Hydrogen bonding and solvent effects on complexation of alkali metal cations by lower rim calix[4]arene tetra(O-[N-acetyl-D-phenylglycine methyl ester]) derivative

Complexation of alkali metal cations with 5,11,17,23-tetra-tert-butyl-26,28,25,27-tetrakis(O-methyl-D-alpha-phenylglycylcarbonylmethoxy) calix[4] arene (L) was studied by means of spectrophotometric, conductometric and potentiometric titrations at 25 degrees C. The solvent effect on the binding ability of L was examined by using two solvents with different affinities for hydrogen bonding, viz. methanol and acetonitrile. Despite the presence of intramolecular NH center dot center dot center dot O=C hydrogen bonds in L, which need to be disrupted to allow metal ion binding, this calix[ 4] arene amino acid derivative was shown to be an efficient binder for smaller Li+ and Na+ cations in acetonitrile (lg K-LiL > 5, lg K-NaL = 7.66), moderately efficient for K+ ( lg K-KL = 4.62), whereas larger Rb+ and Cs+ did not fit in its hydrophilic cavity. The complex stabilities in methanol were significantly lower (lg K-NaL = 4.45, lg K-KL = 2.48). That could be explained by different solvation of the cations and by competition between the cations and methanol molecules (via hydrogen bonds) for amide carbonyl oxygens. The influence of cation solvation on complex stability was most pronounced in the case of Li+ for which, contrary to the quite stable LiL+ complex in acetonitrile, no complexation was observed in methanol under the conditions used.