Cinquefoil Knot Possessing Dynamic and Tunable Metal Coordination

While the majority of knots are made from the metal-template approach, the use of entangled, constrained knotted loops to modulate the coordination of the metal ions remains inadequately elucidated. Here, we report on the coordination chemistry of a 140-atom-long cinquefoil knotted strand comprising five tridentate and five bidentate chelating vacancies. The knotted loop is prepared through the self-assembly of asymmetric “3 + 2” dentate ligands with copper­(II) ions that favor five-coordination geometry. The formation of the copper­(II) pentameric helicate is confirmed by X-ray crystallography, while the corresponding copper­(II) knot is characterized by XPS and LR-/HR ESI-MS. Upon removal of the original template, the knotted ligand facilitates zinc­(II) ions, which typically form four- or six-coordination geometries, resulting in the formation of an otherwise inaccessible zinc­(II) metallic knot with coordinatively unsaturated metal centers. The coordination numbers and geometries of the zinc­(II) cations are undoubtedly determined by X-ray crystallography. Despite the kinetically labile nature and high reversibility of the zinc­(II) complex preventing the detection of 5-to-6 coordination equilibrium in solution, the effects on metal-ion coordination induced by knotting hold promise for fine-tuning the coordination of metal complexes.