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.