Towards squaramide-based coordination cages with catalytic activity
Creators: name: Yalçin, Ertuğrul
Publisher: name: TU Dortmund
Year: 2025
Subjects: subject: Squaramide
subject: Coordination cages
subject: Molecular recognition
subject: 540
subject: Koordinationschemie
subject: Molekulare Erkennung

description: This thesis explores two main topics: the computational study of strain in heteroleptic coordination cages and the incorporation of squaramide functionalities into coordination cages to investigate their host-guest properties. The computational study aimed at expanding the understanding of the thermodynamic driving forces behind the non-statistical self-assembly of cis-[Pd2L2L´2] and trans-[Pd2L2L´2] nanocages. Previous studies on Pd(II)-based homo- and heteroleptic cages composed of a carbazole ligand with either an elongated or non-elongated phenanthrene ligand were analyzed using NMR spectroscopy, ESI-MS and single-crystal X-ray analysis. The findings of these studies raised the question of whether differences in ring strain could be reproduced through gas-phase electronic energy comparisons via DFT geometry optimization. To address this, cage-to-cage transformations were examined, and ligands excised from heteroleptic cages were compared with their fully relaxed counterparts to assess strain in self-assembled coordination cages. This work extends these studies to acridone- and inverted carbazole-based ligands paired with the respective phenanthrene ligands. The calculations revealed that acridone- and inverted carbazole-based ligands preferentially form heteroleptic coordination cages with elongated phenanthrene ligands rather than with the shorter phenanthrene ligands. Additionally, the incorporation of diketopyrrolopyrrole-based ligands in trans-[Pd2L2L´2] cages featuring self-penetrated motifs was examined. Computational studies demonstrated that the release of strain upon transformation of a self-penetrated homoleptic diketopyrrolopyrrole cage to a heteroleptic cage plays a significant role in driving the preferential formation of the heteroleptic structures. Hydrogen bonding is a key noncovalent interaction that influences molecular recognition and catalytic processes. Squaramides are well-known hydrogen bond receptors and catalysts, possessing both hydrogen bond donor and acceptor groups. The second focus of this thesis is the heteroleptic cage formation of squaramidebased ligands with square-planar Pd(II) cations. Poorly soluble squaramide-based ligands assembled into a distinct assembly in DMSO-d6. To enhance the likelihood of observing the effects of hydrogen bonding on host-guest interactions, derivatives of squaramide-based ligands were synthesized and formed analogous heteroleptic cis-[Pd2L2L´2] cages in CD2Cl2. The resulting assemblies were analyzed using NMR spectroscopy, ESI-MS and single-crystal X-ray crystallography. When two squaramide ligands were combined, the formation of a heteroleptic cage was observed. X-ray analysis revealed that the hydrogen bond donor functionalities of one squaramide is oriented outward, with intramolecular hydrogen bonding occurring within the cage. This raised the question of whether such intramolecular hydrogen bonding contributes to a cooperative effect in guest binding. To investigate this, a reference cage was designed by pairing a squaramide-based ligand with a benzothiadiazole-based ligand, forming a distinct heteroleptic cage with the same topology. Computational studies suggested that the cooperative effect of intramolecular hydrogen bonding is more pronounced when two squaramide units are involved in intramolecular hydrogen bonding. To validate this, binding constants for neutral guest molecules were determined via 1H NMR titration experiments and compared across the two different cage systems. The results confirmed a cooperative effect in guest binding, particularly for caffeine and β-nitrostyrene. Additionally, the host-guest properties of the assembled coordination cages were examined with chloride, disulfonate and phosphate anions. However, attempts to determine binding constants for anions to the solely squaramide-based heteroleptic cage in CD2Cl2 revealed broadening in the 1H NMR spectra, suggesting aggregation, likely due to ion pairing effects. Nevertheless, binding constants for two disulfonates with the benzothiadiazole-based cage were successfully obtained, demonstrating a binding constant that is an order of magnitude stronger than for other palladium-based cages. In addition, the incorporation of 1,3-squaramides with a piperazine linker into coordination cages with Pd(II) was investigated. The self-assembled structures and their host-guest complexes were analyzed using NMR spectroscopy, ESI-MS and single-crystal X-ray crystallography. A ringlike topology with the molecular formula [Pd3L6]6+ was characterized. Binding constants for Allura Red and ferrocene-1,1’-disulfonate were determined via 1H NMR titration experiments. Cyclic voltammetry experiments showed that the redox event of Pd3L6 is electrochemically reversible, whereas the free ligand exhibits no reversibility.