Reactions of dimesityldioxoosmium(VI) with bidentate N-heterocycles. Crystal structure of a dioxoosmium(VI) complex containing a molecular square

The interaction of OsO2(mes)(2) (mes=mesityl) with pyz (pyrazine) gave the pyrazine-bridged dimer [OsO2(mes)(2)](2)(mu-pyz) (1), which has been characterized by X-ray crystallography. The structure around Os in 1 is square pyramidal, with the pyrazine, two oxo ligands, and one mesityl group in the square plane. The Os-O distance and O-Os-O angle are 1.71 Angstrom and 147.5 degrees, respectively. The reaction of OsO2(mes)(2) with 4,4'-bpy (4,4'-bipyridyl) and bpe (trans-1,2-bis(4-pyridyl)ethylene) in Et(2)O gave the oligomeric [OsO2(mes)(2)L](n) (L=4,4'-bpy (2), bpe (3)) in good yields. The H-1 NMR spectra of 2 and 3 indicate that L's are coordinated in a symmetric fashion in these oligomers. The reaction of OsO2(mes)(2) with 4,4'-bpy in CHCl3 followed by slow evaporation afforded the crystalline tetramer [OsO2-(mes)(2)(mu-4,4'-bpy)](4) (4). The crystal structure of the tetramer 4 contains the square-planar [Os(4,4'-bpy)](4) core with octahedral local geometry around each Os. The Os-O distance and O-Os-O angle are 1.6 Angstrom and 159 degrees, respectively. Treatment of OsO2(mes)(2) with CNpy (4-cyanopyridine) gave [OsO2(mes)(2)].[OsO2(mes)(2)(CNpy)] (5). The structure of 5 consists of the four-coordinate [OsO2(mes)(2)] and five-coordinate [OsO2(mes)(2)(CNpy)] moieties, which are linked together via the Os=O ... Os interaction. The Os-O distances in the [OsO2(mes)(2)] and [OsO2(mes)(2)(CNpy)] moieties in 5 are 1.69 and 1.71 Angstrom, respectively. The (Os)O ... Os distance is 2.78 Angstrom, and the Os-O ... Os angle is 170 degrees. The Os-O stretching frequency for the above dimesityldioxoosmium(VI) complexes correlates well with the Os-O bond order and the coordination environment around Os.