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Cucurbit[6]uril forms a doubly charged complex with 1,4-butanediammonium cation that is observed using electrospray ionization Fourier transform mass spectrometry. Such 1:1 complexes are not observed for the smaller cucurbit[5]uril, which forms a 2:1 ammonium:cucurbituril complex instead. The 1:1 complex with cucurbit[6]uril is difficult to fragment via collisional activation; when it does fragment, both breakup of the cucurbituril cage and loss of the amine are observed. Further, the complex reacts with tert-butylamine via slow adduction. In contrast, nonrotaxane analogues (such as doubly charged 2:1 complexes of either protonated 1,4-butanediamine or protonated ethylenediamine with cucurbit[6]uril) fragment via easy loss of the intact amine upon collisional activation and react with tert-butylamine via rapid displacement of the original amine. On the basis of stoichiometry, fragmentation behavior, and reactivity, we conclude that the doubly charged complex of cucurbit[6]uril with 1,4-butanediammonium is a gas-phase pseudorotaxane.

Calorimetric and potentiometric studies indicate that a cage-type compound, decamethylcucurbit-[5]uril (MC5), shows very high selectivity for Pb 2+ over alkali, alkaline-earth, NH 4 + , and Cd 2+ cations. The selectivity factor is >10 5.5 . Thermodynamic quantities (log K, ∆H, and ∆S values) for the interactions of MC5 with metal ions were determined at 25 °C by calorimetric titration in 50% formic acid/water (v/v) and by potentiometric titration in an aqueous solution. The log K(H 2 O) value for protonation of MC5 was determined to be 9.56. Pb 2+ forms both 1:1 and 2:1 (Pb 2+ /MC5) complexes in 50% formic acid/water and aqueous solutions, while all other cations studied form only 1:1 complexes with MC5 in the 50% formic acid/water solution. However, both 1:1 and 2:1 complexes have been detected in an aqueous solution for Sr 2+ , indicating that the stoichiometry of MC5 complexes in 50% formic acid and in aqueous solutions could be different.

Values of log Kfor the association of several metal cations with 15-crown-5, 18-crown-6, and NrN'-dibenzyldiaza-18-crown-6, each covalently bonded to silica gel, were found to be nearly the same as those for the association of the same cations with the unbound macrocycles; this observation indicates the possibility of building pre-determined cation selectivity into these permanent macrocycle-bound silica gel systems.