Synthesis of Di-ionizable Calixarene and Calixarene-crown Ether Ligands
Metal cation separations are a major application in the field of host-guest chemistry. Calixarene based ligands can be modified to coordinate efficiently and selectively with specific metal cations. These ligands are designed and modified using systematic structural variation. Optimal complexations and extraction efficiencies can be elucidated by changing one variable at a time on each ligand. The acidities of calixarene-based ligands can be “tuned” by incorporating the N-(X)-sulfonyl carboxamide moiety and varying the “X” group from electron-donating methyl to electron-withdrawing trifluoromethyl with intermediate groups of phenyl and p-nitrophenyl. Presented here are studies of several series of novel “tunable” di-ionizable calixarene based ligands, including two conformationally mobile series, one containing lower rim-functionalizations and the other containing a lower rim crown ether moiety and upper rim di-ionizable groups, and three series of structurally rigid calixcrowns.
The first series presented consists of the synthesis and extraction studies for conformationally mobile di-ionizable 1,2-dimethoxycalixarene ligands. It is beneficial to synthesize a ligand that can attain different conformations when complexing with specific metal cations. The effect of regiochemistry of the alkyl pendant arms in these ligands on binding efficiencies and extraction selectivities with metal cations is examined.
The synthesis and extraction studies of a conformationally mobile calixarene-1,3-crown-6 series with di-ionizable groups on the upper rim are presented next. Relatively few examples of calixcrown-based ligands incorporate upper rim-functionality. Some systematic structural variations presented in this series are the change in crown ether ring size and the regiochemistry of the crown ether ring.
Proton di-ionizable calixarene-1,2-crown-4 ligands with elongated pendant arms were synthesized in the cone conformation. The syntheses of these ligands with butyrate and valerate pendant arms and the extraction studies of the ligands with butyrate pendant arms are discussed followed by the last section in which the syntheses of the corresponding 1,2-alternate and partial cone ligands are presented.