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 ([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-heterocylic imine substituents and a pi-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 [2cyclo]+. 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.