Structural transformations of metal-organic cages through tetrazine-alkene reactivity

The assembly of metal-organic cages follows rules based on the coordination preferences of metal ions and the geometries of their typically rigid and planar precursor ligands. PdnL2n metal-organic cages are amongst the most structurally diverse with subtle differences in the metal-ligand coordination vectors giving rise to drastically different assemblies, however al-most all rely on the rigidity of aromatic linker groups to avoid the formation of intractable mixtures of structures. Here we exploit the inverse electron-demand Diels-Alder (IEDDA) reaction between tetrazine linker groups and alkene reagents to trigger structural changes induced by post-assembly modification. The structure of the 1,4-dihydropyridazine produced by IEDDA (often an afterthought in click chemistry) is crucial, with the presence of two sp3 centers leading to a greater degree of flexibility and non-planarity in this ligand. This drastically changes the range of accessible metal-ligand coordination vec-tors, triggering an initial Pd4L8 tetrahedral cage to transform into different Pd2L4 lantern cages, with both the extent (thermo-dynamics) and rate (kinetics) of this transformation dependent on the alkene dienophile selected. With cyclopentene, the unsymmetrical 1,4-dihydropyridazine ligands undergo integrative sorting in the solid state, with both head-to-tail orientation selection and enantiomer selection, leading to a single lantern isomer from the 39 possible. This preference is rationalized through entropy, symmetry, and the extent of hydrogen bonding. Subsequent oxidation of the 1,4-dihydropyridazine to the aromatic pyridazine rigidifies the ligands and imparts planarity again. The oxidized ligands can no longer fit in the lantern structure, inducing further structural transformations into Pd4L8 tetrahedral cages and Pd3L6 double-walled triangles. The concept of controllable addition of limited additional flexibility and then its removal through well-defined reactivity we en-visage being of great interest for those interested in structural transformations of any class of supramolecular architecture.