OSMIUM(IV) COMPLEXES WITH 2,3-DIAMINO-2,3-DIMETHYLBUTANE (TMEN) - SYNTHESES, PROPERTIES, AND STRUCTURES

(NH4)2OsBr6 reacts smoothly with an excess of 2,3-diamino-2,3-dimethylbutane (tmen) in water to give a brown Os(IV) tris-chelate complex which has lost two protons from two different ligands, [Os(tmen-H)2tmen]2+. The same brown complex is also obtained when OSCl3-3H2O reacts with an excess of tmen in ethanol. In 6 M trifluoromethanesulfonic acid (Htfms), the brown complex accepts one proton and the monodeprotonated pale green [Os(tmen-H)(tmen)2]3+ complex can be isolated. Its pK(a) is 0.04. The pH dependence of the reduction potential of [Os(tmen-H)2tmen]2+ determined by cyclic voltammetry shows that the reduction is accompanied by a gain of two protons, forming the fully protonated [Os(tmen)3]2+ complex. In aerated aqueous acid, the [Os(tmen-H)2tmen]2+ complex can be reduced using granulated Zn to give the fully protonated [Os(tmen)3]3+ complex. The reduction potential of [Os(tmen)3]3+/2+ occurs at -0.35 V (vs NHE). The solution structures of [Os(tmen-H)2tmen]2+ and [Os(tmen-H)(tmen)2]3+ have been determined by high-resolution H-1 and C-13 1D- and 2D-NMR spectroscopy in CD3CN. Both ions show highly deshielded NH protons (delta = 13 and 30 ppm, respectively, relative to TMS) and slow exchange of NH and NH2 protons. In [OS(tmen-H)2tmen]2+, the two NH groups are in a cis arrangement both in solution and in the solid state, as shown by an X-ray structure determination at 100 K. Its toluenesulfonate salt crystallizes in the tetragonal space group P42(1)cBAR with a = b = 13.310(2) angstrom, c = 23.607(4) angstrom, and Z = 4. The structure was refined to R = 0.037 (R(w) = 0.046) for 1528 unique reflections with F(o) > 6-sigma(F(o)). Average Os-N distances are 2.15 angstrom (-NH2) and 1.88 angstrom(-NH). The chemical shifts of the NH protons and the Os=NH distances are both indicative of strong Os-N r-bonding.