Allenylidene Phosphonium Ion: An Isoelectronic Phosphorus Analogue of [3]Cumulene

Herein, we disclose the synthesis, characterization, and bonding analysis of a crystalline allenylidene phosphonium ion ([R₂P═C═C═CAr₂]⁺) [2]⁺ (R = 1,3-diisopropylphenylimidazolin-2-ylidenamino, CAr₂ = 9-fluorenylidene), an isolobal analogue of [3]cumulenes. The cation was prepared via methoxide ion abstraction of an alkynyl phosphine 1. Electron rich N-heterocyclic imine substituents and a π conjugated fluorenyl scaffold together stabilize the resulting three-coordinate phosphonium center of [2]⁺. [2]⁺ features a trigonal planar phosphorus atom which is electrophilic, as evidenced experimentally by the preparation of a Lewis acid-base adduct with 4-dimethylaminopyridine, and fluoride ion affinity calculations. Crystallographic and computational investigations of [2]⁺ support a structure with considerable P═C double bond character. Furthermore, this P═C double bond participates in a thermally reversible intramolecular dearomative [2 + 2] cycloaddition with an arene moiety of one of its supporting 1,3-diisopropylphenylimidazolin-2-ylidenamino substituents to give a tetracyclic isomer [2^cyclo]⁺. The isolation of a free [R₂P═C═C═CAr₂]⁺ draws further parallels between the properties and reactivity of low-coordinate phosphorus compounds and related carbon compounds.