Chromophoric coordination cages

Abstract

One of the principal reasons for the extensive study of coordination cages in the last few decades is the promise they hold for their use as artificial analogues to biological systems, and more specifically artificial enzymes. Indeed, like this class of catalytically active proteins, coordination cages and capsules possess a central cavity able to bind guest molecules. In turn, it has been shown by the supramolecular community that catalytic processes can happen in coordination cages, by the specific arrangement of guest molecules in the cavity. However, unlike enzymes, coordination cages have been seen as rather conformationally or topologically static, in contrast to the vast changes ternary and quaternary structure that may take place upon the binding of a substrate to an enzyme. Moreover, the cavity of non-chiral homoleptic coordination cages do not allow for the asymmetric catalysis enabled by the chirotopic and low-symmetry catalytic pocket of enzymes. Therefore, to better mimic biological systems with coordination cages, the study of low-symmetry systems that are responsive to external stimuli is of interest. In the first project of this thesis, I present an azulene-based Pd2L4 lantern-shaped cage that transforms into a Pd4L8 tetrahedral complex upon addition of benzene disulfonate guests. This geometrical change is clean, efficient, and rapid. The final structure was determined by single crystal X-ray crystallography. The use of azulene, a simple, but coloured two-ring aromatic hydrocarbon allows for an easy monitoring of the progress of the transformation by eye only. Using a second coordination cage based on methylene blue, the transformation was shown to be reversible, thanks to the transfer of the guest from the tetrahedron to the newly added cage, which has a higher affinity for the disulfonates. This experiment demonstrates several key principles of advanced coordination cage chemistry, such as multi-component non-statistical systems of narcissistically self-sorting cages, or guest transfer. Several other azulene-based ligands were synthesised as well, but none of their corresponding homoleptic assemblies could undergo the same cage-to-cage transformation, highlighting the sensitivity of the first system to shape and size. A second azulene-based family of ligands was prepared next, but this time with a chiral amino-biazulene backbone, isomeric to the widely adopted BINOL moiety. Two ligands were thus synthesised, the first of them bearing two pyridine donor groups. Using the racemate of the first ligand for the formation of the Pd(II)-cage ultimately resulted in the formation of a single meso-trans isomer of the Pd2L4 cage in acetonitrile and DMSO, instead of a statistical mixture. X-ray structure analysis of the cage unexpectedly revealed that discrete solvent molecules were responsible for the non-statistical arrangement, by acting as tethers between the amino groups of the ligands, through hydrogen bonding. This is the first time that single solvent molecules were shown to directly influence the outcome of the chiral self-sorting of a coordination cage. This observation was supplemented with experimental and computational models. A larger ligand with 7-isoquinoline groups was also synthesised. Due to the larger donor groups, the amino groups are pulled further away from each-other in the final coordination assembly. This greater distance means that the solvent molecules were not able to bridge the ligands anymore, and therefore a statistical mixture was observed in this case. Finally, thiophene- and and thieno[3,4b]pyrazine-based ligands were used to potentially recreate the cage-to-tetrahedron transformation described in the first part. The transformation was successful due to the similar geometry imparted by the five-membered rings. In addition, the thieno[3,4b]pyrazine-based ligands were observed to be highly luminescent, and the resulting Pd2L4 cages were as well. This is an interesting result, as the fluorescence of palladium(II) assemblies are generally quenched, and therefore their use as fluorescent probes can be limited. Moreover, the emission colour of those ligands could be modulated by electron withdrawing or donating substituents.