Directed assembly of transition-metal-coordinated molecular loops and squares from salen-type components. Examples of metalation-controlled structural conversion

A series of transition-metal-containing molecular "loops" and "squares" has been prepared via a directed-assembly approach and characterized. The molecular loops were prepared from the reaction of cis-(PEt₃)₂Pt(OTf)₂ with bis(4-pyridyl)-functionalized free-base salen-type ligands. Zn(II)-metalation of the salen-type ligands in the molecular loops converts the loops to molecular squares. Alternatively, the squares can be obtained by the directed assembly of cis-(PEt₃)₂Pt(OTf)₂ and bis(4-pyridyl)-functionalized Zn(II)-salen-type ligands. A concentration-dependent dynamic equilibrium between cyclic species was observed when bis(3-pyridyl)-functionalized free-base salen-type ligand was employed in the reaction. Zn(II) or Cr(III) metalation of the free-base ligand shifted the equilibrium to the single dimeric species. The incorporation of multiple reactive metal sites into a single, cavity-containing supramolecular structure points toward catalytic applications for these new assemblies.