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 (1)H 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 Ce(IV)-driven (Ce(IV) = 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.