Development of strongly photoreducing, high triplet energy iridium(III) NHC complexes for use in photocatalysis

Abstract

This thesis mainly describes the development of novel iridium(III) photocatalysts bearing N-heterocyclic carbene ligands, which are more photoreducing and possess higher triplet energies compared to known iridium(III) photocatalysts. Chapter 1 introduces the relevant concepts relating to the photophysical processes of iridium(III) complexes as well as both outer-sphere and inner-sphere photocatalysis. A literature survey is provided, documenting the broad classes of photocatalysis reactions and recent advances in iridium(III) photocatalyst development. Chapter 2 presents the use of the meridional and facial isomers of the homoleptic iridium(III) NHC complex, Ir(pmi)₃, as a photoredox and an energy transfer photocatalyst. Across all of the investigated transformations, the performance of Ir(pmi)₃ is benchmarked against the archetypal, strongly photoreducing photocatalyst, fac-Ir(ppy)₃. The stability of Ir(pmi)₃ photocatalysts under photochemical conditions is also cross-compared with that of fac-Ir(ppy)₃. Based on the promising results in Chapter 2, Chapter 3 investigates the facial and meridional isomers of more conjugated derivatives of Ir(pmi)₃, Ir(pmb)₃ and Ir(pmp)₃, as photocatalysts under blue-light excitation. The best performing photocatalyst, mer-Ir(pmp)₃, is further assessed as an energy transfer and metallaphotoredox catalyst. Its efficacy as a metallaphotoredox catalyst and its stability in the presence of amine substrates and sacrificial reagents is compared against a series of literature known photocatalysts. Chapter 4 explores the photophysical properties of a novel heteroleptic iridium(III) complex, Ir(dFppy)₂(iCzmi), and these are compared to the parent emitter, Ir(dFppy)₂(pmi). The photophysical properties of both complexes are rationalised by a DFT computational study. Finally, Ir(dFppy)₂(pmi) is assessed as a metallaphotoredox catalyst in two dual nickel/photoredox cross-coupling reactions and its performance compared against the literature photocatalyst, [Ir(dF(CF₃)ppy)₂(dtbbpy)]PF₆. Finally, in Chapter 5, the application of photochemical conditions to iridium-catalysed C-H borylation of a scope of aromatic and heteroaromatic substrates is investigated. Both experimental and computational mechanistic analysis is employed to rationalise the source of the observed rate enhancement and broadly higher yields achieved under photochemical conditions.

Date of Award	2 Jul 2026
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Eli Zysman-Colman (Supervisor)