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

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.