Five-coordinate pentafluorobenzothiolate osmium(IV) complexes [Os(SC 6F 5) 4(P(C 6H 4X-4) 3)] (X = OCH 3, CH 3, F, Cl or CF 3): Solid and solution structural characterization

Abstract

The reactions of OsO 4 with excess of HSC 6F 5 and P(C 6H 4X-4) 3 in ethanol afford the five-coordinate compounds [Os(SC 6F 5) 4(P(C 6H 4X-4) 3)] where X = OCH 3 1a and 1b, CH 3 2a and 2b, F 3a and 3b, Cl 4a and 4b or CF 3 5a and 5b. Single crystal X-ray diffraction studies of 1 to 5 exhibit a common pattern with an osmium center in a trigonal-bipyramidal coordination arrangement. The axial positions are occupied by mutually trans thiolate and phosphane ligands, while the remaining three equatorial positions are occupied by three thiolate ligands. The three pentafluorophenyl rings of the equatorial ligands are directed upwards, away from the axial phosphane ligand in the arrangement “3-up” (isomers a). On the other hand, 31P{ 1H} and 19F NMR studies at room temperature reveal the presence of two isomers in solution: The “3-up” isomer ( a) with the three C 6F 5-rings of the equatorial ligands directed towards the axial thiolate ligand, and the “2-up, 1-down” isomer ( b) with two C 6F 5-rings of the equatorial ligands directed towards the axial thiolate and the C 6F 5-ring of the third equatorial ligand directed towards the axial phosphane. Bidimensional 19F– 19F NMR studies encompass the two sub-spectra for the isomers a (“3-up”) and b (“2-up, 1-down”). Variable temperature 19F NMR experiments showed that these isomers are fluxional. Thus, the 19F NMR sub-spectra for the “2-up, 1-down” isomers ( b) at room temperature indicate that the two S-C 6F 5 ligands in the 2-up equatorial positions have restricted rotation about their C–S bonds, but this rotation becomes free as the temperature increases. Room temperature 19F NMR spectra of 3 and 5 also indicate restricted rotation around the Os–P bonds in the “2-up, 1-down” isomers ( b). In addition, as the temperature increases, the 19F NMR spectra tend to be consistent with an increased rate of the isomeric exchange. Variable temperature 31P{ 1H} NMR studies also confirm that, as the temperature is increased, the a and b isomeric exchange becomes fast on the NMR time scale.