Aqueous Synthesis of Strontium Ruthenate(VI) Oxyhydroxides and Their Crystal Structure Solution from Microcrystals

Investigation of the earliest stages of hydrothermal reactions between KRuO₄ and either Sr(NO₃)₂ or SrO₂ allows the isolation of phase-pure SrRuO₃(OH)₂, a hitherto uncharacterized Ru(VI) oxyhydroxide, previously mentioned in the literature as SrRuO₄·H₂O. Further exploration of synthesis conditions with excess of strontium salt yields a second Ru(VI) phase, Sr₃Ru₂O₈(OH)₂ at 200 °C. Both structures were solved by 3D-electron diffraction and refined using powder neutron diffraction (SrRuO₃(OH)₂: monoclinic P2₁/n, a = 9.9903(3) Å, b = 7.7023(2) Å, c = 17.3677(6) Å, β = 89.353(2)°; Sr₃Ru₂O₈(OH)₂: tetragonal P4̅, a = 13.2206(5) Å c = 5.4852(2) Å). The two Ru(VI) materials each contain isolated trigonal bipyramidal ruthenium centers with axially positioned hydroxides with longer Ru-OH bond distances than the equatorial Ru-O distances. Infrared spectroscopy corroborates the presence of hydroxide, with O-H stretching bands that are red-shifted upon deuteration. X-ray absorption near-edge spectroscopy at the Ru K-edge confirms the +6 oxidation state of Ru, and magnetization measurements show two unpaired electrons per Ru associated with the 4d² electronic configuration in both materials. The crystal structures, particularly the local environment of Ru, are compared with other reported Ru(VI) compounds, and derived bond valence sum parameters give consistent results for ruthenate(VI) oxides and oxyhydroxides.