Electrochemical and Resonance Raman Spectroscopic Studies of Water-Oxidizing Ruthenium-Terpyridyl-Bipyridyl Complexes

Abstract

<p> The irreversible conversion of single-site water oxidation catalysts (WOC) into the more rugged catalysts structurally related to [(trpy)(5,5&rsquo;-X₂-bpy)Ru^IV(&mu;-O)Ru^IV(trpy)(O)(H₂O)]⁴⁺ (X = H, <strong>1-dn⁴⁺</strong>; X = F, <strong>2-dn⁴⁺</strong>) represents a critical issue in developing active and durable WOC. In this work, the electrochemical and acid-base properties of <strong>1-dn⁴⁺</strong> and <strong>2-dn⁴⁺</strong> were evaluated. <em>In-situ</em> resonance Raman spectroscopy was employed to characterize species formed upon stoichiometric oxidation of single-site catalysts demonstrating the formation of high oxidation states mononuclear Ru=O and RuO-O complexes. Under turnover conditions, the dinuclear intermediates, <strong>1-dn⁴⁺</strong>and <strong>2-dn⁴⁺</strong>, as well as the previously proposed [Ru^VI(trpy)(O)₂(H₂O)]²⁺ complex (<strong>3²⁺</strong>) are formed. <strong>3²⁺</strong> is a pivotal intermediate that provides access to the formation of dinuclear species. Single crystal X-ray diffraction analysis of the isolated complex [Ru^IV(O)(trpy)(5,5&rsquo;-F₂-bpy)]²⁺ reveals a clear elongation of the Ru-N bond located in the <em>trans</em> position to the oxo group, documenting the weakness of this bond which promotes the release of the bpy ligand and the subsequent formation of <strong>3²⁺</strong>.</p> <p> &nbsp;</p>