Synthesis of Novel Calixarene-Based Ligands for Selective Metal Cation Extraction
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The synthesis of ligands with superior binding efficiency and selectivity toward targeted metal cations is an important area of scientific endeavor. Calixarene-based ligands with interesting properties in binding with various metal cation species have been prepared during the past 20 years. This dissertation research presents the design and synthesis of a variety of novel calixarene-based ligands. To evaluate the metal cation binding behavior of these ligands, solvent extraction was employed for proton-ionizable ligands and an ion-selective electrode (ISE) method for non-ionizable analogues. Proton di-ionizable calixarene-crown-4 ligands in three conformations were synthesized. This is part of continuing research for construction of a comprehensive library of calixarene-crown ethers for various metal cations. The N-(X)sulfonyl carboxamide groups with tunable acidity are employed as novel proton-ionizable groups. By varying X from methyl to trifluoromethyl, a wide pH window was created for the ligands. Spatial arrangement of donor atoms in the macrocyclic ligands was shown to be an important factor that influences metal cation binding properties and selectivities of ligands. To further explore the impact of this factor on calixarene-crown ether ligands, a series of calixarene-crown-4 compounds in the cone conformation with an expanded crown ether ring was synthesized. In solvent extraction, the calixarene-crown-4 ligands with an expanded crown ether ring were found to be better extractants for some heavy metal cations compared to the conventional calixarene-crown-4 analogues. Design and evaluation of ionophores that can selectively bind with heavy metal cations, such as Pb2+ and Hg2+, have important potential applications. According to the soft nature of these metal cations, they are rather thiophilic than oxophilic. A relatively simple approach for enhancing ligand/metal cation interaction is the incorporation of soft donor atoms in the ligand. Compared to thiacalixarene and mercaptocalixarene, less effort has been paid to the design and synthesis of calixarene-crown ethers with thiacrown fragments. A series of di-ionizable p-tert- butylcalixarene-monothiacrown-5 ligands is synthesized and examined by solvent extraction. The ligands were good extractants for Hg2+ and Pb2+. Binding with these soft metal cations is accomplished in a very acidic environment. Some miscellaneous di-ionizable calixarene-based ligands also are prepared. These ligands include di-ionizable calixarene-1,2-crown-5 with elongated proton-ionizable groups and the upper-rim functionalized calixarene-crown-5 ligands. Calixarene-1,2-crowns are less studied compared to 1,3-crown analogues due to their relatively poor metal cation binding behavior. However, some 1,2-crown analogues synthesized recently in the Bartsch Research Group exhibited very good metal cation binding properties and selectivities. The study of 1,2-crowns that incorporated elongated proton-ionizable groups allows us to enhance our understanding of calixarene-1,2-crowns. Some di-ionizable calixarene-crown-5 compounds with proton-ionizable groups on the upper rim are also prepared. The aromatic region of calixarenes could behave as a potential binding site for some soft cations. The modified upper rim has proton-ionizable groups closely located to this potential binding site and possibly an enhanced binding with these soft metal cations could be observed. Finally, some di-benzyl calixarene-dithiacrowns with various crown ether ring sizes, conformations, and para-substituents were synthesized. The increment number of sulfur donor atoms in the crown ether fragment was proposed to improve the ligand binding behavior with heavy metal cations. The binding behaviors of these ligands were evaluated by ion selective electrodes (ISEs). This is the first systematic study of the metal cation binding properties of the calixarene-dithiacrown ligands by this method.