Nuclear magnetic resonance studies of neutral lanthanide(III) complexes with tetraaza-macrocyclic ligands containing three phosphinate and one carboxamide co-ordinating arms

Abstract

Novel, neutral Eu^III, Gd^III and Yb^III complexes have been obtained with tetraaza-macrocyclic ligands containing three phosphinate and one carboxamide co-ordinating arms. The overall results are consistent with a co-ordination geometry based on the octadenticity of these ligands. Among the possible 16 enantiomeric pairs one is largely dominant as ascertained by the high resolution ¹H, ¹³C and ³¹P NMR spectra of the Eu^III and Yb^III complexes. Partial assignment of ¹H and ¹³C resonances was possible for one Eu^III complex on the basis of homo- and hetero-correlated two-dimensional NMR experiments. The evaluation of the Curie contribution to the longitudinal relaxation rates of the ³¹P resonances has provided a route to the determination of the Yb–P distances. The addition of the chiral solvating agent β-cyclodextrin to a solution of the Yb^III complex containing benzylic substituents on the amido nitrogen allowed the chiral resolution of an enantiomeric pair by ³¹P NMR spectroscopy. The measurement of water proton longitudinal relaxation rates of solutions of the Gd^III complexes indicated that these chelates have one inner-sphere co-ordinated water molecule whereas the parent tetraphosphinate complexes have none.