Diruthenium(IV,IV) Complexes with {Ru₂(μ-O)₂} Diamond Core Bridged by Acetato or Hydrogenphosphato Ligands

The reaction centers of higher-valent metals with a doubly oxido-bridged core, referred to as the "diamond core {M₂(μ-O)₂}", have attracted significant attention primarily owing to their role in oxidation reactions. Acetato- or hydrogenphophato-bridged Ru(IV)-Ru(IV) complexes with the doubly oxido-bridged diamond core, {Ru^IV₂(μ-O)₂}, incorporating ethylbis(2-pyridylmethyl)amine (ebpma) [{Ru^IV(ebpma)}₂(μ-O)₂(μ-O₂L)](ClO₄)_m (LO₂^x- = CH₃CO₂^- (acetato); [1(IV,IV)](ClO₄)₃, HPO₄^2- (hydrogenphosphato); [2(IV,IV)](ClO₄)₂), were synthesized as structural models for diiron-containing soluble methane monooxygenase (sMMO). Systematic comparison of crystal structures, electrochemical and spectroscopic properties with analogous complexes in the same tridentate ligand (ebpma) system (LO₂^x- = CO₃^2-; [(CO₃)(IV,IV)]²⁺, HCO₃^-; [(HCO₃)(IV,IV)]³⁺, and SO₄^2-; [(SO₄)(IV,IV)]²⁺) led to obtain new and deeper insights into the electronic structures of the Ru(IV)-Ru(IV) dimers. The structure of the {Ru^IV₂(μ-O)₂} core is dominantly effected by the geometry of the bridging bidentate ligand, whereas the redox features were mainly effected by the electron-donating nature of the bridging bidentate ligands. Furthermore, correlation between the redox potentials and the λ_max of the {Ru₂(μ-O)₂}-{Ru₂(μ-O)₂}* transition band was demonstrated. Notably, the redox reactions of [2(IV,IV)]²⁺ were pH-dependent in water and could be rationalized by considering dimerization of the dimer, in part, through the bridging phosphato ligand; a tetramer formation through dimerization of [2(IV,IV)]²⁺ in water was also proposed. Kinetic and thermochemical analyses of the stoichiometric benzyl alcohol oxidation reactions with [1(IV,IV)]³⁺ suggested the C-H activation process as a PTET process.