New applications of crown ethers. X.¹) ¹³CNMR relaxation times study of cation-binding behavior of monoazacrown ethers and Bis(monoazacrown ether)s

¹³C NMR relaxation times (T₁s) have been determined for monoazacrown ethers (CA15C5 and CA18C6) and bis(monoazacrown ether)s (BCA15C5, BCA18C6, BOA15C5, and BOA18C6) in the absence and presence of Na⁺ and K⁺ ions. For the monoazacrown ethers with heptyl side chain and the bis(monoazacrown ether)s with pentamethylene bridge chain marked changes in T₁ values are observed upon the complexation, and the changes are dependent on the cation and on the structure of the ligands. The complexation with Na⁺ ion can give a larger effect on the T₁ values for the bis(monoazacrown ether)s of 15- and 18-membered rings, especially on the mobility of the bridging chain carbons, which is indicative of the formation of an intramolecular sandwich complex. On the other hand, the present T₁ study again shows the prominent effect of the oxygen in the bridging chain on the complexation of BOA15C5 and BOA18C6. Small differences in T₁ values obtained between Na⁺ complexes and K⁺ complexes of these two bis(crown ether)s suggest that the complexes take an analogous partial structure for the two cations.