Oxidation of ammonia in osmium polypyridyl complexes

The oxidations of cis- and trans-OsIII(tpy)(Cl)2(NH3)](PF6), cis-OsII(bpy)2(Cl)(NH3)](PF6), and OsII(typ)(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 pH is increased, oxidation and proton loss from coordinated ammonia occurs. cis- and trans-OsIII(tpy)(Cl)2(NH3)](PF6) are oxidized by four electrons to give the corresponding OsVI nitrido complexes, OSVI(typ)(Cl)2(N)]+. Oxidation of Os(typ)(bpy)(NH3)](PF6)2 occurs by six electrons to give Os(tpy)(bpy)(NO)](PF6)3. Oxidation of cis-OsII(bpy)2(Cl)(NH3)](PF6) at pH 9.0 gives cis-OsII(bpy)2(Cl)(NO)](PF6)2 and the mixed-valence form of the mu-N2 dimer cis-Os(bpy)2(Cl)2mu-N2)](PF6)3. With NH4+ added to the electrolyte, cis-OsII(bpy)2(Cl)(N2)](PF6) is a coproduct. The results of pH-dependent cyclic voltammetry measurements suggest OsIV as a common intermediate in the oxidation of coordinated ammonia. For cis- and trans-OsIII(tpy)(Cl)2(NH3)]+, OsIV is a discernible intermediate. It undergoes further pH-dependent oxidation to OsVI(tpy)(Cl)2(N)]+. For OsII(tpy)(bpy)(NH3)]2+, oxidation to OsIV is followed by hydration at the nitrogen atom and further oxidation to nitrosyl. For cis-OsII(bpy)2(Cl)-(NH3)]+, oxidation to OsIV is followed by N-N coupling and further oxidation to cis-Os(bpy)2(Cl)2(mu-N2)]3+. At pH 9, N-N coupling is competitive with capture of OsIV by OH- and further oxidation, yielding cis-OsII(bpy)2(Cl)(NO)]2+.