Catalyst development: from transition metal to tethered dual photo/transition metal catalysts

The main focus of the work presented in this thesis is the development of more efficient transition metal catalysts for thermally activated reactions, as well as the design and synthesis of new tethered dual photo- transition metal catalysts that involve both thermal and photo activation. Key approaches to developing highly efficient and selective catalysts include ligand design, metal choice, changing oxidation state and selecting the activation mode (e.g. thermal vs photo activation). In addition to these approaches, dual catalysis is an emerging catalytic strategy where two different catalysts are used in one reaction mixture to achieve enhanced, or new, catalytic performance. Of particular interest is tethering these catalysts together, as the new tethered dual catalysts can be more efficient at promoting previously challenging reactions, as well as imparting new reactivity through increased interactions between the two catalysts.

Firstly, a series of iridium and rhodium catalysts bearing bidentate carbene–triazole ligands were synthesised and characterised, and their catalytic reactivity examined using the hydrogen borrowing reaction between benzyl alcohol and acetophenone. Two major products were formed in this reaction, with the iridium and rhodium catalysts showing substantial differences in product selectivity. Apart from the metal choice, the study showed that the metal oxidation state, counterion and the electronic nature of the ligand also influenced the product selectivity. Controllable product selectivity was achieved via the selection of the appropriate combination of catalyst and base. In addition, mechanistic investigations, including kinetic studies and isotopic labelling experiments, were carried out to provide insight into the origin of the observed product selectivity.

Secondly, the concept of merging traditional transition metal catalysis and photocatalysis was examined using innovative tethered dual catalysts. In fact, most reported dual catalysis strategies utilise two separate catalysts in the reaction mixture, while ‘tethered dual catalysts’ remain underdeveloped, despite the higher likelihood of these tethered species offering novel catalytic reactivity due to the increased potential for synergistic interactions. Intrigued by this, a nonluminous iridium-based transition metal catalyst was tethered to an organic boron-based photocatalyst through different linking modes. Six novel tethered dual catalysts were successfully synthesised and fully characterised. Investigations into their optical and electrochemical properties were undertaken. A detailed catalytic study of these tethered dual catalysts demonstrated that tethering the catalysts together enhanced both transition metal and photo- catalysis. In addition, one of the developed tethered dual catalysts was employed to promote both a sequential and a switchable reaction to demonstrate the utility of these new catalysts, highlighting their applicability in dual catalysis.

Thirdly, a comprehensive investigation into how the mode in which the transition metal catalyst and photocatalyst were tethered together affects catalytic efficiency was performed. These studies highlighted that the ideal tethering mode for transition metal catalysis (intermolecular hydroamination) is different to the optimal tethering mode for photocatalysis (photooxidation of thioanisole). Mechanistic investigations on the intermolecular hydroamination reaction revealed that the observed enhancement in reaction rate relative to the parent catalyst is not simply due to ligand induced electronic effects, but likely due to a combination of steric and electronic effects from the ligand. For the photooxidation of thioanisole, the iridium complex was found to increase the singlet oxygen quantum yield of the photocatalyst, through increased intersystem crossing promoted by the heavy atom effect. In addition, it is likely that the iridium centre in the tethered dual catalyst is able to stabilise the key, charge separated reaction intermediate which resulted in a rate increase. As such, the iridium complex was found to have two roles, as both an intersystem crossing enhancer and a reaction accelerator.

Lastly, a nonluminous palladium transition metal catalyst was tethered to a boron-based photocatalyst. Single crystal X-ray crystallography analysis confirmed that the trans conformation was formed. Interestingly, photophysical studies indicated that this complex exhibited a rare long-lived room temperature phosphorescence. The singlet oxygen quantum yield of the complex was determined, which indicated that the tethered dual catalyst had a significantly higher singlet oxygen quantum yield (64%) than the parent photocatalyst (1%), suggesting that the tethered dual catalyst should be a much more efficient photocatalyst than the parent photocatalyst. By examining the catalytic competency of this dual catalyst, enhanced reaction efficiency was observed in both photo- (oxidation of thioanisole) and transition metal catalysis (Suzuki-Miyaura cross coupling), highlighting the advantage of the tethering strategy. Furthermore, the utility of the dual catalytic character of this palladium-boron catalyst was demonstrated using a representative sequential photocatalytic oxidation – cross coupling reaction.

The results presented in this thesis illustrate that more efficient catalysts can be designed and developed by using the main paradigms of catalyst development, with a particular focus on exploiting a tethered dual catalysis strategy. The detailed catalytic studies performed suggest that the tethered dual catalysts are more efficient than the un-tethered dual catalysts at promoting both the photo- and thermally activated reactions, as well as the sequential reaction, that were examined in this thesis. The insight provided through the mechanistic studies lay the groundwork for developing more efficient tethered dual catalysts, and it is expected that this dual catalysis approach has the potential to lead to advancements in catalytic reaction development.