes, nitriles, phosphaalkynes, and arsaalkynes is studied. The reaction of [cyclo-P5] with the unsaturated compounds results in the formation of novel or barely explored five-membered 6π-electron aromatic heterocycles: 1,2,3-triphospholide anions, 1,2,4-triphospholide anions, 1,2,3,4-tetraphospholide anions, and 1-aza-2,3,4-triphospholide anions. These heterocycles react with [CpFe(η6-Toluene)]PF6 or FeCl2(DME) to give the corresponding (aza)phosphaferrocenes. The novel compounds are characterized by mutinuclear NMR spectroscopy, single-crystal X-ray diffraction analysis (SC-XRD) and cyclic voltammetry. Additionally, the selected species are analyzed quantum chemically.
