GAS-PHASE PROPERTIES AND REACTIVITIES OF PHOSPHOLIDE AND ARSOLIDE ANIONS

The aromatic anions phospholide and arsolide have been generated in the gas phase at room temperature via the reaction of hydroxide ion with 1-tert-butylphosphole and 1-tert-butylarsole, respectively. The acid-base properties and reactivity of the ions were investigated with a flowing afterglow-triple quadrupole instrument. The proton affinities of both phospholide and arsolide have been determined from bracketing experiments to be 338 +/- 3 kcal/mol. Exchange of two hydrogens for deuterium occurs in reactions of phospholide anion with FCH2COOD. In contrast, no H/D exchange occurs in reactions of arsolide ion with FCH2COOD, nor does it occur in reactions between pyrrolide anion and CF3CH2OD. These results are interpreted in terms of kinetic protonation of the heteroatom in all three anions, with exchange in the phospholide anion occurring by slow, reversible [1,5]-hydrogen shifts. All three heterocyclopentadienides are found to be relatively unreactive under flowing-afterglow conditions, exhibiting only slow clustering or addition reactions with polar, electrophilic reagents and electron transfer to NO2. Ab initio molecular orbital calculations performed on the isomeric C(4)H(5)X (X = N, P) species indicate that 2H-phosphole and 3H-phosphole are 7.8 and 4.0 kcal/mol more stable than 1H-phosphole, respectively, while W-pyrrole and 3H-pyrrole are 11.5 and 13.0 kcal/mol less stable than LH-pyrrole. Calculations on the corresponding C(4)H(4)X(-) (X = N, P) conjugate base anions predict proton affinities at the 2-position of phospholide and at the nitrogen atom of pyrrolide of 341.4 and 358.6 kcal/mol, respectively, in good agreement with the experimental data. The gas-phase acidities and relative stabilities of the nitrogen, phosphorus, and arsenic heteroles are discussed in terms of the calculated electronic and geometric structures.