Gas-Phase Synthesis and Characterization of an Azaphosphirenium Ion: The First N,P-Analogue of the Aromatic Cyclopropenyl Cation

Abstract

The 3-(dimethylamino)-1,1-dimethyl-1H-azaphosphiren-1-ium ion (3), the first member of the hitherto unknown class of azaphosphirenium ions and the first N,P-analogue of the aromatic cyclopropenyl cation, is generated in the gas phase via 70 eV electron ionization and demetalation of a ferriphosphaalkene (1) and rapid cyclization of the incipient phosphavinyl cation (2). Spontaneous cyclization of 2 to 3 is predicted by Becke3LYP/6-311++G(d,p) calculations, and the structure and reactivity of 3 is probed via collision-induced dissociation and ion−molecule reactions performed via pentaquadrupole mass spectrometry. Gaseous 3 fails to coordinate efficiently with nitrogen and oxygen nucleophiles, but it forms stable adduct ions with phosphorus and sulfur nucleophiles. The adducts of 3 with P and S nucleophiles are likely favored by the incorporation of relatively strong and rare PS and PP bonds and through extensive charge delocalization involving dimethylamino substituents. With isoprene, an adduct ion is also readily formed presumably via [4 + 2+] cycloaddition (followed by spontaneous ring opening), a reaction which is well documented for phosphenium ions both in solution and in the gas phase.