a Molecular structure of the tris-tridentate ligand (LI) and its self-assembled tetrahedral cage M4L4 (M for metal ions). b Selective self-assembly of LI with M2 over M1 in the mixture of M1/M2 determined by 1H NMR spectroscopy with ±5% error. c Separation factors (with ±5% error) from lanthanide solvent extraction experiment. Reproduced from ref. 17.