Accessing the Different Redox States of Alpha-iminopyridines Within Cobalt Complexes

Cobalt coordination complexes featuring the redox-active alpha-iminopyridine ligand are described. The quite reducing bis(ligand)cobalt monoanion (1(red)) was isolated and characterized by X-ray crystallography. The complex forms a three-membered electron-transfer series along with its neutral and monocationic counterparts, which were previously reported. The electronic structures in this series are all consistent with divalent cobalt and the redox events being ligand-centered. The reactivity profile of 1(red) was briefly explored, and two new compounds were isolated, bis(ligand)methylcobalt (2) and bis(ligand)iodocobalt (3). Though 2 and 3 are isostructural, they are characterized by different electronic structures. The methylcobalt complex is best described by a Co(III) center with two ligand radicals, whereas the iodocobalt species is more aptly assigned as a Co(II) center with only one ligand radical. Further evidence of their different electronic structures is that a low-lying excited state is populated at room temperature in the case of the iodocobalt compound, whereas the methylcobalt complex is an energetically well-isolated spin singlet. Magnetic susceptibility data, structural data, variable temperature NMR spectroscopy, and density functional theory calculations lend support to this proposal.