SYNTHESIS OF NEW CALIXARENE LIGANDS WITH PROTON-IONIZABLE GROUPS
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Metal ion recognition is one of most important processes in supramolecular chemistry. Among the variety of host molecules, modified calixarene ligands have been intensively investigated for specific metal cation complexation due to their unique structure. These ligands can be used to recover precious metals or separate toxic metals from industrial waste. As part of the effort to improve the performance of such ligands in metal ion extraction, N-(X)-sulfonyl carboxamide groups, where X is methyl, phenyl, p-nitrophenyl, and trifluoromethyl, were introduced as proton-ionizable units to tune the acidity of these ligands. Cone di-ionizable calixarene-1,3-crown-7 ligands were prepared to examine the effect of the cavity size for the crown ether ring. The result of solvent extraction experiments indicates that these compounds are highly selective for Ba2+ among the alkaline earth metal cations and good extractants for Pb2+ and Hg2+. The synthesis and extraction studies of p-H-calixarene-1,3-crown-5 and p-tertbutylcalix[ 4]arene-1,3-crown-5 ligands with elongated proton-ionizable side arms are presented next. The influence of elongating the proton-ionizable arms on the lower rim and para substituents on the upper rim of the calixcrown scaffold on the metal cation extraction behavior was evaluated. Incorporation of 2-methoxyethoxy units and various lengths of side arms into p-tert-butylcalixarene in the cone and partial-cone conformations was achieved. In this series, the effects of conformation and elongated pendant arms on extraction efficiencies and selectivities with various metal cations were proved. Proton di-ionizable calixarene-1,2-crown-5 ligands with elongated pendant side arms were synthesized in the partial-cone conformation. Extraction studies probed the influence of elongated side arms and conformation toward various metal cations.