Paramagnetic Lanthanide NMR Probes Signalling Changes in Zinc Concentration by Emission and Chemical Shift: A Proof of Concept Study

A zinc‐selective probe based on a set of rare earth complexes of a modified DO3A macrocyclic ligand incorporating a tris‐pyridylamine (TPA) moiety has been structurally characterised in solution and in the solid‐state. One pyridine group possesses a tert ‐butyl substituent to serve as an NMR reporter group. The mono‐capped square‐antiprismatic Dy complex has a long bond (2.83 Å) to an apical N atom (p K a 5.70 Eu) and binds to one water molecule on zinc binding. Zinc binding is reversible and involves all of the exocyclic ligand N donors; it is signalled by large (ratiometric) changes in Eu emission intensity, and by dramatic changes in the size (>50 ppm) and sign of the chemical shift of the paramagnetically shifted t Bu resonances in Tb, Dy and Tm complexes. Slow trans ‐metallation was observed, leading to formation of an unusual di‐zinc species in which one zinc ion is seven‐coordinate and the other is six‐coordinate.