Switching O−O bond formation mechanism between WNA and I2M pathways by modifying the Ru-bda backbone ligands of water-oxidation catalysts

Abstract Understanding the seven coordination and O−O coupling pathway of the distinguished Ru-bda catalysts is essential for the development of next generation efficient water-oxidation catalysts based on earth-abundant metals. This work reports the synthesis, characterization and catalytic properties of a monomeric ruthenium catalyst Ru-bnda (H2bnda = 2,2'-bi(nicotinic acid)-6,6'-dicarboxylic acid) featuring steric hindrance and enhanced hydrophilicity on the backbone. Combining experimental evidence with systematic density functional theory calculations on the Ru-bnda and related catalysts Ru-bda (H2bda = 2,2ʹ-bipyridine-6,6ʹ-dicarboxylic acid), Ru-pda ((H2pda = 1,10-phenanthroline-2,9-dicarboxylic acid)), and Ru-biqa (H2biqa = (1,1ʹ-biisoquinoline)-3,3ʹ-dicarboxylic acid), we emphasized that seven coordination clearly determines presence of RuV=O with high spin density on the ORuV=O atom, i.e. oxo with radical properties, which is one of the necessary conditions for reacting through the O−O coupling pathway. However, an additional factor to make the condition sufficient is the favorable intermolecular face-to-face interaction for the generation of the pre-reactive [RuV=O···O=RuV], which may be significantly influenced by the secondary coordination environments. This work provides a new understanding of the structure-activity relationship of water-oxidation catalysts and their potential to adopt I2M pathway for O−O bond formation.