NMR SPECTROSCOPIC STUDY OF SOLUTION STRUCTURE AND COMPLEXATIONAL BEHAVIOR OF BIS-BENZO CROWN-ETHERS

The H-1, C-13 and N-15 NMR spectra of a series of polymethylene-bridge d carbonylhydrazone bis(benzo-15-crown-5 ethers) 1-9 (n = 0-8) and bis (benzo-18-crown-6 ethers) 10-11 (n = 1,5) were recorded and assigned b y COSY, HMQC and HMBC 2D NMR experiments. The stereoisomerism of the b ridging R-CH=N-NH-C(=O)-(CH2)(n) moieties was studied by various usefu l NMR parameters (delta(C=O), (3)J(C,H), (1)J(N-H), (1)J(C-H), (2)J(C( =O)-NH)) and intramolecular NOEs. Three isomers (E/E, E/Z and Z/Z, res pectively) with respect to the amide bond proved to exist, NH and the imine lone pair were found in anti position. The -OCH2CH2O- fragments occur as rapidly interconverting gauche conformations and the alpha-CH 2 groups more or less in-plane with the adjacent phenyl ring moiety as concluded from the corresponding C-13 chemical shifts and NOEs, respe ctively. The solvent dependence of the amide isomerism was studied and the complexation of the bis-benzo crown ethers to Na+ and K+ cations carefully investigated. The conformation of the free and complexed bis -crown ethers was obtained. Both the complexation mechanism (sandwich, double-sandwich, inclusion and addition complex) and the conformation al equilibrium about the =CH-Ph bond proved to be strongly dependent o n the bis-benzo crown ethers 1-11 studied. The conformational study is accompanied and corroborated by molecular dynamics and quantum-chemic al calculations. (C) 1997 Elsevier Science B.V.