Two-Electron Redox Tuning of Cyclopentadienyl Cobalt Complexes Enabled by the Phenylenediamide Ligand

Achieving multielectron activity at first-row transition-metal complexes has important implications for homogeneous catalysis using earth-abundant metals. Here, we report a family of cobalt-phenylenediamide complexes that undergo reversible 2e^- oxidation regardless of the ligand substituents, enabling unprecedented multielectron redox tuning over 0.5 V and, in each case, affording the dicationic Co(III)-benzoquinonediimine species. The neutral complexes are best described as delocalized systems with π-bonding in the metallocycle, consistent with a closed-shell singlet ground state predicted by density functional theory (DFT) calculations. Our DFT results also predict an ECE pathway for 2e^- oxidation (ECE = electrochemical step, chemical step, electrochemical step), where the first 1e^- step involves redox-induced electron transfer to yield a Co(II) intermediate. Disruption of the metallocycle bonding in this state enables a change in the coordination geometry through association of an addition ligand, which is critical for accessing the potential inversion. The electronic properties of the phenylenediamide ligand govern whether the second electron is lost from the ligand or metal, providing a remarkable example of tunable 2e^- behavior at first-row systems.