Synthesis, alkali metal picrate extraction, and alkali metal cation binding selectivities of some new cage-annulated polyoxamacrocyclic crown ethers

Five new cage-annulated crown ethers, i.e., 4a, 4b, 6b, 11a, and 11b, have been synthesized and their respective alkali metal picrate extraction profiles along with that of a previously synthesized host molecule, 6a, have been obtained. These results are compared with the corresponding results obtained for electrospray ionization mass spectrometric (ESI-MS) measurements of relative binding selectivities displayed by the same hosts toward a series of alkali metal chlorides. Among the crown-5 hosts studied, 6a displays enhanced avidity toward complexation with K+ picrate in liquid-liquid extraction experiments. Among the three crown-6 hosts, 4b proved to be the best alkali metal picrate extractant and displayed significant levels of avidity toward complexation with the larger alkali metal cations (i.e., K+, Rb+, and Cs+). The trends in the picrate extraction and the ESI-MS results obtained herein show several notable similarities and some differences. The similarities generally stem from size-selective binding properties that are intrinsic to the different cavity sizes of the cage-annulated macrocycles, whereas the differences reflect the important influence of solvation. effects on the binding properties of the macrocycles.