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

Abstract

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, {RuIV2(μ-O)2}, incorporating ethylbis(2-pyridylmethyl)amine (ebpma) [{RuIV(ebpma)}2(μ-O)2(μ-O2L)](ClO4)m (LO2x- = CH3CO2- (acetato); [1(IV,IV)](ClO4)3, HPO42- (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 (LO2x- = CO32-; [(CO3)(IV,IV)]2+, HCO3-; [(HCO3)(IV,IV)]3+, and SO42-; [(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 {RuIV2(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 λmax 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.