Photocatalytic Effect of a Charge-Transfer Complex Formed After Electron Transfer Between a Highly Functionalized Ir(III) Complex and Reductant

Commonly used Ir(III) complexes, such as the commercially available [Ir(ppy)2bpy]+ (ppy = 2-phenylpyridinato, bpy = 2,2’-bipyridine), are prone to degradation and ligand substitution by surrounding molecules (e.g., sacrificial redox agents) when irradiated with light. This results in ambiguity in the mechanism when these Ir complexes are used as photoredox catalysts in organic reactions. In this study, we prepared an [Ir(ppy)2bpy]+ derivative (3) that is functionalized with N-Boc pyrrolidine (NBP) units. Complex 3 is photostable, does not undergo structural modification of the ligands, and acts as an efficient photocatalyst in the hydrodehalogenation of aryl halides (Ar-X). Using optical spectroscopy and electron paramagnetic resonance (EPR), it is shown that the triplet state of 3 accepts an electron from a sacrificial reductant (e.g., N-Boc proline and 1,8-diazabicyclo[5.4.0]undec-7-ene) to form a long-lived (∼1 μs) charge-transfer (CT) complex between the anion radical 3●– and the cation radical of the reductant. Furthermore, the CT complex absorbs over a wide wavelength range (350–700 nm), enabling the creation of a photoexcited 3●– within the complex that can donate an electron to Ar-X. The newly designed Ir complex 3 is therefore an efficient photocatalyst by capitalizing on the formation of a CT complex that is photoexcited to form a highly reducing species.