Nuclear magnetic resonance studies and molecular modelling of the solution structure of some dibenzo crown ethers and their complexes

Abstract

The 1H and 13C NMR spectra of the structurally related crown ethers 1– 7 with further potential coordination groups (OH, COOH) and the alkali metal complexes of the latter were recorded. Structural information was deduced especially from 13C NMR chemical shifts, the salt-induced 13C chemical shifts (as a measure of both the in-plane interactions HC 2C 1OC 8H 2 and HC 5C 6OC 7H 2), vicinal H,H coupling constants and spin-lattice relaxation times, T 1, of protonated 13C nuclei. The complex stability constants ( K A) for the complexes of 1– 7 with LiI were determined by 7Li NMR spectroscopy. Accompanying quantum-mechanical calculations corroborate the spectroscopical conclusions about the preferred conformers of the studied crown ethers both in the free and in the complexed state. New insights into the complexation behaviour of the crown ether carboxylic acids 3– 5 were obtained which are in contrast to the generally discussed “cavity effects”. The results of NMR spectroscopy and molecular modelling prove the complexation of the crown ether carboxylic acid to take place first at the carboxyl group. In a second step, the bound cation is folded over the cavity of the same crown ether or over one of another crown ether ring system.