Mononuclear complexes of a tridentate redox-active ligand with sulfonamido groups: Structure, properties, and reactivity

The design of molecular complexes of earth-abundant first-row transition metals that can catalyze multi-electron C-H bond activation processes is of interest for achieving efficient, low-cost syntheses of target molecules. To overcome the propensity of these metals to perform single-electron processes, redox-active ligands have been utilized to provide additional electron equivalents. Herein, we report the synthesis of a novel redox active ligand, [ibaps]^3-, which binds to transition metals such as Fe^II and Co^II in a meridional fashion through the three anionic nitrogen atoms and provides additional coordination sites for other ligands. In this study, the neutral bidentate ligand 2,2′-bipyridine (bpy) was used to complete the coordination spheres of the metal ions and form NEt₄[M^II(ibaps)bpy] (M = Fe (1) or Co (1-Co)) salts. The Fe^II salt exhibited rich electrochemical properties and could be chemically oxidized by 1 and 2 equiv. of ferrocenium to form singly and doubly oxidized species, respectively. The reactivity of 1 towards intramolecular C-H bond amination of aryl azides at benzylic and aliphatic carbon centers was explored, and moderate to good yields of the resulting indoline products were obtained.