Synthesis and Characterization of Strong Cyclometalated Iridium Photoreductants for Application in Photocatalytic Aryl Bromide Hydrodebromination

A series of potent bis-cyclometalated iridium photoreductants with electron-rich beta-diketiminate (NacNac) ancillary ligands is described. Structure-property analysis reveals that substituent modification of the NacNac ligands has a large effect on the ground-state Ir-IV/Ir-III potential, which shifts cathodically as the NacNac is made more electron-rich. In addition, the excited-state Ir-IV/*Ir-III potentials are ca. 300-500 mV more negative than that of fac-Ir(ppy)(3) (ppy = 2-phenylpyridine), indicating that these compounds have much more reducing excited states. Rate constants for excited-state electron transfer between these photosensitizers and benzophe none are similar to 2-3 times faster than fac-Ir(ppy)(3), demonstrating that these complexes are both kinetically and thermodynamically more potent for excited-state electron transfer. We use these photosensitizers to optimize a simple reaction procedure for photocatalytic debromination of aryl bromide substrates, which requires only the photosensitizer, blue light, and an amine base, without silanes or other additives that are used in previously reported methods.