Unexpected ligand lability in condition of water oxidation catalysis

Abstract

a b s t r a c tIn the search for rational design of improved water oxidation catalysts, enhanced catalytic activities were reported for single site Ru catalysis with Ru-iodide coordination. As these complexes are not initially capable of proton coupled electron transfer (PCET) and Ru@O formation, a proposal was put forward on the generation of catalytically active 7-coordinate Ru species. We tested this hypothesis by EPR and X-ray spectroscopy and found that [Ru II (bpy)(tpy)I] + only serves as a precursor for the formation of [Ru IV (bpy)(tpy)@O] 2+ . Upon oxidation with excess of Ce IV the RuAI bond quickly dissociates with the formation of [Ru III (bpy)(tpy)H 2 O] 3+ and [Ru IV (bpy)(tpy)@O] 2+ complexes. The catalytic steady state was composed of 95% [Ru IV (bpy)(tpy)@O] 2+ species. Thus, introducing the RuAI bond into initial catalysts does not serve to improve catalyst design. This manuscript also shows how EXAFS can directly probe transition metal-halogen interaction for in situ catalysis.

Effect of Quinoline Substitution on Water Oxidation by [Ru(Ql-tpy)(bpy)(OH2 )](PF6 )2

“…This is a clear indication of the turnover limiting step, which is due to the O−O bond formation, since it consumes time for the formation of [Ru III −O−OH] species by means of Ce IV and decomposes afterwards once the concentration of Ce IV decreases in the system. Considering the reaction mechanism proceeds through the generalized sequences for WNA mechanism (Where WNA is meant for Water Nucleophilic attack), such as [Ru II −OH 2 ]→[Ru III −OH]→[Ru IV =O]→[Ru V =O]→[Ru III −O−OH]→[Ru IV −O−O]→[Ru II −OH 2 ] . ReactLab Kinetics version 1.1 was used to find out the intermediate species distribution and their corresponding spectra utilizing the SVD analysis.The analysis of the spectra were considered for three consecutive 1 st order reactions of A→B, B→C, C→D after addition of 4 equivalent of Ce IV , wherein A=[Ru V =O], B=[Ru III −O−OH], C=[Ru IV −O−O], D=[Ru II (OH 2 )].…”

Towards the rational design of the Py5-ligand framework for ruthenium-based water oxidation catalysts

“…This is rather counterintuitive since for similar ruthenium based WOCs, halides such as iodo ligands were found to be especially labile towards ligand exchange reactions. 35 Note, Py5 bearing WOCs do require a halide as spectator ligand, while for many other WOCs the halides are mere placeholders for the substrate to coordinate the metal center. The experimental rates for the chlorido-water exchange were found to be one order of magnitude larger for Py5Me compared to Py5OMe.…”

Mechanistic Analysis of Water Oxidation Catalyst cis-[Ru(bpy)2(H2O)2]2+: Effect of Dimerization

“…Highly oxidized intermediates (containing Ru IV =O fragment) are produced via proton-coupled electron transfer (PCET) which requires the presence of a ligand that can be deprotonated (usually water). [25,27]. This implies at least one Ru-H 2 O fragment is required in order for the complex to be an active catalyst.…”