Thermally Activated Site Exchange and Quantum Exchange Coupling Processes in Unsymmetrical Trihydride Osmium Compounds

Reaction of the hexahydride complex OsH6(PiPr3)2 (1) with pyridine-2-thiol leads to the trihydride derivative
OsH3{κ-N,κ-S-(2-Spy)}(PiPr3)2 (2). The structure of 2 has been determined by X-ray diffraction. The geometry
around the osmium atom can be described as a distorted pentagonal bipyramid with the phosphine ligands occupying
axial positions. The equatorial plane contains the pyridine-2-thiolato group, attached through a bite angle of 65.7(1)°,
and the three hydride ligands. The theoretical structure determination of the model complex OsH3{κ-N,κ-S-(2-Spy)}(PH3)2 (2a) reveals that the hydride ligands form a triangle with sides of 1.623, 1.714, and 2.873 Å,
respectively. A topological analysis of the electron density of 2a indicates that there is no significant electron
density connecting the hydrogen atoms of the OsH3 unit. In solution, the hydride ligands of 2 undergo two different
thermally activated site exchange processes, which involve the central hydride with each hydride ligand situated
close to the donor atoms of the chelate group. The activation barriers of both processes are similar. Theoretical
calculations suggest that the transition states have a cis-hydride−dihydrogen nature. In addition to the thermally
activated exchange processes, complex 2 shows quantum exchange coupling between the central hydride and the
one situated close to the sulfur atom of the pyridine-2-thiolato group. The reactions of 1 with l-valine and
2-hydroxypyridine afford OsH3{κ-N,κ-O-OC(O)CH[CH(CH3)2]NH2}(PiPr3)2 (3) and OsH3{κ-N,κ-O-(2-Opy)}(PiPr3)2 (4) respectively, which according to their spectroscopic data have a similar structure to that of 2. In
solution, the hydride ligands of 3 and 4 also undergo two different thermally activated site exchange processes.
However, they do not show quantum exchange coupling. The tetranuclear complexes [(PiPr3)2H3Os(μ-biim)M(TFB)]2 [M = Rh (5), Ir (6); H2biim = 2, 2‘-biimidazole; TFB = tetrafluorobenzobarrelene] have been prepared
by reaction of OsH3(Hbiim)(PiPr3)2 with the dimers [M(μ-OMe)(TFB)]2 (M = Rh, Ir). In solution the hydride
ligands of these complexes, which form two chemically equivalent unsymmetrical OsH3 units, undergo two
thermally activated site exchanges and show two different quantum exchange coupling processes.