Current work “Heterocyclic electron-rich redox-active self-assembly cages” is devoted to the developing of new organic electron-rich redox-active ligands based on the tetrathiafulvalene (TTF), dithiafulvalene (DTF) and π-extended tetrathiafulvalene scaffolds, as well as their application in construction of new self-assembly cages.
Within the work, 5 new electron-rich redox-active ligands were synthesized on the basis of TTF and DTF scaffold and fully characterized with a particular emphasis on their structural and redox properties. Electrochemical behavior of the TTF ligands demonstrates two characteristic reversible oxidation process with formation of radical cation and dication respectively. DTF ligands possess on oxidation process with formation of a radical cation, changing from irreversible in DMF to fully reversible in the mixture dichloromethane/acetonitrile.
The usage of coordination-driven self-assembly strategy allowed to obtain with high yields 16 new self-assembly cages in combination with Palladium, Ruthenium and Silver acceptors. Formation and structure of corresponding cages were confirmed by 1Н, DOSY, 13С, COSY, NOESY, ROESY NMR, cyclic voltammetry, high resolution mass-spectrometry, for 6 of them x-ray data was obtained.
M8L2 cages using TTF side walls first demonstrated close distance between TTF subunits within the cage, which was confirmed by cyclic voltammetry, spectroelectrochemistry and x-ray data. Ligands based on the DTF scaffold were first used for obtaining redox-active cages in combination with Palladium (resulting in M2L4 and M2L2 self-assemblies) or Ruthenium complexes (resulting in M4L2 self-assemblies). M2L2 Palladium showed effective encapsulation of both large neutral (fullerene and its derivatives) or smaller electron-poor planar molecules. M4L2 Ruthenium-based cages demonstrate dimerization upon increasing the size of bis-ruthenium fragment. Association constant for the corresponding dimer was evaluated using 1H NMR data. Introduction of the electron-poor guest molecules also leads to the formation of a monomer*guest complex and complete dissociation of a dimer.
Special attention was focused on the electrochemical behavior of obtained cages. TTF-based cages exhibit formation of a mix-valence species upon first oxidation to the radical-cation, which correlates well with close distance between TTF panels. Increasing the size of ruthenium sub-unit eliminates this effect. M2L4 lantern-shaped cages using DTF ligands in combination with square-planar palladium retain one distinct oxidation wave to the radical-cation, as well as changing the reversibility of oxidation depending on the solvent. Same behavior is shown for M2L2 cages. In addition, ferrocene sub-unit in M2L2 cages can be used as internal electrochemical reference to further study their behavior. In some M2L2 and in M4L2 formation of a mix-valence species is observed due to the closer distance between the DTF panels. Moreover, cyclic voltammetry using different concentrations allows to attribute it to either inter- or intramolecular processes depending on the structure, as well as to investigate influence of guest molecules.
Continuation of the exTTF ligand led to the new extra-large M12L6 cages in combination with linear acceptors based on the trans-palladium and silver. Theoretical calculations shown both cages possessing big internal cavities of approximately 3658 Å3 and 3954 Å3 respectively. Electrochemical behaviour suggests that palladium-based self-assembly retains its structure upon oxidation, while silver-based cage reversibly disassembles.
The latter cage was used to show first transformation from a discrete cage into supramolecular polymer upon oxidation of the ligand. X-ray analysis also showed important role of the side chains in both oxidation mechanisms and the final structure.
Further research into exTTF-based ligands led to the new compounds using benzo[1,2-b:4,5-b']dithiophene central fragment. It allowed fine control over the general geometry of the ligand, what was demonstrated by cyclic voltammetry and x-ray studies of both neutral and oxidized forms. Obtained data was supported by the theoretical calculations on the B3LYP-D3/6-31G** level.
