Conformational twisting of a formate-bridged diiridium complex enables catalytic formic acid dehydrogenation

We previously reported that iridium complex 1a enables the first homogeneous catalytic dehydrogenation of neat formic acid and enjoys unusual stability through millions of turnovers. Binuclear iridium hydride species 5a, which features a provocative C2-symmetric geometry, was isolated from the reaction as a catalyst resting state. By synthesizing and carefully examining the catalytic initiation of a series of analogues to 1a, we establish here a strong correlation between the formation of C2-twisted iridium dimers analogous to 5a and the reactivity of formic acid dehydrogenation: an efficient C2 twist appears unique to 1a and essential to catalytic reactivity.