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 β-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 IrIV/IrIII potential, which shifts cathodically as
the NacNac is made more electron-rich. In addition, the excited-state
IrIV/*IrIII 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 benzophenone are ∼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.