Multinuclear NMR study of the interaction of the shift reagent lanthanide(III) bis(triphosphate) with alkali-metal ions in aqueous solution and in the solid state

Abstract

The paramagnetic ion-induced relaxation rate enhancements of 6Li in adducts of Li+ and Ln(PPP)J- complexes (Ln = Dy, Tm) in aqueous solution show that up to seven monovalent counterions can coordinate in the second coordination sphere of the Ln(II1) ion to the outer oxygens of the triphosphate ligands. However, the pseudocontact ’Li NMR shift data suggest that in the second coordination sphere some preference of the counterions for the axial region opposite the water ligand may exist. The estimated Ln3+-Li+ distances range from 5.1 to 5.9 A for [Li+]/[Ln(PPP)J-] ratios of 0.2-7. This is supported by a two-dimensional nutation spectrum of a polycrystalline sample of Na,La(PPP),, which indicates coordination of all Na+ ions to phosphate oxygens.