Iridium Multiphoton Photoredox Catalysis; Exploring in situ Catalyst Transformation and it's Effect on Catalytic Capabilities

Transition metal photoredox catalysis is an effective technique that allows organic reactions to proceed under mild experimental conditions. The conventional, single photon mechanism is increasingly under scrutiny as more complex multi-photon mechanisms are discovered and elucidated. In particular, an in situ transformation wherein the photocatalyst [Ir(ppy)2(dtbbpy)]+ undergoes partial hydrogenation to form a new higher-energy photocatalyst. This new species is a more powerful photocatalyst, operating via a tandem catalytic cycle involving both reductive and oxidative quenching cycles. In this work, suitable conditions for achieving this favourable in situ transformation were identified. Electron donors capable of single electron transfer and hydrogen abstraction are necessary, including tertiary amines (TEA, DIPEA) and ascorbic acid. Deprotonation of ascorbic acid to the ascorbate anion inhibits transformation. Increasing the aromaticity of the ancillary ligand also prevents transformation, as seen in the similar photocatalyst analogue [Ir(ppy)2(phen)]+. A water soluble catalyst, [Ir(ppy)2(bpy-TEG2)]+ successfully transformed in aqueous solution, attractive for modifying biomolecules, but at a decreased rate. The multiphoton mechanism exhibits impressive reductive capabilities, reducing aryl halides with peak potentials exceeding its own reduction potential. The non-transforming catalyst, [Ir(ppy)2(phen)]+, saw comparatively decreased catalytic rate across the energy range studied. Substrates that are harder to reduce require a higher influx of photons for successful reduction. A third mechanism was identified wherein very effective reduction occurs directly from a radical carbon dioxide anion (CO2??), produced from the photocatalytic oxidation of oxalate. This in-depth mechanistic understanding allows for greater precision in designing future systems.