Redox Noninnocence in Coordinated 2‑(Arylazo)pyridines: Steric Control of Ligand-Based Redox Processes in Cobalt Complexes

A series
of cobalt complexes of ligands based on the 2-(arylazo)­pyridine
architecture have been synthesized, and the precise structure and
stoichiometry of the complexes depend critically on the identity of
substituents in the 2, 4, and 6 positions of the phenyl ring. The
2-(arylazo)­pyridine motif can support either CoII complexes
with neutral ligands, CoIICl2(La)2 (1), CoIICl2(Lc)2 (3), [CoIICl­(Lb)2]2(PF6)2 (5[PF6]2), or CoIII complexes of reduced 2-(arylazo)­pyridine
ligand radical anions, L•–, CoIIICl­(Lb•–)2 (2), CoIIICl­(Lc•–)2 (4), and CoIIIMe­(Lb•–)2 (6). All three members of the latter class are based
on approximately trigonal-bipyramidal CoX­(L•–)2 architectures [L = 2-(arylazo)­pyridine] with two azo
nitrogen atoms and the X ligand (X = Cl or Me) in the equatorial plane
and two pyridine nitrogen atoms occupying axial positions. Density
functional theory suggests that the electronic structure of the CoIII complexes is also dependent on the identity of X: the strong
σ-donor methyl gives a low-spin (S = 0) configuration,
while the σ/π-donor chloro gives an intermediate-spin
(S = 1) local configuration. In certain cases, one-electron
reduction of the CoIIX2L2 complex
leads to the formation of CoIIIX­(L•–)2; i.e., reduction of one ligand induces a further one-electron
oxidation of the metal center with concomitant reduction of the second
ligand.