The realization of artificial photosynthesis carries the promise of cheap and abundant energy, however, significant advances in the rational design of water oxidation catalysts are required. Detailed information on the structure of the catalyst under reaction conditions and mechanisms of O-O bond formation should be obtained. Here, we used a combination of electron paramagnetic resonance (EPR), stopped flow freeze quench on a millisecond-second time scale, X-ray absorption (XAS), resonance Raman (RR) spectroscopy, and density functional theory (DFT) to follow the dynamics of the Ru-based single site catalyst, [Ru(NPM)(4-pic)(HO)] (NPM = 4-t-butyl-2,6-di(1',8'-naphthyrid-2'-yl)pyridine, pic = 4-picoline), under the water oxidation conditions. We report a unique EPR signal with g-tensor, g = 2.30, g = 2.18, and g = 1.83 which allowed us to observe fast dynamics of oxygen atom transfer from the Ru═O oxo species to the uncoordinated nitrogen of the NPM ligand. In few seconds, the NPM ligand modification results in [Ru(NPM-NO)(4-pic)(HO)] and [Ru(NPM-NO,NO)(4-pic)] complexes. A proposed [Ru(NPM)(4-pic)═O] intermediate was not detected under the tested conditions. We demonstrate that while the proximal base might be beneficial in O-O bond formation via nucleophilic water attack on an oxo species as shown by DFT, the noncoordinating nitrogen is impractical as a base in water oxidation catalysts due to its facile conversion to the N-O group. This study opens new horizons for understanding the real structure of Ru catalysts under water oxidation conditions and points toward the need to further investigate the role of the N-O ligand in promoting water oxidation catalysis.

While the catalytic activity of some Ru-based polypyridine complexes in water oxidation is well established, the relationship between their chemical structure and activity is less known. In this work, the single site Ru complex [Ru(bpy) 2 (H 2 O) 2 ] 2+ (bpy = 2,2 -bipyridine)-which can exist as either a cis isomer or a trans isomer-is investigated. While a difference in the catalytic activity of these two isomers is well established, with cis-[Ru(bpy) 2 (H 2 O) 2 ] 2+ being much more active, no mechanistic explanation of this fact has been presented. The oxygen evolving capability of both isomers at multiple concentrations has been investigated, with cis-[Ru(bpy) 2 (H 2 O) 2 ] 2+ showing a second-order dependence of O 2 evolution activity with increased catalyst concentration. Measurement of the electron paramagnetic resonance (EPR) spectrum of cis-[Ru(bpy) 2 (H 2 O) 2 ] 2+ , shortly after oxidation with Ce IV , showed the presence of a signal matching that of cis,cis-[Ru III (bpy) 2 (H 2 O)ORu IV (bpy) 2 (OH)] 4+ , also known as "blue dimer". The formation of dimers is a concentration-dependent process, which could serve to explain the greater than first order increase in catalytic activity. The trans isomer showed a first-order dependence of O 2 evolution on catalyst concentration. Behavior of [Ru(bpy) 2 (H 2 O) 2 ] 2+ isomers is compared with other Ru-based catalysts, in particular [Ru(tpy)(bpy)(H 2 O)] 2+ (tpy = 2,2 ;6,2 -terpyridine).