Synthesis of new crown ethers and their metal complexes: 1H and 13C NMR study

A new type of crown ethers containing a diphenyl ether unit has been prepared, the ring size ranging from 12 to 36. ¹H and ¹³C NMR spectra of both free ligands and their metal-ion complexes have been recorded. For 18- and 21-membered compounds a general downfield shift was observed for both methylene and aromatic proton resonances on metal-ion complexation. The stoichiometry of K⁺ and Na⁺ complexes was deduced from chemical shift dependence on metal-ion concentration. The K⁺ and Na⁺ complexes of 18- and 21-membered rings have a guest to host ratio of 1:1, whereas the K⁺ salt of the 15-membered ring exists as a 1:2 complex in solution. The ¹H shift observed on salt formation was attributed to electric-field and conformational effects. The ¹³C resonances for the aryl carbons, C-1, C-2 and C-3, and the α-methylene carbon in 15- and 18-membered rings were shifted upfield when an equivalent amount of KSCN was added in CDCI_3?DMSO-d₆. The shift changes were independent of the anion, and similar results were obtained for SCN^?, Br^?, and I^? salts. The upfield shift is explained by conformational factors. The spectral changes were slight for 12- and 36-membered rings. In 15- and 18-membered rings, complexation induces conformational changes which force the C-α carbon into the plane of the benzene ring. The solution conformation of these molecules is discussed.