Computational Analysis of Structure – Activity Relationships in Highly Active Homogeneous Ruthenium-based Water Oxidation Catalysts

Linear free energy scaling relationships (LFESRs) and regression analysis may predict the catalytic performance of heterogeneous and recently, homogenous water oxidation catalysts (WOCs). This study analyses twelve homogeneous Ru-based catalysts – some, the most active catalysts studied: the Ru(tpy-R)(QC) and Ru(tpy-R)(4-pic)(2) catalysts, where tpy is 2,2:6,2-terpyridine, QC is 8-quinolinecarboxylate and 4-pic is 4-picoline. Typical relationships studied among heterogenous and solid-state catalysts cannot be broadly applied to homogeneous catalysts. This subset of structurally similar catalysts with impressive catalytic activity deserves closer computational and statistical analysis of energetics correlating with measured catalytic activity. We report general methods of LFESR analysis yield insufficiently robust relationships between descriptor variables. However, volcano plot-based analysis grounded in Sabatier’s principle reveals ranges of ideal relative energies of the Ru(IV)=O and Ru(IV)-OH intermediates and optimal changes in free energies of water nucleophilic attack on Ru(V)=O. A narrow range of Ru(IV)-OH to Ru(V)=O redox potentials corresponding with the highest catalytic activities suggests facile access to the catalytically competent high-valent Ru(V)=O state, often inaccessible from Ru(IV)=O. Our work introduces experimental oxygen evolution rates into approaches of LFESR and Sabatier principle-based analysis, identifying a narrow yet fertile energetic landscape to bountiful oxygen-evolution activity, leading future rational design.