Tris(1,2-ethanediamine) complexes of osmium(IV), osmium(III) and osmium(II): oxidative dehydrogenation reactions

The reaction of [NH4]2[OsBr6] with neat 1,2-ethanediamine (en) yields cis-[Os(en-H)2(en)]Br2, which is recrystallized as cis-[Os(en-H)2(en)]I·Br in 40–50% yield. [Os(en)3]Br3·2H2O is obtained from the filtrate in ∼ 40% yield by the addition of EtOH, to give a recovery of ∼ 90% of osmium 1,2-ethanediamine complexes. This cis[Os(en-H)2(en)]2+ complex protonates to form [Os(en-H)(en)2]3+ and both OsIV complexes have been characterized by 1H and 13C NMR, IR and UV-Vis spectroscopies. Since the NH exchange rate for the doubly deprotonated OsIV complex is slow at the amido sites their cis relationship is maintained. The OsIV complexes are reduced readily to [Os(en)3]3+ by a variety of reductants. A reversible OsIII/II couple is observed at −0.52 V versus NHE in 0.1 M NaCF3SO3. All of these complexes are oxidized in air to form [Os(en)2(diim)]2+ (diim=ethanediimine) and [Os(en)(diim)2]2+ as the eventual product. The latter were characterized by 1H and 13C NMR, UV-Vis and IR spectroscopy. The mechanism of the dehydrogenation reaction is discussed in detail, in relation to related Fe and Ru amine chemistry.