Anion-pi Interactions-Interactions Between Benzo-Crown Ether Metal Cation Complexes and Counter Ions

摘要

The loss of X radical from [M + Cu + X](+) ions (copper reduction) has been studied by the so called in-source fragmentation at higher cone voltage (M = crown ether molecule, X = counter ion, ClO4, NO3, Cl ). The loss of X+ has been found to be affected by the presence/lack of aromatic ring poor/rich in electrons. Namely, the loss of X+ occurs with lower efficiency for the [NO2-B15C5 + Cu + X](+) ions than for the [B15C5 + Cu + X](+) ions, where NO2-B15C5 - 3-nitro-benzo-15-crown-5, B15C5 = benzo-15-crown-5. A reasonable explanation is that Anion-pi interactions prevent the loss of X from the [NO2-B15C5 + Cu + X](+) ions. The presence of the electrort-withdrawing NO2 group causes the aromatic ring to be poor in electrons and thus its enhances its interactions with anions. For the ion containing the aromatic ring enriched hi electrons, namely [NH2-B15C5 + Cu + ClO4](+) where NH2-B15C5 = 3-amino-benzo-15-crown-5, the opposite situation has been observed. Because of Anion-pi repulsion the loss of X radical proceeds more readily for [NH2-B15C5 + Cu + X](+) than for [B15C5 + Cu + X](+). Iron reduction has also been found to be affected by Anion-pi interactions. Namely, the loss of CH3O radical from the ion [B15C5 + Fe + NO3 + CH3O]+ proceeds more readily than from [NO2 B15C5 + Fe + NO3 + CH3O](+). (J Am Soc Mass Spectrom 2009, 20, 257-262)