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

Abstract Herein, we disclose the synthesis, characterization, and bonding analysis of a crystalline allenylidene phosphonium ion ([R 2 P═C═C═CAr 2 ] + ) [ 2 ] + (R = 1,3‐diisopropylphenylimidazolin‐2‐ylidenamino, CAr 2  = 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 [R 2 P═C═C═CAr 2 ] + draws further parallels between the properties and reactivity of low‐coordinate phosphorus compounds and related carbon compounds.