2 research outputs found
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
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
X‑ray Emission Spectroscopy of Mn Coordination Complexes Toward Interpreting the Electronic Structure of the Oxygen-Evolving Complex of Photosystem II
X-ray emission (XES) spectroscopy
is an attractive technique for
analysis of the electronic structure of molecules, materials, and
metalloproteins. However, a better understanding of XES results is
required. Using a combination of experiment and ground-state density
functional theory analysis, we rationalize differences in the X-ray
emission spectra of multinuclear Mn complexes. Model compounds, including
dinuclear [Mn<sub>2</sub>O<sub>2</sub>L′<sub>4</sub>](ClO<sub>4</sub>)<sub>3</sub> (L′= 2,2′-bipyridyl, [<b>1</b>]) and two examples from the Mn<sub>4</sub>O<sub>4</sub>L<sub>6</sub> “cubane” family of model compounds (L = (p-R-C<sub>6</sub>H<sub>4</sub>)PO<sub>2</sub><sup>−</sup>, R = OCH<sub>3</sub> [<b>2</b>], CH<sub>3</sub> [<b>3</b>] ), were
compared with the Oxygen Evolving Complex of Photosystem II. Our analysis
shows that changes in the structure of the Mn complexes, resulting
in changes to the spin polarization, can introduce significant spectral
shifts in compounds of the same formal redox state. The implications
of changes in spin polarization for understanding photosynthetic water-splitting
catalysis is discussed