Spectroscopic Analysis of Catalytic Water Oxidation by [Ru^II(bpy)(tpy)H₂O]²⁺ Suggests That Ru^VO Is Not a Rate-Limiting Intermediate

Modern chemistry’s grand challenge is to significantly improve catalysts for water splitting. Further progress requires detailed spectroscopic and computational characterization of catalytic mechanisms. We analyzed one of the most studied homogeneous single-site Ru catalysts, [Ru^II(bpy)­(tpy)­H₂O]²⁺ (where bpy = 2,2′-bipyridine, tpy = 2,2′;6′,2″-terpyridine). Our results reveal that the [Ru^V(bpy)­(tpy)O]³⁺ intermediate, reportedly detected in catalytic mixtures as a rate-limiting intermediate in water activation, is not present as such. Using a combination of electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy, we demonstrate that 95% of the Ru complex in the catalytic steady state is of the form [Ru^IV(bpy)­(tpy)O]²⁺. [Ru^V(bpy)­(tpy)O]³⁺ was not observed, and according to density functional theory (DFT) analysis, it might be thermodynamically inaccessible at our experimental conditions. A reaction product with unique EPR spectrum was detected in reaction mixtures at about 5% and assigned to Ru^III-peroxo species with (−OOH or −OO– ligands). We also analyzed the [Ru^II(bpy)­(tpy)­Cl]⁺ catalyst precursor and confirmed that this molecule is not a catalyst and its oxidation past Ru^III state is impeded by a lack of proton-coupled electron transfer. Ru–Cl exchange with water is required to form active catalysts with the Ru–H₂O fragment. [Ru^II(bpy)­(tpy)­H₂O]²⁺ is the simplest representative of a larger class of water oxidation catalysts with neutral, nitrogen containing heterocycles. We expect this class of catalysts to work mechanistically in a similar fashion via [Ru^IV(bpy)­(tpy)O]²⁺ intermediate unless more electronegative (oxygen containing) ligands are introduced in the Ru coordination sphere, allowing the formation of more oxidized Ru^V intermediate

Structure and Electronic Configurations of the Intermediates of Water Oxidation in Blue Ruthenium Dimer Catalysis

Catalytic O₂ evolution with <i>cis</i>,<i>cis</i>-[(bpy)₂(H₂O)­Ru^IIIORu^III(OH₂)­(bpy)₂]⁴⁺ (bpy is 2,2-bipyridine), the so-called blue dimer, the first designed water oxidation catalyst, was monitored by UV–vis, EPR, and X-ray absorption spectroscopy (XAS) with ms time resolution. Two processes were identified, one of which occurs on a time scale of 100 ms to a few seconds and results in oxidation of the catalyst with the formation of an intermediate, here termed [3,4]′. A slower process occurring on the time scale of minutes results in the decay of this intermediate and O₂ evolution. Spectroscopic data suggest that within the fast process there is a short-lived transient intermediate, which is a precursor of [3,4]′. When excess oxidant was used, a highly oxidized form of the blue dimer [4,5] was spectroscopically resolved within the time frame of the fast process. Its structure and electronic state were confirmed by EPR and XAS. As reported earlier, the [3,4]′ intermediate likely results from reaction of [4,5] with water. While it is generated under strongly oxidizing conditions, it does not display oxidation of the Ru centers past [3,4] according to EPR and XAS. EXAFS analysis demonstrates a considerably modified ligand environment in [3,4]′. Raman measurements confirmed the presence of the O–O fragment by detecting a new vibration band in [3,4]′ that undergoes a 46 cm^–1 shift to lower energy upon ¹⁶O/¹⁸O exchange. Under the conditions of the experiment at pH 1, the [3,4]′ intermediate is the catalytic steady state form of the blue dimer catalyst, suggesting that its oxidation is the rate-limiting step