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 Co^II complexes with neutral ligands, Co^IICl₂(L^a)₂ (<b>1</b>), Co^IICl₂(L^c)₂ (<b>3</b>), [Co^IICl­(L^b)₂]₂(PF₆)₂ (<b>5</b>[PF₆]₂), or Co^III complexes of reduced 2-(arylazo)­pyridine ligand radical anions, L^•–, Co^IIICl­(L^b•–)₂ (<b>2</b>), Co^IIICl­(L^c•–)₂ (<b>4</b>), and Co^IIIMe­(L^b•–)₂ (<b>6</b>). 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 Co^III complexes is also dependent on the identity of X: the strong σ-donor methyl gives a low-spin (<i>S</i> = 0) configuration, while the σ/π-donor chloro gives an intermediate-spin (<i>S</i> = 1) local configuration. In certain cases, one-electron reduction of the Co^IIX₂L₂ complex leads to the formation of Co^IIIX­(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