C−H Bond Activation by Iridium(III) and Iridium(IV) Oxo Complexes

Oxidation of an iridium(III) oxo precursor enabled the structural, spectroscopic, and quantum‐chemical characterization of the first well‐defined iridium(IV) oxo complex. Side‐by‐side examination of the proton‐coupled electron transfer thermochemistry revealed similar driving forces for the isostructural oxo complexes in two redox states due to compensating contributions from H+ and e− transfer. However, C−H activation of dihydroanthracene revealed significant hydrogen tunneling for the distinctly more basic iridium(III) oxo complex. Our findings complement the growing body of data that relate tunneling to ground state properties as predictors for the selectivity of C−H bond activation. C−H activation by two isostructural, terminal iridium(III) and iridium(IV) oxo complexes is compared. While the driving forces for hydrogen ion are almost identical due to compensating contributions from H+ and e− transfer, significant differences in hydrogen tunneling were observed. Increased tunneling for the iridium(III) oxo complex is attributed to its higher basicity, which leads to a narrower potential energy barrier. Our results provide guidelines to steer C−H activation under tunneling control.