Electronic Structure Variations of Cu-Based Water Oxidation Catalysts Bearing Redox-Active Ligands

The physical and electrochemical properties of a series of Cu complexes bearing redox-active ligands were investigated. The effect of the geometry and electronic structure on the redox potentials of the CuI/II and ligand-based oxidation was investigated, in both MeCN and aqueous solutions. Here we found that the formation of a delocalized ligand radical is essential to obtain catalytic activity for water oxidation. Furthermore, we found that ligand-based oxidation can be controlled by electronic effects of substituent methoxy groups on various positions on the ligand. This allows us to propose design principles for ligands employed in water oxidation catalysts that operate via a SET-HA mechanism: 1) The redox-active ligand should contain a negative charge of at least -1 or have the ability to oxidize the ligand via PCET whereby a proton is removed from the ligand. 2) The oxidation potential of the redox-active ligands can be controlled by introduction of substituents. To decrease this potential, the electron density should be directed at the position of the ligand where oxidation occurs. 3) Stabilization of the oxidized ligand by electronic or delocalization effects decreases the oxidation potential. The copper mediated water oxidation reaction with the 6,6 '-bis(2-aminopyridyl)-2,2 '-bipyridine framework relies on the presence of at least one amine function in the ligand backbone that can be deprotonated and oxidized. Electron donating and withdrawing substituents directed towards the pi-system do affect the overpotential for water oxidation, those directed at the Cu-center do not.image