Substitution effects on the water oxidation of ruthenium catalysts: A quantum-chemical look

Quantum chemistry has been used to investigate the oxidation of water by a family of seven catalysts based on [Ru(tpy)(bpy)(OH₂)]²⁺ (tpy = 2,2′:6′,2′′-terpyridine, bpy = 2,2′-bipyridine). The electron-donating -OMe and -NH₂ groups (EDG) and electron-withdrawing -COOH and -NO₂ groups (EWG) are installed in the catalyst by replacing hydrogen atoms on the bpy and tpy ligands. The EDG induces an increase in the electron density at the Ru center, whereas the EWG does the opposite. Reduced electron density at the metal center facilitates Ru(N+1)/Ru(N) reduction and thus a higher reduction potential. Catalytic evolution of one oxygen molecule from two water molecules using all catalysts is an exothermic process if driven by Ce^IV. The exothermicity increases from EDG to EWG via parents. Regarding intermediates, the singlet states of 7-coordinated catalysts are slightly more stable than the triplet states of 6-coordinated catalysts for most catalysts. Only for a strong EWG (-NO₂) containing catalyst, the triplet 6-coordinated states complex is the most stable. Calculated Ru-O and O-O distances suggest that oxygen will be liberated favorably from the triplet state of 6-coordinated complexes, whose stability increase (with respect to the singlet of 7-coordinated complexes) with increasing electron-withdrawing nature.