Palladium-based supramolecular assemblies: from complex structures to water-soluble anion-receptors

The combination of palladium salts and bipyridyl ligands can lead to the formation of a large variety of coordination complexes, with different shapes and sizes, displaying a very versatile host-guest chemistry. Increasing their structural complexity remains a central challenge in the field and this thesis describes different approaches to address it.Chapter 2 describes a selection approach, which allowed to identify a novel hexanuclear assembly incorporating two types of dipyridyl ligands. A virtual combinatorial library of PdnL2n2n complexes was prepared by mixing six different ligands with substoichiometric amounts of Pd2+. Equilibrating the reaction mixture resulted in the preferential formation of a heteroleptic Pd6L6L'612 assembly which was then synthesized on a preparative scale. A related but significantly larger Pd6L6L'612 cage was obtained from a pair of metalloligands with a similar combination of bending angles.Chapter 3 describes an investigation on the Li+-binding properties of Pd2+-based hosts. One of the complexes underwent a significant structural rearrangement when LiBF4 was added. Namely, the initial Pd2L4 species was converted to a low-symmetry Pd4L8 assembly, enclosing two solvated LiBF4 ion pairs. The conversion did not occur with other alkali metal ions, indicating highly specific host-guest interactions. Structural analyses revealed the important contributions of π-stacking intramolecular interactions to maintain the highly compact structure of the Pd4L8 receptor.In Chapter 4, the possibility to target the synthesis of intricate Pd-assemblies is investigated. The observations discussed in Chapter 3 were used as a basis to define key characteristics that a ligand should possess to accommodate in such structures. A set of new ligands was designed and prepared following those guidelines. In one of the cases, the complexation with Pd2+ resulted in the formation of a reduced-symmetry Pd2L3 species displaying strong π-stacking interactions between the three adjacent ligands.Chapter 5 describes the preparation of a five-stranded heterometallic helicate incorporating two Pd2+ ions and one La3+ center. Analyses highlighted the low symmetry of the assembly, both in solution and in the solid state. The penta-stranded helicate could be dynamically interconverted with a symmetrical, four-stranded helicate by adjusting the metal-to-ligand ratio.Important structural complexity is, however, not always necessary to achieve strong host-guest interactions. In Chapter 5, the synthesis of a water-soluble Pd2L4 coordination cage from Pd(NO3)2 and a 1,3-di(pyridin-3-yl)benzene ligand, functionalized with a solubilizing side chain, is described. The nitrate anion located in the cage's cavity can be exchanged for halide guests. An apparent association constant of Ka = 1.8(±0.1) x 10^5 M-1 was determined for binding chloride in buffered aqueous solution. This value is significantly higher than what has been reported for other macrocyclic chloride receptors. While heavier halides compete with binding or self-assembly, the receptor displays very good selectivity over common biological anions. The chloride binding affinity was further increased by a factor of three using a fluorinated ligand.