Insights into Ru-Based Molecular Water Oxidation Catalysts: Electronic and Noncovalent-Interaction Effects on Their Catalytic Activities

A series
of Ru-bda water oxidation catalysts [Ru­(bda)­L2] (H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid;
L = [HNEt3]­[3-SO3-pyridine], 1;
4-(EtOOC)-pyridine, 2; 4-bromopyridine, 3; pyridine, 4; 4-methoxypyridine, 5; 4-(Me2N)-pyridine, 6; 4-[Ph­(CH2)3]-pyridine, 7) were synthesized with electron-donating/-withdrawing
groups and hydrophilic/hydrophobic groups in the axial ligands. These
complexes were characterized by 1H NMR spectroscopy, high-resolution
mass spectrometry, elemental analysis, and electrochemistry. In addition,
complexes 1 and 6 were further identified
by single crystal X-ray crystallography, revealing a highly distorted
octahedral configuration of the Ru coordination sphere. All of these
complexes are highly active toward CeIV-driven (CeIV = Ce­(NH4)2(NO3)6) water oxidation with oxygen evolution rates up to 119 mols of O2 per mole of catalyst per second. Their structure–activity
relationship was investigated. Electron-withdrawing and noncovalent
interactions (attraction) exhibit positive effect on the catalytic
activity of Ru-bda catalysts.