NMR Observations of Interligand Interference in the Molecular Motion of Double‐Stranded Dinuclear Helicates

Double‐stranded dinuclear complexes with either twisted or parallel arrangements were obtained by reaction between Schiff‐base ligands and PdII ions. Linear ligands mainly afforded twisted arrangements, while macrocyclic ligands forced the complex to adopt parallel arrangements. In these dinuclear complexes, one isomer could not be converted into the other even upon prolonged heating. The NMR spectra showed that the chemical shifts of the diastereotopic protons can be used as an indicator to distinguish between the arrangements. The NOE cross‐peak characteristics of the complexes with twisted arrangements are interpreted with reference to the crystal structure of a CuII complex as an isostructural model of the PdII complex. Variable‐temperature measurements revealed the rotational interference between the ligands of the counterpart strands, the first time that this has been observed for helically arranged complexes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)