Multielectron Transfer at Cobalt: Influence of the Phenylazopyridine Ligand

The dicationic complex [CpCo(azpy)-(CH3CN)](ClO4)(2) 1 (azpy = phenylazopyridine) exhibits reVer$ible two-electron reduction at a very mild potential (-0.16 V versus Fc(0/+)) in acetoffitrile. This behavior is not observed with the analogous bipyridine and pyrazolylpyridine complexes (3 and 4), which display an electrochemical signature typical of Co-III systems: two sequential one-electron reductions to Co-II at -0.4 V and Co-I at -1.0 to -1.3 V versus Fc(0/+). 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 andelectronic 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 pi* 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.