Water-soluble calixarene hosts that specifically recognize the trimethylammonium group or the benzene ring of aromatic ammonium cations: A combined H-1 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 H-1 NMR spectroscopy and calorimetry In cont rast to the conformationally mobile calix[4]arene tetrasulfonate 3, host 1 specifically binds the TMA-N+(CH3)(3) group, whereas 2 recognizes only the aromatic ring of TMA. The introduction of a spacer between the charged pola r group and the aromatic residue (BTMA) or an electron-withdrawing group on the aromatic moiety (BTMAN) does not alter this selectivity for host 1, wh ereas host 2 selectively recognizes the -N+(CH3)(3) group of BTMAN, but com plexes BTMA unselectively both by the -N+(CH3)(3) group and the aromatic mo iety. The binding constants for all three guests with 1 and 2, as determine d by both H-1 NMR spectroscopy and calorimetric titrations, show that inclu sion is favored by the presence of the sulfonate groups and that the greate r stability observed with 2 mainly arises from a favorable enthalpic contri bution. The factors driving inclusion of the ammonium cation by the two hos ts are discussed. Molecular mechanics calculations allow determination of t he conformational properties of hosts 1 and 2 and of their inclusion comple xes.