Oxidation of Ammonia in Osmium Polypyridyl Complexes

Abstract

The oxidations of cis- and trans-[Os^(III)(tpy)(Cl)_2(NH_3)](PF_6), cis-[Os^(II)(bpy)_2(Cl)(NH_3)](PF_6), and [Os^(II)(tpy)(bpy)(NH_3)](PF_6)_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 pH is increased, oxidation and proton loss from coordinated ammonia occurs. cis- and trans-[Os^(III)(tpy)(Cl)_2(NH_3)](PF_6) are oxidized by four electrons to give the corresponding Os^(VI) nitrido complexes, [Os^(VI)(tpy)(Cl)_2(N)]^+. Oxidation of [Os(tpy)(bpy)(NH_3)](PF_6)_2 occurs by six electrons to give [Os(tpy)(bpy)(NO)](PF_6)_3. Oxidation of cis-[Os^(II)(bpy)_2(Cl)(NH_3)](PF_6) at pH 9.0 gives cis-[Os^(II)(bpy)_2(Cl)(NO)](PF_6)_2 and the mixed-valence form of the μ-N_2 dimer {cis-[Os(bpy)_2(Cl)]_2(μ-N_2)}(PF_6)_3. With NH_4^+ added to the electrolyte, cis-[Os^(II)(bpy)_2(Cl)(N_2)](PF_6) is a coproduct. The results of pH-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(NH_3)]^+, Os^(IV) is a discernible intermediate. It undergoes further pH-dependent oxidation to [Os^(VI)(tpy)(Cl)_2(N)]^+. For [Os^(II)(tpy)(bpy)(NH_3)]_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)(NH_3)]^+, oxidation to Os^(IV) is followed by N−N coupling and further oxidation to {cis-[Os(bpy)_2(Cl)]_2(μ-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+).