HIGH-SPECIFICITY THIACROWN ETHER REAGENTS FOR SILVER(I) OVER BIVALENT MERCURY AND LEAD - THERMODYNAMIC AND C-13 NMR RELAXATION-TIME STUDIES

Log K, DELTA-H, and DELTA-S values for the interactions of Ag+, Hg2+, and Pb2+ with several thiacrown ether ligands were determined in aqueous solution by potentiometry and calorimetry. C-13 NMR spin-lattice relaxation times (T1) and chemical shifts (delta) have been determined for pyridonothia-18-crown-6 (PT18C6) in the absence and presence of Ag+, Hg2+, and Pb2+. A large enhancement of selectivity by PT18C6 for Ag+ over Hg2+ and Pb2+ was found. The interactions of thiacrown ether ligands with the metal ions studied were found to be enthalpy driven. The entropy changes were unfavorable in all cases. The T1 and delta-measurements for PT18C6 interaction with Ag+ and Hg2+ in conjunction with the corresponding thermodynamic data provide a detailed microscopic picture of the macrocycle-metal ion interactions. Both Ag+ and Hg2+ interact strongly with the sulfur portion of PT18C6, but Ag+ interacts much more strongly than Hg2+ with the pyridone portion of the ligand. The influence of preorganization of the binding sites in the macrocycle on binding strength and selectivity was examined. A successful reversal of the common selectivity order of Hg2+ over Ag+ was accomplished by the insertion of a pyridone binding subunit in the thiacrown ether ligand system.