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 Ï -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 à  -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(à ±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 rep