NMR spectroscopic study of solution structure and complexational behaviour of bis-benzo crown ethers

• Published in: Journal of molecular structure Vol. 404; no. 3; pp. 273 - 290
• Main Authors: Kleinpeter, E., Starke, I., Ströhl, D., Holdt, H.-J.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 28.02.1997
• Subjects: Bis-benzo crown ethers; Conformational isomerism; NMR spectroscopy; Conformational isomerism; NMR spectroscopy; Bis-benzo crown ethers
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/S0022-2860(96)09444-6

The 1H, 13C and 15N NMR spectra of a series of polymethylene-bridged 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 by COSY, HMQC and HMBC 2D NMR experiments. The stereoisomerism of the bridging RCHNNHC(O)(CH 2) n moieties was studied by various useful NMR parameters ( δ 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, respectively) with respect to the amide bond proved to exist, NH and the imine lone pair were found in anti position. The OCH 2CH 2O fragments occur as rapidly interconver gauche conformations and the α-CH 2 groups more or less in-plane with the adjacent phenyl ring moiety as concluded from the corresponding 13C chemical shifts and NOEs, respectively. 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 conformational equilibrium about the CHPh bond proved to be strongly dependent on the bis-benzo crown ethers 1–11 studied. The conformational study is accompanied and corroborated by molecular dynamics and quantum-chemical calculations.