Multielectron Transfer at Cobalt: Influence of the Phenylazopyridine Ligand

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.