A High-resolution 13C NMR Study of Some Ethylene Oxide Oligomers, Cryptands and Crown Ethers Complexed with a Variety of Metal Ions in the Solid State. Conformational Change of Ligand Molecules as Viewed from the Conformation-dependent Displacements of 13C Chemical Shifts

Abstract

Abstract We recorded high-resolution solid-state 13C NMR spectra of ethylene oxide oligomers, cryptands, and crown ethers complexed with metal ions, determined by cross polarization-magic angle spinning (CP-MAS) NMR method. It is found that peak-intensities of these complexes are significantly enhanced by the presence of metal ions which act toward conformational fixation of flexible ligand molecules. The 13C chemical shifts of ethylene oxide oligomers complexed with HgCl2 or CdCl2 as open-chain analogues of crown ethers were successfully related with a pair of torsion angles of nearby single bonds. 13C Chemical shifts of cyclic ionophores were displaced (up to 6 ppm) together with conformational changes of ligand molecules. In addition, the 13C chemical shifts of the carbons neighbored with nitrogen atoms of cryptands [2.2.2] and [2.2.1] were well related with a pair of torsion angles of neaby bonds. Further, we found that several kinds of metal complexes with cis-syn-cis and cis-anti-cis isomers of dicyclohexano-18-crown-6 gave rise to substantially different spectral profiles, resulting in the occurrence of significant conformational changes of these ligand molecules as a result of complex formation.