Selective Syntheses and Stability of Palladium and Platinum-Based Coordination Cages

Palladium- and platinum-based coordination cages are hollow supramolecular architectures whose structure can be tuned primarily by ligand design and templation. This thesis presents strategies for selective syntheses of distinct cage species and isomers using geometrical factors, templation, thermodynamic and kinetic effects into account.

In Chapter 2 octahedral coordination cages of the formula [Pd6L12] (BF4)12 were synthesized using low-symmetry heteroditopic N-donor ligands. Due to the low symmetry of the ligands, 112 isomers can potentially be formed. We show that the isomer with cis-coordination at all Pd-centres is thermodynamically favoured. This rule is observed in heteroleptic systems as well. Ligands with different geometries and donor groups (pyridyl-, triazolyl and imidazolyl-donor) are used to discuss the scope and limitations of this selectivity. Furthermore, we provide a geometrical explanation using a coordination vector model for the experimentally observed cis-isomers.

Chapter 3 follows up with the construction of octahedral cages using small low-symmetry N-donor ligands. This time, hexanuclear Pd6L12-type cages were obtained as a mixture of isomers, as evidenced by NMR spectroscopy and single-crystal X-ray crystallography. Attempts to resolve the isomers by cyclic ion mobility-mass spectrometry (cIM-MS) are described. Cages of the general formula [Pt2Pd4Cl8L8] (BF4)4 were obtained in kinetically controlled reactions of homoleptic PtL42 complexes with PdCl2(CH3CN)2.

The relative stability of hexanuclear Pd-based coordination cages was investigated in Chapter 4. For this purpose, eight cages of type [Pd6L12]12+ or [Pd6L8]12+ were synthesized. The stability of the cages was investigated by subjecting cage solutions in DMSO-d6 or CD3CN to different amounts of pyridine. The cage degradation was monitored by 1H NMR. Our findings reveal significant variability in cage stability. The relative thermodynamic stability of the cages varies by nearly two orders of magnitude and the kinetic stability by more than four orders of magnitude. In Chapter 5 describes the templated synthesis of lower-nuclearity imidazole-based cages [PdnL2n]2n+ using radiotherapeutically relevant ReO4â and sulforhodamine dyes. Cages of three different nuclearities were obtained selectively using the same ligand. The structures were characterized by single-crystal X-ray crystallography.

In Chapter 6 the chloride-templated cage [Pd2L4Cl] (BF4)3 was obtained isomer-selectively using tritopic ligands. The resulting tetratopic metallo-ligand was combined with PdCl2(CH3CN)2 to form a dodecanuclear barrel [Pd12L12Cl15] (BF4)9. Using the heterometallic analogue [Pt2L4Cl] (BF4)3, the corresponding heterometallic [Pt6Pd6L12Cl15] (BF4)9 barrel was synthesized. Lastly, in Chapter 7 the assembly of heterotrileptic palladium cages from geometrically distinct ligands was investigated. The flexibility of alkynyl spacer in the ligands was found to be crucial to co