Development of metallosupramolecular photoredox catalysts
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Date
07/07/2017Item status
Restricted AccessAuthor
August, David Philip
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Abstract
Supramolecular chemistry allows the rapid formation of complex systems through self-assembly.
These systems often possess unique properties not observed for conventional
covalent constructs and have potential applications in areas such as sensing, drug delivery
and catalysis. Metallosupramolecular container compounds have been shown to catalyse
reactions with both regio- and stereo-selectivity in methods analogous to enzyme type
catalysis. Separately, visible-light photoredox catalysis has recently gained considerable
interest as an efficient, green and mild method for the rapid synthesis of many chemical
compounds.
In order to combine the favourable properties of both supramolecular catalysis and visible-light
photoredox catalysis, a number of photoredox active metallosupramolecular assemblies
were designed, synthesised and analysed. Initial steps were taken to stabilise a known
iridium-based M6L4 luminescent cage compound to allow guest encapsulation to take place.
The incorporation of isocyanide donors as strong ligands improved the stability of model
compounds but synthesis of an analogous three-dimensional assembly was unsuccessful.
Instead, a “complex-as-ligand” approach was taken that allowed the straight-forward
formation of Pd2L4 systems from a range of photoactive iridium complexes. Importantly,
unlike many other photoactive systems, the complexation to palladium did not drastically
affect the photoredox properties of the constituent iridium complexes. Multiple approaches
were then taken to improve both the stability and guest binding properties of the photoactive
assembly in an effort to achieve supramolecular photoredox catalysis. Utilising a model
system, a general method was thus developed for enhancing the association constants of
neutral guests in organic solvents by switching to large, non-coordinating counter ions that
provided reduced competition for the internal binding site. In combination with this
increased binding affinity, a range of guest properties were adjusted by association with the
hydrogen bond donor environment of the internal cavity. The encapsulation of quinone based
oxidants led to unexpected and novel reaction pathways not observed in the bulk phase. As
such, this work represents a significant advancement in development of
metallosupramolecular systems capable of regio- and stereo-selective photoredox catalysis.