Synthesis and characterisation of small molecule and macromolecular photoredox catalysts for radical thiol-ene reactions
The use of polynuclear complexes is attracting significant attention, offering several advantages over mononuclear analogues. In the context of this project, the presence of several photoactive centres can lead to a multi-electron transfer in a single step and offering other modalities. With the aim to improve the reactivity and stability of photoredox catalysts, multinuclear photoredox catalysts were developed. Consequently, bipyridyl monomeric and trimeric ligands functionalised with imine and amine functionalities were synthesised via Schiff base condensation and reductive amination reactions, respectively. The ligands were then reacted with cis-dichlorobis (2,2'-bipyridine) ruthenium (II) to afford two new mononuclear and two new trinuclear Ru(II)-based complexes. The resulting complexes were then fully characterised by various spectroscopic and analytical techniques such as 1H NMR, 13C{1H} NMR, FT-IR spectroscopy and mass spectrometry. Furthermore, electrochemical, electronic absorption and emission studies for the complexes were conducted and the collected data displayed the effects of ligand modifications on the photophysical and redox properties of the complexes. All the ligand-modified complexes exhibited red shifted emission spectra (614−633 nm) relative to the [Ru(bpy)3](PF6)2 complex (609 nm) and this emission band was attributed to the transition from the triplet MLCT excited state (3MLCT) to the ground state. The excited state redox potentials for the ligand-modified complexes were comparable to that of the known complex thus displaying promising photocatalytic activity. The photoexcited [Ru(bpy)3] 2+ complex had the most positive excited state reduction potential (Ered (2+*/+) = +0.350 V vs Ag/Ag+), making it a more powerful oxidant than the photoexcited states of the ligand-modified complexes. The complexes were then evaluated as photoredox catalysts in the radical hydrothiolation reaction of olefins (thiol-ene reactions) to produce thioethers, which are valuable starting materials in the pharmaceutical industry, material and polymer science, and in the synthesis of various drug molecules. The isolated yields, from the reaction between thiophenol and styrene, were higher for the [Ru(bpy)3](PF6)2 complex (54%) in comparison to the ligand-modified mononuclear complexes (8–24%). This result was expected since [Ru(bpy)3](PF6)2 exhibited the most positive excited state reduction potential (Ered (2+*/+) = +0.350 V vs Ag/Ag+ ), making the photoexcited state of this complex more readily quenched by the thiophenol. Control reactions carried out in the absence of the photocatalyst resulted in either significantly lower yields (6%) or no product formation thus showing the role of the complexes as photoredox catalysts in the reactions. Varying the substrates by using styrene and 1-butanethiol afforded the desired coupled thioether product, while the reaction between thiophenol and allyl alcohol resulted in the formation of the disulfide product. However, when allyl alcohol was reacted with 1- butanethiol, the desired anti-Markovnikov thioether product was formed and this indicated the importance of substrate choice and compatibility with the photoexcited catalyst when designing a photocatalytic system. The reactions carried out using trinuclear complexes (Ered (2+*/+) = +0.300 V vs Ag/Ag+ for each metal centre) resulted in a three-fold increase in isolated yields (26%, 73%) when compared to their respective mononuclear analogues (8%, 22%). This demonstrated the benefits of having a multinuclear photocatalyst with several photoactive centres.
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