From open containers to confined supramolecular architectures

Abstract

The work will investigate the preparation and characterization of charge-neutral Zn(II) metal organic cages based on bis(bidentate) hydroxyquinolate ligands, with focus on how ligand topology and functionality modulate anion recognition and guest-directed assembly. Building on a charge-neutral [Zn2L2] host-complex presented by our group in 2022, various rational designed ligands were synthesized and self-assembled with Zn(OAc)2 to yield discrete [Zn2L2] architectures and others. The first chapter will introduce three new bis(bidentate) ligands LmN3-H2, LN3-H2 and Lcrown3-H2 which are designed and synthesized to increase the functionality of the parent [Zn2L2] cage. Spectroscopic and computational analysis indicate that expanded π-surfaces preserve the metal organic cages integrity while enabling additional host-guest π-interactions, and a crown-ether functionalized derivative demonstrates heteroditopic ion-pair binding. Second, to address the challenge that strongly chelating oxalate can disrupt metal-ligand assemblies, a more flexible ligand LDB3-H2 was designed and synthesized to afford a robust charge-neutral [Zn2LDB32] host-complex. This container forms a well-defined 1:1 oxalate host-guest complex in solution. UV/Vis titrations quantify binding and competition experiments demonstrate selective oxalate recognition over longer dicarboxylates and monocarboxylates. Dicarboxylates form host-guest complexes in a slow-guest exchange behavior, whereas mono-carboxylates demonstrate fast-exchange. Finally, a tripodal bis(bidentate) ligand, LTP3-H3, enables guest-induced control over nuclearity and topology. Tricarboxylates template direct formation of trinuclear and hexanuclear onion-type host-guest complexes with architecturally appealing supramolecular features. 1H NMR competition experiments reveal observable interconversion of a trinuclear species into a hexanuclear species and give insight into the respective binding affinities. Kinetic studies suggest an associative-dissociative exchange-transformation mechanism with a substantial activation barrier.