Ricerc@Sapienza

An integrated FT-ICR and computational approach has been employed to investigate the gas phase structure and reactivity of ESI-formed complexes between organic and inorganic acids (HX) and a K+-containing hexaazamacrocycle (M). Two limiting structures are available to those complexes, either the classical [MK+·XH] or the zwitterionic [MH+·K+·X?] one, with the latter prevailing over the first by increasing the gas-phase acidity of the HX ligand. Both structures undergo the HX displacement when reacting with 2,4-pentanedione (P0) and its 1,1,1-trifluoro (P3) and 1,1,1,5,5,5-hexafluoro (P6) derivatives. The HX displacement efficiency is found to depend not only on the specific structure of the complex, but also on the acid/base properties of the diones. Among them, P3 displays the lowest reactivity towards the [MK+·XH] complexes. These findings are consistent with the co-existence of a direct HA-to-HX and a base-catalyzed A?-to-HX substitution channels. The P0 < P3 < P6 reactivity order, observed towards the zwitterionic [MH+·K+·X?] complexes, points to a HX displacement mechanism whose rate-determining step involves the prototropic transfer from HA to the X?moiety in the encounter complex. The present study provides an extremely rare piece of information on reactive events taking place on contact ion pairs (K+X?) inside a monotopic receptor (MH+).