Structural, Electronic, and Theoretical Description of a Series of Cobalt Clathrochelate Complexes in the Co(III), Co(II) and Co(I) Oxidation States

Encaged hexacoordinated metal complexes have long been a fascinating family of complexes, as they confer the same ligand environment to metal ions in different oxidation states. We recently reported that cobalt dathrochelate complexes behave as hydrogen-producing catalysts at quite modest overpotential (Pantani et al. Angew. Chem., Int. Ed, 2008, 47, 9948). The electrochemical properties evidenced two quasireversible one-electron reduction waves starting from the cobalt(III) derivative, indicating that there are no dramatic changes in the coordination sphere. The intriguing question is the mechanistic pathways for this observed reactivity. In this work, we compare our observed electrochemical and spectroscopic data (UV visible and EPR spectroscopies) with our theoretical findings based on DFT, TD-DFT, and CASSCF calculations. The properties of the Co(III) and Co(II) species can be explained as low-spin complexes. In contrast, the doubly reduced species, the Co(I) form, is a high spin complex and its electronic description involves partial reduction of the ligand cage. This point is of major importance to understand the catalytic activity.