Spectroscopic
Analysis of Catalytic Water Oxidation
by [Ru<sup>II</sup>(bpy)(tpy)H<sub>2</sub>O]<sup>2+</sup> Suggests
That Ru<sup>V</sup>O Is Not a Rate-Limiting Intermediate
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Abstract
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<sup>II</sup>(bpy)(tpy)H<sub>2</sub>O]<sup>2+</sup> (where bpy
= 2,2′-bipyridine, tpy = 2,2′;6′,2″-terpyridine).
Our results reveal that the [Ru<sup>V</sup>(bpy)(tpy)O]<sup>3+</sup> 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<sup>IV</sup>(bpy)(tpy)O]<sup>2+</sup>. [Ru<sup>V</sup>(bpy)(tpy)O]<sup>3+</sup> 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<sup>III</sup>-peroxo species with (−OOH or −OO–
ligands). We also analyzed the [Ru<sup>II</sup>(bpy)(tpy)Cl]<sup>+</sup> catalyst precursor and confirmed that this molecule is not a catalyst
and its oxidation past Ru<sup>III</sup> 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–H<sub>2</sub>O fragment. [Ru<sup>II</sup>(bpy)(tpy)H<sub>2</sub>O]<sup>2+</sup> 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<sup>IV</sup>(bpy)(tpy)O]<sup>2+</sup> intermediate
unless more electronegative (oxygen containing) ligands are introduced
in the Ru coordination sphere, allowing the formation of more oxidized
Ru<sup>V</sup> intermediate