The nature of the exchange of free crowns with a cation crown complex studied by 1H n.m.r.

Abstract

In the ¹H n.m.r. spectra of propeller crown ligands and their salt complexes, the OCH₂CH₂O segments next to the naphthyl rings have ABCD spectra (interpreted in terms of the fluxional process averaging gauche and anti placements of the two oxygens) which have been completely assigned (NUMARIT), defining in detail the stereochemistry of these segments. The exchange of excess ligand with salt complex, studied by dynamic ¹H n.m.r. spectroscopy in a mixed solvent, was found to have a second-order and a first-order component, by the Shchori method, and the results are discussed in relation to the kinetic basis of selectivity of macrocycles for cations. The rearrangement of one OCH₂CH₂O segment between two gauche forms is shown to occur on a KSCN salt complex at a rate much faster than the overall ligand exchange, showing that such a rearrangement is a feasible fast preliminary step for exchange.