Cinquefoil Knot Possessing Dynamic and Tunable Metal Coordination

Abstract

While the majority of knots are made from the metaltemplate 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 140atom-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. ■ INTRODUCTIONKnotting is prevalent at the molecular scale, manifesting in structures such as DNA, RNA, proteins, synthetic polymers, liquid crystals, and subatomic particles. 1−5 Molecular knots constitute constrained closed loops with ordered entanglements, sometimes showcasing inherent topological chirality. 6−8 The substantial progress in knot synthesis 9−20 has enabled chemists to explore knot applications in allostery, strong and selective anion binding, 21 ion transport, 22 information transfer, 23 asymmetric catalysis, 24 and nanotherapeutics 25 and facilitating mechanical function at the molecular level. 26−29 Approximately, 70% of molecular knots are synthesized through metal-templation. 30−38 Organic knots generated in this manner can be regarded as ligands with multiple free chelating sites that can readily coordinate with metal cations. However, the understanding of 'knotting ef fect' on metal-ion coordination 39,40 and exploration of the "function at metalsites" in molecular knots have rarely been discovered.Coordinatively unsaturated metal sites are recognized as key intermediates in catalytic cycles of various organometallic reactions. 41 They also serve as active binding sites for incoming motifs in guest-binding 42,43 and molecular recognition processes. 44 The incorporation of unsaturated coordination sites into diverse architectures allows for the direct observation and analysis of these highly reactive species. 45 It also offers a