Gas-phase study of adduct and exchange complexation reactions between two and tridimensional macrocyclic ligands using ion mobility spectrometry and mass spectrometry

Abstract

A gas-phase study concerning the interaction between macrocyclic ligands and different molecular ion guests (H3O+, NH4+, CH3NH3+) was performed using ion mobility spectrometry and mass spectrometry. 18-crown-6 (18C6), diaza18-crown-6 (DA18C6) and cryptand [222] (C222) ligands were selected for the investigation of structural properties on the nature of complexes formed. Characterization of complexes was performed by ESI-TWIM-TOFMS. In this study, the importance of the type of donating atom in the ring on complex formation was investigated by comparison of the ion mobility spectra of 18C6 and DA18C6 at various temperatures. Different complexation behaviors of ligands showed that the nature of species formed strongly depends on the structural features of the host. DA18C6 does not demonstrate the adduct formation that is observed for 18C6. This is attributed to the high proton affinity of DA18C6, which serves to preferentially remove a proton from any reactant ion upon introduction to the gas phase. In the case of C222 with a three-dimensional cavity, due to the existence of an inclusive site, an interesting behavior was observed. In C222–H3O+ and C222–CH3NH3+ systems, only proton exchange reaction and an inclusion complex formation (C222.H+) are found, while for C222–NH4+ happens inclusive ammonium complex C222·NH4+ in addition to proton exchange reaction and C222.H+ formation. Encapsulation of ammonium ion in the cage of C222 was attributed to size match between ammonium and host cavity that depends on temperature. The exchange constant, enthalpy and entropy of C222·NH4+ complex formation in the gas phase have been determined.