Chemically Induced Cyclometalation of 2-(Arylazo)phenols. Synthesis, Characterization, and Redox Properties of a Family of Organoosmium Complexes

Reaction of 2-(arylazo)phenols (H2ap-R; R = OCH3, CH3, H, Cl, and NO2) with [Os(PPh3)2(CO)2(HCOO)2] affords a family of organometallic complexes of osmium(II) of type [Os(PPh3)2(CO)(ap-R)] where the 2-(arylazo)phenolate ligand is coordinated to the metal center as a tridentate C,N,O-donor. Structure of the [Os(PPh3)2(CO)(ap-H)] complex has been determined by X-ray crystallography. All the [Os(PPh3)2(CO)(ap-R)] complexes are diamagnetic and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. They also show emission in the visible region at ambient temperature. Cyclic voltammetry on the [Os(PPh3)2(CO)(ap-R)] complexes shows a reversible Os(II)−Os(III) oxidation within 0.39−0.73 V vs SCE, followed by a reversible Os(III)−Os(IV) oxidation within 1.06−1.61 V vs SCE. Coulometric oxidation of the [Os(PPh3)2(CO)(ap-R)] complexes generates the [OsIII(PPh3)2(CO)(ap-R)]+ complexes, which have been isolated as the hexafluorophosphate salts. The [OsIII(PPh3)2(CO)(ap-R)]PF6 complexes are one-electron paramagnetic and show axial ESR spectra. In solution they behave as 1:1 electrolytes and show intense LMCT transitions in the visible region. The [OsIII(PPh3)2(CO)(ap-R)]PF6 complexes have been observed to serve as mild one-electron oxidants in a nonaqueous medium.