Host-Guest Chemistry of Acridone-based Coordiantion Cages

Supramolecular coordination cages, assembled from organic ligands and metal cations, are of broad interest due to their versatile topologies and properties. Discrete cages often possess a secluded cavity, which allows the inclusion of various guest molecules (anions, cations and neutral molecules). These structures are stabilized by non-covalent interactions and commonly referred to as host-guest complexes. The here presented thesis entitled “Host-Guest Chemistry of Acridone-based Coordination Cages” focuses on the synthesis, self-assembly and host-guest chemistry of novel coordination cages based on acridone-derived ligands L and square-planar Pd(II) metal ions. In the first part of this thesis, the preparation and characterization of an interpenetrated coordination cage from eight bispyridyl ligands L1 and four Pd(II) cations was studied. This [Pd4L1 8] assembly shows the unique ability to encapsulate neutral guest molecules after activation through addition of halide anions. It is the first example of an interpenetrated coordination cage that shows this feature. In this project, the range of encapsulated neutral molecules was explored. In particular, the size, shape as well as the amount and positions of heteroatoms within the guests were varied and the influence of dispersion interactions in the formation of the hostguest complexes was investigated (Chapter 2). Additionally, the interpenetrated coordination cage has the unique ability to function as a photosensitizer by exciting triplet oxygen into singlet oxygen. The reactivity of the coordination cage was discovered as the guest 1,3-cyclohexadiene was transformed into the Hetero-Diels-Alder product 2,3-dioxabicyclo[2.2.2]oct-5ene in presence of oxygen and light (Chapter 3). The uptake of halide anions in the interpenetrated coordination cage occurs after an allosteric mechanism with positive cooperativity. The chloride- binding ability of the acridone-based [Pd4L1 8] was compared with previously reported interpenetrated coordination cages based on dibenzosuberone and phenothiazine (Chapter 6). Introduction of a bulky adamantyl group in the novel ligand L2 prevents dimerization and results in the formation of the monomeric cage [Pd2L2 4]. Owing to steric crowding, the adamantyl substituent is considerably bent sideways with respect to the ligand backbone and an unprecedented flipping motion of the free ligand was observed. Surprisingly, this unique dynamic also occurs in the coordination cage. Despite the very dense packing within the self-assembled structure, the cage is able to encapsulate a series of bis-anionic guests in an induced-fit fashion. Additionally, electronic structure calculations revealed a substantial contribution from dispersion interactions between the guest and the surrounding adamantyl groups that stabilize the host–guest complex (Chapter 4). X The variation of ligand length, through introduction of different linkers between the acridone backbone and the coordinating pyridyl groups, illustrated the remarkable influence of this ligand feature. Depending on the length, the formation of monomeric or dimeric interpenetrated coordination cages was achieved (Chapter 5). The formation and characterization of these novel structures were verified with the help of NMR spectroscopic studies, HR-MS spectrometric data and X-ray diffraction analysis of several obtained crystal structures. The collected results give a deeper insight in the understanding of supramolecular coordination cages, especially their formation via self-assembly, their ability to form host-guest complexes with a variety of different guest molecules and the influence of dispersion interactions on the stability of the systems. It broadens the scope of supramolecular assemblies and is the basis for further applications in the field of selective recognition, tunable guest uptake and catalysis.