Oxidation of ammonia in osmium polypyridyl complexes

The oxidations of cis- and trans-[Os-III(tpy) (Cl)(2)(NH3)](PF6), cis-[Os-II(bpy)(2)(Cl)(NH3)](PF6), and [Os-II(tpy)(bpy)-(NH3)](PF6)(2) have been studied by cyclic voltammetry and by controlled-potential electrolysis. In acetonitrile or in acidic, aqueous solution, oxidation is metal-based and reversible, but as the DH is increased, oxidation and proton loss from coordinated ammonia occurs, cis- and trans-[Os-III(tpy)(Cl)(2)(NH3)](PF6) are oxidized by four electrons to sive the corresponding Os-VI nitrido complexes, [Os-VI(tpy)(Cl)(2)(N)](+). Oxidation of [Os(tpy)(bpy)-(NH3)](PF6)(2) occurs by six electrons to give [Os(tpy)(bpy)(NO)](PF6)(3). Oxidation of cis-[Os-II(bpy)(2)(Cl)(NH3)]-(PF6) at pH 9.0 gives cis-[Os-II(bpy)(2)(Cl)(NO)](PF6)(2) and the mixed-valence form of the mu-N-2 dimer cis[Os-II(bpy)(2)(Cl)(NH3)]- [Os(bpy)(2)(Cl)](2)(mu-N-2)}(PF6)(3) With NH4+ added to the electrolyte, cis-[Os-II(bpy)(2)(Cl)(N-2)](PF6) is a coproduct. The results of pi-I-dependent cyclic voltammetry measurements suggest Os-IV as a common intermediate in the oxidation of coordinated ammonia. For cis- and trans-[Os-III(tpy)(Cl)(2)(NH3)](+), Os-IV is a discernible intermediate. It undergoes further pH-dependent oxidation to [Os-VI(tpy)(Cl)(2)(N)](+). For [Os-II(tpy)(bpy)(NH3)](2+), oxidation to Os-IV is followed by hydration at the nitrogen atom and further oxidation to nitrosyl. For cis-[Os-II(bpy)(2)(Cl)-(NH3)](+), oxidation to Os-IV is followed by N-N coupling and further oxidation to {cis-[Os(bpy)(2)(Cl)](2)(mu-N-2)(3+) At pH 9, N-N coupling is competitive with capture of Os-IV by OH- and further oxidation, yielding cis-[Os-II- (bpy)(2)(Cl)(NO)](2+).