Doubly or triply protonated? Complexes of cucurbit[n]urils (n = 6–8) with a tripodal ligand tris(2-aminoethyl)amine (TREN)

Abstract

Binding interactions between a tripodal ligand tris(2-aminoethyl)amine (TREN) and a series of cucurbit[n]urils (n = 6–8) in aqueous (H2O or D2O) formic acid solutions were investigated by electrospray mass spectrometry (ESI MS), isothermal titration calorimetry (ITC) and NMR titration. DFT calculations were also performed to rationalize the experimental findings. ESI MS data indicate that the studied cucurbiturils bind TREN amine forming two types of adducts of a 1:1 stoichiometry. Dipositive adducts are formed by CB6 and CB7 homologues, while tripositive adducts are formed by CB7 and CB8 homologues. DFT calculations show that the type of the formed adduct is connected with the dimensions of the cucurbituril portal. The results of NMR and ITC measurements confirm that all three CBn homologues (n = 6–8) bind TREN ligands spontaneously in the liquid phase. According ITC results the CB7 homologue binds this amine more strongly than the CB6 and CB8 homologues.