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

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 earthabundant metals.This work reports the synthesis,characterization and catalytic properties of a monomeric ruthenium catalyst Ru-bnda(H2 bnda=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(H_(2)bda=2,2’-bipyridine-6,6’-dicarboxylic acid),Ru-pda(H_(2)pda=1,10-phenanthroline-2,9-dicarboxylic acid),and Ru-biqa(H_(2)biqa=(1,1’-biisoquinoline)-3,3’-dicarboxylic acid),we emphasized that seven coordination clearly determines presence of Ru^(Ⅴ)=O with high spin density on the ORu^(Ⅴ)=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 faceto-face interaction for the generation of the pre-reactive[Ru^(Ⅴ)=O…O=Ru^(Ⅴ)],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.

Isolation and Identification of Pseudo Seven-Coordinate Ru(Ⅲ)Intermediate Completing the Catalytic Cycle of Ru-bda Type of Water Oxidation Catalysts

Isolation of RuⅢ-bda(17-electron specie)complex with an aqua ligand(2-electron donor)is challenging due to violation of the 18-electron rule.Although considerable efforts have been dedicated to mechanistic studies of water oxidation by the Ru-bda family,the structure and initial formation of the Ru^(Ⅲ)-bda aqua complex are still controversial.Herein,we challenge this often overlooked step by designing a pocketshape Ru-based complex 1.The computational studies showed that 1 possesses the crucial hydrophobicity at the Ru^(V)(O)state as well as similar probability of access of terminal O to solvent water molecules when compared with classic Ru-bda catalysts.Through characterization of single-crystal structures at the Ru^(Ⅱ) and Ru^(Ⅲ) states,a pseudo seven-coordinate“ready-togo”aqua ligand with Ru^(Ⅲ)…O distance of 3.62A was observed.This aqua ligand was also found to be part of a formed hydrogen-bonding network,providing a good indication of how the Ru^(Ⅲ)-OH_(2)complex is formed.