Synthesis and Alkali Metal Ion Binding Properties of Two Rigid Sterochemical Isomers of Calix[6]arene Bis-crown-4

Cone and 1,2,3-alternate stereochemical isomers of 37,40-diallyloxy-(38,42),(39,41)-bis-crown-4 calix[6]arene, 3 and 4 were isolated in moderate yields by bridging the dialkylated calix[6]arene 2 with triethylene glycol di- p -tosylate. The alkali metal complex stoichiometries, association constants, and ion selectivities of 3 and 4 were studied by 1 H NMR titration experiments, liquid−liquid extraction, electrospray ionization mass spectroscopy, and X-ray crystallography. Good agreement between the gas-phase and solution-phase studies regarding these metal binding properties was observed. Both conformers formed 1:1 complexes with all alkali metal ions but were structurally preorganized such that each exhibited a strong preference for the larger cesium ion as evidenced by the “deep-cavity” cesium complex of host 4, wherein π-metal interactions helped to stabilize the complex. The Cs + /Na + selectivity factor for 4 was found to be 1500, while that of 3 was only 140.