Multielectron Transfer at Cobalt: Influence of the Phenylazopyridine Ligand
2017
Waldie, Kate M. | Srinivasan, Ramakrishnan | Kim, Sung-Kwan | Maclaren, Jana K. | Chidsey, Christopher E. D. | Waymouth, Robert M.
The dicationic complex [CpCo(azpy)(CH₃CN)](ClO₄)₂ 1 (azpy = phenylazopyridine) exhibits a reversible two-electron reduction at a very mild potential (−0.16 V versus Fc⁰/⁺) in acetonitrile. This behavior is not observed with the analogous bipyridine and pyrazolylpyridine complexes (3 and 4), which display an electrochemical signature typical of Coᴵᴵᴵ systems: two sequential one-electron reductions to Coᴵᴵ at −0.4 V and Coᴵ at −1.0 to −1.3 V versus Fc⁰/⁺. The doubly reduced, neutral complex [CpCo(azpy)] 2 is isolated as an air-stable, diamagnetic solid via chemical reduction with cobaltocene. Crystallographic and spectroscopic characterization together with experimentally calibrated density functional theory calculations illuminate the key structural and electronic changes that occur upon reduction of 1 to 2. The electrochemical potential inversion observed with 1 is attributed to effective overlap between the metal d and the low-energy azo π* orbitals in the intermediary redox state and additional stabilization of 2 from structural reorganization, leading to a two-electron reduction. This result serves as a key milestone in the quest for two-electron transformations with mononuclear first-row transition metal complexes at mild potentials.
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