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

Abstract

The interaction of OsO2(mes)2 (mes = mesityl) with pyz (pyrazine) gave the pyrazinebridged dimer [OsO2(mes)2]2(μ-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 Å and 147.5°, respectively. The reaction of OsO2(mes)2 with 4,4′-bpy (4,4′-bipyridyl) and bpe (trans-1,2-bis(4-pyridyl)ethylene) in Et2O gave the oligomeric [OsO2(mes)2L]n (L = 4,4′-bpy (2), bpe (3)) in good yields. The 1H 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 CHCls followed by slow evaporation afforded the crystalline tetramer [OsO2-(mes)2(μ-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 Å and 159°, 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 Å, respectively. The (Os)O⋯Os distance is 2.78 Å, and the Os-O⋯Os angle is 170°. 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.