Aggregation of Rare Earth Coordination Complexes in Solution Studied by Paramagnetic and DOSY NMR

The degree of aggregation of neutral, 9-coordinate rare earth coordination complexes has been shown to affect their ligand field, as revealed by diffusion-ordered NMR spectroscopy (DOSY-NMR) measurements on Y(III) complexes, paramagnetic NMR analyses of Yb and Tb analogues and emission spectral studies with the Eu^III systems. In non-polar media a lipophilic tris-isopropyl complex, [Ln.L²] tends to aggregate in chloroform and dichloromethane giving rise to oligomers, whereas in acetic and trifluoroacetic acid the more polar parent complex, [Ln.L¹], also aggregates, profoundly affecting the pseudocontact shift and the form of the Eu emission spectrum. Such behaviour has important implications in the design of responsive spectral probes.