Synthesis, structure and paramagnetic NMR analysis of a series of lanthanide-containing [LnTiO(OPr)(salicylate)] cages.

The influence of paramagnetic Ln ions on the NMR behaviour is investigated via a series of new isostructural lanthanide-containing cages with the general formula [LnTiO(OPr)(salicylate)] (Ln-1, Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho and Er). Compared to conventional coordination compounds containing Ln ions, the peripheral ligands in Ln-1 are separated from the paramagnetic lanthanide centres by oxo-Ti linkages, and therefore experience a weaker paramagnetic influence. As a result, all of the H and C NMR signals of these Ln-1 cages (except for Gd-1) are observed and can be unambiguously assigned, which provides an excellent platform for the in-depth study of the NMR behaviour of paramagnetic Ln ions. Further analysis of the fully assigned paramagnetic signals suggests that the pseudo-contact component dominates the H NMR shifts, whereas both the pseudo-contact and Fermi-contact contributions affect the C shifts, although the majority of the resonance environments are at least four bonds distant from the central Ln ion. Our results also show that the use of Bleaney's factors to describe the pseudo-contact shift is inadequate for this Ln-1 cage series.