Electronic Structure of the Water Oxidation Catalyst cis,cis-[(bpy)₂(H₂O)Ru^IIIORu^III(OH₂)(bpy)₂]⁴⁺, The Blue Dimer

Abstract

The first designed molecular catalyst for water oxidation is the "blue dimer", cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+). Although there is experimental evidence for extensive electronic coupling across the μ-oxo bridge, results of earlier DFT and CASSCF calculations provide a model with magnetic interactions of weak to moderately coupled Ru(III) ions across the μ-oxo bridge. We present the results of a comprehensive experimental investigation, combined with DFT calculations. The experiments demonstrate both that there is strong electronic coupling in the blue dimer and that its effects are profound. Experimental evidence has been obtained from molecular structures and key bond distances by XRD, electrochemically measured comproportionation constants for mixed-valence equilibria, temperature-dependent magnetism, chemical properties (solvent exchange, redox potentials, and pK(a) values), XPS binding energies, analysis of excitation-dependent resonance Raman profiles, and DFT analysis of electronic absorption spectra. The spectrum can be assigned based on a singlet ground state with specific hydrogen-bonding interactions with solvent molecules included. The results are in good agreement with available experimental data. The DFT analysis provides assignments for characteristic absorption bands in the near-IR and visible regions. Bridge-based dπ → dπ* and interconfiguration transitions at Ru(III) appear in the near-IR and MLCT and LMCT transitions in the visible. Reasonable values are also provided by DFT analysis for experimentally observed bond distances and redox potentials. The observed temperature-dependent magnetism of the blue dimer is consistent with a delocalized, diamagnetic singlet state (dπ(1)*)(2) with a low-lying, paramagnetic triplet state (dπ(1)*)(1)(dπ(2)*)(1). Systematic structural-magnetic-IR correlations are observed between ν(sym)(RuORu) and ν(asym)(RuORu) vibrational energies and magnetic properties in a series of ruthenium-based, μ-oxo-bridged complexes. Consistent with the DFT electronic structure model, bending along the Ru-O-Ru axis arises from a Jahn-Teller distortion with ∠Ru-O-Ru dictated by the distortion and electron-electron repulsion.

Molecular Catalysts for Water Oxidation

“…Bending of the Ru−O−Ru unit was described to arise from a Jahn−Teller distortion. 159 The oxidation state changes leading to oxygen evolution have also been discussed extensively, and a 2004 article from Hurst 160 is notable for its insights. In concentrated triflic acid, electrochemical studies 161 show well-resolved features for the various oxidations of the complex from the (II,II) state up to the key (V,V) intermediate that gives rise to oxygen evolution.…”

Syntheses, structures and electrochemical properties of homoleptic ruthenium(III) and osmium(III) complexes bearing two tris(carbene)borate ligands

“…The binding energies of Ru 3d 5/2 and Os 4f 7/2 are 280.1 and 50.3 eV in complexes 1a and 1b, respectively. The binding energy of Ru 3d 5/2 in 1a (280.1 eV) is smaller than that of Ru(III) complex [Ru(bpy) 2 Cl]Cl (281.9 eV) [55], but very close to those of Ru(II) complexes for Cp 2 Ru (280.3, 280.7 eV) [50,51] and Cp ⁎ 2 Ru (279.7, 279.9 eV) [50,51]. Similarly, the binding energy of Os 4f 7/2 in 1b is close to those of Os(II) complexes Cp 2 Os (50.6 eV) and Cp ⁎ 2 Os (49.9 eV) [50].…”

A Self‐Improved Water‐Oxidation Catalyst: Is One Site Really Enough?

“…[13] DFT calculations carried out at the M06-L [14] level of theory (see the Supporting Information for details) to characterize the reaction intermediates as well as the transition states provided a complete catalytic cycle for the dinuclear 1 4+ complex and also for the mononuclear 4 2+ complex for comparative purposes (Scheme 1). [13] DFT calculations carried out at the M06-L [14] level of theory (see the Supporting Information for details) to characterize the reaction intermediates as well as the transition states provided a complete catalytic cycle for the dinuclear 1 4+ complex and also for the mononuclear 4 2+ complex for comparative purposes (Scheme 1).…”