NMR evidence for protonated tetraethyl p-tert-butylcalix[4]arene tetraacetate

In addition to well-known complexes of tetraethyl p-tert-butylcalix[4]arene tetraacetate (abbrev. 1) with alkali metal cations, an equimolar complex of this compound with H3O+, i.e. 1 center dot H3O+, was found to be formed slowly in acetonitrile, however, with a very high equilibrium constant, the value of which is beyond the possibilities of exact determination by NMR. Hydrogen bis(1,2-dicarbollyl) cobaltate (HDCC) was used as a proton source. Further, by a combination of 1D and 2D H-1 NMR with C-13 NMR spectra, it was proved that the ester carbonyls of 1 are not strongly involved in coordination of H3O+. It means that the most probable structure of the 1 center dot H3O+ complex is obviously that with H3O+ bound by strong hydrogen bonds to the phenoxy oxygen atoms of 1.