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Synthesis, Protonation, and Reduction of Ruthenium–Peroxo Complexes with Pendent Nitrogen Bases

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posted on 15.10.2012, 00:00 authored by Tristan A. Tronic, Werner Kaminsky, Michael K. Coggins, James M. Mayer
Cyclopentadienyl and pentamethylcyclopentadienyl ruthenium­(II) complexes have been synthesized with cyclic (RPCH2NR′CH2)2 ligands, with the goal of using these [CpR′′Ru­(PR2NR′2)]+ complexes for catalytic O2 reduction to H2O (R = t-butyl, phenyl; R′ = benzyl, phenyl; R″ = methyl, H). In each compound, the Ru is coordinated to the two phosphines, positioning the amines of the ligand in the second coordination sphere where they may act as proton relays to a bound dioxygen ligand. The phosphine, amine, and cyclopentadienyl substituents have been systematically varied in order to understand the effects of each of these parameters on the properties of the complexes. These CpR″Ru­(PR2NR′2)+ complexes react with O2 to form η2-peroxo complexes, which have been characterized by NMR, IR, and X-ray crystallography. The peak reduction potentials of the O2 ligated complexes have been shown by cyclic voltammetry to vary as much as 0.1 V upon varying the phosphine and amine. In the presence of acid, protonation of these complexes occurs at the pendent amine, forming a hydrogen bond between the protonated amine and the bound O2. The ruthenium–peroxo complexes decompose upon reduction, precluding catalytic O2 reduction. The irreversible reduction potentials of the protonated O2 complexes depend on the basicity of the pendent amine, giving insight into the role of the proton relay in facilitating reduction.