Shining Light on Iridium: Merging Photocatalysis with Transition Metal Hydride Chemistry

Exploiting the ability of visible light to enable new chemical transformations has emerged as an important method in chemistry in recent years since it provides a more sustainable and mild alternative to many thermal processes.
To expand the scope of traditional photocatalysis, combining the light-dependent cycle with a second, co-catalytic cycle has proven to be extremely valuable. Among the many synergistic strategies, merging photocatalysis with transition metal catalysis has gained particular interest since the activation of the organometallic catalyst via energy or electron transfer enables completely new reactivities. In this thesis, two different approaches for the merger of photocatalysis with metal hydride chemistry are investigated in order to enable new light-dependent reactions.
In the first project (Chapter 3), the two catalytic reactivities are merged within one single-component dual photocatalyst. The investigated iridium hydrides are photoactive and can therefore take over the function of the chromophore as well as the metal hydride. While the photohydride or photoacid behavior of iridium hydrides has been reported previously, Chapter 3 focuses on their ability to activate olefins via photoinduced hydrogen atom transfer (photo-HAT). The key to this new reactivity is the weak IrII-H bond (ca. 44 kcal . mol-1), which is formed upon reductive quenching of the triplet-excited Ir(III) hydride in presence of triethylamine. The typical HAT reactivity was observed for a series of 12 different substrates and the radical mechanism was further supported with a radical clock experiment.
The second project (Chapter 4) provides mechanistic insight into the bimolecular processes that are at play when transition metal hydride chemistry is merged with photocatalysis in the context of nucleotide co-factor regeneration. For this purpose, four water-soluble variants of (fac)-[Ir(ppy)3] (ppyH = 2-phenylpyridine) were developed and characterized by steady-state and time-resolved spectroscopy as well as by cyclic voltammetry. Their excited-state reactivity was exploited for the photochemical regeneration of 1-benzyl-1,4-dihydronicotinamide
(1,4-BNAH), a commonly employed nucleotide co-factor mimic. The bimolecular processes that govern this light-dependent transformation were analyzed based on the correlation between the efficiency of the reaction and the excited-state properties of the sensitizers. More mechanistic insight was gained by luminescence-quenching experiments and transient absorption spectroscopy.
All in all, this thesis demonstrates how the synergy between photocatalysis and transition metal hydride chemistry can be utilized to enable new light-dependent reactivities and furthermore provides an insight into the bimolecular processes that govern the interplay between the two reactivities.