Computational Analysis of Structure–Activity Relationships in Highly Active Homogeneous Ruthenium−Based Water Oxidation Catalysts

Abstract

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 thirteen homogeneous Ru−based catalysts—some, the most active catalysts studied: the Ru(tpy−R)(QC) and Ru(tpy−R)(4−pic)2 complexes, where tpy is 2,2’;6’,2”terpyridine, QC is 8−quinolinecarboxylate and 4−pic is 4−picoline. Typical relationships studied among heterogenous catalysts cannot be applied to homogeneous catalysts. The selected group of structurally similar catalysts with impressive catalytic activity deserves closer computational and statistical analysis of multiple reaction step 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 ideal relative energies of the RuIV = O and RuIV−OH intermediates and optimal changes in free energies of water nucleophilic attack on RuV = O. A narrow range of RuIV−OH to RuV = O redox potentials corresponding with the highest catalytic activities suggests facile access to the catalytically competent high−valent RuV = O state, often inaccessible from RuIV = O. Our work incorporates 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 to future rational design.