Water-soluble calixarene hosts that specifically recognize the trimethylammonium group or the benzene ring of aromatic ammonium cations: A combined (1)H NMR, calorimetric, and molecular mechanics investigation

The complexation of N,N,N-trimethylanilinium (TMA), benzyltrimethylammonium (BTMA) and p-nitrobenzyltrimethylammonium (BTMAN) cations by two different water-soluble calix[4]arenes, 1 and 2, both fixed in the cone conformation, was studied at neutral pH by (1)H NMR spectroscopy and calorimetry In contrast to the conformationally mobile calix[4]arene tetrasulfonate 3, host 1 specifically binds the TMA-N(+)(CH(3))(3) group, whereas 2 recognizes only the aromatic ring of TMA. The introduction of a spacer between the charged polar group and the aromatic residue (BTMA) or an electron-withdrawing group on the aromatic moiety (BTMAN) does not alter this selectivity for host 1, whereas host 2 selectively recognizes the -N(+)(CH(3))(3) group of BTMAN, but complexes BTMA unselectively both by the -N(+)(CH(3))(3) group and the aromatic moiety. The binding constants for all three guests with 1 and 2, as determined by both (1)H NMR spectroscopy and calorimetric titrations, show that inclusion is favored by the presence of the sulfonate groups and that the greater stability observed with 2 mainly arises from a favorable enthalpic contribution. The factors driving inclusion of the ammonium cation by the two hosts are discussed. Molecular mechanics calculations allow determination of the conformational properties of hosts 1 and 2 and of their inclusion complexes.