Electronic Structure Assessment: Combined Density Functional Theory Calculations and Ru L2,3-Edge X‑ray Absorption Near-Edge Spectroscopy of Water Oxidation Catalyst

Density functional theory (DFT) is now widely used for analysis of the electronic structure and reactivities of transition metal complexes. However, large variability in how well different combinations of exchange-correlation potentials/basis sets reproduce real molecular geometries and electronic configurations remains a problem. Experimental X-ray absorption near-edge structure (XANES) spectra directly reflect the electronic structure of transition metal complexes. Combined analysis of theoretical calculations and experimental data is highly beneficial for DFT validation as well as for understanding limitations of the DFT. Ruthenium-based molecular water oxidation catalyst cis,cis-[(bpy)2(H2O)­RuIIIORuIVO­(OH)­(bpy)2]4+ is a complex coordination compound with two Ru centers in different oxidation states bound by μ-oxo bridge. Multiple DFT calculations of this catalyst in different oxidation states have been reported previously but it was never clear whether DFT is truly capable of describing its geometry and electronic structure. We tested a variety of DFT potentials with two relativistic models for electronic structure calculations and simulated experimental Ru L2,3-edges XANES spectra. We found that the local density approximation (LDA) exchange-correlation potential reproduces the experimental geometry and XANES spectra, while more advanced potentials fail. We also report combined DFT calculations/Ru L2,3-edges XANES spectra for three protonation forms: cis,cis-[(bpy)2(H2O)­RuIIIORuIVO­(H2O)­(bpy)2]5+, cis,cis-[(bpy)2(H2O)­RuIIIORuIVO­(OH)­(bpy)2]4+, and cis,cis-[(bpy)2(OH)­RuIIIORuIVO­(OH)­(bpy)2]3+. We found that selected DFT technique captures well small modifications of the electronic structure caused by changes in the protonation state of the ligand. Small differences in the Ru L-edges XANES for various protonation states are reflected in the corresponding theoretical spectra. We also observed that overall DFT-based XANES modeling is sensitive to various subtle changes in molecular geometries of Ru complex.