Gas-phase synthesis and characterization of an azaphosphirenium ion: The first N,P-analogue of the aromatic cyclopropenyl cation

The 3-(dimethylamino)-1,1-dimethyl-1H-azaphosphiren-1-ium ion (3), the firs t member of the hitherto unknown class of azaphosphirenium ions and the fir st N,P-analogue of the aromatic cyclopropenyl cation, is generated in the g as phase via 70 eV electron ionization and demetalation of a ferriphosphaal kene (1) and rapid cyclization of the incipient phosphavinyl cation (2). Sp ontaneous cyclization of 2 to 3 is predicted by Beeke3LYP/6311++G(d,p) calc ulations, and the structure and reactivity of 3 is probed via collision-ind uced 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 an d sulfur nucleophiles. The adducts of 3 with P and S nucleophiles are likel y favored by the incorporation of relatively strong and rare P=S and P=P bo nds and through extensive charge delocalization involving dimethylamino sub stituents. With isoprene, an adduct ion is also readily formed presumably v ia [4 + 2(+)] cycloaddition (followed by spontaneous ring opening), a react ion which is well documented for phosphenium ions both in solution and in t he gas phase.