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

Multielectronprocesses at first-row transition-metal complexesare uncommon. Herein, systematically varying the properties of theredox-active o-phenylenediamide ligand enables tuningof a two-electron oxidation process over a large potential range ina series of cobalt complexes. Redox-induced electron transfer andcoordination geometry changes promote this potential inversion, wherethe second electron transfer step is easier than the first. Achievingmultielectron activity at first-row transition-metalcomplexes has important implications for homogeneous catalysis usingearth-abundant metals. Here, we report a family of cobalt-phenylenediamidecomplexes that undergo reversible 2e(-) oxidationregardless of the ligand substituents, enabling unprecedented multielectronredox tuning over 0.5 V and, in each case, affording the dicationicCo(III)-benzoquinonediimine species. The neutral complexes are bestdescribed as delocalized systems with & pi;-bonding in the metallocycle,consistent with a closed-shell singlet ground state predicted by densityfunctional theory (DFT) calculations. Our DFT results also predictan ECE pathway for 2e(-) oxidation (ECE = electrochemicalstep, chemical step, electrochemical step), where the first 1e(-) step involves redox-induced electron transfer to yielda Co(II) intermediate. Disruption of the metallocycle bonding in thisstate enables a change in the coordination geometry through associationof an addition ligand, which is critical for accessing the potentialinversion. The electronic properties of the phenylenediamide ligandgovern whether the second electron is lost from the ligand or metal,providing a remarkable example of tunable 2e(-) behaviorat first-row systems.