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The Os(III) complexes [OsIII(tpy)(bpy)(py)]3+ and [OsIII(bpy)3]3+ (bpy: 2,2′-bipyridine, py: pyridine, tpy: 2,2′,2′′-terpyridine) are unstable toward self-reduction in alkaline solution. Product analysis performed by using UV-Vis spectroscopy, cyclic voltammetry, HPLC and GC show that ca. 90% of the reduced product that appears is the unmodified polypyridyl complex of Os(II), [OsII(tpy)(bpy)(py)]2+ or [OsII(bpy)3]2+. The remaining 10% of the products are accounted for by other Os(II) complexes, including several which appear to have undergone varying degrees of ligand oxidation or substitution. Another product of the reduction of [OsIII(tpy)(bpy)(py)]3+ in basic solution has been identified as [OsII(tpy)(bpy)(OH2)]2+ which results from metal reduction and concomitant pyridine ligand loss. No O2 was detected as a product of the base-catalyzed self-reduction reaction. The Os(III) complexes are stable towards self-reduction and ligand loss in the dark in acetonitrile or aqueous solutions buffered below pH=6. The rates observed for the self-reduction processes are sufficiently rapid at pH values above 7 that useful catalytic rate data cannot be obtained in basic solution unless the substrate of interest is sufficiently reactive and is present in large excess. Triple mixing, pH jump techniques have successfully been employed to observe reaction rates under strongly alkaline conditions. |
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