Accessibility of Formally Six-Coordinate Ruthenium(IV) Complexes Generated by Electrochemical Oxidation of Ruthenium(II) Dimethylglyoxime and Related Complexes Containing Phosphorus, Nitrogen or Oxygen Donor Axial Ligands

Abstract

The chemistry of higher valent ruthenium(IV) complexes has attracted considerable attention because of its possible relevance in catalytic processes and the fact that analogous iron complexes may be biologically important. In this work a range of Ru^II (N₄)(X)(Y) complexes (N₄ = nitrogen-based macrocycle or related ligand ; X, Y = axial ligands) has been prepared. It is shown that the presence of macrocyclic type ligands and suitable axial ligands leads to readily accessible six coordinate formally ruthenium(IV) complexes on the voltammetric time scale as ascertained by studies at platinum, gold, glassy carbon and mercury electrodes. Thus, dimethylgloxime complexes Ru^II ( dmgH )₂(PPh₃)₂ and other complexes containing non- oxidizable macrocyclic type ligands readily undergo two chemically and electrochemically reversible one-electron oxidation processes to produce six coordinate ruthenium(III) and ruthenium(IV) complexes. The ruthenium(III) species are moderately stable on the synthetic time scale at ordinary temperature, whereas -78°C is required to generate low concentrations of the formally ruthenium(IV) complexes with a limited range of complexes. When the axial phosphine ligands are replaced by nitrogen and oxygen donors (N-methylimidazole , dimethyl sulfoxide , pyridine, etc.), the stability of the six-coordinate ruthenium(IV) complexes are significantly lowered as evidenced by the more complex voltammetry for the ruthenium(III)/(IV) oxidation process. Similarly, replacement of the (N₄) macrocyclic type system by (N-O)₂ (e.g. quinolin-8-ol as ligand ) lowers the stability of the ruthenium(IV) complexes. When the macrocyclic type ligand can itself be oxidized (e.g. benzoquinone dioxime as a ligand) four electrons can be transferred reversibly. In this case the oxidation processes are not readily assigned in terms of formal oxidation states although ruthenium(IV) still appears to be an accessible oxidation state. Electrochemical oxidation of the iron complexes, Fe( dmgH)2(P(Obu)₃)₂ and Fe(Pc)(P( OBu ),₃)₂ (Pc = phthalocyanine ), also produces two reversible one-electron oxidation processes, confirming that information derived from the more kinetically inert ruthenium systems may be applied to related iron systems on the voltammetric time scale and that macrocyclic type ligands stabilize both high oxidation state formally ruthenium(IV) and iron(IV) oxidation states.