Reinterpreting the Fate of Iridium(III) Photocatalysts─Screening a Combinatorial Library to Explore Light-Driven Side-Reactions

Abstract

Photoredox catalysts are primarily selected based on ground and excited state properties, but their activity is also intrinsically tied to the nature of their reduced (or oxidized) intermediates. Catalyst reactivity often necessitates an inherent instability, thus these intermediates represent a mechanistic turning point that affords either product formation or side-reactions. In this work, we explore the scope of a previously demonstrated sidereaction that partially saturates one pyridine ring of the ancillary ligand in heteroleptic iridium(III) complexes. Using highthroughput synthesis and screening under photochemical conditions, we identified different chemical pathways, ultimately governed by ligand composition. The ancillary ligand was the key factor that determined photochemical stability. Following photoinitiated electron transfer from a sacrificial tertiary amine, the reduced intermediate of complexes containing 1,10-phenanthroline derivatives exhibited long-term stability. In contrast, complexes containing 2,2′-bipyridines were highly susceptible to hydrogen atom transfer and ancillary ligand modification. Detailed characterization of selected complexes before and after transformation showed differing effects on the ground and excited state reduction potentials dependent on the nature of the cyclometalating ligands and excited states. The implications of catalyst stability and reactivity in chemical synthesis was demonstrated in a model photoredox reaction.

Carbonylative Hydroacylation of Styrenes with Alkyl Halides by Multiphoton Tandem Photoredox Catalysis in Flow

“…Multiphoton photoredox catalysis has emerged as a powerful tool to overcome the thermodynamic and redox limitations of conventional photoredox catalysts; − however, this strategy remains unexplored in carbonylative hydroacylation reaction development. Reported here is a general carbonylative hydroacylation protocol enabling the modular synthesis of unsymmetric dialkyl ketones from alkyl halides with multiphoton photoredox catalysis (Figure d).…”

“…Based on the accumulated mechanistic evidence, we propose that the reaction mechanism is analogous to our previously reported carbonylative functionalization of alkyl halides, 52 in which an organohalide undergoes a single electron reduction by the highly reducing excited-state catalyst [Ir2] 0 * (Figure 3a). The resulting carbon-centered radical (1) is subsequently trapped by carbon monoxide to generate an acyl radical (2), which reacts with alkene (3) to form a benzylic radical (51). Subsequent RPC generates the corresponding anion (52), which is finally protonated by H 2 O (Figure 3b).…”

Blue heteroleptic iridium(iii) complexes for OLEDs: simultaneous optimization of color purity and efficiency

“…3,4 In particular, owing to their high photoluminescence quantum yields (PLQYs), short phosphorescence lifetimes, facile color adjustments and good stability, iridium(III) metal complexes are considered as one of the most promising lightemitting materials for practical applications. [5][6][7] However, there are still some thorny problems in the development of blue phosphorescent iridium(III) complexes with regard to color purity, stability and emission efficiency, as compared to green and red phosphorescent iridium(III) complexes. 8 By increasing the energy gap, the lowest emitting triplet state (T 1 ) approaches the metal-centered d-d* state, resulting in fast nonradiative decay via accessible thermal population and low emission efficiency, frequently accompanied by the decomposition of the iridium complexes due to the breaking of the Ir-N bond.…”

“…3,4 In particular, owing to their high photoluminescence quantum yields (PLQYs), short phosphorescence lifetimes, facile color adjustments and good stability, iridium( iii ) metal complexes are considered as one of the most promising light-emitting materials for practical applications. 5–7…”

Overcoming Photochemical Limitations in Metallaphotoredox Catalysis: Red-Light-Driven C–N Cross-Coupling

“…The current use of high-energy NUV and blue light in metallaphotoredox catalysis also introduces issues of functional group incompatibility and off-target reactivity due to the competitive absorption of this light by substrates and reaction intermediates (Scheme a). − The undesirable hydrodehalogenation of aryl halides and the degradation of nickel complexes to form nickel black have been frequently reported in dual Ni/PC systems with high-energy light. ,− We sought to develop a complementary method which takes advantage of lower-energy wavelengths to decouple photocatalytic reactions from photodegradation pathways, thus enabling more efficient catalysis (Scheme b). A recent report from Hadt and co-workers describes the wavelength dependence of aryl–nickel bond homolysis in bipyridyl and N , N , N ′, N ′,-tetramethyl­ethylenediamine (TMEDA) Ni II complexes commonly used in metallaphotoredox catalysis.…”