Complexes of p-tert-butylcalix[4]arene with mono- and dipositive cations in the gas phase

The host-guest chemistry of p-tert-butyl-calix[4]arene in the gas phase was studied using Fourier transform ion cyclotron resonance mass spectrometry. Gas phase complexes with laser-desorbed alkali metal cations (Na+, K+, Rb+ and Cs+) were generated. The calixarene was observed to complex with up to two metal cations at the same time with the accompanying loss of a proton, to yield species with a net charge of +1. Complexes with Sr2+, and Ba2+, with accompanying loss of a proton to maintain a net charge of +1, were also observed. Relative rate constants and reaction efficiencies for the formation of the 1:1 calixarene/alkali metal complexes vary inversely with cation size, probably reflecting differences in the polarizing capabilities of the cations. Reaction of the alkali metal and alkaline earth complexes with 15-crown-5 results in adduction of the crown in cases where the metals are large, suggesting the larger cations bind in exposed positions on the calixarene. Finally, complexes of the calixarene with benzylammonium cation were observed. Equilibrium measurements indicate Delta G degrees(350) for transfer of benzylammonium from the calixarene to 15-crown-5 is -9.1 +/- 0.4 kJ mol(-1).