​​Bromide as Non‐innocent Ligand to Iron Tames Fenton Chemistry for Chemoselective Non‐degrading Oxidation

It has long been the chemistry dogma that the nitrogen-based ligand of iron complexes determines the redox reactivity: tetra- and/or pentadentate nitrogen-based ligand (N-ligand: PDP, porphyrin, N4Py) enables chemo-selective oxidation through high-valent iron species (FeIV/V=O), while bi- and/or tridentate N-ligand leads to the generation of highly reactive oxygen species (ROS) (i.e., hydroxyl radical) via a Fenton chemistry pathway. The effect of inorganic anionic ligand (i.e., halides, pseudohalides, triflate, nitrate, sulfate, etc) of these iron complexes has rarely been examined and overlooked as an "innocent" anion. Herein, we report our discovery that bromide (Br-) is not an innocent ligand to the iron-BPMA complexes but a decisive factor for taming the Fenton chemistry (ROS) into a mild [HOBr] oxidant, which allows for chemo- and regioselective oxidation of furans, indoles, and sulfides without noticeable degradation. In contrast to the conventional Fenton chemistry pathway by many tridentate N-ligand iron complexes, our Fe(BMPA)Br3 mimics haloperoxidases to generate HOBr by oxidation of bromide ion with hydrogen peroxide. The discovery of the bromide effect on iron complexes bridges the gap of Fenton chemistry and haloperoxidase-catalyzed halogenation and might stimulate interest in reinvestigating the "innocent" ligand of iron complexes for discovery of new reactivity and new applications.