Chemistry of ruthenium. 3. Synthesis, structure, and electron-transfer behavior of trans-dihalobis[(arylazo)oximato]ruthenium(III)

Article abstract of DOI:10.1021/ic50224a003

New haloruthenium(III) (arylazo)oximates of the type RuX₂(HL)(L) are described (X = Cl, Br; HL = RC(=NOH)-N=NAr). The RuX₂ moiety has trans configuration (IR data); the hydrogen-bonded organic part LHL acts essentially as a planar tetradentate ligand. In effect the coordination sphere is trans-RuN₄X₂. The complexes are low spin (t_2g⁵, S = 1/2) and display characteristic EPR spectra in the polycrystalline state at room temperature as well as in frozen benzene. The spectra are sensitive to the nature of R and Ar groups and can be nearly isotropic, axial, or rhombic. The complexes show two LMCT bands near 1000 and 580 nm. They undergo a reversible one-electron transfer at the platinum electrode attributable to the ruthenium(III)-ruthenium(II) couple (cyclic voltammetry and constant potential coulometry). The E°₂₉₈ of this couple is ～0.4 V vs. SCE in acetonitrile. A bromo complex is easier to reduce than the corresponding chloro complex. The interrelationship of E°₂₉₈ with LMCT band energy is noted. The green ruthenium(II) species RuX₂(HL)(L)^- has been generated in solution both electrochemically and chemically (reduction by hydroquinone). It has a characteristic MLCT band near 680 nm. Addition of base (NEt₃) deprotonates RuX₂(HL)(L) quantitatively to RuX₂(L)₂^- with concomitant loss of the electrochemical response which is fully reestablished on addition of acid (HClO₄).