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

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Abstract
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 CDCI3−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.

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