Self-Assembled Main-Group Metal Coordination Cages Directed by Ligand Topology and Stoichiometry

Abstract

This Doctoral Thesis focuses on the design, synthesis, characterization, and study of new coordination-based supramolecular cages incorporating Al3+, Ga3+ and Fe3+. These supramolecular architectures exhibit notable host–guest properties and structural stability in both solid state and solution. Through a systematic approach, the thesis analyses the key factors that govern their self-assembly, stability, geometry, and encapsulation behavior. The work is structured into three interrelated chapters, each addressing distinct families of cages formed using ligands designed to promote specific geometries and functional properties. Chapter I presents the development of helicate or mesocate coordination cages of M2L3 type (M = Ga3+, Fe3+), constructed via coordination of ditopic ligands based on 1,2,3-triazoles with catechol or naphthalene-diol units. These cages are assembled through CuAAC (Cu-catalyzed 1,3-dipolar cycloaddition) reactions. The influence of steric factors such as naphthalene-diol groups as anchoring points and counterions (ammonium cations) is shown to be crucial in modulating cage geometry, solubility, and the ability to encapsulate guest species like K⁺. Stoichiometric and structural control enables selective assembly into helicate or mesocate geometries, highlighting their potential for host–guest applications. Chapter II explores M4L4 tetrahedral cages (M = Al3+, Ga3+) assembled with tritopic ligands derived from 1,2,3-triazole–catechol frameworks, including variants with naphthalene-diol units for cavity modulation. These cages possess well-defined cavities capable of selectively encapsulating organic cations (R4N+), as studied by NMR (1H, NOESY, DOSY) and ITC. Binding constants are in the 103–104 M-1 range, with selectivity depending on guest size: Me4N+ and Et4N+ are efficiently encapsulated, while Bu4N+ is too bulky. Al4L4 prefers smaller guests, while Ga4L4 accommodates larger ones. These results illustrate the precise structural and functional control achieved through ligand design and metal selection, opening opportunities in supramolecular catalysis and molecular recognition. Chapter III investigates triply interlocked [2]catenane coordination cages of M6L4 type (M = Al3+, Ga3+). These topologically complex cages are distinct in geometry and structure from the M2L3 and M4L4 cages described previously and represent the first examples of such assemblies formed with main group metals instead of transition metals. Their assembly occurs in aqueous media and is governed by pH and a ∼3:2 metal-to-ligand stoichiometry. NMR and ITC studies reveal a highly cooperative self-assembly mechanism, with no monomeric M3L2 intermediates detected. The assembly/disassembly processes are reversible and metal-dependent. Thermodynamically, Ga6L4 is ~500 times more stable than its Al6L4 analogue, due to stronger Ga–O coordination and enhanced p–p stacking interactions. These findings demonstrate the possibility of controlling dynamic and complex structures via rational synthetic and structural design. ------------------------------------------------